基于退火路径的中锰钢组织转变与力学性能

 为了更好地指导中锰钢工业试制,利用扫描电镜、电子背散射衍射、透射电镜和拉伸试验机等研究了不同退火路径下低碳中锰钢的组织转变及合金元素配分行为,并评价了其对力学性能的影响。结果表明,热轧中锰钢的显微组织主要由铁素体、板条马氏体、粒状贝氏体和残余奥氏体构成。经冷轧变形后,原组织中的铁素体和马氏体晶粒破碎,残余奥氏体和M/A岛在应变诱导作用下转变为马氏体。两种不同退火路径下,中锰钢的显微组织均由铁素体、残余奥氏体和少量二次马氏体构成,其中一步退火工艺的组织结构呈多边形状,两步退火工艺的组织结构呈板条状。一步退火工艺中,残余奥氏体面积分数为25.6%,平均等效晶粒直径为0.32 μm,残余奥氏体内锰的质量分数为6.36%;两步退火工艺中,残余奥氏体面积分数为27.8%,平均等效晶粒直径为0.38 μm,残余奥氏体内锰的质量分数为5.37%。晶粒尺寸增大及锰元素富集程度降低,导致两步退火工艺得到的残余奥氏体力学稳定性较低。两种工艺下,中锰钢的屈服强度相差不大,但屈服类型有明显差异。一步退火工艺的应力-应变曲线存在屈服平台,应变约为4%,而两步退火工艺提高了中锰钢变形初期的加工硬化和抗拉强度,消除了屈服平台延伸。不过,两步退火工艺下中锰钢塑性损失明显,均匀伸长率和断后伸长率较一步退火工艺分别降低了4.6和6个百分点。

Microstructure evolution and mechanical properties of medium manganese steel based on annealing path

In order to guide the industrial trial production of medium manganese steel better, the microstructure evolution and alloy element distribution behavior of low carbon medium manganese steel under different annealing paths were studied by scanning electron microscope (SEM), electron backscatter diffraction (EBSD), transmission electron microscope (TEM) and tensile testing machine. Moreover, its effect on mechanical properties was evaluated. The results show that the microstructure of hot rolled medium manganese steel is mainly composed of ferrite, lath martensite, granular bainite and retained austenite. After cold rolling, the ferrite and martensite grains were broken, and the retained austenite and M/A island were transformed into martensite under strain induced transformation. The microstructure of medium manganese steel under two different annealing paths both consisted of ferrite, retained austenite and a small amount of secondary martensite. The microstructure of one-step annealing process was polygonal and that of two-step annealing process was lath-like. In one-step annealing process, the area fraction, average equivalent grain diameter and Mn content of retained austenite were 25.6%, 0.32 μm and 6.36%, respectively, and that of two-step annealing process were 27.8%, 0.38 μm and 5.37%, respectively. The increase of grain size and decrease of Mn enrichment degree leaded to lower mechanical stability of retained austenite in two-step annealing process. The yield strength of medium manganese steel under two annealing processes was almost the same, but the yield type was obviously different. There was a yield platform with about 4% elongation on the stress-strain curve of one-step annealing process, while the two-step annealing process improved the work hardening and tensile strength at the initial stage of deformation, which eliminated the yield platform. However, in comparison to the one-step annealing process, the ductility loss of medium manganese steel was obvious under the two-step annealing process, and the uniform elongation and total elongation decreased by 4.6 and 6 percentage points, respectively.