Method of manufacturing high strength steel sheet

Abstract

A method of manufacturing a high strength steel sheet includes subjecting a steel having a chemical composition including C: 0.08% to 0.20%, Si: 0.3% or less, Mn: 0.1% to 3.0%, P: 0.10% or less, S: 0.030% or less, Al: 0.10% or less, N: 0.010% or less, V: 0.20% to 0.80%, and the remainder composed of Fe and incidental impurities on a percent by mass basis to a hot rolling process composed of heating, rough rolling, finish rolling, cooling, and coiling into the shape of a coil at a predetermined coiling temperature, wherein the heating is performed at a temperature of 1,100° C. or higher for 10 min or more, the rough rolling is performed at a finish rough rolling temperature of 1,000° C. or higher, the finish rolling is performed at a finishing temperature of 850° C. or higher, in which a reduction ratio in a temperature range of 1,000° C. or lower is 96% or less, a reduction ratio in a temperature range of 950° C. or lower is 80% or less, the cooling after completion of the finish rolling is performed at an average cooling rate of (30×[V])° C./s or more in relation to the V content [V] (percent by mass) in a temperature range from the finishing temperature to 750° C. and at an average cooling rate of (10×[V])° C./s or more in relation to the V content [V] (percent by mass) in a temperature range from 750° C. to the coiling temperature, and the coiling temperature is 500° C. or higher and (700−50×[V])° C. or lower in relation to the V content [V] (percent by mass).

What is claimed is: 
     
         1 . A method of manufacturing a high strength steel sheet comprising subjecting a steel having a chemical composition comprising C: 0.08% to 0.20%, Si: 0.3% or less, Mn: 0.1% to 3.0%, P: 0.10% or less, S: 0.030% or less, Al: 0.10% or less, N: 0.010% or less, V: 0.20% to 0.80%, and the remainder composed of Fe and incidental impurities on a percent by mass basis to a hot rolling process composed of heating, rough rolling, finish rolling, cooling, and coiling into the shape of a coil at a predetermined coiling temperature,
 wherein the heating is performed at a temperature of 1,100° C. or higher for 10 min or more,   the rough rolling is performed at a finish rough rolling temperature of 1,000° C. or higher,   the finish rolling is performed at a finishing temperature of 850° C. or higher, in which a reduction ratio in a temperature range of 1,000° C. or lower is 96% or less, a reduction ratio in a temperature range of 950° C. or lower is 80% or less,   the cooling after completion of the finish rolling is performed at an average cooling rate of (30×[V])° C./s or more in relation to the V content [V] (percent by mass) in a temperature range from the finishing temperature to 750° C. and at an average cooling rate of (10×[V])° C./s or more in relation to the V content [V] (percent by mass) in a temperature range from 750° C. to the coiling temperature, and   the coiling temperature is 500° C. or higher and (700−50×[V])° C. or lower in relation to the V content [V] (percent by mass).   
     
     
         2 . The method according to  claim 1 , wherein the chemical composition further contains at least one group selected from the group consisting of Group A to Group F on a percent by mass basis:
 Group A: Ti: 0.005% to 0.20%,   Group B: at least one selected from the group consisting of Nb: 0.005% to 0.50%, Mo:   
     
     
         0 . 005% to 0.50%, Ta: 0.005% to 0.50%, and W: 0.005% to 0.50%,
 Group C: B: 0.0002% to 0.0050%,   Group D: at least one selected from the group consisting of Cr: 0.01% to 1.0%, Ni: 0.01% to 1.0%, and Cu: 0.01% to 1.0%,   Group E: Sb: 0.005% to 0.050%, and   Group F: at least one selected from the group consisting of Ca: 0.0005% to 0.01% and REM: 0.0005% to 0.01%.   
     
     
         3 . The method according to  claim 1 , wherein, in subjecting the hot rolled steel sheet to a coating annealing process composed of pickling and coating annealing treatment following the hot rolling process,
 the coating annealing treatment is performed by heating in a temperature range from 500° C. to a soaking temperature at an average heating rate of (5×[C])° C./s or more up to the soaking temperature of (800−200×[C])° C. or lower, in relation to the C content [C] (percent by mass), holding at the soaking temperature for a soaking time of 1,000 s or less, cooling to a zinc coating bath temperature of 420° C. to 500° C. at an average cooling rate of 1° C./s or more, and dipping into the zinc coating bath.   
     
     
         4 . The method according to  claim 3 , wherein, after the coating annealing process is applied, a reheating treatment is further applied by reheating to a temperature of 460° C. to 600° C. and holding at the reheating temperature for 1 s or more. 
     
     
         5 . The method according to  claim 1 , wherein, after the hot rolling process or the coating annealing process, a tempering treatment is further applied by working at a thickness decrease ratio of 0.1% to 3.0%. 
     
     
         6 . The method according to  claim 2 , wherein, in subjecting the hot rolled steel sheet to a coating annealing process composed of pickling and coating annealing treatment following the hot rolling process,
 the coating annealing treatment is performed by heating in a temperature range from 500° C. to a soaking temperature at an average heating rate of (5×[C])° C./s or more up to the soaking temperature of (800−200×[C])° C. or lower, in relation to the C content [C] (percent by mass), holding at the soaking temperature for a soaking time of 1,000 s or less, cooling to a zinc coating bath temperature of 420° C. to 500° C. at an average cooling rate of 1° C./s or more, and dipping into the zinc coating bath.   
     
     
         7 . The method according to  claim 2 , wherein, after the hot rolling process or the coating annealing process, a tempering treatment is further applied by working at a thickness decrease ratio of 0.1% to 3.0%. 
     
     
         8 . The method according to  claim 3 , wherein, after the hot rolling process or the coating annealing process, a tempering treatment is further applied by working at a thickness decrease ratio of 0.1% to 3.0%. 
     
     
         9 . The method according to  claim 4 , wherein, after the hot rolling process or the coating annealing process, a tempering treatment is further applied by working at a thickness decrease ratio of 0.1% to 3.0%.