US2791816A - Method of applying exothermic material to the hot-top of steel - Google Patents

Description

United States Patent METHOD OF APPLYING EXOTHERMIC MATE RIAL TO THE HOT-TOP ()F STEEL Erich C. Pltsch and Michael Bock II, Conneaut, Qlii o, assignors to Exomet Incorporated, Conneaut, Ohio, a corporation of Ohio No Drawing. Application August 3, 1953, Serial No. 372,175

7 Claims. (Cl. 22-216) This invention relates to the casting of large steel ingots weighing at least 3000 pounds from killed'steel in molds having refractory hot-tops, and has for its object the provision of an improved process for casting such-ingots. Our invention is concerned with the casting of the steel and'the treatment of the metal in the hot-top to minimize or eliminate pipe and to materially reduce theloss of metal to be cut from the ingot resulting in a more *efiicient production of steel ingots of improved quality.

In accordance with our invention, we modify the con-. ventional ingot casting technique by pouring much less steel into the hot-top and immediately after pouringiarrest the normal cooling, and then after a relatively long'waiting time release on'the metal in the hot-top a very app re-- ci-able quantity of heat from the reaction-of highly exo=v thermic materials added to the metal in the hot-top to effect a further appreciable delay in the normal cooling rate and thereby feed the ingot and minimize or eliminate the pipe.

ingots of the type with which the invention=is concerned are relatively large and substantially square or circular in cross-section, and it is' the usual practice in casting them to use a refractory hot-top of various shapes and proportions and to pour a very appreciableamounttof steel into the hot-top to feed the shrinking ingot and cause the pipe to form in the hot top which is cutoff. The hot-. topmay be of sr'naller'cross-secti-onal dimensions than the mold and it may taper inwardly towardstthe top. Such hot-tops receive a relatively high level ofsteelp Other hot-tops are virtual extensions of the ingot mold .and receive a relatively low level of metal. Regardless of the kind of hot-top, it is the usual practice to pour from 11% to 22% of the total volume of steel into the hot-top, depending on the size of'the ingot and'the kind of steel. Approximately 3 /z% of the volume of the ingottis required to feed the ingot due to its shrinkage.

In accordance with our invention, we pour the ingots short "and by that we mean from about /3 to /2 the height of metal usually poured into the hot-top. On a volume basis, we pour from Il /2% to 11%, preferably from 5% to 7% of the volume of the ingot, into the hot-. top.

After the ingot has been poured We wait an appreciable time before adding the highly exothermicmaterial to efiect the final heating of the steel in the hot-top. By highly exothermic material, we mean compositions com prising one or more metals which react with an oxygenbearing material of the composition to liberate a large quantity of heat in a reaction ignited by contact of the composition with the steel in the hot-top. In accordance with our present preferred and improved process, we

prolong the waiting time either'by arrestingthe dissipation of heat by use of a thermal insulation'or by the addition. to the steel in the hot-top ofmildlyexothermic material. After the steel in the hot-top has cooled to apoint approaching solidification, we add a highly exothermic material; such as a mixture of iron oxide, preferably Fe3O4, powdered aluminum and slag-forming material, to the steel in the hot-top to release and-add to. the steel from about 250 to 425 kilogram caloriesrper 1.00pounds of steel cast. This reheats'the steel in'the hot-topand enables it to feed the steel in the ingot-mold and also to causeany pipe to form in'thehot-top rather than in-the, ingot.

When alloy steels are being cast, we may substitutefor all or a part of the iron oxide an'oxide ofthef principal alloying j metal, for example manganese oxide, nickel oxide, chromic oxide, etc., so that the reduced metalwill be addedto the steel in the hot-top and thereby maintain the requisite analysis. Special-highly exothermic'materials are sometimes necessary for specialwork.- For example, these can be made to producemet-alrof 12-14% chromium, or of a normal 18-8 stainless steel-composition,.or1 of a 12-14% manganese. steel. It canalso'be made to. produce many other special analyses. The alloy'components are supplied by theroxideof the alloy desired or in some cases by the usetof ferroa1loys,,such as term-manganese, or even pure metaLthe balance arrived at being based on economics and obtaining the proper. heat balance roughly equivalentto that obtained in the iron oxide-aluminum-slag relationship mentioned above. These special highly exothermic products are sometimes necessary to offset -a-tendency towards deleteriousr eifects and/or segregation obtained in theupper portion of large ingots. The smaller the ingot-1 the less necessity for using a special highly exothermic material and also the higher volume of metal used in .thehot-top, the less necessity of this special highly exothermic material. That is to say, on ingots where 10-11% is used in the hot-top there appears to be no necessityinusing the special highly exothermic material, but when a low volume is used in the hot-top and on ingots of20 inches in diameter and larger, this becomes a necessity. In .addi-, tion, on smaller ingots where no iron is desired, such' as on special nickel base super alloys, highly exothermic material can be made using nickel oxide as a .base instead of iron oxide and thus eliminate any possibility of Con-.- tamination. The alloys mentioned should not bacon-t str'ned as'to limit the invention towards, particular alloys but are only given as examples in how special. highlyexo'thermic compositions can be employed in the same manner as the iron oxide-aluminum composition.

Our invention is concerned with the casting of ingots weighing at least 3000 pounds which are either substan-. tially square or circular in cross-section, being atleast 16 inches in diameter for square ingots and at least 19 inches-in diameter for circular ingots, and is applis. cable to all larger ingots including the largest ingots cast, for example, those around 50 inches in diameter. and larger, weighing many tons.

As an example of the practical application of. our in-v vention in normal practice, ingots in the neighborhood of 500 to 625 square inches in which the cross-section is close to being square presently require pouring a hot-top valume of, approximately from 11 to 22%. Small ingots are in the small end of this range, andvery large ingots,

meaningjngots from 36 inches-in diameter-'and'up', are

in the high end of this range. For a specific example, a 22 x 25, 18-8 type 304 stainless steel ingot 1s nor-' going from the liquid to the solid state and so far when our invention is properly practiced even on the largest ingots no more than 11% is required, and on ingots of this type formerly 22% hot-top volume was required. This reduction in hot-top volume amounts to pouring the ingot from /3 to /2 the height inside thehot-top as used in normal practice. Naturally, these pouring heights are based on the normal hot-top cross-section to ingot cross-section. If the hot-top is the same cross-section as the ingot instead of smaller, the pouring height would be on the low side or even less than A; of the volume heretofore mentioned. The height to pour into tapered hot-tops must be calculated on abasisof height versus volume. The pouring volume and/or height will 'also slightly change in relation to the type of steel being poured, i. e. the higher shrinkage steels being poured slightly higher and'the lower shrinkage steels being poured slightly lower. V

The next step in our invention is the waiting time. As heretofore mentioned, a mildly exothermic or insulating material should be added. to the metal surface immediately after pouring to the desired height is completed. It is important to determine the waiting time objectively to have the greatest difference possible in the quantity of'heat between the metal in the hot-top portion and the metal in the ingot mold portion. A limiting'fa'ctor on all waiting time is the bridging eifect of crystals meeting in the center portion of the ingot caused by solidification starting from the outside edges. .These crystals can grow together leaving small pools of liquid metal beneath .them and this latter portion of liquid metal leaves small shrinkage voids when solidification is completed. These small islands of shrinkage would be caused by the feed metal being choked off from them by the bridges solidifying in layers above them as stated before. 7

In relationship to the waiting time, the addition of other materials, such as thermal insulation or heat pro ducing materials, immediately after pouring allows this time to' be greatly extended. The extension of this waiting time is extremely important to the satisfactory results gained by the process of ou'rinvention. Materials to be added to extend'this waiting time can be of the mildly-exothermic type, such as those having carbonaceous, material as a base, or slow heat producing materials, suchas aluminuml drosses with chemical oxidizing agents, such as sodium-nitrate, potassium-nitrate, sodiumchlorate, manganese dioxide, and/or other oxygen producing agents together or individually, or can be of the insulating variety, such' as diatomaceous earth, lime, crushed cork, expanded mica, per-lite, asbestos, rice hulls, pumice, fly ash, and similar inert or relatively inert materials with a fairly good insulating property. In practicing our invention in casting square ingots above 16 inches in cross-section or round ingots above 19 inches in diameter, it is definitely superior to add one of the general types of material mentioned above to the steel surface in'the hot-top'immediately after pouring has been completed. The mildly exothermic and carbonaceous materials should be added in amounts ranging from a inch to 1% inch layer. The insulating materials shouldbe added in amounts ranging from 2 to '5 inches in thick-- ness. The density of the insulating materials would affect the thickness of the layer, the smaller amounts of both materials being used onthe smallersized ingot's'and the larger amounts on the larger ingots. More of these ms 4 1 terials can be used but an excess does not serve any useful purpose.

After the steel has cooled to the desired degree, the remains of the material placed in the hot-top to extend the waiting time may be removed if it is unduly bulky or will'interfere with the subsequent treatment and then the highly exothermic material is added to the steel in the hot-top. While the actual waiting time may vary from 20 minutes to 120 minutes depending on the size of the ingot, we can determine the minimum time by dividing the square of the mean ingot diameter by 1300 when the steel is poured at around 200 F. aboveits melting point. The objective of this determination is to determine the time just before bridging takes place in the ingot. Usually some solidification of steel takes place in contact with the hot-top at the end of the waiting. time, but the time should not be delayed until solidification takes place in the center of the hot-top. When the waiting time is excessive, bridging will have taken place and shrinkage may be encountered in the interior portion of the ingot. In addition, an excessivelylong waiting requires much heat from the highly exothermic material that is wasted in re-melting material already frozen around the hot-top portion of the ingot. A range of the optimum waiting times can be worked out for each size and type of ingot, the larger ingots having much higher optimum waiting times which can extend into hours for ingots above 50 inches in diameter. The pouring temperatures as well as the types of steels will have an effect on the waiting time, i. e. the higher pouring temperatures in relation to the freezing temperatures increase waiting times. Steels which solidify rapidly, tending to form large crystals over a wide solidification range, also have an effect. 'Ingot mold shape also affects the waiting-time.

The amount of highly exothermic material required will vary from around 0.25 pound per square inch in smaller ingots up to about 0.45 pound per square inch for the large ingots, of cross-sectional area in the hottop. Ingots'in the smaller range of from 16 inches to 30 inches would require from 0.25 to 0.38 pound per square inch and larger ingots would require'from 0.35 to 0.45 pound per square inch. The highly exothermic material should all be added at one time. It then will react by itself producing high temperatures and a small amount of liquid metal and slag. Highly exothermic material can also be added based on the weight of the ingot. In this practice approximately 0.75 to 1.25 pounds can be added per 100 pounds of ingot weight. The smallestamount of exothermic material that can be used and still give satisfactory results from a quality standpoint will be found to be most economical but amounts in excess of the minimum requirements may be used without detrimentaleifects.

We: may use any'suitable exothermic material in the final heating of the steel in the hot-top, such as various types of Risotherm, a product of Exomet Incorporated, Conneaut, .Ohio, or mixtures of metals which liberate a large amount of heat on oxidation such as aluminum, silicon, etc.. with oxidizing agents such as metal oxide or sodium nitrate, etc. In producing steels which do not have a large percentage of alloying metal, we prefer to use an exothermic material comprising iron oxide, preferably Fe3O4, aluminum powder,-and slag-forming ingredients, all of which are properly sized and intimately mixed. Exothermic materials in which the iron oxide and aluminum powder are in stoichiometric proportions producesteel containing the exothermic-reaction is aluminurnoxide but the exothermic materiabcomprises' an appreciable amount of other slag-forming materials. While the excess. of iron oxide enters-and becomes a part of .'tlie slag, itis. advantageous; to include in the material other slag-forming materials free of carbon. When using an excess of 12% iron oxide, it is'necessary to add" about 17% of other slag-formingmaterials; As the excessofi'iironioxide is increased, the other slag-forming material is decreased but the excess of iron oxide should not exceed about 23% because the slag will become foamy and unmanageable. As a result of the exothermic reaction, there is produced a relatively deep upper layer of slag, comprising the aluminum oxide, the excess iron oxide, and the added slag-forming materials, having a high heat content and low thermal conductivity which acts as a heat reservoir as well as an insulating top for the ingot.

The exothermic material ignites at about 1900 F.

Aluminum dross 70-75% (-25% aluminum). Iron oxide 8-12%. Clay 8l0%.

Sodium nitrate 8-10% The following are examples of steel ingots produced in accordance with our invention:

Metal Cast Waiting Time Material 1 Added Initially to Increase Delay Time. Amount of Heat Added by Additlon of Highly Exothermic Material. Weightoi Highly Exothermic Material.

1 Mildly exothermic material described above. 1 Highly exothermic materials described above for use on such steels.

creating intense heat, the temperature rising to over .4000 F. The heat greatly increases the temperature of the metal in the hot-top which feeds the ingot as it undergoes shrinkage. Moreover, this heating permits the accumulated gases to escape and dense metal to form at the top of the ingot.

The importance of using an excess of iron oxide in the exothermic mixture is manifested largely by the absence of carbon pick-up by the metal produced and by the lack of violence in the exothermic reactions. The high proportion of iron oxide results in exothermic metal low in carbon and the escape of carbon oxides from the region of the ordinary pipe is not violent.

Thus, there is less heat loss and the exothermic material becomes more efiicient, allowing less material to be used to gain similar quality results. Attempts made heretofore to use exothermic materials in the hot-top have not produced results more beneficial than the use of relatively deep steel in the hot-top. It has been cheaper to use more steel in the hot-top than to use highly exothermic material. In contradistinction, we are able to accomplish surprising results with but a relatively small amount of the exothermic material. The increase in ingot yield has amounted to from 3% to 12% and will vary widely depending on the type and grade of steel used, the size of the ingot mold, the design of the ingot mold, the type of refractory hot-top used, and the general teeming and pouring practice.

The following are three types of highly exothermic material suitable for use in the process of our invention:

For use on 12% manganese steel 10-12% Ferro manganese. 12-15% Slag forming materials.

For use on chromium steel 61-65% FeaO4.

3- 4% CrzOa.

8- 9% Ferro chromium. 22-24% Al.

4- 6% Slag forming materials.

This application is a continuation-in-part of our copending application Serial No. 204,684, filed January 5, 1951.

We claim:

1. In the casting of large steel ingots weighing at least 3000 pounds from killed steel in molds having a refractory hot-top, the improvement which comprises pouring the steel in such amount that the hot-top contains from 3 /2 to 11% of the total steel cast, applying to the steel in the hot-top a material which delays solidification, a1- lowing the ingot to cool for a time varying from 20 minutes for 3000-pound ingots to 120 minutes for large ingots, and then applying to the steel in the hot-top an exothermic material in an amount such as to release on ignition from 250 to 425 kilogram calories of heat per pounds of steel poured.

2. In the process of claim 1, determining the minimum waiting time by dividing the square of the mean diameter of ingot by 1300 when the steel is poured at a temperature about 200 F. above the solidification temperature of the steel.

3. In the process of claim 1, applying to the steel in the hot-top immediately after pouring a thermal insulating material to delay the normal cooling for at least ten minutes.

4. In the process of claim I, applying to the steel in the hot-top an exothermic material which undergoes oxidation with the addition of heat to the steel in the hot-top to delay solidification.

5. In the casting of large steel ingots weighing at least 3000 pounds from killed steel in molds having a refractory hot-top, the improvement which comprises pouring the steel in such amount that the hot-top contains from 3 /2 to 11% of the total steel cast, applying to the steel in the hot-top a material which delays solidification, allowing the ingot to cool to a time just before bridging takes place in the ingot which is in excess of 20 minutes and before the steel solidifies in the hot-top, and then applying to the steel in the hot-top an exothermic material in an amount such as to release on ignition from 250 to 425 kilogram calories of heat per 100 pounds of steel poured.

6. In the improvement of claim 1, using an exothermic material consisting essentially of a metal and an oxygenbearing compound that forms an oxide with the metal,

which material produces temperatures in the hot-top of References Cited in the file of this patent UNITED STATES PATENTS 7. In the improvement of claim 5, using an exothermic I r material consisting essentially of a metal and anoxygen- 2,462,256 Charman et F 22, 1949 hearing compound that forms an oxide with the metal, 5 Lutts, -V----V---- J 24, 1950 which material produces temperatures in the hot-top of 25141793 ,Pletsch, y 1950 about 4000 F. OTHER REFERENCES American Foundryman, August, 1946, page 76.

Claims (1)

1. THE CASTING OF LARGE STEEL INGOTS WEIGHTING AT LEAST 3000 POUNDS FROM KILLED STEEL IN LOLDS HAVING A REFRACTORY HOT-TOP, THE IMPROVEMENT WHICH COMPRISES POURING THE STEEL IN SUCH AMOUNT THAT THE HOT-TOP CONTAINS FROM 31/2 TO 11% OF THE TOTAL STEEL CAST, APPLYING TO THE STEEL IN THE HOT-TOP A MATERIAL WHICH DELAYS SOLIDIFICATION, ALLOWING THE INGOT TO COOL FOR A TIME VARYING FROM 20 MINUTES FOR 300-POUND INGOTS TO 120MINUTES FOR LARGE INGOTS, AND THEN APPLYING TO THE STEEL IN THE HOT-TOP AN EXOTHERMIC MATERIAL IN AN AMOUNT SUCH AS TO RELEASE ON IGNITION FROM 250 TO 425 KILOGRAM CALORIES OF HEAT PER 100 POUNDS OF STEEL POURED.