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

Abstract

A high-carbon hot-rolled steel sheet and a method for manufacturing the steel sheet are provided. The high-carbon hot-rolled steel sheet has a particular chemical composition. The microstructure of the steel sheet includes ferrite, cementite, and pearlite that accounts for 6.5% or less of the entire microstructure by area fraction. The proportion of the number of cementite grains having an equivalent circle diameter of 0.1 μm or less to the total number of cementite grains is 20% or less, the average cementite grain size is 2.5 μm or less, and the cementite accounts for 3.5% or more and 10.0% or less of the entire microstructure by area fraction. The average concentration of solute B in a region extending from a surface layer to a depth of 100 μm is 10 mass ppm or more. The average concentration of N present as AlN in the region is 70 mass ppm or less.

1 . A high-carbon hot-rolled steel sheet having a chemical composition comprising, by mass %,
 C: 0.20% or more and 0.50% or less,   Si: 0.8% or less,   Mn: 0.10% or more and 0.80% or less,   P: 0.03% or less,   S: 0.010% or less,   sol. Al: 0.10% or less,   N: 0.01% or less,   Cr: 1.0% or less,   B: 0.0005% or more and 0.005% or less, and   one or two selected from Sb and Sn in an amount of 0.002% or more and 0.1% or less in total,   with the balance being Fe and unavoidable impurities,   the steel sheet having a microstructure including   ferrite, cementite, and pearlite that accounts for 6.5% or less of the entire microstructure by area fraction,   wherein regarding the cementite, a proportion of a number of cementite grains having an equivalent circle diameter of 0.1 μm or less to a total number of cementite grains is 20% or less, an average cementite grain size is 2.5 μm or less, and the cementite accounts for 3.5% or more and 10.0% or less of the entire microstructure by area fraction,   an average concentration of solute B in a region extending from a surface layer to a depth of 100 μm is 10 mass ppm or more, and   an average concentration of N present as AlN in the region extending from the surface layer to the depth of 100 μm is 70 mass ppm or less.   
     
     
         2 . The high-carbon hot-rolled steel sheet according to  claim 1 , having a tensile strength of 480 MPa or less and a total elongation of 33% or more. 
     
     
         3 . The high-carbon hot-rolled steel sheet according to  claim 1 , wherein the ferrite has an average grain size of 4 to 25 μm. 
     
     
         4 . The high-carbon hot-rolled steel sheet according to  claim 2 , wherein the ferrite has an average grain size of 4 to 25 μm. 
     
     
         5 . The high-carbon hot-rolled steel sheet according to  claim 1 , wherein the chemical composition further comprises, by mass %, one or two groups selected from Group A and Group B:
 Group A: Ti: 0.06% or less, and   Group B: one or two or more selected from Nb, Mo, Ta, Ni, Cu, V, and W each in an amount of 0.0005% or more and 0.1% or less.   
     
     
         6 . The high-carbon hot-rolled steel sheet according to  claim 2 , wherein the chemical composition further comprises, by mass %, one or two groups selected from Group A and Group B:
 Group A: Ti: 0.06% or less, and   Group B: one or two or more selected from Nb, Mo, Ta, Ni, Cu, V, and W each in an amount of 0.0005% or more and 0.1% or less.   
     
     
         7 . The high-carbon hot-rolled steel sheet according to  claim 3 , wherein the chemical composition further comprises, by mass %, one or two groups selected from Group A and Group B:
 Group A: Ti: 0.06% or less, and   Group B: one or two or more selected from Nb, Mo, Ta, Ni, Cu, V, and W each in an amount of 0.0005% or more and 0.1% or less.   
     
     
         8 . The high-carbon hot-rolled steel sheet according to  claim 4 , wherein the chemical composition further comprises, by mass %, one or two groups selected from Group A and Group B:
 Group A: Ti: 0.06% or less, and   Group B: one or two or more selected from Nb, Mo, Ta, Ni, Cu, V, and W each in an amount of 0.0005% or more and 0.1% or less.   
     
     
         9 . A method for manufacturing the high-carbon hot-rolled steel sheet according to  claim 1 , the method comprising:
 subjecting a steel having the chemical composition to hot rough rolling, then performing finish rolling at a finishing temperature equal to or higher than an Ar 3  transformation temperature, and then performing cooling to 650° C. to 750° C. at an average cooling rate of 20° C./sec to 100° C./sec;   performing coiling at a coiling temperature of 500° C. to 700° C. to obtain a hot-rolled steel sheet; and   then heating the hot-rolled steel sheet in a temperature range from 450° C. to 600° C. at an average heating rate of 15° C./h or more and performing annealing that involves holding at an annealing temperature lower than an Ac 1  transformation temperature for 1.0 h or more.   
     
     
         10 . The method for manufacturing the high-carbon hot-rolled steel sheet according to  claim 9 , having a tensile strength of 480 MPa or less and a total elongation of 33% or more. 
     
     
         11 . The method for manufacturing the high-carbon hot-rolled steel sheet according to  claim 9 , wherein the ferrite has an average grain size of 4 to 25 μm. 
     
     
         12 . The method for manufacturing the high-carbon hot-rolled steel sheet according to  claim 9 , wherein the chemical composition further comprises, by mass %, one or two groups selected from Group A and Group B:
 Group A: Ti: 0.06% or less, and   Group B: one or two or more selected from Nb, Mo, Ta, Ni, Cu, V, and W each in an amount of 0.0005% or more and 0.1% or less.   
     
     
         13 . A method for manufacturing the high-carbon hot-rolled steel sheet according to  claim 1 , the method comprising:
 subjecting a steel having the chemical composition to hot rough rolling, then performing finish rolling at a finishing temperature equal to or higher than an Ar 3  transformation temperature, and then performing cooling to 650° C. to 750° C. at an average cooling rate of 20° C./sec to 100° C./sec;   performing coiling at a coiling temperature of 500° C. to 700° C. to obtain a hot-rolled steel sheet; and   then heating the hot-rolled steel sheet in a temperature range from 450° C. to 600° C. at an average heating rate of 15° C./h or more and performing annealing that involves holding at a temperature equal to or higher than an Ac 1  transformation temperature and equal to or lower than an Ac 3  transformation temperature for 0.5 h or more, followed by cooling to a temperature lower than an Ar 1  transformation temperature at an average cooling rate of 1° C./h to 20° C./h, and holding at a temperature lower than the Ar 1  transformation temperature for 20 h or more.   
     
     
         14 . The method for manufacturing the high-carbon hot-rolled steel sheet according to  claim 13 , having a tensile strength of 480 MPa or less and a total elongation of 33% or more. 
     
     
         15 . The method for manufacturing the high-carbon hot-rolled steel sheet according to  claim 13 , wherein the ferrite has an average grain size of 4 to 25 μm. 
     
     
         16 . The method for manufacturing the high-carbon hot-rolled steel sheet according to  claim 13 , wherein the chemical composition further comprises, by mass %, one or two groups selected from Group A and Group B:
 Group A: Ti: 0.06% or less, and   Group B: one or two or more selected from Nb, Mo, Ta, Ni, Cu, V, and W each in an amount of 0.0005% or more and 0.1% or less.