High tensile hot-rolled steel sheet having excellent strain aging hardening properties and method for producing the same

Abstract

The present invention provides a high tensile strength hot-rolled steel sheet having superior strain aging hardenability, which has high formability and stable quality characteristics, and in which satisfactory strength is obtained when the steel sheet is formed into automotive components, thus enabling the reduction in weight of automobile bodies. Specifically, a method for producing a high tensile strength hot-rolled steel sheet having superior strain aging hardenability with a BH of 80 MPa or more, a DELTATS of 40 MPa or more, and a tensile strength of 440 MPa or more includes the steps of heating a steel slab to 1,000° C. or more, the steel slab containing, in percent by mass, 0.15% or less of C, 2.0% or less of Si, 3.0% or less of Mn, 0.08% or less of P, 0.02% or less of S, 0.02% or less of Al, 0.0050% to 0.0250% of N, and optionally 0.1% or less in total of at least one of more than 0.02% to 0.1% of Nb and more than 0.02% to 0.1% of V, the ratio N (mass %) / Al (mass %) being 0.3 or more; rough-rolling the steel slab to form a sheet bar; finish-rolling the sheet bar at a finishing temperature of 800° C. or more; cooling at a cooling rate of 20° C. to 40° C. / s or more within 0.5 second after the finish-rolling; and coiling at a temperature of 650° C. to 450° C. or less.

Description

[0001] The present invention relates to high tensile strength hot-rolled steel sheets having superior strain aging hardenability. More particularly, the invention relates to a high tensile strength hot-rolled steel sheet having a TS (tensile strength) of 440 MPa or more, and relates to a method for producing the same. The high tensile strength hot-rolled steel sheet is mainly used for automobiles as a thin hot-rolled steel sheet having high workability. Furthermore, the high tensile strength hot-rolled steel sheet is-used as a replacement for a thin cold-rolled steel sheet having a thickness of approximately 4.0 mm or less and which was employed because it was difficult to produce a steel sheet with such a small thickness by hot rolling. The applications of the steel sheet in accordance with the present invention extend over a wide range from use for relatively light working, such as slight bending and forming of pipes by roll forming, to relatively heavy working, such as drawing by...

AI Technical Summary

Benefits of technology

[0015] That is, by using N as the strengthening element, by adjusting the content of Al which is a key element in an appropriate range, and by properly setting the hot rolling conditions so that the-microstructure and the dissolved N are optimized, it is possible to obtain a steel sheet (steel sheet of the present invention) having significantly superior formability and strain aging hardenability compared to a conventional solid-solution strengthening type C-Mn steel sheet and a precipitation strengthening steel sheet (conventional steel sheets).

[0136] Skin pass rolling and leveling are usually performed to adjust roughness and to correct shape. In the present invention, in addition thereto, skin pass rolling and leveling are effective in increasing and stabilizing the BH and .DELTA.TS. Such an effect is remarkably caused at an elongation of 1.5% or more. However, if the elongation exceeds 10%, ductility is decreased. Therefore, working after hot-rolling is preferably performed with an elongation of 1.5% to 10%. Additionally, although the working mode is different between skin pass rolling and leveling (the former is rolling and the latter is repeated bending and stretching), the effects of the elongation on the strain aging hardenability of the steel sheet of the present invention in both workings are substantially the same. In the present invention, acid pickling may be performed before or after the working after hot-rolling.

Problems solved by technology

Since large variations in strength occurred when the conventional steel sheets were subjected to plastic deformation under the actual press conditions, the conventional steel sheets could not be applied to components in which high reliability was required even if the conventional steel sheets were evaluated as having desired bake hardenability in the tensile test.

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