Microstructure and mechanical properties of a cast TRIP-assisted multiphase stainless steel

Stainless steels are used in a wide range of complex environments due to their excellent corrosion resistance.Multiphase stainless steels can offer an excellent combination of strength,toughness and corrosion resistance due to the coexistence of different microstructures.The microstructure and mechanical properties of a novel cast multiphase stainless steel,composed of martensite,ferrite,and austenite,were investigated following appropriate heat treatment processes:solution treatment at 1,050℃ for 0.5 h followed by water quenching to room temperature,and aging treatment at 500℃ for 4 h followed by water quenching to room temperature.Results show reversed austenite is formed by diffusion of Ni element during aging process,and the enrichment of Ni atoms directly determines the mechanical stability of austenite.The austenite with a lower Ni content undergoes a martensitic transformation during plastic deformation.The tensile strength of the specimen exceeds 1,100 MPa and the elongation exceeds 24%after solid solution,and further increases to 1,247 MPa and 25%after aging treatment.This enhancement is due to the TRIP effect of austenite and the precipitation of the nanoscale G-phase pinning dislocations in ferrite and martensite.

Influence of Hot Rolling Conditions on the Mechanical Properties of Hot Rolled TRIP Steel

Influence of hot rolling conditions on the mechanical properties of hot rolled TRIP steel was investigated. Thermomechanical control processing （TMCP） was conducted by using a laboratory hot rolling mill, in which three different kinds of finish rolling temperatures were applied. The results show that polygonal ferrite, granular bainite and larger amount of stabilized retained austenite can be obtained by controlled rolling processes. The finer ferrite grain size is produced through the deformation induced transformation during deformation rather than after deformation, which affects the mechanical properties of hot rolled TRIP steel. Mechanical properties increase with decreasing finish rolling temperature due to the stabilization of retained austenite. Ultimate tensile strength （UTS）, total elongation （TEL） and the product of ultimate tensile strength and total elongation （UTS×TEL） reaches optimal values （791 MPa, 36% and 28 476 MPa%, respectively） when the specimen was hot rolled for 50% reduction at finish rolling temperature of 700 ℃.

Effect of Microstructure on Tensile Behavior and Mechanical Stability of Retained Austenite in a Cold-Rolled Al-Containing TRIP Steel

In the present study, a quenching treatment prior to two-stage heat treatment was conducted on a Fe-0.28C-1.55Mn-2.06Al transformation-induced plasticity steel to tailor the final microstructure. Compared with the microstructure of the ferrite, bainite and blocky retained austenite obtained by conventional two-stage heat treatment, the microstructure subjected to quenching plus two-stage heat treatment was composed of the ferrite, lath bainite and film-like retained austenite. The corresponding tensile behavior and mechanical stability of retained austenite were investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The results show that the mechanical stability of blocky retained austenite grains is lower and most of them transform to martensite during the tensile deformation, which leads to higher ultimate tensile strength and instantaneous work hardening exponent. Film-like retained austenite has relatively higher stability, which could cause sustained work hardening and high ductility as well as product of strength and elongation.

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.

Microstructure-based finite element modeling of effect of metastable austenite on mechanical properties of quenching and partitioning (Q&P)980 steel

The microstructure-based finite element modeling was conducted to study the mechanical properties of Q&P 980 steel at the microscopic level.The two-dimensional representative volume elements of real microstructure were obtained from electron backscattered diffraction mapping.Mecking-Kocks equation was used to predict the constitutive strain-stress relationships of individual phases.Mechanical-induced martensitic transformation takes place when the driving force exceeds the critical driving force according to a stress-invariant-based model.The macroscopic stress-strain curves and the work-hardening rate curves obtained from modeling fit well with the experimental results.The simulation results also indicate that the local distributions of stress and strain in constituent phases are dependent on their strength.Soft ferrite carries the highest strain,while hard mechanical-induced martensite carries the highest stress.By comparing the modeling results of the microstructures with and without austenite,it shows that the transformation of retained austenite to hard martensite can increase the .work-hardening ability and hence improve the strength and ductility of the steel.The detailed finite element modeling methods and results are presented and discussed.

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.

Effect of Isothermal Temperature on Microstructure and Mechanical Properties of High Al–Low Si TRIP Steel

In this study, the effect of isothermal temperature on microstructure and mechanical properties of a high Al-low Si TRIP steel was investigated using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron back scattered diffraction, and tensile test. The results show that typical microstructure containing ferrite, bainite, and retained austenite can be obtained when two-stage heat treatment process was utilized. When annealing temperature is 840 ℃ and austempering temperature is 400 ℃, the tensile strength is 542 MPa and the product of strength and elongation is 17,685 MPa%. The morphologies and stability of the retained austenite in low silicon/high aluminum TRIP steel were finally discussed.

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.

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

Both microstrueture 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 19% 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.

Mechanical and Transformation Behaviors of a C-Mn-Si-Al-Cr TRIP Steel under Stress

Transformation induced plasticity （TRIP） steels combine high strength and excellent ductility, making them suited for application in crash-relevant parts in the automotive industry. However, the high Si contents in the conventional TRIP steel will generate surface defects on the hot rolled strip, which is difficult to process in continuous galvanizing lines. In order to solve the above problem the TRIP steel with the addition of Al replacing majority of Si was designed. In the present paper, the volume fraction of various phases in a C-Mn-Si-Al-Cr TRIP steel was determined by metallographic examination and X-ray diffraction analysis, and the multi-phase microstructures were characterized using an atomic force microscope based on their height difference. Tensile tests were performed at different temperatures ranging from -40℃ to 90℃. The results show that transition temperature Ms^σ in the present TRIP steel cannot be determined due to its lower volume fraction of retained austenite, different from the conventional TRIP steel. While the yield stress and tensile strength at different temperatures are higher than those of the conventional TRIP steel, which is attributed to the addition of Cr. In order to evaluate the effect of martensitic transformation on the total elongation, the sample without retained austenite obtained by quenching in liquid nitrogen was carried out under tensile test. The results indicate that the elongation of the original sample containing 9% retained austenite is about 20% higher than that of the sample quenched in liquid nitrogen, which demonstrates that the retained austenite plays an important role in improving the elongation of the TRIP steel.

基于动态相变的热轧TRIP钢组织及性能研究

通过热模拟压缩实验,开展了基于动态相变的热轧C-Mn-Si及C-Mn-Al-Si系TRIP钢组织及性能特征的研究.结果表明,通过动态相变,可以使TRIP钢复相组织由细小铁素体晶粒、尺寸细小及位向混乱的贝氏体束、弥散分布的体积分数较高的颗粒状残余奥氏体等组成.由于铁素体、贝氏铁素体间的大量晶界、贝氏铁素体内的高位错密度以及铁索体晶粒间大量的细小颗粒状残余奥氏体,使实验钢具有连续屈服、屈强比低、强度较高及塑性良好的特点.其中,C-Mn-Si钢的抗拉强度为890 MPa,延伸率为26%;C-Mn-Al-Si钢的强度为760 MPa,延伸率为32%.

低硅TRIP钢的力学性能及残余奥氏体稳定性研究

通过常温拉伸实验研究低SiTRIP钢的力学性能及其加工硬化特点,利用扫描电镜(SEM)、X射线衍射(XRD)、电子背散射衍射(EBSD)、透射电镜(TEM)等手段研究低SiTRIP钢在拉伸变形前后组织变化特别是残余奥氏体特性的变化。结果表明:低SiTRIP钢表现出良好的力学特性,没有屈服平台,低的屈强比,同时又拥有比双相钢更好的延伸率和均匀应变。TRIP钢的残余奥氏体的分布也影响着其稳定性,分布在铁素体晶内的残余奥氏体在拉伸变形后仍然存在。

HSLA-TRIP钢动态拉伸性能和残余奥氏体转变

对成分为 0 .11C- 0 .6 2 Si- 1.6 5 Mn的低硅 TRIP钢不同试验温度下的动态拉伸性能和残余奥氏体的转变行为进行了研究。试验结果表明 ,两相区热处理温度接近 Ac1 可以获得较多的铁素体和残余奥氏体 ,此类试样拉伸试验结果有较高的强塑性配合。 110℃下的抗拉强度较 2 0℃下的降低约 30 0 MPa;均匀伸长率在应变速率 10 0 0s- 1左右达到峰值 ,总伸长率随应变速率提高而单调增加。试验温度为 5 0℃和 75℃下的能量吸收值可达到 2 6 0 0 0MPa· %。试验温度越高 ,残余奥氏体稳定性越好 ,动态拉伸的绝热效应也抑制了残余奥氏体的形变诱发相变。

贝氏体区等温时间对TRIP钢残奥及力学性能影响

为研究贝氏体区等温时间对热轧TRIP钢残余奥氏体和力学性能的影响,采用金相显微镜、X射线衍射、拉伸实验等方法对3种不同贝氏体区等温时间下制备的热轧TRIP钢进行分析.结果表明:随着贝氏体等温时间的延长,残余奥氏体量减少而残余奥氏体碳含量增加,残余奥氏体晶粒尺寸及残余奥氏体形貌变化不大;热轧TRIP钢的力学性能随着贝氏体等温时间的延长,表现为低抗拉强度、高延伸率和高屈服强度.

两相区退火对不同硅含量TRIP钢残留奥氏体和力学性能的影响

研究了硅含量分别为 1 5wt%和 1 0wt %的冷轧低碳硅锰TRIP钢两相区退火温度对残留奥氏体量和力学性能的影响。结果表明 ,随两相区退火温度的升高 ,两种钢的残留奥氏体量和残留奥氏体中的含碳量以及其屈服强度和抗拉强度都上升。当硅含量降至 1 0 %时 ,对钢的残留奥氏体量没有影响 ,但是降低了残留奥氏体中的碳含量 ,同时降低了钢的抗拉强度。两种钢的最大强塑积值相近 ,约为 2 2 0 0 0MPa %。

TRIP-相变诱发塑性钢的研究进展

相变诱发塑性钢是一种汽车用钢 ,通过相变诱发塑性 (TRIP)效应使钢板中残余奥氏体在塑性变形作用下诱发马氏体生核和形成 ,并产生局部硬化 ,继而变形不再集中在局部 ,使相变均匀扩散到整个材料以提高钢板的强度和塑性。典型TRIP钢C含量为 0 .2 % ,Mn 1%～ 2 % ,Si 1%～ 2 % ,通过热轧变形热处理或冷轧 +热处理 ,TRIP钢的组织由 5 0 %～ 6 0 %铁素体 ,2 5 %～ 4 0 %贝氏体或少量马氏体和 5 %～ 15 %残余奥氏体组成。TRIP钢的强度和韧性高于双相钢和微合金钢。介绍了TRIP钢的生产工艺和性能 ,残余奥氏体、合金元素、热处理对TRIP效应的影响和TRIP钢研究趋势。

热处理工艺对吉帕级Q & P钢组织和力学性能的影响

采用光学显微镜、扫描电子显微镜、透射电子显微镜、X射线衍射仪及拉伸试验机等,研究了热处理工艺对冷轧态和预淬火态Q&P钢显微组织和力学性能的影响。结果表明:冷轧态试验钢组织呈等轴状,预淬火态试验钢组织具有层片状特征;对于冷轧态试样,两相区保温温度、淬火温度和配分温度均会对试验钢的力学性能产生明显影响,残留奥氏体的稳定性参数R值与强塑积的变化趋势一致,表明在拉伸载荷作用下残留奥氏体发挥相变诱发塑性(transformation induced plasticity, TRIP)效应是试验钢强塑化的主要原因;对于预淬火态试样,层片状组织有助于残留奥氏体更好地增强TRIP效应,并阻碍裂纹扩展,从而进一步提高其强塑积。

控冷工艺对热轧TRIP钢组织和性能的影响

采用光学显微镜、X射线衍射仪和拉伸试验机等研究了轧后冷却工艺对热轧TRIP钢组织和性能的影响。结果表明:弛豫时间对热轧TRIP钢组织影响较大,随弛豫时间增加,带状组织等级越来越高,铁素体组织也略显粗大;模拟卷取制备的热轧TRIP钢组织中残余奥氏体含量和残余奥氏体中碳含量均较高;贝氏体区停留时间延长,则屈服强度和伸长率增大,但抗拉强度下降。

热处理工艺对1000 MPa级含V-TRIP钢组织和性能的影响

采用扫描电镜(SEM)、X射线衍射仪、膨胀仪和MTS拉伸试验机等实验手段研究了热处理工艺对含V高强度TRIP钢组织和力学性能的影响。结果表明,该TRIP钢热处理后组织细小,细晶强化对机械性能的提高起到了一定作用。两相区退火温度越高,组织中铁素体含量越少,且贝氏体转变速率越快。随着贝氏体等温时间延长,组织中贝氏体量增多,马氏体量减少,残余奥氏体量及其碳含量呈现先增大后减小或趋于平稳的趋势。试验钢经800℃×3 min+400℃×3 min工艺处理后,可获得最大残余奥氏体量,体积分数达到20.6%,其碳含量为1.19 wt%,该工艺下试验钢可获得最佳力学性能,抗拉强度高达1 052 MPa,延伸率24%,强塑积达到25 248 MPa%。

铜元素和退火温度对TRIP冷轧钢板组织和力学性能的影响

将含铜和不含铜的两种TRIP钢板 (0 2 %C、1 5 %Si、1 5 %Mn)冷轧至 1mm厚 ,然后分别在 75 0℃、76 0℃、770℃、780℃下退火 5min ,最后在 4 5 0℃等温 3min ,获得了不同含量和不同稳定性的残留奥氏体。金相和力学性能试验结果表明 ,铜元素能提高残留奥氏体量和钢板的力学性能 ;退火温度影响残留奥氏体量和力学性能 。