Cold rolled steel sheet and hot dipped steel sheet with superior strength and bake hardenability and method for manufacturing the steel sheets

Abstract

Disclosed herein are a bake-hardenable high-strength cold-rolled steel sheet, a hot-dipped steel sheet thereof, and a method for manufacturing the same. The steel sheet comprises 0.0016˜0.01% of C; 0.1% or less of Si; 0.2˜1.5% of Mn; 0.05˜0.15% of P; 0.01% or less of S; 0.08˜0.5% of (soluble) Al; 0.0025% or less of N; 0.003˜0.1% of Nb; 0.003% of less of Ti; 0.01˜0.4% of Mo; 0.0005˜0.005% of B; and the balance of Fe and other unavoidable impurities, in terms of weight %. The steel sheet has fine AlN precipitates, and a grain size (ASTM No.) of 9 or more. The AlN precipitates have a grain size, which can suppress grain growth. The steel sheet has enhanced strength, bake hardenability, aging resistance, and secondary work embrittlement resistance.

Description

TECHNICAL FIELD [0001] The present invention relates to a bake-hardenable cold-rolled steel sheet for automobile bodies, a hot-dipped steel sheet manufactured using the same, and a method for manufacturing the same. More particularly, the present invention relates to a bake-hardenable high-strength cold-rolled steel sheet having excellent bake hardenability, aging resistance at room temperature, and secondary work embrittlement resistance, a hot-dipped steel sheet manufactured using the same, and a method for manufacturing the same. BACKGROUND ART [0002] As used for exterior plates of automobile bodies, bake-hardenable cold-rolled steel sheets typically have a tensile strength of 390 MPa, and exhibit excellent ductility due to the fact that their yield strength is approximately the same as that of mild steel upon press forming. Additionally, the bake-hardenable cold-rolled steel sheets have a characteristic in which the yield strength thereof is increased upon paint baking or coatin...

AI Technical Summary

Benefits of technology

[0027] Therefore, the present invention has been made in view of the above problems, and it is an object of the invention to provide bake-hardenable high-strength cold-rolled steel sheets having bake hardenability, aging resistance, and secondary work embrittlement resistance by adding a small amount of Nb together with Al, Mo and B, while appropriately controlling of the Nb/C ratio and the grain size, hot-dipped steel sheets manufactured using the same, and a method for manufacturing the same.

[0028] In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a bake-hardenable cold-rolled steel sheet manufactured through hot rolling, cold rolling and continuous annealing of a steel, the steel sheet comprising: 0.0016˜0.01% of C; 0.1% or less of Si; 0.2˜1.5% of Mn; 0.05˜0.15% of P; 0.01% or less of S; 0.08˜0.5% of (soluble) Al; 0.0025% or less of N; 0.003˜0.1% of Nb; 0˜0.003% of Ti; 0.01˜0.4% of Mo; 0.0005˜0.005% of B; and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3˜0.7, wherein the steel sheet has fine AlN precipitates formed upon hot rolling the steel, and a grain size(ASTM No.) of 9 or more, the fine AlN precipitates having a size acting as a barrier for suppressing grain growth during annealing of the steel sheet.

[0029] In accordance with another aspect of the invention, a hot-dipped steel sheet manufactured through hot rolling, cold rolling, continuous annealing, and hot dipping of a steel is provided, the steel sheet comprising: 0.0016˜0.01% of C; 0.1% or less of Si; 0.2˜1.5% of Mn; 0.05˜0.15 of P; 0.01% or less of S; 0.08˜0.5% of (soluble)Al; 0.0025% or less of N; 0.003˜0.1% of Nb; 0˜0.003% of Ti; 0.01˜0.4% of Mo; 0.0005˜0.005% of B; and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3˜0.7, wherein the steel sheet has fine AlN precipitates formed u

Problems solved by technology

However, when manufacturing the bake-hardenable cold-rolled steel sheets through batch annealing, although compatibility between the formability and the bake hardenability can be easily ensured, there is a problem of lowering the productivity.

However, when manufacturing the bake-hardenable cold-rolled steel sheets through the continuous annealing method, although the bake hardenability can be easily ensured due to a relatively rapid cooling rate of the method, there is a problem of deterioration in the formability due to rapid heating and short period of annealing.

However, according to the disclosures, since it is necessary to strictly control the amount of Ti, N, and S in the manufacturing process in order to ensure appropriate bake hardenability, there is a problem of increasing manufacturing costs.

Thus, in the case of U.S. Patent Nos. 5,556,485 and 5,656,102, there are problems of deterioration in aging resistance caused by coarse grains as well as an increase of the manufacturing costs due to addition of large amounts of Ti.

However, these methods are designed only to enhance either the bake hardenability or the formability through addition of alloy elements, and do not simultaneously enhance the bake hardenability, the aging resistance, and the secondary work embrittlement resistance.

In the methods of the above disclosures, although boron, known as an element capable of enhancing the secondary work embrittlement resistance, is added in an amount of about 5 ppm to the steel, it is limited in its ability to enhance the secondary work embrittlement resistance due to the presence of excessive P content.

Furthermore, if B is excessively added to the steel to enhance the secondary work embrittlement resistance, deterioration will occur due to an excessive amount of B, and thus, the amount of B which can be added to the steel is limited.

Although electroplated steel sheet has good plating characteristics and excellent corrosion resistance, the electroplated steel sheet is very expensive in comparison to hot dipped steel sheets, and thus the hot dipped steel sheets are mainly used for automobiles.

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