High-carbon hot-rolled steel sheet and method for producing the same

Abstract

A high-carbon hot-rolled steel sheet with excellent width-direction homogeneity is provided. The steel sheet contains 0.2% to 0.7% carbon, 0.01% to 1.0% silicon, 0.1% to 1.0% manganese, 0.03% or less phosphorus, 0.035% or less sulfur, 0.08% or less aluminum, and 0.01% or less nitrogen, and the balance is iron and incidental impurities. The structure is such that the average ferrite grain size of edge parts of the steel sheet is less than 35 μm, the average ferrite grain size of a part closer to the center of the steel sheet than the edge parts is less than 20 μm, and the average carbide grain size is 0.10 μm or more and less than 2.0 μm. The steel sheet is produced by roughly rolling the steel, finish-rolling the steel at a finishing temperature of more than (Ar3+40° C.), cooling the steel at a cooling rate of more than 120° C./s within two seconds after the finish rolling to a cooling termination temperature of more than 550° C. and less than 650° C., coiling the steel at a temperature of 550° C. or less, pickling the steel, and subjecting the steel to spheroidizing annealing at a temperature of 670° C. to the Ac1 transformation point by a batch annealing method.

1 . A high-carbon hot-rolled steel sheet comprising, in percent by mass, 0.2% to 0.7% carbon, 0.01% to 1.0% silicon, 0.1% to 1.0% manganese, 0.03% or less phosphorus, 0.035% or less sulfur, 0.08% or less aluminum, and 0.01% or less nitrogen, the balance being iron and incidental impurities, and having a structure wherein average ferrite grain size of edge parts of the steel sheet is less than 35 μm, the average ferrite grain size of a part closer to a center portion of the steel sheet than the edge parts is less than 20 μm, and average carbide grain size is 0.10 μm or more and less than 2.0 μm, and
 the edge parts of the steel sheet are regions extending from positions 25 mm from both sides of the steel sheet to positions 75 mm from both sides in a width direction of the steel sheet in hot rolling.   
   
   
       2 . The high-carbon hot-rolled steel sheet according to  claim 1 , further comprising, in percent by mass, one or more of 0.005% to 0.5% molybdenum, 0.005% to 0.05% titanium, and 0.005% to 0.1% niobium. 
   
   
       3 . A method for producing a high-carbon hot-rolled steel sheet, comprising:
 roughly rolling a steel having the composition according to  claim 1 ,   finish-rolling the steel at a finishing temperature of more than (Ar3+40° C.),   cooling the steel at a cooling rate of more than 120° C./s within two seconds after the finish rolling to a cooling termination temperature of more than 550° C. and less than 650° C.,   coiling the steel at a temperature of 550° C. or less,   pickling the steel, and   subjecting the steel to spheroidizing annealing at a temperature of 670° C. to the Ac1 transformation point by batch annealing.   
   
   
       4 . A method for producing a high-carbon hot-rolled steel sheet, comprising:
 roughly rolling a steel having the composition according to  claim 2 ,   finish-rolling the steel at a finishing temperature of more than (Ar3+40° C.),   cooling the steel at a cooling rate of more than 120° C./s within two seconds after the finish rolling to a cooling termination temperature of more than 550° C. and less than 650° C.,   coiling the steel at a temperature of 550° C. or less,   pickling the steel, and   subjecting the steel to spheroidizing annealing at a temperature of 670° C. to the Ac1 transformation point by batch annealing.