JP2005154784A - Bearing steel excellent in corrosion resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide bearing steel which is excellent in corrosion resistance, and rolling fatigue life even when adapted for bearings etc., of rolling mills. <P>SOLUTION: The bearing steel has a composition containing, by weight %, 0.10 to 0.3 C, ≤0.5 Si, 0.2 to 1.5 of Mn, ≤0.03 P, ≤0.03 S, 1.0 to 3.5 Ni, 1.0 to 5.0 Cr, 0.03 to 2.5 Mo, 0.005 to 0.050 Al, ≤0.003 Ti, ≤0.0015 O, ≤0.025 N, and substantially the balance Fe, wherein after carburizing or carbonitriding treatment, a surface C concentration is ≥0.7%, the content of the carbide is ≤15% by area and the content of the rod-like carbide 10 having an aspect ratio expressed by the ratio of major diameter to minor diameter of ≥3 and having a minor diameter of ≥2 μm is ≤0.1%. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a bearing steel, and more particularly to a bearing steel excellent in corrosion resistance, surface fatigue strength, and rolling fatigue life suitable for use in large-sized bearing parts such as for rolling mills, thermal power and hydroelectric generators.

Until now, carburized steel of case hardening steel represented by JIS SCr, JIS SCM and JIS SNCM has been used as bearing steel for large bearing parts.
However, in recent years, there has been an increasing demand for longer bearing life. Under such circumstances, rolling fatigue strength has been improved by adding alloy elements such as Si, Ni, and Mo. It was.

  For example, Patent Document 1 below discloses a bearing steel for large-sized bearing parts in which Si and Ni are appropriately added based on JIS SNCM 815.

Incidentally, for example, in the case of bearing parts used in a rolling mill, depending on the rolling mill, there is a case where rusting due to intrusion of rolling water may occur, and there is a problem that the rolling fatigue life is reduced due to this rusting.
For this reason, development of bearing steel with higher corrosion resistance that can be applied to large-sized bearing parts is newly demanded.

  On the other hand, the carbonitriding treatment improves the heat resistance by nitriding compared to the normal carburizing treatment, and the foreign material rolling fatigue resistance (rolling fatigue strength in a dusty environment) is improved by the stability of retained austenite. Therefore, the effectiveness of the bearing steel has been clarified, and therefore, development of bearing steel excellent in carbonitriding is also required.

Japanese Patent Publication No. 2-14416

  The present invention has been made to solve such problems, and has excellent corrosion resistance, excellent surface fatigue strength and rolling fatigue life, and excellent carbonitriding properties, and is particularly suitable as a large bearing component. The purpose is to provide steel.

  Thus, the content of claim 1 is, by weight, C: 0.10 to 0.35%, Si: less than 0.5%, Mn: 0.2 to 1.5%, P: ≦ 0.03%, S: ≦ 0.03%, Ni: 1.0 to 3.5%, Cr: 1.0 to 5.0%, Mo: 0.03 to 2.5%, Al: 0.005 to 0.050%, Ti: ≤0.003%, O: ≤0.0015%, N: ≤0.025%, the balance being substantially composed of Fe After the carburizing or carbonitriding treatment, the surface C concentration is 0.7% or more, the carbide ratio is 15% or less, the aspect ratio expressed by the ratio of the major axis to the minor axis is 3 or more, and the minor axis is 2 μm or more. Carbide content is 0.1% or less.

  According to a second aspect of the present invention, in the first aspect, the alloy component further includes one or two of V and Nb in a range of V: 0.05 to 1.0% and Nb: ≦ 0.1%. To do.

Effects and effects of the invention

  As a result of examining various alloy elements, the present inventors have found that reducing the amount of Si added and improving the appropriate addition of Ni and Cr are effective for improving the corrosion resistance.

It was also found that the effects of surface C concentration, carbide area ratio, and rod-like carbide after carburizing or carbonitriding were significant on corrosion resistance and rolling fatigue life.
In other words, a surface C concentration of a certain level or more is required for improving the rolling fatigue life. On the other hand, when the C concentration is increased, not only the corrosion resistance is deteriorated due to an increase in the carbide area ratio, but also the rolling C It was found that the dynamic fatigue life and impact properties are significantly reduced.

Here, it is considered that the rolling fatigue life and impact characteristics are lowered due to the presence of the rod-shaped carbide because the crack progresses along the interface of the rod-shaped carbide which is an inclusion or the rod-shaped carbide tends to be the starting point of the crack. .
Furthermore, corrosion is likely to proceed along the interface of the rod-like carbide, and it is considered that the corrosion resistance is also lowered.
In the present invention, it has been found that the characteristics deteriorate particularly when a rod-like carbide having an aspect ratio of 3 or more and a minor axis of 2 μm or more is produced.

The present inventors have also found that carbonitriding improves both corrosion resistance and rolling fatigue life, and that appropriate addition of Ni and Mo is effective for improving the rolling fatigue life.
The present invention has been made based on the above findings.

In the present invention, it is desirable that the oxide inclusions in the steel have a maximum diameter of 50 μm or less before carburizing or carbonitriding. That is, the present inventors have found that the corrosion resistance is improved by reducing the size of oxide inclusions present in the steel to 50 μm or less.
It was also clarified that the evaluation by the pore electrical resistance method of the acid-dissolved extract was effective for guaranteeing the size of the oxide inclusions.

  It was also clarified that the corrosion resistance and rolling fatigue life can be improved by carrying out intermediate annealing after carburizing or carbonitriding, followed by secondary quenching and tempering.

  That is, in the present invention, after steel having a maximum diameter of oxide inclusions of 50 μm or less is subjected to carburizing or carbonitriding, intermediate annealing is performed, and then secondary quenching and tempering is performed. As described above, it is desirable that the carbide area ratio is 15% or less, the carbide having an aspect ratio of 3 or more and a minor axis of 2 μm or more is 0.1% or less.

In the present invention, V and Nb can be further contained as alloy components in the ranges of V: 0.05 to 1.0% and Nb: 0.1% or less, respectively.
By containing these components, the crystal grains can be refined and the characteristics as bearing steel can be enhanced.

Next, the reasons for limiting the chemical components and the like in the present invention will be described in detail below.
C: 0.10 to 0.35%
C needs to be 0.10% or more in order to obtain the strength necessary for a bearing and to secure sufficient surface hardness after carburizing or carbonitriding. However, if the content is more than 0.35%, the toughness and machinability are lowered, so the range is 0.10 to 0.35%.

Si: Less than 0.5%
Si is effective in improving the toughness and fatigue resistance of steel by increasing the quenching martensite structure. In this sense, Si is an important component in the present invention. However, when the added amount is 0.5% or more, the corrosion resistance of steel is remarkably deteriorated. Moreover, since toughness and workability deteriorate, the content is set to less than 0.5%.

Mn: 0.2-1.5%
Mn acts as a deoxidizing and desulfurizing element during melting of steel, and is an element effective for enhancing the hardenability of steel. For this reason, it is contained in an amount of 0.2% or more in the present invention. However, if the content is more than 1.5%, the workability and machinability deteriorate, so the upper limit is made 1.5%.

P: ≤0.03%
S: ≦ 0.03%
P and S cause bearing strength deterioration. Therefore, in the present invention, P and S are restricted to 0.03% or less.

Ni: 1.0-3.5%
Ni is an important component in the present invention and has a great effect of improving the corrosion resistance of steel. Ni is an element effective for improving the hardenability of steel and the toughness after quenching and tempering. For this reason, Ni is contained in an amount of 1.0% or more in the present invention. However, if the content is more than 3.5%, the toughness and workability of the steel are lowered, so the upper limit is made 3.5%.

Cr: 1.0-5.0%
Cr is also an important component in the present invention and has a great effect of improving the corrosion resistance of steel. Cr is an element effective for improving the hardenability of steel and the strength and toughness after quenching and tempering. For this reason, it is contained in an amount of 1.0% or more in the present invention. However, if the content exceeds 5.0%, the effect of improving corrosion resistance is saturated, but the hardenability and machinability are impaired, so the upper limit is made 5.0%.

Mo: 0.03-2.5%
Mo is an element useful for improving the strength of steel. Therefore, in the present invention, it is contained in an amount of 0.03% or more. However, if the content is more than 2.5%, the hardenability is lowered and the machinability is also deteriorated, so the upper limit is made 2.5%.

Al: 0.005 to 0.050%
Al has the effect of becoming AlN to refine the crystal grains. Therefore, in the present invention, Al is contained in an amount of 0.005% or more. However, if the content is more than 0.050%, the cleanliness of the steel is lowered, and the effect of preventing the coarsening of the crystal grains is decreased, so the upper limit is made 0.050%.

Ti: ≦ 0.003%
Ti forms hard precipitate TiN and becomes the fracture starting point of rolling fatigue failure, which causes a reduction in rolling fatigue life. Therefore, in the present invention, the Ti content is restricted to 0.003% or less.

O: ≦ 0.0015%
O decreases the cleanliness of the steel and causes the rolling fatigue life to deteriorate. Therefore, in the present invention, the O content is restricted to 0.0015% or less.

N: ≦ 0.025%
N combines with Al to produce AlN, and serves to refine crystal grains. However, if contained in a large amount, the strength of the steel is deteriorated. Therefore, in the present invention, the upper limit of the N content is 0.025%. A more desirable range is 0.01 to 0.02%.

V: 0.05-1.0%
Nb: ≤0.1%
V and Nb are elements that contribute to the refinement of crystal grains. However, if the content of V and Nb is too large, the effect of grain refinement is reduced. Therefore, when adding these as selective elements, V is 0.05 to 1.0%. , Nb is contained in the range of 0.1% or less.

Surface C concentration The surface C concentration after heat treatment is important for securing the strength of the steel, and in order to obtain a predetermined hardness and rolling fatigue life, a surface C concentration of 0.7% or more is required.
Here, the surface C concentration after the heat treatment is desirably 0.7 to less than 0.9% when the heat treatment is carbonitriding and 0.9 to 1.2% when the carburizing treatment is performed.
In particular, in the case of carbonitriding, as the surface C concentration increases, not only the corrosion resistance deteriorates due to an increase in the carbide area ratio, but also the rolling fatigue life and impact characteristics decrease due to the formation of rod-like carbides. It is desirable to make it less than 0.9%.

Carbide Fine carbide is necessary to ensure the rolling fatigue life, but if the carbide area ratio exceeds 15%, the strength of the steel is reduced.
In particular, if the alloy elements and the heat treatment conditions are inappropriate, a rod-like carbide 10 having a ratio of the major axis to the minor axis (aspect ratio) of 3 or more and a minor axis of 2 μm or more is generated as shown in FIG. However, when such a rod-like carbide 10 is produced exceeding 0.1%, both the rolling fatigue life and the impact characteristics are remarkably deteriorated.

Oxide inclusions Oxide inclusions serve as fracture starting points for rolling fatigue and lower the rolling fatigue life.In addition, in the corrosive environment, the interface with the matrix is preferentially corroded. Corrosion resistance also decreases due to the presence.
In order to obtain a bearing steel having excellent corrosion resistance and rolling fatigue life, it is desirable to control the maximum diameter of oxide inclusions to 50 μm or less.

Heat treatment If the amount of alloying elements added is large when carburized or carbonitrided, the martensitic transformation point (Ms point) of the steel will be lowered, a large amount of retained austenite may be generated, and the specified surface hardness may not be obtained It is desirable to perform secondary quenching and tempering.
At this time, it is preferable to perform intermediate annealing before the secondary quenching to optimize the carbide form and enhance the hardenability of the matrix. Nitrogen addition is effective in improving corrosion resistance.

  According to the present invention as described above, the addition of alloy components and the balance of each component, and by optimizing the surface C concentration, carbide area ratio and rod-like carbide after carburizing or carbonitriding, rolling mills and thermal power and A bearing steel that exhibits excellent corrosion resistance even when used in bearing parts for hydroelectric generators, etc., is excellent in surface fatigue strength and rolling fatigue life, and is excellent in carbonitriding ability.

Next, embodiments of the present invention will be described in detail below.
<Material>
Steel with the chemical composition shown in Table 1 is melted in a 150kg vacuum induction melting furnace, round bars with diameters of 32mm and 65mm are produced by hot forging at 1200 ° C, softened after normalizing at 900 ° C. Was subjected to spheroidizing annealing at 760 ° C. to obtain a test material.
As an evaluation of the cleanliness of the material, the particle size distribution of oxide inclusions was measured by acid dissolution extraction-pore electrical resistance method (a method of measuring particle volume by changing electrical resistance when particles pass through the pores).

A φ20 mm round bar was cut out from the R / 2 part of the material, and after quenching from 850 ° C., about 30 g of a 1 mm thick thin plate was cut out and subjected to acid dissolution.
Extraction of oxide inclusions by acid dissolution was performed with sulfuric acid and permanganic acid solution.
The extracted oxide inclusions were dispersed in a 200 cc electrolytic solution, and the particle size distribution in 500 μl of the dispersion solution was measured with an aperture diameter (pore diameter) of 100 μm using a Beckman Coulter Multisizer.

Table 1 shows the maximum diameter of the oxide inclusions obtained from this.
In any of the steels of the present invention, the maximum inclusion diameter is 50 μm or less in any steel.

<Corrosion test>
In order to evaluate the corrosion resistance, a corrosion test was conducted under wet conditions and crevice corrosion.
Specifically, after cutting out a 20 mm diameter and 36 mm long rough specimen from the above materials, carburizing conditions were carburized at 960 ° C for 22 hours in an atmosphere furnace with a carbon potential of 1.2%, and quenched from 860 ° C. Thereafter, intermediate annealing was performed at 660 ° C. for 4 hours, secondary quenching was performed at 790 ° C., and tempering was performed at 180 ° C., and the cylindrical surface was ground and subjected to a corrosion test.

In addition, carbonitriding was performed at 850 ° C. for 7 hours in an atmosphere furnace with 1.2% carbon potential and 5% ammonia after the above carburizing treatment using the same rough-processed test piece as the carbonitriding condition. After the secondary quenching, the same finishing was performed to obtain a test piece.
The corrosion test was carried out using a combined cycle tester, and the corrosion state was investigated after holding at a test temperature of 49 ° C. ± 1 ° C. and a relative humidity of 95% or more for 24 hours.
Moreover, about crevice corrosion, the test piece was left still on V block and the contact part of V block and a test piece was made into crevice corrosion conditions.

<C concentration measurement>
Moreover, the center part of the test piece was cut with a microcutter, and after polishing and finishing, the C concentration distribution from the surface layer was measured using EPMA to determine the surface C concentration.

<Carbide measurement>
For carbide measurement, cut the center of the specimen with a microcutter, and after polishing finish, corrode with picral to reveal carbide, observe 5 views at 5000 times using SEM, and analyze the carbide area by image analysis. The major axis and the minor axis shown in FIG. 1 were measured for the rate and total carbides.

<Rolling fatigue test>
A thrust type rolling fatigue test was conducted to investigate the rolling fatigue strength of bearing parts.
A ring-shaped test piece having an outer diameter of 63 mm, an inner diameter of 28.7 mm, and a thickness of 9 mm was cut out from the material to obtain a roughing test piece.

This test piece was subjected to carburizing, quenching and tempering treatment as a heat treatment.
The carburizing conditions are the same as the corrosion test pieces.
After the heat treatment, one side was 0.15 mm polished and the other test surface was lapped to obtain a test piece for a thrust type rolling fatigue test.

Further, the same roughened test piece was subjected to carbonitriding, quenching and tempering treatment.
The carbonitriding conditions are the same as the corrosion test pieces.
After the heat treatment, the above polishing finish was performed to obtain a test piece.

The test was conducted under the test conditions shown in Table 2 using a thrust type rolling fatigue tester.
A high-speed steel gas atomized powder with a hardness of 750 Hv classified to a particle size of 100 to 180 μm was used for the test in a foreign matter mixed environment.
The rolling fatigue life was evaluated by the number of repetitions (L ₁₀ ) at which the cumulative failure probability in the Weibull probability was 10% (L ₁₀ ) and the number of repetitions (L ₅₀ ) at 50%.

<Charpy impact test>
A Charpy impact test was conducted to investigate the toughness of bearing parts.
A roughing test piece having a width of 12 mm, a height of 14 mm, a length of 55 mm, a notch having a depth of 1.8 mm and a radius of curvature of 10 mm was produced from the above material.

This test piece was subjected to carburizing, quenching and tempering treatment as a heat treatment.
Carburizing conditions are as follows: Carburizing treatment at 930 ° C for 4 hours in an atmosphere furnace with a carbon potential of 1.2%, quenching from 850 ° C, followed by intermediate annealing at 660 ° C for 4 hours, secondary quenching from 790 ° C, 180 ° C Tempering was performed at ° C.
After the heat treatment, it was finished by grinding to a width of 10 mm, a height of 10 mm, a notch curvature radius of 10 mm, and a depth of 2 mm, and subjected to the Charpy test.

Further, the same roughened test piece was subjected to carbonitriding, quenching and tempering treatment.
Carbonitriding conditions are the same as above. After carburizing and nitriding for 5 hours at 850 ° C in an atmosphere furnace with 1.2% carbon potential and 5% ammonia added, intermediate annealing and secondary quenching / tempering are performed. went.
After the heat treatment, the same grinding finish as above was performed to obtain a test piece.
The test was performed using a Charpy tester to measure the energy absorbed when the test piece was broken at room temperature.

<Result>
Table 3 shows the test results of the carburized material.

  In the steel of the present invention subjected to carburizing treatment, the surface C concentration is 0.9% or more, the carbide area ratio is 15% or less, and the rod-like carbide is 0.1% or less. Here, the rod-like carbide is a carbide having a ratio of a major axis to a minor axis (aspect ratio) of 3 or more and a minor axis of 2 μm or more.

  As is clear from the test results, the corrosion resistance is superior to the comparative steel in which the steel of the present invention does not satisfy the component composition range in both wet and crevice corrosion.

  As a result of the rolling fatigue test, it was found that the steel of the present invention has a longer life than the comparative example steel with clean oil. In addition, the rolling fatigue life is reduced by one order or more compared with clean oil under the foreign matter mixing conditions, but the steel of the present invention has a longer life than the comparative example steel.

  It was found that the Charpy impact value of the steel of the present invention is equal to or higher than that of the comparative steel, and the crushing strength as a bearing part is excellent.

Next, Table 4 shows the test results of the carbonitrided material.

The invention steel subjected to carbonitriding treatment has a surface C concentration of 0.7% or more, a carbide area ratio of 15% or less, and a rod-like carbide of 0.1% or less. It was found that the corrosion resistance, rolling fatigue life, and impact value were all superior to that of the example steel.
Further, it was found that the carbonitrided material is superior in corrosion resistance compared to the carburized material, and the rolling fatigue life is improved under the contamination condition.

It is the figure which represented typically the rod-shaped carbide | carbonized_material which exists in steel.

Explanation of symbols

        10 Bar carbide

Claims (2)

% By weight
C: 0.10 to 0.35%
Si: Less than 0.5%
Mn: 0.2-1.5%
P: ≤0.03%
S: ≦ 0.03%
Ni: 1.0-3.5%
Cr: 1.0-5.0%
Mo: 0.03-2.5%
Al: 0.005 to 0.050%
Ti: ≦ 0.003%
O: ≦ 0.0015%
N: ≦ 0.025%
The balance is substantially Fe, and after carburizing or carbonitriding, the surface C concentration is 0.7% or more, the area ratio is 15% or less of carbide, and the aspect ratio expressed by the ratio of the major axis to the minor axis is 3. A bearing steel excellent in corrosion resistance, characterized in that the carbide having a minor axis of 2 μm or more is 0.1% or less. In claim 1, one or two of V and Nb are further added as an alloy component.
V: 0.05-1.0%
Nb: ≤0.1%
Bearing steel excellent in corrosion resistance, characterized by containing in the range of

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