Method And Apparatus For Heating Metallic Strip

Abstract

A method and apparatus for continually heating a metallic strip in a furnace having one or more sections through which the strip passes in a vertical path of travel, and having turn-around rolls and roll chambers baffled from the furnace atmosphere between adjacent vertical sections. Independent heating means are disposed adjacent the surface of each roll and individually controlled so as to maintain the temperature of at least the surface of each roll at substantially the temperature of the strip passing thereabout.

Description

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for continuously heating a ferrous strip in a furnace having rolls about which the strip passes under tension. The invention has great and particular utility in connection with a furnace having one or more sections through which the strip passes vertically, and having turn-around rolls and roll chambers between adjacent sections which are baffled from the furnace atmosphere. Furnaces of the type under consideration are commonly used in connection with a metallic coating operation, wherein the cold reduced strip to be coated is heated continuously in the furnace for annealing while simultaneously preparing the surface to receive a molten metallic coating.

Continuous strip heating furnaces, per se, are very old in the art. The most common continuous strip heating furnace is of a horizontal configuration. That is, the furnace is elongate in the horizontal direction, and the strip to be heated passes through the furnace in a substantially straight, horizontal path of travel. In this type of furnace, the strip is normally maintained under tension primarily applied by roll mechanisms of known configuration outside of the furnace. Various driven supporting rolls will of course be provided within the furnace atmosphere itself.

More recently, continuous strip heating furnaces for metallic coating have been designed which utilize what might be called a vertical configuration. That is, the furnace includes one or more sections through which the strip to be heated passes in a substantially vertical path of travel. Turn-around rolls and perhaps roll chambers are provided at the entrance and exit ends of such a vertical furnace, and between adjacent vertical sections. These rolls normally have water cooled shafts for structural purposes, and are not heated except by the strip. One of the major advantages of such an arrangement is that no rolls are necessary within the hot or cold sections of the furnace. In other words, the turn-around rolls and roll chambers are often baffled from the regular furnace atmosphere.

Under these circumstances, it will of course be apparent that the strip is under considerable tension as it passes about these turn-around rolls.

Commercial experiences with a continuous strip heating furnace of the vertical configuration (as a part of a metallic coating operation) has produced a problem rarely heretofore encountered in connection with horizontal strip heating furnaces. This problem is the formation of "tension buckles" in the furnace. "Tension buckles" are intermittent or continuous longitudinal corrugations in the strip usually occurring in the center area. At the present time, the precise mechanism of tension buckle formation is not known. However, tension buckles have usually been found associated with the passage of a heated strip over a relatively cold roll surface. In any event, the formation of tension buckles during cooling and/or heating clearly results in an unacceptable finished product -- coated strip which must be scrapped. Equally importantly, the formation of tension buckles makes it more difficult to move the strip through the coating line and often results in strip breakage and consequent shut-down of the coating line.

Keeping the above background in mind, this invention is directed to a method and apparatus which has been found empirically to virtually eliminate the formation of tension buckles in a continuous strip heating furnace of the vertical configuration.

SUMMARY OF THE INVENTION

Broadly considered, this invention relates to a method of continuously heating a metallic strip in a furnace having one or more sections through which the strip passes in a substantially straight, vertical path of travel, and having turn-around rolls about which the strip passes under tension located without the primary heating and cooling sections. The invention contemplates the provision of heating elements adjacent each of the rolls, along with means for controlling these heating elements so that the temperature of the roll chambers are independently controlled and the surface of the roll at least will be maintained approximately at the temperature of the strip passing thereabout, and not be unduly influenced by the cooling effect of the water cooled shaft or the adjacent cooling furnace section.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE accompanying this application illustrates schematically a continuous strip heating furnace embodying the apparatus of this invention and which may advantageously be used for the practice of the method of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention has great and particular utility in connection with the continuous heating of a ferrous base metal strand to prepare the surface of the strand to receive a molten metallic coating. The drawings and this specific description will be directed primarily to a continuous furnace utilized in such a process, but it will be understood that other heating applications are contemplated within the scope of this invention.

The strip to be heated is indicated at 10. It enters the first section 12 of the furnace over the rolls 14 and 16. The first section of the furnace in the process under consideration may be a direct fired non-oxidizing heating section. By non-oxidizing it is contemplated that the furnace be supplied with approximately a 5 percent excess of combustibles. The temperature of the furnace in this section may be on the order of 2,300.degree. F. In the metallic coating operation, the function of this direct fired section of the furnace 12 is to quickly burn oil and the like from the surface of the strip. This section of the furnace, at the temperature indicated, will be sufficient to heat the entering strip to a temperature of 1,000.degree. F. to 1,400.degree. F. by the time it passes through the baffle plates 18 at the bottom of the furnace section 12.

The strip then passes about the turn-around rolls 20 and 22, through the opening 24 in the baffle plates, and into the furnace section 26. The strip passes upwardly through this section, through the opening 28 in the upper baffle plates, around the turn-around roll 30, then downwardly through the opening 32 in the furnace section 26, and out through the opening 34 at the bottom of this furnace section. Preferably, this section of the furnace can be of the radiant tube type heating, and will serve to further raise the temperature of the strip. In the embodiment under consideration, maximum strip temperature will be reached at the point the strip passes through the opening 34. This temperature may be on the order of 1,200.degree. to 1,700.degree. F.

After leaving the furnace section 26, the strip passes about the turn-around rolls 36 and 38, through the opening 40 in the roller baffle plates and into the furnace section 42. Within this section, the strip passes upwardly, through the opening 40, about the turn-around roll 46, downwardly through the opening 48 and back through the furnace section 42, and finally out the opening 50 at the bottom. This section of the furnace, in a metallic coating operation may be a tube cooling section of the furnace.

The strip then passes around the turn-around rolls 52 and 54, and upwardly through the opening 56 in the lower baffle plates into the furnace section 58. Once again, the strip passes upwardly in this section of the furnace, through the opening 60 in the upper baffle plates, around the turn-around roll 62, downwardly through the opening 64 and through the furnace section 58, and finally out through the opening 66. This section of the furnace may be of the jet cooling type and will serve to bring the strip down to a temperature of approximately 850.degree. in the case of a metallic coating operation contemplating molten zinc.

Finally, the strip passes about the turn-around rolls 68 and 70, downwardly through the snout 72 and into the bath of molten coating metal, not shown.

Commercial practice utilizing a furnace generally of the configurations described above has resulted in a strip defect which might be called "tension buckles". These tension buckles are non-uniform longitudinal corrugations. Generally, they form in the center portion of the strip and may form intermittently or continuously. Tension buckled strip must be scrapped, with the resulting loss of productive time. Equally importantly, it is extremely difficult to convey buckled strip through the coating line and a serious strip breakage hazard is present.

Investigations have established that the hazard of tension buckles increases significantly with decreasing strip thickness. In addition, the tendency to form tension buckles is relatively dependent upon strip tension in the furnace. For example, when passing light gauge material through a furnace of the type described, the only known way prior to this invention to minimize the hazard of tension buckle formation was to use minimal strip tension and limit the maximum strip temperature to less than about 1,300.degree. F. In other words, it has been impossible to utilize the vertical configuration furnace just described to practice the high drawing quality annealing cycle involving temperatures on the order of 1,500.degree. to 1,700.degree. F. on light gauge strip.

As indicated earlier, one of the nominal advantages of the furnace of a vertical configuration is the fact that the turn-around rolls need not be located in the low or high temperature furnace atmosphere. To this end, it will be observed that each of the turn-around rolls previously described is baffled or otherwise effectively placed in a roll chamber which is outside of the high or low (depending on heating or cooling sections) temperature furnace atmosphere. It is furthermore clear that in the continuous furnace processing of steel strip, the strip temperature is usually significantly different from the temperature of the heating and cooling furnace sections through which it is passing. This invention contemplates the provision of means independent of strip heat to control surface temperature of each of the turn-around rolls. To this end, radiant heating tubes 74 or other heating means are disposed in the roll chambers somewhere adjacent the surface of each of the turn-around rolls. A suitable control structure will be provided so as to maintain the surface of the turn-around roll approximately at the temperature of the strip passing thereabout. Such a control system is largely conventional per se. It would include means for sensing the temperature of the strip as it approaches the roll (indicated schematically at 76), means for sensing the temperature of the roll chamber (such as the zone thermocouples indicated schematically at 78), and means for actuating the radiant tubes so as to control roll chamber temperature thereby controlling roll surface temperature to approximately the temperature of the strip passing thereabout.

Numerous modifications may be made without departing from the scope and spirit of this invention. Accordingly, no limitations are intended except insofar as specifically set forth in the claims which follow.

Claims

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

1. A method of preventing tension buckles during the continuous processing of ferrous strip in an annealing furnace of the type having primary heating and cooling means in vertically oriented heating and cooling sections respectively through which said strip passes in vertically oriented flights and conveyor rolls at the entrance and exit ends of the heating and cooling sections and at the tops thereof baffled or otherwise effectively placed from said primary heating and cooling means and about which the strip passes under longitudinal tension, said method comprising the steps of:

a. heating the roll chambers and at least the surfaces of said rolls by heat sources independent of said primary cooling and heating sources and independent of the heat of the strip, and

b. controlling said independent heat sources so as to maintain the surfaces of said rolls at least approximately at the temperature of said strip passing thereabout.

2. The method claimed in claim 1 wherein said independent heat sources are controlled so as to maintain the temperature of the surfaces of said rolls at least at the temperature of said strip passing thereabout.

3. A method of preventing tension buckles during the continuous processing of ferrous strip by passing said strip through an annealing furnace of the type having at least one primary means for producing a desired temperature of said strip located in at least one vertically oriented section through which said strip passes in vertically oriented flights and conveyor rolls at the entrance and exit ends of said section and at the top thereof about which the strip passes under longitudinal tension, said method comprising the steps of:

a. providing at least one heat source adjacent the surface of each of said rolls, said heat sources being independent of said primary means and independent of the temperature of said strip, and

b. controlling said independent heat sources so as to maintain the temperature of the surfaces of said rolls near the temperature of said strip passing thereabout.

4. The process claimed in claim 3 wherein said independent heat sources are controlled so as to maintain the temperature of the surfaces of said rolls at least at the temperature of said strip passing thereabout.

5. An annealing furnace for continuously processing ferrous strip comprising:

a. at least one vertically oriented heating section through which said strip passes in vertically oriented flights and primary heating means located within said section;

b. at least one vertically oriented cooling section through which said strip passes in vertically oriented flights and primary cooling means located within said section;

c. conveyor rolls at the entrance and exit ends and at the tops of said heating and cooling sections about which said ferrous strip passes under longitudinal tension;

d. means for baffling or otherwise effectively placing said rolls away from said primary heating and cooling means,

e. heating means in addition to said strip independent of said primary heating and cooling means disposed adjacent the surface of each of said rolls, and

f. means for controlling said independent heating means whereby to maintain the surfaces of said rolls at least approximately at the temperature of said strip passing thereabout.

6. An annealing furnace for continuously processing ferrous strip comprising at least one vertically oriented section through which said strip passes in vertically oriented flights, at least one primary means for producing a desired temperature of said strip located within said section, conveyor rolls located at the entrance and exit ends of said section and at the top thereof about which said strip passes in longitudinal tension and means in addition to said strip and independent of said primary means to maintain the surfaces of said rolls at least approximately at the temperature of said strip passing thereabout.

7. The structure claimed in claim 6 wherein said means to maintain said roll surface temperature comprises heating means disposed adjacent said surface of each of said rolls and control means for said heating means.