Steel sheet, member, and methods for producing same

Abstract

A steel sheet and a member with a TS of 1180 MPa or more, and a method for producing them, are disclosed. A base steel sheet has a specified chemical composition and microstructure in predetermined ranges, and in a V-VDA bending test performed to a maximum load point, in a V-bending ridge line portion and a VDA bending ridge line portion, the value obtained by dividing the number of voids at a boundary between a hard phase and a soft phase and the number of voids due to fracture of the hard phase among all voids by the total number of voids is 0.60 or less, in a V-bending flat portion and the VDA bending ridge line portion, the value obtained is 0.20 or less, and carbide has a mean free path of 0.20 μm or more.

1 - 12 . (canceled) 
     
     
         13 . A steel sheet comprising a base steel sheet, wherein the base steel sheet has a chemical composition containing, on a mass percent basis,
 C: 0.050% or more and 0.400% or less,   Si: more than 0.75% and 3.00% or less,   Mn: 2.00% or more and less than 3.50%,   P: 0.001% or more and 0.100% or less,   S: 0.0001% or more and 0.0200% or less,   Al: 0.010% or more and 2.000% or less, and   N: 0.0100% or less,   with the remainder being Fe and incidental impurities,   the base steel sheet has a steel microstructure in which   an area fraction of ferrite: 57.0% or less,   a total area fraction of bainitic ferrite and tempered martensite: 40.0% or more and 90.0% or less,   an area fraction of retained austenite: 3.0% or more and 10.0% or less,   an area fraction of fresh martensite: 10.0% or less, and   a value obtained by dividing an area fraction of tempered martensite by the total area fraction of bainitic ferrite and tempered martensite is 0.70 or more,   a V-VDA bending test is performed to a maximum load point,   in an overlap region of a V-bending ridge line portion and a VDA bending ridge line portion, a value obtained by dividing the number of voids in contact with a hard phase among all voids by the total number of voids is 0.60 or less,   in an overlap region of a V-bending flat portion and the VDA bending ridge line portion, a value obtained by dividing the number of voids in contact with the hard phase among all voids by the total number of voids is 0.20 or less,   carbide has a mean free path L M  of 0.20 μm or more as represented by the following formula (1), and   the steel sheet has a tensile strength of 1180 MPa or more,   
       
         
           
             
               
                 
                   
                     
                       L 
                       M 
                     
                     = 
                     
                       
                         ( 
                         
                           
                             d 
                             M 
                           
                           / 
                           2 
                         
                         ) 
                       
                       × 
                       
                         
                           ( 
                           
                             400 
                             ⁢ 
                             π 
                             / 
                             3 
                             ⁢ 
                             f 
                           
                           ) 
                         
                         
                           1 
                           / 
                           3 
                         
                       
                     
                   
                 
                 
                   
                     formula 
                     ⁢ 
                         
                     
                       ( 
                       1 
                       ) 
                     
                   
                 
               
             
           
         
         wherein L M  denotes the mean free path (μm) of carbide, d M  denotes an average equivalent circular diameter (μm) of carbide, n denotes a circumference ratio, and f denotes a volume fraction (%) of all carbide particles. 
       
     
     
         14 . The steel sheet according to  claim 13 , wherein the base steel sheet has a chemical composition further containing, on a mass percent basis, at least one selected from
 Nb: 0.200% or less,   TI: 0.200% or less,   V: 0.200% or less,   B: 0.0100% or less,   Cr: 1.000% or less,   Ni: 1.000% or less,   Mo: 1.000% or less,   Sb: 0.200% or less,   Sn: 0.200% or less,   Cu: 1.000% or less,   Ta: 0.100% or less,   W: 0.500% or less,   Mg: 0.0200% or less,   Zn: 0.0200% or less,   Co: 0.0200% or less,   Zr: 0.1000% or less,   Ca: 0.0200% or less,   Se: 0.0200% or less,   Te: 0.0200% or less,   Ge: 0.0200% or less,   As: 0.0500% or less,   Sr: 0.0200% or less,   Cs: 0.0200% or less,   Hf: 0.0200% or less,   Pb: 0.0200% or less,   Bi: 0.0200% or less, and   REM: 0.0200% or less.   
     
     
         15 . The steel sheet according to  claim 13 ,
 comprising one or two or more selected from the following (1) to (4):
 (1) comprising a galvanized layer as an outermost surface layer on one or both surfaces of the steel sheet, 
 (2) an average value σ C  of a standard deviation of a distance between a carbide particle A selected from all carbide particles in the steel sheet and a remaining carbide particle other than the carbide particle A is 7.50 μm or less, 
 (3) when a region of 200 μm or less from a surface of the base steel sheet in the thickness direction is defined as a surface layer, the base steel sheet has, in the surface layer, a surface soft layer with a Vickers hardness of 85% or less with respect to a Vickers hardness at a quarter thickness position, and
 when nanohardness is measured at 300 points or more in a 50 μm×50 μm region on a sheet surface at a quarter depth position in the thickness direction and at a half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet, 
 a ratio of a number of measurements with a nanohardness of 7.0 GPa or more on the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet to a total number of measurements at the quarter depth position in the thickness direction of the surface soft layer is 0.10 or less, 
 the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet has a standard deviation σ of 1.8 GPa or less, and 
 the nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet has a standard deviation σ of 2.2 GPa or less, and 
 
   (4) comprising a metal coated layer formed on the base steel sheet on one or both surfaces of the steel sheet.   
     
     
         16 . The steel sheet according to  claim 14 ,
 comprising one or two or more selected from the following (1) to (4):
 (1) comprising a galvanized layer as an outermost surface layer on one or both surfaces of the steel sheet, 
 (2) an average value σ C  of a standard deviation of a distance between a carbide particle A selected from all carbide particles in the steel sheet and a remaining carbide particle other than the carbide particle A is 7.50 μm or less, 
 (3) when a region of 200 μm or less from a surface of the base steel sheet in the thickness direction is defined as a surface layer, the base steel sheet has, in the surface layer, a surface soft layer with a Vickers hardness of 85% or less with respect to a Vickers hardness at a quarter thickness position, and
 when nanohardness is measured at 300 points or more in a 50 μm×50 μm region on a sheet surface at a quarter depth position in the thickness direction and at a half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet, 
 a ratio of a number of measurements with a nanohardness of 7.0 GPa or more on the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet to a total number of measurements at the quarter depth position in the thickness direction of the surface soft layer is 0.10 or less, 
 the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet has a standard deviation σ of 1.8 GPa or less, and 
 the nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet has a standard deviation σ of 2.2 GPa or less, and 
 
 (4) comprising a metal coated layer formed on the base steel sheet on one or both surfaces of the steel sheet. 
   
     
     
         17 . A member comprising the steel sheet according to  claim 13 . 
     
     
         18 . A member comprising the steel sheet according to  claim 14 . 
     
     
         19 . A member comprising the steel sheet according to  claim 15 . 
     
     
         20 . A member comprising the steel sheet according to  claim 16 . 
     
     
         21 . A method for producing a steel sheet, comprising:
 a hot rolling step of hot-rolling a steel slab with the chemical composition according to  claim 13  to produce a hot-rolled steel sheet;   a pickling step of pickling the hot-rolled steel sheet;   an annealing step of annealing the steel sheet after the pickling step at an annealing temperature of (Ac 1 + (Ac 3 −Ac 1 )×¾° C.) or more and 900° C. or less for an annealing time of 20 seconds or more;   a first cooling step of cooling the steel sheet after the annealing step to a first cooling stop temperature of 100° C. or more and 300° C. or less;   a holding step of holding the steel sheet after the first cooling step in a temperature range of 350° C. or more and 550° C. or less for 3 seconds or more and less than 80 seconds;   a second cooling step of cooling the steel sheet after the holding step to a second cooling stop temperature of 50° C. or less, during the cooling, applying a tension of 2.0 kgf/mm 2  or more to the steel sheet once or more in a temperature range of 300° C. or more and 450° C. or less,   then   subjecting the steel sheet to four or more passes, each pass involving contact with a roll with a diameter of 500 mm or more and 1500 mm or less for a quarter circumference of the roll, and   subjecting the steel sheet to two or more passes, each pass involving contact with a roll with a diameter of 500 mm or more and 1500 mm or less for half a circumference of the roll; and   optionally a cold rolling step of cold-rolling the steel sheet after the pickling step and before the annealing step to produce a cold-rolled steel sheet.   
     
     
         22 . A method for producing a steel sheet, comprising:
 a hot rolling step of hot-rolling a steel slab with the chemical composition according to  claim 14  to produce a hot-rolled steel sheet;   a pickling step of pickling the hot-rolled steel sheet;   an annealing step of annealing the steel sheet after the pickling step at an annealing temperature of (Ac 1 + (Ac 3 −Ac 1 )×¾° C.) or more and 900° C. or less for an annealing time of 20 seconds or more;   a first cooling step of cooling the steel sheet after the annealing step to a first cooling stop temperature of 100° C. or more and 300° C. or less;   a holding step of holding the steel sheet after the first cooling step in a temperature range of 350° C. or more and 550° C. or less for 3 seconds or more and less than 80 seconds;   a second cooling step of cooling the steel sheet after the holding step to a second cooling stop temperature of 50° C. or less, during the cooling, applying a tension of 2.0 kgf/mm 2  or more to the steel sheet once or more in a temperature range of 300° C. or more and 450° C. or less,   then   subjecting the steel sheet to four or more passes, each pass involving contact with a roll with a diameter of 500 mm or more and 1500 mm or less for a quarter circumference of the roll, and   subjecting the steel sheet to two or more passes, each pass involving contact with a roll with a diameter of 500 mm or more and 1500 mm or less for half a circumference of the roll; and   optionally a cold rolling step of cold-rolling the steel sheet after the pickling step and before the annealing step to produce a cold-rolled steel sheet.   
     
     
         23 . The method for producing a steel sheet according to  claim 21 , comprising the following one or two selected from the following (1) to (2):
 (1) comprising a galvanizing step of performing a galvanizing treatment on the steel sheet after the holding step and before the second cooling step to form a galvanized layer on the steel sheet, and   (2) comprising a metal coating step of performing metal coating on one or both surfaces of the steel sheet to form a metal coated layer after the pickling step and before the annealing step.   
     
     
         24 . The method for producing a steel sheet according to  claim 22 , comprising the following one or two selected from the following (1) to (2):
 (1) comprising a galvanizing step of performing a galvanizing treatment on the steel sheet after the holding step and before the second cooling step to form a galvanized layer on the steel sheet, and   (2) comprising a metal coating step of performing metal coating on one or both surfaces of the steel sheet to form a metal coated layer after the pickling step and before the annealing step.   
     
     
         25 . The method for producing a steel sheet according to  claim 21 , wherein the annealing in the annealing step is performed in an atmosphere with a dew point of −30° C. or more. 
     
     
         26 . The method for producing a steel sheet according to  claim 22 , wherein the annealing in the annealing step is performed in an atmosphere with a dew point of −30° C. or more. 
     
     
         27 . The method for producing a steel sheet according to  claim 23 , wherein the annealing in the annealing step is performed in an atmosphere with a dew point of −30° C. or more. 
     
     
         28 . The method for producing a steel sheet according to  claim 24 , wherein the annealing in the annealing step is performed in an atmosphere with a dew point of −30° C. or more. 
     
     
         29 . A method for producing a member, comprising a step of subjecting the steel sheet according to  claim 13  to at least one of forming and joining to produce a member. 
     
     
         30 . A method for producing a member, comprising a step of subjecting the steel sheet according to  claim 14  to at least one of forming and joining to produce a member. 
     
     
         31 . A method for producing a member, comprising a step of subjecting the steel sheet according to  claim 15  to at least one of forming and joining to produce a member. 
     
     
         32 . A method for producing a member, comprising a step of subjecting the steel sheet according to  claim 16  to at least one of forming and joining to produce a member.