Manufacturing method of hot-dip metal plated steel strip

Abstract

To provide a manufacturing method of a hot-dip metal plated steel strip for suppressing an occurrence of a splash defect.SOLUTION: In a manufacturing method of a hot-dip metal plated steel strip for continuously producing the hot-dip metal plated steel strip where a steel strip S is continuously dipped in a molten metal bath 4, a gas is blown to the steel strip S lifted from the molten metal bath 4 from a gas injection part of a pair of gas wiping nozzles 10A, 10B arranged sandwiching the steel strip S to adjust an adhesion amount of a molten metal adhered to both surfaces of the steel strip S, the pair of the gas wiping nozzles 10A, 10B is operated in a range of θ:10 - 60, D / B:3 - 12 and D / B:0.1 × θ+9 or less where θ(°) is an angle between a gas injection direction and a horizontal plane, D (mm) is a distance between a gas injection port tip and the steel strip S, and a width of the gas injection part is B(mm).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing hot-dip metal coated steel strip. [Background technology]

[0002] Hot-dip galvanized steel sheets, which are a type of hot-dip metal-coated steel sheets, are widely used in the fields of building materials, automobiles, home appliances, etc. In these applications, hot-dip galvanized steel sheets are required to have excellent appearance. Here, since the appearance after painting is strongly affected by surface defects such as uneven plating thickness, scratches, and foreign matter adhesion, it is important that hot-dip galvanized steel sheets are free of surface defects.

[0003] In a continuous hot-dip metal plating line, a steel strip, which is a metal strip annealed in a continuous annealing furnace in a reducing atmosphere, generally passes through a snout and is introduced into a molten metal bath in a plating tank. The steel strip is then pulled up above the molten metal bath via a sink roll and a support roll in the molten metal bath. After that, a wiping gas is sprayed 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 that adheres to the surface of the steel strip and is pulled up. This adjusts the amount of molten metal attached (hereinafter also referred to as the weight per unit area). Here, the gas wiping nozzle is usually configured to be wider than the width of the steel strip and extends to the outside of the widthwise end of the steel strip in order to accommodate various steel strip widths and to accommodate positional deviations in the width direction when the steel strip is pulled up. In this type of gas wiping method, the molten metal falls downward due to turbulence caused by the jet of water colliding with the steel strip, and splashes around. As the metal splashes, it solidifies and turns into fine metal powder, or splash, which adheres to the steel strip, causing defects (splash defects) and reducing the surface quality of the steel strip.

[0004] In addition, in a continuous process, the production volume can be increased by increasing the steel strip threading speed. However, when controlling the coating weight by gas wiping in a continuous hot dip coating process, the wiping gas pressure must be set higher in order to control the coating weight within a certain range. As a result, splash increases significantly, making it impossible to maintain good quality.

[0005] In order to solve the above problems, the following techniques have been disclosed.

[0006] Patent Document 1 describes a method for preventing molten metal splashes from adhering to the strip surface during hot-dip metal plating. In the method described in Patent Document 1, a metal plate is installed between the main wiping gas supply pipe and the wiping nozzle. Furthermore, a filter is installed along the steel sheet between the main wiping gas supply pipe and the alloying furnace. In the technology 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 toward the steel sheet after wiping is completed, thereby preventing splashes from adhering to the steel sheet.

[0007] Patent Document 2 discloses a method for preventing adhesion of splashes to a plated steel strip by providing a baffle plate protruding behind the wiping nozzle and a dam at the upper front part of the wiping nozzle.

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

[0009] [Patent Document 1] Japanese Patent Application Publication No. 5-306449 [Patent Document 2] JP 2000-328218 A [Patent Document 3] JP 2014-80673 A Summary of the Invention [Problem to be solved by the invention]

[0010] However, it was found that the method disclosed in Patent Document 1 was insufficient in preventing the occurrence of splash defects. In other words, if the mesh of the filter is made larger, the filter effect is lost. On the other hand, if the mesh of the filter is made smaller, the splash that has gone around the outside of the filter is prevented from adhering to the strip surface. However, splash that does not go around the back of the wiping nozzle and directly enters between the filter and the metal plate is difficult to be discharged outside the filter. Therefore, the effect of preventing the occurrence of splash defects is insufficient.

[0011] Moreover, the method disclosed in Patent Document 2 cannot prevent splashes flying upward around the back of the wiping nozzle from adhering to the plated steel strip. Also, splashes (metal powder) accumulated on a straightening plate protruding from the rear of the wiping nozzle during operation are re-scattered due to changes in the wiping gas flow caused by changes in the wiping conditions (wiping gas pressure, nozzle height, etc.). This phenomenon becomes more pronounced as time passes, and it was found that the method disclosed in Patent Document 2 cannot stably prevent splash adhesion.

[0012] The method disclosed in Patent Document 3 can suppress adhesion of splashes to the steel sheet. However, it has become clear that the gas discharged from the side nozzles repels the splashes, which then enter the wiping nozzle slits and cause blockage, resulting in streak-like defects on the steel sheet.

[0013] The present invention has been made in consideration 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 producing a hot-dip metal plated steel strip, comprising the steps of: continuously immersing a steel strip in a molten metal bath; blowing gas onto the steel strip pulled out of the molten metal bath from the gas jets of a pair of gas wiping nozzles disposed on either side of the steel strip, the gas jets having slit-shaped gas jets extending along the width direction of the steel strip and wider than the steel strip; and adjusting the amount of molten metal adhering to both sides of the steel strip, the method comprising the steps of: When a graph is drawn with the angle θ (°) between the direction of gas ejected from the gas ejection nozzle and the horizontal plane as the horizontal axis and the quotient D / B of the distance D (mm) between the tip of the gas ejection nozzle and the steel strip and the width B (mm) of the gas ejection nozzle as the vertical axis, The method for producing a hot-dip metal plated steel strip comprises operating the pair of gas wiping nozzles within a range surrounded 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 jet nozzle of the pair of gas wiping nozzles and the bath surface of the molten metal bath is 50 mm or more and 700 mm or less, The method for producing a hot-dip metal-plated steel strip according to [1], wherein a temperature T (°C) of the gas immediately after being sprayed from the pair of gas wiping nozzles satisfies TM-150≦T≦TM+250 in relation 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, a tip portion of the upper nozzle member and a tip portion of the lower nozzle member are parallel to each other and face each other in a cross-sectional view perpendicular to the width direction of the steel strip to form the gas ejection port; The method for producing a hot-dip metal-plated steel strip according to [1] or [2], wherein the gas passes through the inside of the nozzle header and is sprayed from the gas spray port. [4] The method for producing a hot-dip metal-plated steel strip according to [3], wherein the pressure inside the nozzle header is set to 2 to 70 kPa. [5] A method for producing a hot-dip metal-plated steel strip described in any of [1] to [4], in which a baffle plate is arranged on the outside of both widthwise ends of the steel strip and between the pair of gas wiping nozzles, facing the gas injection port. Effect of the Invention

[0015] According to the present invention, adhesion of splash to a steel strip can be suppressed, and a hot-dip metal plated steel strip in which the occurrence of splash defects is suppressed can be produced.

[0016] According to the present invention, the direction of splash is limited by operating the gas wiping nozzle within a predetermined range with respect to the traveling direction of the steel strip, which results in suppressing the occurrence of splash defects and enabling the stable production of hot-dip galvanized steel strip with excellent surface quality. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram showing a schematic configuration of a continuous hot-dip metal plating facility equipped with a gas wiping nozzle according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic diagram showing a schematic configuration of a gas wiping nozzle used in the continuous hot-dip metal plating facility shown in FIG. [Diagram 3] FIG. 3 is a schematic diagram showing the direction of splash. [Figure 4] FIG. 4 is a schematic diagram illustrating each component according to one embodiment of the present invention. [Diagram 5] FIG. 5 shows the results of investigating the relationship between the angle θ between the gas injection direction and the horizontal plane and the splash defect occurrence rate in one embodiment of the present invention. [Figure 6]FIG. 6 is a schematic diagram showing the splash direction when θ=30° and 65° in one embodiment of the present invention. [Figure 7] FIG. 7 is a schematic diagram showing the velocity distribution of a jet discharged from a gas wiping nozzle. [Figure 8] FIG. 8 shows the results of investigating the occurrence of splash defects when θ=10°, with slit gaps of 1 mm and 2 mm. [Figure 9] FIG. 9 shows the results of investigating the occurrence of splash defects when θ=15°, with slit gaps of 1 mm and 2 mm. [Figure 10] FIG. 10 shows the results of investigating the occurrence of splash defects when θ=30°, with slit gaps of 1 mm and 2 mm. [Figure 11] Figure 11 is a diagram showing the range of the quotient D / B between the angle θ (°) between the gas injection direction and the horizontal plane, 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] FIG. 12 is a schematic diagram (side view) showing one embodiment in which a baffle plate is arranged. [Figure 13] FIG. 13 is a schematic diagram (top view) showing one embodiment in which a baffle plate is arranged. [Figure 14] FIG. 14 is an enlarged view of the vicinity of one width direction end of the steel strip S in FIG. [Figure 15] FIG. 15 is an enlarged view of the tip portion of the gas wiping nozzle. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiments shown below are examples of devices and methods for embodying the technical ideas of the present invention, and the present invention is not limited to the following embodiments.

[0019] In addition, the drawings are schematic, and therefore, it should be noted that the relationship between thickness and planar dimensions, ratios, etc. may differ from the actual relationship between thickness and planar dimensions, and the drawings may include parts in which the relationship between dimensions and ratios differ from one another.

[0020] FIG. 1 shows a schematic configuration of a continuous hot-dip metal plating facility equipped with a gas wiping nozzle according to one embodiment of the present invention.

[0021] The continuous hot-dip metal plating equipment 1 shown in FIG. 1 is equipment for continuously depositing molten metal onto the surface of a steel strip S by immersing the steel strip S as a metal strip in a molten metal bath 4 made of molten metal, and then depositing the molten metal to a predetermined amount.

[0022] The continuous hot-dip metal coating equipment 1 includes a snout 2 , a coating tank 3 , a sink roll 5 , and a support roll 6 .

[0023] The snout 2 is a member that divides a space through which the steel strip S passes. The snout 2 is a member having a rectangular cross section perpendicular to the traveling direction of the steel strip S, and its upper end is connected to, for example, the outlet side of a continuous annealing furnace, and its lower end is immersed in a molten metal bath 4 stored in a coating 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 coating tank 3. Thereafter, the steel strip S is pulled up from the molten metal bath 4 via a sink roll 5 and a support roll 6 in the molten metal bath 4.

[0024] Then, gas (wiping gas) is sprayed onto both sides of the steel strip S that has been pulled up from the molten metal bath 4 from a pair of gas wiping nozzles 10A, 10B (gas nozzles 11, described later) arranged on both sides of the steel strip S, thereby adjusting the amount of molten metal adhering to both sides of the steel strip S. Thereafter, the steel strip S is cooled by cooling equipment (not shown) and is led to a subsequent process, where hot-dip metal-plated steel strips are continuously produced.

[0025] A pair of gas wiping nozzles 10A, 10B (hereinafter simply referred to as "nozzles") are arranged above the molten metal bath 4, facing each other with the steel strip S in between. As shown in FIG. 2, the nozzle 10A sprays gas toward the steel strip S from a gas ejection port 11 (nozzle slit) extending at its tip in the width direction of the steel strip, thereby adjusting the coating weight on the surface of the steel strip. The other nozzle 10B performs the same. This pair of nozzles 10A, 10B scrapes off excess molten metal, adjusting the coating weight on both sides of the steel strip S, and making the coating weight uniform in the width direction and longitudinal direction of the strip.

[0026] The nozzle 10A is usually longer than the width of the steel strip and extends beyond the widthwise end of the steel strip in order to accommodate various steel strip widths and to accommodate positional deviations in the width direction when the steel strip is pulled up. As shown in FIG. 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 ejection ports 11 (nozzle slits) in parallel with each other in a cross section perpendicular to the width direction of the steel strip S (parallel portion in FIG. 2). The gas ejection port 11 extends in the width direction of the steel strip S. Specifically, the gas ejection port 11 is in the form of a slit extending in the width direction of the steel strip S wider than the steel strip S. The longitudinal cross section of the nozzle 10A is tapered toward the tip. The thickness of the tip of the upper and lower nozzle members 13A and 13B (see thickness P in FIG. 15) may be about 1 to 3 mm. The width (opening width) B (slit gap) of the gas ejection port is not particularly limited, but may be about 0.5 to 3.0 mm. Gas supplied from a gas supply mechanism (not shown) passes through the inside of the nozzle header 12, and then passes through a gas flow path defined by the upper and lower nozzle members 13A and 13B, and is ejected from the gas ejection 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] Fig. 15 is an enlarged view of the tip of the nozzle 10A. As shown in Fig. 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] Here, when manufacturing a hot-dip metal-plated steel strip, the thickness of the metal deposit is controlled by blowing pressurized gas onto the surface of the steel strip, which is continuously pulled up from a hot-dip metal plating bath, from gas wiping nozzles arranged on both sides of the steel strip, with the steel strip sandwiched between them. During this process, the molten metal splashes and solidifies during the splash, forming metal powder (splash) that adheres to the steel strip and reduces the surface quality of the steel strip.

[0029] Here, splash defects refer to defects caused by splashes adhering to the steel plate. Specifically, the jets (gas jets) discharged from nozzles installed opposite each other collide near the edge of the steel plate as shown in Figure 3(a), causing the jets to vibrate, which tears off the liquid film of molten metal, and the torn-off liquid film solidifies as droplets while scattering, becoming solids (metal powder) that adhere to the steel plate, resulting in defects.

[0030] In examining a method for suppressing splash defects, the present inventors first investigated the scattering direction of splash (metal powder) using a high-speed camera. As a result, it was found that splash is scattered over a wide area above and below the nozzle as shown in FIG. 3(b) at a nozzle angle θ (angle between the gas injection direction and the horizontal plane) = 0°, which is a general operating condition of a CGL (continuous hot-dip galvanizing line). In order to suppress this splash defect, operators have experienced fine-tuning the nozzle downward (nozzle angle: 0 to 2°). However, since the fine adjustment of the nozzle angle depends on the skill of the operator, splash defects vary depending on the timing of operation and are not stable. Therefore, it was thought that if the nozzle was tilted significantly downward, the situation would change dramatically and splash defects would be improved.

[0031] In an actual CGL line, a 10 ton coil with a width of 1000 mm and a thickness of 1 mm was threaded at a speed of 100 mpm (meters per minute). As shown in Figure 4, under the conditions of a nozzle-to-steel plate distance of 10 mm, a nozzle angle θ = 0 to 80°, and a nozzle tip height of 500 mm, the amount of zinc deposited at the plate width center was 50 ± 5 g / m 2 The pressure indicated by the pressure gauge attached to the nozzle header was adjusted so that the splash defect rate was equal to or less than the nozzle angle. After that, the splash defect rate was checked using a defect meter installed on the CGL outlet side, and the correlation with the nozzle angle was investigated. The splash defect rate is the ratio of the length of the steel strip that was judged to have splash defects in the inspection process to the length of the steel strip that had passed. The slit gap (gas injection port width) was B = 1.0 mm. The experimental results are shown in Figure 5. Here, one coil was used per plot, and the pass standard for the splash defect rate was 0.10% or less. This is because a splash defect rate of 0.10% or less can be said to be of sufficient quality for steel strip with strict surface quality, such as automotive steel sheets.

[0032] In Figure 5, the incidence of splash defects varies widely around the nozzle angle θ = 0°. This suggests that it is difficult to control splash defects by fine-tuning the nozzle angle. As the nozzle was tilted downward, that is, as the nozzle angle became larger, the incidence of splash defects decreased. Then, when θ > 60°, the incidence of splash defects increased again.

[0033] The results of observing the splash conditions with a high-speed camera are shown in Fig. 6. It was found that at a nozzle angle of θ = 30°, where the incidence rate of splash defects was low, splash occurred only below the nozzle, whereas at a nozzle angle of θ = 65°, where the incidence rate of defects began to rise, splash occurred both above and below the nozzle.

[0034] The following is thought to be the cause of this. When the nozzle angle θ = 0°, gas discharged from opposing nozzles collides near the sheet edge as shown in Figure 3. Since the pressure of each nozzle is slightly different and there is also pressure fluctuation over time, the jets colliding at the sheet edge flow both above and below the nozzle. It is presumed that this causes splashes to fly upward and downward.

[0035] Even when the nozzle angle of the nozzle tilted downward is increased, the jet collides with the plate edge. However, since the gas flow rate toward the bath surface (downward) is greater than the gas flow rate toward the upward direction, it is considered that the splash is preferentially scattered downward. As a result, it is considered that the splash toward the upper part of the nozzle is suppressed. This is presumably the reason that the splash range is reduced and the splash defects are reduced. Similarly, it is considered that the occurrence rate of splash defects is close to 0 in the nozzle angle range of θ = 10 to 60 degrees, as a result of almost no splash scattering toward the upper part of the nozzle. By operating within this range, splash scattering toward the upper part of the nozzle is suppressed, and it is also possible to suppress operational troubles such as splash adhesion to the gas injection port and nozzle clogging.

[0036] When the nozzle angle θ is greater than 60°, the gap between the nozzle and the steel sheet becomes smaller as shown in Fig. 6, which makes it difficult for air to escape above the nozzle, and it is believed that a vortex occurs. That is, the gap between the tapered outer portion of the upper nozzle member 13A and the steel strip S becomes narrower, and the upward gas flow caused by collision near the sheet edge is obstructed, which makes it easier for a vortex to occur between the tapered outer portion and the steel strip S. In this case, the splash scattered from the sheet edge is scattered in various directions by the flow of the generated vortex. It is believed that the reason for the increase in splash defects is that the splash scattered above the nozzle due to the influence of the vortex adheres to the steel sheet.

[0037] The nozzle angle θ has a lower limit of 10° because the effect of reducing splash defects appears in the region of 10° or more. Here, the amount of zinc attached varies depending on the collision pressure gradient due to the collision of the gas with the steel strip S and the shear force generated in the zinc film due to the collision of the gas with the steel strip S, and the collision pressure gradient decreases as the downward nozzle angle of the nozzle increases. In this case, the collision pressure gradient refers to the gradient of the collision pressure in the direction corresponding to the direction of the slit gap B when the jet discharged from the nozzle collides with the target (steel strip). In order to obtain the same amount of zinc attached, a larger gas flow rate is required for the same nozzle-steel sheet distance (spacing), and a large-capacity compressor is required, which increases construction costs. In addition, as mentioned above, if a vortex occurs between the outer taper part of the upper nozzle member and the steel sheet, splash defects are induced and splash cannot be suppressed. Furthermore, the outer angle of the nozzle (outer angle α in Figure 15) is about 40 to 50° considering the rigidity of the nozzle. If the nozzle is tilted at 70° or more, the angle becomes 70° + 20° (half the outer angle) = 90°, and the nozzle comes into contact with the steel plate. Taking into account the nozzle-to-steel plate distance, a realistic upper limit for the nozzle angle θ is around 60°. Also, the effect of reducing splash defects appears when the nozzle angle θ is in the range of 60° or less. For these reasons, the upper limit for the nozzle angle θ is set to 60°.

[0038] The preferred range of the nozzle angle θ is 15°≦θ≦45°. Although the effect of reducing splash defects is manifested when θ≧10°, by setting the nozzle angle θ to 15° or more, it is possible to further suppress the reduction in the collision pressure at the end of the steel sheet. That is, when the nozzle angle θ is small, the jets discharged from the opposing nozzles collide with each other on the outside of the sheet edge, causing the jets to vibrate and reducing the pressure acting on the end of the steel sheet. In contrast, by setting the nozzle angle θ to 15° or more, it is possible to suppress the reduction in the pressure acting on the end of the steel sheet. When the collision pressure at the end of the steel sheet decreases, the effect of scraping off the excess molten metal is weakened. By setting the nozzle angle θ to 15° or more, it is possible to suppress edge overcoat defects in which the amount of adhesion at the end of the steel sheet becomes excessive. Therefore, the lower limit of the preferred range of the nozzle angle θ is 15°. In the region of θ>45°, the amount of gas heading toward the bath surface increases, and there is a risk of zinc splashing from the bath surface. Therefore, the upper limit of the preferred range of the nozzle angle θ is 45°. The phenomenon in which zinc splashes from the bath surface is called bath splash. When bath splash occurs, it can cause problems such as defects in the steel sheet and aggravate the environment around the equipment.

[0039] Here, the characteristics of the impinging jet are arranged by D / B, which is the distance (spacing) D from the nozzle tip (tip of the gas ejection port) to the impinging plate (steel strip) divided by the slit gap B, as shown in FIG. 7. In the region where D / B is small, the average velocity on the jet axis is equal to the jet exit velocity, and this region is called the potential core. As D / B increases, the turbulence at the outer edge of the jet reaches the jet axis, the velocity on the jet axis attenuates, 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 impinging pressure of the gas ejected from the opposing nozzle near the plate edge. Then, the incidence rate of splash defects with respect to the nozzle angle was investigated by changing D / B, for nozzle angles θ = 10°, 15°, and 30°, respectively. The results are shown in FIG. 8 to FIG. 10.

[0040] 8 to 10 show that when the nozzle angle is constant, the splash defect occurrence rate can be arranged in terms of D / B, regardless of the slit gap B. In addition, the splash defect occurrence rate differs depending on the nozzle angle. This shows that in order to suppress splash defects, it is important to control D / B, which is the nozzle-to-steel plate distance divided by the slit gap, and the nozzle angle.

[0041] If the nozzle-to-steel plate distance is small, the nozzle may collide with the plate due to plate warpage, 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 (deteriorating jet stability), which also increases splash defects. Therefore, the upper limit of D / B is 10 when the nozzle angle θ=10° (Fig. 8). Increasing the nozzle angle θ suppresses the splash that scatters upward near the plate edge. Therefore, the D / B at which operation can be performed while suppressing splash defects expands, and the upper limit is 12 when the nozzle angle θ=30° (Fig. 10). In the range of 10°≦θ≦30°, operation while suppressing splash defects is possible within the straight line connecting the upper limit of D / B for nozzle angle θ=10° and nozzle angle θ=30°. If D / B exceeds 12, the effect of reducing splash defects is no longer seen because the deterioration of jet stability is greater even if the nozzle angle θ is increased. Therefore, the upper limit of D / B is 12 within the range of 30°≦θ≦60°.

[0042] As described above, the nozzle angle θ and D / B for realizing operation with suppressed splash defects are summarized in (Equation 1) to (Equation 5). Figure 11 summarizes the above ranges for 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 of D / B is D / B≦10. By setting D / B≦10, it is possible to suppress the decrease in collision pressure at the steel plate end caused by the collision of the jets discharged from the opposing nozzles on the outside of the plate edge, and edge overcoat defects can be suppressed. 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 increases when the jets discharged from the opposing nozzles collide on the outside of the plate edge. In order to suppress the decrease in collision pressure at the plate width end caused by this, it is preferable to set it in the above range.

[0044] In the appropriate range of the nozzle angle θ and D / B obtained by dividing the nozzle-steel sheet distance by the slit gap for preventing splash defects, the pressure (gas pressure) inside the nozzle header 12 is preferably 2 to 70 kPa. The pressure is more preferably 3 kPa or more. Also, the pressure is more preferably 60 kPa or less. This is because, if the pressure inside the nozzle header 12 is less than 2 kPa, the turbulence of the jet becomes large before it collides with the steel sheet, making splash defects more likely to occur. If the pressure inside the nozzle header 12 exceeds 70 kPa, the compressor for injecting the gas becomes large, which increases the equipment costs and is not economical.

[0045] In addition, in the above-mentioned appropriate ranges of the nozzle angle θ and D / B, the jet velocity of the gas discharged from the nozzle (gas flow velocity at the nozzle tip) is preferably 100 to 500 m / s. If the flow velocity of the gas discharged from the nozzle is less than 100 m / s, the jet becomes significantly turbulent before it collides with the steel sheet, making splash defects more likely to occur. If the flow velocity of the gas discharged from the nozzle exceeds 500 m / s, the compressor for injecting the gas becomes large, which increases the equipment costs and is not economical.

[0046] Furthermore, the length of the parallel portion of the gas ejection port 11 (length G in FIG. 15) is preferably 10 to 40 mm. If the length of the parallel portion is less than 10 mm, the formation of a potential core of the ejected jet becomes insufficient, and the jet becomes more turbulent before it collides with the steel sheet, making splash defects more likely to occur. If the length of the parallel portion exceeds 40 mm, resistance to the flow of gas passing through the slit gap increases, reducing the efficiency of gas ejection and requiring excessive power.

[0047] In addition, if the nozzle tip height, defined as the distance between the nozzle tip (tip of the gas injection port) and the bath surface of the molten metal (zinc) bath, is too low, a vortex occurs between the nozzle and the bath surface of the molten metal (zinc) bath, which causes a melt wrinkle defect. In other words, the flow (backflow) of the molten metal that is scraped off by the gas injected from the nozzle and flows downward on the surface of the steel sheet becomes uneven, causing the melt wrinkle. On the other hand, if the nozzle tip height is too high, the metal (zinc) starts to solidify locally after the steel strip is pulled up from the molten metal bath until the wiping gas is sprayed, causing the melt wrinkle defect. In other words, the viscosity of zinc becomes uneven on the surface of the steel sheet due to the local solidification of zinc, causing the melt wrinkle. For this reason, in order to suppress the melt wrinkle defect, it is preferable to set the nozzle tip height H (the distance between the tip of the gas injection port and the bath surface of the molten metal bath, see FIG. 4) to 50 mm or more and 700 mm or less. Here, it is more preferable that the nozzle tip height H is more than 150 mm (H>150 mm). Moreover, it is more preferable that the nozzle tip height H is less than 550 mm (H<550 mm).

[0048] Here, the creases are wavy patterns (wrinkles) that appear on the plating surface of hot-dip metal-plated steel sheets. When a plated steel sheet with such creases is used as an exterior panel and its plating surface is used as a base surface for painting, it impairs the surface properties, particularly the smoothness, of the coating film.

[0049] Next, in the production 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 TM-150≦T≦TM+250 in relation to the melting point TM (°C) of the molten metal. If the temperature T (°C) of the wiping gas is controlled within this range, the cooling and solidification of the molten metal can be suppressed, so that viscosity unevenness is less likely to occur and the occurrence of molten crease defects can be suppressed. On the other hand, if the temperature T (°C) of the wiping gas is less than TM-150°C, it does not affect the fluidity of the molten metal, and is therefore ineffective in suppressing the occurrence of molten crease defects. Also, 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 is deteriorated.

[0050] There is no particular limitation on the method for increasing the temperature of the wiping gas supplied to the gas wiping nozzle 10. For example, there is a method of heating the wiping gas in a heat exchanger and then supplying the gas, or a method of mixing the combustion exhaust gas from an annealing furnace with air.

[0051] In this embodiment, a pair of baffle plates 20, 21 are preferably arranged on the outside of both ends of the width direction of the steel strip S, preferably on the extension surface of the steel strip S near the ends of the width direction. Figures 12 and 13 respectively show a side view and a top view of the baffle plates 20, 21 arranged together with a pair of nozzles 10A, 10B. The baffle plates 20, 21 are arranged between the pair of nozzles 10A, 10B. Thus, the front and back surfaces of the baffle plates face the gas ejection ports 11 of the pair of nozzles 10A, 10B. The baffle plates 20, 21 act to avoid direct collision between the gases ejected from the pair of nozzles 10A, 10B, thereby contributing to the reduction of splash. 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 it is preferable that they are rectangular, and two sides of them are preferably arranged parallel to the extending direction of the width direction end of the steel strip S. The plate thickness of the baffle plates 20 and 21 is preferably 2 to 10 mm. If the plate thickness is 2 mm or more, the baffle plate is less likely to be deformed by the pressure of the wiping gas. If the plate thickness is 10 mm or less, the possibility of contact with the wiping nozzle or thermal deformation is reduced. The length of the baffle plates 20 and 21 along the traveling direction of the steel strip S is preferably set so that the upper end is above the position where the gas sprayed from the pair of nozzles 10A and 10B directly collides, and the lower end is located below the position 50 mm above the bath surface. This is because the range where the jets discharged from the opposing nozzles collide on the outside of the strip edge portion can be reduced, thereby suppressing edge overcoat defects. For this reason, the lower end of the baffle plates 20 and 21 may be arranged so as to be immersed in the molten metal bath.

[0053] Fig. 14 is an enlarged view of the vicinity of one widthwise end of the steel strip S in Fig. 13. Referring to Fig. 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 the opposing jets. Furthermore, from the viewpoint of reducing the possibility of the steel strip coming into contact with the baffle plate when meandering, the distance E is preferably 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, it is possible that top dross and splashes may adhere to the baffle plate and be alloyed with the baffle plate and adhere to it. In addition, when the baffle plate is immersed in the bath, it is necessary to consider not only the alloying but also thermal deformation. From this viewpoint, examples of the material of the baffle plate include an iron plate coated with a boron nitride-based spray that easily repels zinc, and SUS316L that does not easily react with zinc. Furthermore, ceramics such as alumina, silicon nitride, and silicon carbide are preferable because they can suppress both alloying and thermal deformation.

[0055] Moreover, the hot-dip metal-plated steel strip manufactured by applying the gas wiping nozzle and the manufacturing method of the hot-dip metal-plated steel strip according to the present embodiment includes a hot-dip galvanized steel strip. This hot-dip galvanized steel strip includes both a plated steel sheet (GI) that is not subjected to alloying treatment after hot-dip galvanizing treatment and a plated steel sheet (GA) that is subjected to alloying treatment. However, the hot-dip metal-plated steel strip manufactured by applying the gas wiping nozzle and the manufacturing method of the hot-dip metal-plated steel strip according to the present embodiment is not limited to this, and includes hot-dip metal-plated steel strips in general that contain other molten metals such as aluminum and tin other than zinc.

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

[0057] [Example 1] Using continuous hot-dip metal plating 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 dipped into a molten zinc bath at a strip passing speed of 1.67 m / s (100 mpm) to produce a hot-dip galvanized steel strip under the conditions shown in Table 1. Regarding the gas wiping nozzles 10A and 10B, the width B of the gas injection port 11 was 1 mm. The temperature of the molten zinc bath during the experiment was 460°C, and the gas temperature T at the tip of the gas wiping nozzle was 100°C or 450°C. Under the conditions shown in Table 1, the coating weight at the strip width center was 50±5 g / m 2 The gas pressure of the wiping nozzle (the pressure inside the nozzle header) was adjusted so that it fell within the range of 100 mm.

[0058] The splash defect occurrence rate is the ratio of the length of steel strip that was judged to have a splash defect to the length of steel strip that had passed the inspection process at the exit of the CGL (continuous hot-dip galvanizing line), and a rate of 0.10% or less was considered to have passed. In addition, the surface of the molten zinc bath was visually observed to evaluate the occurrence of splashes on the bath surface.

[0059] The evaluation of creases in the hot water was carried out at the inspection process on the CGL delivery side according to the following criteria. △: Hot-dip galvanized steel sheet with visible creases ○: Hot-dip galvanized steel sheet with no visible creases

[0060] In addition, cut strips were taken from the coil at the CGL exit, and samples with a diameter of 48 mm were taken from the strip width center and 50 mm inward from the strip width edge for analysis of the amount of adhesion. The obtained samples were analyzed for the amount of adhesion, and the increase rate of the amount of adhesion at the strip width edge relative to the strip width center was calculated as the edge overcoat rate (EOC rate). It is preferable that the water wrinkle evaluation is "good" and the EOC rate is 5.0% or less.

[0061] The experimental results are shown in Table 1. The conditions of Examples 1 to 22 are within the range enclosed by the following (Equation 1) to (Equation 5) in a graph plotted 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) from the tip of the gas injection nozzle to the steel strip and the width B (mm) of the gas injection nozzle as the vertical axis. In other words, Examples 1 to 22 are examples in which gas wiping nozzles 10A and 10B were operated within the above 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 below 0.10%, which was acceptable.

[0062] In addition, in Examples 2, 3, 6, 13, and 14 of the invention, which were operated within the region of the preferred range described below, there was no occurrence of splash on the bath surface, and the EOC rate was 5.0% or less, so that steel sheets were produced without consuming excessive zinc and with suppressed adhesion of splash defects. 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 of Comparative Examples 1 to 16 are outside the range enclosed by (Formula 1) to (Formula 5), ​​and the splash defect occurrence rate exceeded 0.10%, resulting in failure. Comparative Examples 14 to 16 are examples of steel strip production under the conditions described in JP 2018-9220 A. In the conditions of Comparative Examples 14 to 16, the nozzle height is set to 350 mm, which suppresses melt wrinkles, but the operating conditions are outside the above range, which causes splash defects to worsen and results in failure. The edge overcoat also deteriorated.

[0064] [Table 1]

[0065] [Example 2] As another embodiment of the present invention, an example in which a hot-dip galvanized steel strip having a thickness of 1.0 mm and a width of 1200 mm was produced using the continuous hot-dip metal plating equipment 1 having the basic configuration shown in FIG. 1 in the same manner as in embodiment 1 will be described. In this embodiment, the steel strip S was immersed in the molten zinc bath at a sheet passing speed of 0.75 to 2.16 m / s (45 to 130 mpm) to produce a 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, and the length G of the parallel part of the gas injection port 11 was 30 mm. Furthermore, in this embodiment, a pair of baffle plates were arranged on the outside of both ends in the width direction of the steel strip S. The plate thickness of the baffle plate was 5 mm, and the distance E between the width direction end of the steel strip and the baffle plate was 5 mm, and the baffle plate was arranged so that the lower end of the baffle plate was located 30 mm above the bath 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 (the pressure inside the nozzle header) was adjusted so that the adhesion amount at the center of the steel strip S at the plate width was the value shown in Table 2.

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

[0067] Examples 23 to 29 were operated within the range surrounded by the above-mentioned (Equation 1) to (Equation 5) in a graph plotted 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) from the tip of the gas injection nozzle to the steel strip and the width B (mm) of the gas injection nozzle as the vertical axis. Furthermore, Examples 23 to 29 were operated under conditions falling 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 Examples 23 to 29, the distance H between the tip of the gas nozzle and the surface of the molten zinc bath was in the range of 50 mm or more and 700 mm or less, and the temperature T (°C) of the gas immediately after being sprayed from the gas wiping nozzle was operated under conditions that satisfied TM-150≦T≦TM+250 in relation to the melting point TM (°C) of molten zinc.

[0069] From the results in Table 2, in the invention examples 23 to 29, the splash defect occurrence rate was 0.10% or less, which was acceptable. In addition, there was no bath surface splash, and the EOC rate was 5.0% or less. From the above, it was confirmed that this example can suppress adhesion of splash to the steel strip, and can produce a hot-dip galvanized steel strip in which the occurrence of splash defects is suppressed. In addition, it was confirmed that it is possible to produce a hot-dip galvanized steel strip that prevents surface quality defects of the hot-dip galvanized steel strip, such as creases, and improves zinc retention by suppressing edge overcoating.

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

[0071] S steel strip 1. Continuous hot-dip metal plating equipment 2. Snout 3 Plating tank 4 Molten metal bath 5. Sync Roll 6 Support Roll 10A, 10B Gas wiping nozzle 11 Gas nozzle 12 Nozzle Header 13A Upper nozzle member 13B Lower nozzle member 20, 21 Baffle plate 131A Outer tapered portion of upper nozzle member 131B Outer tapered portion of lower nozzle member

Claims

1. A method for producing a hot-dip metal plated steel strip, comprising the steps of: continuously immersing a steel strip in a molten metal bath; blowing gas onto the steel strip pulled out of the molten metal bath from the gas jets of a pair of gas wiping nozzles disposed on either side of the steel strip, the gas jets having slit-shaped gas jets extending in a width direction of the steel strip and wider than the width of the steel strip; and adjusting the amount of molten metal adhering to both sides of the steel strip, the method comprising the steps of: When a graph is drawn with the angle θ (°) between the injection direction of the gas injected from the gas injection port 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 and the steel strip and the width B (mm) of the gas injection port as the vertical axis, A method for producing a hot-dip metal-plated steel strip, comprising operating the pair of gas wiping nozzles within a range surrounded by the following (Equation 1) to (Equation 5): plotting a graph with the angle θ (°) between the gas injection direction and a horizontal plane as the horizontal axis and the quotient D / B of the distance D (mm) between the tip of the gas injection port and the steel strip S and the width B (mm) of the gas injection port as the vertical axis; determining an operation range in the graph drawn in the above step by the following (Equation 1) to (Equation 5); A method for producing a hot-dip metal plated steel strip, comprising a step of operating the pair of gas wiping nozzles within an operating range determined in the steps and within a range surrounded by the following (Equation 1), (Equation 6), (Equation 7), and (Equation 8). 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) D / B=10...(Formula 6) θ=15 (Formula 7) θ=45 (Formula 8)

2. a distance H between the tip of the gas jet port of the pair of gas wiping nozzles and the bath surface of the molten metal bath is 50 mm or more and 700 mm or less; 2. The method for producing a hot-dip metal plated steel strip according to claim 1, wherein a temperature T (°C) of the gas immediately after being sprayed from the pair of gas wiping nozzles satisfies TM-150≦T≦TM+250 in relation to a 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, a tip portion of the upper nozzle member and a tip portion of the lower nozzle member are parallel to each other and face each other in a cross-sectional view perpendicular to the width direction of the steel strip to form the gas ejection port; The method for producing a hot-dip metal plated steel strip according to claim 1 or 2, wherein the gas passes through an inside of the nozzle header and is injected from the gas injection port.

4. The method for producing a hot-dip metal plated steel strip according to claim 3, wherein the pressure inside the nozzle header is set to 2 to 70 kPa.

5. 3. A method for producing a hot-dip metal plated steel strip according to claim 1, further comprising the step of: disposing a baffle plate on the outer side of both widthwise ends of the steel strip and between the pair of gas wiping nozzles so as to face the gas ejection port.

6. 4. The method for producing a hot-dip metal plated steel strip according to claim 3, further comprising disposing a baffle plate on the outer side of both widthwise ends of the steel strip and between the pair of gas wiping nozzles so as to face the gas ejection port.

7. 5. The method for producing a hot-dip metal plated steel strip according to claim 4, further comprising the step of: disposing a baffle plate on the outer side of both widthwise ends of the steel strip and between the pair of gas wiping nozzles so as to face the gas ejection port.

8. The method for producing 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 producing 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 producing 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 producing 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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