JP2019035111A - Steel member and manufacturing method thereof - Google Patents

Abstract

A steel member having high surface fatigue strength and a method for producing the same are provided.
A surface layer 2 and a first martensite layer 4 having a nitrogen element concentration of 0.2 to 1.0% by mass disposed below the surface layer 2 are provided. Includes at least one of lithium iron composite oxide, FeO, and Fe ₃ O ₄ as a main component, and contains at least one selected from the group consisting of solute silicon, silicon oxide, and silicon nitride. The first martensite layer 4 is a steel member 10 containing a γ phase 15 having an area ratio of 30% or less and an ε phase 17 having an area ratio of 10% or less. After applying a denitrification inhibitor to the surface of the nitriding steel, induction heating is performed to produce a steel member.
[Selection] Figure 1

Description

  The present invention relates to a steel member having high surface pressure fatigue strength useful as a power transmission component and the like, and a method for manufacturing the same.

  Steel materials such as power transmission parts for industrial machines are required to have high surface fatigue strength. Conventionally, surface hardening treatments such as carburizing treatment, nitriding treatment, and induction heating hardening are widely known as means for imparting excellent surface pressure fatigue strength to such steel materials. In particular, in recent years, with the increasing demand for noise reduction, high-frequency heat quenching and soft nitriding treatment, which are surface hardening treatments with smaller thermal distortion than carburizing treatment, are attracting attention.

  In the high-frequency heating and quenching, only a necessary portion of the surface layer portion is heated for a short time, so that a quenching distortion is small and a highly accurate surface-hardened component can be obtained. Soft nitriding is frequently applied to steel materials that are required to have a shorter processing time and less distortion than carburizing. In recent years, a method of induction heating and quenching after nitriding has been proposed as a method for producing a steel material having superior mechanical properties. For example, after nitriding a specific steel material, a method of manufacturing a mechanical structural component or the like having increased mechanical strength by induction hardening, and a mechanical structural component manufactured by the method have been proposed ( Patent Documents 1 to 4).

Japanese Patent No. 3145517 Japanese Patent No. 3381738 Japanese Patent No. 5958652 Japanese Patent No. 5994924

  According to the methods proposed in Patent Documents 1 to 4, it has been possible to manufacture a steel material such as a machine structural component having a surface layer with a certain degree of hardness. However, it has been difficult to produce a steel material having a sufficiently high level of surface fatigue strength required in recent years.

  This invention is made | formed in view of the problem which such a prior art has, and the place made into the subject is providing the steel member with high surface-pressure fatigue strength. Moreover, the subject of this invention is providing the manufacturing method of the steel member with high surface pressure fatigue strength.

That is, according to the present invention, the following steel members are provided.
[1] A surface layer, and a first martensite layer having a nitrogen element concentration of 0.2 to 1.0% by mass disposed in a lower layer of the surface layer, wherein the surface layer is lithium Containing at least one of iron composite oxide, FeO, and Fe ₃ O ₄ as a main component, and containing at least one selected from the group consisting of solute silicon, silicon oxide, and silicon nitride, The martensite layer 1 is a steel member containing a γ phase with an area ratio of 30% or less and an ε phase with an area ratio of 10% or less.
[2] The steel member according to [1], wherein the thickness of the surface layer is 0.1 μm or more, and the thickness of the first martensite layer is 10 μm or more.
[3] The above [1] or [1], further comprising a second martensite layer having a thickness of 100 μm or more and having a nitrogen element concentration of less than 0.2 mass%, which is disposed below the first martensite layer. Steel member according to 2].

Moreover, according to this invention, the manufacturing method of the steel member shown below is provided.
[4] A method for producing a steel member according to any one of [1] to [3], wherein the steel member has a step of induction-hardening after applying a denitrification inhibitor to the surface of nitriding steel. Manufacturing method.
[5] The method for producing a steel member according to [4], wherein the denitrification inhibitor contains at least one of a polysiloxane compound and a precursor thereof.
[6] The method for manufacturing a steel member according to [4], wherein a carburization inhibitor is used as the denitrification inhibitor.
[7] The method for producing a steel member according to any one of [4] to [6], wherein the induction heating is performed at 850 ° C. or more for 5 seconds or less.
[8] The method for producing a steel member according to any one of [4] to [7], wherein the nitrided steel has an oxide layer having a thickness of 0.1 μm or more on a surface thereof.

  According to the present invention, a steel member having high surface pressure fatigue strength can be provided. Moreover, according to this invention, the manufacturing method of the steel member with high surface pressure fatigue strength can be provided.

It is sectional drawing which shows typically the layer structure of the steel member of this invention. 2 is an electron micrograph of a cross section of a steel member manufactured in Example 1. FIG. It is a chart (XRD chart) which shows the result of the X-ray diffraction of the steel member manufactured in Example 1. FIG. It is a figure which shows the result of the elemental analysis (silicon (Si) mapping) of the steel member manufactured in Example 1. FIG. It is a figure which shows the result of the elemental analysis (oxygen (O) mapping) of the steel member manufactured in Example 1. FIG.

<Steel members>
Embodiments of the present invention will be described below, but the present invention is not limited to the following embodiments. The steel member of the present invention includes a surface layer and a first martensite layer that is disposed in a lower layer of the surface layer and has a nitrogen element concentration of 0.2 to 1.0 mass%. The surface layer is composed of at least one of lithium iron composite oxide, FeO, and Fe ₃ O ₄ as a main component, and at least one selected from the group consisting of solute silicon, silicon oxide, and silicon nitride. contains. The first martensite layer contains a γ phase with an area ratio of 30% or less and an ε phase with an area ratio of 10% or less.

When nitriding treatment (including soft nitriding treatment) is applied to an iron-based steel material, nitrogen (N) penetrates into the surface of the steel material, and γ ′ phase (Fe ₄ N) or ε is caused by the penetrated nitrogen and iron (Fe). A high hardness oxide layer (compound layer) mainly composed of the phase (ε-Fe _{2 to 3} N) is formed. In the conventional technique, the surface pressure fatigue strength of the steel material is increased by subjecting the steel material after nitriding treatment to induction heating and quenching. However, by applying induction heating and quenching, (i) Nitrogen (N) easily escapes from the oxide layer while decomposition of the oxide layer proceeds; and (ii) Surface pressure fatigue due to nitrogen (N) being released. The present inventors have found out that the strength is not improved so much;

  Under such knowledge, the present inventors applied induction heating quenching after applying a component having a function of preventing or suppressing desorption of nitrogen (N), a so-called denitrification inhibitor, to the surface of the steel material after nitriding treatment. We examined how to do this. As a result, nitrogen (N) is desorbed from the surface of the steel while forming a specific compound layer (surface layer) by promoting the decomposition of the oxide layer, which is not so important from the viewpoint of improving the surface fatigue strength. It has been found that a martensite layer having a high nitrogen concentration (hereinafter also referred to as “N concentration”) that can be suppressed and improved in the surface fatigue strength can be formed under the surface layer, and the present invention has been completed.

  Details of the steel member of the present invention will be described with reference to FIG. FIG. 1 is a cross-sectional view schematically showing a layer structure of a steel member of the present invention. As shown in FIG. 1, a steel member 10 according to an embodiment of the present invention includes a surface layer 2 and a first martensite layer 4 disposed below the surface layer 2.

The surface layer 2 contains at least one of lithium iron composite oxide, FeO, and Fe ₃ O ₄ as a main component. The surface layer 2 is a layer for mainly imparting excellent surface pressure fatigue strength to the steel member 10. The lithium iron composite oxide is, for example, a composite oxide having a crystal structure of α-LiFeO ₂ , Li ₂ Fe ₃ O ₄ , Li ₂ Fe ₃ O ₅ , or Li ₅ Fe _2.5 O ₄ , or a mixture thereof. It is preferable. By providing the surface layer 2 whose main component is at least one of lithium iron composite oxide, FeO (wustite), and Fe ₃ O ₄ (magnetite), the surface pressure fatigue strength of the steel member 10 can be improved. . The surface layer 2 may contain a small amount of iron (III) oxide (Fe ₂ O ₃ ).

  The surface layer 2 contains at least one selected from the group consisting of solute silicon, silicon oxide, and silicon nitride (hereinafter also collectively referred to as “silicon component”). These silicon components are denitrified containing silicon (Si) applied to the surface of a steel material after nitriding treatment (nitriding steel) to suppress the desorption of nitrogen (N) while the decomposition of the oxide layer proceeds. It is a component derived from the inhibitor and formed in the surface layer 2. That is, these silicon components are contained in the surface layer 2 that is formed by permeating silicon (Si) into the nitrided steel by induction-hardening the nitrided steel coated with the denitrification inhibitor. It is the ingredient which became.

  Whether or not silicon components such as solute silicon, silicon oxide, and silicon nitride derived from the denitrification inhibitor are contained in the surface layer 2 is, for example, elemental analysis of the surface layer 2 of the steel member 10. At the same time, it can be identified by mapping a specific element (Si) in an arbitrary cross section of the surface layer 2.

The thickness T _S of the surface layer 2 is preferably 0.1 μm or more, and more preferably 1 μm or more. If the thickness T _S of the surface layer 2 is less than 0.1 μm, the surface pressure fatigue strength of the steel member 10 may be slightly insufficient. The upper limit of the thickness T _S of the surface layer 2 is not particularly limited, but may be substantially 11 μm or less.

The thickness T _S of the surface layer 2 depends on, for example, the thickness of an oxide layer (outermost surface layer) of nitriding steel used as a raw material. Moreover, the thickness of the oxidation layer of nitriding steel can be controlled by appropriately adjusting the processing temperature and the processing time. The thickness T _S of the surface layer 2 can be measured by observing the side surface or cross section of the steel member 10 with an optical microscope or an electron microscope. The thicknesses of other layers to be described later can also be measured by the same method.

  The first martensite layer 4 disposed below the surface layer 2 is a so-called high nitrogen martensite layer containing a relatively high concentration of nitrogen element. Specifically, the nitrogen element concentration (N concentration) of the first martensite layer 4 is 0.2 to 1.0 mass%, preferably 0.30 to 0.65 mass%. By applying a denitrification inhibitor to the surface of the nitriding steel and subjecting it to induction heating and quenching, it is possible to form the first martensite layer 4 that is hard to escape nitrogen and contains a sufficient amount of nitrogen. And it can be set as the steel member 10 with high surface-pressure fatigue strength by providing the 1st martensite layer 4 which contains nitrogen in high concentration in this way. Furthermore, when the first martensite layer 4 contains a sufficient amount of nitrogen, the steel member 10 having high hardness can be obtained.

The first martensite layer 4 contains a γ phase (austenite phase) 15 and an ε phase (ε-Fe _{2 to 3} N) 17. The γ phase 15 and the ε phase 17 are phases that remain due to the nitriding steel used as a raw material. The area ratio of the γ phase 15 in the first martensite layer 4 is 30% or less, preferably 15 to 25% by mass. In addition, the area ratio of the ε phase 17 in the first martensite layer 4 is 10% or less, preferably 5% by mass or less. Since the area ratio (content ratio) of these phases is a predetermined ratio or less, the steel member 10 having excellent surface pressure fatigue strength can be obtained. The area ratio of the γ phase 15 and the ε phase 17 in the first martensite layer 4 is obtained by observing an arbitrary cross section of the steel member 10 with an optical microscope or an electron microscope, and obtaining the total area of the first martensite layer 4. It can be measured and calculated as the occupying ratio.

The thickness T ₁ of the first martensite layer 4 is preferably 10 μm or more, and more preferably 50 μm or more. By setting the thickness T ₁ of the first martensite layer 4 in the above range, the surface pressure fatigue strength of the steel member 10 can be further increased. The thickness T ₁ of the first martensite layer 4 can be controlled, for example, by selecting the thickness of the oxide layer of nitriding steel used as a raw material.

  As shown in FIG. 1, the steel member of the present invention may further include a second martensite layer 6 disposed below the first martensite layer 4. The second martensite layer 6 is a so-called low nitrogen martensite layer having a lower N concentration than the first martensite layer 4. Specifically, the N concentration of the second martensite layer 6 is usually less than 0.2% by mass, and preferably 0% (not substantially contained).

The thickness T ₂ of the second martensite layer 6 is usually 100 μm or more, preferably 120 μm or more. The thickness T ₂ of the second martensite layer 6 depends on, for example, the thickness of nitriding steel used as a raw material.

<Manufacturing method of steel member>
The method for producing a steel member according to the present invention is a method for producing the steel member described above, and includes a step of induction-hardening after applying a denitrification inhibitor to the surface of nitriding steel. The details will be described below.

  As the nitriding steel, it is preferable to use a steel having an oxide layer having a thickness of 0.1 μm or more, preferably 1 μm or more on the surface thereof. By using a nitriding steel having an oxide layer having a thickness of 0.1 μm or more, a steel member having a sufficiently thick surface layer and first martensite layer can be produced.

The nitriding steel can be manufactured, for example, by subjecting an iron-based steel material to a salt bath nitriding treatment (including a salt bath soft nitriding treatment). By subjecting the iron-based steel material to salt bath nitriding treatment, nitriding steel having an oxide layer formed on the surface thereof can be obtained. As the salt bath nitriding treatment, for example, a method of immersing an iron-based steel material in a molten salt bath containing Li ⁺ , Na ⁺ , and K ⁺ as cation components and CNO ⁻ and CO ₃ ²⁻ as anion components Etc. It is preferable to use a molten salt bath to which an alkali metal hydroxide (lithium hydroxide, sodium hydroxide, potassium hydroxide, etc.) is added. The salt bath nitriding treatment can be performed, for example, according to the salt bath nitriding treatment disclosed in Japanese Patent Application Laid-Open Nos. 2002-226963 and 2004-91906.

  Carbon steel, low alloy steel, medium alloy steel, high alloy steel, cast iron, etc. can be used as the iron-based steel material. From the viewpoint of cost, carbon steel and low alloy steel are preferred. Examples of the carbon steel include carbon steel materials for machine structures (S20C to S58C). In addition, as a composition of the carbon steel material for machine structures, for example, C: 0.2-1.2 mass%, Si: 0.05-0.5 mass%, Mn: 0.2-1.8 mass %, P: 0.03% by mass or less (excluding 0% by mass), and S: 0.03% by mass or less (not including 0% by mass), with the balance being iron and inevitable impurities Things can be mentioned.

  Examples of the low alloy steel include nickel chrome steel (SNC236-836), nickel chrome molybdenum steel (SNCM220-815), chrome molybdenum steel (SCM415-445, 822), chrome steel (SCr415-445), Examples thereof include manganese steels for machine structures (SMn420 to 443), manganese chromium steels (SMnC420, 443), and the like.

  In the method for producing a steel member of the present invention, a denitrification inhibitor is applied to the surface of nitriding steel. As the denitrification inhibitor, a component having a function of preventing or suppressing desorption of nitrogen (N) from the oxide layer present on the surface of the nitriding steel is used. Examples of the denitrification inhibitor having such a function include (i) a composition containing at least one of a polysiloxane compound and a precursor thereof; (ii) a so-called carburizing inhibitor; . The denitrification inhibitor is preferably a water-based or organic solvent-based liquid composition from the viewpoint of ease of use. Two or more denitrification inhibitors may be used in combination.

  The polysiloxane compound applied to the surface of the nitriding steel has an appropriate network structure with extremely fine pores capable of suppressing or preventing the permeation of nitrogen element (N) on the surface of the oxidation layer of the nitriding steel. It is presumed to form a coating film having In addition, it is estimated that the coating film as described above can also be formed by using a precursor of a polysiloxane compound, that is, a compound that becomes a polysiloxane compound after coating. By performing high-frequency heating and quenching in a state where such a coating film is formed, desorption of nitrogen from the oxide layer can be suppressed, and the first martensite layer having a high N concentration can be formed in the lower layer of the surface layer. it is conceivable that.

  Examples of the polysiloxane compound include straight silicone oils such as dimethyl silicone oil, methylphenyl silicone oil, and methyl hydrogen silicone oil; modified silicone oils. Furthermore, as modified silicone oil, reactive silicone oil such as amino modification, epoxy modification, carboxy modification, carbinol modification, methacryl modification, mercapto modification, phenol modification; polyether modification, methylstyryl modification, alkyl modification, fatty acid ester modification Non-reactive silicone oils such as; Examples of the precursor of the polysiloxane compound include alkoxysilane and derivatives thereof.

  As the composition containing at least one of the polysiloxane compound and its precursor, a commercially available “nitriding inhibitor” can be used. Specific examples of commercially available anti-nitriding agents include the trade name “Nitrobuster” series (manufactured by Parker Heat Treatment Industry Co., Ltd.).

The carburizing inhibitor is classified into, for example, a type agent containing boric acid as a main component and containing silicon; a type agent containing Na ₂ SiO ₃ (so-called water glass); and the like. It is presumed that the carburizing inhibitor applied to the surface of the nitriding steel forms a coating film on the surface of the oxide layer that can physically suppress or prevent the permeation of nitrogen element (N). By performing high-frequency heating and quenching in a state where such a coating film is formed, desorption of nitrogen from the oxide layer can be suppressed, and the first martensite layer having a high N concentration can be formed in the lower layer of the surface layer. it is conceivable that.

  A commercially available product can be used as the carburizing inhibitor. Specific examples of commercially available carburizing inhibitors include the product name “Carbon Buster” series (manufactured by Parker Heat Treatment Industry); the product name “Condorsal” (manufactured by Nusley); Can be mentioned.

  The steel member of the present invention described above can be obtained by induction-hardening nitriding steel with a denitrification inhibitor applied on the surface thereof. There are no particular limitations on the conditions for induction heating and quenching, and the usual conditions for induction heating and quenching applied to general steel materials can be applied. However, the heating temperature during induction heating and quenching is preferably 850 ° C. or higher, and more preferably 870 ° C. or higher. When the heating temperature is less than 850 ° C., decomposition of the oxidized layer of the nitriding steel becomes slightly insufficient, and the thickness of the formed surface layer (compound layer) tends to be slightly insufficient. Moreover, the heating time in the induction heating and quenching is preferably 5 seconds or less, and more preferably 4 seconds or less. When the heating time is longer than 5 seconds, nitrogen (N) may be easily released, and the N concentration of the formed first martensite layer may be easily lowered.

  According to the method for producing a steel member of the present invention, it is possible to produce a steel member having excellent surface fatigue strength, which is useful as a power transmission component such as a transmission sheave, joint, hub, and shaft, by a simple operation. it can. Furthermore, according to the method for manufacturing a steel member of the present invention, it is possible to manufacture a steel member having uniform characteristics with relatively little variation in surface fatigue strength between individuals.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited to these Examples. In the examples and comparative examples, “parts” and “%” are based on mass unless otherwise specified.

<Manufacture of steel members>
Example 1
Normalized JIS standard S45C equivalent steel (C: 0.42-0.48%, Si: 0.15-0.35%, Mn: 0.6-0.9%, P: 0.03% or less ( 0%) and S: 0.03% or less (not including 0%), the balance being carbon steel made of iron and inevitable impurities, diameter 26 mmφ × 150 mm) was prepared as a cylindrical specimen. . A salt bath soft nitriding treatment was performed in which a cylindrical test piece was immersed in a 600 ° C. molten salt containing cyanate, carbonate, and lithium salt for 2 hours to obtain a nitriding steel. The thickness of the oxide layer formed on the surface of the obtained nitriding steel was 18.8 μm. The thickness of the oxide layer was measured by observing a cross section of the nitriding steel with a scanning electron microscope. After applying the denitrification inhibitor A to the surface of the obtained nitriding steel, induction heating (200 kHz, water quenching) by a single quenching method is performed, and a test piece for measuring surface pressure fatigue strength (steel member) ) The induction heating and quenching temperature was 1,030 ° C. and the time was 1.4 seconds.

(Examples 2-14, Comparative Examples 1-9)
Except that the salt bath soft nitriding treatment was performed at the temperature shown in Table 1 to obtain a nitriding steel, and that the obtained nitriding steel was used for induction heating and quenching under the conditions shown in Table 2. In the same manner as in Example 1, a test piece (steel member) for measuring surface fatigue strength was obtained. Table 1 shows the thickness of the oxide layer formed on the surface of the nitriding steel. Moreover, the kind of denitrification inhibitor used is shown below.

[Denitrification inhibitor]
Denitrification inhibitor A: Antinitriding agent mainly composed of alkoxysilane (trade name “Nitrobuster 202”, manufactured by Parker Heat Treatment Industry Co., Ltd.)
Denitrification inhibitor B: Carburization inhibitor containing boric acid as a main component and silicon (trade name “Carbon Buster 201”, manufactured by Parker Heat Treatment Industry Co., Ltd.)

<Evaluation>
(Layer structure (composition))
Table 2 shows the analysis / measurement results of the layer configuration (composition) of each manufactured test piece (steel member). The thickness of each layer was measured by observing the cross section of the test piece with a scanning electron microscope. As a representative example, an electron micrograph of a cross section of the steel member produced in Example 1 is shown in FIG. The N concentration of the first martensite layer and the second martensite layer was measured by a calibration curve method using a wavelength dispersion X-ray analyzer (WDS) attached to the SEM.

The presence or absence of each component (lithium iron composite oxide, Fe ₃ O ₄ ,...) In the surface layer was confirmed by X-ray diffraction using an X-ray diffractometer (device name “SmartLab”, manufactured by Rigaku Corporation). . In addition, a trade name “PDXL2” (manufactured by Rigaku Corporation) was used as analysis software. In Table 2, “◯” indicates that the presence in the surface layer was confirmed, and “x” indicates that the presence was not confirmed. The chart (XRD chart) which shows the result of the X-ray diffraction of the steel member manufactured in Example 1 is shown in FIG.

  As a representative example, the results of elemental analysis (silicon (Si) mapping) of the steel member produced in Example 1 are shown in FIG. Moreover, the result of the elemental analysis (oxygen (O) mapping) of the steel member manufactured in Example 1 is shown in FIG. The ratio (area%) of each phase (γ phase, ε phase, and γ ′ phase) in the first martensite was measured and calculated by observing the cross section of the test piece with a scanning electron microscope.

(Maximum hardness up to 50μm)
According to the method based on JISZ2244, the maximum hardness (HV) to 50 micrometers of each manufactured test piece (steel member) was measured. The results are shown in Table 3.

(Effective hardened layer depth (513HV depth))
According to the method based on JIS G 0559, the effective hardened layer depth (513HV depth) (mm) of each manufactured test piece (steel member) was measured. The results are shown in Table 3.

(Pitching life)
Each manufactured test piece (steel member) is lubricated with transmission oil maintained at 95 ± 5 ° C., and a roller pitching test (two-cylinder type surface pressure fatigue test) is performed under the following conditions to generate pitching. The number of rotations up to was measured. The results are shown in Table 3.
-Partner material: tempered JIS standard SUJ2 equivalent steel (diameter 130mmφ)
-Surface pressure: 3.91 GPa
・ Slip rate: -40%
・ Rotation speed: 1,500 rpm

  The steel member of the present invention is useful as a power transmission component such as a sheave, a joint, a hub, and a shaft of a transmission that is required to have high surface fatigue strength.

2: Surface layer 4: First martensite layer 6: Second martensite layer 10: Steel member 15: γ phase 17: ε phase

Claims (8)

A surface layer;
A first martensite layer having a nitrogen element concentration of 0.2 to 1.0% by mass disposed in a lower layer of the surface layer;
The surface layer includes at least one of lithium iron composite oxide, FeO, and Fe ₃ O ₄ as a main component, and at least one selected from the group consisting of solute silicon, silicon oxide, and silicon nitride Containing
The first martensite layer is a steel member containing a γ phase with an area ratio of 30% or less and an ε phase with an area ratio of 10% or less. The surface layer has a thickness of 0.1 μm or more;
The steel member according to claim 1 whose thickness of said 1st martensite layer is 10 micrometers or more.   3. The steel according to claim 1, further comprising a second martensite layer having a thickness of 100 μm or more and having a nitrogen element concentration of less than 0.2% by mass, disposed in a lower layer of the first martensite layer. Element. It is a manufacturing method of the steel member according to any one of claims 1 to 3,
The manufacturing method of the steel member which has the process of apply | coating a denitrification inhibitor on the surface of nitriding steel, and carrying out induction heating hardening.   The method for manufacturing a steel member according to claim 4, wherein the denitrification inhibitor contains at least one of a polysiloxane compound and a precursor thereof.   The method for manufacturing a steel member according to claim 4, wherein a carburization inhibitor is used as the denitrification inhibitor.   The manufacturing method of the steel member as described in any one of Claims 4-6 which heat-harden by 850 degreeC or more for 5 second or less.   The method for producing a steel member according to any one of claims 4 to 7, wherein the nitriding steel has an oxide layer having a thickness of 0.1 µm or more on a surface thereof.