JP4168342B2 - Steel material with high hardness and high magnetic properties and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high hardness and high magnetic property steel material and a manufacturing method thereof, and more specifically, a high hardness and high magnetic property steel material having high hardness and high magnetic properties such as a saturation magnetic flux density and a permeability, and such a high hardness and high strength material. The present invention relates to a method of manufacturing magnetic property steel from steel. The steel material with high hardness and high magnetic properties according to the present invention is suitable for use in, for example, a drive motor for an electric vehicle such as a hybrid electric vehicle or a fuel cell vehicle, an iron core that is a component of another type of motor, or the like.

  A drive motor for an electric vehicle such as a hybrid electric vehicle or a fuel cell vehicle is the heart of a drive mechanism that replaces an engine in a conventional vehicle such as a gasoline engine, and an electromagnetic steel plate is generally used as an iron core as a main component of the drive motor. It is used. For this reason, the electromagnetic steel sheet as the iron core material is an important functional material for improving the drive performance and fuel efficiency by reducing the size and weight of the drive motor and increasing the efficiency.

  Also, in other types of motors, high functionality is strongly required for the electromagnetic steel sheet as the core material as the main component.

  As such an electrical steel sheet, an Fe—Si based electrical steel sheet in which silicon is added to iron to improve the magnetic properties by controlling the alignment of crystal orientation and the width of the magnetic domain is generally known.

On the other hand, α ″ -Fe ₁₆ N ₂ ultrafine powder is known as a magnetic material having a high saturation magnetic flux density (see, for example, Patent Document 1).

The α ″ -Fe ₁₆ N ₂ ultrafine powder is directly synthesized by reacting α-Fe ultrafine powder having a particle size of about 20 nm with a nitrogen-containing gas such as ammonia gas at a temperature of 200 ° or less. Can do. The ultrafine powder of α ″ -Fe ₁₆ N ₂ thus obtained is composed of a single phase of α ″ -Fe ₁₆ N ₂ crystal having a bct structure (body-centered tetragonal crystal), and has a high saturation magnetic flux density (saturation). Magnetization).

Also, there is a method of obtaining an α ″ -Fe ₁₆ N ₂ crystal phase as a thin film on the surface of an iron plate by using a vapor deposition method, an MBE method (molecular epitaxial method), an ion implantation method, a sputtering method, an ammonia nitriding method, or the like. is there.
JP 11-340023 A (page 2)

  By the way, in order to increase the efficiency of the motor, it is important to increase the saturation magnetic flux density of the electromagnetic steel sheet as the iron core material. In particular, there is a high demand for high efficiency for a drive motor for an electric vehicle, and there is a strong demand for further improvement of the saturation magnetic flux density in the electromagnetic steel sheet.

  However, an Fe—Si based electrical steel sheet capable of exhibiting a sufficiently high saturation magnetic flux density that can sufficiently meet the demand has not been developed yet.

On the other hand, the above-mentioned conventional α ″ -Fe ₁₆ N ₂ ultrafine powder or α ″ -Fe ₁₆ N ₂ thin film formed on the surface of the iron plate may be used as an iron core material. It is considered difficult to exhibit a saturation magnetic flux density that can sufficiently meet the demand for higher efficiency of a drive motor for an electric vehicle.

That, alpha '' - ultrafine powders Fe ₁₆ N ₂ can, alpha '' - although Fe ₁₆ N ₂ can be expected to improve the saturation magnetic flux density at composed of a single phase of crystal, but since an ultra fine powder form Therefore, it is considered that a sufficiently high saturation magnetic flux density cannot actually be exhibited because the specific surface area increases and the influence of the oxide film on the powder surface is greatly affected.

Further, when obtaining a crystalline phase of α ″ -Fe ₁₆ N ₂ as a thin film, it is difficult to penetrate nitrogen deep inside the iron plate, and therefore α ″ -Fe only to a depth of about several tens of nanometers from the surface. It is considered that a thin film having a ₁₆ N ₂ crystal phase cannot be formed, and that a sufficiently high saturation magnetic flux density cannot be exhibited.

  On the other hand, an increase in motor rotation speed is advantageous for miniaturization of the motor. At this time, when the electromagnetic steel plate is used for the rotor of the motor, if the rotor rotates at a high speed, the centrifugal force acting on the rotor also increases. Therefore, it is necessary to increase the rotor strength in order to prevent the rotor from being damaged. It is necessary to increase the hardness of the electromagnetic steel sheet used for the rotor.

  The present invention has been made in view of the above circumstances, and provides a method for producing a high magnetic property steel material capable of improving the saturation magnetic flux density and hardness, and a high magnetic property steel material having improved saturation magnetic flux density and hardness. This is a technical problem to be solved.

The method of manufacturing a high hardness, high magnetic properties steel of the present invention for solving the above-mentioned problems, by performing shot peening on the surface of the steel material, such finer grain crystal grain size in the range of 10~900nm Riatsu A fine crystallization step of forming a fine crystal layer having a thickness of 100 μm or more on the surface of the steel material, and a nitriding treatment in which the steel material on which the fine crystal layer is formed is reacted with a nitrogen-containing gas within a predetermined temperature range , fine fine crystal layer by nitriding alpha '' - is characterized in that including a nitriding treatment step of the Fe ₁₆ N ₂ to unrealized thickness 100μm or more microcrystalline nitride layer.

In this method of manufacturing a high magnetic property steel material, a range in which a fine crystal layer is formed by forming a fine crystal layer made of fine crystal grains of predetermined nano-order in advance on the surface of the steel material and then performing a predetermined nitriding treatment. In this case, nitrogen easily penetrates deeper into the interior, and the nitriding depth during nitriding can be increased. For this reason, a predetermined nano-order fine crystal layer is formed with a predetermined thickness in the fine crystallization process, and the entire fine crystal layer is nitrided to a depth at which the fine crystal layer is formed in the nitriding treatment process. alpha '' - if such a fine crystalline nitride layer containing Fe ₁₆ N _2, as an internal deeply high iron nitride magnetic properties such as saturation magnetic flux density of the steel alpha '' - to form a Fe ₁₆ N ₂ A thick microcrystalline nitride layer can be formed. Therefore, the saturation magnetic flux density and hardness can be improved.

In a preferred embodiment, when the shot peening process is N: coverage (%), D: shot particle size (mm), and V: shot projection speed (m / sec), N ≧ (3.8 × 10 ⁷ ) Performed under shot conditions satisfying / (D × V ² ). According to this aspect, the fine crystal layer can be formed on the surface of the steel material.

  In a preferred embodiment, the shot peening treatment is performed when the shot particle size (D) is 0.03 to 3.5 mm and the shot projection speed (V) is 50 to 250 m / sec. The shot condition is such that the coverage (N) is 5000% or more.

In a preferred embodiment, in the nitriding step, low temperature nitriding is performed in a temperature range of 150 to 300 ° C. According to this aspect, by performing nitriding at a low temperature in a predetermined temperature range, the fine crystal layer can be nitrided directly to form α ″ -Fe ₁₆ N ₂ having high magnetic properties such as saturation magnetic flux density. it can.

  In a preferred embodiment, the saturation magnetic flux density (σs) in the fine crystal nitride layer is 207 emu / g or more.

High hardness, high magnetic properties steel of the present invention for solving the problems is-than thickness finer crystal grains formed by nitriding the above fine crystalline layer 100μm with grain size in the range of 10～900Nm, α '' - Fe ₁₆ N ₂ with unrealized thickness is characterized in that it has a more fine crystal nitride layer 100 [mu] m.

  In a preferred embodiment, the saturation magnetic flux density (σs) in the fine crystal nitride layer is 207 emu / g or more.

  The manufacturing method of the high hardness and high magnetic property steel material according to the present invention includes a fine crystallization process and a nitriding process.

In the fine crystallization step, by performing shot peening on the surface of the steel material, the crystal grain size in the range-than thickness finer crystal grains within the 10~900nm is steel material of the above fine crystalline layer 100μm Form on the surface. In the subsequent nitriding step, the steel material on which the fine crystal layer is formed is nitrided by reacting the steel material with a nitrogen-containing gas in a predetermined temperature range, thereby nitriding the fine crystal layer to α ′ _'-Fe 16 _{N 2} the unrealized thickness to 100μm or more microcrystalline nitride layer.

In this way, first, a fine crystal layer composed of fine crystal grains having a predetermined nano-order (10 to 900 nm) crystal grain size is formed on the surface of the steel material, and then subjected to a predetermined nitriding treatment, whereby the fine crystal layer is formed. Nitrogen easily penetrates to the depth where the film is formed, and the fine crystal layer can be a fine crystal nitride layer containing α ″ -Fe ₁₆ N ₂ . Therefore, by forming the fine crystal layer with a predetermined thickness, the fine crystal nitride layer with a predetermined thickness can be formed. Therefore, by increasing the thickness of the fine crystal layer, the nitridation depth can be increased to increase the thickness of the fine crystal nitride layer, and α ″ -Fe ₁₆ N ₂ can be further introduced into the steel. It becomes possible to form. Therefore, it is possible to further improve the magnetic characteristics that can be exhibited by the microcrystalline nitride layer containing α ″ -Fe ₁₆ N ₂ having high magnetic characteristics such as saturation magnetic flux density. Specifically, the saturation magnetic flux density (σs) and the magnetic permeability (μ) can be further improved, and the coercive force can be reduced. Further, if the fine crystal nitride layer becomes thicker, the hardness can be improved accordingly.

Here, the permeability (diffusibility) of nitrogen is improved in the fine crystal layer composed of fine crystal grains having a nano-order crystal grain size because nitrogen diffuses inside due to the existence of many crystal grain boundaries. This is thought to be easier. The reason why the magnetic characteristics are improved as the thickness of the fine crystal nitride layer is increased is that the amount of α ″ -Fe ₁₆ N ₂ nitride having a high saturation magnetic flux density is increased. Further, it is considered that the hardness is improved when the thickness of the fine crystal nitride layer is increased because the nitride amount contributing to the increase in hardness is increased as the thickness of the fine crystal nitride layer is increased.

  The steel material is not particularly limited. For example, the steel material may be an Fe material containing a total of about 1 wt% or less of impurity elements such as C, Si, Mn, P, and S.

  According to the shot peening process, the surface of the steel material (shot surface) is repeatedly plastically deformed by continuous collision of shots. For this reason, by performing shot peening treatment under predetermined shot conditions, the metallographic structure of the surface layer of the shot surface can be refined, and a fine crystal layer composed of fine crystal grains having a nano-order crystal grain size is formed. It becomes possible to form on the steel material surface.

In the shot peening process, N ≧ (3.8 × 10 ⁷ ) / (D, where N is coverage (%), D is shot particle size (mm), and V is shot projection speed (m / sec). It is preferable to carry out under shot conditions satisfying × V ² ).

Coverage (N) indicates the degree of shot, and the entire shot surface of the steel material shot uniformly and uniformly is defined as 100%. The larger the coverage (N) value and the larger the shot particle size (D) and shot projection speed (V), the greater the energy applied by the shot per unit area of the shot surface. . As the shot energy per unit area on the shot surface increases, the metallographic structure of the surface layer on the shot surface can be further refined. For this reason, the shot energy per unit area on the shot surface is reduced by performing the shot peening process under the shot condition where the coverage (N) satisfies N ≧ (3.8 × 10 ⁷ ) / (D × V ² ). When the surface structure of the shot surface is set to a predetermined value or more, the metal structure of the surface layer is refined by a predetermined amount, and a fine crystal layer made of fine crystal grains having a crystal grain size of a predetermined nano order (10 to 900 nm) has a predetermined thickness. It can be suitably formed on the steel surface.

  In the shot peening treatment, the shot particle size (D) is 0.03 to 3.5 mm (preferably 0.1 to 3 mm), and the shot projection speed (V) is 50 to 250 m / sec ( Preferably, it is performed under a shot condition where the coverage (N) is 5000% or more when it is preferably 100 to 200 m / sec), more preferably performed under a shot condition where the coverage (N) is 8000% or more, Particularly preferably, it is performed under a shot condition where the coverage (N) is 18000% or more. As the value of coverage (N) is increased, the metallographic structure of the surface layer on the shot surface can be further refined and the thickness of the fine crystal layer can be increased. By performing shot peening under a shot condition where the coverage (N) is 5000% or more, the upper limit of the crystal grain size of the fine crystal grains in the fine crystal layer is 900 nm, and the thickness of the fine crystal layer is about 100 μm or more. Can do. Further, by performing shot peening treatment under a shot condition where the coverage (N) is 8000% or more, the upper limit of the crystal grain size of the fine crystal grains in the fine crystal layer is about 650 nm, and the thickness of the fine crystal layer is about 130 μm or more. The upper limit of the crystal grain size of the fine crystal grains in the fine crystal layer is set to about 300 nm by performing shot peening under a shot condition where the coverage (N) is 18000% or more, and the thickness of the fine crystal layer is Can be about 200 μm or more. Note that, when the coverage (N) exceeds 20000%, the hardness of the shot surface may be lowered, so the upper limit of the coverage (N) can be about 20000%.

  In the shot peening process, the projection pressure can be about 0.3 to 0.5 MPa. Also, the type (material) of the shot grain is not particularly limited, and those having hardness equal to or higher than that of the steel material, specifically, carbon steel of HV600 or higher, alloy steel, or the like can be adopted.

  In the shot peening process, by changing at least one of the shot particle size (D), the shot projection speed (V) and the coverage (N), or changing the shot time, the steel material The thickness of the fine crystal layer formed on the surface and the crystal grain size can be adjusted.

Here, as the shot peening treatment is performed under the condition that the crystal grain size of the fine crystal grains in the fine crystal layer can be reduced, the thickness of the fine crystal layer can be increased, and thus the fine crystal obtained by nitriding the fine crystal layer. The thickness of the nitride layer can be increased. Further, if the thickness of the fine crystal nitride layer containing α ″ -Fe ₁₆ N ₂ is increased, it is advantageous for improving the magnetic properties and hardness.

According to the experiments by the present inventors, the upper limit of the crystal grain size of the fine crystal grains in the fine crystal layer is set to 900 nm, thereby setting the thickness of the fine crystal nitride layer to 100 μm or more (the nitridation depth). ) Can be set to 100 μm or more, and the thickness (nitridation depth) of the fine crystal nitride layer containing α ″ -Fe ₁₆ N ₂ is set to 100 μm or more, thereby effectively improving the magnetic properties and hardness. It has been found that it can be improved.

  Therefore, in the present invention, the upper limit of the crystal grain size of the fine crystal grains in the fine crystal layer is set to 900 nm. Thus, by setting the upper limit of the crystal grain size of the fine crystal grains to 900 nm, as described above, the depth of the fine crystal nitride layer can be about 100 μm or more, and the magnetic characteristics and hardness are effectively improved. It becomes possible.

In addition, if the thickness of the fine crystal nitride layer containing α ″ -Fe ₁₆ N ₂ is increased, it is advantageous to improve the magnetic properties and hardness. Therefore, the crystal grain size of the fine crystal grains in the fine crystal layer Is preferably about 650 nm and the thickness of the fine crystal nitride layer is preferably about 130 μm or more, and the upper limit of the crystal grain size of the fine crystal grains is about 300 nm and the thickness of the fine crystal nitride layer is about 200 μm or more. More preferably.

  In the fine crystallization step, the fine crystal layer may be formed over the entire thickness direction of the steel material, or the fine crystal layer may be formed only on the surface portion of the steel material.

  In addition, the thickness of the fine crystal layer (thickness of the fine crystal nitride layer) is preferably as thick as possible within the range in which it can be formed, but the upper limit that can be formed is about 300 μm.

In the nitriding treatment step, the steel material on which the fine crystal layer is formed is nitrogen-containing gas (for example, a single gas of ammonia or a mixed gas of ammonia gas and other gas (CO or CO ₂ )) within a predetermined temperature range. The fine crystal layer is nitrided to form a fine crystal nitride layer containing α ″ -Fe ₁₆ N ₂ by performing nitriding treatment to react with. In this nitriding treatment step, it is preferable to form a fine crystal nitride layer having a thickness corresponding to the thickness of the fine crystal layer by nitriding the entire fine crystal layer.

In this nitriding process, it is preferable to perform low-temperature nitriding in a temperature range of 150 to 300 ° C. By performing low-temperature nitriding in a temperature range of 150 to 300 ° C., the fine crystal layer can be directly nitrided to directly form α ″ -Fe ₁₆ N ₂ having high magnetic properties such as saturation magnetic flux density. It becomes possible to easily form a fine crystal nitride layer containing ″ -Fe ₁₆ N ₂ . If the temperature in the nitriding treatment in excess of 300 ℃, α '' - becomes nitrides such as Fe ₂ N and Fe ₃ N rather than _{Fe 16 N 2, α ''} - it can not be achieved an improvement of magnetic properties by Fe ₁₆ N ₂ . On the other hand, if nitriding is performed at a temperature lower than 150 ° C., the nitride itself is not formed, and the magnetic characteristics and hardness cannot be improved. Further, the nitriding time in the low temperature nitriding treatment affects the magnetic characteristics such as saturation magnetic flux density and the hardness. The low-temperature nitriding treatment time is preferably about 40 to 200 hours. With this process low temperature nitriding, direct alpha '' - it is possible to form the Fe ₁₆ N ₂ nitride, alpha '' - later need tempering to form the Fe ₁₆ N ₂ nitride Compared with the nitriding heat treatment, it is advantageous in that the number of processing steps can be reduced.

  In the nitriding treatment step, a tempering treatment is performed in which a reaction is performed with a nitrogen-containing gas at a high temperature range of about 500 to 600 ° C. for about 5 to 10 hours, and then for about 1 to 200 hours at 100 to 200 ° C. A nitriding heat treatment may be performed.

  Thus, through the fine crystallization process and the nitriding treatment process, the saturation magnetic flux density (σs) in the fine crystal nitride layer is 207 emu / g or more (more preferably 210 emu / g or more), and the magnetic permeability (μ) is 52 or more. (More preferably 60 or more), the coercive force (Hc) can be 12 Oe or less (more preferably 10 Oe or less), and the hardness of the microcrystalline nitride layer can be 700 mHV or more, so that the saturation magnetic flux density (σs) is High magnetic properties of 207 emu / g or more (more preferably 210 emu / g or more), permeability (μ) of 52 or more (more preferably 60 or more), and coercive force (Hc) of 12 Oe or less (more preferably 10 Oe or less). And obtaining a high hardness and high magnetic property steel having a hardness of 680 mHV or more (more preferably 700 mHV or more). The ability.

  The steel material having high hardness and high magnetic properties thus obtained can be suitably used for, for example, a drive motor for an electric vehicle such as a hybrid electric vehicle or a fuel cell vehicle, an iron core which is a component of another type of motor, or the like.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in more detail, this invention is not limited to these.

(Example 1)
FIG. 1 is a process diagram schematically showing a method of manufacturing a steel material having high hardness and high magnetic properties according to the present embodiment. This manufacturing method includes a fine crystallization process, a nitriding process, and a cutting process.

<Fine crystallization process>
As the steel material 1, a material containing 0.05 wt% of total impurity elements of C, Si, Mn, P, and S and the balance being Fe was prepared. In addition, the shape and dimension of this steel material 1 are thin plates (30 mm × 30 mm) having a thickness of 1 mm.

  Then, the surface of the steel material 1 is subjected to shot peening under the following shot conditions where the coverage is 18000%, the crystal grain size is in the range of 100 to 300 nm, and the average crystal grain size is 200 nm. A fine crystal layer 1a composed of fine crystal grains and having a thickness of 200 μm was formed on the surface of the steel material 1 (see FIG. 2A).

Shot grain type: Carbon steel Shot grain hardness: HV800
Shot particle size: 1.1mm
Shot projection speed: 100 m / sec
Coverage: 18000%
<Nitriding process>
Next, the steel material 1 on which the fine crystal layer 1a is formed is subjected to a low-temperature nitriding treatment in which the steel material 1 is reacted with a nitrogen-containing gas under the following conditions, so that the entire fine crystal layer 1a is nitrided to have a thickness of 200 μm. A microcrystalline nitride layer 1b made of α ″ -Fe ₁₆ N ₂ was formed (see FIG. 2B).

Nitriding temperature: 150 ° C
Nitriding time: 100 hours Nitrogen-containing gas: NH ₃ gas <Cutting step>
The steel material 1 on which the fine crystal nitride layer 1b is formed is cut in cross section so as to cut only the portion of the fine crystal nitride layer 1b, and has a high hardness and high magnetic properties of 200 μm in thickness consisting only of the fine crystal nitride layer 1b. A steel material 10 was manufactured (see FIG. 2C).

A sample was cut out from the obtained high hardness and high magnetic property steel material 10, and the crystal phase in the fine crystal nitrided layer 1b was examined by an X-ray diffractometer. As a result, it was a single crystal phase of α ″ -Fe ₁₆ N ₂ .

(Comparative Example 1)
<Fine crystallization process>
The surface of the same steel material 1 as in Example 1 is subjected to shot peening treatment under the following shot conditions where the coverage is 3000%, the crystal grain size is in the range of 500 to 1000 nm, and the average crystal grain size is A fine crystal layer having a thickness of 65 μm composed of fine crystal grains of 700 nm was formed on the surface of the steel material 1.

Shot grain type: Carbon steel Shot grain hardness: HV700
Shot particle size: 1.1mm
Shot projection speed: 100 m / sec
Coverage: 3000%
<Nitriding process>
Next, the steel material 1 on which the fine crystal layer is formed is subjected to low-temperature nitriding treatment under the same conditions as in the first embodiment, so that the entire fine crystal layer is nitrided to give a fine crystal nitride layer having a thickness of 65 μm. It was.

<Cutting process>
A cross-section was cut from the steel material 1 on which the fine crystal nitrided layer was formed so as to separate only the portion of the fine crystal nitrided layer, thereby producing a magnetic property steel material having a thickness of 65 μm consisting of only the fine crystal nitrided layer.

(Evaluation of magnetic properties and hardness)
About the high hardness and high magnetic property steel material 10 obtained in Example 1 and the magnetic property steel material obtained in Comparative Example 1, the magnetic properties of the saturation magnetic flux density (σs), the coercive force (Hc), and the magnetic permeability (μ), and Vickers. Each hardness was measured. In addition, a sample is cut out from the surface of the steel material 1 after the shot peening treatment (the shot surface on which the fine crystal layer is formed), and the cross section of the sample is observed with an electron microscope, thereby reducing the crystal grain size of the fine crystal layer. It was measured. The results are shown in Table 1 and FIG. The magnetic properties were measured using a vibrating sample magnetometer.

  As is clear from Table 1, the high hardness and high magnetic property steel material 10 of Example 1 has a higher saturation magnetic flux density and permeability than the magnetic property steel material obtained in Comparative Example 1, while having a coercive force. It can be seen that the hardness decreases and the hardness increases.

  Further, from FIG. 3, by setting the coverage during the shot peening process to 5000% or more, the saturation magnetic flux density (σs) in the microcrystalline nitride layer 1b is 207 emu / g or more, the magnetic permeability (μ) is 52 or more, and the coercive force ( It can be seen that Hc) can be set to 12 Oe or less, and a high hardness and high magnetic property steel material having such magnetic properties can be manufactured.

(Relationship between coverage and hardness)
In the fine crystallization process of Example 1, the shot peening treatment was performed by changing the shot peening time in various ways, and the relationship between the coverage, the Vickers hardness and the homogeneity was examined. The result is shown in FIG. The homogeneity is a coating ratio of nanocrystal grains formed on the surface.

  FIG. 4 shows that both the hardness and the homogeneity are low when the coverage is low. Further, by setting the coverage at the time of shot peening to 5000% or more, the Vickers hardness in the fine crystal nitrided layer 1b can be set to 680 mHV or more, and such a high hardness and high magnetic property steel can be manufactured. I understand.

(Relationship between coverage and crystal grain size and nitriding depth of fine crystal layer)
In the fine crystallization process of the first embodiment, the shot peening process is performed by changing the shot peening time in various ways to change the coverage and the nitriding depth (that is, the thickness of the fine crystal nitride layer 1b (the fine crystal And the upper limit of the crystal grain size and the nitriding depth of the fine crystal grains in the fine crystal layer (that is, the thickness of the fine crystal nitride layer 1b (the thickness of the fine crystal layer 1a) Is equal to))). The results are shown in FIGS. In FIG. 5 and FIG. 6, the common logarithm is graduated on the horizontal axis.

  5 and 6, by setting the coverage during the shot peening process to 5000% or more, the upper limit of the crystal grain size of the fine crystal grains in the fine crystal layer 1a is set to 900 nm, and the nitridation depth (of the fine crystal nitride layer 1b) is set. It can be seen that (thickness) can be 100 μm or more. Further, by setting the coverage during the shot peening process to 18000%, the upper limit of the crystal grain size of the fine crystal grains in the fine crystal layer 1a is set to about 300 nm, and the nitridation depth (thickness of the fine crystal nitride layer 1b) is set to about It can be seen that the thickness can be 200 μm.

(Relationship between low temperature nitriding time and magnetic properties)
In the nitriding process of Example 1, the relationship between the low-temperature nitriding time and the magnetic characteristics was examined by variously changing the nitriding time and performing low-temperature nitriding. The result is shown in FIG.

  From FIG. 7, when the low temperature nitridation time exceeds 30 hours, the saturation magnetic flux density (σs) rapidly increases and reaches the maximum value at 45 hours, and when the low temperature nitridation time exceeds 45 hours, the saturation magnetic flux density (σs) gradually decreases. . For this reason, the low temperature nitriding time is preferably 40 hours or more (more preferably 45 hours or more), and preferably 200 hours or less.

(Example 2)
This example is the same as Example 1 except that the nitriding process is changed, such as changing the nitriding temperature to 600 ° C.

<Fine crystallization process>
The surface of the same steel material 1 as in Example 1 is subjected to shot peening treatment under the same shot conditions as in Example 1 with a coverage of 18000%, and as in Example 1, a fine crystal layer having a thickness of 200 μm 1 a was formed on the surface of the steel material 1.

<Nitriding process>
Next, the steel material 1 on which the fine crystal layer 1a is formed is subjected to nitriding heat treatment for nitriding and tempering under the following conditions, thereby nitriding and thickening the entire fine crystal layer 1a. A microcrystalline nitride layer 1b made of α ″ -Fe ₁₆ N _{2 having a} thickness of 200 μm was formed. In addition, after the nitriding treatment, tempering treatment was performed after rapid cooling (water cooling + sub-zero treatment).

Nitriding temperature: 600 ° C
Nitriding time: 6 hours Nitrogen-containing gas: NH ₃ gas Tempering conditions: 150 ° C. × 100 hours <Cutting step>
The steel material 1 on which the fine crystal nitrided layer 1b is formed is cut in cross section so as to cut only the portion of the fine crystal nitrided layer 1b, and has a high hardness and high magnetic characteristics of 200 m in thickness consisting only of the fine crystal nitrided layer 1b. Steel material 10 was manufactured.

A sample was cut out from the obtained high hardness and high magnetic property steel material 10, and the crystal phase in the fine crystal nitrided layer 1b was examined by an X-ray diffractometer. As a result, in addition to the α ″ -Fe ₁₆ N ₂ crystal phase, residual austenite and Martensite crystal phase was also observed.

(Evaluation of magnetic properties and hardness)
With respect to the high hardness and high magnetic property steel material 10 obtained in Example 2, the magnetic properties of saturation magnetic flux density (σs), coercive force (Hc) and magnetic permeability (μ), and Vickers hardness were measured, respectively. Further, the crystal grain size of the fine crystal layer formed on the surface of the steel material 1 after the shot peening treatment was measured. The results are shown in Table 2.

  As is clear from Table 2, the high hardness and high magnetic property steel material 10 of Example 2 has a higher saturation magnetic flux density and a coercive force while a lower coercivity than the magnetic property steel material of Comparative Example 1. In addition, it can be seen that the hardness has increased.

It is process drawing which illustrates typically the manufacturing method of the high hardness and high magnetic property steel materials which concern on a present Example. It is sectional drawing which shows typically the thing obtained at each process of the manufacturing method of the high-hardness high magnetic property steel materials which concern on a present Example, (a) is the thing obtained at the fine crystallization process, (b) is The thing obtained by the nitriding process, (c) shows the thing (high hardness and high magnetic property steel material) obtained by the cutting process, respectively. It is a graph which shows the relationship between the coverage at the time of a shot peening process, and a magnetic characteristic. It is a graph which shows the relationship between the coverage at the time of a shot peening process, Vickers hardness, and homogeneity. It is a graph which shows the relationship between the coverage at the time of a shot peening process, and nitridation depth (thickness of the microcrystal nitrided layer 1b). It is a graph which shows the relationship between the upper limit of the crystal grain diameter of the fine crystal grain in a fine crystal layer, and the nitriding depth (thickness of the fine crystal nitride layer 1b). It is a graph which shows the relationship between the low temperature nitriding time in a nitriding process, and a magnetic characteristic.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Steel material 1a ... Fine crystal layer 1c ... Fine crystal nitride layer 10 ... High hardness and high magnetic property steel material

Claims (7)

By performing shot peening on the surface of the steel material, micro-crystallization grain size-than the thickness finer crystal grains in the range of 10~900nm form more fine crystal layer 100μm on the surface of the steel material Process,
By the nitriding reaction with nitrogen-containing gas at a predetermined temperature range the steel the microcrystalline layer is formed, fine fine crystal layer by nitriding α '' - Fe ₁₆ N ₂ to unrealized thickness Including a nitriding treatment step for forming a fine crystal nitrided layer having a thickness of 100 μm or more . When the shot peening process is N: coverage (%), D: shot particle size (mm), and V: shot projection speed (m / sec),
N ≧ (3.8 × 10 ⁷ ) / (D × V ² )
The method for producing a steel material with high hardness and high magnetic properties according to claim 1, wherein the method is performed under shot conditions that satisfy the following conditions. In the shot peening process, when the shot particle size (D) is 0.03 to 3.5 mm and the shot projection speed (V) is 50 to 250 m / sec, the coverage (N) is The method for producing a steel material with high hardness and high magnetic properties according to claim 2, wherein the method is performed under a shot condition of 5000% or more. The method for producing a steel material with high hardness and high magnetic properties according to any one of claims 1 to 3, wherein in the nitriding step, low-temperature nitriding is performed in a temperature range of 150 to 300 ° C. The method for producing a steel material with high hardness and high magnetic properties according to any one of claims 1 to 4, wherein a saturation magnetic flux density (σs) in the fine crystal nitrided layer is 207 emu / g or more. Crystal grain size-than the thickness finer crystal grains formed by nitriding the above fine crystalline layer 100μm in the range of _{10~900nm, α '' - Fe 16} N 2 unrealized thickness than 100μm the A steel material having high hardness and high magnetic properties, characterized by having a fine crystal nitride layer of The high hardness and high magnetic property steel according to claim 6, wherein a saturation magnetic flux density (σs) in the fine crystal nitrided layer is 207 emu / g or more.

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