TWI448565B - Steel with improved rolling fatigue characteristics - Google Patents

Description

Steel with excellent rolling fatigue characteristics

The present invention relates to a steel material which can be used for a bearing component or a mechanical structural component for an automobile or various industrial machines, and more particularly relates to a steel material which exhibits excellent rotational fatigue characteristics when used as the above various members.

A component such as a bearing or a crankshaft supports an important part of a rotating portion or a sliding portion of a machine, but the contact surface pressure is relatively high and the external force may fluctuate, so that the environment to be used is severe. Therefore, the steel of its material requires excellent durability.

In recent years, the demand for such practices has progressed with the high performance or light weight of machinery, and has become increasingly severe year by year. The durability improvement of the shaft parts is also important for the improvement of the lubricity technique, but the excellent rotational fatigue characteristics of the steel become a particularly important requirement.

The steel used for the bearing is used as a material for bearings used in various fields such as automobiles and various industrial machines, such as SUJ2 and other high-carbon chromium bearing steels specified in the conventional JIS G 4805 (1999). However, the bearing is used in a harsh environment such as a ball bearing or a roller bearing with a very high contact pressure, and an external wheel or a rotating body. Therefore, fatigue defects are easily generated from very fine defects (interventions, etc.). Still exists. For this problem, in order to reduce the number of conservatives, try to increase the improvement of the bearing steel for the rotational fatigue life itself.

For example, in Patent Document 1, it has been proposed to specify a content of Ti and Al in a bearing material, and to perform heat treatment after spheroidizing annealing, and to control fine Ti carbide, Ti carbonitride, and Al nitrogen. The amount of the compound or the like is used to refine the old Worthite iron crystal grains (old γ crystal grains) to improve the rotational fatigue characteristics.

However, in the above-described technique, the Ti content is as high as 0.26% or more, and not only steel is expensive, but also the workability of steel is lowered. Further, the steel material obtained by the above-described technique tends to generate coarse TiN during casting, and thus the formation of precipitates may cause unevenness in fatigue life. In addition, the steel material obtained by the above-mentioned technique has a content of Al of 0.11% or more, and the Al-based nitrogen compound formed during casting and rolling has cracks or scratches, and the manufacturing property is deteriorated. .

[Previous Technical Literature]

[Patent Literature]

[Patent Document 1] Patent No. 3591236

The present invention has been made in view of such circumstances, and an object thereof is to provide a steel material having high manufacturability and improved rolling fatigue characteristics.

The steel material of the present invention which achieves the above object has the following points: C: 0.65 to 1.30% (meaning mass%, the same below), Si: 0.05 to 1.00%, Mn: 0.1 to 2.00%, P: 0.050 % or less (excluding 0%), S: 0.050% or less (excluding 0%), Cr: 0.15 to 2.00%, Al: 0.010 to 0.100%, N: 0.025% or less (excluding 0%), Ti: 0.015 % or less (excluding 0%) and O: 0.0025% or less (excluding 0%), the rest is composed of iron and unavoidable impurities, and the average equivalent diameter of the Al-based nitrogen compound dispersed in the steel is The number density of the Al-based nitrogen compound having a diameter of 25 to 200 nm at 25 to 200 nm is 1.1/μm ² or more and 6.0/μm ² or less.

Further, the above-mentioned "equivalent circle diameter" is intended to mean the diameter of a circle having the same area. In the present invention, the equivalent circular diameter of the Al-based nitrogen compound observed on the observation surface of a transmission electron microscope (TEM) or a scanning electron microscope (SEM) is calculated. In addition, the Al-based nitrogen compound-based AlN which is the object of the present invention is of course included in a part (to a total content of about 30%) containing elements such as Mn, Cr, S, Si, and the like.

In the steel material of the present invention, the average grain size of the old Worthite iron is preferably 11.5 or less, and by satisfying the requirements of such a process, more excellent rotational fatigue characteristics can be obtained.

Further, the steel material of the present invention may further contain (a) Cu: 0.25% or less (excluding 0%), Ni: 0.25% or less (excluding 0%), and other elements as needed. Mo: one or more selected from the group consisting of 0.25% or less (excluding 0%); (b) Nb: 0.5% or less (excluding 0%), V: 0.5% or less (excluding 0%), and B : one or more selected from the group consisting of 0.005% or less (excluding 0%); (c) Ca: 0.05% or less (excluding 0%), REM: 0.05% or less (excluding 0%), Mg: One or more selected from the group consisting of 0.02% or less (excluding 0%), Li: 0.02% or less (excluding 0%), and Zr: 0.2% or less (excluding 0%); (d) from Pb: 0.5 One or more selected from the group consisting of % or less (excluding 0%), Bi: 0.5% or less (excluding 0%), and Te: 0.1% or less (excluding 0%). The properties of steel are improved according to the ingredients contained.

According to the present invention, the chemical composition is appropriately adjusted, and an appropriately sized Al-based nitrogen compound is appropriately dispersed in the steel material, whereby a steel material having further improved rolling fatigue characteristics can be obtained. Therefore, when the steel material of the present invention is applied to a bearing or the like, it can exhibit excellent rotational fatigue characteristics even when used in a severe environment.

[Formation for implementing the invention]

The inventors of the present invention have studied the steel materials with excellent rotational fatigue characteristics (long rotational fatigue life) without deteriorating the manufacturability, and have studied from various angles. Then, in terms of improving the rotational fatigue characteristics of the steel, it is effective to satisfy the requirements of the following (A) to (D).

(A) while reducing the Al content, while dispersing a large number of fine Al-based nitrogen compounds, and suppressing the occurrence of cracks by dispersion strengthening, a good rotational fatigue life can be obtained;

(B) In order to suppress cracking during casting and rolling, it is necessary to specify the amount (number density) and size of the Al-based nitrogen compound;

(C) In order to achieve the dispersion (number density) in the fine Al-based nitrogen compound, it is important to strictly control the content of Al or N in the steel, and, in the manufacturing step, the steel is calendered between heats. Cooling to a temperature range of 850 ° C to 650 ° C of the Al-based nitrogen compound, after which the cooling rate can be accelerated;

(D) If the crystal grains of the old Worthite iron (old γ) are too fine, in order to reduce the hardenability, the incomplete quenching phase is likely to be formed, and the rotational fatigue life tends to be short.

Based on the above findings, the inventors of the present invention have further accumulated intensive research in order to improve the rotational fatigue characteristics of steel materials. As a result, it was found that if the Al or N content in the steel material is strictly specified and the production conditions are controlled, the average equivalent diameter of the Al-based nitrogen compound dispersed in the steel after quenching and tempering is 25 to 200 nm, and is quite round. The number density of the Al-based nitrogen compound having a diameter of 25 to 200 nm is 1.1 / μm ² or more and 6.0 / μm ² or less, and the rotational fatigue characteristics of the steel material can be remarkably improved, and the present invention has been completed.

It is an important requirement to appropriately control the number density of Al-based nitrogen compounds having a diameter of 25 to 200 m in the steel material of the present invention. In other words, by the dispersion strengthening of the Al-based nitrogen compound, the occurrence of cracks is suppressed and the transfer is achieved, and a good rotational fatigue life is achieved. Therefore, the size of the Al-based nitrogen compound must be appropriately controlled. The Al-based nitrogen compound has a size (average equivalent circle diameter) of less than 25 nm, and if it is more than 200 nm, the effect of dispersion strengthening cannot be exhibited. The size of the Al-based nitrogen compound is preferably 40 nm or more (more preferably 50 nm or more), and is preferably 150 nm or less (more preferably 125 nm or less).

The number density of the Al-based nitrogen compound having a diameter of 25 to 200 m is less than 1.1/μm ² , and the effect of improving the rotational fatigue characteristics obtained by the dispersion strength is not effectively exhibited (the rotational fatigue characteristics are deteriorated). Further, when the number density of the Al-based nitrogen compound having a diameter of 25 to 200 m exceeds 6.0 / μm ² , the crystal grains are coarsened to form an incompletely quenched phase (for example, a pinelite or a toughened body (for example) Bainite), so the rotational fatigue life is shortened (rotational fatigue characteristics are deteriorated). The number density of the Al-based nitrogen compound is preferably 1.5 pieces/μm ² or more (more preferably 2.0 pieces/μm ² or more), and is preferably 5.0 pieces/μm ² or less (more preferably 4.0 pieces/μm ² or less).

It is also effective to control the crystal grains of the old Worthite iron (old γ) in the steel of the present invention. The larger the grain size number of the old γ crystal (the smaller the crystal grain), the higher the hardness and the higher the crack transmission property. However, if the crystal grain size number is too large (if the crystal grains are too small), the hardenability is lowered, and an incompletely quenched phase is easily formed, so that the rotational fatigue life is shortened. From such a case, the grain size number of the old γ is preferably 11.5 or less, more preferably 11.0 or less (more preferably 10.5 or less).

The steel material of the present invention contains the above-mentioned content of Al or N, and its chemical composition (C, Si, Mn, P, S, Cr, Al, N, Ti, O) must also be appropriately adjusted. The reasons for limiting the scope of these ingredients are as follows.

[C:0.65~1.30%]

C is an element necessary for increasing the quenching hardness and maintaining the strength at room temperature and high temperature to impart wear resistance. In order to exert such an effect, C must be contained in an amount of 0.65% or more, preferably 0.8% or more (more preferably 0.95% or more). However, if the C content is too large, large carbides are easily formed, which adversely affects the rotational fatigue characteristics. Therefore, the C content is 1.30% or less, and it is preferably suppressed to 1.2% or less (more preferably 1.1% or less).

[Si: 0.05~1.00%]

Si is an element useful for enhancing solid solution strengthening and hardenability of a matrix. In order to exert such an effect, Si must be contained in an amount of 0.05% or more, preferably 0.1% or more (more preferably 0.15% or more). However, if the Si content is too large, the workability or the machinability is remarkably lowered, so the Si content is 1.00% or less, and it is preferably suppressed to 0.9% or less (more preferably 0.8% or less).

[Mn: 0.1~2.00%]

Mn is an element useful for enhancing solid solution strengthening and hardenability of a matrix. In order to exert such an effect, Mn must be contained in an amount of 0.1% or more, preferably 0.15% or more (more preferably 0.2% or more). However, if the Mn content is too large, workability or machinability is remarkably lowered, so the Mn content is 2.00% or less, and it is preferably suppressed to 1.6% or less (more preferably 1.2% or less).

[P: 0.050% or less (excluding 0%)]

P is an element which inevitably contains impurities, but is segregated at the grain boundary, and it is preferable to reduce the workability in order to reduce the workability, but the extreme reduction causes an increase in the cost of steelmaking. From this, the P content is 0.050% or less. It should be reduced to 0.04% or less (more preferably 0.03% or less).

[S: 0.050% or less (excluding 0%)]

The S system inevitably contains an element of impurities, but precipitates as MnS. In order to improve the rotational fatigue characteristics, it is preferable to reduce the force extremely, but the extreme reduction leads to an increase in the cost of steelmaking. From this, the S content is 0.050% or less. It should be reduced to 0.04% or less (more preferably 0.03% or less).

[Cr: 0.15~2.00%]

The Cr system is bonded to C to form a carbide, which imparts wear resistance and contributes to an improvement in hardenability. In order to exert such an effect, the Cr content must be 0.15% or more. It is preferably 0.5% or more (more preferably 0.9% or more). However, if the Cr content is excessive and coarse carbides are formed, the rotational fatigue life is shortened. Therefore, the amount of Cr is 2.00% or less. It should be 1.8% or less (more preferably 1.6% or less).

[Al: 0.010~0.100%]

Al is an element that plays a very important role in the steel material of the present invention, and is an important element in which the Al-based nitrogen compound is finely dispersed in steel and is used to improve the rotational fatigue property of the steel by bonding with N. In order to form a fine Al-based nitrogen compound, it is necessary to contain at least 0.010% or more. However, the Al content is excessive and exceeds 0.100%, and the size and number of precipitated Al-based nitrogen compounds increase, and cracks or scratches are likely to occur during casting or casting. Further, if the Al content is excessive, the crystal grains are too fine, so that the hardenability is lowered, and it is not suitable for a large-sized part, and the rotational fatigue life is shortened. Further, the lower limit of the Al content is preferably 0.013% (more preferably 0.015% or more), and the upper limit is preferably 0.08% (more preferably 0.05% or less).

[N: 0.025% or less (excluding 0%)]

In the same manner as the above-mentioned Al, the N-based element is an element which plays an important role in the steel material of the present invention, and exhibits an effect of improving the rotational fatigue characteristics caused by fine dispersion of the Al-based nitrogen compound. However, the N content is excessive and exceeds 0.025%, and the size and number density of the precipitated Al-based nitrogen compound increase, and cracking is likely to occur during casting or rolling. Further, if the N content is excessive, the crystal grains are too fine, so that the hardenability is lowered, and it is not suitable for a large-sized part, and the rotational fatigue life is shortened. The lower limit of the N content is not particularly limited as long as the specific amount of the Al-based nitrogen compound can be precipitated, and is determined by the cooling rate after rolling or the element (Ti, V, Nb, B, Zr, Te, etc.) bonded to N. The amount and the Al content may be appropriately set. For example, when the N content is 0.0035% or more, a specific amount of the Al-based nitrogen compound is precipitated. Further, the lower limit of the N content is preferably 0.004% (more preferably 0.006% or more), and the upper limit is preferably 0.020% (more preferably 0.022% or less).

[Ti: 0.015% or less (excluding 0%)]

The Ti bond with the N bond in the steel to form TiN not only adversely affects the rotational fatigue characteristics, but also the toxic elements which are harmful to the cold workability or the hot workability, and should be reduced as much as possible, but the extreme reduction is the cost of the steel. Increase. From this, the Ti content must be 0.015% or less. Further, the upper limit of the Ti content is preferably 0.01% (more preferably 0.005% or less).

[O: 0.0025% or less (excluding 0%)]

The form of the impurity in the O-based steel causes a large influence, and forms an intercalation of Al ₂ O ₃ or SiO ₂ which adversely affects the rotational fatigue characteristics, so it is preferable to reduce it as much as possible, but the extreme reduction is the cost of the steel. Increase. From this matter, the O content must be 0.0025% or less. Further, the upper limit of the O content is preferably 0.002% (more preferably 0.0015% or less).

In the case where the element contained in the present invention is as described above, the residue is iron and an unavoidable impurity, and in the case of the unavoidable impurity, the element carried by the raw material, the data, the manufacturing equipment, and the like can be allowed to be mixed. Further, in order to increase the rotational fatigue life, the following elements may be actively contained within a predetermined range.

[One or more selected from the group consisting of Cu: 0.25% or less (excluding 0%), Ni: 0.25% or less (excluding 0%), and Mo: 0.25% or less (excluding 0%)]

Any of Cu, Ni, and Mo functions as an element of the hardenability-improving element of the parent phase, which improves the hardness and contributes to the improvement of the rotational fatigue property. As a result of the above, any one of them is effectively used by containing 0.03% or more. However, if the content of either of them exceeds 0.25%, the workability deteriorates.

[One or more selected from the group consisting of Nb: 0.5% or less (excluding 0%), V: 0.5% or less (excluding 0%), and B: 0.005% or less (excluding 0%)]

Any of Nb, V, and B is bonded to N to form a nitrogen compound, which is an element effective in granulating crystal grains and improving rotational fatigue characteristics. When Nb and B are added in an amount of 0.0005% or more and V-line is added in an amount of 0.001% or more, the rotational fatigue characteristics are improved. However, if Nb or V exceeds 0.5% and B exceeds 0.005%, the crystal grains are finely formed, and an incompletely quenched phase is likely to be formed. Further, the upper limit is preferably 0.3% (more preferably 0.1% or less) of Nb and V, and B is 0.003% (more preferably 0.001% or less).

[Ca: 0.05% or less (excluding 0%), REM: 0.05% or less (excluding 0%), Mg: 0.02% or less (excluding 0%), Li: 0.02% or less (excluding 0%), and Zr: one or more selected from the group consisting of 0.2% or less (excluding 0%)

Any of Ca, REM (rare earth element), Mg, Li, and Zr spheroidizes the oxide-based interposer, and contributes to the element of the improvement of the rotational fatigue property. These effects are effectively exhibited by containing 0.0005% or more of Ca or REM and 0.0001% or more of Mg, Li or Zr. However, even if it is contained excessively, the effect is saturated, and the effect of satisfying the content cannot be expected, which is uneconomical, and therefore should be within the above range. Further, a more preferable upper limit is 0.03% (more preferably 0.01% or less) of Ca or REM, 0.01% (more preferably 0.005% or less) of Mg or Li, and 0.15% (more preferably 0.10% or less) of Zr.

[One or more selected from the group consisting of Pb: 0.5% or less (excluding 0%), Bi: 0.5% or less (excluding 0%), and Te: 0.1% or less (excluding 0%)]

Any of Pb, Bi, and Te is an element that improves the machinability. These effects are effectively exhibited by containing 0.01% or more of Pb and Bi and 0.0001% or more of Te. However, if the content of Pb or Bi exceeds 0.5%, or the content of Te exceeds 0.1%, there is a problem in production such as occurrence of a calendering injury. Further, the upper limit is preferably 0.3% (more preferably 0.2% or less) of Pb and Bi, and 0.075% (more preferably 0.05% or less) of Te.

In the steel material of the present invention, in order to disperse the fine Al-based nitrogen compound in the steel after quenching and tempering, it is important to control the cooling rate after rolling in the production step of the steel material using a cast piece satisfying the above composition. The Al-based nitrogen compound precipitated in the cooling process after rolling is directly left in the same state even after the subsequent spheroidizing annealing, part processing, quenching and tempering. The Al-based nitrogen compound has an average equivalent circular diameter of 25 to 200 nm, and is dispersed in an Al-based nitrogen compound having a circular diameter of 25 to 200 nm of 1.1 pieces/μm ² or more and 6.0 pieces/μm ² or less. The average cooling rate in the precipitation temperature range, that is, the average cooling rate (called the primary average cooling rate) between 850 ° C and 650 ° C for the cooled steel is in the range of 0.10 to 0.90 ° C / sec, so that 650 ° C to room temperature ( The average cooling rate (referred to as secondary cooling rate) of 25 ° C) is 1 ° C / sec or more. Further, the average equivalent circular diameter of the Al-based nitrogen compound precipitated in the cooling process after the rolling, and the number of units per unit area of the Al-based nitrogen compound having a substantially circular diameter of 25 to 200 nm, even after the subsequent spheroidizing annealing, The parts processing, quenching and tempering processes are not directly maintained according to the processing conditions of the steps.

When the primary cooling rate is less than 0.10 ° C / sec, the Al-based nitrogen compound is coarsened. Further, when the primary cooling rate exceeds 0.90 ° C / sec, the average equivalent circular diameter of the Al-based nitrogen compound is less than 25 nm, and the number density of the specific size is less than 1.1 / μm ² , so that the desired size and the size cannot be obtained. number. Moreover, by setting the secondary cooling rate to 1 ° C /sec or more, the coarsening of the Al-based nitrogen compound can be suppressed, and the size can be suppressed.

The steel material of the present invention is formed into a specific part shape, and is quenched and tempered to produce a bearing part or the like. However, the shape of the steel material can be applied to the wire shape, the rod shape, and any other shape thus produced, and the steel material is The size can also be appropriately determined depending on the final product.

Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited to the following examples, and of course, it can be implemented in a range that can be adapted to the meanings of the foregoing and the following description. All of them are included in the technical scope of the present invention.

[Examples]

The steel materials (test Nos. 1 to 51) having the chemical compositions shown in the following Tables 1 and 2 were heated to 1,100 to 1300 ° C in a heating furnace or a soaking furnace, and then subjected to blocking at 900 to 1200 ° C. Calendering. Thereafter, after heating to 900 to 1100 ° C, rolling (including forging by molding) was carried out to prepare a round bar having a diameter of 70 mm. After the end of calendering, the round bar is cooled from 850 ° C to 650 ° C at various average cooling rates (Tables 3 and 4 below) while simultaneously from 650 ° C to room temperature (25 ° C) at 1 ° C / The average cooling rate of seconds is cooled to obtain a rolled material or a forged material.

The rolled material or the forged material was subjected to spheroidizing annealing at 795 ° C (holding time: 6 hours), and then the rolled material or the forged material was peeled off by cutting. Thereafter, a disk having a diameter of 60 mm and a thickness of 5 mm was cut out from the rolled material or the forged material, and oil quenching was performed after heating at 840 ° C for 30 minutes, and tempering was performed at 160 ° C for 120 minutes. Finally, finishing polishing was carried out to prepare a test piece having a surface roughness Ra (arithmetic mean roughness) of 0.04 μm or less.

With respect to the test piece obtained above, the number and size of the Al-based nitrogen compound, and the crystal grain (crystal grain size number) of the old Worthite iron (old γ) were measured under the following conditions, and the fatigue life and the presence or absence of the crack were evaluated. .

[Measurement of the number and size of Al-based nitrogen compounds]

The method for confirming the dispersion state of the Al-based nitrogen compound was a test piece after the heat treatment was cut, and after the cross section was polished, the surface was subjected to carbon deposition, and the replica was observed by FE-TEM (electric field emission type transmission electron microscope). At this time, a component containing an Al-based nitrogen compound of Al or N was specified by EDX (energy dispersive X-ray detector) of TEM, and the field of view was observed at a magnification of 30,000 times. In this case, the first field of view is 16.8 μm ² , and an arbitrary three fields of view (total 50.4 μm ² ) are observed, and the particle analysis software [(particle analysis III for Windows. Version 3.00 SUMITOMO METAL TECHNOLOGY) (trade name)] is used to obtain The number (the average equivalent circle diameter) and the number of Al-based nitrogen compounds having a substantially circular diameter of 25 to 200 nm (the number is converted into a number per μm ² : number density).

[Measurement of crystal grains (crystal size number) of old Worthite (old gamma)]

After the heat-treated test piece was cut and polished, the old Worthfield iron grain boundary corrosion was observed, and the surface of the surface layer was 150 μm in depth, and was carried out in accordance with JIS G 0551 (according to the standard figure method). Particle size determination.

[Measurement of fatigue life]

With a thrust type rotary fatigue tester, the rolling fatigue test was carried out 16 times for each steel (test piece) at a repetition speed: 1500 rpm, surface pressure: 5.3 GPa, and number of stops: 2 × 10 ⁸ times, and fatigue life was evaluated. L ₁₀ (the number of stress repetitions of fatigue failure in 10% of the cumulative damage probability obtained from the Weibull probability paper drawing). At this time, the fatigue life L ₁₀ (L ₁₀ life) was 1.0 × 10 ⁷ or more times as a pass criterion.

[Evaluation of cracks]

After the surface of the sample after rolling and forging, the surface of the sample was visually observed, and when a flaw of 3 mm or more was observed, it was judged to be cracked.

These results are shown in Tables 3 and 4 below together with the production conditions (the primary cooling rate and the presence or absence of secondary cooling).

From the results of these, it can be examined as follows. In other words, it is found that the test Nos. 3 to 5, 8, 10, 11, 14, 16 to 22, and 27 to 32 satisfy the requirements (chemical composition, size and number of Al-based nitrogen compounds) specified in the present invention. Or a preferred component (old gamma crystal grain size number), which does not cause cracking, and achieves excellent rotational fatigue characteristics.

However, Test Nos. 1, 2, 6, 7, 9, 12, 13, 15, 23 to 26, 33 to 51 are beyond any of the requirements specified in the present invention, so either one is rotationally fatigued. Life is shortened.

In Test Nos. 1, 6, 15, 23, 26, 33, 35, 37, and 38, the cooling conditions after rolling were not appropriate, so the size of the Al-based nitrogen compound became too large (where Test No. 23, 26, The 33, 37, and 38 systems also exceeded the old γ crystal grain size number), and either of them had a shortened rotational fatigue life.

In Test Nos. 2, 7, 9, 24, and 25, the cooling rate was fast. Therefore, in Test No. 40, Ti content increased and TiN was formed. Therefore, the number of Al-based nitrogen compounds was insufficient, and Test No. Since the content of Al in the 34-layer is larger than the range specified in the present invention, the number density and size of the Al-based nitrogen compound are excessive, and the rotational fatigue life of either of them is shortened.

In Test Nos. 12 and 13, the number density of the Al-based nitrogen compound was excessive, and the preferred γ crystal grain size number of the preferred material exceeded the range specified by the present invention, and the rotational fatigue life of either of them was shortened.

Test Nos. 36 to 39 and 41 to 51 exceeded the chemical composition of the present invention (test Nos. 37 and 38 also exceeded the above requirements), and either of them had a short rotational fatigue life.

Based on these data, the relationship between the number density of the Al-based nitrogen compound (the Al-based nitrogen compound having a round diameter of 25 to 200) and the fatigue life L ₁₀ is shown in Fig. 1, and the number density of the Al-based nitrogen compound. The relationship with the size (average equivalent circle diameter) is shown in Fig. 2 (above, the chemical composition satisfies the range specified by the present invention). From these figures, it can be seen that the fatigue life L ₁₀ (rotational fatigue life) of growth can be achieved by appropriately controlling the number density or size of the Al-based nitrogen compound.

The relationship between the old γ crystal grain size number and the fatigue life L ₁₀ is shown in Fig. 3. As can be seen from the figure, it is effective to achieve the long fatigue life L ₁₀ (rotational fatigue life) by numbering the old γ crystal grain size into an appropriate range. Further, the relationship between the primary cooling rate (average cooling rate) and the size of the Al-based nitrogen compound (the average equivalent circle diameter of the Al-based nitrogen compound) is shown in Fig. 4 . From this figure, it is understood that adjusting the primary cooling rate to an appropriate range is effective in controlling the size of the Al-based nitrogen compound.

Fig. 1 is a graph showing the relationship between the number density of an Al-based nitrogen compound and the fatigue life L ₁₀ .

Fig. 2 is a graph showing the relationship between the number density and the size of an Al-based nitrogen compound.

Fig. 3 is a graph showing the relationship between the old γ crystal grain size number and the fatigue life L ₁₀ .

Fig. 4 is a graph showing the relationship between the primary cooling rate and the size of the Al-based nitrogen compound.

Claims (7)

A steel material characterized by C: 0.65 to 1.30% (meaning mass%, the same below), Si: 0.05 to 1.00%, Mn: 0.1 to 2.00%, P: 0.050% or less (excluding 0%), S : 0.050% or less (excluding 0%), Cr: 0.15 to 2.00%, Al: 0.010 to 0.100%, N: 0.025% or less (excluding 0%), Ti: 0.015% or less (excluding 0%), and O : 0.0025% or less (excluding 0%), the rest is composed of iron and unavoidable impurities. The average diameter of the Al-based nitrogen compound dispersed in the steel is 25 to 200 nm, and the equivalent diameter is 25~. The number density of the 200 nm Al-based nitrogen compound is 1.1 / μm ² or more and 6.0 / μm ² or less. For example, in the steel of the first application of the patent scope, the average crystal grain size of the old Vostian iron is 11.5 or less. For example, in the steel of the first application of the patent scope, further includes, for other elements, Cu: 0.25% or less (excluding 0%), Ni: 0.25% or less (excluding 0%), and Mo: 0.25% or less. One or more selected from the group consisting of (excluding 0%). For example, the steel of the first application of the patent scope includes further Nb: 0.5% or less (excluding 0%), V: 0.5% or less (excluding 0%), and B: 0.005% or less. One or more selected from the group consisting of (excluding 0%). For example, in the steel of the first application of the patent scope, further including other elements, Ca: 0.05% or less (excluding 0%), REM: 0.05% or less (excluding 0%), Mg: 0.02% or less (excluding 0%), Li: 0.02% or less (excluding 0%), and Zr: 0.2% or less (excluding 0%) More than one selected from the group. For example, the steel material of the first application of the patent scope includes further Pb: 0.5% or less (excluding 0%), Bi: 0.5% or less (excluding 0%), and Te: 0.1% or less. One or more selected from the group consisting of (excluding 0%). The steel material of the first aspect of the patent application further contains one or more elements belonging to the following (a) to (d) as other elements, and (a) Cu: 0.25% or less (excluding 0%), Ni: one or more selected from the group consisting of 0.25% or less (excluding 0%) and Mo: 0.25% or less (excluding 0%); (b) Nb: 0.5% or less (excluding 0%), V : one or more selected from the group consisting of 0.5% or less (excluding 0%) and B: 0.005% or less (excluding 0%); (c) Ca: 0.05% or less (excluding 0%), REM: 0.05% or less (excluding 0%), Mg: 0.02% or less (excluding 0%), Li: 0.02% or less (excluding 0%), and Zr: 0.2% or less (excluding 0%) One or more selected; (d) selected from the group consisting of Pb: 0.5% or less (excluding 0%), Bi: 0.5% or less (excluding 0%), and Te: 0.1% or less (excluding 0%) More than one.