HOT-ROLLED MATERIAL HAVING SUPERIOR PROPERTIES OF PREVENTING CRYSTAL GRAIN FROM COARSENING WHEN CARBURIZED AT HIGH TEMPERATURE

摘要

PROBLEM TO BE SOLVED: To provide a hot-rolled material having superior properties of preventing austenitic crystal grains from coarsening even when carburized at high temperature after having been warm-forged or cold-forged.;SOLUTION: The hot-rolled material has a steel composition comprising 0.10 to 0.30% (which means wt.%, hereinafter the same) C, 0.01 to 1.0% Si, 0.2 to 2.0% Mn, 0.03% or less P, 0.2% or less S, 0.01 to 0.10% Al, 0.003 to 0.030% N, 0.010 to 0.20% Nb and/or Ti, and the balance Fe with unavoidable impurities; and includes precipitates in ferrite, which contain such Nb and/or Ti as to satisfy the following requirements (1) to (4): (1) (average particle size of precipitate (nm)/area rate of precipitate)5.0×10⁴ (nm); (2) average particle size of precipitate: 6 nm or larger; (3) apparent density of precipitate: 20 pieces/μm² or higher; and (4) ratio of (amounts of Nb and/or Ti in precipitate)/(amounts of Nb and/or Ti in steel): 95% or more.;COPYRIGHT: (C)2008,JPO&INPIT