Carbon steel sheet having excellent carburization properties, and method for producing same

Abstract

The invention provides a carbon steel sheet including C: 0.20% to 0.45% by mass, Si: 0.05% to 0.8% by mass, Mn: 0.85% to 2.0% by mass, P: 0.001% to 0.04% by mass, S: 0.0001% to 0.006% by mass, Al: 0.01% to 0.1% by mass, Ti: 0.005% to 0.3% by mass, B: 0.0005% to 0.01% by mass and N: 0.001% to 0.01% by mass, in which a K value that can be obtained from 3C+Mn+0.5Si is greater than or equal to 2.0; surface hardness is less than or equal to 77 on the Rockwell B Scale; and the average content of N in a zone from the surface to a depth of 100 μm is less than or equal to 100 ppm. This carbon steel sheet is configured to be carburized in a carburization atmosphere with a carbon potential of 0.6 or less.

1 . A carbon steel sheet configured to be carburized in a carburization atmosphere with a carbon potential of 0.6 or less, comprising:
 C: 0.20% to 0.45% by mass,   Si: 0.05% to 0.8% by mass,   Mn: 0.85% to 2.0% by mass,   P: 0.001% to 0.04% by mass,   S: 0.0001% to 0.006% by mass,   Al: 0.01% to 0.1% by mass,   Ti: 0.005% to 0.3% by mass,   B: 0.0005% to 0.01% by mass and   N: 0.001% to 0.01% by mass   with a balance including Fe and inevitable impurities,   wherein K value that can be obtained from 3C+Mn+0.5Si is greater than or equal to 2.0;   surface hardness is less than or equal to 77 on Rockwell B Scale; and   an average content of N in a zone from a surface to a depth of 100 μm is less than or equal to 100 ppm.   
     
     
         2 . The carbon steel sheet according to  claim 1 , further comprising one or more components selected from
 Nb: 0.01% to 0.5% by mass,   V: 0.01% to 0.5% by mass,   Ta: 0.01% to 0.5% by mass,   W: 0.01% to 0.5% by mass,   Sn: 0.003% to 0.03% by mass,   Sb: 0.003% to 0.03% by mass, and   As: 0.003% to 0.03% by mass.   
     
     
         3 . A method for producing the carbon steel sheet according to  claim 1 , comprising:
 heating a slab to less than or equal to 1200° C.;   hot-rolling the slab at a final rolling temperature of 800° C. to 940° C. so as to obtain a steel sheet;   cooling the steel sheet at a cooling rate of 20° C./second or more until a temperature of the steel sheet becomes less than or equal to 650° C., as a first cooling;   cooling the steel sheet at a cooling rate of 20° C./second or less, as a second cooling subsequent to the first cooling;   coiling the steel sheet at a temperature of 400° C. to 650° C.;   pickling the steel sheet; and   annealing the steel sheet for 10 hours or more at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C., as a first annealing.   
     
     
         4 . The method for producing the carbon steel sheet according to  claim 3 , wherein the first annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the first annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing. 
     
     
         5 . The method for producing the carbon steel sheet according to  claim 4 , further comprising:
 cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the pickling, as a first cold-rolling.   
     
     
         6 . The method for producing the carbon steel sheet according to  claim 5 , further comprising:
 cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and   annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.   
     
     
         7 . The method for producing the carbon steel sheet according to  claim 6 , wherein
 the second annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.   
     
     
         8 . The method for producing the carbon steel sheet according to  claim 7 , further comprising:
 cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and   annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.   
     
     
         9 . The method for producing the carbon steel sheet according to  claim 8 , wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing. 
     
     
         10 . The method for producing the carbon steel sheet according to  claim 6 , further comprising:
 cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and   annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.   
     
     
         11 . The method for producing the carbon steel sheet according to  claim 10 , wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing. 
     
     
         12 . The method for producing the carbon steel sheet according to  claim 4 , further comprising:
 cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and   annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.   
     
     
         13 . The method for producing the carbon steel sheet according to  claim 12 , wherein the second annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing. 
     
     
         14 . The method for producing the carbon steel sheet according to  claim 13 , further comprising:
 cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and   annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.   
     
     
         15 . The method for producing the carbon steel sheet according to  claim 14 , wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing. 
     
     
         16 . The method for producing the carbon steel sheet according to  claim 12 , further comprising:
 cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and   annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.   
     
     
         17 . The method for producing the carbon steel sheet according to  claim 16 , wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing. 
     
     
         18 . The method for producing the carbon steel sheet according to  claim 3 , further comprising:
 cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the pickling, as a first cold-rolling.   
     
     
         19 . The method for producing the carbon steel sheet according to  claim 18 , further comprising:
 cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and   annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.   
     
     
         20 . The method for producing the carbon steel sheet according to  claim 19 , wherein the second annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing. 
     
     
         21 . The method for producing the carbon steel sheet according to  claim 20 , further comprising:
 cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and   annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.   
     
     
         22 . The method for producing the carbon steel sheet according to  claim 21 , wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing. 
     
     
         23 . The method for producing the carbon steel sheet according to  claim 19 , further comprising:
 cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and   annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.   
     
     
         24 . The method for producing the carbon steel sheet according to  claim 23 , wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing. 
     
     
         25 . The method for producing the carbon steel sheet according to  claim 3 , further comprising:
 cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and   annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.   
     
     
         26 . The method for producing the carbon steel sheet according to  claim 25 , wherein the second annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing. 
     
     
         27 . The method for producing the carbon steel sheet according to  claim 26 , further comprising:
 cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and   annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.   
     
     
         28 . The method for producing the carbon steel sheet according to  claim 27 , wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing. 
     
     
         29 . The method for producing the carbon steel sheet according to  claim 25 , further comprising:
 cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and   annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.   
     
     
         30 . The method for producing the carbon steel sheet according to  claim 29 , wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing. 
     
     
         31 . A carbon steel sheet configured to be carburized in a carburization atmosphere with a carbon potential of 0.6 or less, comprising
 C: 0.20% to 0.45% by mass,   Si: 0.05% to 0.8% by mass,   Mn: 0.85% to 2.0% by mass,   P: 0.001% to 0.04% by mass,   S: 0.0001% to 0.006% by mass,   Al: 0.01% to 0.1% by mass,   Ti: 0.005% to 0.3% by mass,   B: 0.0005% to 0.01% by mass and   N: 0.001% to 0.01% by mass, and further comprising one or more components selected from:   Cr: 0.01% to 2.0% by mass,   Ni: 0.01% to 1.0% by mass,   Cu: 0.005% to 0.5% by mass and   Mo: 0.01% to 1.0% by mass   with a balance including Fe and inevitable impurities,   wherein K′ value that can be obtained from 3C+Mn+0.5Si+Cr+Ni+Mo+Cu is greater than or equal to 2.0;   surface hardness is less than or equal to 77 on Rockwell B Scale; and   an average content of N in a zone from a surface to a depth of 100 μM is less than or equal to 100 ppm.   
     
     
         32 . The carbon steel sheet according to  claim 31 , further comprising one or more components selected from:
 Nb: 0.01% to 0.5% by mass,   V: 0.01% to 0.5% by mass,   Ta: 0.01% to 0.5% by mass,   W: 0.01% to 0.5% by mass,   Sn: 0.003% to 0.03% by mass,   Sb: 0.003% to 0.03% by mass, and   As: 0.003% to 0.03% by mass.   
     
     
         33 . The method for producing the carbon steel sheet according to  claim 31 , comprising:
 heating a slab to less than or equal to 1200° C.;   hot-rolling the slab at a final rolling temperature of 800° C. to 940° C. so as to obtain a steel sheet;   cooling the steel sheet at a cooling rate of 20° C./second or more until a temperature of the steel sheet becomes less than or equal to 650° C., as a first cooling;   cooling the steel sheet at a cooling rate of less than or equal to 20° C./second, as a second cooling subsequent to the first cooling;   coiling the steel sheet at a temperature of 400° C. to 650° C.;   pickling the steel sheet; and   annealing the steel sheet for more than or equal to 10 hours at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C., as a first annealing.