CA2021221A1 - Calcium alloys with improved grindability - Google Patents

Abstract

ABSTRACT
A calcium alloy having sufficient hardness to be mechanically reduced to particulates and powders by grinding, milling or other similar means. The addition of silicon has been found to improve the grindability of calcium metal and calcium-aluminum alloys. A typical alloy consists of Si in the range 1 to 12% with the balance Ca and minor impurities or Si in the range 0.5 to 10%
and Al in the range 0.5 to 10%, the balance Ca and impurities, the total Si and Al content not exceeding about 12%.

Description

This invention relates to calcium alloys having a hardness which aids in the mechanical reduction of the alloy to particulates and powders by grinding, milling or other similar abrasive means. As used herein, the terms grinding and grindability refer to mechanical size reduction by these types of processes.
Calcium metal and alloys are widely used in the treatment of high quality molten steel to modify the morphology of oxide and sulphide nonmetallic inclusions. For example, when added to aluminum deoxidized steel, calcium reacts with dissolved o~ygen and solid alumina nonmetallics in the molten steel to form calcium aluminate inclusions which consist of various proportions of CaO
(lime) and A1203 (alumina).
A1203 iS the principal nonmetallic inclusion present in aluminum deoxidized steel and has a melting point ot 2050C indicating it is solid at typical molten steel processing temperatures (1500 - 1600 GC). Solid A1203 nonmetallics have been reported to cause serious nozzle blockage problems particularly during continuous casting aluminum killed steels. Furthermore, the presence of galaxies of finely divided Al;~03 inclusions can seriously affect the quality of the finished steel product.
The rnelting point of this nonmetallic inclusion is lowered when CaO
is added to A1203 to form a calcium aluminate inclusion. Calcium aluminates containing between about 46% to 55% A1203 the balance being CaO, are liquid below about 1500C. When present in molten steel, liquid calcium aluminates tend to agglomerate and float up into the surface slag thereby resulting in less nozzle blockage and a cleaner steel with fewer quality problems.

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2 ~ ~2:~21 Since the compositional range over which calcium aluminates are liquid is very narrow, the addition of calcium to molten steel must be carefully controlled to achieve the correct balance between A1203 and Ca5:) to obtain the desired liquid nonmetallic inclusions. The addition of too little or, conversely, too much calcium generates solid calcium aluminates which in turn can contribute to nozzle blockage during continuous casting. Because of the narrow compositional range of liquid inclusions, it is often difficult i~or the steelmaker to achieve the desired effects.
Because calcium metal boils at 1484~C, it will readily vaporize when added to molten steel leading to low and inconsistent recoveries which in turn exacerbate the control problems associated with producing liquid calcium aluminates. To achieve the desired control, it is necessary to introduce calcium deep under the surface of the steel pool to take advantage of the ferrostatic pressure to retard ~oiling and to maximize the residence time o~ calcium gas bubbles as they rise to the surface of the steel ~ath.
Submerged injection through a refractory lance of powdered reagents carried by an inert gas or, alternatively, mechanical feeding of a continuous cored wire (that is powdered reagents encapsulated in a steel jacket) are the preferred technologies ~or introducing reagents deep under the surface of a molten steel pool. To utilize either of these technologies, calcium metal and alloys must be available as powder preferably less than 6 mesh for cored wire and less than 20 mesh for injection.
Mechanical size reduction of commercial purity calcium metal by rn/~-3 nt ~
milling, grinding, machining, etc. has proven to be very difficult becausP of a smearing action which acts to clog the various tools employed.
It is known to those skilled in the art that calcium metal can be embrittled by alloying with magnesium and/or aluminum thereby enabling effective mechanical reduction to powders of suitable size for submerged injection or cored wire ~eeding. The best embrittling effects, that is those which yield the highest calcium content in the resulting alloy, are obtained when both magnesium and aluminum are present in the alloy. Alternatively, achieving the same embrittling effect with aluminum alone requires about twice the total magnesium plus aluminum levels when both are present in the alloy, i.e.
aluminum by itself is only about 50% as effective as magnesium and aluminum together.
However, it has been found that the presence of even small amounts of magnesium (i.e. more than 1% Mg) counteracts the beneficial role of calcium by producing solid magnesi-um aluminate inclusions which in tum cause nozzle blocl~age during continuous casting of steel.
As pointed out, in the absence of magnesium, it is necessary to substantially increase the amount of aluminum required to achieve the desired embrittlement for effective grindability. The presence of higher amounts o~
aluminum in the alloy is however detrimental for the following reasons:
(i) the net calcium content contained in the alloy is substantially lower than when magnesium and aluminum are present together;

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(ii) it becomes even more difficult to achieve the naIrow compositional window for liquid calci-um aluminates since the addition of a calcium-aluminum alloy to molten steel produces both CaO and A1~03. Hence, adding a calcium-aluminum alloy richer in aluminum significantly increases the amount of A1203 above that already in the melt thereby complicating control of the mod;fication process; and ~iii) the pyrophoricity of the calcium-aluminum alloy increases with increasing aluminum levels which adversely affects the primary production of the alloy, grinding and subsequent handling of the powders.

This invention relates to a calcium alloy ~having sufficient hardness to be mechanically reduced to particulates and powders by grinding, milling or other similar means. In the present invention the addition of silicon substantially improves the grindability of calcium metal and calcium-aluminum alloys.
Although not totally understood, it is believed that the improved grindability is due to the presence of intermetallic compounds in the eutectic structure of the alloy. Table I lists the amount of intermetallic compound present in various calcium binary alloys.

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s Tahle I
The Proportion of lntermetallic Phase in ~7arious Calcium Alloys AlloyIntermetallic % Intermetallic in Allov 9S% Ca - 5% Mg Mg2Ca 9.2%
95% Ca - 5% Al Al2Ca 8.7%
95% Ca - 55'o Si Ca2Si 19.2%
As indicated in Table I, a 95% Ca - 5% Si alloy contains more than twice as much intermetallic phase than do comparable Ca - Mg or Ca - Al alloys.
The addition of silicon to calcium has the following further advantages:
(i~ silicon is specified in most steel grades and hence its presence is compatible with the majority of steel chemistries;
(ii) unlike magnesium, silicon in the alloy does not exacerbate nozzle blockage during continuous casting;
(iii~ unlike aluminum, silicon in the a]loy does not increase pyrophoricity or complicate the modification process since it does not alter the amount of A1203 in the steel bath; and (iv) when silicon is added in combination with aluminum to the calcium, the amoun~ of aluminum in the alloy can be substantially decreased without deleteriously affecting rn/

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grindability. In addition, this ternary alloy is considerably less pyrophoric than calcium-aluminum binary alloys.
Calcium-sil;con alloys are prepared by melting calcium metal and adding a suitable source of silicon such as silicon metal, ferrosilicon or calcium silicide.
Calcium-silicon-aluminum alloys are prepared in much the same fashion as calcium-silicon alloys except than an appropriate amount of alumimlm metal or alloy are added to the molten bath. Alteratively, a silicon-aluminum rnaster alloy of suitable composition can also be added to molten calcium.
Once prepared, the calcium-silicon or calcium-silicon-aluminum alloys are cast into suitably sized ingots which are subsequently ground by mechanical means to produce appropriately si~ed calcium alloy particulate and powder.
It is noted that if ferrosilicon is used as the silicon source to prepare the final calcium alloy or an intermediate master alloy, that because of its negligible solubility in calcium, iron will not actually dissolve in the alloy but instead will remain as finely divided iron particles held in suspension. The presence of finely divided iron particles in the alloy, typically in the amount of 4% or less~ does not adversely affect either grindability or chemistry since the calcium alloy will be ultimately added to molten steel.
EXAMP_ Rockwell Hardness tests were conducted on various calcium alloys as a means of assessing their grindability. Higher hardness results in the alloy exhibiting less p]astic flow under stress thereby indica~ing a lower ~endency to rn/

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smear during grinding. As previously identified, calcium metal is difficult to grind because of a smearing action which acts to clog the various tools employed.
Table II shows the effec$s of alloy composition on Rockwell Hardness.
Table II
Hardness of Various Calcium Allovs Rockwell H
60 k~: 1~Ball Allov % of Full Scale (1) Commercial grade Metal Ca Metal 50 (2) Ca-Al Allovs 5.4% Al 78 8.0% Al 97 (3) Ca-Si Alloys 4.% Si 7~
10.0% Si 96 (4) Ca-Si-AI Allovs 1.2% Al - 1.6% Si 70 3.5% Al - 3.0% Si 81 4.5% Al - 3.7% Si 91 4.8% Al - 2.9% Si 95 ~s indicated in Table II, calcium-aluminum alloys are substantially harder than pure calcium metal which in turn results in improved grindability.
As discussed previously, however, increasing the aluminum content of the alloy is not desirable because of increased pyrophoricity and aluminum's complicating effects on controlling calcium modification of solid aluminum inclusions to the desired liquid calcium aluminates.
Table II illustrates that comparable improvements in hardness can be rn/

achieved by alloying calcium with silicon or a combination of silicon and aluminum thereby indicating that these alloys will have similar grinding characteristics to the calcium-aluminum binary alloys.
However, these Ca-Si and Ca-Si-AI alloys contain substantially less aluminum which in turn makes them less pyrophoric and more predictable for achieving the narrow compositional window ~or liquid calcium aluminates.
It will be mderstood that these calcium alloys are prepared from commercial grade metals and alloys and hence the final alloy may contain other minor or trace impurities such as, but not exclusively, iron, manganese, zinc, magnesium, oxygen and nitrogen. It is also understood that minor variations in alloy composition which do not significantly alter its performance as resorted to by those skilled in the art are considered to be within the scope of this invention.

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Claims (5)

1. A calcium alloy adapted for comminution and subsequent treatment of steel consisting of Si in the range 1 to 12% with the balance Ca and minor impurities.

2. A calcium alloy as set out in claim 1 consisting of Si in the range 2 to 8%.

3. A calcium alloy adapted for comminution and subsequent treatment of molten steel consisting of Si in the range 0.5 to 10% and Al in the range 0.5 to 10%, the balance Ca and impurities, the total Si and Al content not exceeding about 12%.

4. A calcium alloy as set out in claim 3 consisting of Si in the range 1 to 8% and Al in the range 1 to 8%.

5. A calcium alloy as set out in claim 1, claim 2, claim 3 or claim 4 wherein the alloy also contains up to about 4% Fe.