Hot rolled steel sheet, steel forged part and production method therefor

Abstract

A hot rolled steel sheet having a chemical composition consisting of, in mass %, C: 0.020-0.070%, Si: 0.05-1.70%, Mn: 0.60-2.50%, Al: 0.010-1.000%, N: >0-0.0030%, P≤0.050%, S≤0.005%, Ti: 0.015-0.170%, Nb: 0-0.100%, V: 0-0.300%, Cu: 0-2.00%, Ni: 0-2.00%, Cr: 0-2.00%, Mo: 0-1.00%, B: 0-0.0100%, Mg: 0-0.0100%, Ca: 0-0.0100%, REM: 0-0.1000%, Zr: 0-1.000%, Co: 0-1.000%, Zn: 0-1.000%, W: 0-1.000%, Sn: 0-0.050%, the balance: Fe and impurities, a metal microstructure includes, in area %, ferrite: 5-70%, bainite: 30-95%, retained γ≤2%, martensite≤2%, pearlite≤1%, ferrite+bainite≥95%, a number density of the precipitates in ferrite grains is 1.0×1016−50.0×1016/cm3, an average circle-equivalent diameter of the TiN precipitates in the steel sheet is 1.0-10.0 μm, an average of minimum distances between adjacent TiN precipitates is 10.0 μm or more, and a standard deviation of nano hardness is 1.00 GPa or less.

1 . A hot rolled steel sheet having a chemical composition consisting of, in mass %,
 C: 0.020 to 0.070%,   Si: 0.05 to 1.70%,   Mn: 0.60 to 2.50%,   Al: 0.010 to 1.000%,   N: more than 0% to 0.0030% or less,   P: 0.050% or less,   S: 0.005% or less,   Ti: 0.015 to 0.170%,   Nb: 0 to 0.100%,   V: 0 to 0.300%,   Cu: 0 to 2.00%,   Ni: 0 to 2.00%,   Cr: 0 to 2.00%,   Mo: 0 to 1.00%,   B: 0 to 0.0100%,   Mg: 0 to 0.0100%,   Ca: 0 to 0.0100%,   REM: 0 to 0.1000%,   Zr: 0 to 1.000%,   Co: 0 to 1.000%,   Zn: 0 to 1.000%,   W: 0 to 1.000%,   Sn: 0 to 0.050%, and,   the balance: Fe and impurities, wherein   when a width and a thickness of the steel sheet in a cross section perpendicular to a rolling direction of the steel sheet are defined as W and t, respectively, a metal microstructure includes, in area %, at a position 1/4W or 3/4W from an end face of the steel sheet and 1/4t or 3/4t from a surface of the steel sheet,   ferrite: 5 to 70%,   bainite: 30 to 95%,   retained austenite: 2% or less,   martensite: 2% or less, and,   pearlite: 1% or less, and   a total of ferrite and bainite: 95% or more, and wherein the ferrite contains precipitates including Ti in grains of the ferrite,   a number density of the precipitates including Ti is 1.0×10 16  to 50.0×10 16 /cm 3 ,   the steel sheet includes TiN precipitates,   an average circle-equivalent diameter of the TiN precipitates is 1.0 to 10.0 μm,   an average of minimum distances between adjacent TiN precipitates is 10.0 μm or more, and   a standard deviation of nano hardness is 1.00 GPa or less.   
     
     
         2 . The hot rolled steel sheet according to  claim 1 , wherein
 an average circle-equivalent diameter of the precipitates including Ti is 1.00 to 3.00 nm.   
     
     
         3 . The hot rolled steel sheet according to  claim 1 , wherein
 a tensile strength is 780 MPa or more,   a product of a uniform elongation and a tensile strength is 7000 MPa·% or more, and a product of a hole expansion ratio and a tensile strength is 50000 MPa·% or more.   
     
     
         4 . A method of producing the hot rolled steel sheet according to  claim 1 , comprising:
 subjecting a slab having the chemical composition consisting of in mass %,   C: 0.020 to 0.070%,   Si: 0.05 to 1.70%,   Mn: 0.60 to 2.50%,   Al: 0.010 to 1.000%,   N: more than 0% to 0.0030% or less,   P: 0.050% or less,   S: 0.005% or less,   Ti: 0.015 to 0.170%,   Nb: 0 to 0.100%,   V: 0 to 0.300%,   Cu: 0 to 2.00%,   Ni: 0 to 2.00%,   Cr: 0 to 2.00%,   Mo: 0 to 1.00%,   B: 0 to 0.0100%,   Mg: 0 to 0.0100%,   Ca: 0 to 0.0100%,   REM: 0 to 0.1000%,   Zr: 0 to 1.000%,   Co: 0 to 1.000%,   Zn: 0 to 1.000%,   W: 0 to 1.000%,   Sn: 0 to 0.050%, and,   the balance: Fe and impurities, to a heating process, a continuous hot rolling process, a first cooling process, a second cooling process, and a coiling process, in this order, wherein   in the heating process, the slab is heated to a temperature of SRTmin° C. or more to 1260° C. or less, the SRTmin being represented by a formula (i) below,   the continuous hot rolling process includes rough rolling and multi-stand finish rolling of three stands or more,   an end temperature of the rough rolling is 1100° C. or more,   a cumulative strain of rolling at final three stands in the multi-stand finish rolling is 0.01 to 0.10,   a rolling end temperature of the multi-stand finish rolling is at a temperature of Ar 3 +30° C. or more, the Ar 3  being determined by a formula (ii) below,   in the first cooling process, cooling is started after 1.00 to 5.00 seconds after completion of the multi-stand finish rolling, the cooling is continued at an average cooling rate of 10° C./sec or more from the rolling end temperature down to a temperature range of 650 to 750° C., and thereafter the sheet is held in air for 1 to 10 seconds,   in the second cooling process, after the sheet is held in air, cooling is conducted at an average cooling rate of 10° C./sec or more from a temperature range of 600 to 740° C., and   in the coiling process, the sheet is coiled at a coiling temperature of 450 to 650° C.:
     SRT min=7000/{2.75−log(Ti×C)}−273   (i)
 
   Ar 3 =970−325×C+33×Si+287×P+40×Al−92×(Mn+Mo+Cu)−46×(Cr+Ni)   (ii)
 
   where a symbol of an element in the above formula represents a content (in mass %) of the element in the hot rolled steel sheet and is substituted by zero when the element is not contained.   
     
     
         5 . A steel forged part obtained from the hot rolled steel sheet according to  claim 1 . 
     
     
         6 . A method of producing a steel forged part, wherein the hot rolled steel sheet according to  claim 1  is subjected to at least forging. 
     
     
         7 . The hot rolled steel sheet according to  claim 2 , wherein
 a tensile strength is 780 MPa or more,   a product of a uniform elongation and a tensile strength is 7000 MPa·% or more, and   a product of a hole expansion ratio and a tensile strength is 50000 MPa·% or more.   
     
     
         8 . A steel forged part obtained from the hot rolled steel sheet according to  claim 2 . 
     
     
         9 . A steel forged part obtained from the hot rolled steel sheet according to  claim 3 . 
     
     
         10 . A steel forged part obtained from the hot rolled steel sheet according to claim 
     
     
         11 . A hot rolled steel sheet having a chemical composition comprising, in mass %,
 C: 0.020 to 0.070%,   Si: 0.05 to 1.70%,   Mn: 0.60 to 2.50%,   Al: 0.010 to 1.000%,   N: more than 0% to 0.0030% or less,   P: 0.050% or less,   S: 0.005% or less,   Ti: 0.015 to 0.170%,   Nb: 0 to 0.100%,   V: 0 to 0.300%,   Cu: 0 to 2.00%,   Ni: 0 to 2.00%,   Cr: 0 to 2.00%,   Mo: 0 to 1.00%,   B: 0 to 0.0100%,   Mg: 0 to 0.0100%,   Ca: 0 to 0.0100%,   REM: 0 to 0.1000%,   Zr: 0 to 1.000%,   Co: 0 to 1.000%,   Zn: 0 to 1.000%,   W: 0 to 1.000%,   Sn: 0 to 0.050%, and,   the balance: Fe and impurities, wherein   when a width and a thickness of the steel sheet in a cross section perpendicular to a rolling direction of the steel sheet are defined as W and t, respectively, a metal microstructure includes, in area %, at a position 1/4W or 3/4W from an end face of the steel sheet and 1/4t or 3/4t from a surface of the steel sheet,   ferrite: 5 to 70%,   bainite: 30 to 95%,   retained austenite: 2% or less,   martensite: 2% or less, and,   pearlite: 1% or less, and   a total of ferrite and bainite: 95% or more, and wherein   the ferrite contains precipitates including Ti in grains of the ferrite,   a number density of the precipitates including Ti is 1.0×10 16  to 50.0×10 16 /cm 3 ,   the steel sheet includes TiN precipitates,   an average circle-equivalent diameter of the TiN precipitates is 1.0 to 10.0 μm,   an average of minimum distances between adjacent TiN precipitates is 10.0 μm or more, and   a standard deviation of nano hardness is 1.00 GPa or less.   
     
     
         12 . A method of producing the hot rolled steel sheet according to  claim 11 , comprising:
 subjecting a slab having the chemical composition comprising in mass %,   C: 0.020 to 0.070%,   Si: 0.05 to 1.70%,   Mn: 0.60 to 2.50%,   Al: 0.010 to 1.000%,   N: more than 0% to 0.0030% or less,   P: 0.050% or less,   S: 0.005% or less,   Ti: 0.015 to 0.170%,   Nb: 0 to 0.100%,   V: 0 to 0.300%,   Cu: 0 to 2.00%,   Ni: 0 to 2.00%,   Cr: 0 to 2.00%,   Mo: 0 to 1.00%,   B: 0 to 0.0100%,   Mg: 0 to 0.0100%,   Ca: 0 to 0.0100%,   REM: 0 to 0.1000%,   Zr: 0 to 1.000%,   Co: 0 to 1.000%,   Zn: 0 to 1.000%,   W: 0 to 1.000%,   Sn: 0 to 0.050%, and,   the balance: Fe and impurities, to a heating process, a continuous hot rolling process, a first cooling process, a second cooling process, and a coiling process, in this order, wherein   in the heating process, the slab is heated to a temperature of SRTmin° C. or more to 1260° C. or less, the SRTmin being represented by a formula (i) below,   the continuous hot rolling process includes rough rolling and multi-stand finish rolling of three stands or more,   an end temperature of the rough rolling is 1100° C. or more,   a cumulative strain of rolling at final three stands in the multi-stand finish rolling is 0.01 to 0.10,   a rolling end temperature of the multi-stand finish rolling is at a temperature of Ar 3 +30° C. or more, the Ar 3  being determined by a formula (ii) below,   in the first cooling process, cooling is started after 1.00 to 5.00 seconds after completion of the multi-stand finish rolling, the cooling is continued at an average cooling rate of 10° C./sec or more from the rolling end temperature down to a temperature range of 650 to 750° C., and thereafter the sheet is held in air for 1 to 10 seconds,   in the second cooling process, after the sheet is held in air, cooling is conducted at an average cooling rate of 10° C./sec or more from a temperature range of 600 to 740° C., and   in the coiling process, the sheet is coiled at a coiling temperature of 450 to 650° C.:
     SRT min=7000/{2.75−log(Ti×C)}−273   (i)
 
   Ar 3 =970−325×C+33×Si+287×P+40×Al−92×(Mn+Mo+Cu)−46×(Cr+Ni)   (ii)
 
   where a symbol of an element in the above formula represents a content (in mass %) of the element in the hot rolled steel sheet and is substituted by zero when the element is not contained.