Method for manufacturing molten metal coated steel strip

Abstract

Provided is a method for manufacturing a molten metal plated steel strip that suppresses the generation of splash defects. A method for manufacturing a molten metal plated steel strip in which a steel strip S is continuously immersed in a molten metal bath 4, and gas is blown from gas injection ports of a pair of gas wiping nozzles 10A, 10B disposed across the steel strip S with respect to the steel strip S lifted from the molten metal bath 4, the amount of molten metal adhering to both surfaces of the steel strip S is adjusted, and a molten metal plated steel strip is continuously manufactured, wherein when the angle formed by the gas injection direction and the horizontal plane is θ (°), the interval from the tip of the gas injection port to the steel strip S is D (mm), and the width of the gas injection port is B (mm), the pair of gas wiping nozzles 10A, 10B are operated within the range of θ: 10 to 60, D / B: 3 to 12, and D / B: 0.1 x θ + 9 or less.

Description

Technical Field

[0001] This invention relates to a method for manufacturing steel strip coated with molten metal. Background Technology

[0002] Hot-dip galvanized steel sheets, a type of molten metal coated steel sheet, are widely used in building materials, automobiles, and home appliances. Furthermore, these applications require hot-dip galvanized steel sheets to have an excellent appearance. Since the appearance after coating is strongly affected by surface defects such as uneven coating thickness, scratches, and foreign matter adhesion, it is crucial that hot-dip galvanized steel sheets are free of surface defects.

[0003] In a continuous molten metal plating production line, steel strip annealed in a reducing atmosphere in a continuous annealing furnace typically passes through the furnace nose and is introduced into a molten metal bath within the plating tank. The steel strip is then lifted above the molten metal bath by submerged rollers and support rollers. Next, wiping gas is sprayed onto the surface of the steel strip from gas wiping nozzles positioned on both sides, scraping away excess molten metal adhering to the surface and being lifted. This adjusts the amount of molten metal adhering (hereinafter also referred to as weight per unit area). To accommodate various steel strip widths and to address positional shifts in the width direction during strip lifting, the gas wiping nozzles are typically constructed wider than the steel strip width and extend to the outer side of the strip's width-direction end. In this gas wiping method, the molten metal falling downwards due to the turbulence of the jet stream colliding with the steel strip scatters in all directions, solidifying in the scattering process into fine metal powder, known as spatter, which adheres to the steel strip, creating defects (spatter defects) and resulting in a decrease in the surface quality of the steel strip.

[0004] Furthermore, in continuous processes, the speed of the steel strip can be increased to increase production output. However, in continuous hot-dip galvanizing processes where the coating adhesion is controlled by gas wiping, the wiping gas pressure must be set to a higher level to keep the coating adhesion within a certain range. This results in a significant increase in spatter, making it impossible to maintain good quality.

[0005] To address the aforementioned issues, the following technologies have been disclosed.

[0006] Patent Document 1 discloses a method for preventing molten metal droplets from adhering to the surface of a steel strip during molten metal plating. In the method described in Patent Document 1, a metal plate is disposed between the wiping gas supply main pipe and the wiping nozzle. Furthermore, a filter is disposed between the wiping gas supply main pipe and the alloying furnace, running along the steel plate. In the technology described in Patent Document 1, plating metal droplets generated on the plating bath surface are removed by the filter as they travel around the outside of the wiping nozzle toward the steel plate after wiping, thereby preventing the splashes from adhering to the steel plate.

[0007] Patent document 2 discloses a method for preventing splashes from adhering to the coated steel strip by setting a rectifier plate extending behind the wiping nozzle and a weir at the upper front of the wiping nozzle.

[0008] Patent document 3 proposes a method to suppress splatter defects by providing a side nozzle above the wiping nozzle and blowing gas from the side nozzle into the gas-gas collision zone of the wiping gas through gas turbulence.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: Japanese Patent Application Publication No. 5-306449

[0012] Patent Document 2: Japanese Patent Application Publication No. 2000-328218

[0013] Patent Document 3: Japanese Patent Application Publication No. 2014-80673 Summary of the Invention

[0014] The problem that the invention aims to solve

[0015] However, it is known that the method disclosed in Patent Document 1 is insufficient in preventing the formation of spatter defects. That is, if the mesh size of the filter is increased, the filtering effect disappears. On the other hand, if the mesh size of the filter is decreased, it is possible to suppress spatter that travels around the outside of the filter from adhering to the strip surface. However, spatter that enters directly between the filter and the metal plate without traveling around the back of the wiping nozzle is difficult to discharge outside the filter. Therefore, the effect of preventing the formation of spatter defects is insufficient.

[0016] Furthermore, the method disclosed in Patent Document 2 cannot prevent splatter particles flying upwards around the back of the wiping nozzle from adhering to the plated steel strip. Additionally, splatter particles (metal powder) accumulating on the baffle plate extending behind the wiping nozzle during operation are re-dispersed due to changes in wiping gas flow caused by variations in wiping conditions (wiping gas pressure, nozzle height, etc.). This phenomenon becomes more pronounced over time, and the method disclosed in Patent Document 2 cannot reliably prevent splatter adhesion.

[0017] The method disclosed in Patent Document 3 can suppress the adhesion of spatter to the steel plate. However, the gas ejected from the side nozzle deflects the spatter, which then penetrates and blocks the inside of the wiping nozzle slit, thereby creating strip-shaped defects on the steel plate.

[0018] The present invention was made in view of the above circumstances, and its object is to provide a method for manufacturing molten metal coated steel strip that suppresses the generation of spatter defects by suppressing the adhesion of spatter to the steel strip.

[0019] Methods for solving problems

[0020] The method of the present invention for solving the above problems is described below.

[0021] [1] A method for manufacturing molten metal coated steel strip, wherein a steel strip is continuously immersed in a molten metal bath, and for the steel strip lifted from the molten metal bath, gas is sprayed from the gas injection port having a slit-shaped gas injection port extending wider than the steel strip along its width direction and a pair of gas wiping nozzles disposed across the steel strip, thereby adjusting the amount of molten metal adhering to both sides of the steel strip, and molten metal coated steel strip is continuously manufactured, wherein...

[0022] When plotting a graph with the angle θ (°) between the direction of the gas ejected from the gas nozzle and the horizontal plane as the horizontal axis, and the quotient D / B of the distance D (mm) between the top of the gas nozzle and the steel strip and the width B (mm) of the gas nozzle as the vertical axis,

[0023] Operate the pair of gas wiping nozzles within the range defined by the following formulas (1) to (5).

[0024] D / B = 3…(Equation 1)

[0025] D / B = 0.1 × θ + 9… (Equation 2)

[0026] D / B = 12…(Equation 3)

[0027] θ = 10…(Equation 4)

[0028] θ = 60…(Equation 5)

[0029] [2] According to the manufacturing method of molten metal coated steel strip described in [1], wherein,

[0030] The distance H between the tip of the gas injection port of the aforementioned pair of gas wiping nozzles and the surface of the aforementioned molten metal bath is 50 mm or more and 700 mm or less.

[0031] The temperature T (°C) of the gas immediately after being ejected from the pair of gas wiping nozzles and the melting point TM (°C) of the molten metal satisfy TM-150≤T≤TM+250.

[0032] [3] The manufacturing method of molten metal coated steel strip according to [1] or [2], wherein,

[0033] Each of the aforementioned pair of gas wiping nozzles has a nozzle manifold and an upper nozzle component and a lower nozzle component connected to the nozzle manifold.

[0034] In a cross-sectional view perpendicular to the width direction of the steel strip, the top ends of the upper nozzle member and the lower nozzle member are parallel to each other to form the gas injection port.

[0035] The gas passes through the inside of the nozzle manifold and is ejected from the gas injection port.

[0036] [4] In the manufacturing method of molten metal coated steel strip described in [3], the internal pressure of the nozzle manifold is set to 2 to 70 kPa.

[0037] [5] The method for manufacturing molten metal coated steel strip according to any one of [1] to [4], wherein a baffle is arranged on the outer side of both ends of the steel strip in the width direction and between the pair of gas wiping nozzles in a manner opposite to the gas injection port.

[0038] Invention Effects

[0039] According to the present invention, it is possible to suppress the adhesion of spatter to the steel strip and to manufacture molten metal coated steel strip that suppresses the generation of spatter defects.

[0040] According to the present invention, by operating the gas wiping nozzle within a specified range relative to the direction of travel of the steel strip, the scattering direction of spatter can be limited. As a result, the generation of spatter defects is suppressed, and molten metal coated steel strips with excellent surface quality can be consistently manufactured. Attached Figure Description

[0041] Figure 1 This is a schematic diagram showing the general configuration of a continuous molten metal plating apparatus equipped with a gas wiping nozzle according to one embodiment of the present invention.

[0042] Figure 2 It is shown Figure 1 A schematic diagram of the gas wiping nozzle used in the continuous molten metal plating equipment shown.

[0043] Figure 3 This is a schematic diagram showing the direction of the splash.

[0044] Figure 4 This is a schematic diagram illustrating the various components of one embodiment of the present invention.

[0045] Figure 5 This is a result of examining the angle θ between the gas jet direction and the horizontal plane and the spatter defect generation rate in one embodiment of the present invention.

[0046] Figure 6 This is a schematic diagram showing the direction of splash dispersion when θ = 30° and 65° according to one embodiment of the present invention.

[0047] Figure 7 This is a schematic diagram showing the velocity distribution of the jet stream ejected from the gas wiping nozzle.

[0048] Figure 8 The results are based on the observation of the defects in spatter when the slit gap is 1mm and 2mm and the angle is θ=10°.

[0049] Figure 9 The results are based on the observation of the defects in spatter when the slit gap is 1mm and 2mm and the angle is θ=15°.

[0050] Figure 10 The results are based on the observation of the defects in spatter when the slit gap is 1mm and 2mm and the angle is θ=30°.

[0051] Figure 11 This is a diagram showing the range of the quotient D / B of the angle θ (°) between the gas injection direction and the horizontal plane and the distance D (mm) between the steel strip and the top of the gas injection port.

[0052] Figure 12 This is a schematic diagram (side view) showing one embodiment with a baffle configured.

[0053] Figure 13 This is a schematic diagram (top view) showing one embodiment with a baffle configured.

[0054] Figure 14 It is Figure 13 A magnified view of the vicinity of one width-direction end of the steel strip S.

[0055] Figure 15 This is a magnified view of the area near the tip of the gas wiping nozzle. Detailed Implementation

[0056] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments shown below illustrate apparatus and methods for embodying the technical concept of the present invention, and the present invention is not limited to the following embodiments.

[0057] Furthermore, the accompanying drawings are schematic. Therefore, it should be noted that the relationship between thickness and planar dimensions, ratios, etc., differs from reality, and the accompanying drawings also include parts where the dimensional relationships and ratios differ from each other.

[0058] Figure 1 The diagram 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.

[0059] Figure 1The continuous molten metal coating equipment 1 shown is used to continuously attach molten metal to the surface of the steel strip S by immersing the steel strip S, which is a metal strip, in a molten metal bath 4 composed of molten metal, so that the molten metal is attached to a specified amount.

[0060] The continuous molten metal plating equipment 1 includes a furnace nose 2, a plating tank 3, a submerged roller 5, and a support roller 6.

[0061] The furnace nose 2 is a component that divides the space through which the steel strip S passes. The furnace nose 2 is a rectangular component with a cross-section perpendicular to the direction of travel of the steel strip S. Its upper end is connected, for example, to the outlet side of a continuous annealing furnace, and its lower end is 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 under a reducing atmosphere, passes through the furnace nose 2 and is continuously introduced into the molten metal bath 4 within the plating tank 3. Then, the steel strip S is lifted upwards from the molten metal bath 4 via the submerged roller 5 and the support roller 6 within the molten metal bath 4.

[0062] Then, a pair of gas wiping nozzles 10A and 10B (gas injection ports 11 described later) are arranged on both sides of the steel strip S, and gas (wiping gas) is sprayed onto both sides of the steel strip S as it is lifted upward from the molten metal bath 4, thereby adjusting the amount of molten metal adhering to both sides of the steel strip S. Then, the steel strip S is cooled by a cooling device (not shown) and guided to subsequent processes to continuously produce molten metal coated steel strips.

[0063] A pair of gas wiping nozzles 10A and 10B (hereinafter also referred to as "nozzles") are arranged opposite each other above the molten metal bath 4, separated by a steel strip S. Nozzle 10A is as follows: Figure 2 As shown, gas is blown onto the steel strip S from a gas injection port 11 (nozzle slit) extending at its tip in the width direction of the steel strip, thereby adjusting the amount of coating adhesion on the surface of the steel strip. The other nozzle 10B is treated similarly. Through this pair of nozzles 10A and 10B, excess molten metal is scraped off, adjusting the amount of coating adhesion on both sides of the steel strip S, and homogenizing the coating adhesion in both the width and length directions.

[0064] To accommodate various steel strip widths and to address positional shifts in the width direction during steel strip lifting, the nozzle 10A is typically configured to be longer than the steel strip width and extend to the outer side of the steel strip's width-direction end. Additionally, as... Figure 2 As shown, nozzle 10A has a nozzle manifold 12 and an upper nozzle member 13A and a lower nozzle member 13B connected to the nozzle manifold 12. In a cross-sectional view perpendicular to the width direction of the steel strip S, the top portions of the upper and lower nozzle members 13A and 13B are parallel to each other to form a gas injection port 11 (nozzle slit). Figure 2(The middle section). The gas injection port 11 extends in the width direction of the steel strip S. Specifically, the gas injection port 11 is configured as a slit that extends wider than the steel strip S along the width direction. In addition, the longitudinal cross-sectional shape of the nozzle 10A is a tapered shape that gradually tapers towards the tip. The thickness of the tip portion of the upper and lower nozzle members 13A and 13B (refer to...) Figure 15 The thickness P can be set to approximately 1 mm to approximately 3 mm. Furthermore, the width (opening width) B (slit gap) of the gas injection port is not particularly limited and can be set to approximately 0.5 mm to approximately 3.0 mm. Gas supplied by a gas supply mechanism (not shown) passes through the inside of the nozzle manifold 12, and then through the gas flow paths divided by the upper and lower nozzle members 13A and 13B, and is ejected from the gas injection port 11 onto the surface of the steel strip S. The other nozzle 10B has the same configuration. In this case, the pressure inside the nozzle manifold 12 is measured by a pressure gauge (not shown). The pressure inside the nozzle manifold 12 can be adjusted by the output of the gas supply mechanism.

[0065] Figure 15 This is a magnified view of the area near the tip of nozzle 10A. (See diagram.) Figure 15 As shown, the tapered portion on the outer surface of the upper nozzle member 13A is referred to as the outer shape tapered portion (outer shape tapered portion 131A) of the upper nozzle member 13A, and the tapered portion on the outer surface of the lower nozzle member 13B is referred to as the outer shape tapered portion (outer shape tapered portion 131B) of the lower nozzle member 13B. Furthermore, the angle formed by the outer shape tapered portion 131A of the upper nozzle member 13A and the outer shape tapered portion 131B of the lower nozzle member 13B is referred to as the outer shape angle (outer shape angle α) of the nozzle 10A.

[0066] Here, in the manufacturing of molten metal coated steel strip, pressurized gas is sprayed onto the surface of the steel strip, which is continuously lifted from the molten metal plating bath, from gas wiping nozzles arranged opposite each other on both sides of the steel strip to control the thickness of the adhered metal. However, a problem arises: the molten metal scatters and solidifies during the scattering, becoming metal powder (splatter) that adheres to the steel strip, thus reducing its surface quality.

[0067] Here, spatter defects refer to defects caused by spatter adhering to the steel plate. Specifically, they refer to defects as follows: gas jets (air jets) ejected from nozzles arranged opposite each other... Figure 3 As shown in (a), a collision occurs near the edge of the steel plate, causing jet vibration. As a result, the liquid film of molten metal is torn apart. The torn liquid film solidifies as it is scattered in the form of droplets, thus becoming a solid (metal powder) that adheres to the steel plate, thereby creating a defect.

[0068] In researching methods to suppress spatter defects, the inventors first used a high-speed camera to examine the dispersion direction of the spatter (metal powder). The results showed that, under typical CGL (continuous hot-dip galvanizing) operating conditions, when the nozzle angle θ (the angle between the gas jet direction and the horizontal plane) = 0°, the spatter... Figure 3 (b) shows the splattering widely above and below the nozzle. Empirically, operators fine-tune the nozzle downwards (nozzle angle: 0–2°) to suppress this splatter defect. However, this fine-tuning depends on the operator's skill level, and therefore the splatter defect fluctuates depending on the timing of the operation. Therefore, it is considered whether a significant downward tilt of the nozzle would drastically change the situation and improve the splatter defect.

[0069] In CGL's actual production line, 10 tons of sheet material with a width of 1000mm and a thickness of 1mm are passed through the sheet at a speed of 100mpm (meters per minute). At this time, if... Figure 4 As shown, under the conditions of a nozzle-steel plate spacing of 10 mm, a nozzle angle θ = 0–80°, and a nozzle tip height of 500 mm, the zinc adhesion amount at the center of the plate width is 50 ± 5 g / m. 2 The pressure was adjusted using the method indicated by the pressure gauge installed on the nozzle manifold. Then, the spatter defect generation rate was examined using a defect gauge located on the CGL outlet side, and its correlation with the nozzle angle was investigated. It should be noted that the spatter defect generation rate is the ratio of the length of steel strip identified as having spatter defects during the inspection process to the length of steel strip that passed through. Additionally, the slit gap (width of the gas injection nozzle) was B = 1.0 mm. The experimental results are shown below. Figure 5 In this context, for one point in the diagram using one roll of material, the acceptable standard for the spatter defect rate is below 0.10%. This is because a spatter defect rate below 0.10% can be considered sufficient quality for steel strips with stringent surface quality requirements, such as those used in automotive steel sheets.

[0070] exist Figure 5 In the study, the incidence of spatter defects fluctuated significantly around the nozzle angle θ = 0°. This suggests that spatter defects are difficult to control through fine adjustments to the nozzle angle. As the nozzle is tilted downwards, i.e., as the nozzle angle increases, spatter defects decrease. Then, when θ > 60°, spatter defects increase again.

[0071] The results of observing the scattering of splashes using a high-speed camera are shown below. Figure 6 From this, we can see that when the nozzle angle θ = 30° has a low rate of spatter defects, the spatter only flies downwards from the nozzle. When the nozzle angle θ = 65° has a higher rate of defects, the spatter flies both upwards and downwards from the nozzle.

[0072] The reason for this is as follows. When the nozzle angle θ = 0°, if... Figure 3 As shown, the gases ejected from opposing nozzles collide near the edge of the plate. The pressures at each nozzle are subtly different, and there are also pressure variations over time; therefore, the jets colliding at the plate edge also flow upwards and downwards from the nozzles. It is presumed that the accompanying splashes disperse upwards and downwards.

[0073] Even with an increased nozzle angle for a downward-sloping nozzle, jetting still occurs at the plate edge. However, it is believed that the gas flow rate towards the bath surface (downward) increases compared to the upward flow rate, thus causing spatter to preferentially disperse downwards. This is thought to suppress spatter directed upwards from the nozzle. It is presumed that this reduces the spatter dispersion range and decreases spatter defects. Similarly, it is believed that spatter hardly disperses upwards from the nozzle within the nozzle angle range of θ = 10–60°, resulting in a spatter defect occurrence rate close to 0. By operating within this range, upward dispersion of spatter is suppressed, thus also preventing operational malfunctions such as spatter adhering to the gas injection port and clogging the nozzle.

[0074] When the nozzle angle θ > 60°, it is assumed that... Figure 6 The reduced distance between the nozzle and the steel plate makes it difficult for air to escape upwards from the nozzle, creating vortices. Specifically, the narrowing of the conical portion of the upper nozzle member 13A between itself and the steel strip S obstructs the upward flow of gas near the edge of the plate, making it easier for vortices to form between the conical portion and the steel strip S. In this situation, spatter flying from the edge of the plate is dispersed in various directions due to the flow of the generated vortices. The reason for the increased spatter defects is that the vortex effect causes the spatter flying upwards from the nozzle to adhere to the steel plate.

[0075] Regarding the nozzle angle θ, a reduction in spatter defects occurs in the region above 10°, therefore the lower limit is 10°. Here, the zinc adhesion amount varies depending on the collision pressure gradient generated by the gas impact on the steel strip S and the shear force generated in the zinc film due to the gas impact on the steel strip S. If the downward nozzle angle increases, the collision pressure gradient decreases. 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 ejected from the nozzle collides with the target object (steel strip). Moreover, to obtain the same zinc adhesion amount, if the same nozzle-steel plate distance (gap) is used, a higher gas flow rate is required, necessitating a larger capacity compressor and increasing construction costs. Furthermore, as mentioned above, if vortices are generated between the conical portion of the upper nozzle component and the steel plate, spatter defects are induced, and spatter cannot be suppressed. Furthermore, considering the rigidity of the nozzle, the nozzle's profile angle (… Figure 15The nozzle's profile angle α is approximately 40° to 50°. If the nozzle is tilted to over 70°, then 70° + 20° (half the profile angle) = 90°, and the nozzle contacts the steel plate. Considering the nozzle-steel plate distance, an upper limit of approximately 60° for the nozzle angle θ is realistic. Furthermore, this reduces splatter defects in areas where the nozzle angle θ is below 60°. Therefore, the upper limit for the nozzle angle θ is set at 60°.

[0076] The preferred range for the nozzle angle θ is 15° ≤ θ ≤ 45°. When θ ≥ 10°, a reduction in spatter defects is observed, but by setting the nozzle angle θ to 15° or higher, the reduction in impact pressure at the steel plate end can be further suppressed. That is, if the nozzle angle θ is small, the jet stream ejected from the opposing nozzle collides with the outer edge of the plate, causing vibration and reducing the pressure acting on the steel plate end. Conversely, by setting the nozzle angle θ to 15° or higher, the reduction in pressure acting on the steel plate end can be suppressed. If the impact pressure at the steel plate end decreases, the effect of scraping off excess molten metal weakens. By setting the nozzle angle θ to 15° or higher, edge overcoating defects with excessive adhesion at the steel plate end can be suppressed. Therefore, the lower limit of the preferred range for the nozzle angle θ is 15°. In the region where θ > 45°, the amount of gas facing the bath surface increases, potentially causing zinc spatter to scatter from the bath surface. Therefore, the upper limit of the preferred range for the nozzle angle θ is 45°. It should be noted that the phenomenon of zinc splattering off the bath surface is called bath splatter. If bath splatter occurs, it can cause problems such as defects in the steel plate or deterioration of the surrounding environment of the equipment.

[0077] Here, the characteristics of the collision jet are as follows: Figure 7 The diagram shows the ratio D / B, calculated by dividing the distance (gap) D from the nozzle tip (top of the gas injection port) to the impact plate (steel strip) by the slit gap B. In the region with a small D / B, the average velocity along the jet axis is equal to the jet exit velocity; this region is called the isokinetic core. Then, as D / B increases, turbulence at the outer edge of the jet reaches the jet axis, causing the velocity along the jet axis to decrease, the isokinetic core disappears, and a fully developed region of complete jet turbulence is formed. The inventors believe that the jet turbulence accompanying the disappearance of the isokinetic core affects the variation of the impact pressure of the gas ejected from the opposing nozzles near the plate edge. Then, by varying D / B, the generation rate of spatter defects relative to the nozzle angles θ = 10°, 15°, and 30° was examined. The results are shown in... Figures 8-10 middle.

[0078] from Figures 8-10It can be seen that, with a constant nozzle angle, the spatter defect rate can be controlled using the D / B ratio, independent of the slit gap B. Furthermore, the spatter defect rate varies with the nozzle angle. Therefore, to suppress spatter defects, the management of the D / B ratio (nozzle-plate distance divided by slit gap) and the nozzle angle is crucial.

[0079] If the nozzle-steel plate distance is small, the nozzle may collide with the steel plate due to plate warping; therefore, the lower limit of D / B is set to 3. If D / B increases, the turbulence of the jet stream, which is associated with the disappearance of the isodynamic core, increases (jet stream stability deteriorates), thereby increasing spatter defects. Therefore, the upper limit of D / B is 10 when the nozzle angle θ = 10°. Figure 8 By increasing the nozzle angle θ, splatter that flies upwards near the plate edge is suppressed. Therefore, the D / B ratio is increased to control splatter defects, with an upper limit of 12 when the nozzle angle θ = 30°. Figure 10 Within the range of 10° ≤ θ ≤ 30°, and within the range of the upper limit of the D / B ratio connecting nozzle angles θ = 10° and θ = 30°, operations that suppress spatter defects can be performed. If D / B exceeds 12, even if the nozzle angle θ increases, the effect of jet stability degradation is greater, and therefore the reduction effect of spatter defects is not observed. Therefore, within the range of 30° ≤ θ ≤ 60°, the upper limit of D / B is 12.

[0080] Equations (1) to (5) are the results of rearranging the nozzle angle θ and D / B used above to achieve the operation of suppressing splash defects. Figure 11 This is the result of summarizing the above ranges for D / B and θ.

[0081] D / B = 3…(Equation 1)

[0082] D / B = 0.1 × θ + 9… (Equation 2)

[0083] D / B = 12…(Equation 3)

[0084] θ = 10…(Equation 4)

[0085] θ = 60…(Equation 5)

[0086] The preferred range for D / B is D / B ≤ 10. By setting D / B ≤ 10, the reduction in impact pressure at the steel plate end caused by the collision of the jet streams ejected from opposing nozzles at the outer edge of the plate can be suppressed, thus suppressing edge overcoating defects. That is, if D / B increases, the jet stream turbulence associated with the disappearance of the constant velocity core increases, and the jet stream vibration generated when the jet streams ejected from opposing nozzles collide at the outer edge of the plate also increases. In order to suppress the resulting reduction in impact pressure at the wide end of the plate, it is preferable to set it to the above range.

[0087] Within a suitable range of the nozzle angle θ used to prevent spatter defects and the ratio D / B (nozzle-to-steel plate distance divided by slit clearance), the internal pressure (gas pressure) of the nozzle manifold 12 is preferably 2 to 70 kPa. More preferably, it is 3 kPa or more. Furthermore, it is more preferably 60 kPa or less. This is because when the internal pressure of the nozzle manifold 12 is less than 2 kPa, the turbulence of the jet increases during the period before collision with the steel plate, making spatter defects more likely. This is also because if the internal pressure of the nozzle manifold 12 exceeds 70 kPa, the compressor used for the injected gas becomes larger, increasing equipment costs and making it uneconomical.

[0088] Furthermore, within the appropriate range of the nozzle angle θ and D / B, the jet velocity of the gas ejected from the nozzle (the gas flow velocity at the nozzle tip) is preferably 100–500 m / s. This is because when the gas flow velocity from the nozzle is less than 100 m / s, the turbulence of the jet increases during the period before collision with the steel plate, easily leading to spatter defects. This is because when the gas flow velocity from the nozzle exceeds 500 m / s, the compressor used for injecting the gas becomes large, resulting in high equipment costs, which is uneconomical.

[0089] Furthermore, the length of the parallel portion of the slit gap formed at the gas injection port 11 ( Figure 15 The length G in the slit gap is preferably 10–40 mm. This is because when the length of the parallel portion of the slit gap is less than 10 mm, the formation of the isotropic core of the ejected jet is insufficient, and the turbulence of the jet increases during the period before collision with the steel plate, making it prone to spatter defects. This is because when the length of the parallel portion of the slit gap exceeds 40 mm, the resistance to the flow of gas passing through the slit gap increases, the efficiency of gas injection decreases, and therefore excessive power is required.

[0090] Furthermore, if the nozzle tip height, defined as the distance between the nozzle tip (the tip of the gas jet) and the surface of the molten metal (zinc) bath, is too low, vortices will form between the nozzle and the surface of the molten metal (zinc) bath, resulting in melt wrinkling defects. That is, the flow (backflow) of the molten metal flowing downwards on the surface of the steel plate becomes uneven due to the scraping effect of the gas ejected from the nozzle, thus causing melt wrinkling. Conversely, if the nozzle tip height is too high, local solidification of the metal (zinc) begins during the period from when the steel strip is lifted upwards from the molten metal bath to when the wiping gas is sprayed, resulting in melt wrinkling defects. That is, due to the local solidification of zinc, the viscosity of zinc becomes uneven on the surface of the steel plate, thus causing melt wrinkling. Therefore, to suppress melt wrinkling defects, it is preferable to set the nozzle tip height H (the distance between the tip of the gas jet and the surface of the molten metal bath, referring to...) to a certain value. Figure 4The nozzle tip height H is set to be 50mm or more and 700mm or less. Here, the nozzle tip height H is more preferably greater than 150mm (H > 150mm). Furthermore, the nozzle tip height H is more preferably less than 550mm (H < 550mm).

[0091] Here, melt wrinkles are wavy, flow-like patterns (wrinkles) produced on the coated surface of a steel sheet coated with molten metal. When used as an exterior panel, steel sheets with such melt wrinkles hinder the surface properties of the coating, particularly its smoothness, when the coated surface is used as a coating substrate.

[0092] Next, in the manufacturing of the steel strip S, it is preferable to control the temperature of the wiping gas such that the relationship between the temperature T (°C) of the gas immediately after being ejected from the nozzle slit of the gas wiping nozzle 10 and the melting point TM (°C) of the molten metal satisfies TM-150 ≤ T ≤ TM+250. 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, thus reducing viscosity inhomogeneity and suppressing the formation of melt wrinkling defects. On the other hand, when the temperature T (°C) of the wiping gas is below TM-150°C, it does not affect the fluidity of the molten metal, and therefore has no effect on suppressing the formation of melt wrinkling defects. Furthermore, when the temperature T (°C) of the wiping gas is above TM+250°C, alloying is promoted, leading to a deterioration in the appearance of the steel sheet.

[0093] Furthermore, there are no particular limitations on the method for heating the wiping gas supplied to the gas wiping nozzle 10. Examples include methods such as heating the gas using a heat exchanger and supplying it, and methods such as mixing the combustion exhaust from the annealing furnace with air.

[0094] In addition, in this embodiment, it is preferable to arrange a pair of baffles 20, 21 on the outer side of both ends of the steel strip S in the width direction, preferably on the extended surface of the steel strip near the ends in the width direction of the steel strip S. Figure 12 , Figure 13 Side and top views are shown, respectively, of baffles 20 and 21 configured together with a pair of nozzles 10A and 10B. The baffles 20 and 21 are positioned between the nozzles 10A and 10B. Therefore, the front and back surfaces of the baffles face the gas injection ports 11 of the nozzles 10A and 10B. The baffles 20 and 21 function to prevent the gases ejected from the nozzles 10A and 10B from directly colliding with each other, thereby contributing to the reduction of spatter. Thus, by configuring the baffles, the effect of further suppressing the generation of spatter defects is increased compared to the above embodiment.

[0095] The shapes of the baffles 20 and 21 are not particularly limited, but are preferably rectangular, and preferably arranged with two sides parallel to the extending direction of the ends of the steel strip S in the width direction. The thickness of the baffles 20 and 21 is preferably 2 to 10 mm. When the thickness is 2 mm or more, the baffles are less likely to deform under the pressure of the wiping gas. When the thickness is less than 10 mm, the possibility of contact with the wiping nozzle or thermal deformation is reduced. The length of the baffles 20 and 21 along the traveling direction of the steel strip S is preferably set such that the upper end is above the position where the gas ejected from the pair of nozzles 10A and 10B directly collides, and the lower end is located 50 mm above the bath surface. This is because the range of collision between the jets ejected from the opposing nozzles on the outer side of the plate edge can be reduced, thus suppressing edge overcoating defects. Therefore, the lower end of the baffles 20 and 21 can be configured to be immersed in the molten metal bath.

[0096] Figure 14 It is Figure 13 A magnified view of the vicinity of one width-direction end of the steel strip S. (Refer to...) Figure 14 The distance E between the end of the steel strip in the width direction and the baffle is preferably set to 10 mm or less, more preferably 5 mm or less. This can more reliably prevent direct collisions with opposing jets. In addition, from the viewpoint of reducing the possibility of contact between the steel strip and the baffle during serpentine movement, this distance E is preferably set to 3 mm or more.

[0097] The material of the baffle is not particularly limited. However, in this embodiment, since the baffle is close to the bath surface, it is considered that top slag and splashes may adhere to it and alloy with the baffle. Furthermore, when the baffle is immersed in the bath, not only alloying but also thermal deformation must be considered. From this perspective, materials suitable for the baffle include those coated on iron plates with boron nitride-based coatings that readily repel zinc, and SUS316L, which does not readily react with zinc. In addition, ceramics such as alumina, silicon nitride, and silicon carbide are preferred as they can suppress both alloying and thermal deformation.

[0098] Furthermore, hot-dip galvanized steel strip can be cited as an example of the molten metal coated steel strip manufactured using the gas wiping nozzle and the molten metal coated steel strip manufacturing method of this embodiment. This hot-dip galvanized steel strip also includes either coated steel sheet (GI) that has not undergone alloying treatment after hot-dip galvanizing and coated steel sheet (GA) that has undergone alloying treatment. However, the molten metal coated steel strip manufactured using the gas wiping nozzle and the molten metal coated steel strip manufacturing method of this embodiment is not limited to this, and includes all molten metal coated steel strips containing molten metals other than zinc, such as aluminum and tin.

[0099] As one embodiment of the method for manufacturing the molten metal coated steel strip of the present invention, the method comprises: a step of plotting a graph with the angle θ (°) between the direction of the gas (wiping gas) injection and the horizontal plane as the horizontal axis and the quotient D / B of the distance D (mm) between the top 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; a step of determining the operating range in the graph drawn in the above steps using the above-described (Equations 1) to (Equations 5); and a step of operating the pair of gas wiping nozzles 10A and 10B within the operating range determined in the above steps.

[0100] Example

[0101] [Example 1]

[0102] use Figure 1 The continuous molten metal coating equipment 1 shown has the basic structure used to immerse a steel strip S with a thickness of 1.0 mm and a width of 1200 mm into a molten zinc bath at a throughput speed of 1.67 m / s (100 mpm), producing hot-dip galvanized steel strip under the conditions specified in Table 1. Regarding the gas wiping nozzles 10A and 10B, the width B of the gas injection port 11 is 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 either 100°C or 450°C. Furthermore, under the conditions specified in Table 1, the adhesion amount at the center of the strip width was 50 ± 5 g / m. 2 The method of regulating the gas pressure (the internal pressure of the nozzle manifold) of the wiping nozzle is used.

[0103] The spatter defect rate is the ratio of the length of steel strip with spatter defects determined in the inspection process at the exit side of the CGL (continuous hot-dip galvanizing production line) to the length of steel strip that passes through. A rate of less than 0.10% is considered acceptable.

[0104] In addition, the generation of splashes on the surface of the molten zinc bath is evaluated by visual observation.

[0105] The evaluation of melt wrinkles is carried out in the inspection process on the CGL exit side according to the following criteria.

[0106] △: Hot-dip galvanized steel sheet with visible molten metal wrinkles.

[0107] 〇: Hot-dip galvanized steel sheet where molten metal wrinkles cannot be visually confirmed.

[0108] Additionally, cut sheets were taken from the roll material at the CGL exit side, and 48mm diameter samples for adhesion analysis were cut from the center of the sheet width and 50mm inward from the edge of the sheet width. Adhesion analysis was performed on the obtained samples, and the increase rate of adhesion relative to the center of the sheet width at the edge of the sheet width was taken as the edge overcoating rate (EOC rate) and the results were compiled.

[0109] The preferred melt wrinkling evaluation is "0" and the EOC rate is below 5.0%.

[0110] The experimental results are shown in Table 1. The conditions for Examples 1 to 22 are that the graph plotted with the angle θ (°) between the gas injection direction and the horizontal plane as the horizontal axis and the quotient D / B (the ratio of the distance D (mm) from the top of the gas nozzle to the steel strip) to the width B (mm) of the gas nozzle as the vertical axis falls within the range defined by Equations (1) to (5) below. That is, Examples 1 to 22 are examples of operating the gas wiping nozzles 10A and 10B within the aforementioned range.

[0111] D / B = 3…(Equation 1)

[0112] D / B = 0.1 × θ + 9… (Equation 2)

[0113] D / B = 12…(Equation 3)

[0114] θ = 10…(Equation 4)

[0115] θ = 60…(Equation 5)

[0116] Under the above conditions, a spatter defect rate of less than 0.10% is considered acceptable.

[0117] Furthermore, inventive examples 2, 3, 6, 13, and 14, which operate within the preferred range described below, can produce steel sheets that do not produce surface splatter, have an EOC rate of 5.0% or less, do not consume excessive zinc, and suppress the adhesion of splatter defects.

[0118] D / B = 3…(Equation 1)

[0119] D / B = 10…(Equation 6)

[0120] θ = 15…(Equation 7)

[0121] θ = 45…(Equation 8)

[0122] On the other hand, the conditions of Comparative Examples 1 to 16 deviated from the range defined by (Equations 1) to (Equations 5), and the spatter defect occurrence rate exceeded 0.10%, which is unacceptable. Furthermore, Comparative Examples 14 to 16 are examples of steel strip manufacturing under the conditions described in Japanese Patent Application Publication No. 2018-9220. In the conditions of Comparative Examples 14 to 16, the nozzle height was set to 350 mm, thus suppressing molten metal wrinkling; however, the operating conditions were outside the aforementioned range, resulting in deterioration of spatter defects, which is unacceptable. Furthermore, edge overcoating also deteriorated.

[0123]

[0124] [Example 2]

[0125] As another embodiment of the present invention, for applications using the same methods as in Embodiment 1 Figure 1 An example of manufacturing a hot-dip galvanized steel strip with a thickness of 1.0 mm and a width of 1200 mm using a continuous molten metal coating apparatus 1 with the basic configuration shown will be described. In this embodiment, the steel strip S is immersed in a molten zinc bath at a throughput speed of 0.75 to 2.16 m / s (45 to 130 mpm) and hot-dip galvanized steel strip is manufactured under the conditions shown in Table 2. The width B of the gas injection port 11 of the gas wiping nozzles 10A and 10B is set to 1.0 to 1.4 mm, and the length G of the parallel portion of the slit gap is set to 30 mm. Furthermore, in this embodiment, a pair of baffles are arranged on the outer sides of both ends of the steel strip S in the width direction. The thickness of the baffles is 5 mm, the distance E between the ends of the steel strip in the width direction and the baffles is set to 5 mm, and the baffles are arranged such that the lower end of the baffles is 30 mm above the surface of the molten zinc bath. The temperature of the molten zinc bath is 460°C, and the gas temperature T at the tip of the gas wiping nozzle is 450°C. The gas pressure (internal pressure of the nozzle manifold) of the wiping nozzle is adjusted according to the amount of adhesion at the center of the width of the steel strip S as shown in Table 2.

[0126] The evaluation methods for spatter defect rate, surface spatter, melt wrinkles, and edge overcoating rate are the same as in Example 1. The experimental results are shown in Table 2.

[0127] Examples 23-29 are examples of operations performed within the range defined by Equations 1-5, plotted in diagrams with the angle θ (°) between the gas injection direction and the horizontal plane as the horizontal axis and the quotient D / B (the ratio of the distance D (mm) from the top of the gas nozzle to the steel strip) to the width B (mm) of the gas nozzle as the vertical axis. Furthermore, Examples 23-29 are examples of operations performed under conditions falling within the following preferred range.

[0128] D / B = 3…(Equation 1)

[0129] D / B = 10…(Equation 6)

[0130] θ = 15…(Equation 7)

[0131] θ = 45…(Equation 8)

[0132] Furthermore, Examples 23 to 29 of the invention are examples of operations performed under the condition that the distance H between the top of the gas jet nozzle and the surface of the molten zinc bath is 50 mm or more and 700 mm or less, and the temperature T (°C) of the gas immediately after being ejected from the gas wiping nozzle and the melting point TM (°C) of the molten zinc satisfy TM-150≤T≤TM+250.

[0133] As shown in Table 2, in Examples 23-29 of the invention, the spatter defect occurrence rate was less than 0.10%, which is acceptable. Furthermore, there was no spatter formation on the bath surface, and the EOC rate was less than 5.0%. Based on the above, it can be confirmed that this embodiment can suppress the adhesion of spatter to the steel strip, and can manufacture hot-dip galvanized steel strips with suppressed spatter defects. It is also confirmed that it is possible to manufacture hot-dip galvanized steel strips that prevent surface quality defects such as molten metal wrinkling while suppressing edge overcoating, thereby improving the zinc yield.

[0134]

[0135] Symbol Explanation

[0136] S-shaped steel strip

[0137] 1. Continuous molten metal coating equipment

[0138] 2. Furnace nose

[0139] 3. Plating bath

[0140] 4 Molten metal bath

[0141] 5 Submerged Rollers

[0142] 6 support rollers

[0143] 10A, 10B Gas Wiping Nozzles

[0144] 11 Gas injection port

[0145] 12 Nozzle manifold

[0146] 13A Upper Nozzle Component

[0147] 13B Lower Nozzle Component

[0148] Baffles 20 and 21

[0149] The conical shape of the nozzle component on the 131A

[0150] 131B Lower nozzle component with tapered outer shape

Claims

1. A method for manufacturing a steel strip coated with molten metal, wherein, A steel strip is continuously immersed in a molten metal bath. For the steel strip lifted from the molten metal bath, gas is sprayed from the gas injection port, which has a slit-shaped gas nozzle extending wider than the steel strip in its width direction and a pair of gas wiping nozzles disposed across the steel strip. The amount of molten metal adhering to both sides of the steel strip is adjusted to continuously produce molten metal-coated steel strips. In this method of manufacturing molten metal-coated steel strips... When plotting a graph with the angle θ (°) between the direction of the gas ejected from the gas nozzle and the horizontal plane as the horizontal axis, and the quotient D / B of the distance D (mm) between the top of the gas nozzle and the steel strip and the width B (mm) of the gas nozzle as the vertical axis, The pair of gas wiping nozzles are operated within the range defined by Equations 1 to 5 below. D / B=3 …(Equation 1) D / B = 0.1 × θ + 9 … (Equation 2) D / B=12 …(Equation 3) θ=10 …(Equation 4) θ=60 …(Equation 5) Each of the pair of gas wiping nozzles has a nozzle manifold and an upper nozzle component and a lower nozzle component connected to the nozzle manifold. In a cross-sectional view perpendicular to the width direction of the steel strip, the top ends of the upper nozzle member and the top ends of the lower nozzle member are parallel to each other to form the gas injection port. The gas passes through the inside of the nozzle manifold and is ejected from the gas injection port. The length of the parallel portion of the slit gap is 10–40 mm.

2. The method for manufacturing molten metal coated steel strip according to claim 1, wherein, The distance H between the tip of the gas injection port of the pair of gas wiping nozzles and the surface of the molten metal bath is more than 50 mm and less than 700 mm. The temperature T (°C) of the gas immediately after being ejected from the pair of gas wiping nozzles is related to the melting point TM (°C) of the molten metal, which satisfies the condition TM-150≤T≤TM+250.

3. The method for manufacturing molten metal coated steel strip according to claim 1 or 2, wherein, The internal pressure of the nozzle manifold is set to 2-70 kPa.

4. The method for manufacturing molten metal coated steel strip according to claim 1 or 2, wherein, A baffle is arranged on the outer side of both ends of the steel strip in the width direction and between the pair of gas wiping nozzles, in a manner opposite to the gas injection port.

5. The method for manufacturing molten metal coated steel strip according to claim 3, wherein, A baffle is arranged on the outer side of both ends of the steel strip in the width direction and between the pair of gas wiping nozzles, in a manner opposite to the gas injection port.

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