US3479210A

US3479210A - Method and apparatus for controlling coating metal temperature in a hot-dip coating bath

Info

Publication number: US3479210A
Application number: US797300*A
Authority: US
Inventors: George P Ross
Original assignee: National Steel Corp
Current assignee: National Steel Corp
Priority date: 1968-12-04
Filing date: 1968-12-04
Publication date: 1969-11-18
Anticipated expiration: 1986-11-18
Also published as: LU59748A1; DE1947338A1; GB1272604A; BE741166A; FR2025158A6

Description

Nov. 18, 1969 G. P. ROSS METHOD AND APPARATUS FOR CONTROLLING COATING METAL TEMPERATURE IN A HOT-DIP COATING BATH 5 Sheets-Sheet 1 Filed Dec. 4, 1968 2 M2; 3528 w 538 E :7; l 1111 3528 525%: 2/

INVENTOR GEORGE P. R 0S5 ATTORNEYS Nov. 18, 1969 G. P. ROSS 3,479,210

" METHOD AND APPARATUS FOR CONTROLLING COATING METAL TEMPERATURE IN A HOT-DIP COATING BATH Filed Dec. 4, 1968 5 Sheets-Sheet 2 CONTROL INVENTOR GEORGE R ROSS ATTORNEYS Nov. 18, 1969 G. P. ROSS 3,479,210 METHOD AND APPARATUS FOR CONTROLLING COATING METAL TEMPERATURE IN A HOT-DIP COATING BATH Filed Dec. 4, 1968 3 Sheets-Sheet 3 TEMPERNWE T0 COOLANT r CONTROL I |28. \I"" UN CONTROL V V '40 I58 I60 I42 w/ I A m A l I68 i ff Ii: :2 12 ";'::i iffil bfl? |24 a M ATTORNEYS United States Patent 3,479,210 METHOD AND APPARATUS FOR CONTROLLING COATING METAL TEMPERATURE IN A HOT- DIP COATING BATH George PrRoss, Steubenville, Ohio, assignor to National Steel Corporation, a corporation of Delaware Continuation-impart of application Ser. No. 607,968, Jan. 9, 1967. This application Dec. 4, 1968, Ser. No. 797,300

Int. Cl. C23c 1/14 U.S. Cl. 117114 8 Claims ABSTRACT OF THE DISCLOSURE Method and apparatus for controlling coating metal temperature in a hot-dip coating bath utilizlng an open top trough structure which may have an open bottom 1mmersed in the hot-dip bath so as to confine a liquid coolant in heat exchange with a portion of the exposed upper surface of the bath. Flow of liquid coolant into the trough is regulated to control coating metal temperature. Automatic control of liquid coolant flow can be exercised by sensing coating metal temperature, comparing sensed coating metal temperature to desired coating metal temperature, and responsively controlling liquid coolant flow.

This patent application is a continuation-in-part of my copending patent application Ser. No. 607,968 filed Jan. 9, 1967 and now abandoned.

This invention is concerned with hot-dip coating of continuous metallic strip and in its more specific aspects, with control of coating bath temperature in continuous strip hot-dip coating operations.

In hot-dip metal coating of continuous steel strip, temperature of the strip entering the bath is a determinant factor of the coating bath temperature maintained. In the preheat type of continuous strip hot-dip galvanizing line, for example, gas fired or induction heaters are utilized for initial melting of galvanizing spelter, but they do not control bath temperature during galvanizing operations. The strip entering the bath adds all but a minor fraction of the total heat added to the coating bath. However, temperature of the strip entering the bath is generally determined by processes other than coating, such as in-line heat treatment. As a result, the temperature of the bath can vary widely from optimum coating metal temperature. For example, bath temperatures between 950 F. and 1000" F., and higher can exist because of inline heat treatment of the strip, strip temperature requirements for alloying of galvanizing spelter with a steel base, or for other reasons, whereas a temperature between around 820 F. to 880 F. preferably around 860 F. is considered optimum for general coating metal application and coating weight control.

One method practiced in an attempt to control bath temperature includes use of forced air drafts on the bath to increase heat radiation losses. Teachings on a more effective method can be found in co-pending applications filed by John T. Mayhew entitled Hot-Dip Metal Coating, Ser. No. 696,663 filed Jan. 9, 1968, which is a continuation of abandoned application Ser. No. 374,953 filed June 15, 1964, and Product, Process and Apparatus, Ser. No. 757,522 filed Aug. 27, 1968, which is a continuation of abandoned application Ser. No. 375,264 filed June 15, 1964, which are assigned to the assignee of the present application. These patent applications have generic teachings on control of metal temperature in the vicinity of strip exiting from a coating bath, and specifically teach use of bath submerged conduits for temperature control. While such submerged conduits have ice been successful and elfective in overcoming the above problems, they are difficult to handle and maintain.

The present invention provides novel methods and apparatus for providing desired control of bath temperature and teaches utilization of an evaporating coolant, including a direct contact method, which results in more rapid and more effective control of bath temperature than was available in the prior art.

Other advantages of the present invention will be brought out in describing a specific embodiment as shown in accompanying drawings, in which:

FIGURE 1 is a schematic showing, partially in, section, of a continuous strip hot-dip coating line embodying the invention,

FIGURE 2 is an expanded side view of a portion of the apparatus of FIGURE 1 embodying the invention,

FIGURE 3 is a perspective view of open trough means embodying the invention,

FIGURE 4 is a front view of the apparatus of FIG- URE 2, and

FIGURE 5 is a schematic showing, partially in section, of a continuous strip hot-dip coating line embodying a modification of the embodiment illustrated in FIG- URE 1.

Referring to FIGURE 1, coils 8 and 10 supply steel to form continuous strip 12 by successively welding a trailing end of one coil to the leading end of the next coil. While in continuous form a number of operations are performed on the line. For example, the strip is cleaned in cleaning unit 14- and is then heated, annealed, or otherwise heat treated in heating unit 16. Any of these operations can effect line speed, but usually the heating step is determinant of line speed and, in turn, effects coating bath temperature.

After heat treatment the strip travels through an atmosphere-controlled chute 20 into coating pot 22. Chute 20 includes supplemental means 21 for maintaining heat of the strip at a desired level. Reducing capacity of the atmosphere in the chute 20 has an effect on the temperature required in the chute to prevent oxidation of the strip.

Under ideal conditions the strip would enter the bath slightly above desired coating metal temperature so as to add sufficient heat to the bath to make up for radiation losses and the like. However, with changing line speeds, changing strip dimensions, and changes in required heat treatments, it is seldom possible to maintain these ideal conditions even on sustained runs of the same material. Also when it is desired to produce a product involving alloying a portion or all of the coating metal with the base metal, the temperature of the bath can be about or more, above an ideal temperature for controlling coating weight.

The strip 12 travels through coating metal 24 around submerged sink roll 26 and vertically upwardly from sink roll 26 into coating weight control zone 28 at the exit side of the bath. The present invention teaches location of coating temperature control means contiguous to coating control zone 28 at the exit side of the bath. After passing coating weight control zone 28 the strip travels upwardly around top roll 34, then downwardly around roll 36 and along the processing line, for washing, marking, etc., toward coiler 38.

With a line producing dull coat galvanized, for example, the bath temperature may be at 950 F. to 1000 F. or higher. When it is desired to produce regular galvanized, temperature of coating metal on strip 12 during passage through coating control zone 28 should be between 820 F. to 860 F. for optimum coating weight control. It is also desired to consistently maintain this temperature for better control of coating weight. As set forth above, it

is difiicult to maintain desired temperature with the various processes being performed on the line, especially when rapidly shifting from one product to another, such as from dull coat to regular galvanized, without substantial loss of time or product due to coating metal temperature problems. The present invention makes possible efficient maintenance of desired temperature and provides for rapid control of coating metal temperature when shifting the line from one product to another.

This is accomplished by efficient temperature control of coating metal applied to the strip before entry into the coating weight control zone.

Referirng to FIGURE 2, coating control zone 28 is shown in dotted lines since coating rolls, located at the surface of the bath, may be used, or gas barrier type of coating weight control, located a short distance above the surface of the bath, may be used within the teachings of the invention. Control of coating metal temperature results in smoother coating and better coating weight control with both mechanical contact coating weight control and with gas barrier type pneumatic coating weight control.

Typical positioning for

open troughs

40 and 42, contiguous to coating control zone 28 and parallel to strip 12, is shown in FIGURES 1 and 2. Trough structure is shown in more detail in FIGURE 3. The open troughs include peripheral sidewall means including

elongated walls

44, 46 and endwalls 48, 50.

Rails

52, 53 along the upper elongated side periphery of the open troughs are provided for support of the trough along the rim of a coating pot.

Rails

52, 53 are interconnected by bracing

crossbars

54, 55 with

apertures

56, 57 for securing the open trough in position and adjustment of position to be described later. The trough is open at its upper and lower surfaces.

Peripheral sidewalls

44, 46, 48 and 50 can be formed from steel, orother material having a melting temperature significantly above the melting temperature of the hot-dip coating material.

The'lower peripheries of

troughs

40 and 42 are immersed below the upper surface of the coating metal bath 24 which is exposed to a gaseous atmosphere, e.g. air. The

open troughs

40, 42 confine layers 58, 60 (see FIG. 2) of a liquid coolant, such as water, on the exposed upper surface of the coating bath. The liquid coolant within the

troughs

40, 42 is in direct contact with the bath, extracts heat from the bath, and controls the temperature of a portion of the coating metal bath at its exit side. Baflle means can be used in the bath to further define a controlled temperature zone if desired.

While two coolant troughs are shown in FIGURES l and 2, the number of troughs can be varied as required. Positioning of the troughs, as well as shape of the troughs can also be selected otherwise than as shown to carry out desired objectives.

In the frontal view of FIGURE 4, trough 42 is shown supported by the coating pot. Its elongated dimension is substantially parallel to the strip and its lower periphery is immersed beneath the bath surface sufiiciently to accommodate minor changes in bath level encountered in practice. Threaded bolt means 61 and 62 can be used to secure the trough in position and adjust height or level of the trough as required. I

Liquid coolant is supplied to the trough through conduit 64 under control of valve 66. Heat removal is increased by increasing coolant flow into the trough.

Coolant flow can be regulated manually or can be regulated automatically by apparatus as shown schematically in FIGURES 2 and 4. Thermal sensor 68, or a plurality of such sensors, are positioned to measure temperature in the portion or portions of the bath to be temperature controlled. Temperature readings by sensor 68 are compared to a preselected desired temperature at temperature control unit 70 which delivers an output for opening and closing valve 66 as required. Individual elements used for carrying out such automatic control are well known in the art.

Heat exchange is increased by increasing flow of coolant into a trough and thereby increasing the amount of coolant to be raised to vaporization temperature. However, coolant flow should not be increased into a single trough so as to cause freezing of coating metal to a substantial depth below the bottom of the trough. Efficient heat exchange takes place with a thin solidified layer of coating metal in the bottom of a trough. As heat exchange requirements increase, freezing of metal to a substantial depth below the bottom of a trough should be avoided by adding another cooling trough. More efiicient heat exchange is maintained and possible contact between solidified metal and the strip being coated is avoided. Also, the troughs can be put into use and withdrawn from use more rapidly in this way.

For cleaning purposes, and for removal of a trough, the cooling water flow can be stopped and water in the trough allowed to evaporate completely causing melting of the thin solidified layer of metal in the trough. Cleaning requirements vary depending on the pot and line op eration; return of metal from the trough to the coating bath need only be practiced if it appears that dross or undesired impurities are accumulating in the trough.

With direct contact cooling troughs positioned at the exit side of the bath, a portion of the bath contiguous to the coating control zone can be maintained at a preselected temperature varying by 15 to 30 degrees from other portions of the bath, such as the heated strip entry side of the bath. The over-all temperature of the bath can also be lowered or controlled. In practice water is ordinarily used as the liquid coolant. The amount of water flow required is varied with type of run being conducted. As a representative example, desired coating metal temperature when galvanizing steel strip having a thickness of .092 and a width of 30" has been maintained by using a single cooling trough with water flow of approximately two gallons per minute.

Referring to FIGURE 5, apparatus is shown constituting a modification of the arrangement of FIGURE 1 which modification is preferred under certain conditions and is useful for carrying'out a slightly different method. In this instance, where the temperature of strip- 112 entering the bath is on the high side in producing dull coat galvanized produce, it has been found that the temperature of the bath in the neighborhood of chute may reach such a high temperature that an objectionably thick layer ofzinc-ironalloy may be formed on the strip, or the composition or crystalline structure of this iron-zinc alloy may be altered by the high temperature from that produced at optimum temperatures. One of the desirable products which can be produced using the present inventoin is a galvanized strip which has an iron-zinc alloy on one side with substantially no free zinc and the same alloy layer on the other side but covered by a coating of spelter. It has been found that when the temperature of the bath becomes too high in the neighborhood of chute 120 and the alloy layer is thereby affected, the zinc coating on this type of product does not have good adherence. In such case applicant has found that positioning the troughs and 142 in the back of the bath in the vicinity of the strip entering the bath remedies the situation and a satisfactory product of the type described is then produced. When the modification and method variant of FIGURE 5 is utilized, the bath cooling action carried out by

trough

140 and 142 has a temperature modifying effect on the entire bath by removing from the spelter in the immediate vicinity of each

trough

140 and 142 heat introduced by the entering strip. Of course, where desirable either trough 140 or trough 142 can be omitted.

In FIGURE 5 the reference numerals are the same as those in FIGURE 2, with the addition of 100 in each instance to designate the same components of the combination as those of the combination shown in FIGURE 2.

What is claimed is:

1. In continuous strip hot-dip metal coating, a method for controlling temperature of a hot-dip metal coating bath having an upper surface exposed to a gaseous atmosphere comprising confining a liquid coolant within trough means having wall portions in direct contact with a portion of the exposed upper surface of the hot-dip metal coating bath, and

controlling flow of liquid coolant into the trough means to control temperature of molten metal in the hot-dip metal coating bath, the liquid coolant being preselected to have a vaporization temperature lower than the temperature of molten metal in the hot-dip metal coating bath.

2. The method of claim 1 in which the trough means defines an open bottom and the liquid coolant is in direct contact with the exposed upper surface of the molten metal coating bath.

3. The method of claim 2 in which the bath is molten spelter continuous steel strip is passed into and below the surface of the hot-dip metal coating bath, the continuous steel strip having a temperature substantially above 860 F. at the strip entry side of the bath tending to raise the temperature of the bath substantially above 860 F., and

water is added to the trough means, the flow of water being controlled to maintain the molten spelter at a temperature not greater than about 860 F.

4. The method of claim 1 including the steps of sensing the temperature of molten metal at a point adjacent the trough means,

comparing sensed temperature to desired temperature of molten metal at said point adjacent the trough means,

automatically controlling How of liquid coolant into the trough means in response to the difference in the desired temperature and the sensed temperature.

5. Apparatus for controlling metal temperature in a hot-dip molten metal coating bath comprising means forming a bath of molten coating metal trough means having wall portions in contact with an exposed upper surface portion of the molten metal coating bath for confining a liquid coolant in heat exchange relation with the molten metal coating bath,

means for supplying liquid coolant to the trough means,

and

means for controlling flow of liquid coolant into the trough means.

6. The apparatus of claim 5 in which the trough means defines an open bottom.

7. The apparatus of claim 6 in which the trough means has peripheral sidewalls, and bottom edge portions of the peripheral sidewalls are immersed beneath the exposed upper surface of the molten metal coating bath to define a confined area for receiving the liquid coolant.

8. The apparatus of claim 5 further including means for sensing bath temperature and generating a signal responsive thereto, and 4 means responsive to the generated signal for control of liquid coolant flow into the open trough means.

References Cited UNITED STATES PATENTS 752,768 2/1904 Goodwin 1185 1,907,890 5/1933 Steckel. 2,604,415 7/ 195.2 Whitfield et al. 3,010,844 11/1961 Klein et a1. 3,215,115 11/1965 Knight et a1. 1l85 ALFRED L. LEAVITT, Primary Examiner I. R. BATTEN, JR., Assistant Examiner US. Cl. X.R.