US2068785A - Method of manufacturing low carbon steel - Google Patents

Description

Jan. 26, 1937. E. c. BAlN ET AL 2,068,735

4 METHOD OF MANUFACTURING LOW CARBUN STEEL Filed Nov. 20, 1953 ATTORNEYS Patent ed Jan. 26, 1937 UNITED STATES PATENT OFFICE METHOD or MANUFACTURING Low CARBON scram.

Application November 20, 1933, Serial No. 698,922

'1 Claim.

This invention relates to metallurgy and more particularly to steel metallurgy and to methods of decarburizing molten steel baths. Still more particularly the present invention relates to a method of decarburizing iron and steel alloys containing one'or more of the strong carbide forming elements, chromium, vanadium, tantalum, manganese, columbium, uranium, titanium and the like.

One of the objects of the present invention is to provide an efllcient method for decarburizing iron and steel alloys containing one or more of the strong carbide forming elements.

Another object of the present invention is to provide a method of decarburizing iron and steel baths to a relatively low carbon content without consequent oxidation of the strong carbide forming elements or other metal constituents of the sa1d alloy. 20. Still another object of the present invention is to provide a method for manufacturing low carbon, iron and steel alloys particularly those alloys containing chro'miumwithout oxidizing the said alloy or constituents thereof.

Other objects and advantages will be apparent as the invention is more fully disclosed.

In accordance with the objects of the present invention, we have devised'a method of decarburizing molten iron and steel alloys comprising a succession of treatment steps applied to the molten alloy, each step employing a controlledatmosphere operating to remove carbon and oxygen from the molten metal bath. As a result thereof, we obtain as a final product an extremely low carbon alloy which is substantially free from metal oxide compounds, the said process being highly practical and economical in operation as compared to methods heretofore devised. As a specific embodiment of the practice of the present invention but not as a limitation thereof we will describe the method as it is applied to the manufacture of low carbon, oxide free iron and steel alloys containing a proportion of the stron carbide forming element chromium. The same 45, sequence of steps comprising the present method as is applicable to chromium-containing iron and steel alloys will be in general equally as applicable 'to other iron and steel alloys with slight modifications within each step thereof to accommodate the method to each specificalloyr Before further disclosing the present invention, reference should be made to the accompanying drawing, wherein ,i

The figure is a sectional side elevation of a furnace useful in the practice of the present invention with the associated apparatus used therewith schematically indicated.

The method of the present invention substantially comprises a series of successively applied steps or operations, each operation employing a 6 different and controlled atmosphere. The process involves (I) the maintenance of the metal bath in a molten state throughout the interval of application of the series. of treatment steps thereto preferably by means of induced elecl0 trical currents, the said induction heating means being designed to give relatively high turbulence tothe molten metal of the bath so that all parts of the bath are recurrently brought to the surface for exposure to the controlled atmosphere over the said surface; (2) the maintenance over' the surface of the metal bath of a succession of controlled atmospheres whereby the progressive removal of carbon from the'bath to the low percentage desired is effected; (3) the addition .to the metal bath of a carbon-reducible oxidiz- I ing material to facilitate the removal of carbon from the bath; (4) the application thereto of a final atmosphere 'effective to remove the final residues of carbon and oxygen from the bath; and (5) the degasification of the metal bath before casting and solidification.

As the broad idea is applied to iron and steel alloys containing chromium, the steps of the method of the present invention comprises- (1) A treatment of the molten metal bath in an air atmosphere wherein the carbon content is oxidized to a low'level;

(2) A treatmeritof the molten metal bath in a reducing gas atmosphere in the presence of an oxidizing material wherein the carbon content is still further reduced; I (3) A treatment of the molten metal bath in substantially pure dry hydrogen wherein the carbon content is reduced to a desired low percentage; and (4) A degasiflcation step before the decarburized and deoxidized molten metal bath is removed from the furnace.

As the broadly defined steps of the present invention is applied to iron and steel 'alloys containing chromium, the said alloy is heated to fusion and placed in an induction furnace A of the general type shown in the figure of the drawing. In this type furnace, the metal bath l is held in refractory lined crucible 2 about which is positioned the transformer core 3 including a plurality of induction coils 4 so positioned about the crucible 2 and electrically connected together as to produce in metal bath l a the art by appropriate analysis of the. gas efliuent from the furnace or by observing the appearhigh turbulence or violent agitation. As the specific furnace structure or induction heating means forms no part of the present invention it need not be further described.

Over the top of the furnace A, we place cover B which is hermetically sealed to furnace A. Furnace cover B is provided with inlet and outlet pipes 5 and 6 respectively, inlet pipe 5 being provided with a plurality of connecting leader pipes 1, 8, 9 and I0 having valves ll, I2, l3 and M, which pipes I, 8, 9 and I0 connect respectively to the atmosphere and to gas storage tanks IS, IS and I1. Outlet pipe 6 is connected through valve it to evacuating means It.

In accordance with the present invention, the molten metal bath l comprised of iron or steel and containing chromium is first subjected to decarburization in an air atmosphere at atmospheric pressures which is obtained by opening valve II (valves I2, I: and I4 remaining closed). A high carbon alloy will rapidly oxidize in air at atmospheric pressures but oxidation of the carbon content thereof will proceed only to the extent where the residual carbon of the alloy approximates an equilibrium percentage which varies somewhat with respect to the temperature employed and the amount of chromium present in the alloy. The chromium present thereafter will tend to oxidize. It is advisable, therefore, to terminate this oxidation step before appreciable amounts of chromium have become oxidized. The precise time of termination of this step may be readily ascertained by one skilled in ance of oxide on the bath surface. The evacuating means It is not employed in this step as a means to maintain a reduced pressure in furnace A but is utilized as a means for maintaining a circulation of air at atmospheric pressures through the furnace. Due to the high turbulence in metal bath l, the rate of reaction of the oxidation process is relatively high and a strong free flow of air through furnace A is required.

The low limit of carbon content of the c mium alloy obtainable by the practice of step approximates 30% although dependent in part upon the chromium content of the alloy. With higher chromium, the lower limit of carbon will generally be higher.

In the second step of the present invention, we add to the metal bath through opening 2| provided with sealable cover 2|, a proportion of carbon-reducible "oxidizing material which is preferably comprised of one or more of the metal oxides of iron, chromium, nickel or oxidized ores thereof which are substantially free from associated silica and gangue impurities; We may, however, add to the said oxidizing material a proportion of heretofore known slagging" materials such as fluorspar which will facilitate contacting the said oxidizing material with the molten metal bath. Thereafter, valve II is closed' and the molten metal bath is subjected to a combined decarburizing and deoxidation step atatmospheric pressures utilizing as a controlled atmospher a hydrogenous reducing gas. In this step, valve I2 is opened, thereby permitting a hydrogenous reducing'gas such as water gas, coal gas mixed with blast furnacegas and the like to circulate into furnace l in suflicient volume to substantially maintain the pressure therein' at atmospheric pressures, evacuating means I! being utilized to maintain a strong free flow of such such relative proportions gas through the said furnace A". Alternatively, the gas can be supplied under high pressure suflicient to move it through the furnace in a strong free flow at atmospheric pressure or a little higher. Preferably, the reducing gas employed is comprised of hydrogen, carbon monoxide and nitro' gen. with or without additions thereto of hydrocarbon gases or vapors. The hydrogen and nitrogen content of the reducing gas is preferably in as to be substantially nohexplosive in nature. The carbon monoxide content may vary widely from about 10 to about 40 per cent (by volume) of the hydrogen-nitrogen admixture and the proportion of hydrocarbon vapors or gases added thereto may vary with respect to the specific hydrocarbon gas or vapor employed, the temperature of the bath-and with respect to the composition of the bath. This proportion of hydrocarbon gas or vapor should be below that amount at which deleterious carbon absorption in the metal bath will occur. It is desirable that the reducing gas be substantially free from oxidizing gases such as water vapor, carbon dioxide and the like as well as of hydrogen compounds of sulfur and phosphorus. Otherwise water vapor or carbon dioxide can be com,- pensated by adding an excess of hydrocarbon vapor.

In general, and with most chromium alloys, it is advisable during the circulation of this reducing gas to increase the temperature of the bath to. temperatures above the melting point of the alloy but not in excess of 2100 0., to reduce the absorption of carbon by the bath from the gaseous atmosphere.

This decarburizing and deoxidizing step is continned for a determined time interval which is productive of the result desired. The specific time interval will vary with different alloys and with the degree of deoxidation required. The

- carbon content of the bath by the practice, of

this step may be reduced as low as .08 per cent but this precise per cent is also dependent upon the specific chromium content of the alloy.

The next step, of the present invention, is the flnal decarburizing step and consists in gradually cutting off the volume of the hydrogenous reducing gas of step 3 and in gradually adding thereto substantially pure dry hydrogen until the gas flowing into the furnace consists of substantially puredry hydrogen. The temperature ofthe bath should be maintained at the higher' temperatures of the preceding step during at least the first or initial stages of this hydrogen decarburizing step and thereafter may be dropped to any lower temperatures at which the bath will remain freely fluid. If the chromium content of the bath exceeds -60% or if it be desirable to carry out the process at as low a temperature as possible, a certain amount of hydrocarbon may be added to the pure dry hydrogen the amount of carbon in form of hydrocarbon not exceeding 30 to 50 grams per cubic'meter of hydrogen.

This step of the process is maintained for a time interval required to bring the carbon content of the alloy to as low a per cent as desired onto a low percentage approximating .01 per cent. The precise time interval of treatment will vary with respect to the alloy composition, chromium content and total carbon to be removed (mass of alloy treated). The progress of the step may be followed by analysis of the gas emuent from aoea'ras furnace l or by any other convenient manner as desired.

When the desired low limit of carbon has been obtained in the metal bath l, the hydrogen in furnace A may be displaced by substantially pure dry nitrogen from source I! by closing valve' I3 and opening valve I4. The molten metal bath I may thereafter be degasified by shutting off the nitrogen flow and evacuating the furnace A to a relatively low pressure.

Alternatively, the hydrogen flow may be terminated and the molten bath exposed to the air for a time interval to obtain substantial elimination of excess hydrogen contained therein, in which case it is desirable to add to the molten bath a small proportion of deoxidizing agent such as metallic silicon and the like before casting.

The molten metal thus treated may be poured from furnace A into a ladle, degasified by the addition thereto of gas scavenging elements zirconium, titanium, aluminum, silicon and the like and may be thereafter cast into ingot or other desired shapes. Where a high carbon ferrochromium alloy is decarburized by this process, it may be added directly to a low carbon steel bath to form a desired chromium steel alloy.

From the above descriptionof the method of the present invention, it is apparent that it is adapted to wide use and application in the production of low carbon iron and steel alloys. While asa specific embodiment we have described the application of the method of the present in-' vention to chromium-containing iron and steel alloys, it is apparent that we are not to be limited thereby, as the same sequence of steps of the method described is equally as well applicable to other iron and steel alloys. The only modifications of the process described that is necessary for such other alloys are modifications in the time, temperature and gas compositions in each specific step to adapt the step to produce the desired result of the method as it has herein been described with respect to chromium alloys.

In view thereof, all such modifications and adaptations of the present invention are contemplated as may fall within the scope of the following claims.

What weclaim is:

1. The method of decarburizing, deoxidizing and degasifying molten iron and steel alloys which comprises melting the said alloy and imparting thereto turbulent stirring to recurrently bring all parts of the bath to the surface and thenapplying to the said surface a succession of ents of the bath; second, a reducing gas atmosphere composed principally of hydrogen whereby l the carbon content of the bath is further reduced to a still lower percentage while preventing substantial oxidation of the metal constituents of the bath; thirdly an atmosphere of substantially pure dry hydrogen having a relatively low water vapor content whereby the final carbon content is reduced to an extremely low percentage and the oxygen content of the bath is substantially eliminated; and fourthly, the bath is subjected to heating with turbulent stirring under reduced pressure to remove gases therefrom, the time interval of application of each successive step and the temperature of the molten metal being varied to promote the desired reaction.

2. In the method of claim 1, the addition of carbon reducible solid oxidizing agents to the molten metal bath prior to the said second step of the method.

3. IriQ-tliegmethod of claim 1, the addition to the molten metal bath prior to the application of thesecond atmosphere thereto of'a proportion of solid carbon reducible oxidizing agents and a proportion of fiuxing-materials facilitating the contacting of the said agent with the metal bath.

4. In the method of claim 1, the said succession of atmospheres being supplied in a strong free fiow over the surface of the bath.

5. In the method ofclaim 1, the step of increasing the temperature of the bath to temperatures up to about 2100 C. during the decarburizing 'in the reducing gas atmosphere and during at least the early stages of decarburizing and deoxidizing in substantially pure dry hydrogen, thereafter lowering the temperature of the bath to temperatures at which the bath remains freely fluid.

6. In the method of claim 1, the use of substantially pure dry nitrogen as a flush gas at the conclusion of the hydrogen treating and prior to the final degasification step.

7. In the method of claim 1, the steps of melting and turbulently stirring the molten metal by induced electric currents, said currents inducing vertical rotation of the bath at a plurality of points as well as horizontal rotatory circulation.

- HERBERT GRUBER.

EDGAR C. BAIN.

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