Hot ductility behavior of a Fe-0.3C-9Mn-2Al medium Mn steel

The hot ductility of a Fe-0.3C-9Mn-2Al medium Mn steel was investigated using a Gleeble3800 thermo-mechanical simulator.Hot tensile tests were conducted at different temperatures(600-1300℃)under a constant strain rate of 4×10^(−3)s^(−1).The fracture behavior and mechanism of hot ductility evolution were discussed.Results showed that the hot ductility decreased as the temperature was decreased from 1000℃.The reduction of area(RA)decreased rapidly in the specimens tested below 700℃,whereas that in the specimen tested at 650℃was lower than 65%.Mixed brittle-ductile fracture feature is reflected by the coexistence of cleavage step,intergranular facet,and dimple at the surface.The fracture belonged to ductile failure in the specimens tested between 720-1000℃.Large and deep dimples could delay crack propagation.The change in average width of the dimples was in positive proportion with the change in RA.The wide austenite-ferrite intercritical temperature range was crucial for the hot ductility of medium Mn steel.The formation of ferrite film on austenite grain boundaries led to strain concentration.Yield point elongation occurred at the austenite-ferrite intercritical temperature range during the hot tensile test.

Mechanism of the Interfacial Reaction of AI_2O_3/Medium Mn Steel Containing Nb

The mechanism of the interfacial reaction of Al_2O_3/medium Mn steel containing Nb was studied by means of the observation on the interfacial reaction phenomenon of Al_2O_3/medium Mn steel,and the analyses on the interracial phases.The results show that when T≥1550℃,the interfacial reac- tion of Al_2O_3/medium Mn steel containing Nb happened.In the medium Mn steel matrix,Nb exists in the form of NbC.NbC are the nucleating base of CO gas bubbles.

Accelerating bainite transformation by concurrent pearlite formation in a medium Mn steel:Experiments and modelling

Bainite transformation has yet to be utilized and even thoroughly studied in medium Mn steels.Here,we investigate the isothermal bainite transformation in a 10Mn steel at 450°C experimentally and theoretically,focusing on the effect of dislocations introduced by warm deformation.We show that the bainite transformation in the studied medium Mn steel exhibits extremely sluggish kinetics(on a time scale of days),concurrent with the pearlite formation.The introduced dislocations can significantly accelerate bainite transformation kinetics while also facilitating the pearlite reaction.This is likely the first report on the simultaneous occurrence of these two solid-state reactions in medium Mn steels.With respect to the roles of dislocations in the acceleration of bainite transformation observed in this work,we propose a new‘carbon depletion mechanism’,in which dislocations-stimulated pearlite formation makes a twofold contribution:facilitating the formation of bainitic ferrite sub-units to further enhance the autocatalytic effect and preventing the carbon enrichment in the remaining austenite.On this basis,a physical model is developed to quantitatively understand the bainite transformation kinetics considering the effect of concurrent pearlite formation,revealing good agreements between model descriptions and experiment results.Our findings,herein,offer fundamental insights into the bainite transformation in medium Mn steels and uncover a previously unidentified role played by introduced dislocations in influencing the kinetics of bainite formation,which may guide its future application in manipulating microstructure for the development of advanced high-strength steels.

Tailoring the austenite characteristics via dual nanoparticles to synergistically optimize the strength-ductility in cold rolled medium Mn steel

In this work,we proposed a novel Cu/θdual nanoparticles strategy to tailor the austenite characteris-tics of a medium Mn steel via a tempering-annealing process to optimize the mechanical properties.We explored the effects of Cu-rich particles and cementite precipitated in the tempering process on the austenite reversion during the subsequent annealing process.Both experiments and numerical simula-tions verified that the austenite inherited from cementite had a finer size and a higher Mn enrichment compared with the austenite originating from the tempered martensite matrix.In addition,quantitative evaluations revealed that the pinning effect exerted by the Cu-rich particles could significantly hinder theα/γinterface migration and the recrystallized grain growth,thereby further refining the final mi-crostructure.With contributions from the effects of dual nanoprecipitates on the austenite reversion,the heterogeneous austenite grains inherited from varying nucleation sites ensured the sustained and gradual deformation-induced martensite and twinning formation.Therefore,the Cu-added steels subjected to a tempering-annealing process achieved synergetic enhancement of the tensile strength from 1055 MPa to 1250 MPa and elongation from 33%to 45%.This strategy may provide new guidance for the development and alloy design of high-performance medium Mn steels.

Influences of Austenization Temperature and Annealing Time on Duplex Ultrafine Microstructure and Mechanical Properties of Medium Mn Steel

A duplex ultrafine microstructure in a medium manganese steel （0.2C-5Mn） was produced by austenite re- verted transformation annealing （ART-annealing）. The microstructural evolution during annealing was examined by scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and X-ray diffraction （XRD）. Based on the microstructure examination, it was found that some M3C type carbides appeared in the martensitic matrix at the beginning of the ART-annealing. But with further increasing annealing time, these carbides would be dissolved and finally disappeared. Meanwhile, the austenite lath was developed in the ART-annealing process and the volume fraction of austenite increosed with the increase of the annealing time, which resulted in a duplex microstructure con- sisting of ultrafine-grained ferrite and large fraction of reverted austenite after long time annealing. The mechanical property examinations by uniaxial tensile tests showed that ART-annealing （6 h, 650 ℃） resulted in a superhigh product of strength to elongation up to 42 GPa ·%.

A strong and ductile medium Mn steel manufactured via ultrafast heating process

Ultrafast heating(UFH)at the rates of 10-300℃/s was employed as a new strategy to anneal a coldrolled 7 wt%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.

Yielding behavior of triplex medium Mn steel alternated with cooling strategies altering martensite/ferrite interfacial feature

In this paper, we report the influence of cooling processes on the yielding behavior of a medium Mn steel(MMS) with triplex microstructure, i.e. austenite(γ), ferrite(α) and as-quenched martensite(α’). After the intercritical annealing(IA) at both 725℃ and 750℃, the steel was subjected to the two cooling processes, i.e. air cooling(AC) and water quenching(WQ). It exhibits the discontinuous yielding after the AC following the IA at 750℃ while the continuous yielding after the WQ. Compared with WQ process,both the dilatometry and the microstructural examinations show that the AC process leads to lower Ms temperature, larger retained austenite(RA) fraction and less martensite, the latter is always companied with geometry necessary dislocations(GNDs) generated near the α/α’ interfaces. Considering the complexity of nanosized tri-phases in this steel, the presence of martensite with key features in the resultant specimens was systematically examined by atom probe tomography(APT) on the samples prepared by the specific target lift-out method. The APT results directly revealed the C/Mn co-segregation at the α’/αinterfaces in the AC samples but not in WQ samples. The numerical simulation results further suggest that the segregation of C and Mn at the α’/α interfaces may be due to different mechanisms. We conclude that the yielding of triplex MMS is determined by both the quantity of GNDs generated near theα/α’ interfaces, which increases with martensite fraction, and the extent of their immobilization resulting from the interfacial segregation of solute atoms when the presence of martensite is sufficient. WQ tends to suppress the discontinuous yielding of MMS since the rapid cooling may promote more martensite formed with the increasing quantity of GNDs and prevent the interfacial segregation of both C and Mn.

Ultra-Fine Heterogeneous Microstructure Enables High Strength-Ductility in a Cold-Rolled Medium Mn Steel

The objective of the present study is to develop heterogeneous microstructure in cold-rolled medium Mn steels(MMSs)annealing strategy.The cold-rolled Fe-4.7Mn-0.15C(wt%)steel is annealed twice at different temperatures to produce an ultra-fine heterogeneous microstructure with lath-shaped and granular-shaped retained austenite.Excellent mechanical behavior of significant strength enhancement with negligible ductility loss can be achieved.The high strength-ductility properties are attributed to the active transformation induced plasticity effect over a broad strain range owing to dispersive mechanical stabilities of the heterogeneous austenite.Furthermore,the typical yield point elongation phenomenon which is commonly observed in cold-rolled MMSs can be effectively reduced by this microstructural strategy.

Carbide precipitation behavior and mechanical properties of micro-alloyed medium Mn steel

The carbide precipitation behavior and mechanical properties of advanced high strength steel deformed at different temperatures are investigated by X-ray diffractometer(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM) equipped with an energy dispersing spectroscopy(EDS),and tensile tests.The medium Mn steel was subjected to controlled deformation up to 70% at 750℃,850℃,950℃,and 1050℃,and then quenched with water to room temperature,followed by intercritical annealing at 630℃ for 10 min.In comparison with the undeformed and quenched specimen,it can be concluded that acicular cementite precipitates during the quenching and cooling process,while granular NbC is the deformation induced precipitate and grows during the following annealing process.As the deformation temperature increases from 750℃ to 1050℃,the product of strength and elongation increases at first and then decreases.The smallest average size of second phase particles(20 nm) and the best mechanical properties(32.5 GPa%) can be obtained at the deformation temperature of 950℃.

Carbide precipitates and mechanical properties of medium Mn steel joint with metal inert gas welding

Medium Mn steel was metal inert gas(MIG)welded with NiCrMo-3 and 307 Si filler wires.The effect of filler wires on the microstructure and mechanical properties of joint was investigated,and the carbide precipitates were contrastively discussed.The results revealed that the microstructure of weld metal,heat-affected zone and base metal are austenite.Obvious grain coarsening occurred in the heat-affected zone(HAZ),and the maximum grain size grew up to 160μm.In HAZ,C and Cr segregated at grain boundaries,the carbides was identified as Cr7 C3.The dispersive(Nb,Mo)C phase was also found in weld metal with NiCrMo-3 filler wire.All the welded joints failed in HAZ during tensile tests.The tensile strength of welded joint with NiCrMo-3 filler wire was 675 MPa,which is much higher than that with307 Si filler wire.In comparison to base metal,higher microhardness and lower impact toughness were obtained in HAZ for these two welded joints,which was attributed to the precipitation of Cr7 C3 phase and grain coarsening.The impact toughness around the fusion line is the worst for these two welded joints.

Ultrahigh tensile strength achieved in a lightweight medium Mn steel via prominent work hardening

We develop a new ultrastrong medium Mn steel with a density reduced to 7.39 g cm^(-3).It has a novel tri-phase microstructure comprising a hierarchical martensitic matrix(α’),dispersed ultra-fine-retained austenite grains(γ),and both compressed and{200}orientedδ-ferrite lamellas,the latter’s formation is due to the alloying of high Al and Si contents for reducing density.As a result,both ultrahigh ultimate tensile strength of 2.1 GPa and good ductility of 16%are achieved after an extraordinary plastic strain hardening increment of about 1.4 GPa.The in-situ synchrotron-based high-energy(HE)X-ray diffraction(XRD)examinations during the tensile deformation revealed that the initial presence of residual com-pressive stress inδ-ferrite could increase the stress required to initiate the plastic tensile deformation of the specimen,leading to the isolatedδ-ferrite lamellas mostly deformed elastically to coordinate the plastic deformation of the martensitic matrix during yielding.During the plastic deformation,the gradual release of residual compressive stress inδandα’,the dislocation multiplication in all the three phases and the successiveγ-to-α’transformation all contribute to such a prominent work hardening increment.This study facilitates the development of novel strategies for fabricating ultrastrong but light steels.

Effects of strain states on stability of retained austenite in medium Mn steels

Based on uniaxial tensile and plane strain deformation tests, the effects of strain states on the stability of RA （retained austenite） in medium Mn steels, which were subjected to IA （intercritical annealing） and Q＆P （quenching and partitioning） processing, were investigated. The volume fractions of RA before and after deformation were measured at different equivalent strains. The transformation behaviors of RA were also investigated. The stability of RA differed across two different transformation stages at the plane strain state： the stability was much lower in the first stage than in the second stage. For the uniaxial ten sion strain state, the stability of RA corresponded only to a single transformation stage. The main reason was that there were two types of transformations from RA in the medium Mn steel for the plane strain state. One type was that the martensite originated in the strain-induced stacking faults （SISF）. The other type was the strain-induced directly twin martensite at a certain equivalent strain. However, for the uniax- ial tension state, only the strain-induced twin martensite was observed. Dislocation lines and dislocation tangles were also observed in specimens deformed at different strain states. In addition, complex micro- structures of stacking faults and lath-like phases were observed within a grain at the plane strain state.

Recent developments and perspectives of advanced high-strength medium Mn steel:from material design to failure mechanisms

Advanced high-strength steels are key structural materials for the development of next-generation energy-efficient and environmentally friendly vehicles.Medium Mn steel,as one of the latest generation advanced high-strength steels,has attracted tremendous attentions over the past decade due to its excellent mechanical properties.Here,the state-of-the-art developments of medium Mn steel are systematically reviewed with focus on the following crucial aspects:(a)the alloy design strategies;(b)the thermomechanical processing routes for the optimizations of microstructure and mechanical properties;(c)the fracture mechanisms and toughening strategies;(d)the hydrogen embrittlement mechanisms and improvement strategies.

Enhancing the Hydrogen Embrittlement Resistance of Medium Mn Steels by Designing Metastable Austenite with a Compositional Core-shell Structure

Deformation-induced martensite transformation from metastable retained austenite is one of the most efficient strain-hardening mechanisms contributing to the enhancement of strength-ductility synergy in advanced high-strength steels.However,the hard transformation product(often-martensite)and the H redistribution associated with phase transformation essentially decrease materials’resistance to hydrogen embrittlement.To solve this fundamental conflict,we introduce a new microstructure architecting strategy based on an accurately design of core–shell compositional distribution inside the austenite phase.We employed this approach in a typical medium Mn steel(8 wt.%Mn)with an ultrafine grained austenite-ferrite microstructure.We produced a high Mn content(15–16 wt.%)in the austenite shell region and a low Mn content(~12 wt.%)in the core region,through a thermodynamics-guided two-step austenite reversion treatment.During room-temperature deformation,the austenite core transforms continuously starting from a low strain,providing a high and persistent strain-hardening rate.The transformation of Mn-rich austenite shell,on the other hand,occurs only at the latest regime of the deformation,thus effectively inhibiting the nucleation of H-induced cracks at ferrite/deformation-induced martensite interfaces as well as suppressing their growth and percolation.This step-wise transformation,tailored directly targeted to protect the hydrogen-sensitive microstructure defects(interfaces),results in a significantly enhanced hydrogen embrittlement resistance without sacrificing the mechanical performance in hydrogen-free condition.The design of compositional core–shell structure is expected to be applicable to,at least,other multiphase advanced high-strength steels containing metastable austenite.

Influence of heat treatment processing on the microstructure and properties in medium manganese steel

For the purpose of developing a 1 500 MPa grade steel sheet with excellent strength and ductility, a 0.15C-10Mn-1.5Al steel was employed to study austenite stability and microstructural evolution based on a novel double annealing processing.After a conventional intercritical annealing process, the sample was heated again to a temperature higher than A_(c3) for a very short time to generate austenite grains with different manganese content;thus, the microstructure of martensite plus austenite can be obtained at room temperature.The experimental results show that with increasing annealing temperature, the tensile strength and yield strength increase.When the annealing temperature was higher than 820 ℃,the microstructure consisted of martensite plus austenite, and the tensile strength almost remained invariant with the annealing temperature.A tensile strength of 1 537 MPa and an elong-ation of 25.1% were achieved for the 820 ℃ condition.The volume fractions of austenite and martensite were identified by X-ray diffraction.It was found that with increasing annealing temperature, the volume fraction of the retained austenite decreased, and the ductility also had a gradual downward trend.The related austenite stability was discussed here as well.

Fe-11Mn-4Al-0.2C中锰钢准静态和动态变形行为

本文对比了Fe−11Mn−4Al−0.2C中锰钢变形过程中增塑机制和力学性能的演变规律.随应变速率增加(0.002~200 s^(-1)),中锰钢屈服强度和抗拉强度的变化趋势截然相反,屈服强度从507 MPa增加到649 MPa,但抗拉强度却从1089 MPa降低到876 MPa.准静态加载时增塑机制以强相变诱导塑性(transformation-induced plasticity,TRIP)效应为主;动态加载初期增塑机制以弱TRIP效应为主,加载后期TRIP效应消失,转变为温升软化效应和孪晶诱导塑性(twinning-induced plasticity,TWIP)效应.动态加载初期的位错运动速率远高于准静态的,这导致动态的屈服强度高于准静态的.随着应变的增加,动态加载逐渐累积的绝热温升抑制了马氏体相变,降低了加工硬化能力,而准静态加载则不断产生高硬度马氏体,导致准静态的抗拉强度高于动态的.

退火-回火和回火-退火工艺对冷轧10Mn钢显微组织及力学性能的影响

目的研究退火-回火和回火-退火工艺对冷轧Fe-10.2Mn-2.2Al-0.41C-0.6V(质量分数)中锰钢显微组织和力学性能的影响,明晰上述2种工艺对奥氏体体积分数、晶粒尺寸及稳定性的影响规律,以及显微组织与力学性能的关联机制。方法在退火温度(600℃)、退火时间(30 min)和回火温度(550℃)、回火时间(30 min)一致的情况下,调整退火和回火工艺顺序,分析退火-回火和回火-退火工艺下组织与性能共通性及差异性。结果2种工艺对试验钢的奥氏体体积分数、晶粒尺寸及稳定性影响不大;变形前后的奥氏体转换率分别为88%(退火-回火试样)和86%(回火-退火试样),即奥氏体在变形阶段通过TRIP(Transformation-induced Plasticity)效应对2种试验钢力学性能的贡献无显著区别。退火-回火试样具有1320 MPa屈服强度、1435 MPa抗拉强度以及4.1%断后伸长率;回火-退火试样的屈服强度为1200 MPa、抗拉强度为1360 MPa、断后伸长率为6.6%。除了奥氏体的TRIP效应外,退火-回火试样中预先存在的马氏体是其较高抗拉强度的决定性因素,而回火-退火试样中的大量高角度晶界是其具有较好塑性的主要原因。此外,两试样的屈服强度差异主要与织构强化有关。结论退火-回火工艺有利于中锰钢强度的提高,而回火-退火工艺有利于中锰钢塑性的提升。

回火对冷轧后退火处理中锰钢0.1C-5Mn氢脆敏感性的影响

对不同退火时间处理的冷轧0.1C-5Mn中锰钢进行了不同温度回火处理,利用电化学充氢和慢应变速率拉伸实验（SSRT）研究其氢脆敏感性。结果表明：退火时间从10 min提高到360 min时,实验钢的氢脆敏感性显著增加;随着回火温度的升高,实验钢的氢脆敏感性降低,其中以10 min退火样500℃回火时降低的幅度最为显著;SSRT断口分析表明,实验钢未充氢样的断裂机制为典型的韧窝韧性断裂,而充氢后的氢致起裂区断裂机制为空心韧窝及包含奥氏体（变形后转变为马氏体）晶粒的实心韧窝的混合断裂模式,这种实心韧窝实质上是一种脆性沿晶断裂,因此尽可能抑制或减少这种实心韧窝是降低实验钢氢脆敏感性的一个关键。

ZGMn8CrMo强化机理研究

研究了重载车辆履带板新材料-ZGMn8CrMo水韧处理态的强化机制,发现该材料在常温冲击磨料磨损条件下具有应变诱发 M相变强化特性,在疲劳裂纹扩展过程中也发生应变诱发 M相变;还研究了冲击能量的变化对 M量、硬度和耐磨性影响的规律,发现了ZGMn8CrMo存在 M量的最佳值:1#ZGMn8CrMo在2.0 J/cm2时的饱和值为51.08 %,2#ZGMn8CrMo在2.5 J/cm2时的饱和值为53.16 %,超过 M量的饱和值时将进入失稳磨损而失效。该成果已在重载车辆再制造时应用,使履带的使用寿命提高2.68倍。

预奥氏体化对中Mn TRIP钢组织及力学性能的影响

为了提高钢的综合力学性能,用盐浴法对中Mn TRIP钢进行了热处理.采用SEM、TEM、XRD和拉伸测试研究了预奥氏体化处理对中Mn TRIP钢显微组织及力学性能的影响.实验结果表明:全马氏体冷轧态组织经两相区退火处理后,会析出大量渗碳体颗粒,随着退火时间延长,渗碳体颗粒逐渐溶解,马氏体组织逐渐转变为奥氏体和铁素体双相片层状组织;而在两相区退火处理前添加两相区预奥氏体化处理后,渗碳体析出被有效抑制,双相片层组织迅速形成;相比于常规两相区退火处理,预奥氏体化处理能够提高组织中残余奥氏体体积分数和综合力学性能.