High-strength cold-rolled steel sheet excellent in coating film adhesion

Abstract

A cold-rolled steel sheet of DP (Dual Phase) type with a specific composition meets the requirements: [0001](I) In the surface of the steel sheet, there exist Si—Mn complex oxides no larger than 5 μm in diameter of the equivalent circle as many as 10 or more per 100 μm2 and the coverage of oxides composed mainly of Si on the surface of steel sheet is no more than 10% of surface area, and / or (II) The cross section near the surface of the steel sheet does not show cracks with a width no larger than 3 μm and a depth no smaller than 5 μm in arbitrary ten fields of observation under an SEM with a magnification of 2000. A high-strength cold-rolled steel sheet excellent in coating film adhesion and having a tensile strength no lower than 550 MPa is provided.

Description

BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates to a high-strength cold-rolled steel sheet excellent in coating film adhesion, and more particularly, to a cold-rolled steel sheet which has a tensile strength no lower than 550 MPa and is suitable for use as a steel sheet for automobile parts on account of its excellent coating film adhesion. [0004] 2. Description of the Related Art [0005] There is a growing demand for high-strength steel products necessary for automobiles with less fuel consumption and lower weight than before. This trend is prevailing also in the field of cold-rolled steel sheet. On the other hand, cold-rolled steel sheets are required to have sufficient ductility (such as elongation) because they are formed into automotive parts by pressing. Increase in strength can be effectively achieved by incorporation with alloying elements; however, they adversely affect ductility as their amount increases. [0006] Of these all...

AI Technical Summary

Benefits of technology

[0016] The present invention was completed in view of the foregoing. It is an object of the present invention to provide a cold-rolled steel sheet characterized by a tensile strength no lower than 500 MPa and excellent coating film adhesion and weldability.

[0024] The steel sheet according to the present invention has a high strength in excess of 550 MPa, exhibits good chemical treatability, and / or good coating film adhesion owing to controlled fine cracks, and provides good weldability. It is suitable for automotive parts. It can be produced without cladding or expensive elements.

Problems solved by technology

Increase in strength can be effectively achieved by incorporation with alloying elements; however, they adversely affect ductility as their amount increases.

However, with an increased Si content, the resulting steel sheet is poor in chemical treatability and hence in coating film adhesion.

Moreover, excess Si forms an Si-containing intergranular oxide on the surface of steel sheet, thereby causing cracks to occur and aggravating coating film adhesion.

Unfortunately, cladding needs a complex process which leads to an increased production cost.

(See Japanese Patent No. 2951480 and Japanese Patent No. 3266328) This technology suffers a disadvantage of requiring expensive Ni or Cu, which leads to an increase in production cost.

The steel of this technology has a slightly higher carbon content than the above-mentioned IF steel, but is still unsatisfactory in strength.

Unfortunately, these elements are detrimental to weldability.

Unfortunately, pickling and brushing increase the number of manufacturing steps, which leads to a higher production cost.

In addition, the control of dew point, which is accomplished in a continuous annealing furnace, is not very effective so long as Examples show in the document.

Judging from these results, the disclosed technology will not sufficiently improve chemical treatability.

Therefore, it is poor in weldability.

However, a certain amount of Si is necessary for steel to have both high strength and good ductility, and hence there is a limit of reducing the Si content to keep the Si / Mn ratio below 1.

Further, it turned out that a steel sheet does not always exhibit good chemical treatability even though it has an Si / Mn ratio lower than 1, for a certain Si content and an adequately controlled Mn content.

Images