Process for continuously casting oxidizable metals

Sept 16, 1969

W. E. MAHiN

3,467,167

PROCESS FOR CONTINUOUSLY CASTING OXIDIZABLE METALS

Filed Sept. 19, 1966

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United States Patent Office

3,467,167

Patented Sept. 16, 1969

3,467,167 PROCESS FOR CONTINUOUSLY CASTING OXIDIZABLE METALS William E. Mahin, Oakland, Caiif., assignor to Kaiser Industries Corporation, Oakland, Calif., a corporation of Nevada

Continuation-in-part of application Ser. No. 368,822, May 20,1964. This application Sept. 19,1966, Ser. No. 580,248

Int. Cl. B22d 27/18, 21/02,11/10

U.S. CI. 164—56 19 Claims

ABSTRACT OF THE DISCLOSURE

This disclosure relates to a continuous process for producing improved bodies of cast oxidizable metal substantially free of oxide inclusions. This is accomplished by producing molten metal with a low contamination level and thereafter continuously controlling the environment of that metal until it emerges as a solid cast body which is no longer influenced by exposure to gaseous contaminants.

This is a continuation-in-part of application Ser. No. 368,822 filed May 20, 1964, now abandoned.

This invention relates to a continuous process for producing improved bodies of cast oxidizable metal substantially free of oxide inclusions.

In the casting of iron, steel, copper, brass, nickel etc., hereinafter referred to generally as oxidizable metals in contrast to metals which are not adversely affected by oxidation when in the molten state, the quality of the cast article is affected by the conditions under which it is cast. The usual casting procedure, whether continuous or intermittent, is effected by pouring molten metal into a mold in which it cools and eventually solidifies. Exposure to the atmosphere during production of the molten metal and while it is poured into mold cause it and its alloying ingredients to absorb atmospheric gases, either in the form of oxides and nitrides or dissolved gases. In whatever form they are absorbed, the gases or their reaction products adversely affect the properties of the resultant solid body. Adverse affects are manifested in many ways, among which are the entrapment of solid inclusions, entrapment of gaseous occlusions, surface defects, and others.

It is an object of this invention to provide a process for producing molten metal with a low contamination level and thereafter continuously controlling the environment of that metal until it emerges as a solid cast body which is no longer influenced by exposure to gaseous contaminants.

The process of this invention is initiated by introducing metal as a solid or liquid into a first chamber in which it is maintained as a molten pool or bath. The first chamber preferably is maintained sealed, and when it is the small amount of air that enters with the metal is swept out with innocuous gas passed into the first chamber above the surface of the molten metal without effecting an agitation action through the molten melt to provide an innocuous atmosphere, or at least an atmosphere which is predominately such gas and to reduce or prevent absorption of oxygen and nitrogen in the molten metal. When innocuous gas is employed, the metal in the first chamber should be relatively slag-free either by preventing introduction of slag with the metal or by providing means for removing it.

The molten metal in the first chamber is treated with substantially insoluble solid forming deoxidizers such as aluinum, calcium, magnesium or other active metals or their coponunds or combinations thereof or other known deoxidizers or mixtures of deoxidizers which are effective to remove absorbed oxygen from the molten metal. The three above named deoxidizers and other suitable deoxidizers have certain properties in common. They are very reactive with oxygen so that they will combine with most of the oxygen to form oxides which are solid at the temperature of the liquid metal, have low solubility in most metals and are of lower density than the surrounding liquid metal. Thus when the metal is quiescent these solid oxides tend to float upwards into a surface stratum to become a concentrated mixture of solid particles surrounded by liquid metal, or to some degree they may float above the surface of the metal as a separate surface film or layer. In either case these oxides become concentrted at the upper levels of the metal leaving a highly purified metal in the lower portions of the bath. Quiescence of the bath enhances this separation.

These or other materials may be added to the molten metal in the first chamber with advantage for other purposes. For example calcium, magnesium or rare earth metals may be added to desulfurize; zirconium or titanium may be added to denitrify to produce steels that are nonaging or with other properties; and chromium, vanadium, titanium, nickel or aluminum may be added as alloying elements. It is also desirable to provide a refractory lining for the first chamber which will not react with or decompose in the presence of the molten metal, the alloying ingredients if any, or the metal cleaning materials such as deoxidizers, desulfurizers, etc. It is also desirable to control the temperature of the metal in the first chamber, at least just prior to the time it is passed from the first chamber, to as low a temperature as possible consistent with easy handling and other process considerations. By providing a relatively low teperature holding period, precipitation and separation of nonmetallic impurities is promoted.

The molten metal in the first chamber is withdrawn from beneath the surface of the bath to avoid transferring the insoluble floating material of the surface stratum, and passed into a second chamber so as to form a pool of molten metal therein. The second chamber is a sealed chamber provided at least with a source of inert gas above the surface of the molten metal and if desired a purged inlet for adding solids such as deoxidizers or alloying ingredients. The bottom draw-off from the first chamber in combination with the inert atmosphere in the second chamber creates a metal bath in the second chamber which has little if any insoluble material, and is low in absorbed oxygen and nitrogen. In the second chamber temperature is controlled to keep the metal molten and of the proper temperature for casting, and generally the temperature in the second chamber is higher than the temperature of the metal transferred from the first chamber.

To prevent oxygen contamination, it is also desirable to line the second chamber with refractory material that does not provide available oxygen to the molten metal. Suitable lining material may be, for example, aluminum oxide, magnesium oxide, or mixtures thereof. In this specification and the following claims the term available oxygen is defined as oxygen capable of combining with the molten metal or the deoxidizer whether it is atmospheric oxygen, dissolved oxygen, or chemically combined oxygen that is capable of being released to combine with the metal or deoxidizer.

In this specification and the appended claims the term innocuous gas is defined as a gas or mixture of gases that does not react appreciably to form reaction products that are undesirable in the metal at conditions of use. Some examples of innocuous gases are the noble gases and for steel carbon monoxide and carbon dioxide, and even some mixtures of steam and partly burned natural gas.

3,467,167

In the second chamber, it may also be desirable to add alloying ingredients and in some cases small amounts of deoxidizers in that these ingredients will come in contact with substantially no oxygen, nitrogen or slag and therefore add almost totally to the metal. Such added deoxidizers would provide driving force to aid in the completion of the deoxidation reaction. Since both deoxidizers and alloying ingredients may be used in lesser amounts in the process of this invention, savings in these relatively expensive additives may be realized.

Quiescence is established and maintained in at least the bottom portion of the pool of deoxidized molten metal. This facilitates the further separation into a surface stratum of any insoluble material inadvertently received from the first vessel or formed in the second vessel through completion of any deoxidation reaction occurring in the second vessel.

The quiescent deoxidized molten metal in the second chamber is withdrawn from below the surface of the metal and passed through a passageway or chamber into the top of the mold. The space between the bottom of the second chamber and the top of the mold is an enclosed passageway which is provided with innocuous gas so that the transfer of metal from the second chamber to the mold, and the liquid surface of metal in the mold, are maintained out of contact with oxygen, nitrogen or other undesirable material.

To provide a continuous process, the bath of molten metal in the second chamber must not be depleted except at shutdowns. To maintain such a bath there may be two or more first chambers associated with each second chamber which feed it alternately thereby assuring a supply of molten metal in the second chamber. A continuous quiescent pool of inclusion-free, high purity metal in the second chamber may also be maintained by providing a large capacity second chamber which can hold enough molten metal to supply the mold while more metal is being deoxidized and otherwise prepared in the first chamber.

This invention may be better understood when explained in detail with referance to the accompanying drawing which is a schematic sectional diagram of a device suitable for practicing the invention. The drawing is intended to illustrate the inventive process rather than to limit its scope. The invention will be described with reference to casting steel or other ferrous material, and it should be understood that conditions and material will be different when brass, nickel or other oxidizable metals are cast.

A hot bottom pouring metal ladle 10 discharges molten metal 11 through an opening 12 in a first chamber generally designated 13. The bottom pouring ladle is used to minimize introduction of slag. The chamber is provided with refractory walls 15 and a top closure 16 and the opening 12 is covered and sealed when molten metal is not being charged to the chamber 13. Means 14, such as a rake and chamber shown schematically may also be provided for removing accumulated floating insoluble material from the surface of the metal in chamber 13. The chamber 13 is provided with temperature control means illustrated herein as conductors 17 which are water-cooled and carry electric energy at the proper voltage and frequency to provide induction heating to the molten metal 11 within the heated chamber 13. Induction heating means for chamber 13 is desirable because it causes eddy currents which promote mixing of soluble constituents of the metal, because it provides heat without contaminating the atmosphere and because the cooling water associated with the induction heaters may be employed alone to cool the metal 11 when too much superheat is present. Naturally, when cooling is to be practiced, the electric current is shut off. At this time also the metal will tend to be quiescent. A bin 18 is provided to supply deoxidizer, such as solid particles of aluminum metal, to chamber 13. The bin 18 discharges through double gate valves 20 which have a fixed volume 21 between them and a port 22 entering chamber 13. An innocuous gas line 25 containing control valve 26 introduces inert gas between gate valves 20 so that the aluminum may be stripped of all air before it is introduced into the chamber 13. Innocuous gas also passes through line 27 and control valve 28 into chamber 13 above the surface of the molten metal. A lance or other means may be used to introduce innocuous gas into the space above the surface of the molten metal without effecting an agitation action through the molten melt. A valved vent 24 for gas may also be provided so that gas entering through line 27 can escape from chamber 13. The vent 24 may discharge to the atmosphere or it may lead to means for recovering innocuous gas when an expensive gas such as argon is used. Vent 24 may also be used to control the pressure in chamber 13, during treating and transferring of metals.

A transfer line 33 passes from beneath the surface of the metal 11 in chamber 13 and upwardly to a point higher than the level of the metal in chamber 13 after which it passes downwardly through inlet port 35 into a second chamber generally designated 36 and indicated herein at a lower level than chamber 13. The transfer line 33 advantageously is equipped with drain plugs 37 and 38 for draining chambers 13 and 36 respectively when desired, and it preferably is also provided at its apex with an innocuous gas inlet 40 containing valve 42 so that the flow of molten metal through line 33 may be cut off by breaking the continuity of the liquid stream at a point higher than the level in either of the chambers 13 or 36.

The chamber 36 is provided with refractory walls 43, refractory floor 45 and a sealed refractory cover 46. The sealed cover is provided optionally with an opening 47 connected to bin 48 through double gate valves 50 containing between them a fixed volume 51. The space 51 is purged with innocuous gas passing through line 52 and through control valve 53 to strip air from materials added from bin 48. Heated chamber 36 is also supplied with a source of innocuous gas through line 55 and control valve 56 for introducing gas into the chamber above the surface of the molten metal without effecting an agitation action through the molten melt. It is preferred to maintain a super-atmospheric pressure in chamber 36 to shield the metal from contaminating gases such as air. When gas flow is desired, a vent line 57 is provided.

The chamber 36 is surrounded with conductors 60 which are used for induction heating to maintain the bath 11 molten. Eddy currents from induction heating also cause stirring which tends to keep the molten bath homogeneous with respect to its soluble constituents. Quiescence of the bath may be obtained at the zone of outpouring in many ways, for example through having proper arrangement of the conductors and reducing or stopping the flow of the electric current.

An opening 61 in the floor 45 of the chamber 36, opens from beneath the surface of the bath 11 forming a short passageway which discharges a stream of metal into the top of continuous casting mold 65. The opening 61 preferably is a replaceable insert formed to project into the bath in chamber 36 so that it will not be cooled, thereby avoiding premature cooling of the metal or precipitation of dissolved material. The stream passes through a chamber 67 into which an innocuous gas stream flows, entering through line 68 and valve 70 and venting, when desired, through line 71 and valve 72. The liquid metal 11 is solidified into a body 75 in the mold by conventional means which include sprays of water 76 supplied through line 77. A suitable control means is employed to regulate the flow rate of metal 11 from chamber 36. One suitable means controls flow by adjusting the pressure in vessel 36 responsive to the liquid level in the mold 65. Thus, a sensing means 59, which may be actuated optically, electrically or with radioactive materials, senses the liquid

3,467,167 level in mold 65 and, acting through controller 54, opens or closes valve 58 to control the pressure in chamber 36 and thereby to control the rate at which metal 11 flows through the fixed orifice 61.

In one embodiment of the inveniton, molten low carbon steel is introduced into chamber 13 and when a sufficient quantity of steel has been introduced the opening 12 is closed and sealed. Argon passing through line 27 sweeps air from the chamber and provides a superatmospheric pressure to prevent leakage of air into chamber 13. It may also be desirable to maintain sufficient pressure to reduce vaporization rate of certain deoxidizers or other materials which are volatile at atmospheric pressure at the temperature of the molten metal. The molten metal is treated with argon at least until most of the air is removed from chamber 13. Aluminum pellets and, optionally, alloying material are added from hopper 18 through double gate valves 20 to provide at least an initial deoxidation of the steel and the addition to it of alloying ingredients.

Any insoluble material that is introduced with the steel or produced by the added materials, will tend to float into the surface stratum of the molten metal in chamber 13 and will be raked into chamber 14 from time to time. Such a floating upwards and separation are aided by the quiescence of the bath just before and during the period when the separation and subsequent underpouring of the metal into the second chamber occur. The temperature in chamber 13 preferably is maintained slightly above the melting point of steel, preferably within about 50° C. of the melting point of the steel to promote precipitation of non-metallic material so that the metal passing from the chamber 13 through the outlet 30' will be substantially free of all insoluble material.

The amount of aluminum added preferably is substantially more than the stoichiometric amount needed to combine with the amount of oxygen present in the steel. By adding enough aluminum in vessel 13 to have it carry through the process all the way to the mold, both good deoxidation and maintained low oxygen levels are assured. In other words, in the reversible reaction between aluminum and oxygen, it is desirable in the process of this invention to shift the equilibrium far toward the formation of aluminum oxide in the vessel 13 so that aluminum oxide will be separated from the metal there into a surface stratum and very little, if any, will precipitate in the mold.

It is preferred that flow from chamber 13 be stopped before all of the molten metal 11 is removed to prevent transferring insoluble material to the chamber 36. It may also be desirable to retain a residual amount of molten metal in which deoxidizer may be dissolved for the next charge to chamber 13.

When metal is flowing through line 33, valve 42 is closed and there is no gas flow into the transfer line 33. Instead, transfer line 33 is completely liquid-filled and it transfers molten metal from beneath the surface of the bath in chamber 13 to beneath the surface of the bath in chamber 36.

Argon gas is introduced into the chamber 36 through valve 56 or a combination of valves 56 and 58 so that there is always a positive pressure of inert gas in chamber 36. Since the chamber 36 . is a sealed chamber under pressure there is no way for oxygen or air to enter. Also, there is little if any insoluble phase in chamber 36 and few, if any, inclusions appear in the cast product.

At least the bottom 45 and side lining 43 of chamber 36 are alumina which is unreactive with the aluminum deoxidizer and will not decompose to provide available oxygen. Therefore, any aluminum introduced into the melt reacts with oxygen to form aluminum oxide whether the oxygen is dissolved oxygen or iron oxide. Stable nitrogen compounds may also be formed which tend to produce non-aging steels.

The temperature of the metal in chamber 36 is main6 tained hotter than the metal in chamber 13 thereby preventing precipitation of insoluble material either in chamber 36 or in the nozzles or openings employed in the subsequent transfer of molten metal to a casting mold. ₅ 'Quiescence is established and maintained in at least the bottom portion of the pool of deoxidized molten metal in chamber 36.

The steel passing from the bottom of chamber 36 is highly purified and at the proper temperature for casting. 10 In passing to the mold 65 it is not subjected to any influence that changes its closely controlled state. Chamber 67 which connects chamber 36 to mold 65 is purged with argon to provide a passive atmosphere surrounding the stream entering the mold and the metal surface in the 15 mold. After casting, a solid metal 75 emerges into the air but only after it is no longer subject to influence by oxygen and nitrogen.

Through the method of this invention, the metal is deoxidized to a substantial degree in the first chamber, 20 and may be finally deoxidized in a second chamber in a closely controlled environment. Finally, the metal passes from the molten phase to the relatively invulnerable solid phase without contacting deleterious gases or other sources of oxygen or nitrogen.

The transfer of material between chambers may be accomplished by any suitable means including such simple means as elevation difference, pressure difference, or combinations of both. The innocuous gas source 80 may be at substantially higher pressure than the pressure in any ³⁰ chamber, and valves 28, 56, and 58 may be controlled and regulated in conjunction with the levels in chambers 13 and 36 to regulate the flow rate of liquid 11 through the various passageways.

The process of this invention has many advantages over 35 conventional processes for producing and casting oxidizable metals. Foremost among these are that it permits a truly continuous casting process. Also, the process is capable of casting metals with low general contamination levels, which metals usually are particularly sensitive to ⁴⁰ picking up oxygen. The cast bodies resulting from this process are essentially inclusion-free, porosity-free, and blow-hole free, sound bodies having smooth, clean surfaces. The clean, smooth surfaces also result in wrought bodies that are free of macro defects and having substantially reduced micro defects. Also, in the case of steel, the concurrent reduction of the effect of nitrogen contamination results in non-aging steels.

Many variations of this invention may be practiced without departing from the scope of the invention, and it 50 is intended that the invention be limited only by the appended claims. The well controlled and innocuous or passive environment produced by the process of this invention lends itself to the use of materials for treating metals which may be destroyed or consumed in conventional 55 processes. In addition, the process although adapted for continuous casting of metal may be employed as a semicontinuous or intermittent-continuous process, as when employed to fill multiple molds. In those applications the stream of metal passing from the second chamber may θθ be interrupted intermittently while a series of molds are filled.