1
DESCRIPTION
MANUFACTURING EQUIPMENT FOR GALVANIZED STEEL SHEET, AND
MANUFACTURING METHOD OF GALVANIZED STEEL SHEET
Technical Field
[OOO 11
The present invention relates to manufacturing equipment for a galvanized steel
sheet and a manufacturing method of the galvanized steel sheet. In particular, it relates
to the equipment and the method for the galvanized steel sheet to make dross harmless,
which forms when the galvanized steel sheet is manufactured.
Priority is claimed on Japanese Patent Application No. 20 10- 196796, filed
September 2, 2010, the content of which is incorporated herein by reference.
Background Art
[0002]
Hot dip zinc-aluminum coated steel sheets have been widely used in fields such
as automobiles, consumer electronics, building materials and the like. A representative
category of the coated steel sheets includes the following three types in order from low
aluminum (Al) content in . coating bath. ,
(1) Galvannealed steel sheets (composition of coating bath : for example, 0.125
to 0.14 mass% A1 - Zn)
(2) Galvanized steel sheets (composition of coating bath : for example, 0.1 5 to
0.25 mass% A1 - Zn)
(3) Zinc-aluminum alloy coated steel sheets (composition of coating bath : for
example, 2 to 25 mass% A1 - 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 furthermore 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)". And 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]
15 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 intermetallic compounds of Iron (Fe) dissolved in the coating bath fiom the steel
sheet and A1 or Zn included in the coating bath (molten metal). Specific compositions
of the intermetallic compounds are, for example, Fe2A1s which represents top-dross and
20 FeZn-/ which represents bottom-dross. The top-dross may form in the all coating bath
(for exaniple, GA bath, GI bath) to manufacture 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 of the top-dross is smaller than that of the molten
metal which is the coating bath, the top-dross flows in the coating bath, and finally
comes to top surface of the coating bath. When a large amount of the top-dross flows in
the coating bath, the top-dross accumulates on surface of roll in the coating bath, which
causes occurrence of surface defects on the steel sheets. Also the flowing top-dross
5 accumulates into groove of the roll in the coating bath, which causes occurrence of
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 of the
top-dross adheres to the steel sheet, appearance quality of a product deteriorates and the
product becomes off-grade in some case.
[0006]
On the other hand, since the specific gravity of the 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 bottom of 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 brings about the problems such as the defects of the roll in the coating bath, .
the roll-slipping, the roll-idling, the remarkable deterioration of the appearance quality
which results from its adhesion to the steel sheet, and the like. Moreover, the
bottom-dross does not come to the top surface and does not become 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 the coating tub once reflows in the coating
bath agah by transition of the coating bath flow. Therefore, it can be said that the
bottom-dross is more harmful than the top-dross.
[0007]
In particular, when sheet threading speed of the steel sheet dipped into the
coating bath is accelerated in eider to improve productivity of the coated steel sheets, the
4
bottom-dross which deposits on the bottom of the coating tub is raised up in the coating
bath by the coating bath flow which is derived from high-speed threading of the steel
sheet. The above-mentioned dross adheres to the steel sheet and causes the occurrence
of the dross defects on the steel sheets, which results in a factor of degradation of the
5 coated steel sheet. Therefore, hitherto, the sheet threading speed of the steel sheet was
suppressed and the productivity had to be sacrificed in order to secure the quality of the
coated steel sheets.
To solve the above-mentioned problems caused by the top-dross and the
10 bottom-dross, many suggestions have been made in the past. As shown below, the
suggestions are commonly methods of a sedimentation separation and a flotation
separation of the dross by using the difference in specific gravity between the coating
bath and the dross.
For example, in patent document 1, the dross removal equipment is suggested, ,
in which the molten zinc with the dross is transferred from a coating tub to a storage tub
and the dross is separated by the sedimentation and the flotation by using the difference
in specific gravity between the dross and the coating bath. In the equipment, the
capacity of the storage tub is 10 m3 or more, the transfer volume of the molten zinc is 2
20 m3 1 hour or more, and a baffle plate is installed in the storage tub to divert the coating
bath flo*. iHowever, in patent document 1, dross removal effect is overestimated
because of an adoption of an equation which is applicable to the particle sedimentation in
the case of the relatively slow coating bath flow. In addition, although the harmful
dross is defined as 100 pm or more in patent document 1, the dross defects which
25 become the problem recently include defects which are derived from the dross with size
5
of approximately 50 pm. In fact, a countermeasure with the greater effect than that of
patent document 1 is necessary. On the contrary, in a method described in patent
document 1, in order to remove the dross with the size of approximately 50 pn, the
capacity of the 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 except for patent
document 1 is necessary.
[OO lo]
In patent document 2, the coating equipment is suggested, in which enclosing
parts are installed in a coating tub and the raising up of the bottom-dross is suppressed by
sedimenting and depositing the bottom-dross underneath the enclosing parts. However,
in a method described in patent document 2, the coating bath flow at upper area in the
coating bath becomes rapid with an increase in coating rate, so that the coating bath flow
at lower area in the coating bath also becomes rapid gradually. Thus, since the dross
with small size does not sediment and flows back to the upper area with the coating bath ,
flow, dross removal efficiency is low. Moreover, in the case that the capacity of the
coating tub is practical (for example, 200 ton), the dross with small size flows back
between the upper area and the lower area of the coating bath, grows with time passage,
and finally sediments to 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 ldwbr area of the coating bath, so that the effect as the countermeasure against the
dross defects is low. 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 of the enclosing parts, it can be said that technology described in
c
6
patent document 2 is not practical.
[OO 111
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
5 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
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
10 efficiency of the dross with small size is low and the dross flows back to the coating tub
with the coating bath flow. Moreover, in the case that the capacity of the dross removal
tub is practical (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 finally sediments at the dross removal tub.
15 However, at the time, a large amount of the 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
said that the effect of technology described in patent document 3 is low as the
countermeasure against the dross defects.
[OO 1 21
20 In addition, in the coating equipment suggested in patent document 4, the
coating Vath in a coating pot is transferred to a crystallization pipe, and is cooled and
heated 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 the coating method suggested in patent document 5, a sub pot
25 is additionally installed in a coating pot. The molten metal which includes the
7
bottom-dross is transferred from the coating pot to the sub pot, the bath in the sub pot is
held at higher temperature than that of the coating pot, and A1 concentration is increased
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
Patent Documents
[00 131
[Patent Document 11 Japanese Unexamined Patent Application, First
10 Publication No. H10-140309
[Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. 2003-1 932 12
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. 2008-095207
15 [Patent Document 41 Japanese Unexamined Patent Application, First
Publication No. H05-295507
[Patent Document 51 Japanese Unexamined Patent Application, First
Publication No. H04-99258
20 Summary of Invention
Technical Pj-oblem
[00 141
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
25 the coating bath is not conducted and the dross is separated by the sedimentation and the
8
flotation by simply using the difference in specific gravity between the dross and the
coating bath. However, in the removal methods, there was the problem such that the
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,
5 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
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 of the coated steel sheets was low.
[00 1 51
On the other hand, in the method described in patent document 4, the molten
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
15 described in Example in patent document 4, large flow of bath circulation such that
circulating volume of the coating bath is 0.5 m3 1 min (approximately 200 ton / hour) is
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
20 system of high power are necessary. Moreover, in patent document 4, a method of
removing t k dross which is grown in the crystallization pipe is not disclosed. In the
case that the dross is removed by using a filter, exchange operation thereof is
substantially impossible. And, in the case that the dross is removed by the
sedimentation separation, a sedimentation tub is additionally needed, so that operation is
25 substantially difficult even if being theoretically possible. Therefore, it can be said that
the method described in patent document 4 is not practical.
[00 161
In addition, in the method described in patent document 5, the coating bath in
the sub pot is held at higher temperature than that of the coating pot, A1 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
Example in patent document 5, in the conditions such that bath temperature is heated to
500°C, 550°C and A1 concentration is increased to 0.15 mass% in the coating pot by
using the coating bath from the coating pot (bath temperature of 460°C, A1 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
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 of the dross is dissolved in the coating bath. Namely, since the solubility limit of
Fe of the coating bath increases with an increase in the bath temperature of the coating
bath in the sub pot, most of the dross is dissolved in the coating bath, so that the dross
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 arnouxh 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
cleanup operation of the 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 of the sub pot must be batch processing, dross removal
efficiency is inferior to the case that the dross cleanup processing is consecutively
conducted.
[00 171
As mentioned above, the methods of removing the dross which flows in the
coating bath are investigated for many years, most of the methods are the method which
5 uses the difference in specific gravity between the dross and the coating bath (refer to
patent documents 1 to 3). Among them, in the case of the method of the sedimentation
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
10 capacity of the separating tub.
[00 1 81
On the other hand, the method of the flotation separation of the top-dross is
more advantageous than the method of the sedimentation separation of the bottom-dross.
However, under the general operational condition of the GAY since the dross may form in
15 the state of the bottom-dross only or a mixture of the bottom-dross and the top-dross, the ,
method of transforming the bottom-dross into the top-dross is necessary. Some
technologies are disclosed as the methods (for example, refer to patent document 5).
[00 1 91
However, as described above, since the conventional dross removal methods
20 which were suggested until now are difficult to control A1 concentration of the coating
bath and'thk technical idea thereof may be technical unreasonableness, the methods are
not practicalized. In the conventional methods, the dross removal efficiency and effect
were insufficient, and the dross removal effect itself was much doubtful.
The present invention'is achieved in view of the above-mentioned problems.
11
An object of the present invention is to provide a manufacturing equipment for a
galvanized steel sheet and a manufacturing method of a galvanized steel sheet which are
new and improved, in which the dross which forms inevitably in the coating bath during
the manufacture of the galvanized steel sheet can be removed efficiently and effectively
5 and can be almost-completely rendered harmless.
Solution to Problem
[002 11
The inventors has investigated with singleness of purpose in view of the
10 above-mentioned circumstance, and found the method which almost-completely renders
dross harmless (dross-free) by removing the dross efficiently and effectively within the
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 (1) a process of separating the dross by using the difference in specific
15 gravity by precipitating intentionally the top-dross in the coating bath at the separating
tub where bath temperature thereof is lower than that of the coating tub and (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 of the coating bath to be an unsaturated state in the
adjusting tub where bath temperature thereof is higher than that of the separating tub.
20 [0022]
In qrder to accomplish the aforementioned object, each aspect of the present
invention employs the following.
(a) A manufacturing equipment for a galvanized steel sheet according to an
aspect of the invention, the manufacturing equipment includes:
a coating tub to coat a steel sheet which is dipped in a coating bath, wherein the
12
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 T 1 ;
a separating tub which has a second temperature controller to keep the coating
5 bath transferred through a coating bath outlet of the coating tub to a bath temperature T2
which is lower than the bath temperature TI;
an adjusting tub which 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 of the coating tub, the
separating tub, and the adjusting tub.
[0023]
(b) The manufacturing equipment for the galvanized steel sheet according to (a),
the manufacturing equipment may further include,
15 an aluminum concentration analyzer to measure an aluminum concentration
A1 of the coating bath in the coating tub,
wherein a first zinc-included-metal which includes an aluminum with a
concentration higher than the aluminum concentration A1 of the coating bath in the
coating tub may be supplied to at least one of the separating tub and the adjusting tub
20 depending on a measurement result of the aluminum concentration analyzer.
~ ~ 4 1
(c) In the manufacturing equipment for the galvanized steel sheet according to
(b)Y
the first zinc-included-metal may be supplied to the separating tub, and
a second zinc-included~metawl hich is a zinc-included-metal which includes an
aluminum with a concentration lower than an aluminum concentration A2 of the coating
bath in the separating tub or a zinc-included-metal which does not include an aluminum
may be supplied to the adjusting tub depending on the measurement result of the
aluminum concentration analyzer.
[0025]
(d) In the manufacturing equipment for the galvanized steel sheet according to
(b)Y
the first zinc-included-metal may be supplied to the separating tub, and
a metal may not be supplied to the adjusting tub depending on the measurement
10 result of the aluminum concentration analyzer.
(e) The manufacturing equipment for the galvanized steel sheet according to (b),
the manufacturing equipment may further include,
a premelting tub to melt the first zinc-included-metal or the second
15 zinc-included-metal,
wherein a molten metal of the first zinc-included-metal or the second
zinc-included-metal which is melted in the premelting tub may be supplied to the coating
bath in the adjusting tub.
[0027]
20 (f) In the manufacturing equipment for the galvanized steel sheet according to
(a), ' i
the bath temperature T2 of the separating tub may be controlled by the second
temperature controller to be lower 5°C or more as compared with the bath temperature
T1 of the coating tub and to be higher than a melting point of the molten metal.
[0028]
-
14
(g) In the manufacturing equipment for the galvanized steel sheet according to
(a),
the bath temperature T3 may be controlled by the third temperature controller so
that the bath temperature TI, the bath temperature T2, and the bath temperature T3
5 satisfy a following formula (1) and a following formula (2) in celsius degree, when a
difference of a bath temperature decrease of the coating bath when transferred from the
adjusting tub to the coating tub is ATfall in celsius degree.
T1 +ATfall-10IT3ITl +ATfall+10 **.(I)
T2 + 5 5 T3 -.. (2)
(h) In the manufacturing equipment for the galvanized steel sheet according to
the circulator may include a molten metal transfer apparatus which is installed in
at least one of the coating tub, the separating tub, and the adjusting tub.
(i) In the manufacturing equipment for the galvanized steel sheet according to
(a),
--
the coating bath outlet of the coating tub may be located on a downstream side
of a running direction of the steel sheet so that the coating bath flows out of an upper part
20 of the coating tub by a flow of the coating bath which is derived from a running of the
' i steel sheet.
[003 11
(j) In the manufacturing equipment for the galvanized steel sheet according to
2 5 at least two of the coating tub, the separating tub, and the adjusting tub may be
15
made by dividing one tub with a weir, and
a bath temperature of each tub which is divided by the weir may be controlled
independently.
(k) In the manufacturing equipment for the galvanized steel sheet according to
a storage of the 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.
(1) In the manufacturing equipment for the galvanized steel sheet according to
a storage of the coating bath in the separating tub may be two times or more of a
circulating volume of the coating bath per one hour by the circulator.
[0034]
15 (m) A manufacturing method of a galvanized steel sheet according to an aspect ,
of the invention, the manufacturing method includes:
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
20 which the coating bath transfemed from the adjusting tub is stored at a predetermined
bath temfieiature TI;
separating by a flotation a top-dross which is precipitated 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 T1 of the coating tub; and
dissolving a residual dross at the adjusting tub in which the coating bath
16
transferred from the separating tub is stored at a bath temperature T3 which is higher than
the bath temperature T2 of the separating tub.
[0035]
According to the manufacturing equipment and the manufacturing method for
5 the galvanized steel sheet described in the above (a) and (m), 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 of the circulation bath can be shortened, so that it
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
10 the circulation bath, so that it is possible to precipitate Fe of the coating bath as the
top-dross and to separate by the flotation. Moreover, in the adjusting tub, Fe of the
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.
Advantageous Effects of Invention
[0036]
According to the invention described in the above (a) and (m), the formation and
growth of the dross are suppressed in the coating tub, the top-dross is separated and
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), Zn and A1 which are
consumed by the coating process at the coating tub are supplied by supplying the metal
25 to the separating tub or the adjusting tub. Thereby, it is possible that the dross
17
formation caused by melting the metal at the coating tub is prevented and the coating
bath in the coating tub is controlled to the A1 concentration (fof example, 0.200 mass%)
which is suitable for manufacturing the GI.
According to the invention described in the above (c), the A1 concentration of
5 the coating bath which is stored in the separating tub is controlled to be higher than the
concentration of the coating tub and the adjusting tub. Thereby, it is possible that the
large amount of the top-dross is precipitated and separated by the flotation.
According to the invention described in the above (d), the supply for the bath
element and the adjustment of the A1 concentration are conducted by supplying the metal
10 only to the adjusting tub 3. Thereby, since it is not necessary to supply the metal to the
separating tub 2, it is possible to simplify the equipment configuration.
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 formation of
15 the dross therefor at the adjusting tub.
According to the invention described in the above (f), the solubility limit of Fe
of the 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.
20 According to the invention described in the above (g), the bath temperature of
the coatihg bath which is stored in the adjusting tub is held higher than that of the
separating tub and the bath temperature deviation of the coating bath in the coating tub
decreases. Thereby, it is possible to dissolve the residual dross at the adjusting tub and
to suppress the formation of the dross with harmful size at the coating tub.
25 According to the invention described in the above (h), the circulation of the
coating bath between the coating tub, the separating tub, and the adjusting tub is
conducted by one molten metal transfer apparatus. Thereby, it is possible to simplify
the equipment configuration.
According to the invention described in the above (i), the local stagnation area
5 of the coating bath 10A in the coating tub 1 is hardly formed. Thereby, it is possible to
avoid that the dross grows up to the hamful size at the stagnation area in the coating tub
1.
According to the invention described in the above Cj), two or three tubs of the
coating tub, the separating tub, and the adjusting tub are made as one. Thereby, it is
10 possible to simplify the equipment configuration.
According to the invention described in the above (k), the stagnation time of the
coating bath in the coating tub is shortened. Thereby, it is possible to make the dross
flow out of the coating tub to the separating tub before the dross grows up to the harmful
size.
15 According to the invention described in the above (I), the stagnation time of the ,
coating bath in the separating tub is prolonged. Thereby, it is possible to sufficiently
remove the top-dross at the separating tub.
Brief Description of Drawings
20 [0037]
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
19
in a coating tub.
FIG. 3B is a schematic diagram which illustrates a flowing situation of the dross
in the coating tub.
FIG. 4 is a schematic diagram which illustrates a configuration 1 of
5 manufacturing equipment for a galvanized steel sheet according to an embodiment of the
present invention.
FIG. 5 is a schematic diagram which illustrates a configuration 2 of the
manufacturing equipment for the galvanized steel sheet according to modification 1 of
the embodiment.
FIG. 6 is a schematic diagram which illustrates a configuration 3 of the
manufacturing equipment for the galvanized steel sheet according to modification 2 of
the embodiment.
FIG. 7 is a schematic diagram which illustrates a configuration 4 of the
manufacturing equipment for the galvanized steel sheet according to modification 3 of
15 the embodiment.
FIG. 8 is a schematic diagram which illustrates a configuration 5 of the
manufacturing equipment for the galvanized steel sheet according to modification 4 of
the embodiment.
FIG. 9 is a schematic diagram which illustrates permissible bath temperature
20 range of each tub according to the embodiment when the bath temperature of the coating
tub is 460°@.
FIG. 10 is the ternary phase diagram which indicates state transition of the
coating bath in each tub according to the embodiment.
FIG. 11 is the ternary phase diagram which indicates the state transition of the
25 coating bath in each tub according to modification of the embodiment.
20
FIG. 12 is a graph which indicates a relationship between capacity of the
separating tub and a dross separation ratio according to examples of the present
invention.
FIG. 13 is a graph which indicates a relationship between circulating volume of
5 bath and dross size according to the examples.
FIG. 14 is a graph which indicates a relationship between a bath temperature
deviation of an inflow bath of the coating tub and the dross size according to the
examples.
10 Description of Embodiments
[003 81
Hereinafter, a preferable embodiment of the 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
15 reference sign in the specification and the drawings.
[0039]
[l. Investigation of dross formation and dross removal methods]
First of all, in advance of explanations of manufacturing equipment for a
galvanized steel sheet and a manufacturing method of the galvanized steel sheet
20 according to an embodiment of the present invention, the result of the investigation of
factors of dr,oss formation (top-dross, bottom-dross) in coating bath and the dross
removal methods will be described.
L
[0040]
[I. 1. Dross formation range]
2 5 As mentioned above, the hot dip zinc-aluminum coated steel sheets are the steel
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.
[004 11
The galvannealed steel sheets (GA) are the steel sheets in which the Zn-Fe
intermetallic compound layer is formed by heating for short time at 490 to 600°C just
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 of the GA
includes the alloy of Fe which is dissolved in the coating bath fiom the steel sheet and Zn.
10 Composition of the coating bath (GA bath) for manufacturing the GA includes, for
example, A1 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 A1 is added to Zn bath in order to improve coating
adhesion. When the A1 concentration in the GA bath is excessively high, the alloying of
15 Fe and A1 in the coating layer barely occurs by so-called aluminum barriers, so that the .
A1 concentration in the GA bath is controlled to a predetermined low concentration
(0.125 to 0.14 mass%).
[0042]
The galvanized steel sheets (GI) are frequently utilized as general building
20 materials and the like. Composition of the coating bath (GI bath) for manufacturing the
GI includes) for example, A1 of 0.15 to 0.25 mass% and Zn as the balance. By
controlling the A1 concentration of the 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 of the coating
layer can be suppressed even if the steel sheet is deformed.
[0043]
22
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 of the coating bath for manufacturing the above steel sheets is A1 of 5
mass% and Zn as the balance, A1 of 11 mass% and Zn as the balance, and the like.
5 Since the sufficient amount of A1 is contained in the Zn bath, higher corrosion resistance
is obtained as compared with the GI.
[0044]
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
10 dissolved in the coating bath and A1 or Zn are formed in large amount. The dross
formation in the coating bath depends on temperature of the coating bath (bath
temperature), the A1 concentration in the coating bath, and Fe concentration in the
coating bath (solubility of Fe dissolved in the coating bath from the steel sheet).
[0045]
15 FIG. 1 is a ternary phase diagram which indicates the dross formation range in ,
the various coating baths. In the FIG 1, horizontal axis is the A1 concentration (mass%)
in the coating bath and vertical axis is the Fe concentration (mass%) in the coating bath.
[0046]
As shown in FIG 1, when the Fe concentration in the coating bath exceeds the
20 predetermined concentration which depends on the A1 concentration, the dross is formed.
For exaniplk, in regard to the GA bath where the bath temperature T is 450°C and the A1
concentration is 0.13 mass%, when the Fe concentration in the coating bath becomes
approximately more than 0.025 mass%, the bottom-dross (FeZn-/) is formed. Moreover,
in regard to the GA bath where the bath temperature T is 450°C and the A1 concentration
25 is 0.14 mass%, the top-dross (FeIAlj) is formed when the Fe concentration becomes
23
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.
[0047]
On the other hand, since the A1 concentration of the GI bath (for example, 0.15
to 0.25 mass%) is higher than that of the GA bath, the dross which is formed in the GI
bath is only the top-dross (Fe2A15). 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 0.0 1 mass%, the top-dross is formed. Moreover, in regard to
10 the coating bath for the zinc-aluminum alloy coated steel sheets even though it is not
illustrated, only the top-dross is also formed since the A1 concentration is sufficiently
high (for example, 2 to25 mass%).
[0048]
In addition, as shown in FIG. 1, even if the coating bath is the same, lower limit
15 of Fe concentration where the dross is formed increases with an increase in the bath
temperature T. For example, in regard to the GI bath where the A1 concentration is 0.2
mass%, conditions where the top-dross is formed are as follows: (I) the Fe concentration
is approximately 0.007 mass% or more in case-that the bath temperature T is 450°C, (2)
the Fe concentration is approxinlately 0.014 mass% or more in case that the bath
20 temperature T is 465"C, and (3) the Fe concentration is approximately 0.02 mass% or
more in c'as0 that the bath temperature T is 480°C. Thus, when the Fe concentration in
the GI bath is constant (for example, 0.01 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 top-dross is dissolved in the GI bath and disappears. On the
25 contrary, the unsaturated state is shifted to the supersaturated state in regard to Fe by
24
decreasing the bath temperature T from 465°C to 450°C, so that the top-dross is formed
in the GI bath.
[0049]
C1.2. Factors of dross formation]
Next, the factors of the dross formation in the coating bath will be described.
As the factors of the dross formation, the following factors (1) to (3) are considered, for
example. Hereinafter, each factor will be described.
[0050]
(1) Melting the metal to the coating bath
In order to supply the molten metal which is consumed for coating the steel
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 a liquid state. Although
zinc-included-metal which includes at least Zn for hot dip zinc coating, the
15 zinc-included-metal includes the metal such as A1 and the like besides Zn according to
the composition of the coating bath. Although the melting point of the metal differs
according to the composition of the metal, the melting point is 420°C for example and is
lower than the temperature of the coating bath (for example, 460°C).
[005 11
2 0 When the metal which is dipped into the coating bath is melted, the temperature
of the mdlth 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
25 the saturated state, a large amount of the dross is formed with comparative ease at
2 5
low-temperature area around the metal. The phase of the formed dross is related to the
phase diagram (refer to FIG 1).
[0052]
In general, since the steel sheet is constantly dipped into the coating tub and
5 active iron surface is exposed, the Fe concentration in the coating bath is the saturated
state. Thus, when the temperature of the molten metal around the metal decreases
drastically by supplying the metal in the coating bath where Fe is the saturated state, the
dross is formed by reacting the supersaturated Fe with Zn or A1 in the coating bath.
Moreover, when the metal is preliminarily melted by using a premelting tub and the
10 molten metal is supplied to the coating bath in the coating tub, the dross is hardly formed
because Fe in the premelting tub is the unsaturated state.
[0053]
(2) Fluctuation of the bath temperature T
As the factor of the dross formation following the melt of the metal, the
15 fluctuation of the bath temperature T of the coating bath is considered. Since the
solubility limit of Fe in the coating bath increases with the increase in the bath
temperature T, Fe is hrther dissolved fiom 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 of the coating bath decreases, Fe becomes the
20 supersaturated state all over the coating bath and the dross is promptly formed.
Furthermbri?, even if the low bath temperature T of the coating bath which includes the
dross increases again and the solubility limit of Fe increases, the dross is not decomposed
(does not disappear), because the dissolution rate of Fe from the steel sheet is faster than
that of the decomposition (disappearance) of the dross. In other words, even if the bath
25 temperature of the coating bath'which is low temperature (supersaturated state of Fe)
--
26
increases at the coating tub in which the steel sheet is dipped, the dross hardly
disappears.
[0054]
On the other hand, if the molten metal which is low temperature and includes
5 the dross is transferred to a tub in which the steel sheet in not dipped, is heated, and is
held for long time, the dross can be decomposed (can disappear), because Fe in the
coating bath becomes the unsaturated state. Thus, based on the viewpoint, in the
manufacturing equipment for the galvanized steel sheet according to the embodiment of
the present invention as described later, after forming the dross in the coating bath at a
10 separating tub, the coating bath is transferred to an adjusting tub in which the steel sheet
in not dipped, the bath temperature T increases, and the dross is dissolved (disappears).
(3) Other factors
The fluctuation of the A1 concentration in the coating bath and the temperature
15 deviation in the coating tub are also considered as the factor of the dross formation.
When the A1 concentration in the coating bath increases, the solubility limit of Fe in the
coating bath decreases, so that the top-dross (Fe2A15) which is the intermetallic
compound ofA1 and Fe is readily formed. And, when coating bath flow in the coating
tub decreases and mixing power in the coating tub decreases, temperature of the coating
20 bath at bottom of the coating tub decreases, so that the dross is formed. Thereafter,
when the'cciating bath flow increases again, the dross which deposits on the bottom of the
coating tub is raised up in the coating bath.
[0056]
[I .3. Separation of dross by using the difference in specific gravity]
The methods of the flotation separation of the top-dross and of the sedimentation
27
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 of the bottom-dross is, for example, 7000 to 7200 kg 1 m3 and the specific gravity
of the top-dross is, for example, 3900 to 4200 kg 1 m3. On the other hand, although the
5 specific gravity of the molten zinc bath fluctuates to a certain extent by the temperature
and A1 concentration thereof, it is, for example, 6600 kg / m3.
[0057]
As described above, in case of the separation of the dross by using the difference
in specific gravity, since the difference in specific gravity between the top-dross and the
10 molten zinc bath is large and the top-dross readily comes to top surface, it is relatively
easy 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
15 bottom-dross. Especially, it is difficult to sediment the bottom-dross with small size. ,
Moreover, since the bottom-dross deposits on the bottom of the coating tub and may be
raised up again, it is not easy to remove finally the bottom-dross outside the system
(removing the bottom-dross fiom the bottom of the coating tub).
[0058]
20 As just described, it is difficult to remove the dross in the coating tub, especially,
the bottoh-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.
[0059]
2 5 [I .4. Relation between bath temperature fluctuation and dross growth]
2 8
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 (p).
FIG. 2 indicates the growth of the bottom-dross (FeZn7) which forms in the GA bath and
5 the top-dross (Fe2A15) which forms in the GA bath, the GI bath, and the like.
[0060]
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 of the dross. For example, under
the condition where the bath temperature is constant, the bottom-dross (FeZn7) grows
10 only from approximately 15 pn to 20 pm in the average grain size during 200 hours, and
the top-dross (Fe2A15) grows only from approximately 15 pm to 35 pm during 200 hours.
[006 11
Next, in reference to Table 1, the result of observation of forming behavior of
the dross in case of decreasing the bath temperature will be described. Table 1 shows a
15 state of the dross growth when three types of coating baths A to C in which compositions .
are different are cooled from 460°C to 420°C by a predetermined cooling rate (10 "C /
sec).
[0062]
[Table 11
20 [0063]
Askshown in Table 1, when the bath temperature T decreases from 460°C to
420°C by the predetermined cooling rate of 10 "C / sec and the unsaturated state is
shifted to the supersaturated state in regard to Fe in the coating bath, the rate of formation
and growth of the dross is very fast. For example, in the coating bath A (GA bath) with
25 A1 of 0.13 mass%, the bottom-dross (FeZn,) with the grain size of approximately 50 pm
is formed during only 4 seconds. And, in the coating bath B (GA bath) with A1 of 0.14
mass%, the bottom-dross (FeZn,) with the grain size of approximately 40 pm and the
top-dross (Fe2A15) with the grain size of approximately 10 pm are formed and mixed.
Moreover, in the coating bath C (GI bath) with A1 of 0.18 mass%, three kinds of the
5 top-dross (Fe2A15) with the grain size of approximately 5 pm, 10 pm, and 25 pm are
formed.
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
10 slow. Thus, if the bath temperature T of the coating bath in the coating tub can be kept
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
15 coating tub to the separating tub 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.
[0065]
11.5. Relation between coating rate and dross]
20 FIGS. 3A and 3B are schematic diagrams which illustrate flowing situation of
the dross'in;the GA bath. FIG 3A shows the situation of normal 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).
COO661
2 5 Generally, in the GA bath, the bottom-dross forms and the bottom-dross with
30
large size among them sediments and deposits on the bottom of the coating tub in turn.
When the coating rate (sheet threading speed of the steel sheet) is slow, for example, less
than 100 m / min, the bottom-dross which deposits on the bottom of the tub is not raised
up by the coating bath flow. However, when the coating rate is 100 m / min or more, as
shown in FIG 3A, among the bottom-dross, not only the dross with small size but also
the dross with medium size which has relatively large diameter is raised up from the
bottom of the tub by the bath flow which is derived from the sheet threading, and the
dross flows in the coating bath of the coating tub. Thus, when an amount of the
formation and the deposition of the dross is much in the coating tub, productivity of the
coated steel sheet deteriorates. As described above, when the coating rate is 150 m 1
min or less, the dross with small size and medium size mainly flows in the coating bath.
[0067]
Moreover, when the coating rate, which is conventionally suppressed (for
example, 150 m / min or less) in order to secure the productivity, is changed to 200 m /
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
increase the coating rate.
I00681
r1.6. Dross defects]
The dross defects are defects of the coated steel sheet, are caused by the dross
formed in the coating bath, and include appearance deterioration of the coated steel sheet
which is derived from dross adhesion, surface defects caused by the dross on roll in the
r -
3 1
coating bath, and the like, for example. Although it is said that the diameter of the dross
which cause the dross defects is 100 pm to 300 pm, the dross defects caused by the dross
with very small size such that grain size is approximately 50 pn are observed recently.
Therefore, in order to prevent the occurrence of the small dross defects, the dross-free in
5 coating bath is desired.
[0069]
[2. Configuration of manufacturing equipment for galvanized steel sheet]
Next, in reference to FIGs. 4 to 9, the configuration of the manufacturing
equipment for the galvanized steel sheet according to the embodiment of the present
10 invention will be described. FIG. 4 is a schematic diagram of the manufacturing
equipment for the galvanized steel sheet according to the embodiment, and FIGs. 5 to 8
are schematic diagrams which illustrate modifications 1 to 4 of the embodiment,
respectively. FIG. 9 is a schematic diagram which illustrates permissible bath
temperature range of each tub in case that the bath temperature of the coating bath 1 OA
15 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 TI and A1 respectively. In the same way, the bath
temperature and the aluminum concentration of the coating bath which is stored in the
separating tub 2 are referred to as T2 and A2 respectively, and the bath temperature and
20 the aluminum concentration of the coating bath which is stored in the adjusting tub 3 are
referred to as T3 and A3 respectively.
[0070]
As shown in FIGs. 4 to 8, the manufacturing equipment for the galvanized steel
sheet according to the embodiment (hereinafter, referred to as hot-dip-coating equipment)
25 includes the coating tub1 to coat the steel sheet 11, the separating tub 2 to separate the
3 2
dross, and the adjusting tub 3 to adjust the A1 concentration of the 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 10 is the
molten metal including at least molten zinc and molten aluminum, and is the GI bath for
example. Hereinafter, each configuration of the hot-dip-coating equipment according to
the embodiment will be described.
[007 11
[2.1. Configuration of circulator of coating bath]
First, the circulator will be described. The circulator includes the molten metal
transfer apparatus 5 which is concomitantly installed in at least one of the 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
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.
[0072]
Moreover, the molten metal transfer apparatus 5 may be concomitantly installed
in all the tubs of the coating tub 1, the separating tub 2, and the adjusting tub 3, and may
be concohitantly 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
communicating vessel 6 or 7, the transferring vessel 8, the overflowing vessel 9, and the
f -
3 3
like. In the embodiment of FIGS. 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
the purified coating bath in which the dross is almost removed. Thus, by using the
molten metal transfer apparatus 5 only for the purified coating bath, it is possible to
minimize trouble of the molten metal transfer apparatus 5 such as dross clogging and the
like.
[0073]
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 of the vessel by the bath flow,
15 to prevent a decrease in the temperature and solidification of the bath in the vessel, and ,
the like. Therefor, it is preferable to use the double vessel which equipped with
ceramics inside the vessel, and furthermore, to keep warm or heat outer wall of the vessel.
Especially, before operating the bath circulation, it is preferable to prevent the
solidification of the bath in the vessel by pre-heating the vessel.
2 0 [0074]
€2.2. Overall structure of tubs]
Next, overall configuration of the 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 l),
and FIG. 8 (modification 4), the coating tub 1, the separating tub 2, and the adjusting tub
25 3 may be the configuration in which the tubs are independent respectively. For example,
3 4
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 of the separating tub 2 and the adjusting tub 3 are connected by the
communicating vessel 7, and the adjusting tub 3 and the coating tub 1 are connected by
the transferring vessel 8 with the molten metal transfer apparatus 5. In this way, it is
possible to simplify the overall configuration of the hot-dip-coating equipment by
making the height of the bath surface of the coating bath in each tub the same, by
circulating the coating bath through the vessels such as the communicating vessel, and by
using the mdlten metal transfer apparatus 5 only at the most downstream. Moreover, in
the configuration of the modification las shown in FIG. 5, the overflowing vessel 9 is
installed in upper part side of side wall of the coating tub 1, and the coating bath IOA
which is overflowed from the coating tub 1 flows down into the separating tub 2 through
the overflowing vessel 9.
[0075]
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
21 and 22, which may be the configuration in which the three tubs are seemingly unified.
Moreovei; & shown in the modification 2 in FIG. 6, the separating tub 2 and the
adjusting tub 3 may be composed by partitioning the inside of the 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
possible to simplify the equipm'ent configuration by unifying three or two tubs among the
35
coating tub 1, the separating tub 2, and the adjusting tub 3.
[0076]
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
5 independently control the bath temperature and the A1 concentration of the coating bath
in each tub, respectively. Specifically, the bath temperature T1 and A1 concentration A 1
of the coating bath are controlled at the coating tub 1, the bath temperature T2 and A1
concentration A2 of the coating bath are controlled at the separating tub 2, and the bath
temperature T3 and A1 concentration A3 of the coating bath are controlled at the
10 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 of the coating bath which is stored. The temperature
controllers are equipped with heating apparatus and bath temperature control apparatus.
15 The heating apparatus heats the coating bath of each tub, and the bath temperature
control apparatus controls operation of the heating apparatus. Thus, the bath
temperature of the 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.
20 In addition, although the sample for aluminum concentration measurement of each tub
may be p'erbdically sampled by manpower, it is preferable to respectively equip
aluminum concentration analyzer at each tub, in order to independently control the A1
concentration of the coating bath in each tub. The aluminum concentration analyzer is
composed by sampler for the sample of the aluminum concentration measurement, sensor
25 of the aluminum concentration bf the molten metal or alloy, or the like. The aluminum
36
concentration of the sample which is sampled by the sampler may be periodically
measured by chemical analyzer, or the aluminum concentration of the coating bath may
be continuously measured by the sensor of the aluminum concentration. Based on the
results of the aluminum measurement, the A1 concentration of the coating bath in each
5 tub is independently controlled by controlling the circulating volume or by supplying
first or second zinc-included-metal.
[0077]
Moreover, in all the embodiment of FIGs. 4 to 8, the coating bath 10A flows out
from coating bath outlet which is made by the communicating vessel 6, the overflowing
10 vessel 9, and the weir 2 1 and which is located on the upper part of the coating tub 1 and
downstream side of running direction of the steel sheet 11, and the coating bath 10A
flows into the separating tub 2. This is effective in that the entire coating bath 10A can
be circulated without stagnation of the coating bath 10A in the coating tub 1 by using the
flow of the coating bath 10A which is derived from the running of the steel sheet1 1.
15 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 10B 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 10B in the separating tub 2 contains the top-dross by high density as
20 compared with the lower part. Thus, by transferring the coating bath 10B of the lower
part of the sieparating tub 2 to the adjusting tub 3, the coating bath 10B of the lower part
where the content percentage of the top-dross is low can be transferred to the adjusting
tub 3, so that dross removal efficiency increases.
[0078]
[2.3. Configuration of each bath]
3 7
Next, the configuration of each bath of the coating tub 1, the separating tub 2,
and the adjusting tub 3 will be described.
[0079]
(1) Coating tub
First, the coating tub 1 will be described. As shown in FIGS. 4 to 8, the coating
tub 1 has the functions of (a) storing the coating bath 10A which includes the molten
metal at the predetermined bath temperature T1, and (b) coating the steel sheet 11 which
is dipped in the coating bath 10A. The coating tub 1 is the tub in which the steel sheet
11 is actually dipped in the coating bath 10A and in which the steel sheet 11 is coated by
the molten metal. The composition and the bath temperature T1 of the coating bath 10A
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 1OA is
the GI bath, as shown in FIG 9, the bath temperature T1 of the coating tub 1 is kept at
approximately 460°C by the temperature controller 1.
[OOSO]
In the coating bath 10A of the 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 10A of the coating tub 1, traveling
direction is changed by the sink roll 12, the steel sheet 11 is pulled up vertically upward
from the boding bath 10A, and excessive molten metal on the surface of the steel sheet
11 is wiped by the gas wiping nozzle 13.
[OOS 11
Moreover, it is preferable that storage Q1 [ton] (capacity of the coating tub 1) of
the coating bath 10A in the coat'ing tub 1 is 5 times or less of circulating volume q [ton 1
I
--
hour] of the coating bath 10 per one hour by the circulator. When the storage Q1 of the
coating bath 1OA is more than 5 times of the circulating volume q, stagnation time of the
coating bath 10A in the coating tub 1 is prolonged, so that possibility of the formation
and growth of the dross in the coating bath 10A increases. Thus, by controlling the
5 storage Q1 of the coating bath 10A to be 5 times or less of the circulating volume q, it is
possible that the stagnation time of the coating bath 10A in the coating tub 1 is controlled
to be predetermined time or shorter. In the conditions, when Fe is dissolved in the
coating bath 10A of the coating tub 1 from the steel sheet 11, the dross is not formed in
the coating bath 10A, or, even if the dross is formed, the coating bath 1 OA which contains
10 the dross flows out to the separating tub 2 before the dross grows up to the harmfbl size.
However, it is preferable that the capacity Q1 of the coating tub 1 is as small as possible,
because the coating bath 10A may stagnate in the tub and the dross may grow up to the
harmful size at the stagnation area depending on the shape of the coating tub 1.
15 In addition, during the operation of the hot-dip-coating, part of the coating bath .
10A 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 of the 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
20 preferable that the position where the coating bath 10C flows into the coating tub 1 is
located orl upstream side of the running direction of the steel sheet 11 and that the
position of the coating bath outlet where the coating bath 10A flows out to the separating
tub 2 is located on the upper part of the coating tub 1 and the downstream side of the
running direction of the steel sheet 11. Thereby, the local stagnation area of the coating
25 bath 10A in the coating tub 1 is hard to form. Thus, it can be suppressed that the dross
39
grows up to the harmful size at the local stagnation area in the coating tub 1. Here, the
upstream side of the running direction of the 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 of the steel sheet 11.
5 Similarly, the downstream side of the running direction of 'the steel sheet 11 is the side
including the pulling up position of the steel sheet 11 in case of longitudinally-halving
the coating tub 1.
[0083]
(2) Separating tub
10 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 10B which is transferred
from the coating tub 1 at bath temperature T2 which is lower than the bath temperature
T1 of the coating bath 10A in the coating tub 1, (b) precipitating the top-dross by
supersaturating Fe in the coating bath 10B and removing the precipitated top-dross by the
15 flotation separation. Since the A1 concentration of the GI bath is originally higher than .
that of the GA bath, the state (bath temperature and composition) of the coating bath 10B
in the separating tub 2 becomes the top-dross formation range only by controlling the
bath temperature T2 of the separating tub 2 to be lower than the bathtemperature T1 of
the coating tub 1.
20 [0084]
Pol" example, in case that the coating bath 10 is the GI bath, as shown in FIG. 9,
the bath temperature T2 of the separating tub 2 is kept at the temperature which is lower
5°C or more as compared with the bath temperature T1 of the coating tub 1 and is higher
than the melting point M (for example, melting point of 420°C of the GI bath) of the
25 molten metal which is the coatkg bath 10 (for example, 420°C 5 T2 5 T1-5°C).
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 10B by
transferring the coating bath 10 from the coating tub 1 to the separating tub 2 and by
decreasing the bath temperature T2. Thus, the top-dross can be suitably removed by the
5 flotation separation utilizing the difference in specific gravity.
[OOS5]
The principle will be described in detail. Fe which is dissolved from the steel
sheet 11 is included in the coating bath 10A which flows into the separating tub 2 from
the coating tub 1. The solubility limit of Fe decreases with the decrease in the bath
10 temperature T (from T1 to T2). Thereby, Fe becomes the supersaturated state in the
coating bath 10B of the separating tub 2, so that the dross which is equivalent to the
amount of the supersaturated Fe is precipitated. In case that the coating bath 10 is the
GI bath, the dross which is precipitated in the separating tub 2 by decreasing the bath
temperature T almost becomes the top-dross. As shown in FIG. 1, since the A1
15 concentration of the GI bath is 0.1 5 to 0.25 mass% and is higher than that of the GA bath, .
the state (bath temperature and composition) of the coating bath 10B in the separating tub
2 becomes the top-dross formation range only by controlling the bath temperature T2 of
the separating tub 2 to be lower than the bath temperature T1 of the coating tub 1. Thus,
the dross which is formed in the GI bath is only the top-dross and the bottom-dross
20 hardly forms. In other words, since the A1 concentration A2 of the coating bath 10B (GI
bath) in tlie kparating tub 2 is higher than 0.14 mass% which is the top-dross formation
range, the dross which is formed in the separating tub 2 is only the top-dross.
[OOS6]
As mention above, by precipitating only the top-dross in the coating bath 10B of
4 1
bath 10B becomes smaller than the specific gravity of the 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.
[0087]
In addition, the bath temperature T2 of the separating tub 2 is decreased to be
lower than the bath temperature T1 of the coating tub 1 in order to supersaturate Fe in the
bath, and the bath temperature T2 of the separating tub 2 is controlled to be higher than
the melting point M of the molten metal in order to avoid the solidification of the coating
bath 10B.
[OOSS]
As mentioned above, a large amount of the top-dross is intentionally formed in
the coating bath 10B at the separating tub 2 by decreasing the bath temperature T of the
coating bath 10. Since the top-dross comes to top surface of the coating bath 10B by
the difference in specific gravity compared with the coating bath 10B and is trapped at
the top surface, the flotation separation of the top-dross needs the time to a certain extent. ,
Thus, it is preferable that storage Q2 [ton] (capacity of the separating tub 2) of the
coating bath I OB in the separating tub 2 is 2 times or more of the circulating volume q
[ton / hour] of the coating bath 10 per one hour 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 of the
coating bkth 10 which flows into the separating tub 2 from the coating tub 1 to the
outflow into the adjusting tub 3. When the storage 42 of the coating bath 10B in the
separating tub 2 is less than 2 times of the circulating volume q of the coating bath 10 per
one hour, the time for the flotation separation of the top-dross is not sufficiently obtained,
so that the dross removal efficiency decreases.
In addition, during the operation of the hot-dip-coating, the part of the coating
bath 10A continuously flows into the separating tub 2 from the coating tub 1 through the
communicating vessel 6, the overflowing vessel 9, and the like, and the part of the
5 coating bath 10B in the separating tub 2 continuously flows out to the adjusting tub 3
through the communicating vessel 7 and the like.
(3) Adjusting tub
Next, the adjusting tub 3 will be described. As shown in FIGS. 4 to 8, the
10 adjusting tub 3 has the functioris 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 TI of the coating tub 1 and the bath temperature T2 of the separating tub 2,
(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
15 T3 and the A1 concentration A3 of the coating bath 10C which is transferred to the
coating tub 1 in order to keep constantly the bath temperature T1 and A1 concentration
A1 of the coating tub 1.
1009 11
The adjusting tub 3 is the tub in which a metal (correspond to the first
20 zinc-included-metal or the second zinc-included-metal) is supplied and melted in order to
supply th'e molten metal which is consumed at the coating tub 1. The adjusting tub 3
also has the functions of reheating the bath temperature T which was lowered in the
separating tub 2. Moreover, in case of increasing the A1 concentration A2 of the bath in
the separating tub 2 by supplying the metal with high A1 concentration (first
25 zinc-included-metal) to the separating tub 2 (refer to FIG. 10 as explained below), the
43
adjusting tub 3 also has the functions of decreasing and optimizing the A1 concentration
of the bath by supply the metal with low A1 concentration (second zinc-included-metal).
[0092]
In order to decrease the A1 concentration of the coating bath 10 in the adjusting
5 tub 3, the zinc-included-metal which includes A1 with the concentration lower than the A1
concentration A2 of the coating bath 10B in the separating tub 2 or the
zinc-included-metal which does not include A1 may be supplied and melted in the coating
bath 10C of the adjusting tub 3 as the second zinc-included-metal. By supplying the
metal with low A1 concentration, the A1 concentration A3 of the coating bath 10C which
10 is transferred from the adjusting tub 3to the coating tub 1 is preferably controlled (A2 >
A3 > Al), so that it is possible that the A1 concentration A1 of the coating bath 10A in the
coating tub 1 is kept constantly to the proper concentration which is suitable for the
composition of the intended GI bath. For example, in the GI bath, the A1 concentration
A1 of the coating bath 10A in the coating tub 1 is controlled to the constant concentration
15 within the range of 0.15 to 0.25 mass%.
100931
On the contrary, in case of not supplying any zinc-included-metal to the
separating tub 2 (refer to FIG 11 as explained below), the molten metal (A1 and Zn)
which is consumed at the coating tub 1 may be supplied by supplying the
20 zinc-included-metal (first zinc-included-metal) which includes A1 with the concentration
higher thhn ;the A1 concentration A1 of the coating bath 10A in the coating tub 1. In the
case, the adjusting tub 3 also has the functions of increasing and optimizing the A1
concentration of the bath and of supplying Zn into the system by supply the
zinc-included-metal (first zinc-included-metal) with high A1 concentration.
[0094]
44
Moreover, it is necessary to control the bath temperature T3 of the adjusting tub
3 by the temperature controller 3 to the temperature range which does not become the
problem even if the coating bath 10C flows into the coating tub 1. Thus, as shown in
FIG. 9, it is preferable that the bath temperature T3 is controlled within *lO°C on the
5 basis of the temperature in which the difference of the bath temperature decrease ATfall is
added to the bath temperature T1 of the coating tub 1 (TI + ATfau - 10°C 5 T3 < T1 +
ATfall + 1 O°C). Here, the difference of the bath temperature decrease ATfal, is the value
of the bath temperature decrease of the coating bath 10 which occurs naturally when the
coating bath 10C is transferred from the adjusting tub 3 to the coating tub 1. When the
10 bath temperature T3 of the adjusting tub 3 does not satisfy the temperature range, the
bath temperature deviation in the coating tub lincreases, so that the formation and
growth of the dross in the coating tub 1 are promoted. Moreover, the bath temperature
T4 of the coating bath 10C at the inlet of the coating tub 1 becomes within the range of
*lO°C on the basis of the bath temperature T1 of the coating tub 1 (TI - 10°C < T4 < T1
15 +lO°C).
[0095]
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 10C, it is preferable that the
bath temperature T3 of the adjusting tub 3 is controlled to be higher 5°C or more as
20 compared with the bath temperature T2 of the separating tub 2 (T3 L T2 + 5°C).
Althougfi t k bath temperature TI, T2, and T3 of each tub are controlled by an induction
heating apparatus and the like, the bath temperature fluctuation of approximately *3"C in
general is inevitable because of the limitation of control accuracy. In consideration of
the situation of the control accuracy, that is the maximum (+3"C from the targeted bath
25 temperature) and the minimum'(-3"~ from the targeted bath temperature) of the bath
c
L
45
temperature fluctuation, it is preferable that the bath temperature T3 (targeted value) of
the adjusting tub 3 is higher at least 5OC or more as compared with the bath temperature
T2 (targeted value) of the separating tub 2. Thereby, it is possible that Fe of the coating
bath 10C in the adjusting tub 3 is the unsaturated state. Namely, it is possible that the
5 residual dross with small size which is contained in the coating bath 10B transferred from
the separating tub 2 is certainly dissolved and removed in the adjusting tub 3. When the
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.
10 [0096]
In addition, storage 43 [ton] (capacity of the adjusting tub 3) of the coating bath
10C in the adjusting tub 3 is arbitrary and is not limited in particular, if melting the metal,
keeping the bath temperature T3, and transferring the bath to the coating tub 1 are
possible.
15 [0097]
By the way, when the metal with low A1 concentration (the first
zinc-included-metal or the second zinc-included-metal) is supplied into the adjusting tub
3, the bath temperature decreases locally to the melting point of the metal at minimum
around the metal which is dipped into the coating bath 10C of the adjusting tub 3, so that
20 the dross forms. Since Fe is the unsaturated state in the coating bath 10 of the adjusting
tub 3, the' fdhned dross is dissolved relatively promptly, so that the dross is harmless in
general. However, 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 10C
and may flow out to the coating tub 1.
46
Thus, in the above case, as shown in the modification 4 in FIG 8, the premelting
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 premelting tub 4 may be supplied to the adjusting
tub 3. Thereby, it is possible to supply the molten metal which is preheated to
5 approximately the bath temperature T3 at the premelting tub 4 to the adjusting tub 3 and
to prevent the temperature of the coating bath 10C in the adjusting tub 3 from decreasing
locally. Namely, it is possible to avoid the problem such that the dross forms by
supplying the metal at the adjusting tub 3.
[0099]
10 In addition, during the operation of the hot-dip-coating, the part of the coating
bath 10B continuously flows into the adjusting tub 3 from the separating tub 2 through
the communicating vessel 7 and the like, and the part of the coating bath 10C in the
adjusting tub 3 continuously flows out to the coating tub 1 through the transferring vessel
8 and the like.
15 [0 1001
[3. Manufacturing method of galvanized steel sheet]
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 of the
galvanized steel sheet) will be described. FIG 10 is the ternary phase diagram which
20 indicates state transition of the coating bath 10 (GI bath) in each tub according to the
embodi&n€.
[OlOl]
In the manufacturing method of the galvanized steel sheet according to the
embodiment, the coating bath 10 (GI bath) is circulated by the circulator which includes
25 the molten metal transfer apparatus 5, the vessel, and the like in order of the coating tub I
47
(for example, bath temperature: 460°C, A1 concentration: approximately 0.200 mass%),
the separating tub 2 (for example, bath temperature: 440°C, A1 concentration:
approximately 0.217 mass%), and the adjusting tub 3 (for example, bath temperature:
465"C, A1 concentration: approximately 0.205 mass%). And the following processes
5 are simultaneously and parallelly conducted in each tub of the coating tub 1, the
separating tub 2, and the adjusting tub 3.
[O 1021
(1) Coating process at the coating tub 1
First, in the coating tub 1, the coating bath 10A which is stored in the coating tub
10 1 is kept at the predetermined bath temperature T1, and the steel sheet 11 which is dipped
in the coating bath 10A is coated. In the coating process, the coating bath 10C which is
transferred from the adjusting tub 3 flows into the coating tub 1, and the part of the
coating bath 10A flows out from the coating tub lto the separating tub 2. In the coating
tub 1, since the steel sheet 11 is continuously dipped in the coating bath 10A and Fe is
15 dissolved from the steel sheet 11 and is sufficiently supplied to the coating bath 10A, the .
Fe concentration reaches approximately the saturated concentration.
However, as mentioned above, the stagnation.time of the coating bath 10A 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
20 does not form until the Fe concentration of the coating bath 1 OA reaches the saturation
point. Mokover, even if the dross forms, the dross is only small size and does not grow
up to the large harmful size. Furthermore, since the coating tub 1 is miniaturized as
compared with the conventional coating tub, the stagnation time of the circulating
coating bath 10 in the coating tub 1 is shortened. Therefore, it is possible that the dross
25 growth to the harmful size in the coating tub 1 is certainly avoided.
(2) Separating process at the separating tub 2
Next, the circulation bath which flows out from the coating tub 1 led to the
separating tub 2. In the separating tub 2, the bath temperature T2 of the coating bath
5 10B 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 T1 of the coating tub 1, and the A1
concentration A2 of the coating bath 10B is controlled to the concentration higher than A1
concentration A1 of the coating bath in the coating tub 1. In the separating tub 2, Fe
which is supersaturated in the coating bath 10B is precipitated as the top-dross.
For example, as shown in FIG. 10, when the coating bath 10A of the coating tub
1 is transferred to the separating tub 2, the bath temperature T decreases drastically from
T1 (460°C) to T2 (440°C), and the A1 concentration increases from A1 (approximately
0.200 mass%) to A2 (approximately 0.217 mass%). As the results, Fe becomes the
15 supersaturated state in the coating bath 10B of the separating tub 2, so that the excessive ,
Fe in the coating bath 10B of the separating tub 2 is precipitated as the top-dross (Fe2A15).
As explained in Table 1, the dross forms easily when the bath temperature decreases. In
the embodiment of the GI bath of FIG 10, Fe in the coating bath 10 transferred fiom the
coating tub lto the separating tub 2 becomes the supersaturated state by the decrease in
20 the bath temperature T, so that a large amount of the top-dross is formed in the separating
tub 2, de~jelidingo n the super saturated degree. At the time, the A1 concentration A2 of
the coating bath 1 OB is, for example, 0.14 mass% or more, which is the high
concentration where the state of the coating bath 10B becomes the top-dross formation
range under the condition of the bath temperature T2, so that the top-dross only forms
25 and the bottom-dross hardly fohns. Thus, the top-dross which precipitates in the
49
coating bath 10B of the separating tub 2 comes to top surface of the coating bath 10B of
the separating tub 2 by the difference in specific gravity compared with the coating bath
10B (molten zinc bath), and the dross is separated and removed. In addition, the Fe
concentration of the coating bath 1 OB at the outlet of the separating tub 2 is slightly
5 higher concentration than the saturation point of the Fe concentration, because the
residual dross with small size which is not completely separated in the separating tub 2 is
contained.
[0 1 051
Since the capacity Q2 of the separating tub 2 is sufficiently large as compared
10 with the circulating volume q of the bath and the stagnation time of the coating bath in
the separating tub 2 is 2 hours or more, most of the top-dross is separated by the flotation
and removed outside the system. Moreover, in order to control the A1 concentration A2
of the bath in the separating tub 2 to be, for example, 0.14 mass% or more, small amount
of the metal with high A1 concentration (first zinc-included-metal) which includes A1
15 with the concentration higher than the A1 concentration A1 of the bath in the coating tub ,
1 is supplied and melted in the separating tub 2.
[0 1061
(3) Dissolving process of dross and adjusting process of bath temperature and A1
concentration at the adjusting tub 3
20 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 of the separating tub 2, and the A1 concentration A3 of the
adjusting tub 3 is controlled to be higher than the A1 concentration A1 of the coating tub
25 1 and lower than the A1 concenhation A2 of the separating tub 2. In the adjusting tub 3,
the dross which is contained in the coating bath 10C is dissolved by controlling Fe of the
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.
[0 1071
For example, as shown in FIG. 10, when the coating bath 10B 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 A1
concentration decreases from A2 (approximately 0.2 17 mass%) to A3 (approximately
10 0.205 mass%). As the results, Fe becomes exceedingly the unsaturated state in the
coating bath 10C of the adjusting tub 3, so that the top-dross (FezAls) with small size
which is residual in the bath is decomposed (dissolved) into Fe and A1 relatively
promptly and disappears. In this way, in case of dissolving the residual dross, the
coating bath 10C of the adjusting tub 3 is still the state in which Fe is unsaturated.
15 [0108]
In addition, the metal with low A1 concentration (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 1 oc of the adjusting tub 3. In case that the dross which
forms by melting the metal becomes the problem, as shown in FIG 8, the premelting tub
20 4 may be installed beside the adjusting tub 3, and the molten metal which is melted in the
premeltin'g titb 4 may be supplied to the adjusting tub 3. Moreover, since the metal with
high A1 concentration (first zinc-included-metal) is supplied to the separating tub 2, the
A1 concentration of the circulation bath becomes excessive high concentration. Thus,
the second zinc-included-metal which is supplied to the adjusting tub 3 is the
25 zinc-included-metal with A1 concentration lower than the aluminum concentration A3 of
the coating bath 10B in the separating tub 2 or the zinc-included-metal which does not
include Al. By supplying the second zinc-included-metal with low A1 concentration, the
A1 concentration A3 of the bath in the adjusting tub 3 decreases to be lower than the Al
concentration A2 of the separating tub 2 and is controlled to the concentration which is
5 suitable to keep constantly the A1 concentration A1 of the coating tub 1.
[0 1091
Thereafter,. the coating bath 10C of the 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
10 transferred from the adjusting tub 3 to the coating tub 1, the bath temperature T decreases
naturally by the difference of the bath temperature decrease ATfall as described above.
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 10A from the steel sheet 11 which is dipped in the coating
15 tub 1, the Fe concentration of the bath reaches gradually approximately 0.012 mass%
which is the saturation point at the bath temperature T1 (460°C). Moreover, in the
coating tub 1, A1 is consumed by reacting the steel sheet 11 and the coating bath 10A.
Thus, even if the coating bath 1 OC with relatively high A1 concentration A3
(approximately 0.205 mass%) is transferred from the adjusting tub 3 to the coating tub 1,
20 the A1 concentration A1 of the coating tub 1 hardly increases and keep at nearly constant
value (apprdximately 0.200 mass%).
Moreover, the coating tub 1 is miniaturized as mentioned above, and the
stagnation time of the circulating coating bath 10 in the coating tub 1 is short. Thus,
25 even if the operational fluctuation such as the bath temperature fluctuation occurs to a
certain extent in the coating tub 1, the top-dross is not formed in the coating tub 1 until
the Fe concentration of the coating bath 10A reaches the saturation point (for example,
0.012 mass%). Moreover, even if the Fe concentration of the bath in the coating tub 1
reaches the saturation point and the dross with small size forms, the formed dross does
5 not grow up to the harmhl size (for example, 50 pm or more) during 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.
[Olll]
Moreover, t h e ' ~ceo ncentration of the coating bath 10A in the coating tub 1
varies depending on, for example, the capacity Q1 of the coating tub 1, the circulating
volumes q, dissolvability of Fe, and the like. Thus, Fe of the coating bath IOA can be
the unsaturated state (in case that the Fe concentration is less than 0.012 mass%). In the
15 case, since Fe is unsaturated, the dross hardly forms. Contrary, Fe of the coating bath .
10A also can be slightly the supersaturated state (in case that the Fe concentration is
slightly more than 0.012 mass%). In the case, since the dross which forms in the
coating bath 10A within short time is the small size, the problem such as the dross
defects does not occur.
[0112]
As explained above, by circulating the coating bath 10 in order of the 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 of the galvanized steel sheet
is removed and is almost-completely rendered harmless. Therefore, the coating bath
25 1 OA of the coating tub 1 can be'continuously controlled to the dross-free state.
5 3
Moreover, the problems such as the appearance deterioration of the 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 of the roll in the coating bath, and the
like are solvable. When performing the dross removal by using the manufacturing
5 equipment according to the embodiment, it is unnecessary to stop the sheet threading of
the coated steel sheets. The coating bath 10 is circulated in order of the 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 10A of the coating tub 1 can be continuously controlled to the dross-free and
10 clean state.
Next, in reference to FIG. 10, method of adjusting the A1 concentration of the
coating bath 10 by supplying the metal to the coating bath 10 which is circulated between
the tubs will be described.
15 [0114]
The A1 concentration in the coating layer of the galvanized steel sheets (GI) is,
for example, 0.3 mass% on average, and is higher than the A1 concentration A1 (0.200
mass%) of coating bath 10A in the coating tub 1. Namely, A1 of the coating bath 10A is
concentrated and coated to the coating layer of the steel sheet 11. Therefore, if the A1
20 concentration of the metal which is supplied to the coating bath 10 is 0.200 mass%, the
A1 concehtration of the coating bath 10A decreases gradually. Thus, in the conventional
supply of the metal which is spot-like, A1 concentration is maintained by supplying the
metal with A1 concentration of 0.3 to 0.6 mass% directly to the coating tub.
[0115]
In the hot-dip-coating equipment according to the embodiment, the coating bath
5 4
10 is continuously transferred from the adjusting tub 3 to the coating tub 1. In order to
control the A1 concentration A1 of the coating tub 1 to be 0.200 mass% for example, it is
necessary to keep supplying the coating bath 10 in which the A1 concentration is higher
than 0.200 mass% (for example, 0.205 mass%) to the coating tub 1 from the adjusting
5 tub 3. Thus, in order to control the A1 concentration A3 of the adjusting tub 3 to be
approximately 0.205 mass% which is the target, the A1 concentration A2 of the separating
tub 2 is kept at high concentration (for example, 0.2 17 mass%) which is higher than A3
by supplying intentiopally A1 to the separating tub 2. Moreover, in the separating tub 2,
in order that the large amount of the top-dross is precipitated and separated by the
10 flotation, it is preferable that the A1 concentration A2 of the bath in the separating tub 2 is
controlled to high concentration. Therefore, the metal with high A1 concentration (for
example, 10 mass% A1 - 90 mass% Zn) as the first zinc-included-metal is supplied into
the separating tub 2, and the A1 concentration A2 of the coating bath 1OB in the
separating tub 2 is controlled to high. Here, the amount of A1 supplied to the separating
15 tub 2 is equivalent to the total of the amount of A1 consumed as the top-dross at the
separating tub 2 and the amount of A1 consumed as the coating layer of the steel sheet 11
at the coating tub 1.
[0116]
On the other hand, in the adjusting tub 3, the metal with low A1 concentration
20 and high Zn concentration (for example, the zinc-included-metal which is 0.1 mass% A1 -
Zn or the' zihc-included-metal which does not contain Al) as the second
zinc-included-metal is supplied. Thereby, the A1 concentration of the coating bath 10B
transferred from the separating tub 2 to the adjusting tub 3 decreases, and the A1
concentration A3of the coating bath 1OC in the adjusting tub 3 is controlled to
25 approximately the A1 concentration (for exaniple, 0.205 mass%) which is intermediate
5 5
value of the A1 concentration A2 of the separating tub 2 and the A1 concentration A1 of
the coating tub 1. By transferring the coating bath 10C from the adjusting tub 3to the
coating tub 1, the A1 concentration A1 of the bath in the coating tub 1 can be controlled
to the proper concentration (for example, 0.200 mass%) which is suitable for
5 manufacturing the GI.
[0117]
As described above, in the hot-dip-coating equipment according to the
embodiment, the supply of the coating bath and the composition of the coating bath, for
example, the A1 concentration, are controlled by supplying the metal to the separating tub
10 2 and the adjusting tub 3. Therefore, it is not necessary to supply the metal directly to
the coating tub 1, so that it is possible to prevent the dross from forming by the change of
the bath temperature around the metal.
[0118]
Next, in referenceto FIG 11, the modification of the manufacturing method of
15 the galvanized steel sheet according to the embodiment will be described. FIG. 11 is the ,
ternary phase diagram which indicates the state of the GA bath according to the
embodiment. FIG 11 is the ternary phase diagram which indicates the state transition of
the coating bath 10 (GI bath) in each tub according to modification of the embodiment.
[0118]
20 As shown in FIG 11, in the manufacturing method of the galvanized steel sheet
according tci the modification of the embodiment, the coating bath 10 (GI bath) is
circulated by using the circulator in order of the coating tub 1 (for example, bath
temperature: 460°C, A1 concentration: approximately 0.200 mass%), the separating tub 2
(for example, bath temperature: 440°C, A1 concentration: approximately 0.199 mass%),
25 and the adjusting tub 3 (for example, bath temperature: 465"C, A1 concentration:
approximately 0.205 mass%). In the case, the bath temperature T1, T2, and T3 of the
coating tub 1, the separating tub 2, and the adjusting tub 3 respectively satisfies the
relation of T3 > TI > T2, which is the same as the embodiment of FIG 10 as explained
above. On the other hand, the A1 concentration Al, A2, and A3 of the bath in the
5 coating tub 1, the separating tub 2, and the adjusting tub 3 respectively satisfies the
relation of A3 > A1 2 A2, which is different from the embodiment (A2 > A3 > Al) of FIG
10 as explained above. The A1 concentration A3 of the bath in the adjusting tub 3 is
increased by supplying the metal with high Al concentration (first zinc-included-metal)
only to the adjusting tub 3 and by not supplying any metal to the separating tub 2. The
10 reason will be described below.
[O 1201
In the embodiment of FIG 10 as explained above, by supplying the
zinc-included-metal with high A1 concentration to the separating tub 2, the A1
concentration A2 of the separating tub 2 is controlled to be sufficiently higher than the A1
15 concentration A1 of the coating tub 1 (A2 > Al). Certainly, in case of manufacturing ,
the GAY it is necessary to control the A1 concentration A2 of the separating tub 2 to be
higher than the A1 concentration A1 of the coating tub 1 (for example, 0.14 mass% or
more) in order to precipitate only the top-dross at the separating tub 2. Thereby, since
the dross formation range of the coating bath 10B in the separating tub 2 can be
20 transitioned fiom the bottom-dross and top-dross mixed range to the top-dross formation
range, the fbrmation of the bottom-dross can be prevented in the separating tub 2 (refer to
FIG. I).
[0121]
On the contrary, in case of manufacturing the GI, since the A1 concentration A1
25 of the coating tub 1 is sufficiently high concentration (0.14 mass%or more), it is not
necessary to increase the A1 concentration A2 of the separating tub 2 such as the GAY and
the dross formation range of the GI bath belongs originally to the top-dross formation
range (refer to FIG 1). Thereby, it is possible that all the dross which is precipitated in
the separating tub 2 is to be the top-dross only by controlling the bath temperature T2 of
5 the separating tub 2 to be less than the bath temperature T1 of the coating tub 1.
[O 1221
Thus, in the modification as shown in FIG. 11, it is possible to precipitate the
top-dross at the separating tub 2 by controlling the bath temperature T2 (440°C) of the
separating tub 2 to be less than the bath temperature T1 (460°C) and by not supplying
10 any metal to the separating tub 2. In the case, the A1 concentration A2 of the separating
tub 2 becomes almost the same as the A1 concentration A1 of the coating tub 1 (A2 = Al),
or becomes lower than A1 by the amount of A1 which is equivalent to the formation of
the top-dross (A2 < Al).
[0 1231
15 After separating by the flotation the top-dross in the separating tub 2, the coating ,
bath 10B in the separating tub 2 is transferred to the adjusting tub 3, and the bath
temperature T is increased from T2 (440°C) to T3 (460°C). Thereby, since Fe of the
coating bath 10C in adjusting tub 3 becomes the unsaturated state, the dross with small
size which is contained in the coating bath 10B transferred from the separating tub 2 is
20 dissolved in the coating bath 10C of the adjusting tub 3 and disappears.
Moreover, the first zinc-included-metal is supplied to the adjusting tub 3 in order
to supply the molten metal which is consumed at the coating tub 1. The first
zinc-included-metal is the zinc-included-metal which includes A1 with the concentration
25 higher than the A1 concentration A1 in the coating tub 1 (for example, 10 mass% A1 - 90
mass% Zn). Here, the amount of A1 which is included in the zinc-included-metal
supplied to the adjusting tub 3 is equivalent to the total of the amount of A1 consumed as
the top-dross at the separating tub 2 and the amount of Al consumed as the coating layer
of the GI at the coating tub 1.
By supplying the zinc-included-metal with high A1 concentration to the
adjusting tub 3, t h e ~clo ncentration A3 of the bath in the adjusting tub 3 becomes higher
than the A1 concentration A1 in the coating tub 1 and the A1 concentration A3 of the
separating tub 2 (A3 > A1 2 A2). Thereby, Zn and A1 which are consumed for the
10 coating process at the coating tub 1 are supplied in the adjusting tub 3. Moreover, by
controlling the A1 concentration A3 of the coating bath 10C in the adjusting tub 3 to
approximately the A1 concentration (for example, 0.205 mass%) which is intermediate
value of the A1 concentration A2 of the separating tub 2 and the A1 concentration A1 of
the coating tub 1 and by transferring the coating bath 10C to the coating tub 1, the A1
15 concentration A1 of the bath in the coating tub 1 can be controlled to the proper
concentration (for example, 0.200 mass%) which is suitable for manufacturing the GI.
[0 1261
As described above, in the modification of the embodiment, by supplying the
metal only to the adjusting tub 3, the supply for the bath element and the adjustment of
20 the A1 concentration are conducted. Therefore, it is not necessary to supply the metal
directly tb the coating tub 1, so that it is possible to prevent the dross from forming by
the change of the bath temperature around the metal. Moreover, since it is not
necessary to supply the metal to the separating tub 2, it is possible to simplify the
equipment configuration. When the metal is supplied to the adjusting tub 3, the metal
25 may be preliminarily melted by using the premelting tub 4 and the molten metal may be
supplied to the adjusting tub 3. Thereby, it is possible to prevent the dross from forming
by the change of the bath temperature around the metal in the adjusting tub 3.
[0 1 271
In the above, the manufacturing equipment and the manufacturing method of the
5 galvanized steel sheet according to the embodiment were described in detail.
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
10 (coating rate) of the steel sheet 11 is suppressed and the productivity has to be sacrificed
in order to prevent the dross from raising up in the coating bath 10 is improved, so that
the coating rate can be increased and the productivity of the galvanized steel sheets is
improved.
15 Example
[0128]
[4. Example]
Hereinafter, the examples of the present invention will be described. The
following examples only show the test result concretely for the verification of the effect
20 of the present invention, so that the present invention is not limited to the examples.
[.01?91
[4.1. Test 1 : Coating test of the galvanized steel.sheet (GI)]
The circulation-type hot-dip-coating equipment (correspond to the
hot-dip-coating equipment according to the above described embodiment) was installed
25 in the pilot line, the continuous coating tests which manufactures the galvanized steel
sheet (GI) were conducted. The test conditions of the continuous coating test are 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,
AT1-2 in Table 2 is the bath temperature difference between the bath temperature T1 of
5 the coating tub 1 and the bath temperature T2 of the separating tub 2 (=TI-T2).
[0130]
(1) Conventional hot-dip-coating equipment
Capacity Q1 of coating tub : 60 ton
(2) Circulation-type hot-dip-coating equipment
10 Capacity Q1 of coating tub : 10 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 I hour and 6 ton I hour
[0131]
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 I m2 (both
sides) and the coating rate was 100 m 1 min by using the coil with 0.6 mm in sheet
thickness and 1000 mm in sheet width. And the difference of the bath temperature
decrease ATfan at transferring the bath from the adjusting tub 3 to the coating tub 1 was 2
20 to 3OC.
Thk samples were taken by rapid-cooling the bath of each tub at beginning and
ending of the 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 of the
25 coating tub 1 was drained, and the existence of the sedimented dross was observed at the
8 -
*
bottom of the tub.
Moreover, the A1 concentration and Fe concentration of each tub were measured
every 4 hours.
At the beginning of the 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 of the temperature controller of each tub. The control accuracy of the
bath temperature by the temperature controller of each tub was less than k3"C. In
addition, the circulator of the 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 A1 concentration of the bath in the separating tub 2 and
15 the adjusting tub 3, the metal of 0.38 mass% A1 - Zn was supplied to the separating tub 2 ,
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 coating tub.
[0132]
20 The test results are shown in Table 3 and Table 4. Table 3 shows the A1
concentrdtitidsl and the Fe concentration of the coating tub, the separating tub, and the
adjusting tub as of the lapse of 12 hours, and Table 4 shows the density of the flowed
dross in the coating tub and the visual observed amount of the sedimented dross at the
bottom of the coating tub as of the lapse of 12 hours.
In addition, the targetea values of the 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 of the steel sheet
11 was relative low among the present operational conditions for the GI. Thereby, "0.07
mg / cm3 or less" as the targeted value of the density of the top-dross was obtained.
[0133]
[Table 21
[0134]
[Table 31
[0135]
[Table 41
[0 1361
From the test results as shown in Table 3 and Table 4, in examples 1 to 5, the
density of the top-dross was the targeted value "0.07 mg / cm3" or less, so that the effect
of the dross removal was confirmed. Especially, in example 1, most of the dross was
15 removed, so that the dross-free was almost-completely achieved. In example 1, the
capacity Q2 of the separating tub 2 was 4.0 times (= 40 / 10) of the circulating volume q
of the bath per one hour, which was sufficiently higher than 2 times of the criteria. Thus,
in example 1, since the time for the flotation separation of the top-dross was sufficiently
obtained at the separating tub 2, the density of the top-dross in the coating tub 1 was
20 sufficiently low. Contrary, in example 2, the capacity 42 of the separating tub 2 was 2.0
times (= 12;/ 6) of the circulating volume q of the bath per one hour, which was equal to
2 times of the criteria. Thus, in example 2, since the time for the flotation separation of
the top-dross at the separating tub 2 was shortened as compared with that of example 1,
the dross separation effect decreased. As the result, in example 2, since the small
25 amount of the top-dross which'was formed in the separating tub 2 was flowed back to the
coating tub 1, the density of the top-dross in the coating tub 1 was higher than that of
example 1.
[0137]
On the other hand, in comparative example 1, the large amount of the top-dross
5 existed. The reason seems that, since the bath temperature T2 of the separating tub 2
equalized with the bath temperature TI of the coating tub 1, the dross removal effect
decreased in the separating tub 2. In addition, in comparative example 2 of the
conventional coating tub, the density of the top-dross was excessively larger than the
targeted value "0.07 mg 1 cm3". The reason seems that the coating test was conducted
10 by using only the coating tub without installing the separating tub and the adjusting tub
and that the metal was melted in the coating tub.
[0138]
As shown in Table 2, the bath temperature T2 of the separating tub 2 was 454°C
in example 3, 455°C in example 4, and 456°C in example 5, and thereby the bath
15 temperature difference AT1-2 (=TI -T2) between the bath temperature T1 (460°C) of the .
coating tub 1 and the bath temperature T2 of the separating tub 2 was controlled to 6°C in
example 3, 5°C in example 4, and 4°C in example 5. From examples 3 to 5, the
influence of the bath temperature difference AT1-2 on the dross formation was verified.
As the results, as shown in Table 4, in examples 1 to 4, since the bath temperature
20 difference AT]-2 between the bath temperature T1 of the coating tub 1 and the bath
temperatdreiT2 of the separating tub 2 was 5°C or more (TI - T2 2 5"C), the density of
the flowed dross was notably low and the effect of the present invention was sufficiently
obtained. On the other hand, in example 5, since the bath temperature difference AT1-2
was less than 5°C (T1 - T2 < 5°C) (for example, 4"C), the density of the flowed dross
25 was close to the upper limit (0.07 mg / cm3) which was the target, and the small amount
64
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 AT1-2 between the bath
temperature T1 of the coating tub 1 and the bath temperature T2 of the separating tub 2 is
5 5°C or more.
[0 1391
[4.2. Test 2 : Verification test of separation efficiency of bottom-dross and
top-dross]
Next, the results of the test to verify the separation efficiency of the
10 bottom-dross and the top-dross by using the separation by the difference in specific
gravity will be described.
[0 1401
The specific gravity of the top-dross is 3900 to 4200 kg 1 m3, and the specific
gravity of the bottom-dross is 7000 to 7200 kg / m3.
15 By analyzing the results of the flow simulation which simulated the dross
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 of the separation by the difference in specific
20 gravity of the top-dross and the bottom-dross.
p i a i l
[Table 51
[0 1421
From the test results as shown in Table 5, the separation efficiency of the
25 top-dross was higher than that of the bottom-dross in any case that the grain size was 50
pn, 30 pn, and 10 p.m. Therefore, it is confirmed that the dross separation by the
difference in specific gravity is effective under the condition of the top-dross.
[0 1431
[4.3. Test 3 : Verification test of capacity of separating tub]
Next, the results of the test to investigate, by using the flow analysis, the
capacity 42 of the 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 of the analysis % . were as follows.
. .
[0 1441
10 Circulating volume of bath : 40 ton 1 hour
Capacity of separating tub : 20 to 160 ton
Size of top-dross : 30 pm
[0 1451
The result of the analysis test is shown in FIG 12. As shown in FIG 12, when
15 the capacity Q2 of the separating tub 2 is 2 times or more of the circulating volume q (40 ,
ton / hour) of the coating bath per one hour, the separation efficiency of the dross
becomes 80% or more. When the capacity 42 of the separating tub 2 is less than 2
times of the circulating volume q of the bath, the separation efficiency of the dross
decreases drastically. From the result, it turns out that it is preferable that the capacity
20 42 of the separating tub 2 is 2 times or more of the circulating volume q of the bath ((Q2
iq)?2). .
[0146]
[4.4. Test 4 : Verification test of capacity of coating tub]
Next, the results of the bath circulation test to investigate the stagnation time of
25 the coating bath 10A so that the'dross which is formed in the coating bath 10A (GI bath)
of the coating tub 1 does not grow up to the harmfhl size by using the pilot line of the
galvanizing will be described. The test conditions were as follows.
[0147]
Criteria1 bath temperature T1 of the coating tub (intended bath temperature) :
5 460°C
A1 concentration of bath : 0.20 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
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 : 5 to 60 ton / hour
[0148]
After changing the circulating volume of the 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
of the capacity Q1 of the coating tub 1 was circulated and finished.
The samples were taken from the coating bath which was overflowed from the
coating tdb 1 just before each level of the 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
25 approximately i3"C. Howevk, in order to confirm the conditions to make the dross
67
harmless stably, the test was conducted under the condition where the dross tended to
form and grow as compared with the general condition.
[0 1491
The result of the test is shown in FIG 13. As shown in FIG. 13, when the
5 circulating volume q of the bath per one hour was less than 12 ton I hour (namely, the
capacity Q1 of the coating tub 1 was more than 5 times of the circulating volume q of the
bath per one hour (Q11 q) > 5), the maximum size of the dross which was actually
observed was larger than the harmful size (50 pm), The reason seems that, since the
stagnation time of the coating bath in the coating tub 1 was prolonged, the dross notably
10 grew up to the harmfbl size. Contrary, when the circulating volume q of the bath per
one hour was 12 ton 1 hour or more (namely, the capacity Q1 of the coating tub 1 was 5
times or less of the circulating volume q of the bath per one hour (Q1 1 q) 5 5), the dross
with small size (approximately 27 pm or less) which was sufficiently smaller than the
harmfbl size (50 pm) was only observed. The reason seems that, since the stagnation
15 time of the coating bath in the coating tub 1 was short, the dross did not grow up to the ,
harmful size. Therefore, it turns out that it is preferable that the capacity Q1 of the
coating tub 1 is 5 times or less of the circulating volume q of the bath per one hour.
[0150]
[4.5. Test 5 : Verification test of proper range of inflow bath temperature of
20 coating tub]
Nekt, the results of the test to verify the proper range of the bath temperature T3
of the 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
25 T1 of the coating tub 1, the bath temperature deviation in the coating tub 1 is promoted.
6 8
As the result, it seems that the formation and the growth of the dross in the coating tub 1
are accelerated. Thus, the verification test of proper range of the bath temperature T3 of
the adjusting tub 3 was conducted by using the pilot line of the galvanizing. The test
conditions were as follows.
Criteria1 bath temperature T1 of the coating tub (intended bath temperature) :
460°C
A1 concentration of bath : 0.20 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
Coating weight : 100 g / m2 (both sides)
Bath temperature fluctuation : k5"C (fluctuated intentionally by controlling the
heating output)
Capacity Q1 of coating tub : 60 ton
Circulating volume q of bath : 20 ton / hour
Inflow bath temperature (T3 - ATfall) : 445 to 480°C (ATfan 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)
[0151]
After changing the inflow bath temperature, the circulating volume q of the bath
was kept cbnstant until the coating bath in the coating tub 1 was completely replaced.
Specifically, bath circulation was continued until the coating bath of 3 times of the
capacity Q1 of the coating tub 1 was circulated and finished.
The samples were taken from the coating bath which was overflowed fkom the
coating tub 1 just before each level of the 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 k3"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.
[0 1521
The result of the test is shown in FIG. 14. As shown in FIG 14, when the bath
temperature deviation (T3 - ATfall - T1 : hereinafter, referred to as inflow bath
10 temperature deviation) between the inflow bath temperature (T3 - ATul) of the coating
bath which flows into the coating tub 1 from the adjusting tub 3 and the bath temperature
T1 of the coating tub 1 is not within 10°C (T3 - ATfall - T1 > 10°C or T3 - ATfan - TI <
10°C), 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 pn). Contrary, when the inflow bath
15 temperature deviation is -10°C or more and 10°C or less (-10°C I T3 - ATfall - T1 5
10°C), only the dross (for example, approximately 22 pm 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 -10°C or more and 10°C or less. In other words, it is preferable
20 that the bath temperature T3 of the adjusting tub 3 is within the range off 10°C (TI +
ATfall - 105 X'3 I T1 + ATfal+l 10) on the basis of the temperature (ATfal+l TI) in which
the difference of the bath temperature decrease ATfali at transferring the bath from the
adjusting tub 3 to the coating tub 1 is added to the bath temperature T1 of the coating tub
1. Conventionally, when the bath temperature deviation of the coating bath increases, it
25 has been expected that the formtition and the growth of the dross are accelerated.
70
However, the specific range of the bath temperature deviation which promotes the
formation of the dross with the harmful size has not known. From the test results, in
order to suppress the formation of the dross with the harmful size in the coating tub 1, it
turns out that the bath temperature T3 of the adjusting tub 3 may be within the range of
5 *lO°C on the basis of the temperature in which the difference of the bath temperature
decrease ATfdl is added to the bath temperature T1 of the coating tub 1.
[0153]
As described above, although the preferable embodiment of the present
invention was described in detail with reference to the drawings, the present invention is
10 not limited to the embodiment. It is obvious that a person ordinarily skilled in the art of
the invention can conceive the alterations and the modifications within the technical
ideas used in the scope of claims, so that it is obviously understood that these belong
implicitly to the technical scope of the present invention.
[0 1541
15 The present invention can be widely applied to the hot dip zinc-aluminum
coated steel sheets which are manufactured by using the coating bath 10 whose specific
gravity is higher than the specific gravity of the top-dross (Fe2A15), such as the
galvannealed steel sheets (GA) for which both of the bottom-dross andthe top-dross can
form, the zinc-aluminum alloy coated steel sheets, and the like in addition to the
20 galvanized steel sheets (GI). When the amount of the aluminum increases and the
specific @a+ity of the coating bath 10 is less than the specific gravity of the top-dross,
the dross cannot separated by the 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%.
[0155]
7 1
In addition, in the coated steel sheets which are manufactured by the coating
bath with high aluminum content except for the galvannealed steel sheets, it is not
necessary that the bath composition of the separating tub 2 and the adjusting tub 3 is
intentionally changed like the above mentioned embodiment, and it is possible that the
5 coating bath 10 in which the top-dross is almost not contained by controlling only the
bath temperature T. Thereby, the problems such as the appearance deterioration of the
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 of the roll in the
coating bath, and the like can be solved.
Industrial Applicability
[0156]
According to the present invention, it is possible that the dross which forms
inevitably in the coating bath during the manufacture of the galvanized steel sheet can be
15 removed efficiently and effectively and can be almost-completely rendered harmless.
Accordingly, the present invention has significant industrial applicability.
Reference Signs List
[0157]
20 1 COATINGTUB
2 SEPARATING TUB
3 ADJUSTING TUB
4 PREMELTING TUB
5 MOLTEN METAL TRANSFER APPARATUS
25 6,7 COMMUNICATING VESSEL
C -
8 TRANSFERFUNG VESSEL
9 OVERFLOWING VESSEL
10, 10A, 10B, 10C COATING BATH
11 STEEL SHEET
13 GAS WIPING NOZZLE
TABLE 1
TYPE OF
COATING BATH
COMPOS I T I ON
OF COATING
BATH
FORMED DROSS
AND
SIZE THEREOF
COATING BATH A
0.1 3massOhAl
0.05mass0hFe
BALANCE : Zn
FeZn, : 50 fl m
COATING BATH B COATING BATH C
0.1 4mass0hAl
0.04mass%Fe
BALANCE : Zn
FeZn, : 40 fl m
Fe,AI5 : 1 0 fi m
0.1 8mass%Al
O.O3mass%Fe
BALANCE : Zn
Fe2A15 : 5 p m
Fe2AI5 : 10 fi m
Fe2AI5 : 25 fl m
TABLE 2
I
EXAMPLE 2 10 12 20 460 440 465 6 1.7 2. 0 20
EXAMPLE 3 10 40 20 460 454 465 10 1.0 4. 0 6
EXAMPLE 4
EXAMPLE 5
COMPARAT I VE EXAMPLE 1
COMPARAT I VE EXAMPLE 2
10
10
10
60
40
40
40
-
20
20 -
2 0
-
460
460
-
460
460
455
456
460
-
465
465
465
-
10
10
10
-
1.0
1.0
1.0
-
4.0
4. 0
4. 0
-
5
4
0
-
TABLE 3
0.198
0.201
0.202
-
EXAMPLE 4
EXAMPLE 5
COMPARATIVE EXAMPLE 1
COMPARATIVE EXAMPLE 2
0.009
0.01 1
0.012
-
0.202
0.203
0.206
0.204
0.209
0.21 1
0.212
-
0.01 1
0.01 2
0.01 2
0.013
0.008
0.01 1
0.01 1
-
TABLE 4
TABLE 5
EXAMPLE
EXAMPLE 1
EXAMPLE 2
EXAMPLE 3
EXAMPLE 4
EXAMPLE 5
COMPARAT I VE EXAMPLE 1
COMPARATIVE EXAMPLE 2
SEDIMENTED DROSS
BOTTOM-DROSS
(VISUAL OBSERVAT I ON)
NONE
NONE
NONE
NONE
NONE
NONE
NONE
DENS I TY OF FLOWED DROSS
TOP-DROSS
TOP-DROSS
[mg/cm31
0.025
0.059
0.047
0.051
0.069
0. 171
0.242
SIZE
50p m
3 0m~
10p m
BOTTOM-DROSS
BOTTOM-DROSS
[mg/cm31
NONE
NONE
NONE
NONE
NONE
NONE
NONE
EFF I C I ENCY OF
FLOTATION
SEPARATION
100%
98%
40%
SIZE
50fi m
30p m
10fi m
EFFICIENCY OF
SED I MENTAT I ON
SEPARATION
53%
21%
4%

3 3 9-.a
CLAIMS
1. A manufacturing equipment for a galvanized steel sheet, the manufacturing
equipment comprising:
5 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 T 1 ;
a separating tub which has a second temperature controller to keep the coating
10 bath transferred through a coating bath outlet of the coating tub to a bath temperature T2
which is lower than the bath temperature T1;
an adjusting tub which 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
15 a circulator to circulate the coating bath in order of the coating tub, the
separating tub, and the adjusting tub.
2. The manufacturing equipment for the galvanized steel sheet according to claim
1, the manufacturing equipment hrther comprising
20 an aluminum concentration analyzer to measure an aluminum concentration
Alof the coating bath in the coating tub,
wherein a first zinc-included-metal which includes an aluminum with a
concentration higher than the aluminum concentration A1 of the coating bath in the
coating tub is supplied to at least one of the separating tub and the adjusting tub
25 depending on a measurement result of the aluminum concentration analyzer.
3. The manufacturing equipment for the galvanized steel sheet according to claim
wherein the first zinc-included-metal is supplied to the separating tub, and
5 a second zinc-included-metal which is a zinc-included-metal which includes an
aluminum with a concentration lower than an aluminum concentration A2 of the coating
bath in the separating tub or a zinc-included-metal which does not include an aluminum
is supplied to the adjusting tub depending on the measurement result of the aluminum
concentration analyzer.
4. The manufacturing equipment for the galvanized steel sheet according to claim
2,
wherein the first zinc-included-metal is supplied to the separating tub, and
a metal is not supplied to the adjusting tub depending on the measurement result
15 of the aluminum concentration analyzer.
5. The manufacturing equipment for the galvanized steel sheet according to claim
2, the manufacturing equipment further comprising
a premelting tub to melt the first zinc-included-metal or the second
20 zinc-included-metal,
wherein a molten metal of the first zinc-included-metal or the second
zinc-included-metal which is melted in the premelting tub is supplied to the coating bath
in the adjusting tub.
25 6. The manufacturing equipment for the galvanized steel sheet according to claim
wherein the bath temperature T2 of the separating tub is controlled by the
second temperature controller to be lower 5°C or more as compared with the bath
temperature T1 of the coating tub and to be higher than a melting point of the molten
5 metal.
7. The manufacturing equipment for the galvanized steel sheet according to claim
1,
wherein the bath temperature T3 is controlled by the third temperature controller
10 so that the bath temperature TI, the bath temperature T2, and the bath temperature T3
satisfL a following formula (1) and a following formula (2) in celsius degree, when a
difference of a bath temperature decrease of the coating bath when transferred from the
adjusting tub to the coating tub is ATfa1l in celsius degree.
T1 +ATfall-10ST3ITl+ ATfall+10 ...(1 )
15 T2+5ST3 -..(2)
8. The manufacturing equipment for the galvanized steel sheet according to claim
1,
wherein the circulator includes a molten metal transfer apparatus which is
20 installed in at least one of the coating tub, the separating tub, and the adjusting tub.
9. The manufacturing equipment for the galvanized steel sheet according to claim
1,
wherein the coating bath outlet of the coating tub is located on a downstream
25 side of a running direction of the steel sheet so that the coating bath flows out of an upper
part of the coating tub by a flow of the coating bath which is derived from a running of
the steel sheet.
10. The manufacturing equipment for the galvanized steel sheet according to claim
5 1,
wherein at least two of the 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.
11. The manufacturing equipment for the galvanized steel sheet according to claim
1,
wherein a storage of the coating bath in the coating tub is five times or less of a
circulating volume of the coating bath per one hour by the circulator.
12. The manufacturing equipment for the galvanized steel sheet according to claim
1 ,
wherein a storage of the coating bath in the separating tub is two times or more
of a circulating volume of the coating bath per one hour by the circulator.
20
13. A manufacturing method of a galvanized 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;
25 coating a steel sheet which is dipped in the coating bath at the coating tub in
I)
which the coating bath transferred from the adjusting tub is stored at a predetermined
bath temperature T1;
separating by a flotation a top-dross which is precipitated at the separating tub in
which the coating bath transferred from the coating tub to the separating tub is stored at a
5 bath temperature T2 which is lower than the bath temperature T1 of the coating tub; and
dissolving a residual dross at the adjusting tub in which the coating bath
transferred fiom the separating tub is stored at a bath temperature T3 which is higher than
the bath temperature T2 of the separating tub.