US3307968A

US3307968A - Method and apparatus for controlling the alloying of zinc coatings

Info

Publication number: US3307968A
Application number: US306123A
Authority: US
Inventors: Paul E Schnedler
Original assignee: Armco Inc
Current assignee: Armco Inc
Priority date: 1963-09-03
Filing date: 1963-09-03
Publication date: 1967-03-07
Anticipated expiration: 1984-03-07
Also published as: GB1074305A; BE652482A; DE1298825B; AT272789B; DE1298825C2

Description

METHOD AND APPARATUS FOR CONTROLLING THE ALLOYING OF ZINC COATINGS Filed Sept. 5, .1963

I March 67 P. E. SCHNEDLER 3,

INVENTOR PAUL E. SCHNEDLER,

1- BY h1g3 ATTORNEYS United States Patent Office 3,307,968- Patented Mar. 7, 1967 3,307,968 METHOD AND APPARATUS FOR CONTROLLING THE ALLQYTNG F ZlNC CUATINGS Paul E. Schnedler, Middletown, Ohio, assignor to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio Filed Sept. 3, 1963, Ser. No. 3%,123 8 Claims. (Cl. 117-114) .bination of the two such that the alloying of the zinc with the base metal of the strip would proceed in a more uniform manner and produce a more completely alloyed coating exhibiting a one phase alloy.

The reason alloyed coatings have come into demand in recent times is that they do not present a spangled appearance but a dull surface texture which readily takes paint and is advantageous for various specific purposes. In order to obtain the maximum utility for an alloyed zinc coated steel body the alloying must be as uniform as possible. The strip must be completely alloyed over its entire surface and no bright unalloyed regions must remain; at the same time over-alloying must be avoided.

One of the basic features of the aforesaid patent was the disclosure of heating the strip from within electrically. The advantages of heating from within result from the fact that areas which have completely alloyed have a high emissivity and they will radiate heat rapidly and thus tend to cool. Adjacent regions which have not alloyed and still have a bright surface with a low emissivity will retain the heat so that alloying in these regions will continue to take place. Thus when the strip is heated from within, the phenomena of radiation from bright and dull surfaces produce a self-leveling effect to enhance the uniformity of alloying. When the strip is heated externally, of course these phenomena operate in conflict with each other and accentuate the unevenness of alloying.

It is therefore one of the objects of the present invention to provide for a very accurate control of internal heating and to do this with a minimum of energy input. In this connection it is another object of the invention to provide for heating the strip just after it emerges from the coating bath from the temperature at which it then is up to a temperature at which alloying proceeds rapidly. This means that the electrical heating from within may be more accurately controlled.

In conection with the control of the heating apparatus, it is another object of the invention to provide sensing means for sensing the degree and extent of alloying the strip surface. This sensing is based upon the fact that unalloyed coating will be highly directionally reflective while alloyed coating will not be highly directionally reflective.

It is another object of the invention to utilize photosensitive devices to sense the reflectivity of the coating and to arrange these photosensitive devices in a novel way such that they will be more greatly affected by dispersed light rays reflected from a dull area than by bright rays reflected from a bright surface.

It is still a further object of the invention to provide a plurality of such sensing devices in a row transversely of the strip and to associate with these sensing devices a plurality of supplemental heaters such that if an element of the strip at a particular distance from one edge is not substantially the same distance from the same edge will be energized to increase the heat put into the strip, and thereby increase the rate of alloying.

These and other objects of the invention which will be pointed out in greater detail hereinafter, or which will become apparent as the description proceeds, are accomplished by that series of method steps and by that construction and arrangement of parts of which the following will describe an exemplary embodiment.

Reference is made to the drawings forming a part hereof and in which: i

FIG. 1 is a diagrammatic cross-sectional view through a portion of a coating line.

FIG. 2 is a fragmentary elevation thereof as seen from the right of FIG. 1; and

FIG. 3 is a diagram useful in understanding the interaction of the heating devices.

Briefly, in the practice of the invention there are provided three separate heating means. As the strip emerges from the coating bath it passes through a high intensity booster heater which raises the temperature of the strip to optimum alloying temperature. Since the alloying reaction is a diffusion reaction, it is necessary to maintain the strip at alloying temperature for a period of time necessary to complete the reaction. The second heating step involves electrical resistance heating from within to maintain the strip at the optimum alloying temperature for the period of time necessary to permit the reaction to go to completion. Sensing devices are provided to determine when alloying is complete and to energize the electrical heating means when alloying is not complete.

The third heating means involves a plurality of heating elements disposed transversely of the strip and which may be selectively energized to heat any element of the strip across the width thereof. These selective supplemental heating devices are controlled by photosensitive devices correspondingly positioned transversely of the strip so that if, for example, a portion of the strip at a distance of say 12 inches from one edge is not fully alloyed, supplemental heat is added at a point 12 inches from that edge of the strip.

The longitudinal areas of unalloyed coating may result from a number of causes: generally the coating will tend to be heavier at the strip edges, so that the alloying rate along the edges will be different than in the center; and uneven wear on the exit rolls may cause longitudinal areas of the strip passing over points of greater wear to have a heavier coating, which will require more heat or time for complete alloying. Variations such as those just described are common, and well Within comercial tolerances.

The sensing devices are arranged in two sets; one set to sense dull areas of the strip at the point where the strip should be bright, and the other set arranged to sense bright areas of the strip at a point where alloying should be complete. The light sources and photocells of the two sets are arranged differently as will be described hereinafter to sense, respectively, dull areas and bright areas.

Referring now in more detail to the drawings, there is shown at 11) a coating pot containing molten zinc indicated at 11. A strip 12, which has been subjected to any desired pretreatment, passes around a hold-down roll 13 within the pot and thence upwardly through exit rolls 14. The coated strip 12a then passes upwardly about a turning roll 15 and thence to storage, coiling or other processing.

At 16 there is shown diagrammatically a high intensity booster heater. This heater may be either localized electrical induction or high intensity radiation or a flame type gas heater. The object of the heater 16 is to heat the strip to optimum alloying temperature as soon as possible after leaving the coating bath. Optimum alloying temperature for aluminum-bearing zinc is approximately 925 to 1050 F., although a satisfactory range would extend from about 900 F. to about 1250 F. It should be noted in this connection that since the surface of the freshly coated strip with the molten coating on it is uniformly bright, external heat may be used to raise the temperature of the strip uniformly and this is no contradiction of the principles described in the aforesaid Patent No. 2,986,808. The principles of that patent come into effect when alloying has started; and after alloying has Started, if the heat is applied externally, the uniformity of the alloying rate becomes progressively worse, While if the strip is then heated from within, the self-leveling effect above mentioned comes into play.

The heating means 16 regardless of what type is being used, is controlled by a radiation pyrometer or infra-red detection-device indicated at 17.

At 18 there is disclosed the second heating device, which produces heating from within. In the particular embodiment illustrated this is an inductive resistance heating apparatus and the element 18 is a transformer primary and the entire length of strip 12a from the exit rolls to the roll 15, coupled with a return conductor 22, constitutes the secondary and is thus heated. This heat must be very accurately controlled because the maintenance of alloying temperature is the most critical feature of the alloying operation. In theory, the current flowing through the strip 12a produces a variable controlled cooling effect so that areas of the strip which have completely alloyed and have a high emissivity will radiate heat faster than heat is being supplied internally and actually become cooler; while adjacent regions which have not alloyed and still have a bright surface with a correspondingly low emissivity will retain the heat, and alloying in these regions will continue to take place. It will also be noted that because the booster heater 16 has heated the strip up to alloying temperature, the only requirement of the heating device 18 is to maintain the strip at alloying temperature or to control cooling rate to achieve the proper degree of alloying.

To control the action of the heating device 18 there are provided two series of photocells, the first indicated at 19, and the second at 26. As best seen in FIG. 2, there are a plurality of photocells 19 and a plurality of photocells 20. It will be apparent that the photocells in the row 19 will be positioned to monitor the strip when it is normally still bright; while the photocells 20 are positioned to monitor the strip when it is normally completely alloyed. Variations from normal conditions are indicated by the two series of photocells and, through suitable circuitry operate the induction heating apparatus 18. These connections do not form a part of the present invention and are within the skill of a competent electrical engineer.

A third heating apparatus is provided and this involves a plurality of selective heating devices 21. These are spaced in a row transversely of the strip in positions corresponding to the

photocells

19 and 20. The heaters 21 may be either combustion type or electric heaters, and their purpose is to provide supplemental heat to particular areas transversely of the strip which are not alloying at a sufliciently fast rate. The individual heaters 21 are controlled by a pair of photocells 19 and 2% By examination of FIG. 2 for example, it can be seen that the leftmost pair of

photocells

19 and 20 are aligned with the left-most heater 21, and that correspondingly there is a heater 21 for each additional pair of

photocells

19 and 20 all the way across the strip. Again, the electrical connections or control circuits have not been shown because they are within the skill of a competent engineer.

The disposition of the photocells for proper results is an important feature of this invention. A photocell is normally thought of as being responsive to reflected light and to some degree in proportion to intensity of the reflected light beam. In the present case, it must be borne in mind that when the upwardly moving strip reaches the position of the bank of photocells 19, it is predominantly unalloyed and therefore bright. It is possible, however, that small areas of the strip may have proceeded to alloying and may, therefore, show a dull surface. If the photocells 19 were arranged to respond to bright light, then a small dull area would not produce a signal of significant value which would be useful for control purposes. Similarly and conversely by the time the strip reaches the bank of photocells 20, it is predominantly alloyed and therefore dull. Here again it is possible that a small area of the strip may not be completely alloyed and will therefore be bright and if the photocells of the bank 20 were designed to be responsive to dull light, one small area of brightness would not give a significant signal.

Therefore, the photocells in the bank 19 are arranged to be responsive to the presence of dull areas in a bright strip and those on the bank 20 are arranged to be responsive to small areas of brightness in a predominantly dull strip.

This is accomplished as shown in FIG. 1. The photocells 20 and their associated light sources 20a are dis posed so that the bisector of the angle between the incident light from the source 20a and the reflected light impinging on the photocell Ztl is normal to the strip. Under these circumstances, the dull strip will not affect the photocell 21) but a small bright area resulting from incomplete alloying will produce a reflection which will cause the photocell 20 to put out a signal of significant value. In order to eliminate errors caused by erratic strip movement such as buckling and waving, the strip is caused to pass over a hilly roll 15a and the photocell and the light source are aimed at the strip at the point where it passes over the billy roll 15a.

On the other hand, the photocells 19 and their associated light sources 19a are positioned so that the bisector of the included angle is not normal to the strip. The strip at this point is normally bright and the light beam from the source will not enter the photocell 19 since the angle of reflection is equal to the angle of incidence. The photocell must be positioned so that small buckles and waves in the strip will not cause the bright reflection to enter the photocell. However, the beam from the source 1942 reflected from the dull areas of the strip is broken up and scattered as indicated by the solid arrows in Fig. l and some portions of this reflected light from the dull areas will enter the photocell 19 as shown in the drawing. In this way a dull area in a predominantly bright strip will affect the photocell 19 to produce a signal of significant value.

It has been found experimentally that if a light source 19a and its associated photocell 19 are located as close together as possible, with the light beam and the optical axis of the photocell substantially parallel, the most significant difference in reflectance as between a dull and a bright surface will be obtained if the light beam and the optical axis of the photocell (or their bisector, if they are not parallel) are disposed at an angle of about 25 to the normal to the strip surface. It is at this angle that the difference in reflectance between dull and bright surfaces is greatest, and the amount of this difference is only slightly affected by slight strip movement, which would, in effect, change this angle.

In Fig. 2 there is diagrammatically indicated the way in which the signals from the photocells may operate. So long as, in any given pair of

photocells

19 and 20, the photocell 19 is not putting out a change signal because no dull spots are being encountered, and so long as the photocell Ztl is not putting out a change signal because no bright spots are encountered, it means that the strip passing that particular pair of photocells 19-20 is unalloyed at the point 19 and fully alloyed at the point 20. This means that the heat input from the various heaters is doing the job correctly.

If now, for orie reason or another, the strip as it passes the photocell 19 is dull, it means that alloying has already started and a signal to reduce heat will be put out by the photocell 19. On the other hand, if the strip passing the photocell 20 has a bright area indicating that alloying is incomplete, then the photocell 20 will put out a signal to increase heat, and hence alloying. The signals from the photocell pair 1920 are transmitted to the individual heater 21 which is aligned with the photocell pair 19-20. If the signal to the heaters 21 has come from the photocell 19, the input to the respective heater 21 is reduced whereas if the signal comes from the photocell 20, then the input to the respective heater 21 is increased. In this way uniformity transversely of the strip with regard to alloying process is achieved. Since, however, the heaters 21 are supplemental heaters, a relationship is established between their summation and the induction heating device 18. Thus, in FIG. 2 an input to the various heaters 21 is shown at 30. This input is indicated as entering a kilowatt meter 31 and thence going to the individual heaters 21 subject to the control by the signals from the

photocells

19 or 20. It will be clear that the meter 31 reads the total input to all of the heaters 21. It is desired to keep the input to the various heaters 21 more or less in the middle or a range between minimum and maximum wattage. This range may be indicated diagrammatically by the portion 32 of the meter 31 and so long as the input remains within the range 32, no signal is transmitted to the heater 18. However, if the several pairs of photocells each reading on different portions of the strip cause the total input to the various heaters 21 to rise above a predetermined maximum, then a signal is sent to the heating member 18 to increase the power input 35 to the member 18. This is indicated by the arrow 33 accompanied by the plus sign. If, on the other hand, the signals from the pairs of

photocells

19 and 20 cause the various heaters 21 to be cut off to the point where the total input to them is below the range indicated at 32, then a signal is sent as indicated by the arrow 34, accompanied by the minus sign, to cut down the input 35 to the heater 18. In this manner, the input to the various heaters 21 is maintained within the range indicated at 32 and preferably at about 50% of capacity.

It will be understood that numerous modifications may be made without departing from the spirit of the invention and no limitation is intended other than as specifically set forth in the claims which follow.

The invention having now been fully described, what is claimed is:

1. In a coating apparatus for coating a strip of metal with a layer of another metal, wherein the strip passes through a bath of molten coating metal and then upwardly vertically in an alloying run; booster heating means positioned just above said bath to heat said strip to a temperature at which alloying of said coating metal with the metal of said strip will take place, electrical means for supplying heat to the strip from within over the entire alloying run to maintain alloying temperature in the strip during the alloying reaction, a plurality of supplemental heating devices immediately above said booster heating means to provide heat to longitudinal zones of the strip selectively, and a plurality of control means responsive to the reflectivity of the strip in said longitudinal zones disposed above said supplemental heating devices and in the latter portion of said alloying run for controlling respective ones of said supplemental heating devices.

2. Apparatus according to claim 1, wherein said control means includes a bank of light-sensitive devices and associated light sources positioned adjacent the end of said alloying run at a point in the strip travel where alloying should be complete, and the strip, therefore, uniformly dull, said light-sensitive devices and associated light sources being aimed at the strip such that the bisector of the angle between the incident light rays from said light sources and reflected light rays impinging upon said lightsensitive devices is substantially normal to the strip surface, whereby small bright areas in the generally dull surface will cause said light sensitive devices to give a control signal of significant proportion.

3. Apparatus according to claim 2, wherein said control means includes an additional bank of light-sensitive devices and associated light sources positioned in the latter part of said alloying run at a point in the strip travel where alloying should not have started, and the strip, therefore, should be uniformly bright, said light-sensitive devices and associated light sources being aimed at the strip such that the bisector of the angle between the incident light rays from said sources and their reflection impinging upon said light-sensitive devices is at an angle different than to the strip surface, whereby small dull areas in the generally bright surface will cause said light-sensitive devices to give a control signal of significant proportion.

4. Apparatus according to claim 3, wherein said angle is about 25 to the normal to the strip surface.

5. Apparatus according to claim 1, wherein means are provided to measure the total power input to said supplemental heating devices and means operative when the total input to said supplemental heating devices exceeds a predetermined value to increase the power input to said electrical means, and means operative when the total power input to said supplemental heating devices drops below a predetermined point to reduce the power input to said electrical means.

6. The method of producing on a metallic strip a highly uniform, one phase, alloyed zinc coating, wherein molten zinc is applied to the strip by passing the strip through a bath of molten zinc and thence vertically upwardly in an alloying run; which includes the steps of rapidly heating said strip to alloying temperature as it leaves said coating bath, maintaining said strip at alloying temperature by electrically heating said strip from within throughout said alloying run, sensing the reflectivity of longitudinal zones of the strip adjacent the end of said alloying run at a point at which alloying should be complete, and causing sensings indicating high reflectivity, and therefore incomplete alloying, in a particular longitudinal zone to cause supplemental heating of said particular longitudinal zone after said rapid heating and before said sensing.

7. The method of producing on a metallic strip a highly uniform one phase alloyed zinc coating, wherein molten Zinc is applied to the strip by passing the strip through a bath of molten zinc and thence vertically upwardly in an alloying run; which includes the steps of rapidly heating said strip to alloying temperature as it leaves said coatingbath, maintaining said strip at alloying temperature by electrically heating said strip from within throughout said alloying run, supplementary heating applying to longitudinal zones of said strip immediately after said rapid heating, sensing the reflectivity of said zones adjacent the end of said alloying run at points at which alloying should be complete, causing sensings indicating high reflectivity, and therefore incomplete alloying, in a particular one of said zones to increase the application of supplementary heating in said zone, sensing the reflectivity of said zones in the latter port of said alloying run at points at which alloying should not have started, and causing sensings indicating low reflectivity, and therefore incipient alloying, in a particular one of said zones to decrease the application of supplementary heating in said zone.

8. The method of claim 7, which includes the step of measuring the total amount of supplementary heating supplied to said longitudinal zones, increasing the electrical heating of the strip from within when said total supplementary heating exceeds a predetermined value and decreasing said electrical heating from within when said total supplementary heating drops below a predetermined value.

7 8 References Cited by the Examiner OTHER REFERENCES UNITED STATES PATENTS Burgwin, S. L.: Electronic Regulation of Industrial t 17 6 6 1922188 8/1933 zworykint jPurgseslsgzhln Instrumen s, pp 328 330, 3 9, 2,947,212 8/1960 Woods 8814 5 2 9 0 19 1 Schmdler 117 114 RALPH S. KENDALL, Przmary Examiner.

3,058,840 10/1962 Kerr et al 118-620 X J. R. BATTEN, JR., Assistant Examiner.

Claims

1. IN A COATING APPARATUS FOR COATING A STRIP OF METAL WITH A LAYER OF ANOTHER METAL, WHEREIN THE STRIP PASSES THROUGH A BATH OF MOLTEN COATING METAL AND THEN UPWARDLY VERTICALLY IN AN ALLOYING RUN; BOOSTER HEATING MEANS POSITIONED JUST ABOVE SAID BATH TO HEAT SAID STRIP TO A TEMPERATURE AT WHICH ALLOYING OF SAID COATING METAL WITH THE METAL OF SAID STRIP WILL TAKE PLACE, ELECTRICAL MEANS FOR SUPPLYING HEAT TO THE STRIP FROM WITHIN OVER THE ENTIRE ALLOYING RUN TO MAINTAIN ALLOYING TEMPERATYRE IN THE STRIP DURING THE ALLOYING REACTION, A LURALITY OF SUPPLEMENTAL HEATING DEVICES IMMEDIATELY ABOVE SAID BOOSTER HEATING MEANS TO PROVIDE HEAT TO LONGITUDINAL ZONES OF THE STRIP SELECTIVELY, AND A PLURALITY OF CONTROL MEANS RESPONSIVE TO THE REFLECTIVITY OF THE STRIP IN SAID LONGITUDINAL ZONES DISPOSED ABOVE SAID SUPPLEMENTAL HEATING DEVICES AND IN THE LATTER PORTION OF SAID ALLOYING RUN FOR CONTROLLING RESPECTIVE ONES OF SAID SUPPLEMENTAL HEATING DEVICES.

7. THE METHOD OF PRODUCING ON A METALLIC STRIP A HIGHLY UNIFORM ONE PHASE ALLOYED ZINC COATING, WHEREIN MOLTEN ZINC IS APPLIED TO THE STRIP BY PASSING THE STRIP THROUGH A BATH OF MOLTEN ZINC AND THENCE VERTICALLY UPWARDLY IN