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...

AI Technical Summary

Benefits of technology

[0016]To obtain a high strength steel sheet, it is necessary that a hard phase is a secondary phase, but when the hard phase is present at the grain boundary of a matrix, its interface becomes a starting point of fracture, and local elongation lowers. However, when a hard phase is not present, high strength cannot be obtained. When a hard phase can finely be present in the grains of a matrix, a starting point of fracture can be minimized, and as a result, local elongation of steel can be improved. It is known that local elongation of steel has a correlation with stretch flangeability thereof, and high stretch flangeability can be obtained by improving local elongation.

[0027]In view of the above, the present inventors thought that as the means for improving stretch flangeability while maintaining excellent properties of DP steel (combining strength and uniform elongation), a dual phase structure having a soft phase (ferrite) and a hard phase (martensite) is basically formed, but martensite is finely dispersed in ferrite grains (or small tilt grain boundary). They further thought that even in the case of ensuring the same degree of strength, grain size of the individual martensite can be decreased as compared with the general DP steel, and by further making martensite present in grains (or small tilt grain boundary), it becomes possible to suppress generation of voids in the case that deformation has been generated. By those, it is possible to combine elongation and stretch flangeability, and in addition to this, it is expected that ductility (particularly, uniform elongation) is further improved by utilizing TRIP effect as a structure containing retained austenite. The steel of the present invention developed based on the above-described thoughts has a structure comprising ferrite as a matrix, and martensite and retained austenite, finely dispersed mainly in ferrite grains. Due to this structure, the steel combines elongation and stretch flangeability in addition to strength.

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 become a starting point of fracture.

As a result of verification, cementite which is a hard phase could finely be dispersed among laths, but bainite is slightly poor in ductility, and the desired elongation was not obtained.

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