US2087347A - Method of solidifying molten metals - Google Patents

Description

July 20, 1937. B. M. LARSEN 2,087,347

\ METHOD OF SOLIDIFYING MOLTEN METALS Filed Dec. 21, 1934 2 Sheets-Sheet 1 ATTORNEYS B. M. LARSEN METHOD 0F SOLIDIFYING MOLTEN METALS July 20, 1937.

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REINO .aq guk .0 0.2 04 0.6 0.5 1.0 1.2 1.4 1.6 L ZD 22 24 ATTORNEYS yPatented July 20, 1937 UNITED STATES METHOD OF SOLIDIFYING MOLTEN METALS Bernard M. Larsen, Elizabeth, N. J., United States Steel Corporation,

assignor to New York,

N. Y., a corporation of New Jersey Application December 21, 1934, Serial No. 758,683

4 Claims.

This invention relates to metallurgy and more particularly to methods of casting molten metal baths and has for its object the provision of a method and means for casting molten metal baths to obtain thereby a sound ingot of substantially uniform composition throughout. Another object of the present invention is to provide a method and means for obtaining ingots of substantially uniform composition throughout and substantially free from voids from molten metal baths containing dissolved gases, dissolved metal oxides and carbon and/or containing one or more of the metalloid impurities sulphur, phosphorus, selenium, tellurium.

In the casting of molten metal baths, particularly those of two or more constituents or of complex composition, it is well recognized that in the process of solidication a, tendency to separate a solid phase of different composition from the remainingliquid always results in seg- A reg-ation or variation of composition between different volume portions of the solidified metal. It is also recognized that dissolved or occluded gases tend to be liberated in the process of solidir iication and are evidenced by voids appearing throughout the cross-section of the solidified metal. It is also recognized that in metals, particularly iron and steel, dissolved oxides and carbon interact during the process of solidication to form gaseous carbon oxides which also form voids throughout the cross-section of the solidified metal.

Heretofore in the art it has been proposed to eliminate or minimize segregation by casting under pressure, centrifugal casting, etc. Chill casting and subsequent annealing has also been proposed. It has also been proposed to partially eliminate the deleterious effects of dissolved gases by adding to the molten metal baths certain scavenging agents reactive with such gases. It has also been common practice to partially eliminate the deleterious eifects'of the oxide-carbon reaction by adding to the bathl certain deoxidizing or certain carbon-fixing elements, thereby inhibiting this reaction.

But while each of these heretofore proposed methods is effective to perform the-particular result desired it is apparent that the practice of all three methods upon a single molten bath is tedious and impractical as a general procedure. Inasmuch as the precise amount of reagent required in each case is difficult to ascertain, an excess of reagent is required in each case which excess enters into the final bath composition and markedly effects the physical and vpresent invention;

metallurgical characteristics of the metal. The reagents also precipitate various solid, non-metallic reaction products in the liquid metal thus merely substituting various other deleterious effects on the properties' of the resulting product. 5 Furthermore, in practically the whole of the previous art of metal 'casting it h-as not been possible to adequately control and limit the coarsegrained primary or dendritic crystal formation, with its accompanying dendritic segregation, and various deleterious effects on resulting properties. It is one of the objects of this invention to eliminate these disadvantages of prior art practice.

In accordance with the objects of the present invention, I have devised a method and means for accomplishing the solidication of a molten metal bath to obtain a substantially sound metal of substantially uniform compostion throughout in a simple and expedient manner. Although the present invention is adapted to be widely applied in the solidification of all types of molten metal baths, as a specific embodiment thereof and not in any sense a limitation thereon, I will disclose the same as it has been applied in the solidiiication of molten iron and steel.

In the solidication of molten iron and steel the above noted deleterious features are particul-arly prevalent. I have determined that by accomplishing solidification of a molten metal bath from a moving liquid phase, i. e., from a liquid phase in motion relative to the solid phase at the crystallizing surface, with the solid phase maintained within certain limits in temperature distribution, and with a neutral or reducing gas present in such manner as to effectively absorb gaseous products from liquid or solid phases during freezing, that the deleterious features above noted may be substantially eliminated. Before further disclosing the present invention reference should be made to the accompanying drawings wherein:-

Fig. 1 is a'slde elevation in section of one specific embodiment of the apparatus useful in the present'invention; Fig. 2 is a. cross-sectional view along plane 2-2 of Fig. 1; Figs. 3 and 4 are schematic views illustrating the underlying principles of the solidifying process of the and Figs. 5 to 9 inclusive are representative constitutional diagrams controlling the practice of the present invention in one specific embodiment thereof.

Briefly stated the present invention contemplates effecting the solidication of a molten pouring opening 8.

metal bath progressively in successive layers upon the moving face of a mold, maintaining the temperature of the mold face and solidified metal phase at aftemperature at which a desired metal composition solidifles. Relative motion between liquid and solid phases is maintained-thereby obtaining substantially constant composition in the solid phase and the extent of solidiflcation is carried to a point short of complete solidiiication of the entire liquid phase, the uns'olidified liquid phase containing the undesired lower melting constituents then being removed, to be solidified separately or returned to a melting furnace.

By this manner of solidiflcation I obtain first a substantially uniform composition of solidified metal throughout the cross-section of the solidied phase and the thus solidified metal is substantially free from entrapped or dissolved gases. The lower melting constituents of the molten metal bath together with a portion of the dissolved gases are concentrated in the moving liquid phase and are hence eliminated. To facilitate the removal of dissolved gases and to prevent oxidation of the liquid and solid phases of the metal I preferably employ a reducing gas atmosphere.

The advantages of this method of solidication are that it is not necessary to' add to the liquid phase the various deoxidizing, scavenging and degasifying agents heretofore employed in the art to obtain sound solidified metal, at the same time giving a metal essentially free from dendritic segregation and the weak, coarse-grained primary lcrystal structure almost universally present in cast metal frozen from a quiescent liquid phase.

In accordance with the above method I have devised the apparatus illustrated in Figs. 1 and 2 of the drawings which is effective in the practice of the present invention.

Referring to the drawings, I provide a cylindrical mold I comprised of any suitable material such as cast iron and support the mold with its cylindrical axis lying substantially horizontal in a manner providing for a rotation of the mold about its cylindrical axis. Such a rotation of mold I may be obtained as indicated by providing on the exterior of mold I a pair of tracks 2--2 which ride on rotating driven wheels 3 3 driven by drive shaft 4 in any convenient manner. It is preferable as indicated in Fig. 2 to use two sets of driven wheels 3 3 and 3-3 located on opposite sides of the horizontally disposed mold I.

One end of the mold I is closed by end closure member 5 provided with an axial opening through which tube 6 extends. Tube 6 is connected exteriorly to a source (not shown) of appropriate reducing gas which supplies a iiow of said reducing gas to the interior of mold I.

The opposite end of mold I is closed by end closure member 'I provided with an axially located Each end closure member 5 and 'I is detachably secured to mold I as indicated by means 9. End closure members 5 and 1 are preferably comprised of refractory heat insulating material at least interiorly. Means are provided such as burner I0 to heat the mold I to a desired temperature before molten metal II is supplied to the interior of the mold, and if desired air blast means I3 may be provided to accelerate the rate of heat removal from the solidified metal through the mold. In general after the metal II is poured, the mold burner I0 is extinguished, the heat energy of solidification being sufficient for the purposes of the present invention.

Molten metal II is supplied to the interior of mold I by ladle I2. Before molten metal II is poured into mold I through opening 8 mold'I is brought to a desired temperature by means I0 and mold I is set in rotative motion. The speed of rotative motion imparted to mold I may be widely varied without departing from the present invention. It is` less than that heretofore known in the art in centrifugal casting and preferably should not be in excess of that rate which is sufiicient to maintain the composition of the liquid phase substantially uniform.

The main objective in rotating mold I is to obtain relative motion between the solid and liquid phases in the molten metal bath at the temperature of the mold. As the mold is rotated the solid phase I2 depositing along the cylindrical walls of mold I recurrently passes out of and into the liquid phase thereby building up successive layers of solidified metal along the interior wall face of mold I. The reducing gas atmosphere in the interior of the mold prevents oxidation of the solid and liquid phases, and also aids in sweeping out gases evolving from the liquid phase. This removal of gases can be made relatively complete, because of the large amount of surface exposed to the gas phase, and because the atmosphere can be regulated to give a very low partial pressure of those gases coming out of the metal, and thus give the effect of an eiiicient vacuum treatment of liquid metal. Fig. 3 indicates the type of built up structure of the solid phase obtainable in the practice of the present invention. Solidiiication is continued untilthe liquid phase no longer can separate out the desired metal composition due to the concentration of undesired lower melting constituents therein. Mold I then may be tipped in any desired manner to drain off the remaining liquid phase Il through opening 8 in end 1.

In applying the present invention to the solidifcation of iron and steel alloys, vthe precise modus operandi varies with respect to the specific composition of the molten steel bath.

The most important factor underlying the solidification of iron and steel is the fact that for each pound of steel changed from liquid to solid, heat energy is evolved and must be dissipated by heat conduction away from the surface of crystallization. 'I'his means that the crystallizing surface will tend to advance into the liquid in a direction parallel to the direction of heat ow into the mold. Referring to the schematic diagrams of Figs. 3 and 4, it is assumed in Fig. 4 that the liquid is entirely quiescent, so that no motion ofthe various constituent atoms in the steel can occur except by simple diffusion. .Inl

Fig. 3 it is assumed a rapid turbulent motion of the liquid past the freezing surface is present.

In both cases, the mass of liquid steel is in contact with a metal mold I below, which by absorbing the heat of fusion, causes the crystals to grow into the liquid at right angles to its surface.

' Also in both cases, at the moment the liquid is poured into the mold the rst thin layers of steel are frozen very quickly with a very large number of centers of crystallization, and a thin solid layer freezes first with essentially the composition of the original liquid, and during this period, the interface between liquid and solid will approximate a plane surface parallel with the mold wall.

If steel was a liquid with a single definite freezing temperature, the progress of crystallization would continue to be essentially determined by the rate and direction of heat flow away from the liquid-solid interface. In the case of Fig. 4, even with a quiescent liquid phase the advancing surface of crystallization would still approximate a plane surface parallel with the mold wall, except perhaps for relatively small irregularities caused by variations in crystal orientation. The actual situation is complicated, however, by the fact that all commercial steel and many other commercial metals contain impurities which cause them to freeze over a range of temperatures, and this introduces a diffusion-rate factor into the freezing process. To indicate the effect of this, the simple case o-f an alloy ofiron and phosphorous is illustrative. A portion of the Fe-P equilibrium diagram is shown in Fig. 5. If the liquid alloy at 1600 C. (point D) is poured in a cool mold, the first layers of solid will freeze against a surface which is at temperatures below line BC (the eutectic ternperature in this system), so that this metal is quenched from the liquid state, that is, the liquid can freeze en masse with no tendency for selective crystallization.

As the freezing continues, however, heat flows away from the crystallizing surface more and more slowly and the temperature of this surface soon approaches that of dotted line EF in Fig. 5. When this happens, the so-lid which separates from the liquid at composition E (1.8% P) will have composition F (about 0.2% P). The

adjacent liquid being depleted in Fe and therefore enriched in P, if more solid is to separate at the same plane, from a quiescent liquid phase as in Fig. 3, iron must diffuse in the liquid toward the solidifying surface, and a rate of diffusion enters the process, which tends to oppose the influence of the rate of heat dissipation as the controlling factor in the inode of crystallization. This condition leads to the growth of so-called dendrites.

We can explain this further by imaging a plane surface of crystallization with a small projection of solid metal out into the liquid. 1f heat loss is the only controlling factor, since the thickness of frozen material is greater at this projection, the heat will ow away a little slower at this point, metal will consequently freeze on it a little more slowly, and the projection will tend to be ironed out as freezing proceeds. But if diffusion of atoms in the liquid is the controlling factor, the projection will be closer to more volume portions inside the liquid, and this will favor a more abundant supply of iron-rich liquid to this point. The projection will then tend to obtain more soli-d phase from the liquid than the rest of the crystallizing surface and will thus tend to grow at the expense of adjacent areas of this surface, giving the conditions for dendrite growth.

The growth of dendrites with their tree-like branchings out into the liquid, (as indicated in rough/simplified form in Fig. 4) is essentially the/result of a natural tendency to compromise between the rates of heat loss and of diffusion in the liquid. The main axes of the first dendrite branches (I2) tend to be aligned in the direction of heat flow into the mold walls, but the tree-like growth is the result of a natural tendency to minimize the distances for diffusion in the liquid. Between the dendrite growths, the impurities become more concentrated in a liquid which freezes last advanced f 'rther out into the remaining liquid.

Where a ireasonably strong turbulent movement is set up in the liquid, parallel to the mold wall, as by the apparatusxof Figs. 1 and 2, the essential effect is to give a rate of movement of the constituents in the liquid by convection currents, which, as compared to diffusion in a quiescent liquid, is extremely rapid and effective in mixing all parts of the liquid and thus wiping out the concentration gradients set up by selective crystallization. It should be emphasized again that in the absence of the effect of a limiting diffusion rate, which is the condition obtained by the application of the present invention, the freezing process will be conditioned essentially by the direction of heat ow into the mold, and the crystallizing surface. The crystallizing surface will then tend to advance asa plane at right angles to the direction of heat dissipation. The effect of mixing by turbulent motion in the liquid phase is simply to allow this tedency to become the dominant factor, by removing the diffusion rate limitation. With movement of the liquid past the surface of crystallization, the less pure liquid formed at this surface by the separation of purer solid crystals is carried away and diluted with the main mass of liquid, so that a liquid saturated with the purer solid phase is supplied constantly to all parts of the crystallizing surface. Crystallization can thus occur at all points of an approximately plane surface, so that solid layers should be formed like the layers of an onion parallel to the mold wall and advancing into the liquid, as indicated in Fig. 3.

A large part of the outer zone of an ingot may be frozen in this manner; eventually the temperature of the remaining less pure liquid will become lowered to that temperature at which nuclei for new and less pure crystals begin to form all through this liquid, and if masses of only the purer segregateare desired, the remaining liquid must be separated before this ch-ange in mode of crystallization begins.

The various impurities (or alloying elements) present in commercial steels vary widely in the amount of difference in solubility between liquid and solid phases in the temperature range of crystallization. This naturally affects the tendency of these elements toward segregation in the ingot. A discussion of ingot segregation and a classification of various elements have heretofore been given by Larsen, Metals & Alloys, I, 819-25 (1930), Section XVI.

This classification is somewhat arbitrary, but serves to systematize certain differences in behavior between the various impurities in steel. The iron-rich portions of the binary phase diagrams in Figs. 5 to 9 will serve to illustrate further a few of these differences. Fig. 5 shows this portion of the silicon-iron system, which is typical with respect to the small separation of liquidus (AB) and solidus (AC) lines.4 A 1.8% silicon-iron alloy, for example, at l600 C. (point D), remaining liquid until it reaches point E at 1520 C., will separate out crystals containing about 1.2% Si corresponding topoint F, and this difference between liquid and solid increases only very slightly as freezing continues. The corresponding diagrams for iron-carbon and ironphosphorus in Figs. '7 and 5 show a much greater separation between liquidus and solidus.

The iron-sulphur diagram of Fig. 9 indicates a still greater spread between liquidus and solidus, the very slight solid solubility shown in dotted after the purer dendrites have lines being too small to show accurately on this scale although good data as to the location of this line are not available. In the iron-oxygen system Fig. 8 is a schematic diagram based on indirect evidence but is illustrative of the general situation involved.

In addition to the segregation of alloy constituents above discussed, a low-carbon steel bath contains a certain amount of carbon and iron oxides. Molten steel continuously liberates carbon oxides when these two constituents are present therein, butwhen the steel is poured into a ladle, this gas evolution seems to slow down to an almost zero rate, so that the steel remains quiet in the ladle. Ordinary additions of manganese made at this time to the ladle apparently do .not react with the oxygen content of the steel until the steel starts to freeze and will therefore not lessen the amount of dissolved FeO in the steel.

When this steel is placed in mold I, a thin solid layer of metal is quenched from the liquid on the relatively cool mold surface, but the freezing rate quickly drops and the crystallizing surface rises in temperature, so that the metal freezing out begins to be richer in Fe and therefore lower in C and FeO than the adjacent liquid. 'I'he carbon and oxygen in solution in the remaining liquidus then react and CO bubbles now start to form at this surface of crystallization.

These gas bubbles would naturally tend to rise in the liquid and thus carry up a rapid current of liquid past the solid surface, and if suiciently rapid would prevent the formation of dendrites.

Ordinarily, however, in the prior art practice the intensity of this natural turbulence is not great enough to prevent dendrite formation, and gas bubbles are trapped between the dendrite growths in Fig. 4. Moreover, this natural turbulence cannot be independently controlled in the prior art practice. In the practice of the present invention however, the artificial turbulence imparted to the bath by the rotation of the mold insures the constant removal of these CO bubbles from the solidifying surface and also prevents the dendrite growth, with consequent elimination of blow-holes in the solid metal aggregate.

From the above discussion of the present invention it is believed clear that the same is adapted to be varied widely in the solidication of iron alloys without essentially departing from the nature and scope thereof. The initial temperature of the mold may be arbitrarily selected with respect to the metal bath composition and the desired perfection of approach to constant composition of solid phases. From a knowledge of vthe composition of solid and liquid phases at any given temperature as indicated in Figs. 5 to 9 inclusive, the extent of solidiiication permissible before removing liquidus from the mold I may be approximately estimated.

The resulting product is a tubular ingot of substantially uniform composition throughout and substantially free from voids, which may be utilized directly in the forming of seamless pipes, tubing, etc. or which may be cut up or opened and rolled into sheet, plate, bars, rods, etc.

With the above discussion in mind, it may be seen that in the practice of the present invention in the solidication of molten steel baths, the molten bath composition and the desired solidified metal composition are first to be determined. One skilled in the art can thereafter readily determine the amount of solidified phase permissible before segregation of undesired constituents is obtained.

change this practice in the elimination of these undesired constituents. At the same time however, the metal oxide-carbon reaction during solidication does not result in the deevlopment of gas blow-holes in the solidified metal, even in cases where strong deoxidizers are not used, or are added in amounts less than necessary to fully kill the metal.

This method of solidiiication ofl'ers particular advantages in producing the relatively low carbon iron alloys known in the trade as stainless chromium-iron or chromium-nickel-iron alloys. In this type of alloy, the carbon content is deleterious in that it reacts with the chromium to form carbide compounds thereby removing from the alloy that element imparting corrosion resistance to the alloy. By the practice Iof the present invention the carbon content ofthe solidified metal may be decreased by 4060% of its concentration in the liquid metal, and the deleterious effect of the carbon thereby materially reduced. I

'I'he method described is peculiarly adapted to theproduction of high-grade seamless tubes o1' various metals, particularly of steel. The freedom from large inclusions or groups of inclusions and the characteristic crystal structure of the rim zone of the best rimmed ingots which give maximum plasticity, drawing or forming properties and freedom from outside seams may be produced in combination with the uniformity of composition and freedom from inside seams characteristic of hot-topped and thoroughly killed ingots. 'Ihis combination of desirable properties is practically impossible to obtain either by the conventional methods of ingot casting, or the newer developments in centrifugal casting. This also applies in the production of high-grade sheets such as those for auto bodies, etc., where it is so desirable to combine the properties of freedom from both blisters and seams, together with good deep-drawing properties, freedom from aging, etc.

In the adaptation of this invention in the solidication of other molten metal baths, such as copper, aluminum, nickel alloys, it is only necessary to modify the precise temperatures of solidication with respect to the particular alloy or particular constituent thereof it is desired to eliminate by concentration in the poured oiT liquidus. In solidifying copper and copper alloys for example, one of the constituents most diicult to eliminate is copper oxide which is in solution in the molten copper. By the practice of the present invention, this copper oxide may be concentrated in the poured oil liquidus and nearly oxygen-free copper solidified along the wall of the rotating mold, thus avoiding the use of deoxidizing agents which frequently de'leteriously effect the electrical conductivity of the copper and copper alloys.

In solidifying nickel and nickel alloys the elimination of sulphur is a big problem. By the practice of the present invention this constituent may be substantially eliminated.

Having broadly and specically described the present invention and given one specific embodiment thereof, all modifications and adaptations thereof are contemplated as may fall within the scope of the following claims:

What I claim is:

1. The method' of solidifying molten metals to obtain a solidied mass of metal consisting substantially of a desired solid phase of the said molten me'tal which comprises disposing the molten metal in a cylindrical mold, maintaining the wall of said mold in contact with said molten metal at a temperature favorable to the solidication thereon of the said desired solid phase, maintaining relative motion between the said walls and molten metal to inhibit the occlusion of the liquid phase in the depositing solid phase, maintaining over the surface of the said liquid phase a reducing atmosphere having a relatively low partial pressure of gaseous constituents contained within said liquid phase, and then separating the liquid phase from the said deposited solid phase when the desired extent of deposit of thae11 latter phase has been obtained on the said w 2. The method of solidifying molten metal to obtain a solidified ingot composed substantially of a desired solid phase of said molten metal which comprises placing the said molten metal within a hollow cylindrical mold disposed with its cylindrical axis lying in a substantially horizontal plane, the cylindrical wall temperature of the said mold being maintained at a temperature favorable for the solidication of the said desired solid phase, and rotating the said mold about its said cylindrical axis at a rate adapted to recurrently immerse the inner surface of the mold into the molten metal bath without substantially displacing the bath from the bottom of the mold,

40 circulating through the said mold over theA surface of the molten metal an atmosphere reducing with respect to the said solid phase and the said bath, and continuing the same until the said solid phase has been deposited as a hollow cylindrical shell on the inner surface of the mold to the extent desired and removing from the mold the liquid phase remaining.

3. The method of solidifying molten metal which comprises placing the molten metal within a hollow cylindrical container disposed with its cylindrical axis lying in a horizontal plane and rotating the said mold about its said cylindrical axis at a rate adapted to flow the molten metal over the said rotating surface of the mold building up thereby successive layers of solidifying metal thereon and forming thereby a hollow cylindrical casting, maintaining the temperature of the mold at a temperature adapted for the solidication thereon of a desired solid phase of the said molten metal, and removing the liquid phase remaining in the mold after the solidication of said solid phase from the mold.

4. The method of solidifying molten metal which comprises placing the molten metal within a hollow cylindrical container disposed with its cylindrical axis lying in a horizontal plane, circulating an atmosphere adapted to protect the liquid and solid phases against oxidation through the mold and rotating the said mold about its said cylindrical axis at a rate adapted to flow the molten metal over the said rotating surface of the mold building up thereby successive layers of soli-difying metal thereon and forming thereby a hollow cylindrical casting, maintaining the temperature of the mold at a temperature adapted for the solidication thereon of a desired solid phase of the said molten metal, and removing the liquid phase remaining in the mold after the solidifcation of said solid phase from the mold.

BERNARD M. LARSEN.

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