High-strength steel plate and method for manufacturing the same

Abstract

The invention relates to a high strength steel which has a structure mainly composed of MD structure ( micro diphase structure). The MD structure includes ferrite matrix and martensite or martensite and austenite residue finely dispersed in the matrix as secondary phase, which accounts for more than 90 percent of the whole structure. The secondary phase, which exists in the whole structure, accounts from 10 percent to 60 percent of the whole structure, and the secondary phase, which exists in the MD structure, exists in the ferrite grains and on the grain boundary. The secondary phase which exists in the ferrite grains, accounts for more than 50 percent, and the average grain size of the secondary phase which exists in the whole structure, is less than 3 micrometer. The secondary phase comprises martensite or martensite and austenite residue.

Description

technical field [0001] The present invention relates to a high-strength dual-phase steel sheet having excellent elongation and stretch flangeability. In particular, it relates to a high-strength dual-phase steel sheet having excellent strength-elongation and tensile flangeability in a high-strength region above 590 MPa. Background technique [0002] In industrial fields such as automobiles, electric machinery and equipment, and machinery, steel sheets used by press forming are required to combine excellent strength and formability. In recent years, these demands have been characterized by increasing numbers. Dual-phase steel sheet (DP steel) having a dual phase of ferrite and martensite and TRIP steel containing a structure containing ferrite, bainite, and retained austenite have been established as high-strength steel sheets with excellent ductility. Known. However, dual-phase steel sheets such as DP steel and TRIP steel have a problem of poor tensile flangeability. [...

AI Technical Summary

Problems solved by technology

However, a steel sheet with a martensitic single-phase structure is a steel sheet with a macroscopically uniform structure, so the stretch flangeability is excellent, but the ductility (elongation) is greatly deteriorated (EL<8%)

[0006] 2. Reduce the strength difference of the duplex structure (see patent document 2)

Therefore, there is still an industrial problem of increasing time and cost in this method

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