1
DESCRIPTION
MANUFACTURING EQUIPMENT FOR GALVANNEALED STEEL SHEET, AND
MANUFACTURING METHOD OF GALVANNEALED STEEL SHEET
5
Technical Field
[0001]
The present invention relates to manufacturing equipment for a galvannealed
10 steel sheet and a manufacturing method ofthe galvannea1ed steel sheet. In particular, it
relates to the equipment and the method for the galvan:ilealed steel sheet to make dross,
which forms when the galvannealed steel sheet is manufactured, harmless.
Priority is claimed on Japanese Patent Application No. 2010-196797, filed
September 2,2010, the content of which is incorporated herein by reference.
15
Background Art
[0002]
Hot dip zinc-aluminum coated steel sheets have been widely used in the fields of
automobiles, consumer electronics, building materials and the like. A representative
20 category ofthe coated steel sheets includes the following three types in order of
aluminum (AI) content in coating bath,
(1) Galvannealed steel sheets (composition of coating bath: for example, 0.125
to 0.14 mass% AI - Zn)
(2) Galvanized steel sheets (composition of coating bath: for example, 0.15 to
25 0.25 mass% Al - Zn)
15
• 2
(3) Zinc-aluminum alloy coated steel sheets (composition of coating bath: for
example, 2 to 25 mass% AI - Zn)
[0003]
As described above, the hot dip zinc-aluminum coated steel sheets are steel
5 sheets which are coated by using the coating bath including molten metal such as molten
zinc and molten aluminum. In the coating bath, zinc (Zn) is the main ingredient,
aluminum (Al) is added in order to improve coating adhesion and corrosion resistance,
and substances such as magnesium (Mg), silicon (Si) and the like may be added in order
to improve the corrosion resistance.
10 Hereinafter, the galvannealed steel sheet is referred to as "GA" and the coating
bath for manufacturing the galvannealed steel sheet is referred to as "galvannealed bath
(GA bath)". The galvanized steel sheet is referred to as "GI" and the coating bath for
manufacturing the galvanized steel sheet is referred to as "galvanized bath (GI bath)".
[0004]
When the above-mentioned hot dip zinc-aluminum coated steel sheets are
manufactured, a large amount of inclusions called dross forms in the coating bath. The
dross is made ofintermetallic compounds ofIron (Fe) dissolved in the coating bath from
the steel sheet and Al or Zn included in the coating bath (molten metal). Specific
compositions of the intermetallic compounds are, for example, Fe2Als which represents
20 top-dross and FeZn7 which represents bottom-dross. The top-dross may form in all of
25
the coating bath (for example, GA bath, GI bath) for manufacturing the hot dip
zinc-aluminum coated steel sheets. On the other hand, the bottom-dross only forms in
the galvannealed bath (GA bath).
[0005]
Since the specific gravity ofthe top-dross is smaller than that ofthe molten
•
5
3
metal which is the coating bath, the top-dross flows in the coating bath, and fmally rises
to top surface ofthe coating bath. When a large amount ofthe top-dross flows in the
coating bath, the top-dross accumulates on the surface ofthe roll in the coating bath,
which may cause surface defects on the steel sheets. Also the flowing top-dross
accumulates in grooves ofthe roll in the coating bath, which may cause roll-slipping and
roll-idling because of the decrease in the apparent friction coefficient between the roll
and the steel sheet. In addition, when a relatively large size ofthe top-dross adheres to
the steel sheet, the quality of appearance of a product deteriorates and the product
becomes off-grade in some cases.
10 [0006]
On the other hand, since the specific gravity ofthe bottom-dross is greater than
that of the molten metal which is the coating bath, the bottom-dross flows in the coating
bath, and finally deposits on the bottom of the coating tub. When a large amount of the
bottom-dross flows in the coating bath, in the same way as the top-dross, the
15 bottom-dross causes problems such as defects in the roll in the coating bath, roll-slipping,
roll-idling, remarkable det~rioration ofthe quality ofthe appearance which results from
its adhesion to the steel sheet, and the like. Moreover, the bottom-dross does not rise to
the top surface and is not rendered harmless like the top-dross. The bottom-dross flows
in the coating bath for a long time, and the bottom-dross, which deposits on the bottom of
20 the coating tub once, reflows in the coating bath again by transition ofthe coating bath
flow. Therefore, it can be said that the bottom-dross is more harmful than the top-dross.
[0007]
In particular, when the sheet threading speed ofthe steel sheet dipped into the
coating bath is accelerated in order to improve productivity ofthe coated steel sheets, the
25 bottom-dross which deposits on the bottom of the coating tub rises in the coating bath
4
due to the coating bath flow which is derived from high-speed threading ofthe steel sheet.
The above-mentioned dross adheres to the steel sheet and causes the dross defects on the
steel sheets, which results in a factor of degradation ofthe coated steel sheet. Therefore,
hitherto, the sheet threading speed of the steel sheet was suppressed and the productivity
5 had to be sacrificed in order to ensure the quality ofthe coated steel sheets.
[0008]
To solve the above-mentioned problems caused by the top-dross and the
bottom-dross, many suggestions have been made in the past. As shown below, the
suggestions are commonly methods of sedimentation separation and flotation separation
10 ofthe dross by using the difference in specific gravity between the coating bath and the
dross.
[0009]
For example, in Patent Document 1, dross removal equipment is suggested, in
which molten zinc including the dross is transferred from a coating tub to a storage tub
15 and the dross is separated by sedimentation and flotation by using the difference in
specific gravity between the dross and the coating bath. In the equipment, the capacity
ofthe storage tub is 10 m3 or more, the transfer volume of the molten zinc is 2 m3
/ hour
or more, and a baffle plate is installed in the storage tub to divert the coating bath flow.
However, in Patent Document 1, the dross removal effect is overestimated because of
20 utilization of an equation which is applicable to the particle sedimentation in case of a
relatively slow coating bath flow. In addition, although the harmful size of dross is
defmed as 100 ~ or more in Patent Document 1, the dross defects which are recently
regarded as the problem include defects which are derived from dross with a size of
approximately 50 J.l.m. In fact, a countermeasure with a greater effect than that ofPatent
25 Document 1 is necessary. On the contrary, in a method described in Patent Document
5
1, in order to remove the dross with the size of approximately 50 J.Illl, the capacity ofthe
storage tub needs to be 42 m3 or more, which is not practical because the equipment must
be larger. Moreover, in order to minimize the equipment, since sedimentation velocity
of the bottom-dross is slow, the countermeasure other than Patent Document 1 is
5 necessary.
[0010]
In Patent Document 2, a coating equipment is suggested, in which enclosing
parts are installed in a coating tub and the rise ofthe bottom-dross is suppressed by
sedimenting and depositing the bottom-dross underneath the enclosing parts. However,
10 in a method described in Patent Document 2, the bath flow at an upper area in the coating
bath increases with an increase in coating rate, so that the bath flow at a lower area in the
coating bath also increases gradually. Thus, since the dross with small size does not
sediment and flows back to the upper area with the coating bath flow, the dross removal
efficiency is low. Moreover, in case of the coating tub with practical capacity (for
15 example, 200 ton), the dross with small size flows back between the upper area and the
lower area ofthe coating bath, grows with time passage, and finally sediments in the
lower area. However, at the time, a large amount of the bottom-dross which grows up
to size which is enable to sediment flows in the upper area and the lower area ofthe
coating bath, so that the effect as the countermeasure against the dross defects is low.
20 Moreover, although it is necessary to remove eventually the bottom-dross which
deposited at the lower area, dross cleanup operation is substantially impossible if the
enclosing parts exist. Since considerable time and effort are needed for dismantlement
ofthe enclosing parts, it can be said that technology described in Patent Document 2 is
not practical.
25 [0011]
• 6
In the equipment suggested in Patent Document 3, a coating container is divided
into a coating tub and a dross removal tub, and the molten metal in the coating tub is
transferred to the dross removal tub by using a pump. Moreover, the dross is separated
by the sedimentation in the dross removal tub and the purified bath flows back in the
5 coating tub through opening portion provided for the coating tub. However, since a
method described In Patent Document 3 is the method in which the dross is separated by
simply using the difference in specific gravity between the dross and the bath, separation
efficiency ofthe dross with small size is low and the dross flows back to the coating tub
with the coating bath flow. Moreover, in case ofthe dross removal tub with practical
10 capacity (for example, 200 ton), the dross with small size which is formed in the coating
tub circulates between the coating tub and the dross removal tub with the coating bath
flow, grows with time passage, and fmally sediments at the dross removal tub.
However, at the time, a large amount ofthe bottom-dross which grows up to size which
is enable to sediment flows in the coating tub and the dross removal tub, so that it can be
15 said that the effect oftechnology described in Patent Document 3 is low as the
countermeasure against the dross defects.
[0012]
In addition, in a coating equipment suggested in Patent Document 4, the coating
bath in a coating pot is transferred to a crystallization pipe, and is cooled and heated
20 repeatedly several times in the crystallization pipe. Thereby, the dross is grown and
removed, and the purified bath is reheated in a reheating tub and returned to the coating
pot. Moreover, in a coating method suggested in Patent Document 5, a sub pot is
additionally installed in a coating pot. The molten metal which includes the
bottom-dross is transferred from the coating pot to the sub pot, the bath in the sub pot is
25 held at higher temperature than that ofthe coating pot, and Al concentration is increased
7
0.14 mass% or more. Thereby, the bottom-dross in the coating bath is transformed into
the top-dross, and the top-dross is removed by the flotation separation.
Related Art Documents
5 Patent Documents
[0013]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. H10-140309
[Patent Document 2] Japanese Unexamined Patent Application, First
10 Publication No. 2003-193212
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. 2008-095207
[Patent Document 4] Japanese Unexamined Patent Application, First
Publication No. H05-295507
15
20
[Patent Document 5] Japanese Unexamined Patent Application, First
Publication No. H04-99258
Summary ofthe Invention
Technical Problem
[0014]
As mentioned above, the conventional dross removal methods described in
Patent Documents 1 to 3 are generally the method in which bath temperature control of
the coating bath is not conducted and the dross is separated by the sedimentation and the
flotation by simply using the difference in specific gravity between the dross and the
25 coating bath. However, in the removal methods, there was the problem such that the
8
dross with small size flowed back to the coating tub with the coating bath flow, the dross .
could not be removed completely, and the dross removal efficiency was low. Moreover,
the dross with small size in the coating bath circulates between the separation tub and the
coating tub with the coating bath flow, grows with time passage, and finally sediments at
5 the separation tub. However, at the time, a large amount of the dross which grows up to
size which is enable to sediment flows in the coating bath. Thus the effect as the
countermeasure against the dross defects ofthe coated steel sheets was low.
[0015]
On the other hand, in the method described in Patent Document 4, the molten
10 metal in the coating tub is transferred to the crystallization pipe, the coating bath is
cooled and heated repeatedly several times, and thereby, the dross is grown and removed.
However, in order to utilize the method described in Patent Document 4 effectively, as
described in Example in Patent Document 4, large flow of bath circulation such that
circulating volume of the coating bath is 0.5 m3
/ min (approximately 200 ton / hour) is
15 necessary. In order to conduct continuously the cooling and the heating for 2 hours for
the large flow of the coating bath as described in the Example, the crystallization pipe
with the capacity of 60 m3 (approximately 400 ton) and a cooling system and heating
system ofhigh power are necessary. Moreover, in Patent Document 4, a method of
removing the dross which is grown in the crystallization pipe is not disclosed. In case
20 that the dross is removed by using a filter, exchange operation thereof is substantially
impossible. And, in case that the dross is removed by the sedimentation separation, a
sedimentation tub is additionally needed, so that operation is substantially difficult even
if being theoretically possible. Therefore, it can be said that the method described in
Patent Document 4 is not practical.
25 [0016]
9
In addition, in the method described in Patent Document 5, the coating bath in
the sub pot is held at higher temperature than that ofthe coating pot, AI concentration is
increased, the bottom-dross in the coating bath is transformed into the top-dross, and
thereby the top-dross is removed by the flotation separation. However, as described in
5 Example in Patent Document 5, in the conditions such that bath temperature is heated to
500oe, 5500 e and AI concentration is increased to 0.15 mass% in the coating pot by
using the coating bath from the coating pot (bath temperature of 460oe, AI concentration
of 0.1 mass%), a part of the bottom-dross may be transformed into the top-dross and be
removed by the flotation separation. However, by the method, since solubility limit of
10 Fe of the coating bath increases drastically (saturated concentration of Fe in the coating
pot of 0.03 mass%, saturated concentration of Fe in the sub pot of 0.09 mass% or more),
most ofthe dross is dissolved in the coating bath. Namely, since the solubility limit of
Fe ofthe coating bath increases with an increase in the bath temperature ofthe coating
bath in the sub pot, most ofthe dross is dissolved in the coating bath, so that the dross
15 cannot be separated by the flotation in the sub pot. Thus, when the coating bath in the
sub pot is cooled and transferred to the coating pot, a large amount of the dross is formed,
which is caused by the difference in Fe solubility. As mentioned above, the method
described in Patent Document 5 is much doubtful about the dross removal effect in
actuality. Moreover, in the method described in Patent Document 5, after the dross
20 cleanup operation ofthe sub pot, the coating bath in the sub pot is cooled to the bath
temperature of the coating pot, and the coating bath is reused. Therefore, since the
dross cleanup operation ofthe sub pot must be batch processing, the dross removal
efficiency is inferior to the case that the dross cleanup processing is consecutively
conducted.
25 [0017]
10
As mentioned above, the methods ofremoving the dross which flows in the
coating bath are investigated for many years, most ofthe methods are the method which
uses the difference in specific gravity between the dross and the coating bath (refer to
Patent Documents 1 to 3). Among them, in case ofthe method ofthe sedimentation
5 separation of the bottom-dross, since the difference in specific gravity between the
bottom-dross and the molten zinc bath is small, sedimentation speed is slow. Thus it
was difficult to almost-completely render the dross harmless (dross-free) by the practical
capacity ofthe separating tub.
[0018]
10 On the other hand, the method of the flotation separation ofthe top-dross is
more advantageous than the method ofthe sedimentation separation of the bottom-dross.
However, under the general operational condition ofthe GA, since the dross may form in
the state ofthe bottom-dross only or a mixture ofthe bottom-dross and the top-dross, the
method oftransforming the bottom-dross into the top-dross is necessary. Some
15 technologies are disclosed as the methods (for example, refer to Patent Document 5).
[0019]
However, as described above, since the conventional dross removal methods
which were suggested until now are difficult to control Al concentration ofthe coating
bath and the technical idea thereofmay be technical unreasonableness, the methods are
20 not practicalized. In the conventional methods, the dross removal efficiency and effect
were insufficient, and the dross removal effect itselfwas much doubtful.
[0020]
The present invention is achieved in view ofthe above-mentioned problems.
An object of the present invention is to provide a manufacturing equipment for a
25 galvannealed steel sheet and a manufacturing method of a galvannealed steel sheet which
11
, are new and improved, in which the dross which forms inevitably in the coating bath
during the manufacture ofthe galvannealed steel sheet can be removed efficiently and
effectively and can be almost-completely rendered harmless.
5 Solution to Problem
[0021]
The inventors has investigated with singleness of purpose in view ofthe
above-mentioned circumstance, and fo~d the method which alsnost-completely renders
dross harmless (dross-free) by removing the dross efficiently and effecti~ely within the
10 system. The method, in which coating bath is circulated between the divided and
installed 3 tubs which are a coating tub, a separating tub, and an adjusting tub, utilizes
concurrently (l) a process of separating the dross by using the difference in specific
gravity by precipitating the formed dross in the coating bath as top-dross in the
separating tub where bath temperature thereof is lower than that ofthe coating tub and
15 (2) a process of dissolving and removing the top-dross which is not able to be separated
and removed in the separating tub by controlling Fe ofthe coating bath to be an
unsaturated state in the adjusting tub where bath temperature thereof is higher than that
of the separating tub.
[0022]
20 In order to accomplish the aforementioned object, each aspect ofthe present
invention employs the following.
(a) A manufacturing equipment for a galvannealed steel sheet according to an
aspect ofthe invention, the manufacturing equipment includes:
a coating tub to coat a steel sheet which is dipped in a coating bath, wherein the
25 coating tub has a first temperature controller to keep the coating bath which is a molten
5
12
metal including a molten zinc and a molten aluminum to a predetermined bath
temperature T1;
a separating tub to separate by a flotation a top-dross which is precipitated by
controlling an aluminum concentration A2 ofthe coating bath transferred from the
coating tub to be 0.14 mass% or more by supplying a fIrst zinc-included-metal which
includes an aluminum with a concentration higher than an aluminum concentration Al of
the coating bath in the coating tub, wherein the separating tub has a second temperature
controller to keep the coating bath transferred through a coating bath outlet ofthe coating
tub to a bath temperature T2 which is lower than the bath temperature Tl;
lOanadjusting tub to adjust an aluminum concentration A3 of the coating bath
transferred from the separating tub to a concentration which is higher than the aluminum
concentration Al and is lower than the aluminum concentration A2 by supplying a
,
second zinc-included-metal which includes an aluminum with a concentration lower than
the aluminum concentration A2 or does not include an aluminum, wherein the adjusting
15 tub has a third temperature controller to keep the coating bath transferred from the
separating tub to a bath temperature T3 which is higher than the bath temperature T2; and
a circulator to circulate the coating bath in order ofthe coating tub, the
separating tub, and the adjusting tub.
[0023]
20 (b) The manufacturing equipment for the galvannealed steel sheet according to
(a), the manufacturing equipment may further include,
an aluminum concentration analyzer to measure the aluminum concentration
Alofthe coating bath in the coating tub,
wherein the circulator may control a circulating volume ofthe coating bath
25 depending on a measurement result ofthe aluminum concentration analyzer.
15
t
13
[0024]
(c) In the manufacturing equipment for the galvannealed steel sheet according to
(a),
the bath temperature T2 of the separating tub may be controlled by the second
5 temperature controller to be lower 5°C or more as compared with the bath temperature
Tl of the coating tub and to be higher than a melting point ofthe molten metal.
[0025]
(d) In the manufacturing equipment for the galvannealed steel sheet according to
(a),
10 the bath temperature T3 may be controlled by the third temperature controller so
that the bath temperature Tl, the bath temperature T2, and the bath temperature T3
satisfy a following formula (1) and a following formula (2) in celsius degree, when a
difference of a bath temperature decrease ofthe coating bath when transferred from the
adjusting tub to the coating tub is ~Tfall in celsius degree.
Tl + ~Tfall - 10:::; T3 :::; Tl + ~Tfall + 10 ... (1)
T2 + 5 :::; T3 ... (2)
[0026]
(e) The manufacturing equipment for the galvannealed steel sheet accordhlg to
(a), the manufacturing equipment may further includes,
20 a premelting tub to melt the second zinc-included-metal,
wherein a molten metal ofthe second zinc-included-metal which is melted in the
premelting tub may be supplied to the coating bath in the adjusting tub.
[0027]
(f) In the manufacturing equipment for the galvannealed steel sheet according to
25 (a),
14
• the circulator may include a molten metal transfer apparatus which is installed in
at least one ofthe coating tub, the separating tub, and the adjusting tub.
[0028]
(g) In the manufacturing equipment for the galvannealed steel sheet according to
5 (a),
the coating bath outlet ofthe coating tub may be located on a downstream side
ofa running direction of the steel sheet so that the coating bath flows out of an upper part
ofthe coating tub by a flow ofthe coating bath which is derived from a rUnning ofthe
steel sheet.
10 [0029]
(h) In the manufacturing equipment for the galvannealed steel sheet according to
(a),
at least two of the coating tub, the separating tub, and the adjusting tub may be
made by dividing one tub with a weir, and
15 a bath temperature of each tub which is divided by the weir may be controlled
independently.
[0030]
(i) In the manufacturing equipment for the galvannealed steel sheet according to
(a),
20 a storage ofthe coating bath in the coating tub may be five times or less of a
circulating volume of the coating bath per one hour by the circulator.
[0031]
G) In the manufacturing equipment for the galvannealed steel sheet according to
(a),
25 a storage ofthe coating bath in the separating tub may be two times or more ofa
• 15
circulating volume ofthe coating bath per one hour by the circulator.
25
[0032]
(k) A manufacturinR method ofa galvannealed steel sheet according to an aspect
of the invention, the manufacturing method includes:
5 circulating a coating bath which is a molten metal including a molten zinc and a
molten aluminum in order of a coating tub, a separating tub, and an adjusting tub;
coating a steel sheet which is dipped in the coating bath at the coating tub in
which the coating bath transferred from the adjusting tub is stored at a predetermined
bath temperature Tl;
10 separating by a flotation a top-dross which is precipitated by controlling an
aluminum concentration A2 of the coating bath transferred from the coating tub to be
0.14 mass% or more at the separating tub in which the coating bath transferred from the
coating tub to the separating tub is stored at a bath temperature T2 which is lower than
the bath temperature Tl of the coating tub and a fIrst zinc-included-metal which includes
15 an aluminum with a concentration higher than an aluminum concentration Al ofthe
coating bath in the coating tub is supplied; and
adjusting an aluminum concentration A3 ofthe coating bath transferred from the
separating tub to a concentration which is higher than the aluminum concentration Al
and is lower than the aluminum concentration A2 at the adjusting tub in which the
20 coating bath transferred from the separating tub is stored at a bath temperature T3 which
is higher than the bath temperature T2 ofthe separating tub and a second
zinc-included-metal which includes an aluminum with a concentration lower than the
aluminum concentrationA2 of the coating bath in the separating tub or does not include
an aluminum is supplied.
[0033]
16
According to the manufacturing equipment and the manufacturing method for
the galvannealed steel sheet described in the above (a) and (k), the coating bath is
circulated in order of the coating tub, the separating tub, and the adjusting tub. Thereby,
in the coating tub, the stagnation time ofthe circulation bath can be shortened, so that it
5 is possible to avoid that the dross·forms in the coating tub and grows up to the harmful
size. In the separating tub, Fe is supersaturated by decreasing the bath temperature of
the circulation bath, so that it is possible to precipitate Fe ofthe coating bath as the
top-dross, to also transform the bottom-dross with harmless size which is contained in the
inflow bath into the top-dross, and to separate by the flotation. Moreover, in the
10 adjusting tub, Fe ofthe coating bath is unsaturated by increasing the bath temperature of
the circulation bath, so that it is possible to dissolve and remove the top-dross with small
size which is not able to be separated and removed in the separating tub and to adjust the
composition ofthe coating bath transferred from the adjusting tub to the coating tub by
supplying the metal.
15
Advantageous Effects of Invention
[0034]
According to the invention described in the above (a) and (k), the formation and
growth ofthe dross are suppressed in the coating tub, the top-dross is separatedClfld
20 removed in the separating tub, and the residual dross is dissolved in the adjusting tub.
Thereby, it is possible that the dross which forms inevitably in the coating bath is
almost-completely rendered harmless.
According to the invention described in the above (b), the AI concentration of
the coating bath which is stored in the separating tub can be increased to the
25 concentration which is required to be a top-dross formation range. Thereby, it is
10
5
17
possible that the fonned dross in the separating tub is controlled to be only the top-dross.
According to the invention described in the above (c), the solubility limit ofFe
ofthe coating bath which is stored in the separating tub decreases. Thereby, it is
possible that the dross which is equivalent to the amount of supersaturated Fe is
intentionally precipitated.
According to the invention described in the above (d), the bath temperature of
the coating bath which is stored in the adjusting tub is held higher than that of the
separating tub and the bath temperature deviation ofthe coating bath in the coating tub
decreases. Thereby, it is possible to dissolve the residual dross at the adjusting tub and
to suppress the fonnation ofthe dross with harmful size at the coating tub.
According to the invention described in the above (e), it is not necessary to melt
the metal in the adjusting tub. Thereby, it is possible to suppress the drastic decrease in
the temperature of the molten metal caused by supplying the metal and the fonnation of
the dross therefor at the adjusting tub.
15 According to the invention described in the above (t), the circulating volume of
the coating bath which circulates in order ofthe coating tub, the separating tub, and the
adjusting tub is controlled. Thereby, it is possible that the composition of the coating
bath which is required as the coating bath ofthe coating tub and the composition of the
coating bath which is required as the coating bath of the separating tub are satisfied
20 simultaneously.
According to the invention described in the above (g), the local stagnation area
ofthe coating bath lOA in the coating tub 1 is hardly fonned: Thereby, it is possible to
•
avoid that the dross grows up to the harmful size at the stagnation area in the coating tub
1.
25 According to the invention described in the above (h), two or three tubs of the
15
18
coating tub, the separating tub, and the adjusting tub are made as one. Thereby, it is
possible to simplify the equipment configuration.
According to the invention described in the above (i), the stagnation time of the
coating bath in the coating tub is shortened. Thereby, it is possible to make the dross
5 flow out ofthe coating tub to the separating tub before the dross grows up to the harmful
SIze.
According to the invention described in the above 0), the stagnation time ofthe
coating bath in the separating tub is prolonged. Thereby, it is possible to sufficiently
remove the top-dross at the separating tub.
10
Brief Description ofDrawings
[0035]
FIG. 1 is a ternary phase diagram which indicates a dross formation range in
various coating baths.
FIG. 2 is a graph which indicates dross growth of each phase under condition
where bath temperature is constant.
FIG. 3A is a schematic diagram which illustrates a flowing situation of the dross
in a coating tub.
FIG. 3B is a schematic diagram which illustrates a flowing situation of the dross
20 in the coating tub.
FIG. 4 is a schematic diagram which illustrates a configuration 1 of
manufacturing equipment for a galvannealed steel sheet according to an embodiment of
the present invention.
FIG. 5 is a schematic diagram which illustrates a configuration 2 ofthe
25 manufacturing equipment for the galvannealed steel sheet according to modification 1 of
, 19
the embodiment.
FIG. 6 is a schematic diagram which illustrates a configuration 3 of the
manufacturing equipment for the ga1vannea1ed steel sheet according to modification 2 of
the embodiment.
5 FIG. 7 is a schematic diagram which illustrates a configuration 4 of the
manufacturing equipment for the galvannea1ed steel sheet according to modification 3 of
the embodiment.
FIG. 8 is a schematic diagram which illustrates a configuration 5 ofthe
manufacturing equipment for the galvannealed steel sheet according to modification 4 of
10 the embodiment.
FIG. 9 is a schematic diagram which illustrates permissible bath temperature
range of each tub according to the embodiment when the bath temperature ofthe coating
tub is 460°C.
FIG. 10 is the ternary phase diagram which indicates state transition of the
15 coating bath in each tub according to the embodiment.
FIG. 11 is the ternary phase diagram which indicates a state ofGA bath
according to the embodiment.
FIG. 12 is a graph which indicates bath conditions where all precipitated dross is
to be top-dross in a separating tub according to the embodiment.
20 FIG. 13 is a graph which indicates a relationship between capacity ofthe
separating tub and a dross separation ratio according to examples of the present
invention.
25
FIG. 14 is a graph which indicates a relationship between circulating volume of
bath and dross size according to the examples.
FIG. 15 is a graph which indicates a relationship between a bath temperature
5
10
20
deviation ofan inflow bath ofthe coating tub and the dross size according to the
examples.
Description ofEmbodiments
[0036]
Hereinafter, a preferable embodiment ofthe present invention will be described
in detail with reference to the drawings. Moreover, in regard to the component which . .
has the substantial same function, duplicate explanations are omitted by adding the same
reference sign in the specification and the drawings.
[0037]
[1. Investigation of dross formation and dross removal methods]
First of all, in advance of explanations ofmanufacturing equipment for a
galvannealed steel sheet and a manufacturing method ofthe galvannealed steel sheet
according to an embodiment ofthe present invention, the result ofthe investigation of
15 factors of dross formation (top-dross, bottom-dross) in coating bath and the dross
removal methods will be described.
[0038] . ~.
[1.1. Dross formation range]
As mentioned above, the hot dip zinc-aluminum coated steel sheets are the steel
20 sheets which are coated by using the molten metal in which zinc is the main ingredient
and aluminum is added. For example, (1) the galvannealed steel sheets, (2) the
galvanized steel sheets, and (3) the zinc-aluminum alloy coated steel sheets.
[0039]
The galvannealed steel sheets (GA) are the steel sheets in which the Zn-Fe
25 intermetallic compound layer is formed by heating for short time at 490 to 600°C just
21
after galvanizing and by alloying molten Zn and steel. For example, the GA is
frequently utilized as automobile steel sheets and the like. Coating layer ofthe GA
includes the alloy of Fe which is dissolved in the coating bath from the steel sheet and Zn.
Composition of the coating bath (GA bath) for manufacturing the GA includes, for
5 example, AI of 0.125 to 0.14 mass% and Zn as the balance. The GA bath further
includes Fe which is dissolved in the coating bath from the steel sheet. In the GA bath,
the relatively low-concentration Al is added to Zn bath in order to improve coating
adhesion. When the AI concentration in the GA bath is excessively high, the alloying of
Fe and AI in the coating layer barely occurs by so-called aluminum barriers, so that the
10 Al concentration in the GA bath is controlled to a predetermined low concentration
(0.125 to 0.14 mass%).
[0040]
The galvanized steel sheets (GI) are frequently utilized as general building
materials and the like. Composition of the coating bath (GI bath) for manufacturing the
15 GI includes, for example, AI of 0.15 to 0.25 mass% and Zn as the balance. By
controlling the AI concentration ofthe GI bath to 0.15 to 0.25 mass%, the adhesion of the
coating layer to the steel sheet is particularly improved, so that exfoliation ofthe coating
layer can be suppressed even if the steel sheet is deformed.
[0041]
20 The zinc-aluminum alloy coated steel sheets are frequently utilized as general
building materials in which high durability is required and the like, for example.
Composition ofthe coating bath for manufacturing the above steel sheets is Al of 5
mass% and Zn as the balance, Al of 11 mass% and Zn as the balance, and the like.
Since the sufficient amount ofAl is contained in the Zn bath, higher corrosion resistance
25 is obtained as compared with the GI.
• [0042]
22
5
In the coating bath for manufacturing the hot dip zinc-aluminum coated steel
sheets, the top-dross and the bottom-dross which are the intermetallic compounds of Fe
dissolved in the coating bath and Al or Zn are formed in large amount. The dross
formation in the coating bath depends on temperature ofthe coating bath (bath
temperature), the Al concentration in the coating bath, and Fe concentration in the
coating bath (solubility ofFe dissolved in the coating bath from the steel sheet).
[0043]
FIG. 1 is a ternary phase diagram which indicates the dross formation range in
10 the various coating baths. In the FIG. 1, horizontal axis is the AI concentration (mass%)
in the coating bath and vertical axis is the Fe concentration (mass%) in the coating bath.
[0044]
As shown in FIG. 1, when the Fe concentration in the coating bath exceeds the
predetermined concentration which depends on the Al concentration, the dross is formed.
15 For example, in regard to the GA bath where the bath temperature T is 450°C and the AI
concentration is 0.13 mass%, when the Fe concentration in the coating bath becomes
approximately more than 0.025 mass%, the bottom-dross (FeZn7) is formed. Moreover,
in regard to the GA bath where the bath temperature T is 450°C and the AI concentration
is 0.14 mass%, the top-dross (Fe2AI5) is formed when the Fe concentration becomes
20 approximately more than 0.025 mass%, and the bottom-dross (FeZn7) is formed in
addition to the top-dross when the Fe concentration further increases. As described
above, the top-dross and the bottom-dross are formed and mixed under the conditions.
[0045]
On the other hand, since the AI concentration ofthe GI bath (for example, 0.15
25 to 0.25 mass%) is higher than that of the GA bath, the dross which is formed in the GI
• 23
bath is only the top-dross (Fe2AIs). For example, in regard to the GI bath where the
bath temperature T is 450°C, when the Fe concentration in the coating bath becomes
approximately more than 0.01 mass%, the top-dross is formed. Moreover, in regard to
the coating bath for the zinc-aluminum alloy coated steel sheets even though it is not
5 illustrated, only the top-dross is also formed since the Al concentration is sufficiently
high (for example, 2 t025 mass%).
[0046]
In addition, as shown in FIG 1, even if the coating bath is the same, lower limit
ofFe concentration where the dross is formed increases with an increase in the bath
10 temperature T. For example, in regard to the GA bath where the Al concentration is
0.13 mass%, conditions where the bottom-dross is formed are as follows: (1) the Fe
concentration is approximately 0.025 mass% or more in case that the bath temperature T
is 450°C, (2) the Fe concentration is approximately 0.035 mass% or more in case that the
bath temperature T is 465°C, and (3) the Fe concentration is approximately 0.055 mass%
15 or more in case that the bath temperature T is 480°C. Thus, when the Fe concentration
in the coating bath is constant (for example, 0.03 mass% Fe), the supersaturated state is
shifted to the unsaturated state in regard to Fe by increasing the bath temperature T from
450°C to 465°C, so that the bottom-dross is dissolved in the coating bath and disappears.
On the contrary, the unsaturated state is shifted to the supersaturated state in regard to Fe
20 by decreasing the bath temperature T from 465°C to 450°C, so that the bottom-dross is
formed.
[0047]
[1.2. Factors of dross formation]
Next, the factors ofthe dross formation in the coating bath will be described.
25 As the factors of the dross formation, the following factors (1) to (3) are considered, for
24
• example. Hereinafter, each factor will be described.
[0048]
(1) Melting the metal to the coating bath
In order to supply the molten metal which is consumed for coating the steel
5 sheet in a coating tub to the coating bath, the metal is used. The metal in a solid state is
dipped into the hot coating bath at preferable timing during operation, is melted in the
coating bath, and becomes the molten metal in aliquid state. Although
zinc-included-metal which includes at least Zn for hot dip zinc coating, the
zinc-included-metal includes the metal such as Al and the like besides Zn according to
10 the composition ofthe coating bath. Although the melting point of the metal differs
according to the composition ofthe metal, the melting point is 420°C for example and is
lower than the temperature ofthe coating bath (for example, 460°C).
[0049]
When the metal which is dipped into the coating bath is melted, the temperature
15 ofthe molten metal around the metal decreases lower than the bath temperature T of the
coating bath. Namely, temperature deviation between the temperature (for example,
420°C) around the metal which is dipped into the coating bath and the bath temperature T
(for example, 460°C) of the coating bath arises. Thus, when Fe in the coating bath is
the saturated state, a large amount ofthe dross is formed with comparative ease at
20 low-temperature area around the metal. The phase ofthe formed dross is related to the
phase diagram (refer to FIG. 1).
[0050]
In general, since the steel sheet is constantly dipped into the coating tub and
active iron surface is exposed, the Fe concentration in the coating bath is the saturated
25 state. Thus, when the temperature ofthe molten metal around the metal decreases
5
• 25
drastically by supplying the metal in the coating bath where Fe is the saturated state, the
dross is fonned by reacting the supersaturated Fe with Zn or Al in the coating bath.
Moreover, when the metal is preliminarily melted by using a premelting tub and the
molten metal is supplied to the coating bath in the coating tub, the dross is hardly fonned
because Fe in the premelting tub is the unsaturated state.
[0051]
(2) Fluctuation ofthe bath temperature T
As the factor ofthe dross fonnation following the melt ofthe metal, the
fluctuation ofthe bath temperature T ofthe coating bath is considered. Since the
10 solubility limit of Fe in the coating bath increases with the increase in the bath
temperature T, Fe is further dissolved from the steel sheet which is dipped into the
coating bath and Fe in the coating bath reaches the saturated concentration promptly.
When the bath temperature T ofthe coating bath decreases, Fe becomes the
supersaturated state allover the coating bath and the dross is promptly fonned.
15 Furthennore, even if the low bath temperature T ofthe coating bath which includes the
dross increases again and the solubility limit ofFe increases, the dross is not decomposed
(does not disappear), because the dissolution rate ofFe from the steel sheet is faster than
that ofthe decomposition (disappearance) ofthe dross. In other words, even if the bath
temperature ofthe coating bath which is low temperature (supersaturated state of Fe)
20 increases at the coating tub in which the steel sheet is dipped, the dross hardly
disappears.
[0052]
On the other hand, if the molten metal which is low temperature and includes
the dross is transferred to a tub in which the steel sheet in not dipped, is heated, and is
25 held for long time, the dross can be decomposed (can disappear), because Fe in the
26
coating bath becomes the unsaturated state. Thus, based on the viewpoint, in the
manufacturing equipment for the galvannealed steel sheet according to the embodiment
of the present invention as described later, after forming the dross in the coating bath at a
separating tub, the coating bath is transferred to an adjusting tub in which the steel sheet
5 in not dipped, the bath temperature T increases, and the dross is dissolved (disappears).
[0053]
(3) Other factors
The fluctuation ofthe Al concentration in the coating bath and the temperature
deviation in the coating tub are also considered as the factor ofthe dross formation.
10 When the AI concentration in the coating bath increases, the solubility limit ofFe in the
coating bath decreases, so that the top-dross (Fe2AIs) which is the intermetallic
compound ofAI and Fe is readily formed. And, when coating bath flow in the coating
tub decreases and mixing power in the coating tub decreases, temperature ofthe coating
bath at bottom ofthe coating tub decreases, so that the dross is formed. Thereafter,
15 when the coating bath flow increases again, the dross which deposits on the bottom of the
coating tub rises in the coating bath.
[0054]
[1.3. Separation of dross by using the difference in specific gravity]
The methods of the flotation separation ofthe top-dross and of the sedimentation
20 separation of the bottom-dross by using the difference in specific gravity between the
molten metal which is the coating bath and the dross are known. In general, the specific
gravity ofthe bottom-dross is, for example, 7000 to 7200 kg / m3 and the specific gravity
ofthe top-dross is, for example, 3900 to 4200 kg / m3
. On the other hand, although the
specific gravity ofthe molten zinc bath fluctuates to a certain extent by the temperature
25 and Al concentration thereof, it is, for example, 6600 kg / m3
.
• [0055]
27
As described above, in case ofthe separation of the dross by using the difference
in specific gravity, since the difference in specific gravity between the top-dross and the
molten zinc bath is large and the top-dross readily rises to top surface, it is relatively easy
5 to separate the top-dross by the flotation and to remove the top-dross outside the system.
On the contrary, since the difference in specific gravity between the bottom-dross and the
molten zinc bath is vanishingly small, it is necessary to hold for long time under the
condition where the coating bath flow is low in order to sediment the bottom-dross.
Especially, it is difficult to sediment the bottom-dross with small size. Moreover, since
10 the bottom-dross deposits on the bottom of the coating tub and may rise again, it is not
easy to remove finally the bottom-dross outside the system (removing the bottom-dross
from the bottom ofthe coating tub).
[0056]
As just described, it is difficult to remove the dross in the coating tub, especially,
15 the bottom-dross which deposits on the bottom of the coating tub. Although the various
removal methods were proposed (refer to Patent Documents 1 to 5), the method to
readily separate and remove the dross with high removal efficiency is not yet proposed.
[0057]
[1.4. Relation between bath temperature fluctuation and dross growth]
20 FIG 2 is a graph which indicates the dross growth of each phase under the
condition where the bath temperature is constant. In the FIG 2, horizontal axis is the
time (hours to days) and vertical axis is the average grain size of dross particles (flm).
FIG 2 indicates the growth ofthe bottom-dross (FeZn7) which forms in the GA bath and
the top-dross (FezAls) which forms in the GA bath, the GI bath, and the like.
25 [0058]
• 28
As shown in FIG 2, when the conditions such as the bath temperature T and the
like are constant, a growth rate is slow in each phase ofthe dross. For example, under
the condition where the bath temperature is constant, the bottom-dross (FeZn7) grows
only from approximately 15 ~m to 20 ~ in the average grain size during 200 hours, and
5 the top-dross (FezAls) gtows only from approximately 15 ~ to 35 ~m during 200 hours.
[0059]
Next, in reference to Table 1, the result of observation offonning behavior of
the dross in case of decreasing the bath temperature will be described. Table 1 shows a
state of the dross growth when three types of coating baths A to C in which compositions
10 are different are cooled from 460°C to 420°C by a predetermined cooling rate (10 °C /
sec).
[0060]
[Table 1]
[0061]
15 As shown in Table 1, when the bath temperature T decreases from 460°C to
420°C by the predetennined cooling rate of 10°C / sec and the unsaturated state is
shifted to the supersaturated state in regard to Fein the coating bath, the rate offormation
and growth ofthe dross is very fast. For example, in the coating bath A (GAbath) with
- AI of 0.13 mass%, the bottom-dross (FeZn7) with the grain size ofapproximately 50 ~
20 is formed during only 4 seconds. And, in the coating bath B (GA bath) with Al of 0.14
mass%, the bottom-dross (FeZn7) with the grain size ofapproximately 40.~ and the
top-dross (FezAIs) with the grain size ofapproximately 10 ~m are formed and mixed.
Moreover, in the coating bath C (GI bath) withAl of 0.18 mass%, three kinds ofthe
top-dross (FezAIs) with the grain size of approximately 5 ~, 10~, and 25 ~ are
25 formed.
• [0062]
29
As mentioned above, under the condition where the bath temperature T is
constant (refer to FIG. 2), the growth rates of both the bottom-dross and the top-dross are
slow. Thus, if the bath temperature T ofthe coating bath in the coating tub can be kept
5 constant as much as possible, the dross growth in the coating tub can be suppressed. On
the contrary, if the bath temperature T decreases, the unsaturated state is shifted to the
supersaturated state in regard to Fe in the coating bath, so that the growth rates of the
dross are very fast (refer to FIG. 2). Therefore, by transferring the coating bath of the
coating tub to the separating tub, by increasing the AI concentration in the coating bath,
10 and by decreasing the bath temperature T, the top-dross is intentionally precipitated in the
coating bath of the separating tub, so that it is possible that the top-dross is effectively
separated by the flotation.
[0063]
[1.5. Relation between coating rate and dross]
15 FIGs. 3A and 3B are schematic diagrams which illustrate flowing situation of
the dross in the GA bath. FIG. 3A shows the situation ofnormal operation where the
coating rate is 150 m / min or less and FIG. 3B shows the situation of operation where the
coating rate is high-speed (for example, 200 m / min or more).
[0064]
20 Generally, in the GA bath, the bottom-dross forms and the bottom-dross with
large size among them sediments and deposits on the bottom ofthe coating tub in tum.
When the coating rate (sheet threading speed ofthe steel sheet) is slow, for example, less
than 100 m / min, the bottom-dross which deposits on the bottom ofthe tub does not rise
due to the coating bath flow. However, when the coating rate is 100 m / min or more, as
25 shown in FIG. 3A, among the bottom-dross, not only the dross with small size but also
30
• the dross with medium size which has relatively large diameter rises from the bottom of
the tub due to the bath flow which is derived from the sheet threading, and the dross
flows in the coating bath ofthe coating tub. Thus, when an amount of the formation
and the deposition ofthe dross is much in the coating tub, productivity of the coated steel
5 sheet deteriorates. As described above, when the coating rate is 150 m / min or less, the
dross with small size and medium size mainly flows in the coating bath.
[0065]
Moreover, when the coating rate, which is conventionally suppressed (for
example, 150 m / min or less) in order to ensure the productivity, is changed to 200 m /
10 min or more for example, as shown in FIG 3B, all the bottom-dross flows regardless of
the grain size. Namely, the bottom-dross cannot deposit on the bottom of the tub by the
strong bath flow which is derived from high-speed sheet threading, the dross with large
size also flows in the coating bath. In other words, unless it is possible that the dross in
the coating bath is almost-completely rendered harmless (dross-free), it is difficult to
15 increase the coating rate.
[0066]
[1.6. Dross defects]
The dross defects are defects ofthe coated steel sheet, are caused by the dross
formed in the coating bath, and include appearance deterioration ofthe coated steel sheet
20 which is derived from dross adhesion, surface defects caused by the dross on roll in the
coating bath, and the like, for example. Although it is said that the diameter ofthe dross
which cause the dross defects is 100 JIDl to 300 JIDl, the dross defects caused by the dross
with very small size such that grain size is approximately 50 JIDl are observed recently.
Therefore, in order to prevent the occurrence ofthe small dross defects, the dross-free in
25 coating bath is desired.
• 31
[0067]
[2. Configuration of manufacturing equipment for galvannealed steel sheet]
Next, in reference to FIGs. 4 to 9, the configuration ofthe manufacturing
equipment for the galvannealed steel sheet according to the embodiment of the present
5 invention will be described. FIG. 4 is a schematic diagram ofthe manufacturing
equipment for the galvannealed steel sheet according to the embodiment, and FIGs. 5 to
8 are schematic diagrams which illustrate modifications 1 to 4 ofthe embodiment,
respectively. FIG. 9 is a schematic diagram which illustrates permissible bath
temperature range of each tub in case that the bath temperature ofthe coating bath lOA
10 which is stored in the coating tub 1 according to the embodiment is 460°C. Hereinafter,
the bath temperature and the aluminum concentration of the coating bath which is stored
in the coating tub 1 are referred to as Tl and Al respectively. In the same way, the bath
temperature and the aluminum concentration ofthe coating bath which is stored in the
separating tub 2 are referred to as T2 and A2 respectively, and the bath temperature and
15 the aluminum concentration ofthe coating bath which is stored in the adjusting tub 3 are
referred to as T3 and A3 respectively.
[0068]
As shown in FIGs. 4 to 8, the manufacturing equipment for the galvannealed
steel sheet according to the embodiment (hereinafter, referred to as hot-dip-coating
20 equipment) includes the coating tub1 to coat the steel sheet 11, the separating tub 2 to
separate the dross, and the adjusting tub 3 to adjust the Al concentration ofthe coating
bath 10. In addition, the hot-dip-coating equipment includes circulator to circulate the
molten metal (coating bath 10) for coating the steel sheet 11 in order of the coating tub 1
- the separating tub 2 - the adjusting tub 3 - the coating tub 1. The coating bath lOis the
25 molten metal including at least molten zinc and molten aluminum, and is the GA bath for
32
example. Hereinafter, each configuration ofthe hot-dip-coating equipment according to
the embodiment will be described.
[0069]
[2.1. Configuration of circulator of coating bath]
5 First, the circulator will be described. The circulator includes the molten metal
transfer apparatus 5 which is concomitantly installed in at least one ofthe coating tub 1,
the separating tub 2, or the adjusting tub 3, and the vessel for the molten metal which
connects mutually between the three tubs (for example, communicating vessel 6 or 7,
transferring vessel 8, and overflowing vessel 9). The molten metal transfer apparatus 5
10 may be composed by arbitrary apparatus if the molten metal (coating bath 10) can be
transferred. For example, the molten metal transfer apparatus 5 may be mechanical
pump and magneto-hydrodynamic pump.
[0070]
Moreover, the molten metal transfer apparatus 5 may be concomitantly installed
15 in all the tubs of the coating tub 1, the separating tub 2, and the adjusting tub 3, and may
be concomitantly installed in arbitrary one tub or two tubs among the three tubs.
However, from a viewpoint of simplifying the equipment configuration, it is preferable
that the molten metal transfer apparatus 5 is installed in only one tub and the molten
metal is transferred between the three tubs by connecting the remaining tubs by the
20 communicating vessel 6 or 7, the transferring vessel 8, the overflowing vessel 9, and the
like. In the embodiment ofFIGs. 4 to 8, as the molten metal transfer apparatus 5, the
mechanical pump which transfers the molten metal is installed in the transferring vessel 8
which is the vessel between the coating tub 1 and the adjusting tub 3. As mentioned
later, the coating bath which is transferred from the adjusting tub 3 to the coating tub is
25 the purified coating bath in which the dross is almost removed. Thus, by using the
15
33
.. molten metal transfer apparatus 5 only for the purified coating bath, it is possible to
minimize trouble ofthe molten metal transfer apparatus 5 such as dross clogging and the
like.
[0071]
5 Namely, in the embodiment, the coating tub 1, the separating tub 2, and the
adjusting tub 3 are mutually connected by using the vessel such as the communicating
vessel 6 or 7, the transferring vessel 8, the overflowing vessel 9, and the like, in order to
circulate the coating bath 10. As described above, in case the vessel is used for the bath
circulation, it is preferable to suppress erosion of inner wall ofthe vessel by the bath flow,
10 to prevent a decrease in the temperature and solidification ofthe bath in the vessel, and
the like. Therefor, it is preferable to use the double vessel which equipped with
ceramics inside the vessel and to keep warm or heat outer wall ofthe vessel. Especially,
before operating the bath circulation, it is preferable to prevent the solidification ofthe
bath in the vessel by pre-heating the vessel.
[0072]
[2.2. Overall structure oftubs]
Next, overall configuration ofthe coating tub 1, the separating tub 2, and the
adjusting tub 3 will be described in detail. As shown in FIG 4, FIG 5 (modification 1),
and FIG 8 (modification 4), the coating tub 1, the separating tub 2, and the adjusting tub
20 3 may be the configuration in which the tubs are independent respectively. For example,
in the configuration as shown in FIG 4, the coating tub 1, the separating tub 2, and the
adjusting tub 3 are parallelly installed in the horizontal direction, upper parts of the
coating tub 1 and the separating tub 2 are connected by the communicating vessel 6,
lower parts ofthe separating tub 2 and the adjusting tub 3 are connected by the
25 communicating vessel 7, and the adjusting tub 3 and the coating tub 1 are connected by
34
• the transferring vessel 8 with the molten metal transfer apparatus 5. In this way, it is
possible to simplify the overall configuration ofthe hot-dip-coating equipment by
making the height ofthe bath surface ofthe coating bath in each tub the same, by
circulating the coating bath through the vessels such as the communicating vessel, and by
5 using the molten metal transfer apparatus 5 only at the most downstream. Moreover, in
the configuration ofthe modification las shown in FIG. 5, the overflowing vessel 9 is
installed in upper part side of side wall ofthe coating tub 1, and the coating bath lOA
which is overflowed from the coating tub 1 flows down into the separating tub 2 through
the overflowing vessel 9.
10 [0073]
In addition, the coating tub 1, the separating tub 2, and the adjusting tub 3 may
be functionally independent. For example, as shown in the modification 3 in FIG. 7, the
coating tub 1, the separating tub 2, and the adjusting tub 3 may be composed by
partitioning the inside of single tub with relatively large size into three areas by two weirs
15 21 and 22, which may be the configuration in which the three tubs are seemingly unified.
Moreover, as shown in the modification 2 in FIG. 6, the separating tub 2 and the
adjusting tub 3 may be composed by partitioning the inside ofthe single tub into two
areas by one weir 23, the separating tub 2 and the adjusting tub 3 may be unified, and the
coating tub 1 may be only independent as the tub configuration. In this way, it is
20 possible to simplify the equipment configuration by unifying three or two tubs among the
coating tub 1, the separating tub 2, and the adjusting tub 3.
[0074]
However, in order to achieve the characteristic dross removal method as
mentioned later, in any of the tub component as shown in FIGs. 4 to 8, it is necessary to
25 independently control the bath temperature and the AI concentration of the coating bath
5
15
10
• 35
in each tub, respectively. Specifically, the bath temperature TI and Al concentration Al
ofthe coating bath are controlled at the coating tub 1, the bath temperature T2 and AI
concentration A2 of the coating bath are controlled at the separating tub 2, and the bath
temperature T3 and AI concentration A3 ofthe coating bath are controlled at the
adjusting tub 3. Thus, temperature controller 1, temperature controller 2, and
temperature controller 3 which are not illustrated are respectively installed in each of the
coating tub 1, the separating tub 2, and the adjusting tub 3, in order to control the bath
temperature TI, T2, and T3 ofthe coating bath which is stored. The temperature
controllers are equipped with heating apparatus and bath temperature control apparatus.
The heating apparatus heats the coating bath of each tub, and the bath temperature
control apparatus controls operation ofthe heating apparatus. Thus, the bath
temperature ofthe coating tub 1, the separating tub 2, and the adjusting tub 3 are
respectively controlled to the predetermined temperature TI, T2, and T3, by the
temperature controller 1, the temperature controller 2, and the temperature controller 3.
In addition, although the sample for aluminum concentration measurement of each tub
may be periodically sampled by manpower, it is preferable to respectively equip
aluminum concentration analyzer at each tub, in order to independently control the AI
concentration ofthe coating bath in each tub. The aluminum concentration analyzer is
composed by sampler for the sample ofthe aluminum concentration measurement, sensor
20 ofthe aluminum concentration ofthe molten metal or alloy, or the like. The aluminum
concentration ofthe sample which is sampled by the sampler may be periodically
measured by chemical analyzer, or the aluminum concentration ofthe coating bath may
be continuously measured by the sensor ofthe aluminum concentration. Based on the
results of the aluminum measurement, the Al concentration of the coating bath in each
25 tub is independently controlled by controlling the circulating volume or by supplying
25
36
• fIrst or second zinc-included-metal.
[0075]
Moreover, in all the embodiment ofFIGs. 4 to 8, the coating bath lOA flows out
from coating bath outlet which is made by the communicating vessel 6, the overflowing
5 vessel 9, and the weir 21 and which is located on the upper part ofthe coating tub 1 and
downstream side of running direction of the steel sheet 11, and the coating bath lOA
flows into the separating tub 2. This is effective in that the entire coating bath lOA can
be circulated without stagnation ofthe coating bath lOA in the coating tub 1 by using the
flow of the coating bath lOA which is derived from the running ofthe steel sheetl!.
10 Furthermore, in all the embodiment of FIGs. 4 to 8, the communicating vessel 7 and the
weirs 22 and 23 are installed so that the coating bath lOB which flows out from the lower
part of the separating tub 2 flows into the adjusting tub 3. Since the top-dross is
separated by the flotation at the separating tub 2 as described later, the upper part of the
coating bath lOB in the separating tub 2 contains the top-dross by high density as
15 compared with the lower part. Thus, by transferring the coating bath lOB of the lower
part of the separating tub 2 to the adjusting tub 3, the coating bath lOB of the lower part
where the content percentage ofthe top-dross is low can be transferred to the adjusting
tub 3, so that the dross removal efficiency increases.
[0076]
20 [2.3. ConfIguration of each bath]
Next, the confIguration of each bath ofthe coating tub 1, the separating tub 2,
and the adjusting tub 3 will be described.
[0077]
(1) Coating tub
First, the coating tub 1will be described. As shown in FIGs. 4 to 8, the coating
5
• 37
tub 1 has the functions of (a) storing the coating bath lOA which includes the molten
metal at the predetermined bath temperature Tl, and (b) coating the steel sheet 11 which
is dipped in the coating bath lOA. The coating tub 1 is the tub in which the steel sheet
11 is actually dipped in the coating bath lOA and in which the steel sheet 11 is coated by
the molten metal. The composition and the bath temperature Tl ofthe coating bath lOA
in the coating tub 1 are maintained within the proper range according to the kind of the
coated steel sheets for manufacture. For example, in case that the coating bath lOA is
the GA bath, as shown in FIG 9, the bath temperature Tl ofthe coating tub 1 is kept at
approximately 460°C by the temperature controller 1.
10 [0078]
In the coating bath lOA ofthe coating tub 1, the roll in the coating bath such as
sink roll 12, support roll (not illustrated), and the like is installed, and gas wiping nozzle
13 is installed above the coating tub 1. The steel sheet 11 with strip-shaped to be coated
enters obliquely downward into the coating bath lOA ofthe coating tub 1, traveling
15 direction is changed by the sink roll 12, the steel sheet 11 is pulled up vertically upward
from the coating bath lOA, and excessive molten metal on the surface ofthe steel sheet
11 is wiped by the gas wiping nozzle 13.
[0079]
Moreover, it is preferable that storage Ql [ton] (capacity ofthe coating tub 1) of
20 the coating bath lOA in the coating tub 1 is 5 times or less of circulating volume q [ton /
hour] of the coating bath 10 per one hour by the circulator. When the storage Q1 ofthe
coating bath lOA is more than 5 times ofthe circulating volume q, stagnation time ofthe
coating bath lOA in the coating tub 1 is prolonged, so that possibility ofthe formation
and growth ofthe dross in the coating bath lOA increases. Thus, by controlling the
25 storage Q1 of the coating bath lOA to be 5 times or less of the circulating volume q, it is
• 38
possible that the stagnation time of the coating bath lOA in the coating tub 1 is controlled
to be predetermined time or shorter. In the conditions, when Fe is dissolved in the
coating bath lOA of the coating tub 1from the steel sheet 11, the dross is not formed in
the coating bath lOA, or, even if the dross is formed, the coating bath lOA which contains
5 the dross flows out to the separating tub 2 before the dross grows up to the harmful size.
However, it is preferable that the capacity Q1 of the coating tub 1 is as small as possible,
because the coating bath lOA may stagnate in the tub and the dross may grow up to the
harmful size at the stagnation area depending on the shape ofthe coating tub 1.
[0080]
10 In addition, during the operation ofthe hot-dip-coating, part ofthe coating bath
lOA in the coating tub 1 continuously flows out to the separating tub 2 from the coating
bath outlet which is made by the communicating vessel 6, the overflowing vessel 9, and
the weir 21. And, part ofthe coating bath 10C flows into the coating tub 1 through the
transferring vessel 8 and the like from the adjusting tub 3 as mentioned later. It is
15 preferable that the position where the coating bath 10C flows into the coating tub 1 is
located on upstream side ofthe running direction ofthe steel sheet 11 and that the
position ofthe coating bath outlet where the coating bath lOA flows out to the separating
tub 2 is located on the upper part ofthe coating tub 1 and the downstream side ofthe
running direction ofthe steel sheet 11. Thereby, the local stagnation area of the coating
20 bath lOA in the coating tub 1 is hard to form. Thus, it can be suppressed that the dross
grows up to the harmful size at the local stagnation area in the coating tub 1. Here, the
upstream side ofthe running direction ofthe steel sheet 11 is the side including the
entering position of the steel sheet 11 in case of longitudinally-halving the coating tub 1
so as to separate the entering position and the pulling up position ofthe steel sheet 11.
25 Similarly, the downstream side ofthe running direction ofthe steel sheet 11 is the side
• 39
including the pulling up position of the steel sheet 11 in case oflongitudinally-halving
the coating tub 1.
[0081]
(2) Separating tub
5 Next, the separating tub 2 will be described. As shown in FIGs. 4 to 8, the
separating tub 2 has the functions of (a) storing the coating bath lOB which is transferred
from the coating tub 1 at bath temperature T2 which is lower than the bath temperature
Tl ofthe coating bath lOA in the coating tub 1, (b) precipitating only the top-dross by
supersaturating Fe in the coating bath lOB and by increasing the AI concentration ofthe
10 bath so that the state (bath temperature and composition) ofthe coating bath is controlled
to top-dross formation range, and (c) removing the precipitated top-dross by the flotation
separation.
[0082]
For example, in case that the coating bath 10 is the GA bath, as shown in FIG. 9,
15 the bath temperature T2 ofthe separating tub 2 is kept at the temperature which is lower
5°C or more as compared with the bath temperature Tl ofthe coating tub 1 and is higher
than the melting point M (for example, melting point of420°C ofthe GA bath) of the
molten metal which is the coating bath 10 (for example, 420°C ~ T2 ~ TI-5°C).
Moreover, the Al concentration A2 in the separating tub 2 is controlled to be higher than
20 the AI concentration Al in the coating tub 1. Thereby, it is possible that only the
top-dross is intentionally precipitated in the separating tub 2 without precipitating the
bottom-dross in the coating bath lOB by transferring the coating bath 10 from the coating
tub 1 to the separating tub 2, by decreasing the bath temperature T2, and by increasing
the AI concentration A2. Thus, the top-dross can be suitably removed by the flotation
25 separation utilizing the difference in specific gravity.
5
10
• 40
[0083]
The principle will be described in detail. Fe which is dissolved from the steel
sheet 11 is included in the coating bath lOA which flows into the separating tub 2 from
the coating tub 1. The solubility limit of Fe decreases with the decrease in the bath
temperature T (from Tl to T2). Thereby, Fe becomes the supersaturated state in the
coating bath lOB of the separating tub 2, so that the dross which is equivalent to the
amount ofthe supersaturated Fe is precipitated. At the time, in order that the
precipitated dross is only the top-dross, it is necessary to control the Al concentration A2
ofthe separating tub 2 to higher concentration which is at least 0.14 mass% or more
(refer to FIG 1).
[0084]
For the reason, in case ofmanufacturing the galvannealed steel sheet (GA) with
the relatively low Al concentration, a metal with high AI concentration (correspond to the
fIrst zinc-included-metal) is supplied and melted in the separating tub 2. The metal with
15 high AI concentration includes AI with the concentration higher than the AI concentration
Al (for example, 0.135 mass% AI) ofthe coating tub 1 and zinc. By supplying the
metal with high Al concentration, the AI concentration A2 ofthe separating tub 2 is
controlled to be at least 0.14 mass% or more where the state ofthe coating bath lOB
becomes the top-dross formation range. Since only the top-dross precipitates and the
20 bottom-dross does not precipitate in the coating bath lOB of the separating tub 2 at the
time, the specifIc gravity ofthe dross which precipitates in the coating bath lOB becomes
smaller than the specifIc gravity ofthe molten metal (coating bath 10). Therefore, it is
possible that the top-dross is suitably separated by the flotation and easily removed at the
separating tub 2.
25 [0085]
41
• In addition, the bath temperature T2 ofthe separating tub 2 is decreased to be
lower than the bath temperature Tl ofthe coating tub 1 in order to supersaturate Fe in the
bath, and the bath temperature T2 ofthe separating tub 2 is controlleq to be higher than
the melting point M ofthe molten metal in order to avoid the solidification ofthe coating
5 bath lOB.
[0086]
As mentioned above, a large amount ofthe top-dross is intentionally formed in
the coating bath lOB at the separating tub 2 by decreasing the bath temperature T and by
increasing the AI concentration ofthe coating bath 10. Since the top-dross rises to top
10 surface ofthe coating bath lOB by the difference in specific gravity compared with the
coating bath lOB and is trapped at the top surface, the flotation separation ofthe
top-dross needs the time to a certain extent. Thus, it is preferable that storage Q2 [ton]
(capacity ofthe separating tub 2) ofthe coating bath lOB in the separating tub 2 is 2
times or more ofthe circulating volume q [ton / hour] of the coating bath 10 per one hour
15 by the circulator. Thereby, it is possible to sufficiently remove the top-dross at the
separating tub 2, because the time for the flotation separation which is averagely 2 hours
or more is obtained from the inflow ofthe coating bath 10 which flows into the
separating tub 2 from the coating tub 1 to the outflow into the adjusting tub 3. When
the storage Q2 ofthe coating bath lOB in the separating tub 2 is less than 2 times of the
20 circulating volume q ofthe coating bath 10 per one hour, the time for the flotation
separation ofthe top-dross is not sufficiently obtained, so that the dross removal
efficiency decreases.
[0087]
In addition, during the operation of the hot-dip-coating, the part ofthe coating
25 bath lOA continuously flows into the separating tub 2 from the coating tub 1 through the
42
• communicating vessel 6, the overflowing vessel 9, and the like, and the part of the
coating bath 10B in the separating tub 2 continuously flows out to the adjusting tub 3
through the communicating vessel 7 and the like.
[0088]
5 (3) Adjusting tub
Next, the adjusting tub 3 will be described. As shown in FIGs. 4 to 8, the
adjusting tub 3 has the functions of (a) storing the coating bath 10C which is transferred
from the separating tub 2 at bath temperature T3 which is higher than the bath
temperature Tl of the coating tub 1 and the bath temperature T2 ofthe separating tub 2,
10 (b) dissolving the dross which is contained in the coating bath 10C by controlling Fe of
the coating bath 10C to be the unsaturated state, and (c) adjusting the bath temperature
T3 and the AI concentration A3 ofthe coating bath 10C which is transferred to the
coating tub 1 in order to keep constantly the bath temperature Tl and Al concentration
A1 of the coating tub 1. At the time, the Al concentration A3 ofthe bath in the
15 adjusting tub 3 is controlled to be higher than the AI concentration A1 (for example,
0.125 to 0.14 mass%) of the bath in the coating tub 1 and lower than the AI concentration
A2 (for example, 0.147 mass%) of the bath in the separating tub 2.
[0089]
The adjusting tub 3 is the tub in which a metal with low Al concentration
20 (correspond to the second zinc-included-metal) is supplied and melted in order to supply
the molten metal which is consumed at the coating tub 1. The adjusting tub 3 also has
the functions ofreheating the bath temperature T which was lowered in the separating tub
2 and of decreasing and optimizing the Al concentration ofthe bath in case of increasing
the AI concentration A2 ofthe bath in the separating tub 2.
25 [0090]
•
5
10
43
In order to decrease the AI concentration ofthe coating bath lOin the adjusting
tub 3, the zinc-included-metal which includes AI with the concentration lower than the AI
concentration A2 of the coating bath lOB in the separating tub 2 or the
zinc-included-metal which does not include AI may be supplied and melted in the coating
bath IOC ofthe adjusting tub 3 as the second zinc-included-metal. By supplying the
metal with low AI concentration, the AI concentration A3 ofthe coating bath 10C which
is transferred from the adjusting tub 3to the coating tub I is preferably controlled (A2 >
A3 > AI), so that it is possible that the AI concentrationAI ofthe coating bath lOA in the
coating tub I is kept constantly to the proper concentration which is suitable for the
composition ofthe intended GA bath. For example, in the GA bath, the AI
concentration Al ofthe coating bath lOA in the coating tub I is controlled to the constant
concentration within the range of 0.125 to 0.14 mass%.
[0091]
Moreover, it is necessary to control the bath temperature T3 of the adjusting tub
15 3 by the temperature controller 3 to the temperature range which does not cause the
problem even if the coating bath IOC flows into the coating tub 1. Thus, as shown in
FIG. 9, it is preferable that the bath temperature T3 is controlled within ±IO°C on the
basis ofthe temperature in which the difference ofthe bath temperature decrease 1':1Tfall is
added to the bath temperature TI ofthe coating tub I (TI +1':1Tfall - 10°C::; T3 ::; TI +
20 1':1Tfall + 10°C). Here, the difference ofthe bath temperature decrease 1':1Tfall is the value
ofthe bath temperature decrease ofthe coating bath 10 which occurs naturally when the
coating bath 10C is transferred from the adjusting tub 3 to the coating tub I. When the
bath temperature T3. of the adjusting tub 3 does not satisfy the temperature range, the
bath temperature deviation in the coating tub Iincreases, so that the formation and
25 growth of the dross in the coating tub I are promoted. Moreover, the bath temperature
•
44
T4 of the coating bath lOC at the inlet ofthe coating tub 1 becomes within the range of
±10°C on the basis ofthe bath temperature Tl ofthe coating tub 1 (Tl - 10°C ~ T4 ~ Tl
+ 10°C).
[0092]
5 Furthermore, in order to dissolve the residual dross with small size which is not
able to be removed in the separating tub 2 in the coating bath 1DC, it is preferable that the
bath temperature T3 of the adjusting tub 3 is controlled to be higher 5°C or more as
compared with the bath temperature T2 ofthe separating tub 2 (T3 ~ T2 + 5°C).
Although the bath temperature TI, T2, and T3 of each tub are controlled by an induction
10 heating apparatus and the like, the bath temperature fluctuation of approximately ±3°C in
general is inevitable because ofthe limitation of control accuracy. In consideration of
the situation of the control accuracy, that is the maximum (+3°C from the targeted bath
temperature) and the minimum (-3°C from the targeted bath temperature) ofthe bath
temperature fluctuation, it is preferable that the bath temperature T3 (targeted value) of
15 the adjusting tub 3 is higher at least 5°C or more as compared with the bath temperature
T2 (targeted value) ofthe separating tub 2. Thereby, it is possible that Fe ofthe coating
bath 10C in the adjusting tub 3 is the unsaturated state. Namely, it is possible that the
residual dross with small size which is contained in the coating bath 1DB transferred from
the separating tub 2 is certainly dissolved and removed in the adjusting tub 3. When the
20 temperature difference between the bath temperature T3 and T2 is less than 5°C,
unsaturated degree of Fe is insufficient, so that the residual dross which flows into the
adjusting tub 3 from the separating tub 2 cannot be sufficiently dissolved.
[0093]
In addition, storage Q3 [ton] (capacity of the adjusting tub 3) ofthe coating bath
25 1DC in the adjusting tub 3 is arbitrary and is not limited in particular, if melting the metal,
• 45
keeping the bath temperature T3, and transferring the bath to the coating tub 1 are
possible.
[0094]
By the way, when the metal with low AI concentration (the second
5 zinc-included-metal) is supplied into the adjusting tub 3, the bath temperature decreases
locally to the melting point ofthe metal at minimum around the metal which is dipped
into the coating bath IOC ofthe adjusting tub 3, so that the dross forms. Since Fe is the
unsaturated state in the coating bath 10 ofthe adjusting tub 3, the formed dross is
dissolved relatively promptly, so that the dross is harmless in general. However,
10 depending on the unsaturated degree of Fe in the adjusting tub 3 and the time to melt the
metal, the formed dross may be undissolved in the coating bath IOC and may flow out to
the coating tub 1.
[0095]
Thus, in the above case, as shown in the modification 4 in FIG 8, the premelting
15 tub 4 may be installed in addition to the adjusting tub 3, and the molten metal which is
obtained by melting the metal in the premetting tub 4 may be supplied to the adjusting
tub 3. Thereby, it is possible to supply the molten metal which is preheated to
approximately the bath temperature T3 at the premelting tub 4 to the adjusting tub 3 and
to prevent the temperature of the coating bath IOC in the adjusting tub 3 from decreasing
20 locally. Namely, it is possible to avoid the problem such that the dross forms by
supplying the metal at the adjusting tub 3.
[0096]
In addition, during the operation of the hot-dip-coating, the part ofthe coating
bath lOB continuously flows into the adjusting tub 3 from the separating tub 2 through
25 the communicating vessel 7 and the like, and the part ofthe coating bath IOC in the
• 46
adjusting tub 3 continuously flows out to the coating tub I through the transferring vessel
8 and the like.
[0097]
[3. Manufacturing method of galvannealed steel sheet]
5 Next, in reference to FIG 10, coating method of the steel sheet 11 by using the
hot-dip-coating equipment as mentioned above (that is, the manufacturing method ofthe
galvannealed steel sheet) will.be described. FIG 10 is the ternary phase diagram which
indicates state transition ofthe coating bath 10 (GA bath) in each tub according to the
embodiment.
10 [0098]
In the manufacturing method ofthe galvannealed steel sheet according to the
embodiment, the coating bath 10 (GA bath) is circulated by the circulator which includes
the molten metal transfer apparatus 5, the vessel, and the like in order ofthe coating tub 1
(for example, bath temperature: 460°C, Al concentration: approximately 0.135 mass%),
15 the separating tub 2 (for example, bath temperature: 440°C, AI concentration:
approximately 0.148 mass%), and the adjusting tub 3 (for example, bath temperature:
465°C, Al concentration: approximately 0.143 mass%). And the following processes
are simultaneously and parallelly conducted in each tub ofthe coating tub 1, the
separating tub 2, and the adjusting tub 3.
20 [0099]
(1) Coating process at the coating tub 1
First, in the coating tub 1, the coating bath lOA which is stored In the coating tub
1 is kept at the predetermined bath temperature Tl, and the steel sheet 11 which is dipped
in the coating bath lOA is coated. In the coating process, the coating bath 10C which is
25 transferred from the adjusting tub 3 flows into the coating tub 1, and the part ofthe
47
• coating bath lOA flows out from the coating tub 1to the separating tub 2. In the coating
tub 1, since the steel sheet 11 is continuously dipped in the coating bath lOA and Fe is
dissolved from the steel sheet 11 and is sufficiently supplied to the coating bath lOA, the
Fe concentration reaches approximately the saturated concentration.
5 However, as mentioned above, the stagnation time ofthe coating bath lOA in the coating
tub 1 is short time (for example, 5 hours or less on average). Thus, even if operational
fluctuation such as the bath temperature fluctuation occurs to a certain extent, the dross
does not form until the Fe concentration of the coating bath lOA reaches the saturation
point. Moreover, even if the dross forms, the dross is only small size and does not grow
10 up to the large harmful size. Furthermore, since the coating tub 1 is miniaturized as
compared with the conventional coating tub, the stagnation time ofthe circulating
coating bath lOin the coating tub 1 is shortened. Therefore, it is possible that the dross
growth to the harmful size in the coating tub 1 is certainly avoided.
[0100]
15 (2) Separating process at the separating tub 2
Next, the circulation bath which flows out from the coating tub lIed to the
separating tub 2. In the separating tub 2, the bath temperature T2 of the coating bath
lOB which is stored in the separating tub 2 is kept at the temperature which is lower 5°C
or more as compared with the bath temperature Tl ofthe coating tub 1, and the Al
20 concentration A2 ofthe coating bath lOB is controlled to higher concentration which is at
least 0.14 mass% or more. In the separating tub 2, Fe which is supersaturated in the
coating bath lOB is precipitated as the top-dross, and the bottom-dross with harmless size
which is contained in the inflow bath from the coating bath lOis transformed into the
top-dross.
25 [0101]
48
• For example, as shown in FIG 10, when the coating bath lOA of the coating tub
1 is transferred to the separating tub 2, the bath temperature T decreases drastically from
Tl (460°C) to T2 (440°C), and the AI concentration increases from Al (approximately
0.135 mass%) to A2 (approximately 0.148 mass%). As the results, Fe becomes the
5 supersaturated state in the coating bath lOB pfthe separating tub 2, so that the excessive
Fe in the coating bath lOB ofthe separating tub 2 is precipitated as the top-dross (Fe2AIs).
As explained in Table 1, the dross forms easily when the bath temperature decreases. In
the embodiment of the GA bath ofFIG 10, Fe in the coating bath 10 transferred from the
coating tub 1to the separating tub 2 becomes the supersaturated state by the decrease in
10 the bath temperature T, so that a large amount of the top-dross is formed in the separating
tub 2, depending on the super saturated degree. At the time, the Al concentration A2 of
the coating bath lOB is, for example, 0.14 mass% or more, which is the high
concentration where the state of the coating bath lOB becomes the top-dross formation
range under the condition ofthe bath temperature T2, so that the top-dross only forms
15 and the bottom-dross hardly forms. Thus, the top-dross which precipitates in the
coating bath lOB ofthe separating tub 2 rises to top surface of the coating bath lOB of
the separating tub 2 by the difference in specific gravity compared with the coating bath
lOB (molten zinc bath), and the dross is separated and removed. In addition, the Fe
concentration ofthe coating bath lOB at the outlet of the separating tub 2 is slightly
20 higher concentration than the saturation point ofthe Fe concentration, because the
residual dross with small size which is not completely separated in the separating tub 2 is
contained.
[0102]
Since the capacity Q2 of the separating tub 2 is sufficiently large as compared
25 with the circulating volume q ofthe bath and the stagnation time ofthe coating bath in
• 49
the separating tub 2 is 2 hours or more, most ofthe top-dross is separated by the flotation
and removed outside the system. Moreover, in order to control the Al concentration A2
ofthe bath in the separating tub 2 to be, for example, 0.14 mass% or more, small amount
ofthe metal with high AI concentration (fIrst zinc-included-metal) which includes Al
5 with the concentration higher than the AI concentration A1 ofthe bath in the coating tub
I is supplied and melted in the separating tub 2.
[0103]
(3) Dissolving process of dross and adjusting process of bath temperature and AI
concentration at the adjusting tub 3
10 Furthermore, the circulation bath which flows out from the separating tub 2 is
led to the adjusting tub 3. In the adjusting tub 3, the bath temperature T3 of the
adjusting tub 3 is kept at the temperature which is higher 5°C or more as compared with
the bath temperature T2 ofthe separating tub 2, and the AI concentration A3 of the
adjusting tub 3 is controlled to be higher than the AI concentration A1 ofthe coating tub
15 1 and lower than the AI concentrationA2 of the separating tub 2. In the adjusting tub 3,
the dross which is contained in the coating bath lOC is dissolved by controlling Fe ofthe
coating bath 10C to be the unsaturated state. Thereby, it is possible that the top-dross
with small size (residual dross) which cannot be separated in the separating tub 2 is
dissolved and removed in the coating bath 10C in which Fe is the unsaturated state.
20 [0104]
For example, as shown in FIG. 10, when the coating bath lOB in which the
top-dross is separated in the separating tub 2 is transferred to the adjusting tub 3, the bath
temperature T increases drastically from T2 (440°C) to T3 (465°C), and the AI
concentration decreases from A2 (approximately 0.148 mass%) to A3 (approximately
25 0.143 mass%). As the results, Fe becomes exceedingly the unsaturated state in the
50
• coating bath 10C of the adjusting tub 3, so that the top-dross (Fe2AIs) with small size
which is residual in the bath is decomposed (dissolved) into Fe and Al relatively
promptly and disappears. In this way, in case of dissolving the residual dross, the
coating bath 10C ofthe adjusting tub 3 is still the state in which Fe is unsaturated.
5 [0105]
In addition, the metal (second zinc-included-metal) which is to supply the
molten metal which is consumed at the coating tub 1 is supplied and melted in the
coating bath 10C ofthe adjusting tub 3. In case that the dross which forms by melting
the metal causes the problem, as shown in FIG 8, the premelting tub 4 may be installed
10 ·beside the adjusting tub 3, and the molten metal which is melted in the premelting tub 4
may be supplied to the adjusting tub 3. Moreover, since the metal with high Al
concentration is supplied to the separating tub 2, the AI concentration ofthe circulation
bath becomes excessive high concentration. Thus, the metal which is supplied to the
adjusting tub 3 is the zinc-included-metal with low AI concentration or the
15 zinc-included-metal which does not include AI. By supplying the metal with low AI
concentration, the AI concentration A3 ofthe bath in the adjusting tub 3 decreases to be
lower than the AI concentration A2 ofthe separating tub 2 and is controlled to the
concentration which is suitable to keep constantly the AI concentration Al of the coating
tub 1.
20 [0106]
Thereafter, the coating bath 10C ofthe adjusting tub 3 in which the dross is
almost not contained and Fe is the unsaturated state is led to the coating tub 1 and is
utilized for the coating process as described in above (1). While the coating bath 10C is
transferred from the adjusting tub 3 to the coating tub 1, the bath temperature T decreases
25 naturally by the difference ofthe bath temperature decrease ~Tfall as described above.
• 51
In the coating bath 10C which is transferred from the adjusting tub 3 to the coating tub 1,
the dross is almost not contained and Fe is the unsaturated state. However, since Fe is
dissolved in the coating bath lOA from the steel sheet 11 which is dipped in the coating
tub 1, the Fe concentration of the bath reaches gradually approximately 0.03 mass%
5 which is the saturation point at the bath temperature Tl (460°C). Moreover, in the
coating tub 1, AI is consumed by reacting the steel sheet 11 and the coating bath lOA.
Thus, even if the coating bath 10C with relatively high Al concentration A3
(approximately 0.143 mass%) is transferred from the adjusting tub 3 to the coating tub 1,
the AI concentration Al ofthe coating tub 1 hardly increases and keep at nearly constant
10 value (approximately 0.135 mass%).
[0107]
Moreover, the coating tub 1 is miniaturized as mentioned above, and the
stagnation time of the circulating coating bath lOin the coating tub 1 is short. Thus,
even if the operational fluctuation such as the bath temperature fluctuation occurs to a
15 certain extent in the coating tub 1, neither the top-dross nor the bottom-dross is formed in
the coating tub 1 until the Fe concentration of the coating bath lOA reaches the saturation
point (for example, 0.03 mass%). Moreover, even if the Fe concentration ofthe bath in
the coating tub 1 reaches the saturation point and the dross with small size forms, the
formed dross does not grow up to the harmful size (for example, 50 f..Llll or more) during
20 the short stagnation time (for example, several hours) in the coating tub 1, because the
dross hardly grows under the condition where the bath temperature is constant (refer to
FIG. 2.). The dross with small size which forms in the coating tub 1 is transferred to the
separating tub 2 before the dross grows up to the harmful size, and is removed by the
flotation separation.
25 [0108]
•
5
52
Moreover, the Fe concentration of the coating bath lOA in the coating tub 1
varies depending on, for example, the capacity Q1 ofthe coating tub 1, the circulating
volumes q, dissolvability ofFe, and the like. Thus, Fe ofthe coating bath lOA can be
the unsaturated state (in case that the Fe concentration is less than 0.03 mass%). In the
case, since Fe is unsaturated, the dross hardly forms. Contrary, Fe ofthe coating bath
lOA also can be slightly the supersaturated state (in case that the Fe concentration is
slightly more than 0.03 mass%). In the case, since the dross which forms in the coating
bath lOA within short time is the small size, the problem such as the dross defects does
not occur.
10 [0109]
As explained above, by circulating the coating bath lOin order ofthe coating
tub 1, the separating tub 2, and the adjusting tub 3, it is possible that the dross which
forms inevitably in the coating bath during the manufacture ofthe galvannealed steel
sheet is removed and is almost-completely rendered harmless. Therefore, the coating
15 bath lOA ofthe coating tub 1 can be continuously controlled to the dross-free state.
Moreover, the problems such as the appearance deterioration ofthe surface ofthe steel
sheet caused by the dross adhesion, surface defects caused by the dross, the roll-slipping
caused by the dross precipitation on the surface ofthe roll in the coating bath, and the
like are solvable. When performing the dross removal by using the manufacturing
20 equipment according to the embodiment, it is unnecessary to stop the sheet threading of
the coated steel sheets. The coating bath lOis circulated in order ofthe coating tub 1,
the separating tub 2, and the adjusting tub 3 with the sheet threading. Namely, the dross
is removed by not the batch processing but the consecutive processing. Therefore, the
coating bath lOA of the coating tub 1 can be continuously controlled to the dross-free and
25 clean state.
•
5
53
[0110]
Next, in reference to FIG 10, method of adjusting the AI concentration ofthe
coating bath 10 by supplying the metal to the coating bath 10 which is circulated between
the tubs will be described.
[0111]
The AI concentration in the coating layer ofthe steel sheet 11 is, for example,
;.,...
0.3 mass% on average, and is higher than the AI concentration Al (0.135 mass%) of
coating bath lOA in the coating tub 1. Namely, AI ofthe coating bath lOA is
concentrated and coated to the coating layer ofthe steel sheet 11. Therefore, if the AI
10 concentration ofthe metal which is supplied to the coating bath 10 is 0.135 mass%, the
AI concentration ofthe coating bath lOA decreases gradually. Thus, in the conventional
supply of the metal which is spot-like, Al concentration is maintained by supplying the
metal with Al concentration of 0.3 to 0.5 mass% directly to the coating tub.
[0112]
15 In the hot-dip-coating equipment according to the embodiment, the coating bath
lOis continuously transferred from the adjusting tub 3 to the coating tub 1. In order to
control the AI concentration Al ofthe coating tub 1 to be 0.135 mass% for example, it is
necessary to keep supplying the coating bath lOin which the AI concentration is higher
than 0.135 mass% (for example, 0.143 mass%) to the coating tub 1 from the adjusting
20 tub 3. Thus, in order to control the Al concentration A3 ofthe adjusting tub 3 to be
approximately 0.143 mass% which is the target, the AI concentrationA2 ofthe separating
tub 2 is kept at high concentration (for example, 0.148 mass%) which is higher thanA3
by supplying intentionally Al to the separating tub 2. Moreover, in the separating tub 2,
in order that the large amount ofthe top-dross is precipitated and separated by the
25 flotation, it is preferable that the Al concentration A2 ofthe bath in the separating tub 2 is
5
• 54
controlled to high concentration. Therefore, the metal with high Al concentration (for
example, 10 mass% AI- 90 mass% Zn) as the first zinc-included-metal is supplied into
the separating tub 2, and the Al concentration A2 of the coating bath lOB in the
separating tub 2 is controlled to high. Here, the amount ofAI supplied to the separating
tub 2 is equivalent to the total ofthe amount ofAI consumed as the top-dross at the
separating tub 2 and the amount ofAI consumed as the coating layer ofthe steel sheet 11
at the coating tub 1.
[0113]
On the other hand, in the adjusting tub 3, the metal with low Al concentration
10 and high Zn concentration (for example, the zinc-included-metal which is 0.1 mass% AIZn
or the zinc-included-metal which does not contain AI) as the second
zinc-included-metal is supplied. Thereby, the AI concentration ofthe coating bath lOB
transferred from the separating tub 2 to the adjusting tub 3 decreases, and the Al
concentration A30fthe coating bath 10C in the adjusting tub 3 is controlled to
15 approximately the Al concentration (for example, 0.143 mass%) which is intermediate
value of the Al concentrationA2 of the separating tub 2 and the Al concentration Al of
the coating tub 1. By transferring the coating bath 10C from the adjusting tub 3to the
coating tub 1, the Al concentration A1 ofthe bath in the coating tub 1 can be controlled
to the proper concentration (for example, 0.135 mass%) which is suitable for
20 manufacturing the GA.
[0114]
As described above, in the hot-dip-coating equipment according to the
embodiment, the supply ofthe coating bath and the composition of the coating bath, for
example, the AI concentration, are controlled by supplying the metal to the separating tub
25 2 and the adjusting tub 3. Therefore, it is not necessary to supply the metal directly to
•
5
55
the coating tub 1, so that it is possible to prevent the dross from fonning by the change of
the bath temperature around the metal.
[0115]
[4. Technical meaning of installing the separating tub and the adjusting tub]
Next, the technical meaning of controlling the Al concentration ofthe circulation
bath in addition to the bath temperature T ofthe circulation bath by installing the two
tubs which are the separating tub and the adjusting tub besides the coating tub 1 in the
hot-dip-coating equipment according to the embodiment will be described in detail.
[0116]
10 As mentioned above, in the embodiment, the precipitation and the flotation
separation ofthe top-dross in the bath are promoted by increasing the Al concentration
A2 ofthe bath in the separating tub 2, and the AI concentration ofthe coating bath which
is returned to the coating tub 1 is controlled to the proper concentration by decreasing the
AI concentration A3 of the bath in the adjusting tub 3. In this way, even if the GA is
15 manufactured by using the GA bath (AI concentration: 0.125 to 0.14 mass%) in which
the Al concentration ofthe bath is low as compared with the GI bath, it is possible to
keep the Al concentrationA1 ofthe bath in the coating tub 1 at the intended low
concentration and to increase the AI concentration A3 ofthe separating tub 2 to the high
concentration (for example, 0.147 mass% or more) which is needed for precipitating the
20 top-dross by controlling properly the AI concentration of the circulation bath. Therefore,
it is possible that the top-dross is only precipitated without precipitating the bottom-dross
and is suitably separated by the flotation in the separating tub 2. Namely, since the
bottom-dross is not contained in the circulation bath, it is possible to prevent the
occurrence ofthe dross defects which is caused by the flow back ofthe bottom-dross to
25 the coating tub 1. The principle will be described in detail.
• 56
[0117]
[4.1. Condition ofAl concentrationA2 ofthe bath in the separating tub 2]
First, in reference to FIG 11, the condition in which the precipitated dross is to
be the top-dross in the separating tub 2 (especially the condition ofAl concentration A2
5 of the bath in the separating tub 2) will be described. FIG 11 is the ternary phase
diagram which indicates the state ofthe GA bath according to the embodiment.
[0118]
As shown in FIG 11, the state (bath temperature and composition) ofthe coating
bath is classified into a bottom-dross formation range, a bottom-dross and top-dross
10 mixed range (hereinafter, referred to as "mixed range"), and a top-dross formation range.
In case that the Fe concentration and the bath temperature T ofthe coating bath are
constant, the state of the coating bath transitions in order ofthe bottom-dross formation
range, the mixed range, and the top-dross formation range with the increase in the AI
concentration ofthe bath.
15 [0119]
Here, it is assumed that the state ofthe coating bath lOA (GA bath) in the
coating tub 1 is the state 81 (bath temperature Tl: 460°C, the Fe concentration: 0.03
mass%, AI concentration AI: 0.13 mass%) as shown in FIG 11. In this case, by
transferring the coating bath lOA ofthe state 81 to the separating tub 2, by increasing the
20 Al concentration A2 ofthe bath in the separating tub 2, and by decreasing the bath
temperature T2, the dross which includes the top-dross precipitates in the separating tub
2. However, since the bath state transitions to the mixed range unless the AI
concentration A2 of the bath in the separating tub increases sufficiently, the top-dross and
the bottom-dross are formed and mixed. On the other hand, if the Al concentration A2
25 ofthe bath in the separating tub 2 increases sufficiently so that the bath state becomes the
•
15
57
top-dross fonnation range, only the top-dross fonns and the bottom-dross hardly fonns.
[0120]
When the bottom-dross and the top-dross are fonned and mixed because the AI
concentration A2 of the bath in the separating tub 2 is insufficient, the top-dross can be
5 removed by the flotation separation with comparative ease. However, since the
difference in specific gravity between the bottom-dross and the molten metal is small, the
bottom-dross cannot be effectively separated by the difference in specific gravity. Thus,
the bottom-dross flows in the coating bath ofthe separating tub 2 with the coating bath
flow in the separating tub 2, so that the Fe concentration ofthe separating tub 2 does not
10 decrease. Moreover, the bottom-dross fonned in the separating tub 2 may flow back to
the adjusting tub 3 and further the coating tub 1 with the coating bath flow. Thus, in
order to separate the dross effectively, it is preferable that all the precipitated dross is to
be the top-dross without precipitating the bottom-dross by increasing sufficiently the Al
concentration A2 of the bath to high concentration at the separating tub 2.
[0121]
As the results of the investigation in regard to the condition where all the
precipitated dross is to be the top-dross in the separating tub 2 by using the phase
diagram as showing in FIG 11, the following conclusion was obtained.
[0122]
20 For example, it is assumed that the GA bath in the coating tub 1 is the state 81
(AI concentration Al of the bath: 0.13 mass%, bath temperature Tl: 460°C) as shown in
81 to 85 of FIG 11. The conditions where the bath state becomes the top-dross
fonnation range when the GA bath is transferred to the separating tub 2 of the bath
. temperature T2 need to be as follows. (1) In case that the bath temperature T2 of the
25 separating tub 2 is 450°C, the Al concentration A2 of the bath in the separating tub 2 is to
•
20
58
be 0.147 mass% or more (state 83). (2) In case that the bath temperature T2 is 440°C,
the Al concentrationA2 ofthe bath is to be 0.154 mass% or more (state 85).
[0123]
Moreover, it is assumed that the GA bath in the coating tub 1 is the state 86 (AI
5 concentration Al ofthe bath: 0.14 mass%, bath temperature Tl: 460°C) as shown in 86
to 89 of FIG 11. 8imilarly, The conditions where the bath state becomes the top-dross
formation range need to be as follows. (1) In case that the bath temperature T2 of the
separating tub 2 is 450°C, the Al concentration A2 ofthe bath in the separating tub 2 is to
be 0.143 mass% or more (state 87). (2) In case that the bath temperature T2 is 440°C,
10 the AI concentrationA2 ofthe bath is to be 0.15 mass% or more (state 89).
[0124]
FIG 12 is a graph which summarizes the conditions ofthe Al concentration A2
ofthe bath in the separating tub 2 and which indicates the bath conditions where all the
precipitated dross is to be the top-dross in the separating tub 2. In FIG 12, the boundary
15 lines L1 and L2 indicate the lower limit ofthe AI concentration A2 ofthe bath to make
all the precipitated dross be the top-dross depending on the bath temperature T2 ofthe
separating tub 2. L1 is the boundary line in case that the AI concentration Al ofthe GA
bath is 0.13 mass% and L2 is the boundary line in case that the Al concentration A1 of
the GA bath is 0.14 mass%.
[0125]
As shown in FIG 12, when the AI concentration Al ofthe bath in the coating
tub 1 is 0.13 mass% and the bath state (bath temperature T2, AI concentration A2) ofthe
separating tub 2 belongs to the area which is the upper right side ofthe line L1
connecting four points, 82, 83, 84, and 85, the AI concentrationA2 ofthe bath is higher
25 than the lower limit and the bath state becomes the top-dross formation range, so that
59
only the top-dross precipitates in the separating tub 2. In addition, similarly, when the
AI concentration A1 ofthe bath in the coating tub 1 is 0.14 mass% and the bath state of
the separating tub 2 belongs to the area which is the upper right side ofthe line L2
connecting three points, 87, 88, and 89, the bath state becomes the top-dross formation
5 range, so that only the top-dross precipitates in the separating tub 2.
[0126]
As mentioned above, the conditions of the AI concentration A2 ofthe bath
where all the precipitated dross is to be the top-dross in the separating tub 2 are
determined by the state (AI concentration AI, Fe concentration) ofthe GA bath ofthe
10 coating tub 1 and the bath temperature T2 ofthe separating tub 2. Thus, by increasing
the AI concentration A2 ofthe bath in the separating tub 2 to the high concentration in
accordance with the bath state ofthe coating tub 1 and the bath temperature T2 ofthe
separating tub 2, it is possible that the bath state ofthe separating tub 2 transitions from
the bottom-dross formation range or the mixed range to the top-dross formation range
15 and'that the top-dross is only precipitated in the separating tub 2.
[0127]
[4.2. Necessity of adjusting tub]
As mentioned above, the contribution to precipitate only the top-dross in the
separating tub 2 increases with the increase in the AI concentration A2 ofthe bath in the
20 separating tub 2. However, if the Al concentration A2 ofthe bath in the separating tub 2
increases excessively, the coating bath with high Al concentration flows back to the
coating tub 1. And, if the circulation ofthe coating bath is continued, the AI
concentration Al of the bath in the coating tub 1 increases gradually and is out of the
intended concentration which is suitable for the GA bath. Therefore, in the embodiment,
25 the adjusting tub 3 is installed between the separating tub 2 and the coating tub 1, the
• 60
coating bath lOB with high Al concentration A2 which is transferred from the separating
,
tub 2 is diluted to the suitable AI concentration in the adjusting tub 3, and the coating
bath is transferred to the coating tub 1. By the functions ofthe adjusting tub 3, it is
possible to keep the AI concentration Al ofthe bath in the coating tub 1 at the constant
5 concentration which is suitable for the GA bath and to increase the Al concentration A2
ofthe separating tub 2 to the high concentration.
[0128]
By the way, in the embodiment, the GA bath with low AI concentration ofthe
bath as compared with the GI bath is targeted, so that the necessity of installing the
10 adjusting tub 3 which readjusts the AI concentration of the coating bath increases. The
reason will be described below.
[0129]
Since the Al concentration Al ofthe bath in the coating tub 1 is 0.15 to 0.25
mass% in case that the GI is manufactured by using the GI bath unlike the embodiment,
15 the Al concentration ofthe circulation bath and the AI concentration A2 of the bath in the
separating tub 2 also become at least 0.15 mass% or more consequently. Therefore, the
bath state ofthe GI bath in the separating tub 2 always becomes the top-dross formation
range (refer to FIG. 1). It is possible that the top-dross is precipitated and separated by
the flotation at the tub surface by decreasing the bath temperature T2 to be lower than the
20 bath temperature Tl if the ordinary metal is supplied in the separating tub 2. Therefore,
in case of GI bath, it is not necessary to install the adjusting tub 3 for readjusting the bath
composition.
[0130]
On the other hand, in case that the GA is manufactured by using the GA bath by
25 the method according to the embodiment, it is necessary that the AI concentration Al of
•
5
61
the bath in the coating tub 1 is controlled to be 0.125 to 0.14 mass% which is relative low
concentration in order to ensure the alloying speed at the coating layer ofthe steel sheet
11. Thus, if the AI concentration A2 ofthe bath is not sufficiently high, the bath state of
the GA bath in the separating tub 2 may become the bottom-dross formation range or the
mixed range, so that the risk such that the bottom-dross is precipitated may arise.
[0131]
Therefore, in case ofthe GA bath, it is necessary that the Al concentration A2 of
the bath in the separating tub 2 is increased to the targeted concentration in order to
precipitate only the top-dross in the separating tub 2. For example, as shown in FIG 11
10 and FIG 12, in case that the AI concentration ofthe GA bath is 0.13 mass% and that the
dross is precipitated by decreasing the bath temperature T2 to 450°C in the separating tub
2, it is possible that only the top-dross is precipitated without precipitating the
bottom-dross, only if the AI concentration A2 of the bath in the separating tub 2 is 0.147
mass% or more (requirement 1).
15 [0132]
25
However, when the Al concentration A2 ofthe bath in the separating tub 2 is
excessively high, the amount ofAI in the coating bath which flows back from the
separating tub 2 to the coating tub 1 exceeds excessively the AI consumption in the
coating tub 1. As the result, the Al concentration A1 of the bath in the coating tub 1
20 increases and is outofthe intended concentration. Therefore, in order to control the Al
concentration Al of the bath in the coating tub 1 to the constant concentration which is
suitable for the GA bath, it is necessary to suppress the Al concentration A2 of the bath
transferred from the separating tub 2 to the relatively low concentration in accordance
with the circulating volume q of the bath (requirement 2).
[0133]
• 62
Accordingly, in order to satisfy both the requirements 1 and 2 which are contrary
to each other, the inventors investigate the suitable operational conditions by calculating
the achievable AI concentration A2 ofthe bath in the separating tub 2 under the general
conditions ofthe galvannealed operation. As the result, in case that the operation is .
5 conducted only by the separating tub 2 without installing the adjusting tub 3, it becomes
clear that both the requirements 1 and 2 are not satisfied and the effective GA operation
is not conducted.
[0134]
For example, in the following operational condition A, in case that the adjusting
10 tub 3 is not installed, the Al concentration A2 ofthe bath in the separating tub 2 can only .
increase to 0.145 mass% when the circulating volume q of the bath is 10 ton / hour and
can only increase to 0.140 mass% when the circulating volume q ofthe bath is 15 ton /
hour, by the restriction ofthe requirement 2. Thus, since the AI concentration A2 of the
bath in the separating tub 2 becomes less than 0.147 mass% which is the lower limit
15 required for precipitating only the top-dross, the bottom-dross forms in the separating tub
2. Moreover, when the circulating volume q ofthe bath is excessively small such as 6
ton / hour, the AI concentration A2 ofthe bath in the separating tub 2 becomes 0.155
mass%, which is higher than 0.147 mass% ofthe lower limit. However, the circulating
volume q ofthe bath is excessively small, so that the replacement ofthe coating bath 10A
20 in the coating tub 1 needs time. For example, when the capacity ofthe coating tub 1 is
40 ton, the replacement needs 6.6 hours on average. Therefore, the problem such that
the bottom-dross forms in the coating bath 10A which is stagnated in the coating tub 1
occurs.
25

Metal consumption in the coating tub 1 : 900 kg / m2
•
5
63
Sheet width of the steel sheet 11 : 900 mm
Coating rate : 150 m / min
Bath temperature Tl ofthe coating tub 1 : 460°C
Bath temperature T2 ofthe separating tub 2: 450°C
AI concentration AI ofthe bath in the coating tub 1 : 0.130 mass%
Circulating volume q of bath : 6 ton / hour, 10 ton / hour, and 15 ton / hour
[0135]
In addition, in the following operational condition B, in case that the adjusting
tub 3 is not installed, the AI concentration A2 ofthe bath in the separating tub 2 can only
10 increase to 0.136 to 0.144 mass% when the circulating volume q ofthe bath is any of6
ton / hour, 8 ton / hour, 10 ton / hour, and 15 ton / hour, by the restriction of the
requirement 2. Thus, since the AI concentration A2 ofthe bath in the separating tub 2
becomes less than 0.147 mass% which is the lower limit required for precipitating only
the top-dross, the bottom-dross forms in the separating tub 2.
15
Metal consumption in the coating tub I : 500 kg / m2
Sheet width ofthe steel sheet 11 : 700 mm
Coating rate : 120 m / min
Bath temperature Tl of the coating tub 1 : 460°C
20 Bath temperature T2 ofthe separating tub 2: 450°C
AI concentration Al ofthe bath in the coating tub 1 : 0.130 mass%
Circulating volume q of bath: 6 ton / hour, 8 ton / hour, 10 ton / hour, and 15 ton
/hour
[0136]
25 As mentioned above, in case that the GA bath with low AI concentration as
15
64
• compared with the GI bath is used, if the adjusting tub 3 is not installed, the AI
concentration A2 of the bath in the separating tub 2 cannot be increased sufficiently by
the restriction ofthe requirement 2, so that the requirement 1 cannot be not satisfied.
Thus, the method in which the adjusting tub 3 is not installed has the major problem of
5 applicability to the effective GA operation, so that the method cannot be applied to the
operation ofthe GA bath.
[0137]
On the other hand, in the method according to the embodiment in which the
adjusting tub 3 is installed, it is possible that the AI concentration A3 ofthe coating bath
lOin which the AI concentration increases at the separating tub 2 is [mally adjusted at the
adjusting tub 3. For example, it is possible that the Al concentration A2 which increases
excessively as the separating tub 2 decreases to the AI concentration A3 which is suitable
for return to the coating tub 1.
[0138]
For example, in the operational condition A, it is possible that (1) the Al
concentration A2 of the separating tub 2 increases to 0.182 mass% when the circulating
volume q ofthe bath is 6 ton / hour, (2) the AI concentration A2 ofthe separating tub 2
increases to 0.159 mass% when the circulating volume q ofthe bath is 10 ton / hour, and
(3) the Al concentration A2 of the separating tub 2 increases to 0.149 mass% when the
20 circulating volume q ofthe bath is 15 ton / hour. Namely, the AI concentration A2 of
the separating tub 2 can be controlled to the concentration sufficiently higher than 0.147
mass% which is the lower limit in consideration ofthe requirement 1. Moreover, in the
operational condition B, it is possible that the AI concentration A2 ofthe separating tub 2
increases to 0.157 mass% when the circulating volume q of the bath is 6 ton / hour and
25 the Al concentration A2 ofthe separating tub 2 increases to 0.150 mass% when the
• 65
circulating volume q ofthe bath is 8 ton / hour. Namely, the Al concentrationA2 ofthe
separating tub 2 can be controlled to the concentration sufficiently higher than 0.147
mass% which is the lower limit in consideration ofthe requirement 1.
[0139]
5 As mentioned above, by installing the adjusting tub 3 according to the
embodiment, it is possible that the AI concentration A3 ofthe coating bath 10C decreases
by supplying the second zinc-included-metal (zinc-included-metal with low AI
concentration or zinc-included-metal which does not include AI) to the adjusting tub 3.
Thereby, it is possible that the Al concentration A2 ofthe bath in the separating tub 2
10 increases sufficiently by supplying the zinc-included-metal with high Al concentration to
the separating tub 2. For example, even if the AI concentrationA2 ofthe bath in the
separating tub 2 increases to the high concentration (for example, 0.159 mass%), it is
possible that the Al concentration A3 of the bath decreases to the low concentration (for
example, 0.145 mass%) by readjusting the concentration ofthe coating bath 10C at
15 adjusting tub 3. As the result, by returning the coating bath 10C of the adjusting tub 3
to the coating tub 1, the AI concentration A1 ofthe bath in the coating tub 1 can be
continuously controlled to the constant concentration (for example, 0.13 mass%).
[0140]
As mentioned above, by installing the adjusting tub 3, the effect such that the
20 top-dross is precipitated and separated by the flotation at the separating tub 2 can be
obtained under almost all the GA operational conditions. Moreover, by controlling the
bath temperature T3 ofthe adjusting tub 3 to be higher than the bath temperature T2 of
the separating tub 2, the increase in the solubility limit ofFe, the securement of the
unsaturated degree ofFe, and thereby the acceleration of dissolving the residual dross in
25 the coating bath 10C are effectively performed, so that the combined effect such that the
66
• dross-free is stably achieved is obtained.
[0141]
[4.3. Control of circulating volume of bath in accordance with increase and
decrease in AI concentration of bath in the coating tub]
5 As mentioned above, the operational condition which satisfies both the
requirement 1 and the requirement 2 varies depending on the AI concentration A1 of the
bath in the coating tub 1 and the circulating volume q ofthe bath. Therefore, by
controlling the circulating volume q ofthe bath in accordance with the increase or
decrease in the Al concentrationA1 ofthe bath in the coating tub 1, the AI concentration
10 A2 ofthe bath in the separating tub 2 can be kept to the intended high concentration, and
both the requirement 1 and the requirement 2 can be satisfied.
[0142]
Namely, since the AI consumption per unit time by the coating processing at the
coating tub 1 is constant, there is the restriction such that the AI concentration A2 of the
15 bath in the separating tub 2 cannot increase when the circulating volume q ofthe bath is
large. Therefore, if the operational condition ofthe operation needs to be changed from
the bath state where the AI concentrationA1 ofthe bath in the coating tub 1 is high to the
bath state where the Al concentration is low (for example, in case that the GA is
manufactured by the GA bath with low AI concentration such that the Al concentration is
20 0.125 to 0.13 mass%), the circulating volume q ofthe GA bath may be decreased.
Thereby, Since the volume ofthe GA bath which returns from the adjusting tub 3 to the
coating tub 1 reduces per unit time, it is possible to control the AI concentration of the
GA bath to be the high concentration as compared with that before changing the
operational condition. Therefore, it is possible to keep the AI concentration A2 ofthe
25 bath in the separating tub 2 at the high concentration and to control the bath state of the
67
separating tub 2 to be the top-dross fonnation range.
[0143]
For example, it is known that additive elements such as silicon and manganese
are added to steel when the high tensile steel is manufactured in order to improve the
5 strength and that the alloying speed ofthe GA decreases drastically when the large
amount ofthe additive elements are added. In order to avoid the above situation, the AI
concentration Al ofthe bath in the coating tub 1 may be decreased. For example, when
the operation is conducted under the condition that the Al concentration A1 of the bath in
the coating tub 1 is 0.14 mass%, alloying in the coating layer of the steel sheet 11
10 becomes easier by decreasing Al to 0.13 mass%.
[0144]
Thus, in case that the Al concentration A1 ofthe bath in the coating tub 1 needs
to be decreased for changing the operational condition, the circulating volume q of the
bath may be decreased as compared with that before changing the operational condition
15 in order to precipitate only the top-dross in the separating tub 2. Since the amount ofAI
which is supplied to the coating tub 1 decreases per unit time by decreasing the
circulating volume q ofthe bath, Since the Al quantity supplied per unit time at the
coating tub 1 is reduced by the fall ofthis circulating volume ofbath q, the balance ofthe
consumption and the supply ofAl in the coating tub 1 can be maintained. Namely, even
20 if the AI concentration A2 ofthe bath in the separating tub 2 is kept at the high
concentration which is higher than the lower limit in consideration of the requirement 1,
the AI concentration A1 of the bath in the coating tub 1 does not increase, so that both the
requirement 1 and the requirement 2 can be satisfied. Therefore, it is possible that the
operation is conducted by using the GA bath whose composition is changed in the
25 coating tub 1 and that the top-dross is only precipitated and separated by the flotation in
• 68
the separating tub 2.
[0145]
On the other hand, in case that the Al concentration Al ofthe bath in the coating
tub 1 needs to be increased for changing the operational condition, the circulating volume
5 q of the bath may be increased to the volume which is suitable for the Al concentration
A1 ofthe bath after the increase. Thereby, the balance ofthe consumption and the
supply ofAI in the coating tub 1 are maintained, so that both the requirement 1 and the
requirement 2 can be satisfied.
[0146]
lOInaddition, it is possible to control the circulating volume q of the bath by
controlling the transferring volume per unit time by using the molten metal transfer
apparatus 5 ofthe circulator. The circulating volume q which is suitable for the Al
concentrationA1 ofthe bath in the coating tub 1 may be obtained by the prior experiment
or calculation.
15 [0147]
[4.4. Conclusion]
The above knowledge can be newly obtained by analyzing the ternary phase
diagram of Fe-zinc-aluminum and the temperature dependence thereof, by considering
the actual GA operational condition, the situation ofthe dross defects, and the cause
20 thereof, and by understanding the phenomenon ofthe dross formation, the dross growth,
and the dross disappearance in detail. Therefore, the technical feature, which combines
the conditions (bath temperature T2, Al concentration A2) ofthe separating tub 2 and the
conditions (adjustment ofthe bath temperature T3 and the AI concentrationA3) of the
adjusting tub 3 in order to obtain the coating bath which does not contain the harmful
25 dross, cannot be absolutely obtained only from the publically-known techniques which
• 69
are disclosed in the Patent Documents 1 to 5.
[0148]
20
In the above, the manufacturing equipment and the manufacturing method of the
galvannealed steel sheet according to the embodiment were described in detail.
5 According to the embodiment, it is possible that the dross which forms inevitably during
manufacturing the hot dip zinc-aluminum coated steel sheets is removed efficiently and
effectively at the separating tub 2 and the adjusting tub 3 and is almost-completely
rendered harmless. Thereby, the present situation such that the sheet threading speed
(coating rate) of the steel sheet 11 is suppressed and the productivity has to be sacrificed
lOin order to prevent the dross from rising in the coating bath lOis improved, so that the
coating rate can be increased and the productivity ofthe galvannealed steel sheets is
improved.
Example
15 [0149]
[5. Example]
Hereinafter, the examples ofthe present invention will be described. The
following examples only show the test result concretely for the verification ofthe effect
of the present invention, so that the present invention is not limited to the examples.
[0150]
[5.1. Test 1 : Coating test ofthe galvannealed steel sheet (GA)]
The circulation-type hot-dip-coating equipment (correspond to the
hot-dip-coating equipment according to the above described embodiment) was installed
in the pilot line, the continuous coating tests which manufactures the galvannealed steel
25 sheet (GA) were conducted. The test conditions ofthe continuous coating test are
• 70
shown in Table 2. In addition, as comparative examples, the similar tests were
conducted by using the conventional hot-dip-coating equipment which had only the
coating tub. Here, li.TI-2 in Table 2 is the bath temperature difference between the bath
temperature TI ofthe coating tub 1 and the bath temperature T2 ofthe separating tub 2
5 (=TI-T2).
[0151]
(1) Conventional hot-dip-coating equipment
Capacity Q1 ofcoating tub : 60 ton
(2) Circulation-type hot-dip-coating equipment
10 Capacity Q1 ofcoating tub : 10 ton, 20 ton, and 40 ton
Capacity Q2 of separating tub : 40 ton and 12 ton
Capacity Q3 of adjusting tub : 20 ton
Circulating volume q of bath : 10 ton / hour and 6 ton / hour
[0152]
15 By using the hot-dip-coating equipment, the continuous coating was conducted
for 12 hours under the condition where the intended coating weight was 100 g / m2(both
sides) and the coating rate was 100 m / min by using the coil with 0.6 rom in sheet
thickness and 1000 rom in sheet width. And the difference ofthe bath temperature
decrease li.Tfall at transferring the bath from the adjusting tub 3 to the coating tub 1 was 2
20 to 3°C.
The samples were taken by rapid-cooling the bath ofeach tub at beginning and
ending ofthe coating. The dross type which was contained in the bath and the dross
size and the number per unit observed area were investigated. The dross weight per unit
cubic volume (dross density) was obtained. After finishing the test, the bath ofthe
25 coating tub 1 was drained, and the existence ofthe sedimented dross was observed at the
• 71
bottom of the tub.
Moreover, the AI concentration and Fe concentration ofeach tub were measured
every 4 hours.
At the beginning ofthe coating, since Fe was the unsaturated state in each tub,
5 the dross hardly existed.
All tubs were the ceramic pot, and the induction heating was utilized as the
heating apparatus ofthe temperature controller of each tub. The control accuracy ofthe
bath temperature by the temperature controller of each tub was less than ±3°e. In
addition, the circulator ofthe circulation-type hot-dip-coating equipment was configured
10 by the metal pump for transferring the coating bath from the adjusting tub 3 to the
coating tub 1, by the overflow for transferring the coating bath from the coating tub 1 to
the separating tub 2, and by the communicating vessel 7 for transferring the coating bath
from the separating tub 2 to the adjusting tub 3.
In order to control the Al concentration ofthe bath in the separating tub 2 and
15 the adjusting tub 3, the metal of 10 mass% AI- Zn was supplied to the separating tub 2 in
general in at approximately even intervals. And the metal of 100 mass% Zn was
supplied to the adjusting tub 3 as necessary so as to make the bath surface level
approximately constant with visual observation. On the other hand, for the
conventional hot-dip-coating equipment, the alloyed metal was directly supplied to the
20 coating tub.
[0153]
The test results are shown in Table 3 and Table 4. Table 3 shows the Al
concentration and the Fe concentration ofthe coating tub, the separating tub, and the
adjusting tub as ofthe lapse of 12 hours, and Table 4 shows the density ofthe flowed
25 dross in the coating tub and the visual observed amount of the sedimented dross at the
•
15
72
bottom of the coating tub as ofthe lapse of 12 hours.
In addition, the targeted values ofthe dross density were quantitatively verified
by analyzing the coating bath which was sampled under the operational conditions where
the dross hardly became the problem because the sheet threading speed ofthe steel sheet
5 11 was relative low among the present operational conditions for the GA. Thereby,
"0.15 mg / cm3 or less" as the targeted value ofthe density ofthe top-dross and "0.60 mg
/ cm3 or less" as the targeted value ofthe density ofthe bottom-dross were obtained.
[0154]
[Table 2]
10 [0155]
[Table 3]
[0156]
[Table 4]
[0157]
From the test results as shown in Table 3 and Table 4, in examples 1 to 7, the
density ofthe dross was the targeted value or less, so that the effect of the dross removal
was confirmed. Especially, in examples 1 and 2, most ofthe dross was removed, so that
the dross-free was almost-completely achieved. In addition, in example 3, the
formation and growth ofthe bottom-dross were observed in the coating tub 1. The
20 reason seems that, since the capacity Ql ofthe coating tub 1 was approximately 6.7
times (= 40 / 6) of the circulating volume q ofthe bath per one hour which was higher
than 5 times ofthe criteria, the stagnation time ofthe coating bath in the coating tub 1
with the large size was prolonged, so that the dross formed and grew in the bath ofthe
coating tub in example 3. In addition, in example 4, the flow back of the top-dross to
25 the coating tub 1 was observed. The reason seems that, since the capacity Q2 of the
5
73
separating tub 2 was 1.2 times (= 12/10) ofthe circulating volume q ofthe bath per one
hour which was lower than 2 times ofthe criteria, the time for the flotation separation of
the dross was not sufficiently obtained at the separating tub 2, so that the dross separation
effect was inferior in example 4.
[0158]
On the other hand, in comparative example I, although the dross with large size
did not exist, the large amount ofthe bottom-dross and the top-dross with small size and
medium size existed. The reason seems that, since the bath temperature T2 ofthe
separating tub 2 equalized with the bath temperature TI ofthe coating tub I, the dross
10 removal effect decreased in the separating tub 2. In addition, in comparative example 2
ofthe conventional coating tub, the bottom-dross with large size was observed in
addition to the bottom-dross with small size and medium size, and the density of
top-dross was also high. The reason seems that, since the AI concentration ofthe
coating tub was near the dividing point ofthe top-dross formation range and the
15 bottom-dross formation range, both the bottom-dross and the top-dross were precipitated
by the operational fluctuation.
[0159]
As shown in Table 2, the bath temperature T2 of the separating tub 2 was 454°C
in example 5, 455°C in example 6, and 456°C in example 7, and thereby the bath
20 temperature difference L\TI-2 (=TI-T2) between the bath temperature TI (460°C) ofthe
coating tub 1 and the bath temperature T2 ofthe separating tub 2 was controlled to 6°C in
example 5, 5°C in example 6, and 4°C in example 7. From examples 5 to 7, the
influence ofthe bath temperature difference L\TI-2 on the dross formation was verified.
As the results, as shown in Table 4, in examples 1 to 6, since the bath temperature
25 difference L\TI-2 between the bath temperature TI ofthe coating tub 1 and the bath
10
74
• temperature T2 ofthe separating tub 2 was 5°C or more (Tl - T2 ~ 5°C), the density of
the flowed dross was notably low and the effect ofthe present invention was sufficiently
obtained. On the other hand, in example 7, since the bath temperature difference L\TI-2
was less than 5°C (Tl - T2 < 5°C) (for example, 4°C), the density ofthe flowed dross
5 was close to the upper limit which was the target, and the small amount of sedimented
dross was also formed. In other words, it was confirmed that, although the effect of the
present invention was obtained, the effect decreased in example 7. Therefore, it is
preferable that the bath temperature difference L\TI-2 between the bath temperature Tl of
the coating tub 1 and the bath temperature T2 ofthe separating tub 2 is 5°C or more.
[0160]
[5.2. Test 2 : Verification test of separation efficiency of bottom-dross and
top-dross]
Next, the results ofthe test to verify the separation efficiency ofthe
bottom-dross and the top-dross by using the separation by the difference in specific
15 gravity will be described.
[0161]
The specific gravity ofthe top-dross is 3900 to 4200 kg / m3
, and the specific
gravity ofthe bottom-dross is 7000 to 7200 kg / m3
•
By analyzing the results ofthe flow simulation which simulated the dross
20 separation by the flotation (sedimentation) under the condition where the separating tub 2
was 2.8 m in width x 3.5 m in length x 1.8 m in height (capacity 120 ton) and the
circulating volume of bath was 40 ton / hour, the results as shown in Table 5 were
obtained. Table 5 shows the efficiency ofthe separation by the difference in specific
gravity ofthe top-dross and the bottom-dross.
25 [0162]
15
75
[Table 5]
[0163]
From the test results as shown in Table 5, the separation efficiency ofthe
top-dross was higher than that of the bottom-dross in any case that the grain size was 50
5 J.1Ill, 30 J.1Ill, and 10 J.1Ill. Therefore, it is confirmed that the dross separation by the
difference in specific gravity is effective under the condition ofthe top-dross.
[0164]
[5.3. Test 3 : Verification test ofcapacity of separating tub]
Next, the results of the test to investigate, by using the flow analysis, the
10 capacity Q2 ofthe separating tub 2 which is required to separate effectively and
sufficiently the top-dross by the flotation at the separating tub 2 will be described. The
prerequisites ofthe analysis were as follows.
[0165]
Circulating volume of bath : 40 ton / hour
Capacity of separating tub : 20 to 160 ton
Size of top-dross : 30 J.1Ill
[0166]
The result of the analysis test is shown in FIG 13. As shown in FIG 13, when
the capacity Q2 ofthe separating tub 2 is 2 times or more ofthe circulating volume q (40
20 ton / hour) ofthe coating bath per one hour, the separation efficiency of the dross
becomes 80% or more. When the capacity Q2 ofthe separating tub 2 is less than 2
times of the circulating volume q ofthe bath, the separation efficiency ofthe dross
decreases drastically. From the result, it turns out that it is preferable that the capacity
Q2 ofthe separating tub 2 is 2 times or more ofthe circulating volume q ofthe bath ((Q2
25 / q) ~ 2).
•
10
76
[0167]
[5.4. Test 4: Verification test of capacity of coating tub]
Next, the results ofthe bath circulation test to investigate the stagnation time of
the coating bath 10A so that the dross which is formed in the coating bath 10A (GA bath)
5 of the coating tub 1 does not grow up to the harmful size by using the pilot line of the
galvannealing will be described. The test conditions were as follows.
[0168]
Criterial bath temperature Tl ofthe coating tub (intended bath temperature) :
AI concentration ofbath : 0.136 mass%
Fe concentration ofbath : Saturation (0.3 mass%)
Steel sheet: 0.6 mm in sheet thickness and 1000 mm in sheet width
Coating rate: 100 m / min
Coating weight: 100 g / m2 (both sides)
15 Bath temperature fluctuation: ±5°C (fluctuated intentionally by controlling the
heating output)
Capacity Q1 of coating tub : 60 ton
Circulating volume q of bath : 5 to 60 ton / hour
[0169]
20 After changing the circulating volume ofthe bath, the circulating volume q of
the bath was kept constant until the coating bath in the coating tub 1 was completely
replaced. Specifically, bath circulation was continued until the coating bath of 3 times
ofthe capacity Q1 ofthe coating tub 1 was circulated and finished.
The samples were taken from the coating bath which was overflowed from the
25 coating tub 1just before each level ofthe bath circulation test was finished, and the size
• 77
ofthe dross which existed in the bath was measured.
In addition, the bath temperature fluctuation ofthe coating tub 1 in the actual
operation is generally less than the test condition ofthis time which was ±5°C, and is
approximately ±3°C. However, in order to confirm the conditions to make the dross
5 harmless stably, the test was conducted under the condition where the dross tended to
form and grow as compared with the general condition.
[0170]
The result ofthe test is shown in FIG 14. As shown in FIG 14, when the
circulating volume q ofthe bath per one hour was less than 12 ton / hour (namely, the
10 capacity Ql ofthe coating tub 1 was more than 5 times of the circulating volume q ofthe
bath per one hour (Ql / q) > 5), the maximum size of the dross which was actually
observed was larger than the harmful size (50 J..lIIl). The reason seems that, since the
stagnation time ofthe coating bath in the coating tub 1 was prolonged, the dross notably
grew up to the harmful size. Contrary, when the circulating volume q ofthe bath per
15 one hour was 12 ton / hour or more (namely, the capacity Ql ofthe coating tub 1 was 5
times or less ofthe circulating volume q ofthe bath per one hour (Ql / q):::; 5), the dross
with small size (approximately 27 J..lIIl or less) which was sufficiently smaller than the
harmful size (50 J..lIIl) was only observed. The reason seems that, since the stagnation
time of the coating bath in the coating tub 1 was short, the dross did not grow up to the
20 harmful size. Therefore, it turns out that it is preferable that the capacity Q1 ofthe
coating tub 1 is 5 times or less of the circulating volume q of the bath per one hour.
[0171]
[5.5. Test 5 : Verification test ofproper range ofinflow bath temperature of
coating tub]
25 Next, the results ofthe test to verify the proper range ofthe bath temperature T3
5
78
ofthe coating bath 10C which flows into the coating tub 1 from the adjusting tub 3 will
be described. When the bath temperature T3 of the coating bath 10C which flows into
the coating tub 1 from the adjusting tub 3 deviates excessively from the bath temperature
TI ofthe coating tub 1, the bath temperature deviation in the coating tub 1 is promoted.
As the result, it seems that the formation and the growth ofthe dross in the coating tub 1
are accelerated. Thus, the verification test ofproper range ofthe bath temperature T3 of
the adjusting tub 3 was conducted by using the pilot line ofthe galvannealing. The test
conditions were as follows.
Criterial bath temperature Tl ofthe coating tub (intended bath temperature) :
Al concentration of bath : 0.136 mass%
Fe concentration of bath : Saturation (0.3 mass%)
Steel sheet: 0.6 mm in sheet thickness and 1000 mm in sheet width
Coating rate : 100 m / min
15 Coating weight : 100 g / m2 (both sides)
Bath temperature fluctuation: ±5°C (fluctuated intentionally by controlling the
heating output)
Capacity Q1 of coating tub : 60 ton
Circulating volume q of bath : 20 ton / hour
20 Inflow bath temperature (T3 - ~Tfall) : 445 to 480°C (~Tfall is the difference of
the bath temperature decrease and the bath temperature which decreases naturally when
the coating bath 10C is transferred from the adjusting tub 3 to the coating tub 1)
[0172]
After changing the inflow bath temperature, the circulating volume q ofthe bath
25 was kept constant until the coating bath in the coating tub 1 was completely replaced.
5
• 79
Specifically, bath circulation was continued until the coating bath of 3 times of the
capacity Q1 ofthe coating tub 1 was circulated and finished.
The samples were taken from the coating bath which was overflowed from the
coating tub 1 just before each level ofthe bath circulation test was finished, and the size
of the dross which existed in the bath was measured.
In addition, the bath temperature fluctuation of the coating tub 1 in the actual
operation is generally less than the test condition of this time which was ±5°C, and is
approximately ±3°C. However, in order to confirm the conditions to make the dross
harmless stably, the test was conducted under the condition where the dross tended to
10 form and grow as compared with the general condition.
[0173]
The result ofthe test is shown in FIG 15. As shown in FIG 15, when the bath
temperature deviation (T3 - ~Tfall - Tl : hereinafter, referred to as inflow bath
temperature deviation) between the inflow bath temperature (T3 - ~Tfall) ofthe coating
15 bath which flows into the coating tub 1 from the adjusting tub 3 and the bath temperature
Tl ofthe coating tub 1 is not within lOOC (T3 - ~Tfall - Tl > lOOC or T3 - ~Tfall - Tl <
1DOC), it turns out that the size of the dross which forms in the coating tub 1 may be
larger than the harmful size (for example, 50 Jlm). Contrary, when the inflow bath
temperature deviation is -1DoC or more and 1DoC or less (-1 DoC ~ T3 - ~Tfall - Tl ~
20 lOOC), only the dross (for example, approximately 22 Jlm or less) which is sufficiently
smaller than the harmful size forms. Thus, in order to suppress the formation of the
dross with the harmful size in the coating tub 1, it is preferable that the inflow bath
temperature deviation is -1DoC or more and 1DoC or less. In other words, it is preferable
that the bath temperature T3 ofthe adjusting tub 3 is within the range of±10°C (Tl +
25 ~Tfall - 10 ~ T3 ~ Tl + ~Tfall + 10) on the basis of the temperature (~Tfall + Tl) in which
80
the difference of the bath temperature decrease 1:1Tfall at transferring the bath from the
adjusting tub 3 to the coating tub 1 is added to the bath temperature Tl ofthe coating tub
1. Conventionally, when the bath temperature deviation ofthe coating bath increases, it
has been expected that the formation and the growth ofthe dross are accelerated.
5 However, the specific range ofthe bath temperature deviation which promotes the
formation ofthe dross with the harmful size has not known. From the test results, in
order to suppress the formation ofthe dross with the harmful size in the coating tub 1, it
turns out that the bath temperature T3 ofthe adjusting tub 3 may be within the range of
±10°C on the basis of the temperature in which the difference ofthe bath temperature
10 decrease I:1Tfall is added to the bath temperature Tl ofthe coating tub 1.
[0174]
As described above, although the preferable embodiment ofthe present
invention was described in detail with reference to the drawings, the present invention is
not limited to the embodiment. It is obvious that a person ordinarily skilled in the art of
15 the invention can conceive the alterations and the modifications within the technical
ideas used in the scope ofclaims, so that it is obviously understood that these belong
implicitly to the technical scope ofthe present invention.
[0175]
The present invention can be widely applied to the hot dip zinc-aluminum
20 coated steel sheets which are manufactured by using the coating bath 10 whose specific
gravity is higher than the specific gravity ofthe top-dross (Fe2Als), such as the
galvanized steel sheets (GI) for which only the top-dross forms, the zinc-aluminum alloy
coated steel sheets, and the like in addition to the galvannealed steel sheets (GA).
When the amount ofthe aluminum increases and the specific gravity ofthe coating bath
25 10 is less than the specific gravity ofthe top-dross, the dross cannot separated by the
81
flotation, which is a requirement for the present invention. Therefore, the applicable
scope of the present invention is the hot dip zinc-aluminum coated steel sheets in which
the aluminum content is less than 50 mass%.
[0176]
5 In addition, in the coated steel sheets which are manufactured by the coating
bath with high aluminum content other than the galvannealed steel sheets, it is not
necessary that the bath composition ofthe separating tub 2 and the adjusting tub 3 is
intentionally changed like the above mentioned embodiment, and it is possible that the
coating bath 10 in which the top-dross is almost not contained by controlling only the
10 bath temperature T. Thereby, the problems such as the appearance deterioration ofthe
surface of the steel sheet caused by the dross adhesion, surface defects caused by the
dross, the roll-slipping caused by the dross precipitation on the surface ofthe roll in the
coating bath, and the like can be solved.
15 Industrial Applicability
[0177]
According to the present invention, it is possible that the dross which forms
inevitably in the coating bath during the manufacture ofthe galvannealed steel sheet can
be removed efficiently and effectively and can be almost-completely rendered harmless.
20 Accordingly, the present invention has significant industrial applicability.
Reference Signs List
[0178]
1 COATING TUB
25 2 SEPARATING TUB
.. 82 3 ADJUSTING TUB
4 PREMELTING TUB
5 MOLTEN METAL TRANSFER APPARATUS
6, 7 COMMUNICATING VESSEL
5 8 TRANSFERRING VESSEL
9 OVERFLOWING VESSEL
10, lOA, lOB, lOC COATING BATH
11 STEEL SHEET
12 SINK ROLL
10 13 GAS WIPING NOZZLE
•
TABLE 1
1/4
COATTYIPNEG OBFATH COATING BATH A COATING BATH B COATING BATH C
O.13mass%AI O.14mass%AI 0.18mass%AI
CCOOMAPTOINSGITIOBNATHOF O.05mass%Fe 0.04mass%Fe 0.03mass%Fe
BALANCE : Zn BALANCE : Zn BALANCE : Zn
Fe2A1s : 5tt m
FORMED DROSS AND FeZn7 : 50/1 m
FeZn7 : 40/1 m
SIZE THEREOF Fe2A1s: 10/1m Fe2A1s: 10tt m
Fe2A1s : 25 tt m
TABLE 2
CAPACITY OF EACH TUB BATH TEMPERATURE OF EACH TUB
CIRCULATING
COATING SEPARATING ADJUSTING COATING SEPARATING ADJUSTING VOLUME
EXAMPLE TUB TUB TUB TUB TUB TUB OF BATH Q 1 /q Q2/q AT 1-2
Q 1 [t] Q 2 [t] Q 3 [t] T 1 [OC] T 2 [OC] T 3 [OC] q [t/h] [OC]
EXAMPLE 1 10 40 20 460 460 466 10 1.0 4.0 10
EXAMPLE 2 20 40 20 460 440 465 6 3.3 6. 7 20
EXAMPLE 3 40 40 20 460 450 465 6 6. 7 6. 7 10
EXAMPLE 4 10 12 20 460 450 465 10 1.0 1.2 10
EXAMPLE 5 10 40 20 460 454 465 10 1.0 4.0 6
EXAMPLE 6 10 40 20 460 455 465 10 1.0 4.0 5
EXAMPLE 7 10 40 20 460 456 465 10 1.0 4.0 4
CEOXMAPMAPRLAETIV1E 10 40 20. 460 460 465 10 1.0 4.0 0
CEOXMAPMAPRLAETIV2E 60 - - 460 - - - - - -
J
~
-t: -t>
-1::00
••
TABLE 3
COATING TUB SEPARATING TUB ADJUSTING TUB
EXAMPLE AI CONCENTRATION Fe CONCENTRATION AI CONCENTRATION Fe CONCENTRATION AI CONCENTRATION Fe CONCENTRATION
[mass%] [mass%] [mass%] [mass%] [mass%] [mass%]
EXAMPLE 1 O. 135 0.03 O. 154 0.02 O. 145 0.019
EXAMPLE 2 O. 136 0.029 O. 168 0.012 O. 153 0.01
EXAMPLE 3 O. 134 0.031 O. 166 0.021 O. 149 0.018
EXAMPLE 4 O. 136 0.031 O. 153 0.026 0.144 0.024
EXAMPLE 5 O. 135 0.03 O. 154 0.024 O. 145 0.024
EXAMPLE 6 O. 136 0.031 O. 154 0.026 0.144 0.025
EXAMPLE 7 O. 135 0.032 O. 155 0.029 O. 144 0.029
"EXAMPLEI~t O. 135 0.032 O. 155 0.03 O. 144 0.031
COMPARATIVE O. 133 0.033 - - - - EXAMPLE 2
~ ~
.I:>- ~
~-';·;";'I,M':¥,~!"N~;""_:=~.""",,,=!!!&4OlN:!"""'''''''''Jml&iiG':;;rn;,,,;;:;;,. ~""""""'l ;;;;:;;;;;;;;;;;:;;u.;;;:;;;:;&&& g;;;a:;;;;;:;:;; .&;;;;;iQM!ZM:a;;:;:ui!Alli\At!liiMW....w=&tZL&J : ;::,:;;:;;:;;; hi.,&,;:o::wza;;Z1 ."&4,,,'iJ3&iJlW;;;:aS::;;;W. = = Lam &I&J,ea £ mAX ,z aWL
TABLE 4
4/4
DENSITY OF FLOWED DROSS SEDIMENTED DROSS
EXAMPLE TOP-DROSS BOTTOM-DROSS BOTTOM-DROSS
[mg/cm3
] [mg/cm3
] (VISUAL OBSERVATION)
EXAMPLE 1 0.022 0.014 NONE
EXAMPLE 2 0.047 0.026 NONE
EXAMPLE 3 0.065 0.177 NONE
EXAMPLE 4 O. 112 0.046 NONE
EXAMPLE 5 0.052 0.062 NONE
EXAMPLE 6 0.084 0.206 NONE
EXAMPLE 7 0.141 0.573 SMALL AMOUNT
~EXAMP[E 1~t O. 181 1.388 SMALL AMOUNT
c~~r~~tP~E 0.278 1.749 SMALL AMOUNT
TABLE 5
TOP-DROSS BOTTOM-DROSS
EFFICIENCY OF EFFICIENCY OF
SIZE FLOTATION SIZE SEDIMENTATION
SEPARATION SEPARATION
50lL m 100% 50lLm 53'
30lLrn 98% 30lLrn 21'
10lLrn 40% lOll m 4'

5
8}
CLAIMS
1. A manufacturing equipment for a galvannealed steel sheet, the manufacturing
equipment comprising:
a coating tub to coat a steel sheet which is dipped in a coating bath, wherein the
coating tub has a first temperature controller to keep the coating bath which is a molten
metal including a molten zinc and a molten aluminum to a predetermined bath
temperature Tl;
a separating tub to separate by a flotation a top-dross which is precipitated by
10 controlling an aluminum concentration A2 ofthe coating bath transferred from the
coating tub to be 0.14 mass% or more by supplying a first zinc-included-metal which
includes an aluminum with a concentration higher than an aluminum concentration A1 of
the coating bath in the coating tub, wherein the separating tub has a second temperature
controller to keep the coating bath transferred through a coating bath outlet of the coa~ing
15 tub to a bath temperature T2 which is lower than the bath temperature T1;
an adjusting tub to adjust an aluminum concentration A3 ofthe coating bath
transferred from the separating tub to a concentration which is higher than the aluminum
concentration Al and is lower than the aluminum concentration A2 by supplying a
second zinc-included-metal which includes an aluminum with a concentration lower than
20 the aluminum concentration A2 or does not include an aluminum, wherein the adjusting ..'
tub has a third temperature controller to keep the coating bath transferred from the
separating tub to a bath temperature T3 which is higher than the bath temperature T2; and
a circulator to circulate the coating bath in order ofthe coating tub, the
separating tub, and the adjusting tub.
25
2.
81
The manufacturing equipment for the galvannealed steel sheet according to
5
10
claim I, the manufacturing equipment further comprising
an aluminum concentration analyzer to measure the aluminum concentration
A 1ofthe coating bath in the coating tub,
wherein the circulator controls a circulating volume of the coating bath
depending on a measurement result ofthe aluminum concentration analyzer.
3. The manufacturing equipment for the galvannealed steel sheet according to
claim I,
wherein the bath temperature T2 ofthe separating tub is controlled by the
second temperature controller to be lower 5°C or more as compared with the bath
temperature Tl of the coating tub and to be higher than a melting point ofthe molten
metal.
15 4.
claim I,
The manufacturing equipment for the galvannealed steel sheet according to
wherein the bath temperature T3 is controlled by the third temperature controller
so that the bath temperature TI, the bath temperature T2, and the bath temperature T3
satisfy a following formula (I) and a following formula (2) in celsius degree, when a
20 difference of a bath temperature decrease of the coating bath when transferred from the
adJusting tub to the coating tub is 1:1Tfall in celsius degree.
TI + I:1Tfall - 1O:S T3 :S Tl + I:1Tfall + 10 ... (1)
T2 + 5 :S T3 ... (2)
25 5. The manufacturing equipment for the galvannealed steel sheet according to
•
5
10
85-
claim 1, the manufacturing equipment further comprising
a premelting tub to melt the second zinc-included-metal,
wherein a molten metal ofthe second zinc-included-metal which is melted in the
premelting tub is supplied to the coating bath in the adjusting tub.
6. The manufacturing equipment for the galvannealed steel sheet according to
claim 1,
wherein the circulator includes a molten metal transfer apparatus which is
installed in at least one ofthe coating tub, the separating tub, and the adjusting tub.
7. The manufacturing equipment for the galvannealed steel sheet according to
claim 1,
wherein the coating bath outlet ofthe coating tub is located on a downstream
side of a running direction of the steel sheet so that the coating bath flows out of an upper
15 part of the coating tub by a flow ofthe coating bath which is derived from a running of
the steel sheet.
8. The manufacturing equipment for the galvannealed steel sheet according to
claim 1,
20 wherein at least two ofthe coating tub, the separating tub, and the adjusting tub
are made by dividing one tub with a weir, and
a bath temperature of each tub which is divided by the weir is controlled
independently.
25 9. The manufacturing equipment for the galvannealed steel sheet according to
.. claim 1,
wherein a storage ofthe coating bath in the coating tub is five times or less ofa
circulating volume ofthe coating bath per one hour by the circulator.
5 10.
claim 1,
The manufacturing equipment for the galvannealed steel sheet according to
wherein a storage ofthe coating bath in the separating tub is two times or more
of a circulating volume ofthe coating bath per one hour by the circulator.
10 11. A manufacturing method ofa galvannealed steel sheet, the manufacturing
method comprising:
circulating a coating bath which is a molten metal including a molten zinc and a
molten aluminum in order of a coating tub, a separating tub, and an adjusting tub;
coating a steel sheet which is dipped in the coating bath at the coating tub in
15 which the coating bath transferred from the adjusting tub is stored at a predetermined
bath temperature Tl;
separating by a flotation a top-dross which is precipitated by controlling an
aluminum concentration A2 of the coating bath transferred from the coating tub to be
0.14 mass% or more at the separating tub in which the coating bath transferred from the
20 coating tub to the separating tub is stored at a bath temperature T2 which is lower than
the bath temperature Tl ofthe coating tub and a first zinc-included-metal which includes
an aluminum with a concentration higher than an aluminum concentration Al ofthe
coating bath in the coating tub is supplied; and
adjusting an aluminum concentration A3 ofthe coating bath transferred from the
25 separating tub to a concentration which is higher than the aluminum concentration Al
• and is lower than the aluminum concentration A2 at the adjusting tub in which the
coating bath transferred from the separating tub is stored at a bath temperature T3 which
is higher than the bath temperature T2 ofthe separating tub and a second
zinc-included-metal which includes an aluminum with a concentration lower than the
5 aluminum concentration A2 of the coatirig bath in the separating tub or does not include
an aluminum is supplied.
10
15
Dated this 28/02/2013
~~~\~~,
DEEPAK KdMAR
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANTS