High-yield ratio high-strength steel plate having excellent impact resistance after cold forming and manufacturing method thereof

Abstract

The present invention relates to a high-strength steel plate and a manufacturing method therefor and, more specifically, to a high-strength steel plate having excellent impact resistance after cold forming and having a high-yield ratio, and a manufacturing method therefor.

1 . A steel plate comprising:
 by weight, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.001 to 1.0%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Ti: 0.03 to 0.08%, Nb: 0.01 to 0.05%, a remainder of Fe and inevitable impurities, and Nb+Ti: 0.04 to 0.1%,   wherein a microstructure of a surface portion ranging from a surface to a 50 μm thickness includes, by area, 95% or more equiaxed ferrite and 3% or less pearlite, and includes, by area, 5% or less one or more of bainitic ferrite, bainite, a martensite-austenite constituent (MA) phase, or martensite, in total,   a microstructure of a core portion ranging from a ¼ thickness to a ¾ thickness includes, by area, 80 to 95% bainitic ferrite, 10% or less bainite, 3% or less pearlite, 5 to 10% one or two of a martensite-austenite constituent (MA) phase or martensite, and a remainder including equiaxed ferrite.   
     
     
         2 . The steel plate of  claim 1 , having a thickness of 8 to 25 mm. 
     
     
         3 . The steel plate of  claim 1 , having a tensile strength of 590 MPa or more, a fracture elongation of 25% or more, a yield ratio of 0.75 to 0.9, and an impact toughness of 70 J or more at −20° C. after cold forming. 
     
     
         4 . The steel plate of  claim 1 , wherein a ratio (E/YS) of impact toughness (E) at −20° C. after cold forming and yield strength (YS) before cold forming is 0.15 or more. 
     
     
         5 . The steel plate of  claim 1 , wherein the steel plate comprises edge portions corresponding to 30% regions from both ends, and a central portion of a central 40% region corresponding to a region excluding both of the edge portions, in a width direction,
 wherein, in the edge portions and the central portion, a difference in tensile strength is 10 MPa or less, a difference in fracture elongation is 8% or less, and a difference in impact toughness at −20° C. after cold forming is 20 J or less.   
     
     
         6 . A method of manufacturing a steel plate, comprising:
 reheating a steel slab including, by weight, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.001 to 1.0%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Ti: 0.03 to 0.08%, Nb: 0.01 to 0.05%, a remainder of Fe and inevitable impurities, and Nb+Ti: 0.04 to 0.1%,   hot-rolling the reheated steel slab; and   cooling and coiling the hot-rolled steel plate to a temperature range of 500 to 650° C. at an average cooling rate greater than or equal to a CR value defined in the following relationship 1 within a range of 1 to 30° C./s,   wherein, in the cooling and coiling, edge portions corresponding to 30% regions from both ends in a width direction of a coil are cooled to a temperature (TE) of 550 to 650° C., and a central portion of a central 40% region corresponding to a region excluding both of the edge portions in the width direction is cooled to a temperature (TC) of 500 to 550° C.,   a difference in average temperature between the edge portions and the central portion is 50 to 150° C.:   
       
         
           
             
               
                 
                   
                     CR 
                     = 
                     
                       45 
                       - 
                       
                         16.3 
                         × 
                         
                           [ 
                           C 
                           ] 
                         
                       
                       - 
                       
                         5.6 
                         × 
                         
                           [ 
                           Si 
                           ] 
                         
                       
                       - 
                       
                         16.3 
                         × 
                         
                           [ 
                           Mn 
                           ] 
                         
                       
                       - 
                       
                         2.9 
                         × 
                         
                           [ 
                           Cr 
                           ] 
                         
                       
                       + 
                       
                         15 
                         × 
                         
                           [ 
                           Ti 
                           ] 
                         
                       
                       + 
                       
                         23 
                         × 
                         
                           [ 
                           Nb 
                           ] 
                         
                       
                       - 
                       
                         0.9 
                         × 
                         
                           ( 
                           
                             t 
                             - 
                             8 
                           
                           ) 
                         
                       
                     
                   
                 
                 
                   
                     [ 
                     
                       Relationship 
                       ⁢ 
                           
                       1 
                     
                     ] 
                   
                 
               
             
           
         
       
       Where [C], [Si], [Mn], [Cr], [Ti], and [Nb] are weight percentages of the elements, and t is a thickness (mm) of the steel plate. 
     
     
         7 . The method of  claim 6 , wherein a temperature of the reheating is 1100 to 1350° C., and
 a temperature of the hot-rolling is 800 to 1150° C. 
 
     
     
         8 . The method of  claim 6 , further comprising air-cooling the coiled coil to a temperature range of 200° C. or lower. 
     
     
         9 . The method of  claim 6 , wherein the steel plate has a thickness of 8 to 25 mm.