COLD ROLLED HIGH TENSILE STRENGTH STEEL SHEET EXCELLENT IN RESISTANCE TO SURFACE DISTORTION, BAKE HARDENABILITY, AND STRETCH FLANGE FORMABILITY, AND METHOD FOR PRODUCING THE SAME

摘要

PROBLEM TO BE SOLVED: To provide a cold rolled high tensile strength steel sheet excellent in resistance to surface distortion, bake hardenability, and stretch flange formability, and a method for producing the same.;SOLUTION: The cold rolled high tensile strength steel sheet contains, in mass%, more than 0.015% and less than 0.065% of C, less than 0.4% of Si, 1.0-2.0% of Mn, more than 0.015% and 0.060% or less of P, 0.015% or less of S, 0.01-0.3% of sol.Al, 0.0005-0.0050% of B, 0.005% or less of N, 0.3% or less of Cr, less than 0.15% (including 0) of Mo, and less than 0.15% (including 0) of V, wherein microstructure thereof includes, in a volume fraction, 3.0-12.0% of a secondary phase based on ferrite as a mother phase, the secondary phase includes, in a volume fraction, 0.50-7.0% of martensite, 0.0% or more and less than 4.0% of retained austenite, and 1.0-10.0% of bainite, and a ratio of the volume fraction of the retained austenite to the volume fraction of the secondary phase is less than 40.0%.;COPYRIGHT: (C)2013,JPO&INPIT