US3322558A

US3322558A - Galvanizing

Info

Publication number: US3322558A
Application number: US287887A
Authority: US
Inventors: Jr Charles A Turner
Original assignee: Selas Corp of America
Current assignee: Selas Corp of America
Priority date: 1963-06-14
Filing date: 1963-06-14
Publication date: 1967-05-30
Anticipated expiration: 1984-05-30

Description

y 1967 c. A. TURNER, JR 3,322,558

GALVANIZ ING Filed June 14, 1963 INVENTOR. 22 25 6 CHARLES A. TURNER JR.

4'. Q QSKM ATTORNEY.

United States Patent 3,322,558 GALVANIZING Charles A. Turner, Jr., Flourtown, Pa., assignor to Selas Corporation of America, Dresher, Pa., a corporation of Pennsylvania Filed June 14, 1963, Ser. No. 287,887 Claims. (Cl. 117-46) The present invention relates to the continuous galvanizing of sheet steel, and more particularly to post heating of galvanized strip, or galvannealing, as it is generally known.

When strip metal, usually steel, is galvanized it presents a bright, Spangled surface that is desirable for many purposes. The usual bright, spangled coating will not accept paint, and is too rough to permit a smooth paint finish. There is a demand, however, for galvanized strip that can be painted and can be formed to a finished shape without flaking or peeling of the coating. A suitable finish for painting is commonly produced by extending the period during which the zinc coating remains molten as it exits from the zinc bath during the continuous galvanizing operation. By elevating the strip temperature, the iron-zinc alloying action originating at the coating-base metal interface is accelerated and is extended to the point of appearing at the coating surface. Control of the heating aims to develop a surface alloy analyzing 8% to 12% iron for maximum ductility. This is usually accomplished by passing the strip leaving the pot through a furnace where it is heated to a requisite temperature on a time cycle producing the desired surface alloy layer.

Experience has shown that with ordinary heating equipment it is difficult, if not impossible, to obtain a uniform alloy layer of proper iron content over the full strip width. characteristically, the thin zinc coating above the pot rolls is of irregular thickness. The usual radiant heat transfer method results in slow heating due to the reflectivity of the molten zinc surface. As the alloy first reaches the surface in the thinner areas the altered emissivity (darkening) accounts for an increased heating rate. Continued heating, to develop the surface alloy in the thicker areas, extends the alloying action in the thinner portions to the point of coating embrittlement. This accounts for peeling or flaking of the coating in subsequent severe forming operations.

It is an object of the invention to provide a method of post heating or galvannealing strip metal in such a fashion that the alloyed zinc coating is uniformly ductile over its full width.

It is a further object of the invention to provide apparatus for continuously and uniformly heating a bright strip of metal.

A further object of the invention is to provide a method and apparatus for continuously heating strip material by convection heat, and in which the apparatus has practically no heat inertia.

According to the invention, the strip, as it leaves the I galvanizing pot, is drawn upwardly between rows of open burners. These burners are mounted in an open frame so that there is no refractory to hold and store heat. Therefore, the strip is heated solely with convection heat by the flame, and the effect of the emissivity of the sheet is of no concern.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages and specific objects attained with its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

In the drawings:

FIG. 1 is a diagrammatic view of the apparatus used in the invention,

FIG. 2 is a view taken of one pair of burners on line 2-2 of FIG 1,

FIG. 3 is a section through one of the burners, and

FIG. 4 is an enlarged and exaggerated view of a section of the strip being heated.

Referring to FIG. 1, there is shown a strip of steel 1 moving downwardly into a galvanizing tank 2, around roller 3, between exit rolls 4 and upwardly through the post heating or galvannealing apparatus 5. The heating apparatus includes a frame 6 that is suspended by rollers 7 from a pair of beams 8 along which the apparatus can be quickly moved from a position surrounding the strip to a position beyond one edge thereof according to production requirements.

Mounted on frame 6 and extending horizontally therefrom on opposite sides of strip 1 are a plurality of horizontal burners 9, with the burners on one side of the strip being staggered vertically with respect to those on the opposite side. The burners are of the so-called ribbon type and are of a length at least equal to the maximum Width of the strip to be heated. The burners are identical and may be of various forms. As shown herein, however, each burner includes a casting 11 forming a distributing chamber 12, and a supply chamber 13. These chambers are separated by a porous diffusion screen 14 which is preferably made of a porous ceramic material and which assures a uniform distribution of fuel throughout the length of the burner. Fuel, in the form of a combustible mixture of gas and air, is supplied to chamber 12 and flows through screen 14 evenly to chamber 13. From there it is discharged through a burner screen 15 that may be ceramic or metal, and which is provided with a plurality of accurately formed rows of openings or ports 16, to burn as a substantially continuous flame of uniform length from end to end of the burner. It is noted that the edges of screen 15 are somewhat protected by lips 17 which serve to trap fuel for piloting purposes. It is also noted that casting 11 is provided with passages 18 through which cooling water i circulated. While the casting has been shown as one piece, it will be obvious that it will be made in sections to facilitate assembly.

Each burner casting is attached by flanges 19 to a manifold 21 which is supplied with a fuel mixture through a pipe 22. The fuel mixture may be supplied to the burners from any desired source. As shown herein, each oppositely disposed pair of burners is supplied by gas and air, respectively, through

pipes

23 and 24 leading to a flow mixer 25 for each burner. The vertically displaced gas and air pipes are connected to main supply pipes, and through flexible pipes (not shown) that permit movement of the furnace, to suitable sources of supply. Control valves are provided, in a conventional manner, in each of the

pipes

23 and 24, or in the main air and gas supply pipes or both. With this arrangement a larger or smaller number of burners can be used, or the fuel to all of the burners can be adjusted simultaneously. Also air alone can be supplied through the top burners to chill the strip.

In assembling the parts of the apparatus, the burners facing each other on opposite sides of the strip will be from eight to ten inches apart, or from four to five inches from each side of the strip path. The burners will be separated vertically by about one foot, with the burners on one side of the strip being displaced vertically about six inches from the burners on the opposite side. The burners Will normally be a couple of inches longer than the width of the widest strip to be heated. If narrower strip is to be heated the burners can be provided with flame limiting stops like those shown in Patent 2,199,951.

When using the apparatus to galvanneal strip, the strip is first moved through zinc pot 2, which is maintained at a temperature of about 850 F. While the zinc is still molten, the strip is moved between the vertical rows of burners in the furnace. During this travel the strip is heated by flame impingement from the burners to from 950 F. to 1250" F. depending upon the speed of the strip, strip gauge and the thickness of the zinc coating thereon. The flame striking the strip substantially perpendicularly are deflected above and below the line of impingement to provide a wash of hot gases over the complete surface of the strip. The convection heat transfer, independent of any surface emissivity variation, provides a uniform and rapid heating of the coated strip. In this manner, the development of the desired uniform surface alloy is accomplished in spite of the variable coating thickness. During the process the temperature of the strip is measured by a suitable pyrometer 27 located at the upper end of the furnace. This temperature is used to control, by means of conventional instruments, the supply of fuel to the burners, the number of burners used or the speed of the strip. Since the alloying is a function of time at temperature, any of these variables can be adjusted to produce the desired alloying effect. Ordinarily the strip speed is kept constant and the fuel supply varied. As the strip leaves the burners it is cooled to freeze the zinc and stop the alloying action. The alloying action is arrested when the coating surface has from 8% to 12% iron for maximum ductility. Such a surface has a substantially uniform, dull finish.

FIG. 4 shows in exaggerated form a coating 26 of zinc on steel strip 1 as it leaves zinc pot 2. Some portions are thicker, as shown at a, while others are thinner as indicated at b. Although the difference in thickness is quite small, it does have an effect on the product with ordinary heating equipment. In such equipment, as heat is absorbed by the molten coating, alloying action proceeds and reaches the surface initially in the thinnest coating areas, resulting in a darkening in appearance. The resultant change in surface emissivity then accelerates the heat transfer in these areas. Meanwhile heating proceeds slowly in the thicker areas, but by the time the proper iron content is developed at these surfaces, the alloying action in the thinner areas has proceeded to embrittlement.

With the heating apparatus of the present invention, however, the heat transfer is faster and more uniform because it is of the convection rather than the radiation type and is independent of emissivity considerations. The heating effect of the burners can be changed as quickly as a fuel valve can be adjusted. There is no refractory brickwork to absorb heat and to radiate heat to the strip. The burners will heat the strip from pot temperature of about 850 F. to the maximum desired in from three to six seconds. The time, of course, depends upon the gauge of the strip, but for faster strip speeds more burners can be added.

The method and apparatus of the present invention will produce an improved quality galvannealed strip because of the uniformity of the coating surface that is produced which permits an adherent, smooth paint layer to be applied and which will not peel or flake in severe forming applications. Further-more, maintenance of the apparatus is low because of the lack of refractory in and around the burners. This lack of refractory contributes to the speed of response since there is no radiant refractory wall which requires heating and cooling time. When the heating apparatus is moved into position it is ready to operate at full capacity.

While in accordance with the provisions of the statutes I have illustrated and described the best form of embodiment of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit and scope of the invention set forth in the appended claims, and that in some cases certain features of my invention may be used to advantage without a corresponding use of other features.

What is claimed is:

1. The method of treating the surface of an unenclosed galvanized steel strip to produce an iron-zinc alloy on the surface thereof which comprises moving the strip into a galvanizing pot and upwardly from the pot through a path, heating the strip as it is moving through the path substantially solely by convection heat produced by rows of flames directed entirely against and across the width of the strip from opposite sides thereof as it travels through said path to produce alloying of the zinc, and the iron of the strip, removing the strip from the flames, and cooling the strip when the iron on the suface thereof has reached from 8% to 12%.

2. The method of treating a galvanized steel strip which comprises directing the strip through a galvanizing bath and from the bath through an unenclosed path, directing flames resulting from the burning of fuel transversely against and across the entire width of said strip from a plurality of locations on each side thereof and spaced in the direction of strip travel as said strip moves through said path to heat said strip substantially entirely by convection, varying the fuel supplied to the burners to heat the strip to a temperature where alloying of the zinc and iron will take place, and arresting the alloying action when the iron-zinc alloy on the surface of the strip is a substantially uniform dull finish.

3. The method of treating a galvanized steel strip which comprises moving the strip from a galvanizing bath at a constant speed through an unenclosed path, directing a plurality of flames produced from the burning of fuel against the surface of the strip across its width and from opposite sides at a plurality of points along said path in the direction of strip travel as it moves through said path to heat the strip substantially entirely by convection from the flames, varying the fuel supplied to said flames to heat the strip sufliciently by the time it leaves said path to produce on the strip surface a zinc-iron alloy with a substantially uniform iron content of from between 8% and 12%.

4. The method of galvannealing a strip of steel which comprises moving the strip into a galvanizing bath and vertically upward through an unenclosed path, heating the strip from opposite sides as it moves through said path by directing a plurality of open flames spaced in the direction of strip travel against the strip, the flames extending transversely across the entire width of the strip to raise the temperature of the strip substantially entirely by convection from that of the galvanizing bath to between 900 F. and 1200 F. to produce the growth of an iron-zinc alloy toward the surface of the strip, and removing the heat from the strip when the surface of the strip contains from 8% to 12% iron.

5. The method of treating the surface of a galvanized steel strip to produce an iron zinc alloy on the surface thereof which comprises moving the strip into a galvanizing pot and from the pot upwardly through an enclosed path, supplying a plurality of flames of substantially uniform height and of a length at least equal to the width of the strip, directing the flames against opposite sides of the strip at points spaced along said path in the direction of strip travel, raising the temperature of the strip substantially entirely by convection by direct contact of the flames therewith to produce alloying of the iron and zinc, and removing the strip from the flames when the surface of the strip contains from 8% to 12% iron.

(References on following page) 6 FOREIGN PATENTS 382,274 10/1932 Great Britain.

ALFRED L. LEAVITT, Primary Examiner.

RALPH S. KENDALL, Examiner.

W. L. SOFFIAN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,322,558 May 30, 1967 Charles A. Turner, Jr

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 11, for "an unenclosed" read a line 14 for "a" read an unenclosed line 17 for "entirely against and" read against and entirely line 65, for "enclosed" read unenclosed Signed and sealed this 9th day of January 1968.

(SEAL) Attest:

Edward M. Fletcher, Jr. EDWARD J. BRENNER Attesting Officer Commissioner of Patents

Claims

1. THE METHOD OF TREATING THE SURFACE OF AN UNENCLOSED GALVANIZED STEEL STRIP TO PRODUCE AN IRON-ZINC ALLOY ON THE SURFACE THEREOF WHICH COMPRISES MOVIG THE STRIP INTO A GALVANIZING POT AND UPWARDLY FROM THE POT THROUGH A PATH, HEATING THE STRIP AS IT IS MOVING THROUGH THE PATH SUBSTANTIALLY SOLELY BE CONVECTION HEAT PRODUCED BY ROWS OF FLAMES DIRECTED ENTIRELY AGAINST AND ACROSS THE WIDTH OF THE STRIP FROM OPPOSITE SIDES THEREOF AS IT TRAVELS THROUGH SAID PATH OF PRODUCE ALLOYING OF THE ZINC, AND THE