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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.