High-strength cold-rolled steel sheet, high-strength hot-dip galvanized steel sheet, and high-strength hot-dip galvannealed steel sheet excellent in formability and shape fixability, and methods for manufacturing them

Abstract

A high-strength cold-rolled steel sheet has a specific chemical composition and has a steel microstructure meeting conditions: a total content of bainitic ferrite (BF) and tempered martensite (TM) is 65% (in area percent, hereinafter the same for steel microstructure) or more; a fresh martensite (M) content is 3% to 18%; a retained austenite content is 5% or more; and a polygonal ferrite (F) content is 5% or less. The steel sheet has a specific average KAM<1.00° of 0.50° or more and has a tensile strength of 980 MPa or more. The high-strength cold-rolled steel sheet excels in formability and shape fixability.

Description

TECHNICAL FIELD[0001]The present invention relates to: a high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet, and a high-strength hot-dip galvannealed steel sheet each excellent in formability and shape fixability, and methods for manufacturing them.BACKGROUND ART[0002]In automobiles, a demand has been made to improve fuel efficiency in consideration of environments, and this accelerates body weight reduction more and more. In addition, a demand has been increasingly made to provide steel sheets having a strength (in terms of tensile strength of 980 MPa or more) higher than those of conventional equivalents. Such high-strength steel sheets are for use in various steel members forming automobile bodies so as to maintain safety even when the body weights are reduced. Steels having such high strengths, however, are liable to suffer from deterioration in formability and shape fixability (spring-back control). To prevent this, a demand has been made to p...

AI Technical Summary

Benefits of technology

[0042]The present invention can provide a high-strength (980 MPa or more) cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet, and a high-strength hot-dip galvannealed steel sheet each of which is adapted to have a specific microstructure, is excellent in formability and shape fixability, and is suitable for automobile parts. These steel sheets are hereinafter also generically referred to as “steel sheet(s)”. As used herein the term “excellent in formability (outstanding formability)” refers to “excellent in balance between tensile strength and elongation (TS×EL balance) and balance between tensile strength and bore expandability (TS×λ, balance) at a tensile strength of 980 MPa or more”. Also as used herein the term “excellent in shape fixability” refers to “a low yield ratio (YR)”.

Problems solved by technology

Steels having such high strengths, however, are liable to suffer from deterioration in formability and shape fixability (spring-back control).

The technology disclosed in PTL 1, however, is not aimed at ensuring the properties of a steel sheet having a strength at a still higher level.

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