碳含量对Fe-Mn-Cu-C系TWIP钢组织和力学性能的影响

 采用真空熔炼法制备Fe-20Mn-3.0Cu-XC系高强度高塑性合金钢,通过X射线衍射（XRD）、光学显微镜（OM）和透射电子显微镜（TEM）观察方法研究了碳含量对该系列合金微观组织和力学性能的影响,分析了合金的拉伸变形微观机制。结果表明：Fe-20Mn-3.0Cu-XC系合金拉伸变形前后均为单相奥氏体组织,未发生马氏体相变。随着碳质量分数的增加,合金的屈服强度、抗拉强度和伸长率均显著提高。Fe-20Mn-3.0Cu-1.41C合金的屈服强度为501.62MPa,抗拉强度为1178.4MPa,具有优异的综合力学性能。Fe-20Mn-3.0Cu-XC系合金具有优异的应变硬化能力。随着碳质量分数增大至1.41%,最大应变硬化指数n值达到0.782。Fe-20Mn-3.0Cu-XC系合金拉伸变形过程中,TWIP效应是主要的塑性变形机制,大量位错的塞积、形变孪晶的形成以及位错与孪晶间的交互作用共同引起材料强度和塑性的提高。     

Characterization and Prediction of Flow Behavior in High-Manganese Twinning Induced Plasticity Steels: Part II. Jerky Flow and Instantaneous Strain Rate

The jerky and smooth flow curves in high-manganese twinning induced plasticity (TWIP) steels were investigated by comparing Fe-Mn-C and Fe-Mn-Al-C systems. The pronounced serrations on the flow curves of Fe-Mn-C TWIP steel, produced during tensile testing at 300 K (27 °C) and 373 K (100 °C), were shown to be the result of localized high-temperature Portevin Le-Chatelier (PLC) bands moving across the gage length throughout the deformation. The speed of the PLC bands and their temperature effects were found to be strongly dependent on the applied strain rate, which was controlled by adjusting the cross-head speed of the tensile testing machine. The localized temperature-dependent stacking fault energy (SFE) variations resulting from the PLC effect and adiabatic heating were analyzed and compared for both slow and fast deformation rates. The instabilities in the measured logarithmic strain values caused by jerky flow could cause the local strain rate to deviate systematically from the targeted (applied) strain rate. These instabilities are better observed by calculating the instantaneous strain rate (ISR) values for each instant of deformation along the entire gage length. Finally, a new type of diagram was developed by plotting the true stress against the ISR values. From the diagram, the onset of different mechanisms, such as deformation twinning, nonpronounced, and pronounced serrations, could be marked precisely.     

On the Tensile Behavior of High-Manganese Twinning-Induced Plasticity Steel

High-manganese FeMnC and FeMnAlC austenitic twinning-induced plasticity (TWIP) steel exhibits excellent strain-hardening properties due to the gradual reduction of the mean free path for dislocations glide resulting from deformation twinning. Serrated stress-strain curves are often obtained when this type of steel is tested in a uniaxial tensile test. This phenomenon is due to dynamic strain aging (DSA). It is related to the occurrence of localized Portevin–LeChatelier (PLC) deformation bands. The properties of the PLC bands were accurately determined for a FeMnAlC TWIP steel using a combination of high-sensitivity infrared (IR) thermographic imaging and optical strain analysis carried out in situ during tensile deformation. Strain rate jump tests were conducted at room temperature to measure the instantaneous and steady-state strain rate sensitivity as a function of true stress and true strain. Negative values of the steady-state strain rate sensitivity were measured in both upward and downward jump tests. These measurements explain why FeMnC and FeMnAlC TWIP steels have a limited postuniform elongation. A model for the room-temperature DSA of high-Mn austenitic TWIP steel containing C in solid solution is proposed.     

Fe--22Mn--0.7CTWIP钢的热塑性与断裂机制

基于Gleeble-1500热力模拟试验机测定了Fe-22Mn-0.7C TWIP钢和Q235钢700~1300℃范围内的静态拉伸行为.采用光学显微镜、扫描电子显微镜、能谱仪、电子探针微区分析等技术表征两钢种不同温度下的变形特征和断口形貌.通过分析基体化学成分、相体积分数、晶粒尺寸、凝固缺陷等因素探讨TWIP钢铸态热塑性的变化规律及其影响机制.研究结果表明,Fe-22Mn-0.7C TWIP钢700~1250℃范围内的铸态抗拉强度高于Q235,而其断面收缩率低于40%,且断口均以沿枝晶间断裂方式为主.晶粒细化和控制溶质显微偏析有利于提高TWIP钢热塑性,与基体均质性改善有关.此外,增加应变速率TWIP钢拉伸强度和断面收缩率同时增大.      

Mechanical Properties and Microstructure Evolution During Deformation of Fe–Mn–C TWIP Steel

The mechanical and deformation microstructure properties of the Fe–Mn–C TWIP steel was investigated by means of tensile experiment, in situ scanning electron microscope (SEM) and transmission electron microscope (TEM).The results showed that the sample has excellent mechanical with tensile strength of the steel is about 1140 MPa and the yield strength is higher than 480 MPa, while the elongation is above 57%, the true stress–strain curve from tension tests exhibited repeated serrations and its strain‐hardening rate is constantly changing. It is found that there were different deformation mechanisms at different deformation stages result in the unique true stress–strain curve. Dislocation slip dominated the initial deformation and with the accumulation of deformation stress concentration reached the twin shear stress resulting in twin shear, which lead to TWIP effect. As the strain capacity increased continually, the parallel twins can no longer rotate and shear deformation occurred, which lead to the forming of shear bands. The intercoordination of slip deformation, twin deformation, and shear deformation mechanism make the TWIP steel show high strength and high plasticity.     

铝含量对TWIP钢中夹杂物特征及AlN析出行为的影响

采用扫描电镜、X射线能谱仪以及扫描电镜配置的夹杂物自动扫描统计软件(INCAFeature)表征了Fe-Mn-C(-Al)系TWIP钢中夹杂物的成分、形貌和数量,考察了Al质量分数在0.002%~1.590%的四种TWIP钢中夹杂物的特征和Al含量对AlN析出行为的影响.并在此基础上,采用了适合TWIP钢中高锰高铝特点的热力学参数对AlN夹杂物进行了系统的热力学分析.研究表明,在含有相似N质量分数(0.0078%~0.0100%)的TWIP钢中,当钢中Al质量分数升高至0.75%时,AlN夹杂物开始在钢中析出,并在MnS(Se)-Al₂ O₃上局部析出形成MnS(Se)-Al₂ O₃-AlN复合夹杂;当Al质量分数升高至1.07%时,热力学计算表明AlN已经可以在TWIP钢液相中形成,经不断长大后在MnS(Se)夹杂物表面局部析出形成MnS(Se)-AlN复合夹杂物;在Al质量分数为1.59%的TWIP钢中,AlN的平衡析出温度比其液相线温度高出42℃,在液相中形成的AlN可以作为异质核心,MnS(Se)夹杂在其表面包裹形成MnS(Se)-AlN复合夹杂物.另外,在Fe-18.21% Mn-0.64% C-1.59% Al体系的TWIP钢中,AlN在液相中析出所需的最低氮的质量分数仅为0.0043%.因此,在TWIP钢的冶炼过程中,应尽可能的降低钢中的氮含量,避免生成过量的AlN夹杂.      

Serration Phenomena Occurring During Tensile Tests of Three High-Manganese Twinning-Induced Plasticity (TWIP) Steels

In this study, the serration phenomena of two high-Mn TWIP steels and an Al-added TWIP steel were examined by tensile tests, and were explained by the microstructural evolution including formation of localized Portevin–Le Chatelier deformation bands and twins. In stress–strain curves of the high-Mn steels, serrations started in a fine and short shape, and their height and periodic interval increased with increasing strain, whereas the Al-added steel did not show any serrations. According to digital images of strain rate and strain obtained from a vision strain gage system, deformation bands were initially formed at the upper region of the gage section, and moved downward along the tensile loading direction. The time when the band formation started was matched with the time when one serration occurred in the stress–time curve. This serration behavior was generally explained by dynamic strain aging, which was closely related with the formation of deformation bands.     

Microstructure Characteristics of an Fe-Mn-C TWIP Steel After Deformation

The microstructure characteristics of an Fe-Mn-C TWIP steel after deformation are investigated.The results show that the hot-rolled,cold-rolled and then annealed sample of the Fe-Mn-C TWIP steel has excellent mechanical properties,and the true stress-true strain curve from tension tests exhibits repeated serrations.The deformed microstructure exhibits the typical planar glide characteristics such as no cell formation,dislocation pile-ups on a single slip plane,mechanical twins and stacking faults.There are ...     

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The low cycle fatigue behaviors of Fe- Mn- C TWIP steels with different Ce contents were studied. The research was done with a total strain amplitude of ??0. 8% during cyclic loading. Microstructure evolutions of TWIP steels after fatigue fracture were observed by SEM and TEM. The experimental results indicate that TWIP steels with Ce and without Ce have the same characteristics of cyclic hardening, cyclic saturating, cyclic softening and final failure. The density of inclusions in the TWIP steel is increased by the addition of rare earth element Ce, which results in the absorption capability of fatigue energy of the TWIP steel is reduced. Therefore, Ce plays a negative role in deformation fatigue of TWIP steel.     

Static Tensile Deformation Behaviors of an Fe-30Mn-3Al-3Si TWIP Steel Studied by In-Situ Neutron Diffraction

TWIP (TWinning Induced Plasticity) steel is one of the advanced steels with attractive mechanical properties.The typical composition of TWIP steel includes a large amount of manganese with some aluminum and silicon.Previous study has shown that TWIP steel exhibits high strength with adequate elongation at high strain rates,so that TWIP steel is desired to be applied for automotive use.However,there are few studies concerning the deformation behaviors aimed to make clear the TWIP effect in TWIP steel.In this study,static tensile deformation behaviors of an Fe-30Mn-3Al-3Si TWIP steel and a SUS310S one were studied by in situ neutron diffraction during tensile deformation.In terms of mechanical properties obtained by the static tensile tests,the TWIP steel showed better balance of tensile strength and uniform elongation than the 310S steel.The angular dispersion neutron diffraction with a wavelength of 0.16 nm was performed during stepwise tensile testing by using a neutron diffractometer for residual stress analysis (RESA) at the Japan Atomic Energy Agency.A specimen was extended in a step by step manner and neutron diffraction profiles of (111),(200) and (311) for austenite were obtained at each step.The diffraction peak,lattice plane spacing,lattice plane strain and so on were determined by the profile analysis as a function of applied stress.The changes of lattice plane strain for austenite in the TWIP and 310S steels indicated several deformation stages in the tensile deformation and can be discussed the difference of intergranular stress between the two samples.     

Effects of Aluminum Addition on Tensile and Cup Forming Properties of Three Twinning Induced Plasticity Steels

In the present study, a high Mn twinning induced plasticity (TWIP) steel and two Al-added TWIP steels were fabricated, and their microstructures, tensile properties, and cup formability were analyzed to investigate the effects of Al addition on deformation mechanisms in tensile and cup forming tests. In the high Mn steel, the twin formation was activated to increase the strain hardening rate and ultimate tensile strength, which needed the high punch load during the cup forming test. In the Al-added TWIP steels, the twin formation was reduced, while the slip activation increased, thereby leading to the decrease in strain hardening rate and ultimate tensile strength. As twins and slips were homogeneously formed during the tensile or cup forming test, the punch load required for the cup forming and residual stresses were relatively low, and the tensile ductility was sufficiently high even after the cup forming test. This indicated that making use of twins and slips simultaneously in TWIP steels by the Al addition was an effective way to improve overall properties including cup formability.     

Multiscale Characterization of the Work Hardening Mechanisms in Fe–Mn Based TWIP Steels

When strained in tension, high‐manganese austenitic twinning induced plasticity (TWIP) steels achieve very high strength and elongation before necking. The main hypotheses available in the literature about the origin of their excellent work hardening include deformation twinning and dynamic strain ageing. In order to provide some answers, various experiments at different scales were conducted on Fe–Mn–C steels and the Fe–28 wt%Mn–3.5 wt%Al–2.8 wt%Si alloy. At a macroscopic scale, tensile tests were performed on all the studied grades. It was shown that, though the Fe–Mn–Al–Si based alloy retains very high elongation, the Fe–Mn–C steels properties are even more extraordinary. Tensile tests at different strain rates with the help of digital image correlation were also performed on the Fe–20 wt%Mn–1.2 wt%C steel to study the PLC effect occurring in this type of steel. It is suggested that supplementary hardening could come from reorientation of Mn–C pairs in the cores of the dislocations. At a microscopic scale, the Fe–20 wt%Mn–1.2 wt%C TWIP steel and the Fe–Mn–Al–Si grade were thoroughly investigated by means of in situ TEM analysis. In the Fe–Mn–C steel, the formed twins could also lead to a composite effect, since they contain plenty of sessile dislocations. In the Fe–Mn–Al–Si alloy, mechanical twins are thicker and contain fewer defects, leading to a lower work hardening than the other grade.     

Effect of the Strain Rate on the TRIP–TWIP Transition in Austenitic Fe-12 pct Mn-0.6 pct C TWIP Steel

The strain-rate dependence of the plasticity-enhancing mechanisms in Fe-12 pct Mn-0.6 pct C-0.06 pct N steel was investigated. At low strain rates, deformation-induced ε-martensite was formed. At high strain rate, the strain-induced formation of ε-martensite was inhibited, and mechanical twinning was the dominant plasticity-enhancing deformation mechanism. This transition was associated with an increased work hardening rate and a higher total elongation. Dynamic strain aging (DSA) took place at all strain rates. While propagating type C Portevin-Le Chatelier (PLC) bands were observed at low strain rates, isolated propagating type A PLC bands were observed at high strain rates. The critical strain for the occurrence of DSA had an anomalous negative strain-rate dependence at low strain rates and a normal positive dependence at high strain rates. The transition from negative-to-positive strain-rate dependence was associated with a sharp change in the strain-rate sensitivity of the flow stress. Transmission electron microscopy was used to analyze the relationship between the stacking fault energy (SFE), the strain rate, and the plasticity-enhancing mechanisms. The SFE and critical resolved shear stress for the onset of the twinning and the ε-martensite transformation were calculated and compared with experimental results.     

Fe Mn C系TWIP钢的拉伸应变硬化行为研究

 通过试验研究了Fe Mn C系TWIP钢的拉伸应变硬化行为。结果表明：试验钢在拉伸过程中的应变硬化表现为阶段性多n值行为。其应变硬化机制有两种：在0.4%～3%的应变区间为位错强化阶段,10％～50%的应变区间为孪晶强化阶段。在这两个区间内n值皆为定值,而在4％～10％的应变区间n值呈不断上升趋势。     

Effect of Carbon Content on Stacking Fault Energy of Fe-20Mn-3Cu TWIP Steel

The influence of carbon content on the stacking fault energy(SFE)of Fe-20Mn-3Cu twinning-induced plasticity(TWIP)steel was investigated by means of X-ray diffraction peak-shift method and thermodynamic modeling.The experimental result indicated that the stacking fault probability decreases with increasing carbon addition, the SFE increases linearly when the carbon content in mass percent is between 0.23% and 1.41%.The thermodynamic calculation results showed that the SFE varied from 22.40to 29.64mJ·m-2 when the carbon content in mass percent changes from 0.23%to 1.41%.The XRD analysis revealed that all steels were fully austenitic before and after deformation,which suggested that TWIP effect is the predominant mechanism during the tensile deformation process of Fe-20Mn-3Cu-XC steels.     

含硅镍低密度钢的温拉伸力学行为及强韧机制

 为了分析硅镍合金化奥氏体基低密度钢在中温环境下的拉伸变形行为,采用Instron电子拉力试验机对Fe-28.64Mn-8.99Al-1.68Si-1.39Ni-1.0C(Mn29Al9Si2Ni,质量分数/%)低密度钢在23~300 ℃下进行了温拉伸试验,研究了该钢的温拉伸力学行为,并采用SEM、TEM和热力学计算对该钢的强韧化机制进行了研究。结果表明,随着应变的增加,温拉伸应力-应变曲线主要包括弹性变形、均匀塑性变形和断裂等几个过程,没有明显的屈服现象。随着温度的提高,该钢的强度逐渐降低,塑性(断后伸长率)先增加后减小再升高,于200 ℃时出现塑性低谷,此时该钢的应力-应变曲线和应变硬化率曲线均具有明显的锯齿状特征,应变硬化率随应变的增加变化不大。而该钢在其他温度下的应力-应变曲线和应变硬化率曲线没有发现明显的“锯齿状”特征,应变硬化率随应变的增加而平缓下降。试验钢在23~300 ℃下的主要强韧化机制为κ-碳化物强化、应变强化、孪生诱发塑性和动态应变时效强化。较低温度下位错可动性较差对孪生诱发的促进作用、镍元素和硅元素对孪生的抑制作用、较高温度下孪生现象的减弱和温度对动态应变时效的促进或抑制作用等使得试验钢在23、100和300 ℃时存在明显的孪生诱发塑性,而在200 ℃时存在明显的动态应变时效强化的主要原因。动态应变时效强化是该钢在200 ℃时出现塑性低谷的主要原因。     

逆相变退火处理Fe-Mn-C中锰钢的组织与性能

 为了研究奥氏体逆相变(austenite reverse transformation,ART)退火处理对Fe-Mn-C中锰钢的组织与性能的影响,以ART退火处理1、10和360 min后Fe-5Mn-0.2C中锰钢为基础,利用XRD、SEM等手段对其显微组织进行表征,通过WE-300型拉伸试验机和ML-10型销盘式磨料磨损试验机对其拉伸性能和耐磨性进行测试。结果表明,ART退火过程中,残余奥氏体在原奥氏体板条之间形核并长大,原始马氏体组织逐渐转变为铁素体-奥氏体板条交替分布的复合组织。随着ART退火时间的延长,残余奥氏体体积分数增加(由18.4%提高到 33.6%),Fe-5Mn-0.2C钢的综合力学性能和耐磨性随着残余奥氏体体积分数的增加而显著提高,强塑积由25 613提高到44 496 MPa·%,其耐磨性与目前广泛应用的ZGMn13耐磨钢、Hardox450耐磨钢和中碳马氏体耐磨钢相当。     

中锰钢的研究进展与前景

总结了国内外中锰钢研究现状, 对文献中中锰钢的成分设计、成型工艺、热处理工艺、组织性能调控等进行汇总分析, 得到了合金元素、成型工艺、微观组织结构和热处理对力学性能的影响规律, 并对中锰钢的性能例如lüders带和PLC带对加工硬化率的影响、氢致延迟开裂性能给予了重点关注和讨论; 同时提出借鉴第二代先进高强钢(纯奥氏体相)"层错能"这一控制形变模式的概念, 对中锰钢中奥氏体相的形变模式提出预测; 最后对目前中锰钢研究的争议问题、发展前景及未来可能面对的问题进行阐述.      

Application of a Dislocation Density-Based Constitutive Model to Al-Alloyed TWIP Steel

High Mn steels exhibit an exceptional combination of high strength and large ductility owing to their high strain-hardening rate during deformation. The addition of Al is needed to improve the mechanical performance of TWIP steel by means of the control of the stacking fault energy. In this study, a constitutive modeling approach, which can describe the strain-hardening behavior and the effect of Al on the mechanical properties, was used. In order to understand the deformation behavior of Fe18Mn0.6C and Fe18Mn0.6C1.5Al TWIP steels, a comparative study of the microstructural evolution was conducted by means of transmission electron microscopy and electron backscatter diffraction. The microstructure analysis focused on dislocations, stacking faults, and mechanical twins as these are the defects controlling the strain-hardening behavior of TWIP steels. A comparison of the strain-hardening behavior of Fe18Mn0.6C and Fe18Mn0.6C1.5Al TWIP steels was made in terms of a dislocation density-based constitutive model that goes back to the Kubin–Estrin model. The densities of mobile and forest dislocations are coupled in order to account for the interaction between the two dislocation populations during straining. The model was used to estimate the contribution of dynamic strain aging to the flow stress. As deformation twinning occurred only in a subset of the grains, the grain population was subdivided into twinned grains and twin-free grains. Different constitutive equations were used for the two families of grains. The analysis revealed that (i) the grain size and dynamic recovery effects determine the strain-hardening behavior of the twin-free grains, (ii) the deformation twins, which act as effective barriers to dislocation motion, are the predominant elements of the microstructure that governs the strain hardening of the twinned grains, and (iii) the DSA contribution to strain hardening of TWIP steel is only minor.     

Fe-Mn-Si-Al系和Fe-Mn-C系TWIP钢加工硬化行为

研究了在不同应变量下Fe-Mn-Si-Al系和Fe-Mn-C系孪晶诱导塑性(TWIP)钢的力学性能以及微观组织,分析了TWIP效应在两种不同系列TWIP钢中发挥的作用,阐明了TWIP钢的强化机制.两种系列的TWIP钢都具有高加工硬化能力,但层错能较低的Fe-Mn-C系TWIP钢加工硬化能力更强.两种系列的TWIP钢加工硬化表现为多加工硬化指数行为,这是由多种强化机理在不同阶段起主导作用的结果.微观组织形态与加工硬化强度之间存在着较强的关联性.位错的增殖和形变孪晶的产生对两个系列TWIP钢硬化曲线形态有着明显的影响.在高应变阶段,Fe-Mn-C系TWIP钢大量的第一位向形变孪晶T1和第二位向形变孪晶T2,以及附着在孪晶界旁的高密度位错区域是造成其具有高加工硬化能力的原因,而Fe-Mn-Si-Al系TWIP钢细密的第一位向形变条纹和孪晶片层间的位错是其高加工硬化原因,且其微观组织更为均匀细致.