JP2021195591A - Iron based alloy, method for manufacturing the same and iron base member - Google Patents

Abstract

To provide a new iron based alloy high in specific resistance and excellent in workability and strength.SOLUTION: Iron based alloy satisfies Mn: 24-35%, Al: 13.5-20%, C: 0.55-1.5% and the remainder: Fe and impurities using the total as 100 mass% (referred to as %). The iron based alloy consists of two phase structures of an austenite phase (a γ-phase) and a compound phase; the compound phase consists of Fe, Mn, Al and C; the compound phase of 30-70 vol.% to the total disperses like a network in a matrix consisting of a γ-phase; the iron based alloy achieves both of high specific resistance and high strength at a high level by hot working, heating and cold working; and an eddy current loss can be reduced when using a member consisting of such an iron based alloy in an alternation magnetic field.SELECTED DRAWING: Figure 1A

Description

The present invention relates to an iron-based alloy having a high resistivity.

In order to reduce the eddy current loss generated in the member (electromagnetic member) used in the alternating magnetic field and save energy, the electromagnetic member should be made of a material having a high electrical resistivity (referred to as "specific resistance"). Further, it is more preferable that the electromagnetic member is made of a material having excellent workability and mechanical properties (strength, rigidity, etc.) as well as high resistivity. Such a material may be a magnetic material or a non-magnetic material. The following patent documents describe iron alloys having high resistivity and excellent workability, although their uses are different.

Japanese Unexamined Patent Publication No. 2006-219728

Patent Document 1 proposes an iron alloy for high resistors composed of a quintuple system (Fe-Mn-Al-C-Cr). The iron alloy has a three-phase structure (α phase, γ phase and Cr carbide) controlled by aging heat treatment (Patent Document 1 [0023], etc.).

Further, Patent Document 1 discloses Fe-33.5Mn-15.6Al-0.5C (mass%) as an iron alloy containing no Cr (Comparative Example 12 in Table 1). However, the iron alloy has a two-phase structure of α phase and γ phase, the cold rolling ratio is at most 8%, and the workability is very poor (Patent Document 1 [0022] etc.).

The present invention has been made in view of such circumstances, and an object of the present invention is to provide an iron-based alloy or the like having a new component composition different from the conventional one and having a high resistivity.

As a result of diligent research to solve this problem, the present inventor has found that an iron-based alloy composed of a quaternary system (Fe-Mn-Al-C) having a predetermined composition substantially free of Cr has a high resistivity. In addition, we have newly found that it is also excellent in strength and workability. By developing this result, the present invention described below was completed.

《Iron-based alloy》
(1) The present invention
Satisfy the following component composition with the whole as 100% by mass (simply referred to as "%").
It is an iron-based alloy consisting of a two-phase structure consisting of an austenite phase (simply referred to as "γ phase") and a compound phase.
Mn: 24-35%, Al: 13.5-20%,
C: 0.55-1.5%, balance: Fe and impurities

(2) The iron-based alloy of the present invention exhibits high resistivity. In addition, this iron-based alloy has excellent workability and can exhibit high strength. Such an iron-based alloy can be used, for example, in various electromagnetic members that are required to reduce eddy current loss.

<< Manufacturing method of iron-based alloy >>
The present invention is also understood as a method for producing an iron-based alloy (or iron-based member). For example, the present invention may be a method for producing an iron-based alloy including a cold working step of cold plastic working an iron base material (raw material) having the above-mentioned composition. Further, the iron substrate before the cold working step may be provided with a heat treatment step of subjecting the iron base material to a homogenization treatment and / or a quenching treatment. Further, a hot working step of plastic working the iron substrate in a heated state may be provided before the cold working step (further, before the heat treatment step). The heat treatment step or the hot working step may be performed, for example, by heating the iron substrate to 950 to 1250 ° C. A heat treatment step may be performed after the cold working step. The iron-based alloy obtained through such a process has high resistivity and high strength.

《Iron base member》
The present invention is also understood as a member made of the above-mentioned iron-based alloy (iron-based member). For example, an iron-based member (electromagnetic member or the like) used in an alternating magnetic field can reduce eddy current loss. Further, the iron base member of the present invention is excellent in workability and strength, and therefore has high versatility. Since the iron-based alloy may be a magnetic material or a non-magnetic material, the iron-based member may be a magnetic material (core, yoke, etc.) or a non-magnetic material.

"others"
(1) The iron-based alloy referred to in the present specification may be a raw material (iron base material) before processing or heat treatment, an intermediate product, or a final product as long as it has a predetermined composition and a two-phase structure. good.

(2) Unless otherwise specified, "x to y" in the present specification includes a lower limit value x and an upper limit value y. A range such as "a to b" may be newly established with any numerical value included in the various numerical values or numerical ranges described in the present specification as a new lower limit value or upper limit value.

It is a metal structure photograph of the iron base material which concerns on a sample 11. It is a metal structure photograph of the iron base material which concerns on a sample C1. It is a metal structure photograph of the iron base material which concerns on a sample C4. It is a stress-strain diagram of the iron-based alloy which concerns on a sample 12 and a sample 13. It is a stress-strain diagram of the iron-based alloy which concerns on a sample C4. It is a diffraction pattern of XRD which concerns on a sample 1. It is explanatory drawing which shows the measuring method of a specific resistance.

One or more components arbitrarily selected from the present specification may be added to the components of the present invention described above. The contents described in the present specification apply not only to the iron-based alloy (including the iron base material) of the present invention, but also to the manufacturing method thereof and the iron-based member. Even a methodical component can be a component related to an object. Which embodiment is the best depends on the target, required performance, and the like.

<< Ingredient composition >>
In addition to Fe, the iron-based alloy contains Mn, Al and C as essential elements. The suitable composition range of each element is as follows. Unless otherwise specified, the composition range of each element is shown in the present specification by the mass ratio (indicated by the unit "%") to the entire alloy.

(1) Mn
Mn may be contained, for example, 24-35%, 25-32%, 26-30% and even 27-29%. Mn is an austenite (γ) forming element and stably forms a γ phase in an iron-based alloy even in a normal temperature range. If Mn is too small, the effect will be poor and the resistivity may decrease. If the amount of Mn is excessive, the cold workability can be deteriorated even if the iron-based alloy has a two-phase structure before the cold work.

(2) Al
Al may be contained, for example, 13.5 to 20%, 14.5 to 18%, and further 15 to 17%. Al increases the specific resistance of the iron-based alloy. Therefore, if Al is too small, the resistivity may decrease. When Al is excessive, the cold workability can be deteriorated even if the iron-based alloy has a two-phase structure before cold working, as in Mn.

(3) C
C may be contained, for example, 0.55 to 1.5%, 0.65 to 0.95%, and further 0.7 to 0.85%. C contributes to the formation of the γ phase. If C is too small, it becomes difficult to form a two-phase structure having excellent cold workability. If C is excessive, carbides may increase and cold workability may decrease even if a two-phase structure is formed.

The iron-based alloy may contain impurities (elements other than Fe, Mn, Al and C). The total amount of impurities may be, for example, less than 1%, less than 0.5%, and even less than 0.3%. The cause of impurities is not limited.

《Two-phase organization》
(1) The iron-based alloy is preferably composed of a two-phase structure consisting of a γ phase and a compound phase. Both the γ phase and the compound phase have high resistivity, and the iron-based alloy can also exhibit high resistivity due to their synergistic effect. Further, since the iron-based alloy has a two-phase structure in which the γ phase and the compound phase coexist, the processing rate can be increased without causing cracking. Further, the iron-based alloy can exhibit high mechanical properties (strength (proof stress, fracture (tensile) strength, etc.), ductility, rigidity, etc.) due to the two-phase structure.

In the two-phase structure, for example, the compound phase is dispersed in the form of particles in a matrix phase composed of a γ phase. The compound phase is preferably dispersed in a network, for example. If the two-phase structure is formed before processing, the workability is enhanced. The compound phase in the two-phase structure is formed by plastic working (simply referred to as "working"), heating during processing, heat treatment performed separately, etc., in terms of morphology (size, shape, dispersion, etc.), abundance ratio (volume fraction), etc. May change. For example, the two-phase structure can become a layered structure by processing. The iron-based alloy having a two-phase structure is increased in strength (ratio) by such processing.

Such an iron-based alloy may be used, for example, in a rotor of an electric motor (including a generator). As a result, the power consumption of the unit or system can be reduced, and the performance and efficiency can be improved by reducing the eddy current loss, increasing the rotation speed, and reducing the weight.

(2) The compound phase is usually composed of an intermetallic compound and may have various compositions. The compound phase comprises, for example, Fe, Mn, Al and C. The compound phase may be contained in a total amount of 30 to 70% by volume, more preferably 40 to 60% by volume, based on the whole. For the volume ratio of the compound phase, use ImageJ (free software) for the range (field) of 200 μm × 200 μm of the observation image (500 times) obtained by observing the measurement sample with SEM, and then image → adjust → It was obtained by adding light and shade to the measured part at the threshold.

"Production method"
(1) Iron base material The iron base material used as a raw material may be a molten material or a sintered material. It is preferable that the iron base material has the above-mentioned component composition and has a two-phase structure formed from the stage before heat treatment or processing.

(2) Heat treatment Heat treatment may be appropriately performed before, during, or after processing. The heat treatment is, for example, homogenization treatment, quenching, or the like. The homogenization treatment may be carried out, for example, at 1000 to 1200 ° C. and further to 105 to 1150 ° C. for 0.5 to 5 hours and further to 1 to 3 hours. The heating atmosphere may be an atmospheric atmosphere as well as an inert gas atmosphere and a nitrogen gas atmosphere.

Quenching may be performed by heating to 1000 to 1200 ° C., further to 1050-1150 ° C., and then quenching. The quenching may be water cooling, hot water cooling, oil cooling, etc., but water quenching (WQ) is usually sufficient. Since the iron base material (iron-based alloy) has a large amount of Mn, the martensite phase hardly appears even when quenched, and the two-phase structure is maintained.

(3) Processing The iron base material may be hot-processed or cold-processed. The hot working (process) may be performed, for example, after heating to 950 to 1250 ° C., further to 1100 to 1200 ° C., or in a heated state (hot working step). As a result, the iron base material can be efficiently plastically deformed to a desired form. Hot working is, for example, hot forging. The processing rate can be significantly increased by the multi-step hot processing that repeats heating and processing. For cooling after hot working, for example, air cooling may be performed in the atmosphere.

By cold working (process), the iron base material can be plastically deformed to a desired form. The cold working step is, for example, swaging, cold rolling, and the like, and various general-purpose machining can be used. The cold temperature may be in the room temperature range, and dare to say that it is 70 ° C. or lower. Cold working can also be performed in multiple stages, which can significantly increase the working rate. If the above-mentioned hot working or heat treatment is performed before the cold working, the working rate can be further improved.

"Characteristic"
The iron-based alloy (iron-based member) is excellent in workability and can exhibit high resistivity and high strength. The specific resistance can be, for example, 2.0 μΩm or more, 2.1 μΩm or more, 2.3 μΩm or more, and 2.7 μΩm or more. The strength can be 900 MPa or more, 1200 MPa or more, 1500 MPa or more, 1700 MPa or more, and further 1950 MPa or more. The cold working rate can be 30% or more, 50% or more, and even 80% or more. The processing rate referred to in the present specification is determined by the cross-sectional area ratio before and after processing.

The reason why the iron-based alloy of the present invention exhibits a high specific resistance is considered to be a decrease in the Seebeck coefficient due to a decrease in the concentration of carriers (holes, electrons).

Multiple samples made of iron-based alloys with different composition or processing rates were produced. The electrical properties (specific resistance) and mechanical properties (Young's modulus, tensile strength, elongation) of each sample were evaluated. The present invention will be described in more detail below with reference to such specific examples.

《Manufacturing of samples》
(1) Iron base material The iron base material used as a raw material was melted as follows. First, commercially available pure iron, pure aluminum, pure manganese (electrolytic Mn), pure silicon, and carbon steel (Fe—C alloy) were prepared as raw materials (base materials).

The raw materials blended in the composition shown in Table 1 were dissolved in an argon atmosphere, and the raw materials were poured into a mold to solidify them (casting step). In this way, an iron base material made of ingot (molten material) was obtained.

(2) Hot processing The following hot forging was performed on each iron base material in the atmosphere. An iron substrate heated to 1150 ° C. in a gas furnace was forged (tapped). This heating and forging were performed in 12 steps, and the diameter of the iron base material was reduced from φ50 mm to φ15 mm (hot working step). After this hot forging, it was allowed to cool to room temperature in the atmosphere.

(3) Heat Treatment The hot forged iron substrate was heated in an electric furnace at 1100 ° C. for 2 hours for homogenization. Then, the iron substrate at 1100 ° C. was water-quenched (WQ).

(4) Cold processing The iron base material after the heat treatment was machined to remove the oxide layer to obtain a round bar (φ12 mm × 130 mm). This round bar was swagged at room temperature using a die. This cold working was performed by dividing the die diameter from φ12 mm to φ6 mm into 10 steps and reducing the die diameter in order. The test material of each sample thus obtained was subjected to a tensile test or the like.

For the samples C1 to C5, the cold working was stopped at the stage where cracks and the like occurred during the cold working. Then, the processing rate (1-Aw / Ao) was calculated from the cross-sectional reduction rate (Aw / Ao) in the range where cracks did not occur. Here, Aw is the cross-sectional area after machining, and Ao is the cross-sectional area before the start of machining.

Sample 11 is a case where cold processing is not performed. Sample 12 is a case where the cold working is intentionally stopped in the middle. The sample 13 is a case where the above-mentioned cold working (φ11 mm → φ4 mm) is performed. Hereinafter, the samples 11 to 13 are collectively referred to as "sample 1".

"observation"
(1) Metal structure For the iron substrate of each sample, the metal structure after heat treatment and before cold processing was observed with an OM (Optical Microscope). Observation images (tissue photographs) of Samples 11, C1 and C4 are illustrated in FIGS. 1A, 1B and 1C (collectively referred to simply as "FIG. 1"), respectively.

(2) The volume ratio of the compound phase dispersed in the metal structure was obtained by image analysis of the above observation image with ImageJ. The volume fraction of the compound phase of each sample thus obtained is shown in Table 1.

(3) X-ray diffraction (specification of y)
The test material for each sample was subjected to X-ray diffraction analysis (XRD / Cu-Kα). This confirmed that the matrix phase was the γ phase. It was also confirmed that the compound phase according to sample 1 was composed of Fe-Mn-Al-C. As an example, the diffraction pattern of XRD according to the sample 13 is shown in FIG.

"measurement"
(1) Electrical characteristics (resistivity)
The resistivity of each sample was measured by the DC four-terminal method using a system source meter ((KEITHLEY 2601). Specifically, the following test pieces were prepared and shown in FIG. 4. It was measured.

Mask the central part (between voltage electrodes (L): 10 mm) of the prism (3. mm (t) × 3 mm (w) × 20 mm) manufactured from each of the above-mentioned cold-worked iron-based alloys with masking tape. .. Wrap the terminal wire (silver wire: φ0.20 mm) around the masked both end portions and the two outer portions thereof (see FIG. 4). Silver paste (Dotite D-550 manufactured by Fujikura Kasei Co., Ltd.) is applied to the portion around which each terminal wire is wound and both end faces of the prism. The prism after application is heated in the air at 100 ° C. for 12 hours to dry. In this way, a test piece provided with a current electrode and a voltage electrode was prepared.

A current (I) of 1 A was passed through the current electrode on the outside for 30 seconds, and the voltage (V) of the voltage electrode on the inside was measured. As shown in the formula (1) of FIG. 4, the resistivity (r) was obtained from those values (I, V) and the shape (t, w, L) of the test piece. The floating electromotive force was erased by measuring the natural potential before energization and using the zero adjustment function of the measuring instrument (system source meter). The resistivity of each sample thus obtained is also shown in Table 1.

(2) Mechanical characteristics A tensile test (gauge length: 10 mm) was performed using a test piece (parallel portion: φ2.4 mm × 14 mm, total length: 40 mm) manufactured by machining the above-mentioned test material. The tensile test was carried out using an autograph (AUTOGRAPH AG-1 50 kN manufactured by Shimadzu Corporation) in a room temperature atmosphere at a strain rate of 5 × 10 ^-4 / s. The Young's modulus, tensile strength, and elongation obtained by the tensile test are also shown in Table 1. The tensile strength was calculated based on the load at break and the initial shape of the test piece. Elongation is the strain of the test piece at break.

The stress-strain diagrams obtained in the tensile test for Samples 12, 13 and C4 are shown in FIGS. 2A and 2B (collectively referred to as "FIG. 2"). The stress-strain diagram shown in FIG. 2 was obtained by a video extensometer.

"evaluation"
(1) Electrical characteristics (resistivity)
As is clear from Table 1, all of them had high resistivity except for sample C4 having an excessive amount of Al.

(2) Workability As shown in Table 1, Sample 1 did not crack or the like even when cooled to a processing rate of 75%. That is, the sample 1 was very excellent in processability.

On the other hand, the processing rate of sample C1 and the like was at most 9%, and the processing rate of sample C4 having a small amount of Al remained at 55%. Further, since the samples C1, C2, C3 and C5 were cracked or difficult to process, it was difficult to prepare the tensile test piece itself.

(3) Mechanical properties As can be seen from Table 1 and FIG. 2, the sample 1 has a high strength, and the tensile strength also increases significantly with the increase in the processing rate.

(4) Metallic structure As can be seen from Table 1 and FIG. 1, it was found that the sample 11 before cold working had a two-phase structure in which many compound phases were dispersed in a network.

On the other hand, in the sample C1, the linear compound phase was slightly dispersed. Comparing Sample 11 and Sample C1, it is presumed that the difference in metallographic structure greatly affected the workability.

Incidentally, the sample C4 is a so-called deformed twin-induced steel and can be cold-worked to some extent, but the resistivity is small because the compound phase does not precipitate.

From the above, it was confirmed that the iron-based alloy of the present invention has high resistivity, excellent workability, and high strength.

Claims (8)

Satisfy the following component composition with the whole as 100% by mass (simply referred to as "%").
An iron-based alloy consisting of a two-phase structure consisting of an austenite phase (simply referred to as "γ phase") and a compound phase.
Mn: 24-35%,
Al: 13.5-20%,
C: 0.55-1.5%,
Remaining: Fe and impurities The iron-based alloy according to claim 1, wherein the compound phase is composed of Fe, Mn, Al and C. The iron-based alloy according to claim 1 or 2, wherein the compound phase is contained in an amount of 30 to 70% by volume based on the whole. The iron-based alloy according to any one of claims 1 to 3, which is made of a cold-working material. The iron-based alloy according to any one of claims 1 to 4, wherein the component composition satisfies the following range.
Mn: 25-32%,
Al: 14.5-18%,
C: 0.65 to 0.95%,
Remaining: Fe and impurities A method for producing an iron-based alloy, comprising a cold working step of cold plastic working an iron base material having the component composition according to claim 1. The method for producing an iron-based alloy according to claim 6, further comprising a heat treatment step of subjecting the iron substrate before the cold working step to a homogenization treatment and / or a quenching treatment. An iron-based member made of the iron-based alloy according to any one of claims 1 to 5 and used in an alternating magnetic field.

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