JP2008024970A - Method of manufacturing galvanized steel sheet having excellent appearance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture a galvanized steel sheet having excellent appearance by preventing occurrence of surface defects and surface appearance defects caused by the stagnation of a molten zinc and foreign matters on a plating bath surface. <P>SOLUTION: In the method of manufacturing the galvanized steel sheet, by which a steel sheet is entered into a plating bath by being passed through a snout, the lower end of which is immersed in the plating bath, (a) a metal pump is arranged below a plating bath surface inside the snout across the steel plate, (b) (b-1) a fairing member with its lower end being immersed in the plating bath at least over the width of the steel plate (b-3) is installed while keeping a predetermined space from the metal pump (b-2) with a space equivalent to or smaller than the width of a discharge port or a suction port of the metal pump. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a hot-dip galvanized steel sheet having a beautiful and clear surface appearance.

  Conventionally, hot-dip galvanized steel sheets (including alloyed hot-dip galvanized steel sheets) have been used for general purposes as steel sheets for automobiles, household electrical products, etc. From the viewpoint of differentiation from products, the demand for hot dip galvanized steel sheets with a beautiful and clear appearance tends to increase rapidly.

  In order to produce a hot-dip galvanized steel sheet having a beautiful and clear surface appearance, it is necessary to reduce surface defects as much as possible by reducing strict surface defects as much as possible by strictly controlling the process in the production process.

  Normally, in hot dip galvanizing, as shown in FIG. 1, the steel plate 1 after annealing is connected to an annealing furnace (not shown), and the lower end is immersed in a plating bath 3 of a plating tank 5. Into the plating bath 3, the pot roll 4 in the bath changes the advancing direction and pulls upward, and the gas wiping nozzle 6 ejects gas to adjust the adhesion amount of molten zinc.

  However, liquid foreign matters 7 such as scum and dross are floating on the plating bath surface inside the snout 2, and surface defects are formed when the liquid foreign matters 7 adhere to the steel plate surface when the steel plate is immersed in the plating bath. . Therefore, many methods have been proposed so far for suppressing the generation of scum, dross and the like and for suppressing the adhesion of scum and dross to the steel sheet surface (see Patent Documents 1 to 6).

  Patent Documents 1 to 3 disclose a method in which gas is sprayed onto a steel sheet on the plating bath surface to exclude liquid foreign matters such as scum and dross from the periphery of the steel sheet. According to this method, liquid foreign substances can be eliminated entirely and continuously from the periphery of the steel sheet, so that defects on the surface of the steel sheet can be significantly reduced.

  However, the atmospheric temperature inside the snout is lowered by the jet gas, and this temperature reduction promotes the condensation of the evaporated zinc fume. Therefore, the methods disclosed in Patent Documents 1 to 3 can reduce scum and dross. However, it is not a practical method because it is not possible to suppress the deposit from dropping from the wall surface of the snout due to a decrease in the atmospheric temperature due to gas ejection.

  Moreover, in the method of patent documents 1-3, a plating bath surface is disturbed by jet gas, and this disorder may be transcribe | transferred on the steel plate surface as a flow pattern. This flow pattern on the plated surface is a surface appearance defect, and even if the surface defect due to scum, dross or the like is significantly reduced, the commercial value of the plated steel sheet is greatly impaired.

  Patent Document 4 discloses a device that sucks and removes scum with a metal pump before the scum adheres to the steel plate surface even if scum is generated on the plating bath surface inside the snout. However, in the above-described apparatus, it is necessary to enhance the suction capability of the metal pump and to always operate it, so that the life of the metal pump is short. In addition, when manufacturing conditions such as the width of the steel plate and the plate passing speed change, a new metal pump is required each time, so the apparatus disclosed in Patent Document 4 is not economical.

  In Patent Document 5, a heater is installed at the lower end of the snout, and the inner wall of the stout is heated to a predetermined temperature to prevent the formation of a solidified zinc film on the plating bath surface and the sticking of floating dross to the inner wall of the stout. At the same time, a method for sucking and removing floating dross with a snout pump in a timely manner is disclosed.

  The method disclosed in Patent Document 5 is an efficient method in that the suction and removal of the floating dross may be performed in a timely manner according to the generation amount, and the method also has a long life of the snout pump. However, in the method disclosed in Patent Document 5, it is difficult to grasp the amount of dross in the snout and the amount of stagnation from the outside, the metal pump stops even for a short time, and the flow of the molten metal is interrupted, Since the appearance of the plated surface is greatly affected, it cannot be applied as equipment.

  In addition, the method disclosed in Patent Document 5 is not economical in terms of equipment because it requires a control device that drives the snout pump in a timely manner. Further, when manufacturing conditions such as the width of the steel plate and the plate passing speed change. Since a new metal pump is required each time, it is not economical.

  Further, in Patent Document 6, when forming a lateral flow (flow along the steel plate surface) of molten zinc in the vicinity of the plating bath surface, the discharge of the push pump is made to be two, and the flow on the steel plate surface is discharged. A method and apparatus for removing the scum and dross film as well as preventing the flow pattern on the steel sheet surface from being reduced and preventing the scum and dross film from adhering to the steel sheet surface are disclosed.

  In order to effectively implement the method disclosed in Patent Document 6, it is necessary to increase the discharge capacity of the metal pump on the discharge side and increase the discharge amount. Moreover, in order to ensure a stable flow and cope with fluctuations in the molten metal surface, it is necessary to provide the discharge port and the suction port so as to be movable up and down.

  When trying to carry out the method disclosed in Patent Document 6 using a push pump and a pull pump that are currently used in which the discharge port and the suction port do not move up and down, a stable flow cannot be obtained if there is fluctuation in the molten metal surface, Pattern defects and dross defects occur.

  Furthermore, it is not described in Patent Document 6 to suppress the occurrence of a flow pattern using a push pump and a pull pump that are currently used.

  Therefore, in hot dip galvanizing technology, it is possible to accurately rectify molten zinc at the required flow rate along the surface of the steel sheet without blowing gas on the plating bath surface and without increasing the capacity of the metal pump. It is formed with a width to prevent adhesion of scum, dross, etc. to the surface of the steel sheet, and to prevent pattern defects caused by the flow of molten metal in the snout. It is required to obtain a smooth plating surface.

Japanese Patent Laid-Open No. 07-150323 JP 09-228016 A JP 2000-64015 A JP 2001-049412 A JP 2002-275606 A JP 2003-293107 A

  In view of the above demands, the present invention (i) does not employ a method of blowing gas to the plating bath surface, and uses the current metal pumps (push pump and pull pump) arranged on both sides of the steel plate as much as possible, and in the lateral direction. Assuming that the method of forming the rectification of molten zinc is adopted, (ii) the rectification of the molten zinc in the lateral direction is formed, and thin layers such as scum and dross generated on the plating bath surface enter the plating bath. (Iii) preventing stagnation of molten zinc and pattern defects caused by foreign matter on the plating bath surface, and generating surface defects and surface appearance defects. The objective is to establish technology to prevent this.

  The metal pump used in the present invention is a normal metal pump that is currently used. Molten zinc is supplied from the outside of the snout, and the discharge port of the push-side pump (push pump) has a rectangular or elliptical shape. It has a shape and is located near the surface of the molten zinc in the snout. The discharge port may be entirely immersed or partly positioned above the surface due to fluctuations in the molten metal surface.

  The suction port of the pump on the pull side (pull pump) also has a rectangular or elliptical shape and is located near the surface of the molten zinc in the snout. Similar to the discharge port of the push pump, the suction port is entirely immersed or partly positioned above the surface due to fluctuations in the molten metal surface. Note that the suction port of the pull pump may have a shape that is obliquely cut out so that the scum can be easily sucked and removed.

  Molten zinc sucked from the suction port of the pull pump is discharged out of the snout by the pump.

  The inventor uses the normal metal pump (push pump and metal pump) arranged on both sides of the steel plate inside the snout, and (i) in the vicinity of the discharge port of the push pump and in the vicinity of the pull pump suction port, Zinc or turbulence of molten zinc does not occur, and (ii) laterally molten zinc with a constant flow velocity that is not affected by the reverse flow generated by the accompanying flow that is inevitably generated as the steel plate passes We have intensively studied the method of forming the rectification of

  As a result, the present inventor, on the inner wall of the snout, in parallel with the steel plate surface, the rectification forming member over at least the entire width of the steel plate, in relation to the width of the discharge port of the push pump and the width of the suction port of the pull pump, It has been found that the desired molten zinc rectification can be formed when installed at the required intervals.

  This invention was made | formed based on the said knowledge, and the summary is as follows.

(1) In the method for producing a hot dip galvanized steel sheet, the steel sheet is allowed to enter the plating bath through a snout whose lower end is immersed in the plating bath.
(A) A metal pump is disposed below the plating bath surface inside the snout with the steel plate sandwiched between them,
(B) At the lower end of the snout, (b-1) A rectifying member whose lower end is immersed in the plating bath over at least the width of the steel plate, (b-2) Equal to or less than the width of the discharge port and suction port of the metal pump (B-3) A method for producing a hot-dip galvanized steel sheet having a clean appearance, characterized by being installed with a predetermined distance from the metal pump.

  (2) The method for producing a hot-dip galvanized steel sheet having a clean appearance according to the above (1), wherein the rectification forming member is a flat member.

  (3) An end portion of the rectification forming member is formed as an inclined surface having an inclination of 45 to 90 °. The hot-dip galvanized steel sheet having a clean appearance according to (1) or (2), Production method.

  (4) The method for producing a hot-dip galvanized steel sheet having a clean appearance according to any one of (1) to (3), wherein the interval between the rectification forming members is 250 mm or less.

  (5) The rectification forming member is installed so that an end portion thereof is located in an area of 30 mm or less from the metal pump, and has a clean appearance according to any one of (1) to (4), Manufacturing method of hot dip galvanized steel sheet.

  (6) Melting with a clean appearance according to any one of (1) to (5), wherein the lower end portion of the rectification forming member is immersed in a range of 100 to 650 mm below the bath surface of the plating bath. Manufacturing method of galvanized steel sheet.

  ADVANTAGE OF THE INVENTION According to this invention, the hot dip galvanized steel plate provided with the clear external appearance which does not have the surface defect and surface external appearance defect resulting from the stagnation of the hot dip zinc and the foreign material on the plating bath surface can be manufactured.

  First, the inventor has intensively studied a method for forming a rectification of molten zinc in a lateral direction by using a push pump and a pull pump (hereinafter sometimes collectively referred to as “metal pump”) arranged on both sides of a steel plate.

As a result, the present inventor
(I) At the lower end of the snout, (b-1) A rectifying member whose lower end is immersed in the plating bath over at least the width of the steel plate, (b-2) Equivalent to or less than the width of the discharge port and suction port of the metal pump And with (b-3) the required distance from the metal pump,
(Ii) Without increasing the capacity of the metal pump, the flow of the molten zinc in the lateral direction can be rectified in the snout, and the flow rate can be maintained at or above the "predetermined flow rate"
(Iii) It has been found that a thin layer of foreign matter such as scum and dross on the plating bath surface can be prevented from adhering to the surface of the steel sheet entering the plating bath.

  Here, the rectification means that the flow velocity of the lateral surface flow in the snout is a substantially constant flow between the steel plate and the snout wall. In addition, the predetermined flow rate is a flow rate in the range of 0.5 to 1.5 m / s. In order to suppress the formation of a zinc oxide film that occurs when the plating bath surface in the snout stagnates as described later. Is preferably a flow rate of about 1 m / s or more.

  Hereinafter, the present invention will be described in detail with reference to the drawings.

  In FIG. 2, a push pump P1 and a pull pump P2 are arranged under the bath surface of the plating bath 3 in the snout 2, and a molten metal flow in the lateral direction (P1 → P2) along the surface of the steel plate 1 (hereinafter referred to as “lateral flow”). An embodiment of forming P is shown.

  Usually, in the plating bath 3, a flow in the steel sheet intrusion direction (downward arrow in the figure) (hereinafter also referred to as “accompanied flow”) inevitably occurs with the intrusion of the steel sheet 1. Further, accompanying the occurrence of the accompanying flow, an upward reverse flow (hereinafter sometimes referred to as “reverse flow”) F is generated so as to compensate for the loss of mass of the molten metal drawn into the steel plate.

  The reverse flow F joins the lateral flow P on the push pump P1 side, but disturbs the lateral flow P because the flow velocity is different, and permeates the lateral flow P on the pull pump P2 side to disturb the flow. When the reverse flow is generated, as will be described later, an outflow portion where the molten metal springs out is generated on the surface of the plating bath inside the snout, and the lateral flow P is maintained at a constant flow velocity at least over the entire width of the steel plate. It becomes difficult.

  In view of this, the present inventor has conceived that a rectification forming member is installed at the lower end of the snout to narrow the flow area of the lateral flow P so that it does not flow into the lateral flow P even if a reverse flow occurs.

  However, since the reverse flow itself occurs regardless of the distance between the inner wall of the snout and the steel plate, even if the rectification forming member is simply installed at the lower end of the snout to narrow the basin of the lateral flow P, the constant flow rate is maintained. It is assumed that the lateral flow P cannot be obtained.

  Then, this inventor observed the flow in a snout about the molten metal flow around a steel plate in the case of installing a baffle plate of a predetermined thickness as a baffle forming member at the lower end of the snout. FIGS. 3A and 3B schematically show an example of the result of observing the flow in the snout. 3 (a) and 3 (b) show the snout observed from the top, and the result is shown in the form of a cross section viewed from above.

  As a result of the above observation in the snout, the following was found.

  FIG. 3A shows an observation result of the flow in the snout when there is no current plate.

Molten metal is discharged from the push pump (in the drawing, discharge side Px), and the molten metal is sucked in by the pull pump (in the drawing, suction side Py) (refer to arrows P ₁ , P ₂ , P ₃ in the drawing) However, as the lateral flow (lateral flow) moves away from the push pump and approaches the pull pump, it takes in molten metal around the flow and spreads to the wall surface (in the figure, dotted arrows P ₁ ′, P ₂ ′, P ₃ ', the reference), so lateral flow, as shown in the thickness of the arrow _{P 1, P 2, P 3} , decays as it approaches the Puruponpu.

  Moreover, since the steel plate 1 penetrates into the molten metal in the snout 2, the molten metal around the steel plate is drawn toward the traveling direction of the steel plate (direction perpendicular to the paper surface), and another flow (drawing flow f) is generated. It is formed. The entrainment flow f is generated on the both sides of the steel plate in a direction perpendicular to the steel plate on the snout surface (see “small arrow f” along the steel plate in the figure).

Among these flows, in particular, attenuation of a lateral flow (arrows P ₁ , P ₂ , P _{3 in the} figure) from the push pump (in the figure, discharge side Px) to the pull pump (in the figure, suction side Py). The molten metal fountain Fo, in which the molten metal is sucked from the tip of the snout and flows out to the surface of the molten metal inside the snout by the drawing flow f generated on both surfaces of the steel plate and perpendicular to the steel plate. Oval with an arrow (see).

The flow from the molten metal fountain Fo (hereinafter referred to as “the fountain Fo”) (see “arrow” coming out of the fountain Fo in the figure) is a lateral flow P ₁ , P ₂ , P ₃ , Since it influences the entrainment flow f generated on both sides, a non-uniform flow is generated on the steel plate surface. As a result, a pattern defect occurs on the surface of the steel plate.

Increasing the amount of molten metal discharged from the push pump (discharge side Px in the figure) and increasing the amount of molten metal suction from the pull pump (suction side Py in the figure) by the same amount Since the amount of molten metal taken around when the flow spreads to the wall surface (see dotted arrows P ₁ ′, P ₂ ′, P ₃ ′ in the figure) increases, it occurs on the surface of the molten metal inside the snout 2 The amount of molten metal that springs out from the spring part Fo also increases.

  Therefore, even if a push pump and a pull pump are provided inside a conventional snout and the flow rate (discharge amount and suction amount) of these molten metals is simply increased, pattern defects will further occur on the surface of the steel sheet. Therefore, it is impossible to solve the problem of the present invention that “scum adhesion is prevented without generating a surface pattern”.

  Here, FIG. 3B schematically shows the result of observing the flow in the snout when the rectifying plate 9 is installed on the inner wall of the snout 2.

In this case as well, as in the case where there is no flow straightening plate, the molten metal is discharged from the push pump (discharge side Px in the figure), and the molten metal is sucked in by the pull pump (suction side Py in the figure). P ₁ , P ₂ , P ₃ , and molten metal are drawn into the snout surface, and there is a drawing flow f perpendicular to the steel plate 1 generated on both sides of the steel plate 1, but the rectifying plate 9 is in the snout 2. Is installed, the flow rate at which the lateral flow diffuses toward the wall is reduced, and as a result, the amount of attenuation while the lateral flow flows from the push pump to the pull pump is considerably reduced (in the figure, arrow P _1). , P ₂ , P ₃ thickness, see).

  As a result, the region of the spring portion Fo generated on the surface of the molten metal in the inside of the snout 2 is reduced, and a non-uniform flow is not generated on the surface of the steel plate 1, without causing pattern defects on the surface of the steel plate, In addition, the scum can be removed.

  And the following conclusion can be drawn from the said observation result.

  (A) In order to make up for the molten metal sucked by the pull pump, the inflow from the lower part of the snout and springing out to the surface of the molten metal, the outflow from the spring part formed on the surface of the molten metal near the suction port on the pull pump side is This obstructs the lateral flow of the molten metal and causes pattern defects on the surface of the steel sheet.

  (B) When the interval between the rectifying plates (rectification forming members) installed inside the snout is reduced and the flow width of the lateral flow is reduced, the erupting flow is almost extinguished and is caused on the steel plate surface by the erupting flow. Pattern defects do not occur.

  (C) The spacing between the rectifying plates that suppresses the above-described flow and does not cause pattern defects on the steel plate surface depends on the plate passing speed of the steel plate. (C1) Assuming a general steel plate passing speed of 50 to 150 mpm, the interval between the rectifying plates where pattern defects do not occur is 250 mm or less. (C2) When the distance between the current plates exceeds 250 mm, the above-mentioned rushing current easily occurs, and a pattern defect occurs on the steel plate surface.

  (D) When the distance between the rectifying plates installed in the snout is 250 mm or less, the length of the rectifying plate below the bath surface (the length of the immersion part) is a lateral flow formed by a push pump and a pull pump. If the length of the predetermined section including the portion is not less than 650 mm, the steel plate can be drawn out of the snout without sufficiently developing the flow drawn around the steel plate, thereby further reducing the above-mentioned squirt flow. be able to.

  (E) On the other hand, when the length of the rectifying plate below the bath surface exceeds 650 mm, the accompanying flow of the steel plate increases, and the flow sucked into the snout increases. (E1) As a result, there may be an increase in the flow of the spring flowing out from the spring on the surface of the molten metal. (E2) In that case, if the plate passing speed is high, for example, if the plate passing speed is 120 mpm or more, there is a possibility that pattern defects on the surface of the steel sheet cannot be stably eliminated.

  (F) Use a metal pump to push the gap between the rectifying plate and the rectifying plate between the rectifying plate and the steel plate in order to form a lateral flow with a constant flow rate that is not affected by the outflow of the snout surface. It is effective to make the width equal to or smaller than the width of the discharge port of the pump and the suction port of the pull pump.

  (G) Further, notching the end portion of the rectifying plate at an appropriate angle (inclination) is effective in promoting the rectification of the lateral flow and preventing the generation of eddy currents in the vicinity of the rectifying plate end portion.

  (G1) Sideways when the distance between the current plates is the same as the width of the discharge port and suction port of the metal pump and the end of the current plate is cut out at 45 ° (inclination) (described later) As for the flow velocity of the flow, the width of the discharge port and the suction port of the metal pump is 250 mm, and the current plate is not installed (that is, the distance between the inner wall surfaces of the snout in FIG. 3A is 460 mm). It is about 1.4 times that.

  (G2) If the end of the rectifying plate is notched at 90 ° (described later), the lateral flow velocity should be 1.5 times the lateral flow velocity when no rectifying plate is installed. Can do.

  (H) When the interval between the rectifying plates is narrower than the width of the discharge port of the push-pull pump, the flow velocity of the lateral flow increases.

  Based on the above conclusion obtained by observing the flow of molten metal in the snout, the present inventor has a current plate at the lower end of the snout having an inner wall interval of 460 mm, as shown in FIGS. 4 (a) and 4 (b). The rectifying plate was installed so that the interval W1 was approximately the same as the width W2 of the discharge port of the push pump P1 and the width W2 of the suction port of the pull pump P2.

  Note that the rectifying plate shown in FIG. 4A is a rectifying plate whose end is notched at an angle α. The rectifying plate shown in FIG. 4B is a rectifying plate having its end cut out at α = 90 °.

  As shown in FIG. 4, required gaps are provided on the side surfaces of the discharge port of the push pump P1 and the suction port of the pull pump P2. The reason for this is that during operation, when the steel plate pass line changes, the snout finely moves in a direction perpendicular to the surface of the steel plate, while the push pump P1 and the pull pump P2 are fixed in the bath. This is because the metal pump and the snout wall may come into contact with each other, and this is surely avoided.

  However, if the required gap between the metal pump and the snout is not appropriate, a stagnation region 10 of molten metal is generated in this gap region, as shown in FIG. May develop. In order to avoid the appearance of this pattern, the present inventor examined the proper interval between the rectifying plate and the metal pump and the appropriate shape of the end of the rectifying plate, in which the stagnation region is not generated, in the actual machine test.

  The results of the actual machine test are shown in Table 1 together with other conditions.

  The actual machine test conditions are as follows.

Sheet width: 1500mm or more Sheet thickness: 0.8mm or less Steel type: IF steel Sheet feeding speed: 50 to 135 mpm
Plating bath: Zn—Al (Al: 0.1 to 0.15%)
Intrusion plate temperature: 450-470 ° C
Presence / absence of heating in snout: None Atmosphere temperature in snout: 200 ° C
Presence or absence of alloying process: Yes

  From Table 1, the following knowledge can be obtained.

  (X1) If both ends of the current plate are cut out at an angle (α) of less than 45 °, a stagnation region (see “10” in FIG. 4A) is formed on the surface of the molten metal at the cutout. Generate. When this stagnation region is generated, surface appearance defects due to stagnation of the molten metal appear at the end of the steel plate. Therefore, it is preferable that the end of the current plate is cut out at an angle of 45 to 90 °.

  (x2) The flow rate of the lateral flow when the end of the rectifying plate is notched at an angle of 45 ° is 250 mm for the discharge port and suction port of the metal pump, and no rectifying plate is installed ( That is, it is about 1.4 times the flow velocity of the lateral flow in the case where the distance between both inner wall surfaces of the snout in FIG. 3A is 460 mm.

  (X3) If the end portion of the current plate is formed with a vertical surface (α = 90 °), no stagnation region is generated, so that the flow rate of the lateral flow can be made 1.5, which is the conventional flow rate.

  (Y1) The rectifying plate is installed at the lower end of the snout, and the metal pump is fixed below the plating bath surface, but the snout must be tilted back and forth during operation. It is necessary to provide a necessary gap.

  (Y2) When the interval is wider than 30 mm, a reverse flow flows around the edge of the steel plate or a stagnation region is generated, and the flow velocity of the lateral flow is reduced. Therefore, the interval is 30 mm or less. preferable. In addition, although the one where the said space | interval is narrow is preferable, what is necessary is just to set suitably in the range which does not have trouble in the tilting forward and backward of a snout, and it is not necessary to set a minimum in particular.

  (Z1) The length of the rectifying plate installed in the snout in the plate passing direction is preferably 600 mm or less. If it exceeds 600 mm, the accompanying flow accompanying the movement of the steel sheet develops too much in the snout, and as a result, a squirt flow that swells on the surface of the molten metal in the snout occurs, and a flow pattern appears on the surface of the steel sheet.

  (Z2) Further, the length of the flow straightening plate installed in the snout may be long enough so that the lateral flow formed by the push pump and the pull pump does not diffuse in the direction perpendicular to the width direction of the steel plate. . For example, even if the length of the rectifying plate is 100 mm, the rectifying effect by the rectifying plate can be exhibited as long as the lateral flow does not diffuse in the direction perpendicular to the width direction of the steel plate.

  In the present invention, based on the above requirements, it is possible to produce a hot dip galvanized steel sheet having a clean appearance, free from stagnation of molten zinc and surface defects and surface appearance defects caused by foreign matters on the plating bath surface. However, if the inside of the snout is heated to prevent zinc fume from adhering to the inner wall of the snout and the generation amount of scum, dross, etc. is reduced, a more beautiful appearance without surface defects and surface appearance defects can be obtained. A hot-dip galvanized steel sheet can be manufactured.

  In Table 1, the acceptance of a scum entrainment defect means that the presence or absence of a scum entrainment defect existing on the surface of a coil of 5 to 10 tons is evaluated, and at least one scum entrainment defect exists in the coil. This means that the coil is a rejected coil and tested at the same level for about one day, and as a result, the generation rate of the rejected coil is 8% or less.

  In Table 1, all the plate widths of the steel plates are described as 1500 mm or more, but the steel plates of 1500 mm or more include, for example, steel plates having a width of 1750 mm. Further, when surface pattern defects (surface pattern property defects) occur at one test level, they are stably generated at that test level.

  Next, examples of the present invention will be described. The conditions of the examples are one example of conditions adopted for confirming the feasibility and effects of the present invention, and the present invention is limited to this one example of conditions. Is not to be done. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

  (Example)

  The test method is the same as in Table 1, and the results of testing at a level other than the level shown in Table 1 are shown in Table 2. As shown in Table 2, the generation of scum entrainment defects is acceptable, but the occurrence of surface pattern defects (surface pattern defects) is unacceptable depending on conditions.

  As described above, according to the present invention, a hot-dip galvanized steel sheet having a clean appearance can be produced. Therefore, this invention responds to a severe demand of a customer, expands the use of a hot dip galvanized steel sheet, and has a great applicability in the manufacturing industry using hot dip galvanized steel sheets.

It is a figure which shows the conventional aspect of the hot dip galvanizing apparatus. It is a figure which shows the aspect which forms the molten zinc flow (transverse flow) of the horizontal direction along the surface of a steel plate. It is a figure which shows typically the result of having observed the flow in a snout. (A) shows typically the flow in the case where the current plate is not installed in the snout, and (b) schematically shows the flow in the case where the current plate is installed in the snout. It is a figure which shows the installation aspect of a baffle plate. (A) shows the case where the end of the current plate is cut out at an angle α, and (b) shows the case where the end of the current plate is formed by a vertical surface (angle α = 90 °). Indicates.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Snout 3 Plating bath 4 Pot roll 5 Plating tank 6 Gas wiping nozzle 7 Liquid foreign substance 8 Zinc oxide 9 Current plate 10 Stagnation area P, P ₁ , P ₂ , P ₃ Lateral flow P ₁ ', P ₂ ', P ₃ 'Spreading flow P1 Push pump P2 Pull pump Px Discharge side Py Suction side F Reverse flow Fo Molten metal outflow part f Inlet flow W1 Rectifier plate spacing W2 Push pump discharge port width

Claims (6)

In the method for producing a hot dip galvanized steel sheet, the steel sheet is infiltrated into the plating bath through a snout whose lower end is immersed in the plating bath.
(A) A metal pump is disposed below the plating bath surface inside the snout with the steel plate sandwiched between them,
(B) At the lower end of the snout, (b-1) A rectifying member whose lower end is immersed in the plating bath over at least the width of the steel plate, (b-2) Equal to or less than the width of the discharge port and suction port of the metal pump (B-3) A method for producing a hot-dip galvanized steel sheet having a clean appearance, characterized by being installed with a predetermined distance from the metal pump.   The said rectification | straightening formation member is comprised with the flat member, The manufacturing method of the hot dip galvanized steel plate provided with the clean appearance of Claim 1 characterized by the above-mentioned.   The method for producing a hot-dip galvanized steel sheet having a clean appearance according to claim 1 or 2, wherein an end portion of the rectification forming member is formed with an inclined surface having an inclination of 45 to 90 °.   The method for producing a hot-dip galvanized steel sheet having a clean appearance according to any one of claims 1 to 3, wherein a distance between the rectification forming members is 250 mm or less.   The hot-dip galvanized steel sheet having a clean appearance according to any one of claims 1 to 4, wherein the rectifying member is installed such that an end thereof is located in an area of 30 mm or less from a metal pump. Manufacturing method.   6. The hot-dip galvanized steel sheet having a clean appearance according to claim 1, wherein a lower end portion of the rectification forming member is immersed in a range of 100 to 650 mm below the bath surface of the plating bath. Production method.

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