Patent ID: 7879164
Filing Date: 2011-02-01
Classification: B32B,C21D,C22C,C23C

Abstract:
1. A method of producing a hot-rolled steel sheet comprising: subjecting a carbon steel or low-alloy steel slab, comprising C: 0.01 to 0.25% by mass, to multi-pass hot rolling characterized in that a final rolling pass is finished at a temperature not lower than the Ar cooling the rolled sheet to 720° C. or below within 0.4 second at a cooling rate of not lower than 400° C./second; and maintaining the rolled sheet in a temperature range of 600 to 720° C. for at least 2 seconds to yield: a hot-rolled steel sheet, the main phase of which is ferrite, characterized in that the average ferrite crystal grain diameter D(μm) at the depth of ¼ of the sheet thickness from the steel sheet surface satisfies the relations respectively defined by formulas (1) and (2) given below and the increase rate X (μm/min) in average ferrite crystal grain diameter at 700° C. at the depth of ¼ of the sheet thickness from the steel sheet surface and said average crystal grain diameter D(μm) satisfy the relation defined by the formula (3) given below: wherein C and Mn represent the content (in % by mass) of the respective elements in the steel.