METHOD FOR MANUFACTURING STEEL MATERIAL HAVING UNIFORM STRENGTH IN SHEET THICKNESS DIRECTION AND SUPERIOR FATIGUE-CRACK PROPAGATION RESISTANCE

摘要

PROBLEM TO BE SOLVED: To provide a method for manufacturing a steel sheet having uniform strength in a sheet thickness direction and superior fatigue-crack propagation resistance.;SOLUTION: This steel material comprises, by mass%, 0.02-0.20% C, 0.01-0.50% Si, 0.5-2.0% Mn, 0.05% or less P, 0.02% or less S and the balance substantially Fe, while satisfying Pcm defined by the following expression so as to be 0.25 or less: Pcm = C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B, (wherein elements not to be added are considered to be zero). The method for manufacturing the steel sheet comprises heating the steel material to 1,000 to 1,300°C; then rolling it at 900°C or higher with a cumulative rolling reduction of 50% or higher; and then immediately acceleration-cooling it from an Ar3 point or higher at a cooling rate of 10°C/s or higher, temporarily stopping the cooling, holding it at 750 to 550°C for 5 seconds or longer, and acceleration-recooling it to a lower temperature than 550°C at the cooling rate of 10°C/s or higher.;COPYRIGHT: (C)2005,JPO&NCIPI