Galvanized steel sheet and member, and method of producing same

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: 0.040% or more and 0.200% 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: 0.005% or more and 2.000% or less,

N: 0.0100% or less,

Ti: 0.200% or less, including 0%,

Nb: 0.200% or less, including 0%,

V: 0.100% or less, including 0%,

B: 0.0100% or less, including 0%,

Cu: 1.000% or less, including 0%,

Cr: 1.000% or less, including 0%,

Ni: 1.000% or less, including 0%,

Mo: 0.500% or less, including 0%,

Sb: 0.200% or less, including 0%,

Sn: 0.200% or less, including 0%,

Ta: 0.100% or less, including 0%,

W: 0.500% or less, including 0%,

Mg: 0.0200% or less, including 0%,

Zn: 0.0200% or less, including 0%,

Co: 0.0200% or less, including 0%,

Zr: 0.0200% or less, including 0%,

Ca: 0.0200% or less, including 0%,

Ce: 0.0200% or less, including 0%,

Se: 0.0200% or less, including 0%,

Te: 0.0200% or less, including 0%,

Ge: 0.0200% or less, including 0%,

As: 0.0200% or less, including 0%,

Sr: 0.0200% or less, including 0%,

Cs: 0.0200% or less, including 0%,

Hf: 0.0200% or less, including 0%,

Pb: 0.0200% or less, including 0%,

Bi: 0.0200% or less, including 0%, and

REM: 0.0200% or less, including 0%,

with the balance being Fe and inevitable impurities,

the steel microstructure of the base steel sheet comprises:

area fraction of ferrite: 20.0% or more and 90.0% or less,

area fraction of bainitic ferrite: 4.0% or more and 60.0% or less,

area fraction of tempered martensite: 20.0% or less, including 0%,

area fraction of retained austenite: 3.0% or more,

area fraction of fresh martensite: 20.0% or less, including 0%,

SF+SBF: 55.0% or more and 95.0% or less,

SMA1: 4.0% or more, and

SMA2: 1.5% or more, wherein

tensile strength is 590 MPa or more,

where

S F is area fraction of ferrite,

S BF is area fraction of bainitic ferrite,

S MA1 is area fraction of the sum of island regions of hard secondary phase consisting of retained austenite and fresh martensite having a circularity of 0.40 or less, and

S MA2 is area fraction of the sum of island regions of hard secondary phase consisting of retained austenite and fresh martensite having a circularity of 0.20 or less and an aspect ratio of 4.0 or less, and

wherein the area fractions of ferrite, bainitic ferrite, tempered martensite, retained austenite, and fresh martensite are measured at a ¼ sheet thickness position of the base steel sheet.

2 . A member made of the galvanized steel sheet according to claim 1 .

3 . The galvanized steel sheet according to claim 1 , wherein the galvanized steel sheet satisfies at least one selected from following (a), (b) and (c),

(a) S γ2 /S γ1 is 0.10 or more in the steel microstructure of the base steel sheet, where

S γ1 is area fraction of retained austenite having a solute C concentration of 0.70% or more, and

S γ2 is area fraction of retained austenite having a solute C concentration of 0.85% or more,

(b) diffusible hydrogen content of the base steel sheet is 0.50 mass ppm or less, and

(c) the galvanized layer is a hot-dip galvanized layer or a galvannealed layer.

4 . A member made of the galvanized steel sheet according to claim 3 .

5 . A method of producing the galvanized steel sheet according to claim 1 , the method comprising:

hot rolling a steel slab having the chemical composition according to claim 1 to produce a hot-rolled steel sheet;

cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet;

annealing the cold-rolled steel sheet at an annealing temperature of 760° C. or more and 900° C. or less for an annealing time of 20 s or more;

cooling the cold-rolled steel sheet to a first cooling stop temperature of 350° C. or more and 550° C. or less;

holding the cold-rolled steel sheet at a temperature range of 350° C. or more and 550° C. or less for 3 s or more to 600 s or less;

coating the cold-rolled steel sheet by a galvanizing treatment to produce a galvanized steel sheet, wherein the galvanizing treatment is a hot-dip galvanizing treatment or a galvannealing treatment;

cooling the galvanized steel sheet to a second cooling stop temperature of 280° C. or more and 470° C. or less; and

holding the galvanized steel sheet at a temperature range of 280° C. or more and 470° C. or less for 10 s or more to 2000 s or less,

wherein

the first cooling stop temperature, temperature of a galvanizing bath in the galvanizing treatment, and the second cooling stop temperature satisfy the following Expression (1) and Expression (2)

−120° C. ≤T 0 −T 1 ≤80° C. . . .  (1)

0° C. ≤T 0 −T 2 . . .  (2)

where

T 0 is the first cooling stop temperature in ° C.,

T 1 is the temperature of the galvanizing bath in the galvanizing treatment in ° C., and

T 2 is the second cooling stop temperature in ° C.

6 . A method of producing a member, wherein the galvanized steel sheet according to claim 1 is subjected to at least one of a forming process or a joining process to produce the member.

7 . A method of producing a member, wherein the galvanized steel sheet according to claim 3 is subjected to at least one of a forming process or a joining process to produce the member.