Production method of warm- or hot-formed product

Abstract

Disclosed is a method wherein when a steel sheet is subjected to warm or hot forming to produce a formed product by drawing with a punch and a die, the steel sheet is formed while the forming start temperature is controlled in accordance with the heating temperature of the steel sheet. By this method, when a steel sheet is subjected to warm or hot forming, good formability is obtained without the occurrence of fracture, cracking or the like during forming and also a formed product having good ductility is obtained.

1 . A production method of a warm- or hot-formed product, said production method comprising the steps of: 
 heating a steel sheet to a temperature not lower than the Ac1 transformation temperature;    cooling said steel sheet to a temperature in the range from higher than the martensite transformation start temperature Ms point of said steel sheet to lower than the temperature determined in accordance with said heating temperature; and    forming the cooled steel sheet with a punch and a die.    
   
   
       2 . The production method according to  claim 1 , wherein: said steel sheet is heated to a temperature in the range from not lower than the Ac1 transformation temperature to lower than the Ac3 transformation temperature in said heating step; and said temperature determined in accordance with said heating temperature satisfies the following expression (1):  
       Forming start temperature (° C.)≦0.725×Heating temperature of a steel sheet (° C.)   (1).  
   
   
       3 . The production method according to  claim 1 , wherein: said steel sheet is heated to a temperature not lower than the Ac3 transformation temperature in said heating step; and said temperature determined in accordance with said heating temperature is 600° C.  
   
   
       4 . The production method according to  claim 1 , wherein said step of forming the cooled steel sheet with the punch and the die is completed during the time when the temperature of said steel sheet is higher than said temperature Ms point.  
   
   
       5 . The production method according to  claim 1 , wherein a blank holder is used when said steel sheet is formed in said step of forming the cooled steel sheet with the punch and the die.