JP2006283085A - Method for producing spring material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a spring material excellent in both of working precision and high fatigue strength. <P>SOLUTION: The material for spring composed, by mass%, of ≤0.01% C, 8.0-11.0% Ni, ≤14.0% Co, 2.0-9.0% Mo, ≤2.0% (excluding 0) Ti, ≤1.7% Al, ≤30 ppm O, ≤30 ppm N and the balance inevitably Fe, is solid-solution-treated and after making the hardness to <350 HV, the material is worked into a spring-shape and thereafter, the method for producing the spring material, is performed, by which an aging-hardening treatment is applied at 400-500°C. The method for producing the spring material is desirably performed for improving the fatigue strength, by which after applying the above aging-hardening treatment, a barrel-grinding or a shot-peening is applied, or further, after applying the aging-hardening treatment, a nitriding-treatment is applied, or at the same time as the above aging-hardening treatment, the nitriding treatment are applied. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing a spring material having high fatigue strength characteristics.

Automotive parts and electronics-related parts are required to be small and operate at high speed. As a result, springs used for parts are required to have high accuracy and high fatigue strength.
Conventionally, as spring materials having high fatigue strength, JP-A-10-140294 (see Patent Document 1), JP-A-2003-73783 (see Patent Document 2), JP-A-60-36649 (see Patent Document 3). There are steels disclosed in
Japanese Patent Laid-Open No. 10-140294 JP 2003-73783 A JP 60-36649 A

Since the steel disclosed in Patent Document 1 described above remains in an unstable austenite phase in the processed state, the workability of the remaining austenite phase and the workability of the martensite structure are different, and the processing accuracy is insufficient. is there.
Further, the steel disclosed in Patent Document 2 has a completely martensitic structure in the processed state in order to eliminate the deterioration of processing accuracy due to the unstable austenite phase remaining in the steel shown in Patent Document 1. Residual austenite is lost. However, the hardness in the processed state is as high as 350 HV or more, and the workability itself is poor.
Moreover, although the steel currently disclosed by patent document 3 is excellent in workability, the hardness after an age hardening process is low, and intensity | strength and fatigue strength are inadequate.
The objective of this invention is providing the manufacturing method of the spring material which made the outstanding processing precision and high fatigue strength compatible.

As a result of intensive studies on both the chemical composition and the manufacturing method of the spring material having excellent processing accuracy and high fatigue strength, the inventor limited the alloy components of the spring material and applied a special manufacturing method. The inventors have found that both excellent processing accuracy and high fatigue strength can be achieved, and have reached the present invention.
That is, the present invention is, by mass%, C: 0.01% or less, Ni: 8.0 to 19.0%, Co: 14.0% or less, Mo: 2.0 to 9.0%, Ti: 2 0.0% or less (excluding 0), Al: 1.7% or less, O: 30 ppm or less, N: 30 ppm or less, and the remainder is a solid solution treated at 750 to 950 ° C. at substantially 750 ° C. This is a method for manufacturing a spring material, which is processed into a spring shape after having a hardness of less than 350 HV, and then subjected to age hardening treatment at 400 ° C. to 550 ° C.
Preferably, in order to improve fatigue strength, barrel polishing or shot peening is performed after the age hardening treatment, or nitriding treatment is further performed after the age hardening treatment, or nitriding is performed simultaneously with the age hardening treatment. It is a manufacturing method of a processing spring material.

  By applying the spring material manufacturing method of the present invention, it is possible to obtain a spring material that is used for automobile parts, electronics-related parts, and the like and that is required to be small and operate at high speed.

The present invention is characterized by both a chemical composition that can be used as a spring material having excellent processing accuracy and high fatigue strength, and a manufacturing method.
In the present invention, an alloy having the following composition is subjected to a solution treatment, processed after the solution treatment, and then age-hardened. In this manufacturing method, in order to ensure good workability, the hardness after the solution treatment is set to less than 350 HV and processed into a desired spring shape. Since the hardness is less than 350 HV, excellent processing accuracy can be easily obtained. And it hardens | cures by the low temperature age hardening process which hardly causes the dimensional change of the processed spring shape, and achieves high fatigue strength.
The reason for limiting the alloy necessary for this will be described in detail. The chemical composition is shown as mass%.

C: 0.01% or less Since C increases the hardness after the solution treatment, in order to obtain good workability, the upper limit of C needs to be 0.01% or less. The lower limit is preferably 0.007% or less, and the lower limit is at most 10 ppm as the present technical limit.
Ni: 8.0 to 19.0%
Ni is an important element that forms a substitutional solid solution in the matrix and forms a martensite structure after the solution treatment. However, if it exceeds 19%, austenite is stabilized, and 1% or more of retained austenite is generated after the solution treatment, so that the processing accuracy is lowered. On the other hand, if less than 8.0%, the toughness deteriorates, so Ni was made 8.0 to 19.0%.

Co: 14.0% or less Co increases the solid solubility of Mo during the solution treatment without greatly affecting the stability of the martensitic transformation after the solution treatment, and increases the solid solution of Mo during the age hardening treatment. By reducing the solubility, Mo promotes the formation and precipitation of fine intermetallic compounds, and indirectly contributes to precipitation strengthening. However, even if added over 14.0%, the effect is hardly obtained, so the content was made 14.0% or less.
In addition, when precipitation strengthening is achieved by the main strengthening elements Ni, Mo, and Ti, there is no need for addition, so the Co content is set to 14.0% or less.

Mo: 2.0-9.0%
Mo is an element that contributes to strengthening by forming a fine intermetallic compound (Fe ₂ Mo or Ni ₃ Mo) by age hardening, and precipitating it in the matrix, but its content is less than 2.0%. In that case, the effect is small, and if the content exceeds 9.0%, Fe ₂ Mo which deteriorates ductility and toughness is likely to be coarsened. Therefore, the Mo content is set to 2.0 to 9.0%.
In addition, when adding Mo exceeding 5%, it is preferable to contain 5% or more of Co, and to use together with the above-mentioned effect of Co.
Ti: 2.0% or less (excluding 0)
Ti is an indispensable element that contributes to strengthening by forming a fine intermetallic compound by age hardening treatment, and is added as an essential element, but if its content exceeds 2.0%, it is ductile. Since the toughness deteriorates, the Ti content is set to 2.0% or less (excluding 0). In order to form a nitrided layer and obtain a surface compressive residual stress of 500 MPa or more, a preferable range is 0.4 to 1.0%.

Al: 1.7% or less Since Al has a deoxidizing action, it can be added as a deoxidizing element. However, excessive addition forms oxide inclusions that become the starting point of fatigue failure, so the upper limit is made 1.7%.
O: 30 ppm or less O is limited to 30 ppm or less because it forms oxide inclusions. If the O content exceeds 30 ppm, the fatigue strength in the high cycle region is significantly reduced, so the content was made 30 ppm or less. Preferably it is 20 ppm or less. The lower limit is at most 1 ppm as the current technical limit.
N: 30 ppm or less N is limited to 30 ppm or less in order to form nitrides and carbonitride inclusions. When N exceeds 30 ppm, the fatigue strength in the high cycle region is remarkably reduced, so the content was set to 30 ppm. Preferably it is 20 ppm or less. The lower limit is at most 2 ppm as the current technical limit.

In the present invention, elements other than these specified elements are substantially Fe, but naturally impurities contained unavoidably are included. Among these, inevitably contained impurity elements such as Si and Mn promote the precipitation of coarse intermetallic compounds that cause embrittlement, thereby reducing ductility and toughness, and forming nonmetallic inclusions to reduce fatigue strength. Therefore, both Si and Mn should be 0.1% or less, preferably 0.05% or less, and P and S also become brittle at grain boundaries or form non-metallic inclusions to reduce fatigue strength. Therefore, 0.01% or less is preferable.
Further, other elements having a deoxidizing action, such as Mg and Ca, can be added in place of a part of Al.
In addition to impurities inevitably contained, additional elements can be positively added as long as the gist of the present invention is not impaired. For example, B is an element effective for refining crystal grains, and therefore may be contained in a range of 0.01% or less to the extent that toughness does not deteriorate.

The spring material having the above composition can be adjusted to a hardness of less than 350 HV by applying a solution treatment, but in order to impart excellent workability, processing accuracy and fatigue strength, 750 It is necessary to perform a solution treatment at ˜950 ° C.
When the solution treatment is lower than 750 ° C., coarse Fe ₂ Mo is generated, and not only a sufficiently high hardness cannot be obtained in the age hardening treatment performed after the spring processing, but also the toughness is reduced, so that sufficient fatigue strength is obtained. Cannot be obtained. Further, when the temperature is higher than 950 ° C., the crystal grains of the parent phase are coarsened, and not only a sufficiently high hardness cannot be obtained in the age hardening treatment, but the fatigue strength is lowered, and the workability is also lowered because the ductility is lowered. . Therefore, the solution treatment temperature is in the range of 750 to 950 ° C. The treatment time may be about 3 minutes to 1 hour.
In the present invention, the reason why the hardness after the age hardening treatment is set to less than 350 HV is that this range is determined to be a realistic region based on experience in which excellent workability and processing accuracy can be obtained. . Preferably, it may be in the range of 200 to 340HV.
In this solution treatment, when the amount of retained austenite in the metal structure after the solution treatment is measured with an X-ray diffractometer, it is more preferable if it is less than 1% because excellent processing accuracy can be provided more reliably.

And in this invention, it processes into a spring shape after a solution treatment. As described above, since the hardness after the solution treatment is adjusted to less than 350 HV, a highly accurate shape can be obtained.
For the processing here, stamping, drawing, bending, cutting, or the like can be selected in a timely manner according to the application of the spring material. In particular, since high precision can be obtained by drawing and bending, diaphragms that require drawing and curved leaf spring materials that require bending are suitable.
And by performing an age hardening process at 400 degreeC-550 degreeC, it becomes high hardness of 560HV or more, maintaining the highly accurate shape after a process, and can give a high fatigue strength characteristic to a spring material.
The reason why the age hardening treatment is set to 400 ° C. to 550 ° C. is that when the temperature is lower than 400 ° C., the precipitation of the intermetallic compound does not occur sufficiently and a high strength cannot be obtained. This is because not only is coarsening and high strength cannot be obtained, but also the reverse transformation austenite is generated, resulting in a large dimensional change and a failure to maintain a highly accurate shape. A preferable age hardening treatment temperature is in the range of 450 to 500 ° C., and the treatment time may be about 30 minutes to 5 hours.

Next, a preferable process will be described.
In the present invention, it is preferable to perform barrel polishing or shot peening that can suppress fatigue fracture at the surface starting point after the age hardening treatment.
If the compressive residual stress on the surface of the spring material is set to 100 MPa or more by barrel polishing or shot peening, the fatigue fracture at the surface origin can be more reliably reduced, and higher fatigue strength can be achieved. Regarding the barrel polishing or shot peening conditions in this case, it is more effective to use as small particles as possible because the surface compressive residual stress can be uniformly increased to 100 MPa or more.

In the present invention, as a method for reducing fatigue fracture after age hardening, in addition to the above-described barrel polishing or shot peening, a method of performing nitriding after age hardening, or nitriding simultaneously with age hardening A processing method may be applied.
In addition, although the method of performing barrel polishing or shot peening and the method of performing nitriding treatment may be combined, in that case, the nitriding treatment is preferably a post-process.
By forming a nitride layer on the surface of the spring material according to the present invention, not only the fatigue failure at the surface starting point can be reduced, but also the tensile stress applied to the surface during use of the spring can be relieved and the fatigue strength can be further increased. Can do.
At this time, if the surface compressive residual stress of the spring material is set to 500 MPa or more by nitriding, it is possible to further increase the fatigue strength, and it is more preferable.

As processing conditions for this, if the nitriding temperature is higher than 550 ° C., reverse transformation austenite is generated in the parent phase, resulting in a large dimensional change, making it impossible to maintain a highly accurate shape. When a compound layer is formed on the surface, the compound layer becomes a starting point, and conversely, fatigue strength may be reduced. Therefore, a condition in which the compound layer is not substantially present is good (here, substantially absent) The compound layer is not observed when the cross section of the nitride layer is observed with an optical microscope at a magnification of 1000).
Therefore, a suitable nitriding temperature is 550 ° C. or less, and it is more preferable if it is a time treatment in the range of 15 minutes to 5 hours at a temperature of 400 ° C. or more and 500 ° C. or less. It can be applied.

The following examples further illustrate the present invention.
A 10 kg ingot was prepared in a vacuum induction melting furnace, and after homogenization heat treatment at 1280 ° C. × 20 hours, hot forging and hot rolling were performed at 1100 ° C. Two types of spring material shown as A and B were obtained. Table 1 shows the chemical composition of the spring material. In addition, No. B is a conventional steel disclosed in Patent Document 3.

After removing the oxide scale on the surface of the spring material shown in Table 1 by pickling and grinding, cold rolling and heat treatment for strain removal were repeated to obtain a spring material thin plate having a thickness of 0.3 mm. Here, the heat treatment for removing the strain was 1050 ° C. × 1 minute.
Then, no. The spring material A was rolled to a thickness of 0.2 mm by cold rolling and then subjected to a solution treatment at 900 ° C. for 5 minutes to obtain a material before age hardening.
No. The spring material of B is rolled to a thickness of 0.25 mm by cold rolling, then subjected to a solid solution treatment at 1050 ° C. for 5 minutes, further subjected to a temper rolling to a thickness of 0.2 mm, and a material before age hardening treatment It was.
The Vickers hardness was measured at a longitudinal cross section (cross section along the rolling direction) before the age hardening treatment after mirror polishing with an indenter load of 2.9 N. Table 2 shows the results of electropolishing the surface and measuring the amount of retained austenite (γ) by X-ray diffraction.

Before age hardening, the spring is processed. No. of the material before age hardening treatment. A, No. The hardness of B was less than 350 HV, good workability, the amount of retained austenite (γ) was less than 1%, and the hardness variation was very small.
Using the material before age hardening treatment, a three-point bending test simulating spring processing was performed. In the test, a length of 7.5 mm was supported and the center was pushed in. As a result, the present invention no. A, conventional steel No. About B, it implemented about 10 test pieces, but even if it pushed in to length 10mm, abnormality, such as a crack, did not generate | occur | produce in a test piece, but favorable workability was shown.
Next, the bending angle of the test piece after the test in which the indentation load was removed was measured. As a result, there was almost no difference in the bending angle between the test pieces, and it was confirmed that the processing accuracy was good. This invention No. A, conventional steel No. It was confirmed that both the materials before age hardening treatment of B had excellent workability and processing accuracy.

  Using the test piece subjected to the above three-point bending test, No. A, No. About B, the age hardening process of 480 degreeC x 2 hours was performed. No. About A, the age hardening process of 560 degreeC x 1 hour was performed. And after age-hardening treatment, in the longitudinal section (cross section along the rolling direction), the results of measuring the Vickers hardness with an indenter load of 2.9 N after mirror polishing are shown in Table 3 as samples A1, A2, and B1. Show.

  The hardness after age hardening treatment is 560 HV or higher for sample A1, whereas samples A2 and B1 are less than 560 HV, so sample A1 has high strength and high fatigue strength can be expected. It is considered that A2 and B1 cannot be expected to have high fatigue strength.

From the samples after aging at 480 ° C. for 2 hours (samples A1 and B1), one-way bending fatigue test pieces were cut out and barrel-polished. Further, for sample A1, a test piece that was subjected to nitriding after barrel polishing was also prepared. The cross section of the nitride layer of the specimen of sample A1 subjected to nitriding was observed with an optical microscope at 1000 times, but no compound layer was confirmed on the surface. The nitriding depth was 30 μm and the surface hardness was 850 HV. Sample B1 could not find the nitriding conditions for obtaining a good nitrided layer.
Pulsating bending fatigue test is tested up to 10 ⁷ times, the stress amplitude does not break and the fatigue limit. Table 4 shows the results of measuring the surface compressive residual stress before the test and the fatigue limit.

Sample A1 has high fatigue strength after barrel polishing, and higher fatigue strength is obtained by nitriding. On the other hand, Sample B1 had a low fatigue strength after barrel polishing.
As described above, only the sample A1, which is the steel of the present invention, satisfies all of workability, processing accuracy, and high fatigue strength. Sample A1 can be further increased in fatigue strength by nitriding. Fatigue tests have been evaluated up to ¹⁰⁷ times, in the high cycle fatigue region exceeding 10 ⁷ times, since it is known that fatigue fracture starting from the non-metallic inclusions occur, fine nonmetallic inclusions The steel of the present invention can also achieve high fatigue strength in a high cycle region by making it to an appropriate size.

According to the present invention, since it is possible to provide a spring material that is excellent in workability and processing accuracy and has high fatigue strength, a spring having high accuracy and high fatigue strength can be manufactured.
Since the present invention can be used to manufacture very small springs and highly accurate springs, components using various springs such as flapper valves, shock absorber valves, press plates, leaf springs, diaphragms, etc. This is very effective for reducing the size of a metal mask or the like, or for operating at high speed and accuracy, and for improving reliability.

Claims (3)

In mass%, C: 0.01% or less, Ni: 8.0 to 19.0%, Co: 14.0% or less, Mo: 2.0 to 9.0%, Ti: 2.0% or less ( 0: not included), Al: 1.7% or less, O: 30 ppm or less, N: 30 ppm or less, with the balance being a solid solution treated at 750 to 950 ° C. and a hardness of less than 350 HV Then, it is processed into a spring shape, and then subjected to age hardening at 400 ° C. to 550 ° C. A method for producing a spring material, comprising performing barrel polishing or shot peening after the age hardening treatment according to claim 1. A method for producing a spring material, comprising performing nitriding after the age hardening treatment according to claim 1 or simultaneously with the age hardening treatment.