Method for manufacturing a high strength steel sheet

Abstract

A method of manufacturing a high strength steel sheet. The method including casting a slab consisting essentially of 0.05 to 0.15% C, 0.5% or less Si, 1.00 to 2.00% Mn, 0.09% or less P, 0.01% or less S, 0.005% or less N, 0.01 to 0.1% Sol.Al, and a balance of Fe and inevitable impurities; hot-rolling the slab at a temperature of the Ar3 point or more; beginning cooling the hot-rolled steel sheet within 2 seconds after completing the hot-rolling at a temperature of from 750° C. to 600° C. at a cooling rate of 150° C./s or more; holding the cooled steel sheet at a temperature between 750° C. and 600° C. for 2 to 15 seconds; cooling the steel sheet at a cooling rate of 20° C./s or more; and coiling the cooled steel sheet at a temperature of 400° C. or less.

1 . A method for manufacturing a high strength steel sheet comprising
 casting a slab consisting essentially of 0.05 to 0.15% C, 0.5% or less Si, 1.00 to 2.00% Mn, 0.09% or less P, 0.01% or less S, 0.005% or less N, 0.01 to 0.1% Sol.Al, by mass, and a balance being Fe and inevitable impurities to form a cast slab;   hot-rolling the cast slab, directly or heating thereof, at a temperature of the Ar3 point or more to form a hot-rolled steel sheet;   carrying out a primary cooling step of cooling the hot-rolled steel sheet at a cooling rate of 150° C./s or more to a temperature of from 750° C. to 600° C., beginning cooling thereof within 2 seconds after completing the hot-rolling to form a primary cooled steel sheet;   holding the primary cooled steel sheet at a temperature between 750° C. and 600° C. for 2 to 15 seconds to form a temperature-held steel sheet;   carrying out a secondary cooling step of cooling the temperature-held steel sheet at a cooling rate of 20° C./s or more to form a secondary cooled steel sheet; and   coiling the secondary cooled steel sheet at a coiling temperature of 400° C. or less.   
   
   
       2 . The method according to  claim 1 , wherein the slab further contains at least one element selected from the group consisting of 0.01 to 0.3% Mo, 0.001 to 0.05% Nb, 0.001 to 0.1% Ti, 0.0003 to 0.002% B, and 0.05 to 0.49% Cr, by mass. 
   
   
       3 . The method according to  claim 1 , wherein the cooling rate in the primary cooling step is from 150 to 1000° C./s. 
   
   
       4 . The method according to  claim 3 , wherein the cooling rate in the primary cooling step is from 200 to 700° C./s. 
   
   
       5 . The method according to  claim 1 , wherein the cooling rate in the secondary cooling step is from 20 to 1000° C./s. 
   
   
       6 . The method according to  claim 1 , wherein the coiling temperature is from 0° C. to 400° C.