US3841900A

US3841900A - Apparatus and method for obtaining uniform coating thickness

Info

Publication number: US3841900A
Application number: US00348396A
Authority: US
Inventors: C Parker; W Creek
Original assignee: Bethlehem Steel Corp
Current assignee: Bethlehem Steel Corp
Priority date: 1972-02-04
Filing date: 1973-04-05
Publication date: 1974-10-15
Anticipated expiration: 1991-10-15

Abstract

The weight and distribution of a metal coating on a vertically moving strip are controlled by streams of steam directed downwardly and across the face of the strip as it leaves a bath of molten coating metal.

Description

United States Patent 1191 Parker, Sr. Oct. 15, 1974 APPARATUS AND METHOD FOR [56] References Cited v OBTAINING UNIFORM COATING UNITED STATES PATENTS THICKNESS 3,459,587 8/1969 Hunter et al 1. 117/102 [75] Inventor: Calvin E. Parker, Sr., Walnut Creek, Calif. Primary ExaminerLeon D. Rosdol Assistant Examiner-Edith LI Rollins [73] Asslgnee' g t fi i Corporatlon Attorney, Agent, or Firm-Joseph J. OKeefe; John'l.

' e e Iverson; Robert M. Jones [22] Filed: Apr. 5, 1973 21 Appl. No.; 348,396 [57] ABSTRACT The weight and distribution of a metal coating on a Related Apphcatlon Data vertically moving strip are controlled by streams of [62] Dlvlslon of 223,535 1972, Pat steam directed downwardly and across the face of the 317731013 strip as it leaves a bath of molten coating metal. 52 US. Cl 117/102 M, 117/19, 117/114 A 5 Claims 5 Drawing Figures [51] Int; Cl. B05c 11/10 [58] Field of Search ll7/l02 M, 19,114 A;

BACKGROUND OF THE INVENTION air or steam, for controlling the thickness, or weight, of v a coating on one or both sides of a steel strip as the strip emerges from a bath of molten coating metal, e.g. zinc. The apparatus for directing the stream of gas toward the strip is generally referred to as a coating die, and may comprise conduit means, e.g. a pipe, having a source of gas connected thereto through a manifold. The pipe is provided with an exhaust slot along-its length. Typically, such a prior art coating die was disposed horizontally and was provided with an exhaust slot of constant width, inclined downwardly whereby the stream of fluid emitted from said slot is directed. Coated steel strip wiped by such a die was generally characterized by a non-uniform coating thickness, the thickness being at times as much as greater-at the sides than at the middle of the strip.

It is an object of this invention to provide an improved apparatus and method for obtaining a more uniform coating thickness.

SUMMARY OF THE INVENTION It has been found that the characteristically heavier coatings at the sides of strip wiped by prior art die arrangements are caused by turbulence resulting from gas collisions at the edges of the strip. I have discovered that such turbulence can be minimized by providing a pair of coating dies in registry on opposite sides of a verticallymoving strip as it leaves thecoating bath, each coating die comprising a manifold connected to conduit means extending transversely of, and overlapping the side edges of the strip, the conduit means being angularly disposed relative to the horizontal whereby one end of the die is at a greaterdistance from the bath than the other end. Each of the conduit means is provided with an exhaust slot along the length thereof whereby a narrow stream of fluid is directed out of the exhaust slot downwardly across the face of the strip. The force per unit area with which the stream impinges on the strip progressively decreases as the height of the point of impingement of the stream increases.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view of the coating dies relative to the strip being coated.

FIG. 2 is an endelevational view of the apparatus shown in FIG. 1. g

FIG. 3 is an enlarged sectional view taken along the lines 3-3 of FIG. 1.

FIG. 4 is an enlarged sectional view taken along the lines 4-4 of FIG. I.

2 FIG. 5 is an enlarged bottom plan view of an alternative embodiment of the coating dies of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 1, a steel strip 10 is leaving a bath 12 of molten zinc. The strip is passing vertically upwardly through a pair of coating dies 14, only one of which can be seen in FIG. 1 since the dies 14 are dis? posed in registry on opposite sides of the strip. As can be seen, each die 14 makes an angle of about 20 with a line perpendicular to the edge of the strip.

As is shown in FIGS. 2 and 3, each die 14 comprises a manifold 14a and conduit means 15 provided with an exhaust slot 16 along the length thereof through which a stream of fluid is directed downwardly toward the face of the strip. Preferably, the fluid is steam if the coating metal is molten zinc. Means (not shown) is provided for supplying the steam to each manifold 14a, and preferably the manifold 14a is connected to several points 18 intermediate the ends of the conduit means 15. The exhaust slot 16 is of constant width, e.g. 0.015

in., and the steam is supplied to the manifold 140 at an inlet pressure of 40 psi., for example, for a coating weight of 2 /2 oz. per square ft., although the proper inlet pressure is dependent upon the desired coating weight and the distance of the conduit means 15 from the face of the strip. Typically, inlet pressures may vary from 40 to psi.

Exhaust slot 16 is inclined downwardly so that the steam emitted therefrom is directed downwardly toward the face of the strip. Preferably, as is shown in FIG. 4, the plane passing through the centerline of the stream of steam emitted through the slot 16 makes an angle of about 30 with a plane through the axis of the conduit means 15 and normal to the face of the strip.

With an exhaust slot of constant width, it is essential that the lower end of the die 14 be positioned closer to the strip than is the upper end of the die. The molten zinc runs down the strip as it passes from the bath and, as the distance from the bath increases the coating, until it solidifies, becomes thinner. Hence, the steam is required to have less force per unit area as the distance from the bath increases if the coating is to be of uniform thickness across the strip face. For a strip 48 inches wide, for example, the conduit means 15 is preferably 5% inch from the strip at the lowermost point, and 1 inch from the strip at the uppermost point.

Alternatively, as is shown in FIG. 5, the dies can be positioned parallel to the plane of the strip if the exhaust slot 17 is tapered whereby the width of said slot 17 decreases as the height of said die increases. This arrangement also results in a larger force per unit area against the strip 10 where the steam from the lower end of the die impinges on the strip, the force per unit area progressively decreasing as the point of impingement increases in distance from the surface of the bath. Typically, the width of the slot 17 progressively decreases from 0.025 inch at the lower end of the die to 0.015 inch at the upper endthereof.

Asa result of the above-described dieconstruction and arrangement relative to the strip being coated, the uniformity in coating thickness is much improved over prior art coating dies. For example, I have produced galvanized strip having coatings which varied from 2.12

oz/ft? at the center of the strip to a maximum of 2.40 oz./ft. at the sides thereof with a pair of dies making an angle of 20 with a line perpendicular to an edge of the strip and having a constant widthexhaust slot of 0.020 inch disposed whereby steam emitted therethrough is directed downwardly at 30. The variation in coating'thicknesgwhich is about 1 1.7 percent, was the average of 55 tests. I have also produced galvanized strip having coatings which varied from 2.08 oz./ft. at the center of the strip to a maximum of 2.18 oz./ft. at the sides thereof with a pair of dies making an angle of 20 with a line perpendicular to an edge of the strip and having a tapered exhaust slot of 0015-0025 inches disposed whereby steam emitted therethrough is directed downwardly at 30. This variation in coating thickness, which is about 4.6 percent, was the average of 50 tests. n the other hand, a pair of prior art dies, horizontally disposed, having a constant width exhaust slot of 0.015 inch disposed whereby steam emitted therethrough is directed downwardly at 13, resulted in coatings which varied from 2.03 ore/ft. at the center'of as it leaves a bath of molten coating metal, comprising directing a narrow stream of fluid angularly downwardly across the face of said strip on each side thereof, the streams on opposite sides of said strip being in registry, the height of the point of impingement of said fluid increasing and the force per unit area of said fluid decreasing as said point of impingement progresses from one edge of the strip to the other.

2. An improvement as recited in claim 1, in which the point of impingement of said fluid increases at an angle of about with a line perpendicular to an edge of the strip, as said point progresses from one edge of the strip to the other.

3. An improvement as recited in claim 1, in which the plane passing through the centerline of said stream of fluid makes an angle of about 30 with a plane normal to the face of said strip and passing through the centerthe strip to a maximum of 2.60 ozjftf at the sides thereof. This variation, which is about 20.4 percent, was the average of 35 tests. Thus, as a result of the invention, coating uniformity can be improved by as much as a factor of 4.

While the invention has been described in connection with conventional galvanizing, it also has utility in the one-side galvanizing process. In this case, only one die would be used, and would be positioned so as to direct steam toward the side of the strip having the zinc coating thereon.

I claim:

1. An improvement in the method of controlling the thickness of a metal coating on a vertically moving strip line of the area impinged upon by said stream.

4. An improvement as recited in claim 2, in which the plane passing through the centerline of saidstream of fluid makes an angle of about 30 with a plane normal to the face of said strip and passing through the centerline of the area impinged upon by said stream.

5. An improvement in the method of controlling the thickness of a metal coating on a vertically moving strip as it leaves a bath of molten coating metal, comprising directing a narrow stream of fluid angularly downwardly across the face of said strip, the height of the point of impingement of said fluid progressively increasing and the force per unit area of said fluid progressively increasing and the force per unit area of said fluid progressively decreasing as said point of impingement progresses from one edge 'of the strip to the other. l=

Claims

1. AN IMPROVEMENT IN THE METHOD OF CONTROLLING THE THICKNESS OF A METAL COATING ON A VERTICALLY MOVING STRIP AS IT LEAVES A BATH OF MOLTEN COATING METAL, COMPRISING DIRECTING A NARROW STREAM OF FLUID ANGULARLY DOWNWARDLY ACROSS THE FACE OF SAID STRIP ON EACH SIDE THEREOF, THE STREAMS ON OPPOSITE SIDES OF SIAD STRIP BEING IN REGISTRY, THE HEIGHT OF THE POINT OF IMPINGEMENT OF SAID FLUID INCREASING AND THE FORCE PER UNIT AREA OF SAID FLUID

2. An improvement as recited in claim 1, in which the point of impingement of said fluid increases at an angle of about 20* with a line perpendicular to an edge of the strip, as said point progresses from one edge of the strip to the other.

3. An improvement as recited in claim 1, in which the plane passing through the centerline of said stream of fluid makes an angle of about 30* with a plane normal to the face of said strip and passing through the centerline of the area impinged upon by said stream.

4. An improvement as recited in claim 2, in which the plane passing through the centerline of said stream of fluid makes an angle of about 30* with a plane normal to the face of said strip and passing through the centerline of the area impinged upon by said stream.

5. An improvement in the method of controlling the thickness of a metal coating on a vertically moving strip as it leaves a bath of molten coating metal, comprising directing a narrow stream of fluid angularly downwardly across the face of said strip, the height of the point of impingement of said fluid progressively increasing and the force per unit area of said fluid progressively increasing and the force per unit area of said fluid progressively decreasing as said point of impingement progresses from one edge of the strip to the other.