US3288589A

US3288589A - Process for the production of exceptionally-clean steel

Info

Publication number: US3288589A
Authority: US
Inventors: Thomas E Perry; John A Rinebolt; Roderick J Place; Sidney W Poole
Original assignee: Republic Steel Corp
Current assignee: Republic Steel Corp
Priority date: 1962-06-18
Filing date: 1962-06-18
Publication date: 1966-11-29
Anticipated expiration: 1983-11-29
Also published as: LU43892A1; GB997063A

Description

Nov. 29, 1966 T. E. PERRY ETAL 3,288,589

PROCESS FOR THE PRODUCTION OF EXCEPTIONALLY-CLEAN STEEL Filed June 18, 1962 2 Sheets-Sheet 1 INVENTORS JOHN A. RINEBOLT THOMAS E. PERRY RODERICK J. PLACE BY SIDNEY W. POOLE 4 AT R N E Y Nov. 29, 1966 T. E. PERRY ETAL 3,288,589

PROCESS FOR THE PRODUCTION OF EXCEPTIONALLYCLEAN STEEL Filed June 18, 1962 ZSheets-Sneeta I o2 -22 RI .0 LL! 8 O O Y z 9 *2 0%" "P i N 8mg "-3 'l- O "N Lk -.u, Ian. 55 o|- -w a. T501 0 0 CPU 0 o s w .4 cm 3 -LLI N 0- n: 8 Qca ||||||||||||3 oooo oooooooo'j -uomgvmommvm NNN- INVENTR$- ATTORRNEY United States Patent f 3,288,589 PRUCESS FOR THE PRODUCTION OF EXCEPTHONALLY-CLEAN STEEL Thomas E. Perry, North Canton, John A. Rinebolt, Canton, Roderick J. Place, Massillon, and Sidney W. Poole, Canton, Ohio, assignors to Republic Steel Corporation, Cleveiand, Ohio, a corporation of New Jersey Filed June 18, 1962, Ser. No. 203,091 21 Claims. (Cl. 7512) This invention relates to a process for the production of exceptionally-clean steel. More specifically, it relates to the production of exceptionally-clean steel by a first step of preparing an electrode-ingot of an open steel, such as an air-melt in an electric arc furnace, and thereafter, vacuum melting said electrode-ingot in a conconsumable electrode vacuum furnace.

In various types of steel to be heat-treated to high hardness and strength levels, such as ball-bearing steel, missile steel, etc., cleanliness of the steel, that is freedom from inclusions, is essential. Inclusions cause Weak spots, e.g., internal notches, in the steel. For example, when the steel is used in ball-bearings, cleanliness of the steel is not most critical. Inclusions cannot be tolerated in such steel since considerable pressures are exerted on very small areas on which the ball-bearings must rest or on which they must support considerable Weight.

When inclusions are present in the steel, the resistance to propagation of cracks or notches is lowered. In steels used for missile production, weight is an important factor. Therefore, the ordinary practice of using excess steel as a safety factor to provide for contingencies where a piece of steel may be weakened by inclusions cannot be tolerated. Steels of high strength-to-weight ratios are required. Consequently, it is essential that a steel of extreme cleanliness or freedom from inclusion be used for this purpose to insure good notch properties in the steel.

In missile and jet engines steels it is also essential to have toughness and ductility as well as high tensile strength. High tensile strength can be obtained by increasing the carbon content. However, this generally means a sacrifice in the toughness and ductility. Therefore, it is necessary to have good notch properties by avoiding inclusions in the steel, and to have optimum tensile strength without increasing the carbon content. These properties are effected, together with good toughness and ductility, by the process of this invention for the production of a very clean steel.

In previous attempts to produce exceptionally clean steel or steel substantially free from inclusions, any attempts to effect carbon deoxidation of the steel while in contact with refractories have been unsuccessful. Refractories interfere with the production of clean steel for a number of reasons: (1) spalling of the refractories results in pieces or particles of the refractory material getting into the melt; (2) some carbon acts to reduce the refractories as well as to deoxide in the iron thus making the carbon control difficult; and (3) as the melt is reduced in oxygen content beyond a general level of about 15 p.p.m., oxygen is fed back into the melt from the oxides in the refractories. In the presence of refractories, therefore, the deoxidation cannot be effected below 5-10 parts of oxygen per million. However by the practice of this invention, it is found possible to deoxidize to less than 5 parts of oxygen per million parts of steel.

In accordance with the present invention it has now been found that surprisingly good results are obtained in the production of exceptionally clean steel by the process wherein the electrode for utilimate consumption in a consumable electrode vacuum process is prepared by a first step of melting which results in an open steel, i.e., a

3,2385%? Patented Nov. 29, 1966 non-killed steel, for example by air-melting in an electric arc furnace a charge of scrap iron and any alloying elements as well as any conditioning materials to be used such as slag-forming materials. The steel at tap contains absolute minimum amounts of the strong metallic deoxidizers, such as Al, Ti, etc., and as a result is relatively high in total gas content. The resultant ingot is formed into the desired size and shape either by using an ingot rnold of this desired size and shape, or by having the ingot shaped by forging, rolling or other means into the desired electrode form.

The carbon content of the charge is adjusted by using scrap steel or other iron charge of suflicient carbon content or by the addition of coke or other carbonaceous materail to gives an ingot of sufficient carbon content as to supply carbon for the carbon deoxidation reaction 1n the subsequent consumable electrode vacuum melting step.

If a reduced pressure of microns or less of mercury, advatange-ously 10 microns or less, preferably 3 microns or less, can be maintained in the inital consumable electrode vacuum melting, then the carbon deoxidation reaction is satisfactorily effected to produce satisfactory exceptionally-clean steel. Where very efficient exhaust pumps are available to maintain such reduced pressure on the initial vacuum melting, either by use of a single super-efficient pump or by employing a number of pumps to maintain this condition, then a second vacuum melting step is not necessary. In cases where the exhaust pumps are not sufiiciently efiicient, or the amount of oxygen to be removed is of such great amount that it is difiicult, because of the large amounts of gas being given off, to maintain the reduced pressure below the stated amount, then a second vacuum meltmg step maintained below this limit will produce the desired result. In such case, the first vacuum melting step can be effected at a considerably higher pressure, that is whatever condition is effective in removing excessive gas so as to permit subsequent melting conditions as described above. It is also possible to slow down the rate of gas formation and thereby facilitate maintaining the desired low pressures by slowing down the melting rate, e.g., by lowering the power input and thus reducing the burn-off rate.

In cases where the amount of gas eventually being given off by the carbon deoxidation reaction is the cause for difiiculty in maintaining a reduced pressure of 100 microns or less, it is sometimes possible to reduce the amount of ultimate gas formation by an intermediate partial degassification in the ladle, or by vacuum lift degassing, or by bubbling argon through the molten metal while it is still in the furnace or ladle. Stream and ladle degassifiers suitable for intermediate degassification are illustrated on pages 3689 of Republic Alloy Steels (copyrighted 1961). Suitable vacuum lift degassers are illustrated in US. Patent 3,033,550. Such intermediate treatment can often avoid the necessity for repeating the vacuum melting step. Also as previously indicated the rate of gas formation can be controlled by adjusting the power input or burn-off rate.

By the practice of this invention, it is possible to effect final deoxidation without the addition of strong metallic deoxidizers, such as aluminum or significant amounts of silicon or manganese. By using the conditions of this process, it is possible to effect the final deoxidation solely by the use of carbon. Because of the improved cleanliness of the resultant steel and the lower dissolved gas level, the product has improved transverse ductility, better fatigue resistance, and better toughness. Moreover, this process can be used for most types of alloy steel by the addition of the desired alloying metals, and can also t be used for the production of carbon steel to give a cleaner steel with less non-'metallics and less gases.

In the vacuum melting step, the pressure is advantageously maintained below microns of mercury, preferably below 3 microns. In the event the pressure rises substantially above 100 microns for any substantial period, it becomes necessary to perform a second vacuum melting during which the pressure is maintained below the stated 100 microns.

In the air-melting or first melting step of the process of this invention various methods of melting which will give an open or non-killed steel can be used. However, an electric arc furnace is preferred because of available large capacities, and also because of the capability of maintaining a deoxidizing type of slag and and thus a flexibility of composition. Also it gives better control'of sulfur in the melt as compared to certain other types of furnaces.

The air-melting step, such as performed in electric arc, open-hearth, pneumatic, e.g., Linz-Donawitz process, induction melting, etc., is conducted in accordance with normal practice for the operation of such furnaces or melting operations except that when the melt is tapped, the melt is essentially free of strong metallic deoxidizers.

Carbon deoxidation takes place during the vacuum arc remelting as the metal is transferred across the arc and as it is maintained in the molten state.

It has been found that the process of this invention permits the use of carbon alone for the final deoxidation in the vacuum arc consumable electrode furnace and that the amount of carbon required for such purpose can be calculated and easily controlled so as to give no more than the desired carbon aim in the resultant products.

In the first melting step of this process, a standard electric arc furnace such as presently in common commercial use can be used, such as described on pages 482-484 of the United States Steel Corporation publication, The Making, Shaping and Treating of Steel (copyrighted 1951). To such a furnace, there is charged suitable scrap together with sufficient carbon to insure that the melt contains a carbon content above the required specification level. Alloying elements such as nickel and molybdenum, where desired in the ultimate product, can be charged at this time as these elements are not oxidized in an iron base melt under the oxidizing conditions imposed during meltmg.

After the charge is melted, oxygen is introduced into the molten metal. This can be done either by blowing the melt with a highly concentrated dry oxygen gas using a suitable lance for introducing oxygen at the surface of the molten metal, or by adding iron oxide. The oxygen addition provides temperature uniformity and the elimination of metallic deoxidizers such as aluminum, silicon, etc.

During the finishing period of the first melt a carbide slag is used as the finishing slag in order to refine the steel and to maintain a minimum oxygen level consistent with the carbon content and the bath temperature.

The carbon content of the bath is adjusted to be desired carbon aim just prior to tapping the bath. The carbon can be added in various forms. For example, if chromium is required in the ultimate product, the carbon and chromium can be added in the form of pig iron either by itself or to supplement any ferrochrome that is added.

The amount of carbon added is calculated roughly on the basis of approximately 100150% of the theoretical amount required to convert to carbon monoxide the oxygen present in the iron, and to supply the desired carbon aim for the ultimate product. The calculated amount of carbon to be added should allow also for any carbon already present in the iron. Therefore,'the amount of carbon to be added, regardless of the form of addition, can be calculated roughly as the carbon aim plus 1l.5 times the stoichiometric amount required to reduce the oxygen present to carbon monoxide, minus the amount of carbon already present in the iron.

The molten metal is tapped and poured into suitable ingot molds. Since there are no strong metallic deoxidizers present and there is very little residual manganese, the steel is poured open and gas evolution proceeds during solidification in the ingot mold.

The resultant ingot can be used as such as an electrode or forged or otherwise shaped into the desired electrode shape for use in the consumable electrode vacuum furnace.

FIG. 1 of the accompanying drawings illustrates a schematic arrangement of a typical consumable electrode vacuum furnace that can be used in the practice of this invention.

FIG. 2 shows the Charpy impact transition curves for a steel made by the process of this invention and for steels made by alternate methods.

In operating such a consumable-electrode vacuum furnace, the atmosphere is exhausted, preferably down to the pressure of 1-2 microns. As the electrode is taken to a temperature at which melting starts, the pressure is desirably maintained at less than microns, preferably less than 10 microns, depending upon the capacity of the pumping equipment to remove the gases given off. If the capacity of the pumping equipment is not sufficient to remove the gases as given off, particularly where the amount of oxygen in the steel is excessive or the melting rate is very fast, the pressure may rise as high as 600 or 1,000 microns, or even higher. In such cases a remelting in the vacuum furnace is desirable to produce exceptionally clean steel. As an alternative, the melting rate can be reduced, so as to decrease the rate of gas evolution, by reducing the power input.

The electrode is thus arc-melted into acopper crucible. If the product from this melt is to be remelted, this ingot is forged or otherwise shaped into a new electrode which is subsequently remelted under a reduced pressure of no more than 100 microns (0.1 mm.), advantageously no more than 10 microns (0.01 mm.) of mercury, and preferably 3 microns or less.

In cases where the pressure during the first melting in the vacuum furnace has exceded the desired amount, the resultant ingot may have holes, particularly at the top of the ingot.

In the vacuum melting operation, the vacuum can be effected by a mechanical pump in the early stages for preliminary removal of the atmosphere and then by one or more oil diffusion pumps to more completely exhaust gases and produce and maintain t he desired vacuum.

The type of equipment and method for effecting the vacuum arc melting can be of various types normally used for such purposes. Typical of a type of apparatus suitable for this purpose is that shown in Patents Nos. 2,727,936 issued December 20, 1955; 2,818,461 issued December 31, 1957; and in application Serial No. 698,256, filed by Robert I. Garmy, on November 22, 1957, now Patent No. 2,973,452.

In the apparatus shown schematically in the accompanying drawing, consumable electrode 1 is held in position by supporting means (not shown) but positioned in a region above copper crucible 2. The copper crucible is cooled by water flowing in water inlet 3 and out water outlet 3' and circuilating between the copper crucible and the outer supporting shell 4. This copper crucible acts as a receptacle for the melt 5. Power supply 6 feeds current through conductor 7 and through power tube 8 to electrode 1 and through conductor 9 to the copper crucible. The arcing effect between the melt in the crucible and the consumable electrode is shown by the jagged lines connecting the electrode 1 and the melt 5. The position of the consumable electrode is adjusted upwardly gradually to control the arcing as the level of 5 is raised by additional melt. Vacuum pump 10 creates and maintains a vacuum on the furnace and exhaust gases are forced out through outlet 11.

It has been possible by the process of this invention to 5 6 reduce the oxygen content to levels below 5 parts per TABLE million. The ingot obtained after the vacuum melting at 1355 than 10 microns, Pmferably 1655 than 3 microns, Process Percent Percent Mn Percent Si Percent Al is sound and dense, and is suitable for purposes which require an extremely clean steel having high strength, duc- 0.19 0.28 0.21 0.05 tility, and notch toughness. It has also been found, ap- 016 (124 0.15 Trace parently because of the violent reaction between the car- 0-13 0.16 Q01 Trace bon and the oxides in the metal, and the accompanying sweeping effect of the resultant carbon monoxide, that As shown b the curves of FIG. 2 the Char 1m act i IS an effefztwe fi m1 ogmfrogen i may li strength for a typical steel made acording to ti ie p i'acmany pmsent m the esults m lower i f tice of this invention is much improved over those made levels than would be the case if only vacuum remeiting by standard electric arc furnace as Well as by a corre was emploifedsponding subsequent vacuum melt Where Mn and Si had The particular effectiveness of this invention in producsen added in the preliminary electric arc melt eXcsptiOnilHY clean Steel is demonstrated by results The following examples illustrate various modifications obtained when steels produced according to this invention for practicing h process f hi invention These examare tested according to the 1K inclusion rating described pies ar i te d d erel as illu tration a d re ot to be in, Tentative Recommended Practice for Determining interpreted as limiting the scope of the invention or the the Inclusion Contents of Steel, designation B 45-60 T manner in which the invention can be practiced. Unless hi h ear in paragraph 3, pages 105-118, of the specifically indicated otherwise, parts and percentages are 1960 Supplement for the American Society for Testing glvfin as Parts and Pflcentags y Weight- Material Standards. As illustrated hereinafter in the ex- Example I i the Values for the Chart or table of page 112 i A charge of 385 lbs. of scrap steel low in phosphorus me above supplemnt gefmrany not exceed a Value 0 and sulfur plus 11 lbs. 8 ozs. of nickel and 3 lbs. of ferro 1 for any type of inclusion and in most cases there are molybdenum together with 2 lbs. of Garbo as can no lncmslons or they have a value no greater than bon is added to an electric arc furnace of standard comwhen steels are produced according to the practice of msrcial design This is covered with a layer f 20 this invention. of lime, and 8 lbs. of spar to serve as slag covering. This For example, the following table shows the 1K rating charge i melted by applying 120-140 volts and 1500- f a hinh i ld th steel (A) d by normal l 1800 amps. for 70 to 90 minutes. Then the molten metal tric arc furnace in which Mn, Si and A1 are added as is decafhuriled y l Wing with pure oxygen The reusual, and a similar steel (A,/V) made by an electric sultant slag is run off and replaced with a finishing slag ar-c furnace first melt Without any addition of Mn or Si cpinpnsmg 20 lbs of lime and 8 Spar 4 silica sand plus 3 lbs. coke fines. At this time a sample or Al and a Second Single Vacuum melt of the resultant of the melt is removed'for analysis to determine Whether ingot at a riiduced Pressure of liss than 10 mlcrons of elements need to be added to obtain the ultimate desired mercury, in accordance with the practice of this invenspecification. In this case, the chemical analysis shows m 40 the following percentages:

0 Mn P s Si Ni Cr M0 Cu 1st Prelim .16 .14 .008 .015 .03 2. 95 .05 .48 .04 Spec. Aim .1810 .010 .010 2 80to 1.50 to .45 to .19 Nil Max. Max. Nil 3. 10 1.70 .55

INCLUSION RATING AND GRAIN SIZE Additions of 9 lbs Fe-Cr and .13 lb. Fe-Mo are made to give the desired aims. Then the melt is tapped into an [J-K inclusion rating] open ingot of such dimensions as to give an ingot of 7" RC Sq. x 24" long x 350 lbs. This ingot is forged to Process Sulphlde A Alumna B Silicate G Globulal D a 5%" rd. x 45" long electrode, the skin ground off, and

T H T H T H T H the ingot prepared for remelting as an electrode in a consunTigble eleltctrtzde1 vacuim fur1nac.

e resu an e ec ro e is p ace in a vacuum consum- A 0 L5 0 able electrode furnace of commercial design and the at- I 0 0 0 0 mosphere is exhausted to a reduced pressure of 1-2 microns. Then the electrode is melted by the application FIG. 2 shows a series of Charpy impact transition of 24-26 volts and 3800-4400 amperes for 120 minutes curves for high yield strength steels made by the following while maintaining a reduced pressure of 2 microns of methods: mercury and collecting the melt in the copper crucible of A-Electric arc furnace air melt-conventional practice the furnace. The resultant ingot is dense and has the with Mn and Si added as usual. properties described above for A'/ V.

A/V-Electric arc furnace air melt with Mn and Si E l H added-remelted in consumable-electrode vacuum arc mm? 8 furnace (single vacuum melt at 10 microns). A charge of 350 lbs. of scrap steel low in phosphorus A/VElectric arc furnace air melt without Mn, Si and sulfur plus 36 lbs. of nickel, 16 /2 lbs. of cobalt, 1 lb. or Al addition-rernelted in consumable-electrode vacu- 8 ozs. of ferro molybdenum together with 3 lbs. of Carbo um arc furnace (single vacuum melt at less than 10 mi- 90 as carbon is added to an electric arc furnace of crons). (According to this invention.) standard commercial design. This is covered with a The chemical analyses for C, Mn, Si and A1 of the layer of 20 lbs. of lime, and 8 lbs. of spar to serve as above products, which are also those shown in FIG. 2, slag covering. This charge is melted by applying 120-140 are given in the following table. The analyses (not volts and 1500-1800 amps. for 70 to 90 minutes. Then shown) for P, S, Ni, Cr and Mo are approximately the the molten meta-l is decarburized by blowing with pure same for the three products: oxygen. The resultant slag is run off and replaced with a finishing slag comprising 20 lbs. of lime, and 8 lbs. of spar, 4 lbs. silica sand plus 3 lbs. coke fines. At this time a sample of the melt is removed for analysis to deter- .mine whether elements need to be added to obtain the ultimate desired specification. In this case, the chemical 5 analysis shows the following percentages:

8 Example VI The procedure of Example III is repeated with similar satisfactory results using as the consumable electrode in the vacuum remelting an ingot of open steel produced by an air-melt in an open-hearth furnace.

Mn P S Si Ni Cr Mo 00 Va 1st Pre1im 33 16 006 02 8. 75 04 27 3. 90 2nd Prelim 45 13 006 012 02 9. 05 04 32 4. 00 Specification 44 to 010 010 8. O0 to 25 to 30 to 3. 75 t0 08 to 46 Nil Max. Max. Nil 10. 00 35 35 4. 10 12 1 After refining slag is put on.

Final additions to give the desired aims are 1 lb. 12 02$. 15 FeCr and 12 ozs. Fe-Va. Then the melt is tapped into an open ingot of such dimensions as to give an ingot of 7" RC Sq. X 24" long x 350 lbs. This ingot is forged to a 5 /2" rd. x 45" long electrode, the skin ground ofi, and the ingot prepared for melting in the consumable-electrode vacuum furnace. The electrode is melted under conditions similar to those of Example I with similar results.

Example III A charge of 144,000 pounds of scrap steel low in phosphorus and sulfur plus 15,000 lbs. of nickel and 6300 lbs. of cobalt plus 1200 lbs. molybdenum oxide together with 2400 lbs. of carbon in the form of ground electrodes is added to an electric furnace of standard commercial design. This is covered with a layer of 3000 lbs. of lime and 1500 lbs. of spar which serves as a slag covering. This charge is melted by applying approximately 360 to 380 volts and 10,000 amps which amounts to 62,000 kw. hrs. for the whole heat. Then the molten metal is decarburized by blowing with pure oxygen. The resulting slag is run off and replaced with a finishing slag comprising 3000 lbs. of lime, 1500 lbs. of spar and 500 lbs. of coke fines and 100 lbs. of sand. At this time a sample of the melt is removed for analysis to determine whether elements need to be added to obtain the ultimate desired Example VII The procedure of Example III is repeated with similar satisfactory results using as the consumable electrode in the vacuum remelting an ingot of open steel produced by a first melt in a basic oxygen (pneumatic) furnace wherein oxygen is blown downward vertically into the melt.

When the procedure of Example V is repeated with a vacuum applied in the induction first melting, it is found that carbon deoxidation proceeds to an undesired extent in this first melting with the result that (-a) the refractory with which the melt is in contact is deoxidized simultaneously with the iron deoxidation, and (b) as the oxygen level in the bath is being reduced, additional oxygen is being taken into the bath from the refractory. Consequently, it is desirable to perform this induction melting at approximately atmospheric pressures or at least pressures not sufficiently reduced as to cause the disadvantages recited above.

While certain features of this invention have been described in detail with respect to various embodiments thereof, it will, of course, 'be apparent that other modifications can be made within the spirit and scope of this invention and it is not intended to limit the invention to the exact details shown except insofar as they are despecification. In this case, the chemical analysis shows the following percentages: fined 1n the following claims.

0 Mn P I s Si Ni Cr M0 00 Va 1st Prelim .366 .15 .009 .008 02 7. 70 .16 .29 3.14 o. 0 Specification .42 to 15 010 007 10 8. 75 to 25 to 30 to 3. to 08 to 44 Max. Max. Max. Max. 0. 25 35 .35 4. 00 12 Final additions to give the desired aims are 2475 lbs. of nickel, 1240 lbs. cobalt, 145 lbs. ferro-molybdenum, 400 lbs. of ferrochromium, 175 lbs. of vanadium plus 204 lbs. of carbon. Then the melt is tapped into open ingots of such dimensions as to give ingots of 24" diameter and approximately 165" long x 20,000 lbs. These ingots are then used as the electrodes for consumable vacuum melting using amperages between 5,000 and 30,000 amps. at 20-25 volts at a reduced pressure of less than 3 microns for a sufiicient time in each case to complete the melt. Similar results are obtained as in Example I with respect to JK micro inclusion ratings. The oxygen content of the product in each case is in the range of 1-4 parts per Example V The procedure of Example III is repeated with similar satisfactory results using as the consumable electrode in the vacuum remelting an ingot of open steel produced by an air-melt in an induction furnace.

The invention claimed is:

1. A process for the preparation of an exceptionally clean steel comprising the steps of:

(a) melting a charge of iron at atmospheric pressure in the absence of an added metallic deoxidizer selected from the class consisting of strong deoxidizers of the class consisting of aluminum and titanium and significant amounts of weaker deoxidizers of the class consisting of manganese and silicon;

(b) adjusting the carbon content of the resultant melt while still in molten form to an amount sufiicient to react With the oxygen content of said melt and also to supply an additional amount of carbon required for the ultimate carbon aim in the resultant steel,

(c) thereafter casting said melt into an ingot;

(d) melting said ingot as the consumable electrode in a vacuum, consumable-electrode electric arc furnace while the electrode region of said furnace is maintained at a reduced pressure of no more than microns of mercury.

2. A process of claim 1, in which said reduced pressure is no more than 10 mircons of mercury.

3. A process of claim 1, in which the carbon adjustment comprises the addition of carbon to said melt of an amount of carbon approximately 1-1.5 times the stoichiometric amount required to react with the oxygen content of said melt and also to supply any additional amount of carbon required for the ultimate carbon aim in the resultant steel.

4. The process of claim 3 in which said carbon is added as a high carbon ferrochrome.

5. The process of claim 3 in which said carbon is added in the form of pig iron.

6. A process of claim 3 in which said reduced pressure is no more than microns of mercury.

7. A process of claim 1 in which the melt of said airmelting is degasified under a reduced pressure of no more than 1,000 microns of mercury, the resultant melt thereafter being cast into an ingot, and the resultant ingot remelted as a consumable electrode in a vacuum consumable electrode furnace under a reduced pressure of no more than 100 microns of mercury.

8. A process of claim 7 in which said reduced pressure in said vacuum consumable electrode furnace is no more than 10 microns of mercury.

9. A process for the preparation of an exceptionally clean steel comprising the steps of:

(a) air-melting a charge of scrap iron in an electric arc furnace in the absence of an added metallic deoxidizer selected from the class consisting of strong deoxidizers of the class consisting of aluminum and titanium and significant amounts of weaker deoxidizers of the class consisting of manganese and silicon;

(b) adjusting the carbon content of the resultant melt while still in molten form to an amount suflicient to react with the oxygen content of said melt and also to supply an additional amount of carbon required for the ultimate carbon aim in the resultant steel;

(c) thereafter casting said melt into an ingot; and

(d) melting said ingot as the consumable electrode in a vacuum, consumable-electrode electric arc furnace While the electrode region of said furnace is maintained at a reduced pressure of no more than 100 microns of mercury.

10. A process of claim 9, in which the carbon adjust ment comprises the addition of carbon to said melt of an amount of carbon approximately 1-1.5 times the stoichiometric amount required to react with the oxygen content of said melt and also to supply any additional amount of carbon required for the ultimate carbon aim in the resultant steel.

11. The process of claim 10, in which said carbon is added as a high carbon ferrochrome.

12. A process of claim 10, in which said carbon is added in the form of pig iron.

13. A process of claim 9, in which said reduced pressure is no more than 10 microns of mercury.

14. A process of claim 9, in which said reduced pressure is no more than 3 microns of mercury.

15. A process of claim 9, in which the initial melting of said scrap iron is completed under a layer of a carbide slag.

16. A process of claim 9, in which the initial melt of said scrap is treated by blowing with concentrated oxygen introduced under the surface of said melt.

17. A process of claim 9, in which the melting of said scrap iron is completed under a layer of a carbide slag, with the melted iron being treated with oxygen by blowing a jet of concentrated oxygen gas at the surface of said melt.

18. A process of claim 9, in which the melt of said air-melting is degasified under a reduced pressure of no more than about 1,000 microns of mercury, the resultant melt thereafter being cast into an ingot, and the resultant ingot remelted as a consumable electrode in a vacuum consumable electrode furnace under a reduced pressure of no more than 100 microns of mercury.

19. A process of claim 1, in which said melting at atmospheric pressure is effected in an induction furnace.

20. A process of claim 1, in which said melting at atmospheric pressure is effected in an open-hearth furnace.

21. A process of claim 1, in which said melting at atmospheric pressure is effected in a basic oxygen furnace.

References Cited by the Examiner UNITED STATES PATENTS 3/1965 Dagan 43 OTHER REFERENCES DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, HYLAND BIZOT,

Examiners.

H. F. SATIO, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,288,589 November 29, 1966 Thomas E. Perry et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 24, strike out "not"; line 57, strike out in"; column 2, line 15, for "materail" read material column 4, line 41, for "exceded" read exceeded Signed and sealed this 12th day of September 1967.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNER Commissioner of Patents

Claims

1. A PROCESS FOR THE PREPARATION OF AN EXCEPTIONALLY CLEAN STEEL COMPRISING THE STEPS OF: (A) MELTING A CHARGE OF IRON AT ATMOSPHERIC PRESSURE IN THE ABSENCE OF AN ADDED METALLIC DEOXIDIZER SELECTED FROM THE CLASS CONSISTING OF STRONG DEOXIDIZERS OF THE CLASS CONSISTING OF ALUMINUM AND TITANIUM AND SIGNIFICANT AMOUNTS OF WEAKER DEOXIDIZERS OF THE CLASS CONSISTING OF MANGANESE AND SILICON; (B) ADJUSTING THE CARBON CONTENT TO THE RESULTANT MELT WHILE STILL IN MOLTEN FORM TO AN AMOUNT SUFFICIENT TO REACT WITH THE OXYGEN CONTENT OF SAID MELT AND ALSO TO SUPPLY AN ADDITIONAL AMOUNT OF CARBON REQUIRED FOR THE ULTIMATE CARBON AIM IN THE RESULTANT STEEL, (C) THEREAFTER CASTING SAID MELT INTO AN INGOT; (D) MELTING SAID INGOT AS THE CONSUMABLE ELECTRODE IN A VACUUM, CONSUMABLE-ELECTRODE ELECTRIC ARC FURNANCE WHILE THE ELECTRODE REGION OF SAID FURNANCE IS MAINTANED AT A REDICED MICRONS OF MERCURY.