High-ductility steel sheet excellent in press formability and strain age hardenability, and method for manufacturing the same

Abstract

A steel sheet composition contains appropriate amounts of C, Si, Mn, P, S, Al and N and 0.5 to 3.0% Cu. A composite structure of the steel sheet has a ferrite phase or a ferrite phase and a tempered martensite phase as a primary phase, and a secondary phase containing retained austenite in a volume ratio of not less than 1%. In place of the Cu, at least one of Mo, Cr, and W may be contained in a total amount of not more than 2.0%. This composition is useful in production of a high-ductility hot-rolled steel sheet, a high-ductility cold-rolled steel sheet and a high-ductility hot-dip galvanized steel sheet having excellent press formability and excellent stain age hardenability as represented by a ΔTS of not less than 80 MPa, in which the tensile strength increases remarkably through a heat treatment at a relatively low temperature after press forming.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates mainly to steel sheets for automobiles, and more particularly, to high-ductility steel sheets having very high strain age hardenability and excellent press formability such as ductility, stretch-flanging formability, and drawability, in which the tensile strength increases remarkably through a heat treatment after press forming, and to methods for manufacturing the same. The term “steel sheets” as herein used shall include hot-rolled steel sheets, cold-rolled steel sheets, and hot-dip galvanized steel sheets. The term “steel sheets” as herein used shall also include steel sheets and steel strips. 2. Description of the Related Art In recent years, weight reduction in automobile bodies has become a very important issue in relation to emission gas control for the purpose of preserving global environments. More recently, efforts are made to achieve higher strength of automotive steel sheets and to re...

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Benefits of technology

The present invention was made in view of the fact that, in spite of the strong demand as described above, a technique for industrially stably manufacturing a steel sheet satisfying these properties has never been found. The present invention solves the problems described above. It is an object of the present invention to provide is directed to high-ductility and high-strength steel sheets suitable for automobiles and having excellent press formability and excellent strain age hardenability, in which the tensile strength increases considerably through a heat treatment at a relatively low temperature after press forming. It is also an object of the present invention to provide a manufacturing method capable of stably manufacturing the high-ductility and high-strength steel sheets.

A detailed mechanism of improvement in hole expanding formability with content of Cu has not yet been clarified, as in the hot-rolled steel sheet and the cold-rolled steel sheet, but it is considered that the contained Cu reduces the difference in hardness among the ferrite, the tempered martensite / retained austenite, and the martensite formed by strain induced transformation.

Problems solved by technology

In recent years, weight reduction in automobile bodies has become a very important issue in relation to emission gas control for the purpose of preserving global environments.

In general, however, a higher strength of steel sheet tends to result in a lower ductility and a lower hole-expanding ratio, thus leading to poor press formability.

In a bake treatment performed after the press forming of this steel sheet, the solute C is fixed to a dislocation introduced during the press forming and inhibits the movement of the dislocation, thus resulting in an increase in yield stress.

In this bake hardenable type automotive steel sheet, the yield stress can be increased, but the tensile strength cannot be increased.

Even in this steel sheet, however, it is yet difficult to increase tensile strength after the bake treatment, and an improvement in crashworthiness cannot still be achieved.

In the steel sheet manufactured by the technique disclosed in Japanese Examined Patent Application Publication No. 8-23048, the tensile strength and the yield stress increase by strain age hardening; however, a serious problem is posed in that coiling of the steel sheet at a very low coiling temperature as less than 150° C. results in large variations in mechanical properties.

Another problem includes a large variation in increment of yield stress after press forming and bake treatments, as well as poor press formability due to a low hole-expanding ratio (λ) and decreased stretch-flanging workability.

For this purpose, it is essential to complete hot finish rolling at a temperature above the Ar3 transformation point, air-cool the sheet within the temperature region of Ar3 to Ar1 for 1 to 10 seconds, cool the sheet at a cooling rate of not less than 20° C. / second, and coil the cooled sheet at a temperature of not more than 350° C. A low coiling temperature of not more than 350° C. causes serious deformation of the shape of the hot-rolled steel sheet, thus inhibiting industrially stable manufacture.

In the steel sheet manufactured by this method, however, the heat treatment temperature must be high as not less than 500° C., in order to obtain remarkable hardening from the heat treatment after the forming, and this has a problem in practice.

The tensile strength of the steel sheet is increased by heating the sheet at a temperature within the range of 200 to 450° C. However, the resultant steel sheet involves a problem in that the microstructure comprises a ferrite single phase, a ferrite and pearlite composite structure, or a ferrite and bainite composite structure; hence, high ductility and low yield strength are unavailable, resulting in low press formability.

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