30SiMnCrB5热成形钢的微观组织和力学性能

Microstructure and mechanical properties of 30SiMnCrB5 hot stamping steel

为了提高热成形钢的综合性能,设计了一种C-Si-Mn-Cr-B系热成形钢,采用热膨胀仪测定并研究了30SiMnCrB5热成形钢的连续冷却转变曲线和相变规律.分析了经轧制、退火及热成形模拟后钢板的微观组织形貌和力学性能,结合等密度线极图的方法,判定了热成形模拟后钢板中马氏体变体与母相的取向关系.30SiMnCrB5热成形钢具有较好的淬透性,临界冷速为5℃·s-1,有效抑制了珠光体和贝氏体的形成,完全马氏体组织的硬度可达600 HV以上.热成形模拟后的微观组织由板条马氏体和残余奥氏体构成,残余奥氏体主要以薄膜状分布在马氏体板条间,质量分数为6%~8%,抗拉强度为1800 Mpa左右,总伸长率可达10%以上,强度和塑性的匹配较好.热成形模拟后30SiMnCrB5热成形钢板中马氏体变体与母相的取向关系更接近N-W关系,12种变体没有都出现在原始奥氏体内.

A C-Si-Mn-Cr-B hot stamping steel was designed to improve its comprehensive properties. The continuous cooling transformation （CCT） curves and phase transformation rules of 30SiMnCrB5 hot stamping steel were measured and studied by thermal dilatometry. The microstructure morphology and mechanical properties of 30SiMnCrB5 steel sheet after rolling, annealing and hot stamping simulated were analyzed by scanning electron microscopy, transmission electron microscopy and tensile testing. The orientation relationship between martensite variants and parent phases in the hot-stamped steel sheet was determined by pole figure contouring. It is found that the critical cooling rate of 30SiMnCrB5 hot stamping steel is 5℃·s-1 with good hardenability which inhibits the formation of pearlite and bainite effectively. The hardness of full martensite is more than 600HV. The microstructure of the hot-stamped steel sheet is composed of lath martensite and retained austenite, which accounts for 6% to 8% and distributes between martensite laths by film. The strength and ductility of the hot-stamped steel sheet are matched well with a tensile strength of about 1800MPa and a total elongation up to 10%. The orientation relationship between martensite variants and parent phases in the hot-stamped steel sheet is closer to the N-W orientation relationship, which the 12 variants do not appear wholly in prior austenite.