To provide dies, where a high strength steel sheet is pressed by a die quenching process and a molding having excellent fatigue strength can be obtained.
The hot forming die is a pair of upper and lower hot forming dies operating to a vertical direction. The face in contact with a work in at least either the upper die or the lower die is provided with a plurality of almost spherical dents forming air layers in a space with the work, and the diameter r [mm] of each dent and the center distance d [mm] satisfy the inequalities of 2r+0.2≥d≥2r-0.6, 1.1≥r≥0.6 and d≥1.2. In the hot forming method, a steel sheet comprising, by mass, 0.1 to 0.4% C, ≤0.5% Si and 0.2 to 4% Mn, and the balance Fe with inevitable impurities is heated at 800 to 1,100°C, and is pressed by the hot forming die.
COPYRIGHT: (C)2007,JPO&INPIT
Manabu Takahashi
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Mizuno Katsufumi
Hiroshin Takano