Process for manufacturing a steel tube for air bags

Abstract

A seamless steel tube for air bag use is formed from a steel comprising, in mass percent, C: 0.04-0.20%, Si: 0.10-0.50%, Mn: 0.10-1.00%, P: at most 0.025%, S: at most 0.005%, Al: at most 0.10%, Cr: 0.01-0.50%, Cu: 0.01-0.50%, Ni: 0.01-0.50%, a remainder of Fe and unavoidable impurities. The tube is cold drawn at least one time with a working ratio so that reduction in area is greater than 40% to obtain predetermined dimensions. The tube is then quench hardened by heating to at least the Ac 3 point at a temperature rate increase of at least 50° C. per second followed by cooling at a cooling rate of at least 50° C. per second at least in a temperature range of 850-500° C. Tempering is done at a temperature of at most the Ac 1 point.

1 . A process for manufacturing a steel tube for air bags characterized by including:
 a tube forming step in which a seamless steel tube is produced by hot tube forming from a steel comprising, in mass %, C: 0.04-0.20%, Si: 0.10-0.50%, Mn: 0.10-1.00%, P: at most 0.025%, S: at most 0.005%, Al: at most 0.10%, Cr: 0.01-0.50%, Cu: 0.01-0.50%, Ni: 0.01-0.50%, and a remainder of Fe and unavoidable impurities,   a cold drawing step in which the resulting seamless steel tube is subjected to cold drawing at least one time with a reduction in area of at least 40% in one time of cold drawing to obtain a steel tube having predetermined dimensions, and   a heat treatment step in which the cold drawn steel tube is subjected to quench hardening by heating it to a temperature of at least the Ac 3  point at a rate of temperature increase of at least 50° C. per second followed by cooling at a cooling rate of at least 50° C. per second at least in a temperature range of 850-500° C. and then to tempering at a temperature of at most the Ac 1  point.   
     
     
         2 . A process for manufacturing a steel tube for air bags as set forth in  claim 1  wherein the steel further contains less than 0.10% of Mo. 
     
     
         3 . A process for manufacturing a steel tube for air bags as set forth in  claim 1  wherein the steel contains at least one of Nb: at most 0.050%, Ti: at most 0.050%, and V: at most 0.20%. 
     
     
         4 . A process for manufacturing a steel tube for air bags as set forth in  claim 1  wherein the steel contains at least one of Ca: at most 0.005% and B: at most 0.0030%. 
     
     
         5 . A process for manufacturing a steel tube for air bags as set forth in  claim 1  wherein the contents of Cu, Ni, Cr and Mo in the steel satisfy the following Equation (1):
   Cu+Ni≧(Cr+Mo) 2 +0.3  (1)
 
 wherein the symbols for elements in Equation (1) mean the values of the content of the respective elements in mass percent, and Mo=0 when the steel does not contain Mo. 
 
     
     
         6 . A process for manufacturing a steel tube for air bags as set forth in  claim 1  wherein the wall thickness of the steel tube after completion of the cold drawing step is at most 2.0 mm. 
     
     
         7 . A process for manufacturing a steel tube for air bags as set forth in  claim 6  wherein the cold drawing step is carried out by performing cold drawing one time. 
     
     
         8 . A process for manufacturing a steel tube for air bags as set forth in  claim 1  wherein heating for quench hardening in the heat treatment step is carried out by high-frequency induction heating. 
     
     
         9 . A process for manufacturing a steel tube for air bags as set forth in  claim 8  wherein the steel tube obtained in the cold drawing step is straightened before heating for the quench hardening.