CARBON COLD-ROLLED SHEET AND GALVANIZED SHEET BOTH EXCELLENT IN FATIGUE CHARACTERISTICS AND PROCESS FOR PRODUCING BOTH)

摘要

The present invention provides a cold-rolled steel sheet or a galvanized steel sheet for deep drawing improved in fatigue properties of a base metal and fatigue properties of a spot welding portion. The steel sheet according to any one of claims 1 to 3, wherein the cold-rolled steel sheet or the zinc-coated steel sheet contains 0.0001 to 0.0026% by weight of carbon and not more than 1.2% by weight of silicon, 0.03 to 3.0% by weight of manganese, 0.015 to 0.15% 0.005 to 0.1 wt.%, Nitrogen: 0.0005 to 0.0080 wt.%, And boron: 0.0003 to 0.0030 wt.%, And iron and inevitable impurities. The method of producing the steel sheet may further comprise:₃Cooling the hot rolled strip at 750C at a cooling rate of at least 50C / sec within 1.5 seconds after completion of the hot rolling, and Cold annealing the cold rolled strip at a temperature of from room temperature to 750C; cold stripping the coiled strip at a reduction of at least 70%; subjecting the cold rolled strip to continuous annealing at 600 to 900C, Subjecting the steel sheet to continuous galvanization by a jamming method and subjecting the steel sheet to a temper rolling at a reduction rate of at least 1.5 x (1-400 x C)% and at least 2080 x (C-0.0015) Where C represents the carbon content in weight%.