High strength cold rolled steel sheet and high strength galvanized steel sheet having excellent ductility and bendability, and methods for producing same

Abstract

Provided is a high strength cold rolled steel sheet having a tensile strength of 980 MPa or higher and excellent ductility and bendability. The high strength cold rolled steel sheet has a specified component composition. The structures at a position of ¼ sheet thickness in the steel sheet has certain area ratio of ferrite relative to the entire structure, certain area ratio of a mixed structure of fresh martensite and retained austenite relative to the entire structure, certain region in which the concentration of Mn is enriched, certain standard deviation of the fraction of a region in which the concentration of Mn is enriched, and certain concentration of Mn in a ferrite phase.

1 . A high strength cold rolled steel sheet having a tensile strength of 980 MPa or higher and excellent ductility and bendability,
 the steel sheet satisfying a component composition comprising in percent by mass:   C: 0.10% to 0,30%,   Si: 1.2% to 3%;   Mn: 0.5% to 3.0%;   P: more than 0% to 0.1% or less;   S: more than 0% to 0.05% or less;   Al: 0.005% to 0.2%;   N: more than 0% to 0.01% or less; and   O: more than 0% to 0.01% or less,   the balance being iron and inevitable impurities, and structures at a position of ¼ sheet thickness in the steel sheet satisfying all of (1) to (5) below:   (1) the area ratio of ferrite relative to the entire structure is from 5% or more to less than 50%, and the balance is a hard phase, when observed under a scanning electron microscope;   (2) the area ratio of a mixed structure of fresh martensite and retained austenite relative to the entire structure is more than 0% to 30% or less, when observed under an optical microscope after LePera etching;   (3) a region in which the concentration of Mn is enriched to 1.2 times or more the concentration of Mn in the steel sheet is present at 5 area% or more, when analyzed with an electron probe microanalyzer;   (4) when the fraction of a region in which the concentration of Mn is enriched to 1.2 times or more the concentration of Mn in the steel sheet is measured in 2-μm square sections, a standard deviation in measurements for 100 sections is 4.0% or more; and   (5) the concentration of Mn in a ferrite phase is 0.90 times or less the concentration of Mn in the steel sheet, when analyzed with an electron probe microanalyzer.   
     
     
         2 . The high strength cold rolled steel sheet according to  claim 1 , wherein the volume ratio of retained austenite relative to the entire structure is 5% or more, when measured by an X-ray diffraction method. 
     
     
         3 . The high strength cold rolled steel sheet according to  claim 1 , wherein the hard phase comprises a mixed structure of fresh martensite and retained austenite, and at least one structure selected from the group consisting of bainitic ferrite, bainite, and tempered martensite. 
     
     
         4 . The high strength cold rolled steel sheet according to  claim 1 , wherein the component composition further comprises one or more of (A) to (E) below, in percent by mass, as another element:
 (A) at least one selected from the group consisting of Cr: more than 0% to 1% or less and Mo: more than 0% to 1% or less;   (B) at least one selected from the group consisting of   Ti: more than 0% to 0,15% or less,   Nb: more than 0% to 0.15% or less, and   V: more than 0% to 0.15% or less;   (C) at least one selected from the group consisting of Cu: more than 0% to 1% or less and Ni: more than 0% to 1% or less;   (D) B: more than 0% to 0.005% or less; and   (E) at least one selected from the group consisting of Ca: more than 0% to 0.01% or less, Mg: more than 0% to 0.01% or less, and REM: more than 0% to 0,01% or less.   
     
     
         5 . A high strength electrogalvanized steel sheet in which an electrogalvanized layer is formed on the surface of the high strength cold rolled steel sheet according to  claim 1 . 
     
     
         6 . A high strength galvanized steel sheet in which a galvanized layer is formed on the surface of the high strength cold rolled steel sheet according to  claim 1 . 
     
     
         7 . A high strength galvannealed steel sheet in which a galvannealed layer is formed on the surface of the high strength cold rolled steel sheet according to  claim 1 . 
     
     
         8 . A method for producing a high strength cold rolled steel sheet having a tensile strength of 980 MPa or higher and excellent ductility and bendability, the steel sheet being the high strength cold rolled steel sheet according to  claim 1 , comprising:
 in hot rolling a steel sheet having the component composition,   coiling at a coiling temperature of 500° C. to 800° C., then holding for 3 hours or more at 500° C. to 800° C., then cooling to room temperature, soaking and holding in a temperature range from (Ac 1  point+20° C.) to less than Ac 3  point after cold rolling, then cooling to a temperature range of 500° C. or less at an average cooling rate of 10° C./s or more to 500° C. and at an average cooling rate of 10° C./s or more at 500° C. or less,   then reheating to a temperature range of 250° C. to 500° C., holding for 30 seconds or more, and cooling to room temperature.   
     
     
         9 . A method for producing a high strength electrogalvanized steel sheet, wherein the high strength cold rolled steel sheet obtained by the producing method according to  claim 8  is fluffier subjected to electrogalvanizing. 
     
     
         10 . A method for producing a high strength galvanized steel sheet having a tensile strength of 980 MPa or higher and excellent ductility and bendability, the steel sheet being the high strength galvanized steel sheet according to  claim 6 .
 comprising:   in hot rolling a steel sheet having the component composition,   coiling at a coiling temperature of 500° C. to 800° C., then holding for 3 hours or more at 500° C. to 800° C., then cooling to room temperature, soaking and holding in a temperature range from (Ac 1  point+20° C.) to less than Ac 3  point after cold rolling, then cooling to a temperature range of 500° C. or less at an average cooling rate of 10° C./s or more to 500° C. and at an average cooling rate of 10° C./s or more at 500° C. or less,   then reheating to a temperature range of 250° C. to 500° C., holding for 30 seconds or more, galvanizing within the holding period, and then cooling to room temperature.   
     
     
         11 . The method for producing a high strength galvanized steel sheet according to  claim 10 , wherein alloying in a temperature range from 450° C. to 550° C. is performed after the galvanization. 
     
     
         12 . A steel sheet comprising percent by mass:
 C: 0.10% to 0.30%,   Si: 1.2% to 3%;   Mn: 0.5% to 3.0%;   P: more than 0% to 0.1% or less;   S: more than 0% to 0.05% or less;   Al: 0.005% to 0.2%;   N: more than 0% to 0.01% or less; and   O: more than 0% to 0.01% or less,   the balance being iron and inevitable impurities;   wherein structures at a position of ¼ sheet thickness in the steel sheet satisfy (1) to (5) below:   (1) an area ratio of ferrite relative to the entire structure is from 5% or more to less than 50%, and the balance is a hard phase, when observed under a scanning electron microscope;   (2) an area ratio of a mixed structure of fresh martensite and retained austenite relative to the entire structure is more than 0% to 30% or less, when observed under an optical microscope after LePera etching;   (3) a region in which the concentration of Mn is enriched to 1.2 times or more the concentration of Mn in the steel sheet is present at 5 area% or more, when analyzed with an electron probe microanalyzer;   (4) when the fraction of a region in which the concentration of Mn is enriched to 1.2 times or more the concentration of Mn in the steel sheet is measured in 2-m square sections, a standard deviation in measurements for 100 sections is 4.0% or more; and   (5) a concentration of Mn in a ferrite phase is 0.90 times or less the concentration of Mn in the steel sheet, when analyzed with an electron probe microanalyzer.