Steel plate subjected to heat treatment and process for producing the same

Abstract

The present invention provides a steel sheet for heat treatment consisting essentially of, by mass %, 0.05-0.09% C, below 1% Si, 1.6-2.4% Mn, 0.02% P or less, 0.02% S or less, 0.01-0.1% sol. Al, 0.005% N or less, 0.0003-0.003% B, Ti in a range satisfying (48/32)S+(48/14)N≦Ti≦2[(48/32)S+(48/14)N], and the rest of Fe, wherein the average diameter of iron carbides precipitating in steel is 2 μm or smaller. The steel sheet for heat treatment according to the present invention has high strength and excellent hydrogen embrittlement resistance after being press formed and quenched, so that the steel sheet is well suited for automobile construction members.

1 . A steel sheet for heat treatment consisting essentially of, by mass %, 0.05-0.09% C, below 1% Si, 1.6-2.4% Mn, 0.02% P or less, 0.02% S or less, 0.01-0.1% sol.Al, 0.005% N or less, 0.0003-0.003% B, Ti satisfying formula (1), and the rest of Fe:  
         (48/32)S+(48/14)N≦Ti≦2[(48/32)S+(48/14)N]  (1)  (the content of each element shown by the element symbol in formula (1) is represented by mass %),    wherein the average diameter of iron carbides precipitating in steel is 2 μm or smaller.    
     
     
         2 . The steel sheet for heat treatment according to  claim 1 , further comprising at least one element selected from 0.1-2% Cr and 0.1-2% Mo.  
     
     
         3 . A method for manufacturing a steel sheet for heat treatment comprising the steps of: 
 hot rolling a steel slab having composition as defined in  claim 1  or  2  into a steel sheet;    cooling the hot rolled steel sheet at an average cooling rate of 30° C./s or less; and    coiling the cooled steel sheet at a coiling temperature of 500° C. or above.    
     
     
         4 . A method for manufacturing a steel sheet for heat treatment comprising the steps of: 
 hot rolling a steel slab containing composition defined in  claim 1  or  2  into sa steel sheet;    cold rolling the hot rolled steel sheet; and    annealing the cold rolled steel sheet for recrystallization,    wherein the annealed cold rolled steel sheet is cooled at an average cooling rate of 30° C./s or less to 400° C. after annealing.