JP4857906B2 - 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 a continuous hot dipping process, a steel strip is generally immersed in a plating bath filled with molten metal, and the steel strip is pulled vertically upward from the plating bath, and then a gas provided opposite to the steel strip. Gas wiping in which gas is blown from the wiping nozzle onto the steel strip surface is performed. By this gas wiping, excess molten metal is scraped off and the amount of plating adhesion is controlled, and the molten metal adhering to the steel strip surface is made uniform in the plate width direction and the plate longitudinal direction. The gas wiping nozzle is usually configured to be longer than the width of the steel strip and to the outside of the width end of the steel strip in order to cope with various widths of the steel strip as well as misalignment in the width direction when the steel strip is pulled up. It extends.

In such a gas wiping method, a so-called splash is generated in which molten metal falling below the steel strip is scattered around due to the turbulence of the gas jet that collided with the steel strip, which adheres to the surface of the steel strip and adheres to the surface of the plated steel strip. There is a problem that the quality is degraded.
In order to increase the production amount in the continuous processing process of the steel strip, the steel strip passing speed (line speed) may be increased. However, when controlling the coating amount by gas wiping method in the continuous hot dipping process, if the line speed is increased, the initial coating amount immediately after passing through the plating bath of the steel strip increases due to the viscosity of the molten metal. In order to control within a certain range, it is necessary to set the gas pressure blown from the gas wiping nozzle to the steel strip surface to a higher pressure, which greatly increases the splash and makes it impossible to maintain good surface quality.

In order to solve the above problems, for example, the following techniques have been proposed.
In Patent Document 1, a baffle plate is provided, and direction changing means for changing an external force acting on a hot dipped metal attached to the metal plate in a direction toward the center of the width of the metal plate in the vicinity of the metal plate edge below the gas wiping nozzle. The technology to provide is shown.
Patent Document 2 discloses a technique of providing an air pad having a header chamber having a slit for generating a static pressure between a gas wiping nozzle and a plating bath surface and a steel strip.

In Patent Document 3, the flow velocity of the airflow that goes around from the lower side of the gas wiping nozzle to the upper side and faces the front of the steel plate is weakened by a rectifying plate that protrudes behind the gas wiping nozzle and / or rides on the airflow. The technique which stops the splash conveyed by the weir provided in the gas wiping nozzle upper front part is shown.
In Patent Document 4, an auxiliary nozzle (sub nozzle) that is divided into three or more in the width direction on the upper and lower sides of the main nozzle, and each of the divided portions can independently control the pressure, and injects the gas from the main nozzle. A technique for suppressing the spread of the main jet and stabilizing the gas flowing along the steel plate after the collision is shown.

JP 2003-321757 A JP 2002-173750 A JP-A-200-328218 JP-A-10-204599

  However, these conventional techniques cannot stably reduce the occurrence of splash. The slit gap of a gas wiping nozzle for molten metal generally has a very small aspect ratio (aspect ratio = 1: 2000 or so), so the influence of nozzle processing accuracy and mounting accuracy on the suitability of gas wiping is very large. . 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 above prior art is used, it is difficult to stably suppress the occurrence of splash.

  Therefore, the object of the present invention is to solve the above-mentioned problems of the prior art and to make the steel strip pass at high speed in the manufacturing method of the molten metal plated steel strip in which the amount of plating adhesion is controlled using the gas wiping nozzle. However, an object of the present invention is to provide a manufacturing method that can appropriately suppress the occurrence of plating surface defects due to splash and stably manufacture a high-quality molten metal-plated steel strip.

The gist of the production method of the present invention for solving the above problems is as follows.
[1] Generated at the gas wiping part in the manufacturing method of the molten metal plated steel strip, which controls the amount of plating 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. A method for producing a molten metal-plated steel strip, comprising: measuring a sound wave, converting it into a frequency spectrum, and adjusting a position of a gas wiping nozzle so that the frequency spectrum satisfies a predetermined condition.
[2] In the manufacturing method of [1], the position of the gas wiping nozzle is adjusted so that the sound pressure intensity in the specific frequency region of the frequency spectrum or the integrated value of the sound pressure intensity is equal to or less than a reference value. A method for producing a molten metal-plated steel strip.
[3] A method for producing a molten metal-plated steel strip, characterized in that, in the production method of [1] or [2], an inclination angle in the vertical direction of the gas wiping nozzle is adjusted.

  According to the present invention, the correlation between the frequency spectrum of the sound wave generated by the gas wiping unit and the occurrence of the splash is utilized, and the gas frequency spectrum generated by the gas wiping unit satisfies the predetermined condition. By adjusting the position of the wiping nozzle, it is possible to effectively suppress the occurrence of splash even when the steel strip is passed at high speed regardless of the processing accuracy, mounting accuracy, operating conditions, etc. of the gas wiping nozzle.

  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 the edge of the steel strip. Generated from (edge splash). This is due to the following reason. 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, When the gas jets from the opposing gas wiping nozzles collide with each other, the speed fluctuation in the part becomes very large, and therefore, splash easily occurs at the steel strip edge part.

  Here, when the gas wiping portion of the molten metal is observed, the sound may change greatly. This change in sound is the timing at which the welding point of the steel strip passes, or the wiping conditions such as the gas wiping pressure, the distance between the gas wiping nozzle and the steel strip, and the tilt angle of the gas wiping nozzle in the vertical direction are changed. Very often it is timing. As a result of investigations and studies on the relationship between the sound and the occurrence of splash by the present inventors, it was found that there is a clear correlation between the frequency spectrum of the sound and the occurrence of splash. Therefore, as a result of further investigation, it is possible to stabilize the occurrence of splash by adjusting the position of the gas wiping nozzle so that the frequency spectrum of the sound wave generated in the gas wiping unit satisfies a predetermined condition. It was found that it can be suppressed.

For this reason, in the present invention, a gas wiping unit is used in a method for manufacturing a molten metal plated steel strip in which a gas wiping nozzle blows gas onto the surface of a steel strip that is continuously pulled up from a molten metal plating bath to control the amount of plating. The sound wave generated in is measured and converted into a frequency spectrum, and the position of the gas wiping nozzle is adjusted so that the frequency spectrum satisfies a predetermined condition.
In the present invention, various conditions of the predetermined frequency spectrum are conceivable. For example, (1) the maximum value of the sound pressure intensity in the specific frequency region, and (2) the sound pressure intensity in the specific frequency region. A reference value is determined in advance for the integral value, etc., so that the measured value is equal to or less than the reference value. In addition, in the wide frequency range of the frequency spectrum, since the correlation between the high frequency region and the occurrence of splash is particularly high, it is preferable to set the reference values (1) and (2) in the high frequency region. For example, in the case of (1) above, the maximum value of the sound pressure intensity in the frequency region of 1000 to 5000 Hz in the frequency spectrum is set to be a reference value (for example, −20 dB) or less, and in the case of (2) above. Is set so that the integrated value of the sound pressure intensity in the frequency region of 3000 Hz or higher of the frequency spectrum is equal to or lower than a reference value (for example, −20 dB).

  Examples of the fluctuation factors of the frequency spectrum of the sound wave generated by the gas wiping unit include, for example, the line speed and the plate shape (for example, the plate shape based on the pushing amount of the support roll in the bath, and the shape control amount using electromagnetic force or gas force) The wiping conditions such as the gas wiping pressure, the distance between the gas wiping nozzle and the steel strip, and the inclination angle of the gas wiping nozzle in the vertical direction can be considered. However, when the line speed, the distance between the gas wiping nozzle and the steel strip, the gas wiping pressure, etc. are changed, the plating adhesion amount changes, and the control becomes complicated. For this reason, in the present invention, the position of the gas wiping nozzle is a control (adjustment) target. Further, among the adjustment of the position of the gas wiping nozzle, it is particularly possible to adjust the inclination angle θ in the vertical direction with respect to the horizontal direction of the gas wiping nozzles 1a and 1b as shown in FIG. 1 (S: steel strip in the figure). It has been found that this is effective in controlling the frequency spectrum, and that the effect of the adjustment on the amount of plating is very small. Therefore, as the position adjustment of the gas wiping nozzle, it is particularly preferable to adjust the inclination angle θ in the vertical direction with respect to the horizontal direction of the gas wiping nozzle.

  In practicing 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 this sound wave is converted into a frequency spectrum by a known real-time sound wave analysis device. Based on the spectrum and the reference value as described above, the position of the gas wiping nozzle is adjusted (preferably, the tilt angle θ in the vertical direction with respect to the horizontal direction of the gas wiping nozzle). Further, 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 the production line for hot-dip galvanized steel strip, a sound pressure detection microphone is installed at a position 3 m away from the side of the gas wiping nozzle, and the sound wave from the gas wiping part is measured. The spectrum was converted into a spectrum, and the relationship between the frequency spectrum and the number of splash defects was investigated. The number of splash defects generated was counted by identifying defects by image processing using an online defect detector.
The production conditions of the hot dip galvanized steel strip were as follows: steel strip size: plate thickness 1.0 mm × plate width 1000 mm, line speed: 150 mpm, hot dip galvanizing bath temperature: 460 ° C., target plating adhesion amount 50 g / m ² . The gas wiping conditions were nozzle slit size: 1.0 mm × 2000 mm, nozzle gas pressure: 70 kPa, nozzle-steel strip distance: 8 mm, and nozzle height from the plating bath surface: 400 mm.
In this embodiment, the tilt angle θ in the vertical direction with respect to the horizontal direction of the gas wiping nozzle as shown in FIG.

In the operation of the comparative example before application of the present invention, the inclination angles θ of the gas wiping nozzles 1a and 1b shown in FIG. 1 were set to the nozzle 1a: 0 ° and the nozzle 1b: 0 °. In this operation, the number of occurrences of splash defects was 3.4 / km.
On the other hand, in the present invention example, a reference value of -8 dB is set as the maximum value of the sound pressure intensity in the frequency region of 100 Hz or higher, and the frequency spectrum of the measured sound wave is equal to or lower than this reference value. The inclination angle θ of the gas wiping nozzles 1a and 1b shown was adjusted. The frequency spectrum when the inclination angle θ is adjusted to nozzle 1a: −0.4 °, nozzle 1b: −0.7 ° (minus means that the tip of the nozzle has a downward inclination) 2 together with the frequency spectrum. In the frequency spectrum of this example of the present invention, the sound pressure intensity decreases as a whole, and the maximum value of the sound pressure intensity in the frequency region of 100 Hz or higher is lower than the reference value of -8 dB. As a result, the number of occurrences of splash defects was drastically reduced to 0.1 / km.

Explanatory drawing which shows inclination-angle (theta) with respect to the horizontal direction of a gas wiping nozzle. The graph which shows the frequency spectrum of the sound wave which generate | occur | produced in the gas wiping part in the Example of this invention of an Example with the frequency spectrum of the comparative example before this invention application

Claims (3)

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,
A molten metal plated steel characterized by measuring a sound wave generated in a gas wiping unit, converting it to a frequency spectrum, and adjusting a position of the gas wiping nozzle so that the frequency spectrum satisfies a predetermined condition. Manufacturing method of the belt.   2. The molten metal plated steel according to claim 1, wherein the position of the gas wiping nozzle is adjusted so that the sound pressure intensity in a specific frequency region of the frequency spectrum or the integrated value of the sound pressure intensity is equal to or less than a reference value. Manufacturing method of the belt.   The method for producing a molten metal-plated steel strip according to claim 1 or 2, wherein an inclination angle in a vertical direction of the gas wiping nozzle is adjusted.

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