JP5316495B2 - Bearing steel - Google Patents

Description

  The present invention relates to a bearing steel having improved rolling fatigue life by controlling the form of sulfides in the steel.

  Parts such as “ball bearings” and “roller bearings”, which are important for automobiles and industrial machinery, are repeatedly subjected to high surface pressure loads, and therefore there is a need for improved rolling fatigue life. Conventionally, materials such as “high carbon chromium bearing steel” described in JIS G 4805 (1999) are used as these parts.

  Since oxide inclusions are found on the fracture surface of the specimens broken in the rolling fatigue test, the correlation between the rolling fatigue life and the oxide inclusions has been investigated so far. For this reason, conventionally, the rolling fatigue life has been improved by reducing the size and number of oxide inclusions. As a result, the total oxygen concentration in steel, which is widely used as an index of oxide inclusions in steel, has recently become less than 10 ppm.

  However, as a result of the reduction of the total oxygen concentration, breakage due to sulfide inclusions appears remarkably instead of the oxide inclusions so far. On the other hand, there is an increasing demand for further improvement in rolling fatigue life. In addition to conventional oxide inclusions, a method for reducing the number and size of sulfide inclusions, in addition to this, Techniques for controlling the shape of inclusions are being studied. Examples of elements that can control the shape of sulfide inclusions include Te and Ca.

  Te is known as a free cutting element of the same group 16 element as S, and forms MnTe in molten steel. When MnTe is precipitated, it is said that extension of sulfide inclusions can be suppressed during rolling and forging, and it is put into practical use as Te free-cutting steel.

  Ca is one of the elements widely used for controlling the form of inclusions, and since Ca has a strong affinity for S, it forms CaS in molten steel. Like Te, it is said that extension of sulfide inclusions can be suppressed, and it is put into practical use as Ca free-cutting steel.

  For example, in Patent Document 1, as a method of “adjusting L / W (length / thickness ratio) of MnS in a steel bar as a raw material to an appropriate value or less”, a specific amount of C, Si, Mn, P, Al In addition to O, Ni, and Cr, steel containing “S: 0.005 to 0.030%” contains “Ca: 5 to 30 ppm” or “Ca: 5 to 30 ppm and Te: 5 to 30 ppm” A technique relating to “a method for producing a hot forging bar” is disclosed. The technique proposed in Patent Document 1 is that the steel having the specific chemical composition, in which steel containing only Ca in addition to the elements from C to S is selected, the rolling ratio from slab to bar steel For steel containing both Ca and Te in addition to the elements from C to S, the shape deformation of MnS can be reduced by reducing the rolling ratio from slab to bar steel to 63 or less. Minimize it.

Patent Document 2 discloses a technique for “dispersing MnS finely and maintaining its shape in a spherical shape” in addition to a specific amount of C, Si, Mn, P, Al, O, and N as “S: 0. 0.003 to 0.5% "is made to contain" one or two of Zr: 0.0003 to 0.01% and Te: 0.0003 to 0.005% " A technique relating to the characteristic “steel excellent in forgeability and machinability” is disclosed. In this document, Zr uses ZrO ₂ as a MnS precipitation site to uniformly disperse MnS or forms a composite sulfide to reduce deformability, and Te reduces the deformability of MnS. It is supposed to be

Japanese Patent No. 3399780 Japanese Patent No. 3954751

Katsunari Oikawa, 3 others, "Iron and Steel", Vol. 80, 1994, no. 8, p623-628

  However, in Patent Document 1, the technical idea is to adjust the shape of MnS present in a portion relatively close to the surface of the steel bar where cracking occurs during hot forging, that is, in a portion where the cooling rate is high during casting. No mention is made of the form of MnS crystallized inside. In addition, there is no clear explanation about the mechanism by which the form of MnS changes with the addition of Ca alone or both of Ca and Te, and there is no description about the ratio of Ca and Te added to the amount of S in steel. Therefore, this is not a technique for stably obtaining the effects of the invention. Furthermore, in the invention described in Patent Document 1, it is limited to manipulating the deformability of sulfide inclusions due to Te addition, and has not yet reached the idea of controlling the solidification form that has the greatest influence on the inclusion shape. .

  Further, in Patent Document 2, the technical idea is to suppress the deformability of the generated MnS, and MnS is not spheroidized by controlling the form of MnS at the stage of the slab where MnS is deposited. For this reason, there is a problem that the effect of the invention cannot be obtained in the case where long MnS extending in a rod shape that affects the product performance has already been generated at the slab stage. Further, in the invention described in Patent Document 2, as in the invention described in Patent Document 1, it is limited to manipulating the deformability of sulfide inclusions due to Te addition, and the idea of controlling the solidification form is considered. Has not reached.

  The present invention has been made as a result of investigations based on the problems as described above, and an object of the present invention is to provide a bearing steel material that stably improves the rolling fatigue life.

  In order to solve the above problems, the present inventors added Te alone or both Te and Ca to molten steel having a bearing steel composition, and the shape of sulfide inclusions in the slab at that time Were investigated in detail. As a result, the following knowledge was obtained. When neither Te nor Ca is added, the sulfide inclusions are solidified and formed as rod-like eutectics and have a high probability of coarsening, and are elongated by rolling to become long (hereinafter simply referred to as “long "It is also called" Longer "), which is the starting point for destruction at the product stage.

  On the other hand, when an appropriate amount of Te is added, MnS-MnTe having a melting point lower than that of MnS is generated as the sulfide-based inclusions. Therefore, the sulfide-based inclusions are generated by monotectic solidification, and the inclusion shape is spherical. It becomes close, and coarsening is suppressed. That is, by adding an appropriate amount of Te, it is possible to solidify the sulfide inclusions and to control the shape of the sulfide. For this reason, even if the slab is rolled, the probability that the sulfide inclusions become longer and become the starting point of fracture is reduced.

  At this time, although the form of the sulfide inclusions in the slab changes with the addition of Te, the form of the sulfide changes when Te is added above a certain threshold according to the S concentration in the molten steel, Below the threshold, the sulfide form hardly changes. Further, the amount of Te added in which the form of sulfide inclusions changes is different between the case where Ca is added before the addition of Te and the case where Ca is not added.

  Moreover, in addition to the above-mentioned sulfide form investigation at the slab stage, the correlation between the rolling fatigue life of the steel material after rolling and the sulfide form was investigated. As a result, the steel material to which Te is added alone or to which both Te and Ca are added in an appropriate amount has a long rolling fatigue life, and the sulfide inclusions found at the base point of fracture are reduced in size compared to the case of no addition. In other words, it turned out that it was not long.

The present invention has been made on the basis of such knowledge, and the gist thereof lies in the following bearing steel materials (1) and ( 2 ).

( 1 ) By mass%, C: 0.5-1.2%, Si: 0.1-0.9%, Mn: 0.1-2.0%, P: ≦ 0.03%, S: 0.0003-0.03%, O: ≦ 0.005% , N: ≦ 0.02% , Al: 0.0005-0.04% and Cr: 0.05-2.0% , Further, Te: 0.0001 to 0.016% and Ca: 0.0001 to 0.005% are contained, the balance is Fe and impurities, and the content of S, Te and Ca in mass% is (Te + Ca) / S> satisfies 0.4 conditions, have a sulfide inclusions monotectic solidified, the sulfide-based inclusions, the ratio of Mn and S is 70 atomic% or more In the slab that is an inclusion and is continuously cast, the following ( Bearing steels, wherein a) fraction of what the 3 or higher degree of unevenness in the expression is less than 5.4%.
Concavity and convexity = (sulfide circumference) ² / (4 × π × sulfide area) (1)

( 2 ) In place of part of Fe, in mass%, Ti: 0.005 to 0.1%, V: 0.008 to 0.3%, Mo: 0.008 to 1.0%, Nb: 0.008 to 0.1%, Ni: 0.008 to 2.0% and REM (rare earth element): 0.002 to 0.01% of one or two or more of the above (1 ) Bearing steel materials .

  The above-mentioned “sulfide inclusion” is an inclusion composed of Mn and S as main constituent elements. When measured with an energy dispersive X-ray analyzer attached to a scanning electron microscope, Mn and S This refers to an inclusion in which the proportion occupied by is 70 atomic% or more. At this time, Te may be contained in the sulfide type inclusion, and both Te and Ca may be contained. The sulfide inclusions may include oxide inclusions. This sulfide-based inclusion is observed with an optical microscope, a scanning electron microscope or the like after mirror-polishing a sample cut from a slab 1 / 2W, 1 / 4T portion (W is the width, T is the thickness direction). It can be observed by using an instrument.

  Since the bearing steel material of the present invention can suppress the formation of coarse and long sulfide inclusions that have been the starting point of fracture, the rolling fatigue life can be extended. For this reason, when used as a material for parts such as “ball bearings” and “roller bearings” which are important for automobiles and industrial machines, safety can be improved and costs can be reduced.

It is a figure which shows the relationship between Te / S or (Te + Ca) / S, and the ratio of the sulfide whose unevenness | corrugation degree is 3 or more among the sulfides 5 micrometers or more in a slab. The ratio of the sulfide having an unevenness degree of 3 or more in the sulfides of 5 μm or more in the slab, and the ratio of the sulfide-type inclusions having an aspect ratio of 4 or less in the sulfide-type inclusions of 5 μm or more in the steel bar It is a figure which shows the relationship. The figure which shows the relationship between the ratio of the sulfide type inclusion whose aspect-ratio is 4 or less among the sulfide type inclusions more than 5 micrometers in steel bar, and the improvement rate of the fatigue life at the time of adding Te or Te and Ca It is.

The present invention (including embodiments) will be described in detail below.
(A) Chemical composition of steel The effect of each component contained in the bearing steel material of the present invention and the reason for limiting the content thereof will be described. In addition, all “%” of each component means “mass%”.

C: 0.5 to 1.2%
C is an element that secures the strength of the base material, affects the surface hardness and machinability during quenching, and also affects the rolling fatigue life. Considering the application as bearing steel, C is required to be contained by 0.5% or more. On the other hand, when the C content is excessively high, the hardness of the base material becomes too high and the workability is lowered, and the tool life at the time of cutting is significantly reduced. For this reason, the upper limit of the C content is set to 1.2%. The desirable C content is in the range of 0.55 to 1.1%.

Si: 0.1-0.9%
Si has a solid solution strengthening action, and is an effective element for enhancing the hardenability and improving the rolling fatigue life. Si is a deoxidizing element, and it is necessary to contain 0.1% or more in order to control the O (oxygen) content described later to 0.005% or less. However, if Si is contained in excess of 0.9%, the solid solution strengthening becomes excessive, causing a reduction in tool life during cutting. For this reason, the upper limit of the Si content is set to 0.9%. A desirable Si content is 0.15 to 0.7%.

Mn: 0.1 to 2.0%
Mn is an element that improves the hardenability of the steel material, and is an element that forms MnS that improves the machinability, and is actively added. If the content is too small, the effect is poor, so it is necessary to contain 0.1% or more. However, if the Mn content is excessively high, the surface hardness after quenching becomes excessively high and the workability is lowered, and the tool life at the time of cutting is significantly reduced. Furthermore, it can cause burn cracking. For this reason, the upper limit of the Mn content is set to 2.0%. A desirable Mn content is 0.2 to 1.2%.

P: ≦ 0.03%
P is an element that precipitates at the grain boundaries and causes a reduction in the toughness and ductility of the steel material. When the content of P exceeds 0.03%, a decrease in rolling fatigue life appears remarkably, so the content of P is set to 0.03% or less. A desirable P content is 0.01% or less.

S: 0.0003 to 0.03%
S is a harmful element that reduces the strength of steel, while it is a free-cutting element and improves machinability as MnS. The smaller the addition amount, the better. However, in order to ensure machinability, a content of 0.0003% or more is necessary. On the other hand, the higher the S content, the higher the probability that coarse sulfide inclusions will be generated. However, considering the range in which the effect of sulfide morphology control associated with the addition of Te or Te and Ca can be reliably obtained. The upper limit of the S content was 0.03%.

O: ≦ 0.005%
O is an element inevitably contained in the manufacturing process of the steel material, and may be an oxide inclusion or may be dissolved. Since it is difficult to separate oxygen from oxide inclusions and dissolved oxygen, the O content in the present invention is the total O concentration of both. Oxide inclusions greatly affect the strength, machinability, and rolling fatigue life of steel materials. When dissolved, it affects the morphology of sulfide inclusions. When the total O concentration is increased, the probability that coarse oxide inclusions are generated increases, and in addition, the above Ca addition effect cannot be sufficiently obtained. For this reason, the upper limit of the content of O is set to 0.005%. A desirable O content is 0.002% or less.

N: ≦ 0.02%
N is an element that constitutes a nitride and refines the crystal grains to improve the strength of the base material. When N is contained in excess of 0.02%, coarse nitrides are generated, and the rolling fatigue life is reduced. For this reason, the upper limit of the content of N is set to 0.02%.

Te: 0.0002 to 0.021% (in the case of single addition)
Te: 0.0001 to 0.016% (when combined with Ca)
Te is an important element that changes the crystallization form of sulfide inclusions by forming MnTe having a low melting point by combining with Mn. That is, Te forms MnS-MnTe having a melting point lower than that of MnS during the solidification process, thereby changing the crystallization form of sulfide from eutectic to orthorhombic, and the inclusion shape is nearly spherical as described above. Is possible. It also has an effect of suppressing the extension of sulfide inclusions.

  In order to obtain the shape control effect of sulfide inclusions accompanying the addition of Te, a content of 0.0002% or more is necessary in the case of adding Te alone. On the other hand, when Te is added in combination with Ca, since Ca forms CaS, the amount of Te necessary to form MnS-MnTe is small as will be described later. For this reason, the lower limit of the Te content when combined with Ca is set to 0.0001%.

  Further, if the Te content becomes too high, problems such as hot embrittlement during casting and cracking during rolling occur. Therefore, in the case of adding Te alone, the upper limit of the Te content is 0.021%. did. On the other hand, when compounding with Ca, the amount of Te addition can be reduced by the amount of Ca addition, so the upper limit of the Te content when compounding with Ca is 0.016%.

Ca: 0.0001 to 0.005%
Ca dissolves in sulfide inclusions to form CaS, thereby relaxing supercooling during sulfide crystallization and suppressing the coarsening of sulfide inclusions. Moreover, when it adds simultaneously with Te, the production | generation of FeTe which causes the hot embrittlement at the time of casting can be suppressed by forming CaTe. However, when Ca alone is added, the crystallization form of sulfide does not change greatly, so it is necessary to add it together with Te. Since the effect is obtained even if the Ca content is very small, the lower limit was made 0.0001%. However, even if Ca is contained in excess of 0.005%, the effect is saturated, and the refining time is extended with the addition of Ca, so the melting cost increases. For this reason, the upper limit of the Ca content is set to 0.005%.

  The bearing steel material of the present invention is positively added with Al or / and any one or more of Cr, Ti, V, Mo, Nb, Ni and REM (rare earth elements) as necessary. You may make it contain.

Al: ≦ 0.04%
Al is a strong deoxidizing element and forms Al ₂ O ₃ in steel. It is preferable to sufficiently deoxidize to obtain the Ca addition effect described above, but excessive Al addition should be avoided because coarse Al ₂ O ₃ reduces the rolling fatigue life. For this reason, the content of Al when added is preferably 0.04% or less.

Cr: 0.05-2.0%
Cr is an element effective for improving the hardenability and improving the rolling fatigue life, and the effect becomes remarkable when the content is 0.05% or more. However, when Cr is contained in excess of 2.0%, the surface hardness after quenching becomes too high and the workability is lowered, and the tool life at the time of cutting is significantly reduced. For this reason, when Cr is added, the Cr content is preferably 0.05 to 2.0%. The desirable Cr content in this case is 0.9 to 1.6%.

Ti: 0.005 to 0.1%
Ti is a deoxidizing element and is an element that improves the strength of the base metal by forming carbides and nitrides in the steel, and its effect can be obtained by adding 0.005% or more. However, if the Ti content exceeds 0.1%, the rolling fatigue life decreases due to the coarsening of hard TiN. Therefore, when Ti is added, the Ti content is preferably 0.005 to 0.1%.

V: 0.008 to 0.3%
V is an element that forms carbides and nitrides in the steel and improves the strength of the base material by strengthening ferrite, and the effect can be obtained by adding 0.008% or more. However, if V is contained in excess of 0.3%, the machinability is lowered. Therefore, when V is added, the V content is preferably 0.008 to 0.3%.

Mo: 0.008 to 1.0%
Mo is an element that improves the hardenability and improves the strength of the base material, and its effect can be obtained by adding 0.008% or more. However, when Mo is contained in excess of 1.0%, the tool life during cutting is reduced. Therefore, the Mo content when added is preferably 0.008 to 1.0%.

Nb: 0.008 to 0.1%
Nb is an element that forms carbides or carbonitrides in steel and improves the strength of the base metal by strengthening ferrite, and its effect can be obtained by adding 0.008% or more. However, when Nb exceeds 0.1%, coarse nitrides are formed and the rolling fatigue life is reduced, and the tool life during cutting is reduced. Therefore, the Nb content when added is preferably 0.008 to 0.1%.

Ni: 0.008 to 2.0%
Ni is an element that improves the hardenability and improves the strength of the base material, and its effect can be obtained by adding 0.008% or more. However, if the Ni content exceeds 2.0%, the tool life at the time of cutting will be reduced, and it will cause burn cracking. Therefore, the Ni content when added is preferably 0.008 to 2.0%.

REM (rare earth element): 0.002 to 0.01%
REM refers to 17 elements of scandium (Sc), yttrium (Y) and lanthanoid. REM has strong affinity with S and O, and forms REM oxide or REM sulfide. REM is considered to be chemically equivalent in steel materials, and the total content thereof is taken as the REM content. When misch metal is used as the REM source, the sum of La, Ce and Nd contents can be regarded as the REM content for convenience. By adding 0.002% or more of REM, REM inclusions are finely dispersed in the steel material, and the machinability of the steel material is improved. However, if the REM content exceeds 0.01%, the amount of hard REM oxide generated increases, leading to a reduction in tool life during cutting. Therefore, the REM content when added is preferably 0.002 to 0.01%.

  The bearing steel material of the present invention is composed of the above-described components, the remaining Fe and impurities. Mg is an inevitable element mixed into molten steel by elution from a refractory, and is generally handled as an impurity. When the Mg concentration in the molten steel is high, hard MgO is formed and the machinability of the steel material is lowered. Further, when MgS is formed, it becomes a crystallization starting point of sulfide inclusions, and the spheroidization of inclusions is inhibited. Therefore, the Mg content is 0.002% or less, and further 0.001% or less. It is desirable that

(B) Te / S and (Te + Ca) / S
The steel materials of the present invention shown in the above (1) and ( 2 ) adjust the amount of Te or (Te + Ca) to be introduced according to the S content in the molten steel in order to control the form of sulfide inclusions. There is a need. The basis of this technology is that by adding Te, MnTe having a melting point lower than that of MnS is formed in the solidification process of the molten steel, so that sulfide inclusions are changed to MnS-MnTe having a melting point lower than that of MnS. The object is to solidify the crystals by suppressing the bar-shaped eutectic formation of the inclusions.

  According to the Fe-MnS pseudo binary phase diagram reported in the aforementioned Non-Patent Document 1, normally, MnS is a eutectic in which a solid phase of Fe and a solid phase of MnS are formed from a liquid phase mainly composed of Fe. Produced by reaction. The MnS form produced at this time is a long bar-shaped eutectic, and when it is rolled, it becomes longer and can be the starting point of fracture. However, when the liquid phase of MnS is prepared so that the liquid phase can exist metastable, MnS is generated by a monotectic reaction in which a solid phase of Fe and a liquid phase of MnS are formed from a liquid phase mainly composed of Fe. The form of MnS produced by the monotectic solidification is spherical and has a low probability of being elongated even when rolled. Te / S> 0.4 needs to be satisfied in order to reliably obtain the effect of suppressing the bar-like eutecticization, that is, the effect due to the monotectic solidification.

  Here, when Ca is added in addition to Te at the end of solidification of the molten steel, the sulfide inclusions are composed of CaS, MnS, and MnTe under the condition that the molten steel is sufficiently deoxidized. . That is, the amount of Te necessary to form MnS-MnTe having a low melting point is reduced by the amount of S that forms CaS. Therefore, when Te and Ca are added in combination, it is necessary to satisfy (Ca + Te) / S> 0.4 in order to reliably obtain the rod-like eutectic suppression effect.

  Te and Ca described above can be added as a metal lump or alloy lump in the refining stage of molten steel. Moreover, it is also possible to add as an alloy wire mixed with other elements. Since these elements have a low addition yield, it is desirable to add them from the latter half of the refining to the last stage. Furthermore, it is desirable to add to the fully deoxidized molten steel.

  On the other hand, the rod-like eutectic suppression effect of sulfide inclusions is obtained by satisfying the above Te / S> 0.4 or (Ca + Te) / S> 0.4, and beyond this, Te or Ca is contained. The effect is saturated even if added excessively. Therefore, it is desirable that Te / S or (Ca + Te) / S is less than 0.6, more preferably less than 0.5.

(C) Form of sulfide inclusions The steel material of the present invention satisfies the chemical composition shown in (A) above and satisfies the relationship of Te, Ca, and S shown in (B). Through a general ladle refining process, it is produced by a continuous casting method or an ingot-making method. The ladle refining process here refers to a refining facility using a ladle refining facility having a heating device called LF (Laddle Furnace) and VAD (Vacuum Arc Degassing), and a refining facility using a vacuum degassing device called RH. Used to refer to the process of adjusting molten steel temperature and composition. It is necessary that the sulfide in the steel material is generated by the twin crystal solidification by the above process.

  The bearing steel material of the present invention is obtained by subjecting the slab or steel ingot produced by the continuous casting method or the ingot-making method to ingot rolling in a temperature range exceeding 1000 ° C. It can be manufactured by applying rolling. Through the above process, the sulfide inclusions in the steel material are extended in the longitudinal direction. At this time, the sulfide-based inclusion appearing in the longitudinal cross section of the steel material needs to be an inclusion having an aspect ratio (length / thickness ratio) as small as possible, that is, an inclusion generated by monotectic solidification. When there are many sulfide inclusions that are formed as rod-like eutectic and are elongated in the steel, the rolling fatigue test increases the probability of sulfide appearing on the raceway surface on the specimen, and the rolling fatigue life Cause a significant drop in On the other hand, when the formation of rod-shaped eutectic is suppressed and sulfide inclusions are generated by the monotectic solidification, the probability that sulfide inclusions appear on the raceway surface can be greatly reduced.

The sulfide inclusions in the slab or steel produced by the above continuous casting method can be observed with an observation device such as an optical microscope or a scanning electron microscope. A sulfide having a circle equivalent diameter of 5 μm or more present in the observation field of 100 mm ² , which is likely to be a starting point of fracture in a rolling fatigue test (hereinafter referred to as “sulfide of 5 μm or more”). All) are the objects of observation.

  In addition, the solidification form of the sulfide can be determined by quantifying the sulfide shape based on a microscope observation image in the present invention. That is, while the solidified sulfide is present in an isolated shape in a nearly spherical shape, in the case of eutectic solidification, it exists in the shape of a bar that extends long. When this feature is quantified and compared with the case where Te alone or both of Te and Ca are added and the case where neither of them is added, when the ratio of sulfides close to spherical is large, On the other hand, when the existence ratio of sulfides close to a sphere is small, that is, when the existence ratio of long sulfides is large, the sulfide is produced as a rod-like eutectic. judge.

  Steels having the compositions shown in Table 1 were melted. At this time, the amount of molten steel was about 80 tons, and the amounts of Te and Ca were changed for each steel type to produce the steel.

  In the refining stage, the hot metal discharged from the blast furnace was de-S treated with a hot metal pretreatment facility, de-P and de-C treated with a converter-type refining vessel, and then received into a ladle. Next, a lime-fluorite-based flux was blown by a flux injection method, and the molten steel was stirred for about 40 minutes using Ar gas by VAD. In addition, after adjusting the molten steel composition in VAD, Te was added to the molten steel. Moreover, when adding Ca, after adding Ca with a wire addition apparatus, Te was added. Then, it cast by the mold of 300 mm x 400 mm size by the continuous casting method, and obtained the slab.

A block sample for observing inclusions was cut out from the 1/2 W and 1/4 T parts (W is the width and T is the thickness direction) of the slab obtained here, embedded in resin, and mirror-polished. Inclusions in the obtained sample were observed with an optical microscope or a scanning electron microscope, and the morphology of the sulfide inclusions was investigated. In addition, in the observation of inclusions at this time, for the purpose of determining whether the sulfide-type inclusions are rod-like eutectics or monocrystals, sulfide-type inclusions of 5 μm or more existing in 100 mm ² are used. All of the objects were examined for size and shape. Here, the shape of the sulfide inclusions was quantitatively evaluated by calculating the degree of unevenness shown in the following formula (1) by image processing.
Concavity and convexity = (sulfide circumference) ² / (4 × π × sulfide area) (1)

  In other words, at the slab stage, the solidified sulfides in the form of spheroids have a nearly spherical shape, and the sulfides produced by the eutectic reaction exist in the shape of elongated rods. From the ratio of sulfides with a degree of unevenness exceeding 3, the solidification form of the monotectic or eutectic was determined.

  These cast slabs were held in a soaking furnace at 1200 ° C. and then cracked and rolled at a temperature range of 1020 to 1100 ° C. to obtain steel pieces having a cross-sectional dimension of 160 mm × 160 mm. Further, this steel slab was heated to 1050 ° C., and then rolled in a temperature range of 800 to 930 ° C. to obtain a steel bar having a diameter of 70 mm (hereinafter referred to as “φ70 mm”). At this time, the reduction was applied so that the total reduction ratio was 15 or more and the reduction ratio in the temperature range of 1000 ° C. or more was 4 or more.

A block sample for observing inclusions was cut out from the 1/2 W, 1/4 T part of the steel bar obtained here, embedded in resin, and mirror polished. Similar to the investigation at the slab stage, the inclusions in the obtained sample were observed with an optical microscope or a scanning electron microscope, and the form of the sulfide inclusions was investigated. In addition, in the inclusion observation at this time, for the purpose of investigating the aspect ratio of sulfide inclusions of 5 μm or more, the observation area of 100 mm ² in the longitudinal direction was observed 10 times, and the average of the sulfide type was observed. The aspect ratio of inclusions was calculated. If the aspect ratio was 4 or less, it was evaluated as a sulfide having a nearly spherical shape, and the solidification form of the monotectic or eutectic was determined from the ratio of the sulfide having an aspect ratio of 4 or less.

  A spheroidizing annealing treatment (furnace cooling after holding at 780 ° C. for 6 hours) is applied to the φ70 mm steel bar obtained by the above method, and then the diameter is 60 mm so that the longitudinal direction becomes the thickness of the shaped material. The raw material with a thickness of 5.5 mm was sliced and collected. The shaped material was heated at 830 ° C. for 30 minutes, then quenched with oil, further heated at 180 ° C. for 1 hour, and then tempered by being allowed to cool in the air. Further, a rolling fatigue test piece was prepared by lapping the surface of the shaped material and subjected to a rolling fatigue test.

  Table 2 shows the rolling fatigue test conditions.

The rolling fatigue test was performed using a thrust type rolling fatigue tester under conditions of a repetition rate of 1800 cpm (cycle per minute) and a maximum contact surface pressure of 5230 MPa. Rolling fatigue test results were plotted on paper Weibull distribution probability was evaluated L ₁₀ life showing a 10% failure probability as a rolling contact fatigue life. At this time, in the rolling fatigue life in the same steel type, a condition in which an improvement in the rolling fatigue life of 15% or more was recognized compared with the condition in which Te was not added was determined to have an improvement effect.

  Table 3 shows the ratio (%) of the number of sulfides having an unevenness degree of 3 or more to the total number of sulfides of 5 μm or more, and the ratio (%) of the number of sulfides having an aspect ratio of 4 or less to the total number of sulfides of 5 μm or more. The rolling fatigue life and the effect of improving fatigue life are shown. In the “Fatigue life improvement effect” column, a mark “◯” indicates that there was an improvement effect, and a mark “X” indicates that there was no improvement effect. Further, “book” in the “classification” column means an example of the present invention, and “ratio” means a comparative example.

  The results of Table 3 are displayed in FIGS. 1 to 3 so that the relationship between the survey items shown in Table 3 can be visually recognized in association with the present invention example and the comparative example.

  FIG. 1 is a diagram showing the relationship between Te / S or (Te + Ca) / S and the ratio of sulfides having a degree of unevenness of 3 or more among sulfides of 5 μm or more in a slab. As shown in FIG. 1, the form of sulfide inclusions changes abruptly when Te / S or (Te + Ca) / S is around 0.4. This is because the formation mechanism of sulfide is from eutectic solidification. This is due to the change to the monotectic solidification. The generation of sulfide is performed by monotectic solidification (the scope of the present invention in FIG. 1), so that the ratio of the number of sulfides having a degree of unevenness of 3 or more is drastically reduced. It can be seen that it has a small, nearly spherical shape.

  FIG. 2 shows the ratio of sulfides having a degree of unevenness of 3 or more in sulfides of 5 μm or more in a slab, and sulfide-based inclusions having an aspect ratio of 4 or less in sulfide steel inclusions of 5 μm or more in steel bars. It is a figure which shows the relationship with the ratio of a thing. As is clear from the figure, the steel of the present invention has few long sulfides at the slab stage, and the ratio of sulfides having a large aspect ratio is low even after rolling. On the other hand, in the comparative example, there are many long sulfides from the slab stage, and the ratio of sulfides having a large aspect ratio after rolling is high. A clear difference appears between the two, suggesting that this is a difference in the solidification form of sulfide.

  FIG. 3 shows the ratio of sulfide inclusions having an aspect ratio of 4 or less among sulfide inclusions of 5 μm or more in steel bars, and Te or Te and Ca with respect to the Te non-addition condition of each steel type. It is a figure which shows the relationship with the improvement rate of the fatigue life at the time of adding. As shown in FIG. 3, the fatigue life improvement effect is correlated with the ratio of sulfide inclusions having an aspect ratio of 4 or less, and the high fatigue life improvement rate means that the sulfide inclusions are stretched. It turns out that it has become a condition to have the form which is not.

  From the above investigation results, by solidifying with Te / S or (Te + Ca) / S being 0.4 or more, the sulfide inclusions can control the solidification form to be monotectic, and the aspect ratio after rolling It can be seen that it can be kept low.

  The bearing steel material of the present invention is a bearing steel that has improved rolling fatigue life by controlling the form of sulfides in the steel. When used as a material for parts such as “ball bearings” and “roller bearings” which are important for automobiles and industrial machines, safety can be improved and costs can be reduced, and the social contribution of the present invention is very large.

Claims (2)

In mass%, C: 0.5-1.2%, Si: 0.1-0.9%, Mn: 0.1-2.0%, P: ≦ 0.03%, S: 0.0003 -0.03%, O: ≦ 0.005% , N: ≦ 0.02% , Al: 0.0005-0.04% and Cr: 0.05-2.0% ,
And Te: 0.0001 to 0.016% and Ca: 0.0001 to 0.005%, the balance being Fe and impurities, and
The content of S, Te and Ca in mass% satisfies the condition of (Te + Ca) / S> 0.4,
Have a monotectic solidified sulfide inclusions, the sulfide-based inclusions,
Inclusions in which the proportion of Mn and S is 70 atomic% or more, and
In the continuously cast slab, the ratio of the unevenness shown in the following formula (1) is 3 or more is 5.4% or less for all the inclusions having a size of 5 μm or more. Bearing steel material characterized by.
Concavity and convexity = (sulfide circumference) ² / (4 × π × sulfide area) (1) Instead of a part of Fe, by mass%, Ti: 0.005 to 0.1%, V: 0.008 to 0.3%, Mo : 0.008 to 1.0%, Nb: 0.008 It is characterized by containing one or more of -0.1%, Ni: 0.008-2.0% and REM (rare earth element): 0.002-0.01%. The bearing steel material according to 1 .