US3337330A - Treatment of molten metal - Google Patents

Description

United States Patent 3,337,330 TREATMENT OF MOLTEN METAL Charles W. Fink], Chicago, Ill., assignor to A. Fink] & Sons Company, Chicago, 11]., a corporation of Illinois No Drawing-Filed Aug. 14, 1964, Ser. No. 389,779

9 Claims. (Cl. 75-49) This application is a continuation-in-part application of the following co-pending applications: Ser. No. 777,- 664, filed Dec. 2, 1958, which issued as Patent 3,236,635 on Feb. 22, 1966; Ser. No. 340,594, filed Jan. 20, 1964; Ser. No. 176,493, filed Feb; 26, 19 62, which issued as Patent 3,236,636 on Feb. 22, 1966; Ser. No. 186,569, filed Mar. 6, 1962, which issued as Patent 3,292,915 on Dec. 20, 1966; Ser, No. 113,419, filed May 29, 1961, which issued as Patent 3,206,302 on Sept. 14, 1965; and Ser. No. 108,513, filed May 8, 1961, and Ser. No. 197,275, filed May 24, 1962, both of which are now abandoned.

My invention relates to a method of making steel having improved ductility and cleanliness and a reduced tendency to flake. It is particularly directed to a method of making large tonnage batches of such steel on a produc tion basis. Batches of over 30 tons have been made up to the present time, and it is contemplated the invention is applicable to batches of up to 150 tons and over.

In recent years great effort has been directed to the obtaining of improved ductility without loss of strength. This effort has been particularly marked in connection with steels intended for use in the aerospace industry, but the demand has been nearly as insistent for conventional applications. Ductility is in part related to the amount of hydrogen and oxygen in the final product. This invention is therefore directed, among other things, to improving ductility by removing substantial amounts of hydrogen and oxygen from the steel while it is in a molten condition.

Steel makers have also directed increasing attention to the problem of cleanliness and residuals. Steels which are dirty in the sense they contain an undesirably high quantity of inclusions such as silicates and oxides have poor properties including, particularly, poor transverse properties.

The formation of oxides can be materially reduced by reducing the dissolved oxygen in the melt available for combination with such elements as manganese, silicon and aluminum. These elements combine with oxygen to yield ferric and manganese oxides, silica and alumina, a certain portion of Which will remain in the steel and be therefore present in the final product as undesirable inclusions. Accordingly, an object of the invention is to reduce the quantity of oxygen available for combination with materials which will form, as an end product, undesirable inclusions.

The art has long recognized that undesirably high amounts of hydrogen in steel causes flaking. This problem is particularly troublesome to makers of large, one-of-a kind parts such as forge shops who make crank shafts and other machine components on a custom basis. In recent years the requirements that metals withstand higher speeds such as in rotors, high temperatures, and high pres sures has made the problem of flaking one of primary concern in many industries, particularly the aerospace industry.

Another object of the application is to remove hydrogen from the steel in an eifective manner while in a molten condition.

Attempts have ben made to remove deleterious gases prior to final solidification by workers in the art, but generally, there, attempts have been confined to the use of the vacuum melting procedure, or some variation of it. Good results have been obtained but the vacuum melting process is so costly the end product is prohibitively expensive.

Accordingly, one of the objects of the invention is to provide a method of removing deleterious gases, and thereby lower the final inclusion count and improve ductility, which does not require that the: metal be subjected to vacuum melting, or any variation thereof, at any time.

Another object of the invention is to provide a method of making improved quality steel as above described which enables the metal to be naturally cooled. This feature has the great advantage of enabling the special process of the invention to be incorporated in a conventional production schedule of large tonnage heats, whereby delays are not encountered and additional facilities, such as extensive annealing or slow cooling, are not necessary. The cost of treatment is thereby maintained at a minimal, economically feasible level.

In a preferred embodiment of the invention, steel is first prepared in an electric furnace or other vessel from suitable starting materials, such as scrap. The melt or batch of molten steel formed thereby invariably contains hydrogen and oxygen as included deleterious gases. Nitrogen is likewise invariably included but at the moment opinion is divided in the steel industry as to Whether nitrogen is a harmful gas, and, if so, to what extent.

The molten steel contains as is generally understood in the industry, a substantial quantity oi alloying material of which carbon is the most common. The invention has been particularly applied to .what is generally referred to as medium carbon steels, specifically, heats containing from .33 to .64 carbon. It is contemplated however, that the invention is applicable to steels having carbon in the range of from .15 to 2.25%. The field of most common application, and in which the process can be expected to be most effective, encompasses steels having carbon in the range of .20 to 1.00 or more. For purposes of this description the medium carbon range is considered to extend from about .30 to about .70 carbon. For SAE 52100 steel such carbon range will be from about 0.95 to about 1.10. Other alloying elements such as nickel, chromium, molybdenum, vanadium, or manganese or a combination of one or more of these elements will likewise be present. Other elements may be present in signficant amounts with lesser frequency than the above mentioned elements.

At this point in the process the molten steel is then exposed to a vacuum. The vacuum to which the molten metal should be exposed and the time of exposure to the vacuum are important in the achieving of satisfactory gas levels. It has been discovered that final included gas results of l1.5 -p.p.m. hydrogen, 2030 ppm. nitrogen and 1550 p.p.m. oxygen can be consistently achieved. The higher carbon steels such as 52100 will have lower oxygen levels of about 8 p.p.m. These results are very comparable to the degassing of the same steel by processes in which both purging gas and a vacuum are employed.

On the basis of investigations it has been discovered that the steel should be subjected to a vacuum on the order of about 2 mm. of Hg or below if purging is not to be employed. Investigations have also confirmed that for alloy steels the time at which the steels should be subjected to the vacuum approaching 2 mm. Hg or below should fall within the range of times given by the following formula:

T in minutes (Pepth of bath in inches) 1030 Further investigations on steels of the FX and 4340 type have shown that a length of time derived from the equation 2 T in Depth of bggh 1n inches) will give adequate final included gas contents. For convenience the nominal compositions of FX and 4340 (electric furnace specification) are given below:

4- melt after the removal of a substantial quantity of the included deleterious gases. Under certain conditions the above considerations likewise apply to the addition of manganese, vanadium and chrome.

FX 4340 5 It is impossible to define with accuracy the precise moment when the deoxidizers should be added. As those .50.60 .33-.43 skilled in the art of vacuum treatment of steels readily @3122 3322? appreciate, the pumping curve begins to taper off and 30-100 1. 05-200 a relatively constant vacuum tends to become established 331- 5; 'ggjgg 10 in most instances after several minutes at a low vacuum. l It is when this condition prevails that addition of deoxidizers may be advantageously made. A gra hic ex- IP &S max'each' ample of this condition may be seen in FIGUIEE 4 of .Llsted 1n the fPHOWmg table are tunes P l m Patent 3,071,458. The most advantageous moment will mmutes) for venous depths of baths for venous slzed vary from installation to installation and will depend on heats of PX type Steels: many factors, such as size of the heat and capacity of the vacuum system. In any event it is obvious to those Depth Time Time $g skilled in the art that when the relatively stable vacuum has been reached a substantial quantity of the included 064 M L 4 deleterious gases have been removed, and this holds true 45':. 2. 20 5.0 35 even if the charge is melted or added under vacuum. It

Z3 5% 23 is thought that over 50%, and perhaps as much as 67%, of the deleterious gases have been removed but these figures are impossible of confirmation in the resent state If tune t the .Steel Should be w l Vacuum 25 of the art because of the difiiculties associate zl with samfer apprexmlately tWlee calculate? e i pling for gas analysis under these conditions. It will also If the equlpment and penmts 1t 15 deslmble that be obvious that should the deoxidizing materials be added the Steel be exposed to a vaeuum the of abeut earlier it may still be said that a substantial quantity of of Hg or less Actual eperanon on medlum carbon the deleterious gases have been removed, using the word Steels h taken place m the 200 to substantial in the sense of more than an insi nificant 500 microns of Hg for a substantial port1on of the time amount a the Steel was under 1 which in turn was a As soon as the vacuum treatment has been completed Stantiel of the e the Steel was exposed to the material may be immediately cast into a mold. One vaeuum conditions h of the great advantages of my process is that no special At the eengnefiieement of the Y l e l t e handling in flake prevention treatment is needed after molten Steel 5 P 5 e z stripping the ingot from its mold. Preferably the degassed y a metena 01 meter S SH 9 y ae We emetal is teemed directly into a chill mold such as an ingot oxldize the molten steel to an appreciable extent pr1or to mold The ingot mold may be reheated as is usual in further processing. In other words, the steel should conmany melt Shops commonl i molds are reheated tain insufficient quantities of elements which combine 40 to temperatures to .3 and Occasionafi some with oxygen more avidly than does carbon at atmospheric What higher 0 R is a coon preheating tgmperw and redueed. pressures ture. Whether or not the metal is teemed into a receptacle eependmg aPPhcatwl 5 e fact 18 which has been preheated it is considered that the metal dlselesed that the degassmg 1S eensldereely undergoes a fast cool. The the term fast cool is used in hindered by the presence of t t (or other aetlve contrast to a slow cool as illustrated for example by cooldeoxidizers). Aluminum is a deoxidlzer. It tends to coming the met al in a receptacle in a Vacuum environment bine with oxygen to form A1203 If the oxygen in the or in an inert atmos here Teemin into a sand mold a melt combines with aluminum, the carbon monoxide boil, ra hits mold or 5 c'onventioial re 6 t 1e h which is an important stirring agent, is considerably reg p h d f h p ae S W duced and consequently the degassing effect is considerm e e some ee pre eating app led to them are ably reduced It Should be understood for example, also hkewise 1ncluded within the amblt of a fast or natural as disclosed in Patent 3,236,635, that degassing is pro- COOL moted by the carbon monoxide boil which results from Set out below 15 a table Se'mng forth VENOUS examples the Combination of f ee Oxygen in the l A the c0 of commercial sized heats of medium carbon low alloy bubbles upwardly it creates a stirring effect which resteels which have been treated in accordance with the duces stratification in the melt. invention.

Pre-Vac. Made 01' H B O a b O 11 0 Heat Grade Weight, Final (3% After Ladle Anal- (p.p.m.) (p.p.m.) (p.p.m.)

Tons 0% Additions ysis 0% Final Bfgzge Final 134, 732 FX 32. 0 54 .551 58 1. 5 10 134, 002 4340 33. 0 330 .330 40 1. 5 37 13 3:23 22:3 at a 13 a 234, 924 FX 32. 0 595 .614 040 1. 2 24 b Pin tube samples. 0 Solid core samples.

Accordingly, degassing should be carried out in the absence of aluminum, silicon or other active deoxidizers. When deoxidizers sufficiently active to deoxide the molten metal to an appreciable extent, such as aluminum or The Final C% figures are based on furnace analysis, the Pre-Vacuum are calculated based on carbon additions, and the Made figures are based on ladle analysis at the end of vacuum treatment. It will be noted from the above silicon, or both, are added, they should be added to the examples that the carbon content of the batch was mainrained no lower than substantially the same level as existed prior to the addition of the deoxidizing material. That is, the carbon content of the bath at the end of furnace treatment was substantially the same as the carbon content of the treated melt. In heat number 134,980 about 5 points of carbon had been added to the ladle to compensate for the slight loss of carbon which occurs during the vacuum treatment for example. It will be noted how ever that in all other instances the carbon content Was maintained relatively uniform between end of furnace treatment and finish of the vacuum treatments.

Various other steps may be employed to advantage in the process. Thus, the degassing action may be enhanced by reducing the thickness of the slag layer on the bath before exposure to the vacuum all as more fully set forth in the aforesaid co-pending applications. Further a synthetic slag may advantageously be added to the bath at the conclusion of the vacuum treatment and the process may be carried out within the permissible temperature drop limitation of the melt, all as further amplified in the copending applications, of which this application is a continuation-in-part.

Further, the effectiveness of the process may be enhanced by (a) conserving as much heat as possible during subjection to the vacuum, irrespective of whether or not additional heat is supplied from an external source, (b) using a metal holding receptacle having the most efficient physical configuration, which may be a thin flat bath if heat loss is not a problem, or a spherical configuration if heat loss is a problem, and (c) freeboard. All these factors are further amplified in the aforementioned copending applications to which reference is made for a more complete understanding.

Although a broad disclosure relating to a good many alloy steels and a preferred disclosure relating to particular types of low alloy steels have been disclosed, it will be understood that the invention is described in illustrative terms only. Accordingly, the scope of the invention should only be limited by the scope of the following appended claims.

I claim:

1. In a method of making steel having a final carbon content of from about .15 to 2.25, at least one additional alloying element and a low inclusion content, the steps comprising providing a melt which contains oxygen as an undesirable element in a treatment vessel,

said melt containing from about .15 to about 2.25 carbon, but insufficient quantities of a substance selected from a group consisting of vanadium, silicon and manganese to materially inhibit the C+O- CO reaction at treatment pressures,

subjecting the said melt to an absolute pressure sufficiently low to promote the C+O CO reaction, maintaining said low absolute pressure, and consequently the C+O- CO reaction, until the oxygen available for inclusion formation has been lowered to a level at which the desired final low inclusion content Will be attained, and thereafter adding a substance selected from the group consisting of vanadium, silicon and manganese.

2. The method of claim 1 further including the steps of forming the molten melt in a furnace and, thereafter transferring the melt to a degassing ladle.

3. The method of claim 1 further including the step of decreasing the thickness of the slag layer on the melt in the treatment vessel prior to subjection to the low absolute pressure.

4. The method of claim 3 further including the step of adding a synthetic slag at the conclusion of the vacuum treatment of the melt in its decreased slag cover condition.

5. The method of claim 1 further characterized in that the melt is subjected to a vacuum approaching 2 mm. Hg absolute or below for a substantial portion of the time the molten metal is subjected to vacuum.

6. The method of claim 5 further including the step of decreasing the thickness of the slag layer on the melt in the treatment vessel prior to subjection to the vacuum.

7. The method of claim 6 further including the step of adding a synthetic slag at the conclusion of the vacuum treatment of the melt in its decreased slag cover condition.

8. The method of claim 1 further characterized in that the melt is subjected to a vacuum approaching 500 microns Hg absolute or below for a substantial portion of the time the molten metal is subject to vacuum.

9. The method of claim 8 further including the step of decreasing the thickness of the slag layer on the melt in the treatment vessel prior to subjection to the vacuum.

References Cited UNITED STATES PATENTS 963,652 7/1910 Reynolds 49 1,277,523 9/1918 Yensen 75-49 1,554,368 9/1925 Rackoif et a1. 7549 2,068,785 1/1937 Bain et a1. 7549 2,093,666 9/1937 VOgt 7549 2,726,952 12/1955 Morgan 75*-49 2,776,204 1/1957 Moore 7549 2,929,704 3/1960 Harders 75--49 2,993,780 7/1961 Allard 7549 3,084,038 4/1963 Finkl 7549 3,125,440 3/1964 Hornak et al 7549 OTHER REFERENCES Hornak et al.: Journal of Metals, vol. 10, July 1958, pages 471 and 475.

Ziegler: Transactions, AIMME, Iron and Steel Div., 1929, pages 428-440.

DAVID L. RECK, Primary Examiner. HYLAND BIZOT, Examiner. H. TARRING, Assistant Examiner.

Claims (1)

1. IN A METHOD OF MAKING STEEL HAVING A FINAL CARBON CONTENT OF FROM ABOUT .15 TO 2.25, AT LEAST ONE ADDITIONAL ALLOYING ELEMENT AND A LOW INCLUSION CONTENT, THE STEPS COMPRISING PROVIDING A MELT WHICH CONTAINS OXYGEN AS AN UNDERSIRABLE ELEMENT IN A TREATMENT VESSEL, SAID MELT CONTAINING FROM ABOUT .15 TO ABOUT 2.25 CARBON, BUT INSUFFICIENT QUANTITIES OF A SUBSTANCE SELECTED FROM A GROUP CONSISTING OF VANADIUM, SILICON AND MANGANESE TO MATERIALLY INHIBIT THE C+O-->CO REACTION AT TREATMENT PRESSURES, SUBJECTING THE SAID MELT TO AN ABSOLUTE PRESSURE SUFFICIENTLY LOW TO PROMOTE THE C+O-->CO REACTION, MAINTAINING SAID LOW ABSOLUTE PRESSURE, AND CONSEQUENTLY THE C+O-->CO REACTION, UNTIL THE OXYGEN AVAILABLE FOR INCLUSION FORMATION HAS BEEN LOWERED TO A LEVEL AT WHICH THE DESIRED FINAL LOW INCLUSION CONTENT WILL BE ATTAINED, AND THEREAFTER ADDING A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF VANADIUM, SILICON AND MANGANESE.

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