Excellent thickness steel and its manufacturing method to crack initiation characteristics-out resistance to fatigue

摘要

PPROBLEM TO BE SOLVED: To provide a method for manufacturing a thick steel material superior in fatigue-crack generation resistance. PSOLUTION: A base steel material has a composition containing, by mass%, 0.02-0.5% C, 0.01-0.55% Si, 0.1-3.0% Mn, 0.2% or less P, 0.05% or less S, 0.1% or less Al and 0.005% or less N. The manufacturing method includes sequentially: a solution treatment step of solution-treating the base steel material so that a solution treatment temperature T(K) and a solution treatment time t(s) satisfy the relation (2): t≥XSP2/SP/exp(-24438/T), wherein X represents ((thickness (m) of base steel material)/2); a hot rolling step of reheating the steel material to a temperature of (ASBc3/SBtransformation point +100°C) or higher, hot-rolling the steel material with a cumulative rolling reduction of 50% or more in a temperature range of higher than the ASBc3/SBtransformation point, and subsequently air-cooling the hot-rolled steel material to a temperature of the MSBs/SBpoint or lower; and a reheating step of reheating the steel material to a temperature range of the ASBc3/SBtransformation point to the ASBc1/SBtransformation point, and then cooling the steel material to the MSBs/SBpoint or lower at a cooling rate of 10°C/s or more. PCOPYRIGHT: (C)2010,JPO&INPIT