Apparatus for forming nitrogen cloud to produce hot dip coated steel sheet

Abstract

A device installed between a surface of a coating bath and an air knife equipment to produce a hot dip metal coated steel sheet to form a non-oxidation atmosphere in a surface of a coated steel sheet ascending from the coating bath, the device comprising: lower gas discharge bars spaced apart from the surface of the coating bath by a predetermined distance and discharging a non-oxidation gas in a direction of the surface of the coating bath along the surface of the coated steel sheet; a side cover extending upwardly slopingly in a direction of the coated steel sheet from the sides of the lower gas discharge bars; and upper gas discharge bars formed at an upper end of the side cover and discharging a non-oxidation gas downwardly.

Claims

1. A device to form a non-oxidation atmosphere around the perimeter of a coated steel sheet (1) ascending from a coating bath (3), the device comprising: lower gas discharge bars (38, 39, 41 and 42) spaced apart from the surface of the coating bath (3) by a predetermined distance and discharging a non-oxidative gas in a direction downwards toward the surface of the coating bath (3); side covers (43a and 43b) extending upwardly from upper ends of the lower gas discharge bars (41 and 42), said side covers (43a and 43b) extending along a width direction of the coated steel sheet on opposite sides of the coated steel sheet (1), and having upper ends formed to be spaced apart from the coated steel sheet (1) by a predetermined distance, the side covers (43a and 43b) disposed slantingly in a direction traveled by the coated steel sheet when ascending from the coating bath; and upper gas discharge bars (44 and 45) formed at the upper ends of the side covers (43a and 43b) and discharging a non-oxidative gas in a direction downwards toward the surface of the coated bath (3), wherein the device is disposed between the surface of the coating bath (3) and air knife equipment (2) for adjusting a coating thickness of the coated steel sheet, wherein the lower gas discharge bars have four sides (38, 39, 41 and 42) which are configured to surround the coated steel sheet (1), wherein two sides (38 and 39) of the lower gas discharge bars face each other in a width direction of the coated steel sheet (1) and two other sides (41 and 42) of the lower gas discharge bars face each other in a thickness direction of the coated steel sheet (1).

2. The device according to claim 1, further comprising: a non-oxidative gas supply pipe (46) supplying a non-oxidative gas to the lower gas discharge bars (38, 39, 41 and 42); and a plurality of holes (nozzles) formed on a lower surface of the lower gas discharge bars (38, 39, 41 and 42) for erupting the non-oxidative gas.

3. The device according to claim 1, further comprising a lifting unit (5) configured to move the device in a vertical direction between the surface of the coating bath (3) and the air knife equipment (2).

4. The device according to claim 2, further comprising a lifting unit (5) configured to move the device in a vertical direction between the surface of the coating bath (3) and the air knife equipment (2).

5. The device according to claim 1, wherein the upper gas discharge bars (44 and 45) has a cross-section having a rectangular shape.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is images (magnification of 5,000) showing surface of GI coating bath-based MgAlBa added coated steel sheets generated by the device of the present invention;

(3) FIG. 2 is images (magnification of 2,000) showing the cross-section of GI coating bath-based MgAlBa added coated steel sheets generated by the device of the present invention;

(4) FIG. 3 is a plan schematic view illustrating a device for forming a nitrogen dam (device for forming a nitrogen cloud) according to an embodiment of the present invention;

(5) FIG. 4 is a sectional view taken along A-A section of FIG. 3;

(6) FIG. 5 is a schematic view of the device shown in FIG. 4; and

(7) FIG. 6 is a perspective view of the device shown in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

(8) Hereinafter, the present invention will be described in more detail. In the present invention, an oxidation suppressing atmosphere is formed through installation of a nitrogen curtain at a lower end portion of an air knife (non-oxidation atmosphere device=nitrogen cloud (tent) forming device) of a coating bath, and a top dross floater is prevented from being adsorbed to a base steel sheet (strip). After the strip is deposited in the coating bath, a reaction of oxide generated when the strip ascends to an interface of the coating bath with oxygen is blocked to be minimized, and the generated oxide is prevented from accessing the strip to prevent surface adsorption. Also, in a case in which the strip is integrally formed with the air knife, a gap between a molten metal and the knife is minimized in controlling a coating attachment amount to facilitate controlling of a coating attachment amount, and due to a supplementary effect of reducing a temperature of a lower end portion by a predetermined portion, an effect of enhancing surface quality according to an increase in a cooling speed after coating may be realized.

(9) FIGS. 3 to 6 schematically illustrate the device of the present invention. The device of the present invention is installed to be spaced apart from a surface of the coating bath 3 by a predetermined distance and vertically moved up and down between the surface of the coating bath 3 and an air knife 2 by a lifting unit 5.

(10) The device of the present invention includes lower end nitrogen discharge bars 38, 39, 41 and 42 formed to have a rectangular shape surrounding four sides of the coated steel sheet 1 in a width direction and a thickness direction, ascending from a surface of the coating bath 3. The lower end nitrogen discharge bars 38, 39, 41 and 42 receive nitrogen from a nitrogen supply pipe 46 and discharge a nitrogen gas in a direction of the surface of the coating bath 3. Although not shown, a plurality of holes (nozzles) for erupting a nitrogen gas are formed at a predetermined interval on a lower surface of each of the lower end nitrogen discharge bars 38, 39, 41 and 42.

(11) Although the lower end nitrogen discharge bars 38, 39, 41 and 42, which are pipes having a rectangular shape, may be integrally formed, as illustrated in the drawing, they may be separated as a first bar 41 and a second bar 42 and spaced apart from one another to be open in a thickness direction (in a vertical direction of the drawing) of the coated steel sheet 1).

(12) Also, the device of the present invention includes side covers 43a and 43b extending upwardly slantingly in a direction of the coated steel sheet 1 from the upper ends of the lower end gas discharge bars 41 and 42 surrounding both sides of the coated steel sheet 1 in a width direction and having upper ends formed to be spaced apart from the surface of the coated steel sheet 1 by a predetermined distance, and upper nitrogen discharge bars 44 and 45 formed at the upper ends of the side covers 43a and 43b and discharging a non-oxidation gas in a direction of the surface of the coating bath 3.

(13) The upper nitrogen discharge bars 44 and 45, having a pipe shape formed to have a nitrogen discharge hole (not shown) in a direction of the surface of the coating bath, erupt a nitrogen gas in a direction of the surface of the coating bath while facing each other at the upper ends of the side covers 43a and 43b with the coated steel sheet 1 interposed therebetween. The upper nitrogen discharge bars 44 and 45 receive nitrogen from the nitrogen supply pipe 46.

(14) Meanwhile, the side covers 43a and 43b extend upwardly slantingly in a direction of the coated steel sheet 1 from the upper ends of the lower end gas discharge bars 41 and 42 surrounding both sides of the coated steel sheet 1 in a width direction and having upper ends formed to be spaced apart from the surface of the coated steel sheet 1 by a predetermined distance. Accordingly, the discharged nitrogen gas 10 may be seized to the vicinity of the coated steel plate 1, rather than being dispersed.

(15) Advantages of the device for forming a nitrogen cloud of the present invention described above will be described compared with the related art apparatuses D1 to D4.

(16) 1) Since the device of the present invention forms the nitrogen cloud only in a partial space at a lower end of the air knife, the structure is not deformed due to latent heat generated by the related art box type and there is no factor hindering micronization of spangles due to a degradation of a cooling rate after coating.

(17) The method and device of the present invention relate to a method (or structure) of forming nitrogen DAM by forming a nitrogen curtain (nitrogen cloud) using a nozzle in a section of a lower end portion of the air knife in which oxidation may first occur or in which a dross may adsorbed to the strip on the surface of the coating molten metal, rather than having such a box type as that in the cited inventions in which the entirety of an air knife for controlling a coating attachment amount from the surface of the coating molten metal is covered. Since the nitrogen cloud 47 is formed using the nitrogen nozzle at upper and lower portions of the section of the lower end portion of the air knife and the interior thereof is maintained under a nitrogen atmosphere, rather than the method of filling the closed space with nitrogen, a gas may be smoothly flow from the interior of the apparatus outwardly, and thus, latent heat is not maintained. As can be seen in the drawings, since the nitrogen cloud 47 of the present invention is formed only in the partial space of the lower end of the air knife, it does not affect any structure (component) other than the surface of the coated molten metal or the strip on which coating is performed. Thus, a possibility of deforming the structure due to heat generated by the related art box type or generating an error due to heat of an electric device for driving the air knife such as various sensors or a motor is low.

(18) 2) The top dross may be easily removed

(19) Since the manufacturing apparatus of the present invention is spaced apart from the surface of the coating molten metal by a predetermined distance, rather than an atmosphere of being directly in contact with the surface of the coating molten metal or deposited, dross may be removed by personnel or a robot through the space without being interfered. Also, since the cloud in the form of a nitrogen curtain sprayed through the nozzle is constantly maintained even when the apparatus or a tool is inserted into the separated space to remove the top dross, it may also be effective in maintaining the nitrogen atmosphere.

(20) 3) Effect of preventing adsorption of top dross of the upper portion of the coated molten metal to strip

(21) Even though the port portion of the coated molten metal is filled with nitrogen in manufacturing an Mg-added alloy coated steel sheet, it is not possible to actually perfectly prevent a fine oxide film by the partial top dross and Mg having high oxidation. However, since the amount can be considerably reduced, the manufacturing method of spraying the nitrogen gas is applied.

(22) In the present invention, in order to suppress a fine oxide film at the upper portion of the coated molten metal and the top dross, the nitrogen atmosphere is formed and adsorption of the top dross and fine oxide film to the strip may also be physically prevented.

(23) In the present invention, when a nitrogen is sprayed downwardly from the lower nitrogen discharge bars 41 and 42, a nitrogen cloud is formed in a side direction of the coating port (see FIGS. 3 to 6). This generates an effect of physically preventing movement of the top dross and fine oxide film floating in an upper portion of the coating bath to the vicinity of the strip to thus prevent adsorption thereof to the strip.

(24) Thus, the method of the present invention obtaining the effect of preventing adsorption to the strip after coating simultaneously when the nitrogen atmosphere is formed is different from the related art device for suppressing an oxide by forming only the nitrogen atmosphere.

(25) 4) Reduction in cost for nitrogen gas

(26) Since the device of the present invention forms the nitrogen atmosphere only at the required partial space at the lower end of the air knife, the nitrogen cloud may be maintained only with a small amount of nitrogen coming from the lower end nitrogen discharge bars 41 and 42, and is more effective compared with the box type for supplying nitrogen while maintaining a pressure higher than normal pressure.

(27) Thus, the manufacturing method of the present invention is able to reduce a nitrogen usage amount, compared with the related art method for filling the interior of the box type with nitrogen. Also, the manufacturing method of the present invention is a manufacturing method which can exhibit a considerably effective oxide generation suppressing and adsorption preventing effect, compared with the related art method, even with the same amount of nitrogen.

(28) Hereinafter, the present invention will be described in more detail through comparison between Examples and comparative Examples. These examples are provided only to illustrate the present invention in more detail and should not be construed as limiting the scope and spirit of the present invention.

(29) A cold-rolled steel sheet with a thickness of 0.8 mm, a width of 120 mm and a length of 250 mm was coated using a melt-coating simulator. As shown in Table 1, a zinc-aluminum-based alloy-coated steel sheet was produced by changing a composition of the coating bath. In addition, a nitrogen cloud was formed using the nitrogen cloud formation device shown in FIGS. 3 to 6.

(30) The amount of adhered coating was controlled using an air knife and the amount of coating of the produced zinc-aluminum-based alloy-coated steel sheet evaluated based on one side is shown in Table 1.

(31) Evaluation items were corrosion resistance and workability. Corrosion resistance was compared with an initial rust generation time (5%) under a 35 C. NaCl salt spray test atmosphere in accordance with KSD 9504 and evaluated. Workability was compared and evaluated by observing a width (fracture width) of cracks generated after 180 OT bending test in accordance with a KSD 0006 test method using a 30 to 50 stereomicroscope and measuring the width of the fracture surface. Observation of alloy phase was carried out using an X-ray diffraction.

(32) Detailed test results obtained by the test method are given below.

(33) 1. Dross level: an amount of dross generated in an upper part of coating bath after molten coating specimens according to coating composition.

(34) : generation of 5% or less of dross with respect to coating bath

(35) : generation of 10 to 20% less of dross with respect to coating bath

(36) X: generation of 20% or more of dross with respect to coating bath

(37) 2. Surface appearance: visibility (clearance) and formation level of spangles of surface appearance of coating layer observed by the naked eye

(38) : Clear formation of spangles with high gloss

(39) : Non-clear formation of spangles

(40) X: Little formation of spangles with bad appearance

(41) 3. Corrosion resistance of shear surface: ratio of rust generated after salt spray test for 1,000 hours

(42) : rust ratio of 5% or less

(43) : rust ratio of 10 to 20%

(44) X: rust ratio of 30% or more

(45) 4. Corrosion resistance of flat portion: a ratio of rust generated after salt spray test for 2,500 hours.

(46) : rust ratio of 5% or less

(47) : rust ratio of 20 to 30%

(48) X: rust ratio of 30% or more

(49) TABLE-US-00001 TABLE 1 Coating bath composition Appln. (% by weight) of Ba nitrogen Items Mg Al (ppm) Balance MgZn.sub.2 AA B C D dam Exams. 1 1 3 10 Zn and Applied of 2 1 4 20 impurity present 3 1 5 80 invention 4 2 3 10 5 2 4 20 6 2 5 40 7 3 3 20 8 3 4 40 9 3 5 60 Comp. 1 2 X X X X Not Exams. 2 1 7 10 X X X Applied 3 3 7 10 X 4 0.5 4 5 X X X 5 4 4 X X X 6 4 5 100 X X * A: Dross level, B: Surface appearance, C: Corrosion resistance of shear surface, D: Corrosion resistance of flat portion

(50) As illustrated in Table 1, it can be seen that the manufacturing example employing the device of the present invention has excellent surface appearance and corrosion resistance.