Abstract Several decades have passed since the discovery of the transformation-induced plasticity effect in low-alloy steel. Many studies were published during its development, indicating that this steel received common interest from industries, universities, and research institutes. This article discusses transformation-induced plasticity (TRIP) steels, including their morphology, microstructures, internal and external factors that influence austenite stability and the mechanical properties of steels, measurements to detect their constituents, processing control in preparation and production, and developments in advanced high-strength steels and welding techniques. It describes application performance of TRIP automotive steels.

This study systematically explored the effects of pre-quenching (PQ) treatment on the microstructure evolution, mechanical properties and the underlying regulation mechanisms of the transformation-induced plasticity (TRIP) steel. PQ treatment transformed the initial cold-rolled ferrite-pearlite microstructure into a lath martensitic structure with high dislocation density and fine subgrain boundaries. This reconstructed microstructure provided abundant nucleation sites for reverted austenite during subsequent intercritical annealing. After isothermal bainitic transformation, the pre-quenched samples developed a refined multiphase microstructure alongside optimized retained austenite (RA) characteristics, including increased volume fraction, enhanced carbon (C) concentration and a lath morphology. The reduction in soft ferrite content, elevated dislocation density within the recovered martensite, and subgrain refinement collectively contributed to a significant improvement in yield strength (YS). Meanwhile, the enhanced stability of RA, along with the load redistribution effect from the refined bainite grains, prevented its premature martensitic transformation. This allowed the TRIP effect to be sustained over a broader strain range, thereby significantly enhancing ductility and work hardening capacity. For the 890P-300-TRIP sample, the product of strength and elongation (PSE) of 26.2 GPa·% was achieved despite a decrease in ultimate tensile strength (UTS), exhibiting a favorable strength-ductility balance. Furthermore, the microstructural response was sensitive to PQ parameters, which in turn altered the multi-scale deformation coordination mechanism. In the 990P-600-TRIP sample, for instance, a lower RA content and insufficient C concentration limited the sustainability of the TRIP effect, while its coarsened matrix structure weakened dislocation recovery capacity. The synergistic interplay between constrained RA transformation and suppressed matrix recovery ultimately enabled the prolonged maintenance of the work hardening plateau. This work demonstrates that PQ treatment is an effective strategy for tailoring the microstructure and properties of TRIP-assisted steels, with precise control of process parameters being crucial for optimizing their strength-ductility balance.

Hot-forming, as a typical representative forming technology of high-strength steel (HSS), is one of the most effective ways to manufacture structural components for achieving automotive lightweighting goal. In this paper, a newly-developed commercial microalloyed hot-formed steel is selected and its hot-forming is studied by experiments and simulations. The new steel has a wide undercooled austenite region, providing more suitable condition for the manufacturing of one-piece large-sized integrated parts. The high-temperature mechanical behaviors of the investigated steel show that the flow stress obviously decreases with the increase in deformation temperature, and it increases with the increasing strain rate. An integrated component assembly of the rear floor and longitudinal beam is selected as a typical one-piece integrated part when performing the hot-forming simulation to evaluate the formability. The influences of the key process parameters, namely forming velocity and frictional coefficient, on formability are further analyzed. Finally, the Latin Hypercube Sampling (LHS) method is used to generate the parameter combination and the Response Surface Method (RSM) is adopted in optimization. As a result, an optimal process parameter combination is obtained and its predicted result matches the simulated one very well, with a relative error of only 2.57%. The research results of this paper are favorable for understanding the mechanical behaviors of the hot-formed steel at elevated temperatures, improving the formability and providing a reference for the development of large-sized integrated hot-formed parts.

With the increasing demand for machine learning models to predict the mechanical properties of steel, model interpretability has been widely concerned. This is crucial for regulating the chemical composition and processing parameters of steel plates. In this study, process parameters and composition were considered as features, while ultimate tensile strength (UTS) and total elongation (TE) were served as the target variables. The mechanical properties of DH auto-steel plates were predicted using a machine learning method based on data from an industrial production line. The model trained with the Gradient Boosting Regression (GBR) algorithm demonstrated good prediction accuracy. In contrast, the symbolic regression expression obtained by the Sure Independence Screening and Sparsifying Operator (SISSO) algorithm exhibited a clearer relationship between the features and the targets but with lower predictive accuracy. To further analyze the effect of features on the properties, thermodynamic parameters were introduced. A simplified model was developed by extracting key feature combinations. The relationships between the extracted features and the mechanical properties of DH steels were then interpreted using SHapley Additive exPlanations (SHAP) values, Individual Conditional Expectation (ICE), and Partial Dependence Plots (PDPs). The value ranges for over-aging temperature, austenite fraction (AF), and C content in austenite (AC) that favor UTS and TE were determined. This can provide a theoretical reference for improving DH steel plates.

A comparative study on microstructures and mechanical properties of low-carbon low-alloy automotive steels with and without quenching pretreatment was conducted to investigate the strength-ductility enhancement mechanisms. Pre-quenching at 900 °C and 990 °C was performed on two steels. The two steels treated by quenching and partitioning (Q&P) and bainitic austempering (BAT) were named as “QP” and “CB” steels, respectively. The pre-quenched samples obtained lamellar microstructures with higher amounts of retained austenite (RA). Both steel samples reached the highest product of ultimate tensile strength and total elongation (PSE) when pre-quenched at 900 °C, while pre-quenching at 990 °C led to the coarser microstructure and performance deterioration. The coupled transformation-induced plasticity (TRIP) effect and dislocation strengthening caused the greatly enhanced work hardening of the pre-quenched QP sample displaying the best strength-ductility combination. The enhanced strain hardening capacity of RA and the promoted deformation capability mainly attributed to the more persistent strain hardening of the pre-quenched CB sample showing relatively high uniform elongation (UEL). For the two TRIP-assisted steels, the intensive TRIP effect was found to be harmful to crack initiation fracture toughness (i.e. lower necking ratio).

This study investigated the hydrogen embrittlement behaviors of three laser welding joints for the 1500 MPa AlSi coated steel using slow strain rate tensile and hydrogen concentration experiments. The results show that LWHT is fractured in the fusion zone without hydrogen charging because the S-ferrite reduces the mechanical properties. With the increase in hydrogen concentration, the fracture location is still FZ. The fusion zone of LWFHT is composed of martensite and retained austenite, and when the hydrogen concentration is 3.4 ppm, retained austenite traps many hydrogen atoms. The newly formed martensite during tensile inheriting the high hydrogen concentration in retained austenite causes cleavage in the fusion zone. When the hydrogen concentration is 13.6 ppm, most hydrogen segregates at the prior austenite grain boundaries, causing an intergranular fracture in the fusion zone. The fusion zone of LWF-HS is composed of martensite and carbide, and grain refinement and nanoscaled Fe3C can reduce HE susceptibility. With the increase in hydrogen concentration, the fracture location is still base materials. The significantly increased hydrogen concentration compared to LWF-HT is mainly trapped in carbides without reducing the banding force of dislocations and grain boundaries. This work provides a scientific basis and technical direction for realizing high-quality laser wire-filling welding of Al-Si coated steel.

In the framework of integrated computational materials engineering (ICME), combined with high-throughput thermodynamic and kinetic calculations, the composition and intercritical annealing temperature were designed for a 1180 MPa grade low-carbon and low-alloy transformation-induced plasticity (TRIP) steel. The steel with a composition of Fe–0.26C–2.19Mn–0.91Si–0.76Al (wt

Resistance spot welding (RSW) of press-hardened steel (PHS) with Al-Si coatings is challenging due to brittle fracture. In this paper, the joint alloying is achieved through the preset Ni foil welding method, and the microstructure, hardness, and strength of the joint are investigated. The results show that the microstructure of the quenched Al-Si-coated steel RSW nugget tongue consists of Fe-Al IMCs and delta-ferrite. Softening is observed at the boundary between the HAZ and the nugget. Cracks are generated at the interface of the Fe-Al IMCs and the HAZ during the tensile process. The fracture mode is mixed ductile-brittle fracture. With Ni addition, the microstructure transformation changes to lath martensite (LM) + austenite (gamma) + Fe-Ni-Al solid solution + Ni. The brittle Fe-Al IMCs in the solder joints are eliminated. Boundary layer softening is eliminated. The microhardness at the boundary between the nugget and the HAZ increases from 362.2 HV to 441.8 HV due to Ni solid solution strengthening. The fracture mode is ductile fracture, and the fracture force value has increased to 23.33 kN. The mechanical properties of the solder joints are improved.

The effects of isothermal treatment and rolling on the microstructure and mechanical properties of low carbon manganese steel were investigated using optical microscopy, scanning electron microscopy and transmission electron microscopy. The isothermal temperature affects the precipitation particle size, morphology, quantity and distribution characteristics. As the isothermal temperature increases from 500 to 700 °C, the microstructure was gradually transformed from bainite and ferrite to ferrite and pearlite, and further transformed to ferrite and martensite. The contribution of precipitation strengthening exceeds 120 MPa at an isothermal temperature of 600 °C. With the increase in the cooling rate, the lath characteristics of bainite becomes more obvious, and the transformation of granular bainite to lath bainite is accelerated. Under a certain rolling process, the strength of the high nitrogen steel is 10-50 MPa higher than that of the low-nitrogen steel. 180° cold bending test results show that there is no crack on the surface and sides of the specimen, indicating good cold work formability.

Al-Si coated 22MnB5 steel has been widely used in automotive manufacturing applications in the press hardened state and has become a key material to achieve lightweight and safety improvement. In the process of laser welding, the weld mechanical properties are deteriorated due to δ-ferrite precipitation caused by the Al-Si coating. To solve this problem, the common solution is to remove the Al-Si coating by laser ablation before welding. Despite its high cost and time involved, it is still widely adopted by the tailored blank manufacturers. In this work, laser welding with filled wire with three different chemical composition combinations has been developed to manufacture Al-Si coated steel tailored blanks. With this new manufacturing process, the formation of δ-ferrite can be inhibited without removing the coating. The experimental results show that Al element segregation exists in the fusion zone (FZ) of the welded joint, and the microstructure is a multiphase microstructure of δ-ferrite, α-ferrite and lath martensite (LM). When C-containing filler wire is used, there is still a certain amount of δ-ferrite in FZ, and the martensite type is changed from LM to high hardness and brittle twin martensite (TM). When Mn-containing filler wire is used, the full LM is obtained in the FZ, but there are shrinkage holes. Therefore, the mechanical properties of the above three kinds of welds still can’t meet the standards. When Ni-containing filler wire is used, Al segregation in the FZ is reduced, no δ-ferrite is detected in the FZ, and the microstructure is high density LM. In the tensile shear test, the tensile pattern is broken at the base metal, accompanied by obvious necking fracture. The tensile strength and microhardness of welded joints are increased by 50.6 and 26.3 pct, respectively, compared with the welded joints without filler wires.

Phase relations in the C-Fe-Zr ternary system were investigated using the experimental data obtained through the combination of X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA) techniques. Isothermal sections of the C-Fe-Zr system were experimentally determined at 1173 K, 1273 K, 1373 K, and 1473 K, and no ternary compound was found in this system. A self-consistent set of thermodynamic parameters of the C-Fe-Zr system were obtained for the first time using the CALculation of PHAse Diagram (CALPHAD) method, and the calculated results showed good consistency with the experimental data. It can provide reliable thermodynamic information of molten corium composition as one of necessary subsystems.

Given the developing high-performance and cost-effective heavy steel plate, the quenching-lamellarizingtempering (QLT) treatment was conducted on a designed low-Ni steel. Results revealed that the introduction of the "L" process can achieve impact absorbed energy of up to -130 J in the transverse direction, while ensuring GPa-grade high strength. Microstructural analysis showed that high-temperature lamellarizing promoted matrix recovery and recrystallization, facilitated the formation of reversed austenite, and resulted in a composite structure predominantly featuring tempered martensite (TM), intercritical ferrite (IF), and a considerable volume of retained austenite (RA). However, the DICTRA simulations indicated that with increasing lamellarizing temperatures, the C content within the reversed austenite decreased, compromising its thermal stability and leading to the transformation of some unstable reversed austenite into fresh martensite (FM). Consequently, samples treated at 700 degrees C exhibited a comparable RA content to those lamellarized at 675 degrees C, while the dislocation density increased appreciably due to FM formation, culminating in elevated ultimate tensile strength (UTS) and reduced total elongation (TEL). After tempering, the intensive precipitation of Cu-rich particles in FM compensated softening associated with high-temperature lamellarizing through precipitation strengthening and increased the yield strength (YS) by 87 MPa. Notably, tempering did not alter the RA content in the experimental steel; instead, it enhanced the enrichment of C elements in the RA, thus the sustained transformation-induced plasticity (TRIP) effect can be obtained by stabilizing RA. In addition, the excellent cryogenic toughness of QLT samples was attributed to the pronounced plastic deformation of the soft TM/IF matrix and the inhibition of crack propagation by RA. Despite the minimal impact of tempering on low-temperature toughness, the elastic strain energy was noticeably improved due to the precipitation of nano Cu-rich particles.

In this paper, the influence of Al distribution in the fusion zone (FZ) on microstructures and properties of welded joints was studied under 13 different laser powers (300 W -> 3000 W). The results show that Al can quickly accumulate at the upper surface of the FZ and the fusion line. Due to the stirring action of the beam and the dilution of the molten pool metal, Al becomes gradually uniform with the increase of laser power. This work guides the welding process parameters of lap welding the second layer steel plate to obtain uniform weld. In addition, with the decrease of Al content, the microstructure and hardness of the FZ change as follows: Al-based solid solution (Al > 60.2 wt%, 80 similar to 120 HV) -> Fe-Al intermetallic compound (IMC) (10 wt% < Al < 60.2 wt%, 340 similar to 980 HV) -> Fe-based solid solution (Al < 10 wt%, 350 similar to 530 HV). Therefore, this work revealed the relationship between the distribution-microstructure-hardness of Al and gave the possible microstructures and properties database of Fe/Al welding joints.

A comparative study on microstructures and mechanical properties of a low carbon Fe-Mn-Si steel with and without quenching pretreatment is performed to investigate the strength and plasticity enhancement mechanism. Microstructural analysis and 3D APT observation show that the lamellar microstructure obtained by introducing pre-quenching at 990 degrees C for 20 min prior to quenching and partitioning (Q&P) process mainly consists of ferritic laths, Mn-enriched tempered martensite and retained austenite with high mechanical stability. Mn enrichment in tempered martensite helps maintain a relatively high tensile strength. The refined alternative laths consisting of ferrite and tempered martensite share the same crystallographic orientation, which should be easier to hinder the crack propagation and delay the fracture. Compared to the polygonal-structured sample treated without quenching pretreatment, both higher frequencies of S3 CSL boundaries and higher volume fraction of stable retained austenite allow the pre-quenched sample exhibit an excellent strength-plasticity balance. The plasticity of the pre-quenched sample increases by nearly 90% and the product of strength and elongation (PSE) reaches higher than 24 GPa$%.(c) 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

In this paper, the quasi-static and dynamic tensile tests with a gauge length of 20 mm for dual-phase (DP) 780 steel were conducted under strain rates ranging from 10−3 to 103/s. The neglected details and differences between the quasi-static and dynamic tensile test conditions were investigated to develop an accurate numerical modeling method. A high-precision combination method for the empirical constitutive model was proposed and utilized in the developed numerical model to verify the ability of the constitutive model to reproduce the experimental data at different strain rates. The excellent results indicate that the adjusted determination coefficient of the constitutive model, the adjustable range of the extrapolation curves, and the proportion factor P in the combined constitutive model are the key parameters for accurately describing the quasi-static and dynamic mechanical behavior before and after the necking.

将材料热、动力学与中试试验相结合,针对商用淬火配分(Q&P,Quenching and Partitioning)钢的贝氏体进行优化设计.基于试验膨胀数据建立Bohemen切变模型,充分考虑淬火配分工艺下过时效等温阶段的组织演变复杂性,特别是回火阶段马氏体中碳化物析出特性以及对膨胀结果的影响,通过合理化数据对Bohemen模型进行修订,建立等温贝氏体相变动力学模型,实现贝氏体相变的精准预测.结合修订的Bohemen模型计算结果分析淬火配分钢中贝氏体/马氏体交互作用对残余奥氏体保留及稳定性的影响.结果显示,低淬火温度下马氏体板条间的残余奥氏体含量高,占据变形过程中的主导地位;反之,高淬火温度下贝氏体中的残余奥氏体含量高.由此形成基于材料计算下淬火配分钢的贝氏体优化设计.

Brittle intermetallic compound (IMC) is easily formed at the interface in resistance spot welding (RSW) of aluminum alloy and steel—heat significantly impacts the morphology of melt nugget and the distribution of IMC at the interface. This study studied the effects of process impact on morphology, IMC, and joint performance. The effects of process parameters on morphology, IMC, and performance under asymmetric heat input created by asymmetric electrodes were investigated. It was found that an IMC composed of Fe 2 Al 5 and FeAl 3 with a maximum thickness of about 5 μm was formed at the aluminum/steel interface in the welded joint. The thickness of IMC increased with welding current and welding time. The thickness of IMC was more relevant to welding time. Meanwhile, excessive heat input caused the aluminum nugget to be easily welded through. The Cu in the electrode diffused into the aluminum nugget and increased its hardness, but it did not improve the joint performance. The melting range of aluminum, thickness, and internal defects of the aluminum side was the main factors that affected the joint strength. This study shows that using asymmetric heat input is an effective method to improve the performance of steel and aluminum in RSW.

以DP980钢为研究对象,利用双向拉伸试验机采用载荷控制、应变控制和位移控制的加载方式开展了拉伸比例为4 ∶ 1、4 ∶ 2、4 ∶ 3、4 ∶ 4、3 ∶ 4、2 ∶ 4 和 1 ∶ 4 的双向拉伸试验.获得了 3 种控制方式 7 种拉伸比例下的双向拉伸应力-应变曲线,利用视频引伸计采集了粘贴在试样中心区域标记点的偏移量的变化过程,分析了材料在 3 种稳定控制方式下的加载历史及双向拉伸曲线.结果表明,3 种控制方式下获得的双向拉伸曲线之间差异非常明显,即使材料在两个加载方向的拉伸比例相同,由于各向异性的影响,其应力和应变也并不相同,应变控制试验的标记中心点偏移量最小,位移控制试验过程中的载荷和应变的变化趋势较大.

Aldehyde condensation is a reaction step in the oxidization of a lubricant base stock into high-molecular-weight products, forming sludge and a paint film, which lead to the failure of lubricating oil. Calculations on the basis of the density functional theory (DFT) were employed to investigate the reaction mechanism of the acid-catalyzed aldol condensation of a lubricant base stock. Carbonyl compounds could be converted into their resonant enol structures. However, the activation energy of the process was relatively high, and it was difficult to initiate. The existence of the acid could obviously decrease the activation energy of the reaction from 269.17–287.82 kJ/mol to 177.10–177.63 kJ/mol, and it significantly reduced the difficulty of initiating this reaction. The carbocation formed by the carbonyl compounds and acid could further react with the enol and produce an intermediate reaction product in which the chain of molecules grew longer. This process was not difficult to initiate, with a reaction activation energy of 65.10 kJ/mol. The intermediate product with a larger molecular weight could be converted into carbonyl compounds containing a β-hydroxy by removing a hydrogen proton from it. The energy barrier for this process was 193.15 kJ/mol, and it was not easy to initiate the reaction.

A salt spray test was used to evaluate the corrosion resistance of pickling high-strength steel JSH440W to phosphating film on its surface, and scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) were used to analyze the micro-morphology and element content of the surface defects area of the sample after salt spray test. The results show that the enrichment of Si, Mn, and O elements on the surface of pickling high-strength steel JSH440W badly influences the film-forming performance of phosphating film. The enrichment of Si, Mn, and O elements on the strip steel surface will slow down the film-forming speed of the phosphating film and make the thickness of the phosphating film thinner, which will lead to the reduction of corrosion resistance in this area, and it is easy to cause pitting corrosion behavior, to cause the paint film on the sample surface to fall off. Reducing the enrichment of Si, Mn, and O elements on the surface of acid-washed high-strength steel and increasing the roughness uniformity of strip steel is beneficial to improving the quality of phosphating film and the coating property of pickling high-strength steel.