Hot dip coating device and method

Abstract

A hot dip coating device to provide a metal coating on a moving metal sheet, containing a liquid bath of metal coating material in use. The metal coating material on the moving metal sheet in use, including a container for the liquid bath, a guide or sink roll in the container below liquid bath surface level in use to guide the moving metal sheet through the bath, and a gas knife, above the liquid bath in use, having an outlet to project wiping gas on the metal coating on the metal sheet. At least one supporting roll with the guide or sink roll in use influence metal sheet shape in its width direction at the gas knife location. The one supporting roll is above liquid bath surface level to operate on the metal sheet before the metal sheet enters the bath. A method for use of the device.

Claims

1. A method of coating a moving metal sheet using a hot dip coating device containing a liquid bath of metal coating material, wherein the metal coating material is to be provided on the moving metal sheet, said hot dip coating device comprising: a container for the liquid bath, at least a guide or sink roll provided in the container below a surface of the liquid bath to guide the moving metal sheet through the bath, and a gas knife provided above the liquid bath, said gas knife having an outlet to project wiping gas on the metal coating provided on the metal sheet that passes along the gas knife, and wherein at least four supporting rolls are provided which together with the guide or sink roll influence a shape of the metal sheet in its width direction at least at the location of the gas knife, wherein the at least four supporting rolls are placed above the surface of the liquid bath on a position to operate on the metal sheet before the metal sheet enters the liquid bath, wherein at least three of the supporting rolls move to and fro in the direction of the moving metal sheet to influence crossbow of the moving metal sheet as the moving metal sheet is moving; wherein the metal sheet is moved over at least four of the supporting rolls before entering the liquid bath of metal in the container of the coating device, wherein the moving metal sheet has opposed first and second sides and at least two of the supporting rolls contacts the first side of the moving metal sheet and at least two of the supporting rolls contacts the second side of the moving metal sheet, wherein the at least four supporting rolls comprises a first supporting roll, a second supporting roll, a third supporting roll, and a fourth supporting roll, wherein the fourth supporting roll is the supporting roll closest to the guide or sink roll and contacts the side of the moving metal sheet opposite to the side of the moving metal sheet that has contact with the guide or sink roll, and wherein the supporting rolls contact of the opposed first and second sides of the moving metal sheet consists of successively alternately contact of the opposed first and second sides of the moving metal sheet relative to each other said supporting roll; wherein the second supporting roll contacts the moving metal sheet successively alternately relative to the first supporting roll, wherein the third supporting roll contacts the moving metal sheet successively alternately relative to the second supporting roll, wherein the fourth supporting roll contacts the moving metal sheet successively alternately relative to the third supporting roll; wherein the third supporting roll is sufficiently separated from the second roll such that the third roll and the second roll are each movable to and fro in a direction of the moving metal sheet to exert bending forces on the moving metal sheet to influence crossbow of the moving metal sheet; wherein the fourth supporting roll is sufficiently separated from the third roll such that the fourth roll and the third roll are each movable to and fro in a direction of the moving metal sheet to exert bending forces on the moving metal sheet to influence crossbow of the moving metal sheet; wherein the second, third and fourth supporting rolls are sufficiently separated to independently simultaneously exert penetration on the moving metal sheet; then the metal sheet moves into the liquid bath and contacts the guide or sink roll provided in the container below the surface of the liquid bath which guides the moving metal sheet through the bath, then the metal sheet moves out of the liquid bath and passes between a pair of said gas knives above the surface of the liquid bath, each gas knife outlet projecting wiping gas on the metal coating provided on the metal sheet that passes along the gas knifes; wherein there is an absence of supporting rolls in the liquid bath of metal, wherein the metal sheet moves along a path and a portion of the path of the metal sheet is inclined at an angle such that the metal sheet moves into the liquid bath at the inclined angle, and the supporting rolls are located along the inclined portion of the path of the metal sheet.

2. The method according to claim 1, wherein the liquid bath of metal is a liquid bath of zinc or zinc alloy, or wherein the liquid bath of metal is a liquid bath of aluminium or aluminium alloy; wherein the second supporting roll is sufficiently separated from the first roll such that the second roll and the first roll can each move to and fro in a direction of the moving metal sheet to exert bending forces on the moving metal sheet to influence crossbow of the moving metal sheet.

3. The method according to claim 1, further comprising controlling movement of at least one of the supporting rolls to or from the moving metal sheet by measuring the traverse shape of the coating when the metal sheet moves downstream of the gas knife and using this as input for moving one or more of the supporting rolls, to introduce a feed-back loop by which the thickness distribution of the coating over the width direction of the metal sheet is controlled.

4. The method according to claim 1, further comprising controlling movement of at least one of the supporting rolls to or from the moving metal sheet by measuring the traverse shape of the metal sheet when the metal sheet moves downstream of the gas knife and using this as input for moving one or more of the supporting rolls, to introduce a feed-back loop by which the form of the metal sheet is controlled.

5. The method according to claim 1, wherein the liquid bath of metal is a liquid bath of zinc aluminium alloy, zinc magnesium alloy or zinc aluminium magnesium alloy.

6. The method according to claim 1, wherein the liquid bath of metal is a liquid bath of aluminium silicon alloy.

7. The method according to claim 1, wherein the metal sheet moves through a snout into the liquid bath, wherein the supporting rolls are placed within the snout.

8. The method according to claim 7, wherein the supporting rolls are removably mounted within the snout to accommodate replacement and/or maintenance.

9. The method according to claim 1, wherein at least one said supporting roll is provided with a motor drive.

10. The method according to claim 1, wherein the gas knife is movable in length direction of the coating device, that is the direction of the moving metal sheet.

11. The method according to claim 10, further comprising monitoring the position of the passing metal sheet and aligning the gas knife with the moving metal sheet to position the gas knife in relation to the moving metal sheet during the coating of the metal sheet.

12. The method according to claim 1, wherein the guide or sink roll is movable vertically and/or horizontally in the liquid bath.

13. The method according to claim 1, wherein the guide or sink roll is the only roll in the liquid bath of metal.

14. The method according to claim 1, wherein the first supporting roll is the supporting roll farthest from the guide or sink roll and contacts the side of the moving metal sheet that has contact with the guide or sink roll; wherein the second supporting roll contacts the side of the moving metal sheet opposite to the side of the moving metal sheet that has contact with the guide or sink roll; wherein the third supporting roll contacts the side of the moving metal sheet that has contact with the guide or sink roll; wherein the fourth supporting roll contacts the side of the moving metal sheet opposite to the side of the moving metal sheet that has contact with the guide or sink roll.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will hereinafter be further elucidated with reference to the drawing of an exemplary embodiment of an hot dip coating device according to the invention that is not limiting as to the appended claims.

(2) In the drawing:

(3) FIG. 1 shows a hot dip coating device according to the prior art;

(4) FIG. 2 shows a first embodiment of a hot dip coating device according to the invention;

(5) FIG. 3 shows a second embodiment of a hot dip coating device according to the invention;

(6) FIG. 4 shows graphs representing the shape of the metal sheet with different settings of the supporting rolls in the embodiment of FIG. 2; and

(7) FIG. 5 shows graphs representing the shape of the metal sheet with different settings of the supporting rolls in the embodiment of FIG. 3.

(8) Whenever in the figures the same reference numerals are applied, these numerals refer to the same parts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) Making reference first to FIG. 1 it shows a hot dip coating device 10 to provide a metal coating on a moving metal sheet 9, comprising a liquid bath 8 of metal coating in a container 7 to be provided on the metal sheet 9, wherein a guide or sink roll 1 is provided below surface level 8′ of the liquid bath 8 to guide the moving metal sheet 9 through the bath 8. A gas knife 4 is provided above the liquid metal bath 8. Said gas knife 4 has an outlet 4′ to project wiping gas on the metal coating provided on the metal sheet 9 while it passes along the gas knife 4. The wiping by the gas knife 4 determines the thickness of the coating on the metal sheet 9.

(10) It is of prime importance that the coating process executed with the hot dip coating device 10 as shown in the drawing, results into an uniform coating thickness for appearance quality and corrosion resistance of the coated metal sheet 9. The coating thickness depends inter alia on the distance between the gas knife 4 and the metal sheet 9. As the distance of the gas knife 4 to the metal sheet 9 increases, also coating thickness increases. The coating thickness variations on the metal sheet 9 can be found either in travelling direction of the metal sheet 9 or in its width direction or both. The variation in the travelling direction is usually attributed to vibration of the metal sheet 9, while the variation of the coating thickness in the width direction is attributed to a phenomenon that is called crossbow.

(11) The coating thickness variations due to crossbow are normally counteracted by using a set of supporting rolls 2, 3 located behind the sink roll 1 when seen in processing direction of the metal sheet 9. In FIG. 1 this is shown in that supporting rolls 2, 3 that are embodied as a correction roll 2 and a stabilizer roll 3 are provided in the liquid metal bath 8. Together with the guide or sink roll 1, said correction roll 2 and stabilizer roll 3 influence the crossbow or shape of the sheet 9 in its width direction at the location of the gas knife 4.

(12) The shape of the metal sheet 9 or crossbow resulting from the effect of the sink roll 1 is a negative bow. A metal sheet 9 is said to have a negative bow if the bottom side of the metal sheet B is the concave side. When this happens, the coating thickness on the bottom side B of the metal sheet 9 is thicker in the middle than at its outer sides. Since the correction roll 2 bends the metal sheet 9 in the opposite direction of the sink roll 1, it has the tendency to create positive crossbow; while the stabilizer roll 3 bends the metal sheet 9 in the same direction as the sink roll 1 and therefore it has the tendency to create negative crossbow again. The final bow of the metal sheet 9 at the gas knife 4 is the combined result of these three consecutive bending operations.

(13) FIG. 2 and FIG. 3 show two embodiments of the hot dip coating device 10 of the invention wherein the at least one supporting roll 2, 3 is positioned outside of the liquid bath 8, in particular wherein the rolls 2, 3 are positioned above surface level 8′ of the liquid bath 8 on a position enabling it to operate on the metal sheet 9 before the metal sheet 9 enters the liquid metal bath 8.

(14) The metal sheet 9 enters the liquid bath at an inclined angle “A”. The position of the supporting rolls 2, 3 corresponds to where normally a snout 20 is provided through which the metal sheet 9 is moved before it enters into the liquid bath 8. The application of such a snout is entirely known to the skilled person and requires no further elucidation with reference to the drawing. Anyway, when such a snout is present it is preferable that the supporting roll or rolls 2, 3 are placed within the snout.

(15) Schematically shown in the drawing is a preferred option wherein the at least one supporting roll 2, 3 is provided with a motor drive 24. The application of such a motor drive 24 for the supporting rolls 2, 3 requires no further elucidation with reference to the drawing since the manner in which this can be implemented is entirely clear for the skilled person.

(16) Other preferable features are that the guide or sink roll 1 is movable vertically and/or horizontally in the liquid bath 8. In this way the positioning relative to the gas knife 4 and the positioning relative to the snout can be adjusted when the guide or sink roll wears. Likewise the gas knife 4 is preferably movable in the direction of the metal sheet 9. In this latter situation the gas knife 4 is advantageously provided with a positioning system 11 as shown in FIG. 2 to monitor the position of the passing metal sheet 9 and to align the gas knife 4 with the moving metal sheet 9.

(17) When applying a snout it is further preferable that the at least one supporting roll 2, 3 is removably mounted within the snout to accommodate its replacement and/or maintenance.

(18) As both FIG. 2 and FIG. 3 show the invention results in only the guide or sink roll 1 being present below surface level 8′ of the liquid bath 8.

(19) With reference again to FIG. 2 it is shown that the hot dip coating device 10 has two supporting rolls 2, 3, at least one of which should be arranged to be movable to and fro the metal sheet 9.

(20) Conversely FIG. 3 shows that it is also possible that the dip coating device 10 of the invention has four supporting rolls 2,3,5,6. The application of four supporting rolls as shown in FIG. 3 instead of (one or) two as shown in FIG. 2 is not arbitrary, but results in better achievements in terms of controllability of the crossbow of the metal sheet 9 as will be explained hereinafter.

(21) Though not shown in a figure, it is also possible to use three supporting rolls 2, 3, 5, of which at least one should be movable to and fro the moving metal sheet 9. The number of rolls that have to be used may be determined by the thickness of the metal sheet 9 and the velocity of the metal sheet 9.

(22) According to the invention at least one of the supporting rolls 2, 3, 5, 6 has to be movable in the direction of the moving metal strip 9. In this way the crossbow of the metal strip 9 is influenced, and together with the influence by the guide or sink roll 1 the crossbow of the steel strip 9 at the position of the gas knife 4 is determined. The crossbow of the metal strip determines the thickness of the coating in travers direction, as elucidated above.

(23) With reference to FIG. 4 and FIG. 5 the resulting crossbow for a different adjustment of the supporting rolls in the embodiments of FIG. 2 and FIG. 3 is analysed. Two different thicknesses of the metal sheet 9 are also considered, namely 0.7 mm and 1 mm. The results for the embodiment of FIG. 2 are shown in FIG. 4. As it is shown in this figure, the supporting rolls 2, 3 are capable to correct the crossbow created by the sink roll 1. With a particular position of the stabilization roll 3 there is always a range for the correction roll 2 adjustment that can be used to correct the crossbow caused by the sink roll 1. It can however be noted that the available range to provide corrective action is relatively narrow. This narrow range of the usable adjustment by correction roll 2 is indicated by the steep gradient in the graphs, wherein the horizontal upper and lower interrupted lines of the crossbow define the range wherein the metal strip 9 is deemed flat.

(24) In comparison FIG. 5 shows the corresponding results when four supporting rolls 2, 3, 5, 6 are applied as shown in FIG. 3. With a position of the supporting roll 2 at 15 mm and 20 mm for a thickness of the metal sheet 9 of 0.7 mm and 1.0 mm respectively, and different adjustments of the supporting roll 3 as depicted by the graphs in FIG. 3, the graphs reveal that in combination therewith roll 6 can always be set at an adjustment value that ensures that the metal sheet 9 will be within the upper and lower limits of the range wherein the metal strip 9 is deemed flat.

(25) The invention can be used for all types of coating using hot dip techniques, and is particularly useful for coating a metal sheet with zinc or zinc alloy, preferably a zinc aluminium alloy, zinc magnesium alloy or zinc aluminium magnesium alloy, or with aluminium or aluminium alloy, preferably an aluminium silicon alloy.

(26) In an embodiment a device 21 for measuring the traverse shape of the coating is measured downstream of the gas knife 4. The measuring results can be used to control the adjustment of at least one of the rolls 2, 3, 5, 6, such that the coating thickness in transverse direction of the metal sheet 9 in improved in a closed loop, for instance using P, PI, PID or smith predictive control. Alternatively, it is possible to use a device 21 for measuring the traverse shape of the metal sheet 9 itself, and with the measuring results to control the crossbow of the metal sheet in a closed loop.

(27) Although the invention has been discussed in the foregoing with reference to an exemplary embodiment of the hot dip coating device of the invention, the invention is not restricted to these particular embodiments which can be varied in many ways without departing from the invention. The discussed exemplary embodiments shall therefore not be used to construe the appended claims strictly in accordance therewith. On the contrary the embodiments are merely intended to explain the wording of the appended claims without intent to limit the claims to these exemplary embodiments. The scope of protection of the invention shall therefore be construed in accordance with the appended claims only, wherein a possible ambiguity in the wording of the claims shall be resolved using these exemplary embodiments.