Abstract: Effect of tempering time (10-30 min) at 200 ℃ on microstructure and mechanical properties of a 1800 MPa hot stamping steel after flat forming at 930 ℃ for 4 min was studied by scanning electron microscope, electron backscattering diffraction and tensile test at room temperature. The results show that with the extension of tempering time, the tensile strength of the tested steel changes a little, while the yield strength and elongation after fracture both increase first and then decrease. The martensite sub-grain size is the smallest when tempered for 20 min, while the low angle grain boundary is the most when tempered for 10 min. After tempering at 200 ℃ for 10 min, the comprehensive mechanical properties of the hot stamping steel are the best due to the release of residual stress, the decrease of martensite sub-grain size and the increase of low angle grain boundary, of which the tensile strength is 1844 MPa, the elongation increases from 8.27% of quenched tested steel to 11.78%, and the product of strength and elongation reaches more than 21 GPa·%. It is concluded that short-time tempering is beneficial to the improvement of comprehensive properties of the tested ultra-high strength steel and the reliable application of hot stamping parts.

Key words: 1800 MPa hot stamping steel, low temperature tempering, microstructure, mechanical properties