WO2011013559A1

WO2011013559A1 - Method of combined heat treatment and quench-hardened steel member

Info

Publication number: WO2011013559A1
Application number: PCT/JP2010/062269
Authority: WO
Inventors: 剣吾深沢; 佳孝三阪; 一博川嵜; 芳宏池田; 正昭別府; 知義小西
Original assignee: 高周波熱錬株式会社; 日本パーカライジング株式会社
Priority date: 2009-07-31
Filing date: 2010-07-21
Publication date: 2011-02-03
Also published as: JP2011032536A

Abstract

A steel base (1) is nitrided to form a nitrogen compound layer (2) on the surface thereof and to diffuse nitrogen into a surface layer part (1d) of the steel base (1) coated with the nitrogen compound layer (2). This steel base (1) is subjected to induction hardening in an ammonia gas atmosphere, under vacuum, or in a low-oxygen atmosphere or the like so that the nitrogen compound layer (2) remains unoxidized after the hardening in a thickness of at least 1 µm and that a hardened layer including a fine martensite structure containing nitrogen is formed in the surface layer part (1d) of the steel base (1) over an effective case depth of 200 µm or more. Thus, a quench-hardened steel member is obtained in which, even when a high areal pressure exceeding 2 GPa is applied to the nitrogen compound layer (2) present on the surface of the steel base (1), the nitrogen compound layer (2) has high peel strength with respect to adhesion to the steel base (1) and in which it is possible to sufficiently take advantage of the effects of the nitrogen compound layer (2), i.e., excellent sliding properties, high wear resistance, and high seizing resistance.

Description

Composite heat treatment method and hardened steel member

The present invention relates to a composite heat treatment method and a hardened steel member for producing a hardened steel member used as a machine structural part having excellent mechanical strength such as surface pressure strength, wear resistance, and bending fatigue strength.

Conventionally, for example, steel and cast iron mechanical structural parts such as shafts, gears, pistons, shafts, and cams are nitrided to improve mechanical strength such as surface pressure strength, wear resistance, and bending fatigue strength. Surface hardening treatment such as treatment, soft nitriding treatment, carburizing quenching, induction hardening is applied.

Among these, a nitrogen compound layer formed on the surface of a steel substrate by nitriding or soft nitriding is known to have excellent slidability, resistance to wear, and high seizure resistance (hereinafter referred to as this). This will be referred to as effect I by the nitrogen compound layer.)

This nitrogen compound layer is formed by diffusing and penetrating nitrogen into the steel substrate by performing nitriding treatment or soft nitriding treatment on the surface of the steel substrate. Compared to steel materials that do not contain nitrogen, the steel materials that contain diffused nitrogen have a fine martensite structure that is obtained after quenching in a state containing diffused nitrogen, which increases the hardness and improves hardenability. Are known. That is, the formation of a nitrogen diffusion layer by nitriding treatment or soft nitriding treatment can also be used as a nitrogen diffusion penetrating treatment that increases the hardness of the steel material (hereinafter referred to as effect II by the nitrogen compound layer). ). This effect II is not due to the action of the nitrogen compound layer itself, but due to the action of diffused nitrogen in the steel material immediately below the nitrogen compound layer generated when the nitrogen compound layer is formed.

Formation of a nitrogen diffusion layer by nitriding or soft nitriding causes an increase in surface pressure strength and fatigue strength in addition to an increase in steel hardness. The martensitic structure containing diffusion nitrogen obtained by quenching is due to the fact that it has resistance to temper softening and resistance to crack initiation / growth in addition to the above-mentioned increase in hardness and improvement in hardenability. It is known to have high surface pressure strength and high fatigue strength.

By the way, when induction hardening is performed after nitriding, the quenching temperature needs to be at least the temperature Ac3 transformation point at which an austenite structure is formed, and is usually selected from a temperature range of 750 to 1050 ° C. The nitrogen compound layer formed at a nitriding temperature of 570 ° C. is a combination of iron and nitrogen, and is oxidized and decomposed when reheated to 650 ° C. or higher in the air atmosphere. Then, it is released as nitrogen gas and the nitrogen compound layer disappears. This has been reported for a long time (Non-Patent Document 1).

The combined heat treatment technology by nitriding and quenching usually uses only the effect II caused by the nitrogen diffusion layer obtained in the surface layer portion of the steel material by nitriding treatment, and nitrogen formed in the surface layer portion of the steel material by nitriding treatment The characteristic (effect I) of excellent slidability, high wear strength, and high seizure resistance resulting from the fact that the compound layer has diffused nitrogen is not utilized. That is, the nitrogen compound layer does not stop disappearing during quenching, which is a subsequent process of nitriding. There are many disclosure examples for this technology, and for example, the composite heat treatment described in Patent Documents 1 to 5 can be mentioned.

Patent Document 6 discloses a composite heat treatment method in which a nitriding treatment is performed at a temperature of 600 ° C. or higher to form a nitrogen compound layer having a thickness of 5 μm or less, followed by induction hardening to obtain a quenched member having a nitrogen compound layer having a thickness of 2 μm or less. ing. The reason why the nitriding conditions are set to a high temperature of 600 ° C. or higher in this composite heat treatment method is that a higher concentration of nitrogen diffusion can be expected toward the deeper side of the steel material at higher temperatures. However, a nitrogen compound layer obtained at a nitriding temperature exceeding 600 ° C. is a nitrogen compound layer having a low hardness and having no effect I. That is, this composite heat treatment method is only expected to have the effect II by the nitrogen compound layer, and the remaining nitrogen compound layer of 2 μm or less may be omitted.

In the composite heat treatment method disclosed in Patent Document 7, after forming the nitrogen compound layer on the surface of the steel material by nitriding treatment, the nitrogen compound layer is not oxidized or decomposed during induction hardening. A hardened steel material is manufactured by coating with a protective film having a thickness of 1 to 3 mm comprising a gas nitriding / ion nitriding inhibitor, carburizing inhibitor and antioxidant containing silicon oxide as a component, followed by quenching. This method solves the problem of damage and disappearance of the nitride layer caused by high-temperature heating by induction hardening of the nitride layer formed on the surface of the steel material as it is, and is a method of combining effects I and II. However, in the composite heat treatment method disclosed in Patent Document 7, even though the oxidation phenomenon at the time of heating can be prevented, the thermal conductivity is low because the antioxidant film is a thick film of 1 mm or more. In practice, it was difficult to obtain the desired fine martensite because the quenching cooling rate required for martensitic transformation was insufficient. That is, even if the effect I can be obtained, the effect II cannot be obtained.

The hardened steel member disclosed in Patent Document 8 is formed by forming a hard nitride layer on the surface of a steel material by nitriding, and then converting the hard nitride layer into Ti, Zr, Hf, V, Nb, Ta, Cr, It is manufactured by coating with an inorganic nitrogen compound layer (protective film) containing at least one metal oxide selected from the group consisting of W, Mo and Al, followed by quenching. In this hardened steel member, after a hard nitride layer is formed on the steel by nitriding, a protective film is formed on the hard nitride layer so that the hard nitride layer is not oxidized or decomposed during induction hardening. The steel member is provided with a deep hardening depth as well as a nitrogen compound layer. This hardened steel member can have both effects I and II.

Japanese Patent No. 3193320 Japanese Patent No. 3327386 Japanese Patent No. 3145517 Japanese Patent Laid-Open No. 7-90364 JP 2007-154254 A JP 2007-77411 A JP 58-96815 A JP 2008-038320 A

However, in general, nitriding or soft nitriding is inferior in surface pressure strength, fatigue strength, and the like as compared with carburizing and induction hardening. For example, when a roller pitching test is performed, the nitrogen compound layer may peel off from the steel substrate. If the hard nitrogen compound layer is peeled off, the debris can be fatally damaged in gear parts. For this reason, it has been widely believed that the nitrogen compound layer has a rather adverse effect in the fatigue test at a high surface pressure exceeding 2 GPa. For this reason, in the case of the hardened steel member disclosed in Patent Document 8, in the gear part, in order to avoid a possibility that the hard nitrogen compound layer may be peeled off and cause fatal damage, the nitrogen compound layer is formed after induction hardening. It has been considered necessary to peel off.

As a result of earnest research on the fact that the hard nitrogen compound layer is peeled off from the steel substrate and cannot have the effect I, the nitrogen compound layer is formed as long as the base of the nitrogen compound layer is hard and the hard part is deep. It was confirmed by a fatigue test, a gear single unit test, and the like that it could not be peeled even at a high surface pressure exceeding 2 GPa, and it was confirmed that the nitrogen compound layer was not an essential damage factor for gear parts and the like. The present inventors do not lie in the fact that the nitrogen compound layer is peeled off because the surface pressure strength and fatigue strength of the nitrogen compound layer itself are inferior to those of carburizing and induction hardening, but support the nitrogen compound layer. It was found that the effective hardened layer depth of the substrate was shallow. That is, the cause of the peeling of the outermost nitrogen compound layer formed by nitriding is that the effective hardened layer depth immediately below the layer is insufficient.

In view of the above problems, the present invention is a composite heat treatment method for producing a hardened steel member that combines nitriding treatment and induction hardening, and is used for protection against oxidation on a nitrogen compound layer formed on the surface of a steel substrate. Without covering the film, a good nitrogen compound layer remains after induction hardening, and has the above effects I and II, and the effective hardened layer of the steel substrate covered with the nitrogen compound layer has high surface pressure strength, high Combined heat treatment method for manufacturing hardened steel members with fatigue strength and high mechanical strength in terms of surface pressure strength, wear resistance and bending fatigue strength, and mechanical structural parts such as shafts, gears, pistons, shafts, cams, etc. It is an object of the present invention to provide a suitable hardened steel member.

In order to achieve the above object, the present invention includes a step of forming a nitrogen compound layer on the surface of a steel substrate by nitriding and diffusing nitrogen into a surface layer portion of the steel substrate covered with the nitrogen compound layer, and quenching. The atmosphere is an ammonia gas atmosphere, an inert gas atmosphere, a reducing gas atmosphere, a combination gas atmosphere thereof, a low-oxidation atmosphere, or a high-frequency quenching in a vacuum, leaving an unoxidized nitrogen compound layer of 1 μm or more and a nitrogen compound. Forming a hardened layer containing a diffused nitrogen and containing a fine martensite structure on the surface side of the surface layer portion of the steel substrate immediately below the layer, having an effective hardened layer depth of 200 μm or more.

According to the present invention, the oxidation of the nitrogen compound layer is suppressed during induction hardening without covering the nitrogen compound layer formed by nitriding the surface of the steel base material with an anti-oxidation protective film, and nitrogen is added. In the compound layer, an unoxidized region remains in the depth direction by 1 μm or more. Therefore, the presence of this nitrogen compound layer makes it possible to produce a hardened steel member that is excellent in slidability, resistant to wear, and has high seizure resistance (effect I by the nitrogen compound layer). In addition, the formation process of the nitrogen compound layer is also used as a nitrogen diffusion pretreatment for forming a nitrogen diffusion layer for improving the hardenability in the surface layer portion of the steel substrate (effect II by the nitrogen compound layer). Can diffuse and infiltrate nitrogen. Therefore, according to the present invention, the hardenability is improved, and the effective hardened layer depth of 200 μm or more including the diffused nitrogen on the surface side and the fine martensite structure is increased immediately after the nitrogen compound layer by induction hardening performed thereafter. It can be provided in the surface layer portion of the substrate. The effective hardened layer depth and the fatigue strength correlate, and the fatigue strength increases as the effective hardened layer depth increases. In addition, the nitrogen-containing martensite structure obtained by quenching has high surface pressure strength and high resistance due to its resistance to temper softening and crack initiation / growth in addition to increased hardness and improved hardenability. Has fatigue strength. According to the present invention, it has been demonstrated that even if a high surface pressure exceeding 2 GPa is applied to the nitrogen compound layer, the peel strength of the nitrogen compound layer with respect to the steel substrate can be kept large, and therefore it has excellent slidability, wear resistance, and seizure. The effect of the nitrogen compound layer having a high resistance characteristic can be fully utilized.

That is, according to the present invention, a thick effective hardened layer depth can be obtained under the layer by induction hardening while keeping the nitrogen compound layer healthy. Combined with the fact that it is not martensite but fine martensite containing nitrogen, it can withstand high surface pressure fatigue tests.

According to the present invention, a combined heat treatment combining nitriding treatment and induction hardening can leave a nitrogen compound layer formed on the surface of a steel substrate in a good state that is not oxidized even after induction hardening in an excellent state. And the effective hardening depth of a martensite structure | tissue can be deeply formed with 200 micrometers or more in the outermost layer part of the surface layer part of the steel base material which is a foundation | substrate of a nitrogen compound layer. This martensite structure includes a fine martensite structure containing diffusion nitrogen of 50 μm or more. Therefore, according to the present invention, the peel strength of the nitrogen compound layer with respect to the steel substrate can be ensured to be large compared with the case where the effective hardened layer depth can be obtained only in the conventional composite heat treatment, and the nitrogen compound layer on the outermost surface. It was confirmed that the good slidability can be fully utilized, and no adverse effect is exerted even in a fatigue test under a high surface pressure exceeding 2 GPa.

In order to achieve the above object, the present invention provides a surface layer portion in which a nitrogen compound layer having a hardness of HV550 or more and not oxidized remains 1 μm or more on the surface of the steel substrate and is covered with the nitrogen compound layer of the steel substrate. A fine martensite structure containing nitrogen is included, and the effective hardened layer depth exceeding HV550 is 200 μm or more in terms of the distance from the surface of the steel substrate.

According to the above configuration, a nitrogen compound layer that is not oxidized after induction hardening remains 1 μm or more. Therefore, the presence of the nitrogen compound layer in the quenched steel member is excellent in slidability, resistance to wear, and seizure resistance. Have high characteristics (effect I by nitrogen compound layer I). The nitrogen compound layer is supported by a thick effective hardened layer including a martensite structure as a base, and the surface side portion of the hardened region is not a simple martensite structure but a fine martensite structure containing nitrogen, In addition to improving hardness and hardenability, it has high tempering softening resistance, high surface pressure strength and high fatigue strength due to resistance to crack initiation and growth, and fatigue performed under high surface pressure exceeding 2 GPa In the test, it was proved that there is no adverse effect, and the effect I of the nitrogen compound layer can be fully utilized as compared with the conventional product in which the peel strength of the nitrogen compound layer with respect to the steel substrate is shallow in the effective hardened layer. Therefore, according to the above configuration, since the effect I and the effect II are provided and the high contact pressure strength and the high fatigue strength are provided, high mechanical strength is required for the contact pressure strength, wear resistance, and bending fatigue strength. It is suitable as mechanical structural parts such as shafts, gears, pistons, shafts and cams.

According to the composite heat treatment method for producing a quenched steel member combining nitriding treatment and induction hardening according to the present invention, the nitrogen compound layer formed on the surface of the steel substrate is coated with an anti-oxidation protective film. Therefore, it can be protected from oxidation during induction hardening, and a good nitrogen compound layer remains 1 μm or more after induction hardening, and the mechanical strength, sliding resistance and anti-resistance based on the characteristics of the nitrogen compound layer remain. In addition to maintaining wearability (Effect I), the formation of a nitrogen compound layer increases the hardness of the surface portion of the steel substrate and improves hardenability (Effect II), as well as resistance to temper softening and cracking. A hardened steel member having high surface pressure strength and high fatigue strength resulting from resistance to generation / growth can be produced.

Further, according to the present invention, even if a high surface pressure exceeding 2 GPa is applied to the nitrogen compound layer, the peel strength of the nitrogen compound layer with respect to the steel substrate is kept large, and thus it has excellent slidability, wear resistance, and seizure resistance. Therefore, it is possible to provide a hardened steel member that can fully utilize the characteristics of the nitrogen compound layer that have high properties.

Furthermore, according to the present invention, since it has the above effects I and II, and further has high surface pressure strength and high fatigue strength, high mechanical strength is required for surface pressure strength, wear resistance, and bending fatigue strength. A hardened steel member suitable as a mechanical structural part such as a shaft, a gear, a piston, a shaft, and a cam can be provided.

It is a typical manufacturing process figure of the hardened steel member concerning the embodiment of the present invention. It is a figure of the optical microscope photograph image which shows the cross-sectional state after quenching using the test material by Example 1. FIG. It is a graph which shows the cross-sectional hardness distribution of the cross-sectional hardness measurement result after quenching using the test material by Example 2. It is a figure which shows the optical microscope photograph image which shows the cross-sectional state after quenching using the test material by the comparative example 1. FIG. It is a figure which shows the optical microscope photographic image after quenching using the test material by Example 3, and a SEM micrograph image. It is a figure of the optical microscope photograph image and SEM microscope photograph image which concern on the comparative example 2.

DESCRIPTION OF SYMBOLS 1 Steel base material 1a Steel base surface 1b Nitrogen diffusion layer 1c Effective hardened layer 1d Surface layer part of steel base material 2 Nitrogen compound layer

Hereinafter, a method for manufacturing a hardened steel member and a hardened steel member according to an embodiment of the present invention will be described with reference to the drawings.

The hardened steel member according to this embodiment is a hardened steel member manufactured by a combined heat treatment method combining nitriding treatment and induction hardening, as shown in FIGS. 1 (a) to 1 (c). In this hardened steel member, as shown in FIG. 1 (c), after induction hardening, a nitrogen compound layer 2 having a hardness of HV550 or higher that is not oxidized remains on the surface 1a of the steel base 1 at 1 μm or more, and the nitrogen compound layer The effective hardened layer 1c exceeding HV550 having a distance of 200 μm or more from the surface is formed on the surface layer portion 1d of the steel substrate 1 covered with 2, and the fine martens containing diffusion nitrogen of 50 μm or more from the surface It is a highly cured layer that greatly exceeds HV550 including the site structure.

The method for manufacturing the quenched steel member is based on a combined heat treatment method combining nitriding treatment and induction hardening, as shown in FIGS. 1 (a) to 1 (c).

In this manufacturing method, first, the steel substrate 1 shown in FIG. 1 (a) is placed in a nitriding facility and heated to 350 ° C. to 600 ° C. to nitride the surface 1a of the steel substrate 1. The nitriding treatment is any one of salt bath soft nitriding treatment, gas nitriding treatment, gas soft nitriding treatment or plasma nitriding treatment.

By this nitriding treatment, as shown in FIG. 1B, a nitrogen diffusion layer 1b having a distance of 50 μm or more from the surface is formed on the surface layer portion 1d of the steel substrate 1, and the surface 1a of the steel substrate 1 has a hardness. A nitrogen compound layer 2 of HV550 or higher is formed.

Next, the steel substrate 1 on which the nitrogen compound layer 2 is formed is placed in an induction hardening facility and induction hardened. In this case, as shown in FIG. 1C, the quenching atmosphere is an ammonia gas atmosphere, an inert gas atmosphere, a reducing gas atmosphere or a combination gas atmosphere thereof, or a low oxidation atmosphere or a vacuum. Then, the surface layer portion 1d of the steel substrate 1 can be heated to 750 ° C. to 860 ° C. instantaneously with respect to the surface layer portion 1d of the specific steel substrate by using the high frequency coil 1 to 2 seconds at the longest. Heat to 750 ° C. to 860 ° C. per second, immediately quench, and finish induction hardening.

By this induction quenching, as shown in FIG. 1 (c), the nitrogen compound layer 1b that is not oxidized after quenching remains 1 μm or more, and the surface is formed on the surface layer portion 1d of the steel substrate 1 immediately below the nitrogen compound layer 2. Effective hardening layer 1c exceeding HV550 of 200 μm or more at a distance from the surface, and high hardening layer greatly exceeding HV550 as a fine martensite structure containing diffusion nitrogen with respect to the nitrogen diffusion layer 1b of 50 μm or more from the surface It will cause.

Hereinafter, the present invention will be described in more detail.

[Base material steel]
The steel substrate 1 to which the present embodiment is applied is not particularly limited, and examples thereof include carbon steel, low alloy steel, medium alloy steel, high alloy steel, cast iron and the like. Preferred materials from the viewpoint of cost are carbon steel, low alloy steel, and the like. For example, carbon steel for machine structure (S20C to S58C) is suitable as carbon steel, and nickel chrome steel (SNC236 to SNC836), nickel chrome molybdenum steel (SNCM220 to SNCM815), chrome as low alloy steel. Molybdenum steel materials (SCM415 to SCM445, SCM822), chromium steel materials (SCr415 to SCr445), manganese steel materials for mechanical structures (SMn420 to SMn443), manganese chromium steel materials (SMC420 to SMC443) and the like are suitable. These steel materials do not necessarily need to use a tempered steel material (H material) that guarantees hardenability by tempering, and may use a tempered steel material that remains in an untempered ferrite-pearlite structure. .

[Nitriding method and thickness of nitrogen compound layer]
In the present embodiment, the nitrogen compound layer 2 on the surface 1a of the steel substrate 1 diffuses and infiltrates active nitrogen from the surface 1a of the steel substrate 1 and is hard and stable on the surface 1a of the steel substrate 1. Obtained by nitriding to form nitride. Although it is not particularly limited as long as it is a nitrogen compound layer, it is usually made from a nitride mainly containing Fe as a base material component and containing Ti, Zr, Mo, W, Cr, Mn, Al, Ni, C, B, Si and the like. It is preferable that it is a layer. As a method for forming a nitrogen compound layer, the above effect I can be achieved, such as salt bath nitriding treatment such as tuftride (registered trademark) treatment, isonite (registered trademark) treatment, pulsonite (registered trademark) treatment, gas soft nitriding treatment, plasma nitriding treatment, etc. Any nitriding method can be used as long as it has a method in which a nitrogen-diffused region is formed in the nitrogen compound layer and the steel substrate portion immediately below.

The heating temperature of the nitriding treatment for forming the nitrogen compound layer having the above effect I is preferably 350 ° C. to 600 ° C. When nitriding is performed at a temperature lower than 350 ° C., a nitrogen compound layer necessary for exhibiting good performance cannot be sufficiently formed on the surface of the steel substrate, and nitriding is performed at a temperature exceeding 600 ° C. The higher the temperature, the higher the concentration of nitrogen diffusion toward the deeper side of the steel material can be expected. However, the hardness of the resulting nitrogen compound layer is low, and the nitrogen compound layer has no effect I due to the nitrogen compound layer. The heating temperature on the upper limit side is preferably 580 ° C. or lower, more preferably 570 ° C. or lower, from the viewpoint of obtaining high hardness. When nitriding is performed by heating to 350 ° C. to 600 ° C., a nitrogen compound layer having sufficient hardness and thickness and exhibiting good performance can be obtained.

When nitriding is performed at a heating temperature of 350 ° C. to 600 ° C. and taking an appropriate time according to the nitriding method used, a nitrogen diffusion layer 1b having a distance of 50 μm or more from the surface is formed on the surface layer portion 1d of the steel substrate 1. Can be generated. When the nitrogen diffusion layer 1b of 50 μm or more is generated, this portion can be a highly hardened layer that greatly exceeds the Vickers hardness HV550 after induction hardening, and can have sufficient high surface pressure strength and high fatigue strength. . In addition, even if high surface pressure exceeding 2 GPa is applied to the nitrogen compound layer 2 that remains after induction hardening, the peel strength of the nitrogen compound layer to the steel substrate is kept large, so that it is excellent in slidability, wear resistance, and seizure resistance. It is possible to make full use of the characteristics of the nitrogen compound layer, which has high properties.

When the nitriding treatment is performed at a heating temperature of 350 ° C. to 600 ° C. and taking an appropriate time according to the nitriding method to be used, the nitrogen compound layer 2 can be formed on the surface layer portion 1d of the steel substrate 1. The thickness of the nitrogen compound layer 2 obtained by nitriding before induction hardening is not particularly limited, but the nitrogen compound layer 2 when the nitrogen diffusion layer 1b of 50 μm or more is formed is formed with a thickness of 1 μm or more. For example, the nitriding process may be terminated. By performing nitriding treatment, it is usually formed with a thickness of 1 μm to 30 μm. The thickness of the nitrogen compound layer 2 is preferably 2 μm to 20 μm, and more preferably 3 μm to 15 μm. If processing time is lengthened, it can form in thickness of 30 micrometers.

When performing induction hardening in a quenching atmosphere such as an ammonia gas atmosphere other than a low oxidation atmosphere, the oxidation of the surface of the nitrogen compound layer can be completely suppressed. In this case, the nitrogen compound layer may be formed to a thickness of 1 μm or more. When induction hardening is performed in a low-oxidation atmosphere, the surface portion of the nitrogen compound layer is oxidized, so the thickness of the nitrogen compound layer is determined in consideration of the oxidation depth, and the oxidized portion is scraped off. In this step, a nitrogen compound layer having a thickness of at least 1 μm is left.

[Induction hardening]
In the present embodiment, following the nitriding treatment, induction hardening is performed in a gas atmosphere, a low-oxidation atmosphere, or in a vacuum that can prevent oxidation of the nitrogen compound layer.

Here, examples of the gas atmosphere that can prevent oxidation of the nitrogen compound layer include an ammonia gas atmosphere, an inert gas atmosphere, a reducing gas atmosphere, or a combination thereof. These gas atmospheres are preferable to the low oxidation atmosphere. A vacuum is preferable because it takes equipment and running time to form an ultra-high vacuum. Examples of the reducing gas atmosphere include petroleum gases such as hydrogen, propane, and butane, modified gases thereof, alcohols, esters, and ketones. As an inert gas, neutral gas, such as nitrogen and argon, or those combinations are mentioned. By induction hardening in this atmosphere, oxidation and decomposition of the nitrogen compound layer are sufficiently suppressed at the quenching temperature of the present embodiment.

The high-frequency heating is performed by a high-frequency heating coil that reaches the heating temperature set to 750 ° C. to 860 ° C. on the surface layer of the object to be processed (steel base material). After reaching a temperature of 750 ° C. to 860 ° C., a fine martensite structure containing nitrogen can be obtained by immediately cooling with a coolant.

When the heating temperature during induction hardening is set to 750 ° C. to 860 ° C., the nitrogen diffusion layer 1b becomes a fine martensite structure due to rapid cooling, and no excessive residual austenite exists, and good induction hardening can be performed. As for the heating temperature, a more preferable heating temperature is 770 ° C. to 840 ° C., and a further preferable heating temperature is 780 ° C. to 830 ° C.

When the heating temperature is lower than 750 ° C., nitrogen is contained and it is easy to quench, but at this temperature, the steel base material is not sufficiently austenitized, so that quenching becomes insufficient. A heating temperature exceeding 860 ° C. is not preferable because excessive retained austenite is easily generated in the martensitic structure immediately below the nitrogen compound layer.

The high frequency heating is preferably performed for about 1 second by a high frequency heating coil that reaches a heating temperature set at 750 ° C. to 860 ° C. It is preferable that the limit is about 4 seconds at the longest. When the treatment is large during high-frequency heating, a multistage temperature raising method including preheating can be appropriately performed. After quenching by high frequency heating, a tempering treatment may be performed under appropriate conditions in the same manner as a normal quenching technique.

[Influence of induction hardening on nitrogen compound layer]
Even a nitrogen compound obtained by nitriding by heating to 350 ° C. to 600 ° C. is oxidized and decomposed when reheated to 650 ° C. or higher in an air atmosphere, and the nitrogen in the nitrogen compound layer is Then, for example, it is released as nitrogen gas and the nitrogen compound layer disappears. The induction hardening is performed in an ammonia gas atmosphere, an inert gas atmosphere, a reducing gas atmosphere, a combination gas atmosphere thereof, a low oxidation atmosphere, or under vacuum, and the quenching time is long and is about 4 seconds. The nitrogen compound layer 2 is heated by heat transfer from the surface 1a of the steel substrate 1 within a short quenching time, and the inner surface is high and the surface has a low temperature gradient. Therefore, the disappearance of nitrogen on the surface of the nitrogen compound layer 2 due to oxidation or decomposition is sufficiently suppressed.

[Cross section hardness / characteristics]
By the composite heat treatment as described above, the surface of the steel substrate is provided with a nitrogen compound layer having HV550 or more in terms of Vickers hardness and having a thickness of 1 μm to 30 μm, and immediately below the nitrogen compound layer of the steel substrate, A martensitic structure having an effective hardened layer depth of 200 μm or more (depth of layer having HV550 or more in terms of Vickers hardness) at a distance from the surface can be obtained, and 50 μm of the outermost layer portion of the effective hardened layer depth. In the above depth, a highly hard layer including not only a normal martensite structure but also a fine martensite structure containing diffusion nitrogen and having a Vickers hardness greatly exceeding HV550 can be obtained.

The effective hardened layer of HV550 or higher has a hardness distribution that is not uniform in the depth direction and gradually decreases from the surface to the inside. The hardened layer containing a fine martensite structure containing nitrogen greatly exceeds the hardness of HV550. For example, a hardness of HV630 or higher can be obtained. The effective hardened layer depth of the martensite structure including the fine martensite structure containing nitrogen is 400 μm or more depending on the condition settings such as the depth of the fine martensite structure containing diffusion nitrogen, the induction hardening temperature, and the type of the steel substrate. Furthermore, it is possible to obtain a steel material having a hardness distribution so that it is 600 μm or more.

[Nitrogen compound layer after quenching]
According to the present embodiment, the nitrogen compound layer remains after high-frequency heating, but the nitrogen compound layer does not necessarily remain 100% with respect to the state of the nitrogen compound layer before high-frequency heating, and the minimum film thickness is 1 μm or more. It is sufficient that the compound layer thickness remains. More preferably, it is 2 μm or more, and more preferably 3 μm or more. Regarding the nitrogen compound layer, the surface layer portion that has been oxidized or decomposed may be removed as necessary, and even if the thickness of the nitrogen compound layer is reduced thereby, the minimum film thickness may be 1 μm or more. The surface layer portion of the nitrogen compound layer that has been oxidized or decomposed is brittle and has low hardness, so that it can be easily removed. For example, lapping treatment, emery paper polishing, buffing, shot blasting, shot peening can be used.

[Characteristics of steel parts by processing of this embodiment]
By the manufacturing method of the above embodiment, a mechanical component having both the effects I and II of the nitrogen compound layer and the high surface pressure strength and the high fatigue strength can be obtained. That is, the machine part subjected to the treatment of this embodiment has high slidability and seizure resistance due to the nitrogen compound layer formed on the outermost surface (Effect I), and high mechanical strength due to the nitrogen-containing fine martensite structure. Hardness, high tempering softening resistance, crack initiation / crack growth resistance, high surface pressure resistance, high fatigue strength, and deep effective hardened layer depth.

In the composite heat treatment according to the present embodiment, the heating temperature for quenching by high-frequency heating is 750 to 860 ° C., and the quenching temperature is sufficiently lower than the induction quenching and carburizing quenching usually performed at a temperature exceeding 900 ° C. This is extremely advantageous in terms of thermal deformation and cracking, and enables a significant reduction in the post-cutting process for adjusting dimensional accuracy performed after general induction hardening or carburizing and quenching.

As described above, the steel material to which the present embodiment is applied does not need to use a tempered steel for the effect of improving the hardenability of the effect II by nitrogen, and has a ferrite-pearlite structure that is a non-tempered steel. Sufficient mechanical strength can be obtained even with steel. Further, although alloy steel tends to have a slightly higher surface hardness, a sufficiently deep effective hardened layer depth can be obtained even with inexpensive carbon steel due to the effect II of nitrogen. For example, a carbon steel for mechanical structure such as S45C is a heat treatment material having a hardness profile with sufficient hardness and sufficient depth. Further, even S45C does not need to be a tempered material, and even if the heat treatment of this embodiment is applied to a steel member with a non-tempered ferrite-pearlite structure, it becomes a heat-treated machine part having sufficient mechanical strength. obtain.

As described above, the application of this embodiment improves the mechanical strength of parts, reduces the cutting process, and switches to inexpensive materials, thereby reducing the size and weight of the entire machine parts by reducing the size of the parts, and nitriding and induction hardening. It is possible to reduce the actual cost by surplus to compensate for the cost increase due to the combined processing.

[Use]
The hardened steel member according to the present invention is suitable for use as a mechanical structural component having excellent mechanical strength such as surface pressure strength, wear resistance, bending fatigue strength, etc., particularly in a high load / high surface pressure region. Suitable for what is used. The hardened steel member according to the present invention is not particularly limited with respect to the shape and part type of the steel member. Examples of hardened steel members include shafts, gears, pistons, shafts, cams, and the like, which are suitable for transmission-related parts and powertrain parts for automobiles and construction equipment.

Using an SCM440 tempered material with a diameter of 8 mm and a length of 50 mm as a steel substrate, this surface was degreased and washed, and then in a molten salt bath at 560 ° C. for 1 hour, salt bath soft nitriding treatment (Isonite treatment: Nippon Parkerizing Co., Ltd.) And a nitrogen compound layer mainly composed of iron nitride having a thickness of 7 μm was formed on the surface of the steel substrate.

Next, a high frequency is applied to the surface of the steel substrate using an induction hardening apparatus in an ammonia gas atmosphere, the surface is heated to 860 ° C. for 0.8 seconds, and immediately cooled (water cooled) without taking a holding time. ) And quenched.

The following evaluation test was performed on the obtained hardened steel member.

The hardened steel member was cut with a microcutter, embedded in a resin, and subjected to cross-sectional observation with a metal microscope. As a result, a micrograph image shown in FIG. 2 was obtained. From this micrograph image, it was confirmed that iron nitride having a thickness of 7 μm that was not oxidized remained on the surface of the steel substrate. Moreover, cross-sectional hardness measurement was performed using a micro Vickers hardness tester using this embedded sample. As a result, the Vickers hardness measured at a depth of 0.1 mm from the surface of the steel substrate was 816 Hv. Moreover, when the depth of Vickers hardness 550Hv used as an effective hardened layer was measured, it was 1.25 mm from the surface.

Using an S45C tempered material with a diameter of 8 mm and a length of 50 mm as the steel substrate, this surface was degreased and washed, and then salt bath soft nitriding treatment at 560 ° C. for 2 hours in a molten salt bath (Isonite treatment: Nippon Parkerizing Co., Ltd.) And a nitrogen compound layer mainly composed of iron nitride having a thickness of 13 μm was formed on the surface of the steel substrate.

Next, a high frequency is applied to the surface of the steel substrate using an induction hardening apparatus in an argon gas atmosphere, the surface is heated to 820 ° C. over 1.0 second, and immediately cooled without taking a holding time ( It was quenched with water.

The quenched steel member was cut with a microcutter, embedded in resin, and cross-section was observed with a metal microscope. As a result, it was confirmed that iron nitride with a thickness of 10 μm that was not oxidized remained on the surface of the steel substrate.

Moreover, the cross-sectional hardness was measured with a micro Vickers hardness tester using this embedded sample. FIG. 3 shows the cross-sectional hardness distribution of the measurement results. As the cross-sectional hardness distribution, the Vickers hardness at a depth of 0.1 mm from the surface of the steel material substrate was 720 Hv, and the depth of the Vickers hardness 550 Hv serving as an effective hardened layer was 0.74 mm from the surface.

[Comparative Example 1]
SCM440 tempered material having a diameter of 8 mm and a length of 50 mm was used as the base material, and after degreasing and cleaning the surface, salt bath soft nitriding treatment at 560 ° C. in a molten salt bath (Isonite treatment: Nippon Parkerizing Co., Ltd. And a nitrogen compound layer mainly composed of iron nitride having a thickness of about 7 μm was formed on the surface of the steel material.

Next, a high frequency is applied to the surface of the steel substrate using an induction hardening apparatus in the air atmosphere, this surface is heated to 860 ° C. for 0.8 seconds, and immediately cooled (water cooling) without taking a holding time. And quenching. The following evaluation tests were performed on the obtained hardened steel member.

The hardened steel member was cut with a microcutter, embedded in resin, and observed with a metal microscope to obtain a micrograph image shown in FIG. From this micrograph image, it was confirmed that the iron nitride remaining in the non-oxidized state on the surface of the steel substrate was less than 1 μm in thickness. Moreover, cross-sectional hardness measurement was performed using a micro Vickers hardness tester using this embedded sample. As a result, the Vickers hardness measured at a depth of 0.1 mm from the surface of the steel substrate was Hv815. Moreover, when the depth of Hv550 or more used as an effective hardened layer was measured, it was 1.28 mm from the surface.

Table 1 shows a list of numerical values of the measurement results of Example 1, Example 2, and Comparative Example 1.

[Comparative evaluation of Example 1, Example 2 and Comparative Example 1]
In Example 1 and Example 2, as shown in FIG. 2, the nitrogen compound layer on the surface remained without significant damage even after induction hardening. In the quenched steel members of Examples 1 and 2, the non-oxidized nitrogen compound layer remains with a sufficient thickness larger than 1 μm, so that effects I and II can be obtained, and the effective hardened layer has a thickness of 200 μm or more. Since there are 0.1 mm or more highly hardened layers that greatly exceed Hv550, the nitrogen compound layer has high surface pressure strength and high fatigue strength.

In Comparative Example 1, it was observed from FIG. 4 that the entire nitrogen compound layer was oxidized. Therefore, since the hardened steel member of Comparative Example 1 does not have a nitrogen compound layer, the effect I cannot be obtained, and only the effect II can be obtained.

Using a test material made of SCM440 having a diameter of 3 mm and a length of 10 mm and having a chemical composition shown in Table 2, this surface was degreased and cleaned, and then plasma-nitrided at 500 ° C. for 8 hours to cool with oil. A nitrogen compound layer mainly composed of iron nitride having a thickness of about 7 μm was formed on the steel material surface.

Next, using an induction hardening apparatus in a vacuum (10 ⁻⁴ Torr or less, ie 1.3 × 10 ⁻⁹ Pa or less), applying a high frequency to the surface of the test material and applying this surface for 3 seconds, various temperatures are applied. The sample was immediately quenched (water-cooled) and quenched without taking a holding time. As a result, it was confirmed that the degree of decomposition of the surface of the nitrogen compound layer 2 increases as the heating temperature increases. In the induction hardening in vacuum, it was confirmed that the decomposition of the surface of the nitrogen compound layer 2 was most suppressed when heated to 830 ° C. and rapidly cooled.

FIG. 5 shows an optical microscope photograph image obtained by cutting a quenched specimen obtained when the heating temperature during quenching is 830 ° C. with a microcutter, embedding in a resin, and observing a cross section with a metal microscope. And SEM micrograph images.

Table 2 shows a table comparing the chemical composition of the SCM440 specimen of Example 3 with the chemical composition of the SCM440 JIS standard.

[Comparative Example 2]
Using the test material in which the same nitrogen compound layer as in Example 3 was used, a high frequency was applied to the surface of the test material in an air atmosphere which was evacuated and purged to atmospheric pressure in the air, and the surface was applied for 3 seconds. The sample was heated to 820 ° C., and immediately quenched (water cooled) and quenched without taking a holding time. About the obtained quenching test material, it cut | disconnected with the micro cutter, embedded in resin, the cross-sectional observation was performed with the metal microscope, and the optical microscope photograph image and the SEM microscope photograph image were obtained. As a result, as shown in FIG. 6, it was observed that the nitrogen compound layer was oxidized and decomposed to the deepest part.

[Other Embodiments and Examples]
The present invention is not limited to the above-described embodiments and examples, and the technical scope of the claims includes various design changes within the scope of the invention.

Claims

Forming a nitrogen compound layer on the surface of the steel substrate by nitriding and diffusing nitrogen into a surface layer portion of the steel substrate covered with the nitrogen compound layer;
An ammonia gas atmosphere, an inert gas atmosphere, a reducing gas atmosphere, a combination gas atmosphere thereof, a low-oxidation atmosphere, or a high-frequency quenching in a vacuum leaves an unoxidized nitrogen compound layer at least 1 μm, and the nitrogen compound layer Forming a hardened layer containing diffusion nitrogen on the surface side of the surface layer portion of the steel substrate directly below and containing a fine martensite structure at an effective hardened layer depth of 200 μm or more from the surface;
A combined heat treatment method.
2. The composite heat treatment method according to claim 1, wherein the heating temperature during induction hardening is set to 750 ° C. to 860 ° C.
The nitriding treatment is performed by any of salt bath soft nitriding, gas nitriding, gas soft nitriding or plasma nitriding, so that the nitrogen compound layer is formed on the surface of the steel substrate to a depth of 1 to 30 μm. The composite heat treatment method according to claim 1, wherein the composite heat treatment method is formed.
The composite heat treatment method according to claim 3, wherein a temperature during the nitriding treatment is set to 350 ° C to 600 ° C.
1 μm or more of a non-oxidized nitrogen compound layer having a hardness of HV550 or more remains on the surface side of the steel substrate,
On the surface side of the surface layer portion covered with the nitrogen compound layer of the steel substrate, contains a fine martensite structure containing nitrogen,
And the hardening steel member whose effective hardened layer depth of the martensitic structure exceeding HV550 is 200 micrometers or more in the distance from the surface of the said steel base material.