JP2000073140A - Axle for railway car - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an axle for a railway car high in fretting fatigue strength. SOLUTION: This axle for a railway car is composed of steel contg. 0.3 to 0.48% C, 0.05 to 1% Si, 0.5 to 2% Mn, 0 to 1.5% Cr, 0 to 0.3% Mo and 0 to 2.4% Ni, from the surface to the inside, the region of tempered martensite or bainite is formed, at least in the fitted edge parts 3 or the peripheral regions thereof, a hardened layer 4 having >=400 Vicker's hardness, the ratio (K/D) of the thickness (K) of the hardened layer to the diameter (D) of the fitted parts 2 is 0.005 to 0.05 is formed, on the upper side of the hardened layer, a plating layer 5 contg. by weight, 0.02 to 2% B and the balance Ni with inevitable impurities is formed and on the lower side of the hardened layer, the region of tempered martensite or bainite is formed. Preferably, the ratio (M/K) of the thickness (M) of the plating layer to the thickness (K) of the hardened layer is controlled to 0.005 to 0.2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an axle for a railway vehicle, and more particularly to an axle for a railway vehicle which exhibits high fatigue strength at a fitting portion where wheels, brake discs, etc. are fitted.

[0002]

2. Description of the Related Art Axles used in railway vehicles are required to have high reliability because breakage thereof leads to extremely serious accidents. In particular, in the fitting area where wheels, brake discs, gears, etc. are fitted, fretting fatigue occurs due to repeated high load stress and slight relative sliding with mating members such as wheels, resulting in significant fatigue strength. It is known to decrease.

[0003] A typical axle for a railway vehicle is used without surface hardening using carbon steel as a raw material. However, 1
With the introduction of Shinkansen rolling stock in the 960s, highly reliable axles were required under severe conditions such as increased load capacity due to higher speeds and lighter weight to maintain ride comfort, and carbon steel was subjected to induction hardening. An axle with high fretting fatigue strength has been developed and used as an axle having excellent fatigue reliability.

In recent years, further speeding up of the Shinkansen has been planned,
The technological development is progressing, and a higher axle with higher fretting fatigue strength corresponding to high speed is required.

The present applicant has disclosed in Japanese Patent Application Laid-Open No. Hei 10-8202 an axle obtained by induction hardening a low alloy steel containing Si, Cr, Mo and the like. This axle is characterized in that a hardened layer is formed at the fitting portion.

[0006]

The present applicant has obtained an axle with high fatigue strength by the means disclosed in the above publication. However, in order to further increase the speed of the vehicle and pursue safety, there is a demand for an axle having more excellent fatigue strength characteristics.

An object of the present invention is to provide an improved axle for a railway vehicle having a high fretting fatigue strength by improving a surface treatment method of a fitting end portion and a peripheral region thereof.

[0008]

It is known that the fatigue strength of the fitting part depends on the hardness and residual stress of the surface, and that the induction hardening forms a compressive residual stress in the hardened layer, thereby improving the fatigue strength. ing. Damage due to fretting fatigue is more likely to occur as the frictional force generated at the fitting portion due to the minute slip increases. Therefore, the present inventors further apply a plating process to the fitting end portion where the hardened layer is formed by induction hardening and the surrounding area, to form a compressive residual stress in the plating layer, and to reduce friction in the fitting portion. With the aim of improving the fatigue strength by reducing the force, trial production and evaluation tests of the axle were repeated, and it was confirmed that the following a and b were effective in improving the fatigue strength.

A. A hardened layer having a Vickers hardness of 400 or more is formed at the fitting end and its peripheral region, and the ratio (K / D) of the thickness (K) to the fitting diameter (D) is formed.
To 0.005 to 0.05.

B. On the upper side (surface side) of the cured layer, weight%
To form a plating layer containing B: 0.02 to 2%, the balance being Ni and unavoidable impurities, and the thickness (M)
In the ratio (M / K) to the thickness (K) of the cured layer is 0.005 to
0.2.

FIG. 1 is a schematic view showing the procedure for measuring the friction coefficient of an axle. The friction coefficient is measured by a strain gauge under the action of a pad pressing force P and an axial tension / compression force σ. And F / P from the tangential force F in the axial direction.
Table 1 shows the friction coefficient obtained by the measurement procedure shown in FIG.
As shown in the table, forming the plating layer on the axle significantly reduces the coefficient of friction.

[0012]

[Table 1]

The present invention is based on the above findings, and the gist is as follows (1) and (2).

(1) C: 0.3 to 0.48% by weight,
Si: 0.05-1%, Mn: 0.5-2%, Cr: 0
-1.5%, Mo: 0-0.3%, Ni: 0-2.4%
Having a region of tempered martensite or bainite from the surface to the inside, and having a hardened layer having a Vickers hardness of 400 or more at least at the fitting end and its peripheral region, Has a thickness (K) of 0.005 to 0.05 in terms of a ratio (K / D) to the diameter of the fitting portion (D), and contains 0.02 to 2% by weight of B on the upper side of the cured layer. An axle for a railway vehicle, comprising a plating layer having a balance of Ni and unavoidable impurities, and a tempered martensite or bainite region below the hardened layer.

(2) The thickness (M) of the plating layer is 0.005 to 0.2 as a ratio (M / K) to the thickness (K) of the hardened layer. An axle for a railway vehicle according to the paragraph.

In the above item (1), the steel may contain other elements such as AI, Ca, Ti, Nb, etc., and the balance is Fe and inevitable impurities.

In the above item (1), the "peripheral region" means a fitting end portion which is a contact end between a fitting portion and a boss inner surface of a wheel, a brake disc or a gear fitted to the fitting portion. From the width of the fitting part on both sides in the axial direction
It indicates the range including the surface layer portion within about 5%. The Vickers hardness measurement is a hardness at a test load of 1 kgf. A cured layer having a Vickers hardness of 400 or more may be simply referred to as a cured layer in the following description.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the chemical composition of steel will be described. Hereinafter, all percentages indicate weight%.

A. C: 0.3 to 0.48% C is an element that increases the strength of the base material and improves the hardness of the surface by induction hardening, and the fatigue strength monotonously increases with the increase of C. If C is less than 0.3%, the fatigue strength is insufficient, and if it exceeds 0.48%, sintering cracks occur. Therefore, the lower limit is set to 0.3% and the upper limit is set to 0.48%.

B. Si: 0.05 to 1% Si is effective as a deoxidizing element and also in improving fatigue strength. For sufficient deoxidation, 0.05% or more of Si must remain in the steel solidified after deoxidation. However, if it exceeds 1%, the fatigue strength is not improved, but rather the toughness is remarkably reduced, so the upper limit is 1%.

C. Mn: 0.5 to 2% Mn is an element necessary for enhancing hardenability, and must contain at least 0.5%. However, even if it is contained excessively, its effect is saturated and toughness is deteriorated, so the upper limit is 2%.

D. Cr: 0 to 1.5% Cr is an element effective for improving the hardenability, but may not be contained if fatigue strength can be ensured. 1.
If it exceeds 5%, during induction hardening, the thickness of the hardened layer becomes excessive, the compressive residual stress decreases, and the fatigue strength decreases.
The upper limit is 1.5%.

E. Mo: 0 to 0.3% Mo is an element that is effective for improving the hardenability and the strength of the base material, but may not be contained when the fatigue strength can be ensured. If it exceeds 0.3%, the fatigue strength decreases, so 0.3% is made the upper limit.

F. Ni: 0 to 2.4% Ni is an element effective for increasing the strength of the base material,
If the fatigue strength can be ensured, it may not be contained.
Even if the content exceeds 2.4%, the fatigue strength is almost saturated and the temper becomes brittle, so the upper limit is made 2.4%.

As other components, AI, Ca, Ti,
Elements such as Nb may be contained in a total of 0.2% or less. The balance is Fe and inevitable impurities.

The structure, hardened layer and plated layer will be described.
2A and 2B are schematic views showing an example of the shape of the axle according to the present invention. FIG. 2A is an overall view of the axle, and FIG.
5 is a schematic partial cross-sectional view of a Y portion of FIG. Reference numeral 1 denotes an axle, 2 denotes a fitting portion, 3 denotes a fitting end, 4 denotes a hardened layer, and 5 denotes a plating layer.

2 (a) and 2 (b), the surface of the axle 1 other than the hardened layer 4 in the peripheral area around the fitting end 3 and the inside thereof has a structure of tempered martensite or bainite. This is to ensure sufficient tensile strength and sufficient toughness. This structure does not need to exist up to the center of the axle, and the inside may be a structure of ferrite and pearlite.

In FIG. 2B, at least the fitting end 3 and its surrounding area have a Vickers hardness of 40.
It has a hardened layer 4 transformed into martensite of 0 or more, and its thickness (K) is a ratio (K /
D) is set to 0.005 to 0.05. At the fitting end and the surrounding area, compressive residual stress is generated due to volume expansion due to martensitic transformation, which suppresses crack generation and propagation and improves fatigue strength. K / D is 0.00
If it is less than 5, the region where the compressive residual stress occurs becomes shallow,
When it is larger than 05, the maximum value of the compressive residual stress becomes small, and in any case, the fatigue strength decreases. The hardened layer may be formed as long as it is "the fitting end and the surrounding area",
The whole axle may be used.

Under the hardened layer, there is a region having a structure of tempered martensite or bainite. This is to ensure sufficient tensile strength and toughness. Before the induction hardening described later, this structure was also present in the surface layer where the hardened layer was formed, and during the induction hardening, the fine carbides dispersed in these structures were removed for a short time. And assists in setting the hardness of the cured layer to Vickers hardness of 400 or more. Note that the region of the structure does not have to be in contact with the hardened layer. It is common to be in contact with the induction hardened portion, which is slightly lower in hardness than the hardened layer.

As shown in FIG. 2B, a plating layer 5 containing 0.02 to 2% by weight of B on the upper side of the hardened layer 4 and a balance of Ni and inevitable impurities is provided. The formation of the plating layer reduces the frictional force between the fitting portion and the inner surface of the boss, and generates a compressive residual stress in the plating layer due to volume expansion during plating. Since the formation of this compressive residual stress lowers the compressive residual stress just below the plating layer, from this viewpoint, the formation of the plating layer has an effect of slightly reducing the fatigue strength. The fatigue strength is remarkably improved due to the reduction in the frictional force at the contact surface, and therefore, the fatigue strength is improved by forming the plating layer.
When the content of B is less than 0.02%, the compressive residual stress formed in the plating layer becomes small and the fatigue strength is insufficient, and when it exceeds 2%, the fatigue strength is almost saturated and the cost increases. Therefore, the lower limit is set to 0.02% and the upper limit is set to 2%.
Preferably, it is 0.05% or more and 1.5% or less.

In a preferred embodiment of the present invention, the ratio (M / K) of the thickness (M) of the plating layer to the thickness (K) of the hardened layer is 0.0
05 to 0.2. M / K is 0.0
If it is less than 05, the compressive residual stress formed in the plating layer is insufficient, and if it is more than 0.2, the compressive residual stress in the hardened layer is reduced due to volume expansion of the plated layer, and the fatigue strength of the hardened layer is reduced. .

Next, an example of the method for manufacturing an axle according to the present invention will be described. After smelting the steel in the above-described component range and roughly forming it into an axle shape by hot forging, a quenching and tempering process is performed. It is desirable that the heating temperature before quenching is in the range of Ac _{3 to} 950 ° C. and the tempering temperature is in the range of 450 to 675 ° C. The Ac ₃ transformation point of the steel having the above composition is about 800 ° C. If the tempering temperature is lower than 450 ° C, sufficient toughness cannot be obtained,
If it exceeds 675 ° C., sufficient tensile strength cannot be obtained. By quenching and tempering, a structure of tempered martensite or bainite is obtained.

Thereafter, semi-finished machining is performed, and induction fitting quenching is performed on the fitting end and its peripheral region. In induction hardening, after rapid heating by an induction heating (frequency 1 to 30 kHz) coil, rapid cooling is performed by water injection. The high-frequency heating conditions at this time are as follows: current 200 to 250 A, voltage 800
~ 900V, heating time 10 ~ 20 seconds, cooling delay time 0 ~
The cooling can be performed within a range of 5 seconds and a cooling time of 10 to 20 seconds. After induction hardening, low-temperature tempering is performed at a heating temperature of 150 ° C to 300 ° C. If the temperature is lower than 150 ° C., the required toughness cannot be secured. If the temperature is higher than 300 ° C., the compressive residual stress generated by induction hardening disappears, and the expected fatigue strength may not be obtained. A hardened layer transformed into martensite is obtained by the induction hardening.

Next, in order to remove black scale on the surface, finish machining is performed, and then the axle is immersed in a plating bath containing a B source and a Ni source to perform electroplating. As for the electroplating conditions, the plating bath temperature can be 40 to 80 ° C., and the current density can be 0.5 to 5 A / dm ² . After the electroplating, a baking treatment may be performed, but the temperature is set to be equal to or lower than the tempering temperature after induction hardening.

[0035]

EXAMPLES (Example 1) A small axle having a diameter of 40 mm, a length of 835 mm, and a fitting portion with a width of 90 mm at the center of the shaft was manufactured using low-alloy steel having a chemical composition shown in Table 2, and
The fatigue strength was evaluated.

[0036]

[Table 2]

The low alloy steel is melted in a vacuum electric melting furnace, cast into a steel ingot, roughly formed into a round bar by hot forging, and then oil quenched at 850 ° C. and tempered at 550 ° C. sequentially. did. Next, semi-finishing machining was performed to make the fitting portion diameter 40.8 mm, and then induction hardening was performed. In the induction hardening, the fitting end and its surrounding area were rapidly heated by an induction heating coil, rapidly cooled by water injection, and then tempered at 200 ° C. The induction heating conditions were adjusted so that the depth of the hardened layer was 0.5 mm, and the range of formation of the hardened layer in the axial direction was a range of a fitting portion (width 90 mm) and 105 mm on both sides thereof (a total of 300 mm). After induction hardening, finish machining was performed, and the diameter of the fitting portion was 40 mm. After finishing machining, the axle was immersed in a plating bath containing a B source and a Ni source, and electroplating was performed so that the composition of the plating layer was 1% B and the balance substantially consisted of Ni. The temperature of the plating bath was 60 ° C., and the current density was 3 A / dm ² . A rotary bending fatigue test was performed using the small axle manufactured as described above.

FIG. 3 is a schematic diagram of a small axle fatigue test apparatus. As shown in the figure, the test was performed with both supports rotating and bending with a stress applied to the fitting part at the center of the shaft. Even if the number of repetitions was 2 × 10 ⁷ times, the maximum stress amplitude that was not broken was the fatigue limit. And

Table 3 shows the fatigue limit in the above fatigue test.
It is shown together with the thickness of the hardened layer and the thickness of the plating layer.

[0040]

[Table 3]

As shown in Table 3, in Example 1 of the present invention, the thickness of the hardened layer was 0.5 mm (K / D: 0.0125), and the thickness of the plating layer was 20 μm (M / K: 0.3 mm). 04).
In addition, as shown in the same table, in each condition of the presence or absence of plating, the plating component, the plating thickness, and the thickness of the hardened layer, Comparative Example 1
No plating, Comparative Example 2 plating components, Comparative Example 3
And 4, the thickness of the hardened layer, Comparative Examples 5 and 6, the plating thickness,
The other conditions were the same as in Example 1 of the present invention.

As shown in Table 3, the fatigue strength of Example 1 of the present invention was 370 MPa, which was improved by 15% or more as compared with Comparative Example 1 showing the highest fatigue strength of 320 MPa among Comparative Examples 1 to 6.

Example 2 A small axle having the same dimensions as that shown in Example 1 was manufactured using carbon steel having the chemical composition shown in Table 4, and the fatigue limit was evaluated. The axle manufacturing method and the fatigue test method were performed in the same manner as described in Example 1. Table 5 shows the results of the fatigue test.

[0044]

[Table 4]

[0045]

[Table 5]

As shown in Table 5, in Example 2 of the present invention, the thickness of the hardened layer was 0.5 mm (K / D: 0.0125), and the thickness of the plating layer was 20 μm (M / K: 0.3 mm). 04).
As shown in the same table, Comparative Example 7 was the same as Example 2 of the present invention except for the condition where plating was not performed.

As shown in Table 5, the fatigue limit of Example 2 of the present invention was 310 MPa, which was improved by about 15% as compared with Comparative Example 7.

(Example 3) Using alloy steel and carbon steel shown in Tables 1 and 4, respectively, a large axle having a length of 2600 mm, a fitting portion having a width of 180 mm at the shaft end and a diameter of 200 mm was manufactured, The fatigue strength was evaluated.

The above steel was melted in a vacuum electric melting furnace, cast into a steel ingot, and roughly formed into a round bar by hot forging.
Oil quenching at 550 ° C and tempering at 550 ° C were sequentially performed.
Next, semi-finished machining is performed, the diameter of the fitting portion is 202 mm, and the diameter of the other portion (non-fitting portion) is 1 mm.
After 92 mm, induction hardening, then 200 ° C
Tempered. The induction heating conditions were adjusted so that the thickness of the cured layer was 5 mm and the range of formation of the cured layer in the axial direction was 40 mm or more on both sides from the fitting end. After induction hardening, finish machining was performed, and the diameter of the fitting portion was set to 200 mm. After finishing machining,
The axle was immersed in a plating bath containing a B source and a Ni source, and electroplated to a thickness of 50 μm so that the composition of the plating layer was 1% B and the balance substantially consisted of Ni. The temperature of the plating bath was set to 60 ° C.

Next, a rotary bending fatigue test was performed using the large axle.

FIG. 4 is a schematic diagram of a large axle fatigue test apparatus. As shown in FIG.
In a single support rotary bending press-fitted at 0 MPa, the nominal bending stress (bending moment / cross-sectional modulus of the fitting portion) at the fitting portion was set to 200 MPa, and 2 × 1
0 ⁷ times of repeated bending given was carried out. The above 2
00MPa is 147MP which is the allowable stress of the fourth type (induction hardened axle) specified in JIS-E4501.
This is a value that is sufficiently higher than a and is judged to be sufficient for use when the axle is mounted on an actual railway vehicle. Table 6 shows the results of the fatigue test together with the thickness of the hardened layer and the thickness of the plating layer.

[0052]

[Table 6]

As shown in Table 6, Examples 3 and 4 of the present invention were all good without being broken by repeated bending of 2 × 10 ⁷ times.

[0054]

According to the present invention, it is possible to obtain an axle for a railway vehicle having higher fatigue strength than the conventional one.
This makes it possible to cope with speeding-up of a Shinkansen vehicle or the like.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a procedure for measuring a friction coefficient of an axle.

FIGS. 2A and 2B are schematic views showing an example of an axle according to the present invention. FIG. 2A is an overall view of the axle, and FIG. 2B is a schematic partial cross-sectional view of a portion Y in FIG.

FIG. 3 is a schematic diagram of a small axle fatigue test device.

FIG. 4 is a schematic diagram of a large axle fatigue test device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Axle 2 Fitting part 3 Fitting end 4 Hardened layer 5 Plating layer

Continued on front page (72) Inventor Yoshinari Yamamura 5-1-1, Shimaya, Konohana-ku, Osaka Sumitomo Metal Industries, Ltd. Kansai Works Steel Works, F-term (reference) 4K024 AA03 BA03 BB04 BC10 DA01 DA10 DB01 GA16

Claims (2)

[Claims] C .: 0.3 to 0.48% by weight, S
i: 0.05 to 1%, Mn: 0.5 to 2%, Cr: 0 to 0
It is made of steel containing 1.5%, Mo: 0 to 0.3%, and Ni: 0 to 2.4%, and has a tempered martensite or bainite region from the surface to the inside. In the peripheral region, a hardened layer having a Vickers hardness of 400 or more has a thickness (K) of the hardened layer.
The ratio (K / D) to the diameter (D) of the fitting portion is 0.0
And a plating layer containing B in an amount of 0.02 to 2% by weight on the upper side of the hardened layer and a balance of Ni and unavoidable impurities. An axle for a railway vehicle having a tempered martensite or bainite region. 2. The method according to claim 1, wherein a thickness (M) of the plating layer is 0.005 to 0.2 in a ratio (M / K) to a thickness (K) of the hardened layer. Axle for railway vehicles.