METHOD FOR MANUFACTURING COLD-ROLLED STEEL SHEET SUPERIOR IN RECRYSTALLIZATION SOFTENING RESISTANCE, AND COLD-ROLLED STEEL SHEET FOR AUTOMATIC TRANSMISSION

摘要

PROBLEM TO BE SOLVED: To provide a cold-rolled steel sheet which is used for an automatic transmission member of automobiles and the like, and is superior in hardness, surface roughness, recrystallization softening resistance and the like.;SOLUTION: The cold-rolled steel sheet is manufactured by the steps of: producing a slab having a component composition including, by mass%, 0.15-0.25% C, 0.25% or less Si, 0.3-0.9% Mn, 0.03% or less P, 0.015% or less S, 0.01-0.08% Al, 0.008% or less N, 0.05-0.5% Cr, 0.01-0.05% Ti, 0.002-0.005% B and the balance Fe with unavoidable impurities; heating the slab to a temperature higher than 1,230°C in a heating furnace, and extracting the slab; hot-rolling the slab at a hot-rolling-finishing temperature of the Ar₃ transformation temperature or higher, and a winding temperature of 500-600°C to form a hot-rolled steel plate having a ferrite-pearlite mixture structure containing ferrite crystals with grain sizes of 5-15 μm and a pearlite+cementite fraction of 40% or more; pickling the hot-rolled steel plate; and cold-rolling the hot-rolled steel plate at a rolling reduction of 30% or more while skipping annealing treatment.;COPYRIGHT: (C)2010,JPO&INPIT