Method for predicting splash defects in hot-dip metal coated steel strip, method for manufacturing hot-dip metal coated steel strip, device for predicting splash defects in hot-dip metal coated steel strip, and hot-dip metal coated steel strip manufacturing equipment

Abstract

To provide a method for predicting splash defects of a hot-dip metal coated steel strip in a method for manufacturing the hot-dip metal coated steel strip, capable of controlling a coating weight using a gas wiping nozzle.SOLUTION: A method for predicting splash defects of a hot-dip metal coated steel strip in a facility for manufacturing the hot-dip metal coated steel strip, capable of spraying gas on both surfaces of the steel strip from a pair of gas wiping nozzles facing each other to control a coating weight on the surface of a steel strip continuously pulled from a hot-dip metal plating bath comprises: a reference frequency band setting step, a power-spectrum specific step, a spectral intensity specific step and a defect prediction step of predicting the occurrence of splash defects.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a hot-dip metal coated steel strip in which gas is blown from a gas wiping nozzle onto the surface of a steel strip being continuously pulled up from a hot-dip metal coating bath, thereby controlling the coating weight on the steel strip surface. [Background technology]

[0002] Hot-dip galvanized steel sheets are widely used in fields such as building materials, automobiles, and home appliances. These applications require hot-dip galvanized steel sheets to have excellent appearance. Since the appearance after painting is strongly affected by surface defects such as uneven coating thickness, scratches, and foreign matter adhesion, it is important that hot-dip galvanized steel sheets are free from surface defects.

[0003] Hot-dip metal coated steel strips are generally produced in a continuous hot-dip metal coating line as shown in Figure 1. Specifically, a steel strip S annealed in a continuous annealing furnace in a reducing atmosphere passes through a snout 2 and is continuously introduced into a molten metal bath 4 in a coating tank 3. The steel strip S is then drawn above the molten metal bath 4 via a sink roll 5 and a support roll 6. The steel strip S is adjusted to a predetermined coating thickness by a pair of wiping nozzles 10, after which it is cooled and introduced to a subsequent process. The pair of wiping nozzles 10 are positioned above the coating tank 3, facing each other across the steel strip S, and the gas jet nozzles consist of slits 11 extending in the width direction of the steel strip S. To accommodate various steel strip widths and to accommodate widthwise shifts during drawing up, the wiping nozzles 10 are typically longer than the width of the steel strip S and extend beyond the widthwise ends of the steel strip S. In such a gas wiping device, gas is injected from the slit 11 and sprayed onto the surface of the steel strip S, resulting in the excess molten metal being scraped off, adjusting the amount of plating applied to the steel strip surface and making the molten metal applied to the steel strip surface uniform in the width direction and longitudinal direction of the plate.

[0004] Such a wiping nozzle 10 has a problem in that droplets P of molten metal (hereinafter referred to as "splash") scattered by the gas jet adhere to the steel strip surface, resulting in a deterioration in the surface quality of the coated steel strip. Most of the molten metal splash occurs at the edge of the steel strip (edge splash). This is due to the following reasons: At the center of the steel strip, gas jets discharged from the gas wiping nozzles located on both sides of the steel strip remain as single jets and become wall jets after impacting the steel strip. At the edge of the steel strip, however, the gas jets from the opposing gas wiping nozzles collide with each other and vibrate significantly up and down, as shown in Figure 2 (see, for example, S.J. Kim, J., W. Cho, K., J. Ahn and M.K. Chung: ISIJ Int., 43 (2003)). Therefore, gas velocity fluctuations (turbulence) become very large at the edge of the steel strip, making splashes more likely to occur. In order to increase production volume in a continuous steel strip processing process, the steel strip threading speed can be increased. However, when controlling the coating weight using the wiping method in a continuous hot-dip coating process, if the steel strip passing speed is increased, the initial coating weight immediately after the steel strip S passes through the coating bath increases due to the viscosity of the molten metal.Therefore, in order to control the coating weight within a certain range, the gas G sprayed onto the steel strip surface from the wiping nozzle 10 (10a, 10b) must be set at a higher pressure, which significantly increases splashes caused by collisions between jets at the edge of the steel strip, making it impossible to maintain good surface quality.

[0005] To solve the above problems, the technology of Patent Document 1 places baffle plates on both sides of the steel strip as it passes through, and provides an inclined guide at the bottom corner of the baffle plate facing the steel strip to redirect the flow of injected gas inward near the edge of the steel strip.

[0006] Furthermore, the technology in Patent Document 2 involves providing a sub-nozzle (auxiliary nozzle) adjacent to the main nozzle (wiping nozzle), making the nozzle tip of the partition plate between the main nozzle and the sub-nozzle at an acute angle, and slightly tilting the sub-jet from the sub-nozzle relative to the main jet from the main nozzle.According to this document, as a result of the lengthening of the potential core, the controllability of the amount of adhesion is improved and the gas jet is stabilized, thereby reducing noise.

[0007] Patent Document 3 proposes the following method: This method utilizes the correlation between the frequency spectrum of sound waves generated in the gas wiping unit and the occurrence of splashes, measures the sound waves generated in the gas wiping unit, converts them into a frequency spectrum, and adjusts the position of the gas wiping nozzle so that the sound pressure intensity or the integrated value of the sound pressure intensity in a specific frequency range of this frequency spectrum is equal to or less than a reference value.

[0008] Patent document 4 proposes a technology that utilizes the correlation between the frequency spectrum of the sound waves generated in the gas wiping section and the occurrence of splashes, and adjusts the setting conditions of the gas wiping nozzle so that peaks in the frequency spectrum do not appear in specific frequency ranges. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-321756 [Patent Document 2] Japanese Patent Application Publication No. 10-204599 [Patent Document 3] Japanese Patent Application Laid-Open No. 2007-308778 [Patent Document 4] Japanese Patent Application Laid-Open No. 2011-32526 Summary of the Invention [Problem to be solved by the invention]

[0010] While the techniques described in Patent Documents 1 and 2 can temporarily reduce splashing, they cannot consistently reduce splashing during operation. Specifically, the slit gap of a gas wiping nozzle for molten metal generally has a very small aspect ratio (approximately 1:2000), meaning that the nozzle's machining accuracy and installation accuracy have a significant impact on the suitability of gas wiping. Therefore, the optimal conditions for reducing splashing vary depending on the machining accuracy and installation accuracy of the gas wiping nozzle itself, as well as other operating conditions such as line speed and warpage of the steel strip. Therefore, even if the techniques described in Patent Documents 1 and 2 are used, it is difficult to consistently suppress splashing.

[0011] In the method of Patent Document 3, the "specific frequency range" to be evaluated spans a wide frequency range, and the method is unable to respond to slight changes in the gas vibration frequency spectrum related to splash generation. As a result, it is not possible to properly detect changes in the splash generation situation due to changes in steel strip size or other various operating conditions, which can result in unstable operation.

[0012] The method of Patent Document 4 calculates the frequency band in which the spectral peak should be suppressed based on the gas pressure and the nozzle distance, but the frequency band to be suppressed is almost constant for lines with a nearly constant standard of deposition amount, which is problematic in that it lacks practicality. Also, there are cases where broad peaks appear, as shown in Figure 3, and in such cases, changing the nozzle angle or nozzle height does not reduce the peak in the frequency band to be suppressed.

[0013] An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a method for predicting splash defects in a hot-dip metal-coated steel strip in a hot-dip metal-coated steel strip manufacturing method in which coating weight is controlled using a gas wiping nozzle. Another object of the present invention is to provide a hot-dip metal-coated steel strip manufacturing method using the hot-dip metal-coated steel strip splash defect prediction method, a hot-dip metal-coated steel strip splash defect prediction device, and hot-dip metal-coated steel strip manufacturing equipment including the hot-dip metal-coated steel strip splash defect prediction device. [Means for solving the problem]

[0014] The present invention, which has been made to solve the above problems, has the following configuration. [1] A method for predicting splash defects in a hot-dip metal coated steel strip in a hot-dip metal coated steel strip manufacturing facility in which a coating weight is controlled by spraying gas onto both surfaces of a steel strip that is continuously pulled up from a hot-dip metal coating bath from a pair of opposing gas wiping nozzles, a reference frequency band setting step of setting a reference frequency band HB including a frequency that becomes a peak in the power spectrum of sound waves generated in the gas wiping section when gas is sprayed onto the steel strip from one side of the gas wiping nozzle; a power spectrum determining step of determining the power spectrum of sound waves generated in the gas wiping section when gas is sprayed from the pair of gas wiping nozzles onto both sides of the steel strip during the production of the hot-dip metal coated steel strip; a spectral intensity specifying step of specifying, from the power spectrum specified in the power spectrum specifying step, a spectral intensity PB in the reference frequency band HB and a spectral intensity PA that appears in a frequency band different from the reference frequency band HB and that is caused by interference of opposing jets that occur outside the steel strip path when gas is sprayed from the pair of gas wiping nozzles onto both sides of the steel strip; a defect prediction step of predicting occurrence of a splash defect using the spectral intensity PB and the spectral intensity PA. A method for predicting splash defects in hot-dip coated steel strips. [2] A method for predicting splash defects in hot-dip metal-plated steel strips as described in [1], wherein the peak is the maximum value of the power spectrum of the sound waves generated in the gas wiping section when gas is sprayed onto the steel strip from one side of the gas wiping nozzle. [3] The spectral intensity PB specified in the spectral intensity specifying step is a maximum spectral intensity in a reference frequency band HB, and the spectral intensity PA is a maximum spectral intensity in a frequency band lower than the reference frequency band HB. The method for predicting splash defects in hot-dip metal coated steel strip according to [1] or [2]. [4] In the reference frequency band setting step, the reference frequency band HB is set from a peak of a spectrum intensity at a frequency of 2000 Hz or more and less than 3000 Hz; The maximum spectral intensity PA identified in the spectral intensity identification step is the maximum spectral intensity in a frequency band of 1000 Hz or more and less than 2000 Hz. The method for predicting splash defects in a hot-dip metal coated steel strip according to any one of [1] to [3]. [5] The method for predicting splash defects in a hot-dip metal-plated steel strip according to any one of [1] to [4] is used to predict the occurrence of splash defects in the hot-dip metal-plated steel strip, and the operating conditions of the hot-dip metal-plating equipment are set so as to suppress the occurrence of splash defects, thereby producing a hot-dip metal-plated steel strip. Manufacturing method of hot dip metal coated steel strip. [6] The operating conditions of the hot-dip metal coating equipment are at least one selected from the group consisting of a nozzle angle, which is the angle between the jet direction of the gas jetted from the gas jet port of the gas wiping nozzle and a horizontal plane; a nozzle height, which is the distance from the liquid surface of the hot-dip metal coating bath to the nozzle port of the gas wiping nozzle; an internal pressure of the nozzle header of the gas wiping nozzle; a nozzle-steel-plate distance, which is the distance between the tip of the gas jet port and the steel strip; and a threading speed at which the steel strip is threaded. [5] A method for producing a hot-dip metal-plated steel strip according to [5]. [7] A splash defect prediction device for a hot-dip metal coated steel strip in a hot-dip metal coated steel strip manufacturing facility that controls the coating weight by spraying gas from a pair of opposing gas wiping nozzles onto both sides of the surface of the steel strip being continuously pulled up from a hot-dip metal coating bath, a power spectrum acquisition unit that acquires a power spectrum of a sound wave generated in the gas wiping portion of the gas wiping nozzle; a reference frequency band setting unit that sets a reference frequency band HB that includes a frequency that becomes a peak in the power spectrum of sound waves generated in the gas wiping unit when gas is sprayed onto the steel strip from one side of the gas wiping nozzle; a spectral intensity specifying unit that specifies a spectral intensity PB in the reference frequency band HB and a spectral intensity PA that appears in a frequency band different from the reference frequency band HB from a power spectrum obtained when gas is sprayed onto both surfaces of the steel strip from the pair of gas wiping nozzles in a manufacturing process of the hot-dip metal coated steel strip; a defect prediction unit that predicts the occurrence of a splash defect using the spectral intensity PB and the spectral intensity PA. A device for predicting splash defects in hot-dip coated steel strips. [8] A splash defect prediction device for hot-dip metal-plated steel strips described in [7], wherein the peak in the reference frequency band setting unit is the maximum value of the power spectrum of the sound wave generated in the gas wiping unit when gas is sprayed onto the steel strip from one side of the gas wiping nozzle. [9] The spectral intensity PB specified by the spectral intensity specifying unit is a maximum spectral intensity in a reference frequency band HB, and the spectral intensity PA is a maximum spectral intensity in a frequency band lower than the reference frequency band HB. [7] or [8]. A splash defect prediction device for hot-dip metal coated steel strips.

[10] A hot-dip metal-plated steel strip manufacturing facility, comprising the hot-dip metal-plated steel strip splash defect prediction device according to any one of [7] to [9]. [Effects of the Invention]

[0015] The present invention provides a method for predicting splash defects in hot-dip metal-coated steel strips caused by gas wiping, which is a source of splash. It also provides a method for manufacturing hot-dip metal-coated steel strips using the method for predicting splash defects in hot-dip metal-coated steel strips, a device for predicting splash defects in hot-dip metal-coated steel strips, and hot-dip metal-coated steel strip manufacturing equipment including the device for predicting splash defects in hot-dip metal-coated steel strips. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a diagram showing an example of continuous hot-dip metal plating equipment. [Figure 2] FIG. 10 is a diagram showing how gas jets from opposing gas wiping nozzles collide with each other. [Figure 3] FIG. 10 is a diagram showing that there is a broad peak in the relationship between frequency and power spectrum. [Figure 4] FIG. 2 is a diagram showing the configuration of a gas wiping nozzle. [Figure 5] 1 is a schematic diagram of a splash defect detection device in a hot-dip galvanizing line. [Figure 6] FIG. 2 is a diagram showing the positional relationship between a pair of gas wiping nozzles and a steel strip. [Figure 7] FIG. 10 is a diagram showing the results of frequency analysis of the sound waves of gas wiping vibration noise when the gas injection angle in the vertical direction of the gas wiping nozzle is changed under specified conditions, and converting them into a frequency spectrum. [Figure 8(a)(b)] (a) shows the relationship between frequency and spectrum of each frequency, and (b) shows the relationship between PA and splash defect contamination rate. [Figure 8(c)(d)] (c) is a graph showing the relationship between PB and the splash defect contamination rate, and (d) is a graph showing the relationship between PA / PB and the splash defect contamination rate. [Figure 9] FIG. 1 is a top view of the facility layout of the present invention. [Figure 10] FIG. 2 is a data processing block diagram. [Figure 11]FIG. 10 is a graph showing the change in splash defect rate over 10 days for an example of the present invention and a conventional example. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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 concept of the present invention, and the technical concept of the present invention is not limited to the following embodiments in terms of the materials, shapes, structures, arrangements, etc. of the components. Furthermore, the drawings are schematic. Therefore, it should be noted that the relationships and ratios between thicknesses and planar dimensions differ from the actual ones, and the drawings also include portions where the relationships and ratios of dimensions differ from one another.

[0018] The present invention relates to a method for predicting splash defects in hot-dip metal-plated steel strips in a hot-dip metal-plated steel strip manufacturing facility that controls the coating weight by spraying gas from a pair of opposing gas wiping nozzles onto both sides of the surface of the steel strip as it is continuously pulled up from a hot-dip metal plating bath. First, the hot-dip metal-plated steel strip manufacturing facility and the splash defect measurement method will be described, and then a description will be given using a flow chart of the processing steps.

[0019] The continuous hot-dip metal coating equipment 1 shown in Figure 1 is equipment for continuously depositing molten metal on the surface of the 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 a predetermined amount of molten metal on the surface of the steel strip S. 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.

[0020] The snout 2 is a member having a rectangular cross section perpendicular to the traveling direction 7 of the steel strip S, which defines a space through which the steel strip S passes. Its upper end is connected to the outlet side of a continuous annealing furnace, for example, 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. The steel strip S is then drawn upward from the molten metal bath 4 via a sink roll 5 and a support roll 6 in the molten metal bath 4. Wiping gas is then sprayed onto both sides of the steel strip S drawn upward from a pair of gas wiping nozzles 10a, 10b 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. The steel strip S is then cooled by cooling equipment (not shown) and introduced to a subsequent process, where a hot-dip metal-coated steel strip S is continuously produced.

[0021] The configuration of a gas wiping nozzle is shown in Figure 4. Here, a pair of gas wiping nozzles 10 arranged on both sides of the steel strip S includes, as shown in Figure 4, a nozzle header 12, a first nozzle member 13 arranged on the upper side connected to the nozzle header 15, and a second nozzle member 14 arranged on the lower side. The first nozzle member 13 and the second nozzle member 14 are arranged opposite each other, and a slit 11 serving as a gas injection port is formed so as to extend elongatedly in the longitudinal direction X. The width of the opening of the slit 11 (nozzle outlet) is called the slit gap 16. The nozzle height refers to the distance from the liquid surface of the molten metal coating bath to the nozzle opening of the gas wiping nozzle. The gas wiping nozzles 10a, 10b are arranged on each side of the steel strip S so that the length direction X of the slit 11 is aligned with the width direction of the steel strip S, the width direction Z perpendicular to the length direction X of the slit 11 is aligned with the length direction (threading direction) of the steel strip S, and the depth direction Y perpendicular to the length direction X of the slit 11 is aligned with the thickness direction of the steel strip S. The width direction Z of the slit is the same as the up-and-down direction of the gas wiping nozzles 10a, 10b. One of the gas wiping nozzles 10a, 10b sprays wiping gas from the slit 11 toward one side of the steel strip S. The other gas wiping nozzle 10 also sprays wiping gas from the slit 11 toward the other side of the steel strip S. This scrapes off excess molten metal on both sides of the steel strip S, adjusting the amount of coating (molten metal) and making it uniform in the width and length directions of the steel strip S. The gas wiping nozzles 10a, 10b are configured to be longer than the width of the steel strip S so that they can accommodate a variety of steel strip S widths and to accommodate widthwise positional deviations when the steel strip S is pulled up, and the length of the slit 11 is longer than the width of the steel strip S, extending outward from the widthwise end of the steel strip S.

[0022] Next, splash phenomena were verified in an actual hot-dip galvanizing line equipped with the above equipment. The gas wiping nozzle shape was a slit gap of 1.0 mm, a distance between nozzles of 14-20 mm, and a slit width of 1700 mm. Zinc was used for plating, the plating bath temperature was 460°C, and steel strips with thicknesses of 0.6-1.2 mm and widths of 900-1500 mm were threaded at a speed of 1.6-2.0 m / s, with a coating weight of 45-50 g / m 2The nozzle gas pressure was adjusted within a range of 55 to 65 kPa so that the nozzle pressure was 0.01 to 0.05 kPa. The number of splash defects was counted using a defect meter installed at the exit of the continuous hot-dip galvanizing line. A schematic diagram of the defect detection device 20 is shown in FIG. 5. The defect detection device 20 includes a projector 21 and a camera 22. The projector 21 is a device that irradiates the steel sheet surface with white light or monochromatic light at a certain angle relative to the traveling direction of the hot-dip galvanized steel strip S. The projector 21 preferably irradiates parallel light onto the steel sheet surface. It is preferable that multiple cameras 22 (e.g., 20 or more) are arranged in the width direction of the hot-dip galvanized steel strip S so as to acquire images from a predetermined angle relative to the traveling direction 7 of the hot-dip galvanized steel strip S. If there are no defects on the steel sheet (steel strip) surface, the light irradiated from the projector 21 is specularly reflected by the steel sheet surface. On the other hand, if there are defects, the irradiated light is diffusely reflected by the steel sheet surface. Splash defects can be detected by receiving this diffusely reflected light with the camera 22. In the example of Figure 7, cameras 22 arranged along the width direction of the hot-dip galvanized steel strip S detect splash defects on the inspection line L. It is preferable to use cameras that can detect splash defects of 100 µm or more in size.

[0023] The experimental results are shown below, where the gas wiping nozzle vertical gas injection angle was varied under the above conditions: gas pressure 60 kPa, nozzle separation distance 20 mm, sheet thickness 0.6 mm, and sheet width 1000 mm. As shown in Figure 6, the experiment was conducted with an angle difference θ (θa-θb) of -0.8° and -1.1° between the vertical gas injection angles of a pair of gas wiping nozzles 10a and 10b. Figure 7 shows the results of measuring the gas wiping vibration noise, frequency analyzing the measured sound waves, and converting them into a frequency spectrum. Figure 7 reveals the appearance of two characteristic peaks, near 1600-1800 Hz and near 2000-2200 Hz. As will be described later, the peak in band A is due to the interference of opposing jets generated outside the steel strip path when gas is sprayed from a pair of gas wiping nozzles onto both sides of the steel strip. The peak in band B is due to the gas injection noise from the wiping nozzles. Note that the reference frequency bands HB and HA being different means that even if the frequency bands A and B overlap, the frequencies at which peaks appear in each frequency band are different, and so the reference frequency bands HB are different. When setting the reference frequency band HB, it is necessary to set it to a frequency band that includes a frequency at which a peak occurs in the power spectrum of the sound waves generated in the gas wiping section when gas is sprayed onto the steel strip from one side of the gas wiping nozzle. In this case, it is preferable that the frequency band includes the maximum spectral intensity in the power spectrum of the sound waves generated in the gas wiping section.

[0024] In Figure 7, the peak value of band B remains roughly the same regardless of the nozzle angle, but the peak value of band A varies greatly. It is presumed that the degree of collision of the opposing jets changes depending on the nozzle angle, causing the peak value to vary greatly. As will be described later, when frequency analysis was performed on the sound waves measured when gas was being discharged from only one wiping nozzle (either 10a or 10b) (when no opposing jets were generated), a spectral peak was observed in the vicinity of 2000 to 2200 Hz, and this was determined to be the peak caused by the sound of gas being discharged from the wiping nozzle, i.e., the peak of band B.

[0025] Including these conditions, various operating conditions were varied to investigate the audio features (peak values in bands A and B) and splash defects per unit area. The splash defect contamination rate, which is the number of splash defects per unit area, was used as an indicator of the extent of splash defects. As a result, for example, when the slit gap 16 was 0.8 to 1.5 mm, the gas pressure was 50 to 70 kPa, and the nozzle parallel length was 20 to 30 mm, the spectral intensity PA of the peak appearing due to the interference of the opposing jets was between 800 and 2500 Hz, and the spectral intensity PB of the peak appearing due to the gas discharge sound was between 1500 and 3500 Hz. The preferred range of the spectral intensity PA is the maximum spectral intensity in a frequency band smaller than the reference frequency band HB. It was found that the peak PA, which is caused by the interference of opposing jets, appears frequently in the frequency band below 1000 to 2000 Hz, and the peak PB, which is caused by the sound of gas discharge, appears frequently in the frequency band below 2000 to 3000 Hz. As a result, the maximum values of the spectra appearing in these frequency bands were searched for and the data organized as an example. The results are shown in Figures 8(a)(b) and 8(c)(d). Figure 8(a) shows the relationship between frequency and the spectrum at each frequency, (b) shows the relationship between PA and the splash defect contamination rate, Figure 8(c) shows the relationship between PB and the splash defect contamination rate, and (d) shows the relationship between PA / PB and the splash defect contamination rate.

[0026] Figures 8(b) and 8(c) show that the results cannot be organized by looking only at the peak PA caused by the interference of opposing jets or the peak PB caused by the gas discharge noise; in other words, there is a large amount of variation and the trend is not clear. In particular, even when the PA caused by the interference of jets has the same sound pressure, there are differences in the splash defect contamination rate. However, by calculating PB and PA and taking the ratio PA / PB, it became possible to stratify the results. The pressure inside the nozzle header of a gas wiping nozzle (hereinafter referred to as gas pressure) is the gas pressure measured in the nozzle header. The nozzle header is a space in front of the nozzle where gas is stored to ensure uniform gas release across the width. The greater the gas pressure, the more zinc is scraped off by wiping, and the more zinc is scattered as droplets. PB is the parameter that changes depending on the gas pressure, and this parameter is thought to correlate with the amount of base zinc that is scattered. In contrast, PA is the parameter that indicates the degree of interference of the jet at the nozzle edge. Whether a splash defect occurs is thought to be determined by how much zinc is scraped off and how much of that zinc is scattered above the nozzle due to interference with the jet. For this reason, the occurrence of defects cannot be explained simply by each parameter, and it is thought that it is only by taking the ratio that it becomes possible to sort them out.

[0027] The layout (top view) of the equipment used in the present invention and a data processing block diagram are shown in Figures 9 and 10. As shown in Figure 9, a sound pressure detection microphone (sound collection microphone) 30 is installed near the gas wiping section (5 m away from the side of the gas wiping nozzle), and sound waves from the gas wiping section are measured. These sound waves are then converted into a frequency spectrum by a sound wave analyzer (frequency analyzer) 31. As shown in Figure 10, first, in S1, to identify the peaks due to gas discharge noise from the wiping nozzle 10, gas is discharged from only one of the opposing nozzles 10 within the gas pressure range expected during operation, and sound is collected during this process to identify the peaks. This process corresponds to the step of setting a reference frequency band that includes the frequencies that become peaks in the power spectrum of sound waves generated in the gas wiping section when gas is sprayed onto the steel strip from one side of the gas wiping nozzle. In S2, the frequency band of the peaks due to the opposing jets is identified by discharging gas from both opposing nozzles 10 within the gas pressure range expected during operation, and sound is collected during this process. This step corresponds to a power spectrum identification step for identifying the power spectrum of sound waves generated in the gas wiping section when gas is sprayed from the pair of gas wiping nozzles onto both sides of the steel strip during the production of hot-dip metal-coated steel strip. In S3, the process control device calculates PA / PB, and if it is equal to or greater than a set threshold, changes the gas wiping operating conditions to adjust PA / PB to be equal to or less than the threshold. This step corresponds to a spectral intensity identification step for identifying the spectral intensity PB in the reference frequency band HB and the spectral intensity PA appearing in a frequency band other than the reference frequency band HB from the power spectrum identified in the power spectrum identification step, and a defect prediction step for predicting the occurrence of splash defects using the spectral intensity PB and the spectral intensity PA. Variable gas wiping conditions include nozzle angle, nozzle height, line speed, gas pressure, and nozzle distance. The PA / PB threshold is set appropriately based on the splash defect control range required for each line.

[0028] In the spectral intensity specifying step of specifying the spectral intensity PB in the reference frequency band HB and the spectral intensity PA appearing in a frequency band different from the reference frequency band HB, it is preferable to specify the maximum spectral intensity PB and the maximum spectral intensity PA, but the above can be performed even if neither is the maximum value.

[0029] The analysis of the frequency spectrum of the sound waves from the gas wiping unit and the adjustment of the position of the gas wiping nozzle based on the analysis may be performed continuously during operation, or at appropriate time intervals, or may be performed as needed when the plating conditions are changed.

[0030] The operating conditions of the hot-dip metal plating equipment are at least one selected from the nozzle angle, which is the angle between the spray direction of the gas sprayed from the gas nozzle of the gas wiping nozzle and the horizontal plane; the nozzle height, which is the distance from the liquid surface of the hot-dip metal plating bath to the nozzle nozzle of the gas wiping nozzle; the pressure inside the nozzle header of the gas wiping nozzle; the nozzle-steel plate distance, which is the distance between the tip of the gas nozzle and the steel strip; and the passing speed at which the steel strip is passed.

[0031] In addition, the method for producing a hot-dip metal-plated steel strip of the present invention is a method for producing a hot-dip metal-plated steel strip by predicting the occurrence of splash defects in the hot-dip metal-plated steel strip using the above-described method for predicting splash defects in the hot-dip metal-plated steel strip, and setting the operating conditions of the hot-dip metal plating equipment so as to suppress the occurrence of splash defects.

[0032] The present invention also provides a splash defect prediction device for a hot-dip metal coated steel strip, which is used in a hot-dip metal coated steel strip manufacturing facility that controls the coating weight of a steel strip by spraying gas onto both surfaces of the steel strip from a pair of opposing gas wiping nozzles as the steel strip is continuously pulled up from a hot-dip metal coating bath, and includes: a power spectrum acquisition unit that acquires a power spectrum of a sound wave generated in the gas wiping portion of the gas wiping nozzle; a reference frequency band setting unit that sets in advance a reference frequency band HB including a peak of a spectral intensity from a power spectrum acquired when gas is ejected from one of the gas wiping nozzles; a spectral intensity specifying unit that specifies a spectral intensity PB in the reference frequency band HB and a spectral intensity PA that appears in a frequency band different from the reference frequency band HB from a power spectrum obtained when gas is sprayed onto both surfaces of the steel strip from the pair of gas wiping nozzles in a manufacturing process of the hot-dip metal coated steel strip; and a defect prediction unit that predicts the occurrence of a splash defect by using the spectrum intensity PB and the spectrum intensity PA. The reference frequency band setting unit preferably sets a reference frequency band HB that includes the frequency at which the spectral intensity is maximum in the power spectrum of the sound waves generated in the gas wiping unit when gas is sprayed from the gas wiping nozzle onto the steel strip. Furthermore, the spectral intensity specifying unit that specifies the spectral intensity PB in the reference frequency band HB and the spectral intensity PA that appears in a frequency band different from the reference frequency band HB preferably specifies the maximum spectral intensity PB and the maximum spectral intensity PA, but the above can be implemented even if neither is a maximum value.

[0033] Furthermore, the hot-dip metal coated steel strip manufacturing facility of the present invention includes the above-mentioned device for predicting splash defects in hot-dip metal coated steel strip. [Example]

[0034] In the continuous hot-dip metal coating equipment 1 having the basic configuration shown in Figure 1, a steel strip S having a thickness of 0.6 mm and a width of 900 to 1500 mm is coated on one side with a coating weight of 45 to 50 g / m. 2 The strip was threaded at a speed of 1.67 to 2 m / sec. to produce a hot-dip galvanized steel strip.

[0035] The gas wiping nozzle 10 used had a slit width of 2000 mm, a slit gap of 1 mm, a nozzle gas pressure of 55 to 70 kPa, a nozzle-to-steel sheet distance of 7 to 10 mm, and a nozzle height from the plating bath surface of 200 to 700 mm. A microphone was installed 5 m away from the side of the wiping nozzle to measure the sound waves from the gas wiping section, and the sound waves were converted into a frequency spectrum by frequency analysis every 50 seconds using a sound wave analyzer (sampling frequency 50 kHz, sampling time 2 seconds). PA / PB was calculated from the maximum value PA in the frequency A band of 1000 to 2000 Hz and the maximum value PB in the frequency B band of 2000 to 3000 Hz, and the operating conditions were changed so that this value was below a threshold value.

[0036] Figure 11 shows the change in splash defect rate over 10 days for each of the examples of the present invention and the conventional examples (methods used in Patent Documents 3 and 4). The splash defects were counted per unit threading area of the steel strip using the defect detection device shown in Figure 8, with defects of 100 μm or larger being considered splash defects. A splash defect rate that does not pose a problem in practice is 1 defect / 100 m. 2 It was confirmed that the splash defect contamination rate was significantly reduced compared to Conventional Example 1 (Patent Document 3) and was stable at a low level compared to Conventional Example 2 (Patent Document 4). As an implementation method of the above conventional method, Patent Document 3 controls the sound pressure intensity and the integral value of the sound pressure intensity in a specific frequency range to control the wiping conditions. In Patent Document 4, the wiping conditions are controlled so that no peak appears in a specific frequency range. [Explanation of symbols]

[0037] 1. Continuous hot-dip metal plating equipment 2 Snout 3 Plating tank 4. Molten metal bath 5. Sink Roll 6 Support Roll 7 Direction of steel strip S 10, 10a, 10b Gas wiping nozzle 11 Slit 12 nozzle header 13 First nozzle member 14 Second nozzle member 15 nozzle header 16 Slit Gap 20 Defect detection device 21 Floodlight 22 Camera 30 Sound pressure detection microphone (sound collection microphone) 31 Sound wave analyzer (frequency analyzer) 32 Wiping nozzle S steel strip P Molten metal droplets (splash) L inspection line G Gas X length direction Y depth direction Z width direction PA Spectral intensity of peaks appearing due to the interference of opposing jets Spectral intensity of the peak caused by the sound of PB gas being discharged

Claims

1. A method for predicting splash defects in a hot-dip metal coated steel strip in a hot-dip metal coated steel strip manufacturing facility in which a coating weight is controlled by spraying gas onto both surfaces of a steel strip that is continuously pulled up from a hot-dip metal coating bath from a pair of opposing gas wiping nozzles, comprising: a reference frequency band setting step of setting a reference frequency band HB including a frequency that becomes a peak in the power spectrum of sound waves generated in the gas wiping section when gas is injected onto the steel strip from one side of the gas wiping nozzle; a power spectrum determining step of determining the power spectrum of sound waves generated at the gas wiping section when gas is sprayed from the pair of gas wiping nozzles onto both surfaces of the steel strip during the production of the hot-dip metal coated steel strip; a spectral intensity specifying step of specifying, from the power spectrum specified in the power spectrum specifying step, a maximum spectral intensity P B in the reference frequency band HB having a frequency of 2000 Hz or more and less than 3000 Hz set in the reference frequency band setting step, and a spectral intensity PA which is a maximum spectral intensity in a frequency band of 1000 Hz or more and less than 2000 Hz, which appears in a frequency band different from the reference frequency band HB and is caused by interference of opposing jets generated outside the steel strip path when gas is sprayed from the pair of gas wiping nozzles onto both sides of the steel strip; a defect prediction step of predicting the occurrence of a splash defect using the spectral intensity P B and the spectral intensity P A , calculating P A / P B and adjusting the ratio P A / P B to be equal to or less than a threshold value.

2. 2. A method for predicting splash defects in hot-dip metal-plated steel strips according to claim 1, wherein the peak is the maximum value of the power spectrum of sound waves generated in the gas wiping section when gas is sprayed onto the steel strip from one side of the gas wiping nozzle.

3. The method for predicting splash defects in hot-dip metal-plated steel strips according to claim 1 or 2 is used to predict the occurrence of splash defects in hot-dip metal-plated steel strips, and the operating conditions of the hot-dip metal-plating equipment are set so as to suppress the occurrence of splash defects, thereby producing hot-dip metal-plated steel strips. Manufacturing method of hot dip metal coated steel strip.

4. The operating conditions of the hot-dip metal coating equipment are at least one selected from a nozzle angle, which is the angle between the injection direction of the gas injected from the gas injection port of the gas wiping nozzle and a horizontal plane; a nozzle height, which is the distance from the liquid surface of the hot-dip metal coating bath to the nozzle port of the gas wiping nozzle; a pressure inside a nozzle header of the gas wiping nozzle; a nozzle-steel-plate distance, which is the distance between the tip of the gas injection port and the steel strip; and a threading speed at which the steel strip is threaded. The method for producing a hot-dip metal coated steel strip according to claim 3.

5. A splash defect prediction device for a hot-dip metal coated steel strip in a hot-dip metal coated steel strip manufacturing facility in which a coating weight is controlled by spraying gas from a pair of opposing gas wiping nozzles onto both surfaces of a steel strip that is continuously pulled up from a hot-dip metal coating bath, a power spectrum acquisition unit that acquires a power spectrum of a sound wave generated in the gas wiping portion of the gas wiping nozzle; a reference frequency band setting unit that sets a reference frequency band HB that includes a frequency that becomes a peak in the power spectrum of sound waves generated in the gas wiping unit when gas is injected onto the steel strip from one side of the gas wiping nozzle; a spectral intensity specifying unit that specifies, from a power spectrum acquired when gas is sprayed onto both surfaces of the steel strip from the pair of gas wiping nozzles in a manufacturing process of the hot-dip metal coated steel strip, a maximum spectral intensity PB in the reference frequency band HB of 2000 Hz or more and less than 3000 Hz set by the reference frequency band setting unit, and a spectral intensity PA that is the maximum spectral intensity in a frequency band of 1000 Hz or more and less than 2000 Hz that appears in a frequency band caused by interference of opposing jets generated outside the steel strip path when gas is sprayed onto both surfaces of the steel strip from the pair of gas wiping nozzles; A splash defect prediction device for hot-dip metal coated steel strips, comprising: a defect prediction unit that predicts the occurrence of splash defects using the spectral intensity PB and the spectral intensity PA, calculates PA / PB, and adjusts PA / PB so that it is equal to or less than a threshold value.

6. 6. The splash defect prediction device for hot-dip metal-plated steel strips according to claim 5, wherein the peak in the reference frequency band setting unit is the maximum value of the power spectrum of sound waves generated in the gas wiping unit when gas is sprayed onto the steel strip from one side of the gas wiping nozzle.

7. A hot-dip metal coated steel strip manufacturing facility comprising the apparatus for predicting splash defects in hot-dip metal coated steel strip according to claim 5 or 6.

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