High strength member, method for manufacturing high strength member, and method for manufacturing steel sheet for high strength member

Abstract

A high strength member according to the present invention is the high strength member having a bending ridge line portion formed from a steel sheet, the member having a tensile strength of 1470 MPa or higher, a residual stress of 300 MPa or lower in an end surface of the bending ridge line portion, and a Vickers hardness (HV) of 200 or higher and 450 or lower in the end surface of the bending ridge line portion.

What is claimed: 
     
         1 . A method for manufacturing a high strength member, the method comprising
 a bending work process of cutting a steel sheet having a tensile strength of 1470 MPa or higher and performing bending work on the cut steel sheet and   an end surface treatment process of heating an end surface, which has been formed by cutting, at a temperature of 400° C. or higher and 900° C. or lower for more than 0 seconds and 10 seconds or less after the bending work process.   
     
     
         2 . The method according to  claim 1 , wherein the steel sheet has a chemical composition containing, by mass %,
 C: 0.17% or more and 0.35% or less,   Si: 0.001% or more and 1.2% or less,   Mn: 0.9% or more and 3.2% or less,   P: 0.020% or less,   S: 0.0010% or less,   Al: 0.010% or more and 0.20% or less,   N: 0.010% or less, and a balance of Fe and incidental impurities, and   a microstructure, in which a total area fraction of one or both of bainite containing carbides having an average grain size of 50 nm or less and martensite containing carbides having an average grain size of 50 nm or less is 90% or more.   
     
     
         3 . The method according to  claim 1 , wherein the steel sheet has a chemical composition containing, by mass %,
 C: 0.17% or more and 0.35% or less,   Si: 0.001% or more and 1.2% or less,   Mn: 0.9% or more and 3.2% or less,   P: 0.020% or less,   S: 0.0010% or less,   Al: 0.010% or more and 0.20% or less,   N: 0.010% or less,   Sb: 0.001% or more and 0.10% or less, and a balance of Fe and incidental impurities, and   a microstructure, in which a total area fraction of one or both of bainite containing carbides having an average grain size of 50 nm or less and martensite containing carbides having an average grain size of 50 nm or less is 90% or more.   
     
     
         4 . The method according to  claim 2 , wherein the steel sheet has the chemical composition further containing at least one selected from the groups of, by mass %,
 group A:
 B: 0.0002% or more and less than 0.0035%, 
   group B:
 at least one selected from
 Nb: 0.002% or more and 0.08% or less, and 
 Ti: 0.002% or more and 0.12% or less, 
 
   group C:
 at least one selected from
 Cu: 0.005% or more and 18 or less, and 
 Ni: 0.005% or more and 1% or less, 
 
   group D:
 at least one selected from
 Cr: 0.01% or more and 1.0% or less, 
 Mo: 0.01% or more and less than 0.3%, 
 V: 0.003% or more and 0.5% or less, 
 Zr: 0.005% or more and 0.20% or less, and 
 W: 0.005% or more and 0.20% or less, 
 
   group E:
 at least one selected from
 Ca: 0.0002% or more and 0.0030% or less, 
 Ce: 0.0002% or more and 0.0030% or less, 
 La: 0.0002% or more and 0.0030% or less, and 
 Mg: 0.0002% or more and 0.0030% or less, and 
 
   group F:
 Sn: 0.002% or more and 0.1% or less. 
   
     
     
         5 . The method according to  claim 3 , wherein the steel sheet has the chemical composition further containing at least one selected from the groups of, by mass %,
 group A:
 B: 0.0002% or more and less than 0.0035%, 
   group B:
 at least one selected from
 Nb: 0.002% or more and 0.08% or less, and 
 Ti: 0.002% or more and 0.12% or less, 
 
   group C:
 at least one selected from
 Cu: 0.005% or more and 18 or less, and 
 Ni: 0.005% or more and 18 or less, 
 
   group D:
 at least one selected from
 Cr: 0.01% or more and 1.08 or less, 
 Mo: 0.01% or more and less than 0.3%, 
 V: 0.003% or more and 0.5% or less, 
 Zr: 0.005% or more and 0.20% or less, and 
 W: 0.005% or more and 0.20% or less, 
 
   group E:
 at least one selected from
 Ca: 0.0002% or more and 0.0030% or less, 
 Ce: 0.0002% or more and 0.0030% or less, 
 La: 0.0002% or more and 0.0030% or less, and 
 Mg: 0.0002% or more and 0.0030% or less, and 
 
   group F:
 Sn: 0.002% or more and 0.1% or less. 
   
     
     
         6 . A method for manufacturing a high strength member, the method comprising
 an end surface treatment process of cutting a steel sheet having a tensile strength of 1470 MPa or higher and heating an end surface, which has been formed by cutting, at a temperature of 400° C. or higher and 900° C. or lower for more than 0 seconds and 10 seconds or less and   a bending work process of performing bending work on the steel sheet, which has been subjected to the end surface treatment process.   
     
     
         7 . The method according to  claim 6 , wherein the steel sheet has a chemical composition containing, by mass %,
 C: 0.17% or more and 0.35% or less,   Si: 0.001% or more and 1.2% or less,   Mn: 0.9% or more and 3.2% or less,   P: 0.020% or less,   S: 0.0010% or less,   Al: 0.010% or more and 0.20% or less,   N: 0.010% or less, and a balance of Fe and incidental impurities, and   a microstructure, in which a total area fraction of one or both of bainite containing carbides having an average grain size of 50 nm or less and martensite containing carbides having an average grain size of 50 nm or less is 90% or more.   
     
     
         8 . The method according to  claim 6 , wherein the steel sheet has a chemical composition containing, by mass %,
 C: 0.17% or more and 0.35% or less,   Si: 0.001% or more and 1.2% or less,   Mn: 0.9% or more and 3.2% or less,   P: 0.020% or less,   S: 0.0010% or less,   Al: 0.010% or more and 0.20% or less,   N: 0.010% or less,   Sb: 0.001% or more and 0.10% or less, and a balance of Fe and incidental impurities, and   a microstructure, in which a total area fraction of one or both of bainite containing carbides having an average grain size of 50 nm or less and martensite containing carbides having an average grain size of 50 nm or less is 90% or more.   
     
     
         9 . The method according to  claim 7 , wherein the steel sheet has the chemical composition further containing at least one selected from the groups of, by mass %,
 group A:
 B: 0.0002% or more and less than 0.0035%, 
   group B:
 at least one selected from
 Nb: 0.002% or more and 0.08% or less, and 
 Ti: 0.002% or more and 0.12% or less, 
 
   group C:
 at least one selected from
 Cu: 0.005% or more and 18 or less, and 
 Ni: 0.005% or more and 1% or less, 
 
   group D:
 at least one selected from
 Cr: 0.01% or more and 1.0% or less, 
 Mo: 0.01% or more and less than 0.3%, 
 V: 0.003% or more and 0.5% or less, 
 Zr: 0.005% or more and 0.20% or less, and 
 W: 0.005% or more and 0.20% or less, 
 
   group E:
 at least one selected from
 Ca: 0.0002% or more and 0.0030% or less, 
 Ce: 0.0002% or more and 0.0030% or less, 
 La: 0.0002% or more and 0.0030% or less, and 
 Mg: 0.0002% or more and 0.0030% or less, and 
 
   group F:
 Sn: 0.0028 or more and 0.1% or less. 
   
     
     
         10 . The method according to  claim 8 , wherein the steel sheet has the chemical composition further containing at least one selected from the groups of, by mass %,
 group A:
 B: 0.0002% or more and less than 0.0035%, 
   group B:
 at least one selected from
 Nb: 0.002% or more and 0.08% or less, and 
 Ti: 0.002% or more and 0.12% or less, 
 
   group C:
 at least one selected from
 Cu: 0.005% or more and 18 or less, and 
 Ni: 0.005% or more and 18 or less, 
 
   group D:
 at least one selected from
 Cr: 0.01% or more and 1.0% or less, 
 Mo: 0.01% or more and less than 0.3%, 
 V: 0.003% or more and 0.5% or less, 
 Zr: 0.005% or more and 0.20% or less, and 
 W: 0.005% or more and 0.20% or less, 
 
   group E:
 at least one selected from
 Ca: 0.0002% or more and 0.0030% or less, 
 Ce: 0.0002% or more and 0.0030% or less, 
 La: 0.0002% or more and 0.0030% or less, and 
 Mg: 0.0002% or more and 0.0030% or less, and 
 
   group F:
 Sn: 0.002% or more and 0.1% or less. 
   
     
     
         11 . A method for manufacturing a steel sheet for the high strength member manufactured by using the method for manufacturing a high strength member according to  claim 1 , the method for manufacturing a steel sheet comprising
 a hot rolling process of performing hot rolling on a steel material,   a cold rolling process of performing cold rolling on a hot rolled steel sheet, which has been obtained by performing the hot rolling process, and   an annealing process of heating a cold rolled steel sheet, which has been obtained by performing the cold rolling process, to an annealing temperature equal to or higher than an A c3  temperature, cooling the heated steel sheet to a cooling stop temperature of 350° C. or lower at an average cooling rate of 3° C./sec or higher in a temperature range from the annealing temperature to a temperature of 550° C., and holding the cooled steel sheet in a temperature range of 100° C. or higher and 260° C. or lower for 20 seconds or more and 1500 seconds or less.   
     
     
         12 . A method for manufacturing a steel sheet for the high strength member manufactured by using the method for manufacturing a high strength member according to  claim 2 , the method for manufacturing a steel sheet comprising
 a hot rolling process of performing hot rolling on a steel material,   a cold rolling process of performing cold rolling on a hot rolled steel sheet, which has been obtained by performing the hot rolling process, and   an annealing process of heating a cold rolled steel sheet, which has been obtained by performing the cold rolling process, to an annealing temperature equal to or higher than an A c3  temperature, cooling the heated steel sheet to a cooling stop temperature of 350° C. or lower at an average cooling rate of 3° C./sec or higher in a temperature range from the annealing temperature to a temperature of 550° C., and holding the cooled steel sheet in a temperature range of 100° C. or higher and 260° C. or lower for 20 seconds or more and 1500 seconds or less.   
     
     
         13 . A method for manufacturing a steel sheet for the high strength member manufactured by using the method for manufacturing a high strength member according to  claim 3 , the method for manufacturing a steel sheet comprising
 a hot rolling process of performing hot rolling on a steel material,   a cold rolling process of performing cold rolling on a hot rolled steel sheet, which has been obtained by performing the hot rolling process, and   an annealing process of heating a cold rolled steel sheet, which has been obtained by performing the cold rolling process, to an annealing temperature equal to or higher than an A c3  temperature, cooling the heated steel sheet to a cooling stop temperature of 350° C. or lower at an average cooling rate of 3° C./sec or higher in a temperature range from the annealing temperature to a temperature of 550° C., and holding the cooled steel sheet in a temperature range of 100° C. or higher and 260° C. or lower for 20 seconds or more and 1500 seconds or less.   
     
     
         14 . A method for manufacturing a steel sheet for the high strength member manufactured by using the method for manufacturing a high strength member according to  claim 4 , the method for manufacturing a steel sheet comprising
 a hot rolling process of performing hot rolling on a steel material,   a cold rolling process of performing cold rolling on a hot rolled steel sheet, which has been obtained by performing the hot rolling process, and   an annealing process of heating a cold rolled steel sheet, which has been obtained by performing the cold rolling process, to an annealing temperature equal to or higher than an A c3  temperature, cooling the heated steel sheet to a cooling stop temperature of 350° C. or lower at an average cooling rate of 3° C./sec or higher in a temperature range from the annealing temperature to a temperature of 550° C., and holding the cooled steel sheet in a temperature range of 100° C. or higher and 260° C. or lower for 20 seconds or more and 1500 seconds or less.   
     
     
         15 . A method for manufacturing a steel sheet for the high strength member manufactured by using the method for manufacturing a high strength member according to  claim 5 , the method for manufacturing a steel sheet comprising
 a hot rolling process of performing hot rolling on a steel material,   a cold rolling process of performing cold rolling on a hot rolled steel sheet, which has been obtained by performing the hot rolling process, and   an annealing process of heating a cold rolled steel sheet, which has been obtained by performing the cold rolling process, to an annealing temperature equal to or higher than an A c3  temperature, cooling the heated steel sheet to a cooling stop temperature of 350° C. or lower at an average cooling rate of 3° C./sec or higher in a temperature range from the annealing temperature to a temperature of 550° C., and holding the cooled steel sheet in a temperature range of 100° C. or higher and 260° C. or lower for 20 seconds or more and 1500 seconds or less.   
     
     
         16 . A method for manufacturing a steel sheet for the high strength member manufactured by using the method for manufacturing a high strength member according to  claim 6 , the method for manufacturing a steel sheet comprising
 a hot rolling process of performing hot rolling on a steel material,   a cold rolling process of performing cold rolling on a hot rolled steel sheet, which has been obtained by performing the hot rolling process, and   an annealing process of heating a cold rolled steel sheet, which has been obtained by performing the cold rolling process, to an annealing temperature equal to or higher than an A c3  temperature, cooling the heated steel sheet to a cooling stop temperature of 350° C. or lower at an average cooling rate of 3° C./sec or higher in a temperature range from the annealing temperature to a temperature of 550° C., and holding the cooled steel sheet in a temperature range of 100° C. or higher and 260° C. or lower for 20 seconds or more and 1500 seconds or less.   
     
     
         17 . A method for manufacturing a steel sheet for the high strength member manufactured by using the method for manufacturing a high strength member according to  claim 7 , the method for manufacturing a steel sheet comprising
 a hot rolling process of performing hot rolling on a steel material,   a cold rolling process of performing cold rolling on a hot rolled steel sheet, which has been obtained by performing the hot rolling process, and   an annealing process of heating a cold rolled steel sheet, which has been obtained by performing the cold rolling process, to an annealing temperature equal to or higher than an A c3  temperature, cooling the heated steel sheet to a cooling stop temperature of 350° C. or lower at an average cooling rate of 3° C./sec or higher in a temperature range from the annealing temperature to a temperature of 550° C., and holding the cooled steel sheet in a temperature range of 100° C. or higher and 260° C. or lower for 20 seconds or more and 1500 seconds or less.   
     
     
         18 . A method for manufacturing a steel sheet for the high strength member manufactured by using the method for manufacturing a high strength member according to  claim 8 , the method for manufacturing a steel sheet comprising
 a hot rolling process of performing hot rolling on a steel material,   a cold rolling process of performing cold rolling on a hot rolled steel sheet, which has been obtained by performing the hot rolling process, and   an annealing process of heating a cold rolled steel sheet, which has been obtained by performing the cold rolling process, to an annealing temperature equal to or higher than an A c3  temperature, cooling the heated steel sheet to a cooling stop temperature of 350° C. or lower at an average cooling rate of 3° C./sec or higher in a temperature range from the annealing temperature to a temperature of 550° C., and holding the cooled steel sheet in a temperature range of 100° C. or higher and 260° C. or lower for 20 seconds or more and 1500 seconds or less.   
     
     
         19 . A method for manufacturing a steel sheet for the high strength member manufactured by using the method for manufacturing a high strength member according to  claim 9 , the method for manufacturing a steel sheet comprising
 a hot rolling process of performing hot rolling on a steel material,   a cold rolling process of performing cold rolling on a hot rolled steel sheet, which has been obtained by performing the hot rolling process, and   an annealing process of heating a cold rolled steel sheet, which has been obtained by performing the cold rolling process, to an annealing temperature equal to or higher than an A c3  temperature, cooling the heated steel sheet to a cooling stop temperature of 350° C. or lower at an average cooling rate of 3° C./sec or higher in a temperature range from the annealing temperature to a temperature of 550° C., and holding the cooled steel sheet in a temperature range of 100° C. or higher and 260° C. or lower for 20 seconds or more and 1500 seconds or less.   
     
     
         20 . A method for manufacturing a steel sheet for the high strength member manufactured by using the method for manufacturing a high strength member according to  claim 10 , the method for manufacturing a steel sheet comprising
 a hot rolling process of performing hot rolling on a steel material,   a cold rolling process of performing cold rolling on a hot rolled steel sheet, which has been obtained by performing the hot rolling process, and   an annealing process of heating a cold rolled steel sheet, which has been obtained by performing the cold rolling process, to an annealing temperature equal to or higher than an A c3  temperature, cooling the heated steel sheet to a cooling stop temperature of 350° C. or lower at an average cooling rate of 3° C./sec or higher in a temperature range from the annealing temperature to a temperature of 550° C., and holding the cooled steel sheet in a temperature range of 100° C. or higher and 260° C. or lower for 20 seconds or more and 1500 seconds or less.