TECHNICAL FIELD
[0001] The present invention relates to a hot dip galvanized steel sheet (including galvannealed
steel sheet) which has excellent press-formability, bondability, and phosphatability,
and which is used for thin steel sheet for automobile and the like, and also to a
method for manufacturing thereof.
BACKGROUND ART
[0002] Hot dip galvanized steel sheets are widely used in automobiles, electrical appliances,
and other apparatuses owing to their good corrosion resistance compared with ordinary
cold-rolled steel sheets. The hot dip galvanized steel sheets in these uses are often
press-formed. The hot dip galvanized steel sheets have, however, a drawback of inferiority
in press-formability compared with the cold-rolled steel sheets because the galvanizing
components in the hot dip galvanized steel sheet adhere with the press die thus making
the sliding resistance between the steel sheet and the die large and instable compared
with that for the cold-rolled steel sheets. That is, for a hot dip galvanized steel
sheet, the steel sheet becomes difficult in sliding into the die during the press-forming
stage at a portion such as bead part where the sliding resistance increases, which
likely induces fracture of the steel sheet-.
[0003] A common practice to improve the press-formability of zinc-based plated steel sheet
is a method of coating a high viscosity lubricant oil. The method, however, has problems
such as the generation of defects during the painting stage caused by insufficient
degreasing, and the instable press-formability during the press-forming stage caused
by absence of the lubricant oil. To solve these problems, minimization of the quantity
of lubricant oil is an effective means. To do this, however, the improvement in the
press-formability of zinc-based plated steel sheet is required.
[0004] The galvannealed steel sheet is a hot dip galvanized steel sheet which formed an
Fe-Zn alloy layer thereon after heating thereof. The alloy layer is normally composed
of Γ phase, δ
1 phase, and ζ phase. When the Fe concentration decreases, the alloy layer tends to
decrease in hardness and melting point in an order of Γ phase - δ
1 phase - ζ phase. From the viewpoint of sliding performance, the Γ phase with high
Fe concentration is effective because of the.high hardness, the high melting point,
and the hardly-inducing adhesion. Accordingly, thegalvannealed steel sheet which emphasizes
the press-formability is manufactured so as to have a high average Fe concentration
in the alloy layer.
[0005] When, however, the average Fe concentration in the alloy layer increases, the Γ phase
which is hard and brittle is likely formed at the interface between the plating and
the steel sheet, thereby likely inducing a phenomenon of peeling of plating (what
is called the "powdering") in the vicinity of the interface during the press-forming
stage.
[0006] JP-A-1-319661, (the term "JP-A" referred to herein signifies the "Unexamined Japanese Patent Publication"),
discloses a method of forming a hard iron-based alloy as the secondary layer on ordinary
alloy layer using electroplating method or the like to attain both the sliding performance
and the powdering resistance. The double plating layer, however, increases the manufacturing
cost.
[0007] Further low cost methods are disclosed in
JP-A-53-60332 and
JP-A-2-190483. According to these disclosed technologies, the weldability and the press-formability
are improved by forming an oxide film composed mainly of ZnO on the surface of a zinc-based
plated steel sheet applying electrodeposition treatment, dipping treatment, coating
oxidation treatment, or heat treatment.
[0008] JP-A-4-88196 discloses a technology to improve the press-formability and the phosphatability by
forming an oxide film composed mainly of a P oxide on the surface of a zinc-based
plated steel sheet by dipping the steel sheet in an aqueous solution of pH 2 to 6,
containing 5 to 6 g/liter of sodium phosphate, by applying electrodeposition treatment
in the aqueous solution, or by spraying the aqueous solution onto the steel sheet.
[0009] Furthermore,
JP-A-3-191093 discloses a technology to improve the press-formability and the phosphatability by
forming a Ni oxide film on the surface of a zinc-based plated steel sheet by applying
electrodeposition treatment, dipping treatment, coating treatment, coating oxidation
treatment, or heat treatment.
[0010] However, the inventors of the present invention applied the technologies, disclosed
in the respective
JP-A-53-60332,
JP-A-2-190483,
JP-A-4-88196, and
JP-A-3-191093 to hot dip galvanized steel sheets, and found that these technologies cannot improve
stably the press-formability. Detail study for the cause of failing in attaining the
stable improvement has revealed the following. A hot dip galvanized steel sheet contains
Al oxide, and a galvannealed steel sheet contains an irregularly distributed Al oxide
and has an increased roughness on the surface of plating layer, thus a desired film
cannot be stably formed for both cases even by electrodeposition treatment, dipping
treatment, coating oxidation treatment, heat treatment, and the like. Specifically
for the galvannealed steel sheet, several micrometers or larger irregular profile
on the surface thereof is created owing to the non-uniformity of alloying reaction
and to the shape of Fe-Zn alloy phase, thereby increasing the sliding resistance at
the surface of plateau to deteriorate the press-formability. Furthermore, the inventors
of the present invention determined the friction factor of ZnO film formed on each
of the hot dip galvanized steel sheet and the galvannealed steel sheet by a physical
method, and found that sufficient press-formability cannot be attained. The findings
lead to a conclusion that the conventional technologies of forming a ZnO film on the
surface of plating layer cannot expect the sufficient improvement in the press-formability
even when a uniform film is formed
[0011] To this point, the inventors of the present invention disclosed a technology to improve
the sliding performance in
JP-A-2001-323358. According to the disclosure, a plateau is formed on a planting layer on a galvannealed
steel sheet, and a film composed of an oxide or a hydroxide containing Zn, Fe, Al,
and the like is formed on the plateau, and further a fine irregular profile is formed
on the surface of plateau including the film.
[0012] Although the technology disclosed in
JP-A-2001-323358 improves the press-formability more than the technologies disclosed in the above
patent publications, there was occurred insufficient improvement in the press-formability
in some cases.
[0013] In recent years, the bonding method of hot dip galvanized steel sheets increases
the cases of applying adhesives to bonding the steel sheets together. To do this,
however, the hot dip galvanized steel sheets have to have strong bonding strength,
or have excellent bondability.
[0014] The above-described conventional technologies, however, decrease the bondability
and the phosphatability, in some cases, by forming a film on the hot dip galvanized
steel sheet.
[0015] JP 2002 256448 A discloses a method for manufacturing a galvanized steel sheet, wherein said method
comprises the steps of subjecting the steel sheet to hot-dip galvanizing, whereafter
alloying said hot-dip galvanized steel with a heat treatment, subsequent temper rolling
it and then contacting it with an acid solution, whereby the steel sheet is left for
one to 30 seconds after finishing contacting. Then, the steel sheet is washed with
water and dried. The acidic solution used for the above method contains Fe- and Zn-ion.
Moreover, the pH of the acidic solution is set in the range of 1 to 5 and the coating
weight of the acidic solution is adjusted to smaller than 3 g/m
2.
[0016] JP 2003 138362 A discloses a galvannealed steel sheet having excellent sliding characteristics at
press-forming, wherein said galvannealed steel sheet is formed by a method comprising
steps of plating a steel sheet in a galvanization bath, performing subsequently alloying
treatment and temper rolling. Thereafter, the temper-rolled steel sheet is immersed
in an acidic solution having a pH of 1.5 and containing ferrous sulfate in an amount
of 400 g/l, zinc sulfate in an amount of 15 g/l and sodium acetate in an amount of
30 g/l. After immersing the steel sheet in the acidic solution, the steel sheet is
rinsed and dried. The thus obtained alvanneaied steel sheet has a layer of oxides
of Fe, Zn and Al of 1 to 400µm in thickness, which layer is formed on a flat part
of a plating surface of said steel sheet. Further, on this oxide layer, projections
composed of the oxides or hydroxides of Fe and Zn are formed, wherein said projections
have a maximum height of 5 to 300 µm wherein, from a cross-sectional view, the number
of said projections is 20 to 1000 pieces for a length of 10µm.
[0017] JP 2003 138364 A refers also to galvannealed steel sheets having excellent sliding characteristics
at press forming. The galvannealed steel sheet is formed by a method comprising steps
of plating a carbon steel sheet in a galvanization both and performing an alloying
treatment following by the step of temper rolling to flatten the crowning of the heights
of the plating layer, thereby forming a flat part in the plating surface of the temper
rolled steel sheet. Thereafter, the temper-rolled steel sheet is immersed for at least
one second in a sulfuric acid solution containing 300 g/l of FeSO
4 and 7 H
2, 30 g/l of ZnSO
4 and 7 H
2, 20 g/l sodium acetate and 15 g/l sodium-acid-citrate. Said acidic solution has a
pH of 2.2. Thereafter, the steel sheet is rinsed and dried. As further disclosed in
Tables 1 and 2 of D5, the Fe concentration in the plating coat ranges from 8.6 mass
% to 10.9 mass% and 8.5 mass% to 10.8 mass%, respectively.
[0018] JP 2003 306781 A also deals with a method for manufacturing a galvannealed steel sheet having excellent
slidability characteristic at the time of press forming, comprising steps of hot dip
zincing a steel plate followed by heat treatment to apply alloying treatment to the
plating layer and temper rolling said alloyed plate to smoothening the plating surface.
Thereafter, the steel plate is immersed in an acidic solution and then neglecting
for one to 30 seconds. The acidic solution has a pH buffer action obtained by adding
thereto phthalate such as sodium acetate and acid potassium phthalate, citrate such
as sodium acid citrate and citrate 2-potassium, succinate such as sodium succinate,
and lactate such as sodium lactate. The pH of said acidic solution is set in the range
of 1.0 to 5.0. The acidic solution contains 50 g/l of zinc sulfate, 50 g/l of ferrous
sulfate, 30 g/l of sodium sulfate and 20 g/l of sodium acetate. The Fe concentration
in the plating coat ranges from 9.3 mass % to 11.4 mass %.
DISCLOSURE OF THE INVENTION
[0019] An object of the present invention is to provide a hot dip galvanized steel sheet
having excellent press-formability, bondability, and phosphatability, and to provide
a method for manufacturing thereof.
[0020] The above object is attained by a hot dip galvanized steel sheet as defined in claim
1 and a method for manufacturing the same as defined in parallel claim 5. Further
embodiments of the hot dip galvanized steel sheet and the method for manufacturing
the same are defined in dependent claims 2 to 4 and 6 to 10, respectively.
[0021] The hot dip galvanized steel sheet according to the present invention can be manufactured
by a manufacturing method having the steps of: hot-dip-galvanizing a steel sheet;
temper-rolling the hot dip galvanized steel sheet to form a plateau on a surface of
the galvanized layer; bringing the temper-rolled hot dip galvanized steel sheet into
contact with an acidic solution containing Fe ion and having a pH buffering effect
to form a film being composed of a compound containing Zn, Fe, and O on a surface
of the plating layer; and allowing to standing the hot dip galvanized steel sheet
for 1 to 30 seconds after contacting with the acidic solution, followed by washing
thereof with water.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022]
Figure 1 is a schematic drawing of flat sliding test apparatus.
Figure 2 shows an example of the shape of bead for determining the coefficient of
friction.
Figure 3 shows another example of the shape of bead for determining the coefficient
of friction.
Figure 4 illustrates a test piece for determining bondability.
Figure 5 illustrates the bondability test.
Figure 6 is a schematic drawing of draw-bead tester.
Figure 7 shows the structure of film-forming apparatus.
EMBODIMENTS OF THE INVENTION
[0023] An effective means to improve the press-formability of hot dip galvanized steel sheet
is to decrease the sliding resistance of the surface of plating layer contacting directly
with the die during the press-forming stage.
[0024] To do this, a plateau is formed on the surface of plating layer, and a film of an
O-containing compound such as an oxide, which can decrease the sliding resistance,
is formed on the plateau, thereby limiting most part of the surface contacting with
the die during the press-forming stage to the plateau , and effectively reducing the
sliding resistance.
[0025] A presumable reason of decreasing the sliding resistance by forming a film of O-containing
compound, or of attaining good lubrication, is that the O-cortaining compound such
as an oxide is generally hard and has high melting point, thus the adhesion of the
plating layer with the die can be suppressed.
[0026] For the case of galvannealed steel sheet, when a plateau is formed on the surface
of plating layer by a mechanical means such as temper-rolling, the oxide containing
Al, existing on the surface of plating layer, can be destroyed locally, thereby efficiently
and uniformly providing the film of O-containing compound.
[0027] The percentage of the plateau on the surface of plating layer is preferably in a
range from 30 to 70% by area.
[0028] For attaining a film having further high lubrication and having excellent bondability
and phosphatability, it is effective to add Zn and Fe to the compound. Since the ion
radius of Zn (II) differs from that of Fe, the growth of Zn oxide and Fe oxide interferes
with each other, thereby refining the compound. Actually, according to a finding of
the inventors of the present invention, the film of an oxide containing Zn and Fe
likely becomes fine lamellar-like oxides compared with the film of oxide only of Zn,
which likely forms coarse plate-like oxides. Although the reason of attaining that
high lubrication by that type of film is not fully analyzed, a presumable reason is
that Fe varies the electron state of the oxide of Zn to increase the adsorption of
the lubricant oil on the film, or that the O-containing compound is refined to an
appropriate compound size to increase the adsorption area of the lubricant oil on
the film. Since, that fine compounds are formed, the surface of plating layer has
strong adhesion, thus not decreasing the bondability. The non-decrease in the bondability
is attained presumably by a large number of contact points between the compounds and
the surface of plating layer, and by not-concentrated external force to a specific
compound. The refinement of the compound is expected to improve the bonding strength
with the adhesives on bonding the steel sheets using adhesives and the like, thereby
contributing to the improvement of the bondability of the hot dip galvanized steel
sheets. Since the refined compounds readily dissolve during the phosphatization treatment
even if they remain until immediately before the phosphatization treatment, they do
not adversely affect the formation of phosphatized film. Therefore, good phosphatability
is attained.
[0029] As described above, with the film composed of a compound containing Zn, Fe, and O,
high lubrication and excellent bondability and phosphatability are attained. To do
this, it is necessary to regulate the ratio of the quantity of Fe to the sum of the
quantity of Zn, [Zn](% by atom), and the quantity of Fe, [Fe] (% by atom), or {[Fe]/([Zn]
+ [Fe]), in the film to a range from 0.002 to 0.15. If the ratio of the quantity of
Fe is smaller than 0.002, a plate-like oxide composed mainly of Zn, which has weak
bondability between the surface of plating layer and the O-containing compound, is
formed, thereby decreasing the adhesion of film and further decreasing the bondability.
In addition, no effect of Fe addition to oxides is obtained, and it is not insufficient
to improve the lubrication. On the other hand, if the ratio becomes larger than 0.15,
the efficiency to form the O-containing compound decreases, thus, in an ordinary chemical
method for film-forming using a solution fails to form stably a film having large
film thickness necessary to decrease the sliding resistance. Furthermore, that excessively
large quantity of Fe gives excessively refined film, which results in insufficient
effect to improve the lubrication. Consequently, if the ratio {[Fe]/([Zn] + [Fe]}
is within a range from 0.002 to 0.15, further high lubrication and excellent adhesion
are attained.
[0030] The {[Fe]/([Zn] + [Fe]) in the film was determined by a transmission electron microscope
(TEM) and an energy-dispersive X-ray spectrometer (EDS). That is, a cross section
sample of the surface layer was cut to prepare from the plateau of the surface of
plating layerusing the focused ionbeamprocessing (FIB) method, and electron beams
were radiated onto the film on the sample, then the element analysis was applied on
5 to 10 points along the film thickness using EDS, followed by determining the atom
concentration using the approximation as film. Since the percentages of Fe in the
film may be non-uniform in the depth direction in some cases, the [Fe] is an average
value of the Fe quantities determined at the respective analytical points. The judgment
inside the film was given by defining the point where the X-ray intensity of Zn becomes
half the intensity on the surface of plating layer as the interface at the steel sheet
side, and by defining the point where the X-ray intensity of Zn in the film becomes
half as the surface. Alternatively, scanning Auger, microscope (SAM) can be used to
conduct the element analysis at the surface of the plateau of the plating layer to
determine the {[Fe]/([Zn] + [Fe]} value. Nevertheless, if the percentages of Fe in
the film are non-uniform in the depth direction, the TEM method gives more correct
determination.
[0031] The average film thickness A which is determined by the element analysis of the film
composed of a compound containing Zn, Fe, and 0 has to be 10 nm or larger to sufficiently
decrease the sliding resistance. On the other hand, if the average film thickness
A becomes larger than 100 nm, the film is fractured during the press-forming stage
to increase the sliding resistance, to decrease the film adhesion, and to deteriorate
the weldability of hot dip galvanized steel sheet. Therefore, the average film thickness
A determined by the film element analysis of the film is required to enter the range
from 10 to 100 nm.
[0032] The average film thickness A determined from the element analysis of the f ilmwas
derived by SAM combined with Ar
+ sputtering. That is, the secondary electron image observation function in SAM identified,
(readily identifiable), the plateau on the surface of plating layer, and the sputtering
and the observation were repeated down to a depth where the O concentration becomes
almost unchanged applying the Ar
+ sputtering at 3 kV of acceleration voltage over a region of about 3 µm x 3 µm on
the surface of plateau down to a specified depth, then the composition at the depth
was determined from the detected element peak intensity applying a relative sensitivity
factor correction. After the O content in the film became the maximum value at a certain
depth, (the depth may be the uppermost layer in some cases), the O content decreased
to give a constant value. The film thickness A was determined by converting a sputtering
time, when the sum of the maximum value and the reached constant value becomes half
at a depth deeper than the depth that gives the maximum value of O content, into the
depth, based on the sputter rate of, for example, a SiO
2 film having a known film thickness. The observation was given to at least three flat
parts per a single sample, and the average of the three observed values was derived.
[0033] If the average film thickness B determined by observation of film thickness cross
section is in a range from 20 to 1000 nm, and if the film thickness ratio B/A is 1.
5 or larger, further high lubrication is attained, and further low sliding resistance
is attained. A film having large ratio of the average film thickness B to the average
film thickness A means a film having a large void fraction therein. Larger B/A value
provides higher lubrication because the positions for adsorbing the lubricant oil
increase and because the lubricant oil easily enters the void, thus larger B/A provides
higher lubrication.
[0034] If the average, film thickness B is smaller than 20 nm, or if the ratio B/A is smaller
than 1.5, the void fraction in the film becomes small, which fails to attain high
lubricant, If the average film thickness B exceeds 1000 nm, the weldability deteriorates,
and the manufacturing cost increases.
[0035] Formation of a film having the average film thickness B in a range from 20 to 1000
nm and having the film thickness ratio B/A of 1.5 or larger is attained by decreasing
the value of {[Fe]/([Zn] + [Fe]} within the range of the present invention, or by
decreasing the quantity of Fe in the film.
[0036] The average film thickness B determined by observation of film thickness cross section
was derived from the observation of bright field image of TEM. The TEM observation
sample was prepared by forming a carbon layer on the surface of plating layer using
a carbon coater to protect the surface, and then by cutting the cross section at the
plateau of the surface of plating using FIB method, thus obtaining the cross section
sample of the surface of plating layer containing the film. The bright field image
on the cross section of plating layer was observed and photographed under a defocus
condition slightly offset from the just-focus point (focused state). Then, straight
lines were drawn between the individual peak points on the film over about 10 µm length
parallel to the film, and the lengths of these lines were averaged to obtain the average
film thickness B.
[0037] Applicable compound containing Zn, Fe, and O, forming the film includes an oxide,
a hydroxide, and a mixture thereof.
[0038] The present invention is also applicable to a galvannealed steel sheet on which the
hot dip galvanizing layer is processed by alloying treatment.
[0039] The hot dip galvanized steel sheet according to the present invention can be manufactured,
as described before, by a method having the steps of: hot-dip-galvanizing a steel
sheet; temper-rolling the hot dip galvanized steel sheet to form a plateau on a surface
of the galvanized plating layer; bringing the temper-rolled hot dip galvanized steel
sheet into contact with an acidic solution containing Fe ion and having a pH buffering
effect to form a film composed of a compound containing Zn, Fe, and 0 on a surface
of the plating layer; and allowing the hot dip galvanized steel sheet to standing
for 1 to 30 seconds after contacting with the acidic solution, followed by washing
thereof with water.
[0040] When a hot dip galvanized steel sheet is brought into contact with an acidic solution,
zinc in the plating layer dissolves. The dissolution of zinc is considered to accompany
the generation of hydrogen so that the hydrogen ion concentration in the acidic solution
decreases along with the progress of zinc dissolution, and pH of the acidic solution
increases, thereby forming a film of O-containing compound composed mainly of Zn on
the surface of zinc plating layer. When the acidic solution has a pH buffering effect,
the pH increase in the acidic solution becomes mild even if zinc dissolves and even
if hydrogen generation reaction begins, thus the zinc dissolution positively proceeds
to form a film of O-containing compound, sufficient to improve the sliding performance..
When Fe ion exists in the acidic solution, the Fe ion reduction reaction begins to
precipitate trace amount of Fe on the surface of plating layer , which suppresses
the excess growth of the film of O-containing compound composed mainly of Zn, thereby
forming a film of very fine compound.
[0041] The hot dip galvanized steel sheet after contacting with the acidic solution is washed
with water. If the time for allowing to standing prior to the washing with water is
less than 1 second, the acidic solution is removed before forming the film of O-containing
compound composed mainly of Zn. If the time therefor is more than 30 seconds, the
film thickness saturates. Therefore, the hot dip galvanized steel sheet after contacting
with the acidic solution has to be washed with water after allowing to standing for
a period from 1 to 30 seconds.
[0042] When the hot dip galvanized steel sheet is brought into contact with the acidic solution,
the acidic solution is preferably retained on the surface thereof as a thin film.
Excess acidic solution retained on the surface of the steel sheet does not increase
the pH of the solution even when the zinc dissolution occurs, and the formation of
an O-containing compound composed mainly of Zn may take a long time, and further the
plating layer may be significantly damaged to lose the rust-preventive performance
inherent in the plating layer. Accordingly, the quantity of acidic solution retained
on the surface of hot dip galvanized steel sheet is preferably 3 g/m
2 or smaller. The adjustment of the quantity of acidic solution, can be done by squeeze-rolling,
air-wiping, and the like.
[0043] The Fe ion being added to the acidic solution has two kinds : Fe
2+ and Fe
3+. Both of these Fe ions are effective to form a film of a fine compound containing
Zn, Fe, and O. However, presence of Fe
3+ generates large amount of sludge in the acidic solution to likely cause bruising
on the surface of the steel sheet. Accordingly, smaller Fe
3+ concentration is better. Since, however, Fe
2+ is actually oxidized with time to increase Fe
3+, an acidic solution free from Fe
3+ cannot be attained. Therefore, the control of Fe
3+ concentration in the acidic solution is important, and the Fe
3+ concentration is preferably limited to 2 g/liter or smaller to prevent the occurrence
of bruising. The control, of Fe
3+ concentration can be done by renewing the acidic solution when the Fe
3+ concentration exceeds 2 g/liter, or by dissolving Fe in the acidic solution to utilize
the Fe
3+ reduction reaction.
[0044] To stably form the film of a compound containing Zn, Fe, and O, it is preferable
to use an acidic solution having pH buffering effect within a region of pH from 2
to 5. An index for the evaluation of the pH buffering effect is the degree of pH increase,
which is defined by the quantity of an aqueous solution of 1 mole/liter sodium hydroxide
solution (ml) necessary to increase the pH of 1 liter of the acidic solution from
2 to 5. Specifying the degree of pH increase in a range from 3 to 20 is preferred
to stably form the film of a compound containing Zn, Fe, and O at thicknesses of 10
nm or more in a plateau on the surface of plating layer. The specification of the
pH increase region in a range from 2 to 5 is adopted because the pH larger than 5
triggers the generation of Zn oxide and becomes difficult to form the film of a compound
containing Zn, Fe, and O having the thicknesses of 10 nm or larger even if the steel
sheet is allowed to standing for a long time after contacting with the acidic solution,
and because the pH smaller than 2 fails to substantially contribute to the easiness
of forming the film of a compound containing Zn, Fe, and O. If the pH increase degree
is smaller than 3, the pH increase proceeds rapidly to flail in sufficient zinc dissolution,
which results in insufficient formation of the film of a compound containing Zn, Fe,
and O. If the pH increase degree exceeds 20, the zinc dissolution is enhanced to take
a long time for forming the film of a compound containing Zn, Fe, and O, and also
the plating layer may be seriously damaged to lose the rust-preventive performance
inherent in the plating layer. Regarding the pH increase degree of the acidic solution
having pH larger than 2, the evaluation is given by decreasing the pH of the acidic
solution to 2 by adding an inorganic acid having very little pH buffering effect,
such as sulfuric acid, to the acidic solution within a pH range from 2 to 5.
[0045] Applicable acidic solution having the pH buffering effect includes the one having
pH from 1 to 5 and containing 5 to 50 g/liter of pH buffer of at least one of: acetic
acid salt such as sodium acetate (CH
3COONa) ; phthalic acid salt such as potassium hydrogen phthalate ((KOOC)
2C
6H
4); citric acid salt such as sodium citrate (Na
3C
6H
5O
7) and potassium dihydrogen citrate (KH
2C
6H
5O
7) succinic acid salt such as sodium succinate (Na
2C
4H
4O
4); lactic acid salt such as sodium lactate (NaCH
3CHOHCO
2); tartaric acid salt such as sodium tartarate (Na
2C
4H
4O
6); boric acid salt; and phosphoric acid salt. If the concentration of the pH buffer
is smaller than 5 g/liter, the pH increase begins relatively early along with the
dissolution of zinc, which fails to form the film of a compound containing Zn, Fe,
and O, sufficient to improve the sliding performance. If the concentration of the
pH buffer exceeds 50 g/liter, the zinc dissolution is enhanced to take a long time
for forming the film of a compound containing Zn, Fe, and O, and the plating layer
may be seriously damaged to lose the rust-preventive performance inherent in the plating
layer. If the pH of the acidic solution is smaller than 1, formation of the film of
a compound containing Zn, Fe, and O becomes difficult, though the zinc dissolution
is enhanced. If the pH of the acidic solution exceeds 5, the dissolution rate of zinc
decreases. Consequently, the pH of acidic solution is preferably in a range from 1
to 5. If the pH of acidic solution is larger than 5, the pH can be adjusted by an
inorganic acid having no pH buffering effect, such as sulfuric acid, or by an acidic
solution of the applying salt such as the salt of acetic acid, phthalic acid, and
citric acid.
[0046] To add Fe ion to the acidic solution, it is preferred to add at least one of sulfuric
acid salt, nitric acid salt, and chloride of Fe, and further to adjust the Fe ion
concentration to a range from 0.1 to 100 g/liter. If the Fe ion concentration is smaller
than 0.1 g/liter, the film of a compound containing Zn, Fe, and O is formed solely
by the above salts having the pH buffering effect, and the film thickness control
and the refinement of compound may become difficult. If the Fe ion concentration exceeds
100 g/liter, the growth of the film of a compound containing Zn, Fe, and O is significantly
suppressed, and the film necessary to improve the sliding performance may not be formed.
Although the addition of Fe ion is effective in the film thickness control and the
refinement of compound, the Fe ion in the acidic solution enhances the dissolution
of the plating layer to bring the plating layer weak, thus more likely inducing the
peeling of plating, or what is called the "powdering" , during the press-forming stage.
From this viewpoint, the Fe ion is preferably 10 g/liter or smaller. When the application
to a position being subjected to severe bending/unbending deformation is expected,
the Fe ion concentration is more preferably 5 g/liter or smaller. The term "Fe ion
concentration" referred to herein signifies the total concentration of Fe
2+ and Fe
3+.
[0047] Before the hot dip galvanized steel sheet is brought into contact with the acidic
solution, it is preferable to bring the steel sheet into contact with an alkaline
solution to activate the surface thereof. The contacting with alkaline solution is
adopted by the reason described below. For a galvannealed steel sheet, although the
oxide containing Al, formed on the surface of plating layer after plating, is fractured
and removed by the roll during the temper-rolling stage, a part thereof still remains
on the surface of plating layer, which makes the reactivity with the acidic solution
non-uniform, thereby may failing in forming a homogeneous film of a compound containing
Zn, Fe, and O. For the case of non-alloyed hot dip galvanized steel sheet, the surface
of plateau has a portion which does not contact with the roll face of the temper-rolling
and which retains the oxide containing Al, thus the surface activation is specifically
preferred to be performed by applying alkali treatment or the like to remove a part
or all of the oxide.
[0048] There is no specific limitation of the method for contacting with alkaline solution,
and dipping method, spray method, and the like may be applied. If the pH of alkaline
solution is low, the reaction becomes slow to take a long time for the treatment.
Accordingly the pH of alkaline solution is preferably 10 or larger. Applicable alkaline
solution includes sodium hydroxide.
[0049] If the acidic solution is retained on the surface of hot dip galvanized steel sheet
after water-washing and drying, the steel sheet coil likely generates rust during
a long time of storage. To prevent the rust generation, it is preferred to bring the
hot dip galvanized steel sheet after contacting with the acidic solution dip in an
alkaline solution or to spray an alkaline solution to neutralize the acidic solution
remained on the surface of the steel sheet. In this case, the pH of the alkaline solution
is preferably 12 or smaller to prevent the dissolution of the film of a compound containing
Zn, Fe, and O, formed on the surface of plating layer. Applicable alkaline solution
includes sodium hydroxide and sodium phosphate.
[0050] Similar effect is attained by heating the steel sheet after hot-dip-galvanizing to
process the plating layer by alloying treatment.
[0051] As described above, since the present invention uses an acidic solution containing
Fe ion and having pH buffering effect, a film of a compound containing Zn, Fe, and
O, providing excellent sliding performance, bondability, and phosphatability can be
stably formed. Even when the acidic solution contains other metallic ions and inorganic
compounds as impurities or as intentional additives, the effect of the present invention
is not deteriorated. In particular, when a hot dip galvanized steel sheet contacts
with the acidic solution, although the Zn ion is dissolved to increase the Zn concentration
in the acidic solution, the increase in the Zn ion concentration does not affect the
effect of the present invention.
[0052] The zinc plating bath for manufacturing the hot dip galvanized steel sheet according
to the present invention is required to contain Al. Even when elements other than
Al, such as Fe, Pb, Sb, Si, Sn, Mn, Ni, Ti, Li, and Cu exist in the zinc plating bath,
the effect of the present invention is not deteriorated.
[0053] Contacting the hot dip galvanized steel sheet with the acidic solution can be done
by dipping the hot dip galvanized steel sheet in the acidic solution, by spraying
the acidic solution thereto, by coating the acidic solution thereon using a roll,
and the like.
[0054] Even when the film composed of a compound containing Zn, Fe, and O contains elements
such as F, Mg, Al, Si, P, S, Cl, K, Ca, and Ba, existing in the acidic solution, or
contains adsorbed water, the effect of the present invention is not deteriorated.
The film is not necessarily formed continuously, and the film covering not the whole
area of plateau is also effective. Nevertheless, to decrease the friction resistance,
the film preferably covers 60% or more of the plateau.
Example 1
[0055] Galvannealed layer was formed on each of cold-rolled steel sheets having 0.8 mm of
thickness using an ordinary method, which plated steel sheets were then processed
by temper-rolling. After that, a film was formed on the surface of zinc plating layer
under the respective treatment conditions given in Table 1 to prepare the sample Nos.
1 to 22.
[0056] With the treatment X in Table 1, a ZnO coating was formed by the reactive sputtering
method.
[0057] With the treatments Y, Z, and A to E, a liquid film was formed on the surface of
each steel sheet by spraying and roll-squeezing an acidic solution onto the surface
of the steel sheet. The acidic solution at temperatures from 25°C to 40°C contained
a pH buffer composed of sodium acetate and sodium citrate at the respective quantities
given in Table 1, further contained iron (II) sulfate by 2 g/liter or smaller Fe
2+ concentration, and had the respective Fe
2+ concentrations given in Table 1. The formed liquid film was allowed to standing for
a period given in Table 1, and then was immediately washed by spraying water at 50°C,
followed by drying using a drier to form the film containing Zn, Fe, and O. The quantity
of liquid film was adjusted by varying the pressure of squeeze-rolls. The pH of acidic
solution was adjusted by adding sulfuric acid.
[0058] With thus prepared samples, average film thickness A, average film thickness B, and
{[Fe]/([Zn] + [Fe])} in the film were determined using the respective above-described
methods. Using the method described below, the coefficient of friction as an index
of press-formability was determined, and the bondability, then phosphatability, and
the powdering resistance of the plating layer having the film were investigated.
(1) Determination of coefficient of friction
[0059] Figure 1 is a schematic drawing of the flat sliding test apparatus used in the examples.
[0060] A comparative sample 11 for determining the coefficient of friction is fixed on a
sample table 12 which is fixed on the upper face of a horizontally movable slide table
13. At the lower face of the slide table 13, there is located a vertically movable
slide table support 15 equipped with a roller 14 contacting with the slide table 13.
A first load cell 17 is attached to the slide table support 15. The first load cell
determines the pressing load N applied from a bead 16 to the sample 11 by pushing-up
the slide table support 15. A second load cell 18 is attached to an end of the slide
table 13. The second load cell 18 determines the sliding resistance F by moving the
slide table 13 in the horizontal direction in a state of pressing the bead 16 against
the comparative sample 11. The tests were conducted by coating a lubricant oil on
the surface of the comparative sample 11. The applied lubricant oil was PRETON R352L,
a washing oil for press-work manufactured by Sugimura Chemical Industrial Co., Ltd.
[0061] Figure 2 and Fig. 3 show the shapes of applied beads.
[0062] The bead 16 shown in Fig. 2 has the dimensions of 10 mm in width, 12 mm in the length
in the sliding direction, 3 mm in the length in the sliding direction to which the
sample is pressed, and 4.5 mm in the radius at each end in the sliding direction.
[0063] The bead 16 shown in Fig. 3 has the dimensions of 10 mm in width, 69 mm in the length
in the sliding direction, 60 mm in the length in the sliding direction to which the
sample is pressed, and 4.5 mm in the radius at each end in the sliding direction.
[0064] The sample slides under a condition that the flat part of the lower face of the bead
16 is pressed against the surface of the sample.
[0065] The flat sliding tests were conducted under the two conditions given below, and the
coefficient of friction µ=F/N between the sample and the bead was calculated.
[0066] Condition 1: The bead shown in Fig. 2 ; 400 kgf of the pressing load N; and 100 cm/min
of the sample sliding speed (the horizontal moving speed of the slide table 13).
[0067] Condition 2 : The bead shown in Fig. 3 ; 400 kgf of the pressing load N; and 20 cm/min
of the sample sliding speed.
(2) Bondability test
[0068] As illustrated in Fig. 4, two sheets of test pieces 21, each having 25 mm in width
and 200 mm in length, were cut from each sample. An adhesive 23 was injected between
the test pieces 21 via a spacer 22 having 0.12 mm in thickness, thus preparing an
bondability test piece 24 which had a non-bonding portion at an end thereof. After
baking the bondability test piece 24 at 150 °C for 10 minutes, the non-bonding portion
was folded vertically to the bondability test piece as shown in Fig. 5. The folded
portions were drawn by a tensile tester at a testing speed of 200 mm/min to conduct
the peeling test. The applied adhesive 23 was an adhesive for hemming of vinyl chloride
resin group.
[0069] The peeling occurs at the weakest position in terms of strength. If the adhesion
between the test piece and the adhesive is sufficient, the peeling occurs by cohesive
failure inside the adhesive. If the adhesion therebetween is insufficient, the peeling
occurs at the interface between the test piece and the adhesive. The bondability was
evaluated by the peeling mode, giving "○" rank to the peeling caused by the cohesive
failure inside the adhesive as "superior bondability", and giving "X" rank to the
peeling occurred at interface between the test piece and the adhesive as "interior
bondability". For the case of galvannealed steel sheet, particularly the case of forming
Γ phase at the interface between the plating layer and the steel sheet gave weak strength
at the interface between the plating layer and the steel sheet, thereby generating
peeling at the interface in some cases. However, also that case was judged as "good
bondability" between the sample piece and the adhesive, and gave the evaluation of
"○" rank.
(3) Phosphatability test
[0070] Each sample was treated under an ordinary condition using a dip-type zinc phosphate
treatment solution for automobile surface treatment for coating, (PBL3080, manufactured
by Nihon Parkerizing Co., Ltd.), and then a zinc phosphate film was formed thereon.
The crystal state of the zinc phosphate filmwas observed by scanning electron microscope
(SEM). The film formed in uniform state was evaluated as "○", and the film formed
in non-uniform state was evaluated as "X".
(4) Powdering resistance test
[0071] Figure 6 is a schematic drawing of the draw-bead tester used in the examples.
[0072] First, the plating layer on a face of a square test piece cut from the sample face
not contacting with the bead was peeled using hydrochloric acid, and the weight W
1 g of the test piece was determined. Then, the test piece was attached to the tester
given in Fig. 6. After pressing the triangle bead having 0.5 mm in tip radius against
the test piece at 500 kgf load into 4 mm of penetration depth, the test piece was
drawn out at a constant speed of 200 mm/min. For the drawn-out test piece, the contact
face with the bead was forcefully peeled using an adhesive tape, then the weight W
2 g was determined. By dividing the (W
1 - W
2) by the drawn-out area, the quantity of peeling per unit area was derived, and the
powdering resistance was evaluated by the peeled quantities.
[0073] The result is given in Table 2.
[0074] For the Examples according to the present invention, where the film thickness A was
10 nm or larger and where the quantity ratio of Fe in the film, {[Fe]([Zn] + [Fe])},
was in a range from 0.002 to 0.15, the friction factor was lower than that for the
cases of Comparative Examples 1 and 2, where no treatment was applied and no film
was formed. The fact shows that Examples of the present invention have high lubrication.
[0075] Examples 17 and 19 of the present invention having 10 nm or larger film thickness
A and having 1. 5 or larger film thickness ratio B/A gave lower coefficient of friction
than that of Comparative Example 6 having smaller than 1.5 of B/A, though having similar
film thickness A. The fact shows that, even with a same degree of film thickness,
large 8/A provides high lubrication. In particular, larger B/A provides lower coefficient
of friction stably. It is also understood that the increase in the film thickness
ratio B/A is attained by the reduction in the percentage of Fe in the film.
[0076] Since the bondability, the phosphatability, and the powdering resistance of Examples
of the present invention were similar with those of Comparative Examples 1 and 2 using
ordinary galvannealed steel sheets, the results are not given in Table 2. There was
identified, however, a tendency of somewhat decreasing the adhesion between the film
and the surface of plating layer when the percentage of Fe decreases. That is, with
the ratio of Fe at 0.002 or larger, all the bondability tests gave cohesive failure
inside the adhesive, thus no practical problem arises. However, in Comparative Example
11 with the ratio of Fe smaller than 0.002, the bondability test gave mixed occurrence
of interfacial peeling, though the coefficient of friction was relatively decreased,
thus failing to attain good bondability. When considering that Comparative Examples
6 to 8 gave insufficient formation of zinc phosphate crystals during phosphatizatior,
it is shown that Examples 17 to 22 of the present invention having a similar film
thickness A with each other give excellent phosphatability owing to the refining the
film or to the increase in void fraction in the film.
[0077] Since the treatment solution applied was an acidic solution containing iron (II)
sulfate, sulfur was detected by several percentages by weight in some cases. The presence
of sulfur by that amount, however, did not affect the effect of the present invention.
Table 1
Treatment |
pH buffer (g/liter) |
Fe2+ concentration (g/liter) |
Time for allowing to standing (sec) |
X |
- |
- |
- |
Y |
0 |
60 |
7.5 |
Z |
40 |
20 |
7.5 |
A |
35 |
7 |
0.5 - 15 |
B |
16 |
0.3 |
2.8 - 7.5 |
C |
35 |
0.6 |
5 |
D |
30 |
1 |
9 - 30 |
E |
35 |
0 |
5 |
Table 2
Sample No. |
Treatment |
{[Fe]/([Zn] + [Fe])} |
Film thickness A (nm) |
Film thickness B (nm) |
Film thickness ratio B/A |
Friction factor µ |
Remark |
Condition 1 |
Condition 2 |
1 |
- |
- |
|
- |
- |
0.179 |
0.250 |
Comparative example |
2 |
- |
- |
6 |
- |
- |
0.181 |
0.251 |
Comparative example |
3 |
X |
0 |
5 |
6 |
1.2 |
0.178 |
- |
Comparative example |
4 |
X |
0 |
10 |
11 |
1.1 |
0.172 |
- |
Comparative example |
5 |
X |
0 |
23 |
28 |
1.2 |
0.163 |
- |
Comparative example |
6 |
X |
0 |
35 |
42 |
1.2 |
0.153 |
- |
Comparative example |
7 |
X |
0 |
49 |
64 |
1.3 |
0.141 |
- |
Comparative example |
8 |
X |
0 |
99 |
121 |
1.2 |
0.141 |
- |
Comparative example |
9 |
Y |
0.53 |
12 |
15 |
1.3 |
0.161 |
0.234 |
Comparative example |
10 |
Z |
0.24 |
23 |
31 |
1.3 |
0.145 |
0.223 |
Example |
11 |
A |
0.16 |
8 |
26 |
3.3 |
0.165 |
0.250 |
Comparative example |
12 |
A |
0.22 |
27 |
47 |
1.7 |
0.133 |
0.202 |
Comparative example |
13 |
A |
0.15 |
25 |
102 |
4.1 |
0.128 |
0.168 |
Example |
14 |
B |
0.14 |
20 |
53 |
2.6 |
0.132 |
0.182 |
Example |
15 |
B |
0.13 |
27 |
98 |
3.7 |
0.134 |
0.172 |
Example |
16 |
B |
0.086 |
27 |
195 |
7.3 |
0.130 |
0.170 |
Example |
17 |
B |
0.072 |
33 |
204 |
6.3 |
0.128 |
0.171 |
Example |
18 |
C |
0.024 |
29 |
300 |
10.5 |
0.130 |
0.169 |
Example |
19 |
D |
0.005 |
36 |
515 |
14.5 |
0.129 |
0.165 |
Example |
20 |
D |
0.003 |
43 |
606 |
14.1 |
0.125 |
0.167 |
Example |
21 |
D |
0.008 |
97 |
976 |
10.1 |
0.123 |
0.160 |
Example |
22 |
E |
< 0.002 |
22 |
58 |
2.6 |
0.143 |
0.200 |
Comparative example |
Example 2
[0078] Galvannealed layer was formed on each of cold-rolled steel sheets having 0.8 mm of
thickness using an ordinary method, which plated steels sheet were then processed
by temper-rolling. After that, a film was formed on the surface of zinc plating layer
using a film-forming apparatuses given in Fig. 7 under the respective treatment conditions
given in Table 3 to prepare the sample Nos. 1 to 20.
[0079] First, with an acidic solution tank 2 in Fig. 7, the steel sheet was dipped into
an acidic solution at 50°C and pH 2.0 to form a liquid film on the surface of the
steel sheet using squeeze rolls 3. The formed liquid film was washed in a washing
tank 5 by spraying hot water at 50 °C against the steel sheet, and then the steel
sheet was passed through a neutralization tank 6 without applying neutralization.
The steel sheet was washed by spraying water at 50°C thereto in a washing tank 7,
followed by drying in a drier 8, thus forming the film on the surface of plating layer.
The quantity of liquid film was adjusted by varying the pressure of squeeze rolls
3.
[0080] The acidic solution in an acidic solution tank 2 was an acidic solution which contained
a pH buffer prepared by mixing 30 g/liter of disodium hydrogenphosphate and 20 g/liter
of citric acid, adding a specific amount of iron (II) sulfate to add Fe ion thereto,
and further adding sulfuric acid to adjust pH. For comparison, an acidic solution
containing only iron (II) sulfate, not containing pH buffer, was used, (Sample Nos.
3 to 5).
[0081] The period of allowing to standing before water-washing is the time between the adjustment
of quantity of liquid film by the squeeze rolls 3 and the start of washing in the
washing tank 5. The period thereof was adjusted by varying the line speed. For some
of the samples, washing was applied immediately after adjustment of the quantity of
liquid film using a shower-water-washing apparatus 4 at exit of the squeeze rolls
3.
[0082] Other than the above samples, sample Nos. 15 to 17 were prepared, which samples were
treated by: applying activation treatment by dipping the sample in an aqueous solution
of sodium hydroxide at pH 12 in an activation tank 1 before dipping the sample in
the acidic solution ; and then spraying an aqueous solution of sodium hydroxide at
pH 10 in a neutralization tank 6 to neutralize the acidic solution remained on the
surface of the steel sheet.
[0083] For thus prepared samples, determination of the coefficient of friction, and evaluation
of the bondability, the phosphatability, and the powdering resistance were given using
similar methods with those applied in Example 1.
[0084] After coating the rust preventive oil, the steel sheets were allowed to standing
outdoors for about 6 months while preventing external influences such as those of
dust, and the generation of spot-rusts was examined. The evaluation was given as "○"
rank for no generation of spot-rusts, and as "X" rank for generation of spot-rusts.
[0085] The result is given in Table 3.
[0086] Sample Nos. 9 to 14 and Nos. 18 to 20, which were Examples of the present invention,
having pH buffering effect and processed by the treatment of Fe ion-containing acidic
solution provided low coefficient of friction and showed excellent bondability and
phosphatability.
[0087] Sample Nos. 15 to 17, which were Examples of the present invention, processed by
alkali treatment in the activation tank prior to the acidic solution treatment showed
lower coefficient of friction than that of the sample Nos. 12 to 14, which were treated
by the same acidic solution and were allowed to standing for the same period prior
to the water-washing. The sample Nos. 15 to 17 generated no spot-rusts owing to the
alkali treatment in the neutralization tank after the acidic solution treatment, which
samples are advantageous also for a long period of storage.
[0088] Regarding the powdering resistance, the sample Nos. 9 to 17 which were treated by
acidic solutions containing 5 g/liter or smaller Fe concentration showed a tendency
of decreased quantity of peeling of plating in the draw-bead test, thus these samples
provided excellent powdering resistance.
[0089] On the other hand, the sample Nos. 1 and 2 of Comparative Examples which were not
treated by acidic solution gave high coefficient of friction because they have no
film to improve the sliding performance.
[0090] The sample Nos. 3 to 5 of Comparative Examples which were treated by acidic solution
containing no pH buffer gave higher coefficient of friction than that of the samples
of Examples of the present invention, though giving lower coefficient of friction
than that of the sample Nos. 1 and 2, thus the sample Nos. 3 to 5 are expected to
insufficiently form the film.
[0091] The sample Nos. 6 to 8 of Comparative Examples which were treated by acidic solution
containing no Fe ion, though containing pH buffer, showed poor bondability or phosphatability,
though providing low coefficient of friction.
Table 3
Sample No. |
Acidic solution |
pH increase degree |
Quantity of liquid film (g/m2) |
Time for allowing to standing (sec) |
Use of activation tank |
Use of neutralization tank |
Friction factor |
Powdering resistance (g/m2) |
Bondability |
Phosphatability |
Presence/ absence of spot-rusts |
Remark |
pH buffer |
Fe2) concentration |
Condition 1 |
Condition 2 |
1 |
- |
- |
- |
- |
- |
- |
- |
0.179 |
0.250 |
1.6 |
○ |
○ |
○ |
Comparative Example |
2 |
- |
- |
- |
- |
0.181 |
0251 |
12 |
○ |
○ |
○ |
Comparative example |
3 |
- |
5 g/liter |
0.6 |
3.0 |
5.0 |
- |
- |
0.156 |
0.217 |
1.6 |
○ |
○ |
× |
Comparative example |
4 |
3.0 |
10.0 |
- |
- |
0.152 |
0.209 |
1.2 |
○ |
○ |
× |
Comparative example |
5 |
3.0 |
30.0 |
- |
- |
0.150 |
0.210 |
1.7 |
○ |
○ |
× |
Comparative example |
6 |
|
- |
7.5 |
3.0 |
5.0 |
- |
- |
0.39 |
0.191 |
1.4 |
× |
○ |
× |
Comparative example |
7 |
|
3.0 |
10.0 |
- |
- |
0.132 |
0.182 |
2.1 |
× |
○ |
× |
Comparative example |
8 |
|
3.0 |
30.0 |
- |
- |
0.135 |
0.196 |
1.8 |
× |
× |
× |
Comparative example |
9 |
|
0.5 g/liler |
7.6 |
3.0 |
5.0 |
- |
- |
0.137 |
0.190 |
1.6 |
○ |
○ |
× |
Examples |
10 |
|
3.0 |
10.0 |
- |
- |
0.137 |
0.187 |
1.0 |
○ |
○ |
× |
Example |
11 |
|
3.0 |
30.0 |
- |
|
0.134 |
0.184 |
1.3 |
○ |
○ |
× |
Example |
12 |
Disodium |
5 g/liter |
8.0 |
3.0 |
5.0 |
- |
- |
0.131 |
0.193 |
1.8 |
○ |
○ |
× |
Example |
13 |
hydrogenphosphate (30 g/liter) + Citric |
3.0 |
10.0 |
- |
- |
0.130 |
0.186 |
2.3 |
○ |
○ |
× |
Example |
14 |
acid (20 g/liter) |
3.0 |
30.0 |
- |
- |
0.129 |
0.179 |
0.8 |
○ |
○ |
× |
Example |
15 |
|
3.0 |
5.0 |
○ |
○ |
0.125 |
0.164 |
1.2 |
○ |
○ |
○ |
Example |
16 |
|
3.0 |
10.0 |
○ |
○ |
0.127 |
0.165 |
1.1 |
○ |
○ |
○ |
Example |
17 |
|
3.0 |
30.0 |
○ |
○ |
0.126 |
0.164 |
1.7 |
○ |
○ |
○ |
Example |
18 |
|
50 g/liler |
8.3 |
3.0 |
5.0 |
- |
- |
0.135 |
0.192 |
6.5 |
○ |
○ |
× |
Example |
19 |
|
3.0 |
10.0 |
- |
- |
0.133 |
0.182 |
8.6 |
○ |
○ |
× |
Example |
20 |
|
3.0 |
30.0 |
- |
- |
0.133 |
0.191 |
7.1 |
○ |
○ |
× |
Example |
Example 3
[0092] Galvannealed layer was formed on each of cold-rolled steel sheets having 0. 8 mm
of thickness using an ordinary method, which plated steels sheet were processed by
temper-rolling. After that, a film was formed on the surface of zinc plating layer
using a film-forming apparatuses having the structure given in Fig. 7 under the respective
treatment conditions given in Table 4 to prepare the sample Nos. 1 to 26.
[0093] First, with the acidic solution tank 2 in Fig. 7, the steel sheet was dipped into
the acidic solution at 50°C and pH 2. 0 to form a liquid film on the surface of the
steel sheet using the squeeze rolls 3. The formed liquid film was washed in the washing
tank 5 by spraying hot water at 50°C against the steel sheet, and then the steel sheet
was passed through the neutralization tank 6 without applying neutralization. The
steel sheet was washed by spraying water at 50°C thereto in the washing tank 7, followed
by drying in the drier 8, thus forming the film on the surface of plating layer. The
quantity of liquid filmwas adjusted by varying the pressure of squeeze rolls 3.
[0094] The acidic solution in the acidic solution tank 2 was an acidic solution which contained
a pH buffer prepared by mixing 30 g/liter of disodium hydrogenphosphate and 20 g/liter
of citric acid, adding a specific amount of iron (II) sulfate to add Fe ion thereto,
and further adding sulfuric acid to adjust pH. For comparison, an acidic solution
containing only iron (II) sulfate, not containing pH buffer, was used, (Sample Nos.
3 to 5). To identify the influence of Fe
3+, an acidic solution containing Fe(III) sulfate was also applied to some of the samples,
(sample Nos. 18 to 23).
[0095] The period of allowing to standing before water-washing is the time between the adjustment
of quantity of liquid film by the squeeze rolls 3 and the start of washing in the
washing tank 5. The period thereof was adjusted by varying the line speed. For some
of the samples, washing was applied immediately after adjustment of the quantity of
liquid film using a shower-water-washing apparatus 4 at exit of the squeeze rolls
3.
[0096] Other than the above samples, sample Nos. 15 to 17 were prepared, which samples were
treated by: applying activation treatment by dipping the steel sheet in an aqueous
solution of sodium hydroxide at pH 12 in the activation tank 1 before dipping the
steel sheet in the acidic solution; and then spraying an aqueous solution of sodium
hydroxide at pH 10 in the neutralization tank 6 to neutralize the acidic solution
remained on the surface of the steel sheet.
[0097] For thus prepared samples, determination of the coefficient of friction, and evaluation
of the bondability, the phosphatability, the powdering resistance, and the generation
of spot-rusts were given using similar methods with those applied in Example 1.
[0098] The result is given in Table 4.
[0099] Other than the sample Nos. 18 to 23 for investigating the influence of Fe
3+, the samples gave results almost equal to those in Example 2.
[0100] The sample Nos. 18 to 23, which were treated by acidic solution varying the Fe
3+ concentration by adding iron (III) sulfate, gave low coefficient of friction and
excellent bondability and phosphatability. Although the sample Nos. 18 to 20 having
2 g/liter or smaller Fe
3+ concentration showed no bruise caused by sludge, the sample Nos. 21 to 23 having
larger than 2 g/liter of Fe
3+ concentration showed bruise.
Table 4
Sample No. |
Acidic solution |
pH Increase degree |
Quantity of liquid film (g/m2) |
Time for allowing to standing (sec) |
Use of activation tank |
Use of neutralization tank |
Friction factor |
Powdering resistance (g/m2) |
Bondability |
Phosphatability |
Presence/absence of spot-rusts |
Presence/absence of bruise |
Remark |
pH buffer |
Fe2+ concentration |
Fe3+ concentration |
Condition 1 |
Condition 2 |
1 |
- |
|
- |
- |
- |
- |
- |
- |
0.179 |
0.250 |
1.7 |
○ |
○ |
○ |
○ |
Comparative example |
2 |
- |
- |
- |
- |
0.181 |
0.251 |
1.2 |
○ |
○ |
○ |
○ |
Comparative example |
3 |
- |
5 g/liter |
5 g/liter |
0.6 |
3.0 |
5.0 |
- |
- |
0.156 |
0.217 |
1.1 |
○ |
○ |
× |
○ |
Comparative example |
4 |
3.0 |
10.0 |
- |
- |
0.152 |
0.209 |
1.1 |
○ |
○ |
× |
○ |
Comparative example |
5 |
3.0 |
30.0 |
- |
- |
0.150 |
0.210 |
0.9 |
○ |
○ |
× |
○ |
Comparative example |
6 |
|
- |
- |
7.5 |
3.0 |
5.0 |
- |
- |
0.131 |
0.191 |
1.2 |
× |
○ |
× |
○ |
Comparative example |
7 |
|
3.0 |
10.0 |
- |
- |
0.130 |
0.193 |
1.1 |
× |
○ |
× |
○ |
Comparative example |
8 |
|
3.0 |
30.0 |
- |
- |
0.126 |
0.184 |
1.8 |
× |
× |
× |
○ |
Comparative example |
9 |
|
0.5 g/liter |
- |
7.6 |
3.0 |
5.0 |
- |
- |
0.134 |
0.195 |
1.6 |
○ |
○ |
× |
○ |
Example |
10 |
|
3.0 |
10.0 |
- |
- |
0.130 |
0.191 |
1.3 |
○ |
○ |
× |
○ |
Example |
11 |
|
3.0 |
30.0 |
- |
- |
0.135 |
0.195 |
1.3 |
○ |
○ |
× |
○ |
Example |
12 |
|
5 g/liter |
- |
8.0 |
3.0 |
5.0 |
- |
- |
0.131 |
0.179 |
1.2 |
○ |
○ |
× |
○ |
Example |
13 |
|
3.0 |
10.0 |
- |
- |
0.138 |
0.192 |
1.2 |
○ |
○ |
× |
○ |
Example |
14 |
Disodium hydrogen- |
3.0 |
30.0 |
- |
- |
0.140 |
0.186 |
1.0 |
○ |
○ |
× |
○ |
Example |
15 |
phosphate |
3.0 |
5.0 |
○ |
○ |
0.138 |
0.171 |
1.0 |
○ |
○ |
○ |
○ |
Example |
16 |
(30 g/lifer) + Citric |
3.0 |
10.0 |
○ |
○ |
0.133 |
0.165 |
1.8 |
○ |
○ |
○ |
○ |
Example |
17 |
acid (20 |
3.0 |
30.0 |
○ |
○ |
0.130 |
0.164 |
1.3 |
○ |
○ |
○ |
○ |
Example |
18 |
g/liter) |
1 g/liter |
8.1 |
3.0 |
5.0 |
- |
- |
0.140 |
0.198 |
2.5 |
○ |
○ |
× |
○ |
Example |
19 |
|
3.0 |
10.0 |
- |
- |
0.138 |
0.199 |
2.3 |
○ |
○ |
× |
○ |
Example |
20 |
|
3.0 |
30.0 |
- |
- |
0.133 |
0.195 |
2.6 |
○ |
○ |
× |
○ |
Example |
21 |
|
5 g/liter |
8.1 |
3.0 |
5.0 |
- |
- |
0.138 |
0.187 |
3.5 |
○ |
○ |
× |
× |
Example |
22 |
|
3.0 |
10.0 |
- |
- |
0.133 |
0.182 |
3.9 |
○ |
○ |
× |
× |
Example |
23 |
|
3.0 |
30.0 |
- |
- |
0.130 |
0.185 |
3.3 |
○ |
○ |
× |
× |
Example |
24 |
|
50 g/liter |
- |
8.3 |
3.0 |
5.0 |
- |
- |
0.139 |
0.191 |
8.2 |
○ |
○ |
× |
○ |
Example |
25 |
|
3.0 |
10.0 |
- |
- |
0.135 |
0.190 |
7.2 |
○ |
○ |
× |
○ |
Example |
26 |
|
3.0 |
30.0 |
- |
- |
0.131 |
0.193 |
8.5 |
○ |
○ |
× |
○ |
Example |