High-strength steel sheet and method for manufacturing same

Abstract

According to the present invention, a predetermined steel component is included; a steel plate composition is made to have 5-70% of a martensite, 5-40% of a residual austenite, and 5% or more of a bainitic ferrite in an upper bainite, by area in relation to the entire steel plate composition; the total area of the martensite, the residual austenite, and the bainitic ferrite is made to be 40% or more; 25% or more of the martensite is made to be tempered martensite; the area of a polygonal ferrite in relation to the entire steel plate composition is more than 10% and less than 50%, and the average grain size thereof is made to be 8 mum or less; and when a group of ferrite grains comprising adjacent polygonal ferrite grains is made to be a polygonal ferrite grain group, the average diameter thereof is 15 mum or less, the average amount of C in the residual austenite is made to be 0.70 mass% or more, and the tensile strength is made to be 780 MPa or more; whereby it is possible to obtain a high-strength press member having excellent ductility and stretch flangeability, as well as a tensile strength of 780-1400 MPa.

Description

technical field [0001] The present invention relates to a high-strength steel sheet having a tensile strength (TS) of 780 MPa to 1400 MPa, which is used in the industrial fields of automobiles and electrical equipment, and has excellent workability, especially ductility and stretch flangeability, and a method for producing the same. Background technique [0002] In recent years, reducing the fuel consumption of automobiles has become an important issue from the viewpoint of protecting the global environment. Therefore, studies are being actively conducted to reduce the thickness of vehicle body parts and reduce the weight of the vehicle body itself by increasing the strength of vehicle body materials. [0003] Generally, in order to increase the strength of a steel sheet, it is necessary to increase the ratio of hard phases such as martensite and bainite to the entire structure of the steel sheet. However, increasing the strength of a steel sheet by increasing the proportio...

AI Technical Summary

Problems solved by technology

However, as long as the main phase is polygonal ferrite, it is difficult to achieve both high strength and workability with a tensile strength (TS) of 780 MPa or more

In addition, even if the workability of the hard phase itself is improved by forming carbides in the hard phase, the workability of the polygonal ferrite deteriorates, so in order to achieve high strength with a tensile strength (TS) of 780 MPa or more When the amount of polygonal ferrite is reduced, sufficient workability cannot be obtained

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