JP5723233B2 - Steel material for spheroidized heat-treated bearings with excellent rolling fatigue life - Google Patents

Description

  The present invention relates to a steel material for a spheroidized heat-treated bearing that exhibits an excellent rolling fatigue life when used as a bearing component.

  Conventionally, high carbon chromium bearing steel such as SUJ2 defined in JIS G 4805 (1999) has been used as a bearing material used in various fields such as automobiles and various industrial machines. . However, since bearings are used in harsh environments such as inner and outer rings and rolling elements such as ball bearings and roller bearings with extremely high contact surface pressure, fatigue failure is likely to occur due to very fine defects (inclusions, etc.). There is a problem. In order to solve this problem, attempts have been made to improve the steel for bearings in order to increase the rolling fatigue life itself and reduce the number of maintenance operations.

For example, in Patent Document 1, the life of the bearing material is increased by strictly defining the number of oxide-based nonmetallic inclusions that become defects. On the other hand, in Patent Document 2, by observing a test area of 30000 mm ² that is much larger than the evaluation area of Patent Document 1, in particular, the maximum size of the sulfide affects the rolling fatigue life. Is indicated.

  However, there are many bearing steel inclusions that are currently used industrially that are very strictly controlled, and it is difficult to further improve the rolling fatigue life by controlling these inclusions alone. It has become.

  Therefore, as shown in Patent Documents 3 and 4, techniques for improving the rolling fatigue life by reducing stripe segregation have been proposed. Among these, in Patent Document 3, the rolling temperature is lowered and the forging pressure ratio is increased (60 or more) to reduce the hardness variation due to the striped segregation. Further, in Patent Document 4, the rolling temperature is relatively high, and the forging speed is slowed to improve the stripe segregation, thereby reducing the variation in the area ratio of carbides after the subsequent spheroidizing heat treatment, thereby rolling fatigue. It will improve the service life.

  In the techniques of Patent Documents 3 and 4, it can be said that the effect of improving the rolling fatigue life is exhibited. However, there are restrictions on the rolling method and the rolling size, and it cannot be said that the methods are industrially highly flexible. Further, the effect of improving the rolling fatigue life cannot always be said to sufficiently meet the increasing demand for longer life.

Japanese Patent No. 3889931 JP 2006-63402 A JP 2009-84647 A JP 2010-47832 A

  The present invention has been made in view of such circumstances, and an object thereof is to provide a steel material for a spheroidized heat-treated bearing for obtaining a bearing having a further improved rolling fatigue life.

The steel material for spheroidizing heat-treated bearings with excellent rolling fatigue life according to the present invention is C: 0.95 to 1.10% (meaning of mass%, the same applies hereinafter), Si: 0.15 to 0.90% , Mn: 1.2% or less (excluding 0%), Cr: 0.90 to 1.60%, P: 0.025% or less (excluding 0%), S: 0.025% or less ( When the EPMA (Electron Probe Micro Analyzer) line analysis in the direction perpendicular to the rolling direction is performed on the plane parallel to the rolling direction of the steel material, the balance is made of iron and inevitable impurities. The standard deviation σ ₁ of the X-ray intensity value and the average value a ₁ satisfy the relationship of the following equation (1), and the standard deviation σ ₂ of the spheroidized cementite particle size and the average value a ₂ are expressed by the following equation (2): The point is that the relationship is satisfied.
(Standard deviation σ _{1 of} Cr X-ray intensity value / average value a _{1 of} Cr X-ray intensity value)
≦ 0.25 (1)
(Standard deviation σ _{2 of} spheroidized cementite particle size / average value a ₂ of spheroidized cementite particle size)
≦ 0.15 (2)

  In the steel material for bearings of the present invention, (1) the total content of P and S is suppressed to 0.020% or less (not including 0%), and (2) as another element, Ni: 0 1 selected from the group consisting of less than .25% (not including 0%), Cu: less than 0.25% (not including 0%), and Mo: less than 0.08% (not including 0%) It is also useful to contain more than seeds, and the properties of the steel material are further improved depending on the components to be suppressed or contained.

  According to the present invention, since the steel material for a spheroidized heat-treated bearing having a further improved rolling fatigue life can be realized, excellent rolling fatigue even when used in a harsh environment when the bearing steel material is applied to a bearing. Life can be demonstrated, and unnecessary maintenance (replacement, inspection, etc.) can be reduced.

Is a graph showing the relationship between (the average value of the X-ray intensity of the standard deviation sigma ₁ / Cr of X-ray intensity value a ₁ of Cr) and the L ₁₀ life. Is a graph showing the relationship between the L ₁₀ life (the average value a ₂ of the standard deviation sigma ₂ / spheroidal cementite grain size of spheroidized cementite grain size).

The present inventors examined the influence of striped segregation in more detail as a viewpoint different from inclusion control with the aim of improving the rolling fatigue life. As a result, when EPMA line analysis is performed in a direction perpendicular to the rolling direction on a plane parallel to the rolling direction of the steel material, the standard deviation and average value of the X-ray intensity value of Cr satisfy the relationship of the following formula (1). In addition, when the standard deviation σ ₂ of the spheroidized cementite particle diameter and the average value a ₂ satisfy the relationship of the following formula (2), it has been found that the rolling fatigue life can be remarkably improved. completed.
(Standard deviation σ _{1 of} Cr X-ray intensity value / average value a _{1 of} Cr X-ray intensity value)
≦ 0.25 (1)
(Standard deviation σ _{2 of} spheroidized cementite particle size / average value a ₂ of spheroidized cementite particle size)
≦ 0.15 (2)

  Segregation of Cr leads to non-uniform carbide size and area ratio when spheroidizing heat treatment (spheroidizing annealing process), resulting in a decrease in rolling fatigue life when used as a bearing. When the relationship of the above expression (1) is satisfied, Cr segregation is remarkably reduced, and the rolling fatigue life is extremely excellent.

  The steel for bearings of the present invention assumes a steel material after spheroidizing heat treatment and before quenching and tempering, but the relationship of the above formula (1) is substantially the same as the steel material before spheroidizing heat treatment and the quenching and tempering material. The provisions of the present invention can be applied to any of them. The value on the right side of the formula (1) is preferably 0.23 or less, and more preferably 0.20 or less.

  Moreover, although it is known that the ratio of the area ratio of the spheroidized carbides in the Cr segregation part and the non-segregation part affects the rolling fatigue life (for example, Patent Document 4), the present inventors examined. However, it has been found that the variation in the spheroidized cementite particle size has a great influence on the rolling fatigue life. That is, when the relationship of the above formula (2) is satisfied, the variation in the spheroidized cementite particle size is remarkably reduced, and the rolling fatigue life is extremely excellent.

  Note that the relationship of the above formula (2) does not change greatly even after quenching and tempering after spheroidizing heat treatment, and therefore the provisions of the present invention can be applied to both spheroidizing heat treatment materials and quenching and tempering materials. . The value on the right side of the formula (2) is preferably 0.13 or less, and more preferably 0.10 or less.

  The steel material of the present invention is based on the chemical composition of SUJ2-4 specified in JIS G4805 (1999), C: 0.95 to 1.10%, Si: 0.15 to 0.90% , Mn: 1.2% or less (excluding 0%), Cr: 0.90 to 1.60% is satisfied. The reasons for limiting the ranges of these elements are as follows.

[C: 0.95 to 1.10%]
C is an essential element for increasing the quenching hardness and maintaining the strength at room temperature and high temperature to impart wear resistance. Therefore, it must be contained in an amount of 0.95% or more, and preferably 0.98% or more of C is contained. However, if the C content is excessively large, giant carbides are likely to be generated, which adversely affects the rolling fatigue characteristics. Therefore, the C content is 1.10% or less, preferably 1.05% or less. Should be suppressed.

[Si: 0.15-0.90%]
Si is a solid solution strengthening element, and when it is finally used as a component such as a bearing, it exhibits the effect of suppressing temper softening and also the effect of making carbide finer in the quenching / tempering step. If the Si content is less than 0.15%, these effects are not exhibited, and if it exceeds 0.90%, adverse effects such as deterioration of cold forgeability and hot workability occur, so 0.90% It is necessary to do the following. The preferable lower limit of the Si content is 0.20% or more (more preferably 0.25% or more), and the preferable upper limit is 0.8% or less (more preferably 0.7% or less).

[Mn: 1.2% or less (excluding 0%)]
Mn is an effective element for improving the hardenability of bearing steel, but if its content is excessive, the hardness after hot working becomes too high, which hinders industrial production, In order to reduce the fatigue life by generating a large amount of retained austenite phase, it is necessary to be 1.2% or less. A preferred lower limit of the Mn content is 0.3% or more (more preferably 0.35% or more), and a preferred upper limit is 1.0% or less (more preferably 0.8% or less).

[Cr: 0.90 to 1.60%]
Cr is an element that combines with C to form fine carbides, imparts wear resistance, and contributes to improving hardenability. In order to exert such an effect, the Cr content is set to 0.90% or more. Preferably it is 1.0% or more. However, if Cr is present excessively, coarse carbides are generated, and the rolling fatigue life is decreased. Therefore, the Cr content is 1.60% or less. Preferably it is 1.5% or less.

  As for P and S, as indicated in the chemical composition of SUJ2-4 specified in JIS G 4805 (1999), P: 0.025% or less (excluding 0%), S: 0.025% or less However, these elements tend to promote stripe segregation, and as a result, also tend to promote Cr segregation. For these reasons, the total content (P + S) of P and S is preferably 0.020% or less (not including 0%). The total content of P and S is more preferably 0.015% or less, and still more preferably 0.010% or less. However, from the viewpoint of manufacturing in a mass production process, the lower limit is about 0.006%. Become.

  The contained elements defined in the present invention are as described above, and the balance is iron and inevitable impurities, and elements that are brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. (for example, Al, O, N) Etc.) can be allowed to be mixed. In order to increase the rolling fatigue life, the following elements can be positively contained within a specified range.

[From the group consisting of Ni: less than 0.25% (not including 0%), Cu: less than 0.25% (not including 0%), and Mo: less than 0.08% (not including 0%) One or more selected]
Ni, Cu, and Mo are all elements that act as a hardenability improving element of the parent phase and contribute to improving the rolling fatigue characteristics by increasing the hardness. These effects are preferably exhibited by containing 0.03% or more of Ni, 0.03% or more of Cu, and 0.01% or more of Mo. However, when the Ni content is 0.25% or more, or the Mo content is 0.08% or more, workability deteriorates, and when the Cu content is 0.25% or more, cracking during hot rolling is promoted. Therefore, each should preferably be within the above range.

  The bearing steel material of the present invention is obtained by heating a steel material satisfying the above-mentioned chemical composition in, for example, a soaking furnace, and then hot rolling and spheroidizing heat treatment, but satisfies the relationship of the above formula (1). Thus, in order to reduce the segregation of Cr, it is preferable to control the manufacturing conditions as closely as possible.

In order to obtain the spheroidized heat treated bearing steel of the present invention, it is necessary to heat a steel material satisfying the above chemical composition at a certain stage from casting to a bearing part under appropriate conditions. The conditions greatly depend on the processing history after the steel material is cast. That is, the problem of the present invention is that the microsegregation inevitably generated during casting is a striped segregation that is striped in subsequent rolling, but the primary forging ratio in rolling or forging causes the striped segregation. The width changes. Therefore, in order to improve striped segregation by heat treatment, it is necessary to set heating conditions according to the width of the striped segregation.

  In the past, in order to reduce segregation, the cast slab was heated to diffuse Cr and C, but the segregation element was not sufficiently homogenized at a practical heat treatment temperature and time. Met. Therefore, it was necessary to reduce the segregation in the subsequent rolling process, but the heating temperature was too low in the rolling process, and the segregation could not always be reduced.

  Therefore, in the present invention, not only as a slab, but after rolling or forging to some extent, by performing a heat treatment to reduce segregation, the segregation is greatly improved, and the subsequent rolling conditions are reduced, Regardless of the processing conditions including general processing conditions, the rolling fatigue life when the bearing parts were made through the subsequent spheroidizing heat treatment, quenching and tempering could be improved.

  As specific conditions, after the casting, the primary forging pressure ratio ("cross-sectional area perpendicular to the casting direction of the slab / cross-sectional area perpendicular to the processing direction of the rolled material or forged material") is rolled to 3 or more. Alternatively, at the stage of forging, heat treatment (diffusion heat treatment) is performed at 1200 to 1350 ° C. to improve microsegregation, and then rolling and forging are performed under arbitrary conditions, so that the steel material for the bearing has a size suitable for each part. Can be realized. The heating time at that time is based on satisfying the relationship of the later-described formula (3) based on the diffusion of Cr, and an efficient production condition can be provided industrially without waste.

The point of the above idea is to set the heating temperature and time at which Cr is sufficiently diffused according to the width of the stripe segregation that varies depending on the primary forging pressure ratio. When the primary forging pressure ratio is Rf, the heating temperature is T (K), and the heating time is t (hours), the following relationship is established.
Stripe segregation width ∝ √ (1 / Rf)
Diffusion distance ∝ √ (Diffusion coefficient xt)
Diffusion coefficient ∝ exp (-Q / RT)
However, Q: Activation energy of diffusion R: Gas constant Necessary heating time t ∝ (1 / Rf) × (exp (−Q / RT))

Based on the above concept, the inventors of the present invention have confirmed the diffusion heat treatment conditions through experiments and obtained recommended heat treatment conditions in order to reduce Cr microsegregation. That is, the heating time (diffusion heat treatment time) t is set so as to satisfy the following formula (3), and even if it is longer than that time by about 20% or more, the improvement range is saturated, so the upper limit is from an industrial viewpoint. Were determined. In addition, [Right side of equation (3)] × 1.2 is recommended.
t> 8 × 10 ⁻⁹ × (1 / Rf) × exp [69.7 / (0.00198242 × T)]
... (3)

The diffusion heat treatment temperature is set to 1200 ° C. or higher, which is a lower limit set for finishing the treatment within an industrially reasonable time, and 1350 ° C. or lower is heated above this temperature. This is because the heating equipment cost increases industrially, and a thick oxide film is generated on the surface of the steel material, thereby increasing the oxide film removal process cost for the next rolling process. Also, the larger the primary forging pressure ratio, the shorter the diffusion heat treatment time, but it is better to industrially heat treat in a continuous furnace, but when performing heat treatment in a batch furnace, it matches the size of the furnace. Therefore, productivity is significantly reduced. Accordingly, the primary forging pressure ratio is preferably about 3 or more (more preferably 5 or more) and 10 or less (more preferably 8 or less).

  Moreover, in order to reduce the variation of the cementite particle size after the spheroidizing heat treatment and satisfy the relationship of the above formula (2), the following method may be followed. First, as a material before spheroidizing heat treatment, a steel material that satisfies the relationship (1) above by the above-described manufacturing method is used, and further processing conditions (heating temperature of secondary rolling or secondary forging) and spheroidizing heat treatment conditions are appropriate. It is necessary to.

  At this time, the processing conditions (secondary processing conditions) are preferably lower than a general temperature (1000 ° C. or lower, preferably 980 ° C. or lower) (preferably lower limit is about 800 ° C.). In addition, the spheroidizing heat treatment is performed at a low temperature (for example, 730 to 755 ° C. × 5 to 10 hours) as much as the segregation of Cr is suppressed, and the cooling rate is set faster than usual in the subsequent cooling (for example, 10 ˜50 ° C./hour), it is possible to obtain a spheroidized cementite having a relatively fine particle size. When the cooling rate becomes too fast, softening becomes insufficient, and cold forgeability deteriorates. The cooling rate at this time is preferably 20 ° C./hour or more and 40 ° C./hour or less.

  The steel material for bearings of the present invention is formed into a predetermined part shape and then subjected to spheroidizing heat treatment, and further quenched and tempered to produce a bearing part. The shape of the steel material stage can be applied to such production. Both linear and rod-like shapes are included, and the size can be appropriately determined according to the final product.

  Hereinafter, the present invention will be described in more detail by way of examples.However, the present invention is not limited by the following examples as a matter of course, and may be implemented with modifications within a range that can meet the gist of the preceding and following descriptions. Of course, they are all possible and are included in the technical scope of the present invention.

  Steels having various chemical composition shown in Table 1 below were melted by 150 kg vacuum melting to produce ingots having a diameter of 230 mm (round bar).

Using the ingot obtained above, after hot forging to a predetermined primary forging ratio once under the conditions shown in Table 2 below (forging temperature, primary forging pressure ratio), heat treatment is performed under various conditions [diffusion heat treatment temperature, Diffusion heat treatment time and the value on the right side of equation (3) (8 × 10 ⁻⁹ × (1 / Rf) × exp [69.7 / (0.00186242 × T)]), and further hot forging (secondary forging Temperature, total forging pressure ratio) to obtain a round bar having a diameter of 65 mm.

The obtained round bar was subjected to spheroidizing heat treatment, further quenched and tempered, and a thrust rolling fatigue test was performed. The conditions for the spheroidizing heat treatment at this time were heating at 750 ° C. for 6 hours and gradually cooling to 680 ° C. over 5 hours (average cooling rate: 14 ° C./hour). The conditions for quenching and tempering were heating at 840 ° C. × 30 minutes followed by oil quenching and tempering at 160 ° C. × 120 minutes. At this time, as a comparative example, a sample prepared by heating at 760 ° C. for 6 hours under general conditions and gradually cooling to 680 ° C. over 8 hours (average cooling rate 10 ° C./hour) was prepared (Test No. 10). ).

A thrust test piece is prepared from each of the obtained steel materials (quenched / tempered material), and a thrust rolling fatigue test is performed 10 times each at a surface pressure of 5.3 GPa to obtain a fatigue life L ₁₀ (cumulative failure probability 10%). The number of stress repetitions until fatigue failure was evaluated, and the fatigue life L ₁₀ was 9 × 10 ⁶ times or more as an acceptance criterion.

For each steel material, EPMA line analysis (acceleration voltage: 15 kV) in the direction perpendicular to the rolling direction in a state before spheroidizing annealing on a plane parallel to the rolling direction (longitudinal section of the steel material) was 1 mm long (2 μm). The average value and standard deviation are calculated using the X-ray intensity of Cr at each data point, and the standard deviation σ ₁ / Cr X-ray intensity of Cr is calculated. The average value a ₁ ) was evaluated.

  In addition, the spheroidized heat-treated material was quenched and tempered, and was subjected to Picral corrosion, and with a scanning electron microscope (SEM), photographs of arbitrary locations at 2000 magnifications were taken for each of five fields of view, and the spheroidized cementite particle size (circle) Equivalent diameter: Diameter when converted to a circle of the same area) Image analysis, obtain standard deviation of spheroidized cementite particle size, average value of spheroidized cementite particle size, (standard deviation of spheroidized cementite particle size / spherical The average value of the cementite particle size was evaluated.

These results [(standard deviation σ _{1 of} Cr X-ray intensity value / average value a ₁ of X-ray intensity value of Cr), (standard deviation σ _{2 of} spheroidized cementite particle diameter / average value of spheroidized cementite particle diameter Table 2 below shows a ₂ ) and fatigue life L ₁₀ ].

  From these results, it can be considered as follows. That is, test no. 1 to 9 satisfy the requirements defined in the present invention, and all of them have excellent rolling fatigue life. In particular, one using a steel material (steel type F) having a total content of P and S (indicated by “P + S”) of 0.020% or less (test No. 6) or one containing a predetermined amount of Mo (test) In Nos. 8 and 9), it can be seen that a more excellent rolling fatigue life is exhibited.

In contrast, test no. Those of 10 to 14, since the outside the requirements defined in the present invention, both fatigue life L ₁₀ is low. That is, test no. In No. 10, the diffusion heat treatment conditions are appropriate, but since the spheroidization heat treatment conditions are normal conditions (temperature is high), the variation in the spheroidized cementite particle size deviates from the requirement defined in the present invention, and the fatigue life L ₁₀ is low.

Test No. 11 and 12, the diffusion heat treatment conditions are not appropriate, and the secondary forging temperature is high. As a result, the variation in the spheroidized cementite particle size is large (segregation is not improved), and fatigue is caused. the life L ₁₀ is low. Test No. In No. 13, since the primary forging was not performed, the diffusion heat treatment conditions were inadequate. 11 and 12 are obtained. Test No. 14 intended, for not performing diffusion heat treatment, the segregation is not improved, also large variations in spheroidal cementite grain size, the fatigue life L ₁₀ is low.

Based on these data, in Figure 1 the relationship between the L ₁₀ life (the average value a ₁ of the X-ray intensity of the standard deviation sigma ₁ / Cr of X-ray intensity values of Cr), (spheroidal cementite grain size The standard deviation σ ₂ / average value of spheroidized cementite particle size a ₂ ) and the L ₁₀ lifetime are shown in FIG. 2, respectively. (Standard deviation σ _{1 of} Cr X-ray intensity value / X-ray intensity of Cr By controlling the average value a ₁ ) and (standard deviation σ _{2 of} spheroidized cementite particle size / average value a ₂ of spheroidized cementite particle size) within an appropriate range, excellent fatigue life (rolling fatigue life) ) Is achieved.

Claims (3)

C: 0.95 to 1.10% (meaning mass%, the same shall apply hereinafter)
Si: 0.15-0.90%,
Mn: 1.2% or less (excluding 0%),
Cr: 0.90 to 1.60%,
P: 0.025% or less (excluding 0%),
S: each including 0.025% or less (excluding 0%),
The balance consists of iron and inevitable impurities,
When EPMA line analysis is performed in a direction perpendicular to the rolling direction on a plane parallel to the rolling direction of the steel material, the standard deviation σ ₁ and the average value a ₁ of the X-ray intensity value of Cr satisfy the relationship of the following formula (1): In addition, a steel material for a spheroidized heat-treated bearing having an excellent rolling fatigue life, wherein the standard deviation σ ₂ of the spheroidized cementite particle size and the average value a ₂ satisfy the relationship of the following formula (2).
(Standard deviation σ _{1 of} Cr X-ray intensity value / average value a _{1 of} Cr X-ray intensity value)
≦ 0.25 (1)
(Standard deviation σ _{2 of} spheroidized cementite particle size / average value a ₂ of spheroidized cementite particle size)
≦ 0.15 (2)   The steel material for a spheroidized heat-treated bearing according to claim 1, wherein the total content of P and S is suppressed to 0.020% or less (not including 0%). Further, as other elements, Ni: less than 0.25% (not including 0%), Cu: less than 0.25% (not including 0%), and Mo: less than 0.08% (not including 0%) The steel material for spheroidizing heat-treated bearings according to claim 1 or 2, comprising at least one selected from the group consisting of:

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