JPH07828B2 - Carburized parts - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is applied to parts for machine structures constituting automobiles, agricultural machines, industrial machines, etc., and is particularly required to have surface wear resistance and fatigue strength. The present invention relates to improvement of carburized parts used as parts.

(Prior Art) In recent years, for example, power transmission parts have tended to be miniaturized in response to a demand for weight reduction of automobiles, and therefore, higher strength is required for each component. There is. In this case, the content of high strength required in the power transmission component is roughly classified into improvement of fatigue strength and surface pressure strength, but the present invention relates particularly to improvement of surface pressure strength. Pitting resistanc
It aims to improve e). Here, the term "pitting" means a phenomenon in which peeling from the surface as a starting point occurs under high surface pressure load including slack acting on the gear.

By the way, as a means for increasing the surface hardness in order to improve the surface pressure strength, there is so-called excess carburization in which the carbon potential is considerably increased during carburization so that more carbides are precipitated. A special pot designed to be used is being developed. However, this special steel is expensive because it contains a large amount of Cr and Mo, and at the same time has the drawback that the hardenability becomes excessive.
Therefore, there is a problem that the application is limited.

(Object of the invention) The present invention has been made by paying attention to the above-mentioned conventional problems, in which a carbon potential is slightly increased in a part which is contacted and rotated under a high surface pressure (for example, Hertz stress of 200 kgf / mm ² or more). It is intended to provide a carburized component which can obtain high hardness even at a high level (for example, CP <2%) and can particularly improve pitting resistance.

(Structure of the invention) According to the above-mentioned object, in order to improve the pitting resistance of the parts that are contacted and rotated under a high surface pressure, as a result of a detailed examination of the mechanism of pitching generation, it is found that pitching occurs from the surface layer part and the former austenite. Grain boundaries can serve as crack generation and propagation paths.In particular, grain boundary oxidation that occurs during carburization and carbides that precipitate at the grain boundaries weaken the grain boundaries, and a certain hardening depth is required to improve pitting resistance. It has been clarified that it is preferable to secure the hardness and further increase the surface hardness, and the present invention has been completed.

That is, the carburized part according to the invention has a C: 0.10% by weight.
~ 0.30%, Si: 0.50% or less, Mn: 0.20 to 1.50%, Cr: over 1.6 to 3.0%, and if necessary, Mo: 0.05 to 0.50%, V: 0.0
5 to 0.50%, Ti: 0.05 to 0.50%, Nb: 0.05 to 0.50%, Ta: 0.05
~ 0.50%, Zr: 0.05 ~ 0.50%, one or more of them, and the balance is made of steel consisting essentially of Fe. Hv550
The effective hardened layer depth is 0.4 mm or more, the surface hardness is Hv800 or more, the area ratio of carbides formed within 0.1 mm from the surface is 10 to 40%, and the major axis is 5 μm or less, and It is characterized in that the ratio of major axis / minor axis of carbides of 80% or more is 2 or less.

Next, the reason for limiting the component range (% by weight) of the material constituting the carburized component according to the present invention will be described.

C: 0.10 to 0.30% C is an element necessary to secure the core strength of carburized parts, and for this purpose, 0.10% or more is added. However, if it is too large, the toughness deteriorates, so the content was made 0.30% or less.

Si: 0.50% or less Si is an element that promotes grain boundary oxidation during carburization and deteriorates pitting resistance, so it is set to 0.50% or less. And
In particular, when performing normal gas carburization, the lower the Si content, the better. More preferably, it is 0.15% or less, further preferably 0.10% or less.

Mn: 0.20 to 1.50% Mn is an element added as a deoxidizing agent during melting, and for this purpose, 0.20% or more is added. However, if the amount is too large, the hardenability becomes excessive, depending on the balance with other elements, and it may cause quenching strain, so it was set to 1.50% or less.

Cr: over 1.6% to 3.0% Cr is an element that precipitates carbides (cementite) during carburization and increases the hardness of the surface layer to improve the pitting resistance of carburized parts. And, in order to obtain such an effect, it is necessary to add more than 1.6%. However, if the addition amount is too large, the hardenability becomes excessive, so the content was limited to 3.0% or less.

Mo: 0.05 to 0.50%, V: 0.05 to 0.50%, Ti: 0.05 to 0.50%, Nb:
One or more of 0.05 to 0.50%, Ta: 0.05 to 0.50%, Zr: 0.05 to 0.50% Mo, V, Ti, Nb, Ta, Zr are all carbide forming elements,
Each of them is an element effective in forming carbides that are harder than cementite and contributing to an increase in the hardness of the surface layer and further improving the pitting resistance. 0.05% for each species or two or more species
It is also possible to add the above. However, if added excessively, the carbides will become coarse and rather the pitting resistance will be reduced, so it is necessary to make each 0.50% or less.

The carburized component according to the present invention is made of steel having the above composition as a raw material, has an effective hardened layer depth of Hv550 or more of 0.4 mm or more, a surface hardness of Hv800 or more, and is formed within a range of 0.1 mm or less from the surface. Area ratio of carbides to be used is 10-40%, major axis is 5 μm
It is characterized in that the ratio of the major axis / minor axis of the carbide is 80% or more, that is, the aspect ratio is 2 or less, and the reason for such limitation is described in detail below.

First, regarding the effective hardened layer depth and the surface hardness, when the effect of the hardening characteristics on the pitting resistance was examined using a pitching tester, the effective hardened layer depth was
The results shown in the figure are obtained, and the surface hardness is
The results shown in the figure were obtained.

As shown in Fig. 1, the effective hardened layer depth above Hv550 is 0.4.
It was confirmed that when the thickness was not less than mm, the pitting resistance was remarkably improved, and as shown in FIG. 2, when the surface hardness was HV800 or more, the pitting resistance was remarkably improved similarly to the above.

Next, regarding carbides, if the area ratio of carbides formed within the range of 0.1 mm from the surface is less than 10%, the above surface hardness Hv 800 or more cannot be obtained, and if it exceeds 40%, coarse carbides When a rod-shaped carbide having a major axis length of more than 5 μm and a major axis / minor axis ratio of carbide of 80% or more, that is, an aspect ratio of more than 2, and a reticulated carbide precipitated at a grain boundary are formed, pitching occurs. It is not preferable because it can be the starting point.

(Example) Pitching test pieces were prepared from each steel after the steel having the chemical composition shown in Table 1 was melted. Then, each of the pitching test pieces was subjected to a surface hardening treatment under the conditions shown in Table 2. In the surface hardening treatment, the carbon potential shown in Table 2 was set in consideration of the temperature and the balance between the carburizing period and the diffusion period during carburization. In addition, in addition to carburizing, a part of them was subjected to induction hardening and laser hardening as a composite heat treatment. In this case, after the carburizing treatment, heating was performed for a short time so that the surface temperature was 900 to 950 ° C., and then cooling was performed.

Then, when the properties of the carburized layer in each test piece were examined, the same results as shown in Table 2 were obtained.

As shown in Table 2, the carburized layers of the test pieces of the present invention A and B all satisfy the above-mentioned conditions, and the surface hardness of the carburized B ′ is further hardened by induction hardening. Was higher, the effective hardened layer depth was larger, and the carbides were also finer.

Further, the amount of carbides increased in D, E, F, and G to which the carbide-forming element was added, and the surface hardness was higher and the effective hardened layer depth was larger in D'and F'which were subjected to the composite heat treatment. And the carbides were also becoming finer.

On the other hand, Comparative H (SCM 420) had a low surface hardness and a small area ratio of carbides. When the carbon potential during carburization is as low as 0.80 (A '), the surface hardness is low, the surface hardened layer depth is small, and the area ratio of carbides is small. On the contrary, the carbon potential during carburization is 2.30. When it was high (E '), the effective hardened layer depth was considerably large, but a large amount of carbides on the rod was generated, and the proportion of carbides having an aspect ratio of 2 or less was low. From this, it was found that the carbon potential during carburization is preferably about 1.0 to 2.0.

Next, as shown in FIG. 3, for each pitching test piece after the above heat treatment, the diameter d ₁ = 130 mm and the width W ₁ = 18.
A large roller 1 of mm and a small roller (test piece) 2 with a boss having a diameter d ₂ = 26 mm and a length l ₁ = 130 mm are brought into contact rotation under high surface pressure to perform a pitching test, and a small roller (test piece) 2 To size
The life was defined when 0.5 mm pitching occurred. The results are shown in Table 3. The surface pressure in the pitching test was 300 and 350 kgf / mm ² , the slip ratio was 40%, and the oil temperature was 40-5.
The temperature was 0 ° C and the rotation speed was 1000 rpm.

As shown in Table 3, it is clear that in the present inventions (1) and (2), the life until pitting occurs is longer than that of the comparative examples, and the steel containing the carbide-forming element is used as the raw material. It is obvious that pitching is less likely to occur in the case of the above. Further, in the case of B ', D', and F'which has been subjected to the composite heat treatment, a new austenite grain boundary is formed by quenching during induction hardening or laser hardening, so cracks along the grain boundary are less likely to occur. ,
It was confirmed that the pitting life was also good. On the other hand, the conventional SCM420 material (H) and the low carbon potential during carburization (A ') have a low surface hardness and a small effective hardened layer depth, resulting in a short life until pitting occurs. In the case of carburizing with a high carbon potential (E '), the surface hardness is high and the effective hardened layer depth is sufficient, but since the carbide is coarse and precipitates at the grain boundaries, cracks follow this. Was easily generated, and the life before pitching was short.

The carburizing treatment applied in the present invention includes not only ordinary gas carburizing but also vacuum carburizing and carbonitriding.
Also in this case, it is preferable that the carbon potential is in the range of 1.0 to 2.0 so that carbides are formed.

Further, when performing the composite heat treatment, the same effect can be obtained not only by the induction hardening or the laser hardening described above but also by other hardening for a short time. For example, electron beam heating or plasma heating can be adopted.

In addition, when smelting steel, remelting is performed by electroslag remelting (ESR), vacuum arc melting (VAR), or a combination of these at the time of smelting to reduce the size of initial carbides (unmelted carbides). It is also desirable to do so. Further, as described above, it is also desirable to minimize the amount of Si that promotes grain boundary oxidation during carburization and deteriorates pitting resistance.

(Effects of the Invention) As described above, the carburized component according to the present invention is, by weight%, C: 0.10 to 0.30%, Si: 0.50% or less, and Mn: 0.20 to 1.
50%, Cr: over 1.6 to 3.0%, and, if necessary, Mo: 0.05
~ 0.50%, V: 0.05 ~ 0.50%, Ti: 0.05 ~ 0.50%, Nb: 0.05 ~
0.50%, Ta: 0.05 to 0.50%, Zr: 0.05 to 0.50%, containing 1 or 2 or more types, the balance is made of steel consisting essentially of Fe, and the effective hardened layer depth of Hv550 or more is 0.4. mm or more, surface hardness is Hv800 or more, and the area ratio of carbides formed within 0.1 mm from the surface is 10 to 40% and the major axis is 5
The ratio of major axis / minor axis of carbides with a diameter of less than μm and 80% or more is 2
Since it is as follows, it is possible to obtain a high hardness even in the ordinary carburizing treatment with a slightly increased carbon potential, and it is possible to provide a component with high pitting resistance. This is a very excellent effect that can meet the demands for downsizing and high strength of structural parts such as gears used as transmission parts.

[Brief description of drawings]

Fig. 1 is a graph showing an example of the result of examining the effect of the effective hardened layer depth of the carburized part on the pitting resistance, and Fig. 2 is the effect of the surface hardness of the carburized part on the pitting resistance. FIG. 3 is a graph showing an example of the results, and FIG. 3 is an explanatory view showing the procedure of the pitching test.

Claims (2)

[Claims] 1. A steel material containing C: 0.10 to 0.30%, Si: 0.50% or less, Mn: 0.20 to 1.50%, Cr: more than 1.6 to 3.0%, and the balance substantially consisting of Fe, as a raw material. , Hv550 or more effective hardened layer depth is 0.4 mm or more, surface hardness is Hv800 or more, and the area ratio of carbides formed within the range of 0.1 mm from the surface is 10 to 40%, and the major axis is 5 μm or less. And a ratio of major axis / minor axis of carbides of 80% or more to 2 or less, a carburized part. 2. In weight%, C: 0.10 to 0.30%, Si: 0.50% or less, Mn: 0.20 to 1.50%, Cr: more than 1.6 to 3.0%, and Mo: 0.0.
5 to 0.50%, V: 0.05 to 0.50%, Ti: 0.05 to 0.50%, Nb: 0.05 to
0.50%, Ta: 0.05 to 0.50%, Zr: 0.05 to 0.50%, containing 1 or 2 or more types, the balance is made of steel consisting essentially of Fe, and the effective hardened layer depth of Hv550 or more is 0.4. mm or more, surface hardness is Hv800 or more, and the area ratio of carbides formed within 0.1 mm from the surface is 10 to 40% and the major axis is 5
The ratio of major axis / minor axis of carbides with a diameter of less than μm and 80% or more is 2
Carburized parts characterized by the following.