METHOD FOR MANUFACTURING STEEL MEMBER, JIG USED FOR MANUFACTURING STEEL MEMBER, AND STEEL MEMBER

Abstract

To improve the fatigue strength of a steel member having a hole by imparting residual compressive stress to an opening edge of the hole in a quenching process without an additional plastic working process.SOLUTION: A method for manufacturing a steel member includes a step of: preparing a steel material 1 having a hole on the outer surface; a step of heating the material 1 to carburize it; and a quenching step of hardening the heated material 1 by cooling it. The quenching is performed in a state in which a tool 2, which is in contact with an opening edge of the hole 1a and suppresses the contraction of the inner diameter of the opening of the hole 1a, is disposed in the hole 1a.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a steel member, a jig used in manufacturing a steel member, and a steel member. [Background technology]

[0002] An example of a steel member having a hole on its outer surface is a shaft member having an oil hole. In such a steel member having a hole on its outer surface, the area around the hole may be a weak part in terms of strength. For example, WO2018 / 056333A1 (Patent Document 1) describes that after carburizing and quenching a steel intermediate member, the hole is cut to transform the retained austenite in the surface layer of the hole into hard processing-induced martensite, thereby increasing the hardness near the hole. Here, it is proposed to control the volume fraction of martensite and retained austenite in the surface layer near the hole within a predetermined range by optimizing the chemical composition, heat treatment conditions, and cutting conditions of the steel material.

[0003] In addition, Japanese Patent Laid-Open Publication No. 2006-111962 (Patent Document 2) discloses striking the inner surface of an oil hole with an ultrasonic vibrator to form striking marks on the inner surface. This plastic working causes work hardening, which improves fatigue strength. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] WO2018 / 056333A1 [Patent Document 2] JP 2006-111962 A Summary of the Invention [Problem to be solved by the invention]

[0005] The inventors found that when a repeated load is applied to a steel member having a hole on its outer surface, stress may concentrate on the surface of the hole, which may become the starting point of fracture. Therefore, the inventors considered applying residual compressive stress to the vicinity of the opening of the hole in the steel member to increase the strength against fatigue fracture. The method of WO2018 / 056333A1 requires a cutting process after carburizing and quenching. In addition, JP 2006-111962 A requires a striking process using an ultrasonic vibrator.

[0006] Therefore, an object of the present invention is to provide a manufacturing method for a steel member, a jig used in manufacturing a steel member, and a steel member, which can impart residual compressive stress to the edge of the hole opening in the hardening process without the need for additional plastic processing processes such as cutting or ultrasonic impact, thereby improving the fatigue strength of a steel member having a hole. [Means for solving the problem]

[0007] A method for manufacturing a steel member according to an embodiment of the present invention includes the steps of preparing a steel material having a hole on its outer surface, heating the material to subject it to a carburizing treatment, and quenching the heated material by cooling it. In the quenching step, quenching is performed in a state where a jig that comes into contact with the edge of the opening of the hole and suppresses shrinkage of the inner diameter of the edge of the opening of the hole is placed in the hole. Effect of the Invention

[0008] According to the present invention, the fatigue strength of a steel member having a hole can be improved by imparting residual compressive stress to the edge of the opening of the hole in the hardening process without adding a plastic processing process. [Brief description of the drawings]

[0009] [Figure 1A] FIG. 1A is a front view showing an example of a steel member applicable to one embodiment of the present invention. [Figure 1B] FIG. 1B is a cross-sectional view taken along line BB in FIG. 1A. [Figure 1C] FIG. 1C is an enlarged view of the vicinity of the opening of the hole 1a in FIG. 1B. [Diagram 2] FIG. 2 is a flow diagram showing an example of a method for manufacturing a steel member in this embodiment. [Diagram 3] FIG. 3 is a cross-sectional view showing a state in which a jig 2 is placed on the steel material shown in FIG. 1B. [Figure 4] FIG. 4 is an enlarged view of the vicinity of the opening of the hole 1a in FIG. [Diagram 5] FIG. 5 is a cross-sectional view showing an example of a state in which the jig comes into contact with the edge of the opening of the hole. [Figure 6] FIG. 6 is a cross-sectional view taken along line CC in FIG. [Figure 7] FIG. 7 is a diagram showing an example of a configuration for connecting a first jig and a second jig. [Figure 8] FIG. 8 is a diagram showing a three-dimensional model used in the analysis of the examples. [Figure 9] FIG. 9 shows the heat pattern of carburizing and quenching in the embodiment. [Figure 10] FIG. 10 shows the heat transfer coefficient of oil in the examples. [Figure 11] FIG. 11 is a graph showing the residual stress distribution in the oil hole obtained by analysis. [Figure 12] FIG. 12 is a diagram showing the deformation before and after quenching obtained by analysis. [Figure 13] FIG. 13 shows an analytical model of a shaft member including a jig. [Figure 14] FIG. 14 is a graph showing a comparison of residual stress distributions in oil holes obtained by analysis. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] 1A to 1C are diagrams showing an example of a steel member having a hole applicable to one embodiment of the present invention. In the example shown in Figs. 1A to 1C, the steel member 1 having a hole is a shaft member having an oil hole 1a. Fig. 1A is a front view of the steel member. Fig. 1B is a cross-sectional view taken along line BB in Fig. 1A. Fig. 1C is an enlarged view of the vicinity of the opening of hole 1a in Fig. 1B.

[0011] The steel member 1 shown in Figs. 1A to 1C is a hollow cylinder, but the steel member may be a solid member. That is, the steel member may be, for example, a solid column or a hollow cylinder. The hole 1a is formed on the outer surface of the steel member 1. In the example shown in Figs. 1A to 1C, the hole 1a is formed extending in a direction perpendicular to the axis (axis J1) of the shaft of the steel member 1. That is, the direction of the axis J2 of the hole 1a, i.e., the depth direction of the hole 1a, is perpendicular to the axial direction of the steel member 1. In this way, the configuration in which the axial direction of the hole 1a is inclined at an angle with respect to the axial direction of the steel member 1 is a typical configuration of an oil hole of a shaft member. Note that the hole 1a is a through hole, but the hole 1a may be a non-through hole, i.e., a recessed hole in the outer surface.

[0012] When a steel component with a hole is carburized and quenched using conventional methods, the circumferential compressive residual stress on the inner surface of the hole is greatest at the center of the hole's axial direction (depth direction) and is minimal at the end of the hole, i.e., the opening. When a torsional load around the axial direction of the steel component is repeatedly applied to a steel component with such a residual stress distribution, cracks may occur starting from the end of the hole. In this case, the crack surface is perpendicular to the oil hole.

[0013] The inventors have studied how to improve fatigue strength by increasing the residual stress in the compressive direction at the end of the inner surface of the hole, i.e., at the opening. As a method for imparting compressive residual stress, it is possible to carry out a treatment involving plastic processing such as shot peening, ultrasonic impact, or cutting after heat treatment. However, such a treatment involving plastic processing requires the preparation of equipment for carrying out the treatment. This leads to an increase in costs. In addition, the surface of the opening of the hole becomes rough due to plastic processing, which reduces fatigue strength and increases the risk of it becoming a starting point of fracture. In order to improve the surface roughness after plastic processing, an additional process such as etching is required.

[0014] Therefore, the inventors investigated a method of imparting residual stress to the opening of a hole without plastic processing. In the investigation, they focused on the fact that the inner diameter of the opening of a hole shrinks during quenching. They then came up with the idea of ​​increasing the inelastic tensile strain in the part that is likely to become the starting point of fracture (fracture starting point part) by preventing the inner diameter of the opening of the hole from shrinking during quenching. They found that this makes it possible to increase the residual stress in the fracture starting point part in the compression direction after quenching is completed.

[0015] As a result of further investigation, the inventors came up with the idea of ​​inserting a jig shaped to contact the edge of the hole opening into the hole and performing quenching as a means for preventing the inner diameter of the hole in the steel member from shrinking during cooling in the quenching process. This makes it possible to increase the residual stress in the end of the hole in the compressive direction after quenching. The following embodiments are based on this finding.

[0016] A method for manufacturing a steel member according to an embodiment of the present invention includes the steps of preparing a steel material having a hole on its outer surface, heating the material to subject it to a carburizing treatment, and quenching the heated material by cooling it. In the quenching step, quenching is performed in a state where a jig that comes into contact with the edge of the opening of the hole and suppresses shrinkage of the inner diameter of the edge of the opening of the hole is placed in the hole.

[0017] In the above manufacturing method, the jig in contact with the edge of the hole opening prevents the inner diameter of the hole opening from shrinking during quenching. This increases the inelastic tensile strain at the edge of the hole opening, i.e., the end of the hole. As a result, the residual stress at the edge of the hole opening after quenching increases in the compression direction. That is, the compressive residual stress increases at the edge of the hole opening. This makes it possible to impart compressive residual stress to the end of the hole, which is likely to become the starting point of fracture. In this manufacturing method, plastic processing for imparting residual stress to the fracture starting point is not necessary. That is, even without adding a plastic processing process, residual compressive stress is imparted to the edge of the hole opening in the quenching process, thereby improving the fatigue strength of the steel member having a hole.

[0018] In the quenching, the jig is preferably disposed so as to contact the edge of the opening of the hole, but not the inner surface of the axial center of the hole. This makes it possible to prevent the contraction of the inner diameter of the opening of the hole during quenching without preventing the contraction of the inner diameter of the axial center of the hole. Therefore, the effect of inhibiting the contraction of the inner diameter is not distributed throughout the entire axial direction of the hole, but is concentrated at the opening. This increases the effect of imparting compressive residual stress to the fracture origin.

[0019] From a similar viewpoint, during hardening, the jig may be arranged so as to be in contact with the edge of the opening of the hole and not in contact with the inner surface of the hole other than the edge of the opening of the hole.

[0020] In the hardening, the jig may have an inclined surface that is inclined with respect to the axial direction of the hole. In the hardening, the jig may be disposed so that the inclined surface of the jig contacts the edge of the opening of the hole. This results in a state in which the inclined surface of the jig contacts the edge of the opening of the hole and does not contact the inner surface of the hole deeper than the edge of the opening of the hole. Therefore, during hardening, the prevention of the contraction of the inner diameter of the hole by the jig can be concentrated in the opening portion of the hole.

[0021] The inclined surface of the jig may be inclined so as to move away from the inner surface of the hole as it goes from the opening of the hole to the axial depth of the hole. For example, the jig may have a tapered portion formed so as to become thinner from the opening of the hole to the axial depth. The outer peripheral surface of the tapered portion of the jig may be an inclined surface and may be arranged to contact the edge of the opening of the hole during hardening.

[0022] The hole may penetrate the steel material. In this case, during the quenching, a first jig in contact with an edge of the opening of the hole on one outer surface and a second jig in contact with an edge of the opening of the hole on the other outer surface opposite to the one outer surface may be placed in the hole while being connected to each other by a screw. This allows compressive residual stress to be imparted to both one opening and the other opening of the hole functioning the steel member.

[0023] A jig in an embodiment of the present invention is a jig for inserting and placing in a hole when hardening a steel material having a hole on its outer surface. The jig may have a tapered portion whose thickness changes in the axial direction. In the tapered portion, the maximum cross section of a surface perpendicular to the axial direction can be larger than the area of ​​the opening of the hole, and the minimum cross section of the surface perpendicular to the axial direction can be smaller than the area of ​​the opening of the hole. The jig can be placed in the hole with the outer peripheral surface of the tapered portion in contact with the edge of the opening of the hole. By hardening the steel material with the jig placed in the hole in this manner, compressive residual stress can be imparted to the opening of the hole.

[0024] A steel member according to an embodiment of the present invention has a hole on an outer surface thereof, and a portion of the outer surface of the steel member at an edge of the opening of the hole has a larger residual compressive stress than a portion of the outer surface on the outside thereof. The surface roughness Ra of the inner surface of the hole in a region within 2 mm from the edge of the opening is 0.2 to 5.0 μm.

[0025] Residual stress is measured by X-ray stress measurement. The residual stress at the edge of the hole opening is the residual stress on the outer surface within a distance of 1 mm from the edge of the hole. Here, the residual stress is the residual stress in the circumferential direction of the hole.

[0026] The surface roughness Ra is the arithmetic mean surface roughness, and is measured using a stylus-type surface roughness measuring instrument according to the method specified in JIS B 0601:2013.

[0027] In the steel member having the above configuration, the residual compressive stress at the edge of the hole opening is greater than that of the surrounding outer surface. In other words, the configuration is such that compressive residual stress is imparted to the edge of the hole opening. This makes the hole opening less likely to become the starting point of fracture due to fatigue. Furthermore, since the surface roughness of the inner surface near the hole opening is 0.2 to 5.0 μm, the decrease in fatigue strength caused by the roughness of the inner surface near the hole opening is suppressed. Therefore, the hole opening is less likely to become the starting point of fracture due to fatigue.

[0028] The steel member having the above configuration can be manufactured by the above manufacturing method. After carburizing a steel material having a hole on its outer surface, the material is quenched while a jig is placed in contact with the edge of the hole opening. This prevents the inner diameter of the hole opening from shrinking during quenching, and imparts compressive residual stress to the hole opening. The edge of the hole opening has a larger residual compressive stress than the outer surface outside it. In this way, compressive residual stress is imparted to the hole opening without plastic processing the inner surface of the hole. In addition, the inner surface of the hole does not become rough due to the impartation of compressive residual stress. Therefore, the surface roughness of the inner surface near the hole opening is set to 0.2 to 5.0 μm, and the decrease in fatigue strength due to the rough surface can be suppressed.

[0029] The steel member may have a portion in which the compressive residual stress decreases with increasing distance from the edge of the opening in the axial direction of the hole in a region on the inner surface of the hole within 2 mm from the edge of the opening. In this configuration, the inner surface near the opening of the hole has a portion in which the compressive residual stress increases with increasing distance from the edge of the opening. In other words, the configuration is such that compressive residual stress is imparted to the edge of the opening of the hole. Therefore, the opening of the hole is less likely to become the starting point of fracture.

[0030] Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals and their description will not be repeated. The dimensional ratios between the components shown in each drawing do not necessarily represent the actual dimensional ratios.

[0031] Fig. 2 is a flow diagram showing an example of a method for manufacturing a steel member in this embodiment. The manufacturing method shown in Fig. 2 includes a step (S1) of preparing a steel material having a hole on its outer surface, a step (S2) of placing a jig in the hole of the material, a step (S3) of carburizing the material, and a step (S4) of quenching the material with the jig placed in it. The steel member is not limited to one for a specific application, but here, as an example, an example of manufacturing a shaft having an oil hole as the steel member will be described.

[0032] (S1: Prepare the steel material to be heat treated) Although there is no limitation on the steel material to be used as the raw material, in this embodiment, as an example, a low-carbon steel material with a carbon content of 0.10 to 0.40 mass % is prepared.

[0033] In this embodiment, as an example, in mass%, C: 0.10-0.40%, Si: 0.05 to 2.5%, Mn: 0.2-3%, P: 0.05% or less, S: 0.2% or less, Al: 0.005-0.1%, N: Contains 0.001 to 0.02% Optionally, further comprising, in weight percent: Cr: 0.1-2%, Ni: 0.1-2%, Mo: 0.1-2%, Cu: 0.1-2%, B: 0~0.02%, Ti: 0.003 to 0.05%, V: 0.05~0.5%, Nb: 0 to 0.1% of one or more types; A steel is prepared, the balance of which is Fe and unavoidable impurities.

[0034] The steel material may be, for example, a solid or hollow round bar or steel pipe. When the material is a round bar or steel pipe, the holes are formed, for example, on the outer circumferential surface. For example, as shown in Figs. 1A and 1B, a material having holes penetrating in a direction perpendicular to the axial direction of the round bar is prepared. In this example, a pair of holes is provided in thick-walled parts facing each other in a direction perpendicular to the axis of the round bar. The pair of holes are coaxial and aligned in a direction perpendicular to the axis of the round bar. In this example, the thickness direction of the steel member (shaft) is the same as the axial direction of the holes. The steel material may be a steel pipe instead of a round bar. The holes may be through holes or bottomed recesses (recessed holes).

[0035] The size of the steel member is not particularly limited. In the present embodiment, as an example, a hollow round bar having an outer diameter of 15 to 40 mm, an inner diameter of 5 to 20 mm, and a length of 8 to 200 mm is prepared as the steel material.

[0036] The manufacturing process of the material includes, but is not limited to, a process of melting steel and performing continuous casting or blooming to produce a billet. The process of producing a billet may include hot working, cold working, or heat treatment in addition to continuous casting or blooming. The billet may be machined after hot working to produce a solid or hollow round bar. Heat treatment may be performed after hot working. Steel pipes may be produced by hot working. This hot working is, for example, the Mannesmann process or the Ugine-Sejournet process. The round bar or steel pipe after hot working may be cut to a predetermined length. The steel pipe is not limited to a seamless pipe, and may be, for example, a forged steel pipe, an electric resistance welded steel pipe, or an arc welded pipe.

[0037] (S2: Place the jig in the hole in the steel material) The jig is placed so as to contact the edge of the opening of the hole in the steel material. FIG. 3 is a cross-sectional view showing the state in which the jig 2 is placed on the steel material 1 shown in FIG. 1B. FIG. 4 is an enlarged view of the vicinity of the opening of the hole 1a in FIG. 3. In the example shown in FIG. 3 and FIG. 4, the jig 2 includes a tapered portion 2t whose thickness (area of ​​the cross section perpendicular to the axis J3) changes in the axis J3 direction of the jig 2, and a thin shaft portion 2h extending in the axis J3 direction from the thin end of the tapered portion 2t. The thickness of the thin shaft portion 2h is the same as the thickness of the thin end of the tapered portion 2t. In the tapered portion 2t, the cross-sectional area of ​​the cross section D1 where the cross section perpendicular to the axis J3 direction is maximum is larger than the area of ​​the opening of the hole 1a (the area of ​​the region surrounded by the inner diameter of the edge 1c of the opening in the plane perpendicular to the axis J2 of the hole 1a). In the tapered portion 2t, the cross-sectional area of ​​a cross section D2 perpendicular to the axis J3 direction that is the smallest is smaller than the area of ​​the opening of the hole 1a.

[0038] The jig 2 is placed so that the axis J3 of the jig 2 and the axis J2 of the hole 1a are coaxial. When the jig 2 is placed, the tapered portion 2t contacts the edge 1c of the opening of the hole 1a, and the thin shaft portion 2h is located inside the hole 1a. A part of the tapered portion 2t is located outside the opening of the hole 1a. The outer surface 2a of the tapered portion 2t is an inclined surface that is inclined with respect to the axial direction of the hole 1a. The inclined surface is inclined so as to move away from the inner surface of the hole 1a as it goes from the opening of the hole 1a to the back of the hole. The outer surface 2a of the tapered portion 2t, which is this inclined surface, contacts the edge 1c of the opening of the hole 1a. In this example, the jig 2 is placed so that it contacts the edge 1c of the opening of the hole 1a, and does not contact the inner surface of the hole other than the edge 1c of the opening of the hole 1a. In the subsequent carburizing treatment (S3) and quenching (S4), the jig 2 prevents the inner diameter of the opening of the hole 1a from shrinking.

[0039] FIG. 5 is a cross-sectional view showing an example of a state in which a jig contacts the edge of the opening of a hole. FIG. 5(a) and FIG. 5(b) are examples of a case in which a corner of the opening of a hole 1a is chamfered. In the example of FIG. 5(a), the inclined surface (outer surface 2a) of the jig 2 contacts the end of the chamfered surface 1c2 on the inside (inner diameter side) of the hole 1a. In the example of FIG. 5(b), the inclined surface (outer surface 2a) of the jig 2 contacts the end of the chamfered surface 1c2 on the outside (outer diameter side) of the hole 1a. Although not shown, the outer surface 2a may be in surface contact with the entire or part of the chamfered surface 1c2.

[0040] In the example of Fig. 5(c), the cross-sectional shape of the edge 1c of the opening of the hole 1a in a plane passing through the axis J3 is curved. That is, the edge 1c of the opening of the hole 1a is rounded (has R). In this case, the inclined surface (outer surface 2a) of the jig 2 contacts the curved surface of the edge 1c of the opening of the hole 1a. Note that the form in which the jig contacts the edge of the opening of the hole is not limited to the examples shown in Figs. 5(a) to 5(c).

[0041] Fig. 6 is a cross-sectional view taken along line CC in Fig. 4. Fig. 6 shows a cross-section of the edge 1c of the opening of hole 1a that contacts the jig 2. In the example shown in Fig. 6(a), the jig 2 contacts the entire edge 1c of the opening of hole 1a. By having the jig 2 contact the entire edge 1c, the effect of the jig 2 in suppressing diameter reduction is enhanced.

[0042] In the example shown in FIG. 6(b), the jig 2 contacts the edge 1c of the opening of the hole 1a at a portion of the circumferential direction. The jig 2 contacts the edge 1c of the opening of the hole 1a at two points. The axis J3 of the hole 1a is located between the two portions of the edge 1c with which the jig 2 contacts. In other words, the jig 2 contacts the edge 1c of the opening at positions that face each other across the axis J3 of the hole 1a. The jig 2 and the edge 1c of the opening of the hole 1a may contact each other at three or more points.

[0043] In the example shown in FIG. 3, a first jig 21 that contacts the edge of the opening of the hole 1a on one outer surface and a second jig 22 that contacts the edge of the opening of the hole 1a on the other outer surface are connected to each other and arranged. The other outer surface is the outer surface opposite to the one outer surface. As a result, the jig 2 suppresses the reduction in diameter of the opening edge of the outer surface at both ends of the hole 1a. Note that the configuration of the jig that suppresses the opening edge at both ends of the hole 1a is not limited to the example shown in FIG. 3. For example, the first jig 21 and the second jig 22 may be integrally formed. In this case, the edges at both ends of the hole 1a can be suppressed by the integrally formed jig. Also, the first jig 21 and the second jig 22 may not be connected to each other and may independently suppress the opening edge at both ends of the hole 1a.

[0044] FIG. 7 is a diagram showing an example of a configuration for connecting the first jig 21 and the second jig 22. FIG. 7(a) and FIG. 7(b) show an example in which the first jig 21 and the second jig 22 are connected by a screw. In the example of FIG. 7(a), a female thread is formed on the end of the first jig 21 and the second jig 22 opposite to the tapered portion. The first jig 21 and the second jig 22 are connected by a connecting shaft having a male thread formed on the outer periphery. The male thread on one end of the connecting shaft 3 meshes with the female thread of the first jig 21, and the male thread on the other end of the connecting shaft 3 meshes with the female thread of the second jig 22.

[0045] 7(b), a female thread is formed on the end of the first jig 21 opposite the tapered portion, and a male thread is formed on the end of the second jig 22 opposite the tapered portion. The female thread of the first jig 21 and the male thread of the second jig 22 are engaged with each other to connect them. The female thread of the first jig 21 and the male thread of the second jig 22 may have a stopper that limits the amount of screwing, or may not have a stopper.

[0046] As shown in Figures 7(a) and 7(b), in a configuration in which the first jig 21 and the second jig 22 are connected by a screw, the jig can more reliably suppress the opening of the hole by the tightening torque of the screw. Also, the relative positional relationship between the first jig 21 and the second jig 22 can be adjusted by the amount of screwing in the screw. Note that the form of connection by the screw is not limited to the example shown in Figures 7(a) and 7(b).

[0047] In the example shown in FIG. 7(c), the first jig 21 and the second jig 22 are connected by a connecting pin. In this example, the inner diameter of the end of the first jig 21 opposite the tapered portion is larger than the outer diameter of the end of the second jig 22 opposite the tapered portion. The end of the first jig 21 and the end of the second jig can overlap in the radial direction. The end of the first jig 21 and the end of the second jig 22 are each provided with a hole for passing the connecting pin 4. With the ends of the first jig 21 and the second jig 22 overlapping, the connecting pin 4 is inserted into these holes.

[0048] The material of the jig 2 is not limited to a specific one, but for example, steel that undergoes martensitic transformation in the carburizing and quenching process of the steel material may be used as the material of the jig 2. The jig 2 may be a hollow cylinder or a solid rod. From the viewpoint of strength, it is preferable that the jig 2 is a solid body.

[0049] (S3: Carburizing treatment) In the carburizing process, the steel material is heated to cause carbon to penetrate into the surface layer of the steel material. The carburizing process is not limited to a specific form, and can be performed by, for example, solid carburizing, liquid carburizing, gas carburizing, or vacuum carburizing. The carburizing process may include a carburizing step for causing carbon to penetrate into the surface layer of the steel material, and a soaking step. The carburizing process is performed on the steel material on which a jig is placed.

[0050] The carburizing temperature in the carburizing step is, for example, 800 to 1100°C. The lower limit of the carburizing temperature is preferably 900°C. The upper limit of the carburizing temperature is preferably 1000°C, more preferably 950°C.

[0051] The carbon potential (Cp1) in the carburizing step is not limited to this, but may be set to 0.7 to 1.1%. The carbon potential (Cp1) may be varied in the carburizing step.

[0052] After the carburizing step, the atmosphere of the steel material is held at a predetermined temperature for a predetermined time to perform soaking. The temperature in the soaking is, for example, 800 to 900°C, but is not limited thereto. The lower limit of the soaking temperature is preferably 820°C, and the upper limit of the soaking temperature is preferably 870°C. The temperature holding time (constant temperature holding time) in the soaking is, for example, 5 to 80 minutes. The lower limit of the soaking temperature holding time is preferably 20 minutes, and the upper limit of the soaking temperature holding time is preferably 60 minutes.

[0053] (S4: Quenching) The steel material that has been heated and soaked in the carburizing process is quenched by cooling. Quenching is performed with a jig placed in the hole of the steel material. The steel material with the jig placed in the hole is heated to a temperature of Ac3 point or higher to cause austenite transformation, and then quenched to cause martensite transformation. Quenching may be, for example, oil quenching. Tempering may also be performed after quenching.

[0054] In the manufacturing method shown in Fig. 2, in the quenching step (S4), quenching is performed with the jig 2 inserted in the hole 1a as shown in Fig. 3. When the inner diameter of the hole 1a shrinks during quenching, the jig 2 prevents the inner diameter of the edge 1c of the opening of the hole 1a from shrinking. This increases the compressive residual stress near the edge 1c of the opening of the hole 1a.

[0055] In this embodiment, the shape of the jig 2 is a shape that can restrain the shrinkage of the edge of the opening of the hole 1a. In addition, in the example shown in FIG. 3, the jig 2 is shaped so that it contacts only the edge 1c of the opening of the hole 1a, which may be the starting point of the fracture, and does not contact the center of the hole 1a in the axial J2 direction. That is, the thickness of the thin shaft portion 2h of the jig 2 is smaller than the inner diameter of the hole 1a. The shrinkage of the inner diameter of the hole 1a after quenching is the largest in the center of the hole 1a in the axial J2 direction. By arranging the jig 2 so that it contacts only the edge 1c of the opening of the hole 1a, the jig 2 can efficiently suppress the shrinkage of the opening of the hole 1a. In addition, by providing an inclined surface such as the outer surface 2a of the tapered portion 2t at the contact portion of the jig 2 with the edge 1c of the opening of the hole 1a, it is possible to prevent the formation of a gap at the contact portion between the opening of the hole 1a and the jig 2. This makes it possible to more efficiently exert the effect of suppressing the shrinkage of the opening of the hole 1a.

[0056] According to this embodiment, the diameter of the opening of the hole is suppressed by the jig during quenching, so that the opening of the hole can be given a compressive residual stress. Therefore, a process of giving compressive residual stress by plastic processing such as shot peening, ultrasonic impact, or cutting is not required. The inner surface of the hole of the steel material after quenching maintains the surface roughness of the inner surface after carburizing. Therefore, the surface roughness of the inner surface of the hole of the steel member manufactured by the manufacturing method of this embodiment is equal to or greater than the surface roughness Ra obtained by applying carburizing and quenching, and is equal to or less than the surface roughness Ra caused by plastic processing. For example, the surface roughness Ra in a region within 2 mm from the edge of the inner surface of the opening of the hole can be set to 0.2 to 5.0 μm. This can provide a steel member that minimizes the risk of fatigue failure due to surface roughness and gives compressive residual stress to the opening of the hole of the steel member.

[0057] The present invention is not limited to the above-described embodiment. For example, the manufacturing method does not exclude shot peening, ultrasonic impact, cutting, or other plastic processing, and may include these steps.

[0058] The steel member is not particularly limited, and may be, for example, a shaft member having an oil hole on the outer circumferential surface. For example, shaft members used in automobiles and industrial machines are repeatedly exposed to high stress. Such shaft members are required to have excellent fatigue resistance. A torsional torque is applied to the shaft member to transmit rotational power. Therefore, high static torsional strength and torsional fatigue strength are required. Since the circumferential stress generated in a shaft member loaded with a torsional load is proportional to the radius, the stress on the outer circumferential side of the shaft member becomes more critical than the inner circumferential side. When a repeated load is applied to a shaft member having an oil hole, stress may concentrate on the outer circumferential surface of the oil hole, i.e., the edge of the opening, which may become the starting point of destruction. By applying the above embodiment to a shaft member, residual compressive stress can be added to the edge of the opening of the oil hole on the outer circumferential surface of the shaft member without complicating the process. Therefore, the fatigue strength of the shaft material can be improved. EXAMPLES

[0059] A shaft member with oil holes was modeled as a steel member with holes and a simulation was performed. The shaft member was analyzed using a carburizing and quenching simulation. In the analysis, the residual stress due to heat treatment was evaluated. Figure 8 shows the three-dimensional model used in the analysis. The shaft member was cylindrical with an outer diameter of 24 mm and an inner diameter of 8 mm. An oil hole with a diameter of 4 mm was drilled through the shaft member in a direction perpendicular to its axis. Both ends of the oil hole (the edges of the opening) were chamfered. To reduce calculation costs, a model was created that was 1 / 8 the size of the shaft member by utilizing symmetry. Hexahedral elements were used for the elements that make up the shaft member. JIS SCM420, a common steel for carburizing machines, was used for the material of the shaft member. The chemical composition of the material is shown in Table 1 below.

[0060] [Table 1]

[0061] Figure 9 shows the heat pattern of carburizing and quenching. In this analysis, typical semi-hot oil cooling conditions were used. Figure 10 shows the heat transfer coefficient of the oil. The oil temperature was set to 130°C. Actual measured data for SCM420 was used for mechanical properties such as Young's modulus and stress-strain diagram. Thermal properties such as specific heat and thermal conductivity were calculated using a prediction formula based on the chemical components.

[0062] Figure 11 shows the residual stress distribution in the circumferential direction of the oil hole on the outermost surface of the inner surface of the oil hole from the outer diameter end of the shaft (edge ​​of the opening) to the inner diameter end of the shaft, obtained by analysis. Note that Figure 11 also shows the residual stress distribution when carburizing and quenching are performed without placing a jig in the oil hole. In Figure 11, the distance from the outer surface of the shaft is the distance from the edge of the oil hole opening in the axial direction of the oil hole. In the example of Figure 11, compressive residual stress is distributed throughout the entire axial direction of the oil hole. It can be seen that the compressive residual stress is greatest in the center of the oil hole in the axial direction (thickness direction) and is smallest at the ends.

[0063] Figure 12 shows the deformation before and after quenching obtained by analysis. The deformation is displayed at a magnification of 50 times. It can be seen that the inner diameter of the oil hole has been reduced by quenching.

[0064] Next, a quenching analysis of a shaft member was performed under the conditions of this embodiment. Fig. 13 shows an analysis model of a shaft member including a jig. The shape of the jig is not limited to the example shown in Fig. 13. The shape of the jig may be changed depending on the shape of the shaft.

[0065] Fig. 14 is a graph showing a comparison of residual stress distribution in the circumferential direction of the oil hole at the outermost surface of the inner surface of the oil hole from the shaft outer diameter end (edge ​​of the opening) to the shaft inner diameter end after quenching without a jig and quenching with a jig obtained by analysis. From the results in Fig. 14, it was confirmed that the compressive residual stress increases at the shaft outer diameter end (near the edge of the opening), which is the starting point of fracture (crack). In the quenching without a jig, the residual stress at the shaft outer diameter end was -292 MPa, whereas in the quenching with a jig, the residual stress was -391 MPa. By using a jig as in this embodiment, the compressive residual stress can be increased by 30% or more.

[0066] From the above results, it was found that quenching using a jig is expected to increase the residual stress in the compressive direction at the end (edge ​​of the opening) of the oil hole on the outer periphery of the shaft, which is the starting point of fracture, and to improve fatigue strength. Specifically, it is thought that by inserting a jig into the oil hole and performing quenching, the edge of the opening of the oil hole, which is trying to shrink, and the jig are restrained, generating inelastic tensile strain at the edge of the opening. This makes it possible to increase the compressive residual stress at the end (edge ​​of the opening) of the oil hole, which is the starting point of fracture after quenching. [Explanation of symbols]

[0067] 1. Steel material 1a hole 1c Edge of hole opening 2 Jig 2t tapered section

Claims

1. Providing a steel blank having a hole on an outer surface thereof; A step of heating the material to perform a carburizing treatment; and a quenching step of quenching the heated material by cooling the material. In the quenching step, quenching is performed in a state where a jig is placed in a position in contact with at least a portion of the entire circumference of the edge of the opening of the hole and prevents contraction of the inner diameter of the edge of the opening of the hole, The method for manufacturing a steel component, wherein the hole is excluding a hole having an internal thread.

2. A method for producing a steel member according to claim 1, A method for manufacturing a steel member, wherein during the hardening, the jig is positioned so as to be in contact with the edge of the opening of the hole and not in contact with any inner surface other than the edge of the opening of the hole.

3. A method for producing a steel member according to claim 1 or 2, A method for manufacturing a steel component, wherein the jig has an inclined surface that is oblique to the axial direction of the hole, and during the hardening, the jig is positioned so that the inclined surface of the jig contacts the edge of the opening of the hole.

4. Providing a steel blank having a hole on an outer surface thereof; A step of heating the material to perform a carburizing treatment; and a quenching step of quenching the heated material by cooling the material. In the quenching step, quenching is performed in a state where a jig is placed in a position in contact with at least a portion of the entire circumference of the edge of the opening of the hole and prevents contraction of the inner diameter of the edge of the opening of the hole, the hole extends through the steel blank; A manufacturing method for a steel member, wherein during the hardening, a first jig in contact with the edge of the opening of the hole on one outer surface and a second jig in contact with the edge of the opening of the hole on the other outer surface opposite the one outer surface are placed in the hole while being connected to each other by screws.

5. A method for producing a steel member according to any one of claims 1 to 4, the steel member is a shaft member, A method for manufacturing a steel member, wherein an axial direction of a hole in the steel member is inclined at an angle with respect to an axial direction of the shaft member.

6. A jig for inserting and arranging a steel material having a hole on an outer surface thereof into the hole when hardening the steel material, the jig comprising: The jig includes a first jig and a second jig; The first jig and the second jig are A first jig that contacts an edge of the opening of the hole on one outer surface of the steel material and a second jig that contacts an edge of the opening of the hole on the other outer surface opposite to the one outer surface are configured to be able to be placed in the hole while being connected to each other by a screw; Each of the first jig and the second jig is A tapered portion whose thickness changes in the axial direction is provided. A jig wherein, in the tapered portion, the maximum cross-section of a surface perpendicular to the axial direction is greater than the area of ​​the opening of the hole, and, in the tapered portion, the minimum cross-section of the surface perpendicular to the axial direction is smaller than the area of ​​the opening of the hole.

7. A steel member having a hole on an outer surface thereof, a portion of the outer surface within 1 mm from the edge of the opening of the hole has a larger residual compressive stress than a portion of the outer surface beyond 1 mm from the edge of the opening of the hole; The surface roughness Ra of the inner surface of the hole in a region within 2 mm from the edge of the opening is 0.2 to 5.0 μm; A steel member wherein a corner of the opening is an edge of the opening, and if the corner of the opening is chamfered, the chamfered surface is the edge of the opening, and if the cross-sectional shape of the corner of the opening is curved, the curved part is the edge of the opening.

8. A steel member according to claim 7, A steel member, wherein in a region of the inner surface of the hole within 2 mm from the edge of the opening, there is a portion in which compressive residual stress decreases with increasing distance from the edge of the opening in the axial direction of the hole.

9. A steel member according to claim 7 or 8, the steel member is a shaft member, A steel member, wherein an axial direction of a hole in the steel member is inclined at an angle with respect to an axial direction of the shaft member.

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