<p>Ultrahigh strength steels are highly competitive materials for vehicles to concurrently meet the increasing demand of the weight reduction and passenger safety. Hot stamping is the key forming technology to manufacture automobile components with high strength. Hot stamping steel and its manufacturing technology experienced a fast development in the past decade. This paper reviewed the state of the art of the manufacturing and applications of hot stamping steels/components in the following aspects: (1) hot stamping steels (from traditional MnB steels to recently newly developed hot stamping steels); (2) forming technologies (from traditional hot stamping process to industry 4.0 intelligent production); (3) novel hot stamping + quenching & partitioning (Q&P) process and fundamentals of deformation assisted heat treatments; (4) simulation techniques for hot stamping process (modeling of the temperature-stress field, microstructure field and simulation of the manufacturing process); (5) the assessments of in-service performance of hot stamped components. Finally, the trends of the development of hot stamping steels and related forming technologies in the future will be discussed.</p>

汽车采用超高强钢是实现轻量化兼顾安全性的必由之路，热冲压成形是高强汽车零件成形的关键工艺。近10年来，热成形钢及其零件制造技术迅速发展。本文从以下几方面对热成形钢/零件制造与使用现状进行了综述：(1) 热成形钢材料(从传统MnB钢到最近新发布的一些热成形新钢种)；(2) 工艺(热成形传统工艺到工业4.0智能化生产)；(3) 热成形淬火配分(Q&P)创新工艺研究现状及形变热处理基本原理；(4) 热成形过程的仿真模拟(热/力场、组织场、工艺等的模拟)；(5) 热成形零件的使用服役评价。并对今后热成形汽车钢制造与使用前景作出展望。

Steel industries are always trying to manufacture advanced high strength steel (AHSS) product with higher efficiency and better mechanical properties. Over the past decades, replacing batch annealing by continuous annealing led to much improved production efficiency but not as much in the improvement of mechanical properties. Recently, a novel flash processing (FP), also termed as ultrafast heating (UFH) process, under which steel is heated at a rate over 100℃/s to a peak temperature and instantly cooled, has attracted more attentions due to its unprecedented high efficiency. The FP can significantly shorten the heat treatment duration to avoid grain growth, thus bringing significant strengthening effect without deteriorating ductility and toughness. The mechanical properties of typical AHSS grades subjected to FPs were summarized. The influence of process parameters, including initial microstructure, preheating, heating rate were analyzed. Moreover, the accurate microstructure tailoring via the ultrafast heating to an intercritical temperature was also briefly discussed. Finally, the industrial application of current FP technology was prospected.

对于先进高强钢而言，实现热处理工艺的简化与高效，并保证良好的力学性能一直为从业者所追求。过去数十年间，罩式退火发展到连续退火，虽然一定程度上提升了生产效率，但效果仍然有限。近年来，一种新型的闪速加热工艺（超快速加热工艺）因其极高的工艺效率而逐渐为业内所熟知，该工艺典型特点为以高于100℃/s的热速加热至目标温度并快速冷却。该技术的突出优势在于大幅缩短热处理周期、提升效率的同时，能够抑制晶粒的长大，从而获得显著的强化效果并能保证塑韧性。总结了多种先进高强钢经超快速加热工艺处理后的力学性能，并分析讨论了一些关键影响因素，如起始组织、预热温度、加热速率等的作用机制；同时，对该工艺下的精准组织调控进行了分析讨论，并对其工业化应用前景进行了展望。

Ultrafast heating (UFH) at the rates of 10-300 °C/s was employed as a new strategy to anneal a cold-rolled 7wt% Mn steel, followed by the immediate cooling. Severely deformed strain-induced martensite and lightly-deformed thermal martensite, both had been already enriched with C and Mn before, transformed to fine and coarse austenite grains during the UFH, leading to the bimodal size distribution. Compared with the long intercritical annealing (IA) process, the UFH processes produced larger fraction of RA grains (up to 37%) with a high density of dislocation, leading to the significant increase in yield strength by 270 MPa and the product of strength and elongation up to 55 GPa% due to the enormous work hardening capacity. Such a significant strengthening is first attributed to high density dislocations preserved after UFH and then to the microstructural refinement and the precipitation strengthening; whilst the sustainable work hardening is attributed to the successive TRIP effect during deformation, resulting from the large fraction of RA instantly formed with the bimodal size distribution during UFH. Moreover, the results on the microstructural characterization, thermodynamics calculation on the reverse transformation temperature and the kinetic simulations on the reverse transformation all suggest that the austenitization during UFH is displacive and involves the diffusion and partition of C. Therefore, we propose that it is a bainite-like transformation.

轻量化是支撑汽车电动化和智能化的重要赋能技术之一。抗拉强度为1500 MPa的热冲压成形用硼钢(22MnB5)是目前最经济有效的车身轻量化技术解决方案，而汽车工业对轻量化需求的日益提高正引领热冲压成形钢向着更高强度、更高塑性及更高断裂应变的方向发展。本文首先分析了车身轻量化对碰撞过程中构件变形抗力和断裂抗力的要求，解释了强度与塑性及断裂应变等材料的力学性能参量对碰撞变形抗力和断裂抗力的影响，然后介绍了作者及其他研究人员在研究开发更高强度、更高延伸率、更高断裂应变的新一代热冲压成形钢的最新进展：(1) 提出了一种新的强韧化方法，在热冲压成形钢的马氏体基体组织内引入大量纳米级的VC析出物，从而在获得2000 MPa强度的同时保持了与常规1500 MPa热冲压钢22MnB5相当的延伸率和断裂应变。(2) 在热冲压成形钢的微观组织中引入残余奥氏体，利用相变诱导塑性效应显著提升热冲压成形钢的延伸率；具体的工艺实施途径包括模具外淬火-配分工艺、淬火-闪配分工艺、淬火-回火配分工艺等。(3) 介绍了热冲压成形钢的新一代Al-Si镀层技术以及Al-Si镀层板断裂应变改善方面的最新研究进展；通过降低Al-Si镀层与钢板基体之间的合金化扩散来减少界面C富集，从而达到显著提高Al-Si镀层热冲压成形钢断裂应变的目的。

<p>It is very important to find out the mechanism of composition in steel. Many efforts have been put on the study of the effect of Si and Mn elements on the microstructure and mechanical properties of middle Mn steel, transformation induced plasticity (TRIP) steel and quenching and partitioning (Q&P) steel. But fewer studies were focused on the mechanism of Si and Mn contents in a press hardening steel. In this work, the microstructures after hot rolled and the fine martensite structure after hot stamping in ultra-high strength press hardening steel (PHS) with different Si and Mn contents were studied by OM, SEM, EBSD and TEM. The results showed that there are a great influence of Si and Mn contents on the microstructure and mechanical properties of PHS after hot rolled. The yield strength of the PHS increases from 552 MPa to 751 MPa, the ultimate tensile strength (UTS) increases from 757 MPa to 1124 MPa, and the microstructures are different with the Mn content rose from 0.57% to 1.21% and the other components remained the same. The UTS of the steels goes up as the Si content goes up from 0.25% to 0.38%, and the yield strength and the elongation show a fluctuation trend. After simulating hot stamping process at 950 ℃ and holding 5 min, the microstructure of the steels with different compositions is martensite, but it is different in the fine martensite structure and the average size of sub-grain; after hot stamping process, the comprehensive mechanical properties of the steel B with 0.30%C, 0.34%Si and 1.21%Mn are the most outstanding, the yield strength is 1161 MPa, the UTS is 1758 MPa, and the elongation is 6.5%; after hot stamping process, the microstructure of the steel B is fine lath martensite, and there is a large amount of dislocation in the martensite lath, and precipitates a small number of carbide. The mechanical properties of the ultra-high strength press hardening steels designed in this work is not obvious correlation before and after hot stamping process, and it is just a slight difference in martensite fine structure which is beneficial to controlling the performance stability of the mass industrial production.</p>

采用OM、SEM、EBSD和TEM等技术,研究了Si、Mn含量对超高强度热成形钢在相同的轧制和模拟热冲压成形工艺处理后的组织和性能的影响。结果表明,Si、Mn含量对热成形前轧制态钢的组织和性能有较大影响,在其它成分相同的情况下,随着Mn含量(质量分数)由0.57%增加到1.21%,实验用钢的屈服强度由552 MPa提高到751 MPa,抗拉强度由757 MPa提高到1124 MPa,组织由贝氏体+铁素体+珠光体转变为马氏体+贝氏体。随着Si含量由0.25%增加到0.38%,实验用钢的抗拉强度逐渐升高,屈服强度和伸长率呈波动趋势。在950 ℃保温5 min相同的工艺条件下模拟热冲压淬火实验后,4种钢的组织均为马氏体,但马氏体的精细结构各不相同,平均亚晶粒尺寸大小不一;含0.34%Si和1.21%Mn的钢B的综合力学性能最优,其屈服强度为1161 MPa,抗拉强度为1758 MPa,伸长率为6.5%,且热冲压成形后的组织为细小的板条马氏体,马氏体板条上有大量的位错,且只有少量的碳化物析出。基于本研究成分设计的超高强度热成形钢,其热冲压成形前的组织和性能与热成形后的力学性能无明显相关性,只是最终的马氏体精细结构略有差别,有利于工业化批量试制零件的性能稳定性控制。

The hierarchical martensitic features in ultra-high strength stainless steel (UHSSS), including the prior austenite grains, martensite packets, blocks and laths with the descending size, were refined to various extents by employing different thermomechanical processes and then carefully characterized. Their relation to yield strength and impact toughness was analyzed. We conclude that the refinement of martensitic structures could lead to the significant increase of yield strength, which follows the Hall-Petch relation with the effect grain size defined by high angle boundaries (HABs). Impact toughness of UHSSS depends on the frequency and capability for retained austenite (RA) grains at both HABs and martensite lath boundaries to trap the propagating cracks via strain-induced transformation, in which the film-like RA grains at lath boundaries appear to make the greater contribution.

Austenite formation from a ferrite-cementite mixture is a crucial step during the processing of advanced high strength steels (AHSS). The ferrite-cementite mixture is usually inhomogeneous in both structure and composition, which makes the mechanism of austenite formation very complex. In this contribution, austenite formation upon continuous heating from a designed spheroidized cementite structure in a model Fe-C-Mn alloy was investigated with an emphasis on the role of heating rate in kinetic transitions and element partitioning during austenite formation. Based on partition/non-partition local equilibrium (PLE/NPLE) assumption, austenite growth was found alternately contribute by PLE, NPLE and PLE controlled interfaces migration during slow-heating, while NPLE mode predominately controlled the austenitization by a synchronous dissolution of ferrite and cementite upon fast-heating. It was both experimentally and theoretically found that there is a long-distance diffusion of Mn within austenite of the slow-heated sample, while a sharp Mn gradient was retained within austenite of the fast-heated sample. Such a strong heterogeneous distribution of Mn within austenite cause a large difference in driving force for ferrite or martensite formation during subsequent cooling process, which could lead to various final microstructures. The current study indicates that fast-heating could lead to unique microstructures which could hardly be obtained via the conventional annealing process.

In the engineering design of machines and vehicles, the technologies involving vibration and noise reduction receive considerable attention as they can prevent undesirable fatigue failures and offer more comfort to the users. Compared with other damping alloys, high Mn steel has been intensively studied because its high strength and excellent damp capacity can be achieved simultaneously at a low cost. Particularly, it has a strong potential of circumventing the long-existing trade-off of strength and damping capacity for more applications in the new circumstance. In this study, the microstructures and damping and mechanical properties of a novel hot-rolled Nb-alloyed high Mn steel annealed at the temperature range of 750-1050oC were investigated. Heterogeneous recrystallization occurred during hot rolling, resulting in the formation of recrystallized bands along the rolling direction, in which fine lamellar ε martensite grains were interwoven with austenite; the coarse unrecrystallized ε martensite blocks were located between the bands. The latter was reversely transformed to the coarse austenite with a single orientation, remained unrecrystallized, and transformed back to the coarse highly-dislocated ε martensite blocks during the annealing below 850oC. Conversely, the ε martensite blocks reversely transformed the austenite grains partially and completely recrystallized to form even more refined grains with multi-orientations during annealing at 950 and 1050oC, respectively. Thus, the recrystallized austenite grains transformed to fine ε lamellar martensite, and the austenite grains were retained with multi-orientations, both having a low density of dislocation. Consequently, the higher annealing temperature led to higher damping capacity but lower strength; whereas, the partial recrystallization occurring during annealing at 950oC resulted in the best combination of mechanical and damping properties. Therefore, this indicates that tailoring the extent of recrystallization via the Nb-alloying and annealing process for high Mn steel can be an effective method to achieve different combinations of strength and damping capacity.

研究了回火温度对含15% δ铁素体(体积分数)的热轧高铝中锰钢显微组织和力学性能的影响。结果表明，钢中长300 μm的δ铁素体经热轧再结晶可被细化分割至大量长3 μm左右竹节状晶粒，且在回火温度高达700℃时尺寸不变。400~500℃回火后，马氏体基体依然维持较高的位错密度且析出细小渗碳体和纳米级VC粒子，屈服强度最高；同时C由马氏体向奥氏体的大量配分提高了奥氏体稳定性，获得了相对持久的加工硬化，最终获得了屈服强度约1500 MPa、抗拉强度1800 MPa和断后延伸率14%的最佳力学性能组合。该钢δ铁素体相一方面由于析出强化、晶粒细化和位错强化而得到显著硬化；另一方面δ铁素体体积分数不高且呈孤岛状嵌在马氏体基体中，这些特征导致该钢屈服强度由马氏体基体而非δ铁素体所决定，因此相比于其他类似Al含量的含δ铁素体中锰钢，屈服强度大幅度提高。