[Technical Field of the Invention]
[0001]
The present invention relates to a hot-dip zinc-plated steel sheet.
Priority is claimed on Japanese Patent Application No. 2020-057273, filed
March 27, 2020, the content of which is incorporated herein by reference.
[Background Art]
[0002]
In recent years, a need for increasing the strength of vehicle members has
increased from the viewpoint of stricter collision safety criteria for vehicles and the
improvement of fuel efficiency. The application of hot stamps has been extended in
order to achieve an increase in the strength of vehicle members. Hot stamping is a
technique for pressing a blank that is heated to a temperature at which the single-phase
region of austenite is formed (Ac3 point) or more (for example, heated to about 900°C)
and then rapidly cooling the blank in a die at the same time as forming to perform
quenching. According to this technique, it is possible to manufacture a press-formed
product having high shape fixability and high strength.
[0003]
Since a Zn component remains on the surface layer of a formed product
obtained after hot stamping in a case where hot stamping is applied to a zinc-plated steel
sheet, an effect of improving corrosion resistance is obtained as compared to a formed
product obtained from the hot stamping of an unplated steel sheet. For this reason, the
application of hot stamping to a zinc-plated steel sheet is being extended.
[0004]
- 1 -
Patent Document 1 discloses a hot-press formed steel member manufactured by
a method which includes a heating step of heating a zinc-plated steel sheet to a
temperature equal to or higher than an Ac3 transformation point and a hot press forming
step of performing hot press forming at least twice after the heating step and in which
all the hot press forming performed in the hot press forming step are performed to
satisfy a predetermined equation.
[0005]
In a case where the zinc-plated steel sheet is subjected to hot stamping,
electrode sticking (a phenomenon in which a copper electrode and plating provided on
the surface of the formed product are melted and adhered to each other) may occur
during spot welding in a formed product obtained after hot stamping. Electrode
sticking is not preferable because it could cause a poor weld or it will inevitably cause
manufacturing downtime to replace the electrode. Electrode sticking during spot
welding is not considered in Patent Document 1.
[Prior Art Document]
[Patent Document]
[0006]
[Patent Document 1] PCT International Publication No. W02013/147228
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0007]
The present invention has been made in consideration of the above-mentioned
circumstances and an object of the present invention is to provide a hot-dip zinc-plated
steel sheet from which a hot-stamping formed body excellent in spot weldability can be
obtained. Further, another object of the present invention is to provide a hot-dip zinc-
- 2 -
plated steel sheet from which a hot-stamping formed body having the above-mentioned
property and having strength generally required for a hot-stamping formed body can be
obtained.
[Means for Solving the Problem]
[0008]
The present inventor investigated the cause of electrode sticking during spot
welding. As a result, the present inventor found that electrode sticking during spot
welding is further suppressed as the number of voids present in a zinc-plated layer is
smaller since electrode sticking during spot welding is greatly affected by voids
(vacancy) present in the zinc-plated layer (a hot-dip zinc-plated layer obtained after hot
stamping) of a hot-stamping formed body. The present inventor thought that an
overcurrent occurs by a narrow electric current path caused by the voids in the zincplated
layer, and the overcurrent causes overheating which makes electrode sticking
between an electrode and zinc plating.
[0009]
Further, although a detailed mechanism is uncertain, the present inventor
thought that voids formed in the hot-stamping formed body are caused by a difference
in thermal contraction between a base metal and a hot-dip zinc-plated layer and a
difference in thermal contraction between different phases present in a plating layer
during hot stamping forming. The present inventor investigated a method of reducing
a difference in thermal contraction during hot stamping forming. As a result, the
present inventor found that the occurrence of voids can be suppressed in a case where,
in a hot-dip zinc-plated steel sheet, an average grain size in a surface layer region of the
steel sheet is set to 4.0 J.lm or less and a standard deviation of grain sizes is set to 2.0 J.lm
or less and the maximumAl concentration in a boundary layer present between the steel
- 3 -
sheet and the hot-dip zinc-plated layer is set to 0.30 mass %or more.
[0010]
The present inventor supposes a mechanism, in which the formation of voids in
the hot-dip zinc-plated layer is suppressed in a case where the surface layer region of
the steel sheet and the boundary layer are formed as described above, as follows. Al is
uniformly diffused and concentrated (a Fe-Al alloy layer is formed) in the boundary
layer due to the grain refining and grain size regulation of crystal grains of the surface
layer region of the steel sheet. It is considered that Al, of which the linear expansion
coefficient is intermediate between those of Fe and Zn, is concentrated in the boundary
layer and a difference in thermal contraction between the base metal and the hot-dip
zinc-plated layer is alleviated, which suppresses the formation of voids.
[0011]
Further, it is considered that a boundary between a r phase and a Fe-Zn solid
solution serves as the origin of the occurrence of voids due to a difference in thermal
contraction between different phases in the plating layer, that is, the r phase having a
high Zn concentration (the Fe concentration is in a range of 10 mass % to 30 mass %)
and the Fe-Zn solid solution having a high Fe concentration (the Fe concentration is in a
range of 50 mass % to 80 mass % ). However, in a case where Al is concentrated in the
boundary layer, a Fe-Zn alloying reaction during heating at the time of hot stamping is
suppressed, so that an increase in the occurrence origin of voids (a boundary between
the r phase and the Fe-Zn solid solution) is suppressed. Accordingly, it is supposed
that the number of voids formed in the zinc-plated layer of the hot-stamping formed
body is reduced.
[0012]
The present inventor found that it is effective to control hot rolling condition in
- 4 -
order to perform the grain refining and grain size regulation of crystal grains of the
surface layer region of the steel sheet. The present inventor found that, in a case where
water pressure for descaling performed on the inlet side of the finish rolling is
controlled in the finish rolling of hot rolling, the temperature distribution of the surface
layer region of the steel sheet can be controlled, which makes it possible to perform the
grain refining and grain size regulation of crystal grains of the surface layer region of
the steel sheet.
[0013]
The gist of the present invention made on the basis of the above-mentioned
knowledge is as follows.
[1] A hot-dip zinc-plated steel sheet according to an aspect of the present
invention includes a steel sheet, a boundary layer that is provided on a surface of the
steel sheet, and a hot-dip zinc-plated layer that is provided on a surface of the boundary
layer,
a chemical composition of the steel sheet contains, by mass %,
C: 0.18% to 0.50%,
Si: 0. 10% to 1.50%,
Mn: 0.5% to 2.5%,
sol.Al:O.OOl% to 0.100%,
Ti: 0.010% to 0.100%,
S: 0.0100% or less,
P: 0.100% or less,
N: 0.010% or less,
Nb: 0% to 0.05%
V: 0% to 0.50%,
- 5 -
Cr: 0% to 0.50%,
Mo: 0% to 0.50%,
B: 0% to 0.010%,
Ni: 0% to 2.00%, and
a total of REM, Ca, Co, and Mg: 0% to 0.0300%,
a remainder consists of Fe and impurities,
in a surface layer region of the steel sheet, an average grain size is 4.0 11m or
less and a standard deviation of grain sizes is 2.0 11m or less, and
in the boundary layer, a maximumAl concentration is 0.30 mass% or more.
[2] In the hot-dip zinc-plated steel sheet according to [1], the chemical
composition may contain, by mass %, one or two or more selected from the group
consisting of
Nb: 0.02% to 0.05%,
V: 0.005% to 0.50%,
Cr: 0.10% to 0.50%,
Mo: 0.005% to 0.50%,
B: 0.0001% to 0.010%,
Ni: 0.01% to 2.00%, and
a total of REM, Ca, Co, and Mg: 0.0003% to 0.0300%.
[3] In the hot-dip zinc-plated steel sheet according to [1] or [2], the chemical
composition may contain, by mass %, 0.24% to 0.50% of C.
[Effects ofthe Invention]
[0014]
According to the aspect of the present invention, it is possible to provide a hotdip
zinc-plated steel sheet from which a hot-stamping formed body excellent in spot
- 6 -
weldability and having strength generally required for a hot-stamping formed body can
be obtained.
[Brief Description of the Drawings]
[0015]
FIG. 1 is a schematic diagram showing the GDS profile of a hot-dip zinc-plated
steel sheet according to an embodiment.
[Embodiments ofthe Invention]
[0016]
A hot-dip zinc-plated steel sheet according to an embodiment will be described
in detail below. The hot-dip zinc-plated steel sheet according to this embodiment
includes a steel sheet, a boundary layer that is provided on the steel sheet, and a hot-dip
zinc-plated layer that is provided on the boundary layer.
First, a steel sheet of the hot-dip zinc-plated steel sheet according to this
embodiment will be described. The reason why the chemical composition of the steel
sheet of the hot-dip zinc-plated steel sheet according to this embodiment is to be limited
will be described below. All percentages (%) related to the chemical composition
mean mass %.
[0017]
The chemical composition of the steel sheet of the hot -dip zinc-plated steel
sheet according to this embodiment includes, by mass%, C: 0.18% to 0.50%, Si: 0.10%
to 1.50%, Mn: 0.5% to 2.5%, sol.Al: 0.001% to 0.100%, Ti: 0.010% to 0.100%, S:
0.0100% or less, P: 0.100% or less, N: 0.010% or less, and a remainder consisting of Fe
and impurities. Each element will be described below.
[0018]
C: 0.18% to 0.50%
- 7 -
Carbon (C) increases the strength of a hot-stamping formed body obtained after
hot stamping. In a case where the C content is excessively low, the above-mentioned
effect is not obtained. For this reason, the C content is set to 0.18% or more. The C
content is preferably 0.20% or more, 0.24% or more, or 0.25% or more. On the other
hand, in a case where the C content is excessively high, the toughness of the hot-dip
zinc-plated steel sheet deteriorates. Accordingly, the C content is set to 0.50% or less.
The C content is preferably 0.45% or less or 0.40% or less.
[0019]
Si: 0.10% to 1.50%
Si is an element that improves the fatigue property of the hot-stamping formed
body. Further, Si is also an element that improves a hot-dip galvanizing property,
particularly plating wettability, by forming a stable oxide film on the surface of the steel
sheet during recrystallization annealing. In order to obtain these effects, the Si content
is set to 0.10% or more. The Si content is preferably 0.15% or more or 0.18% or more.
On the other hand, in a case where the Si content is excessively high, Si contained in
steel is diffused during heating at the time of hot stamping and forms oxide on the
surface of the steel sheet. The oxide formed on the surface of the steel sheet
deteriorates a phosphate treatment property. Further, Si is also an element that raises
the AC3 point of the hot-dip zinc-plated steel sheet. In a case where the Ac3 point of
the hot-dip zinc-plated steel sheet is raised, a heating temperature during hot stamping
needs to be raised in order to sufficiently austenitize the steel sheet and a heating
temperature during hot stamping exceeds the evaporation temperature of the hot-dip
zinc-plated layer. For this reason, the Si content is set to 1.50% or less. The Si
content is preferably 1.40% or less, 1.20% or less, or 1.00% or less.
[0020]
- 8 -
Mn: 0.5% to 2.5%
Mn is an element that improves the hardenability of steel. The Mn content is
set to 0.5% or more to improve hardenability and obtain the desired strength of the hotstamping
formed body. The Mn content is preferably 1.0% or more or 1.5% or more.
On the other hand, even though the Mn content exceeds 2.5%, an effect of improving
hardenability is saturated and steel is embrittled, so that quenching cracks are likely to
occur during casting, hot rolling, and cold rolling. For this reason, the Mn content is
set to 2.5% or less. The Mn content is preferably 2.1% or less or 2.0% or less.
[0021]
sol.Al: 0.001% to 0.100%
Al is an element that deoxidizes molten steel to suppress the formation of oxide
serving as the origin of fracture. Further, Al is also an element that has an action of
suppressing an alloying reaction between Zn and Fe and an action of improving the
corrosion resistance of the hot-stamping formed body. In order to obtain these effects,
the sol.Al content is set to 0.001% or more. The sol.Al content is preferably 0.005%
or more. On the other hand, in a case where the sol.Al content is excessive, the Ac3
point of the steel sheet is raised, a heating temperature needs to be rai sed in order to
sufficiently austenitize the steel sheet, and a heating temperature during hot stamping
exceeds the evaporation temperature of the hot-dip zinc-plated layer. For this reason,
the sol.Al content is set to 0.100% or less. The sol.Al content is preferably 0.090% or
less, 0.070% or less, or 0.050% or less.
In this embodiment, sol.Al means acid-soluble Al, and indicates solute Al that
is present in steel in the state of a solid solution.
[0022]
Ti: 0.010% to 0.100%
- 9 -
Ti is an element that increases oxidation resistance after hot-dip galvanizing.
Further, Ti is also an element that improves the hardenability of the steel sheet by
combining with N present in steel to form nitride (TiN) and suppressing the formation
of nitride (BN) from B. In order to obtain these effects, the Ti content is set to 0.010%
or more. The Ti content is preferably 0.020% or more. On the other hand, in a case
where the Ti content is excessive, the Ac3 point is raised and a heating temperature
during hot stamping is raised. For this reason, productivity may deteriorate, and it may
be difficult to secure a r phase since formation into a Fe-Zn solid solution may be
facilitated. Further, in a case where the Ti content is excessive, a large amount of Ti
carbide is formed and the amount of solute C is reduced, so that the strength of the hotstamping
formed body is reduced. Furthermore, the wettability of plating may
deteriorate, and the toughness of the hot-stamping formed body may deteriorate due to
the excessive precipitation ofTi carbide. For this reason, the Ti content is set to
0.100% or less. The Ti content is preferably 0.070% or les s.
[0023]
S: 0.0100% or less
S is an element that is contained as an impurity and is an element that forms
sulfide in steel to cause the deterioration of the toughness of the hot-stamping formed
body and to deteriorate a delayed fracture resistance property. For this reason, the S
content is set to 0.0100% or less. The S content is preferably 0.0050% or les s. It is
preferable that the S content is 0%. However, since cost required to removeS is
increased in a case where the S content is to be excessively reduced, the S content may
be set to 0.0001% or more.
[0024]
P: 0.100% or less
- 10 -
Pis an element that is included as an impurity, and is an element that
segregates at a grain boundary to deteriorate the toughness and delayed fracture
resistance property of steel. For this reason, the P content is set to 0.100% or less.
The P content is preferably 0.050% or less. It is preferable that the P content is 0%.
However, since cost required to remove Pis increased in a case where the Pcontent is to
be excessively reduced, the P content may be set to 0.001% or more.
[0025]
N: 0.010% or less
N is an impurity element, and is an element that forms coarse nitride in steel to
deteriorate the toughness of steel. Further, N is also an element that facilitates the
occurrence of blow holes during spot welding. Furthermore, in a case where B is
contained, N combines with B to reduce the amount of solute B and deteriorates the
hardenability of the steel sheet. For this reason, theN content is set to 0.010% or les s.
TheN content is preferably 0.007% or less. It is preferable that theN content is 0%.
However, since manufacturing cost is increased in a case where the N content is to be
excessively reduced, theN content may be set to 0.0001% or more.
[0026]
The remainder of the chemical composition of the steel sheet of the hot-dip
zinc-plated steel sheet according to this embodiment is Fe and impurities. Elements,
which are unavoidably mixed from a steel raw material or scrap and/or during the
manufacture of steel and are allowed in a range where the properties of the hot-stamping
formed body obtained from the hot stamping of the hot-dip zinc-plated steel sheet
according to this embodiment do not deteriorate, are exemplified as the impurities.
[0027]
The steel sheet of the hot -dip zinc-plated steel sheet according to this
- 11 -
embodiment may contain the following elements as arbitrary elements instead of a part
of Fe. The contents of the following arbitrary elements in a case where the following
arbitrary elements are not contained are 0%.
[0028]
Nb: 0% to 0.05%
Nb has an action of forming carbide to refine crystal grains during hot
stamping. In a case where crystal grains are refined, the toughness of steel is
increased. In order to reliably obtain this effect, it is preferable that the Nb content is
set to 0.02% or more. However, in a case where the Nb content is excessively high,
the above-mentioned effect may be saturated and the hardenability of steel may
deteriorate. Accordingly, the Nb content is set to 0.05% or less.
[0029]
V: 0% to 0.50%
Vis an element that finely forms carbonitride in steel to improve strength. In
order to reliably obtain this effect, it is preferable that the V content is set to 0.005% or
more. On the other hand, in a case where the V content exceeds 0.50%, the toughness
of steel deteriorates during spot welding and cracks are likely to occur. For this
reason, the V content is set to 0.50% or less.
[0030]
Cr: 0% to 0.50%
Cr is an element that improves the hardenability of steeL In order to reliably
obtain this effect, it is preferable that the Cr content is set to 0. 10% or more. On the
other hand, in a case where the Cr content exceeds 0.50%, Cr carbide is formed in steel
and it is difficult for Cr carbide to be dissolved during heating of hot stamping, so that
hardenability deteriorates. For this reason, the Cr content is set to 0.50% or less.
- 12 -
[0031]
Mo: 0% to 0.50%
Mo is an element that improves the hardenability of steeL In order to reliably
obtain this effect, it is preferable that the Mo content is set to 0.005% or more.
However, in a case where the Mo content is excessively high, the above-mentioned
effect is saturated. Accordingly, the Mo content is set to 0.50% or less.
[0032]
B: 0% to 0.010%
B is an element that improves the hardenability of steeL In order to reliably
obtain this effect, it is preferable that the B content is set to 0.0001% or more. On the
other hand, even though the B content exceeds 0.010%, an effect of improving
hardenability is saturated. For this reason, the B content is set to 0.010% or less.
[0033]
Ni: 0% to 2.00%
Ni is an element that has an effect of improving the toughness of steel, an
effect of suppressing the embrittlement of steel caused by liquid Zn during heating of
hot stamping, and an effect of improving the hardenability of steeL In order to reliably
obtain these effects, it is preferable that the Ni content is set to 0.01% or more. On the
other hand, even though the Ni content exceeds 2.00%, the above-mentioned effects are
saturated. For this reason, the Ni content is set to 2.00% or less.
[0034]
a total of REM, Ca, Co, and Mg: 0% to 0.0300%
REM, Ca, Co, and Mg are elements that suppress the occurrence of cracks
during spot welding by controlling sulfide and oxide in a preferred shape and
suppressing the formation of coarse inclusions. In order to reliably obtain this effect, it
- 13 -
is preferable that the total content of REM, Ca, Co, and Mg is set to 0.0003% or more.
In order to reliably obtain the above-mentioned effect, the content of even any one of
REM, Ca, Co, and Mg may be 0.0003% or more. On the other hand, in a case where
the total content of REM, Ca, Co, and Mg exceeds 0.0300%, inclusions are excessively
generated and cracks are likely to occur during spot welding. For this reason, the total
content of REM, Ca, Co, and Mg is set to 0.0300% or less.
[0035]
The chemical composition of the steel sheet described above may be measured
by a general analysis method. For example, the chemical composition of the steel
sheet described above may be measured using inductively coupled plasma-atomic
emission spectrometry (ICP-AES). C and S may be measured using a combustioninfrared
absorption method and N may be measured using an inert gas fusion-thermal
conductivity method. Further, sol.Al may be measured by ICP-AES using a filtrate
that is obtained in a case where a sample is decomposed with an acid by heating. The
chemical composition may be analyzed after the hot-dip zinc-plated layer provided on
the surface of the hot-dip zinc-plated steel sheet is removed by mechanical grinding.
[0036]
The steel sheet of the hot -dip zinc-plated steel sheet according to this
embodiment has the above-mentioned chemical composition, and has an average grain
size of 4.0 Jlm or less and a standard deviation of grain sizes of 2.0 Jlm or less in a
surface layer region thereof. The surface layer region of the steel sheet of the hot -dip
zinc-plated steel sheet according to this embodiment will be described below.
[0037]
Surface layer region: an average grain size of 4.0 Jlm or less and a standard
deviation of grain sizes of 2.0 Jlm or less
- 14 -
In this embodiment, the surface layer region refers to a region between the
surface of the steel sheet and a position that is away from the surface of the steel sheet
in a depth direction by a distance of 25 )liD. In a case where an average grain size in
this surface layer region exceeds 4.0 )liD or the standard deviation of grain sizes exceeds
2.0 )liD, it is not possible to suppress the evaporation of zinc present in the hot-dip zincplated
layer in the heating during hot stamping. Accordingly, a lot of voids are formed
in the hot-stamping formed body. As a result, desired spot weldability cannot be
obtained in the hot-stamping formed body. For this reason, in the surface layer region
of the steel sheet, an average grain size is set to 4.0 )liD or less and the standard
deviation of grain sizes is set to 2.0 )liD or less. Since a smaller average grain size in
the surface layer region of the steel sheet is more preferable, an average grain size in the
surface layer region of the steel sheet may be set to 3.5 )liD or less or 3.0 )liD or less.
Further, since a smaller standard deviation of grain sizes in the surface layer region of
the steel sheet is more preferable, the standard deviation of grain sizes in the surface
layer region of the steel sheet may be set to 1. 8 )liD or less or 1.5 )liD or less.
[0038]
The lower limit of an average grain size in the surface layer region of the steel
sheet does not need to be particularly limited, but may be set to 1.5 )liD. Further, the
lower limit of the standard deviation of grain sizes in the surface layer region of the
steel sheet does not need to be particularly limited, but may be set to 1.0 )liD.
[0039]
Method of measuring average grain size and standard deviation of grain sizes
in surface layer region
An average grain size and the standard deviation of grain sizes in the surface
layer region are measured using electron back scatter diffraction pattern-orientation
- 15 -
image microscopy (EBSP-OIM). EBSP-OIM is performed using a device in which a
scanning electron microscope and an EBSP analysis device are combined with each
other and OIM Analysis (registered trademark) manufactured by AMETEK, Inc.
In a region between the surface of the steel sheet and a position, which is away
from the surface of the steel sheet in the depth direction by a distance of 25 J.lm, in a
cross section parallel to a rolling direction, an analysis is made in at least 5 visual fields
in a region having a size of 40 J.lm x 30 J.lm with a magnification of 1200. A spot
where an angle difference between adjacent measurement points is 5° or more is defined
as a grain boundary, and the equivalent circle diameters of crystal grains are calculated
and are regarded as grain sizes. The average value of the obtained grain sizes of
crystal grains is calculated, so that an average grain size in the surface layer region is
obtained. Further, a standard deviation is calculated from the obtained grain sizes of
crystal grains, so that the standard deviation of grain sizes in the surface layer region is
obtained.
The steel sheet, the boundary layer, and the hot-dip zinc-plated layer may be
specified using a method to be described later, and the above-mentioned measurement
may be performed for the steel sheet and the surface layer region of the specified region.
[0040]
A method of specifying the steel sheet, the boundary layer, and the hot-dip
zinc-plated layer will be described below.
At an arbitrary position on the hot-dip zinc-plated steel sheet, the
concentrations (mass %) of Fe, Zn, and Al are measured using glow discharge optical
emission spectrometry (GDS) up to a depth of 50 J.lm from the surface of the hot-dip
zinc-plated steel sheet in the depth direction (sheet thickness direction). In a case
where the hot-dip zinc-plated steel sheet according to this embodiment is measured
- 16 -
using GDS, a GDS profile shown in FIG. 1 can be obtained. In this embodiment, a
depth range in which the Fe concentration is 85 mass %or more is defined as the steel
sheet and a depth range in which the Zn concentration is 90 mass % or more is defined
as the hot-dip zinc-plated layer. Further, a depth range between the steel sheet and the
hot-dip zinc-plated layer is defined as the boundary layer.
[0041]
The metallographic structure of the steel sheet is not particularly limited as
long as desired strength and desired spot weldability can be obtained after hot stamping.
However, the metallographic structure of the steel sheet may consist of, by area %, 20%
to 90% of ferrite, 0% to 100% of bainite and martensite, 10% to 80% of pearlite, and
0% to 5% of residual austenite. The metallographic structure of the steel sheet may be
measured using the following methods.
[0042]
(Method of measuring area ratios of ferrite and pearlite)
The measurement of the area ratios of ferrite and pearlite is performed using
the following method. A cross section parallel to the rolling direction is finished as a
mirror surface and is polished for 8 minutes at room temperature using colloidal silica,
which does not contain an alkaline solution, to remove strain introduced into the surface
layer of the sample. A region, which has a length of 50 11m and is present between a
depth corresponding to 1/8 of the sheet thickness from the surface and a depth
corresponding to 3/8 of the sheet thickness from the surface, is measured at a
measurement interval of 0.1 11m with an electron backscatter diffraction method at an
arbitrary position in a longitudinal direction on the cross section of the sample so that a
region having a depth corresponding to 1/4 of the sheet thickness from the surface can
be analyzed. As a result, crystal orientation information is obtained. A device, which
- 17 -
includes a schottky emission scanning electron microscope (JSM-7001F manufactured
by JEOL Ltd.) and an EBSP detector (DVC5 detector manufactured by TSL Solutions),
is used for the measurement. In this case, the degree of vacuum in the device is set to
9.6 x 10·5 Pa or less, an accelerating voltage is set to 15 kV, an irradiation current level
is set to 13, and the irradiation level of an electron beam is set to 62. Further, a
reflected electron image is taken in the same visual field.

CLAIMS
1. A hot-dip zinc-plated steel sheet comprising:
a steel sheet;
a boundary layer that is provided on a surface of the steel sheet; and
a hot-dip zinc-plated layer that is provided on a surface of the boundary layer,
wherein a chemical composition of the steel sheet contains, by mass %,
C: 0.18% to 0.50%,
Si: 0.10% to 1.50%,
Mn: 0.5% to 2.5%,
sol.Al: 0.001% to 0.100%,
Ti: 0.010% to 0.100%,
S: 0.0100% or less,
P: 0.100% or less,
N: 0.010% or less,
Nb: 0% to 0.05%,
V: 0% to 0.50%,
Cr: 0% to 0.50%,
Mo: 0% to 0.50%,
B: 0% to 0.010%,
Ni: 0% to 2.00%, and
a total of REM, Ca, Co, and Mg: 0% to 0.0300%,
a remainder consi sting of Fe and impurities,
in a surface layer region of the steel sheet, an average grain size is 4.0 11m or
less and a standard deviation of grain sizes is 2.0 11m or less, and
in the boundary layer, a maximumAl concentration is 0.30 mass % or more.
- 38 -
2. The hot-dip zinc-plated steel sheet according to claim 1,
wherein the chemical composition contains, by mass%, one or two or more
selected from the group consisting of
Nb: 0.02% to 0.05%,
V: 0.005% to 0.50%,
Cr: 0.10% to 0.50%,
Mo: 0.005% to 0.50%,
B: 0.0001% to 0.010%,
Ni: 0.01% to 2.00%, and
a total of REM, Ca, Co, and Mg: 0.0003% to 0.0300%.
3. The hot-dip zinc-plated steel sheet according to claim 1 or 2,
wherein the chemical composition contains, by mass%,
C: 0.24% to 0.50%.