采用OM、SEM、EBSD和TEM等技术,研究了Si、Mn含量对超高强度热成形钢在相同的轧制和模拟热冲压成形工艺处理后的组织和性能的影响。结果表明,Si、Mn含量对热成形前轧制态钢的组织和性能有较大影响,在其它成分相同的情况下,随着Mn含量(质量分数)由0.57%增加到1.21%,实验用钢的屈服强度由552 MPa提高到751 MPa,抗拉强度由757 MPa提高到1124 MPa,组织由贝氏体+铁素体+珠光体转变为马氏体+贝氏体。随着Si含量由0.25%增加到0.38%,实验用钢的抗拉强度逐渐升高,屈服强度和伸长率呈波动趋势。在950 ℃保温5 min相同的工艺条件下模拟热冲压淬火实验后,4种钢的组织均为马氏体,但马氏体的精细结构各不相同,平均亚晶粒尺寸大小不一;含0.34%Si和1.21%Mn的钢B的综合力学性能最优,其屈服强度为1161 MPa,抗拉强度为1758 MPa,伸长率为6.5%,且热冲压成形后的组织为细小的板条马氏体,马氏体板条上有大量的位错,且只有少量的碳化物析出。基于本研究成分设计的超高强度热成形钢,其热冲压成形前的组织和性能与热成形后的力学性能无明显相关性,只是最终的马氏体精细结构略有差别,有利于工业化批量试制零件的性能稳定性控制。

It is very important to find out the mechanism of composition in steel. Many efforts have been put on the study of the effect of Si and Mn elements on the microstructure and mechanical properties of middle Mn steel, transformation induced plasticity (TRIP) steel and quenching and partitioning (Q&P) steel. But fewer studies were focused on the mechanism of Si and Mn contents in a press hardening steel. In this work, the microstructures after hot rolled and the fine martensite structure after hot stamping in ultra-high strength press hardening steel (PHS) with different Si and Mn contents were studied by OM, SEM, EBSD and TEM. The results showed that there are a great influence of Si and Mn contents on the microstructure and mechanical properties of PHS after hot rolled. The yield strength of the PHS increases from 552 MPa to 751 MPa, the ultimate tensile strength (UTS) increases from 757 MPa to 1124 MPa, and the microstructures are different with the Mn content rose from 0.57% to 1.21% and the other components remained the same. The UTS of the steels goes up as the Si content goes up from 0.25% to 0.38%, and the yield strength and the elongation show a fluctuation trend. After simulating hot stamping process at 950 ℃ and holding 5 min, the microstructure of the steels with different compositions is martensite, but it is different in the fine martensite structure and the average size of sub-grain; after hot stamping process, the comprehensive mechanical properties of the steel B with 0.30%C, 0.34%Si and 1.21%Mn are the most outstanding, the yield strength is 1161 MPa, the UTS is 1758 MPa, and the elongation is 6.5%; after hot stamping process, the microstructure of the steel B is fine lath martensite, and there is a large amount of dislocation in the martensite lath, and precipitates a small number of carbide. The mechanical properties of the ultra-high strength press hardening steels designed in this work is not obvious correlation before and after hot stamping process, and it is just a slight difference in martensite fine structure which is beneficial to controlling the performance stability of the mass industrial production.