48 巻 (2007) 555 号 p. 318-322
Oxidation in hot stamping of ultra high strength steel sheets was prevented by coating the sheets with an oxidation preventive oil. Solid films having oxidation barriers are generated on the surfaces of steel sheets by drying the coated sheets. For four types of oxidation preventive oil, the amount of oxide scale under natural cooling of each heated sheet without forming was first measured. The oil that forms a liquefied film at elevated temperatures exhibited high oxidation prevention during heating, and this oil was chosen for a hot bending experiment. Hot hat-shaped bending of the coated sheets was carried out to examine the properties of the products. For the bent products, the shape accuracy was very high, the surface roughness was similar to that of the sheet, and the hardness was 1.5 times larger than that of the sheet before the bending. In addition, the load required for hot bending decreases markedly. The film remaining on the surface of the formed product could be removed using phosphoric acid. It was found that the hot stamping operation using the oxidation preventive oil is effective in the precision forming of ultra high strength steel sheets.