Method of production of steel sheet semi-finished products by press hardening with locally-modified structure in spots for welding

Abstract

A method for producing steel sheet semi-finished products (P) by press hardening is provided that utilizes locally-modified structure in spots for welding. During this method, a steel semi-finished product (P) is heated to a temperature at which complete transformation of ferritic-pearlitic structure to austenite occurs. Afterwards, the semi-finished product (P) is cooled locally in spots for welding to a temperature close to the Mf, after which heat conduction from the surroundings causes reheating and consequently annealing of the hardening structure. Subsequently, the semi-finished product (P) is formed into a drawn part which, at the same time, is hardened in the tool, after which the sheet semi-finished product (P) is retrieved from the tool, once the required temperature was reached.

1 . A method for producing steel sheet semi-finished products by press hardening with locally-modified structure in spots for welding comprising:
 (a) heating a steel semi-finished product to a first temperature at which complete transformation of ferritic-pearlitic structure to austenite occurs to thereby form a semi-finished product;   (b) locally cooling the semi-finished product in spots for welding to a second temperature close to the martensite finish temperature, wherein generated heat causes temperature in the spots to rise to a third temperature at which a hardening structure becomes tempered; and   (c) transferring the semi-finished product to a tool, in which a sheet blank is formed into a drawn part which, at the same time, is hardened in the tool, after which a sheet semi-finished product is retrieved from the tool once a required temperature is reached.   
     
     
         2 . The method according to  claim 1 , wherein the semi-finished product is heated to a temperature within the range of 800° C. to 980° C., wherein the energy supplied for heating and austenitizing is used in part for tempering hardened spots in locations of future welds. 
     
     
         3 . The method according to  claim 1 , wherein the semi-finished product is locally and intensively cooled by a cooling medium in the spots for welding for 1 to 10 seconds.