Method for manufacturing a steel sheet

Abstract

The method for manufacturing steel sheet comprises the steps of: rough-rolling to form a sheet bar; finish-rolling the sheet bar to form a steel strip; applying primary cooling and secondary cooling to the finish-rolled steel strip; and coiling the secondary-cooled steel strip. The primary cooling is conducted at cooling speeds of 120° C./sec or more down to the temperatures of from 500 to 800° C. The secondary cooling is conducted at cooling speeds of less than 60° C./sec.

What is claimed is:  
     
         1 . A method for manufacturing steel sheet comprising the steps of: 
 rough-rolling a continuously cast slab consisting essentially of 0.8% or less C, by weight to form a sheet bar;    finish-rolling the sheet bar at finishing temperatures of (Ar 3  transformation point −20° C.) or more to form a steel strip;    rapid cooling the steel strip after completed the finish-rolling at cooling speeds of higher than 120° C./sec down to temperatures of from 500 to 800° C.; and    coiling the steel strip after completed the rapid cooling at coiling temperatures of from 400 to 750° C.    
     
     
         2 . The method of  claim 1 , wherein the continuously cast slab contains 0.8% or less C, 2.5% or less Si, and 3.0% or less Mn, by weight.  
     
     
         3 . The method of  claim 1 , wherein the continuously cast slab contains 0.8% or less C, 2.5% or less Si, 3.0% or less Mn, and 0.01 to 0.2% of at least one element selected from the group consisting of Ti, Nb, V, Mo, Zr, and Cr, by weight.  
     
     
         4 . The method of  claim 1 , wherein the continuously cast slab contains 0.8% or less C, 2.5% or less Si, 3.0% or less Mn, and 0.005% or less of at least one element selected from the group consisting of Ca and B, by weight.  
     
     
         5 . The method of  claim 1 , wherein the continuously cast slab contains 0.8% or less C, 2.5% or less Si, 3.0% or less Mn, 0.01 to 0.2% of at least one element selected from the group consisting of Ti, Nb, V, Mo, Zr, and Cr, and 0.005% or less at least one element selected from the group consisting of Ca and B, by weight.  
     
     
         6 . The method of  claim 1 , wherein the rough-rolling of the continuously cast slab is carried out by direct hot-rolling.  
     
     
         7 . The method of  claim 1 , wherein the rough-rolling of the continuously cast slab is carried out by reheating the slab to temperatures of 1,200° C. or below before cooling thereof to room temperature.  
     
     
         8 . The method of  claim 1 , further comprising the step of heating the sheet bar by an induction heating unit immediately before the finish-rolling or during the finish-rolling.  
     
     
         9 . The method of  claim 1 , wherein the rapid cooling of the steel strip is started within a time ranging from more than 0.1 second and less than 1 second after completed the finish-rolling.  
     
     
         10 . The method of  claim 1 , further comprising the steps of: cold-rolling the coiled steel strip; and annealing the cold-rolled steel strip.  
     
     
         11 . The method of  claim 1 , wherein the rapid cooling step is carried out so as the temperature difference between the maximum value and the minimum value in width direction and in longitudinal direction of the steel strip after the rapid cooling to become 60° C. or less.  
     
     
         12 . The method of  claim 1 , wherein the rapid cooling step is carried out by cooling the steel strip at heat transfer coefficients of 2,000 kcal/m 2 h ° C. or more.  
     
     
         13 . A steel sheet prepared by the method for manufacturing steel sheet of  claim 1  and having variations of tensile strength in width direction and in longitudinal direction thereof within ±8% of average value of the tensile strength in a coil.  
     
     
         14 . A method for manufacturing steel sheet comprising the steps of: 
 rough-rolling a continuously cast slab consisting essentially of more than 0.8% and 1% or less C, by weight to form a sheet bar;    finish-rolling thus obtained sheet bar at finishing temperatures of (Acm transformation point −20° C.) or more to form a steel strip;    rapid cooling the steel strip after completed the finish-rolling at cooling speeds of higher than 120° C./sec down to temperatures of from 500 to 800° C.; and    coiling the steel strip after completed the rapid cooling at coiling temperatures of from 400 to 750° C.    
     
     
         15 . The method of  claim 14 , wherein the rough-rolling of the continuously cast slab is carried out by direct hot-rolling.  
     
     
         16 . The method of  claim 14 , wherein the rough-rolling of the continuously cast slab is carried out by reheating the slab to temperatures of 1,200° C. or less before cooling thereof to room temperature.  
     
     
         17 . The method of  claim 14 , further comprising the step of heating the sheet bar by an induction heating unit immediately before the finish-rolling or during the finish-rolling.  
     
     
         18 . The method of  claim 14 , wherein the rapid cooling of the steel strip is started within a time ranging from more than 0.1 second and less than 1 second after completed the finish-rolling.  
     
     
         19 . The method of  claim 14 , further comprising the steps of: cold-rolling the coiled steel strip; and annealing the cold-rolled steel strip.  
     
     
         20 . The method of  claim 14 , wherein the rapid cooling step is carried out so as the temperature difference between the maximum value and the minimum value in width direction and in longitudinal direction of the steel strip after the rapid cooling to become  60 ° C. or less.  
     
     
         21 . The method of  claim 14 , wherein the rapid cooling step is carried out by cooling the steel strip at heat transfer coefficients of 2,000 kcal/m 2 h° C. or more.  
     
     
         22 . A steel sheet prepared by the method for manufacturing steel sheet of  claim 14  and having variations of tensile strength in width direction and in longitudinal direction thereof within ±8% of average value of the tensile strength in a coil.  
     
     
         23 . A method for manufacturing steel sheet comprising the steps of: 
 forming a slab consisting essentially of 0.05 to 0.14% C, 0.5% or less Si, 0.5 to 2.5% Mn, 0.05% or less P, 0.01% or less S, 0.005% or less O, and less than 0.0005% Ca, by weight, by continuous casting conducting treatment to reduce segregation;    hot-rolling the slab at finishing temperatures of finish-rolling of Ar 3  transformation point or more to form a hot-rolled steel sheet;    starting primary cooling within 2 seconds after completed the hot-rolling at cooling speeds of from 100 to 2,000° C./sec to cool the hot-rolled steel sheet to a temperature range of from 600 to 750° C.;    applying secondary cooling, after cooling to the temperature range, at cooling speeds of less than 50° C./sec; and    coiling the secondary-cooled hot-rolled steel sheet at temperatures of from 450 to 650° C.    
     
     
         24 . The method of  claim 23 , further comprising the step of reheating the slab before the hot-rolling thereof.  
     
     
         25 . The method of  claim 23 , further comprising the steps of: pickling the coiled hot-rolled steel sheet; and annealing the pickled hot-rolled steel sheet.  
     
     
         26 . The method of  claim 23 , further comprising the steps of: pickling the coiled hot-rolled steel sheet; cold-rolling the pickled hot-rolled steel sheet; and annealing the cold-rolled steel sheet.  
     
     
         27 . The method of  claim 23 , wherein the slab further contains 0.01 to 0.3% at least one element selected from the group consisting of Ti, Nb, V, Mo, Zr, and Cr, by weight.  
     
     
         28 . A method for manufacturing steel sheet comprising the steps of: 
 hot-rolling a steel consisting essentially of 0.03 to 0.12% C, 1% or less Si, 0.5 to 2% Mn, 0.02% or less P, 0.01% or less S, further at least one element selected from the group consisting of 0.005 to 0.1% Nb, 0.005 to 0.1% V, and 0.005 to 0.1% Ti, by weight, at temperatures of 1,070° C. or below to accumulated reductions in thickness of 30% or more; and    cooling the hot-rolled steel sheet within 6 seconds after completed the rolling at average cooling speeds of not less than 80° C./sec to temperatures of from above 500° C. to not more than 700° C.    
     
     
         29 . The method of  claim 28 , wherein the steel further contains 0.05 to 0.5% Mo.  
     
     
         30 . A method for manufacturing steel sheet comprising the steps of: 
 hot-rolling a steel consisting essentially of 0.03 to 0.12% C, 1% or less Si, 0.5 to 2% Mn, 0.02% or less P, 0.01% or less S, and 0.05 to 0.5% Mo, by weight, at temperatures of 1,070° C. or below to accumulated reductions in thickness of 30% or more; and cooling the hot-rolled steel sheet within 6 seconds after completed the rolling at average cooling speeds of not less than 80° C./sec to temperatures of from above 500° C. to not more than 700° C.