JP4117170B2 - Carburizing steel and carburized parts - Google Patents

Description

【００５９】
【表２】

【００６０】
【発明の効果】
以上述べたように、本発明の浸炭用鋼材および浸炭処理部品は、良好な被削性を維持しつつねじり疲労強度に優れるので、歯車と一体になったシャフト類など、複雑形状部分を有する自動車の駆動形部品等を安価に提供できるようになった。
【図面の簡単な説明】
【図１】き裂の種類を説明する図であり、（ａ）はモードＩき裂、（ｂ）はモードIIき裂、（ｃ）はモードIIIき裂を示す。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel material for carburizing or carburizing / nitriding treatment used for drive system parts such as automobiles, particularly shafts, which require high torsional fatigue strength.
[0002]
[Prior art]
Torsional fatigue strength is often a problem for parts used in automobile drive systems, particularly shafts. In order to improve torsional fatigue strength, induction-hardened parts are often used (for example, Patent Document 1). However, parts with extremely complex shapes such as shafts integrated with gears need to be processed into the required shape by cutting and grinding before quenching the material, but the steel for induction hardening has a C content. Is about 0.4 to 0.7% (hereinafter,% representing the component ratio is mass%) and is inferior in machinability, so that it is difficult to apply to such parts. Therefore, for parts that require higher machinability, a carburized steel material having a lower C content (about 0.1 to 0.25%) and improved machinability than steel for induction hardening. After this is cut into a required shape, carburizing (or carburizing and nitriding) treatment is performed to increase the surface hardness or impart rolling fatigue properties (for example, Patent Document 2). . Furthermore, it is known that carburized parts can obtain higher bending fatigue strength and the like than induction-hardened parts, and there is an increasing need for improving the torsional fatigue strength of carburized parts.
[0003]
However, although many proposals have been made for improving the torsional fatigue strength of induction-hardened parts (for example, Patent Document 1), there are a lot of knowledge about bending fatigue and pitching improvement for carburized parts, but about torsional fatigue strength. The present situation is that there is almost no knowledge of this.
[0004]
[Patent Document 1]
Japanese Patent No. 2774118 [Patent Document 2]
Japanese Patent No. 3081927 gazette
[Problems to be solved by the invention]
Accordingly, an object of the present invention is to provide a carburizing (or carburizing and nitriding) process that has high machinability and has excellent torsional fatigue strength, which can be used for driving system parts (particularly, shafts) of automobiles having complicated shapes. The object is to provide a part and a carburizing steel for producing the part.
[0006]
[Means for Solving the Problems]
In order to achieve the above-mentioned object, the inventors have intensively studied to elucidate the fracture mechanism due to torsional fatigue of carburized steel, and have completed the invention based on the knowledge obtained thereby.
[0007]
That is, while the stress state in the rotating bending fatigue is repeated tension-compression, the torsional fatigue is subjected to a shearing stress. Therefore, it is said that crack generation and propagation behavior differ from general fatigue failure due to repeated tension-compression. Several findings have been reported on torsional fatigue of induction-hardened steel, and it has been reported that mode I and mode III crack propagation affects fatigue life. However, with regard to torsional fatigue of carburized steel, the actual condition is that the crack propagation behavior has hardly been investigated. As a result of the investigation by the inventors this time, the torsional fatigue of the carburized steel is different from the torsional fatigue of the induction hardening steel, and the mode I crack propagates in the carburized part with high strength, but the steel material in the core part with low strength. It was found that the failure occurred by mode II cracks propagating on the plane parallel to the rolling direction. It was also found that mode II crack propagation rate has an important effect on fatigue life, and that mode II crack propagation rate reduction is effective in extending torsional fatigue life.
[0008]
The mode I to III cracks are names of cracks generally used in fracture mechanics. The mode I crack is also called an open crack, and tensile stress acts to open the crack. In addition, mode II cracks are also called in-plane shear cracks, and mode III cracks are also called out-of-plane shear cracks, both of which are subjected to shear stress (see Fig. 1; 2 is the direction of stress, and 3 is the direction of crack propagation.) When torsional stress is applied to shaft parts such as shafts, the mode I crack is inclined at 45 ° to the axis, the mode II crack is parallel to the axis, and the mode III crack is perpendicular to the axis. Progress in any direction.
[0009]
As a result of investigating the factors affecting the mode II crack propagation rate that determines the torsional fatigue life, it was found that the shape, amount and distribution of inclusions present in the steel material greatly influence. That is, generally used steel material contains about 0.002 to 0.02% of S as an impurity, and there are sulfides (there is much MnS, but there are also sulfides such as Ti, Zr, and REM). ) Is a little present. In addition to sulfides, oxides (oxides such as Al, Ca, and Mg) and composites of sulfides and oxides also exist. In the present invention, these are collectively referred to as inclusions. Of the particles composed of these inclusions, those having a major axis of 3 μm or more are mostly sulfides or composites of sulfides and other compounds (such as oxides). In many cases, the Vickers hardness is lower than that of the parent phase, so that the steel is stretched during rolling. Such inclusion particles tend to be a fatigue crack propagation path because of a large hardness difference from the parent phase. And, it was found that the more the inclusion particles are flattened, the more the amount of inclusion particles is, and the more the inclusion particles are unevenly distributed in the matrix, the greater the propagation speed of fatigue cracks. . The inventors have reduced the propagation rate of fatigue cracks by controlling the form, amount and distribution of such inclusion particles having a major axis of 3 μm or more by adjusting the chemical composition and production conditions of the steel material, It has been found that the torsional fatigue strength of carburized steel can be improved.
[0010]
On the other hand, it was found that fine inclusion particles having a major axis of less than 3 μm have little influence on the torsional fatigue strength regardless of the form, distribution, and amount because the size itself is small.
[0011]
The present invention has been made based on the above findings, and the gist thereof is as follows.
[0012]
The invention of claim 1 has a chemical composition of C: 0.1 to 0.4%, Mn: 0.1 to 2.0%, Si: 0.6% or less, P: 0.015% or less, S : 0.1% or less, Al: 0.01 to 0.1%, N: 0.003 to 0.02% , balance Fe and unavoidable impurities , and long axis in any cross section parallel to the rolling direction In which the average aspect ratio of inclusion particles having a particle size of 3 μm or more is 6.0 or less and the area ratio of the inclusion particles is 0.6 or less.
[0013]
In the invention of claim 2, the chemical composition is C: 0.1 to 0.4%, Mn: 0.1 to 2.0%, Si: 0.6% or less, P: 0.015% or less, S : 0.1% or less, Al: 0.01 to 0.1%, N: 0.003 to 0.02% , balance Fe and unavoidable impurities , and long axis in any cross section parallel to the rolling direction The average aspect ratio AR of inclusion particles having a particle size of 3 μm or more, the area ratio Af (unit:%) of the inclusion particles, and the distribution index F of the inclusion particles satisfy the following formula (1): Steel material.
−21.4 × AR + 414.2 × F−80.8 × Af−30 ≧ 0 Formula (1)
Here, F = X ₁ / (A / n) ^1/2 Formula (2),
A: Area of an arbitrary observation visual field in the cross section, n: Number of inclusion particles existing in the observation visual field, X ₁ : Each inclusion for all the inclusion particles existing in the observation visual field This is a value obtained by actually measuring the distance to another inclusion particle present closest to each particle and arithmetically averaging the measured distance.
[0014]
In the first and second aspects of the present invention, the “rolling direction” is a direction in which the steel material is stretched by rolling, forging, or the like.
[0015]
According to the invention of claim 3, the chemical composition further includes, in mass proportion, Cr: 0.1 to 3.0%, Ni: 0.1 to 3.0%, Mo: 0.1 to 1.0 %. The carburizing steel material according to claim 1, comprising one or more kinds.
[0016]
The invention according to claim 4 is the carburized steel material according to any one of claims 1 to 3, wherein the chemical composition further contains Ti: 0.005 to 0.1 % by mass ratio. is there.
[0017]
In the invention of claim 5, the chemical composition further includes Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, Zr: 0.0005 to 0.05%, Te: 0.0005. It is steel material for carburizing of any one of Claims 1-4 including 1 type, or 2 or more types in -0.05%.
[0019]
The invention of claim 6 is a carburized component produced by carburizing or carburizing and nitriding the carburizing steel according to any one of claims 1 to 5 by a conventional method.
[0020]
[Action]
Hereinafter, the reason why the chemical composition of steel material and the form, amount and distribution of inclusion particles in the present invention are limited as described above will be described.
[0021]
(A) Chemical composition a) C
C is an indispensable element for securing the core strength of the finally obtained carburized (or carburized and nitrocarburized) processed part, and needs to be 0.1% or more. On the other hand, when C is contained exceeding 0.4%, the toughness of the core part is deteriorated, and machinability and cold forgeability are lowered to deteriorate the workability. Therefore, the C content is 0.1 to 0.4%. In addition, the more preferable range of C content is 0.14-0.3%.
[0022]
b) Si
Si works effectively as a strengthening element or deoxidizing element, but if Si exceeds 0.6%, it promotes grain boundary oxidation during carburizing and deteriorates fatigue properties and adversely affects cold forgeability. Effect. Therefore, the Si content is set to 0.6% or less. In particular, when high fatigue strength is required, the Si content is preferably 0.1% or less.
[0023]
c) Mn
Mn is an element effective for deoxidation of steel materials, and it is necessary to make it 0.1% or more in order to exhibit the effect effectively. On the other hand, when Mn is contained exceeding 2.0%, generation of a striped structure is promoted to cause cracking, and cold workability and machinability are deteriorated. Therefore, the Mn content is 0.1 to 2.0%. In addition, the more preferable range of Mn content is 0.3 to 1.0%.
[0024]
d) S
S generates sulfides and contributes to improvement of machinability, but if it exceeds 0.1%, the torsional fatigue strength is reduced. Therefore, the S content is 0.1% or less. A more preferable range of the S content is 0.035% or less, and a more preferable range is 0.025% or less.
[0025]
e) P
If P exceeds 0.015%, it segregates at the grain boundaries and lowers toughness and fatigue strength. Therefore, the P content is 0.015% or less. In addition, the more preferable range of P content is 0.010% or less, More preferably, it is 0.007% or less.
[0026]
f) Al
Al has a deoxidizing action of the steel material, and also has an action of generating AlN by combining with nitrogen and preventing the coarsening of the crystal grains. In order to effectively exhibit the effect, 0.005 However, the effect is saturated at 0.06%, and if it exceeds that, it combines with oxygen to become non-metallic inclusions and lowers toughness, fatigue strength, and the like. Therefore, the Al content is 0.005 to 0.06%. A more preferable range is 0.015 to 0.04%.
[0027]
g) N
N combines with Al, V, Ti, Nb, etc. in the steel to form nitrides, and has the effect of suppressing the coarsening of the crystal grains. Although it is necessary to make it contain more than 003%, the effect is saturated at 0.03%, and if it exceeds that, nitride becomes inclusions and lowers toughness and fatigue strength. Therefore, the N content is set to 0.003 to 0.03%. A more preferable range is 0.005 to 0.02%.
[0028]
h) Cr, Ni, Mo
Cr, Ni, M o, respectively since it has an effect of refining the increase or hardened tissue hardenability preferable to contain one or more of these elements. Among these elements, especially Cr has an excellent effect of improving hardenability, and in order to exhibit the effect effectively, it is necessary to contain 0.1% or more, but when exceeding 3.0% Cr carbide is generated to cause grain boundary segregation, thereby reducing the grain boundary strength and lowering the toughness and fatigue strength. Therefore, the Cr content is set to 0.1 to 3.0%. Ni contributes to the improvement of impact resistance of the structure after quenching, and in order to exert its effect effectively, it is necessary to contain 0.1% or more, but the effect is saturated at 3.0%. Excessive addition is completely useless economically. Therefore, the Ni content is 0.1 to 3.0%. Mo effectively works to reduce the incompletely quenched structure and improve the grain boundary strength, and in order to exert its effect effectively, it is necessary to contain 0.1% or more, but the effect is 1.0%. Addition beyond that is saturated economically. Therefore, the Mo content is set to 0.1 to 3.0% . Contact name more preferable content range of each element, Cr: 0.5~1.5%, Ni: 0.3~1.5%, Mo: 0.2 to 0.8%.
[0029]
i) T i
T i is combined with charcoal Motoya nitrogen and have a effect of improving the toughness and fatigue strength by refining the crystal grains generate carbides and nitrides, in order to effectively exhibit the effect, 0 . It is necessary to 005% or more over including chromatic, but the effect is zero. Saturates at 1 %, and beyond that, large inclusions are formed, which in turn reduces toughness. Therefore, the Ti content is 0 . 005 to 0.1 % . In addition, good Ri preferably Ihan circumference is 0.01 to 0.05%.
[0030]
j) Ca, Mg, Zr, Te
Ca, Mg, Zr, and Te each have a function of spheroidizing sulfide inclusions, and are effective in improving torsional fatigue strength. It is preferable to contain one or more of these elements, since the mechanical properties such as toughness when added are also improved. In order to exhibit the effect effectively, it is necessary to contain 0.0005% or more of each element, but the effect is 0.01% for Ca, 0.01% for Mg, and 0.5% for Zr. , Te is saturated at 0.5%, and beyond that, large inclusions are formed, which in turn reduces toughness. Therefore, the content of each element is as follows: Ca: 0.0005-0.01%, Mg: 0.0005-0.01%, Zr: 0.0005-0.05%, Te: 0.0005-0. 05%.
[0032]
(B) Form, amount and distribution of inclusion particles As described above, fine inclusion particles having a major axis of less than 3 μm hardly affect the torsional fatigue strength, and therefore, inclusion particles having a major axis of 3 μm or more ( Hereinafter, the shape, amount and distribution are defined only for “inclusion particles” or “particles”).
[0033]
a) When the average aspect ratio AR of the morphological particles of inclusion particles exceeds 6.0, the flatness of the particles becomes excessive, the fatigue crack propagation rate increases, and the torsional fatigue strength decreases. Therefore, AR ≦ 6.0 It is desirable to do. Here, the aspect ratio is a value defined by the major axis / minor axis of the particle, and the average aspect ratio is a value obtained by arithmetically averaging the aspect ratio of all particles present in an arbitrary observation field.
[0034]
b) Amount of inclusion particles When the area ratio Af (unit:%) of the particles exceeds 0.6%, the number of particles present in the matrix increases, the fatigue crack propagation rate increases, and the torsional fatigue strength increases. Since it decreases, it is desirable that Af ≦ 0.6. Here, the area ratio represents a value defined by the total area of all particles present in an arbitrary observation visual field in an arbitrary steel cross section / the area of the observation visual field in%. It should be noted that it is more desirable to satisfy AR ≦ 6.0 and Af ≦ 0.6 at the same time in order to maintain high torsional fatigue strength from the examples described later (see Tables 1 and 2).
[0035]
c) Increasing the distribution index F of the particles even if the average aspect ratio AR of the inclusion particles exceeds 6.0 or the area ratio Af of the particles exceeds 0.6%. By satisfying the formula (1) reprinted below, the particles are uniformly distributed without being unevenly distributed in the matrix, so that the fatigue crack propagation rate is suppressed and the torsional fatigue strength is reduced. It remains high (see Test Nos. 11, 116, 118, 120 in Table 2). Note that the value of the distribution index F takes any value from 0 to 1 according to the definition of the formula (2) reprinted below, and all particles existing in the observation field are average distances (A / n). ) Equal to 1 when arranged at equal intervals at intervals equal to ^1/2, and due to the uneven distribution of particles, the distance to the nearest particle becomes smaller and becomes closer to 0 as the number of particles increases. It is. Therefore, increasing the value of F so as to be close to 1 so as to satisfy the formula (1) means that the particles are uniformly dispersed in the matrix.
[0036]
[Repost]
−21.4 × AR + 414.2 × F−80.8 × Af−30 ≧ 0 Formula (1)
_{Here, F = X 1 / (A} / n) 1/2 ... Equation (2),
A: Area of an arbitrary observation visual field in the cross section of the steel material, n: Number of particles existing in the observation visual field, X ₁ : For all particles existing in the observation visual field, each of the particles present closest to each other This is a value obtained by actually measuring the distance to the particles and arithmetically averaging the measured distance.
[0037]
As described above, by controlling the chemical composition of the carburizing steel material and the average aspect ratio (form), area ratio (amount), and distribution index (distribution) of inclusion particles having a major axis of 3 μm or more in the steel material The torsional fatigue strength of carburized steel produced by carburizing (or carburizing and nitriding) this steel material can be improved. In the following embodiments of the invention, a method for controlling the morphology, amount, and distribution of inclusion particles will be described.
[0038]
DETAILED DESCRIPTION OF THE INVENTION
The carburized component of the present invention is usually produced through the following steps (1) to (4).
(1) Molten steel production process: Molten steel whose components are adjusted to be within the chemical composition range of the present invention is produced in a refining furnace or a melting furnace.
(2) Billet casting process: This molten steel is cast to produce a billet.
(3) Processing step: This billet is processed into a carburized member having a required shape by rolling, forging or machining.
(4) Carburizing (carburizing and nitriding) treatment process: This carburizing member is subjected to carburizing (carburizing and nitriding), and further subjected to quenching and tempering to obtain a carburized component.
[0039]
Hereinafter, manufacturing conditions and the like in each process will be described in detail.
[0040]
(1) Molten steel production process Using a converter, a high-frequency induction melting furnace, an arc furnace, etc. as the refining furnace or melting furnace, the components of the molten steel are adjusted by the usual component adjustment means so that the chemical composition is within the range of the present invention. can do. For example, the S content may be adjusted to 0.1% or less by adding an appropriate amount of a desulfurizing agent such as quick lime, but it is desirable that the S content be within a range of about 0.005% or more where the desulfurizing agent cost does not increase excessively. Ca, Mg, Zr, and Te are representative elements that easily form oxides in molten steel. When these elements are added to the molten steel, fine oxides are formed, which become sulfide nucleation sites, so that inclusion particles mainly composed of sulfides can be uniformly dispersed in the billet after casting. Further, these elements are dissolved in the inclusions, and have an effect of suppressing the extension of the inclusion particles. When these elements are added alone, these effects increase as the amount added increases. However, excessive addition increases the cost, so 0.02% or less is more preferable. More preferably, the content is 0.01%. Moreover, these effects can be further increased by adding two or more of these elements in combination. In this case, the addition amount is more preferably about 0.02% or less, and further preferably 0.001 to 0.01%, for the same reason as the case of adding alone.
[0041]
(2) Billet casting process When the cooling rate at the time of billet casting is slow, inclusion particles crystallized in the liquid phase aggregate and become coarse, the average aspect ratio increases, and the torsional fatigue strength may decrease. . Therefore, the cooling rate needs to be 150 ° C./h or more, and is preferably 200 ° C./h or more (see Test Nos. 1, 11, and 12 in Table 1). The cooling rate here means one defined by (solidification start temperature−solidification end temperature) / (time from the start of solidification to the end of solidification).
[0042]
(3) Machining process Machining is performed by appropriately combining machining methods such as hot rolling, hot forging, and machining according to the size and shape complexity of the parts. For example, in the case of shafts integrated with gears etc., after the billet is hot-rolled or hot-forged to be processed into a roughly required shape, at least the gear part having a complicated shape needs to be further shaped by cutting or grinding Become. The heating temperature of the billet during hot rolling or hot forging is 1100 to 1300 ° C, preferably 1150 to 1200 ° C (see test Nos. 1 to 4 in Table 1), and the rolling or forging finishing temperature is 1000 to 1150. ° C, preferably 1050 to 1150 ° C (see Test Nos. 1 to 7 in Table 1). In general, the higher the temperature, the softer the steel structure, which is the parent phase of the steel, compared to the inclusion particles, so the higher the billet heating temperature and the rolling or forging finishing temperature, the more difficult the inclusion particles expand. Since the average aspect ratio of inclusion particles in the steel material after rolling or forging becomes small, the torsional fatigue strength increases. However, if the temperature is excessively high, the mechanical structure of the steel material after rolling or forging decreases due to the coarsening of the steel structure, and the fuel consumption of the heating furnace increases. Is desirable. The rolling reduction by rolling or forging is 98% or less, more preferably 96% or less, and still more preferably 92% or less (see Test Nos. 1 and 8 to 10 in Table 1). Here, the rolling reduction is defined as S ₁ when the billet cross-sectional area perpendicular to the rolling direction before rolling (or forging) is S ₁ , and S ₂ is the member cross-sectional area perpendicular to the rolling direction after rolling (or forging). ₁₋ S ₂ ) / S ₁ × 100 (unit:%). As the rolling reduction increases, the extension of the inclusion particles becomes more significant. Therefore, it is desirable to limit the rolling reduction to the above range.
[0043]
(4) Carburizing (nitrocarburizing) treatment process Carburizing treatment is usually carried out using a gas carburizing method, and carburizing containing CO or CH ₄ in a heating furnace in which the machined member is heated to about 800 to 1000 ° C. It is held for 1 to 5 hours in a gas atmosphere, and C is diffused and penetrated from the member surface to a depth of about 1 mm. Alternatively, a liquid carburizing method or a solid carburizing method may be used instead of the gas carburizing method. Also, carburizing and nitriding can be performed in which NH ₃ or the like is added to the carburizing gas and nitriding is performed simultaneously with carburizing. The member that has been carburized (carburized and nitrocarburized) is quenched in water or oil, and further tempered at a temperature of about 170 to 200 ° C. Then, if necessary, shot peening may be further performed to impart compressive stress to the surface of the carburized component to further improve the mechanical properties.
[0044]
The carburized parts manufactured by the above steps have a chemical composition defined by the present invention, and are controlled so that the form, amount and distribution of inclusion particles satisfy the predetermined conditions defined by the present invention. As a result, an excellent torsional fatigue strength can be obtained.
[0045]
【Example】
In order to confirm the effects of the present invention, a billet prepared by variously changing the chemical composition is rolled or forging temperature, the rolling reduction is changed variously to create a steel material, and the form and amount of inclusions in the steel material And the distribution state was measured. Further, the steel material was machined to produce a torsional fatigue test piece, which was subjected to normal carburizing treatment and shot peening treatment, and then subjected to a torsional fatigue test to measure torsional fatigue strength. This will be described in more detail below.
[0046]
The billet was melted by casting molten steel produced in a converter or a high-frequency induction melting furnace. The billet melted in the converter has a prism shape of □ 155 mm × about 10 mL. The billet melted in the high frequency induction melting furnace has a truncated cone shape with a top surface of φ245 mm, a bottom surface of φ210 mm, a height of 350 mm, and a weight of about 150 kg.
[0047]
In addition, in order to investigate the influence which the coarsening of the inclusion particle | grains had on the fall of the torsional fatigue strength, the cooling rate at the time of casting was variously changed to 120-1600 degreeC / h, and the billet was cast.
[0048]
In addition, in order to investigate the influence on the shape of inclusion particles and the torsional fatigue strength due to hot rolling or hot forging conditions, the billet cast as described above is heated at a temperature of 1050 to 1200 ° C. and a finishing temperature of 900 to 1120 ° C. Then, various reductions were made in the range of 86 to 99%, and hot rolling or hot forging was performed to produce a round bar having a diameter of about 50 to 80 mm.
[0049]
The round bar was cut into a thickness of 30 mm, and then a test piece having a cut surface parallel to the direction in which the inclusion particles were extended was cut out to produce a sample capable of measuring the major axis and minor axis of the inclusion particles. The test piece cut-out position was determined in accordance with JIS G 0303. An image analysis apparatus (Nireco Corporation) in which an optical microscope and an image capturing device / image analysis software are integrated so that the cut surface of the test piece can be observed in parallel with the direction in which the inclusion particles are extended. Manufactured, model: LUZEX F (LUZEX is a registered trademark)), and the morphology, amount, and distribution of inclusion particles were measured. The measurement magnification is 100 times, and the observation area is in the range of 5.5 mm × 5.5 mm centering on the position of 1/4 of the diameter of the round bar, and all the inclusion particles having a major axis of 3 μm or more existing in this observation area. Measurements were made. Image capture is performed in RGB, and the binarization levels are R / 125/180, G / 110/180, B / 120/180 (each denominator value 180 means full scale), and inclusion particles The brightness (gray level) and contrast of the image were adjusted so that can be sufficiently distinguished from the matrix.
[0050]
On the other hand, the test piece for the torsional fatigue test was formed by cutting a round bar having a diameter of 52 mm into an appropriate length, and then forming it into a tube having a total length of 230 mm, an outer diameter of a parallel portion of 20 mm, and an inner diameter of 10 mm by machining. A horizontal hole having a diameter of 3 mm was formed in the center (that is, the center in the longitudinal direction of the parallel part). In order to avoid stress concentration in the side hole portion, a taper-shaped reamer process having a reamer depth of 0.8 mm and 45 ° with respect to the depth direction was performed. Then, this test piece was carburized under the condition of 925 ° C. × 150 min, further held at 850 ° C. for 10 min, followed by quenching, and then tempered at about 180 ° C. for 120 min. Further, the test piece after the tempering treatment was subjected to shot peening treatment. Shot peening was performed on the entire outer surface of the parallel part of the test piece centering on the side hole part under the conditions of the projection material hardness HRC60, the average shot particle size 0.6 mm, and the arc height 0.85 mA. Based on JIS Z 2273, the torsional fatigue test was performed by swinging, changing the torque to 4 levels (690, 780, 880, 1180 N · m) at a frequency of 5 Hz, and determining the swinging torsional fatigue limit.
[0051]
The test results are shown in Tables 1 and 2.
Table 1 shows the morphology, amount, and distribution of inclusion particles when the chemical composition of the molten steel is constant and the cooling rate during casting, the heating temperature during hot rolling or hot forging, the finishing temperature, and the reduction rate are changed. It summarizes the results of investigating the effects on (average aspect, area ratio, distribution function) and torsional fatigue strength (one-sided torsional fatigue limit). In the table, ◯ indicates that the specified value of the present invention is satisfied, and x indicates that it is not satisfied. Since it can be said that the chemical composition of the molten steel does not substantially change even after casting, the chemical composition of the billet cast from the molten steel in Table 1 is the chemical composition of the carburized steel defined in the present invention (claims 1 and 2). Satisfies the range. In addition, the case where AR ≦ 6 and Af ≦ 0.6 is satisfied is indicated by “◯”, and the case where it is not satisfied is indicated by “×”. When the value of the left side of the formula (1) is 0 or more, “◯” is indicated. Represents. And when the swing torsional fatigue limit is 400 N / mm ² or more, “◯” is shown, and when it is less than 400 N / mm ² , “×” is shown.
[0052]
As is apparent from Table 1, the chemical composition of the molten steel satisfies the range defined by the present invention, and further satisfies the test No. 1 satisfying AR ≦ 6 and Af ≦ 0.6. In the case of 1, 2, 3, 5, 9, 10, and 12 (corresponding to the invention of claim 1), it was confirmed that a high torsional fatigue strength with a one-sided torsional fatigue fatigue limit of 400 N / mm ² or more was obtained. .
[0053]
Of the above tests, Test No. In the cases of 1, 2, 9, 10, and 12, the chemical composition of the molten steel satisfies the range defined by the present invention, and further satisfies the formula (1), which corresponds to the invention of claim 2. Yes.
[0054]
Next, Table 2 changes the chemical composition of the molten steel and the reduction ratio during hot rolling or hot forging while keeping the cooling rate during casting, the heating temperature during hot rolling or hot forging, and the finishing temperature constant. This is a summary of the results of investigations on the effect of inclusion particles on the morphology, amount, distribution, and torsional fatigue strength. The meanings of ○ and X in the table are the same as in Table 1. In addition, Test No. 101 is a test No. in Table 1. The same test as 1.
[0055]
As is apparent from Table 2, the chemical composition of the molten steel satisfies the range defined by the present invention, and further satisfies the test No. 1 satisfying AR ≦ 6 and Af ≦ 0.6. In the case of 101 to 110 and 112 to 114 (corresponding to the invention of claim 1), it was confirmed that a high torsional fatigue strength with a single swing torsional fatigue fatigue limit of 400 N / mm ² or more was obtained.
[0056]
Further, even when the conditions of AR ≦ 6 and Af ≦ 0.6 are not satisfied, the chemical composition of the molten steel satisfies the range defined by the present invention, and further satisfies the formula (1) (Claim 2). Test No.) In the case of 111, 116, 118, and 120, it was confirmed that a high torsional fatigue strength with a swing-torsional fatigue limit of 400 N / mm ² or more was obtained.
[0057]
On the other hand, test No. in which the chemical composition of the molten steel deviated from the range defined by the present invention. In the case of 122 to 128, the test No. Except for 123, the swing torsional fatigue limit did not reach 400 N / mm ² , and sufficient torsional fatigue strength could not be obtained. In addition, Test No. In the case of 123, although the C content of the molten steel is 0.50% and is outside the range of C: 0.1 to 0.4% defined by the present invention, the one-side torsional fatigue fatigue limit is 400 N / mm. ^{Although 2} or more were obtained, the machinability was inferior, and machining from a billet to a torsional fatigue test piece was difficult.
[0058]
[Table 1]

[0059]
[Table 2]

[0060]
【The invention's effect】
As described above, the carburizing steel and carburized parts of the present invention have excellent torsional fatigue strength while maintaining good machinability, and therefore, automobiles having complicated shapes such as shafts integrated with gears. The drive type parts can be provided at low cost.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram for explaining types of cracks, where (a) shows a mode I crack, (b) shows a mode II crack, and (c) shows a mode III crack.

Claims (6)

The chemical composition is C: 0.1 to 0.4% by mass ratio, Mn: 0.1 to 2.0%, Si: 0.6% or less, P: 0.015% or less, S: 0.00. 1% or less, Al: 0.01-0.1%, N: 0.003-0.02%, balance Fe and inevitable impurities, and
For carburizing, wherein an average aspect ratio of inclusion particles having a major axis of 3 μm or more in an arbitrary cross section parallel to the rolling direction is 6.0 or less, and an area ratio of the inclusion particles is 0.6% or less Steel material. The chemical composition is C: 0.1 to 0.4% by mass ratio, Mn: 0.1 to 2.0%, Si: 0.6% or less, P: 0.015% or less, S: 0.00. 1% or less, Al: 0.01-0.1%, N: 0.003-0.02%, balance Fe and inevitable impurities, and
An average aspect ratio AR of inclusion particles having a major axis of 3 μm or more in an arbitrary cross section parallel to the rolling direction, an area ratio Af of the inclusion particles, and a distribution index F of the inclusion particles satisfy the following formula: Carburizing steel.
Formula −21.4 × AR + 414.2 × F−80.8 × Af−30 ≧ 0
Here, the unit of Af is%,
^{_{F = X 1 / (A /}} n) 1/2,
A: Area of an arbitrary observation visual field in the cross section, n: Number of inclusion particles existing in the observation visual field, X ₁ : Each inclusion for all the inclusion particles existing in the observation visual field This is a value obtained by actually measuring the distance to another inclusion particle present closest to each particle and arithmetically averaging the measured distance. The chemical composition further includes one or more of Cr: 0.1 to 3.0%, Ni: 0.1 to 3.0%, and Mo: 0.1 to 1.0 % in terms of mass ratio. The steel material for carburizing according to claim 1, wherein the steel material for carburizing is included. The steel composition for carburizing according to any one of claims 1 to 3, wherein the chemical composition further contains Ti: 0.005 to 0.1 % by mass ratio.   The chemical composition further includes Ca: 0.0005-0.01%, Mg: 0.0005-0.01%, Zr: 0.0005-0.05%, Te: 0.0005-0. The steel material for carburizing according to any one of claims 1 to 4, comprising one or more of 05%. A carburized part manufactured by carburizing or carburizing and nitriding the carburizing steel according to any one of claims 1 to 5 in a conventional manner.

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