JP7469596B2 - Bearing Steel - Google Patents

Description

The present invention relates to bearing steel.

Generally, the steel used for automobile bearings, gears, shafts, pulleys, etc. is machined into the shape of the part by cutting, so excellent machinability is required. In addition, the steel used for these parts needs to have excellent properties against impact and friction, so excellent hardness, toughness, rolling fatigue strength, and wear resistance are required. Generally, the higher the surface hardness, the better the rolling fatigue strength, so machine parts that require excellent rolling fatigue strength are manufactured by hot working or cold working the raw material bearing steel to form the part shape, and then heat treating it by quenching, tempering, etc.

In recent years, there has been a trend to reduce the amount or viscosity of lubricating fluid (oil, etc.) between mechanical parts as described above in order to improve fuel efficiency of automobiles. Reducing the amount of lubricating fluid reduces the cooling effect, causing the surface temperature to rise, and mechanical parts will be used in a harsher environment. Furthermore, reducing the viscosity of the lubricating fluid not only reduces the cooling effect by thinning the liquid film, but also increases friction, causing the surface temperature of the mechanical parts to rise. If the surface temperature of a mechanical part rises, the internal structure of the mechanical part will change as it is used, and the area where the structure has changed may become the starting point for destruction. For this reason, mechanical parts used in high-temperature environments need to have properties that make it difficult for their structure to change even when the surface temperature rises.

Patent Document 1 discloses a steel with excellent toughness and wear resistance, which contains, by mass%, C: 0.30-0.65%, Si: 0.20-1.00%, Mn: 0.20-0.60%, P: 0.030% or less, S: 0.030% or less, Cr: 1.00-3.00%, Al: 0.005-0.200%, N: 0.0200% or less, O: 0.0030% or less, with the balance being Fe and unavoidable impurities, and further, the value of 4×Si+3Cr-Mn calculated from the contents of Si, Mn, and Cr in the above composition is 6.00% or more, and the prior austenite grain size is 8.0 μm or less.

Patent Document 2 discloses a steel for induction hardening having a predetermined chemical composition, a steel structure consisting of ferrite and pearlite, an area ratio of the pearlite being 85% or more, a ratio of the number of the composite inclusions to the total number of Al ₂ O ₃ inclusions and composite inclusions in the steel being 20% or more, and the composite inclusions containing, by mass%, 2.0% or more of SiO ₂ and 2.0% or more of CaO, with the remainder being 99% or more of Al ₂ O ₃ .

Patent Document 3 discloses a steel with excellent rolling fatigue life, which has a specified chemical composition and is characterized in that, when EPMA line analysis is performed in a direction perpendicular to the rolling direction, the standard deviation and average value of the Cr X-ray intensity values satisfy (standard deviation of the Cr X-ray intensity values/average value of the Cr X-ray intensity values) ≦ 0.25.

However, Patent Documents 1 to 3 do not take into consideration the microstructural changes that occur as the surface temperature of the steel increases. When the steels disclosed in Patent Documents 1 to 3 are used in machine parts used in high-temperature environments, the Mn contained in the steel can cause hydrogen contained in lubricating fluids, etc., to penetrate and diffuse into the steel, causing microstructural changes and resulting in destruction.

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

The present invention aims to provide a bearing steel that has excellent machinability after normalizing, and has excellent hardness, toughness, rolling fatigue strength, wear resistance, and resistance to microstructural change after quenching and tempering.

The gist of the present invention is as follows.
[1] A bearing steel according to one embodiment of the present invention has a chemical composition, in mass%,
C: 0.70 to 1.20%,
Si: 0.20 to 0.80%,
Mn: 0.20% or more and less than 0.40%;
Cr: 1.60 to 1.95%,
V: 0.21 to 0.30%,
Al: 0.005 to 0.060%,
N: 0.0020 to 0.0080%,
P: 0.020% or less,
It contains S: 0.020% or less, O: 0.0015% or less, the balance being Fe and impurities, and satisfies the following formulae (1) and (2).
1.100≦C+Si/7+Mn/5+Cr/9+Mo/2.5≦1.700 … (1)
(Cr/Mn)+V≧5.00 (2)
In the above formula, the symbol for an element indicates the content of the element in mass %, and 0 is substituted when the element is not contained.
[2] The bearing steel according to the above [1], wherein the chemical composition is, in mass%,
Ca: 0.0050% or less,
B: 0.0040% or less,
Mo: 0.80% or less,
Ti: 0.050% or less,
The steel may contain one or more elements selected from the group consisting of Nb: 0.050% or less and Ni: 0.30% or less.

The present invention provides bearing steel that has excellent machinability after normalizing, and excellent hardness, toughness, rolling fatigue strength, wear resistance, and resistance to microstructural change after quenching and tempering.

FIG. 2 is a diagram showing a Mori-type thrust rolling fatigue test piece used in the examples. FIG. 2 is a diagram illustrating the Mori-type thrust rolling fatigue test carried out in the examples.

When mechanical parts used in high-temperature environments are repeatedly subjected to contact pressure that involves sliding, the structure of the surface layer of the part changes, and the area where the structure has changed may become the starting point for fracture. The inventors speculate as follows about the changes in the structure of mechanical parts used in high-temperature environments.

When mechanical parts are subjected to repeated surface pressure while sliding in a high-temperature environment, local deformation occurs on the surface of the part, and large strain is introduced at that location, resulting in the formation of white structure. When white structure occurs inside the steel, this structure can become the starting point for the formation of microcracks, which can ultimately lead to destruction. The inventors speculate that one of the causes of local destruction in high-temperature environments is the penetration of hydrogen, which is generated by the decomposition of lubricating fluid (oil, etc.), into the steel.

The inventors have therefore conducted extensive research into the chemical composition of steel with excellent resistance to microstructural change. "Excellent resistance to microstructural change" means that the steel is unlikely to change to white even when used in a high-temperature environment. As a result, the inventors have discovered that by controlling the contents of Mn, Cr, and V within a predetermined range and by controlling the contents of these elements to satisfy a predetermined relationship, steel with excellent resistance to microstructural change can be obtained. More specifically, the inventors have discovered that by adding V to refine the crystal grains, the yield strength can be improved and the occurrence of local deformation can be suppressed, by reducing the Mn content to strengthen the crystal grain boundaries, the occurrence of cracks can be suppressed, and by adding Cr, steel with excellent resistance to microstructural change can be obtained.

The bearing steel according to this embodiment, which has been made based on the above findings, has a chemical composition, in mass%, of C: 0.70 to 1.20%, Si: 0.20 to 0.80%, Mn: 0.20% or more and 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, with the balance being Fe and impurities, 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)
In the above formula, the symbol for an element indicates the content of the element in mass %, and 0 is substituted when the element is not contained.

The preferred embodiment of the present invention will be described in detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the present invention. A numerical range described with "to" includes the lower limit and the upper limit. A numerical value indicated as "less than" does not include the numerical range. All percentages for chemical composition indicate mass %.

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

Si: 0.20 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 during tempering and in high-temperature environments during use. If the Si content is low, the above effects cannot be obtained, so the Si content is set to 0.20% or more. It is preferably set to 0.22% or more, 0.30% or more. On the other hand, if 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 set to 0.75% or less, 0.70% or less, or 0.65% or less.

Mn: 0.20% or more, less than 0.40% Manganese (Mn) improves hardenability during hardening. 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 set to 0.25% or more. On the other hand, if the Mn content is high, Mn segregates at the grain boundaries, deteriorating the resistance to structural change of the steel. Therefore, the Mn content is set to less than 0.40%, and preferably set to 0.35% or less.

Cr: 1.60 to 2.00%
Chromium (Cr) imparts hardenability to steel and increases its strength. In addition, Cr improves temper softening resistance and inhibits softening during tempering and in high-temperature environments during use. Furthermore, Cr inhibits the hydrogen sensitivity of steel, thereby improving its resistance to structural change. If the Cr content is low, the above effects cannot be obtained, so the Cr content is set to 1.60% or more. Preferably, it is set to 1.65% or more, 1.70% or more, or 1.80% or more. On the other hand, if the Cr content is high, the toughness decreases and the hardness of the steel increases, deteriorating the machinability. Therefore, the Cr content is set to 2.00% or less. Preferably, it is set to 1.95% or less.

V: 0.02 to 0.30%
Vanadium (V) forms fine V nitrides, V carbides, or V carbonitrides to suppress the coarsening of crystal grains during quenching, thereby improving the yield strength of the steel and suppressing local deformation on the surface of the part under pressure. V also improves the rolling fatigue strength and microstructural change resistance of the steel. One of the reasons for this is thought to be that V precipitates trap hydrogen that has penetrated into the steel, thereby suppressing embrittlement due to hydrogen penetration. If the V content is low, the above effect cannot be obtained, so the V content is set to 0.02% or more. Preferably, it is 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, if the V content is high, coarse V precipitates are formed in the steel, and the toughness of the steel decreases. Therefore, the V content is set to 0.30% or less. Preferably, it is 0.25% or less, 0.21% or less.

Al: 0.005 to 0.060%
Aluminum (Al) deoxidizes molten steel. In addition, Al combines with N in steel to form AlN, suppressing the 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 set to 0.008% or more, 0.015% or more. On the other hand, if the Al content is high, a large amount of coarse Al ₂ O ₃ inclusions and/or Al ₂ O ₃ clusters formed by agglomeration of multiple Al ₂ O ₃ inclusions are generated in the steel, and the rolling fatigue strength of the steel after quenching and tempering is reduced. Therefore, the Al content is set to 0.060% or less. It is preferably set to 0.050% or less, 0.045% or less.
In the present embodiment, the Al content means the total Al content.

N: 0.0020 to 0.0080%
Nitrogen (N) combines with Al to form AlN in the steel, suppressing the 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, 0.0028% or more. On the other hand, if the N content is high, N dissolves excessively in ferrite, causing strain aging, and the cold workability of the steel is reduced. Furthermore, if the N content is high, coarse nitrides are generated in the steel, and the machinability and rolling fatigue strength of the steel are reduced. Therefore, the N content is set to 0.0080% or less. It is preferably 0.0070% or less, 0.0060% or less.

P: 0.020% or less Phosphorus (P) is an impurity element. P segregates at the grain boundaries and embrittles the grain boundaries, reducing 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, 0.011% or less. Although it is preferable to set the P content to 0%, since an excessive reduction in the P content does not provide an effect commensurate with the increase in refining costs, the P content may be set to 0.003% or more, 0.005% or more.

S: 0.020% or less Sulfur (S) is an impurity element. S forms coarse inclusions (MnS) in steel, which 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, or 0.010% or less. Although it is preferable to set the S content to 0%, since an effect commensurate with the increase in refining costs cannot be obtained even if the S content is excessively reduced, the S content may be set to 0.003% or more, or 0.005% or more.

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, 0.0011% or less. Although it is preferable to set the O content to 0%, since an effect commensurate with the increase in refining costs cannot be obtained even if the O content is reduced excessively, the O content may be set to 0.0003% or more, 0.0005% or more.

The balance of the chemical composition of the bearing steel according to this embodiment is Fe and impurities. In this embodiment, impurities refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment when the bearing steel is industrially manufactured, and are acceptable to the extent that they do not adversely affect the bearing steel according to this embodiment.

The bearing steel according to this embodiment may contain one or more of the following optional elements in place of the remaining Fe. If the optional elements listed below are not included, the content is 0%.

Ca: 0.0050% or less Calcium (Ca) modifies Al ₂ O ₃ inclusions to form composite inclusions (Al ₂ O ₃ -CaO-SiO ₂ ). Ca modifies Al ₂ O ₃ inclusions to form composite inclusions, thereby further improving the rolling fatigue strength of the steel after quenching and tempering. In order to reliably obtain this effect, it is preferable that the Ca content be 0.0005% or more. On the other hand, if the Ca content is high, the number of coarse inclusions in the steel increases, and the rolling fatigue strength of the steel after quenching and tempering may decrease. Therefore, it is preferable that the Ca content be 0.0050% or less. It is more preferable that the Ca content be 0.0040% or less.

B: 0.0040% or less Boron (B) dissolves in steel to increase the hardenability of the steel, thereby further improving the rolling fatigue strength of the steel after quenching and tempering. In order to reliably obtain this effect, the B content is preferably 0.0005% or more. More preferably, it is 0.0010% or more. On the other hand, if the B content is high, the above effect saturates 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 steel and further improves the rolling fatigue strength of steel after quenching and tempering. In order to reliably obtain this effect, the Mo content is preferably 0.01% or more. More preferably, it is 0.07% or more, 0.12% or more. On the other hand, if the Mo content is high, the hardness after normalizing may increase and the machinability may decrease, so the Mo content is preferably 0.80% or less. More preferably, it is 0.60% or less, 0.50% or less.

Ti: 0.050% or less Titanium (Ti) forms Ti nitrides or Ti carbides in steel, suppressing the coarsening of crystal grains during quenching, thereby further improving the rolling fatigue strength of the steel after quenching and tempering. In order to reliably obtain this effect, the Ti content is preferably 0.010% or more. More preferably, it is 0.015% or more, 0.020% or more. On the other hand, if the Ti content is high, coarse Ti nitrides and/or Ti carbides may be generated 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, 0.040% or less.

Nb: 0.050% or less Niobium (Nb) forms Nb nitrides, Nb carbides, or Nb carbonitrides in steel, and suppresses the coarsening of crystal grains by the pinning effect. In order to reliably obtain this effect, the Nb content is preferably 0.010% or more. More preferably, it is 0.020% or more, or 0.025% or more. On the other hand, if 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) dissolves in steel and further improves the rolling fatigue strength of steel after quenching and tempering. In order to reliably obtain this effect, the Ni content is preferably 0.001% or more, and more preferably 0.01% or more. On the other hand, if the Ni content is high, the above effect saturates and the alloy cost increases. Therefore, the Ni content is preferably 0.30% or less, and more preferably 0.25% or less.

The bearing steel according to this 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)
In the above formulas, the element symbols indicate the content of the element in mass %, and when the element is not contained, 0 is substituted. The above formulas (1) and (2) will be described below.

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

(Cr/Mn)+V≧5.00 (2)
The left side of the above formula (2) indicates an index of the microstructural change resistance of the bearing steel according to this embodiment. If the left side of the above formula (2) is less than 5.00, the microstructural change resistance of the steel deteriorates, and white structure is generated when used in a high-temperature environment, making it more likely to break. Therefore, the left side of the above formula (2) is set to 5.00 or more. The left side of the above formula (2) is preferably 5.50 or more, and more preferably 6.00 or more. The left side of the above formula (2) may be set to 10.30 or less, based on the possible values of the Cr content, Mn content, and V content of the bearing steel according to this embodiment.

The chemical composition of the above-mentioned bearing steel can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-non-dispersive infrared absorption method.

The steel according to this embodiment is a bearing steel, and the metal structure to be controlled is the metal structure after quenching, so the metal structure of the bearing steel according to this embodiment (before quenching) is not particularly limited. The metal structure of the steel according to this embodiment may have a total area ratio of ferrite (proeutectoid ferrite), pearlite, and cementite of 90% or more, and an area ratio of the remaining structure of 10% or less. The remaining structure may be 0%, the total area ratio of ferrite and cementite may be 100%, or the total area ratio of ferrite, pearlite, and cementite may be 100%. The remaining structure in the metal structure is, for example, bainite. However, the bearing steel according to this embodiment can obtain the properties of the bearing steel according to this embodiment even if it does not have the above-mentioned metal structure.

The bearing steel according to this embodiment can be selected appropriately depending on the machine parts to which it is applied, and may be a steel bar or wire rod.

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

One example of the manufacturing method includes a steelmaking process in which molten steel is refined and cast to produce a material (a slab or an ingot), and a hot working process in which the material is hot worked to produce bearing steel. Each process is explained below.

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

In the refining process, first, molten iron produced by a well-known method is refined in a converter (primary refining). The molten steel tapped from the converter is then subjected to secondary refining. In the secondary refining, alloying elements are added to adjust the composition, producing molten steel that satisfies the above-mentioned chemical composition.

The molten steel produced in the above refining process is used to manufacture materials (cast pieces or ingots) (casting process). Specifically, the molten steel is used to manufacture cast pieces by a continuous casting method, or the molten steel is used to manufacture ingots by an ingot casting method.

[Hot processing process]
The produced material is hot worked to produce bearing steel (steel bar or wire rod). The hot working process includes a rough rolling process and a finish rolling process. In the rough rolling process, the material is hot worked to produce billets. In the rough rolling process, for example, a blooming mill is used. The material is bloomed using the blooming mill to produce billets.

When blooming is carried out in the rough rolling process, the heating temperature and holding time in the heating furnace are as follows.
Heating temperature: 1150 to 1300°C
Holding time at the 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) at a furnace temperature of 1150 to 1300°C.

If the heating temperature is less than 1150°C or the holding time at 1150-1300°C is less than 1.5 hours, the V carbides and V composite carbides in the material will not be fully dissolved. As a result, the V nitrides, V carbides, or V carbonitrides will coarsen, and it will not be 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 1150-1300°C exceeds 40.0 hours, the unit consumption will be excessively high, resulting in high manufacturing costs.

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, the V nitrides, V carbides, or V carbonitrides in the material will be sufficiently dissolved.

In the finish rolling process, the billet is first heated in a heating furnace. The heated billet is then hot-rolled in a continuous rolling mill to produce a steel bar or wire rod 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 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) at a furnace temperature of 1100 to 1300°C.

In the finish rolling process, precipitation of V carbides and V composite carbides during the process is suppressed as much as possible. If the heating temperature in the heating furnace in the finish rolling process is less than 1100°C or the holding time at 1100-1300°C is less than 0.5 hours, the load on the rolling mill during finish rolling will be excessively large. On the other hand, if the heating temperature exceeds 1300°C or the holding time at 1100-1300°C exceeds 5.0 hours, the unit consumption will be excessively high, resulting in high production costs.

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, the V carbides and V composite carbides in the material will be sufficiently dissolved.

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

The manufactured bearing steel may be subjected to normalizing treatment (normalizing treatment) or spheroidizing annealing treatment as necessary. The bearing steel according to this embodiment can be manufactured by the manufacturing method described above as an example.

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

First, the steel for bearing is hot forged or cold forged to manufacture an intermediate product. If necessary, the intermediate product is subjected to stress relief annealing. After hot forging or cold forging or stress relief annealing, the intermediate product is subjected to cutting or drilling to be processed into a part shape to obtain a crude product. Then, the crude product is quenched. The heating temperature of the quenching process is 800 to 900°C. Then, an oil quenching process is performed. In this embodiment, a low-temperature tempering process (for example, a heating process at 130°C to 200°C for about 30 to 120 minutes) may be further performed as necessary. After quenching or quenching and tempering, a grinding process is further performed to manufacture a bearing part.
Bearing parts manufactured using the bearing steel according to this 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 explained in more detail using an example. However, the conditions in the example are an example of conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to this example of conditions. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

The steel bars for bearings were manufactured by the following method.
First, primary refining was performed in a converter on molten iron produced by a known method. Secondary refining was performed on the molten steel tapped from the converter. In the secondary refining, alloy elements for component adjustment were added to adjust the chemical composition. Using the molten steel after secondary refining, a cast piece having a cross section of 400 mm x 300 mm was produced by a continuous casting method. This cast piece was heated to 1250 ° C. and held for 1.5 to 40.0 hours, and then a steel piece having a cross section of 162 mm x 162 mm was produced by blooming. The produced steel piece was 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 piece was hot-rolled (finish rolling) using a continuous rolling mill, and then cooled to room temperature (25 ° C.) to obtain a bearing bar having a diameter of 70 mm. After the finish rolling, the average cooling rate for cooling to room temperature was 1.0°C/sec or less until the surface temperature of the bearing steel bar reached 800 to 500°C. The bearing steel bar was then subjected to spheroidizing annealing. After being held at 840°C for 6 hours, it was cooled to 690°C at an average cooling rate of -10°C/h and then allowed to cool.

The chemical composition of the resulting bearing steel bar is shown in Table 1. The chemical composition of the bearing steel bar was measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S were measured using the combustion-infrared absorption method, N was measured using the inert gas fusion-thermal conductivity method, and O was measured using the inert gas fusion-non-dispersive infrared absorption method.
In addition, Ceq in Table 1 is the middle part 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 bearing steel bar, a test specimen simulating a machine part (bearing part) was prepared by the following method. The chemical composition of the test specimen was the same as that of the bearing steel bar.
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 with a diameter of 70 mm. The round bar with a diameter of 60 mm was machined to produce a plurality of intermediate products with a diameter of 70 mm and a thickness of 5.5 mm. The thickness direction of the intermediate products was the longitudinal direction of the round bar.

The intermediate product thus produced was quenched and tempered. Specifically, it was heated at 850°C for 30 minutes, and then quenched in oil at 60°C. It was then tempered for 1 hour at 160°C using a normal heat treatment furnace. Grinding and lapping were performed on the intermediate product after the tempering process to produce a Mori-type thrust-type rolling fatigue test piece with a diameter of 58 mm and a thickness of 5.0 mm, as shown in Figure 1. Using the above method, a Mori-type thrust-type rolling fatigue test piece simulating a bearing part was produced.

The Rockwell hardness of the cylinder top surface of the Mori-type thrust rolling fatigue test piece was measured. Specifically, a Rockwell hardness HRC test was performed on three arbitrary points on the top surface of the Mori-type thrust rolling fatigue test piece in accordance with JIS Z 2245:2016. The test force was 1471 N. The average value of the obtained Rockwell hardness at the three points was defined as the surface Rockwell hardness (HRC1). If the surface Rockwell hardness was more than 58HRC1 and less than 65HRC1, it was judged as having excellent hardness and passed, and if the surface Rockwell hardness was less than 58HRC1 or more than 65HRC1, it was judged as having poor hardness and failed. The measurement results are shown in Table 2.

[Cutting test]
The machinability of the steel was evaluated by a cutting test using drilling. Specifically, the maximum cutting speed capable of drilling a total of 1000 mm in SUJ2 steel was first obtained, and the steels of each steel number were used to evaluate whether or not the steels could be machined to a hole depth of 1000 mm at this maximum cutting speed. More specifically, SUJ2 steel with a diameter of 35 mm specified in JIS G 4805:2008 was heated at a temperature of 820°C for 10 hours, then slowly cooled to a temperature of 690°C at a rate of 1.0°C/min or less, and then allowed to cool in the air. This round bar was cut into a length of 130 mm and used as a material. A TiC-coated carbide drill with a diameter of 10 mm and a length of 300 mm was used to drill a hole to a depth of 100 mm at a feed rate of 0.2 mm/rev while lubricating with water-soluble cutting oil, and the maximum cutting speed VL ₁₀₀₀ (m/min) capable of drilling a hole to a depth of 1000 mm was obtained.

The maximum cutting speed VL ₁₀₀₀ is usually used as an evaluation index of tool life, and it can be judged that the larger the maximum cutting speed VL ₁₀₀₀ , the better the tool life. Each steel number was normalized in the same manner as SUJ2 described above, and drilling was performed at the maximum cutting speed VL ₁₀₀₀ obtained with SUJ2. If the drilled hole depth was 1000 mm, it was judged to have excellent machinability and pass, and is recorded as "Good" in Table 2, and if the drilled hole depth was not able to be machined to 1000 mm, it was judged to have poor machinability and fail, and is recorded as "Bad" in Table 2. For the comparative examples that could not be machined to a drilled hole depth of 1000 mm, the evaluation tests (rolling fatigue evaluation test, wear resistance evaluation test, and microstructural change resistance evaluation test) described later were not performed.

[Rolling fatigue evaluation test]
Mori-type thrust rolling fatigue test pieces were prepared in the same manner as in the Rockwell hardness test. The obtained Mori-type thrust rolling fatigue test pieces and a Mori-type thrust rolling fatigue tester were used to carry out a rolling fatigue evaluation test. The Mori-type thrust rolling fatigue test was carried out in the following manner. FIG. 2 is a diagram for explaining the Mori-type thrust rolling fatigue test. The Mori-type thrust rolling fatigue test pieces 1, a thrust bearing race with a designation #51305 as an upper race 2, and a steel ball 3 with a diameter of 9.525 mm were combined to carry out the Mori-type thrust rolling fatigue test. The steel ball 3 was made of steel that satisfied the SUJ2 standard specified in JIS G 4805:2008, and was prepared by a general manufacturing process, that is, the manufacturing process of spheroidizing annealing, test piece processing, oil quenching, low-temperature tempering, and polishing. Quenching was carried out under the condition of 840°C for 30 minutes, and quenching was oil quenching at 60°C. The tempering was carried out in a normal heat treatment furnace at 160° C. for 1 hour. The tempered steel ball was subjected to grinding and lapping to manufacture a steel ball 3 having a diameter of 9.525 mm.

In the Mori-type thrust rolling fatigue test, the test load was 400 kgf, the maximum surface pressure was 5.23 GPa, the rotation speed was 1200 rpm, and the lubricant was immersed in Crisef H8, and the test was repeated 10 times with a durability of 200 x ^106. The test results were plotted on a Weibull probability paper, and the L50 life, which indicates a 50% probability of failure, was regarded as the "rolling fatigue life". In addition, a similar test was performed using SUJ2 steel specified in JIS G 4805:2008 to determine the "rolling fatigue life" of SUJ2 steel, which was regarded as the "rolling fatigue life of conventional steel".
From the obtained "rolling fatigue life" and the "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 as excellent rolling fatigue strength and passed. When the rolling fatigue strength was less than 120%, it was judged as poor rolling fatigue strength and failed.

[Toughness evaluation]
First, the steel bar for bearings 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 with a diameter of 60 mm. The 60 mm round bar was machined to produce a plurality of intermediate products with a diameter of 40 mm.
The intermediate product thus obtained was subjected to quenching and tempering treatment. Specifically, the intermediate product was heated at 850°C for 30 minutes, and then quenched in oil at 60°C. Then, tempering was performed at 160°C for 1 hour using a normal heat treatment furnace. Charpy test pieces having a U-notch were prepared from the intermediate product thus treated. The Charpy test pieces were 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 specimen, a Charpy impact test was performed at room temperature in accordance with JIS Z 2242:2018. The absorbed energy obtained by the Charpy impact test was divided by the original cross-sectional area of the notch (the cross-sectional area of the notch of the Charpy impact specimen before the Charpy impact test) to calculate the impact value _vE20 (J/ ^cm2 ). When the impact value _vE20 (J/ ^cm2 ) was 30 J/ ^cm2 or more, it was judged to be excellent in toughness and passed, and when it was less than 30 J/ ^cm2 , it was judged to be poor in toughness and failed.

[Wear resistance evaluation test]
The wear resistance was evaluated by preparing a Mori-type thrust rolling fatigue test piece in the same manner as in the rolling fatigue evaluation test, and performing a Mori-type thrust rolling fatigue test using this Mori-type thrust rolling fatigue test piece. The shape of the test piece is as shown in FIG. 1. The test conditions were a maximum contact pressure of 5.23 GPa, a rotation speed of 1200 rpm, and a repetition rate of 1×10 ⁶ times in a lubricated environment. The other conditions were the same as those in the rolling fatigue evaluation test. After performing the Mori-type thrust 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 90° pitch in the circumferential direction. The wear depth was obtained from the roughness profile, and the average value of the wear depths at the four measured points (hereinafter, the average wear depth) was used as an evaluation index for wear resistance. When the average wear depth was 5 μm or less, the specimen was judged to have excellent wear resistance and passed the test, and this is recorded as “Good” in Table 2. When the average wear depth was more than 5 μm, the specimen was judged to have poor wear resistance and passed the test, and this is recorded as “Bad” in Table 2.

[Evaluation test of resistance to structural change]
The microstructural change resistance evaluation test was performed by preparing Mori-type thrust rolling fatigue test pieces in the same manner as in the rolling fatigue evaluation test, introducing hydrogen by cathodic electrolytic charging under the following conditions, and then performing the Mori-type thrust rolling fatigue test. Hydrogen was introduced into the steel using cathodic electrolytic charging. Specifically, the Mori-type thrust rolling fatigue test pieces were subjected to cathodic electrolytic charging for 48 hours in a 3% NaCl aqueous solution containing 3 g/l of NH ₄ SCN at a current density of 0.2 mA/cm ² , and then left at room temperature for 24 hours so that the hydrogen concentration in the steel became uniform. Thereafter, in order to prevent hydrogen release from the steel, the pieces were stored in liquid nitrogen until the start of the Mori-type thrust rolling fatigue test.

After hydrogen introduction, the Mori-type thrust rolling fatigue test pieces stored in liquid nitrogen were returned to room temperature with running water, and immediately subjected to a Mori-type thrust rolling fatigue test under the following conditions. The structure of the cross section of the contact surface layer of the Mori-type thrust rolling fatigue test pieces was then observed with an optical microscope to evaluate the changes in structure. The Mori-type thrust rolling fatigue test was conducted under the same conditions as the rolling fatigue evaluation test, except that the number of tests was 5 and the number of repetitions was 1 x ^106. For test pieces that broke before the number of repetitions reached 1 x ¹⁰⁶ , the test was stopped midway and the presence or absence of white structure was observed by the method described below.

The presence or absence of white structure generation was judged on a cross section obtained by cutting the center of the contact part of the Mori-type thrust rolling fatigue test piece in the thickness direction of the test piece. After polishing the observed cross section, it was etched with 3% nitric acid alcohol (nital etchant). The entire circumference in the circumferential direction was observed at a depth of 0 to 500 μm from the sliding surface on the etched observation surface with an optical microscope at 500 times magnification. When no white structure with a maximum length exceeding 5 μm was observed in the observation surface, it was judged to have excellent resistance to microstructural change and passed, and was recorded as "absent" in Table 2. When a white structure with a maximum length exceeding 5 μm was observed, it was judged to have poor resistance to microstructural change and passed, and was recorded as "present" in Table 2. In addition, all test pieces that were destroyed before reaching 1 × 10 ⁶ repetitions had white structure with a maximum length exceeding 5 μm observed.

Test numbers 1 to 9, which used steels 1 to 9, have chemical compositions within the range of the present invention, and therefore meet the pass criteria for machinability, surface Rockwell hardness, toughness, rolling fatigue properties, wear resistance, and microstructural change resistance. On the other hand, test numbers 10 to 24, which used steels 10 to 24, did not meet the pass criteria for one or more properties.

Test No. 10 using Steel 10 was an example in which the surface Rockwell hardness, rolling fatigue strength and wear resistance were poor due to the low C content.
Test No. 11 using Steel 11 is an example in which Ceq was high and machinability was poor.
Test No. 12 using Steel 12 had a high Mn content, a low V content, and a low value on the left side of formula (2), and therefore was an example in which the rolling fatigue strength and resistance to microstructural change were poor.

Test No. 13 using Steel 13 was an example in which the left side of formula (2) was low and therefore the rolling fatigue strength and resistance to microstructural change were poor.
Test No. 14 using Steel 14 was an example in which the surface Rockwell hardness, rolling fatigue strength and wear resistance were poor due to the high C content.

Test No. 15 using Steel 15 was an example in which the surface Rockwell hardness and rolling fatigue strength were poor due to the low Si content.
Test No. 16 using Steel 16 was an example of poor machinability due to the high Si content.

Test No. 17 using Steel 17 was an example in which the surface Rockwell hardness, rolling fatigue strength and wear resistance were poor due to the low Mn content.
Test No. 18 using Steel 18 was an example in which the rolling fatigue strength and resistance to microstructural change were poor due to the high Mn content.

Test No. 19 using Steel 19 was an example in which the rolling fatigue strength and resistance to microstructural change were poor due to the low Cr content.
Test No. 20 using Steel 20 was an example of poor machinability due to the high Cr content.

Test No. 21 using Steel 21 is an example in which the V content was high and therefore the toughness was poor.
Test No. 22 using Steel 22 was an example in which the Ceq was low and the surface Rockwell hardness, rolling fatigue strength and wear resistance were poor.

Test No. 23 using Steel 23 did not contain V, and was therefore an example in which the rolling fatigue strength and resistance to microstructural change were poor.
Test No. 24 using Steel 24 had a low Cr content and did not contain V, and therefore the left side of formula (2) was low, and therefore this is an example in which the rolling fatigue strength, wear resistance, and resistance to microstructural change properties were poor.

The present invention provides bearing steel that has excellent machinability after normalizing, and has excellent hardness, toughness, rolling fatigue strength, wear resistance, and resistance to microstructural change after quenching and tempering.

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

Claims (2)

The chemical composition, in mass%, is
C: 0.70 to 1.20%,
Si: 0.20 to 0.80%,
Mn: 0.20% or more and less than 0.40%;
Cr: 1.60 to 1.95%,
V: 0.21 to 0.30%,
Al: 0.005 to 0.060%,
N: 0.0020 to 0.0080%,
P: 0.020% or less,
A bearing steel comprising S: 0.020% or less, O: 0.0015% or less, the balance being Fe and impurities, and satisfying 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)
In the above formula, the symbol for an element indicates the content of the element in mass %, and 0 is substituted when the element is not contained. The chemical composition, in mass%,
Ca: 0.0050% or less,
B: 0.0040% or less,
Mo: 0.80% or less,
Ti: 0.050% or less,
2. The bearing steel according to claim 1, further comprising one or more elements selected from the group consisting of Nb: 0.050% or less and Ni: 0.30% or less.

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