JP2010185478A - Torsion bar and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a torsion bar having high strength against fatigue which can cope with a large stress load, and its manufacturing method. <P>SOLUTION: High-frequency quenching is applied to a steel material which is a raw material of the torsion bar in a state that a torsional stress is imposed in a 45°-direction (generation direction of tension main stress) which is the use direction of the torsion bar, and after that, a compression residual stress is imparted in the 45°-direction by releasing a load of the torsional stress. In the torsion bar, the fatigue generated by the tension main stress generated in the 45°-direction which is the use direction causes breakage, but when the compression residual stress is imparted in the 45°-direction, the tension main stress generated in the 45°-direction is offset by the compression residual stress at the use of the torsion bar. When the torsional stress imposed during the high-frequency quenching is lower than 900 MPa, a limit of the fatigue of a sample is almost the same as that not imposed with the torsional stress, but when the torsional stress is greater than or equal to 900 MPa, the limit of the fatigue of the sample is increased. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a torsion bar to which a torsional stress is applied during use and a method for manufacturing the torsion bar, and more particularly to improvement of a technique for improving fatigue strength.

  A torsion bar which is a linear spring is used for suspension of automobiles. The torsion bar uses a spring action due to torsional deformation when its one end is fixed and the other end is twisted, and since the elastic energy stored per unit volume is large, it is lightweight and compact.

  In such a torsion bar manufacturing method, various techniques have been proposed to improve the fatigue strength. For example, in Non-Patent Document 1, a tempered torsion bar tempered to have a predetermined hardness is obtained by quenching and tempering using SUP9 as the material of the torsion bar. Further, the induction hardening material is often changed from a tempered alloy to carbon steel, but the induction hardening torsion bar is also changed to carbon steel such as S45C. In this case, the material made of S45C is induction-quenched so that the ratio of the depth of the cured layer formed on the surface (= the depth of the cured layer / the radius of the torsion bar) is 45 to 75%. By tempering at a low temperature (150 ° C.), higher fatigue strength than that of a normal torsion bar is obtained.

  On the other hand, techniques for improving the fatigue strength of spring products include the application of compressive residual stress by shot peening or induction hardening. In addition, stress shot peening and stress induction hardening, which have been developed for application to leaf springs, are techniques for applying these techniques to obtain a larger compressive residual stress.

  For example, in the stress shot peening of Patent Document 1, shot peening is performed on the surface of the leaf spring in a state where bending stress is applied in the direction of use. In the leaf spring after shot peening, a compressive residual stress larger than that in normal shot peening is generated in the surface layer due to a restoring force (a force that the leaf spring tries to return) when removing it from the jig. For example, in the stress induction hardening of Patent Document 2, induction hardening is performed on the surface in a state where bending stress is applied to the leaf spring in the direction of use. In the leaf spring after induction hardening, a compressive residual stress larger than that of normal induction hardening occurs due to a restoring force that acts when the leaf spring is removed from the jig.

JP-A-60-227929 JP 2006-71082 A

Research on induction hardening torsion bar, Yasuo Sato, Akira Tange, Katsuyoshi Shirai, Spring Technology Study Group

  By the way, in automobiles, it is required to reduce the weight of each part in order to improve fuel consumption. A reduction in diameter is effective for reducing the weight of the torsion bar. However, in this case, since a larger stress is applied to the torsion bar, the fatigue strength of the conventional torsion bar as described above is insufficient.

  Accordingly, an object of the present invention is to provide a torsion bar having a high fatigue strength that can cope with a large stress load due to a reduction in diameter and a method for manufacturing the torsion bar.

  In the torsion bar, as shown in FIG. 11, when one end is fixed and the other end is twisted in the arrow direction T, a tensile main stress is generated in the 45 ° direction with respect to the axis, and the main compression is performed in the −45 ° direction with respect to the axis. Stress is generated. The present inventor considers the state of stress generation when using such a torsion bar, and improves the fatigue strength by applying torsional stress as follows during induction hardening to the steel material that is the material of the torsion bar. I found out that I can plan.

  In a state where a torsional stress is applied in the 45 ° direction, which is the direction of use of the torsion bar, induction hardening is performed on the steel material. Thereafter, when the torsional stress is released, compressive residual stress is applied in the 45 ° direction. In a torsion bar, fatigue due to tensile principal stress generated in the direction of 45 °, which is the direction of use, causes failure. However, when compressive residual stress is applied in the direction of 45 ° as described above, the direction of 45 ° The tensile principal stress generated in the above is offset with the compressive residual stress, and as a result, the fatigue strength can be improved. In this case, in order to obtain such an effect, it is necessary to set the torsional stress applied to the steel material during induction hardening to 900 MPa or more.

  The method for producing a torsion bar according to the present invention is characterized in that induction hardening is performed on a steel material in a state where a torsional stress of 900 MPa or more is applied to the steel material. In the torsion bar manufacturing method of the present invention, by setting the torsional stress applied to the steel material during induction hardening to 900 MPa or more, the fatigue strength can be improved, and as a result, it is possible to cope with a large stress load. Thus, the diameter of the torsion bar can be reduced.

  The manufacturing method of the torsion bar of the present invention can use various configurations. For example, torsional stress can be continuously applied during induction hardening. For example, in the case where the torsional stress of the steel material is kept constant and the torsional stress is not continuously applied, when induction hardening is started, the strain is relaxed by heating, and the torsional stress is removed during the quenching. As a result, since the above effect due to the torsional stress becomes insufficient, it is important to continuously apply the torsional stress. By applying a continuous torsional stress during high-frequency moving quenching, compressive residual stress can be applied to the entire surface region parallel to the axis.

  The 1st torsion bar of this invention is a torsion bar obtained by the said manufacturing method. That is, the torsion bar according to the present invention is characterized in that the fatigue limit is 450 MPa or more in a one-way stress amplitude. In the torsion bar of the present invention, the same effect as that of the method of manufacturing the torsion bar of the present invention can be obtained.

  In the second torsion bar of the present invention, a cured layer is formed on the surface, and the radius r of the torsion bar and the depth t from the surface of the cured layer satisfy 0.05 <t / r <0.17. It is characterized by. When t / r is 0.05 or less, since the cured layer is thin, it is difficult to control the thickness, and it is difficult to form a stable and uniform cured layer. On the other hand, when t / r is 0.17 or more, the effect of the restoring force at the time of unloading does not reach the vicinity of the surface, so that sufficient fatigue strength cannot be obtained. By satisfying 0.05 <t / r <0.17, the above problem can be solved.

  The hardness of the steel material is preferably 430 to 510 HV. When the hardness of the steel material is 430 HV or less, sufficient fatigue strength cannot be ensured. On the other hand, when the hardness of the steel material is 510 HV or more, the steel material is brittlely broken, which is not preferable. By setting the hardness of the steel material to 430 to 510 HV, the above problems can be solved.

  In the torsion bar, when a torsional stress is applied in the use direction T, as shown in FIG. 11, a tensile main stress is generated in the 45 ° direction with respect to the axis, and a compressive main stress is generated in the −45 ° direction with respect to the axis. It is preferable that the compressive residual stress applied in the 45 ° direction relative to the axis is sufficiently larger than the compressive residual stress applied in the −45 ° direction. In this case, as the residual stress difference increases, the fatigue limit of the torsion bar increases. When the maximum value of the compressive residual stress difference is 850 MPa or more, a sufficient fatigue limit can be obtained.

  According to the torsion bar or the manufacturing method thereof of the present invention, the fatigue strength can be improved by setting the torsional stress to be applied to the steel material at the time of induction hardening to 900 MPa or more. This makes it possible to reduce the diameter of the torsion bar.

The high frequency induction heating apparatus used in the Example of this invention is represented, (A) is an expansion perspective view showing the sample used for the apparatus, (B) is a perspective view showing the schematic structure of an apparatus. (A) is a graph showing the relationship between the distance from the surface of the sample of Experimental Example 11 of the present invention and the residual stress, and (B) shows the relationship between the distance from the surface of the sample of Comparative Experimental Example 11 and the residual stress. It is a graph. It is a graph showing the relationship between the stress amplitude in the fatigue test to the sample of Experimental example 11 of this invention, and Comparative experimental example 11 and 12, and the frequency | count of durability. It is a graph showing the relationship between the torsional stress and the fatigue limit in the case of induction hardening of the samples of Experimental Examples 21 and 22 and Comparative Experimental Examples 21 to 24 of the present invention. It is a graph which shows the relationship between the distance from the surface of the 45 degree direction of the sample of Experimental example 22 of this invention, and a -45 degree direction, and a residual stress. It is a graph which shows the relationship from the distance from the surface of the 45 degree direction of the sample of the comparative experiment example 23 of this invention, and -45 degree direction, and a residual stress. It is a graph which shows the relationship between the distance from the surface of the 45 degree direction of the sample of the comparative experiment example 24 of this invention, and -45 degree direction, and a residual stress. It is a graph which shows the relationship between the surface compressive residual stress difference in the 45 degree direction and -45 degree direction of the sample of Experimental Examples 21 and 22 of this invention, and Comparative Experimental Examples 21-24, and a fatigue limit. It is a graph which shows the relationship between the ratio (t / r) of the depth t from the surface of the hardened layer with respect to the radius r of a sample, and a fatigue limit. It is a graph showing the relationship between the distance from the surface of a sample when t / r is 0.17, and the residual stress of 45 degree direction and -45 degree direction. It is a conceptual diagram which shows the 45 degree direction (tensile principal stress generation | occurrence | production direction) and -45 degree direction (compression main stress generation | occurrence | production direction) at the time of torsional stress load in use of a torsion bar.

(1) Test Method Hereinafter, embodiments of the present invention will be described in more detail with reference to specific examples. In the Example, the high frequency induction heating apparatus 100 shown in FIG. 1 was used. In the high frequency induction heating apparatus 100, high frequency transfer quenching was performed on the surface of the sample 10 in a state where a torsional stress was applied to the sample 10 by the torque generating unit 102 fixed to the jig 101.

  In the test piece, a small-diameter portion 11 is formed at the center, a large-diameter portion 12 is formed at both ends, and a linear sample 10 is formed in which a tapered portion 13 that connects the small-diameter portion 11 and the large-diameter portion 12 is formed. Was used. The diameter of the small diameter part 11 was 12 mm. High frequency heating was performed by the coil 103, and the frequency and output were set to 200 KHz and 45 kW. A hatched portion 10A in FIG. 1A is a portion to be heated by the coil 103, the arrow T indicates the direction of torsional stress applied to the sample 10, and the arrow M indicates the moving direction of the sample 10.

(2) Test results (2-1) Effect of presence or absence of torsional stress during induction hardening As sample 10, SUP9 conditioned to HV480 was used, and the influence of presence or absence of torsional stress during induction hardening was examined. In Experimental Example 11, high-frequency transfer quenching was performed by the high-frequency induction heating apparatus 100 in a state where a torsional stress corresponding to a shear stress τ of 1000 MPa on the outermost surface of the sample 10 was applied to the sample 10. In Comparative Experiment Example 11, high-frequency transfer quenching was performed by the high-frequency induction heating device 100 in a state where no torsional moment was applied to the sample 10. The results are shown in FIGS. 2 (A) and 2 (B).

  2A and 2B are graphs showing the relationship between the distance from the surface of the sample of Experimental Example 11 and Comparative Experimental Example 11 of the present invention and the residual stress. 2A and 2B, the solid line indicates the residual stress in the 45 ° direction (tensile principal stress generation direction) with respect to the axis, and the broken line indicates the −45 ° direction (compression main stress generation direction) with respect to the axis. The compressive residual stress is shown.

  As can be seen from FIGS. 2A and 2B, in the sample of Experimental Example 11 in which induction hardening was performed with a torsional stress applied, a comparative experiment in which induction hardening was performed without applying torsional stress. A larger compressive residual stress is obtained in the direction of 45 °, which is the tensile principal stress generation direction in use, than in the sample of Example 11 in the direction of −45 °, which is the compression main stress generation direction. The maximum value of the difference was 850 MPa or more.

  Fatigue tests were performed on the samples of Experimental Example 11 and Comparative Experimental Example 11. In the fatigue test, the stress ratio was set to 0.1. The stress that did not break after 500,000 times was defined as the fatigue limit. The result is shown in FIG. FIG. 3 is a graph showing the relationship between the stress amplitude and the durability count. The plot □ in FIG. 3 shows the data of the sample of Experimental Example 11 of the present invention, the plot Δ shows the data of the sample of Comparative Experimental Example 11, and the plot × shows the sample of Comparative Experimental Example 12 that was not induction hardened. The data of (samples in a tempered state) are shown.

  The alternate long and short dash line indicates conventional data of Comparative Sample 1 described in Non-Patent Document 1 (a torsion bar tempered to have a predetermined hardness by quenching and tempering using SUP9 as a torsion bar material). 1 is shown. The solid line indicates Comparative Sample 2 described in Non-Patent Document 1 (S45C is used as the material of the torsion bar, and the torsion bar is induction-quenched so that the depth of the hardened layer is 45 to 75%. The conventional data 2 of the torsion bar tempered in FIG. In the fatigue test of Non-Patent Document 1, the average stress is set to 490 MPa.

  As can be seen from FIG. 3, the sample of Experimental Example 11 in which induction hardening was performed with a torsional stress applied had higher fatigue strength than the conventional data 1 and 2 described in Non-Patent Document 1. Further, the sample of Experimental Example 11 of the present invention had a larger stress amplitude when viewed at the same durability number than the sample of Comparative Experimental Example 11, and the durability frequency was higher when viewed at the same stress amplitude. This confirmed that the sample of Experimental Example 11 of the present invention had higher fatigue strength than the sample of Comparative Experimental Example 11. As shown in FIGS. 2 (A) and 2 (B), in the sample of Experimental Example 11 of the present invention, when the torsion bar is used, in the 45 ° direction (tensile principal stress generation direction), the −45 ° direction (compression) This is probably because a compressive residual stress larger than that in the main stress generation direction) or the axial direction is obtained.

(2-2) Relationship between Torsional Stress and Fatigue Limit during Induction Hardening Using SUP9 conditioned to HV480 as sample 10, the magnitude of torsional stress applied to the sample is set to 1200 MPa, 1000 MPa, 890 MPa, 667 MPa, and 333 MPa Then, high-frequency transfer quenching was performed by the high-frequency induction heating apparatus 100. Then, the sample of Experimental Example 21 in which the torsional stress during induction hardening was 1200 MPa, the sample of Experimental Example 22 set to 1000 MPa, the sample of Comparative Experimental Example 21 set to 890 MPa, the sample of Comparative Experimental Example 22 set to 780 MPa, and 667 MPa A fatigue test was performed on the sample of comparative experimental example 23 and the sample of comparative experimental example 24 set to 333 MPa. The result is shown in FIG.

  FIG. 4 is a graph showing the relationship between torsional stress and fatigue limit during induction hardening in Experimental Examples 21 and 22 and Comparative Experimental Examples 21 to 24 of the present invention. 5 to 7 are graphs showing the relationship between the distance from the surface of the sample of Experimental Example 22 and Comparative Experimental Examples 23 and 24 of the present invention and the residual stress in the 45 ° direction and the −45 ° direction. In FIG. 4, the data of the sample of Comparative Experimental Example 11 (the torsional stress magnitude of 0 MPa) in which no torsional stress was applied to the sample during induction hardening are also shown.

  As can be seen from FIGS. 5 to 7, it was confirmed that as the torsional stress applied during induction hardening increases, the compressive residual stress in the 45 ° direction, which is the tensile principal stress generation direction during use, increases. And, as can be seen from FIG. 4, when the torsional stress applied during induction hardening is less than 900 MPa, the fatigue limit of the sample is substantially the same as that to which no torsional stress is applied. . Thereby, it was confirmed that the torsional stress applied during induction hardening needs to be set to 900 MPa or more.

(2-3) Relationship between Surface Compressive Residual Stress Difference and Fatigue Limit in 45 ° Direction and −45 ° Direction FIG. 8 shows 45 ° of samples of Experimental Examples 21 and 22 and Comparative Experimental Examples 21 to 24 of the present invention. It is a graph showing the relationship between the surface compressive residual stress difference in a direction and -45 degree direction, and a fatigue limit. The plot (2) in the figure shows the data of the samples of Experimental Examples 21 and 22 and Comparative Experimental Examples 21 to 24 of the present invention in order from the right side on the horizontal axis.

  As can be seen from FIG. 8, when the difference in surface compressive residual stress between the 45 ° direction and the −45 ° direction is less than 850 MPa, the fatigue strength of the sample is substantially the same as that to which no torsional stress is applied, but 850 MPa or more. Then it became bigger. Thereby, it was confirmed that the difference in surface compressive residual stress between the 45 ° direction and the −45 ° direction is preferably set to 850 MPa or more.

(2-4) Influence of loading state of torsional stress at induction hardening Using S45C as sample 10, with a constant torsional stress applied to the sample, high-frequency transfer quenching is performed by high-frequency induction heating apparatus 100, and A sample of Experimental Example 31 was obtained. Further, S45C was used as the sample 10, and high-frequency transfer quenching was performed by the high-frequency induction heating device 100 in a state where a constant torsional stress was not continuously applied to the sample, and a sample of Comparative Experimental Example 31 was obtained. The residual stress of the cross section of each part of the sample of Experimental Example 31 and Comparative Experimental Example 31 of the present invention on the quenching start side, the intermediate portion, and the quenching end side was measured.

  As a result, in the experimental example 31 of the present invention in which the continuous load of torsional stress was performed, the compressive residual stress difference was obtained in all the above-mentioned locations, and in the comparative experimental example 31 in which the continuous load of torsional stress was not performed, the present invention The difference in compressive residual stress was smaller in all the above locations than in Experimental Example 31. As a result, it was confirmed that when a torsional stress was continuously applied, a large compressive residual stress difference was obtained over the entire surface region parallel to the axis.

(2-5) Depth from surface of hardened layer SUP9 tempered to HV480 is used as sample 10, and the sample is subjected to high-frequency transfer quenching with high-frequency induction heating device 100 while torsional stress is applied to the sample. Samples having different depths of the hardened layer were obtained by changing. About these samples, the relationship between the depth of a hardened layer and fatigue strength was investigated. The result is shown in FIG. FIG. 9 is a graph showing the relationship between t / r and the fatigue limit. FIG. 10 is a graph showing the relationship between the distance from the surface of the sample and the residual stress when t / r is 0.17. In FIG. 9, the depth of the hardened layer is indicated by the ratio (t / r) of the depth t from the surface of the hardened layer to the radius r of the sample.

  As can be seen from FIG. 9, when t / r was 0.05 or less, the fatigue strength was greatly reduced. In this case, since the cured layer is thin, it is difficult to control the thickness, and as a result, it is difficult to form a stable and uniform cured layer. On the other hand, when t / r was 0.17 or more, the fatigue strength was small. In this case, although the cause is unknown, it is considered that the effect of the restoring force at the time of unloading the torsional stress does not reach the vicinity of the surface as can be seen from FIG. As described above, it was confirmed that the fatigue limit can be improved when the ratio of the depth t from the surface of the hardened layer to the radius r of the sample satisfies 0.05 <t / r <0.17.

  DESCRIPTION OF SYMBOLS 10 ... Sample, 10A ... Heated part, 11 ... Small diameter part, 12 ... Large diameter part, 13 ... Tapered part, 100 ... High frequency induction heating apparatus, 101 ... Jig, 102 ... Torque generation part, 103 ... Coil

Claims (8)

  A method for producing a torsion bar, comprising subjecting the steel material to induction hardening in a state where a torsional stress of 900 MPa or more is applied to the steel material.   The torsion bar manufacturing method according to claim 1, wherein the torsional stress is continuously applied during the induction hardening. A torsion bar manufactured by the manufacturing method according to claim 1 or 2,
A torsion bar characterized by having a fatigue limit of 450 MPa or more in a single swing stress amplitude. A torsion bar manufactured by the manufacturing method according to claim 1 or 2,
A hardened layer is formed on the surface,
A torsion bar satisfying 0.05 <t / r <0.17, where r is the radius of the torsion bar and t is the depth of the hardened layer from the surface.   The torsion bar according to claim 4, wherein residual stresses in the 45 ° direction and the −45 ° direction with respect to the axis of the steel material are different.   The torsion bar according to claim 5, wherein the maximum value of the difference in compressive residual stress between the 45 ° direction and the −45 ° direction with respect to the axial direction in the region where the compressive residual stress is applied is 850 MPa or more.   The torsion bar according to claim 4, wherein the steel material has a hardness of 430 to 510 HV.   The torsion bar according to any one of claims 4 to 7, wherein a compressive residual stress is applied to the entire surface region parallel to the axis of the steel material.

Images