Steel sheet and method for producing same

Abstract

This steel sheet has a predetermined chemical composition, a tensile strength is 780 MPa or greater, a total of the area ratio of ferrite and an area ratio of bainite is 10% or greater and 90% or less, a total of an area ratio of martensite and an area ratio of tempered martensite is 10% or greater and 90% or less, and a total of an area ratio of pearlite and an area ratio of residual austenite is 0% or greater and 10% or less, a number proportion of crystal grains of ferrite and bainite having an area of 6 μm2 or less is 40% or greater to a total number of crystal grains of the ferrite and the bainite, and a number proportion of crystal grains of ferrite and bainite having an area of 50 μm2 or greater is 5% or less to the total number of crystal grains of the ferrite and the bainite, and a maximum Mn content in a region up to 0.5 μm from an interface between the ferrite and the martensite or the tempered martensite, in a direction perpendicular to the interface and toward an inside of the ferrite grains, is 0.30 mass % or more lower than an average Mn content of the steel sheet

1 . A steel sheet consisting of, as a chemical composition, by mass %:
 C: 0.07% to 0.15%;   Si: 0.01% to 2.00%;   Mn: 1.5% to 3.0%;   P: 0% to 0.020%;   S: 0% to 0.0200%;   Al: 0.001% to 1.000%;   N: 0% to 0.0200%;   O: 0% to 0.0200%;   Co: 0% to 0.500%;   Ni: 0% to 1.000%;   Cu: 0% to 0.500%;   Mo: 0% to 1.000%;   Cr: 0% to 2.000%;   Ti: 0% to 0.5000%;   Nb: 0% to 0.50%;   V: 0% to 0.500%;   W: 0% to 0.100%;   Ta: 0% to 0.100%;   B: 0% to 0.0100%;   Mg: 0% to 0.050%;   Ca: 0% to 0.050%;   Zr: 0% to 0.050%;   REM: 0% to 0.100%;   Sn: 0% to 0.050%;   Sb: 0% to 0.050%;   As: 0% to 0.050%; and   a remainder: Fe and impurities,   wherein a tensile strength is 780 MPa or greater,   in a microstructure,
 an area ratio of ferrite is 5% or greater, 
 a total of the area ratio of ferrite and an area ratio of bainite is 10% or greater and 90% or less, 
 a total of an area ratio of martensite and an area ratio of tempered martensite is 10% or greater and 90% or less, and 
 a total of an area ratio of pearlite and an area ratio of residual austenite is 0% or greater and 10% or less, 
   a number proportion of crystal grains of ferrite and bainite having an area of 6 μm 2  or less is 40% or greater to a total number of crystal grains of the ferrite and the bainite, and   a number proportion of crystal grains of ferrite and bainite having an area of 50 μm 2  or greater is 5% or less to the total number of crystal grains of the ferrite and the bainite, and   a maximum Mn content in a region up to 0.5 μm from an interface between the ferrite and the martensite or the tempered martensite, in a direction perpendicular to the interface and toward an inside of the ferrite grains, is 0.30 mass % or more lower than an average Mn content of the steel sheet.   
     
     
         2 . The steel sheet according to  claim 1 ,
 wherein an average aspect ratio of the crystal grains of the ferrite and the bainite having an area of 6 μm 2  or less is 1.0 or greater and 2.0 or less.   
     
     
         3 . The steel sheet according to  claim 1 ,
 wherein a coating layer containing zinc, aluminum, magnesium, or an alloy of these metals is provided on a surface.   
     
     
         4 . A method for producing a steel sheet, the method comprising:
 hot-rolling a slab to obtain a hot-rolled steel sheet, the slab including, as a chemical composition, by mass %, C: 0.07% to 0.15%, Si: 0.01% to 2.00%, Mn: 1.5% to 3.0%, P: 0% to 0.020%, S: 0% to 0.0200%, Al: 0.001% to 1.000%, N: 0% to 0.0200%, O: 0% to 0.0200%, Co: 0% to 0.500%, Ni: 0% to 1.000%, Cu: 0% to 0.500%, Mo: 0% to 1.000%, Cr: 0% to 2.000%, Ti: 0% to 0.5000%, Nb: 0% to 0.50%, V: 0% to 0.500%, W: 0% to 0.100%, Ta: 0% to 0.100%, B: 0% to 0.0100%, Mg: 0% to 0.050%, Ca: 0% to 0.050%, Zr: 0% to 0.050%, REM: 0% to 0.100%, Sn: 0% to 0.050%, Sb: 0% to 0.050%, As: 0% to 0.050%, and a remainder: Fe and impurities;   cooling the hot-rolled steel sheet to a coiling temperature of 650° C. or lower and 450° C. or higher at an average cooling rate of 30° C./sec or higher, and coiling the hot-rolled steel sheet at the coiling temperature;   holding the hot-rolled steel sheet after the coiling so that a holding time in a temperature range from the coiling temperature to the coiling temperature—50° C. is 2 to 8 hours;   cooling the hot-rolled steel sheet after the holding to a temperature of 300° C. or lower at an average cooling rate of 0.1° C./sec or higher;   cold-rolling the hot-rolled steel sheet after the cooling at a ratio of sheet thickness reduction of 20% to 80% to obtain a cold-rolled steel sheet; and   annealing the cold-rolled steel sheet by heating the cold-rolled steel sheet to an annealing temperature of 740° C. to 900° C. at an average temperature rising rate of 5° C./sec or higher and holding the cold-rolled steel sheet at the annealing temperature for 60 to 300 seconds,   wherein in the hot rolling,
 finish rolling is performed using a rolling mill having four or more stands, and in a case where an initial stand is defined as a first stand and a final stand is defined as an n-th stand, a ratio of sheet thickness reduction in each of stands ranging from an (n−3)-th stand to the n-th stand is set to 30% or greater, and a rolling temperature in the n-th stand is set to 900° C. or lower. 
   
     
     
         5 . The method for producing a steel sheet according to  claim 4 ,
 wherein in the annealing, a coating layer containing zinc, aluminum, magnesium, or an alloy of these metals is formed on a surface of the steel sheet.   
     
     
         6 . The steel sheet according to  claim 2 ,
 wherein a coating layer containing zinc, aluminum, magnesium, or an alloy of these metals is provided on a surface.