Technical Field
[0001]
The present invention relates to a zinc-based plated steel sheet.
Background Art
[0002]
These days, to protect the environment and prevent global warming, the
suppression of the consumption of fossil fuel is increasingly demanded, and the
demand influences various manufacturing industries. For example, automobiles,
15 which are indispensable to daily life and activity as a moving means, are no
exception, and improvements in fuel efficiency etc. by the weight reduction of car
bodies etc. are required. However, for automobiles, simply achieving a weight
reduction of the car body is not permitted in terms of the functionality of the product,
and it is necessary to ensure proper safety.
20 [0003]
Most of the structure of the automobile is formed of iron-based materials, in
particular steel sheets, and the reduction in the weight of the steel sheet is important
to the weight reduction of tbe car body. However, as described above, simply
reducing the weight of the steel sheet is not permitted, and ensuring the mechanical
25 strength of the steel sheet is required at the same time. Such a demand on the steel
sheet is placed not only in the automobile manufacturing industry but also in various
manufacturing industries similarly. Hence, research and development are being
made to enhance the mechanical strength of the steel sheet and thereby obtain a steel
sheet in which the mechanical strength can be maintained or improved even when the
30 wall thickness is made smaller than those of conventionally used steel sheets.
[0004]
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In general, a material having high mechanical strength tends to decrease in
shape fixability in molding such as bending, and is difficult to mold into a
complicated shape. As a means for solving snch a problem with moldability, what
is called "the hot pressing method (also called the hot stamping method or the die
5 quenching method)" is given. In the hot pressing method, a material to be molded
is once heated to high temperature, the steel sheet softened by heating is pressed to
be molded, and then cooling is performed. By the hot pressing method, the material
of the object can be easily pressed becanse the material is once heated to high
temperature and softened. Furthermore, the mechanical strength of the material can
10 be enhanced by the quenching effect by the cooling after molding. Thns, a molded
product in which both good shape fixability and high mechanical strength are
achieved can be obtained by the hot pressing method.
[0005]
However, when the hot pressing method is nsed for a steel sheet, the snrface
15 of the steel sheet is oxidized by the steel sheet being heated to a high temperature of
800°C or more, and scales (compounds) are produced. Hence, the process of
removing the scales (what is called a descaling process) is needed after hot pressing
is performed, and productivity is reduced. In addition, in a member etc. requiring
corrosion resistance, it is necessary to perform anti~rust treatment or metal covering
20 on the surface of the member after processing, and a surface cleaning process and a
surface treatment process are needed; consequently, productivity is further reduced.
[0006]
As a method to suppress such a reduction in productivity, for example, a
method in which a steel sheet to be hot pressed is provided with a covering in
25 advance is given. Various materials such as organic-based materials and inorganicbased
materials are generally used as the covering on the steel sheet. Among these,
plated steel sheets based on zinc (Zn), which has a sacrificial anti-corrosion action on
the steel sheet, are widely used as automotive steel sheets etc. from the viewpoints of
the anti-corrosion capacity and the steel sheet production technique.
30 [0007]
By providing a Zn-based metal covering, the production of scales on the
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surface of the steel sheet can be prevented, and processes such as descaling become
unnecessary; thus, the productivity of molded products is improved. In addition,
the Zn-based metal covering has also an anti-rust effect, and therefore also corrosion
resistance is improved. Patent Literature 1 to Patent Literature 4 below disclose a
5 method of hot pressing a plated steel sheet that is obtained by providing a Zn-based
metal covering to a steel sheet having a prescribed component composition.
[0008]
In Patent Literature 1 to Patent Literature 3 below, a Zn-hot-dipped steel
sheet or an alloyed Zn-hot-dipped steel sheet is used as a steel sheet for hot pressing.
10 By using a Zn-hot-dipped steel sheet or an alloyed Zn-hot-dipped steel sheet for hot
pressing, a structure member can be molded without iron oxides (that is, scales)
being formed on the surface. Further, in view of the fact that, when a Zn oxide
layer is formed thick on the surface of a heat-treated steel material obtained by hot
pressing a Zn-based plated steel sheet, the coating adhesiveness and the post-coating
15 corrosion resistance of the heat-treated steel material are adversely affected, Patent
Literature 4 below discloses an invention in which a heat-treated steel material is
20
subjected to shot blasting to remove a Zn oxide layer or is subjected to coating after
the thickness of a Zn oxide layer is reduced.
[0009]
Patent Literature 5 and Patent Literature 6 below disclose an invention that
improves the coating adhesiveness and the post-coating corrosion resistance of a
heat-treated steel material obtained by hot pressing a Zn-based plated steel sheet.
Patent Literature 5 below discloses an invention in which a Zn-hot-dipped steel sheet
with its surface covered with a silicone resin coating film is used as a steel sheet for
25 hot pressing, and Patent Literature 6 below discloses an invention in which a Zn-hot-
30
dipped steel sheet covered with a barrier layer containing phosphorus ( P) and silicon
(Si) (a phosphate is given as an example of P, and colloidal silica is given as an
example of Si) is used as a steel sheet for hot pressing.
[0010]
Patent Literature 7 below discloses a technology in which elements that are
easier to oxidize than Zn (easily oxidizable elements) are added into a Zn plating
5
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layer and an oxide layer of these easily oxidizable elements is formed on the outer
layer of the Zn plating layer during the temperature increase in hot pressing, and
thereby the volatilization of zinc is prevented.
[0011]
According to the inventions disclosed by Patent Literature 5 to Patent
Literature 7 below, since a Zn plating layer is covered with the barrier layer
described above, the vaporization of Zn is suppressed, and thus the adhesiveness of
an intermediate coating film and an over-coating film and post -coating corrosion
resistance are good.
10 [0012]
15
20
25
Patent Literature 8 below mentions that, when a material in which a coating
film containing ZnO is applied to the upper layer of an AI plating layer is used for
hot pressing, temperature increase characteristics, lubricity, and coating adhesiveness
are improved.
Patent Literature
[0013]
Patent Literature 1:
Patent Literature 2:
Patent Literature 3:
Patent Literature 4:
Patent Literature 5:
Patent Literature 6:
Patent Literature 7:
Patent Literatme 8:
Technical Problem
Citation List
JP 2003-73774A
JP 2003-129209A
JP 2003-126921A
JP 2004-323897A
JP 2007-63578A
JP 2007-291508A
JP 2004-270029A
JP 2011-129084A
Summary ofinvention
30 [0014]
However, when a Zn-based plated steel sheet, in particular a Zn-hot-dipped
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steel sheet or an alloyed Zn-hot-dipped steel sheet, is hot pressed, there is a case
where a phosphate coating film formed by phosphate treatment adheres less easily
(that is, phosphate treatability is low). The Zn-based hot dipping dealt with by the
present invention contains AI in a plating bath and a plating layer even in cases other
5 than Zn-Al-based alloy plating containing aluminum (Al) as a main component
The reason is as follows. That is, the temperature of the plating bath is
approximately 440 to 480°C; in this temperature range, when Zn and Fe come into
contact, Fe and Zn are continuously alloyed, and consequently dross occurs. By
putting Al in the plating bath, the reaction between Fe and AI occurs before the
10 reaction between Fe and Zn occurs, and consequently the occurrence of dross is
suppressed. For this reason, usually AI is contained in a Zn hot dipping bath.
[0015]
In general, in Zn hot dipping, AI is contained at 0.2 to 0.3% in the plating
ba~ and 0.2 to 1.0 mass% of AI is contained in the plating layer; in alloyed Zn hot
15 dipping, AI is contained at 0.1 to 0.2% in the plating ba~ and 0.1 to 0.5 mass% of AI
is contained in the plating layer.
----
[0016]
The Al in the plating layer diffuses and moves to the outer layer of the
plating layer not only during the formation of a plating coating film but also during
20 the heating of hot pressing, and forms an AI oxide film. Since the AI oxide film
does not dissolve in phosphoric acid, the reaction between Zn and a phosphate (zinc
phosphate etc.) is inhibited, and a phosphate coating film is less likely to be formed
in the area where the AI oxide film is formed. Consequently, phosphate treatability
is low in the area where the Al oxide film is formed. In particular, phosphate
25 treatability is significantly reduced in the case where, in the hot pressing process, the
steel sheet is rapidly heated to the Ac3 point or more by energization heating or
induction heating and then press molding is quickly performed. In this case, also
coating adhesiveness is reduced.
30
[0017]
Examples of the Zn-based plated steel sheet include, as well as the zinc hot
dipping mentioned above, electroplating, vapor deposition plating, etc. Examples
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of the plating of the plated steel sheet produced by such a method include zinc
electroplating, zinc nickel electroplating, zinc cobalt electroplating, and the like, and
include plating not containing AL These have no concern of reduction in phosphate
treatability caused by an Al oxide film, but may have poor coating adhesion
5 depending on the amount of plating attached, heating conditions, etc.; thus, an
improvement in coating adhesiveness after hot pressing is desired like in Zn-hotdipping-
based materials.
[0018]
In addition, when the present inventors conducted a check experiment on a
10 heat-treated steel material disclosed by Patent Literature 5 above that was obtained
by using, as a steel sheet for hot pressing, a Zn-hot-dipped steel sheet with its surface
covered with a silicone resin coating film, it has been found that, although postcoating
corrosion resistance in a cycle corrosion test in which a dry and a wet
environment are repeated is good, coating adhesiveness is not always good. Hence,
15 a heat-treated steel material obtained by the invention disclosed in Patent Literature 5
above is not suitable for use as it is for a part or a member in which water is likely to
- ------------------- -- ---- ---
collect because of the structure (for example, a bag-like structural part below the
door, a member with a closed cross section in the engine compartment, etc.), for
example.
20 [0019]
25
On the other hand, the addition of easily oxidizable elements into a zinc
plating layer disclosed in Patent Literature 7 above requires new operational actions,
such as the temperature control of the plating bath and dross measures.
[0020]
Phosphoric acid treatability after hot pressmg can be improved by the
stacking of a ZnO-containing coating film on a plating layer disclosed in Patent
Literature 8 above. In the case where the underlying layer is Al-based plating, the
ZnO in the coating film may react with the plating layer by heating and adhesiveness
may be maintained. However, in the case where the underlying layer is Zn-based
30 plating, the reaction between ZnO and Zn cannot be expected, and it is feared that the
adhesiveness between the stacked ZnO coating film and the underlying plating will
5
PCT/JP2016/060800
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be reduced; consequently, it is presumed that a new action is needed with the
composition of the coating film etc. in order to improve the adhesiveness between
the oxide coating film and the underlying plating.
[0021]
Thus, the present invention has been made in view of the issue mentioned
above, and an object of the present invention is to provide a zinc-based plated steel
sheet excellent in coating adhesiveness after hot pressing more conveniently.
Solution to Problem
10 [0022]
On the basis of the findings obtained by extensive studies on the plated steel
sheet for hot pressing of the object mentioned above, the present inventors have
thought up the following zinc-based plated steel sheet.
Main points of the present invention are as follows.
15 [0023]
(I)
A zinc-based plated steel sheet including:
a zinc-based plated steel sheet that is a base metal; and
a surface treatment layer formed on at least one surface of tl1e zinc-based
20 plated steel sheet and containing one or more magnesium compounds,
25
wherein the amount of the one or more magnesium compounds contained is
not less than 0.2 g/m2 and not more than 5.0 g/m2 per one surface on a magnesium
oxide basis.
(2)
The zinc-based hot-dipped steel sheet according to (1),
wherein the surface treatment layer further contains at least one of one or
more phosphorus-containing compounds, one or more vanadium-containing
compounds, one or more aluminum-containing compounds, one or more siliconcontaining
compounds, and one or more chromium-containing compounds in the
30 following range as the contained amount per one surface,
the one or more phosphorus-containing compounds: not less than 0.0 g/m2
PCT/JP2016/060800
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and not more than 0.01 g/m2 on a P basis,
the one or more vanadium-containing compounds: not less than 0.0 g/m2
and not more than 0.01 g/m2 on a V basis,
the one or more aluminum-containing compounds: not less than 0.0 g/m2
5 and not more than 0.005 g/m2 on an AI basis,
the one or more silicon-containing compounds: not less than 0.0 g/m2 and
not more than 0.005 g/m2 on a Si basis, and
the one or more chromium-containing compounds: not less than 0.0 g/m2
and not more than 0.01 g/m2 on a Cr basis.
10 (3)
The zinc-based plated steel sheet according to (1 ), wherein the one or more
magnesium compounds are magnesium oxide.
(4)
The zinc-based plated steel sheet according to (3 ), wherein the amount of
15 the magnesium oxide contained is not less than 0.4 g/m2 and not more than 2.5 g/m2
per one surface on a magnesium oxide basis.
(5)
The zinc-based plated steel sheet according to (3) or (4), wherein a particle
size of the magnesium oxide is not less than 5 nm and not more than 100 nm.
20 (6)
25
The zinc-based plated steel sheet according to any one of(3) to (5), wherein
a particle size of the magnesium oxide is not less than 10 nm and not more than 50
nm.
(7)
The zinc-based plated steel sheet according to (I) or (2), wherein the one or
more magnesium compounds are one or two compounds selected from the group
consisting of magnesium nitrate and magnesium sulfate.
(8)
The zinc-based plated steel sheet according to (7), wherein the amount of
30 the one or two compounds selected from the group consisting of magnesium nitrate
and magnesium sulfate contained is not less than 0.4 g/m2 and not more than 2.5
5
10
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g/m2 per one surface on a magnesium oxide basis.
(9)
The zinc-based plated steel sheet according to any one of (I) to (8), wherein
the zinc-based plated steel sheet is a zinc-based plated steel sheet for hot pressing.
Advantageous Effects oflnvention
[0024]
As described above, according to the present invention, it becomes possible
to improve the coating adhesiveness to a coating film provided after hot pressing.
Description of Embodiments
[0025]
Hereinbelow, preferred embodiments of the present invention are described
in detail.
15 [0026]
<1. Zinc-based plated steel sheet>
A Zn-based plated steel sheet according to an embodiment of the present
invention includes a Zn-based plating layer on a ground steel sheet, and further
includes a surface treatment layer described in detail below on at least one surface of
20 the Zn-based plating layer. The surface treatment layer contains one or more
magnesium compounds. The Zn-based plated steel sheet having such a
configuration can be suitably used for the hot pressing method described above; after
the hot pressing method is performed, magnesium oxide is formed on the outer layer.
The configuration of the Zn-based plated steel sheet will now be described in detail.
25 [0027]
( 1) Ground steel sheet
The ground steel sheet used for the Zn-based plated steel sheet according to
the present embodiment is not particularly limited, and various steel sheets having
known characteristics and chemical compositions may be used. The chemical
30 composition of the steel sheet is not particularly limited, but is preferably a chemical
composition with which high strength is obtained by quenching. For example,
PCT/JP2016/060800
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when it is attempted to obtain a heat-treated steel material with a tensile strength of
980 MPa or more, an example of the ground steel sheet is made of steel for
quenching having a chemical composition of, in mass%, C: 0.05 to 0.4%, Si: 0.5% or
less, Mn: 0.5 to 2.5%, P: 0.03% or less, S: 0.01% or less, sol. AI: 0.1% or less, N:
5 0.01% or less, B: 0 to 0.005%, Ti: 0 to 0.1 %, Cr: 0 to 0.5%, Nb: 0 to 0.1 %, Ni: 0 to
1.0%, Mo: 0 to 0.5%, and the balance: Fe and impurities.
[0028]
When it is attempted to obtain a heat-treated steel material with a relatively
low strength in which the strength becomes less than 980 MPa during quenching, the
10 chemical composition of the ground steel sheet may not be in the range described
above.
[0029]
The total amount of Mn and Cr contained is preferably 0.5 to 3.0% from the
viewpoint of quenchability during the quenching described above and the viewpoint
15 of forming Mn oxides and Cr oxides contained in a zinc oxide layer after heating.
The total amount ofMn and Cr contained is more preferably 0.7 to 2.5%.
---- -- -------- --
[0030]
When Mn and Cr are contained as the chemical composition of the steel
sheet, part of the zinc oxide layer formed on the outer layer after hot pressing
20 becomes composite oxides containing Mn and Cr. Coating adhesiveness after
phosphate-based chemical conversion treatment IS further improved by these
composite oxides containing Mn and Cr being formed. Although details are
unknown, it is presumed that, by these composite oxides being formed, the alkali
resistance of the phosphate-based chemical conversion treatment coating film formed
25 is improved as compared to zinc oxide, and good coating adhesiveness is exhibited.
[0031]
In the case where Mn and Cr are contained as the chemical composition of
the steel sheet, the total amount of Mn and Cr contained is preferably in the range of,
in mass%, not less than 0.5% and not more than 3.0%, and more preferably in the
30 range of, in mass%, not less than 0.7% and not more than 2.5%. When the total
amount of Mn and Cr contained is less than 0.5%, zinc oxide that is fonned on the
PCT/JP2016/060800
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outer layer after hot pressing and composite oxides that contain Mn and Cr are
insufficient, and it may be difficult to bring out better coating adhesiveness. On the
other hand, when the total amount of Mn and Cr contained is more than 3.0%,
although there is no problem with coating adhesiveness, the cost is increased, and
5 furthermore the toughness of the spot welded portion may be significantly reduced
and the wettability of plating may be significantly degraded.
[0032]
(2) Zn-based plating layer
The Zn-based plating layer according to the present embodiment is not
10 particularly limited, and commonly known Zn-based plating may be used.
Specifically, examples of the Zn-based plating layer according to the present
embodiment include Zn hot dipping, alloyed Zn hot dipping, Zn-55% Al-1.6% Si hot
dipping, Zn-11% Al hot dipping, Zn-11% Al-3% Mg hot dipping, Zn-6% Al-3% Mg
hot dipping, Zn-11% Al-3% Mg-0.2% Si hot dipping, Zn electroplating, Zn-Ni
15 electroplating, Zn-Co electroplating, and the like_ It is also effective to form a
covering of plating of the components mentioned above by a method such as vapor
deposition; thus, the method of plating is not particularly limited.
[0033]
In the present embodiment, as a specific plating operation in the case of
20 using Zn-based hot dipping, an operation in which a steel sheet is dipped in a plating
bath in which Zn or a Zn alloy in a molten state is retained and the steel sheet is
pulled up from the plating bath is performed. The amount of plating attached to the
steel sheet is controlled by adjusting the speed of the pulling-up of the steel sheet, the
flow rate and the flow velocity of wiping gas jetted from a wiping nozzle provided
25 above the plating bath, etc. Alloying treatment is performed by, after plating
treatment like the above, additionally heating the plated steel sheet using a gas
furnace or an induction heating furnace, a heating furnace in which these are
combined, or the like. The plating operation may also be performed by the method
of continuously plating a coil or the method of plating a cut sheet single body.
30 [0034]
In the present embodiment, as a specific plating operation in the case of
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using electroplating, electrolysis treatment is performed in an electrolyte solution
containing Zn ions, using the steel sheet, as a negative electrode, and a counter
electrode. The amount of plating attached to the steel sheet is controlled by the
composition of the electrolyte solution, the current density, and the electrolysis time.
5 [0035]
The thickness of the Zn-based plating layer (that is, the amount of the Znbased
plating layer attached) is preferably in the range of 20 g/m2 to 100 g/m2 per
one surface. When the thickness of the Zn-based plating layer is less than 20 g/m2
per one surface, the effective amount of Zn after hot pressing cannot be ensured and
10 corrosion resistance is insufficient; thus, this is not preferable. When the thickness
of the Zn-based plating layer is more than 100 glm2 per one surface, the
processability and the adhesiveness of the Zn-based plating layer are teduced; thus,
this is not preferable. A more preferred thickness of the Zn-based plating layer is in
the range of 30 g!m2 to 90 g/m2 per one surface.
15 (0036]
20
(3) Surface treatment layer
A surface treatment layer containing one or more magnesmm (Mg)
compounds is further formed on a Zn-based plating layer like the above.
[0037]
Here, the "magnesium compound" is a compound that, after hol pressing,
can be present as magnesium oxide (MgO) on the outer layer of the surface treatment
layer. The magnesium compound may be magnesium oxide itself. The
magnesium compound may also be a substance in which one or two or more
compounds or the like selected from the group consisting of magnesium chloride,
25 magnesium nitrate, and magnesium sulfate, which change to magnesium oxide after
hot pressing, are dissolved in a treatment liquid. By magnesium oxide being
present on the outer layer of the surface treatment layer after hot pressing, it becomes
possible to provide a heat-treated steel material (hot pressed steel material) excellent
in durability even in an environment of dipping in salt water.
30 [0038]
By magnesium oxide being present on the outer layer of the surface
PCT/JP2016/060800
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treatment layer after hot pressing, phosphate treatability is improved. As a reason
for the improvement in phosphate treatability, it is presumed that the chemical
conversion reaction with a phosphate is accelerated by magnesium oxide being
dissolved in the phosphate treatment liquid. Furthermore, magnesium oxide formed
5 after hot pressing has also good adhesiveness to the underlying Zn-based plating
layer. As a reason for the good adhesiveness to the Zn-based plating layer, it is
presumed that, during heating in the hot pressing method, part of the magnesium
compound(s) reacts with Zn and AI in the Zn-based plating layer and changes to a
composite oxide. It is presumed that, as a result of these, excellent coating
10 adhesiveness is exhibited even in a salt water dipping environment.
[0039]
The amount of the surface treatment layer attached is preferably not less
than 0.2 g/m2 and not more than 5.0 g/m2 per one surface on a magnesium oxide
basis both in the case where magnesium oxide is contained and in the case where a
15 treatment liquid containing one or two or more compounds selected from the group
consisting of magnesium chloride, magnesium nitrate, and magnesium sulfate is used.
When the amount of the surface treatment layer attached is less than 0.2 g/m2 per one
surface on a magnesium oxide basis, sufficient magnesium oxide is not present after
hot pressing; consequently, the effect of improving phosphate treatability by the
20 dissolving-out of Mg during phosphate treatment is reduced, and coating
adhesiveness after hot pressing cannot be ensured sufficiently. On the other hand,
when the amount of the surface treatment layer attached is more than 5.0 g/m2 per
one surface on a magnesium oxide basis, the cost of the Zn-based plated steel sheet
according to the present embodiment is increased, and it is presumed that the
25 cohesive force of the surface treatment layer is weakened and a coating film that is
formed on the surface treatment layer after hot pressing is likely to peel off. The
amount of the surface treatment layer attached is preferably not less than 0.4 g/m2
and not more than 2.5 g/m2 per one surface on a magnesium oxide basis.
30
[0040]
Here, the amount of the magnesium compound( s) contained in the surface
treatment layer can be measured by a known method; for example, the fact that the
PCT/JP2016/060800
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vanous compounds are magnesiUm compounds is checked beforehand by crosssectional
energy dispersive X-ray (EDX) analysis or the like, and then the coating
film is dissolved; thus, the measurement can be made using inductively coupled
plasma (ICP) emission spectrometric analysis or the like.
5 [0041]
When forming the surface treatment layer containing magnesiUm oxide,
magnesium oxide in the surface treatment layer is preferably in a particulate form
with a particle size (primary particle size) of not less than 5 nm and not more than
I 00 nm. For the particle size of magnesium oxide, a smaller size is advantageous in
10 terms of post-coating corrosion resistance, but those with a particle size of less than 5
nm are difficult to obtain and are disadvantageous in terms of cost. When the
particle size of magnesium oxide is more than I 00 nm, the surface area is reduced,
and accordingly it is feared that the reactivity with the underlying plating during
heating will be reduced; thus, this is not preferable. The particle size of magnesium
15 oxide is preferably not less than 10 nm and not more than 50 nnL
[0042]
The particle size (primary particle size) of particulate magnesium oxide like
the above can be measured by a known method; for example, the measurement can
be made by a method in which a cross section-embedded sample is prepared after
20 coating, several particle sizes of magnesium oxide in the coating film are measured,
and the average of the obtained measurement results is taken as the particle size.
[0043]
In the case where magnesium oxide is used to form the surface treatment
layer, a treatment liquid in which a powder of magnesium oxide is mixed with a resin
25 and a crosslinker, and water or any of various solvents is used as the solvent is used.
On the other hand, in the case where one or two or more compounds selected from
the' group consisting of magnesium chloride, magnesium nitrate, and magnesium
sulfate are used, a treatment liquid in which one or two or more of these magnesium
compounds are dissolved in water or any of various solvents, and a resin and a
30 crossiinker are mixed is used.
[0044]
5
10
PCT/JP2016/060800
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Examples of the resin include a polyurethane resin, a polyester resin, an
epoxy resin, a (meth)acrylic resin, a polyolefin resin, and a phenol resin, modified
products of these resins, and the like.
[0045]
Examples of the crosslinker include a zirconium carbonate compound, an
organic titanium compound, an oxazoline polymer, a water-soluble epoxy compound,
a water-soluble melamine resin, a water-dispersible blocked isocyanate, a waterbased
aziridine compound, etc.
[0046]
Examples of the other component that is preferably further contained in the
surface treatment layer according to the present embodiment include one or two or
more selected from zirconia, lanthanum oxide, cerium oxide, and neodymium oxide.
[0047]
When ztrcoma, lanthanum oxide, cenum oxide, or neodymium oxide
15 mentioned above is contained in the surface treatment layer, during heating, zirconia,
lanthanum oxide, cerium oxide, or neodymium oxide in the surface treatment layer
-----· ·-~----------- -- -----
makes harmless an Al oxide that is present before hot pressing and is formed during
hot pressing. Thereby, the formation of zinc oxide during hot pressing is
accelerated; thus, phosphate treatability after hot pressing is enhanced, and coating
20 adhesiveness is improved. Although details of the fact that an Al oxide is made
harmless during heating by zirconia, lanthanum oxide, cerium oxide, or neodymium
oxide are unknown, it is presumed that zirconia, lanthanum oxide, cerium oxide, or
neodymium oxide dissolves an Al oxide formed on the surface of the steel sheet,
thereby Zn, which is relatively easy to oxide after Al, is oxidized during hot pressing,
25 and consequently the production of zinc oxide (ZnO), which is excellent in chemical
convertibility, is accelerated. To obtain this effect more efficiently, the particle size
of the oxide mentioned above is preferably not less than 5 nm and not more than 500
30
nm.
[0048]
The amount of the one or two or more selected from zirconia, lanthanum
oxide, cerium oxide, and neodymium oxide contained in the snrface treatment layer
PCT/JP2016/060800
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is preferably in the range of not less than 0.2 g/m2 and not more than 2 g/nl per one
surface. When the amount of the one or two or more selected from zirconia,
lanthanum oxide, cerium oxide, and neodymium oxide contained in the surface
treatment layer is less than 0.2 g/m2 per one surface, sufficient zirconia, lanthanum
5 oxide, cerium oxide, and neodymium oxide are not present after hot pressing;
consequently, the effect of making harmless an AI oxide of the plated surface is
reduced, and it may be difficult to sufficiently ensure coating adhesiveness after hot
pressmg. On the other hand, when the amount of zirconia etc. contained in the
surface treatment layer is more than 2 g/m2 per one surface, the cost of the Zn-based
10 hot-dipped steel sheet according to the present embodiment is increased, and it is
presumed that the cohesive force of the surface treatment layer is weakened and a
coating film that is formed on the surface treatment layer after hot pressing is likely
to peel off.
[0049]
15 The amount of the one or two or more selected from zirconia, lanthanum
oxide, cerium oxide, and neodymium oxide contained in the surface treatment layer
-----------~· ------ - -------------- --------
is preferably not less than 0.4 g!d and not more than 1.5 g/m2 per one surface.
[0050]
Typical examples of the treatment liquid containing z1rcoma, lanthanum
20 oxide, cerium oxide, and neodymium oxide include a zirconia sol, a lanthanum oxide
sol, a cerium oxide sol, and a neodymium oxide sol, and specific examples of the
commercially available product include NanoUse (registered trademark) series
produced by Nissan Chemical Industries, Ltd. and Seramesu series produced by Taki
Chemical Co., Ltd.
25 [0051]
30
Examples of the other component that is preferably further contained in the
surface treatment layer according to the present embodiment include one or two or
more selected from titanium oxide, nickel oxide, and tin(IV) oxide.
[0052]
When the one or two or more selected from titanium oxide, nickel oxide,
and tin(IV) oxide mentioned above are contained in the surface treatment layer, these
PCT/JP2016/060800
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oxides are present on the surface of the steel sheet after hot pressing; thereby, some
influence is given to the cohesion deposition of an electrodeposition coating film
during electrodeposition coating, and the oxides and the electrodeposition coating
film adhere strongly; thus, strong adhesiveness can be exhibited even when chemical
5 conversion treatment (phosphate treatment or FF chemical conversion treatment) is
not sufficient. To obtain this effect more efficiently, the particle size of the oxide
mentioned above is preferably not less than 2 mn and not more than 100 mn.
[0053]
In addition, among these oxides, titanium oxide not only has the feature
10 mentioned above but also can suppress excessive oxidation and vaporization of Zn
during hot pressing, and can enhance not only coating adhesiveness after hot pressing
but also corrosion resistance after hot pressing. It is surmised that titanium oxide
usually exists in a state of a metal oxide stably, but reacts with zinc oxide formed
during heating in hot pressing and forms a composite oxide with zinc oxide, and
15 thereby suppresses excessive oxidation and vaporization of Zn. To obtain this
effect more efficiently, the particle size of titanium oxide mentioned above is
------------
preferably not less than 2 nm and not more than 100 mn.
[0054]
The particle size of the one or two or more selected from titanium oxide,
20 nickel oxide, and tin(IV) oxide mentioned above is more preferably not less than 5
mn and not more than 50 mn.
[0055]
In the case where the surface treatment layer contains titanium oxide, nickel
oxide, and tin(IV) oxide, these are contained preferably in the range of not less than
25 0.2 g/m2 and not more than 2 g/m2 per one surface, and more preferably in the range
of not less than 0.4 g/m2 and not more than 1.5 g/m2 per one surface. When the
amount of titanium oxide, nickel oxide, and tin(IV) oxide contained is less than 0.2
glm2 per one surface, these oxides are not present sufficiently after hot pressing, and
consequently it may be difficult to bring out still better adhesiveness to the
30 electrodeposition coating film.
[0056]
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On the other hand, when the amount of titanium oxide, nickel oxide, and
tin(IV) oxide contained is more than 2 g/m2 per one surface, the cost of the Zn-based
plated steel sheet according to the present embodiment is increased, and it is
presumed that the cohesive force of the surface treatment layer is weakened and a
5 coating film that is formed on the surface treatment layer after hot pressing is likely
to peel off.
[0057]
In addition to the above, when the amount of titanium oxide contained is
less than 0.2 g/m2 per one surface, a sufficient amount of a composite oxide with zinc
10 oxide cannot be formed, and it may be difficult to efficiently suppress the oxidation
and vaporization of Zn.
15
[0058]
The surface treatment layer according to the present embodiment may
contain, in addition to oxides like the above, at least one of one or more P-containing
compounds, one or more V-containing compounds, one or more Cu-containing
compounds, one or more Al-containing compounds, one or more Si-containing
---~ ~~--~ -____ __ _
compounds, and one or more Cr-containing compounds described in detail below in
the range of a prescribed content.
[0059] -
20 The P-containing compound is a compound containing phosphorus as a
constituent element. Examples of the P-containing compound include compounds
such as phosphoric acid, phosphorous acid, phosphonic acid, phosphonous acid,
phosphinic acid, phosphinous acid, a phosphine oxide, and phosphine, an ionic
compound containing any of these compounds as an anion, and the like. All these
25 P-containing compounds are commercially available as reagents or products, and can
be easily obtained. These P-containing compounds exist in a state of being
dissolved in a treatment liquid or in a state of being dispersed as powder in a
treatment liquid, and exist, in the surface treatment layer, in a state of being dispersed
as solid.
30 [0060]
The V-containing compound IS a compound containing vanadium as a
PCT/JP20 16/060800
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constituent element. Examples of the V-containing compound include vanadium
oxides such as vanadium pentoxide, metavanadic acid-based compounds such as
ammonium metavanadate, vanadium compounds such as sodium vanadate, other Vcontaining
compounds, and the like. All these V-containing compounds are
5 commercially available as reagents or products, and can be easily obtained. These
V-containing compounds exist in a state of being dissolved in a treatment liquid or in
a state of being dispersed as powder in a treatment liquid, and exist, in the surface
treatment layer, in a state of being dispersed as solid.
10
[0061]
The surface treatment layer according to the present embodiment preferably
contains one or two or more compounds selected from one or more P-containing
compounds and one or more V-containing compounds like the above individually in
the range of not less than 0.0 g/m2 and not more than 0.01 g/m2 per one surface on a
P and V basis.
15 [0062]
One or two or more compounds selected from one or more P-containing
------ ------- ------
compounds and one or more V-containing compounds like the above are oxidized
into an oxide during hot pressing, and the oxide exists locally at the interface
between the Zn-based plating layer and the surface treatment layer and forms an
20 oxide layer that contains at least one of P and V and has weak cohesive force. Since
the amount of the one or two or more compounds selected from one or more Pcontaining
compounds and one or more V-containing compounds contained is
individually in the range of not less than 0.0 g/m2 and not more than 0.01 g/m2 per
one surface on a P and V basis, the thickness of an-oxide layer like the above that is
25 formed during hot pressing and has weak cohesive force is reduced, and the
adhesiveness between the Zn-based plating layer and the surface treatment layer after
hot pressing is further improved.
[0063]
When the amount of the one or two or more selected from one or more P-
30 containing compounds and one or more V-containing compounds contained in the
surface treatment layer is more than 0.01 g/m2 per one surface, the thickness of the
PCT/JP2016/060800
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oxide layer that is formed during hot pressmg and has weak cohesive force is
increased; consequently, the adhesiveness between the Zn-based plating layer and the
surface treatment layer is reduced, and as a result also adhesiveness after
electrodeposition coating is reduced. From the viewpoint of the adhesiveness
5 between the Zn-based plating layer and the surface treatment layer after hot pressing,
the amount of the one or two or more compounds selected from one or more Pcontaining
compounds and one or more V-containing compounds contained in the
surface treatment layer is more preferably individually not less than 0.0 g/m2 and not
more than 0.003 g/m2 per one surface on a P and V basis.
10 [0064]
The Al-containing compound is a compound containing aluminum as a
constituent element. Examples of the Al-containing compound include metal Al,
aluminum oxide, aluminum hydroxide, an ionic compound containing an aluminum
ion as a cation, and the like. All these A)-containing compounds are commercially
15 available as reagents or products, and can be easily obtained. These Al-containing
compounds exist in a state of being dissolved in a treatment liquid or in a state of
being dispersed as powder in a treatment liquid, and exist, in the surface treatment
layer, in a state of being dispersed as solid.
[0065]
20 The Si-containing compound is a compound containing silicon as a
constituent element. Examples of the Si-containing compound include Si simple
substance, silica (silicon oxide), organic silane, a silicone resin used also as a binder
resin, and other Si-containing compounds. All these Si-containing compounds are
commercially available as reagents or products, and can be easily obtained. These
25 Si-containing compounds exist in a state of being dissolved in a treatment liquid or in
a state of being dispersed as powder in a treatment liquid, and exist, in the surface
treatment layer, in a state of being dispersed as solid.
[0066]
The surface treatment layer according to the present embodiment preferably
30 contains one or two or more compounds selected from one or more A)-containing
compounds and one or more Si-containing compounds like the above individually in
PCT/JP2016/060800
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the range of not less than 0.0 g/m2 and not more than 0.005 g/m2 per one surface on
an AI and Si basis.
[0067]
One or two or more compounds selected from one or more Al-containing
5 compounds and one or more Si-containing compounds like the above are oxidized
into an oxide during hot pressing, and the oxide concentrates on the surface of the
surface treatment layer. Since the amount of the one or two or more compounds
selected from one or more A)-containing compounds and one or more Si-containing
compounds contained is individually in the range of not less than 0.0 g/m2 and not
10 more than 0.005 g/m2 per one surface on an AI and Si basis, the existence ratio of the
oxides containing AI or Si that are formed on the surface of the surface treatment
layer during hot pressing is reduced, and the adhesiveness between the surface
treatment layer and the electrodeposition coating film after hot pressing is further
improved.
15 [0068]
When the amount of the one or two or more selected from one or more AIcontaining
compounds and one or more Si-containing compounds contained in the
surface treatment layer is more than 0.005 g/m2 per one surface, the existence ratio of
the oxides containing AI or Si that are formed during hot pressing is increased.
20 These oxides containing AI or Si inhibit the formation of a chemical conversion
treatment coating film, and reduce the adhesiveness between the surface treatment
layer and the electrodeposition coating film after hot pressing; therefore, when the
existence ratio of the oxides containing AI or Si that are formed during hot pressing
is increased, the adhesiveness between the surface treatment layer and the
25 electrodeposition coating film is reduced. From the viewpoint of the adhesiveness
between the surface treatment layer and the electrodeposition coating film after hot
pressing (that is, post-coating adhesiveness), the amount of the one or two or more
compounds selected from one or more A)-containing compounds and one or more Sicontaining
compounds contained in the surface treatment layer is more preferably
30 individually not less than 0.0 g/m2 and not more than 0.002 g/m2 per one surface on
an AI and Si basis.
PCT/JP2016/060800
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[0069]
The Cr-containing compound is a compound containing chromium as a
constituent element. Examples of the Cr-containing compound include metal Cr,
chromium compounds having various valences, an ionic compound containing a
5 chromium ion having any of various valences as a cation, and the like. These Crcontaining
compounds exist in a state of being dissolved in a treatment liquid or in a
state of being dispersed as powder in a treatment liquid, and exist, in the surface
treatment layer, in a state of being dispersed as solid.
10
[0070]
The Cr-containing compound vanes m performance and properties m
accordance with the valence, and many hexavalent chromium compounds are
harmful. · In view of the cunent tendency of attention to environmental protection
being strongly required, the surface treatment layer according to the present
embodiment preferably contains as little amount of Cr-containing compounds like
15 the above as possible, and is more preferably chromium-free.
[0071]
From the above point of view, the surface treatment layer according to the
present embodiment preferably contains one or two or more compounds selected
from one or more Cr-containing compounds like the above in the range of not less
20 than 0.0 g/m2 and not more than 0.01 g/m2 per one surface on a Cr basis, and more
preferably contains no Cr-containing compound.
[0072]
The surface treatment layer may contain pigments such as carbon black and
titania, various anti-rust pigments used for coated steel sheets, and the like as long as
25 the effect of the present invention based on containing a magnesium compound is not
inhibited.
[0073]
As the method for forming the surface treatment layer, a treatment liquid
containing a magnesium compound may be applied to the surface of a zinc-plated
30 steel sheet, and drying and baking may be performed.
[0074]
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The coating method is not limited to a specific method, and examples
include a method in which a ground steel sheet is dipped in a treatment liquid or a
treatment liquid is sprayed to the surface of a ground steel sheet, and then the
attached amount is controlled by a roll or gas spraying so as to obtain a prescribed
5 attached amount, and a method of coating using a roll coater or a bar coater.
[0075]
The method of drying and baking is not limited to a specific method, either,
as long as it is a method that can volatilize a dispersion medium (mainly water).
Here, if heating is performed at an excessively high temperature, it is feared that the
10 uniformity of the surface treatment layer will be reduced; conversely, if heating is
performed at an excessively low temperature, it is feared that productivity will be
reduced. Thus, to produce a surface treatment layer having excellent characteristics
stably and efficiently, the surface treatment layer after coating is preferably heated at
a temperature of approximately 80°C to 150°C for approximately 5 seconds to 20
15 seconds.
[0076]
The formation of the surface treatment layer is preferably performed in-line
in the production line of the plated steel sheet because this is economical; but the
surface treatment layer may be formed also in another line, or may be formed after
20 blanking for molding is performed.
[0077]
Here, the amount of the contained various oxides mentioned above that are
preferably contained in the surface treatment layer can be measured by a known
method; for example, the fact that the various compounds are the oxides of attention
25 is checked beforehand by cross-sectional energy dispersive X-ray (EDX) analysis or
the like, and then the coating film is dissolved; thus, the measurement can be made
using inductively coupled plasma (ICP) emission spectrometric analysis or the like.
Also the amount of the above-mentioned one or more P-containing compounds, Vcontaining
compounds, Cu-containing compounds, Al-containing compounds, Si-
30 containing compounds, and Cr-containing compounds contained in the surface
treatment layer can be measured by a similar method.
PCT/JP2016/060800
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[0078]
<2. With regard to hot pressing process>
In the case where the hot pressing method is used for a Zn-based plated steel
sheet like that described above, the Zn-based plated steel sheet is heated to a
5 prescribed temperature, and is then press-molded. In the case of the Zn-based
plated steel sheet according to the present embodiment, heating is usually performed
to 700 to 1 000°C because hot press molding is performed; but in the case where a
martensite single phase is formed after rapid cooling or martensite is formed at a
volume ratio of 90% or more, it is important that the lower limit of the heating
10 temperature be the Ac3 point or more. In the case of the present invention, also the
case where a two-phase region of martensite/ferrite is formed after rapid cooling is
included, and therefore the heating temperature is preferably 700 to 1 000°C as above.
[0079]
Examples of the hot pressing method include two methods of hot pressing
15 by slow heating and hot pressing by rapid heating. Examples of the heating method
used include heating with an electric furnace or a gas furnace, flame heating,
energization heating, high-frequency heating, induction heating, etc., and the
atmosphere during heating is not particularly limited; as a heating method to obtain
the effect of the present invention significantly, energization heating, induction
20 heating, and the like, which are rapid heating, are preferably used.
[0080]
In the hot pressing method by slow heating, the radiation heating of a
heating furnace is used. First, the Zn-based plated steel sheet according to the
present embodiment that is used as a steel sheet for hot pressing is placed in a
25 heating furnace (a gas furnace, an electric furnace, etc.). The steel sheet for hot
pressing is heated at 700 to 1 000°C in the heating furnace, and is, depending on the
condition, kept at this heating temperature (soaking). Thereby, Zn in the Zn-based
plating layer is combined with Fe and forms a solid phase (an Fe-Zn solid solution
phase). After the molten Zn in the Zn-based plating layer is combined with Fe and
30 forms a solid phase, the steel sheet is taken out of the heating furnace. Alternatively,
by combining Zn in the Zn-bascd plating layer with Fe by soaking, the solid phase
PCT/JP2016/060800
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may be formed as an Fe-Zn solid solution phase and a ZnFe alloy phase; and then the
steel sheet may be taken out of the heating furnace.
[0081]
Alternatively, the Zn-based plated steel sheet may be heated to 700 to
5 I 000°C while no keeping time is provided or the keeping time is set to a short time,
and the steel sheet may be taken out of the heating furnace. In this case, after the
steel sheet is heated to 700 to I 000°C, cooling is performed without applying stress
to the steel sheet by press molding or the like until Zn in the Zn-based plating layer is
combined with Fe and forms a solid phase (an Fe-Zn solid solution phase or a ZnFe
10 alloy phase). Specifically, cooling is performed until at least the temperature of the
steel sheet becomes 782°C or less. After the cooling, as described below, cooling is
performed while the steel sheet is pressed using a mold.
[0082]
Also in hot pressing by rapid heating, similarly, the Zn-based plated steel
15 sheet according to the present embodiment that is used as a steel sheet for hot
pressing is rapidly heated to 700 to 1000°C. The rapid heating is performed by, for
example, energization heating or induction heating. The average heating rate in this
case is 20°C/second or more. In the case of rapid heating, after the Zn-based plated
steel sheet is heated to 700 to I 000°C, cooling is performed without applying stress
20 to the steel sheet by press molding or the like until Zn in the Zn-based plating layer is
combined with Fe and forms a solid phase (an Fe-Zn solid solution phase or a ZnFe
alloy phase). Specifically, cooling is performed until at least the temperature of the
steel sheet becomes 782°C or less. After the cooling, as described below, cooling is
performed while the steel sheet is pressed using a mold.
25 [0083]
The taken-out steel sheet is pressed using a mold. When pressing the steel
sheet, the steel sheet is cooled by the mold. A cooling medium (e.g., water etc.) is
circulated through the mold, and the mold removes heat from the steel sheet and
cools it. By the above process, a hot pressed steel material is produced by nmmal
30 heating.
[0084]
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The hot pressed steel material produced using the Zn-based plated steel
sheet including the surface treatment layer according to the present embodiment has
excellent phosphate treatability and coating adhesiveness. In particular, the Znbased
plated steel sheet according to the present embodiment exhibits the effect
5 significantly in the case where heating is performed at 700 to 1000°C by hot pressing
by rapid heating or hot pressing by slow heating while no keeping time is provided or
the keeping time is set to a short time.
[0085]
In the case where hot pressing by normal heating is performed using a
10 conventional plated steel sheet, the steel sheet is soaked in a heating furnace. In this
case, although an AI oxide film is formed on the outer layer of the plating layer of the
steel sheet for hot pressing, the AI oxide film is broken and divided to some degree
due to long time soaking, and therefore the adverse effect on chemical conversion
treatability is smalL On the other hand, in the case where hot pressing by rapid
15 heating is performed, the soaking time is very short. Hence, the AI oxide film
formed on the outermost surface is less likely to be broken. Thus, in hot pressing
by rapid heating in the case where a conventional plated steel sheet is used, the
phosphate treatability and the coating adhesiveness of the hot pressed steel material
are low as compared to hot pressing by normal heating.
20 [0086]
On the other hand, the Zn-based plated steel sheet for hot pressing according
to the present embodiment contains one or more magnesium compounds in the
surface treatment layer, and thereby makes the AI oxidization harmless and
accelerates the production of zinc oxide during hot pressing; ·and can thus exhibit
25 good phosphate treatability and coating adhesiveness.
[0087]
The action and effect of the Zn-based plated steel sheet according to an
embodiment of the present invention will now be described still more specifically
with reference to Examples. Examples shown below are only examples of the Zn-
30 based plated steel sheet according to the present invention, and the Zn-based plated
steel sheet according to the present invention is not limited to Examples below.
PCT/JP2016/060800
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[0088]

In the following, first, p1eces of molten steel having the chemical
compositions shown in Table 1 below were produced. After that, the produced
5 pieces of molten steel were used to produce slabs by the continuous casting method.
The obtained slab was hot rolled to produce a hot rolled steel sheet. Subsequently,
the hot rolled steel sheet was pickled, and then cold rolling was performed to produce
a cold rolled steel sheet; thus, steel sheets of steel #1 to #8 having the chemical
compositions described in Table 1 were prepared. As shown in Table 1, the sheet
10 thicknesses of the steel sheets of all the steel types were 1.6 mm.

Claim 1
A zinc-based plated steel sheet comprising:
a zinc-based plated steel sheet that is a base metal; and
a surface treatment layer formed on at least one surface of the zinc-based
plated steel sheet and containing one or more magnesium compounds,
wherein the amount of the one or more magnesium compounds contained is
not less than 0.2 g/m2 and not more than 5.0 g/m2 per one surface on a magnesium
oxide basis.
Claim2
The zinc-based plated steel sheet according to claim 1,
wherein the surface treatment layer further contains at least one of one or
more phosphorus-containing compounds, one or more vanadium-containing
15 compounds, one or more aluminum-containing compounds, one or more siliconcontaining
compounds, and one or more chromium-containing compounds in the
following range as the contained amount per one surface,
the one or more phosphorus-containing compounds: not less than 0.0 g/m2
and not more than 0.0 I g/m2 on a P basis,
20 the one or more vanadium-containing compounds: not less than 0.0 g/m2
and not more than 0.01 g/m2 on a V basis,
the one or more aluminum-containing compounds: not less than 0.0 g/m2
and not more than 0.005 g/m2 on an AI basis,
the one or more silicon-containing compounds: not less than 0.0 g/m2 and
25 not more than 0.005 g/m2 on a Si basis, and
30
the one or more chromium-containing compounds: not less than 0.0 g/m2
and not more than 0.01 g/m2 on a Cr basis.
Claim 3
The zinc-based plated steel sheet according to claim 1 or 2, wherein the one
or more magnesium compounds are magnesium oxide.
PCT/JP2016/060800
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Claim4
The zinc-based plated steel sheet according to claim 3, wherein the amount
of the magnesium oxide contained is not less than 0.4 g/m2 and not more than 2.5
5 g/m2 per one surface on a magnesium oxide basis.
10
15
20
ClaimS
The zinc-based plated steel sheet according to claim 3 or 4, wherein a
particle size of the magnesium oxide is not less than 5 nm and not more than 100 nm.
Claim6
The zinc-based plated steel sheet according to any one of claims 3 to 5,
wherein a particle size of the magnesium oxide is not less than 10 nm and not more
than 50 nm.
Claim 7 ------- -<- -------
The zinc-based plated steel sheet according to claim 1 or 2, wherein the one
or more magnesium compounds are one or two compounds selected from the group
consisting of magnesium nitrate and magnesium sulfate.
ClaimS
The zinc-based plated steel sheet according to claim 7, wherein the amount
of the one or two compounds selected from the group consisting of magnesium
nitrate and magnesium sulfate contained is not less than 0.4 g/m2 and not more than
25 2.5 g/m2 per one surface.
Claim 9
The zinc-based plated steel sheet according to any one pf claims 1 to 8,
wherein the zinc-based plated steel sheet is a zinc-based plated steel sheet for hot
30 pressmg.