US3039158A

US3039158A - Highly thermally efficient hot top and preformed protective refractory and exothermic unit therefor

Info

Publication number: US3039158A
Application number: US57782A
Authority: US
Inventors: John W Mueller
Original assignee: Oglebay Norton Co
Current assignee: Oglebay Norton Co
Priority date: 1960-09-22
Filing date: 1960-09-22
Publication date: 1962-06-19
Anticipated expiration: 1979-06-19
Also published as: GB919512A

Description

June 19, 1962 J. w. MUELLER 3,

HIGHLY THERMALLY EFFICIENT HOT TOP AND PREFORMED PROTECTIVE REFRACTORY AND EXOTHERMIC UNIT THEREFOR Filed Sept. 22, 1960 INVENTOR. JW-IA/ W Ml/EZLER BYM ,4rromvsys 4''! time 3,039,158 Patented June 19, 1962' nice This invention relates to a hot top and particularly to a highly thermally efiicient hot top.

As is Well understood in the art, hot tops are used' in conjunction with ingot molds in the casting of metal ingots such as steel ingots. The purpose of the hot tops is to provide a reservoir of molten metal therein which feeds the pipe or shrinkage cavity as it tends to form in the solidifying ingot in the mold and thus enable the production of solid ingots which can be further processed without necessitating expensive scrapping of metal. In order that the hot tops perform their function efiiciently and economically it is a requisite that the hot tops be thermally efiicien-t so that the molten metal therein does not solidify as quickly as the molten metal in the mold and remains in a flowable molten condition to feed the pipe or shrinkage cavity forming in the solidifying ingots.

Although single use or all refractory hot tops are employed to some extent, substantial numbers of composite hot tops are used. These composite hot tops have metal casings which are provided with refractory linings. The composite hot tops can be reused for a number of pouring operations although, as is known in the art, they require a measure of preparation for each pouring operation.

The metal casings of the composite hot tops have been lined heretofore with hard dense high quality firebrick linings which, in the use of the hot tops, is covered with a thin paint-like slurry to fill up cracks in the lining and to facilitate the stripping of the hot tops from the sinkhead of the ingot. These high quality hard dense firebrick linings are semipermanent linings in that they can be used for a number of pouring operations. This is due to the fact that such linings have good mechanical strength and heat resistant characteristics. However, the high quality hard dense firebrick linings, because of their density, do not have good heat insulating characteristics. Also, these linings due to their density and mass are highly heat absorbing and absorb so much heat from the molten metal in the hot tops in the time period that hot tops function that they produce a chill in this molten metal causing the same to solidify too quickly and hence to lessen the efliciency of their function of feeding the pipe or shrinkage cavity forming in the solidifying ingots.

In order to compensate for the poor heat insulating characteristics and the high heat absorption of these high quality hard dense firebrick linings, it has been necessary to have the hot tops of a size such that they contain a volume of molten metal equal to or 16% of the total and combined volume of molten metal in the molds and hot tops even though only 3% to 5% of the molten metal in the hot tops is required to feed the pipes or shrinkage cavities that tend to form in the solidifying ingots.

Inasmuch as the sinkheads on the ingots, formed by the solidified metal remaining in the hot tops, are cropped later from the ingots and scrapped, it will be realized that a substantial loss of metal occurs with substantial increase in the cost of ingots.

In an effort to improve the thermal efiiciency of hot tops to permit the use of lower volumes of metal in the hot tops, the metal casings of composite hot tops have been lined with insulating firebrick which are distinguished from the high quality hard dense firebrick by being rather porous, fragile, of poor mechanical strength, but having good heat insulating properties and low heat absorption characteristics. However, these insulating firebrick linings, unless protected, are quickly destroyed by the heat of the molten metal which usually is 2800 F. to 2950 F., that is, the insulating firebrick linings by themselves do not constitute semipermanent linings capable of being used for a substantial number of pouring operations before being replaced. These commercial insulating firebrick linings have temperature resisting use ratings of from 1600 F. to 2600 F. which are less than the temperature of the molten metal in the hot tops. In order to make the insulating firebrick linings semipermanent ones capable of repeated use, they have been covered with a protective refractory veneer applied to the inner surf-ace of the linings and dried in situ, or have been covered by preformed refractory inserts inserted into the hot tops. The protective veneers or inserts are onlysingle use expedients and are replaced for each pouring operation. These veneers or inserts need only be sufficiently thick to provide the required temperature drop or gradient between the temperature of the molten metal and the rated use temperature of the insulating firebrick linings. As an example, if the rated use temperature of the insulating firebrick lining is 2600 F., then the temperature drop needed would only be 200 F. to 350 F. It has been found that hot tops provided with insulating firebrick linings and the protective veneers or inserts have substantial increased thermal efficiency since the insulating firebrick linings prevent heat losses due to their good heat insulating properties while the protective refractory veneers or inserts are not sufliciently thick to cause high heat absorption therein from the molten metal in the hot tops during the time period the latter function.

The hot tops having the insulating firebrick linings protected by the veneers or inserts require a relatively low volume of metal therein and since the heat losses from the metal are reduced the metal remains molten and flowable a suflicient length of time and functions satisfactorily to feed the pipes or shrinkage cavities in the solidifying ingots. It has been found that these more thermally eflicient hot tops require a volume of metal therein which is only 10% to 12% of the total molten metal in the molds and hot tops. Also, it has been found that these more thermally efiicient hot tops produce an increase of 3% to 5% in sound ingot yields as compared to the sound ingot yields when using hot tops lined with the high quality hard dense firebrick linings first described above.

The present invention contemplates the provision of hot tops having a still higher thermal efiiciency such that they will require a volume of molten metal which is only approximately 7% or less of the total volume of molten metal in the hot tops and molds and will thereby further increase the sound ingot by an additional 3% or more as compared to the sound ingot yields of the previously describe-d hot top constructions.

The invention further contemplates that this hot top of increased thermal efiiciency will be so constructed internally as to produce the advantageous results referred to and at the same time will enable its insulating firebrick lining to be a semipennanent one capable of being reused for a number of pouring operations, by the use of an improved protective single use protection which can be readily applied to the insulating firebrick lining during the preparation of the hot top for a pouring operation.

More specifically the invention contemplates a composite hot top which has a semipermanent insulating fire-' brick lining covered with a preformed, dried, relatively dense, mechanically strong refractory panel insert which has applied to and integrated with its exposed surface a facing layer of exothermic material. The protective panel inserts are relatively thin so as not to have great heat obsorption quality, and they provide the desired temperature drop or gradient between the temperature of the molten metal and the rated use temperature of the insulating firebrick linings and, in addition, they protect the latter from the heat and fluxing actions of the exothermic layers. The exothermic layers on the relatively thin protective panel inserts avoid chills of the molten metal in the hot tops due to heat absorption therefrom by the refractory panels since said exothermic layers function to reduce heat absorption by the panel from the molten metal in the hot top.

The use of hot tops constructed as contemplated by the present invention eifects substantial savings and increases the efiiciency of ingot production in that the size of the sink-head which is subsequently scrapped is greatl reduced thereby substantially increasing the yield of sound steel from the ingot.

The invention contemplates a preformed relatively dense, essentially nonfragile, self-sustaining, mechanically strong refractory insert that supports a facing layer of exothermic material and acts as a heat buffer between the exothermic facing layer and the insulating firebrick lining. This concept produces a low volume hot top requiring a minimum amount of exothermic material as the facing layer and hence a minimum volume hot top for minimum cost. Hot tops embodying the invention are economically competitive with other hot tops used for producing ingots of all grades of steel as distinguished from known exothermic hot tops which are prohibitive cost-wise except for use in connection with production of high grade high cost alloy and stainless steels.

During the following detailed description of an embodiment of the invention further advantages resulting therefrom will become apparent such description being taken in conjunction with the accompanying drawing forming a part of this specification and wherein:

FIG. 1 is a view partly in elevation and partly in section of a hot top of the composite floating type constructed in accordance with the present invention and shown mounted in the open upper end of an ingot mold, a portion of the latter being illustrated in section.

FIG. 2 is an elevational view of the preformed panel insert having a refractory layer and an exothermic facing layer thereon which are used to protect the insulating firebrick lining for the side walls of the casing.

FIG. 3 is a view similar to FIG. 2, but shows the protective panel insert having a refractory layer and an exothermic facing layer thereon and used to protect the insulating firebrick lining at the corners of the casing of the hot top, and

FIG. 4 is a sectional view of a portion of the hot top shown in FIG. 1 and is taken substantially on line 4-4 of FIG. 1 looking in the direction of the arrows.

The hot top shown in the drawing insofar as its metal casing and its manner of use are concerned may be said to be conventional. The metal casing of the hot top is illustrated as formed of an upper section and a lower section 11 suitably secured together. The hot top is shown as being of rectangular configuration, but it will be understood it might be of round, oval or other configuration.

The upper section 1 0 of the metal casing is shown as provided at its upper end with an integral inwardly extending lip or flange 12, which may extend into the inside face of the insulating brick lining as shown or it may extend into the inside face of the insert. The lower section or part 11 of the metal casing has at its lower end an integral inwardly extending lip or flange 13, which is of less width than the flange 12.

The metal casing is lined with porous, fragile, insulating firebrick 21 of poor mechanical strength but good heat insulating properties and low heat absorption characteristics. These insulating fibebrick constituting the lining are distinguished in this way from high grade, hard, dense firebrick which, because of their density and mass do not have good heat insulating characteristics and low heat absorption property, although they do have good mechanical strength. The insulating firebrick lining of the hot top embodying the invention is mounted between and is supported by the

flanges

12 and 13 of the casing. The insulating firebrick lining in combination with the preformed panel inserts having a refractory layer and an exothermic facing layer thereon constitute a novelty of the present invention and will be described in greater detail hereinafter.

The hot top illustrated has a downwardly divergent or tapered interior although they may also be made with no taper and in preparing for a pouring operation the hot top is placed in a suitable stand in inverted position as is well understood in the art. The insulating firebrick lining of the hot top casing is semipermanent in that it is used for a number of pouring operations. The

preformed panel inserts

23 and 24 are. inserted into the interior of the hot top and placed in correct position in contact with the insulatinglining 21 and inside of ledge 12 as shown, or they may be stopped off and fit under the wider ledge 12 as explained above. Then the refractory bottom ring 14 is placed on the lower end of the hot top as will be well understood. This bottom ring is provided with a raised rib portion 15 extending upwardly past the inner end of flange 13 and underlying the insulating firebrick lining. The refractory bottom ring 14 is shown as provided on its inner periphery with a recessed portion 16 furnishing a shoulder that supports the' if preformed panel inserts. It will be understood that the rib 15 might be wider and the recessed portion 16 m'iitted and the panel inserts then would engage the uppersurface of the wider rib- 15 of the bottom ring. The usual wiper strip 17 may be positioned between the lower end of the casing and the upper side of the bottom ring as is well understood in the art. Also any space or clearance between the bottom ring, the metal casing and the lower end of the insulating firebrick lining can be sealed with a suitable sealing compound as indicated at 18. The refractory bottom ring 14 and the wiper strip 17 are secured to the casing in the usual way by suitable attaching clips (not shown) as is well understood in the art.

When the hot top thus equipped is used for a pouring operation, it is positioned a predetermined distance in the upper open end of an ingot mold l9 and initially held in such position by suitable. blocks such as the wooden blocks 20; After the pouring of the molten metal into the mold and the hot top, the blocks 20 are knocked out or destroyed and thereafter the hot top floats on the molten metal in the ingot mold. The preparation and use of the hot top as broadly described above is well known in the art.

The insulating firebrick lining is shown as formed of side portions 21 and corner portions 22. The

portions

21 and 22 have their contacting surfaces shaped to closely interfit one another and to closely interlock in position in the hot top between the lips or

flanges

12 and 13 of the casing. The insulating firebrick lining extends inwardly beyond the inner end of the lower lip or flange 13 and overlies the raised rib 15 of the bottom rim 14, as clearly shown in FIG. i1.

As previously stated, the insulating firebrick lining is rather porous, fragile and of poor mechanical strength but has good heat insulating properties and low heat absorption characteristics. However, insulating firebrick of this type have a rated temperature use of between 1600 F. and 2600" F .and since the temperature of the molten metal is between 2800" F. and 295G F., the lining would be destroyed or so severely damaged by the thermal shock of direct contact with the molten metal it would have to be replaced for each pouring operation. This destruction or damaging of the lining also would be increased because of the mechanical abuses to which it would be subjected during the stripping of the hot top as? w vimsert is to provide a suflicient tempertaure drop or gradient between the temperature of the molten metal and the temperature to which the insulating firebrick lining would be subjected to one within the rated temperature use of the lining.

The present invention contemplates protecting the insulating firebrick lining of the hot top casing so that it Will be a semiperrnanent lining capable of being reused for a number of pouring operations and at the same time greatly increasing the thermal efiiciency of the hot top enabling a lower volume of metal to be used therein than is ordinarily the case and thereby improving the yield of sound steel from the ingot.

The preformed protective inserts for the insulating firebrick lining of the hot top in accordance with the invention comprise a dense, essentially nonfragile, selfsustaining refractory supporting layer having integrated therewith on one side an exothermic facing layer. The protective covering for the insulating firebrick lining may be preformed as a single insert unit or they may be preformed as separate side panel inserts 23 and corner panel inserts 24, which have their contacting edge surfaces shaped to closely interfit and mutually interlock. The protective panel inserts 23 and 24 when positioned in the hot top have their refractory supporting layers 25 and 25a contacting the insulating firebrick lining while the exothermic facing layers 26 and 26a of the panel inserts 23 and 24 bound the interior of the hot top.

The inner surfaces of the insulating firebrick lining may have a slurry made from sea coal or other suitable material sprayed or otherwise applied thereto or this slurry may be applied to the surfaces of the refractory supporting layers of the panel inserts which contact the lining. The purpose of the slurry is to prevent penetration of molten metal onto the insulating brick lining should the metal accidentally seep through the joints between the panel insert units and to prevent fusing of the panel inserts to the insulating firebrick lining which would hinder the stripping of the hot top from the sinkhead.

The refractory supporting layers 25 and 25a of the preformed protective panel inserts are formed of a high silica heat resistant composition. This composition may vary considerably in its constitutent makeup but a suitable one has been found to consist in five to eight parts by weight of silica sand to which is added one part by weight of liquid sodium silicate of approximately 40 Baum specific gravity. These materials are thoroughly mixed and the composition is placed in molds and formed to the desired shapes after which the panel is removed from the mold and dried either before or after the exothermic facing layer is molded thereon depending on the materials being used.

The exothermic facing layers 26 and 26a of the panel inserts 23 and 24 may vary in their composition and while not wishing to be limited to particular exothermic compositions, a number of suitable ones will be referred to. The exothermic composition may include sources of carbon, such as coal, coke breeze, tar and graphite; sources of silicon such as ferrosilicon; sources of aluminum such as dross, or any other materials which, when suitably compacted, will in the presence of entrained air or oxygen or oxygen obtained from an included oxidizing agent, oxidize or burn with the liberation of heat. Also various farm or forest products or bi-products which will function exothermically may be used such as straw, vegetable hulls like oats or rice. In addition, the exothermic composition will contain a suitable heat destructible binder. The exothermic composition after being thoroughly mixed, is placed in the panel molds on top of the refractory composition which may or may not have been previously dried and then the exothermic composition or the refractory composition and the superimposed exothermic composition are thoroughly dried as, for instance, by being baked at temperatures not high enough to ignite the exothermic composition. In either event the refractory composition and the exothermic composition become attached or integrated together when dried.

Although the thickness of the refractory layers and of the exothermic layers of the preformed protective panels may vary, it is contemplated that each layer will be approximately one-quarter to one-half inch thick so that the integrated layers of the panels will have a total thickness of approximately one-half inch to one inch.

The refractory supporting layers of the protective panels provide the required temperature drop or gradient between the temperature of the molten metal and the tem perature to which the insulated firebrick lining is subjected, that is, the insulating firebrick lining will not be subjected to a temperature higher than its rated use temperature. While the refractory supporting layers of the panels are hard, dense, essentially nonfragile and selfsustaining, they do not because of their relative thinness provide sufficient mass to absorb high quantities of heat from the molten metal such as would be enough to produce a chill in the molten metal and lessen the efficiency of the hot top. Also the refractory supporting layers of the preformed panels keep the exothermic facing layers thereof out of direct contact with the insulating firebrick lining and hence the latter is protected from and is not directly subjected to heat generated by the exothermic layers.

The exothermic facing layers 26 and 26a of the preformed

protective panels

23 and 24 when ignited by the molten metal during the use of the hot top serve to compensate for heat losses and heat absorption from the molten metal and thus to increase the thermal efficiency of the hot top.

It will be seen that the refractory supporting layers of the protective panels not only act as a supporting means for the facing exothermic layers thereof, but provide an effective heat buffer between the molten metal and the insulating firebrick lining and between the exothermic facing layers and the insulating firebrick lining. The refractory supporting layers of the panels also protect the insulating firebrick lining from the fluxing action produced by the reaction of the exothermic layers, and thus prolong the life of the insulating firebrick lining. The use of the refractory supporting layers in combination with the exothermic facing layers of the protective panels and with said panels covering the insulating firebrick lining make it possible to produce an extremely low volume hot top while utilizing a minimum amount of exothermic material, thus providing such a low volume hot top at a minimum cost. A hot top construction in accordance with the invention need only contain a volume of molten metal which is approximately 7% of the total volume of the molten metal in the hot top and mold and hence even though a hot top embodying the invention may be somewhat more expensive than other types of hot tops, it is economically competitive with the latter, particularly in connection with the production of lower carbon and nonalloy grades of steel. There are in use hot tops including exothermic agents in their construction but such hot tops have not been economically competitive or practical except possibly for high grade, high cost alloys and stainless steels.

The protective panel inserts being preformed can be produced accurately and uniformly as to size and composition in a plant properly equipped for such production. Also the protective panel inserts being self-sustaining and essentially nonfragile can be handled, shipped, stored and used with a minimum of breakage. In addi tion, the preformed protective panel inserts facilitate the preparation of the hot tops for a pouring operation, as compared to applying protective veneer compositions and exothermic compositions to the brick lining of the hot tops by troweling or otherwise applying the same thereto and the-n drying such compositions in situ.

Furthermore the preformed protective panel inserts are thoroughly dried prior to their insertion into the hot tops and hence the danger is obviated of pouring molten metal into the hot tops when the protective coverings of the brick linings are not completely dried.

Although a specific structural embodiment of the invention has been illustrated and described and certain refractory and exothermic compositions referred to, it will be understood that the invention is not to be limited thereto but is to be construed as including such variations as come within the scope of the appended claims.

Having thus described my invention, I claim:

1. A highly thermally efficient low volume hot top comprising a metal casing provided at its lower end with an inwardly extending flange, a lining in said casing contacting and supported on said flange and formed of porous rather fragile insulating firebrick having high insulating and low heat absorption properties, a single use refractory bottom ring on the lower end of said casing having a first portion underlying and contacting said flange and a second portion extending inwardly of the hot top beyond the inner end of said flange, and a preformed insertable protective unit covering the inner surface of said lining throughout its vertical length and including a hard dense essentially nonfragile, self-sustaining refractory layer contacting said lining, said layer having a lower end portion overlapping the inner side of said second portion of said ring, said lower end portion of said layer and said second portion of said ring having interengaging and interfitting surfaces acting to support said unit in the hot top and to seal against metal flowing between the bottom ring and said unit.

2. A highly thermally eflicient low volume hot top as defined in claim 1 wherein said interengaging and interfitting surfaces include a shouldered recess formed in one of said portions and interfitting with the other of said portions.

3. A highly thermally efficient low volume hot top as defined in claim 1 and wherein said interengaging and interfitting surfaces include a shouldered recess on the inner periphery of said ring and in which inter'fits said lower end portion of said layer.

4. A highly thermally efficient low volume hot top as defined in claim 1 wherein said layer of said unit is a supporting and heat buffer layer and a facing layer of exothermic material is integrated with said first layer over the entire surface of the latter remote from said lining with both of said layers having a thickness substantially less than the thickness of said lining.

References Cited in the file of this patent UNITED STATES PATENTS 2,821,000 Nouveau Jan. 28, 1958 FOREIGN PATENTS 565,154 Great Britain Oct. 30, 1944 525,428 Canada May 22, 1956