JP2007077411A - Machine structural component having excellent fatigue strength and wear property, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a machine structural component made steel to form into a product by being subjected to surface treatment composed of nitriding treatment and the subsequent induction hardening, which has excellent bending fatigue strength and wear properties, particularly, has excellent pitting fatigue resistance and facial pressure strength. <P>SOLUTION: A steel having an alloy composition comprising, by weight, 0.1 to 0.6% C, 0.03 to 1.5% Si, 0.01 to 2.0% Mn, 0.005 to 0.025% S and 0.1 to 3.0% Cr, and the balance Fe is formed, and is subjected to nitriding treatment at ≥600°C, so as to form a nitrided layer in which the thickness of the produced compound layer is ≤5 μm, depth is ≥0.2 mm, and the concentration of nitrogen in the position of a 0.05 mm depth from the surface is 0.3 to 2.5 wt.%. Then, induction hardening is performed under the conditions where the part of the nitrided layer is austenitized. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a mechanical structural component having excellent fatigue strength and wear characteristics, and a method for manufacturing a mechanical structural component involving nitriding and induction hardening.

Of the mechanical properties of mechanical structural parts, the surface of parts is often hardened for the purpose of improving properties related to surface properties such as surface pressure strength, bending fatigue strength, and torsional fatigue strength. . For example, by carburizing or carbonitriding a case-hardened steel whose C content is suppressed to a low value of 0.13 to 0.23%, it is possible to obtain a part with improved wear resistance and fatigue strength. In addition, nitriding treatment (including methods such as tuftride, gas soft nitriding, and ion nitriding) and induction hardening are also employed.

Each of these surface treatment methods has merits and demerits, and should be selected according to the purpose. However, the desired characteristics may not be improved by one treatment. For example, in the conventional nitriding treatment, the bending fatigue strength is not sufficiently high because the hardened layer depth is shallow. On the other hand, only by induction hardening, the surface hardness is low and the temper softening resistance is also low, so that the rolling fatigue strength is insufficient. Although carburizing treatment gives high surface hardness by induction hardening, it is the same as induction hardening in that rolling fatigue strength is limited because of low resistance to temper softening.

Therefore, an attempt has been made to combine two or more surface treatments. For example, the proposal is that induction hardening is performed after nitriding treatment (Patent Document 1), and the hardness and depth of the hardened layer that cannot be achieved by either of them are realized, and excellent mechanical properties are obtained. However, even in this combination, there is a problem that the compound layer after nitridation generates a large amount of retained austenite on the surface layer by induction hardening and cannot exhibit sufficient mechanical properties.
Japanese Patent Laid-Open No. 7-90364 (Patent 3405468)

The object of the present invention is to eliminate the disadvantages of known techniques in the surface treatment of mechanical structural parts made of steel consisting of induction hardening following nitriding, and to solve bending fatigue strength and wear characteristics, in particular pitting fatigue resistance. The object is to provide a machine structural component having characteristics or surface pressure strength superior to those of the prior art, and to provide a method for producing such a machine structural component.

The mechanical structural parts having excellent fatigue strength and wear characteristics according to the present invention are C: 0.1-0.6%, Si: 0.03-1.5%, Mn: 0.01-2. Steel having an alloy composition containing 0%, S: 0.005 to 0.025% and Cr: 0.1 to 3.0% with the balance being Fe and inevitable impurities is formed into the shape of a mechanical structural component. It is obtained by molding, nitriding and induction hardening, and the compound layer thickness is 2 μm or less, the curing depth is 0.5 mm or more, and the hardness at the position of 0.05 mm from the surface is 800 HV. The above is characterized in that the retained austenite of the layer reaching a depth of 0.2 mm from the surface is 20% by volume or less.

In the mechanical structural component of the present invention, the thickness of the compound layer generated by nitriding is set to 5 μm or less, so that the amount of N existing in the surface layer is limited. The amount of retained austenite in the layer reaching the depth becomes a small amount of 20% by volume or less, and the hardness at a depth of 0.05 mm from the surface is kept high at 800 HV or more. Since the hardening depth is as deep as 0.5 mm or more and a high N concentration is secured up to a deep position, the bending fatigue strength is high and the surface pressure strength is also high. Combined with these actions, the mechanical structural component of the present invention exhibits excellent fatigue strength and wear characteristics.

In addition to the above-mentioned components, this mechanical structural part is made of, in weight percent, Al: 0.02-1.0%, V: 0.05-1.0%, Mo: 0.05-1 Manufactured using steel having an alloy composition containing one or more of 0.0%, Ti: 0.005 to 1.0%, Nb: 0.1% or less, and Zr: 0.1% or less can do.

The method of manufacturing a machine structural part having excellent surface pressure strength and bending fatigue strength according to the present invention is a steel having the above basic alloy composition, that is, by weight%, C: 0.1 to 0.6%, Si : 0.03-1.5%, Mn: 0.01-2.0%, S: 0.005-0.025% and Cr: 0.1-3.0%, with the balance being Fe and Even if the steel having an alloy composition composed of inevitable impurities is used as the material, the steel having a modified alloy composition, that is, in addition to the above-mentioned components, in addition, by weight%, Al: 0.02 1.0%, V: 0.05 to 1.0%, Mo: 0.05 to 1.0%, Ti: 0.005 to 1.0%, Nb: 0.1% or less, and Zr: 0.00. Even if the material is steel having an alloy composition containing 1% or less of 1% or less, nitriding treatment is performed at a temperature of 600 ° C or less. The thickness of the compound layer formed by nitriding is 5 μm or less, the depth of the nitrided layer is 0.2 mm or more, and the nitrogen concentration at a position 0.05 mm deep from the surface is 0.3 to 2.5 wt% A nitride layer is formed, and then induction hardening is performed under the condition that the nitride layer portion becomes austenite.

In the present invention, the reason why the alloy composition of the steel used as the material of the machine structural component is selected as described above is as follows.
C: 0.1 to 0.6%
C is a component that gives an internal hardness to ensure the strength required for the part, and for this purpose, it is necessary to be present at 0.1% or more. On the other hand, a large amount of C excessively increases the hardness after forging or rolling-solution treatment and decreases workability, so the upper limit is 0.6%.

Si: 0.03-1.5%
Si acts as a deoxidizing and desulfurizing agent during the melting of steel, and has the effect of increasing the temper softening resistance and improving the rolling life. In order to obtain this effect, it is necessary to add 0.03% or more of Si. If the amount is too large, the workability and toughness are reduced, so the addition is limited to 1.5% or less.

Mn: 0.01 to 2.0%
Mn also acts as a deoxidizing and desulfurizing agent during the melting of steel and helps to improve hardenability. This function can be obtained by adding 0.01% or more of Mn. A large amount of addition results in a decrease in toughness, so the upper limit is 2.0%.

S: 0.005-0.025%
S is a free-cutting element and is a useful component in terms of enhancing machinability during machining. This effect is obtained by addition of 0.005% or more, but a large amount of S lowers the hot workability and fatigue limit of steel, so an addition amount within 0.025% is selected.

Cr: 0.1-3.0%
Cr not only increases the depth of the nitrided layer and increases its nitrogen content, but also helps improve hardenability. In order to ensure this effect, 0.1% or more is added. However, if the Cr content is increased, the nitriding properties are impaired, so the limit is 3.0%.

In the present invention, the meaning of the alloy component to be arbitrarily added and the reason for limiting the composition range are as follows.
Al: 0.02-1.0%, V: 0.05-1.0%, Mo: 0.05-1.0%, Ti: 0.005-1.0%, Nb: 0.1% 1 or 2 or more of Zr: 0.1% or less These components have a strong influence on the nitriding properties of steel. Therefore, one or more of them are added depending on the properties required after induction hardening. Good. Among the above, Al and Ti are effective in increasing the nitrogen concentration on the surface of the component, and also suppress the growth of crystal grains by the alloy nitride during high-frequency quenching and make the structure finer. Contributes to improvement. In order to obtain this benefit, 0.02% of Al and 0.005% or more of Ti must be added. V not only increases the nitrogen concentration on the surface, but also helps increase the nitrogen concentration in the depth direction. This effect is obtained by adding 0.05% or more of V. Mo and Zr increase the depth of the nitride layer and also help improve the hardenability. Zr is effective in a small amount, but Mo is effective when added in an amount of 0.05% or more. Nb increases the nitrogen concentration on the surface and contributes to the refinement of crystal grains during induction hardening, similar to Al and Ti. The effect of each of the above elements is saturated when the addition amount is increased. Therefore, before saturation, that is, Al, Ti and V are 1.0% or less, Mo is 2.0% or less, Nb and Zr are Select an addition amount of 0.1% or less.

The feature of the method for manufacturing a mechanical structural component of the present invention is that the thickness of the compound layer generated by nitriding is regulated. Since the compound layer contains a large amount of nitrogen, it decomposes during induction quenching to produce an austenite (hereinafter referred to as “γ”) phase, and this γ phase contains a large amount of nitrogen. Since the baking does not occur and the γ phase exists stably up to room temperature, desired surface characteristics cannot be obtained. The thicker the compound layer, the greater the amount of remaining γ phase. Since the hardness of the γ phase generated and remaining on this surface is lower than the hardness of the hardened layer obtained by induction hardening, the mechanical properties cannot be enhanced.

For this reason, the thickness of the compound layer produced by the nitriding treatment must be regulated below a certain limit, which is 5 μm as described above. If it is 5 μm or less, it has been found that the amount of γ phase generated during induction hardening is very small and does not substantially affect the mechanical properties of the product. In order to regulate the thickness of the compound layer after the nitriding treatment, for example, in the case of ion nitriding, the mixing ratio of N ₂ gas may be controlled, and even if the compound layer becomes thick due to the nitriding atmosphere, The compound layer can be thinned by stopping, and a compound layer having a thickness of 5 μm or less can be formed by performing nitriding according to a technique known to those skilled in the art.

The reason why the nitriding treatment is performed at 600 ° C. or more is to make the nitrogen concentration at a position of 0.05 mm depth from the surface 0.2 to 2.5 wt%. In this type of alloy steel, the eutectoid temperature is higher than 600 ° C., so it is advantageous to employ a nitriding temperature of 600 ° C. or higher in order to dissolve a large amount of N into a solid solution. When the nitriding is performed at a high temperature, the generated nitride becomes coarse. Therefore, in combination with the increase in the amount of dissolved N, the condition that the N concentration is 0.3 to 2.5% by weight at a depth of 0.05 mm from the surface. Is easy to achieve. On the other hand, when nitriding is performed at a high temperature of 600 ° C. or higher, the N concentration of the compound layer is lower than in the case of nitriding at a temperature of about 550 ° C., which is usually performed. It is possible to reduce the amount of residual γ phase generated, and to prevent the surface hardness from being lowered. However, nitriding at an excessively high temperature is not appropriate because N is diffused to the core more than necessary. Usually, a processing temperature up to 680 ° C. is selected.

The condition that the depth of the nitrided layer is 0.2 mm or more and the N concentration at the position of 0.05 mm from the surface is 0.3 to 2.5% by weight is that the hardness of the induction-hardened portion is sufficiently high. It must be fulfilled. As a result, the mechanical component of the present invention has excellent bending fatigue strength and high surface pressure strength.

Steels having the alloy compositions shown in Table 1 were melted using a high-frequency induction furnace having a capacity of 50 kg. The ingot was forged and forged into two types of steel bars having a diameter of 30 mm and 22 mm, and normalized.

Table 1% by weight, balance Fe

The normalized steel was machined to produce roller pitting test pieces and rotating bending fatigue test pieces, which were subjected to gas soft nitriding followed by induction hardening. The hardness at a depth of 0.05 mm was measured, and the amount of residual γ phase was examined. A roller pitting test and an Ono-type rotary bending fatigue test were performed using each test piece. The conditions of gas soft nitriding and induction hardening are shown in Table 2. The thickness of compound layer and depth of nitride after soft nitriding, hardness after induction hardening, amount of residual γ phase, roller pitting test Table 3 shows the results of the rotating bending fatigue test.

Table 2

Table 3

Comparative Example No. 1 is data in the case where only a known soft nitriding steel is subjected to induction hardening. Therefore, the characteristics of this steel were used as standards, and the characteristics of the examples and other comparative examples were shown as relative values. Example No. 5 of the present invention In each of Nos. 1 to 16, the thickness of the compound layer is 5 μm or less, and it is clear that the results of the roller pitting test and the rotating bending fatigue test are superior to the standard. The steel used in the comparative example is within the scope of the invention as an alloy composition, but is out of the scope of the invention in the subsequent processing. Comparative Examples Nos. 2 to 4 are the same steels with different thicknesses of the compound layers produced by nitriding, and Comparative Examples Nos. 5 to 7 are different steels with different compound layer thicknesses. The thickness is approximately 10 μm. These comparative examples show equivalent or better results compared to the standard comparative example No. 1, but the examples outperform all of the comparative examples. Comparative Example No. 8 indicates that the nitriding temperature is less than 600 ° C., and No. 9 indicates that the nitriding is performed at an excessively high temperature, and the desired characteristics cannot be obtained.

Claims (3)

By weight, C: 0.1-0.6%, Si: 0.03-1.5%, Mn: 0.01-2.0%, S: 0.005-0.025% and Cr: A compound layer obtained by forming steel having an alloy composition containing 0.1 to 3.0% and the balance of Fe and inevitable impurities into the shape of a mechanical structural part, and performing nitriding treatment and induction hardening Has a thickness of 2 μm or less, a curing depth of 0.5 mm or more, a hardness at a position of 0.05 mm depth from the surface of 800 HV or more, and a residual austenite of a layer reaching a depth of 0.2 mm from the surface is 20 Mechanical structural parts with excellent fatigue strength and wear characteristics that are less than volume%. In addition to the components defined in claim 1, the steel is in weight percent, Al: 0.02-1.0%, V: 0.05-1.0%, Mo: 0.05-1.0. %, Ti: 0.005 to 1.0%, Nb: 0.1% or less, and Zr: 0.1% or less, having an alloy composition containing one or more kinds. Structural parts. It is a method of manufacturing the machine structural component of Claim 1 or 2, Comprising: By weight%, C: 0.1-0.6%, Si: 0.03-1.5%, Mn: 0.01 Steel having an alloy composition containing ~ 2.0%, S: 0.005 to 0.025% and Cr: 0.1 to 3.0%, the balance being Fe and inevitable impurities, or Furthermore, by weight, Al: 0.02-1.0%, V: 0.05-1.0%, Mo: 0.05-1.0%, Ti: 0.005-1.0%, A steel having an alloy composition containing one or more of Nb: 0.1% or less and Zr: 0.1% or less is formed into the shape of a machine structural part and subjected to nitriding at a temperature of 600 ° C. or less. The thickness of the compound layer formed by nitriding is 5 μm or less, the depth of the nitride layer is 0.2 mm or more, and the depth from the surface is 0.05 mm. Nitrogen concentration to form a nitrided layer of 0.3 to 2.5 wt%, then manufacturing method characterized by performing induction hardening under conditions nitride layer portion is austenitized.