Production method of warm- or hot-formed product

Abstract

Disclosed is a method wherein when a steel sheet is subjected to warm or hot forming to produce a formed product by drawing with a punch and a die, the steel sheet is formed while the forming start temperature is controlled in accordance with the heating temperature of the steel sheet. By this method, when a steel sheet is subjected to warm or hot forming, good formability is obtained without the occurrence of fracture, cracking or the like during forming and also a formed product having good ductility is obtained.

Description

BACKGROUND OF THE INVENTION [0001] The present invention relates to, in the field of producing a steel sheet formed product that is mainly applied to an automobile body: a method for producing a formed product by heating a steel sheet (blank) as the raw material to a temperature of not lower than the austenite and ferrite formation temperature (Ac1 transformation temperature) and subjecting the heated steel sheet to press forming; and a formed product obtained by the production method. In particular, the present invention relates to: a method for producing a formed product that assures good formability without causing fracture, cracking, etc. during press forming; and a formed product thereof. [0002] In the field of automobile parts, the strengthening of the materials for the parts is promoted with the aim of securing both collision safety performance and weight reduction at the same time. Those parts are generally produced by press-forming a steel sheet. However, when cold forming ...

AI Technical Summary

Benefits of technology

[0008] The present invention has been established in view of the above situation and the object thereof is to provide: a method for producing a formed product by hot stamping, the formed product having a wider application range by securing good formability and good ductility without the occurrence of fracture and cracking during forming when a steel sheet is subjected to hot or cold forming; and the formed product that can exhibit such advantages.

[0014] By the aspect of the present invention, since the forming start temperature is made to be controllable in accordance with the heating temperature of a steel sheet when the steel sheet is hot formed or warm formed, good formability can be secured without the occurrence of fracture and cracking during forming, thus a formed product that shows good ductility can be produced, and thereby the application range of the steel sheet is expected to expand.

Problems solved by technology

However, when cold forming is applied to a highly strengthened steel sheet, the material is hardly formed particularly if it exceeds 980 MPa.

However, since the timing when a heated blank touches a tool set varies by the sites of the heated blank, temperature difference appears in the blank and thus unevenness of the material strength caused by the temperature difference is likely to appear in the blank.

Since the flow stress of the material increases with such temperature drop, the material tends to fracture during forming.

Therefore, the problem has been that, even when a blank is heated and softened intentionally, deep drawing can not be applied because of the above reasons.

As a result, a part having an ultra-high strength of 1,470 MPa or more can be obtained but, since the microstructure of the part is composed of martensite, the ductility of the part is inferior.

This means that the part may have the possibility of fracture in some circumstances, for example, when an automobile collision occurs and the part is deformed.

When the part fractures, the part cannot absorb collision force at the moment and resultantly the damage to a passenger may possibly increase.

For those reasons, it cannot be said that a part formed by hot stamping always has a wider range of application and it is the present situation that the advantages of both high strength and good dimensional accuracy cannot be utilized enough.

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