Effects of different tempering temperature on the microstructure and mechanical properties of the directly-quenched steel were investigated by SEM,TEM and other experimental methods. The results indicate that the tested steel obtain the best comprehensive mechanical properties with tensile strength of 805MPa, yield strength of 719MPa, elongation of 25.8％ and impact energy of 106J at -20℃. The recovery of lath bainite become even more clear with increasing tempering temperature. The small-angle grain boundaries in lath bainite gradually disappear through motion, merging and reorganization of dislocation, which cause lath wider. It was found that bainite lath boundaries blurred, the dislocation network located in bainite and the precipitation of (Nb, Ti )(C, N) composite phase with the average size of 10-20nm after tempering at 620℃. Mechanical properties with tempering temperature show non-monotonic nature, which is ascribed to the comprehensive mechanism including the recovery softening process of dislocation substructure, and the strengthening mechanism due to the dissolution of carbon and precipitation of the second phase in bainite during tempering.%利用SEM,TEM等实验方法,研究了不同回火温度对直接淬火Fe-Mn-Mo-Nb-Ti系低碳贝氏体组织和力学性能的影响.结果表明,580℃回火后钢板具有最佳综合力学性能,抗拉强度805MPa,屈服强度719MPa,延伸率25.8%,-20℃冲击功106J.随回火温度的升高,板条贝氏体回复作用逐渐加强,位错通过运动、合并、重组,使得某些板条之间的小角度晶界逐渐消失,相邻板条合并,组织粗化.620℃回火后板条贝氏体边界模糊不清,位错呈现网状形态,同时发现析出(Nb,Ti)(C,N)复合相,平均尺寸10-20nm.回火后力学性能变化趋势的非单调性,归因于回火过程贝氏体中位错亚结构的回复软化与碳的脱溶及析出第二相的强化机制综合作用.
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