JP5035137B2 - Bearing steel and manufacturing method thereof - Google Patents

Description

  The present invention relates to a bearing steel material and a manufacturing method thereof, and more particularly, to a bearing steel material excellent in rolling fatigue life and cold workability and a manufacturing method thereof.

  Conventionally, "high carbon chromium bearing steel" described in JIS G 4805 (1999) has been used for rolling bearings such as "ball bearings" and "roller bearings" used in various industrial machines and automobiles. I came.

  The rolling bearings described above are generally made from a steel material such as a steel bar or wire manufactured by hot working such as hot rolling. First, the rolling bearing is first subjected to a softening heat treatment called “spheroidizing annealing” for a long time. Then, it is cut, formed into a predetermined shape by hot forging or cold forging, and manufactured by quenching and tempering. In the case of forming by hot forging, turning is performed after spheroidizing annealing is performed again before quenching and tempering.

  In addition, since a high surface pressure acts repeatedly on a rolling bearing, a long rolling fatigue life is required. It is known that rolling fatigue characteristics (rolling fatigue life) are deteriorated by 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.

  On the other hand, in recent years, for example, with higher engine output and smaller peripheral components, the usage environment of rolling bearings has become increasingly severe due to higher surface pressure and higher temperatures. Longer rolling fatigue life has been demanded.

  However, simply reducing the oxygen content does not ensure the desired good rolling fatigue life. For this reason, reducing the size of the oxide in the steel improves the rolling fatigue life. Are proposed in Patent Document 1 and Patent Document 2.

  That is, in Patent Document 1, “in weight%, C: 0.15 to 1.10%, Si: 0.15 to 0.70%, Cr: 0.50 to 1.60%, Mo: 0.00. 10 to 1.00%, Mn: 0.10% or less, O: 8ppm or less, and further, if necessary, containing Ni: 0.4-5.0%, from the balance Fe and inevitable impurity elements Thus, a technique relating to “super clean bearing steel by electron beam melting method, wherein the oxide inclusions have a particle size of 15 μm or less” is disclosed.

Patent Document 2 states that “the chemical composition of steel satisfies JIS G 4805, O: 0.0009 mass% or less, Al: 0.005 mass% or less, and S: 0.005 mass% or less. The number of oxides having a size of 3 μm or more present in a cross-sectional area 160 mm ² parallel to the rolling direction is 100 or less, of which 2 or less is 10 μm or more. As composition ratio by composition,
Alumina system: (MgO) and (SiO ₂ ) are less than 3%, and (CaO) and (CaO) / ((CaO) + (Al ₂ O ₃ )) are not more than 0.08.
Spinel system: (MgO) in the range of 3% to 20% and binary system with the balance being (Al ₂ O ₃ ) contain 15% (CaO) and / or 15% (SiO ₂ ). It may have a spinel crystal structure.
A technique relating to a “high carbon chrome bearing steel characterized in that the total number of alumina-based and spinel-based in accordance with the above definition is less than 60% of the total number of oxides” is disclosed.

Patent Document 2 discloses a process in which the high carbon chromium bearing steel uses a deoxidizer that does not substantially contain Al during deoxidation after oxidation refining by a converter or an electric furnace and subsequent component adjustment. Including a step of controlling the basicity of slag ((CaO)% / (SiO ₂ )%) in ladle scouring to be 0.8 or more and less than 3.0, followed by a vacuum degassing treatment step of 35 minutes or more It is shown to be obtained by taking a manufacturing process.

  The high carbon chromium bearing steel is a hypereutectoid steel containing about 1.0% C by mass% as shown in JIS G 4805 (1999). Steel materials usually exhibit a structure composed of plate-like pro-eutectoid cementite and pearlite. Therefore, the hardness is Vickers hardness (hereinafter also referred to as “Hv hardness”) of about 350 to 400, and when the hot-worked steel material is cut or cold forged, as a pretreatment Softening heat treatment such as spheroidizing annealing is required.

  For this reason, in recent years, steel materials having cold workability to such an extent that the softening heat treatment such as spheroidizing annealing can be simplified by controlling the shape of cementite as hot-worked have been proposed.

  Specifically, Patent Document 3 states that “in mass%, C: 0.6 to 1.5%, Mn: 0.2 to 1.5%, Si: 0.05 to 1.2%, Cr: 0.5 to 2.5%, P: 0.03% or less, S: 0.02% or less, balance: steel composition composed of iron and inevitable impurity elements, or, in addition, by mass, Al: 0.01 ~ 0.03%, Cu: 0.2% or less, Ni: 0.2% or less and Mo: 0.1% or less steel composition containing one or more selected from cementite, and of cementite Among them, a technology relating to “wires and steel bars for bearings having a spheroidized carbide structure as hot rolled, characterized in that the ratio of those having an aspect ratio (major axis / minor axis) of 2 or less is 70% or more” is disclosed. Yes.

Patent Document 3 discloses that the above-mentioned “bearing wire rod / bar” reduces surface finish of hot rolling in the temperature range of (Ar ₁ −50 ° C.) to (Ar ₁ + 50 ° C.) of the steel material. It is shown that it can be obtained by immediately cooling to 500 ° C. or less at a cooling rate of 0.5 ° C./s or less.

JP-A-7-109541 JP 2006-200027 A JP 2004-190127 A

The technique proposed in Patent Document 1 described above reduces the oxides such as Al ₂ O ₃ by using a steel produced by an ordinary mass production steel production method as a base material and remelting it by electron beam melting. It is a method to make it. For this reason, the manufacturing cost becomes extremely high and it is difficult to apply to mass production on an industrial scale. Moreover, the rolling fatigue life of rolling bearings in recent years has not always been sufficient. Furthermore, this technique did not give any consideration to the cold workability when cutting or cold forging a hot-worked steel material.

  The technique proposed in Patent Document 2 focuses only on oxide inclusions, and, in the same way as the technique disclosed in Patent Document 1 described above, under the severe usage environment of rolling bearings in recent years. The rolling fatigue life was not always sufficient. Furthermore, although the high carbon chromium bearing steel has good machinability, it cannot be said that other cold workability such as cutting or cold forging is sufficient.

  Although the wire rods and steel bars for bearings proposed in Patent Document 3 have a spheroidized carbide structure while being hot-rolled, they are rolling in recent years as in the case of the techniques disclosed in Patent Document 1 and Patent Document 2 above. The rolling fatigue life is not always sufficient in the severe use environment of the bearing.

  Therefore, the present invention has good durability against damage due to rolling fatigue even under the severe usage environment of rolling bearings in recent years, and can ensure excellent rolling fatigue life and cold workability. Therefore, it is an object of the present invention to provide a bearing steel material capable of simplifying a softening heat treatment over a long period of time, such as spheroidizing annealing performed after hot working, and a method for manufacturing the same.

The inventors of the present invention previously examined oxides,
(A) The main constituents of slag in the process of so-called “secondary refining” of steel are mainly CaO and SiO _2, and further, strict control is performed so that the amount of Al ₂ O ₃ is as small as possible. That this oxide can be obtained, and that this soft oxide can be refined by applying a reduction,
(B) In the case of steel manufactured by the method of refining as described in (a) above, there is a tendency that an oxide that seems to be MnO tends to be contained in the sulfide. Unlike sulfides in bearing steel subjected to deoxidation treatment, it is difficult to be drawn and divided by reduction, but the S content is 0.010% or less in mass%, and reduction ratio, processing temperature, etc. If the rolling conditions are properly controlled, not only oxides but also sulfides can be stretched, broken and refined, resulting in excellent rolling fatigue life even in harsh usage environments. Bearing steel material can be obtained,
And proposed “Bearing Steel and its Manufacturing Method” in Japanese Patent Application No. 2007-204872.

  The inventors then further studied the cold workability improvement of the bearing steel proposed in the above patent application. As a result, in the final reduction step of the two or more reduction steps in which the total reduction ratio is 15 or more, if the heating conditions of the reduction material and the surface temperature of the reduction material during the reduction step are controlled more strictly, It has been found that a simple steel material can be obtained.

  The present invention has been completed based on the above findings, and the gist of the present invention resides in a bearing steel material shown in the following (1) and a manufacturing method of the bearing steel materials shown in (2) and (3).

(1) By mass%, C: 0.85 to 1.2%, Si: 0.1 to 0.5%, Mn: 0.05 to 0.6%, P: 0.03% or less, S: 0.010% or less, Cr: 1.2 to 1.7%, Al: 0.005% or less, Ca: 0.0005% or less and O: 0.0020% or less, with the balance being Fe and impurities Non-metallic inclusions having an average composition of oxides by mass%, CaO: 10 to 60%, Al ₂ O ₃ : 35% or less, MnO: 35% or less, and MgO: 15% or less The arithmetic average value of the maximum thickness of the oxide and the arithmetic average value of the maximum thickness of the sulfide, which is composed of the remainder SiO ₂ and impurities, and is present in the area of 100 mm ² in 10 longitudinal sections of the steel material Is 8.5 μm or less, respectively, and further from the surface of the steel material to the R / 2 part position. Bearing steels, wherein the average cross-sectional hardness of 290 or less in Vickers hardness.
However, “R” represents the radius of the bearing steel.

  (2) A bearing steel material is produced by applying a reduction to a total reduction ratio of 15 or more to a slab or steel ingot having the average composition of chemical components and oxides described in (1) above by two or more reduction steps. In the final reduction step of the two or more reduction steps, the reduction is performed so as to satisfy all of the following [1] to [3], and after the completion of the reduction in the final reduction step, 400 A method for producing a bearing steel material, wherein the temperature range up to 0 ° C. is cooled at a cooling rate of 5 ° C./s or less.

[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.
However, the total reduction ratio refers to the value obtained by dividing the cross-sectional area of the slab or steel ingot by the cross-sectional area of the bearing steel obtained by the final reduction in the final reduction process, and the reduction in the final reduction process. The ratio refers to a value obtained by dividing the cross-sectional area of the material to be reduced before the reduction is applied in the final reduction process by the cross-sectional area of the bearing steel obtained by the final reduction in the final reduction process.

(3) After the slab or steel ingot is subjected to oxidative refining as primary refining, secondary refining is performed by using a flux that does not substantially contain Al without performing Al deoxidation treatment. Final slag basicity CaO / SiO ₂ value after refining is 0.8 to 2.0, and slag composition is mass%, MgO: 15% or less, F: 10% or less, Al ₂ O ₃ : The method for producing a bearing steel material according to (2) above, wherein the steel material is controlled to be 20% or less and subsequently cast.

Note that the “impurity” in the average composition of the oxide refers to Cr ₂ O ₃ , Na ₂ O, ZrO _{2 and the} like.

  The “longitudinal longitudinal section” (hereinafter referred to as “L section”) refers to a surface cut in parallel to the longitudinal direction of the steel material.

  The cooling rate in the temperature range up to 400 ° C. after finishing the rolling refers to the average cooling rate on the surface of the material to be rolled in the temperature range.

The “remainder” in the slag composition is MnO, FeO, TiO ₂ , Cr ₂ O ₃ or the like.

Further, “a flux that does not substantially contain Al” means that Al ₂ O ₃ in the flux is less than 3%.

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

  Hereinafter, the invention relating to the bearing steel material of (1) and the invention relating to the manufacturing method of the bearing steel material of (2) and (3) are respectively referred to as “present invention (1)” to “present invention (3)”. That's it. Also, collectively referred to as “the present invention”.

  The bearing steel material of the present invention has various durability against the damage caused by rolling fatigue and has a long rolling fatigue life even in the severe usage environment of recent rolling bearings. It can be used as a material for rolling bearings such as “ball bearings” and “roller bearings” used in automobiles and automobiles. Moreover, since the bearing steel material of the present invention is excellent in cold workability, it is possible to simplify softening heat treatment such as spheroidizing annealing and reduce manufacturing costs. This bearing steel can be manufactured by the method of the present invention.

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

(A) Chemical composition of steel:
C: 0.85-1.2%
C is an element that secures the hardness at the time of quenching and improves the rolling fatigue life, and needs to be 0.85% or more. However, if the C content increases and exceeds 1.2% in particular, the wear resistance is improved, but the hardness of the base material becomes too high and the cold forgeability deteriorates, and the tool life during cutting is increased. Cause deterioration and burning cracks. Therefore, the content of C is set to 0.85 to 1.2%. In addition, the minimum with preferable C content is 0.9%. Moreover, a preferable upper limit is 1.1%.

Si: 0.1 to 0.5%
Si is an element effective for improving the hardenability and improving the rolling fatigue life, and must be contained by 0.1% or more. However, when Si is contained exceeding 0.5%, the hardness of the base material becomes high, and the tool life during cutting and the cold forgeability are deteriorated. Therefore, the Si content is set to 0.1 to 0.5%. In addition, the minimum with preferable Si content is 0.15%. Moreover, a preferable upper limit is 0.35%.

Mn: 0.05 to 0.6%
Mn is an element effective for enhancing the hardenability and improving the rolling fatigue life, and must be contained by 0.05% or more. However, if Mn is contained in excess of 0.6%, the hardness of the base material is increased, resulting in a decrease in tool life at the time of cutting and deterioration of cold forgeability, and further causes cracking. . Therefore, the Mn content is set to 0.05 to 0.6%. In addition, the minimum with preferable Mn content is 0.1%. Moreover, a preferable upper limit is 0.5%.

P: 0.03% or less P segregates at the grain boundary and shortens the rolling fatigue life. In particular, when the content exceeds 0.03%, the rolling fatigue life is significantly reduced. Therefore, the content of P is set to 0.03% or less. The range of preferable P content is 0.02% or less.

S: 0.010% or less S is an element that forms sulfides. If the content exceeds 0.010%, coarse sulfides remain and the rolling fatigue life is shortened. Therefore, the content of S is set to 0.010% or less. From the viewpoint of improving the rolling fatigue life, the lower the S content, the better. However, S has an effect of improving the machinability, and when the content is 0.005% or more, the machinability is improved. The improvement effect is surely obtained. For this reason, when importance is attached to machinability, the S content is preferably set to 0.005% or more.

Cr: 1.2-1.7%
Cr is an element effective for improving the hardenability and improving the rolling fatigue life, and must be contained by 1.2% or more. However, when Cr is contained exceeding 1.7%, the hardness of the base metal becomes high, the tool life at the time of cutting is reduced and the cold forgeability is deteriorated, and further, when cracking occurs. There is. Therefore, the Cr content is set to 1.2 to 1.7%. In addition, the minimum with preferable Cr content is 1.3%. Moreover, a preferable upper limit is 1.6%.

Al: 0.005% or less Al is an undesirable element, and in the present invention, it is necessary to reduce Al as much as possible. Therefore, as will be described later, deoxidation treatment is not performed by adding Al after oxidative refining as the primary refining, and the flux used when stirring the newly generated slag and molten steel is also Al ₂ O ₃ A material having a low content of Al and containing substantially no Al is used. However, when the Al content increases, especially when it exceeds 0.005%, the amount of hard oxides mainly composed of Al ₂ O ₃ increases, and the coarse oxides after the reduction. As a result, the rolling fatigue life is shortened. Therefore, the Al content is set to 0.005% or less. The Al content is preferably 0.003% or less, and the lower the better.

Ca: 0.0005% or less In the present invention, as will be described later, after the removal of slag produced by oxidative refining as primary refining, a flux whose main component is CaO is introduced to newly produce slag. And stir the molten steel. At this time, Ca is mixed as a soft oxide from the flux into the steel. However, if the Ca content increases and exceeds 0.0005%, the ratio of CaO in the oxide composition becomes too high, resulting in a coarse oxide. Therefore, the Ca content is set to 0.0005% or less. The preferable Ca content is 0.0003% or less, and more preferably 0.0002% or less. In addition, the lower limit of the amount of Ca contained is not particularly specified, and it is sufficient that CaO in the average composition of oxides in the steel material is 10% or more.

O: 0.0020% or less O is an undesirable impurity element. When the content of O increases, and particularly exceeds 0.0020%, it remains as a coarse oxide after rolling, leading to a decrease in rolling fatigue life. Therefore, the content of O is set to 0.0020% or less. In addition, the range of preferable O content is 0.0015% or less.

  For the above reasons, the bearing steel according to the present invention (1) contains C, Si, Mn, P, S, Cr, Al, Ca, O in the above-described range, and the balance is the chemical component of Fe and impurities. It was defined as consisting of

  Also in the present invention (2), a slab or steel ingot containing C, Si, Mn, P, S, Cr, Al, Ca, O in the above-described range, and the balance having chemical components of Fe and impurities. It was decided to use.

  In addition, since rolling fatigue life will be reduced when coarse TiN is generated, it is preferable that Ti in the impurity is 0.003% or less and N in the impurity is 0.010% or less.

(B) Non-metallic inclusions:
(B-1) Average composition of oxide:
In the present invention, for non-metallic inclusions, first, the average composition of oxides is, by mass, CaO: 10 to 60%, Al ₂ O ₃ : 35% or less, MnO: 35% or less, and MgO: 15%. In the following, it must consist of the remainder SiO ₂ and impurities. Hereinafter, the content in the average composition of oxide in mass% is also referred to as “concentration”.

The “oxide” in the present invention is mainly composed of a ternary system of CaO, SiO ₂ , Al ₂ O ₃ , MnO and MgO, and the average composition of the oxide is in the above range. In general, oxides are soft as a whole, and are easily stretched, divided, and made finer in a rolling process such as rolling, so that the rolling fatigue life is not reduced, and therefore, even under severe use environments. This is because an excellent rolling fatigue life can be secured.

  The reasons for limiting each oxide composition are shown below.

CaO: 10 to 60%
An oxide having a basic composition of SiO ₂ , which is an acidic oxide, contains CaO, which is basic, so that the liquidus temperature of the oxide is lowered and becomes ductile in a rolling temperature range such as rolling. The above effect is obtained when the CaO concentration in the average composition of the oxide is 10% or more. However, when it exceeds 60%, the SiO ₂ concentration is relatively lowered and the ductility is not exhibited. Therefore, the CaO concentration in the average composition of the oxide is set to 10 to 60%. In addition, the preferable upper limit of the said CaO density | concentration for making the ductility stable in rolling reduction temperature ranges, such as rolling, is 50%.

Al ₂ O ₃ : 35% or less When the concentration in the average composition of the amphoteric oxide Al ₂ O ₃ exceeds 35%, an Al ₂ O ₃ (corundum) phase is crystallized or MgO together with MgO described later.・ Al ₂ O ₃ (spinel) phase crystallizes out. These solid phases are hard and maintain the thickness at the time of crystallization without stretching even under rolling or the like. Therefore, the Al ₂ O ₃ concentration in the average composition of the oxide needs to be 35% or less. A preferable upper limit of the concentration of Al ₂ O ₃ to stably and reliably suppress generation of the hard phase is 25%.

MnO: 35% or less MnO is basic as an oxide and promotes softening of the SiO ₂ system, so that a relatively high concentration is acceptable. However, MnO is a so-called lower oxide that is stable when the steel is in a weakly deoxidized state. If the MnO concentration is high, the content of O (oxygen) in the steel also increases. That is, if the MnO concentration in the average composition of the oxide exceeds 35%, the O content may not be 0.0020% or less. Therefore, the MnO concentration in the average composition of the oxide is set to 35% or less. In order to make the above-described O content 0.0015% or less, the MnO concentration in the average composition of the oxides is preferably 25% or less.

MgO: 15% or less MgO is a basic oxide, and a small amount can soften a SiO ₂ -based oxide, but its solubility is low, while it has a low MgO (periclase) phase and Al ₂ O ₃ together with MgO. The Al ₂ O ₃ (spinel) phase crystallizes out. If MgO in the average composition of the oxide exceeds 15%, the probability of crystallizing the hard phase described above increases. Therefore, the MgO concentration in the average composition of the oxide is set to 15% or less. In order to more reliably suppress the crystallization of the hard phase described above, the MgO concentration in the average composition of the oxide is preferably 10% or less.

The “oxide” as used in the present invention is mainly composed of a ternary system of CaO, SiO ₂ , Al ₂ O ₃ , MnO and MgO, but is composed of Cr ₂ O ₃ , Na ₂ O and ZrO _2. It is preferable that the sum total of impurities in oxides such as 5% or less.

The average composition of the oxide is CaO: 10 to 50%, Al ₂ O ₃ : 25% or less, MnO: 25% or less, and MgO: 10% or less, with the balance being SiO ₂ and 5% or less impurities. Is preferred.

Further, in the average composition of the oxide, the lower limit of Al ₂ O ₃ , MnO and MgO does not need to be specified.

For the reasons mentioned above, the average composition of oxides of bearing steels according to the present invention (1), by mass%, CaO: 10~60%, Al 2 O 3: 35% or less, MnO: 35% or less and MgO : 15% or less and the balance being SiO ₂ and impurities.

  Also in the present invention (2), a slab or a steel ingot having an average composition of the oxide is used.

  In addition, the average composition of the oxide is, for example, a plurality of arbitrary oxides having a thickness of 3 μm or more by energy dispersive X-ray spectroscopy after mirror-polishing the L cross-section obtained by cutting the steel material parallel to the longitudinal direction, For example, 20 compositions may be obtained by arithmetically averaging the measured compositions.

  In addition, the average composition of the above oxide employs, for example, the steelmaking method described in the following <1> and <2>, and thereafter, a slab or a steel ingot is produced by a conventional continuous casting method or a casting method. Can be obtained.

  <1> In the steelmaking process of bearing steel, deoxidation treatment with Al addition that is usually performed to remove oxygen contained as impurities after oxidation refining in converters and electric furnaces that are so-called “primary refining furnaces” Not performed.

<2> About the final slag after the completion of secondary refining, the basicity (CaO / SiO ₂ ) is 0.8 to 2.0, the composition is mass%, MgO: 15% or less, F: 10% Hereinafter, control is performed so that Al ₂ O ₃ : 20% or less. The above F (fluorine) is derived from CaF ₂ as the main component of the fluorite as Zokasu agent.

For the final slag after the completion of secondary refining, in order to obtain the composition of <2> above, after the steel is discharged from the primary refining furnace to the ladle, slag composition control in “secondary refining” can be easily performed. In order to achieve this, first, slag produced by oxidative refining flowing out from the primary smelting furnace is removed, and after removal, the main component is CaO and substantially free of Al, Al ₂ O ₃ and MgO. What is necessary is just to stir the newly produced | generated slag and molten steel by throwing in the flux with little content of. As already stated, the “remainder” in the slag composition is MnO, FeO, TiO ₂ , Cr ₂ O ₃ or the like.

  In addition, as means for obtaining strong stirring, for example, stirring under reduced pressure, stirring by injection, bottom blowing stirring from the bottom of the ladle, or the like may be applied. When stirring by injection, it is preferable to blow the above-mentioned flux at the same time. Moreover, when implementing a pressure reduction process, it is necessary to stop at the pressure reduction process for stirring to the last. This is because, if the decompression process for a long time is performed, the inclusion of hard inclusions from the refractory and the entrainment of slag are caused, leading to a decrease in cleanliness.

  In addition, as long as the Ca content of the steel does not exceed 0.0005%, Ca may be further added to the molten steel during the secondary refining process.

(B-2) Maximum thickness of oxide and maximum thickness of sulfide:
When both the oxide and sulfide are thick, the rolling fatigue life is reduced. What has the greatest influence on the rolling fatigue life is the coarsest inclusions existing below the raceway surface. In particular, if oxides and sulfides with a maximum thickness exceeding 8.5 μm in an area of 100 mm ² of the L cross-section of the steel material are present at a number of sites in the steel material, the probability of existing on the raceway surface increases. As a result, the rolling fatigue life is significantly reduced.

For the reasons described above, the bearing steel according to the present invention (1) has an arithmetic average value of the maximum oxide thickness and the maximum thickness of sulfide existing in an area of 100 mm ² at 10 locations on the L cross section of the steel material. It was specified that the arithmetic average values were 8.5 μm or less, respectively.

  The arithmetic average value of the maximum thickness of the oxide and the arithmetic average value of the maximum thickness of the sulfide are both preferably 7 μm or less.

  As already described, the “L cross section” refers to a surface cut in parallel to the longitudinal direction of the steel material.

(C) Average cross section hardness from the surface of the steel material to the R / 2 part position:
In general, since the Hv hardness of the high carbon chromium bearing steel as hot worked is about 350 to 400, when performing cold working such as cutting or cold forging, spheroidizing annealing after hot working, etc. It was necessary to perform a softening heat treatment for a long time. However, the cold workability of the steel material is largely governed by the deformability from the surface of the steel material to the R / 2 part position, and even in the case of a high carbon chrome bearing steel material, the average cross-section at the above-mentioned site as hot-worked If the hardness is 290 or less in terms of Hv hardness, good cold workability can be ensured even with hot working or shortening the softening heat treatment time.

  Therefore, about the bearing steel material which concerns on this invention (1), it was decided that the average cross-section hardness from the surface to R / 2 part position is 290 or less in Hv hardness.

  The preferred average cross-sectional hardness in Hv hardness at the above part is 270 or less, more preferably 250 or less. The lower the average cross-sectional hardness at the above part, the better the cold workability that can be obtained, so the lower limit of the average cross-sectional hardness is not particularly specified.

  In addition, for the slab or steel ingot having the chemical composition described in the item (A) and the average composition of the oxide described in the item (B), for example, as described in the following item (D) In the final reduction step of two or more reduction steps with a reduction ratio of 15 or more, if the heating temperature of the material to be reduced and the surface temperature of the material to be reduced during the reduction step are optimized, the deformability is greatly affected. Since the spheroidization of cementite from the above surface to the R / 2 part position is promoted, the average cross-sectional hardness at the above portion can be made 290 or less in terms of Hv hardness.

  The average cross-sectional hardness from the surface of the steel material to the R / 2 part position is, for example, after mirror-polishing a “cross section” (hereinafter referred to as “C section”) obtained by cutting the steel material perpendicularly to the longitudinal direction. Ten points measurement from about 0.5mm below the surface to R / 2 part position using Vickers hardness tester at almost equal intervals to satisfy the conditions described in JIS Z 2244 (2003) After that, the arithmetic average may be obtained.

  As described above, “R” represents the radius of the bearing steel.

(D) Manufacturing method of bearing steel:
The bearing steel of the present invention (1) comprises, for example, the method of the present invention (2), specifically, the chemical components described in the above section (A), and the nonmetallic inclusion (B-1). When a reduction in which the total reduction ratio is 15 or more is applied to a slab or steel ingot having the average composition of the oxide described in the section by two or more reduction steps, the final reduction step of the two or more reduction steps is performed. In order to satisfy all of the following [1] to [3], further, the temperature range up to 400 ° C. is cooled at a cooling rate of 5 ° C./s or less after finishing the reduction in the final reduction step. Can be manufactured by.

[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.

  As already stated, the total reduction ratio refers to the value obtained by dividing the cross-sectional area of the slab or the steel ingot by the cross-sectional area of the bearing steel obtained by the final reduction in the final reduction process, The reduction ratio in the final reduction step refers to a value obtained by dividing the cross-sectional area of the material to be reduced before the reduction is applied in the final reduction step by the cross-sectional area of the bearing steel obtained by the final reduction in the final reduction step. .

  Further, the slab or steel ingot having the average composition of the chemical component and oxide according to the present invention (2), that is, the chemical component described in the item (A), and the non-metallic inclusion (B-1) For the slab or steel ingot having the average composition of the oxide described in the item (1), for example, after adopting the method described in the items (1) and (2) in the item (B-1), It can be obtained by casting by a continuous casting method or a mold method.

  Hereinafter, as an example of the bearing steel material of the present invention (1), a steel bar and a wire are specifically taken up, and a method for producing these steel materials will be described in detail.

  The slab or steel ingot having an average composition of the chemical components and oxides described above is subjected to reduction at a total reduction ratio of 15 or more by two or more reduction steps. In addition to this, using a steel slab obtained by split rolling in a temperature range exceeding, a hot forging process, a process consisting of hot rolling and forging, or hot bar steel continuous rolling or Reduction is applied in a process such as continuous rolling of the wire rod, so that the final bar steel or wire rod is processed so that the total reduction ratio is 15 or more.

  In the above process, if the total reduction ratio is less than 15 when the slab or steel ingot is processed into the final bar or wire, it consists of the chemical components described in the above section (A), and (B-1 The conditions of the maximum thickness of the oxide and the maximum thickness of the sulfide described in the above (B-2) are applied to the bearing steel material even if the slab or the steel ingot having the average composition of the oxide described in the section) is used. Therefore, the desired excellent rolling fatigue life cannot be ensured in a severe use environment.

  In addition, the larger the total rolling reduction ratio, the smaller the maximum oxide thickness and sulfide maximum thickness described in the above section (B-2), and the rolling fatigue characteristics (rolling fatigue life) are improved. To do. 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 and wire obtained by processing them, and the equipment surface. There may be.

  The total reduction ratio is preferably 30 or more.

  However, the steel bar or wire rod as the bearing steel material satisfies the conditions of the maximum oxide thickness and sulfide maximum thickness described in the above section (B-2), and the steel material described in the above section (C). In order to satisfy the condition that the average cross-sectional hardness from the surface to the R / 2 part position is 290 or less in terms of Hv hardness, it is not sufficient to satisfy the total reduction ratio of 15 or more by two or more reduction processes. It is.

  That is, when the average composition of the oxide is as described in the above section (B-1), the sulfide present at the same time contains an oxide that seems to be MnO, and the deoxidation treatment is performed by adding Al. Since the sulfide is hardened compared to the above, it is difficult to be stretched and divided by processing. Therefore, first of all, the maximum thickness of the sulfide described in the section (B-2) is applied to the steel bar or wire as the bearing steel material. In order to satisfy the conditions, the total reduction ratio should satisfy 15 or more, and as proposed in the patent application of Japanese Patent Application No. 2007-204872, the reduction ratio in a temperature range of 1000 ° C. or less is included in the reduction. It is necessary to reduce the pressure to 4 or more.

  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 above temperature range, the sulfide is hardly stretched and divided, and the maximum thickness condition of the sulfide described in the above section (B-2) cannot be satisfied. In this case, the desired excellent rolling fatigue life cannot be ensured 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, and the sulfide is easily stretched and divided (B-2). This is because the conditions for the maximum thickness of the sulfide described in the section are satisfied.

  However, in order to simultaneously satisfy the condition of the average cross-sectional hardness from the surface of the steel material described in the above item (C) to the R / 2 part position in the steel bar or wire material as the bearing steel material, Japanese Patent Application No. 2007-204872 The total reduction ratio proposed in the patent application satisfies 15 or more, and it is not sufficient to reduce the reduction ratio in the temperature range of 1000 ° C. or less to 4 or more, and the total reduction is performed by two or more reduction processes. When applying the reduction at a ratio of 15 or more, the final reduction step of the two or more reduction steps is reduced so as to satisfy all of the above [1] to [3], and further in the final reduction step. After finishing the reduction, it is necessary to cool the temperature range up to 400 ° C. at a cooling rate of 5 ° C./s or less.

That is, according to the condition [1] in which the material to be pressed is heated to the temperature range of Ae ₁ point to Aem point to start the reduction, fine particles in the pearlite existing in the material to be reduced before the final reduction process Spherical cementite can be left in the prior austenite grains at the start of rolling in the final rolling step. And the condition [2] that the surface temperature of the material to be reduced during the final reduction step is in the temperature range of 680 ° C. to (Aem point−30 ° C.) and the condition that the reduction ratio at that time is 4 or more [ 3], and when the temperature range up to 400 ° C. after cooling is cooled at a cooling rate of 5 ° C./s or less, the fine granular or spherical cementite (in the above-mentioned prior austenite grains ( (Hereinafter referred to as “residual cementite”). Since strain accumulates in the vicinity, fine proeutectoid cementite can be further uniformly induced by processing in the vicinity of the residual cementite. Can be obtained. Thereby, it becomes possible to simultaneously satisfy the condition of the average cross-sectional hardness from the surface of the steel material to the R / 2 part position described in the section (C).

When the heating temperature of the material to be reduced in the final reduction process is lower than Ae ₁ point, the pearlite itself of the reduction material remains at the reduction start stage in the final reduction process, and a large amount of plate-like cementite remains in the structure after reduction. Therefore, a cementite structure having a nearly spherical shape cannot be obtained. For this reason, the condition of the average cross-section hardness from the surface of the steel material described in the above (C) to the R / 2 part position cannot be satisfied.

  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 it remains as a precipitation site in the rolling start stage in the final rolling process. There is no cementite. For this reason, even if the surface temperature of the material to be pressed during the final reduction 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 Since it cannot be uniformly induced by processing in the grains, austenite undergoes pearlite transformation in the cooling process after completion of the reduction, and plate-like cementite is precipitated in the prior austenite grains, thereby obtaining a cementite structure having a nearly spherical shape. I can't. Therefore, also in this case, the condition of the average cross-section hardness from the surface of the steel material to the R / 2 part position described in the above (C) item cannot be satisfied.

  When the surface temperature of the material to be reduced during the final reduction step is less than 680 ° C., many dislocations can be introduced, but by maintaining at that temperature, austenite in the two-phase structure of austenite and cementite is transformed into pearlite. Will start. And when the pearlite produced | generated by transformation is reduced, although some plate-like cementite in pearlite is partly divided, the aspect ratio of cementite does not become so small. For this reason, the condition of the average cross-sectional hardness from the surface of the steel material described in the above (C) to the R / 2 part position cannot be satisfied. In addition, since the deformation resistance of pearlite is extremely large, the load on the reduction equipment is extremely increased.

  On the other hand, when the surface temperature of the material to be pressed during the final reduction process exceeds (Aem point −30 ° C.), the processing strain introduced under reduction easily disappears, and fine proeutectoid cementite is processed uniformly. It is impossible to induce precipitation. For this reason, a cementite structure having a nearly spherical shape cannot be obtained. As a result, in this case as well, the condition of the average cross-sectional hardness from the surface of the steel material to the R / 2 part position described in the above (C) section should be satisfied. I can't.

In the present invention, when the surface temperature of the pressed material during the final reduction process is likely to exceed (Aem point −30 ° C.) due to processing heat generation, intermediate cooling at an intermediate stage in the final reduction process is performed. That is, cooling in an intermediate cooling zone between rolling mills in hot steel bar continuous rolling or wire rod 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 a short time so as to continue the reduction in the temperature range of (Aem point−30 ° C.), the inner quality is hardly affected, and from the surface of the steel material described in the above (C), R / 2 The condition that the average cross-sectional hardness up to the part position is 290 or less in terms of Vickers hardness can be satisfied. 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 reduction ratio in the final reduction 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 material described in the section (C) is obtained. Although the condition of the average cross-sectional hardness from the surface to the R / 2 part position can be satisfied, in order to obtain the maximum thickness of the sulfide described in the item (B-2), the reduction in the final reduction step The ratio needs to be 4 or more of the condition [3]. The greater the reduction ratio in the final reduction step, the smaller the maximum thickness of the sulfide. Therefore, it is preferably 6 or more, and more preferably 8 or more.

  After finishing the reduction in the final reduction step, it is necessary to cool the temperature range up to 400 ° C. 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 exceeds the growth of pro-eutectoid cementite and residual cementite at the time of cooling, and pearlite transformation occurs and plate-like cementite precipitates. Accordingly, a cementite structure having a nearly spherical shape is not obtained, and therefore, the condition of the average cross-sectional hardness from the surface of the steel material to the R / 2 part position described in the section (C) is determined. It is because it cannot be satisfied.

  It should be noted that the temperature range for cooling at the cooling rate of 5 ° C./s or less described above is sufficient to be 400 ° C. after finishing the reduction in the final reduction step, and the temperature range below 400 ° C. needs to be specified in particular. There is no. 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.

  As already stated, the total reduction ratio refers to the value obtained by dividing the cross-sectional area of the slab or the steel ingot by the cross-sectional area of the bearing steel obtained by the final reduction in the final reduction process, The reduction ratio in the final reduction step refers to a value obtained by dividing the cross-sectional area of the material to be reduced before the reduction is applied in the final reduction step by the cross-sectional area of the bearing steel obtained by the final reduction in the final reduction step. .

  For the reasons described above, in the present invention (2), a slab or steel ingot having the average composition of the chemical components and oxides described in the present invention (1), in other words, the chemistry described in the above section (A). The slab or steel ingot having the components and having the average composition of the oxide described in the above (B-1) for the non-metallic inclusions is subjected to reduction at a total reduction ratio of 15 or more by two or more reduction steps. When adding, the final reduction step of the two or more reduction steps is reduced so as to satisfy all of the above [1] to [3], and further, after finishing the reduction in the final reduction step, up to 400 ° C. The temperature range was defined as cooling at a cooling rate of 5 ° C./s or less.

Further, in the present invention (3), after oxidative refining as primary refining, secondary refining is performed using a flux that does not substantially contain Al, without performing Al deoxidation treatment, and after secondary refining is completed. Final slag basicity CaO / SiO ₂ value is 0.8-2.0, and slag composition is mass%, MgO: 15% or less, F: 10% or less, Al ₂ O ₃ : 20% It was regulated to use slabs and steel ingots that were subsequently cast and controlled to be as follows.

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

Example 1
Cast steel slabs 1 to 21 having various chemical compositions shown in Table 1 were produced.

  In Table 1, Steels 1 to 12 and Steels 14 to 17 are steels whose chemical compositions are within the range defined by the present invention, and Steels 13 and 18 to 21 are chemical compositions defined by the present invention. It is a comparative steel that is out of the conditions. Among the steels of the comparative examples, steel 20 and steel 21 are steels corresponding to conventional Al killed steel.

Table 1 also 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.

  Among the above steels, steels 1 to 19 were subjected to oxidative refining as primary refining in a converter, then removed and charged with flux. And after blowing the flux by the flux injection method, in a state in which the flux is mixed in the molten steel, in an Ar atmosphere by a vacuum molten steel stirring equipment with an arc heating device (hereinafter referred to as “VAD”), Molten steel stirring was performed for 40 minutes at an Ar flow rate of 40 to 60 L / min. Thereafter, continuous casting was performed to obtain a slab of 300 mm × 400 mm. In addition, about steel 19, Al was added slightly at the time of steel extraction from a converter, and it deoxidized lightly, but about steel 1-18, the deoxidation process of Al addition was not performed.

Steels 20 and 21 were subjected to oxidative refining as primary refining in a converter, then deoxidized by addition of Al at the time of steel removal from the converter, and then removed and charged with flux. Then, by VAD, molten steel is stirred for 40 minutes under an Ar atmosphere in an Ar atmosphere for 40 minutes, and further processed by an RH vacuum degassing apparatus for 40 minutes, so that hard oxidation mainly composed of Al ₂ O ₃ is performed. The thing was removed. Thereafter, continuous casting was performed to obtain a slab of 300 mm × 400 mm.

  Table 2 shows the composition of the flux introduced after removing steels 1 to 21 and the composition of the flux used in the flux injection method for steels 1 to 19.

Also, it is shown in Table 3, the slag composition and basicity by mass percent after VAD processing of steel 1-21 of (CaO / SiO _2).

The “remainder” in the composition of the flux used in the flux injection method in Table 2 is C, S, P, etc., and the “remainder” in the slag composition in Table 3 is MnO, FeO, TiO ₂ , Cr ₂ O ₃ and the like.

  From the T / 4 part of the slabs of steels 1 to 21 obtained in this way (where "T" represents the thickness of the slab), that is, from the intermediate part between the outer surface and the center of the slab, the oxide After cutting out a block for composition measurement, embedding the block in a resin, and mirror-polishing the L cross section, select 20 arbitrary oxides with a thickness of 3 μm or more by energy dispersive X-ray spectroscopy. It was measured.

  And the composition measured about 20 oxides was arithmetic-averaged, and the "average composition" of the oxide in each slab was calculated | required.

In Table 4, the average composition of the oxide measured as mentioned above about each slab of steel 1-21 is shown. The balance in the average composition of the oxide refers to “impurities”, that is, Cr ₂ O ₃ , Na ₂ O, ZrO _{2 and the} like.

  About the slabs of the above steels 1 to 19, these were soaked at 1250 ° C., and then rolled at a temperature range of 1150 to 1100 ° C. to form a steel slab of 160 mm × 160 mm. After heating, the steel bar is rolled in a temperature range of 830 to 750 ° C., and the temperature range up to 400 ° C. is cooled at a cooling rate of 0.4 ° C./s after the rolling, and the diameter is 70 mm (hereinafter referred to as “φ70 mm”). .) Steel bar was manufactured. The cooling in the temperature range below 400 ° C. was allowed to cool in the atmosphere.

  On the other hand, for the slabs of Steel 20 and Steel 21, these were soaked at 1250 ° C., and then rolled at a temperature range of 1150 to 1100 ° C. to obtain a steel slab of 160 mm × 160 mm. After heating to ° C., steel bars were rolled in a temperature range of 1150 to 1050 ° C., and after the end of rolling, the temperature range up to 400 ° C. was cooled at a cooling rate of 0.4 ° C./s to produce φ70 mm bar steel. The cooling in the temperature range below 400 ° C. was allowed to cool in the atmosphere.

  A block for measuring the oxide composition was cut out from R / 2 part (where “R” represents the radius of the steel bar) of a steel bar of φ70 mm of steels 1 to 21 obtained as described above, and the block was resinized. After embedding in L and mirror-polishing the L cross section, 20 arbitrary oxides having a thickness of 3 μm or more were selected by energy dispersive X-ray spectroscopy, and the respective compositions were measured.

  And the composition measured about 20 oxides was arithmetically averaged, and the "average composition" of the oxide in each steel bar of (phi) 70mm was calculated | required.

In addition, 10 blocks of 100 mm ² are cut out from the R / 2 part of φ70 mm steel bars of steels 1 to 21 and embedded in resin so that the L cross section becomes the test surface, and then mirror polished, and then 100 mm The maximum thickness of oxide and the maximum thickness of sulfide existing in each L cross section of ² were measured using an optical microscope, and each was arithmetically averaged.

Specifically, assuming that the magnification of observation with an optical microscope is 400 times, first, oxides and sulfides having the largest thickness are detected in the L cross section of 100 mm ² , and then each magnification is 1000 times. Was measured for 10 blocks, and 10 arithmetic average values were obtained for each of the 10 blocks.

In addition, when the oxide and the sulfide are combined without separation, the thicknesses of the oxide and the sulfide are respectively measured, and when the thicknesses are the largest in the measured L cross section, Was arithmetically averaged as the oxide or sulfide having the largest thickness in the 100 mm ² L cross section.

Table 5 shows the average composition of oxides measured as described above for each 70 mm steel bar of steels 1 to 21 and the arithmetic average value of the maximum thickness of oxides present in 10 100 mm ² L cross sections. And the arithmetic mean value of the maximum thickness of sulfide. The balance in the average composition of the oxide refers to “impurities”, that is, Cr ₂ O ₃ , Na ₂ O, ZrO _{2 and the} like. In Table 5, the arithmetic average value of the maximum thickness of the oxide and the arithmetic average value of the maximum thickness of the sulfide are respectively expressed as “maximum oxide thickness” and “maximum sulfide thickness”. "". Also in the following description, the arithmetic average value of the maximum oxide thickness and the arithmetic average value of the maximum sulfide thickness are referred to as “maximum oxide thickness” and “maximum sulfide thickness”, respectively. .

  Moreover, after embedding C cross section cut perpendicularly to the longitudinal direction from φ70 mm steel bars of steels 1 to 21 obtained as described above in a resin, and specularly polishing, from the surface of the steel bars using a Vickers hardness tester The average cross-section hardness up to the R / 2 part position was measured.

  Specifically, the surface of the C cross section is set at a test force of 9.807 N at substantially equal intervals so as to satisfy the conditions described in “Vickers hardness test-test method” described in JIS Z 2244 (2003). After measuring the Hv hardness from the lower 0.5 mm position to the R / 2 part position at equal intervals, the average cross-sectional hardness was obtained by arithmetic averaging.

  In Table 5, the average cross-sectional hardness from the bottom surface to the R / 2 part position determined as described above is also shown as “average hardness”.

  Furthermore, from steel bars 1 to 21 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 so as to coincide with the rolling direction, and cold workability was evaluated.

  That is, using a press machine, compressing each of the above test pieces at a compression rate of 10 to 65% and determining the relationship between the crack generation rate and the compression rate, and when cracks occur on the three test pieces The cold workability was evaluated with the minimum compression ratio as the critical compression ratio.

  In addition, said "compression rate" means the value represented by {(21-h) / 21} * 100 by making the height of the test piece after compressing using a press machine into h (mm). Say.

  Table 5 also shows the limit compression ratio obtained as described above.

  Next, in order to evaluate the rolling fatigue characteristics in the state of being completely spheroidized and annealed, each of the steels 1 to 21 obtained as described above was maintained at 780 ° C. for 6 hours after being held at 780 ° C. for 6 hours. Perform normal spheroidizing annealing for 20 hours in total furnace time, and then slice the shaped material having a diameter of 60 mm and a thickness of 5.5 mm so that the longitudinal direction is the thickness of the shaped material. And collected.

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

  The surface of the shaped material thus quenched and tempered was lapped to produce a rolling fatigue test piece, which was subjected to a 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).

  Table 6 shows the detailed conditions of the rolling 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”.

  Table 5 also shows the rolling fatigue life determined as described above.

From Table 5, the chemical composition of steel, non-metallic inclusions (that is, the average composition of oxide and the maximum thickness of oxide and the maximum thickness of sulfide existing in 10 areas of 100 mm ^{2 in} the L cross section of steel) ) And an average cross-section hardness from the surface of the steel material to the R / 2 part position is test numbers 1 to 12 satisfying the provision of the present invention (1), a long rolling fatigue life of 6.29 × 10 ⁷ or more. In addition, a high critical compression ratio of 50% or more is also obtained.

  On the other hand, even if the chemical composition of steel and the average cross-sectional hardness from the surface of the steel material to the R / 2 part position satisfy the provisions of the present invention (1), it is manufactured by a method deviating from the provisions of the present invention (3) However, in the case of test numbers 14 to 17 in which the nonmetallic inclusions deviate from the conditions specified in the present invention (1), the rolling fatigue life is short.

That is, in the case of each of the above test numbers, although the maximum thickness of the sulfide satisfies the conditions specified in the present invention (1), the average composition of the oxides deviates from the conditions specified in the present invention (1). Since the product becomes hard and as a result, the maximum thickness of the oxide is increased and deviates from the conditions defined in the present invention (1), the rolling fatigue life is 2.57 × 10 ⁷ , 2.88, respectively. × 10 ⁷ , 3.29 × 10 ⁷ and 2.21 × 10 ⁷ are short.

  Also, the rolling fatigue life is short when the chemical composition of steel deviates from the provisions of the present invention.

That is, the test number 13 is used for the average composition of oxide, the maximum thickness of the oxide, and the average cross-sectional hardness from the surface of the steel material to the R / 2 part position satisfy the conditions specified in the present invention (1). Since the S content of the steel 13 is as high as 0.016% and exceeds the value specified in the present invention (1), the maximum thickness of the sulfide increases and deviates from the conditions specified in the present invention (1). Therefore, the rolling fatigue life is as short as 2.76 × 10 ⁷ .

Test No. 18 shows that the average composition of oxide, the maximum thickness of sulfide, and the average cross-sectional hardness from the surface of the steel material to the R / 2 part position satisfy the conditions specified in the present invention (1), but the steel used Since the O content of 18 is as high as 0.0021% and exceeds the value specified in the present invention (1), the maximum thickness of the oxide is increased and deviates from the conditions specified in the present invention (1). The dynamic fatigue life is as short as 2.56 × 10 ⁷ .

Test No. 19 shows that the maximum content of sulfide and the average cross-sectional hardness from the surface of the steel material to the R / 2 part position satisfy the conditions specified in the present invention (1), but the Al content of the steel 19 used is , 0.008%, which exceeds the value specified in the present invention (1), so that the average composition of the oxide deviates from the conditions defined in the present invention (1) and becomes a hard oxide. Since the maximum thickness of the object becomes larger and deviates from the conditions specified in the present invention (1), the rolling fatigue life is as short as 1.97 × 10 ⁷ .

Test number 20 and test number 21 are rolled steel materials produced by conventional methods of steel 20 and steel 21 corresponding to conventional Al killed steel, and the Al contents are 0.019% and 0.021, respectively. %, Exceeding the value specified in the present invention (1), and the maximum thickness of the sulfide satisfies the condition defined in the present invention (1), but the average composition of the oxide is defined in the present invention (1). When the condition is not met, the oxide becomes a hard oxide, and as a result, the maximum thickness of the oxide is increased and deviates from the condition defined in the present invention (1), so that the rolling fatigue life is 3.10 × 10 ^7. And 2.15 × 10 ⁷ . Furthermore, since the average cross-sectional hardness from the surface of the steel material to the R / 2 part position is also outside the conditions defined in 362 and 353 and the present invention (1), both of the critical compression ratios are as low as 15%.

(Example 2)
A 300 mm × 400 mm slab of Steel 3, Steel 11, Steel 13, Steel 17, and Steel 20 produced in Example 1 was soaked at 1250 ° C., and then subjected to block rolling in a temperature range of 1150 to 1100 ° C. to 160 mm. A steel piece of × 160 mm was used.

  Next, using the steel pieces, the steel bars were rolled under the conditions shown in the following << 1 >> to << 6 >> to produce steel bars having a diameter of 70 mm (φ70 mm) or a diameter of 100 mm (φ100 mm).

<< 1 >> After heating the steel slab to 1200 ° C., the steel bar is 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, φ70 mm Steel bar manufacturing,
<2> After heating the steel slab to 830 ° C., the steel bar is rolled in a temperature range of 830 to 750 ° 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, φ70 mm Steel bar manufacturing,
<3> After the steel slab is heated to 1000 ° C., the steel bar is rolled in a temperature range of 830 to 780 ° 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, φ70 mm Steel bar manufacturing,
<4> After heating the steel slab to 780 ° C., the steel bar is rolled in a temperature range of 720 to 600 ° 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, φ70 mm Steel bar manufacturing,
<5> After heating the steel slab to 850 ° C., the steel bar is rolled in a temperature range of 920 to 810 ° 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, φ70 mm Steel bar manufacturing,
<6> After the steel slab is heated to 830 ° C., the steel bar is rolled in a temperature range of 830 to 750 ° C., and after the end of rolling, the temperature range up to 400 ° C. is cooled at a cooling rate of 0.3 ° C./s, and φ100 mm Steel bar manufacture.

  In addition, 300 mm × 400 mm slabs of Steel 3, Steel 11, Steel 13, Steel 17, and Steel 20 produced in Example 1 above were soaked at 1250 ° C., and then splitted in a temperature range of 1150 to 1100 ° C. The steel pieces were rolled into 240 mm × 240 mm steel pieces, and further steel bars were rolled using the steel pieces under the conditions shown in the following << 7 >> to produce a steel bar having a diameter of 120 mm (φ120 mm).

  <7> After heating the steel slab to 860 ° C., the steel bar is rolled in a temperature range of 830 to 780 ° C., and after the end of rolling, the temperature range up to 400 ° C. is cooled at a cooling rate of 0.2 ° C./s, φ120 mm Steel bar manufacture.

  In any case of the above << 1 >> to << 7 >>, the cooling in the temperature range below 400 ° C. was allowed to cool in the air.

  Table 7 shows the details of the manufacturing conditions for each steel bar described above.

  A block for measuring the oxide composition was cut out from the R / 2 part of the steel bars of steel 3, steel 11, steel 13, steel 17, and steel 20 manufactured as described above, and the blocks were made of resin. After embedding in L and mirror-polishing the L cross section, 20 arbitrary oxides having a thickness of 3 μm or more were selected by energy dispersive X-ray spectroscopy, and the respective compositions were measured.

  And the composition measured about 20 oxides was arithmetic-averaged, and the "average composition" of the oxide in the bar steel of (phi) 70mm, (phi) 100mm, and (phi) 120mm was calculated | required.

Further, 10 blocks of 100 mm ² are cut out in the longitudinal direction from the R / 2 portion of the steel 3, steel 11, steel 13, steel 17, and steel 20 φ70 mm, φ100 mm, and φ120 mm steel bars, and the L cross section is the test surface. Embedded in the resin so as to be mirror-polished, and then the maximum thickness of the oxide and the maximum thickness of the sulfide existing in each L cross section of 100 mm ² were measured using an optical microscope. did.

Specifically, assuming that the magnification of observation with an optical microscope is 400 times, first, oxides and sulfides having the largest thickness are detected in the L cross section of 100 mm ² , and then each magnification is 1000 times. Was measured for 10 blocks, and 10 arithmetic average values were obtained for each of the 10 blocks.

In addition, when the oxide and the sulfide are combined without separation, the thicknesses of the oxide and the sulfide are respectively measured, and when the thicknesses are the largest in the measured L cross section, Was arithmetically averaged as the oxide or sulfide having the largest thickness in the 100 mm ² L cross section.

Table 8 shows the average composition of oxides measured as described above for each of the steel bars, the arithmetic average value of the maximum thickness of oxides present in 10 100 mm ² L cross sections, and the maximum of sulfides. The arithmetic average value of thickness is shown. As described above, the balance in the average composition of the oxide refers to “impurities”, that is, Cr ₂ O ₃ , Na ₂ O, ZrO _{2 and the} like. The “maximum oxide thickness” and “maximum sulfide thickness” refer to the arithmetic average value of the maximum oxide thickness and the arithmetic average value of the maximum sulfide thickness, respectively.

  In addition, steel C, steel 11, steel 13, steel 17 and steel 20 obtained as described above were cut from φ70 mm, φ100 mm, and φ120 mm steel bars perpendicularly to the longitudinal direction, embedded in resin, and specularly polished. Thereafter, the average cross-sectional hardness from the surface of the steel bar to the R / 2 part position was measured using a Vickers hardness tester.

  Specifically, the surface of the C cross section is set at a test force of 9.807 N at substantially equal intervals so as to satisfy the conditions described in “Vickers hardness test-test method” described in JIS Z 2244 (2003). After measuring the Hv hardness from the lower 0.5 mm position to the R / 2 part position at approximately equal intervals, the average cross-sectional hardness was obtained by arithmetic averaging.

  In Table 8, the average cross-sectional hardness from the surface to the R / 2 part position determined as described above is also shown as “average hardness”.

  Further, from the above steel 3, steel 11, steel 13, steel 17 and steel 20 of φ70 mm, φ100 mm and φ120 mm steel bars, all are R / 4 part positions of the C cross section, that is, intermediate between the surface and R / 2 part positions. Based on the position, a compression test piece having a diameter of 14 mm and a height of 21 mm was cut out so as to coincide with the rolling direction, and cold workability was evaluated.

  That is, using a press machine, compressing each of the above test pieces at a compression rate of 10 to 65% and determining the relationship between the crack generation rate and the compression rate, and when cracks occur on the three test pieces The cold workability was evaluated with the minimum compression ratio as the critical compression ratio.

  As described above, the above-mentioned “compression rate” means {(21−h) / 21} × 100 where h (mm) is the height of the test piece after compression processing using a press machine. The value represented by

  Table 8 also shows the limit compression ratio obtained as described above.

  Next, in order to evaluate the rolling fatigue characteristics in the state of complete spheroidizing annealing, the steel 3, steel 11, steel 13, steel 17 and steel 20 obtained as described above are φ70 mm, φ100 mm and φ120 mm steel bars. Both are held at 780 ° C. for 6 hours and then cooled in the furnace, followed by normal spheroidizing annealing for a total furnace time of 20 hours, and then the diameter is adjusted so that the longitudinal direction is the thickness of the shaped material. A shaped material having a thickness of 60 mm and a thickness of 5.5 mm was sliced and collected.

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

  The surface of the shaped material thus quenched and tempered was lapped to produce a rolling fatigue test piece, which was subjected to a 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.

  The detailed conditions of the rolling fatigue test are as shown in Table 6 above.

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”.

  Table 8 also shows the rolling fatigue life determined as described above.

From Table 8, a test number 23 in which a slab in which the chemical composition and non-metallic inclusions (that is, the average composition of oxides) of the steel satisfy the provisions of the present invention (2) was reduced by the method of the present invention (2) and In the case of test number 30, it can be seen that a long rolling fatigue life of 7.13 × 10 ⁷ or more and a high critical compression ratio of 50% or more are obtained.

  In contrast, when the steel chemical composition and non-metallic inclusions (that is, the average composition of oxides) satisfy the provisions of the present invention (2), a reduction with a total reduction ratio of 15 or more is applied. Even when the steel bar is rolled as the final reduction step, the test in which the steel slab heating temperature and the surface temperature during the steel bar rolling deviate from at least one of the conditions [1] and [2] defined in the present invention (2). In the case of No. 22, Test Nos. 24-26, Test No. 29, and Test Nos. 31-33, the average cross-sectional hardness from the surface of the steel material to the R / 2 part position deviates from the conditions specified in the present invention (1). ing. For this reason, the critical compression ratio in these test numbers is as low as 15 to 25%, which is inferior in cold workability.

Among the above test numbers, in the case of test number 22 and test number 29 in which both the steel slab heating temperature and the surface temperature during steel bar rolling exceed 1000 ° C., the maximum thickness of the sulfide is also defined in the present invention (1). It is out of the condition to do. For this reason, the rolling fatigue life is as short as 3.11 × 10 ⁷ and 3.24 × 10 ⁷ , respectively.

Further, using a slab in which the chemical composition of steel and non-metallic inclusions (that is, the average composition of oxides) satisfy the provisions of the present invention (2), the total pressure of 15 or more satisfying the provisions of the present invention (2). When rolling steel bars as a final reduction process, the steel slab heating temperature and the surface temperature during steel bar rolling in the conditions [1] and [2] defined in the present invention (2) are satisfied. In the case of test number 27 and test number 34 where the rolling reduction ratio in the steel bar rolling process deviates from the condition [3] defined in the present invention (2), the maximum thickness of the sulfide is defined in the present invention (1). It is off. For this reason, the rolling fatigue life is as low as 2.51 × 10 ⁷ and 2.76 × 10 ⁷ , respectively.

Further, the present invention (2) is used for rolling steel bars as a final reduction process using a slab in which the chemical composition of steel and non-metallic inclusions (that is, the average composition of oxides) satisfy the provisions of the present invention (2). Even if the steel slab heating temperature, the surface temperature during rolling of the steel bar and the rolling ratio are satisfied, the total rolling ratio is as low as 10.6, which is defined by the present invention (2). In the case of test number 28 and test number 35 that deviate from the above conditions, the maximum oxide thickness and the maximum sulfide thickness deviate from the conditions defined in the present invention (1). For this reason, the rolling fatigue life is as low as 2.09 × 10 ⁷ and 1.87 × 10 ⁷ , respectively.

Furthermore, even if the non-metallic inclusions (that is, the average composition of the oxide) satisfy the provisions of the present invention (2), the slab whose S content as a chemical component of steel deviates from the provisions of the present invention (2) is used. In the case of the test numbers 36 to 42, the maximum thickness of the sulfide was increased regardless of the reduction condition and deviated from the condition defined in the present invention (1). Moreover, depending on the reduction condition, Since the maximum thickness of the oxide also increases and deviates from the conditions specified in the present invention (1), the rolling fatigue life is 2.43 × 10 ⁷ , 2.76 × 10 ⁷ , 1.89 × 10 ⁷ , respectively. It is as short as 2.55 × 10 ⁷ , 2.12 × 10 ⁷ , 2.81 × 10 ⁷ and 1.64 × 10 ⁷ .

  Among the above test numbers, the steel strip heating temperature and the surface temperature during the steel bar rolling are determined from at least one of the conditions [1] and [2] defined in the present invention (2) during the steel bar rolling as the final reduction step. In the case of the test number 36 and the test numbers 38 to 40 which are deviated, the average cross-sectional hardness from the surface of the steel material to the R / 2 part position is also out of the conditions defined in the present invention (1). For this reason, the limit compression rate in these test numbers is as low as 15 to 25%, and the cold workability is also inferior.

Test Nos. 43 to 49 using slabs in which the non-metallic inclusions (that is, the average composition of oxides) deviate from the definition of the present invention (2) even if the chemical composition of the steel satisfies the provision of the present invention (2) In this case, the maximum thickness of the oxide becomes large regardless of the rolling condition and deviates from the condition defined in the present invention (1). In addition, depending on the rolling condition, sulfidation like test number 43 and test numbers 48 and 49 occurs. Since the maximum thickness of the object also increases and deviates from the conditions specified in the present invention (1), the rolling fatigue life is 2.42 × 10 ⁷ , 2.21 × 10 ⁷ , 1.78 × 10 ⁷ , 2 .39 × 10 ⁷ , 1.96 × 10 ⁷ , 2.37 × 10 ⁷ and 1.55 × 10 ⁷ .

  Among the above test numbers, the steel strip heating temperature and the surface temperature during the steel bar rolling are determined from at least one of the conditions [1] and [2] defined in the present invention (2) during the steel bar rolling as the final reduction step. In the case of the test number 43 and the test numbers 45 to 47 which have been removed, the average cross-sectional hardness from the surface of the steel material to the R / 2 part position is also outside the conditions defined in the present invention (1). For this reason, the limit compression rate in these test numbers is as low as 15 to 25%, and the cold workability is also inferior.

In the case of test numbers 50 to 56 using slabs in which the chemical components and non-metallic inclusions (that is, the average composition of oxides) corresponding to conventional Al killed steel deviate from the provisions of the present invention (2) In addition, the maximum thickness of the oxide is increased regardless of the rolling condition and deviates from the conditions specified in the present invention (1), so that the rolling fatigue life is 3.10 × 10 ⁷ , 3.31 × 10 ⁷ , respectively. It is as short as 2.99 × 10 ⁷ , 2.89 × 10 ⁷ , 2.14 × 10 ⁷ , 2.53 × 10 ^7, and 1.99 × 10 ⁷ .

  Among the above test numbers, the steel strip heating temperature and the surface temperature during the steel bar rolling are determined from at least one of the conditions [1] and [2] defined in the present invention (2) during the steel bar rolling as the final reduction step. In the case of the test number 50 and the test numbers 52 to 54 deviated, the average cross-sectional hardness from the surface of the steel material to the R / 2 part position is also outside the conditions defined in the present invention (1). For this reason, the limit compression rate in these test numbers is as low as 15 to 25%, and the cold workability is also inferior.

  The bearing steel material of the present invention has various durability against the damage caused by rolling fatigue and has a long rolling fatigue life even in the severe usage environment of recent rolling bearings. It can be used as a material for rolling bearings such as “ball bearings” and “roller bearings” used in automobiles and automobiles. Moreover, since the bearing steel material of the present invention is excellent in cold workability, it is possible to simplify softening heat treatment such as spheroidizing annealing and reduce manufacturing costs. This bearing steel can be manufactured by the method of the present invention.

Claims (3)

In mass%, C: 0.85 to 1.2%, Si: 0.1 to 0.5%, Mn: 0.05 to 0.6%, P: 0.03% or less, S: 0.010 %: Cr: 1.2-1.7%, Al: 0.005% or less, Ca: 0.0005% or less, and O: 0.0020% or less, with the balance being Fe and impurities. With respect to non-metallic inclusions, the average composition of oxides is mass%, CaO: 10 to 60%, Al ₂ O ₃ : 35% or less, MnO: 35% or less, and MgO: 15% or less, and the balance SiO. ₂ and impurities, and the arithmetic average value of the maximum thickness of the oxide and the arithmetic average value of the maximum thickness of the sulfide existing in the area of 100 mm ² in the longitudinal longitudinal section of the steel material are 10 respectively. 8.5 μm or less, and further, an average break from the steel surface to the R / 2 part position Bearing steels, wherein the hardness is 290 or less in Vickers hardness.
However, “R” represents the radius of the bearing steel. A method for producing a bearing steel by applying a reduction of a total reduction ratio of 15 or more to a slab or steel ingot having an average composition of chemical components and oxides according to claim 1 by two or more reduction steps. In the final reduction step of the two or more reduction steps, the reduction is performed so as to satisfy all of the following [1] to [3], and the temperature is reduced to 400 ° C. after completion of the reduction in the final reduction step. The region is cooled at a cooling rate of 5 ° C./s or less.
[1] Heating the pressed material to a temperature range of Ae ₁ point to Aem point and starting the rolling [2] The surface temperature of the pressed material during the rolling process is 680 ° C. to (Aem point−30 ° C.) [3] The reduction ratio is 4 or more. However, the total reduction ratio is a bearing steel material obtained by final reduction in the final reduction step in the cross-sectional area of a slab or steel ingot. The reduction ratio in the final reduction step is obtained by the final reduction in the final reduction step in the cross-sectional area of the material to be reduced before the reduction is applied in the final reduction step. The value divided by the cross-sectional area of bearing steel. After the slab or steel ingot is oxidatively smelted as primary smelting, Al deoxidation treatment is not performed, secondary smelting is performed using a flux that does not substantially contain Al, and after secondary smelting is completed The final slag has a basicity CaO / SiO ₂ value of 0.8 to 2.0 and a slag composition of mass%, MgO: 15% or less, F: 10% or less, Al ₂ O ₃ : 20 The method for producing a bearing steel material according to claim 2, wherein the steel material is controlled to be not more than% and subsequently cast.