KR100811912B1 - High Strength Cr-Mo alloy steel for the high temperature vacuum carburizing - Google Patents

Abstract

The present invention relates to a high-strength chromium-molybdenum alloy steel for high temperature vacuum carburizing, and more particularly, to a high-strength chromium-molybdenum alloy steel for vacuum carburizing suitable for vacuum carburizing for the purpose of maximizing the advantages obtained in vacuum carburizing, unlike conventional carburizing. will be.

To this end, the present invention has iron (Fe) as a main component, carbon (C) 0.22 to 0.24% by weight, silicon (Si) 0.70 to 0.90% by weight, manganese (Mn) 0.50 to 0.80% by weight, phosphorus (P) ) 0.030 wt%, sulfur (S) 0.020 wt%, chromium (Cr) 1.90-2.20 wt%, molybdenum (Mo) 0.20-0.40 wt%, titanium (Ti) 0.008-0.012 wt%, niobium (Nb) 20-40 It provides a high-strength chromium-molybdenum alloy steel for high-temperature vacuum carburizing, characterized in that the dissolved oxygen amount of 15ppm ppm, nitrogen (N ₂ ) 70-120ppm, alloy-based components for the cleanliness of the alloy.

Vacuum carburizing, high strength chromium-molybdenum alloy steel, transmission, gears

Description

High Strength Cr-Mo alloy steel for the high temperature vacuum carburizing}

1 is a process diagram illustrating a process of manufacturing a gear and shaft for a conventional vehicle transmission.

The present invention relates to a high-strength chromium-molybdenum alloy steel for high temperature vacuum carburizing, and more particularly, to a high-strength chromium-molybdenum alloy steel for vacuum carburizing suitable for vacuum carburizing for the purpose of maximizing the advantages obtained in vacuum carburizing, unlike conventional carburizing. will be.

Recently, in accordance with the trend of high performance and high output of automotive engines, securing durability of transmission parts has emerged as one of the major challenges. To improve the durability of transmission, improvement of durability of gears, which is a major member, is the first priority.

In general, transmission gears are used by performing surface hardening treatment to improve durability. Currently, heat treatment that is commonly used is general gas carburizing heat treatment. Particles are formed at 15 to 25 μm, and there are potential elements that deviate from environmentally friendly trends such as generation of carbon dioxide, salt bath or oil in the manufacturing process.

Therefore, development of a new heat treatment method to cope with the problem of gas carburization is required, one of which is the vacuum carburization method.

The vacuum carburizing method is a new heat treatment method for carburizing in a vacuum atmosphere and quenching with a high pressure gas, and since the surface abnormality layer is not generated, durability (fatigue resistance and abrasion resistance) is dramatically improved, and when carburizing It is known as a new carburizing method that can reduce production time because it does not generate carbon dioxide, mainly use gas as a quenching medium, and has little waste oil / waste water treatment, and it is eco-friendly. have.

For this reason, the vacuum carburizing method is applied by some automobile companies in Europe and the United States, and active development is being carried out mainly in Japanese automobile companies.

Typically, transmission gears for automobiles are manufactured through a manufacturing process as shown in FIG. 1, wherein carburization mainly refers to general gas carburization heat treatment.

When commercially available alloy steel for transmission gears is replaced by vacuum carburization instead of general gas carburization, the thermal deformation characteristics that can be predicted during quenching can improve NVH (Noise, Vibration, Harshness) performance, and the process itself is environmentally friendly. Therefore, there is an advantage that it is possible to respond to environmental regulations in the future.

However, the durability of the parts cannot be said to be necessarily improved, because the difference in the surface residual stress distribution according to the carburizing method and the cooling rate difference during the hardening is mentioned, but the obvious reason is not known yet.

However, if the development of a dedicated material suitable for vacuum carburizing, it is possible to take advantage of the above-mentioned vacuum carburizing and to increase the durability.

In addition, since all of the vacuum carburizing furnaces developed at this time can be subjected to high temperature carburizing, if the vacuum carburizing material is developed to be processed at a high temperature, it is possible to improve productivity by shortening the carburizing time, and the ripple effect will be doubled.

Therefore, in the present invention, vacuum carburization is an element that is limited to use in the conventional carburization, although the carburization occurs in an anhydrous atmosphere of vacuum, although it is a beneficial element in the physical property improvement when designing alloy elements, the element is mainly chromium and Considering the fact that it is silicon, it is actively adopting the content of chromium and silicon, and combined with Ti and Nb to enable high-temperature carburizing at 1050 ℃, and is suitable for vacuum carburizing based on the constituent elements of existing Cr-Mo alloy steel. Designed to provide high strength chromium-molybdenum alloy steel for high temperature vacuum carburizing as an exclusive alloy steel suitable for the vacuum carburizing method, which is a vacuum carburizing material that can be carburized without grain coarsening even at a high temperature of 1050 ° C, and is suitable for the vacuum carburizing method that is expected. There is a purpose.

The present invention for achieving the above object is iron (Fe) as a main component, carbon (C) 0.22 to 0.24% by weight, silicon (Si) 0.70 to 0.90% by weight, manganese (Mn) 0.50 to 0.80% by weight , Phosphorus (P) 0.030 wt%, sulfur (S) 0.020 wt%, chromium (Cr) 1.90 to 2.20 wt%, molybdenum (Mo) 0.20 to 0.40 wt%, titanium (Ti) 0.008 to 0.012 wt%, niobium (Nb) ) 20 to 40 ppm, nitrogen (N ₂ ) 70 to 120ppm, alloy-based components to provide a high-strength chromium-molybdenum alloy steel for high temperature vacuum carburizing, characterized in that the dissolved oxygen content of 15ppm contained for the cleanliness of the alloy.

Hereinafter, the present invention will be described in more detail.

Alloy-based components of the chromium-molybdenum alloy steel according to the present invention are as described with Comparative Steel 1 and Comparative Steel 2 in Table 1 below.

As shown in Table 1, the chromium-molybdenum alloy steel of the present invention, unlike conventional chromium-molybdenum alloy steel (SCM920HVSi) and nickel-chromium-molybdenum alloy steel (SNCM518H), the iron (Fe) as a main component, 0.22 to 0.24 wt% of carbon (C), 0.70 to 0.90 wt% of silicon (Si), 0.50 to 0.80 wt% of manganese (Mn), 0.030 wt% of phosphorus (P), 0.020 wt% of sulfur (S), and chromium (Cr) 1.90 to 2.20 wt%, molybdenum (Mo) 0.20 to 0.40 wt%, titanium (Ti) 0.008 to 0.012 wt%, niobium (Nb) 20 to 40 ppm, nitrogen (N ₂ ) 70 to 120 ppm, alloy-based components of the alloy For cleanliness (to reduce the impurity content) is characterized in that the dissolved oxygen content of 15ppm.

In order to improve the workability, the chromium-molybdenum alloy steel of the present invention was not added with nickel, and the chemical composition was greatly controlled so that fatigue strength, torsional fatigue strength, contact fatigue properties (fitting resistance), and the like were significantly improved compared to conventional alloy steels. It was.

In order to stabilize the high-temperature carburization, a fine precipitate element was added in combination, and Mo content, which is an expensive element, was slightly reduced in terms of quenchability control and cost reduction.

Since chromium and silicon added to the chromium-molybdenum alloy steel of the present invention greatly improve the softening resistance, it minimizes the occurrence of tooth surface fitting, which is a form of damage to the gear surface that occurs after a long time of use, thereby providing a great advantage to increase the vehicle warranty life. Despite the fact that chromium and silicon are very effective elements for improving the properties of automobile gears, their use was limited because they were very good at making intergranular oxide layers during carburization.

However, since the chromium-molybdenum alloy steel of the present invention is applied to the vacuum carburizing method, which is an oxidation-free atmosphere carburizing process, it is possible to input chromium and silicon in a content suitable for fatigue strength and softening resistance without concern for grain boundary oxide layer.

Referring to the main members of the present invention and the reason for limitation of the content in more detail as follows.

1) Carbon (C): 0.22 to 0.24 wt%

At least 0.22% by weight is required in order to obtain the desired core hardness from about 440 to 495, and when it exceeds 0.24% by weight, the core hardness is increased so that the compressive residual stress on the surface cannot be sufficiently introduced after quenching. The content is limited to 0.22 to 0.24 wt%.

2) Silicon (Si): 0.70 to 0.90 wt%

Silicon is used in the base to help improve the fatigue strength and greatly improve the softening resistance during tempering, thereby significantly increasing the contact fatigue property (fitting resistance). If the content is less than 0.70% by weight, softening resistance If it does not become large, on the other hand, if it exceeds 0.90% by weight, the solid solution strengthening effect of the base is so large that the moldability is extremely reduced, so that forging and processing are difficult, it is preferable to limit it to 0.70 to 0.90% by weight.

3) Manganese (Mn): 0.50 to 0.80 wt%

An amount of at least 0.50% by weight should be added to ensure hardenability of the steel. However, Mn is susceptible to particle oxidation, and should not exceed 0.80% by weight in order to reduce it.

4) Phosphorus (P): 0.030 wt%

In general, it is preferable to control the content to 0.03% by weight within a non-hazardous range such as chromium-molybdenum alloy steel.

5) Sulfur (S): 0.020 wt%

In general, it is preferable to control the amount by 0.02% by weight within a range that is not more harmful than chromium-molybdenum alloy steel.

6) Chromium (Cr): 1.90 to 2.20 wt%

As a component for alloy design for the purpose of vacuum carburizing and improving fatigue strength, chromium is a very effective element for improving the hardenability of steel. It is very useful for improving the bending fatigue strength by increasing the core hardness. It promotes and makes carburizing suitable, and also greatly improves softening resistance during tempering, and significantly improves contact fatigue characteristics (fitting resistance). When the content is less than 1.90% by weight, it is usually small in high strength chromium-molybdenum alloy steel. No significant difference compared to the grain size, the fatigue strength property improvement is not large, and the softening resistance is not significantly increased during tempering, so that the contact fatigue property is insufficient. On the other hand, when it exceeds 2.20% by weight, it is processed and forged by a large amount of fine carbide precipitation. Since the problem of rapid dropping of the formation occurs, it is limited to 1.90 to 2.20% by weight.

7) Molybdenum (Mo): 0.20 to 0.40 wt%

The purpose of adding Molybdenum (Mo) is to prevent formation of a large amount of harmful carbides due to the increase in chromium content, to increase toughness (to reduce embrittlement), and to obtain a desired microstructure during gas cooling after vacuum carburizing by properly adjusting the hardenability. In general, the gas cooling method after vacuum carburizing has a problem that the hardening ability of the material is low due to low cooling rate compared to conventional oil quenching or salt bath quenching, and thus has a low core hardness.

Thus, molybdenum (Mo) is usually contained 0.20 ~ 0.40% by weight less than the high-strength chromium-molybdenum alloy steel it is possible to reduce the cost.

8) Titanium (Ti), Niobium (Nb), Nitrogen

In the chromium-molybdenum alloy steel of the present invention, in order to prevent grain coarsening during high temperature carburization (to increase GCT), Ti and Nb, which are elements capable of promoting grain refinement, are added.

In order to prevent grain growth at high temperature, a lot of fine precipitates that are stable at high temperature should be distributed evenly. Titanium (Ti) is an element that makes TiN precipitates and effectively makes grain refinement, and the effect is very excellent, and in case of niobium (Nb) NbN or a large amount of Nb (CN) is an element that effectively makes grain refinement, and if the titanium (Ti) content is properly managed, other niobium (Nb) and other elements need not be added, but in the present invention niobium (Nb) The reason for the complex addition is that it is very difficult to find a suitable titanium (Ti) content, so as to obtain a stable high temperature stable fine precipitate.

Therefore, the chromium-molybdenum alloy steel of the present invention was added in an amount of 0.008 to 0.012% by weight of titanium (Ti), 20 to 40 ppm of niobium (Nb), and 70 to 120 ppm of nitrogen for an appropriate amount of TiN precipitation.

At this time, titanium (Ti) is an element that makes fine carbide (mainly TiN) in the alloy steel to refine the crystal grains, the effect is not great when the content is less than 0.008% by weight, the carbide is determined when it exceeds 0.012% by weight Excessive precipitation or large precipitation at grain boundaries not only increases the brittleness but also lowers the grain refining effect, so it is limited to 0.008 to 0.012 wt%.

Niobium (Nb), like titanium (Ti), is an element that prevents grain coarsening by making fine niobium carbon and nitride in alloy steel. If the content is less than 20ppm, the effect is not significant. The problem of excessive precipitation at this grain boundary resulting in increased brittleness is limited to 20 to 40 ppm.

Hereinafter, the embodiment of the present invention will be described in more detail with a comparative example, but the present invention is not limited by the following examples.

Example

Iron (Fe) as the main component, carbon (C) 0.22 to 0.24% by weight, silicon (Si) 0.70 to 0.90% by weight, manganese (Mn) 0.50 to 0.80% by weight, phosphorus (P) 0.030% by weight, sulfur (S) 0.020 wt%, Chromium (Cr) 1.90-2.20 wt%, Molybdenum (Mo) 0.20-0.40 wt%, Titanium (Ti) 0.008-0.012 wt%, Niobium (Nb) 20-40 ppm, Nitrogen (N ₂ ) 70-120ppm, chromium-molybdenum alloy steel material containing 15ppm of dissolved oxygen in the alloy components for carburizing, carburizing at 1050 ℃ for 40 minutes, gas quenching for 20 minutes, and then at 170 ℃ for 2 hours Tempering was performed to prepare a specimen.

Comparative Example 1

Chromium-molybdenum alloy steel (SCM920HVSi) according to Comparative Example 1 is the representative high strength transmission gear steel of the applicant of the present application, by weight% of carbon (C) 0.17 to 0.21, silicon (Si) 0.15 or less, manganese (Mn) 0.60 to 0.85, phosphorus (P) 0.020 or less sulfur (S) 0.030 or less, chromium (Cr) 1.25 to 1.45, molybdenum (Mo) 0.55 to 0.65, niobium (Nb) 15 to 35 ppm, dissolved oxygen of 15 ppm or less added chromium-molybdenum alloy steel material Carburized at 900 ° C for 2 hours, salt quenched at 220 ° C in a salt bath, and then tempered at 170 ° C for 2 hours to prepare a specimen.

Comparative Example 2

Nickel-chromium-molybdenum alloy steel (SNCM518H) according to Comparative Example 2 is a representative high-strength transmission gear steel made by Mitsubishi Steel, Japan, in terms of weight% of carbon (C) of 0.15 to 0.18, silicon (Si) of 0.15 or less, and manganese (Mn) of 0.60 to 0.85, phosphorus (P) 0.025 or less, sulfur (S) 0.01 to 0.02, nickel (Ni) 1.55 to 1.65, chromium (Cr) 0.50 to 0.65, molybdenum (Mo) 0.55 to 0.65, niobium (Nb) 15 to 35 ppm, dissolved Nickel-chromium-molybdenum alloy steel material to which oxygen content of 15 ppm was added was carburized at 900 ° C. for 2 hours, salt quenched at 220 ° C. in a salt bath, and then tempered at 170 ° C. for 2 hours to prepare a specimen.

Test Example 1

The hardness measurement and the fatigue test, the impact test, the contact fatigue test and the torsion fatigue test for Examples and Comparative Examples 1-2 were carried out as described in the footnotes described in Table 2 below. As described.

As shown in Table 2 above, the chromium-molybdenum alloy steel according to the present invention was found to have a significant improvement in physical properties compared to the existing high strength Cr-Mo alloy steel, despite the fine carburizing heat treatment at 1050 ℃ high temperature, The grain distribution was shown, and the fatigue characteristics also showed a significant increase.

In particular, it was found that the contact fatigue and the torsion fatigue showed a significant increase in response to the alloy design.

Test Example 2

For the alloy steels according to Examples and Comparative Examples 1 and 2, a comparative evaluation of the temperature (GCT) at which grain coarsening takes place was carried out to reconfirm the possibility of high temperature carburizing at 1050 ° C., and the results are shown in Table 3 below. same.

As shown in Table 3 above, the alloy steel of the present invention was confirmed that the high temperature grain coarsening temperature is significantly higher than the existing high strength Cr-Mo alloy steel.

That is, the existing high-strength Cr-Mo alloy steels can not be carburized at 1050 ℃ with a GCT of 1000 ℃ or less, the alloy steel of the present invention was found to be carburized more than 1050 ℃ with a GCT of 1100 ℃ or more.

As seen above, according to the high-strength chromium-molybdenum alloy steel for high-temperature vacuum carburization according to the present invention, it exhibits very excellent fatigue properties (durability) compared to the existing high-strength nickel-chromium-molybdenum alloy steel and chromium-molybdenum steel during vacuum carburization. High temperature carburizing of 1050 ℃ or higher is also possible, and thus, it is possible to obtain profitability improvement by shortening the heat treatment time.

Claims (1)

Iron (Fe) as the main component, carbon (C) 0.22 to 0.24% by weight, silicon (Si) 0.70 to 0.90% by weight, manganese (Mn) 0.50 to 0.80% by weight, phosphorus (P) 0.030% by weight, sulfur (S) 0.020 wt%, Chromium (Cr) 1.90-2.20 wt%, Molybdenum (Mo) 0.20-0.40 wt%, Titanium (Ti) 0.008-0.012 wt%, Niobium (Nb) 20-40 ppm, Nitrogen (N ₂ ) High strength chromium-molybdenum alloy steel for high-temperature vacuum carburizing, comprising 70 to 120 ppm of dissolved oxygen in the alloy component.

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