JPH0368109B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface hardened steel with excellent hardness. More specifically, the steel contains a nitride-forming metallic element in addition to iron, and has an internal nitrided layer in which a high concentration of nitrogen penetrates and diffuses from the surface of the steel toward the inside, The present invention relates to a surface hardened steel having a small nitrogen concentration decreasing gradient toward the inner nitrided layer, that is, the nitrogen concentration distribution in the inner nitrided layer is substantially uniform, and the inner nitrided layer has excellent hardness. The invention also relates to a method for surface hardening of steel to obtain said surface hardened steel.

BACKGROUND ART Conventionally, steel whose surface is hardened by nitriding the steel is known. Regarding the nitriding treatment, there are known liquid nitriding methods in which steel is treated in a molten cyanide bath, and gas nitriding methods in which steel is treated in an ammonia gas atmosphere. [Edited by Toru Araki, Steel Engineering Course 8,
Heat treatment technology for steel, Asakura Shoten (1972), 173-179
and Imao Tamura et al., Steel Materials Science, Asakura Shoten (1981), pp. 219-228]. Liquid nitriding methods include liquid pressure nitriding and tuftride treatment, and gas nitriding methods include single-stage nitriding, two-stage nitriding, and gas pressure nitriding. The nitriding method has the advantage of being able to harden the surface of stainless steel, which cannot be hardened by quenching, and various studies are being conducted on it. All of the surface-hardened steels obtained by the above nitriding method have a so-called compound layer (hereinafter often referred to as the "white layer") with a thickness of several μm to several tens of μm on the outermost surface, mainly an Fe _{2 to 3} N layer or an Fe ₄ The compound layer has a so-called diffusion layer that solidly dissolves nitrogen (N, or a mixture of both), but the compound layer is very brittle and impractical, so it needs to be scraped off, and it requires a complicated process. The disadvantage is that it requires In addition, the concentration distribution of nitrogen contained in the diffusion layer is not uniform,
The nitrogen concentration rapidly decreases from the surface side toward the inside of the steel, and the hardness also decreases rapidly in proportion to this, making it impossible to obtain a sufficient surface hardness. Furthermore, among the above nitriding methods, the liquid nitriding method uses a molten salt bath containing highly toxic cyanide as the main component, so thorough pollution countermeasures are required, and the gas nitriding method has the disadvantage of requiring a long time for nitriding. be. Furthermore, in order to harden the surface of stainless steel using the conventional nitriding method as described above, it is necessary to remove the strong oxide film on the surface of the stainless steel by pickling in advance. For example, industrially, stainless steel is cleaned with fluoronitric acid and then treated in a high-temperature ammonia gas atmosphere, but it is not easy to remove the strong oxide film on the surface of stainless steel and is a complicated process. The disadvantage was that it required . Therefore, there is an ion nitriding method as a method for nitriding stainless steel without using toxic cyanide compounds, in a relatively short time required for nitriding, and without pickling like other steel materials.
The ionization chamber method ionizes N ₂ by continuously applying a voltage between a pair of electrodes with a desired steel as the cathode in a mixed gas atmosphere of N ₂ gas and H ₂ gas to cause a discharge. This is a method of infiltrating and diffusing steel. The principle of the ion nitriding method is explained in detail as follows. When electrodes are placed in a sealed container, the pressure inside the container is reduced, and a DC voltage of 300V to 1000V is applied, a glow discharge occurs between the electrodes. When the steel product to be surface hardened is made the same polarity as the cathode and a voltage is applied between the two electrodes in a mixed gas atmosphere of nitrogen and hydrogen, a sudden voltage drop occurs several hundred to several millimeters in front of the steel product. The _N2 gas in the glow discharge is ionized near the cathode, and the N ⁺ ions are accelerated at high speed in the cathode space due to the potential difference and collide with the steel surface. At this time, most of the high kinetic energy of the ions becomes thermal energy and heats the steel, and at the same time nitrogen permeates. The ion nitriding method is characterized in that the formation of a white layer on the steel surface as shown in FIG. 1 can be controlled by adjusting the nitrogen gas partial pressure, voltage, mixed gas pressure, temperature and time. However, in the ion nitriding method, as the discharge continues, N ⁺ ions continue to collide with the steel, and the thermal energy generated at that time causes the temperature of the steel to continue to rise. Generally, the higher the nitriding temperature, the more nitrogen penetrates and diffuses, but a homogeneous nitride layer with a high nitrogen concentration is not formed and the hardness becomes lower. Therefore, in order to obtain a substantially homogeneous hardened surface layer with a high nitrogen concentration and good hardness, it is necessary to adjust the temperature so that it does not rise too much. In the conventional ion nitriding method, in order to adjust the temperature, the temperature of the steel is adjusted by adjusting the kinetic energy of the nitrogen ions and the thermal energy provided by the nitrogen ions by increasing or decreasing the DC output, specifically the voltage, applied to the electrode. was under control. However, with the temperature control method described above, the nitrogen ion energy in the nitrogen ion atmosphere increases or decreases in proportion to the increase or decrease of the applied voltage, making it unstable. It was difficult to obtain a hardened surface layer with good hardness.

The present inventors conducted intensive research to obtain a steel with a homogeneous hardened surface layer with excellent hardness by simultaneously controlling the nitrogen ion energy in the nitrogen ion atmosphere and the nitriding temperature of the steel.As a result, the voltage By intermittently applying a substantially constant voltage to the electrode without any change, the nitrogen ion energy and the temperature of the steel are simultaneously adjusted and kept at substantially constant levels. It was surprisingly discovered that the steel has a nitrided layer as a surface hardening layer, which is different from the nitrided layer of steel obtained by the method, and that this layer is harder and stronger than conventional ones. That is,
As mentioned above, steel obtained by conventional nitriding has a compound layer on its outermost layer that is too brittle to be practical, and below that a nitrogen diffusion layer that is a practical surface hardening layer. However, the nitrogen concentration on the outermost layer side is about 0.1% at most, and the nitrogen concentration decreases rapidly from the surface side to the inside of the steel in the diffusion layer, and the hardness also decreases accordingly. On the other hand, steel obtained by nitriding under substantially constant nitrogen ion energy and substantially constant temperature conditions as described above does not have a brittle compound layer (white layer), and the outermost layer is Nitrogen concentration is 0.1-10
% by weight, and the decreasing gradient of nitrogen concentration from the surface side to the inside of the steel is small, that is, there is little difference in nitrogen concentration throughout the nitrided layer, and therefore the hardness also decreases as the depth from the surface of the steel increases. It has a nitrided layer that is almost constant without any oxidation. Also, its hardness is superior to conventional ones. The present invention has been completed based on these findings.

That is, one object of the present invention is to provide a surface hardened steel having an excellent hardness nitrided layer with a high surface nitrogen concentration and a small nitrogen concentration decreasing gradient from the surface to the inside as a surface hardening layer. be.

Another object of the present invention is to obtain a surface hardened steel having an excellent hardness nitrided layer with a high surface nitrogen concentration and a small nitrogen concentration decreasing gradient from the surface to the inside. The object of the present invention is to provide a method for surface hardening steel.

According to the present invention, the surface hardened steel is made of steel containing at least one nitride-forming metal element in addition to iron, and has an internal nitrided layer in which nitrogen penetrates and diffuses from the surface of the steel toward the inside. The thickness of the internal nitrided layer is 0.01 mm or more, the nitrogen concentration at the outermost surface of the steel is 0.1 to 10% by weight, and the nitrogen concentration of the internal nitrided layer increases from the surface side to the inside of the steel. The thickness (t) in μm from the surface of the steel in the internal nitrided layer and the nitrogen concentration (c) satisfy the following formula: |Δc/Δt|≦0.5(%/μm) () A surface hardened steel is provided.

The surface-hardened steel of the present invention does not have a compound layer that is brittle and impractical as seen in steels obtained by conventional nitriding processes, and a high concentration of nitrogen penetrates and diffuses into the steel from the surface side. It has an internal nitrided layer with a very small lateral nitrogen concentration decreasing gradient. The nitrogen concentration at the outermost surface of the surface hardened steel of the present invention is 0.1 with respect to the weight of the internal nitrided layer at the outermost surface.
-10% by weight, preferably 3-8% by weight. In the above formula (), |Δc/Δt| represents the absolute value of the nitrogen concentration decreasing gradient from the surface side of the steel toward the inside in the nitrided layer, and in the surface hardened steel of the present invention, the nitrogen concentration decreasing gradient is 0.5% per 1μm thickness of nitride layer
It is as follows. Further, in the present invention, when the thickness of the internal nitrided layer exceeds 100 μm, the nitrogen concentration in the deepest part of the internal nitrided layer from the surface of the steel is never less than 50% of the nitrogen concentration in the outermost part of the steel. The thickness of the internal nitrided layer of the surface hardened steel of the present invention is required to be at least 0.01 mm from a practical point of view. In view of the cost of nitriding treatment and obtaining a hardened layer with a practically sufficient thickness, the thickness of the internal nitrided layer is preferably 0.01 mm to 0.5 mm, more preferably 0.05 mm to 0.2 mm.
mm, particularly preferably 0.1 mm to 0.15 mm.

In addition to iron, the surface hardened steel of the present invention includes chromium (Cr), aluminum (Al),
Molybdenum (Mo), titanium (Ti), tungsten (W), boron (B), vanadium (V), manganese (Mn), zirconium (Zr), niobium (Nb), tantalum (Ta), silicon (Si), etc. Contains at least one element selected from the following. The hardness of the internal nitrided layer of the surface hardened steel of the present invention is provided by the formation of nitrides of these metal elements. Examples of steels containing at least one metal element include austenitic, martensitic, and ferrite stainless steels (JIS
G4303), heat-resistant steel (JIS G4311), maraging steel [Imao Tamura et al., Steel Materials Science, Asakura Shoten (1981)
), 135 pages], high-speed tool steel (JIS G4403), corrosion-resistant and heat-resistant superalloy (JIS G4901), and alloy tool steel (JIS
G4404) etc.

The internal nitrided layer of the surface-hardened steel of the present invention can be easily identified when observing the cross section of the steel due to its visual difference from the base material in which nitrogen has not penetrated and diffused.
The thickness of the internal nitrided layer can be directly measured by observing the internal nitrided layer in a cross section obtained by cutting surface hardened steel perpendicular to its surface under an optical microscope. In the present invention, the nitrogen concentration at the outermost surface of surface hardened steel is 5 μm from the surface of surface hardened steel.
It refers to the ratio (wt%) of the amount of nitrogen contained in the surface layer of the scraped steel to the weight of the surface layer of the scraped steel. It can be measured according to the prescribed method for determining nitrogen in iron and steel. In addition, in the present invention, the nitrogen concentration decreasing gradient in the internal nitrided layer can be determined by scraping off a layer with a thickness of 5 μm around the arbitrary different depth of the internal nitrided layer, and reducing the nitrogen concentration in the layer to the above-mentioned value. The relationship between the depth of the layer from the surface and the nitrogen concentration is measured according to the nitrogen content measurement method, and the relationship between the depth of the layer from the surface and the nitrogen concentration is expressed on a graph, and the unit thickness in the internal nitrided layer from the surface of the steel toward the inside is read from the graph. It is expressed as the absolute value of the reduction rate (%) of nitrogen concentration per (μm).

The fact that the nitrogen concentration distribution in the internal nitrided layer of the surface-hardened steel of the present invention is much more uniform than that of conventional ones means that when a vertical cross-section of any location on the surface of the surface-hardened steel of the present invention is Probe Micro Analyzer [Electron Probe]
Microanalyzer (EPMA)] or X-ray Microanalyzer (XMA)
This can be easily determined by measuring by X-ray emission spectrometry, which is known to those skilled in the art (see FIG. 5).

As mentioned above, the surface-hardened steel of the present invention is different from the conventional ion nitriding method in that the steel is treated in a way that keeps the nitrogen ion energy and treatment temperature constant at the same time by intermittently performing electrical discharge in the reactor. It can be obtained by

That is, further according to the present invention, a method of hardening the surface of steel by penetrating and diffusing nitrogen ions from the surface of steel containing at least one nitride-forming metal element in addition to iron in a nitrogen ion atmosphere. The steel is placed in a reactor having an anode and a cathode so as to be in contact with the cathode so as to be homopolarized, and a gas mixture containing nitrogen gas is introduced into the reactor, and the anode and the cathode are A method for surface hardening steel is provided, which comprises intermittently applying a substantially constant voltage between the anode and the cathode homopolarized with copper to cause an electric discharge intermittently.

In carrying out the method of the present invention, first, the steel to be surface hardened is placed in a sealable reactor having both electrodes, an anode and a cathode, so that the steel is the same polarity as the cathode. To make the steel the same polarity as the cathode, the cathode can be made into a shelf and the steel can be placed on top of it, or the steel can be suspended from the cathode with an electrically conductive wire. . Next, after sealing the reaction container, the gas in the reaction container is exhausted using a vacuum pump to create a vacuum, and then a mixed gas containing nitrogen gas is introduced. In addition to nitrogen gas, the mixed gas contains at least one auxiliary gas selected from hydrogen gas, argon gas, helium gas, and neon gas.
Contains seed gas. The nitrogen gas concentration in the mixed gas is 0.1 to 95% by volume, preferably 20 to 50% by volume. As the auxiliary gas in the mixed gas, the above-mentioned hydrogen gas, argon gas, helium gas, and neon gas can be used alone, or
They can also be used in combination. When using a combination of auxiliary gases, the composition ratio of each auxiliary gas may be in any case and is not limited. Among the above-mentioned auxiliary gases, it is preferable to use only hydrogen gas because nitridation is likely to occur, or a combination of hydrogen gas and argon gas because nitridation is likely to occur, discharge is likely to occur, and hydrogen gas and argon gas are available at low cost.
The mixed gas is introduced into the reactor at a pressure of 0.01 to 200 mmHg, preferably 0.1 to 20 mmHg.

A substantially constant voltage is then applied intermittently between the anode and cathode in the reactor to cause an intermittent discharge between the anode and the cathode homopolarized to the steel. In the present invention, it is essential that the applied voltage be kept substantially constant. The voltage to be applied is DC voltage 200
~1200V, preferably 300-600V. When applying a voltage at a constant level intermittently in the method of the present invention, it is preferable to apply the voltage for a constant time at regular intervals, since this can be easily done by automatic control. At that time, if voltage application and subsequent voltage application pause are considered as one cycle,
In order to prevent the temperature from rising too much, the ratio of the voltage application time to the time of one cycle (hereinafter abbreviated as "trigger rate") is set not to exceed about 70%. The time for one voltage application varies depending on the material, shape, and size of the steel, but it is generally desirable to be about 10 msec or less in order to prevent the temperature from rising too much. As a lower limit for the voltage application time, if the nitrogen ion energy obtained by the discharge in the reactor can be substantially maintained at the required level, the voltage application, that is, the discharge can be applied instantaneously, for example, with a voltage application time of 0.1 msec or less. It is not limited as it can be repeated. From a practical point of view, 3-5m
Seconds are preferred. Additionally, the voltage application pause time can be extended to the extent that a substantially constant required level of nitrogen ion energy can be maintained. Therefore, the lower limit of the trigger rate can also be set small within a range that can maintain a substantially constant required level of nitrogen ion energy. In the method of the present invention, the temperature of the steel is 200-1200°C, preferably 400-1200°C.
Preferably, the discharge is carried out to maintain a substantially constant temperature within the range of 1000°C. This temperature is achieved by the thermal energy obtained when the nitrogen ions generated by the discharge impinge on the steel, and can be maintained at a substantially constant temperature by adjusting the trigger rate of voltage application. However, the material, shape, size,
or a level sufficient to maintain a substantially constant voltage and trigger rate of said voltage application to maintain a substantially constant nitrogen ion energy at the level necessary to obtain an internal nitrided layer of desired thickness and hardness. If a constant temperature cannot be obtained, the temperature of the steel can be adjusted by heating the reactor with an auxiliary heater. The temperature of the steel can be measured by placing a thermocouple in the reactor. In the method of the present invention, as mentioned above, by setting the gas pressure, voltage, temperature conditions, etc. in the reactor, the nitrogen ion energy in the reactor can be adjusted to 6 × 10 ^-2 ~
Nitriding treatment is performed with the voltage set in the range of 1.2×10 ³ eV.
Nitrogen ion energy can be measured by a method known to those skilled in the art using an ion density measuring probe.

The time for treating steel in the present invention varies depending on the material, shape and size of the steel to be treated and the thickness of the internal nitrided layer to be obtained.
The longer the treatment time, the deeper the inside of the steel can be nitrided, but considering the cost of treatment and the thickness of the nitrided layer that is practically sufficient, the treatment time should be 15 minutes or more, preferably 5 to 48 minutes. It's time.

The surface-hardened steel of the present invention does not have a compound layer that is brittle and impractical unlike surface-hardened steels obtained by conventional nitriding methods, but has an internal nitrided layer with excellent hardness that can be clearly distinguished from the base metal. However, the internal nitrided layer maintains a substantially constant hardness throughout, regardless of its depth. Moreover, the hardness varies slightly depending on the nitriding conditions and the steel material, but the hardness is 100g under a load of 100g.
Bits hardness [HV (0.1)] of about 1000 to about 1600
The surface hardness of conventional nitrided steel is 100 to 200 on the Vickers hardness. For example, the Bitkers hardness of the surface of surface-hardened steel is determined by the Japanese Industrial Standard JIS.
It can be measured by the Bitkers hardness test method according to Z2244 (1974). The thickness and hardness of the internal nitrided layer having excellent hardness of the surface hardened steel of the present invention can be changed by various combinations of nitriding conditions such as temperature and time, as described above.

It can be seen from the nitrogen concentration distribution and the results of X-ray diffraction analysis that the surface-hardened steel of the present invention uniformly forms metal nitrides of nitride-forming metal elements other than iron and nitrogen in the internal nitrided layer. In the present invention, when steel containing a large amount of carbon is used as the steel to be treated, metal nitrides are not simply precipitated in the internal nitrided layer of the surface-hardened steel obtained, but metal carbonitrides are added to the entire internal nitrided layer. It is also possible to precipitate over a period of time.
That is, in a practical steel that contains at least one carbide-forming metal element such as chromium, vanadium, tungsten, molybdenum, titanium, etc. and a large amount of carbon, the carbide-forming element and carbon form metal carbides inside the steel. However, when this steel is internally nitrided by the method of the present invention, not only metal nitrides are simply precipitated, but also some nitrogen is solidly dissolved in the metal carbide, and as a result, metal carbonitrides are also formed. It will precipitate out. Examples of such steel include martensitic stainless steel such as SUS420 steel and austenitic stainless steel with a high carbon content. As described above, if steel with a high carbon content is subjected to surface hardening treatment by the method of the present invention described above, a surface hardened steel whose internal nitrided layer is a carbonitride layer containing carbon can be obtained. It is also possible to surface harden steel by nitriding to obtain surface hardened steel in which the internal nitrided layer is a carbonitrided layer formed of metal carbonitrides. In that case, as the mixed gas containing nitrogen gas in the method of the present invention, a mixed gas containing hydrocarbon gas such as methane gas, propane gas, butane gas, acetylene gas, etc. in addition to nitrogen gas and the above-mentioned auxiliary gas is used. This can be done by surface hardening the steel. In this case, the above-mentioned hydrocarbon gas can be mixed at a ratio of 0.01 to 10% by volume based on the amount of nitrogen gas in the mixed gas.

EXAMPLES Hereinafter, the present invention will be explained in detail with reference to Examples, but the present invention is not limited thereto.

Example: An austenitic stainless steel ion nitriding reactor made of austenitic stainless steel SUS304 has an exhaust port and a mixed gas inlet, and is equipped with an anode and a table-shaped cathode.
After placing a SUS304 steel sample (15 cm x 15 cm x 10 cm) and sealing the reactor, the air inside the reactor was exhausted from the exhaust port to create a vacuum. Next, a mixed gas consisting of 25% by volume nitrogen gas and 75% by volume hydrogen gas was introduced into the reactor through the mixed gas inlet and the gas pressure was 5 mm.
It was introduced to become Hg. Next, a voltage of 800V was applied between the anode and cathode of the reactor for 2.4 msec, and the application pause time was repeated for 3.1 msec, that is, the trigger rate
Apply at 43.6% and discharge intermittently.
The temperature of the steel can be controlled only by the thermal energy brought about by the nitrogen ion energy ionized by the discharge.
The temperature was kept at 550°C ± 5°C and the steel was treated under these conditions for 30 hours. The temperature of the steel was measured using a thermocouple.

The obtained nitrided steel was cut perpendicular to its surface, and the cut surface was observed under an optical microscope at 400 times magnification. As a result, as shown in FIG. 2, it was found that an internal nitrided layer 4 of a substantially constant thickness was formed which was visually distinguishable from the solid steel matrix 5. The thickness of the internal nitride layer 4 is approximately 135μ
It was m. In addition, when the boundary between the internal nitrided layer 4 and the steel matrix 5 in which nitrogen has not penetrated and diffused was observed with a scanning electron microscope at a magnification of 2000 times, the internal nitrided layer 4 and the steel matrix 5 were observed as shown in Figure 3. It was observed that there was a clear interface with the matrix 5. When the nitrogen concentration in the outermost layer of steel was measured using the method described above,
It was 6.1% by weight. Further, the decreasing gradient of nitrogen concentration from the surface side of the steel toward the inside in the internal nitrided layer was measured by the method described above and was found to be 0.11%/μm. The hardness of the internal nitrided layer was measured using a micro-Vickers hardness meter with a measuring load of 100 g on the same cross section of the surface-hardened steel as above. As shown in Figure 4, the hardness of the internal nitrided layer from the surface was measured. Bitskers hardness is almost constant regardless of depth [HV (0.1)]
It had an excellent hardness of approximately 1150. Also,
EPMA (Hitachi X-650 type, acceleration voltage
As a result of measuring the relative nitrogen concentration distribution and the chromium concentration distribution contained in the steel for comparison in the same cross section of the surface hardened steel using It can be seen that the nitrogen concentration is very low in the parent phase of the steel that has not been hardened by nitriding in the lower layer, but the internal nitrided layer contains a very high concentration of nitrogen. still,
The chromium concentration was constant from the surface to the interior. Furthermore, the internal nitride layer was analyzed using an X-ray diffraction device.
NORELCO [manufactured by Philips (USA)] (Co-
As shown in Figure 6, CrN, a nitride with chromium contained in the SUS304 steel, which is the base material of the sample, was precipitated in the internal nitrided layer. I realized that I was doing it.

Comparative Example 1 Using the same reactor as in Example, a sample (20 cm
x 20 cm x 15 cm), and under a mixed gas having the same composition and pressure as in the example, a voltage was continuously applied between the anode and the cathode to cause discharge. The temperature of the steel is increased or decreased by increasing or decreasing the DC output [voltage (volts) x current (amperes) (VA)] applied between the two poles between 250 and 450V.
The temperature was kept at 570°C and the steel was treated for 12 hours.

When the obtained surface-hardened steel was cut perpendicular to its surface and the cut surface was observed with an optical microscope at a magnification of 400 times, the surface of the steel 1 was observed as shown in Figure 1.
A brittle compound layer (white layer) 2 with a thickness of approximately 6 μm was observed from the inside toward the inside. Further, in the diffusion layer 3, there was no internal nitrided layer whose boundary could be clearly distinguished from the parent phase in which nitrogen had not penetrated and diffused, as was obtained in the examples.

Comparative Example 2 JIS steel type SCM440 was treated under the ion nitriding conditions shown below using a DC glow discharge device equipped with a heater.
The surface was hardened by ion nitriding. The intermittent discharge conditions were as described in Japanese Patent Application Laid-Open No. 54-29845, in which discharge was repeated for 30 seconds at 30 second intervals as described below. Ion nitriding conditions: Nitriding temperature 520℃ Nitriding time 4hr Gas used N ₂ : 250c.c./min H ₂ : 750c.c./min Total pressure 4Torr Discharge mode 30 seconds voltage application (ON) + 30 seconds voltage application pause (OFF) ) with a repeated discharge voltage of 400V.The obtained ion nitrided sample was cut perpendicular to its surface, and the cut surface was observed for its structure using an optical microscope, and the Bitkers hardness distribution was measured using a micro Bitkers hardness meter with a measurement load of 25 g. and EPMA
(Electron Prove Microanalysis) (Accelerating voltage
Nitrogen concentration analysis was performed using 5kV). The results obtained are shown in FIGS. 7, 8, and 9. As is clear from the metallographic micrograph of the cross section of the steel sample in FIG. 7, a white compound with a width of approximately 1 μm is formed in the surface hardened steel obtained. Furthermore, as is clear from FIG. 8, the Vickers hardness of the surface of the ion-nitrided steel decreases rapidly as it goes inward from the surface of the steel. Furthermore, as shown in Figure 9, as a result of the analysis of the nitrogen concentration distribution by EPMA, the surface hardened steel obtained under the ion nitriding conditions of this comparative experiment has an extremely low nitrogen concentration on the surface, which is almost undetectable by EPMA. It was hot. This low surface nitrogen concentration supports the formation of a compound layer on the surface.
The nitrogen concentration distribution from the surface to the inside of the sample could not be observed directly with EPMA due to its low concentration; The nitrogen concentration rapidly decreases toward

[Brief explanation of drawings]

FIG. 1 is a micrograph showing the metal structure of a cross section of steel treated by a conventional ion nitriding method. FIG. 2 is a micrograph showing the metallographic structure of a cross section of the surface hardened steel according to the present invention. FIG. 3 is a scanning electron micrograph showing the metallographic structure of a cross section of the surface hardened steel according to the present invention. Figure 4 shows the depth (μm) from the surface and the Vickers hardness [HV
(0.1)]. Figure 5 shows the depth (μ) from the surface of the surface hardened steel according to the present invention.
2 is a graph showing the relationship between chromium concentration distribution in steel and nitrogen concentration distribution that penetrates and diffuses from the steel surface. FIG. 6 is a graph showing the results of X-ray diffraction analysis of the internal nitrided layer of the surface hardened steel according to the present invention. FIG. 7 is a microscopic photograph of a cross section of the metallographic structure of a steel sample. FIG. 8 is a distribution diagram of the Vickers hardness of the surface of the ion-nitrided steel. 9th
The figure shows the results of EPMA analysis of nitrogen concentration distribution. 1...Surface, 2...Compound layer, 3...Diffusion layer,
4... Internal nitrided layer, 5... Mother phase.

Claims (1)

[Scope of Claims] 1. A steel comprising at least one nitride-forming metal element in addition to iron, and there is no compound layer on the surface of the steel, and nitrogen penetrates and diffuses from the surface of the steel toward the inside. A surface hardened steel having an internal nitrided layer, the thickness of the internal nitrided layer is 0.01 mm or more, and the nitrogen concentration at the outermost surface part of the steel is 0.1 to 10% by weight,
The nitrogen concentration in the internal nitrided layer decreases from the surface side of the steel toward the inside, and the thickness (t) in μm from the surface of the steel in the internal nitrided layer and the nitrogen concentration
A surface hardened steel characterized in that (c) satisfies the following formula: |Δc/Δt|≦0.5 (%/μm) (). 2. The surface hardened steel according to claim 1, wherein the internal nitrided layer has a Vickers hardness [HV (0.1)] that is substantially constant over the entire layer from the surface side of the steel. 3. The surface hardened steel according to claim 1, wherein the internal nitrided layer has a thickness of 0.01 to 0.5 mm. 4. The surface hardened steel according to claim 3, wherein the internal nitrided layer has a thickness of 0.05 to 0.2 mm. 5 Steel containing at least one nitride-forming metal element, where the at least one nitride-forming metal element is selected from the group consisting of chromium, aluminum, molybdenum, titanium, tungsten, boron, vanadium, manganese, zirconium, niobium, tantalum, and silicon. The surface hardened steel according to claim 1, wherein the metal element is selected. 6. The surface hardened steel according to claim 5, wherein the steel is stainless steel, heat resistant steel, maraging steel, high speed tool steel, corrosion resistant heat resistant superalloy or alloy tool steel. 7. The surface hardened steel according to claim 1, wherein the internal nitrided layer is a carbonitride layer containing carbon. 8 A method of hardening the surface of steel by penetrating and diffusing nitrogen ions from the surface of the steel containing at least one nitride-forming metal element in addition to iron in a nitrogen ion atmosphere, and having an anode and a cathode. A steel is placed in a reactor so as to be in contact with the cathode to achieve homopolarization, and a gas mixture containing nitrogen gas is introduced into the reactor, and the steel is intermittently substantially in contact with the cathode. Applying a constant voltage to intermittently cause a discharge between an anode and a cathode homopolarized with steel, and applying a substantially constant voltage intermittently between the anode and the cathode. And when the subsequent pause in voltage application is considered as one cycle, the time of voltage application does not exceed about 70% of the time of one cycle, and the time of voltage application in one cycle is about 10 msec or less. A steel surface hardening method characterized by applying a voltage so that the surface hardens. 9. The method for surface hardening steel according to claim 8, wherein the voltage application time in one cycle of voltage application and subsequent voltage application stop is 3 to 5 msec. 10 The voltage applied between the anode and the cathode is 200~
The surface hardening method according to claim 8, which is 1200V. 11. The method for surface hardening steel according to claim 8, wherein the electric discharge is carried out while maintaining the temperature of the steel at a substantially constant temperature in the range of 200 to 1200°C. 12. The method for surface hardening steel according to claim 8, wherein the nitrogen gas concentration in the mixed gas containing nitrogen gas is 0.1 to 95% by volume. 13. The steel surface hardening method according to claim 8, wherein the mixed gas contains at least one gas selected from hydrogen gas, argon gas, helium gas, and neon gas in addition to nitrogen gas. 14 The pressure of the mixed gas in the reactor is 0.01~
A method for surface hardening steel according to claim 8, wherein the surface hardening temperature is 200 mmHg. 15. The method according to claim 8, wherein the discharge is performed intermittently for 15 minutes or more.