US3887721A

US3887721A - Metallic coating method

Info

Publication number: US3887721A
Application number: US316826A
Authority: US
Inventors: Roman A Schwieterman
Original assignee: Armco Inc
Current assignee: Armco Steel Co LP
Priority date: 1972-12-20
Filing date: 1972-12-20
Publication date: 1975-06-03
Anticipated expiration: 1992-06-03
Also published as: SE399076B; GB1448726A; IN141188B; FR2211543A1; ES421643A1; ZA739073B; FR2211543B1; JPS5524499B2; IT1000281B; DE2363222A1; CA1028580A; DE2363222C2; NL7316571A; BE808583A; AU6307773A; JPS4990637A; BR7309666D0

Abstract

Method for applying a metallic coating to a ferrous base metal strand wherein the surface of the strand is first thoroughly cleaned to render it receptive to a molten coating metal. During the cleaning step, the strand may be heated to a temperature less than the melting point of the metallic coating metal. A nonconductive coating pot is provided which has a primary coil within its side walls. Power is supplied to the primary coil which induces heavy secondary currents in the coating metal within the pot. This current is converted into heat by resistance of the coating metal itself, thereby supplying a high quantity of heat directly in the coating metal. The induced secondary current constantly stirs the molten coating metal so as to help provide a bright bath surface where the strand exits from the bath, and minimizes accumulation of oxides and dross.

Description

United States Patent [1 1 Schwieterman 1 1 METALLIC COATING METHOD [75] Inventor: Roman A. Schwieterman,

Middletown, Ohio [22] Filed: Dec. 20, 1972 [21] Appl. No.: 316,826

[451 June 3,1975

Primary Examiner-Leon D. Rosdol Assistant Examiner-Edith L. Rollins Attorney, Agent, or FirmMelvil1e, Strasser, Foster & Hoffman [5 7] ABSTRACT Method for applying a metallic coating to a ferrous base metal strand wherein the surface of the strand is first thoroughly cleaned to render it receptive to a molten coating metal. During the cleaning step, the strand may be heated to a temperature less than the melting point of the metallic coating metal. A nonconductive coating pot is provided which has a primary coil within its side walls. Power is supplied to the primary coil which induces heavy secondary currents in the coating metal within the pot. This current is converted into heat by resistance of the coating metal itself, thereby supplying a high quantity of heat directly in the coating metal. The induced secondary current constantly stirs the molten coating metal so as to help provide a bright bath surface where the strand exits from the bath, and minimizes accumulation of oxides and dross.

5 Claims, 2 Drawing Figures [52] US. Cl 427/45; 219/1065 [51] Int. Cl. 1305c 3/0; B44d 1/20 [58] Field of Search 117/71 M, 114 R, 114 A, 117/114 B, 114 C, 93.2,113;148/6.11, 6.14 R; 13/27, 26 X [56] References Cited UNITED STATES PATENTS 2,110,893 3/1938 Sendzimir 117/71 M 2,322,618 6/1943 DeMare. 13/26 2,643,201 6/1953 Chadsey et a1. 117/93.2 2,698,810 1/1955 Stauffer 117/93.2 2,824,021 2/1958 Cook et a1. 117/114 A 3,203,824 8/1965 McQuaid et a1. 117/114 R 3,478,156 11/1969 Segsworth 13/27 3,508,977 4/1970 Basche 148/6.11 3,666,537 5/1972 Williams 117/114 C FOREIGN PATENTS OR APPLICATIONS 849,503 9/1952 Germany 117/114 C METALLIC COATING METHOD BACKGROUND OF THE INVENTION This invention relates to the hot dip coating of a ferrous base metal strand. Such processes have advantageously been used for many years in the production of iron or steel strip having a thin coating of zinc, aluminum, terne, and various other combinations.

Generally considered, the processes of the prior art all involve as a first step the thorough cleaning of the surface of the strand to be coated. This cleaning operation may be carried out by successive oxidizing and reducing heat treatments as taught by Sendzimir, by chemical cleaning, or by various other processes.

The clean strand is then passed into a bath of molten coating metal, and withdrawn in a generally upwardly path of travel. The molten coating metal adhering to the surface of the strand and drawn upwardly from the path is then finished by means of coating rolls, air knives, or the like, and the molten coating is subsequently solidified. According to the prior art, the molten coating metal bath is generally maintained in an externally heated iron pot. Long experience with these coating metal pots has disclosed several disadvantages, particularly in the case of pots for molten aluminum. First of all, the pot has a relatively short life. This short life is caused by several factors, including the rapid build-up of dross on the pot bottom, and creep or bulging of the walls caused by high temperature of the externally applied heat and the weight of the coating metal contained,

In addition, the quantity of heat which can be supplied externally is of course limited. The prior art has found it necessary with coating pots of this type to heat the strip to a temperature in excess of the melting point of the coating metal prior to passing the strip into the bath. In other words, enough heat could not be supplied to the coating metal bath to maintain the coating metal molten and at the same time heat a relatively cool (cool with respect to the melting point of the coating metal) strand up to coating temperature.

Finally, the large, uncovered surface of molten coating metal in the pot leads to a rapid formation of oxides and dross at the surface. This accumulation of oxides and dross at the surface of the bath is one of the most significant problems encountered today in the hot dip coating processes. That is, upon emergence from the bath, the strand tends to pick up particles of dross and oxide from the surface of the bath, resulting in heavy edges or other imperfections in the coating applied.

Keeping the foregoing comments in mind, it is an object of this invention to provide an improved method for applying a metallic coating to a ferrous base metal strand.

More specifically, one object of this invention is to provide a hot dip coating method wherein the base metal strand can enter the bath at a temperature below the melting point of the coating metal.

Another object of the invention is to provide a hot dip coating method which can generate sufficient heat within the coating metal bath that additional coating metal may be melted directly in the pot.

Still another object of the invention is to provide a hot dip coating method which will greatly enhance coating pot life.

Still a further object of this invention is to provide a hot dip metallic coating method which permits very accurate control over the temperature throughout the molten coating metal bath.

Still a further object of the invention is the provision of a metallic coating method wherein the accumulation of dross and oxides in the bath are minimized.

A further object of this invention is to provide a coating method which maintains a bright, oxide and dross free bath surface at the point where the base strand exits the coating metal bath.

SUMMARY OF THE INVENTION Broadly considered, this invention relates to the hot dip coating of a ferrous base metal strand with one of the conventional metallic coating metals. The process includes the steps of thoroughly cleaning the surface of the strand to render it receptive to a molten coating metal. The cleaned strand is then passed into a bath of the molten coating metal. The bath is maintained in a vessel or pot constructed of a non-conductive material having a primary coil within its side walls. Power will be applied to the primary coil which will induce secondary currents in the charge of coating metal in the pot. The secondary currents are converted to heat by the resistance of the charge itself.

The size of the pot and the induced secondary currents serve to continuously agitate the molten coating metal in the bath so as to prevent dross accumulation on the bottom of the pot, and help provide a bright bath surface where the coated strand exits from the bath.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of a coating method according to this invention.

FIG. 2 is a schematic diagram showing the coating pot utilized in the method of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, a complete coating process embodying the teachings of this invention has been illustrated schematically. A coil of a suitable ferrous base metal strand is indicated at 10. The continuous strip or sheet passes from the rolls 11 and 12 as indicated to enter the top of the first furnace section 14. This first section of the furnace 14 may be of the direct fired, non-oxidizing type. That is, there is approxi mately a 5 percent excess of combustibles introduced into this section. Furnace temperature may be on the order of 2300F., so that the base metal strand will be rapidly heated to a temperature on the order of at least 1 F. This is effective to almost instantaneously burn surface contaminants such as oil and the like from the surface of the strip. The vertical configuration is desirable in that it eliminates the necessity for supporting rollers in the hot sections of the furnace.

The second section of the furnace indicated generally at 16 may be of the radiant heating type. In this section of the furnace, the temperature of the base strand will be raised to a temperature on the order of 1350F. to 1550F., reaching its maximum temperature at the point 18. A reducing atmosphere will be supplied to this portion of the furnace, as well as to the succeeding portions of the furnace described below.

The third section of the furnace indicated generally at 20 is a tube cooling zone.

The final section of the furnace 22 may include means for jet cooling of the strand, in some cases to a temperature below the melting point of the coating metal being used.

The strip passes out of the furnace portion 22, over the turn down roll 24 and through the snout 26 into the bath of molten coating metal indicated generally at 28. This bath is explained in more detail hereinafter and shown in FIG. 2.

The strip is withdrawn from the bath 28 in a generally vertical path of travel past the jet finishing knives indi cated generally at 30, and after allowing time for solidification, over the turning roll 32 and coiled for storage and shipment as at 34.

Portions of the process described are entirely conventional and well known in the art. For example, the furnace configuration is known per se. Similarly, the jet finishing techniques are known per se. The primary aspect of the method of this invention centers about the bath of molten coating metal 28 which will be described hereinafter. One very important advantage of the particular bath arrangement to be described is that the base metal strand may be cooled in the

furnace portions

20, 22, and in the snout 26 to a temperature below the melting point of the coating metal. In other words, in the case of aluminum, the strip may be cooled to a temperature on the order of l200 prior to entering the molten metal bath. The utilization of lower strip temperatures upon entering the bath has been found to reduce substantially the interface alloy formation; that is, the alloy formation at the interface between the base strand and the coating metal. Of course, the reduction in thickness of alloy formation greatly improves adherence of the coating metal.

Turning now to FIG. 2, the molten metal bath of this invention will be described in more detail. The molten metal bath is maintained in the pot indicated generally at 40. Hereinafter, the pot 40 will be referred to as a coreless induction coating." During use, the coreless induction coating pot 40 will be filled with molten metal to the level 42.

As previously indicated, the base metal strand to be coated passes through the snout 24, over the turn down roll 26, and into the molten metal bath. It will be observed that the lower end of the snout 24a is immersed in the bath of molten coating metal, so that the proper atmosphere (i.e. a reducing or non-oxidizing atmosphere) may be maintained at all times within the snout 24.

Suitably mounted for rotation with in the molten metal bath are the pot rolls 44 and 46, and the stabilizer roll 48. These aspects of the coating machinery are entirely conventional.

The coreless induction coating pot indicated generally at 40 includes an interior wall and bottom section of refractory material indicated at 50. This interior structure may be formed in a variety of ways. For example, an exemplary pot in commercial use includes two layers of ceramic brick. These bricks are precision laid with thin joints between adjacent bricks. On the inside of the two layers of ceramic brick may be one or more layers of a ceramic feltmaterial. The innermost layer may be a layer of ceramic grout material on the order of one quarter inch thick. As previously indicated, a variety of materials may be used for the interior pot portions 50. It is important that whatever material is used be electrically non-conductive and that the material be chosen for compatability under high temperature with the molten coating metal.

Surrounding the interior wall portion 50 will be the primary induction coil indicated generally at 52. The primary coil may be constructed of water cooled copper tubing. The coil 52 will be connected in any suitable manner to a source of power.

Cooling coils 53a and 5312 will preferably be provided above and below the primary coil 52. These coils are used and controlled to keep temperatures in the wall portion 50 evenly dispersed, and prevent cracking and spalling of the pot walls.

Surrounding the primary coil 52 will be the structural support for the coreless induction melting pot indicated generally at 54. This structural framework may be of steel or the like, and will provide the strength necessary to retain the large quantity of molten metal within the pot.

If desired, the pot can be mounted on wheels 56 which in turn operate on the track 58. By this expedient, a plurality of coating pots may be interchanged with the rest of the coating equipment. For example, pots containing different coating metals such as aluminum and zinc may be used interchangeably.

In operation of the coating pot, alternating current at a frequency of 60 cycles will be transmitted over watercooled cables to the primary coil 52 described earlier. The power applied to this primary coil creates a magnetic flux which passes through the material within the pot. The material in the pot acts as the secondary winding of a transformer having a single turn.

The high density, rapidly changing magnetic flux generated by the primary coil induces heavy secondary currents in the material in the pot. These heavy secondary currents are converted into heat by the electrical resistance of the material in the pot.

These induced secondary currents provide a continuous stirring or agitation of the molten material in the pot as indicated by the arrows 60. This stirring action is extremely important to the method of this invention. First of all, commercial practice according to the teachings of this application has established that the stirring action will help maintain a bright, oxide and dross free bath surface at the point where the strip exits the coating metal bath. The induced currents at the bath surface flow from the center radially to the periphery and are believed to create an additive effect to the washing action of the jet nozzles to keep oxides formed in jet finishing pushed away from the strip, thereby maintaining a bright area around the strip. The maintaining of a bright bath surface in this area virtually eliminates the pickup of oxides and resulting coating imperfections encountered with conventional processes.

Secondly, this stirring action apparently maintains oxide and dross in a suspension uniformly distributed throughout the coating pot. This of course substantially eliminates the build up of oxides and dross at the bottom of the pot. Prior experience with aluminum would lead one to expect a significant and troublesome oxide build up on the pot wall in a band at mid coil elevation. Oxide build up has been insignificant probably because of complex current patterns caused by high diameter/- depth ratio and submerged pot equipment.

The coreless induction pot design just described has several very important additional advantages in a metallic coating operation. First of all, it will be apparent that the heat is generated within the pot itself. This permits very accurate temperature control of the molten material in the pot.

Secondly, this permits the more rapid generation of heat in the pot. This, in turn, provides at least two important advantages. It will permit the melting of additional coating metal directly in the pot. That is, pigs of solid coating metal 62 may be transported by the conveyor or chute 64 directly into the coating pot. They will be maintained in a single location in the pot by the baffle plate 66. The deflector plate 68 will prevent molten metal from splashing directly out of the pot.

In addition, the rapid generation of heat within the coating pot is what in fact makes it possible for the strip to enter the pot at a temperature below melting point of the coating metal. Sufficient heat can be generated within the pot to raise the strip to coating temperature without causing coating metal to freeze on the surface of the strip at entry.

The coreless induction pot of this invention embodies what might be called a short coil design. That is, the ratio between the diameter of the pot and the depth of the pot is very different from previously known coreless induction furnaces. In the commercial unit referred to earlier, the pot utilized has a bath diameter of ten feet and a depth of nine feet.

It is believed that the foregoing constitutes a full and complete disclosure of this invention, and no limitations are intended except insofar as specifically set forth in the claims that follow.

The embodiment of the invention in which an exclusive property or privilege is claimed are defined as follows,

1. In a hot dip metallic coating method including the steps of thoroughly cleaning the surface of a ferrous base metal strand to prepare said surface to be wet by the coating metal, passing said cleaned base metal strand into a bath of molten coating metal, withdrawing said strand from said bath in an upward path of travel whereby a quantity of said molten coating metal will be carried from said bath by said strand, finishing said molten metal adhering to said strand, and solidifying said molten coating; the improved steps of:

a. providing a nonconductive coreless induction coating pot for containing said bath of molten coating metal;

b. providing a primary coil within the side walls of said pot; and

c. supplying power to the primary coil of said coreless induction pot whereby to induce heavy secondary currents in the coating metal in said pot which are converted into heat by the resistance of said coating metal, said induced secondary currents serving to continuously agitate the molten coating metal whereby to help provide a bright bath surface where said strand exists from said bath and to substantially eliminate the accumulation of oxides and dross on the bottom of said pot and to minimize the accumulation of oxides and dross on the walls of said pot.

2. The method claimed in claim 1 wherein the depth of said coreless induction coating pot is less than the diameter of said coreless induction coating pot.

3. In a hot dip metallic coating method including the steps of thoroughly cleaning the surface of a ferrous base metal strand to prepare said surface to be wet by the coating metal, passing said cleaned base metal strand into a bath of molten coating metal, withdrawing said strand from said bath in an upward path of travel whereby a quantity of said molten coating metal will be carried from said bath by said strand, finishing said molten metal adhering to said strand, and solidifying said molten coating; the improved steps of:

a. providing a non-conductive coreless coating pot;

b. supplying said pot with a charge of the desired coating metal;

c. providing a primary coil within the side walls of said pot; and

d. supplying power to said primary coil of said pot whereby to induce heavy secondary currents in said charge of coating metal in said pot, said secondary currents being converted into heat by the resistance of said charge and serving to continuously agitate the molten coating metal whereby to provide a bright bath surface where said strand exits said bath and to minimize the formation and accumulation of oxides and dross.

4. The method claimed in claim 3 including the step of cooling sasid strand to a temperature less than the melting point of said metallic coating after said cleaning of said strand and prior to the passing of said strand into said bath.

5. The method claimed in claim 1 wherein the coating metal is aluminum.

Claims

1. IN A HOT DIP METALLIC COATING METHOD INCLUDING THE STEPS OF THOUROUGHLY CLEANING THE SURFACE OF A FERROUS BASE METAL STRAND TO PREPARE SAID SURFACE TO BE WET BY THE COATING METAL, PASSING SAID CLEANED BASE METAL STRAND INTO A BATH OF MOLTEN COATING METAL, WITHDRAWING SAID STRAND FROM SAID BATH IN AN UPWARD PATH OF TRAVEL WHEREBY A QUANTITY OF SAID MOLTEN COATING METAL WILL BE CARRIED FROM SAID BATH BY SAID STRAND, FINISHING SAID MOLTEN METAL ADHERING TO SAID STRAND, AND SOLIDIFYING SAID MOLTEN COATING; THE IMPROVED STEPS OF: A. PROVIDING A NONCONDUCTIVE CORELES INDUCTION COATING POT FOR CONTAINING SAID BATH OF MOLTEN COATING METAL; B. PROVIDING A PRIMARY COIL WITHIN THE SIDE WALLS OF SAID POT; AND C. SUPPLYING POWER TO THE PRIMARY COIL OF SAID CORELESS INDUCTION POT WHEREBY TO INDUCE HEAVY SECONDARY CURRENTS IN THE COATING METAL IN SAID POT WHICH ARE CONVERTED INTO HEAT BY THE RESISTANCE OF SAID COATING METAL, SAID INDUCED SECONDARY CURRENTS SERVING TO CONTINUOUSLY AGITATE THE MOLTEN COATING METAL WHEREBY TO HELP PROVIDE A BRIGHT BATH SURFACE WHERE SAID STRAND EXISTS FROM SAID BATH AND TO SUBSTANTIALLY ELIMINATE THE ACCUMULATION OF OXIDES AND DROSS ON THE BOTTOM OF SAID POT AND TO MINIMIZE THE ACCUMULATION OF OXIDES AND DROSS ON THE WALLS OF SAID POT.

1. In a hot dip metallic coating method including the steps of thoroughly cleaning the surface of a ferrous base metal strand to prepare said surface to be wet by the coating metal, passing said cleaned base metal strand into a bath of molten coating metal, withdrawing said strand from said bath in an upward path of travel whereby a quantity of said molten coating metal will be carried from said bath by said strand, finishing said molten metal adhering to said strand, and solidifying said molten coating; the improved steps of: a. providing a nonconductive coreless induction coating pot for containing said bath of molten coating metal; b. providing a primary coil within the side walls of said pot; and c. supplying power to the primary coil of said coreless induction pot whereby to induce heavy secondary currents in the coating metal in said pot which are converted into heat by the resistance of said coating metal, said induced secondary currents serving to continuously agitate the molten coating metal whereby to help provide a bright bath surface where said strand exists from said bath and to substantially eliminate the accumulation of oxides and dross on the bottom of said pot and to minimize the accumulation of oxides and dross on the walls of said pot.

3. In a hot dip metallic coating method including the steps of thoroughly cleaning the surface of a ferrous base metal strand to prepare said surface to be wet by the coating metal, passing said cleaned base metal strand into a bath of molten coating metal, withdrawing said strand from said bath in an upward path of travel whereby a quantity of said molten coating metal will be carried from said bath by said strand, finishing said molten metal adhering to said strand, and solidifying said molten coating; the improved steps of: a. providing a non-conductive coreless coating pot; b. supplying said pot with a charge of the desired coating metal; c. providing a primary coil within the side walls of said pot; and d. supplying power to said primary coil of said pot whereby to induce heavy secondary currents in said charge of coating metal in said pot, said secondary currents being converted into heat by the resistance of said charge and serving to continuously agitate the molten coating metal whereby to provide a bright bath surface where said strand exits said bath and to minimize the formation and accumulation of oxides and dross.