US3516819A - Environmental control process for gaseously removing oxygen from liquid metals - Google Patents

Description

as 9 m STEEL June 23, 1970 PARLEE ETAL 3,516,819

ENVIRONMENTAL CONTROL PROCESS FOR GASEOUSLY REMOVING OXYGEN FROM LIQUID METALS Filed NOV. 25, 1966 LEGEND ppm=PART$ PER MILLION H m ATMOSPHERE SHADED AREA INDICATES on H2O CONTENT D 0.02 f k 0 IO 20 a0 so so a0 I00 Hg m ATMOSPHERE 32 22 20 y 7 28 I 34 U9] INVENTORS NORMAN AD. PARLEE BY WILLIA M E. MAHIN United States Patent Oflice 3,516,819 Patented June 23, 1970 ABSTRACT OF THE DISCLOSURE The use of a hydrogen containing innocuous gas stream to remove dissolved oxygen from molten metal. This can be made feasible only if certain very critical criteria are met. These criteria concern maintaining a rather specific ratio of water vapor concentration in the innocuous atmosphere to the percentage of hydrogen present and lowering the hydrogen vapor. content of the circulating gas stream in the final stages to remove dissolved hydrogen from the molten metal.

The metal industry and the steel industry in particular have long felt a need for improved methods of lowering the oxygen content of molten metal to reduce the oxygen content of the final solidified metal product. The steel industry has been quite concerned with this problem particularly in connection with low carbon steels. One of the problems with the lower carbon steels is that they have a correspondingly higher oxygen content which can lead to a seamy product. Other problems involving the internal soundness and cleanliness of the steel are also related to the high oxygen content. With increased interest in the application of continuous casting techniques to low carbon steels, these problems have become increasingly important. The use of hydrogen gas to remove oxygen from molten metal has been tried many times but has never become a significant commercial process. Other approaches have been preferred, for example vacuum degassing or the use of slag forming metals and minerals. It has now been discovered that hydrogen deoxidation can be successfully used if certain critical conditions are met.

Gaseous deoxidation processes have a number of advantages over other methods of deoxidation such as by use of slag forming metals and minerals. Where iron or steel is being deoxidized, the use of a gaseous atmosphere can protect the iron from further oxidation and loss as iron oxide while various other processes are being carried out. As used herein the terms environmental control processes refers to a family of processes for improved metal making in a controlled innocuous atmosphere environment. The gaseous process substitutes cheaper gases as deoxidizers for more expensive deoxidizing metals. The gaseous deoxidation in environmental control processes can prevent loss of valuable alloying elements, such as manganese, silicon, chromium, etc. Gaseous deoxidation makes possible the reduction or substantial elimination of losses of deoxidizers, like aluminum, which losses can be extremely heavy when operations are done in the open atmosphere. Gaseous deoxidation processes can replace vacuum processes which require heavy vessels and cumbersome equipment and which are generally only adaptable to batch processes. The use of gaseous processes can make continuous or near continuous steel making possible. The gaseous deoxidation processes can be tied in very nicely with continuous casting to result in an overall system. Gaseous deoxidation systems can greatly reduce internal inclusions and surface defects and surface conditioning of the metal because they leave no liquid or solid deoxidation products behind. In summation, it may be said that gaseous deoxidation makes it possible to take ordinary steel and through the use of environmental control processing systems, to convert it into almost any other kind of alloy or special steel desired with near perfect control of composition and steel cleanliness.

Removal of impurities from metal by gaseous processes may involve gas-liquid metal interactions and reactions of three types. The first of these is simple evaporation. An example of simple evaporation is the removal of lead from liquid steel where the dissolved lead simply volatilizes and forms gaseous lead. Pure lead at steel making temperatures of 1600" C. and steel saturated with lead (approximately 0.25%) have a gaseous lead vapor pressure of about 0.5 atmosphere. Either evacuation or sweeping the lead vapor away from the surface of the stirred bath with another gas can reduce the lead to a low level if the operation is carried on long enough.

Another method of removal of impurities from metal by gaseous processing involves gas-metal reactions promoted by simply lowering the partial pressure over the bath. These reactions differ from evaporation in that they are chemical reactions, i.e., they involve the making or breaking of chemical bonds. Some of these are simple reactions: two atoms of dissolved hydrogen (H) will form one molecule of gaseous hydrogen (H (g)). Two atoms of dissolved nitrogen (N) will form one molecule of gaseous nitrogen (N (g)). The direction and possible extent of the reactions are determined at any temperature, for example 1600 C., by the equilibrium constants. Their equilibrium constants are such that the percent dissolved hydrogen or percent dissolved nitrogen at equilibrium is controlled by the square root of the partial pressure of the hydrogen or nitrogen. Thus, the levels of dissolved hydrogen or dissolved nitrogen in the metal can be controlled by vacuum or by sweeping an innocuous gas over the metal which causes the reaction to tend to go to completion.

It is theoretically possible to remove oxygen directly from steel by the reaction of the dissolved oxygen forming gaseous oxygen (2O=O (g)). However, since the partial pressure of oxygen gas in equilibrium with oxygen saturated steel (0.23% O at 1600 C.) is about 10* atmospheres, it is not practical to remove it directly by vacuum or by sweeping with a noble gas.

Other important reactions and the equations controlling their equilibria in low carbon (not more than about 0.15% carbon by weight) steels are:

Thus vacuum or noble gas sweeping over the metal can remove C and O by the first two of these reactions and H and 0 by the third. Removal of both components simultaneously to low levels can only occur if proper stoichiometric ratios are present.

The third method involves gas-metal reactions using gaseous reagents. In this method, gaseous reducing agents, such as hydrogen, can be added to remove oxygen through the reaction: H (g)+O=H O(g).

From the foregoing, it will be realized that there are quite a number of reactions and combinations of reactions that may be used to develop practical processes for removal of oxygen. However, for a process to be commercial, it must be reasonably fast. Some kinetic factors promoting rapid reactions are summarized as follows:

(A) Freedom from floating slag layers which would effectively prevent gas-metal contact.

(B) Rapid removal of the product gas from the gas-metal surface.

(C) The rapid feeding of fresh reacting gasesto the gasmetal interface.

(D) The rapid presentation of new metal surface to the reacting gasmetal surface.

(E) The rapid removal of surface-active impurity atoms, that is, atoms which tend to concentrate at the surface and thus interfere with the escape of other atoms from the surface.

The first of these factors is achieved in environmental control and in some vacuum processes. The second is achieved in vacuum processes by good vacuum and is achieved in sweeping and flushing processes by rapid movement of the product gas away from the surface. The third may be achieved by rapidly feeding to the metal a gas with a properly controlled ratio of reactants to products at a high concentration of reactants. This requires efiicient expulsion or stripping or gettering of the product gas and rapid movement of the reacting gas onto or into the metal. The fourth, that is rapid presentation of new metal surface, is extremely important and, in many processes, is the condition most diflicult to obtain. It is likely to be relatively easy to provide for rapid removal of product gas and for feeding fresh gas. Thus, the rate at which the impurity to be removed can get to the surface of the bath may be the controlling factor.

In many gas-metal processes for removal of impurities and where the surface inhibiting factors are absent, the rate of removal can be described by C.,C, 2.303 V a where C C and C are bulk concentrations, surface concentrations and initial concentrations respectively, of the impurity being removed. A is surface area and V is volume of the metal. D is the diffusion coefficient of the impurity in iron, I is time and 6 is the surface mass transfer boundary layer thickness.

, When the supply of fresh reacting gas and the efficiency ofv gas product removal from the surface are good, the

rate of impurity removal from the bath (rate of change of the concentration fraction C -C's) depends on the amount of gas-metal surface, A and the boundary layer thickness 8. The smaller is, the faster the impurity will be removed. Small 6s or small diffusion layer thicknesses bring about fast removal because the rate is often controlled by the diffusion of the impurity through the boundarylayer to the surface. Fast stirring brings about small values of 6. In other words, rapid impurity removal rates are effected mainly by bringing about rapid presentations of new surface to the reacting gasmetal surface.

It should be emphasized that rates of removal of different impurities will-.be different depending upon their diffusion coefficients. Hydrogen is fastest, perhaps ten times faster than carbon. Carbon is intermediate, and then oxygen which is perhaps one-half as fast as carbon to reach the surface where reaction and escape take place. The rapid removal of surface'active impurities such as oxygen and sulfur is important in many cases. However, hydrogen escapes rapidly under almost any surface condition. This is partly because of its small size and partly because it reacts quickly with the surface active impurities oxygen and sulfur. Environmental control processes involving hydrogen sweeping are believed to remove surface active oxygen and allow the reactions including the reaction between dissolved carbon and dissolved oxygen to proceed faster in the direction of dissolved oxygen removal.

The components to be removed from the circulated or cycled gases in the instant deoxidizing process may comprise oxygen, water, carbon monoxide, carbon dioxide, nitrogen, hydrogen sulfide and hydrogen. Some possible gettering or stripping processes only remove one of these gases. Others remove several or possibly all to some degree. When noble gases like argon are involved, the simple expulsion of the mixture containing unwanted gases, such as carbon monoxide from the vessel would in most cases be too expensive. In these cases, stripping of the undesired components and returning the gettered gases with addition of fresh Wanted gas is the preferred method. If pure hydrogen is used in a closed or sealed vessel, then simple expulsion While adding make-up hydrogen can be used. Or, some expulsion of unwanted gases with some added hydrogen and some recirculation to a stripping or gettering system, can be used. The gettering system or stripping system may consist of nothing more than water-cooled coils for condensing water to approximately 10,000 parts per million since pure hydrogen (no argon) even at this level of water can deoxidize steel to a content of under 0.005% by weight dissolved oxygen. Gettering or stripping might conceivably be done in the furnace chamber, as for example. by circulating the gas over water condensing coils at the chamber walls. Or, gettering systems may involve a circulation pump for removing the gas from the chamber and circulating it through gettering or stripping columns of various kinds before returning the treated gas to the furnace.

Gettering or stripping of water may be accomplished conveniently by use of materials known in the trade as molecular sieves. Molecular sieves are synthetic crystalline metal alumina-silicates belonging to a class of minerals known as zeolites. Other possible methods may be use calcium oxide or magnesium oxide at appropriate temperatures.

Hydrogen treatment with water stripping or gettering involves the following bath reactions:

Hydrogen is added to the gettered gas as needed. This system is very simple. Even more important, hydrogen is very reactive and diifuses rapidly making for rapid reaction as far as allowed by the stirring and contact conditions and the ratio of hydrogen to water that is maintained in the gas in contact with the metal. It is believed to react with and pick off surface oxygen atoms which under appropriate conditions encourages the fourth of the above reactions. Extremely rapid reducing conditions can be set up by using very high percentages of or even essentially hydrogen. Actually, as the percentage of hydrogen increases, the degree of water stripping or gettering becomes less important and simple water cooling condensing coils can be used.

The equilibrium relationship between hydrogen content in the gas and oxygen dissolved in the steel is shown in Table I.

TABLE I.EQUILIBRIUM LEVEL OF OXYGEN DISSOLVED IN IRON AT l,600 C. AND ONE ATMOSPHERE AT DIFFER- OF DEWPOINT AND HYDRO GEN CONTENT Dewpoint +5 C. 7 C. 15 C. -50 C. 71.6 C.

Percent H2O (Volume) 0. 86 0. 33 0. 16 0. 004 0. 0002 (Volume) Percent (weight) 0 Dissolved 1 Very low.

Further data extending the effects of hydrogen up to essentially 100% by volume have been summarized in a family of curves in FIG. 1. The data shown in Table I and in FIG. 1 show the effectiveness of hydrogen in removing oxygen from steel. They also show that the importance of gettering or stripping efficiency is lessened as hydrogen levels in the atmosphere increase up to essentially 100% so long as the water vapor concentration is controlled. At very high percentages of hydrogen, it is anticipated that effective gettering or stripping can be done with simple water coolant coils to reduce oxygen in the steel to a relatively low value of perhaps 0.005% or less. Then a still lower oxygen level can be reached for the final step by efficient water gettering such as by using a molecular sieve. column. It is interesting to note the instantaneous level of water in the atmosphere which is in equilibrium with 0.065% by weight oxygen in the steel at several levels of hydrogen in the atmosphere as shown in Table II below.

TABLE II Atmosphere in equilibrium with 0.065% dissolved oxygen in steel at l600 C.

Percent (vol.) hydrogen: Percent (vol) water Here again, the advantage of high hydrogen contents with respect to simplicity of gettering is quite evident. Generally speaking the speed with which oxygen can be removed will be roughly proportional to the hydrogen content at a fixed rate of hydrogen to water. This is a further advantage of use of essentially 100% hydrogen as the initial gaseous reductaut.

The harmful effects of hydrogen, such as embrittlement or bubbles formed during solidification, can be avoided by reducing the hydrogen level in the atmosphere above the melt and allowing the excess hydrogen to escape from the melt before pouring the ingot or slab.

In using rich concentrates of hydrogen in contact with liquid metals there is always a possibility that dissolved hydrogen may cause undesirable side effects such as brittleness of the solid metal or undesirable discontinuities such as cracks or bubbles. However it is well known that for each metal there is a critical level of dissolved hydrogen which causes such serious effects but below which level of dissolved hydrogen there may be no harmful effects whatsoever.

The level of dissolved hydrogen that can be considered harmless in liquid steel may depend also on the chemical nature of the steel and on the size of the body into which it is cast. It has been known for some years that large steel forgings made of certain grades of alloy steels are particularly prone to brittleness or cracking near the center especially as the size of these forgings increases to several feet in diameter. Although hydrogen is known to diffuse rather rapidly through solid steel, the hydrogen tends to concentrate in the center of a large ingot during the freezing process and since it would have to diffuse considerable distances to reach the outside, such large masses have to be cooled exceedingly slowly through certain critical ranges of temperature and even then some brittleness or cracking may result.

In the case of low carbon steels to be cast in slabs of, say, not over 10 inches in thickness which are to be subsequently rolled into sheets or relatively thin plates the problem of hydrogen concentration that can be tolerated without any harmful effects is principally one of avoiding bubble formation during freezing. Although bubbles in cast slabs or ingots which are produced by a reducing gas such as hydrogen or carbon monoxide are known to be harmless so far as the rolled product is concerned providing that they are buried relatively deeply inside a solid gas-tight shell of steel, still it may be desired to avoid the formation of such bubbles. In that case the critical limit of hydrogen concentration in liquid steel would be the maximum concentration that would go into solution in the solid steel at the freezing point. Since low carbon steels freeze in the form of delta iron in which hydrogen has a solubility of about 0.0007% by weight, this would indicate that probably the liquid steel could contain up to a like amount just before freezing without forming bubbles to any degree. Table III shows how the equilibrium level of hydrogen dissolved in Fe depends upon concentration of hydrogen in the gas atmosphere and is independent of the dewpoint. It would appear that in most instances a concentration of hydrogen in the gas of 3.2% by volume or perhaps as much as 4% by volume, might be tolerated without forming bubbles. As shown in Table III, even lower Concentrations of hydrogen in the metal can be attained simply by further lowering the concentration of hydrogen in the gas atmosphere.

TABLE III.EQUILIB RIUM LEVEL OF HYDRO GEN DIS- SOLVED IN Fe Al 1,600 C. AND ONE ATMOSPHERE TOTAL PRESSURE AT DIFFERENT LEVELS OF DEWPOINT AND HYDROGEN CONTENT OF ARGON Since hydrogen diffuses extremely rapidly into and out of liquid steel, a two-step process may consist of initial deoxidation with up to hydrogen and simple water stripping by use of water cooled coils followed by a change in the atmosphere to not more than about 4% by volume hydrogen for example by transferring the metal to a second vessel with stripping of water being carried out to very low dewpoint levels for example, with molecular sieives and, if necessary, stripping of hydrogen to a suitable low level. During the period of exposure to the low hydrogen concentrations some aluminum may be added to complete the deoxidation and provide non-aging and grain-control effects which are well recognized as further benefits of the use of aluminum in steels. During the first step make-up hydrogen may be added not only to replace that consumed by formation of water through reaction with oxygen in the liquid steel but also to furnish the excess gas needed for some expulsion. The principal purpose of expulsion would be for removal of excess carbon monoxide which otherwise would accumulate and if the amount were excessive could slow down or even stop the reaction by the dissolved carbon with dissolved oxygen.

Some gettering or stripping of hydrogen from the system gas in the second step obviously would be required as the hydrogen level tends to build up above, say, 4% in the atmosphere. This may readily be accomplished for example by circulating some of the system innocuous gas over a hot catalyst after adding a small amount of pure oxygen. Through this means hydrogen is converted to Water and any carbon monoxide present to carbon dioxide. The gases would then be passed through gettering or stripping columns for removal of the water and the carbon dioxide and return of the purified innocuous gas to the system.

The question of explosion hazards must be considered. Explosion diaphragms and other safety features to protect personnel as well as equipment should be provided. However, hydrogen has been used for years in large scale metallurgical operations, such as in annealing furnaces for silicon steels in the electrical industry. One of the safety principles that has been used is to introduce all hydrogen into the system in such a way that it is immediately heated and reacts with any oxygen present.

Other objects and advantages of this invention will be apparent from the following description taken in conjunction With the accompanying drawings.

This invention relates to a process for removing oxygen impurities from molten metal. The process comprises providing a body of molten metal in a chamber maintained above the melting point of the metal. A hydrogen containing innocuous gas is passed into contact with the surface of the metal so as to maintain an atmosphere in the chamber substantially free of undesired constituents. The hydrogen vapor reacts with the oxygen in'the metal as it circulates into sweeping contact with the surface of the molten metal and forms water vapor. The water vapor is stripped from the gas stream so as to establish a water vapor concentration in the innocuous atmosphere at a ratio to the percent hydrogen of not more than 0.14, preferably not more than 0.10. In this way, metal substantially free of undesired oxygen impurities is produced. The desired oxygen level may vary somewhat as the car bon content varies. When the oxygen content of the molten metal has been reduced to the desired level, the hydrogen content of the circulating gas stream is lowered for example by transferring the metal to a second vessel, so as to remove hydrogen gas from the molten metal until the hydrogen content of the metal is at the desired level. Preferably, the initial innocuous gas can be substantially 100% hydrogen. This process, although useful for removing oxygen and impurities from metal in general, is particularly applicable to ferrous metals, more specifically, steels.

From time to time some of the gas containing undesired gaseous constituents, for example carbon monoxide may be expelled by an addition of a corresponding volume of excess hydrogen. When reducing the hydrogen vapor content of the circulating gas stream, it has been found that if the hydrogen vapor content is lowered to not more than about 4% by volume hydrogen and the water vapor concentration in the circulating gas stream is lowered to not more than 1,000 parts per million, the hydrogen gas will be removed from the molten steel until the hydrogen content of the steel is not more than about 0.0005 by weight and at the same time a suitable low oxygen concentration will be maintained.

If desired, after the treatment with hydrogen containing innocuous gas, the molten metal may be thereafter treated with a metallic deoxidizer to further lower the oxygen content of the molten metal and to provide the usual effects of grain refinement and non-aging characteristics. A suitable metallic deoxidizer is aluminum. When the molten metal has been deoxidized to the desired level, it is passed from below the surface of the metal in the chamber through a passageway wherein it is not exposed to available oxygen and thereafter cast in a mold without being exposed to available oxygen. In this way, metal substantially free of undesired oxygen impurities is produced.

In this specification and the appended claims, the term innocuous gas is defined as a gas or mixture of gases that does not react appreciably to form reaction products that are undesirable in the metal at conditions of use. Some examples of innocuous gases are the noble gases and for steel in the instant process hydrogen and in some instances certain concentration ranges of carbon monoxide and carbon dioxide, and even some mixtures of steam and partly burned natural gas.

FIG. 2 is a schematic diagram of an apparatus arrangement used in testing some of the principles of this invention.

As shown in FIG. 2, a melting furnace 10, induction heated, is provided in a furnace chamber 12 which is sealed from the atmosphere except for gas inlets and discharge lines as will be explained more fully hereinafter. As shown in the figure, the innocuous gas stream can enter furnace chamber 12 in two ways. One way is through line 14 which enters the chamber directly above the melting furnace through a lance. An optical pyrometer, not shown, is used to measure the temperature of the melt in furnace 10. Furnace 10 is electrically induction heated. The alternate method for introduction of innocuous gas into the chamber is via line 16 which enters the furnace chamber 12 at a point removed from melting furnace 10. For the previously mentioned safety considerations this line is only used for part of the recirculated gas. The innocuous gas leaves furnace chamber 12 via line 18. The gaseous atmosphere as it leaves the furnace chamber 12 via line 18 has provision for a vent line 20 for control of buildup of undesired gas stream constituents. The exit gas line 18 then feeds the gas stream into cooler 22 where the temperature of the gas which has been elevated due to the temperature of the molten metal in the furnace chamber 12 is lowered. The cooled gas is pumped from cooler 22 by means of pump 24 through a flow meter 26 to a molecular sieve column 28. Make-up gas is introduced into the gas stream by means of make-up gas line 30. The flow meter is used to control the rate of flow of the cooled exit gas from furnace chamber 12 and molecular sieve column 28 is used to getter or strip the desired quantity of water vapor from the gas stream. The gas stream then passes from molecular sieve column 28 via gas line 32 with provision for instrument line 34 to a junction point with lines 16 and 14. Valve 36 and flow meter 38 are used to control the distribution of the circulating innocuous gas stream between the two gas lines 14 and 16.

Induction heated furnaces stir the metal and this is desirable for the previously mentioned reasons. Some stirring effect can also be obtained by impingement of the gas stream in a slant-wise fashion on the molten metal surface.

The melting stock used in melting furnace 10 was commercial iron containing on the average 0.08% by weight oxygen and 0.02% by weight carbon. In all tests, this material was either sand blasted or machined to remove rust, scale or paint. The innocuous atmospheres used in the tests were comprised of varying proportions of 99.9% pure argon and 99.995% pure hydrogen. The argon was certified to contain not more than 8.5 parts per million water, and the hydrogen was certified to contain less than 10 parts per million water. In all tests, power was turned on in the melting furnace 10 only after oxygen had been removed from furnace chamber 12 due to displacement by adding argon to the circulating innocuous gas stream. The oxygen level was, in all cases, less than 1% in the furnace chamber 12 before the electrical power was turned on in melting furnace 10. After the oxygen had been purged from the furnace chamber 12 and the metal was heated to a red heat the desired quantity of hydrogen for the particular test was introduced in the gas stream and the power was turned up to melt the steel in melting furnace 10 quickly. Temperature of all tests was controlled by an optical pyrometer. The standard temperature for all tests shown herein was 1,650 C. with the exception of test 68 where the temperature was 1,638 C. In those tests wherein an aluminum addition was made, the aluminum was added by plunging a packet of aluminum into the molten melt at the end of a rod of iron of the same composition as that already molten in melting furnace 10. The results of the tests are summarized in Table IV below.

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00 0 .0 5 5 2 5 .0 0000 Z 0 00 3 Z 52 500500 808 0500 208 000 0 20 0 35 0 0 000 0012 000 05 0 wmm-m 0 WW A 0000 0000 8 D 00 a N 11 about 4% by volume thereby lowering the dissolved hydrogen content of the metal to 0.0007 by weight or less.

2. The process of claim 1 wherein the innocuous gas is substantially pure hydrogen.

3. The process of claim 1 wherein the ferrous metal is steel.

4. The process of claim 3 wherein when the oxygen content of the molten steel has been reduced to the desired level, the hydrogen vapor content of the circulating gas stream is lowered to not more than about 4% by volume hydrogen and the water vapor concentration in the circulating gas stream is lowered to not more than 1000 parts per million by volume so as to remove hydrogen gas from the molten steel until the hydrogen content of the steel is not more than about 0.0005 by Weight hydrogen.

5. The process of claim 1 including the following addition step:

(a) thereafter treating the molten metal with a metallic deoxidizer to further lower the oxygen content of the molten metal.

6. The process of claim 5 wherein the metallic deoxidizer is aluminum.

7. The process of claim 5 wherein the further deoxidized molten metal is passed from below the surface of the metal in the chamber through a passageway in which it is not exposed to available oxygen.

8. The process of claim .7 wherein the metal is there after cast in a mold without being exposed to available oxygen.

References Cited UNITED STATES PATENTS RICHARD o. DEAN, Primary Examiner Us. c1. X.R.

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