CA1106621A - Method for producing improved metal castings by pneumatically refining the melt - Google Patents
Method for producing improved metal castings by pneumatically refining the meltInfo
- Publication number
- CA1106621A CA1106621A CA300,003A CA300003A CA1106621A CA 1106621 A CA1106621 A CA 1106621A CA 300003 A CA300003 A CA 300003A CA 1106621 A CA1106621 A CA 1106621A
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- Prior art keywords
- melt
- castings
- gas
- argon
- refining
- Prior art date
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C1/00—Refining of pig-iron; Cast iron
- C21C1/04—Removing impurities other than carbon, phosphorus or sulfur
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/04—Removing impurities by adding a treating agent
- C21C7/068—Decarburising
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/04—Removing impurities by adding a treating agent
- C21C7/068—Decarburising
- C21C7/0685—Decarburising of stainless steel
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Treatment Of Steel In Its Molten State (AREA)
- Manufacture And Refinement Of Metals (AREA)
- Continuous Casting (AREA)
- Heat Treatment Of Strip Materials And Filament Materials (AREA)
- Electrical Discharge Machining, Electrochemical Machining, And Combined Machining (AREA)
Abstract
10.943 METHOD FOR PRODUCING IMPROVED METAL CASTINGS BY PNEUMATICALLY
REFINING THE MELT
ABSTRACT
Castings of superior surface quality and internal quality can be produced by:
(1) transferring the melt from the furnace into a separate refining vessel provided with submerged tuyeres, and (2) refining the melt by (a) injecting into the melt through the tuyeres an oxygen-containing gas which may contain up to 30% of a dilution gas, and (b) thereafter injecting a sparging gas into the melt through the tuyeres.
Preferably, the oxygen-containing gas is surrounded by an annular stream of a protective fluid. Argon is pre-ferred for dilution, protection as well as sparging.
REFINING THE MELT
ABSTRACT
Castings of superior surface quality and internal quality can be produced by:
(1) transferring the melt from the furnace into a separate refining vessel provided with submerged tuyeres, and (2) refining the melt by (a) injecting into the melt through the tuyeres an oxygen-containing gas which may contain up to 30% of a dilution gas, and (b) thereafter injecting a sparging gas into the melt through the tuyeres.
Preferably, the oxygen-containing gas is surrounded by an annular stream of a protective fluid. Argon is pre-ferred for dilution, protection as well as sparging.
Description
lO,g~3 EACKGROUND
This application relates in general to the manu~ac-ture of metal ca3tings, and more particularly to a method for improving the quality of castings by pneumatically refining the melt prior to casting.
Metal articles are generally divided into two product classifications depending on their method of manufac-ture, wrought products and cast products. Wrought products are made by first teeming molten metal into a mold, and then mechanically working or deforming the intermediate product by rolling, drawing, extruding or forging. In contrast, cast products are made without the second step, i.e. without the mechanical deformation of the solidified product. While cast products are generally heat treated, and may also be mechani-cally cleaned9 machined or repaired subsequent to casting, they are not subject to plastic deformation.
This di~ference between a ~rought and a cast product, i.eO the presence or absence of mechanical deforma-tion, is extremely important because it offers the manu-facturer o~ wrou~ht products opportunities to correct oreliminate various defec~s which may have occurred during solidification~ For example, it is well known that while ~olidified ingots of rimmed steel have very good surface characteristics, they contain many small blow holes beneath the surace. Similarly, in most continuously cast steel shapes, there is a center region containing shrinkage poroslty.
Nonetheless, these blow holes and regions of porosity are
This application relates in general to the manu~ac-ture of metal ca3tings, and more particularly to a method for improving the quality of castings by pneumatically refining the melt prior to casting.
Metal articles are generally divided into two product classifications depending on their method of manufac-ture, wrought products and cast products. Wrought products are made by first teeming molten metal into a mold, and then mechanically working or deforming the intermediate product by rolling, drawing, extruding or forging. In contrast, cast products are made without the second step, i.e. without the mechanical deformation of the solidified product. While cast products are generally heat treated, and may also be mechani-cally cleaned9 machined or repaired subsequent to casting, they are not subject to plastic deformation.
This di~ference between a ~rought and a cast product, i.eO the presence or absence of mechanical deforma-tion, is extremely important because it offers the manu-facturer o~ wrou~ht products opportunities to correct oreliminate various defec~s which may have occurred during solidification~ For example, it is well known that while ~olidified ingots of rimmed steel have very good surface characteristics, they contain many small blow holes beneath the surace. Similarly, in most continuously cast steel shapes, there is a center region containing shrinkage poroslty.
Nonetheless, these blow holes and regions of porosity are
2.
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almost entirely eliminated during subsequent rolling, and the final wrought product contains virtually no evidence of the original porosity.
Similarly~ surace defects in ingots, slabs and billets are not a problem to the producer of wrought products, because these are intermediate products which undergo con-siderable mechanical reworking and plastic deformation prior to shipment. Furthermore, when surface defects occur, they can readily be removed by grinding or scarfing before further mechanical processing. In contrast, the surface quality of castings is very important because castings are a final product and any defect must be removed by costly and time consuming manual grinding, gauging or chipping. Then the cavity so caused must be rebuilt by welding or overlaying of metal. In addition, surface repair may diminish the dimension-al accuracy and mechanical properties o~ the cas~ing.
It is evident, therefore, that since ingots, slabs and billets are intermediate products, certain surface and internal defects can be tolerated in them, while in castings such defects cannot, because castings are poured directly into their final shape.
The metal ~ounding industry has long been plagued with a number of dificult problems caused by unsatisfactory castings. These problems are due both to surface defects and to internal defects. While many surface deects can be remedied by the costly inishing operations mentioned above, internally defective castings frequently have to be scrapped, remelted and , lO,g4~
cast over. Some of the common surface flaws in castings in-clude: hot tears, surface cracks, rough surface, and holes ranging in size from pinholes to gross blow holes. In general, the ultimate causes of these defects are not well understood.
Consequently, melting and casting practices to produce satis-factory castings require a large amount of experience and empirical evaluation. Internal defects are due mainly to porosity and inclusions which adversely effect the mechanical properties of castings, i.e. its strength, ductility, tough-ness and impact resistance. The above-mentioned defects, as well as others such as embrittlement, age-hardening and the presence of fish-eyes or white spots, are believed to be related to the presence of uncontrolled amounts of oxygen, nitrogen, hydrogen, phosphorous and sulfur in ~he melt.
Consequently, it has long been an objective of the foundry industry to produce sound cas~ings with low or controlled levels of these five elements. In the production of stainless castings, where corrosion resistance is of paramount impor-tance, it is often an additional objective to produce sound castings with low carbon levels.
Casting defects are conventionally remedied during the so called finishing operations. Most of these operations are highly labor intensive and consequently very costly. In addition, much of the finishing consists of grinding which causes dust that can be harmful to health. Some castings, however, cannot be repaired because the critical application for the part does nat allow it. In such case, the defective 0 3 g43 ~ 6 ~ ~ ~
casting must be scrapped. Consequently, the foundry art has long sought a method which would improve castings both in terms of their surface quality and physical properties.
Various techniques have been used in the foundry art to refine melts prior to casting in order to improve the quality of the resultant castings. The final stage of melt-ing often includes some form of purification or refining treat-ment intended to influence the microstructure and cleanliness of the casting. Such treatments usually involve the blowing of ga~ses or the addition of certain reagents to the furnace or transfer ladle. These treatments may include decarburiza-tion, dephosphorization, deoxidation, desulfurization and degassing.
Prior to the present invention, decarburization of molten steel for ca~tings, was generally accomplished by blowing oxygen into the melt through a consumable lance inserted through an opening in the furnace. This technique of decar burization is, in the irst place, dangerous to the operator because it exposes him to hot metal and sparks, and because the operator usu~lly holds the lance which is in itself hazardous. Secondly, this technique of decarburization is often inaccurate because all the oxygen does not always react with the bath. Hence, it is often necessary to reblow the molten steel because insufficient carbon was removed initially.
Lastly, such prior art methods of decarburization tend to generate a great deal of fume and smoke which is hazardous to health and damaging to the environment.
lO,g43 ~ ~ 6 ~ 2~
Because the presence of oxygen is known to be detrimental to the properties of the castings, foundries generally deoxidize the molten me~al prior to pouring. In addition, deoxidation is generally required to prevent the formation of blow holes during solidification. This is normally accomplished by the addition of well-known deoxidants such as silicon or aluminum, and also by the addition of special deoxidants, such as "Calcibar" and "Hypercal." The attainment of a well deoxidized melt prior to casting is essential for the production of sound, tough castings.
Desulfurization of molten steel for castings, prior to this invention, has generally been accomplished by the ormation of basic slags in the furnace, i.e. slags containing a high ratio of lime to silica or lime to alumina, and by subsequently mixing the slags with well deoxidized metal.
Equilibrium between the slag and the metal causes the sulfur to be transerred from the metal to the slag. This process is very slow, often requiring several hours, particularly when very low (i.e. under .005%) sulfur is desired. Indeed, it i8 o~ten necessary to remove the slag and to produce a new ; one. Sometimes this step has to be repeated several times in order to reach the desired low level of sulfur. T;liS process is very laborious and time consuming, and unnecessarily exposes the furnace operators to molten metal and to unhealthy fumes.
An alterna~ive, and much more costly desulfurization technique is to add expensive sulfur scavenging elemen~s, such as calcium, magnesium or the rare earth elements, to the furnace l~,g~3 immediately prior to tapping or to the transfer ladle. The expense of this technique, as well as its non-reproducibility, mitigates against its general use.
Known degassing treatments include vacuum melting, vacuum degassing, as well as degassing by bubbling scavenging gases such as argon through the melt. While argon degassing in the ladle prior ~o casting can improve the quality of castings by lowering the hydrogen and oxygen content of the melt, it does not remove all impurities or achieve low hydro-gen levels in the limited time available. Because the timeavailable for degassing is strictly limited by heat loss from the degassing vessel, it has been found that it is not possible to lower the dissolved gas content sufficiently for many applications. Furthermore, degassing by itself does not re-move sulfur and may necessitate reheating the melt in order to obtain sufficient fluidity or casting.
Prior to the present invention, therefore, the foundry art utilized the above-described techniques in an effort to produce deect-free castings. However9 these prior art tech~iques are expensive, often inaccurate or non-reproduc-ible, time-consuming, generally hazardous to the health of the operators, and by-and-large inadequate to the needs o the industry. Cansequently, e~tensive post-solidification repair of castings is usually still required. In fact, in castings, for example, destined for nuclear applications, the cost of inspection and repair often exceeds the material value of the - castings themselves.
During the past twenty five years, the manufacturers 7.
l~,g~3 of wrought steel products have made large gains in upgrading their molten metal processing tecnniques through the adoption of one of several now well known refining processes such as the BOF, AOD, OB~ or Q-BOP and LWS processes. U.S. Patents illustrative of these processes, respectively, are Nos.: -2,800,631; 3,252,790; 3,706,549; 3,930,843 and 3,844 768.
The production of wrought steels containing controlled levels of carbon, phosphorous, sulfur, oxygen, nitrogen and hydrogen is now readily and economically achievable through judicious selection of one, or a combination of more than one, of the above yrocesses. In the foundry or cast metal industry, however, comparable advances have been absent. While the industry has, at various times, produced products with low or controlled levels of one or perhaps two of the above six elements, the manufacture of castings with low or controlled levels of all six elements has hitherto not been possible, and consequently, the value or advantages of being able to control all six elements have hitherto not been known.
The pneumatic ~reatment of molten stainless steel for the production of wrought steel by the simultaneous injec-tion of argon and oxygen into the melt~ commonly referred to as the AOD process, has achieved wide commercial acceptance in stainless steel mills for the manufacture of wrought products. The basic AOD refining process is disclosed by Krivsky in UOS~ Patent No. 3 9 752,793. An improvement on Krivsky relating to the programmed blowing of the gases is : disclosed in Nelson et al, U.S. Patent No. 3,046,107. The use of nitrogen in combination with argon and oxygen to lO,g4 ~ ~ 6 6Zl achieve predetermined nitrogen contents is disclosed in Saccomano et al in U.S. Patent No. 3,754,894. A modification of the AOD process is also shown by Johnsson et al in U.S.
Patent NoO 3,867,135 which utilizes steam or ammonia in combination with oxygen to refine molten me~al.
It is worthy of note that none of the above-men-tioned pneumatic melt refining techniques have, prior to this invention, been used by the foundry art for the production of castings.
OBJECTS
_ It is an object of the present invention to improve the surface quality, internal quality and physical properties of castings.
It is another object of the present invention to improve the method of producing castings by pneumatically refining the melt prior to casting.
It is still another object of this invention to increase the yield of acceptable castings.
SJMMAR~
It has now been discovered that by pneumatically refining the melt in a separate vessel prior to casting, castings of a quality superior to that heretofore obtainable can be produced. Such castings have unexpectedly superior surface quality, internal quality and physical properties.
The above, and other objects which will be apparent to those skilled in the art are achieved by the present invention which comprises:
10,943 In a process for producing metal castings having improved surface quality and internal quality by melting selected charge materials in a furnace, teeming the melt into a mold, permitting the melt to solidify in the mold, and removing the casting from the mold, the improvement com-prising:
(1) transferring the melt from the melting furnace into a refining ves~el provided with at least one submerged tuyere, and 10 - (2) refining said melt by (a) injecting into the melt through said tuyere(s) an oxygen-containing gas con- -taining up to 90% of a dilution gas, and (b) thereafter injecting a ~sparging gas into the melt through said tuyere(s).
Preferably, the oxygen-containing gas stream is surrounded by an annular stream of protective fluid.
The term "refining" as used in the present specifi-cation and claims is meant to include any one or more oE the following e~fects: decarburization, dephosphorization, de-sulfurization~ degassing, deoxidation, gaseous alloying, impurity oxidation, impurity volatilization, slag reduction and flotation and homogenization of non-metallic impurities.
The present invention is applicable to refining of any iron, cobalt or nickel based alloy, and the term "metal" is used in that sense.
The term "dilution gas" as used herein is intended to mean one or more gases that are added to the oxygen stream for the purpose of reducing the partial pressure of the 10 .
10,443 ~ 2 ~
carbon monoxide in the gas bubbles formed during decarburiza-tion of the melt, and/or for the purpose of altering the feed rate of oxygen to the melt without substantially altering the total injected gas flow rate. Suitable dilution gases include: argon, helium, hydrogen, nitrogen, carbon monoxide, carbon dioxide, steam and hydrocarbon gases, for example, methane, ethane~ propane and natural gas. Argon is the most preferred dilution gas.
The term "protective fluid" as used herein is meant to include one or more fluids which surround the oxygen con-taining gas and protect the tuyere and surrounding refractory lining from e~cessive wear. Suitable protective fluids in-clude: argon, helium, nitrogen, hydrogen, carbon monoxide, carbon dio~ide, hydrocarbon fluids (gas or liquid) and steam.
Methane, ethane, propane or natural gas are suitable hydro-carbon gases. No. 2 diesel oil is a suitable hydrocarbon liquid. Argon is the most preferred protective fluid.
The term "sparging gas" as used herein is intended to mean one or more gases which remove thos~ impurities from ~he melt by volatilization or transfer to the slag by entrap-ment or reaction with the slag. Suit~ble sparging gases include- argon, helium, nitrogen and steam. Argon is also the preferred sparging gas.
Castings ha~ting improved surface quality are defined as castings which when compared to the prior art require re-duced rleaning, grinding, chippinga welding or other repair.
Such improved surface quality can be evidenced by a reduced 11, 10,94 ~ 2 ~
level of defects determined during dye penetrant or magna-flux testing.
Castings having improved internal quality are de-fined as castings which when compared to the prior art dis-play one or more of the following characteristics: a lower level of inclusions, finer as-cast grain size, reduced internal porosity, reduced tendency for hydrogen flaking during machin-ing, reduced evidence of deects when inspected by X-ray techniques or better physical properties such as toughness.
THE DRAWING
Figure 1 represents a cross-sectional view of a preerred refining vessel or converter for use in carrying out the process of the present invention.
DETAILED DESCRIPTION
It was expected that utilization of pneumatic refin-ing for the treatment of steel melts for castings would pro-duce most of the chemical benefits obtained by refining molten steel for the production of wrought steel products. In particular, it was expected that some improved internal q1~ality would be obtained by better deoxidation of thP melt, by better separation of deoxidation products, and by the attainment of lower sulfur levels and Lower hydrogen content.
However, it was unexpectedly discovered that pneumatic refining in accoxdance with this invention produces improvements in the surace quality of the castings beyond any expectations, that it produces castings with greatly improved stxength, ; ductility and toughness, and that it makes possible the pro-duction of castings of far superior quality than previously 10,94 ~ 2 ~
possible from low alloy steels and carbon steels.
As a result of the present invention, foundries are now able to cast with significantly increased assurance of obtainlng satisfactory castings, as well as of obtaining castings of higher quality. More specifically, the surface quality of the resultant castings have fewer cracks and re-duced hot tears. In addition7 it has been found that use of the present invention produces a smoother casting surface, believed to result from reduced interaction of the sand mold with the melt. It has also been found that the physical properties of the castings have been unexpectedly improved.
The improvements are believed to be related to the lower levels of inclusions, lower hydrogen flaking, as well as lower porosity found in castings made in accordance with this in-vention. Molten steel treated in accordance with the present invention has a higher flowability or fluidity at the same temperature than untreated metal, resulting in superior castings, since the metal will ~low into smaller and more intricate crevices than unrefined melt. Alternatively, the same fluidity may be achieved at a lower casting temperature.
This again contributes to improved casting surface quality.
The pneumatic refining treatment of the present invention may be advantageously employed on any type of iron or steel melt, and also on cobalt and nickel alloys, normally used for the manufacture of metal castings. It has, however, been found to be particularly beneficial in the treatment of ferritic and austenitic stainless steels, low alloy steels and carbon s~eels. Special benefits are obtained in castings made ; 13.
10, g~
6Z~
steels such as WC~ and HY80 which are sensitive to hydrogen flaking as well as hot tearing. High strengt'n steels such as HY130 which normally require extensive chipping, grinding and welding in order to repair as-cast defects, are significantly improved by the present invention, resulting in considerable finishing cost savings. Austenitic stainless grades such as CN7M, CH20, CK20, 310L, and 347L, which, prior to the present invention, were extremely difficult to cast without cracking or microfissuring, can now by means of the present invention, be readily cast without fear of cracking.
The advantages of the present invention while appli-cable to small, simple castings as well as to complex or large ones, are of particular significance when producing high quality castings such as required, for example, ~or pumps and turbines used in the aircraft, shipbuilding and nuclear power industries.
In addition to the unexpected results of the present invention described above, other benefits resulting from use of the present invention include raw material savings due to minimized oxidation of molten metal and the ability to use lower grade charge materials. Increased production also results from greater accuracy in achieving desired aim melt chemistries and fewer rejects due to improved casting quality.
In practicing the present invention, melting of the charge materials may be accomplished by any means known in the art. The most common foundr~ melting furnaces include ~uel fired ~urnaces of the hearth or crucible type, as well as electric furnaces of the resistance, induction or arc type.
The last two are preferred. Following melting of the charge 1~ .
~ 43 Z~
ma~erials, the melt is transferred by a ladle or otherY7ise poured into the pneumatic coverter shown in Figure 1.
Figure 1 is a cross-sectional view of a preferred refining vessel 1 for use in practicing the present invention.
Vessel 1 comprises an outer steel shell 2, removably attached to a trunion ring 3. The trunion ring and consequently the vessel is tiltable by being fixedly attached by drive means (not shown), in order to facilitate charging, sampling, slag removal and tapping. Shell 2 is lined with basic refractory bricks 4. A removable shell arrangement is preferred, since several shells are necessary to maintain uninterrupted opera-tions. While one shell is in use, the spare or spares are being relined. A horizontally disposed concentric tube tuyere 5 is located in the side-wall of the vessel near the bottom of the vessel for injection of the fluids. If desired, the tuyeres can be located in the bottom of the vessel in place of or in addition to the sides. Preferably, however, at least two tuyeres are used, and positioned in the side~wall of the vessel, near the bot~om and horizontally disposed in such manner as to be asymmetric. That is, no two tuyeres should be positio~ed so that their axes, and consequently the fluid streams injected diametrically opposed to each other.
Asymmetric positioning of the tuyeres improves mixing of the melt by the injected gases. The tuyere 5 consists of an inner tube 6 and a conce~tric outer tube 7. Oxygen alone or ad-mixed with a dilution gas is injected through the inner tube 6, and the protective gas is injected through the outer tube 7 of the ~uyere. The latter forms a protective annular .
lO,g43 shroud around the oxygen stream which pro~ects the refractory lining from ra~id deterioration. The pressure of the fluids must be sufficiently great to penetrate into the melt. Prefer-ably, the absolute pressures of the fluids at the tuyere in-lets, of both the central and annular passages, are at least two times greater than the absolute pressures of the ~luids at the outlets.
A detailed description of a suitable vessel and tuyeres for carrying out the present invention is shown by Saccomano and Ellis in U.S. Patent No. 3,703,279. The sparging gas may be injected into the melt either through the same tuyere or tuyeres as used for the oxygen stream or through separate tuyeres; the former is preferred. Preferably, after the oxygen blow is completed, the sparging gas is injected ; through the center passage of the tuyere as well as through the annular passage in order to prevent molten metal from flowing back into the tuyere where it would ~reeze.
In general, the molten metal refining step of the present process is carried out by injecting oxygen and a dilu-tion gas, as well as a protective fluid (both of which may beargon) into the melt through the submerged tuyeres. The decar-burization, i.e. the reaction o~ the injected oxygen with car-bon in the melt, produces controlled oxidation o~ the bath components) as well as heat which maintains bath temperature.
The melt is initially blown with a high ratio o oxygen to dilution and protec~ive gases. Depending on the steel composi-tion being re~ined, as the carbon content of the melt decreases, the ratio of oxygen to dilution gas and protective fluid may 16.
10,943 be lowered, generally in several steps, in order to maintain ~avorable thermodynamic conditions throughout the blow.
Since the oxygen and other gases are introduced below the level of the melt and at high velocity, excellent mixing takes place within the melt and intimate gas-metal and slag-metal contact occurs. As a result, the reaction kinetics of all chemical processes which take place within the vessel are greatly improved. This permits desulfuriza-tion to very low levels (under 0.005%) generally, in less than ten minutes o~ blowing and without addition of expensive desulfurizing agents such as calcium, magnesium or rare earths. Dephosphorization of alloys containing less than approximately one percent chromium can readily be achieved by decarburizing the bath to below 0.1% carbon by using a gas mixture containing at least 75% oxygen. The phosphorous bearing slag so formed must then be decanted prior to blowing with a sparging gas or adding any reducing agents, deoxidants, or ~esul~urizing agents.
Other major benefits o~ the inven~ion are very close control o~ the end point carbon and very low residual values of oxygen9 nitrogen and hydrogen. Typical residual values ~or these three elements o~tained by practicing the inven-tion are shown in Table I.
TABLE I
Stainless Steel Low Alloy Steel .T . :e-- 7 :.
Oxygen 40-70 ppm 20-50 ppm Hydrogen2-4 ppm 1-3 ppm Nitrogen150-200 ppm 20-50 ppm 17.
lO,g43 In addition, lead, and zinc in the melt are reduced to levels that are metallurgically harmless.
The synergistic results obtained by the present invention, i.e. low gas content (oxygen, nitrogen and hydro-gen) together with low sulfur and increased fluidity of the melt have combined to produce castings of unprecedented sur-face quality, internal cleanliness and improved mechanical properties. Table II below compares the chemical and physical properties of two castings of stainless steel grade CA6NM, one made by conventional practice and the other by the present invention with ASTM specification A296.
TABLE II
Chemistry ASTM Spec.
(C/o) A296 Conventional Invention C 0.06 max .05 0.026 Mn 1.00 max .60 0.47 Si 1.00 max .55 0.96 Cr 11.5 - 14.0 12.70 12.81 Ni 3.5 - 4.5 3.80 4O00 Mo 0.40- 1.00 0.50 0.57 S 0.03 max 0.025 0.022 P 0.04 max 0.020 0.025 Mechanical _ Tensile (ksi) 110 min. 115 122.8 Yield (ksi~ 80 min. 100 108.3 Elongation (%) 15 min. 20 21 Red. of area (%) 35 min. 60 67 Impact Strength none 65 77-80 Charpy V-notch (at R.T.3 It can be seen from Table II that the casting made in accordance with the present invention is superior in all respects~ and particularly in impact res1stance. The difference in toughness is even more impressive when one recognizes that in this particular casting the sulfur level 18.
1~,943 ~ 2~
was 0~022~/o rather than the customary value of le~s than O.OlV/5 obtainable with pneumatic refining. In this case no special desulfurizing treatment was employed.
On high strength alloys such as ~-130 an 85%
improvement in impact strength has been obtained on a casting made from HY-130 in accordance with this invention when com-pared to a casting o:E the same alloy made from vacuum de-gassed metal. Such high impact strength far exceeds any previously obtained impact strength on castings made from this alloy.
Example 1 An electric arc furnace was charged with 6290 lbs.
of HY-80 scrap, 5869 lbs. of mild steel scrap and 300 lbs. of lime. Power was applied to the electrodes and the charge was melted in approximately one hour. Following melt down, the composition was adjusted, in accordance with conventional practice, to have the furnace tap composition shown below, and a temperature of about 3100F.
The above melt was tapped from the arc furnace into - 20 a transfer ladle, and then charged into the refining vessel.
500 lbs. of lime, 100 lbs~ of MgG and 60 lbs. of aluminum were added to the charge. At the start of the pneumatic refining period the temperature of the melt was 2900~F. The ~ melt was blown through two submerged~ horizontal, concentric-- tube tuyeres, asymmetrically positioned in the lower side-wall of a refractory-lined refining vessel such as shown in Figure 1 .
19 .
lO,g43 The blowing gas, consisting of oxygen diluted with argon, was injected through the center tube of the tuyeres.
Argon was used as the protective fluid, and injected through the annular passage of the tuyeres. The ratio of the oxygen flow rate to that of the combined argon flows was 3 to 1.
A total of 2150 t.3 of oxygen was injected. The comhined gas flow rate of the injected gases was about 6000 SCFH.
~bout 9 minutes after the flow began, 11 lbs. of charge chrome and 18 lbs. of standard manganese were added to the melt. At the end of the blow the temperature of the melt was 3080F and the carbon content was 0.10%
Following the addition of 100 lbs. of 50% FeSi, the melt was sparged and stirred by injecting argon at a rate of about 4000 SCFH for 4 minutes through both passages of both tuyeres. The melt temperature at this time was 3000F. The melt was then conventionally deoxidized and sparged with argon for 2 more minutes before being tapped into a bottom pouring ladle for subsequent teeming into molds. The furnace tap composition and the final composition of the refined melt at tap are tabulated below.
~e~lY~ %C ~/~n %Si %Cr %Ni %MO %P %S
~ , ~
Furnace Tap 0.32 0.54 0.55 1.29 2.85 0.43 0.014 0.004 Refined Melt 0.10 0.61 0.35 1.49 2.97 0.42 0.017 0.001 For purposes of comparison, a conventionally pro-cessQd heat of HY-80 was prepared as follows. An electric 20.
10,943 ~ 2 ~
arc furnace was charged with 15,000 lbs. of HY-8Q scrap, 55 lbs. of charge chrome, 14,082 lbs. of mild steel scrap and 600 lbs. of lime. Power was applied to the electrodes and the charge was melted and heated to 2790F in approximately 75 minutes. About 4000 SCF of oxygen was then injected into the bath by means of a hand-held consumable lance. The slag formed thereby was skimmed off, and the bath temperature was measured to be 2850F.
The following additions were then made to the melt:
200 lbs. carbon, 500 lbs. 50% FeSi, 500 lbs. lime, 220 lbs.
charge chrome, 285 lbs. Ni, and 66 lbs. Mo O3.
Power was again applied to the electrodes and the bath temperature was increased during a period of 45 minutes to 3020F. At this point, a preliminary sample was taken which had the analysis shown below. Thereafter, additions of 500 lbs. lime, 200 lbs. charge chrome, 135 lbs. Ni and 28 lbs.
FeMo were made, and the melt was further decarburized by injecting 6700 SCF of oxygen into the bath by means of a hand-held consu~able lance. After about 20 minutes of blowing, the carbon was measured to be 0.07% 275 lbs. of SiMn and 131 l~s. of 75% FeSi were added, and the heat was immediately tapped and sampled. The final tap eomposition is also shown below.
AnalYsis %C ~!~n %Si %Cr %Ni %MO %P %S
Preliminary 0.63 0.26 1.06 0.93 2.32 0O34 0.016 0.006 Furnace Tap 0.10 0.63 0.47 1.40 2.79 0.40 0.015 0.007 Table III below compares the physical properties of 10,9~3 the castings produced from the melts prepared in Examples 1 and 2 above, both of which were heat treated in substantially the same manner in accordance with conventional techniques TABLE III
Example 1Example 2 Tensile Strength (psi) 1~2,750 102,325 Yield Strength (psi) 87,20087,900 Elongation (/Q) 22 21 Reduction of Area (%) 55 53 10 Impact Strength (ft. lbs) 58,100,108 44,45,37 at-100F (Charpy "V"-notch) It can be seen from Table III that all of the properties of the castings, other than greatly improved impact strength of the castings made by the present invention, are substantially the sameO One would expect, to obtain similar properties since both the chemical composition and heat treatment of the castings were substantially the same.
The improved impact strength is believed to reflect the improved internal cleanliness of the melt produced in accor-dance with the present invention. While this increase intoughness is, in itself3 a considerable improvement in the ~ quality of the casting, an additional improvement of great ; significance was observed in the cleaning and finishing of the castings. The castings made from the melt of Example 1, required substantially less cleaning, grinding, welding and other repair than the prior art casting made from the melt ; of Example 2. This improvement was unexpected and not predictable rom past experience, and is of great importance to the foundry industry since the labor savings involved 22.
~ 43 represent a significant portion of the value of ,he casting.
In addition to the unexpected improvements described above, other improvements on HY-80 castings made in accordance with this invention have also been found. For example, the welds re~uired to repair an experimental casting made by the present invention numbered only 5, as compared to 95 repair welds required on the same casting made by conventional prac-tice. Further, castings made by the present invention dis-played no hydrogen flaking even in 13" sections.
Example 3 An electric arc furnace was charged with 8947 lbs.
of 18-8 stainless steel scrap 3 40 lbs. of carbon and 500 lbs.
of lime. Power was applied to the electrodes and the charge was melted. Following melt down, the composition was conven-tionally adjusted to have a furnace tap composition shown below and a temperature of about 3100F.
The above melt was tapped from the arc furnace into a transfer ladle and then charged into the refining vessel.
500 lbs. of lime was added to the charge. At the start of the pneumatic refining period the te~perature of the melt was 2910F. The melt was blown through two submerged, hori~ontal, concentric-tube ~uyeres, asymmetrically positioned in the lower side~wall of a refining vessel as shown in Figure 1.
The blowing gas consisted of o~ygen diluted with argon injected through the center tubes. Argon was injected as the protective fluid through the annular passage of the tuyeres. The ratio of oxygen to the combined argon flow rates was 3 to l. A
23.
10,943 6~
total of 1800 Et.3 of oxygen was injected. The combined flow rate of the injected gases (i.e. oxygen plus ar~;on) was about 7000 SCFH. After 21 minutes of blowing at the 3:1 ratio, the melt temperature was 3120F and the carbon content was 0.15%. The ratio of the oxygen flow rate to that of the combined ægon flows was then changed to 1:1. At this ratio the injection was continued for about 15 minutes during which time 1000 ft. o:E total oxygen was injected. Thereafter, the ratio of oxygen to combined argon flows was again changed to 1:3, and 100 ft. of oxygen was lnjected over about 4 minutes time. 400 lbs. of FeCrSi, 100 lbs. lime and 215 lbs. of 50%
FeSi was then added, and the melt stirred and sparged for 17 minutes with argon alone injected through both passages oE
both tuyeres. The tap temperature was 2920F. The heat was then tapped into a botl:orn pouring ladle for subsequent teeming into molds.
%C ~/OMn %Si %Cr ~/oNi %Cu ~I/oMO %P C~/oS
Furnace Tap 0.35 0.75 0.34 19.29 8.950.34 0.65 0.029 0.00 Refined Melt 0.02 0.70 1.47 20.09 9.540.33 0.63 0.028 0.00 ~
For purposes of comparison, a conventionally pro-cessed hea~ of 18-8 stainless steel was prepared as follows.
Arl electric arc furnace was charged with 18,702 lbs. of 18-8 scrap, 374 lbs. FeNi, 150 lbs. carbon and 2500 lbs. of lime.
Power was applied to the electrodes and the charge was melted and heated to 2850F in approximately 118 minutes. A pre-liminary sample taken at this time had the composition shown 24.
0 7 ~43 ~ ~ 6 ~ ~ ~
below. About 12,000 SCF o~ oxygen was then injected into the bath via a hand-held consumable lance. The slag formed there-by was skimmed off, and the following additions were made to the melt: 2278 lbs. FeCrSi, 300 lbs. low CFeCr, 800 lbs.
lime, 80 lbs. Ni.
Power was again applied to the electrodes and the heat was tapped into a ladle for subsequent teeming into molds.
The preliminary sample composition and the final tap composi-tion are shown below.
AnalYsis %C V/~n %Si %Cr %Ni /~o %P /OS
Preliminary 0.45 0.58 0.42 17.65 8.78 0.83 0.028 0.010 Tap 0.05 0.63 1.21 19.84 8.85 0.78 0.033 0.005 The mechanical properties of the castings made from the melts of Examples 3 and 4, i.e. the invention and the prior art respectively, were substantially the same. However~
the average time required for cleaning and repair, based on 6 castings~ made by the invention was approximately 30% less than the average time required for cleaning and repair of 7 like castings made by the prior art.
25.
10~9~3 ~ 6 2~
almost entirely eliminated during subsequent rolling, and the final wrought product contains virtually no evidence of the original porosity.
Similarly~ surace defects in ingots, slabs and billets are not a problem to the producer of wrought products, because these are intermediate products which undergo con-siderable mechanical reworking and plastic deformation prior to shipment. Furthermore, when surface defects occur, they can readily be removed by grinding or scarfing before further mechanical processing. In contrast, the surface quality of castings is very important because castings are a final product and any defect must be removed by costly and time consuming manual grinding, gauging or chipping. Then the cavity so caused must be rebuilt by welding or overlaying of metal. In addition, surface repair may diminish the dimension-al accuracy and mechanical properties o~ the cas~ing.
It is evident, therefore, that since ingots, slabs and billets are intermediate products, certain surface and internal defects can be tolerated in them, while in castings such defects cannot, because castings are poured directly into their final shape.
The metal ~ounding industry has long been plagued with a number of dificult problems caused by unsatisfactory castings. These problems are due both to surface defects and to internal defects. While many surface deects can be remedied by the costly inishing operations mentioned above, internally defective castings frequently have to be scrapped, remelted and , lO,g4~
cast over. Some of the common surface flaws in castings in-clude: hot tears, surface cracks, rough surface, and holes ranging in size from pinholes to gross blow holes. In general, the ultimate causes of these defects are not well understood.
Consequently, melting and casting practices to produce satis-factory castings require a large amount of experience and empirical evaluation. Internal defects are due mainly to porosity and inclusions which adversely effect the mechanical properties of castings, i.e. its strength, ductility, tough-ness and impact resistance. The above-mentioned defects, as well as others such as embrittlement, age-hardening and the presence of fish-eyes or white spots, are believed to be related to the presence of uncontrolled amounts of oxygen, nitrogen, hydrogen, phosphorous and sulfur in ~he melt.
Consequently, it has long been an objective of the foundry industry to produce sound cas~ings with low or controlled levels of these five elements. In the production of stainless castings, where corrosion resistance is of paramount impor-tance, it is often an additional objective to produce sound castings with low carbon levels.
Casting defects are conventionally remedied during the so called finishing operations. Most of these operations are highly labor intensive and consequently very costly. In addition, much of the finishing consists of grinding which causes dust that can be harmful to health. Some castings, however, cannot be repaired because the critical application for the part does nat allow it. In such case, the defective 0 3 g43 ~ 6 ~ ~ ~
casting must be scrapped. Consequently, the foundry art has long sought a method which would improve castings both in terms of their surface quality and physical properties.
Various techniques have been used in the foundry art to refine melts prior to casting in order to improve the quality of the resultant castings. The final stage of melt-ing often includes some form of purification or refining treat-ment intended to influence the microstructure and cleanliness of the casting. Such treatments usually involve the blowing of ga~ses or the addition of certain reagents to the furnace or transfer ladle. These treatments may include decarburiza-tion, dephosphorization, deoxidation, desulfurization and degassing.
Prior to the present invention, decarburization of molten steel for ca~tings, was generally accomplished by blowing oxygen into the melt through a consumable lance inserted through an opening in the furnace. This technique of decar burization is, in the irst place, dangerous to the operator because it exposes him to hot metal and sparks, and because the operator usu~lly holds the lance which is in itself hazardous. Secondly, this technique of decarburization is often inaccurate because all the oxygen does not always react with the bath. Hence, it is often necessary to reblow the molten steel because insufficient carbon was removed initially.
Lastly, such prior art methods of decarburization tend to generate a great deal of fume and smoke which is hazardous to health and damaging to the environment.
lO,g43 ~ ~ 6 ~ 2~
Because the presence of oxygen is known to be detrimental to the properties of the castings, foundries generally deoxidize the molten me~al prior to pouring. In addition, deoxidation is generally required to prevent the formation of blow holes during solidification. This is normally accomplished by the addition of well-known deoxidants such as silicon or aluminum, and also by the addition of special deoxidants, such as "Calcibar" and "Hypercal." The attainment of a well deoxidized melt prior to casting is essential for the production of sound, tough castings.
Desulfurization of molten steel for castings, prior to this invention, has generally been accomplished by the ormation of basic slags in the furnace, i.e. slags containing a high ratio of lime to silica or lime to alumina, and by subsequently mixing the slags with well deoxidized metal.
Equilibrium between the slag and the metal causes the sulfur to be transerred from the metal to the slag. This process is very slow, often requiring several hours, particularly when very low (i.e. under .005%) sulfur is desired. Indeed, it i8 o~ten necessary to remove the slag and to produce a new ; one. Sometimes this step has to be repeated several times in order to reach the desired low level of sulfur. T;liS process is very laborious and time consuming, and unnecessarily exposes the furnace operators to molten metal and to unhealthy fumes.
An alterna~ive, and much more costly desulfurization technique is to add expensive sulfur scavenging elemen~s, such as calcium, magnesium or the rare earth elements, to the furnace l~,g~3 immediately prior to tapping or to the transfer ladle. The expense of this technique, as well as its non-reproducibility, mitigates against its general use.
Known degassing treatments include vacuum melting, vacuum degassing, as well as degassing by bubbling scavenging gases such as argon through the melt. While argon degassing in the ladle prior ~o casting can improve the quality of castings by lowering the hydrogen and oxygen content of the melt, it does not remove all impurities or achieve low hydro-gen levels in the limited time available. Because the timeavailable for degassing is strictly limited by heat loss from the degassing vessel, it has been found that it is not possible to lower the dissolved gas content sufficiently for many applications. Furthermore, degassing by itself does not re-move sulfur and may necessitate reheating the melt in order to obtain sufficient fluidity or casting.
Prior to the present invention, therefore, the foundry art utilized the above-described techniques in an effort to produce deect-free castings. However9 these prior art tech~iques are expensive, often inaccurate or non-reproduc-ible, time-consuming, generally hazardous to the health of the operators, and by-and-large inadequate to the needs o the industry. Cansequently, e~tensive post-solidification repair of castings is usually still required. In fact, in castings, for example, destined for nuclear applications, the cost of inspection and repair often exceeds the material value of the - castings themselves.
During the past twenty five years, the manufacturers 7.
l~,g~3 of wrought steel products have made large gains in upgrading their molten metal processing tecnniques through the adoption of one of several now well known refining processes such as the BOF, AOD, OB~ or Q-BOP and LWS processes. U.S. Patents illustrative of these processes, respectively, are Nos.: -2,800,631; 3,252,790; 3,706,549; 3,930,843 and 3,844 768.
The production of wrought steels containing controlled levels of carbon, phosphorous, sulfur, oxygen, nitrogen and hydrogen is now readily and economically achievable through judicious selection of one, or a combination of more than one, of the above yrocesses. In the foundry or cast metal industry, however, comparable advances have been absent. While the industry has, at various times, produced products with low or controlled levels of one or perhaps two of the above six elements, the manufacture of castings with low or controlled levels of all six elements has hitherto not been possible, and consequently, the value or advantages of being able to control all six elements have hitherto not been known.
The pneumatic ~reatment of molten stainless steel for the production of wrought steel by the simultaneous injec-tion of argon and oxygen into the melt~ commonly referred to as the AOD process, has achieved wide commercial acceptance in stainless steel mills for the manufacture of wrought products. The basic AOD refining process is disclosed by Krivsky in UOS~ Patent No. 3 9 752,793. An improvement on Krivsky relating to the programmed blowing of the gases is : disclosed in Nelson et al, U.S. Patent No. 3,046,107. The use of nitrogen in combination with argon and oxygen to lO,g4 ~ ~ 6 6Zl achieve predetermined nitrogen contents is disclosed in Saccomano et al in U.S. Patent No. 3,754,894. A modification of the AOD process is also shown by Johnsson et al in U.S.
Patent NoO 3,867,135 which utilizes steam or ammonia in combination with oxygen to refine molten me~al.
It is worthy of note that none of the above-men-tioned pneumatic melt refining techniques have, prior to this invention, been used by the foundry art for the production of castings.
OBJECTS
_ It is an object of the present invention to improve the surface quality, internal quality and physical properties of castings.
It is another object of the present invention to improve the method of producing castings by pneumatically refining the melt prior to casting.
It is still another object of this invention to increase the yield of acceptable castings.
SJMMAR~
It has now been discovered that by pneumatically refining the melt in a separate vessel prior to casting, castings of a quality superior to that heretofore obtainable can be produced. Such castings have unexpectedly superior surface quality, internal quality and physical properties.
The above, and other objects which will be apparent to those skilled in the art are achieved by the present invention which comprises:
10,943 In a process for producing metal castings having improved surface quality and internal quality by melting selected charge materials in a furnace, teeming the melt into a mold, permitting the melt to solidify in the mold, and removing the casting from the mold, the improvement com-prising:
(1) transferring the melt from the melting furnace into a refining ves~el provided with at least one submerged tuyere, and 10 - (2) refining said melt by (a) injecting into the melt through said tuyere(s) an oxygen-containing gas con- -taining up to 90% of a dilution gas, and (b) thereafter injecting a ~sparging gas into the melt through said tuyere(s).
Preferably, the oxygen-containing gas stream is surrounded by an annular stream of protective fluid.
The term "refining" as used in the present specifi-cation and claims is meant to include any one or more oE the following e~fects: decarburization, dephosphorization, de-sulfurization~ degassing, deoxidation, gaseous alloying, impurity oxidation, impurity volatilization, slag reduction and flotation and homogenization of non-metallic impurities.
The present invention is applicable to refining of any iron, cobalt or nickel based alloy, and the term "metal" is used in that sense.
The term "dilution gas" as used herein is intended to mean one or more gases that are added to the oxygen stream for the purpose of reducing the partial pressure of the 10 .
10,443 ~ 2 ~
carbon monoxide in the gas bubbles formed during decarburiza-tion of the melt, and/or for the purpose of altering the feed rate of oxygen to the melt without substantially altering the total injected gas flow rate. Suitable dilution gases include: argon, helium, hydrogen, nitrogen, carbon monoxide, carbon dioxide, steam and hydrocarbon gases, for example, methane, ethane~ propane and natural gas. Argon is the most preferred dilution gas.
The term "protective fluid" as used herein is meant to include one or more fluids which surround the oxygen con-taining gas and protect the tuyere and surrounding refractory lining from e~cessive wear. Suitable protective fluids in-clude: argon, helium, nitrogen, hydrogen, carbon monoxide, carbon dio~ide, hydrocarbon fluids (gas or liquid) and steam.
Methane, ethane, propane or natural gas are suitable hydro-carbon gases. No. 2 diesel oil is a suitable hydrocarbon liquid. Argon is the most preferred protective fluid.
The term "sparging gas" as used herein is intended to mean one or more gases which remove thos~ impurities from ~he melt by volatilization or transfer to the slag by entrap-ment or reaction with the slag. Suit~ble sparging gases include- argon, helium, nitrogen and steam. Argon is also the preferred sparging gas.
Castings ha~ting improved surface quality are defined as castings which when compared to the prior art require re-duced rleaning, grinding, chippinga welding or other repair.
Such improved surface quality can be evidenced by a reduced 11, 10,94 ~ 2 ~
level of defects determined during dye penetrant or magna-flux testing.
Castings having improved internal quality are de-fined as castings which when compared to the prior art dis-play one or more of the following characteristics: a lower level of inclusions, finer as-cast grain size, reduced internal porosity, reduced tendency for hydrogen flaking during machin-ing, reduced evidence of deects when inspected by X-ray techniques or better physical properties such as toughness.
THE DRAWING
Figure 1 represents a cross-sectional view of a preerred refining vessel or converter for use in carrying out the process of the present invention.
DETAILED DESCRIPTION
It was expected that utilization of pneumatic refin-ing for the treatment of steel melts for castings would pro-duce most of the chemical benefits obtained by refining molten steel for the production of wrought steel products. In particular, it was expected that some improved internal q1~ality would be obtained by better deoxidation of thP melt, by better separation of deoxidation products, and by the attainment of lower sulfur levels and Lower hydrogen content.
However, it was unexpectedly discovered that pneumatic refining in accoxdance with this invention produces improvements in the surace quality of the castings beyond any expectations, that it produces castings with greatly improved stxength, ; ductility and toughness, and that it makes possible the pro-duction of castings of far superior quality than previously 10,94 ~ 2 ~
possible from low alloy steels and carbon steels.
As a result of the present invention, foundries are now able to cast with significantly increased assurance of obtainlng satisfactory castings, as well as of obtaining castings of higher quality. More specifically, the surface quality of the resultant castings have fewer cracks and re-duced hot tears. In addition7 it has been found that use of the present invention produces a smoother casting surface, believed to result from reduced interaction of the sand mold with the melt. It has also been found that the physical properties of the castings have been unexpectedly improved.
The improvements are believed to be related to the lower levels of inclusions, lower hydrogen flaking, as well as lower porosity found in castings made in accordance with this in-vention. Molten steel treated in accordance with the present invention has a higher flowability or fluidity at the same temperature than untreated metal, resulting in superior castings, since the metal will ~low into smaller and more intricate crevices than unrefined melt. Alternatively, the same fluidity may be achieved at a lower casting temperature.
This again contributes to improved casting surface quality.
The pneumatic refining treatment of the present invention may be advantageously employed on any type of iron or steel melt, and also on cobalt and nickel alloys, normally used for the manufacture of metal castings. It has, however, been found to be particularly beneficial in the treatment of ferritic and austenitic stainless steels, low alloy steels and carbon s~eels. Special benefits are obtained in castings made ; 13.
10, g~
6Z~
steels such as WC~ and HY80 which are sensitive to hydrogen flaking as well as hot tearing. High strengt'n steels such as HY130 which normally require extensive chipping, grinding and welding in order to repair as-cast defects, are significantly improved by the present invention, resulting in considerable finishing cost savings. Austenitic stainless grades such as CN7M, CH20, CK20, 310L, and 347L, which, prior to the present invention, were extremely difficult to cast without cracking or microfissuring, can now by means of the present invention, be readily cast without fear of cracking.
The advantages of the present invention while appli-cable to small, simple castings as well as to complex or large ones, are of particular significance when producing high quality castings such as required, for example, ~or pumps and turbines used in the aircraft, shipbuilding and nuclear power industries.
In addition to the unexpected results of the present invention described above, other benefits resulting from use of the present invention include raw material savings due to minimized oxidation of molten metal and the ability to use lower grade charge materials. Increased production also results from greater accuracy in achieving desired aim melt chemistries and fewer rejects due to improved casting quality.
In practicing the present invention, melting of the charge materials may be accomplished by any means known in the art. The most common foundr~ melting furnaces include ~uel fired ~urnaces of the hearth or crucible type, as well as electric furnaces of the resistance, induction or arc type.
The last two are preferred. Following melting of the charge 1~ .
~ 43 Z~
ma~erials, the melt is transferred by a ladle or otherY7ise poured into the pneumatic coverter shown in Figure 1.
Figure 1 is a cross-sectional view of a preferred refining vessel 1 for use in practicing the present invention.
Vessel 1 comprises an outer steel shell 2, removably attached to a trunion ring 3. The trunion ring and consequently the vessel is tiltable by being fixedly attached by drive means (not shown), in order to facilitate charging, sampling, slag removal and tapping. Shell 2 is lined with basic refractory bricks 4. A removable shell arrangement is preferred, since several shells are necessary to maintain uninterrupted opera-tions. While one shell is in use, the spare or spares are being relined. A horizontally disposed concentric tube tuyere 5 is located in the side-wall of the vessel near the bottom of the vessel for injection of the fluids. If desired, the tuyeres can be located in the bottom of the vessel in place of or in addition to the sides. Preferably, however, at least two tuyeres are used, and positioned in the side~wall of the vessel, near the bot~om and horizontally disposed in such manner as to be asymmetric. That is, no two tuyeres should be positio~ed so that their axes, and consequently the fluid streams injected diametrically opposed to each other.
Asymmetric positioning of the tuyeres improves mixing of the melt by the injected gases. The tuyere 5 consists of an inner tube 6 and a conce~tric outer tube 7. Oxygen alone or ad-mixed with a dilution gas is injected through the inner tube 6, and the protective gas is injected through the outer tube 7 of the ~uyere. The latter forms a protective annular .
lO,g43 shroud around the oxygen stream which pro~ects the refractory lining from ra~id deterioration. The pressure of the fluids must be sufficiently great to penetrate into the melt. Prefer-ably, the absolute pressures of the fluids at the tuyere in-lets, of both the central and annular passages, are at least two times greater than the absolute pressures of the ~luids at the outlets.
A detailed description of a suitable vessel and tuyeres for carrying out the present invention is shown by Saccomano and Ellis in U.S. Patent No. 3,703,279. The sparging gas may be injected into the melt either through the same tuyere or tuyeres as used for the oxygen stream or through separate tuyeres; the former is preferred. Preferably, after the oxygen blow is completed, the sparging gas is injected ; through the center passage of the tuyere as well as through the annular passage in order to prevent molten metal from flowing back into the tuyere where it would ~reeze.
In general, the molten metal refining step of the present process is carried out by injecting oxygen and a dilu-tion gas, as well as a protective fluid (both of which may beargon) into the melt through the submerged tuyeres. The decar-burization, i.e. the reaction o~ the injected oxygen with car-bon in the melt, produces controlled oxidation o~ the bath components) as well as heat which maintains bath temperature.
The melt is initially blown with a high ratio o oxygen to dilution and protec~ive gases. Depending on the steel composi-tion being re~ined, as the carbon content of the melt decreases, the ratio of oxygen to dilution gas and protective fluid may 16.
10,943 be lowered, generally in several steps, in order to maintain ~avorable thermodynamic conditions throughout the blow.
Since the oxygen and other gases are introduced below the level of the melt and at high velocity, excellent mixing takes place within the melt and intimate gas-metal and slag-metal contact occurs. As a result, the reaction kinetics of all chemical processes which take place within the vessel are greatly improved. This permits desulfuriza-tion to very low levels (under 0.005%) generally, in less than ten minutes o~ blowing and without addition of expensive desulfurizing agents such as calcium, magnesium or rare earths. Dephosphorization of alloys containing less than approximately one percent chromium can readily be achieved by decarburizing the bath to below 0.1% carbon by using a gas mixture containing at least 75% oxygen. The phosphorous bearing slag so formed must then be decanted prior to blowing with a sparging gas or adding any reducing agents, deoxidants, or ~esul~urizing agents.
Other major benefits o~ the inven~ion are very close control o~ the end point carbon and very low residual values of oxygen9 nitrogen and hydrogen. Typical residual values ~or these three elements o~tained by practicing the inven-tion are shown in Table I.
TABLE I
Stainless Steel Low Alloy Steel .T . :e-- 7 :.
Oxygen 40-70 ppm 20-50 ppm Hydrogen2-4 ppm 1-3 ppm Nitrogen150-200 ppm 20-50 ppm 17.
lO,g43 In addition, lead, and zinc in the melt are reduced to levels that are metallurgically harmless.
The synergistic results obtained by the present invention, i.e. low gas content (oxygen, nitrogen and hydro-gen) together with low sulfur and increased fluidity of the melt have combined to produce castings of unprecedented sur-face quality, internal cleanliness and improved mechanical properties. Table II below compares the chemical and physical properties of two castings of stainless steel grade CA6NM, one made by conventional practice and the other by the present invention with ASTM specification A296.
TABLE II
Chemistry ASTM Spec.
(C/o) A296 Conventional Invention C 0.06 max .05 0.026 Mn 1.00 max .60 0.47 Si 1.00 max .55 0.96 Cr 11.5 - 14.0 12.70 12.81 Ni 3.5 - 4.5 3.80 4O00 Mo 0.40- 1.00 0.50 0.57 S 0.03 max 0.025 0.022 P 0.04 max 0.020 0.025 Mechanical _ Tensile (ksi) 110 min. 115 122.8 Yield (ksi~ 80 min. 100 108.3 Elongation (%) 15 min. 20 21 Red. of area (%) 35 min. 60 67 Impact Strength none 65 77-80 Charpy V-notch (at R.T.3 It can be seen from Table II that the casting made in accordance with the present invention is superior in all respects~ and particularly in impact res1stance. The difference in toughness is even more impressive when one recognizes that in this particular casting the sulfur level 18.
1~,943 ~ 2~
was 0~022~/o rather than the customary value of le~s than O.OlV/5 obtainable with pneumatic refining. In this case no special desulfurizing treatment was employed.
On high strength alloys such as ~-130 an 85%
improvement in impact strength has been obtained on a casting made from HY-130 in accordance with this invention when com-pared to a casting o:E the same alloy made from vacuum de-gassed metal. Such high impact strength far exceeds any previously obtained impact strength on castings made from this alloy.
Example 1 An electric arc furnace was charged with 6290 lbs.
of HY-80 scrap, 5869 lbs. of mild steel scrap and 300 lbs. of lime. Power was applied to the electrodes and the charge was melted in approximately one hour. Following melt down, the composition was adjusted, in accordance with conventional practice, to have the furnace tap composition shown below, and a temperature of about 3100F.
The above melt was tapped from the arc furnace into - 20 a transfer ladle, and then charged into the refining vessel.
500 lbs. of lime, 100 lbs~ of MgG and 60 lbs. of aluminum were added to the charge. At the start of the pneumatic refining period the temperature of the melt was 2900~F. The ~ melt was blown through two submerged~ horizontal, concentric-- tube tuyeres, asymmetrically positioned in the lower side-wall of a refractory-lined refining vessel such as shown in Figure 1 .
19 .
lO,g43 The blowing gas, consisting of oxygen diluted with argon, was injected through the center tube of the tuyeres.
Argon was used as the protective fluid, and injected through the annular passage of the tuyeres. The ratio of the oxygen flow rate to that of the combined argon flows was 3 to 1.
A total of 2150 t.3 of oxygen was injected. The comhined gas flow rate of the injected gases was about 6000 SCFH.
~bout 9 minutes after the flow began, 11 lbs. of charge chrome and 18 lbs. of standard manganese were added to the melt. At the end of the blow the temperature of the melt was 3080F and the carbon content was 0.10%
Following the addition of 100 lbs. of 50% FeSi, the melt was sparged and stirred by injecting argon at a rate of about 4000 SCFH for 4 minutes through both passages of both tuyeres. The melt temperature at this time was 3000F. The melt was then conventionally deoxidized and sparged with argon for 2 more minutes before being tapped into a bottom pouring ladle for subsequent teeming into molds. The furnace tap composition and the final composition of the refined melt at tap are tabulated below.
~e~lY~ %C ~/~n %Si %Cr %Ni %MO %P %S
~ , ~
Furnace Tap 0.32 0.54 0.55 1.29 2.85 0.43 0.014 0.004 Refined Melt 0.10 0.61 0.35 1.49 2.97 0.42 0.017 0.001 For purposes of comparison, a conventionally pro-cessQd heat of HY-80 was prepared as follows. An electric 20.
10,943 ~ 2 ~
arc furnace was charged with 15,000 lbs. of HY-8Q scrap, 55 lbs. of charge chrome, 14,082 lbs. of mild steel scrap and 600 lbs. of lime. Power was applied to the electrodes and the charge was melted and heated to 2790F in approximately 75 minutes. About 4000 SCF of oxygen was then injected into the bath by means of a hand-held consumable lance. The slag formed thereby was skimmed off, and the bath temperature was measured to be 2850F.
The following additions were then made to the melt:
200 lbs. carbon, 500 lbs. 50% FeSi, 500 lbs. lime, 220 lbs.
charge chrome, 285 lbs. Ni, and 66 lbs. Mo O3.
Power was again applied to the electrodes and the bath temperature was increased during a period of 45 minutes to 3020F. At this point, a preliminary sample was taken which had the analysis shown below. Thereafter, additions of 500 lbs. lime, 200 lbs. charge chrome, 135 lbs. Ni and 28 lbs.
FeMo were made, and the melt was further decarburized by injecting 6700 SCF of oxygen into the bath by means of a hand-held consu~able lance. After about 20 minutes of blowing, the carbon was measured to be 0.07% 275 lbs. of SiMn and 131 l~s. of 75% FeSi were added, and the heat was immediately tapped and sampled. The final tap eomposition is also shown below.
AnalYsis %C ~!~n %Si %Cr %Ni %MO %P %S
Preliminary 0.63 0.26 1.06 0.93 2.32 0O34 0.016 0.006 Furnace Tap 0.10 0.63 0.47 1.40 2.79 0.40 0.015 0.007 Table III below compares the physical properties of 10,9~3 the castings produced from the melts prepared in Examples 1 and 2 above, both of which were heat treated in substantially the same manner in accordance with conventional techniques TABLE III
Example 1Example 2 Tensile Strength (psi) 1~2,750 102,325 Yield Strength (psi) 87,20087,900 Elongation (/Q) 22 21 Reduction of Area (%) 55 53 10 Impact Strength (ft. lbs) 58,100,108 44,45,37 at-100F (Charpy "V"-notch) It can be seen from Table III that all of the properties of the castings, other than greatly improved impact strength of the castings made by the present invention, are substantially the sameO One would expect, to obtain similar properties since both the chemical composition and heat treatment of the castings were substantially the same.
The improved impact strength is believed to reflect the improved internal cleanliness of the melt produced in accor-dance with the present invention. While this increase intoughness is, in itself3 a considerable improvement in the ~ quality of the casting, an additional improvement of great ; significance was observed in the cleaning and finishing of the castings. The castings made from the melt of Example 1, required substantially less cleaning, grinding, welding and other repair than the prior art casting made from the melt ; of Example 2. This improvement was unexpected and not predictable rom past experience, and is of great importance to the foundry industry since the labor savings involved 22.
~ 43 represent a significant portion of the value of ,he casting.
In addition to the unexpected improvements described above, other improvements on HY-80 castings made in accordance with this invention have also been found. For example, the welds re~uired to repair an experimental casting made by the present invention numbered only 5, as compared to 95 repair welds required on the same casting made by conventional prac-tice. Further, castings made by the present invention dis-played no hydrogen flaking even in 13" sections.
Example 3 An electric arc furnace was charged with 8947 lbs.
of 18-8 stainless steel scrap 3 40 lbs. of carbon and 500 lbs.
of lime. Power was applied to the electrodes and the charge was melted. Following melt down, the composition was conven-tionally adjusted to have a furnace tap composition shown below and a temperature of about 3100F.
The above melt was tapped from the arc furnace into a transfer ladle and then charged into the refining vessel.
500 lbs. of lime was added to the charge. At the start of the pneumatic refining period the te~perature of the melt was 2910F. The melt was blown through two submerged, hori~ontal, concentric-tube ~uyeres, asymmetrically positioned in the lower side~wall of a refining vessel as shown in Figure 1.
The blowing gas consisted of o~ygen diluted with argon injected through the center tubes. Argon was injected as the protective fluid through the annular passage of the tuyeres. The ratio of oxygen to the combined argon flow rates was 3 to l. A
23.
10,943 6~
total of 1800 Et.3 of oxygen was injected. The combined flow rate of the injected gases (i.e. oxygen plus ar~;on) was about 7000 SCFH. After 21 minutes of blowing at the 3:1 ratio, the melt temperature was 3120F and the carbon content was 0.15%. The ratio of the oxygen flow rate to that of the combined ægon flows was then changed to 1:1. At this ratio the injection was continued for about 15 minutes during which time 1000 ft. o:E total oxygen was injected. Thereafter, the ratio of oxygen to combined argon flows was again changed to 1:3, and 100 ft. of oxygen was lnjected over about 4 minutes time. 400 lbs. of FeCrSi, 100 lbs. lime and 215 lbs. of 50%
FeSi was then added, and the melt stirred and sparged for 17 minutes with argon alone injected through both passages oE
both tuyeres. The tap temperature was 2920F. The heat was then tapped into a botl:orn pouring ladle for subsequent teeming into molds.
%C ~/OMn %Si %Cr ~/oNi %Cu ~I/oMO %P C~/oS
Furnace Tap 0.35 0.75 0.34 19.29 8.950.34 0.65 0.029 0.00 Refined Melt 0.02 0.70 1.47 20.09 9.540.33 0.63 0.028 0.00 ~
For purposes of comparison, a conventionally pro-cessed hea~ of 18-8 stainless steel was prepared as follows.
Arl electric arc furnace was charged with 18,702 lbs. of 18-8 scrap, 374 lbs. FeNi, 150 lbs. carbon and 2500 lbs. of lime.
Power was applied to the electrodes and the charge was melted and heated to 2850F in approximately 118 minutes. A pre-liminary sample taken at this time had the composition shown 24.
0 7 ~43 ~ ~ 6 ~ ~ ~
below. About 12,000 SCF o~ oxygen was then injected into the bath via a hand-held consumable lance. The slag formed there-by was skimmed off, and the following additions were made to the melt: 2278 lbs. FeCrSi, 300 lbs. low CFeCr, 800 lbs.
lime, 80 lbs. Ni.
Power was again applied to the electrodes and the heat was tapped into a ladle for subsequent teeming into molds.
The preliminary sample composition and the final tap composi-tion are shown below.
AnalYsis %C V/~n %Si %Cr %Ni /~o %P /OS
Preliminary 0.45 0.58 0.42 17.65 8.78 0.83 0.028 0.010 Tap 0.05 0.63 1.21 19.84 8.85 0.78 0.033 0.005 The mechanical properties of the castings made from the melts of Examples 3 and 4, i.e. the invention and the prior art respectively, were substantially the same. However~
the average time required for cleaning and repair, based on 6 castings~ made by the invention was approximately 30% less than the average time required for cleaning and repair of 7 like castings made by the prior art.
25.
Claims (11)
1. In a process for producing metal castings having improved surface quality and internal quality by melting selected charge materials in a furnace, teeming the melt into a mold, permitting the melt to solidify in the mold, and removing the casting from the mold, the improvement comprising:
(1) transferring the melt from the melting furnace into a refining vessel provided with at least one submerged tuyere, and (2) refining said melt by (a) injecting into the melt through said tuyere(s) an oxygen-containing gas con-taining up to 90% of a dilution gas, and (b) thereafter injecting a sparging gas into the melt through said tuyere(s).
(1) transferring the melt from the melting furnace into a refining vessel provided with at least one submerged tuyere, and (2) refining said melt by (a) injecting into the melt through said tuyere(s) an oxygen-containing gas con-taining up to 90% of a dilution gas, and (b) thereafter injecting a sparging gas into the melt through said tuyere(s).
2. The process of claim 1 wherein the oxygen-con-taining gas stream is surrounded by an annular stream of a protective fluid.
3. The process of claim 1 wherein the dilution gas is selected from the group consisting of argon, helium, hydrogen, nitrogen, carbon monoxide, carbon dioxide, steam and a hydrocarbon gas.
4. The process of claim 1 wherein the dilution gas is argon.
26.
10,943
26.
10,943
5. The process of claim 1 wherein the sparging gas is selected from the group consisting of argon, helium, nitrogen and steam.
6. The process of claim 1 wherein the sparging gas is argon.
7. The process of claim 2 wherein the protective fluid is selected from the group consisting of argon, helium, hydrogen, nitrogen, carbon monoxide, carbon dioxide, steam and a hydrocarbon fluid.
8. The process of claim 2 wherein the protective fluid is argon.
9. The process of claim 1 wherein the refining vessel is provided with at least two submerged tuyeres.
10. The process of claim 9 wherein the tuyeres are located in the side wall of the vessel near the bottom, dis-posed horizontally, and positioned such that the tuyere axes are asymmetric.
11. The process of claim 1 wherein the absolute pressure of the injected fluids at the tuyere inlets is at least two times the absolute pressure of the fluids at the tuyere outlets.
27.
27.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US78343177A | 1977-03-31 | 1977-03-31 | |
| US783,431 | 1977-03-31 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1106621A true CA1106621A (en) | 1981-08-11 |
Family
ID=25129226
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA300,003A Expired CA1106621A (en) | 1977-03-31 | 1978-03-30 | Method for producing improved metal castings by pneumatically refining the melt |
Country Status (21)
| Country | Link |
|---|---|
| JP (1) | JPS6039729B2 (en) |
| AR (1) | AR216787A1 (en) |
| AT (1) | AT376454B (en) |
| AU (1) | AU518013B2 (en) |
| BE (1) | BE865518A (en) |
| BR (1) | BR7801963A (en) |
| CA (1) | CA1106621A (en) |
| CH (1) | CH621953A5 (en) |
| DE (1) | DE2813717A1 (en) |
| ES (1) | ES468347A1 (en) |
| FI (1) | FI780978A (en) |
| FR (1) | FR2385467A1 (en) |
| GB (1) | GB1603135A (en) |
| IT (1) | IT1102498B (en) |
| MX (1) | MX5384E (en) |
| NL (1) | NL179300C (en) |
| NO (1) | NO157606C (en) |
| NZ (1) | NZ186831A (en) |
| SE (1) | SE443373B (en) |
| TR (1) | TR20268A (en) |
| ZA (1) | ZA781814B (en) |
Families Citing this family (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4187102A (en) * | 1978-08-24 | 1980-02-05 | Union Carbide Corporation | Method for controlling the temperature of the melt during pneumatic refining of steel |
| FR2448572B1 (en) * | 1979-02-09 | 1985-10-18 | Pennsylvania Engineering Corp | PROCESS FOR OBTAINING LOW-CARBON STEEL WITH THE OXYGEN-ARGON CONVERTER |
| DE3347718C2 (en) * | 1983-09-02 | 1986-05-28 | Ulrich Dipl.-Ing. 5905 Freudenberg Glasmeyer | Process for the production of steels with a high degree of purity and low gas content in steelworks and steel foundries |
| JPS62138975U (en) * | 1986-02-25 | 1987-09-02 | ||
| DE4328045C2 (en) * | 1993-08-20 | 2001-02-08 | Ald Vacuum Techn Ag | Process for decarburizing carbon-containing metal melts |
| DE102015105307A1 (en) * | 2015-04-08 | 2016-10-13 | Sms Group Gmbh | converter |
Family Cites Families (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3252790A (en) * | 1956-06-27 | 1966-05-24 | Union Carbide Corp | Preparation of metals and alloys |
| FR1309507A (en) * | 1960-11-18 | 1962-11-16 | Union Carbide Corp | Process for decarburizing steels with high chromium content |
| GB1253581A (en) * | 1968-02-24 | 1971-11-17 | Maximilianshuette Eisenwerk | Improvements in processes and apparatus for making steel |
| FR2173717A2 (en) * | 1972-02-29 | 1973-10-12 | Creusot Loire | Decarburising chrome steels by blowing with diluted carbon - monoxide through double/triple feed tuyeres |
| FR2146900A1 (en) * | 1971-07-23 | 1973-03-09 | Creusot Loire | Decarburising chrome steels by blowing with diluted carbon - monoxide through double/triple feed tuyeres |
| DE2160999A1 (en) * | 1971-12-09 | 1973-06-28 | Maximilianshuette Eisenwerk | DUESE FOR ADDING OXYGEN WITH A PROTECTIVE MEDIUM INTO CONVERTER VESSELS |
| FR2216362B1 (en) * | 1973-02-07 | 1975-10-31 | Creusot Loire | |
| US3861888A (en) * | 1973-06-28 | 1975-01-21 | Union Carbide Corp | Use of CO{HD 2 {B in argon-oxygen refining of molten metal |
| JPS51130604A (en) * | 1975-05-08 | 1976-11-13 | Nisshin Steel Co Ltd | A steelmaking process with ar recycle-recovery system |
-
1978
- 1978-03-30 NZ NZ186831A patent/NZ186831A/en unknown
- 1978-03-30 ES ES468347A patent/ES468347A1/en not_active Expired
- 1978-03-30 FI FI780978A patent/FI780978A/en not_active Application Discontinuation
- 1978-03-30 MX MX786982U patent/MX5384E/en unknown
- 1978-03-30 SE SE7803592A patent/SE443373B/en not_active IP Right Cessation
- 1978-03-30 DE DE19782813717 patent/DE2813717A1/en not_active Ceased
- 1978-03-30 NO NO781100A patent/NO157606C/en unknown
- 1978-03-30 CA CA300,003A patent/CA1106621A/en not_active Expired
- 1978-03-30 ZA ZA00781814A patent/ZA781814B/en unknown
- 1978-03-30 AR AR271604A patent/AR216787A1/en active
- 1978-03-30 NL NLAANVRAGE7803393,A patent/NL179300C/en not_active IP Right Cessation
- 1978-03-30 BE BE186429A patent/BE865518A/en not_active IP Right Cessation
- 1978-03-30 AT AT0226478A patent/AT376454B/en not_active IP Right Cessation
- 1978-03-30 BR BR7801963A patent/BR7801963A/en unknown
- 1978-03-30 GB GB12399/78A patent/GB1603135A/en not_active Expired
- 1978-03-30 JP JP53036186A patent/JPS6039729B2/en not_active Expired
- 1978-03-31 CH CH349678A patent/CH621953A5/en not_active IP Right Cessation
- 1978-03-31 FR FR7809601A patent/FR2385467A1/en active Granted
- 1978-03-31 AU AU34646/78A patent/AU518013B2/en not_active Expired
- 1978-03-31 TR TR20268A patent/TR20268A/en unknown
- 1978-03-31 IT IT48708/78A patent/IT1102498B/en active
Also Published As
| Publication number | Publication date |
|---|---|
| IT7848708A0 (en) | 1978-03-31 |
| SE7803592L (en) | 1978-10-01 |
| NL179300C (en) | 1986-08-18 |
| AU518013B2 (en) | 1981-09-10 |
| NZ186831A (en) | 1981-12-15 |
| SE443373B (en) | 1986-02-24 |
| MX5384E (en) | 1983-07-04 |
| IT1102498B (en) | 1985-10-07 |
| ES468347A1 (en) | 1978-12-01 |
| TR20268A (en) | 1980-12-08 |
| CH621953A5 (en) | 1981-03-13 |
| NL7803393A (en) | 1978-10-03 |
| DE2813717A1 (en) | 1978-10-05 |
| AT376454B (en) | 1984-11-26 |
| GB1603135A (en) | 1981-11-18 |
| NO157606B (en) | 1988-01-11 |
| AU3464678A (en) | 1979-10-04 |
| JPS6039729B2 (en) | 1985-09-07 |
| FI780978A (en) | 1978-10-01 |
| ATA226478A (en) | 1984-04-15 |
| NO157606C (en) | 1988-04-20 |
| FR2385467A1 (en) | 1978-10-27 |
| AR216787A1 (en) | 1980-01-31 |
| NO781100L (en) | 1978-10-03 |
| BR7801963A (en) | 1978-10-24 |
| FR2385467B1 (en) | 1982-03-05 |
| BE865518A (en) | 1978-10-02 |
| JPS53131916A (en) | 1978-11-17 |
| ZA781814B (en) | 1979-03-28 |
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