High-strength steel plate and manufacturing method thereof

Abstract

Disclosed is a high-strength steel plate having superior ductility and stretch flangeability and a tensile strength (TS) of 980 MPa or higher, and having 0.17-0.73% C, 3.0% or less Si, 0.5-3.0 or less Mn, 0.1% or less P, 0.07% S, 3.0% or less Al, 0.010% or less N, and 0.7% or more Si + Al, an area ratio of martensite of 10-90% with respect to the entire steel plate composition, a residual austenite amount of 5-50%, and an area ratio of bainitic ferrite in the upper bainite of 5% or less with respect to the entire steel plate composition. Twenty-five percent or more of the aforementioned martensite is tempered martensite, and the total of the area ratio of the aforementioned martensite with respect to the entire steel plate composition, the aforementioned residual austenite amount and the area ratio of the aforementioned bainitic ferrite in the upper bainite with respect to the entire steel plate composition is 65% or more. The area ratio of polygonal ferrite with respect to the entire steel plate composition is 10% or less (including 0%), and the average amount of C in the aforementioned residual austenite is 0.70% or more.

Description

technical field [0001] The present invention relates to a high-strength steel sheet having a tensile strength (TS) of 980 MPa or more and excellent formability, especially ductility and stretch-flangeability, used in industrial fields such as automobiles and electric equipment, and a method for producing the same. Background technique [0002] In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of global environmental protection. Therefore, studies are being actively conducted to reduce the weight of the vehicle body itself by reducing the thickness of vehicle body parts 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 the steel sheet by increasing the proportion of the hard phas...

AI Technical Summary

Problems solved by technology

However, since the main phase is made of polygonal ferrite, it is difficult to simultaneously achieve high strength and workability with a tensile strength (TS) of 980 MPa or more

In addition, even if the workability of the hard phase itself is improved by forming carbides in the hard phase, it is not as good as the excellent level of workability of polygonal ferrite. Therefore, in order to achieve a tensile strength (TS) of 980MPa If the amount of polygonal ferrite is reduced for the above high strength, sufficient workability cannot be obtained.

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