JP2013112861A - Steel bar for bearing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel bar for bearings, which is excellent in rolling fatigue life and can be cold-forged as rolled.SOLUTION: The steel bar for bearings of a chemical composition comprising C, Si, Mn, P, S, Cr, Al, Ca, and O each by a specified amount with the balance being Fe and impurities, is characterized in that the predicted maximum width of the inclusion is not greater than 20 μm and predicted maximum length thereof is not greater than 800 μm in the predicted volume of 144 mm, each obtained by applying statistics of extremes to the inclusion present at the starting point of fracture appearing in an ultrasonic fatigue test; if the inclusion present at the start point of fracture is an oxide, the oxide is one of specific binary, ternary and quaternary oxides of a mean composition comprising CaO: 2.0 to 20%, MgO: 0 to 20%, and SiO: 0 to 10% with the balance being AlO; if the inclusion present at the starting point of fracture is a sulfide, the sulfide is a mono-component sulfide, namely CaS of a mean composition comprising CaS: 100%, or a specific binary sulfide or ternary sulfide of a mean composition comprising CaS: at least 1.0% and MgS: 0 to 20% with the balance being MnS; and the maximum hardness from the surface of the steel bar to the position of R/2 is not greater than 290 by Vickers hardness.

Description

  The present invention relates to a steel bar for bearings, and in particular, mainly cold forging, which has excellent cold forgeability as it is rolled and excellent in rolling fatigue life without performing heat treatment such as spheroidizing annealing. The present invention relates to a steel bar for bearings used in applications.

  Bearing parts used in automobiles and industrial machines are generally made of rolled steel of high carbon chrome bearing steel represented by SUJ2-5 specified in JIS G 4805 (2008), and hot forging, cutting, etc. Through a so-called “secondary processing” step, the desired shape is finished.

  In recent years, from the viewpoint of reducing the manufacturing cost of bearing components and improving the yield, there is an increasing need for a manufacturing process for cold forging or cutting a bearing steel bar (hereinafter sometimes simply referred to as “bar steel”).

  However, the hot-rolled microstructure of high carbon chromium bearing steel is generally a single-phase structure of pearlite and has high hardness. Therefore, if the hot-rolled bearing steel bar is directly used for cold forging or cutting, problems such as cracking during cold forging and reduced tool life may occur. It has been considered difficult.

  For this reason, conventionally, as a pretreatment for cold forging and cutting, heat treatment for a long time exceeding 20 h called “spheroidizing annealing” is performed to change the microstructure to a mixed structure of ferrite and spherical cementite. I came.

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

  Therefore, a request to improve productivity by omitting the spheroidizing annealing as a pre-treatment or reducing the time even if it cannot be omitted to reduce energy consumption and equipment cost. Is getting bigger.

  In addition, a long rolling fatigue life is required as the performance of the bearing component as the final product. It is known that the above-mentioned rolling fatigue life (rolling fatigue characteristics) decreases due to non-metallic inclusions in steel (hereinafter also simply referred to as “inclusions”), particularly oxides. Therefore, conventionally, attempts have been made to reduce the oxygen content in steel by a steel making process. As a result, in recent years, it has become possible to stably produce a steel material in which the oxygen content is less than 10 ppm by mass, and the rolling fatigue life has been improved accordingly.

  However, with the recent increase in engine output and downsizing of peripheral parts, the demand for rolling fatigue life has become strict, and simply reducing the oxygen content ensures the required good rolling fatigue life. For this reason, there is an increasing demand for a steel material that can realize a longer rolling fatigue life than a rolling bearing.

  Thus, the present situation is that there is a demand for a steel bar for bearings that can be cold forged even if the spheroidizing annealing is omitted or simplified, and that has a significantly improved rolling fatigue life.

  In order to meet these demands, for example, Patent Document 1 discloses a technique that can omit or simplify the spheroidizing annealing.

  Specifically, in Patent Document 1, in mass%, C: 0.6 to 1.5%, Mn: 0.2 to 1.5%, Si: 0.05 to 1.2%, Cr: 0 0.5 to 2.5%, P: 0.03% or less, S: 0.02% or less, balance: iron and inevitable impurities, if necessary, Al: 0.01 to 0.03% , Cu: 0.2% or less, Ni: 0.2% or less, and Mo: 0.1% or less, a steel composition containing one or more, and an aspect ratio (major axis) of cementite The ratio of those having a (minor axis) of 2 or less is 70% or more, and “the wire rod / bearing steel having a spheroidized carbide structure as it is hot-rolled” and its manufacturing method are disclosed.

  Patent Documents 2 and 3 disclose techniques for improving the rolling fatigue life.

That is, in Patent Document 2, in steels for machine parts such as machine structural steels and bearing steels that have a surface hardness of 58 HRC or more when used for machine parts, non-metallic inclusions in a steel material cross section of 100 mm ² are used. The maximum inclusion diameter √area max of sulfide in 30000 mm ² calculated using extreme value statistical processing after measuring the maximum inclusion diameter of an object at 30 locations is 40 μm or less. “Steel used for machine parts having excellent rolling fatigue life” is disclosed.

Patent Document 3 is a steel for machine structural use in which the surface hardness of the steel when used for machine parts is 58 HRC or more, and O is 20 ppm or less and Al is less than 0.010% by mass ratio, When the inclusion diameter is defined as (longitudinal x lateral) ^1/2 , oxide-based nonmetallic inclusions having a maximum inclusion diameter existing in the speculum area of 3000 mm ² existing in the steel or inclusions of 15 μm or more Disclosed is a “steel used for machine parts having excellent rolling fatigue life” characterized in that the composition of all oxide-based nonmetallic inclusions having a diameter is SiO ₂ : 30% or more by mass%. ing.

  Furthermore, in the patent document 4, the present inventors are also able to ensure a microstructure that is inferior to that of conventional spheroidizing annealing in a bearing steel material containing a high amount of carbon and chromium with continuous rolling. Proposed.

Specifically, in the patent document 4, the present inventors, in mass%, C: 0.7-1.2%, Cr: 0.8-1.8%, Mn: 0.2-1. 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 all possible and 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 (3) The total reduction rate is 30% or more We have proposed a "Production Method of Bearing Steel" characterized by satisfying

JP 2006-63402 A JP 2009-242923 A JP 2008-240019 A JP 2009-275263 A

Zhou Seei et al .: Iron and Steel, 87 (2001) 12, p. 22 Seto Kozo: Series Iron and Steel Technology Flow Series 2 Volume 9 Steel for Bearings (1999), p. 79 [Supervision / Issue: Japan Iron and Steel Institute] Misao Nagao et al .: Sanyo Technical Report Vol. 12 (2005) No. 1 1, p. 38

  The bearing wire and steel bar disclosed in Patent Document 1 described above has a spheroidized carbide structure as it is in hot working, but the composition of inclusions cannot be controlled. For this reason, there are cases where the stretched inclusions cannot be avoided, and it cannot always be said that the rolling fatigue life can be stably extended.

  The steel disclosed in Patent Document 2 is locally excellent in rolling fatigue life, but is coarse when the total volume of a portion where fatigue damage such as actual bearing parts may occur is large. Inclusions may be present and may lead to early peeling.

  The steel disclosed in Patent Document 3 may not have an excellent rolling fatigue life because stretched coarse oxides and sulfides may exist.

  Therefore, the technologies disclosed in Patent Documents 2 and 3 do not always have sufficient rolling fatigue life under the severe usage environment of rolling bearings in recent years. In addition, these technologies do not give any consideration to the omission or simplification of the spheroidizing annealing after the steel bar rolling.

  Further, the specific purpose of the technique proposed by the present inventors in Patent Document 4 is to have a spheroidized structure as it is in rolling such as hot wire rod rolling and steel bar rolling, and to reduce the spheroidizing annealing time to half that of the prior art. It is to provide a method for producing a high carbon chromium bearing steel that can be shortened to the extent that it can be obtained, and furthermore, spherical cementite obtained by conventional spheroidizing annealing can be obtained without performing spheroidizing heat treatment. It is providing the manufacturing method of a high carbon chromium bearing steel material. For this reason, sufficient consideration is given to the moldability required for cold forging. However, in terms of product performance required as a final bearing part, particularly in terms of extending the rolling fatigue life, there are some cases where it is not always sufficient.

  The present invention has been made in view of the above situation, has good durability against damage due to rolling fatigue, can ensure an excellent rolling fatigue life, and omits spheroidizing annealing after rolling. Or it aims at providing the steel bar for bearings which can be cold forged even if it simplifies.

  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 at least reducing the length of inclusions observed in the longitudinal direction of the steel material. It is necessary to realize a long life, and it has been found that the length of inclusions can be shortened by controlling the composition of oxides and sulfides. In application 2010-207644, "rolling bearing steel" suitable for use as rolling bearing materials such as "ball bearings" and "roller bearings" can be stably obtained. did.

  After proposing the above technique, the present inventors have further investigated the extension of the rolling fatigue life.

  As a result, to increase the rolling fatigue life, it is very effective to control the length of inclusions to be short, but not only the length of inclusions but also the width of inclusions (inclusions) It has been found that the rolling fatigue life can be greatly extended by controlling the "length" and the dimension perpendicular to the "length" to be short.

  First, it has been found that the rolling fatigue life has a good correlation with the fatigue crack growth rate. The growth rate of this fatigue crack depends on the size of the initial crack. Therefore, it has become clear that in order to improve the rolling fatigue life, it is necessary to suppress the occurrence of the initial crack itself or to reduce the size of the initial crack as much as possible.

  The occurrence of the initial crack is affected by the width and length of the inclusions that are the starting point of the crack. For this reason, it came to the conclusion that controlling the width in addition to the length of the inclusions has the effect of suppressing the initial crack generation and reducing the size of the generated initial crack.

  Next, based on the technical idea described above, the present inventors have examined the width and length of inclusions necessary for extending the rolling fatigue life.

As a result, the maximum inclusion width W predicted in the estimated predicted volume 144 mm ³ for 10 test pieces in the thrust type rolling fatigue test for evaluating the rolling fatigue life is 20 μm or less, and the maximum expected inclusion is It was confirmed that if the object length L is 800 μm or less, the rolling fatigue life can be further extended.

  As a result of further studies, the following important findings (a) to (c) were obtained regarding the adjustment of the width and length of inclusions.

  (A) By controlling the composition of the oxide and sulfide, that is, by controlling the composition so that an appropriate amount of CaO is contained in the oxide and CaS is contained in the sulfide, Can be shortened.

  (B) By containing a trace amount of Ca in the steel, not only the composition of the inclusions in (a) can be controlled, but also the inclusions can be dispersed. Usually, when a small amount of Ca is not contained, inclusions are unevenly distributed and clustered. Therefore, even if the length of each inclusion can be shortened, the clustering is not effective for fatigue initial crack formation. The inclusions exhibit the same behavior as one coarse inclusion, and the rolling fatigue life is significantly reduced. However, by including a trace amount of Ca, the width of inclusions can be reduced by two effects of dispersion of inclusions and shortening of the length of inclusions.

(C) When a steel bar with controlled inclusions is manufactured, when evaluating whether the desired inclusion composition and form are obtained, particularly when evaluating the form of inclusions, the inclusions have a three-dimensional shape. Therefore, it is conceivable to underestimate in a so-called two-dimensional evaluation in which an arbitrary cross section of the steel bar is microscopically observed. Therefore, it is necessary to perform an evaluation in which a sufficient volume in three dimensions is secured. For this purpose, the fatigue test piece collected so that the diameter direction of the steel bar and the axial direction of the test piece are parallel to each other is subjected to a tempering treatment, and an ultrasonic fatigue test is performed using the fatigue test piece to perform the fracture. In accordance with the method shown in Non-Patent Document 1, it is effective to evaluate the inclusion form that has become a fatigue start point in a state where a sufficient volume is secured in three dimensions by extreme value statistical processing. It is confirmed whether the maximum inclusion width W predicted in the estimated predicted volume 144 mm ³ in the rolling fatigue test is 20 μm or less and the predicted maximum inclusion length L is 800 μm or less. That's fine.

  The present inventor further performed the spheroidizing annealing after rolling based on the technical idea of omitting the spheroidizing annealing described above and the technical idea of extending the rolling fatigue life which is the product performance of the bearing component. The microstructure of steel bars that can be cold forged even if omitted is investigated.

  As a result, it was found that the microstructure of the rolled material and the crack generation (formability) during cold forging have a correlation, and the following findings (d) to (h) were obtained.

  (D) The microstructure of the rolled steel bars for bearings usually has a pro-eutectoid cementite in a very small portion. However, when the microstructure is observed with an optical microscope or the like, the area occupancy ratio of pearlite (hereinafter simply referred to as a simple structure). “Area ratio”) is almost 100%. Since this pearlite is hard and has a reduced ductility, so-called cracking occurs when cold forging is performed.

  (E) In order to suppress the occurrence of the above cracks, the ductility decreases from the surface of the steel bar where cracking is a problem to the R / 2 part (“R” represents the radius of the steel bar). It is necessary to suppress pearlite as much as possible. Specifically, in order to suppress the occurrence of cracks during cold forging, it is necessary to suppress at least the pearlite area ratio to 40% or less, to suppress lamellar cementite generated during the cooling process of steel bar rolling, May be formed. That is, the microstructure of R / 2 part from the surface of the steel bar may be a structure mainly composed of spherical cementite.

  (F) As conditions in the steel bar rolling process that is the final reduction process among the two or more reduction processes in which the total reduction ratio is 15 or more, the heating conditions of the material to be reduced and the surface temperature of the material to be reduced during the reduction process By controlling more strictly, the steel bar having the structure described in the item (e) can be stably built.

  (G) There is a correlation between the area ratio of pearlite and the hardness of the rolled steel bar. When the pearlite area ratio is 40% or less, the hardness is 290 or less in terms of Vickers hardness. The Vickers hardness when the pearlite area ratio is 0% is about 200.

  (H) In order to stably suppress the occurrence of cracks during cold forging, the hardness is preferably 270 or less in terms of Vickers hardness. At that time, the pearlite area ratio is 30% or less.

  The present invention has been completed based on the above technical idea and knowledge based on the technical idea, and the gist thereof is a bearing bar shown in the following (1).

(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 0.0015%, and O: 0.0010% or less And the balance is a steel bar for bearings having a chemical component consisting of Fe and impurities,
The fatigue test piece collected so that the diameter direction of the steel bar and the axial direction of the test piece are parallel to each other is subjected to a tempering treatment, and an ultrasonic fatigue test is performed using the fatigue test piece to be destroyed.
In the case of the fracture origin inclusions oxide, oxide has an average composition, in mass%, CaO: 2.0~20%, MgO : 0~20% and SiO _2: with 0 to 10% And the balance is Al ₂ O ₃ , a binary oxide of CaO and Al ₂ O _3, a ternary oxide of CaO, MgO and Al ₂ O ₃ , CaO, SiO ₂ and Al ₂ O ₃ It consists of any one of a ternary oxide and a quaternary oxide of CaO, MgO, SiO ₂ and Al ₂ O ₃ ,
And when the said fracture origin inclusion is a sulfide, this sulfide has an average composition of mass%, CaS: 100% of CaS univalent sulfide, or CaS: 1.0% or more. MgS: 0 to 20%, and the balance is MnS, consisting of a binary sulfide of CaS and MnS or a ternary sulfide of CaS, MgS and MnS, and
The maximum inclusion width W predicted in the estimated predicted volume 144 mm ³ when the extreme value statistical processing is performed on the width and length of the above-described fracture origin inclusions without distinguishing between oxides and sulfides. 20 μm or less and the maximum inclusion length L predicted in the estimated predicted volume 144 mm ³ is 800 μm or less,
Furthermore, the maximum hardness from the surface of the steel bar to the R / 2 part position is 290 or less in terms of Vickers hardness,
A steel bar for bearings with excellent cold forgeability.
“R” represents the radius of the steel bar for bearing.

  The “impurities” in the remaining “Fe and impurities” are mixed from ore, scrap, or production environment as raw materials when industrially producing steel, and have an adverse effect on the present invention. It means what is allowed in the range.

In the following description, the above-mentioned “maximum inclusion width W predicted in the estimated predicted volume 144 mm ³ ” is omitted and referred to as “predicted maximum inclusion width W”, and similarly, “evaluated predicted volume 144 mm”. ³ is abbreviated as “predicted maximum inclusion length L”.

  Hereinafter, the invention related to the bearing steel bar (1) is referred to as “the present invention”.

  The bearing steel bar of the present invention has good durability against damage due to rolling fatigue and has a long rolling fatigue life, so that "ball bearings" and " It can be suitably used as a material for a rolling bearing such as a “roller bearing”. Moreover, since the steel bars for bearings of the present invention are excellent in cold forgeability, spheroidizing annealing after rolling can be omitted, and the manufacturing cost can be reduced.

It is a figure explaining the relationship between the area ratio of pearlite, and the Vickers hardness of a steel bar rolled material. It is a figure explaining the method which extract | collected the ultrasonic fatigue test piece used in the Example from the steel bar of diameter 70mm. The unit of the dimension in the figure is “mm”. It is a figure which shows the rough shape as cut out from the board | plate material of the ultrasonic fatigue test piece used in the Example. The unit of the dimension in the figure is “mm”. It is a figure which shows the finishing shape of the ultrasonic fatigue test piece used in the Example. The unit of the dimension in the figure is “mm”. It is a figure which shows the composition analysis method of the fracture origin inclusion observed on the fracture surface after an ultrasonic fatigue test. In the figure, (a) shows the case where the starting inclusion is an oxide, and (b) shows the case where the starting inclusion is a sulfide type. The 46 mm ³ in the range of up to 90% of the maximum stress of the ultrasonic fatigue test pieces used in Examples is a diagram for explaining a reference volume.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" of content of each element, oxide, and sulfide means "mass%".

(A) Chemical composition of steel bar for bearing:
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 cause cracking. 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, when Si is contained exceeding 0.35%, the hardness of the base material becomes high and the tool life at the time of cutting after forging 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, when Mn is contained exceeding 0.5%, the hardness of the base material becomes high, and the tool life at the time of cutting after forging 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%.

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%.

Cr: 0.80 to 1.80%
Cr has the effect of enhancing the hardenability of the steel and thermally stabilizing the cementite and inhibiting solid solution of the 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 during the quenching process after making the final shape, it is easy to cause cracking, fatigue resistance, etc. The mechanical properties are degraded. 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 0.0015%
Ca has the effect of suppressing stretching and coarsening of sulfide inclusions by forming a solid solution in the sulfide inclusions and forming CaS. Furthermore, since the crystallization form is changed by forming CaS, there is an effect that the sulfide inclusions are uniformly dispersed. Moreover, by forming oxide inclusions, there is an effect of uniformly dispersing oxide inclusions, and these effects can suppress a decrease in rolling fatigue life. These effects are exhibited when the Ca content is 0.0003% or more. However, if the Ca content exceeds 0.0015%, not only the above effects are saturated, but also coarse oxide inclusions are produced, leading to a reduction in rolling fatigue life. Therefore, the content of Ca is set to 0.0003 to 0.0015%. In addition, the minimum with preferable Ca content is 0.0004%. A preferred upper limit is 0.0013%.

O: 0.0010% or less O 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.

  For the above reasons, the steel bar for bearings according to the present invention contains C, Si, Mn, P, S, Cr, Al, Ca and O in the above-described range, and the balance is composed of chemical components of Fe and impurities. Stipulated.

  In addition, as impurities of the bearing steel bar described above, mixing of Cu, Ni, Mo and Ti from scrap cannot be avoided, but the content of elements such as Cu and Ni that do not form carbides is Cu: 0.00. If it is about 2% or less and Ni: about 0.25% or less, it does not affect the rolling fatigue life or the microstructure formation during forging. On the other hand, in the case of an element that forms a carbide among impurities, it is preferable that the content of Mo in particular be 0.08% or less.

  Further, when coarse TiN is generated, the rolling fatigue life is remarkably lowered, so that the Ti content in the impurities is preferably 0.003% or less.

(B) Average composition of inclusions of fracture origin when ultrasonic fatigue test is performed:
(B-1) When the fracture origin inclusion is an oxide:
The bearing steel bar according to the present invention is subjected to a tempering treatment on a fatigue test piece collected so that the diameter direction of the steel bar is parallel to the axial direction of the test piece, and an ultrasonic fatigue test is performed using the fatigue test piece. in the case of a fracture origin inclusions oxides when performing the oxide has an average composition, in mass%, CaO: 2.0~20%, MgO : 0~20% and SiO _2: 0 in 10%, and is an Al ₂ O ₃ balance, binary oxide of CaO and Al ₂ O _3, CaO, 3 elemental oxides of MgO and Al ₂ O _3, CaO, SiO ₂ and Al It must be made of any one of ₂ O ₃ ternary oxides and CaO, MgO, SiO ₂ and Al ₂ O ₃ quaternary oxides. Hereinafter, the content in the average composition of oxide in mass% is also referred to as “concentration”.

“Oxide” in the present invention is based on a quaternary system of CaO, MgO, SiO ₂ and Al ₂ O ₃ , and when the average composition of the oxide is in the above range, Generation of a long stretched or point-row-like coarse oxide is suppressed, and an excellent rolling fatigue life can be ensured even in a severe use environment.

  The reasons for limiting each oxide composition are shown below.

CaO: 2.0-20%
CaO, which is a basic oxide, is one of the main components of slag and is used as a solvent medium during desulfurization. When the CaO concentration is 2.0% or more, an effect of suppressing the generation of Al ₂ O ₃ and spinel that are long stretched or dot-like is obtained. On the other hand, when the CaO concentration exceeds 20%, coarse oxides mainly composed of large-scale CaO are generated. Therefore, the CaO concentration in the average composition of the oxide is set to 2.0 to 20%.

MgO: 0 to 20%
Since MgO is a basic oxide and has low solubility, it is crystallized as a hard MgO (periclase) phase, and further together with Al ₂ O ₃ as an MgO.Al ₂ O ₃ (spinel) phase. Since these may become coarse oxides in the form of point sequences and remain in the steel material and reduce the rolling fatigue life, an upper limit is set for the MgO concentration and limited to 20% or less. Note that MgO may not be present in the oxide. For this reason, the MgO concentration in the average composition of the oxide is set to 0 to 20%.

SiO ₂ : 0 to 10%
SiO ₂ , which is an acidic oxide, is one of the main components of slag, and may be contained in the oxide. Up to 10% is acceptable, but if it exceeds 10%, the oxide stretches. It may become coarse and a rolling fatigue life may fall. Note that SiO ₂ may not be present in the oxide. Therefore, the SiO ₂ concentration in the average composition of the oxide is set to 0 to 10%.

When the CaO concentration is 2.0% or more, Al ₂ O ₃ which is the remainder of the oxide is suppressed from being stretched for a long time or to be in a dotted line shape. Therefore, the concentration of Al ₂ O ₃ as the balance, and a binary oxide of CaO and Al ₂ O _3, may be 98.0% for CaO concentration of 2.0%.

For the reasons described above, the bearing steel according to the present invention has an oxide having an average composition of mass% and a CaO: 2.0 to 20% MgO: 0 to 20% and SiO _2: 0-10% and the balance is a Al ₂ O _3, 2 elemental oxides of CaO and Al ₂ O _3, CaO, MgO one of Al 3 elemental oxides of ₂ O _3, CaO, ₃ elemental oxides of SiO ₂ and Al ₂ O ₃ and CaO, MgO, 4 elemental oxides of SiO ₂ and Al ₂ O ₃ and It was defined as consisting of

  In addition, what is necessary is just to obtain | require the average composition of the said oxide as follows.

  With respect to the fracture origin inclusions observed on the fracture surface during the ultrasonic fatigue test, first, whether the fracture origin inclusion is an oxide or a sulfide is determined by energy dispersive X-ray spectroscopy. Check. After confirming that it is an oxide, as shown in FIG. 5A, the oxide is subjected to composition analysis by point analysis at an arbitrary plurality of locations (for example, 5 locations). The same applies to all the test pieces in which the fracture origin inclusion was confirmed to be an oxide in the ultrasonic fatigue test (for example, seven in the case of test number 1 shown in Table 3 of Examples described later). The composition analysis is performed, and the obtained composition is arithmetically averaged (for example, in the case of test number 1 in Table 3 above, 5 points × 7 pieces = 35 arithmetic average).

(B-2) When the fracture starting inclusion is a sulfide:
The bearing steel bar according to the present invention is subjected to a tempering treatment on a fatigue test piece collected so that the diameter direction of the steel bar is parallel to the axial direction of the test piece, and an ultrasonic fatigue test is performed using the fatigue test piece. In the case where the fracture origin inclusion at the time of carrying out is sulfide, the sulfide has a mean composition of mass% and CaS: 100% of CaS unidirectional sulfide, or CaS: 1.0 % Or more, MgS: 0 to 20%, and the balance is MnS, and must be made of a binary sulfide of CaS and MnS or a ternary sulfide of CaS, MgS and MnS. Hereinafter, the content in the average composition of sulfide in mass% is also referred to as “concentration”.

  The term “sulfide” as used in the present invention is based on a ternary system of CaS, MgS and MnS, and when the average composition of the sulfide is in the above range, a stretched coarse sulfide Is suppressed, and an excellent rolling fatigue life can be ensured even in a severe use environment.

  Below, the reason for limitation of each sulfide composition is shown.

CaS: 1.0% or more CaS is a sulfide generated by a desulfurization reaction. When the CaS concentration is 1.0% or more, an effect of suppressing the formation of stretched coarse sulfide is obtained. Even if only CaS exists as a sulfide, that is, the CaS concentration may be 100%. Therefore, the CaS concentration in the average composition of sulfide is set to 1.0% or more.

  Note that the CaS concentration in the case where the sulfide is made of a binary sulfide of CaS and MnS or a ternary sulfide of CaS, MgS and MnS may be a value close to 100%.

MgS: 0 to 20%
In the refining stage, Mg may be taken into the steel and MgS may be mixed into the sulfide. If the MgS concentration exceeds 20%, the MgO concentration in the above-described oxide increases, leading to the generation of coarse oxides in the form of dotted lines, so the MgS concentration is limited to 20% or less. Note that MgS may not be present in the sulfide. Therefore, the MgS concentration in the average composition of sulfides is set to 0 to 20%.

  Note that when the CaS concentration is 1.0% or more, the formation of stretched coarse sulfides is suppressed. Therefore, the concentration of MnS as the balance may be 99.0% when the CaS concentration is 1.0% in the case of a binary sulfide of CaS and MnS. In the case of ternary sulfides of CaS, MgS and MnS, the CaS concentration may be 1.0% and the value close to 99.0% when the MgS concentration is close to 0%. .

  For the reasons described above, the bearing steel according to the present invention has an average composition of mass% when the fracture starting inclusions in the ultrasonic fatigue test are sulfides. 100% CaS monosulfide, or CaS: 1.0% or more, MgS: 0 to 20% and the balance is MnS, and CaS and MnS binary sulfide or CaS, MgS And ternary sulfides of MnS.

  In addition, what is necessary is just to obtain | require the average composition of the said sulfide as follows similarly to the average composition of an oxide.

  With respect to the fracture origin inclusions observed on the fracture surface during the ultrasonic fatigue test, first, whether the fracture origin inclusion is an oxide or a sulfide is determined by energy dispersive X-ray spectroscopy. Check. After confirming that it is a sulfide, as shown in FIG. 5 (b), the sulfide is subjected to composition analysis by point analysis at an arbitrary plurality of locations (for example, 5 locations). The same applies to all the specimens in which the fracture origin inclusion was confirmed to be sulfide in the ultrasonic fatigue test (for example, 12 specimens in the case of test number 1 shown in Table 3 of Examples described later). The composition analysis is performed, and the obtained composition is arithmetically averaged (for example, in the case of test number 1 in Table 3 above, arithmetically averaged at 5 locations × 12 = 60).

(C) Predicted maximum inclusion width W and predicted maximum inclusion length L of the fracture origin inclusion when performing an ultrasonic fatigue test:
The bearing steel bar according to the present invention is subjected to a tempering treatment on a fatigue test piece collected so that the diameter direction of the steel bar is parallel to the axial direction of the test piece, and an ultrasonic fatigue test is performed using the fatigue test piece. When the width and length of the fracture starting inclusion are subjected to extreme value statistical processing, the predicted maximum inclusion width W must be 20 μm or less and the predicted maximum inclusion length L must be 800 μm or less. I must.

  The “tempering treatment” referred to in the present invention (1) is a “quenching-tempering” heat treatment for adjusting to a Rockwell hardness (HRC) using a C scale within a range of 58 to 66. . In order to obtain this hardness, for example, as described in Table 4.2 of Non-Patent Document 2, in the case of high-carbon chromium bearing steel SUJ2, a quenching temperature of 810 to 850 ° C. and a tempering temperature of 120 What is necessary is just to perform at -180 degreeC.

  By the method described in Non-Patent Document 1, specifically, for example, extreme value statistical processing is performed by the following procedure, and the inclusion form is evaluated.

Determined <1> In the ultrasonic fatigue test pieces, as reference volume V ₀ which per one specimen range up to 90% of the maximum stress acting on the test piece during the fatigue test, the reference volume V ₀ a (mm ³⁾ . When one ultrasonic fatigue test piece is subjected to fatigue fracture, the observed fracture starting inclusion is the maximum inclusion in the reference volume V ₀ .
<2> Therefore, ultrasonic fatigue fracture is performed on n ultrasonic fatigue test specimens, and the width and length of each fracture starting inclusion are measured. Here, the width of the fracture starting inclusion measured with the i-th (i = 1 to n) ultrasonic fatigue test piece is W _i (μm) and the length is L _i (μm).
<3> The widths W _i (i = 1 to n) and lengths L _i (i = 1 to n) of the measured n fracture origin inclusions are rearranged in ascending order, and W _j (j = 1 to n), respectively. ) And L _j (j = 1 to n).
<4> The following normalization variable y _j is calculated for each _j .
y _j = −ln [−ln {j / (n + 1)}].
<5> With W _j on the horizontal axis of the extreme probability probability sheet and the standardized variable y _j on the vertical axis, the plot is made for j = 1 to n, and the width of the fracture origin inclusion is calculated by the least square method. Find an approximate line. Further, the L _j in the coordinate abscissa similarly extreme value probability paper, taking reference variables y _j to coordinate the vertical axis, the length of the plotted for j = 1 to n, starting points of fracture inclusions by the least square method Find the approximate line for.
<6> Assume that the estimated predicted volume is V (mm ³ ) and T = (V + V ₀ ) / V ₀ , and obtain the value of the standardization variable y in the estimated predicted volume V from the following equation.
y = −ln [−ln {(T−1) / T}]
<7> In the approximate straight line for the width of the fracture starting inclusion obtained in <5>, the value on the coordinate horizontal axis when the value on the coordinate vertical axis is y is the predicted maximum inclusion in the estimated predicted volume V. It becomes the object width W. Similarly, in the approximate straight line for the length of the fracture starting inclusion determined in <5> above, the value on the coordinate horizontal axis when the value on the coordinate vertical axis is y is the predicted value in the estimated predicted volume V. The maximum inclusion length L is obtained.

  If the extreme value statistical processing is performed in the above procedure and the predicted maximum inclusion width W exceeds 20 μm, even if the predicted maximum inclusion length L is shortened, a rolling initial crack is generated starting from the inclusion, Since the generated crack size increases, the rolling fatigue life may decrease as a result. A desirable predicted maximum inclusion width W is 10 μm or less.

  When the predicted maximum inclusion width W is 20 μm or less, the occurrence of the initial rolling crack is delayed and the rolling fatigue life is improved. However, if the predicted maximum inclusion W is only 20 μm or less, sufficient rolling is achieved. A fatigue life may not be achieved. This is because the inclusion length has an influence on the occurrence of rolling initial cracks. For example, even if the predicted maximum inclusion width W is 20 μm or less, if the predicted maximum inclusion length L exceeds 800 μm, the occurrence of rolling initial cracks is not sufficiently suppressed and the rolling life is reduced. There is. The desired predicted maximum inclusion length L is 780 μm or less.

  In addition, regarding the measurement method of the width and length of inclusions, the measurement method using an optical microscope described in Non-Patent Document 3 is referred to, and the minor axis of the inclusion, which is the fracture starting point of the ultrasonic fatigue test, is defined as the width (W). The major axis may be measured as the length (L). In addition, for inclusions that exist in a crowded manner, if the inclusion group at the part that affected the fracture start point of the ultrasonic fatigue test is considered to be one inclusion, the maximum width and maximum length can be evaluated. Good.

Specifically, the major axis of the inclusion is the largest side connecting the ends of inclusions existing in a single group or a plurality of inclusions, and is the maximum of the inclusion sandwiched by a line parallel to the side of the major axis. The major diameter may be a short diameter. For inclusions present in groups, the distance between inclusions and the size of the smaller inclusion (√AREA = (L × W) ^1/2 ) are compared, and the size of the smaller inclusion is determined. If the value of √AREA is larger than the distance between the inclusions, the two may be determined as one, while the value of √AREA, which is the size of the smaller inclusion, is the distance between the inclusions. If it is smaller, both may be judged as separate inclusions.

  Note that, for example, the predicted maximum inclusion width W and the predicted maximum inclusion length L of the fracture starting inclusion can be obtained by the method described in the later section (E).

(D) Hardness from the surface of the steel bar to the R / 2 part position:
In the steel bar for bearing according to the present invention, the maximum hardness from the surface of the steel bar to the R / 2 part position must be 290 or less in terms of Vickers hardness.

  In general, the microstructure of a rolled steel bar for bearings represented by SUJ2, for example, is a single-phase structure of pearlite and has a Vickers hardness of about 400. Therefore, in order to perform cold forging and cutting, it is necessary to soften by performing spheroidizing annealing after rolling.

  However, as shown in FIG. 1, by suppressing lamellar cementite generated in the cooling process of the steel bar rolling process, forming spherical cementite and controlling the pearlite area ratio to 40% or less, the hardness of the steel bar rolled material is Vickers hardness is 290 or less. And, from the surface of the steel bar where cracks are a problem to the R / 2 part position, if the maximum hardness is 290 Vickers hardness or less, the occurrence of cracks during cold forging can be suppressed, Furthermore, cutting is also facilitated.

  Therefore, in the steel bar for bearing according to the present invention, the maximum hardness from the surface to the R / 2 part position is 290 or less in terms of Vickers hardness.

  In addition, in order to suppress stably the crack at the time of cold forging, it is preferable that the average cross-sectional hardness in the said site | part is 270 or less in terms of Vickers hardness. In this case, the pearlite area ratio is 30% or less. The average cross-sectional hardness is more preferably 250 or less in terms of Vickers hardness.

  In addition, although the better cold forgeability is obtained as the average cross-sectional hardness at the above-mentioned part is lower, the average cross-sectional hardness when the pearlite area ratio is 0% in order to suppress cracking during cold forging is In order to make the average cross-section hardness 200 or less in terms of Vickers hardness by 200 or less in terms of Vickers hardness, in order to grow larger spheroidized cementite, industrialized spheroidizing annealing is performed. Further, it is necessary to carry out for a long time, which causes a significant increase in manufacturing cost.

  Therefore, it is preferable that the average cross-sectional hardness from the surface of the bearing steel bar according to the present invention to the R / 2 part position exceeds 200 in terms of Vickers hardness.

In addition, the steel bar in the case where a steel bar for bearings is manufactured by applying a reduction that gives a total reduction ratio of 15 or more to the slab or the steel ingot, as described in the following item (E) In the rolling process,
Heating temperature of the pressed material,
By optimizing the surface temperature and the reduction ratio of the material to be reduced during the reduction process, spheroidization of cementite from the above surface to the R / 2 part position is promoted, so that the maximum hardness at the above part can be stabilized. Vickers hardness can be 290 or less.

  The maximum hardness and average cross-sectional hardness from the surface of the steel bar to the R / 2 part position are, for example, a “cross section” (hereinafter referred to as “C cross section”) that is a surface obtained by cutting the steel bar perpendicularly to the longitudinal direction. After mirror polishing, from the position of about 0.5 mm below the surface to the R / 2 part position, using a Vickers hardness tester, almost equally spaced so as to satisfy the conditions described in JIS Z 2244 (2009) Then, after measuring 10 points, the maximum value of the measurement points and the arithmetic average may be obtained respectively. As described above, the above “R” refers to the radius of the steel bar for the bearing.

(E) Manufacturing method of bearing steel bar:
The bearing steel bar according to the present invention can be obtained, for example, by the manufacturing method described below.

  First, after performing oxidation refining in a converter, Al is added at the time of steel output from the converter, deoxidation treatment is performed, and then further demetalization treatment is performed.

_{Then, CaO: 30~70%, Al 2} O 3: 5~40%, SiO 2: 10% or less (not including 0%), MgO: 0~10% , CaF 2: 0~30%, CaO / A slag containing SiO ₂ : 6 or more and CaO / Al ₂ O ₃ : 1.5 to 15 is adjusted in a range of 5 to 20 kg per ton of molten steel, and a vacuum molten steel stirring device with an arc heating device (hereinafter, "VAD")), stirring and refining treatment with Ar gas is performed, treatment is performed for 30 minutes with an RH vacuum degassing apparatus, and molten steel having the chemical components described in the above section (A) is continuously cast. The slab has a cross section of 300 mm × 400 mm.

  Alternatively, after performing the treatment for 30 minutes in the RH vacuum degassing apparatus, the molten steel having the chemical component described in the above section (A) is cast into a mold to form a 3-ton steel ingot.

  The slag composition of the above components and the treatment with VAD are intended to control the composition of oxides and sulfides. And the process in RH vacuum degassing apparatus is a process implemented in order to reduce the total amount of oxide inclusions.

Subsequently, all the following [1] to [3] are performed in the steel bar rolling process in the case where a reduction in which the total reduction ratio is 15 or more is applied to the slab or the steel ingot by the split rolling process and the steel bar rolling process. It can be stably produced by reducing the temperature so as to satisfy the conditions, and further cooling the temperature range up to 400 ° C. at a cooling rate of 5 ° C./s or less after finishing the reduction in the bar rolling process.
[1] Start the reduction by heating the material to be pressed to a temperature range of Ae ₁ point to Aem point;
[2] The surface temperature of the material to be rolled during the rolling step is within a temperature range of 680 ° C. to (Aem point−30 ° C.),
[3] The reduction ratio is 4 or more.

  The “total reduction ratio” refers to a value obtained by dividing the cross-sectional area of a slab or steel ingot by the cross-sectional area of a bearing steel bar obtained by the final reduction in the steel bar rolling process. The “reduction ratio at” indicates a value obtained by dividing the cross-sectional area of the material to be reduced before the reduction is applied in the steel bar rolling process by the cross-sectional area of the steel bar for the bearing obtained by the final reduction in the steel bar rolling process.

Furthermore, “Ae ₁ point” and “Aem point” refer to the eutectoid temperature in the equilibrium state and the temperature at which cementite completely dissolves in austenite in the equilibrium state, respectively.

  Hereinafter, the method for manufacturing these steel bars will be described in detail by taking the steel bars for bearing according to the present invention as an example.

  To the slab or steel ingot having the chemical component described in the item (A), a reduction in which the total reduction ratio is 15 or more is added by the ingot rolling process and the steel bar rolling process. Using a steel slab obtained by rolling in the temperature range exceeding 1000 ° C, further reduction is applied in the hot steel bar continuous rolling process so that the total reduction ratio becomes 15 or more. To steel bar.

  In the above process, when the total reduction ratio when the slab or the steel ingot is processed into the final steel bar is less than 15, even if it is a molten steel by the refining method described above, it is described in the item (B) in the steel bar for bearing. In addition, the predicted maximum inclusion width W cannot be satisfied, and therefore it may not be possible to ensure a desired excellent rolling fatigue life in a severe use environment.

  As the total rolling reduction ratio increases, the predicted maximum inclusion width W described in the above item (B) decreases and the rolling fatigue life is improved. For this reason, the upper limit of the total reduction ratio does not need to be specified in particular, and is the maximum value determined from the dimensions of the slab and the steel ingot, the dimensions of the final bar obtained by processing them, and the equipment surface. Also good.

  The total rolling reduction ratio is more preferably 30 or more.

  However, the bearing steel bar satisfies the condition of the predicted maximum inclusion width W described in the item (B), and the maximum hardness from the surface of the steel bar described in the item (D) to the R / 2 part position is high. In order to satisfy the condition of 290 or less in the Vickers hardness, it may not be sufficient to satisfy the total reduction ratio of 15 or more by the block rolling process and the bar rolling process.

  This is because the deformation resistance of the matrix (base material) is smaller than that of sulfides, and therefore, the pressurization applied at a high temperature, particularly the pressurization applied in a temperature range exceeding 1000 ° C., preferentially deforms the matrix. Therefore, under the pressure in the temperature range, the sulfide is not easily stretched and divided, and it is difficult to satisfy the condition of the predicted maximum inclusion width W of the sulfide. In this case, it is difficult to ensure a desired excellent rolling fatigue life in a severe use environment.

  On the other hand, if the temperature range to which the reduction is applied is lowered to 1000 ° C. or less, the difference in deformation resistance between the matrix and the sulfide is reduced, so that the sulfide is easily stretched and divided, and the (B The condition of the predicted maximum inclusion width W described in the section) is satisfied.

  And the above-mentioned total reduction ratio by the block rolling process and the steel bar rolling process satisfies 15 or more, and in the steel bar rolling process, the steel sheet is reduced so as to satisfy all of the above-mentioned [1] to [3]. By cooling the temperature range up to 400 ° C. after finishing the rolling in the rolling process at a cooling rate of 5 ° C./s or less, in addition to the condition of the predicted maximum inclusion width W, the item (D) It is possible to stably satisfy the condition of the maximum hardness from the surface of the steel bar to the R / 2 part position described in 1.

According to the condition [1] in which the reduction material is heated to the temperature range of Ae ₁ point to Aem point and the reduction starts, the fine granular or spherical shape in the pearlite existing in the reduction material before the bar rolling process Cementite can be left in the prior austenite grains at the start of rolling in the bar rolling process.

  And the condition [2] that the surface temperature of the material to be rolled during the rolling process in the steel bar rolling process is in the temperature range of 680 ° C. to (Aem point−30 ° C.), and the rolling ratio at that time is 4 or more In addition, by satisfying the above condition [3] and further cooling the temperature range up to 400 ° C. after finishing the reduction at a cooling rate of 5 ° C./s or less, the fine particles remaining in the above prior austenite grains and Since strain accumulates in the vicinity of spherical cementite (hereinafter referred to as “residual cementite”), fine proeutectoid cementite can be further uniformly induced by processing in the vicinity of the residual cementite, and the aspect ratio, It is possible to obtain cementite having a shape close to a sphere with a very small “long diameter / short diameter”.

  Thereby, it becomes possible to stably satisfy the maximum hardness condition from the surface of the steel bar to the R / 2 part position described in the section (D).

When the heating temperature of the pressed material in the steel bar rolling process is lower than Ae ₁ point, the pearlite itself of the pressed material remains at the rolling start stage in the steel bar rolling process, and a large amount of plate-like cementite remains in the structure after the rolling. Therefore, it is difficult to obtain a cementite structure having a nearly spherical shape. For this reason, it becomes difficult to stably satisfy the maximum hardness condition from the surface of the steel bar to the R / 2 part position described in the section (D).

  On the other hand, when the heating temperature exceeds the Aem point, the material to be pressed becomes austenite, and the remaining cementite is completely dissolved in the matrix, so that the remaining site to be a precipitation site at the rolling start stage in the bar rolling process. There is no cementite. For this reason, even if the surface temperature of the pressed material during the steel bar rolling process is controlled within the temperature range of 680 ° C. to (Aem point−30 ° C.) of the condition [2], fine proeutectoid cementite and the prior austenite grain boundaries and Processing-induced precipitation cannot be uniformly performed in the grains, and austenite undergoes pearlite transformation in the cooling process after the completion of rolling, and plate-like cementite is precipitated in the prior austenite grains, thereby obtaining a cementite structure having a nearly spherical shape. It becomes difficult. Therefore, in this case as well, it is difficult to stably satisfy the maximum hardness condition from the surface of the steel bar to the R / 2 part position described in the section (D).

  When the surface temperature of the material to be reduced during the rolling process of the steel bar rolling is below 680 ° C., many dislocations can be introduced, but by maintaining at that temperature, austenite in the two-phase structure of austenite and cementite is reduced. Perlite transformation starts. And when the pearlite produced | generated by transformation is crushed, although some plate-like cementite in pearlite is partly parted, the area ratio of pearlite does not change so much. For this reason, it becomes difficult to stably satisfy the maximum hardness condition from the surface of the steel bar to the R / 2 part position described in the section (D). Moreover, since the deformation resistance of pearlite is extremely large, it is difficult to avoid an extremely large load on the reduction equipment.

  On the other hand, when the surface temperature of the material to be pressed during the steel bar rolling process exceeds (Aem point -30 ° C), the processing strain introduced by the reduction is easily lost, and fine proeutectoid cementite is uniformly processed. It is impossible to induce precipitation.

  For this reason, a cementite structure having a nearly spherical shape cannot be obtained. As a result, the condition of the maximum hardness from the surface of the steel bar to the R / 2 part position described in the above section (D) is stably satisfied in this case as well. It becomes difficult to make it.

In the above-described manufacturing method, if the surface temperature of the material to be pressed during the steel bar rolling process is likely to exceed (Aem point −30 ° C.) due to processing heat generation, Intermediate cooling, that is, cooling in an intermediate cooling zone between rolling mills in hot steel bar continuous rolling may be performed. In this case, although the surface temperature of the pressed material may temporarily fall below 680 ° C., it is reheated to 680 ° C. or more by the start of the subsequent rolling, and the surface temperature of the pressed material is 680 ° C .- ( If the cooling is performed in such a short time as to continue the reduction in the temperature range of (Aem point−30 ° C.), the inner quality is hardly affected. From the surface of the steel bar described in the above (D), R / 2 The maximum hardness up to the part position can satisfy the condition of 290 or less in terms of Vickers hardness. In this case, it is preferable that the time Δt from the start of cooling in the intermediate cooling zone to the reheating of the surface temperature of the material to be rolled after the end of cooling to Ae ₁ point or more is 10 s or less.

  If the rolling reduction ratio in the steel bar rolling step is 2 or more, fine proeutectoid cementite is uniformly processed and induced to obtain a cementite structure having a nearly spherical shape, and the steel bar described in the section (D) is obtained. Although the conditions of the maximum hardness from the surface to the R / 2 part position can be satisfied, satisfying the conditions of the predicted maximum inclusion width W and the predicted maximum inclusion length L described in the above section (B) It can be difficult. However, when the rolling reduction ratio in the bar rolling process is 4 or more of the condition [3], the condition of the predicted maximum inclusion width W and the predicted maximum inclusion length L described in the above (B) can be stably satisfied. it can. In addition, the prediction maximum inclusion width W and the prediction maximum inclusion length L become small, so that the reduction ratio in a steel bar rolling process is large. For this reason, it is preferable that the reduction ratio in the said steel bar rolling process is 6 or more, and if it is 8 or more, it is more preferable.

  After finishing the rolling in the steel bar rolling process, the temperature range up to 400 ° C. is cooled at a cooling rate of 5 ° C./s or less. The final cooling rate in the temperature range up to 400 ° C. is 5 ° C./s. If exceeded, the growth of pro-eutectoid cementite and residual cementite at the time of cooling is inhibited, and pearlite transformation occurs, and plate-like cementite precipitates, and the pro-eutectoid cementite is networked along the prior austenite grain boundaries. Thus, a cementite structure having a nearly spherical shape cannot be obtained, and therefore, the condition of the maximum hardness from the surface of the steel bar to the R / 2 part position described in the above section (D) is stably satisfied. This is because it becomes difficult.

  It should be noted that the temperature range for cooling at the cooling rate of 5 ° C./s or less is sufficient if the temperature range is 400 ° C. after the reduction in the steel bar rolling process, and the temperature range below 400 ° C. is particularly limited. It doesn't reach. For this reason, it may be determined as appropriate from, for example, air cooling (cooling), forced air cooling, mist cooling, etc. in consideration of manufacturing equipment and productivity.

  In addition, the lower limit of the final cooling rate in the temperature range up to 400 ° C. is that if the cooling rate is slowed down, the effect of suppressing pearlite increases, but a temperature control facility for slowing down the cooling rate is necessary, and as a result Since it causes an increase in manufacturing cost, it is preferably 5 ° C./h.

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

  Bearing steels 1 to 20 having various chemical compositions shown in Table 1 were melted.

  Steels 1 to 11 and Steels 17 to 20 in Table 1 are steels whose chemical compositions are within the range defined by the present invention, and Steels 12 to 16 are comparative examples whose chemical compositions deviate from the conditions defined by the present invention. Example steel.

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.

  First, after oxidative refining in a 70-t converter, Al was added at the time of steel removal from the converter to perform a deoxidation treatment, and then a denitrification treatment was further performed.

  Next, slag was adjusted to the conditions shown in Table 2 under an Ar atmosphere by VAD, and stirring with Ar gas was performed under conditions of an Ar gas flow rate of 200 L / min and a processing time of 30 min.

  Then, after further processing for 30 minutes by the RH vacuum degassing apparatus, continuous casting was performed to obtain a slab of 300 mm × 400 mm.

  About each said slab, after soaking these at 1250 degreeC, it slabbly rolled in the temperature range of 1150-1100 degreeC, and it was set as the steel piece of 160 mm x 160 mm.

  Subsequently, the steel slabs of steels 1 to 19 were heated to 830 ° C. and then rolled into a steel bar at a temperature range of 830 to 750 ° C., and the temperature range up to 400 ° C. after the rolling was completed at a cooling rate of 0.4 ° C./s. After cooling, a steel bar having a diameter of 70 mm (hereinafter referred to as “φ70 mm”) was produced. The cooling in the temperature range below 400 ° C. was allowed to cool in the atmosphere.

  On the other hand, the steel slab of steel 20 is heated to 1200 ° C., and then rolled in a temperature range of 1150 to 1050 ° C., and after the end of rolling, the temperature range up to 400 ° C. is cooled at a cooling rate of 0.4 ° C./s. Thus, a steel bar having a diameter of 70 mm was manufactured. The cooling in the temperature range below 400 ° C. was allowed to cool in the atmosphere.

  With respect to the C cross-section (that is, the surface obtained by cutting the steel bar perpendicular to the rolling direction, which is the longitudinal direction) of the steels 1 to 20 obtained as described above, the R / R is an intermediate position between the surface and the center. A plate material having a thickness of 14 mm, a width of 45 mm, and a length of 200 mm was cut out in a direction parallel to the rolling direction with 2 parts as a reference.

  Next, both ends in the width direction of the plate material were “flattened” by milling, and then the same steel material as the plate material was welded to both ends by electron beam welding to finish a plate having a width of 70 mm.

  Next, as shown in FIG. 2, 19 pieces of the coarse-shaped ultrasonic fatigue test pieces shown in FIG. 3 were collected from all the plate materials from the width direction.

  As a tempering treatment, the sampled rough shaped ultrasonic fatigue test piece was heated at 830 ° C. for 30 minutes, then quenched with oil, further heated at 180 ° C. for 1 hour, and then allowed to cool to room temperature in the atmosphere. And tempered. Next, finishing was performed to produce an ultrasonic fatigue test piece shown in FIG.

  An ultrasonic fatigue test was carried out using the ultrasonic fatigue test piece that had been finished.

Specifically, a fatigue test was performed using an ultrasonic fatigue tester USF-2000 manufactured by Shimadzu Corporation under the conditions of a frequency of 20 kHz, a stress amplitude of 900 MPa, and a stress ratio of −1 until failure occurred. In addition, when it did not break even if the number of repetitions became 1.0 × 10 ⁷ , the fatigue test was carried out until the fracture occurred by increasing the stress by 20 MPa.

  With respect to the destroyed specimen, the inclusions at the origin of destruction are observed using a scanning electron microscope (hereinafter referred to as “SEM”), and the minor axis (ie, width) and major axis (ie, length) of the inclusion are measured. At the same time, the inclusion composition of the fracture starting point was determined by energy dispersive X-ray spectroscopy. The minor axis (width), major axis (length), and composition of the fracture origin inclusions were obtained by evaluating the inclusions of all 19 fracture start points for each test number.

  The width of inclusions is the maximum width connecting the ends of inclusions existing in a single or plural group, and the length of inclusions is the number of inclusions existing in a single or plural group. Maximum length. In addition, the procedure which calculated | required the inclusion composition of the fracture origin by energy dispersive X-ray spectroscopy is shown below.

・ First, check whether the inclusions are oxides or sulfides.
-Next, the composition analysis by point analysis was implemented with respect to arbitrary five places of this inclusion (refer FIG. 5).
・ Analysis of the composition of inclusions at the origin of fracture was conducted in the same manner for all 19 of the ultrasonic fatigue tests.
Finally, arithmetically average the obtained composition for each oxide or sulfide.

In the ultrasonic fatigue test piece, as shown in FIG. 5, the range up to 90% of the maximum stress acting on the test piece during the fatigue test was defined as the reference volume V ₀ per test piece. In this test piece shape, the reference volume V ₀ is 46 mm ³ . Then, assuming that the estimated predicted volume V is 144 mm ³ , the predicted maximum inclusion width W and the predicted maximum inclusion length L at y = 1.28 were obtained by the extreme value statistical processing described above with reference to Non-Patent Document 1. .

  Table 3 shows the type, average composition, predicted maximum inclusion width W, and predicted maximum inclusion length L of the fracture origin inclusions measured as described above for each of the steels 1 to 20 having a diameter of 70 mm.

  In Test No. 12, since no sulfide was present at the fracture start point of the ultrasonic fatigue test, the columns of CaS, MgS, and MnS of the average sulfide composition were all represented as “−”.

Similarly, in Test No. 13, since no oxide was present at the fracture starting point of the ultrasonic fatigue test, all the columns of CaO, MgO, SiO ₂ and Al ₂ O ₃ of the average oxide composition were “-”. It was written.

  Moreover, after cutting the steel 1-20 steel bars obtained as described above in a diameter of 70 mm perpendicularly to the longitudinal direction, embedding in a resin so that the C cross section becomes a test surface, and microscopically polishing, Vickers hardness The maximum hardness and average cross-sectional hardness from the surface of the steel bar to the R / 2 part position were measured using a testing machine.

  Specifically, the test force is set to 9.807 N at equal intervals so as to satisfy the conditions described in “Vickers Hardness Test—Test Method” described in JIS Z 2244 (2009), and the surface below the C cross section. After measuring 10 points of Vickers hardness from the position of 0.5 mm to the R / 2 part position, the maximum value and the arithmetic average value were determined, and the maximum hardness and the average cross-sectional hardness were respectively determined.

  Table 3 shows the maximum hardness and average cross-sectional hardness from the above surface to the R / 2 part position. In Table 3, the average cross-sectional hardness is expressed as “average hardness”, and the Vickers hardness is expressed as “HV”.

  In addition, after the above embedded sample was mirror-polished again, it was corroded with picric alcohol (picral liquid), and 10 fields of view using an SEM at a magnification of 5000 at any location from the surface to the R / 2 part position. Microstructure images were taken for. The area of each visual field is 25 μm × 20 μm.

  Next, the pearlite area ratio was calculated by the image processing software using the photographed image. In Table 3, the pearlite area ratio calculated as described above is also shown as “perlite ratio”.

  Furthermore, from steel bars 1 to 20 with a diameter of 70 mm, a compression test with a diameter of 14 mm and a height of 21 mm based on the R / 4 part position of the C cross section, that is, the intermediate position between the surface and the R / 2 part position. The piece was cut out to coincide with the rolling direction. Cold forgeability was evaluated using this compression test piece.

  Specifically, in a method of gradually increasing the compression ratio using a crank press, 10 compression test pieces are cold-compressed for each steel and visually confirmed on 5 or more of the 10 test pieces. The cold forgeability was evaluated based on the compression ratio at which cracking occurred (hereinafter, this value is referred to as the critical compression ratio). The target for cold forgeability is that the critical compression ratio is 70% or more.

  Table 3 shows the results of the cold compression test. In the evaluation column of cold forgeability, “◯” indicates that the limit compression ratio has reached the target of 70% or more, while “×” indicates that the target has not been reached.

  Further, in order to evaluate rolling fatigue characteristics, the diameter is 60 mm and the thickness is 5 so that the longitudinal direction becomes the thickness of the shaped material from the center of the φ70 mm steel bar of the steels 1 to 20 obtained as described above. A 5 mm shaped material 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.

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

  As shown in Table 3, in the case of test numbers 1 to 11 of the present invention examples, the chemical compositions of the steels 1 to 11 satisfy the conditions defined in the present invention, and the diameter direction of the steel bar and the axial direction of the test piece are parallel. The fatigue test piece collected as described above is subjected to a tempering treatment, and the predicted maximum inclusion width W of the inclusion, which is a fracture starting point when the fatigue test piece is used to perform an ultrasonic fatigue test, is 20 μm. The predicted maximum inclusion length L is as short as 800 μm or less and satisfies the conditions defined in the present invention. The average composition of the oxide that is the fracture starting point and the average composition of the sulfide that is the fracture starting point all satisfy the conditions defined in the present invention. Moreover, the maximum hardness from the surface of the steel bar to the R / 2 part position is 260 to 280, which satisfies the conditions specified in the present invention.

For this reason, in the case of test numbers 1 to 11 of the present invention example, the rolling fatigue life is 3.5 × 10 ⁷ or more, and the rolling fatigue life can be extended. It achieves the target at 71% or more and is excellent in cold forgeability.

  On the other hand, in the case of test numbers 12 to 16 of comparative examples in which the chemical compositions of the steels 12 to 16 used deviate from the conditions specified in the present invention, the rolling fatigue life was short and did not reach the target.

That is, the test number 12 is 0.0021% in which the O content of the steel 12 used exceeds the value specified in the present invention, and is a fracture starting inclusion when the ultrasonic fatigue test is performed and the steel 12 is broken. The predicted maximum inclusion width W and the predicted maximum inclusion length L of the oxide exceed the values specified in the present invention. For this reason, the L ₁₀ life is as short as 8.0 × 10 ⁶ .

The test number 13 is 0.016% in which the S content of the steel 13 used exceeds the value specified in the present invention, and the predicted maximum inclusion length L of sulfide which is a fracture starting inclusion is the present invention. The CaS concentration in the sulfide is lower than the value specified in the present invention. For this reason, the L ₁₀ life is as short as 5.0 × 10 ⁶ .

Test No. 14 is 0.0001% in which the Ca content of the steel 14 used is lower than the value specified in the present invention, and the CaS concentration in the sulfide is lower than the value specified in the present invention. The predicted maximum inclusion length L of inclusions exceeds the value specified in the present invention. For this reason, the L ₁₀ life is as short as 7.0 × 10 ⁶ .

Test No. 15 is 0.0023% in which the Ca content of the steel 15 used exceeds the value specified in the present invention, and the CaO concentration in the oxide exceeds the value specified in the present invention. The predicted maximum inclusion width W of inclusions exceeds the value specified in the present invention. For this reason, the L ₁₀ life is as short as 1.2 × 10 ⁷ .

The test number 16 is 0.011% and 0.0015% in which the S content and the O content of the steel 16 used exceed the values specified in the present invention, respectively, and the predicted maximum inclusion of the fracture origin inclusion Both the width W and the predicted maximum inclusion length L exceed the values specified in the present invention. For this reason, the L ₁₀ life is as short as 6.0 × 10 ⁶ .

  Further, even when the chemical composition of the steels 17 to 19 used satisfies the conditions specified in the present invention, the inclusion that is the starting point of fracture when the ultrasonic fatigue test is performed is the present invention. Test numbers 17 to 19 that do not satisfy all four conditions regarding the average composition of oxide, the average composition of sulfide, the predicted maximum inclusion width W, and the predicted maximum inclusion length L defined in Rolling fatigue life is short.

That is, in Test No. 17, the SiO ₂ concentration in the oxide exceeds the value specified in the present invention, and both the predicted maximum inclusion width W and the predicted maximum inclusion length L of the fracture starting inclusion are both It exceeds the value specified in the present invention. For this reason, the L ₁₀ life is as short as 7.5 × 10 ⁶ .

In Test No. 18, the SiO ₂ concentration in the oxide exceeds the value specified in the present invention, and the predicted maximum inclusion length L of the fracture starting inclusion exceeds the value specified in the present invention. For this reason, the L ₁₀ life is as short as 6.5 × 10 ⁶ .

In Test No. 19, the MgO concentration in the oxide exceeds the value specified in the present invention, and the predicted maximum inclusion length L of the fracture starting inclusion exceeds the value specified in the present invention. For this reason, the L ₁₀ life is as short as 5.0 × 10 ⁶ .

In addition, the test number 20 is the inclusion that is the starting point of fracture when the chemical composition of the steel 20 used satisfies the conditions specified in the present invention and is destroyed by performing the ultrasonic fatigue test. The L ₁₀ life is 3.8 × 10 because all of the four conditions regarding the average composition of oxide, the average composition of sulfide, the predicted maximum inclusion width W, and the predicted maximum inclusion length L are satisfied. ⁷ and the target has been reached. However, the maximum hardness from the surface of the steel bar to the R / 2 part position is 410 in terms of Vickers hardness, which exceeds the range specified in the present invention. For this reason, the evaluation of cold forgeability is “x” and the target is not reached. Therefore, cold forging cannot be performed by omitting or simplifying spheroidizing annealing after rolling.

The bearing steel bar of the present invention has good durability against damage due to rolling fatigue and has a long rolling fatigue life, so that "ball bearings" and " It can be suitably used as a material for a rolling bearing such as a “roller bearing”. Moreover, since the steel bars for bearings of the present invention are excellent in cold forgeability, spheroidizing annealing after rolling can be omitted, and the manufacturing cost can be reduced. The bearing bar of the present invention is suitable as a material for cold forging, but can also be used for a material for warm forging disclosed in Japanese Patent Application Laid-Open No. 2009-24218.

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 %: Cr: 0.80-1.80%, Al: more than 0.005% and 0.040% or less, Ca: 0.0003-0.0015% and O: 0.0010% or less The balance is a steel bar for bearings having a chemical component consisting of Fe and impurities,
The fatigue test piece collected so that the diameter direction of the steel bar and the axial direction of the test piece are parallel to each other is subjected to a tempering treatment, and an ultrasonic fatigue test is performed using the fatigue test piece to be destroyed.
In the case of the fracture origin inclusions oxide, oxide has an average composition, in mass%, CaO: 2.0~20%, MgO : 0~20% and SiO _2: with 0 to 10% And the balance is Al ₂ O ₃ , a binary oxide of CaO and Al ₂ O _3, a ternary oxide of CaO, MgO and Al ₂ O ₃ , CaO, SiO ₂ and Al ₂ O ₃ It consists of any one of a ternary oxide and a quaternary oxide of CaO, MgO, SiO ₂ and Al ₂ O ₃ ,
And when the said fracture origin inclusion is a sulfide, this sulfide has an average composition of mass%, CaS: 100% of CaS univalent sulfide, or CaS: 1.0% or more. MgS: 0 to 20%, and the balance is MnS, consisting of a binary sulfide of CaS and MnS or a ternary sulfide of CaS, MgS and MnS, and
The maximum inclusion width W predicted in the estimated predicted volume 144 mm ³ when the extreme value statistical processing is performed on the width and length of the above-described fracture origin inclusions without distinguishing between oxides and sulfides. 20 μm or less and the maximum inclusion length L predicted in the estimated predicted volume 144 mm ³ is 800 μm or less,
Furthermore, the maximum hardness from the surface of the steel bar to the R / 2 part position is 290 or less in terms of Vickers hardness,
A steel bar for bearings with excellent cold forgeability.
“R” represents the radius of the steel bar for bearing.