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

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 (22MnB5), 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.