Nb添加肌焼き鋼の強切削加工後のガス浸炭における表面炭素濃度の異常低下現象

摘要

歯車部品や軸部品など自動車に用いられる動力伝達部品の多くは，機械加工により成形され，さらにその後の熱処理により疲労強度や耐摩耗性などの機能が付与されている。これら動力伝達部品に多く用いられる熱処理方法として，高周波焼入れやガス浸炭焼入れ，真空浸炭焼入れなどがある。特にガス浸炭焼入れは大量生産性に優れ，表面炭素濃度の調整，操業が容易であることなどから工業的に広く利用されている。%The surface carbon concentration (Cs) of carburized Nb-bearing steel (SCM420Nb) was found to decrease substantially when carburized after severe machining. It was observed that the effect of machining condition is small in normal case hardening steel (SCM420), in which the Cs was in the range of 0.9～1.1%. However, the Cs in carburized SCM420Nb was reduced to 0.61% by mild machining, and 0.38% by severe machining. Since the Cs of severely machined SCM420Nb recovered to > 0.8% after the removal of surface layer by polishing, it was concluded that the abnormal decrease in Cs of SCM420Nb is mainly due to severely deformed surface layer produced by machining. Discussion was made on the abnormally low Cs observed in severely machined SCM420Nb steel with regard to the effect of solute Nb on the carbon activity and the effect of surface roughness. It was found that these effects are small compared with the observed reduction in Cs. Comparing the experimental and calculated carbon concentration profiles, Cs was expected to be reduced immediately after the start of carburizing. Finally, it was proposed that the oxide layer created on the surface during carburizing might be the cause of the abnormally low Cs.