KR101144516B1 - Alloy Steel for Low Temperature Vacuum Carburizing - Google Patents

Abstract

The present invention relates to a low-temperature vacuum carburizing alloy steel, more specifically, a sufficient ferrite phase (α) is secured when the carburizing in about 810 ℃, the minimum available carburizing temperature of the conventional vacuum carburizing furnace to ensure that the heat treatment of the annular gear According to the present invention, the present invention relates to an alloy steel for low-temperature vacuum carburizing, which has a large effect of improving heat deformation and can satisfy the shape regulation of roundness, cylinder degree, etc. of the annular gear to be manufactured.

The low-temperature vacuum carburizing alloy steel of the present invention contains Fe as a main component, C 0.17 to 0.24 wt%, Cr 0.8 to 1.2 wt%, Mn 0.4 to 0.8 wt%, Si 0.80 to 1.20 wt%, and P 0.020 wt% or less. , S 0.020% by weight or less, V 0.015 to 0.045% by weight, the balance of dissolved oxygen in the alloy component comprising the remaining weight% of Fe characterized in that it is configured to be 10ppm or less.

Description

Alloy Steel for Low Temperature Vacuum Carburizing {Alloy Steel for Low Temperature Vacuum Carburizing}

The present invention relates to a low-temperature vacuum carburizing alloy steel, more specifically, a sufficient ferrite phase (α) is secured even at about 810 ° C., which is the lowest available carburizing temperature of a conventional vacuum carburizing furnace, to provide thermal deformation due to heat treatment of the annular gear. It relates to an alloy steel for low temperature vacuum carburizing, which has a significant improvement effect and can satisfy the shape regulation of roundness, cylinder degree, etc. of the annular gear to be manufactured.

Automobile transmission gears are usually subjected to carburization. Deformation of parts is involved in the heat treatment of carburization, and when the amount of deformation is excessive, unexpected problems such as deterioration of assemblability between parts and abnormal noise are generated.

1 is a diagram illustrating an example of a general annular gear.

As shown, the annular gear which is the main part of the planetary gear device, which is an important power transmission device of the automatic transmission, is a cylindrical part and is toothed through broaching as an internal gear. Therefore, it is one of the parts that is difficult to process after heat treatment, unlike general external gear, which is not only sensitive to thermal deformation, but also easy to process after grinding such as grinding because of its large diameter and thin thickness.

There are various methods of heat treatment of the annular gear in consideration of thermal deformation.

For example, in order to minimize the thermal deformation of the annular gear, a method that mainly involves gas carburizing, quenching, high frequency heating jig quenching is applied. The final jig quenching process, that is, by combining the jig after high frequency heating and applying quenching, the shape regulation such as roundness, cylinder degree, etc. could be satisfied.

However, the above-described method has a problem that the process is not only long and complicated, but also has to be processed one by one for each part, thereby reducing work efficiency. Also, the shape of the gear teeth is unevenly deformed by local uneven cooling during jig cooling. There was also a problem that caused noise.

On the other hand, in recent years, productivity has been improved, and vacuum carburization and gas quenching, which can be uniformly cooled, have contributed greatly to the improvement of noise-induced problems caused by non-uniform deformation, but in terms of roundness, cylinder degree, etc. It is disadvantageous compared with the room for improvement.

Therefore, many studies have been made to apply the low temperature vacuum carburizing method comparable to the existing jig quenching method in terms of roundness, cylinder degree, etc. while applying the vacuum carburizing method. 2 is a conceptual diagram illustrating a low temperature vacuum carburizing method, which is a state diagram of iron-carbon.

Referring to the drawings, the low-temperature carburization method is a temperature raising and carburizing process of the treated product by carburizing in the temperature range between A1 and A3, which is a dual phase in which the austenitic phase (γ) and the ferrite phase (α) coexist. The ferrite phase (α) is maintained without phase change at, and is maintained at the time of quenching, so the thermal deformation improvement effect is very large. However, in the case of vacuum carburizing furnaces that are mass-produced at present, due to the characteristics of carburizing and mass production at 800 ~ 810 ° C or more, sufficient ferrite phase (α) is obtained when carburizing is carried out at the carburizing temperature of 810 ° C. As it is not secured, there is a disadvantage that the effect of improving thermal deformation is insufficient.

The present invention is to solve the above-mentioned conventional problems, the technical problem to be solved by the present invention is a low temperature that can ensure a sufficient ferrite phase (α) even at about 810 ℃, the lowest available carburizing temperature of the conventional vacuum carburizing furnace An alloy steel for vacuum carburizing is provided.

Low-temperature vacuum carburizing alloy steel of the present invention for achieving the above object is based on Fe, the C 0.17 ~ 0.24 wt%, Cr 0.8 ~ 1.2 wt%, Mn 0.4 ~ 0.8 wt%, Si 0.80 ~ 1.20 wt% It is characterized in that the amount of dissolved oxygen is 10ppm or less in the alloy component, including P 0.020% by weight or less, S 0.020% by weight or less, V 0.015 to 0.045% by weight, and Fe remaining weight%.

According to the low-temperature vacuum carburizing alloy steel according to the present invention, a sufficient ferrite phase (α) is secured when the carburizing is carried out even at about 810 ° C., which is the lowest available carburizing temperature of a conventional vacuum carburizing furnace. The improvement effect is great and the effect which can satisfy shape regulation of roundness, cylinder degree, etc. of the annular gear to manufacture is exhibited.

Hereinafter, a suitable embodiment of the low-temperature vacuum carburizing alloy steel according to the present invention will be described in detail.

The low-temperature vacuum carburizing alloy steel of the present invention contains Fe as a main component, as shown in Table 1 below, and C 0.17 to 0.24 wt%, Cr 0.8 to 1.2 wt%, Mn 0.4 to 0.8 wt%, and Si 0.80 to 1.20 wt%. To the purity of the alloy (to reduce the impurity content) in the alloying components including%, P 0.020 wt% or less, S 0.020 wt% or less, V 0.015 to 0.045 wt%, and the rest of Fe by weight, the amount of dissolved oxygen is 10 ppm or less. It is characterized by being configured.

<Table 1> Alloy composition of invention steel and comparative steel (content unit: weight%)

division C Cr Mn Si P S V O
(ppm) Fe Invention steel 0.17
~ 0.24 0.8
~ 1.2 0.4
~ 0.8 0.8
~ 1.2 0.02
Below 0.02
Below 0.015
~ 0.045 10
Below Remainder Comparative steel
(SCr420H) 0.17
~ 0.23 0.85
~ 1.25 0.55
~ 0.9 0.15
~ 0.35 0.03
Below 0.03
Below - 25
Below Remainder

Design core of the low-temperature vacuum carburizing alloy steel according to the present invention is as follows.

The alloy steel of the present invention is designed to ensure a sufficient ferrite phase (α) within about 810 ℃, the lowest available carburizing temperature of the existing vacuum carburizing furnace. That is, the alloy is designed to ensure that the A3 temperature of the alloy steel of the present invention to 850 ℃ or more, 30-30% of the ferrite phase (α) can be present in the core portion through the carburizing and quenching process 810 ~ 830 ℃.

Referring to the design of the alloy steel of the present invention in more detail as follows.

Carbon (C) contains 0.17 to 0.24% by weight to have a proper core hardness after high frequency carburization. In this case, when the content is less than the prescribed amount, the ferrite phase (α) is too large and the hardness of the core is too low to decrease the strength.

Chromium (Cr), an important element to increase the fatigue strength of gear steels, was added in an amount of 0.8 to 1.2% by weight. Chromium (Cr) not only lowers the A1 temperature, but also lowers the A3 temperature, limiting the amount. In order to lower the temperature of A1 constantly and add 0.8% or more to secure the fatigue strength, limiting it to 1.2% or less is difficult to secure sufficient amount of ferrite phase (α) due to the A3 temperature decrease.

Manganese (Mn) was reduced from 0.4 to 0.8% by weight compared to the existing carburized steel.

This is because manganese has an advantage of increasing strength, but it is an element that causes segregation during steelmaking and causes non-uniform thermal deformation. More than 0.4% was added to lower the A1 temperature and less than 0.8% was used to reduce segregation.

The silicon (Si) content was increased in order to raise the A3 temperature to secure a sufficient amount of ferrite phase (α), and V was added in a sufficient amount.

Silicon (Si) is 0.80 to 1.20 wt%, which not only increases the temperature of A3 but also significantly increases the contact fatigue property (fitting resistance). At this time, if the content is less than the specified amount, it is difficult to secure sufficient amount of ferrite phase (α) at the carburizing temperature because the temperature increase effect of A3 is not large. If the content exceeds 1.2% by weight, the solid solution strengthening effect is so large that the formability is extremely forged and forged. And make processing difficult. Since the amount of silicon is very high compared to the existing steel, the alloy steel of the present invention is suitable for vacuum carburizing. This is because, when carburizing in the existing gas carburizing furnace, there is a risk that the surface oxide layer is formed by the oxidizing atmosphere, thereby reducing the durability.

Vanadium (V) was 0.015 to 0.045 wt%, which not only gave the A3 temperature increase effect but also maximized the grain refinement effect. Since V is an element that enhances fine precipitation strengthening, the amount thereof is limited to 0.015 or more for not only the core strength enhancement effect due to the fine precipitation strengthening but also the A3 synergistic effect, and the limit to 0.045 or less is very expensive. to be.

Dissolved oxygen was 10 ppm or less. Since the alloy steel of the present invention has a relatively lower core hardness than the existing carburizing alloy steel, the required characteristics of the surface layer (carburizing layer) are more important, so the amount of dissolved oxygen is limited to 10 ppm or less. This is because when the dissolved oxygen is present, there is a problem that a performance degradation problem occurs due to surface contact contact caused by the large oxide.

&Lt; Examples and Comparative Examples &

Table 2 below shows the chemical composition and the important phase transformation temperature of the alloy steel and comparative steel of the present invention.

TABLE 2

division Chemical composition (% by weight) Phase transformation temperature (℃) Low temperature carburizing
Temperature
(℃) C Cr Mn Si P S V O
(ppm) Fe Al A3 Invention steel
0.22 1.0 0.6 1.1 0.015 0.01 0.04 8 Remainder 761 852 770-840 Comparative steel
(SCr42OH) 0.2 1.1 0.8 0.2 0.015 0.015 - 17 Remainder 743 803 755-790

In the case of the comparative steel, which is an existing steel, when the carburizing is performed at about 810 ° C, which is the lowest available carburizing temperature of the existing vacuum carburizing furnace, it is predicted that there is no amount of α in the core because it is carburized in the γ single phase region above the A3 temperature. It is expected that the amount of thermal deformation will be greatly improved.

Actually carburized at 810 ℃ compared example is shown in Table 3 below.

TABLE 3

division Heat treatment condition Heat treatment properties After heat treatment
Average dimension (㎛) Remarks Hardness (Hv)
(Surface / deep) Core α amount (%) Roundness Cylinder Example
Inventive Lecture Vacuum carburization (810 ℃) → high pressure gas cooling
(130 bar, helium) 750/340 Approximately 35 38 42 Invention steel
Low temperature carburizing Comparative example
(Existing steel) Comparison 1
760/390 About 2 53 72 Existing Steel
Low temperature carburizing Comparison 2 Gas carburizing (920 ° C) → furnace cooling → high frequency heating → jig quenching (oil cooling) 750/400 0 36 40 Jig Hardening * Test part used: Rear annular gear for 6-speed automatic transmission (gear inner diameter IBD: 122.45mm (gear of Figure 1))
* The average dimension after heat treatment is the average of 20 products tested

When the present invention steel was vacuum carburized at 810 ° C., deformation was improved after heat treatment superior to that of conventional steels subjected to the same heat treatment. This is because, in the case of the existing carburizing material, sufficient ferrite phase α is not secured in the core at the carburization at this temperature, whereas sufficient ferrite phase α is secured in the case of the inventive steel.

In addition, compared with the existing jig annealing method, the results were excellent enough to show the same level of results.

As described above, the low temperature vacuum carburizing-only alloy steel according to the present invention ensures sufficient ferrite phase (α) upon carburizing even in the vicinity of 810 ° C., which is the lowest available carburizing temperature of the conventional vacuum carburizing furnace, resulting in heat treatment of the annular gear. The heat deformation effect can be improved, and the shape regulation of roundness, cylinder degree, etc. of the annular gear to be manufactured can be satisfied.

1 is a perspective view showing the structure of a conventional general annular gear,

Figure 2 is a conceptual diagram that can explain low-temperature vacuum carburizing, it is a state diagram of iron-carbon.

Claims (1)

Fe is the main component, and C 0.17 to 0.24 wt%, Cr 0.8 to 1.2 wt%, Mn 0.4 to 0.8 wt%, Si 0.80 to 1.20 wt%, P 0.020 wt% or less, S 0.020 wt% or less, V 0.015 Low-temperature vacuum carburizing alloy steel, which is composed of dissolved oxygen less than 10ppm to reduce the purity or impurity content of the alloy to ˜0.045% by weight, remaining Fe and other unavoidable impurities.

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