JPH03199340A - High frequency hardened parts - Google Patents

Abstract

PURPOSE:To obtain long-life high frequency hardened parts by forming a product by the use of a steel stock having a composition which contains respectively prescribed amounts of C, Si, and Mn and in which oxygen content is limited to a specific value or below, subjecting the above product to high frequency hardening and to tempering, and regulating the amount of retained austenite and hardness, respectively. CONSTITUTION:A stock which has a composition which consists of, by weight ratio, 0.5-0.7% C, 0.5-1.2% Si, 0.5-1.5% Mn, and the balance iron and in which O content is regulated to <=13ppm is prepared. Subsequently, a product formed of the above stock is subjected to high frequency hardening and to tempering, by which the desired high frequency hardened parts in which the amount of retained austenite and hardness are regulated to <=15% and >=HRC58, respectively, is obtained, that is, the above method can contribute toward hardenability, taking the improvement of the service life of the high frequency hardened material into account, and further, by utilizing the elements reduced in affinity for carbon, the long-life high frequency hardened parts can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) This invention relates to components such as rolling bearings and ball screws that are induction hardened and used.

(Prior Art) Recently, induction hardened bearing parts have been increasingly used. After heat treatment, the materials used as bearing parts are
It is essential to have a high surface hardness of HRC58-64, and to obtain a hardness of HRC58 or higher by induction hardening,
A carbon content of 0.4% or more is required. However, the rolling life of bearing parts made of conventional carbon steel for mechanical structures and induction hardened is not as long as that of bearing parts made of conventional bearing steel such as 5UJ2 or carburized steel. This is the current situation. In the heat treatment process of steel, the more alloying elements there are, the better the hardenability and the harder the hardening depth can be obtained, but it takes time for the carbon to become a solid solution, and short-term treatments such as induction hardening are difficult. This is because it is difficult to obtain high hardness by heating for a long time. Therefore, in alloy steels such as SCM445, heat treatment,
Pre-treatment such as grilling is required. There is also a method of heating to a higher temperature, but in that case the structure becomes coarser. Therefore, increasing the amount of alloying elements in induction hardened materials has a negative effect on obtaining high hardness. In addition to high hardness, it has recently been revealed that the cooling rate during quenching is an important factor in the rolling life of bearing parts. That is, by reducing the cooling rate,
The crack sensitivity of martensite is reduced and the rolling life is improved. In other words, hardening steel with good hardenability over time will lead to a longer life.

Considering the above points, conventional induction-hardened bearing parts cannot achieve high hardness due to short-time heating called high-frequency heating, which does not allow sufficient diffusion of alloying elements, and the high cooling rate. It is assumed that this is the cause.

(Problem to be solved by the invention) This invention takes into account the above-mentioned improvement in the lifespan of the induction hardened material, contributes to hardenability, and has an affinity for carbon (bonds with carbon to form carbides). The purpose is to provide long-life induction hardened parts by utilizing elements that are easy to form and have a small force.

(Means for Solving the Problems) This invention provides a method for solving the problems in which C0.5 to 0.7% by weight and Si
It is made of a material containing 0.5-1.2% Mn, 0.5-1.5% Mn, and the balance iron with an oxygen content of 13 ppm or less, and the product formed from this material is induction hardened and then tempered. Retained austenite 15% or less, hardness 1 (
It relates to induction hardened parts of RC58 or higher.

(Examples and Effects) The reason why the composition range of the induction hardened parts such as rolling bearings and ball screws of the present invention is limited to the above range is as follows.

Since induction hardening requires heating for a short time, the alloying elements present as carbides must quickly separate from the carbon by heating and diffuse into the matrix (austenite phase) to contribute to hardenability.

Therefore, if we compare the same content, among the in-shell alloying elements such as Mn, Mo, Cr, Si, and Ni, Mn has the highest contribution rate to hardenability, followed by Mo, Cr, and Si. ,
It decreases in the order of Ni. In addition, the affinity with carbon is Mo,
The order is Cr, Mn, Ni, and Si.

In the earlier application, JP-A-59-129752, a long-life induction hardened material was invented by focusing on the Mn content, but as a result of intensive research on the Si content, it was discovered that there is an appropriate amount of Si for the rolling fatigue life. , which led to the present invention. S
The i amount is 1.50 to 2 for spring steel such as 5UP6.5UP7.
．． Although it contains about 20% of steel, its transfer fatigue life was not sufficient compared to bearing steel and carburized steel. In the course of the present invention, a transfer fatigue life test was conducted with the amount of Si varied over a wide range, and it was found that the life peaked at <0.5 to 1.2% Si, as shown in FIG. The life in this case is equivalent to that of through-hardened bearing steel. As mentioned above, the reason for the long life is that Si has a low affinity with carbon, which facilitates the incorporation of carbon into austenite in a short period of time through high-frequency induction, as well as the resistance to tempering caused by the addition of Si. One example is improved sexual performance. Improved tempering resistance makes it possible to perform sufficient tempering without reducing hardness, and can be expected to improve lifespan by improving crack sensitivity.

The reason why the life decreases when the amount of Si exceeds 1.2% is that when the amount of Si is too large, the amount of carbon and carbide formed increases, so the amount of carbon solid solution in austenite decreases for the same heating temperature. And it's probably not desirable. In this case, increasing the heating time will increase the amount of carbon in solid solution, but this will also undesirably coarsen the microstructure.

Mn is an element that is effective in increasing the hardenability of steel.
Since it has a small affinity with carbon, it is a preferable element as a material for induction hardening. It is necessary to add the necessary amount depending on the wall thickness of the product, and the practical range of the value is 0.5 to 1.
It is 5%.

Note that C'r is an element that tends to form carbides, so it is not preferable to contain a large amount, but if it is 0.5% or less, there is no actual harm.

Carbon is an essential component to increase the hardness, for example, the hardness of induction hardened parts (RC58).
To achieve this, 0.5% or more is required. On the other hand, C
If it exceeds 0.7%, short-term heating such as induction hardening,
When using the water cooling process, it is difficult to adjust the amount of carbon that dissolves into the austenite (the higher the heating temperature, the more carbon in the austenite), and if more than 0.7% of carbon is dissolved, , the risk of quench cracking increases.

Nowadays, as the cleanliness of steel has improved, the effect of retained austenite on rolling life has diminished, but compared to bearing steel, materials with lower carbon content such as induction hardened materials are the source of inclusions in steelmaking. It is difficult to reduce a certain oxygen content6, so the influence of retained austenite still remains.

As shown in parentheses in Fig. 1, the amount of retained austenite is 8% or more when 5iji is 0.5 to 1.2%, which is within the range of the present invention. It has become. However, if the amount of retained austenite exceeds 15%, it is estimated that the life will be reduced, so the upper limit is set to 15%.

Reducing the oxygen content is essential for reducing nonmetallic inclusions and improving rolling life and cracking fatigue strength.The effect is stable at 13 ppm or less, so the upper limit is set at 13 ppm.
Set as ppm.

(Effects of the Invention) As understood from the descriptive explanation, according to the present invention, during high-frequency heating, which is a short-time treatment, insufficient solid solution of carbon can be prevented by placing Mn and Si, which have low affinity for carbon, into the solid solution. By selecting and using the carbon within the range shown below, it is easy to inject carbon into austenite within a short time, and the hardness can be increased to HRC58.
In addition to maintaining a rolling life equivalent to that of deep-hardened bearing steel, by adjusting the amount of retained austenite and oxygen content within the specified range, a decrease in rolling life can be prevented, and at the same time cracking can occur. Effects such as helping to improve fatigue strength are obtained, and because of these effects, the parts of the present invention are suitable for use not only in rolling bearing raceways but also in ball screws and the like.

[Brief explanation of drawings]

Figure 1 shows the influence of Si content and retained austenite amount on 10% rolling life in the present invention at 0.6%G, 0.
．． It is a characteristic graph showing the results obtained for 8% Mn steel. one or more one

Claims (1)

[Claims] 1. C0.5-0.7%, Si0.5-1 in weight ratio
．． 2% Mn, 0.5 to 1.5% Mn, and the balance iron, and the oxygen content is 13 ppm or less, and the product formed from this material is induction hardened and tempered to have a residual austenite content of 15% or less, and induction hardened parts with hardness of HRC58 or higher.