Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2001-05-11
2002-09-17
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C420S590000
Reexamination Certificate
active
06451087
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for producing a mother alloy for an iron-based amorphous alloy containing a sufficiently small amount of Al, which is harmful to the surface conditions and toughness of the amorphous alloy.
BACKGROUND OF THE INVENTION
Amorphous alloy ribbons are widely used for cores for transformers, choke coils, etc. Nanocrystalline alloy ribbons having microstructures having crystal grain sizes of several tens of nanometers are also widely used for various magnetic parts because of their excellent soft magnetic properties. The nanocrystalline alloy ribbons are produced by heat-treating the amorphous alloy ribbons.
These magnetic alloy ribbons are generally produced by melt-quenching methods such as a single-roller method, a double-roller method, etc. and wound to form toroidal magnetic cores. The details of the melt-quenching methods are described in the chapter of “Melt-Quenching Method” in Masumoto, et. al., “Amorphous Alloys.” For instance, the single-roller method is a method of ejecting a molten alloy onto a rapidly rotating cooling roller, so that the molten alloy is quenched to continuously form a ribbon.
Iron-based magnetic alloys having various compositions have been developed depending on their applications. Among them, iron-based magnetic alloys containing Nb are known to have excellent soft magnetic properties. For instance, Japanese Patent Publication No. 60-38454 discloses amorphous Fe—Nb—Si—B alloys having excellent effective magnetic permeability. As described in Japanese Patent Publication No. 4-4393, nanocrystalline, magnetic Fe—Cu—Nb—Si—B alloys have excellent soft magnetic properties. Nb functions as an element for decreasing a magnetostriction to improve soft magnetic properties in the amorphous alloys, and an element effective for making crystal grains finer in the nanocrystalline alloys. In these soft magnetic alloys containing Nb, high-purity Nb has conventionally been used as a raw material, because high-purity Nb contains small amounts of impurity elements such as Al harmful to magnetic properties and toughness of the amorphous alloys. However, because the high-purity Nb is extremely expensive as a raw material, it makes the production cost of alloy ribbons higher.
From the industrial point of view, ferroniobium is preferable, because it is as cheap as {fraction (1/10)} of the high-purity Nb. However, the ferroniobium contains impurity elements, particularly Al in an amount of about 0.1-2 mass %, because the ferroniobium is produced by a Thermit method. The Thermit method is a refining method using fine Al powder for reducing metal oxides such as Nb
2
O
5
. Thus, there remains a relatively large amount of Al in ferroalloys (iron alloys) such as ferroniobium produced by the Thermit method.
Al contained as an impurity is oxidized to Al
2
O
3
during melting. In the case of producing an amorphous alloy ribbon by a single-roller method, a slag or inclusion based on Al
2
O
3
are accumulated in a nozzle for ejecting a molten alloy, resulting in streaks on the ribbon surface in its longitudinal direction. If there were streaks on the surface of the ribbon, a toroidal coil formed from the ribbon would have a low packing factor because of surface roughness, resulting in failure to the miniaturization of the toroidal coil and extremely poor magnetic properties. Also, because Al
2
O
3
remains as a non-metallic inclusion in the alloy ribbon, the ribbon is brittle, because the non-metallic inclusion acts as sites for fracture. If the ribbon were brittle, it would be cut when wound to a toroidal coil, resulting in extremely disturbed processes and poor yield.
OBJECT OF THE INVENTION
Accordingly, an object of the present invention is to provide a method for producing a mother alloy suitable for iron-based amorphous alloys having a low Al content, capable of providing amorphous alloy ribbons with excellent magnetic properties free from streaks and brittleness, even when inexpensive ferroalloys such as ferroniobium containing a relatively large amount of Al are used as raw materials.
DISCLOSURE OF THE INVENTION
The method for producing a mother alloy for an iron-based amorphous alloy according to one embodiment of the present invention comprises the steps of (a) melting raw materials for elements constituting the amorphous alloy together with at least one oxide of an element constituting the amorphous alloy, the raw materials containing aluminum as an inevitable impurity, and the oxide having a smaller standard free energy of formation than that of Al
2
O
3
in an absolute value; and (b) removing the resultant Al
2
O
3
from the melt, thereby reducing the content of aluminum to 50 ppm or less in the melt.
The method for producing a mother alloy for an iron-based amorphous alloy according to another embodiment of the present invention comprises the steps of (a) melting raw materials for elements constituting the amorphous alloy to form a melt, the raw materials containing aluminum as an inevitable impurity; (b) blowing an oxygen-based gas into or onto the melt; and (c) removing the resultant Al
2
O
3
from the melt, thereby reducing the content of aluminum to 50 ppm or less in the melt. In the method according to another embodiment of the present invention, at least one oxide of an element constituting the amorphous alloy is preferably used together with the raw materials for elements constituting the amorphous alloy, the oxide having a smaller standard free energy of formation than that of Al
2
O
3
in an absolute value.
Ferroniobium is preferably used as one of raw materials for elements constituting the amorphous alloy. The oxide of an element constituting the amorphous alloy is preferably at least one of iron oxide, copper oxide, silicon oxide and boron oxide, more preferably iron oxide (Fe
2
O
3
).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
One of the important features of the present invention is to use at least one oxide of an element constituting the amorphous alloy, which has a smaller standard free energy of formation than that of Al
2
O
3
in an absolute value. The standard free energy of formation of a substance is defined as the free energy increase of the reaction in a standard state at 25° C., in which the substance is formed from elements. The standard free energies of formation are described, for instance, in the column of “Standard Free Energies of Formation for Oxides” in “Metal Data Book,” edited by the Japan Institute of Metals, and in the figures of standard free energies of formation for oxides in “New Lecture on Metallurgy, Nonferrous Metal Refining” of the Japan Institute of Metals.
For instance, when ferroalloys such as ferroniobium and ferroboron, which contain a relatively large amount of Al, are combined with Fe
2
O
3
, one of the oxides having a smaller standard free energy of formation than that of Al
2
O
3
in an absolute value, Fe
2
O
3
is reduced to iron, a main element constituting the iron-based amorphous alloy, by the oxidation-reduction reaction of Fe
2
O
3
+2Al=Al
2
O
3
+2Fe. On the other hand, Al contained as an inevitable impurity in the ferroalloys are oxidized to Al
2
O
3
, which floats as a slag on a surface of the resultant melt. By removing Al
2
O
3
, the melt is purified, with a reduced amount of Al harmful to the surface conditions and toughness of an amorphous alloy ribbon, which is to be formed from the mother alloy. Metal elements formed as by-products of Al
2
O
3
can be used as elements for constituting the amorphous alloy. For instance, Fe formed from Fe
2
O
3
by the above oxidation-reduction reaction constitutes a main element for the iron-based amorphous alloy.
Oxides usable in the present invention are iron oxide, copper oxide, silicon oxide and boron oxide. The iron oxide may be Fe
2
O
3
, FeO, Fe
3
O
4
or mixtures thereof. The copper oxide may be CuO, Cu
2
O or mixtures thereof. The silicon oxide may be SiO
2
, and the boron oxide may be B
2
O
3
.
The amount of the oxide added to the melt may be adjusted depending on the Al contents in the
Bizen Yoshio
Meguro Takashi
Mishima Setsuo
Hitachi Metals Ltd.
King Roy
McGuthry-Banks Tima
Sughrue & Mion, PLLC
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