Method for manufacturing a steel sheet

Abstract

A method for manufacturing a steel sheet comprising rough-rolling a steel consisting essentially of 0.04 to 0.12 weight % C, 0.25 to 2 weight % Si, 0.5 to 2.5 weight % Mn, 0.1 weight % or less Al, and a balance of substantially Fe and inevitable impurities to form a sheet bar; finish-rolling the sheet bar at a rolling end temperature of the Ar 3 transformation point or more; primary-cooling the finish-rolled steel sheet within 1 second after completing the finish-rolling at a cooling speed of higher than 200° C./sec through a cooling range where-the difference between the start temperature of the cooling and the end temperature of the cooling is 100° C. to below 250° C.; slowly cooling the primary-cooled steel at a cooling speed of 10° C./sec or less, for 2 seconds to less than 20 seconds, at a temperature of 580° C. to 720° C.; and coiling the slowly cooled steel sheet at a temperature of 400° C. to below 540° C.

1 . A method for manufacturing a steel sheet comprising 
 (a) rough-rolling a steel consisting essentially of 0.04 to 0.12 weight % C, 0.25 to 2 weight % Si, 0.5 to 2.5 weight % Mn, 0.1 weight % or less Al, optionally at least one element selected from the group consisting of Ti, Nb, V, Zr, Cr and Mo, and a balance of substantially Fe and inevitable impurities to form a sheet bar;    (b) finish-rolling the sheet bar at a rolling end temperature of the Ar 3  transformation point or more to form a finish-rolled steel;    (c) primary-cooling the finish-rolled steel sheet within 1.0 second after completing the finish-rolling at a cooling speed higher than 20° C./sec through a cooling range where the difference between a start temperature of the cooling and an end temperature of the cooling is in a range of 100° C. to below 250° C. to form a primary-cooled steel;    (d) slowly cooling the primary-cooled steel at a cooling speed of 10° C./sec or less, for a period of from 2 seconds to less than 20 seconds, at a temperature range of 580° C. to 720° C. to form a slowly cooled steel sheet; and    (e) coiling the slowly cooled steel sheet at a temperature of 400° C. to below 540° C.    
     
     
         2 . The method for manufacturing a steel sheet according to  claim 1 , further comprising heating the sheet bar at a heating unit disposed at an inlet side of a continuous hot finish-rolling mill or between stands of the continuous hot finish-rolling mill.  
     
     
         3 . The method for manufacturing a steel sheet according to  claim 1 , wherein the steel further contains 0.01 to 0.2 weight % of at least one element selected from the group consisting of Ti, Nb, V and Zr.  
     
     
         4 . The method for manufacturing a steel sheet according to  claim 1 , wherein the steel further contains at least one of 1 weight % or less Cr and 0.5 weight % or less Mo.  
     
     
         5 . The method for manufacturing a steel sheet according to  claim 2 , wherein a reduction in thickness at a final stand of the continuous hot finish-rolling mill is less than 30%.  
     
     
         6 . The method for manufacturing a steel sheet according to  claim 1 , wherein the primary-cooling begins more than 0.5 seconds after completing the finish-rolling.  
     
     
         7 . (canceled)  
     
     
         8 . The method for manufacturing a steel sheet according to  claim 3 , wherein the steel further contains at least one of 1 weight % or less Cr and 0.5 weight % or less Mo.  
     
     
         9 . The method for manufacturing a steel sheet according to  claim 1 , wherein the steel has a composition of 0.084 weight % C, 1.08 weight % Si, 1.53 weight % Mn and 0.047 weight % Al, with the balance being Fe and inevitable impurities.  
     
     
         10 . The method for manufacturing a steel sheet according to  claim 1 , wherein the steel has a composition of 0.068 weight % C, 0.95 weight % Si, 1.58 weight % Mn, 0.045 weight % Al, and 0.07 weight % Ti, with the balance being Fe and inevitable impurities.  
     
     
         11 . The method for manufacturing a steel sheet according to  claim 9 , wherein the finish-rolling is carried out at 850° C.; the primary-cooling begins 0.6 seconds after completing the finish-rolling; the primary-cooling speed is 240° C./sec; the difference between the start temperature and the end temperature of the primary-cooling is 115° C.; the slow cooling is carried out for 8 seconds; and the cooling is carried out at 470° C.  
     
     
         12 . The method for manufacturing a steel sheet according to  claim 9 , wherein the finish-rolling is carried out at 850° C.; the primary-cooling begins 1.0 seconds after completing the finish-rolling; the primary-cooling speed is 45° C./sec; the difference between the start temperature and the end temperature of the primary cooling is 190° C.; the slow cooling is carried out for 10 seconds; and the cooling is carried out at 470° C.