US3201224A

US3201224A - Method of making cleaner alloy steels or the like

Info

Publication number: US3201224A
Application number: US239669A
Authority: US
Inventors: Howard G Grim
Original assignee: Heppenstall Co; Midvale Heppenstall Co
Current assignee: Heppenstall Co; Midvale Heppenstall Co
Priority date: 1962-11-23
Filing date: 1962-11-23
Publication date: 1965-08-17
Anticipated expiration: 1982-08-17

Description

1965 H. G. GRIM 3,201,224

METHOD OF MAKING CLEANER ALLOY STEELS OR THE LIKE Filed NOV. 23, 1962 INVENTOR Howard G. Grim United States Patent 0,

3,201,224 METHOD OF MAKING CLEANER ALLOY STEELS OR THE LIKE Howard G. Grim, Munhall Borough, Pa., assignor, by mesne assignments, to Midvale-Heppenstall Company, Nicetown, Pa., a corporation of Pennsylvania, and Heppenstall Company, Pittsburgh, Pa., a corporation of Pennsylvania Filed Nov. 23, 1962, Ser. No. 239,669 11 Claims. (Cl. 75-13) This invention relates to method of making cleaner alloy steels or the like, utilizing vacuum degassing. More particularly, this invention pertains to making super-alloy steels or the like by furnace working the metal in two stages with removal of oxygen and other gases as such under vacuum between such stages to inhibit formation and residence of inclusions in the specification products.

In the manufacture, for example, of high temperature super-alloy steels for rotors for jet engines and generators, the specifications imposed upon the steelmaker may involve a sonic test to determine the soundness of the casting. That test is adversely alfected by the presence in high quality alloy steels of inclusions, usually oxides of metallics and/or non-metallics, in varying distribution arrangements depending upon the way the steel is made and finished. That portion of the steelmaking industry producing such super-alloys has turned in many cases to the usage of conventional electric arc furnaces, prefer ably with a double-slag operation, to achieve as close quality control as possible. Sometimes, a consumable electrode melting operation has been resorted to in an effort to improve the quality of the final product, but is subject to whatever impurities are contained in the electrodes. Before tapping the furnace in such prior practices, the steel was wholly or partially killed in advance by conventional deoxidizers producing inclusions reflected in the final products, or foregoing desirable alloying ingredients, to the detriment of the quality of the product metal.

Practices of this invention may be employed utilizing conventional alloying addition agent metallurgical principles and conventional equipment such as a tilting electric arc furnace with carbon electrodes in a unique combination with novel tap degassing equipment of the kind offered for installation in the United States by Bochumer Verein of Germany. Such practices of this invention provide super-alloy steels or the like to finish specifications with radically reduced inclusions, being oxides for the most part, therein and consequently higher quality for the severe high quality services in which such superalloy steels or the like are employed, thereby overcoming deficiencies of prior practices caused by the presence of such inclusions.

Other objects, features and advantages of this invention will be apparent from the following description of one operational embodiment, which may be performed in apparatus such as that schematically shown in the accompanying drawings, in which:

FIGURE 1 is a view in side elevation of a tilting electric arc furnace in cooperative combination with a tap degassing ladle during a tapping step in the course of an operational embodiment of this invention;

FIGURE 2 is a view in side elevation of the apparatus shown in FIGURE 1 in a position'suitable for the return of degassed molten metal to the furnace; and

FIGURE 3 is a view in vertical cross section through a tap degassing ladle such as that shown in FIGURES 1 and 2.

An illustrative operation of one embodiment of the method of this invention for the production of cleaner alloy steels or the like will now be described, with ref- BZdLZZd Patented Aug. 17, 1965" ICE erence to such drawings. Thus, a tiltable electric arc furnace ltl may be utilized. Such furnace may be provided with carbon electrodes 11, a removable cover 12, a furnace proper 13 having a charging door 14 and slagging lip 15 on the front thereof and a tapping spout 16 at the rear thereof. The inside of the furnace is provided with a working hearth 17, which is basic in the accordance with the movements of tilting cylinders Zll.

A trunnioned slag pot 21 may be mounted in a rest below the level of working floor 22 for use when furnace It) is slagged off in the course of a heat. It is unnecessary to further describe furnace 10 inasmuch as it with its operation is that of a conventional electric arc furnace save for the combinative aspects disclosed herein.

For the degassing step, preferably in tap degassing equipment, intermediate the furnace stages of this invention, a tap degassing ladle 23 is shown of the kind supplied by Bochumer Verein and to which no claim per se is made herein. Ladle 23 comprises a refractory-lined shell 24 having a bottom discharge nozzle 25 controlled by an insulated stopper 26. Stopper 26 is provided with a goose neck 27 and a slide 28 which is non-circular in cross section to pass slidably through a guide barrel 29 afiixed to the outside of ladle 24. The upper end of slide 28 passes through a bushing 30 to maintain a vacuum seal relative to the interior 31 of ladle 23. The lower end of slide 28 is pivotally connected to a manipulating handle 32 which rests on the inside bottom of a U-shaped fulcrum shackle 33 pivotally connected at its upper end to bracket 34 which also atlixes the lower end of barrel 29 to shell 24. Hence if a pipe extension placed over handle 32 is moved in the direction of arrow 35 it raises stopper 26 and opens nozzle 25 to the desired extent depending upon the distance toward the head of arrow 35 that handle 32 is moved; or, any suitable remote control may be used for the operation of the stopper handle 32, or slide 28. 'When handle 32 is in the position shown in FIGURE 3, the stopper 26 is fully down and nozzle 25 is closed.

Shell 24 is provided with annular flange 3d affixed thereto and an annular double sealing ring 37 of any suitable material to engage the lower edge of a domed cover 33 and provide a vacuum seal in the course of an opera tion. Cover 38 is readily removable whenever interior 31 is notunder vacuum and is provided with a normally closed opening 39 which may be used for supplying addition agents, or a stirrer in the event that the contents of the ladle are to be stirred, or protective slag, or for inspection if and whenthe pressure tight cover of tube 39 is removed. Interior 31 of ladle 23 may be evacuated through pipe 4% communicating with the interior of the cover 38 and ladle 23. Pipe 4d has a valve 41 therein and is adapted to be removab-ly connected to a vacuumproducing device such as a steam ejector, a vacuum gauge 42 being utilized in pipe if desired.

The rate of flow of molten metal from furnace It through tapping spout 16 may be regulated within reasonable limits by selecting the tilt of the furnace. It also is desirable to provide an intermediate pouring box 43 which, as shown, is an integral part of cover 38. Box 43 has a refractory-lined metal shell open at the top and discharging through cover 38 by means of a nozzle 44 which is subject to being opened and closed by movement of an insulated stopper rod 45. Box 43 inhibits introduction of air into interior 31 and may be used to further regulate the rate of metal flow thereinto by controlling the separation between stopper 45 and nozzle. 44. Stopper rod 45 is operated in the manner described in connection with stopper 26 and there is therefore no need to redescribe it here. The outside of shell 24 is provided with trunnion studs 46 adapted to be engaged by the hooks at the lower ends of the legs of yoke bail 47. A clevis 48 is provided at the top of bail 47 to be engaged by a crane hook 49 which has a crane hook rotator 5t) incorporated in the lower block at the lower end of the crane cable 51 for facing movement of the load in any desired direction about the vertical axis of the crane hook 49.

In a basic steelmaking operation of this invention utilizing equipment of the type described above, furnace is charged with any conventional charging materials when it is in horizontal position, the electrodes are lowered and the heat begins with the melting down of any scrap and other solid materials that might be utilized in the charge. Such solid materials preferably include materials for a suitable basic oxidizing slag, such as a lime-silica slag, the needed oxygen being obtained for any melt down and the oxidizing phase from conventional sources therefor (see The Making, Shaping and Treating of Steel, Seventh Edition, 1957, pages 352 to 435). The described operation then is switched over to a reducing slag, preferably a basic carbidic slag, by pulling out the oxidizing slag over the lip so that it will fall in the slag pot 21 and substituting the carbide slag over the molten metal bath in the first stage of the whole heat. While Working molten steel on hearth 17 in such first stage, phosphorus and sulphur in excess of the specified maximum are eliminated from the metal on the hearth and the carbon is reduced in the bath generally and usually to within about five to about ten points (0.050.10%) in excess of final carbon specification; the excess of carbon purposely left in the first stage refining being the quantity conducive to most complete removal of oxygen in combined and uncombined form in the following degassing step. Normally, there Will be enough manganese in the charge itself to promote removal of sulphur, for example, to specification by the slag of the first stage bath. Certain addition agents which are relatively resistant to oxidation by oxygen in the first stage metal bath, such as ferrochromium, nickel and ferromolybdenum may be added for their alloying effect to the first stage bath in the furnace in the usual way during the working thereof. However, addition agents required for final specification which may also have a higher affinity than carbon has for oxygen should be added in the second, rather than in the first, stage of the heat. Substantially no deoxidizers are added to the first stage bath and when the temperature thereof attains a desired tapping temperature, furnace 10 is tilted, either with or Without first pulling the carbide slag.

Such tilting to the desired inclination of the furnace 10 will discharge the molten metal of the first stage bath at a selected rate with at least some slag thereon into the interior of the preheated box 43 atop the preevacuated and preheated tap degassing ladle 23. The rate of flow of the metal from the first stage bath is further controlled as desired by the extent to which the outer end of pouring box handle 48 is depressed, remotely or manually, allowing unskilled metal of the first stage bath, without material loss of heat, to flow into evacuated interior 31 of ladle 23, the stopper 26 in which remains closed. In the course of such tapping, material oxidation of the liquid metal flow by the air is inhibited by the closeness of the respectively cooperating equipment parts and by such slag layer as accompanies the tapped metal, the slag covering the top of the metal in pouring box 43. The pro-evacuation of interior 31, preferably to a vacuum as low as 100 microns at the beginning of the tapping operation, causes rapid evolution and exhaustion through pipe and openedvalve 41 of occluded gases from out of the first stage molten metal including carbon monoxide, oxygen, nitrogen and hydrogen. The steam ejector or vacuum pump (not shown) which is attached to pipe 40 by a flexible pipe connection to produce a selected vacuum preferably is left running to keep the vacuum as high as possible during such tapping operation, which, however, will be lowered in the course thereof. Further, although the liquid metal of the first stage bath has not been killed to any extent by the use of conventional deoxidizers in furnace 10, or as it flows into box 43, the ebullition of the metal during tapping will not be excessive because of the controlled rate of pouring and the rapid evolution and evacuation of gases from the metal in interior 31. In such degassification, virtually all of the oxygen (and other gases.) will be drawn out of the molten metal in uncombined form and in combined form because of its uniting with the extra points of carbon, thus ridding the degassed first stage liquid steel in ladle 23 of oxygen and bringing the carbon content of the degassed liquid steel down substantially to final specification. Stopper 45 will be operated to close when all of the liquid metal contents of hearth 17 have been received by box 43 and discharged into ladle 23. Normally, a few additional minutes will be provided after stopper 45 is closed for completion of the degassing of the first stage liquid metal in ladle 23, but the total time preferably should not exceed about fifteen minutes.

When degassification is deemed complete, valve is closed and pipe 49 is disconnected from the vacuumproducing device so that the crane may operate to lift ladle 23, rotate it and pour the degassed first stage liquid metal into a furnace for the second or completion stage of the heat. In such second stage, the same furnace 10 may be used again, or another furnace may be used. Furnace 10 itself is preferred for use in the second stage because of its nearness to ladle 23 with consequent less likelihood of loss of heat by the liquid metal therein. in returning the degassed first stage liquid steel to furnace 10 for refining to complete its specification, the tilting cylinders 29 are retracted to tip the furnace slightly for ward as shown in FIGURE 2 so that the runner in tapping spout 16 inclines toward hearth 17. Then, when ladle 23 is positioned as shown in FIGURE 2, rotated through about a vertical axis by rotator 5t nozzle 25 will be above the runner channel in spout 16. Hence, when handle 32 is depressed to the selected extent, the degassed first stage liquid steel will run into furnace 1t and onto hearth 17 for the second stage of the refining operation, such two stages together with the intermediate tap degassing normally comprising a complete heat for the production of specification super-alloy steel product. Or, the return of degassed first stage molten metal to furnace 10 may be faster with greater conservation of heat and temperature if furnace 13 is kept horizontal and cover 12 opened enough to position spout 25 over the opening so that when stopper 26 is raised, such degassed first stage molten metal can discharge into furnace It) at a faster rate. In some cases, ladle 23 may not be a bottom-pouring ladle, in which case the degassed first stage molten metal can be lip poured into the top of furnace 19 after cover 12 has been opened and ladle cover 33 has been removed.

When ladle 23 has finished discharging liquid steel being returned to hearth 17, or in the course of such return, a synthetic inert slag is preferably placed on top thereof by means of shoveling or a charging machine operated through the front door 14 and/or side door 52 of the furnace. Such a synthetic inert slag, which may have a lime-silica ratio of about three to one and a suitable flux, without any added carbon, will protect the second stage bath against oxidation and reduction during the specification alloying thereof in furnace It in the course of the second stage of the heat. In such second stage, the remaining needed addition agents, if any, such as, e.g., ferrosilicon, ferromanganese and ferrovanadium, as selected and desired in kind and quantity, are added to be melted by the second stage bath and finish the alloying of the steel. Normally, the second stage bath requires some furtner heating, so that after making certain cover 12 and electrodes 11 are in proper position, the power is turned on and electrodes 11 quickly bring the temperature of the second stage bath to the desired level for final tapping. It is then preferable to perform a further degassification of the second stage fully refined liquid metal to remove at least hydrogen resulting from the electrode arcs in heating the second stage bath. Such further degassification is obtained by repeating the tapping and tap degassing steps described above in connection with the first stage, utilizing, for example, the same or similar apparatus such as a preheated and pro-evacuated empty ladle 23 sealed by a cover such as cover 33 and having a pouring box such as pouring box 43. After such further degassification of the second stage liquid metal, the cover 38 and box 43 may be removed therefrom so that the fully refined and further degassed sec- 0nd stage liquid metal can be teemed into molds or other receivers previously prepared therefor. Preferably, either before (through tube 39) or as soon as the liquid metal in the ladle is exposed, a synthetic inert slag is spread over the top thereof to protect it against oxidation or other change until such discharge thereof from the pouring ladle.

\ Results of practices of this invention are that cleaner alloy steels or other metals may be obtained by a practice of the method combination of this invention, which by a new combination of steps removes gas such as oxygen which otherwise would be converted into unwanted inclusions, yet does not hinder or alter the selected metallurgical reactions which are desired. Thus, super-alloy steels made hereunder may have a range of inclusions as low as to 40 parts per million, which is of particular importance for the future of very high quality steels such as alloy steels having a specification falling, for example, within the following range:

Specification range (in weight percentage) C a- 0.18 to 0.40

Mn 0.20 to 0.80 P and S nil to 0.64 Si nil to 0.35

li 1.75 to 3.75 Cr nil to 2.00 Mo 0.30 to 0.60 V 0.04 to 0.30

Fe Balance Although the foregoing illustrative method practice has been described in connection with the making of a super-alloy steel in a basic electric furnace operation with intermediate tap degassing, the invention is also applicable to steels or" other specifications and to other metals, whether or not alloys, subject to the oxide and other gascombining inclusion problem, to the usage of other furnace, whether or not tilting, and other degassing equipment. Further, in some cases, it may not be necessary to reheat the degassed first stage molten metal after it has been returned to the furnace, in which event final tapping of the second stage fully refined liquid metal can be made into a conventional tapping ladle, preferably with a deposit of a protective slag on top of the tapped metal in the ladle until the tapping ladle is discharged. In addition, practices of this invention are applicable to the production of alloys in which at least some of the addition agents may be added other than in the furnace and/ or in which some deoxidizer may be added to the molten metal in the first stage to react with a part of the oxygen therein, leaving, however, oxygen remaining in sign ficant quantity to be removed in the course of an intermediate degassing treatment as disclosed above. Still further, the invention may be used in an acid steelmaking process provided that the charging metal composition has a sufficiently low phosphorus content, assuming that a low phosphorus analysis is desired in the end product.

While one embodiment and practice of this invention has been disclosed and described herein, it will be understood that they may be otherwise embodied within the scope of the following claims.

I claim:

1. Method of making alloy metals, comprising, in combination, conventionally charging an electric arc furnace, melting and refining the charge in said furnace to provide a slag-covered bath of liquid metal, adding selected alloying addition agents, removing said slag, tapping oif said. molten metal in said bath substantially at finish temperature by tilting said furnace to a predetermined extent, collecting said tapped metal in a pouring box having a regulatable bottom opening, discharging metal from the bottom of said box into a previously evacuated pouring ladle, continuing the evacuation of said ladle during said discharge to remove occluded gases from said metal, returning said degassed metal to an electric arc furnace, supplying a non-oxidizing slag to said degassed metal in said last-named furnace, adding further alloying addition agents to said degassed metal to complete the refining of said degassed metal and bring it to finish specification, and tapping said degassed metal at selected tapping temperature.

2. Method of making alloy metals, comprising, in combination, conventionally charging an electric arc furnace with carbon electrodes, melting and refining the charge in said furnace to provide a bath of liquid metal successively covered by a basic oxidizing and a basic carbide slag, adding selected alloying addition agents, successively removing said slags, tapping off said molten metal in said bath substantially at finish temperature by tilting said furnace to a predetermined extent, collecting said tapped metal in a pouring box having a regulatable bottom opening, protecting said metal while in said box against oxidation, discharging metal from the bottom of said box into a previously preheated and evacuated pouring ladle, said pouring ladle having a cover to which said pouring box is aifixed, continuing the evacuation of said ladle during said discharge to remove occluded gases from said metal, rotating said ladle, returning said degassed metal to said electric arc furnace from said ladle, supplying an inert slag to said degassed metal in said last-named furnace, adding further alloying addition agents to said degassed metal to complete the refining of said degassed metal and bring it to finish specification, and tapping said degassed metal at selected tapping temperature.

3. Method as set forth in claim 1 for the production of alloy steels, comprising, performing said first-named refining stage to end up in said bath of liquid metal with from 0.05% to 0.10% more carbon than the final carbon specification, reacting said excess carbon with oxygen for evacuation as a gas in said ladle, reheating said degassed molten metal in said second-named electric arc furnace, conducting said degassed metal during said second-named tapping directly into a pouring box having a regulatable bottom opening, protecting said degassed metal while in said box against oxidation, discharging metal from the bottom of said box into a previously evacuated pouring ladle for further degassing, said pouring ladle having a cover to which said pouring box is aifixed, thereafter raising the pressure in said ladle above said further degassed metal substantially to atmospheric pressure, and teeming said further degassed metal into ingot molds or other receiving means.

4. In a method of producing metal to specification substantially free of inclusions caused by reaction of gases in such metals, steps comprising, in combination, preparing refined molten metal on the hearth of a tilting furnace, adding addition agents to said molten metal to introduce alloying components having a lower afiinity for oxygen than carbon in said molten metal, tilting said furnace to tap said molten metal Without the addition of deoxidizers and with its carbon content at least five one-hundredths of one percent higher than the final carbon specification for said metal, pouring said tapped molten metal into a vacuum Zone at a controlled rate for the evolution therefrom of oxygen and other gases including carbon monoxide sufiicient to bring the carbon content of said degassed molten metal substantially to specification, returning and further refining said degassed molten metal on the hearth of a furnace, adding urther addition agents to said last-named molten metal having a higher afiinity for oxygen than carbon in said degassed molten metal, and tapping said returned molten metal from said lastnamed furnace as specification molten metal substantial- 1y free of normal inclusion.

5. in a method of producing metal to specification substantially free of inclusions caused by reaction of gases in such metals, steps comprising, in combination, preparing refined molten metal on the hearth of a tilting furnace, tilting said furnace to tap said molten metal Without the addition of deoxidizers, pouring said tapped molten metal into a vacuum zone at a controlled rate for the evolution and evacuation therefrom of oxygen and other gases, returning and further refining said'degassed molten metal on the hearth of a furnace and tapping said returned molten metal from said last-named furnace as specification molten metal substantially free of normal inclusions.

6. In a method of making specification metals substantially free of inclusions, steps comprising, in combination, refining molten metal on the hearth of a basic furnace with successively basic and carbidic slags thereon, tapping said molten metal without the addition of deoxidizers, conducting said tapped molten metal substantially directly into a vacuum zone for the evolution therefrom of oxygen and other gases, protecting said molten metal against oxidation in the course thereof, returning and further refining said degassed molten metal on the hearth of a furnace under an inert slag to bring it substantially to final specification, and tapping said returned degassed molten metal from said last-named furnace specification as molten metal with an abnormally low inclusion content.

7. In a method of making specification alloy metals substantially free of inclusions, steps comprising, in combination, refining molten metal on the hearth of a basic" furnace with successively basic and carbidic slags thereon, adding alloying addition agents during said refining, tapping said molten metal without the addition of deoxidizers and conducting said tapped molten metal at a controlled rate into a vacuum Zone for the evolution therefrom of oxygen and other gases, protecting said molten metal against oxidation in the course thereof, returning and further refining said degassed molten metal on the hearth of a furnace under an inert slag to bring it substantially to final specification, completing the addition of alloying addition agents during said further refining, and tapping said returned degassed molten metal from said lastnamed furnace as specification molten metal With an abnormally low inclusion content.

8. In a method of making metals substantially free 23 of inclusions caused by reaction of gases in such metals, steps comprising, in combination, preparing refined molten metal on the hearth of a furnace, adding addition agents to said molten metal to introduce alloying components, pouring said molten metal from said furnace into a vacuum zone at a controlled rate Without the addition of deoxidizers for the evolution and evacuation therefrom of oxygen and other gases, returning and further refining said degassed molten metal on the hearth of a furnace, adding further addition agents to said last-named molten metal, and pouring said returned molten metal from said last-named furnace as specification molten metal substantially free of inclusions.

9. Method as set forth in claim 8, comprising, regulating the flow of molten metal at least during said firstnamed pouring, reheating said degassed molten metal when the temperature thereof is below the desired temperature for said second-named pouring, and conducting said returned molten metal during said second-named pouring into a vacuum zone for further evolution and evacuation of gases.

lib. Method of making an alloy steel substantially free of inclusions, comprising, in combination, working a heat of ferrous molten metal in a tilting furnace substantially to tapping readiness with selected alloying ingredients having lesser afiinities for oxygen than carbon, "-roviding at least one appropriate slag for said molten metal in said furnace, tapping said heat by tilting said furnace into an intermediate bottom stoppered pouring box, discharging said molten metal at a selected rate into a pouring ladle in an evacuated vacuum chamber below said pouring box to remove substantially all gases from said molten metal, returning said degassed molten metal to the hearth of a tilting furnace, providing a substantially neutral slag over said bath, completing the addition to said degassed molten metal of alloying ingredients having greater afiinities for oxygen than carbon to bring it to specification, and tapping said completed degassed molten metal.

11. Method as set forth in claim 19, comprising, performing said first-named tapping When said molten metal has a carbon content higher than the carbon specification for said alloy steel, removing some oxygen in said gases by reaction thereof with excess carbon for evacuation in said vacuum chamber.

References Cited by the Examiner UNITED STATES PATENTS DAVID L. RECK, Primary Examiner.

BENJAMIN I-IENKIN, Examiner.

Claims

1. METHOD OF MAKING ALLOY METALS, COMPRISING, IN COMBINATION, CONVENTIONALLY CHARGING AN ELECTRIC ARC FUNACE, MELTING AND REFINING THE CHARGE IN SAID FURNANCE TO PROVIDE A SLAG-COVERED BATH OF LIQUD METAL, ADDING SELECTED ALLOYING ADDITION AGENTS, REMOVING SAID SLAG, TAPPING OFF SAID MOLTEN METAL IN SAID BATH SUBSTANTIALLY AT FINISH TEMPERATURE BY TILTING SAID FURNACE TO PREDETERMINED EXTENT, COLLECTING SAID TAPPED METAL IN A POURING BOX HAVING A REGULATABLE BOTTOM OPENING, DISCHARGING METAL FROM THE BOTTOM OF SAID BOX INTO A PREVIOUSLY EVACUATED POURING LADLE, CONTINUING THE EVACUATION OF SAID LADLE DURING SAID DISCHARGE TO REMOVE OCCLUDED GASES FROM SAID METAL, RETURNING SAID DEGASSED METAL TO AN ELECTRIC ARC FURNACE, SAID LAST-NAMED FURNACE, ADDING FURTHER ALLOYING ADDITION AGENTS TO SAID DEGASSED METAL TO COMPLETE THE REFINING OF SAID DEGASSED METAL AND BRING IT TO FINISH SPECIFICATION, AND TAPPING SAID DEGASSED METAL AT SELECTED TAPPING TEMPERATURE.