JP2016166398A - Bearing steel and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bearing steel capable of achieving excellent rolling fatigue life.SOLUTION: There is provided a bearing steel containing C:0.95 to 1.2%, Si:0.15 to 0.35%, Mn:0.05 to 0.5%, P:0.025% or less, S:0.0005 to 0.01%, Cr:0.80 to 1.80%, Al:0.005 to 0.04%, Ca:0.0003 to 0.0030%, Mg:0.0001 to 0.003%, O:0.0030% or less, Cu:0 to 1.0%, Ni:0 to 3.0%, Mo:0 to 0.15%, V:0 to 0.30%, Nb:0 to 0.10%, B:0 to 0.0030% and Ti:0 to 0.10%, where ultrasonic fatigue failure origin (A) has predictive maximum inclusion diameter √areaof 45 μm or less, (B) of which 50% or more contains both of oxide and sulfide of 5 mass% or more and has (C) the content when making three dimension oxide of CaO-AlO-MgO and (D) CaS and MnS contents in an average composition in specific range respectively.SELECTED DRAWING: None

Description

  The present invention relates to a high carbon chromium bearing steel excellent in rolling fatigue life.

  Bearing steel is used for rolling bearings such as “ball bearings” and “roller bearings”. In recent years, in bearings for automobiles, the use conditions have become increasingly severe due to higher surface pressures and higher temperatures due to the need for higher engine output and lighter parts. Rolling fatigue life has been demanded.

  It is known that the rolling fatigue life of bearing steel is reduced by non-metallic inclusions in the steel (hereinafter also simply referred to as “inclusions”), particularly oxides and sulfides. Therefore, the rolling fatigue life has been improved by reducing the amount of oxide and sulfide in the steel. However, simply reducing the content of oxides and sulfides does not ensure the desired good rolling fatigue life. In addition, the size of the oxides and sulfides in the steel is reduced to reduce rolling. It has been proposed to improve fatigue life.

In Patent Document 1, in steels for machine parts such as steels for machine structures and bearings that use a surface hardness of steel as 58 HRC or more when used for machine parts, non-metallic inclusion diameters are measured using an optical microscope. Measured and calculated by using extreme value statistical processing, the predicted value √area _max of the maximum inclusion diameter of sulfide in 30000 mm ² is 40 μm or less, “for a machine with excellent rolling fatigue life” Steel used in parts "is disclosed.

The inventors of the present invention also disclosed in Patent Document 2 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 0.0012% And O: 0.0010% or less, with the balance being Fe and impurities, and proposed bearing steel having chemical components satisfying the following [1] to [3] formulas.
0.70 ≦ Ca / O ≦ 1.80 [1]
Ca / O ≧ 1250S-5.80 [2]
20 ≦ Mn / S ≦ 170 [3]

In Patent Document 3, a steel material of high carbon chrome bearing steel in which mass%, O is 0.0010% or less, and S is 0.004% or less, in a longitudinal longitudinal section of the steel material, Inclusions whose maximum inclusion diameter √AREA predicted in 144 mm ³ is 45 μm or less when the distribution of the size of inclusions, which is the starting point of ultrasonic fatigue testing, is subjected to extreme value statistical processing. In the case where the average aspect ratio is 7 or less and the inclusion that is the starting point of fracture is an oxide or a sulfide, a “rolling bearing steel” is disclosed in which the inclusion composition is limited.

  As a method for evaluating the size of coarse inclusions that can be the starting point of fatigue fracture of these steels, Non-Patent Document 1 describes a method using extreme value statistics.

JP 2006-63402 A JP 2014-15632 A JP2012-62526A

Takayoshi Murakami: Metal fatigue Effect of minute defects and inclusions (1993), [Yokendo]

The steel disclosed in Patent Document 1 limits the maximum inclusion diameter expected in 30000 mm ² , but there is a possibility that coarse inclusions may exist in a larger region where fatigue failure may occur. Not considered.

Further, the bearing steel of Patent Document 2 proposed by the present inventors is one in which the amounts of Ca, O and S are controlled in order to suppress the aggregation and coarsening of Al ₂ O ₃ that occurs in the molten steel stage. This invention steel improves the rolling fatigue life by suppressing the formation of coarse Al ₂ O ₃ . However, the inclusion composition affecting the rolling fatigue life has not been examined in detail.

  The steel disclosed in Patent Document 3 has improved rolling fatigue characteristics by controlling the composition of oxide-based and sulfide-based inclusions that can be the starting point of fatigue fracture. However, there is a problem in that inclusions that are the actual starting points of destruction do not necessarily exist separately as oxides or sulfides. In addition, the control of the composition and form of inclusions in the molten steel stage and the solidification stage, which is necessary for rendering coarse inclusions harmless to rolling fatigue characteristics, is not considered.

  The present invention has been made in view of the above situation, and its purpose is to appropriately control the composition and form of non-metallic inclusions in steel, particularly the oxysulfide contained in the molten steel stage and the steel material after solidification. Accordingly, an object of the present invention is to provide a bearing steel that has good durability against breakage due to rolling fatigue and can ensure an excellent rolling fatigue life even under a severe use environment of the rolling member.

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.
Therefore, the present inventors have studied paying attention to the composition and form of inclusions, and obtained the following important findings (a) to (f).

(A) The composition of sulfide can be controlled by Ca. Specifically, when Ca is added to the molten steel, Ca is combined with S in the molten steel or at the solidification stage of the steel to generate (Mn, Ca) S and CaS. Due to the generation of (Mn, Ca) S and CaS, the diameter of the sulfide is reduced and dispersed, so that coarse MnS sulfide that becomes a stress concentration source of rolling fatigue is reduced.

(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. 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 deoxidized product Al ₂ O ₃ reacts with Ca, resulting in 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) When steel containing S is solidified, S is concentrated in the unsolidified molten steel, and at the same time, inclusions tend to be pushed into the molten steel. In the Al killed steel to which Ca is added, as the solidification progresses and S is concentrated in the molten steel, CaO contained in the spherical (Al, Ca) O becomes Al, Mg, S in the steel and 3CaO + 2 Al +. 3 S → Al ₂ O ₃ + 3CaS
CaO + Mg + S → MgO + CaS
Cause the reaction. Therefore, when the Al killed steel containing Ca, S, and Mg is solidified, the proportion of CaO in the oxide composition decreases, and the proportion of Al ₂ O ₃ and MgO increases. In particular, Al ₂ MgO ₄ , which is a highly stable oxide of Al and Mg, increases. Further, CaS generated by this reaction adheres to the periphery of the oxide and forms a complex inclusion with the oxide.

(E) The ratio of CaO in the oxide is reduced by the reaction shown in (d), and the Ca + S-added steel in which a complex inclusion with CaS is formed does not cause the reaction (d) (Al, Ca) O. However, the rolling fatigue life is improved as compared with the Ca-added steel that remains as a low melting point composition.

(F) Based on the above knowledge, in order to improve the rolling fatigue life, after changing the Al ₂ O ₃ to (Al, Ca) O by Ca in the molten steel stage to suppress aggregation, CaS formation in the solidification stage Thus, it is desirable to change the oxide rich in (Al, Ca) to the oxide rich in Al ₂ O ₃ and MgO to form a composite inclusion with CaS.
As the oxide, SiO ₂ , MnO, and the like are also produced, but the ratio to the whole oxide is low.

  The present invention has been completed based on the above technical idea and knowledge based thereon, and the gist of the present invention resides in the bearing steel shown in the following (1) and the manufacturing method of the bearing steel shown in (2).

(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: 0.0005-0.01%, Cr: 0.80-1.80%, Al: 0.005-0.04%, Ca: 0.0003-0.0030%, Mg: 0.0001-0. 003%, O: 0.0030% or less, N: 0.003 to 0.030%, Cu: 0 to 1.0%, Ni: 0 to 3.0%, Mo: 0 to 0.15%, V : 0 to 0.30%, Nb: 0 to 0.10%, B: 0 to 0.0030% and Ti: 0 to 0.10%, the balance having a composition consisting of Fe and impurities,
And the bearing which the inclusion which exists in a fracture starting point satisfy | fills the following (A)-(D) when carrying out the fatigue fracture of the ultrasonic fatigue test piece extract | collected perpendicularly to the rolling direction of steel materials is characterized by the above-mentioned steel.
(A) When the distribution of the inclusion diameter is subjected to extreme value statistical processing, the maximum inclusion diameter √area _max predicted in the test volume 144 mm ³ is 45 μm or less. (B) 50% or more of the inclusions (C) The oxide in the inclusion, which is a composite inclusion of oxide and sulfide, containing 5% or more by mass of both oxide and sulfide, and CaO—Al ₂ O ₃ —MgO ternary oxidation When it is regarded as an object, the content in mass% in the average composition is CaO: 0 to 20%, MgO: more than 10% and 40% or less. (D) CaS and MnS occupying the average composition of the entire inclusion The content in terms of mass% is in the range of 10 to 60% and 0 to 20%, respectively.

(2) In the deoxidation step in the refining of the steel having the composition described in (1) above, the addition order of deoxidation elements is C, Al, Ca, and when the molten steel is solidified in the mold, the mold Manufacture of bearing steel characterized in that the average cooling rate from the liquidus temperature to the solidus temperature is 50 ° C./min or less at ½ part of the distance from the inner surface of the short side center to the mold center Method.

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

In the Example of this invention, it is a figure which shows the position which cuts out a board | plate material from the round bar of (phi) 80, and the position which performs plate laser welding. In the Example of this invention, it is a figure which shows the position which cuts out the rough processed goods of an ultrasonic fatigue test piece from a board | plate material. It is a figure which shows the shape of the rough processed goods of the ultrasonic fatigue test piece in the Example of this invention. It is a figure which shows the finishing shape of the ultrasonic fatigue test piece in the Example of this invention.

  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 hardness during quenching and improves the rolling fatigue life, and needs to be 0.95% or more. However, if the C content exceeds 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, and the cold forgeability deteriorates. Invite. In addition, the hardness is increased, resulting in a decrease in tool life during cutting and burning cracks. A preferable lower limit of the C content is 0.97%. A preferable upper limit is 1.1%.

Si: 0.15-0.35%
Si is an element necessary for increasing the temper softening resistance and improving the rolling fatigue life, and must be contained by 0.15% or more. However, when Si is contained exceeding 0.35%, the hardness of the base material becomes high and the tool life at the time of cutting is reduced. A preferable lower limit of Si is 0.18%. A preferable upper limit is 0.30%.

Mn: 0.05 to 0.5%
Mn is an element necessary for improving the hardenability and improving the rolling fatigue life, and must be contained by 0.05% or more. However, even if Mn is contained in excess of 0.5%, the above effect is saturated, the base material becomes harder, and the tool life during cutting is reduced. Furthermore, it also causes burn cracking. A preferable lower limit of Mn is 0.10%. Moreover, a preferable upper limit is 0.45%.

P: 0.025% or less P is contained as an impurity in steel and segregates at grain boundaries to reduce the rolling fatigue life. In particular, when the content exceeds 0.025%, the rolling fatigue life is significantly reduced. The P content is preferably as low as possible, and is preferably 0.020% or less.

S: 0.0005% or more and less than 0.010% S is an important element that characterizes the present invention. S reacts with CaO in the spherical oxide during solidification of the steel to form a composite inclusion containing CaS, thereby increasing the ratio of Al ₂ O ₃ and MgO in the spherical oxide. Therefore, the content of S needs to be 0.0005% or more. However, when S is contained in an amount of 0.010% or more, it becomes excessive with respect to Ca, and S that remains without forming CaS forms Mn and MnS in the molten steel at the final stage of solidification. Since this tends to be coarse inclusions, it should be avoided from the viewpoint of improving the rolling fatigue life. The preferable S content is 0.006% or less, more desirably 0.004% or less.

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 content of Cr exceeds 1.80%, not only the above effects are saturated, but also cracking is likely to occur during the quenching process after making the final shape, and the rolling fatigue life is increased. Cause a decline. A preferable lower limit of the Cr content is 0.90%. Moreover, a preferable upper limit is 1.60%.

Al: 0.005-0.040%
Al has a deoxidizing action. In order to acquire this effect, it is necessary to contain Al 0.005% or more. However, if its content exceeds 0.040%, it tends to remain as a coarse oxide, and the rolling fatigue life is significantly reduced. The minimum with preferable Al content is 0.01%, and a preferable upper limit is 0.038%.

Ca: 0.0003 to 0.0030%
Ca forms an appropriate amount of (Al, Ca) O as an oxide. If (Al, Ca) O is formed, the interfacial energy between the molten steel and inclusions is reduced, and the cohesive strength of the oxide is reduced. Therefore, the coarsening of the oxide in steel is suppressed and the rolling fatigue life is increased. Further, Ca is combined with S in the steel to form CaS or (Mn, Ca) S, thereby suppressing S from collecting in the final solidified portion and forming coarse MnS. Each effect of Ca described above is exhibited when the Ca content is 0.0003% or more. However, if the Ca content exceeds 0.0030%, not only the above effects are saturated, but also the oxide has a composition that easily forms coarse inclusions containing a large amount of CaO, resulting in a rolling fatigue life. May be reduced. The upper limit with preferable Ca content is 0.0025%, More preferably, it is 0.0020%.

Mg: 0.0001% or more and 0.0030% or less Mg is combined with O in the oxide in the solidification stage, and promotes the reaction to change the oxide composition from one containing a lot of CaO to one containing a lot of (Al, Mg) O. To do. The effect of Mg is exhibited when the Mg content is 0.0001% or more. However, if Mg exceeds 0.0030%, a large amount of an oxide having a single composition of MgO is formed, which may cause a reduction in rolling fatigue life. The upper limit with preferable Mg amount is 0.0010%, More preferably, it is 0.0008%.

O: 0.0030% or less O is contained in steel as an impurity. When the content of O increases, and particularly exceeds 0.0030%, it remains as a coarse oxide in the steel, leading to a reduction in rolling fatigue life. The O content is preferably as low as possible, and the preferred upper limit is 0.0020%, more preferably 0.0015%.

N: 0.003-0.030%
N combines with Al in the steel to form AlN, and has the effect of suppressing crystal grain coarsening in the quenched portion. In order to obtain this effect, the N content needs to be 0.003% or more. However, if its content exceeds 0.030%, coarse nitrides are produced, which may lead to a decrease in rolling fatigue life.

  In addition, in order to obtain the effect of suppressing the coarsening of crystal grains in the quenched portion, when one or more of V and Nb described later are contained, nitrides of these elements must be generated. Therefore, when it contains 1 or more types of V and Nb, the minimum with preferable N content is 0.0050%.

  Moreover, in order to obtain the hardenability improvement effect at the time of induction hardening, when containing B and Ti so that it may mention later, it is necessary to suppress the coupling | bonding of B and N as much as possible. Therefore, when it contains B and Ti, the upper limit with preferable N content is 0.010%, More preferably, it is 0.008%.

  In addition, in order for Ca to form oxide (Al, Ca) O and to suppress aggregation and coarsening of the oxide more stably, a balance between the Ca content and the O content is important. Therefore, it is preferable to control so that 0.7 ≦ Ca / O ≦ 2.0.

  Further, S reacts with Ca in the oxide (Al, Ca) O in the solidification stage, thereby lowering CaO in the oxide. Therefore, it is preferable that S exists in an amount sufficient to form CaS and CaS in the molten steel and oxide. On the other hand, if S is present exceeding the amount that reacts with Ca, it causes coarse MnS generation. From the above, it is preferable that the relationship between the Ca content and the S content satisfies −0.0030 <Ca−1.25S <0.0005.

  The bearing steel of the present invention contains each of the above-mentioned elements, and the balance consists of Fe and impurities. The “impurities” are those that are mixed in from the ore, scrap, or production environment as a raw material when steel is industrially manufactured, and are allowed within a range that does not adversely affect the present invention. Means.

  The bearing steel material of the present invention may have 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. .

Cu: 1.0% or less Cu, like C and Mn, has the effect of ensuring the necessary hardness for the rolling part after quenching. Therefore, Cu may be contained. However, if the Cu content exceeds 1.0%, fatigue strength may be reduced, and hot workability may be reduced.
On the other hand, the effect of Cu described above is stably obtained when the content is 0.05% or more.

Ni: 3.0% or less Ni, like C and Mn, has the effect of ensuring the necessary hardness for the rolling part after quenching. Therefore, Ni may be included. However, if the Ni content exceeds 3.0%, fatigue strength may be reduced.
On the other hand, the effect of Ni described above can be stably obtained when the content is 0.05% or more.

Mo: 0.15% or less Mo, like C and Mn, also has an effect of ensuring the necessary hardness for the rolling part after quenching. Therefore, you may contain Mo. However, even if the Mo content exceeds 0.15%, the above effect is saturated and the cost is increased.
On the other hand, the effect of Mo described above is stably obtained when the content is 0.02% or more.

  Said Cu, Ni, and Mo can be contained only in any one of them, or 2 or more types of composites. The maximum amount when these elements are contained in combination may be up to 4.15%.

  Since the bearing steel according to the present invention is heated to a high temperature during quenching, it may contain one or more of V and Nb having an effect of suppressing grain coarsening within the following range.

V: 0.30% or less Since V combines with N to form a nitride, it has an effect of suppressing coarsening of crystal grains in the quenched portion. 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 it contains V exceeding 0.30%, the effect of suppressing the coarsening of the crystal grains in the quenched portion is saturated, and further the strength of the base material becomes too high and the machinability may be lowered. is there. The upper limit of the V content for securing sufficient machinability is preferably 0.20%.
On the other hand, the effect of V described above is stably obtained when the content is 0.01% or more.

Nb: 0.10% or less Since Nb combines with N to form a nitride, it has the effect of suppressing crystal grain coarsening in the quenched portion. 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 an amount exceeding 0.10%, the effect of suppressing the coarsening of the crystal grains in the quenched portion is saturated, and the strength of the base material becomes too high, and the machinability may be reduced. is there. The upper limit of the Nb content for ensuring sufficient machinability is preferably 0.05%.
On the other hand, the effect of Nb described above can be stably obtained when the content is 0.01% or more.

  Said V and Nb can be made to contain only any 1 type in them, or 2 types of composites. The total amount when these elements are contained in combination may be 0.40%.

In order to ensure better hardenability, the bearing steel material 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 small amount of inclusion and further increase the depth of the hardened layer necessary for the rolling part after quenching. Therefore, B may be contained. However, even if the B content exceeds 0.0030%, the effect is saturated.
On the other hand, the above-described effect of B can be stably obtained when the content is 0.0005% or more.

Ti: 0.10% or less 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 if Ti is contained in an amount exceeding 0.10%, 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. .
On the other hand, the effect of Ti described above can be stably obtained when the content is 0.005% or more.

(B) Non-metallic inclusions:
(B-1) Size of non-metallic inclusions In the present invention, when an ultrasonic fatigue test specimen collected in the direction perpendicular to the rolling direction of steel material is subjected to fatigue failure, the distribution of the diameters of inclusions present at the fracture starting point is determined. When the extreme value statistical processing is performed, the maximum inclusion diameter √area _max predicted in the test volume 144 mm ³ must be 45 μm or less. The test volume 144 mm ³ is the test volume of the rolling fatigue test piece as described below.

Hereinafter, a method for predicting the maximum inclusion diameter √area _max in the test volume of the rolling fatigue test piece by the extreme value statistical processing shown in Non-Patent Document 1 will be described.

First, a fatigue test using ultrasonic fatigue test specimens taken in the direction perpendicular to the rolling direction of the steel material is performed, and √area, which is the diameter of the inclusion existing at the fracture starting point after fatigue fracture, is tested for each steel type. Evaluate with a piece. n is an integer of 10 or more. √ When evaluating the area, observe the inclusions with a scanning electron microscope (SEM), measure the major axis and minor axis of the inclusion, and √area = (major axis x minor axis) ^1/2 The starting inclusion diameter √area is determined for each piece. When a plurality of inclusions exist in a row, they are collectively evaluated as one inclusion. However, if the gap between inclusions is larger than the smaller √area among the continuous inclusions, both are regarded as separated inclusions, and if the gap is smaller, both are combined into one It is a thing.

The test volume of the rolling fatigue test that is the predicted volume is V, and the volume of the ultrasonic fatigue test piece that is the inspection reference volume is V ₀ . The diameters √area of inclusions existing at the fracture starting points of n ultrasonic fatigue test specimens are arranged from the smaller one, and are set as √area _j (j = 1 to n), respectively. For these, the cumulative distribution function F _j and the standardization variable y _j are expressed as F _j = j / (n + 1)
y _j = −ln (−ln (F _j ))
Calculate with the following formula. Regarded as a linear relationship is established between} area _j and y _j, obtained relation} area _j and y _j by the least squares method, the distribution of the maximum inclusion diameter straight √area = a × y + b ··· (1)
Calculate

Then, from the predicted volume and the inspection reference volume, the recursion period T and the corresponding standardization variable y are set to T = (V + V ₀ ) / V _0.
y = −ln [−ln [(T−1) / T]]
√area when y is substituted into the maximum inclusion distribution line of the above equation (1) is taken as the predicted maximum inclusion diameter √area _max in the test volume of the rolling fatigue test piece.

The test volume (V) of the rolling fatigue test is the test volume of 10 test pieces of the Mori type rolling fatigue test used in the examples described later. The rolling fatigue characteristics are evaluated by L ₁₀ life, when its is for testing in ten rolling fatigue test piece. Since the rolling element used in the test has a diameter of 9.53 mm and a maximum contact stress of 5230 MPa, the width of the track on which the Hertz stress is applied is 0.7 mm, and the depth on which 90% or more of the maximum stress is applied is 0.17 mm. . Further, since the radius of the orbit is 19.25 mm, the test volume for one test piece is 2π × 19.25 mm × 0.7 mm × 0.17 mm = 14.4 mm ³ . Thus the test volume V of the rolling fatigue test is 14.4mm ³ × 10P = 144mm ^3.

Also, the ultrasonic fatigue test piece used in the examples described later has the shape shown in FIG. 3, and the test volume to which 90% or more of the maximum stress is applied is 46.1 mm ³ , so that V ₀ = 46.1 mm ^3. It is. When the standardized variable y corresponding to the recursion period of the embodiment of the present invention is obtained from V and V ₀ , y = 1.28.

(B-2) Ratio of composite inclusions of oxide and sulfide Below, the mass of both oxides and sulfides of n inclusions appearing at the fracture start point in the ultrasonic fatigue test piece of each steel type is mass. The ratio of oxide-sulfide composite inclusions containing 5% or more in% is called “composite inclusion ratio”. In the steel of the present invention, the composite inclusion ratio must be 50% or more. The fact that the ratio of the composite inclusions is out of this range is the inclusion control that characterizes the present invention, that is, the suppression of the aggregation of Al ₂ O ₃ oxide by Ca or the change of the composition due to the reaction with S does not occur. It means that there are inclusions remaining as clustered Al ₂ O ₃ or (Al, Ca) O. These inclusions are more harmful to rolling fatigue properties than composite inclusions whose oxide composition is controlled by reaction with Ca and S and whose predicted maximum inclusion diameter √area _max is equivalent. The rolling fatigue life is reduced.

(B-3) in the average composition invention of the oxide, while looking the oxide contained in the inclusions appearing in fracture origin of the ultrasonic fatigue test pieces and ternary oxides of CaO-Al ₂ O ₃ -MgO When it does, content in the mass% in the average composition must be in the range of CaO: 0-20%, MgO: more than 10% and 40% or less.

The reasons for limiting each oxide composition are shown below.
CaO: 0 to 20%
In the inclusions of Al killed steel, CaO forms (Al, Ca) O, which is a low melting point spherical inclusion. When CaO is present in an amount of 20% or more, the proportion of (Al, Ca) O in the oxide is as high as half or more, but (Al, Ca) O is an intervening having a lower Young's modulus than the steel matrix. Therefore, the load stress locally increases around the inclusion during rolling fatigue, and the generation and propagation of fatigue cracks is promoted, resulting in a decrease in rolling fatigue characteristics. Therefore, the CaO finally remaining in the oxide in the steel of the present invention is 20% or less.

MgO: More than 10% and 40% or less At the solidification stage of steel, concentrated dissolved S and CaO react to produce CaS, and the oxide composition is changed in a direction in which CaO decreases. This reaction is most accelerated under the conditions that the oxide Al ₂ MgO ₄ forms. Under the condition that MgO in the oxide is 10% or less in the stage after solidification, CaO in the oxide is not sufficiently reduced, and (Al, Ca) O harmful to rolling fatigue characteristics remains. ing. In addition, when the proportion of MgO in the total oxide greatly exceeds 28%, which is the proportion of MgO in Al ₂ MgO ₄ , particularly in excess of 40%, single MgO is likely to be formed, and these are agglomerated. It tends to be a coarse cluster.

(B-4) Content of sulfide in inclusions Furthermore, in the present invention, the content in mass% of CaS and MnS in the average composition of all inclusions appearing at the starting point of the ultrasonic fatigue test piece is Each must be in the range of 10-60% and 0-20%. The reasons for limiting the content of each sulfide are shown below.

CaS: 10-60%
In the steel specified in the present invention, the aggregation of Al ₂ O ₃ is suppressed by Ca at the stage of molten steel, and after the oxide becomes (Al, Ca) O, Ca in the oxide is changed to S at the stage of solidification. It is a steel characterized in that it reacts to become a composite inclusion of an oxide and CaS with a high proportion of Al ₂ O ₃ and MgO. A part of CaS generated in the solidification stage adheres to the oxide and becomes a composite inclusion. Therefore, when the composition and form of the inclusion are appropriately controlled, the composite inclusion appears at the starting point of ultrasonic fatigue. Contains CaS. Even if the final oxide composition is included in the range specified by (B-3), if the content of CaS in the entire inclusion is less than 10%, after solidification from the molten steel stage Until then, the amount of Ca contained in the oxide remains small, and it is considered that the suppression of the aggregation of Al ₂ O ₃ by Ca is insufficient. Moreover, when the content of CaS exceeds 60%, the content of Ca in the oxide is high at the stage of molten steel, and there is a single CaO, so that the suppression of the aggregation of the oxide becomes insufficient. .

MnS: 0 to 20%
Among the S in the molten steel, those that did not form Ca and CaS in the oxide form MnS in the final solidification stage. In the case where the average composition of inclusions that have become the starting point of fracture in the ultrasonic fatigue test contains 20% or more of MnS, not only oxides and CaS, and composite inclusions thereof, but mainly MnS Inclusions exist as coarse inclusions. These coarse inclusions mainly composed of MnS are detrimental to the rolling fatigue characteristics, and therefore, the amount of MnS produced needs to be suppressed to 20% or less. MnS may not be contained at all in the inclusion appearing at the starting point.

  In addition, the determination of the composite inclusion shown in (B-2) and the measurement of the oxide composition and the content of sulfide in the inclusion shown in (B-3) and (B-4) are as follows. (1) It carries out by calculating | requiring the quantity of each component of an inclusion by the method shown to (7).

(1) A surface analysis of energy dispersive X-ray spectroscopy is performed so as to include all the inclusions appearing at the fracture starting point of the ultrasonic fatigue test piece, and a spectrum corresponding to the average composition of the inclusions is obtained.
(2) The molar fraction of Mg, Al, S, Ca, Mn in inclusions is determined from the obtained spectrum. Hereinafter, the molar fraction of each element is [Mg], [Al], [S], [Ca], and [Mn].
(3) Since Mn is preferentially associated with S, the smaller of [S] and [Mn] is defined as the molar fraction of MnS in the entire inclusion. Hereinafter, this is referred to as {MnS}.
(4) Since the remaining S is combined with Ca, the smaller one of [S]-{MnS} and [Ca] is the molar fraction of CaS. Hereinafter, this is referred to as {CaS}.
(5) Ca that has not formed CaS forms an oxide. Therefore, [Ca]-{CaS} is the molar fraction of CaO.
(6) Since Mg and Al form oxides with inclusions starting from ultrasonic fatigue, [Mg] and [Al] / 2 are set to the molar fractions of MgO and Al ₂ O ₃ , respectively.
(7) About the inclusion of the fracture starting point of each test piece, the mass fraction of each compound was calculated | required from the molar fraction of each compound calculated | required by (3)-(6), and this was obtained from each test piece. By averaging the starting inclusions, the oxide composition and the CaS and MnS contents in the inclusions are determined.

  The bearing steel having the above-described chemical composition and inclusions is manufactured by the following refining process and casting process.

[Refining process]
In the refining process, molten steel is refined. The refining is, for example, a vacuum degassing process using RH (Ruhrstahl-Heraeus). In the bearing steel manufacturing method according to the present embodiment, the order in which deoxidizers are charged when refining molten steel is important. In the present invention, the raw materials for C, Al, and Ca, which are elements added during deoxidation, are added in this order.

The molten steel is deoxidized with C, and the generated CO is discharged out of the system, and then Al is introduced into the molten steel to deoxidize the molten steel. Preferably, the O content (total oxygen content) in the molten steel after Al deoxidation is set to 0.0030% or less. And after Al deoxidation, Ca is thrown into molten steel and deoxidation is performed. For the deoxidation with Ca, for example, addition of a Ca—Si alloy or CaO—CaF ₂ flux can be used. After deoxidation with Ca, refining including vacuum degassing may be further performed. Through the above steps, molten steel having the above chemical components and inclusion composition is produced.

The reason why C deoxidation is performed first is to reduce the amount of oxide generated by Al deoxidation by discharging oxygen out of the system as CO before Al deoxidation.
The reason why the deoxidation with Ca is performed after the Al deoxidation is that the Al ₂ O ₃ oxide is changed to (Al, Ca) O and aggregation is suppressed. Moreover, if Ca is added prior to Al, single CaO is generated and becomes a coarse cluster, which tends to remain in the steel material, which is harmful to rolling fatigue characteristics.

  The method and order of adding a trace amount of Mg, which is a strong deoxidizing element that forms MgO, is not particularly specified. For example, a part of the furnace wall containing MgO may be reduced so that Mg is mixed into the molten steel and the Mg amount condition defined in the claims of the present invention is satisfied.

[Casting process]
A slab is manufactured using the refined molten steel.
In this embodiment, it is preferable that the cooling rate RC of the slab during casting is 50 ° C./min or less. The cooling rate is an average cooling rate from the liquidus temperature to the solidus temperature at 1/2 part of the distance from the inner surface of the mold short side center to the mold center when the molten steel is solidified in the mold. . If the cooling rate RC exceeds 50 ° C./min, during the casting, the generated coarse oxide inclusions are trapped before floating, and as a result, the inclusion diameter tends to become coarse. Further, since the surrounding steel solidifies without sufficiently reacting with S concentrated in the unsolidified portion in the molten steel, the oxide remains containing a large amount of CaO.

The produced slab or steel ingot is hot-worked to produce a billet. The billet is made into a steel bar by hot working. At this time, it is desirable to perform rolling with a reduction ratio of 6 or more. Increasing the rolling reduction ratio tends to stretch relatively soft inclusions. However, if the rolling reduction ratio is 6 or more, a large difference occurs in the predicted maximum inclusion diameter √area _max measured in a cross section parallel to the rolling direction. Absent. The reduction ratio is a value obtained by dividing the cross-sectional area of the slab by the cross-sectional area of the rolled bearing steel obtained by the final reduction.

Steels 1 to 35 having the chemical composition of high carbon chromium bearing steel SUJ2 described in JIS G 4805 (2008) shown in Table 1 are melted using a vacuum melting furnace, cast into a SiO ₂ crucible, and steel ingot Was made. Table 2 shows the order of the elements added during deoxidation of the steel and the cooling rate RC during the subsequent casting. In the melting of steels 1 to 29 and 32 to 35, after deoxidizing C, deoxidation treatment was performed with Al, and then Ca—Si alloy was added to adjust the amount of Ca. Steel 30 was added with Al after adding C as a deoxidizing element after C deoxidation. Steel 31 was added with Al as a deoxidizing element, then C was added, and then Ca treatment was performed.

  The slab obtained by the above process was once cooled to room temperature, then heated to a temperature range of 1200 to 1300 ° C., and hot forged at a finishing temperature of 1000 ° C. or more to obtain a round bar having a diameter of 80 mm. The cooling after hot forging was carried out in the atmosphere.

Thickness (t) is 14 mm from the position shown in FIG. 1 of the R / 2 part, which is the intermediate position between the surface and the center of the round bar of the steel 1 to 35 obtained as described above with respect to the longitudinal cross section. After the plate material was cut out, the SM490 plate material was welded with an electron beam in the width direction of the plate material to make the width (W) 90 mm. In order to eliminate the influence at the time of welding, the plate material was normalized at 860 ° C. for 1 hour, heated to 795 ° C., held for 1.5 hours, cooled to 650 ° C. over 4 hours, and spheroidized. Annealed. Thereafter, from the position where the center of the test portion of the plate shown in FIG. 2 overlaps the center in the width direction of the plate, the ultrasonically processed fatigue test piece of the shape shown in FIG. After holding for a minute, it was oil-quenched and tempered at 180 ° C. for 1 hour. The rough processed test piece was finished into the shape shown in FIG. 4 to obtain an ultrasonic fatigue test piece.
The units of the dimensions shown in FIGS. 1 to 3 are all mm.

An ultrasonic fatigue test was performed on the ultrasonic fatigue test piece subjected to the above finishing process. Specifically, a fatigue test was performed until fracture occurred under the conditions of a frequency of 20 kHz, a stress amplitude of 800 MPa, and a stress ratio of -1. When the fracture did not occur until the number of repetitions reached 10 ⁷ , a fatigue test was performed with the stress ratio increased by 100 MPa. The √area, which is the starting inclusion diameter of the fractured specimen, was determined by observing the inclusion with a scanning electron microscope (SEM) and by the method described above.

The test volume of the ultrasonic fatigue test piece is 46 mm ^{3 to} which a stress of 90% or more of the maximum stress is applied. Then, with reference to Non-Patent Document 1, the predicted maximum inclusion diameter √area _max in 144 mm ³ corresponding to the rolling fatigue test piece 10P was determined by extreme value statistics.

  Surface analysis of energy dispersive X-ray spectroscopy was performed so as to include all inclusions appearing at the fracture starting point of the ultrasonic fatigue test specimen, and a spectrum corresponding to the average composition of the inclusions was obtained. The content of CaS and MnS contained in these inclusions and the average composition of the oxide were determined at each starting point, and the ratio of composite inclusions of each steel type was determined from the results. And the arithmetic average of these values was taken with the starting inclusion 10P of each steel type, and the content of CaS and MnS for each steel type and the average composition of the oxide were determined.

  Table 3 shows the results of the inclusion evaluation. Steels 1 to 19 are steels in which the chemical composition shown in Table 1 and the composition and form of inclusions shown in Table 3 are within the range defined in the present invention. Steels 20 to 29 are steels of comparative examples in which the chemical composition shown in Table 1 deviates from the range specified in the present invention. Steels 30 to 32 are shown in Table 3 because the chemical composition shown in Table 1 is within the range defined by the present invention, but the order of addition or cooling rate of the deoxidizing elements shown in Table 2 is not a preferred value. It is steel in which the composition or form of inclusions is out of the range defined in the present invention. Steels 33 to 35 have the chemical composition shown in Table 1 within the range specified in the present invention, but the relationship between the Ca and O amounts and the relationship between the Ca and S amounts are not preferable values. Steel whose composition or form is out of the conditions defined in the present invention.

  Moreover, the center part of the round bar with a diameter of 80 mm of steel 1-35 was cut | disconnected, and after hold | maintaining at 795 degreeC for 6 hours, furnace cooling was performed and spheroidizing annealing was performed. Thereafter, the shaped material having a diameter of 60 mm and a thickness of 6 mm was sliced and collected so that the longitudinal direction was the thickness of the shaped material. This shaped material was heated at 830 ° C. for 30 minutes, then oil-quenched and tempered at 180 ° C. for 1 hour.

  One side of the φ60 circular surface of this shaped material is mirror-finished and the opposite surface is ground to produce a rolling fatigue test piece having a thickness of 5 mm. The mirror-finished surface becomes the test surface. Thus, it used for the rolling fatigue test.

  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). The test part was an annular region having a radius of 19.25 mm from the center of the test piece. As the steel ball, SUJ2 tempered material defined in JIS G 4805 was used. Table 4 shows the detailed conditions of the fatigue test.

The rolling fatigue test results were plotted on a Weibull distribution probability paper, and the L ₁₀ life showing 10% failure probability was evaluated as “rolling fatigue life”. Note that the determination of the lifetime of the rolling fatigue life, the case where L ₁₀ life satisfied the 50 × 10 ⁶ or more as a long life, which was targeted. Table 5 shows the rolling fatigue life determined by the above method.

As shown in Tables 1 and 3, in the case of test numbers 1 to 19 using steels 1 to 19 satisfying the chemical composition and inclusions defined in the present invention, the rolling fatigue life (L ₁₀ ) is all A long rolling fatigue life is obtained at 51.4 × 10 ⁶ or more.

  On the other hand, in the case of test numbers 20 to 35, the chemical compositions or inclusions of the steels 11 to 26 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. 20 has a high Ca content of the used steel 20 of 0.0032%, which exceeds the value specified in the present invention. Therefore, even in the steel after solidification, the CaO fraction in the oxide and the predicted maximum inclusion The diameter √area _max deviates from the conditions specified by the present invention, and as a result, the L ₁₀ life is as short as 16.6 × 10 ⁶ .

In Test No. 21, the Ca content of the used steel 21 is as low as 0.0002%, which is lower than the value specified in the present invention. Therefore, the aggregation of Al ₂ O ₃ and the generation of coarse MnS can be suppressed by Ca. In addition, the ratio of the composite inclusion, the CaS content in the inclusion, and the predicted maximum inclusion diameter √area _max deviate from the conditions defined by the present invention, and as a result, the L ₁₀ life is as short as 13.8 × 10 ⁶ .

In Test No. 22, since the S content of the steel 22 used was as high as 0.0105% and exceeded the value specified in the present invention, a large amount of coarse MnS was generated and appeared at the ultrasonic fatigue starting point. Therefore, the ratio of composite inclusions, the contents of CaS and MnS in the inclusions, and the predicted maximum inclusion diameter √area _max deviate from the conditions defined by the present invention, and as a result, the L ₁₀ life is 6.5 × 10 ^6. And short.

In test number 23, the S content of the steel 23 used is as low as 0.0003%, which is lower than the value specified in the present invention. Therefore, the composition change due to the reaction of CaO with S in the solidification stage of the steel does not occur. The ratio of the composite inclusion, the CaS content in the inclusion, and the CaO content in the oxide deviate from the conditions defined by the present invention. As a result, the L ₁₀ life is as short as 12.5 × 10 ⁶ .

In Test No. 24, the O content of the used steel 24 is as high as 0.0032% and exceeds the value specified in the present invention, so that a large amount of oxide is generated and coarsened. Therefore, the predicted maximum inclusion diameter √area _max deviates from the conditions specified by the present invention, and as a result, the L ₁₀ life is as short as 10.1 × 10 ⁶ .

In Test No. 25, the Mg content of the steel 25 used was as high as 0.0033% and exceeded the value specified in the present invention, so that a large amount of oxide of MgO alone was generated to form coarse inclusions. Therefore, the ratio of composite inclusions, the fraction of MgO in the oxide, and the predicted maximum inclusion diameter √area _max deviate from the conditions defined by the present invention, and the L ₁₀ life is as short as 13.2 × 10 ⁶ .

Test No. 26 does not contain 1 ppm of Mg in the steel 26 used, and is lower than the value specified in the present invention. Therefore, no composition change occurs due to the reaction of CaO with S during the solidification stage of the steel. The ratio of inclusions, the content of CaS in the inclusions, and the fraction of CaO and MgO in the oxide deviate from the conditions defined by the present invention. As a result, the L ₁₀ life is as short as 12.1 × 10 ⁶ .

Test No. 27, as high as 1.23% C content of the steel 27 used, which exceeds the value regulated by the present invention, pro-eutectoid cementite cause quench cracking, resulting in L ₁₀ life 20.5 × 10 ⁶ and short.

In test number 28, the C content of the steel 28 used was as low as 0.91%, which is lower than the value specified in the present invention, so that the hardness of the steel material after quenching was not sufficiently obtained, and as a result, the L ₁₀ life Is as short as 16.4 × 10 ⁶ .

In Test No. 29, the Cr content of the steel 29 used was as high as 1.83% and exceeded the value specified in the present invention, so that cracking was likely to occur. As a result, the L ₁₀ life was 19.1 × 10 ^6. And short.

Although the test number 30 is included in the range of values specified in the present invention for the chemical components of the steel 30 used, the fraction of CaO in the oxide exceeds the range specified in the present invention. This is because Ca was added prior to the addition of Al, so CaO was produced and grew at the molten steel stage, and became coarse inclusions without suppressing oxide aggregation. In this steel, the predicted maximum inclusion diameter √area _max is also outside the conditions defined by the present invention, and the L ₁₀ life is as short as 8.8 × 10 ⁶ .

The test number 31 is included in the range of values defined in the present invention for the chemical components of the steel 31 used. However, since Al deoxidation was carried out before C deoxidation, Ca was insufficient with respect to Al ₂ O ₃ produced and remained in large quantities, so that the subsequent reaction of CaO and S did not occur sufficiently, and composite intervention The ratio of the product and the CaS content in the inclusions are below the range defined in the present invention. Further, in this steel, the aggregation and coarsening of Al ₂ O ₃ were not sufficiently suppressed by Ca, and the predicted maximum inclusion diameter √area _max deviated from the conditions stipulated by the present invention, so the L ₁₀ life was 12.3 × As short as 10 ⁶ .

In the test number 32, the chemical composition of the steel 32 used is included in the value range defined in the present invention. However, in this steel, since the cooling rate RC is high, CaO in the oxide remains without reacting with S in the solidification stage, and S remains as MnS. Therefore, the ratio of the composite inclusions, the contents of MnS and CaS in the inclusions, and the CaO fraction in the oxides deviate from the conditions defined in the present invention, and the L ₁₀ life is as short as 19.3 × 10 ^6. .

In the test number 33, the chemical composition of the steel 33 used is included in the range of values defined in the present invention. However, since Ca / O is higher than the preferred range, CaO becomes coarse in the molten steel stage, and after solidification, the content of CaS in inclusions, the fraction of CaO in the oxide, and the predicted maximum inclusion The diameter √area _max deviates from the conditions specified by the present invention, and as a result, the L ₁₀ life is as short as 8.4 × 10 ⁶ .

In the test number 34, the chemical composition of the steel 34 used is included in the value range defined in the present invention. However, since Ca / O is lower than the preferred range, the suppression of Al ₂ O ₃ aggregation by Ca does not occur sufficiently in the molten steel stage, and the ratio of composite inclusions and the predicted maximum inclusion diameter √area _max are It deviates from the conditions specified by the invention, and as a result, the L ₁₀ life is as short as 9.2 × 10 ⁶ .

In the test number 35, the chemical composition of the steel 35 used is included in the value range defined in the present invention. However, since Ca-1.25S was higher than the preferred range, Ca in the oxide remained even when CaO in the oxide reacted with S in the solidification stage. Therefore, the CaO fraction in the oxide deviates from the conditions specified by the present invention, and as a result, the L ₁₀ life is as short as 17.1 × 10 ⁶ .

  The bearing steel of the present invention has good durability against damage due to rolling fatigue and has a long rolling fatigue life. Therefore, it should be used suitably as a material for rolling bearings such as “ball bearings” and “roller bearings”. Can do.

Claims (2)

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: 0.0005 -0.01%, Cr: 0.80-1.80%, Al: 0.005-0.04%, Ca: 0.0003-0.0030%, Mg: 0.0001-0.003%, O: 0.0030% or less, Cu: 0 to 1.0%, Ni: 0 to 3.0%, Mo: 0 to 0.15%, V: 0 to 0.30%, Nb: 0 to 0. 10%, B: 0 to 0.0030% and Ti: 0 to 0.10%, the balance having a composition consisting of Fe and impurities,
And the bearing which the inclusion which exists in a fracture starting point satisfy | fills the following (A)-(D) when carrying out the fatigue fracture of the ultrasonic fatigue test piece extract | collected perpendicularly to the rolling direction of steel materials is characterized by the above-mentioned steel.
(A) When the distribution of the inclusion diameter is subjected to extreme value statistical processing, the maximum inclusion diameter √area _max predicted in the test volume 144 mm ³ is 45 μm or less. (B) 50% or more of the inclusions (C) The oxide in the inclusion, which is a composite inclusion of oxide and sulfide, containing 5% or more by mass of both oxide and sulfide, and CaO—Al ₂ O ₃ —MgO ternary oxidation When it is regarded as an object, the content in mass% in the average composition is CaO: 0 to 20%, MgO: more than 10% and 40% or less. (D) CaS and MnS occupying the average composition of the entire inclusion The content in terms of mass% is in the range of 10 to 60% and 0 to 20%, respectively.   In the deoxidation step in the refining of the steel having the composition according to claim 1, the deoxidation element is added in the order of C, Al, Ca, and when the molten steel is solidified in the mold, An average cooling rate from a liquidus temperature to a solidus temperature is 50 ° C./min or less at a half part of the distance from the inner surface to the mold center.