US3802911A

US3802911A - Method for controlling the coating thickness of a coated metal strip

Info

Publication number: US3802911A
Application number: US00255403A
Authority: US
Inventors: Camera A La; J Semer
Original assignee: Youngstown Sheet and Tube Co
Current assignee: Ltv Steel Co Inc; Jones and Laughlin Steel Inc
Priority date: 1971-01-28
Filing date: 1972-05-22
Publication date: 1974-04-09
Anticipated expiration: 1991-04-09

Abstract

Method for continuously coating an advancing metal strip substrate with molten treating material and controlling the coating thickness by a fluid jet stream. The stream is projected through a discharge nozzle opening which is greater than the strip width but the effective width of the jet stream is controlled to generally correspond to the strip width by means of tracking baffles. The stream is also projected a substantially uniform distance across the effective width, as opposed to projecting the outside edges or zones of the stream a shorter distance than the middle stream zone.

Description

United States Patent 1 La Camera et al.

[ METHOD FOR CONTROLLING THE COATING THICKNESS OF A COATED METAL STRIP [75] lnventors: Alfred F. La Camera, Youngstown; Joseph R. Semer, Columbiana, both of Ohio [73] Assignee: Youngstown Sheet and Tube Company, Youngstown, Ohio [22] Filed: May 22, 1972 [21] Appl. No.: 255,403

Related US. Application Data [62] Division of Ser. No. 110,534, Jan. 28. 1971, Pat. No.

[52] US. Cl. 117/102 M, 117/102 L, 117/131, l18/8,118/63 [51] Int. Cl B44d 1/06 [58] Field of Search 15/307; 117/64 R, 102 R, 117/102 M, 102 L, 131; 118/8, 63

[56] References Cited UNITED STATES PATENTS 3,032,006 Kahn et al. 118/8 I EFFECTIVE NOZZLE [111 3,802,911 [451 Apr.9, 1974 Fowells et al. 117/102 L Senwedler et a1. 118/8 Primary Examiner-Edward G. Whitby Attorney, Agent, or Firm-John Stelmah 5 7 ABSTRACT Method for continuously coating an advancing metal strip substrate with molten treating material and controlling the coating thickness by a fluid jet stream. The stream is projected through a discharge nozzle opening which is greater than the strip width but the effective width of the jet stream is controlled to generally correspond to the strip width by means of tracking baffles. The stream is also projected a substantially uniform distance across the effective width, as opposed to projecting the outside edges or zones of the stream a shorter distance than the middle stream zone.

5 Claims, 3 Drawing Figures DIS CH/l RGE WIDTH PATENTEDAPR 9 1914 $802.9 1 1 EFFECT/VE NOZZLE DISCHARGE W/DTH EFFECTIVE NOZZLE DISCHARGE W/DTH METHOD FOR CONTROLLING THE COATING THICKNESS OF A COATED METAL STRIP CROSS-REFERENCE TO RELATED APPLICATION This is a division of application Ser. No. I 10,534 filed Jan. 28, 1971, now US. Pat. No. 3,687,103.

BACKGROUND OF THE INVENTION This invention relates generally to methods for controlling continuous coating of metal strip with molten treating material, and more particularly to methods for continuous galvanize coating of steel strip, and most particularly to novel and improved method for controlling the coating thickness across the width of the strip.

Edge build-up on galvanized strip is a problem which has plagued the galvanizing industry. Such build-up is considered to be excessive when the zinc deposit at the edges creates a spooling effect when the strip is coiled.

Others in the art have recognized this edge build-up problem and have sought to solve the problem in various ways. Considerable previous art is directed to various techniques of increasing the impact pressure at the strip edges. Some attempt to solve the problem by providing specially contoured discharge openings in the jet dies through which pressurized fluid is directed toward the strip. This is undesirable on galvanizing lines where strips of different widths are processed. In such cases a separate jet die set is required for each width size with a concomitant loss in production because of additional jet die set-up time. Others, as exemplified by the disclosure in U.S. Pat. No. 2,894,856, provide means for directing separate fluid streams at the strip edges and against the molten metal pool surface.

The art has recognized that, with the use of air knife" fluid streams and in the absence of compensating devices, there occurs build-up of the coating at the strip edges. It is believed that in the narrow zones adjacent the edges of the strip, the vertical components of the fluid streams are less than in the center portions of the strip, and that this reduction in vertical component permits the heavier build-up at the strip edges. In US. Pat. No. 3,526,204, it is disclosed that the wiping action of the fluid stream be increased in the narrow zones adjacent the edges of the strip by means of a nozzle extension which brings the jet stream closer to the strip and increases the impact pressure.

In the present invention, more simple and facile methods are provided and which present a different mode of operation. Rather than increase the wiping action at the edges, the method of this invention provides a fluid stream which has a substantially uniform distribution and impact across the width of the strip, including the narrow zones adjacent the edges.

OBJECTS AND SUMMARY OF THE INVENTION It is an object of this invention to provide method for improving the distribution of coating material across the width of a metal substrate.

A specific object of this invention is to provide more simple and facile methods for controlling a fluid stream directed toward an advancing coated strip to deter excessive build-up at the strip edges.

A more specific object of this invention is to provide a generally uniform fluid stream impact across the width of a coated metal strip, as distinguished from increased impact at the strip edges, for controlling the thickness of the coating material.

Briefly, the objects are attained by and this invention comtemplates baffling the discharge opening of a fluid stream nozzle for defining an effective width of the fluid stream which corresponds generally to the width of the strip, and without the necessity of bringing the edges of the fluid stream closer, than other portions of the stream, to the strip. Tracking is also provided so that the baffles may follow the strip when the strip moves transversely across the width of the nozzle and to maintain a relatively constant relation between the strip width and the effective fluid stream width.

DESCRIPTION OF THE DRAWING The invention will be more fully understood and further objects and advantages thereof will become apparent when reference is made to the following detailed description and to the accompanying drawing, in which:

FIG. 1 is front elevational view of a fragmentary portion of a continuous galvanizing line showing a steel strip exiting from the molten zinc bath together with a schematic representation of a jet nozzle forming part of the control apparatus of this invention;

FIG. 2 is side elevational view taken along line 2-2 of FIG. 1 together with jet nozzles positioned on both sides of the strip; and

FIG. 3 is cross-sectional plan view, taken along line 33 of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT This invention is particularly adapted for use in conjunction with a continuour galvanizing line and will be described in that connection. However, it will be understood that the principles can be applied to other metal coating operations.

Referring to the drawing, a molten bath of coating material 10, such as zinc, is contained in tank 12. The numeral 14 designates a strip of material, such as steel, advancing out of the bath and carrying on its surface some of the coating material.

Above the bath, the coating on the strip 14 is screeded, on opposing sides, by a pair of fluid streams emanating from

discharge nozzles

16 and 18. As the strip 14 advances vertically, sometimes it also has a tendency to move sideways, in a direction corresponding to the lateral extent of the discharge nozzles, i.e., transverse to the advancing path or movement of the strip. This transverse movement may be as great as six or eight inches in some cases. To compensate for this transverse movement, the

nozzles

16 and 18 are made wider than the widest strip to be processed and thus insure coverage of the entire strip width by the fluid streams.

In a representative embodiment of this invention, a pair of baffles 20 is provided for each of the

nozzles

16 and 18. For the purpose of description, the baffles are separately designated as 20 R and 20 L. The corresponding support structure for each baffle is likewise designated by similar numerals but with R and L suffixes.

Tracking means is provided for each baffle 20 of a pair to enable the baffles to follow the transverse movement of the strip 14 and maintain the space relationship therewith hereinafter described. The tracking means as illustrated comprises a bracket 24, reciprocable sup- 5 spring or a motor fluid or electric. The support 26 also carries bracket 24 with the baffle thereon. Thus, viewing the strp face as in FIG. 1, when the strip 14 shifts to the left the urging means 30 L will be compressed by the strip edge X acting on follower 28 L; concomitantly, the urging means 30 R will extend causing foll5 lower 28 R to follow edge Y.

In a preferred embodiment, the follower 28 and baffle are relatively positioned so that the distance d, from the inside baffle edge 20 e (edge nearest the strip) to the strip 14, is in the range of one-eighth to three- 20 fourths inch. Stated in another way, when the baffles comprising a pair on one side of a strip width face, are positioned generally equally about the strip width, the baffles define therebetween an effective fluid stream discharge width which is greater than the strip width generally in the range of one-fourth inch to 1V2 inches. While the reasons are not fully known, it has been found that if the distance d is closer than one-eighth inch considerable turbulence occurs at the edge zones.

On the other hand, if the distance d is greater than three-fourths inch, the baffles do not appear to be effective toward equalizing the distribution of the fluid stream at the edge zones. The baffles have been found to be most effective when the distance is in the order of one-eighth to three-eighths inch.

The discharge opening of each nozzle may be spaced from the strip, in a direction transverse to the face of the strip, a distance s up to 1% inches, however the preferred general range is one-fourth to three-fourths inch. The baffles 20 are positioned as close to the face of the nozzle defining the discharge opening as possible without interferring with the slidable transverse movement of the baffles across the nozzle face.

The following table of data has been prepared from trials that have been conducted. The table sets forth examples where: no baffle (NB) was used at either strip width edge (X and Y); no baffle at one edge and a baffle B at the other edge; and a baffle B at each edge. The

line speed of the strip was in the range of to 5 feet per minute.

EDGE (l STRIP GAcE WIDTH MID: mcnes WIDTH 1. The strip edges are designated as X and Y; l I B des ignates that No Baffle was used at the respective edge; B designates the use of a Baffle" at the respective edge. I

2. d with the respective numerals designates the distance, in inch fractions, between the edge of the strip and the inside edge of the corresponding baffie.

3. Ratio of coating weight at designated zones, oneeighth inch and one-fourth inch from respective strip edges, as compared with mid-width coating weight.

From the table it will be observed that at the edges where no baffle was used, these edges were always thicker than the center in each case. The variation was from a +0.8 to +3.8 mils. In contrast, the variation in gage comparing baffled edge to center of strip was 0.8 to +0.5 mil; this represents a substantial reduction in edge build-up and in the concomitant propensity for spooling when the strip is coiled.

It will also be noted that the coating weightratio of non-baffled edge zone compared to outer zone is substantial, i.e., at one-eighth inch from edge the ratio varies from 2.16 to 3.10 and at one-eighth inch the ratio varies from 1.52 to 2.28. In contrast, the baffled edge zones varied, at one-eighth inch, from 1.08 to 2.02 and at one-fourth inch from 0.80 to 1.05. This evidences substantial improvement in uniformity of coating thickness on the strip surface, and particularly in comparing the strip edge zones with the strip mid-width.

What is claimed is:

1. In a coating process wherein a fluid stream is directed through a first nozzle at a first side of a vertically advancing coated strip to screed excess coating therefrom, the steps of:

maintaining an effective fluid stream width corresponding to, but greater than, the strip width by means of baffles which are spaced apart a distance corresponding to said effective fluid stream width;

tracking transverse shifting of said strip, transverse to its advancing movement; and

automatically and correspondingly shifting said baffles in a manner whereby said effective fluid stream width is maintained.

2. ln a coating process as described in claim 1,

wherein:

a fluid stream is directed at said strip through a nozzle on the side opposing said first side, and which further comprises:

maintaining an effective tluidstream width, from the comma WEIGHT RATlOtZi) ap ears ;satire;susstsmianyzaaai raise effective fluid stream width from said first nozzle.

tracking movement of said strip transverse to the advancing movement; and

maintaining a generally uniform spacing between the discharge opening of said nozzle and the strip face being screeded, across said effective fluid stream width.

5. The process as described in claim 4, wherein:

said effective fluid stream is generally equally spaced across and is greater than said strip width in the range of one-fourth inch to l /2 inches.

Claims

2. In a coating process as described in claim 1, wherein: a fluid stream is directed at said strip through a nozzle on the side opposing said first side, and which further comprises: maintaining an effective fluid stream width, from the opposing nozzle, substantially equal to the effective fluid stream width from said first nozzle.
3. In a coating process as described in claim 2, wherein a separate pair of baffles is provided for each of said first side and opposing side of the strip, and which further comprises: simultaneously shifting each pair of baffles in response to tracking of a transverse shifting of said strip.
4. In a coating process wherein a fluid stream is directed through a nozzle at an advancing coated metal strip to screed excess coating therefrom, the steps of: maintaining an effective fluid stream width corresponding generally to, but greater than, the strip width; tracking movement of said strip transverse to the advancing movement; and maintaining a generally uniform spacing between the discharge opening of said nozzle and the strip face being screeded, across said effective fluid stream width.
5. The process as described in claim 4, wherein: said effective fluid stream is generally equally spaced across and is greater than said strip width in the range of one-fourth inch to 1 1/2 inches.