Galvanized steel sheet and member, and method for manufacturing same

Abstract

Provided is a galvanized steel sheet having a TS of 980 MPa or more, high YS, excellent ductility, strain hardenability, and hole expansion formability. A base steel sheet has a defined chemical composition and a steel microstructure as follows: area ratio of ferrite: 65.0% or less (including 0%), area ratio of bainitic ferrite: 5.0% or more and 40.0% or less, area ratio of tempered martensite: 0.5% or more and 80.0% or less, area ratio of retained austenite: 3.0% or more, area ratio of fresh martensite: 20.0% or less (including 0%), S BF +S TM +2×S MA : 65.0% or more, S MA1 /S MA : 0.80 or less, and S MA2 /S MA : 0.20 or more.

1 . A galvanized steel sheet comprising a base steel sheet and a galvanized layer on a surface of the base steel sheet, wherein
 the base steel sheet comprises:   a chemical composition containing, in mass %,   C: 0.050% or more and 0.400% or less,   Si: 0.20% or more and 3.00% or less,   Mn: 1.00% 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,   N: 0.0100% or less,   optionally at least one selected from the group consisting of   Ti: 0.200% or less,   Nb: 0.200% or less,   V: 0.100% or less,   B: 0.0100% or less,   Cu: 1.000% or less,   Cr: 1.000% or less,   Ni: 1.000% or less,   Mo: 0.500% or less,   Sb: 0.200% or less,   Sn: 0.200% 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: 0.0200% or less,   Ca: 0.0200% or less,   Ce: 0.0200% or less,   Se: 0.0200% or less,   Te: 0.0200% or less,   Ge: 0.0200% or less,   As: 0.0200% 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,   where carbon equivalent Ceq is 0.540% or more, and the balance is Fe and inevitable impurity,   further, the base steel sheet has a microstructure wherein   area ratio of ferrite: 65.0% or less (including 0%),   area ratio of bainitic ferrite: 5.0% or more and 40.0% or less,   area ratio of tempered martensite: 0.5% or more and 80.0% or less,   area ratio of retained austenite: 3.0% or more,   area ratio of fresh martensite: 20.0% or less (including 0%),   S BF +S TM +2×S MA : 65.0% or more,   S MA1 /S MA : 0.80 or less, and   S MA2 /S MA : 0.20 or more, and   tensile strength is 980 MPa or more,   wherein   S BF  is area ratio of the bainitic ferrite,   S TM  is area ratio of the tempered martensite,   S MA  is area ratio of a hard secondary phase consisting of the retained austenite and fresh martensite,   S MA1  is a total area ratio of island regions among island regions constituting the hard secondary phase that have an equivalent circular diameter of 2.0 μm or more where 20% or less of the circumference is in contact with tempered martensite, and   S MA2  is a total area ratio of island regions among island regions constituting the hard secondary phase where 1% or more of the circumference is in contact with bainitic ferrite.   
     
     
         2 . (canceled) 
     
     
         3 . The galvanized steel sheet of  claim 1 , wherein S MA3 /S MA  is 0.05 or more in the microstructure of the base steel sheet,
 wherein   S MA3  is a total area ratio of island regions among island regions constituting the hard secondary phase where 1% or more of the circumference is in contact with bainitic ferrite and more than 20% of the circumference is in contact with tempered martensite.   
     
     
         4 . The galvanized steel sheet of  claim 1 , wherein the galvanized steel sheet satisfies at least one selected from following (a), (b), (c), (d) and (e),
 (a) an amount of diffusible hydrogen in the base steel sheet being 0.50 mass ppm or less,   (b) the galvanized steel sheet further comprising a decarburized layer,   (c) the galvanized steel sheet further comprising a metal coating or plating layer at least on one side between the base steel sheet and the galvanized layer,   (d) the galvanized steel sheet further comprising a metal coating or plating layer at least on one side between the base steel sheet and the galvanized layer, the metal coating or plating layer being an Fe-based coating or plating layer, and   (e) the galvanized layer being a hot-dip galvanized layer or a galvannealed layer.   
     
     
         5 - 8 . (canceled) 
     
     
         9 . A member made using the galvanized steel sheet of  claim 1 . 
     
     
         10 . A method of producing a galvanized steel sheet,
 the method comprising:   a hot rolling process, in which a steel slab having the chemical composition of  claim 1  is hot rolled to make a hot-rolled steel sheet;   a cold rolling process, in which the hot-rolled steel sheet is cold rolled into a cold-rolled steel sheet;   an annealing process, in which the cold-rolled steel sheet is annealed at an annealing temperature of 760° C. or more and 900° C. or less for an annealing time of 20 s or more;   a first cooling process, in which the cold-rolled steel sheet is cooled to a first cooling stop temperature of 300° C. or more and 550° C. or less;   a holding process, in which the cold-rolled steel sheet is held at a temperature range of 300° C. or more and 550° C. or less for 3 s or more and 600 s or less;   a coating process, in which the cold-rolled steel sheet is subject to a galvanizing treatment to become the galvanized steel sheet;   a second cooling process, in which the galvanized steel sheet is cooled to a second cooling stop temperature of 100° C. or more and less than 300° C.; and   a reheating process, in which the galvanized steel sheet is reheated to a reheating temperature of (the second cooling stop temperature+50° C.) or more and 500° C. or less, and the galvanized steel sheet is held at a temperature range of (the second cooling stop temperature+50° C.) or more and 500° C. or less for 10 s or more and 2,000 s or less, wherein   a relationship between the first cooling stop temperature and a temperature of a galvanizing bath in the galvanizing treatment satisfies the following Formula (1),
   −150° C.  T   0   −T   1 ≤50° C.  (1)
 
   where T 0  is the first cooling stop temperature (° C.) and T 1  is the temperature of the galvanizing bath in the galvanizing treatment (° C.).   
     
     
         11 . The method of producing a galvanized steel sheet of  claim 10 , wherein the method satisfies at least one selected from following (f), (g), (h) and (i),
 (f) a dew point of the annealing process being more than −30° C.,   (g) the method further comprising a metal coating or plating treatment process after the cold rolling process and before the annealing process, in which a metal coating or plating layer is formed on at least one surface of the cold-rolled steel sheet,   (h) the method further comprising a metal coating or plating treatment process after the cold rolling process and before the annealing process, in which a metal coating or plating layer is formed on at least one surface of the cold-rolled steel sheet and the metal coating or plating layer is an Fe-based coating or plating layer, and   (i) the galvanizing treatment being a hot-dip galvanizing treatment or a hot-dip galvannealing treatment.   
     
     
         12 - 14 . (canceled) 
     
     
         15 . A method of producing a member, wherein the galvanized steel sheet of  claim 1  is subjected to at least one of a forming process and a joining process to make the member. 
     
     
         16 . The galvanized steel sheet of  claim 3 , wherein the galvanized steel sheet satisfies at least one selected from following (a), (b), (c), (d) and (e),
 (a) an amount of diffusible hydrogen in the base steel sheet being 0.50 mass ppm or less,   (b) the galvanized steel sheet further comprising a decarburized layer,   (c) the galvanized steel sheet further comprising a metal coating or plating layer at least on one side between the base steel sheet and the galvanized layer,   (d) the galvanized steel sheet further comprising a metal coating or plating layer at least on one side between the base steel sheet and the galvanized layer, the metal coating or plating layer being an Fe-based coating or plating layer, and   (e) the galvanized layer being a hot-dip galvanized layer or a galvannealed layer.   
     
     
         17 . A member made using the galvanized steel sheet of  claim 3 . 
     
     
         18 . A member made using the galvanized steel sheet of  claim 4 . 
     
     
         19 . A member made using the galvanized steel sheet of  claim 16 . 
     
     
         20 . A method of producing a member, wherein the galvanized steel sheet of  claim 3  is subjected to at least one of a forming process and a joining process to make the member. 
     
     
         21 . A method of producing a member, wherein the galvanized steel sheet of  claim 4  is subjected to at least one of a forming process and a joining process to make the member. 
     
     
         22 . A method of producing a member, wherein the galvanized steel sheet of  claim 16  is subjected to at least one of a forming process and a joining process to make the member.