High-strength cold rolled steel sheet having excellent deep drawability and bake hardenability and method for manufacturing the same

Abstract

A high-strength cold rolled steel sheet having excellent deep drawability and bake hardenability, with a tensile strength ≧440 MPa, an average r-value ≧1.20 and a bake hardening value ≧40 MPa, is obtained by subjecting a steel raw material having a chemical composition including C: 0.010-0.06 mass %, Si: more than 0C:\Users\ejensen\Desktop\!TEMP\.5 mass % but not more than 1.5 mass %, Mn: 1.0-3.0 mass %, Nb: 0.010-0.090 mass %, Ti: 0.015-0.15 mass % and satisfying

(Nb/93)/(C/12)<0.20 to hot rolling, cold rolling and then to annealing including steps of heating to a temperature of 800-900° C. while a temperature region of 700-800° C. is an average heating rate <3° C./s, and soaking and thereafter cooling at a rate ≧5° C./s from the soaking temperature to a cooling stop temperature ≦500° C. to thereby form a microstructure including ferrite phase with an area ratio ≧70% and martensite phase with an area ratio ≧3%.

Description

TECHNICAL FIELD[0001]The present invention relates to a high-strength cold rolled steel sheet suitable for use in an outer panel and the like of an automobile body and having a tensile strength TS of not less than 440 MPa, an average r-value of not less than 1.20 and a Bake hardening (BH) value of not less than 40 MPa and excellent deep drawability and bake hardenability as well as a method for manufacturing the same.BACKGROUND ART[0002]Recently, it is strongly demanded to reduce the weight. of the automobile body for improving a fuel consumption of an automobile and reducing CO2 discharge amount from a viewpoint of the protection of global environment. At the same time, it is strongly demanded to increase the strength of the automobile body in view of ensuring safeness of crew member(s) at the time of impact. In order to meet these demands, it is required to simultaneously satisfy the weight reduction and the increase in strength of the automobile body. To this end, it is effective...

AI Technical Summary

Benefits of technology

[0042]According to the present invention, it is possible to manufacture a high-strength cold rolled steel sheet having excellent deep drawability and bake hardenability with a tensile strength TS of not less than 440 MPa, an average r-value of not less than 1.20 and a BH value of not less than 40 MPa by limiting C content to 0.010-0.06 mass % and restricting a relation of (Nb/93)/(C/12) between Nb addition amount and C content to less than 0.20 so as to render the reduction of solute C badly exerting on the deep drawability as attained in the conventional extremely-low carbon IF steel to a certain level and further controlling solute C (C*) amount not fixed by Nb and Ti to a given range.

[0043]According to the present invention, it is further possible to cheaply and stably manufacture a high-strength cold rolled steel sheet having excellent deep drawability and bake hardenability with an average r-value of not less than 1.20 and a BH value of not less than 40 MPa by reducing expensive Nb as far as possible and positively utilizing Ti even in high-strength steel sheets having not only a tensile strength of not less than 440 MPa but also not less than 500 MPa, particularly not less than 590 MPa.

[0044]Therefore, when the high-strength cold rolled steel sheet of the present invention is applied to automobile parts, it is possible to increase the strength of the part, which has been difficult to conduct press forming, so that the invention largely contributes to improve collision safety of the automobile body and reduce the weight thereof.

[0045]A basic technic

Problems solved by technology

As previously mentioned, the conventional techniques utilizing solid solution enhancement for increasing the strength of mild steel sheets having an excellent deep drawability are necessary to add a greater amount or excessive amount of alloying elements and have problems in not only r-value and BH property but also raw mater

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