WO2019208549A1

WO2019208549A1 - Sliding member and manufacturing method thereof and power steering device and manufacturing method thereof

Info

Publication number: WO2019208549A1
Application number: PCT/JP2019/017164
Authority: WO
Inventors: 波東　久光; 小室　又洋; 佐通　祐一; 高太郎椎野; 幸二沼倉; 信太郎辻見
Original assignee: 日立オートモティブシステムズ株式会社
Priority date: 2018-04-27
Filing date: 2019-04-23
Publication date: 2019-10-31
Also published as: JP6998265B2; JP2019189925A

Abstract

The present invention addresses the problem of providing a sliding member less susceptible to decreased durability and having improved hardness and a manufacturing method thereof, and a power steering device and manufacturing method thereof. This power steering device has a sliding member (10a) characterized by containing a steel base material (3), granular iron carbide (2) contained in the base material, and metallic carbide (1) contained in the base material and containing metal and carbon as constituent elements, wherein the volume ratios of the iron carbide (2) and the metallic carbide (1) decrease along the depth direction from the surface of the base material toward the inside, and the metal is one or more selected from the group consisting of Ti, V, Nb, Ta, Zr, W, Hf, and Mo.

Description

Sliding member and manufacturing method thereof, power steering device and manufacturing method thereof

The present invention relates to a sliding member, a manufacturing method thereof, a power steering device, and a manufacturing method thereof.

It is known that the surface of sliding members such as bearings, ball screws, chains, cams, gears, and splines may cause fatigue damage called pitting or flaking as they slide. Since these fatigue damages reduce the life of the sliding member, various methods for improving the fatigue strength of the sliding portion have been proposed. Moreover, there is also an advantage that parts can be reduced in size and cost by improving the fatigue strength of the sliding portion.

In Patent Document 1, in a steel sliding member, a large number of hard particles having a hardness equal to or higher than the surface hardness of the hardened layer are scattered in a crystalline state on the surface layer portion of the hardened layer obtained by heat treatment. Proposed sliding members have been proposed. These hard particles are oxides, carbides or nitrides of any of the elements Cr, V, Ti and Nb originally contained in the sliding member. The thickness of the hardened layer particles included in the martemper treatment is 20 to 30 μm. It is shown to form to the extent. A large number of hard particles exposed on the surface of the hardened layer exert a so-called “anchor effect” and try to stay there, so that the surface of the hardened layer around the hard particles is moved by the sliding of the sliding member. It is said that it is possible to effectively prevent plastic flow, reduce adhesive wear on the surface of the sliding member, and improve wear resistance.

Japanese Patent Laying-Open No. 2015-14300

Since sliding members such as bearings, ball screws, chains, cams, gears, and splines are composed of a combination of two or more parts facing each other, only the parts that are easily damaged (hereinafter referred to as reinforced parts) are surfaces. Even if a strengthening process is performed to harden, depending on the hardness of the opposing parts (hereinafter referred to as relative parts), there is a possibility that the reinforcing parts may attack and damage the relative parts. For this reason, when there is a possibility that the relative part may be damaged, it is necessary to perform a strengthening process so that the relative part also has a life equivalent to that of the strengthened part.

From this background, it may be necessary to adjust the hardness of one or both members to the extent that the relative parts are not damaged. In order to increase the surface hardness of the reinforced parts to such an extent that the relative parts are not damaged, it is considered that this can be achieved by reducing the amount of metal carbides described as “hard particles” in Patent Document 1, for example. By reducing the interstitial amount of the metal carbide, the distance between the metal carbides is increased, so there is a concern that the anchor effect of the metal carbide is reduced and the durability is lowered.

In view of the circumstances described above, an object of the present invention is to provide a sliding member with enhanced hardness while suppressing a decrease in durability, a manufacturing method thereof, a power steering device, and a manufacturing method thereof.

A first aspect of the present invention that solves the above problems includes a steel base material, granular iron carbide contained in the base material, a metal carbide contained in the base material and containing metal and carbon as constituent elements, and The volume fraction of iron carbide and metal carbide decreases along the depth direction from the surface to the inside of the substrate, and the metal is a group consisting of Ti, V, Nb, Ta, Zr, W, Hf and Mo It is a sliding member characterized by being one or more types selected from.

According to a second aspect of the present invention, there are provided a nut having a female thread groove formed on an inner peripheral surface, a rack screw disposed on an axial center of the nut and having a male screw groove opposed to the female screw groove, and a female screw. A ball screw mechanism having a plurality of circulating balls interposed between the groove and the male screw groove; a steering gear case that houses the ball screw mechanism; an electric motor that is used to drive the ball screw mechanism; And a power transmission mechanism for transmitting the rotational driving force of the motor to the nut, wherein the rack screw or the circulating ball is the sliding member of the present invention.

According to a third aspect of the present invention, there is provided a metal infiltration step in which one or more kinds of metals selected from the group consisting of Ti, V, Nb, Ta, Zr, W, Hf, and Mo are infiltrated into a steel substrate; A carbon permeation step for permeating carbon into the base material, and the carbon permeation step is a method for producing a sliding member characterized in that the base material is permeated with carbon at a concentration that causes hypereutectoid.

According to a fourth aspect of the present invention, there are provided a nut having a female screw groove formed on the inner peripheral surface, a rack screw disposed on the axial center of the nut and having a male screw groove opposed to the female screw groove on the outer peripheral surface, and a female screw. A ball screw mechanism having a plurality of circulating balls interposed between the groove and the male screw groove; a steering gear case that houses the ball screw mechanism; an electric motor that is used to drive the ball screw mechanism; In a method of manufacturing a power steering apparatus having a power transmission mechanism that transmits a rotational driving force of a motor to a nut, a rack screw or a circulating ball is constituted by the sliding member of the present invention, and the sliding member is of the present invention. It is a manufacturing method of a power steering device characterized by being manufactured by a manufacturing method of a sliding member.

More specific configurations of the present invention are described in the claims.

According to the present invention, it is possible to provide a sliding member with enhanced hardness while suppressing a decrease in durability, a manufacturing method thereof, a power steering device, and a manufacturing method thereof.

Issues, configurations, and effects other than those described above will be clarified by the following description of embodiments.

Schematic diagram of the sliding member of Example 1 Observation photograph of the cross section of Example 1 SEM observation photograph of the part corresponding to part A (part near the surface) in FIG. 2A and the result of elemental analysis by EPMA SEM observation photograph of the part corresponding to B part (the part 500 μm deep from the surface) in FIG. 2A and the result of elemental analysis by EPMA Schematic diagram of the sliding member of Example 2 Laser microscope image of the dotted line in Fig. 3A Schematic diagram of the sliding member of Example 3 Laser microscope image of the dotted line in Fig. 4A Schematic diagram of the equipment used in the thrust fatigue test Schematic diagram of the test piece of Comparative Example 1 Observation photograph of the tissue in FIG. 6A Schematic diagram of the test piece of Comparative Example 2 SEM observation photograph of Fig. 7A Mapping by EPMA in Fig. 7B Schematic diagram of test piece of Comparative Example 3 Observation photograph of the tissue in Fig. 8A The graph which shows the result of the thrust fatigue test of Examples 4-6 and Comparative Examples 1-3 The graph which shows the result of the thrust fatigue test of Example 4, 7-10 and the comparative example 1 The graph which shows the relationship between the hardness difference of the test piece 5 of Example 4, 7-10, and the ball 6, and a life ratio. Cross-sectional schematic diagram of an electric power steering device Enlarged view of the ball screw mechanism of FIG. Schematic diagram of apparatus for applying slurry to rack screw 13 Schematic diagram of heat treatment apparatus used in Example 11 The graph which shows the durability test result of the electric power steering apparatus of Example 11 and Comparative Example 4 Schematic diagram showing an example of equipment for laser alloying of thrust fatigue test specimens Schematic diagram of the structure of Example 12, a macro photograph taken with a microscope showing a laser alloying region, and a detailed structure photograph composed of laser alloying Schematic diagram of an apparatus that performs laser alloying treatment on the external thread of the rack screw 13 The flowchart which shows an example of the manufacturing method of the sliding member of this invention Iron-carbon equilibrium diagram The enlarged view of the location where the rack screw 13 which comprises the ball screw mechanism of the electric power steering apparatus of Example 14 and the ball | bowl 6 contact. The figure which expands the surface of the rack screw 13 of FIG.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, it is not limited to the embodiment taken up here, and combinations and improvements can be appropriately made without departing from the scope of the invention.

The present invention will be described in detail based on the following Examples 1 to 14.

[Example 1]
FIG. 1 is a schematic diagram of a sliding member according to the first embodiment. As shown in FIG. 1, the sliding member 10 a has a steel base 4 and a metal carbide 1 and an iron carbide 2 contained in the base 4. The metal carbide 1 is scattered inside the base material 4, and the iron carbide 2 is scattered inside the base material 4 in the form of particles. The volume ratios of the metal carbide 1 and the iron carbide 2 are reduced along the depth direction from the surface (surface on the gas phase side) 40 of the substrate 4 to the inside.

The base material 4 is made of steel. In FIG. 1, a portion (also referred to as a hard layer, a surface reinforcing layer, etc.) where the metal carbide 1 and the iron carbide 2 of the substrate 4 are present has a martensite layer 3 having a martensite structure.

The metal carbide (hard particles) 1 contains metal and carbon (C) as constituent elements, and the metals are titanium (Ti), vanadium (V), niobium (Nb), tantalum (Ta), zirconium ( One or more selected from the group consisting of Zr), tungsten (W), hafnium (Hf), and molybdenum (Mo), and in this embodiment, vanadium. The metal carbide is preferably a one-to-one bond between metal and carbon. Such metal carbide has a particularly high hardness.

2A is an observation photograph of a cross section of Example 1. FIG. FIG. 2B shows the SEM (Scanning Electron Microscope) observation photograph in the vicinity of the surface corresponding to the A portion (part near the surface) in FIG. 2A and the result of elemental analysis by EPMA (Electron Probe Micro Analyzer). FIG. 2C is a SEM observation photograph of a portion corresponding to portion B in FIG. 2A (portion deep by 500 μm from the surface) and the result of elemental analysis by EPMA.

From FIG. 2B, vanadium carbide (VC) 50 formed by combining vanadium 50 and carbon as metal elements and iron carbide 2 formed by combining iron and carbon are present in the martensite layer 3 in the vicinity of the surface of the substrate. It can be seen that it is scattered in granular form. Such granular iron carbide can inhibit the growth of martensite and suppress the coarsening of the martensite layer 3. On the other hand, in the case of acicular iron carbide, it is difficult to suppress the coarsening of the martensite layer 3. FIG. 2C shows that vanadium carbide 50 and iron carbide 2 are scattered in the martensite layer 3 at a depth of about 500 μm from the surface. Moreover, from FIG. 2B and FIG. 2C, the volume ratio of the vanadium carbide 50 and the iron carbide 2 is decreasing along the depth direction from the surface 40 of the base material 4 to the inside. That is, it can be seen that the concentrations of vanadium carbide 50 and iron carbide 2 in the substrate 4 decrease from the surface of the substrate 4 to the inside.

With the above-described configuration, the hardness difference of the parts constituting the sliding member is suppressed to a certain value or less, the thickness of the hardened layer is sufficiently deeper than the depth at which the maximum shear stress derived from Hertz's contact theory occurs, and A sliding member having a hardened layer sufficiently exceeding the grinding allowance can be obtained.

The sliding member can be used for, for example, a rack screw, a ball and a nut of a power steering device which will be described later. Since the rack screw and the ball, and the ball and the nut are in contact with each other with a force applied thereto, if the difference in hardness between the two members in contact is large, one of the members may damage the other member. In other words, the maximum Hertz stress derived from Hertz's contact theory generated by receiving a load is the largest for the rack screw and ball and the nut is smaller than that due to the geometry of the ball screw. For this reason, when the surface treatment which makes a rack screw a reinforcement | strengthening part and to which a hard particle is scattered is given and hardness is made high, the ball | bowl which is a relative part may be damaged.

In view of the above background, it is necessary to be able to arbitrarily adjust the hardness of the hard layer in order to suppress the hardness difference of the parts constituting the sliding member to a certain value or less. The sliding member 10a of this example includes a martensite layer (Vickers hardness: 500 Hv to 800 Hv) serving as a base material of a hard layer, and a metal carbide 1 (Vickers hardness: about 2500 Hv) dispersed in the martensite layer. Also, containing iron carbide 2 (Vickers hardness: about 1200 Hv) dispersed in the martensite layer, adjusting the content of metal carbide 1 and iron carbide 2 and the content ratio of metal carbide 1 and iron carbide 2 Thus, the hardness of the hard layer can be adjusted. Specifically, in the manufacture of the sliding member 10a, the metal element 1 and the carbon infiltration conditions are controlled by adjusting the metal element infiltration conditions and the carbon infiltration conditions. The content of iron carbide 2 can be adjusted.

It is possible to adjust the hardness by simply reducing the amount of metal carbide, but in this case, the anchor effect is reduced because the distance between the metal carbides increases. Here, the anchor effect is an effect of suppressing the growth and coarsening by causing the granular particles to act as anchors when the steel structure grows. By suppressing the coarsening of the tissue, it is possible to prevent a portion having a weak strength from being partially generated. While reducing the amount of metal carbide, it is necessary to add iron carbide 2 having intermediate hardness between metal carbide 1 and martensite layer 3 of base material 4 while suppressing deterioration in durability due to reduction in anchor effect. The hardness can be adjusted.

Here, in order to exhibit the anchor effect, the iron carbide needs to be present in a granular form. For example, pearlite and cementite, which are layered and needle-like structures that may be originally included in steel materials, are insufficient. . The diameter of the granular iron carbide is, for example, 0.5 μm or more and 5 μm or less (1 μm or more and 5 μm or less if the grain boundary of the steel structure, 0.5 μm or more and 1 μm or less if it is within the grain), and the aspect ratio is 1 or more and 5 or less. It is as follows.

In addition, in order to produce | generate iron carbide 2, it is necessary to osmose | permeate the carbon more than the density | concentration (carbon is about 0.8 weight% or more) used as hypereutectoid. FIG. 21 is an iron-carbon equilibrium diagram. As shown in FIG. 21, granular iron carbide is obtained by carburizing from the austenite state of hypereutectoid or higher, cooling to Acm line or lower, precipitating carbon that cannot be completely dissolved, and fixing by a quenching process. Can be provided. Here, a method of lowering to a point (for example, (1) in FIG. 21) higher than the A1 transformation point (about 727 ° C.) (for example, (1) in FIG. 21) and once lowering to a point below the A1 transformation point (for example, 600 ° C.). Any of the methods of reheating to 850 ° C. ((2) in FIG. 21) may be used. Each step will be described in Example 1 and Example 11 described later. Here, the metal elements Ti, Nb, V, Zr, and W are more preferable because they constitute the metal carbide 1 having high hardness. The hardness difference of the hard layer of the sliding member is preferably about 200 Hv or less in terms of Vickers hardness.

FIG. 20 is a flowchart showing an example of the manufacturing method of the sliding member of the present invention. As shown in FIG. 20, the manufacturing method of the sliding member of the present invention includes a step (S1) of applying a slurry containing metal to a base material, and a step (permeation) of the metal to the base material by heat treatment ( S3), a step of introducing a gas containing carbon (S3), and a step of heat-treating the base material with carbon (S4). Hereinafter, an example of a method for manufacturing a sliding member having the structure shown in FIGS. 1 to 2C will be described.

The material constituting the base material 4 is a carbon steel material S45C for mechanical structure defined by Japanese Industrial Standards JIS G 4501, and is a steel material containing about 0.45 mass percent carbon C. A round bar sample having an outer diameter of 10 mm and a length of 50 mm was used as the substrate 4.

In 2-butanone, vanadium powder having an average particle diameter of 50 μm and benzotriador were mixed with 20% by mass and 5% by mass of epoxy, respectively, to prepare a slurry. An S45C round bar sample was immersed in the prepared slurry, and a slurry film was formed on the outer surface of the S45C round bar sample. The outer surface of the S45C round bar sample was heated and held at a high frequency so that vanadium diffused by about 500 μm. In this embodiment, the temperature is 1300 ° C. and the holding time is 5 minutes. Thereafter, carburizing gas was flowed to diffuse about 500 μm of carbon on the outer surface of the S45C round bar sample. In the case of this example, the temperature was 1200 ° C., the holding time was 5 minutes, and acetylene was used as the carburizing gas.

After that, the temperature of the S45C round bar test piece was lowered to about 850 ° C., and water-soluble quenching oil was sprayed onto the outer surface of the S45C round bar test piece to perform quenching. The tempering was performed using an electric furnace at a temperature of 180 ° C. and a holding time of 90 minutes. The toughness can be improved by tempering. This S45C round bar sample was cut and polished to prepare the observation sample described above.

Since the penetration path of vanadium and carbon is mainly a grain boundary, in order to prevent the selective formation of vanadium carbide and iron carbide 2 at the grain boundary, the vanadium is heated at a temperature of 1300 ° C., a holding time of 5 minutes, and carbon. , 1200 ° C., holding time: 5 minutes, and then the temperature is lowered to a temperature below the A1 transformation point (about 727 ° C.) of the iron-carbon parallel phase diagram (eg, about 727 ° C.), for example, 600 ° C. It is more preferable to reheat and quench.

Moreover, the Vickers hardness of the cross section at a position about 50 μm deep from the surface was 850 Hv to 950 Hv. In this example, vanadium was used as the metal element. However, as in the case of vanadium, 1 selected from the group consisting of titanium, niobium, tantalum zirconium, tungsten, hafnium, and molybdenum constituting a high-hardness metal carbide. More than one kind of metal element may be used.

[Example 2]
3A is a schematic diagram illustrating an example of the sliding member of Example 2, and FIG. 3B is a laser microscope observation photograph of the dotted line portion of FIG. 3A. The sliding member 10b shown in Example 2 has the metal carbide 1 and the iron carbide 2 scattered in the martensite layer 3 of the base material 4 similarly to Example 1, and the metal carbide 1 and the iron carbide 2 The volume ratio decreases along the depth direction from the surface 40 of the substrate 4 to the inside. Further, the iron carbide 2 exists in a portion deeper than the metal carbide 1 (a portion surrounded by a dotted line in FIG. 3A) inside the base material 4. In the sliding member 10b, the hard layer extends from the surface to the layer where the iron carbide 2 exists.

FIG. 3B shows a state where the iron carbide 2 has penetrated to the inside of the base material more than the metal carbide 1. FIG. 3B is a laser micrograph obtained by etching the dotted line portion of the base material 4 of FIG. 3A with sodium picrate soda that the iron carbide 2 is easily etched. As shown in FIG. 3B, it can be seen that iron carbides 2 etched in black are scattered in the martensite layer 3.

When the metal element has a shorter penetration distance into the substrate than carbon, it is not practically practical to make the hard layer containing metal carbide particles and iron carbide particles generated by the metal element penetration sufficiently thick There is. Therefore, the hard layer containing both metal carbide particles and iron carbide particles is suppressed to the minimum necessary thickness, and iron carbide particles composed of iron element and carbon bonded in martensite are dispersed inside the hard layer. It is industrially realistic to provide the hardened layer. In other words, metal carbide that takes a long time to penetrate is retained near the surface, and iron carbide that is easy to penetrate penetrates deep into the interior to achieve the desired strength, thereby allowing the metal carbide particles and iron carbide particles to penetrate to the same depth. Manufacturing time can be shortened.

It is desirable that the hardened layer containing metal carbide particles and iron carbide is sufficiently deeper than the depth at which the maximum shear stress derived from Hertz's contact theory is generated, and has a surface enhancement layer thickness sufficiently exceeding the grinding allowance, A thickness obtained by adding about 2 times or more of the depth at which the maximum shearing stress derived from the grinding allowance and Hertz's contact theory occurs is preferable.

Hereinafter, an example of a method for producing a sliding member having the structure shown in FIGS. 3A and 3B will be described. The material which comprises the base material 4 is the steel material S45C for mechanical structures prescribed | regulated by Japanese Industrial Standards JISG4501, and is a steel material containing about 0.45 mass percent carbon. A round bar sample having an outer diameter of 10 mm and a length of 50 mm was used as the substrate.

In 2-butanone, vanadium powder having an average particle diameter of 50 μm and benzotriador were mixed with 20% by mass and 5% by mass of epoxy, respectively, to prepare a slurry. An S45C round bar sample was immersed in the prepared slurry, and a slurry film was formed on the outer surface of the S45C round bar sample. The outer surface of the S45C round bar sample was heated and held at a high frequency so that vanadium diffused by about 500 μm. In this example, the temperature is 1300 ° C. and the holding time is 5 minutes. Then, carburizing gas was flowed, and carbon C was diffused by about 1000 μm on the outer surface of the S45C round bar sample. In the case of this example, the temperature was 1200 ° C., the holding time was 20 minutes, and acetylene was used as the carburizing gas. Compared to Example 1, the carbon penetration time was increased from 5 minutes to 20 minutes. This is a treatment for allowing the iron carbide 2 to penetrate deeper than the metal carbide 1.

After that, the temperature of the S45C round bar test piece was lowered to about 850 ° C., and water-soluble quenching oil was sprayed onto the outer surface of the S45C round bar test piece to perform quenching. The tempering was performed using an electric furnace at a temperature of 180 ° C. and a holding time of 90 minutes. This S45C round bar sample was cut and polished to prepare the observation sample described above.

Since the penetration path of vanadium and carbon is mainly a grain boundary, in order to prevent the selective formation of vanadium carbide and iron carbide at the grain boundary, vanadium is heated at a temperature of 1300 ° C., a holding time of 5 minutes, Temperature: 1200 ° C., holding time: 20 minutes After soaking, the temperature is lowered to a temperature below the A1 transformation point (about 727 ° C.) of the iron-carbon parallel phase diagram (about 727 ° C.), for example, 600 ° C. It is more preferable to carry out heating and quenching.

Moreover, the Vickers hardness of the cross section at a position about 50 μm deep from the surface was 850 Hv to 950 Hv. The Vickers hardness of the cross section at a position about 600 μm deep from the surface where the iron carbide 2 penetrates to the inside of the base material than the metal carbide 1 was 750 Hv to 850 Hv.

[Example 3]
4A is a schematic diagram illustrating an example of the sliding member of Example 3, and FIG. 4B is a laser microscope observation photograph of the dotted line portion of FIG. 4A. As shown in FIG. 4A, the sliding member 10c has a martensite layer 3 constituting the base material, and a metal carbide 1 and an iron carbide 2 contained in the martensite layer 3, and the metal carbide 1 and the iron carbide 2 The volume ratio decreases along the depth direction from the surface of the substrate to the inside. Furthermore, the martensite layer 3 is formed to a deeper part than the layer in which the metal carbide 1 and the iron carbide 2 exist. In the sliding member 10c, the hard layer extends from the surface to the martensite layer 3 formed to a deeper portion than the layer in which the metal carbide 1 and the iron carbide 2 are present.

FIG. 4B is a laser micrograph obtained by etching the dotted line portion of the base material 4 in FIG. 4A with sodium picrate acid so that the iron carbide 2 is easily etched. As shown in FIG. 4B, it can be seen that there is a needle-like martensite layer 3 etched in black.

A hardened layer having metal carbide 1 and iron carbide 2 by providing a hardened layer having metal carbide 1 and iron carbide 2, a layer having iron carbide inside thereof, and a hardened layer made of martensite inside thereof. And the freedom degree of the thickness of the layer which has the iron carbide 2 inside it increases. Moreover, the manufacturing process can be simplified and the strength can be increased by adjusting the required hardness while leaving a portion where the metal carbide and iron carbide do not penetrate.

That is, in order to construct a hardened layer having metal carbide 1 and iron carbide 2, it is necessary to infiltrate the metal element and carbon, and the amount of penetration and the depth of penetration are governed by temperature and time. And it is industrial to keep it to the minimum necessary thickness. Similarly, in order to construct a hardened layer having iron carbide 2, it is necessary to infiltrate carbon, and since the amount of penetration and the depth of penetration are governed by temperature and time, the required hardness and the minimum thickness required It is industrial to keep it down.

Therefore, when the hardened layer having the metal carbide 1 and the iron carbide 2 and the internal structure of the hardened layer having the iron carbide 2 are as soft as the base material, the hardening having the metal carbide 1 and the iron carbide 2 on the soft layer. Even if the hardened layer having the layer and the iron carbide 2 is configured, and the hardened layer having the metal carbide 1 and the iron carbide 2 and the hardened layer having the iron carbide 2 resist the load acting on sliding, It can happen that a soft structure is fatigued and destroyed like a base material inside a hardened layer having iron carbide 2. For example, an iron plate is placed on tofu and a heavy object is placed on the iron plate and slid.

From this, considering the industrial feasibility, a configuration in which a hardened layer having metal carbide 1 and iron carbide 2 and a hardened layer made of martensite inside the hardened layer having iron carbide 2 is preferable. . A hardened layer made of martensite can be easily formed by quenching. The main element constituting the hardened layer made of martensite is carbon. For example, if the steel contains 0.15% by mass or more of carbon, the hardened layer made of martensite can be formed by quenching.

In addition, the thickness of the hardened layer made of martensite is a steel base material containing a predetermined amount of an alloy element such as manganese, chromium, nickel and molybdenum for improving the hardenability. The thickness of the hardened layer composed of martensite can be adjusted. Moreover, if it is a steel base material in which carbon is less than 0.15 mass percent, the hardening layer which consists of a martensite can be comprised by infiltrating carbon to the required depth of a steel base material.

Hereinafter, an example of a method for producing a sliding member having the structure shown in FIGS. 4A and 4B will be described. The material constituting the base material 4 is a carbon steel material S45C for mechanical structure defined by Japanese Industrial Standards JIS G 4501, and is a steel material containing about 0.45 mass percent carbon C. A round bar sample having an outer diameter of 10 mm and a length of 50 mm was used as the substrate.

In 2-butanone, vanadium powder having an average particle diameter of 50 μm and benzotriador were mixed with 20% by mass and 5% by mass of epoxy, respectively, to prepare a slurry. An S45C round bar sample was immersed in the prepared slurry, and a slurry film was formed on the outer surface of the S45C round bar sample. The outer surface of the S45C round bar sample was heated and held at a high frequency so that vanadium diffused by about 500 μm. In this embodiment, the temperature is 1300 ° C. and the holding time is 5 minutes. Thereafter, a carburizing gas was flowed to diffuse about 2000 μm of carbon on the outer surface of the S45C round bar sample. In this example, the temperature was 1200 ° C., the holding time was 80 minutes, and acetylene was used as the carburizing gas.

Compared with Example 2, the carbon penetration time was increased from 20 minutes to 80 minutes. This is a treatment for penetrating the iron carbide 2 deeper than the metal carbide 1 and further forming the martensite layer 3 therein. Here, the carbon concentration of the martensite layer 3 can be prevented from generating particulate iron carbide by permeating carbon below eutectoid (carbon is about 0.8 mass% or less). The carbon concentration can be controlled by adjusting the amount of carburizing gas.

Since the permeation path of vanadium and carbon is mainly a grain boundary, in order to prevent the selective formation of vanadium carbide and iron carbide at the grain boundary, vanadium V is maintained at a temperature of 1300 ° C. and a holding time of 5 minutes. Was allowed to permeate at a temperature of 1200 ° C. and a holding time of 80 minutes, and then the temperature was lowered to a temperature below the A1 transformation point (about 727 ° C.) of the iron-carbon parallel phase diagram, for example, 600 ° C. It is more preferable to reheat and quench.

The Vickers hardness at a position about 50 μm deep from the surface was 850 Hv to 950 Hv. The Vickers hardness of the cross section at a position about 600 μm deeper from the surface in which the iron carbide 2 penetrated to the inside of the substrate than the metal carbide 1 was 750 Hv to 850 Hv. The Vickers hardness of the cross section of the martensite layer 3 at a position 1100 μm deep from the surface was 550 Hv to 700 Hv.

[Examples 4 to 6 and Comparative Examples 1 to 3]
The effects of Examples 1 to 3 described above were confirmed by a thrust fatigue test. FIG. 5 is a schematic view of an apparatus used for the thrust fatigue test. The thrust fatigue test is a test method that simulates a thrust bearing. The test piece 5 is a test piece for evaluating the fatigue strength of the surface treatment, and is subjected to the surface treatment to be evaluated. The test piece 5 has an outer diameter of 57 mm and a thickness of 5 mm. The ball 6 is a ball used in a thrust bearing. In this test, the ball diameter was 3/8 inch and the number of balls was 3 and arranged every 120 °. The diameter of the track on which the ball 6 rolls is 38.5 mm. The material of the ball 6 is high carbon chrome bearing steel SUJ2 defined by Japanese Industrial Standard JIS G 4805.

The Vickers hardness of the cross section at a position 50 μm deep from the surface of the SUJ2 ball 6 used in this test was in the range of 720 Hv to 780 Hv. The outer ring 7 was a thrust bearing outer ring and a commercially available product was used. The test piece 5, the ball 6 and the outer ring 7 are set in the test tank 8, the lubricating oil 9 is put in the test tank 8, set in the test apparatus main body (not shown), and a load 10 so as to obtain a predetermined test surface pressure. Then, rotation 11 was applied using a motor (not shown) of the test apparatus main body, and the relationship between the number N of times the test piece 5 was damaged and the test surface pressure P was obtained as fatigue characteristics. As the lubricating oil 9, a naphthenic kinematic viscosity of 8.46 mm2 / S (40 ° C.) was used. The test surface pressure is the maximum Hertz stress determined from Hertz's contact theory. The thrust fatigue test was performed on the following six types of test pieces in total.

Example 4: Specimen corresponding to Example 1 Example 5: Specimen corresponding to Example 2 Example 6: Specimen corresponding to Example 3 Comparative example 1: Induction-hardened test piece having a reference life Comparative example 2: Test piece in which vanadium was infiltrated in Example 4 but not infiltrated with carbon Comparative Example 3: Test piece in which vanadium was not infiltrated in Example 5 and only carbon was infiltrated
The base material of the test piece is a steel material S45C for machine structural use specified by Japanese Industrial Standard JIS G 4501, and is a steel material containing about 0.45 mass percent carbon C.

Example 4 is a test piece corresponding to Example 1. A slurry in which vanadium powder having an average particle size of 50 μm and benzotriador were mixed in an amount of 20% by mass and 5% by mass of epoxy in a solvent was immersed in the surface of the test piece 5 to form a slurry film. did. By high frequency heating, the temperature was set to 1300 ° C. and the holding time was set to 5 minutes so that vanadium diffused by about 500 μm. The temperature was lowered to 1200 ° C., and maintained for 5 minutes in a state where a carburizing gas was supplied so that carbon C diffused by about 500 μm. Acetylene was used as the carburizing gas. After the temperature was lowered to 600 ° C., the sample was reheated to 850 ° C., and quenching was performed by spraying water-soluble quenching oil on the test piece 5. The tempering was performed using an electric furnace at a temperature of 180 ° C. and a holding time of 90 minutes.

After the heat treatment described above, both sides of the test piece 5 were ground, and the parallelism, thickness, and surface roughness of the test piece were adjusted to specified values. The Vickers hardness of the cross section at a depth of about 50 μm from the surface was 850 Hv to 950 Hv, and the hardened layer depth (hardened layer thickness) was 2 mm.

Example 5 is a test piece corresponding to Example 2. A slurry in which vanadium powder having an average particle size of 50 μm and benzotriador were mixed in an amount of 20% by mass and 5% by mass of epoxy in a solvent was immersed in the surface of the test piece 5 to form a slurry film. did. By high frequency heating, the temperature was set to 1300 ° C. and the holding time was set to 5 minutes so that vanadium diffused by about 500 μm. The temperature was lowered to 1200 ° C. and kept for 20 minutes in a state in which a carburizing gas was supplied so that carbon C diffused by about 1000 μm. Acetylene was used as the carburizing gas. After the temperature was lowered to 600 ° C., the sample was reheated to 850 ° C., and water-quenched quenching oil was sprayed onto the test piece 5 to perform quenching. The tempering was performed using an electric furnace at a temperature of 180 ° C. and a holding time of 90 minutes.

After the heat treatment described above, both sides of the test piece 5 were ground, and the parallelism, thickness, and surface roughness of the test piece were adjusted to specified values. The Vickers hardness of the cross section at a depth of about 50 μm from the surface is 850 Hv to 950 Hv, and the Vickers hardness of the cross section at a position about 600 μm deep from the surface of the portion in which the iron carbide 2 penetrates to the inner side of the base material than the metal carbide 1 is The hardened layer depth was 2.5 mm from 750 Hv to 850 Hv.

Example 6 is a test piece corresponding to Example 3. A slurry in which vanadium powder having an average particle size of 50 μm and benzotriador were mixed in an amount of 20% by mass and 5% by mass of epoxy in a solvent was immersed in the surface of the test piece 5 to form a slurry film. did. By high-frequency heating, the temperature was set to 1300 ° C. and the holding time was set to 5 minutes so that vanadium diffused by about 500 μm. The temperature was lowered to 1200 ° C., and maintained for 80 minutes in a state where a carburizing gas was supplied so that carbon was diffused by about 2000 μm. Acetylene was used as the carburizing gas. After the temperature was lowered to 600 ° C., the sample was reheated to 850 ° C., and quenching was performed by spraying water-soluble quenching oil on the test piece 5. The tempering was performed using an electric furnace at a temperature of 180 ° C. and a holding time of 90 minutes.

After the heat treatment described above, both sides of the test piece 5 were ground, and the parallelism, thickness, and surface roughness of the test piece were adjusted to specified values. The Vickers hardness of the cross section at a depth of about 50 μm from the surface is 850 Hv to 950 Hv, and the Vickers hardness of the cross section at a depth of about 600 μm from the surface of the portion in which the iron carbide 2 penetrates to the inner side of the base material than the metal carbide 1 is The Vickers hardness of the cross section of the martensite layer 3 1750 μm from the surface at 750 Hv to 850 Hv was 550 Hv to 700 Hv, and the hardened layer depth was 3.0 mm.

[Comparative Example 1]
Comparative Example 1 is a test piece having a reference life, the test surface side of the test piece 5 is heated at a frequency of 30 kHz and a heating time of 10 seconds, and a temperature higher by about 30 ° C. than the A3 transformation point of the iron-carbon phase diagram. Quenching was performed by setting the temperature to about 900 ° C. and spraying a water-soluble quenching oil onto the test piece 5. The tempering was performed using an electric furnace at a temperature of 180 ° C. and a holding time of 90 minutes.

After the heat treatment described above, both sides of the test piece 5 were ground, and the parallelism, thickness, and surface roughness of the test piece were adjusted to specified values. The Vickers hardness of the cross section at a depth of about 50 μm from the surface was 600 Hv to 720 Hv, and the hardened layer depth was 2 mm.

6A is a schematic diagram of a test piece of Comparative Example 1, and FIG. 6B is an observation photograph of the structure of FIG. 6A. FIG. 6B is a laser micrograph of the structure of Comparative Example 1 etched with nital. As shown in FIG. 6B, it can be seen that the vicinity of the surface exhibits the entire martensite structure by induction hardening. Since metal element and carbon are not permeated, metal carbide 1 and iron carbide 2 are not observed.

[Comparative Example 2]
Comparative Example 2 is a comparative example with respect to Example 4, and is a test piece that does not infiltrate carbon but infiltrate vanadium with respect to Example 4. On the surface of the test piece 5, vanadium powder having an average particle diameter of 50 μm and benzotriador in 2-butanone were mixed with 20% by mass of each solvent and 5% by mass of epoxy, respectively, and the slurry was immersed to form a slurry film. . Heat treatment was performed at a temperature of 1300 ° C. and a holding time of 5 minutes so that vanadium diffused by about 500 μm by high frequency heating. Thereafter, the temperature was lowered to 1200 ° C. and held for 5 minutes. After the temperature was lowered to 600 ° C., the sample was reheated to 850 ° C., and water-quenched quenching oil was sprayed onto the test piece 5 to perform quenching. The tempering was performed using an electric furnace at a temperature of 180 ° C. and a holding time of 90 minutes.

After the heat treatment described above, both sides of the test piece 5 were ground, and the parallelism, thickness, and surface roughness of the test piece were adjusted to specified values. The Vickers hardness of the cross section at a depth of about 50 μm from the surface was 850 Hv to 950 Hv, and the hardened layer depth was 2 mm.

7A is a schematic diagram of a test piece of Comparative Example 2, FIG. 7B is a SEM observation photograph of FIG. 7A, and FIG. 7C is a mapping by EPMA of FIG. 7B. As shown in FIGS. 7A and 7B, in Comparative Example 2, carbon penetration was not performed, and thus no iron carbide 2 was observed. Moreover, the vanadium carbide 50 accompanying vanadium permeation is observed.

[Comparative Example 3]
The comparative example 3 is a comparative example with respect to the example 1, and is a test piece in which the vanadium is not infiltrated and the carbon is infiltrated with respect to the example 1. The surface of the test piece 5 was heat-treated at a temperature of 1300 ° C. and a holding time of 5 minutes by high-frequency heating without forming a slurry film. The temperature was lowered to 1200 ° C. and held for 5 minutes in a state where a carburizing gas was supplied so that carbon diffused by about 500 μm. Acetylene was used as the carburizing gas. After the temperature was lowered to 600 ° C., the sample was reheated to 850 ° C., and water-quenched quenching oil was sprayed onto the test piece 5 to perform quenching. The tempering was performed using an electric furnace at a temperature of 180 ° C. and a time of 90 minutes.

After the heat treatment described above, both sides of the test piece 5 were ground, and the parallelism, thickness, and surface roughness of the test piece were adjusted to specified values. The Vickers hardness of the cross-sectional hardness at a depth of about 50 μm from the surface was 800 Hv to 900 Hv, and the hardened layer depth was 2 mm.

FIG. 8A is a schematic view of a test piece of Comparative Example 3, and FIG. 8B is an observation photograph of the structure of FIG. 8A. FIG. 8B is a laser micrograph of the structure of Comparative Example 3 etched with sodium picrate. As shown in FIG. 8B, iron carbide 2 accompanying carbon permeation is observed in the martensite layer 3 of the base material. In Comparative Example 3, no vanadium permeation was performed, so no vanadium carbide was observed.

FIG. 9 is a graph showing the results of the thrust fatigue test of Examples 4 to 6 and Comparative Examples 1 to 3. The test surface pressure was 6 GPa. The life evaluation method is set so that the test is stopped when the value of the acceleration sensor installed at a predetermined location of the test apparatus exceeds three times the value at the start of the test. The surface of No. 5 was confirmed, and the time when flaking with a large sliding width (a dent caused by damage) was confirmed was defined as the life. Even when the value of the acceleration sensor exceeded the set value, if flaking damage did not occur on the surface of the test piece 5, the test was repeated, and the test was continued until flaking damage occurred. In the Hertzian contact theory at a test surface pressure of 6 GPa, the maximum shear stress generation depth is about 180 μm, which is sufficiently shallower than the hardened layer depths of Examples 4 to 6 and Comparative Examples 1 to 3.

The vertical axis in FIG. 9 indicates the ratio when the number of times of life of the test piece subjected to the treatment of Comparative Example 1 is 1, and is the life improvement rate under the same test surface pressure. In addition, the test was implemented 3 conditions for each condition, and evaluated by the average value.

As shown in FIG. 9, it was confirmed that each of Examples 4 to 6 had a life of 10 times or more that of Comparative Example 1. The Vickers hardness at 50 μm from the surface of Comparative Example 1 was 600 Hv to 720 Hv, and the life of Examples 4 to 6 was increased because the surface hardness was improved by the surface treatment of the present invention. This is thought to be due to an increase in resistance to dynamic damage.

In each of Examples 4 to 6, the Vickers hardness of the cross section at a depth of about 50 μm from the surface is the same at 850 Hv to 950 Hv, but the life may be increased in the order of Example 4, Example 5, and Example 6. confirmed. From this, it is suggested that not only the hardened layer of metal carbide 1 and iron carbide 2 scattered in the martensite layer 3 but also the configuration and thickness of the internal hardened layer affect the sliding damage.

On the other hand, in Comparative Example 2, the life was about half that of Comparative Example 1. This is because vanadium permeates to generate vanadium carbide by reacting with the carbon in the substrate S45C, so that the carbon in the substrate S45C is consumed, and the vanadium has not penetrated the carbon after infiltrating, It is presumed that a region having a small amount of carbon was partially formed and a region having a low hardness was partially formed. Therefore, it was suggested that it is necessary to permeate more carbon so as not to cause carbon deficiency of the base material. This phenomenon can be sufficiently prevented by infiltrating hypereutectoid (carbon concentration of about 0.8% by weight or more) as in the present invention.

In Comparative Example 3, the life was about three times that of Comparative Example 1, but the life was shorter than in Examples 4, 5, and 6 times or more. This is presumably because Comparative Example 3 did not disperse the metal carbide 1 but dispersed only the iron carbide 2, and thus the resistance to sliding damage was not sufficiently improved.

From the above experiments, it was found that the resistance to sliding damage can be increased by interspersing the metal carbide 1 and the iron carbide 2 in the martensite layer 3. Moreover, it turned out that not only the hardened layer by the metal carbide 1 and the iron carbide 2 but the structure and thickness of an internal hardened layer also influence a sliding damage.

[Examples 7 to 10]
Next, a test piece in which the diffusion concentration of vanadium was changed with respect to Example 4 (average particle diameter of vanadium: 50 μm) was prepared, the dispersion amount of metal carbide, sliding damage, and the hardness of the test piece 5 and the ball 6. The effects of difference and sliding damage were evaluated. The amount of vanadium dispersed can be adjusted, for example, by changing the diffusion path by changing the particle size of vanadium. Moreover, since the hardness of the test piece 5 also changes according to the amount of vanadium dispersed, the hardness difference between the test piece 5 and the ball 6 can be adjusted. The average particle size of vanadium in Examples 7 to 10 is as follows.

Example 7: Average particle diameter of vanadium 10 μm
Example 8: Average particle diameter of vanadium 25 μm
Example 9: Average particle size of vanadium 100 μm
Example 10: Average particle diameter of vanadium V 200 μm
Examples 7 to 10 have the same heat treatment conditions and test conditions as Example 4 except for the particle size of vanadium.

FIG. 10 is a graph showing the results of the thrust fatigue test of Examples 4, 7 to 10 and Comparative Example 1. As shown in FIG. 10, in Example 7 where the average particle size of vanadium is the smallest and the penetration of vanadium is predicted to be the largest, the test piece 5 is undamaged, but the test is completed with a life of 1 time due to the ball 6 damage. did. The average value of the Vickers hardness of the cross section at a depth of about 50 μm from the surface of the test piece 5 of Example 7 was 1350 Hv, which was about 600 Hv harder than the average value of 750 Hv of the Vickers hardness of the cross section of the ball 6. For this reason, it is considered that the surface of the ball 6 is damaged by wear or excessive sliding on the surface of the test piece 5.

Example 8 in which vanadium permeation is estimated to be next to Example 7 ended the test with a life of 7 times due to the damage of the ball 6 although the test piece 5 was not damaged. The average value of the cross-section Vickers hardness at a depth of about 50 μm from the surface of the test piece 5 of Example 8 was 1000 Hv, which was about 250 Hv harder than the average value of 750 Hv of the Vickers hardness of the cross section of the ball 6. For this reason, although not as much as Example 7, it is thought that the surface of the ball 6 was finally damaged on the surface of the test piece 5 due to wear or excessive sliding.

In Example 9, the average particle diameter of vanadium is larger than that in Example 4, and the penetration amount of vanadium is less than that in Example 4. Damaged (no damage to the ball was observed). The average value of the Vickers hardness of the cross section at a depth of about 50 μm from the surface of the test piece 5 of Example 9 was 850 Hv, which was about 100 Hv harder than the average value of 750 Hv of the Vickers hardness of the cross section of the ball 6.

Example 10 which is considered to have a larger average particle diameter of vanadium than Example 9 and less permeation of vanadium, was damaged by a life of twice that of Example 9 which was shorter than that of Example 9 (the ball was damaged). It was not possible.) The average value of the Vickers hardness of the cross section at a depth of about 50 μm from the surface of the test piece 5 of Example 10 was 750 Hv, which was equivalent to the average value of the Vickers hardness of the cross section of the ball 6 750 Hv. This is because the interstitial amount of the metal carbide 1 in the martensite layer 3 of the base material is reduced, and the distance between the metal carbides 1 that are hard materials is increased, thereby reducing the anchor effect of the hard particles, It is estimated that the durability has decreased.

FIG. 11 is a graph showing the relationship between the hardness difference between the test piece 5 and the ball 6 of Examples 4 and 7 to 10 and the life ratio. It can be seen that the life ratio increases as the hardness difference between the test piece 5 and the ball 6 increases (the hardness of the test piece 5 increases). However, it can be seen that when the hardness difference between the test piece 5 and the ball 6 exceeds 200 Hv and further increases, the life ratio decreases due to damage to the ball. Thus, it can be seen that in the sliding of the relative parts, the service life is regulated by the hardness difference between the relative parts.

From the above experiment, it can be seen that the dispersion amount of the metal carbide 1 in the martensite layer 3 and the hardness difference of the relative parts must be appropriately controlled.

[Example 11]
Based on the thrust fatigue test results described above, the present invention was applied to the ball screw portion of the electric power steering apparatus. FIG. 12 is a schematic sectional view of the electric power steering apparatus, and FIG. 13 is an enlarged view of the ball screw mechanism of FIG. As shown in FIG. 12, the electric power steering apparatus has a nut 12 having a female screw groove formed on the inner peripheral surface, and a male screw groove that is disposed on the shaft center of the nut 12 and faces the female screw groove on the outer peripheral surface. A rack screw 13, a ball screw mechanism including a plurality of circulating balls 6 interposed between the female screw groove and the male screw groove, a steering gear case 14 that houses the ball screw mechanism, and a ball screw mechanism. The electric motor 15 is provided for driving, and a power transmission mechanism 16 that transmits the rotational driving force of the electric motor to the nut 12.

When the present invention is applied to the ball screw portion of the electric power steering device, the rolling distance can be shortened because the durable surface pressure can be increased as compared with the conventional surface treatment. The ability to shorten the rolling distance leads to a reduction in the number of turns of the ball screw and a reduction in the diameter of the rack screw, which can contribute to light weight and downsizing.

Also, the electric power steering device is almost 100% popular in general vehicles, and its application to large vehicles is being studied. A problem in application to a large vehicle is that the load applied to the ball screw increases, the surface pressure of the ball screw portion increases, and the life is shortened due to fatigue damage accompanying rolling. When applying the same surface-strengthening treatment used for ordinary vehicles to large vehicles, increase the pressure-receiving area by increasing the diameter of the ball screw or increasing the length of the ball screw. It is necessary to make the surface pressure at the same level as that of ordinary cars, but because the sliding distance becomes longer, the load transmission efficiency decreases due to the increase in friction, and it becomes necessary to install an extra capacitor for assistance. There is a downside.

Therefore, by applying a highly durable surface treatment to the ball screw part, the diameter of the ball screw part can be reduced or the length of the ball screw part can be shortened accordingly. In addition to suppressing the reduction in transmission efficiency, it can also be expected to reduce weight.

The manufacturing method of the power steering device described above will be described below. Here, since the nut 12 is an inner surface screw and the rack screw 13 is an outer surface screw, the surface pressure calculated from Hertz's contact theory is relatively larger than the nut 12 due to its geometric shape, so the rack screw 13 The present invention was applied to the outer surface thread portion. The material of the rack screw 13 was a carbon steel material S45C for machine structure defined by Japanese Industrial Standard JIS G 4501. The round bar material was finished in the shape of a predetermined rack screw 13 through processes such as turning and rolling.

The surface enhancement treatment of the present invention was performed on the rack screw 13 (Example 11). Moreover, in order to confirm the lifetime improvement effect of the surface reinforcement | strengthening process of this invention, the conventional process was also implemented as the comparative example 4. FIG. The contents of the surface strengthening treatment of Example 11 are shown below.

FIG. 14 is a schematic diagram of an apparatus for applying a slurry to the rack screw 13. Slurry application to the rack screw 13 was performed by a slurry application device 17 shown in FIG. In the slurry raw material charging device 18, 2-butanone, vanadium powder having an average particle diameter of 50 μm, and benzotriazole are respectively charged in an amount of 20% by mass and 5% by mass of epoxy. These raw materials are agitated and mixed by the slurry agitating fan 20 in the slurry agitating device 19 to become the adjusted slurry 21.

The adjusted slurry 21 is transferred to the slurry tank 23 by the pump 22. The rack screw 13 is masked in advance so that the slurry 21 is not applied except for the external thread portion. The rack screw 13 is immersed in the slurry tank 23 in the rack screw conveying device 24, pulled upward while rotating to homogenize the coating thickness, and dried in a drying device (not shown) to form a slurry film. Is done.

A series of processes from raw material input to slurry application and drying are controlled by the slurry dipping device control panel 25. FIG. 15 is a schematic view of a heat treatment apparatus used in Example 11. The rack screw 13 having the slurry film is subjected to heat treatment by a heat treatment apparatus shown in FIG. The rack screw 13 is loaded into the heat treatment device 28, the inside of the heat treatment device 28 is evacuated and replaced with an inert gas, and heated by the high frequency coil 26. About 500 μm vanadium is infiltrated at a heating temperature of T1: 1300 ° C. and an infiltration time t1: 5 min.

After the heat treatment described above, the temperature is lowered to T2: 1200 ° C. while injecting acetylene gas from the carburizing gas nozzle 27 in a state where the pressure inside the heat treatment apparatus 28 is reduced, and carbon is infiltrated by about 500 μm at an infiltration time t2: 5 min. . After the temperature was lowered to 600 ° C., the mixture was reheated to 850 ° C., and water-quenching quenching oil was injected into the rack screw 13 to perform quenching. The tempering was performed using an electric furnace at a temperature of 180 ° C. and a holding time of 90 minutes. Thereafter, the external thread portion and the like were finished to a predetermined dimension by grinding. The Vickers hardness at a depth of 50 μm from the surface of the sliding portion of the rack screw 13 was the same as that of Example 4 confirmed with the thrust fatigue test piece.

The durability test was conducted by incorporating this rack screw 13 into the electric power steering device. The ball 6 was made of high carbon chrome bearing steel SUJ2 defined in Japanese Industrial Standard JIS G 4805 as in the thrust fatigue test. The Vickers hardness at a depth of 50 μm from the surface of the sliding portion of the ball was about 720 Hv to 780 Hv. As the nut 12, an alloy steel SCM420 for machine structure defined by Japanese Industrial Standard JIS G 4053 was used. The round bar material was processed into a predetermined nut 12 shape through a process such as turning.

After this, gas carburization treatment was performed, and the internal threaded parts were finished by grinding. As described above, since the surface pressure acting on the nut 12 is lower than that of the rack screw 13, the carburizing gas is widely used in consideration of industrial feasibility, not the surface treatment of the present invention. The Vickers hardness at a depth of 50 μm from the surface of the sliding portion of the nut 12 was about 660 Hv to 760 Hv.

In the durability test, repeated durability tests were performed at the maximum load assumed during actual operation. The life evaluation method is that the value of the acceleration sensor attached to the electric power steering at a predetermined location exceeds twice the value at the start of the test, or the value of the acoustic sensor attached to the electric power steering at the predetermined location The test was set to stop when it exceeded twice the time. After the test was stopped, the ball screw portion of the electric power steering was disassembled, and the sliding portions of the rack screw 13, the ball 6 and the nut 12 were observed to confirm that damage had occurred. However, when the test stop condition was not reached even when exceeding 3 times the life of Comparative Example 4, the test was terminated at that time, assuming that a life of 3 times or more was confirmed. In Comparative Example 4, the nut 12 and the ball 6 are the same as in the present invention, and the surface treatment of the rack screw 13 is induction hardening that has been conventionally employed.

FIG. 16 is a graph showing the durability test results of the electric power steering devices of Example 11 and Comparative Example 4. In Example 11, compared to Comparative Example 4, it was confirmed that the lifetime was 3 times or more. Further, as a result of observing the sliding surfaces of the rack screw 13, the ball 6 and the nut 12 after the life test, no significant damage was confirmed.

[Example 12]
In Examples 1 to 11, metal elements and carbon were diffused using diffusion, but in this example, powder containing metal and carbon is fed, and the powder and substrate are alloyed by laser. We evaluated and evaluated the method to make it.

FIG. 17 is a schematic view showing an example of an apparatus for laser alloying a thrust fatigue test piece. The base material of the test piece 5 is a carbon steel material S45C for mechanical structure defined by Japanese Industrial Standards JIS G 4501, and is a steel material containing about 0.45 mass percent carbon C. Powder 31 consisting of 1% by mass of carbon, 2% by mass of vanadium, 4% by mass of chromium, 5% by mass of molybdenum, and the remainder of iron on the surface of the test piece 5 is fed from the powder feeding device 32 into the laser welding nozzle 29, Using the laser transmitter 30, the powder 31 and the substrate S45C were simultaneously melted and alloyed. The powder 31 is a gas atomized powder having an average particle diameter of 70 μm and a powder feeding amount of about 4 g / min. The laser alloying conditions were as follows: laser output: 2500 W, focal diameter: 4 mm, alloying speed: 1000 mm / min. This condition is a condition that the alloying depth is about 0.5 mm and the alloying width is about 3 mm.

After laser irradiation, the test surface side of the test piece 5 is heated at a frequency of 30 kHz and a heating time of 10 seconds to a temperature about 30 ° C. higher than the A3 transformation point of the iron-carbon system phase diagram, about 900 ° C. Was sprayed on the test piece 5 for quenching. The tempering was performed using an electric furnace at a temperature of 180 ° C. and a holding time of 90 minutes.

The base material, metal, and carbon are melted and alloyed with a laser, and then quenched, whereby the base material is martensitic and a structure in which metal carbide and iron carbide particles are dispersed in the martensite layer can be formed. Moreover, toughness can be improved by tempering. Durability can be improved even with a structure that has been melted and alloyed with a laser, but the structure that has been melted and alloyed with a laser is a solidified structure, and a heat-affected zone is formed in the surrounding area that has been melted and alloyed. Therefore, considering the stabilization of the structure, it is more preferable to perform quenching and tempering.

After the heat treatment described above, both sides of the test piece 5 were ground, and the parallelism, thickness, and surface roughness of the test piece were adjusted to specified values. The Vickers hardness of the cross section at a depth of about 50 μm from the surface was 880 Hv to 980 Hv, and the total hardened layer depth was 2 mm.

FIG. 18 is a schematic diagram of the structure of Example 12, a macro photograph taken with a microscope showing a laser alloying region, and a detailed structure photograph composed of laser alloying. A laser alloying region 33 in which the metal carbide 1 and the iron carbide 2 were dispersed in the martensite layer 3 was confirmed in a range of 500 μm in depth from the surface. Further, a heat affected zone 34 accompanying laser alloying was observed inside the laser alloying region 33. The Vickers hardness of the cross section at a depth of 50 μm from the surface was 880 Hv to 980 Hv. The thrust fatigue test of Example 11 was performed on the test piece 5 thus produced. The test conditions are the same as in FIG. As a result, the lifetime of Example 12 was confirmed to be 10 times longer than that of Comparative Example 1 in FIG.

[Example 13]
Based on the results of the thrust fatigue test described above, the laser alloying of the present invention shown in Example 12 was applied to the ball screw portion of the electric power steering apparatus. The object of application is the external thread portion of the rack screw 13 as in Example 11, and is the same as Example 11 except for laser alloying.

FIG. 19 is a schematic view of an apparatus for applying a laser alloying process to the external thread portion of the rack screw 13. On the surface of the external thread portion of the rack screw 13, a powder 31 consisting of 2% by mass of carbon C1% by mass, 4% by mass of chromium, 5% by mass of molybdenum, and the remainder of iron Fe is fed from the powder feeding device 32 to the laser welding nozzle 29. The powder 31 and the substrate S45C were simultaneously melted and alloyed using a laser transmitter 30. The rack screw 13 is supported by the rotary feed device 35, controlled by the rotary feed device control panel 36, and fed in the axial direction of the rack screw 13 while rotating, so that the rack screw 13 is moved to the sliding portion of the external thread portion of the rack screw 13. Along with this, laser alloying is performed.

Powder 31 is a gas atomized powder having an average particle diameter of 70 μm and a powder feeding amount of about 4 g / min. The laser alloying conditions were a laser output of 2500 W, a focal diameter of 4 mm, and an alloying speed of about 1000 mm / min. This condition is a condition that the alloying depth is 0.5 mm and the alloying width is 3 mm. After that, the rack screw 13 is heated at a frequency of 30 kHz and a heating time of 10 seconds to reach a temperature about 30 ° C. higher than the A3 transformation point of the iron-carbon phase diagram, about 900 ° C., and water-soluble quenching oil is injected onto the test piece 5 And quenched. The tempering was performed by high frequency heating at a temperature of 200 ° C. for 1 minute. The Vickers hardness of the cross section at a depth of about 50 μm from the surface was 880 Hv to 980 Hv, and the hardened layer depth was 2 mm.

The durability test was conducted by incorporating the above-described rack screw 13 into the electric power steering apparatus. The ball 6 was made of high carbon chrome bearing steel SUJ2 defined in Japanese Industrial Standard JIS G 4805 as in the thrust fatigue test. The Vickers hardness at a depth of 50 μm from the surface of the sliding portion of the ball was about 720 Hv to 780 Hv. As the nut 12, an alloy steel SCM420 for machine structure defined by Japanese Industrial Standard JIS G 4053 was used. The round bar material was processed into a predetermined nut 12 shape through a process such as turning. Thereafter, a gas carburizing process was performed, and the inner surface screw part and the like were finished by grinding. As described above, since the surface pressure acting on the nut 12 is lower than that of the rack screw 13, the carburizing gas is widely used in consideration of industrial feasibility, not the surface treatment of the present invention. The Vickers hardness at a depth of 50 μm from the surface of the sliding portion of the nut 12 was about 660 Hv to 760 Hv.

As a result of the durability test with the electric power steering, it was confirmed that the life was three times longer than that of Comparative Example 4. Further, as a result of observing the sliding surfaces of the rack screw 13, the ball 6 and the nut 12 after the life test, no significant damage was confirmed.
[Example 14]
In Example 14, the metal carbide and iron carbide layers are provided only on the rack screw 13 at positions where the balls 6 are in contact with each other.

FIG. 22 is an enlarged view of a portion where the rack screw 13 and the ball 6 constituting the ball screw mechanism of the electric power steering apparatus of the fourteenth embodiment come into contact with each other. As shown in FIG. 22, the rack screw 13 and the ball 6 are in contact with each other at two locations (37a, 37b) (in operation, depending on the moving direction, they are in contact with either one side).

FIG. 23 is an enlarged view of the surface of the rack screw 13 of FIG. As shown in FIG. 23, on the surface 38 of the rack screw 13, metal carbide and iron carbide are formed in two lines along the groove of the rack screw 13. By providing metal carbide and iron carbide only on the surface 38 corresponding to the two wires in this way and leaving an untreated part around the surface, it is hard and excellent in strength and excellent in toughness but low in strength. Since these parts coexist, a structure that is resistant to impact can be obtained.

In order to provide metal carbide and iron carbide in two places, the laser alloying shown in Example 13 can be used. A metal carbide or iron carbide layer can be provided only in the laser-treated portion.

In Examples 11 to 14, metal carbide and iron carbide are added to the rack screw 13, but metal carbide and iron carbide may be added to the surface of the ball 6.

As described above, according to the present invention, it has been proved that a sliding member having high durability and a manufacturing method thereof, a power steering device and a manufacturing method thereof can be provided. According to the present invention, it is possible to improve the durability of a sliding member composed of a combination of two or more parts facing each other, to achieve downsizing and cost reduction of the sliding member, and to protect earth resources and the global environment. It is also beneficial for conservation. In addition, when the present invention is used for a ball screw portion of an electric power steering device, a durable surface pressure can be increased as compared with a conventional surface treatment. Can contribute to

In addition, this invention is not limited to the above-mentioned Example, Various modifications are included. For example, the above-described embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described. Further, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. Further, it is possible to add, delete, and replace other configurations for a part of the configuration of each embodiment.

DESCRIPTION OF SYMBOLS 1 ... Metal carbide, 2 ... Iron carbide, 3 ... Martensite, 4 ... Vanadium carbide, 5 ... Test piece, 6 ... Ball, 7 ... Outer ring, 8 ... Test tank, 9 ... Lubricating oil, 10 ... Load, 11 ... Rotation , 12 ... nuts, 13 ... rack screw, 14 ... steering gear case, 15 ... electric motor, 16 ... power transmission mechanism, 17 ... slurry coating device, 18 ... slurry raw material charging device, 19 ... slurry stirring device, 20 ... slurry stirring Fan, 21 ... adjusted slurry, 22 ... pump, 23 ... slurry tank, 24 ... rack screw conveying device, 25 ... slurry dipping device control panel, 26 ... high frequency coil, 27 ... carburizing gas nozzle, 28 ... heat treatment device, 29 ... laser welding Nozzle, 30 ... laser transmitter, 31 ... powder, 32 ... powder feeding device, 33 ... laser alloying region, 34 ... heat affected zone, 3 ... rotary feeder 36 ... rotary feeder control panel, 37a, 37b ... contact portion between the ball and the rack screw, 38 ... rack screw surface metal carbide and iron carbide.

Claims

A steel base material, a granular iron carbide contained in the base material, and a metal carbide contained in the base material and containing metal and carbon as constituent elements,
The volume fraction of the iron carbide and the metal carbide decreases along the depth direction from the surface to the inside of the base material,
The sliding member according to claim 1, wherein the metal is at least one selected from the group consisting of Ti, V, Nb, Ta, Zr, W, Hf, and Mo.
2. The sliding member according to claim 1, wherein the iron carbide exists up to a portion deeper than the metal carbide inside the base material.
The said base material has a martensite layer which has a martensite structure | tissue, The said martensite layer exists in the deeper part than the said iron carbide and the said metal carbide inside the said base material. The sliding member according to 1.
A nut having a female thread groove formed on the inner peripheral surface;
A rack screw disposed on the axis of the nut and having an external thread groove formed on the outer peripheral surface thereof opposed to the internal thread groove;
A ball screw mechanism having a plurality of circulating balls interposed between the female screw groove and the male screw groove;
A steering gear case that houses the ball screw mechanism;
An electric motor used to drive the ball screw mechanism;
A power transmission mechanism that transmits the rotational driving force of the electric motor to the nut;
The power steering device according to any one of claims 1 to 3, wherein the rack screw or the circulating ball is a sliding member according to any one of claims 1 to 3.
The power steering device according to claim 4, wherein the iron carbide and the metal carbide are included in a surface of a male screw groove of the rack screw or a surface of the nut.
The rack screw and the circulating ball are in contact at least at two points,
6. The power steering apparatus according to claim 5, wherein the iron carbide and the metal carbide are provided along the male screw groove corresponding to the two points.
The power steering apparatus according to any one of claims 4 to 6, wherein the iron carbide has a diameter of 0.5 µm to 5 µm and an aspect ratio of 1 to 5.
A metal permeation step for permeating one or more metals selected from the group consisting of Ti, V, Nb, Ta, Zr, W, Hf, and Mo into a steel substrate;
A carbon infiltration step for infiltrating carbon into the substrate,
In the carbon infiltration step, the carbon is infiltrated into the base material at a concentration that causes hypereutectoid deposition.
In the carbon infiltration step, iron carbide and metal carbide are formed on the base material,
The volume ratio of the iron carbide and the metal carbide decreases along the depth direction from the surface to the inside of the base material.
The method for producing a sliding member according to claim 9, wherein the concentration of the carbon that causes hypereutectoid is 0.8% by mass of the carbon with respect to the base material.
A nut having a female thread groove formed on the inner peripheral surface;
A rack screw disposed on the axis of the nut and having an external thread groove formed on the outer peripheral surface thereof opposed to the internal thread groove;
A ball screw mechanism having a plurality of circulating balls interposed between the female screw groove and the male screw groove;
A steering gear case that houses the ball screw mechanism;
An electric motor used to drive the ball screw mechanism;
In a method for manufacturing a power steering apparatus, including a power transmission mechanism that transmits a rotational driving force of the electric motor to the nut,
The rack screw or the circulating ball is configured by the sliding member according to any one of claims 1 to 3,
A method for manufacturing a power steering device, wherein the sliding member is manufactured by the method for manufacturing the sliding member according to any one of claims 8 to 10.
12. The method of manufacturing a power steering device according to claim 11, further comprising an application step of applying a slurry containing the metal to the surface of the base material before the metal infiltration step.
The method for manufacturing a power steering apparatus according to claim 12, further comprising a quenching and tempering step after the carbon infiltration step.
The method for manufacturing a power steering device according to claim 13, wherein the metal infiltration step and the carbon infiltration step diffuse the metal or the carbon by thermal diffusion, respectively.
12. The metal infiltration step and the carbon infiltration step are alloying steps in which powder containing the metal and the carbon is fed and the powder and the base material are alloyed by laser. The manufacturing method of the power steering apparatus as described in any one of.
The method for manufacturing a power steering device according to claim 15, further comprising a quenching and tempering step after the alloying step.