Thick-walled high-strength hot rolled steel sheet with excellent low-temperature toughness and method for producing same

Abstract

A thick-walled high-strength hot rolled steel sheet has a high tensile strength TS of 521 MPa or more and excellent low-temperature toughness. The steel material forming the sheet contains 0.02%-0.08% C, 0.01%-0.10% Nb, and 0.001%-0.05% Ti and is heated; C, Ti, and Nb satisfies (Ti+(Nb/2))/C<4.

1 . A thick-walled high-strength hot rolled steel sheet comprising, on a mass percent basis:
 0.02%-0.08% C, 0.01%-0.50% Si,   0.5%-1.8% Mn, 0.025% or less P,   0.005% or less S, 0.005%-0.10% Al,   0.01%-0.10% Nb, 0.001%-0.05% Ti,   the balance being Fe, and incidental impurities, and   a microstructure, wherein C, Ti, and Nb are contained satisfy expression (1):
   (Ti+(Nb/2))/C<4  (1)
 
   where Ti, Nb, and C each represent the proportion thereof (percent by mass), and   wherein in the microstructure, a difference ΔD between average grain size (μm) of a ferrite phase serving as a main phase at a position 1 mm from a surface of the steel sheet in the thickness direction and the average grain size (μm) of the ferrite phase serving as the main phase at a middle position of the steel sheet in the thickness direction is 2 μm or less, and the difference ΔV between a fraction (percent by volume) of a second phase at a position 1 mm from the surface of the steel sheet in the thickness direction and the fraction (percent by volume) of the second phase at the middle position of the steel sheet in the thickness direction is 2% or less.   
     
     
         2 . The steel sheet according to  claim 1 , further comprising:
 mill scale having a thickness of 3 to 30 μm on the surface of the steel sheet.   
     
     
         3 . The steel sheet according to  claim 1 ,
 wherein the difference ΔHV between Vickers hardness HV 1mm  at a position 1 mm from the surface of the steel sheet in the thickness direction and Vickers hardness HV 1/2t  at a middle position of the steel sheet in the thickness direction is 50 points or less.   
     
     
         4 . The steel sheet according to  claim 1 ,
 wherein in the microstructure, a minimum lath spacing of a bainite phase or a tempered martensitic phase at a position 1 mm from the surface of the steel sheet in the thickness direction is 0.1 μm or more.   
     
     
         5 . The steel sheet according to  claim 1 , further comprising, on a mass percent basis:
 one or more selected from 0.01%-0.10% V, 0.01%-0.50% Mo, 0.01%-1.0% Cr, 0.01%-0.50% Cu, and 0.01%-0.50% Ni.   
     
     
         6 . The steel sheet according to  claim 1 , further comprising, on a mass percent basis:
 0.0005%-0.005% Ca.   
     
     
         7 . A method for producing a thick-walled high-strength hot rolled steel sheet comprising:
 heating a steel material containing, on a mass percent basis,   0.02%-0.08% C, 0.01%-0.50% Si,   0.5%-1.8% Mn, 0.025% or less P,   0.005% or less S, 0.005%-0.10% Al,   0.01%-0.10% Nb, 0.001%-0.05% Ti,   the balance being Fe, and incidental impurities, C, Ti, and Nb being contained to satisfy expression (1):
   (Ti+(Nb/2))C<4  (1);
 
   performing hot rolling including rough rolling and finish rolling;   performing accelerated cooling at an average cooling rate of 10° C./s or more at a middle position of the steel sheet in the thickness direction to a cooling stop temperature of BFS or lower at a middle position of the steel sheet in the thickness direction, wherein BFS is defined by expression (2):
   BFS (° C.)=770−300C−70Mn−70Cr−170Mo−40Cu−40Ni−1.5CR  (2);
 
   
       and
 performing coiling at a coiling temperature of BFS0 or lower at the middle position of the steel sheet in the thickness direction, the BFS0 being defined by expression (3):
   BFS0 (° C.)=770−300C−70Mn−70Cr−170Mo−40Cu−40Ni  (3)
 
 
 
       where in expressions (1), (2), and (3), C, Ti, Nb, Mn, Cr, Mo, Cu, and Ni each represent a proportion (percent by mass) thereof, and CR represents a cooling rate (° C./s) at the middle position of the steel sheet in the thickness direction. 
     
     
         8 . The method to  claim 7 , further comprising:
 performing scale removal treatment with a scale breaker before the rough rolling and before the finish rolling, wherein in the hot rolling, the finish entry temperature (FET) is set in the range of 800° C. to 1050° C., and finish delivery temperature (FDT) is set in the range of 750° C. to 950° C.   
     
     
         9 . The method according to  claim 7 ,
 wherein in the accelerated cooling, when a carbon equivalent Ceq is 0.37% or less, n average cooling rate at a position 1 mm from a surface of the steel sheet in the thickness direction is set to 10° C./s or more, and when the carbon equivalent Ceq exceeds 0.37%, the average cooling rate is set to 10 to 200° C./s, the carbon equivalent Ceq being defined by expression (4):
   Ceq (%)=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 (4) 
   
       (where C, Ti, Mn, Cr, Mo, V, Cu, and Ni each represent the proportion thereof (percent by mass)). 
     
     
         10 . The method according to  claim 7 ,
 wherein the accelerated cooling is performed at an average cooling rate of 100° C./s or more at a position 1 mm from a surface of the steel sheet in the thickness direction, and the coiling is performed at a coiling temperature of 300° C. or higher at a middle position of the steel sheet in the thickness direction.   
     
     
         11 . The method according to  claim 7 , further comprising, on a mass percent basis, one or more selected from 0.01%-0.10% V, 0.01%-0.50% Mo, 0.01%-1.0% Cr, 0.01%-0.50% Cu, and 0.01%-0.50% Ni. 
     
     
         12 . The method according to  claim 7 , further comprising, on a mass percent basis, 0.0005%-0.005% Ca. 
     
     
         13 . The steel sheet according to  claim 2 , further comprising, on a mass percent basis:
 one or more selected from 0.01%-0.10% V, 0.01%-0.50% Mo, 0.01%-1.0% Cr, 0.01%-0.50% Cu, and 0.01%-0.50% Ni.   
     
     
         14 . The steel sheet according to  claim 3 , further comprising, on a mass percent basis:
 one or more selected from 0.01%-0.10% V, 0.01%-0.50% Mo, 0.01%-1.0% Cr, 0.01%-0.50% Cu, and 0.01%-0.50% Ni.   
     
     
         15 . The steel sheet according to  claim 4 , further comprising, on a mass percent basis:
 one or more selected from 0.01%-0.10% V, 0.01%-0.50% Mo, 0.01%-1.0% Cr, 0.01%-0.50% Cu, and 0.01%-0.50% Ni.   
     
     
         16 . The method according to  claim 8 , further comprising, on a mass percent basis, one or more selected from 0.01%-0.10% V, 0.01%-0.50% Mo, 0.01%-1.0% Cr, 0.01%-0.50% Cu, and 0.01%-0.50% Ni. 
     
     
         17 . The method according to  claim 9 , further comprising, on a mass percent basis, one or more selected from 0.01%-0.10% V, 0.01%-0.50% Mo, 0.01%-1.0% Cr, 0.01%-0.50% Cu, and 0.01%-0.50% Ni. 
     
     
         18 . The method according to  claim 10 , further comprising, on a mass percent basis, one or more selected from 0.01%-0.10% V, 0.01%-0.50% Mo, 0.01%-1.0% Cr, 0.01%-0.50% Cu, and 0.01%-0.50% Ni.