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. In a steel microstructure of the base steel sheet, bainitic ferrite, tempered martensite, retained austenite, and fresh martensite are in specified ranges. The concentration of carbon in retained austenite and the density of carbides in tempered martensite are in specified ranges. The amount of diffusible hydrogen is 0.50 ppm by mass or less. A V-VDA bending test is performed to the maximum load point. In an L cross section, a crack has a length of 400 μm or less. In a specified region of VDA bending, a change in the grain size of bainitic ferrite in the thickness direction due to processing is 5.0 or less.

1 - 11 . (canceled) 
     
     
         12 . 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 bainitic ferrite: 3.0% or more and 20.0% or less,   an area fraction of tempered martensite: 40.0% or more and 90.0% or less,   an area fraction of retained austenite: more than 3.0% and 15.0% or less,   a concentration of carbon in retained austenite: 0.60% by mass or more and 1.30% by mass or less,   an area fraction of fresh martensite: 10.0% or less, and   a density of carbides in tempered martensite: 8.0 particles/μm 2  or less,   an amount of diffusible hydrogen in the base steel sheet is 0.50 ppm by mass or less,   a V-VDA bending test is performed to a maximum load point,   in an L cross section,   a crack has a length of 400 μm or less,   in a region formed from each position on a starting line present from a starting point of a bending peak on an outside of a VDA bend to a position of 50 μm in a thickness direction up to a position of 50 μm on each side of the starting line perpendicular to the starting line,   with respect to an average grain size of bainitic ferrite in the thickness direction, a ratio of the average grain size before processing to the average grain size after the processing is 5.0 or less, and   the steel sheet has a tensile strength of 1180 MPa or more.   
     
     
         13 . The steel sheet according to  claim 12 , 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.   
     
     
         14 . The steel sheet according to  claim 12 ,
 comprising one or two or more selected from the following (1) to (3):   (1) comprising a galvanized layer as an outermost surface layer on one or both surfaces of the steel sheet,   (2) 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 
   (3) comprising a metal coated layer formed on the base steel sheet on one or both surfaces of the steel sheet.   
     
     
         15 . The steel sheet according to  claim 13 ,
 comprising one or two or more selected from the following (1) to (3):   (1) comprising a galvanized layer as an outermost surface layer on one or both surfaces of the steel sheet,   (2) 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 
   (3) comprising a metal coated layer formed on the base steel sheet on one or both surfaces of the steel sheet.   
     
     
         16 . A member comprising the steel sheet according to  claim 12 . 
     
     
         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 method for producing a steel sheet, comprising:
 a hot rolling step of hot-rolling a steel slab with the chemical composition according to  claim 12  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 +0.4×(Ac 3 −Ac 1 ))° 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 400° C. or more and 600° C. or less;   a second cooling step of cooling the steel sheet after the first cooling step to a second cooling stop temperature of 100° C. or more and 300° C. or less at an average cooling rate of 25.0° C./s 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;   a reheating step of performing a reheat treatment of heating the steel sheet after the second cooling step to a tempering temperature range of more than 300° C. and 500° C. or less, holding the steel sheet in the temperature range for a tempering time of 20 seconds or more and 900 seconds or less, and setting a carbide control parameter CP represented by the following formula (1) to 10,000 or more and 15,000 or less during the reheat treatment; 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,   
       
         
           
             
               
                 
                   
                     CP 
                     = 
                     
                       
                         ( 
                         
                           T 
                           + 
                           273 
                         
                         ) 
                       
                       × 
                       
                         ( 
                         
                           k 
                           + 
                           
                             1.2 
                             × 
                             log 
                             ⁢ 
                             t 
                           
                         
                         ) 
                       
                     
                   
                 
                 
                   
                     formula 
                     ⁢ 
                         
                     
                       ( 
                       1 
                       ) 
                     
                   
                 
               
             
           
         
         wherein T denotes the tempering temperature (° C.), k denotes a material constant depending on a C content, and t denotes the tempering time (second), 
       
       
         
           
             
               
                 k 
                 = 
                 
                   
                     
                       - 
                       6 
                     
                     × 
                     
                       C 
                       M 
                     
                   
                   + 
                   17.8 
                 
               
               , 
             
           
         
         wherein C M  denotes a carbon concentration (% by mass) of martensite formed in the second cooling step. 
       
     
     
         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 +0.4×(Ac 3 −Ac 1 ))° 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 400° C. or more and 600° C. or less;   a second cooling step of cooling the steel sheet after the first cooling step to a second cooling stop temperature of 100° C. or more and 300° C. or less at an average cooling rate of 25.0° C./s 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;   a reheating step of performing a reheat treatment of heating the steel sheet after the second cooling step to a tempering temperature range of more than 300° C. and 500° C. or less, holding the steel sheet in the temperature range for a tempering time of 20 seconds or more and 900 seconds or less, and setting a carbide control parameter CP represented by the following formula (1) to 10,000 or more and 15,000 or less during the reheat treatment; 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,   
       
         
           
             
               
                 
                   
                     CP 
                     = 
                     
                       
                         ( 
                         
                           T 
                           + 
                           273 
                         
                         ) 
                       
                       × 
                       
                         ( 
                         
                           k 
                           + 
                           
                             1.2 
                             × 
                             log 
                             ⁢ 
                             t 
                           
                         
                         ) 
                       
                     
                   
                 
                 
                   
                     formula 
                     ⁢ 
                         
                     
                       ( 
                       1 
                       ) 
                     
                   
                 
               
             
           
         
         wherein T denotes the tempering temperature (° C.), k denotes a material constant depending on a C content, and t denotes the tempering time (second), 
       
       
         
           
             
               
                 k 
                 = 
                 
                   
                     
                       - 
                       6 
                     
                     × 
                     
                       C 
                       M 
                     
                   
                   + 
                   17.8 
                 
               
               , 
             
           
         
         wherein C M  denotes a carbon concentration (% by mass) of martensite formed in the second cooling step. 
       
     
     
         22 . The method for producing a steel sheet according to  claim 20 , 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 to form a galvanized layer on the steel sheet after the first cooling step and before the second cooling step, 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.   
     
     
         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 to form a galvanized layer on the steel sheet after the first cooling step and before the second cooling step, 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 20 , wherein the annealing in the annealing step is performed in an atmosphere with a dew point of −30° C. or more. 
     
     
         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 . A method for producing a member, comprising a step of subjecting the steel sheet according to  claim 12  to at least one of forming and joining to produce a member. 
     
     
         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.