【作者】 姚鑫；

【导师】 黄兴民；

【作者基本信息】 西南交通大学 ， 材料工程（专业学位）， 2021， 硕士

【摘要】 中锰钢(Medium Manganese Steel,MMS)兼具高强度与高塑性,在汽车板材等领域具有良好的应用前景。研究表明,含铝中锰钢在单轴拉伸过程中发生明显塑性失稳,可能成为限制其批量生产和广泛应用的关键问题。此外,在中锰钢各相间观察到明显的非均匀应变分布,而奥氏体与铁素体之间的应变分配可能是中锰钢强塑性提高的原因之一。因此,厘清含铝中锰钢单轴拉伸过程中的塑性失稳现象和组织演变机制具有重要意义。本文以临界退火态热轧含铝中锰钢为研究对象,结合红外热成像(Infrared Thermography,IRT)、数字图形相关(Digital Image Correlation,DIC)、扫描电镜(Scanning Electron Microscopy,SEM)和有限元(Finite Element Method,FEM)仿真等方法对温度场、微区应变和显微组织进行准原位观察和系统分析,并尝试通过预变形与二次退火工艺改善实验钢的塑性失稳,得出以下结论:(1)随着应变量增加,PLC(Portevin Le-Chatelier)带逐渐由A型(连续)转化为C型(随机),主要特征为:PLC带随机形核,各微区应变、温度场异常起伏;局部应变和温度场演化过程具有同一性;PLC带内集中应变诱导残余奥氏体转变为马氏体(Strain Induced Martensite Transformation,SIMT),可能是导致锯齿状应力起伏的主要原因;(2)临界退火态实验钢(750℃保温60min,简称IA750)显微组织由铁素体和奥氏体组成,屈服强度、抗拉强度和总延伸率分别为629 MPa、930 MPa和64.6%。少量预变形(3%)能减小残余奥氏体平均晶粒尺寸(由0.52降至0.41μm,下降24.5%),迟滞马氏体相变,屈服强度、抗拉强度和总延伸率分别为703 MPa、1058 MPa和67.3%;(3)IA750经预变形后二次退火,能提高再结晶分数,降低位错密度;预变形20%+700℃退火30 min可有效消除塑性失稳,同时获得优异的综合力学性能,屈服强度、抗拉强度和总延伸率分别为636 MPa、1197 MPa和58%;(4)PLC带与奥氏体稳定性密切相关;块状奥氏体大多位于多重晶界处,与相邻相有较大的结晶取向差,机械稳定性较低;等轴奥氏体具有相对较高机械稳定性,相同应变条件更趋于沿着特定晶向旋转;显微DIC分析和仿真模拟结果表明,空间排布与拉伸方向垂直的条状奥氏体具有更高局部应变与应力,与拉伸方向呈45°夹角的奥氏体优先出现应变集中,由此更易发生马氏体转变。更多还原

【Abstract】 Medium manganese steel(MMS) has both high strength and high plasticity,which has a good application prospect in the field of automobile plate.The results show that the obvious plastic instability of aluminum-containing MMS during uniaxial tension may become a key problem limiting its mass production and wide application.The strain distribution between austenite and ferrite may be the reason for the strength and plasticity enhancement of medium manganese steel.Therefore,to understand deformation behavior and the microstructure evolution during tensile process is very crucial.In addition,obvious heterogeneous strain distribution was observed among phases of medium manganese steel.The strain distribution between austenite and ferrite may be the reason for the strength and plasticity enhancement of MMS.Therefore,it is of great significance to clarify the mechanism of plastic instability and microstructure evolution of aluminum-containing medium manganese steel during uniaxial tension.In this paper,the uniaxial tensile process of 5Mn hot rolled critical annealed medium manganese steel was studied.The temperature field,micro zone strain and microstructure were observed and analyzed by means of Infrared Thermography(IRT),Digital Image Correlation(DIC),Scanning Electron Microscopy(SEM)and Finite Element Method(FEM).The plastic instability of experimental steel was improved by pre-strain and secondary annealing,and the following conclusions were obtained:(1)With the increase of strain,the PLC(Portevin Le-Chatelier)band gradually transforms from A-type(continuous)to C-type(random),which is characterized by the randomness of nucleation position and the abnormal fluctuation of strain and temperature fields in each micro region;the evolution process of strain and temperature in PLC band is uniform and identical;the strain concentration in PLC band promotes the transformation of retained austenite into martensite,which leads to local strain hardening;SIMT(Strip Induced Martensite Transformation)in PLC band causes serrated stress fluctuation;(2)The microstructure of the critical annealed experimental steel(heat preservation at 750 ℃ for 60 min,referred to as IA750)consists of ferrite and austenite.The yield strength,tensile strength and total elongation are 629 MPa,930 MPa and 64.6%respectively.A small amount of pre-strain(3 %)can reduce the average grain size of retained austenite(from 0.52 to 0.41 μm,decreased by 24.5%)and retard martensitic transformation.The yield strength,tensile strength and total elongation are 703 MPa,1058 MPa and 67.3 % respectively;(3)The recrystallization fraction of IA750 can be increased and dislocation density can be reduced by pre-strain and secondary annealing;the plastic instability can be effectively eliminated by pre-strain of 20% and annealing at 700 ℃ for 30 min,the yield strength,tensile strength and total elongation are 636 MPa,1197 MPa and 58 %respectively;(4)PLC band is closely related to austenite stability,blocky austenite is mostly located at multiple grain boundaries,which has a large crystal orientation difference with adjacent phases and low stability;equiaxed austenite is relatively stable,which is not easy to transform into martensite under the same strain condition,but rotates along the specific crystallization direction;The μ-DIC and simulation results show that the lath austenite with spatial arrangement perpendicular to the tensile direction has higher local strain and stress,and the strain concentration occurs preferentially in austenite with 45 °tensile direction,so martensite transformation is more likely to occur.更多还原