JP4379315B2 - Mechanical structure member and shaft using the same - Google Patents

Description

  The present invention relates to a mechanical structural member and a shaft using the same, and more particularly to a mechanical structural member that can be used for all parts in which torsional fatigue strength is particularly important and a shaft using the mechanical structural member.

  As a conventional carburized shaft, SCr420H, SCM420H, etc. defined by JIS are used. However, with the recent trend toward higher output and lighter weight for transportation equipment such as automobiles, there is a high need for improving torsional strength in power transmission parts, and the above JIS standard steel is not sufficient.

From such a background, the inventor of the present application discloses a technique for improving torsional strength using a case-hardened steel to which boron is added (see, for example, Patent Document 1). However, this method has a drawback in that mass production manufacturability is not good because the carburizing heat treatment conditions are limited in order to avoid a decrease in hardness due to incomplete quenching.
JP 2003-239039 A

  The present invention has been made in view of such problems of the prior art, and the object of the present invention is that there is no significant increase in parts cost compared to carburized members that have been used in the past. An object of the present invention is to provide a mechanical structural member having excellent properties and torsional fatigue strength and a shaft using the same.

As a result of intensive studies to solve the above problems, the present inventors have found that the above problems can be solved by adding molybdenum in addition to boron and optimizing the depth of the carburized layer. The invention has been completed.
In addition, the present inventors have found that an improvement in torsional fatigue strength can be expected by efficiently projecting small diameter shot peening onto the fracture starting point portion, and have completed the present invention.

  According to the present invention, since the toughness of the carburized layer is improved by molybdenum added together with boron and the carburized layer having a certain depth, a mechanical structural member excellent in torsional fatigue strength and a shaft using the same are obtained.

  Hereinafter, the mechanical structural member of the present invention and the shaft using the same will be described in detail. In the present specification and claims, “%” indicates a mass percentage unless otherwise specified.

The mechanical structural member of the present invention has C: 0.10 to 0.30%, Si: 1.00% or less, Mn: 0.30 to 1.70%, P: 0.015% or less, S: 0.00. 020% or less, Cr: 0.10 to 1.60%, Mo: 0.10 to 1.00%, B: 0.005 to 0.0030%, N: 0.03% or less, and Al: 0.04 %, Nb: 0.010 to 0.200% and / or Ti: 0.010 to 0.100%, and the balance is composed of Fe and inevitable impurities. With such a configuration, the toughness of the carburized layer is improved, and if it is adopted for a structural component that receives torsional input, it is possible to increase the strength and reduce the weight.
Below, the reason for limitation of each containing component is shown.

C: 0.10 to 0.30%
C has the effect of improving the hardness after carburizing and quenching and improving the strength after carburizing. Further, it is an element necessary for obtaining the strength of the core. When the amount of C is less than 0.10%, the effect becomes small, and when it exceeds 0.30%, the toughness and workability of the core part are lowered.

Si: 1.00% or less Since Si promotes grain boundary oxidation after carburizing and brings about a decrease in strength, it is desirable to make it 0.25% or less. However, in a process capable of suppressing grain boundary oxidation such as vacuum carburizing and plasma carburizing, it is not necessarily limited to 0.25% or less. However, excessive addition causes deterioration of machinability and cold forgeability, so the upper limit is made 1.00% or less.

Mn: 0.30 to 1.70%
Mn is an element effective for improving hardenability. The lower limit is set to 0.3 because moderate austenite remains after carburizing in order to improve toughness. Moreover, when it contains exceeding 1.70%, cold forgeability will be reduced and the grain boundary oxidation after carburizing will be promoted.

P: 0.015% or less P is an element that easily segregates at grain boundaries and degrades the toughness of the carburized layer. When the content exceeds 0.015%, the decrease in fatigue strength becomes significant. Further, since P is an impurity element, the content is preferably as close to 0% as possible.

S: 0.020% or less S is an element that deteriorates the toughness of the carburized layer. When the content exceeds 0.020%, the decrease in fatigue strength becomes significant. Further, since S is an impurity element, the content is preferably as close to 0% as possible.

Cr: 0.10 to 1.60%
Cr is added in order to improve the hardenability of the base material and improve the strength of the core. However, if excessively added, brittleness of crystal grain boundaries may be caused, so the content is made 1.60% or less. Moreover, in order to ensure hardenability, content is added 0.10% or more.

Mo: 0.10 to 1.00%
Mo improves the hardenability, suppresses the formation of abnormal structures in the carburized layer, and improves the toughness by improving the grain boundary strength of the carburized layer, thereby delaying crack propagation. If the content is less than 0.10%, the effect is not sufficient. If the content exceeds 1.00%, the hardness of the material increases and the workability and machinability deteriorate.

B: 0.0005 to 0.0030%
B is an element effective for hardenability, and has an effect of improving the strength by segregating at the grain boundaries and strengthening the grain boundaries. If the content is less than 0.0005%, the effect is not sufficiently exhibited, and if it exceeds 0.0030%, the effect is saturated.

Al: 0.04% or less Al reacts with N in steel to form AlN, and has an action of preventing coarsening of austenite crystal grains during carburizing. However, the effect is saturated when it exceeds 0.04%.

N: 0.03% or less N reacts with Al in copper to form AlN and has an effect of preventing coarsening of austenite crystal grains during carburizing. However, if added over 0.03%, the effect is saturated.

Nb: 0.010-0.200%
Case-hardened steel subjected to carburizing and quenching treatment tends to coarsen the crystal grains. In order to suppress this, pinning of grain boundaries by fine precipitates is effective, and Nb carbonitride is used. In this case, if there is little content of Nb, the effect which suppresses grain growth is scarce, and if it adds excessively, the hardness of the steel materials after rolling will rise and cold forgeability will deteriorate.

Ti: 0.010 to 0.100%
In order to exhibit the result by adding B, it is necessary to exist in a solid solution B in the steel material. However, B has a strong affinity for N and forms BN. Therefore, N is fixed with Ti to ensure solid solution B. In that case, if it is less than 0.010%, the addition amount is small and N remains. On the other hand, if it exceeds 0.100%, it is excessively added, and if it is coarse, precipitates are generated and the strength is adversely affected.

Further, the machine structural member of the present invention is subjected to a carburizing quenching and tempering treatment as a surface hardening treatment, and the effective hardened layer depth (reference hardness: 513 HV) after the treatment is 0 with respect to the thickness of the mechanical structural member. 0.5 to 3.0%. For example, in the case of a shaft using this machine structural member, the ratio of the effective hardened layer depth to the shaft diameter satisfies 0.5 to 3.0%.
The carburizing and tempering treatment is an effective treatment for hardening the surface and improving the torsional strength, but if the ratio of the effective hardened layer depth to the member thickness is not more than 0.5%, the effect cannot be exerted. If it exceeds 3.0%, the toughness of the carburized layer is reduced. More preferably, it is good to set it as 1.0 to 2.5%.

Ni: 0.25% or less Ni is an element that improves hardenability and contributes to increasing the internal hardness after carburizing and quenching. Moreover, it exists as an unremovable impurity in the steelmaking stage. However, in the case of addition exceeding 0.25%, the hardness of the steel material is increased, which causes adverse effects such as deterioration of cold forgeability and cutting workability at the time of manufacture and a significant increase in material cost. . Therefore, the upper limit is set to 0.25.

Furthermore, the mechanical structural member preferably contains Pb: 0.3% or less, Bi: 0.1% or less, or Ca: 0.1% or less, and any combination thereof.
These elements are effective elements for improving the machinability. However, if the content exceeds the above content, not only the machinability improving effect is saturated but also the toughness may be lowered.

Furthermore, the mechanical structural member is subjected to shot peening treatment in which a steel ball of 40 to 200 μm is projected at a projection pressure of 4 to 6 kg / cm ² , and the compressive residual stress on the surface of the projection part satisfies 1000 to 1500 MPa. Is preferred.
Shot peening is effective in suppressing the occurrence of initial cracks in torsional fatigue loads. In order to reinforce the stress concentration part that becomes the starting point of the initial crack, a steel ball of 200 μm or less is desirable. On the other hand, if the thickness is less than 40 μm, sufficient energy cannot be given to the steel ball, and it becomes difficult to give compressive residual stress. Moreover, although it is desirable that the applied compressive residual stress is higher, 1000 to 1500 MPa is sufficient by the above-described method.

  The above-mentioned mechanical structural member can be used for all parts in which torsional fatigue strength is regarded as important, but a typical example is a transmission shaft. Since the torsional fatigue strength of such a shaft is improved, it is possible to reduce the weight by increasing the strength of the component and reducing the shaft.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further in detail, this invention is not limited to these Examples.

(Example 1 and Comparative Examples 1 to 4)
150 kg of steel having a chemical structure shown in Table 1 was vacuum-melted by a conventional method. Next, these steels were hot forged and normalized by a normal method, and then processed into a shaft specimen shown in FIG. Each test piece was carburized, quenched, and tempered, and the surface was ground.
In Example 1 and Comparative Examples 1 and 2, shot peening was performed under the conditions shown in Table 2. Table 1 shows all specifications of the manufactured shaft.

  Using the obtained shaft test piece, a fatigue test was performed with a torsion tester. The relationship between the load torque and the number of breakage was obtained from the test results shown in FIG. Table 3 shows strength ratios based on 100,000 times strength and Comparative Example 4.

From Table 1 and Table 3, in Example 1, since molybdenum is added, the hardened layer is shallow, and the shot peening treatment is performed, the highest torsional fatigue strength is shown.
On the other hand, in Comparative Example 1, boron is added, the hardened layer is shallow, and the shot peening treatment is performed, but the strength is low because the material is not added with molybdenum. In Comparative Example 2, the hardened layer is shallow and shot peening is performed, but the strength is low because it is a material to which boron and molybdenum are not added. Furthermore, in Comparative Example 3, boron is added, but molybdenum is not added. The cured layer is deep and the shot peening treatment is not performed, so the strength is low. Furthermore, Comparative Example 4 is a material to which boron and molybdenum are not added, the hardened layer is deep, and since the shot peening treatment is not performed, the strength is low.

It is sectional drawing which shows a shaft test piece. It is a graph which shows a torsional fatigue test result.

Claims (5)

C: 0.10 to 0.30%, Si: 1.00% or less, Mn: 0.30 to 1.70%, P: 0.015% or less, S: 0.020% or less, Cr: 0.00. 10 to 1.60%, Mo: 0.10 to 1.00%, B: 0.0005 to 0.0030%, N: 0.03% or less and Al: 0.04% or less, and Nb : A mechanical structural member composed of a steel material containing 0.010 to 0.200% and / or Ti: 0.010 to 0.100%, the balance being Fe and inevitable impurities,
An effective hardened layer depth (standard hardness: 513 HV) by carburizing and quenching and tempering processing satisfies 0.5 to 3.0% with respect to the thickness of the member.   The mechanical structural member according to claim 1, containing Ni: 0.25% or less.   3. The composition according to claim 1, comprising at least one element selected from the group consisting of Pb: 0.3% or less, Bi: 0.1% or less, and Ca: 0.1% or less. Mechanical structural member. The shot peening process which projects a 40-200 micrometer steel ball with a projection pressure of 4-6 kg / cm < ² > is performed, and the compressive residual stress of the surface of a to-be-projected part satisfies 1000-1500 MPa, It is characterized by the above-mentioned. The machine structural member according to any one of the above.   A shaft comprising the mechanical structural member according to any one of claims 1 to 4.

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