第三代汽车钢的热塑性及断裂机理

Hot plasticity and fracture mechanism of the third generation of automobile steel

采用Gleeble-1500热模拟试验机,对第三代汽车钢(TG钢)在不同的变形温度下进行了热拉伸试验,研究其热塑性的变化运用光学显微镜和扫描电镜分析了实验钢热变形的断口形貌及断裂机理.发现实验钢的强度随温度的升高而降低,热塑性曲线分为第Ⅰ脆性区、高温塑性区和第Ⅲ脆性区三个区域,其中第Ⅲ脆性区存在两个塑性极小值.在1300～800℃时实验钢的组织为奥氏体,断裂方式为连孔延性断裂,动态再结晶使韧窝分离前发生了较大的塑性变形,断口为大而深的韧窝;750℃时实验钢沿奥氏体晶界析出铁素体,断裂方式为界面断裂,断口既存在着铁素体内聚失效形成的小的孔洞,也存在由于裂纹沿奥氏体晶界扩展形成的石块状形貌;650℃由于出现了铁素体的准解理,实验钢的塑性下降,热塑性曲线再次出现极小值.

Tensile tests at different temperatures were performed on a Gleeble-1500 hot simulator to investigate the hot plasticity of the third generation of automobile steel （TG steel）. The fracture morphology and fracture mechanism were analyzed by optical microscopy （OM） and scanning electron microscopy （SEM）. It is found that the strength of the steel decreases with temperature rise. The curve of hot plasticity consists of three segments： the first brittle zone, the high-temperature plastic zone and the third brittle zone. The curve in the third brittle zone contains two relative minimum points. In the temperature range of 800 to 1300℃, the microstructure of the steel is austenite, and the fracture mode is ductile fracture. Due to dynamic recrystallization, large plastic deformation occurs before the material breaks off, big and deep dimples form on the fracture surface of samples. At 750 ℃ there is a uneven distribution of ferrite along austenite grain boundaries, the fracture mode is interface fracture, and the fracture surface includes both holes caused by cohesive failure and the part like stones caused by crack enlargement along austenite grain boundaries. The plasticity of the steel at 650 ℃ deteriorates, and the quasi-cleavage of ferrite leads to a minimal value in the curve of hot plasticity again.