JP4734081B2 - Method for producing hot-dip galvanized steel sheet - Google Patents

Description

  The present invention relates to a method for manufacturing a hot-dip galvanized steel sheet, and more particularly, to a method for manufacturing a hot-dip galvanized steel sheet while keeping the hot-dip galvanized bath clean by efficiently removing dross generated in the hot-dip galvanized bath. .

  At present, as steel plates to which rust prevention is imparted, hot dip galvanized steel plates (hereinafter abbreviated as GI) and zinc alloyed hot dip galvanized steel plates (hereinafter abbreviated as GA) in which zinc (Zn) is plated. ) Is manufactured and used frequently in various applications.

  Usually, the above-mentioned GI and GA are manufactured using the same molten zinc bath. It is known that when manufacturing these GI and GA, it is necessary to add a predetermined concentration of aluminum (Al) to the molten zinc bath. For example, when manufacturing GA, the Al concentration in the bath is about 0.10% by mass to 0.15% by mass, and when manufacturing GI, the Al concentration in the bath is 0.15% by mass to 0.15% by mass. Operation is carried out at about 0.30% by mass.

However, in the GA or GI manufacturing method in which Al is added to the molten zinc bath as described above, when the Al concentration exceeds 0.15% by mass, the steel in the molten zinc bath reacts with Al. While Fe ₂ Al ₅ is generated as top dross, when the Al concentration is less than 0.15% by mass, the steel sheet and molten zinc react with each other, and FeZn ₇ is generated as bottom dross. There was a point.

  As described above, GA and GI are usually manufactured using the same molten zinc bath, and it is necessary to change the amount of Al to be added between GA and GI. Therefore, when the operation is switched from the manufacture of GA to the manufacture of GI, or when the operation is switched from manufacture of GI to manufacture of GA, the generated dross changes. The specific gravity of dross also changes greatly, and a large amount of dross produced remains in the molten zinc bath without rising or sinking. Therefore, conventionally, when the production is switched from GA to GI, or from GI to GA, the production reaction of these dross subsides, and it takes about 3 hours until it completely floats or completely sinks. Needed.

  Therefore, it was necessary to examine the above dross countermeasures in order to enable efficient operation.

For example, in the invention disclosed in Patent Document 1, by adding a certain amount or more of nickel (Ni) to the molten zinc bath, dross generated anywhere in the molten zinc bath is less than that of Zn. A top dross mainly composed of Ni ₂ Al ₃ having a light specific gravity is used, and this is recovered to reduce the above-described calming time.

  In addition, dross produced in any of the molten zinc baths as described above will adversely affect the surface shape of the plated steel sheet if left in the molten zinc bath as it is. Need to be removed. As such a dross removal method, for example, in Patent Document 2 and Patent Document 3, in order to remove dross generated by the molten metal existing in the snout, a supply nozzle for supplying the molten metal into the snout, A method for removing dross adhered to the surface of a steel sheet by providing a nozzle for sucking the molten metal is disclosed.

  In addition, since it is difficult to remove dross because GA and GI are manufactured using the same molten zinc bath, for example, a molten zinc bath dedicated to GA manufacturing and a molten zinc dedicated to GI manufacturing are used. Another possible countermeasure against dross is the provision of multiple molten zinc baths such as baths, so that the generated dross can be unified with either the top dross or the bottom dross, and the dross can be easily removed.

Japanese Patent No. 2978947 JP 2004-59942 A JP 2004-99922 A

  However, the method of Patent Document 1 described above is a method of unifying the generated dross into a top dross, and Patent Documents 2 to 3 are methods of removing dross existing in the snout. As to where it is generated in the bath, it has not been elucidated phenomenologically or analytically using simulation, so a more effective removal method could not be developed.

  In addition, according to the method described in Patent Document 3, the generated dross having a size of about 50 μm can be effectively removed, but a dross having a smaller size, for example, a size of about 10 to 50 μm. Could not be removed effectively.

  In addition, for the method of providing a hot dip galvanizing bath for each type of hot dip galvanized steel sheet to be manufactured, there are places where multiple hot dip galvanizing baths can be installed and the funds for the installation. Because it is necessary, it is hard to say that it is a realistic method.

  Therefore, despite the use of the same hot-dip galvanizing bath, the production of efficient hot-dip galvanized steel sheets that can control the type of dross produced and effectively remove the various types of dross produced. A method was sought after.

  Therefore, the present invention has been made in view of such problems, and its purpose is to limit the generation location of dross only to the inside of the snout and to effectively remove the generated dross, thereby eliminating surface defects. The object is to provide a method for producing a small number of hot-dip galvanized steel sheets.

  In order to solve the above-described problems, the inventors of the present application have conducted intensive research. In the present invention, unless otherwise specified, the term “hot dip galvanized steel sheet” is used to mean both hot dip galvanized steel sheet GI and galvannealed steel sheet GA.

  As a result of research, it is clear that most of the dross generated in the molten zinc bath can be made AlNi by plating the steel sheet to be plated beforehand with Ni (hereinafter abbreviated as Ni pre-plating). became. This AlNi dross was found to be a top dross because a part of plated Ni was dissolved in molten zinc and reacted with Al in the molten zinc, and its specific gravity was lighter than Zn. .

  Moreover, when Ni is added to the molten zinc bath in an amount of 0.01 to 0.05% by mass, for example, when the Ni pre-plated steel sheet is immersed in the molten zinc bath from the snout, the AlNi dross is molten zinc in the snout. It was clarified that AlNi dross was not generated in places other than snout, but only in the vicinity of the bath surface of the bath.

  Therefore, based on the above knowledge, the present inventors have come up with the following invention.

That is, in order to solve the above-mentioned problem, according to one aspect of the present invention, a step of heating a Ni pre-plated steel sheet; and the above-mentioned Ni pre-plated steel sheet is subjected to snout and 0.13 to 0.30 mass% Al. And 0.01 to 0.05% by mass of Ni, the bath is produced only in the vicinity of the bath surface of the molten zinc bath in the snout. And a step of recovering AlNi dross existing in the vicinity of the surface by a recovery means. A method of manufacturing a hot dip galvanized steel sheet is provided.

  By using the manufacturing method as described above, most of the dross produced is AlNi which is the top dross, and the dross is also produced in the molten zinc in the snout where the Ni pre-plated steel sheet is immersed in the molten zinc bath. It is almost limited to the vicinity of the bath surface. For this reason, dross can be collected effectively.

  In the step of recovering the dross, the molten zinc discharge means and the molten zinc suction means respectively disposed on one side and the other side of the Ni pre-plated steel sheet are brought close to the bath surface of the molten zinc bath in the snout. The bath flow along the width direction of the Ni pre-plated steel sheet may be generated, and the dross may be recovered by the molten zinc suction means.

  By providing a molten zinc discharge means as described above and a molten zinc suction means corresponding to the recovery means, and generating a bath flow in and near the bath surface of the molten zinc bath in the snout, In addition, dross existing near the bath surface can be recovered more efficiently.

  The step of heating the Ni pre-plated steel plate can also heat the Ni pre-plated steel plate by a current induced in a circuit including the Ni pre-plated steel plate.

  This method is a method of heating the Ni pre-plated steel plate without directly applying a voltage to the Ni pre-plated steel plate, and the Ni pre-plated steel plate can be heated at a much lower voltage than the conventional heating method. it can.

  Moreover, it is also possible to use what used the transformer effect type | formula energization heating (Ultra Rapid Transformer Type Heater: URTH) method as a heating apparatus used for said heating process.

  By using a heating apparatus employing the above heating method, the Ni pre-plated steel sheet can be rapidly heated with a short heating length.

  In the step of recovering the dross, the suction amount of the molten zinc suction means is (discharge amount of the molten zinc discharge means) ≦ (suction amount of the molten zinc suction means) ≦ 1.5 × (of the molten zinc discharge means) Discharge amount). However, this does not include the case of (discharging amount of molten zinc discharging means) = (suction amount of molten zinc sucking means) = 1.5 × (discharging amount of molten zinc discharging means).

  Moreover, as said molten zinc suction means, it is also possible to use a gas lift pump, for example.

  Thus, by setting the suction amount of the molten zinc suction means, it is possible to more effectively remove the generated AlNi dross.

  Further, in the step of recovering the dross, molten zinc is applied to the front and back surfaces of the Ni pre-plated steel sheet in the direction opposite to the traveling direction of the Ni pre-plated steel sheet from a plurality of spraying means provided in the molten zinc bath. It is also possible to spray.

  Here, at least one spraying means can be provided on the front and back surfaces of the Ni pre-plated steel sheet.

  By providing such spraying means, AlNi dross generated in the vicinity of the molten zinc bath surface in the snout rides on the flow generated when the Ni pre-plated steel sheet is immersed in the molten zinc bath and enters the molten zinc bath. Diffusion can be suppressed, and AlNi dross can be kept near the molten zinc bath surface in the snout. Moreover, the flow of molten zinc from the spraying means causes a flow in the vicinity of the Ni pre-plated steel sheet in the molten zinc bath, and this flow becomes a separation flow for separating the AlNi dross attached to the Ni pre-plated steel sheet.

  Further, in the step of recovering the dross, the Ni pre-plated steel sheet is formed so that the molten zinc ejected from the plurality of spraying means provided in the molten zinc bath forms an acute angle with respect to the traveling direction of the Ni pre-plated steel sheet. It is also possible to be sprayed on.

  As described above, by spraying molten zinc so as to form an acute angle with the Ni pre-plated steel sheet, the ejecting flow generated when the spraying means ejects molten zinc interferes with the above-described separation flow, and the separation of AlNi dross It is possible to suppress the phenomenon that the effect is reduced.

  When the spraying means as described above is arranged, the suction amount of the molten zinc suction means is set to (discharge amount of molten zinc discharge means + discharge amount of spraying means) ≦ (suction amount of molten zinc suction means) ≦ 1 5 × (discharge amount of molten zinc discharge means + discharge amount of spraying means) is also possible. However, (Discharge amount of molten zinc discharge means + Discharge amount of spraying means) = (Amount of suction of molten zinc suction means) = 1.5 × (Discharge amount of molten zinc discharge means + Discharge amount of spray means) Is not included.

  By setting the suction amount of the molten zinc suction means in this way, it is possible to effectively remove AlNi dross while well interlocking with the spraying means.

In order to solve the above-mentioned problems, according to another aspect of the present invention, a step of heating a Ni pre-plated steel sheet; and the Ni pre-plated steel sheet contains 0.01 to 0.05 mass% of Ni through a snout. , A molten zinc bath whose Al concentration is changed from 0.13 to 0.16% by mass to 0.25 to 0.30% by mass, or 0.01 to 0.05% by mass of Ni, When immersed in a molten zinc bath changed from 0.25 to 0.30 mass% to 0.13 to 0.16 mass%, it is generated only in the vicinity of the bath surface of the molten zinc bath in the snout. And a step of recovering AlNi dross existing in the vicinity of the bath surface by a recovery means. A method of manufacturing a hot dip galvanized steel sheet is provided.

  By using the above manufacturing method, for example, when the Al concentration of the molten zinc bath is changed from the Al concentration for GA production to the Al concentration for GI production, or conversely, the GA production from the Al concentration for GI production. When the Al concentration of the molten zinc bath is changed to the Al concentration for use, most of the dross produced becomes AlNi, and the place where the dross is produced is in the snout where the Ni pre-plated steel sheet is immersed in the molten zinc bath. Because it is almost limited, the generated dross can be effectively removed and the operation can be performed without waiting for the dross generation reaction to settle down.

  The details of this manufacturing method can be performed in the same manner as in the above manufacturing method.

  ADVANTAGE OF THE INVENTION According to this invention, the production | generation method of the hot dip galvanized steel plate which can limit the production | generation place of dross only in a snout and can remove the produced | generated dross effectively can be provided.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the present specification and drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant description is omitted.

  First, a heating device for heating the hot dip galvanized steel sheet according to the first embodiment of the present invention and a hot dip galvanizing device will be described. FIG. 1 is an explanatory diagram showing an outline of the configuration of the heating device and the hot dip galvanizing device according to the first embodiment.

  In the present embodiment, as the heating device 50, for example, a device employing a heating method of the URTH method can be used. The heating device 50 includes, for example, a conductive roll 10, a seal roll 11, and a turn roll 12 in order from the upstream side along the Ni pre-plated steel sheet S. The exit side of the turn roll 12 communicates with the turn roll 14 in the molten zinc bath 13 of the hot dipping apparatus 60.

  The periphery of the Ni pre-plated steel sheet S from the seal roll 11 to the molten zinc bath 13 is surrounded by, for example, an enclosure 15, and in particular, the enclosure 15 from the downstream side of the turn roll 12 to the molten zinc bath 13 includes a snout 19. be called. The envelope 15 can maintain the Ni pre-plated steel sheet S in a predetermined inert atmosphere (for example, in a nitrogen atmosphere) from heating to plating.

  Between the seal roll 11 and the turn roll 12, for example, a ring-shaped iron core 16 surrounding the Ni pre-plated steel sheet S and the surrounding body 15 is provided. A primary coil 17 is wound around the iron core 16, and an AC power supply 18 is connected to the primary coil 17.

  The tip of the snout 19 is in contact with molten zinc (not shown) in the molten zinc bath 13. A bus bar 20 that is electrically connected to the conductive roll 10 is connected to the tip of the snout 19. By the bus bar 20, the secondary coil 21 around the iron core 16 is formed by the conductive roll 10, the Ni pre-plated steel sheet S, the molten zinc in the molten zinc bath 13, the snout 19, and the bus bar 20. By applying a voltage to the primary coil 17 and causing a current to flow from the AC power source 18, power can be induced on the secondary coil 21 side, and a high current can be caused to flow through the Ni pre-plated steel sheet S. The Ni pre-plated steel sheet S can be heated by the current flowing through the Ni pre-plated steel sheet S and heated. As described above, the URTH method is a method that can heat the Ni pre-plated steel sheet S without directly applying a voltage to the Ni pre-plated steel sheet S.

  As described above, the Ni pre-plated steel sheet S introduced from the conductive roll 10 is heated while proceeding in the direction of the arrow in FIG. 1, and the Ni pre-plated steel sheet S heated to about 470 ° C. through the snout 19 is obtained. , It is immersed in the molten zinc bath 13 of the hot dipping apparatus 60. The turn roll 14 in the molten zinc bath 13 changes the traveling direction of the Ni pre-plated steel sheet S to the vertical direction of the molten zinc bath 13 (upward in FIG. 1). The surface is galvanized. The time for which the Ni pre-plated steel sheet S is immersed in the molten zinc bath is about several seconds.

  Examples of important parameters for producing a good quality hot dip galvanized steel sheet include the temperature of the hot galvanizing bath, the components of the hot galvanizing bath, and the component ratio of the hot dip galvanizing bath. The temperature of the hot dip galvanizing bath is a factor that affects the wettability of the steel sheet, and greatly affects the properties of the resulting hot dip galvanized steel sheet. In order to make GI and GA different, it is needless to say that the component and component ratio of the molten zinc bath are important because it is necessary to change the component and component ratio of the molten zinc bath.

  In addition, the pot used for the molten zinc bath 13 is preferably a ceramic pot. This is because if iron is used, Fe (Fe), which is a component of the pot, reacts with Al in the molten zinc bath, and FeAl dross may be generated in the bath.

  In the conventional method for producing a hot dip galvanized steel sheet that does not use a Ni pre-plated steel sheet, the temperature of the hot dip zinc bath needs to be set to a high temperature of, for example, 450 to 480 ° C. in order to improve the wettability of the steel sheet (eg Japanese Patent No. 3557810). In addition, when a steel sheet is immersed in a molten zinc bath, for example, when GA is produced with an Al concentration in the molten zinc bath of less than 0.15 mass%, bottom dross, ZnFe dross, is generated. When GI was produced with an excess of 0.15% by mass, FeAl dross, which was a top dross, was generated.

  However, as in this embodiment, by using a Ni pre-plated steel sheet heated by the URTH method, the wettability between the steel sheet and the molten zinc is improved due to Ni present on the surface of the steel sheet, and the temperature of the molten zinc bath is increased. For example, in the range of 445 to 455 ° C. In addition, the adhesion of plating has been improved, and it has become possible to prevent the occurrence of unplated (unplated) parts. When the temperature of the molten zinc bath is lower than 445 ° C., the solidification temperature of zinc is about 419 ° C., so that unplating due to wettability is likely to occur, and when it exceeds 455 ° C., the molten zinc bath When the fume from the surface increases and adheres to the snout wall and falls into the molten zinc bath, it causes plating surface defects.

  Also, by applying Ni pre-plating to the steel sheet, when the steel sheet comes into contact with the molten zinc bath 13 in the snout 19, a part of the Ni pre-plating melts and reacts with Al in the bath to produce AlNi dross. It became clear. Since the ratio of Al to Ni in the generated AlNi dross is about 1: 1 in terms of moles, it is considered that a compound having a structure as AlNi is mainly used. In addition, the specific gravity of AlNi is 5.8, which is lighter than the specific gravity of zinc, 7.13, and it has been clarified that this AlNi dross is a top dross.

  In addition, the fact that the temperature of the molten zinc bath 13 can be lowered to 445 to 455 ° C. also has the effect of being able to strictly control the adhesion thickness of the plating and the alloying behavior between the steel plate and the plated metal. it can. Further, the temperature range of 445 to 455 ° C. generally has an effect that AlNi dross easily precipitates because the solubility is lowered when the bath temperature of the molten zinc bath is lowered.

  Here, when the Ni pre-plated steel sheet is heated using a normal heating method instead of heating the steel sheet by the URTH method used in this embodiment, the pre-plated Ni is alloyed with iron during the heating. As a result, a part becomes Fe-Ni alloy. In this case, iron is also eluted in the molten zinc bath 13, and the produced dross is mainly Al-Fe dross. The URTH method can shorten the heating time of the steel sheet as compared with the conventional method, and the heating temperature can be lowered as compared with the conventional method, so that the above-mentioned Fe-Ni alloying is difficult to proceed. There is. As described above, in addition to the temperature of the molten zinc bath 13, the type of dross generated in the molten zinc bath 13 is also controlled by selecting the URTH method as the heating method.

  As a component of the molten zinc bath 13 in the present embodiment, for example, Al can be added at a predetermined concentration in addition to the molten zinc as the main component. The Al concentration needs to be changed depending on the hot-dip galvanized steel sheet to be manufactured. For example, when manufacturing GA, GI is manufactured with an Al concentration of 0.13 to 0.16% by mass. In this case, the Al concentration can be 0.25 to 0.30% by mass.

  According to the present embodiment, if Ni is, for example, 0.01 to 0.05 mass%, preferably 0.02 to 0.03 mass% in the molten zinc bath 13, almost AlNi dross except in the snout 19. It was revealed that does not generate. When the Ni concentration is less than 0.01% by mass, dross other than AlNi dross may be generated, which is not preferable. Further, when the Ni concentration exceeds 0.05% by mass, AlNi dross is also generated outside the snout 19, which is not preferable.

  The method described in Patent Document 1 is also a method in which Ni is added to the molten zinc bath so that the dross is unified with the top dross. However, in the method described in the same document, the Ni concentration in the bath is set in the bath. It is adjusted to a value of 4 times or more of the Al concentration. When the Ni concentration is calculated using the formula described in the document, in the case of the Al concentration used in the embodiment of the present invention, the Ni concentration in the bath is at least 0.08% by mass or more. Also from this, it is clear that the present invention and the invention described in Patent Document 1 are completely different.

  Thus, by using Ni pre-plated steel sheet heated by the URTH method and adding Ni to the molten zinc bath 13 at the above concentration, the generated dross becomes AlNi dross, which is the top dross, and the snout Outside of 19, AlNi dross can be hardly generated. This makes it possible to limit the location of dross generation in the snout 19 where it has not been known where it occurs in the past, and to study an effective dross removal method.

  In this embodiment, the AlNi dross produced is almost 10 to 50 μm in size, but it is less than about 10 μm produced in the snout and drawn into the molten zinc bath by the steel plate. Since the Ni concentration in the molten zinc bath is lower than the Ni concentration at the site where the AlNi dross is generated inside the snout, there is a possibility that the AlNi dross of the size is re-dissolved in the molten zinc bath.

  However, when the temperature of the molten zinc bath 13 is low and the Ni concentration is high as described above, AlNi is liable to precipitate, so the Ni concentration in the molten zinc bath 13 is likely to change. Therefore, when the temperature is strictly controlled, it is necessary to adjust the Ni concentration in the molten zinc bath 13 to be 0.05% by mass or less.

  Further, if the operation is continued, the bath surface of the molten zinc bath 13 is lowered, so that the bath surface needs to be kept constant. For this purpose, about 1 ton of an alloy containing Zn and Al is added, for example, once or more per hour. In this way, by adding an alloy containing Zn and Al, the Al concentration in the molten zinc bath 13 is adjusted, and the Ni concentration is adjusted to be 0.05% by mass or less.

  FIG. 2 is an explanatory diagram illustrating an outline of the configuration by enlarging the region 2 of FIG. FIG. 3 is a plan view of FIG. 2 taken along the bath surface. With reference to FIG. 2 and FIG. 3, a method for effectively removing AlNi dross produced only in the snout 19 will be described in detail.

  Corresponding to a plurality of spraying means for spraying molten zinc onto the surface of the Ni pre-plated steel sheet S and a spray nozzle 22A corresponding to a plurality of spraying means for spraying the molten zinc onto the surface of the Ni pre-plated steel sheet S. A spray nozzle 22B is disposed. The spray nozzles 22A and 22B are disposed so as to face substantially opposite directions with respect to the traveling direction of the Ni pre-plated steel sheet S, that is, the direction from the entrance side to the exit side of the snout 19 (direction A in FIG. 2). Here, the surface of the Ni pre-plated steel sheet S means the surface on the side where the angle formed by the Ni pre-plated steel sheet S and the hot dip zinc bath surface is an obtuse angle. Further, the back surface of the Ni pre-plated steel sheet S means a surface on the side where the angle formed between the Ni pre-plated steel sheet S and the hot dip zinc bath surface is an acute angle.

As the Ni pre-plated steel sheet S travels in the A direction in FIG. 2, a flow (associated flow) in the A direction can occur in the molten zinc bath 13. When this accompanying flow occurs, AlNi dross floating on the surface of the molten zinc bath in the snout 19 rides on the accompanying flow and diffuses into the molten zinc bath 13. To prevent such diffusion, blowing from the blowing hole of the nozzle 22A in B ₁ direction in FIG. 2, and, by ejecting molten zinc B ₂ direction in FIG. 2 from the blowing hole of the spray nozzle 22B (ejection Flow), the accompanying flow as described above can be eliminated. Further, by adjusting the spraying amount of the spray nozzles 22A and 22B, not only the associated flow is erased but also a separation flow for separating the generated AlNi dross from the Ni pre-plated steel sheet S is generated in the direction C in FIG. It is possible to make it.

As shown in FIG. 2, a blowing direction B ₁ of the molten zinc from spray nozzles 22A, the angle between the traveling direction A of Ni Puremekki steel sheet S, and theta _1. Similarly, the direction B ₂ spraying molten zinc from the nozzle 22B blowing, the angle between the traveling direction A of Ni Puremekki steel sheet S, and theta _2.

Since the Ni pre-plated steel sheet S is immersed obliquely with respect to the molten zinc bath surface, the jet flow from the spray nozzle and the separation flow are likely to interfere with each other. Therefore, the magnitudes of the angles θ ₁ and θ ₂ need to be acute angles smaller than at least 90 degrees. Here, the magnitude of θ ₁ is preferably 15 to 35 degrees, and the magnitude of θ ₂ is preferably 35 to 55 degrees. When the magnitude of each angle is less than the lower limit, the separation flow interferes with the flow shown in FIG. 2C, and a sufficient effect cannot be obtained. Moreover, when the magnitude | size of each angle exceeds said upper limit, a part of jet flow which collided with the steel plate S may become a downward flow, and is unpreferable.

As a result of reduced theta ₁ and theta ₂ as described above, since the reach jet speed of the jet flow is lowered, it is necessary to increase the release speed from spray nozzles 22A and 22B.

  According to FIG. 3, a discharge pump 23 corresponding to molten zinc discharge means for blowing molten zinc is provided on one side of the snout 19, and the other side corresponds to molten zinc suction means for sucking molten zinc. A suction pump 24 is provided. That is, the discharge pump 23 and the suction pump 24 are provided on both sides of the Ni pre-plated steel plate along the direction b in FIG. In FIG. 3, the discharge pump 23 is provided with two nozzles. However, the number of nozzles provided in the discharge pump in the present invention is not limited to the above, and the size of the snout 19 in the a direction. It is possible to arrange according to. The suction pump 24 is provided with a suction port of a predetermined size.

  The molten zinc is discharged from the nozzle of the discharge pump 23 along the b-axis positive direction, that is, along the width direction of the Ni pre-plated steel sheet, thereby causing a flow toward the b-axis positive direction. AlNi dross (not shown) floating on the bath surface along this flow is pushed away to the suction pump 24 installed on the downstream side of the flow. At this time, the floating AlNi dross does not adhere to the Ni pre-plated steel sheet S due to the separation flow by the spray nozzles 22A and 22B provided in the bath. The floating dross that has been swept away is removed from the inside of the snout 19 by the suction pump 24.

  Further, as shown in FIG. 3, the spray nozzles 22A and 22B provided in the molten zinc bath 13 are only a predetermined angle with respect to the normal direction of the Ni pre-plated steel sheet S (a-axis direction in FIG. 3). The separation flow generated by the spray nozzles 22A and 22B is inclined to flow in the positive direction of the b-axis as a whole.

  Various pumps currently used can be applied to the suction pump 24. Further, as the suction pump 24, for example, a gas lift pump adopting a gas lift system can be used.

  The suction amount of the suction pump 24 is set to a predetermined value so that the floating dross can be effectively removed. This suction amount is also changed by the presence or absence of the spray nozzles 22A and 22B provided in the molten zinc bath 13.

  For example, when the spray nozzles 22A and 22B are not installed in the molten zinc bath 13, the suction amount of the suction pump 24 can be set to a value in the following range. However, the following formula does not include the case where (discharge amount of the discharge pump 23) = (suction amount of the suction pump 24) = 1.5 × (discharge amount of the discharge pump 23).

  (Discharge amount of the discharge pump 23) ≦ (Suction amount of the suction pump 24) ≦ 1.5 × (Discharge amount of the discharge pump 23)

  For example, when the spray nozzles 22A and 22B are installed in the molten zinc bath 13, the suction amount of the suction pump 24 can be set to a value in the following range. When a plurality of spray nozzles are provided, the “discharge amount of the spray nozzle” shown below means the total discharge amount of all the spray nozzles, and in this embodiment, the spray nozzle The total of the discharge amount of 22A and the discharge amount of the spray nozzle 22B is the total discharge amount. In the equation shown below, (discharge amount of discharge pump 23 + discharge amount of blowing nozzle) = (suction amount of suction pump 24) = 1.5 × (discharge amount of discharge pump 23 + discharge amount of blowing nozzle) ) Is not included.

  (Discharge amount of the discharge pump 23 + discharge amount of the blowing nozzle) ≦ (Suction amount of the suction pump 24) ≦ 1.5 × (Discharge amount of the discharge pump 23 + discharge amount of the spray nozzle)

  When the suction amount of the suction pump 24 is less than “the discharge amount of the discharge pump 23” or less than “the discharge amount of the discharge pump 24 + the discharge amount of the spray nozzle”, the floating dross cannot be efficiently removed. AlNi dross that is floating dross accumulates in the snout 19.

  Further, when the suction amount of the suction pump 24 exceeds 1.5 times “the discharge amount of the discharge pump 23” or 1.5 times of “the discharge amount of the discharge pump 24 + the discharge amount of the spray nozzle”, A flow that flows from the molten zinc bath 13 outside the snout 19 is generated in the snout 19, and a vortex is generated in the snout 19, so that the floating dross cannot be efficiently removed.

  By disposing the suction pump 24 having a flow rate set as described above and the discharge pump 23 for supplying molten zinc, 10-50 μm AlNi dross produced in the snout 19 is effectively removed, It becomes possible to keep the inside of the snout 19 clean.

The parameters of the spray velocity of the spray nozzles 22A and 22B, the angles θ ₁ and θ ₂ , the angle between the spray nozzle and the a-axis direction in FIG. 3, and the position where the spray flow collides with the Ni pre-plated steel sheet S are Ni It can be set according to the moving speed of the steel sheet S.

  In particular, the ejection speed of the spray nozzles 22A and 22B is proportional to the moving speed of the Ni pre-plated steel sheet S. This is because the jet flow from the spray nozzle must push back the accompanying flow accompanying the movement of the steel sheet S.

In order to set the jet velocity, the angles θ ₁ and θ ₂ , the angle formed by the spray nozzle and the a-axis direction in FIG. 3, and the position where the jet flow collides with the Ni pre-plated steel sheet S, each model is used. It is possible to determine the optimum conditions for the parameters.

  For example, Table 1 shows the values of each parameter when the moving speed of the Ni pre-plated steel sheet S is 60 m / min.

  Moreover, not only when manufacturing GI or GA as described above, but also when changing the hot-dip galvanized steel sheet manufactured from GI to GA or from GA to GI, the Ni pre-plated steel sheet as described above. By using S and adding Ni of the above concentration into the molten zinc bath 13, the dross produced is limited to the AlNi dross that is the top dross. Therefore, it is not necessary to wait for the time for the dross to calm down as in the prior art, and it is possible to efficiently produce a hot dip galvanized steel sheet.

  Below, the hot dip galvanized steel sheet produced using the manufacturing method of the hot dip galvanized steel sheet according to the first embodiment of the present invention will be described with reference to examples.

Table 2 shows the evaluation results of the plating properties of the hot-dip galvanized steel sheets prepared under conditions 1 to 13. In Table 2 below, the plate passing speed means the moving speed of the steel plate, and the plate passing width means the lateral width of the steel plate, that is, the width of the steel plate in the direction parallel to the b direction in FIG. . The suction amount means the suction amount of the suction pump 24, and the discharge amount means the discharge amount of the discharge pump 23. The nozzle discharge amount 1 means the discharge amount of the spray nozzle 22A, and the nozzle discharge amount 2 means the discharge amount of the spray nozzle 22B. Here, the angles θ ₁ and θ ₂ are respectively 15 degrees and 35 degrees when the sheet passing speed is 100 to 120 m / min, and 25 degrees when the sheet passing speed is 70 to 100 m / min. And 45 degrees, and when the plate passing speed is 50 to 70 m / min, they are set to 35 degrees and 55 degrees, respectively, but other angle combinations are possible within this range.

Here, the unit of the Ni pre-plating amount in Table 2 is g / m ² . Further, the unit of the plate passing speed is m / min, and the unit of the plate passing width is mm. The unit of Al concentration in the bath and Ni concentration in the bath is mass%. The unit of bath temperature is ° C. The units of suction amount, discharge amount, and discharge amount of each nozzle are liters / minute.

In addition, regarding the evaluation of the plating properties in Table 2, six types of results shown in Table 3 below were obtained.

  Conditions 1 to 6a in Table 2 are the setting values and evaluation results of each parameter when GA is manufactured under various conditions. Conditions 1 to 3 are examples according to the present invention, and conditions 4 to 6a are comparative examples.

  As is clear from Table 2, under conditions 1 to 3, a good GA free from surface defects due to dross entrainment could be produced. On the other hand, when the Ni concentration in the molten zinc bath was 0.06% by mass, surface defects due to AlNi dross had occurred. Further, even under condition 5 in which the discharge pump, suction pump, and spray nozzle were not used, surface defects caused by AlNi dross occurred, Ni pre-plated steel sheet was used, and the Ni concentration in the molten zinc bath was in a range suitable for the present invention. It became clear that the occurrence of surface defects could not be suppressed by simply using the Further, under condition 6 in which Ni pre-plating was not performed and Ni was not added to the molten zinc bath, FeZn dross different from AlNi dross was generated, and surface defects due to this dross were generated. For this reason, it is not possible to suppress surface defects caused by dross simply by providing a discharge pump, a suction pump, and a spray nozzle. Ni plating is applied to the steel sheet and Ni is added to the molten zinc bath. It became clear that it was essential. Further, under the condition 6a in which the Al concentration in the molten zinc bath was lower than the predetermined concentration, the Fe concentration in the hot dip galvanizing was increased. This means that the alloying reaction between Zn and Fe has progressed too much, suggesting that a GA that does not meet the specifications has been manufactured.

  Conditions 7 to 11 in Table 2 are setting values and evaluation results of each parameter when GI is manufactured under various conditions. Conditions 7 and 8 are examples according to the present invention, and conditions 9 to 11 are comparative examples.

  As is apparent from Table 2, under conditions 7 and 8, a good GI free from surface defects due to dross entrainment could be produced. Under condition 9 in which the Ni concentration in the molten zinc bath was 0.06% by mass, surface defects due to AlNi dross had occurred. Further, under condition 10 in which the discharge pump, the suction pump, and the spray nozzle were not used, surface defects caused by AlNi dross occurred, Ni pre-plated steel plate was used, and the Ni concentration in the molten zinc bath was in a range suitable for the present invention. It became clear that the occurrence of surface defects could not be suppressed by simply using the In addition, under the condition 11 in which a steel plate not subjected to Ni pre-plating was used and Ni was not added to the molten zinc bath, non-plating that would cause an unplated place occurred. This is because it is not possible to suppress surface defects caused by dross simply by providing a discharge pump, suction pump, and spray nozzle. It is essential to apply Ni pre-plating to the steel sheet and to add Ni to the molten zinc bath. It is suggested that.

  Conditions 12 and 13 in Table 2 are the setting values and evaluation results of each parameter when the hot-dip galvanized steel sheet to be manufactured is changed from GA to GI. Condition 12 is an example according to the present invention, and condition 13 is a comparative example.

  In condition 12, it can be seen that the manufactured GI did not cause surface defects due to dross entrainment, and a good hot-dip galvanized steel sheet could be manufactured. Further, under condition 13 using a steel plate not subjected to Ni pre-plating, surface defects caused by AlNi dross occurred, and it became clear that Ni pre-plating is indispensable.

  Conditions 14 to 16 in Table 2 are set values and evaluation results of each parameter when the discharge pump and the suction pump are used but the spray nozzles 22A and 22B are not used. Conditions 14 to 16 are examples according to the present invention.

Under conditions 14 to 16, it was possible to produce a good GA in which the number of surface defects due to dross entrainment was within ² per 1 m ^{2 of the} steel sheet. This is because a pre-plated steel plate is used, and an appropriate concentration of Ni is added to the molten zinc bath, and if the discharge pump and suction pump flow rates are set appropriately, This suggests that a good hot-dip galvanized steel sheet can be manufactured without a spray nozzle.

  Conditions 17 to 19 in Table 2 are the setting values and evaluation results of each parameter when the galvanized steel sheet to be manufactured is changed from GI to GA. Conditions 17 and 18 are examples according to the present invention, and condition 19 is a comparative example.

Under condition 17, it can be seen that surface defects due to dross entrainment did not occur in the manufactured GA, and a good galvannealed steel sheet could be manufactured. Under the condition 18 in which the spray nozzles 22A and 22B in the molten zinc bath were not used, surface defects were generated due to the dross entrainment, but the number of surface defects was within ² per 1 m ² of the steel sheet. It can be seen that the plated steel sheet has been manufactured. Further, in condition 19 using a steel plate not subjected to Ni pre-plating, surface defects caused by AlNi dross occurred, and it became clear that Ni pre-plating is indispensable.

  As mentioned above, although preferred embodiment of this invention was described referring an accompanying drawing, it cannot be overemphasized that this invention is not limited to this example. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the claims, and these are naturally within the technical scope of the present invention. Understood.

  For example, a discharge pump that sprays molten zinc onto a Ni pre-plated steel sheet and a suction pump that sucks the sprayed molten zinc are used, but the hot-dip galvanized steel sheet is manufactured without using a spray nozzle in the molten zinc bath. Is also possible.

  The present invention is applicable to a manufacturing method of a hot dip galvanized steel sheet and an alloyed hot dip galvanized steel sheet.

It is explanatory drawing which shows the outline of a structure of the heating apparatus and hot dip galvanizing apparatus which concern on the 1st Embodiment of this invention. It is explanatory drawing which expanded the area | region 2 of FIG. 1, and demonstrated the outline of the structure. It is a top view at the time of cut | disconnecting FIG. 2 by the bath surface.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Conductive roll 11 Seal roll 12 Turn roll 13 Molten zinc bath 14 Turn roll 15 Enclosure 16 Iron core 17 Primary coil 18 AC power supply 19 Snout 20 Busbar 21 Secondary coil 22A, 22B Spray nozzle 23 Discharge pump 24 Suction pump 50 Heating device 60 Hot dipping equipment S Ni pre-plated steel sheet

Claims (16)

Heating the Ni pre-plated steel sheet;
When the Ni pre-plated steel sheet is immersed in a molten zinc bath prepared to contain 0.13 to 0.30 mass% Al and 0.01 to 0.05 mass% Ni via a snout. And recovering AlNi dross produced only in the vicinity of the bath surface of the molten zinc bath in the snout and present in the vicinity of the bath surface by a recovery means;
A method for producing a hot dip galvanized steel sheet, comprising:   In the step of recovering the dross, the molten zinc discharge means and the molten zinc suction means respectively disposed on one side and the other side of the Ni pre-plated steel sheet, near the bath surface of the molten zinc bath in the snout, The method for producing a hot dip galvanized steel sheet according to claim 1, wherein a bath flow along the width direction of the Ni pre-plated steel sheet is generated, and the dross is recovered by the hot dip zinc suction means.   3. The hot-dip galvanized steel sheet manufacturing method according to claim 1 or 2, wherein the step of heating the Ni pre-plated steel sheet heats the Ni pre-plated steel sheet by a current induced in a circuit including the Ni pre-plated steel sheet. Method.   The method for manufacturing a hot-dip galvanized steel sheet according to claim 3, wherein the step of heating the Ni pre-plated steel sheet heats the Ni pre-plated steel sheet by a transformer effect type electric heating method. In the step of recovering the dross, the suction amount of the molten zinc suction means is:
(Discharge amount of molten zinc discharge means) ≦ (Amount of suction of molten zinc suction means) ≦ 1.5 × (Discharge amount of molten zinc discharge means)
The method for producing a hot-dip galvanized steel sheet according to any one of claims 2 to 4, wherein   In the step of recovering the dross, from the plurality of molten zinc spraying means provided in the molten zinc bath, the molten zinc is applied to the front and back surfaces of the Ni preplated steel sheet in a direction opposite to the traveling direction of the Ni preplated steel sheet. The method for producing a hot dip galvanized steel sheet according to any one of claims 1 to 5, wherein   The molten zinc according to claim 6, wherein in the step of recovering the dross, the molten zinc is sprayed on the Ni pre-plated steel sheet so as to form an acute angle with respect to the traveling direction of the Ni pre-plated steel sheet. Manufacturing method of plated steel sheet. In the step of recovering the dross, the suction amount of the molten zinc suction means is:
(Discharge amount of molten zinc discharge means + Amount of spray of molten zinc spray means) ≦ (Amount of suction of molten zinc suction means) ≦ 1.5 × (Discharge amount of molten zinc discharge means + Amount of spray of molten zinc spray means)
The method for producing a hot dip galvanized steel sheet according to claim 6 or 7, characterized in that: Heating the Ni pre-plated steel sheet;
The Ni pre-plated steel sheet was subjected to snout and contained 0.01 to 0.05% by mass of Ni, and the Al concentration was changed from 0.13 to 0.16% by mass to 0.25 to 0.30% by mass. A molten zinc bath or a molten zinc bath containing Ni in an amount of 0.01 to 0.05% by mass and having an Al concentration changed from 0.25 to 0.30% by mass to 0.13 to 0.16% by mass A step of recovering AlNi dross produced only in the vicinity of the bath surface of the molten zinc bath in the snout and existing in the vicinity of the bath surface by a recovery means when immersed;
A method for producing a hot dip galvanized steel sheet, comprising:   In the step of recovering the dross, the molten zinc discharge means and the molten zinc suction means respectively disposed on one side and the other side of the Ni pre-plated steel sheet, near the bath surface of the molten zinc bath in the snout, The method for producing a hot-dip galvanized steel sheet according to claim 9, wherein a bath flow along the width direction of the Ni pre-plated steel sheet is generated, and the dross is recovered by the hot-dip zinc suction means.   The hot-dip galvanized steel sheet manufacturing method according to claim 9 or 10, wherein the step of heating the Ni pre-plated steel sheet heats the Ni pre-plated steel sheet by a current induced in a circuit including the Ni pre-plated steel sheet. Method.   The method of manufacturing a hot dip galvanized steel sheet according to claim 11, wherein the step of heating the Ni pre-plated steel sheet heats the Ni pre-plated steel sheet by a transformer effect type electric heating method. In the step of recovering the dross, the suction amount of the molten zinc suction means is:
(Discharge amount of molten zinc discharge means) ≦ (Amount of suction of molten zinc suction means) ≦ 1.5 × (Discharge amount of molten zinc discharge means)
The method for producing a hot-dip galvanized steel sheet according to any one of claims 10 to 12, wherein   In the step of recovering the dross, from the plurality of molten zinc spraying means provided in the molten zinc bath, the molten zinc is applied to the front and back surfaces of the Ni preplated steel sheet in a direction opposite to the traveling direction of the Ni preplated steel sheet. The method for producing a hot-dip galvanized steel sheet according to any one of claims 9 to 13, wherein   The molten zinc according to claim 14, wherein in the step of recovering the dross, the molten zinc is sprayed on the Ni pre-plated steel sheet so as to form an acute angle with respect to a traveling direction of the Ni pre-plated steel sheet. Manufacturing method of plated steel sheet. In the step of recovering the dross, the suction amount of the molten zinc suction means is:
(Discharge amount of molten zinc discharge means + Amount of spray of molten zinc spray means) ≦ (Amount of suction of molten zinc suction means) ≦ 1.5 × (Discharge amount of molten zinc discharge means + Amount of spray of molten zinc spray means)
The method for producing a hot-dip galvanized steel sheet according to claim 14 or 15, wherein

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