CASE HARDENED STEEL EXCELLENT IN THE PREVENTION OF COARSENING OF PARTICLES DURING CARBURIZING THEREOF, METHOD OF MANUFACTURING THE SAME, AND RAW SHAPED MATERIAL FOR CARBURIZED PARTS

摘要

A method of manufacturing case hardened steel, capable of minimizing stably the coarsening of particles therein in the carburizing and quenching step, comprising heating steel containing 0.015-0.04 % of Al, 0.005-0.04 % of Nb, 0.006-0.020 % of N and a specific range of percentage of other specific components at 1150 DEG C or above for 10 minutes or more, hot rolling the resultant steel at a finishing temperature of 920-1000 DEG C, and then gradually cooling the resultant product from 800 DEG C to 500 DEG C at a rate of 1 DEG C/sec or above: a case hardened steel manufactured by this method and having an amount of deposition of Nb (CN) after the hot rolling step of at least 0.005 %, an amount of deposition of AlN of at most 0.005 %, the number of Nb (CN) particles with diameters of at most 0.1 mu m in a mother phase of the steel of at least 20/100 mu m<2>, a content of a bainite structure of at most 30 % and a crystal grain number of ferrite of 8-11; and a raw shaped material for carburized parts made by utilizing the case hardened steel.