WO2025191998A1 - Apparatus for manufacturing hot-dip galvanized steel sheet and method for manufacturing same - Google Patents

Abstract

Provided is a technology for manufacturing a hot-dip galvanized steel sheet with few surface defects. This apparatus for manufacturing a hot-dip galvanized steel sheet comprises: a molten zinc container for storing molten zinc; a snout which is provided so that an end part thereof is immersed in a molten zinc bath in the molten zinc container and which supplies a steel sheet into the molten zinc bath; a sink roll which is provided so as to be immersed in the molten zinc bath in the molten zinc container and on which the steel sheet is rolled up; a support roll which is provided above the sink roll so as to be immersed in the molten zinc bath and which is disposed so as to be in contact with one side or both sides of the steel sheet on the side pulled up; and a flow regulating plate which is provided above the sink roll and which has a flow regulating mechanism for regulating a molten zinc flow in the molten zinc container, wherein the flow regulating plate is installed so that the shortest distance from the lower end position to the sink roll surface falls within the range of more than 0 mm and 50 mm or less, and the lower end position is located in the range from the end part of the sink roll on the snout side in the horizontal direction to the center axis of the sink roll in the top view.

Description

Manufacturing apparatus for hot-dip galvanized steel sheet and manufacturing method thereof

The present invention relates to a manufacturing apparatus for hot-dip galvanized steel sheets, and more specifically to a manufacturing apparatus for hot-dip galvanized steel sheets that produces plated steel sheets with fewer surface defects, and to a manufacturing method for hot-dip galvanized steel sheets using the apparatus.

Hot-dip metal coated steel sheets, especially 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, whether painted or not. Since the appearance of unpainted steel sheets, as well as painted steel sheets, is strongly affected by surface defects such as scratches and foreign matter, it is important that these surface defects are absent.

Hot-dip galvanized steel sheets are generally produced in continuous hot-dip galvanizing equipment by immersing the steel sheet in a molten zinc bath (hereinafter referred to as the "bath" or "bath"). The bath contains a sink roll and support rolls, which come into contact with the steel sheet. The steel sheet to be plated is annealed in an annealing furnace, then immersed in the molten zinc bath via a snout. The coating amount is controlled using a gas wiping device, and the plating process is carried out. During this process, oxides of the molten zinc generated in the snout can adhere to the material, resulting in defects. These defects are referred to as "ash defects." To prevent these ash defects, it is necessary to control the flow of the molten zinc bath and prevent zinc oxides from adhering to the material. Patent Document 1, for example, is an example of a technology for controlling the flow of the molten zinc bath. Patent Document 1 discloses a device installed above the sink roll, which controls the flow of molten zinc in the molten zinc vessel and prevents dross from being entrained by the support rolls.

JP 2013-224457 A

However, the above-mentioned conventional techniques have the following problems to be solved.
That is, the technology described in Patent Document 1 controls the flow of bottom dross, which has a higher density than molten zinc and is more likely to settle, on the surface of the coating bath. Here, the flow generated by the collision between the accompanying flow of the steel sheet entering the molten zinc bath and the accompanying flow caused by the rotation of the sink roll is called the "impingement flow." Patent Document 1 does not take into consideration ash generated in the snout and carried by the impingement flow. In particular, the distance between the lower end of the straightening plate and the surface of the sink roll is large, and the impingement flow cannot be prevented from flowing toward the support roll relative to the straightening plate. This results in the problem of ash coming into contact with the steel sheet being pulled up, resulting in surface defects.

The present invention was made to solve the above problems, and its purpose is to provide an apparatus for manufacturing hot-dip galvanized steel sheets with fewer surface defects and reduced ash defect generation. Furthermore, it also aims to propose a method for manufacturing hot-dip galvanized steel sheets using this manufacturing apparatus.

The inventors conducted a flow analysis of a manufacturing device for hot-dip galvanized steel sheets. As a result, they discovered that the flow within the molten zinc bath changes depending on the relative positions of the flow straighteners and sink rolls. In particular, they discovered that it is possible to suppress the flow that causes zinc oxide generated within the snout to adhere to the steel strip surface.

The manufacturing apparatus for hot-dip galvanized steel sheets of the present invention, which advantageously solves the above-mentioned problems, is a manufacturing apparatus for hot-dip galvanized steel sheets in which a steel sheet to be plated is immersed in a molten zinc bath and molten zinc is deposited on the surface to form a coating layer, and is equipped with a molten zinc container for storing molten zinc, a snout which is arranged so that its end is immersed in the molten zinc bath in the molten zinc container and supplies the steel sheet to be plated into the molten zinc bath, a sink roll which is arranged so that it is immersed in the molten zinc bath in the molten zinc container and around which the steel sheet to be plated is wrapped, a support roll which is arranged above the sink roll so that it is immersed in the molten zinc bath and is positioned in contact with one or both sides of the steel sheet to be plated on the pull-up side, and a straightening plate which is arranged above the sink roll and has a straightening mechanism for controlling the flow of molten zinc in the molten zinc container, and is characterized in that the shortest distance from the lower end position to the sink roll surface is in the range of more than 0 mm and not more than 50 mm, and the lower end position of the straightening plate is installed so that it is in the range from the horizontal end of the snout side of the sink roll to the central axis of the sink roll when viewed from above.

The manufacturing apparatus for hot-dip galvanized steel sheet according to the present invention comprises:
(a) the width of the straightening plate is equal to or greater than the width of the steel sheet to be plated;
(b) the rectifying plate has a plate thickness of 9 mm or more and has ribs;
would be a more preferable solution.

The method for manufacturing hot-dip galvanized steel sheet according to the present invention, which advantageously solves the above-mentioned problems, is characterized by including the steps of immersing a steel sheet to be plated in a hot-dip galvanizing bath using any of the hot-dip galvanized steel sheet manufacturing apparatuses described above, and depositing molten zinc on the surface of the steel sheet to form a coating layer.

According to the present invention, the lower end of the straightening plate installed above the sink roll of the molten zinc bath is positioned closer to the sink roll, thereby suppressing the flow that causes zinc oxide generated in the snout to adhere to the surface of the steel strip at the exit of the molten zinc bath. As a result, zinc oxide can be prevented from adhering to the steel strip, and the incidence of ash defects can be significantly reduced, improving productivity and providing industrial usefulness.

1 is a schematic conceptual diagram showing a manufacturing apparatus for a hot-dip galvanized steel sheet according to an embodiment of the present invention. FIG. 2 is a schematic conceptual diagram illustrating the flow of a molten zinc bath according to the embodiment. FIG. 1 is a schematic conceptual diagram illustrating the flow of a molten zinc bath on a conventional flow straightening ramp.

The following describes in detail the embodiments of the present invention. Note that the drawings are schematic and may differ from the actual product. Furthermore, the following embodiments exemplify devices and methods that embody the technical concepts of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concepts of the present invention can be modified in various ways within the technical scope set forth in the claims.

FIG. 1 shows a schematic diagram of a hot-dip galvanized steel sheet manufacturing apparatus according to one embodiment of the present invention. The hot-dip galvanized steel sheet manufacturing apparatus 10 comprises a molten zinc container 1 for storing molten zinc Z, a snout 2 with its end immersed in the molten zinc bath, and a sink roll 4 and support rolls 5 and 6 immersed in the molten zinc bath. In this embodiment, the apparatus is equipped with a straightening plate 7 having a straightening mechanism for controlling the flow of molten zinc within the molten zinc container 1. The hot-dip galvanized steel sheet manufacturing apparatus 10 may also be equipped with a coil payoff device, payoff reel, annealing furnace, gas wiping device, alloying furnace, coil winding device, and other accessories not shown.

The steel sheet 3 to be plated is continuously supplied from inside the snout 2 to the molten zinc bath stored in the molten zinc vessel 1. The steel sheet 3 to be plated is wrapped around the sink roll 4, changing its course from the approach direction to the pull-up direction. In the example of Figure 1, the back side support roll 5, which is on the inward winding side of the sink roll 4 of the steel sheet 3 to be plated being pulled up, and the front side support roll 6, which is on the outside, are arranged to contact the steel sheet 3 to be plated. The back side of the steel sheet to be plated is referred to as side B, and the front side is also referred to as side F. The support rolls 5 and 6 are arranged above the sink roll 4, and may be on one side of the steel sheet 3 to be plated.

In this embodiment, the current plate 7 installed above the sink roll 4 is characterized in the following manner.
1) The shortest distance a from the lower end position of the straightening plate 7 to the surface of the sink roll 4 is in the range of more than 0 mm and not more than 50 mm.
2) When viewed from above, the lower end position of the straightening plate 7 is in the range from the horizontal end of the snout side of the sink roll to the central axis CL of the sink roll 4. In other words, the distance b between the horizontal lower end position of the straightening plate 7 and the horizontal end of the snout side of the sink roll 4 is greater than or equal to 0 and less than the radius of the sink roll 4, with the direction toward the central axis CL being positive.

The inventors observed the actual equipment and conducted flow simulations because conventional straightening vanes such as those described in Patent Document 1 were unable to suppress ash defects. As a result, they discovered that the lower end position of the straightening vane 7 is related to the suppression of ash defects. The mechanism behind this is thought to be as follows: Figure 2 is a schematic diagram conceptually showing the flow of the molten zinc bath with the straightening vane 7 positioned according to this embodiment. Figure 3 is a schematic diagram conceptually showing the flow of the molten zinc bath with the conventional straightening vane position, where ash defects occur.

Zinc oxide generated within the snout 2 enters the molten zinc bath on the accompanying flow of the steel sheet 3 to be plated as it enters the bath. This zinc oxide has a lower specific gravity than molten zinc, so it floats in the molten zinc bath and moves with the flow. It is thought that the zinc oxide then adheres to the steel sheet to be plated and is pressed against the roll, becoming an ash defect. As shown in Figures 2 and 3, the flow accompanying the steel sheet 3 to be plated supplied from the snout 2 collides with the flow accompanying it on the upper surface as the sink roll 4 rotates, forming a collision flow that sprays upward. This collision flow contains zinc oxide.

As shown in Figure 3, the lower end of the conventional straightening plate 7A is in contact with the impinging flow, distributing the flow of the molten zinc bath toward the support roll. In this embodiment, by installing the straightening plate 7 in the above range, as shown in Figure 2, the impinging flow is not distributed toward the support roll, and zinc oxide can be floated to the bath surface 8 between the snout 2 and the straightening plate 7 on an extension line. This reduces ash defects.

The conditions for the lower end position of the straightening plate 7 satisfied by this embodiment must be met throughout the entire period of operation related to the production of galvanized steel sheets. For example, the sink roll 4 thins during operation, reducing its radius by approximately 15 mm. Furthermore, because the straightening plate 7 is installed close to the sink roll 4 so as not to come into contact with it, it is constantly under load due to the accompanying flow of molten zinc, which can cause it to deform approximately 40 mm toward the snout. There are many limitations to installing a moving device for the straightening plate 7. In other words, there is a lot of ancillary equipment around the molten zinc vessel 1 of the hot-dip galvanized steel sheet manufacturing apparatus 10, making installation of a moving device for the straightening plate 7 physically difficult and requiring excessive investment. Therefore, taking into account the thinning of the sink roll 4 and deformation of the straightening plate 7, it is preferable to move the lower end position of the straightening plate 7, which is initially installed, horizontally by a predetermined amount toward the center axis CL of the sink roll 4. Designing the straightening plate 7 in this manner enables the production of maintenance-free galvanized steel sheets with few surface defects.

The material for the straightening plate 7 is preferably SUS316L. The thickness of the straightening plate 7 is preferably 9 mm or more from the viewpoint of preventing deformation. On the other hand, an excessively heavy straightening plate 7 may make installation difficult, so a thickness of 15 mm or less is preferable. From the viewpoint of preventing deformation, it is preferable for the straightening plate 7 to have reinforcing ribs extending in the width direction of the plated steel sheet 3.

Furthermore, the thickness of the steel sheet 3 to be plated is not particularly limited, but is, for example, 0.4 to 3.5 mm. The thinner the thickness of the steel sheet 3 to be plated, the faster the passing speed of the steel sheet 3 to be plated and the faster the speed of the impinging flow, making it more likely that ash defects will occur. Therefore, the effects of the present invention are more easily achieved when the thickness of the steel sheet 3 to be plated is 0.4 to 1.2 mm.

Example 1
Using a manufacturing apparatus for hot-dip galvanized steel sheets configured according to Figure 1, the rate of ash defect occurrence on the steel sheet surface was investigated by changing the lower end position of the straightening plate. The results are summarized in Table 1. The rate of ash defect occurrence was calculated using flow analysis software to calculate the trajectory of the zinc oxide simulant particles, and the rate was calculated as the percentage of particles that reached the steel sheet to be plated relative to the total number of particles. The results in Table 1 show that the invention examples were able to significantly reduce the rate of ash defect occurrence compared to the comparative examples.

Example 2
Next, the deformation of the plate, i.e., the amount of warpage, was investigated for each plate thickness under the arrangement of the straightening plates under condition No. 3 in Table 1. The amount of warpage of the plated steel plate after passing through 20,000 tons is summarized in Table 2. By setting the plate thickness to 9.0 mm, the deformation of the straightening plates was significantly reduced.

In this specification, the unit of mass "t" is 1000 kg.

10 (hot-dip galvanized steel sheet) manufacturing apparatus 1 molten zinc container 2 snout 3 (to be plated) steel sheet (material to be plated)
4 Sink roll 5 (back side) support roll 6 (front side) support roll 7 Straightening plate 7A (conventional) Straightening plate 8 Bath surface CL (of sink roll) Central axis Z Molten zinc

Claims (5)

A manufacturing apparatus for hot-dip galvanized steel sheets, which immerses a steel sheet to be plated in a molten zinc bath and deposits molten zinc on the surface of the steel sheet to form a coating layer,
a molten zinc container for storing molten zinc;
a snout having an end immersed in the molten zinc bath in the molten zinc container, for supplying a steel sheet to be plated into the molten zinc bath;
a sink roll that is provided so as to be immersed in the molten zinc bath in the molten zinc container and around which the steel sheet to be plated is wound;
a support roll provided above the sink roll so as to be immersed in the molten zinc bath and placed in contact with one or both surfaces of the steel sheet to be plated on the lifting side;
A flow straightening plate provided above the sink roll and having a flow straightening mechanism for controlling the flow of molten zinc in the molten zinc container;
Equipped with
The straightening plate is installed in a manufacturing apparatus for hot-dip galvanized steel sheets so that the shortest distance from its lower end position to the surface of the sink roll is in the range of more than 0 mm and not more than 50 mm, and its lower end position, when viewed from above, is in the range from the horizontal end of the snout side of the sink roll to the central axis of the sink roll. The hot-dip galvanized steel sheet manufacturing apparatus of claim 1, wherein the width of the straightening plate is equal to or greater than the width of the steel sheet to be plated. The hot-dip galvanized steel sheet manufacturing apparatus of claim 1, wherein the straightening plate has a thickness of 9 mm or more and has ribs. The hot-dip galvanized steel sheet manufacturing apparatus of claim 2, wherein the straightening plate has a thickness of 9 mm or more and has ribs. A method for producing hot-dip galvanized steel sheet, comprising the steps of immersing a steel sheet to be plated in a hot-dip galvanizing bath using the hot-dip galvanized steel sheet manufacturing apparatus described in any one of claims 1 to 4, and depositing molten zinc on the surface of the steel sheet to form a coating layer.