JP2010007134A - Surface treatment method for steel material, surface treatment device, and steel obtained thereby - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface treatment method for a steel material having excellent wear resistance and heat resistance by forming a nitrogen compound layer consisting mainly of a chromium nitride having high adhesion on the surface of the steel material. <P>SOLUTION: In the surface treatment method for a steel material which immerses the steel into a molten salt, an inert gas layer for intercepting the liquid face of the molten salt from the atmospheric air is formed in the vicinity of the liquid face of the molten salt, during the immersion of a nitriding steel into a chromium-containing molten salt, and a chromium nitride is formed on the surface layer part. In this way, the reaction between the components in the molten salt with an oxygen component or moisture is substantially prevented, the stabilization of the diffusion components in the molten salt is attained, and a compound layer as a treatment layer having excellent properties can be formed on the surface part of the object to be treated. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention provides a surface treatment method and a surface treatment apparatus for a steel material that has excellent wear resistance and heat resistance by forming a nitrogen compound layer mainly composed of chromium nitride having high adhesion on the surface of the steel material, and It relates to the steel material obtained by them.

  As a method for improving the wear resistance of steel parts, for example, a nitriding method, a carburizing method, a carbonitriding method, and the like in which N or C penetrates the surface portion thereof have been applied to a wide range of machine parts.

  On the other hand, due to higher precision of machine parts and rising steel prices, engine parts and molds are becoming more compact and more loaded, and various parts have higher wear resistance and longer life than before. There is a need to make it easier. For this reason, coating processing methods such as the PVD method, the PCVD method, and the CVD method have been widely used.

  Among these, the PVD method and the PCVD method have a relatively low processing temperature similar to the nitriding treatment, and can form a film having various characteristics at a temperature lower than the transformation point of steel. Widely used as a surface coating film for molds and the like.

  However, in order to improve the adhesion to the substrate to be coated, these coatings are laminated with coating materials having different properties, or the surface of the substrate to be coated is nitrided to minimize the difference in hardness from the coating. Although it has been made smaller, its adhesion to the base material is never sufficiently high, and it is easy to cause peeling in parts that are subjected to high surface pressure, making it difficult to use (For example, Patent Documents 1, 2, 3, and 4).

  In addition, when high adhesion with a base material is required, a CVD method that easily forms an alloy layer with the base material is also used, but this method has a temperature higher than the transformation point of steel. Therefore, the range of applicable members is naturally limited (for example, Patent Document 5). 6).

  Because of these, it does not require large deformations or special heat treatment after processing, it has excellent wear resistance, and also has high adhesion that does not cause peeling even when high surface pressure is applied. In order to form the element, it is considered effective to diffuse the element from the surface at a temperature lower than the transformation point of the steel.

  As such a method, after forming a nitride layer on the surface, a method of diffusing chromium from the surface in a salt bath is disclosed (Patent Document 7). Moreover, the metal mold | die used for forging and casting using the surface treatment method using chromium diffusion by a salt bath is also disclosed (patent documents 8 and 9).

Japanese Patent No. 3514963 JP 2003-245738 A Japanese Patent No. 3971293 JP 2007-224805 A JP 2003-10958 A JP 2008-126334 A Japanese Patent No. 3939451 JP 2001-25843 A Japanese Patent Laid-Open No. 2001-25856

  With respect to the treatment method of Patent Document 7, a chromium nitride can be formed by diffusing chromium on the surface of the article to be treated, and a relatively good wear-resistant treated article can be obtained. Is difficult to maintain stably. For this reason, it is difficult to stably form chromium nitride and to stabilize the surface roughness important for the sliding member. Therefore, for example, the surface roughness is adjusted by polishing or the like after the treatment, but it is difficult to polish the hard nitrogen compound layer itself, and the nitrogen compound layer contributing to wear resistance is removed by polishing. It is difficult to adjust the surface roughness by polishing. On the other hand, when it is applied to a sliding component or the like with a large surface roughness, there is a problem in that the wear of the counterpart material proceeds even if the processed product itself has good wear resistance.

  The molds and the like used at high temperatures in Patent Documents 8 and 9 have a compound layer mainly composed of chromium nitride formed on the surface. The chromium nitride not only has excellent wear resistance but also heat resistance and oxidation resistance, and is excellent in wear resistance and seizure resistance. However, heat cracks are generated and propagated due to repeated thermal cycles. In some cases, it is easy, and it is not always sufficient as a measure for improving the life of the mold.

  As described above, members that are used in harsh environments such as sliding members that are subjected to high surface pressure and dies that are subjected to repeated thermal cycles are processed below the transformation point of steel with less deformation, and their durability It is necessary to develop a treatment method sufficient to satisfy the properties and a member having such properties.

  The present invention has been made to solve such problems, and by forming a nitrogen compound layer mainly composed of chromium nitride having high adhesion on the steel surface, excellent wear resistance and heat resistance are achieved. It is an object of the present invention to provide a surface treatment method and a surface treatment apparatus for a steel material that will have the following, and a steel material obtained thereby.

  As a result of diligent research to solve the above problems, the main reason for destabilizing the properties of the product to be treated when the steel material is salt bathed and infiltrated with diffusing components such as chromium is during salt bath processing. It was found that oxygen content and moisture were taken into the salt bath from the liquid surface and reacted with the diffusing component to form an oxide, which did not contribute to the diffusion reaction to the treated product. . Moreover, by preventing this, it becomes possible to stably supply the diffusion component to the surface of the processed product, and as a result, it is possible to form a compound layer having stable characteristics such as surface roughness. Was found.

  In order to achieve the above object, a surface treatment method for a steel material of the present invention is a surface treatment method for a steel material in which the steel material is immersed in a molten salt, and at least during the immersion of the steel material in the molten salt, near the liquid surface of the molten salt. The gist is to form an inert gas layer for blocking the liquid surface from the atmosphere.

  In order to achieve the above object, the steel material surface treatment apparatus of the present invention is a steel material surface treatment apparatus for immersing a steel material in a molten salt, and at least during the immersion of the steel material in the molten salt, the molten salt liquid The gist is that an inert gas layer forming means for forming an inert gas layer for blocking the liquid surface from the atmosphere in the vicinity of the surface is provided.

In order to achieve the above object, the steel material of the present invention has a chromium and iron alloy layer in which a compound layer in which chromium nitride and iron nitride are mixed is formed in a surface layer portion, and nitrogen is contained under the compound layer. A diffusion layer in which nitrogen diffuses in the base steel is formed under the alloy layer,
The gist is that the surface hardness of the compound layer is Hv 700 or more and Hv 1000 or less.

  In the steel material surface treatment method of the present invention, at least during immersion of the steel material in the molten salt, an inert gas layer for blocking the liquid surface from the atmosphere is formed in the vicinity of the liquid surface of the molten salt. In this way, at least during the immersion treatment of the steel material in the molten bath, the surface of the salt bath is covered with an inert gas, thereby substantially preventing the reaction of the components in the molten salt with oxygen and moisture. Thus, it is possible to stabilize the diffusion component in the molten salt and form a compound layer that is a treatment layer having excellent properties on the surface portion of the article to be treated. Furthermore, since the fluctuation of the diffusion component during processing is reduced, the surface roughness of the steel material surface can be kept low, and a steel material having excellent characteristics as a sliding member can be obtained.

  In the surface treatment method for steel according to the present invention, when the molten salt contains chromium and the nitrided steel is immersed in the molten salt to form chromium nitride in the surface layer portion, diffusion during the immersion treatment in the molten salt Prevents deterioration due to oxidation of chromium, which is a component, to stably form chromium nitride, and to form a compound layer containing extremely high adhesion of chromium nitride and iron nitride on the surface layer, for example, high surface It is possible to form a surface layer that can be used for a long time without causing problems such as peeling like PVD coating even in a sliding environment where pressure is applied and oil runs out. It becomes.

  In the steel material surface treatment method of the present invention, after the immersion treatment in the molten salt, when the shot peening treatment is performed with a shot media of 0.2 mm or less, fine irregularities are formed on the surface of the article to be treated, and the contact is made. The sliding property with the mating material can be improved, and seizure with the mating material can be prevented. Furthermore, a high compressive stress can be applied to the surface layer portion, and it is possible to improve not only wear resistance and heat resistance but also fatigue characteristics and seizure resistance. Therefore, it is possible to improve durability when used for a mold that is repeatedly used, for example.

  The steel material surface treatment apparatus of the present invention is an inert gas layer forming means for forming an inert gas layer for shielding the liquid surface from the atmosphere in the vicinity of the liquid surface of the molten salt at least during immersion of the steel material in the molten salt. It has. In this way, at least during the immersion treatment of the steel material in the molten bath, the surface of the salt bath is covered with an inert gas, thereby substantially preventing the reaction of the components in the molten salt with oxygen and moisture. Thus, it is possible to stabilize the diffusion component in the molten salt and form a compound layer that is a treatment layer having excellent properties on the surface portion of the article to be treated. Furthermore, since the fluctuation of the diffusion component during processing is reduced, the surface roughness of the steel material surface can be kept low, and a steel material having excellent characteristics as a sliding member can be obtained.

  In the steel material surface treatment apparatus of the present invention, when the molten salt contains chromium and the nitrided steel material is immersed in the molten salt to form chromium nitride in the surface layer portion, diffusion during the immersion treatment in the molten salt Prevents deterioration due to oxidation of chromium, which is a component, to stably form chromium nitride, and to form a compound layer containing extremely high adhesion of chromium nitride and iron nitride on the surface layer, for example, high surface It is possible to form a surface layer that can be used for a long time without causing problems such as peeling like PVD coating even in a sliding environment where pressure is applied and oil runs out. It becomes.

  In the steel material surface treatment apparatus of the present invention, the inert gas layer forming means forms an inert gas layer at 400 ° C. or higher by supplying the inert gas heated by the heating means to the vicinity of the liquid surface of the molten salt. In this case, by supplying a high-temperature inert gas to the vicinity of the liquid surface, solidification of the molten salt is prevented, and problems such as difficulty in taking out the processed product do not occur, and workability is improved. It becomes possible.

  In the steel material of the present invention, a compound layer in which chromium nitride and iron nitride are mixed is formed on the surface layer portion, and an alloy layer of chromium and iron containing nitrogen is formed under the compound layer. In addition, a diffusion layer in which nitrogen diffuses in the base steel is formed. In this way, a compound layer in which chromium nitride and iron nitride, which are excellent in wear resistance and heat resistance, are stably formed on the surface layer part, and the base material side is important for adhesion to the base material. An alloy layer which is an interdiffusion layer of chromium and iron in the base material is formed, and a diffusion layer is further formed on the base material side to contribute as a hardness gradient layer. As a result, even in a use environment where a high load is applied, the compound layer formed on the surface of the steel material hardly peels. Furthermore, when the surface hardness of the compound layer is set to Hv 700 or more and Hv 1000 or less, the wear of the counterpart material is prevented from being seized or oxidized, and excellent wear resistance is exhibited. For example, a small surface roughness is required. However, it is possible to adjust the surface roughness by a simple polishing process that hardly reduces the thickness of the nitrogen compound layer mainly composed of chromium nitride.

  In the steel material of the present invention, when the nitrided steel material is immersed in the molten salt to form chromium nitride on the surface portion thereof, and the surface roughness after the surface treatment immersed in the molten salt is 0.5 μm or less in Ra In addition to suppressing the wear of the steel material itself of the present invention by the presence of a compound layer with high surface adhesion, the surface roughness of the steel material is reduced and the wear of the mating material to be slid is also suppressed. Even if it is used as a sliding member that can be loaded with a particularly high surface pressure, it is possible to form a highly durable mechanical component or the like.

  In the steel material of the present invention, when the hardness at a depth of 50 μm from the surface is Hv 800 or less, the diffusion layer not only has compressive stress but also has toughness by appropriately controlling the hardness of the diffusion layer supporting the compound layer. Therefore, in order to demonstrate the effect of suppressing the development of heat cracks and improve the heat crack resistance, for example, when applied to a hot mold or the like that is repeatedly subjected to a heat cycle, the heat crack resistance is improved. It can be used as a long-life mold material having an excellent surface layer.

  As described above, mechanical parts and molds with excellent wear resistance, peel resistance, and heat resistance can be obtained by performing surface treatment on parts and molds that have been difficult to apply in the past. As a result, excellent durability can be expressed in these members.

  Furthermore, in the present invention, when the nitriding treatment includes a fluorination treatment step, it can be applied to a wide variety of steel types including not only carbon steel and low alloy steel but also high alloy steel, tool steel, stainless steel, etc. In addition to this, a nitrided layer can be formed according to the intended use, and as a result, a treated layer can be formed according to the purpose.

  Furthermore, in the present invention, when a sufficient amount of inert gas is supplied so as to keep the inside of the treatment tank at a positive pressure during at least the immersion treatment of the steel material in the molten salt, the oxygen content in the treatment tank is further reduced. It is possible to prevent the moisture from being mixed and the diffusion component in the molten salt from being deteriorated or reduced by the reaction.

  Next, the best mode for carrying out the present invention will be described.

  Steel materials targeted by the present invention are carbon steel, low alloy steel, high alloy steel, structural rolled steel, high tensile steel, mechanical structural steel, carbon tool steel, alloy tool steel, high speed steel, bearing steel, spring. Various steel materials such as steel, case-hardened steel, nitrided steel, stainless steel, and heat-resistant steel can be applied.

In the steel material surface treatment method of the present invention, first, if necessary, the steel material is subjected to nitriding treatment, whereby the surface layer of the steel material is an iron nitrogen compound layer mainly composed of Fe ₃ N, which is iron nitride, A nitrogen diffusion layer in which nitrogen is diffused in the base material is formed in the lower layer.

At this time, even when only the nitrogen diffusion layer is formed, it is possible to form a compound layer mainly composed of chromium nitride by the subsequent salt bath treatment, but in that case, it is formed by the salt bath treatment. Since the compound layer cannot be made sufficiently thick, it is preferable to form an iron nitrogen compound layer mainly composed of Fe ₃ N by nitriding treatment, and the thickness is preferably 3 μm or more, more preferably 5 μm or more. .

  As the nitriding treatment, various nitriding treatments such as gas nitriding treatment, salt bath nitriding treatment, ion and plasma nitriding treatment can be applied, but it is preferable to apply gas nitriding treatment including a fluorination treatment step. In this way, when gas nitriding treatment including fluorination treatment process is applied, it can be applied to a wide range of steel types including not only carbon steel and low alloy steel but also high alloy steel, tool steel, stainless steel, etc. It becomes. Furthermore, since it is possible to select a wide range of nitriding temperatures and gas compositions so as to form a nitride layer according to the intended use, it is most preferable as a nitriding treatment applied to the surface treatment of the present invention.

For example, in a mold or the like in which repeated thermal cycles are loaded and heat cracks are likely to occur, it is desirable to form a deep tough nitrogen diffusion layer with suppressed hardness. In this case, it is preferable to perform two-stage nitriding after the fluorination. That is, in the first half of the nitriding treatment, nitriding treatment is performed in an extremely low NH ₃ concentration atmosphere to form a uniform nitrogen diffusion layer that does not excessively increase the hardness, and in the latter half of the nitriding treatment, nitriding in a high NH ₃ concentration atmosphere is performed. By performing the treatment, it is possible to increase only the N concentration of the surface layer and form an iron nitrogen compound layer mainly composed of Fe ₃ N.

  Then, by performing a salt bath treatment to be described later, a compound layer mainly composed of chromium nitride can be formed on the surface, and the hardness of the diffusion layer having a depth of 50 μm from the surface can be made Hv 800 or less. In this way, not only the wear resistance by the compound layer mainly composed of chromium nitride is improved, but also a diffusion layer having compressive stress and toughness is obtained, and the effect of suppressing the development of heat cracks To improve heat crack resistance.

The fluorine source gas in the fluorination treatment is not particularly limited as long as it contains fluorine gas or a fluorine compound, but it is gaseous at room temperature and has high chemical stability. A gas containing NF ₃ gas that is easy to handle is preferably used. The normal fluorination treatment process depends on the material and surface condition of the article to be treated, but for example, a gas diluted with N ₂ gas so that the NF ₃ gas concentration becomes 1000 to 100000 ppm is used as the fluorine source gas, It is carried out by holding at 600 ° C. for 1 minute to 180 minutes. By performing this fluorination treatment step, it is possible to replace the oxide film on the steel material surface, which is an obstructive factor for forming a uniform nitride layer, with a fluoride film.

Subsequent to the above fluorination treatment, in a gas atmosphere containing NH ₃ , hold at 400 to 600 ° C. for 30 minutes to 50 hours, more preferably at 500 to 600 ° C. for 1 hour to 10 hours, and form Fe on the surface of the article to be treated. ₃ An iron nitrogen compound layer mainly containing N and a nitrogen diffusion layer are formed. At this time, the fluoride film covering the surface of the steel material to be processed has the property of being easily reduced and removed even in a weak reducing atmosphere with a low NH ₃ gas concentration. The optimum processing time and the like can be adjusted in consideration of the material quality and intended use.

  The heat treatment method of the present invention forms a compound layer by immersing a steel material that has undergone nitriding treatment in a molten salt, if necessary, performing a salt bath treatment, and diffusing and infiltrating the diffusion component in the molten salt from the surface of the steel material. To do.

  As the molten salt, those containing an alkali metal chloride and / or an alkaline earth metal chloride as a main component can be used. The molten salt preferably contains chromium. By including chromium in the molten salt, the nitrided steel material can be immersed in the molten salt to form chromium nitride in the surface layer portion.

Examples of the alkali metal chloride include LiCl, NaCl, and KCl. Examples of the alkaline earth metal chloride include MgCl ₂ , CaCl ₂ , and BaCl ₂ . These can be used singly or in combination, but it is also possible to lower the melting point of the molten salt by mixing, and in order to ensure the uniformity of the components in the salt bath by stirring, the viscosity is to some extent. Since lowering is also necessary, it is desirable to lower the melting point, that is, the viscosity, using a mixed salt.

  Further, as chromium added to the molten salt, industrial metal chromium can be used, and it is desirable that it is in a powder form so that it can be easily added and dissolved in the molten salt. The amount of chromium contained in the molten salt is preferably about 5 to 20% by weight although it depends on the surface area of the article to be treated.

  In addition, the molten salt not only leads to stabilization of the components in the salt bath, but also has the effect of adjusting the viscosity of the salt bath and making the components in the salt bath uniform during stirring. It is desirable to add an appropriate amount of the compound. The amount of silicon or silicon compound added in this case is preferably about 5 to 20% by weight.

  The above-mentioned salt is heated to 500 to 700 ° C. to form a molten salt, a steel material is immersed in this molten salt bath, and a diffusion component is diffused and permeated from the surface to form a compound layer. By immersing the nitrided steel material in a molten salt containing chromium, a nitrogen compound layer mainly composed of chromium nitride can be formed on the surface layer portion.

  FIG. 1 is a view showing an example of a salt bath treatment furnace in which the steel material is immersed in a molten salt.

  In this salt bath furnace, a treatment tank 7 into which a salt bath treatment agent 8 to be a molten salt is introduced is disposed inside a furnace body 5 covering the outside of the furnace, and the treatment tank 7 is heated from the outside. Is provided with a heater 6 that heats and melts the salt bath treating agent 8 to form a molten salt. Further, an impeller 9 for agitating an internal salt bath treating agent (molten salt) 8 is inserted into the treatment tank 7.

Further, an inert gas layer is formed to block the liquid surface of the molten salt 8 from the atmosphere so that the molten salt 8 absorbs oxygen and moisture from the outside air from its surface and does not react with components in the molten salt. A pipe 10 and an annular pipe 11 are provided as means for forming an inert gas layer. In other words, in this example, an annular pipe 11 connected to the pipe 10 and having a large number of holes is disposed in the upper part of the processing tank 7. The annular tube 11 is disposed along the inner peripheral surface of the processing tank 7 on the liquid surface side with respect to the upper opening of the processing tank 7. For example, N ₂ gas is jetted as an inert gas from a number of holes in the annular tube 11, and the N ₂ gas is supplied near the liquid surface of the molten salt 8, and the liquid surface is covered with the N ₂ gas. A gas layer is formed.

  In this way, an inert gas layer for blocking the liquid surface of the molten salt 8 from the atmosphere is formed by supplying an inert gas near the liquid surface below the upper opening in the treatment layer 7. Is done.

The annular tube 11 is disposed in a region where the temperature is maintained high by the heater 6, and the inert gas layer forming unit supplies the inert gas heated by the heating unit to the vicinity of the liquid surface of the molten salt 8. . Thereby, high-temperature N ₂ gas is ejected from the annular tube 11, and an inert gas layer of 400 ° C. or higher, preferably equal to or higher than the temperature of the molten salt 8 is formed on the liquid surface of the molten salt 8. It is possible. Thereby, it can prevent that the molten salt 8 solidifies on the surface.

The method for supplying N ₂ gas into the processing tank 7 is not limited to the above-described structure, and N ₂ gas preheated by a preheating device (not shown) may be supplied, or the heater 6 and the processing tank 7 may be preheated by passing the pipe 10 through the gap 7. Moreover, N ₂ gas can be bubbled into the molten salt 8 as an auxiliary, and the N ₂ gas can be supplied as an auxiliary to the liquid surface of the molten salt 8. However, if the N ₂ gas is supplied only by bubbling, the high-temperature molten salt 8 is scattered around and the inert gas layer in the vicinity of the liquid level is disturbed by the gas floating on the liquid surface of the molten salt 8 by bubbling. Because the liquid level comes into contact with the atmosphere, it is used only as an auxiliary.

In addition, in order to suppress the amount of N ₂ gas to be used and to prevent the turbulence caused by the air flow of the formed inert gas layer and maintain the state, the treatment tank 7 is provided with a lid 12, which is more than the liquid level of the treatment tank 7. It is desirable to maintain the upper space at a positive pressure.

  Using the above apparatus, a molten salt 8 mainly composed of an alkali metal chloride and / or an alkaline earth metal chloride is formed in the treatment tank 7, and preferably chromium is added to the molten salt 8, More preferably, silicon having a higher affinity for oxygen than chromium or a compound of silicon is added to stabilize the state of the salt bath, particularly the activity of chromium. The steel material nitrided in the molten salt 8 is immersed, and a salt bath treatment is performed by heating and holding at 500 to 700 ° C. for 1 to 50 hours, more preferably 2 to 20 hours.

  When the temperature of the salt bath treatment is less than 500 ° C., the formation rate of the compound layer mainly composed of chromium nitride is slowed down, so that the treatment takes a long time and the thickness thereof is also increased. It tends to be uneven. Furthermore, since the viscosity of the salt bath also increases, it is difficult to make the salt bath components uniform by stirring, so the treatment temperature is set to 500 ° C. or higher. More preferably, it is 520 ° C. or higher. In addition, when the processing temperature exceeds 700 ° C., not only the surface roughness of the product to be processed is deteriorated, but also the softness of the material of the product to be processed becomes large, and the necessary strength cannot be obtained. The treatment temperature is 700 ° C. or lower. More preferably, it is 650 degrees C or less.

  Further, the treatment time must be determined in consideration of the structure of the target surface treatment layer and the characteristics such as hardness to be obtained, but the chromium nitride having sufficient wear resistance is mainly used in less than 1 hour. It is difficult to obtain a compound layer to be processed. Conversely, if the processing time is too long, softening of the material itself of the product to be processed increases, and there is also a problem in productivity, so the upper limit is about 50 hours. To do. More preferably, it is about 2 to 20 hours.

  After the immersion treatment in the molten salt 8, a shot peening treatment is performed with a shot media of 0.2 mm or less as necessary.

  The shot peening can be performed by a shot peening process in which a hard iron ball having a diameter of 0.2 mm or less, more preferably 0.1 mm or less, is collided at a high speed. Thus, in the surface layer portion, a compound layer mainly composed of chromium nitride is formed to 2 μm or more, an interdiffusion layer (alloy layer) of chromium and iron of 2 μm or more and a nitrogen diffusion layer are formed, and further, by shot peening, Form minute irregularities on the surface. Thus, by forming minute irregularities on the surface, for example, when used as a die for forging, in the plastic flow during forging, the contact area with the mating material to be contacted becomes smaller and slipperiness is reduced. And seizure resistance is further improved. And it becomes possible to exhibit a favorable performance in the use environment where very high surface pressure is loaded.

  In addition, since the article to be treated to which the shot peening treatment is applied is applied with a higher compressive stress on the surface portion, not only the wear resistance but also the fatigue characteristics can be improved. Therefore, for example, when it is used for a mold that is used repeatedly, durability can be improved.

  At this time, the shot peening conditions such as the material of the shot material and the projection speed can be adjusted so that the surface roughness (Ra) is 0.3 to 0.8 μm. When Ra is less than 0.3 μm, it works in a direction to suppress wear and damage of the mating material to be slid, but minute unevenness is not formed sufficiently, so that the slipperiness with the mating material in contact is important. In use, the contact area increases, and slipping tends to cause seizure, and conversely, when Ra exceeds 0.8 μm, the unevenness becomes coarse and wear and damage of the counterpart material increase.

  FIG. 2 is a diagram showing an example of a cross-sectional view of a surface-treated steel material according to the present invention.

  In this steel material, a compound layer 1 in which chromium nitride and iron nitride are mixed is formed in the surface layer portion, and a chromium-iron alloy layer 2 containing nitrogen is formed under the compound layer 1, and the alloy layer 2 A diffusion layer 3 is formed in which nitrogen diffuses in the base steel.

  That is, the steel material has little fluctuation of the diffusion component in the molten salt being processed, and in particular, the stable supply of chromium from the molten salt to the surface of the steel material results in wear resistance and heat resistance on the surface portion. The compound layer 1 mainly composed of chromium nitride, which is excellent in the above, is stably formed. Further, an alloy layer 2 is formed on the base material 4 side in which chromium important for adhesion to the base material 4 and iron in the base material 4 are interdiffused. Further, the diffusion layer 3 is formed on the base material 4 side and functions as a hardness gradient layer.

  Thereby, even in a use environment where a high load is applied, the compound layer 1 mainly composed of chromium nitride formed on the steel material surface does not peel off. Furthermore, since the fluctuation | variation of the chromium which is a spreading | diffusion component during a process becomes small, it becomes possible to suppress the surface roughness of the steel material surface low, and it becomes an excellent thing as a sliding member.

  As described above, by performing the nitriding treatment and the salt bath treatment using the salt bath treatment apparatus of the present invention, chromium having excellent wear resistance, peeling resistance, and the like without significantly deteriorating the surface roughness. The compound layer 1 mainly composed of nitride can be formed on the steel material surface.

  It is preferable that the nitrided steel material is immersed in the molten salt to form chromium nitride on the surface layer, and the surface roughness after the surface treatment immersed in the molten salt is 0.5 μm or less in terms of Ra. If Ra exceeds 0.5 μm, the wear resistance of the product to be processed will not be greatly deteriorated, but the wear and damage of the counterpart material will increase, so the processing conditions should be adjusted so that the value does not exceed 0.5 μm. Is preferred.

  Further, the surface hardness of the compound layer 1 was set to Hv 700 or more and Hv 1000 or less by adjusting the treatment conditions such as the components of the molten salt and the treatment time. That is, a steel material that is nitrided and has an iron nitride layer and a nitrogen diffusion layer is immersed in molten salt 8 containing chromium to diffuse and infiltrate chromium, thereby generating chromium nitride in the iron nitride layer of the surface layer portion. At this time, the chromium nitride is produced so that iron nitride remains so that the compound layer of the surface layer portion finally contains chromium nitride and iron nitride and the surface hardness becomes Hv 700 or more and Hv 1000 or less. Thereby, for example, even when a smaller surface roughness is required, it is possible to adjust the surface roughness by a simple polishing method that hardly reduces the thickness of the compound layer 1. It becomes.

  As described above, the surface after the salt bath treatment has a micro Vickers hardness of 700 Hv or more and 1000 Hv or less, so that it has not only wear resistance but also workability. The surface roughness can be further improved by adding a polishing process by a simple method using a lapping machine using a compound containing diamond grains in the shot material. By setting the surface roughness (Ra) to 0.3 μm or less by such a polishing treatment, it is possible to further reduce the wear and damage of the material to be slid as well as the product to be treated.

  The thickness of the compound layer 1 is at least 2 μm. Although the wear resistance is improved even when the thickness is less than 2 μm, the thickness is not less than 2 μm because it may not always be sufficient in terms of durability. More preferably, it is 3 μm or more.

  At this time, the alloy layer 2 which is an interdiffusion layer of chromium and iron containing nitrogen is formed on the base material 4 side of the compound layer 1 by controlling the treatment temperature and time, the chromium concentration in the salt bath, and the like. Can be formed. The thickness of the interdiffusion layer (alloy layer) 2 is set to 2 μm or more. If the thickness is 1 μm or more, a certain degree of adhesion can be obtained, but if it is less than 2 μm, there is a risk of peeling when a high load is applied. Therefore, in the present invention, in order to ensure sufficient adhesion, Processing is performed so that the thickness is 2 μm or more.

  Further, in addition to the presence of the interdiffusion layer (alloy layer) 2, the adhesion of the compound layer 1 is due to a synergistic effect with the diffusion layer 3 contributing as a hardness gradient layer on the base material 4 side. Can be greatly improved.

  On the other hand, in a usage environment where a very high surface pressure is applied, by performing the above-described shot peening treatment, micro unevenness is formed on the surface, improving the slipperiness with the mating material in contact with it, and improving seizure resistance. This can be further improved.

  Furthermore, by adjusting the hardness of the diffusion layer 3 including the nitriding conditions, the micro Vickers hardness of the diffusion layer 3 at a depth of 50 μm or more from the surface can be Hv 800 or less, more desirably Hv 750 or less. . In this way, heat cracks that occur on the surface of a hot mold or the like due to repeated loading of the thermal cycle are easy to propagate in areas where the hardness is high, i.e., the toughness is reduced. Since the layer 3 suppresses the development of heat cracks, it is possible to obtain a steel material having a surface structure excellent in heat crack resistance as well as wear resistance.

  Examples of the present invention will be described below.

  A 30 mm × 30 mm × 5 mm test piece subjected to quenching and tempering treatment on SCM435, SACM645, and SKD61 materials was prepared, and the surface was polished to a surface roughness (Ra) of 0.1 to 0.2 μm.

Each test piece was degreased and washed, and then subjected to fluorination treatment in a mixed gas of NF ₃ and N ₂ at 300 ° C. for 60 minutes, and these were treated at 570 ° C. for 120 minutes, NH ₃ gas being 70% by volume and N _2. Nitriding treatment was performed in a mixed gas having a gas content of 30% by volume to form a nitrogen compound layer mainly composed of 8 to 12 μm of Fe ₃ N and a nitrogen diffusion layer of 100 to 200 μm.

Thereafter, each test piece, respectively 20 wt% NaCl, KCl, CaCl ₂ and a 40 wt%, were mixed at a ratio of 40 wt%, 10 wt% and silicon chromium powder to molten salt melted by heating to 550 ° C. The powder was contained in an amount of 5% by weight, and the treatment was carried out by dipping for 20 hours while supplying N ₂ gas near the surface of the molten salt using the salt bath treatment furnace shown in FIG.

On the other hand, as a comparative example, a test piece was also produced by a method in which N ₂ gas was not supplied to the salt bath surface.

The surface hardness of each test piece of the present invention example (salt bath treatment while supplying N ₂ gas to the surface of the salt bath) and the thickness of the compound layer mainly composed of chromium nitride have a surface hardness of 900 regardless of the steel type. ± 50 Hv, the nitrogen compound layer thickness was in the range of 6 ± 1 μm. On the other hand, in the comparative example (N2 gas is not supplied to the surface of the salt bath), the surface hardness was 900 ± 100 Hv and the nitrogen compound layer thickness was in the range of 5 ± 3 μm.

  Table 1 below shows the results of measuring the surface roughness (Ra) after the salt bath treatment of the present invention example and the comparative example for each of five sheets. In addition, SKD61 no. FIG. 3 shows the cross-sectional metal structures of (a) the present invention example and (b) the comparative example in which the surface portion of the test piece 1 was corroded with the night liquid.

As is clear from Table 1, even when a long-time salt bath treatment was performed, the surface roughness of which was likely to deteriorate, the treatment was performed while supplying N ₂ gas to the salt bath surface as in the present invention example. When performed, it can be seen that the surface roughness is stable at a relatively low value regardless of the steel type. On the other hand, it can be seen that in the comparative example in which N ₂ gas is not supplied, the variation in surface roughness is very large.

  Further, in the cross-sectional structure of FIG. 3, the nitrogen compound layer mainly composed of non-corroded chromium nitride formed on the surface is formed with a stable thickness in the present invention example of (a), In the comparative example of (b), it can be seen that the thickness is not stable, and as a result, the surface roughness is deteriorated. Therefore, when the surface of the comparative example of (b) was polished to improve its surface roughness, the nitrogen compound layer mainly composed of chromium nitride was left with a thickness that had sufficient wear resistance. It turns out that it is very difficult to implement so that it may be in a state.

  FIG. 4 shows the result of measuring the element concentration distribution of the cross section of the surface portion of the example of FIG. 3 (a) using EPMA.

  In the region (i) in FIG. 4 which is the outermost surface portion, a compound layer 1 mainly composed of chromium nitride is formed with a thickness of about 5 μm. It can be seen that the compound layer 1 contains chromium, iron and nitrogen, respectively, and chromium nitride and iron nitride are mixed. The abundance ratio of chromium nitride and iron nitride in the compound layer 1 is approximately 4: 1 to 5: 1, and the abundance ratio of iron nitride gradually increases from the surface side toward the base material 4 side. ing.

  Further, an alloy layer 2 which is an interdiffusion layer of about 3.5 μm of chromium and iron containing nitrogen is formed in the region (ii) on the base material 4 side of the compound layer 1. The alloy layer 2 has a concentration gradient in which the abundance ratio of iron increases and the abundance ratio of chromium decreases as it approaches the base material 4 side from the surface side.

  Further, in the region (iii) on the base material 4 side of the alloy layer 2, since the nitrogen concentration is not high, a diffusion layer 3 that is difficult to understand from FIG. 4 and contributes as a hardness gradient layer is formed. By forming this layer structure, the compound layer 1 mainly composed of chromium nitride on the outermost surface has a very high peel resistance.

  In addition, as a result of adding a polishing process by a simple method using a lapping machine using a compound in which a test piece of the present invention contains a compound containing diamond grains in a shot material, the surface roughness (Ra) is All were in the range of 0.1 to 0.3 μm, and the thickness of the nitrogen compound layer mainly composed of chromium nitride on the surface was 4 μm or more. Therefore, when this surface treatment is applied to a machine part or the like, it is possible to further reduce the wear and damage of the material to be slid as well as the object to be treated.

  A pin having a diameter of 10 is prepared using SKD61 as a raw material, tempered to HRC50 by heat treatment, and then subjected to fluorination treatment and nitridation treatment under the same conditions as in Example 1, and then the same conditions as in Example 1 except for the treatment time A bath treatment was performed to obtain a test piece. The salt bath treatment time was 8 hours.

  At this time, on the surface portion, a compound layer 1 mainly composed of chromium nitride of about 4.5 μm, an alloy layer 2 which is an interdiffusion layer of about 3 μm of chromium and iron containing nitrogen, and a diffusion layer of about 100 μm 3 was formed, and its surface roughness (Ra) was 0.31 μm. In addition, a test piece was also prepared on the surface of which a polishing treatment using a compound containing diamond particles in a shot material was performed. The surface roughness (Ra) was 0.17 μm.

In addition, as a comparative material, a test piece that was subjected to fluorination treatment and nitriding treatment, and a test piece that was treated by a method in which N 2 gas was not supplied during the salt bath treatment were also produced. Note the N ₂ surface roughness of the gas was treated in a manner that does not supply the test specimen (Ra) of 0.58 .mu.m.

  FIG. 5 shows the results of performing a pin-on-disk type abrasion test (surface pressure 1.3 MPa, sliding speed 1.5 m / sec, sliding distance 50000 m, non-lubricated, mating material SUJ2) on these test pieces.

From this, it can be seen that the comparative example (e) in which the N ₂ gas is not supplied during the salt bath treatment is clearly improved in wear resistance as compared with the case where only the nitriding treatment is performed. The present invention example (c) in which the N ₂ gas was supplied to the liquid surface during the salt bath treatment and the present invention example (d) in which the above polishing treatment was performed on the surface thereof were compared with the comparative example (e It can be seen that it has much better wear resistance than. Further, in the comparative example (e), the wear of the SUJ2 which is the sliding counterpart material is intense, whereas in the present invention example (d) having a particularly small surface roughness, noticeable wear and surface roughness of the counterpart material are recognized. However, it is considered to have excellent durability when applied as a sliding member.

A forging die that was tempered to HRC45 by heat treatment using SKD61 as a raw material was prepared, and fluorinated in a mixed gas of NF ₃ and N ₂ for 120 minutes at 400 ° C., and then for 120 minutes at 580 ° C., NH ₃ gas Is nitrided in a mixed gas of 5% by volume and N ₂ gas of 95% by volume, and further in a mixed gas of 70% by volume of NH ₃ gas and 30% by volume of N ₂ gas at 580 ° C. Nitrided.

Thereafter, each test piece NaCl, KCl, and the CaCl _2, respectively 20 wt%, 40 wt%, were mixed at a ratio of 40 wt%, 15 wt% and silicon chromium powder in the molten salt melted by heating at 570 ° C. The powder was contained at 5% by weight, and the treatment was carried out by immersing for 10 hours while supplying N ₂ gas to the surface of the salt bath using the salt bath treatment furnace shown in FIG. FIG. 6 shows the result of measuring the cross-sectional hardness of the test piece subjected to the same tempering as this mold and the surface treatment of the present invention.

  Also, the surface of the above mold is shot peened using a hard iron ball of 0.2 mm or less, and fine irregularities are formed so that the surface roughness (Ra) is about 0.6 μm. Also prepared. As a comparative example, a mold was also prepared in which a salt bath nitronitriding treatment, which is widely used for forging mold processing, was performed at 570 ° C. for 180 minutes. The results of measuring the cross-sectional hardness of the test piece subjected to the salt bath nitronitriding treatment are also shown in FIG.

  Table 2 below shows the results of hot forging S50C connecting rods using these molds and stamping them up to the end of their lifetime up to 15000 times.

  From the results shown in Table 2, it can be seen that when the present invention is applied to a hot forging die, a significant improvement in life is observed. The reason for the improvement in the life is that the nitrogen compound layer mainly composed of chromium nitride formed on the surface is excellent in heat resistance, so that a significant improvement in wear resistance is observed.

  In addition to the compressive stress of the nitrogen diffusion layer, the hardness of the nitrogen diffusion layer is greatly reduced as compared with the conventional nitriding treatment as shown in FIG. It is thought that the progress of cracks is suppressed. At this time, a compound layer 1 mainly composed of about 6 μm of chromium nitride and an alloy layer 2 which is an interdiffusion layer of chromium and iron containing about 4 μm of nitrogen are formed on the surface portion. Was confirmed by elemental analysis by EPMA.

  Further, when minute irregularities are formed on the mold using a shot material of 0.2 mm or less, the compound layer 1 mainly composed of chromium nitride on the surface has a thickness of 4 to 5 μm. Even after 15000 times of stamping, the stamping was continued. The reason for this is considered to be that it is difficult for stress to concentrate locally and cracks are difficult to further progress due to the improved slipperiness of the material to be forged on the mold surface.

  As described above, even in a hot forging die loaded with a thermal cycle with a large temperature difference, not only the wear resistance but also seizure resistance and heat crack resistance are good, so that not only the forging die but also, for example, Similarly good results can be obtained when applied to casting molds and die casting molds.

  From the above results, by applying the present invention to various machine parts, various molds and the like, it is possible to greatly improve the durability particularly when high surface pressure or high temperature is applied.

  The treatment method of the present invention can be used for surface treatment of steel materials, and by using the surface-treated steel materials, components that are used particularly in environments where high surface pressure, high temperature and lubrication are not good. It can be suitably used for types and molds.

It is sectional drawing of an example of the salt bath processing furnace of this invention. It is the schematic diagram which showed the surface layer structure formed by the surface treatment of this invention. It is an example of the cross-sectional corrosion structure | tissue after the salt bath process of this invention example (a) and a comparative example (b). It is the figure which showed element concentration distribution of the surface part of the example of this invention of Fig.3 (a). It is a pin on disk type abrasion test result of the test piece made from SKD61. It is a cross-sectional hardness measurement result of this invention example (f) and a comparative example (g).

Explanation of symbols

1 Compound layer 2 Interdiffusion layer (alloy layer)
3 Diffusion layer 4 Base material 5 Furnace body 6 Heater 7 Treatment tank 8 Salt bath treatment agent (molten salt)
9 Impeller 10 Piping 11 Ring 12 Lid

Claims (9)

  A surface treatment method for a steel material in which the steel material is immersed in the molten salt, and at least during the immersion of the steel material in the molten salt, an inert gas layer is formed in the vicinity of the liquid surface of the molten salt to block the liquid surface from the atmosphere. A method for surface treatment of a steel material, comprising:   The surface treatment method for a steel material according to claim 1, wherein the molten salt contains chromium and the nitrided steel material is immersed in the molten salt to form chromium nitride in the surface layer portion.   The steel material surface treatment method according to claim 1 or 2, wherein a shot peening treatment is performed with a shot media of 0.2 mm or less after the immersion treatment in the molten salt.   A steel surface treatment apparatus for immersing a steel material in a molten salt, wherein at least the steel material is immersed in the molten salt and an inert gas layer is formed in the vicinity of the liquid surface of the molten salt to block the liquid surface from the atmosphere. A surface treatment apparatus for a steel material, comprising an inert gas layer forming means.   The steel material surface treatment apparatus according to claim 4, wherein the molten salt contains chromium, and the nitrided steel material is immersed in the molten salt to form chromium nitride in the surface layer portion.   The steel material according to claim 4 or 5, wherein the inert gas layer forming means forms an inert gas layer at 400 ° C or higher by supplying the inert gas heated by the heating means to the vicinity of the liquid surface of the molten salt. Surface treatment equipment. A compound layer in which chromium nitride and iron nitride are mixed is formed on the surface layer part, and an alloy layer of chromium and iron containing nitrogen is formed under the compound layer, and the base steel is formed under the alloy layer. A diffusion layer in which nitrogen diffuses is formed,
A steel material characterized in that the surface hardness of the compound layer is Hv700 or more and Hv1000 or less.   The steel material according to claim 7, wherein the nitrided steel material is immersed in a molten salt to form chromium nitride on a surface layer portion thereof, and the surface roughness after finishing the surface treatment immersed in the molten salt is 0.5 μm or less in Ra.   The steel material according to claim 7 or 8, wherein the hardness at a depth of 50 µm from the surface is Hv 800 or less.