JP2003166015A - Method for modifying surface of steel material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for modifying the surface of a steel material, which can introduce the maximum compression residual-stress on the top surface of the steel material. <P>SOLUTION: The objective method for modifying the surface of the steel material comprises hardening the steel material 11, and then subjecting the steel material 11 to subzero treatment in a state of giving the tensile stress P, while giving the tensile stress P to the hardened steel material 11, to introduce the compression residual stress into the steel material 11. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for modifying the surface of a steel material, and more particularly to a method for modifying the tooth surface of a gear.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of global environment and economic aspects,
BACKGROUND ART Automotive mechanical parts made of steel, such as gears, are required to be smaller and lighter by improving fatigue strength (particularly bending fatigue strength).

As a means for improving the fatigue strength of steel materials,
First, improvement of surface hardness is mentioned. The surface hardness is improved by surface modification treatment (carburizing, nitriding, nitrocarburizing, carbonitriding,
Induction hardening, etc.). However, when the surface hardness of the steel material becomes 700 HV or more by the surface modification treatment, the improvement of the surface hardness does not contribute to the improvement of the fatigue strength. Therefore, in this case, it is necessary to introduce the compressive residual stress, which is another means for improving the fatigue strength of the steel material.

Examples of means for introducing the compressive residual stress include shot peening, laser peening, and cavitation peening. Among them, impeller type shot peening is generally often used. Here, it is difficult to introduce a compressive residual stress in a steel material having a hardness of 700 HV or more because the deformation resistance is too high, but by performing shot peening in a state where tensile stress is applied,
Compressive residual stress can be introduced into steel materials of 700 HV or more.

[0005]

By the way, in order to further improve the fatigue strength of the steel material, it is effective to maximize the compressive residual stress on the outermost surface of the steel material.

However, in the method of introducing the compressive residual stress by shot peening, the position of the maximum compressive residual stress is 30 to 60 μm from the surface, so that the maximum compressive residual stress cannot be obtained on the outermost surface of the steel material. there were. That is, since the compressive residual stress on the outermost surface of the steel material is small, there has been a problem that a large improvement in fatigue strength cannot be expected.

Further, when shot peening is applied to a steel material, the surface roughness of the steel material becomes rough, which leads to a decrease in fatigue strength, and the work environment is affected by noise and dust generated when shot particles collide with the steel material. There was a problem of getting worse.

Further, the method of introducing the compressive residual stress by shot peening has a problem that the manufacturing cost of the product becomes high because the equipment and construction cost is high.

An object of the present invention, which was conceived in consideration of the above circumstances, is to provide a method for modifying the surface of a steel material capable of introducing maximum compressive residual stress on the outermost surface of the steel material.

[0010]

In order to achieve the above object, the surface modification method of a steel material according to the present invention is a method of applying a tensile stress to a hardened steel material after subjecting the steel material to a quenching treatment. The steel material in the applied state is subjected to sub-zero treatment to introduce compressive residual stress into the steel material. Also,
After quenching a gear made of steel, insert a shaft with a diameter larger than the inner diameter of the ring into the ring of the gear to apply tensile stress to the gear and to apply tensile stress to the gear. Sub-zero treatment is applied to the gear to introduce compressive residual stress to the gear.

Further, the steel material before the quenching treatment may be subjected to a carburizing treatment or a carbonitriding treatment.

According to the above surface modification method for steel materials, the maximum compressive residual stress can be introduced to the surface of steel materials.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the accompanying drawings.

When the above-mentioned shot peening is applied to the quenched steel material, the retained austenite present in the steel material undergoes a phase transformation to martensite, but at that time, since the volume expansion is accompanied, the compressive residual stress is also caused by this volume expansion. be introduced. Here, even if thermal energy is applied instead of kinetic energy by shot peening, the retained austenite undergoes a phase transformation into martensite, so the present invention has a feature in adopting a sub-zero treatment to provide thermal energy. There is.

In the method for modifying the surface of a steel material according to the first embodiment, after quenching the steel material, a tensile stress is applied to the quenched steel material, and a tensile stress is applied (prestress state). After subjecting the steel material to the sub-zero treatment, the application of the tensile stress to the steel material is released to introduce a high compressive residual stress into the steel material, especially on the surface of the steel material. The term "steel material" used here does not indicate only the material, but also includes steel members.

As a method of applying tensile stress to the hardened steel material, for example, as shown in FIG. 1, the upper pressing pins 13a and 13b are arranged on the upper surface of the plate material (steel material) 11 and the lower pins 14a and 14b are arranged on the lower surface thereof. At the same time, the tightening bolt 15 is passed through the plate member 11 and the upper pressing pins 13a and 13b.
By arranging a and 15b, fixing the plate material 11 to the base 12 with the tightening bolts 15a and 15b, and performing 4-point bending, a tensile stress P is applied to the plate material 11. At this time, by attaching the strain gauge 16 to the plate material 11, the value of the applied tensile stress P can be confirmed and adjusted. The sub-zero process is performed while the plate material 11 is fixed to the base 12.

Here, the steel material is not particularly limited as long as it is a steel material that can be hardened, but a steel material having a large content of retained austenite, that is, a steel material having a large content of C is preferable, for example, JIS standard. Carbon tool steel Steel (SK
Material), various alloy tool steel materials (SKS material, SKD material, SKT material),
High speed tool steel (SKH) and high carbon chrome bearing steel (SUJ). In addition to these, steel materials with a low carbon content, for example, JIS standard carbon steel steel for machine structure (SC material) or various structural alloy steel steel (SCr material, SCM material, SN
The CM material may be carburized (or carbonitrided).

The stress value of the tensile stress applied to the hardened steel material is appropriately selected according to the type of the steel material and the value of the compressive residual stress introduced into the steel material, and is not particularly limited, but is 700 to 1400 MPa. Is preferred.

The treatment temperature of the sub-zero treatment is appropriately selected according to the desired production amount of martensite (described later), that is, according to the balance of hardness and toughness of the steel material to which compressive residual stress is introduced. , About 203K (-70 ℃)
The following is acceptable.

Next, the operation of this embodiment will be described.

In the surface modification method of the present embodiment,
By subjecting the steel material after quenching to sub-zero treatment, residual austenite, which was largely left in the quenched steel material, transforms to form martensite, and carbon atoms are solid-soluted in supersaturation to cause volume expansion and compressive residual stress. A steel material (surface modified steel material) in which is introduced is obtained. Further, at the time of this sub-zero treatment, by performing the sub-zero treatment in a state in which tensile stress is applied to the steel material after quenching (prestress state), a steel material in which a larger compressive residual stress is introduced can be obtained. this is,
It is assumed that this is due to the phenomenon shown below.

By subjecting the steel material after quenching to the sub-zero treatment in a state where tensile stress is applied, the c-axes of the body-centered tetragonal crystals constituting the martensite phase to be formed are aligned in one direction,
In addition, a martensite phase crystal is formed in a state of being stretched in the c-axis direction. When the application of the tensile stress is released after the sub-zero treatment, a large stress is generated in the compression direction with respect to the martensite phase crystal that has been stretched in the c-axis direction. As a result, a large compressive residual stress is introduced into the steel material, and the steel material has excellent fatigue strength, particularly bending fatigue strength. here,
Carburizing (or carbonitriding) as steel material after quenching
By using a material that has been subjected to quenching treatment later, the carbon content on the surface of the steel material increases due to carburizing treatment (or carbonitriding treatment), and the amount of retained austenite on the surface of the steel material increases due to subsequent quenching treatment. After that, by performing sub-zero treatment on the steel material after this carburizing and quenching treatment (or carbonitriding and quenching treatment) while applying tensile stress, it is possible to introduce a large compressive residual stress to the surface portion of the steel material. The compressive residual stress is maximized on the outermost surface. As a result, the fatigue strength of the steel material can be significantly improved.

Further, the introduction of the compressive residual stress in the surface modification method of the present embodiment is carried out by the sub-zero treatment in the state where the tensile stress is applied, and is not by the shot peening as in the conventional case. The surface roughness of the steel does not become rough, that is, the fatigue strength of the steel material is not reduced, and the work environment is not deteriorated by noise and dust generated when the shot particles collide with the steel material.

Further, the equipment / construction cost for the sub-zero treatment in the surface modification method of the present embodiment is cheaper than that of the shot peening in the conventional surface modification method. It is possible to reduce the manufacturing cost of the modified steel material (product).

Next, another embodiment of the present invention will be described with reference to the accompanying drawings.

FIGS. 4A to 4 are schematic sectional views for explaining the method for modifying the surface of the steel material according to the second embodiment.
It shows in (c).

In the method for modifying the surface of steel according to the second embodiment, first, as shown in FIG. 4 (a), a gear 41 made of steel is carburized and quenched.

Next, as shown in FIG. 4B, the shaft 4 having a diameter larger than the inner diameter of the ring portion 41a of the quenching gear 41.
2 by a hydraulic press, the ring portion 41 of the quenching gear 41
It is inserted (press-fitted) into a and a tensile stress P is applied in the radial direction of the gear 41 (left-right direction in FIG. 4B). The gear 41 integrated with the shaft 42 is immersed in liquid nitrogen of about 123K for a predetermined time to perform a sub-zero treatment.

Next, after pulling up the gear 41 and the shaft 42 from the liquid nitrogen, the gear 41 and the shaft 42 are left to stand until they return to room temperature. After that, the shaft 42 is removed (pressed out) from the ring portion 41a of the gear 41 by a hydraulic press with pressure, and as shown in FIG. 4C, the surface-modified gear in which the maximum compressive residual stress is introduced. 43 is obtained.

It is needless to say that the method of modifying the surface of steel according to the present embodiment also has the same effects as those of the method of modifying the surface of steel described above.

Further, since the shaft 42 uniformly applies the tensile stress P to the entire gear 41, a large compressive residual stress is introduced to the entire surface modified gear 43.

Further, in the second embodiment, the gear 41 and the shaft 42 are integrated by press fitting, but the gear 41 and the shaft 42 are integrated by shrink fitting instead of press fitting. Good. As a result, the gear 4
1, a larger tensile stress P can be applied, and a larger compressive residual stress can be introduced into the surface modified gear 43.

It is needless to say that the embodiments of the present invention are not limited to the above-mentioned embodiments, and various other embodiments are possible.

[0034]

EXAMPLE A round bar made of Cr-Mo steel (JIS structural steel alloy steel SCM822H) was subjected to hot forging treatment to obtain a width of 4
After being formed into a plate material having a length of 0 mm, a length of 100 mm, and a thickness of 8 mm, the plate material is annealed. After that, the annealed plate is machined to a width of 30 mm and a length of 90 m.
A smooth material having a thickness of 5 mm and a thickness of 5 mm is formed.

With respect to this smooth material, first, a hydrocarbon gas atmosphere, a temperature of 1203 K (930 ° C.), and a time of 1.26 k
Vacuum carburizing is applied under the condition of s (21 minutes), and N
₂ + NH ₃ gas atmosphere, temperature 1143K (870 ℃), vacuum nitriding treatment under the conditions of time 1.8ks (30 minutes),
After that, oil cooling was performed.

Example 1 A state in which a tensile stress of 1000 MPa was applied to the smooth material after oil cooling using the method shown in FIG. 1 and the tensile stress was applied (prestressed state)
Leave the smoothing material in liquid nitrogen at 123K for 900s (15
Min) Dip and sub-zero treatment.

After the sub-zero treatment, the smoothing material is pulled up from the liquid nitrogen and left to stand until the smoothing material returns to room temperature. afterwards,
Remove the tightening bolts 15a and 15b and attach the smooth material to the base 1.
Sample 1 (surface-modified plate material) is prepared by removing from 2.

(Prior Art Example 1) Impeller-type shot peening is applied to a smooth material after oil cooling to prepare a sample 2 (surface-modified plate material). Here, various conditions of the impeller type shot peening are as follows: shot diameter φ0.8 mm, shot hardness about 560 HV, shot speed 73 m / s, shot distance 350 mm, coverage about 300%, arc height about 0. It is 5 mmA.

(Comparative Example 1) Sample 3 is a smooth material that has been cooled with oil.

The residual stress of the samples 1 to 3 was measured. Residual stress distribution of each sample, that is, depth from the surface (μ
The relationship between m) and residual stress (MPa) is shown in FIG.

Here, the measurement of the residual stress was carried out by using a fine part X-ray measuring machine. The internal residual stress was measured by masking each sample by the window method and then polishing by electrolytic polishing to a predetermined depth. The measurement position of the residual stress in each sample is the position where the strain gauge 16 is attached in the sample 1, and the condition of the X-ray stress measurement is Cr-K as X-ray.
_{Using α} rays, the diameter of the incident X-ray beam is 2 mm, and the stress is calculated by the 2θ-sin ² ψ method (ψ = 0 °, 10 °, 20 °, 3
0 °, 40 °) was used.

As shown in FIG. 2, the residual stress (σ _rs ) on the surface of the plate material was measured in the sample 3 which had been carbonitrided and quenched.
Was about 50 MPa, that is, tensile residual stress. The maximum compressive residual stress (σ _rmax ) is about −400 MPa,
The generation position was a position where the depth from the surface was about 410 μm.

In sample 2 obtained by subjecting sample 3 to surface modification treatment by impeller type shot peening, σ
_{The rmax} was −869 MPa, and the generation position was a position where the depth from the surface was about 20 μm. Further, σ _{rs on the} surface of the plate material was −628 MPa.

On the other hand, in the sample 1 obtained by subjecting the sample 3 to the surface modification treatment by the prestress + subzero treatment, the position where σ _rmax occurs is the plate surface, and σ _rmax is −1.
It was 179 MPa.

When the bending fatigue strength of each sample was measured, the bending fatigue strength of sample 1 in which σ _rmax was generated on the surface of the plate material was significantly improved as compared with the bending fatigue strength of samples 1 and 2.

Next, the samples 1 to 3 were observed using a microscopic X-ray measuring machine to measure the amount of retained austenite. FIG. 3 shows the distribution of the retained austenite of each sample, that is, the relationship between the depth (μm) from the plate material surface and the retained austenite amount (volume%).

As shown in FIG. 3, in the sample 3 that had been carbonitrided and quenched, the amount of retained austenite (γ _Rs ) on the plate surface was 27.6 wt%, and the maximum amount of retained austenite (γ _Rmax ) was 47.4 wt. %Met.

In sample 2, the amount of γ _Rs is 22.2 wt.
%, Γ _Rmax amount was 39.6 wt%. From this, it can be seen that the amount of γ _Rs does not decrease much even if the surface modification treatment is performed by impeller shot peening.

On the other hand, in sample 1, the γ _Rs amount was 14.4 wt% and the γ _Rmax amount was 18.3 wt%. From this, it can be seen that the amount of γ _Rs is greatly reduced, that is, the amount of martensite phase generated is large, by performing the surface modification treatment by the prestress + subzero treatment. As a result, in Sample 1, a large compressive residual stress is introduced.

[0050]

In summary, according to the present invention, the following excellent effects are exhibited. (1) By performing sub-zero treatment in a state where tensile stress is applied to the steel material after quenching, maximum compressive residual stress can be introduced to the surface of the steel material. (2) According to (1), a steel material having excellent fatigue strength can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing a method for imparting tensile stress to a hardened steel material by a surface modification method for a steel material according to a first embodiment.

FIG. 2 is a diagram showing the relationship between the depth from the surface of the steel material and the residual stress.

FIG. 3 is a diagram showing the relationship between the depth from the surface of the steel material and the amount of retained austenite.

FIG. 4 is a schematic cross-sectional view for explaining the method for surface modifying a steel material according to the second embodiment.

[Explanation of symbols]

11 Plate material (steel material) P tensile stress

Continued front page    (72) Inventor Pillar Ando             79-5 Tokiwadai, Hodogaya-ku, Yokohama-shi, Kanagawa             Yokohama National University Faculty of Engineering F-term (reference) 3J030 BC02 BC10                 4K042 AA18 BA04 BA09 DA01 DA06

Claims (3)

[Claims] Claims: 1. After quenching a steel material, a tensile stress is applied to the quenched steel material, and a sub-zero treatment is applied to the steel material to which the tensile stress is applied to introduce a compressive residual stress into the steel material. Method for surface modification of steel material. 2. After quenching a gear made of steel, a shaft having a diameter larger than the inner diameter of the ring is inserted into the ring of the gear to give tensile stress to the gear and A surface reforming method for a steel material, which comprises subjecting a gear in a state of being subjected to a subzero treatment to introducing compressive residual stress to the gear. 3. The method for surface modification of a steel material according to claim 1, wherein the steel material before quenching is subjected to carburizing treatment or carbonitriding treatment.