Cold-rolled steel sheet and manufacturing method thereof

Abstract

This cold-rolled steel sheet has a predetermined chemical composition, in which, in a case where a range of ⅛ to ⅜ of a sheet thickness from a surface in a sheet thickness direction is defined as a t/4 portion and a range of 20 μm from the surface in the sheet thickness direction is defined as a surface layer portion, a microstructure at the t/4 portion includes, by volume percentage, 0% or more and 10.0% or less of retained austenite and 90.0% or more and 100% or less of one or two of martensite and tempered martensite, a ratio of a dislocation density of the surface layer portion to a dislocation density of the t/4 portion is 0.80 or more, a ratio of a hardness of the surface layer portion to a hardness of the t/4 portion is 0.90 or more, and a tensile strength of the cold-rolled steel sheet is 1,310 MPa or more.

What is claimed is: 
     
         1 . A cold-rolled steel sheet comprising, as a chemical composition, by mass %:
 C: 0.150% to 0.500%;   Si: 0.01% to 2.00%;   Mn: 0.50% to 3.00%;   P: 0.0200% or less;   S: 0.0200% or less;   Al: 0.100% or less;   N: 0.0200% or less;   O: 0.020% or less;   Ni: 0% to 1.000%;   Mo: 0% to 1.000%;   Cr: 0% to 2.000%;   B: 0% to 0.010%;   As: 0% to 0.050%;   Co: 0% to 0.500%;   Ti: 0% to 0.500%;   Nb: 0% to 0.500%;   V: 0% to 0.500%;   Cu: 0% to 0.500%;   W: 0% to 0.100%;   Ta: 0% to 0.100%;   Ca: 0% to 0.050%;   Mg: 0% to 0.050%;   La: 0% to 0.050%;   Ce: 0% to 0.050%;   Y: 0% to 0.050%;   Zr: 0% to 0.050%;   Sb: 0% to 0.050%;   Sn: 0% to 0.050%; and   a remainder: Fe and impurities,   wherein, in a case where a range of ⅛ to ⅜ of a sheet thickness from a surface in a sheet thickness direction is defined as a t/4 portion and a range of 20 μm from the surface in the sheet thickness direction is defined as a surface layer portion,   a microstructure at the t/4 portion includes, by volume percentage, 0% or more and 10.0% or less of retained austenite and 90.0% or more and 100% or less of one or two of martensite and tempered martensite,   a ratio of a dislocation density of the surface layer portion to a dislocation density of the t/4 portion is 0.80 or more,   a ratio of a hardness of the surface layer portion to a hardness of the t/4 portion is 0.90 or more, and   a tensile strength of the cold-rolled steel sheet is 1,310 MPa or more.   
     
     
         2 . The cold-rolled steel sheet according to  claim 1 ,
 wherein a coating layer made of zinc, aluminum, or magnesium, or an alloy of one or more of these metals is provided on the surface.   
     
     
         3 . A manufacturing method of a cold-rolled steel sheet, comprising:
 a hot rolling process of performing hot rolling on a slab having the chemical composition according to  claim 1  to obtain a hot-rolled steel sheet, and coiling the hot-rolled steel sheet in a state in which a temperature of a center portion in a width direction is higher than 600° C. and 700° C. or lower and a temperature of an edge portion at a position 20 mm away from an end portion in the width direction is 600° C. or lower;   a cold rolling process of pickling the hot-rolled steel sheet after the hot rolling process and performing cold rolling on the hot-rolled steel sheet at a rolling reduction of 30% to 90% to obtain a cold-rolled steel sheet;   an annealing process of heating the cold-rolled steel sheet to an annealing temperature of higher than Ac3° C., holding the cold-rolled steel sheet at the annealing temperature, and cooling the cold-rolled steel sheet after the holding to a cooling stop temperature so that an average cooling rate to 400° C. is 10° C./seC or faster and an average cooling rate from 400° C. to the cooling stop temperature of 100° C. or lower is 15° C./seC or faster;   a heat treatment process of heating the cold-rolled steel sheet after the annealing process to a temperature range of 200° C. to 350° C. and holding the cold-rolled steel sheet in the temperature range; and   a skin pass rolling process of performing skin pass rolling on the cold-rolled steel sheet after the heat treatment process at a rolling reduction of 0.1% or more,   wherein a difference between a sheet thickness of the center portion in the width direction and a sheet thickness of the edge portion of the cold-rolled steel sheet after the cold rolling process is 10 μm or more.   
     
     
         4 . The manufacturing method of a cold-rolled steel sheet according to  claim 3 ,
 wherein, in the annealing process, a coating layer made of zinc, aluminum, or magnesium, or an alloy of one or more of these metals is formed on front and rear surfaces of the cold-rolled steel sheet.