US2755181A - Process of introducing boron into ferrous metal - Google Patents

Description

July 17, 1956 PROCESS OF m'monucmc BORON INTO FERROUS METAL Filed Oct. 9. 1952 E.' SPIRE 2,755,181

2 Sheets-Sheet 1 I0 74 I8 TIME (no/1v.)

INVENTOR.

ie n pin A RNEY July 17, 1956 Filed 001:. 9. 1952 R o c/rwsu "c ma RD N588 E. SPIRE 2,755,181

PROCESS OF INTRODUCING BORON INTO FERROUS METAL 2 Sheets-Sheet 2 DISTIANCE FROM Ql/E/VCHED END 0; STANDARD BAR- SIXTEENT/IS 0F INC/I INVENTOR:

Ejz'enne Spine,

- 2,755,181 Patented July 17, 1956 PROCESS OF INTRODUCING BORON INTO FERROUS METAL Etle e S ire lVlontreal, Quebec, Canada, asslgnor to L A lr Lll nlile, Societe Anonyme pour lEtude et lExploltation des l'rocetles Georges Claude, Parls, France Application October 9, 1952, Serial No. 313,934

3 Claims.

This invention concerns a new process for making ferrous metals containing boron.

Boron is used in steel, for example, to increase hardenability of the metal, and thereby replace other alloys such as manganese, nickel, chromium, molybdenum. The ability of boron to replace several hundred t1mes its own weight of any of these metals makes possible the use of plain carbon steels or lower alloyed steels for heat treated mechanical parts. Boron steel is used mostly as wrought steel and much less as cast steel.

PRIOR ART The normal procedure to add boron to steel, is to use either ferro-boron, or complex ferro-alloys. A number of boron-containing alloys are now available for this purose.

p Those alloys contain from .5% to boron. The ferro-boron contains only boron, silicon and iron. The complex ferro-alloys contain boron and also titanium, zirconium, aluminum and manganese. Finally, boron can be introduced into steel by the addition of borax to the ladle or ingot mold, especially if a suitable reducing agent is mixed with the borax.

However, the results have not been satisfactory, because there is much variation in the recovery and an everpresent danger of hot-shortness and brittleness resulting from overdosing the steel with boron. In other words, it is a known fact that it is detrimental to the steel to add more than .006% boron, and in using borax, more boron than desirable may be introduced.

Another difiicult problem is the control of nitrogen in the steeland the proper proportioning of boron to nitroen. g THE INVENTION The applicant has now found that these difiiculties and problems can be avoided and good boron steel produced by simultaneously reducing the acid soluble nitrogen content in a ferrous metal, for example, steel, while introducing boron from a reducible borating agent, for example, alkali metal borate or an alkaline earth metal borate or boron oxide, reduced with a reducing agent, for example, aluminum or carbon. This operation is according to the invention performed outside the furnace where the steel is made, in a suitable container, for example, a ladle. The

nitrogen is continuously eliminated by flushing the metal with a non-oxidizing nitrogen-free gas, for example, argon or helium, i. e., by passing through the molten metal gas bubbles. This flushing action promotes a strong agitation of the free surface of the metal, which is then utilized to stir the highly basic slag to which the borating agent and reducing agent have been added.

The invention will be described in greater detail in the following description and reference will also be made to the accompanying drawings, in which,

Fig. 1 is a graph illustrating the results of Example 1 in.

terms of simultaneous reduction of the acid soluble nitrogen existing in the steel and the introduction of boron into the steel.

Fig.2 is a graph illustrating the results of Example 2 in tests of hardenability of the steel as measured by the end-quench test.

A preferred procedure, according to the invention, is as follows:

v The steel is made as usual in any type of furnace, for example, open hearth, basic or acid, electric, basic or acid.

Before tapping the heat in the preheated ladle, the furnace slag is carefully eliminated from the steel. This operation is done preferably in the furnace but apart from the inconvenience could be done in the ladle. The heat is then deoxidized or blocked," following the usual practice, with a deoxidizing agent, for example, ferro-silicon, ferro-manganese and aluminum, .and tapped immediately in the ladle. This deoxidation can also be done while filling up the ladle. I

llefore or after tapping into the ladle a solid highly basic flux having the preferred following composition is put in the ladle:

Percent 0:10 50 to 60 A: 10 to 20 ggfizx 10 to 15 5 to 10 T102ZrO2 10 to 20 The total weight of this flux should preferably be maintained between 1% and 6% by weight of the metal. The ladle' Wlll. be filled up preferably to half of its content, first to let the special prepared slag take the temperature of the steel, usually from about 1550 C. to about 1620" C. depending on its carbon content, and the rest of the metal contained in the furnace is tapped.

During the interval, more deoxidizer (apart from that used to reduce the borate) is added to the steel in the form of aluminum: .5 1b.? ton to 3 lbs/ton of steel. Some of this can be added in the form of powder to induce an exothermic reaction either with the iron oxide existing in the slag or with specially added iron oxide.

Then, using any convenient means, a nitrogen-free gas, for example, argon, is introduced through the metal in the ladle. The preferred means is the porousbottom ladle described in U. S. patent applications Serial No. 100,699, filed June 22, 1949, and Serial No. 193,581, filed Novemher 2, 1950, both now abandoned. The flow of the gas is regulated to give a strong boiling action of the whole mass of metal.

Using the above-mentioned ladle up to a 30-ton ladle, the rate of flow, for a porous plug four inches in diameter, will be within 2 literslminjton and 20 liters/min./ ton. The flow can be continuous or pulsating. This means flowing the gas in, then stopping, then starting again, thus avoiding undue turbulence which may have undesirable cooling effect.

By way of further example, the following test results are given:

one pound'of aluminum and 2.5 calcium silicide per ton had been added, was tapped in a special ladle with a 3 porous plugs, as described in U. S. patent application Ser. No. 100,699 and Ser. No. 193,581. In this ladle was placed 2% by weight of a slag having the following ditional 2 lbs. per ton of aluminum. The injection of argon started, and the rate :of flow regulated at 7 liters per minute, thus creating a vigorous mixing action of composition: x the flux which melted in less than 45 seconds.

Percent The experiment lasted 4% minutes, at which time C 0 g f 54 the steel remaining in the ladle was poured into ingots. A1 0; 22.5 From the keel blocks cast during this experiment, chemip; 13.5 cal and mechanical tests were made which gave the fol- Borax 9.1 10 lowing results: C .9

. TABLE 2 While filling up the ladle with the molten metal, more deoxidizer was added, i. e., 2 lbs. per ton of aluminum. Percent Through the porous refractory, argon was introduced at oimnmewnh minute, a rate of flow of about 60 liters per minute, for five one Total Boron minute periods spaced apart three minutes. The plug was a silicon carbide plug 3% inches top diameter and 4 0 m inches high. 2: :oos I2 The following results were obtained, which are also .005 .013 shown in the graph of Figure 1.

TABLE 3 TABLE 1 Percent Percent P. s. 1. Sample No. Typeolstesl Ultimate Reduc- Impact Time iiigiiif' m iii e? Acid Acid Total Cm mm, Tnanlnhh mm 13 tlcn 0! Area Kegge nitrogen Nitrogen 31-3 bom 3312%:1133113:1331:: 1% 1% 33% 2% 111-16 3% 3 35 minutes .0010 .ooss .0055 30 is minu .010

HARDENABILITY Although the steel obtained contains 010% boron, Em 2 mm the a mm mm which is very much above the usual amount accepted, by the n m the mechanical tests gave the following results: 36 T sample B14 with no how has a hardenabnity Ummatc strength a. 32,000 w i h i quite inferior-to sample 31-15 which has boron Elongation "mm" mtroduced by the apphcant's procedure. ROdllCllOl! of 81138 d0--.-- 46-5 DISCUSSION OF THE RESULTS It can be seen from Table l and Figure 1 that the acid It can be m b y making steel according to the zgg g g gz z i Mum! and has been present invention, extremely good mechanical properties Consequently, even without titanium and mco nium, 2: zz ggg g a qmte. mm'kabk Wha is particularly to be noted the applicant has avoided, to the maximum, the formation is the fact that hm is almost no reduction in the im of boron nitride by flushing out the acid soluble nitrogen. PM Open, of me ml mu With a minimum of boron nitride present, there seems while the invention been b w t to be no over-dosing of boron liable to take place, and example primnfly in term, of steel muse L is proven by the very good results of the mechanical preferred application, it an be .ppued to other f ms Exam k 2 5 metals for analogous purposes, for example, boron can P be added to cast iron in order to accelerate the malle- In a 500 pound basic lined induction furnace, approxiabllllml treatment mately 450 lbs. of steel were melted. This steel had the The reducible 50115118 8 which hive een found following analysis: suitable for use in the invention are alkali metal boratea Percent or alkaline earth metal borates, or both, or boron oxides c b 24 or mixtures with the other borates. Preferred borates Manganese 1,00 are sodium borate and potassium borate. sil 0 The applicant's process is applicable to all steels includ After melting, this steel was poured into a special ladle mt at-eel toned M mm mum for gaseous agitation with a porous plug assembly located with an n is 'pphuble to t th bott fth t descn :10. 1331639 .fh ie N... 193,'?8i.fi Z1"?".i.'e M which is following analysis was prepuedz. vention operates, the applicant furnishes the following Percent discussion of his theory. Limc 5s 5 65 The hardenability of steel is controlled by the rate of Alumina transformations of austenite to martensite. The slower Fluompar the rate, the deeper the steel hardens. This property is 9 convemcf'uy measured l! the End-quench" test Carbon 1 gr Jominy .hndgmboron occurs in steel in two 7o orms-one sou in me sulphuric acid, the other This slag was preheated at 700' C. The metal was insoluble in the same acid. The acid insoluble form is tapped at 1650 C. into the ladle and was killed with 1 boron nitride. It a apparently only the acid soluble $12151; gfelaltlgrlilnnum per ton and 2% pounds of calcium boron in steel that retards the rate of transformation austenite-martensite, or increases After filling up the ladle with the steel, the preheated material. the hudmbmty of the slag was placed on the surface of the metal with an ad- Ifthesteelcontainsmochnitrogemitwillcombine with boron to form acid "insoluble" boron (boron nitride).

Consequently, in order to have the maximum amount of acid soluble boron, which is the only one having an influence on the transformation of the steel and by way of consequence of its hardenability, one must limit as much as possible the nitrogen content of the steel, or combine this nitrogen (or stabilize it) with the addition of elements having greater affinity for nitrogeni than boron, for instance aluminum, zirconium, titanium,1*etc.

It is also necessary to protect boron from oxidation, by adding a certain amount of a-strong deoxidizen like aluminum, in order to avoid the formation of boron pp a oxide which would have no efiect on the transforma- 'tion characteristics of the steel (this form of boro n, as

oxide, is also acid soluble). That is why complex ferro-alloys containing not only Ti and Zr, but also A1, are used more frequently than simplerferro-alloys. However, for steel con "ning above .4% carbon, where the oxygen content is low, ferro-boron has "been used, preferably with addition of a small amount of ferrotitanium.

The amount of boron required to assume'optimum results in the steel, lies between .0008% and .003% for the intermediate range of steels, and may be above this range for the low carbon steels.

The presence of a minimum of .0012 acid soluble boron is required. However, small excess of boron over the optimum amount causes brittleness and hot shortness. For instance, with .003%, the steel would be perfect, but with .006%, the steel would be quite brittle. This effect is even more noticeable in cast steels where on account of brittleness, it is not recommended to use boron.

For protection of boron against oxidation, it is usually recommended to have at least .02% Al residual in the steel.

NITROGEN IN STEEL The occurrence of nitrogen in steel is also to be divided in two: acid soluble nitrogen and acid insoluble" nitrogen.

The acid soluble nitrogen is the portion which will have a tendency to form boron nitride, and which is to be stabilized by such agents as zirconium, titanium, aluminum.

INCONVENIENCES OF THE PRIOR ART In open hearth, the limits are .003 -.008 In electrical steel .007 -.0l2 In Bessemer steel .00l2-.020

It is a known fact that nitrogen control in steel is difficult, and that there is an appreciable variation of its content in the furnace while filling the ladle, and even when pouring the ingots.

From a private communication to the inventor, the following figures have been given:

In a certain 20-ton basic electric furnace steel, the total N2 was, after the second slagging off, .006% and .013% at tap.

Sims, in his Paper No. 2347 presented at the AIME in 1948, says: In the later period of the heat, an important increase in nitrogen occurred with the alloys additions and on tapping. In open hearth heats, there Finally Massinon (Revue Universelle des Mines, August 1950, page 261) giyes the following results be- Itlween the first and the lastyingot of a low carbon steel Na total, percent 1st ingo .0059 2nd ingo .0070 5th ingo .0095 6th ingot .0090

.003 N2%, ne must add X 1000=2 lbs. per ton .006 N 2%. one must add X 1000=8 lbs. per ton As the nitrogen analysis cannot be made rapidly on the furnace fioor, it is a hit and miss proposition to add to the steel the right amount of ferro-alloy.

(c) Fading eflect.-When boron is added to the ladle, the effect, as measured by hardenability, tends to decrease or fade towards the last ingot poured (because of the nitrogen variation). It would seem simple to compensate by adding excess boron.

Unfortunately, excessive amounts of boron have, as said before, .undesirable effects, so the addition has to be carefully controlled.

(d) Costa-It can be seen, from paragraph (b), that the cost of theaddition can vary widely according to the mtrogen content of the steel.

Assuming $0.50 a pound for the alloy mentioned, the price, for the addition only, can vary from $1.00 to $4.00 per ton of steel treated.

The process of the invention is preferably carried out similarly to processes as described in United States patent application Serial Number 217,909, filed March 28, 1951, now abandoned.

That application again preferably uses apparatus described in more detail in the aforesaid now abandoned earlier United States patent applications Serial No. 100,- ,699, filed June 22, 1949, and Serial No. 193,581, filed November 2, 1950.

Preferably, therefore, the applicant flushes with the inert gas by using a vessel, for example, a ladle, whose bottom is equipped with a porous plug for introducing the inert gas in finely divided form. Preferably, the plug has a working area of at least 3 square inches and a preferable porosity of between about .1 and about 5 liters per minute, per square centimeter of surface, per centimeter of thickness, per centimeter mercury of pressure calculated when measured on a brucite type refractory at room temperature with air, while various sized vessels, for example, ladles, can be employed to carry out the process of the invention.

The mass of molten metal preferably has a horizontal cross-sectional area of at least about 25 square inches and a depth from the surface to the point of introduction of the inert gas of at'least about 10 inches. The depth of the container is preferably not less than one-half the diameter.

Flushing may, however, be accomplished by lancing or other convenient means to insure contact between a substantial volume of the mass of molten metal and the inert gas, so as to give ample opportunity for the inert gas to perform its function as described above.

Therefore, the term flushing is used to indicate the action of introducing the gas at a point below the sur- .012 N2%, one must add face, preferably .well below the surface of the molten metal, and better still, at the bottom, or close to the bottom, of the vessel in which it is contained, and allowing the gas to pass through the metal and leave it at the top surface.

I claim:

1. A process of introducing boron into a ferrous metal containing normal nitrogen and maintained in the molten state in a container, consisting in simultaneously putting a slag comprising boron oxide and a reducing agent into contact with the molten metal bath and flushing said bath with a nonoxidizing nitrozen-free inert gas.

2. A process of introducing boron into a ferrous metal containing normal nitrogen and maintained in the molten state in a container, consisting in simultaneously supplying to the surface of the molten metal bath a slag containing a borating agent and a reducing agent, and flushing said bath with a non oxidizing nitrogen-free inert gas.

3. A process of introducing boron into a ferrous metal containing normal nitrogen and maintained in the molten state in a container provided in its bottom with a porous 8 plug for introducing a gas in a finely divided form through the molten metal bath, consisting in simultaneously supplying to the surface of the molten bath a slagkomprismg a borating agent and a reducing agent, and flushing said bath through said porous plug with a non oxidizing, nitrogen-free inert gas.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Transactions of the ASM, vol. 40, 1948, 111 1122, 1127. pm

Claims (1)

1. 3. A PROCESS OF INTRODUCING BORON INTO A FERROS METAL CONTAINING NORMAL NITROGEN AND MAINTAINED IN THE MOLTEN STATE IN A CONTAINER PROVIDED IN ITS BOTTOM WITH A POROUS PLUG FOR INTRODUCING A GAS IN A FINELY DIVIDED FORM THROUGH THE MOLTEN METAL BATH, CONSISTING IN SIMULTANEOUSLY SUPPLYING TO THE SURFACE OF THE MOLTEN BATH A SLAG COMPRISING A BORATING AGENT AND A REDUCING AGENT, AND FLUSHING SAID BATH THROUGH SAID POROUS PLUG WITH A NON OXIDIZING NITROGEN-FREE INERT GAS.

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