Steel pipe for an airbag system and a method for its manufacture

Abstract

A high strength steel pipe for an airbag system has a steel composition comprising C: 0.05-0.20%, Si: 0.1-1.0%, P; at most 0.025%, S: at most 0.010%, Cr: 0.05-1.0%, Al: at most 0.10% at least one of Ti and Mn satisfying (1) Ti≦0.02% and (2) 0.4%≦Mn+40Ti≦1.2%, and a remainder of Fe. The composition may further include one or more of (i) at least one of Mo: 0.05-0.50%, Ni: 0.05-1.5%, V: 0.01-0.2%, and B: 0.0003-0.005%, (ii) at least one of Cu: 0.05-0.5% and Nb: 0.003-0.1%, and (iii) at least one of Ca: 0.0003-0.01%, Mg: 0.0003-0.01%, and REM: 0.0003-0.01%. The steel pipe can be manufactured by forming a pipe from the above-described steel composition to obtain prescribed dimensions, heating to at least the Ac 1 transformation point and quenching, and then tempering at the Ac 1 transformation point or below.

1 . A steel pipe for an airbag system having a steel composition which consists essentially, in mass %, of: C: 0.05-0.20%, Si: 0.1-1.0%, P: at most 0.025%, S; at most 0.010%, Cr: 0.05-1.0%, Al; at most 0.10%, at least one of Ti and Mn in amounts satisfying the following equations (1) and (2), Mo: 0-0.50%, Ni: 0-1.5%, V: 0-0.2%, B: 0-0.005%, Cu: 0-0.5%, Nb: 0-0.1%, Ca: 0-0.01%, Mg: 0-0.01%, REM: 0-0.01%, and a remainder of Fe and impurities,  
         Ti≦0.02%  (1) 0.4%≦Mn+40Ti≦1.2%  (2).  
       wherein the symbols for elements in the above equation indicate the mass % of the elements, the steel pipe having a tensile strength of at least 1000 MPa.  
     
     
         2 . A steel pipe for an airbag system as claimed in  claim 1  wherein the steel composition contains at least 0.20 mass % of Mn.  
     
     
         3 . A steel pipe for an airbag system as claimed in  claim 1  wherein the steel composition contains, in mass %, at least one of Mo: 0.05-0.50%, Ni: 0.05-1.5%, V: 0.01-0.2%,and B: 0.0003-0.005%.  
     
     
         4 . A steel pipe for an airbag system as claimed in  claim 1  wherein the steel composition contains, in mass %, at least one of Cu: 0.05-0.5% and Nb: 0.003-0.1%  
     
     
         5 . A steel pipe for an airbag system as claimed in  claim 1  wherein the steel composition contains, in mass %, at least one of Ca: 0.0003-0.01%, Mg: 0.0003-0.01%, and REM: 0.0003-0.01%.  
     
     
         6 . A steel pipe for an airbag system as claimed in  claim 1  wherein the steel has a microstructure having a γ grain size number which is 11 or larger.  
     
     
         7 . A method of manufacturing a steel pipe for an airbag system comprising forming a pipe from a steel having a composition as recited in  claim 1  by a process including pipe making and subsequent cold working, then heating the cold-worked pipe to a temperature of at least the Ac 1  transformation point of the steel followed by quenching, and then tempering the quenched pipe at a temperature no higher than the Ac 1  transformation point.  
     
     
         8 . A method as claimed in  claim 7  wherein the heating temperature in the heating step of the cold-worked pipe is at least the Ac 3  transformation point of the steel.  
     
     
         9 . A method as claimed in  claim 8  wherein the heating temperature is in the range of 900-1000° C.  
     
     
         10 . A method as claimed in  claim 7  wherein the heating is performed by rapid heating at a rate of temperature increase of at least 10° C./sec.  
     
     
         11 . A method as claimed in  claim 10  wherein the heating is performed by high frequency induction heating.  
     
     
         12 . A method as claimed in  claim 7  wherein the quenching is performed such that the cooling rate at least in the temperature range of from 850° C. to 500° C. is at least 20° C./sec.