WARN FORMING METHOD AND HIGH TENSILE STRENGTH STEEL SHEET EXCELLENT IN WARM FORMABILITY

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°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°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 ≤6 μm and the balance of a ferrite phase, the tensile strength at ≥150°C is 90% of the tensile strength at 25°C, is suitable for warm forming.;COPYRIGHT: (C)2001,JPO