JPS6160874A - Surface hardened steel and surface hardening method of steel - Google Patents

Abstract

PURPOSE:To produce easily a surface hardened steel having high hardness by bringing a steel contg. nitride forming metallic elements into contact with a cathode in a nitrogen ion atmosphere, impressing a specified voltage between the cathode and anode and executing intermittently electric discharge. CONSTITUTION:The steel contg. nitride forming metallic elements of >=1 kinds of Cr, Al, etc. except Fe is brought into contact with the cathode and is thus homopolarized. A gaseous mixture contg. gaseous nitrogen is introduced into a reaction vessel and the specified voltage is substantially intermittently im pressed between the above-mentioned cathode and anode in the above-mentioned reaction vessel to execute intermittently the electric discharge between the two electrodes. The nitride layer in which the nitrogen concn. in the surface is as high as 0.1-10wt% and that the decreasing gradient ¦DELTAc/DELTAt¦ of the nitrogen concn. (t; thickness mu, c; nitrogen concn. %) from the surface toward the inside is as small as <=0.5 and which has >=0.01mm thickness is formed on the steel surface. The surface hardened steel having the high hardness is thus easily and quickly obtd.

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 in which the nitrogen concentration decrease gradient toward the inner nitrided layer is small, 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. [Toru Araki and others, Steel Engineering Pressure Regulation 8, Steel Heat Treatment Technology, Asakura Shoten (19
72), pp. 173-179, and Imao Tamura et al., Steel Materials Science, Asakura Shoten (1981), pp. 219-228].

Liquid nitriding methods tend to include liquid pressure nitriding and tuftride treatment9, while gas nitriding methods include single-stage chamber method, two-stage nitriding method,
There are technologies such as 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 μm on the outermost surface, mainly an Fe 2-3N layer or Fe, 4 , N, or a mixture of both), and has a so-called diffusion layer in which nitrogen is solidly dissolved in the lower layer, but this compound layer is extremely brittle and impractical, so it is necessary to scrape it off, which is complicated. The disadvantage is that it requires a long process. In addition, the concentration distribution of nitrogen contained in the diffusion layer is not uniform, and the nitrogen concentration decreases rapidly from the surface side to the inside of the steel, and the hardness also decreases rapidly in proportion to this. Sufficient hardness has not been obtained. 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 drawback of long-term cleavage during nitriding. There is. Furthermore, in order to harden the surface of stainless steel using the conventional nitriding method as described above, it was 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. It had the disadvantage of requiring .

Therefore, ion nitriding is a method for electrifying stainless steel GA without using toxic cyanide compounds, requiring a relatively short time for nitriding, and without pickling like other steel materials. In the ion nitriding method, in a mixed gas atmosphere of N2 gas and N2 gas, a voltage is continuously applied between a pair of electrodes with desired scrap as a cathode to cause a discharge, thereby ionizing N2, and converting nitrogen into steel by the potential difference. This is a method of infiltrating and spreading the virus. The principle of the ion nitriding method is explained in detail as follows. Place an O electrode in a sealed container, reduce the pressure inside the container, and
When a DC voltage of V to 1000 V 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 seconds before the steel product. The N2 gas present during 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 penetrates. The ion nitriding method has the characteristic that the formation of white spots 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 temperature of the steel continues to rise due to the thermal energy generated at that time. 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 kinetic energy of the nitrogen ions and the thermal energy brought about by the nitrogen ions are adjusted by increasing or decreasing the DC output applied to the electrode, specifically the voltage. It controlled the temperature. 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. Therefore, even if the temperature of the steel is held constant, it is not homogeneous. It was difficult to obtain a hardened surface layer with good recarburization.

The inventors of the present invention conducted intensive research to obtain a steel with a homogeneous and hardened surface layer by simultaneously controlling the nitrogen ion energy in the nitrogen ion atmosphere and the nitriding temperature of the steel. By intermittently applying a substantially constant voltage to the electrode without causing any 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, by the conventional nitriding method.
As mentioned above, the steel obtained by this process has a compound layer on its outermost layer that is too brittle for practical use, and a nitrogen diffusion layer that is a practical surface hardening layer below it. The nitrogen concentration on the surface 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 the conditions of substantially constant nitrogen ion energy and substantially constant temperature as described above has no brittle compound layer (white layer), and the outermost layer of nitrogen The concentration is 0.1 to 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 the nitrogen concentration on the nitrided layer, and therefore the hardness is also the same as that of the steel. It has a nitrided layer that does not decrease as the depth from the surface increases and remains almost constant. 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 as a surface hardening layer, which has a high surface nitrogen concentration and a small nitrogen concentration decreasing gradient from the surface to the inside. It is in.

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

According to the present invention, the 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 plated steel to the inside. The thickness of the internal nitrided layer is 0.01u 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 is directed from the surface side to the inside side of the steel. The thickness in μm from the surface of the steel in the internal nitrided layer 1) and the nitrogen concentration (c) are determined by the following formula:
Provided is a surface hardened steel that satisfies the following.

The surface-hardened steel of the present invention does not have a compound layer that is brittle and unsuitable for practical use, unlike steel obtained by conventional nitriding, and a high concentration of nitrogen penetrates and diffuses from the surface side of the steel. It has an internal nitrided layer with a very small decreasing gradient of nitrogen concentration toward the inside. The nitrogen concentration in the outermost surface portion of the surface hardened steel of the present invention is 01 to 10% by weight, preferably 3 to 8% by weight, based on the weight of the internal nitrided layer in the outermost surface portion.

In the above formula (1), 1ΔC/Δt1 represents the absolute value of the nitrogen concentration decreasing gradient in the nitrided layer from the surface 1jj of the steel toward the inside, and in the surface hardened steel of the present invention, the nitrogen concentration decreasing gradient is The thickness of the layer is 1 μm! +0.5% or less. Further, in the present invention, the thickness of the internal nitrided layer is 100 mm.
If it exceeds μm, the nitrogen concentration at the deepest part of the internal nitrided layer from the surface of the steel is 5 μm of the nitrogen concentration at the surface of the steel.
It will never be less than 0%. From a practical point of view, the thickness of the internal nitrided layer of the surface hardened steel of O's invention should be at least Q.01.
m is necessary. 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 rnrn to 0.5 am, more preferably 0.05+ni to 0.2 am. , particularly preferably 0.1 m to 0.15 m.

The surface hardened steel of the present invention contains chromium (Cr) and aluminum (At>) in addition to iron as nitride-forming metal elements.

Molybdenum (MO), titanium (Ti), tungsten/(
W), boron (B), vanadium (V), mango/(M
n), norconium (Zr), niobium (Nb), mental (Ta), and silicon (Sl). 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 ferritic stainless steels (JIS 04303), heat-resistant steels (JIS 0431
1), Maraging steel [Imao Tamura et al., Steel Materials Science,
Breakfast Shoten (1981), 135 pages], High Speed Tool Steel (
JIS G41103), corrosion-resistant and heat-resistant superalloys (JIS
G4901) and alloy tool steel (JIS c4404
) etc.

The internal nitrided layer of the surface-hardened steel of the present invention can be easily identified by its visual difference from the base material in which nitrogen has not penetrated and diffused when observing the cross section of the steel, so the thickness of the internal nitrided layer is as follows: Surface-hardened steel is cut perpendicularly to its surface, and the internal nitrided layer portion of the cross section can be directly observed under an optical microscope.

In the present invention, the nitrogen concentration in the outermost surface part of surface hardened steel means that the nitrogen concentration at the outermost surface part of surface hardened steel is
9 Refers to the ratio of the amount of nitrogen contained in the surface layer of the scraped steel to the weight of the surface layer (double %). and Nitrogen Determination Method in Steel VC Therefore Ho 11
be able to determine 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 width of 5 μm around the arbitrary different depths of the internal nitrided layer, and reducing the nitrogen concentration in the layer to the above-mentioned level. Nitrogen content measurement method ν Accordingly, the relationship between the depth of the layer from the surface and the amount of nitrogen is expressed on a graph, and the unit in the internal nitrided layer from the surface of the steel toward the inside is read from the graph. Thickness μ
m) rate of decrease in nitrogen concentration (expressed as the absolute value of @).

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 steels means that when a vertical section of any location on the surface of the surface-hardened steel of the present invention is taken by electron beam Globe Ma Micro Analyzer (El
ectron Probe MICrOanal")'
:','q+(Eplm)] or X-ray Microanalyzer (
This can be easily determined by measurement using a well-known X-ray emission spectrometer (see Figure 5).

As mentioned above, the surface hardening method of the present invention differs from the conventional ion nitriding method in that it requires intermittent discharge in the reactor.
This can be obtained by treating steel in a manner that simultaneously maintains constant nitrogen ion energy and treatment temperature.

That is, further according to the present invention, a method of hardening the surface of plated 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, which is homopolarized to the steel, to cause an electric discharge intermittently.

In carrying out the method of the present invention, first, both an anode and a cathode are placed in a sealable reactor having poles in such a manner that they are homopolar to the entire cathode of the steel to be surface hardened. To make the steel the same polarity as the cathode, the cathode may be made into a shelf and the steel may be placed on top of the cathode, or the steel may be suspended from the cathode with an electrically conductive wire or the like. Next, after sealing the anti-Z container, the gas in the reaction container is evacuated using a vacuum pump to create a vacuum, and then a mixed gas containing nitrogen gas is introduced. The mixed gas contains, in addition to nitrogen gas, at least one gas selected from hydrogen gas, argon gas, helium gas, and neon gas as an auxiliary gas. The nitrogen gas concentration in the mixed gas is 01 to 95% by volume, preferably 20 to 50% by volume. As the auxiliary gas in the mixed gas, the above hydrogen gas, argon gas, helium gas, and neon gas can be used alone or 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 in the reactor at a pressure of 0.01 to 200 mmHg.
Preferably, it is introduced so that the pressure becomes 01~2Ch+tHg.

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 a DC voltage of 200 to 1200V, preferably 300V.
~600■. 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, when voltage application and the subsequent voltage application pause are considered to be one cycle, the ratio of the voltage application time to the time of the one cycle (hereinafter abbreviated as "trigger rate") is determined by the temperature rise too high. In order to prevent this from occurring, it should not exceed approximately 70%. Line 72: U.1
The voltage application time for each cycle varies depending on the material, shape, and size of the steel, but is generally desirably about 101 seconds 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 kept substantially constant at the required level, the voltage can be applied instantaneously, for example, for 0,1 m seconds or less. This is not a limitation as it is possible to repeat the voltage application, that is, the discharge.From a practical standpoint, 3 to 5 msec is preferable.The voltage application pause time is also sufficient to maintain a substantially constant required level of nitrogen ion rays. It can be extended as long as the energy can be maintained.

Therefore, the lower limit of the bird 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 400-100°C.
Preferably, the discharge is carried out to maintain a substantially constant temperature within the range of 0°C. The above temperature is achieved by the thermal energy obtained when the nitrogen ions generated by the discharge impinge on the steel, and can be kept at a substantially constant temperature by adjusting the trigger rate of voltage application.

However, depending on the material, shape, and size of the steel to be surface hardened, and the level of nitrogen ion energy necessary to maintain a substantially constant level of nitrogen ion energy to obtain an internal nitrided layer of desired thickness and hardness, If the substantially constant voltage and trigger rate of the voltage application do not provide a sufficient level of constant temperature, an auxiliary heater can heat the reactor to control the temperature of the steel.

The temperature of the steel can be measured by placing a thermocouple in the reactor. In the method of the present invention, the nitrogen ion energy in the reactor is adjusted to 6 x 10-2 to 1.2 x 10 e by setting the gas pressure, voltage, temperature conditions, etc. in the reactor as described above.
The nitriding process is performed by setting the voltage within the range of V. Nitrogen ion energy can be measured by a method known to those skilled in the art using an ion density measuring glove.

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 cannot be used in practical use unlike surface-hardened steel obtained by conventional nitriding methods, but has an internal nitrided layer with excellent hardness that can be clearly distinguished from the base material. 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 Vickers hardness is approximately 1 at a load of 100ro (HV(0,1)).
The surface hardness of conventional nitrided steel is 100 to 1600 on Vickers hardness. The Vickers hardness of the surface of surface-hardened steel is determined by Nippon Kogyo, for example.
It can be measured by the Vickers hardness test method according to the standard JIS Z2244 (1974). The thickness and hardness of the internal nitrided layer having excellent hardness of the surface hardened steel of the present invention are as follows:
As mentioned above, it can be changed by various combinations of nitriding conditions such as temperature and time.

The surface-hardened steel Vi of the present invention also contains #4 containing at least one nitride-forming metal element in addition to iron in a mixed gas flow of nitrogen gas + hydrogen gas, ammonia gas + hydrogen gas, etc. at 800 ° C. By heating and holding at 1200℃,
Also, ion implantation (which is used in fields such as semiconductor technology)
It can also be obtained by implanting nitrogen into the steel using the technique of ion implantation.

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 a steel containing a large amount of carbon is used as the steel to be treated, metal nitrides are not only precipitated in the internal nitrided layer of the surface hardened steel obtained, but also metal carbonitrides are deposited over the entire internal nitrided layer. It is also possible to precipitate over the entire range.

In practical steels containing at least one type of carbon and a large amount of carbon, the carbide-forming elements and carbon form metal carbides inside the steel, but when this steel is internally nitrided by the method of the present invention, it simply becomes In addition to precipitating metal nitrides, a small amount of nitrogen also dissolves in the metal carbide, resulting in the precipitation of metal carbonitrides as well. Examples of such steel include multi-site stealth steel such as SUS 420 steel and austenitic stainless steel with a high carbon content. If 'iA with a high carbon content is subjected to surface hardening treatment by the method of the present invention as described above,
A surface-hardened steel whose internal nitrided layer is a carbonitride layer containing carbon can be obtained, but when steel with a low carbon content is surface-hardened by nitriding, the internal nitrided layer is a carbonitride layer containing metal carbonitrides. Certain surface hardened steels can also be obtained. In that case, in the method of the present invention, the mixed gas containing nitrogen gas is a mixture containing hydrocarbon gas such as methane gas, grosmetal gas, butane gas, acetylene gas, etc. in addition to nitrogen gas and the above-mentioned auxiliary gas. This can be done by surface hardening the steel using gas. In this case, the above-mentioned hydrocarbon gas is 0.0% relative to the amount of nitrogen gas in the mixed gas.
They can be mixed at a ratio of 1 to 10 volumes.

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

Example Austenitic stainless steel MSU having an exhaust port and a mixed gas inlet, and equipped with an anode and a tabular cathode.
A specimen made of austenitic stainless steel 5US304 steel (
15,: IILX 15cmX 10cm)
After the reactor was sealed, the air inside the reactor was exhausted from the exhaust port to create a vacuum. Next, a mixed gas consisting of 25 volumes of nitrogen gas and 75 volumes of hydrogen gas was introduced into the reactor through the mixed gas inlet so that the gas pressure was 5 mm. Next, a voltage of 800 V was repeatedly applied between the anode and cathode of the reactor for 2.4 ms and a pause time of 3.1 ms, that is, at a trigger rate of 43.6%, to intermittently discharge. The temperature of the plated steel was maintained at 550°C±5°C using only the thermal energy provided by the nitrogen ion energy ionized by the discharge, 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 is cut perpendicular to its surface,
The cut surface was observed under an optical microscope at a magnification of 400 times. As a result, as shown in FIG. 2, it was found that an internal nitrided layer 4 with a substantially constant thickness was formed that could be visually clearly distinguished from the steel matrix 5. The thickness of the internal nitride layer 4 was approximately 135 μm. 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 FIG. It was observed that there was a clear interface with the matrix 5. The nitrogen concentration in the outermost surface layer of the steel was measured using the method described above and was found to be 6.1% by weight. In addition, the nitrogen concentration decreasing gradient from the surface side of the steel in the internal nitrided layer to the inside was measured using the above method and was found to be 0.11.
%/μm. The hardness of the internal nitrided layer is measured at a load of 100 on the same cross section as above of surface hardened steel. As shown in Figure 4, the Vickers hardness of the inner nitrided layer is approximately constant at Hv(0,1)) of approximately 1150 regardless of the depth from the surface. It had excellent hardness. In addition, EPMA (
The results of measuring the relative nitrogen concentration distribution and the chromium concentration distribution contained in the steel for comparison on the same cross section of the surface hardened steel using Hitachi X-650 model (accelerating voltage 10 kV). As shown in Figure 5, the nitrogen concentration in the matrix of steel that has not been hardened by nitriding below the internal nitrided layer is very low, but the internal nitrided layer contains a very high concentration of nitrogen. I understand. Note that the chromium concentration was constant from the surface to the inside. Furthermore, the internal nitrided layer was analyzed using an
As shown in Figure 6, CrN, a nitride with chromium contained in the SUS 304 steel, which is the base material of the sample, was precipitated in the internal nitrided layer. I understand.

Comparative Example The same reactor as in the example was used, and a sample (20
(1 m×20 cm×15 cIIL) was placed, 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. DC output applied between the two poles [voltage (melt) x current (ampere) (VA):
The temperature of the steel was maintained at 570° C. while increasing and decreasing the voltage of 1 between 250 and 450 μ, 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 400 times magnification, it was found that the surface of the steel was cut from surface 1 toward the inside, as shown in Figure 1. A brittle compound layer (white layer) 2 with a thickness of about 6 μm was observed.

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.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional micrograph of steel treated by a conventional ion nitriding method. FIG. 2 is a cross-sectional micrograph of surface hardened steel according to the present invention. FIG. 3 is a scanning electron micrograph of a cross section of a hardened steel according to the invention. Figure 4 is a graph showing the relationship between the depth from the surface (μm) and Vickers hardness HV (o, 1) of the surface-hardened steel according to the present invention, the chromium concentration distribution, and the nitrogen concentration distribution that penetrates and diffuses from the steel surface. be. FIG. 6 is a graph showing the results of xI-gland diffraction analysis of the internal nitrided layer of the surface-hardened steel according to the present invention. 1...Surface 2...Compound layer 3...Diffusion layer 4,
...Internal nitrided layer 5...Matrix patent applicant Institute of Applied Science 25/J□
25 pm Fig. 3 Fig. 4 Surface sea urchin captive Olm) Fig. 5 Surface force / sea urn 5 strokes (pm) Fig. 6 Double-grained degree 2e (0) Hand-set correction 8 (method) % formula % 1. Indication of the case Patent Application No. 179265 filed in 1988 2. Name of the invention Surface hardened steel and surface hardening method for steel 3. Relationship with the amendment case Patent applicant 4, agent January 9, 1985 day (shipment date January 29, 1985) 6
, Contents of Correction Subject to Correction (Japanese Patent Application No. 59-179265) The column of the brief description of the drawings in the specification is amended as follows. (1) Insert J indicating "JilMm" in front of "Microphotograph" on page 26, line 12 of Ming Jl11'B. Insert "showing the metallographic structure of" before the competition photo. (3) Insert "indicates metallographic structure" before "scanning electron micrograph" on page 26, line 14 of Specification V.

Claims (1)

[Claims] 1. A hardened surface made of steel containing at least one nitride-forming metal element in addition to iron, and having an internal nitrided layer in which nitrogen penetrates and diffuses from the surface of the steel toward the inside. In steel, the thickness of the internal nitrided layer is 0.01 mm or more, the nitrogen concentration at the outermost surface part of the steel is 0.1 to 10% by weight, and the nitrogen concentration in the internal nitrided layer is from the surface side of the steel. The μ from the surface of the steel in the internal nitrided layer decreases toward the inside.
A surface hardened steel characterized in that the thickness (t) in m and the nitrogen concentration (c) satisfy the following formula: |Δc/Δt|≦0.5 (%/μm) (I). 2. The surface hardened steel according to claim 1, wherein the internal nitrided layer has a thickness of 0.01 to 0.5 mm. 3. The surface hardened steel according to claim 2, wherein the internal nitrided layer has a thickness of 0.05 to 0.2 mm. 4. In steel containing at least one nitride-forming metal element, the at least one nitride-forming metal element is chromium, aluminum, molybdenum, titanium, tungsten, boron, vanadium, manganese, zirconium, niobium, tantalum, and silicon. The surface hardened steel according to claim 1, which is a metal element selected from: 5. The steel is stainless steel, heat-resistant steel, maraging steel,
The surface-hardened steel according to claim 4, which is a high-speed tool steel, a corrosion-resistant and heat-resistant superalloy, or an alloy tool steel. 6. The surface hardened steel according to claim 1, wherein the internal nitrided layer is a carbonitride layer containing carbon. 7. In a nitrogen ion atmosphere, nitrogen ions are introduced and diffused from the surface of the steel containing at least one nitride-forming metal element in addition to iron to harden the surface of the steel, and the anode and cathode are hardened. The steel is placed in a reactor having a structure such that the steel is brought into contact with the cathode so as to be homopolarized, and a mixed gas containing nitrogen gas is introduced into the reactor, and the steel is intermittently substantially heated between the anode and the cathode. A steel surface hardening method characterized by applying a constant voltage to cause intermittent discharge between an anode and a cathode that is homopolarized to the steel. 8. When applying a substantially constant voltage intermittently between the anode and the cathode, if the voltage application and the subsequent suspension of voltage application are one cycle, the voltage application time is relative to the time of one cycle. 8. A method according to claim 7, characterized in that the surface hardening of steel does not exceed about 70%. 9. The voltage applied between the anode and the cathode is 200 to 120
The method for surface hardening steel according to claim 7, wherein the voltage is 0V. 10. The method for surface hardening steel according to claim 7, 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. 11. The method for surface hardening steel according to claim 7, wherein the nitrogen gas concentration in the mixed gas containing nitrogen gas is 0.1 to 95% by volume. 12. The method for surface hardening steel according to claim 7, 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. 13. The pressure of the mixed gas in the reactor is 0.01 to 20
The method for surface hardening of steel according to claim 7, wherein the surface hardening temperature is 0 mmHg. 14. The method according to claim 7, wherein the discharge is performed intermittently for 15 minutes or more.