High-strength composite steel sheet having excellent moldability and delayed fracture resistance

Abstract

The present invention provides a high-strength composite steel sheet which has a tensile strength of 980 MPa class as well as excellent and excellent anti-delayed fraction property, and also has excellent spot-weldability. The high-strength composite steel sheet comprises a steel satisfying: C: 0.10 to 0.25% (% by mass in case of a chemical component, the same shall apply hereinafter), Si: 1.0 to 3.0%, Mn: 1.5 to 3.0%, P: 0.15% or less, S: 0.02% or less, Al: 0.4% or less, and comprising the remnant made from iron and unavoidable impurities; the contents of Si, Al, Mn and Cr satisfy the relationship of “(Si+Al) / Mn or (Si+Al) / (Mn+Cr)=0.74 to 1.26”; and microstructure is specified.

Description

TECHNICAL FIELD

[0001] The present invention relates to a high-strength composite steel sheet which has a tensile strength of 980 MPa or higher class as well as excellent formability and excellent anti-delayed fraction property, and also has excellent spot-weldability and is useful as automotive structural parts (body flame members such as pillar, member and reinforcement; bumper, door guard bar, sheet parts, suspension parts, and other reinforcing members).BACKGROUND ART

[0002] In recent years, for the purpose of reducing fuel consumption due to saving body weight of automobiles and ensuring safety upon collision, demands for high-strength steels have increased more and more. Accordingly, steel sheets having a tensile strength of 980 MPa or higher class have been required in place of those having a tensile strength of 590 MPa class. However, in the case of high-strength steel sheets having a tensile strength of 980 MPa or higher class, deterioration of formability cannot be avoided and...

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