As a representative of steels available in the market, medium-Mn steel shows vast application prospects in lightweight automobile fields. This review details the research progress of medium-Mn steels, focusing on the following aspects. The roles of common adding elements, rolling technologies, and various heat treatments on the microstructure and mechanical properties of medium-Mn steel are analyzed, thus providing references for designing tailored medium-Mn steel with excellent performance. Considering that hydrogen embrittlement is a challenge faced in the development of high-strength steel, the hydrogen embrittlement behavior of medium-Mn steel is also discussed, particularly emphasizing the influence of microstructure, hydrogen concentration, strain, etc. Furthermore, practical strategies to improve resistance to hydrogen embrittlement are summarized. Finally, this review provides prospects for the development and research prospects of medium-Mn steel.

Steels with medium manganese (Mn) content (3∼12 wt-%) have emerged as a new alloy class and received considerable attention during the last decade. The microstructure and mechanical response of such alloys show significant differences from those of established steel grades, especially pertaining to the microstructural variety that can be tuned and the associated micromechanisms activated during deformation. The interplay and tuning opportunities between composition and the many microstructural features allow to trigger almost all known strengthening and strain-hardening mechanisms, enabling excellent strength-ductility synergy, at relatively lean alloy content. Previous investigations have revealed a high degree of microstructure and deformation complexity in such steels, but the underlying mechanisms are not adequately discussed and acknowledged. This encourages us to critically review and discuss these materials, focusing on the progress in fundamental research, with the aim to obtain better understanding and enable further progress in this field. The review addresses the main phase transformation phenomena in these steels and their mechanical behaviour, covering the whole inelastic deformation regime including yielding, strain hardening, plastic instability and damage. Based on these insights, the relationships between processing, microstructure and mechanical properties are critically assessed and rationalized. Open questions and challenges with respect to both, fundamental studies and industrial production are also identified and discussed to guide future research efforts.

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.

Hot rolled plates of a low-carbon medium-manganese test steel 4Mn (Fe-4Mn-3.5Ni-2Cu-0.05C-0.018Nb-0.018Ti, in mass fraction) were heated at 860 oC for 1 h and then water quenching, followed by tempering treated at 600, 640, and 670 °C for 2 h respectively. Next, the effect of tempering process on the microstructure and mechanical property of the steel plates was assessed via SEM+EBSD, XRD, TEM, pendulum impact testing machine and hydraulic tensile testing machine. The results indicate that the microstructure of the steels tempered at different temperatures is composed of tempered martensite/ferrite + reversed austenite + fresh martensite. With the increasing tempering temperature, the ultimate tensile strength and work hardening index increase sequentially, while the yield strength and low-temperature impact toughness decreases sequentially. For tempering at 600 and 640 oC, the increase in ultimate tensile strength is primarily due to the transformation-induced plasticity effect formed by a fresh martensite, with the improving work hardening capability of the steel gradually as the temperature rises. For tempering at 670 oC, the content of fresh martensite significantly increases, enhancing the work hardening capacity and further boosting the ultimate tensile strength. However, the steel becomes excessively hard and brittle, leading to premature necking and a reduction in elongation. The decrease in low-temperature impact toughness is due to two factors, on one hand the mechanical stability of reversed austenite decreases, weakening its ability to mitigate stress concentrations through transformation, resulting in a reduction in the energy required for crack initiation; on the other hand, the increase of twins and blocky reversed austenite leads to a shift in the fracture austenite grain boundaries, thereby reducing the crack propagation energy. The precipitation of Nb/TiC in steel can help refine the grain size. In addition, the Cu-rich phase coarsens with the increase of tempering temperature, significantly reducing the yield strength.

将低碳中锰钢Fe-4Mn-3.5Ni-2Cu-0.05C-0.018Nb-0.018Ti在860 ℃淬火1 h后水冷，再分别在600、640和670 ℃回火2 h，然后分别用JBN-300C摆锤冲击实验机和WE-300液压拉伸实验机进行冲击实验和拉伸实验。结果表明，在不同温度回火的实验钢，其微观组织均由回火马氏体/铁素体+逆转变奥氏体+新鲜马氏体构成。随着回火温度的提高实验钢的抗拉强度和加工硬化指数随之提高，屈服强度和低温冲击功降低。在600和640 ℃回火的实验钢，其抗拉强度的提高主要依赖相变诱导塑性效应生成的新鲜马氏体，其加工硬化性能随着回火温度的提高而改善。回火温度提高到670 ℃新鲜马氏体的含量显著提高，虽然加工硬化能力和抗拉强度进一步提高，但是实验钢变得硬而脆和颈缩提前，从而使其延伸率降低。低温冲击功降低的原因，一个是逆转变奥氏体机械稳定性降低和相变缓解应力集中的效果减弱，使裂纹形成功降低；另一个是孪晶和块状逆转变奥氏体数量的增多导致断裂模式转变为晶间断裂，裂纹易沿着孪晶界和原奥氏体晶界高速扩展而降低了裂纹扩展功。实验钢中Nb/TiC的析出，有助于晶粒细化。同时，回火温度的提高使富Cu相粗化，使实验钢的屈服强度显著降低。

By aging treatment of a medium manganese lightweight steel at 550oC, the evolution of its microstructure and mechanical properties was analyzed. The results indicate that aging treatment has a significant impact on precipitates. When the aging time is less than 30 minutes, a large number of intragranular κ'-carbides formed by spinodal decomposition, which distributed dispersedly in the austenite matrix. As the aging time increases, besides the intragranular κ'-carbides, intergranular κ-carbides are also formed at grain boundaries through eutectoid reactions as a layered structure of α ferrite and κ-carbide. Intragranular κ'-carbides greatly increase the strength, but intergranular κ-carbides significantly reduce the ductility of the steel. Compared with long-time aging, the strength of the steel can be significantly increased by short-time aging, while the steel still maintains a high elongation with better overall mechanical properties. Among them, the good performance is achieved for the steel after aging for 30 minutes, namely a tensile strength of 1031 MPa, yield strength of 784 MPa, an elongation of 41.08%, and a product of strength and elongation of 42.35 GPa·%.

对中锰轻质钢进行不同时间的时效处理，研究了时效时间对其微观组织演变和力学性能的影响。结果表明，时效处理时间对析出物有显著的影响。时效时间短于30 min时，在奥氏体基体内析出大量调幅分解生成的弥散分布的晶内κ'碳化物。随着时效时间的延长，除了在晶内析出κ'碳化物，在晶间也发生共析反应在晶界处形成κ-碳化物，构成(α-铁素体+晶间κ-碳化物)片层组织。晶内κ'碳化物的生成使钢的强度提高，但是晶间κ-碳化物的析出使其塑性显著降低。与长时时效相比，短时时效使钢的强度显著提高，还使其保持较高的伸长率，使其具有更优异的综合力学性能。时效30 min的钢，其抗拉强度为1031 MPa，屈服强度为784 MPa，伸长率为41.08%，强塑积为42.35 GPa·%。

The effects of heat treatment on the microstructure evolution was studied in regards to austenite nucleation and grain growth. It was found that the austenite nucleation and matrix recrystallization kinetics of samples annealed at 675 °C for different times were revealed, implying a strong interaction between the ferrite matrix and austenite. The recrystallization of the matrix during annealing provided favorable conditions for austenite nucleation and growth, and the formation of austenite during this process reduced the matrix recrystallization kinetics, thus delaying the recrystallization process of the matrix around the austenite grains. The statistical results for the austenite grain size under different annealing temperatures indicated that the average grain size of the austenite slightly increases with increasing of the annealing temperature, but the austenite with the largest grain size grows faster at the same temperature. This difference is attributed to the strict Kurdjumov Sachs (KS) orientation relationship (OR) between the austenite grains and the matrix, because the growth of austenite with a strict KS OR with the matrix is often inhibited during annealing. In contrast, the austenite maintains a non-strict KS OR with the matrix and can grow preferentially with increasing annealing temperature and time.

The mechanical properties of a novel heterogeneous cold-rolled medium Mn steel were investigated by means of mechanical testers, in situ EBSD (electron back-scattered diffraction) and SDTEM (spherical differential transmission electron microscope). The results show that the sample annealed at 680°C consists of multiple microstructure of austenites (granular shape, blocky shape, and lath-like shape) and fine ferrite grains. The heterogeneous steel has ultimate tensile strength of 1.27 GPa, total elongation of 54.5% and product of strength and elongation of 69.3 GPa·%. During tensile deformation the granular-shape austenite with a low C/Mn content preferentially transforms into martensite ahead of the blocky-shape and lath-like austenite with high C/Mn content, and the multi-type microstructure of austenite with various stability lead to a continuous TRIP effect in a large strain region, which is responsible for the excellent properties of the heterogeneous medium Mn steel. In addition, austenite grain boundaries or austenite/ferrite interfaces are the preferred nucleation zone of martensite during deformation. The effect of Mn/C content on austenite stability readily overrides those of grain size.

使用原位电子背散射衍射(EBSD)和球差透射电镜(ACTEM)等手段，研究了新型异质结构中锰TRIP钢在拉伸过程中微观组织的演变机制和力学性能。结果表明，在680℃退火后的实验钢中生成了多形貌、多尺度的异质奥氏体结构(颗粒状、块状、片层状奥氏体)和铁素体组织，其抗拉强度为1272 MPa，总延伸率为54.5%，强塑积高达69.3 GPa·%。在拉伸过程中C/Mn含量较低的颗粒状奥氏体先发生相变，而C/Mn含量较高的块状和片层状奥氏体在较大的应变范围内逐渐发生相变，从而导致高强度与高塑性的良好匹配。结果还表明，马氏体相变优先在奥氏体晶界/相界附近的区域形核。与晶粒尺寸相比，C/Mn元素对奥氏体稳定性的作用更重要。

高强度、高塑性是汽车钢的重要发展方向，本文综述了高强度高塑性第三代汽车钢的“多相(multiphase)、亚稳(metastable)和多尺度(multiscale)” M³组织性能调控理论和技术，以及面临的新挑战。M³组织与性能调控理论为高强度高塑性钢提供了理论支持，亚稳奥氏体的相变诱发塑性(TRIP)效应能够提高加工硬化率并推迟颈缩的发生，从而提高了钢的强度与塑性，同时产生了剪切边裂纹敏感性提高，氢致延迟断裂性能下降，循环载荷下亚稳奥氏体的转变行为复杂等新的问题和挑战。当前，含亚稳奥氏体高强度高塑性钢的质量一致性和应用基础研究缺乏，而汽车钢作为量大面广的产品，需要从它的成分设计和组织调控-冲裁切割-成形制造-连接涂装-服役评价等全链条环节中开展组织演变和性能评估，充分考虑产品的技术适用性和成本，进而为组织调控理论和技术的完善提供依据。

Owing to the excellent combination of specific strength and ductility, medium Mn steels (MMSs) with Mn contents of 3%-12% (mass fraction) are considered the most promising candidates for the third-generation advanced high-strength steel. The combination of excellent strength-ductility is mainly attributed to the active transformation-induced plasticity effect of the metastable retained austenite during deformation. Therefore, producing a considerable amount of retained austenite with reasonable stabilities in the steel by various heat treatment schedules is always important. In this study, granular- and lamellar-structured retained austenites were developed in a cold-rolled 0.15C-5Mn MMS by introducing a technical process of precontrolling ferrite recrystallization in the annealing schedule. The microstructures of the annealed samples were analyzed using SEM, EBSD, and TEM. The results show that duplex microstructures comprising various amounts of recrystallized ferrite and fresh martensite can be obtained in the cold-rolled MMS when controlling the occurrence of recrystallization at different intercritical temperatures by a preannealing process. When this microstructure is used for the final austenite reverted transformation annealing, the resultant ultrafine duplex microstructure with recrystallized ferrite and two types of heterogeneous retained austenite, i.e., lamellar and granular, is produced. The heterogeneous-structured austenite shows more sensitivity to increasing strain, i.e., various mechanical stabilities, which enable an excellent strength-ductility combination and reduced Lüders strain in the cold-rolled medium Mn steel.

为探究铁素体再结晶对冷轧中锰钢微观组织与力学性能的影响规律，以0.15C-5Mn (质量分数，%)冷轧中锰钢为研究对象，采用两步临界区退火的热处理方法，利用SEM、TEM和EBSD等表征手段和力学性能测试方法，研究了铁素体再结晶调控对冷轧中锰钢多样化残余奥氏体形成及其力学性能的影响。结果表明，通过在不同温度预先调控冷轧中锰钢中的铁素体再结晶，可获得由不同比例的等轴状再结晶铁素体和马氏体组成的双相细晶组织。经常规退火处理后，在终态组织中形成了不同体积分数的超细晶再结晶铁素体和呈等轴状/板条状形貌的多样化细晶残余奥氏体，使中锰钢在拉伸变形过程中表现出多样化的TRIP效应，在提升冷轧中锰钢强塑性能的同时，其Lüders变形也获得改善。

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.

A medium-Mn steel (Fe-12Mn-3Al-0.05C wt%) was designed using Thermo-Calc (R) simulations to balance the fraction and stacking fault energy of reverted austenite. lntercritical annealing for 0.5, 8 and 48 h was carried out at 585 degrees C to investigate the microstructural evolution. X-ray diffraction (XRD), electron backscatter diffraction (EBSD), 3-dimensional EBSD, energy-dispersive spectroscopy via scanning transmission electron microscopy (STEM-EDS) and atom probe tomography (APT) enable characterization of phase fraction, grain area, grain morphology and alloy partitioning. An increase in annealing time from 0.5 h to 48 h increases the amount of ultrafine-grained (UFG) reverted austenite from 3 to 40 vol %. EBSD and TEM reveal multiple morphologies of UFG austenite (equiaxed, rod-like and plate-like). In addition, most of the remaining microstructure consists of recovered alpha'-martensite that resembles the cold-rolled state, as well as a relatively small fraction of UFG ferrite (i.e., only a small amount of martensite recrystallization occurs). Multi-scale characterization results show that the location within the cold-rolled microstructure has a strong influence on boundary mobility and grain morphology during austenite reversion. Results from APT reveal Mn-decoration of dislocation networks and low-angle lath boundaries in the recovered alpha'-martensite, but an absence of Mn-decoration of defects in the vicinity of austenite grains, thereby promoting recovery. STEM-EDS and APT reveal Mn depletion zones in the ferrite/recovered alpha'-martensite near austenite boundaries, whereas gradients of C and Mn co-partitioning are visible within some of the austenite grains after annealing for 0.5 h. Relatively flat C enriched austenite boundaries are present even after 8 h of annealing and indicate certain boundaries possess low mobility. At later stages the growth of austenite followed the local equilibrium (LE) model such that the driving force between two equilibrium phases moves the mobile interface, as confirmed by DICTRA simulations (a Thermo-Calc (R) diffusion module). The sequence of austenite reversion is: (i) formation of Mn- and C-enriched face-centered-cubic nuclei from decorated dislocations and/or particles; (ii) co-partitioning of Mn and C and (iii) growth of austenite controlled by the LE mode. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd.