JP5640340B2 - Manufacturing method of molten metal plated steel strip - Google Patents

Description

  The present invention relates to a method for producing a molten metal plated steel strip in which a gas wiping nozzle is sprayed with a gas onto the surface of a steel strip that is continuously pulled up from a molten metal plating bath to control the amount of coating on the surface of the steel strip. is there.

In the continuous hot dip plating process of the steel strip, as shown in FIG. 6, the steel strip S is immersed in the plating bath 2 filled with the molten metal, the direction is changed by the sink roll 3, and then vertically upward from the plating bath 2. Then, above the plating bath 2, gas wiping is performed in which gas (pressurized air or the like) is blown onto the steel strip surface from the gas wiping nozzles 1a and 1b provided facing each other across the steel strip S. By this gas wiping, excess molten metal adhering to the surface of the steel strip is scraped to control the amount of plating adhesion, and the adhering molten metal is made uniform in the plate width direction and the plate longitudinal direction.
In order to stabilize the steel strip traveling position in the gas wiping unit as described above, the support roll 4 is usually disposed under the bath surface above the sink roll, and when performing alloying treatment or the like as necessary. A support roll 5 is installed above the gas wiping nozzles 1a and 1b.

  The gas wiping nozzles 1a and 1b are usually configured to be longer than the steel strip width in order to cope with various steel strip widths and at the same time to shift in the width direction when the steel strip is pulled up. It extends to the outside. In such a gas wiping apparatus, a so-called splash is generated in which molten metal falling below the steel strip is scattered by the disturbance of the gas jet colliding with the steel strip S, which adheres to the surface of the steel strip and adheres to the plated steel strip. There is a problem that the surface quality is degraded.

  Further, in order to increase the production amount in the continuous treatment process of the steel strip, the steel strip passing speed may be increased. However, when controlling the coating amount by gas wiping method in the continuous hot dipping process, increasing the steel strip passing speed increases the initial adhesion amount immediately after passing through the plating bath of the steel strip due to the viscosity of the molten metal, In order to control the coating amount within a certain range, it is necessary to set the gas sprayed from the gas wiping nozzle to the steel strip surface at a higher pressure, which greatly increases the splash and cannot maintain good surface quality. .

In order to solve the above-mentioned problems, Patent Documents 1 and 2 describe a technique in which gas is blown to a steel strip before it reaches the gas wiping section after leaving the plating bath, and the excess molten metal is removed to some extent. It is shown.
In the technique of Patent Document 1, a baffle plate is arranged on both sides of a steel strip to be passed, and an inclined guide that changes the flow of a jet gas in the vicinity of the steel strip edge inwardly at the bottom of the steel strip side corner of the baffle plate. Is provided.
In addition, the technique of Patent Document 2 is provided with a sub nozzle (auxiliary nozzle) adjacent to the main nozzle (wiping nozzle) to make the tip of the outlet of the partition plate of the main nozzle and the sub nozzle an acute angle, and from the main nozzle. According to the document, the potential core is lengthened, resulting in an increase in adhesion amount controllability and stabilization of the gas jet, thereby reducing noise. It is said.

  However, although the techniques of Patent Documents 1 and 2 can achieve a temporary splash reduction, the occurrence of splash cannot be stably reduced during operation. In other words, the slit gap of a gas wiping nozzle for molten metal generally has a very small aspect ratio (aspect ratio = 1: 2000), so that the processing accuracy and mounting accuracy of the nozzle have a great influence on the suitability of gas wiping. Big. Therefore, the optimum conditions in which splash is unlikely to occur vary depending on the operating conditions such as the processing accuracy and mounting accuracy of the gas wiping nozzle itself, as well as the line speed and the warp amount of the steel strip. For this reason, even if the techniques of Patent Documents 1 and 2 are used, it is difficult to stably suppress the occurrence of splash.

  In order to solve this problem, Patent Document 3 proposes the following method. This method uses the correlation between the frequency spectrum of the sound wave generated by the gas wiping unit and the splash generation, measures the sound wave generated by the gas wiping unit and converts it to a frequency spectrum, and in a specific frequency region of this frequency spectrum. The position of the gas wiping nozzle is adjusted so that the sound pressure intensity or the integrated value of the sound pressure intensity is below the reference value.

JP 2003-321756 A JP-A-10-204599 JP 2007-308778 A

  However, in the method of Patent Document 3, since the “specific frequency region” to be evaluated extends over a wide frequency region, and cannot cope with a slight change in the gas vibration frequency spectrum related to the occurrence of splash, A change in the occurrence of splash due to a change or a change in various other operating conditions cannot be properly detected, and the operation may not be stable.

  Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art and relates to the sheet feeding speed of the steel strip in the manufacturing method of a hot-dip metal-plated steel strip in which the amount of plating adhesion is controlled using a gas wiping nozzle. Another object of the present invention is to provide a production method that can appropriately suppress the occurrence of plating surface defects due to splash and stably produce a high-quality hot-dip metal-plated steel strip.

  The inventors of the present invention limited the wiping gas vibration frequency band to be evaluated as much as possible with respect to a method using the correlation between the frequency spectrum of the sound wave generated in the gas wiping unit and the occurrence of splash as in Patent Document 3. As a result of further investigations, splashes are likely to occur at the edge of the steel strip in the first place because the gas jets injected from a pair of opposing wiping nozzles collide with each other in the absence of the steel strip. It was found that the main cause was the occurrence of a vibration phenomenon. Therefore, it was found that the gas wiping vibration sound frequency can be clearly measured by subtracting the frequency component due to background noise when performing sound wave measurement (noise measurement) of the gas wiping unit and obtaining the frequency spectrum. As a result of further investigation, by adjusting the gas wiping nozzle setting conditions so that the frequency spectrum peak does not appear in a specific frequency range determined by the gas wiping nozzle conditions, the occurrence of plating surface defects due to splash can be prevented. It was found that it can be effectively suppressed.

The present invention has been made on the basis of such findings and has the following gist.
[1] Generated at the gas wiping section in the manufacturing method of the molten metal plated steel strip, in which the amount of coating is controlled by blowing gas from the gas wiping nozzle onto the surface of the steel strip that is continuously pulled up from the molten metal plating bath. transformed into the frequency spectrum by measuring the sound waves, when the peak of the frequency spectrum in a predetermined frequency region appears, it adjusts the setting condition of the gas wiping nozzle as eliminated by the peak relatively small melt A method for producing a metal-plated steel strip, wherein the predetermined frequency region is a frequency region in which St obtained by the following formula satisfies 0.1 ≦ St ≦ 0.15: Manufacturing method of the belt.
St = fd / U
Where f: vibration frequency
d: Ratio of the distance L between the gas wiping nozzles to the slit gap B of the gas wiping nozzles (= L / B * 0.001)
U: Gas outlet nozzle gas velocity (m / s)
[2] In the production method of [1], the gas wiping nozzle setting condition to be adjusted is a nozzle height from the bath surface or / and a gas injection angle in a vertical direction with respect to the steel strip. A method for producing a molten metal-plated steel strip .

  According to the present invention, it is possible to suppress the gas wiping vibration that becomes a splash generation source, so that the amount of splash generation can be stably reduced regardless of the steel strip passage speed, and a plated steel strip without surface defects can be produced with high productivity. It can be manufactured stably.

Schematic diagram showing the state of the opposed gas jet near the steel strip edge in the gas wiping section Explanatory drawing which shows angle difference (theta) of the gas injection angle in the up-down direction of a pair of gas wiping nozzle used in the experiment of FIG. Drawing which shows the frequency spectrum of a gas wiping vibration sound in the experiment conducted by adding an angle difference θ of 0.2 ° and 1.2 ° to the gas injection angle in the vertical direction of a pair of gas wiping nozzles, respectively. The graph which shows the relationship between the power spectrum average value and splash amount in 1600-2300Hz of a gas wiping vibration sound In an Example, the graph which shows the splash defect rate of a prior art example and this invention example Explanatory drawing schematically showing continuous hot dip plating equipment for steel strip

  In the method of controlling the coating amount by blowing gas from the gas wiping nozzle onto the surface of the steel strip that is continuously pulled up from the molten metal plating bath, most of the splash of molten metal generated by gas wiping is Generated from (edge splash). This is due to the following reasons. That is, the gas jet discharged from the gas wiping nozzles arranged on both sides of the steel strip becomes a wall jet as a single jet after colliding with the steel strip at the steel strip center, whereas at the steel strip edge, It has been found that a phenomenon in which the gas jets from the opposing gas wiping nozzles collide with each other causes a large vibration in the vertical direction as shown in FIG. For this reason, the gas velocity fluctuation (disturbance) becomes very large at the steel strip edge portion, and splash is likely to occur.

  This phenomenon was experimentally verified again using a hot dip galvanizing wiping simulator that can reproduce the actual hot dip galvanizing process. The gas wiping nozzle had a slit gap of 1.0 mm, a distance between nozzles of 10 mm, a slit width of 400 mm, and a nozzle gas pressure of 70 kPa. The hot dip galvanizing bath temperature is 460 ° C, and a steel strip with a plate thickness of 0.4 mm and a plate width of 200 mm is passed at 2.5 m / s. At that time, measurement of gas wiping vibration noise and gas wiping nozzle are performed with a noise meter. Splashes scattered at the bottom were collected at the same time. In addition, it has been confirmed in advance that there is a correlation between the splash defect that occurs in the actual continuous hot dipping line and the amount of splash splashed under the gas wiping nozzle.

  The experimental results when the gas injection angle in the vertical direction of the gas wiping nozzle is changed under the above conditions are shown below. As shown in FIG. 2, the experiment was performed with an angle difference θ of 0.2 ° and 1.2 ° added to the vertical gas injection angles of the pair of gas wiping nozzles 1a and 1b. The distance from the nozzle nozzle tip to the nozzle turning center was 200 mm. FIG. 3 shows the result of measuring the gas wiping vibration sound and converting it to a frequency spectrum. According to this, when the angle difference θ between the gas injection angles is 0.2 ° and 1.2 °, the singular frequency (frequency at which the power spectrum value shows a peak) is different. Including this condition, FIG. 4 shows the relationship between the amount of splash when the angle difference θ is variously changed and the power spectrum average value of the gas wiping vibration sound at 1600 to 2300 Hz. According to this, the decrease in the power spectrum value contributes to the reduction of the splash amount, more specifically, when there is a peak of the frequency spectrum in a specific frequency region (1550 to 2325 Hz in the case of FIG. 4) ( For example, when there is no peak of the frequency spectrum in the same frequency region (for example, when the angle difference θ is 1.2 °), the splash amount is significant. It turns out that it is reducing.

  For the above reasons, in the present invention, the sound wave generated in the gas wiping unit is measured and converted into a frequency spectrum, and the setting condition of the wiping nozzle is adjusted so that the peak of the frequency spectrum does not appear in the predetermined frequency region A. As a result, it is possible to suppress the gas wiping vibration that becomes a splash generation source, and to significantly reduce the amount of splash.

  Here, the frequency region A may be determined in advance as follows. That is, it is known that the vibration phenomenon of the jet as described above can be generally arranged by the Strouhal number St (= fd / U) (for example, Toshihiko Shauchi, “Jet Engineering”, Morikita Publishing Co., Ltd.) , March 2004, p.37-38). Here, f: vibration frequency, d: representative length, U: representative speed. In the case of a system such as a gas wiping unit, d is the ratio of the distance L between the gas wiping nozzles to the slit gap B of the gas wiping nozzle (= L / B * 0.001), U is the nozzle outlet gas velocity, 0.1 It has been found that the frequency region satisfying ≦ St ≦ 0.15 may be the frequency region A, that is, the peak of the frequency spectrum may be prevented from appearing in the frequency region. Thus, since the frequency region A is determined by the slit gap of the gas wiping nozzle, the distance between the nozzles, and the nozzle outlet gas velocity (nozzle gas pressure), when applied to actual operation, the slit gap of the gas wiping nozzle in advance. Further, the frequency region A needs to be obtained from the distance between nozzles and the nozzle gas pressure. For example, when the slit gap of the gas wiping nozzle is 0.9 mm, the distance between nozzles is 16 mm, and the nozzle gas pressure is 60 kPa (nozzle outlet gas velocity: 290 m / s), the frequency region A is 1631 to 2447 Hz.

  In addition, as the setting conditions of the adjustable gas wiping nozzle, for example, nozzle gas pressure, nozzle-steel strip distance, gas injection angle in the vertical direction (nozzle angle), nozzle height from the bath surface, etc. can be considered. If the nozzle gas pressure and the nozzle-steel strip distance are changed, the amount of plating adhesion also changes, so that it is generally difficult to change (adjust) during operation. Therefore, when changing (adjusting) during operation, it is particularly desirable to adjust either or both of the gas injection angle in the vertical direction and the nozzle height from the bath surface.

In carrying out the present invention, a sound pressure detection microphone is installed in the vicinity of the gas wiping unit to measure the sound wave from the gas wiping unit, and the sound wave is converted into a frequency spectrum by a known real-time sound wave analyzer. In the process control device, based on the frequency spectrum, it is determined whether or not a frequency spectrum peak appears in the frequency region A determined from the gas wiping nozzle condition. If the frequency spectrum peak appears in the frequency region A, the gas wiping is performed. Nozzle setting conditions are adjusted (for example, the gas injection angle in the vertical direction of the gas wiping nozzle on the front and back of the steel strip is changed so as to increase), so that the peak of the frequency spectrum does not appear in the frequency region A.
The analysis of the frequency spectrum of the sound wave from the gas wiping unit and the position adjustment of the gas wiping nozzle based on the analysis may be performed continuously during the operation or may be performed at appropriate time intervals. Good. Moreover, you may perform suitably at the time of change of plating conditions.

In a continuous hot dip galvanizing facility, a steel strip having a plate thickness of 0.7 to 1.0 mm, a plate width of 1000 to 1200 mm, a single-sided plating adhesion amount of 45 to 50 g / m ² , and a plate passing speed of 2 to 2.5 m. A hot dip galvanized steel strip was produced by continuous hot dip galvanizing under the conditions of / s.
The gas wiping nozzle used had a slit width of 2000 mm and a slit gap of 0.9 mm, a nozzle gas pressure of 60 to 80 kPa, a nozzle-steel strip distance of 7 to 10 mm, and a nozzle height from the plating bath surface of 400 mm. . A baffle plate was used under fixed conditions.

  In the example of the present invention, a microphone is installed at a location about 5 m away from the side of the wiping nozzle and the sound wave from the gas wiping unit is measured, and the sound wave is converted into a frequency spectrum by a real-time sound wave analysis device (real-time FFT, sampling frequency 20 kHz). Conversion processing (always processing) is performed, and 10 frequencies determined as spectrum peaks are output. In the process control device that performs calculation calculation of the gas wiping condition, the current frequency region A calculated from the gas wiping nozzle condition is compared with 10 points output from the real-time FFT, and if there is a numerical value in the frequency region A, The gas injection angle in the direction was changed.

  FIG. 5 shows the transition of the splash defect rate for 10 days for each of the invention example and the conventional example (each method of Patent Document 1 and Patent Document 3). The amount of splash generated is the ratio of the steel strip length determined to have a splash defect in the inspection process to the steel strip length passed under each manufacturing condition, and includes a slight splash defect that does not cause a practical problem. . According to FIG. 5, the splash defect rate of the example of the present invention is significantly reduced (86% reduction in average) compared to the conventional example 1 (the method of Patent Document 1), and the conventional example 2 (the method of Patent Document 3). ), It was confirmed that it was stable at a low level (a 45% decrease in the overall average).

1a, 1b Gas wiping nozzle 2 Plating bath 3 Sink roll 4, 5 Support roll S Steel strip

Claims (2)

In the manufacturing method of the molten metal plating steel strip, which controls the amount of plating by blowing gas from the gas wiping nozzle on the surface of the steel strip continuously pulled up from the molten metal plating bath,
The sound wave generated in the gas wiping unit is measured and converted into a frequency spectrum. When a frequency spectrum peak appears in a predetermined frequency region , the gas wiping nozzle A method of manufacturing a molten metal-plated steel strip that adjusts the setting conditions ,
The method for producing a hot-dip metal-plated steel strip, wherein the predetermined frequency region is a frequency region in which St obtained by the following equation satisfies 0.1 ≦ St ≦ 0.15 .
St = fd / U
Where f: vibration frequency
d: Ratio of the distance L between the gas wiping nozzles to the slit gap B of the gas wiping nozzles (= L / B * 0.001)
U: Gas outlet nozzle gas velocity (m / s)   The molten metal-plated steel strip according to claim 1, wherein the set condition of the gas wiping nozzle to be adjusted is a nozzle height from the bath surface or / and a gas injection angle in a vertical direction with respect to the steel strip. Production method.

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