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: 0.005-0.020%, N: >0.0030-0.0060%, 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%, Ca: 0-0.0100%, Mg: 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, wherein Ca+Mg+REM≥0.0005, 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: 0.005 to 0.020%,   N: more than 0.0030% to 0.0060% or less,   P: 0.050% or less,   S: 0.005% or less,   Ti: 0.015 to 0.170%,   O: 0.0010 to 0.0100%,   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%,   Ca: 0 to 0.0100%,   Mg: 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   the chemical composition satisfies a formula (i) below, and   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 ¼ W or ¾ W from an end face of the steel sheet and ¼ t or ¾ t 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:
   Ca+Mg+REM≥0.0005   (i)
 
   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.   
     
     
         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:
 after melting process, casting a slab having the chemical composition according to  claim 1 , and subjecting the slab 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 melting process, an Si content of molten steel is set to 0.05 to 0.20% and a dissolved oxygen concentration is set to 0.0020 to 0.0080%, then deoxidation treatment is performed,   in the deoxidation treatment, Ti and thereafter Al are added, followed by one or more selected from Ca, Mg, and REM,   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.:
   SRTmin=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 7 . 
     
     
         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: 0.005 to 0.020%,   N: more than 0.0030% to 0.0060% or less,   P: 0.050% or less,   S: 0.005% or less,   Ti: 0.015 to 0.170%,   O: 0.0010 to 0.0100%,   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%,   Ca: 0 to 0.0100%,   Mg: 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   the chemical composition satisfies a formula (i) below, and   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 ¼ W or ¾ W from an end face of the steel sheet and ¼ t or ¾ t 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:
   Ca+Mg+REM≥0.0005   (i)
 
   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.   
     
     
         12 . A method of producing the hot rolled steel sheet according to  claim 11 , comprising:
 after melting process, casting a slab having the chemical composition according to  claim 11 , and subjecting the slab 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 melting process, an Si content of molten steel is set to 0.05 to 0.20% and a dissolved oxygen concentration is set to 0.0020 to 0.0080%, then deoxidation treatment is performed,   in the deoxidation treatment, Ti and thereafter Al are added, followed by one or more selected from Ca, Mg, and REM,   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.:
   SRTmin=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.