METHOD FOR MANUFACTURING CURBURIZED STEEL SUPERIOR IN GRAIN-COARSENING RESISTANCE AFTER HIGH TEMPERATURE CARBURIZING

摘要

PROBLEM TO BE SOLVED: To provide a method for manufacturing a high-temperature curburized steel superior in grain-coarsening-preventing characteristics, which prevents crystal grains from coarsening during carburizing at a high temperature and can skip normalizing treatment after hot forging.;SOLUTION: A raw material of the carburized steel comprises, by mass%, 0.1-0.35% C, 0.05-0.5% Si, 0.2-2.0% Mn, 0.005-0.05% Al, 0.1-0.3% Ti and the balance Fe with unavoidable impurities. The method for manufacturing the carburized steel, which can skip the normalizing treatment, comprises hot-forging the raw material at a heating temperature of 1,200°C, and cooling it at 800 to 500°C at a cooling rate of 2°C/sec. Thus hot-forged steel has TiC or TiCN precipitates with sizes of 10 to 100 nm in an amount of 30 particles/μm² or more; and a structure including a bainitic structure of 5% or less and the balance a ferrite+pearlite structure. When the steel is carburized at 1,000°C or a higher temperature, it has austenitic grains with sizes of grain size number 7 or greater, and less than 20% of such coarsened grains as to have a grain size number difference of 3 or greater.;COPYRIGHT: (C)2005,JPO&NCIPI