| 摘要 |
PROBLEM TO BE SOLVED: To provide a warm forming method suitable for deep drawing and a high tensile strength steel sheet. SOLUTION: A relation between a steel sheet temperature and tensile strength is obtained beforehand, a temperature at a punch part of a steel sheet is a reference temperature, a temperature to have 90% of the tensile strength at the reference temperature is obtained as a lower limit temperature of a blank holder part, press working is conducted while controlling a temperature of a die and/or a blank holder so that a steel sheet temperature at the blank holder is the lower limit temperature or more of the blank holder part. A steel sheet, in which the tensile strength at >=150 deg.C is <=90% of a normal temperature, is used, a steel sheet temperature of the punch part is a normal temperature or less, a temperature is controlled so as to have the blank holder part of >=150 deg.C, the high tensile strength steel, which contains, by weight, 0.05-0.20% C, 0.1-1.0% Si, 0.70-2.5% Mn, further, has martensite of 5-40 vol.%, an average grain diameter of <=6μm and the balance of a ferrite phase, the tensile strength at >=150 deg.C is 90% of the tensile strength at 25 deg.C, is suitable for warm forming.
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