Middle/high carbon steel sheet and method for manufacturing same

Abstract

A middle/high carbon steel sheet according to an aspect of the present invention is a steel sheet, in which composition thereof contains, by mass %, C: 0.10% to 1.50%, Si: 0.01% to 1.00%, Mn: 0.01% to 3.00%, P: 0.0001% to 0.1000%, and S: 0.0001% to 0.1000%, and a remainder consisting of Fe and impurities, in which the steel sheet has a structure in which a total volume percentage of a martensite, a bainite, a pearlite, and a residual austenite is equal to or lower than 5.0%, and a remainder thereof is a ferrite and carbides, in which a spheroidizing ratio of carbide particles is 70% to 99%, and in which a proportion of a number of the carbide particles including a crystal interface at which an orientation difference is equal to or greater than 5° in the carbide particles is equal to or lower than 20% of a total number of the carbide particles.

1 - 3 . (canceled) 
     
     
         4 . A middle/high carbon steel sheet, wherein
 composition thereof comprises, by mass %:   C: 0.10% to 1.50%,   Si: 0.01% to 1.00%,   Mn: 0.01% to 3.00%,   P: 0.0001% to 0.1000%,   S: 0.0001% to 0.1000%, and   a remainder comprising Fe and impurities,   wherein the steel sheet has a structure in which a total volume percentage of a martensite, a bainite, a pearlite, and a residual austenite is equal to or lower than 5.0%, and a remainder thereof includes a ferrite and carbides,   wherein a spheroidizing ratio of carbide particles is 70% to 99%, and   wherein a proportion of a number of the carbide particles including a crystal interface at which an orientation difference is equal to or greater than 5° in the carbide particles is equal to or lower than 20% of a total number of the carbide particles.   
     
     
         5 . The middle/high carbon steel sheet according to  claim 4 ,
 wherein the composition of the steel sheet further includes, by mass %, one or more selected from the group consisting of   Al: 0.001% to 0.500%,   N: 0.0001% to 0.0500%,   O: 0.0001% to 0.0500%,   Cr: 0.001% to 2.000%,   Mo: 0.001% to 2.000%,   Ni: 0.001% to 2.000%,   Cu: 0.001% to 1.000%,   Nb: 0.001% to 1.000%,   V: 0.001% to 1.000%,   Ti: 0.001% to 1.000%,   B: 0.0001% to 0.0500%,   W: 0.001% to 1.000%,   Ta: 0.001% to 1.000%,   Sn: 0.001% to 0.020%,   Sb: 0.001% to 0.020%,   As: 0.001% to 0.020%,   Mg: 0.0001% to 0.0200%,   Ca: 0.001% to 0.020%,   Y: 0.001% to 0.020%,   Zr: 0.001% to 0.020%,   La: 0.001% to 0.020%, and   Ce: 0.001% to 0.020%.   
     
     
         6 . A method for manufacturing a middle/high carbon steel sheet,
 wherein, when a billet having the composition according to  claim 4  is hot-rolled directly after continuously-casting or is temporary cooled, heated, and hot-rolled after the continuously-casting, finish hot rolling is completed in a temperature region of 600° C. to 1,000° C.,   the hot-rolled steel sheet is coiled at 350° C. to 700°,   the hot-rolled steel sheet is box-annealed,   the hot-rolled steel sheet is cold-rolled at a cold-rolling reduction ratio of 10% to 80%,   and then a cold-rolled sheet is annealed at an annealing temperature of 650° C. to 780° C. for a retention time at the annealing temperature of 30 to 1,800 seconds in a continuous annealing line.   
     
     
         7 . The middle/high carbon steel sheet according to  claim 4 ,
 wherein a grain size of the ferrite is 5 to 60 μm.   
     
     
         8 . The method for manufacturing a middle/high carbon steel sheet according to  claim 6 ,
 wherein, in a case in which the billet is temporary cooled and heated before hot-rolling, a heating temperature of the billet is 950 to 1250° C. and a heating time is 0.5 to 3 hours.   
     
     
         9 . The method for manufacturing a middle/high carbon steel sheet according to  claim 6 ,
 wherein the hot-rolled steel sheet after the finish hot rolling and before coiling is cooled at a cooling rate of 10 to 100° C./second.   
     
     
         10 . The method for manufacturing a middle/high carbon steel sheet according to  claim 6 ,
 wherein a box-annealing temperature is 670° C. to 770° C. and a retention time at the box-annealing temperature is 1 to 100 hours in box-annealing for the hot-rolled steel sheet.