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

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

[0001] The present invention relates to a manufacturing apparatus for hot-dip galvanized steel sheets, and more particularly to a manufacturing apparatus for hot-dip galvanized steel sheets that manufactures plated steel sheets with fewer surface defects, and to a manufacturing method for hot-dip galvanized steel sheets using the apparatus.

[0002] Hot-dip metal-coated steel sheets, particularly hot-dip galvanized steel sheets, are widely used in fields such as building materials, automobiles, and home appliances. In these applications, hot-dip galvanized steel sheets are required to have excellent appearance regardless of whether they are painted or not. Here, the appearance of unpainted steel sheets as well as that of painted steel sheets are strongly affected by surface defects such as scratches and foreign matter adhesion, so it is important that these surface defects are absent.

[0003] Hot-dip galvanized steel sheets are generally produced by immersing a steel sheet in a molten zinc bath (hereinafter sometimes referred to as a coating bath or bath) in a continuous hot-dip galvanizing facility. A sink roll and a support roll are present in the coating bath and are in contact with the steel sheet. The steel sheet to be coated is annealed in an annealing furnace, and then immersed in the molten zinc bath via a snout. The coating amount is controlled by a gas wiping device to perform the coating process. During this process, oxides of the molten zinc generated in the snout adhere to the material to be coated, resulting in defects. These defects are hereinafter referred to as ash defects. To prevent these ash defects, it is necessary to control the flow of the molten zinc bath and prevent the flow of zinc oxides from adhering to the material to be coated. Patent Document 1, for example, is an example of a technology for controlling the flow of the molten zinc bath. Patent Document 1 discloses an apparatus that is installed above a sink roll and has a rectifying mechanism that controls the flow of molten zinc in a molten zinc vessel, and that can prevent dross entrainment in the support rolls.

[0004] JP 2013-224457 A

[0005] However, the above-mentioned conventional technology has the following problems that must be solved. Specifically, the technology described in Patent Document 1 controls the flow of so-called 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. As a result, ash comes into contact with the steel sheet being pulled up, causing surface defects.

[0006] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an apparatus for producing a hot-dip galvanized steel sheet with fewer surface defects by reducing the occurrence of ash defects, and further to propose a method for producing a hot-dip galvanized steel sheet using the production apparatus.

[0007] The inventors conducted a flow analysis of a manufacturing apparatus for hot-dip galvanized steel sheets. As a result, they found that the flow in the molten zinc bath changes depending on the positional relationship between the flow straightening vanes and the sink roll. In particular, they found that it is possible to suppress the flow that causes zinc oxide generated in the snout to adhere to the steel strip surface.

[0008] and a rectifying plate having a rectifying mechanism for controlling the flow of molten zinc in the molten zinc container. The rectifying plate is characterized in that the shortest distance from its lower end to the surface of the sink roll is between 0 mm and 50 mm, and the lower end of the rectifying plate is located between the horizontal end of the sink roll on the snout side and the central axis of the sink roll when viewed from above.

[0009] In addition, in the manufacturing apparatus for hot-dip galvanized steel sheet according to the present invention, (a) the width of the straightening plate is equal to or greater than the width of the steel sheet to be plated, and (b) the straightening plate has a plate thickness of 9 mm or more and has ribs, etc., which are more preferable solutions.

[0010] The method for producing a hot-dip galvanized steel sheet according to the present invention, which advantageously solves the above-mentioned problems, is characterized by including a step of immersing a steel sheet to be plated in a hot-dip galvanizing bath using any of the above-mentioned hot-dip galvanized steel sheet manufacturing apparatuses, and depositing molten zinc on the surface of the steel sheet to form a coating layer.

[0011] According to the present invention, the lower end of the current plate provided above the sink roll of the molten zinc bath is positioned close 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, adhesion of zinc oxide to the steel strip can be prevented, and the incidence of ash defects can be significantly reduced, improving productivity and being industrially useful.

[0012] 1 is a schematic conceptual diagram illustrating 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 a flow of a molten zinc bath according to the embodiment; FIG. 3 is a schematic conceptual diagram illustrating a flow of a molten zinc bath in a conventional flow straightening device;

[0013] The following describes in detail embodiments of the present invention. Note that the drawings are schematic and may differ from the actual embodiments. Furthermore, the following embodiments exemplify devices and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

[0014] Figure 1 shows a schematic diagram of an apparatus for manufacturing a hot-dip galvanized steel sheet according to one embodiment of the present invention. The apparatus 10 for manufacturing a hot-dip galvanized steel sheet includes a molten zinc container 1 for storing molten zinc Z, a snout 2 provided so that its end is 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 in the molten zinc container 1. The apparatus 10 for manufacturing a hot-dip galvanized steel sheet may be equipped with a coil payoff reel, an annealing furnace, a gas wiping device, an alloying furnace, a coil winding device, and the like, all of which are not shown.

[0015] 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 container 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, a 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 a front-side support roll 6, which is on the outer side, are arranged so as 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.

[0016] In this embodiment, the straightening plate 7 installed above the sink roll 4 is characterized by the following features: 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 0 and less than the radius of the sink roll 4, with the direction toward the central axis CL being positive.

[0017] The inventors conducted observations of an actual machine and flow simulations because the conventional straightening vanes 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 in the arrangement of the straightening vane 7 according to this embodiment. Figure 3 is a schematic diagram conceptually showing the flow of the molten zinc bath in the conventional straightening vane arrangement in which ash defects occur.

[0018] Zinc oxide generated in the snout 2 enters the molten zinc bath on the accompanying flow of the steel sheet 3 to be plated entering the molten zinc bath. The zinc oxide has a smaller specific gravity than the molten zinc, so it floats in the molten zinc bath and moves along 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, resulting in ash defects. 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 the upper surface of the sink roll 4 as it rotates, forming a collision flow that sprays upward. This collision flow contains zinc oxide.

[0019] As shown in Figure 3, the lower end of the conventional straightening vane 7A is in contact with the impinging flow, and the flow of the molten zinc bath is distributed toward the support roll. In this embodiment, by installing the straightening vane 7 within 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 vane 7 on an extension line. This reduces ash defects.

[0020] The condition for the lower end position of the current plate 7 satisfied by this embodiment must be satisfied throughout the entire operation period for producing galvanized steel sheets. For example, the sink roll 4 undergoes thinning during operation, resulting in a reduction in radius of approximately 15 mm. Furthermore, because the current plate 7 is installed close to the sink roll 4 without contacting 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 current plate 7. In other words, there are many ancillary facilities around the molten zinc vessel 1 of the hot-dip galvanized steel sheet production apparatus 10, making installation of a moving device for the current plate 7 physically difficult and requiring excessive investment. Therefore, taking into account the thinning of the sink roll 4 and the deformation of the current plate 7, it is preferable to move the lower end position of the current plate 7, which is initially installed, horizontally by a predetermined amount toward the center axis CL of the sink roll 4. Designing the current plate 7 in this manner enables maintenance-free production of galvanized steel sheets with few surface defects.

[0021] The material of the rectifying plate 7 is preferably SUS316L. The thickness of the rectifying plate 7 is preferably 9 mm or more from the viewpoint of preventing deformation. On the other hand, an excessively heavy rectifying plate 7 may make installation difficult, so the thickness is preferably 15 mm or less. From the viewpoint of preventing deformation, it is preferable that the rectifying plate 7 has reinforcing ribs extending in the width direction of the plated steel sheet 3.

[0022] 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 ash defects more likely to occur. Therefore, when the thickness of the steel sheet 3 to be plated is 0.4 to 1.2 mm, the effects of the present invention are more easily exhibited.

[0023] Example 1 Using a hot-dip galvanized steel sheet manufacturing apparatus 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 flow straightening plate. The results are summarized in Table 1. The rate of ash defect occurrence was determined by calculating the trajectories of zinc oxide simulant particles using flow analysis software and defining it as the ratio of particles that reach the steel sheet to all particles. The results in Table 1 show that the inventive example can significantly reduce the rate of ash defect occurrence compared to the comparative example.

[0024]

[0025] (Example 2) Next, the deformation of the plate, i.e., the amount of warpage, was investigated for each plate thickness using the straightening plate arrangement of 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 plate can be significantly reduced.

[0026]

[0027] In this specification, the unit of mass "t" is 1000 kg.

[0028] 10 (hot-dip galvanized steel sheet) manufacturing device 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 (sink roll) central axis Z molten zinc

Claims

1. 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, comprising: 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 which 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 wound; a support roll which is arranged above the sink roll so that it is immersed in the molten zinc bath and is placed in contact with one or both sides of the steel sheet to be plated on the pull-up side; and a rectifying plate which is arranged above the sink roll and has a rectifying mechanism for controlling the flow of molten zinc in the molten zinc container, wherein the shortest distance from the lower end position of the rectifying plate to the surface of the sink roll is in the range of more than 0 mm but not more than 50 mm, and the lower end position of the rectifying plate is installed so that, when viewed from above, the shortest distance from the lower end position to the surface of the sink roll is in the range from the horizontal end of the snout side of the sink roll to the central axis of the sink roll.

2. The apparatus for manufacturing hot-dip galvanized steel sheets according to claim 1, wherein the width of the straightening plate is equal to or greater than the width of the steel sheet to be plated.

3. The manufacturing apparatus for hot-dip galvanized steel sheet according to claim 1, wherein the straightening plate has a thickness of 9 mm or more and has ribs.

4. The manufacturing apparatus for hot-dip galvanized steel sheet according to claim 2, wherein the straightening plate has a thickness of 9 mm or more and has ribs.

5. A method for producing hot-dip galvanized steel sheets, 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 according to any one of claims 1 to 4, and depositing molten zinc on the surface of the steel sheet to form a coating layer.