High-strength cold-rolled steel sheet, high-strength plated steel sheet, and methods for their manufacture

Abstract

A high-strength cold-rolled steel sheet providing a product with a good surface condition after press forming, having excellent bake hardenability and anti room temperature aging property, and having a dual phase structure with a tensile strength of at least 340 MPa is provided.A high-strength cold-rolled steel sheet has a structure comprising a main phase which is a ferrite and a secondary phase which is a low temperature transformation product including a martensite and has a hardness distribution of the ferrite phase in an arbitrary cross section having a length of 10 mm in the widthwise direction of the sheet which satisfies the relationship prescribed by (Hv(max)−Hv(ave))<0.5×(Hv(ave). Hv(max) is the maximum Vickers hardness of ferrite grains in a region at a distance of from (⅛)t to (¼)t in the thickness direction from the surface when the thickness of the high-strength cold-rolled steel sheet is t, and Hv(ave) is the average Vickers hardness of ferrite grains in this region.

Description

TECHNICAL FIELD[0001]This invention relates to a high-strength cold-rolled steel sheet and a high-strength plated steel sheet which can be formed into various shapes by press forming or the like and methods for their manufacture. Specifically, the present invention relates to a high-strength cold-rolled steel sheet and a high-strength plated steel sheet capable of providing a product having a good surface condition after press forming, a good bake hardenability, and a good anti room temperature aging property.BACKGROUND ART[0002]As technical fields in industry become highly specialized, special high-level performance is now required of the materials used in each technical field. For example, cold-rolled steel sheet which is used after being formed into various shapes by press forming or the like is often required to have a high strength. Therefore, the use of high-strength cold-rolled steel sheet is now being envisaged. In particular, increasing the fuel efficiency of automobiles by...

AI Technical Summary

Benefits of technology

[0023]The inventions disclosed in Patent Documents 6 and 7 decrease variations in hardness inside a steel sheet caused by segregation of P by suppressing the segregation of P and adding suitable amounts of Si and Mn, whereby the occurrence of surface defects is prevented. However, due to the addition of P, Mn, and Si in those inventions, the yield stress increases, so it is not possible to avoid a deterioration in shape fixability and resistance to surface distortion. Giving a steel sheet a dual phase structure is effective at decreasing the yield stress. However, according to the results of investigations by the present inventors, although the inventions disclosed in Patent Documents 6 and 7 can suppress surface defects in a steel sheet having a single phase structure of ferrite, the occurrence of surface defects in a dual phase steel sheet cannot be suppressed.

[0024]In the inventions disclosed in Patent Documents 8 and 9, the yield stress at the time of cooling subsequent to annealing is predicted based on the yield stress and tensile strength at room temperature, and by controlling the cooling rate, surface defects are prevented. However, according to the results of investigation by the present inventors, these inventions cannot suppress the occurrence of surface defects in a dual phase steel sheet.

[0025]The present invention was made in light of the above-described problems of the prior art, and its object is to provide a high-strength cold-rolled steel sheet and a high-strength plated steel sheet which can be used after forming into various shapes by press forming or the like, which can form a product having a good surface condition after press forming, and which has both excellent bake hardenability and anti room temperature aging property as well as to provide methods for their manufacture.

Problems solved by technology

However, due to the addition of P, Mn, and Si in those inventions, the yield stress increases, so it is not possible to avoid a deterioration in shape fixability and resistance to surface distortion.

However, according to the results of investigations by the present inventors, although the inventions disclosed in Patent Documents 6 and 7 can suppress surface defects in a steel sheet having a single phase structure of ferrite, the occurrence of surface defects in a dual phase steel sheet cannot be suppressed.

However, according to the results of investigation by the present inventors, these inventions cannot suppress the occurrence of surface defects in a dual phase steel sheet.

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