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。     

碳含量对Fe-Ni-Mn-Si-C系TWIP钢应变硬化行为的影响

采用单向拉伸、金相组织观察、TEM等方法,研究了3种不同碳含量Fe-Ni-Mn-Si-C系TWIP钢的拉伸应变硬化行为。研究结果表明,随着碳含量的增加,合金的强度和伸长率均提高,当碳质量分数为1.0%时,该TWIP钢具有较好的综合力学性能,强塑积达到83 160 MPa.%。3种TWIP钢的真应力-真应变曲线均不完全遵...     

TWIP钢的拉伸应变硬化行为

采用静态拉伸、金相组织观察方法研究了5种不同锰含量的TWIP钢的拉伸应变硬化行为.结果表明:5种钢的真应力与真应变不遵循Hollomond的线性关系,其中1号钢的应变硬化指数n值随真应变的增大先升后降,其它4种成分钢的n值随真应变的增大而提高.对于同一成分的钢,其n值均随应变速率的增大而减小.其微观变形机制是:随着锰含量的增加,孪晶形成逐渐起主导作用;拉伸前组织中有退火孪晶;随着变形的进行,产生大量的形变孪晶,孪晶与位错之间的交互作用与硬化率相协调,从而延迟了颈缩的产生,导致TWIP钢具有很高的均匀变形能力.     

Effects of Twin-Dislocation and Twin-Twin Interactions on the Strain Hardening Behavior of TWIP Steels

 In the present paper, tensile tests of Fe-30Mn-5Si -2Al steel were carried out for different strains of 0.05, 0.14, 0.26, and up to the strain-to-failure in order to observe the evolution of microstructure during deformation and investigate the strain hardening behavior. Three-stage strain hardening behavior was observed in this steel during tensile test. In stage I, planar dislocation structure was observed by TEM to be the main deformation mechanism, and low strain hardening rate exponent was exhibited. Primary deformation twinning occurred in stage II, and the strain hardening rate exponent increased due to the blockage of dislocations’ motion by twin boundaries. In stage III, the strain hardening rate exponent had increased to be higher than 0.5. The obstacle effect of twin boundaries and twin-twin interaction had been observed by TEM, and the interactions between primary and secondary twins were found to cause the additional hardening in addition to the obstacle effect on dislocations’ motion, which led to the twinning induced plasticity effect in the later stage of deformation.     

高锰奥氏体TRIP/TWIP钢的组织和力学性能

研究了两种不同锰含量的高锰奥氏体钢在室温拉伸变形过程中力学性能和组织的变化.结果表明,随着钢中锰含量的变化,实验钢在流变应力的作用下出现相变诱导塑性的TRIP效应和孪晶诱导塑性的TWIP效应.在1×10-3 s-1的初始应变速率条件下,锰的质量分数为23.8%的实验钢可达到666 MPa的抗拉强度和67%的伸长率,而锰的质量分数为33%的实验钢可达到540 MPa的抗拉强度和97%的伸长率.并且在10-3～10-1 s-1的初始应变速率范围内,实验钢的抗拉强度对于流变应力不敏感,而实验钢的塑性则表现出一定的应变速率敏感性.由于该钢具有较好的综合力学性能,有望作为新一代高强度、高塑性汽车用钢.     

高锰合金钢的SME、TRIP、TWIP效应

 由于锰的价格低廉以及在材料中的重要作用而成为钢铁工业常用的合金元素。锰含量高时,可使Fe Mn合金形成的奥氏体在较低温度下存在。加入Si、Al元素可对合金中奥氏体的稳定性产生不同程度的影响,从而使材料在承受外界载荷时呈现出不同的反应。研究表明：Si可降低奥氏体层错能,有利于A→ε M相变,从而使合金易产生形状记忆效应。加大变形量,由于大量的奥氏体转变为α′ M时体积膨胀,在使材料伸长率提高的同时,强度也得到提高(相变诱发塑性效应),因此可用作高性能结构件。Al和Mn是提高奥氏体层错能的合金元素。对于Al、Mn含量高的钢,在外力作用下则可通过孪生诱发塑性变形产生孪晶诱发塑性效应,因而材料在具有较高强度的前提下,还具有60%~80%的伸长率。     

固溶温度对节约型双相不锈钢TRIP/TWIP行为的影响

 通过微拉伸、电子背散射（EBSD）、透射电子显微镜（TEM）等手段,研究了具有亚稳奥氏体相的节约型双相不锈钢在1 000～1 200 ℃范围内不同固溶温度下的组织与性能的演变规律;探讨了固溶温度对形变诱导塑性（TRIP/TWIP）的作用机制。结果表明,随着固溶温度的升高,抗拉强度与伸长率均先升高后降低,而亚稳奥氏体相比例由74%（1 000 ℃）降低到37%（1 200 ℃）;1 050 ℃固溶时,试验钢表现出最佳综合性能,抗拉强度达到960 MPa,伸长率达到62%,强塑积达到60 GPa·%。在经拉伸变形的微观结构中形变诱导马氏体与形变孪晶共存,表明试验钢中亚稳奥氏体相的变形机制主要受TRIP及TWIP共同控制,从而导致其塑性变形过程呈现多阶段应变硬化特征,而钢中铁素体相的变形机制主要变形为位错的滑移。     

Finite Element Modelling of TRIP Steels

A constitutive model that describes the mechanical behaviour of steels exhibiting “Transformation Induced Plasticity” (TRIP) during martensitic transformation is presented. Multiphase TRIP steels are considered as composite materials with a ferritic matrix containing bainite and retained austenite, which gradually transforms into martensite. The effective properties and overall behaviour of TRIP steels are determined by using homogenization techniques for non‐linear composites. The developed constitutive model considers the different hardening behaviour of the individual phases and estimates the apportionment of plastic strain and stress between the individual phases of the composite. A methodology for the numerical integration of the resulting elastoplastic constitutive equations in the context of the finite element method is developed and the constitutive model is implemented in a general‐purpose finite element program. The prediction of the model in uniaxial tension agrees well with the experimental data. The problem of necking of a bar in uniaxial tension is studied in detail.     

Strain Rate Dependent Flow Stress and Energy Absorption Behaviour of Cast CrMnNi TRIP/TWIP Steels

Modern steel developments often use additional deformation mechanisms like the deformation induced martensitic transformation (TRIP‐effect) and mechanical twinning (TWIP‐effect) to enhance elongation and strength. Three high‐alloyed cast CrMnNi‐steels with different austenite stabilities were examined. Dependent on the austenite stability, TRIP‐effect and TWIP‐effect were found. A low austenite stability causes a distinctive formation of deformation induced α'‐martensite and therefore a strong strain hardening. The increase of strain rate leads to an increase in yield strength and flow stress, but also to a counteractive adiabatic heating of the specimen. Dependent on the degree of deformation, low austenite stabilities and high strain rates lead to excellent values in specific energy absorption.     

Effects of Pre-strain and Temperature on Bake Hardening of TWIP900CR Steel

The effects of pre-strain and baking temperature on bake hardening behaviour of TWIP900 CR steel were investigated.The results reveal that the bake hardening process contributes to an increase in yield strength up to 65 MPa at the baking temperature of 200℃.The difference in yield strength between baking temperatures of 170 and 200℃is almost insignificant.It is clearly observed that baking at a high temperature does not result in a significant increase in yield strength.For a reasonable bake hardening,agood combination of pre-strain and baking temperature is necessary.Besides,the toughness of the material is found to decrease with increasing pre-strain.     

Martensite Transformation and Its Control in DP,TRIP and TWIP Steels

Designing of alloy concept and process for DP,TRIP and TWIP steels stressing at martensite transformation are analyzed.For DP steel,austenite volume percent and its carbon content at different intercritical temperatures are calculated as well as the tensile strength of the steel,which meet well with the experimental result.The condition for dissolution of carbide is discussed by experiments and predicted by kinetic estimation.Several sample TRIP steels are prepared and their concentration profiles are calculated showing different diffusion characteristics of elements.Calculation also shows carbon enrichment is successful in this stage through the quick diffusion of carbon from ferrite to austenie.In order to maintain the austenite stability or to prevent precipitation of cementite,minimum cooling rate from the intercritical zone to over aging stage is obtained through kinetic simulation.Bainite transformation is estimated,which indicates the carbon rerichment from ferrite of bainite structure to austenite in this stage is also successful.Thermal HCP martensite transformation and the strain induced martensite transformation in TWIP steel is introduced.Relationship between transformation and mechanical properties in the steel is also mentioned.     

碳含量对新型Fe-Ni-Mn-Si-C系TWIP钢组织和力学性能的影响

通过光学显微镜、X射线衍射和透射电镜等方法研究了碳含量对Fe-Ni-Mn-Si-C系合金微观组织和力学性能的影响。结果表明:Fe-Ni-Mn-Si-C系合金的主要塑性变形机制为孪生诱发塑性(TWIP)效应。碳的质量分数由0.70%增加至0.98%,合金的屈服强度和抗拉强度分别由391 MPa和860 MPa增大到458 MPa和974 MPa,伸长率由63.6%提高到69.2%。随着碳含量的提高,Fe-Ni-Mn-Si-C系合金出现明显的动态应变时效现象。Fe-15Ni-12Mn-2.5Si-XC合金具有良好的应变硬化能力,随着碳的质量分数提高至0.98%,最大应变硬化指数达到0.73。     

Cyclic Deformation Behaviour of Three Austenitic Cast CrMnNi TRIP/TWIP Steels with Various Ni Content

This study presents the cyclic deformation behaviour of three high‐alloyed austenitic cast steels which are characterized by different chemical compositions leading to different austenite stabilities and stacking fault energies. Thus, depending on the chemical composition different deformation mechanisms arise which have a significant influence on the cyclic deformation behaviour and life time relations. The materials were characterized under total‐strain control. The fatigue life relations of Basquin and Manson‐Coffin are applied successfully for all steel variants. The cyclic stress‐strain response is described using the Ramberg‐Osgood relationship. It is shown that the parameters n' and K' depend strongly on the accumulated plastic strain λ_p. The mechanical properties are discussed together with microstructural investigations of deformation structures and martensitic transformations as well as twinning, respectively.     

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.     

Study on Microstructures and Mechanical Properties of High Manganese Steels with TRIP/TWIP Effects

In the present work, advanced high strength and high ductility TRIP/TWIP steels with different manganese concentrations were studied. The microstructures of these steels were evaluated prior to and after deformation and the mechanical properties of these steels were determined. The microstructure analysis indicated that both TRIP and TWIP effects appeared in the steel with lower Mn content, while the TWIP effect was the dominant deformation mechanism in the steel with a higher Mn content, with many deformation twins formed during the deformation. In addition, the forming limit diagrams of these steels were recorded and the results showed an excellent formability.     

Fatigue Behavior of Dual‐Phase and TWIP Steels for Lightweight Automotive Structures

Car bodies are increasingly made with high‐strength steels, for both lightweighting and safety purposes. Steel sheets, made by continuous casting, hot rolling, cold rolling, and continuous heat treating, are used to deep draw the car body parts, which are then joined by resistance spot welding (RSW). Two high‐strength automotive steels, with similar tensile strength, are studied here. The low alloy, dual‐phase steel consist of ferrite and martensite, obtained by an intercritical heat treatment, followed by fast cooling. The innovative, high‐Mn TWIP steel exhibits a promising combination of strength and toughness, arising from the austenitic structure, strengthened by C, and from the twinning induced plasticity effect. Tensile specimens are fatigue tested at room temperature with zero load ratio, both in the as‐fabricated (unnotched) condition and after the RSW of an homologous sheet square. Moreover, pre‐cracked compact tension specimens are tested with load ratio 0.1 to determine the fatigue crack growth behavior. These results are completed with crystallographic, microstructural, tensile, and fractographic examinations, and the influence of the microstructure and of the welding process is discussed.     

低碳Si-Mn系TRIP钢的动态拉伸性能

在气动式间接杆杆型冲击拉伸实验机上对工业生产的两种低碳Si-Mn系TRIP钢不同应变率下的高速冲击拉伸性能进行了研究,并和静态拉伸性能进行了比较.结果表明,两种钢的室温拉伸性能随应变率变化具有相同趋势,但动态下的应变率敏感性比静态下的要高得多.由于TRIP钢组织中残余奥氏体的变形诱发向马氏体的转变显著改善了材料的塑性.     

Microstructures and Mechanical Properties of Fe-Mn-（Al, Si） TRIP/TWIP Steels

The mechanical properties and microstructure of two low carbon high manganese steels with 23.8% （No. 1） and 33% （No. 2） （mass percent） of manganese were investigated. The results showed that No. 1 steel possesses high strength and high plasticity, and No. 2 steel has a relatively high strength and extraordinary plasticity. The No. 1 steel exhibits both TRIP （transformation induced plasticity） and TWIP （twin induced plasticity） effects during the deformation; while only TWIP effect appeared under the same deformation condition for No. 2 steel. The comparison between the microstructures and mechanical properties of two steels was made, and the strengthening mechanisms were also analyzed.     

A Material Model for TRIP‐Steels and its Application to a CrMnNi Cast Alloy

A material model is presented that accounts for strain rate dependent inelastic deformation and strain‐induced phase transformation in TRIP‐steels. Modifications for the kinetics equations of the strain‐induced phase transformation, introduced by Stringfellow, are proposed to overcome a drawback of Stringfellow's model. A parameter identification strategy that relies on Gauss‐Markov estimates is used to determine the model parameters from experimental data of a recently developed cast TRIP‐steel. Good agreement is observed between experimental results of the compression test and the corresponding finite element simulation employing the proposed model. This forms the basis for future applications of the material model in the design of composites and structures.