肌焼鋼におけるMn量適正化によるオーステナイト結晶粒粗大化抑制とそのメカニズム(相変態・材料組織)  [in Japanese] Prevention of Austenite Grain Coarsening of Carburizing Steel during Carburizing by Mn Content Optimization and its Improvement Mechanism(Transformations and Microstructures)  [in Japanese]

Carburizing steels are often used for automotive drive train parts which require high fatigue strength and impact value. Prevention of austenite grain coarsening of these steel parts during carburizing is an important issue for exerting high performance. In this study, with a view to preventing the grain coarsening during carburizing, influence of Mn content on grain coarsening temperature was investigated for Ti-modified carburizing steel with ferrite and pearlite microstructure. It was found that grain coarsening temperature was elevated more than 50K by reducing Mn content from 0.8 mass% to 0.4 mass%. This is attributed to the elevated Ar_3 point due to lower Mn content, where ferrite and pearlite microstructure is coarser prior to the carburizing and initial austenite grain is preferably coarse during carburizing. This result showed that the reduction of Mn content in carburizing steel was effective to prevent grain coarsening during carburizing when the microstructure prior to carburizing was controlled as ferrite and pearlite by hot forging, controlled cooling after hot forging, normalizing or full annealing.