Hot press-formed product, process for producing same, and thin steel sheet for hot press forming

Abstract

There is provided a hot press-formed product, including a thin steel sheet formed by a hot press-forming method, and having a metallic structure that contains martensite at 80% to 97% by area and retained austenite at 3% to 20% by area, the remainder structure of which is at 5% by area or lower, whereby balance between strength and elongation can be controlled in a proper range and high ductility can be achieved.

Description

TECHNICAL FIELD[0001]The present invention relates to a hot press-formed product required to have high strength, such as used for structural members of automobile parts, a process for producing the same, and a thin steel sheet for hot press forming. In particular, the present invention relates to a hot press-formed product that can be provided with a prescribed shape and at the same time heat treated to have prescribed strength when a preheated steel sheet (blank) is formed into the prescribed shape, a process for producing such a hot press-formed product, and a thin steel sheet for hot press forming.BACKGROUND ART[0002]As one of the measures for fuel economy improvement of automobiles beginning from global environmental problems, automobile body lightening has proceeded, and steel sheets to be used for automobiles need to be strengthened as highly as possible. However, highly strengthening of steel sheets for automobile lightening lowers elongation EL or r value (Lankford value), r...

AI Technical Summary

Benefits of technology

The invention allows for the control of the metallic structure of a hot press-formed product by retaining an appropriate amount of austenite. This results in a more ductile product compared to conventionally used 22MnB5 steel. Additionally, strength and elongation can be controlled through a combination of heat treatment conditions and pre-forming steel sheet structure.

Problems solved by technology

However, highly strengthening of steel sheets for automobile lightening lowers elongation EL or r value (Lankford value), resulting in the deterioration of press formability or shape fixability.

According to such a hot pressing method, a steel sheet is formed in a state of low strength, and therefore, the steel sheet has decreased springback (favorable shape fixability).

However, there is almost no application of steel grades other than 22MnB5 steel.

The techniques disclosed in Non-patent Documents 1 and 2 provide a tensile strength of not higher than 600 MPa and an elongation of about 12% to 18% at an energy-absorbing portion side, in which techniques, however, laser welding (to prepare a tailor welded blank, abbreviated as TWB) is needed previously, thereby increasing the number of steps and resulting in high cost.

In addition, it results in the heating of energy-absorbing portions, which need not to be hardened originally.

Therefore, these techniques are not preferred from the viewpoint of energy consumption.

The technique disclosed in Non-patent Document 3 is based on 22MnB5 steel, in which boron addition, however, adversely affects the robustness of strength after quenching against heating to a temperature within two-phase region, making difficult the control of strength at an energy-absorbing portion side, and further making it possible to obtain only an elongation as low as 15%.

The technique disclosed in Non-patent Document 4 is based on 22MnB5 steel, and therefore, this technique is not economic in that control is made in such a manner that 22MnB5, which originally has excellent hardenability, is not hardened (control of press tool cooling).

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