Work Hardening Behavior and Stability of Retained Austenite for Quenched and Partitioned Steels

Both microstructure and mechanical properties of low alloy steels treated by quenching and partitioning (Q&P)process were examined.The mixed microstructure of martensite and large-fractioned retained austenite (about 27?3%)was characterized and analyzed,excellent combinations of total elongation of 1 9% and tensile strength of 1 835 MPa were obtained,and three-stage work hardening behavior was demonstrated during tensile test.The en-hanced mechanical properties and work hardening behavior were explained based on the transformation-induced plas-ticity effect of large-fractioned austenite.     

Behaviour Model of Austenitic Stainless Steels for Automotive Structural Parts

One of the most important preoccupation of car manufacturers is to reduce emissions and hence to reduce weight of cars. One of the outstanding materials able to reduce weight while at the same time keeping the same crash absorption and hence safety, is austenitic steel. Austenitic stainless steels are used in crash relevant parts of cars. Moreover, designers can use their very good corrosion resistance and their well known surface aspect for structural visible parts like wheels, cross members, roof panels or tailgates. In this paper, stainless steels for automotive use are presented in detail. First, their chemical composition and tensile properties are explained. Then, a model for forming and crash behaviour is described. Using this model, stainless steels can be engineered into automotive parts and thus stainless steel can be considered as a workable and predictable material for the automotive industry.     

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.     

Microstructure and Mechanical Properties of TRIP Steel with Annealed Martensite

The microstructure and mechanical properties of cold rolled TRIP steel containing C 0.2, Si 0.5, Mn 1.5, A1 1.3, and Nb-kV 0.13 （mass%） with annealed martensite （TAM steel） were investigated using optical microscopy, field emission gun scanning electronic microscope （FEG SEM）, transmission electron microscopy （TEM）, X-ray diffraction （XRD）, and mechanical testing. The mierostructure of the TAM steel mainly consisted of polygonal ferrite, bainite, annealed martensite and retained austenite. The martensite after annealing did not spheroidize, which consisted of annealed lath martensite structure and interlath second phase. Compared with the traditional TRIP steel with polygonal ferrite matrix （TPF steel）, the TAM steel has more excellent elongation rate over 32%. The TAM steel also has better strain hardening behavior than the TPF steel. The excellent elongation and strain harden- ing behavior of TAM steel result from high retained austenite stability of the TAM steel, which is attributed to its fine distribution and medium strength ratio of second phase to matrix.     

铌含量和热处理工艺对TRIP钢组织和力学性能的影响

通过两相区退火和贝氏体转变区等温处理,研究了铌含量和贝氏体等温处理温度对低碳 TRIP钢(w(Mn)=1.38%,w(Si)=0.6%,w(Al)=0.5%)组织和力学性能的影响.实验结果表明:增加铌含量,实验钢的残余奥氏体量减少,抗拉强度和屈服强度增加;当铌的质量分数为0.014%时,实验钢的伸长率和强塑积较高;贝氏体等温处理温度为400 ℃时,实验钢的残余奥氏体量较多,力学性能较好.     

氮、碳、锰对介稳奥氏体锰钢力学性能及耐磨性的影响

 研究了氮、碳、锰等合金元素对介稳奥氏体锰钢力学性能及耐磨性的影响。结果表明,在中、小冲击磨损工况条件下,由于应力波效应,决定锰钢耐磨性的因素有2个：加工硬化性和机械强度,而且2个因素相互制约,只有当这2个因素的提高幅度都较大时,其耐磨性才能大幅度提高。将15C 127Mn奥氏体锰钢的碳的质量分数大约降至1%,锰的质量分数降至≤8%,同时加入质量分数约01%的氮时,可得到一种新型耐磨材料——氮微合金化介稳奥氏体锰钢。     

Contrary Effects of Microstructure and Cleanliness on Tensile and Toughness Properties in Precipitation Hardening Stainless Steels

A detailed microstructural analysis on two precipitation hardening steels of similar chemical composition and of identical heat treatment but with different microstructures and mechanical properties was performed to reveal the constituents that are of major importance for strength and toughness. Light optical microscopy, magnetic saturation measurements, energy dispersive X‐ray analysis, scanning and transmission electron microscopy were used to identify and quantify the internal cleanliness, the secondary phases (δ‐ferrite, retained austenite), and the copper‐rich precipitates. The high purity and homogeneity of the steel's microstructure proved to be beneficial for upper shelf toughness and general isotropic behavior. In contrast, good elongation depended on local element segregation, allowing the stabilization of austenite to ambient temperature. The precipitation of copper‐enriched inter‐metallic phase was documented both in the martensitic matrix and in the δ‐ferrite; it ensured – for given heat treatment – sufficient yield strength independent of other microstructural constituents.     

Effect of Thermomechanical Control Processing on Microstructure and Mechanical Properties of Fe-0.2C-1.44Si-1.32Mn Hot Rolled TRIP Steel

The effect of thermomechanical control processing(TMCP)on microstructure and mechanical properties of Fe-0.2C-1.44Si-1.32Mn hot rolled TRIP steel was investigated through experiments.Strain-induced transformation and transformation-induced plasticity behavior of retained austenite were analyzed.The results show that with multipass deformation,reduction per pass of more than critical deformation in austenite recrystallization region and total reduction of more than 58% in non-recrystallization region and high temperature section of two-phase region,austenite can be refined before γ→α transformation.It is beneficial to obtain refined ferrite grain in final microstructure.To obtain fine and uniform microstructure and excellent strength-ductility balance,a three-stage cooling process(laminar cooling-air cooling-ultra-fast cooling)after hot rolling was conducted.The ultimate tensile strength and elongation of the investigated steel can reach 663 MPa and 41%,respectively.     

Microstructure and Mechanical Properties of High Manganese TRIP Steel

Microstructure evolution and mechanical properties of newly designed 0.1C-6Mn-0.5Si-1Al TRIP-aided steels under different annealing conditions and the effects of matrix microstructure before intercritical annealing on the final microstructure were studied by means of X-ray diffraction（XRD）,scanning electron microcopy（SEM）,dilatometric simulation,optical microstructure（OM） and tensile testing in this work.The experimental results indicate that the TRIP steel with Mn of 6% could form a considerable amount of retained austenite with good TRIP effect after a simple intercritical annealing treatment,and the matrix microstructure before intercritical annealing treatment can greatly affect the final microstructure.The original microstructure of the ferritic matrix steel was eliminated,while annealed martensite was remained from the martensite matrix steel under the same intercritical annealing conditions     

Effect of Cooling Method on Microstructure and Mechanical Properties of Hot-Rolled C-Si-Mn TRIP Steel

The controlled cooling technology following hot rolling process is a vital factor that affects the final micro- structure and mechanical properties of the hot-rolled transformation induced plasticity （TRIP） steels. In the present study, low alloy C-Si-Mn TRIP steel was successfully fabricated by hot rolling process with a 4450 hot roiling mill. To maximize the volume fraction and stability of retained austenite of the steel, two different cooling methods （aircooling and ultra-fast cooling ＂AC-UFC＂ and ultrmfast cooling, air cooling and ultra-fast cooling ＂UFC-AC-UFC＂） were conducted. The effects of the cooling method on the microstructure of hot-rolled TRIP steel were investigated via optical microscope, transmission electron microscope and conversion electron Mossbauer spectroscope. The mechanical properties of the steel were also evaluated by conventional tensile test. The results indicated that ferrite and bainite in the microstructure were refined with the cooling method of UFC-AC-UFC. The morphology of retained austenite was also changed from small islands distributing in bainite district （obtained with AC-UFC） to granular shape locating at the triple junction of the ferrite grain boundaries （obtained with UFC-AC-UFC）. As a result, the TRIP steel with a content of retained austenite of 11. 52%, total elongation of 32% and product of tensile strength and total elongation of 27 552 MPa·% was obtained.     

The Effect of Size and Shape of Austenite Grains on the Mechanical Properties of a Low‐Alloyed TRIP Steel

The effects of size and shape of austenite grains on the extraordinary hardening of steels with transformation induced plasticity (TRIP) have been studied. The deformation and transformation of austenite was followed by interrupted ex situ bending tests using electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM). A finite element model (FEM) was used to relate the EBSD based results obtained in the bending experiments to the hardening behavior obtained from tensile experiments. The results are interpreted using a simple rule of mixture for stress partitioning and a short fiber reinforced composite model. It is found that both, the martensite transformation rate and the flow stress difference between austenite and martensite significantly influence the hardening rate. At the initial stage of deformation mainly larger grains deform, however, they do not reach the same strain level as the smaller grains because they transform into martensite at an early stage of deformation. A composite model was used to investigate the effect of grain shape on load partitioning. The results of the composite model show that higher stresses develop in more elongated grains. These grains tend to transform earlier as it is confirmed by the EBSD observations.     

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.     

Effect of Cold Deformation on Phase Evolution and Mechanical Properties in an Austenitic Stainless Steel for Structural and Safety Applications

The effects of cold deformation on the formation of strain induced α’ martensite and mechanical properties of an austenitic stainless steel have been examined.X-ray diffraction analysis has revealed that 30% and 40% cold rolling have resulted in the formation of 24% and 31.5% martensite respectively.Microstructural investigation has demonstrated that the formation of martensite is enhanced with increase in the percent deformation at 0 ℃.Investigation of mechanical properties reveals that hardness,yield strength and tensile strength values increase where as percent elongation drops with increasing deformation.The fractographic observation corroborates the tensile results.Examination of sub-surface at the fractured end of the tensile sample manifests that void/microcrack nucleation occurs in the interfacial regions of the martensite phase as well as at the austenite-martensite interface     

奥氏体耐磨锰钢的研究现状与进展

在综述奥氏体耐磨锰钢的特点、微观结构、加工硬化机理、磨损与耐磨机理的基础上，提出应深入开展其微观结构与组织及性能的关系以及变形与磨损行为的系统研究，为该类钢的生产和应用提供了更充分的理论和实验依据。     

The Effects of Strain Rate and Temperature on the Deformation Behavior of Cold‐Rolled TRIP800 Steel

In the present study, the influences of temperature and strain rate on the deformation behavior of cold‐rolled TRIP800 steel were investigated. Microstructural observation and tensile tests were performed and volume fractions of retained austenite were measured at various temperatures and strain rates. The results reveal that both temperature and strain rate affect the volume fractions of retained austenite that transforms into martensite. The strain‐induced transformation of retained austenite is retarded with increasing temperature and the retained austenite becomes more stable against straining. The amount of retained austenite that transforms into martensite is not influenced significantly by strain rate. The variation in mechanical properties with temperature and strain rate was related to the effects of dynamic strain aging, tempering of banite, high temperature softening, and the volume fractions of retained austenite.     

铜含量对高碳TWIP钢组织和力学性能的影响

 采用真空熔炼法制备了Fe-20Mn-XCu-1.3C系高强度高塑性合金钢。通过单向拉伸试验和OM观察,研究了铜含量的变化对该合金微观组织和力学性能的影响。结果表明：Fe-20Mn-XCu-1.3C系合金拉伸变形前后均为单相奥氏体组织。随着铜含量的增加,合金的屈服强度和伸长率提高,而抗拉强度降低,Fe-20Mn-3.0Cu-1.3C合金的抗拉强度为1256MPa,伸长率为77.6%,强塑积达到97465.6MPa·%,具有优异的综合力学性能。铜含量的增加提高合金的层错能,推迟了变形过程中孪晶的形成并降低了孪晶的形成速率,使位错滑移更容易发生。Fe-20Mn-XCu-1.2C系合金具有较高的加工硬化速率水平,其加工硬化速率随着铜含量的增加而降低。