MXPA00006229A - Production of hot dip metal plated metallic strip - Google Patents

Abstract

A method of manufacturing a hot dip coated metal strip includes the steps of depositing a molten metal coating solution on the surfaces of the metal strip by continuously dipping the metal strip in a coating bath, lifting the metal strip at a constant speed while supporting it with a pair of upper and lower support rolls for clamping the surfaces of the metal strip in the coating bath, adjusting the coating weights of the molten metal deposited on the surfaces of the metal strip by wiping the molten metal with gases from gas wiping nozzles disposed above the surface of the coating bath, and advancing the metal strip while supporting it with a pair of upper and lower touch rolls for clamping the surfaces thereof, wherein the metal strip is advanced by setting the distance L between the upper support roll disposed in the coating bath and the lower touch roll disposed outside the coating bath within the range determined by a formula L ? 80 x T x W<2>/V, where L:distance between the uppe r support roll in the coating bath and the lower touch roll outside the coating bath (mm), V:line speed of the metal strip (m/min), T:tension imposed on the metal strip (kgf/mm<2>), and W:target coating weight per one side of the metal strip (g/m<2>). According to the invention, the stable quality of the metal strip can be obtained by reducing the variation of the coasting weights of the molten metal deposited on the surfaces of the metal strip by reducing the variation of the coating weights of the molten metal deposited on the surfaces of the metal strip at all times regardless of the change of the operating conditions under which continuous hot dip galvanizing operation is carried out. Further, a coating cost can be greatly reduced by preventing the excessive deposition of the molten metal.

Description

METHOD TO MANUFACTURE METALLIC STRIPS COATED BY HOT IMMERSION BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a metal strip coated by hot dip. More particularly, the present invention relates to a method for manufacturing a hot-dip coated metal strip having a coating layer of uniform thickness by reducing the vibration of the metal strip that rises from a coating bath of hot dip and travel vertically at an approximately constant speed. 2. Description of the Related Art In general, hot-dip galvanizing is applied to the surfaces of a steel strip using a continuous, hot-dip galvanizing apparatus (also referred to as a line) as described below. . First, as shown in Figure 2, a strip of steel 1 as a material to be coated is introduced into a dip bath 2 by hot dip, the travel direction of the steel strip 1 is deflected upwards by a sinking roller 3 placed in the galvanizing bath 2, the spring of the steel strip 1 is corrected by a pair of support rollers 4, upper and lower, placed in the galvanizing bath 2 to hold both surfaces of the steel strip 1, and then the steel strip 1 rises vertically from the galvanizing bath 2. During that time, molten zinc is deposited on the surfaces of the steel strip 1. A gas 6 (referred to as the gas) is blown. of rinsing) on the surfaces of the steel strip 1, in which the molten zinc has been deposited and traveling upwards, through the nozzles 5 (referred to as rinsing nozzles because they rinse the coated metal), so that the amount of molten metal in positioned on the steel strip 1 is adjusted to a desired amount (so that the molten metal can be deposited in a manner to join the entire surface of the steel strip 1). A pair of touch roller 7, which hold the surfaces of the steel strip 1 in a similar manner to the support rollers 4, are placed above the rinsing nozzles 5 to stabilize the travel of the steel strip 1. The steel strip 1, which has passed through the touch rollers 7, can be subjected to an alloy treatment when traveling through an alloy furnace 8 placed above the touch rollers 7, so that the layer of coating it is subjected to alloy when necessary. In this way, it has recently become very important to make a high speed steel strip hot dip galvanized with a low coating weight (referred to as a light coating). In accordance with the reduced coating weight, a technology has been required to manufacture a strip of hot dipped galvanized steel while preventing vibration thereof due to an increase in the pressure of the rinse gas 6 and the like. This is because the coating weight of the molten zinc deposited on the surface of the steel strip is greatly varied by an increase in the vibration of the steel strip and thus deteriorates the quality of the product. Commonly, when the steel strip 1 coated by hot dip, which has a particularly low coating weight (the coating weight on the one hand is 45 g / m 2 or less) is manufactured at a high speed, the strip of Steel 1 is vibrated in the position where the rinsing nozzles 5 are placed in a vertical direction to the surfaces thereof at a total vibration amplitude of 1-2 mm at all times. Since the rinsing can not be carried out smoothly when this vibration occurs, at present, the standard deviation of the variation of the coating weights on the surface of a steel strip is adjusted to a large value of 2 - 4 g / m2 (s = 2 - 4 g / m2) with respect to the coating weight on the one hand of 45 g / m2. Nevertheless, since it is generally required by customers to guarantee the lower limit of the coating weight, when the guarantee for a lower limit is maintained, the molten zinc is excessively deposited. This means that a large amount of zinc is consumed uneconomically from the point of view of the manufacturers. When a hot dipped galvanized steel strip is manufactured, the large variation of the coating weight directly leads to the variation of the coating weight of the annealing by hot dip. In this way, when a steel strip 1 is manufactured, the coating frequently undesirably exfoliates in a powdery state (referred to as dusting) from a portion of the steel strip 1 where the zinc is deposited in coarse form, furthermore. , a defect is present such as a non-uniform alloy and the like in the manufacture of the steel strip 1. Technologies have been vigorously developed to prevent vibration and many of these have been published. For example, publications Nos. 5-320847 and 5-078806 of unexamined Japanese patent applications describe technologies for depositing a static pressure pad to maintain the pressure of a gas being blown to the rinsing nozzles at a constant pressure. In addition, publication number 6-322503 of the Japanese Unexamined Patent Application describes a technology for separately depositing nozzles to blow a protective gas above the rinsing nozzles and depositing gas protection plates between the blowing nozzles of protective gas and rinsing nozzles. However, technologies for preventing the vibration of a steel strip by means of the static pressure pad or by blowing another gas are not of practical use because a high power must be specially provided to generate a desired pressure and The desired flow velocity of the gas as well as the effect of the technologies is little when the steel strip has a relatively large thickness. In addition, publications Nos. 52-113330, 6-179956 and 6-287736 of the Japanese patent applications not discussed describe technologies for preventing vibration of a steel strip using magnetic force or electromagnetic force. However, these technologies are not yet of practical use because not only do they require an expensive magnetic force generator separately and the operation becomes complex, but also because there is little effect of the technologies on a steel strip that It has a relatively large thickness.

BRIEF DESCRIPTION OF THE INVENTION In view of the above circumstances, an object of the present invention is to provide a method for manufacturing a hot-dip coated metal strip that can provide the metal strip with stable quality by reducing the variation of the coating weight of molten metal to be deposited on the surfaces of the metal strip even if the operating conditions of the hot dip coating are thus changed which can greatly reduce the cost of coating by preventing the excessive deposition of molten metal. In order to achieve the above object, the inventors examined the influences of the tension of a metallic strip in travel, the weight of the target coating, the linear velocity of the metal strip, the pressure of a rinse gas, the distance between a roller of touch placed above the rinsing nozzles and a support roller placed in a bath, and the like, in the vibration of the metal strip in a position of rinsing by gas in many test operations. Then, the inventors have finished the present invention based on the discovered knowledge of the analysis of the data obtained in the examination, that the vibration of a metal strip can be greatly reduced when the operation is carried out by adjusting the distance between the rod. illo of touch and the support roller placed in the bathroom within a certain interval. That is, according to the present invention there is provided a method for manufacturing a hot-dip coated metal strip that includes the steps of depositing the molten metal on the strip surfaces.

? When the metallic strip is continuously immersed in a hot dip coating bath, lift the metal strip at a constant speed while supporting it with a pair of upper and lower support rollers to hold the surfaces of the metal strip in the metal strip. coating bath, adjusting the coating weights of the molten metal deposited on the surfaces of the metal strip by rinsing the molten metal with gases from gas flushing nozzles placed above the surfaces of the coating bath, and advancing the metal strip so that it is supported with a pair of upper and lower touch rollers positioned outside the coating bath to hold the surfaces thereof, wherein the metal strip is advanced by adjusting the distance L between the upper support roller placed on the 'coating bath and the lower touch roller placed outside the coating bath within the range of determined by the following formula: L < 80 x T x W2 / V wherein, L: distance between the upper support roller in the coating bath and the lower stop roller outside the coating bath (mm); V: linear speed of the metal strip (m / min); T: tension imposed on the metal strip (gf / mm2); and W: target coating weight on one side of the metal strip (g / m2) Furthermore, according to the present invention, it is preferable that the metal strip is composed of a steel strip and that the solution of molten metal coating in the bath of coating by hot immersion it is molten zinc. Still further, it is preferred that the metal strip is subjected to an alloy treatment downstream of the top touch roller. According to the present invention, the total vibration amplitude of the metal strip having the molten metal deposited on the surfaces thereof is greatly reduced in the gas rinse positions compared to the total vibration amplitude, conventional, and The coating weights can be applied smoothly and ideally. As a result, a metal strip having molten metal deposited on the entire surface thereof, with a uniform coating weight, can be stably manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a view showing how the support rollers and the touch rollers are placed inside and outside a bath, respectively, and how a steel strip is vibrated; Figure 2 is a view showing an ordinary continuous hot injection galvanizing apparatus. Figure 3 is a graph showing the relationship between a distance L between an upper support roller in the bath and a lower stop roller outside. of the bath and a total vibration amplitude of a steel strip, Figure 4 is a graph showing the relationship between a pressure of a gas expelled from the gas flushing nozzles and a total vibration amplitude of a steel strip; Figure 5 is a graph showing the relationship between the tension of a steel strip and a total vibration amplitude thereof; Figure 6 is a graph showing the relationship between a pressure of a gas expelled from the gas flushing nozzles and a coating weight on one side of a steel strip; Figure 7 is a graph showing the relationship between linear velocity and a steel strip and a coating weight on one side thereof; Figure 8 is a graph showing the relationship between a total vibration amplitude of a steel strip and the variation of a coating weight on one side thereof; Figure 9 is a graph comparing the variation of a coating weight in a conventional coating method and that in the method of the present invention; Figure 10 is a graph comparing a quantity of metal consumption in the conventional coating method and that in the method of the present invention; and Figure 11 is a graph comparing an occurrence ratio of a defective product due to dusting in the conventional coating method and that in the method of the present invention.

DESCION OF THE PREFERRED MODALITIES The inventors carried out several test operations using the hot-dip galvanizing apparatus, shown in Figure 2. and described below. At that time, the support rollers 4 and the touch rollers are arranged as pairs of rollers, upper and lower, respectively as shown in Figures 1 and 2. In the figures, each upper roller is denoted by "a" and each lower roller is denoted by "b". A distance L (reference number 10, units of mm) was measured between an upper support roller 4a and a lower touch roller 4b parallel to the line of passage 9 of the steel st1. In addition, a total vibration amplitude B (reference number 11, units of m) 'of the steel stwas measured to be measured with an interval gap between the surfaces of the steel st1 and the front edges of the rinsing nozzles (subsequently, referred to simply as nozzle) 5 perpendicular to the line of passage 9. First, the inventors examined the influence of the distance L between the upper support roller 4a placed in the bath and the lower touch roller 7b in the total amplitude of the vibration b of the steel st1 when the tension of the steel st1 was adjusted to 1.5 kgf / mm2 and a linear speed thereof was adjusted to 90 m / min. As a result, the relationship shown in Figure 3 was found. That is, the total amplitude of vibration was reduced by a decrease in distance L, whenever a coating weight on one side was 30 g / m2 and 45 / m2. The relationship is represented by the following formula (1). B oc L ... (1) In addition, the inventors paid attention to the pressure p of a rinse gas 6 and the tension T relative to steel 1 as factors that influenced the total vibration amplitude B of the steel st1 and tested them. Figure 4 shows the results of the measurement of the pressure p and the total vibration amplitude B of the steel stwhen the distance L was adjusted to 1000 mm and the distance between the front edges of the nozzles and the surfaces of the stof Steel was adjusted to be approximately 6-8 mm. In addition, Figure 5 shows the result of the measurement of the total amplitude of the vibration B of the steel st1 when the voltage T was changed variedly. It can be seen from Figures 4 and 5 that the total amplitude of vibration B of the steel st1 is approximately in proportion to the gas pressure p and approximately in proportion to the tension T of the steel st1. This relation can be expressed simply by a formula (2).

B oc p / T (2) In addition, the relationship between the gas pressure of the nozzles, the linear velocity of the steel st1 and the coating weight thereof was examined. Figure 6 shows the relationship between the gas friction p and the coating weight on one side of the steel st1 when the distance between the front edges of the nozzles 5 and the steel st1 was adjusted to 6-8 mm and the linear velocity of the steel st1 was adjusted to 90 m / min and the gas pressure p changed in a varied manner. In this case, the coating weight on one side is approximately in proportion to the inverted square root of the pressure P. In contrast, Figure 7 shows the relationship between the linear velocity of the steel st1 and the coating weight per one side, when the distance between the front edges of the nozzles and the steel st1 was adjusted to be approximately 6-8 mm, the pressure P was kept constant and the linear velocity changed in a varied manner. As a result, one can see the coating weight on one side is approximately in proportion to the square root of the linear velocity of the steel st1. Therefore, the following formula (3) will be established, where the coating weight on the one hand it is represented by W (g / m2), the linear velocity of the steel st1 is represented by V (m / min) and the gas pressure P is represented by P (kgf / cm2).

P oc v / W (3) It is noted that the coating weight on one side W was measured with a coating weight gauge showing the value of the coating weight on one side of the steel strip 1. In addition, while the ratio between the linear velocity relative to the steel strip 1 and the total vibration amplitude B thereof was examined with the other conditions held constant in the test, the total vibration amplitude B of the steel strip 1 almost completely was not influenced by the speed linear. In this way, the inventors have found that the following formula will be established by arranging the formulas (1), (2) and (3) obtained in the previous tests.

B oc L x V / (T x W2) (4) Next, the expression L x V / (T x W2), which was referred to as a vibration coefficient, was used to arrange the test data. The inventors subsequently examined the relationship between the total amplitude of the vibration B of the steel strip 1 and the variation of the coating weight (the evaluation was carried out based on the standard deviation c (g / m2) of the coating weight) . Conventionally, the variation of the coating weight is evaluated on both sides of a steel strip and the Japanese Industrial Standards (JIS) also employ the so-called "two-sided guarantee" which evaluates the variation based on the total coating weight of both sides of the steel strip. The applicant describes a coating technology of both sides in Publication No. 10-306356 of Patent Application Not Examined. In the variation of the total coating weight of both sides when the steel strip 1 reaches one of the rinsing nozzles 5 by vibration, the coating weight of the side of the steel strip 1 near the nozzle is reduced, while increases the coating weight on the side of the same and away from the nozzle.

In addition, a "total coating weight of both sides" that is obtained by adding the coating weights of both sides of the steel strip 1 does not vary greatly in many cases, and thus the standard deviation s is made to a value little. Therefore, the "both sides guarantee" is used for convenience in the technology, and the coating weight deviation must be evaluated naturally based on the coating weight on the one hand from the point of view of the characteristics of the coating. coating, an anti-dusting property and the like. As a natural result, automakers recently require "one-sided warranty" beyond the stipulation of the JIS. In this way, when the inventors reviewed the coating weights used in their company at present on the base of one side, it was found that the standard deviation s of them was 2-3 g / m. proposed to establish a coating operation method to obtain the standard deviation s smaller than the previous value, specifically a standard deviation s of 1.5 g / m2 or less As a result, the inventors have found that the method of operation can be established when a Total vibration amplitude B of a steel strip is adjusted to 0.5 mm or less despite the change in operating conditions in the coating as shown in Figure 8. When many tests were carried out to minimize the stable the total amplitude of vibration, it was found that the vibration coefficient must satisfy the following formula: L x V / (T x W2) < 00 The present invention has been terminated by employing this condition. That is, the steel strip 1 is advanced by the upper limit of the distance L between the upper support roller 4a and the lower touch roller 7b which is adjusted to satisfy the following formula: L < 80 x V / (T x W In addition, it is even better to adjust the upper limit to satisfy L < 80 x V / (T x W2). It is noted that the lower limit of the distance L is not particularly critical in the present invention. In a current coating device, however, the upper support roller 4a ordinarily has diameters of approximately 250 mm f, each support roller has an immersion depth of approximately 150-200 mm in the center thereof, a height of each rinsing nozzle 5 above the bath is about 150-600 mm, and a distance of at least about 300 mm is necessary from each rinsing nozzle 5 to the lower touch roller 7b above the bath from a viewpoint of the structure of the apparatus of coating. As a result, in practice the lower limit of the distance L is expected to be approximately 600 m. Further, it is preferred to move the touch roller 7b to actually change the distance L. This is because it is easier to move the lower touch roller 7b than to move the support roller 4a placed in the bath from the viewpoint of the structure of the coating apparatus.

Example: A cold-rolled steel strip 1 having a thickness of 0.65-0.90 mm was galvanized by the hot-dip galvanizing apparatus, continuous, shown in Figure 2. At that time, the operation was carried out using a method of manufacturing a metal strip coated by hot dip according to the present invention in which the restriction was imposed on the adjustment of the distance between the previous rolls (examples of the present invention) and by a conventional method in which no restriction is imposed on them (comparative examples). A coating weight was measured in-line while the steel strip 1 is advanced. The measurement was made by a fluorescent X-ray coating weight gauge (not shown) positioned above the steel strip 1 in travel for give down. Accordingly, the variation s of the measured coating weights represents the variation thereof on one side of the steel strip 1. Further, the pressure of a rinse gas used under the conditions of the respective examples is a value measured in the side of the steel strip 1 where the coating weight was measured.

Table 1 shows the operating conditions and the result of the measurements collectively. It is apparent from Table 1 that in the specimens numbers 1-18, which were manufactured by the manufacturing method according to the present invention, the total vibration amplitudes of the steel strip 1 are 0.5 mm or less because L x V / (T x W2) < 80 is satisfied in them. As a result, the variation s of the coating weights is made at 1.5 g / m2 or less in all the examples (reference to Figure 9). This suggests that an objective value of the coating weight can more closely reach a lower limit value in the operation and the consumption of metal can be greatly reduced in this way. Figure 10 shows the comparison of a quantity of coating metal actually consumed in the conventional manufacturing method with that actually consumed in the manufacturing method according to the present invention. When consumption in the conventional manufacturing method is represented by 100%, the consumption of manufacturing method of the present invention is approximately 90%. This means that the consumption of the coating metal can be greatly reduced. On the other hand, in the specimens numbers 19-29 manufactured by the conventional method of manufacture, the steel strip 1 has a total amplitude of great duration and the variation s of the coating weights thereof is 2.0 g / m2 or plus .

Table 1 Then, a so-called "hot dipped galvanized steel strip" is made by placing an alloy furnace 8 above the touch rollers 7 in Figure 2 and by heating the steel strip 1 on which the steel was deposited. zinc melted in the alloy furnace 8, so that the Fe content in the zinc coating layer of the steel strip 1 is made at 8-13% by weight. Then, an anti-dusting property, which was one of the most important quality characteristics, of the steel strip 1 was examined. Dusting is a defect where a coating layer deposited in a powder state is exfoliated from a portion of the hot dipped galvanized steel sheet, which deteriorates from the intimate contact property of the coating during press forming thereof. When this phenomenon occurs during press formation, the coating powder falls between a die of the press and the steel sheet to thereby cause a regularity effect to the steel sheet. In this way, it is desired that the dusting is not present. The operation was carried out paying attention to the dusting under the conditions of a target coating weight on one side adjusted to 45-55 g / m2, a linear velocity of the steel strip 1 adjusted to 100 m / min - 150 m / min, and a tension of the steel strip 1 adjusted to 1.5 kgf / mm2 - 2.0 kgf / mm2. Table 332 shows examples of the operating conditions different from the different operating conditions and the result of the operation collectively. It is noted that the anti-dusting property was evaluated by a known method of placing a tape of adhesive on the coating layer of a specimen shown from a strip of hot dip galvanized steel. • under pressure, detaching the adhesive strip 5 after the specimen was bent at 90 ° and returned to its original state and then measuring an amount of exfoliation of the coated layer with fluorescent x-rays. That is, the anti-dust property is represented by the number of accounts, that is counted on the x-rays of zinc contained in • the exfoliated coating layer. Usually, when the number of accounts is 1500 or less, there is no defect due to dusting, in the current pressing process. However, when the number of counts exceeds 1500, a defect due to dusting frequently occurs. It is apparent from Table 2 that since the variation of a coating weight can be greatly reduced according to the method of the present invention, the number of beads is stable to a low degree, whereby the galvanized steel strip 1 can be stably manufactured by hot dip, with excellent anti-dusting property. In In contrast, in the conventional method, a product was elaborated in which the number of quanta was increased and it was made to 1500 or more in the same portions and in which the defect due to the dusting that could arise frequently when the process was processed. product. This is because a coating weight varied greatly in the product. Figure 11 shows an occurrence relationship of the defective products after they were braided. It is apparent from Figure 11 that there were almost no defective products by the method of the present invention. In the above examples, the steel strip was used as a metal strip and the molten zinc was used as the molten metal. However, in spite of saying that the present invention is not limited from this medium to the same and is applicable to other kinds of metal strips and to another molten metal different from molten zinc.

Shows the Maximum and Minimum Measured Values As described, a metal strip having molten metal deposited on the entire surface thereof, at a uniform coating weight can be manufactured by the present invention. As a result, it is possible to get closer to a lower objective coating weight during a coating operation, whereby the consumption of the coating metal can be greatly reduced compared to normal consumption

Claims (6)

RE IV I N D I CAC IONE S

1. A method for manufacturing a metal strip by hot dip coating, comprising the steps of: immersing a metal strip in a hot dip coating bath to continuously deposit molten metal on the surfaces of the metal strip; transporting the metal strip at a substantially constant speed while supporting the metal strip with a pair of upper and lower support rollers that hold the surfaces of the metal strip in the coating bath; adjusting a coating weight of the molten metal deposited on the surfaces of the metal strip by rinsing the molten metal with gases from gas flushing nozzles positioned above a surfaces of the coating bath; and advancing the metal strip while being supported with a pair of upper and lower touch rollers positioned outside the coating bath to hold the surfaces of the same, wherein the metal strip is advanced by adjusting the distance L between the roller of top support placed in the coating bath and the lower touch roller placed outside the coating bath within the range determined by the following formula:

L < 80 x T x 2 / V wherein, L: distance between the upper support roller in the coating bath and the lower stop roller outside the coating bath (mm); V: linear speed of the metal strip (m / min); T: tension imposed on the metal strip (kgf / mm2); and: target coating weight on one side of the metal strip (g / m2) 2. The method according to claim 1, wherein the metal strip is composed of a steel strip and the hot dip coating bath is filled by molten zinc.

3. The method according to claim 1, wherein the metal strip is subjected to an alloying treatment downstream of the top touch roller. A method for manufacturing a hot-dip coated metal strip, comprising the steps of: transporting a metal strip through a dip coating bath to continuously deposit molten metal on the surface of the metal strip; supporting the metal strip with a pair of support roll submerged in the coating bath; blowing gas into the metal strip as the coating bath emerges with gas rinse nozzles deposited above a surface of the coating bath, to thereby adjust a coating weight of the molten metal in the strip; and further transporting the metal strip while it is supported by a pair of touch rollers positioned outside the coating bath, wherein a distance L between the upper support roller placed in the coating bath and a lower touch roller placed outside the coating bath. Bath coating is maintained according to the following formula: L < 80 x T x W2 / V wherein, L: distance between the upper support roller in the coating bath and the lower stop roller outside the coating bath (mm); V: linear speed of the metal strip (m / min); T: tension imposed on the metal strip (kgf / mm2); and W: target coating weight on one side of the metal strip (g / m2). 5. The method according to claim 4, wherein the metal strip is composed of a steel strip and the dip coating bath is hot. filled with molten zinc. The method according to claim 4, wherein the metal strip is subjected to an alloying treatment downstream of the top touch roller. SUMMARY OF THE INVENTION A method for manufacturing a hot-dip coated metal strip includes the steps of depositing a molten metal coating solution on the surfaces of the metal strip by continuously immersing the metal strip in a coating bath, lifting the metal strip at a speed constant while being supported with a pair of upper and lower support rollers to support the surfaces of the metal strip in the coating bath, adjusting the coating weights of the molten metal deposited on the surfaces of the metal strip by rinsing the molten metal with gases from gas flushing nozzles placed above the surfaces of the coating bath, and advancing the metal strip while supports with a pair of upper and lower touch roller to hold the surfaces thereof, wherein the metal strip is advanced by adjusting the distance L between the upper support roller placed in the coating bath and the lower touch roller placed outside the coating bath within the range determined by the formula L <; 80 x T x W2 / V, where L: distance between the upper support roller in the coating bath and the lower stop roller outside the coating bath (mm), V: linear speed of the metal strip (m / min ), T: tension imposed on the metal strip (kgf / mm2), W: target coating weight on one side of the metal strip (g / m2). According to the invention, a stable quality of the metal strip can be obtained by reducing the variation of the coating weights of the molten metal deposited on the surfaces of the metal strip by reducing the variation of the coating weights of the molten metal deposited on the metal strip. the surfaces of the metal strip at all times despite the change in the operating conditions under which the hot-dip galvanizing operation is carried out. In addition, the coating cost can be greatly reduced by preventing the excessive deposition of molten metal.