High-strength cold-rolled steel sheet of excellent bending workability and hole-expanding property, hot-dip galvanized steel sheet and manufacturing method thereof

Abstract

The invention relates to a high-strength cold-rolled steel sheet of excellent bending workability and hole-expanding property. The high-strength cold-rolled steel sheet comprises the following components in weight percentage: 0.06 to 0.1% of C, 0.05 to 0.5% of Si, 2.0 to 2.5% of Mn, 0.001 to 0.10% of P, 0.010% or less of S, 0.01 to 0.10% of soluble Al, 0.010% or less of N, more than one selected from 0.003 to 0.08% of Ti and Nb, the balance of Fe and impurities. The fine structure of the steel material is composed of 40% or less of martensite, 30 to 40% of bainite and 20 to 30% of ferrite in area percentage. The hardness ratio of bainite to ferrite is over 0.7. The ferrite contains precipitates, wherein the density of the precipitates is 1.5*10<6> per mm2 and the size of the precipitates is smaller than 10 nm. In this way, the hardness ratio of bainite to ferrite is 2 or less.

Description

technical field [0001] The present invention relates to a method for manufacturing an ultra-high-strength steel plate with a tensile strength of 980 MPa or higher, which is mainly used as a structural part of an automobile. More specifically, it relates to a cold-rolled steel sheet excellent in formability and bendability obtained by appropriately controlling the area content of ferrite, bainite, and martensite, and controlling the hardness ratio, size, etc. of transformation phases A hot-dip galvanized steel sheet and a manufacturing method thereof. Background technique [0002] In recent years, in order to protect the global environment, while the regulations on fuel efficiency have been strengthened, the weight reduction of automobile bodies has also been actively carried out. As a countermeasure, people are reducing the weight of automotive materials by increasing the strength of steel plates. Generally, high-strength automotive materials can be classified into precipi...

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Problems solved by technology

This will lead to a significant change in yield strength and decrease in elongation. Therefore, it is necessary to develop a new product that can ensure a stable material in a wider annealing temperature range.

However, in known technologies such as Japanese Patent Publication No. 3729105, Japanese Patent Publication 2010-90432 and Japanese Patent Application Laid-Open No. 2005-105367, the above-mentioned content has not been fully studied.

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