HIGH-STRENGTH STEEL SHEET SUPERIOR IN DEEP DRAWABILITY, AND MANUFACTURING METHOD THEREFOR

摘要

PROBLEM TO BE SOLVED: To provide a high-strength steel sheet which satisfies the expressions of 500 MPa≤TS, 1.2≤r value and 1.0≤r90, and to provide a manufacturing method therefor. SOLUTION: The high-strength steel sheet which has such a superior deep drawability as to satisfy 1.2≤r value and 1.0≤r90comprises: by mass%, 0.030 to 0.050 C, 1.0 or less Si, 2.0 to 3.0 Mn, 0.005 to 0.1 P, 0.01 or less S, 0.005 to 0.5 Al, 0.01 or less N, 0.010 to 0.30 Nb while satisfying the expression (1): (Nb/93)/(C/12)=0.2 to 0.7 and the balance Fe with unavoidable impurities. The steel sheet has also a structure including, a ferrite phase of 50% or more and a martensite phase of 3% or more, by an area rate; and has such a crystal orientation distribution in a sheet thickness direction as to satisfy the expressions (2) to (5); where (2): fsur to 1/4ä111}≥40% (3): fsur to 1/4ä110}≤5% (4):Δfä111}≥10% and (5):Δfä110}≤3%, wherein fsur to 1/4ä111}and fsur to 1/4ä110}represent average area rates of crystal grains of which the ä111} and ä110} faces are parallel to the surface of the sheet, and which are determined at four positions that are 1/4 deep from the surface layer in the sheet thickness direction; f1/2ä111}and f1/2ä110}represent average area rates of crystal grains of which the ä111} and ä110} faces are parallel to the surface of the sheet, and which are determined at two positions in the vicinity of a 1/2 deep portion from the surface in the sheet thickness direction; andΔfä111}is equal to fsur to 1/4ä111}-f1/2ä111}, andΔfä110}is equal to fsur to 1/4ä110}-f1/2ä110}. COPYRIGHT: (C)2009,JPO&INPIT