High strength steel sheet and method for producing the same

Abstract

The present invention relates to A high strength steel sheet having a structure which is mainly composed of MD structure (Micro Duplex structure) comprising a ferrite matrix, and as a secondary phase, martensite or martensite and retained austenite, finely dispersed in said matrix, wherein the proportion that the MD structure occupies in the whole structure is 90% or more, wherein the proportion that the secondary phase present in the whole structure occupies in the whole structure is from 10 to 60%, wherein the secondary phase in the MD structure is present in ferrite grains and at grain boundary, in which the proportion of the secondary phase present in the ferrite grains is 50% or more, and wherein the average grain size of the secondary phase in the whole structure is 3 μm or less. The secondary phase is constituted of martensite, or martensite and retained austenite.

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 stretch flangeability in high strength region of 590 MPa or more.BACKGROUND OF THE INVENTION[0002]In industrial fields of automobiles, electrical machinery and appliances, machinery and the like, steel sheets used by press forming are required to combine excellent strength and formability. In recent years, such demand characteristics are increasing more and more. Dual phase steel sheets (DP steel) having dual phase of ferrite and martensite, and TRIP steels comprising a structure containing ferrite, bainite and retained austenite are known as high strength steel sheets having excellent ductility. However, dual phase steel sheets such as DP steel and TRIP steel have the problem that stretch flangeability is poor.[0003]To improve st...

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Benefits of technology

[0026]To combine strength and ductility, it is advantageous to be a dual phase structure in which a soft phase and a hard phase are combined, and the steel of the present invention has a dual phase structure comprising ferrite as a soft phase and martensite as a hard phase. However, even though a dual phase structure merely comprising soft ferrite and hard martensite is formed as in DP steel as described before, it is effective to combine strength and uniform elongation, but it is not sufficient to ensure excellent stretch flangeability. The reason that DP steel is poor in stretch flangeability includes size and existence position of martensite as a hard phase. Specifically, martensite in DP steel is generally large as about several ten μm, and grain boundary between martensite and ferrite is large tilt grain boundary. When deformation is added to a steel sheet, stress concentrates at grain boundary (large tilt grain boundary) as compared with the inside of grains. When interface having strength difference is present here, voids are generated in a moment, which is liable to lead to fracture. Such generation of voids and fracture have a correlation with local elongation, and a steel sheet that is liable to cause generation of voids and propagation of cracks is poor in local elongation. Grain boundary between ferrite and martensite in DP steel is just an interface having strength difference, and excellent stretch flangeability cannot be possessed for this reason. Furthermore, alt

Problems solved by technology

However, when a hard phase is not present, high strength cannot be obtained.

The reason that the matrix is bainite is that lath of bainitic ferrite constituting bainite is not grain boundary, and is therefore difficult to b

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