High carbon thin steel sheet and method for producing same

Abstract

A steel sheet having a composition containing C: 0.20% to 0.50%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.02% or less, sol.Al: 0.08% or less, N: 0.02% or less, and Fe and incidental impurities, and a microstructure composed of ferrite and cementite, wherein each of the average grain size ds of the ferrite in the region from the surface of the steel sheet to the position at one-quarter of the sheet thickness and the average grain size dc of the ferrite in the region from the position at one-quarter of the sheet thickness of the steel sheet to the sheet thickness center is 20 to 40 μm, 0.80≦ds/dc≦1.20 is satisfied, the average grain size of the cementite is 1.0 μm or more, the spheroidizing ratio is 90% or more, and 90% or more of cementite is present inside ferrite grains.

1 . A mild high carbon steel sheet characterized by having a chemical composition containing C: 0.20% to 0.50%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.02% or less, sol.Al: 0.08% or less, N: 0.02% or less, and the remainder composed of Fe and incidental impurities, on a percent by mass basis, and a microstructure composed of ferrite and cementite, wherein each of the average grain size ds of the ferrite in the region from the surface of the steel sheet to the position at one-quarter of the sheet thickness and the average grain size dc of the ferrite in the region from the position at one-quarter of the sheet thickness of the steel sheet to the sheet thickness center is 20 to 40 μm, 0.80≦ds/dc≦1.20 is satisfied, the average grain size of the cementite is 1.0 μm or more, the spheroidizing ratio is 90% or more, and 90% or more of cementite, on a ratio of the number of grains basis, is present inside ferrite grains. 
     
     
         2 . The mild high carbon steel sheet according to  claim 1 , further comprising at least one selected from the group consisting of Cr: 0.1% to 1.5%, Mo: 0.1% to 0.5%, Ni: 0.1% to 1.0%, Ti: 0.01% to 0.03%, Nb: 0.01% to 0.03%, V: 0.01% to 0.03%, and B: 0.0005% to 0.0050% on a percent by mass basis. 
     
     
         3 . The mild high carbon steel sheet according to  claim 1 , wherein the Si content is 0.1% or more and 0.5% or less on a percent by mass basis. 
     
     
         4 . The mild high carbon steel sheet according to  claim 1 , wherein the Mn content is 0.2% or more and 1.0% or less on a percent by mass basis. 
     
     
         5 . The mild high carbon steel sheet according to  claim 1 , wherein the cementite has an average grain size of 1.0 μm or more and 3.0 μm or less. 
     
     
         6 . The mild high carbon steel sheet according to  claim 1 , wherein the spheroidizing ratio of the cementite is a ratio of the number of cementite grains having a cross-sectional aspect ratio, which is major axis/minor axis, of 3 or less to the number of all cementite grains. 
     
     
         7 . The mild high carbon steel sheet according to  claim 1 , having a Vickers hardness of 150 or less. 
     
     
         8 . A method for manufacturing a mild high carbon steel sheet comprising the steps of hot-rolling a steel slab having a chemical composition containing C: 0.20% to 0.50%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.02% or less, sol.Al: 0.08% or less, N: 0.02% or less, and the remainder composed of Fe and incidental impurities, on a percent by mass basis, at a finishing temperature higher than or equal to the Ar 3  transformation point, performing primary cooling to a primary cooling stop temperature of 550° C. to 650° C. at an average cooling rate of 25° C./sec to 50° C./sec, performing secondary cooling to a secondary cooling stop temperature of 500° C. to 600° C. at an average cooling rate of 120° C./sec or more for a cooling time of 1 s or less, performing coiling, removing scale on the steel sheet surface layer, performing cold rolling at a reduction ratio of 20% to 30%, and performing annealing by holding for 20 hours or more at an annealing temperature of 680° C. or higher and lower than the Ac 1  transformation point. 
     
     
         9 . The method for manufacturing a mild high carbon steel sheet according to  claim 8 , wherein the steel slab further contains at least one selected from the group consisting of Cr: 0.1% to 1.5%, Mo: 0.1% to 0.5%, Ni: 0.1% to 1.0%, Ti: 0.01% to 0.03%, Nb: 0.01% to 0.03%, V: 0.01% to 0.03%, and B: 0.0005% to 0.0050% on a percent by mass basis. 
     
     
         10 . The method for manufacturing a mild high carbon steel sheet according to  claim 8 , wherein the average cooling rate of the secondary cooling is 120° C./sec or more and 300° C./sec or less. 
     
     
         11 . The method for manufacturing a mild high carbon steel sheet according to  claim 8 , wherein the holding time of the annealing temperature is 30 hours or more and 50 hours or less.