Technical Field
[0001] The present invention relates to a surface
treatment solution giving a surface treatment coating
excellent in waterproofness, solvent resistance,_and
10 adhesion with a plated steel sheet.
Background Art
[0002] In recent years, to protect the environment and
prevent global warming, there has been rising demand for
keeping down the consumption of chemical fuels. This
15 demand has influenced various manufacturing industries.
For example, automobiles, essential as transportation for
daily life and activities, are no exceptions. Improvement
of fuel efficiency and the like through weight reduction
of car bodies and the like is being sought. However, with
20 automobiles, simple weight reduction of car bodies is not
allowed in terms of product quality. Suitable safety has
to be secured.
[0003] Most of the structure of a car body is formed
by iron, in particular steel sheet. Reducing the mass of
25 the steel sheet is important in the weight reduction of
car body. However, as explained above, it is not allowed
to just reduce the mass of the steel sheet. It is
demanded that the mechanical strength of the steel sheet
be secured. Similar demands on steel sheet are being made
30 in various manufacturing industries besides the
automobile manufacturing industry.
[0004] For this reason, R&D is underway on raising the
mechanical strength of steel sheet so as to obtain steel
sheet enabling the mechanical strength to be maintained
35 or further raised even if making the sheet thinner than
steel sheet used up to now.
[0005] In general, material having a high mechanical
- 2 -
strength often has a low shape freezability in bending or
other forming processes, so when working it into
complicated shapes, the working process itself becomes
difficult. As one of the means for solving this problem
5 relating to formability, there is the so-called "hotpressing"
(also called the 11 hot-stamping" 1 "hot-press",
or ''die quenching'').
[0006] In the hot-pressing, the material to be shaped
is heated once to a high temperature to make it soften,
10 is press-formed, and then is cooled. According to this
hot-pressing, since the material is heated once to a high
temperature to make it soften, it can be easily pressed
by a die. Furthermore, due to the hardening by the
contact cooling with the die after shaping, the
15 mechanical strength of the material rises. Therefore,
hot-pressing enables products with both good shape
freezability and high mechanical strength to be obtained.
[0~07] However, when applying the hot-pressing to
steel sheet, the steel sheet is heated to, for example, a
20 800°C or more high temperature, so the iron at the steel
sheet surface oxidizes to form scale (oxides). Therefore,
after hot-pressing, a process to remove the scale
(descaling process) is necessary, so the productivity
falls. Further, in the case of a member requiring
25 corrosion resistance, it is necessary to treat the
surface of the member after being shaped so as to prevent
rust or to cover the metal. For this reason, a surface
cleaning process and a surface treatment process is
further necessary and the productivity falls.
30 [0008] As such a method of suppressing the fall in
productivity, there is the method of covering the steel
sheet in advance. In general, as normal materials for
covering steel sheet, organic materials, inorganic
materials, and various other materials are being used.
35 Among these, steel sheet covered by zinc plating, which
has a sacrificial corrosion action against steel sheet,
is being widely used for steel sheet for automobiles and
- 3 -
the like from the viewpoint of corrosion prevention
performance and production technology.
[0009] However, the heating temperature in hotpressing
(700 to 1000°C) is higher than the decomposition
5 temperature of organic materials and the melting point
and boiling point of the metal materials such as Znbased,
so when heated at the time of hot-pressing,
sometimes the plating layer of the steel sheet surface
melts _and evaporates and the surface properties
10 remarkably deteriorate.
[0010] Accordingly, as steel sheet for hot-pressing,
for example, steel sheet covered by metal containing Al
having a higher melting point and boiling point compared
with a covering of an organic material or a covering of a
15 Zn-based metal, so-called Al plated steel sheet, is often
used.
[0011] If covering a steel sheet with a metal
containing Al, it is possible to prevent the formation of
scale at the steel sheet surface and a descaling process
20 and other processes become unnecessary, so the
productivity is improved. Further, a covering of metal
containing Al also has a rustproofing effect, so the
corrosion resistance after painting is also improved~
[0012] Patent Literature 1 discloses the technique of
25 using a wurtzite type compound (ZnO) to cover the surface
of anAl plated steel sheet andimprove the hot-pressing
lubricity and chemical convertability (chemical coating
formability) after hot-pressing. This technique is
effective for improvement of the hot-pressing lubricity,
30 gives excellent chemical convertability after hotpressing,
and improves the corrosion resistance after
painting as well.
[0013] However, in Patent Literature 1, a watersoluble
resin, silane coupling agent, and the like is
35 used as a binder for the wurtzite type compound of the
ZnO particles, so the coating is inferior in
waterproofness and solvent resistance. Further, the
- 4 -
adhesion between the coating and Al plated steel sheet is
not sufficient. Therefore, at locations where the ZnO
coating drops off, hot-pressing lubricity, chemical
convertability after hot-pressing, and corrosion
5 resistance after painting are also not stably expressed.
Further, the ZnO particles in the treatment solution are
not sufficiently stable, so the ZnO particles easily
aggregate and precipitate and the productivity falls.
Citation List
10 Patent Literature
[0014] Patent Literature
Summary of Invention
Technical Problem
1: W02009/131233A
[0015] As explained above, Al plated steel sheet
15 having high melting point Al (Al plated steel sheet) is
considered promising as steel sheet for automobile where
corroslon resistance is demanded. Technique is proposed
for using the wurtzite type compound of ZnO particles and
water-soluble resin to cover the surface of Al plated
20 steel sheet to thereby improve the hot-pressing lubricity
and chemical convertability after hot-pressing. However,
if using a water-soluble resin, since a sufficient
performance cannot be obtained as a binder, the coating
becomes inferior in waterproofness and solvent
25 resistance. Further, adhesion of the ZnO coating with
respect to the plated steel sheet cannot be obtained. As
a result, in actuality, the hot-pressing lubricity, the
chemical convertability after hot-pressing, and the
corrosion resistance after painting cannot be stably
30 obtained. Further, if using a silane coupling agent as a
binder, silica remains after heating and the chemical
convertability at the time of painting is obstructed, so
the corrosion resistance after painting cannot be
obtained.
35 [0016] Therefore, the present invention, in
consideration of the above problem, has as its problem
the improvement of the waterproofness, solvent
- 5 -
resistance, and adhesion with the plated steel sheet of
surface treatment coating in a plated steel ~heet so as
to stably secure formability at the time of hot-pressing,
the chemical convertability after hot-pressing, the
5 corrosion resistance after painting, and the spot
weldability and has as its object the provision of a
surface treatment solution of plated steel sheet for hotpressing
solving this problem.
10
15
Solution to Problem
[0017] The inventors engaged in intensive studies to
solve the above problem and as a result found that by
using a surface treatment solution containing a specific
ZnO aqueous dispersion and water dispersible organic
resin to form a surface treatment coating containing ZnO
particles and a water dispersible organic resin in a
specific mass ratio on the surface of a plated steel
sheet, the waterproofness, solvent resistance, and
adhesion with the plated steel sheet of the coating are
improved and further a hot-pressing lubricity, chemical
20 convertability after hot-pressing, corrosion resistance
after painting, and spot weldability are stably secured.
That is, the gist of the present invention is as follows:
[0018] (1) A surface treatment solution for a plated
steel sheet to be hot-pressed comprising a ZnO aqueous
25 dispersion (A) and a water dispersible organic resin (B) ,
wherein
the ZnO aqueous dispersion (A) comprises water and ZnO
particles having an average particle size of 10 to 300
nm,
30 the water dispersible organic resin (B) has a 5 to 300 nm
emulsion average particle size, and
35
a mass ratio (WA/WB) of a mass (WA) of ZnO particles in
the ZnO aqueous dispersion to a mass (WB) of solid content
in the water dispersible organic resin is 30/70 to 95/5.
[0019] (2) The surface treatment solution for a plated
steel sheet to be hot-pressed according to (1), wherein
the water dispersible organic resin (B) is one or more
- 6 -
types of resin selected from a group consisting of a
- water dispersible polyurethane resin, a water dispersible
epoxy resin, a water dispersible acryl resin, and a water
dispersible polyester resin.
5 [0020] (3) The surface treatment solution for a plated
steel sheet to be hot-pressed according to (1) or (2),
wherein the water dispersible organic resin (B) has a
number average molecular weight of 10000 or more.
[0021] (4) The surface treatment solution for a plated
10 steel sheet to be hot-pressed according to any one of (1)
to (3), wherein the water dispersible organic resin (B)
is one or more types of resin having a 5 to 45 mgKOH/g
acid value and selected from a group consisting of a
water dispersible polyurethane resin, a water dispersible
15 acryl resin, and a water dispersible polyester resin.
[0022] (5) The surface treatment solution for a plated
steel sheet to be hot-pressed according to any one of (1)
to (4), comprising at least one type of additive (C)
selected from compounds comprising elements selected from
20 the group consisting of B, Mg, Si, Ca, Ti, V, Zr, W, and
Ce.
[0023] (6) The surface treatment solution for a plated
steel sheet to be hot-pressed according to (5), wherein a
mass ratio (Wc/WA) of a mass (WA) of the ZnO particles in
25 the ZnO aqueous dispersion to a mass (We) of the additive
(C) is 0.05 to 0.6.
[0024] (7) The surface treatment solution for a
steel sheet to be hot-pressed according to any one
plated
of ( 1)
to (6), wherein the ZnO particles have an aspect ratio
30 (long axis/short axis) of 1 to 2.8.
[0025] (8) The surface treatment solution for a plated
steel sheet to be hot-pressed according to any one of (1)
to (7), wherein the ZnO aqueous dispersion (A) comprises
one or both of an anionic dispersant and nonionic
35 dispersant.
Advantageous Effects of Invention
[0026] According to the present invention, it is
- 7 -
possible to provide a specific surface treatment
solution, excellent in stabilitr at the time of storage,
containing a ZnO aqueous dispersion and water dispersible
organic resin. Further, it is possible to use this
5 surface treatment coating to secure waterproofness,
solvent resistance, and adhesion with a plated steel
sheet of the coating and to stably obtain a plated steel
sheet excellent in hot-pressing lubricity, chemical
convertability after hot-pressing, corrosion resistance
10 after painting, and spot weldability.
Brief Description of Drawings
[0027] FIG. 1 is a schematic view of a hot Bowden
tester used in the examples.
Description of Embodiments
15 [0028]
The steel sheet which is able to use the surface
treatment solution of the present invention is a plated
steel sheet, for example, steel sheet on one side or both
sides of which a plating layer containing at least Al is
20 formed.
[0029] I Steel Sheet)
The steel sheet is preferably steel sheet having the
required mechanical characteristics after hot-pressing
(meaning various characteristics including tensile
25 strength, yield point, elongation, drawing ability,
hardness, impact value, fatigue strength, creep strength,
and the like relating to mechanical deformation and
fracture). The above steel sheet, for example, contains,
by mass%, at least one or more of C: 0.1 to 0.4%, Si:
30 0.01 to 0.6%, Mn: 0.5 to 3%, Ti: 0.01 to 0.1%, and B:
0.0001 to 0.1% and has a balance of Fe and unavoidable
impurities. "Hot pressingn means, for example, making the
steel sheet 900°C by raising the temperature in the
atmosphere to 900°C, then holding the sheet there for 1
35 minute and rapidly cooling it through the die.
[0030] (Plating Layer)
The plating layer of the plated steel sheet, as explained
- 8 -
above, is formed on one side or both sides of the steel
sheet. The plating layer is, for example, formed by the
hot dip coating method, but the method of forming the
plating layer is not limited to the hot dip coating
5 method. The object of forming the plating layer includes
the prevention of formation of scale (iron oxides) at the
steel sheet surface at the time of the heating in the
hot-pressing, so the plating layer is preferably formed
at both sides of the steel sheet.
10 [0031] The plating layer should contain at least Al.
The content of Al in the plating layer is at least 10
mass%, typically is 80 mass% or more~ The components
other than Al are not particularly limited, but Si may be
added for the following reasons.
15 [0032] Si is an element suppressing the formation of a
Fe-Al alloy layer at the time of hot dip coating. If the
amount of addition of Si in the plating layer containing
Al is less than 3 mass%, at the time of hot dip coating,
the Fe-Al alloy layer thickly grows. Further, at the time
20 of working, there is a possibility of fracture of the
plating layer being aggravated and the corrosion
resistance being impaired. Further, if the amount of
addition of Si in the plating layer containing Al is over
15 mass%, the plating layer containing Al will fall in
25 workability and corrosion resistance. For this reason,
the amount of addition of Si in the plating layer
containing Al is preferably 3 to 15 mass%.
[0033] The plating layer containing Al of the above
chemical composition prevents the corrosion of steel
30 sheet during transport before hot-pressing and during
transport after hot-pressing and when plated steel sheet
is used as an auto-part. Further, at the time of heating
during hot-pressing, it prevents the formation of scale
(iron oxides) at the steel sheet surface. These effects
35 are obtained when the content of Al in the plating layer
is 10 mass% or more, more preferably are obtained at 80
mass% or more.
- 9 -
[0034] By the presence of a plating layer containing
Al at one side or bGth sides of steel sheet, the
descaling process after hot-pressing, the surface
cleaning process, the surface treatment process, and the
5 like may be omitted or simplified, so the productivity is
improved. Further, the plating layer containing Al has a
higher melting point and boiling point than covering by
an organic material or covering by another metal material
(for example, a Zn-based material), so at the time of
10 hot-pressing, high temperature working becomes possible.
[0035] Part of the Al contained in the plating layer
containing Al becomes alloyed with the Fe in the steel
sheet at the time of hot dip coating and the time of hotpressing.
Accordingly, the plating layer containing Al is
15 not necessarily formed by a single layer with a constant
chemical composition, but is a plating layer including
partially alloyed layers (alloy layers).
[0036]
The surface treatment solution for the plated steel sheet
20 to be hot-pressed of the present invention is a surface
treatment solution for a plated steel sheet to be hotpressed
containing a ZnO aqueous dispersion (A) comprised
of average particle size 10 to 300 nm ZnO particles
dispersed in water and a water dispersible organic resin
25 (B) having an emulsion average particle size of 5 to 300
nm and having a mass ratio (WA/W8 ) of the mass of the ZnO
particles in the ZnO aqueous dispersion (WAI to the mass
of the solid content in the water dispersible organic
resin (W8 ) of 30/70 to 95/5.
30 [0037] The "mass ratio (WA/W8)" referred to here, for
example, may be obtained by taking a predetermined amount
of the surface treatment solution (mass W) and heating it
in the atmosphere at 900°C to cause the water content to
evaporate, burning off the water dispersible organic
35 resin, designating the remaining ZnO amount as WA, and
calculating Ws from W-WA to find (Wn/Ws)
[0038] (ZnO Aqueous Dispersion (A))
- 10 -
The ZnO particles contained in the ZnO aqueous dispersion
(A) used in the present invention has an average particle
size of 10 to 300 nm. In the ZnO aqueous dispersion (A) ,
it is considered that the ZnO particles may be partially
5 aggregated. Therefore, the "average particle size"
referred to here is the average particle size including
also aggregated particles. If the ZnO particles exceed
300 nm in average particle size, they precipitate and
caking easily occurs, so the storage stability of the
10 surface treatment solution cannot be secured. Further, if
the ZnO particles are less than 10 nm in average particle
size, the particles easily aggregate and precipitate
along with the elapse of time making it impossible to
secure the storage stability of the surface treatment
15 solution. In this case, it sometimes becomes necessary to
blend in a large amount of dispersant to stabilize the
dispersion of the surface treatment solution, so this is
not preferable.
[0039] The average particle size of the ZnO particles
20 in the ZnO aqueous dispersion can, for example, be
measured by a dynamic light scattering type particle size
distribution measuring apparatus (Microtrac UPA-EX150,
manufactured by Nikkiso Co., Ltd.). Alternatively, it may
be similarly found by the method of coating the surface
25 treatment solution on the surface of the plated steel
sheet, drying it to obtain a coating, examining the
surface by a field emission type scan electron microscope
(FE-SEM), measuring 100 or more ZnO particles for the
long axis and short axis, and finding the average value
30 or the method of coating the surface treatment solution
on the surface of the plated steel sheet, drying it to
obtain a coating, and examining a cross-section of the
surface by a transmission type electron microscope (TEM)
[0040] The aspect ratio (long axis/short axis) of the
35 ZnO particles contained in the ZnO aqueous dispersion (A)
used in the present invention is preferably 1 to 2.8. If
the aspect ratio is over 2.8, a good hot-pressing
- 11 -
lubricity cannot be obtained. If the aspect ratio is 1 to
2.8, the shape of the ZnO particles is sufficiently close
to a spherical shape and the particle surface is low in
flatness, so the contact area at the time of hot-pressing
5 becomes small, the coefficient of friction becomes small,
and the hot-pressing lubricity becomes excellent.
[0041] The aspect ratio of ZnO particles can be
obtained by coating the surface treatment solution on the
surface of the plated steel sheet, drying it to obtain a
10 coating, examining the surface by a field emission type
scan electron microscope (FE-SEM), measuring 100 or more
ZnO particles for the long axis and short axis, and
finding the average value. Alternatively, it can be
similarly obtained by coating the surface treatment
15 solution on the surface of the plated steel sheet, drying
it to obtain a coating, and examining a cross-section of
the surface by a transmission type electron microscope
(TEM) •
[0042] To secure the storage stability of the surface
20 treatment solution, as a dispersant of a ZnO aqueous
dispersion, one or both of an anionic dispersant and
nonionic dispersant is preferably added. If using a
cationic dispersant, the stability as a treatment
solution when blended with a water dispersible organic
25 resin cannot be secured. As examples of the anionic
dispersant able to be used, ammonium
naphthalenesulfonate, sodium naphthalenesulfonate,
ammonium polyacrylate, sodium polyacrylate, and the like
may be mentioned. As examples of the nonionic dispersant
30 able to be used, polyoxyethylenenaphthyl ether,
polyoxyethylenealkyl ether, and the like may be
mentioned.
[0043] (Water Dispersible Organic Resin (B) )
The coating obtained from the water dispersible organic
35 resin (B) used in the present invention is excellent in
waterproofness and solvent resistance and furthermore
improves the adhesion with the plated steel sheet as the
- 12 -
binder component of ZnO particles. Here, the "water
dispersible organic resin11 is an aqueous-dispersion of an
organic resin having an emulsion average particle size of
5 nm or more. The average particle size (median size) can
5 be measured, for example, by a dynamic light scattering
type particle size distribution measuring apparatus
(Microtrac UPA-EX150, manufactured by Nikkiso Co., Ltd.)
Note that, for example, a water soluble resin such as
polyacrylic acid or polyvinyl alcohol or a water soluble
10 polymer precursor such as a hydrolyzed product of a
silane coupling agent does not form an emulsion, so is
not a "water dispersible organic resin".
[0044] The number average molecular weight of the
water dispersible organic resin (B) is preferably 10000
15 or more. If the number average molecular weight of the
water dispersible resin (B) is 10000 or more, the barrier
property when made into a coating rises, so the oxygen
barrier property and water vapor barrier property rise
and, further, resistance to water and solvents can be
20 easily obtained. The number average molecular weight can
be measured using gel permeation chromatography (GPC) and
the like.
[0045] The acid value of the water dispersible organic
resin (B) is preferably 5 to 45 mgKOH/g, more preferably
25 10 to 25 mgKOH/g in range. If the acid value of the water
dispersible organic resin (B) is less than 5 mg KOH/g,
the affinity with respect to water becomes lower and
dispersion of the resin in a liquid as an emulsion
becomes difficult, so a sufficient storage stability
30 cannot be secured and further adhesion with a plated
steel sheet sometimes cannot be obtained. If the acid
value is over 45 mg KOH/g, the hydrophilic groups of the
anion groups become excessive and water is easily taken
in when forming the coating, the waterproofness becomes
35 insufficient, and adhesion between the ZnO particles and
plated steel sheet sometimes cannot be obtained. If the
acid value is 5 to 45 mg KOH/g, the storage stability as
- 13 -
a treatment solution can be secured and waterproofness at
the time formed into a coating can-be simultaneously
obtained.
[0046] The acid value was measured by a method based
5 on JIS K0070-1992. A sample was dissolved in
methylethylketone and a phenolphthalein solution was
added to prepare a sample solution. The sample solution
was titrated by a 0.1N potassium hydroxide ethanol
solution and the acid value was calculated from the end
10 point. Further, a sample solution consisting of only
methylethylketone was titrated to obtain a blank value.
The acid value was calculated by formula 1.
Acid value [mgKOH/g] ~ {0.1[N] x (titrated amount of
sample solution- blank value) [ml] x 56.11[g/mol]} I
15 amount of sample [g] Formula 1
[0047] If the water dispersible organic resin becomes
less than 5 nm in emulsion average particle size, the
organic resin forming the emulsion often becomes too high
in acid value and/or becomes too low in molecular weight.
20 For this reason, the barrier property of the coating and
the resistance to water and solvents become lower, the
waterproofness and solvent resistance become
insufficient, and adhesion between the ZnO particles and
the plated steel sheet cannot be obtained, therefore this
25 is not suitable as the water dispersible organic resin
(B) of the present invention. On the other hand, _if the
emulsion average particle size exceeds 300 nm, poor
coating-forming is caused and bumpy uneven coating is
formed, so sufficient waterproofness and solvent
30 resistance cannot be obtained. Further, the emulsion
particles become larger than ZnO particles, so in the
process of coating and drying the surface treatment
solution, sometimes the uniformity of coating of the ZnO
particles is impaired, the ZnO particles form an uneven
35 coating, and the formability at the time of hot-pressing,
the chemical convertability after hot-pressing, the
corrosion resistance after painting, and the spot
- 14 -
we1dability cannot be stably obtained.
[0048] (Solid Content Mass Ratio (WA/W8))
In the surface treatment solution of plated steel sheet
for hot-pressing of the present invention, the mass of
5 the ZnO particles used is 30 mass% to 95 mass% with
respect to the total solid content of the surface
treatment solution, more preferably 35 mass% to 80 mass%.
Further, the mass of the water dispersible organic resin
used is 5 mass% to 70 mass% as resin solid content with--
10 respect to the total solid content of the surface
treatment solution, more preferably 20 mass% to 65 mass%.
Note that, here, the "solid content" means the solid
component forming the coating and does not include the
solvent and the like. The mass ratio (WA/W8 ) of the solid
15 content mass (WA) of the ZnO particles to the solid
content mass (W8 ) in the water dispersible organic resin
is 30/70 to 95/5.
[0049] If the mass ratio (WA/WB) is less than 30/70,
the ratio of the solid content of the water dispersible
20 organic resin in the coating becomes greater, so resin
remains on the plated steel sheet after heating and the
chemical convertability becomes insufficient, so
corrosion resistance after painting cannot be obtained.
On the other hand, if (WA/W8 ) is over 95/5, the water
25 dispersible resin becomes insufficient in binder effect,
the adhesion of the ZnO particles and plat~d steel sheet
becomes insufficient, ZnO particles drop off, and the
hot-pressing lubricity, chemical convertability after
hot-pressing, and the corrosion resistance after painting
30 are not stably expressed.
[0050] The water dispersible organic resin (B) is not
particularly limited in type so long as having a 5 to 300
nm emulsion average particle size. As specific examples 1
a water dispersible polyurethane resin, water dispersible
35 epoxy resin, water dispersible acrylic resin, and water
dispersible polyester resin may be mentioned.
[0051] The type of the water dispersible polyurethane
- 15 -
resin is not particularly limited so long as a non-watersoluble
polyurethane r~sin dispersed in water. A water
dispersion of a general polyurethane resin such as a
condensation polymer obtained from a diisocyanate or
5 polyisocyanate having two or more isocyanate groups in
one molecule and a diol or polyol having two or more
hydroxyl groups in one molecule may be used. As the
method of dispersion, any of the forced emulsification
type using _a surfactant or water-soluble polymer or other
10 dispersant for water dispersion and a self emulsification
type incorporating hydrophilic groups in the structure
can be used.
[0052] The type of the water dispersible epoxy resin
is not particularly limited so long as a non-water
15 soluble epoxy resin dispersed in water. For example, a
polyglycidyl ether compound of a mononuclear polyhydric
phenol compound, a polyglycidyl ether compound of a
polynuclear polyhydric phenol compound, a polyglycidyl
ether of polyvalent alcohols, or other water dispersions
20 may be used. Note that, part of the epoxy resin may be
vinyl-modified, acrylic-modified, phosphoric acidmodified,
isocyanate-modified, amine-modified, or silylmodified.
As the method of dispersion, a forced
emulsification type using a surfactant, water-soluble
25 polymer, or other dispersant for water dispersion or a
soap-free self emulsification type introducing
hydrophilic groups by modification may also be used.
[0053] The type of the water dispersible acrylic resin
is not particularly limited so long as a non-water
30 soluble acrylic resin dispersed in water. Further, it may
also be one obtained by any polymerization method. For
example, a homopolymer or copolymer of a water
dispersible acrylic monomer obtained by emulsion
polymerization, solution polymerization, core-shell type
35 seed polymerization, and the like in the presence of a
polymerization initiator and emulsifier may be mentioned.
[0054] The type of the water dispersible polyester
- 16 -
resin is not particularly limited so long as a non-water
soluble polyest&r resin dispersed in water. For example,
a water dispersion of a condensation resin obtained from
polybasic acids selected from a maleic acid, fumaric
5 acid, itaconic acid, succinic acid, glutaric acid,
suberic acid, adipic acid, azelaic acid, sebacic acid,
dimer acid, trimer acid, isophthalic acid, terephthalic
acid, trimellitic acid, trimesic acid, pyromellitic acid,
naphthalene dicarboxylic acid, and other polybasic acids,
10 and polyols selected from ethyleneglycol,
diethyleneglycol, trimethylolpropane, neopentylglycol,
1,4-CHDM, polymer polyol, polycaprolactone polyol,
polycarbonate diol, polybutadiene polyol,
neopentylglycol, methylpentadiol, and other polyols may
15 be mentioned.
[0055] (Corrosion Resistance Improving Additive (C))
The corrosion resistance improving additive (C) used in
the present invention is at least one type of additive
selected from compounds including elements selected from
20 B, Mg, Si, Ca, Ti, V, Zr, W, and Ce and further improves
the corrosion resistance after painting. The content of
the corrosion resistance improving additive (C) is
preferably 5 mass% to 60 mass% with respect to the mass
of the ZnO particles in the surface treatment solution,
25 more preferably 5 mass% to 20 mass% in range. If less
than 5 mass%, the effect of improvement of the corrosion
resistance after painting, after hot-pressing is not
sufficiently obtained. If over 60 mass%, the additive (C)
obstructs the etching effect of the acid on the ZnO
30 particles. Further, the etching effect of the acid on the
additive (C) is poor. For this reason, sometimes the
etching by the acid component required at the time of
chemical conversion treatment is suppressed, the chemical
convertability after hot-pressing becomes insufficient,
35 and conversely the corrosion resistance after painting
falls. Note that as is generally known, chemical
conversion treatment such as treatment for forming zinc
- 17 -
phosphate coating forms a coating by bringing an acidic
chemical conversion treatment solution into contact with
a base material of a metal and the like to etch it.
During this time, a pH value of the chemical conversion
5 treatment solution near the contact surface rises,
whereby a coating is formed. Therefore, when the etching
effect is low, the chemical convertability tends to fall.
[0056] As the corrosion resistance improving additive
(C), for example 1 boric acid, zinc bo~ate, ammonium
10 magnesium phosphate, magnesium hydrogen phosphate,
magnesium oxide, magnesium acetylacetonate, magnesium
citrate, magnesium silicate, colloidal silica, sodium
silicate, lithium silicate, potassium silicate, 3-
glycidoxypropyltrimethoxysilane, 3-
15 glycidoxypropyltriethoxysilane, 3-
aminopropyltriethoxysilane, N-2-(aminoethyl)-3-
aminopropyltrimethoxysilane, calcium oxide, calcium
phosphate, titanium oxide, titanium hydrofluoric acid,
ammonium hexafluorotitanate, di-i-propoxytitanium bis
20 (acetylacetonate), di-i-propoxytitanium
bis(triethanolaminate), ammonium metavanadate, vanadium
fluoride, vanadyl phosphate, vanadium oxalate, vanadium
oxyacetylacetonate, vanadium oxytriisopropoxide,
zirconium oxide, zircon hydrofluoric acid, ammonium
25 hexafluorozirconate, ammonium tungstate, sodium
tungstate, cerium nitrate, cerium oxide, and the like may
be mentioned.
[0057] (pH)
The surface treatment solution of the present invention
30 is preferably a pH of 6 to 12, more preferably 8 to 11.
If the pH is in this range, the storage stability of the
surface treatment solution is also good. If the pH is too
low or too high, the storage stability of the surface
treatment solution sometimes cannot be obtained.
35 [0058] As a pH adjuster for adjusting the pH, it is
possible to use a conventionally known one. For example,
phosphoric acid, hydrofluoric acid, nitric acid, formic
- 18 -
acid, acetic acid, lactic acid, glycolic acid, phosphonic
acid-, citric acid, tartaric acid, ammonia, -sodium
hydroxide, monoethanolamine, diethanolamine,
triethanolamine, ethylenediamine, and the like may be
5 mentioned.
[0059] (Solvent)
As the solvent of the surface treatment solution of the
present invention, water is used, but in accordance with
need, such as improvement of the dryability of the
10 coating, it may also be an aqueous solvent to which an
alcohol-, ketone-, cellusolve-based water soluble organic
solvent is added. The amount of solvent in the surface
treatment solution of the present invention is not
particularly limited, but 30 mass% to 99 mass% with
15 respect to the total mass of the treatment solution is
preferable, and 50 mass% to 90 mass% is more preferable.
[0060] (Other Additives)
Additives of a wax, pigment, surfactant, anti foaming
agent, leveling agent, thickener, and the like can be
20 added to the surface treatment solution of the present
invention, in a range not outside the gist of the present
invention or detracting from the coating performance.
[0061] (Method of Preparation of Surface Treatment
Solution)
25 The method of preparation of the surface treatment
solution of the present invention is not particularly
limited. The solution may be produced by a known method.
For example, it may be prepared by adding to a ZnO
aqueous dispersion (A) a predetermined amount of a water
30 dispersible organic resin (B) , and mixing and stirring
them.
[0062] A preferable mode of a steel sheet treated
using the surface treatment solution of the present
invention will be explained.
35
In the above-mentioned plated steel sheet for hot-
19 -
pressing having a surface treatment coating of the
present invention, the above-mentioned ZnO coating amount
is preferably, as Zn, 0.3 to 2 g/m2 per side, more
preferably 0.5 to 1.5 g/m2
• If the ZnO coating amount is
5 less than 0.3 g/m2
, the lubricity at the time of hotpressing
is not obtained, while if over 2 g/m2 as well,
the lubricity at the time of hot-pressing is not further
improved and conversely the spot weldability falls.
[0063] (Method of Production of Plated Steel Sheet To
10 be Hot-Pressed)
The method of production of the plated steel sheet-to be
hot-pressed having a surface treatment coating of the
present invention is a method of production comprising a
coating process of coating the surface treatment solution
15 of_the present invention on the surface of plated steel
sheet containing Al, and a heat drying process of heat
drying the surface treatment solution coated on the
surface of the plated steel sheet to obtain a coating.
Note that, before coating the surface treatment solution
20 of the present invention on the plated steel sheet
containing Al, it is also possible to perform
pretreatment aimed at removing oil or dirt. The
pretreatment is not particularly limited. For example,
hot water washing, solvent washing, alkali degreasing and
25 washing, and the like may be mentioned.
[0064] The means for coating in the above coating
process is not particularly limited. For example, the
generally used roll coating, shower coating, air
spraying, airless spraying, curtain coating, brushing,
30 dipping, and the like may be mentioned.
[0065] As the means for heat drying in the above heat
drying process, a dryer, hot air furnace, high frequency
induction heating furnace, infrared furnace, and the like
may be mentioned. The heat drying temperature in the heat
35 and drying process is a plate maximum temperature of
preferably 50 to 200°C, more preferably 60 to 150°C. If
the plate maximum temperature is in this range, the
- 20 -
moisture evaporation speed becomes fast and the drying
efficiency becomes better.
[0066] According to the present invention, a specific
surface treatment coating containing ZnO particles is
5 formed on the surface of the plated steel sheet, so steel
sheet excellent in waterproofness, solvent resistance,
adhesion with a plated steel sheet, hot-pressing
lubricity, and further chemical convertability after hotpressing,
corrosio~_resistance after coating, and spot
10 weldability is obtained. The reason is not necessarily
clear, but it is believed to be because of the following
actions and effects.
[0067] The waterproofness, solvent resistance, and
adhesion with the plated steel sheet of the surface
15 treatment coating obtained from the water dispersible
organic resin depend on the molecular weight and acid
value of the organic resin. If the water dispersible
organic resin has an average particle size of 5 nm or
more, the number average molecular weight of the organic
20 resin exceeds 10000, so the resistance to water or a
solvent is high and a tough coating can be obtained.
Further, it is believed that the acid value of the water
dispersible organic resin gives rise in adhesion due to
not only the affinity with water, but also the
25 interaction between the ZnO particles and plating
surface.
[0068] As the reason why the surface treatment coating
containing ZnO particles enables a high lubricity, it is
believed that in crystal structure, ZnO becomes particles
30 closer to spherical shapes than other substances and has
a smaller frictional resistance to the die used for the
press forming. Further, it is believed ZnO has a high
melting point (about 1975°C) and does not melt even at the
high temperature in hot-pressing (about 800°C or more) and
35 the like.
[0069] As the reason why the provision of ZnO
particles on the plated steel sheet containing Al causes
- 21 -
the chemical conversion coating to form well, the change
in the reactivity with acid may be considered. In the
chemical conversion reaction, due to the acid etching the
material, the pH at the material interface rises and a
5 chemical conversion coating forms. It is guessed that if
heating the plated steel sheet containing Al to 800°C or
more, the plating surface becomes an Al-Fe alloy layer
extremely inert with respect to an acid and a chemical
conversion reaction becomes harder to occur. On the other
10 hand, if the plated steel sheet containing Al has a
coating containing ZnO particles, the heated plating
surface becomes an Al-Zn oxide layer whereby the material
becomes able to be etched by chemical conversion, so the
chemical conversion coating is formed well and corrosion
15 resistance after electrodeposition painting can be
expressed.
Examples
[0070] Below, examples will be given to explain the
present invention, but the present invention is not
20 limited to these examples.
[0071] (1) Test Material
A steel strip having the steel components shown in Table
1 was adjusted to a plate thickness of 1.4 mm through the
normal hot rolling, pickling, and cold rolling processes
25 then was applied Al plating on a nonoxidizing furnace
type continuous hot dip plating line. The Al plating bath
at this time contained 9% of Si and 2% of Fe. The coating
amount was adjusted to 40 g/m2 per side.
[0072] Table 1. Steel Components of Test Material
30 (mass%)
[0073] (2) Pretreatment (Washing)
As the production method of the test plate, first, the
test plate surface was treated by PALKRINTM N364S
35 manufactured by Nihon Parkerizing Co., Ltd. to remove the
- 22 -
oil and dirt from the surface. Next, the surface was
rinsed by tap wate± and it was confirmed that the metal
material surface was 100 percentage-wet by water. After
that, the surface was flushed with pure water (deionized
5 water) then was dried in an oven at 70°C atmosphere. This
was used as a test plate.
[0074] (3) Production Method of ZnO Aqueous Dispersion
To a beads mill, 300 g of zinc oxide powder having an
initial particle size 500 nm and 30 g of dispersant were
10 charged into 670 g of water. Zirconia beads having
diameter 0.3 mm were used to disperse this to a
predetermined average particle size. The particle size
distribution was measured by dynamic light scattering
type particle size distribution measuring device
15 (Microtrac UPA-EX150, manufactured by Nikkiso Co., Ltd.).
The median size (dSO) was made the average particle size
of the zinc oxide.
[0075] (4) Water Dispersible Organic Resin
(Water Dispersible Polyurethane Resin)
20 Polyurethane Resin A
Superflex 150 (manufactured by DKS Co. Ltd.)
[0076] (Water Dispersible Acrylic Resin)
Synthesis Method of Acrylic Resin A
As the monomer composition, 72 parts of methyl
25 methacrylate, 27 parts of butyl acrylate, and 1 part of
methacrylic acid were used. As the synthesis method, the
above monomer was blended with 100 parts of a 10 mass%
emulsifier aqueous solution containing the reactive
nonionic emulsifier and polyoxyethylene octyl phenyl
30 ether (HLB17.9) blended in 6:4 ratio, then a homogenizer
was used for emulsification at 5000 rpm for 10 minutes to
obtain a monomer emulsified solution. Next, to a fournecked
flask equipped with a stirrer, reflux cooler,
thermometer, and monomer feed pump, 150 parts of the
35 emulsifier aqueous solution was added and the result held
at 40 to 50°C. A 5 mass% aqueous solution of ammonium
persulfate (50 parts) and the above monomer emulsified
- 23 -
solution were added to dropwise addition funnels. These
funnels were attached to another openings of the flask.
The solution was added dropwise about 2 hours and was
raised temperature to 60°C where it continued to be
5 stirred for about 1 hour and made to react. The reaction
solution was cooled to room temperature while stirring to
obtain an aqueous dispersion of the Acrylic Resin A. This
acrylic resin is anionic.
[0077] Synthesis Method of Acrylic Resins B to G
10 A similar synthesis method as the Acrylic Resin A was
used to obtain aqueous dispersions of the Acrylic Resins
B toG by monomer compositions shown in Table 2.
15
[0078] Table 2
Monomer
Acrylic Acrylic Acrylic Acrylic Acrylic Acrylic
Resin B Resin c Resin D Resin E Resin F Resin G
Methyl
72 parts 71 parts 70 parts 69.5 parts 72.5 parts 68.5 parts methacrylate
N butyl 26.5 parts 25.5 parts 25 parts 24 parts 27 parts 23 parts
acrylate
Methacrylic
1.5 parts 3.5 parts 5 parts 6.5 parts 0.5 pare 8.5 parts
acid
[0079] (Water Dispersible Polyester Resin)
Synthesis Method of Polyester Resin A
An anionic polyester resin obtained by a condensation
reaction of an alcohol component comprised of
ethyleneglyco1: 3.2 parts and neopentylglycol: 30.9 parts
20 and an acid component comprised of isophthalic acid: 16.5
parts, terephthalic acid: 38.2 parts, adipic acid: 7.8
parts, and trimellitic anhydride: 3.4 parts was
synthesized by the following method: To a 1000 ml round
bottom flask provided with a Claisen tube and an air
25 cooler, the total acid component, total alcohol
component, and catalyst (calcium acetate: 0.25 g, N-butyl
titanate: 0.1 g) were inserted, the inside of the flask
was replaced with nitrogen, and the content was heated to
180°C for melting. Further, the bath temperature was
30 raised to 200°C then the content was heated with stirring
for about 2 hours to cause an esterification or esterexchange
reaction. Next, the bath temperature was raised
- 24 -
to 260°C then, after about 15 minutes, the inside of the
flask was decompressed to 0.5 mmHg to cause a-reaction
for about 3 hours (polycondensation reaction) . After
completeing reaction, the system was allowed to cool
5 while introducing nitrogen and the content was taken out.
10
15
20
To the content taken out, suitable amount of ammonia
water was added to give a final pH of 6 to 7. The mixture
was heated with stirring in an autoclave at 100°C for 2
hours to obtain a water dispersible polyester resin.
[0080] Synthesis Methods of Polyester Resins B to G
Synthesis methods of similar to the Polyester Resin A
were used to obtain aqueous dispersions of the Polyester
Resins B to G by the monomer compositions shown in Table
3.
[0081]
Monomer
Terephthalic
acid
Isophthalic
acid
Adipic
acid
Trimelli tic
anhydride
Ethylene
glycol
Neopentyl
glycol
Table 3
Polyester Polyester
Resin B Resin c
38.4 parts 38.4 parts
16.5 parts 16.6 parts
7.8 parts 7.9 parts
3.1 parts 2. 7 parts
3.2 parts 3.2 parts
31.0 parts 31.2 parts
Polyester Polyester
Resin D Res.in E
38.6 parts 39.2 parts
16.6 parts 16.9 parts
7. 9 parts 8. 0 parts
2.5 parts 0. 9 part
3.2 parts 3. 3 parts
31.2 parts 31.7 parts
[0082] (Water Dispersible Epoxy Resin)
Epoxy Resin A
Polyester Polyester
Resin F Resin G
38.0 parts 39.2 parts
16.4 parts 16.9 parts
6. 9 parts 8.0 parts
4.6 parts 0. 8 part
3.2 parts 3.3 parts
30.9 parts 31.8 parts
ADEKA Resin EM-0430 (manufactured by ADEKA Corporation)
[0083] (Water-Soluble Resin)
Polyacrylic Acid A
Jurymer AC-lOSL (manufactured by Toagosei Co., Ltd.)
[0084] (5) Composition of Surface Treatment Solution
A ZnO aqueous dispersion and a water dispersible organic
25 resin were blended in water so that the components became
the composition ratio shown in Table 4 (mass ratio
(WA/WB)) to obtain a surface treatment solution. Further,
the water dispersible organic resin was measured for
average particle size (median size) by a dynamic light
- 25 -
scattering type particle size distribution measuring
apparatus (Microtrac UPA-EX150, manufactured by Nikkiso
Co., Ltd.). The results are shown in Table 4.
[0085] (6) Treatment Method
5 One side of a pretreated test plate was coated with a
surface treatment solution by a bar coater and heated to
dry to obtain a surface treatment coating. The ZnO
coating amount (value as Zn) of the formed surface
treatment coating and heating temperature (PMT: plate
10 maximum temperature) are shown in Table 4. The obtained
surface treated steel plate was evaluated by the
following methods.

CLAIMS
Claim 1. A surface treatment selution for a plated
steel sheet to be hot-pressed comprising a ZnO aqueous
dispersion (A) and a water dispersible organic resin (B),
5 wherein
the ZnO aqueous dispersion (A) comprises water
and ZnO particles having an average particle size of 10
to 300 nm,
the water dispersible organic resin (B) has a 5
10 to 300 nm emulsion average particle size, and
15
a mass ratio (WA/WB) of a mass (WA) of ZnO
particles in the ZnO aqueous dispersion to a mass (WB) of
solid content in the water dispersible organic resin is
30/70 to 95/5.
Claim 2. The surface treatment solution for a plated
steel sheet to be hot-pressed according to claim 1,
wherein the water dispersible organic resin (B) is one or
more types of resin selected from a group consisting of a
water dispersible polyurethane resin, a water dispersible
20 epoxy resin, a water dispersible acryl resin, and a water
dispersible polyester resin.
Claim 3. The surface treatment solution for a
plated steel sheet to be hot-pressed according to claim 1
or 2, wherein the water dispersible organic resin (B) has
25 a number average molecular weight of 10000 or more.
Claim 4. The surface treatment solution for a plated
steel sheet to be hot-pressed according to any one of
claims 1 to 3, wherein the water dispersible organic
resin (B) is one or more types _of resin having a 5 to 45
30 mgKOH/g acid value and selected from a group consisting
of a water dispersible polyurethane resin, a water
dispersible acryl resin, and a water dispersible
polyester resin.
Claim 5. The surface treatment solution for a plated
35 steel sheet to be hot-pressed according to any one of
claims 1 to 4, comprising at least one type of additive
(C) selected from compounds comprising elements selected
- 33 -
from the group consisting of B, Mg, Si, Ca, Ti, V, Zr, W,
and Ce.
Claim 6. The surface treatment solution for a plated
steel sheet to be hot-pressed according to claim 5,
5 wherein a mass ratio (W 0 /WA) of a mass (WA) of the ZnO
particles in the ZnO aqueous dispersion to a mass (We) of
the additive (C) is 0.05 to 0.6.
Claim 7. The surface treatment solution for a plated
steel sheet to be _hot-pressed according to any one of
10 claims 1 to 6, wherein the ZnO particles have an aspect
ratio (long axis/short axis) of 1 to 2.8.
Claim 8. The surface treatment solution for a plated
steel sheet to be hot-pressed according to any one of
claims 1 to 7, wherein the ZnO aqueous dispersion (A)
15. comprises one or both of an anionic dispersant and
nonionic dispersant.