Hot-dip galvanized steel sheet
A hot-dip galvanized steel sheet includes: a predetermined chemical composition; and a steel structure represented by: in terms of area ratio, polygonal ferrite: 10% or less; upper bainite: 20% or less; retained austenite: 5% or less; and martensite: 70% or more, in which: martensite having Fe carbides at a number density of 1×10 6 /mm 2 or more is contained by 50% or more, in terms of area ratio, with respect to the entire amount of martensite; and the steel structure has an average effective crystal grain diameter of 5.0 μm or less.
1. A hot-dip galvanized steel sheet, comprising:
a chemical composition represented by:
in terms of mass %,
C: 0.14 to 0.3%;
Si: 0.001 to 2.0%;
Mn: 2.0 to 4.0%;
P: 0.05% or less;
S: 0.01% or less;
N: 0.01% or less;
Al: 0.001 to 1.0%;
Ti: 0.001 to 0.10%;
B: 0.0001 to 0.01%;
Mo: 0 to 0.50%;
Cr: 0 to 0.80%;
Ni: 0 to 1.00%;
Cu: 0 to 1.00%;
V: 0 to 0.50%;
Nb: 0.0 to 0.10%;
Ca: 0.00 to 0.01%;
Mg: 0.00 to 0.01%;
REM: 0.00 to 0.01%;
Bi: 0.00 to 0.01%; and
the balance: Fe and impurities; and
a steel structure represented by:
in terms of area ratio,
polygonal ferrite: 10% or less;
upper bainite: 20% or less;
retained austenite: 5% or less; and
martensite: 70% or more,
when the area ratio is calculated from a microscopic photograph of a cross section in a rolling direction of the steel sheet for polygonal ferrite, upper bainite, pearlite, cementite, martensite, and tempered martensite and is calculated from a peak area ratio of bcc and fcc by an X-ray diffraction for retained austenite, wherein:
martensite having Fe carbides at a number density of 1×10 6 /mm 2 or more is contained by 50% or more, in terms of area ratio, with respect to the entire amount of martensite, when the number density is calculated from a microscopic photograph of a cross section in a rolling direction of the steel sheet; and
the steel structure has an average effective crystal grain diameter of 5.0 μm or less, when measured by an electron back scatter diffraction pattern-orientation image microscopy (EBSP-OIM) method.
2. The hot-dip galvanized steel sheet according to claim 1 , wherein
an amount of solid-solution B is 0.0010 mass % to 0.0100 mass %, and a prior austenite grain diameter is 1.0 μm to 7.0 μm, when
the amount of solid-solution B is calculated by subtracting a mass of B contained in a precipitate from a total mass of B contained in the hot-dip galvanized steel sheet, and
the prior austenite grain diameter is calculated from a microscopic photograph of a cross section in a rolling direction of the steel sheet.
3. The hot-dip galvanized steel sheet according to claim 1 , wherein
a product of an amount of solid-solution B and a prior austenite grain diameter is 0.0010 mass %·μm or more, when
the amount of solid-solution B is calculated by subtracting a mass of B contained in a precipitate from a total mass of B contained in the hot-dip galvanized steel sheet, and
the prior austenite grain diameter is calculated from a microscopic photograph of a cross section in a rolling direction of the steel sheet.
4. The hot-dip galvanized steel sheet according to claim 1 , wherein
in the chemical composition, Mo: 0.001 to 0.50% is satisfied.
5. The hot-dip galvanized steel sheet according to claim 1 , wherein
in the chemical composition,
Cr: 0.001 to 0.80%,
Ni: 0.001 to 1.00%, or
Cu: 0.001 to 1.00%, or
any combination thereof is satisfied.
6. The hot-dip galvanized steel sheet according to claim 1 , wherein
in the chemical composition,
V: 0.001 to 0.50%, or
Nb: 0.001 to 0.10%, or
both thereof are satisfied.
7. The hot-dip galvanized steel sheet according to claim 1 , wherein
in the chemical composition,
Ca: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%,
REM: 0.0001 to 0.01%, or
Bi: 0.0001 to 0.01%, or
any combination thereof is satisfied.