WO2012160677A1

WO2012160677A1 - Bearing steel with excellent cold workability

Info

Publication number: WO2012160677A1
Application number: PCT/JP2011/062002
Authority: WO
Inventors: 正樹貝塚; 亮廣松ヶ迫; 智一増田
Original assignee: 株式会社神戸製鋼所
Priority date: 2011-05-25
Filing date: 2011-05-25
Publication date: 2012-11-29

Abstract

This bearing steel, which can exhibit good cold workability in cold working carried out after spheroidizing annealing and also can ensure good rolling fatigue life, contains C:0.8-1.3%, Si:0.05-0.8%, Mn:0.1-2%, P:0.05% or less, S:0.05% or less, Cr: 1-2%, Al:0.01-0.1%, N:0.015% or less, Ti:0.015% or less, and O:0.0025% or less, the remainder being iron and unavoidable impurities, wherein the amount of a solid solution N is 0.003% or less (including 0%), a pro-eutectoid cementite has an aspect ratio of 10 or less and an average long diameter of 1.5-5μm, and the area ratio of the pro-eutectoid cementite with a 1.5-5μm long diameter is 0.6% or greater.

Description

Bearing steel with excellent cold workability

The present invention relates to bearing steel used for bearing parts and machine structural parts used in automobiles and various industrial machines, and in particular, when manufacturing bearing parts such as balls, rollers, needles and races by cold working. In particular, the present invention relates to a bearing steel that can exhibit good cold workability.

The various bearing parts as described above are processed into final shapes by cutting, forging, cutting and the like of wire rods and steel bars. Especially for cold working (cold rolling or cold forging), the rolled material is too hard and cold working is difficult, so spheroidization prior to cold working for the purpose of improving cold workability. In general, annealing is performed.

Ensuring good cold workability is important from the viewpoint of improving productivity and saving energy, and reducing costs and reducing CO ₂ emissions. In order to ensure good cold workability, it is necessary that the spheroidizing annealing material has a good degree of spheroidization of cementite and low hardness and deformation resistance. Under such circumstances, various techniques have been proposed so far in order to secure an excellent spheroidized annealing structure and to reduce hardness.

For example, in Patent Document 1, a rolling material of bearing steel is specified with a rolling temperature and a cooling rate, and by reducing network proeutectoid cementite and enlarging pearlite lamella spacing, uniform cold workability is excellent. A technique for obtaining a spheroidized structure has been proposed. However, since this technique is based on the uniform and fine dispersion of spheroidized cementite, the hardness does not decrease so much and it cannot be said that good cold workability can be secured.

On the other hand, in Patent Document 2, by increasing the pearlite colony size by carrying out the batch rolling and hot rolling at a low temperature, and by controlling the cooling conditions after rolling, the proeutectoid cementite is increased, It proposes that even if spheroidizing annealing is omitted, a steel material that is hard to break by wire drawing can be obtained. However, this technology has a problem in that since the proeutectoid cementite is too large, the rolling fatigue characteristics (rolling fatigue life) that are basically required for bearing parts are reduced. In addition, since this technique is characterized by not performing spheroidizing annealing, there is a problem that the use of parts is limited due to the properties of the obtained steel material and the versatility is poor.

By the way, various bearing parts are important parts that support rotating parts and sliding parts of machinery. However, the contact surface pressure is considerably high and the external force may fluctuate. In many cases. Therefore, excellent durability is required for the steel material which is the material of such bearing parts. These requirements are becoming stricter year by year as the performance and weight of machinery are improved. In order to improve the durability of bearing parts, it is particularly important that the rolling fatigue characteristics be excellent.

JP-A-5-84405 Japanese Patent No. 4008320

The present invention has been made in view of such circumstances, and its purpose is to exhibit good cold workability in cold working performed after spheroidizing annealing, and to provide good rolling fatigue life. The object is to provide steel for bearings that can be secured.

The bearing steel according to the present invention that has achieved the above-mentioned object includes C: 0.8 to 1.3% (meaning of mass%, hereinafter the same), Si: 0.05 to 0.8%, Mn : 0.1 to 2%, P: 0.05% or less (not including 0%), S: 0.05% or less (not including 0%), Cr: 1 to 2%, Al: 0.01 ~ 0.1%, N: 0.015% or less (not including 0%), Ti: 0.015% or less (not including 0%), and O: 0.0025% or less (not including 0%) The balance is made of iron and inevitable impurities, the amount of dissolved N is 0.003% or less (including 0%), and the proeutectoid cementite has an aspect ratio of 10 or less and an average major axis of 1 The main point is that the area ratio of pro-eutectoid cementite having a major axis of 1.5 to 5 μm is 0.6 to 5%.

In the bearing steel of the present invention, when the major axis of the Al-based nitrogen compound dispersed in the steel is a and the minor axis is b, the average grain size represented by (a × b) ^1/2 is 70 nm or more and 200 nm. The number density of Al-based nitrogen compounds having a particle size of 70 to 200 nm is preferably 0.5 to 4.0 / μm ² . By satisfying this requirement, it is possible to secure a better rolling fatigue life as well as cold workability. The Al-based nitrogen compounds targeted by the present invention include not only AlN but also those containing elements such as Mn, Cr, S, Si in part (total content up to about 30%). Is intended.

In the bearing steel of the present invention, if necessary, as another element, (a) Nb: 0.5% or less (not including 0%), V: 0.5% or less (not including 0%) And B: one or more selected from the group consisting of 0.005% or less (excluding 0%), (b) Ca: 0.05% or less (not including 0%), REM: 0.05% (Excluding 0%), Mg: 0.02% or less (not including 0%), Li: 0.02% or less (not including 0%), and Zr: 0.2% or less (0% (C) Pb: 0.5% or less (not including 0%), Bi: 0.5% or less (not including 0%), and Te: 0 It is also useful to include one or more selected from the group consisting of .1% or less (not including 0%). Depending on the components contained, the properties of the steel material can be further improved.

According to the present invention, it is possible to realize a bearing steel capable of exhibiting good cold workability by appropriately adjusting the chemical component composition and appropriately dispersing appropriately-sized proeutectoid cementite in the steel material. . The present invention is useful also from the viewpoint of improving productivity and energy saving, reducing costs, and reducing CO ₂ emissions. Further, by appropriately dispersing an Al-based nitrogen compound having an appropriate size in the bearing steel, it is possible to ensure a better rolling fatigue life as well as cold workability. When such a steel material is applied to a bearing component, an excellent rolling fatigue life can be exhibited even when used in a harsh environment.

It is a graph which shows the relationship between pro-eutectoid cementite area ratio and deformation resistance. It is a graph which shows the relationship between solid solution N amount and deformation resistance. Is a graph showing the relationship between the number density and fatigue life L ₁₀ of the Al-based nitrogen compounds.

The present inventors examined from various angles with the aim of realizing a bearing steel that exhibits excellent cold workability. And, in order to improve the cold workability of the steel material and improve the rolling fatigue life as necessary, it has been found that it is effective to satisfy the following requirements (A) to (D): Obtained.

(A) A good cold workability can be obtained by dispersing proeutectoid cementite of a predetermined size,
(B) In order to keep the area ratio, size (major axis), and aspect ratio of pro-eutectoid cementite within a predetermined range, the steel material is rolled at a low temperature (800 to 950 ° C.), and the finish rolling temperature is set to Ar _1. It is effective to do below the transformation point,
(C) If solid solution N (solid nitrogen) in steel is suppressed to a predetermined amount or less, dynamic strain aging is suppressed and cold workability is improved.
(D) In order to suppress the solid solution N (solid nitrogen) in the steel to a predetermined amount or less, a steel material containing a predetermined amount of Al and N is rolled in the above temperature range, whereby a preferred form of Al-based nitrogen. Since a compound is obtained, the amount of solute N can be suppressed to an appropriate value (0.003% or less).

Based on the above findings, the present inventors have made further studies to improve the cold workability of the steel material and improve the dynamic fatigue life. As a result, the amount of solute N in the steel material is reduced, the content of Al and N is strictly defined, and the production conditions are controlled, so that after spheroidizing annealing, the aspect ratio is 10 or less and the average major axis is Precipitate cementite with a thickness of 1.5 to 5 μm is deposited, and if the area ratio of pro-eutectoid cementite with a major axis of 1.5 to 5 μm is 0.6% or more, the cold workability of steel is good. The present invention has been completed.

The bearing steel of the present invention has an aspect ratio of pro-eutectoid cementite of 10 or less, an average major axis of 1.5 to 5 μm, and a major axis of 1.5 to 5 μm even after the usual spheroidizing annealing treatment. By dispersing a predetermined amount (0.6% or more by area ratio) of pro-eutectoid cementite, the hardness of the steel material can be reduced and the deformation resistance can be reduced. Also, by reducing the amount of solute N, dynamic strain aging during cold working can be suppressed and deformation resistance can be reduced.

In the bearing steel of the present invention, the form of pro-eutectoid cementite is defined. This pro-eutectoid cementite is obtained from a hypereutectoid steel (C content: 0.8% or more) like the steel material of the present invention from the austenite state. When cooling, it is cementite that precipitates from austenite prior to the eutectoid transformation and precipitates along the grain boundaries. The cementite produced thereafter is a layered cementite and can be distinguished.

[Size of proeutectoid cementite (average major axis)]
The deformation resistance after spheroidizing annealing, which is one of the indexes of cold workability, can be reduced by increasing the size of pro-eutectoid cementite. From such a viewpoint, the size of pro-eutectoid cementite needs to be 1.5 μm or more in terms of average major axis. However, if the size of proeutectoid cementite becomes too large, cracks are likely to occur during cold working. Further, such coarse inclusions become a stress concentration source during rolling, and cause a reduction in rolling fatigue life. From such a viewpoint, the size of proeutectoid cementite needs to be 5 μm or less in terms of the average major axis. The major axis means the length (major axis) of the largest portion of pro-eutectoid cementite.

[Aspect ratio of proeutectoid cementite: 10 or less]
The proeutectoid cementite as described above needs to have an aspect ratio (major axis / minor axis) of 10 or less. When this aspect ratio (major axis / minor axis) is larger than 10, cracks are generated at the interface, and cracks are likely to occur during cold working.

[Area ratio of pro-eutectoid cementite with a major axis of 1.5 to 5 μm: 0.6% or more]
In order to improve the cold workability, the pro-eutectoid cementite needs to be appropriately dispersed. From this viewpoint, it is necessary to make the area ratio 0.6% or more. However, if this area ratio becomes too large, there is a possibility that the rolling fatigue life may be reduced, so that it is preferably 7% or less.

[Solution N content: 0.003% or less (including 0%)]
In the bearing steel of the present invention, it is also an important requirement to reduce the amount of solute N in order to improve the cold workability. When the amount of solute N exceeds 0.003%, dynamic strain aging occurs during cold working, increasing deformation resistance and reducing cold workability. In addition, since solid solution C (solid solution carbon) interacts with Cr (Cr lowers the activity of C), dynamic strain aging does not occur.

The inventors have also studied the requirement for further improving the rolling fatigue life of the steel material. As a result, it has also been found that better rolling fatigue life can be secured if an Al-based nitrogen compound having a predetermined size is appropriately dispersed. That is, the Al and N contents in the steel material are strictly defined, the production conditions are controlled, and an Al-based nitrogen compound of a predetermined size dispersed in the steel after quenching and tempering is dispersed in the number density. If the dispersion is 0.5 to 4.0 / μm ² , the cold workability can be further improved and a better rolling fatigue life can be secured.

In the steel material of the present invention, appropriately controlling the number density of the Al-based nitrogen compound reduces the amount of solute N in the steel and suppresses dynamic strain aging during cold working. Thereby, deformation resistance can be reduced and cold workability can be made still better. Further, by dispersing the Al-based nitrogen compound in the matrix (in the steel base material), accumulation of strain during rolling fatigue can be alleviated and a better rolling fatigue life can be exhibited.

In order to exert the above effects, it is necessary to appropriately control the size and density of the target Al-based nitrogen compound. When the major axis of the Al-based nitrogen compound is a and the minor axis is b, the average particle size represented by (a × b) ^1/2 is smaller than 70 nm, or the number density of the Al-based nitrogen compound is 4.0. When it becomes larger than pieces / μm ² , the hardness becomes too high due to dispersion strengthening, and the deformation resistance increases. If the number density of the Al-based nitrogen compound is smaller than 0.5 / μm ² , the amount of solute N cannot be reduced sufficiently, and it does not contribute to the reduction of deformation resistance. On the other hand, when the average particle size is larger than 200 nm, it does not contribute to the relaxation of strain accumulation and the rolling fatigue life cannot be improved.

The size (average particle diameter) of the Al-based nitrogen compound is more preferably 90 nm or more, and more preferably 180 nm or less. The number density of the Al-based nitrogen compound is more preferably 1.0 piece / μm ² or more, and more preferably 3.0 pieces / μm ² or less.

In the steel material of the present invention, the chemical component composition (C, Si, Mn, P, S, Cr, Al, N, Ti, O) including the above-described Al and N contents needs to be appropriately adjusted. However, the reasons for limiting the ranges of these components are as follows.

[C: 0.8 to 1.3%]
C is an essential element for increasing quenching hardness, maintaining strength at room temperature and high temperature, dispersing cementite to impart wear resistance, and improving cold workability. In order to exert such an effect, C must be contained by 0.8% or more (hypereutectoid steel), preferably 0.9% or more (more preferably 0.95% or more). desirable. 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 1.3% or less, preferably 1.2% or less. (More preferably, 1.1% or less).

[Si: 0.05 to 0.8%]
Si is an element useful for improving the solid solution strengthening and hardenability of the matrix. In order to exert such effects, it is necessary to contain Si by 0.05% or more, preferably 0.1% or more (more preferably 0.15% or more). However, if the Si content is excessively increased, the cold workability and the machinability are remarkably lowered. Therefore, the Si content is 0.8% or less, preferably 0.7% or less (more preferably 0.6%). Should be kept below.

[Mn: 0.1-2%]
Mn is an element useful for strengthening the solid solution of the matrix and improving the hardenability. In order to exert such an effect, it is necessary to contain Mn in an amount of 0.1% or more, preferably 0.15% or more (more preferably 0.2% or more). However, if the Mn content is excessively increased, the cold workability and the machinability are remarkably lowered. Therefore, the Mn content is 2% or less, preferably 1.6% or less (more preferably 1.2% or less). Should be suppressed to.

[P: 0.05% or less (excluding 0%)]
P is an element inevitably contained as an impurity, but it is desirable to reduce it as much as possible because it segregates at the grain boundary and lowers the cold workability. However, extremely reducing causes an increase in steelmaking cost. For these reasons, the P content is set to 0.05% or less. Preferably 0
. It may be reduced to 04% or less (more preferably 0.03% or less).

[S: 0.05% or less (excluding 0%)]
S is an element that is inevitably contained as an impurity, but is precipitated at the grain boundary as FeS and decreases the cold workability. Moreover, although it is desirable to reduce as much as possible in order to precipitate as MnS and to reduce a rolling fatigue life, extreme reduction leads to the increase in steelmaking cost. For these reasons, the S content is set to 0.05% or less. Preferably, it is good to reduce to 0.04% or less (more preferably 0.03% or less).

[Cr: 1 to 2%]
Cr combines with C to form carbides, improves wear resistance and cold workability, contributes to improving hardenability, and further reduces the activity of C. It is an element that suppresses strain aging. In order to exert these effects, the Cr content needs to be 1% or more. Preferably it is 1.1% or more (more preferably 1.2% or more). However, when the Cr content is excessive, coarse carbides are generated and the rolling fatigue life is decreased. Therefore, the Cr amount is 2% or less. Preferably it is 1.8% or less (more preferably 1.6% or less).

[Al: 0.01 to 0.1%]
Al is an element that plays an important role in the steel material of the present invention, and by binding with N, it is finely dispersed in the steel as an Al-based nitrogen compound, reducing the solute N in the steel and reducing the cooling of the steel material. It is an important element for improving hot workability and rolling fatigue life. In order to produce a fine Al-based nitrogen compound, it is necessary to contain at least 0.01% or more. However, if the Al content becomes excessive and exceeds 0.1%, the size and number of Al-based nitrogen compounds that precipitate will increase, and cracks and scratches are likely to occur during casting and rolling. On the other hand, if the Al content is excessive, the crystal grains become too fine and the hardenability deteriorates, so that it cannot be applied to large parts and the rolling fatigue life is reduced. The preferable lower limit of the Al content is 0.013% (more preferably 0.015% or more), and the preferable upper limit is 0.08% (more preferably 0.05% or less).

[N: 0.015% or less (excluding 0%)]
N, like Al, is an element that plays an important role in the steel material of the present invention, and is an important element for exerting an effect of improving the rolling fatigue life due to fine dispersion of an Al-based nitrogen compound. However, if the N content becomes excessive and exceeds 0.015%, solid solution N tends to remain in the steel material manufacturing process, and the deformation resistance increases due to dynamic strain aging during cold working. Moreover, when N content becomes excess, the magnitude | size and number density of the Al-type nitrogen compound to precipitate will increase, and it will become easy to produce a crack at the time of casting or rolling. Furthermore, if the N content is excessive, the crystal grains become too fine, so that the hardenability is lowered, cannot be applied to large parts, and the rolling fatigue life is lowered. The lower limit of the N content is not particularly limited as long as a predetermined amount of Al-based nitrogen compound can be precipitated, and the cooling rate after rolling and the amount of elements (Ti, V, Nb, B, Zr, Te, etc.) that bind to N are reduced. And what is necessary is just to set suitably according to Al content. For example, when the N content is 0.0035% or more, a predetermined amount of an Al-based nitrogen compound can be precipitated. The preferable lower limit of the N content is 0.004% (more preferably 0.006% or more), and the preferable upper limit is 0.013% (more preferably 0.010% or less).

[Ti: 0.015% or less (excluding 0%)]
Ti combines with N in steel to produce TiN, which not only adversely affects rolling fatigue characteristics, but is also a harmful element that harms cold workability and hot workability. For this reason, it is desirable to reduce Ti as much as possible, but extremely reducing causes an increase in steelmaking cost. For these reasons, the Ti content needs to be 0.015% or less. In addition, the upper limit with preferable Ti content is 0.010% (more preferably 0.005% or less).

[O: 0.0025% or less (excluding 0%)]
O has a great influence on the form of impurities in the steel and forms inclusions such as Al ₂ O ₃ and SiO ₂ that adversely affect the rolling fatigue characteristics, so it is preferable to reduce it as much as possible. However, extremely reducing O causes an increase in steelmaking costs. For these reasons, the O content needs to be 0.0025% or less. In addition, the upper limit with preferable O content is 0.002% (more preferably 0.0015% or less).

The contained elements specified in the present invention are as described above, with the balance being iron and inevitable impurities. As the inevitable impurities, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. can be allowed. In order to increase the dynamic fatigue life, the following elements can be positively contained within a specified range.

[Nb: 0.5% or less (not including 0%), V: 0.5% or less (not including 0%) and B: 0.005% or less (not including 0%) One or more types]
Nb, V, and B are all effective elements for bonding with N to form a nitrogen compound to adjust the grain size and improve the rolling fatigue life. If Nb and B are added at 0.0005% or more and V is added at 0.001% or more, rolling fatigue characteristics can be improved. However, if Nb or V exceeds 0.5%, or B exceeds 0.005%, the crystal grains become finer and an incompletely quenched phase tends to be formed. A more preferable upper limit is 0.3% (more preferably 0.1% or less) for Nb and V, and 0.003% (more preferably 0.001% or less) for B.

[Ca: 0.05% or less (not including 0%), REM: 0.05% or less (not including 0%), Mg: 0.02% or less (not including 0%), Li: 0.0. 02% or less (not including 0%) and Zr: one or more selected from the group consisting of 0.2% or less (not including 0%)]
Ca, REM (rare earth element), Mg, Li, and Zr are all elements that spheroidize oxide inclusions and contribute to improving the rolling fatigue life. These effects are more effectively exhibited by containing 0.0005% or more in Ca and REM and 0.0001% or more in Mg, Li and Zr. However, even if it is contained excessively, the effect is saturated, and an effect commensurate with the content cannot be expected, which is uneconomical. A more preferable upper limit is 0.03% (more preferably 0.01% or less) for Ca or REM, 0.01% (more preferably 0.005% or less) for Mg or Li, and 0.15 for Zr. % (More preferably 0.10% or less).

[Pb: selected from the group consisting of 0.5% or less (not including 0%), Bi: 0.5% or less (not including 0%), and Te: 0.1% or less (not including 0%) One or more types]
Pb, Bi, and Te are all machinability improving elements. These effects are more effectively exhibited by containing 0.01% or more of Pb and Bi and 0.01% or more of Te. However, if the content of Pb or Bi exceeds 0.5% or the content of Te exceeds 0.1%, production problems such as generation of rolling flaws occur. A more preferable upper limit is 0.3% (more preferably 0.2% or less) for Pb and Bi, and 0.075% (more preferably 0.05% or less) for Te.

In the steel material of the present invention, after spheroidizing annealing, it is cold worked into a predetermined part shape, but in order to ensure the proeutectoid cementite in the form as described above before spheroidizing annealing, the production conditions ( In particular, it is necessary to appropriately control the hot rolling conditions of the slab. Usually, the process of hot rolling a bearing steel from a slab is performed at about 900 to 1100 ° C. In the present invention, by performing this rolling at a low temperature of 800 to 950 ° C., prior austenite (old γ) By reducing the crystal grain size and increasing the grain interface area, the precipitation sites of proeutectoid cementite can be dispersed. If the temperature at the time of rolling is less than 800 degreeC, the load to a rolling mill will increase and it will cause failure etc. On the other hand, when the rolling temperature exceeds 950 ° C., the crystal grain size becomes coarse (causing coarsening of pro-eutectoid cementite), and the required amount of pro-eutectoid cementite of a predetermined size cannot be dispersed.

Note that the rolling temperature (800 to 950 ° C.) corresponds to a temperature at which the Al-based nitrogen compound is likely to precipitate (precipitation temperature range). For this reason, by rolling a steel material containing a predetermined amount of Al and N in this temperature range, a preferred form of an Al-based nitrogen compound is obtained, and the amount of solid solution N is also suppressed to an appropriate value (0.003% or less). it can.

By setting the finishing temperature of the hot rolling to 900 ° C. or less at which pro-eutectoid cementite starts to precipitate, pro-eutectoid cementite can be precipitated during rolling. Then, since the precipitated cementite is destroyed in the subsequent finish rolling process, the aspect ratio of the pro-eutectoid cementite can be made 10 or less. When the finish rolling temperature is higher than 900 ° C., the pro-eutectoid cementite precipitates along the grain boundary after rolling, and tends to grow in the longitudinal direction.

Furthermore, after rolling under the above conditions, the average cooling rate (referred to as the primary average cooling rate) during the cooling of the steel material from 850 ° C. to 650 ° C. is in the range of 0.10 to 0.85 ° C./second. As a result, pro-eutectoid cementite can be grown and its size (average major axis) can be controlled to 1.5-5 μm, and the area ratio of pro-eutectoid cementite with a major axis of 1.5-5 μm can be increased to 0.6% or more. it can. When the primary cooling rate is less than 0.10 ° C./second, the growth of pro-eutectoid cementite is suppressed and a predetermined area ratio cannot be obtained. On the other hand, when the primary cooling rate exceeds 0.85 ° C./second, the growth of proeutectoid cementite is suppressed and a predetermined area ratio cannot be obtained.

In the steel material of the present invention, in order to disperse the fine Al-based nitrogen compound in the steel after quenching and tempering, a slab satisfying the above component composition is used in the steel material production process, and the cooling rate after rolling is controlled. This is very important. The Al-based nitrogen compound that precipitates in the cooling process after rolling remains in the same state even after the subsequent spheroidizing annealing, parts processing, quenching / tempering process. Therefore, the cooling rate at 850 to 650 ° C., which is also the precipitation temperature range of the Al-based nitrogen compound, that is, the primary cooling rate is in the range of 0.10 to 0.85 ° C./second, and the steel material is moved from 650 ° C. to room temperature (25 ° C.). By cooling at an average cooling rate (called the secondary cooling rate) of 1 ° C./second or more when cooling to 10%, the major axis of the Al-based nitrogen compound dispersed in the steel even after quenching and tempering is a, short When the diameter is b, the number density of the Al-based nitrogen compound having an average particle size represented by (a × b) ^1/2 of 70 nm to 200 nm and a particle size of 70 to 200 nm is 0.5. Up to 4.0 pieces / μm ² can be achieved.

When the primary cooling rate is less than 0.10 ° C./second, the Al-based nitrogen compound becomes coarse, and when it exceeds 0.85 ° C./second, the average particle size of the Al-based nitrogen compound is less than 70 nm, The number density of the size becomes less than 0.5 / μm ² , and the desired size and number cannot be obtained. Further, by setting the secondary cooling rate from less than 650 ° C. to room temperature to 1 ° C./second or more, coarsening of the Al-based nitrogen compound can be suppressed and the size thereof can be controlled.

The steel material of the present invention is subjected to spheroidizing annealing, and then cold-worked into a predetermined part shape and subsequently quenched and tempered to be manufactured into a bearing part or the like. Both the linear shape and the rod shape are included, and the size is 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, and can of course be implemented with modifications within a range that can be adapted to the purpose described above and below. They are all included in the technical scope of the present invention.

The steel materials (test Nos. 1 to 36) having various chemical compositions shown in Tables 1 and 2 below were heated to 1100 to 1300 ° C. in a heating furnace or a soaking furnace, and then subjected to block rolling at 900 to 1200 ° C. Thereafter, hot rolling was performed in a temperature range of 800 to 1050 ° C., and finish rolling was performed at a temperature of 950 ° C. or less (including forging simulating rolling) to produce a round bar having a diameter of 70 mm (see the following table). 3, 4). After completion of processing, 850 to 650 ° C. is cooled at various average cooling rates (Tables 3 and 4 below), and from 650 ° C. to room temperature (25 ° C.) is cooled at an average cooling rate of 1 ° C./second. Rolled material or forged material was obtained.

The above rolled material or forged material was subjected to spheroidizing annealing at 795 ° C. (holding time: 6 hours). A disk having a diameter of 10 mm and a thickness of 16 mm was cut out from the center of this steel material, and used as a test piece for evaluating workability. Further, the steel material after spheroidization was cut by cutting. Thereafter, a disk having a diameter of 60 mm and a thickness of 5 mm was cut out and subjected to oil quenching after heating at 840 ° C. for 30 minutes and tempering at 160 ° C. for 120 minutes. Finally, finish polishing was performed to prepare a test piece having a surface roughness Ra (arithmetic average roughness) of 0.04 μm or less.

Using the test piece obtained above, the size, aspect ratio, area ratio, solid solution N amount, number density of Al-based nitrogen compounds, size of the pro-eutectoid cementite were measured under the following conditions, and cold. Workability (deformation resistance) and rolling fatigue life were evaluated.

[Measurement of pro-eutectoid cementite size, aspect ratio, and area ratio]
(A) The test piece after rolling was cut perpendicular to the longitudinal direction.
(B) The resin was embedded in a resin so that the cross section could be observed, and polishing with emery paper, diamond buffing, and electrolytic polishing were sequentially performed to finish the observation surface as a mirror surface.
(C) Corroded with nital (3% nitric acid ethanol solution).
(D) The position of D / 4 (D is the diameter) of the test piece (diameter: 70 mm, thickness: 10 mm disk) was observed at a magnification of 6000 times with a scanning electron microscope, and four positions were photographed.
(E) From the photograph taken, the pro-eutectoid cementite precipitated along the grain boundaries was traced, and the particle analysis software [“Particle Analysis III for Windows. Version 3.00 SUMITOMO METAL TECHNOLOGY” (trade name)] was used. The area ratio and aspect ratio of pro-eutectoid cementite having a major axis of 1.5 to 5 μm were obtained, and the average value of the four fields of view was defined as the area ratio (average area ratio) and aspect ratio (average aspect ratio) of pro-eutectoid cementite. Further, the major axis was calculated from the size of each proeutectoid cementite, and the average value of the four fields of view was obtained (adopted as “average major axis”).

[Measurement of solute N content]
The solid solution N amount of each test piece is a value calculated by subtracting the N amount in all N compounds from the total N amount in steel in accordance with JIS G 1228.

(A) The total N amount is a value determined by using an inert gas melting method-thermal conductivity method. A sample cut from the test steel material is placed in a crucible, melted in an inert gas stream, extracted N, transported to a thermal conductivity cell, and the change in thermal conductivity is measured. Were determined.
(B) The total amount of N compounds is a value determined using ammonia distillation separation indophenol blue absorptiometry. In 10% AA-based electrolyte (non-aqueous solvent-based electrolyte that does not generate a passive film on the steel surface, specifically, a methanol solution in which 10% acetylacetone and 10% tetramethylammonium chloride are dissolved) Constant current electrolysis was performed using a sample cut out from the test steel material as an electrode. About 0.5 g of the sample was dissolved, and the insoluble residue (N compound) was filtered through a polycarbonate filter having a hole size of 0.05 μm. The insoluble residue was decomposed by heating in sulfuric acid, potassium sulfate, and pure Cu chips and combined with the filtrate. After making this solution alkaline with sodium hydroxide, steam distillation was performed, and the distilled ammonia was absorbed in dilute sulfuric acid. Phenol, sodium hypochlorite, and sodium pentacyanonitrosyl iron (III) were added to form a blue complex, and the absorbance was measured using a photometer to determine the total amount of N compounds.

[Measurement of number density and size of Al-based nitrogen compounds]
As a method for confirming the dispersion state of the Al-based nitrogen compound, the rolled material is cut, the cross section is polished, carbon is vapor-deposited on the surface, and replica observation is performed with an FE-TEM (field emission transmission electron microscope). Carried out. At this time, components of an Al-based nitrogen compound containing Al and N were specified by TEM EDX (energy dispersive X-ray detector), and the field of view was observed at a magnification of 30000 times. At this time, 1 field of view was set to 16.8 μm ^2, and arbitrary 3 fields of view were observed (total 50.4 μm ² ), and particle analysis software [“Particle Analysis III for Windows. Version 3.00 SUMITOMO METAL TECHNOLOGY” (trade name) used, determine the number density of the size [(a × ^b) an average particle diameter] and a particle size of 70 ~ 200 nm Al-based nitride compound represented by ^1/2 (number density is converted to per [mu] m ²⁾ It was.

[Cold workability]
After cutting out a disk (test piece) having a diameter of 10 mm and a thickness of 16 mm from the central part of the steel material after spheroidization, and using a press tester, cold working at a working rate (compression rate): 80%, The side surface of the test piece was observed with an optical microscope (magnification: 20 times), the presence or absence of cracking was confirmed, and the deformability was evaluated. Moreover, the deformation resistance (MPa) at that time was measured, and cold workability was evaluated. The processing rate is indicated by [{1- (L / L ₀ )} × 100 (%)] (where L: test piece length before processing, L ₀ : test piece length after processing). At this time, the deformation resistance of 850 MPa or less was regarded as acceptable.

[Measurement of rolling fatigue life]
Rolling fatigue test was performed 16 times for each steel material (test piece) under the conditions of repetition rate: 1500 rpm, surface pressure: 5.3 GPa, number of cancellations: 2 × 10 ⁸ times with a thrust type rolling fatigue tester. Fatigue life L ₁₀ (the number of stress repetitions until fatigue failure at a cumulative failure probability of 10% obtained by plotting on Weibull probability paper) was evaluated. At this time, the fatigue life L ₁₀ (L ₁₀ life) was set to 1.0 × 10 ⁷ times or more as an acceptance criterion.

These results are shown in Tables 5 and 6 below.

From these results, it can be considered as follows. That is, test no. Those of 3 to 6, 9 to 17, 20 to 22, 24, 25, 27, 28, 31, 36 are the requirements specified in the present invention (chemical composition, average major axis of proeutectoid cementite, aspect ratio, area ratio) , Solid solution N amount) and preferable requirements (form of Al-based nitrogen compound), both show no cracking, low deformation resistance, and excellent rolling fatigue life is achieved. .

Test No. No. 30 satisfies the requirements specified in the present invention (chemical component composition, size of pro-eutectoid cementite, aspect ratio, area ratio, solute N amount), but preferable requirements (form of Al-based nitrogen compound) Come off. That is, test no. No. 30 has a large average particle diameter of the Al-based nitrogen compound (coarse) and a low rolling fatigue life.

In contrast, test no. Those of 1, 2, 7, 8, 18, 19, 23, 26, 29, and 32 to 35 have poor cold workability because any of the requirements defined in the present invention is not met.

Test No. For Nos. 1 and 29, the cooling rate after rolling was too high. 2, 19 and 34 are not suitable for the rolling temperature (test No. 19 has pro-eutectoid cementite coarsened), and the area ratio of pro-eutectoid cementite is insufficient (test No. 19). No. 29 also lacks the average particle size of pro-eutectoid cementite) and has a large deformation resistance. In particular, test no. No. 29 does not satisfy the preferable requirements of the form of the Al-based nitrogen compound, and has a short rolling fatigue life.

Test No. Those of 7, 8, 33 and 35 have a high deformation resistance because of a large amount of dissolved N. In addition, the cooling rate after rolling is too fast, a preferred form of the Al-based nitrogen compound is not formed, and the rolling fatigue life is short.

Test No. Nos. 18 and 23 have a high finish rolling temperature, so that the aspect ratio of cementite is large and cracking occurs.

No. In the case of No. 26, since the cooling conditions after rolling are too slow, cementite is coarsened and the rolling fatigue life is short. Test No. No. 32 has a large amount of solute N as the N content increases, and has a high deformation resistance.

Based on these data, the relationship between the pro-eutectoid cementite area ratio and the deformation resistance is shown in FIG. 1 (a sample with the same amount of dissolved N) is plotted. It turns out that it is effective for reduction of resistance.

Similarly, the relationship between the amount of dissolved N and deformation resistance is shown in FIG. 2 (a sample having the same area ratio of pro-eutectoid cementite is plotted), but appropriately controlling the amount of dissolved N can reduce deformation resistance. It turns out that it is effective.

FIG. 3 shows the relationship between the number density of the Al-based nitrogen compound and the fatigue life L ₁₀ (L ₁₀ life). By appropriately controlling the number density of the Al-based nitrogen compound, a long fatigue life L ₁₀ (rolling fatigue) It can be seen that (lifetime) is achieved.

Claims

C: 0.8 to 1.3% (meaning of mass%, the same applies hereinafter), Si: 0.05 to 0.8%, Mn: 0.1 to 2%, P: 0.05% or less (0% ), S: 0.05% or less (excluding 0%), Cr: 1 to 2%, Al: 0.01 to 0.1%, N: 0.015% or less (including 0%) No), Ti: 0.015% or less (not including 0%) and O: 0.0025% or less (not including 0%), the balance being made of iron and inevitable impurities, and the amount of dissolved N is 0 0.003% or less (including 0%),
Pro-eutectoid cementite has an aspect ratio of 10 or less and an average major axis of 1.5 to 5 μm.
A steel for bearings characterized in that the area ratio of pro-eutectoid cementite having a major axis of 1.5 to 5 μm is 0.6% or more.
When the major axis of the Al-based nitrogen compound dispersed in the steel is a and the minor axis is b, the average grain size represented by (a × b) 1/2 is 70 nm or more and 200 nm or less, and the grain size is 70 to The bearing steel according to claim 1, wherein the number density of the 200 nm Al-based nitrogen compound is 0.5 to 4.0 pieces / µm 2 .
Further, as other elements, Nb: 0.5% or less (not including 0%), V: 0.5% or less (not including 0%), and B: 0.005% or less (not including 0%) The bearing steel according to claim 1, comprising at least one selected from the group consisting of:
Further, as other elements, Ca: 0.05% or less (not including 0%), REM: 0.05% or less (not including 0%), Mg: 0.02% or less (not including 0%) And Li: 0.02% or less (excluding 0%) and Zr: 0.2% or less (excluding 0%), and one or more selected from the group consisting of: steel.
Further, as other elements, Pb: 0.5% or less (not including 0%), Bi: 0.5% or less (not including 0%), and Te: 0.1% or less (not including 0%) The bearing steel according to claim 1, comprising at least one selected from the group consisting of: