JP2021109993A - Steel for bearings - Google Patents

Abstract

To provide a steel for bearings that has excellent machinability after normalizing and has, after hardening and tempering, excellent hardness, toughness, rolling fatigue strength, wear resistance and structure change resistance.SOLUTION: A steel for bearings has a predetermined chemical composition and satisfies the following (1) formula and the following (2) formula. 1.100≤C+Si/7+Mn/5+Cr/9+Mo/2.5≤1.700 (1) and (Cr/Mn)+V≥5.00 (2) where each element symbol denotes the content of the element in mass% and, if the element is absent, 0 is substituted.SELECTED DRAWING: None

Description

The present invention relates to bearing steel.

In general, steel used for bearings, gears, shafts, pulleys, etc. of automobiles is processed into a part shape by cutting, so that excellent machinability is required. Further, since the steel used for these parts needs to have excellent properties against impact, friction and the like, excellent hardness, toughness, rolling fatigue strength and wear resistance are required. In general, the higher the surface hardness, the higher the rolling fatigue strength. Therefore, for machine parts that require excellent rolling fatigue strength, the steel for bearings, which is the raw material, is subjected to hot or cold working. It is manufactured in the shape of a part and subjected to heat treatment such as quenching and tempering.

In recent years, there has been a tendency to reduce the amount of lubricating liquid (oil, etc.) between mechanical parts as described above, or to reduce the viscosity, for the purpose of improving the fuel efficiency of automobiles. When the amount of lubricating liquid is reduced, the surface temperature rises due to the decrease in the cooling effect, and the mechanical parts are used in a harsher environment. Further, when the viscosity of the lubricating liquid is lowered, not only the cooling effect is lowered due to the thinning of the liquid film, but also the surface temperature of the machine part is raised due to the increase in friction. When the surface temperature of a machine part rises, the structure inside the machine part changes during use, and destruction may occur starting from the changed structure. Therefore, mechanical parts used in a high temperature environment need to have characteristics that the structure does not easily change even when the surface temperature rises.

Patent Document 1 describes in terms of mass%, C: 0.30 to 0.65%, Si: 0.25 to 1.00%, Mn: 0.25 to 0.60%, P: 0.030% or less. , S: 0.030% or less, Cr: 1.00 to 3.00%, Al: 0.005 to 0.200%, N: 0.0200% or less, O: 0.0030% or less. The balance is composed of Fe and unavoidable impurities, and the steel satisfies the value of 4 × Si + 3Cr—Mn calculated from the contents of Si, Mn, and Cr in the above composition of 6.00% or more, and the former austenite. A steel having excellent toughness and abrasion resistance, which is characterized by having a particle size of 8.0 μm or less, is disclosed.

Patent Document 2 describes that the steel structure has a predetermined chemical composition, the steel structure is made of ferrite and pearlite, the area ratio of the pearlite is 85% or more, and Al ₂ O ₃ inclusions and composite inclusions are contained in the steel. The ratio of the number of the composite inclusions to the total number of the composite inclusions is 20% or more, and the composite inclusions contain 2.0% or more of SiO ₂ and 2.0% or more of CaO in mass%. , Induction hardening steel in which 99% or more of the balance is Al ₂ O _{3 is disclosed.}

Patent Document 3 states that the standard deviation and average value of the X-ray intensity value of Cr are (the standard of the X-ray intensity value of Cr) when the EPMA line is analyzed in the direction perpendicular to the rolling direction and has a predetermined chemical composition. A steel having an excellent rolling fatigue life is disclosed, which satisfies (deviation / average value of X-ray intensity values of Cr) ≤0.25.

However, Patent Documents 1 to 3 do not consider any structural changes caused by an increase in the surface temperature of steel. When the steel disclosed in Patent Documents 1 to 3 is applied to machine parts used in a high temperature environment, hydrogen contained in the lubricating liquid or the like penetrates into the steel and diffuses due to the action of Mn contained in the steel. This may cause structural changes and destruction.

Japanese Patent No. 5868099 International Publication No. 2018/016502 Japanese Patent No. 5723232

The present invention provides a bearing steel having excellent machinability after normalizing and excellent hardness, toughness, rolling fatigue strength, wear resistance and structure change resistance after quenching and tempering. The purpose.

The gist of the present invention is as follows.
[1] The bearing steel according to one aspect of the present invention has a chemical composition of mass%.
C: 0.70 to 1.20%,
Si: 0.25 to 0.80%,
Mn: 0.20% or more, less than 0.40%,
Cr: 1.60 to 2.00%,
V: 0.02 to 0.30%,
Al: 0.005 to 0.060%,
N: 0.0020 to 0.0080%,
P: 0.020% or less,
S: 0.020% or less and O: 0.0015% or less are contained, the balance is composed of Fe and impurities, and the following equations (1) and (2) are satisfied.
1.100 ≤ C + Si / 7 + Mn / 5 + Cr / 9 + Mo / 2.5 ≤ 1.700 ... (1)
(Cr / Mn) + V ≧ 5.00 ・ ・ ・ (2)
However, the element symbol in the above formula indicates the content of the element in mass%, and when the element is not contained, 0 is substituted.
[2] The bearing steel according to the above [1] has a chemical composition of mass%.
Ca: 0.0050% or less,
B: 0.0040% or less,
Mo: 0.80% or less,
Ti: 0.050% or less,
It may contain one or more selected from the group consisting of Nb: 0.050% or less and Ni: 0.30% or less.

According to the present invention, there is provided a bearing steel having excellent machinability after normalizing and excellent hardness, toughness, rolling fatigue strength, wear resistance and structure change resistance after quenching and tempering. Can be done.

It is a figure which shows the forest type thrust type rolling fatigue test piece used in an Example. It is a figure explaining the forest type thrust type rolling fatigue test performed in an Example.

Mechanical parts used in a high-temperature environment may be destroyed due to changes in the structure of the surface layer of the parts as the contact surface pressure accompanied by slippage is repeatedly applied, starting from the changed structure. The present inventors infer the structural changes of machine parts used in a high temperature environment as follows.

When a mechanical part is repeatedly subjected to surface pressure while slipping in a high temperature environment, local deformation occurs on the surface layer of the part, and a large strain is introduced at that location to form a white structure. When a white structure is formed inside the steel, this structure may be the starting point for microcracks, which may eventually cause fracture. The present inventors presume that the invasion of hydrogen generated by the decomposition of the lubricating liquid (oil, etc.) in a high temperature environment into the inside of the steel is one of the causes of local fracture in a high temperature environment. is doing.

Therefore, the present inventors have diligently studied the chemical composition of steel having excellent structure change resistance. The excellent tissue change resistance means that the structure is unlikely to change to a white structure even when used in a high temperature environment. As a result, the present inventors control the respective contents of Mn, Cr and V within a predetermined range, and control the contents of these elements so as to satisfy a predetermined relationship, thereby making the structure resistant. It was found that steel with excellent change characteristics can be obtained. More specifically, the present inventors improve the yield strength and suppress the occurrence of local deformation by adding V to refine the crystal grains, and reduce the Mn content to reduce the grain boundaries. It was found that the occurrence of cracks can be suppressed by strengthening the steel, and further, by containing Cr, a steel having excellent structure change resistance can be obtained.

The steel for bearings according to the present embodiment made based on the above findings has a chemical composition of C: 0.70 to 1.20%, Si: 0.25 to 0.80%, Mn: in mass%. 0.20% or more, less than 0.40%, Cr: 1.60 to 2.00%, V: 0.02 to 0.30%, Al: 0.005 to 0.060%, N: 0.0020 It contains ~ 0.0080%, P: 0.020% or less, S: 0.020% or less and O: 0.0015% or less, and the balance consists of Fe and impurities, and the following formula (1) and the following ( 2) Satisfy the equation.
1.100 ≤ C + Si / 7 + Mn / 5 + Cr / 9 + Mo / 2.5 ≤ 1.700 ... (1)
(Cr / Mn) + V ≧ 5.00 ・ ・ ・ (2)
However, the element symbol in the above formula indicates the content of the element in mass%, and when the element is not contained, 0 is substituted.

Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the spirit of the present invention. The numerical limit range described with "~" in between includes the lower limit value and the upper limit value. The value indicated as "less than" does not include the value in the numerical range. All% of the chemical composition indicates mass%.

C: 0.70 to 1.20%
Carbon (C) increases the hardness of bearing steel after quenching and tempering. Since this effect cannot be obtained when the C content is low, the C content is set to 0.70% or more. It is preferably 0.80% or more. On the other hand, when the C content is high, the retained austenite content becomes too high and the hardness of the bearing steel after quenching decreases. Therefore, the C content is set to 1.20% or less. Preferably, it is 1.10% or less.

Si: 0.25 to 0.80%
Silicon (Si) imparts hardenability to steel and enhances rolling fatigue strength. Furthermore, Si improves temper softening resistance. That is, Si suppresses softening in a high temperature environment during tempering treatment and use. Since the above effect cannot be obtained when the Si content is low, the Si content is set to 0.20% or more. It is preferably 0.22% or more and 0.30% or more. On the other hand, when the Si content is high, the hardness of the steel increases and the machinability deteriorates. Therefore, the Si content is set to 0.80% or less. It is preferably 0.75% or less, 0.70% or less, and 0.65% or less.

Mn: 0.20% or more and less than 0.40% Manganese (Mn) enhances hardenability during quenching. If the Mn content is low, the above effect cannot be obtained, so the Mn content is set to 0.20% or more. It is preferably 0.25% or more. On the other hand, when the Mn content is high, Mn segregates at the grain boundaries, which deteriorates the structure change resistance of the steel. Therefore, the Mn content is set to less than 0.40%. It is preferably 0.35% or less.

Cr: 1.60 to 2.00%
Chromium (Cr) imparts hardenability to steel to increase its strength. In addition, Cr improves tempering softening resistance and suppresses softening in a high temperature environment during tempering treatment and use. Furthermore, Cr improves the structure change resistance by suppressing the hydrogen sensitivity of steel. Since the above effect cannot be obtained when the Cr content is low, the Cr content is set to 1.60% or more. It is preferably 1.65% or more, 1.70% or more, and 1.80% or more. On the other hand, when the Cr content is high, the toughness decreases and the hardness of the steel increases, resulting in deterioration of machinability. Therefore, the Cr content is set to 2.00% or less. It is preferably 1.95% or less.

V: 0.02 to 0.30%
Vanadium (V) improves the yield strength of steel by forming fine V-nitrides, V-carbides, or V-carbonitrides and suppressing coarsening of crystal grains during quenching, and is applied to the surface layer of parts under pressure. Suppresses the occurrence of local deformation. In addition, V improves the rolling fatigue strength and the structure change resistance of the steel. It is considered that one of the causes is that the V precipitate traps the hydrogen that has entered the inside of the steel and suppresses the embrittlement due to the hydrogen intrusion. Since the above effect cannot be obtained when the V content is low, the V content is set to 0.02% or more. It is preferably 0.05% or more, 0.10% or more, and 0.15% or more. On the other hand, when the V content is high, coarse V precipitates are formed in the steel and the toughness of the steel is lowered. Therefore, the V content is set to 0.30% or less. Preferably, it is 0.25% or less and 0.21% or less.

Al: 0.005 to 0.060%
Aluminum (Al) deoxidizes molten steel. Further, Al combines with N in the steel to form AlN, which suppresses coarsening of crystal grains during quenching and tempering. If the Al content is low, the above effect cannot be obtained, so the Al content is set to 0.005% or more. It is preferably 0.008% or more and 0.015% or more. On the other hand, if the Al content is high, coarse _Al 2 _{O 3} inclusions in steel, and / or a plurality of _Al 2 _{O 3} inclusions are aggregated _Al 2 _{O 3} clusters in large amounts produced, after quenching and tempering The rolling fatigue strength of steel decreases. Therefore, the Al content is set to 0.060% or less. It is preferably 0.050% or less and 0.045% or less.
The Al content in the present embodiment means the total Al content.

N: 0.0020 to 0.0080%
Nitrogen (N) combines with Al to form AlN in the steel and suppresses coarsening of crystal grains during quenching, thereby increasing the rolling fatigue strength of the steel after quenching and tempering. If the N content is low, the above effect cannot be obtained, so the N content is set to 0.0020% or more. It is preferably 0.0025% or more and 0.0028% or more. On the other hand, when the N content is high, N is excessively dissolved in ferrite to cause strain aging, and the cold workability of the steel is lowered. Further, when the N content is high, coarse nitrides are generated in the steel, and the machinability and rolling fatigue strength of the steel are lowered. Therefore, the N content is set to 0.0080% or less. It is preferably 0.0070% or less and 0.0060% or less.

P: 0.020% or less Phosphorus (P) is an impurity element. Since P segregates at the grain boundaries and embrittles the grain boundaries, it reduces the rolling fatigue strength of the steel after quenching and tempering. Therefore, the P content is limited to 0.020% or less. The P content is preferably 0.015% or less and 0.011% or less. The P content is preferably 0%, but even if the P content is excessively reduced, the effect corresponding to the increase in the refining cost cannot be obtained. Therefore, the P content is 0.003% or more and 0.005%. The above may be applied.

S: 0.020% or less Sulfur (S) is an impurity element. S forms coarse inclusions (MnS) in the steel and reduces the rolling fatigue strength of the steel after quenching and tempering. Therefore, the S content is limited to 0.020% or less. The S content is preferably 0.016% or less, 0.013% or less, and 0.010% or less. The S content is preferably 0%, but even if the S content is excessively reduced, the effect corresponding to the increase in the refining cost cannot be obtained. Therefore, the S content is 0.003% or more and 0.005%. The above may be applied.

O: 0.0015% or less Oxygen (O) is an impurity element. O combines with Al, Si and Ca to form oxides (or oxide-based inclusions), which reduces the rolling fatigue strength of steel after quenching and tempering. Therefore, the O content is limited to 0.0015% or less. The O content is preferably 0.0014% or less and 0.0011% or less. The O content is preferably 0%, but even if the O content is excessively reduced, the effect corresponding to the increase in the refining cost cannot be obtained. Therefore, the O content is 0.0003% or more and 0.0005%. The above may be applied.

The rest of the chemical composition of the bearing steel according to this embodiment is Fe and impurities. In the present embodiment, the impurities are those mixed from ore, scrap, manufacturing environment, etc. as a raw material when the bearing steel is industrially manufactured, and are mixed in the bearing steel according to the present embodiment. It means something that is acceptable as long as it does not adversely affect it.

The bearing steel according to the present embodiment may contain one or more of the following optional elements instead of the remaining Fe. When the following optional elements are not contained, the content is 0%.

Ca: 0.0050% or less Calcium (Ca) modifies Al ₂ O ₃ inclusions to form complex inclusions (Al ₂ O _3- CaO-SiO ₂ ). Ca further improves the rolling fatigue strength of steel after quenching and tempering by modifying _{Al 2} O _{3 inclusions to form composite inclusions.} In order to surely obtain this effect, the Ca content is preferably 0.0005% or more. On the other hand, when the Ca content is high, coarse inclusions may increase in the steel and the rolling fatigue strength of the steel after quenching and tempering may decrease. Therefore, the Ca content is preferably 0.0050% or less. The Ca content is more preferably 0.0040% or less.

B: 0.0040% or less Boron (B) dissolves in the steel to improve the hardenability of the steel, thereby further improving the rolling fatigue strength of the steel after quenching and tempering. In order to surely obtain this effect, the B content is preferably 0.0005% or more. More preferably, it is 0.0010% or more. On the other hand, when the B content is high, the above effects are saturated and the alloy cost increases. Therefore, the B content is preferably 0.0040% or less. More preferably, it is 0.0030% or less.

Mo: 0.80% or less Molybdenum (Mo) dissolves in the steel to further improve the rolling fatigue strength of the steel after quenching and tempering. In order to surely obtain this effect, the Mo content is preferably 0.01% or more. More preferably, it is 0.07% or more and 0.12% or more. On the other hand, when the Mo content is high, the hardness after normalizing may increase and the machinability may decrease. Therefore, the Mo content is preferably 0.80% or less. More preferably, it is 0.60% or less and 0.50% or less.

Ti: 0.050% or less Titanium (Ti) forms Ti nitrides or Ti carbides in the steel to suppress coarsening of crystal grains during quenching, thereby rolling fatigue of the steel after quenching and tempering. Further improve the strength. In order to surely obtain this effect, the Ti content is preferably 0.010% or more. More preferably, it is 0.015% or more and 0.020% or more. On the other hand, when the Ti content is high, coarse Ti nitrides and / or Ti carbides may be formed in the steel, which may reduce the machinability of the steel. Therefore, the Ti content is preferably 0.050% or less. More preferably, it is 0.045% or less and 0.040% or less.

Nb: 0.050% or less Niobium (Nb) forms Nb nitride, Nb carbide or Nb carbonitride in steel, and suppresses coarsening of crystal grains by the pinning effect. In order to surely obtain this effect, the Nb content is preferably 0.010% or more. More preferably, it is 0.020% or more and 0.025% or more. On the other hand, when the Nb content is high, coarse Nb precipitates may be formed in the steel and the toughness of the steel may decrease. Therefore, the Nb content is preferably 0.050% or less. More preferably, it is 0.040% or less.

Ni: 0.30% or less Nickel (Ni) is dissolved in steel to further improve the rolling fatigue strength of steel after quenching and tempering. In order to surely obtain this effect, the Ni content is preferably 0.001% or more. More preferably, it is 0.01% or more. On the other hand, when the Ni content is high, the above effects are saturated and the alloy cost increases. Therefore, the Ni content is preferably 0.30% or less. More preferably, it is 0.25% or less.

The bearing steel according to the present embodiment has the above chemical composition and satisfies the following formulas (1) and (2).
1.100 ≤ C + Si / 7 + Mn / 5 + Cr / 9 + Mo / 2.5 ≤ 1.700 ... (1)
(Cr / Mn) + V ≧ 5.00 ・ ・ ・ (2)
However, the element symbol in the above formula indicates the content of the element in mass%, and when the element is not contained, 0 is substituted. The above equation (1) and the above equation (2) will be described below.

1.100 ≤ C + Si / 7 + Mn / 5 + Cr / 9 + Mo / 2.5 ≤ 1.700 ... (1)
The middle side of the above equation (1) indicates a hardenability index (Ceq). If Ceq is less than 1.100, the hardenability of the steel becomes insufficient and the desired hardness cannot be obtained. Therefore, Ceq is set to 1.100 or more. It is preferably 1.200 or more and 1.300 or more. On the other hand, when Ceq exceeds 1.700, the hardenability of the steel becomes excessively high, and hard bainite is formed in a part of the metal structure of the rolled bearing steel. This deteriorates the cold workability of steel. Therefore, Ceq is set to 1.700 or less. Ceq is preferably 1.600 or less and 1.500 or less.

(Cr / Mn) + V ≧ 5.00 ・ ・ ・ (2)
The left side of the above equation (2) shows an index of the structure change resistance characteristics of the bearing steel according to the present embodiment. If the left side of the above equation (2) is less than 5.00, the structure change resistance property of the steel deteriorates, a white structure is formed when the steel is used in a high temperature environment, and fracture is likely to occur. Therefore, the left side of the above equation (2) is set to 5.00 or more. The left side of the above equation (2) is preferably 5.50 or more and 6.00 or more. The left side of the above equation (2) may be 10.30 or less from the values that the Cr content, Mn content, and V content of the bearing steel according to the present embodiment can be taken.

The chemical composition of the bearing steel described above may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy). C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas melting-thermal conductivity method, and O may be measured using the inert gas melting-non-dispersion infrared absorption method.

Since the steel according to the present embodiment is a steel for bearings and the metal structure to be controlled is the metal structure after quenching, the metal structure of the steel for bearings (before quenching) according to the present embodiment is not particularly limited. In the metal structure of the steel according to the present embodiment, the total area ratio of ferrite (initialized ferrite), pearlite and cementite may be 90% or more, and the area ratio of the residual structure may be 10% or less. Further, the residual structure may be 0%, the total area ratio of ferrite and cementite may be 100%, and the total area ratio of ferrite, pearlite and cementite may be 100%. The residual structure in the metal structure is, for example, bainite. However, the bearing steel according to the present embodiment can obtain the characteristics of the bearing steel according to the present embodiment even if it does not have the above-mentioned metal structure.

The bearing steel according to the present embodiment may be appropriately selected depending on the applicable machine parts and the like, and may be steel bars or wire rods.

Next, an example of a method for manufacturing bearing steel according to the present embodiment will be described.
In this embodiment, as an example of bearing steel, a method for manufacturing a steel bar or wire rod for quenching and tempering will be described. However, the bearing steel according to the present embodiment is not limited to steel bars or wire rods.

An example of the manufacturing method includes a steelmaking process in which molten steel is smelted and cast to produce a material (slab or ingot), and a hot processing process in which the material is hot-processed to produce steel for bearings. Hereinafter, each step will be described.

[Steelmaking process]
The steelmaking process includes a refining process and a casting process.

In the refining process, first, the hot metal produced by a well-known method is refined in a converter (primary refining). Secondary refining is carried out on the molten steel discharged from the converter. In the secondary refining, an alloy element for adjusting the composition is added to produce molten steel satisfying the above-mentioned chemical composition.

A material (casting piece or ingot) is manufactured using the molten steel produced by the above refining step (casting step). Specifically, a slab is produced by a continuous casting method using molten steel, or an ingot is produced by an ingot method using molten steel.

[Hot working process]
The manufactured material is hot-worked to produce bearing steel (steel bar or wire). The hot working step includes a rough rolling step and a finish rolling step. In the rough rolling process, the material is hot-processed to produce billets. In the rough rolling step, for example, a bulk rolling mill is used. Billets are manufactured by performing slab rolling on the material with a slab rolling mill.

When performing bulk rolling in the rough rolling step, the heating temperature and holding time in the heating furnace are as follows.
Heating temperature: 1150 to 1300 ° C
Holding time at the above heating temperature: 1.5 to 40.0 hours Here, the heating temperature is the furnace temperature (° C.) of the heating furnace. The holding time is the holding time (hours) when the furnace temperature of the heating furnace is 1150 to 1300 ° C.

If the heating temperature is less than 1150 ° C. or the holding time at 1150 to 1300 ° C. is less than 1.5 hours, the V-carbide and V-composite carbide in the material are not sufficiently dissolved. Therefore, the V-nitride, V-carbide, or V-carbonitride becomes coarse, and it is not possible to suppress the coarsening of crystal grains during quenching and tempering. On the other hand, if the heating temperature exceeds 1300 ° C. or the holding time at 1150 to 1300 ° C. exceeds 40.0 hours, the basic unit becomes excessively high and the manufacturing cost becomes high.

If the heating temperature in the rough rolling process is 1150 to 1300 ° C. and the holding time at 1150 to 1300 ° C. is 1.5 to 40.0 hours, V nitride, V carbide, or V charcoal in the material. The nitride dissolves sufficiently.

In the finish rolling process, the billet is first heated using a heating furnace. The billet after heating is hot-rolled using a continuous rolling machine to produce steel bars or wire rods, which are steel materials for bearing parts. The heating temperature and holding time in the heating furnace in the finish rolling process are as follows.
Heating temperature: 1100 to 1300 ° C
Holding time at the above heating temperature: 0.5 to 5.0 hours Here, the heating temperature is the furnace temperature (° C.) of the heating furnace. The holding time is the holding time (hours) when the furnace temperature of the heating furnace is 1100 to 1300 ° C.

In the finish rolling step, the precipitation of V carbides and the like and V composite carbides and the like during the finish rolling step is suppressed as much as possible. If the heating temperature in the heating furnace of the finish rolling process is less than 1100 ° C. or the holding time at 1100 to 1300 ° C. is less than 0.5 hours, the load applied to the rolling mill during finish rolling becomes excessively large. .. On the other hand, if the heating temperature exceeds 1300 ° C. or the holding time at 1100 to 1300 ° C. exceeds 5.0 hours, the basic unit becomes excessively high and the manufacturing cost becomes high.

If the heating temperature in the finish rolling process is 1100 to 1300 ° C. and the holding time at 1100 to 1300 ° C. is 0.5 to 5.0 hours, V carbides and the like and V composite carbides and the like in the material will be present. It dissolves sufficiently.

The bearing steel after finish rolling is cooled to room temperature. At this time, the average cooling rate until the surface temperature of the bearing steel reaches 800 to 500 ° C. is set to 1.0 ° C./sec or less, and ferrite, pearlite and cementite are mainly used (90% or more in total of the area fraction). It is preferable to have a metal structure.

If necessary, the produced bearing steel may be subjected to a normalizing treatment (normalizing treatment) or a spheroidizing normalizing treatment. The bearing steel according to the present embodiment can be manufactured by the manufacturing method of the example described above.

The steel for bearings according to this embodiment is suitably used for automobile parts such as bearings, gears, shafts and pulleys. Bearing parts used as these automobile parts are generally manufactured by the following methods.

First, hot forging or cold forging is performed on the bearing steel to manufacture an intermediate product. If necessary, stress relief annealing is performed on the intermediate product. A crude product is obtained by cutting or drilling an intermediate product after hot forging, cold forging, or stress relief annealing, and processing it into a part shape. After that, the crude product is hardened. The heating temperature of the quenching treatment is 800 to 900 ° C. Subsequently, an oil quenching process is carried out. In this embodiment, if necessary, low-temperature tempering treatment (for example, heat treatment at 130 ° C. to 200 ° C. for about 30 to 120 minutes) may be further performed. Bearing parts can be manufactured by further performing grinding after quenching or quenching and tempering.
Bearing parts manufactured using the bearing steel according to the present embodiment can be suitably used as automobile parts such as bearings, gears, shafts, and pulleys.

Next, the effect of one aspect of the present invention will be described more specifically by way of examples. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention. The present invention is not limited to this one-condition example. The present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

A steel bar for bearings was manufactured by the following method.
First, primary refining was carried out in a converter for hot metal produced by a well-known method. Secondary refining was carried out on the molten steel discharged from the converter. In the secondary refining, the chemical composition was adjusted by adding an alloy element for adjusting the composition. Using the molten steel after the secondary refining, a slab having a cross section of 400 mm × 300 mm was produced by a continuous casting method. This slab was heated to 1250 ° C. and held for 1.5 to 40.0 hours, and then slab rolling was performed to produce a steel piece having a cross section of 162 mm × 162 mm. The produced steel pieces were air-cooled to room temperature (25 ° C.), and then heated again under the conditions of a heating temperature of 1100 to 1300 ° C. and a holding time at the heating temperature of 0.5 to 5.0 hours. The heated steel pieces were hot-rolled (finish-rolled) using a continuous rolling mill and then cooled to room temperature (25 ° C.) to obtain bearing steel bars having a diameter of 70 mm. After finish rolling, when cooling to room temperature, the average cooling rate until the surface temperature of the bearing steel bar reached 800 to 500 ° C. was 1.0 ° C./sec or less. This bearing steel bar was further spheroidized and annealed. After holding at 840 ° C. for 6 hours, the mixture was cooled to 690 ° C. at an average cooling rate of −10 ° C./h, and then allowed to cool.

Table 1 shows the chemical composition of the obtained steel bars for bearings. The chemical composition of the steel bars for bearings was measured using ICP-AES (Inductively Coupled Plasma-Atomic Measurement Spectrometry). C and S were measured using the combustion-infrared absorption method, N was measured using the inert gas melting-thermal conductivity method, and O was measured using the inert gas melting-non-dispersion infrared absorption method.
Ceq in Table 1 is the middle side of the following equation (1).
1.100 ≤ C + Si / 7 + Mn / 5 + Cr / 9 + Mo / 2.5 ≤ 1.700 ... (1)

[Rockwell hardness test]
Using the obtained steel bars for bearings, test pieces simulating mechanical parts (bearing parts) were produced by the following method. The chemical composition of the test piece is the same as that of the steel bar for bearings.
First, the obtained bearing steel bar was heated at 1200 ° C. for 30 minutes and then hot forged at a finishing temperature of 950 ° C. or higher to produce a round bar having a diameter of 70 mm. A round bar having a diameter of 60 mm was machined to produce a plurality of intermediate products having a diameter of 70 mm and a thickness of 5.5 mm. The thickness direction of the intermediate product was the longitudinal direction of the round bar.

The produced intermediate product was quenched and tempered. Specifically, it was heated at 850 ° C. for 30 minutes and then quenched with oil at 60 ° C. Then, tempering was performed at 160 ° C. for 1 hour using a normal heat treatment furnace. The intermediate product after the tempering treatment was subjected to grinding and lapping to prepare a forest-type thrust type rolling fatigue test piece having a diameter of 58 mm and a thickness of 5.0 mm as shown in FIG. By the above method, a forest type thrust type rolling fatigue test piece simulating a bearing part was produced.

The Rockwell hardness of the upper surface of the cylinder of the obtained forest-type thrust type rolling fatigue test piece was measured. Specifically, a Rockwell hardness HRC test conforming to JIS Z 2245: 2016 was carried out on any three points on the upper surface of the forest-type thrust type rolling fatigue test piece. The test force at this time was 1471N. The average value of the obtained Rockwell hardness at three points was defined as the surface Rockwell hardness (HRC1). When the surface Rockwell hardness was more than 58HRC1 and 65HRC1 or less, it was judged to be excellent, and when the surface Rockwell hardness was 58HRC1 or less or more than 65HRC1, it was judged to be inferior in hardness. The measurement results are shown in Table 2.

[Cutting test]
The machinability of steel was evaluated by a cutting test by drilling. Specifically, first, for SUJ2 steel, find the maximum cutting speed at which a total of 1000 mm can be drilled, and then use the steel of each steel number to machine at this maximum cutting speed until the drilled hole depth reaches 1000 mm. It was evaluated by whether or not. More specifically, the steel of SUJ2 having a diameter of 35 mm specified in JIS G 4805: 2008 is heated at a temperature of 820 ° C. for 10 hours, slowly cooled to a temperature of 690 ° C. at 1.0 ° C./min or less, and then in the air. It was allowed to cool down. Using a material obtained by cutting this round bar to a length of 130 mm, using a TiC-coated carbide drill with a diameter of 10 mm and a length of 300 mm, the depth is 0.2 mm / rev while lubricating with water-soluble cutting oil. Drilling was performed up to 100 mm, and the maximum cutting speed VL ₁₀₀₀ (m / min) capable of drilling a machined hole depth of 1000 mm was determined.

The maximum cutting speed VL ₁₀₀₀ is usually used as an evaluation index of the tool life, and it can be judged that the larger the _{maximum cutting speed VL 1000 is, the better the tool life is.} The steel of each steel number is subjected to the same normalizing as SUJ2 described above, drilling is performed at the maximum cutting speed VL ₁₀₀₀ obtained by SUJ2, and the work is performed until the drilled hole depth reaches 1000 mm. It was judged to be acceptable because it was excellent in properties, and "Good" was described in Table 2. If it could not be machined until the drilled hole depth was 1000 mm, it was judged to be inferior in machinability and it was judged to be unacceptable. It was described as "Bad". For the comparative examples that could not be machined until the machined hole depth became 1000 mm, the evaluation tests (rolling fatigue evaluation test, abrasion resistance evaluation test, and structure change resistance evaluation test) described later were not performed.

[Rolling fatigue evaluation test]
A forest-type thrust type rolling fatigue test piece was prepared by the same method as in the Rockwell hardness test. A rolling fatigue evaluation test was conducted using the obtained forest-type thrust type rolling fatigue test piece and the forest-type thrust type rolling fatigue tester. The forest-type thrust type rolling fatigue test was carried out by the following method. FIG. 2 is a diagram illustrating a forest type thrust type rolling fatigue test. A forest-type thrust type rolling fatigue test was conducted by combining a forest-type thrust type rolling fatigue test piece 1, a thrust bearing race with a nominal number of # 51305 as the upper race 2, and a steel ball 3 having a diameter of 9.525 mm. .. The steel ball 3 is made of steel that meets the SUJ2 standard specified in JIS G 4805: 2008, and is a general manufacturing process, that is, spheroidizing annealing, test piece processing, oil quenching, low temperature tempering and polishing. , Made by the manufacturing process of. Quenching was carried out under the condition of 840 ° C. for 30 minutes, and quenching was performed by oil quenching at 60 ° C. Tempering was carried out at 160 ° C. for 1 hour using a normal heat treatment furnace. The tempered steel ball was ground and wrapped to produce a steel ball 3 having a diameter of 9.525 mm.

The Mori thrust-type rolling fatigue test, the test load 400 kgf, 5.23GPa the maximum surface pressure, 1200 rpm rotation speed, while immersing the lubricant Kurisefu H8, the test of endurance and 200 × 10 ⁶ Repeated 10 times. The test results were plotted on the Weibull probability paper, and the L50 life showing a 50% failure probability was regarded as the "rolling fatigue life". Further, the same test was performed using the SUJ2 steel specified in JIS G 4805: 2008, and the "rolling fatigue life" of the SUJ2 steel was obtained, which was regarded as the "rolling fatigue life of the conventional steel".
From the obtained "rolling fatigue life" and "rolling fatigue life of conventional steel", the rolling fatigue strength (= "rolling fatigue life" / "rolling fatigue life of conventional steel" x 100) was calculated. .. When the rolling fatigue strength was 120% or more, it was judged to be acceptable as having excellent rolling fatigue strength. When the rolling fatigue strength was less than 120%, it was judged to be inferior in rolling fatigue strength and rejected.

[Toughness evaluation]
First, the bearing steel bar was heated at 1200 ° C. for 30 minutes and then hot forged at a finishing temperature of 950 ° C. or higher to produce a round bar having a diameter of 60 mm. A round bar having a diameter of 60 mm was machined to produce a plurality of intermediate products having a diameter of 40 mm.
The obtained intermediate product was subjected to quenching and tempering treatment. Specifically, it was heated at 850 ° C. for 30 minutes and then quenched with oil at 60 ° C. Then, tempering was carried out at 160 ° C. for 1 hour using a normal heat treatment furnace. A Charpy test piece having a U notch was prepared from the intermediate product subjected to the above treatment. The Charpy test piece was prepared so that the longitudinal direction of the test piece was parallel to the longitudinal direction of the round bar.

Using the above Charpy test piece, a Charpy impact test was performed at room temperature in accordance with JIS Z 2242: 2018. By dividing the absorbed energy obtained by the Charpy impact test by the original cross-sectional area of the notch (the cross-sectional area of the notch of the Charpy impact piece before the Charpy impact test), the impact value vE ₂₀ (J / cm ²⁾ ) Was calculated. When the impact value vE ₂₀ (J / cm ² ) was 30 J / cm ² or more, it was judged to be acceptable as having excellent toughness, and ^{when it was less than 30 J / cm 2} , it was judged to be rejected as being inferior to toughness.

[Abrasion resistance evaluation test]
For wear resistance, a forest-type thrust type rolling fatigue test piece is prepared by the same method as the rolling fatigue evaluation test, and the forest-type thrust type rolling fatigue test piece is used to perform a forest-type thrust type rolling fatigue test. Evaluated by doing. The shape of the test piece is as shown in FIG. The test conditions were a maximum contact surface pressure of 5.23 GPa, a rotation speed of 1200 rpm, and a repetition rate of 1 × 10 ⁶ times in a lubricated environment. Other conditions were the same as in the rolling fatigue evaluation test. After performing the Mori-type thrust type rolling fatigue test, the roughness of the contact portion of the test piece was measured. The roughness of the contact portion of the test piece after the test was measured at four points at a pitch of 90 ° with respect to the circumferential direction. The wear depth was obtained from the roughness profile, and the average value of the wear depths at the four measurement points (hereinafter referred to as the average wear depth) was used as an evaluation index for wear resistance. When the average wear depth is 5 μm or less, it is judged as excellent in wear resistance, and it is judged as “Good” in Table 2. When the average wear depth is more than 5 μm, it is judged as inferior in wear resistance and rejected. However, it is described as "Bad" in Table 2.

[Structural change resistance evaluation test]
In the tissue change resistance evaluation test, a forest-type thrust type rolling fatigue test piece is prepared by the same method as the rolling fatigue evaluation test, hydrogen is introduced by the cathode electrolytic charge method under the following conditions, and then the forest-type thrust is used. It was evaluated by performing a mold rolling fatigue test. Hydrogen introduction into steel was carried out using cathodic electrolysis. Specifically, after performing a cathode electrolytic charge with a ^{current density of 0.2 mA / cm 2} for 48 hours in a 3% NaCl aqueous solution to which a forest-type thrust type rolling fatigue test piece was _{added with a concentration of 3 g / l of NH 4 SCN.} , The steel was left at room temperature for 24 hours so that the hydrogen concentration in the steel became uniform. After that, in order to prevent hydrogen release from the steel, it was stored in liquid nitrogen until the start of the forest-type thrust type rolling fatigue test.

After introducing hydrogen, the forest-type thrust type rolling fatigue test piece stored in liquid nitrogen was returned to room temperature with running water, and immediately after the forest-type thrust type rolling fatigue test was performed under the following conditions, the forest-type thrust type rolling fatigue test piece was performed. The structure of the surface layer cross section of the contact portion of the rolling fatigue test piece was observed with an optical microscope to evaluate the structure change. In the Mori-type thrust type rolling fatigue test, the number of tests was 5, and ^{the conditions other than the number of repetitions were 1 × 10 6} times were the same as those in the rolling fatigue evaluation test. For the test pieces that were destroyed before the number of repetitions ^{reached 1 × 10 6} times, the test was stopped in the middle and the presence or absence of white tissue was observed by the method described later.

The presence or absence of white tissue formation was determined on a cross section obtained by cutting the central portion of the contact portion of the forest-type thrust type rolling fatigue test piece in the test piece thickness direction. After polishing the observation cross section, it was etched with 3% alcohol nitrate (Nital corrosive liquid). The entire circumference in the circumferential direction was observed with an optical microscope at a depth of 0 to 500 μm from the sliding surface on the etched observation surface. If no white tissue having a maximum length of more than 5 μm is observed on the observation surface, it is judged to be acceptable as having excellent tissue change resistance, and is described as “None” in Table 2, and the maximum length is more than 5 μm. When the tissue was observed, it was judged to be unacceptable because it was inferior in the tissue change resistance, and was described as "Yes" in Table 2. A white tissue having a maximum length of more than 5 μm was observed in all the test pieces that were destroyed without reaching 1 × 10 ^{6 times.}

Test numbers 1 to 9 using steels 1 to 9 have machinability, surface Rockwell hardness, toughness, rolling fatigue characteristics, wear resistance and structure resistance because the chemical composition is within the range of the present invention. All of the change characteristics meet the acceptance criteria. On the other hand, in the test numbers 10 to 24 using steels 10 to 24, any one or more of the characteristics did not satisfy the acceptance criteria.

Test No. 10 using steel 10 is an example in which the surface Rockwell hardness, rolling fatigue strength, and wear resistance are inferior because the C content is low.
Test No. 11 using steel 11 is an example in which machinability is inferior because Ceq is high.
Test No. 12 using steel 12 is an example in which the rolling fatigue strength and the structure change resistance are inferior because the Mn content is high, the V content is low, and the left side of the formula (2) is low.

Test No. 13 using steel 13 is an example in which the rolling fatigue strength and the structure change resistance are inferior because the left side of the formula (2) is low.
Test No. 14 using steel 14 is an example in which the surface Rockwell hardness, rolling fatigue strength and wear resistance are inferior due to the high C content.

Test No. 15 using steel 15 is an example in which the surface Rockwell hardness and rolling fatigue strength were inferior because the Si content was low.
Test No. 16 using steel 16 is an example in which the machinability is inferior because the Si content is high.

Test No. 17 using steel 17 is an example in which the surface Rockwell hardness, rolling fatigue strength, and wear resistance are inferior because the Mn content is low.
Test No. 18 using steel 18 is an example in which the rolling fatigue strength and the structure change resistance are inferior because the Mn content is high.

Test No. 19 using steel 19 is an example in which the rolling fatigue strength and the structure change resistance are inferior because the Cr content is low.
Test No. 20 using steel 20 is an example in which the machinability is inferior because the Cr content is high.

Test No. 21 using steel 21 is an example in which the toughness is inferior because the V content is high.
Test No. 22 using steel 22 is an example in which the surface Rockwell hardness, rolling fatigue strength, and wear resistance are inferior due to the low Ceq.

Test No. 23 using steel 23 did not contain V, and thus was an example in which the rolling fatigue strength and the structure change resistance were inferior.
Test No. 24 using steel 24 is an example in which the rolling fatigue strength, wear resistance, and structure change resistance are inferior because the Cr content is low, V is not contained, and the left side of the formula (2) is low. ..

According to the present invention, there is provided a bearing steel having excellent machinability after normalizing and excellent hardness, toughness, rolling fatigue strength, wear resistance and structure change resistance after quenching and tempering. be able to.

1 Mori-type thrust type rolling fatigue test piece 2 Upper race 3 Steel ball

Claims (2)

The chemical composition is mass%,
C: 0.70 to 1.20%,
Si: 0.25 to 0.80%,
Mn: 0.20% or more, less than 0.40%,
Cr: 1.60 to 2.00%,
V: 0.02 to 0.30%,
Al: 0.005 to 0.060%,
N: 0.0020 to 0.0080%,
P: 0.020% or less,
For bearings containing S: 0.020% or less and O: 0.0015% or less, the balance being Fe and impurities, and satisfying the following equations (1) and (2). steel.
1.100 ≤ C + Si / 7 + Mn / 5 + Cr / 9 + Mo / 2.5 ≤ 1.700 ... (1)
(Cr / Mn) + V ≧ 5.00 ・ ・ ・ (2)
However, the element symbol in the above formula indicates the content of the element in mass%, and when the element is not contained, 0 is substituted. When the chemical composition is mass%,
Ca: 0.0050% or less,
B: 0.0040% or less,
Mo: 0.80% or less,
Ti: 0.050% or less,
The bearing steel according to claim 1, further comprising one or more selected from the group consisting of Nb: 0.050% or less and Ni: 0.30% or less.

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