JP2019131867A - Slurry for surface hardening and structural material manufactured using the same - Google Patents

Abstract

To provide, when forming a hardened layer including metal carbide on a treated material as a base material of a structural material, slurry for surface hardening which facilitates the treatment and prevents the base material from decreasing in carbon concentration, and to obtain a structural material improved in fatigue lifetime by the hardened layer formed using the slurry for surface hardening.SOLUTION: Slurry for surface hardening is slurry which is applied over a surface of a treated material and used to harden the treated material, and includes metal powder, a carbonizing agent as a compound containing carbon and nitrogen, and a solvent. The structural material has a base material and a hardened layer, which includes particles of carbide including metal and carbon as constituent elements. The hardened layer has a higher concentration of carbide than the base material, the carbide monotonously decreasing in concentration from a vapor phase-side surface of the hardened layer toward the center part of the base material.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a slurry for surface hardening treatment and a structural material produced using the slurry.

  There is a surface hardening method as a technique for controlling the structure in order to improve mechanical properties, corrosion resistance or functionality in an iron-based material.

  Patent Document 1 discloses a carbide containing a base material formed of steel whose carbon content decreases from the surface toward the inside, and a transition metal element of Group 4A, Group 5A or Group 6A in the periodic table. A sliding-resistant member having a film made of is described. It is described that the film made of the carbide has high hardness, and therefore, the effect that the surface of the sliding-resistant member can be made high in hardness is described. In Patent Document 1, a film is formed on the surface of the base material by immersing the base material subjected to carburizing treatment in a high-temperature molten salt bath. As an example of the molten salt bath agent, one obtained by adding Fe-V metal powder to borax is mentioned.

Patent Document 2 describes a surface-hardened steel material having an iron compound containing granular carbon and nitrogen on the surface side of an iron-based material for the purpose of improving the reliability of the steel material having a surface hardened layer. In this document, as an example, a process of carbonitriding the surface of an iron plate at 600 ° C. using an NH ₃ —CH-based mixed gas is described.

  Patent Document 3 describes a composite roll for hot rolling in which the outer layer has a structure in which graphite and MC-based carbide containing V or the like are dispersed in a metal structure of an Fe-based alloy. Yes. This outer layer is formed using molten metal.

  Non-Patent Document 1 discloses a carburizing method by applying a paste in which water glass, graphite or the like is mixed to a base material made of a metal such as steel and performing a plasma treatment.

JP, 2006-041830, A Japanese Patent Laying-Open No. 2015-206061 JP 2004-009063 A

Balanovskii et al .: IOP Conf. Series: Earth and Environmental Science 87 (2017) 092003

  In the vanadium carbide film formed on the sliding-resistant member described in Patent Document 1, vanadium is supplied from the molten salt bath, whereas carbon is supplied from the carburized substrate. For this reason, a region where the carbon concentration is lowered is generated at the boundary portion between the vanadium carbide film and the substrate side, and there is a possibility that the mechanical characteristics as the structural material may vary.

  Since the surface-hardened steel described in Patent Document 2 is carbonitrided using gas, there remains room for improvement in that the processing apparatus may become large, such as sealing a furnace used at that time.

  Since the outer layer of the hot-rolling composite roll described in Patent Document 3 is formed using molten metal, there remains room for improvement in that the processing device may become large.

  The carburizing method described in Non-Patent Document 1 does not include the concept of adding a nitrogen component.

  In the method of forming a metal carbide layer (MC layer) on the surface of the structural material, the MC layer continuous on the surface of the structural material may not be desirable from the viewpoint of adhesion to the structural material and reliability. For this reason, it is desirable that this method can cope with a case where it is desirable to have a structure in which metal carbide (MC) is dispersed without being continuous. Further, it is necessary to prevent oxidation as much as possible in the process of diffusing the metal element M and carbon C on the surface of the structural material.

  An object of the present invention is to provide a surface hardening treatment for simplifying the treatment and preventing a decrease in the carbon concentration of the base material when forming a hardened layer containing a metal carbide on the material to be treated which is a base material of the structural material. An object of the present invention is to provide a structural material having a fatigue life improved by a hardened layer formed using the slurry.

  The slurry for surface hardening treatment of the present invention is a slurry that is applied to the surface of a material to be treated and used for the hardening treatment of the material to be treated, and is a metal powder, a carbonizing agent that is a compound containing carbon and nitrogen, and a solvent. And including.

  The structural material of the present invention has a base material and a hardened layer, the hardened layer contains carbide particles containing metal and carbon as constituent elements, and the hardened layer has a carbide concentration higher than that of the base material. The carbide concentration is monotonously decreasing from the gas phase side surface of the hardened layer toward the center of the substrate.

  According to the present invention, when forming a hardened layer containing a metal carbide on a material to be treated which is a base material of a structural material, the treatment can be simplified and a decrease in the carbon concentration of the base material can be prevented. The fatigue life of the structural material can be improved.

3 is a schematic diagram showing a structure of a hardened layer formed on the structural material of Example 1. FIG. 6 is a schematic diagram showing a structure of a hardened layer formed on the structural material of Example 2. FIG. It is a graph which shows an example of distribution of the hardness in the hardened layer of a structural material. It is a graph which shows an example of distribution of the carbon concentration in the hardened layer of a structural material. It is a graph which shows an example of distribution of the vanadium density | concentration in the hardened layer of a structural material. It is a schematic cross section which shows the example of the structural material of this invention.

  The present invention relates to a slurry used for surface modification for improving fatigue strength, such as low alloy steel, and a structural material produced using the slurry.

  FIG. 6 is a schematic cross-sectional view showing the structural material of the present invention having a hardened layer.

  As shown in this figure, the structural material 50 is obtained by forming a hardened layer 54 (carbonized layer) on the surface of a base material 52 (material to be treated).

  In order to prevent a decrease in the carbon concentration of the base material 52 when forming a hardened layer 54 (hereinafter also referred to as a “metal carbide layer”) on the base material 52, a material having a higher carbon concentration than the base material 52 is used. It is effective to use and supply carbon from the outside of the substrate 52. In the structural material 50 manufactured by such a method, the hardened layer 54 is a composition gradient layer having a distribution in the ratio of elements such as carbon.

  Here, it is desirable that the material to be treated before forming the hardened layer 54 is steel and has a carbon concentration of less than 0.6 mass%.

  For example, when the material to be treated is S45C, a material having a carbon concentration of 0.45% by mass or more is brought into contact with the outside of the material to be treated and heated, so that carbon is utilized from the outside of the material to be treated using a carbon concentration gradient. Can be supplied.

  Regarding the continuity and reliability of the metal carbide layer (hereinafter, metal carbide is also referred to as “MC”), it is desirable that MC be dispersed in the material to be processed in a particle form rather than a layer form. Therefore, it is desirable that the MC is a material system in which the MC dissolves or dissolves in the material to be treated when heated. If the MC can be dissolved in the material to be processed, the MC is hardly layered and remains. As such an element M, vanadium (V) is optimal. This is because V is an element that is most solid-solved as an MC forming element in γ-Fe produced by heating a steel material.

  In order to prevent oxidation during the process, it is desirable to use an additive element effective for preventing oxidation when V is selected as the metal element M, as will be described later.

In the above case, the material used outside the material to be treated contains carbon and vanadium, and the carbon concentration of the material needs to be higher than the carbon concentration of the material to be treated. Here, with respect to the material containing carbon (C) and vanadium (V), V changes to a plurality of types of carbides V _x C _y (x and y are positive numbers), and therefore changes depending on the difference between x and y. It is necessary to consider the nature of the respective carbides to be made. The carbide having the lowest carbon concentration in the vanadium carbide system is V ₂ C. The temperature at which V ₂ C is in equilibrium with the liquid phase is about 1630 ° C. For this reason, V ₂ C can coexist with the liquid phase at a low temperature among metal elements capable of forming MC. V ₂ C (11-14 mass% carbon) has a carbon concentration higher than the carbon concentration of the material to be treated, and if the γ phase is formed by heating the material to be treated, V and C are γ It is desirable to use V ₂ C because it is easily dissolved in the phase.

In order to form V ₂ C, it is necessary to suppress formation of an oxide such as V ₂ O ₃ . When using V powder, carbon is supplied from the surface of the material to be treated while suppressing surface oxidation, and a powder containing V ₂ C is produced at low cost, and this is applied to the surface of the material to be treated. Alternatively, close contact and heat. Thereby, V and C can be diffused from the surface of the material to be processed.

  In order to provide the low-cost manufacturing process, it is necessary to employ a process in which special equipment is not required as much as possible and processing is performed in a short time. For this purpose, it is effective to mix a carbon source substance and V powder to form a slurry that can be applied, and also to mix a nitrogen-containing substance in the slurry to prevent oxidation. More specifically, alcohol, ketone or the like is used for the slurry, and an additive containing C and N (nitrogen) such as benzotriazole and imidazole is added together with the V powder. It is desirable to use such an additive containing C and N and not containing O (oxygen). This is because a film combined with V is formed on the surface of the V powder, and oxidation is prevented.

  Note that benzotriazole also serves as corrosion prevention and carbonization of metal powder.

  When measures against the above problems are adopted in the mass production process, the following results.

  The material to be treated is a material in which a γ phase is formed by heating, and any of low alloy steel and tool steel can be treated. The slurry used for processing contains 2-butanone, methanol, and ethanol. The element M selected to disperse and grow MC on the material to be processed is preferably vanadium. V powder is pulverized, benzotriazole or imidazole is added, and mixed with alcohol, ketone or the like to form a slurry.

In order to ensure the applicability of the slurry to the material to be treated, an epoxy resin or acrylic acid is added as a binder. After applying such a slurry to the surface of the material to be treated, drying is performed, the solvent is removed, and heating is performed in a temperature range where a γ phase is formed on the material to be treated. A part of carbon in benzotriazole or imidazole becomes a carbon supply source to vanadium, and a part or the whole surface of the V powder becomes V ₂ C before the temperature is raised to the γ phase formation temperature. A part of such vanadium carbide is dissolved and diffused in the γ phase of the material to be treated. When the temperature is lowered from the temperature at which solid solution and diffusion proceed, solid solution V and C precipitate as VC. This VC-based precipitate has a hardness (Vickers hardness) of 1500 Hv or more, and contributes to improved wear resistance, improved fatigue strength, and reduced friction coefficient.

  Since the metal such as vanadium and carbon are supplied to the material to be processed in this way by diffusion, both the concentration of the metal and carbon in the manufactured structural material are on the gas phase side of the metal carbide layer, except for variations. It decreases substantially monotonically from the surface toward the center (bulk) of the substrate. In other words, the metal carbide concentration decreases monotonously.

  In the present specification, “metal carbide” or “carbide” includes cases where nitrogen is contained as a constituent element in metal carbide, such as carbonitride in which part of carbon of metal carbide is substituted with nitrogen. ".

  Next, main parameters in the manufacturing process will be described.

  The maximum particle size of vanadium powder (V powder) used for the slurry is 100 μm, and the average particle size is preferably 1 to 50 μm. When the particle diameter exceeds 100 μm, the contact area with the material to be treated is small, so the amount of V and C that diffuses decreases, and the effect of increasing the hardness of the surface of the material to be treated becomes small. On the other hand, when the particle size is less than 1 μm, not only the pulverization time becomes long, but also local oxidation easily proceeds, and the particle shape tends to become non-uniform by pulverization. For this reason, the supply amount of V and C tends to be uneven.

The amount of benzotriazole or imidazole added is 1 to 20 parts by volume with respect to 100 parts by volume of the solvent. When the addition amount is less than 1 part by volume, an oxide such as V ₂ O ₃ grows on the surface of the V powder, and V ₂ C is hardly formed. When the addition amount exceeds 20 parts by volume, the viscosity of the slurry tends to increase, and the applicability decreases.

  The coating film thickness after coating and drying is preferably 10 to 200 μm. When the coating thickness of the V powder is less than 10 μm on average, the volume fraction of VC that diffuses and precipitates in the material to be processed is low, and the fatigue life improvement effect is small. When the coating film thickness exceeds 200 μm, the V powder after treatment remains undiffused and the use efficiency of V is lowered.

  Hereinafter, main parameters of the heat treatment in the manufacturing process will be described.

As for the fatigue strength improvement effect by VC, it is desirable that VC is dispersed in the matrix of martensite and retained austenite. The particle size of the dispersed VC is in the range from 10 nm to 10 μm. Here, VC includes V _x C _y (x and y are positive numbers) such as V ₈ C ₇ , V ₆ C ₅ , V ₄ C ₃ , V ₃ C ₂ , and V ₂ C. In the present specification, these are referred to as “VC compounds” or “VC carbides”.

  The volume ratio of the VC compound in the region from the surface on the gas phase side to the depth of 50 μm is preferably 1 to 30%. When the volume ratio is less than 1%, the effect of increasing fatigue strength is small. When the volume ratio exceeds 30%, particles of the VC compound are easily connected to each other, tend to become a starting point of a crack, and fatigue strength tends to decrease.

Such a VC compound is precipitated by lowering the solid solubility limit in the γ phase by holding or cooling at a temperature lower than the temperature at which V and C dissolved in the γ phase are dissolved. If the temperature at which the solid solution is formed is 1100 ° C, the VC compound is precipitated by maintaining the temperature at 900 ° C. Furthermore, by rapidly cooling from 900 ° C., the matrix can be made martensite and retained austenite, and the martensite block size can be reduced by the VC compound. Further, the additive element can be mixed into the VC compound by other additive elements. Thereby, carbides such as M ₃ C, M ₇ C ₃ , M ₂₃ C ₆ , and M ₅ C ₂ may be formed. The carbon source is V ₂ C grown from the slurry, and a VC compound can be formed on the material to be treated without using a carburizing gas such as acetylene.

  When V diffuses from the surface of the material to be processed to about 100 μm, γ crystal grain growth of the material to be processed can be suppressed. Further, carbide containing V grows at the grain boundaries and in the grains, contributing to the refinement of martensite laths and packets, and crack propagation can be suppressed.

  In the present invention, the layer structure changes from the outermost surface (surface on the gas phase side) of the structural material to the inside as follows.

The concentration of V and C decreases in the depth direction from the surface. For this reason, the volume ratio of the VC compound decreases in the depth direction from the surface. Structure in which VC compound is grown on martensite and retained austenite matrix from surface side, structure in which VC compound and M ₃ C compound are grown on martensite and retained austenite, M ₃ C compound on martensite and retained austenite Becomes a mixed structure of ferrite and pearlite.

  By forming the VC compound, the life of the sliding component can be greatly improved.

  In the following, embodiments with different processing methods will be described.

  Example 1 is an example in which a structural material was produced by applying a slurry to a material to be treated and then performing a heat diffusion treatment.

  The material to be treated is carbon steel S45C (Fe-0.2Cr-0.45C alloy).

  This material to be treated was washed with acetone to remove the oil component on the surface.

  The slurry to be applied was obtained by mixing 20 parts by mass of vanadium powder (V powder) having an average particle size of 10 μm and 20 parts by mass of benzotriazole as a carbonizing agent with 100 parts by mass of 2-butanone as a solvent.

  In order to reduce the maximum particle size of the V powder to 50 μm or less by pulverization, the above slurry or mixed solution was treated with a ball mill. In this treatment, zirconia balls having a diameter of 0.5 mm were used. The processing condition was 600 rpm for 10 hours. The slurry pulverized by this treatment was centrifuged. Thereby, impurities such as oxides were separated and removed. 5 parts by mass of an epoxy resin was added to this mixed solution to adjust the viscosity so that it could be applied to the material to be treated.

  The coating method was an immersion method. The material to be treated was immersed in the slurry whose viscosity was adjusted, and the slurry was applied to the surface of the material to be treated by pulling it up at a constant speed. The coating film thickness was about 200 μm.

  The treated material to which the slurry was applied was dried and the solvent was evaporated, followed by heating in an Ar gas atmosphere. The heating conditions were a temperature rising rate of 10 ° C./min, and a temperature of 1100 ° C. was maintained for 2 hours. After holding, it was cooled below the A1 transformation temperature and heated again. The heating holding temperature and time were 850 ° C. and 1 hour. After this heating and holding, oil hardening was performed to form a hardened layer, which was used as a test piece.

  After tempering at 180 ° C. for 2 hours, the fatigue life of the test piece was evaluated. In the life test, the life was obtained from the ratio of the pitching area using a two-cylinder fatigue test.

  About the test piece produced on said conditions, the volume ratio of VC type compound was measured by EPMA analysis. As a result, the volume ratio of the VC compound in the region from the surface to a depth of 50 μm was 20%. Similarly, the hardness in the region from the surface to a depth of 50 μm was measured and found to be 900 Hv. Here, the EPMA analysis refers to analysis by an electron probe microanalyzer (Electron Probe Micro Analyzer). According to the EPMA analysis, hardness, carbon concentration, vanadium concentration, etc. in a minute region can be measured.

  It was confirmed that the fatigue life was 10 times as a result of obtaining a relative value with a carburized material having a carbon concentration of about 0.6 mass% on the surface not using vanadium as 1.

  The process conditions for increasing the fatigue life by 10 times as described above will be described below.

  Life improvement is achieved by dispersing VC carbide particles in the base material of martensite and retained austenite, thereby reducing the crystal grain size of the base material in the range of 0.2 to 10 μm, and fine martensite blocks and packets. This is achieved. Furthermore, crack growth can be suppressed by the high hardness VC-based carbide. The VC compound includes 1 to 100 nm particles precipitated on martensite and 0.2 to 2 μm particles precipitated on grain boundaries. The former contributes to the strengthening of martensite and the latter contributes to the refinement of martensite.

  In order to disperse and grow the VC compound as in this example, an external carbon source is required. Benzotriazole, which is added to the slurry and serves as a carbon supply source, does not use an expensive carburizing furnace and forms a VC compound inside the structural material by diffusion from the slurry, and has an antioxidant effect.

  Table 1 shows the specifications of the slurry containing vanadium powder according to this example and the evaluation results of the structural material (test piece) produced using the slurry.

  In this table, Sample 1-1 shows the specifications of the material to be processed and the slurry, which are raw materials of the structural material produced by the above method, the physical properties of the obtained structural material, and the like.

  Samples 1-2, 1-3, and 1-4 are obtained by reducing the amount of benzotriazole added.

  From this table, it can be seen that in the case of Samples 1-2, 1-3, and 1-4, the volume ratio of the VC-based compound formed after the curing treatment is decreased. In particular, in the case of Sample 1-4, which is an additive-free comparative example, the fatigue life improvement effect is not recognized. This shows that the addition amount of benzotriazole is required to be 5 parts by mass or more.

  Moreover, although not shown in this table | surface, when the addition amount of benzotriazole exceeds 30 mass parts, carbon will become excess and piece-like carbide will grow on the surface of a to-be-processed material. In this case, the relative ratio of fatigue life is less than 1.

  Therefore, a desirable range of the addition amount of benzotriazole is 5 parts by mass or more and less than 30 parts by mass.

  Samples 1-5, 1-6, 1-7, 1-8, and 1-9 are cases where imidazole was used instead of benzotriazole. Sample 1-9 is an additive-free comparative example.

  Even in the case of imidazole, since the fatigue life improvement effect cannot be obtained without addition, carbonizing agents containing carbon and nitrogen are effective in forming VC compounds, and the fatigue life of structural materials is increased more than twice. It was found that it was possible to

  Next, the case where the material to be processed is SCM420 will be described.

  Samples 1-10, 1-11, 1-12, and 1-13 are cases where methanol was used as the solvent. The coating thickness is 150 μm. Sample 1-13 is an additive-free comparative example.

  In this case, when the benzotriazole is in the range of 5 parts by mass or more and 20 parts by mass or less, the volume ratio of the VC compound is 7 to 18%, and the fatigue life is 3 to 12 times.

  Samples 1-14, 1-15, 1-16, 1-17, and 1-18 are cases in which the average particle size of the V powder is 5 μm and imidazole is used as the carbonizing agent. Sample 1-18 is an additive-free comparative example.

  In this case, when the imidazole is in the range of 5 parts by mass or more and 20 parts by mass or less, the volume ratio of the VC compound is 9 to 25%, and the fatigue life is 3 to 17 times.

As described above, it has been clarified that benzotriazole and imidazole act as a carbonizing agent. Since the slurry contains a carbonizing agent, the vanadium powder is carbonized and V ₂ C grows by applying and heating the slurry of this example. That is, when the slurry is heated, vanadium contained in the slurry is carbonized and becomes a VC compound. The V and C derived from the VC compound are diffused and supplied to the inside of the material to be processed, so that the VC compound grows inside the material to be processed.

V ₂ C grows before being diffused as V and C in the material to be treated, and becomes a component necessary for the curing treatment of the surface of the material to be treated. V ₂ C grows before reaching the holding temperature of 1100 ° C., and V and C diffuse into the material to be processed while holding at 1100 ° C. The diffused V and C are solid-dissolved in the γ phase and start to precipitate when the solid solubility limit is reached. At 850 ° C., part of the solid solution V and C is precipitated. The precipitated VC compound is spherical and dispersed within the grain boundaries and grains. This VC compound contains a part of Cr and Mo added to the material to be treated.

  FIG. 1 schematically shows a typical structure of a cured layer produced in this example.

In this figure, the matrix 13 is martensite or retained austenite. The VC-based carbide 11 is dispersed and distributed in the form of particles, and grows in the grains of the grain boundaries 12 and the matrix 13. Since a part of martensite is divided by the VC-based carbide 11, the martensite structure itself becomes fine. Although not shown, M ₃ C-based carbides also exist as carbides other than the VC-based carbides 11. In the martensite, VC-based carbide 11 and other carbides are also observed. The carbide having the largest volume ratio is the VC-based carbide 11. This VC-based carbide 11 has a higher volume ratio of VC, V ₈ C ₇ , V ₄ C ₃ and V ₃ C _{2 having} a higher carbon concentration and higher hardness than V ₂ C, and such a high carbon concentration VC-based compound. The fatigue life is improved by dispersing and growing.

  In addition, even if a part of V of the VC-based carbide 11 is replaced with a metal element (Cr, Mo, Ni, Cu, Al, etc.) added in advance, and a part of carbon is replaced with nitrogen or boron, If the hardness is maintained and the dispersed state is maintained, there is no problem because there is no significant effect on the life enhancement effect.

  Example 2 is an example in which a structural material was produced by applying a slurry to a material to be treated and then performing a laser heat treatment.

  The material to be treated is S45C (Fe-0.2Cr-0.45C alloy).

  This material to be treated was subjected to ultrasonic cleaning using acetone to remove oil and fat components on the surface.

  The slurry to be applied is 100 parts by mass of 2-butanone as a solvent, 20 parts by mass of vanadium powder (V powder) having an average particle diameter of 10 μm, 5 parts by mass of titanium powder (Ti powder) having an average particle diameter of 10 μm, and a carbonizing agent. A mixture of 10 parts by mass of a certain benzotriazole was used.

  In order to reduce the maximum particle size of the V powder and Ti powder to 30 μm or less by pulverization, the above slurry or mixed solution was processed by a bead mill. In this treatment, zirconia balls having a diameter of 0.5 mm were used. The processing condition was 600 rpm for 10 hours. The slurry pulverized by this treatment was centrifuged. Thereby, impurities such as oxides were separated and removed. 5 parts by mass of carbon powder having a particle size of 2 μm and 5 parts by mass of epoxy resin were added to this mixed solution to adjust the viscosity so as to be applied to the material to be treated.

  The coating method was an immersion method. The material to be treated was immersed in the slurry whose viscosity was adjusted, and the slurry was applied to the surface of the material to be treated by pulling it up at a constant speed. The coating film thickness was about 200 μm.

  The material to which the slurry was applied was dried, the solvent was evaporated, and then locally dissolved by laser heating in an Ar gas atmosphere (laser melting and quenching). The heating conditions were 800 W, laser beam diameter 1.5 mm, and feed rate 5 mm / second.

  What was produced in this way was used as a test piece.

  For the test piece of this example as well, the volume ratio of the VC compound was measured by EPMA analysis in the same manner as in Example 1. As a result, the volume ratio of the VC compound in the region from the surface to a depth of 50 μm was 20%. The hardness in the region from the surface to a depth of 50 μm was 1020 Hv. The fatigue life was 20 times on the same basis as in Example 1.

  The process conditions for increasing the fatigue life by 20 times as described above will be described below.

  The life improvement is achieved by dispersing VC carbide particles in the matrix of martensite and retained austenite, thereby refining the crystal grain size of the matrix in the range of 0.2 to 10 μm, and using VC compound particles and TiC particles. This is achieved by miniaturizing martensite blocks and packets. Furthermore, crack growth can be suppressed by the highly hard TiC and VC compounds. TiC and VC compound particles include 1-100 nm particles precipitated in martensite and 0.2-2 μm particles precipitated in grain boundaries. The former contributes to the strengthening of martensite and the latter contributes to the refinement of martensite.

  In order to disperse and grow MC-based compounds (M is a plurality of MC-based compound-forming elements) as in this example, an external carbon source is required. Benzotriazole, which is added to the slurry and serves as a carbon supply source, disperses the VC-based compound inside the structural material by diffusion from the slurry without using an expensive carburizing furnace, and also has an antioxidant effect.

  Table 2 shows the specifications of the slurry containing vanadium powder and metal powder other than vanadium according to this example and the evaluation results of the structural material (test piece) produced using the slurry.

  In this table, Sample 2-1 shows the specifications of the material to be processed and the slurry, which are the raw materials of the structural material produced by the above method, and the physical properties of the obtained structural material.

  Samples 2-2 to 2-9 show cases where Nb, Ta, W, Zr, Mo, Cr, Ni, and Co are used as the element M instead of Ti. As a method for supplying element M, metal powder of element M was mixed with the slurry instead of Ti powder, and a structural material was produced by the same method as in Sample 2-1. In this case, the second MC compound is formed, or a part of V of the VC compound is substituted.

  As shown in this table, the fatigue life of Sample 2-1 is the highest among Samples 2-1 to 2-9. Therefore, it can be seen that when Ti is used, the lifetime improvement effect is the highest.

  Sample 2-10 is a case where Ti is used, and the ratio of V powder and metal powder (Ti powder) is changed. The solvent was changed to methanol. The coating thickness was 150 μm.

  In this case, as shown in this table, the hardness is 1050 Hv, which is slightly higher than that of Sample 2-1. On the other hand, the fatigue life is 18 times that of Sample 2-1.

  Sample 2-11 is obtained by increasing the ratio of carbon powder compared to sample 2-1. The solvent was methanol. The coating thickness was 150 μm.

  In this case, as shown in this table, the hardness is 1030 Hv, which is slightly higher than that of Sample 2-1. On the other hand, the fatigue life is 8 times, which is lower than that of Sample 2-1.

  Sample 2-12 is a comparative example showing the case where no carbonizing agent (benzotriazole) is used. The solvent was methanol. Since Sample 2-12 is mixed with a slurry in which carbon powder is mixed, the fatigue life is higher than that of Sample 1-4 in Table 1. However, the effect of improving the fatigue life is small and not sufficient. It is presumed that this is because part of the surface of the V or Ti powder is oxidized, part of oxygen is mixed into the molten alloy, and part of the molten alloy becomes bubbles.

  Samples 2-13 to 2-18 are cases where Ti is used, and the carbonizing agent is imidazole, and the coating thickness is changed in the range of 30 to 300 μm. From these results, it was found that when the coating thickness was increased, the volume ratio of the MC compound increased. The volume ratio of the MC compound was confirmed to improve the life in the range of 2 to 22%. In particular, it was found that the volume ratio of MC carbides having a fatigue life of 10 times or more is 10 to 19%. In addition, when sample thickness was 300 micrometers like sample 2-18, it turned out that although the volume ratio of MC type carbide | carbonized_material is high, a fatigue life becomes 5 times.

  From the results of Samples 2-13 to 2-18, when the volume ratio of the MC-based carbide is 10 to 19%, it is particularly effective for reducing the weight of the structural material.

  Whether to perform laser melting continuously or discontinuously with pulses can be selected according to the required specifications regarding deformation strain and deformation. In addition, by cooling after laser melting and then induction hardening, the curing depth can be increased to 1 to 2.5 mm without changing the form of MC-based particles on the surface of the structural material. is there.

  FIG. 2 schematically shows a typical structure of the cured layer produced in this example.

In this figure, the matrix 23 is martensite or retained austenite. The MC carbide 21 is dispersed and distributed in the form of particles, and grows in the grains of the grain boundaries 22 and the matrix 23. The VC carbide 24 is dispersed and distributed in the form of particles, and grows in the grains rather than the grain boundaries 22. A part of the martensite is divided by the VC carbide 24 or the MC carbide 21, so that the martensite structure itself becomes fine. The MC carbide 21 has a larger particle size than the VC carbide 24 and grows more at the grain boundaries 22. MC type carbides and other carbides are also recognized in M ₃ C type carbides and martensite as carbides other than VC type and MC type compounds, but the carbides with the highest volume fraction are MC type carbides 21.

The VC carbide 24 has a higher volume ratio of VC, V ₈ C ₇ , V ₄ C ₃ , and V ₃ C ₂ compounds having a higher carbon concentration and higher hardness than V ₂ C, and a VC system having such a high carbon concentration. The fatigue life is improved by dispersing and growing a high concentration carbon-containing MC-based compound such as a compound and TiC. Note that a part of V or M of the VC carbide 24 or the MC carbide 21 is substituted with a metal element (Cr, Mo, Ni, Cu, Al, etc.) added in advance, and a part of carbon is nitrogen or boron. As long as the hardness is maintained and the dispersed state is maintained even if it is replaced by the above, there will be no problem with no significant effect on the life enhancement effect.

  Next, other features related to the structure in the hardened layer will be described.

V grows as a plurality of carbides having a higher carbon concentration than V ₂ C as a VC compound. Its crystal structure is cubic, orthorhombic or hexagonal. Since these carbides have carbon defects, the carbon concentration is lower than VC (V: C = 1: 1) and have various lattice constants. For this reason, it has a matching interface or a non-matching interface with the base.

  In contrast, Ti has some carbon and nitrogen defects as TiC or TiCN carbonitride compounds, but has fewer carbon defects than VC compounds. The TiC type has a higher carbon concentration (a value including nitrogen) than the VC type compound. Further, Ti is difficult to dissolve in the γ phase. Ti that does not form a solid solution than V tends to grow larger as a precipitate. This is because a part of VC dissolves in the γ phase.

  Therefore, V is also contained in martensite and retained austenite. By optimizing the tempering temperature, a part of V in the martensite forms a VC compound with carbon and contributes to strengthening of the martensite. Ti contributes to grain growth inhibition and martensite packet refinement. TiC-based compounds grow and carbon in γ tends to be deficient. However, since V is solid-solved with carbon in γ, the carbon concentration is high in the V peripheral portion. Since V that is solid-solved with carbon becomes a carbon supply source to TiC, carbon deficiency is suppressed. For this reason, by selecting or diffusing or dissolving multiple elements with different solid solubility limits for the γ phase of carbide, suppression of carbon deficiency, suppression of flake carbide growth along grain boundaries, and high fatigue life and high fatigue It is possible to control the carbide structure for strengthening.

  FIG. 3 shows the hardness distribution of Sample 2-1 in Table 2. The horizontal axis represents the depth from the surface (gas phase side) of the hardened layer of the structural material, and the vertical axis represents the hardness at each depth.

  As shown in the figure, the hardness on the gas phase side is 1000 Hv, and the hardness gradually decreases toward the inside of the cured layer. The hardness is 600 Hv at a position 2 mm deep from the surface.

  FIG. 4 shows the carbon concentration distribution of Sample 2-1. The horizontal axis represents the depth from the surface (vapor phase side) of the hardened layer of the structural material, and the vertical axis represents the carbon concentration at each depth.

  As shown in the figure, the carbon concentration is 1.4% by mass on the surface on the gas phase side, and the carbon concentration decreases toward the inside of the hardened layer. In the surface layer portion from the surface to a depth of 0.5 mm, the carbon concentration is 1% by mass or more.

  FIG. 5 shows the vanadium concentration distribution of Sample 2-1. The horizontal axis represents the depth from the surface (vapor phase side) of the hardened layer of the structural material, and the vertical axis represents the V concentration at each depth.

  As shown in the figure, the V concentration is 8% by mass on the gas phase side surface, and the V concentration decreases toward the inside of the cured layer. In the surface layer portion having a depth of 0.5 mm from the surface, the V concentration is 5% by mass or more.

  Example 3 is an example in which a structural material was manufactured by performing laser heat treatment while injecting slurry.

  The material to be treated is S45C (Fe-0.2Cr-0.45C alloy).

  This material to be treated was subjected to ultrasonic cleaning using acetone to remove oil and fat components on the surface.

  The slurry injected into the heating part at the time of laser irradiation of the laser heat treatment is 20 parts by mass of vanadium powder (V powder) having an average particle diameter of 5 μm and titanium powder having an average particle diameter of 5 μm in 100 parts by mass of 2-butanone as a solvent. A mixture of 5 parts by mass of (Ti powder) and 10 parts by mass of benzotriazole as a carbonizing agent was used.

  In order to reduce the maximum particle size of the V powder and Ti powder to 10 μm or less by pulverization, the above slurry or mixed solution was processed by a bead mill. In this treatment, zirconia balls having a diameter of 1.0 mm were used. The processing condition was 600 rpm for 10 hours. The slurry pulverized by this treatment was centrifuged. Thereby, impurities such as oxides were separated and removed.

  At the same time as the slurry was injected into the heated portion of the material to be processed, laser melting was performed in an Ar gas atmosphere to cause local dissolution (laser melting and quenching). The heating conditions were 800 W, laser beam diameter 1.5 mm, and feed rate 10 mm / second.

  What was produced in this way was used as a test piece.

  For the test piece of this example as well, the volume ratio of the VC compound was measured by EPMA analysis in the same manner as in Example 1. As a result, the volume ratio of the MC compound in the region from the surface to a depth of 50 μm was 15%. The hardness in the region from the surface to a depth of 50 μm was 1100 Hv. The fatigue life was 20 times on the same basis as in Example 1.

  The process conditions for increasing the fatigue life by 20 times are the same as in Example 2.

  Example 4 is an example in which a structural material was produced by applying a slurry to a material to be treated and then performing a heat diffusion treatment. In this example, unlike Example 1, a case using titanium powder (Ti powder) instead of vanadium powder (V powder) will be described as a representative example.

  The material to be treated is S45C (Fe-0.2Cr-0.45C alloy).

  This material to be treated was washed with acetone to remove the oil component on the surface.

  The slurry to be applied was a mixture of 100 parts by mass of 2-butanone as a solvent, 20 parts by mass of titanium powder (Ti powder) having an average particle size of 20 μm, and 10 parts by mass of benzotriazole as a carbonizing agent. In this example, the metal powder such as Ti powder was different from those shown in Tables 1 and 2 and had an average particle diameter of 20 μm or 50 μm.

  In order to reduce the maximum particle size of the Ti powder to 50 μm or less by pulverization, the above slurry or mixed solution was treated with a ball mill. In this treatment, zirconia balls having a diameter of 0.5 mm were used. The treatment condition was 600 rpm for 1 hour. The slurry pulverized by this treatment was centrifuged. Thereby, impurities such as oxides were separated and removed. 5 parts by mass of an epoxy resin was added to this mixed solution to adjust the viscosity so that it could be applied to the material to be treated.

  The coating method was an immersion method. The material to be treated was immersed in the slurry whose viscosity was adjusted, and the slurry was applied to the surface of the material to be treated by pulling it up at a constant speed. The coating film thickness was about 100 μm.

  The treated material to which the slurry was applied was dried and the solvent was evaporated, followed by heating in an Ar gas atmosphere. The heating conditions were a temperature rising rate of 10 ° C./min and a temperature of 1150 ° C. and then held for 2 hours. After holding, it was cooled below the A1 transformation temperature and heated again. The heating holding temperature and time were 850 ° C. and 1 hour. After this heating and holding, oil hardening was performed to form a hardened layer, which was used as a test piece.

  After tempering at 180 ° C. for 2 hours, the fatigue life of the test piece was evaluated. In the life test, the life was obtained from the ratio of the pitching area using a two-cylinder fatigue test.

  Also for the test piece of this example, the volume ratio of the TiC compound was measured by EPMA analysis. As a result, the volume ratio of the TiC compound in the region from the surface to a depth of 50 μm was 20%. The hardness in the region from the surface to a depth of 50 μm was 900 Hv. The fatigue life was 10 times.

  Table 3 summarizes the above conditions and results.

  In this table, Sample 3-1 shows the specifications of the material to be processed and the slurry, which are raw materials of the structural material produced by the above method, and the physical properties of the obtained structural material.

  This table describes the case where Ti, V, W, Zr, Ta, Nb, Hf, or an alloy powder containing these metal elements is used as the metal powder. From this table, it can be seen that the structural material containing carbides containing these elements has a significantly improved life compared to the structural material not containing carbides.

  In this table, benzotriazole was used as the carbonizing agent, but instead of this, imidazole, 1,2,4-triazole (124T), 3-amino-1,2,4- Triazole (3AT), m3-mercapto-1,2,4-triazole (3MT), 3-carboxamide-1,2,4-triazole (3CT), 2-amino-1,3,4-thiadiazole (2ATZ), 3-aminopyrazole (3APZ) and 3-aminopyrrolidine (3AP) can be used. As the carbonizing agent, a substance not containing oxygen is suitable.

  Such a life improvement effect is remarkable even in SCM415 containing 0.15% of carbon and S50C containing 0.5% of carbon, and the base material of the material to be treated has a carbon concentration of 0.1 to 0.8%. It was found that the lifetime would be improved. If the carbon concentration of the matrix exceeds 0.8%, the carbon diffuses to the slurry application side, the layered carbide tends to grow, and the inclined layer tends to become thin. In this case, the lifetime improvement effect is 2 to 5 times.

  In Examples 1 to 4, 2-butanone or methanol was used as the solvent for the slurry, but the present invention is not limited to this, and the functional group having 6 or less carbon atoms is used as the solvent for the slurry. Can be selected from ketones having two carbon atoms and alcohols having 6 or less carbon atoms.

  Example 5 is an example of a method using plasma.

  The material to be treated is S45C (Fe-0.2Cr-0.45C alloy).

  This material to be treated was washed with acetone to remove the oil component on the surface.

  5 parts by mass of an epoxy resin was mixed with 100 parts by mass of carbon powder having a particle diameter of 5 μm, and this was applied to the surface of the object to be processed. The coating thickness was about 100 μm.

  Further, a material mainly composed of water glass containing sodium silicate (silicate) or the like was applied. The thickness of the water glass layer was about 100 μm. Water glass and carbon powder were locally mixed.

After drying in the atmosphere, a mixed gas of Ar and N ₂ was flowed, and a high voltage pulse was applied to generate plasma. The feed rate of the plasma torch was 50 to 200 mm / second.

  The surface of the material to be treated is rapidly heated and rapidly cooled by the plasma irradiation. By rapid heating, carbon is supplied from the surface, and the carbon concentration on the surface of the material to be treated can be in the range of 0.7 to 1.5 mass%.

  From the result of composition analysis, it was confirmed that the carbon concentration was hypereutectoid in the region from the surface of the material to be treated to a depth of 1000 μm.

  The state of the surface layer of the material to be treated will be further described.

Part of N _{2 in} the gas phase is decomposed by plasma irradiation and diffuses from the surface of the material to be processed. Thereby, nitrogen is supplied to the surface layer in a range where the diffusion depth is smaller than that of carbon. Residual austenite and martensite are generated by such diffusion supply of carbon and nitrogen.

  In addition, a part of hydrocarbon molecules or carbon is changed into radicals or ions by plasma. The diffusion of radicals or ions is accelerated by the electric field.

  These conditions can be set by controlling the distance between the plasma torch and the material to be treated, the thickness of the dielectric (silicate), the relative dielectric constant of the silicate and carbon powder, the gas pressure, and the like. . Thereby, the supply amount to the to-be-processed material of the carburizing, nitriding, or the metal element to add can be designed.

  The supplied carbon is changed to carbide by heating after the plasma treatment in the atmosphere. The carbide is spheroidized and then quenched to reduce the volume fraction of retained austenite in the vicinity of a depth of 100 μm to a range of 10 to 30%. This structural material has a hardness of 750 to 850 Hv at a depth of 100 μm.

The above treatment (plasma carburizing and nitriding treatment) does not require evacuation before the treatment, the plasma irradiation part is partially heated, can supply carbon and nitrogen in a short time, and select a gas type There are advantages such as being able to diffuse various elements. As the gas species, a mixed gas of Ar and N ₂ (Ar + N ₂ ), Ar + NH ₃ , Ar + CO ₂ + N ₂ , Ar + CO ₂ + NH ₃ , Ar + NH ₃ + H ₂ , acetylene, or other CH group-containing gas and N-containing gas A mixed gas, a mixed gas of He and the above gas species, or the like can be used.

As a coating material, various metallic elements (Ti, V, Cr, Co, Ni, Cu, Zr, Nb, Mo, Ta, Hf, W, etc.) are added to carbon powder and diffused together with carbon. It is possible. Some of the metal elements diffused in the surface layer are carbides such as M ₃ C, M ₇ C ₃ , M ₄ C ₃ , M ₈ C ₇ , MC (M is Ti, V, Cr, Co, Ni, Cu, Zr, Nb, Mo, Ta, Hf, W, and Fe).

Furthermore, carbonitrides in which a part of the carbon of the carbide is substituted with nitrogen can also be generated. The formation of such carbides and carbonitrides can confirm an increase in hardness, a decrease in friction coefficient, a decrease in wear, and an increase in softening resistance. Moreover, fatigue strength and bending strength increase. These elements suppress the diffusion of oxygen to the material to be processed during plasma irradiation. By forming various oxides such as TiO ₂ and Cr ₂ O _3, it is possible to combine with oxygen and suppress entry of oxygen into the material to be processed. In addition, the effect of preventing impurities such as Na from diffusing into the material to be treated has been confirmed.

  As the carbon powder, activated carbon, carbon black, carbon nanofiber, carbon nanotwist, graphene, fullerene, diamond-like carbon (DLC), or the like can be used. Further, by using a nitrogen compound such as BN instead of carbon powder, nitrogen can be infiltrated and diffused in the material to be treated.

  After the plasma carbonitriding process, the curing depth or the curing width can be adjusted in a wider range by a combination of laser quenching or induction quenching with a large curing range or processing range. Further, a wide range of carburizing treatment, nitrogen-nitriding treatment, laser alloying, high-frequency carburizing, high-frequency nitrogen nitriding, etc. can be used together. The material to be treated can be applied to steel types other than S45C, such as low carbon steel, high carbon steel, carburized steel, stainless steel, and heat resistant steel.

  This method can be carburized and nitrocarburized in a short time, and does not require vacuum exhaust equipment, so it is less expensive than ordinary carburizing / nitrogenizing equipment in terms of equipment costs. Therefore, it can be applied to the surface treatment of a structural material such as a sliding material.

  The short-time surface treatment using the atmospheric pressure discharge phenomenon as in this method can be applied to the surface modification of functional materials as well as sliding materials. Specifically, as an increase in magnetic flux by carburizing and nitriding the grain boundaries of soft magnetic materials, improving the insulation of grain boundaries such as powdered iron, increasing the magnetization of electromagnetic steel sheets, and surface treatment of permanent magnet materials , FeCo-based materials and NiFe-based materials increase coercivity, increase the magnetic transformation point, increase NdFeB-based magnet coercive force (the metal element to be diffused is at least one of various rare earth elements). Can be obtained. Moreover, it can apply as a surface treatment process to the modeling material by 3D printer (3DP).

  Example 6 is an example in which a torch for feeding and applying a mixture of water glass and carbon powder and a torch for generating plasma by flowing a plasma gas are used.

  The material to be processed is SCM415.

The carburizing source is carbon powder shielded by water glass. The surface of the material to be processed is irradiated with plasma at a frequency of 1 MHz and 20 kW. Carrier gas is a mixed gas of Ar and CO _2. The feeding speed of the plasma torch is 20 to 200 mm / second. Carburizing to a depth of 0.2 to 2 mm and quenching can be performed at a feed rate in this range. Oxidation can be prevented by flowing an Ar + H ₂ sheath gas from the outer peripheral side of the carrier gas.

  Furthermore, as an antioxidant measure, the water glass component is changed from sodium silicate to a mixture of sodium silicate and nitride or fluoride, or a mixture of sodium silicate and nitride, fluoride or carbide. It is possible to suppress the entry of decomposed oxygen from sodium silicate. Also, carbon powder is mixed with rare earth elements whose free energy of oxide formation is more negative than iron, and metal powder or carbide such as Ti, V, Mo, Nb, Zr, W, Cr, Ta, Y, etc. By doing so, it is possible to prevent oxygen from entering the SCM 415.

  As described above, it is possible to carburize the surface of a component having a complicated shape using a carbon source to be carbonized and a torch as a heat source for carburizing using a plurality of torches.

In addition to the above torch, the coating material is dried and laser-quenched using a laser-irradiated torch. In addition to carburizing, a plasma torch dedicated to nitriding is added to independently control carburizing and nitriding, and Nitrogenation can be performed around the carburized portion on the surface of the treatment material. Further nitriding a part of the carburized surface, performing carburizing and nitriding with a plurality of torches at the same time, utilizing various metal carbides (M ₃ C, M ₇ C ₃ , Carbide such as M ₂ C, M ₂₃ C ₆ , MC, etc. M is a metal element used for powder feeding or a metal element in the material to be treated. It is.

  In addition, after carburizing under a high frequency condition where a plurality of frequencies, which are plasma irradiation conditions, are carburized under a low frequency condition, it is possible to control the surface hardness and the hardness distribution.

  As high electrical resistance materials other than water glass, various oxides, oxyfluorides, and nitrides can be used. Plasma is generated in at least three kinds of materials, that is, a material to be treated, a high carbon-containing material, and a dielectric material. Using the discharge phenomenon, carbon is diffused into the heated material to be treated. By controlling the thermal energy, electric field (electric field gradient), predetermined frequency, and input voltage, the amount and depth of carbon that diffuses can be controlled.

  Usable gas species are those described in Example 5. By setting the plasma power to 100 kW to 1 MW and adjusting the frequency to an appropriate range, diffusion of the metal element supplied from the outside of the material to be processed is promoted. In this way, it is possible to disperse the various carbides, carbonitrides, or nitrides of the above elements in layers in the carburized or nitrogenated structure inside the material to be treated. Further, by adjusting the plasma output in addition to the plasma frequency, the diffusion of carbon and nitrogen and the diffusion of metal elements can be promoted.

  In order to perform such a curing process, it is necessary to perform construction with an apparatus including a control system for controlling the gas flow rate, powder feeding and power, and frequency of a plurality of torches, a positioning system, a feeding system, and a thermometer.

  With the above equipment, in addition to the structural material curing treatment, it can be applied to functional materials such as magnetic materials, thermoelectric materials, conductive materials, biomaterials, etc., such as increased magnetization, increased retention, improved corrosion resistance, increased thermoelectric treatment I can confirm.

  11: VC carbide, 12: grain boundary, 13: matrix, 21: MC carbide, 22: grain boundary, 23: matrix, 24: VC carbide, 50: structural material, 52: substrate, 54: Hardened layer.

Claims (13)

A slurry applied to the surface of the material to be treated and used for curing the material to be treated,
Metal powder,
A carbonizing agent which is a compound containing carbon and nitrogen;
A slurry for surface curing treatment, comprising a solvent.   The slurry for surface hardening treatment according to claim 1, wherein the carbonizing agent does not contain oxygen.   The metal powder is a metal composed of one or more metal elements selected from the group consisting of Ti, V, Cr, Co, Ni, Cu, Zr, Nb, Mo, Ta, Hf, W, and Fe. The slurry for surface hardening treatment of Claim 1 which is a powder.   The slurry for surface hardening treatment according to claim 1, wherein the metal powder contains V.   The slurry for surface hardening treatment according to claim 1, wherein the solvent is selected from a ketone having two functional groups having 6 or less carbon atoms and an alcohol having 6 or less carbon atoms.   The slurry for surface hardening treatment according to claim 5, wherein the solvent is 2-butanone or methanol.   The carbonizing agent is benzotriazole, imidazole, 1,2,4-triazole, 3-amino-1,2,4-triazole, m3-mercapto-1,2,4-triazole, 3-carboxamide-1,2, The slurry for surface hardening treatment of Claim 1 which is 1 or more types selected from the group which consists of 4-triazole, 2-amino-1,3,4-thiadiazole, 3-aminopyrazole, and 3-aminopyrrolidine.   Furthermore, the slurry for surface hardening treatment of Claim 1 containing carbon powder. A substrate;
A cured layer,
The hardened layer includes carbide particles including metal and carbon as constituent elements,
The hardened layer has a higher concentration of the carbide than the base material,
The structural material in which the concentration of the carbide monotonously decreases from the gas phase side surface of the hardened layer toward the center of the base material.   The structural material according to claim 9, wherein a volume ratio of the carbide in the hardened layer is 1 to 30%.   10. The carbide according to claim 9, wherein the carbide includes one or more metal elements selected from the group consisting of Ti, V, Cr, Co, Ni, Cu, Zr, Nb, Mo, Ta, Hf, W, and Fe. Structural material.   The structural material according to claim 9, wherein the carbide includes V.   The structural material according to claim 9, wherein the carbide includes nitrogen.

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