Process and apparatus for producing molten metal coated steel sheets

A steel sheet is plated with a molten metal in a plating tank. A buffer member is provided between a sink roll submerged in the melt in the plating tank and the side wall of tank at its delivery end from which the plated steel sheet emerges. During plating, the buffer member decelerates the melt flows coming from below the sink roll so as to settle the suspended dross in the melt. The buffer member may be in the form of a rod or a plate. If desired, it may be combined with a shield plate provided below the sink roll or a raised portion formed on the bottom of the plating tank.

BACKGROUND OF THE INVENTION 
The present invention relates to a process and apparatus for producing 
steel sheets provided with molten metal coatings by the continuous hot-dip 
plating method. More specifically, the invention relates to a process and 
apparatus for producing coated steel sheets in which the sedimentation of 
dross or impurities that are formed and suspended in the melt during 
hot-dipping is effectively promoted to yield coatings of good quality 
without dross defects. 
The conventional continuous hot-dip plating method for producing steel 
sheets coated with molten metal (referred to below as molten metal coated 
steel sheets) consists of applying a preliminary treatment to a steel 
sheet through a continuous furnace, dipping the steel sheet in a plating 
bath, and passing it through the bath. A brief description of this method 
is given below with molten zinc as an example of a molten metal. 
FIG. 1 is a schematic diagram showing the conventional production process. 
As shown, a plating tank 1 is continuously supplied with a steel sheet 2 
via a snout 6 after the sheet 2 has been subjected to a surface activation 
treatment in a continuous furnace (not shown). The steel sheet 2 is passed 
around a sink roll 3 submerged in the plating bath so that it ascends 
through the bath and passes between snap rolls 4 and is subsequently drawn 
out of the bath. 
When a molten metal is molten zinc, this process is called hot galvanizing. 
In this process of hot galvanizing, Fe dissolving out of the steel sheet 
combines chemically with Al and Zn to form dross (or impurities) 
principally composed of FeZn.sub.7 and Fe.sub.2 Al.sub.5 as indicated by 
dots in FIG. 1. Generally speaking, FeZn.sub.7 deposits as bottom dross 5 
on the bottom of the plating tank, whereas Fe.sub.2 Al.sub.5 floats as top 
dross 9 on the surface of the melt in the plating bath. In actual 
operations, the rotation of the sink roll 3 and other submerged members 
creates currents, which cause part of the bottom dross 5 to become 
suspended in the plating bath or melt as indicated by arrows in FIG. 1. 
Top dross 9 floats on the surface of the melt and can be removed with a 
screen, which is the most common method currently used in practice. On the 
other hand, the dross suspended in the plating bath is difficult to remove 
and unavoidably adheres to the surface of the steel sheet, thereby 
reducing the quality of a molten metal coating. In an extreme case, the 
coated steel sheet has to be discarded as scrap because of unacceptable 
quality. 
However, the very nature of the continuous hot galvanizing method makes it 
impossible to prevent the formation of dross. Instead, recent efforts in 
the plating industry are directed to reducing the suspension of dross due 
to the currents created by the rotation of the sink roll and other 
submerged members. To this end, the depth of the plating tank is 
sufficiently increased to reduce the adverse effects of the currents on 
the bottom dross. This approach has proved reasonably successful in 
reducing the suspension of dross. 
Increasing the depth of the plating tank is indeed effective in reducing 
the suspension of dross, but on the other hand, a greater amount of 
plating bath is necessary and the energy costs for the maintenance and 
control of the bath increase accordingly, as does the operational 
difficulty in removing the bottom dross. Another problem with this 
approach is that it cannot be applied to existing plating tanks, which 
must be reconstructed at significant cost to increase their depth. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method for producing 
molten metal coated steel sheets that employs convenient and effective 
means to reduce the deposition of dross on the final product. 
Another object of the invention is to provide an apparatus for implementing 
this method. 
The present inventors conducted intensive studies of hydrodynamic phenomena 
in plating tanks in order to reduce the suspension of bottom dross and 
made the following observations, as shown in detail in FIG. 1. 
(1) A dross defect is a phenomenon that develops when the dross suspended 
in the melt in the plating tank adheres to the surface of a steel sheet 
and remains in a coating even after finishing hot-dipping by gas wiping 
and other methods. 
(2) Dross held between the sink roll and the steel sheet is pressed in the 
decreasing nip so that it adheres firmly enough to prevent effective 
removal by wiping. 
(3) The problems produced by bottom dross are mainly due to bottom dross of 
a comparatively large size (approximately 100 .mu.m), which settles on the 
bottom during shutdown. When the plating operation is restarted, however, 
this dross becomes suspended in the melt by the lifting action of the 
currents that are created by the motion of the sink roll and other 
submerged rotating members, as well as by the steel sheet. 
(4) The concomitant, submerged currents primarily move from the feed end of 
the plating tank to the delivery end in the direction of sheet movement. 
After the dross is lifted below the sink roll, it repeatedly bumps against 
the side wall of the plating tank, goes up through the melt, and is 
stirred by the submerged rotating members together with the suspended 
dross, so that it circulates through the melt in the plating tank. 
The present invention, which is capable of performing continuous plating 
with molten metals, has been accomplished on the basis of these findings. 
According to a first aspect, the present invention provides a process for 
producing a molten metal coated steel sheet by applying a molten metal 
coating to a steel sheet that is continuously admitted through a hot-dip 
plating bath as the sheet is passed around a submerged sink roll. The 
adhesion of dross to the surface of the coated steel sheet is reduced by 
decelerating melt flows coming from below the sink roll to allow suspended 
dross to settle by means of a buffer member provided between the sink roll 
and a side wall of the molten metal plating tank at the delivery end from 
which the coated steel sheet emerges. 
In a preferred embodiment, the buffer member may be combined with a current 
suppressing member such as a shield plate provided between the sink roll 
and the bottom of the molten metal plating tank and/or a raised portion 
formed on the bottom of the molten metal plating tank in an area located 
below the sink roll. As a result, melt flows coming from below the sink 
roll and, hence, the lifting and suspension of the bottom dross are 
suppressed during the hot-dipping of the steel sheet. 
According to a second aspect, the present invention provides an apparatus 
for producing a molten metal coated steel sheet which comprises a molten 
metal plating tank, a sink roll submerged in a molten metal plating bath, 
and a buffer member provided between the sink roll and the side wall of 
the molten metal plating tank at the delivery end from which the coated 
steel sheet emerges and which is capable of decelerating melt flows coming 
from below the sink roll. 
In a preferred embodiment, the apparatus may further include a current 
suppressing member such as a shield plate provided between the sink roll 
and the bottom of the molten metal plating tank and/or a raised portion 
formed on the bottom of the molten metal plating tank in an area located 
below the sink roll. These additional members suppress melt flows coming 
from below the sink roll and, hence, the lifting and suspension of the 
bottom dross collecting on the bottom of the plating tank.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The operation of the present invention is described below more specifically 
with reference to accompanying drawings. 
FIG. 2 is a schematic diagram outlining the motion of currents created in a 
molten metal plating bath used in the invention. As shown, bottom dross 5 
generated in the plating bath is lifted by currents created by the 
movements of steel sheet 2. The movements include dipping in the plating 
bath (chiefly made of molten Zn), changing the direction of sheet pass, 
and recovering the steel sheet from the plating bath. Currents are also 
created by the rotation of submerged rolls, namely, sink roll 3 and snap 
rolls 4, and the thus lifted bottom dross is suspended in the plating 
bath. The suspended dross will adhere to the surface of the steel sheet 
(also see FIG. 1). The numerals in FIG. 2 other than those used to 
identify various parts and components refer to dimensions in millimeters. 
According to the invention, a buffer member 7 of an appropriate shape 
(which may be a cylinder or a polygonal prism) is provided between the 
side wall of the plating tank at the steel sheet delivery end and the sink 
roll so that the direction of ascending currents is changed to promote the 
sedimentation, i.e., settling of the suspended dross. The currents that 
create the movement of suspended dross occur in various directions as 
indicated by arrows in FIG. 2, while settling dross is indicated by dots 
and dashed lines. 
FIGS. 3a to 3d are schematic perspective views showing four examples of 
shapes of the buffer member used in the present invention. The numerals in 
these figures represent the dimensions of the buffer members in 
millimeters. 
FIG. 3a shows the case where the buffer member is comprised of a rod having 
a circular cross section. The buffer member may be a hollow part having 
two open ends or two closed ends or one closed and one open end. FIG. 3b 
shows the case where the buffer member is a rod having a polygonal cross 
section, e.g., hexagonal cross section. FIG. 3c shows the case where the 
buffer member is a rod having a rectangular cross section. FIG. 3d shows 
the case where the buffer member is a rod having a triangular cross 
section. 
FIG. 4 is a schematic diagram showing the directions of currents created in 
a molten metal plating bath used in another embodiment of the invention, 
in which the buffer member 7 is plate-shaped. Such a buffer member may be 
installed in such a way that the angle of its upper and lower surfaces is 
variable, as shown in FIG. 5. 
FIGS. 6a to 6d are schematic perspective views showing four examples of 
shapes of a plate-shaped buffer member. The numerals in those figures 
represent the dimensions of the buffer members in millimeters. 
FIG. 6a shows an example where the buffer member is a simple flat plate. 
FIG. 6b shows an example where the buffer member is a plate with flanges 
extending in opposite directions from opposite lengthwise edges. FIG. 6c 
shows an example in which the buffer member is a channel-shaped plate with 
flanges extending in the same direction from opposite lengthwise edges. 
FIG. 6d shows an example in which the buffer member is a curved plate 
having an S-shaped transverse cross section. 
According to a preferred embodiment of the present invention, a buffer 
member of an appropriate shape selected from the examples shown in FIGS. 
3a to 3d and FIGS. 6a to 6d is provided between the side wall of the 
plating tank at the steel sheet delivery end and the sink roll. 
As above-mentioned, the buffer member is preferably provided somewhere in 
the area between the sink roll and the side wall of the molten metal 
plating tank at the delivery end from which the plated steel sheet 
emerges. If the buffer member is rod-shaped, the position of the sink 
roll, i.e., its height from the bottom of the plating tank and its 
distance from the side wall should be taken into account in determining 
the location of the buffer member, and preferably it is positioned between 
the sidewall of the plating tank and the sink roll, with the top surface 
of the sink roll being in substantial alignment with the center of the 
buffer member. If the buffer member is plate-shaped, it is preferably 
positioned in such a way that its center of area is in substantial 
alignment with the top surface of the sink roll. 
However, depending on the speed at which the steel sheet is passed, best 
results may not be attained even if the center of the buffer member 7 is 
brought into alignment with the top surface of the sink roll. For 
instance, in Examples 2 and 5, described later, the best results were 
attained when the center of the buffer member 7 was 100 mm higher than the 
top surface of the sink roll at a sheet pass speed of 120 m/min. 
Thus, as indicated by arrows in FIG. 2, currents rising from below the sink 
roll are decelerated when their direction is changed temporarily by the 
buffer member 7. Even if the directional change is abrupt, the melt 
(plating bath) has no difficulty in creating currents, but the dross is 
unable to follow the abrupt change and separates from the current and 
descends in the direction indicated by dashed lines in FIG. 2. 
The dross-containing currents that are ascending at the steel sheet 
delivery end impinge against the buffer member, which is in the form of 
either a rod or a plate. If the buffer member is installed in such a way 
that its major surfaces are parallel to the melt level, the direction of 
these currents will change so sharply (through an acute angle) that they 
interfere with other currents created by the rotating sink roll, and the 
resulting local enhancement of the stirring action will cause part of the 
suspended dross in the plating tank to ascend. Another possibility is the 
shortening of the time (or distance) of contact between the buffer member 
and the currents. Whichever the case may be, the sedimentation of the 
suspended dross is accelerated. 
As it settles the suspended dross passes along the side wall of the plating 
tank and rests on the bottom of the tank at the steel sheet feed end. As a 
result, the quantity of the dross that is suspended by the currents 
decreases sharply. 
The state of dross suspension varies with factors such as the speed of 
sheet pass, the formulation of the plating bath, and its temperature. 
Therefore, if fine adjustments of the vertical position of the buffer 
member are made based on the amount of dross deposits on the surface of 
the coated steel sheet or on the concentration of dross in the plating 
bath, the cleanliness of the molten metal bath can be further increased. 
According to the invention, further improvements can be achieved by 
providing a shield plate, i.e., a straightening vane between the sink roll 
and the bottom of the plating tank and/or by forming a raised portion on 
the tank bottom in an area below the sink roll. The shield plate hereunder 
sometimes referred to as "straightening vane". 
FIG. 7 shows an embodiment in which a straightening vane 8 is provided 
above the bottom of the plating tank 1. The vane, or shield plate 8 
assures complete isolation of the bottom dross 5 from the area where the 
steel sheet passes. Therefore, the bottom dross will not be lifted by 
currents, and the possibility that the bottom dross that has experienced 
accelerated sedimentation by the buffer member 7 will be lifted again by 
the currents due to the rotation of the sink roll is reduced. 
While FIG. 7 shows an embodiment having a straightening vane 8, the same 
results can be attained by forming a raised portion on the bottom of the 
plating tank as shown in FIG. 8, which is a simplified sectional view. A 
raised portion 13 having a rectangular cross section is formed in the 
plating tank 1 below the sink roll. Three examples of shapes of the raised 
portion 13 are shown in FIGS. 9a, 9b, and 9c. In these examples, the cross 
section of the raised portion is respectively trapezoidal, 
semi-elliptical, and the frustum of a part of a sphere. The numerals in 
FIGS. 9a-9c refer to the dimensions of the raised portions in millimeters. 
The advantages of the present invention are described below more 
specifically by means of examples, which are given here for illustrative 
purposes only and should not be taken as limiting. 
EXAMPLES 
In these examples, steel sheets were subjected to continuous hot 
galvanizing under the conditions set forth below using an apparatus of the 
type shown in FIG. 2. In each example, a buffer member according to the 
present invention was used either alone or in combination with a 
straightening vane or a raised portion according to the present invention. 
To verify the advantages of the invention, the results of hot dipping were 
compared with those obtained by a prior art method not using any buffer 
member. 
______________________________________ 
HOT GALVANIZING CONDITIONS 
______________________________________ 
Steel sheet to be plated 
0.60 mm.sup.T .times. 1200 mm.sup.W .times. L 
Bath formulation 
0.10-0.12% Al and a balance of Zn 
and impurities 
Bath temperature 
460 .+-. 10.degree. C. 
Sheet pass speed 
100 m/min 
Plating tank layout 
See FIG. 2 
Diameter of sink roll 
600 mm; distance from the tank bottom 
to the roll center = 750 mm (height of 
the top surface of roll = 1050 mm) 
______________________________________ 
Hot galvanizing was carried out under the conditions set forth above. After 
gas wiping, heat treatment was performed to yield hot galvanized steel 
sheets with alloying. 
EXAMPLE 1 
In this example, hot galvanizing was conducted under the conditions set 
forth above using buffer members in the plating tank. For the shapes and 
dimensions of the respective buffer member, see FIGS. 3a-3d. The 
galvanized steel sheets thus produced are designated cases a to d, 
respectively. Each buffer member was mounted at such a height that the 
center of the buffer member was 1050 mm from the tank bottom and in 
substantial alignment with the top surface of the sink roll. 
For comparison, hot galvanizing was conducted under the same conditions as 
above, except that no buffer member was used (prior art case). 
The results are shown graphically in FIG. 10 in terms of the number of 
dross deposits. For data collection, 20 pieces each measuring 1000 mm in 
length were cut from each sample of galvanized steel sheet across its 
width and the number of dross deposits on the surface of each piece in 
contact with the sink roll was counted. A total number of dross deposits 
for 20 pieces was measured. 
As can be seen from FIG. 10, the number of dross deposits on the surface of 
galvanized steel sheets was at least reduced to one half in the invention, 
and when a cylindrical buffer member was used as in case a, the decrease 
was almost to a third. 
EXAMPLE 2 
In this example, buffer members of the types shown in FIGS. 3a-3d were used 
as in Example 1. However, the sheet pass speed was increased to 120 m/min, 
and the distance from the tank bottom to the center of each buffer member 
was varied in the vertical direction. The numbers of dross deposits on the 
surfaces of the respective samples of galvanized steel sheets were 
referred to as Cases a to d, in which the buffer members of FIGS. 3a-3d 
were employed, respectively. 
The results are shown graphically in FIG. 11, from which it can be seen 
that each of the buffer members could be adjusted in vertical position to 
obtain an optimal height. It was also clear that each of the four buffer 
members used in accordance with the present invention contributed to a 
marked decrease in the number of dross deposits on the surfaces of coated 
steel sheets as compared to the prior art case. 
EXAMPLE 3 
The procedure of Example 1 was repeated, except that a buffer member of the 
cylinder type was used in combination with either a straightening vane 
provided below the sink roll or a raised portion formed on the bottom of 
the plating tank. The number of dross deposits on the surfaces of the hot 
galvanized steel sheets was counted. 
The results are shown graphically in FIG. 12, from which it is clear that 
the combined use of the buffer member with the straightening vane or the 
raised portion produced a synergistic effect which helped further reduce 
the quantity of dross deposition compared to when the buffer member was 
used alone. 
The case of "cylinder + straightening vane" mentioned in FIG. 12 refers to 
the tank design illustrated in FIG. 7. The four different shapes of raised 
portion illustrated in FIGS. 8 and 9a-9c were used in Example 3. 
EXAMPLE 4 
The procedure of Example 1 was repeated except that plate-shaped buffer 
members like those illustrated in FIGS. 6a-6d were used in a plating tank 
having the construction shown in FIG. 4. 
The results are shown graphically in FIG. 13. Cases a to d, refer to the 
use of the buffer members shown in FIGS. 6a-6d, respectively, and the 
prior art case refers to a case in which no buffer member was used. The 
plate-shaped buffer members could clearly achieve the same results as the 
rod-shaped buffer members. 
EXAMPLE 5 
Hot galvanizing was performed under the same conditions as in Example 4, 
except that the sheet pass speed was increased to 120 m/min, and the plate 
shown in FIG. 6a was used as a buffer member with the angle of its 
installation varied from zero to 180 degrees. The number of dross deposits 
is shown graphically in FIG. 14 as a function of the angle setting. FIG. 
14 also shows that 80 dross deposits occurred in the prior art case, in 
which no buffer member was used. 
EXAMPLE 6 
To verify the synergistic effect of the combination of a plate-shaped 
buffer member with a straightening vane, hot galvanizing was performed 
under the same conditions as in Example 4, except that a plate-shaped 
buffer member shown in FIG. 4 was used in combination with the 
straightening vane 8 shown in FIG. 7. The number of dross deposits on the 
galvanized steel sheet was counted, and the result is shown graphically in 
FIG. 15, which also shows the result of the prior art case which employed 
neither a straightening vane nor a buffer member. The dross deposition was 
negligible in the present invention. 
According to the present invention, the number of dross deposits that occur 
in the continuous plating of steel sheets with molten metals can be 
reduced without increasing the depth of existing plating tanks. Hence, 
plated steel sheets of good quality can be produced with great ease.