通过光学显微镜分析、扫描电子显微镜分析、透射电子显微镜分析以及力学性能测试，研究了不同热处理工艺及Nb 元素的加入对NM550 耐磨钢的组织和力学性能的影响． 结果表明: 在200℃回火时，随着淬火温度的提高，不含Nb 钢的晶粒长大比较明显，强度和硬度下降，韧性提高; 含Nb 钢中Nb 的析出物抑制了晶粒的长大，同时Nb 的回溶提高了奥氏体中的合金浓度、减小了析出物尺寸，在固溶强化、析出强化和细晶强化的共同作用下，含Nb 钢展示了更优异的力学性能．当回火温度从200℃升高到250℃时，由于碳化物的粗化，两种钢的力学性能都出现不同程度的下降，应当避免在此温度回火．

The influences of both the addition of Nb and the heat treatment process on the microstructure and mechanicalproperties of NM550 wear-resistant steel were investigated by means of LM ( Light Microscopy) ,SEM andTEM and by mechanical tests.The results show that ( 1) with the increase of the quenching temperature at a temperingtemperature of 200℃,the grain growth of the Nb-free steel is more obvious with the decreases of the tensilestrength and Brinell hardness as well as with a toughness improvement; ( 2) however,the addition of Nb can effectivelyretard the growth of austenite grain,and the re-dissolution of NbC particles increases the alloying elementconcentration in austenite and decreases the size of NbC particles; ( 3) under the combined action of the solutionstrengthening,the precipitation strengthening and the grain refinement strengthening,the Nb-bearing steel showsbetter mechanical properties; and ( 4) when the tempering temperature increases from 200℃ to 250℃,the mechanicalproperties of the two kinds of steels decrease to varying degrees due to the coarsening of carbide,which indicatesthat the tempering at those temperatures should be avoided.