In recent years, there has been a demand for decreases in the thickness and increases in the strength of structural parts for automobiles out of consideration of global environmental problems and collision safety.
In order to meet this demand, structural parts for automobiles are increasingly using high-strength steel sheet as a base material. However, when structural parts for automobiles are manufactured by press forming of a high-strength steel sheet used as a base material, forming defects in the shape of wrinkles and spring back easily develop. Therefore, it is not easy to manufacture structural parts for automobiles by press forming of high-strength steel sheets.
So-called hot press working is known as a method of solving such problems. hot press working is a method of manufacturing high-strength formed articles by press forming a steel sheet which has been heated to a high-temperature range over 700° C. and then carrying out quench hardening either inside or outside the press dies.
In hot press working, because forming is carried out in a high-temperature region in which the strength of a steel sheet is decreased, the above-described forming defects can be suppressed. Furthermore, it is possible to proved the formed article with a high strength by carrying out quench hardening after forming. Accordingly, hot press working can manufacture formed articles such as structural parts for automobiles having a high strength such as 1500 MPa or above, for example.
Concerning hot press working, Patent Document 1, for example, discloses a steel sheet for hot press forming which is purported to make it possible to carry out successful forming without the occurrence of fractures or cracks at the time of forming by hot press working.
Recently, new techniques are being proposed which make it possible to manufacture high-strength formed articles by methods other than hot press working.
For example, Patent Document 2 discloses a technique for push-through bending of a metal material. In this technique, while the a heating apparatus and a cooling apparatus undergo relative movement with respect to a metal material, the metal material is locally heated by the heating apparatus, and a bending moment is imparted to a location where the resistance to deformation has been greatly decreased by heating so as to perform bending to a desired shape which is bent two-dimensionally or three-dimensionally. Quench hardening is then performed by cooling with the cooling apparatus. (In this description, this technique will be referred to as hot three-dimensional bending and direct quench).
The hot three-dimensional bending and direct quench technique can efficiently manufacture a high-strength formed article with a high bending accuracy. Accordingly, the hot three-dimensional bending and direct quench technique can manufacture formed articles such as structural parts for automobiles having a high strength of the 900 MPa grade or above, for example.