JP6241839B2 - Hardening method for low alloy steel - Google Patents

Description

  The present invention relates to a method for hardening a low alloy steel.

  Conventionally, steel materials such as carbon steel have been used in a wide range of industrial fields because they are more versatile than other metals.

  Such a steel material is subjected to a hardening treatment for its surface layer to extend its life. For example, in the automobile industry, automobile piston rings, crankshafts, various gears, cams, die forgings, It is used for press-molded products, die-cast extrusion-molded products, injection-molded products, cutting tool drills, taps, and reamers.

  As the surface layer hardening treatment method, a surface treatment method is disclosed in which the surface of a metal material shielded by nitrogen gas, such as Fe, Ti, Al, Si, Zr, etc., is irradiated with nitrogen gas plasma for nitriding (patent) Reference 1).

  Also, when supplying nitrogen-containing gas to the plasma arc generated in the plasma torch and irradiating the preheated material surface with the nitrogen plasma jet from the outer nozzle of the plasma torch, the center axis of the outer nozzle A nitriding method using a plasma jet is disclosed, in which preheated nitrogen gas is ejected from two or more symmetrical locations in the direction of the plasma jet, the plasma jet is widened, and the surface of the material to be treated is nitrided. (Patent Document 2).

  Further, there is disclosed a nitriding method characterized by irradiating a material with an atmospheric pressure plasma jet generated using a source gas containing at least nitrogen, thereby nitriding the surface of the material (Patent Document 3). ).

JP-A-62-211365 Japanese Patent Laid-Open No. 10-265937 JP 2009-202087 A

  However, the above-described conventional surface nitriding treatment is a method in which nitrogen atoms are diffused in the surface layer of an alloy steel containing rare metals such as Cr, V, and Mo, and the rare metal nitrogen compounds are finely precipitated and hardened. Therefore, hardening requires rare metals such as Ni, Cr, Mo, etc. contained in alloy steel, and in the case of inexpensive low alloy steels with little or no rare metal content in the conventional surface nitriding treatment described above It was difficult to cure.

  Accordingly, there is a demand for a hardening treatment method capable of carrying out a surface hardening treatment of a low alloy steel having a low rare metal content or no rare metal.

The present invention irradiates the surface of a low alloy steel with a nitrogen gas plasma jet added with hydrogen gas under atmospheric pressure, and heats the surface layer of the low alloy steel to a temperature above the austenite transformation point temperature. Diffusing nitrogen atoms into the surface layer of the steel and quenching the low alloy steel to martensite the surface layer of the low alloy steel to form a nitrogen martensite phase,
Is a method for hardening a low alloy steel.

  According to the present invention, a low alloy steel whose surface is nitrogen hardened can be obtained.

It is a cross-sectional schematic diagram showing the nitriding process of the low alloy steel using the atmospheric pressure plasma jet generator by the one aspect | mode of the method which concerns on this invention. It is a cross-sectional schematic diagram showing the blast cooling of the low alloy steel by one aspect | mode of the method based on this invention. It is a cross-sectional schematic diagram showing the nitriding mechanism of the surface layer of the low alloy steel by the method according to the present invention. It is a cross-sectional schematic diagram showing the composition in the surface layer of the low alloy steel nitrocarburized and quenched by the method according to the present invention. It is a cross-sectional hardness distribution map of the SPCC material which carried out the nitriding and the water cooling quenching by one mode of the method concerning the present invention. It is a cross-sectional hardness distribution map of the SPCC material which carried out the nitriding and the blast quenching by one mode of the method concerning the present invention. It is a cross-sectional hardness distribution map of the SPCC material which carried out the nitriding and the water cooling quenching by one mode of the method concerning the present invention. FIG. 8 is a cross-sectional hardness distribution diagram in which the cross-sectional hardness distribution diagram of FIG. 7 is expanded in the depth direction. It is a metallographic photograph of the iron-nitrogen martensite phase that is the most hardened part of the SPCC material that has been subjected to nitriding and water-cooling quenching according to one embodiment of the method according to the present invention. It is a metal structure photograph of three types of hardened layers of a eutectoid phase, a lath martensite phase, and a ferrite phase formed immediately under plasma jet irradiation of an SPCC material that has been nitrogen-nitrided and water-cooled and quenched by one embodiment of the method according to the present invention. . It is a cross-sectional hardness distribution map of the SPCC material which carried out the nitriding and the water cooling quenching by one mode of the method concerning the present invention. It is a metallographic photograph of the iron-nitrogen martensite phase that is the most hardened part of the SPCC material that has been subjected to nitriding and water-cooling quenching according to one embodiment of the method according to the present invention.

  The method according to the present invention provides a new surface hardening treatment method for low alloy steel. The method according to the present invention heats the surface layer of a low alloy steel to a temperature above the austenite transformation point by blowing an atmospheric pressure plasma jet generated using nitrogen gas added with hydrogen gas to the surface of the low alloy steel, It involves diffusing atoms into the surface layer of the low alloy steel, and then quenching the low alloy steel to cause martensitic transformation to produce a nitrogen martensite phase in the surface layer of the low alloy steel.

  According to the method of the present invention, a martensite phase can be formed in a low alloy steel regardless of the presence or absence of a rare metal, and thus surface hardening treatment of a low alloy steel, which has been difficult in the past, can be performed.

  The low alloy steel used in the method according to the present invention refers to a carbon steel that contains a rare metal in a small amount or does not contain a rare metal other than inevitable impurities.

  For comparison, the alloy steel generally refers to a special steel containing Ni, Cr, Mo or the like, for example, an alloy steel such as chrome steel or chrome molybdenum steel (JIS standard: SCr, SCM). And special tool steels (JIS standards: SUS, SUJ, SUP, etc.) such as alloy tool steel (JIS standards: SKS, SKD, etc.), stainless steel, bearing steel, spring steel, etc. it can.

  With respect to the alloy steel, the low alloy steel used in the method according to the present invention is preferably a rare metal of 5% by mass or less, more preferably 0.5% by mass or less, and further preferably 0.1% by mass or less. Carbon steel that contains carbon, and even more preferably carbon steel that is substantially free of rare metals, that is, carbon steel that does not contain rare metals other than unavoidable impurities or does not contain them at all. As described above, the low alloy steel used in the method according to the present invention has a rare metal content that is low or substantially free, so that the cost can be reduced.

In the present application, the rare metal refers to a non-ferrous metal used in the industry other than a base metal such as iron, copper, zinc, and aluminum or a noble metal. Specific examples are as follows:
Lithium (Li), beryllium (Be), boron (B), scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), cobalt (Co), nickel (Ni), Gallium (Ga), germanium (Ge), selenium (Se), rubidium (Rb), strontium (Sr), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), indium (In), Antimony (Sb), tellurium (Te), cesium (Cs), barium (Ba), lanthanoid series 15 elements (lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm) , Samarium (Sm), Europium (Eu), Gadolinium (Gd), Terbium (Tb), Dyspro Um (Dy), Holmium (Ho), Erbium (Er), Thulium (Tm), Ytterbium (Yb), Lutetium (Lu)), Hafnium (Hf), Tantalum (Ta), Tungsten (W), Rhenium (Re) , Thallium (Tl), bismuth (Bi). The low alloy steel used in the method according to the present invention includes these rare metals in the above content range or substantially does not contain these rare metals.

  Moreover, the low alloy steel used in the method according to the present invention is more preferably a low carbon steel having a low carbon content. The low alloy steel used in the method according to the present invention is preferably 0.5 to 0.3% by mass, more preferably 0.3 to 0.2% by mass, and still more preferably 0% with respect to the entire low alloy steel. Low carbon steel containing 2 to 0.15% by mass of carbon, even more preferably 0.15 to 0.005% by mass or less. Further, the low alloy steel used in the method according to the present invention may be a low alloy steel substantially free of carbon.

  The low alloy steel used in the method according to the present invention is preferably a carbon steel containing rare metal in the above-mentioned content range or substantially free of carbon, and the carbon content is in the above content range. It is a low carbon steel.

  The low alloy steel used in the method according to the present invention also preferably contains a base metal such as copper, zinc, and aluminum other than iron in an amount of 0.1% or less, more preferably 0.05% or less, and more preferably iron. It is a carbon steel that does not substantially contain any base metal other than the above, that is, contains no base metal other than unavoidable impurities or does not contain it at all.

  Low alloy steels used in the method according to the present invention are also gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), osmium. (Os) or other noble metal is preferably contained in an amount of 0.1% or less, more preferably 0.05% or less, and further preferably contains no noble metal substantially, ie, contains no noble metal other than inevitable impurities, or not at all. Carbon steel not included.

  In the low alloy steel that can be used in the method according to the present invention, the total amount of rare metal, base metal, and noble metal is preferably 5% by mass or less, more preferably 1% by mass or less, and further preferably 0.5% by mass or less. More preferably, the carbon steel is substantially free of rare metals, base metals, and noble metals, that is, carbon steel containing no rare metals, base metals, and noble metals other than unavoidable impurities or not containing them at all.

  The low alloy steel that can be used in the method according to the present invention can be, for example, an SPCC material, an SPHC material, an S10C material, or the like.

  The shape of the low alloy steel that can be used in the method according to the present invention is not particularly limited. For example, any shape such as a plate shape, a disk shape, a column shape, a prism shape, a truncated cone shape, or a truncated pyramid shape can be used. It can be in shape.

  Further, the thickness of the low alloy steel that can be used in the method according to the present invention is not particularly limited, but when using a plate-like or disc-like low alloy steel, for example, a low alloy steel having a thickness of 1 to 100 mm. Can be used.

  Hereinafter, the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic cross-sectional view showing a nitriding treatment of a low alloy steel using an atmospheric pressure plasma jet generator 100 that can be used in the method according to the present invention.

  The atmospheric pressure plasma jet generator 100 supplies high-frequency power to a substantially conical internal electrode 1, a cylindrical external electrode 2 disposed around the internal electrode 1, and the internal electrode 1 and the external electrode 2. A high voltage pulse power supply 3 is provided. The external electrode 2 is connected to ground and can be fixed at 0V. A discharge region 19 can be formed between the internal electrode 1 and the external electrode 2.

  The external electrode 2 has a jet nozzle 10 at its lower end for injecting the generated plasma jet to the outside.

  In the atmospheric pressure plasma jet generator 100, the mixed gas 4 containing nitrogen gas added with hydrogen gas can be supplied from the opening 18 into the atmospheric pressure plasma jet generator 100 (discharge region 19). The opening 18 is provided with a hole inclined obliquely, and the supplied mixed gas 4 can flow spirally around the internal electrode 1 toward the jet nozzle 10. The mixed gas 4 is blown out as a plasma jet from the jet nozzle 10, and a plasma jet plume 7 directed toward the surface of the low alloy steel 8 can be generated.

  In the method according to the present invention, a pulse arc type atmospheric pressure plasma jet can be used. In the discharge region 19 between the internal electrode 1 and the external electrode 2, the discharge is repeated according to the high voltage pulse, and the pulse arc plasma 5 can be generated.

  A high voltage pulse power supply 3 can be used to apply a predetermined voltage, voltage pulse width, pulse repetition frequency, and discharge current to the internal electrode 1 to form a pulse arc type atmospheric pressure plasma jet.

  The pulsed arc type plasma jet that can be used in the method according to the present invention can be used at atmospheric pressure, and the surface layer of the low alloy steel 8 is heated above the austenite transformation temperature in the Fe-N system by self-heating of the atmospheric pressure plasma jet. Of 590C to 1400C, more preferably 590C to 1000C, and even more preferably 590C to 910C.

  Further, in the method according to the present invention, without requiring a heater for heating the low alloy steel, the ferrite structure of the surface layer of the low alloy steel can be austenitized and simultaneously nitrogen atoms can be diffused into the surface layer. it can.

  According to the method of the present invention, since the treatment temperature is high, the diffusion rate of nitrogen atoms into the low alloy steel is very high, and the treatment rate can be greatly improved as compared with the conventional method. In addition, a heater for heating the low alloy steel is unnecessary, the equipment can be simplified, and power consumption can be reduced.

  Furthermore, since the method according to the present invention uses atmospheric pressure plasma as described above, large-scale facilities such as a vacuum apparatus and a processing furnace are unnecessary, and initial investment and maintenance costs can be reduced.

  The flow rate of the mixed gas 4 containing nitrogen gas added with hydrogen gas is preferably 5 to 100 liters / minute, more preferably 10 to 30 liters / minute, and further preferably 15 to 25 liters / minute. . When the mixed gas 4 is in the above flow rate range, the low alloy steel is easily nitrided while being heated to a desired temperature.

  The ratio of hydrogen gas to nitrogen gas contained in the mixed gas 4 is preferably 0.1 to 10% by volume, more preferably 0.2 to 3% by volume, and more preferably 0.5 to 1.5% by volume. %. Nitrogen atoms can be supplied to the surface of the low alloy steel by adding hydrogen gas to nitrogen gas in a pulse arc type plasma jet at the above ratio.

  The mixed gas 4 can be supplied from the opening 18, but a gas in which nitrogen gas and hydrogen gas are mixed in advance may be supplied from the opening 18, or when there are a plurality of openings 18, separate openings may be provided. Nitrogen gas and hydrogen gas may be separately supplied from the unit 18.

  The surface of the low alloy steel 8 can be disposed at a position away from the tip of the jet nozzle 10 by preferably 1 to 10 mm, more preferably 2 to 6 mm, and even more preferably 3 to 5 mm. When the distance between the tip of the jet nozzle 10 and the surface of the low alloy steel 8 is within the above range, the surface layer of the low alloy steel 8 is heated to a desired temperature that is equal to or higher than the austenite transformation point temperature. It becomes easy to reach the surface of the low alloy steel 8 before the nitrogen atoms therein recombine with the nitrogen molecules.

  In the method according to the present invention, high concentration nitrogen atoms are generated by high voltage pulse discharge inside the atmospheric pressure plasma jet. When the material surface is irradiated with an atmospheric pressure plasma jet using a mixed gas 4 containing nitrogen gas added with hydrogen gas, the surface of the low alloy steel is immersed in a short time by the high-density nitrogen atoms contained in the plasma plume. Can be nitrogened.

  As described above, according to the method of the present invention, the hardening treatment by surface nitriding of the low alloy steel can be efficiently performed in a short time. Further, for example, it is not necessary to use a harmful gas such as ammonia used in the prior art, so the environmental load is small.

  A cover 6 can be arranged around the jet nozzle periphery 10 so that a large amount of external oxygen does not enter the processing atmosphere. Thereby, the production | generation of a nitrogen oxide can be reduced. As the cover, for example, a quartz cover can be used. The plasma jet plume 7 is irradiated on the low alloy steel 8 and can be exhausted outside the cover 6 as exhaust 9.

  After heating and nitriding the low alloy steel 8, the application to the internal electrode 1 is stopped and the plasma is turned off, and the low alloy steel 8 is quenched by blast cooling 12 using a cooling gas 11 as shown in FIG. be able to. When the plasma generation voltage is turned off, only the gas is blown to the low alloy steel 8, so that blast cooling is performed, and the process can be shifted to the quenching process very easily, and the processing process and processing time can be reduced.

  The cooling gas 11 may be the same as the mixed gas 4 containing nitrogen gas added with hydrogen gas, or may be only nitrogen gas. In this case, the blast cooling of the low alloy steel 8 is possible only by stopping the application to the internal electrode 1 or by stopping the supply of the hydrogen gas accordingly. The cooling rate of the low alloy steel 8 may be changed by adjusting the flow rate of the cooling gas 11. Further, another gas other than nitrogen gas and hydrogen gas may be used as the cooling gas 11.

  The cooling gas 11 used for blast cooling is preferably nitrogen gas, hydrogen gas, helium gas, or a combination thereof.

  When increasing the cooling rate and quenching from the surface of the low alloy steel 8 deeply, it is preferable to use hydrogen gas or to combine hydrogen gas with nitrogen gas or the like. The thermal conductivity of the nitrogen gas is 26 mW / (m · K), the hydrogen gas is 180 mW / (m · L), and the thermal conductivity of the hydrogen gas is high. The heat can be taken away more and the quenching depth equivalent to liquid cooling can be obtained.

  The low alloy steel 8 may be quenched by liquid cooling. The liquid cooling is preferably performed using water, liquid nitrogen, oil, or a polymer quenching agent, and is performed by immersing the low alloy steel 8 in the liquid or spraying the liquid on the low alloy steel 8. be able to.

  Quenching of the low alloy steel 8 may be performed by combining blast cooling and liquid cooling, or may be performed by spraying mist such as water or liquid nitrogen that can be used for liquid cooling together with blast cooling. .

  FIG. 3 is a schematic cross-sectional view showing the nitriding mechanism of low carbon steel by the method according to the present invention. FIG. 3 is a schematic diagram of the surface hardening treatment of the low alloy steel 8 having the ferrite phase 15. The surface layer of the low alloy steel 8 is irradiated with the plasma jet plume 7, and the austenite phase in which nitrogen atoms 13 are diffusion-nitrogenized. 14 can be formed.

  Next, by quenching the low alloy steel 8 in which the austenite phase 14 is formed, the martensite transformation can be performed to generate the nitrogen martensite phase 17 as shown in FIG. In this way, the surface layer of the low alloy steel 8 can be hardened.

  In the quenching, immediately after heating and nitriding the surface layer of the low alloy steel, the surface layer of the low alloy steel is preferably rapidly cooled to 550 ° C. or less by blast cooling, liquid cooling, or a combination thereof. Done. The cooling rate is preferably 25 to 700 ° C./second, more preferably 50 to 600 ° C./second, further preferably 100 to 600 ° C./second, and even more preferably 100 to 400 ° C./second. Yes, even more preferably 150-400 ° C / sec. By quenching in the range of the cooling rate, it is easy to prevent damage such as cracking while obtaining a nitrogen martensite phase in the surface layer of the low alloy steel.

  The present invention is also directed to a low alloy steel containing a nitrogen martensite phase on the surface. The low alloy steel is a carbon steel that contains or substantially does not contain the above-described types and contents of rare metals. The low alloy steel is preferably a low carbon steel having a carbon content in the above-described range.

  The low alloy steel according to the present invention is also a carbon steel in which the content of base metals such as copper, zinc, and aluminum other than iron is preferably in the above-described range, and more preferably is substantially free of base metals other than iron. It is.

  Low alloy steels used in the method according to the present invention are also gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), osmium. Carbon steel containing noble metals such as (Os) in the above-described content range, and more preferably substantially free of noble metals.

  In the low alloy steel according to the present invention, the total amount of the rare metal, the base metal, and the noble metal is preferably 5% by mass or less, more preferably 1% by mass or less, still more preferably 0.5% by mass or less, and still more preferably rare metal, Carbon steel substantially free of base metal and precious metal.

  The low alloy steel that can be used in the method according to the present invention can be, for example, an SPCC material, an SPHC material, an S10C material, or the like.

  The nitrogen martensite phase contained in the surface layer of the low alloy steel according to the present invention has a high hardness, preferably a Vickers hardness of 150 to 800 Hv. The nitrogen martensite phase contained in the surface layer of the low alloy steel preferably has a thickness from the surface of the low alloy steel of preferably 10 μm or more, more preferably 50 μm or more, still more preferably 100 μm or more, and even more preferably 150 μm or more. Even more preferably, it is 200 μm or more. Although the upper limit of the thickness of the nitrogen martensite phase contained in the surface layer of the low alloy steel is not particularly limited, it can be, for example, 1000 μm or less.

Example 1
The low alloy steel was hardened by using the atmospheric pressure plasma jet generator 100 shown in FIG. The distance between the internal electrode 1 and the external electrode 2 disposed around the internal electrode 1 in the atmospheric pressure plasma jet generator 100 was 18 mm, and the hole diameter of the jet nozzle 10 was 4 mm. The external electrode 2 was connected to ground and fixed at 0V.

  As shown in FIG. 1, a square SPCC material having a width of 2 cm and a thickness of 1 mm was disposed as the low alloy steel 8 at a position where the surface was 4 mm away from the tip of the jet nozzle 10. The SPCC material used has iron as a main component and contains 0.02 mass% carbon, 0.09 mass% Mn, 0.017 mass% P, and 0.004 mass% S as minor components. Was.

  Under atmospheric pressure, nitrogen gas of 20 L / min and hydrogen gas of 0.2 L / min were respectively supplied from the separate openings 18 to the inside of the atmospheric pressure plasma jet generator 100 (discharge region 19).

  By applying a high voltage pulse from the high voltage pulse power source 3 to the internal electrode 1 at a voltage of 5 kV, a voltage pulse width of 5 μs, a pulse repetition frequency of 21 kHz, and a discharge current of 1 A, a discharge region 19 is formed to excite the discharge plasma. did.

  The SPCC material was irradiated with the plasma jet plume 7 from the jet nozzle 10 for 30 minutes, the SPCC material was heated and nitrogenated, and then the SPCC material was placed in water for water quenching. The surface temperature of the SPCC material when the SPCC material was irradiated with the plasma jet plume 7 was 900 ° C., and the cooling rate to 550 ° C. of the surface of the SPCC material by water quenching was 400 ° C./second.

  About the SPCC material which carried out the nitriding quenching, the cross section was cut out, mirror polishing was performed, and the cross-sectional hardness distribution was measured with the Vickers hardness meter (weight 10g, the product made from Akashi, HM-102). FIG. 5 shows a cross-sectional hardness distribution diagram of the surface layer portion of the SPCC material that has been subjected to nitrogen quenching.

  The nitrogen martensite phase of the SPCC material sample subjected to nitriding and water quenching was obtained with a thickness of about 250 μm from the surface of the SPCC material, and exhibited a Vickers hardness of 150 Hv to 600 Hv. This is a hardness that cannot be achieved by conventional nitriding.

(Example 2)
After heating and nitriding the SPCC material for 30 minutes in the same manner as in Example 1, the application of the high voltage pulse and the supply of hydrogen gas were stopped, and quenching was performed with blast cooling using nitrogen gas of 20 L / min. Except that, the SPCC material was subjected to nitrogen quenching in the same manner as in Example 1. The cooling rate to 550 ° C. of the surface of the SPCC material by blast cooling was 150 ° C./second.

  About the SPCC material which carried out the nitriding quenching, the cross section was cut out, mirror-polished, and the cross-sectional hardness distribution was measured with the Vickers hardness meter. FIG. 6 shows a cross-sectional hardness distribution diagram of the surface layer portion of the SPCC material that has been subjected to nitrogen quenching.

  The nitrogen martensite phase of the SPCC material sample subjected to nitriding and blast quenching was obtained with a thickness of about 100 μm from the surface of the SPCC material, and exhibited a Vickers hardness of 150 Hv to 600 Hv.

(Example 3)
Using a square SPCC material having a width of 6 cm and a thickness of 1 mm, the SPCC material was heated in the same manner as in Example 1 except that the heating and nitriding time was 5 minutes, 10 minutes, 20 minutes, and 30 minutes. Nitrogen quenching of the material was performed.

  About the SPCC material which carried out the nitriding quenching, the cross section was cut out, mirror-polished, and the cross-sectional hardness distribution was measured with the Vickers hardness meter. FIG. 7 shows a cross-sectional hardness distribution diagram of the surface layer portion (surface to 250 μm depth) of the SPCC material subjected to nitriding and quenching with heating and nitriding times of 5, 10, 20, and 30 minutes. .

  On the surface of the SPCC material sample that had been subjected to nitriding and water quenching, the most hardened portion was formed in an annular shape within a radius of about 20 mm from the plasma jet irradiation center, particularly in a range of about 5 mm to about 15 mm. As the treatment time increased, the radius of the ring forming the most cured portion gradually increased. The most hardened part shows a Vickers hardness of 700 to 800 Hv, which is a hardness that cannot be reached by a conventional nitriding treatment.

  FIG. 8 shows a cross-sectional hardness distribution diagram when the depth direction of the cross-sectional hardness distribution diagram of FIG. 7 is expanded (surface to 500 μm to 1000 μm depth).

  The SPCC material sample subjected to nitriding and water quenching exhibited a Vickers hardness of 400 to 500 Hv up to a depth of 500 μm to 1000 μm just below the irradiation center of the plasma jet. This is a hardness and hardening depth that cannot be achieved by conventional nitriding.

  FIG. 9 shows a metallographic photograph of the hardest part of the SPCC material that has been quenched by heating and nitriding for 30 minutes. This most hardened portion is the most hardened portion showing 750 Hv at a depth of 10 μm and a position 10 mm in the radial direction from the plasma jet irradiation center shown in FIG. 7. The cross section of the sample is etched with 3% nital solution.

  As shown in FIG. 9, the most hardened part of the SPCC material sample subjected to nitriding and water quenching was a martensite phase 20 having a typical bamboo leaf-like structure. Since the SPCC material does not contain sufficient carbon for martensite formation, this martensite phase is an iron-nitrogen martensite phase.

  FIG. 10 shows a photograph of the metallographic structure of the SPCC material that has been quenched by heating and nitriding for 30 minutes, depending on the depth position immediately below the plasma jet irradiation center. The cross section of the sample is etched with 3% nital solution.

  Immediately under the plasma jet irradiation center of the SPCC sample subjected to nitriding and water quenching, the eutectoid phase is near the surface 100 μm from the surface, the lath martensite phase is 500 μm below the surface, and the ferrite is 900 μm below the surface. As a result, it was found that there are three phases of a eutectoid phase, a lath martensite phase, and a ferrite phase.

Example 4
A SPCC material was used in the same manner as in Example 1 except that a square SPCC material having a width of 6 cm and a thickness of 1 mm was used, and the surface of the sample was placed at a position 7 mm away from the tip of the jet nozzle 10 and the surface temperature was 600 ° C Nitrogen quenching was performed.

  About the SPCC material which carried out the nitriding quenching, the cross section was cut out, mirror-polished, and the cross-sectional hardness distribution was measured with the Vickers hardness meter. FIG. 11 is a cross-sectional hardness distribution diagram of the surface layer portion (surface to 1000 μm depth) of the SPCC material that has been subjected to nitriding and quenching.

  On the surface of the SPCC material sample that has been subjected to nitriding and water quenching, a most hardened portion is formed in an annular shape within a radius of 15 mm from the plasma jet irradiation center, particularly within a radius of 5 mm to 10 mm, and a Vickers of 600 to 700 Hv. The hardness was shown. This is a hardness that cannot be achieved by conventional nitriding. Further, the SPCC material sample subjected to nitriding and water quenching exhibited a Vickers hardness of 200 to 300 Hv from the surface to a depth of 1000 μm immediately below the plasma jet irradiation center.

  FIG. 12 shows a metallographic photograph of the most hardened part of the SPCC material that has been subjected to nitriding and quenching. This most hardened portion is the hardest portion showing 750 Hv at a depth of 10 μm and a position 10 mm in the radial direction from the plasma jet irradiation center shown in FIG. 11. The cross section of the sample is etched with 3% nital solution.

  The most hardened part of the SPCC material sample subjected to nitriding and water quenching was the martensite phase 20 having a typical bamboo leaf-like structure. Since the SPCC material does not contain sufficient carbon for martensite formation, this martensite phase is an iron-nitrogen martensite phase.

DESCRIPTION OF SYMBOLS 100 Atmospheric pressure plasma jet generator 1 Internal electrode 2 External electrode 3 High voltage pulse power supply 4 Mixed gas containing nitrogen gas and hydrogen gas 5 Pulse arc plasma 6 Cover 7 Plasma jet plume 8 Low alloy steel 9 Exhaust 10 Jet nozzle 11 Cooling gas 12 Blast cooling 13 Nitrogen atoms 14 Austenitic phase 15 Ferrite phase 17 Nitrogen martensite phase 18 Aperture 19 Discharge region 20 Leaf-like martensite phase

Claims (10)

The surface of the low alloy steel is irradiated with a plasma jet of nitrogen gas added with hydrogen gas under atmospheric pressure, and the surface layer of the low alloy steel is heated above the austenite transformation point temperature in the Fe-N system , Diffusing nitrogen atoms into the surface layer of low alloy steel ,
And quenching the previous SL low alloy steel, wherein generating the surface layer of the low alloy steel is martensitic transformation nitrogen martensite phase, and
The quenching is performed by blast cooling, liquid spray cooling, or a combination thereof;
A method for hardening a low alloy steel, comprising:   The low alloy steel hardening treatment method according to claim 1, wherein the low alloy steel contains 0 to 0.5 mass% of a rare metal.   The method for hardening a low alloy steel according to claim 1 or 2, wherein the low alloy steel contains 0 to 0.3 mass% of carbon.   The method of hardening a low alloy steel according to any one of claims 1 to 3, wherein a ratio of the hydrogen gas to the nitrogen gas is 0.1 to 10% by volume. The method of hardening a low alloy steel according to any one of claims 1 to 4, wherein the blast cooling is performed using nitrogen gas, hydrogen gas, helium gas, or a combination thereof. The liquid in the liquid spray cooling water, liquid nitrogen, oil, polymer quenching agent, or Ru combinations thereof der, hardening method of the low alloy steel according to any one of claims 1 to 5. The low-alloy steel according to any one of claims 1 to 6 , wherein the quenching includes cooling the surface layer of the low-alloy steel to 550 ° C or lower at a cooling rate of 100 to 600 ° C / second. Curing method. A low alloy steel in which the surface layer from the surface to a depth of 500 μm is a nitrogen martensite phase and has a Vickers hardness of 400 Hv or more . The low alloy steel according to claim 8 , wherein the low alloy steel contains 0 to 0.5 mass% of a rare metal. The low alloy steel according to claim 8 or 9 , wherein the low alloy steel contains 0 to 0.3 mass% of carbon.