US2493394A - Process of pouring metals and products produced thereby - Google Patents

Description

Patented Jan. 3, 1950 UNITED STATES PATENT PRODUCTS PRODUCED- Holbert Earl Dunn, Crafton, and Heinrich Wilhelm Rathmann, Pittsburgh, Pa., and Howard Calvin Parkman, Niagara Falls, N; Y., assignora to Vanadium Corporation of America, New York, N. Y., a corporation of Delaware Application August 27, 1946, Serial No. 693,198

14 Claims.

The present invention relates to a process of pouring metals and products produced thereby, and more specifically to a process of pouring or casting refractory metals which have smelting slags characterized by high viscosity and from which the metals do not readily separate. The invention has been developed by us with particular reference to the casting of ferro-chromium, and will therefore be described with particular reference thereto.

Figure 1 of the accompanying drawing is an elevation partly in section showing diagrammatically the casting, of high-carbon ferro-chromium and Figure 2 is a similar view showing the casting of low-carbon ferro-chromium.

The invention relates particularly to the pouring of molten metal through a body of slag of such a character and in such a manner that metal of the cast regulus or ingot shows an improved cleanliness and is particularly free from high melting point non-metallic inclusions. One of the principal objects of the invention is to obtain by the process herein described such an improved cleanliness in the cast metal. Another object is the eflicient production of sound ingots having an improved skin distinguished by freedom from cracks. Other objects of the invention will be apparent from the following description and claims.

The invention will first be described with particular reference to its application in the production of high-carbon ferro-chromium. Highcarbon ferro-chromium is a notable example of a refractory metal which has a smelting slag characterized by high viscosity and from which the molten high-carbon ferro-chromium does not readily separate. High-carbon ferro-chromium is produced by the smelting of chromite ore in an electric furnace, usually an electric arc furnace. Chromite ore, together with coke, coal, and sometimes a small amount of silica as a flux, are added to the top of the furnace as the charge. The carbonaceous material reduces the chromite ore to form molten high-carbon ferro-chromium which, together with the slag produced by the smelting operation, descends into the bottom part of the furnace from which the metal and slag are periodically tapped. High-carbon ferrochromium has a melting point in the neighborhood of 2800 F. and is usually tapped at a temperature from about 2900 F. to 3200 F. Even at such tapping temperatures the slag is sluggish or highly viscous to dry cokey consistency and quickly chills. The ferro-chromium metal does not readily separate from such slags but appreciable quantities of the metal are entrapped as shot in the usual pouring operation. The usual practice has been to tap the metal from the furnace, together with as much slag as will run out with the metal, into a refractory lined ladle in which the metal is allowed to freeze or from which it may be-drained from a clay-plugged hole in the bottom of the ladle into a suitable chill mold. The smelting slag is so viscous and refractory that it is readily entrapped in the ferro-chromium, not only in microscopic inclusions but in inclusions of considerable size. It forms slag attachments to the surface of the metal which are very diflicult to separate after freezing. The inclusions are characterized by the presence of numerous highly refractory polyhedral inclusions of the chromite type. which are particularly difficult to remove in steelmaking operations and are particularly objectionable if carried through to the finished steel product. The fracture of the cold metal is usually of a dull, silvery lustre, often heat-tinted to bluish oxidized surfaces caused by infiltration of the air through numerous cracks in the surface of the ingot, particularly when cast in a chill mould. When the metal is allowed to freeze in the ladle the ingot is characterized by a rough surface containing large slag attachments which are difficult to remove from the ingot. When the metal is tapped from the ladle into a chill mold these slag attachments still persist to some extent, and in addition, the rapid chilling of the metal results in surface strains and surface cracking so that when removed from the mold the ingot tends to break into undesirably small pieces.

We have found that by pouring the ferrochromium through a body of casting slag, as hereinafter described, the objectionable inclusions which resulted from the refractory smelting slag can be largely, if not completely eliminated. Not only are the large inclusions of entrapped slag eliminated, but microscopic inclusions of various objectional types including those of the chromite type which have been particularly objectionable in high-carbon ferro-chromium, as heretofore made, are largely, if not completely, eliminated. The following is a specific example of our process as applied to the making of high-carbon ferrochromium.

The high-carbon ferro-chromium is smelted in the usual electric arc furnace and the metal and slag are tapped in the usual way, since our process does not require any change in the usual smelting and tapping operations. The metal and accompanying slag, however, are tapped into a ladle containing a previously prepared casting slag. The casting slag is a relatively fluid slag and has a melting point considerably lower than that of the smelting slag. The casting slag may be made synthetically or may be formed by suitable additions from slags of other metallurgical operations. We have utilized a. waste slagof another metallurgical operation which had the following composition:

Per cent S10: 27.54 CaO 46.44 MgO 18.25 FeO 0.94

CrzOs 0.90

This slag was adjusted and made more fluid by the addition of about 275 to 350 pounds of silica sand to about 20 cubic feet of the original waste slag. The slag was put into an electric furnace and melted to form a fluid adjusted slag of the following composition:

As we have carried out the process in the making of high-carbon ferro-chromium, a furnace tapping high-carbon ferro-chromium at the rate of 3400 pounds per tap (about 8 cubic feet) every one and two-thirds hours was employed and was serviced by a cast iron ladle of about 34 cubic feet capacity and weighing about 9300 pounds. About 16 cubic feet of adjusted slag was poured into the ladle, filling it to a depth of about 25 inches. The temperature of the adjusted slag was 3000 to 3100 F., having a fluidity of to 9.5 inches as measured by the Herty viscosimeter. The ladle was then transferred to tapping position at the spout of the ferro-chromium smelting furnace about 5 to minutes before the tap hole was scheduled to open. This period of time allowed the slag next to the inner surface of the cast iron ladle to chill, forming a thin frozen slag skull over the bottom and sides of the ladle. The ladle in tapping position was placed as high as possible beneath the tapping spout so that the surface of the receiving pool of casting slag was about 35 inches below the level of the spout runner. The furnace was then tapped and the melted term-chromium, together with as much of the smelting slag as would flow out with the ferro-chromium, was run into the ladle. The average time between tapping and plugging of the tap hole was about 8 to 12 minutes with a range of 4 to 20 minutes. The ferro-chromium was tapped at an average temperature of about 3000 to 3100" F., but ranged from between 2820 and 3240 F. The thick, viscous smelting slag ran out at a temperature about to less than the temperature of the ferro-chromium. Approximately the same volume of metal as slag flowed from the furnace, the combined volume of metal and smelting slag averaging about 14 to 17 cubic feet, depending on the furnace burden. This casting practice is diagrammatically illustrated in Figure 1 of the drawings in which reference numeral l indicates the cast iron ladle, 2 the smelting furnace, 3 the spout of the furnace, 4 the body of casting slag. 5 the pool of molten metal forming in the bottom of the ladle, 6 the auaaec 4 thin skull of chilled slag between the ladle and the ferro-chromium, and l and 8 the metal and the accompanying smelting slag respectively flowing from the spout into the ladle.

The deep pool 4 of casting slag into which the metal is teemed has a triple function. It washes the metal and removes slag inclusions which are present in the metal as tapped from the furnace. It serves to strip the smelting slag away from the teemed stream of metal and to dissolve the smelting slag, thus reducing its viscosity and preventing inclusions of refractory smelting slag, and also releasing particles of metal entrapped in the smelting slag. It serves to check the velocity of the metal as poured from the furnace into the ladle and causes a gentle descent of the metal onto the top of the quiescent accumulating pool of metal in the bottom of the ladle.

The slag layer is deep enough so that it has a substantial washing action on the metal to remove from it slag inclusions present in the metal as it is teemed from the furnace. These inc1usions are high melting point inclusions characteristic of the high-melting smelting slag with which the metal was in contact in the furnace. The fluid casting slag tends to dissolve these inclusions and remove them from the metal.

When the stream of metal and smelting slag enters the layer of casting slag the stream of smelting slag is stripped away from the stream of metal, and because of its greater density the metal descends through the casting slag, leaving the smelting slag entrapped by the casting slag. The casting slag dissolves and fluidifies the smelting slag as shown by an analysis of the slag in the ladle at the end of the pouring operation. In the specific example referred to above, the smelting slag from the ferro-chromlum furnace had an analysis as follows:

Per cent S102 27.73 CaO 0.75 MgO 33.95 FeO 2.15 CrzOa 6.82 A1203 23.72

This slag was a very refractory, viscous slag. The slag in the ladle at the completion of the This final ladle slag was a fairly fluid slag, its viscosity ranging from 3.4 to 7 inches on the Herty viscosimeter. It will thus be seen that the body of slag, by dissolving and fluidifying the entering smelting slag, remains fluid through the entire casting operation, thus minimizing the chance of inclusions that might otherwise be entrapped in the metal as it passed through the slag. The fluidifying of the smelting slag by its mixture with the casting slag releases metal shot which may be entrapped in the melting slag and thus increases the yield of ferro-chromium.

The thick layer of casting slag checks the velocity 0f the stream of metal poured from the furnace and causes a gentle descent of the metal onto the top of the pool of metal accumulating in the bottom of the ladle. Any violent intermixing of the metal with the contents of the ladle is thus avoided and the pool of accumulating metal is allowed to remain quiescent. The gently descending entering metal stream can have but a limited penetration into the accumulating pool of ingot metal, thus shortening either the distance an entering particle of any non-metallic substance must rise to meet the slag-metal interface in case it has evaded the filtering action of the'slag layer, or shortening the time required if it is released below that interface by precipitation from the cooling metal.

The elimination of inclusions by the shortened distance of ascent to the slag-metal interface is helped by the slow cooling of the metal in our process. Before the metal is tapped, the ladle is allowed to stand for .5 or minutes until the cast iron metal of the ladle is heated and until a chilled layer of insulating slag is formed over the bottom and sides of the ladle, which prevents the pool of metal coming in direct contact with the cast iron. This insulating layer of slag serves to keep the metal fluid for a longer period than where the metal is poured immediately into a cast iron ladle. The heavy body of slag on top of the pool of metal also serves to insulate it. Measurements of the temperature of the ladle after pouring indicate that the freezing of the metal is considerably retarded and is not completed for some hours after pouring, allowing the maximum time for inclusions to rise to the surface of the metal and thus be eliminated. These various factors combined in producing in an enhanced degree a micrographic cleanliness in the metal, particularly with respect to non-metallic inclusions of a size order of 200 microns or less, and more particularly 50 microns or less, which are so slow in rising through a column of liquid metal that they have been entrapped in the highcarbon ferro-chromium ingots cast in the usual manner.

After a cooling period of 6 to 8 hours the ladle is rip-ended and the entire cast skidded to the cleaning floor where the 12 to 15-inch top layer of slag is easily split clean from the metal regulus, whose surface is otherwise completely encased in a thin shell of chilled slag, usually between A; to inches in thickness, but occasionally reaching a thickness of 1% inches, which peels off readily from the bright, smooth, dense ingot or regulus skin. The presence of the thin shell or skull of slag over the sides and bottom of the regulus is evidence of the formation of such thin skull of slag during the period between the filling of the ladle with the casting slag and the pouring of the metal, as otherwise the metal would freeze in contact with the cast iron body of the ladle.

The surface of the regulus is smooth and without sharp corners or projections, showing that the metal solidified in an envelope of slag, which allowed the metal to slowly solidify and take on a rounded surface contour because of the action of the surface tension of the molten metal before solidification. This slow solidification allows the surface of the metal to free itself of surface inclusions of slag which have been characteristic of high-carbon ferro-chromium ingots as cast according to the usual practice. When crushed the regulus is found free from pipe, gas holes or shrinkage cavities with a bright, lustrous, silvery fracture of remarkably even texture throughout, and a notable absence of shrinkage strains is found, which ordinarily cause excessive shattering and spalling, resulting in considerable loss microns or less in size and are principally of the duplex type, which are readily removable from the steel to which ferro-chromium is added during the steelmaking process. Tests indicate that the ferro-chromium made according to our process contains less than of l per cent of nonmetallic inclusions as determined by acid dissolution, which is less than half of the content of non-metallic inclusions characteristic of highcarbon ferro-chromium made in the usual way. The nitrogen content of the ferro-chromium made by our process has been found to be consistently lower of the order of A; of that of ferro-chromium cast in the ordinary way; for example, less than .025 per cent nitrogen as compared to the ordinary .06 to .10 per cent nitrogen.

The polyhedral inclusions of the chromite type are claimed to be insoluble in steelmaking slags at steelmaking temperatures up to 3100 F. and have created difiiculty in certain steelmaking processes, whereas the small inclusions of the type which remain in our ferro-chromium are not damaging to steels in that they are soluble in steelmaking slags at steelmaking temperatures. The inclusions, therefore, are removable from the steel bath after the addition of form-chromium and during the course of the steelmaking operation and, consequently, do not contaminate the steel product as has been the case with the polyhedral inclusions of the chromite type. Such polyhedral inclusions are insoluble or for other reasons not removed from the steelmaking slag and, therefore, contaminate the steel product. Such inclusions as remain in our ferro-chromium are of advantageous size and character and are either dissolved by the steelmaking slag or otherwise are removed in the course of the steelmaking process and thus result in a steel product of a more desirable character.

In the specific procedure above described, the pool of casting slag was approximately inches deep and the metal was poured from a height of about 3 feet. This depth of slag is sufficient to check and control the velocity of the entering metal and to provide sufficient washing action. The depth of the casting slag may, however, be varied. The casting slag should not be less than 60 about 12 to 15 inches thick in order to get proper control of the velocity of the metal and proper washing action. On the other hand, it is not necessary in general to increase the depth of the slag beyond about inches in orderto check 65 the velocity of the metal and get proper washing action, although the slag layer may be some-'- what deeper. Moreover, if the depth of the slag is too great, there is danger of granulation of the liquid metal, particularly if the temperature of 70 the slag is lower than that of the metal.

The height of the pour will be governed some what by operative conditions. We prefer to keep the height of the pour to a minimum; for example, in the operation above described, the

75 height of pouring was inches. The height of aceasu pouring should not be high enough so that the impact of the molten metal causes a turbulent intermixing of the metal and the contents of the ladle. In general. pouring above a height of or 6 feet should be avoided. In general. the higher the height of pouring the deeper should be the layer of slag. The-depth of the slag and the height of pour should be so correlated that turbulent intermixing of the metal and contents of the ladle is avoided.

The thickness of the chilled skull of slag formed around the bottom and sides of the regulus depends on the mass and temperature of the metal ladle or mold and the length of time between filling the ladle or mold with the casting slag and the pouring of the metal described specifically above. The chilled layer or skull of slag should be thick enough to prevent the metal from coming into direct contact with the iron and to serve as an insulating layer between the iron of the mold or ladle and the solidifying metal, which allows the surface tension of the molten metal to form a smooth skin and which also contributes to the slow freezing of the metal to give greater time for inclusions to rise to the top metal-slag interface. The skull of slag should be thin enough so that it remains solid and its surface does not become too mushy or fluid in contact with the metal. If it is of too great a thickness, it will not remain frozen under the heating of the metal by escape of the heat to the mold but the inner surface of such thick layer of slag will become quite mushy or even molten and the fingers of slag will extend into and be entrapped within the metal surface. The ideal slag skin thickness appears to lie between and inch, although it may be as thick as 1 inch in some cases without harmful effect; in other cases, especially in small ingots, as thin as ,3 inch.

If desired, instead of pouring the metal into the casting ladle directly from a smelting furnace, the metal can be tapped from a smelting furnace or furnaces into a transfer ladle or ladles and poured into a casting ladle or mold, thus producing a larger ingot or regulus when the metal is collected from two or more furnaces. In such case the smelting slag may largely freeze in the transfer ladle so that the metal as teemed from the transfer ladle, particularly from a bottom pour ladle, may be accompanied by little, if any, of the original smelting slag.

While it is much preferred to allow the metal to freeze into an ingot or regulus in the bottom of the ladle containing the casting slag, it is possible to pour the molten metal accumulating beneath the casting slag into a separate mold or molds before it freezes, obtaining some of the advantages of increased micrographic cleanliness but to impaired degree, since we believe that it is highly desirable, if not necessary, in obtaining the full advantages of our invention to allow the metal to freeze slowly enclosed in an envelope of casting slag.

We will now describe, as a second example, the application of our process to the casting of low carbon ferro-chromium. The low carbon ferrochromium was smelted in the usual way to produce a low carbon term-chromium of the following composition:

Per cent Chromium 70.21 Silicon .68 Carbon .58

8 The heat weighed about 2,400 lbs. As poured from the furnace the metal had a temperature of about 3130 degrees F. and the slag as poured from the furnace had a temperature of about 3100 degrees F. with a fluidity of 8 to 10 plus inches on the Herty viscosimeter.

The process as we carried it out is illustrated diagrammatically in Figure 2 of the drawings. The tilting smelting furnace is indicated at reference numeral I 0. A casting mold H was provided of the character shown in the drawing. This mold consisted of a cast iron stool [2 carried on a car l3 and a heavy mold frame or ring I of cast iron. The stool I! was about six feet square outside dimensions and ten inches in thickness. The cast iron ring II had an inside dimension of about fifty-two inches square and a thickness of ten inches. The Joint between the stool l2 and the ring M was luted with magnesite, as indicated at [5. Mounted on top of the ring I 4 was a cast iron ring or frame is which had a spout ll normally closed with a graphite plug l8. On top of the frame is there was a similar frame l9 provided with an overflow spout 20.

4 Each frame was about fourteen inches high and had an inside diameter of about fifty-two inches square. In carrying out our Process with this apparatus as illustrated, the furnace was tilted and the greater part of the charge of smelting slag run into the composite mold which filled it to a depth of about twenty-five inches, the excess running out through the overflow spout 20. The pouring was then stopped for about five minutes to allow a thin layer of chilled slag to form against the surface of the massive cold cast iron stool l2 and ring i4. After this period of Walting, the furnace was tilted further and the metal poured into the mold through the layer of slag which was maintained at a thickness of about twenty-five to nineteen inches, the excess overfiowing through the spout 20. The metal formed a slab about six inches thick in the bottom of the mold indicated by reference numeral 2 l which was surrounded on its sides and bottom by a thin layer of chilled slag 22. Within ten or fifteen minutes after the metal pour was completed, the plug l8 was removed and the lower side spout l1 tapped to drain the slag layer to a thickness of about six inches above the top of the metal, so that the metal might freeze quicker than with the original thickness of slag. This slag discard left the hot top l6 at a temperature of about 2850 degrees F. and a fluidity of about 2% to 3% inches on the Herty viscoslmeter. In the course of five or six hours the ingot was stripped from the mold and the layer of slag split from the top of the ingot or regulus. The sides and bottom of the ingot or regulus were encased in a thin slag skin or skull about ya to V4 inch in thickness, which quickly disintegrated and crumbled from the ingot skin while the thick top layer would have required eight to twelve hours to disintegrate. The slab was afterward crushed in the usual way to form the saleable low carbon ferro-chromium.

The action of the slag which was poured into the mold and allowed to partially chili before the metal was teemed was similar to that described in the first example except that in this case the smelting slag was used as the casting slag since it did not have the high viscosity of the smelting slag in a high carbon ferro-chromium smelting operation. The slag, however, had the same action in washing the term-chromium which passed through it and in checking the flow of and the slag envelope served to surround the regulus as it froze and allowed the surface tension of the metal to form a smooth, well-rounded skin as described in connection with the first example. The ingots cast in this manner had an equiaxial 1-2 millimeter granular crystal structure throughout.

While the process has been carried out suc-' cessfully as above described, using the smelting slag as the casting slag, it is preferred to use a special casting slag which has a higher silica content and is freer of iron and chromium oxides. Such slag is preferably prepared by taking the smelting slag from a previous low carbon ferrochromium smelting operation and treating it with either chromium silicide or ferro-silicon or aluminum to reduce and recover the chromium and iron oxide contents. An addition of silica sand is then made to the slag to make it less basic and more fluid. A typical low carbon ferrochromium smelting slag had the following composition:

Per cent S102 28.55 (29.0 46.10 MgO 16.48 Feo 0.94 CrzOs 4.24 A1203 7.50 Ratio CaO/SiOz 1.61

Such a slag after adjustment, as above described had approximately the following analysis:

Percent SiO: 32.50 38.0 44.73 MgO 17.13 ZFeO .50 CrzOa 1.00 A1203 5.60 Ratio CaO/SiOz 1.37

Such an adjusted slag instead of disintegrating and crumbling from the ingot, formed a vitreous skin about to inch in thickness which shelled off readily from the ingot skin as it cooled to atmospheric temperature.

The metal cast according to our method using either slag described above is characterized by an unusually brilliant, lustrous fracture with a marked improvement in macrographic and micrographic cleanliness and lowered nitrogen content as compared with low carbon ferro-chromium cast by the ordinary methods. The ingot skin is smooth and bright as if burnished, the burnished appearance being more pronounced, however, when the ingot is cast in the adjusted slag which forms a vitreous skin.

While the present invention has been developed and has been described with particular reference to the manufacture of high carbon and low carbon ferro-chromiums, our process is applicable to the treatment of other refractory metals. By refractory metals we mean metals such as chromium, vanadium, titanium, zirconium, columbium, tantalum, uranium, etc. either in relatively pure form or in the form of alloys. A distinguishing characteristic of such refractory metals is that their reduction temperatures are higher than steelmaking temperatures, being appreciably above 3000 F. Refractory metals are also characterized by the presence of refractory high melting point viscous slags produced in their smelting operations. Such slags tend to produce in the metals refractory high melting point inclusions which persist when the metals are added to steels. The refractory metals to which this invention relates (and by "refractory metals" we mean to include their alloys) are used as intermediates in the metallurgical industry; especially, though not exclusively, in steelmaking. Usually they are crushable and are supplied in the crushed form in numerous size specifications. While such metals are used as intermediate alloys for steelmaking, they are also used in the non-ferrous industry as hardeners. Examples of refractory metals particularly useful for the steelmaking industry are high-carbon ferro-chromium, low-carbon ferro-chromium, ferro-silicon, ferrochromium-silicon, ferro-manganese, ferro-manganese-silicon, ferro-vana'dium (both low and high carbon with both high and low silicon content), as well as vanadium, chromium and titanium metals of commercial purity. When such refractory metals are treated with suitable slags, in accordance with our process, the advantages set forth in greater detail in the above discussion of the manufacture of high-carbon and low-carbon ferro-chromiums are obtained.

Slags of the character above described can be used in the treatment of such refractory metals. While we prefer for the sake of economy to employ waste slags from other metallurgical operations adjusted to the proper fluidity, other slags may be used. The suitable slags may be, for example, of either silicate or aluminate types, essentially calcium silicates or aluminates, with relatively small amounts of other slag-making oxides or they may be of compositions of the quaternary system, SlO2-AlzO3CaO-MgO, or in the three-component systems from which it is constituted. The principal requirement is that the slag shall develop the proper fluidity, and 45 particularly a sufflciently high fluidity within the operating ranges of the pouring practice, and that the content of heavy metal oxides shall be low. namely under 5% and preferably under 2%, in order to avoid reaction with the reducing ele- 50 ments present in the metal being cast; this is especially desirable if a sound ingot or regulus, extremely low in its content of minute nonmetallic particles is to be obtained.

Slags containing sufficient lime and silica con- 5 tents may usually be caused to disintegrate into fine powder upon solidifying and cooling by adjusting their. composition to insure a substantial calcium ortho-sillcate (2CaO.SiO2) component, while providing for the alumina as pentacalcium trialuminate (ECaOBAhOs) and the magnesia as Fosterite (2MgO.SiO2), adding 5 to 15% excess of the theoretrical lime requirements to insure rapid disintegration of the ingot slag skin upon stripping and cooling to atmospheric temperature.

By adjusting their composition in such a manner that a deficiency of lime exists, below the theoretrical requirements for 2CaO.SlOz after other components are satisfied as above, the slag will not disintegrate upon cooling but will result in a cold-short, vitreous slag skin which peels readily from the ingot upon cooling below 800 to 900 F. .The latter type of casting slag is in many cases, though not necessarily always, more desirable from the standpoint of ingot metal qual- 76 ity. Calcium aluminate slags of the 5Ca0.3AlzOa type, with or without the partial substitution of fluorspar up to 30 or 40% of their lime content, have been found to make excellent casting slags for chromium, vanadium and titanium, aluminum-vanadium and aluminum-titanium as well as low carbon ferro-alloys of vanadium-titanium.

The cooling rate or freezing of the regulus or ingot can be varied and controlled in our process by the control of the depth of the casting slag pool as well as by the character of the molds themselves and by the time allowed for the slag to heat the mold and to form an insulating skull or skin. For example, in the above described procedure, in casting slabs of low carbon ferrochromium, it is possible by leaving on a heavy layer of casting slag, to cause the slab to crystallize largely, if not wholly, as columnar or prismatic dendritic crystals extending through the entire thickness of the slab. However, by tapping ofl the casting slag to leave a relatively thin layer and secure more rapid cooling of the slab, low carbon ferro-chromium can be made to crystallize entirely in the form of. relatively small granular equiaxed crystals. The same control of the crystalline qualities of the metal can be had with other metals and alloys. Also, as described in connection with-the manufacture of high carbon ferro-chromium, the pouring of the slag into the mold followed by a period in which the slag heats the mold and forms an insulating thin layer of slag along the bottom and sides of the mold, inhibits the formation of the initial thin skin of minute "chill crystals" which are formed immediately upon contact of molten metals with relatively cold walls of a chilled mold, and thereby permits the natural forces of surface tension of the liquid metal to adjust the area of its surface skin before solidification. When the initial chili is thus delayed, the sub-skin crystalline structure of the ingot may be more readily controlled within wider limits as the rate of cooling is varied by suitable ratio limits of the ingot surface area to cross-section, suitable limits of mold ratio (cross-sectional area of the mold per unit area of ingot) and correspondingly suitable heat diffusivity Conductivity Specific Heat X Density of the superimposed layer of casting slag and of the ingot mold. For example, in the casting of high carbon ferro-chromium as set forth in the specific example above, the cast iron mold ladle weighed about 9,300 lbs. and would accommodate an ingot as heavy as 6,300 lbs. at a chill ratio by weight of 1.48 and having a mold ratio ranging from .41 to .56, while the ratio of ingot surface to ingot cross-sectional area was 9.2. In the casting of low-carbon ferro-chromium with the arrangement shown in Figure 2, a chill ratio as high as 10 or 12-1 may be used to advantage of optimum mold life, with a mold ratio of .9 to 1.5.

Although we have specifically illustrated and described a preferred embodiment of our invention with particular reference to the making of ferro-chromium, it is to be understood that the invention may be otherwise embodied and practised within the scope of the following claims.

We claim:

1. The process of casting high carbon ferrochromium which comprises forming in a furnace a charge of molten ferro-chromium having the usual supernatant viscous smelting slag, forming in a casting mold a body of molten casting slag at a temperature approximating that of the pouring temperature of the ferro-chromium and considerably more fluid than the smelting slag, allowing the casting slag to stand in the mold until a thin skull of chilled slag forms against the bottom and sides of the mold, pouring the molten ferro-chromium together with at least a part of its smelting slag into the mold whereby the ferrochromium passes through the casting slag to form a. regulus in the bottom of the mold and the smelting slag is entrapped by and dissolved in the casting slag, and allowing the ferro-chromium to solidify as a regulus in the bottom of the mold.

2. The process of casting refractory metals which have refractory smelting slags characterized by high viscosity and from which the metals do not readily separate, which comprises forming in a casting mold a body of molten casting slag considerably more fluid than the smelting slag and having a temperature sufficiently high so that the main body of the casting slag remains fluid during the metal pouring operation, allowing the slag to stand in the mold until a thin skull of chilled slag forms against the bottom and sides 01' the mold, and then pouring the molten metal together with at least a part of its smelting slag into the mold whereby the metal passes through the casting slag to form a regulus in the bottom of the mold and the smelting slag is entrapped by and dissolved in the casting slag, and allowing the metal to solidify as a regulus in the bottom of the mold.

3. The process of casting refractory metals which comprises forming in a casting mold a body of molten fluid casting slag at a temperature above the melting point of the metal to be cast and sufficiently high so that the main body of the slag remains fluid during the metal pouring operation, allowing the slag to stand in the mold until a layer of chilled slag forms against the bottom and sides of the casting mold sufficiently thick to form a slag skull around the bottom and sides of the metal regulus when cast of about one-sixteenth to one-half inch thick, pouring into the mold and through the body of molten casting slag the metal to be cast without turbulent intermixing of the metal and slag, and allowing the metal to solidify in the mold to form a regulus enclosed in an envelope of slag.

4. The process of casting refractory metals which comprises forming in a casting mold a body of molten fluid casting slag at a temperature above the melting point of the metal to be cast and sufliciently high .to remain fluid during the metal pouring operation, allowing the slag to stand until a thin skull of chilled slag is formed over the bottom and sides of the mold, and then pouring into the mold and through the body of molten casting slag the molten metal to be cast so as to form-a regulus having a smooth skin, and

allowing the metal to solidify in the mold.

5. The process of casting low carbon ferrochromium which comprises providing a slab casting mold having heavy chilled bottom and sides, pouring into such mold a body of molten casting slag at a temperature above the melting point of the ferro-chromium to be cast and sufilciently high so that the main body of the slag remains fluid during the metal pouring operation, allowing the slag to stand in the mold until a thin skull of chilled slag forms against the bottom and sides of the mold, pouring molten ferro-chromium into the mold and through the body of casting slag without turbulent intermixing of the metal and slag, and allowing the metal to solidify in the 13 mold to form a slab surrounded by an envelope of $188. V

6. The process of casting low carbon ferrochromium which comprises forming in a casting mold a body of molten casting slag at a temperature above the melting point of the ferro-chromium and sufficiently high to remain fluid during the metal pouring operation, allowing the casting slag to stand in the mold until a thin skull of chilled slag forms against the bottom and sides of the mold, pouring the molten ferrochromium into the mold and through the body of casting slag and allowing the excess slag to be tapped from the mold, allowing the mold with the casting slag to stand for a short time and thereafter tapping ofi more of the casting slag, and allowing the ferro-chromium to solidify in the mold. I

'1. A high carbon ferro-chromium regulus having a smooth, lustrous metallic skin substantially devoid of chill crystallization and strains resulting therefrom and of the character formed by the solidification of such a regulus in an envelope of slag, the ferro-chromium containing less than 1/10 of 1% of non-metallic inclusions as determined by acid dissolution, said inclusions being predominantly 50 microns or less in size and being readily removable during the steel-making process from steel to which the ferro-chromium is added, the ferro-chromium being substantially free of inclusions of the polyhedral refractory type characteristic of chromite.

8. As a new product of manufacture, high carbon ferro-chromium consisting of crushed reguli which has smooth, lustrous metallic skins substantially devoid of chill crystallization and strains resulting therefrom and of the character formed by the solidification of such a reguli in envelopes of slag, the ferro-chromium containing less than of 1% of non-metallic inclusions as determined by acid dissolution, said inclusions being predominantly 50 microns or less in size and being readily removable during the steel-making process from steel to which the ferro-chromium is added, the ferro-chromium being substantially free of inclusions of the polyhedral refractory type characteristic of chromite.

9. A regulus of refractory metal having a reduction temperature appreciably above 3000 F., said regulus having a smooth, lustrous metallic skin substantially devoid of chill crystallization and strains resulting therefrom and of the character formed by the solidification of such a regulus in an envelope of slag, the metal of said regulus containing less than 1 6 of 1% of non-metallic inclusions as determined by acid dissolution, said inclusions being predominantly 50 microns or less in size and being readily removable during the steel-making process from steel to which the metal is added.

10. As a new product of manufacture, a refractory metal having a reduction temperature appreciably above 3000 F., and consisting of crushed reguli whichhave smooth, lustrous metallic skins substantially devoid of chill crystallization and strains resulting therefrom and of the character formed by the solidification of such reguli in envelopes of slag, the metal containing less than 1 of 1% of non-metallic inclusions as determined by acid dissolution, said inclusions being predominantly microns or less in size and being readily removable during the steelmaking process from steel to which the metal is added, the metal being substantially free from inclusions of the refractory type characteristic of the smelting slags of such refractory metal.

, 11. A high carbon ferro-chromium regulus having a smooth, lustrous metallic skin substantially devoid of chili crystallization and strains resulting therefrom and of the character formed by the solidification of such a regulus in an envelope of slag.

12. A regulus of refractory metal having a reduction temperature appreciably above 3000 F., said regulus having a smooth, lustrous metallic skin substantially devoid of chili crystallization and strains resulting therefrom and of the character formed by the solidification of such a regulus in an envelope of slag.

13. A regulus of refractory metal having a reduction temperature appreciably above 3000 F., said regulus having a smooth, lustrous metallic skin substantially devoid of chili crystallization and strains resulting therefrom and containing less than of 1% of non-metallic inclusions as determined by acid dissolution, said inclusions being predominantly 50 microns or less in size.

14. The process of casting refractory metals which comprises forming in a casting mold a body of molten fluid casting slag, allowing the slag to stand until a thin skull of chilled slag is formed over the bottom and sides of the mold, then pouring molten metal into the mold through a body of molten casting slag, and allowing the metal to solidify in the mold.

HOLBERT EARL DUNN.

HEINRICH WILHELM RATHMANN.

HOWARD CALVIN PARKMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 944,371 Monnot Dec. 28, 1909 1,753,891 Jones Apr. 8, 1930 2,445,670 Hopkins July 20, 1948 FOREIGN PATENTS Number Country Date 416,228 Great Britain Sept. 18, 1934

Claims (1)

1. THE PROCESS OF CASTING HIGH CARBON FERROCHROMIUM WHICH COMPRISES FORMING IN A FURNACE A CHARGE OF MOLTEN FERRO-CHROMIUM HAVING THE USUAL SUPERNATANT VISCOUS SMELTING SLAG, FORMING IN A CASTING MOLD A BODY OF MOLTEN CASTING SLAG AT A TEMPERATURE APPROXIMATING THAT OF THE POURING TEMPERATURE OF THE FERRO-CHROMIUM AND CONSIDERABLY MORE FLUID THAN THE SMELTING SLAG, ALLOWING THE CASTING SLAG TO STAND IN THE MOLD UNTIL A THIN SKULL OF CHILLED STAG FORMS AGAINST THE BOTTOM AND SIDES OF THE MOLD, POURING THE MOLTEN FERRO-CHROMIUM TOGETHER WITH AT LEAST A PART OF ITS SMELTING SLAG INTO THE MOLD WHEREBY THE FERROCHROMIUM PASSES THROUGH THE CASTING SLAG TO FORM A REGULUS IN THE BOTTOM OF THE MOLD AND THE SMELTING SLAG IS ENTRAPPED BY AND DISSOLVED IN THE CASTING SLAG, AND ALLOWING THE FERRO-CHROMIUM TO SOLIDIFY AS A REGULUS IN THE BOTTOM OF THE MOLD.

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