Steel sheet, member, and methods for producing same

Abstract

A base steel sheet is disclosed having a specified chemical composition and a steel microstructure at a quarter thickness position containing specified ranges of ferrite, fresh martensite, retained austenite, bainite, tempered bainite, and tempered martensite, the value obtained by dividing the total area fraction of isolated island-like fresh martensite and isolated island-like retained austenite by the sum of the area fraction of fresh martensite and the volume fraction of retained austenite in a ferrite grain is 0.65 or more, the isolated island-like fresh martensite and the isolated island-like retained austenite in the ferrite grain has an average grain size of 2.0 μm or less, and the amount of diffusible hydrogen in the base steel sheet is 0.50 ppm by mass 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: 0.030% or more and 0.250% or less,   Si: 0.01% or more and 0.75% or less,   Mn: 2.00% or more and less than 3.50%,   P: 0.001% or more and 0.100% or less,   S: 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,   and has a steel microstructure,   as a microstructure at a quarter thickness position of the base steel sheet, in which   an area fraction of ferrite: 20.0% or more and 80.0% or less,   an area fraction of fresh martensite: 15.0% or less,   an area fraction of retained austenite: 3.0% or less,   a value obtained by dividing a total area fraction of island-like fresh martensite and island-like retained austenite in a ferrite grain by a sum of an area fraction of fresh martensite and an area fraction of retained austenite in the entire steel sheet: 0.65 or more,   an area fraction of bainite and tempered bainite: 10.0% or less,   an area fraction of tempered martensite: 10.0% or more and 70.0% or less, and   the island-like fresh martensite and the island-like retained austenite in the ferrite grain has an average grain size of 2.0 μm or less, and   an amount of diffusible hydrogen in the base steel sheet is 0.50 ppm by mass or less, and the steel sheet has a tensile strength of 780 MPa or more.   
     
     
         13 . The steel sheet according to  claim 12 , wherein the chemical composition further contains, on a mass percent basis, at least one element 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  under a condition of a finish rolling temperature of 820° C. or more to produce a hot-rolled steel sheet;   a heating step of heating the steel sheet after the hot rolling step in a temperature range of 350° C. or more and 600° C. or less at an average heating rate of 7° C./s or more;   an annealing step of annealing under conditions of an annealing temperature: 750° C. or more and 900° C. or less and an annealing time: 20 seconds or more;   after the annealing step, a first cooling step of cooling under conditions of an average cooling rate of 7° C./s or more from (the annealing temperature −30° C.) to 650° C. and an average cooling rate of 14° C./s or less from 650° C. to 500° C.;   after the first cooling step, a second cooling step of applying a tension of 2.0 kgf/mm 2  or more to the steel sheet in a temperature range of 300° C. or more and 450° C. or less, then subjecting the steel sheet to five 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 then cooling the steel sheet to a cooling stop temperature of 250° C. or less;   a reheating step of reheating the steel sheet to a temperature range of the cooling stop temperature or more and 440° C. or less and holding the steel sheet for 20 seconds or more after the second cooling step; and   optionally a cold rolling step of cold-rolling the steel sheet after the hot rolling step and before the heating step at a rolling reduction of 20% or more and 80% or less to produce a cold-rolled steel sheet.   
     
     
         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  under a condition of a finish rolling temperature of 820° C. or more to produce a hot-rolled steel sheet;   a heating step of heating the steel sheet after the hot rolling step in a temperature range of 350° C. or more and 600° C. or less at an average heating rate of 7° C./s or more;   an annealing step of annealing under conditions of an annealing temperature: 750° C. or more and 900° C. or less and an annealing time: 20 seconds or more;   after the annealing step, a first cooling step of cooling under conditions of an average cooling rate of 7° C./s or more from (the annealing temperature −30° C.) to 650° C. and an average cooling rate of 14° C./s or less from 650° C. to 500° C.;   after the first cooling step, a second cooling step of applying a tension of 2.0 kgf/mm 2  or more to the steel sheet in a temperature range of 300° C. or more and 450° C. or less, then subjecting the steel sheet to five 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 then cooling the steel sheet to a cooling stop temperature of 250° C. or less;   a reheating step of reheating the steel sheet to a temperature range of the cooling stop temperature or more and 440° C. or less and holding the steel sheet for 20 seconds or more after the second cooling step; and   optionally a cold rolling step of cold-rolling the steel sheet after the hot rolling step and before the heating step at a rolling reduction of 20% or more and 80% or less to produce a cold-rolled steel sheet.   
     
     
         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 hot rolling 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 hot rolling 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.