Nb-Ti微合金化热冲压成形用钢的微观组织与力学性能

Microstructure and mechanical properties of Nb-Ti micro-alloy hot stamping steels

利用扫描电镜(SEM)、透射电镜(TEM)对Nb-Ti微合金化热成形钢的微观组织进行观察,采用Kahn撕裂试验对其韧性和撕裂性能进行了研究,并利用Thermo-Calc热力学软件对其析出行为和析出粒子成分进行分析计算.结果表明,含碳质量分数0.13%的热成形钢在Nb-Ti微合金化后的组织为马氏体,和传统热成形钢(22MnB5)相比其奥氏体晶粒、板条块和板条束都得到细化,并且其抗拉强度达到1500 MPa以上,撕裂强度和单位面积裂纹扩展能分别达到1878 MPa、436 kN·m^(-1).在950℃奥氏体化时,Nb-Ti合金元素几乎全部以析出粒子形式存在,能有效阻止奥氏体晶粒长大.另外在基体中主要存在两种析出物,一种是尺寸在100~200 nm的Ti(C,N);另一种是纳米级别的钛铌复合碳氮化物,能有效强化基体,提高强度.

The microstructure of Nb-Ti micro-alloy hot stamping steels was observed and investigated by scanning electron microscopy（SEM） and transmission electron microscopy（TEM）.The toughness and tear properties were investigated by Kahn tear test.The thermodynamic software Thermo-Calc was used to calculate and analyze the behavior and composition of precipitates.The results show that Nb-Ti micro-alloy hot stamping steels with mass fraction 0.13% C are composed of martensite.Compared with traditional hot stamping steel（22 MnB5）,the Nb-Ti micro-alloy steel has finer prior austenite grains,martensite packets,and martensite laths.The tensile strength is more than 1500 MPa,tear strength is 1878 MPa,and the unit propagation energy is 436 kN·m^（-1）.Nearly all Nb-Ti elements exist as precipitates during austenization at 950 ℃ and they can hinder the growth of austenite grains effectively.Two types of precipitates of different sizes are dispersed in the martensite matrix.The sizes of Ti（C,N） precipitates range from 100 nm to 200 nm,and their quantity is low.The nanometer-sized precipitates are carbides or nitrides of Ti and Nb,which can strengthen the martensite matrix and improve the intensity.