Fe-Al ALLOY PRODUCTION METHOD

Abstract

A method for producing Fe—Al alloy includes: ingot-producing by casting Fe—Al alloy and taking out the Fe—Al alloy from a mold to obtain an ingot, the Fe—Al alloy containing Al: 2.0 to 9.0 mass % and a balance of Fe and impurities; hot-forging the ingot to form a hot-forged material; hot-rolling the hot-forged material to form a hot-rolled material; oxide-film removing by removing an oxide film of the hot-rolled material to form a material for cold rolling; cold-rolling the material for cold rolling to form a cold-rolled material; and annealing the cold-rolled material. Heating of the ingot in the hot-forging starts before a surface temperature of the ingot taken out from the mold is cooled to less than 250° C. in the ingot-producing.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation, filed under 35 U.S.C. §111(a), of International Patent Application No. PCT/JP2013/059517, filed Mar. 29, 2013, which claims foreign priority benefit to Japanese Patent Application No. 2012-084504, filed Apr. 3, 2012, both of which are hereby incorporated by reference in their entireties.

BACKGROUND

1. Field

The present invention relates to a method for producing Fe—Al alloy.

2. Description of Related Art

For example, as noise and vibrations from an automobile or similar means has become an increasingly urgent problem, needs for metal excellent in vibration damping characteristics (hereinafter referred to as vibration-damping alloy) have been increased. As the vibration-damping alloy, Fe—Cr—Al alloy, Fe—Co—V alloy, Mn—Cu alloy, Mg alloy, or similar alloy is known and is employed for various applications. Among these materials, the Fe—Al alloy is known as a metal whose raw material cost is inexpensive and that exhibits excellent vibration damping characteristics. Fe—Al alloy containing Al of 2 to 12 mass % is also known as exhibiting excellent soft magnetic property (Patent Document 1). The following method for producing the Fe—Al alloy excellent in the above-described vibration damping characteristics is disclosed (Patent Document 1). Plastic forming is performed on alloy containing Al of 2 to 12 mass % and a balance of Fe and inevitable impurities. The alloy that has been subjected to the plastic forming is cold-rolled. Finally, the alloy on which the cold rolling process has been performed in such a manner that a reduction in area becomes 5% or higher is annealed at a temperature condition of 400 to 1200° C.

Patent Literature

Patent Document 1: PCT Patent Publication No. WO 2006/085609

SUMMARY

As described above, usefulness of the Fe—Al alloy is expected. However, according to examinations by the present inventors, toughness of the Fe—Al alloy at an ordinary temperature is extremely low. It was apprehended that if the ingot size became large, for example, 100 kg or more, due to a difference in a cooling rate between the inside of and the surface of the ingot and a handling operation such as removal and agglomeration generated a crack. Since the Fe—Al alloy contains a large amount of Al, which is likely to be oxidized, the skin of the ingot is likely to be rough and generation of a crack was apprehended also due to low toughness. The object of the present invention is to provide a method for producing the Fe—Al alloy that allows effectively preventing a crack during production.

The present inventors have obtained the following knowledge. Ingot produced by casting Fe—Al alloy has a columnar structure of coarse structure. Moreover, while a risk of crack caused by the property of the ingot is high, once performing a hot-forging step allows reducing a risk of the crack. Then, based on the knowledge, the inventors proceeded the examinations. Consequently, the inventors have found that the toughness of the Fe—Al alloy was able to be significantly improved at a specific temperature or more, and transition to the hot-forging step in a state where the ingot was not cooled to less than a predetermined temperature allowed reducing the risk of the crack. Thus, the inventors have achieved the present invention.

That is, the present invention is a method for producing Fe—Al alloy includes an ingot-producing step, a hot-forging step, a hot-rolling step, an oxide-film removing step, a cold-rolling step, and an annealing step. The ingot-producing step casts Fe—Al alloy and taking out the Fe—Al alloy from a mold to obtain an ingot. The Fe—Al alloy contains Al: 2.0 to 9.0 mass % and a balance of Fe and impurities. The hot-forging step hot-forges the ingot to form a hot-forged material. The hot-rolling step hot-rolls the hot-forged material to form a hot-rolled material. The oxide-film removing step removes an oxide film of the hot-rolled material to form a material for cold rolling. The cold-rolling step cold-rolls the material for cold rolling to form a cold-rolled material. The annealing step anneals the cold-rolled material. Heating of the ingot in the hot-forging step starts before a surface temperature of the ingot taken out from the mold is cooled to less than 250° C. in the ingot-producing step. In the present invention, the Fe—Al alloy may further contain Nb of 1.0 mass % or less. In the present invention, an ingot surface removing step of removing an oxide film of the ingot surface may be performed before cooling the surface temperature of the ingot obtained by the ingot-producing step to less than 250° C., and subsequently the step may proceed to the hot-forging step. In the present invention, the annealing step may be performed in non-oxidizing gas atmosphere.

According to the present invention, a crack generated during producing Fe—Al alloy can be effectively prevented.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained in greater detail below with reference to exemplary embodiments in conjunction with the figures in the drawing, in which:

FIG. 1 illustrates an example of a Charpy impact test of Fe—Al alloy; and

FIG. 2 illustrates an example of the Charpy impact test of Fe—Al alloy.

DESCRIPTION OF EMBODIMENTS

An important feature of the present invention is to maintain ingot after casting so as not to be cooled to less than a predetermined temperature and to transition to a hot-forging step. The following describes the details. Fe—Al alloy of the present invention that contains Al of 2.0 to 9.0 mass % is alloy providing excellent vibration damping characteristics. On the other hand, it has been confirmed that ingot produced by casting Fe—Al alloy formed a coarse columnar structure and therefore bonding force of the crystal grain boundary surface was small, likely to cause grain boundary fracture. Additionally, it has been confirmed that since the strength of the Fe—Al alloy at an ordinary temperature is low and the toughness is extremely low, even a slight impact was likely to generate a crack. The inventors examined the toughness of the Fe—Al alloy. Specifically, a specimen for a Charpy impact test was extracted from the casted ingot to conduct the Charpy impact test. The composition of the ingot from which the specimen for the Charpy impact test was extracted contained Al of 8 mass % and the balance of Fe and impurities.

FIG. 1 illustrates the result of the Charpy impact test on the specimen. As illustrated in FIG. 1, the following has been confirmed. The toughness of the specimen for the Charpy impact test was rapidly increased at 225° C. and a high impact value of 300 J/cm2 was able to be obtained at a temperature range of 250° C. or more. From the results of fractographic observation and the impact test of the specimen for the Charpy impact test, it has been confirmed that brittle fracture was dominant at less than 250° C. From these test results, the inventors have found the following. Even if the crystal grain boundary surface of the ingot is small bonding force and therefore the crystal grain boundary surface is likely to cause the grain boundary fracture, transitioning the ingot to the hot forging without cooling the ingot to the temperature range of less than 250° C. allows reducing the risk of crack. The simplest method of avoiding cooling the ingot to less than the predetermined temperature is to remove and agglomerate the ingot at a high temperature range at which the ingot can be handled so as to shorten transition time to the hot forging. Needless to say, the ingot may be kept warm or may be heated in the transition period. The reason for specifying the surface temperature of the ingot taken out from a mold in the present invention is that at this temperature, the surface of the ingot is likely to be cooled most. The surface temperature of the ingot, for example, can be easily measured using a simple thermometer such as a thermography.

In the present invention, as described above, the surface temperature of the ingot taken out from the mold is not cooled to less than 250° C. and is transitioned to the hot-forging step. This allows reducing the risk of a crack. This restricts fine cracks, allowing preventing actualization of the crack during the hot forging. As the condition of the hot forging process, for example, it is preferred that the ingot be heated to a temperature of 1000° C. to 1200° C. and hot-forged at a temperature of 850° C. or more to produce a hot-forged material. Since the hot-forged material has a metal structure where coarse crystal grains are broken (extended), after the hot forging, the hot-forged material does not generate a crack and can be cooled up to a normal temperature. To further reliably reduce the above-described fine cracks, removing an oxide film on the surface of the Fe—Al alloy ingot is preferred. Comparatively large unevenness is present on the surface of the Fe—Al alloy ingot, and the oxide film is formed on a surface with unevenness. Accordingly, such surface may be a starting point of the above-described fine crack. Therefore, removing the oxide film on the surface of the Fe—Al alloy ingot is preferred. To remove the oxide film on the surface of the ingot as well, the removal needs to be performed at a temperature range of not being less than 250° C. This is because, as illustrated in FIG. 1, in the state of ingot, the toughness at the temperature range of less than 250° C. is low, and therefore this causes a risk of crack. For removal of the oxide film from the ingot, for example, using a grinder polishing during hot work is preferred. This is because of the following reason. As described above, the comparatively large unevenness is present on the surface of the Fe—Al alloy ingot. Since the oxide film is formed on a surface with unevenness, the method allows removing the unevenness on the ingot simultaneously with removal of the oxide film.

Hot rolling is performed on the above-described hot-forged material. As the condition of the hot rolling, it is preferred that the hot-forged material be heated to a temperature of 1000 to 1200° C. and be hot-rolled at a temperature of 800° C. or more to form the hot-rolled material. Since an oxide film is formed on the surface of the hot-rolled material, the oxide film is removed to produce a material for cold rolling. If the oxide film remains on a strip after the cold rolling, for example, vibration damping characteristics at the part on which the oxide film remains may be degraded. The method for removing the oxide film from the hot-rolled material includes a method of physical removal, pickling, or a similar method. Since the oxide film of the hot-rolled material has a thickness of approximately 50 to 100 μm, for example, the oxide film is preferable to be removed by pickling or a similar method.

Using the material for cold rolling, which is obtained by the above-described oxide-film removing step, the cold rolling is performed. The cold rolling is performed to thicken the material as desired and to provide strength by applying a strain by the cold rolling and micronizing the crystal grains at the subsequent annealing step. For example, insofar as the crystal grain diameter is micronized to 50 to 300 μm by the annealing to provide strength to the Fe—Al alloy, a rolling reduction of the cold-rolling step is only necessary to be 50% or more. Afterwards, the annealing adjusts the crystal grain diameter so as to provide required vibration damping characteristics to the Fe—Al alloy. At this time, hard, thin oxide film is likely to be formed on the surface of the Fe—Al alloy, annealing the Fe—Al alloy in non-oxidizing atmosphere is preferred. The non-oxidizing atmosphere includes reduced-pressure atmosphere, gas atmosphere, or similar atmosphere. However, since its capability of continuous annealing, selecting hydrogen, nitrogen, AX gas, Ar gas, or similar gas is preferred. The Fe—Al alloy that can be obtained by the production method specified in the present invention, which is described above, can prevent the problem of a crack during the production, allowing efficient production of thin Fe—Al alloy.

The Fe—Al alloy referred to in the present invention means Fe—Al alloy that indispensably contains Al of 2.0 to 9.0 mass % and the balance of Fe and impurities and beside alloy that further contains a third element of 1 mass % or less. The present invention designs Al to 2.0 to 9.0 mass %. This is because in the case where the Fe—Al alloy strip specified in the present invention is employed as vibration-damping alloy, solidifying Al in Fe increases magnetostriction, contributing the vibration damping characteristics. If Al is less than 2.0 mass %, the vibration damping characteristics cannot be sufficiently provided. On the other hand, the excess of Al of 9.0 mass % precipitates Fe3Al, deteriorating processability. As the third element selectively added, an element that forms a compound with Fe and provides an effect of micronization of the crystal grain in a casting structure is selected. Specifically, the third element includes Nb, Ti, Mo, W, or a similar element, among them, particularly, addition of Nb is preferable. Nb is a comparatively inexpensive element and also is likely to form the compound with Fe. This allows precipitating the Fe2Nb compound to the crystal grain boundary surface of the casting structure and inhibiting formation of coarse crystal grains, contributing reduction of the grain boundary fracture. Further, addition of 1 mass % or less does not inhibit processability of hot work and cool work. The balance other than the above-described elements is Fe and impurities. The representative impurities of the above-described impurities include C, Si, Mn, P, S, Cr, Ni, N, and O. These impurity elements may be C≦0.01 mass %, Si≦0.2%, Mn≦0.2 mass %, P≦0.01 mass %, S≦0.005 mass %, Cr≦0.05 mass %, Ni≦0.05 mass %, N≦0.01 mass %, and O≦0.01 mass %.

WORKING EXAMPLE

First, the preparatory experiment was conducted. The ingot with the composition illustrated in Table 1 was produced by vacuum melting. Specimens for the Charpy impact test were extracted from the casted ingot to conduct the Charpy impact test. In the Charpy impact test, the specimens were heated to a predetermined temperature, the temperature was retained, and then hot processability was evaluated. The evaluation results were illustrated in FIG. 2.

TABLE 1
(mass %)
	Al	Nb	Balance
	No. 1	5.03	—	Fe and inevitable impurities
	No. 2	5.90	—	Fe and inevitable impurities
	No. 3	8.09	—	Fe and inevitable impurities
	No. 4	9.15	—	Fe and inevitable impurities
	No. 5	8.13	0.16	Fe and inevitable impurities
	No. 6	8.17	0.95	Fe and inevitable impurities
	The “—” sign means additive-free.

As illustrated in FIG. 2, it can be seen that the impact property depends on an amount of added Al and an amount of added Nb. Alloys of No. 1, No. 2, No. 3, and No. 5 obtained the high impact value of 300 J/cm2 at a temperature range of 250° C. or more. On the other hand, No. 4 that exceeds an Al amount specified in the present invention (9.15%) obtained the low impact value of 100 J/cm2 even at the temperature range of 250° C., and a part of a configuration of the fracture surface was brittle fracture. No. 3 and No. 5 of the same Al amount obtained the high impact value of 300 J/cm2 at a temperature range of 200° C. or more because of an effect of addition of a trace Nb. Meanwhile, the No. 6 alloy whose Nb was increased up to 0.95% was the low impact value of 200 J/cm2 at the temperature range of 250° C.; however, since the aspect of the fracture surface was ductile fracture, this is considered as a level for practical application.

Based on the above-described results, a large-sized steel ingot was produced. The vacuum melting melted and produced ingot made of Fe—Al alloy of 2600 kg. Table 2 illustrates the chemical composition.

TABLE 2
(mass %)
C	Al	Si	Mn	P	S	Cr	Ni	N	O	Balance
0.007	7.93	0.13	0.13	0.007	0.002	0.01	0.01	0.0012	0.0003	Fe and inevitable impurities

Before the surface temperature of the ingot extracted from the mold is reduced to less than 250° C., the ingot was stored in a heat holding furnace, heated such that the surface temperature of the steel ingot became 250° C. or more, and then was managed. The surface temperature of the ingot before put into the heat holding furnace was around 500° C. with a radiation thermometer. Then, the ingot was taken out from the heat holding furnace, and then was heated to 1000° C. by a different heating furnace for hot forging, thus the hot-forged material at a thickness of 55 mm was able to be obtained. A defect such as a large crack was not able to be especially found on the surface of the hot-forged material. Then, to find surface flatness of the hot-forged material and to remove minor surface flaw, the surface of the hot-forged material was polished with a grinder, thus obtaining the hot-forged material at a thickness of 53 mm. Next, the above-described hot-forged material was heated to 1000° C. and was hot-rolled at 1000° C., thus obtaining the hot-rolled material at a thickness of 1.7 mm. A defect such as a crack was not able to be especially found on the surface of the hot-rolled material. Then, the oxide film formed on the hot-rolled material was removed by pickling, thus forming a material for cold rolling. The thickness of the material for cold rolling was 1.65 mm. The material for cold rolling was cold-rolled, thus obtaining the cold-rolled material at the thickness of 0.8 mm. The obtained cold-rolled material was finally annealed at 800° C. To avoid the surface to be oxidized, atmosphere of the annealing was set to inert gas atmosphere.

A test specimen for measurement of crystal grains was extracted from the cold-rolled material of Fe—Al alloy annealed as described above, and the crystal grains were checked. The crystal grains were a fine, uniform metal structure at an average crystal grain of 100 μm. Afterwards, for evaluation on the vibration damping characteristics, an internal friction of the cold-rolled material of the Fe—Al alloy at the average crystal grain of 100 μm was measured. The internal friction was measured using a high-temperature elastic modulus coincidence measurement apparatus manufactured by Nihon Techno-Plus Co, Ltd. Consequently, the internal friction of 0.1 or more was able to be obtained at room temperature, it was confirmed that the cold-rolled material had excellent vibration damping characteristics. According to the method for producing the Fe—Al alloy of the present invention, which is described above, a crack during production can be effectively prevented and also can produce a cold-rolled steel strip of Fe—Al alloy at a thickness of 0.8 mm or less containing crystal grains required for excellent vibration damping characteristics.

Claims

1. A method for producing Fe—Al alloy, comprising:

ingot-producing by casting Fe—Al alloy and taking out the Fe—Al alloy from a mold to obtain an ingot, the Fe—Al alloy containing Al: 2.0 to 9.0 mass % and a balance of Fe and impurities;

hot-forging the ingot to form a hot-forged material;

hot-rolling the hot-forged material to form a hot-rolled material;

oxide-film removing by removing an oxide film of the hot-rolled material to form a material for cold rolling;

cold-rolling the material for cold rolling to form a cold-rolled material; and

annealing the cold-rolled material, wherein

heating of the ingot in the hot-forging starts before a surface temperature of the ingot taken out from the mold is cooled to less than 250° C. in the ingot-producing.

2. The method for producing Fe—Al alloy according to claim 1, wherein

the Fe—Al alloy further contains Nb of 1.0 mass % or less.

3. The method for producing Fe—Al alloy according to claim 1, wherein

ingot surface removing by removing an oxide film of the ingot surface is performed before cooling the surface temperature of the ingot obtained by the ingot-producing to less than 250° C., and

subsequently proceeding to the hot-forging.

4. The method for producing Fe—Al alloy according to claim 1, wherein

the annealing is performed in non-oxidizing gas atmosphere.

5. The method for producing Fe—Al alloy according to claim 2, wherein

ingot surface removing by removing an oxide film of the ingot surface is performed before cooling the surface temperature of the ingot obtained by the ingot-producing to less than 250° C., and

subsequently proceeding to the hot-forging.

6. The method for producing Fe—Al alloy according to claim 2, wherein

the annealing is performed in non-oxidizing gas atmosphere.

7. The method for producing Fe—Al alloy according to claim 3, wherein

the annealing is performed in non-oxidizing gas atmosphere.