High-strength steel plate and manufacturing method thereof

Abstract

According to the present invention, by containing predetermined steel components, the structure of the steel sheet is such that the area ratio of martensite is 5 to 70% and the area ratio of retained austenite is 5 to 40% based on the area ratio of the entire steel sheet structure. , the area ratio of bainitic ferrite in upper bainite is 5% or more, and the area ratio of the above-mentioned martensite, the above-mentioned area ratio of retained austenite and the above-mentioned area ratio of bainitic ferrite 40% or more in total, 25% or more of the above-mentioned martensite is tempered martensite, the area ratio of polygonal ferrite to the entire steel plate structure is more than 10% and less than 50%, and the average particle size is 8 μm or less, When a group of ferrite grains consisting of adjacent polygonal ferrite grains is used as a polygonal ferrite grain group, its average diameter is 15 μm or less, and the average C content in the above-mentioned retained austenite is 0.70% by mass or more, and, By setting the tensile strength at 780 MPa or more, a high-strength pressurized member having excellent ductility and stretch flangeability and a tensile strength of 780 to 1400 MPa can be obtained.

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...

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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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