Lighter, Stronger, Durable: Advances in Zn-Coated Sheet Steel
The International Conference on Zinc and Zinc Alloy Coated Steel Sheet (Galvatech) is a premier international conference on cutting-edge technologies for the processing and performance of coated steel sheets for automotive, electrical appliance and construction applications.
Deadline for uploading oral recorded presentation: May 20, 2021
Deadline for uploading poster: May 20, 2021
Last author registration: May 20, 2021
Considering our frame conditions for developing and producing high quality corrosion protected steels, we cannot oversee significant changes. Environmental consciousness has risen not only since the Fridays for Future movement. Resulting legal regulations may find an echo in the way we will produce steel in the future soon, reducing our carbon footprint. Obviously, environment protection will change transportation, too, including individual travel. Which consequences are to be expected for car production in terms of figures and design, and consequently, for steel application? Covid-19 is governing our present every day’s working conditions. Which reactions to these will we keep in the future? And eventually, what could be the most interesting product and process developments we are looking at in this conference?
Zn and Zn alloy coated steel sheets are widely used because of their good corrosion resistance. In
particular, galvannealed steel sheets are mostly used for automobile manufacturing in Japan because
of their good corrosion resistance after painting, weldability, and stampability. Furthermore, several
innovative high Mg added Zn alloy coatings with superior corrosion resistance have been originally
developed in Japan primarily for construction use. To clarify the mechanism of the excellent
properties of these coatings, a series of consortiums has been organized in Japan, wherein a lot of
innovative and epoch-making results were obtained.
Automotive industry requirements for steels capable of enabling increasingly fuel-efficient vehicles continue to drive process technology developments for North American hot-dip and electro-galvanizing lines. Third-generation zinc and zinc alloy-coated grades have reached commercialization; higher-capability grades continue to be developed. Zinc-coated hot press forming grades have reached commercialization, tailor-processed hot press formed grades permit increasingly efficient use of steel in automobiles. A key barrier to adoption of advanced zinc-coated steels are incidences of liquid metal embrittlement (LME) that is associated in many cases with higher levels of steel retained austenite or silicon alloying and ameliorative practices are under development. Quality and productivity benchmarks also continue to be pushed to ever-higher levels. Dual phase and multi-phase grades with expanded combinations of formability and strength continue to be developed. Such steel grades should satisfy automaker requirements and ensure the dominance of steel-based vehicle manufacturing for many years to come. The processing of these steels creates challenges for steel suppliers, and the approaches being taken to meet these challenges are described. These include reactive wetting, IR pyrometry during heat treating of advanced high strength steels (AHSS), hydrogen-induced delayed cracking and cold rolling harder substrates. A new census of North American production coating lines is provided. Challenges for the future include further improved productivity, quality and process efficiency, and flexibility in line scheduling to minimize costs.
With China's economic expansion having moderated to a "new normal" pace and manufacturing industry upgrading, the total amount of zinc and zinc alloy coated steel sheets in China has remained stable, but the demand for high-end zinc and zinc alloy coated steel sheets has gradually increased. This paper gives overviews of development and application of zinc and zinc alloy coated steel sheets in China in recent years, and briefly introduces recent progress in galvanized advanced high strength steels (AHSS), galvanized 3rd generation automobile steels. Developments of Zn-Al-Mg coatings, continuous PVD coatings, functional coatings and environmentally friendly post-treatments are also described. The paper finally discusses the development trend of zinc and zinc-alloy coated steel sheet in China.
Hot dipped zinc coated material was introduced for corrosion protection with Volvo Cars 240 series in the early 1980. Including the use of conventional hot-dipped galvanized material (GI), electro-zinc coated material (EG) and galvannealed (GA) experience of coated sheet material for automotive use has been collected over the past 40 years.
Lately there has been new additional coatings added with AlSi for hot-formed material and recently also ZM (Zink Magnesium).
This presentation will give you an overview of the coating use at Volvo Cars from the introduction to the current status and also an outlook for the future.
In recent years, steel industries have been various studies to apply ZnMgAl coatings to automobiles. Initially, the solidification experiment of ZnMgAl coating on steel was investigated by using in-situ real time x-ray scattering, we confirmed that the solidification prceeds in the order of Zn, Zn/MgZn2 binary eutectic and Zn/MgZn2/Al ternary eutectic phases. Also, we evaluated the properties such as wettability, surface roughness, fluidity and corrosion resistance according to Mg and Al with varying contents. The difference in corrosion products between the general hot-dip galvanizing(GI) and ZnMgAl coating was confirmed in detail. In the case of ZnMgAl coating, the dense corrosion products were formed on surface, thus the corrosion resistance was superior than GI material. In addition, the ZnMgAl coated steel sheet was excellent in formability due to its low coefficient of friction. Phosphate treatment, weldability and adhesion properties were similar to those of general hop-dip galvanized steel sheets to apply for automotive.
Zn alloy-coated steels such as ZnMg and ZnMgAl have been recognized as a potential automotive materials replacing traditional Zn-coated steels such as galvanized (GI) and galvannealed (GA) steel with enhanced corrosion resistance under harsh corrosive environment. There are two representative Zn alloy coatings, PosMAC®1.5 (ZnMgAl coating) and PosPVD® (ZnMg coating) for automotive application in POSCO steel products. These have been developed to meet the requirements for automotive parts for weldability, surface quality, phosphating and paintability, and corrosion resistance with low alloy contents in Zn-based coating. Because applications of newly developed steel product require careful consideration, POSCO makes efforts to obtain performance data from various corrosive conditions and find solutions for proper applications.
In this study, the anti-corrosion performance of PosMAC®1.5 and PosPVD® were investigated from the viewpoint of automotive body structures. Corrosion resistances of flat surface, bimetallic joint and lap-joint surface were compared with conventional Zn-coated steels by cyclic corrosion tests (CCTs) and proving ground (P/G) test. In the results of these tests, the Zn alloy-coated steels exhibited better corrosion resistance compared to that of GI and GA steels despite the lower coating weights and prove its potential as a high corrosion resistant automotive steels. Finally, the corrosion life of the coatings were quantitatively predicted based on the vehicle corrosion (P/G) test results.
In the recent years the Kelvin probe technique found a steadily and quickly increasing usage in corrosion research. The main application initially was for through-paint monitoring of corrosion driven delamination. Soon after that, with the development and availability of Scanning Kelvin Probe Microscopy (SKPFM), an Atomic Force Microscopy based Kelvin probe technique, followed the application of the technique for the screening of possible galvanic elements on alloy surfaces and more recently Kelvin probe techniques are applied for detection of hydrogen. In this paper an overview will be given about the potential of the Kelvin probe techniques for investigating corrosion performance and the hydrogen uptake and release characteristics of zinc and zinc alloy coatings and steel. It will be shown that Kelvin probe techniques, if applied in combination with an experimental test protocol that is tailored for the question to be addressed, can provide information that would be difficult to obtain with other techniques.
Under certain conditions of oxygen shortage and high temperatures as could occur after coiling a hot-rolled strip, alloying elements with high oxygen affinity can diffuse and oxidise preferably along the grain boundaries. This selective internal oxidation can be increased by additional heat induced from phase transformations in the coiled state and depends on steel composition. This diffusion to the grain boundaries and subsequent oxidation causes a depletion of solute alloying elements in a surface near area.
When oxides are present along grain boundaries, pickling can alter the surface topography by selective dissolution of these oxides on the µm-scale (µ-topography). This altered topography and the depletion of alloying elements influence diffusion properties during intercritical annealing prior hot dip galvanizing, leading to changed surface segregation of alloying elements.
This effect was exemplified by pickling and subsequently cold rolling a hot rolled high strength steel strip with and without internally oxidized grain boundaries, using an adapted process. Samples with and without altered µ-topography were galvanized in a hot-dip simulator with varying annealing parameters. The annealed and galvanized samples where characterized in terms of surface enrichment of alloying elements by GDOES, inhibition layer density by potentiostatic dissolution and SEM measurements, as well as crash relevant automotive zinc adhesion tests.
Surfaces with altered µ-topography show higher reactivity through lower surface segregation of alloying elements and can build more dense inhibition layers during hot-dip galvanising under identical annealing conditions. This effect could be helpful to enhance galvanisability of future difficult steel grades.
Houssem Eddine CHAIEB a,*, Vincent MAUREL a, Samuel FOREST a, Kais AMMAR a, Alain KÖSTER a, Franck NOZAHIC b, Joost DE STRYCKER b, Jean-Michel MATAIGNE c
a Le Centre des Matériaux, MINES ParisTech, 63-65 Rue Henri Auguste Desbruères, 91100 Corbeil-Essonnes, France
b ArcelorMittal Global R&D; Gent, J.F. Kennedylaan 3, B-9060 Zelzate, Belgium
c ArcelorMittal Global R&D; Maizières, Voie Romaine, Maizières-lès-Metz, France
Zinc based coatings deposited on steel by hot-dip galvanizing are well known to present a strong basal orientation. This has been highlighted in several previous works [1, 2]. This orientation tendency is less pronounced in Zn-5Al coating and orientation heterogeneities are witnessed. Understanding the origin of such heterogeneities and linking them to the solidification process requires the development of new guidelines.
Figure 1. SEM image of Zn-5Al coating
Figure 2. EBSD of Zn-5Al coating
An epitaxial relationship between certain zinc dendrites and binary eutectic appears to be present. In this study, we aim at describing the microstructure and the texture of the near-eutectic Zn-5Al hot-dip galvanized coating. Several experimental techniques (SEM, EBSD, Optical Microscopy) and preparation methods are applied in order to investigate the consequences of the solidification process of this grade of coatings and to link it to the observed crystallographic orientations.
The microstructure analysis was done both on the surface and the cross-section of the sample. A link between the microstructure and orientations’ heterogeneity is to be established. The observed complex morphologies of different phases yield a multiscale aspect of the coating microstructure that will be discussed.
[ 1 ] R. Parisot, S. Forest, A. Pineau, F. Grillon, X. Demonet, J.-M. Mataigne, Deformation and damage mechanisms of zinc coatings on hot-dip galvanized steel sheets: Part I. Deformation modes, Metall. Mater. Trans. A. 35 (2004) 797–811. doi:10.1007/s11661-004-0007-x.
[ 2 ] R. Parisot, S. Forest, A. Pineau, F. Grillon, X. Demonet, J.-M. Mataigne, Deformation and damage mechanisms of zinc coatings on hot-dip galvanized steel sheets: Part II. Damage modes, Metall. Mater. Trans. A. 35 (2004) 813–823. doi:10.1007/s11661-004-0008-9.
In a hot-dip galvanizing line for the production of zinc-coated steel strips, the heat treatment of the strip is an important process step. In order to achieve the desired material and surface properties, the strip has to be heated according to a predefined temperature trajectory. From the viewpoint of control, this is a challenging task, in particular in transient furnace operation, e.g. when a welded joint traverses the furnace or when the strip velocity changes. Because the product diversity, the demands on the product quality, and the desire to minimize the energy consumption are steadily increasing, there is a need for an advanced process control concept that accounts for all these challenges. In this paper, a nonlinear model predictive controller for the strip temperature in a combined direct- and indirect-fired annealing furnace is described. The controller was implemented at a hot-dip galvanizing line of voestalpine Stahl GmbH, Linz, Austria and measurement results from three years operational experience are presented.
The basis for the model predictive controller is a first-principles dynamical model of the furnace, which is characterized by moderate complexity and which captures the essential dynamical behavior of the real furnace. The model incorporates sub-models describing the flue gas, the wall, the radiant tubes, the rolls, the strip, and the relevant heat transfer mechanisms. Using this furnace model and the estimated current system state, the model predictive controller selects optimal trajectories for the fuel supply so that the strip temperature reaches its desired target temperature. In the control algorithm, a tailored constrained nonlinear optimization problem is numerically solved by the Levenberg-Marquardt method. The gradient and the approximated Hessian matrix of the objective function are analytically computed using an adjoint-based approach. This computationally highly efficient algorithm ensures that the controller can be executed in real time.
Measurements from the implementation at the industrial plant of voestalpine demonstrate the excellent performance of the developed control concept. It ensures compliance with all temperature constraints and achieves accurate strip temperature control in both steady-state and non-steady-state furnace operation. A long-term analysis shows the significant improvement of control performance in terms of accuracy and homogeneity of the strip temperature compared to the previous implemented PI temperature controllers. Encouraged by excellent feedback from three years operational experience, the developed control concept is currently transferred to the other hot-dip galvanizing lines of voestalpine.
The main focus of the proposed paper is on the design and efficient operation of metallic recirculating radiant tubes installed with recuperative or regenerative burners in vertical strip galvanizing lines. Premature tube failures may result in loss of production capacity arising from reduced heat input or, worse, unplanned plant shutdowns. As a galvanizing line furnace equipped with around 200 radiant tubes is not an uncommon case, the factors influencing tube life need to be thoroughly examined. From the perspective of the suppliers and end users of both the radiant tubes and the galvanizing line furnaces, the following aspects influencing tube life form the central themes of the paper:
- burner control and tube structural design,
- tube installation and process conditions in the furnace, and
- tube and burner maintenance.
Creep and the corresponding thermal stresses play a dominant role as a limiting factor in the lifetime of these radiant tubes. This is not only due to their exposure to high temperatures (> 900 °C) during operation, but also because of the resulting inhomogeneous surface temperature distributions and also, their own dead-weight. Recent investigations conducted in the Department for Industrial Furnaces and Heat Engineering in the field of radiant tube technology show that it is possible to determine an optimal burner position and tube geometry in order to maximise the recirculation of gases inside the tube and thereby, minimize the thermal stresses in the tube arising from temperature gradients on it. In addition, these tubes sustain alternating temperatures during the furnace operation, e. g. during burner on/off-firing or process-related changes such as change of the strip’s speed or cross-section. The effects of these temperature changes are much more detrimental than previously thought. Furthermore, the influence of the tube’s surroundings, i.e. neighbouring tubes or strip, is seen to exacerbate the material weakening phenomena and lead to a further shortening of its service life.
The results are from multiple public-funded research projects as well as collaborations with different burner manufacturers and furnace operators. Numerical simulations have been validated against experimental measurements from in-house test benches. Particular emphasis is on best-practices for furnace operation and on the tubes’ structural design (including its end support) for the improvement of its service life. Concluding remarks also include an outlook on future research topics.
Stress corrosion cracking (SCC) of uncoated and AlSi-coated hot forming steel grades of 1500 and 1900 MPa tensile strength is investigated under different corrosive conditions using the VDA 238-201 step load test and the VDA 238-202 constant load test. Hydrogen charging was realized by cathodic polarization with an attached zinc coated sheet or by acid solutions.
The 1900 MPa grades show a significantly higher susceptibility to SCC compared to the well established 1500 MPa hot forming steel 22MnB5. In general, e-coated CR1900T specimen with a controlled damage of the paint surface by stone chipping show higher failure loads and thus a reduced SCC risk compared to bare specimen. This positive effect of e-coating is even stronger for AlSi-coated material.
Since many of the 1900 MPa specimen fail during the first 24 h / 50% load step, a modified step load test sequence with a lower minimum stress level and a longer step duration is recommended for 1900 MPa steel grades. To simplify test preparation, specimen without attached zinc coated sheet are tested in an aqueous acid solution with pH1. These conditions lead to similar failure loads than under cathodic polarization with attached zinc coated sheet in neutral 5% NaCl solution.
As a conclusion, due to their relatively high susceptibility to stress corrosion cracking, the 1900 MPa hot forming grades are not recommended under wet corrosive conditions in automotive body applications, especially when they are in contact with zinc coated parts.
In order to meet the increasing demand for a flawless and attractively painted car, surface topography of sheet metal is becoming more and more into focus of optimization. Besides inhomogeneous painting, disturbing surface texture with long wavelength proportions can make the appearance of a coating disturbed and inferior. In order to measure and display these inhomogeneity’s, new characteristic values such as the Wsa1-5 value or the Wa0.8 value were introduced with the aim of characterizing surfaces and evaluating them to predict the resulting effect on the final painting. The measured values consider the proportion of long wavelength structures in the topography and provide information about the wave height. However, not only the surface, but also the material plays an important role in meeting the highest painting requirements. In addition, forming conditions during component production can create and reinforce visible textures. This effect is associated with an increase in waviness and is a result of material inhomogeneity, which can be found along the sheet metal cross-section and inside the sheet metal plane. Thereby a direct dependence between the waviness values and material isotropy is deduced. This results in a direct dependence between the waviness values and material isotropy. The article explains the identified relationships between texture characteristic and material isotropy in a typical steel for the automotive industry.
Quality specifications for high grade textured steel and aluminium strip continue to increase. In line with its strategy of continued research and development in texturing technology, Sarclad has developed the next advance in Electrical Discharge Texturing to push roll texturing capabilities above and beyond the current requirements of the industry and into the next generation.
This paper details the development of the Next-generation EDT technology. It presents information on the revolutionised, high-efficiency electrical power delivery units and how these are combined with advanced digital closed-loop position control of each texturing electrode.
The effect of this development is to provide improved control of the cratering process that generates the texture on the roll. With this improved control the EDT machine is able to deliver high-quality textures benefiting from increased peak density and greater uniformity of texture when considering both the surface roughness (Ra) and peak count (RPc).
Additionally, the new technology will reduce the time required to texture a given roll, enabling higher throughput of rolls compared to any equivalent sized machine on the market, using any other texturing process.
The prevention of intermetallic dross particles like Fe2Al5 in strip galvanising is paramount in making good quality exposed automotive panels. These intermetallic dross particles can create small surface features which can adversely affect the final painting quality. The formation of intermetallic dross seems inevitable during the galvanising of steel as aluminium additions are necessary to improve the coating adhesion and to prevent the Fe-Zn alloying reaction. Based on the results of an initial laboratory study and on the outcome of several industrials trials, will be shown that adding up to 0.8 % Al to the zinc bath results in a large reduction in the amount of supersaturated Fe. The study also shows that the thickness of the inhibition layer does no longer depend on the Strip Entry Temperature (SET) nor on small variations in the Al %. The microstructure of the coating is characterised by small zinc grains with some multiphase regions. Laboratory and semi industrial tests demonstrate that this new product has a better resistance against tool pollution and galling. All other application properties like waviness, spot welding and adhesive bonding are similar or better than conventional GI. Hence this product is the new benchmark for exposed automotive GI panels.
ArcelorMittal Gent is one of the first steel suppliers to have invested in a fully versatile furnace with the objective to produce a high amount of third generation Advanced High Strength Steels, such as Q&P; steels. ArcelorMittal chose the ANDRITZ Group to supply the necessary equipment, in particular the ANDRITZ Selas furnace, to insure that the specific thermal and process cycles needed for these steel grades can be realized.
The furnace performances allowed to produce new grades such as Q&P; steels after 12 weeks of operation, respecting specific heating paths and extreme cooling patterns, necessary for these high-end products, without jeopardizing coating quality.
While mechanical properties of zinc coatings are well-known and have been studied in detail during the last decades [1,2], the fundamental behaviour of coatings of zinc- or aluminium-based alloys is much less understood. Understanding the contribution of the different micro-phases in their complex solidification structure requires new approaches.
Nanoindentation coupled with data post-processing has been proven to be a powerful technique to draw two-dimensional mechanical mappings and to obtain the mean elastic modulus and hardness of constituent microscale phases in heterogenous materials [3,4]. The microstructure of low-alloy zinc-based coatings consists mainly of phases having hexagonal close-packed (HCP) crystal structure, like η-Zn in the form of dendrites or eutectic phases (Zn/Al or Zn/Al/MgZn2) and MgZn2 in the form of eutectic phases (Zn/MgZn2 or Zn/Al/MgZn2). The Al-rich phase in the Zn/Al and Zn/Al/MgZn2 eutectic phases, is a solid solution of Al and Zn having a face-centered cubic (FCC) structure. The elastic behaviour of HCP single crystals is usually highly anisotropic, meaning that the stiffness of the material strongly depends on the direction of loading. In addition, the hardness of Zn crystal has been reported to be also highly anisotropic .
In this study, the microstructure of Zn-5Al and Zn-3.7Al-3.0Mg coatings was investigated using scanning electron microscope (SEM) coupled with an electron probe microanalyzer (EPMA). Mechanical property maps were built using a grid nanoindentation technique [3,4] and were quantitatively correlated to the microstructure map. Electron backscatter diffraction (EBSD) was used to determine the crystallographic orientation (Euler angles) of each indented phase in order to determine the angle between the direction of indentation and the hexagonal orientation  of the phase (η-Zn or MgZn2). By combining all these techniques, the effect of the anisotropy of η-Zn and MgZn2 crystals on the mechanical properties of Zn-Al and Zn-Al-Mg coatings was quantified.
1. R. Parisot, S. Forest, A. Pineau, F. Grillon, X. Demonet, J.-M. Mataigne, Deformation and damage mechanisms of zinc coatings on hot-dip galvanized steel sheets: Part I. Deformation modes, Metall. Mater. Trans. A. 35 (2004) 797–811. doi:10.1007/s11661-004-0007-x.
2. R. Parisot, S. Forest, A. Pineau, F. Grillon, X. Demonet, J.-M. Mataigne, Deformation and damage mechanisms of zinc coatings on hot-dip galvanized steel sheets: Part II. Damage modes, Metall. Mater. Trans. A. 35 (2004) 813–823. doi:10.1007/s11661-004-0008-9.
3. N.X. Randall, M. Vandamme, F.-J. Ulm, Nanoindentation analysis as a two-dimensional tool for mapping the mechanical properties of complex surfaces, J. Mater. Res. 24 (2009) 679–690. doi:10.1557/jmr.2009.0149.
4. D. Mercier, J.-F. Vanhumbeeck, M. Caruso, X. Vanden Eynde, M. Febvre, Microstructural and mechanical characterisation of electroplated nickel matrix composite coatings, Surf. Eng. 35 (2019) 177–188. doi:10.1080/02670844.2018.1433270.
5. Y.T. Pei, G.M. Song, W.G. Sloof, J.T.M. De Hosson, A methodology to determine anisotropy effects in non-cubic coatings, Surf. Coatings Technol. 201 (2007) 6911–6916. doi:10.1016/J.SURFCOAT.2006.11.044.
With the increasing use of galvanized high-strength steel sheets in automobiles, the surface quality requirements of galvanized sheets are also increasing. When hot-dip galvanized C-Si-Mn dual-phase steel (such as DP600~DP1000) is produced, a serious orange peel-like rough surface defect appears on the surface of the steel sheet, which is mainly found in the middle area of the middle section of the coil. The orange peel-like defect samples are investigated and analyzed by Keynes microscope, scanning electron microscope (SEM) and glow discharge spectrometer (GDS). It was found that the zinc grains on the surface of orange peel defect area was abnormally large, about 300 microns, while it was only about 50 microns in normal area. After the zinc coating was etched by hydrochloric acid, the morphology of the inhibition layer was irregular, and there were transverse microcracks perpendicular to the rolling direction on the surface of the substrate and the full hard coil. The Al content at the interface between zinc layer and matrix is lower than that in the normal region. The analysis shows that the microcracks on the surface of the full hard coil are closely related to the type and composition of the scale of the hot rolled steel sheet. The hot-rolling coiling temperature is lowered to 580 °C or less, avoiding the eutectoid transformation of FeO, changing the surface oxide scale and composition, and eliminating orange peel defects. The analysis shows that the microcracks on the surface of the full hard coil are closely related to the type and composition of the scale of the hot rolled steel sheet. When the coiling temperature is lower than 580 °C, the orange peel-like defect is eliminated. Due to the avoidance of the eutectoid transformation of FeO, the type of scale is changed, and the surface enrichment of Si during hot rolling is suppressed.
In order to characterise the maximum possible cooling rate for AHSS with non oxidizing gas a new test bench has been constructed at VKI, combining 1 central round jet surrounded by 6 jets. The facility allows jet Reynolds number up to 200.000, value for which very few experimental data are available in the literature. Results are obtained for different normalised standoff distances Z/D (ratio between the outlet of the jet and the plate divided by the diameter of the round jet) and for different tilt angles of the plate with respect to the array of jets. The heat transfer coefficient is obtained by application of the active quantitative infrared thermography. Plate at constant and uniform heat flux is used. The thermograms are analysed by an in-house DIP program allowing the determination of the heat transfer coefficient mapping. A mean heat transfer coefficient is defined on a characteristic cell. The evolution of the Nu-number obtained by increasing the Reynolds number to this high value will be presented.
The experimental data are compared with numerical simulation for the validation of turbulent heat transfer. The numerical simulations are afterwards performed taking into account the industrial temperature (higher temperature difference between the jets and the metal sheet). A comparison of the heat transfer coefficient and the Nusselt number is made between the laboratory and industrial context.
The presentation will address some important elements that are leading to a change in the automotive industry. In particular, technological developments such as electrification, connectivity, autonomous driving and smart mobility have the potential to change the automotive industry in the long term. Potential implication will be highlighted.
The key ways to reduce CO2 emissions in iron and steelmaking can be summarized under the general terms “Smart Carbon Usage” (SCU) and “Carbon Direct Avoidance”. SCU covers on the basis of carbon carriers as reductant incremental measures at the conventional blast furnace converter route and the CO2 mitigation measures by applying so-called “end-of-pipe” technologies like CCS (CO2 Capture and Storage) and CCU (Carbon Capture and Usage). CDA covers the scrap based electric arc furnace route and the iron ore based steelmaking route direct reduction plant and electric arc furnace by the use of natural gas and/or hydrogen as reducing agent, which means the complete avoidance of coal and coke for the reduction of iron ores. The application of CCU at the conventional blast furnace converter route, which means the conversion of process gases into chemical raw materials, as well as the implementation of the direct reduction technology with hydrogen and subsequent smelting of the DRI (Direct Reduced Iron) to steel in an electric arc furnace require an immense amount of hydrogen and CO2-free electric energy.
ZnAlMg coatings produced by hot-dip galvanization process have shown superior corrosion resistant, anti-galling and wear performances. Nevertheless, currently these coatings exhibit lower cracking resistance and ductility compared to conventional galvanized zinc (GI) coatings on steel sheets during forming processes. In this study, mechanical properties and cracking behavior of ZnAlMg galvanized steels have been investigated thoroughly. Microstructure, mechanical properties and key causes of cracking initiation and propagation have been scrutinized by utilizing scanning electron microscopy (SEM), orientation imaging microscopy, nanoindentation and in-situ SEM tensile/bending tests. Ultimately, effective plastic deformation-based factors are obtained to understand and explain the cracking behavior and consequently link the microstructural features to cracking tendency of these coatings. The findings of this study are employed in designing new microstructure controlled ZnAlMg coatings with superb cracking resistance.
The work studied the effect that processing parameters have on microstructure and mechanical properties of dual-phase steel grade 980. This was achieved by subjecting steel coupons to various annealing cycles featuring different heating rates, cooling rates and intercritical annealing temperatures using a dilatometer. Following heat treatments, the microstructures and mechanical properties of the coupons were compared. Experimental results showed that when partial austenitisation is performed, the cooling rate does not affect phase transformation, microstructure or microhardness if kept within the studied range (-2.1 °C/s to -7.1 °C/s). By contrast, the intercritical austenitisation temperature affects the phase transformations significantly during cooling. Thus, the amount of austenite formed during intercritical annealing influences the phase transformation kinetics during cooling.
Iron is corroded by various factors such as moisture, salt and air exposure, causing problems in the building's exterior and structure. To solve this problem, studies have been continued about rust-resistant iron. Most of the galvanized steel used for outdoor applications are coated again painting on the galvanizing layer. More than that, Magnesium-added Zn-Al-Mg coated steel sheet is excellent for corrosion resistance of cut surfaces. In this study, adhesion was analysis using Scanning Kelvin Probe(SKP) and corrosion resistance of coated steel sheet was evaluated by Salt spray test and Electrochemical Impedance Spectroscopy(EIS) analysis.
A fast and non-destructive method based on X-ray diffraction for phase quantification in zinc alloyed coatings is presented. Post-annealed hot-dip galvanized steel sheets may contain various Fe/Zn alloys (Γ-Fe3Zn10, Γ1-Fe11Zn40, δ-Fe13Zn126, ζ-FeZn13) arranged in a layered conformation and with a predictable stacking order. Each phase within the coating has a different crystalline structure implying a heterogeneous mechanical behaviour: the phase quantification is therefore crucial for controlling further product behaviour, i.e. formability under press.
The determination of the individual layer thicknesses remains a challenging, time consuming and typically destructive task. Compared other methods, X-ray diffraction provides a unique fingerprint for each alloy constituting the coating. The so-called Rietveld method typically applies to an infinitely thick (compared to x-ray penetration depth) and homogenous mixture of phases. In the software TOPAS, the industry standard for quantitative phase analysis, the Rietveld method can be adjusted to a conformation of stacked phases by applying additional corrections related to (i) limited layer thicknesses and (ii) x-ray absorption through successive layers. A calibration of the beam flux allows for consideration of layer thicknesses as refinable parameters, acting then as unique scaling factors for each phase. Consequently, TOPAS delivers fast (few seconds of refinement), meaningful and absolute quantification results. Further coating parameters like coating weight, chemical alloying or phase ratios can be derived from the refined layer thicknesses.
A dedicated industrial diffractometer, D8 ENDEAVOR, equipped with an optimized diffraction geometry (DBO) provides high quality data in short measurement times (few minutes). No sample preparation, fast data acquisition, a large sample magazine with 66 positions and a fully automated evaluation contribute to an ideal solution in QC environment with high sample throughput.
In continuous galvanizing lines (CGL), strip annealing temperature is, though not the only one, the most significant physical parameter in the annealing process and therefore is used as the main indicator for product qualification. Annealing furnaces today are mostly regulated by Proportional Integral Derivative (PID) controllers or with the help of static based models. Those methods, though applicable, are providing non optimal control during transitions due to their limited ability in forecasting the furnace behaviour. To address this issue Fives has built comprehensive dynamic models of annealing furnaces and used them in Model Predictive Control (MPC). This paper shows how MPC helps reacting in advance of a process change and provides better strip quality as well as enhanced plant productivity. For two different annealing furnaces, Radiant Tube Furnace (RTF) and Direct Fired Furnace (DFF), the main components of the furnace models are first given. Then the MPC and examples of its application within Virtuo® Level 2 system are detailed.
Through-Process Quality Control is a quality system intended to cover the entire production line, from the liquid phase to the finishing and packaging lines of the final end product. The term “Through-Process” has been coined to specifically emphasize this idea. TPQC is a quality management system for steel production, which helps to continuously monitor and to manage quality across all production processes throughout the entire production chain, from steelmaking to sheet galvanizing. The system collects all process parameters and measured production data, starting in the liquid phase until the final product leaves the packaging line for shipment.
• Intelligent storage of high resolution data, encompassing the entire product genealogy and ensuring fast access to all quality-relevant production data of the final strip
• Product Explorer: in-depth analysis of process data based on time or length of product
• Quality and process assistance: rule-based quality checkpoints, root cause analyses, and suggestions for corrective actions
• Automatic rule-based surface grading
• Surface defect density map generated by the integration of surface inspection systems
• Tailor-made reporting
• Statistical Process Control (SPC) & Key Performance Indicators (KPIs)
• Data Analysis: TPQC data can be accessed using data-mining tools to deepen the knowledge about quality & process
• Primetals Technologies provides training and service packages for the different TPQC features
• Efficient troubleshooting and fast access to the required information in one system
• Quality management & quality certification support
• Reduced manual product inspection, rework, and downgrades
• Reduced influence of human factor
• Digitalization of know-how
• Big data analysis and self-learning systems—leverage your own production data
• Suggestions of root causes, and for corrective, and compensational actions
• Supporting system to achieve conformity with ISO9001/IATF16949 and other automotive standards
• To ensure continuous improvement and stable, well understood production processes, new know-how is created
This paper describes how to use the TPQC integrated know-how-based IT solution to improve efficiency and quality for processing lines.
With ever increasing targets on productivity, unpredictable down times caused by a weld break can become an important problem. Lately, partly because of drivers like the rise of multiple high strength steel grades and the retirement of highly experienced operators, we have seen CGL lines experience an increase in weld break events. Technologies like pressure sensors, current sensors, cameras and pyrometers are used to help characterize the weld quality. Nonetheless, these instruments cannot be trusted for an automatic rejection or acceptance of the weld. Recently, a new approach was proposed using laser-ultrasonics to characterise the weld in-line with the welding process. It uses ultrasounds through the weld itself, an approach broadly accepted in the world of non-destructive testing. The topic of this paper is about the industrial implementation of a laser-ultrasonic instrument optimized for weld inspection. Results on a long term trial in a North American plant are presented. The impact of steel alloy, weld preparation and surface conditions on the acceptance threshold are discussed.
Advanced high-strength steel grades (AHSS-grades) of the third generation offer significant ecological and economic potential in the automotive industry for reducing automotive weight, improving fuel efficiency, and thus decreasing emissions. Their properties are based on special chemical compositions and microstructures. The annealing and galvanizing process of these grades challenges the plant technology.
This paper presents solutions for the industrial production of coated AHSS-grades and shows new production aspects with advanced furnace and coating technology. It will introduce the intelligent furnace concept with X-CAP® technology for a precise closed-loop controlled heat treatment process based on material properties. Furthermore, it presents equipment technologies for the galvanizing process and surface quality. Finally, the most recent and innovative references for hot-dip galvanizing lines will be introduced.
A large range of digital solutions and equipment exists in order to manage quality requirements of high end products on strip processing lines. But in the end day-to-day users have to juggle between dozens of solutions and finally get lost in trying to link data from different software or finding the right tool adapted to their situation. Solving defect crises requires rather applying the right methodology and using a limited set of tools in order to be efficient. Fives’ Eyeron™ advanced quality management software is an all-in-one integrated solution designed by and for metallurgists that gathers all necessary tools in one single place to efficiently solve any defect crisis faced during production. By providing easy and clear visualization of all plant data such as process, surface inspection, order or claim information, as well as powerful data analysis and control features, Eyeron™ allows its users to tackle any quality issue directly from one interface. Eyeron™ gives then the possibility to perform automatically product quality certification and even provides recommendations to reallocate products at any production step from slab down to finished rolled product, considering the quality required by each customer. This paper will presents how Fives’ Eyeron™ software can be used by process and quality engineers to resolve defect crisis on hot dip galvanizing line, to assess daily progress in terms of quality yield and evaluate overall achieved quality performance.
Efficient and effective operation of a hot dip galvanizing line depends from several technological aspects: mechanical, thermal, chemical, automation and control. The synergy and the harmonization among all aspects is the key to get the best performance in terms of productivity and quality.
Danieli is recently achieving outstanding results in their new lines, thanks to a specific focus on the integration among all technological equipment installed in a hot dip galvanizing line, with a special regard to furnace, automation and air knife.
Based on this experience and driven by the latest requirements coming from the market, it is possible to produce thin and wide strips, arriving to an high ratio width/thickness with quite interesting results in terms of final products. This achievement is the result of an extremely accurate control of strip speed, tension and furnace zones temperatures, involving mechanical, thermal and automation aspects.
Danieli is providing the fully integrated automation, from the level 1 up to the level 3 system, allowing a complete control of the production. Particular attention is put on the introduction of intelligent process models, data analysis and the introduction of virtual sensor to retrofit the process models of the galvanizing line and the reheating furnace. A strong integration with all the automation levels allows to manage the process in an innovative way driven by the collected data.
The latest design of Danieli zinc pot air knives is achieving excellent results in terms of maximum speed versus low zinc coating weight, targeting high productivity and enhanced quality.
To ensure a constant high productivity and quality for the newest AHSS, Andritz developed a new generation of its Metris Advanced Furnace Control. It consists of a modular design with a newly developed cooling model and a brand new multi model predictive control.
The precise physical model for all heating and cooling sections act as virtual sensors , which gives a good insight of the temperature distribution within the furnace and acts as a huge source for Big Data.
The multi model predictive control ensures collaboration between all models to enable a constant high quality and productivity. A method to compensate the varying strip emissivity over the coil length is shown.
The system was successfully installed at a Continuous Annealing Line of SSAB, Borlänge, Sweden. The presented results show a significant increase in quality.
Resistance spot welding (RSW) is the most common joining process in car body manufacturing and is frequently used to join components made of advanced high strength steels (AHSS). These steels are typically applied with a zinc coating to improve their resistance against corrosion. During the RSW-process the liquefied zinc is reported to infiltrate the grain boundaries of the steel substrate causing a phenomenon referred to as liquid metal embrittlement (LME). In cases where LME is egregious, joint performance might be affected; therefore, prevention or mitigation of LME is desirable. To allow a practical implementation of avoidance strategies, it is favorable to interfere with the parametrization of the established spot welding processes as little as possible. This study investigates methods suitable for the practical avoidance of liquid metal embrittlement during the resistance spot welding of AHSS. At first, the effect of various process influences is investigated and major influence factors are identified. Occurring effects are analyzed in detail and correlated with an FE-simulation to generate a better understanding of the mechanisms causing LME in RSW. In this context the impacts of excessive energy input by high weld currents and elongated weld times on LME crack formation are discussed. Successively the avoidance of LME is achieved by a combined adaption of the electrode geometry and hold-time, with respect to the welding boundary conditions. Finally, the robustness of the proposed methods is validated in a more complex welding scenario.
In zinc coated 3rd Generation advanced high strength steels (3G-AHSS), liquid metal embrittlement (LME) occurs due to liquid zinc or zinc alloy penetration along the grain, grain boundaries and pre-existing cracks of the steel sub-surface area in the presence of critical amount of external tensile stress during resistance spot welding. Due to penetration of liquid metal, cohesion between grains of steel decreases and causes embrittlement. Most of the zinc coated 3G-AHSS are used in the automotive industry where multiple layers of coated steel sheets are joined together using resistance spot welding (RSW) technique. High temperature exposure, presence of tensile stress during welding and liquid metal are the three key parameters responsible for LME during spot welding of zinc coated AHSS. The LME crack initiation and propagation depend on the states and condition of the above three factors. Present study deals with the determination of onset of LME crack formation and the propagation during interrupted spot welding of Zn coated 3G-AHSS. All the spot welding has been carried out at constant electrode force along with constant welding current, only the welding cycle (time) has been varied. In addition, to evaluate the effect of external stress along with the stress required for RSW on LME crack formation and propagation, interrupted RSW has been carried out in the presence of external tensile stress. Results show that LME crack formation and propagation are very much dependent on the welding cycle which eventually controls the total heat input and stress condition. It has been also observed that with Zn coated 3G-AHSS the welding cycle (time) as well as stress affects the structural change at the steel/coating interfacial area.
Liquid Metal Embrittlement (LME) is a complex phenomenon which may occur during spot welding of zinc-coated Advanced High Strength Steels (AHSS) developed for the Automotive market.
Zinc or zinc alloy in liquid state penetrates along the steel grain boundaries and may cause, depending on conditions, grains’ decohesion and cracks’ initiation and propagation under given mechanical stresses.
LME sensitivity depends on the steel mechanical properties and surface / sub-surface characteristics, the coating composition and microstructure and the spot welding process itself.
In this study, a comparative assessment of conventional GalvanIzed (GI) and GalvAnnealed (GA) coatings in terms of LME behavior has been done on the same sensitive AHSS material with the same welding process configuration and parameters. The intrinsic effect of the type of coating (GI vs GA) on the LME performance could therefore be properly evaluated.
Corrosion protected galvanized advanced high strength steels with high ductility (AHSS HD) like the investigated dual phase steel (DP1200HD) tend to show an elevated risk of liquid metal cracking (LMC) during resistance spot welding (RSW). LMC is an intergranular cracking mechanism driven by temperature, tensile stresses, plastic deformation and the presence of liquid zinc. Crack initiation and growth is caused by liquid metal embrittlement (LME) where the material experiences a drastic loss of ductility of up to 95 percent depending on the loading conditions. The aim of this work is to experimentally investigate LME and to develop a model to predict the local LMC during RSW.
The prediction of LMC is addressed with laboratory scale hot tensile tests with uncovered and galvanized steel sheets at a Gleeble 3800 thermomechanical simulator and extensive laboratory scale resistance spot-welding tests. All gained material and process data are subsequently used for creating a physically based and validated LMC indicator, which is dependent on temperature, plastic strain and strain rate. Newly developed and validated finite element models of the welding process accounting for all relevant multi-physical phenomena provide deep insight in the RSW process and help to understand the influence of different conditions leading to LMC. The LMC indicator enhances the capabilities of these models and provides predictions for the onset of LMC. Taking advantage of the LMC indicator allows modifying the welding process such that a significant reduction of LMC during RSW can be achieved.
The resistance spot welding is widely employed by automotive manufacturers because of its low cost and fast process. However, when the galvanized TRIP steel plates are joined by resistance spot welding, cracks are generated by liquid metal embrittlement (LME) because the substrate gets exposed to liquid zinc. These cracks deteriorate mechanical reliability of welds and limit the application of TRIP steel.
The present study investigated the microstructural evolution and LME sensitivity of differently Zn-coated TRIP steels. Three Zn-coated TRIP steel sheets were prepared by applying different coating process: (i) continuous galvanizing, (ii) galvannealing, and (iii) electrogalvanizing. Hot tensile testing and microstructural analysis exhibits that the coating method influences Fe-Zn reaction which controls the contact between steel substrate and liquid Zn alloy at the high temperature. The electrogalvanized TRIP steel exhibits the highest resistance to LME, while the galvanized one shows the lowest resistance.
Third generation advanced high strength steels (AHSS) were developed as automotive structural materials capable of enhancing vehicle fuel efficiency and crashworthiness. The use of zinc (Zn) coated AHSS is limited by Zn-assisted liquid metal embrittlement (LME) that leads to surface cracking during high temperature processing. It has been reported that AHSSs are generally more sensitive to Zn-assisted LME compared to conventional mild and high strength low alloy (HSLA) steels; the factors controlling the LME sensitivity of AHSSs are not precisely established. AHSS grades have tailored multi-phase microstructures and relatively rich alloy compositions; microstructural and alloying variations may influence LME susceptibility. In this work, the influence of starting microstructure variations on Zn-LME sensitivity is studied using a 0.25C-2.7Mn-1.45Si steel alloy, continuous annealed to generate different AHSS microstructures: martensitic, quench and partitioned (Q&P;), dual-phase (DP), and transformation-induced plasticity (TRIP)-assisted bainitic ferrite (TBF). High temperature tensile tests were conducted on electrogalvanized (EG) sheets of different starting microstructure variants to compare Zn penetration characteristics, and the critical temperatures and stresses required for inducing LME. The results are interpreted in the context of the specific influence of starting microstructure on LME behavior during resistance spot welding of Zn-coated AHSS sheets.
Nowadays, automotive steel customers are more and more strict on the coating quality, especially for the exposed parts that should not have any aspect defect.
To satisfy this requirement at the exit of galvanising line, one important parameter is the cleanliness of the substrate surface after the cleaning section at the entry of the line in terms of carbon pollution and iron fines.
Indeed, if carbon (as component of the mill oil) is still present at the surface when entering the furnace, furnace pollution will occur in the long term by production of soot that will not only stay on the furnace walls but will eventually lead to dirt falling on the rolls and/or on the strip.
Besides, the iron fines can also lead to roll pick-up defects inside the furnace, to an increase of dross in the bath by combination with zinc and aluminium and also to a drift of the bath composition.
These phenomena will eventually result in aspect defects on the final product.
A direct on-line measurement is so mandatory to fully evaluate and control the surface cleanliness all along the coil and is more and more required by customers as an assurance of final product quality.
Using this cleanliness measurement, the cleaning section can be optimised by adjusting relevant parameters, such as the brushes pressure, the solution composition, the current applied if electrolytic cleaning is used.
On the market, no on-line methods are able to separately measure the mentioned pollutants. CRM has developed an innovating method based on LIBS principle which succeeds in an on-line independent measurement of both pollutants. This will be realised by using a groundbreaking approach that will allow reaching the measurement sensitivity required after the cleaning section sensitivity (i.e. 10-15 mg/m² for carbon, 20 mg/m² for iron fines) thanks to atmosphere conditioning around the plasma and use of adequate spectral lines for carbon and iron fines levels measurement.
This system can be used at a later stage of development in a control loop for the optimisation of the final cleaning line and of the production speed.
Over the past two years, four CGLs have installed an online, surface cleanliness monitor system utilizing a non-contact, laser ablation technique to evaluate the contamination layer on the moving sheet. The CGL operators using the system obtain unprecedented detail about the sheet’s surface cleanliness along the entire length of each coil.
The system has been improved over the last two years with customer feedback and hard lessons learned about laser quality. The most notable system enhancements are the addition of a different industrial laser technology and an all-new, instrument head design enabling field serviceability of lasers using interchangeable laser modules.
CGLs using the system have immediate feedback of how adjustments to the cleaning section affect surface cleanliness out of the cleaning section, or sometimes, seeing the adjustments have no effect on cleanliness. With an instrument head before the cleaning section, the incoming cold-rolled sheet can be monitored. The system allows cleaning section performance to be bench marked before and after significant changes: for example, changing brushing technology; electrolytic cleaning phase change or power protocols; evaluating new solution types or operating temperatures; etc.
For one CGL operator the continuous, online, surface cleanliness data has launched an Industry 4.0 project towards automating cleaning section control using artificial intelligence to evaluate the surface cleanliness data along with other inputs. The prospects of advancement in process control around cleaning are promising with many anecdotes and data to share.
In summary, this non-commercial paper will explain details about the benefits and challenges found by CGL operators using the surface cleanliness evaluation system by showing detailed data, plots, and explaining anecdotal use-cases from CGL operators who have implemented the system. A very brief, contextual overview about surface cleanliness will be included to make the session an applicable as possible; why cleanliness is important, key historical methods used to measure surface cleanliness – with pros & cons, along with some fundamentals of cleaning section troubleshooting.
During the manufacturing of the modern high strength steels, it is very important to know the austenite level compared to ferrite at given positions in the line during the steel production. Indeed, ensuring the constancy of the austenite fraction is mandatory for the constancy of the mechanical properties of the final product.
There are several positions in the line where it is essential to know the fraction of austenite, namely for example, the exit of zinc bath or several intermediate locations in the furnace in steel galvanising lines.
These positions induce many constraints which are very difficult to address. Indeed, this new sensor aims to overcome the limitations of current measurement devices of the austenite level in steels. In particular, the sensor allows the measurement at low or high temperature of the steel strip (850°C). It allows the measurement at a distance of several tens of millimetres of the strip while keeping a sufficient sensitivity. The measurement is not influenced by the vibrations of the strip as well as by abrupt change of the distance sensor – strip. This sensor is also able to work above and below the Curie temperature. The modularity of this sensor allows characterising not only the full width but different zones on the width and, particularly, the sides and borders of the strip which can show different properties due to different behaviour during the heating or cooling phase.
The surface quality of a finished product on a galvanizing line is a very important criteria, especially for automotive and home appliance exposed products. To control it, automatic surface inspection systems (ASIS) have been developed since many years and are installed on all galvanizing lines dealing with high product quality. A highly performant ASIS from Primetals Technologies has been developed for many years and is especially protected under the worldwide known Trademark SIAS®.
In the last ten years, ASIS performance improvements are mostly due to the integration of new developments in lighting and camera technologies. Deep-learning and artificial intelligence are fast developing technologies. Convolutional neural networks have proved to be the most efficient tool to address many image processing problems like image retrieval and classification. It remains still very challenging to use these technologies industrially for real-time applications on large video streams.
A new online real time defect detection/classification system using full convolutional network has been developed and the prototype of this system is installed in Liège on the EUROGAL galvanizing line (ArcelorMittal Belgium). The added value is expected to be threefold:
Improved defect detection/classification performances (for textured product in particular).
Provide an easier tuning on multi-camera systems integrating several lighting conditions.
Decrease the sensitivity to the tuning parameters and provide a more generic detection configuration, easier to transfer from one product or one line to another.
This paper presents the principle, the first results of the prototype which is installed at EUROGAL. We will present also the vision of the future of surface inspection system, dealing with training the neural network on multiple sites, and the SIAS Fleet Management System.
Insights into the structure of surface oxides on Zn-coated press-hardened steel (PHS) are indispensable for further processing like welding or adhesive bonding. The main oxide on top of galvanized steel is Al2O3 from the primary galvanizing process. This oxide is strongly altered during final austenitization annealing before hot-forming respectively press-hardening, depending on the annealing time. The role of steel alloying elements on the oxide composition is investigated by the means of scanning and transmission electron microscopy with energy dispersive X-ray spectroscopy as well as Auger electron spectroscopy.
Four different steel grades are investigated, whereupon two are standard galvanized and the other two are galvannealed. For both coatings, we can show that the main oxides after austenitization heat treatment are native ZnO and the initial Al2O3. Moreover, our results indicate that the main alloying component Mn forms (Mn,Zn)Mn2O4 spinel, which also contains traces of Fe and is embedded in the upper ZnO layer. Also noticeable was a varying, nanometre thick Cr enrichment at the initial Al2O3 layer depending on the availability of Cr in the steel alloy. Additionally, small precipitates of further alloy elements can be found at this (Cr,Al)2O3 layer. Further experiments with time-of-flight secondary ion mass spectrometry attached to a Helium Ion Microscope allowed to reliably distinguish between ZnO and Zn(OH)2, which are both present in the oxide layers. All specimen show high local differences related to skin-passing prior to final heat treatment. This is especially the case for shorter annealing times, where thermodynamic equilibrium is not yet reached.
Thus, we can conclude that the oxide composition does change with the chemical composition of the steel alloy. Therefore, alloying agents do not only affect the mechanical respectively metallurgical properties of the used steel grades, but also influence surface oxides, and can have an impact on processing steps like welding or bonding.
The study presented here focuses on the selective oxidation of Fe-Mn (1wt.%) binary alloy. The samples were annealed by means of a laboratory furnace with a temperature profile relevant to the galvanizing line practice. The sample was first heated to 800°C at a rate of approximately 6°Cs-1. It was kept at that temperature for 60 s before being cooled to room temperature. The gas atmosphere consists in a mixture of N2 - 5 vol.%H2 with a dew point of -40°C. To obtain information on the nucleation and growth of oxide particles, we interrupted annealing at 700°C during the heating phase. Some specimens were held at 800°C during 0, 60, 120 and 300 seconds. The rapid cooling of the samples that occurs when the annealing furnace is stopped is considered to act like a dip for selective oxidation reactions.
During annealing the native iron oxides are reduced and the manganese diffuses to the surface where it is preferentially oxidized and forms MnO oxide particles. These selective oxide particles were characterized using several complementary analysis techniques. The surface of the samples was observed in a Field Emission Gun Scanning Electron Microscope (FEG-SEM) to obtain high resolution images and analyzed by Electron Back-Scattered Diffraction (EBSD). Image analysis was used to measure the geometric parameters that characterize oxide particles in two dimensions. Thin cross-section were extracted from the oxidized samples using a Focused Ion Beam Microscope (FIB) and characterized in a Transmission Electron Microscope (TEM).
We studied the oxide particles present on three ferrite grain orientations: Fe(100), Fe(110) and Fe(111) as a function of annealing time. It has been demonstrated that, on our model material, MnO particles are monocrystalline. Their shape, size, nucleation and growth depend on the ferrite grain orientation where they are formed. Elementary mechanisms of the oxidation reaction are proposed and discussed to explain this behavior.
The selective oxidation of Boron in bake hardening steel sheet is investigated in a hot dip process simulator for a typical DFF RTF cycle with two levels of pre-oxidation. The time in the RTF (85, 125 and 220 seconds), the RTF dew point (-40, -25 and -10 oC) and the hydrogen content (1, 3 and 5 %) of the RTF atmosphere are varied. The samples were analysed with FEG-SEM, EDX and GDOES. A few cases were analysed with Auger to get more information about the condition of the outer surface.
Both the annealed substrate as the inhibition layer formed in a MagiZinc bath containing 1.6 % Aluminium and 1.6 % Magnesium was analysed.
The amount of Boron enrichment at the surface increases when the dewpoint is decreased from -10 to -40 oC which is an indication of more external oxidation of Boron. Pre oxidation leads to a lower Boron enrichment at the surface. The effect of pre oxidation is biggest for the cases with lower dewpoints (-40 and -25 oC) and it is limited for a dewpoint of -10 oC which is in accordance with results from literature. The longer times in the RTF and soak showed saturation of Boron at the surface for a dewpoint of -40 oC, this did not happen for the higher dewpoints.
The inhibition layer showed small holes where we could still detect oxides on the substrate. In some cases evidence of an inhibition layer was still found that can indicate reduction of oxides by the MagiZinc bath.
This paper has the aim at presenting a methodology of defect root cause analysis for hot-dip galvanized coatings. It has been developed to provide a global overview of all aspects related to the Materials Science pyramid – the processing route, the material properties, the structure and in-use performance – all linked by the material’s characterization. Its final aim is to provide not a single parameter leading to failure or poor performance, but to identify the space of parameters and their combinations leading to the undesired outcomes for a given coating system. In order to perform such an integrated analysis a minimal set of process parameters must be identified, generally through brainstorming.
Management of data acquisition and communication between production site, characterization team, quality surveillance, and engineering must be carefully carried out with full traceability of data to allow the assembly of the Materials Science pyramid database. Process parameters shall be acquired for a statistically significant population (comprising accepted and rejected coils) and each coil’s coating microstructure must be analyzed to provide the base of the dataset. A quality control parameter related to the coating being studied is required to finish the triangle of process-property-structure.
The method is composed by three main steps. First, establishing a set of parameters representing the structure-related features of the coating material, what is followed by data clean-up for ensuring stability of conditions for each coil. Finally, the application of clustering techniques, proves more reliable and efficient than classical statistical analysis, for interpreting the parametric effects over the quality issue being analyzed rather than studying the tendencies promoted by individual parameters.
Surface defects on galvanized strips are a key factor for the product quality in the flat-rolled steel industry. Some of the most notorious examples are slivers and scale defects. Correct classification and reduction of sliver and scale defects are essential for high end-product quality, lower downgrading rates, less CO2 emissions, and consequently lower costs.
Steelmakers currently use automated surface inspection systems (ASIS) from various vendors to inspect the products after hot rolling, pickling, and galvanizing. These typical inspection systems use conventional classifiers and as a result, their accuracy is limited. To circumvent this limitation the manufacturers are increasingly switching to defect classifiers that are based on neural networks.
Smart Steel Technologies (SST) uses multiple deep neural networks that are specialized for each process step. In addition to the surface inspection data, SST also utilizes the Level 1 and Level 2 production data to improve classification accuracy.
Neural networks are known for requiring a lot of hand-labeled images to perform well. In this paper, we focus on different aspects of effective data collection and emphasize the importance of the continuous involvement of domain experts in this process. We also highlight the effectiveness of the cross-process, multi-view approach to improve classification accuracies.
Steel manufacturers are continuously developing higher performance steel grades by optimizing the chemistry at the steelwork and by changing the process parameters, in particular the heat cycles to be applied in the annealing furnaces. In hot dip galvanizing lines, the production of the latest Gen3 AHSS require higher annealing temperatures which are difficult to be achieved by using gas fired radiant tubes. The new thermal cycles also require a quenching followed by an induction rapid heating where the strip may still have a big fraction of austenite which results in poor magnetic properties for a heating by a conventional inductor. This also raises concerns in the case of the galvannealing induction heating process for the same reasons. To fulfil the requirements of the new annealing cycles, i.e. heating the strip at high temperature (above Curie point) or rapid heating the strip with poor magnetic properties, the transverse flux induction is the unique industrial solution.
Fives has been developing the CELES EcoTransFlux™ technology since more than 20 years, first for rapid heating of stainless steel in strip processing lines. Today, this advanced technology is becoming a key equipment for processing the latest Gen3 AHSS and also for developing higher performance electrical steel products.
This paper presents the main features and performances of Celes transverse flux induction technology as well as different industrial applications and references.
Jet Vapour Deposition (JVD) technology is industrialized at ArcelorMittal since mid of 2016. This technology, developed in partnership with CRM Group, constitutes an attractive technical and economic alternative to Electro-Galvanizing (EG) deposition process. JVD is a vacuum deposition process fully compatible with high line speeds, which allows the deposition on all kinds of steels of pure zinc coatings with in-use product properties similar to EG coatings ones. Furthermore, thanks to its flexibility, a wide range of coating thickness that can be different on each side, is accessible according to the request of customers. For automotive market, the current production concerns Advanced / Ultra High Strength Steels (AHSS / UHSS) higher than 1000 MPa used for structural parts in vehicles. This request is motivated by the fact that JVD is an hydrogen free process, with no risk of embrittlement by hydrogen for these steels. This is a major advantage of JVD compared to EG, since EG requires a post-thermal treatment to degas hydrogen trapped in steel during Zn deposition. But JVD may also be suitable for drawing steels used for external / exposed parts of vehicles, which require a very good aspect quality. JVD has a limited impact on the steel topography parameters. Using automotive exposed formats leads to JVD Zn coated steels fulfilling exposed specifications. Lastly, JVD is a green process compared to EG: less energy and water are required to produce a similar coating.
Physical Vapor Deposition have now entered the panel of technologies for fabricating metallic Zn-rich coatings on steel. We have recently demonstrated that General Vapor-Phase Galvanizing can coat complex-shape steel articles after fabrication with a fully alloyed Fe-Zn coating.
Surface activation is a key step of the process because the coating results from the reaction of zinc vapor with steel in a range of temperature between 350°C and 550°C. We have tested several industrial protocols able to clean the surface and stimulate its reactivity. Our results show the importance to produce surface sites providing a sufficient diffusive flux of iron and where zinc atoms can adsorb for nucleating Fe-Zn aggregates. The influence of roughness on reactivity will also be discussed. The other genuine aspect of our process is to maintain an adsorption-desorption equilibrium of zinc atoms to and from the surface so that complex-shape articles can be coated at once without making shadows, nor producing over-thickness. Then, complex-shape articles can be coated at once without requiring a complex handling device into the evaporation chamber. These conditions depart from the one of standard (deposition-condensation) PVD or reactive-PVD (deposition followed by the diffusive reaction), and produce uncommon specific microstructures. We will discuss the surface mechanisms that drive and stabilize the process.
The phase composition of coatings fabricated by GVPD is similar to the one of GA steel. However, coatings produced by GVPD are usually two to three time thicker than the GA coatings. GVPD coatings are especially efficient for the fabrication of high standard duplex coatings. Fabrication in the low range of fabrication temperature (350°C – 390°C) can treat high strength steels without producing a significant tempering of martensite. Coating in vacuum after the fabrication of parts bypass the problem of producing a coating compatible with hot stamping.
Arcelor Mittal Cleveland Hot Dip Galvanizing line produces 260,000 metric tons of Galvannealed AHSS products (GA) annually for automotive customers. New AHSS grades and thickness gauges are under constant development for automotive applications to meet the future automotive fuel efficiency standards. The use of alloy elements such as Manganese, Silicon and Aluminum in Dual Phase and TRIP chemistries can lead to challenges to many aspects of the continuous galvanizing process, including galvannealing or alloying.
Under galvannealing, or under alloy, of AHSS is one of the major quality concerns. Based on several case studies at the industrial coating line, the present work reviewed various mechanisms that have caused under alloy on both Dual Phase and TRIP steel. Coated steel samples with under alloyed appearances were examined by SEM/EDS, XRD and GDS. Metallography results showed that non-uniform zinc coating and silicon segregation at the edge were two major contributing factors for edge under alloy on TRIP and Dual Phase steel. It was found that unpickled hot band scales and micro flaps were responsible for the underalloyed streaks and patches in Dual Phase steel. It is suggested that selective internal oxidation in the radiant tube furnace can help galvannealing of AHSS, to a limited extent. DFF pre-oxidation can also enhance galvannealing of AHSS but it can lead to other processing issues such as low surface roughness and high dew point in radiant tube furnace. Both internal oxidation and DFF pre-oxidation practices could produce abnormal heavy coating buildup on strip in zinc pot during line stop which could lead to safety and quality issues. It is believed that when tensile strength for dual phase is over 1180 MPa, ferrite volume fraction plays a critical role in the galvannealing process. The high resistivity and low magnetic permeability of the steel could potentially lead to traditional galvannealing furnace tripping off causing steel not alloyed. The investigations concluded that the homogeneity of alloy elements, incoming steel substrate cleanliness, pot equipment reliability, steel microstructure and galvanneal inductor design are all important aspects to ensure full galvannealing of AHSS.
In industrial electro galvanizing lines, the performance of the dimensionally stable anodes (Ti + IrOx) is a crucial factor for product quality. Ageing of the anodes causes worsened zinc coating distribution on the steel strip and a significant increase in production costs due to a higher resistivity of the anodes. Up to now, the end of the anode lifetime has been detected by visual inspection every several weeks. The voltage of the rectifiers increases much earlier, indicating the deterioration of anode performance. Therefore monitoring rectifier voltage has the potential for a premature determination of the end of anode lifetime.
Anode condition is only one of many parameters affecting the rectifier voltage. In this work we employed machine learning to predict expected baseline rectifier voltages for a variety of steel strips and operating conditions at an industrial electro galvanizing line. In the plating section the strip passes twelve “Gravitel” cells and zinc from the electrolyte is deposited on the surface at high current densities.
Data, collected on one exemplary rectifier unit equipped with two anodes, have been studied for a period of two years. The dataset consists of one target variable (rectifier voltage) and nine predictive variables describing electrolyte, current and steel strip characteristics. For predictive modelling, we used selected Random Forest Regression. Training was conducted on intervals after the plating cell was equipped with new anodes. Our results show a Normalized Root Mean Square Error of Prediction (NRMSEP) of 1.4 % for baseline rectifier voltage during good anode condition. When anode condition was estimated as bad (by manual inspection), we observe a large distinctive deviation in regard to the predicted baseline voltage. The gained information about the observed deviation can be used for early detection resp. classification of anode ageing to recognize the onset of damage and reduce total operation cost.
The surface quality of galvanized steel sheets is considerably influenced by intermetallic dross particles, which inevitably form during operation. η-phase Fe2Al5 top dross and δ-phase FeZn7 bottom dross particles have been reported to be entrapped into the coating causing functional and optical defects. Moreover, particles are frequently mentioned to adhere to the stationary and the moving bath equipment. Particularly, dross particles which are adhered to sink and stabilizer rolls can cause imprints on the surface of the steel strip. Furthermore, the cleaning of the rolls is related to down times of the mill. An efficient management of dross particle dynamics is thus a key criterion for running galvanizing mills.
CFD-DPM models give a comprehensive insight into the dynamics of dross particles and their tendency to come in contact with relevant surfaces. Buoyancy and settling effects of dross particles, which lead to the accumulation of top and bottom dross in regions of the galvanizing bath are predicted with the model. A special focus is put on a sub-model investigation of the surrounding of the sink roll. Factors of influence on the dross dynamics and accumulation such as particle type, particle size, wall roughness and line speed are elucidated with the DPM model. In addition, a new approach in modelling dross based on scalar transport equations is presented, which enables the inclusion of the thermodynamics and the reaction kinetics of dross formation.
During production of galvanized products, the Hot Dip Galvanizing bath is managed with additions of ingots allowing to guaranty the stability of the bath level and composition. Hot Dip Galvanizing pot productivity (in kg/h) is then defined as the amount of materials melted in the pot during a given time. The pot productivity is generally monitored to maintain the HDG bath at a stable level and vary depending on the production conditions (coating weight, line speed and strip width). In case of extreme process conditions (high coating weight, high line speed and wide strip), the pot productivity may become bottleneck, resulting on the necessity to limit the line speed to maintain the stability of the bath level, and thus affecting the HDG line productivity.
In the present paper, the different parameters affecting the pot productivity are highlighted and discussed considering the constraints induced by the ingots loading technology, but also by the ingots melting kinetic itself. Different ways to improve the pot productivity are then proposed and were investigated based on laboratory ingots melting trials, numerical simulations and on-line industrial trials with instrumented ingots.
The main objective of the present work was to investigate the forced wetting of a partly oxidized steel by a liquid Zn - Al (0.2 wt. %) alloy. The wetting experiments are performed by means of the dispensed drop technique. The wetting is shown to be reactive with the formation of Fe2Al5Znx. The evolution of the contact angle and spreading diameter is determined as a function of spreading time.
The final contact angle lies between the receding and the advancing contact angle and is a decreasing function of the initial kinetic energy of the droplet. The liquid zinc drop remains pinned in a metastable position, due to the contact angle hysteresis.
During the first ms, the spreading diameter increases up to its maximal value Dmax. In good agreement with previous studies in the field of wetting at low temperatures, the maximal spreading diameter scales as , We being the Weber number which compares inertial and capillary forces.
The kinetic energy of the liquid metal droplet needed to reach the minimum receding contact angle was predicted from the model based on the Weber number to describe Dmax. This kinetic energy is in good agreement with the experimental results.
As a final conclusion, an increase in the initial kinetic energy of the droplet leads to a decrease in the final contact angle. It is therefore expected that forced wetting could improve the galvanizability of steels by liquid Zn-Al alloys.
In order to improve fuel efficiency and to reduce carbon emissions, automakers are using advanced high strength steels (AHSS) to lightweight vehicles without compromising passenger safety. In manufacturing, the steel sheet is annealed in a continuous galvanizing line (CGL) to achieve the desired mechanical properties. During annealing, alloying elements such as Si, Mn, Cr, Al, etc. in the steel can form oxides, which pose coatability challenges. Knowing the morphology and the type of these oxides can help pinpoint the defect source and improve AHSS coatability. The present work was conducted to determine the oxide phases formed as a result of annealing temperature and annealing furnace atmosphere. The annealed steel surface was evaluated using various surface analytical techniques. The oxide type and coverage were predicted by combining GDOES chemical depth profile with thermodynamic calculations using the Gibbs Energy Minimization (GEM) principle.
In advanced high strength automotive steels, small amounts of diffusible hydrogen can lead to a deterioration of mechanical performances, especially a loss of ductility, in the simultaneous presence of internal stresses and of a sensitive microstructure. In the hot-dip galvanizing process, hydrogen is mainly absorbed during high temperature operations in hydrogen-containing atmospheres before hot-dipping, when the solubility of hydrogen in steel is the highest. After hot-dipping, the metallic zinc overlay coating can act as a hydrogen barrier, and also the Fe-Zn intermetallic layer inhibits hydrogen diffusion out of the metal. As a result, an excess of diffusible hydrogen remains in the steel substrate and can subsequently leads to a possible embrittlement.
In this work, the effects of the coating nature on the hydrogen diffusion and embrittlement of a 980MPa dual phase steel (DP980) are investigated. The attention is focused on three Zn-based coating alloys: galvanized (Zn-0.23%Al), galvannealed (Fe-Zn based on Zn-0.12%Al) and Zn1.2Al1.2Mg. Indeed, if the behaviour of classical galvanized zinc coating with respect to hydrogen is well known, it is however not the case for its alternatives.
The hydrogen permeability of the coatings is first assessed through degassing experiments at room temperature on samples prepared with a hot dip galvanizing process simulator. The influence of cracks developed in the coating of the galvannealed samples is investigated, as these are not present in GI and Zn1.2Al1.2Mg coatings. Secondly, hydrogen uptake of these coated samples due to their corrosion in cyclic SAE J2334 corrosion testing is analysed. Finally, mechanical performances of corroded samples are evaluated. In all cases, the diffusive hydrogen content is measured through a thermal desorption analysis, with a critical assessment of the methodology.
One of the most important aspects for a material used in automotive industry is the reliability in service regarding corrosion resistance, safety and functionality. However, hydrogen embrittlement can cause a dramatic deterioration of mechanical properties, especially in case of advanced high strength steels (AHSS). Thus, it is essential to clarify the mechanisms of hydrogen insertion into the material as well as the effect of hydrogen on the mechanical behavior. Corrosion is one source for hydrogen in steel. Therefore, the present work is based on a comprehensive approach using electrochemical and thermal desorption techniques to study the hydrogen insertion into hot-dip galvanized dual-phase steel during corrosion. The effect of hydrogen from corrosion at defects as well as at cut edges is investigated and discussed. Results from hydrogen determination experiments are compared with those from mechanical tests to reveal the impact on the mechanical properties.
One focus of the present work is the influence of the corrosion conditions: pH sensitive hydrogels are used to visualize pH changes at these special areas where galvanic corrosion between steel and coating contributes to the overall corrosion process. A second focus is put on the influence of thermal and mechanical sample pre-treatment on the hydrogen entry into the material. Therefore, coated samples were pre-strained to uniform elongation and/or annealed, to simulate the heating cycle in the cathodic dip painting process, before immersion into sodium chloride solution to induce corrosion. Surface analysis of the differently pre-treated samples before and after corrosion was performed via scanning electron microscopy (SEM). Via thermal desorption mass spectrometry (TDMS) the amount of diffusible hydrogen after immersion of coated samples in aqueous sodium chloride solution was determined. Constant load test (CLT) and step load test (SLT) revealed the effect of corrosion and inserted hydrogen on the mechanical properties of the steel. Scanning Kelvin probe (SKP) and scanning Kelvin probe force microscopy (SKPFM) enabled in situ detection of inserted hydrogen during corrosion. In this manner, the role of defects in the coating as well as diffusion pathways of hydrogen within the steel microstructure was studied.
Hydrogen embrittlement of advanced high strength steels (AHSSs) in atmospheric exposure conditions is of utmost importance for automotive industry as the application of AHSSs grows steadily due to desirable mechanical properties. Different aspects of the hydrogen embrittlement phenomenon such as hydrogen entry caused by atmospheric corrosion reactions are currently under intensive research.
This study was focused on understanding the effect of atmospheric climatic and exposure conditions on atomic hydrogen formation, entry and diffusion in bare and zinc coated AHSSs. Several complementary techniques allowing for in situ investigation of hydrogen entry and permeation through complex phase and dual phase AHSSs and their mechanical properties have been employed on bare steel and zinc coated specimens with artificial defects. KircTec sensor is a new device for hydrogen permeation measurements based on monitoring of electric resistance changes. One side of a steel specimen was exposed to wet-dry cycling conditions in a corrosion chamber after application of sodium chloride solution in order to induce atmospheric corrosion, while hydrogen content was measured by the KircTec sensor attached to the opposite side. Scanning Kelvin Probe Force Microscopy (SKPFM) measurements with lateral resolution allowing for identification of permeation paths and the effect of microstructure as well as scanning Kelvin probe (SKP) measurements were conducted using a similar setup, recording changes in contact potential difference in controlled atmospheres. The effect of atmospherically induced hydrogen on mechanical properties of AHHSs was assessed by a Slow Strain Rate Test (SSRT) in real time.
The paper will present results on the effect of wet-dry cycling, presence of corrosion products, zinc coating as well as on the mechanism of atomic hydrogen entry.
To improve the surface feature of high-Mn steel, such as wettability with liquid Zn, a functionally graded multilayer was fabricated on the top of as-cast high-Mn steel slab by laser cladding process. For the alloying material, low carbon steel powder (0.07C, 1.83Mn) was deposited. To investigate the optimum condition for wettability, four different samples were prepared: bare high-Mn steel sample (LC-0), once, twice and three times cladded samples (LC-1, 2, 3). The thickness of clad layers was 2~4mm and that of slab was 140mm. Due to substrate re-melting effect of laser cladding, compositional gradient along the deposition direction appeared, resulting in different microstructure by layer. The first layer with 9% Mn had a dual-phase microstructure (martensite + interdentiric austenite) followed by bainitic and fully ferrite layers along the deposition direction. All samples were hot and cold rolled, and hot-dip galvanized in molten zinc bath. Defects in Zn coating and oxides, which were formed during the annealing prior to hot dipping, were investigated using SEM Energy Dispersive Spectrometry and X-ray Photoelectron Spectroscopy. As the number of deposited layer increased from LC-0 to LC-3, Mn content of the top surface of the sample decreased and the fraction of ferrite increased. MnO was dominant in LC-0, but as the number of deposited layer increased, the fraction of MnO-SiO2 oxides and Al2O3 increased. It is because Mn/Si and Al decreased as the number of clad layer increased and the diffusivity of Si and Al in ferrite is much higher than that of Mn in austenite. Because the laser cladding process turned coarse grains of the substrate into finer grains, oxides in LC-1, 2, 3 were smaller and finer, and evenly distributed than that of LC-0. When the surface was modified by laser cladding, internal oxidation along grain boundary also decreased, but there was almost no difference between sample LC-1, 2, 3. The reason would be attributed to the melt pool convection flow and the evolved solidification structure during the laser cladding process.
The lifetime of pot bearings is one of the major sources of maintenance shutdowns on a continuous galvanizing line. However the lifetime between different lines can be very different whereas the general process windows may look the same
A simple model is proposed to quantify the expected wear rate, considering the bearing geometry and process parameters. A viable model would help operators understand if a decrease of the bearing life is linked to variations in the process window.
The model has been used to explain the differences in bearing wear on several CGL using the same bearing materials. Validation showed that the observed trend is in relation with the model prediction. Next step will consist in comparing various lines and maybe set some coefficients to include the differences in bearing materials used (e.g. superalloys and ceramic).
This model approach attempts to put a little more science into bearing lifetime prediction.
Modern automotive grade continuous galvanising lines (CGL) are presented with a significant risk when considering online trials of new galvanising bath submerged journal bearings. Such trials of submerged bearings can pose significant financial loss should untested bearing components fail, resulting in unplanned maintenance stops. This limits opportunity for hardware improvements on modern CGLs where the cost of unplanned maintenance is high and as a result conventional materials and geometric designs of bearings remain unchanged for several years with limited optimisation effort.
To optimise new galvanising bath journal bearings an online trial is essential to ensure confidence in the technology. However, to mitigate the risk of testing new journal bearings materials and designs, a bespoke bearing test rig has been developed. This paper describes the development of an off-line bearing test rig and also explores the financial comparison between the development of the machine and unplanned online maintenance.
The experimental rig allows test samples to be rotated in a 200 kg bath of molten zinc or zinc alloy at a maximum rate of 300 RPM. This converts to a maximum simulated line speed of 200 m/min. A self-aligning test bearing housing ensures that the rotating journal specimen maintains a constant angle of abrasion with the bushing component. Displacement sensors which share a common linear velocity to the journal will provide real time measurement of wear rate and are capable of recording displacements as small as 10 µm. Acoustic emission is measured and bearing friction can be calculated through comparison of the torque meter and signal outputs from the motor. Initial tests have shown that this novel rig permits the safe and accurate testing of new bearing materials and designs. It can test small scale bars or full bearing assemblies, affording flexibility and more adventurous testing. Automation of the testing rig is permissible through an incorporated process control system which permits the rig to run continuously and safely for five weeks, simulating a galvanising campaign.
Mathematical modelling and measurement of the galvanising bath hardware in service conditions provide key information for designing and constructing the offline testing rig. The design process of the new bearing testing rig is then explored, with an emphasis on simulation of accurate line. The financial incentive of offline testing are outlined which draw comparisons between the development cost of the bearing rig compared to the predicted cost of a bearing failure and line stop.
This study experimentally investigated the ability of a novel multi-slot air-knife design to reduce high-intensity tonal noise and produce thinner coating weights in the continuous galvanizing gas jet wiping process. Coating weight measurements were carried out over a wide range of operating conditions to correlate the effect of the multi-slot jet operating parameters on the final coating thicknesses. These experiments showed that the resulting coating weight agreed with the predictions of the Elsaadawy et al.  analytical model and could produce thinner coatings versus the conventional single-slot geometry for the same main jet velocity with relatively low auxiliary jet velocities.
Further experiments showed that the novel multi-slot air-knife design reduced the tonal noise and jet oscillations of the aeroacoustics feedback mechanism exhibited by conventional single slot air-knifes, where the use of the auxiliary jets resulted in an average reduction in the acoustic tone intensity by 85%. Furthermore, the oscillation of the air-knife due to the large vortices of the aeroacoustics feedback mechanism was reduced by 45%, which resulted in a decrease in the fluctuating pressure at the substrate by 75%. The coating weight experiments also confirmed a correlation between suppressing the aeroacoustic feedback mechanism and lighter coating weights.
The findings of this work indicate that the multi-slot design can be a more effective wiping actuator in the continuous hot-dip galvanizing line, producing more consistent and lighter coating weights with less intense tonal noise when compared to the single jet design. This paper will discuss the results of these investigations in detail.
Galvanized press hardened steel (PHS) is in serial production more than 15 years. Zinc provides excellent cathodic corrosion protection for PHS parts, but can cause Liquid Metal Embrittlement (LME) and micro cracks. One can dispose of this LME risk by using the indirect hot forming process and avoiding tensile stress at high temperatures. To eliminate LME for the direct hot forming process there are two approaches. One approach is to use thin zinc coatings so that the whole coating after the PHS annealing furnace is pure zinc ferrite but the lower zinc content in the coating also reduces the cathodic corrosion protection. Another approach is pre-cooling (< 650°C) to avoid liquid zinc due to transformation to ZnFe-intermetallic phases before starting hot forming. Serial production of several hundred thousand tons has proved that all this countermeasures are successful.
In contrary to micro cracks caused by liquid zinc and forming at high temperatures, (depths up to several 100 µm) at forming temperatures below 650 °C and under special conditions small micro cracks (depths up to several 10 µm) may occur. Those micro cracks are also known as 2nd order micro cracks. They sometimes arise in parts with high frames and are often located in the frame near to the flange. Several mechanisms have been discussed about the origin of those 2nd order micro cracks. The weakening of austenitic grain boundaries near to the interface by zinc penetration and LME by residues of liquid zinc or LME by friction induced hot spot temperatures, were the most intensively discussed explanations so far. We could prove by several experiments resp. lab investigations and real part production, that 2nd order micro cracks are due to vapor metal embrittlement (VME). This results from the high zinc vapor pressure of the ZnFe-intermetallic phases. The cracking of the ZnFe-coating during hot forming generates high moveable Zn. If there is not sufficient oxygen to form immediately zinc oxide, there is a chance that Zn atoms reach the steel surface and weaken the austenite grain and/or austenite grain boundary. Special hot forming conditions, for example close local contact between part and tool together with high tensile stress may cause 2nd order micro cracks. Because of the limited amount of zinc atoms created by this mechanism, these micro cracks are limited to several 10 µm in depth. Measures to eliminate also these small micro cracks are proposed.
Zinc coatings offer the advantage of a galvanic corrosion protection instead of a conventional barrier corrosion protection. Thus zinc coatings prevent the corrosion of the steel substrate even in the case of a damaged coating layer. It is therefore of interest to use zinc coatings in the automobile industry particularly for components of the body in white. These components are often produced by the direct press hardening process consisting of a hot forming and a subsequent quenching step in the die. Zinc is known to have an embrittling effect on the sheet’s base material at elevated temperatures leading to surface cracks in the steel substrate during forming. To be able to numerically simulate the press hardening process including a reliable damage indicator model it is necessary to understand the crack mechanism of zinc induced cracks in detail. To this end metallographic investigations of galvannealed press-hardening steel sheets (20MnB8) using a light optical microscope as well as a scanning electron microscope comprising Electron-Backscattering-Diffraction (EBSD) and Energy Dispersive X-Ray Analysis (EDX) were conducted on selected cracks appearing during press hardening experiments of simple hat shaped components. The crack depth and distribution along the formed component’s cross sections were determined by light optical microscopy for establishing a subsequent correlation to the simulated load and temperature conditions. The crack propagation path in the substrate was correlated to the lattice orientation obtained from EBSD measurements. Thereby it was determined if cracks propagated along prior austenite grain boundaries or martensite block boundaries to analyze whether or not zinc penetration in prior austenite grain boundaries is an influencing factor for crack formation. The presence of zinc in the vicinity of the crack was detected by means of EDX scans. The investigations showed that the cracks propagate along prior austenite grain boundaries as well as martensite block boundaries. Hence zinc penetration in prior austenite grain boundaries does not seem to be the only relevant factor to zinc induced crack formation.
Knowledge of the crack distribution, the cracks’ propagation paths and the zinc distribution in the crack near regions enables to better understand the mechanisms leading to zinc induced crack formation, which is crucial for developing a valid damage criterion taking into account the most important influencing factors.
Zn-coated press hardened steels are in high demand in the automotive industry because their high strength enhances passenger safety while supplying robust cathodic corrosion protection. However, micro-crack formation after thermomechanical processing is an issue that limits full deployment. Thereby, the objective of this research was to determine the mechanism for micro-cracking in Zn-coated PHS by focusing on the relationship between the origin of micro-cracks and the diffusion-driven coating microstructural evolution as a function of annealing time.
Galvanized 22MnB5 steel sheets were annealed at 900°C for a variety of annealing times ranging from 30 – 780s and were then planar die-quenched with an average cooling rate of 100°Cs-1, resulting in a fully martensitic substrate microstructure. In order to precisely determine the degree of Zn penetration into the bulk substrate, two sets of samples were examined. The first set of samples were annealed for 30s (the shortest time) and 780s (the longest time) and die-quenched while the second set comprised tensile specimens from the 30s and 780s annealing times which were subsequently pulled to failure. The substrate prior austenite grain boundaries (PAGBs) and grain boundaries (GBs) of the Zn-ferrite (α-Fe(Zn)) coating were studied before and after tensile testing to determine if zinc diffusion in these regions contributed to micro-crack formation and propagation. Scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) indicated significant zinc enrichment in the PAGBs, GBs and at the micro-crack tip in the PAGB region for both the 30s and 780s planar die DHPF samples. Based upon the mentioned results, a new micro-cracking mechanism was proposed which clarified the importance of solid-state grain boundary diffusion and zinc enrichment in micro-crack formation and propagation in Zn-coated press hardened steels.
ZnAlMg (ZM) metallic coatings are already in use since the start of their development in the late 2000’s, while the production still benefits from a steady volume increase. It has been proven already that this type of metallic coatings is having an excellent corrosion resistance in divers harsh environments.
One of the highly used markets for this type of metallic coatings is the solar panel market, where different Zn-based metallic coatings are used as supporting structures for solar parks. These parks are typically installed in environments where the conditions can be quite harsh. Supporting poles need to be buried into the soil, where different soil parameters are influencing the corrosion. Another way of burying profiles in soil is by embedding them in concrete, which can also cause important corrosion, mainly during the initial drying phase after installation.
In the exposed part of the structures, several mechanisms of corrosion will take place, depending on the location. Atmospheric corrosion is the main one, but in desert areas (with a high amount of sunny hours, making them a preferred location to install large solar parks) also sand abrasion needs to be taken into account. It will be shown in this work to be an important factor influencing the lifetime of the protecting metallic coatings.
In this study, the performance of ZnAlMg was compared to other well-known Zn-based coatings in several of these harsh environments.
X-ray diffraction is a technique, which is widely used in material analysis. It is typically utilized in the laboratory to reveal information about the microstructure of materials.
IMS developed in cooperation with SMS group and Drever International a new measuring system, which applies the x-ray diffraction technology to the harsh environment of a production line. The new system provides a continuous online measurement of the fraction of the austenite content during heat treatment in an annealing furnace. The result is used as main control variable for the X-CAP® (X-Ray Controlled Annealing Process).
This presentation will describe the measures, which are necessary to manage the challenges of online measurement like temperatures and measuring geometry. We will discuss the performance of the system and the compensation of environmental influences. Furthermore, we would like to sum up the experiences which we achieved during the first year of operation in a CGL of Tata Steel SEGAL in Belgium.
Direct hot press forming (DHPF) of Zn-coated steels presents significant challenges associated with avoiding liquid metal embrittlement (LME) while maintaining robust cathodic protection. As identified by previous studies, a minimum amount of the Γ-Fe3Zn10 phase (15 vol%) is necessary to provide robust cathodic protection for Zn-coated press hardening steels (PHS). The Γ-Fe3Zn10 phase is liquid at typical forming temperatures, and the combination of a liquid metal phase present on the surface of the steel and an applied strain creates conditions known to cause LME. Therefore, the dual objectives of avoiding LME and providing robust cathodic protection are ostensibly incompatible under typical hot stamping conditions.
To this end, two prototype PHS steels which have increased manganese contents relative to 22MnB5 were created to enable stamping below the peritectic temperature of 783°C, thus potentially eliminating the liquid phase essential for LME. Experiments have shown that generating fully martensitic microstructures is possible while avoiding the presence of Zn-rich liquids during deformation and providing a sufficient fraction of the Fe3Zn10 phase to provide robust cathodic protection. The microstructural development, mechanical testing results, coating analysis and fractography of these novel direct press-hardening steels will be discussed as a function of the imposed processing routes.
Grain boundary weakening caused by grain boundary wetting is a potential precursor for liquid metal embrittlement in a Zn coated press hardening 20MnB8 steel. Galvanized press hardened steel samples, deformed by bending, and not deformed (flat) steel samples were analyzed by means of electron backscatter diffraction (EBSD), Auger electron spectroscopy (AES), energy dispersive X-ray analysis (EDX) and transmission electron microscopy (TEM) on the nanometer scale. Sample cross sections were prepared by Ar ion milling and subsequently analyzed via EBSD. Measurement of all possible phases such as bcc iron, different Zn/Fe phases (Zn-ferrite, gamma, delta) and ZnO was successful. We showed micro cracks which were formed between prior austenite grains and identified structures developed after micro crack formation. Zn as well as oxygen was detected by Auger electron spectroscopy on top of the micro crack surface. Zn/Fe phases were present at the wedge shaped crack tips, smaller than 100 nm in size. Zn distribution indicated that Zn penetrated from the crack tip further into the martensite bulk. For a complete picture, including the material state before micro cracking, we used electrolytic galvanized resp. Zn coated and not deformed samples. The thermal press hardening treatment was equally to hot dip galvanized 20MnB8. Cross sections were prepared by breaking the sample in a fracture stage and characterizing the interface coating steel by Auger spectroscopy. A Zn signal could be detected up to a depth of 25 µm near to the interface steel coating by those Auger measurements. By TEM EDX measurements, Zn could be found at prior austenite grain boundaries near to the interface coating steel. The quantity was one atomic layer or even less. The effect of Zn at prior austenitic grain boundaries on micro crack formation, due to grain boundary weakening, can not be ruled out from a physical characterization point of view.
The hot-stamping (HS) process has been widely used to produce automobile components, because of the great demand for higher strength steel sheets. Moreover, coated steel sheets, including galvannealed steel sheets (GA), are being applied as HS material [1,2]. The structural change of GA coating during the HS heating process has been researched [2,3]. However, the mechanical properties of this Fe-Zn solid solution has not yet been clearly understood. In this study, the hardness and the compressive deformation property of Fe-Zn solid-solution were investigated.
Hot stamped GA samples were prepared by heating them in an air furnace at 900°C for 4 minutes and then cooling them rapidly by pressing with flat mold tools. The hardness was measured by the micro-Vickers hardness test. The compressive deformation property was evaluated in the micropillar compression test. These micropillar specimens of Fe-Zn solid-solution single crystal were prepared with FIB. The crystal structure was measured by using XRD.
The hardness of Fe-Zn solid-solution was about 300 Hv. This is about six tenths of that substrate after hot-stamping (martensite), and about one-and-a-half times as hard as the substrate before hot-stamping (ferrite). In the compression test, the yield stress of Fe-Zn solid-solution single crystal was about 800 MPa, about half of that of the substrate (martensite). Furthermore, Fe-Zn solid-solution could be deformed by 9 percent. In these results, it was revealed that a Fe-Zn solid-solution is softer than the substrate of martensite, and that Fe-Zn solid-solution has a certain amount of plastic deformation capacity of compression. By analyzing the lattice spacing with XRD pattern, it was revealed that Zn atoms are located randomly in the Fe-Zn solid-solution lattice. Therefore it is presumed the mechanical properties of Fe-Zn solid solution are derived from those of ferrite.
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