JP3446420B2 - Continuous hot-dip plating method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous hot-dip galvanizing method, in which a floating substance such as a Zn oxide floating on a plating bath surface in a snout is coated on a steel strip surface during plating. The present invention relates to a continuous hot-dip galvanizing method capable of producing a plated steel sheet of good quality without plating defects by preventing contact with the hot-dip galvanized steel sheet. [0002] The Sendzimir method is a general method for continuously producing hot-dip coated steel sheets. FIG. 3 is a side view of a horizontal plating line of the Sendzimir method (or the non-oxidizing furnace method). In the Sendzimir method in which a steel strip is annealed in a plating line, as shown in FIG. 3, a steel strip 1 supplied from a pay-off reel 2 passes through a preheating zone 3 and is heated and recrystallized in a solitary state. After entering the reduction zone 4, then enter the plating bath 8 through the cooling zone 5, the low-temperature holding zone 6, and the snout 7, and after passing through the plating adhesion amount control device 9 such as gas wiping, the cooling device or the alloying treatment furnace 10 is activated. It passes and becomes a product. When cooled by the cooling device 10 after passing through the plating adhesion amount control device, it becomes a so-called hot-dip coated steel sheet, and when heat-treated in the alloying furnace 10 without cooling, it becomes an alloyed hot-dip coated steel sheet. [0005] On the plating bath surface in the snout, a floating substance of an oxide (eg, zinc oxide powder) in which a molten metal is vaporized and solidified or a compound of Fe and a component in the bath (eg, Fe ₂ Al ₅ ) eluted from a steel sheet. And tends to increase in proportion to the plating processing amount. [0006] These suspended matters floating over the entire surface of the plating bath are attracted to the bath flow created by the penetration of the steel strip into the plating bath and come into contact with the steel strip. If this floating material comes into contact with the steel strip surface, it will be plated in an attached state, so that an uneven pattern is generated on the surface of the plating film, the floating material is left in the film, and It becomes unplated. FIG. 2 is a side view of a conventional plating bath.
(A) is a diagram showing the flow of the plating bath and the plating bath around the snout outlet, and (b) is an enlarged view of the snout outlet. After passing through the continuous furnaces 3, 4, 5, and 6, the snout 1
The steel strip 1 that has exited 4 enters the plating bath 11 and is plated.
It is carried out of the bath via the sink roll 12 and the support roll 13. The flow of the plating bath is generally as indicated by the arrow in FIG. Further, in the snout, there are accompanying flows formed by running of the steel strip 1 as indicated by solid arrows in FIG. 3B, and a gentle upward flow generated with the accompanying flows indicated by dotted arrows. A floating substance 16 floats on the bath surface in the snout, and the floating substance is entrained in an accompanying flow as shown in FIG. It is considered that once the steel strip touches the suspended matter, a change occurs at the plating interface, and a difference occurs in the crystal growth of the plating film, so that a pattern is generated as a gloss difference.
Furthermore, suspended matter remains in the film, and a convex defect is generated by pressing, an oxide is peeled off during plating, and a plated metal is not partially adhered. As a method of preventing the generation of suspended matter, there is a method of installing a heater on the outer wall of the snout. In other words, in order to prevent the molten metal from evaporating, zinc oxide adheres to the inner wall of the snout, and the cooled and solidified material is peeled off and falls to the bath surface to become a floating substance, so that the wall of the snout is heated by the heater. Is heated to prevent solidification of the vaporized zinc oxide. There is also a method of suppressing oxidation by increasing the hydrogen concentration in the snout. However, since these methods do not provide a sufficient effect, a method of pumping suspended matter with a pump is generally used. However, since suspended matter spreads and floats over the entire surface of the plating bath, one place is used. It is difficult to collect and remove.
In addition, the bath surface undulates at the time of pumping, and a large amount of suspended matter may temporarily come into contact with the surface of the steel sheet, thereby impairing the quality. Japanese Patent Application Laid-Open No. 7-54115 discloses a method for preventing non-plating by forcibly flowing a molten metal at a flow rate of 1 m / sec in a direction opposite to a traveling direction of a steel strip in a snout. ing. This method can enhance the reduction action of the oxide of the steel strip surface by aluminum in the plating bath by the forced flow of the molten metal, thereby ensuring the reaction between the steel strip and the molten metal and preventing non-plating. It is. [0013] However, although this method has the effect of enhancing the reducing action, since the molten metal is forcibly flowed by the circulating pump, turbulence is generated and suspended matter on the surface of the plating bath is caught in the bath and scattered. Resulting in. Therefore, it is not possible to prevent the floating material from contacting the steel strip surface,
Thereby, non-plating may occur. In addition, the number of man-hours required for collection increases due to the scattering of suspended matters. Japanese Patent Application Laid-Open No. Hei 6-330271 discloses that the inside of a snout is made to have a mixed atmosphere of hydrogen and nitrogen,
A method for preventing occurrence of plating appearance defects and improving adhesion of a plating film is disclosed. However, although the effect can be expected, it is difficult to maintain such an atmosphere. SUMMARY OF THE INVENTION The present invention is to prevent the floating material of the plating bath from coming into contact with the surface of the steel strip during plating, and to collect and discharge the floating material on the inner surface of the snout at a predetermined position. And to produce a hot-dip coated steel sheet free from defects such as non-plating. Means for Solving the Problems The inventors of the present invention have developed a water model for a method for preventing the suspended matter on the plating bath surface in the snout from contacting the steel strip surface and a method for facilitating the discharge of the suspended matter. The following findings were obtained as a result of various studies of hydrodynamics in the tank for use. (1) Immediately above the sink roll, there is a bath flow generated in the snout direction by the rotation of the sink roll and the support roll, and most of the flow flows in the plating bath and flows into the snout. Is less. (2) By setting the distance between the sink roll and the snout outlet (FIG. 2L1) to 1100 to 1500 mm, the steel strip melts from the snout outlet on the surface side (hereinafter referred to as the steel strip surface) in contact with the sink roll. The flow of metal can be efficiently introduced. (3) With this introduction, an upward flow is formed in the snout, and a barrier is formed between the upward flow and the accompanying flow of the steel strip running in the opposite direction, so that the floating material comes into contact with the surface of the steel strip. Can be prevented. (4) Most of the upward flow turns away from the steel strip at the surface layer of the bath, and passes through both edges of the steel sheet to the back side of the steel sheet. Due to the flow, suspended matter on the surface of the bath is moved to the back side of the steel strip and aggregated. (5) The partition plate is partially immersed in a hot-dip plating bath in a snout, and is provided so as to face the surface (rear surface) of the steel strip that does not contact the sink roll, so that the floating material is applied to the rear surface of the steel plate. Contact can also be prevented. The present invention has been completed on the basis of such findings, and the gist of the invention is that "a position where a steel strip which has passed through a continuous hot-dip continuous furnace and a snout sequentially contacts a sink roll in a plating bath for the first time. 11 from the snout exit to
While installing the sink roll and the snout so as to be 00 to 1500 mm, a partition plate for preventing the floating material on the hot-dip plating bath surface in the snout from contacting the steel strip,
A continuous hot-dip plating method characterized in that a steel strip in a snout is provided so as to face a surface of the steel strip that does not contact the sink roll and is plated. DESCRIPTION OF THE PREFERRED EMBODIMENTS The distance between the position where the steel strip which has exited the snout comes into contact with the sink roll for the first time and the outlet of the snout, ie, the distance L1 between the point P and E in FIG. Is less than about 110 mm for conventional devices. In such a case, the bath 18 immediately above the sink roll formed by the rotation of the sink roll and the support roll hardly flows into the snout. FIGS. 1A and 1B are side views showing the periphery of a plating bath for explaining the method of the present invention. FIG. 1A shows the distance between a position where a steel strip first contacts a sink roll in the plating bath and a snout outlet. The figure which shows the approximate bath flow at the time of arrange | positioning a sink roll and a snout so that L2 (it is only hereafter described as the space | interval of a sink roll and a snout) becomes 1100-1500 mm, (b) is an enlarged view of a snout part, (C) is a figure which shows the bath flow in the AA cross section of (b) figure. As shown in FIG. 1A, by setting the distance L2 between the sink roll and the snout from 1100 mm to 1500 mm, the flow of the bath generated immediately above the sink roll due to the rotation of the sink roll and the support roll can be efficiently reduced. Can be introduced in the direction opposite to the threading direction. Therefore, as shown in FIG. 2B, the barrier B can be formed at the boundary between the ascending flow 19 and the ascending flow in the sink roll direction immediately near the steel strip. The barrier thus formed prevents suspended matter present in the snout from adhering to the sink roll contact surface. The preferred range is 1300
１４1450 mm. If the interval is below or above this range, the flow of molten metal into the snout decreases (the amount of escape to the snout increases) and the barrier becomes incomplete,
As described below, it becomes difficult to collect the suspended matter in a part of the snout. The stream 21 immediately above the sink roll enters into the snout and rises, and most of the stream 21 flows away from the steel sheet on the bath surface layer on the front side of the steel strip, and as shown in FIG. It moves around the both edges of the steel strip and moves to the back side of the steel strip to gather and form a suspended matter pool 16 '. Therefore, it is easy to catch and discharge the liquid with a pump or the like. In addition, there is no diffusion of suspended matter due to undulation when the suction port moves when the pump is used. However, in this state, a floating substance adheres to the back side of the steel strip. In order to prevent this, FIG.
Partition plate 1 partially immersed in plating bath as shown in (b)
5 is provided so as to face the back side of the steel strip, so that contact of the floating material on the hot-dip plating bath surface with the back surface of the steel sheet can be prevented. Although the size and shape of the partition plate are not particularly limited, the width of the partition plate is 40 to 50 slightly longer than the width of the steel strip.
mm is preferable. The material is preferably a material having heat resistance such as ceramics that does not melt at the plating bath temperature and poor wettability with the plating bath. Further, when the partition plate is partially immersed in the plating bath, the movement of suspended matter can be prevented well, and the non-immersed portion is desirably about 50 to 100 mm. FIG. 4 is a view showing an example of the shape of the partition plate.
Numeral 4 indicates a snout cross section, 1 indicates a steel strip cross section, and 15 indicates a cross section of a partition plate. (A) is a flat partition plate, (b) is a U-shaped cross section, (c) is a C-shaped cross section, and (d) is a C-shaped cross section. As a result of the test, any of these shapes could prevent the suspended matter from contacting the steel strip surface. The hot-dip plating includes plating of zinc, tin, aluminum, copper and the like. However, the method of the present invention is effective regardless of the type of plating bath, and the type of plating metal is not limited. EXAMPLES A carbon steel strip having a thickness of 0.6 mm and a width of 1200 mm was hot-dip galvanized under the following conditions. Bath composition: Al 0.10 to 0.12
%, Other Zn and impurity bath temperature: 460 ± 10 ° C. Sink roll diameter: 750φ. Passing speed: 100 m / min Alloying treatment temperature: 520 ° C. The distance between the sink roll and the snout is adjusted by adjusting the depth of immersion of the snout in the bath, 1000 mm as a conventional example, 1100 mm, 1200 mm, 1300 mm as an example of the present invention.
Various changes were made to 1400 mm, 1450 mm, 1500 mm, and 1150 mm and 1550 mm as comparative examples. In the examples of the present invention and the comparative examples, a flat plate-shaped ceramic shown in FIG. 4A was used as a partition plate, and in the conventional example, no partition plate was used. Ten hours after the start of plating, the bath surface on the front side of the steel strip in the snout, that is, a, b, and b in FIG.
The percentage of the area occupied by suspended matters in c and d was measured by image processing in video shooting. The result is shown in FIG. From the steel strip after the alloying treatment, a width of 1200 mm
20 samples each of 1000 mm length are collected,
The number of unplated portions on both surfaces was examined by visual inspection.
Further, the number of suspended matters remaining in the plating film was polished with a grindstone and visually examined. Table 1 shows the results. [Table 1] As is clear from FIG. 5 and Table 1, when the distance between the sink roll and the snout is within the range specified in the present invention, the area ratio of the suspended matter on the bath surface is extremely small as 5% or less. If it is less than 1100 mm and more than 1500 mm, the amount of suspended matter increases. Particularly, in the case of the prior art example, the number is as large as 70. As shown in Table 2, when the distance between the sink roll and the snout is within the range specified in the present invention, the number of unplated portions and the number of remaining suspended matter are extremely small on both the front and back surfaces of the steel strip. I understand. According to the method of the present invention, by slightly changing the arrangement of the conventional equipment, plating defects due to suspended matter in the snout in continuous hot-dipping can be drastically reduced, and a plated steel sheet having good surface quality can be obtained. It can be easily manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a side view around a conventional plating bath. FIG. 2 is a side view around a plating bath for explaining the method of the present invention. FIG. 3 is a side view of a horizontal plating line of the Sendzimir method. FIG. 4 is a diagram showing an example of the shape of a partition plate. FIG. 5 is a diagram showing a relationship between an interval between a sink roll and a snout and an area ratio occupied by suspended matters. [Description of Signs] 1 steel plate 11 plating bath 12 sink roll 13 support roll 14 snout 15 partition plate 16 float

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims 1] The distance from the position where the steel strip passing through the snout and first contacting the sink roll in the plating bath to the snout outlet passes through the continuous hot-dipping furnace. 150
A sink roll and a snout are installed so as to be 0 mm, and a partition plate for preventing floating material on a hot-dip plating bath surface in the snout from coming into contact with the steel strip is attached to a sink roll of the steel strip in the snout. A continuous hot-dip plating method characterized in that plating is performed by providing a surface facing a non-contact surface.