JPH0625823A - Parts made of carburized steel excellent in pitting resistance - Google Patents

Abstract

PURPOSE:To provide parts made of carburized steel excellent in pitting resistance and most suitable for parts for machine structural use. CONSTITUTION:A steel having a composition which consists of, by weight, 0.05-0.3% C, 0.05-2% Si, 0.3-2% Mn, 2-8% Cr, <=0.03% S, 0.015-0.06% Al, 0.005-0.02% N, and the balance Fe with inevitable impurities and where the contents of P and O among the inevitable impurities are controlled to <=0.02% and <=0.002%, respectively, is used as a stock. The parts made of this stock are subjected to carburizing or carbonitriding treatment and then to quench-and- temper treatment, by which carbides or carbonitrides of <=5mum average grain size are precipitated in the surface layer part. Further, the area ratio of these carbides or carbonitrides is regulated to 2-60%, and further, those having a composition of M7C3 comprise >=30% of these carbides and carbonitrides.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carburized steel part having excellent pitting resistance. More specifically, it is subjected to carburizing or carbonitriding treatment and quenching / tempering treatment. Precipitating granular carbides with M ₇ C ₃ carbonitrides of 30% or more, especially with a surface hardness of HV80.
The present invention relates to a carburized steel component having a tempering softening resistance of 0 or higher.

[0002]

2. Description of Related Art High fatigue resistance and wear resistance are required for mechanical structural parts such as gears, shafts and bearings in automobiles, construction machines and industrial machines, which are subjected to high repetitive stress and surface pressure. It It is said that application of a surface strengthening method such as carburizing or carbonitriding is effective for extending the life of such a machine structural part, and is widely applied. In addition, as a material for machine structural parts, JIS G4104 is used.
Carburizing steels such as chrome steel and chrome molybdenum steel specified in G4105 and G4105 are used.

However, in recent years, mechanical structure parts have been required to have higher strength in response to reduction in size and weight of parts and higher output of engines. Under these circumstances, it is necessary to further improve the bending fatigue strength and the pitting resistance of the gears, shafts, etc., which are mechanical structural parts.

By the way, in order to improve the bending fatigue strength of carburized steel parts, it is effective to increase the compressive residual stress in the surface layer, and a technique such as that disclosed in JP-A-1-306521 has been proposed. This technology adjusts the chemical composition of carburized steel parts to suppress the occurrence of incompletely hardened layers in the surface layer after carburizing, and then strongly improves bending fatigue strength by applying strong shot peening. .

On the other hand, high carbon carburization is known as a means for improving the pitting resistance. High carbon carburizing has a carbon content in the surface layer of the carburized material higher than the conventional 0.8 to 1% by weight.
It is a surface hardening treatment method in which carbides are generated and dispersed in the martensite in the surface layer portion of the carburized material at about 3% by weight. However, when high-carbon carburizing is performed using conventional chrome steel or chrome molybdenum steel, carbide is not spheroidized in the carburized layer and precipitates in a mesh shape, which easily causes quenching cracks and rather reduces pitting resistance. There is a drawback that.

[0006]

The present invention has been made under these circumstances, and an object thereof is to provide a carburized steel part which is excellent in pitting resistance and which is most suitable as a part for machine structural use. is there.

[0007]

The present invention capable of achieving the above object is that C: 0.05 to 0.3% by weight, Si: 0.0
5 to 2% by weight, Mn: 0.3 to 2% by weight, Cr: 2 to 8
% By weight, S: 0.03% by weight or less, Al: 0.015-
0.06% by weight, N: 0.005 to 0.02% by weight, respectively, and the balance Fe and unavoidable impurities, wherein P is 0.02% by weight or less and O is 0.002.
A steel made by controlling the content of each to be less than or equal to wt% is used as a material, and parts made of the material are subjected to carburizing or carbonitriding treatment and quenching / tempering treatment, and the surface layer portion has an average grain size of 5 μm.
m or less carbides or carbonitrides are deposited, and the area ratio of these carbides or carbonitrides is 2 to 60%.
And 30% or more of the carbide or carbonitride is M ₇
The gist is that it has a composition of C ₃ (where M is a metal element contained). Further, the carburized steel component of the present invention has the above-mentioned elements as basic components, but may contain Nb, V, Ni, Mo, W, etc., if necessary. Furthermore, as a means to further enhance the pitting resistance,
The parts may be carburized or carbonitrided, cooled to a point of Ar ₁ or lower, reheated in a carburizing or carbonitriding atmosphere at a temperature of Ar ₁ or higher, and then quenched and tempered.

[0008]

The present invention is constructed as described above, but in short,
In order to improve the surface hardness that affects pitting resistance, by adjusting the amount of various alloying elements and performing carburizing or carbonitriding treatment to deposit a fine predetermined amount of spherical carbide on the surface layer, the surface layer The hardness of the part is HV80
The present invention has been completed by finding that it can be achieved at 0 or more and extremely excellent pitting resistance can be obtained. First, the reasons for limiting the chemical composition in the carburized steel part of the present invention are as follows.

C: 0.05 to 0.3% by weight C must be contained in an amount of 0.05% by weight or more in order to secure the hardness of the core. However, when the content of C exceeds 0.3% by weight, the hardness becomes excessive, the toughness decreases, and the machinability also decreases.

Si: 0.05 to 2% by weight Si is an element effective in deoxidizing molten steel, and for this purpose, it should be 0.
It is necessary to contain at least 05% by weight. But 2% by weight
If it is contained in excess of 10%, the carburizing property is hindered, the surface carbon concentration is reduced, and the formation of carbides is hindered. In addition, the grain boundary oxide layer becomes deep and bending fatigue strength also decreases. Therefore, the Si content is set to 0.05 to 2% by weight.

Mn: 0.3-2% by weight Mn is a deoxidizing element of molten steel and is an element for improving hardenability. To obtain this effect, 0.3% by weight
It is necessary to add the above. However, when the content of Mn exceeds 2% by weight, the hardenability becomes excessive, the hardness of the core portion becomes too high, the toughness is lowered, and the machinability is lowered.
Therefore, the Mn content is set to 0.3 to 2% by weight.

Cr: 2 to 8 wt% Cr is an element effective for improving the hardness of the core and forming carbides when carburized with high carbon to secure high surface hardness. When the Cr content is 2% by weight or more, spheroidization of the carbide is promoted, but when it is less than 2% by weight, these effects are insufficient. On the other hand, the effect is saturated even if the content exceeds 8% by weight. Therefore, the Cr content is set to 2 to 8% by weight.

S: 0.03 wt% or less Most of S is contained as a sulfide-based inclusion in steel and is an element effective for improving machinability. However, if the S content exceeds 0.03% by weight, MnS serves as a starting point to cause pitting and deteriorate toughness. Therefore, the S content is 0.03% by weight or less.

Al: 0.015 to 0.06% by weight Al is an element effective for deoxidizing and refining the crystal grain size. A
If the 1 content is less than 0.015% by weight, such effects are small, and if it exceeds 0.06% by weight, the effect of refining the crystal grain size is saturated. Therefore, the Al content is 0.015 to 0.
It is set to 06% by weight.

N: 0.005 to 0.02% by weight N is an element effective in combining with Al, V, etc. to form a nitride and refining the grain size to improve the toughness of the steel. N
If the content is less than 0.005% by weight, such an effect is small, and if the content exceeds 0.02% by weight, the effect is saturated. Therefore, the N content is 0.005-0.02
Weight%

The carburized steel part of the present invention contains the above elements as basic components and the balance iron and unavoidable impurities. Among these unavoidable impurities, P and O should be kept within the following ranges.

P: 0.02% by weight or less P is an element that reduces toughness, and the content must be reduced as much as possible. Therefore, the P content is suppressed to 0.02% by weight or less.

O: 0.002% by weight or less O is combined with Al and Si to form oxide inclusions (Al ₂ O) in the steel.
₃ , a harmful element that produces SiO ₂ . When the O content is high, a large amount of the above hard oxide-based inclusions are generated, which becomes a starting point of fatigue fracture and adversely affects machinability. Therefore, the O content is 0.002% by weight or less.

The carburized steel parts of the present invention may contain N if necessary.
The elements such as b, V, Ni, Mo, and W may be contained as described above, but the amounts when these elements are contained are as follows.

Nb: 0.01 to 0.5% by weight, V: 0.
05 to 2 wt% Nb and V are both effective elements for forming carbonitrides by combining with C and N in steel to refine the crystal grains and increase the toughness. In addition, Nb and V form a composite carbide like Mo and increase the surface hardness. If the Nb content is less than 0.01% by weight and the V content is less than 0.05% by weight, such an effect is small, and if the Nb content exceeds 0.5% by weight and the N content exceeds 2% by weight, respectively. Even when added, these effects saturate. Therefore, the Nb content is 0.01 to 0.5% by weight, and the V content is 0.05 to 2% by weight.

Ni: 0.5-4 wt% Ni is an element effective for increasing the toughness of the carburized layer and ensuring the hardenability of the carburized layer and the core. If the Ni content is less than 0.5% by weight, these effects are small, and if added in excess of 4% by weight, the effects are saturated. Therefore, the Ni content is 0.5 to 4
Weight%

Mo: 0.05 to 1 wt% Mo is an element effective for significantly increasing the hardenability of the carburized layer and suppressing the formation of the incompletely hardened layer. It is also a carbide forming element and forms a compound carbide with Cr to increase the hardness of the carbide and further increase the surface hardness of the carburized material. For this purpose, Mo must be added in an amount of 0.05% by weight or more. However, even if added in excess of 1% by weight, these effects saturate. Therefore, the Mo content is 0.05 to 1% by weight.
To

W: 0.3 to 1% by weight W is an element effective in increasing the surface hardness by forming a hard compound of WC in the carburized layer, and if less than 0.3% by weight, such an effect is produced. However, the effect is saturated when it is added in an amount of more than 1% by weight. Therefore, the W content is set to 0.3 to 1% by weight.

Next, the reasons for limiting the structure of the carburized steel part of the present invention will be described. First, the average particle size of the carbide or carbonitride deposited on the surface layer of the carburized steel part of the present invention must be 5 μm or less. That is, when the average grain size of the carbide or carbonitride exceeds 5 μm, it becomes the starting point of bending fatigue and reduces the fatigue strength. When the area ratio of the carbide or carbonitride is less than 2%, high surface hardness cannot be obtained and the softening resistance is low, and when it exceeds 60%, quench cracking and spalling are likely to occur. Furthermore, the composition M in the carbide or carbonitride
_{If 7} C ₃ is less than 30%, the particles do not become granular and serve as a stress concentration source to lower the bending fatigue strength.

In the carburized steel part of the present invention, as a means for increasing the surface hardness and improving the pitting resistance, the part is carburized or carbonitrided and then cooled to Ar ₁ point or lower, and further Ar ₁ point or higher. It is effective to employ a configuration in which the material is reheated in a carburizing or carbonitriding atmosphere at the temperature of 1, and then quenched and tempered. Setting the amount of retained austenite (residual γ) in the surface layer portion to 30 area% or less is also effective from the viewpoint of increasing surface hardness and bending fatigue strength to improve pitting resistance. The atmosphere for carrying out the carburizing or carbonitriding treatment is not particularly limited, but the equilibrium carbon concentration is preferably 1% by weight or more in order to stably generate the carbide.

The present invention will be described in more detail with reference to the following examples, but the following examples are not intended to limit the present invention, and any modification of the design can be made in view of the gist of the preceding and the following. Are included in the technical scope of.

[0027]

【Example】

Example 1 Inventive Steel Nos. 1 to 14 and Comparative Steel No. 1 having the chemical compositions shown in Table 1. Melt 15 to 23 in a small furnace, hot forge and normalize, then from a steel bar with a diameter of 30 mm, a diameter of 25 mm and a length of 100
The test piece was machined to mm, tempered after high carbon carburizing and tempered. Regarding the test piece, the carbon concentration in the surface layer, the hardness, and the state of carbide formation of 0.05 mm from the surface were investigated.

In the pitching test, a cylindrical test piece (diameter 70 mm × length 24 mm) was machined from a steel bar having a diameter of 80 mm,
Carburizing treatment similar to the above was carried out, and evaluation was made by the median value of repetition until three pitching occurred in three test pieces under each condition under the test conditions of rotation speed of 1364 rpm, slip ratio of -40% and surface pressure of 4165 MPa. . These results are shown in Table 2.
Will be shown collectively.

The high carbon carburizing and quenching treatment is carried out by holding the machined test piece at 870 ° C. for 5 hours in a carburizing atmosphere having an equilibrium carbon concentration of 1% by weight or more and oil quenching the same, and then carrying out the same carburizing. Adjust the gas composition so that the atmosphere is 850
After oil quenching by holding at 1 ° C for 1 hour, tempering was performed by holding at 180 ° C for 2 hours.

[0030]

[Table 1]

[0031]

[Table 2]

From these results, the following can be considered.
No. 1 to 14 are examples of the present invention, all of which have a surface carbon concentration of 1% by weight or more, a surface hardness of HV 800 or more, and a carbide average particle size of 5 μm or less and a granular carbide area ratio of 8 to 39. a%, further M ₇ C ₃ type carbide is 3
By occupying 3% or more, the pitting resistance is remarkably excellent.

On the other hand, in No. Nos. 15 to 23 are comparative examples in the case where one of the requirements specified in the present invention is not satisfied. No. No. 15 has a high Mn content and a low Cr content. No. 16 is No. 16 when the Cr content is low. 1
No. 7 has a high C content and a low Cr content. 18
Are comparative examples when the Si content is high. In each case, the surface carbon concentration is low, the precipitation amount of carbide is small, and the pitting resistance is 1/2 or less of that of the steel of the present invention. No. 19 is N
i content, No. No. 20 is Mo content, No. No. 21 is the case where the W content is high, respectively, and the effect of improving the pitting resistance is steel No. 21 of the present invention. 6, No. 8, No. There is almost no difference compared with 14. In addition, No. 22 is the S content,
No. No. 23 is the case where the O content is high, and the pitting resistance is steel No. It is inferior to 1.

Comparative Example Steel No. 1 of the present invention shown in Table 1. 2, 8, 14 and comparative steel Nos. Using 5 steels of 16 and 20, in a carburizing atmosphere with an equilibrium carbon concentration of 0.8%, held at 925 ° C for 3 hours for oil quenching, and held at 180 ° C for 2 hours for tempering treatment. A test piece was obtained in the same manner as in Example 1. A pitching test was performed on the test piece by examining the carbon concentration in the surface layer, the hardness, the state of carbide formation of 0.05 mm from the surface, and the amount of residual γ. The results are shown in Table 3, which are the cases where the carburizing treatment is not carried out in a high carbon atmosphere having an equilibrium carbon concentration of 1% by weight or more. In both cases, the surface carbon concentration is less than 1% by weight and the surface hardness is It can be seen that the pitting resistance is inferior because it is low and the generation of granular carbides is small.

[0035]

[Table 3]

Example 2 The steel No. of the present invention shown in Table 1 was used. 2, 8, 14 and comparative steel Nos. The test piece is machined using five kinds of steels of 15 and 20,
In a carburizing atmosphere in which the equilibrium carbon concentration is 1% by weight or more, oil quenching is performed by holding at 925 ° C. for 5 hours, and further 850
After holding in an atmosphere of (RX gas + NH ₃ ) at 1 ° C. for 1 hour for oil quenching, it was held at 180 ° C. for 2 hours for tempering treatment. The test piece was processed in the same manner as in Example 1. For the test piece, the carbon concentration of the surface layer, the surface hardness, the carbonitride production state of 0.05 mm from the surface, and the amount of residual γ were examined, and a pitching test was performed. The results are shown in Table 4.
Shown in.

[0037]

[Table 4]

The above-mentioned steels are those obtained by carbonitriding,
Each of the steels of the present invention has a surface carbon concentration of 2% by weight or more,
The surface hardness is as high as HV 800 or more, the area ratio of granular carbonitrides having an average particle size of carbonitrides of 5 μm or less is 34 to 43%, and further, 30% or more of M ₇ C ₃ type carbides are occupied and remain. The amount of γ is 20 to 27%, and the pitting resistance is remarkably excellent. On the other hand, comparative steel No. 15 is the surface carbon concentration,
The surface hardness is low, the carbide is reticulated and the average particle size is large, the residual γ amount exceeds 60%, and the pitting resistance is poor. In addition, No. No. 20 shows no difference in pitting resistance from the steel of the present invention.

[0039]

The present invention is configured as described above,
By depositing fine carbides or carbonitrides on the surface layer by carburizing or carbonitriding treatment, carburized steel parts capable of significantly improving pitting resistance were obtained.

Claims (6)

[Claims] 1. C: 0.05 to 0.3% by weight, Si:
0.05-2% by weight, Mn: 0.3-2% by weight, Cr:
2 to 8% by weight, S: 0.03% by weight or less, Al: 0.0
15 to 0.06% by weight, N: 0.005 to 0.02% by weight, respectively, and the balance Fe and unavoidable impurities, wherein P is 0.02% by weight or less and O is 0.002. Each of steels controlled to be less than or equal to wt% is used as a material, and parts made of the material are subjected to carburizing or carbonitriding treatment and quenching / tempering treatment, and carbide or carbonitride having an average particle diameter of 5 μm or less in the surface layer portion. The area ratio of these carbides or carbonitrides is 2 to 60%, and the content of these carbides or carbonitrides is 30%.
% Carburized steel parts having excellent pitting resistance, characterized by having a composition of M ₇ C ₃ (where M is a metal element contained). 2. The carburized steel part according to claim 1, wherein
Further, Nb: 0.01 to 0.5% by weight and V: 0.05
Carburized steel parts made of steel containing at least one selected from 2 to 2% by weight. 3. A carburized steel part according to claim 1 or 2, which is made of steel containing Ni: 0.5 to 4% by weight. 4. A carburized steel part according to any one of claims 1 to 3, which is made of a steel further containing Mo: 0.05 to 1% by weight. 5. The carburized steel part according to claim 1, further comprising a steel containing W: 0.3 to 1% by weight. 6. The carburized steel part according to claim 1, wherein the part is carburized or carbonitrided, cooled to Ar ₁ point or lower, and further carburized at a temperature of Ar ₁ point or higher. Reheat in carbonitriding atmosphere, then quench,
Carburized steel parts that have been tempered.