Method and apparatus for manufacturing hot-dip metal plated steel strip

Abstract

A method for manufacturing a hot-dip metal plated steel strip having a plated layer formed on the outer surface thereof includes continuously immersing a steel strip in a bath tub that accommodates a molten metal plating bath, causing a sink roll disposed in the bath tub to change the traveling direction of the steel strip, then causing the steel strip to pass through two support rolls formed of a stabilizing roll and a correcting roll disposed above the sink roll but below the surface of the plating bath, and extracting the steel strip out of the bath. A flow regulation plate is disposed so as to cover at least the upper side of a roll body of the stabilizing roll out of the support rolls with a gap between the flow regulation plate and the stabilizing roll, and the flow of the molten metal plating bath directed toward a portion of the steel strip located from the exit of the support rolls to the surface of the plating bath is led toward the lower side of the stabilizing roll via the gap.

Claims

1. A method for manufacturing a hot-dip metal plated steel strip having a plated layer formed on an outer surface thereof comprising: continuously immersing a steel strip in a bath tub that accommodates a molten metal plating bath; causing a sink roll disposed in the bath tub to change a traveling direction of the steel strip; causing the steel strip to pass through two support rolls, wherein the two support rolls consist of a stabilizing roll, a correcting roll disposed above the sink roll but below a surface of the plating bath, and an entirety of a flow regulation plate, having an inner wall, is disposed along an outer surface of a roll body of the stabilizing roll, so as to cover at least an upper side of a roll body of the stabilizing roll with a gap between the flow regulation plate and the stabilizing roll; and extracting the steel strip out of the bath, characterized in that a flow of the molten metal plating bath directed toward a portion of the steel strip located from an exit of the support rolls to the surface of the plating bath is led toward a lower side of the stabilizing roll via the gap.

2. The method for manufacturing a hot-dip metal plated steel strip according to claim 1, wherein the gap has a gap dimension continuously or intermittently increasing from an entrance of the gap toward an exit thereof.

3. The method for manufacturing a hot-dip metal plated steel strip according to claim 1, wherein the gap has a gap dimension continuously or intermittently increasing from an entrance of the gap toward a center thereof while continuously or intermittently decreasing from the portion where the gap dimension is increased toward the exit of the gap.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic view (a side view) illustrating an embodiment of a manufacturing apparatus preferably used to implement the present invention.

(2) FIG. 2 is an enlarged view of main parts illustrating the manufacturing apparatus shown in FIG. 1.

(3) FIG. 3 is a schematic view illustrating an example of the cross-sectional shape of a gap.

(4) FIG. 4 is a schematic view illustrating another example of the cross-sectional shape of the gap.

(5) FIG. 5 is a view illustrating a state of the flow of a plating bath.

(6) FIG. 6 a schematic view illustrating an example of a related-art apparatus for manufacturing a hot-dip metal plated steel strip.

(7) FIG. 7 is a schematic view illustrating another example of a related-art apparatus for manufacturing a hot-dip metal plated steel strip.

(8) FIG. 8 is a schematic view illustrating another example of the manufacturing apparatus preferably used to implement an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(9) The embodiments of the present invention will be more specifically described below with reference to the drawings.

(10) FIG. 1 schematically illustrates (in the form of a cross section) an apparatus for manufacturing a hot-dip metal plated steel strip preferably used to implement an embodiment of the present invention, and FIG. 2 is an enlarged view of main parts of the apparatus shown in FIG. 1.

(11) In FIG. 1, reference character 1 denotes a bath tub that accommodates a molten metal plating bath M, and reference character 2 denotes a snout that has a front end portion immersed in the plating bath and leads a steel strip S to be plated into the bath tub 1, and reference character 3 denotes a sink roll that is disposed in the bath tub 1 and changes the traveling direction of the steel strip S, and reference character 4 denotes support rolls that are disposed above the sink roll 3 but below the surface of the molten metal plating bath, guides the steel strip S, and corrects the shape thereof. The support rolls 4 are formed of two rolls, a stabilizing roll 4a ((upper) support roll closer to bath surface) and a correcting roll 4b ((lower) support roll farther from bath surface).

(12) Reference character 5 is a wiping nozzle disposed above the surface of the molten metal plating bath so as to sandwich the steel strip S. The wiping nozzle 5 is located at the exit of the plating bath and has the function of spraying a gas onto the outer surface of the steel strip S and adjusting the thickness of the plated layer.

(13) Reference character 6 is a flow regulation plate that covers at least the upper side of the stabilizing roll 4a with a gap t (see FIG. 2). The flow regulation plate 6 has an inner wall 6a formed along the outer surface of a roll body of the stabilizing roll 4a and has the following dimensions: the size of the gap t formed between the flow regulation plate 6 and the stabilizing roll 4a ranges from 5 to 60 mm; a separation distance L (see FIG. 2) in the horizontal direction from an upper front end 6b to the steel strip S ranges from 50 to 100 mm; and a length H (see FIG. 2), over which the roll body of the stabilizing roll 4a is covered over an angular range θ from 60 to 135° around a rotational center P of the roll 4a from the upper front end 6b toward the bottom of the bath tub 1. The flow regulation plate 6 is preferably as wide as the roll body of the roll 4a or wider than the roll body of the roll 4a. The flow regulation plate 6 only may be resistant to hot-dip zinc and is preferably made, for example, of SUS or ceramic coated steel material having a thickness of not less than 15 mm.

(14) The gap t is preferably, for example, the gap shown in FIG. 3, which has a gap dimension continuously or intermittently increasing from an entrance t1 of the gap t toward an exit t2 thereof, or the gap shown in FIG. 4, which has a gap dimension continuously or intermittently increasing from the entrance t1 of the gap t toward a center t3 thereof while continuously or intermittently decreasing from the center t3, where the gap dimension is maximized, toward the exit t2 of the gap t. The thus set gap t allows dross in the vicinity of the steel strip in the plating bath to be efficiently removed. It is noted that FIGS. 3 and 4 described above show an example of the gap t, and that the gap t does not necessarily have the shape shown in FIGS. 3 and 4. In particular, an apparatus having the gap t shown in FIG. 4 has a maximum gap dimension at the center t3 of the gap t, but the portion where the gap dimension is maximized may be shifted toward the entrance t1 or the exit t2 of the gap t.

(15) In a surface layer region of the plating bath, the plating bath typically flows toward the steel strip S (flows from the side facing the wall surface of the bath tub 1 toward the steel strip S), as shown in FIG. 5. Therefore, even in an apparatus in which a flow regulation plate 7 or 8 is disposed on the downstream side of the stabilizing roll 4a ((upper) support roll closer to the bath surface) (conventional apparatus) in the plating bath facing the roll 4a, as shown in FIG. 6 or 7, it is difficult to avoid adhesion of the top dross present in the region to the steel strip S. On the other hand, in the present invention, since the flow of the plating bath can be led toward the lower side of the stabilizing roll 4a via the gap t, as shown in FIG. 2, adhesion of the top dross to the steel strip S can be suppressed.

(16) In an embodiment of the present invention, the gap t is set to a value ranging from 5 to 60 mm, the reason of which is that the gap t ranging from 5 to 60 mm allows the flow of the plating bath to be efficiently led to the entrance of the gap t. The gap t more preferably ranges from 10 to 40 mm.

(17) Further, in an embodiment of the present invention, the separation distance L from the upper front end 6b of the flow regulation plate 6 to the surface of the steel strip S is set to a value ranging from 50 to 100 mm. The reason thereof is that the separation distance L ranging from 50 to 100 mm allows the flow of the plating bath to be smoothly introduced into the gap t. When the separation distance L exceeds 100 mm, the distance from the upper front end 6b to the steel strip S is too large, and it is therefore difficult to lead the dross into the gap t and the dross may undesirably be caught on the steel strip S, whereas when the separation distance L is less than 50 mm, the dross may adhere to the steel strip S when the dross is led into the gap t.

(18) Further, in an embodiment of the present invention, the flow regulation plate 6 covers the roll body of the stabilizing roll 4a over the angular range from 60 to 135°. The reason thereof is that when the range over which the roll body of the stabilizing roll 4a is covered is less than 60°, the flow of the plating bath cannot be efficiently led to the lower side of the stabilizing roll 4a, whereas when the range over which the roll body of the stabilizing roll 4a is covered exceeds 135°, the flow of the plating bath reaches the steel strip S located below the stabilizing roll 4a, and the dross may be undesirably caught on the steel strip S.

(19) The upper end of the immersed flow regulation plate 6 is preferably below the bath surface by not more than 10 mm so that surface layer flow is not hindered. The upper end of the immersed flow regulation plate 6 is also set in a position above the upper surface of the support rolls 4 (position close to liquid surface). The upper end of the immersed flow regulation plate 6 is automatically determined by the position where the stabilizing roll 4a of the support rolls 4 is disposed. For example, in the following conditions: the upper surface of the stabilizing roll 4a of the support rolls 4 is located in a position below the bath surface by a depth of 60 mm; the thickness of the flow regulation plate 6 is 10 mm; and the gap t is 10 mm, the upper end of the immersed flow regulation plate 6 is located in the position below the bath surface by a depth of 40 mm.

(20) The embodiment of the present invention is described with reference to the case where the stabilizing roll 4a is disposed on the rear side of the steel strip S (the same side as the steel strip in positional relationship between the sink roll and the steel strip) and the correcting roll 4b is disposed on the front side of the steel strip S (the same side as the sink roll in positional relationship between the sink roll and the steel strip). Alternatively, an apparatus shown in FIG. 8, in which the correcting roll 4b is disposed on the rear side of the steel strip S and the stabilizing roll 4a is disposed on the front side of the steel strip, can also be employed. Also in this case, the flow regulation plate 6 is provided along a part of the upper roll, that is, the stabilizing roll 4a.

EXAMPLES

Example 1

(21) A cold-rolled steel strip having a width ranging from 800 to 1900 mm and a thickness ranging from 0.4 to 4.0 mm is placed in the manufacturing apparatus shown in FIG. 1 described above, in which a flow regulation plate is disposed in a setting condition shown in Table 1. Hot-dip galvanizing is performed on the cold-rolled steel strip under the following conditions: the bath temperature: 450 to 460° C.; the adhesion amount of plating: 45 to 90 g/m.sup.2 per one side; and the linear speed: 60 to 150 mpm, and five sample material plates each having a size of 500 mm by 500 mm are collected from a hot-dip galvanized steel strip (coil) resulting from 300 tons of the processed cold-rolled steel strip. Hat-shape processing (punch diameter: 300 mm ϕ (R: 21 mm), wrinkle preventing pressure: 320 kN, punch speed: 320 mm/min, and product height: 27 mm) is performed on the five collected sample material plates. After the upper surface of each of the five resultant hat-shaped plates is ground with a #150 grindstone over one reciprocal motion, the number of foreign matter objects resulting from the dross is measured, and the number of foreign matter objects in the five hat-shaped plates were averaged. Table 1 shows the number of foreign matter objects per hat-shaped plate. The reason why the upper surface of each of the hat-shaped plates is ground is that the foreign matter objects is thus easily visually recognizable.

(22) TABLE-US-00001 TABLE 1 Average number Flow regulation plate of foreign Upper end of matter objects immersed flow Separation resulting from regulation plate Gap t Angle θ distance L dross (number per Number (mm) (mm) (°) (mm) hat-shaped plate) 1 — — — — 24.9 2 20 40 90 100 16.7 3 10 40 90 100 19.5 4 10 10 90 100 17.9 5 40 10 90 100 3.3 6 40 10 135 100 2.1 7 40 40 135 100 5.6 8 30 10 90 100 5.8 9 30 20 120 50 7.6 10 40 10 120 20 28.5 11 40 10 60 50 21.8 12 5 45 135 100 16.1 13 45 5 135 50 18.5 14 40 40 90 200 23.1 15 40 10 170 50 31.5 16 40 10 135 50 2.6

(23) In the case where the steel strip is plated in accordance with the embodiments of the present invention, Table 1 shows that adhesion of dross is suppressed and a product having good quality can be manufactured.

Example 2

(24) A manufacturing apparatus having a structure shown in FIG. 3 (upper end of immersed flow regulation plate: 10 mm, angle θ: 90°, and separation distance L: 100 mm) having a tapered gap having the gap dimension continuously increasing from the entrance to the exit of the gap (gap dimension at entrance t1: 10 mm, gap dimension at exit t2: 20 mm) and a manufacturing apparatus having a structure shown in FIG. 4 (upper end of immersed flow regulation plate: 40 mm, angle θ: 135°, and separation distance L: 100 mm) having a crescent-shaped gap having a gap dimension continuously increasing from the entrance to the center of the gap (gap dimension at entrance t1: 10 mm, gap dimension at center t3: 40 mm) and decreasing from the center to the exit of the gap (gap dimension at exit t2: 10 mm) are used to perform hot-dip galvanizing on the same cold-rolled steel strip as in Example 1, and the average number of foreign matter objects resulting from the dross is examined (the plating conditions and the examination conditions under which average number of foreign matter objects resulting from dross are the same as those in Example 1). As a result, in the case of the apparatus having the tapered gap, the average number (number per hat-shaped plate) of foreign matter objects resulting from the dross is 13.5, and the quality is further improved as compared with the number 4 in Table 1. In the case of the apparatus having the crescent-shaped gap, the average number of foreign matter objects resulting from the dross is 1.1, and the quality is further improved as compared with the number 6 in Table 1.

(25) The present invention can provide a method and apparatus capable of manufacturing a hot-dip metal plated steel strip with surface defects resulting from adhesion of dross suppressed.

REFERENCE SIGNS LIST

(26) 1 bath tub

(27) 2 snout

(28) 3 sink roll

(29) 4 support roll

(30) 4a stabilizing roll

(31) 4b correcting roll

(32) 5 wiping nozzle

(33) 6 flow regulation plate

(34) 6a inner wall

(35) 6b upper front end

(36) 7 flow regulation plate

(37) 8 flow regulation plate

(38) S steel strip