Method for manufacturing hot-dip metal-plated steel strip

Abstract

Provided is a molten metal-plated steel strip production method which suppresses the occurrence of a splash defect.　This molten metal-plated steel strip production method is for continuously producing a molten metal-plated steel strip by continuously immersing a steel strip S in a molten metal bath 4 and jetting a gas to the steel strip S pulled up from the molten metal bath 4 from gas jetting ports of a pair of gas wiping nozzles 10A, 10B disposed across the steel strip S so as to adjust the amount of molten metal adhering to both surfaces of the steel strip S, wherein when an angle formed by a gas jetting direction and a horizontal plane is defined as θ (˚), a distance from the tip of the gas jetting port to the steel strip S is defined as D (mm), and a width of the gas jetting port is defined as B (mm), the pair of gas wiping nozzles 10A, 10B are operated within the ranges of θ: 10 to 60, D / B: 3 to 12, and D / B: 0.1 x θ + 9 or less.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a molten metal plated steel strip.

Background Art

[0002] Galvanized steel sheets, which are a type of molten metal plated steel sheets, are widely used in fields such as building materials, automobiles, and household appliances. In these applications, it is required that the galvanized steel sheets have excellent appearance. Here, the appearance after painting is strongly affected by surface defects such as uneven plating thickness, scratches, and foreign matter adhesion. Therefore, it is important that there are no surface defects in the galvanized steel sheets.

[0003] In a continuous molten metal plating line, generally, a steel strip as a metal strip annealed in a continuous annealing furnace in a reducing atmosphere passes through a snout and is introduced into a molten metal bath in a plating bath. Then, the steel strip is pulled up above the molten metal bath through sink rolls and support rolls in the molten metal bath. Thereafter, wiping gas is blown onto the surface of the steel strip from gas wiping nozzles arranged on both sides of the steel strip to scrape off the excess molten metal adhering to and pulled up on the surface of the steel strip. Thereby, the adhesion amount of the molten metal (hereinafter, also referred to as basis weight) is adjusted. Here, since the gas wiping nozzles correspond to various steel strip widths and also correspond to misalignment in the width direction when the steel strip is pulled up, they are usually configured wider than the steel strip width and extend outside the width direction ends of the steel strip. In such a gas wiping method, the molten metal that falls downward due to the turbulence of the jet impinging on the steel strip scatters around and solidifies during scattering to become fine metal powder, so-called splash, which adheres to the steel strip, resulting in defects (splash defects) and causing deterioration of the surface quality of the steel strip.

[0004] Furthermore, in a continuous process, increasing production volume can be achieved by increasing the steel strip feeding speed. However, when controlling the amount of plating adhesion using a gas wiping method in a continuous hot-dip galvanizing process, the wiping gas pressure must be set to a higher pressure in order to control the amount of plating adhesion within a certain range. As a result, splash increases significantly, making it impossible to maintain good quality.

[0005] To solve the above problems, the following technologies are disclosed.

[0006] Patent Document 1 describes a method for preventing molten metal splashes from adhering to a strip surface during molten metal plating. In the method described in Patent Document 1, a metal plate is placed between the main wiping gas supply pipe and the wiping nozzle. Furthermore, a filter is placed between the main wiping gas supply pipe and the alloying furnace, aligned with the steel plate. In the technique described in Patent Document 1, plating metal splashes generated on the plating bath surface are removed by the filter as they travel around the outside of the wiping nozzle towards the steel plate after wiping is complete, thereby preventing splashes from adhering to the steel plate.

[0007] Patent Document 2 discloses a method for preventing splash from adhering to a plated steel strip by providing a rectifier plate extending from the rear of the wiping nozzle and a weir in front of the upper part of the wiping nozzle.

[0008] Patent Document 3 proposes a method for suppressing splash defects by installing a side nozzle above the wiping nozzle and blowing gas from the side nozzle into the gas turbulence in the gas-gas collision zone of the wiping gas. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 5-306449 [Patent Document 2] Japanese Patent Publication No. 2000-328218 [Patent Document 3] Japanese Patent Publication No. 2014-80673 [Overview of the project] [Problems that the invention aims to solve]

[0010] However, it was found that the method disclosed in Patent Document 1 is insufficient in preventing the occurrence of splash defects. In other words, increasing the mesh size of the filter eliminates the filter's effectiveness. On the other hand, decreasing the mesh size of the filter suppresses splashes that go around the outside of the filter from adhering to the strip surface. However, splashes that enter directly between the filter and the metal plate without going around the back of the wiping nozzle are less likely to be discharged outside the filter. Therefore, the method is insufficient in preventing the occurrence of splash defects.

[0011] Furthermore, the method disclosed in Patent Document 2 cannot prevent splashes that travel around the back of the wiping nozzle and fly upward from adhering to the plated steel strip. In addition, splashes (metal powders) that accumulate on the rectifier plate extended behind the wiping nozzle during operation become scattered again due to changes in the wiping gas flow caused by changes in wiping conditions (wiping gas pressure, nozzle height, etc.). This phenomenon becomes more pronounced over time, and it was found that the method disclosed in Patent Document 2 cannot reliably prevent splash adhesion.

[0012] The method disclosed in Patent Document 3 can suppress the adhesion of splash to the steel plate. However, it has become clear that the gas discharged from the side nozzle repels the splash, which then enters the inside of the wiping nozzle slit, causing blockage and resulting in streaky defects on the steel plate.

[0013] This invention has been made in view of the above circumstances, and aims to provide a method for manufacturing a hot-dip metal-plated steel strip that suppresses the occurrence of splash defects by suppressing the adhesion of splash to the steel strip. [Means for solving the problem]

[0014] The means of the present invention for solving the above problems are as follows. [1] A method for manufacturing a molten metal-plated steel strip, comprising continuously immersing a steel strip in a molten metal bath, and blowing gas onto the steel strip as it is pulled out of the molten metal bath from the gas nozzles of a pair of gas wiping nozzles, each having a slit-shaped gas nozzle extending wider than the steel strip along the width direction of the steel strip and positioned on either side of the steel strip, thereby adjusting the amount of molten metal adhering to both sides of the steel strip, thereby continuously manufacturing a molten metal-plated steel strip, When a graph is drawn with the angle θ(°) between the direction of gas injection from the gas nozzle and the horizontal plane as the horizontal axis, and the quotient D / B (the distance D(mm) between the tip of the gas nozzle and the steel strip and the width B(mm) of the gas nozzle) as the vertical axis, A method for manufacturing a hot-dip metal-plated steel strip, comprising operating the pair of gas wiping nozzles within the range enclosed by the following equations (1) to (5). D / B=3 (Formula 1) D / B=0.1×θ+9 (Formula 2) D / B=12 (Formula 3) θ=10 (Formula 4) θ=60 (Formula 5) [2] The distance H between the tip of the gas nozzle of the pair of gas wiping nozzles and the surface of the molten metal bath is 50 mm or more and 700 mm or less. A method for manufacturing a hot-dip metal-plated steel strip according to [1], wherein the temperature T (°C) of the gas immediately after being ejected from the pair of gas wiping nozzles satisfies the relationship TM-150 ≤ T ≤ TM+250 with respect to the melting point TM (°C) of the molten metal. [3] Each of the pair of gas wiping nozzles has a nozzle header and an upper nozzle member and a lower nozzle member connected to the nozzle header, The tip portion of the upper nozzle member and the tip portion of the lower nozzle member form the gas injection port, facing each other parallel to each other in a cross-sectional view perpendicular to the width direction of the steel strip. The method for manufacturing a molten metal plated steel strip according to [1] or [2], wherein the gas passes through the inside of the nozzle header and is ejected from the gas ejection port. [4] The method for manufacturing a molten metal plated steel strip according to [3], wherein the pressure inside the nozzle header is 2 to 70 kPa. [5] The method for manufacturing a molten metal plated steel strip according to any one of [1] to [4], wherein a baffle plate is disposed between the outer sides of both ends in the width direction of the steel strip and between the pair of gas wiping nozzles so as to face the gas ejection port. [Advantages of the Invention]

[0015] According to the present invention, it is possible to suppress the adhesion of splashes to the steel strip and manufacture a molten metal plated steel strip in which the occurrence of splash defects is suppressed.

[0016] According to the present invention, by operating the gas wiping nozzle within a predetermined range with respect to the traveling direction of the steel strip, the scattering direction of the splashes is limited. As a result, the occurrence of splash defects is suppressed, and a molten metal plated steel strip having excellent surface quality can be stably manufactured. [Brief Description of the Drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram showing a schematic configuration of a continuous molten metal plating facility provided with a gas wiping nozzle according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing a schematic configuration of a gas wiping nozzle used in the continuous molten metal plating facility shown in FIG. 1. [Figure 3] FIG. 3 is a schematic diagram showing the scattering direction of splashes. [Figure 4] FIG. 4 is a schematic diagram for explaining each configuration according to an embodiment of the present invention. [Figure 5] FIG. 5 shows the results of investigating the angle θ formed by the gas ejection direction and the horizontal plane and the splash defect occurrence rate in an embodiment of the present invention. [Figure 6]Figure 6 is a schematic diagram showing the splash scattering direction at θ = 30° and 65° in one embodiment of the present invention. [Figure 7] Figure 7 is a schematic diagram showing the velocity distribution of the jet discharged from the gas wiping nozzle. [Figure 8] Figure 8 shows the results of splash defect occurrence at θ=10° for slit gaps of 1 mm and 2 mm. [Figure 9] Figure 9 shows the results of splash defect occurrence at θ=15° for slit gaps of 1 mm and 2 mm. [Figure 10] Figure 10 shows the results of splash defect occurrence at θ=30° for slit gaps of 1 mm and 2 mm. [Figure 11] Figure 11 shows the range of the angle θ (°) between the gas injection direction and the horizontal plane, and the quotient D / B between the distance D (mm) from the tip of the gas injection nozzle to the steel strip and the width B (mm) of the gas injection nozzle in the present invention. [Figure 12] Figure 12 is a schematic diagram (side view) showing one embodiment in which a baffle plate is arranged. [Figure 13] Figure 13 is a schematic diagram (top view) showing one embodiment in which a baffle plate is arranged. [Figure 14] Figure 14 is an enlarged view of the vicinity of one of the widthwise ends of the steel strip S shown in Figure 13. [Figure 15] Figure 15 is a magnified view of the area near the tip of the gas wiping nozzle. [Modes for carrying out the invention]

[0018] Embodiments of the present invention will be described below with reference to the drawings. The embodiments shown below are illustrative of apparatus and methods for realizing the technical concept of the present invention, and the present invention is not limited to these embodiments.

[0019] Furthermore, drawings are schematic representations. Therefore, it should be noted that the relationship and ratios between thickness and planar dimensions may differ from those in reality, and there may be differences in dimensional relationships and ratios between drawings themselves.

[0020] Figure 1 shows a schematic configuration of a continuous molten metal plating apparatus equipped with a gas wiping nozzle according to one embodiment of the present invention.

[0021] The continuous molten metal plating equipment 1 shown in Figure 1 is a device for continuously depositing molten metal onto the surface of a steel strip S by immersing it in a molten metal bath 4 made of molten metal, and then depositing a predetermined amount of molten metal onto it.

[0022] The continuous molten metal plating equipment 1 comprises a snout 2, a plating tank 3, a sink roll 5, and a support roll 6.

[0023] The snout 2 is a component that demarcates the space through which the steel strip S passes. The snout 2 is a component with a rectangular cross-section perpendicular to the direction of travel of the steel strip S, with its upper end connected to, for example, the outlet side of a continuous annealing furnace, and its lower end immersed in the molten metal bath 4 stored in the plating tank 3. In this embodiment, the steel strip S, annealed in a continuous annealing furnace in a reducing atmosphere, passes through the snout 2 and is continuously introduced into the molten metal bath 4 in the plating tank 3. Subsequently, the steel strip S is pulled up from the molten metal bath 4 via the sink roll 5 and support roll 6 in the molten metal bath 4.

[0024] Then, gas (wiping gas) is blown onto both sides of the steel strip S, which has been lifted out of the molten metal bath 4, from a pair of gas wiping nozzles 10A and 10B (gas injection port 11, described later) located on both sides of the steel strip S, thereby adjusting the amount of molten metal adhering to both sides of the steel strip S. After that, the steel strip S is cooled by a cooling system (not shown) and led to a subsequent process, where molten metal-plated steel strips are continuously manufactured.

[0025] A pair of gas wiping nozzles 10A and 10B (hereinafter also simply referred to as "nozzles") are positioned above the molten metal bath 4, facing each other with the steel strip S in between. As shown in Figure 2, nozzle 10A blows gas from a gas injection port 11 (nozzle slit) extending in the width direction of the steel strip at its tip toward the steel strip S, thereby adjusting the amount of plating deposited on the surface of the steel strip. The other nozzle 10B does the same. These pair of nozzles 10A and 10B scrape away excess molten metal, adjusting the amount of plating deposited on both sides of the steel strip S, and ensuring that the amount of plating deposited is uniform in both the width direction and the length direction of the steel strip.

[0026] The nozzle 10A is typically longer than the width of the steel strip and extends outward from the end of the steel strip in the width direction to accommodate various steel strip widths and to compensate for positional displacement in the width direction when the steel strip is pulled up. As shown in Figure 2, the nozzle 10A has a nozzle header 12 and an upper nozzle member 13A and a lower nozzle member 13B connected to the nozzle header 12. The tip portions of the upper and lower nozzle members 13A and 13B form gas injection ports 11 (nozzle slits) that are parallel to each other in a cross-sectional view perpendicular to the width direction of the steel strip S (parallel portions in Figure 2). The gas injection ports 11 extend in the width direction of the steel strip S. Specifically, the gas injection ports 11 are slit-shaped and extend wider than the steel strip S along the width direction of the steel strip S. The longitudinal cross-sectional shape of the nozzle 10A is tapered, narrowing towards the tip. The thickness of the tips of the upper and lower nozzle members 13A and 13B (see thickness P in Figure 15) should be approximately 1 to 3 mm. The width (opening width) B (slit gap) of the gas injection port is not particularly limited, but can be approximately 0.5 to 3.0 mm. Gas supplied from a gas supply mechanism (not shown) passes inside the nozzle header 12, then through the gas flow path partitioned by the upper and lower nozzle members 13A and 13B, and is injected from the gas injection port 11 and sprayed onto the surface of the steel strip S. The other nozzle 10B has a similar configuration. In this case, the pressure inside the nozzle header 12 is measured by a pressure gauge (not shown). The pressure inside the nozzle header 12 can be adjusted by the output of the gas supply mechanism.

[0027] Figure 15 is a magnified view of the area near the tip of the nozzle 10A. As shown in Figure 15, the tapered portion on the outer surface of the upper nozzle member 13A is called the outer tapered portion of the upper nozzle member 13A (outer tapered portion 131A), and the tapered portion on the outer surface of the lower nozzle member 13B is called the outer tapered portion of the lower nozzle member 13B (outer tapered portion 131B). The angle formed by the outer tapered portion 131A of the upper nozzle member 13A and the outer tapered portion 131B of the lower nozzle member 13B is called the outer angle of the nozzle 10A (outer angle α).

[0028] In the process of manufacturing molten metal-plated steel strips, pressurized gas is sprayed onto the surface of the steel strip as it is continuously pulled from the molten metal plating bath, using gas wiping nozzles positioned opposite each other on both sides of the strip to control the thickness of the deposited metal. However, this process can cause molten metal to scatter, solidify during the scattering process, and become metal powder (splash) that adheres to the steel strip, degrading its surface quality.

[0029] Here, a splash defect refers to a defect caused by splashes adhering to a steel plate. Specifically, as shown in Figure 3(a), a jet of gas discharged from opposing nozzles collides near the edge of a steel plate, causing the jet to vibrate. This vibrates the jet, tearing apart the liquid film of molten metal, and as the torn-off film scatters as droplets, it solidifies into a solid (metal powder) that adheres to the steel plate, resulting in a defect.

[0030] In investigating a method to suppress splash defects, the inventors first investigated the scattering direction of splash (metal powder) using a high-speed camera. As a result, it was found that under typical CGL (continuous hot-dip galvanizing line) operating conditions, with a nozzle angle θ (angle between the gas injection direction and the horizontal plane) = 0°, the splash was scattered over a wide area both upward and downward from the nozzle, as shown in Figure 3(b). To suppress these splash defects, operators have empirically made fine adjustments to the nozzle downward (nozzle angle: 0~2°). However, since the fine adjustment of the nozzle angle depends on the operator's skill level, the splash defects varied depending on the timing of the operation and were not stable. Therefore, the inventors hypothesized that if the nozzle was tilted downward significantly, the situation would change dramatically and the splash defects would be improved.

[0031] In the CGL production line, a 10t coil with a plate width of 1000mm and a plate thickness of 1mm was passed through at a speed of 100mpm (meters per minute). As shown in Figure 4, under conditions of a nozzle-to-steel plate distance of 10mm, nozzle angle θ=0~80°, and nozzle tip height of 500mm, the zinc deposition amount at the center of the plate width was 50±5g / m². 2 The pressure indicated by the pressure gauge attached to the nozzle header was adjusted accordingly. Subsequently, the splash defect rate was examined using a defect meter installed on the CGL outlet side, and its correlation with the nozzle angle was investigated. The splash defect rate is the ratio of the length of steel strip determined to have splash defects in the inspection process to the total length of steel strip that passed through. The slit gap (width of the gas injection port) was B = 1.0 mm. The experimental results are shown in Figure 5. Here, one coil was used for each plot, and the acceptance standard for the splash defect rate was 0.10% or less. This is because a splash defect rate of 0.10% or less indicates sufficient quality for steel strips with strict surface quality requirements, such as those used for automotive steel sheets.

[0032] In Figure 5, the incidence of splash defects varies greatly around the nozzle angle θ = 0°. This suggests that controlling splash defects is difficult by fine-tuning the nozzle angle. As the nozzle was tilted downwards, i.e., as the nozzle angle increased, the number of splash defects decreased. Then, at θ > 60°, the number of splash defects increased again.

[0033] Figure 6 shows the results of observing the splash dispersion using a high-speed camera. It was found that at a nozzle angle θ=30°, where the incidence of splash defects was low, the splash dispersed only downwards from the nozzle, while at a nozzle angle θ=65°, where the incidence of defects began to increase, the splash dispersed both upwards and downwards from the nozzle.

[0034] The following is considered to be the cause of this: When the nozzle angle θ = 0°, as shown in Figure 3, the gases discharged from opposing nozzles collide near the edge of the plate. Because the pressures of each nozzle are slightly different and there are pressure fluctuations over time, the jets colliding at the edge of the plate flow both above and below the nozzles. Consequently, it is presumed that splashes scatter both upward and downward.

[0035] Even under conditions where the nozzle angle of the downward-tilted nozzle is increased, the jet still collides with the plate edge. However, since the flow rate of gas directed toward the bath surface (downward) increases compared to the flow rate of gas directed upward, it is thought that the splash preferentially scatters downward. As a result, it is thought that the splash directed toward the nozzle is suppressed. This reduces the range of splash scattering and is estimated to have reduced splash defects. Similarly, in the range of nozzle angle θ = 10 to 60°, it is thought that the splash did not scatter much toward the nozzle, resulting in a splash defect rate close to zero. By operating within this range, the scattering of splash toward the nozzle is suppressed, which can also suppress operational problems such as splash adhering to the gas injection port and blocking the nozzle.

[0036] When the nozzle angle θ > 60°, as shown in Figure 6, the gap between the nozzle and the steel plate becomes smaller, making it difficult for air to escape upwards from the nozzle, which is thought to cause vortices to form. In other words, the space between the outer tapered portion of the upper nozzle member 13A and the steel strip S becomes narrower, obstructing the flow of gas that collides near the edge of the plate and moves upwards, making it easier for vortices to form between the outer tapered portion and the steel strip S. In this case, the splash scattered from the edge of the plate is scattered in various directions by the flow of the generated vortices. The reason for the increase in splash defects is thought to be that the splash scattered upwards from the nozzle due to the influence of these vortices adhered to the steel plate.

[0037] Regarding the nozzle angle θ, the effect of reducing splash defects appears in the region of 10° or more, so the lower limit is 10°. Here, the amount of zinc deposition changes due to the impact pressure gradient caused by the collision of gas with the steel strip S and the shear force generated in the zinc film by the collision of gas with the steel strip S, and as the downward nozzle angle increases, the impact pressure gradient decreases. In this case, the impact pressure gradient refers to the gradient of impact pressure in the direction corresponding to the direction of the slit gap B when the jet discharged from the nozzle collides with the target object (steel strip). Furthermore, in order to obtain the same amount of zinc deposition, a larger gas flow rate is required for the same nozzle-steel plate distance (spacing), which requires a large-capacity compressor and increases construction costs. Also, as mentioned earlier, if vortices are generated between the outer tapered portion of the upper nozzle member and the steel plate, splash defects are induced and splash cannot be suppressed. In addition, the outer diameter angle of the nozzle (outer diameter angle α in Figure 15) is about 40-50° considering the rigidity of the nozzle. If the nozzle is tilted more than 70°, the angle becomes 70° + 20° (half of the outer diameter angle) = 90°, and the nozzle will come into contact with the steel plate. Considering the nozzle-to-steel plate distance, a realistic upper limit for the nozzle angle θ is around 60°. Furthermore, a reduction in splash defects appears in the region where the nozzle angle θ is 60° or less. Therefore, the upper limit for the nozzle angle θ is set to 60°.

[0038] The preferred range for the nozzle angle θ is 15° ≤ θ ≤ 45°. While a reduction in splash defects is observed at θ ≥ 10°, setting the nozzle angle θ to 15° or higher further suppresses the reduction in impact pressure at the edge of the steel plate. Specifically, when the nozzle angle θ is small, the jets discharged from opposing nozzles collide outside the plate edge, causing the jets to oscillate and reducing the pressure acting on the edge of the steel plate. In contrast, setting the nozzle angle θ to 15° or higher suppresses the reduction in pressure acting on the edge of the steel plate. A decrease in impact pressure at the edge of the steel plate weakens the effect of scraping off excess molten metal. Setting the nozzle angle θ to 15° or higher suppresses edge overcoat defects where the amount of molten metal adhering to the edge of the steel plate is excessive. Therefore, the lower limit of the preferred range for the nozzle angle θ is 15°. In the region θ > 45°, the amount of gas directed towards the bath surface increases, and there is a risk of zinc splashing from the bath surface. Therefore, the upper limit of the preferred range for the nozzle angle θ is 45°. The phenomenon of zinc splashing from the surface of the bath is called bath surface splashing. When bath surface splashing occurs, problems can arise such as defects in the steel plates and deterioration of the surrounding environment of the equipment.

[0039] Here, the characteristics of the impinging jet are summarized by D / B, which is the distance (spacing) D from the nozzle tip (tip of the gas injection port) to the impinging plate (steel strip) divided by the slit gap B, as shown in Figure 7. In the region where D / B is small, the average velocity on the jet axis is equal to the jet outlet velocity, and this region is called the potential core. Subsequently, as D / B increases, the turbulence at the outer edge of the jet reaches the jet axis, the velocity on the jet axis decreases, the potential core disappears, and the jet becomes a fully developed region where it is completely turbulent. The inventors considered that the turbulence of the jet due to the disappearance of the potential core affects the fluctuation of the impingement pressure near the plate edge of the gas discharged from opposing nozzles. Then, they varied D / B and investigated the occurrence rate of splash defects with respect to the nozzle angle for nozzle angles θ = 10°, 15°, and 30°, respectively. The results are shown in Figures 8 to 10.

[0040] Figures 8-10 show that, when the nozzle angle is constant, the splash defect rate can be expressed as D / B regardless of the slit gap B. Furthermore, the splash defect rate differs depending on the nozzle angle. Therefore, it is clear that controlling the D / B ratio (nozzle-to-steel plate distance divided by the slit gap) and the nozzle angle is crucial for suppressing splash defects.

[0041] If the nozzle-to-steel plate distance is small, the nozzle may collide with the steel plate due to the effect of plate warping, so the lower limit of D / B is set to 3. If D / B increases, the turbulence of the jet increases due to the disappearance of the potential core (deterioration of jet stability), and this also increases splash defects. For this reason, the upper limit of D / B is 10 at a nozzle angle θ = 10° (Figure 8). As the nozzle angle θ increases, upward scattering of splash near the plate edge is suppressed. Therefore, the range of D / B at which operation is possible while suppressing splash defects expands, and the upper limit is 12 at a nozzle angle θ = 30° (Figure 10). In the range of 10° ≤ θ ≤ 30°, operation while suppressing splash defects is possible within the range of the straight line connecting the upper limits of D / B for nozzle angles θ = 10° and θ = 30°. When D / B exceeds 12, even if the nozzle angle θ increases, the effect of deterioration in jet stability becomes greater, and the effect of reducing splash defects is no longer observed. Therefore, within the range of 30° ≤ θ ≤ 60°, the upper limit of D / B is 12.

[0042] The nozzle angle θ and D / B required to achieve operation with suppressed splash defects are summarized in (Equations 1) to (5). Figure 11 summarizes the above range in terms of D / B and θ. D / B=3 (Formula 1) D / B=0.1×θ+9 (Formula 2) D / B=12 (Formula 3) θ=10 (Formula 4) θ=60 (Formula 5)

[0043] The preferred range for D / B is D / B ≤ 10. By setting D / B ≤ 10, it is possible to suppress the decrease in impact pressure at the edge of the steel plate caused by the jet discharged from opposing nozzles colliding outside the plate edge, thereby suppressing edge overcoat defects. In other words, as D / B increases, the turbulence of the jet increases due to the disappearance of the potential core, and the vibration of the jet that occurs when the jet discharged from opposing nozzles collides outside the plate edge also increases. In order to suppress the decrease in impact pressure at the edge of the plate width that occurs as a result, it is preferable to set it within the above range.

[0044] In order to prevent splash defects, the internal pressure (gas pressure) of the nozzle header 12 is preferably 2 to 70 kPa within the appropriate range of the nozzle angle θ and the nozzle-to-steel plate distance divided by the slit gap (D / B). More preferably, the pressure is 3 kPa or higher. Furthermore, it is even more preferable that the pressure is 60 kPa or lower. This is because if the internal pressure of the nozzle header 12 is less than 2 kPa, the turbulence of the jet becomes large before impact with the steel plate, making splash defects more likely. If the internal pressure of the nozzle header 12 exceeds 70 kPa, the compressor for injecting the gas becomes larger, increasing equipment costs and making it uneconomical.

[0045] Furthermore, within the appropriate range of the nozzle angle θ and D / B described above, the jet velocity of the gas discharged from the nozzle (gas flow velocity at the nozzle tip) is preferably 100 to 500 m / s. This is because if the gas flow velocity discharged from the nozzle is less than 100 m / s, the jet becomes turbulent before impacting the steel plate, making splash defects more likely. If the gas flow velocity discharged from the nozzle exceeds 500 m / s, the compressor required to inject the gas becomes larger, increasing equipment costs and making it uneconomical.

[0046] Furthermore, gas nozzle 1 1 The length of the parallel section (length G in Figure 15) is preferably 10 to 40 mm. The aforementionedIf the length of the parallel section is less than 10 mm, the formation of the potential core of the discharged jet becomes insufficient, leading to increased turbulence in the jet before it hits the steel plate, and making splash defects more likely to occur. The aforementioned If the length of the parallel section exceeds 40 mm, the resistance to the flow of gas passing through the slit gap increases, reducing the efficiency of gas injection and thus requiring excessive power.

[0047] Furthermore, if the nozzle tip height, defined as the distance between the nozzle tip (the tip of the gas injection port) and the surface of the molten metal (zinc) bath, is too low, vortices will be generated between the nozzle and the surface of the molten metal (zinc) bath, resulting in molten metal wrinkle defects. In other words, molten metal wrinkles occur because the flow of molten metal (backflow) that is scraped away by the gas injected from the nozzle and flows downwards along the surface of the steel plate becomes uneven. Conversely, if the nozzle tip height is too high, local solidification of the metal (zinc) will begin between the time the steel strip is pulled upwards from the molten metal bath and the time the wiping gas is blown onto it, resulting in molten metal wrinkle defects. In other words, molten metal wrinkles occur because the viscosity of zinc becomes uneven on the surface of the steel plate due to the localized solidification of zinc. For this reason, in order to suppress molten metal wrinkle defects, it is preferable to set the nozzle tip height H (the distance between the tip of the gas injection port and the surface of the molten metal bath, see Figure 4) to 50 mm or more and 700 mm or less. Here, a nozzle tip height H greater than 150 mm (H > 150 mm) is more preferable. Furthermore, a nozzle tip height H of less than 550 mm (H < 550 mm) is more preferable.

[0048] Here, "molten metal ridges" refer to the wave-like, flow-like patterns (wrinkles) that appear on the plated surface of molten metal-plated steel sheets. When plated steel sheets with such molten metal ridges are used as exterior panels, their plated surface impairs the surface properties of the paint film, particularly its smoothness, when the plated surface is used as a base surface for painting.

[0049] Next, in the manufacturing of the steel strip S, it is preferable to control the temperature of the wiping gas so that the temperature T (°C) of the gas (wiping gas) immediately after being sprayed from the nozzle slit of the gas wiping nozzle 10 satisfies the relationship TM-150 ≤ T ≤ TM+250 with respect to the melting point TM (°C) of the molten metal. By controlling the temperature T (°C) of the wiping gas within this range, the cooling and solidification of the molten metal can be suppressed, making viscosity unevenness less likely and suppressing the occurrence of molten metal wrinkle defects. On the other hand, if the temperature T (°C) of the wiping gas is below TM-150°C, it does not affect the fluidity of the molten metal and is therefore ineffective in suppressing the occurrence of molten metal wrinkle defects. Furthermore, if the temperature T (°C) of the wiping gas is higher than TM+250°C, alloying is promoted, and the appearance of the steel sheet deteriorates.

[0050] Furthermore, the method for raising the temperature of the wiping gas supplied to the gas wiping nozzle 10 is not particularly limited. For example, one method is to heat and raise the temperature of the wiping gas in a heat exchanger before supplying it, or to mix the combustion exhaust gas of an annealing furnace with air.

[0051] Furthermore, in this embodiment, it is preferable that a pair of baffle plates 20 and 21 be arranged on the outside of both ends in the width direction of the steel strip S, preferably on the extended surface of the steel strip near the width direction ends of the steel strip S. Figures 12 and 13 show a side view and a top view, respectively, of the baffle plates 20 and 21 arranged together with a pair of nozzles 10A and 10B. The baffle plates 20 and 21 are arranged between the pair of nozzles 10A and 10B. Therefore, the front and back surfaces of the baffle plates face the gas injection ports 11 of the pair of nozzles 10A and 10B. The baffle plates 20 and 21 contribute to reducing splash by acting to avoid direct collision between the gases injected from the pair of nozzles 10A and 10B. As a result, by arranging the baffle plates, the effect of further suppressing the occurrence of splash defects is enhanced compared to the above embodiment.

[0052] The shape of the baffle plates 20 and 21 is not particularly limited, but is preferably rectangular, and preferably two sides are arranged parallel to the direction of extension of the widthwise end of the steel strip S. The thickness of the baffle plates 20 and 21 is preferably 2 to 10 mm. If the thickness is 2 mm or more, the baffle plates will be less likely to deform due to the pressure of the wiping gas. If the thickness is 10 mm or less, the possibility of contact with the wiping nozzle or thermal deformation will be reduced. The length of the baffle plates 20 and 21 along the direction of travel of the steel strip S is preferably set such that the upper end is above the position where the gas injected from the pair of nozzles 10A and 10B directly impacts, and the lower end is below a position 50 mm above the bath surface. This is because the area in which the jets discharged from opposing nozzles collide outside the plate edge can be reduced, thereby suppressing edge overcoat defects. For this reason, the lower ends of the baffle plates 20 and 21 may be arranged to be immersed in the molten metal bath.

[0053] Figure 14 is an enlarged view of the vicinity of one widthwise end of the steel strip S in Figure 13. Referring to Figure 14, the distance E between the widthwise end of the steel strip and the baffle plate is preferably 10 mm or less, and more preferably 5 mm or less. This makes it possible to more reliably prevent direct collision of opposing jets. Furthermore, from the viewpoint of reducing the possibility of contact with the baffle plate when the steel strip meanders, it is preferable that the distance E be 3 mm or more.

[0054] The material of the baffle plate is not particularly limited. However, in this embodiment, since the baffle plate is close to the bath surface, there is a possibility that top dross and splash may adhere to it and alloy with the baffle plate, causing it to solidify. Furthermore, if the baffle plate is immersed in the bath, not only alloying but also thermal deformation must be considered. From this viewpoint, suitable materials for the baffle plate include iron plates coated with a boron nitride-based spray that repels zinc, and SUS316L, which does not react well with zinc. In addition, ceramics such as alumina, silicon nitride, and silicon carbide are desirable because they can suppress both alloying and thermal deformation.

[0055] Furthermore, a hot-dip galvanized steel strip manufactured by applying the gas wiping nozzle and hot-dip galvanized steel strip manufacturing method according to this embodiment is a hot-dip galvanized steel strip. This hot-dip galvanized steel strip includes both plated steel sheets (GI) that are not alloyed after hot-dip galvanizing (GI) and plated steel sheets (GA) that are alloyed. However, the hot-dip galvanized steel strip manufactured by applying the gas wiping nozzle and hot-dip galvanized steel strip manufacturing method according to this embodiment is not limited to these, and includes all hot-dip galvanized steel strips that contain other molten metals other than zinc, such as aluminum and tin.

[0056] In one embodiment, the method for manufacturing a molten metal-plated steel strip of the present invention includes the steps of: drawing a graph with the angle θ (°) between the gas (wiping gas) injection direction and the horizontal plane as the horizontal axis, and the quotient D / B of the distance D (mm) between the tip of the gas injection port 11 and the steel strip S and the width B (mm) of the gas injection port 11 as the vertical axis; determining the operating range in the graph drawn in the above step using the above-mentioned (Equations 1) to (Equations 5); and operating the above-mentioned pair of gas wiping nozzles 10A and 10B within the operating range determined in the above step. [Examples]

[0057] [Example 1] Using the continuous hot-dip galvanizing equipment 1 with the basic configuration shown in Figure 1, a steel strip S with a thickness of 1.0 mm and a width of 1200 mm was immersed in the molten zinc bath at a speed of 1.67 m / s (100 mpm) to produce a hot-dip galvanized steel strip under the conditions shown in Table 1. For gas wiping nozzles 10A and 10B, the width B of the gas injection port 11 was 1 mm. During the experiment, the temperature of the molten zinc bath was 460°C, and the gas temperature T at the tip of the gas wiping nozzle was 100°C or 450°C. Furthermore, under the conditions shown in Table 1, the amount of galvanized material adhering to the center of the strip width was 50 ± 5 g / m². 2 The gas pressure of the wiping nozzle (the pressure inside the nozzle header) was adjusted to fit within the gas requirements.

[0058] The splash defect rate is the ratio of the length of steel strip determined to have splash defects to the total length of steel strip that passed through the inspection process at the exit of the CGL (Continuous Hot-Dip Galvanizing Line), and a rate of 0.10% or less was considered acceptable. In addition, the surface of the molten zinc bath was visually observed to evaluate the occurrence of splashes on the bath surface.

[0059] The water flow wrinkle evaluation was performed according to the following criteria in the inspection process on the CGL exit side. △: Hot-dip galvanized steel sheet where molten metal wrinkles can be seen with the naked eye. ○: Hot-dip galvanized steel sheet in which no molten metal streaks can be seen with the naked eye.

[0060] Furthermore, cut sheets were taken from the coil at the CGL exit, and 48mm diameter samples for adhesion analysis were taken at the center of the sheet width and 50mm inward from the edge of the sheet width. The adhesion analysis of the obtained samples was performed, and the increase rate of adhesion at the edge of the sheet width relative to the center of the sheet width was defined as the edge overcoat rate (EOC rate), and the results were summarized. It is preferable that the water wrinkle evaluation is "○" and the EOC rate is 5.0% or less.

[0061] The experimental results are shown in Table 1. The conditions for Invention Examples 1 to 22 fall within the range enclosed by the following equations (Equation 1) to (Equation 5) in a graph plotted with the angle θ (°) between the gas injection direction and the horizontal plane on the horizontal axis and the quotient D / B (mm) between the distance D (mm) from the tip of the gas injection nozzle to the steel strip and the width B (mm) of the gas injection nozzle on the vertical axis. In other words, Invention Examples 1 to 22 are examples of operating the gas wiping nozzles 10A and 10B within the aforementioned range. D / B=3 (Formula 1) D / B=0.1×θ+9 (Formula 2) D / B=12 (Formula 3) θ=10 (Formula 4) θ=60 (Formula 5) Under the above conditions, the splash defect rate was 0.10% or less, resulting in a passing grade.

[0062] Furthermore, in Invention Examples 2, 3, 6, 13, and 14, which were operated within the range described below, no splash occurred on the bath surface, the EOC rate was 5.0% or less, and steel sheets were produced without consuming excessive zinc and with the adhesion of splash defects suppressed. D / B=3 (Formula 1) D / B=10 (Formula 6) θ=15 (Formula 7) θ=45 (Formula 8)

[0063] On the other hand, the conditions for Comparative Examples 1 to 16 fell outside the range enclosed by (Equation 1) to (Equation 5), resulting in a splash defect rate exceeding 0.10% and failure to pass inspection. Comparative Examples 14 to 16 are examples of steel strip manufacturing under the conditions described in Japanese Patent Publication No. 2018-9220. Under the conditions for Comparative Examples 14 to 16, the nozzle height is set to 350 mm, which suppresses molten metal streaks, but the operating conditions fall outside the above range, resulting in worsened splash defects and failure to pass inspection. Edge overcoating also deteriorated.

[0064] [Table 1]

[0065] [Example 2] As another embodiment of the present invention, similar to Example 1, an example will be described in which a hot-dip galvanized steel strip with a thickness of 1.0 mm and a width of 1200 mm was manufactured using a continuous hot-dip metal plating equipment 1 with the basic configuration shown in Figure 1. In this embodiment, the steel strip S was immersed in the molten zinc bath at a feed speed of 0.75 to 2.16 m / s (45 to 130 mpm) to manufacture the hot-dip galvanized steel strip under the conditions shown in Table 2. The width B of the gas injection port 11 of the gas wiping nozzles 10A and 10B was 1.0 to 1.4 mm. Gas nozzle 11The length G of the parallel section was set to 30 mm. Furthermore, in this embodiment, a pair of baffle plates were placed on the outside of both ends in the width direction of the steel strip S. The thickness of the baffle plates was 5 mm, and the distance E between the width direction end of the steel strip and the baffle plates was set to 5 mm, so that the lower end of the baffle plates was positioned 30 mm above the surface of the molten zinc bath. The temperature of the molten zinc bath was 460°C, and the gas temperature T at the tip of the gas wiping nozzle was 450°C. The gas pressure of the wiping nozzle (pressure inside the nozzle header) was adjusted so that the amount of coating in the center of the width of the steel strip S was the value shown in Table 2.

[0066] The evaluation methods for the splash defect rate, bath surface splash, water streaks, and edge overcoat rate are the same as in Example 1. The experimental results are shown in Table 2.

[0067] Examples 23 to 29 of the inventions involve operation within the range enclosed by the above-mentioned equations (1) to (5) in a graph plotted with the angle θ (°) between the gas injection direction and the horizontal plane on the horizontal axis and the quotient D / B (mm) of the distance D (mm) from the tip of the gas injection nozzle to the steel strip and the width B (mm) of the gas injection nozzle on the vertical axis. Furthermore, Examples 23 to 29 of the inventions involve operation under conditions that fall within the following preferred range. D / B=3 (Formula 1) D / B=10 (Formula 6) θ=15 (Formula 7) θ=45 (Formula 8)

[0068] Furthermore, in Invention Examples 23 to 29, the distance H between the tip of the gas injection nozzle and the surface of the molten zinc bath is in the range of 50 mm to 700 mm, and the operation is carried out under conditions where the temperature T (°C) of the gas immediately after injection from the gas wiping nozzle satisfies the relationship TM-150 ≤ T ≤ TM+250 with respect to the melting point TM (°C) of molten zinc.

[0069] Table 2 shows that in Invention Examples 23-29, the splash defect occurrence rate was 0.10% or less, which was acceptable. Furthermore, there was no splash on the galvanizing bath surface, and the EOC rate was 5.0% or less. From the above, it was confirmed that this embodiment can suppress the adhesion of splash to the steel strip and produce hot-dip galvanized steel strip with suppressed splash defects. In addition, it was confirmed that it is possible to produce hot-dip galvanized steel strip that prevents surface quality defects such as molten metal wrinkles and improves zinc yield by suppressing edge overcoating.

[0070] [Table 2] [Explanation of symbols]

[0071] S steel strip 1. Continuous molten metal plating equipment 2 Snout 3 Plating tank 4. Molten metal bath 5 Syncroll 6 Support Roles 10A, 10B Gas Wiping Nozzles 11 Gas nozzles 12 Nozzle Header 13A Upper nozzle component 13B Lower nozzle component 20, 21 Baffle Plate 131A Outer tapered portion of upper nozzle member 131B Lower nozzle member outer tapered section

Claims

1. In a method for manufacturing a molten metal-plated steel strip, in which a steel strip is continuously immersed in a molten metal bath, and as the steel strip is pulled out of the molten metal bath, gas is blown onto the steel strip from the gas nozzles of a pair of gas wiping nozzles, each nozzle having a slit-shaped gas nozzle extending wider than the steel strip along the width direction of the steel strip and positioned on either side of the steel strip, thereby adjusting the amount of molten metal adhering to both sides of the steel strip, the amount of molten metal adhering to both sides of the steel strip is adjusted to continuously produce a molten metal-plated steel strip. When a graph is drawn with the horizontal axis representing the angle θ (°) between the direction of gas injection from the gas nozzle and the horizontal plane, and the vertical axis representing the quotient D / B of the distance D (mm) between the tip of the gas nozzle and the steel strip and the width B (mm) of the gas nozzle, A method for manufacturing a hot-dip metal-plated steel strip, wherein the pair of gas wiping nozzles are operated within the operating range enclosed by the following formulas (1) to (5), The steps include drawing a graph with the angle θ (°) between the gas injection direction and the horizontal plane as the horizontal axis, and the quotient D / B of the distance D (mm) between the tip of the gas injection nozzle and the steel strip and the width B (mm) of the gas injection nozzle as the vertical axis, In the graph drawn in the above step, the operating range is determined by the following (Equation 1) to (Equation 5), A method for manufacturing a hot-dip metal-plated steel strip, comprising the step of operating the pair of gas wiping nozzles within the operating range defined in the above step. D / B=3...(Formula 1) D / B=0.1×θ+9 (Formula 2) D / B=12 (Formula 3) θ=10 (Formula 4) θ=60 (Formula 5)

2. The distance H between the tip of the gas nozzle of the pair of gas wiping nozzles and the surface of the molten metal bath is 50 mm or more and 700 mm or less. The method for manufacturing a molten metal-plated steel strip according to claim 1, wherein the temperature T (°C) of the gas immediately after being ejected from the pair of gas wiping nozzles satisfies the relationship TM-150 ≤ T ≤ TM+250 with respect to the melting point TM (°C) of the molten metal.

3. Each of the pair of gas wiping nozzles comprises a nozzle header and an upper nozzle member and a lower nozzle member connected to the nozzle header. The tip portion of the upper nozzle member and the tip portion of the lower nozzle member form the gas injection port, facing each other parallel to each other in a cross-sectional view perpendicular to the width direction of the steel strip. The method for manufacturing a molten metal-plated steel strip according to claim 1 or 2, wherein the gas passes through the inside of the nozzle header and is injected from the gas injection port.

4. The method for manufacturing a molten metal-plated steel strip according to claim 3, wherein the internal pressure of the nozzle header is set to 2 to 70 kPa.

5. A method for manufacturing a hot-dip metal-plated steel strip according to claim 1 or 2, wherein a baffle plate is arranged on the outside of both ends of the steel strip in the width direction and between the pair of gas wiping nozzles, so as to face the gas injection port.

6. The method for manufacturing a hot-dip metal-plated steel strip according to claim 3, wherein a baffle plate is arranged on the outside of both ends in the width direction of the steel strip and between the pair of gas wiping nozzles so as to face the gas injection port.

7. The method for manufacturing a hot-dip metal-plated steel strip according to claim 4, wherein a baffle plate is arranged on the outside of both ends in the width direction of the steel strip and between the pair of gas wiping nozzles so as to face the gas injection port.

8. The method for manufacturing a hot-dip metal-plated steel strip according to claim 1 or 2, wherein the length of the parallel portion of the gas injection port is 10 to 40 mm.

9. The method for manufacturing a hot-dip metal-plated steel strip according to claim 3, wherein the length of the parallel portion of the gas injection port is 10 to 40 mm.

10. The method for manufacturing a hot-dip metal-plated steel strip according to claim 4, wherein the length of the parallel portion of the gas injection port is 10 to 40 mm.

11. The method for manufacturing a hot-dip metal-plated steel strip according to claim 5, wherein the length of the parallel portion of the gas injection port is 10 to 40 mm.

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