Steel sheet and method of producing the same

Abstract

A steel sheet of the present invention has a steel structure obtained by performing a soaking at a dual phase region temperature of Ac1 temperature or higher and lower than Ac3 temperature for a soaking time of 15 seconds or longer and 35 seconds or shorter, next, performing a primary cooling to a temperature range of 250° C. or higher and 380° C. or lower within 3 seconds at a cooling rate of 0.5 ° C./s or more and 30° C./s or less, and performing a retention in a temperature range of 260° C. or higher and 370° C. or lower for 180 seconds or longer and 540 seconds or shorter, in which a yield ratio is 65% or less and tensile strength is 590 MPa or more after the primary cooling.

1 . A method of producing a steel sheet using a continuous annealing line, the method comprising:
 a first retention process of retaining a base steel sheet having the component composition according to  claim 1  at a dual phase region temperature of Ac1 temperature or higher and lower than Ac3 temperature for 15 seconds or longer and 35 seconds or shorter;   a primary cooling process of primarily cooling the steel sheet to a temperature range of 250° C. or higher and 380° C. or lower within 3 seconds at a cooling rate of 0.5° C./s or more and 30° C./s or less after the first retention process; and   a second retention process of retaining the steel sheet while passing through an overaging section arranged in the continuous annealing line whose temperature is set to 260° C. or higher and 370° C. or lower for a retention time of 180 seconds or longer and 540 seconds or shorter after the primary cooling.   
     
     
         2 . The method of producing a steel sheet according to  claim 1 ,
 wherein, in the second retention process, y that is a product of an overaging section passing temperature which is the retention temperature when the steel sheet passes through the overaging section, and an overaging section passing time which is the retention time, and x that is the cooling rate in the primary cooling process satisfy a following Expression (6),
     y≤ 796700× x   (−0.971)    (6).
 
   
     
     
         3 . The method of producing a steel sheet according to  claim 1 , further comprising:
 a preliminary sheet passing process of, before starting the primary cooling process, passing a required amount or more of a temperature adjusted steel sheet whose primary cooling stop temperature is set to 330° C. or lower through the continuous annealing line.   
     
     
         4 . The method of producing a steel sheet according to  claim 3 ,
 wherein the required amount is 30 tons.   
     
     
         5 . The method of producing a steel sheet according to  claim 1 ,
 wherein the base steel sheet further contains, by mass %, any one or two or more of   Cr: 0.01% or more and 0.5% or less,   Mo: 0.01% or more and 0.5% or less, and   B: 0.0005% or more and 0.005% or less.   
     
     
         6 . The method of producing a steel sheet according to  claim 5 ,
 wherein the base steel sheet further contains, by mass %, one or two or more of Nb, Ti, and V of 0.005% or more and 0.05% or less in total.   
     
     
         7 . The method of producing a steel sheet according to  claim 1 ,
 wherein the base steel sheet further contains, by mass %, one or two or more of Nb, Ti, and V of 0.005% or more and 0,05% or less in total.