Patent ID: 7828912
Filing Date: 2010-11-09
Classification: C21D,Y10T

Abstract:
1. A high-strength hot-rolled steel sheet having ferrite structure and a strength of at least 1049 N/mm 2 excellent in hole expandability and ductility, consisting essentially of, in terms of a mass %: C: 0.01 to 0.09%, Si: 1.2 to 1.5%, Mn: 0.5 to 3.2%, Al: 0.003 to 0.04%, P: 0.03% or below, S: 0.005% or below, Ti: 0.10 to 0.25%, Nb: 0.01 to 0.05%, the balance consisting of iron and unavoidable impurities; and satisfying all of the following formulas <1> to <3>: wherein said hot rolled steel sheet is produced by the steps comprising: finishing hot rolling at rolling end temperature from an Ar cooling the steel sheet to 650 to 800° C. at a cooling rate of at least 20° C./sec; air cooling the steel sheet for 0.5 to 0.8 seconds; further cooling the steel sheet to 300 to 600° C. at a cooling rate of at least 20° C./sec; and coiling the steel sheet, whereby ferrite structure is strengthened by TiC and/or NbC precipitates, Mn and C without adding Mg.