US2040604A - Method of manufacturing low carbon iron and steel alloys - Google Patents

Description

Patented May 12, 1936 UNITED STATES PATENT OFFICE mrrnon or ACTURING LOW MANUF CARBON IRON AND STEEL ALLOYS No Drawing. Application September '1, 1934, Serial No. 743,157. In Germany October 9,

1981 Claims.

This invention relates to metallurgyand more particularly -to methods of manufacturing alloys and still more particularly to methods of removing carbon from alloys without oxidizing the metallic '5 constituents of the alloy. This application is a continuation in part of application Serial No. 604,315 filed April 9, 1932.

One of the objects of the present invention is to provide an eflicient and commercially practical 10 method of decarburizing alloys-without oxidizing the metallic constituents thereof.

Another object of the present invention is to provide a method of decarburizing iron base al-' loys containing at least one of the so-called l5 stronger carbide forming elements.

Still another object of the'present invention is to provide a method of forming low carbon chromium-containing iron alloys free from substantial amounts of oxygen.

Other objects and advantages will become apparent as the present invention is further disclosed.

Heretofore in the art it has been proposed to remove the carbon content of a molten iron or steel bath by circulating hydrogen 'therethrough or thereover. The diiflculty experienced in thus removing the carbon is that while hydrogen and carbon will react to form methane at temperatures approximating 400 to 600 C. at higher temperatures the reverse reaction occurs and at the temperatures of molten metal baths carbon and hydrogen do not react at all to form methane. Accordingly, it has been found necessary to incorporate in the hydrogen a proportion of an oxidizing gas which reacts with the carbon to form carbon oxides. These oxidizing gases and vapors in the hydrogen promote oxidation of the metal constituents of the bath particularly as the carbon content is lowered which is accentuated when the metal bath contains one or more of the so-called stronger carbide forming elements, Cr,

Mo, W, Zr, Ta, Cb, A], V and the like which form carbide compounds that are more stable and more difficult to reduce to their elemental constituents than iron or manganese carbides, and

particularly those elements whose oxides are difflcult to reduce or are not reducible by hydrogen.

We have found that these diiilculties and disadvantages may be. overcome in the following manner:-

We place the molten iron alloy in a refractory lined crucible, superpose thereon a proportion of a solid oxidizing material reducible by carbon such as a metal oxide, enclose the surface of the bath and circulate thereover a reducing gas comprised in major part of hydrogen and in minor part of other reducing gases, which gas is substantially free from oxidizing gases and vapors. During the circulation of the said gas we heat the metal bath to a temperature at least approxi- 1 mating 1800 C. and turbulently stir the-bath to effectively increase the surface area of the bath exposed to the action of the gas and to maintain the distribution of the oxidizing material and the composition of the bath substantially uniform throughout. T

In this manner the carbon content of the bath is'rapidly lowered to a relatively low percentage. We have found, however, that the precise low limit of carbon obtainable in this manner depends upon the particular alloy constituents contained therein and upon the relative percentages thereof present. We have also found that to obtain substantially complete decarburization in this manner a relatively large excess of oxidizing material must be added to the bath, the metal bath must be heated to as high a temperature as is practicable with refractory materials available, preferably as close to 1900" C. as is practicable. We have further found that the composition of the reducing gas atmosphere as the carbon content becomes lowered below about .10% must be limited to substantially pure hydrogen substantially free from associated oxidizing gases, in order to effect substantially complete elimination of carbon and substantially complete elimination of residual oxidizing material. For practical purposes at the relatively high temperatures employed in the present invention the water vapor content of the reducing gas should not be in excess of that amount retained therein after cooling the gas to temperatures approximating- The most common iron alloy illustrative of the practice of the present invention is the chromium-containing iron alloy, with or without associated alloy elements. Chromium is one of the stronger of the carbide-forming elements andforms an oxide which is difllcultly reduced by hy-' drogen. The elimination of carbon from chromium-containing iron and steel alloys to below .01%

without resultant oxidation of the chromium is a tedious and expensive process by methods heretofore proposed in the art. In accordance with.

the present invention, however, we accomplish the removal of carbon to a relatively low percentage in a relatively short time and in a relatively inexpensive manner and obtain a substantially oxidefree product.

In the manufacture of low-carbon and oxidefree iron-chromium alloys in accordance with the present invention, we first form a molten iron bath preferably of one of the lower carbon steels and add thereto the desired amount of ferrochromium alloy which may contain a certain amount of carbon and which will bring the chromium content within the range desired. The thus formed alloy is disposed in an electric furnace of the type described and claimed in our copending application Serial No. 557,234 filed August 15, 1931, now issued as Patent No. 1,983,242, December 4, 1934. To the said bath is then added a proportion of a metal oxide such as iron, chromium or nickel oxide and the bath is heated by induced electric'currents with turbulent stirring to temperatures approximating 1800 C. A substantially non-oxidizing and non-carburizing reducing gas composed in major part of hydrogen and in part of other reducing gases such as CO, methane, ethane and the like and if desired a proportion of nitrogen is then circulated over the enclosed surface of the bath and the temperature of the bath and the turbulent stirring thereof is maintained until the carbon content of the alloy has been lowered as far below 1.0% as is practicable in an economically practical time interval.

This turbulent stirring of the bath is essential in the practice of the present invention as it greatly accelerates the rate of carbon removal by increasing the effective surface area of the bath in contact with the metal oxide and maintains a substantially uniform distribution of the oxide throughout the bath and a substantially uniform bath composition. The induction furnace of the above-identified patent is peculiarly adapted for the purpose of the present invention as it provides means to heat the bath by induced electric currents, the said currents turbulently stirring the molten metal bath in a plurality of radially vertical planes as well as rotating the bath horizontally, thereby recurrently bringing all parts of the bath to the surface and maintaining the metal oxide added thereto thoroughly dispersed and uniformly distributed throughout the bath.

The non-oxidizing and non-carburizing reducing gas composition may be widely varied without departing essentially from the scope of the present invention. We have found that the reducing gas is substantially non-oxidizing for the purposes of the present invention when the gas is substantially free from oxygen and carbon dioxide 7 and when the water vapor content thereof is not in excess of that which is retained therein at temperatures approximating -'60 to 70" C.

We have also found that the gas is substantially non-carburizing when the same contains a small percentage of one or more of the stable hydrocarbon gases methane, ethane, etc., or a small percentage of the vapor of one or more oi the, liquid hydrocarbons such as benzol. The precise proportion of each of these hydrocarbon and vapors permissible is variable with respect to the moisture and other oxidizing-gas content of the gas as in effect these hydrocarbons neutralize the deleterious.;.eflect of moisture and other oxidizing gases. We have found that these hydrocarbon gases and vapors may be employed in such percentages as are insuflicient to produce a clouding of the gas in the furnace. This clouding" eifect is produced as a result of the decomposition of these gases with liberation of finely divided carbon. I

Accordingly, for the purposes of removing carbon to a relatively low percentage by the action of metal oxides in presence of reducing gases we =ment in hydrogen obviously will widely vary with may employ a reducing gas comprised in major part of hydrogen but containing relatively large amounts of carbon'monoxide with or without one or more of the stable hydrocarbon gases above identified, provided the oxygen, carbon dioxide and water vapor content is substantially limited I as above noted. This mixture of gases may be most conveniently obtained by appropriately mix A ing together water gas", blast furnace gases, producer gases, commercial hydrogen and the like. The hydrogen content of the mixture may vary widely except that the proportion should be sufficiently high to insure that the gas is strongly reducing towards most metal oxides.

We have found, however, that whereas the above identified reducing gas in presence of a suitable metal oxide is effective in removing carbon from the bath without substantial oxidation of the metal constituents of the bath, and is moreover effective in reducing a number ofv the metal oxides, it is not eifective in reducing some oxides, particularly chromium oxide at the temperature of rapid carbon removal from the bath when the chromium content of the bath is above a relatively few per cent.

We therefore follow the initial decarburizing treatment as above described with a deoxidizlng treatment at temperatures ranging from 1800 to 2000 C. depending upon the chromium content of the alloy in an atmosphere of hydrogen substantially free from oxidizing gases CO1) and nitrogen and having a water vapor content not in excess of that retained therein on cooling to -60 to -'l0 C.

This final high temperature treatment in hydrogen in addition to deoxidizing the alloy also is effective in lowering the carbon content of the bath to relatively low percentages at an economically practical rate. The time interval of treatrespect to the total amount of'oxide and carbon remaining in the alloy.

At the conclusion of this step of the method the metal bath may be cooled down to a desired pouring temperature and cast into open air molds. For some types of alloys it may be desirable to insure the complete removal of the hydrogen gas from the metal before pouring. This may be accomplished by heating the metal under gradually reducing pressures until the hydrogen is evolved from the molten bath or by replacing the hydrogen with nitrogen. Alternatively by exposing the molten bath to an oxidizing atmosphere for a time interval adapted to effect an oxidation of the hydrogen the hydrogen thereby may be eflectively removed. This latter method is not preferred as metal oxides particularly chromium oxides will be formed and thereafter a metallic reducing agent (Al, Mg, an alkali or alkaline earth metal) must be added before casting.

The above method has been described particularly with respect to alloys containingairon and chromium. Such alloys commonly contain additional alloying elements such as silicon, manganese, nickel, copper or one or more of the elements of the titanium, vanadium or chromium groups of elements. The presence of any one or more of these elements does not depart from the practice of the present invention and does not oxidizing and non-carburizing reducing gas disclosed, that a rapid decarburization of the metal bath will be effected provided that a solid metal oxide has been incorporated in the bath and that the turbulence of the bath is sufficient to maintain a thorough and uniform dispersion of the oxide throughout the bath.

Having broadly and specifically described the present invention and given one specific embodiment thereof, it is apparent that the same is adapted to be extensively adapted in the removal of carbon and oxygen from a wide variety of alloys, and all such modifications and adaptations are contemplated as may fall within the scope of the following claims.

What we claim is:

l. The method of decarburizing and deoxidizing iron alloys containing at least one of the stronger carbide forming elements which com- .prises melting said alloy, adding a metal oxide thereto inan amount in excess of that empirically necessary to react with the carbon content of the bath, heating the bath to elevated temperatures approximating 1800 C., turbulently stirring the bath, enclosing the surface of the bath from the atmosphere and circulating a hydrogencontaining reducing gas over the heated and turbulently stirred surface of thebath, said gas being substantially free from oxidizing gases and vapors,

for a time interval adapted to effect the removal of the major part of the carbon from said alloy and thereafter heating the bath to a higher temperature approximating 2000 C. in an atmosphere of substantially pure dry hydrogen fora time interval adapted to effect a removal of the desired proportion of the remaining carbon and substantially all of the said oxygen content of said alloy.

2. The method of decarburizing chromium-iron alloys which'yomprises melting said alloy, adding thereto a proportion of chromium oxide in an amount in excess of that necessary to empirically combine with thecarbon content of the bath, heating the bath to elevated temperatures approximating 1800 C., turbulently stirring the bath to uniformly disperse said oxide throughout said bath, enclosing the surface of the bath from the atmosphere and circulating over the heated and f turbulently stirred surface of the bath a reducing gas comprised in major part of hydrogen substantially free from oxidizing gases and vapors, for a time interval adapted to effect the lowering of the carbon content of said alloy to the desired proportion.

3. The method of decarburizing and deoxidizing chromium-iron alloys which comprises melt ing said alloy, adding thereto a proportion of chromium oxide in an amountin excess of that necessary to empirically combine with the carbon content of the bath, heating the same to elevated temperatures approximating 1800 C turbulently stirring the bathto uniformly disperse said oxide throughout said bath, enclosing the surface of the bath from the atmosphere, and circulating over the turbulently stirred surface a reducing gas comprised in major part of hydrogen and in part of other reducing gases, said reducing gas being substantially free from oxidizing gases and vapors, for a time interval adapted to effect the removal of the major portion of the carbon from said alloy and thereafter heating the bath to a higher temperature in an atmosphere of substantially pure dry hydrogen for a time interval adapted to effect a removal of the desiredproportion of the remaining carbon and oxygen content of said alloy.

4. In the decarburizing of molten iron alloys the steps of adding to the bath a proportion of a solid oxidizing agent and of heating the alloy to temperatures ranging between 1600 to 1800 C. in a reducing gas atmosphere comprised in major part of hydrogen and in minor part of other reducing gases, said gas being substantially free from oxygen and carbon dioxide and a water vapor content not in excess of that retained therein at -60 to -.-'l0 0., and of turbulently stirring the bath to maintain a thorough and. uniform dispersion of said oxide throughout the bath and a substantially uniform composition in the bath during the circulation of said gas. i

5. In the deoxidizing and decarburizing of molten iron alloys, the steps-of incorporating a proportion of a metal oxide in said bath, heating the bath to temperatures ranging between 1800 to 2000 C. in an atmosphere of substantially pure dry hydrogen free from deleterious amounts of oxidizing gases and nitrogen, and turbulently stirring the molten alloy to uniformly distribute said oxide throughout the bath during the circulation of said gas thereover.

6. The method of decarburizing alloys which comprises melting said alloy, heating the same to elevated temperatures approximating 1800 C., Y

incorporating within said molten bath a proportion of a solid oxidizing agent and a proportion of a metallic carbidecompound, the metal constituents of said oxide and said carbide being desired in the said alloy, enclosing the surface of the bath from the-atmosphere, circulating over said surface a reducing gas comprised in major part of hydrogen and substantially free from oxidizing gases and vapors, maintaining the bath at the said elevated temperatures and-turbulently stirring the said bath'during the circulation of said gas to accelerate the rate;of carbon removal therefrom.

'7. The method of decarburizing iron alloys which comprises melting thesaid alloy, adding metal oxides thereto, the amount of said oxide being in excess of that required to empirically combine'with' the said carbon, disposing the said alloy and oxidemixture on a hearth, turbulently stirriwthe melt by induced electric currents to thoroughly disseminate the said oxide through the' melt, enclosing the surface of the'melt from the atmosphere, circulating over the surface of the turbulently stirring melt a strong free flow of reducing gases comprised at least in major part of hydrogen, the said bath being maintained meanwhile at a temperature favorable to metal oxide reduction by the carbon in said bath, and

continuing said treatment for a time interval required' to eliminate the said carbon to a desired low percentage.

8.- The method of eliminating carbon iron temperatures approximately 1800 0., incorporating therein. a carbon reducible metal oxide in j excess of that needed to empirically combine with the said carbon; turbulently stirring the molten alloy to thoroughly distribute the said oxide therethrough, enclosing the surface 01 the melted alloy from the atmosphere, circulating a reducing gas comprised in major part of hydrogen over the surface of the bath and continuing the circulation of said gas and the turbulent stirring oi the bath at the said high temperature until the carbon content has been reduced to below .10%, thereafter replacing the said reducing gas with substantially pure hydrogen having a water vapor content not in excess of that amount retained therein at temperatures approximating -60 to -70 C., and continuing the turbulent stirring of the bath at more elevated temperatures approximating 1900 C; for a time interval required to eliminate the major portion of remaining oxide and to lower the carbon to-a desired low percentage. I

9. The method or eliminating carbon from molten iron alloys which comprisesmelting the said alloy, adding metal oxide thereto in excess of that empirically needed to combine with the I carbon content of the alloy, thoroughly disseminating and distributing the metal oxide through out the said bath and heating the bath to a temperature favorable to the reduction of said oxide by the carbon'in said bath and protecting the metallic constituents of the bath from oxidation by circulating a reducing gas over the surface ofthebath.

10. The method or claim 9, in which the metal oxide remaining from the practice 01 the method of claim 9 is eliminated by replacing the said reducing gas with substantially pure dry hydrogen having a water vapor content not in excess of that retained therein after exposure to temperatures approximating 60 to 70 C. and the bath is heated to temperatures favorable to the reduction of the said oxide by the said lwdrogen and the said bath is turbulently stirred during said heating to recurrently bring all parts 01' the bath into contact with the said hydrogen. 7

HERBERT GRUBER. WILHELM ROHN.