High-strength cold-rolled steel sheet, high-strength hot-dip galvanized steel sheet, and high-strength hot-dip galvannealed steel sheet excellent in formability and shape fixability, and methods for manufacturing them

Abstract

A high-strength cold-rolled steel sheet has a specific chemical composition and has a steel microstructure meeting conditions: a total content of bainitic ferrite (BF) and tempered martensite (TM) is 65% (in area percent, hereinafter the same for steel microstructure) or more; a fresh martensite (M) content is 3% to 18%; a retained austenite content is 5% or more; and a polygonal ferrite (F) content is 5% or less. The steel sheet has a specific average KAM <1.00° of 0.50° or more and has a tensile strength of 980 MPa or more. The high-strength cold-rolled steel sheet excels in formability and shape fixability.

1 . A cold-rolled steel sheet, comprising, in mass percent, based on the chemical composition:
 C in a content of 0.1% to 0.3%;   Si in a content of 1.0% to 3.0%;   Mn in a content of 0.5% to 3.0%;   P in a content of 0.1% or less;   S in a content of 0.03% or less;   Al in a content of 0.01% to 1.0%; and   N in a content of 0.01% or less,   with a remainder being iron and inevitable impurities,   the steel sheet having a steel microstructure meeting conditions given as follows:
 a total content of bainitic ferrite (BF) and tempered martensite (TM) of 65% in area percent or more; 
 a content of fresh martensite (M) is from 3% to 18%; 
 a content of retained austenite is from 5% or more; and 
 a content of polygonal ferrite (F) is 5% or less, 
   the steel microstructure further meeting a condition given as follows:   an average KAM <1.00°  is 0.50° or more,   wherein the “average KAM <1.00° ” denotes is an average of kernel average misorientation (KAM) values (misorientation values, in “degree (°)”) in a region of less than 1.00° as measured at two or more points; and   the steel sheet having a tensile strength of 980 MPa or more.   
     
     
         2 . The steel sheet according to  claim 1 , further comprising at least one element selected from the group consisting of:
 Ti in a content of 0.01% to 0.1%;   Nb in a content of 0.01% to 0.1%; and   V in a content of 0.01% to 0.1%.   
     
     
         3 . The steel sheet according to  claim 1 , further comprising at least one element selected from the group consisting of:
 Cr in a content of 0.01% to 1%;   Mo in a content of 0.01% to 1%; and   B in a content of 0.0001% to 0.005%.   
     
     
         4 . The steel sheet according to  claim 1 , further comprising at least one element selected from the group consisting of:
 Cu in a content of 0.01% to 1%; and   Ni in a content of 0.01% to 1%.   
     
     
         5 . The steel sheet according to  claim 1 , further comprising at least one element selected from the group consisting of:
 Ca in a content of 0.0005% to 0.005%; and   Mg in a content of 0.0005% to 0.005%.   
     
     
         6 . A hot-dip galvanized steel sheet, comprising:
 the cold-rolled steel sheet of  claim 1 ; and   a hot-dip galvanized layer formed on or over a surface of the cold-rolled steel sheet.   
     
     
         7 . A hot-dip galvannealed steel sheet, comprising:
 the cold-rolled steel sheet of  claim 1 ; and   a hot-dip galvannealed layer formed on or over a surface of the cold-rolled steel sheet.   
     
     
         8 . A method for manufacturing the cold-rolled steel sheet of  claim 1 , the method comprising, in an order as follows:
 heating a cold-rolled steel sheet having the chemical composition to a temperature range (T1) of Ac 3  to 960° C.;   cooling the cold-rolled steel sheet from the temperature range (T1) down to 500° C. at an average cooling rate (CR1) of 5° C./s or more;   further cooling the cold-rolled steel sheet from 500° C. down to a temperature range (T2) of (Ms-200)° C. to 420° C. at an average cooling rate (CR2) of 10° C./s or more; and   holding the cold-rolled steel sheet in the temperature range (T2) for a time period (t2) of 10 to 70 seconds.   
     
     
         9 . A method for manufacturing the hot-dip galvanized steel sheet of  claim 6 , the method comprising, in an order as follows:
 heating a cold-rolled steel sheet having the chemical composition to a temperature range (T1) of Ac 3  to 960° C.;   cooling the cold-rolled steel sheet from the temperature range (T1) down to 500° C. at an average cooling rate (CR1) of 5° C./s or more;   further cooling the cold-rolled steel sheet from 500° C. down to a temperature range (T2) of (Ms-200)° C. to 420° C. at an average cooling rate (CR2) of 10° C./s or more;   holding the cold-rolled steel sheet in the temperature range (T2) for a time period (t2) of 10 to 70 seconds; and   immersing the cold-rolled steel sheet in a zinc bath.   
     
     
         10 . A method for manufacturing the hot-dip galvannealed steel sheet of  claim 7 , the method comprising, in an order as follows:
 heating a cold-rolled steel sheet having the chemical composition to a temperature range (T1) of Ac 3  to 960° C.;   cooling the cold-rolled steel sheet from the temperature range (T1) down to 500° C. at an average cooling rate (CR1) of 5° C./s or more;   further cooling the cold-rolled steel sheet from 500° C. down to a temperature range (T2) of (Ms-200)° C. to 420° C. at an average cooling rate (CR2) of 10° C./s or more;   holding the cold-rolled steel sheet in the temperature range (T2) for a time period (t2) of 10 to 70 seconds;   immersing the cold-rolled steel sheet in a zinc bath; and   subjecting the cold-rolled steel sheet to an alloying treatment at an alloying treatment temperature (T3) of 450° C. to 560° C.