Methods for the production of hot-dip coated steel strips and continuous hot-dip coating equipment.

Abstract

The present invention provides a method for manufacturing a molten metal plated steel strip, the method being capable of sufficiently suppressing the generation of flow lines and manufacturing a high quality molten metal plated steel strip at a low cost. The method for manufacturing a molten metal plated steel strip of the present invention is characterized in that when blowing gas from a pair of gas wiping nozzles 20A, 20B onto a steel strip S drawn up from a molten metal bath 14 and adjusting the adhesion amounts of the molten metal on both surfaces of the steel strip S, the angle formed between the ejection port portions of the gas wiping nozzles 20A, 20B and a horizontal plane is between 10 degrees and 75 degrees inclusive, and the header pressure of the gas wiping nozzles 20A, 20B is less than 30 kPa.

Description

Method for manufacturing a molten metal-plated steel strip and continuous molten metal plating facility The present invention relates to a method for manufacturing a molten metal-plated steel strip and a continuous molten metal plating facility, and particularly relates to gas wiping for adjusting the amount of molten metal adhered to the surface of the steel strip (hereinafter also referred to as "plating adhesion amount"). In a continuous molten metal plating line, as shown in FIG. 2, a steel strip S annealed in a continuous annealing furnace in a reducing atmosphere passes through a snout 10 and is continuously introduced into a molten metal bath 14 in a plating bath 12. Thereafter, the steel strip S is pulled up above the molten metal bath 14 via a sink roll 16 and a support roll 18 in the molten metal bath 14, adjusted to a predetermined plating thickness by gas wiping nozzles 20A and 20B, and then cooled and led to a subsequent process. The gas wiping nozzles 20A and 20B are arranged opposite to each other above the plating bath 12 with the steel strip S interposed therebetween, and gas is blown from their injection ports toward both surfaces of the steel strip S. By this gas wiping, excess molten metal is scraped off, the plating adhesion amount on the surface of the steel strip is adjusted, and the molten metal adhered to the surface of the steel strip is made uniform in the plate width direction and the plate longitudinal direction. The gas wiping nozzles 20A and 20B are configured to correspond to various steel strip widths and to cope with positional deviations in the width direction when the steel strip is pulled up, and thus are usually configured to be wider than the steel strip width and extend beyond the width direction ends of the steel strip. In such a gas wiping method, wavy flow marks (molten metal sags) are likely to occur on the plated surface of the manufactured molten metal-plated steel strip due to one or both of (1) vibration of the impact pressure of the wiping gas and (2) viscosity unevenness due to oxidation / cooling of the molten metal. When the plated steel sheet with such molten metal sags is used as the surface for coating in the application of an exterior panel, the surface properties of the coating film, particularly smoothness, are impaired. Therefore, the plated steel sheet with molten metal sags cannot be used for exterior panels that require excellent appearance coating treatment, which has a great impact on the yield of the plated steel sheet. As methods for suppressing plating surface defects such as scum, the following methods are known. Patent Document 1 describes a method of making scum less noticeable by changing the surface properties of the temper rolling roll and the rolling conditions during temper rolling, which is a process after plating. Patent Document 2 describes a method of suppressing the occurrence of scum by adjusting the roughness of the steel sheet surface according to the plating adhesion amount using a skin pass mill, a tension leveler, etc. before introducing the steel sheet into a hot-dip galvanizing bath. JP-A-2004-27263 JP-A-55-21564 However, according to the study by the present inventors, in the method disclosed in Patent Document 1, minor scum was improved, but no effect was observed on severe scum. Also, in the method disclosed in Patent Document 2, there are cost problems due to the necessity of installing a skin pass mill, a tension leveler, etc. in the previous process of the hot-dip galvanizing bath. Further, even when these are installed, it is difficult to obtain the desired surface roughness due to chemical and physical changes of the galvanized film accompanying pickling and recrystallization in the pretreatment equipment and the annealing furnace, and it is considered difficult to sufficiently suppress the occurrence of scum. Therefore, in view of the above problems, an object of the present invention is to provide a method for manufacturing a hot-dip metal-plated steel strip and a continuous hot-dip metal plating facility that can sufficiently suppress the occurrence of scum and manufacture a high-quality hot-dip metal-plated steel strip at low cost. In order to solve the above problems, the present inventors focused on the installation angle of the gas wiping nozzles. Usually, the gas wiping nozzles are installed such that the gas injection direction is substantially perpendicular to the steel strip (i.e., in the horizontal direction), but the present inventors have found that by installing the gas wiping nozzles so that the gas injection direction is downward at a predetermined angle or more with respect to the horizontal direction, the occurrence of scum can be sufficiently suppressed. The gist configuration of the present invention completed based on the above findings is as follows. (1) Continuously dipping a steel strip into a hot-dip metal bath, blowing gas from a pair of gas wiping nozzles arranged with the steel strip therebetween onto the steel strip pulled up from the hot-dip metal bath to adjust the adhesion amount of the hot-dip metal on both sides of the steel strip, A method for manufacturing a molten metal plated steel strip by continuously manufacturing a molten metal plated steel strip, wherein the gas wiping nozzle is installed downward with respect to the horizontal plane such that an angle θ formed by an injection port portion thereof and the horizontal plane is 10 degrees or more and 75 degrees or less, and a header pressure P of the gas wiping nozzle is less than 30 kPa. A method for manufacturing a molten metal plated steel strip is provided. (2) The method for manufacturing a molten metal plated steel strip according to (1) above, wherein the components of the molten metal contain Al: 1.0 to 10% by mass, Mg: 0.2 to 1% by mass, Ni: 0 to 0.1% by mass, and the balance is composed of Zn and inevitable impurities. (3) The temperature T (°C) of the gas immediately after being discharged from the tip of the gas wiping nozzle satisfies the relationship with the melting point T M (°C) of the molten metal, T M −150 ≤ T ≤ T M +250. The method for manufacturing a molten metal plated steel strip according to (1) or (2) above is controlled so as to satisfy the above condition. (4) The method for manufacturing a molten metal plated steel strip according to any one of (1) to (3) above, wherein the gas is an inert gas. (5) A plating bath that contains a molten metal and forms a molten metal bath, a pair of gas wiping nozzles that are arranged with a steel strip continuously pulled up from the molten metal bath interposed therebetween, blow gas toward the steel strip, and adjust the plating adhesion amount on both sides of the steel strip, and the gas wiping nozzle is installed downward with respect to the horizontal plane such that an angle θ formed by an injection port portion thereof and the horizontal plane is 10 degrees or more and 75 degrees or less, and a header pressure P of the gas wiping nozzle is set to less than 30 kPa. A continuous molten metal plating facility is provided. (6) A memory that records the relationship between the header pressure P and a suitable angle θ within a range where the header pressure P is less than 30 kPa, an angle detector that detects the angle θ, a nozzle driving device for changing the angle θ, a control device for the nozzle driving device, further comprises, and when the operating conditions are changed and the header pressure P is changed, the control device reads out a suitable angle θ corresponding to the changed pressure P from the memory, and when the detected angle detected by the angle detector does not satisfy the suitable angle θ, controls the nozzle driving device to make the detected angle the suitable angle θ. The continuous molten metal plating facility according to (5) above. (7) A surface appearance detector for observing the surface appearance of the steel strip after wiping; A nozzle driving device for changing the angle θ; A control device for the nozzle driving device; further comprises, and the control device controls the nozzle driving device based on the output from the surface appearance detector to finely adjust the angle θ. The continuous molten metal plating facility according to (5) above. According to the method for manufacturing a molten metal-plated steel strip and the continuous molten metal plating facility of the present invention, the generation of bath dents can be sufficiently suppressed, and a high-quality molten metal-plated steel strip can be manufactured at low cost. It is a schematic diagram showing the configuration of a continuous molten metal plating facility 100 according to an embodiment of the present invention. It is a schematic diagram showing the configuration of a conventional continuous molten metal plating facility. (A) and (B) are cross-sectional views perpendicular to the steel strip S of the gas wiping nozzle 20A in an embodiment of the present invention. It is a graph showing the collision pressure distribution curves at various nozzle angles θ. It is a cross-sectional view perpendicular to the steel strip S of the gas wiping nozzle 20A showing the case where the nozzle angle θ is 80 degrees. Referring to FIG. 1, a method for manufacturing a molten metal-plated steel strip and a continuous molten metal plating facility 100 (hereinafter, also simply referred to as "plating facility") according to an embodiment of the present invention will be described. Referring to FIG. 1, the plating facility 100 of the present embodiment includes a snout 10, a plating bath 12 for containing molten metal, a sink roll 16, and a support roll 18. The snout 10 is a member having a rectangular cross-section perpendicular to the advancing direction of the steel strip S that partitions the space through which the steel strip S passes, and its tip is immersed in the molten metal bath 14 formed in the plating bath 12. In one embodiment, the steel strip S annealed in a continuous annealing furnace in a reducing atmosphere passes through the snout 10 and is continuously introduced into the molten metal bath 14 in the plating bath 12. Thereafter, the steel strip S is pulled up above the molten metal bath 14 via the sink roll 16 and the support roll 18 in the molten metal bath 14, adjusted to a predetermined plating thickness by a pair of gas wiping nozzles 20A and 20B, and then cooled and led to a subsequent process. In addition to FIG. 1, referring also to FIGS. 3(A) and 3(B), a pair of gas wiping nozzles 20A and 20B (hereinafter also simply referred to as "nozzles") are arranged opposite to each other above the plating bath 12 with the steel strip S interposed therebetween. The nozzle 20A blows gas toward the steel strip S from an injection port 26 (nozzle slit) extending in the plate width direction of the steel strip at its tip to adjust the amount of plating adhered to the surface of the steel strip. The other nozzle 20B is the same, and by these pair of nozzles 20A and 20B, excess molten metal is scraped off, the amount of plating adhered to both surfaces of the steel strip S is adjusted, and it is made uniform in the plate width direction and the plate longitudinal direction. The nozzle 20A is configured to be longer than the steel strip width and extend beyond the widthwise ends of the steel strip in order to accommodate various steel strip widths and to address misalignment in the width direction during steel strip lifting. Also, as shown in FIG. 3(B), the nozzle 20A includes a nozzle header 22, and an upper nozzle member 24A and a lower nozzle member 24B connected to the nozzle header 22. The tip portions of the upper and lower nozzle members 24A and 24B face each other in parallel in a cross-sectional view perpendicular to the steel strip S, forming a gas injection port 26 (nozzle slit) (the parallel portion in FIG. 3(B)). The injection port 26 extends in the plate width direction of the steel strip S. The longitudinal cross-sectional shape of the nozzle 20A is a tapered shape that tapers toward the tip. The thickness of the tip portions of the upper and lower nozzle members 24A and 24B may be about 1 to 3 mm. Also, the opening width of the injection port (nozzle gap) is not particularly limited, but can be about 0.5 to 3.0 mm. The gas supplied from a gas supply mechanism (not shown) passes through the inside of the header 22, further passes through the gas flow path partitioned by the upper and lower nozzle members 24A and 24B, and is injected from the injection port 26 and blown onto the surface of the steel strip S. The other nozzle 20B has a similar configuration. In the method for manufacturing a molten metal-plated steel strip according to this embodiment, the steel strip S is continuously immersed in the molten metal bath 14, and gas is blown from a pair of gas wiping nozzles 20A and 20B disposed with the steel strip S therebetween onto the steel strip S pulled up from the molten metal bath 14 to adjust the amount of molten metal adhering to both surfaces of the steel strip S, thereby continuously manufacturing a molten metal-plated steel strip. Here, the causes of the generation of the above-described molten metal puddles include the generation of initial unevenness at the point where the wiping gas collides with the surface of the molten metal (the stagnation point). The cause of the generation of the initial unevenness is considered to be that the molten metal flows irregularly on the steel strip due to either or both of (1) the vibration of the impact pressure of the wiping gas and (2) the viscosity unevenness due to the oxidation / cooling of the molten metal. Therefore, suppressing this phenomenon (1) and / or (2) is considered to lead to the suppression of the generation of molten metal puddles. From this perspective, in the present invention, it is important that the injection port portions of the gas wiping nozzles 20A and 20B are installed downward with respect to the horizontal plane such that the angle θ formed by the injection port portion and the horizontal plane is 10 degrees or more. By setting the angle θ to 10 degrees or more, the generation of hot water unevenness can be sufficiently suppressed. On the other hand, if the angle θ exceeds 75 degrees, the generation of hot water unevenness cannot be suppressed due to the generation of an unstable pressure accumulation described later, so the angle θ should be 75 degrees or less. Here, in this specification, the "angle θ formed by the injection port portion and the horizontal plane" means, as shown in FIGS. 3(A) and 3(B), the portion (parallel portion) where the upper nozzle member 24A and the lower nozzle member 24B face each other to form a slit, and when viewed in a cross-section perpendicular to the steel strip, it means the angle formed by the extending direction of the parallel portion and the horizontal plane. In the present invention, the header pressure P of the wiping nozzle is set to less than 30 kPa. This is because if the header pressure P is 30 kPa or more, the wind speed when the wiping gas collides with the bath surface becomes high, and bath surface splashes occur frequently. When the target plating adhesion amount is large, the header pressure P will be small, but in that case, the above-mentioned hot water unevenness is likely to occur. However, by setting the angle θ of the gas wiping nozzle as described above, even with a small header pressure P of less than 30 kPa, the generation of hot water unevenness can be sufficiently suppressed. When the header pressure P is less than 10 kPa, the collision pressure at the edge portion of the steel strip becomes particularly weak, so there is a possibility that the adhesion amount at the edge portion becomes too thick and the adhesion amount becomes non-uniform in the steel strip width direction. Therefore, the header pressure P is preferably 10 kPa or more. In the present invention, by controlling the angle θ of the wiping nozzle in this way, the range of the collision pressure acting on the steel strip S is widened, and the generation of hot water unevenness is suppressed. Usually, the wiping nozzle is installed such that the gas injection direction is substantially perpendicular to the steel strip S, so the collision pressure becomes large. Therefore, when measuring the collision pressure under the condition where hot water unevenness occurs, it was found that the collision pressure vibrates over time. This is considered to be because, particularly in the case of low gas pressure, the potential core does not develop sufficiently in the parallel portion inside the nozzle (see FIG. 3(B)), and the potential core is disturbed by the outside air after jetting from the nozzle. When the impact pressure is vibrating, if the acting range of the impact pressure is local, the vibration will directly result in uneven plating adhesion. On the other hand, even when the impact pressure is vibrating, if the acting range is wide, the unevenness of the liquid film caused by the vibration will overlap, resulting in less likelihood of adhesion unevenness. As an easy method to expand the acting range of the impact pressure, a method of controlling the angle θ of the wiping nozzle was implemented. Wiping was carried out while changing the angle θ, and the surface appearance after wiping was inspected. At θ = 0°, hot water groove defects occurred, but an improvement trend was observed at θ = 10° or more. Fig. 4 compares the distribution curves of the impact pressure measured under the conditions of θ = 0°, 10°, 30°, and 80°. In Fig. 4, (a) is the impact pressure distribution curve when θ = 0°, (b) is the impact pressure distribution curve when θ = 10°, (c) is the impact pressure distribution curve when θ = 30°, and (d) is the impact pressure distribution curve when θ = 80°. In Fig. 4, b is the opening width of the nozzle slit (nozzle gap), y is the vertical distance from the gas jet center (y = 0), and y / b on the horizontal axis indicates the ratio of the two. y < 0 means the lower side (molten plating bath side) from the gas jet center, and y > 0 means the upper side (anti-molten plating bath side) from the gas jet center. Also, the impact pressure ratio on the vertical axis indicates the ratio of the impact pressure under other conditions when the maximum pressure of the impact pressure distribution curve at the set nozzle angle θ is used as the reference (1.0). Note that the "gas jet center" means the vertical center of the vertical range where the gas impinges on the steel strip. As shown in Fig. 4, the impact pressure distribution at θ = 10° in (b) has an FWHM of the impact pressure ratio that is 1.2 times larger than that of the impact pressure distribution at θ = 0° in (a), indicating that wiping is carried out over a wider range. Also, the impact pressure distribution at θ = 30° in (c) has an even larger FWHM of the impact pressure ratio compared to the impact pressure distribution at θ = 10° in (b). Thus, by setting the angle θ within an appropriate range and widening the FWHM for wiping, the influence of the vibration of the impact pressure can be suppressed, and it is considered that a hot water groove suppression effect can be obtained. On the other hand, when the angle is further increased to θ = 80°, the collision pressure distribution (d) has a more gentle pressure distribution than (b) and the half-value width is enlarged, but the appearance of the steel strip after plating deteriorates again. The reason for the deterioration of the appearance at this time is that, with the distance d between the tip of the wiping nozzle and the steel strip being constant, when the angle θ of the wiping nozzle is increased, the gap between the upper part of the wiping nozzle and the steel strip S becomes extremely narrow, so the wiping gas cannot be discharged smoothly from the gap between the wiping gas and the steel strip S, resulting in an unstable pressure accumulation (see Fig. 5). Therefore, when the angle exceeds a certain value, the influence of the generated pressure accumulation becomes stronger than the effect of increasing the half-value width of the collision pressure distribution, and it is considered that the appearance gradually deteriorates. In addition, when the angle θ is increased, the distance between the wiping nozzle and the steel strip S becomes shorter, and when the steel strip S vibrates, there is a risk of contact with the wiping nozzle. From the above, the angle θ should be 75 degrees or less. Furthermore, regarding the upper limit of the angle θ, from the viewpoint of more sufficiently suppressing the generation of the hot water groove, it is preferably set as follows in relation to the header pressure P. That is, when the header pressure P is 0 to 10 kPa, it is preferably θ ≤ 75 degrees, when the header pressure P exceeds 10 kPa and is 20 kPa or less, it is preferably θ ≤ 60 degrees, and when the header pressure P exceeds 20 kPa and is 30 kPa or less, it is preferably θ ≤ 50 degrees. Also, the temperature T (°C) of the gas immediately after being discharged from the tip of the gas wiping nozzle is related to the melting point T M (°C) of the molten metal, and it is preferably controlled to satisfy M T - 150 ≤ T ≤ T M + 250. When the gas temperature T is controlled within the above range, the cooling and solidification of the molten metal can be suppressed, so uneven viscosity is less likely to occur, and the generation of the hot water groove can be suppressed. On the other hand, when the gas temperature T is lower than T M - 150°C and too low, since it does not affect the fluidity of the molten metal, it is not effective in suppressing the generation of the hot water groove. Also, when the temperature of the wiping gas is higher than T M + 250°C and too high, alloying is promoted and the appearance of the steel plate deteriorates. The gas ejected from nozzles 20A and 20B is preferably an inert gas. By using an inert gas, oxidation of the molten metal on the surface of the steel strip can be prevented, and thus unevenness in the viscosity of the molten metal can be further suppressed. Examples of the inert gas include, but are not limited to, nitrogen, argon, helium, carbon dioxide, etc. In the present embodiment, the components of the molten metal preferably contain Al: 1.0 to 10% by mass, Mg: 0.2 to 1% by mass, Ni: 0 to 0.1% by mass, with the balance being Zn and inevitable impurities. When Mg is contained in this way, it has been confirmed that unevenness in viscosity due to oxidation / cooling of the molten metal is likely to occur, and that liquid level dips are likely to occur. Therefore, when the molten metal has the above component composition, the effect of suppressing liquid level dips of the present invention is remarkably manifested. Also, when the composition of the molten metal is 5% by mass Al-Zn or 55% by mass Al-Zn, the effect of suppressing liquid level dips of the present invention can also be obtained. Examples of the hot-dip metal-coated steel strip manufactured by the manufacturing method and plating equipment of the present invention include hot-dip galvanized steel sheets, which include both plated steel sheets (GI) that are not subjected to an alloying treatment after the hot-dip galvanizing treatment and plated steel sheets (GA) that are subjected to an alloying treatment. In the present embodiment, while setting the angle θ within the above range, it is preferable to further perform control for finely adjusting the angle θ. As a first control example, according to the value of the header pressure P of the gas wiping nozzle, the angle θ of the wiping nozzle is controlled to be within a more preferable range or value within the range of 10 to 75 degrees. As described above, the preferable range within the range of 10 to 75 degrees of the angle θ of the wiping nozzle changes according to the value of the header pressure P. Therefore, by adjusting the angle θ as follows, liquid level dips can be suppressed more reliably and sufficiently. Referring to FIG. 1, the angle detector 40 is a device that detects the angle θ of the nozzles 20A and 20B, and the nozzles 20A and 20B are adjusted to display 0 degrees in a state parallel to the bath surface. Examples of the angle detector 40 include a physical method such as a graduated scale, a type using a laser, and a type applying the electrical characteristics of a special liquid, but it is not particularly limited thereto. The nozzle driving device 42 includes a motor for rotating the nozzle and can change the angle θ. The memory 44 stores a correspondence table between the header pressure P and the nozzle angle θ, that is, information regarding the range of suitable nozzle angles θ corresponding to the header pressure P. For example, as described above, when the header pressure P is 0 to 10 kPa, the angle θ is set to 10 to 75 degrees; when the header pressure P exceeds 10 kPa and is 20 kPa or less, the angle θ is set to 10 to 60 degrees; and when the header pressure P exceeds 20 kPa and is 30 kPa or less, a correspondence table in which the angle θ is set to 10 to 50 degrees is recorded in the memory 44. The header pressure P can be appropriately determined from operating conditions such as the line speed, the thickness of the steel strip, the target plating adhesion amount, and the distance between the tip of the wiping nozzle and the steel strip. Therefore, during operation under predetermined operating conditions or when changing the operating conditions, the control device 46 reads out a suitable angle θ (suitable range or target value) corresponding to the determined header pressure P from the memory 44. The control device 46 determines the necessary angle change amount from the angle θ read from the memory 44 and the output value of the angle detector 40, and controls the nozzle driving device 42. The nozzle driving device 42 rotates the nozzles 20A and 20B to a predetermined angle according to the output value of the control device 46. Specifically, when the operating conditions are changed and the header pressure P is changed, the control device 46 reads out a suitable angle θ corresponding to the changed pressure P from the memory 44, and when the detected angle detected by the angle detector 40 does not satisfy the suitable angle θ, controls the nozzle driving device 42 to make the detected angle the suitable angle θ. As a second control example, the appearance of the steel strip surface after wiping is observed, and based on the result, the angle θ is finely adjusted. Referring to FIG. 1, the surface appearance detector 48 is a device that detects the appearance of the steel strip surface after passing through the gas wiping nozzle, for example, the arithmetic mean waviness Wa, and is provided, for example, above the gas wiping nozzle 20A. The surface appearance detector 48 continuously photographs the steel strip surface after passing through the gas wiping nozzle and inputs the information to the control device 46. The type of the surface appearance detector 48 includes, but is not particularly limited to, a non-contact 3D roughness meter using a laser. The control device 46 controls the nozzle drive device 42 based on the output of the surface appearance detector 48 to finely adjust the angle θ. Specifically, the following control is performed. Regarding the surface appearance of the steel strip, it is determined whether it is acceptable or not based on the following criteria. ××: Non-conforming = Galvanized steel sheet with a large amount of splash defects (0 < Wa, 1.30 ≦ S) ×: Non-conforming = Galvanized steel sheet with large visible hot dents (1.50 < Wa, S < 1.30) △: Non-conforming = Galvanized steel sheet with small visible hot dents (1.00 < Wa ≦ 1.50, S < 1.30) ○: Conforming = Beautiful galvanized steel sheet with no visible hot dents (0.50 < Wa ≦ 1.00, S < 1.30) ◎: Conforming = Very beautiful galvanized steel sheet with no visible hot dents (0 < Wa ≦ 0.50, S < 1.30) Note that Wa is the value of the arithmetic mean waviness Wa [μm] measured based on the standard of JIS B0601-2001. The splash mixing rate S is the ratio [%] of the length of the steel strip determined to have splash defects in the inspection process to the length of the steel strip passing through under each manufacturing condition. When Wa measured by the detector is 0.50 < Wa ≦ 1.00 (that is, conforming "○"), fine adjustment is performed so that the wiping nozzle angle θ becomes larger, and then the measured Wa becomes 0 < Wa ≦ 0.50 (that is, conforming "◎"). This is because when the wiping nozzle angle θ is increased, the vibration of the impact pressure of the wiping gas further decreases. The measurement location by the surface appearance detector 48 is preferably the position where the steel strip S passes through the wiping nozzle and the molten metal on the surface of the steel strip has solidified. In the case directly above the wiping nozzle, since the molten metal has not solidified, the measured arithmetic average waviness Wa will vary. Therefore, the position where the molten metal on the surface of the steel strip has solidified, for example, a position 40 m or more downstream of the wiping nozzle, is desirable. Incidentally, since the responsiveness deteriorates, the measurement position is preferably immediately after the molten metal has solidified. Therefore, for example, a position 70 m or less downstream of the wiping nozzle is desirable. If the nozzle height H is too low, a large amount of bath surface splash will occur, so a height of 200 mm or more is desirable. The nozzle height H and the distance d between the tip of the gas wiping nozzle and the steel strip shown in Fig. 3(A) do not necessarily need to be interlocked with the wiping nozzle angle θ, but it is preferable to appropriately change them according to the target coating amount and the amount of bath surface splash. In the production line of hot-dip galvanized steel strip, a production test of hot-dip galvanized steel strip was carried out. In each inventive example and comparative example, the plating equipment shown in Fig. 1 was used. As the gas wiping nozzle, one with a nozzle gap of 1.2 mm was used. In each inventive example and comparative example, the composition of the plating bath, the temperature T of the plating bath, the melting point T M of the plating bath, the angle θ of the nozzle, the wiping gas pressure P, the gas type, and the temperature T of the wiping gas were as shown in Table 1. The distance d between the tip of the nozzle and the steel strip was 15 mm. The height H of the nozzle from the bath surface was 350 mm. As a method of supplying gas to the gas wiping nozzle, a method of supplying gas pressurized to a predetermined pressure by a compressor was adopted. In this way, a steel strip with a thickness of 1.2 mm × a width of 1000 mm was passed through at a steel strip speed L (line speed) of 2 m / s to produce a hot-dip galvanized steel strip. In addition, the appearance of the produced hot-dip galvanized steel strip and the total coating adhesion amount on both sides were evaluated. Regarding the appearance evaluation of the steel plate, the pass or fail was judged according to the following criteria. The results are shown in Table 1. ××: Fail = Galvanized steel plate with a large amount of splash defects (0 < Wa, 1.30 ≦ S) ×: Fail = Galvanized steel plate in which large thermal dimples can be visually confirmed (1.50 < Wa, S < 1.30) △: Non - conforming = Galvanized steel sheet with small hot water wrinkles visible by visual inspection (1.00 < Wa ≤ 1.50, S < 1.30) ○: Conforming = Beautiful galvanized steel sheet with no visible hot water wrinkles by visual inspection (0.50 < Wa ≤ 1.00, S < 1.30) ◎: Conforming = Extremely beautiful galvanized steel sheet with no visible hot water wrinkles by visual inspection (0 < Wa ≤ 0.50, S < 1.30) Note that Wa is the value of the arithmetic mean waviness Wa [μm] measured based on the standard of JIS B0601 - 2001. The splash mixing rate S is the ratio [%] of the length of the steel strip determined to have splash defects in the inspection process to the length of the steel strip passed under each manufacturing condition. As is clear from Table 1, when the nozzle angle θ is 10 - 75 degrees and the wiping gas pressure P is less than 30 kPa, a low Wa and a beautiful surface appearance can be obtained. On the other hand, when the nozzle angle θ or the gas wiping pressure P is outside the scope of the present invention, Wa or the splash mixing rate S becomes large. In particular, for plating types B, E, and F, the effects are significantly obtained when the nozzle angle θ and the wiping gas pressure P are within the scope of the present invention. According to the method for manufacturing a molten metal - plated steel strip and the continuous molten metal plating equipment of the present invention, the generation of hot water wrinkles can be sufficiently suppressed, and high - quality molten metal - plated steel strips can be manufactured at low cost. 100 Continuous molten metal plating equipment 10 Snout 12 Plating tank 14 Molten metal bath 16 Sink roll 18 Support roll 20A, 20B Gas wiping nozzles 22 Nozzle header 24A Upper nozzle member 24B Lower nozzle member 26 Injection port 40 Angle detector 42 Nozzle drive device 44 Memory 46 Control device 48 Surface appearance detector S Steel strip

Claims

Page 1 of 2 pages of Claims 1. A method for the production of hot-dip coated steel strips consisting of continuous immersion of a steel strip in a molten metal bath, and gas wiping from a pair of gas-wiping nozzles arranged between the nozzles to the steel strip as it is pulled up from the molten metal bath to apply a heavy coating of molten metal on both sides of the steel strip. By this method, continuous hot-dip coated steel strips are produced, in which each nozzle of the gas-wiping nozzle consists of an injection port section mounted downward relative to the horizontal plane so that the angle formed between the injection port section and the horizontal plane is 10 degrees or more and 75 degrees or less, and the hexor pressure P is less than 30 kPa.

2. The method of producing hot-dip coated steel strips according to claim 1, in which the molten metal is composed of the following chemical elements: aluminum 1.0 percent by mass to 10 percent by mass, magnesium 0.2 percent by mass to 1 percent by mass, and nickel 0 percent by mass.1 percent by mass, with the presence of zinc balancing agent and unavoidable impurities.

3. The method of hot-dip galvanizing steel strip production according to one of the claims of claim 1 or claim 2, where the temperature 1 (°C) of the gas immediately after being immediately vented from the tip of each gas wipe nozzle is controlled to conform to Tm-150 < T < Tm+250°C relative to the melting point TM (°C) of the molten metal.

4. The method of hot-dip galvanizing steel strip production according to one of the claims of claim 1 to claim 3, where the gas is an inert gas. 5.A continuous hot-dip coating apparatus consists of a coating bath configured to hold the molten metal and to form the molten metal bath, and a pair of two gas wipe-off nozzles arranged with a steel bar between them, and configured to blow gas onto the steel bar to apply a heavy coating on both sides of the steel bar, which is continuously pulled up from the molten metal bath. Each gas wipe-off nozzle is equipped with an injection port that is mounted below the horizontal plane so that the angle formed between the injection port and the horizontal plane is 10 degrees or more and 75 degrees or less, and the header pressure P is specified to be less than 30 kPa.The continuous hot-dip coating equipment under claim 5, which includes a memory unit where the relationship between the header pressure P and the chemical formula optimal angle is recorded in the range where the header pressure P is below 30 kPa, an angle detector configured to detect the chemical formula angle, a nozzle driver configured to change the chemical formula angle, and a controller for the nozzle driver, where the controller is configured to read from the memory the chemical formula optimal angle corresponding to the pressure P after being changed in response to changes in operating conditions, and is configured to control the nozzle driver to set the detection angle to the chemical formula optimal angle when the detection angle detected by the angle detector does not correspond to the chemical formula optimal angle.7.The continuous hot-dip coating equipment under claim 5, comprising: a surface appearance detector configured to observe the surface appearance of the steel strip after wiping; a nozzle driver configured to change the 0-angle and a controller for the nozzle driver, where the controller is configured to control the nozzle driver based on the outlet from the surface appearance detector to fine-tune the chemical formula angle;