US1757298A - Process of producing low carbon alloys - Google Patents

Description

Patented May 6, l$30 UNITED. STATES PATENT OFFICE PROCESS OF PRODUCING LOW CARBON ALLOYS No Drawing.

This invention relates pri irily to a method of eliminating from metals or alloys of metals the excess of carbon which remains therein after the step of reduction of the ore 5 by the carbon method, and has for its ob- 15 metals of the chromium series, or two alloying metals, such, for instance, as chormium and nickel. it being understood, however that the invention is not limited in its application to these particular metals.

In the production of chromium for alloying purposes, the principal source of which is the iron-chromium mineral known as chromite, it is common to reduce the oxides of iron and chromium by heating the ore in an electric furnace in the presence of an excess of carbon. But this leaves the metallic iron and chromium with sucha high carbon content that it is useless for most alloying or addition purposes, for which reason the resultant alloy must be further treated by what is usually termed the oreing down process, which consists in strongly heating the ferro alloy and carbon with additional chromite ore and thereby oxidizing a substantial portion of the remaining carbon. F erro-alloys of chromium and the like are also produced by the socalled thermit process, according to which metallic silicon or aluminum is used as a reducing agent. But these methods are both ob- .jectionable because of high metal losses or expense of reagents employed.

The present invention provldes a method of further reducing, or reducing to an unobjection ab'le proportion, the carbon constit1ipent of ferro-alloys produced by the usual earbon method of reduction, and it is based n the discovery' th'at if the high carbon roalloyjbe melted in a silica lined electric furnace, and the bath be then heated to a term perature somewhat higher than is ordinarily Application flled May 29, 1926. Serial No. 112,692.

required for ingot or casting production, for some cause, possibly catalytic action induced by the alloying metal, the order of affinity of oxygen, as between silicon and carbon, is reversed and a violent reaction sets up between the carbon of the alloy and the silica of the furnace lining and slag (such reaction being represented probably by the formula 2C plus SiO- equals Si plus 2C0) and the ebullition of the carbon monoxide gas. produces violent boiling that insures good contact between the furnace lining and slag and all parts of the metal bath. By this procedure I have found it possible to reduce carbon from a proportion of more than 1% to a proportion of about 15% in less than three minutes from the time the bath commences to boil; and by this method I am enabled to produce a superior low carbon alloy from a charge averaging considerably higher than the permissible proportion of carbon, with very much less expense than if the low carbon alloy be produced by the use of alloying metals that were of low carbon content before addition. For instance, if it be proposed to produce high percentage chrome-nickel steel, the present process admits of the use of a very much cheaper grade of ferro chrome, with respect to the proportion of contained carbon impuritygthan can possibly be used with the ordinary methods of producing high percentage chrome-nickel steel, the reason for this being that with the process of the present invention the excess of carbon can be eliminated from the bath after addition of the alloying metals, and this elimination can a be accomplished by the relatively inexpensive expedient of using a silica lining and slag, and running up the temperature of the bath sufiiciently beyond mere fusion to set up the 7 (probably in accordance with the reaction .plus SiO equals 2C0 plus Si), which point of temperature can readily be recognized by violent boiling effect which will be produced by the ebullition of CO gas; and continue this temperature until the reduction of the carbon is completed, the attainment of which will be manifest from thedi'sappearance of the violent ebullition.

The process of the present invention can be used for the treatment of high carbon iron-chromium or nickel-iron-ehromium or other ferro alloys to produce low carbon ferro alloys suitable for use in steel making. For instance, take ferro-chrome containing 65% of chromium, 32% of iron, and 3% of carbon; charge the same into an electric furnace having a silica lining; heat the bath to the fusing point of the ferro-alloy; then raise the temperature of the bath to about, say, 3000 F. or more; maintain this temperature until ocular indication of the reaction (violent ebullition followed by a subsidence of reaction) indicates completion of the reaction, or until analysis shows a carbon content of 1% and silicon content of 3 or such variation of the silicon content as will be caused by losses; and then draw off the bath.

The process of the present invention may also be used in the production of heat or corrosion-resisting ingots or castings, wherein the content of nickel, chromium, or the like is high, by charging high carbon ferro-alloysinto a silica furnace and causing reduction of the carbon by the described high temperature treatment, just prior to tapping the furnace.

WVhile I have herein referred to the electric furnace as the preferred instrumentality for developing heat required, the invention is not limited to the use of such a furnace, but may be carried on in an acid or silica lined induction furnace, or even in an acid open hearth furnace.

Metal coming from the process herein described naturally carries a relatively high silicon content, the percentage corresponding closely in most cases to that which was necessary to satisfy the carbon in the reaction described, but if the resultant proportion of silicon be objectionable, the alloy produced by this acid process may be reduced by oxidizing the excess of silicon with ore or air, or the alloy may be transferred to a furnace with a basic lining, and the silicon very rapidly and completely eliminated without the re-introduction of carbon. Thus, by the double process it'becomes possible to produce alloy castings or ingots or pigs practically free from both carbon and silicon. Moreover, the double treatment renders the alloy extremely low in impurities such as phosphorus and sulphur. But most molten heat-resisting and corrosion-resisting steels, of which chromium and nickel are substantial constituents, use appreciable percentages of silicon, hence, in many instances the material obtained by the first process, in which the carbon is eliminated, will be ready for its final use.

An important condition of the process herein described is that it is essentially re ducing and strongly so, whereas processes heretofore employed for reducing the carbon content of ferro alloys are distinctly oxidizing and involve a waste of alloying metal.

' While I have described my process as used in connection with chromium and nickel alloys, it may, of course, be employed in lowering the carbon content of any alloy having the proper ingredients to effect the reduction of SiO by carbon at the relatively low temperatures possible in metal melting practice.

I claim:

1. The process of producing low carbon silicon containing ferro-alloys and incorporating the same into a mass of steel to be alloyed, which consists in subjecting the mass of steel together with the high carbon ferroalloy to a temperature materially in excess of the fusing point of the bath while in the presence of silica, and thereby causing the oxygen -of the silica to combine with carbon in the bath and the silicon to be added to the bath, and to cause ebullition throughout the mass of steel; oxygen from external sources being excluded from the bath during the procedure.

2. The process of reducing the carbon content of a ferro alloy, which consists in heating the ferro alloy to its fusing point and then heating the ferro alloy to a materially greater degree and up to the point at which oxygen reverses its afiinity as between silicon and carbon; the ferro alloy being in contact with silica at least at a time when it is at the last named temperature and for a period sufiicient to efiect the desired degree of reduction in the carbon content, but being substantially free from contact with other oxides during the procedure.

Signed at Chicago Heights, Illinois, this 19th day of May, 1926.

FRANK ALFRED FAHRENWALD.