稀土对低碳钢马氏体相变的影响

INFLUENCE OF RARE EARTH ELEMENTS ON MARTENSITIC TRANSFORMATION IN ALOW CARBON STEEL

０．２７Ｃ－１Ｃｒ钢中加入稀土，细化奥氏体晶粒，降低Ｍ＿ｓ，缩小马氏体条宽，减少残余奥氏体量，并抑制自回火过程。稀土偏聚在原奥氏体晶界，不因马氏体相变而改变。根据以往证明：低碳钢马氏体相变中存在碳的扩散，则低碳钢淬火后的残余奥氏体量不但决定于Ｍ＿ｓ和淬火介质温度（Ｔ＿ｑ），还受合金元素对碳扩散的影响。由此将Ｍａｇｅｅ公式修改为：ｒ（％）＝ｅｘｐ［ａ（Ｃ＿１－Ｃ。）－β（Ｍ＿ｓ－Ｔ＿ｑ）〕其中Ｃ＿０和Ｃ＿１分别为淬火前和淬火后奥氏体中的碳浓度。

The addition of rare earth elements(RE)in 0.2 7C -1Cr steel refines austenite grain size,lowers M_s,diminishes the size of lath martensite,decreases the amount of retained austenite and retards the auto-tempering process. RE segregate at austenite grain boundary and this kind of distribution will not be changed during the martensitic transformation. According to the previous evidence that there may exist the carbon diffusion during the martensite transformation in low carbon steels,the amount of the retained austenite in quenched low carbon steel depends not only on the M_s and the temperature of quenching medium,T_q, but also on the influence of alloying elements on carbon diffusion. Subsequently, the Magee’s equation expressing the amount of retained austenite(r%)in quenched steels is modified as:r(%)=exp[α(C_1-C_0)-β(M_s -T_q) ,where C_0 and C_1 are carbon contents in austenite before and after quenching respectively.