JP6575785B2 - Method for producing hot-dip steel strip - Google Patents

Description

  The present invention relates to a method of manufacturing a hot-dip galvanized steel strip that prevents condensate from adhering to the steel strip in a snout of a continuous hot dip plating line.

  In a continuous hot dip plating line for steel strip, a steel strip annealed in an annealing furnace is introduced into a hot dip bath through a snout having a cylindrical shape. The snout has its tip (bottom end) immersed in a plating bath and has the function of blocking the annealed steel strip from the atmosphere. However, such a snout has the following problems. The solution was desired.

  That is, metal vapor of the plating bath is generated inside the snout, and condensate (ash) is generated by the metal vapor contacting and cooling the wall surface of the snout. And if this condensate adheres to the surface of the steel strip passing through the snout, the condensate will be plated with the steel strip adhering to the steel strip, which causes a negative effect on the surface quality of the plated steel sheet. It was.

  As a conventional technique for improving the surface quality of a steel sheet caused by the adhesion of condensate, for example, in Patent Document 1, metal vapor evaporating from a hot dipping bath in a snout is heated by a heater provided on the snout wall surface. In addition, there is disclosed a technique for removing atmospheric gas containing metal vapor by passing it through an atmospheric gas processing circuit.

  Patent Document 2 discloses that the temperature and inner wall temperature in the turn-down roll chamber and the snout are within the range of -150 ° C. within the range from the plating bath directly to the turn-down roll chamber on the continuous furnace side. In Patent Document 3, the atmospheric gas in the snout is sucked by a blower, the sucked atmospheric gas is cooled by a fume solidification recovery device, and further, metal fine powder in the atmospheric gas is passed through a filter. The technique of heating the atmospheric gas again to the atmospheric temperature after being separated and introducing it into the snout is further disclosed in Patent Document 4, in which the ceiling of the turndown roll chamber of the continuous hot dip galvanizing line is heated and heated. A warming technique is disclosed.

JP-A-7-316760 JP-A-8-176773 JP-A-7-233457 JP-A-8-246120

  However, in Patent Document 1, it is necessary to separately provide a large-scale facility, and an increase in facility cost and maintenance cost is inevitable. Further, since it is necessary to cool and reheat the atmospheric gas, the energy loss is large. In some cases, evaporation of the plating metal may be promoted, and the basic unit loss of the plating metal is large.

  Further, in Patent Document 2, the atmosphere temperature and inner wall temperature in the turn-down roll chamber and the snout are in the range from the plating bath directly to the turn-down roll chamber on the continuous furnace side, and the temperature difference with the plating bath is within −150 ° C. This promotes natural convection of the metal fume, thereby forcibly transferring the metal fume upstream from the snout mounting portion, avoiding deterioration of the basic unit of the plated metal due to an increase in the evaporation amount of the plated metal. I can't. Moreover, in order to promote the metal condensation in a place other than the heating unit, it is necessary to remove the condensate, and the steel strip may be scratched due to contact and adhesion of the condensate. Furthermore, in this method, since the metal fume that is the root cause of the condensate is actively convected, the condensate generated at each part of the inner wall of the snout cannot be prevented. The problem of surface defects caused by adhesion to the steel plate from the inner wall on the side in contact with the sink roll remains.

  Moreover, in patent document 3, like patent document 1, it is necessary to provide a large-scale installation separately, and an increase in an installation cost and the cost concerning a maintenance cannot be avoided, and it is also necessary to cool and reheat atmospheric gas. The merit is small from the viewpoint of energy loss and basic unit loss of plated metal.

  Furthermore, in the technique of Patent Document 4, there is a problem similar to that of Patent Document 2 because it prevents adhesion of condensate by promoting convection of the atmospheric gas, and the technique disclosed here is It is limited to the turn-down roll chamber, and no consideration is given to preventing the deterioration of the quality of the steel strip due to the adhesion of condensate in the snout.

  The object of the present invention is to avoid the adhesion of the condensate to the steel strip in the snout of the continuous hot dipping line without requiring large-scale equipment or consuming wasteful energy, and has good quality. The present invention proposes a method for producing a hot-dip galvanized steel strip that can be stably produced.

The present invention relates to a method of manufacturing a hot dipped steel strip in which a steel strip that has been annealed is continuously introduced into a molten metal plating bath through a snout and plated, and the lower temperature of the snout and the upper temperature of the snout are determined. ,
A method for producing a hot dipping steel strip, characterized in that the lower temperature ≧ plating bath surface temperature of the snout of the upper temperature> scan Nauto.

  In the method having the above configuration, it is desirable that the lower temperature of the snout is a plating bath surface temperature + 50 ° C. or less, and the upper temperature of the snout is a plating bath surface temperature +50 to 100 ° C.

  The lower temperature of the snout and the upper temperature of the snout are preferably adjusted using a heating means provided around the snout.

  If the vertical distance from the top of the plating bath to the upper wall surface of the turn-down roll chamber is defined as the height of the snout, in the present invention, the upper 50% of the height is the upper part of the snout and the lower 50% is the lower part of the snout. To do. The upper temperature of the snout and the lower temperature of the snout shall mean the average temperature of the wall surface in that range. The method of obtaining the average temperature is not particularly limited, but it is possible to measure the temperature at multiple points in the height direction and circumferential direction of the snout, or at a place that shows the average temperature derived by measuring the temperature at multiple points in advance. A method of measuring temperature is conceivable. Further, although it is preferable to directly measure the temperature of the inner wall surface of the snout, if the temperature difference between the inner wall surface and the outer wall surface can be predicted, the temperature may be measured on the outer wall surface. When a snout having a box shape composed of four side walls is applied as the snout, four points at the midpoint of each wall portion on the outer periphery of the snout at the upper part of the snout and the lower part of the snout, respectively. It is desirable to measure the average value from eight points at regular intervals in the height direction of the snout (using a thermocouple, etc.) from there, and to calculate the average values as the lower temperature and upper temperature of the snout, respectively. . The temperature measuring position closest to the plating bath surface at the bottom of the snout is about bath surface +200 mm.

  By setting the lower temperature and the upper temperature of the snout such that the upper temperature of the snout ≧ the lower temperature of the snout ≧ the plating bath surface temperature, a temperature gradient is imparted to the wall surface of the snout, and the atmospheric temperature in the snout is made uniform. Further, the convection of the metal vapor is less likely to occur due to the uniformity of the ambient temperature, and the vapor pressure of the gas near the plating bath surface is maintained at a high level. As a result, the evaporation of the molten metal is restricted, which causes surface defects. The production of objects is suppressed. In particular, the effect of suppressing the formation of aggregates can be further enhanced by setting the lower temperature of the snout to the plating bath surface temperature + 50 ° C. or less and the upper temperature of the snout to the plating bath surface temperature +50 to 100 ° C.

It is the figure which showed typically the suitable continuous hot dipping equipment used for implementation of this invention. It is the figure which expanded and showed the cross section of the principal part of a snout. It is the figure which showed the side surface of snout typically.

Hereinafter, the present invention will be described more specifically with reference to the drawings.
FIG. 1 is a diagram schematically showing a molten metal plating facility suitable for use in the practice of the present invention.

  In the figure, reference numeral 1 denotes a bathtub in which molten metal is accommodated, 2 is a sink roll placed in the bathtub 1 and changes the traveling direction of the steel strip S to be plated, 3 is above the sink roll 2 A support roll 4 for guiding and correcting the shape of the steel strip S; 4 is a wiping nozzle that adjusts the thickness of the plating layer by blowing gas to the steel strip S placed on the bath surface and pulled up from the bathtub 1; 5 is a snout whose lower end is immersed in the molten metal of the bathtub 1 and whose upper end is connected to the equipment of the annealing furnace. The snout 5 has a passage for introducing the steel strip S that has been annealed into the molten metal of the bathtub 1 without touching the atmosphere.

  Reference numeral 6 is a heating means provided on the outer wall of the snout 5, and 7 is a turn-down roll chamber provided on the exit side of the annealing furnace. The heating means 6 is composed of a lower heating means 6a for heating the lower part of the snout 5 and an upper heating means 6b for heating the upper part so that appropriate temperature adjustment can be performed at the lower and upper parts of the snout 5. As the heating means 6, a heater, a heat exchanger, or the like can be applied. However, the heating method is not particularly limited as long as it can efficiently heat the atmospheric temperature in the snout 5. About the heating means 6, it can also provide in the turndown roll chamber 7 as needed.

  FIG. 2 is an enlarged view of the cross section of the snout 5 in the vicinity of the bath surface of the bathtub 1. Usually, when molten zinc (plating bath) is used as the molten metal, the bath temperature is set to about 440 to 480 ° C., and the temperature of the steel strip S subjected to the plating treatment is also set to the same level, The wall temperature of the snout 5 is often much lower than the bath surface temperature, especially when the wall surface of the snout is not heated, and generally the temperature close to the bathtub 1 is higher.

  In this situation, zinc evaporates from the plating bath and diffuses into the passage of the snout 5 and into the furnace of the annealing furnace, and the vapor is cooled by the wall surface of the snout 5 to precipitate as ash (ZnO). It accumulates. Then, the deposited ash is supposed to fall on the surface of the steel strip S passing through the passage of the snout 5, and the surface quality of the plated steel strip is impaired.

  In the present invention, the lower temperature and the upper temperature of the snout 5 are set by the heating means 6 so that the upper temperature of the snout ≧ the lower temperature of the snout ≧ the plating bath surface temperature. According to this, the ambient temperature in the snout is It becomes uniform and the convection of the metal vapor is less likely to occur due to the homogenization of the ambient temperature, and the metal vapor does not precipitate at any location in the snout 5 because the wall surface has a temperature equal to or higher than the melting point of zinc. Therefore, the vapor pressure of the gas in the vicinity of the plating bath surface is maintained at a high level (the formation of aggregates such as ash that causes the surface defects by limiting the evaporation of the molten metal).

  In the present invention, the lower temperature of the snout 5 is preferably set to the plating bath surface temperature + 50 ° C. or lower, because the effect is saturated even if the temperature is higher than this, resulting in energy loss. .

Further, the upper temperature of the snout 5 needs to be equal to or higher than the lower temperature of the snout, and it is preferable to set the plating bath surface temperature +50 to 100 ° C. The reason is that the upper temperature of the snout 5 is the same as that of the snout 5. This is because a temperature gradient is provided by maintaining the temperature higher than the lower temperature, and the temperature range is appropriate for limiting the convection of the atmospheric gas. As for the upper temperature of the snout 5, too high temperature is not preferable from the viewpoint of energy loss due to temperature rise. According to the present invention, the rate of occurrence of surface defects due to aggregates can be reduced regardless of the thickness, width, material, etc. of the steel sheet.

  FIG. 3 is a diagram schematically showing the side surface of the snout 5. For example, as shown in FIG. 3, the upper temperature and lower temperature of the snout 5 are divided into four regions (i) to (d) in the height direction, and the wall portion of the snout is divided into each region. Measure the temperature with a thermocouple, etc., find the average value at the lower part (b), (b) and upper part (c) of the snout 5, respectively, and use the average as the lower and upper temperatures of the snout 5. Good. In particular, when the snout 5 is divided into four regions (a) to (d) in the height direction, (a) is set to a temperature range of the plating bath surface temperature +0 to 50 ° C. (B) may be a temperature range of plating bath surface temperature +20 to 50 ° C., (c) may be a temperature range of plating bath surface temperature +50 to 80 ° C., and (d) is a plating bath surface. It is preferable that the temperature is in a temperature range of +50 to 100 ° C. When measuring the temperature of the wall portion of the snout 5, when the snout 5 has a box shape formed of four side wall portions, the intermediate points (back side, front side, 4 points on the OP side and DR side) are preferably measured at equal intervals in the height direction of the snout 5 from the plating bath surface +200 mm, for example, at intervals of 500 mm.

Using a continuous hot-dip galvanizing facility with a snout passage area of 0.9 m ² and a length of 8 m from the plating bath to the upper wall on the exit side of the turn-down roll chamber, the plating bath temperature: Steel strip having a thickness of 0.4 to 3.3 mm and a width of 800 to 1850 mm under the conditions of 420 to 500 ° C., plating bath Al concentration of 0.1 to 0.3 mass%, and line speed of 20 to 180 mpm (material: SPCC ( JIS G 3141), annealing temperature (heating zone outlet temperature): 500 to 1000 ° C.), the lower temperature of the snout and the upper temperature of the snout are the conditions shown in Table 1, and the obtained plating The surface quality of the steel strip was investigated. The results are shown in Table 1 together with the lower and upper temperatures of the snout. In addition, the surface quality of the steel strip was evaluated by the rate of occurrence of surface defects due to aggregates (ash). The temperature inside the snout was adjusted using a tube heater.

  As is apparent from Table 1, by controlling the wall temperature of the snout according to the present invention, that is, the upper temperature of the snout ≧ the lower temperature of the snout ≧ the plating bath surface temperature, and the lower temperature of the snout is the plating bath surface temperature + 50 ° C. Hereinafter, when the convection control of the metal vapor was performed with the upper temperature of the snout being the plating bath surface temperature +50 to 100 ° C., it was confirmed that the adhesion of the condensate (ash) was suppressed and the surface quality was good. . The steel strips with good surface quality were also investigated for their mechanical properties (yield point, tensile strength, elongation), but it was confirmed that they were within acceptable limits and had no problems. .

  ADVANTAGE OF THE INVENTION According to this invention, the method which can manufacture a plated steel strip with favorable surface quality stably can be provided, without requiring a large-scale installation or consuming useless energy.

1 Bath 2 Sink roll 3 Support roll 4 Wiping nozzle 5 Snout 6 Heating means 7 Turn-down roll chamber S Steel strip

Claims (3)

About the manufacturing method of the hot dip galvanized steel strip which continuously introduces the steel strip which finished the annealing treatment into the hot dip metal plating bath through the snout, and performs the plating treatment,
The lower temperature of the snout and the upper temperature of the snout,
Method for producing a hot dipping steel strip, characterized in that the lower temperature ≧ plating bath surface temperature of the snout of the upper temperature> scan Nauto.   2. The hot dipped steel strip according to claim 1, wherein the lower temperature of the snout is a plating bath surface temperature + 50 ° C. or less, and the upper temperature of the snout is a plating bath surface temperature +50 to 100 ° C. 3. Production method.   The method for producing a hot-dip galvanized steel strip according to claim 1 or 2, wherein the lower temperature of the snout and the upper temperature of the snout are adjusted using a heating means provided around the snout.

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