Development of Warm and Hot Press Forming of Ultra High Strength Steel Sheets Using Resistance Heating

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A warm and hot press forming process of ultra high tensile strength steel sheets using resistance heating was developed to improve springback and formability. In this process, the decrease in temperature of the sheet before the forming is prevented by directly heating the sheets set into the dies by means of the electrical resistance. Since the heating time up to 800 ℃ is only 2 seconds, the resistance heating is rapid enough to synchronize with a press. The reduction of springback in warm bending and the quenching in hot press forming were examined. The oxidation in hot stamping was prevented by coating sheets with an oxidation preventive oil. In addition, a warm and hot shearing process using the resistance heating was developed to reduce the sharing load for the ultra high tensile strength steel sheets.