JP2015129335A - Steel for carburized shaft bearing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for carburized shaft bearing having good durability to breakage due to rolling motion fatigue even under severe use environment and capable of achieving stable and excellent rolling motion fatigue life.SOLUTION: There is provided a steel for carburized shaft bearing containing, by mass%, C:0.1% or more and less than 0.4%, Si:0.02 to 1.3%, Mn:0.2 to 2.0%, P:0.05% or less, S:less than 0.010%, Cr:0.50 to 2.00%, Al:0.01 to 0.10%, Ca:0.0003 to 0.0030%, O:0.0030% or less and N:0.002 to 0.030% and the balance Fe with inevitable impurities and satisfying [0.7≤Ca/O≤2.0] and [Ca/O≥1250S-5.8]. The steel for carburized shaft bearing may contain one or more kind of element selected from Cu, Ni, Mo, V and Nb at a specific amount and/or B and Ti at specific amounts.

Description

  The present invention relates to a carburized bearing steel, and more particularly to a carburized bearing steel excellent in rolling fatigue characteristics (rolling fatigue life).

  High surface pressure repeatedly acts on mechanical structural parts such as bearings and automotive parts such as “constant velocity joints” and “hub units”. Therefore, the above parts (hereinafter collectively referred to as “rolling members”) require excellent rolling fatigue characteristics.

  In recent years, the usage environment of rolling members has become increasingly severe due to higher surface pressures and higher temperatures, for example, due to needs for higher engine output, smaller parts, and lighter weight. For this reason, the rolling member has been required to have a longer rolling fatigue life.

In response to the above requirements, the rolling member material is generally a non-metallic inclusion typified by Al ₂ O ₃ (hereinafter simply referred to as “inclusion”) that causes peeling of the rolling site. In some cases, it has been attempted to improve the rolling fatigue life by reducing as much as possible.

  However, for example, as described in Non-Patent Document 1, as a result of the reduction in the diameter of oxides due to recent progress in steelmaking technology, the size of sulfides may be relatively increased. In measures taken as an index, the variation in rolling fatigue life may increase.

  Therefore, recently, attempts have been made to reduce the content of O (oxygen) and S (sulfur) in steel, to reduce the size of oxides and sulfides, and to control the form of inclusions by a steelmaking process. For example, Patent Documents 1 and 2 disclose various techniques.

Specifically, in Patent Document 1, in mass%, C: 0.1 to 0.4%, Si: 0.4 to 3.0%, Mn: 0.3 to 1.5%, P: 0 0.03% or less, Ni: 0.25 to 3.5%, Cr: 0.3 to 5.0%, Al: 0.005 to 0.050%, O: 0.0015% or less, N: 0.00. It is 025% or less, and further contains a specific amount of Mo, V, Nb, “sol. B + Ti”, Se, Te, Pb, S, Ca, Bi, or two or more as necessary. In addition, the “rolling fatigue strength” is characterized in that the balance is an alloy composition substantially consisting of Fe and the abundance of alumina clusters having a length of 0.5 mm or more is 10 ⁻³ pieces / mm ³ or less. Has been disclosed.

In Patent Document 2, in mass%, C: 0.1-0.5%, Si: 0.01-1.5%, Mn: 0.3-1.8%, S: 0.001-0. 15%, Cr: 0.4-2.0%, Ti: 0.05-0.2%, Al: 0.04% or less, N: 0.0050% or less, P: 0.025% Hereinafter, it is limited to O: 0.0025% or less, and further contains one or two or more of Mg: 0.003% or less, Zr: 0.01% or less, and Ca: 0.005% or less. In addition, it contains any one or more of a specific amount of Nb, Mo, Ni, V, and B, the balance is made of iron and unavoidable impurities, and the precipitation amount of AlN is 0.01% or less. The equivalent circle diameter is over 20 μm, the aspect ratio is over 3, the sulfide density d (pieces / mm ² ), and the S content [S] (mass%) are d ≦ A “skin-hardened steel excellent in cold workability, machinability, and fatigue characteristics after carburizing and quenching” characterized by satisfying 1700 [S] +20 and its manufacturing method are disclosed.

JP 2000-297346 A International Publication No. 2010/116555

Misao Nagao et al .: Sanyo Technical Report Vol. 12 (2005) No. 1 1, p. 38

  Since the technique disclosed in Patent Document 1 described above is not considered for sulfides, coarse sulfides may exist, and an excellent rolling fatigue life may not be obtained.

  The technique disclosed in Patent Document 2 is not considered for oxides, and there is a possibility that stretched coarse oxides may exist, and therefore, excellent rolling fatigue life cannot be obtained. There is.

  This invention is made | formed in view of the said present condition, The objective is to provide the steel for carburized bearing which can improve a rolling fatigue characteristic more stably.

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

  Therefore, in order to solve the above-mentioned problems, the present inventors have studied focusing on the composition and form of inclusions, and obtained the following important findings (a) to (d).

  (A) By controlling the composition of the sulfide, specifically, for example, Ca is added to the molten steel, and the composition is controlled so that (Mn, Ca) S and CaS are contained in the sulfide. As a result, coarse sulfides that become a stress concentration source of rolling fatigue can be dispersed and reduced in diameter.

(B) Even when the content of O is reduced, when the oxide is mainly composed of Al ₂ O ₃ , it may agglomerate and coalesce and exist as coarse inclusions. There is a possibility that a good rolling fatigue life cannot be obtained.

(C) When the above-described addition of Ca is performed in the molten steel stage of aluminum killed steel (hereinafter referred to as “Al killed steel”), the deoxidation product Al ₂ O ₃ reacts with Ca to produce a low melting point composition. It changes to oxide (Al, Ca) O. At this time, the oxide is spheroidized in the molten steel, and aggregation coarsening is suppressed.

(D) Ca added to the molten steel of the above item (c) reacts with both S in the molten steel and Al ₂ O ₃ which is a deoxidation product of Al killed steel, and the two reactions compete. Therefore, the amount of Ca that reacts with Al ₂ O ₃ varies depending on the amount of S contained in the molten steel. Therefore, it is necessary to control the amounts of Ca, O, and S to an appropriate relationship in order to appropriately change the oxide composition at the molten steel stage to suppress coarsening and improve the rolling fatigue life.

  Therefore, the inventors next investigated the influence of Ca addition on the composition and morphology of oxides and sulfides.

  As a result, as shown in (e) and (f) below, if the [Ca / O] and S content, which is the ratio of the Ca content to the O content in the steel, is controlled within an appropriate range, the coarseness It has been found that the formation of oxides can be suppressed.

(E) The composition of the oxide after adding Ca can be expressed using [Ca / O] as an index. And
[0.7 ≦ Ca / O ≦ 2.0]
If this is satisfied, formation of coarse oxides can be suppressed.

  (F) However, when a large amount of S is contained, Ca also reacts with S. For this reason, even if the Ca amount and O amount contained in the steel satisfy the relational expression described in the above (e), depending on the amount of S present in the steel, it is not possible to appropriately control the composition of the oxide. In some cases, coarse oxides are formed. However, the Ca amount, O amount and S amount contained in the steel satisfy the above [0.7 ≦ Ca / O ≦ 2.0] and further satisfy [Ca / O ≧ 1250S−5.8]. Thus, the formation of stable and coarse oxides and point-sequence oxides can be suppressed.

  The present invention has been completed based on the above technical idea and knowledge based thereon, and the gist thereof is the following steel for carburized bearings.

(1) The chemical composition is mass%,
C: 0.1% or more and less than 0.4%, Si: 0.02 to 1.3%, Mn: 0.2 to 2.0%, P: 0.05% or less, S: less than 0.010% Cr: 0.50-2.00%, Al: 0.01-0.10%, Ca: 0.0003-0.0030%, O: 0.0030% or less, and N: 0.002-0. 030%,
The balance: Fe and impurities,
Carburized bearing steel that satisfies the following formulas [1] and [2].
0.7 ≦ Ca / O ≦ 2.0 ... [1]
Ca / O ≧ 1250S-5.8 [2]
However, the element symbol in a formula means content (mass%) of each element.

  (2) In place of a part of Fe, in mass%, Cu: 1.0% or less, Ni: 3.5% or less, and Mo: 1.5% or less, containing one or more of the above ( Steel for carburized bearing as described in 1).

  (3) It replaces with a part of Fe, and contains 1 or more types of V: 0.10% or less and Nb: 0.03% or less by mass%, as described in said (1) or (2) Steel for carburized bearings.

  (4) Carburizing according to any one of the above (1) to (3), which contains B: 0.0030% or less and Ti: less than 0.01% by mass% instead of part of Fe Steel for bearings.

  The rolling member made of the steel for carburized bearing according to the present invention has good durability against damage due to rolling fatigue, and has a stable and long rolling fatigue life even under the severe usage environment in recent years. Have For this reason, the carburized bearing steel of the present invention can be suitably used as a material for various rolling members such as mechanical structural parts such as bearings, and further, automotive parts such as constant velocity joints and hub units.

It is a figure explaining the influence which the amount of Ca / O and S in steel has on the formation of coarse oxides and dot-sequence-like oxides, and the region surrounded by a thick frame in the figure is for carburized bearings of the present invention This is the area that steel should fill. In an Example, it is a figure which shows the heat pattern of the "carburization quenching-tempering" performed to the shaped material whose diameter is 60 mm and thickness is 5.5 mm. “Cp” in the figure represents “carbon potential”. “OQ” represents “oil quenching”. Cooling after tempering was allowed to cool in the atmosphere, and was indicated as “AC” in the figure.

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

C: 0.1% or more and less than 0.4% C is an important element that affects the strength of the steel material of the present invention. In order to ensure the core strength of the component (the strength of the fabric of the component) when carburized and quenched, it is necessary to contain 0.1% or more of C. On the other hand, when 0.4% or more of C is contained, toughness and machinability are lowered. Therefore, the C content is set to 0.1% or more and less than 0.4%. The C content is preferably 0.12% or more, and more preferably 0.15% or more. The C content is preferably 0.35% or less, and more preferably 0.30% or less.

Si: 0.02-1.3%
Si is an element that has a large effect of improving hardenability and resistance to temper softening and also has an effect of improving rolling fatigue strength. However, when the Si content is less than 0.02%, the above effects are insufficient. On the other hand, when the Si content exceeds 1.3%, not only the effect of increasing the rolling fatigue strength is saturated, but also the toughness and machinability are significantly lowered. Therefore, the Si content is set to 0.02 to 1.3%. The Si content is preferably 0.10% or more, and more preferably 0.15% or more. Further, the Si content is preferably 0.70% or less, and more preferably 0.35% or less.

Mn: 0.2 to 2.0%
Mn is an element that dissolves in steel and increases the rolling fatigue strength of the steel and improves the hardenability of the steel. Further, Mn combines with S in the steel to form MnS and enhances the machinability of the steel. In order to obtain these effects, it is necessary to contain 0.2% or more of Mn. However, if the Mn content is excessive, the surface hardness after quenching becomes too high, and the toughness and machinability deteriorate. For this reason, an upper limit is set and the Mn content is set to 0.2 to 2.0%. When it is desired to improve hardenability and strength, the Mn content is preferably 0.6% or more. The Mn content is preferably 0.9% or less.

P: 0.05% or less P is contained in the steel as an impurity and segregates at the grain boundary to reduce the rolling fatigue life. In particular, when the content exceeds 0.05%, the rolling fatigue life is significantly reduced. Therefore, the content of P is set to 0.05% or less. The P content is preferably as low as possible, and is preferably 0.02% or less.

S: Less than 0.010% S is contained in steel as an impurity. S is an element that forms sulfides, and when the content is 0.010% or more, coarse sulfides remain, so that the rolling fatigue life is reduced. Therefore, the S content is less than 0.010%. From the viewpoint of improving the rolling fatigue life, the S content is preferably as low as possible, preferably 0.006% or less, more preferably 0.002% or less. Note that the S content also needs to satisfy the formula [2] described below.

Cr: 0.50 to 2.00%
Cr is an element effective for improving the hardenability of steel, strength and toughness after quenching and tempering. In order to obtain these effects, a Cr content of 0.50% or more is necessary. However, if Cr is contained in excess of 2.00%, the toughness is lowered and the machinability is also lowered. Therefore, the content of Cr is set to 0.50 to 2.00%. The Cr content is preferably 0.60% or more, and more preferably 1.50% or less.

Al: 0.01-0.10%
Al is an element used for deoxidizing in the refining process, and is an element having an effect of forming AlN to refine crystal grains. However, when the Al content is less than 0.01%, the above effect is insufficient. On the other hand, when Al is contained in excess of 0.10%, it tends to remain as a coarse oxide, leading to a reduction in rolling fatigue life. Therefore, the Al content is set to 0.01 to 0.10%. The Al content is preferably 0.02% or more, and preferably 0.04% or less.

Ca: 0.0003 to 0.0030%
Ca forms an appropriate amount of (Al, Ca) O as an oxide, and forms a solid solution in the sulfide to form (Mn, Ca) S and CaS. By forming (Al, Ca) O, the interfacial energy is reduced, and the cohesive strength of the oxide is reduced, thereby suppressing the coarsening of the oxide. This effect can suppress a decrease in rolling fatigue life. Further, for sulfides, there is an effect of suppressing stretching and coarsening by forming (Mn, Ca) S and 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 0.0030%, not only the above-mentioned effect is 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 0.0030%. The Ca content is preferably 0.0005% or more, and is preferably 0.0025% or less. It should be noted that the Ca content also needs to satisfy the following formulas [1] and [2].

O: 0.0030% or less Since O is an element that is contained in steel as an impurity and generates an oxide, its content needs to be reduced as much as possible. When the content of O increases, especially exceeding 0.0030%, it tends to remain as a coarse oxide, leading to a decrease in rolling fatigue life. Therefore, the content of O is set to 0.0030% or less. The O content is preferably 0.0025% or less, more preferably 0.0020% or less. In addition, although it is preferable to make content of O as small as possible, when the cost in steel manufacture is considered, the minimum becomes about 0.0005%. Note that the O content needs to satisfy the following formulas [1] and [2].

N: 0.002 to 0.030%
N combines with Al to produce AlN and serves to refine crystal grains. However, if the N content is less than 0.002%, the above effect is insufficient. On the other hand, when N is contained exceeding 0.030%, the strength of the steel is lowered. Therefore, the N content is set to 0.002 to 0.030%. In addition, in order to acquire the hardenability improvement effect, as mentioned later, when containing B and Ti, it is necessary to suppress the coupling | bonding of B and N as much as possible. Therefore, when B and Ti are contained, it is desirable that the upper limit of the N content be further lower than 0.030%. In this case, the more desirable upper limit is 0.004%.

Ca / O: 0.7 to 2.0, and (1250S-5.8) or more The carburized bearing steel of the present invention must have a chemical composition in which Ca / O satisfies the following formula [1].
0.7 ≦ Ca / O ≦ 2.0 ... [1]
However, the element symbol in a formula means content (mass%) of each element.

Ca / O is an index of the oxide composition after adding Ca. When Ca / O is less than 0.7, Al ₂ O ₃ does not completely change to (Al, Ca) O, which is a low-melting oxide, and is a coarse spinel or dot sequence mainly composed of Al ₂ O _3. This forms a group of Al ₂ O ₃ oxides and causes a reduction in rolling fatigue life. On the other hand, when Ca / O exceeds 2.0, a coarse oxide having a high melting point mainly composed of CaO or an oxide of dotted CaO is likely to be formed, resulting in a decrease in rolling fatigue life. Therefore, Ca / O was made into the range of 0.7-2.0.

The carburized bearing steel of the present invention must further have a chemical composition in which Ca / O satisfies the following formula [2].
Ca / O ≧ 1250S-5.8 [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 0.7 to 2.0, the added Ca reacts not only with the oxide but also with S in the steel. This is because it may not be possible to suppress alignment.

That is, in the case of [Ca / O <1250S−5.8], the amount of Ca consumed for the formation of CaS is increased, and the amount of Ca used for changing the oxide composition is insufficient. Therefore, spinel containing Al ₂ O ₃ as a main component is generated, resulting in oxide coarsening or formation of a group of point-like Al ₂ O ₃ oxides, which reduces the rolling fatigue life. To do. Therefore, [Ca / O ≧ 1250S−5.8] was set. In addition, 1250 which is a coefficient with respect to S means the decrease coefficient of Ca accompanying CaS formation.

  FIG. 1 shows the relationship between the aforementioned Ca / O and the amount of S in an organized manner. In addition, the area | region enclosed with the thick frame in a figure is an area | region which the steel for carburized bearings of this invention should satisfy | fill.

  In order to satisfy the above formulas [1] and [2], for example, the molten steel is deoxidized with Al, and then a Ca—Si alloy is added to adjust the amount of Ca.

  The carburized bearing steel of the present invention has a chemical composition composed of the above-described elements, with the balance being Fe and impurities.

  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 carburized bearing steel of the present invention may contain one or more elements selected from Cu, Ni, Mo, V and Nb and / or B and Ti instead of a part of the above-mentioned Fe. Good.

Cu: 1.0% or less Cu has an effect of increasing rolling fatigue strength. Cu also has the effect of improving hardenability. For this reason, you may contain Cu. However, even if the Cu content exceeds 1.0%, the above effect is saturated, so that the production cost of steel is increased. Therefore, an upper limit is set for the amount of Cu in the case of inclusion, and the amount is set to 1.0% or less. When Cu is contained, the amount of Cu is preferably 0.5% or less.

  On the other hand, in order to stably obtain the effect of Cu described above, the Cu content is preferably 0.05% or more, and more preferably 0.07% or more.

Ni: 3.5% or less Ni has an effect of increasing rolling fatigue strength. Ni also has the effect of improving hardenability and toughness. For this reason, Ni may be contained. However, even if the Ni content exceeds 3.5%, the above effect is saturated, so that the production cost of steel is increased. Therefore, an upper limit is set for the amount of Ni in the case of inclusion, and it is set to 3.5% or less. When Ni is contained, the amount of Ni is preferably 2.0% or less.

  On the other hand, in order to stably obtain the above-described effect of Ni, the Ni content is preferably 0.05% or more, and more preferably 0.07% or more.

Mo: 1.5% or less Mo has the effect of enhancing the hardenability of steel and increasing the rolling fatigue strength. Mo also has an effect of suppressing an incompletely quenched layer in the quenching process after carburizing. For this reason, you may contain Mo. However, when the Mo content is excessive, the machinability of the steel is lowered, and the manufacturing cost of the steel is increased. Therefore, an upper limit is set for the amount of Mo in the case of inclusion, and it is set to 1.5% or less. When Mo is contained, the amount of Mo is preferably 0.5% or less, and more preferably 0.3% or less.

  On the other hand, in order to stably obtain the effect of Mo described above, the Mo content is preferably 0.02% or more, and more preferably 0.03% or more.

  Said Cu, Ni, and Mo can be contained only in any one of them, or 2 or more types of composites. In addition, the upper limit of the total amount in the case of containing two or more of these elements in combination is 6.0%.

V: 0.10% or less V combines with N to form a nitride, and has the effect of suppressing coarsening of crystal grains. Furthermore, V also has an effect of increasing the strength of the base material by combining with C. Therefore, V may be contained. However, even if V is contained in excess of 0.10%, the effect of suppressing crystal grain coarsening is saturated, and the strength of the base material becomes too high, and the machinability may be lowered. Therefore, when V is contained, the content is set to 0.10% or less.

  On the other hand, the effect of V described above is stably obtained when the content is 0.01% or more. The lower limit of the V content is preferably 0.015%.

Nb: 0.03% or less Nb combines with N to form a nitride, and has the effect of suppressing coarsening of crystal grains. Furthermore, Nb also has the effect of increasing the strength of the base material by combining with C. Therefore, you may contain Nb. However, even if Nb is contained in excess of 0.03%, the effect of suppressing the coarsening of crystal grains is saturated, and a coarse compound may be generated, resulting in a decrease in rolling fatigue strength. Therefore, when Nb is contained, the content is set to 0.03% or less. The upper limit of the Nb content for ensuring sufficient rolling fatigue strength is preferably 0.025%.

On the other hand, the effect of Nb described above can be stably obtained when the content is 0.01% or more. The lower limit of the Nb content is preferably 0.012%.
Said V and Nb can be made to contain only any 1 type in them, or 2 types of composites. In addition, the upper limit of the total amount when these elements are contained in combination is 0.13%.

  In order to ensure better hardenability, the carburized bearing steel according to the present invention may contain a combination of the following amounts of B and Ti.

B: 0.0030% or less B is an element that can greatly improve the hardenability of steel with a trace amount, and can further increase the depth of the hardened layer necessary for the rolling part. Therefore, B may be contained. However, even if the B content exceeds 0.0030%, the effect is saturated. Therefore, when it contains B, the content was made into 0.0030% or less. The upper limit of the B content is preferably 0.0020%.

  On the other hand, the above-described effect of B can be stably obtained when the content is 0.0005% or more. The lower limit of the B content is preferably 0.0007%.

Ti: Less than 0.01% By containing B, the hardenability is improved when B is not a compound but exists in a solid solution state. Therefore, when B is combined with N to form BN, the effect of improving hardenability by B cannot be expected. Therefore, when the above amount of B is contained, Ti having a higher affinity with N than B and a stronger nitride forming ability is contained in combination. However, even when Ti is contained in an amount of 0.01% or more, not only the effect of fixing N is saturated, but also a large amount of coarse TiN is generated, so that rolling fatigue characteristics may be deteriorated. . Therefore, the content of Ti when combined with B is set to less than 0.01%.

  On the other hand, the effect of Ti described above is stably obtained when the content is 0.007% or more. The lower limit of the Ti content is preferably 0.008%.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Steels 1 to 28 having the chemical composition shown in Table 1 were melted using a vacuum melting furnace to produce a 150 kg steel ingot. At the time of melting, deoxidation treatment was performed with Al, and then Ca-Si alloy was added to adjust the Ca content.

  Steels 1 to 18 in Table 1 are steels whose chemical compositions are within the range defined by the present invention, and Steels 19 to 28 are comparative examples of steels whose chemical compositions deviate from the conditions defined by the present invention. .

  Each steel ingot is once cooled to room temperature, and then heated to a temperature in the temperature range of 1200 to 1300 ° C. according to the chemical composition, then hot forged at a finishing temperature of 1000 to 1150 ° C., and a round bar having a diameter of 80 mm did. The cooling after hot forging was allowed to cool in the atmosphere.

  Subsequently, each round bar having a diameter of 80 mm was heated at 850 ° C. for 30 minutes, and then subjected to a normalizing treatment in which it was allowed to cool to room temperature in the atmosphere.

  A shaped material having a diameter of 60 mm and a thickness of 5.5 mm so that the longitudinal direction of the round bar is the thickness of the shaped material from the center of the round bar having a diameter of 80 mm of steels 1 to 28 obtained as described above. Were sliced and collected.

  After rolling the above-mentioned shaped material having a diameter of 60 mm and a thickness of 5.5 mm with the heat pattern shown in FIG. Was prepared and subjected to a rolling fatigue test so that the lapped surface became the test surface. 2, “Cp” represents “carbon potential”, and “OQ” represents “oil quenching”. Cooling after tempering was allowed to cool in the atmosphere, and was represented as “AC” in FIG.

  The rolling fatigue test was performed using a thrust type rolling fatigue tester with a maximum contact surface pressure of 5230 MPa and a repetition rate of 1800 cpm (cycle per minute), with 10 tests. The test part was an annular region having a radius of 19.25 mm from the center of the test piece. As a steel ball (an opponent ball), a SUJ2 quenching-tempering ball defined in JIS G 4805 (2008) was used.

  Table 2 shows the detailed conditions of the rolling fatigue test.

Rolling contact fatigue test results are plotted on paper Weibull distribution probability, it was to evaluate the L ₁₀ life that shows a 10 percent probability of failure as a "rolling contact fatigue life". Note that the determination of the lifetime of the rolling fatigue life, the case where L ₁₀ life satisfies a 10.0 × 10 ⁶ or more and long life, which was targeted.

  Table 3 shows the rolling fatigue life determined as described above.

As shown in Table 3, in the case of test numbers 1 to 18 using steels 1 to 18 that satisfy the chemical composition defined in the present invention, the rolling fatigue life (L ₁₀ life) is 10.0 × 10 ⁶ or more. Thus, a long rolling fatigue life is obtained.

  On the other hand, in the case of test numbers 19 to 28, the chemical compositions of the steels 19 to 28 used deviate from the conditions defined 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.

Test No. 19 is because the steel 19 used does not satisfy the formula [1], that is, Ca / O is as high as 2.18 and exceeds the value specified in the present invention. It was easy to form a dot-like oxide, and the L ₁₀ life was as short as 6.06 × 10 ⁶ , which did not reach the target.

In Test No. 20, since the steel 20 used does not satisfy the formula [1], that is, Ca / O is as low as 0.56 and is lower than the value specified in the present invention, the oxide easily aggregates with a high melting point. As a result, Al ₂ O ₃ -based coarse oxides and dot-sequence oxides were easily formed, and the L ₁₀ life was as short as 3.16 × 10 ⁶ , which was not achieved.

Test No. 21 is because the steel 21 used does not satisfy the formula [2], that is, Ca / O is 1.14, [1250S-5.8] is 1.58, and [Ca / O <1250S. -5.8], a coarse oxide or dot-sequence oxide of Al ₂ O ₃ system is formed, and the L ₁₀ life is as short as 3.30 × 10 ^6, and the target has not been achieved. there were.

In the test number 22, the Ca content of the steel 22 used is as high as 0.0031%, which exceeds the value specified in the present invention. For this reason, coarse oxides and dot-sequence oxides are easily formed, and the L ₁₀ life is as short as 4.47 × 10 ^6, which is not achieved.

In Test No. 23, since the Ca content of the steel 23 used is as low as 0.0001% lower than the value specified in the present invention, Ca / O is as low as 0.10 and does not satisfy the formula [1]. , oxide becomes that easy aggregation of the refractory, so that coarse, L ₁₀ life is as short as 4.99 × 10 ^6, were not achieved the goal.

In the test number 24, the O content of the steel 24 used is as high as 0.0036%, which exceeds the value specified in the present invention. Therefore, it tends to remain much coarse oxides, L ₁₀ life is as short as 2.26 × 10 ^6, it was not achieved the goal.

In test number 25, the S content of the steel 25 used is as high as 0.0114%, which exceeds the value specified in the present invention, and does not satisfy the formula [2], that is, Ca / O is 0.80. , [1250S-5.8] is 8.45 and [Ca / O <1250S-5.8], so that a large amount of coarse sulfides are produced, and a large amount of Ca and S produces CaS. As a result, the amount of Ca that reacts with Al ₂ O ₃ is reduced, the oxide mainly composed of Al ₂ O ₃ having a high melting point is aggregated, and the L ₁₀ life is 1.54 × 10 ^{6, which} is short of the target. Met.

In Test No. 26, the steel 26 used did not satisfy the formula [1]. That is, although the steel 26 contained Cu and Ni in order to increase the rolling fatigue life, the Ca / O was 0. It is as low as 48, which is below the value specified in the present invention. Therefore, the oxide easily aggregates with a high melting point, and as a result, an Al ₂ O ₃ -based coarse oxide or dot-sequence oxide is easily formed, and the L ₁₀ life is 5.21 ×. It was as short as 10 ⁶ and the target was not achieved.

In Test No. 27, the steel 27 used does not satisfy the formula [1]. That is, although the steel 27 contains Nb in order to suppress the coarsening of crystal grains, its Ca / O is 2. 29, which is higher than the value specified in the present invention. Thus, the CaO-based, is likely to be formed is coarse oxides and point rows oxide, L ₁₀ life is as short as 7.32 × 10 ^6, were not achieved the goal.

In test number 28, the Nb content of the steel 28 used was as high as 0.0342%, exceeding the value specified in the present invention. Therefore, Nb is, coarse Nb (C, N) combines with C and N is _{produced, L 10} life is as short as 5.06 × 10 ^6, were not achieved the goal.

The rolling member made of the steel for carburized bearing according to the present invention has good durability against damage due to rolling fatigue, and has a stable and long rolling fatigue life even under the severe usage environment in recent years. Have For this reason, the carburized bearing steel of the present invention can be suitably used as a material for various rolling members such as mechanical structural parts such as bearings, and further, automotive parts such as constant velocity joints and hub units.

Claims (4)

Chemical composition is mass%,
C: 0.1% or more and less than 0.4%, Si: 0.02 to 1.3%, Mn: 0.2 to 2.0%, P: 0.05% or less, S: less than 0.010% Cr: 0.50-2.00%, Al: 0.01-0.10%, Ca: 0.0003-0.0030%, O: 0.0030% or less, and N: 0.002-0. 030%,
The balance: Fe and impurities,
Carburized bearing steel that satisfies the following formulas [1] and [2].
0.7 ≦ Ca / O ≦ 2.0 ... [1]
Ca / O ≧ 1250S-5.8 [2]
However, the element symbol in a formula means content (mass%) of each element.   It replaces with a part of Fe and contains 1 or more types of Cu: 1.0% or less, Ni: 3.5% or less, and Mo: 1.5% or less by the mass%. Steel for carburized bearings.   The carburized bearing steel according to claim 1 or 2, which contains at least one of V: 0.10% or less and Nb: 0.03% or less in mass% instead of a part of Fe. The carburized bearing steel according to any one of claims 1 to 3, which contains, by mass%, B: 0.0030% or less and Ti: less than 0.01% in place of part of Fe.