DESCRIPTION
Title of Invention: Aluminum Plated Steel Sheet Having
Excellent Corrosion Resistance With Respect to Alcohol or
Mixed Gasoline of Same and Appearance and Method of
Production of Same
Technical Field
[ 0001 ] The present invention relates to aluminum
plated steel sheet having excellent corrosion resistance
and appearance which does not rust even if used for tanks
of fuel containing methanol, ethanol, or other alcohol or
gasoline in which these alcohols are mixed etc. (below,
in the present application, these being referred to all
together as "flex fuels)") and a method of production of
the same.
Background Art
[0002] In recent years, for automobile fuel, the ratio
of introduction of methyl alcohol, ethyl alcohol, methylt-
butyl ether, and other alcohols and gasoline in which
these alcohols are mixed (flex fuels), so-called
"gasohol", has been increasing as a general trend.
[0003] However, these flex fuels have the following
defects :
(a) Water is easily contained
(b) Phase separation easily occurs due to an increase in
the moisture content and a drop in temperature.
(c) Further, oxidation causes deterioration resulting in
production of organic acids and easy phase separation.
For example, methanol changes to formic acid and ethanol
changes to acetic acid. As a result, phase separation
into mainly alcohol and/or organic acid and water occurs,
so there is much stronger corrosiveness compared with
ordinary gasoline.
[OOOS] In general, as the material for automobile fuel
tanks, Pb-Sn alloy plated steel sheet, called "ternecoated
sheet", and zinc (Zn)-plated steel sheet treated
with thick chromate has been used. However, Pb-Sn-alloy
plated steel sheet and zinc-plated steel sheet are
corroded by the moisture which is contained in alcohol,
formaldehyde, acetoaldehyde, and other alcohol oxides,
formic acid, acetic acid, or other impurities and elute
plating ingredients into the fuel. For this reason,
conventional plated steel sheet is not suited to a fuel
tank for flex fuels.
[OOOS] Further, in recent years, art relating to hot
dip aluminum coated steel sheet has been proposed. PLT 1
describes aluminum plated steel sheet for fuel tank use
which has excellent corrosion resistance comprised of
steel sheet on the surface of which an aluminum plating
layer comprised of, by wt%, Mg: 0.5 to 15%, Si: 2 to 15%,
and a balance of A1 and unavoidable impurities is
provided, the aluminum plating layer containing an Mg2Si
phase.
[0006] PLT 2 describes hot dip aluminum coated steel
sheet which has excellent corrosion resistance and
appearance characterized by comprising steel sheet on the
surface of which an aluminum plating layer comprised of,
by wt%, Mg: 1 to 15%, Si: 2 to 15% and also an alkali
earth metal element other than Mg in 0.02 to 5% is
provided, the plating layer containing an Mg2Si phase.
[0007] PLT 3 proposes hot dip aluminum coated steel
sheet comprised of steel sheet on the surface of which a
metal structure which contains, by wt%, Mg: 3 to lo%, Si:
1 to 15%, and Ca: 0.01 to 2% and which has an Mg2Si phase
with a long axis of 10 pm or less is provided.
[ 0 0 0 8 ] Even if these plated steel sheets are used for
fuel tanks for petroleum-derived gasoline or diesel oil,
the inside surfaces of the tanks will not be corroded, so
pitting will not occur. Further, no floating corrosion
products are formed, so no filter clogging occurs in the
fuel circulation system either. Furthermore, the outside
surface of the tank is protected against salt corrosion
by paint in addition to the plating layer, so again no
pitting occurs.
[0009] However, if using flex fuels for automobile
fuel, the fuel tanks will be corroded. That is, with
aluminum plated steel sheet, the alcoholate reaction (see
5 following formula) causes the aluminum plating to
dissolve in the alcohol, so sufficient corrosion
resistance is not exhibited against alcohol or mixed
gasoline. Al-Mg-Si-based plated steel sheet is resistant
to an alcoholate reaction, but if the alcohol
10 concentration is high, again a sufficient corrosion
resistance is not obtained:
2A1+6 (R-OH) +2 (A1 (OR) 3) +3H2
2 (A1 (OR) 3) +6H2+2Al (OH) 3+6 (R-H)
[ 0 0 10 ] As art for solving these problems relating to
15 alcohol and flex fuels, several arts have been proposed.
PLT 4 describes steel sheet for a high corrosion
resistance fuel tank for alcohol or flex fuels
characterized by comprising steel sheet on the surface of
which is provided an organic bonded solid lubricant film
20 on which aluminum or an A1-Si alloy plating which
contains Si: 3 to 12% is deposited to 30 to 400 g/m2,
which is treated by chromate to 10 to 40 mg/m2 converted
to chrome, and which contains metal powder over that.
[ 0 0 11 ] PLT 5 describes steel sheet for fuel tank use
25 comprised of steel sheet on the surface of which a zincbased
plating layer is provided, over which an organic
film which is formed by a bisphenol type epoxy resin,
phosphoric acid-modified epoxy resin, modified epoxy
resin which has a primary hydroxyl group, a glycoluril
30 resin and a metal compound is provided, the metal
compound containing at least a V compound, the glycoluril
resin being contained in 5 to 20 mass% with respect to
the total solid content of the organic film, and the
metal compound being contained in 10 to 20 mass%.
35 However, these steel sheets for fuel tank use are coated
on their surfaces after plating (surface treated).
Plating along is not enough to improve the corrosion
resistance to alcohol or flex fuels.
[0012] As explained above, plated steel sheet which
has sufficient corrosion resistance with respect to
alcohol or mixed gasoline of the same has not yet been
5 commercialized.
Citations List
Patent Literature
[0013] PLT 1: Japanese Patent Publication No. 2000-
290763A
10 PLT 2: Japanese Patent Publication No. 2001-73108A
PLT 3: Japanese Patent Publication No. 2000-328168A
PLT 4: Japanese Patent Publication No. 6-306637A
PLT 5: Japanese Patent Publication No. 2007-186745A
Summary of Invention
15 Technical Problem
[0014] As explained above, the arts of PLT's 4 and 5
describe steel sheets which are coated (surface treated)
after plating - not steel sheets without coatings which
have sufficient corrosion resistance as plated steel
20 sheet for flex fuels. The steel sheet described in PLT 3
was of an extent exhibiting relatively good corrosion
resistance even to flex fuels. In general, flex fuels
easily contain moisture. The steel materials of fuel
tanks which contact them, in particular the vertical wall
25 parts which are heavily press formed, are exposed to a
corrosive environment. Therefore, steel materials for
fuel tanks for flex fuels are required to have higher
corrosion resistance than the past.
[OOlS] The plated steel sheet according to the present
invention aims at giving a corrosion resistance and
appearance against flex fuels equal to or better than
steel sheet due to conventional coating. Specifically, in
the evaluation of the corrosion resistance, it aims at
prevention of excessive corrosion products, red rust,
35 etc. in the later explained corrosion tests. Further, in
appearance, it aims at no wrinkles being visually
confirmable.
Solution to Problem
[0016] The inventors took note of the effect of
suppression of corrosion due to flex fuels in the MgaSi
intermetallic compounds present in the aluminum plating
layer and engaged in research to solve the above
problems. As a result, they discovered that Mg2Si with a
long axis of 10 pm or less inhibits an alcoholate
reaction and improves the corrosion resistance against
flex fuels.
[0017] Further, Mg2Si is a brittle intermetallic
compound which easily breaks upon being press formed. In
particular, an automobile fuel tank has a complicated
shape, so when produced by press forming, sometimes the
rate of reduction of thickness at the vertical wall parts
15 reaches close to 20%. At this time, due to press forming,
stress locally concentrates at the A1 plating layer
around Mg2Si which has a large aspect ratio, that is, an
elongated shape. As a result, the inventors discovered
that the A1 plating layer cracked and the corrosion
20 resistance deteriorated.
[ 0 0 18 ] From the above, the inventors discovered that
the Mg2Si particles should be as close to a spherical
shape (aspect ratio=l) as possible and that the aspect
ratio should be made 3 or less.
25 [0019] That is, they discovered that by making the
long axis of the Mg2Si particles less than 10 pm and
making the aspect ratio 3 or less and finely dispersing
the particles, plated steel sheet which has excellent
corrosion resistance to flex fuels is obtained and
30 thereby completed the present invention.
[0020] The inventors also intensively studied the
method of finely dispersing Mg2Si particles which have the
above shape features in an aluminum plating layer. As a
result, they discovered that to make the Mg2Si particles
35 finely disperse and make them shapes close to spherical
shapes, it is effective to add Ti in the plating metal
and control the cooling method after plating so that the
Ti-A1 peritectic reaction is promoted.
[0021] Furthermore, the inventors discovered that to
prevent the formation of MgO (oxide film) and
deterioration of the plating appearance even when Mg does
5 not form Mg2Si particles and remains in the aluminum
plating layer, it is important to limit the amount of Mg
and add a fine amount of Ca with a stronger oxidizing
power than Mg and give priority to the formation of CaO.
Due to this, they discovered a range of the plating
composition which has excellent corrosion resistance with
respect to flex fuels and which is excellent in
appearance.
[0022] That is, by making the contents of Si and Mg
and Ca in the Al-Mg-Si-Ca-based hot dip coating layer
specific ranges by further by adding a suitable amount of
Ti so as to make Mg2Si particles with a long axis of 10 p
or less and close to spherical shapes finely disperse in
the plating layer, plated steel sheet which is excellent
in both corrosion resistance and appearance with respect
20 to flex fuels is obtained. Steel sheet which has this
plating layer can be produced by an existing process, so
is steel sheet with good flexibility in production.
Furthermore, it is equal to ordinary steel sheet in
workability, so fuel tanks can be produced while making
25 use of conventional processes.
[0023] The present invention was made based on these
discoveries and has as its gist the following:
[0024] (1) Aluminum plated steel sheet which is
excellent in corrosion resistance and appearance which is
30 comprised of steel sheet on at least one surface of which
there is a plating layer comprised of, by mass%,
Si: 2% to 11%,
Mg: 3% to 9%,
Ca: 0.1% to 5%,
35 Ti: 0.005% to 0.05%, and
a balance of A1 and unavoidable impurities,
the plating layer containing Mg2Si particles with a long
axis of 10 pm or less and a ratio of a long axis and a
short axis, that is, an aspect ratio, of 1 to 3.
(2) Aluminum plated steel sheet which is excellent in
corrosion resistance and appearance as set forth in (1)
characterized by having an Al-Si-Fe-based alloy layer of
a thickness of 5 pm or less at the interface of the
plating layer and the steel sheet.
(3) Aluminum plated steel sheet which is excellent in
corrosion resistance and appearance as set forth in (1)
or (2) characterized in that the plating layer has a
thickness of 9 to 35 pm.
(4) A fuel tank which is excellent in corrosion
resistance and appearance characterized by using aluminum
plated steel sheet which has a plating layer comprised
of, by mass%,
Si: 2% to 11%,
Mg: 3% to 9%,
Ca: 0.1% to 5%,
Ti: 0.005% to 0.05%, and
a balance of A1 and unavoidable impurities,
the plating layer containing Mg2Si particles with a long
axis of 10 pm or less and a ratio of a long axis and a
short axis, that is, an aspect ratio, of 1 to 3.
(5) A method of production of aluminum plated steel sheet
which is excellent in corrosion resistance and appearance
characterized by dipping and running steel sheet in a hot
dip aluminum coating bath comprised of, by mass%,
Si: 2% to 11%,
Mg: 3% to 9%,
Ca: 0.1% to 5%,
Ti: 0.005% to 0.05%, and
a balance of A1 and unavoidable impurities and
having a temperature of 670°C or more,
cooling in a first stage cooling process up to a
temperature range of 600 to 640°C by a cooling rate of
25OC/sec or more, then further cooling in a second stage
cooling process up to a temperature range of 550 to 590°C
by a cooling rate of lG°C/sec or more.
Advantageous Effects of Invention
[0025] According to the present invention, it is
5 possible to provide plate steel sheet which can handle
methanol or ethanol or other alcohol alone or gasoline
containing these alcohols or other flex fuel. The plated
steel sheet according to the present invention has a
corrosion resistance which can handle various fuels and
is excellent in appearance. Further, the press
formability, weldability, and coatability are no
different from conventional aluminum plated steel sheet.
There is no need to change the fuel tank production
process. That is, the flexibility of production is high
15 and existing processes can be utilized for production.
Brief Description of Drawings
[0026] FIG. 1 is a view which shows the relationship
of the long axis of Mg2Si which is contained in the
plating layer of the present invention and the corrosion
20 resistance with respect to alcohol-containing gasoline.
FIG. 2 is a view which shows the relationship of the
aspect ratio of Mg2Si which is contained in the plating
layer of the present invention and the corrosion
resistance with respect to alcohol-containing gasoline.
Description of Embodiments
[0027] Below, embodiments of the present invention
will be explained. Note that, in the present invention,
the steel sheet which forms the base material is not
particularly limited. This is because the corrosion
30 resistance and appearance are determined by the aluminum
plating layer on the surface of the steel sheet. First,
the reasons for numerical limitation of the ratios of the
hot dip coating ingredients of the present invention will
be explained. Here, the % in the ingredients mean mass%.
35 [0028] Si: 2 to 11%,
In the present invention, it is important to add Mg and
Si in combination and make MgaSi sufficiently precipitate
in the plating layer. Si is an ingredient which is
required for reacting with Mg to cause precipitation of
Mg2Si. To make Mg2Si precipitate enough to contribute to
the improvement of the corrosion resistance with respect
5 to flex fuels, the content of Si has to be 2% or more.
From the viewpoint of obtaining a sufficient amount of
precipitation of Mg2Si, the lower limit of Si is
preferably 48, more preferably 6%.
[0029] On the other hand, to include Si in over 118,
10 the plating bath has to be raised in melting point. This
is disadvantageous economically. If the Si further
increases, the workability is degraded, so 11% was made
the upper limit. From the viewpoint of the operating
stability of the plating bath (prevention of shortening
15 of life of equipment inside high temperature bath), the
upper limit of Si is preferably lo%, more preferably 8%.
[ 0 0 3 0 1 Mg: 3 to 9%
Mg is an ingredient which is necessary for reacting with
Si to cause precipitation of Mg2Si in the plating layer.
20 To make Mg2Si sufficiently precipitate to contribute to
improvement of the corrosion resistance, the content of
Mg has to be 3% or more. From the viewpoint of obtaining
a sufficient amount of precipitation of Mg2Si, the lower
limit of Mg is preferably made 4%, more preferably 5%.
25 [0031] On the other hand, to include Mg in more than
9%, the melting point of the plating bath has to be
raised. This is disadvantageous economically.
Furthermore, if Mg increases, excessive Mg causes serious
wrinkling due to the oxide film and degrades the plating
30 appearance, so the upper limit was made 9%. From the
viewpoint of making the appearance of the plating layer
more beautiful, the upper limit of Mg is more preferably
made 7%.
[0032] Ca: 0.1 to 5%,
35 Ca is stronger in oxidizing power than even Mg, so is
added to suppress the formation of an Mg oxide film and
improve the plating appearance. The content of Ca to
obtain this effect depends on the content of Mg. If in
the above range of content of Mg, 0.1% or more is
necessary. From the viewpoint of appearance, the lower
limit of Ca is preferably 0.12%, more preferably 0.15%.
[ 0 0 3 3 ] On the other hand, if adding Ca in over 5%, the
plating bath becomes higher in melting point and the
plating rises in viscosity - which becomes
disadvantageous economically, so the upper limit was made
5%. From the viewpoint of the stability of the operation,
the upper limit of the concentration of Ca is preferably
made 4.8%, more preferably 4.2%.
[ 0 0 3 4 ] Ti: 0.005 to 0.05%
Ti causes a peritectic reaction of L (liquid phase ("L"
means liquid phase metal) ) +TiA13-+ (Al) at 665OC. TiA13
forms the nuclei for formation of Mg2Si in the process of
solidification of the plating layer, so if making the
TiA13 finely disperse, Mg2Si also finely disperses.
Further, since TiA13 becomes nuclei for formation of
Mg2Si, the Mg2Si which easily grows into a plate shape
easily becomes particles. To make the TiA13 finely
disperse in a plating bath of 665°C or more, the Ti
content has to be 0.005% or more. From the viewpoint of
securing fine dispersion of Mg2Si, the lower limit is
preferably made 0.008%, more preferably 0.01%.
[0035] On the other hand, if adding Ti in 0.05% or
more, the liquidus temperature of the Al-Si-Mg-Ca alloy
is raised, so this is not preferable. Further, the
reduction of dross by oxidation of Ti and other aspects
of operational stability are also impaired. If
considering the stability in operation, the upper limit
of the Ti content is preferably 0.02%, more preferably
0.018%.
[0036] [Si mass%] 20.6~[M g mass%]
To prevent Mg from remaining in the A1 plating layer
alone and make it Mg2Si to suppress an alcoholate
reaction, it is preferable to make [Si mass%]>0.6x[Mg
mass%]. Mg2Si suppresses the first stage reaction in the
alcoholate reaction which is shown by the following
formulas. Due to this, dissolution of the main ingredient
A1 in the plating is inhibited and the role of
5 sacrificial corrosion prevention is performed.
2A1+6 (R-OH) +2 (A1 (OR) 3 ) +3H2
2 (A1 (OR) 3) +6H2+2Al (OH) 3+6 (R-H)
[0037] Further, if [Si mass%] becomes less than
0.6[~M g mass%], Mg remains, so there is no effect of
10 addition of Ca and the plating appearance is degraded.
[0038] 0 . 6[~M g mass%] 2 [Ca mass%]>O. 03x [Mg mass%]
Ca is an element which suppresses the oxidation of the
remaining Mg. Ca suppresses the formation of MgO, which
causes wrinkles, and improves the plating appearance. If
15 [Ca mass%] becomes less than 0.03~[M g mass%], the effect
of suppression is not sufficiently obtained. This is
because the remaining Mg oxidizes and forms an oxide film
which becomes the cause of wrinkling. On the other hand,
if [Ca mass%] exceeds 0 . 6[~M g mass%], the plating melting
20 point becomes higher, which is not economical, and the
effect of improvement of the plating appearance becomes
saturated. Therefore, the range of content of Ca is
preferably made the range of the following formula:
O.Gx[Mg mass%]>[Ca mass%]20.03x[Mg mass%]
25 [0039] Long Axis of Mg2Si of 10 pm or Less
An Mg2Si intermetallic compound is hard and brittle. For
this reason, if there is an intermetallic compound with a
large long axis present, the plating layer itself
remarkably falls in workability. For this reason, cracks
30 occur in the press forming of steel sheet and corrosion
easily occurs at the cracked parts. The inventors ran a
draw bead test (sheet thickness reduction rate 20%) on
plated steel sheet different in the long axis of Mg2Si
while envisioning press forming of a fuel tank and
35 discovered that if the long axis of the Mg2Si exceeds 10
pm, cracks easily occur. Therefore, the long axis of the
Mg2Si which causes precipitation dispersed in the plating
layer was made 10 pm or less.
[0040] If making the long axis of Mg2Si even finer, a
larger amount of the Mg2Si finely disperses, the plating
layer becomes more uniform in structure, and the
corrosion resistance is further improved. Further, even
if the plating layer corrodes, the corrosion products do
not become coarser and clogging of the filter of the fuel
circulation system is remarkably reduced. For this
reason, the upper limit of the long axis of Mg2Si is
preferably made 8 p or less, more preferably 6 pm or
less, if possible 5 pm or less. FIG. 1 shows the
relationship between the long axis of Mg2Si and the
indicator of the corrosion resistance constituted by the
15 amount of corrosion products. It is learned that if the
long axis of Mg2Si becomes larger, the corrosion products
increase (that is, the corrosion resistance
deteriorates).
[ 0 0 4 1 ] On the other hand, the lower limit of Mg2Si is
20 not particularly limited. However, if too small, the
specific surface area becomes larger, the dissolution
speed of Mg2Si becomes larger, and the lifetime becomes
shorter. Further, to make the Mg2Si size smaller, the
cooling rate after plating has to be remarkably raised.
25 This is also disadvantageous economically. From this, as
the lower limit of the long axis of Mg2Si, it is
preferable to secure 1 pm or more. The lower limit of the
long axis of Mg2Si is preferably 1.5 p, if possible 2 pm
or more.
30 [0042] Aspect Ratio of Mg2Si: 1 to 3
As explained above, Mg2Si is hard and brittle, so easily
forms starting points of crack propagation at the time of
press forming. In particular, if there are pin-shaped
particles two-dimensionally (planarly) with a long axis
35 longer than the short axis, they induce cracks in the
plating layer and cause deterioration of the corrosion
resistance. For this reason, a smaller aspect ratio
(ratio of long axis and short axis of Mg2Si particles
(long axis/short axis)) is preferable. Ideally, the
particles are spherical in shape and the aspect ratio is
1. This is the lower limit. To prevent any detrimental
effect from being recognized in the corrosion resistance,
from the experiments of the inventors, 3 is made the
upper limit of the aspect ratio. Of course, the smaller
the aspect ratio, the better. Making it 2.4 or less is
preferable.
[ 0 0 4 3 ] Ordinary Mg2Si intermetallic compounds are
elliptically shaped plates (pieces) under ordinary
solidification conditions (cooling by standing). To make
the Mg2Si intermetallic compounds ones with small aspect
ratios, that is, as close to spherical shapes as
possible, it is necessary to control the cooling
conditions. For this reason, the inventors discovered
that to prevent the Mg2Si particles from becoming coarser,
it is possible to rapidly cool the sheet to a certain
extent so as to cool it while the particles are fine and
make the aspect ratio small. The cooling conditions will
be explained later.
100441 The long axis and aspect ratio of the Mg2Si
particles can be found by examining the cross-section of
the plating layer by an EPMA. The EPMA is used to examine
the Al-Mg-Si-Ca-based plating layer, the clumps of
precipitates including Mg and Si in the plating layer are
judged to be MgaSi, and the precipitates are measured for
their long axis and short axis. The aspect ratio is found
by dividing the long axis by the short axis.
[0045] Next, the method of making MgaSi particles with
a long axis of 10 pm or less and an aspect ratio of 1 to
3 disperse and precipitate in the plating layer in large
numbers will be explained. To make Mg2Si with a long axis
35 of 10 pm or less and an aspect ratio of 1 to 3 disperse
in the plating layer in large numbers, as explained in
the section on Ti, it is important to make TiA13 finely
disperse in the liquid phase in the process of
solidification of the plating.
The reason is that TiA13 forms nuclei for formation of
Mg2Si and, as a result, Mg2Si can be made to finely
5 disperse uniformly in the plating layer. For this reason,
the method of production of the plated steel sheet
according to the present invention is characterized by
two stages of cooling for precipitation of TiA13 after hot
dip coating and precipitation of Mg2A1 after that.
100461 First, the hot dip aluminum coating process
will be explained. The temperature of the plating bath,
which has the same composition of ingredients as the
plating layer, should be 670°C or more. Therefore, the
steel sheet is dipped and passed through the bath in the
same way as an ordinary hot dip aluminum coating process
to plate aluminum.
[0047] The plating bath temperature is more preferably
given a lower limit of 680°C from the viewpoint of
securing a margin from the peritectic reaction
20 temperature (665°C). Furthermore, 685°C is more
preferable. Further, if the plating bath temperature
becomes too high, the sheet will be rapidly cooled before
the TiA13 precipitates after plating and the lifetime of
the various equipment inside the bath will become
25 shorter, so the upper limit should be made 700°C or so.
Making it 695°C is more preferable. If possible, making it
690°C is still more preferable. The temperature of the
steel sheet at the time of entering the plating bath
should be made slightly higher than the plating bath
30 temperature (plating bath temperature to plating bath
temperature+30°C or so). More preferably, it is desirably
made 5 to 15°C higher than the plating bath temperature.
[0048] Next, the aluminum plated steel sheet is cooled
in the first stage of the cooling process. The first
35 stage of the cooling process cools the hot dip coated
steel sheet all at once down to the peritectic reaction
temperature of L (liquid phase) +TiA13+ (Al) (665OC) to
620°C by a 25°C /sec or faster cooling rate. At this time,
the plating layer has TiA13 remaining in it due to the
overcooling. The target temperature of the first stage of
cooling is 620°Cf but in the actual process, it is
sufficient to cool down to 600 to 640°C in range of
temperature.
[0049] After that, as the second stage of cooling, the
plated steel sheet is cooled down to 570°C by a cooling
rate of lG°C/sec or more to make Mg2Si precipitate and
grow. At this time, the larger the cooling rate, the
smaller the aspect ratio of the Mg2Si particles tends to
become. The target temperature of the second stage of
cooling is 570°C, but in the actual process, it is
sufficient to cool down to a temperature of 550 to 590°C
in range.
[0050] Note that, the hot dip aluminum coating
process, first stage cooling process, and second stage
cooling process are preferably consecutive. This is to
maintain the cooling rate in the process of
solidification of the plating layer in the above range.
[0051] The first stage of cooling, that is, the
cooling when passing the peritectic reaction temperature
(665OC ) , is necessary in order for the TiA13 to finely
disperse in the plating layer. At this time, the greater
the cooling rate, the more the TiA13 is finely dispersed,
so the Mg2Si is also finely dispersed and precipitates.
That is, it is important that the cooling rate from the
temperature of the hot dip coating metal (670°C or more)
to 620°C be 25"C/sec or more.
[0052] This is because if the cooling rate during this
time is 19"C/sec or so, the aspect ratio of the Mg2Si
particles sometimes becomes 3 or less, but if made
25"C/sec or more, the aspect ratio of the Mg2Si particles
reliably becomes 3 or less.
[0053] The upper limit of the cooling rate need not be
particularly set, but if the cooling rate becomes too
large, for example, in the case of a jet cooler, the
increase in the flow rate may cause patterns. In the case
of mist cooling as well, similar patterns may form.
Further, while inefficient economically as well, 35"C/sec
or 40°C/sec or so may also be made the upper limit.
[0054] The second stage cooling, that is, the cooling
from when the plating metal is in the overcooling state
(620°C) to complete solidification (570°C), is necessary
for the Mg2Si to precipitate and grow. At this time, if
the cooling rate is too slow, the Mg2Si grows, the
particle size becomes larger, and the aspect ratio
becomes greater. For this reason, it was learned that the
15 cooling rate during this should be made 16OC/sec or more.
Preferably, it is made 18"C/sec or more, if possible, it
is made 20°C/sec or more.
[0055] On the other hand, if the cooling rate is too
fast, the particles become finer, the aspect ratio
20 becomes smaller, and the Mg2Si sometimes does not
sufficiently precipitate. However, the Mg2Si starts to
precipitate after precipitation of TiA13, so no upper
limit need particularly be provided. If forced to say
something, the first stage cooling rate should be made
the upper limit.
[0056] Further, the plated steel sheet according to
the present invention is produced by running steel sheet
in the hot dip coating bath in a similar ray to normal
hot dip coated steel sheet, so inevitably, an alloy layer
comprised of intermetallic compounds is formed between
the plating layer and base iron. At this time, the
composition of the alloy layer becomes Al-Si-Fe. If the
amounts of addition of Mg and Ca increase, sometimes
formation of intermetallic compounds in which these
elements are included is observed.
[0057] The thickness of the alloy layer is preferably
5 pm or less. The reason is that the alloy layer is hard
and brittle, so if thick, the workability of the steel
sheet is greatly impaired. Further, if the alloy layer is
thick, cracks first occur in the alloy layer at the press
forming etc. These are propagated through the plating
layer. As a result, there is a possibility of causing the
corrosion resistance to deteriorate.
[0058] If the time of dipping in the plating bath is
long, a thick alloy layer is formed. Therefore, it is
necessary to adjust the time of dipping in the plating
bath. Further, it is known that by adding Mg in the
plating bath, the effect of reduction of the thickness of
the alloy layer is also obtained.
[0059] Note that, the Fe from the steel sheet enters
the plating bath, so Fe unavoidably enters the plating
layer. Further, the plating layer sometimes also includes
Cu, Ni, Mn, etc. However, even if Fe, Cu, Ni, and Mn are
included in a total of 2 mass% or less, the
characteristics of the steel sheet according to the
present invention are not affected.
[0060] In hot dip coating, the wiping for controlling
the amount of plating deposition is most generally gas
wiping. The type of the wiping gas is usually N2, air,
etc., but in addition, C02, combustion gas, etc. may be
used. The concentration of oxygen on the bath surface may
also be controlled.
[0061] If the amount of plating deposition increases,
in general the corrosion resistance is improved.
Conversely, the workability and weldability fall. The
present invention enables this effect to be obtained even
with a little amount of deposition since the plating
composition is excellent in corrosion resistance. The
amount of plating deposition (total amount of deposition
of plating layer and alloy layer of intermetallic
compound) preferably is a thickness of 9 to 35 p (25 to
100 g/m2 per side). If the amount of plating deposition is
less than 9 pm per side (25 g/m2), the corrosion
resistance becomes insufficient. If the amount of plating
deposition exceeds 35 p (100 g/rn2), the corrosion
resistance becomes saturated and further the press
formability, plating adhesion, and resistance weldability
become poor. From the viewpoint of the corrosion
resistance, the lower limit of the amount of deposition
is more preferably 10.5 pm (30 g/m2), still more
preferably 12.3 pm (35 g/m2) .
10 [0062] The composition of the plated sheet which is
used in the present invention is not particularly
limited. For applications where a high degree of
workability is demanded, use of IF steel excellent in
workability is desired. Further, to secure air-tightness
15 after the welding and secondary workability, steel sheet
to which several ppm of B is added is preferable.
Conversely, for applications where strength is demanded,
low carbon steel, high tension steel, etc. naturally may
be used.
20 [0063] In the present invention, the post-treatment of
the plated steel sheet is not particularly limited. Of
course, the sheet may also have a post-treatment film
aimed at improvement of the workability. For example,
chromate-free chemical conversion using Zr, V, cr3+, etc.
and resin coating are also possible. For the chemical
conversion, phosphoric acid, silica, etc. may be used.
100641 Further, an Mg-based compound may be added. As
the type of resin, for example, there are an acrylic acid
or methacrylic acid ester, vinyl carboxylic acid ester,
30 vinylether, styrene, acrylamide, acrylonitrile,
halogenated vinyl, or other ethylenic unsaturated
compound and epoxy, urethane, polyester, etc.
[0065] Furthermore, in addition to chemical conversion
and resin coating, treatment for making the appearance
35 uniform after hot dip coating such as zero spangle
treatment, treatment for modifying the plating such as
annealing, temper rolling for adjusting the surface
conditions and properties, etc. may be applied.
[0066] To make the steel sheet and plating bath
uniformly react and obtain a stable appearance,
pretreatment such as preplating, washing, etc. may be
applied before hot dip coating. For example, preplating
by Ni, Co, Sn, and Zn may be considered. Next, a fuel
tank which uses the plated steel sheet according to the
present invention will be explained.
[0067] The fuel tank of the present invention is not
particularly limited in application. It may be used in
automobiles, motorcycles, buses, trucks, etc.
Furthermore, the methcd of working the plated steel sheet
according to the present invention is also not
particularly limited. This is because the sheet has
workability equivalent to conventional plated steel
sheet. As a result, the fuel tank according to the
present invention is also not particularly limited in
shape.
[0068] Summarizing the above, the fuel tank according
to the present invention can be produced by an existing
design technique while using an existing production
process. However, only naturally, in the case of plated
steel sheet which is plated on only one surface, the
plated surface has to be made the surface which contacts
the flex fuels.
[0069] The plated steel sheet according to the present
invention itself has excellent corrosion resistance to
flex fuels, so fuel tanks which use the same also have
excellent corrosion resistance.
Examples
[0070] Below, examples of the present invention will
be explained. Cold rolled steel sheet (sheet thickness
0.8 mrn) was used as the base material for hot dip
aluminum coating. The composition of the plating bath was
based on A1 and was changed in the amounts of Si, Mg, Ca,
and Ti in various ways. In addition to these, as an
impurity element, the Fe which enters from the steel
sheet etc. was contained in an amount of 1 to 2% or so
(mass%).
[0071] The sheet temperature of the steel sheet when
5 entering the plating bath and the bath temperature were
both made 670°C. The time of dipping in the bath was
changed to control the thickness of the alloy layer. The
amount of plating deposition was adjusted after plating
by N2 gas wiping. The cooling was controlled from 670°C to
10 620°C (first stage cooling) and from 620°C to 570°C
(second stage cooling). The thus produced samples were
checked for plating composition, long axis (pm) and
aspect ratio of Mg2Si, thickness of alloy layer, and
plating thickness.
15 [0072] The plating layer was evaluated by peeling off
only the plating layer and evaluating it in accordance
with the test methods of properties of plating layers
which are described with reference to JIS H 8672. That
is, the aluminum coating layer was electrolytically
20 peeled off in a 241 g/l solution of A1Cl3-6H20. The peeled
off area at this time was 25 cm2, and the current density
was 20 m24/cm2. When reaching the potential which shows the
alloy layer, the current was immediately stopped, the
alloy layer was exposed, and the solution of the
25 dissolved plating layer was quantitatively analyzed by
ICP (inductively coupled plasma). The thickness of the
alloy layer was measured by a 400X cross-section
micrograph.
[0073] Further, the hot dip coated steel sheet was
30 evaluated for appearance, corrosion resistance, and
overall condition by the methods explained below. The
results are shown in Tables 1 to 4. (Note that, Table 1-1
and Table 1-2 will together be called Table 1.)
[0074] Evaluation of Appearance
35 E (Excellent): Visibly beautiful with no wrinkles
F (Fair): Visible slight wrinkles
P (Poor): Visible wrinkles
[0075] Corrosion Resistance
Using an Erichsen testing machine, diameter 50 mm, depth
40 mm, flat bottom cupped samples were used to seal in
the following three types (Degraded E10, Degraded E25,
and Degraded E100) of ethanol-mixed gasoline 40 ml at 45OC
for 1500 hours and examined for appearance.
[0076] A sample with corrosion products was washed by
ultrasonically waves while sealing in fuel. The corrosion
products deposited on the sample and the corrosion
products floating free in the fuel were filtered using a
0.8 pm mesh membrane filter.
[0077] A membrane filter on which corrosion products
were trapped was used to further filter normal heptane
100 ml, then the membrane filter was dried at 80°C for 1
hour and the dry weight of the corrosion products was
measured.
[0078] Degraded El0 is a mixed solution of 99.5 vol%
of a mixed gasoline containing 10 vo1% of ethanol and 0.5
vol% of corrosive water (to which acetic acid 100 pprn and
chlorine 100 pprn are added)
Degraded E25 is a mixed solution of 98 vol% of a mixed
gasoline containing 25 vol% of ethanol and 2 vol% of
corrosive water (to which acetic acid 100 pprn and
chlorine 100 pprn are added)
Degraded El00 is a mixed solution of 93 vol% of ethanol
and 7 vol% of corrosive water (to which acetic acid 100
pprn and chlorine 100 pprn are added)
[0079] Evaluation of Corrosion Resistance
E (Excellent): No change in appearance and less than 12
mg/liter of corrosion products
G (Good): No change in appearance and less than 24
mg/liter of corrosion products
F (Fair): White rust present and 24 mg/liter or more of
corrosion products
P (Poor): Red rust and white rust confirmed
[ 0 0 8 0 1 Overall Evaluation
E (Excellent): No problem in appearance and corrosion
resistance
G (Good) : Usable level
F (Fair): Usable level, some problem in corrosion
resistance or appearance
P (Poor): Problem in appearance or corrosion resistance -
unusable level
[ 0 0 8 11 Table 1 shows the results of study of the
chemical composition of the plating layer. Comparative
Example No. 1 had a low amount of Mg outside of the
present invention, so red rust and white rust were
confirmed and the corrosion resistance was inferior.
[0082] Comparative Example No. 9 had a high amount of
Mg outside of the present invention, so the appearance
was inferior.
[ 0 0 8 3 1 Comparative Example No. 10 had a low amount of
Si outside of the present invention, so the amount of
formation of Mg2Si was insufficient and the corrosion
resistance was inferior. Conversely, Comparative Example
No. 20 had a high amount of Si outside of the present
invention, so the plating bath was too high in melting
point and the plating bath could not be prepared.
[ 0 0 8 4 ] Comparative Example No. 21 had a low amount of
Ca outside of the present invention, so wrinkles occurred
and the appearance was inferior. As the amount of Ca
became greater as in Nos. 22 and 23, wrinkling was
suppressed. With the amount of Ca of No. 24, a beautiful
appearance was obtained. However, if the amount of Ca
became further greater as in No. 31 to No. 34, the
plating bath became viscous and operation became
difficult. Comparative Example No. 35 had a high amount
of Ca outside of the present invention, so the makeup of
the bath became difficult and a sample which could be
evaluated could not be prepared.
[0085] No. 36 to No. 46 changed the amount of Ti. The
tendency was recognized that as the amount of Ti
increases, the Mg2Si becomes smaller in long axis and the
Mg2Si becomes smaller in aspect ratio (that is, becomes
close to spherical).
[0086] Comparative Example No. 36 had no Ti added,
5 while Comparative Example No. 37 had an insufficient
amount of Ti. The corrosion resistance was inferior. On
the other hand, when the amount of Ti is over 0.02%, the
liquidus temperature of the Al-Si-Mg-Ca alloy is raised,
so this is not preferable. Further, the reduction of
10 dross by oxidation of Ti and other aspects of operational
stability are also impaired. Comparative Example No. 46
had too great a Ti. The makeup of the bath became
difficult and a sample which could be evaluated could not
be prepared.
15 [ 0 0 8 7 ] As opposed to this, the invention examples were
all excellent in appearance and corrosion resistance. The
overall evaluations were also excellent.
[0088] Next, the inventors took the plating
composition of Invention Example No. 5 and changed the
20 first and second cooling rates after hot dip coating. The
results are shown in Table 2. In No. 47 to No. 53, the
first stage cooling rate was changed. If the first stage
cooling rate increases, a tendency is observed of the
Mg2Si becoming smaller in aspect ratio (that is, becoming
25 closer to a sphere).
[0089] In Comparative Example No. 47 and Comparative
Example No. 48, the first stage cooling rate was
insufficient, the rate was insufficient for fine
dispersion and precipitation of Mg2Si, and the aspect
30 ratio could not be controlled to 3 or less. Note that
like in No. 53, if making the cooling rate over 30°C/sec,
a pattern is formed due to the increase in flow rate of
the jet cooler. This is also inefficient economically.
[0090] In No. 54 to No. 62, the second stage cooling
35 rate was changed. If the second stage cooling rate
increases, a tendency is observed of the Mg2Si becoming
smaller in long axis.
[0091] In Comparative Example No. 54, the second stage
cooling rate is insufficient, so the Mg2Si becomes larger
in long axis and the corrosion resistance becomes
insufficient. On the other hand, if making the cooling
5 rate excessive such as in No. 61 and No. 62, the Mg2Si
becomes smaller in long axis and the amount of
precipitation becomes insufficient, so a sufficient
corrosion resistance becomes hard to obtain.
[0092] Next, the test results on the effects of the
10 thickness of the alloy layer are shown in Table 3. The
inventors took the plating composition of Invention
Example No. 5 and changed the time for dipping in the hot
dip coating bath and the thickness of the alloy layer for
evaluation. The results are shown in Table 3.
[0093] Invention Example No. 62 to No. 66 were
obtained by a 670°Cx2 second bath dipping time and gave a
2 to 5 pm alloy layer thickness. The appearance and
corrosion resistance were excellent and the overall
evaluation was excellent.
[0094] Invention Example No. 67 was obtained by a
620°Cx4 second bath dipping time and gave a 6.0 pm alloy
layer thickness. It had visible slight wrinkles in
appearance, white rust in the corrosion resistance test
with Degraded E100, corrosion products of 24 mg/l or
25 more, and somewhat poor El00 corrosion resistance, but
was evaluated overall as being usable.
[0095] No. 68 was obtained by a 620°Cx6 second bath
dipping time and gave a 7.0 pm alloy layer thickness. It
had visible slight wrinkles in appearance, white rust in
the corrosion resistance test with Degraded E25 and
Degraded E100, corrosion products of 24 mg/l or more, and
poor El00 corrosion resistance.
[0096] It is believed that as the alloy layer
thickness becomes greater, the uneven thickness of the
alloy layer caused fine wrinkles in the surfacemost
plating layer. Further, a thick alloy layer is poor in
workability. Cracks occurred which propagated to the
plating layer in the process of cupping.
[0097] From the above test results, it was confirmed
that when the thickness of the alloy layer is over 5 pm,
5 use is possible, but the appearance becomes poor and the
workability falls.
[0098] Next, the effects of the plating thickness will
be evaluated. The inventors took the plating composition
of Invention Example No. 5 and changed the wiping after
10 hot dip coating to run a test on the effects of the
plating thickness. The results are shown in Table 4.
[0099] As shown in Invention Example Nos. 69 to 71 of
Table 4, when the plating thickness is less than 12.3 pm
(amount of plating deposition per side of 35 g/m2), the
corrosion resistance becomes poorer. Conversely, as shown
in Comparative Example No. 82 to No. 84, when plating
thickness is more than 35 pm (amount of plating
deposition per side of 100 g/m2), not only is the result
uneconomical, but also the separately evaluated press
20 formability, plating adhesion (plating internal
breakage), and resistance weldability fall.
[ 0 10 0 ] From the above test results, the plating
thickness does not have much an effect on the corrosion
resistance or appearance. If in the above-mentioned
range, it was confirmed that hot dip coated steel sheet
which has excellent appearance and corrosion resistance
can be obtained.
[ 0 10 1 ] Above, the present invention will be explained
by examples. Note that, the embodiments of the present
30 invention are not limited to these examples.
[0102] Table 1-1
composition Corrosion resistance
[0103] Table 1-2
N o
Overall
evaluation
Remarks
44
45
46
Plating
composition
mass%
Mg
6
6
6
lSt
cooling
"C/s
M ~ ~ s ~
aspect
ratio
Si
8
8
8
2nd
cooling
OC/s
Alloy
layer
thickness
P
Mg, S i
long
axis
Ca
0.2
0.2
0.2
Ti
Plating
thickness
P
0.04
0.048
0.052
Appearance
28
28
-
Corrosion resistance
2 6
2 6
-
Degraded
El0
4
4
-
Degraded
E25
Degraded
El00
1.1
1.1
-
3
3
-
15
15
-
E
E
-
E
E
-
E
E
-
E
E
- -
Operation
difficult
Operation
difficult
Comp. ex., bath
makeup difficult
[0104] Table 2
Corrosion resistance

[0106] Table 4
No
Overall
evaluation
Remarks
80 6 8 0.2 0.015 28 2 6 6 1.2 3 32 E E E E E
Plating
composition
mass%
81
82
83
84
lSt
cooling
OC/s
Mg
6
6
6
6
2nd
cooling
OC/s
Si
8
8
8
8
Mg2 S i
long
axis
Ca Ti w
0.2
0.2
0.2
0.2
aspect
ratio
0.015
0.015
0.015
0.015
Alloy
layer
thickness
Pm
28
28
28
28
Plating
thickness
w
2 6
2 6
2 6
2 6
Appearance
6
6
6
6
Corrosion resistance
Degraded
El0
1.2
1.2
1.2
-
1.2
Degraded
E25
3
3
3
-
3
Degraded
El00
3 5
3 7
4 0
--
4 5
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
Press
formability,
plating adhesion,
and resistance
weldability fall
Press
formability,
plating adhesion,
and resistance
weldability fall
Press
formability,
plating adhesion,
and resistance
weldability fall
Industrial Applicability
[01071 The plated steel sheet according to the present
invention can not only be utilized in automobiles,
motorcycles, buses, trucks, etc., in particular as fuel
5 tanks of flex fuels, but can also be utilized as members
in which resistance to corrosion by flex fuels etc. is
demanded.

- 32 -
CLAIMS
Claim 1. Aluminum plated steel sheet which is excellent
in corrosion resistance and appearance which is comprised
of steel sheet on at least one surface of which there is
a plating layer comprised of, by mass%,
Si: 2% to 11%,
Mg: 3% to 9%,
Ca: 0.1% to 5%,
Ti: 0.005% to 0.05%, and
a balance of A1 and unavoidable impurities,
said plating layer containing Mg2Si particles with a long
axis of 10 pm or less and a ratio of a long axis and a
short axis, that is, an aspect ratio, of 1 to 3.
Claim 2. Aluminum plated steel sheet which is excellent
in corrosion resistance and appearance as set forth in
claim 1 characterized by having an Al-Si-Fe-based alloy
layer of a thickness of 5 pm or less at the interface of
said plating layer and said steel sheet.
Claim 3. Aluminum plated steel sheet which is excellent
20 in corrosion resistance and appearance as set forth in
claim 1 or 2 characterized in that said plating layer has
a thickness of 9 to 35 pm.
Claim 4. A fuel tank which is excellent in corrosion
resistance and appearance characterized by using aluminum
25 plated steel sheet which has a plating layer comprised
of, by mass%,
Si: 2% to 11%,
Mg: 3% to 9%,
Ca: 0.1% to 5%,
30 Ti: 0.005% to 0.05%, and
a balance of A1 and unavoidable impurities,
said plating layer containing Mg2Si particles with a long
axis of 10 pm or less and a ratio of a long axis and a
short axis, that is, an aspect ratio, of 1 to 3.
35 Claim 5. A method of production of aluminum plated steel
sheet which is excellent in corrosion resistance and
appearance characterized by dipping and running steel
sheet in a hot dip aluminum coating bath comprised of, by
mass%,
Si: 2% to 11%,
Mg: 3% to 9%,
Ca: 0.1% to 5%,
Ti: 0.005% to 0.05%, and
a balance of A1 and unavoidable impurities and
having a temperature of 670°C or more,
1-0 cooling in a first stage cooling process up to a
-
temperature range of 600 to 640°C by a cooling rate of
25OC/sec or more, then 'further cooling in a second stage
cooling process up to a temperature range of 550 to 590°C
by a cooling rate of lG°C/sec or more.
Dated this 241 12/20 1 3
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANTS