Manufacturing method of hot-dip galvanized steel strip

Abstract

To provide a method for manufacturing a hot-dip metal coated steel strip, capable of preventing clog of a wiping nozzle from occurring only by changing a pressure of a wiping gas without moving the wiping nozzle and preventing a poor appearance of the coated steel strip from occurring.SOLUTION: A method for manufacturing a hot-dip metal coated steel strip comprises: continuously conveying a steel strip having a tail end of a preceding steel strip and a head end of a following steel strip connected by a weld in a longitudinal direction; dipping the steel strip into a molten metal bath while tracking a position of the weld on a conveying path of the steel strip to pull the steel strip from the molten metal bath; and blowing a wiping gas having a predetermined pressure on both surfaces of the steel strip from a wiping nozzle to remove an excessive deposition of molten metal from the steel strip. The wiping nozzle blows the wiping gas having a pressure higher than the predetermined pressure on the weld.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a method for producing hot-dip galvanized steel strip. [Background technology]

[0002] In a continuous hot-dip plating apparatus for continuously plating a steel strip, the steel strip is immersed in a molten metal bath to deposit molten metal on the surface of the steel strip. After the steel strip is pulled out of the molten metal bath, a wiping gas is sprayed onto the surface of the steel strip with the molten metal deposited thereon to remove excess molten metal.

[0003] When removing excess molten metal using wiping gas, it is necessary to spray gas uniformly across the width of the steel strip without disrupting the gas flow in order to ensure a uniform amount of molten metal adheres across the width of the steel strip.

[0004] In the continuous hot dip plating apparatus, the tail end of the steel strip discharged first (leading steel strip) is welded to the tip end of the steel strip discharged after the preceding steel strip (trailing steel strip). The location where the preceding material and the trailing material are connected by welding is called a weld point. In the vicinity of the weld point (weld part), in order to track the weld point, a puncher hole may be provided near the center of the width of the steel strip or a notch (notcher part) may be provided at the end of the width. In addition, the weld part is an unsteady part where the thickness and width of the steel strip change. In such a weld part, the conveying state becomes unstable, the flow of the wiping gas becomes turbulent, and the molten metal may splash toward the wiping nozzle. The molten metal splashed toward the wiping nozzle may adhere to the wiping gas outlet (nozzle lip part) of the wiping nozzle. The adhesion of the molten metal to the nozzle lip part is called clogging. This clogging disrupts the flow of the wiping gas, reducing the ability to remove excess molten metal uniformly across the width of the steel strip, resulting in linear areas of excess molten metal remaining on the strip in the longitudinal direction, resulting in a defective appearance. This defective appearance is called a linear overcoat.

[0005] As a technique for suppressing linear overcoat, for example, Patent Document 1 discloses a hot-dip plating method in which a metal steel strip having a weld in the longitudinal direction is continuously transported, and excess molten metal is gas-wiped from the surface of the metal strip after it has been immersed in a molten metal bath and then pulled out, characterized in that the vibrating nozzle is vibrated based on the tracking position of the weld.

[0006] Patent document 2 also discloses a method for manufacturing a hot-dip metal plated steel strip, which is characterized by including a step of controlling at least one of the distance between the wiping nozzle and the steel sheet, the angle of the wiping nozzle, and the gas pressure so that the Weber number, which is an evaluation index for the scattering of molten metal from the surface of the steel strip, is below a preset threshold value within a predetermined range in the longitudinal direction of the steel strip, including the position of the weld point of the steel strip. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2007-231350 A [Patent Document 2] JP 2019-167553 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, in the method described in Patent Document 1, the strain-induced actuator is expensive and has a short lifespan due to the high temperature and harsh environment, so the strain-induced actuator needs to be frequently replaced. In addition, in the method described in Patent Document 2, the wiping nozzle is retracted from a predetermined position before the welding point passes the wiping position, and the wiping nozzle is returned after the welding point passes the wiping position. That is, the method described in Patent Document 2 controls the distance between the wiping nozzle and the steel sheet so that the Weber number is equal to or less than a preset threshold. When controlling the pressure of the wiping gas, it is only suggested that the pressure be reduced before and after the passage of the welding point in order to make the Weber number equal to or less than a preset threshold. In controlling the movement and pressure of the wiping nozzle, the response speed is low, so that the area where the desired plating adhesion amount is not obtained becomes large, resulting in a decrease in yield.

[0009] The present invention has been made in consideration of the above-mentioned problems, and an object of the present invention is to provide a method for producing molten metal plated steel strip that can suppress the occurrence of clogging of the wiping nozzle and thereby suppress the occurrence of poor appearance of the plated steel strip by simply changing the pressure of the wiping gas without moving the wiping nozzle. [Means for solving the problem]

[0010] At the welded portion, the conveying state becomes unstable, the flow of the wiping gas becomes disturbed, and the molten metal may scatter from the puncher hole, the notcher portion, etc. toward the nozzle lip portion. The inventors have come up with the idea of ​​preventing clogging by increasing the pressure of the wiping gas. Conventionally, when controlling the pressure of the wiping gas to prevent clogging, the pressure of the wiping gas is reduced. However, if the pressure of the wiping gas is high, the energy of the wiping gas discharged from the nozzle lip portion becomes greater than the energy of the molten metal toward the nozzle lip portion. Therefore, the molten metal scattered toward the nozzle lip portion of the wiping nozzle does not reach the nozzle lip portion, and the adhesion of the molten metal to the nozzle lip portion can be prevented. On the other hand, if the pressure of the wiping gas is low, the energy of the wiping gas discharged from the nozzle lip portion becomes smaller than the energy of the molten metal toward the nozzle lip portion. Therefore, the molten metal scattered toward the nozzle lip portion reaches the nozzle lip portion, and the molten metal adheres to the nozzle lip portion. Therefore, in the conventional method of preventing clogging by reducing the pressure of the wiping gas, the distance between the nozzle lip and the steel strip had to be increased.

[0011] The gist of the present invention, which has been completed based on the above findings, is as follows. [1] A method for producing a hot-dip metal plated steel strip according to one embodiment of the present invention includes: continuously transporting a steel strip, in which a tail end of a preceding steel strip and a head end of a following steel strip are connected by a weld, along its longitudinal direction; tracking the position of the weld on the transport path of the steel strip; immersing the steel strip in a molten metal bath and then lifting the steel strip from the molten metal bath; and then spraying a wiping gas having a predetermined pressure from a wiping nozzle onto both sides of the steel strip to remove excess molten metal from the steel strip. A pressure at which the molten metal scattered by the wiping gas does not reach the nozzle lip of the wiping nozzle. The above wiping gas is sprayed a step of transmitting a signal to an adjustment valve that adjusts the spray pressure of the wiping gas before the weld passes over the spray position of the wiping gas, to change the spray pressure of the wiping gas to a pressure at which the molten metal scattered by the spray of the wiping gas does not reach a nozzle lip of the wiping nozzle; and a step of transmitting a signal to the adjustment valve that changes the spray pressure to a low pressure lower than the spray pressure adjusted by the signal transmitted in the step, after the step and before the weld passes over the spray position of the wiping gas. . [ 2 ] Above [1 ]In the manufacturing method of the described hot-dip metal plated steel strip, it is preferable to calculate the amount of molten metal adhering to the surface of the steel strip after the wiping gas is sprayed, and based on the calculated value of the amount of molten metal adhering, cut the steel strip to remove the welded portion, and separate the steel strip into the leading steel strip and the trailing steel strip. [ 3 ] the above[ 2] In the described method for manufacturing a hot-dip galvanized steel strip, it is preferable that the calculated adhesion weight is calculated based on the pressure of the wiping gas, the conveying speed of the steel strip, the distance between the wiping nozzle and the steel strip, and the height of the spraying position of the wiping gas from the bath surface of the molten metal bath. Effect of the Invention

[0012] According to the present invention, by simply changing the pressure of the wiping gas without moving the wiping nozzle, it is possible to suppress the occurrence of clogging of the wiping nozzle and thereby suppress the occurrence of defective appearance of the plated steel strip. [Brief description of the drawings]

[0013] [Figure 1] 1 is a diagram showing an example of a continuous hot-dip metal coating apparatus to which a method for producing a hot-dip metal coated steel strip according to an embodiment of the present invention can be applied. [Diagram 2] 5 is a schematic diagram for explaining the mechanism by which molten metal adheres to a nozzle lip portion of a wiping nozzle. FIG. [Diagram 3] 1 is a schematic diagram for explaining the distance D between the tip of the nozzle lip and the steel strip, and the height H of the wiping gas blowing position from the bath surface of the molten metal bath. [Figure 4] 11 is a graph showing the timing of changing the wiping gas pressure and the change over time in the wiping gas pressure at the wiping gas blowing position. FIG. [Diagram 5] FIG. 5 is a graph showing the relationship between the distance from a welding point on a steel strip and the calculated coating weight of plating when wiping gas is blown onto the steel strip at the wiping gas pressure shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The dimensions and ratios of each component in the drawings do not represent the actual dimensions and ratios of each component.

[0015] <Outline of the continuous hot-dip metal coating equipment> First, prior to describing the method for producing a hot-dip metal-plated steel strip according to the present embodiment, a continuous hot-dip metal coating apparatus to which the method for producing a hot-dip metal-plated steel strip according to the present embodiment can be applied will be described with reference to Figures 1 and 2. Figure 1 is a diagram showing an example of a continuous hot-dip metal coating apparatus to which the method for producing a hot-dip metal-plated steel strip according to one embodiment of the present invention can be applied.

[0016] In the continuous hot-dip metal coating apparatus 1 of Fig. 1, a steel strip S is annealed in an annealing furnace (not shown) and then introduced into a molten metal bath 2. The steel strip S introduced into the molten metal bath 2 is turned upward by a sink roll 3 provided in the molten metal bath 2, and after warping is corrected by a support roll 4, it is pulled out of the molten metal bath 2 with the molten metal still attached thereto. Wiping gas is sprayed from the nozzle lip 51 of the wiping nozzle 5 toward both sides of the steel strip S, and the coating weight of the steel strip S coated with the molten metal is adjusted. The steel strip S with the coating weight adjusted is appropriately subjected to post-treatment and then cut by a cutter (not shown). At this time, the welded portion is removed.

[0017] The continuous hot-dip metal coating apparatus 1 is equipped with multiple weld detectors (not shown) on the transport path along which the steel strip is transported. The weld detector is composed of, for example, a light projector and a light receiver, and detects the weld by the light receiver receiving light from the light projector that passes through a puncher hole present in the weld of the steel strip. The continuous hot-dip metal coating apparatus 1 tracks the weld by integral calculation of the transport speed (line speed). Tracking of the weld is performed by performing tracking correction every time the weld passes through multiple weld detectors, and continues until the weld is wound onto a reel on the steel strip outlet side.

[0018] The pressure of the wiping gas is controlled by a coating weight control device 6. For example, a molten metal coating weight control device described in JP 2015-148006 A is used as the coating weight control device 6. The coating weight control device 6 includes a supply pipe (not shown) for supplying wiping gas to the wiping nozzle 5 and an adjustment valve (not shown) provided in the supply pipe, and controls these. In the coating weight control device 6, there is no turbulence in the flow of the wiping gas in the pipe located upstream of the adjustment valve, so there is no delay in the pressure change of the wiping gas. In addition, the adjustment valve responds at high speed. Since there is no delay in the pressure change of the wiping gas and the adjustment valve responds at high speed, the pressure of the wiping gas can be changed in a short time. As a result, the timing of changing the pressure of the wiping gas can be controlled with high accuracy.

[0019] <Method of manufacturing hot-dip metal-coated steel strip> Next, a method for producing a hot-dip metal coated steel strip according to this embodiment will be described. Fig. 2 is a schematic diagram for explaining the mechanism by which molten metal adheres to the nozzle lip of the wiping nozzle.

[0020] In the manufacturing method of the hot-dip metal plated steel strip according to the present embodiment, a steel strip S, in which the tail end of a leading steel strip and the front end of a trailing steel strip are connected by a weld, is continuously transported along its longitudinal direction, and the steel strip S is immersed in a hot-dip metal plating bath while tracking the position of the weld on the transport path of the steel strip S. The manufacturing method of the hot-dip metal plated steel strip includes pulling up the immersed steel strip S from the hot-dip metal plating bath, and then spraying a wiping gas having a predetermined pressure from a wiping nozzle 5 onto both sides of the steel strip S to remove excess molten metal from the steel strip S, and the wiping nozzle 5 sprays the wiping gas at a pressure higher than the predetermined pressure onto the weld.

[0021] As described above, by increasing the wiping gas pressure, as shown in FIG. 2(A), the molten metal scattered from the welded portion does not reach the nozzle lip portion 51 of the wiping nozzle 5, and adhesion of the molten metal to the nozzle lip portion 51 can be prevented. As a result, the occurrence of linear overcoating starting from the welded portion can be prevented. On the other hand, when the wiping gas pressure is low, as shown in FIG. 2(B), the molten metal scattered from the welded portion may adhere to the nozzle lip portion 51. When the molten metal adheres to the nozzle lip portion 51, linear overcoating occurs.

[0022] The above-mentioned predetermined pressure is a pressure of the wiping gas within a range in which a desired plating adhesion weight can be obtained, and is appropriately adjusted.

[0023] In the manufacturing method of the hot-dip metal-coated steel strip according to the present embodiment, when the wiping gas is sprayed onto the steel strip S, a signal for changing the spray pressure of the wiping gas to a pressure higher than a predetermined pressure is sent to an adjustment valve for adjusting the spray pressure before the wiping gas spray position passes the welded portion of the steel strip S. This allows the wiping gas at a high pressure to be sprayed onto the welded portion. As a result, clogging of the nozzle lip portion 51 can be prevented.

[0024] If the timing of sending the signal to change the wiping gas blowing pressure to a higher pressure than the predetermined pressure is significantly earlier than the time when the weld passes the wiping gas blowing position, the area with low plating adhesion may increase, and the yield may decrease. Therefore, it is preferable to control the timing of sending the increase signal to an extent that does not cause a significant decrease in the yield.

[0025] Furthermore, it is preferable to have a step of sending a signal to an adjusting valve to change the spray pressure from a high pressure to a low pressure lower than the high pressure after the above step and before the weld passes the wiping gas spraying position. By sending a signal to the adjusting valve to change the spray pressure from a high pressure to a low pressure before the weld passes the wiping gas spraying position, it is possible to reduce the area in the steel strip S where the coating weight is reduced due to an increase in the wiping gas pressure. As a result, it is possible to improve the yield.

[0026] The low pressure here refers to a wiping gas pressure at which a desired coating weight can be obtained. Therefore, when the coating weight is changed before and after the weld, the low pressure may be a pressure according to the coating weight of the succeeding material, and may be the same pressure before and after the weld, or may be different pressures.

[0027] In the manufacturing method of the hot-dip metal-coated steel strip according to the present embodiment, it is preferable to calculate the coating weight adhered to the surface of the steel strip S after the wiping gas is sprayed, and based on the calculated coating weight, the steel strip S is cut to remove the welded portion and separate it into a leading steel strip and a trailing steel strip. Figure 3 is a diagram showing a schematic view of the position of the wiping nozzle 5.

[0028] The calculated adhesion amount is preferably calculated based on the pressure of the wiping gas, the conveying speed of the steel strip S, the distance between the tip of the nozzle lip portion 51 and the steel strip S, and the height of the wiping gas blowing position from the bath surface of the molten metal bath 2. The height of the wiping gas blowing position from the bath surface of the molten metal bath 2 is the distance from the bath surface of the molten metal bath 2 to the center position of the nozzle lip portion 51 in the height direction.

[0029] The calculated adhesion amount is calculated, for example, based on the following formula (1). W = exp(y + p × lnP + v × lnV + d × lnD + h × lnH) (1) W: Calculated adhesion amount (g / m 2 ) P: Nozzle pressure (kPa) V: conveying speed of steel strip S (m / min) D: Distance between the tip of the nozzle lip 51 and the steel strip S (m) H: Height of wiping gas blowing position from the surface of the molten metal bath (m) y, p, v, d, h: coefficients

[0030] The actual coating weight can be determined from a correlation equation with the calculated coating weight, which is previously determined, and the calculated coating weight and the correlation equation. EXAMPLES

[0031] Next, examples of the present invention will be shown, but the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to the conditions used in the following examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0032] The pressure of the wiping gas sprayed on the welded portion was increased to investigate whether a linear overcoat occurred. Figure 4 is a graph showing the timing of changing the wiping gas pressure and the time change in the wiping gas pressure at the wiping gas spraying position. As shown in Figure 4, in the present invention example, a signal to increase the wiping gas was sent 2 seconds before the welded portion passed the spraying position, and a signal to return the wiping gas pressure to the original pressure was sent 1.2 seconds before the welded portion passed the spraying position. The steel strip transport speed V was set to 85 m / min, as an example.

[0033] As shown in Figure 4, when a signal to increase the wiping gas was sent 2 seconds before the weld passed the spray position, and a signal to return the wiping gas pressure to the original pressure was sent 1.2 seconds before the weld passed the spray position, the wiping gas pressure increased before and after the weld passed the spray position. As a result, no linear overcoat was generated on the obtained steel strip. In this way, by increasing the wiping gas pressure when the weld passes the spray position, clogging of the wiping nozzle is prevented and linear overcoat is suppressed.

[0034] The calculated adhesion amount was calculated based on the following formula (1). W = exp(y + p × lnP + v × lnV + d × lnD + h × lnH) (1) W: Calculated adhesion amount (g / m 2 ) P: Nozzle pressure (kPa) V: conveying speed of steel strip S (m / sec) D: Distance between the tip of the nozzle lip and the steel strip S (m) H: Height of wiping gas blowing position from the surface of the molten metal bath (m) y, p, v, d, h: coefficients

[0035] Figure 5 shows the relationship between the distance from the weld point on the steel strip and the calculated coating weight when wiping gas is blown onto the steel strip at the wiping gas pressure shown in Figure 4. Figure 5 also shows the minimum coating weight required for the steel strip before and after the weld point.

[0036] It was found that the calculated coating weight falls below the required lower limit in the range where the wiping gas pressure is increased, as shown in Fig. 5. Since the actual plating coating weight can be obtained from a correlation equation with the calculated coating weight, it was found that the cutting area can be determined based on the calculated coating weight.

[0037] In the comparative example in which the manufacturing method of the hot-dip galvanized steel strip according to the present embodiment was not applied, a steel strip with a linear overcoat was generated, and the portion with the linear overcoat had to be removed at the exit side of the manufacturing line. It was also necessary to eliminate clogging of the wiping nozzle. As a result, the manufacturing cost per unit mass of the hot-dip galvanized steel strip in the example of the present invention was 60% of the manufacturing cost per unit mass of the hot-dip galvanized steel strip in the comparative example.

[0038] Although the embodiment of the present invention has been described above, the present invention is not limited to such examples. It is clear that a person skilled in the art can come up with various modified or altered examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally belong to the technical scope of the present invention. [Explanation of symbols]

[0039] 1. Continuous hot-dip metal plating equipment 2 Molten metal bath 3. Sink Roll 4 Support Roll 5 Wiping nozzle 6. Coating weight control device 51 Nozzle lip

Claims

1. A method for manufacturing a hot-dip galvanized steel strip, comprising the steps of: continuously transporting a steel strip, the tail end of a preceding steel strip and the front end of a following steel strip connected by a welded portion, along its longitudinal direction; tracking the position of the welded portion on the transport path of the steel strip, immersing the steel strip in a molten metal bath, and then pulling the steel strip out of the molten metal bath; and then spraying a wiping gas having a predetermined pressure from a wiping nozzle onto both sides of the steel strip to remove excess molten metal from the steel strip, The wiping nozzle blows the wiping gas to the welded portion at a pressure such that the molten metal scattered by the blowing of the wiping gas does not reach a nozzle lip of the wiping nozzle, a step of transmitting a signal to an adjustment valve for adjusting the spray pressure of the wiping gas to a pressure at which the molten metal scattered by the spraying of the wiping gas does not reach a nozzle lip of the wiping nozzle before the weld passes through a spray position of the wiping gas; A method for manufacturing a hot-dip galvanized steel strip, comprising the step of sending a signal to the adjusting valve after the step and before the weld passes the spraying position of the wiping gas, to change the spraying pressure to a low pressure lower than the spraying pressure adjusted by the signal sent in the step.

2. 2. A method for manufacturing a hot-dip galvanized steel strip as described in claim 1, further comprising the steps of: calculating an amount of molten metal adhering to the surface of the steel strip after the wiping gas is sprayed; and cutting the steel strip to remove the weld based on a calculated amount of molten metal, which is the calculated amount of molten metal adhering, and separating the steel strip into the leading steel strip and the trailing steel strip.

3. 3. The method for producing a hot-dip galvanized steel strip according to claim 2, wherein the calculated adhesion amount is calculated based on the pressure of the wiping gas, the conveying speed of the steel strip, the distance between the wiping nozzle and the steel strip, and the height of the spraying position of the wiping gas from the bath surface of the molten metal bath.

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