Patent Grant
11162166
The present disclosure relates to an apparatus for continuous molten metal coating treatment for continuously molten metal coating a travelling metal strip, and to a method for molten metal coating treatment using the apparatus.
Conventionally, hot-dip metal coating on a metal strip, for example, hot-dip galvanizing on a steel strip, is generally performed in a continuous hot-dip galvanizing line as illustrated in
In this continuous hot-dip metal coating line, the molten zinc, adhered to surfaces of the steel strip and being pulled up, is wiped from the steel strip so as to control to a desired coating weight, by discharging heated gas or gas at ordinary temperature from the gas wiping nozzles 85 to blow the gas onto the surfaces of the steel strip S. This gas wiping method is widely used at present.
If impinging pressure of the gas on the steel strip is increased when controlling the coating weight of the molten zinc by the above method, however, there arises a problem that a coated surface will have a defective appearance. This is due to splattering of the molten zinc, called splashing, occurred by increase in gas flow rate, resulting in that the splattered molten zinc adheres to the steel strip surfaces again. In addition, zinc entrains air to build up as a lumpy mass of oxide (dross) on the bath surface, as the zinc bath is in contact with atmospheric air. There is therefore another problem that the dross adheres to the steel strip to cause the defective appearance on the coated surface. Furthermore, while increase in the impinging pressure of the gas is required in order to obtain thin coatings, a coating amount for the hot-dip galvanizing of about 30 g/m2 will be the lower limit for now because a warp or vibration of the steel strip makes it difficult to reduce a distance between the nozzle and the steel strip.
Techniques as described in PTL 1 to 3 are known as means for solving these problems. PTL 1 discloses a method for controlling a hot-dip coating weight in which the coating weight is controlled by blowing burner exhaust gas from a wiping nozzle toward a surface of a metal strip being continuously pulled up from a hot-dip metal coating bath.
PTL 2 discloses a method for wiping a molten metal using an electromagnetic force by disposing a pair of electromagnetic coils to face both surfaces of a steel strip being continuously pulled up from a hot-dip metal coating bath. In PTL 2, such a method for controlling a coating weight is disclosed as a method to replace the gas wiping method.
PTL 3 discloses a method for molten metal coating where a steel strip is spray coated by spraying fine particles of a molten metal, from a pair of spray nozzles provided to face each other with the steel strip in between, on surfaces of the steel strip continuously travelling to be supplied. In PTL 3, such a method for coating treatment is disclosed as a method to replace a method where the metal strip is immersed in the molten metal.
PTL 1: JP 2009-263698 A
PTL 2: JP 2007-284775 A
PTL 3: JP H8-165555 A
However, problems of the splashing and the dross remain after all in the method described in PTL 1, as it is still the gas wiping method using the gas impinging pressure even if the method of PTL 1 allows to reduce amount of gas by enhancing wiping efficiency using the exhaust gas burnt to a high temperature.
The method described in PTL 2 needs to send a large electric current to the electromagnetic coils in order for thin coatings, resulting in a problem that the steel strip will be heated. Moreover, the method requires a zinc bath, thus leaving the problem, of the dross formed in the bath or on the bath surface due to contact with air, unsolved.
Groups of fine particles of the molten metal diffuse to reach the steel strip surfaces in the spray coating method described in PTL 3. Issues therefore arise such that flow rate density of the fine particles mass varies on the steel strip surfaces to produce distribution in coating thickness, and such that the fine particles of the molten metal are sprayed on outside of edges of the steel strip as well to worsen a throughput yield for the molten metal. Moreover, other issues also occur such that extremely fine mist, as a result of variation in fine particle sizes, floats within a furnace without adhering to the steel strip, resulting in poorer throughput yield for the molten metal or contamination within the furnace.
In view of the aforementioned problems, it could be helpful to provide a totally new method for molten metal coating treatment as a method for treating surfaces of a metal strip by molten metal coating, the new method avoiding inherent issues involved in conventional immersion coating processes and spray coating processes. It could also be helpful to provide an apparatus for continuous molten metal coating treatment capable of carrying out such a method.
With the aim to solve the aforementioned problems, we have reached discoveries of a method and an apparatus by which it is possible to produce a coated metal strip having a quality surface by utilizing an electromagnetic force (the Lorentz force) to discharge droplets of a molten metal from a nozzle onto the metal strip. We thus provide the followings.
(1) An apparatus for continuous hot dip molten metal coating treatment including:
a coating furnace defining a space of a non-oxidizing atmosphere in which a metal strip continuously travels; and
a nozzle system configured to discharge a molten metal droplet toward a surface of the metal strip,
the nozzle system including:
(2) The apparatus for continuous molten metal coating treatment according to the foregoing (1), the apparatus further including:
a heating mechanism configured to heat the metal strip; and
a controller of the heating mechanism configured to control a temperature of the metal strip to (Tu −20° C.) or more (in other words, a controller of the heating mechanism configured to control the metal strip to a temperature equal to or higher than Tu −20° C.), where a melting point of the molten metal is expressed in Tu (° C.).
(3) The apparatus for continuous molten metal coating treatment according to the foregoing (1) or (2), the apparatus further including a sealing device configured to separate the space of the non-oxidizing atmosphere from air, the sealing device disposed at a side in the coating furnace where the metal strip leaves.
(4) The apparatus for continuous molten metal coating treatment according to any one of the foregoings (1) to (3), the apparatus further including a damping-straightening mechanism configured to suppress the metal strip from vibrating or warping, the damping-straightening mechanism set on at least one of an upstream side or a downstream side of the nozzle system with respect to a travelling direction of the metal strip.
(5) The apparatus for continuous molten metal coating treatment according to any one of the foregoings (1) to (4), wherein the nozzle on the tip of the nozzle cartridge has a plurality of the discharge ports disposed in a transverse direction of the metal strip.
(6) The apparatus for continuous molten metal coating treatment according to the foregoing (5), wherein a plurality of the nozzle cartridges is disposed in the transverse direction of the metal strip, so that the discharge ports are arranged at given intervals across an entire range of the transverse direction of the metal strip.
(7) The apparatus for continuous molten metal coating treatment according to any one of the foregoings (1) to (6), wherein a plurality of the nozzle cartridges is disposed in a travelling direction of the metal strip.
(8) The apparatus for continuous molten metal coating treatment according to the foregoing (7), the apparatus capable of forming a multi-layered coating by controlling a type of the molten metal supplied to the chamber of each nozzle cartridge to be different, among the nozzle cartridges disposed at different positions in the travelling direction of the metal strip.
(9) A method for molten metal coating treatment comprising: coating a surface of a metal strip by discharging a droplet of a molten metal toward the surface of the metal strip while the metal strip is continuously travelling, by means of the apparatus for continuous molten metal coating treatment according to any one of the foregoings (1) to (8).
The disclosed apparatus for continuous molten metal coating treatment allows to perform a totally new method for molten metal coating treatment as a method for treating surfaces of a metal strip by molten metal coating, the new method avoiding inherent issues involved in conventional immersion coating processes and spray coating processes.
And by means of the disclosed method for molten metal coating treatment, it is possible to treat surfaces of a metal strip by molten metal coating while avoiding inherent issues in conventional immersion coating and spray coating processes.
In the accompanying drawings:
Each apparatus for continuous molten metal coating treatment 100, 200 according to one embodiment of the present disclosure as respectively illustrated in
The disclosure characteristically discharges droplets of the molten metal toward surfaces of the metal strip S utilizing an electromagnetic force (the Lorentz force) by the nozzle system 10. The nozzle system 10 will now be described with reference to
Firstly, the nozzle system 10 includes a nozzle cartridge 20 as illustrated in
While
In this embodiment, the chamber 21, defined in the vicinity of the tip of the nozzle cartridge 20, consists of a first chamber 21A in a cuboid shape; a third chamber 21C in a cuboid shape and being smaller in size than the chamber 21A; and a second chamber 21B joining these chambers 21A and 21C as well as having a tapered shape as in sectional views of
Heat resistant graphite, various kinds of ceramics, and the like can be suitably used as materials for the nozzle cartridge 20 and the nozzle 23. It is preferable to wind an electromagnetic coil (not depicted) around the nozzle cartridge 20 so that the molten metal can be remained at high temperature by induction heating.
The nozzle system 10 includes a magnetic flux generation mechanism and a current generation mechanism. The magnetic flux generation mechanism is for generating magnetic flux in a given direction in at least a part of the chamber 21, while the current generation mechanism is for sending an electric current, in a direction perpendicular to the above given direction, to the molten metal positioned in the at least a part of the chamber where the magnetic flux is applied. The current generation mechanism according to the present embodiment will now be described with reference to
As illustrated in
As illustrated in
In the present embodiment, the pulsed current is applied to the molten metal in the third chamber 21C for either a right side or a left side of
A principle of this ejecting will be briefly described with reference to
As a second aspect, when the pulsed current is directed opposite to the direction illustrated in
Techniques for discharging the molten metal using the Lorentz force are already known as disclosed in WO2010/063576 and WO2015/004145. The former publication describes a discharging technique corresponding to the aforementioned first aspect. And the latter publication describes discharging techniques corresponding to the first aspect and the second aspect in detail along with their discharging principles. In general, finer droplets can be obtained in the second aspect than in the first aspect. One of the aspects may be selected depending on a desired droplet diameter of the molten metal.
The present disclosure applies this technique for discharging the molten metal utilizing the Lorentz force to a continuous molten metal coating treatment, and achieves uniform coatings. Although a method for controlling to discharge the molten metal by means of a piezoelectric element as in inkjet technologies might be an option, such a method is not suitable for use in high temperature environment due to problems related to heat resistance. This method therefore requires heat protection measures with a combination of heat insulations and cooling mechanisms. In addition, the method has problems such as shorter maintenance or replacement cycles because of a shorter head lifetime. On the other hand, improved heat resistance as well as a longer head lifetime are obtainable in the method for discharging the molten metal from a nozzle by utilizing the electromagnetic force. Preferable conditions for achieving the uniform coatings in the present disclosure will be described below.
With reference to
Atmosphere in the coating furnace 1 needs to be the non-oxidizing atmosphere. An oxygen concentration in the furnace is preferably less than 200 ppm, and more preferably 100 ppm or less, from the perspective of sufficiently preventing non-coating from occurring due to oxidized surfaces of the metal strip and consequent deteriorated wettability. Furthermore, the oxygen concentration in the furnace is preferably 0.001 ppm or more from the perspective of cost restriction in removing oxygen. Although no particular limitations are placed on atmosphere gases in the coating furnace 1 as long as the gases are of non-oxidizing, examples of which that can be suitably used include one or more of the gases selected from an inert gas such as N2 and Ar; and a reducing gas such as H2.
While the metal strip S and the nozzle system 10 are arranged for coating both faces of the metal strip in a vertical furnace according to
In order to suppress the oxidation of the metal strip and the molten metal, it is preferable to provide a sealing device 2, for separating the space of the non-oxidizing atmosphere from air, on a side in the coating furnace 1 where the metal strip leaves. Examples of the sealing device include partitions such as a gas curtain and a slit; or sealing rollers as illustrated in
Dimension of the nozzle 23 is not particularly limited, however, preferred is a rectangle of about 1 to 10 mm for a longitudinal direction of the metal strip and about 1 to 200 mm for a transverse direction of the metal strip, with reference to
Referring to
For discharging the molten metal droplets, the pulsed current requires to be managed for controlling droplet diameters and discharging amount according to line speed, desired coating thickness or resolution. And in managing the pulsed current, frequency needs to be set high to a certain degree in order to form small droplets. In that sense the frequency of the pulsed current is preferably 100 Hz or more. More preferred is 500 Hz or more. In addition, the frequency of the pulsed current is preferably 50000 Hz or less because of a limit of speed at which the molten metal can be filled into the nozzle. Further, strong magnetic field and current output are necessary for the molten metal, having high specific gravity, to be discharged such that the molten metal can speed up enough to land onto the metal strip. These will be parameters that need to be appropriately adjusted according to shapes of the discharge ports, required droplet diameters, types of molten metals to be used, and so on. In general, droplet volume V is given by the following formula.
In the formula, r is a radius of the discharge port, v is a discharging velocity, and f is a resonance frequency of a pressure wave in the chamber. The radius of the discharge port can be reduced for reducing the droplet diameter (the droplet volume). Or the resonance frequency can be set high for the smaller droplet diameter.
Our various studies also found that the droplet diameter was almost the same as or slightly larger than the size of the discharge port. In the present embodiment, it is preferable to design an average droplet diameter to be 100 μm or less in terms of achieving the uniform coatings. In order to stably discharge fine droplets having the droplet diameter of 100 μm or less, it is preferable to set the discharge port to have a diameter of 60 μm or less, and more preferably 50 μm or less. Moreover, the diameter of the discharge port is preferably set to 2 μm or more in terms of maintaining stable filling and discharging of the molten metal droplets. Therefore, a preferred range of the average droplet diameter is 2 μm or more as well. As described herein, “droplet diameter” refers to a diameter of a sphere when taking the droplet as the sphere having the volume equal to that of the droplet. A method for measuring the droplet diameter is as follows. The measurement was begun with discharging droplets of the molten metal onto a metal plate. One of the discharged and then solidified droplets was measured by a laser microscope to obtain a 3D height distribution. The obtained 3D height distribution was used for calculating the droplet volume. Finally, the droplet diameter was resulted by converting into a diameter of a sphere having the equivalent volume as the calculated droplet volume. The average droplet diameter is defined as an arithmetic mean of the droplet diameters calculated for freely and randomly selected 10 or more droplets discharged onto the metal plate.
It is preferable to set the interval between adjacent discharge ports (a distance between centers of the discharge ports) in a range of 10 to 250 μm in terms of obtaining the uniform coatings under the aforementioned conditions.
In addition, magnetic field strength is preferably 10 mT or more, and more preferably 100 mT or more, in order to discharge the droplets so that the droplets can land fast onto the metal strip. Moreover, the magnetic field strength is preferably 1300 mT or less because of a limit of magnetic force of the permanent magnet.
In order to uniformly coat a wide metal strip subjected to sheet passing at high speed, it is necessary to dispose multiple nozzle cartridges in the transverse direction of the metal strip, so that the discharge ports are arranged at given intervals across an entire range of the transverse direction of the metal strip. In addition, disposing multiple nozzle cartridges in a travelling direction of the metal strip is also preferable. A coating speed can be improved with these arrangements. As an example of the nozzle cartridge arrangement, the nozzle cartridges may be disposed in multiples rows along the transverse direction as well as the travelling direction of the metal strip, so that the nozzles 23 are disposed having relative positions as illustrated in
It is desirable to configure the facilities in such a way that the nozzle replacement will not affect an overall atmosphere in the furnace by providing an additional sealing device on an upstream side of the nozzle as well with respect to the travelling direction of the metal strip. This facilitates the replacement of the nozzles and nozzle cartridges.
As for a temperature of the metal strip S to be coated, the desired temperature is (Tu −20° C.) or more, i.e., the desired temperature is equal to or higher than Tu −20° C., where a melting point of the molten metal used for coating is expressed in Tu (° C.). This desired temperature is for coating a surface smoothly and uniformly. It is possible to obtain smooth coated faces when the temperature of the metal strip is (Tu −20° C.) or more, as the droplets landed onto the metal strip surface do not solidify immediately and exert its leveling ability. For that reason, while not illustrating in
In contrast to above, the temperature of the metal strip surface is set below Tu −20 (° C.) if one desires to maintain the shape of the droplets to obtain a predetermined surface texture without leveling of the molten metal after landing. And in a case where the surface is coated with some added patterns, formed fine shapes, or printed text and so on, the temperature of the metal strip surface is set less than Tu −20 (° C.), and more desirably (Tu −40° C.) or less, i.e., a temperature equal to or lower than Tu −40° C. In such a case, the temperature of the metal strip is preferably set to 10° C. or more because the metal strip of excessively low temperature will become a brittle material having difficulty in the sheet passing.
Further referring to
Furthermore, the facilities may be configured to have a separate system capable of injecting different types of the molten metals so as to be able to alter the type of the molten metal injected into the chamber of each nozzle cartridge. This configuration allows to obtain a multilayer coating or composite coating formed of different types of the molten metals. As a specific example, such multi-layered coatings can be formed by controlling the type of the molten metal supplied to the chamber of each nozzle cartridge to be different among the nozzle cartridges 20 disposed at different positions in the travelling direction of the metal strip, as illustrated in
In some cases, the metal strip travelling in the furnace may warp as a result of effects of vibration or defective shapes. For such a reason, it is preferable to install a damping-straightening mechanism, for suppressing the metal strip from vibrating or warping, on at least one of the upstream side or the downstream side of the nozzle system with respect to the travelling direction of the metal strip. For example,
A distance from the nozzle surface (a tip of the discharge port) to the metal strip is preferably set to greater than 0.2 mm and less than 10 mm. With the distance of 0.2 mm or less, there is a risk that the metal strip possibly contacts with the nozzle if the metal strip could not be damped sufficiently. And with the distance of 10 mm or more, gaps occurred in the landing positions of the metal droplets, as a result of effects of gas flows around the nozzle, will make the uniform coatings difficult.
According to the embodiment described above, it is possible to apply the molten metal coating treatment to surfaces of the metal strip continuously travelling, while averting the problems inherent in conventional immersion coating processes and spray coating processes. Examples of the metal strip include, without particularly limiting to, a steel strip. And examples of the molten metal to be discharged as droplets include, again, without particularly limiting to, molten zinc. The preferred conditions described in the embodiment may be adopted individually or may be adopted in any combination.
One face of a steel strip having a sheet thickness of 0.4 mm and a sheet width of 100 mm was galvanized using the apparatus illustrated in
Appearance of the coating was judged according to the following criteria.
Good: No unevenness in the appearance or discoloration is visually observed.
Fair: Tolerable as a product though minor unevenness in the appearance and/or minor discoloration is visually observed.
Poor: Obvious unevenness in the appearance and/or obvious discoloration is visually observed.
The non-coating was judged according to the following criteria.
Good: No non-coating is visually observed.
Fair: Tolerable as a product though minor non-coating is visually observed.
Poor: Obvious non-coating is visually observed.
The splashing was judged according to the following criteria.
Good: No splashing is visually observed.
Fair: Tolerable as a product though minor splashing is visually observed.
Poor: Obvious splashing is visually observed.
As reported in Table 1, coating treatments without defects of the splashing or the dross were possible in the present Examples. Under the condition 6 where the temperature of the steel strip is low out of the preferred range of the present disclosure, the minor unevenness in the leveling occurred on the molten metal, resulting in slightly unfavorable appearance though it was still within the acceptable range. Reducing in the frequency of the electric current was found to make the stable discharging of the fine droplets difficult, and as a result the coating thickness was increased. Furthermore, under the condition 13 where the oxygen concentration in the furnace is 200 ppm, the non-coating in a very small area was barely confirmed though it was still within the acceptable range as a product.
In addition, the coating was performed using a nozzle head respectively made to have the nozzle diameter of 50 μm and 60 μm under the conditions 1 to 5 of Table 1 for the purpose of comparison. As a result, coating treatments without defects of the splashing or the dross were possible though the coating thickness was increased to 10 to 11 μm and 16 to 17 μm, respectively. The average droplet diameter of randomly selected 10 droplets by means of the aforementioned method was 52 μm and 62 μm, respectively.
Gas wiping was performed as illustrated in
While a zinc-aluminum alloy containing 0.2% by mass of Al is used as the molten metal in the Example, the present method is applicable to various types of the molten metals.
The present disclosure provides a fully new method for molten metal coating treatment as well as an apparatus for continuous molten metal coating treatment capable of carrying out such a method. The method, as a method for treating surfaces of a metal strip by molten metal coating, avoids inherent issues involved in conventional immersion coating processes and spray coating processes. The disclosure is therefore industrially highly useful.
| Number | Date | Country | Kind |
|---|---|---|---|
| JP2017-034018 | Feb 2017 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2018/004731 | 2/9/2018 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2018/155245 | 8/30/2018 | WO | A |
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| Entry |
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| Number | Date | Country | |
|---|---|---|---|
| 20200232082 A1 | Jul 2020 | US |
