ROLLED BAR STEEL FOR CASE HARDENING HAVING EXCELLENT FATIGUE PROPERTY AND CRYSTAL GRAIN COARSENING RESISTANCE, AND METHOD FOR PRODUCING THE SAME

摘要

PROBLEM TO BE SOLVED: To provide a rolled bar steel for case hardening giving a case hardened component which has excellent crystal grain coarsening prevention effect even in the case carburizing is performed at a temperature higher than that in the conventional example for performing carburizing or carbonitriding treatment in a shorter period of time as the stock for a bar-shaped machine component such as a pulley for a CVT (Continuously Variable Transmission) requiring carburizing depth, has reduced dimensional strain upon case hardening treatment, thus has excellent dimensional precision, and has excellent fatigue properties as well.;SOLUTION: The rolled bar steel for case hardening is composed of a steel in which the contents of N, Al, Nb, Ti, Ca or the like are specified, and, the steel conyains, as oxide based inclusions, the three multiple oxide based inclusions of CaO, Al₂O₃, SiO₂. Among the multiple inclusions, the number of the ones satisfying the compositional ratio of 5 to 50% CaO, 5 to 50% Al₂O₃ and 25 to 75% SiO₂ accounts for ≥70% of the number of all the oxide based inclusions, and the number of the inclusions satisfying 50 to 100% Al₂O₃ is ≤10% by an occupancy ratio in the number of all the oxide based inclusions.;COPYRIGHT: (C)2007,JPO&INPIT