JP2006104551A - High strength machine part having excellent fatigue property and method for improving its fatigue property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide high strength machine parts having a notched part with the radius of curvature of ≤25 mm and further having a surface HV hardness of ≥250, and to provide a method for improving its fatigue strength. <P>SOLUTION: Regarding the high strength machine parts having excellent fatigue strength, in machine parts having a composition of, by mass, 0.1 to 1.2% C, and the balance Fe with inevitable impurities, and having a notched part with the radius of curvature of ≤25 mm, the HV hardness of the surface is ≥250, and further, the compressive residual stress of the surface layer in the notched part is -100 to -1,500 MPa, and to provide a method for improving its fatigue strength. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a high-strength mechanical component having excellent fatigue characteristics such as a shaft and gears widely used in automobiles and various industrial machines, and a method for improving fatigue characteristics.
Specifically, the present invention relates to a high-strength mechanical component having a notch portion with a radius of curvature of 25 mm or less and having a surface HV hardness of 250 or more and excellent fatigue properties, and a method for improving fatigue properties.

In order to reduce the weight and performance of automobiles and various industrial machines, there is a growing need for increasing the strength of various machine parts. The increase in strength of these machine parts is achieved by adjusting the chemical composition of steel materials and various heat treatment methods.・ It can be achieved under certain conditions.
However, even if the strength of the machine part can be increased, the fatigue characteristics may not improve as the strength of the machine part increases. In particular, in a machine part having a notch, the effect of improving fatigue strength is small even if the strength is increased. This is because stress concentrates on the notch.
Generally, as described in Non-Patent Document 1, as means for increasing fatigue strength, there are surface hardening treatment such as induction hardening, carburizing, nitriding, carbonitriding, and tuftride treatment, shot peening and surface roll processing treatment, etc. . However, in a machine part having a notch with a small radius of curvature, there is a limit to the means for increasing the fatigue strength as described above, and there is a problem that a significant improvement in fatigue characteristics cannot be realized.

Further, in a crankshaft of an internal combustion engine, a joint portion between an arm and a pin or a journal is a fillet portion having a rounded cross section. This fillet portion is a location where twisting stress and bending stress during crankshaft rotation tend to concentrate. For this reason, it is practiced to reinforce the fillet part, and examples thereof include a roll processing method and an induction hardening method.
The roll processing method is a technique for improving the strength by cold working the boundary between the pin / flange and the journal / flange with a fillet roll. As a conventional technique, for example, Patent Document 1 discloses a technique for reducing the stress concentration by increasing the radius of curvature of a portion where tensile stress is applied. Techniques to ease concentration are introduced.
On the other hand, the induction hardening method is a technique in which the surface of the pin, the journal portion, the pin / flange, and the fillet portion at the boundary of the journal / flange is martensite by induction hardening to increase the strength. As a prior art, for example, Patent Document 3 introduces an induction hardening method and apparatus that hardly cause quenching cracks.
"Metallic Material Fatigue Design Handbook" (edited by the Society of Materials Science, Yokendo, August 20, 1981, 2nd edition) 95-105 JP 2002-122126 A JP 2002-224920 A JP 2002-173711 A

  The present invention has been made in view of the actual situation as described above, and has a high-strength mechanical component having a fatigue characteristic having a notch portion with a radius of curvature of 25 mm or less and a surface HV hardness of 250 or more, and its fatigue. It is an object to provide a strength improvement method.

  The present inventors have made various studies on methods for improving the fatigue properties of high-strength mechanical parts having notches subjected to various heat treatments such as quenching / tempering treatment, carburizing treatment, and induction hardening treatment. As a result, in order to improve fatigue characteristics, it is essential to apply compressive residual stress to the notch, and in conventional shot peening, compressive residual stress is efficiently applied to the notch having a small radius of curvature. Clarified that it is difficult to do. Therefore, as a result of examining various means as an application method of compressive residual stress in place of shot peening, it was found that ultrasonic hitting treatment is extremely effective for introduction of compressive residual stress, and fatigue characteristics are greatly improved. Furthermore, the manufacturing technology of the optimum ultrasonic hitting process was established. Based on the above examination results, it is concluded that if a compression residual stress and compressive residual stress applying method by ultrasonic impact treatment are optimally selected, a high-strength mechanical part having a notch with excellent fatigue characteristics can be realized. And the present invention has been made.

The present invention has been made on the basis of the above findings, and the gist thereof is as follows.
(1) It is a machine part having a notch portion with a mass radius of C: 0.1 to 1.2%, the balance being made of steel consisting of Fe and inevitable impurities, and a radius of curvature of 25 mm or less. A high-strength mechanical component having excellent fatigue properties, wherein the steel material has an HV hardness of 250 or more and a compressive residual stress of the notch surface layer is -300 to -1500 MPa.
(2) The aspect ratio, which is the ratio of the length in the major axis direction to the minor axis direction of the crystal grains, is 1.5 or more in a region where the depth from the surface layer of the notch is at least 30 μm or less. A high-strength mechanical component having excellent fatigue properties as described in (1).
(3) A method for improving fatigue characteristics of a high-strength mechanical component that applies a compressive residual stress by hitting a notch of the high-strength mechanical component according to (1) or (2) with an ultrasonic vibrator,
Hardness ratio of the ultrasonic vibrator to mechanical parts: 1.1 or more, ultrasonic vibrator frequency: 10-60 kHz, ultrasonic output: 500-5000 W, pressing force to the notch of the ultrasonic vibrator : A method for improving the fatigue characteristics of a high-strength mechanical component, characterized by subjecting the notched portion of the high-strength mechanical component to ultrasonic striking treatment under a condition of 10 to 1000 N.

  According to the present invention, the fatigue characteristics of a high-strength mechanical component having a notch with a radius of curvature of 25 mm or less can be significantly improved by applying an ultrasonic impact treatment to the notch and introducing a compressive residual stress. Thus, it is possible to provide a high-strength mechanical part excellent in fatigue characteristics and a method for improving fatigue characteristics, and thus, there are significant industrially useful effects.

The best mode for carrying out the present invention will be described below.
First, the reasons for limiting the components of the steel material that is the subject of the present invention will be described.
C is an essential element for securing the strength of the machine part. However, if it is less than 0.1%, it is difficult to obtain the target HV hardness of 250 or more, while it exceeds 1.2%. Since the ductility of machine parts is reduced, the content is limited to a range of 0.1 to 1.2%.
Alloy elements such as Si, Mn, Cr, Mo, and Ni can be added depending on the heat treatment conditions and applications of various machine parts. Preferred ranges are: Si: 0.01-2%, Mn: 0.2-2%, Cr: 0.05-2%, Mo: 0.05-3%, Ni: 0.05-2%, Cu : 0.05 to 1%, B: 0.0003 to 0.005%. Furthermore, the preferable range of Al, Ti, Nb, and V is a range of 0.002 to 0.5% for all elements. The preferable ranges of P and S are P: 0.015% or less and S: 0.05% or less.
In addition, the present invention can be applied to any steel material such as ferrite-pearlite steel, bainite steel, martensitic steel, etc. It can be widely applied when it is cut after forging and subjected to carburizing, induction hardening, quenching or tempering.

Next, the reasons for limiting the radius of curvature, surface HV hardness, and compressive residual stress of machine parts will be described. When there is a large notch with a radius of curvature exceeding 25 mm, the fatigue strength is not significantly reduced, and thus is excluded from the scope of the present invention. In addition, when compressive residual stress is applied to a part having a notch exceeding 25 mm, the shot peening process of the prior art is more economical than the ultrasonic hitting process, so the upper limit of the radius of curvature is limited to 25 mm. did. The technique of the present invention is sufficiently effective even when the surface HV hardness of the machine part is less than 250. However, in general, high fatigue strength is not required in the case of low-strength steel, so the lower limit of the surface HV hardness is 250. Restricted to. If the compressive residual stress of the surface layer of the notch is less than −300 MPa, the effect of improving the fatigue strength is small. On the other hand, even if a compressive residual stress exceeding −1500 MPa is applied, the effect of improving the fatigue characteristics is saturated. The range of compressive residual stress was limited to -300 to -1500 MPa. The residual stress of the present invention is measured by the X-ray method.

When the ultrasonic hitting treatment of the present invention is not performed, the aspect ratio in the major axis direction and the minor axis direction of the crystal grains is less than 1.5, and the effect of improving fatigue characteristics is insufficient. Limited to 1.5. In addition, if the region having an aspect ratio of 1.5 or more is less than 30 μm from the notched surface layer, it is difficult to obtain a sufficient fatigue strength improvement effect, so the lower limit was limited to 30 μm. In the present invention, the aspect ratio of the crystal grains is measured with an optical microscope of 500 times. When the structure of the machine part is ferrite pearlite, it is the average value of ferrite grains and pearlite grains, when pearlite is the main component, it is the average value of pearlite grains, and when it is martensite or tempered martensite, it is the average value of prior austenite grains. is there.
Next, conditions for the ultrasonic hitting process will be described. In the present invention, after finishing the final machine part through various processes such as hot forging, cold forging, various machining and quenching / tempering treatment, carburizing treatment, induction hardening treatment according to the specifications of the machine parts, Finally, an ultrasonic hitting process is performed on the notch.

  If the hardness of the ultrasonic vibrator is less than 1.1 times the hardness of the notch surface, it is difficult to efficiently apply compressive residual stress to the notch by the ultrasonic hitting process. And the hardness ratio of machine parts was limited to 1.1 or more. Although the radius of curvature of the tip of the ultrasonic transducer is not particularly limited, it is not possible to efficiently apply compressive residual stress if the radius of curvature is larger than the notch radius of the machine part. It is a preferable condition that the tip radius is equal to or less than the curvature radius of the notch. If the frequency of the ultrasonic vibrator is less than 10 kHz, the compressive residual stress cannot be applied efficiently, so the lower limit is limited to 10 kHz. On the other hand, since the effect of introducing the compressive residual stress is saturated even if the ultrasonic impact treatment is performed at a frequency exceeding 60 kHz, the upper limit of the frequency is limited to 60 kHz. A preferable range of the frequency is 20 to 40 kHz. If the output of the ultrasonic wave is less than 500 W, the ultrasonic striking treatment time for applying the predetermined compressive residual stress becomes long and not economical, so the lower limit is limited to 500 W. Even if the ultrasonic output exceeds 5000 W, the effect is saturated, so 5000 W was set as the upper limit. If the pressing force to the notch of the ultrasonic transducer is less than 10N, it is not economical because compressive residual stress cannot be efficiently applied, so the lower limit is limited to 10N. On the other hand, since the effect is saturated even if the pressing force exceeds 1000 N and the ultrasonic impact treatment is performed, the upper limit is limited to 1000 N.

The application of compressive residual stress by ultrasonic hitting treatment has better fatigue characteristics than the application of compressive residual stress by shot peening. The reason is
1) The compressive residual stress by ultrasonic hitting treatment is higher than that of shot peening 2) The compressive residual stress by ultrasonic hitting treatment is applied to the inside of the steel material rather than shot peening 3) The site of ultrasonic hitting treatment is plastically deformed 4) It is estimated that the surface roughness due to the ultrasonic hitting process is smaller than that of shot peening.

Hereinafter, the effects of the present invention will be described more specifically with reference to examples.
A round bar-shaped part (shaft) as shown in FIG. 1 was manufactured by hot forging using steel materials having chemical components shown in Table 1. The hot forging temperature is 1200 ° C. Then, it machine-finished into the machine part which has a notch shown in FIG. Furthermore, quenching / tempering treatment, high-frequency heat treatment, and carburization treatment were performed using these mechanical parts, and the HV hardness of the surface was measured. The quenching / tempering treatment was performed under the conditions of quenching temperature: 850 to 950 ° C. and tempering temperature: 180 to 650 ° C. In the high-frequency heat treatment, after heating to 950 ° C., a quenching treatment was performed, and then a tempering treatment was performed at 150 ° C. The carburizing process was performed under the conditions of a carburizing temperature of 950 ° C. and a tempering temperature of 160 ° C. In addition, ultrasonic hitting treatment was finally applied to the notch. The residual stress at the notch was measured by the X-ray method after the ultrasonic impact treatment. The aspect ratio of the crystal grains was determined by observing 10 fields or more in a 30 μm region from the surface layer of the notch with a 500 × optical microscope. The fatigue strength (10 ⁷ cycles) of machine parts was investigated by a rotating bending fatigue test. The above production conditions and measurement results are shown in Table 2.

Test Nos. 3, 5, 7, 8, 9, 12, 16, 18, 20, 21, 23, 25, 28, 32, 34, and 37 in Table 2 are examples of the present invention, and other examples are comparative examples. . As can be seen from the table, in all of the examples of the present invention, a high compressive residual stress is imparted to the notch, and the aspect ratio of the crystal grains is 1.5 or more. As a result, a high-strength component having higher fatigue strength and superior fatigue characteristics than the comparative example is realized.
On the other hand, test Nos. 1, 4, 6, 8, 11, 14, 17, 19, 22, 24, 27, 30, 33, and 35, which are comparative examples, are all notched after manufacturing the machine part. This is a case where the residual stress control process is not performed. Since the compressive residual stress is low or is a tensile residual stress, both are examples in which the fatigue strength is lower than that of the present invention.

Test Nos. 2, 15, 31, and 36, which are comparative examples, are all subjected to conventional shot peening treatment after component manufacture. By performing the shot peening process, although the residual stress in the notch part moves to the compressive residual stress side, a large compressive residual stress cannot be efficiently applied because the radius of curvature of the notch part is small. As a result, the fatigue strength is inferior to the examples of the present invention in any of the examples.
Test Nos. 10, 13, 26, 29, and 38, which are comparative examples, are all examples in which the conditions of the ultrasonic impact treatment are inappropriate. That is, No. 10 has a low hardness ratio between the ultrasonic vibrator and machine parts, and No. 13 has a low frequency of the ultrasonic vibrator, so No. 26 goes to the notch of the ultrasonic vibrator. No. 29 is low in hardness ratio and the vibration frequency of the ultrasonic transducer is low, and No. 38 is low in ultrasonic output. This is an example in which the effect of improving fatigue strength is low.

It is a figure which illustrates the machine component which has a notch used in the Example of this invention.

Claims (3)

  It is a mechanical part having a notch part with a curvature radius of 25 mm or less, comprising C: 0.1-1.2% by mass, the balance being made of a steel material composed of Fe and inevitable impurities, A high-strength mechanical component having excellent fatigue characteristics, wherein the steel material has an HV hardness of 250 or more and a compressive residual stress of the notch surface layer is -300 to -1500 MPa.   The aspect ratio, which is the ratio of the length in the major axis direction to the minor axis direction of the crystal grains, is 1.5 or more in a region where the depth from the surface layer of the notch is at least 30 μm or less. A high-strength mechanical component having excellent fatigue characteristics as described in 1. A method for improving fatigue characteristics of a high-strength mechanical component by applying a compressive residual stress by hitting a notch portion of the high-strength mechanical component according to claim 1 or 2 with an ultrasonic transducer, wherein the ultrasonic vibration Hardness ratio of child to mechanical parts: 1.1 or more, ultrasonic vibrator frequency: 10-60 kHz, ultrasonic output: 500-5000 W, pressing force on notch of ultrasonic vibrator: 10-1000 N A method for improving fatigue characteristics of a high-strength mechanical component, comprising subjecting the notched portion of the high-strength mechanical component to an ultrasonic impact treatment under the conditions described above.

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