Thin gauge steel sheet excellent in surface conditions, formability, and workability and method for producing the same

Abstract

The present invention provides ultralow carbon thin gauge steel sheet and a method for producing the same where coalescence and growth of inclusions in the molten steel are prevented and the inclusions are finely dispersed in the steel sheet, whereby surface defects and cracks at the time of press forming are prevented, growth of recrystallized grains at the time of continuous annealing is promoted, and a high r value (r value≧2.0) and elongation (total elongation≧50%) are exhibited, that is, ultralow carbon thin gauge steel sheet excellent in surface conditions, formability, and workability comprised of, by mass %, 0.0003%≦C≦0.003%, Si <0.01%, Mn ≦0.1%, P≦0.02%, S≦0.01%, 0.0005%≦N≦0.0025%, 0.01%≦acid soluble Ti≦0.07%, acid soluble Al ≦0.003%, and 0.002%≦La+Ce+Nd≦0.02% and a balance of iron and unavoidable impurities, said steel sheet characterized by containing at least cerium oxysulfite, lanthanum oxysulfite, and neodymium oxysulfite.

1 . Ultralow carbon thin gauge steel sheet excellent in surface conditions, formability, and workability comprised of, by mass %, 0.0003%≦C≦0.003%, Si≦0.01%, Mn ≦0.1%, P≦0.02%, S≦0.01%, 0.0005%≦N≦0.0025%, 0.01%≦acid soluble Ti≦0.07%, acid soluble Al≦0.003%, and 0.002%≦La+Ce+Nd≦0.02% and a balance of iron and unavoidable impurities, said steel sheet characterized by containing at least cerium oxysulfite, lanthanum oxysulfite, and neodymium oxysulfite.  
   
   
       2 . Ultralow carbon thin gauge steel sheet excellent in surface conditions, formability, and workability comprised of, by mass %, 0.0003%≦C≦0.003%, Si≦0.01%, Mn ≦0.1%, P≦0.02%, S≦0.01%, 0.0005%≦N≦0.0025%, 0.01%≦acid soluble Ti≦0.07%, acid soluble Al≦0.003%, and 0.002%≦La+Ce+Nd≦0.02% and a balance of iron and unavoidable impurities, said steel sheet characterized in that an average grain size of recrystallized grains is 15 μm or more and an average value of an aspect ratio of the recrystallized grain size is 2.0 or less.  
   
   
       3 . Ultralow carbon thin gauge steel sheet excellent in surface conditions, formability, and workability as set forth in  claim 1 , characterized in that said thin gauge steel sheet further contains, by mass %, 0.0004%≦Nb≦0.05%.  
   
   
       4 . Ultralow carbon thin gauge steel sheet excellent in surface conditions, formability, and workability as set forth in claims  1 , characterized in that said thin gauge steel sheet further contains, by mass %, 0.0004%≦B≦0.005%.  
   
   
       5 . A method for producing ultralow carbon thin gauge steel sheet excellent in surface conditions, formability, and workability comprising casting molten steel comprised of, by mass %, 0.0003%≦C≦0.003%, Si≦0.01%, Mn≦ 0 . 1%, P≦0.02%, S≦0.01%, 0.0005%≦N≦0.0025%, 0.01%≦acid soluble Ti≦0.07%, acid soluble Al≦0.003%, and 0.002%≦La+Ce+Nd≦0.02% and a balance of iron and unavoidable impurities, heating the obtained cast slab, hot rolling and coiling it to obtain a hot rolled steel strip, cold rolling it by a cold rolling rate of 70% or more, then continuously annealing it during which recrystallization annealing it at 600 to 900° C.  
   
   
       6 . A method for producing ultralow carbon thin gauge steel sheet excellent in surface conditions, formability, and workability as set forth in  claim 5 , characterized in that said molten steel further contains, by mass %, 0.0004%≦Nb≦0.05%.  
   
   
       7 . A method for producing ultralow carbon thin gauge steel sheet excellent in surface conditions, formability, and workability as set forth in  claim 5  characterized in that said molten steel further contains, by mass %, 0.0004%≦B≦0.005%.