Abstract: Thermodynamic software Thermo-Calc was used to calculate the equilibrium phase diagram, precipitation temperature of each phase, element content in the phase, growth law of carbides at different temperatures and the influence of Nb and V content on precipitation temperature and amount of carbides in the microalloyed 2000 MPa hot stamping steel. A steel with selected composition was smelted by 50 kg vacuum furnace, and hot rolling and cold rolling were carried out. The hot forming process was simulated by plate die quenching, the mechanical properties and three-point bending properties were tested, and the field emission scanning electron microscope and EBSD were used to characterize the microstructure. The results show that the main carbides in Nb and V microalloyed 2000 MPa hot stamping steel are NbC and VC, and the precipitation temperatures are above 1150 ℃ and 880 ℃, respectively, and the precipitation temperatures increase with the increase of Nb and V content, respectively. The tensile strength and elongation of hot stamping steel plate after plate die quenching are more than 2000 MPa and 8%, respectively. The tensile fracture is ductile fracture and the three-point bending angle is more than 66°. The results of SEM and EBSD show that the martensite structure is composed of packet, block and lath. The prior austenite grain size is about 10 μm, and the size of martensite block is <5 μm. The martensite block is composed of martensite lath, and the small angle grain boundary between the martensite lath is discontinuous, and most of the grain boundary orientations are less than 5°. Fine prior austenite grain and martensite block structure are the main reasons for the high strength and high plasticity of microalloyed 2000 MPa hot stamping steel.

Key words: 2000 MPa hot stamping steel, microalloying, precipitate, martensite, strength and toughness