US3496280A - Method of refining steel in plasma-arc remelting - Google Patents

Description

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Feb.'17 ,1 970 v ID.A:IDUKI E'LOW ETAL 3,496,280

Mlii'll-IOD OF REFINING STEEL IN PLASMA-ARC REMELTI NG Fi led Aug. 15, 1968 W/THDRAWAL DRIVE INVENTORS. DONALD A. DUKELOW, KEITH K. KAPPMEYER 8 ROBERT J. KING Attorney 3,496,280 METHOD OF REFINING STEEL IN PLASMA-ARC REMELTING Donald A. Dukelow, Mount Lebanon Townshop, Allegheny County, Keith K. Kappmeyer, Franklin Township, Westmoreland County, and Robert J. King, Churchill Borough, Pa., assignors to United States Steel Corporation, a corporation of Delaware Filed Aug. 15, 1968, Ser. No. 752,934

Int. Cl. H051) 7/18 US. Cl. 13-9 3 Claims ABSTRACT OF THE DISCLOSURE Method involves adding fiuxing material to metal being remelted in a plasma-arc furnace to effect refinement, e.g., desulfurization, of the metal being remelted while at the same time decreasing the oxygen content thereof.

The present invention relates generally to metal processing and, more particularly, to an improved method of rfining steel during plasma-arc cold mold remelting.

Conventional plasma-arc cold mold remelting practice, such as is described in Gage Patent No. 3,147,330, dated Sept. 1, 1964, improves cleanliness of metals and alloys buft' 'has little or no other effect on the chemical compositions of the metals and alloys except to decrease the oxygen contents thereof.

it is the primary object of the present invention to provide an improved plasma-arc cold mold remelting process wherein the metal being remelted is chemically refined in addition to having its oxygen content lowered as it is remelted by the plasma-arc process.

It is a more specialized object of the present invention to provide an improved metal processing method as set forth in the object above wherein a plasma-arc is directed onto a pool of previously melted molten metal, and heat is i radiated and convected from the plasma-arc to a feed bar of metal which is suspended above a mold and causes droplets of molten feed bar metal to fall into the pool of molten metal in the mold, and fiuxing material is added to the molten metal of the pool to bring about refinement, e.g.', desulfurization, of the molten metal.

These and other objects will become more apparent after referring to the following specification and drawing in'which:

The single figure is a diagrammatic vertical sectional view illustrating an apparatus which can be utilized for practicing the method of the invention.

' Referring more particularly to the drawing, reference numeral 2 designates generally a plasma-arc cold mold furnace consisting of a water-cooled furnace chamber 4, and a water-cooled mold assembly 6. The bottom of the mold assembly is closed by a starter bar 8. One or more plasma-arc torches 10, each consisting of a constricted nozzle containing a cathode, which may be made of tungsten or similar material, is disposed in the upper part of the chamber 4 with its lower end directed toward a pool of molten metal 16 in the mold. A feed bar 12 of the metal to be remelted is suspended in a mechanism in the upper portion of the chamber 4 whereby it is incrementally advanced toward the mold as it becomes fused by heat from the plasma-arc.

In the melting process, the transferred arc technique is used where an are 14 is struck with the molten metal in the mold, which metal has been previously melted by the plasma-arc torch 10. The molten metal pool 16 then serves as the anode. Gas is forced through the nozzle in the annular area around the arc and is raised to a very high temperature. The gas temperature, which may be as high as 10,000 to 20,000 K., depends on the nozzle size,

nitcd States atent amperage and gas flow. The high temperature causes the gas to become ionized an dthe resulting material is referred to as plasma. Any gas which does not attack the cathode can be used. for example, argon, helium, nitrogen or hydrogen. For metallurgical melting operations, argon is usually used.

Utilizing one or more plasma-arc torches, the metal is remelted in an inert atmosphere. The inert gas protects the metal from oxidation. During remelting, oxide inclusions tend to separate from the metal. During the remelting operation after the arc has been struck, the feed bar 12 is fed toward the pool 16. The pool 16 is contained at its sides by the water-cooled mold assembly 6 which is open at both ends and which serves as a caster. The feed bar 12 is heated by radiation and convection and fused, and the resulting molten metal drips into the pool which is supported at its bottom by the starter bar 8. The solidified product 18 which is formed from the molten metal pool 16 is withdrawn through a cooling chamber 20 which is filled with an inert gas such as argon, at a rate governed by the melting rate.

The plasma-arc cold mold remelting process thus far described is conventional and similar to the process disclosed in Gage Patent No. 3,147,330.

In conventional plasma-arc cold mold remelting practice such as described above, the product shows decreased oxygen and improved cleanliness. The chemical composition, except for oxygen, shows little or no change. However, for many grades of steel it is necessary that the sulfur content be lowered as well as the oxygen content so as to gain full benefits to improved physical quality, such as increased notch toughness.

By the method of the present invention, such desulfurization is accomplished by use of a prepared particulate flux 22, which is added to the furnace preferably with ad ditions beginning when the molten metal pool 16 is first established. As the feed bar 12 melts and drips through the molten flux 24, which floats on top of the molten metal pool 16, sulfur is transferred from the molten metal to the flux 24.

The flux 22 may be fed into the furnace from a hopper 26 through a vibrating chamber 28- and a flux chute 30 which is directed toward the mold assembly 6. The chamber 28 may be vibrated by means of an air vibrator 32.

In the process of the present invention, sulfur transferred between the molten metal and the molten flux is greatly favored by the low oxygen potential and high temperature in the plasma-arc furnace. Nevertheless, the flux should have a composition which promotes desulfurization, i.e., be basic in nature, have a fairly low melting point and have high fluidity. Two flux compositions which meet these requirements are CaF -CaO-AI O and CaF -CaO-SiO Simpler compositions such as CaF -CaO would also be effective. The flux can be added to the furnace intermittently or continuously and these techniques are preferred in order to keep fresh flux available for the metal droplets. By replenishing the flux, the effects of CaF vaporization and loss of flux by seeping down between the product and mold are minimized.

The following examples are illustrative of heats of steeLremelted with flux additions in a plasma-arc cold mold remelting furnace such as shown in the drawing.

EXAMPLE 1 Prefused flux consisting of approximately CaF 15% CaO, and 15% A1 0 was added to the mold 6 on top of the starter bar 8. Two torches were utilized with an argon flow of one s.c.f.m. and the power set at 600 amps at 40 volts on each torch. A feed bar of A181 4340 was remelted in 22 minutes. The sulfur content ,of the metal decreased from 0.007% in the feed bar to 0.004% in the remelted product, while the oxygen decreased from 20 p.p.m. in the feed bar to 8 ppm, in the remelted product.

EXAMPLE 2 A flux material similar to that described in Example 1 was added intermittently as 12% Ni, 5% Cr and 3% Mo steel alloy was remelted, One plasma-arc torch was used. The power utilized in the single torch was 800 amps at 40 volts. The remelting time was 31 /2 minutes. The sulfur content of the steel decreased from 0.009% in the feed bar to 0.004% in the remelted product, and the oxygen decreased from 14 p.p.m. in the feed bar to 7 p.p.m. in the finished product.

EXAMPLE 3 Flux consisting of 60% CaF 30% CaO and 10% A1 0 was added intermittently to the furnace while a feed bar of AISI 4340 steel was remelted. The sulfur content was reduced from 0.022% in the feed bar to 0.008% in the remelted product, and the oxygen decreased from 36 p.p.m. in the feed bar to 13 p.p.m, in the remelted product.

The amount of flux used in the examples above amounted to from to 80 pounds per ton of steel. In commercial practice, the amount of flux to be used would be determined empirically according to product requirements.

For purposes of comparison, a feed bar of AISI 4340 steel was remelted by the plasma-arc cold mold remelting process with the addition of flux and another feed bar of the same steel was remelted without the addition of flux, Where the flux was use, the sulfur content was reduced from 0.022% in the feed bar to 0.008% in the remelted product, and the oxygen content decreased from 23 p.p.m. in the feed bar to 8 p.p.m. in the remelted product. When the other AISI 4340 steel feed bar was remelted in the plasma-arc cold mold remelting furnace without the use of flux, the sulfur content was virtually unchanged from 0.021% in the feed bar to 0.023% in the remelted product while the oxygen content was reduced from 20 p.p.m. in the feed bar to 7 p.p.m. in the remelted product.

Two feed bars of 12Ni-5Cr-3Mo steel alloy were remelted by the plasma-arc cold mold remelting process with the use of flux in one instance and without flux in the other. Where the flux was used, the sulfur content decreased from 0.009 in the feed bar to 0.004 in the remelted product and the oxygen content decreased from 13 p.p.m. in the feed bar to 7 p.p.m. in the remelted product. In the instance where no flux was used, the sulfur content remained unchanged from 0.009 in the feed bar to 0.009 in the remelted product and the oxygen content decreased from 15 p.p.m. in the feed bar to 9 p.p.m. in the remelted product.

Although the method of the invention has been described in connection with a plasma-arc cold mold operation, it will be understood that it can be used in plasmaarc remelting processes conducted in skull type or refractory-lined furnaces.

We have demonstrated that by use of the method of the invention wherein flux is added during plasma-arc cold mold remelting, desulfurization can be effected While oxygen removal takes place.

While we have shown but one embodiment of our invention, other adaptations and modifications may be made without departing from the scope of the following claims.

We claim:

1. In a method of refining metal, the steps comprising supporting a bar of feed metal above a mold, heating the lower end of said bar by radiation and convection from a plasma-arc until it fuses and droplets thereof fall into said mold in the form of molten metal, advancing said heated bar toward said mold as said bar melts, and adding particulate fiuxing material to the metal in said pool.

2. In a method as defined by claim 1 in which said fluxing material is added intermittently.

3. In a method as defined by claim 1 in which said fluxing material is added continuously.

References Cited UNITED STATES PATENTS 2,189,387 2/1940 Wisslcr 139 2,207,746 7/1940 Maier 139 2,686,822 8/1954 Evans et al. 13-9 X 3,042,513 7/1962 Crome --11 X 3,147,329 9/1964 Gage 139 3,267,529 8/1966 Gruber et a1. 75-10 X 3,280,295 10/1966 Mondain-Monval et a1.

3,380,904 4/ 1968 Goldberger 139 3,404,078 10/1968 Goldberger 139 X 3,422,206 1/1969 Baker et a1. 1334 3,429,691 2/ 1969 McLaughlin 7510 BERNARD A. GILI-IEANY, Primary Examiner H. B. GILSON, Assistant Examiner US. Cl. X.R.

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