Steel sheet and manufacturing method therefor

Abstract

This steel sheet has a prescribed chemical composition, wherein a steel structure in the interior of the steel sheet contains, by volume fraction, 0-30% soft ferrite, 3-40% residual austenite, 0-30% fresh martensite, and a total of 0-10% pearlite and cementite, the remainder including hard ferrite. The number percentage of residual austenite with an aspect ratio of 2.0 or greater included among all of the residual austenite is 50% or greater, a soft layer having a thickness of 1-100 [mu]m in the sheet thickness direction from the surface exists, of the ferrite contained in the soft layer the volume fraction of crystal grains with an aspect ratio of 3.0 or greater is 50% or greater, the volume fraction of the residual austenite in the soft layer is 80% or less of the volume fraction of theresidual austenite in the interior of the steel sheet, and the peak emission intensity of a wavelength representing Si appears in a range of greater than 0.2 [mu]m from the surface and 10.0 [mu]m or less from the surface.

Description

technical field [0001] The present invention relates to a steel plate and a manufacturing method thereof. Background technique [0002] In recent years, awareness of environmental issues is increasing. Therefore, in the automobile industry, it is important to reduce the weight of a vehicle body for the purpose of improving fuel efficiency. On the other hand, there is also a need to increase the body strength of automobiles and ensure collision safety. In order to take into account the lightweight and crash safety of the car body, it is only necessary to use high-strength steel plates as the material of the car body. However, the higher the strength of the steel sheet, the more difficult the press forming becomes. This is because, in general, the higher the strength of the steel sheet, the lower the formability such as ductility, bendability, and hole expandability. [0003] In addition, in the case of an ultra-high-strength steel sheet having a tensile strength exceeding...

AI Technical Summary

Problems solved by technology

[0008] However, it is difficult to simultaneously improve the ductility and hole expandability of conventional high-strength steel sheets and the hydrogen embrittlement resistance of bent parts.

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