Steel plate for hot forming and manufacturing method of hot press formed steel member

Abstract

Provided is a method for manufacturing a steel member through hot press forming using a steel sheet that contains a comparatively large amount of Si. The method allows hot press forming to be carried out satisfactorily by stably and reliably suppressing shedding/peeling of oxide scale during hot press forming, even without restricting hot press forming conditions to a narrow range. The steel sheet includes, in mass ratio, C: 0.15% to 0.35%, Si: 1.0% to 3.0%, Mn: 1.0% to 3.0%, Al: more than 0% up to 0.10%, Ti: ([N]×48/14) % to 0.10% (where [N] denotes the amount of N in the steel sheet), B: 5 ppm to 50 ppm, P: more than 0% to less than 0.015%, S: more than 0% up to 0.010%, and N: more than 0% up to 0.010%, the balance being iron and unavoidable impurities. The average oxygen concentration from an outermost surface of the steel sheet down to a depth of 10 μm in a sheet thickness direction is 0.70 mass % or higher.

Description

TECHNICAL FIELD[0001]The present invention relates to a steel sheet for hot forming, and to a method for manufacturing a hot press formed steel member using the steel sheet for hot forming.BACKGROUND ART[0002]For instance, materials that make up steel parts for automobiles are designed to be ever stronger, in order to achieve steel sheets that exhibit both crashworthness and light weight. To manufacture such steel parts, the steel sheet that is used is required to have high formability. However, when a steel sheet having been imparted with enhanced strength is subjected to cold working, for instance cold press forming, the press forming force increases and dimensional accuracy degrades significantly, which is problematic.[0003]Methods for solving the above problem include hot press forming in which a steel sheet, as a base material, is press-formed in a heated state, and forming and imparting of high strength are realized simultaneously, to achieve a steel member. This method involv...

AI Technical Summary

Benefits of technology

[0036]The present invention allows reliably suppressing shedding / peeling of scale during the above hot press forming, and allows for good hot press forming, without restricting hot press forming conditions to a narrow range, in hot press forming where there is used a steel sheet of comparatively high Si content. As a result there is obtained a steel member that is free of press-scratches or the like that boasts both beautiful appearance and high strength. Moreover, die contamination is suppressed, which allows curtailing significantly the number of die repairs.

Problems solved by technology

However, when a steel sheet having been imparted with enhanced strength is subjected to cold working, for instance cold press forming, the press forming force increases and dimensional accuracy degrades significantly, which is problematic.

In the above method, however, a problem arises in that scale that is formed on the steel sheet surface sheds off during hot press forming and adheres to dies, which become thus contaminated; further, the shed scale is pressed against the steel sheet surface, and scratches occur in the surface of the steel member.

However, Patent Literature 1 to Patent Literature 4 above are problematic as regards the hardness stability of the manufactured member, given the low content of Si in the steel sheet.

When Mn content is low, however, variability in the hardness of the member after hot pressing is large, which is problematic.

In particular, a problem arises in that shedding / peeling of scale cannot be prevented reliably, by resorting to the above technologies alone, at certain sites where the hot press forming condition need to be controlled rigorously.

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