材料期刊网

基于以Nb替代V的设想, 采用OM, SEM, EDS, 相分析、硬度测定及冲击韧性实验, 研究了不同Nb-V含量对Cr8型冷作模具钢微观组织和力学性能的影响. 结果表明,Nb提高MC共晶碳化物的析出温度, 包晶反应和包共晶反应的温度也随之提高,初生γ相中C含量的分布更不均衡. 当Nb含量(质量分数)达到1.32%时,
初生γ相界附近更易形成粒状碳化物; 不同Nb-V含量会改变MC共晶碳化物的类型, 随Nb含量的升高, MC共晶碳化物由以VC为主变成VC和少量(Nb, V)C,
再变成以NbC为主和(Nb, V)C复合, 使钢中莱氏体形态更平直; 当Nb含量达到1.32%时,以Nb替代V可显著提高低温淬火硬度, 使淬火硬度峰值向低温区移动, 同时Nb替代V对二次硬化有利, 可以获得更高的回火硬度和抗回火软化性; 实验钢的冲击韧性随Nb含量的升高而降低.

Cr8–type cold work die steels have been widely used in recent years. These steels have obvious secondary hardening effect due to high V content. Now it has been successfully used in high speed steel that Nb is substituted for V. However, it has been not widely applied in cold work die steel yet. In order to investigate the effect of different Nb–V contents on the microstructure and mechanical properties of this kind steel and the role of Nb in tool and die steels, in this paper the microstructure and morphology of three Cr8–type cold work die steels with different Nb–V contents were investigated by OM, SEM and EDS analysis, the mechanical properties including the hardness and impact toughness were measured at room temperature. Experimental results indicate that the precipitation temperature of MC–eutectic carbides increase with the increase of Nb content, and the improvement of their properties is closely related with Nb content. Moreover, the increase of Nb content causes the increase of metatectic and eutectic temperature and the disequilibrium of C distribution in primary γ phase. The granular carbides appear near the grain boundary of primary  phase when Nb content reaches to 1.32%; the variation in Nb/V ratio with the increase of Nb content will change the type of MC–eutectic carbides, the type of MC–eutectic carbides from mainly VC change to VC and a small amount of (Nb, V)C and then to mainly NbC and (Nb, V)C. So the shape of ledeburite is more straight. If Nb content reaches to 1.32%, the substitution of Nb for V increases quenching hardness, the peak of quenching hardness moves to low temperature region; the substitution of Nb for V is beneficial for secondary hardening in tested steels, and higher hardness and temper softening resistance can be obtained in them. In addition, their impact toughness decreases with the increase of Nb content.