Advanced steels for lightweight automotive structures

Sun X., Stephens E., Khaleel M.

This paper examines the effects of fusion zone size on failure modes, static strength and energy absorption of resistance spot welds (RSW) of advanced high strength steels (AHSS). DP800 and TRIP800 spot welds are considered. The main failure modes for spot welds are nugget pullout and interfacial fracture. Partial interfacial fracture is also observed. The critical fusion zone sizes to ensure nugget pull-out failure mode are developed for both DP800 and TRIP800 using the limit load based analytical model and the micro-hardness measurements of the weld cross sections. Static weld strength tests using cross tension samples were performed on the joint populations with controlled fusion zone sizes. The resulted peak load and energy absorption levels associated with each failure mode were studied using statistical data analysis tools. The results in this study show that the conventional weld size of 4 t can not produce nugget pullout mode for both the DP800 and TRIP800 materials. The results also suggest that performance based spot weld acceptance criteria should be developed for different AHSS spot welds.

Walp M.S., Wurm A., Siekirk J.F., Desai A.K.

Speer J.G., Assunção F.C., Matlock D.K., Edmonds D.V.

Cornette D., Cugy P., Hildenbrand A., Bouzekri M., Lovato G.

Energy savings are among the most important goals of steel users. But generally, the increase of Tensile Strength for a given metallurgy is obtained to the detriment of ductility. Arcelor develops new ultra high strength steel with TWinning Induced Plasticity (TWIP) effect for weight reduction and impact resistance. This product, based on manganese (Mn) alloying metallurgy, has a tensile stress higher than 1000 MPa for a total elongation superior to 50%. Mechanical properties of these new steel grades are reported, in terms of tensile and forming behaviour, weldability and fatigue. Optimised designs are presented for safety parts in FeMn TWIP 1000 and assessed against conventional UHS steel. Tensile and formability properties are presented first by means of basic tests (stretching, bending, etc.) in order to classify the different steels. Crash resistance is investigated by using a dynamic axial compression test and a dynamic three point bending test on structural components with closed and open cross sections. For each steel grade, the estimated weight saving potential is compared with respect to parts manufactured in high drawing ability steel. The exceptional mechanical characteristics of this product permit to propose innovative steel design solutions for automotive safety component.

Chen X.M., Shi M.F., Chen G., Kamura M., Watanabe K., Omiya Y.

Yan B., Kantner C., Zhu H., Nadkarni G., Horvath C.

Wüebbels T., Matlock D.K., Speer J.G.

Shaw J.R., Zuidema B.K.

Cornette D., Hourman T., Hudin O., Laurent J.P., Reynaert A.

McCormick M.A., Fekete J.R., Meuleman D.J., Shi M.F.

DiCostanzo G.P., Matlock D.K., Foley R.P.

Campbell M.P.

Hsia H., Kidd J.A.

This paper describes a study to evaluate the potential of weight reduction by material substitution for passenger cars and light trucks. Production vehicles considered to be optimally designed for a given functional size were selected as baseline vehicles for the study. Four alternative materials were used in the analysis: high strength steels, aluminum alloys, fiberglass reinforced plastics, and hybrid reinforced plastics. Applying equal stiffness as the structural requirement, weight savings by direct material substitution were computed for each baseline vehicle.

Ruxanda R.

Kaya Ö., Cavusoglu O., Güral A.

The aging of commercial DP600 dual-phase steel under different pre-strain levels was conducted up to three times at 140 °C and 190 °C to investigate the effects of cyclic static strain aging (CSSA) on the material's mechanical properties. As the number of CSSA processes increased, the yield stress values exhibited a notable increase compared to those of the raw material. However, the yield strength demonstrated a tendency to decrease when the material was subjected to three cycles of strain aging at 140 °C. The yield stress values of dual-phase steels subjected to strain aging at 190 °C were higher than those of materials aged at 140 °C. As the CSSA temperature increased, the tensile stress values demonstrated a corresponding rise. However, the maximum tensile stress values of dual-phase steel demonstrated a decline with an increase in the number of CSSA repeats. Moreover, a reduction in Brinell hardness values was noted concurrently with an increase in ultimate tensile strength values. However, an inverse relationship was identified between the data points and the yield stress values. It was observed that the continuous yield property of dual-phase steel was lost following two and three CSSA processes at 190 °C.

Songthong T., Panich S., Chongbunwatana K., Primee S.

The modern automotive industry has highlighted a significant trend towards lighter automobiles. They frequently use a conventional forming limit curve (FLC) to assess the formability of widely used DP440 sheet steel. However, when the steel sheet undergoes multiple production steps, the curve becomes less effective in estimating the onset of necking, likely due to the twisting of the strain trajectory the sheet experiences. Therefore, engineers need an improved formability prediction tool to better handle such manufacturing elaboration and develop top-quality dies. This work proposes a phenomenological approach capable of detecting the onset moments of localized necking in DP440 sheets, which undergo progressive deformation under varying strain ratios. The approach begins with the establishment of an experimental bilinear failure strain database. Therein, six Marciniak tests are first carried out to pre-stretch rectangular sheet specimens by three different strain ratios, each with two different deformation degrees, to cover the whole deformation range encountered regularly in metal forming processes. To complete the bilinear failure strain database, all the specimens that have already been stretched are stretched again until breakage, following the Nakajima standard procedure. This produces six FLCs out of DP440 sheets previously pre-strained with six different strain values. Four displacement functions can then be derived from such a database, and a nonlinear FLC expressed in the form of a 3D forming limit surface is eventually composed. A nonlinear strain path up to failure, used to verify the developed FLC, is crafted by conducting a hole expansion test on prepared square specimens followed by a cup draw test on the same specimens both experimentally and simulatively. The nonlinear FLC reliably captures the onset of local necking of the test DP440 sheet deformed under a complex strain path, whereas the conventional FLC does not.

Ito K., Yokoi T., Hyodo K., Mori H.

AbstractTo advance the development of high-strength polycrystalline metallic materials towards achieving carbon neutrality, it is essential to design materials in which the atomic level control of general grain boundaries (GGBs), which govern the material properties, is achieved. However, owing to the complex and diverse structures of GGBs, there have been no reports on interatomic potentials capable of reproducing them. This accuracy is essential for conducting molecular dynamics analyses to derive material design guidelines. In this study, we constructed a machine learning interatomic potential (MLIP) with density functional theory (DFT) accuracy to model the energy, atomic structure, and dynamics of arbitrary grain boundaries (GBs), including GGBs, in α-Fe. Specifically, we employed a training dataset comprising diverse atomic structures generated based on crystal space groups. The GGB accuracy was evaluated by directly comparing with DFT calculations performed on cells cut near GBs from nano-polycrystals, and extrapolation grades of the local atomic environment based on active learning methods for the entire nano-polycrystal. Furthermore, we analyzed the GB energy and atomic structure in α-Fe polycrystals through large-scale molecular dynamics analysis using the constructed MLIP. The average GB energy of α-Fe polycrystals calculated by the constructed MLIP is 1.57 J/m2, exhibiting good agreement with experimental predictions. Our findings demonstrate the methodology for constructing an MLIP capable of representing GGBs with high accuracy, thereby paving the way for materials design based on computational materials science for polycrystalline materials.

Guan X., Qu S., Wang H., Cao G., Feng A., Chen D.

In advanced engineering applications, there has been an increasing demand for the service performance of materials under high-strain-rate conditions where a key phenomenon of adiabatic shear instability is inevitably involved. The presence of adiabatic shear instability is typically associated with large shear strains, high strain rates, and elevated temperatures. Significant plastic deformation that concentrates within a adiabatic shear band (ASB) often results in catastrophic failure, and it is necessary to avoid the occurrence of such a phenomenon in most areas. However, in certain areas, such as high-speed machining and self-sharpening projectile penetration, this phenomenon can be exploited. The thermal softening effect and microstructural softening effect are widely recognized as the foundational theories for the formation of ASB. Thus, elucidating various complex deformation mechanisms under thermomechanical coupling along with changes in temperatures in the shear instability process has become a focal point of research. This review highlights these two important aspects and examines the development of relevant theories and experimental results, identifying key challenges faced in this field of study. Furthermore, advancements in modern experimental characterization and computational technologies, which lead to a deeper understanding of the adiabatic shear instability phenomenon, have also been summarized.

Liu M., Chen Q., Tang H., Han L.

Automotive lightweighting has been a goal pursued by the automotive industry in recent years, and hot formed steel is the material support for achieving automotive safety lightweighting. In the current research process of hot formed steel in high-strength steel for automobiles, whether it is the QP steel series that obtains residual austenite through carbon partitioning, or the medium Mn steel that stabilizes austenite by adding Mn content, its strengthening and toughening mechanism is enhanced and plasticized through residual austenite. This study, represented by 20 Mn2Cr steel, systematically analyzed the microstructure of the material at different aging temperatures after continuous annealing at 830 °C using characterization methods such as XRD, EBSD, SEM, and TEM. The results showed that there was no obvious austenite peak in the XRD spectrum when the aging temperature was 270 °C; when the aging temperature is 300 °C and 360 °C, only at 2θ, a weak austenite peak appeared at an angle of about 74°, indicating that the residual austenite content in the experimental steel after continuous annealing treatment was very low, only 2.38 % and 4.09 %. This new generation of hot formed steel uses the strengthening and toughening mechanism of a small amount of fine high dislocation density ferrite, Cr carbide particles and refined ferrite flat noodles to achieve ultra-high strength and good plasticity, but uses the strengthening and toughening mechanism that combines high dislocation density ferrite and Cr carbide second-phase particles with refined ferrite flat noodles to improve the high performance of the new generation of automotive steel.

Yang Y.G., Su Z., Zuo W., Ni D., Hu Y., Wei Z., Mi Z.

Abstract In this study, the effect of welding currents and welding times on the microstructure and mechanical properties of the dissimilar welded joints of Q&P980 and Q&P1180 steel was investigated. The macrostructure and microstructure of the dissimilar welded joints were characterized and the relationship between the welding parameters and the mechanical performance was analyzed using confocal laser scanning microscope, scanning electron microscope, and mechanical properties testers. Results show that with the increasing welding current and welding time, the nugget diameter, indentation rate, and maximum shear force of the dissimilar joint increase. The absorption energy of the dissimilar joint increases when the welding current rises, while it increases first and then decreases with elevating welding time. All the hardness distributions of the dissimilar Q&P980/Q&P1180 joints exhibit the highest hardness value in the fusion zone and a gradually decreasing hardness value in the heat-affected zone. Moreover, with increasing current and time, much higher hardness occurs at the FZ/HAZ boundary. The microstructure characterization illustrates the martensite fraction in the intercritical heat-affected zone of the Q&P1180 side is higher than that of the Q&P980 side after the welding process. With the increase of welding current and time, the lath martensite in the fusion zone gradually coarsens. The coarsening martensite and the nugget diameter are responsible for the change in the shear force and energy absorption of the dissimilar Q&P980/Q&P1180 joints.

Stovpchenko G., Medovar L., Stepanenko D., Jiang Z., Dong Y., Liu Y.

Kwok T.W., Dye D.

Medium Mn steels are an emerging class of 3rd generation advanced high-strength steels. These steels have received significant attention due to their high strengths, large ductilities and also lower cost compared to their predecessor high Mn Twinning Induced Plasticity (TWIP) steels. Additionally, medium Mn steels have been found to exhibit TWIP and/or Transformation Induced Plasticity (TRIP) effects which can be harnessed to give a high strain hardening rate. Many thermomechanical processing concepts in the literature have been developed, producing multiple microstructure types with differentmechanical properties. The present review therefore aims to summarise the current knowledge of medium Mn steel alloy design especially on the processing, microstructure and property relationships in medium Mn steels. It complements the review of Sun et al. [Physical metallurgy of medium-Mn advanced high-strength steels, Int Mater Rev. 2023.], written independently and in parallel, which focusses more on the phase interfaces and thermodynamics.

Muñiz L., Trinidad J., Galdos L.

The quality and complexity demands of manufactured parts in sectors such as automotive and aeronautics lead to narrower process windows. This affects the repeatability and stability of the process, where material properties and process variations have a major impact. In bending processes, the bending angle is affected by variability in mechanical and microstructural properties, especially in high-strength materials. To address this, mechanical and microstructural characterization is crucial. This study conducted mechanical and microstructural characterization on five high-strength steels from different suppliers: three DP980 and two CP980. These materials are currently used by an industrial company in the automotive sector to manufacture a real product by means of U-bending, where a real issue of variability exists. Tensile tests were performed to quantify mechanical fluctuations. Microstructural analysis was also performed to determine the grain size and volume fractions of martensite and ferrite in the case of DP980, and ferrite, bainite, and retained austenite in the case of CP980. The largest variations were found for the hardening exponent, mean grain size, and elongation. To analyze their variability in an industrial process, U-bending tests were carried out using the five materials and the bending angle after the springback was measured. A total of 250 pieces were bent for the different materials and press strokes. Variations up to 1.25° in bending angle were found between the five batches for the same press stroke. A quantitative correlation analysis was performed to estimate the influence of the different parameters on the bending angle, where sheet thickness and tensile strength were shown to be two of the most influential parameters. Knowing this influence based on the variability of the properties, a control approach can be developed to reduce defects.

Abd Al Al S.A., Meilinger Á., Gáspár M., Lukács J.

Resistance spot welding (RSW) is one of the most common welding methods for steel sheets, as it is mainly used to join the automotive body structure parts. Different types of ultra-high strength steels (UHSS) have become widely used in the automotive body to obtain the required demands such as lower car weight, improving crashworthiness behavior, and enhancing strength–ductility combination. Martensitic UHSS belong to the highest grades width their tensile strength above 1000 MPa. During the lifetime of the vehicle cyclic loading generally occurs, therefore the optimization of welding technology should be performed considering the fatigue resistance of the welded joints. In our research 1 mm thick standardized lap shear sheets of martensitic MS1400 steel were welded by a TECNA 8007 RSW equipment with two different welding parameter combinations. The idea was to analyze the effect of welding and pulsation parameters on joint properties under static and cyclic loading. The welding parameters have been calibrated to produce the same weld nugget size for both technological combinations. Macroscopic, hardness, and tensile-shear tests were carried out to determine the fundamental mechanical characteristics of the RSW joints. The relation between the weld nugget microstructure and mechanical properties was explored. The high cycle fatigue (HCF) tests were performed on an MTS 810.23 universal electro-hydraulic materials testing system. A statistical approach was applied during the preparation and evaluation of the investigations, which increased their reliability. Measured and analyzed data of the lap shear welded joints, prepared by different technological parameters, were compared and discussed. The parameters of the HCF experiments were calculated considering the Japanese testing method (JSME S 002-1981). In most of the samples it was observed from both welding parameter combinations that the fatigue cracks initiate and grow in curvature shape in the softened part of the heat-affected zone towards the base metals in both directions symmetrically. A slight difference was observed in the HCF resistance of the welded joints prepared by different welding parameters.

Cheng L., Lin H., Zhang Y.

In order to realize the lightweight design of mobile pump truck, this paper takes the frame of a certain type of mobile pump truck as the research object. The response surface method is used to carry out lightweight design of the longitudinal beam structure of the frame, and the finite element method is used to establish the finite element model to compare and analyze the optimized and original designs. The results show that the height, width and thickness of the optimized longitudinal beam section are reduced by 10mm, 11mm, and 0.8mm respectively, and the weight of the whole frame is reduced by 35.8kg. Before and after optimization, the displacement and stress changes of the frame are small in four motion situations, which meet the lightweight requirements of optimization design.

Ding W., Zhang N., Zhang G., Li Y., Zhang M.

In this work, the phase transformation, microstructure, and mechanical properties of medium manganese steels containing 1.0 and 2.5% aluminum (mass%) were investigated at different intercritical annealing (IA) temperatures (670-820 °C) after very short IA times (1 min) using thermodynamic simulations, scanning electron microscopy, transmission electron microscopy, x-ray diffraction and uniaxial tensile tests. The results show that with the addition of aluminum, the temperature range between A1 and A3 increases from 338 °C for 1.0% Al to 506 °C for 2.5% Al. The retained austenite (RA) has two different morphologies, namely polygonal and lath. Most of the RA transformed into martensite during deformation. Two types of martensite were observed: the α’-martensite and ε-martensite. The steel with 2.5% aluminum (mass%), after IA at 790 °C, shows the best combination of tensile properties, including a tensile strength of 982.5 MPa, an elongation of 42.96%, and tensile strength × total elongation greater than 42 GPa%.

Cao Z.H., Wang Z., Ngiam Y., Luo Z.C., Geng Z.Y., Wang J.J., Zhang Y., Huang M.

Subedi U., Poudel S., Gyanwali K., Amorim Coutinho Y., Matula G., Kunwar A.

Though the martensitic transformation has been a commonly investigated topic in the field of experimental and computational materials science, the understanding of this mechanism in a variety of alloys is yet far from complete. In this era of Industry 4.0, there have been ongoing trends on employing machine learning (ML) techniques for the study of the martensitic alloys, and such data-driven approaches are expected to unravel a great amount of information about the process-structure-property behaviour relationship in this class of materials. However, with the availability of a large variety of datasets and with an option to use different ML models, a bulk amount of information has already been generated with regard to martensitic alloys. The discovery and design of shape memory alloys can be accelerated if the multi-principal element functional alloys and martensitic transformation phenomenon are studied extensively using machine learning techniques. Thus, it is necessary to highlight the major categories or aspects of these alloys that have been predicted with ML. The present work performs a state-of-the-art review on the machine learning models developed for the quantification of aspects such as martensitic start temperature (Ms), materials properties, microstructure, mechanisms etc., on the alloys.