JP2022186017A - Fe-based alloy for solid phase nitrogen absorption and Fe-based alloy member - Google Patents

Abstract

To provide an Fe-based alloy for solid phase nitrogen absorption which can materialize high magnetic permeability, high electric resistance, high saturation magnetic flux density and high hardness, and a member using the Fe-based alloy for solid phase nitrogen absorption.SOLUTION: An Fe-based alloy for solid phase nitrogen absorption contains 0.020 mass% or less C, 0.030 mass% or less N, and 5.0 or more and 18.0 mass% or less Cr, and further contains one or two or more elements of 0.5 or more and 3.0 mass% or less Si, 0.1 or more and 3.0 mass% or less Al, and 0.05 or more and 3.0 mass% or less Ti, and the balance composed of Fe and inevitable impurities, and has an area ratio of a ferrite phase at 23°C of 95% or more. An Fe-based alloy member comprises: a base portion composed of the Fe-based alloy for solid phase nitrogen absorption; and a nitrogen absorption layer formed on the surface of the base portion, wherein hardness of the nitrogen absorption layer is 400 HV or more, and an area ratio of the ferrite phase of the base portion at 23°C is 95% or more.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to an Fe-based alloy for solid-phase nitrogen absorption and an Fe-based alloy member, and more specifically, an Fe-based alloy for solid-phase nitrogen absorption suitable for surface hardening treatment by a solid-phase nitrogen absorption method, and an Fe-based alloy using the same. It relates to an Fe-based alloy member.

Parts (e.g., injectors) that make up a fuel injection device for a vehicle with an internal combustion engine are required to have good magnetic properties to enable highly accurate fuel injection control in addition to corrosion resistance to fuel. . Electromagnetic stainless steel has excellent corrosion resistance and soft magnetism, and is therefore used for many parts that constitute fuel injection devices.

Various proposals have been made in the past regarding parts that constitute such a fuel injection device or materials that are suitable for such parts. For example, Patent Document 1 discloses an electromagnetic valve including a fixed core, a coil wound around the outer circumference of the fixed core, a movable core arranged so as to oppose the fixed core, and a valve housing that accommodates the movable core. A type fuel injector is disclosed.
In the document, shot peening was applied to the surface of an annular guide provided on the inner peripheral surface of the valve housing (the portion where the movable core slides when the movable core reciprocates along the inner surface of the valve housing). and that a hardened layer is formed by chrome plating.

Although it is not intended to be applied to parts constituting a fuel injection device in Patent Document 2, it contains predetermined amounts of C, Si, Mn, P, S, Cr, N, Al, and Ti, and the surface layer A ferritic stainless steel material is disclosed in which nitrides are precipitated in the part.
In the same document,
(A) When ferritic stainless steel is annealed (bright annealing) in a mixed gas atmosphere containing N ₂ and H ₂ , nitrogen atoms permeate from the surface to a depth of about 0.05 mm. Fine nitrides are dispersed and precipitated on the surface layer,
(B) By dispersing and precipitating nitrides, the surface hardness of the steel material can be improved without reducing the corrosion resistance, and
(C) It is stated that the hardness of only the surface layer portion is increased by bright annealing, and the interior has a soft ferrite structure, so press workability is good.

Furthermore, although not intended to be applied to parts constituting a fuel injection device, Patent Document 3 discloses a compound containing predetermined amounts of C, Si, Mn, Cr, N, and Ni, and the balance being Fe and Ni. A composite material is disclosed that includes a ferromagnetic steel material containing unavoidable impurities and a non-magnetic portion formed on the surface layer of the ferromagnetic steel material.
In the same document,
(A) When a ferromagnetic steel material is subjected to nitrogen atom diffusion treatment (solid-phase nitrogen absorption method), a non-magnetic portion in which N is enriched in solid solution can be formed in the surface layer portion;
(B) It is stated that such a composite material is suitable as a magnetic circuit component used in an electromagnetic actuator or the like.

In particular, in the case of injectors, energy saving has conventionally been achieved through precise injection control of fuel. In recent years, there is a tendency to increase the driving pressure of injectors in order to further improve fuel efficiency. Therefore, the materials used for these parts include:
(a) high magnetic permeability and high electrical resistance for precise control of fuel;
(b) high saturation flux density for high pressure drive (≈high load torque) and
(c) High hardness is required to suppress material deterioration due to collisions accompanying sliding motion.

For example, 13% chromium electromagnetic stainless steel exists as an existing material that achieves both magnetic properties and corrosion resistance at a high level. However, 13% chromium electromagnetic stainless steel has a low hardness when used alone, and has a problem of durability against valve collision due to high-pressure driving. In order to solve this problem, in injector applications, the surface of the material is generally plated with chrome to ensure surface hardness (see Patent Document 1). However, since chromium is non-magnetic, the chromium plating treatment inevitably deteriorates the magnetic properties.

On the other hand, Patent Document 3 describes a method of forming a non-magnetic portion on the surface layer portion of a ferromagnetic steel material using a solid-phase nitrogen absorption method. Here, the "solid-phase nitrogen absorption method" is a heat treatment method in which nitrogen atoms are diffused from the surface of the steel material into the interior of the material by holding the steel material in nitrogen at a high temperature, thereby increasing the nitrogen concentration near the surface of the material or in the entire material. Say. The solid state nitrogen absorption method differs from the nitridation method in that it does not form nitrides.
Solid-solution strengthening of the material surface can be achieved by causing nitrogen to form a solid solution on the surface of the material using the solid-phase nitrogen absorption method. However, when a non-magnetic layer is formed on the surface layer by the solid-phase nitrogen absorption method, deterioration of the magnetic properties is unavoidable as in the case of chromium plating.

JP-A-2002-081356 JP-A-11-350088 JP 2013-028825 A

The problem to be solved by the present invention is to provide an Fe-based alloy for solid-phase nitrogen absorption that can achieve high magnetic permeability, high electrical resistance, high saturation magnetic flux density, and high hardness.
Another problem to be solved by the present invention is to provide a member made of such an Fe-based alloy for solid-phase nitrogen absorption.

In order to solve the above problems, the Fe-based alloy for solid-phase nitrogen absorption according to the present invention comprises:
C ≤ 0.020 mass%,
N ≤ 0.030 mass%, and
5.0≦Cr≦18.0 mass %
and further
0.5≦Si≦3.0 mass%,
0.1 ≤ Al ≤ 3.0 mass%, and
0.05≤Ti≤3.0mass%
containing one or more elements of
The balance consists of Fe and unavoidable impurities,
The area ratio of the ferrite phase at 23°C is 95% or more.

The Fe-based alloy member according to the present invention is
a base made of the Fe-based alloy for solid-phase nitrogen absorption according to the present invention;
A nitrogen absorption layer formed on the surface of the base,
The nitrogen absorption layer has a hardness of 400 HV or more,
The area ratio of the ferrite phase in the base at 23° C. is 95% or more.

In an Fe-based alloy containing a predetermined amount of Cr, the ferrite phase is stabilized by reducing C and N, which have high austenite-forming ability, as much as possible and adding a small amount of Ti, which has high ferrite-forming ability. Further, adding appropriate amounts of Al and Si improves magnetic properties such as electrical resistance and magnetic anisotropy. As a result, the ferrite phase is stable not only at room temperature but also at high temperature (temperature range where solid-phase nitrogen absorption treatment is performed), and exhibits high magnetic permeability, high electrical resistance, and high saturation magnetic flux density at room temperature. An Fe-based alloy is obtained.

When a member made of such an Fe-based alloy is subjected to a solid-phase nitrogen absorption treatment, the nitrogen concentration in the surface layer portion becomes high, and eventually only the surface layer portion becomes the austenite phase. When rapidly cooled from this state, the austenite phase in the surface layer part transforms to martensite. As a result, a member can be obtained in which the core portion is made of a ferrite phase with high magnetic properties and the surface layer portion is made of a martensite phase with high hardness. The martensite phase in the surface layer exhibits soft magnetism, although its magnetic properties are inferior to those of the ferrite phase in the core. Therefore, the Fe-based alloy member according to the present invention exhibits higher magnetic properties than conventional members having a non-magnetic layer formed on the surface layer.

An embodiment of the present invention will be described in detail below.
[1. Fe-based alloy for solid phase nitrogen absorption]
[1.1. Main constituent element]
The Fe-based alloy for solid-phase nitrogen absorption (hereinafter also simply referred to as "Fe-based alloy") according to the present invention contains the following elements, with the balance being Fe and unavoidable impurities. The types of additive elements, their component ranges, and the reasons for their limitations are as follows.

(1) C ≤ 0.020 mass%:
C deteriorates the magnetic properties of the Fe-based alloy and promotes austenitization of the parent phase. Therefore, the smaller the amount of C, the better. In order to suppress deterioration of magnetic properties and austenitization of the parent phase, the amount of C should be 0.020 mass% or less. The amount of C is preferably 0.010 mass% or less, more preferably 0.00510 mass% or less.

(2) N ≤ 0.030 mass%:
N promotes austenitization of the parent phase. Therefore, the smaller the amount of N before the solid-phase nitrogen absorption treatment, the better. In order to suppress the austenitization of the matrix phase, the N content needs to be 0.030 mass% or less. The amount of N is preferably 0.010 mass% or less, more preferably 0.00510 mass% or less.

(3) 5.0≦Cr≦18.0 mass%:
Cr has the effect of improving the corrosion resistance of the Fe-based alloy and the effect of improving the equilibrium nitrogen concentration during solid-phase nitrogen absorption. In order to obtain such effects, the Cr content should be 5.0 mass % or more. The Cr content is preferably 6.0 mass% or more, more preferably 11.0 mass% or more.
On the other hand, when the amount of Cr becomes excessive, the maximum magnetic flux density decreases. Therefore, the Cr content should be 18.0 mass% or less. The Cr content is preferably 15.0 mass% or less, more preferably 13.0 mass% or less.

(4) 0.5≦Si≦3.0 mass%:
Si has the effect of promoting ferritization of the matrix and the effect of increasing the hardness and electrical resistance of the matrix. In order to obtain such effects, the amount of Si needs to be 0.5 mass % or more.
On the other hand, when the amount of Si becomes excessive, the maximum magnetic flux density and workability are lowered. Therefore, the Si content should be 3.0 mass% or less. The Si content is preferably 2.30 mass% or less, more preferably 1.55 mass% or less.

(5) 0.1 ≤ Al ≤ 3.0 mass%:
Al has the effect of promoting ferritization of the matrix and the effect of increasing the electrical resistance of the matrix. In order to obtain such effects, the amount of Al needs to be 0.1 mass % or more. The amount of Al is preferably 0.3 mass% or more.
On the other hand, when the amount of Al is excessive, the surface layer portion does not become martensitic in the quenching after solid phase nitrogen absorption, and the hardness of the surface layer portion decreases. Therefore, the Al content should be 3.0 mass% or less. The Al content is preferably 2.5 mass% or less, more preferably 1.1 mass% or less.

(6) 0.05≦Ti≦3.0 mass%:
Ti has the effect of promoting ferritization of the mother phase when added in a small amount. In order to obtain such effects, the amount of Ti should be 0.05 mass % or more. The amount of Ti is preferably 0.10 mass% or more.
On the other hand, when the amount of Ti is excessive, the surface layer portion does not become martensite during quenching after absorbing solid-phase nitrogen, and the hardness of the surface layer portion decreases. Therefore, the Ti content should be 3.0 mass% or less. The Ti content is preferably 2.5 mass% or less, more preferably 0.5 mass% or less.

The Fe-based alloy according to the present invention contains at least (a) Cr,
(b) 0.5 ≤ Si ≤ 3.0 mass%, 0.1 ≤ Al ≤ 3.0 mass%, and 0.05 ≤ Ti ≤ 3.0 mass%, containing one or more elements I wish I had.
Here, "contains one or more elements of 0.5≦Si≦3.0 mass%, 0.1≦Al≦3.0 mass%, and 0.05≦Ti≦3.0 mass%" means that when the content of at least one element of Si, Al, and Ti is equal to or higher than the above-described lower limit, the content of the remaining elements may be less than the above-described lower limit, or It means that it can be greater than or equal to the value.

[1.2. inevitable impurities]
"Inevitable impurities" refer to trace components mixed in from raw materials and refractories during production of Fe-based alloys. As unavoidable impurities, for example,
(a) Mn of 0.10 mass% or less,
(b) P less than or equal to 0.010 mass%;
(c) 0.005 mass% or less S,
(d) 0.05 mass% or less of Cu;
(e) 0.10 mass% or less of Ni,
(f) 0.005 mass% or less O;
and so on.

In particular, both Ni and Mn have the effect of promoting austenitization of the parent phase. Therefore, the less Ni and Mn, the better. In order to suppress the austenitization of the parent phase, each of the Ni amount and the Mn amount is preferably 0.01 mass% or less.

[1.3. metal structure]
Since the Fe-based alloy according to the present invention has optimized components, the ferrite phase area ratio at 23° C. before the solid-phase nitrogen absorption treatment is 95% or more. The “area ratio of the ferrite phase” will be described later.
In the Fe-based alloy according to the present invention, the matrix of the metal structure (region excluding inclusions) is usually a ferrite phase single phase not only at room temperature but also at high temperature (temperature range where solid phase nitrogen absorption treatment is performed). becomes. However, depending on the composition, a trace amount of austenite phase may be generated at high temperatures.

[1.4. Ferrite Stabilization Index]
The Fe-based alloy according to the present invention preferably satisfies the following formula (1).
([Cr]+2.6[Si]+7[Al]+7[Ti])/(5[C]+5[N]+0.35)≧33 (1)
However, [X] represents the mass ratio (mass%) of the element X contained in the Fe-based alloy for solid-phase nitrogen absorption.

The variable on the left side of equation (1) represents the "ferrite stabilization index." If the ferrite stabilization index is too small, the matrix of the core may not become a ferrite single phase in the temperature range (900 to 1100° C.) in which the solid-phase nitrogen absorption treatment is performed, and an austenite phase may form. When rapid cooling is performed from this state, a martensite phase is formed also in the core portion, and the magnetic flux density is lowered.

On the other hand, the larger the ferrite stabilization index, the easier it is for ferrite to stabilize at high temperatures. The ferrite stabilization index is preferably 33 or more in order to bring the core matrix closer to a single ferrite phase in the temperature range in which the solid-phase nitrogen absorption treatment is performed.
On the other hand, if the ferrite stabilization index is too large, the nitrogen absorption layer will not turn into an austenite phase at high temperatures, and surface hardness may not be obtained by quenching. Therefore, the ferrite stabilization index is preferably 80 or less.

[2. Fe-based alloy member]
The Fe-based alloy member according to the present invention is
a base made of the Fe-based alloy for solid-phase nitrogen absorption according to the present invention;
and a nitrogen absorption layer formed on the surface of the base.

[2.1. base]
[2.1.1. material]
The base is made of Fe-based alloy for solid phase nitrogen absorption. Since the details of the Fe-based alloy are as described above, the description is omitted.

[2.1.2. Area ratio of ferrite phase]
“Ferrite phase area ratio (%)” means the ratio of the area of the ferrite phase to the area of the cross section of the Fe-based alloy at 23°C.
In other words, the "area ratio (%) of the ferrite phase" is
(a) cutting the Fe-based alloy member after the solid phase nitrogen absorption treatment in a direction perpendicular to the surface, polishing and etching the cross section;
(b) A region near the center of the cross section (region not including the nitrogen absorption layer) is observed with an optical microscope at room temperature, and the area of the field of view (S ₀ ) and the area of the ferrite phase included in the field of view (S) are calculated. death,
(c) A value obtained by dividing S by S0 (= S _x 100/ _S0 )
Say.
In the present invention, S ₀ is 1 mm×4 mm.

Since the composition of the Fe-based alloy according to the present invention is optimized, the area ratio of the ferrite phase in the base is 95% or more. By optimizing the composition of the Fe-based alloy, the area ratio of the ferrite phase becomes 98% or more.

[2.1.3. Crystal grain size]
"Crystal grain size" refers to the crystal grain size measured by a cutting method using a straight test line described in JIS G0551 from an optical microscope photograph, and shown in Table A.1 in JIS G0551 Annex A. A value obtained by converting from the crystal grain size using 1.

The crystal grain size of the base affects the magnetic properties of the Fe-based alloy member. In general, the larger the crystal grain size of the base, the higher the magnetic properties of the Fe-based alloy member. Since the Fe-based alloy member according to the present invention is subjected to solid-phase nitrogen absorption treatment, grain growth occurs during the treatment. If the manufacturing conditions are optimized, the crystal grain size of the base will be 150 μm or more. If the manufacturing conditions are further optimized, the crystal grain size of the base will be 300 μm or more.
On the other hand, if the crystal grain size of the base portion is increased more than necessary, the mechanical properties of the Fe-based alloy member may deteriorate. Therefore, the crystal grain size of the base is preferably 1000 μm or less.

[2.2. Nitrogen absorption layer]
The term "nitrogen absorption layer" refers to a layer formed by solid-solution of nitrogen in the surface layer of the base material by subjecting the base material to solid-phase nitrogen absorption treatment.

[2.2.1. Solid phase nitrogen absorption treatment and quenching]
The solid-phase nitrogen absorption treatment is performed by heating the substrate to a prescribed temperature in a prescribed nitrogen atmosphere.

The heating temperature affects the nitrogen concentration on the substrate surface. Therefore, it is preferable to select an optimum heating temperature according to the composition of the substrate. In general, the lower the heating temperature, the higher the equilibrium nitrogen concentration on the substrate surface. However, if the heating temperature is too low, Cr ₂ N may precipitate and deteriorate the magnetic properties. Therefore, the heating temperature is preferably 900° C. or higher.
On the other hand, if the heating temperature is too high, the equilibrium nitrogen concentration on the surface of the base material may decrease and the hardness may decrease. Therefore, the heating temperature is preferably 1100° C. or less.

Nitrogen partial pressure affects the nitrogen concentration on the substrate surface. Therefore, it is preferable to select an optimum value for the nitrogen partial pressure according to the composition of the base material. Generally, the higher the nitrogen partial pressure, the higher the equilibrium nitrogen concentration at the substrate surface. In order to obtain such effects, the nitrogen partial pressure is preferably 0.035 MPa or more. In particular, when the matrix of the surface layer has a composition that makes it difficult to form a martensite single phase (for example, a composition with less Cr), it is recommended to increase the nitrogen partial pressure to make more nitrogen solid solution and austenitize the surface layer. preferable.
On the other hand, if the nitrogen partial pressure becomes too high, the nitrogen concentration will increase excessively and the martensite transformation start temperature will decrease. As a result, the austenite phase tends to remain after quenching, and the hardness may decrease. Therefore, the nitrogen partial pressure is preferably 0.7 MPa or less.

The nitrogen concentration on the substrate surface is determined by the heating temperature and the nitrogen partial pressure. On the other hand, the treatment time affects the thickness of the nitrogen absorption layer. Therefore, it is preferable to select an optimum treatment time according to the required thickness of the nitrogen absorption layer. The treatment time is usually about 1 minute to 300 minutes.

After diffusing a predetermined amount of nitrogen into the surface layer, the member is quenched. As a result, the austenite phase in the surface layer part undergoes martensite transformation. Quenching conditions are not particularly limited as long as they are conditions that allow martensitic transformation of the surface layer portion.
For example, in the solid phase nitrogen absorption treatment, quenching may be performed by rapidly cooling the base material from the treatment temperature. Alternatively, quenching may be performed by heating to a quenching temperature and quenching after solid-phase nitrogen absorption treatment. For rapid cooling, gas cooling, water cooling, ice water cooling, oil cooling, or the like can be used.

Further, before hardening, a nitrogen diffusion treatment for diffusing nitrogen inside the member may be performed. Specifically, following the solid phase nitrogen absorption treatment, the member is held at a high temperature of about 900 to 1100° C. in an inert atmosphere such as argon gas, thereby diffusing nitrogen inside the member. When the nitrogen diffusion treatment is performed, the nitrogen absorption layer becomes thick, so that the surface hardness of the member after quenching can be stably obtained. However, if the nitrogen absorption layer becomes too thick, the magnetic properties (magnetic flux density) will decrease. Therefore, the holding temperature and holding time of the nitrogen diffusion treatment should be set in consideration of the magnetic properties (magnetic flux density) required for the member.
Further, sub-zero treatment may be performed to keep the material at 0° C. or lower after quenching.

[2.2.2. Hardness of nitrogen absorption layer]
"Hardness of nitrogen absorption layer"
(a) cutting the Fe-based alloy member after the solid phase nitrogen absorption treatment in a direction perpendicular to the surface and polishing the cross section;
(b) Vickers hardness is measured at any five points at a depth of 20 μm ± 5 μm from the surface of the member,
(c) Refers to a value obtained by calculating the average value of five points of Vickers hardness.

In the Fe-based alloy member according to the present invention, the hardness of the nitrogen absorbing layer becomes 400 HV or more by optimizing the composition of the base material and the conditions of solid-phase nitriding treatment and quenching. By optimizing the composition of the base material and the manufacturing conditions, the hardness of the nitrogen absorbing layer becomes 500 HV or more.

[2.2.3. Thickness of nitrogen absorption layer]
The interface between the ferrite phase in the core and the nitrogen-absorbing layer in the surface layer is usually not flat but has a highly uneven shape. In the present invention, the "thickness of the nitrogen absorption layer"
(a) cutting the Fe-based alloy member after the solid phase nitrogen absorption treatment in a direction perpendicular to the surface and polishing the cross section;
(b) Observe the vicinity of the surface of the cross section at any three points at a magnification that allows observation of a region with a width (distance in the direction parallel to the surface of the base material) of about 500 μm,
(c) calculating the shortest distance from the surface of the member to the ferrite phase in each field of view;
(d) A value obtained by calculating the average value of the shortest distances at a total of three locations.

In the present invention, the thickness of the nitrogen absorption layer is not particularly limited. In general, the thicker the nitrogen absorbing layer, the better the wear resistance. On the other hand, if the thickness of the nitrogen absorption layer is too thick, the magnetic properties of the member may deteriorate. Therefore, it is preferable to select an optimum thickness of the nitrogen absorption layer in consideration of these points.
As mentioned above, the thickness of the nitrogen absorption layer mainly depends on the treatment time of the solid phase nitrogen absorption treatment. If the manufacturing conditions are optimized, the thickness of the nitrogen absorption layer will be about 20 to 200 μm.

[2.3. Application]
The Fe-based alloy member according to the present invention can be applied to various uses. Examples of members to which the present invention is applied include:
(a) a movable core of an electromagnetic fuel injector;
(b) rotor cores of motors for electric vehicles;

[3. action]
In an Fe-based alloy containing a predetermined amount of Cr, the ferrite phase is stabilized by reducing C and N, which have high austenite-forming ability, as much as possible and adding a small amount of Ti, which has high ferrite-forming ability. Further, adding appropriate amounts of Al and Si improves magnetic properties such as electrical resistance and magnetic anisotropy. As a result, the ferrite phase is stable not only at room temperature but also at high temperature (temperature range where solid-phase nitrogen absorption treatment is performed), and exhibits high magnetic permeability, high electrical resistance, and high saturation magnetic flux density at room temperature. An Fe-based alloy is obtained.

When a member made of such an Fe-based alloy is subjected to a solid-phase nitrogen absorption treatment, the nitrogen concentration in the surface layer portion becomes high, and eventually only the surface layer portion becomes the austenite phase. When rapidly cooled from this state, the austenite phase in the surface layer part transforms to martensite. As a result, a member can be obtained in which the core portion is made of a ferrite phase with high magnetic properties and the surface layer portion is made of a martensite phase with high hardness. The martensite phase in the surface layer exhibits soft magnetism, although its magnetic properties are inferior to those of the ferrite phase in the core. Therefore, the Fe-based alloy member according to the present invention exhibits higher magnetic properties than conventional members having a non-magnetic layer formed on the surface layer.

Since the martensite phase in the surface layer of the member has magnetism like the ferrite phase, deterioration of magnetic properties is less than that of non-magnetic plating. In addition, since the Fe-based alloy according to the present invention has optimized components, high hardness can be obtained, and magnetic properties equal to or better than those of conventional materials can be obtained.

(Examples 1 to 21, Comparative Examples 1 to 8)
[1. Preparation of sample]
In a vacuum induction furnace, 50 kg of steel having the composition shown in Table 1 was melted and cast into an ingot. Thereafter, a round bar of φ35 mm was forged under conditions of a start temperature of 1100° C. and an end temperature of 900° C., and then air-cooled. Further, a ring test piece with an outer diameter of 28 mm, an inner diameter of 20 mm, and a thickness of 3 mm and a square bar test piece of 3 mm×3 mm×50 mm were cut out from the obtained round bar.

Next, solid-phase nitrogen absorption treatment and quenching were performed on each test piece. That is, first, the test piece was placed in the processing chamber, and then the processing chamber was evacuated by the depressurizing means. Next, nitrogen gas was introduced into the processing chamber to maintain the pressure in the processing chamber at a predetermined value. The pressure inside the processing chamber is
(a) 0.1 MPa (Examples 1-2, Examples 4-19, Comparative Examples 3-5), or
(b) 0.7 MPa (Example 3, Example 20, Comparative Examples 1 and 2)
and
In this state, the test piece was heated at a temperature of 900 to 1100° C. for a predetermined time. The heating time was adjusted so that the thickness of the nitrogen absorption layer after quenching was 10 μm or more. Furthermore, after the solid-phase nitrogen absorption treatment was completed, the test pieces were quenched. Quenching was performed by gas cooling, and then subzero treatment was performed at −70° C. for 2 hours.

[2. Test method]
[2.1. Thickness of nitrogen absorption layer]
After the solid-phase nitrogen absorption treatment, the specimen was cut and the thickness of the nitrogen absorption layer was measured.
[2.2. Surface nitrogen concentration (nitrogen concentration of nitrogen absorption layer)]
Using an electron probe microanalyzer (EPMA), the nitrogen concentration in the surface layer of the test piece was measured.

[2.3. Surface hardness (hardness of nitrogen absorption layer)]
After the solid-phase nitrogen treatment, the test piece was cut, and the Vickers hardness was measured at a depth of 20 μm±5 μm from the surface.
[2.4. Magnetic flux density B ₃₀₀₀₀ ]
Put the quenched ring test piece in a resin case, wrap 500 coils for magnetic field application and 100 coils for detection around the case, and use a DC magnetization property analyzer to measure the magnetic flux when the magnetic field is 30 kA / m. A density B of ₃₀₀₀₀ was measured.

[2.5. electrical resistance]
Using the square bar test piece after quenching, the electrical resistance of the test piece was measured by the four-probe method.
[2.6. Area ratio of ferrite phase in the core]
The test piece after the solid-phase nitrogen absorption treatment was cut, and the area ratio of the ferrite phase in the core was calculated.

[3. result]
Table 2 shows the results. Table 2 shows the following.
(1) Comparative Example 1 has a low surface hardness. It is considered that this is because the surface layer portion did not martensite because the amount of Al was excessive.
(2) Comparative Example 2 has low surface hardness. It is considered that this is because the surface layer portion did not martensite because the amount of Si was excessive.
(3) Comparative Example 3 has a low B ₃₀₀₀₀ . This is probably because the C content was excessive, so austenite was formed in the core in the high temperature range, and the matrix of the core did not become a ferrite single phase at room temperature.
(4) Comparative Example 4 has a low B ₃₀₀₀₀ . It is considered that this is because the amount of Cr is excessive.

(5) Comparative Example 5 has a low surface hardness and a low _B30000 . The low surface hardness is considered to be due to the low content of Cr, the low concentration of dissolved nitrogen, and the absence of alloying elements such as Si that improve the hardness. The reason why B ₃₀₀₀₀ is low is considered to be that the matrix of the core part was almost martensitic during quenching because it did not contain Al, Si and Ti, and the matrix of the core part did not become a ferrite single phase at room temperature.
(6) Comparative Example 6 has a low surface hardness and a low _B30000 . The reason for the low surface hardness is considered to be that the surface layer did not turn into martensite due to the excessive amount of Ti. The reason why B ₃₀₀₀₀ is low is considered to be that the Laves phase is formed due to the excessive amount of Ti, and the core matrix does not become a ferrite single phase at room temperature.
(7) Comparative Example 7 has a low B ₃₀₀₀₀ . This is probably because austenite was formed in the core at high temperatures due to the small amount of Si, and the matrix of the core did not become a ferrite single phase at room temperature.
(8) Comparative Example 8 has low magnetic properties. This is probably because austenite was generated in the core portion due to the excessive amount of N, and the core portion did not become a ferrite single phase.

(9) In all of Examples 1 to 21, the surface hardness was 400 HV or more, and the area ratio of the ferrite phase in the core was 95% or more at room temperature.
(10) Examples 3 and 20 have higher surface hardness than other materials having substantially the same composition. This is probably because a larger amount of nitrogen dissolved in the surface layer by setting the nitrogen partial pressure to 0.7 MPa. On the other hand, in Comparative Examples 1 and 2, the surface hardness was less than 400 HV even at a nitrogen partial pressure of 0.7 MPa. It is considered that this is because the surface layer did not turn into martensite because the amount of Al or Si was excessive.
(11) B ₃₀₀₀₀ increased as the amount of Al and/or Si decreased. On the other hand, the electrical resistance increased as the amount of Al and/or Si increased.

Although the embodiments of the present invention have been described in detail above, the present invention is by no means limited to the above-described embodiments, and various modifications are possible without departing from the gist of the present invention.

The Fe-based alloy according to the present invention can be used for a movable core of an electromagnetic fuel injection device, a rotor core of a motor for an electric vehicle, and the like.

Claims (5)

C ≤ 0.020 mass%,
N ≤ 0.030 mass%, and
5.0≦Cr≦18.0 mass %
and further
0.5≦Si≦3.0 mass%,
0.1 ≤ Al ≤ 3.0 mass%, and
0.05≤Ti≤3.0mass%
containing one or more elements of
The balance consists of Fe and unavoidable impurities,
An Fe-based alloy for solid-phase nitrogen absorption having a ferrite phase area ratio of 95% or more at 23°C. The Fe-based alloy for solid-phase nitrogen absorption according to claim 1, which satisfies the following formula (1).
([Cr]+2.6[Si]+7[Al]+7[Ti])/(5[C]+5[N]+0.35)≧33 (1)
However, [X] represents the mass ratio (mass%) of the element X contained in the Fe-based alloy for solid-phase nitrogen absorption. a base made of the Fe-based alloy for solid-phase nitrogen absorption according to claim 1 or 2;
A nitrogen absorption layer formed on the surface of the base,
The nitrogen absorption layer has a hardness of 400 HV or more,
An Fe-based alloy member, wherein the ferrite phase area ratio of the base portion at 23°C is 95% or more. 4. The Fe-based alloy member according to claim 3, wherein the crystal grain size of said base portion is 150 [mu]m or more. 5. The Fe-based alloy member according to claim 3, comprising a movable core of an electromagnetic fuel injection device.