JP5867324B2 - Bearing steel - Google Patents

Description

  The present invention relates to bearing steel. Specifically, the present invention relates to a high carbon chrome bearing steel having excellent rolling fatigue life.

  Bearing parts used for automobiles and industrial machines are generally hot forging, cold forging, cutting using high-carbon chromium bearing steel materials represented by SUJ2-5 specified in JIS G 4805 (2008). Through a process such as processing, it is finished in a desired shape.

  In addition, the high carbon chromium bearing steel as hot rolled usually has a microstructure that is a single phase structure of pearlite or a mixed structure of a hard phase such as bainite and pearlite, and has a high hardness. Accordingly, when cutting or cold forging is performed on a hot-rolled high carbon chrome bearing steel, it is difficult to avoid problems such as a decrease in tool life and occurrence of cracks during cold forging.

  Therefore, conventionally, for the purpose of improving workability such as cutting workability and cold forgeability, a spheroidizing heat treatment for longer than 20 hours called “spheroidizing annealing” is performed as a pretreatment, The structure has been changed to a mixed structure of ferrite and spherical cementite.

  However, the long spheroidizing heat treatment requires expensive heat treatment equipment, consumes a lot of energy, and lowers productivity, leading to an increase in cost.

  Therefore, from the industry, the time for spheroidizing heat treatment as a pretreatment for cutting and cold forging process is omitted, or even if it can not be omitted, the time is greatly shortened to reduce energy consumption and equipment cost. In addition, there is an increasing demand for improving productivity by simplifying the process.

  Therefore, in order to meet these demands, the present inventors have proposed a technique that can omit or simplify the spheroidizing annealing in Patent Document 1.

Specifically, the inventors of the present invention described in Patent Document 1 in terms of mass%, C: 0.7 to 1.2%, Cr: 0.8 to 1.8%, Mn: 0.2 to 1. A material to be rolled containing 2% and S: 0.015% or less, and having a chemical composition in which the value of Mn / S, which is the ratio of the content of Mn and S, is 20 to 170, Ae ₁ point to Aem After heating to a temperature range of points, rolled by an all-continuous hot rolling method comprising two or more rolling steps and one or more intermediate cooling steps between the first rolling step and the last rolling step, , After the end of rolling, a method for producing a bearing steel material that is finally cooled in a temperature range up to 400 ° C. under a cooling rate of 5 ° C./s or less, wherein the all-continuous hot rolling method includes [1] each The surface temperature of the material to be rolled during the rolling process is in the range of 680 ° C. to (Aem point−30 ° C.), [2] Intercooler In, that the time Δt from the start of cooling to the surface temperature of the cooling after the end the rolled material is recuperated more than _one point Ae is 10s or less, and (3) a total reduction rate is 30% or more, In addition to the “basic steel manufacturing method” which satisfies all of the above conditions and the above basic conditions, heating in the temperature range of Ae ₁ point to Aem point is further performed as [4] material to be rolled. The time when the temperature of 750 to 850 ° C. is 60 minutes or less in cumulative time, and [5] the time when the temperature of the material to be rolled exceeds 850 ° C. and below the Aem point is 120 min or less in cumulative time. We proposed a method of manufacturing bearing steels that is characterized.

  With the above technology, it is possible to build a microstructure with a spheroidized structure as it is, such as hot wire rolling and steel bar rolling, omitting the spheroidizing annealing or reducing the spheroidizing annealing time to about half or less of the conventional It is possible to shorten it.

  As already described, the bearing steel finally becomes a bearing component through a machining process such as cutting and cold forging. In recent years, the demand for longer bearing life for product characteristics, particularly rolling fatigue characteristics, as bearing parts is becoming increasingly severe.

  The rolling fatigue characteristics of bearing parts, that is, the rolling fatigue life, is determined by the nonmetallic inclusions (hereinafter also simply referred to as “inclusions”), particularly oxide inclusions (hereinafter referred to as “inclusions”). It is known that it is lowered simply by “oxide”. Therefore, in order to prolong the rolling fatigue life, it is necessary to reduce the oxygen content in the steel material, reduce the total amount of oxides, and adjust the composition and form of inclusions.

  In order to meet these demands, for example, Patent Document 2 discloses a technique capable of extending the rolling fatigue life by adding a small amount of Ca and further limiting the amounts of sulfur, aluminum, and oxygen to be low.

  Specifically, in Patent Document 2, in mass%, C: 0.80 to 1.20%, Si: 1.0% or less, Mn: 2.0% or less, Cr: 0.50 to 2.0 In a bearing steel containing, as necessary, Mo or V in an amount of 0.80% or less and the balance being substantially Fe, Ca: 0.0005 to 0.0100%, S: “Bearing steel with good rolling life characteristics and machinability” characterized by adjusting to 0.025% or less, Al: 0.040% or less, and O: 0.0020% or less is disclosed.

JP 2009-275263 A JP-A-54-128418

  The specific purpose of the technique proposed by the present inventors in Patent Document 1 is to have a spheroidized structure as it is rolled, such as hot wire rod rolling and steel bar rolling, and the spheroidizing annealing time is about half or less than the conventional one. It is to provide a method for producing a high-carbon chromium bearing steel that can be shortened to a high level, and furthermore, it is possible to obtain spherical cementite obtained by conventional spheroidizing annealing without performing spheroidizing heat treatment. It is to provide a method for producing a carbon chromium bearing steel material.

For this reason, it is possible to simplify and omit a part of the manufacturing process to realize reduction of CO ₂ emission amount, improvement of productivity, and reduction of manufacturing cost. However, there have been some cases where the product performance required as a final bearing part, which has been strongly demanded in recent years, particularly in terms of extending the rolling fatigue life, is not always sufficient.

The steel disclosed in Patent Document 2 adjusts the oxide composition from the Al ₂ O ₃ system to the Al ₂ O ₃ —SiO ₂ —CaO system by adding a small amount of Ca, and has a long rolling fatigue life. It aims to make it easier. However, even if the composition of the oxide is adjusted by simply adding Ca, a coarse oxide is formed according to the amount of O and S contained in the steel, premature peeling occurs, and the rolling fatigue life is reversed. May be shortened.

  This invention is made | formed in view of the said present condition, and it aims at providing the bearing steel which can implement | achieve the outstanding rolling fatigue life as a bearing component.

  In general, rolling fatigue is a phenomenon in which repeated loads are applied to inclusions present in steel, cracks are generated due to stress concentration, and then the cracks gradually develop due to repeated loads, eventually leading to delamination. Is understood.

  Accordingly, the inventors have studied focusing on the form and composition of inclusions for extending the rolling fatigue life, and regarding the adjustment of the composition and form of inclusions, first, the following (a) and (b) ) Obtained important findings.

  (A) By controlling the composition of the oxide and sulfide, specifically, by controlling the composition so that an appropriate amount of CaO is contained in the oxide and CaS is contained in the sulfide, rolling Coarse inclusions that become harmful to the fatigue life (becomes a stress concentration source with repeated loading in rolling fatigue) can be suppressed.

(B) In order to control the composition of the inclusions in (a) above, it is necessary to contain a trace amount of Ca at least in the molten steel after the deoxidation treatment. For example, when a trace amount of Ca is not added and Ca is not contained in the molten steel, the oxide becomes mainly Al ₂ O _{3 in the} molten steel, and the oxide aggregates and coarsens during solidification. However, when Ca is added to the molten steel and Ca is contained in the molten steel, the interfacial energy is lowered by changing the composition of the oxide produced in the molten steel, and as a result, the cohesive force is lowered, thereby suppressing coarsening. Is done. On the other hand, when Ca is contained in the molten steel, in the case of sulfide, the MnS main body is changed to a sulfide containing CaS, which is spheroidized and hardly deformed during rolling, so that the diameter can be reduced.

  Ca added in a small amount in the molten steel stage after the deoxidation treatment reacts with both oxides and sulfides to change their composition. That is, since the reaction behavior with Ca varies depending on the amount of oxide and the amount of S in the molten steel immediately before Ca addition, the composition of the oxide and sulfide after the reaction changes. And a form will also change according to this composition change. Therefore, in order to extend the rolling fatigue life, it is necessary to appropriately adjust the composition of oxide and sulfide by addition of Ca to reduce the diameter.

  Then, next, the inventors conducted a detailed examination on the reaction between Ca added in the molten steel stage after deoxidation treatment and oxides and sulfides, and the oxide and sulfide forms in the slab. went.

  As a result, as shown in the following (c) and (d), the form of oxides and sulfides can be managed by using the [Ca / O] and S amount, which is the ratio of the O amount and the Ca amount. (See FIG. 1).

(C) The reaction between Ca and oxide added in the molten steel stage after the deoxidation treatment can be organized by the amount of O and the amount of Ca contained in the molten steel after the deoxidation treatment. That is, the amount of O represents the total amount of oxide formed in the molten steel immediately before Ca addition after deoxidation treatment, and the composition of the oxide after the addition of Ca is the amount of Ca and O contained in the molten steel. The ratio “Ca / O” can be expressed as an index. And
[0.70 ≦ Ca / O ≦ 1.80]
If this condition is satisfied, the formation of coarse oxides or dot-sequence oxides can be suppressed.

  (D) However, Ca added at the molten steel stage also reacts with S. For this reason, even if the Ca content and the O content satisfy the relational expression described in (c) above, depending on the S content, CaS is formed at the molten steel stage, and the formation of coarse oxides cannot be suppressed. There is. However, if the Ca content, O content and S content satisfy the above [0.70 ≦ Ca / O ≦ 1.80] and further satisfy [Ca / O ≧ 1250S-5.80], then the The coarsening of the oxide can be suppressed.

  The present invention has been completed based on the above technical idea and knowledge based thereon, and the gist of the present invention is the following bearing steel.

(1) By mass%, C: 0.95 to 1.2%, Si: 0.15 to 0.35%, Mn: 0.05 to 0.5%, P: 0.025% or less, S: 0.010% or less, Cr: 0.80 to 1.80%, Al: more than 0.005% to 0.040% or less, Ca: 0.0003 to 0.0010 % (however, 0.0010% And O: 0.0010% or less, with the balance being Fe and impurities, bearing steel having a chemical composition satisfying the following formulas [1] to [3].
0.70 ≦ Ca / O ≦ 1.80 [1]
Ca / O ≧ 1250S-5.80 ... [2]
20 ≦ Mn / S ≦ 170 [3]
However, the element symbol in a formula means content (mass%) of each element.

  The “impurities” are those that are mixed from ore, scrap, or production environment as raw materials when industrially producing steel, and are allowed within a range that does not adversely affect the present invention. Means.

  The bearing steel of the present invention has good durability against damage due to rolling fatigue and has a long rolling fatigue life. Therefore, the ball bearings and rollers used in various industrial machines and automobiles are used. It can be suitably used as a material for rolling bearings such as “bearings”.

It is a figure explaining the influence which the amount of Ca / O and S in a slab has on the formation of a coarse oxide and a point sequence-like oxide. It is a figure explaining the spheroidization process conditions performed using the box-type electric heating furnace of an atmospheric atmosphere. It is a figure explaining the spheroidization process conditions performed using the box-type electric heating furnace of an atmospheric atmosphere, and is an example of the long-time process generally used as the spheroidization process conditions.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of each element means “mass%”.

(A) Chemical composition of bearing steel:
C: 0.95-1.2%
C is an element that secures the hardness at the time of quenching and improves the rolling fatigue life, and the content needs to be 0.95% or more. However, if the C content increases, especially exceeding 1.2%, the wear resistance is improved, but a large amount of coarse pro-eutectoid cementite is dispersed in the heating stage in the steel bar rolling process. It causes deterioration of the forgeability. In addition, the hardness is increased, resulting in a decrease in tool life during cutting and burning cracks. Therefore, the content of C is set to 0.95 to 1.2%. In addition, the minimum with preferable C content is 0.97%. Moreover, a preferable upper limit is 1.1%.

Si: 0.15-0.35%
Si is an element effective for enhancing the hardenability and improving the rolling fatigue life, and must be contained by 0.15% or more. However, if Si is contained in excess of 0.35%, the hardness of the base material increases and the tool life during cutting is reduced. Therefore, the Si content is set to 0.15 to 0.35%. In addition, the minimum with preferable Si content is 0.20%. Moreover, a preferable upper limit is 0.32%.

Mn: 0.05 to 0.5%
Mn is an element effective for enhancing the hardenability and improving the rolling fatigue life, and must be contained by 0.05% or more. However, if Mn is contained exceeding 0.5%, the hardness of the base material becomes high, and the tool life during cutting is reduced. Furthermore, it also causes burning cracks. Therefore, the Mn content is set to 0.05 to 0.5%. In addition, the minimum with preferable Mn content is 0.10%. Moreover, a preferable upper limit is 0.45%. Note that the Mn content also needs to satisfy the following formula [3].

P: 0.025% or less P segregates at the grain boundary and shortens the rolling fatigue life. In particular, when the content exceeds 0.025%, the rolling fatigue life is significantly reduced. Therefore, the content of P is set to 0.025% or less. The range of preferable P content is 0.020% or less.

S: 0.010% or less S is an element that forms sulfides, and when the content exceeds 0.010%, coarse sulfides remain, so cold forgeability deteriorates and rolling fatigue life. Is significantly shortened. Therefore, the content of S is set to 0.010% or less. In addition, from the viewpoint of improving the rolling fatigue life, the lower the S content, the better. The preferable upper limit is 0.001%. Note that the S content must satisfy the following formulas [2] and [3].

Cr: 0.80 to 1.80%
Cr has the effect of enhancing the hardenability of the steel, thermally stabilizing cementite, and inhibiting solid solution of cementite in the matrix at high temperatures. This effect is exhibited when the Cr content is 0.80% or more. However, if the Cr content exceeds 1.80%, not only the above effects are saturated, but also cracking is likely to occur during the quenching process after the final shape is obtained, and the rolling fatigue life is increased. Cause a decline. Therefore, the Cr content is set to 0.80 to 1.80%. In addition, the minimum with preferable Cr content is 0.90%. Moreover, a preferable upper limit is 1.60%.

Al: more than 0.005% to 0.040% or less Al is an element used for deoxidation in the refining process, and if it does not contain more than 0.005%, the deoxidation effect by Al I can't get it. However, if the Al content exceeds 0.040%, it tends to remain as a coarse oxide, leading to a reduction in rolling fatigue life. Therefore, the Al content is more than 0.005% and not more than 0.040%. In addition, the minimum with preferable Al content is 0.007%. A preferred upper limit is 0.038%.

Ca: 0.0003 to 0.0012%
Ca forms an appropriate amount of CaO in the oxide and forms a solid solution in the sulfide to form CaS. By forming CaO in the oxide, the interfacial energy is reduced, and the cohesive strength of the oxide is reduced, thereby suppressing coarsening. This effect can suppress a decrease in rolling fatigue life. Also, for sulfides, there is an effect of suppressing stretching and coarsening by forming CaS. Furthermore, since the crystallization form changes, sulfide inclusions are uniformly dispersed. By these effects, it is possible to suppress a decrease in rolling fatigue life. Each effect of Ca described above is exhibited when the Ca content is 0.0003% or more. However, when the Ca content exceeds 0.0012%, not only the above effects are saturated, but also, in particular, the oxide becomes coarse, and as a result, the rolling fatigue life may be reduced. Therefore, the content of Ca is set to 0.0003 to 0.0012%. In addition, the minimum with preferable Ca content is 0.0004%. Moreover, a preferable upper limit is 0.0010%. It should be noted that the Ca content also needs to satisfy the following formulas [1] and [2].

O: 0.0010% or less O (oxygen) is an element that forms an oxide and needs to be reduced as much as possible. When the content of O increases and exceeds 0.0010% in particular, it tends to remain as a coarse oxide, leading to a decrease in rolling fatigue life. Therefore, the content of O is set to 0.0010% or less. The O content is preferably 0.0008% or less. Note that the O content needs to satisfy the following formulas [1] and [2].

Ca / O: 0.70 to 1.80 and (1250S-5.80) or more The bearing steel of the present invention must have a chemical composition in which Ca / O satisfies the following formula [1].
0.70 ≦ Ca / O ≦ 1.80 [1]
However, the element symbol in a formula means content (mass%) of each element.

Ca / O is an index representing the ratio of Ca to the total amount of oxide. When Ca / O is less than 0.70, coarse or dotted-lined spinel containing Al ₂ O ₃ as a main component is generated. It is difficult to obtain a spherical oxide. On the other hand, when Ca / O exceeds 1.80, it becomes easy to form coarse oxides or dot-sequence oxides mainly composed of CaO, and it becomes difficult to suppress coarse oxides. Therefore, Ca / O was set to a range of 0.70 to 1.80.

In the bearing steel of the present invention, Ca / O must have a chemical composition satisfying the following formula [2].
Ca / O ≧ 1250S-5.80 ... [2]
However, the element symbol in a formula means content (mass%) of each element.

  This is because even if Ca / O is in the range of 0.70 to 1.80, depending on the amount of S present in the steel, coarsening of the oxide may not be suppressed due to CaS formation. It is.

That is, in the case of [Ca / O <1250S-5.80], the amount of Ca used for controlling the oxide form is insufficient with the formation of CaS, so coarse or dot-like Al ₂ O ₃ is the main component. As a result, spinel is formed and the coarsening of the oxide cannot be suppressed. Therefore, [Ca / O ≧ 1250S-5.80] was set. In addition, 1250 which is a coefficient with respect to S means a Ca reduction coefficient accompanying CaS formation.

Mn / S: 20-170
The bearing steel of the present invention must further have a chemical composition that satisfies the following formula [3].
20 ≦ Mn / S ≦ 170 [3]
However, the element symbol in a formula means content (mass%) of each element.

  First, in order to suppress the occurrence of cracks during hot working, in addition to the content of S being 0.010% or less already described, at least S to reduce the amount of solute S. Is required to be present as MnS (sulfide), and if the value of Mn / S, which is the ratio of the content of Mn and S, is 20 or more, S will not cause ductility reduction as solute S. is there. On the other hand, if the value of Mn / S is too large, solid solution S will not exist and S will not cause a decrease in ductility, but if the value of Mn / S exceeds 170, the size of MnS produced will increase. This is because coarse MnS may remain. Therefore, the value of Mn / S was set to 20 to 170.

(B) Bearing steel manufacturing method:
The bearing steel according to the present invention can be manufactured, for example, by making steel melted in a converter into a slab of 300 mm × 400 mm by continuous casting.

  Hereinafter, the present invention will be described more specifically with reference to examples.

  Bearing steels 1 to 13 having various chemical compositions shown in Table 1 were melted in a 70 t converter.

  In addition, after performing the deoxidation process by Al with a converter, the CaSi compound was added and Ca amount was adjusted.

  Steels 1 to 6 in Table 1 are steels whose chemical compositions are within the range defined by the present invention, and Steels 7 to 13 are comparative steels whose chemical compositions deviate from the conditions defined by the present invention. .

Table 1 shows the Ae ₁ point and Aem point of each steel determined by the phase diagram calculation software “Pandat ver. 6.0” developed and sold by Materials Design Technology Laboratory Co., Ltd. “Ae ₁ point” and “Aem point” are the eutectoid temperature in the equilibrium state and the temperature at which cementite completely dissolves in austenite in the equilibrium state, respectively.

  The steel melted in the converter is made into a slab of 300 mm × 400 mm by continuous casting, further soaked at 1250 ° C., and then subjected to block rolling in a temperature range of 1150 to 1100 ° C. to obtain a steel of 160 mm × 160 mm. It was a piece.

Next, the steel slab was subjected to the “total area reduction ratio” in the conditions shown as test numbers 1 to 14 in Table 2 by an all-continuous hot rolling line equipped with cooling equipment between each rolling mill row having the following configuration. Was hot rolled to 88.96% and processed into a steel bar having a diameter of 60 mm. The “total area reduction ratio” means that when the cross-sectional area before rolling of the material to be rolled in the all continuous hot rolling method is A ₀ and the area after leaving the final rolling mill is A _f , {( A ₀ −A _f ) / A ₀ } × 100 (%).
・ Rough rolling mill row: Consists of 8 rolling mills,
Intermediate rolling mill row: Consists of four rolling mills,
-Finish rolling mill row: Consists of two rolling mills.

  In addition, the hot rolling of the test numbers 1-13 in Table 2 is the manufacturing method of invention of patent document 1. FIG. Moreover, the hot rolling of test number 14 in Table 2 is a conventional all-continuous hot rolling method in which the austenite single phase region is heated and hot rolled.

  The surface temperature of the material to be rolled at the time of rolling the steel bar was measured using a radiation thermometer.

  After the end of continuous rolling, that is, after the end of rolling by the second rolling mill in the finish rolling mill row, the sheet was allowed to cool in the atmosphere.

  In Table 2, the rough rolling mill row, the intermediate rolling mill row, and the finish rolling mill row are expressed as “rough row”, “intermediate row”, and “finishing row”, respectively. The cooling facility between the intermediate rolling mill row and the finishing rolling mill row is denoted as “cooling facility 1”. Furthermore, when it cooled using the cooling equipment 1 and 2, the temperature which became the lowest between the cooling start and completion | finish in each equipment was described as "minimum temperature", respectively.

  The rolling start temperature, entry side temperature, exit side temperature and rolling end temperature described in Table 2 are the surface temperature of the material to be rolled measured using a radiation thermometer, and the minimum temperature in each cooling facility is as follows: The surface of the material to be rolled obtained by numerical analysis using the differential method, taking into account the surface temperature measurement value measured with a radiation thermometer, cooling conditions in the cooling facility, cooling conditions in the air after leaving the cooling facility, and rolling conditions From the temperature history, the minimum temperature on the surface of the material to be rolled in each cooling facility is calculated and described.

  These rolled steel bars were examined for the microstructure by the following method.

  That is, first, a test piece having a length of 20 mm is cut out from each steel bar having a diameter of 60 mm, and a cross section (hereinafter referred to as a “longitudinal cross section”) cut out parallel to the rolling direction through the central axis of these test pieces. After embedding in resin so that it becomes the inspection surface, mirror polishing, it was corroded with picric alcohol (picral liquid), and a microstructure was taken for 10 fields of view using a scanning electron microscope (SEM) at a magnification of 5000 times did. The area of each visual field is 25 μm × 20 μm.

  Next, the long diameter L and the short diameter W of each cementite are individually measured by the image processing software using the photographed image, and the ratio of the cementite whose L / W is 5.0 or less and the L / W is The ratio of the cementite which is 2.0 or less was calculated, respectively.

  Further, an equivalent circle diameter of each cementite having an L / W of 2.0 or less was derived by image processing software, and the average particle diameter of the cementite having an L / W of 2.0 or less was obtained by arithmetic averaging. . In the following description, the ratio of cementite whose L / W determined as described above is 2.0 or less is referred to as “spheroidization rate”.

  The above microstructure investigation results are shown in Table 2 together.

  As is clear from Table 2, in Test Nos. 1 to 13 manufactured by the manufacturing method of the invention of Patent Document 1, it was confirmed that the spheroidization rate was about 80% regardless of the steel type.

  Moreover, the spheroidized structure was not obtained in Test No. 14 of the conventional all continuous hot rolling method in which the austenite single-phase region was heated and hot rolled.

  Next, a test piece having a length of 300 mm was cut out from each bar. Next, spheroidizing heat treatment was performed on the test pieces of Test Nos. 1 to 13 using a box-type electric heating furnace in an air atmosphere under the condition that the total furnace time shown in FIG.

  In the heat treatment pattern shown in FIG. 2, the total furnace time is halved compared to the long-time treatment used as a general spheroidizing heat treatment.

  Moreover, the test number 14 processed with the heat processing pattern of FIG. 3 shown as an example of the long time processing used as a general spheroidization heat processing.

  Next, the microstructure of each test piece having a diameter of 60 mm subjected to the spheroidizing heat treatment was examined by the following method.

  First, a so-called “longitudinal section” cut through the central axis of each spheroidized heat treatment specimen and parallel to the rolling direction is embedded in a resin so as to be a test surface, mirror-polished, and then picric alcohol (picral) The microstructure was corroded at a magnification of 5000 times and a microstructure image was taken with respect to 10 visual fields in the center using a scanning electron microscope (SEM). The area of each visual field is 25 μm × 20 μm.

  Next, using the microstructure image, the major axis L and the minor axis W of each cementite are individually measured by image processing software, and the ratio of cementite in which L / W is 2.0 or less, that is, “ The spheroidization rate was calculated.

  Table 2 also shows the spheroidizing heat treatment pattern and the microstructural examination results after the spheroidizing heat treatment.

  As shown in Table 2, in the case of test numbers 1 to 13, the spheroidization rate is 95% or more, and in the case of test number 14, it can be confirmed that the normally required spheroidization rate of 85% is obtained. It was.

  Therefore, in order to evaluate rolling fatigue characteristics, the diameter of the steel 1 to 14 obtained by performing the spheroidizing heat treatment is 60 mm in diameter so that the longitudinal direction is the thickness of the shaped material from the center of the steel bar having a diameter of 60 mm. The sliced material with a thickness of 5.5 mm was sliced and collected.

  The above-mentioned shaped material having a diameter of 60 mm and a thickness of 5.5 mm was heated at 830 ° C. for 30 minutes, then oil-quenched, and then further heated at 180 ° C. for 1 h and then tempered to cool in the atmosphere. .

  A rolling fatigue test piece was prepared by wrapping the surface of a shaped material having a diameter of 60 mm and a thickness of 5.5 mm subjected to the tempering treatment of “quenching-tempering” in this way. Provided.

  The rolling fatigue test was performed using a thrust type rolling fatigue tester under conditions of a maximum contact surface pressure of 5230 MPa and a repetition rate of 1800 cpm (cycle per minute) in lubricating oil. In addition, the tempered material of SUJ2 prescribed | regulated to JISG4805 (2008) was used as a steel ball.

  Table 3 shows the detailed conditions of the rolling fatigue test. In addition, “quenching-tempering” in Table 3 is synonymous with the above “tempering”.

The rolling fatigue test results were plotted on the Weibull distribution probability paper, and the rolling fatigue characteristics were evaluated by setting the L ₁₀ life indicating 10% failure probability as “rolling fatigue life”. In addition, regarding the judgment of extending the rolling fatigue life, the life was extended when the L ₁₀ life satisfied 2.0 × 10 ⁷ or more, and this was the target.

  Table 2 also shows the rolling fatigue life determined as described above.

As shown in Table 2, in the case of test numbers 1 to 3 and 5 using steels 1 to 3 and 5 satisfying the chemical composition defined in the present invention and test number 14, the rolling fatigue life (L ₁₀ (Life) is 5.9 × 10 ⁷ or more, and a long rolling fatigue life can be achieved.

  On the other hand, in the case of test numbers 7 to 13, the chemical compositions of the steels 7 to 13 used deviate from the conditions specified in the present invention. For this reason, in the case of each said test number, a rolling fatigue life is short and has not reached the target.

In the case of test number 7, steel 7 used does not satisfy the formula [1], that is, the value of Ca / O calculated from the contents of Ca and O is lower than the value specified in the present invention. Therefore, coarse spinel oxide inclusions are generated. For this reason, in the case of test number 7, the L ₁₀ life is as short as 1.1 × 10 ⁷ .

In the case of test number 8, since the steel 8 used does not satisfy the equation [2], that is, [Ca / O <1250S-5.80], coarse oxide inclusions are generated. For this reason, in the case of test number 8, the L ₁₀ life is as short as 9.0 × 10 ⁶ .

In the case of test number 9, the steel 9 used does not satisfy the formula [1], that is, the value of Ca / O calculated from the contents of Ca and O exceeds the value specified in the present invention. Since it is 00, coarse inclusions mainly composed of CaO are generated. For this reason, in the case of test number 9, the L ₁₀ life is as short as 1.2 × 10 ⁷ .

In the case of test number 10, the used steel 10 has a Ca content of 0.0015% which exceeds the value specified in the present invention, and thus produces coarse oxide inclusions. For this reason, in the case of test number 10, the L ₁₀ life is as short as 8.0 × 10 ⁶ .

In the case of test number 11, the used steel 11 is 0.0001% in which the Ca content is lower than the value specified in the present invention, and the value of Ca / O is 0.11 lower than the value specified in the present invention. Further, since [2] is not satisfied [Ca / O <1250S-5.80], coarse and elongated sulfide inclusions and coarse oxide inclusions are generated. Therefore, in the case of Test No. 11, L ₁₀ life 5.0 × 10 ⁶ short.

In the case of test number 12, the used steel 12 has an S content of 0.0110% exceeding the value specified in the present invention, and does not satisfy the formula [2] [Ca / O <1250S-5.80]. Therefore, coarse and elongated sulfide inclusions and coarse oxide inclusions are generated. Therefore, in the case of Test No. 12, L ₁₀ life is short and 7.0 × 10 ^6.

In the case of test number 13, the used steel 13 is 0.0015% in which the O content exceeds the value specified in the present invention, and the Ca / O value is 0.47 lower than the value specified in the present invention. Therefore, coarse oxide inclusions are generated. For this reason, in the case of test number 13, the L ₁₀ life is as short as 1.2 × 10 ⁷ .

  The bearing steel of the present invention has good durability against damage due to rolling fatigue and has a long rolling fatigue life. Therefore, the ball bearings and rollers used in various industrial machines and automobiles are used. It can be suitably used as a material for rolling bearings such as “bearings”.

Claims (1)

In mass%, C: 0.95 to 1.2%, Si: 0.15 to 0.35%, Mn: 0.05 to 0.5%, P: 0.025% or less, S: 0.010 % Or less, Cr: 0.80 to 1.80%, Al: more than 0.005% to 0.040% or less, Ca: 0.0003 to 0.0010 % (excluding 0.0010%) and O: Bearing steel containing 0.0010% or less, the balance being Fe and impurities, and having a chemical composition satisfying the following formulas [1] to [3].
0.70 ≦ Ca / O ≦ 1.80 [1]
Ca / O ≧ 1250S-5.80 ... [2]
20 ≦ Mn / S ≦ 170 [3]
However, the element symbol in a formula means content (mass%) of each element.

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