JP3996386B2 - Carburizing steel with excellent torsional fatigue properties - Google Patents

Description

【００５１】
【表２】

【００５２】
表２及び図３より明らかなように、［Ｃｏ］＋２．１×［Ｎｉ］＋２．８×［Ｃｕ］−２．４［Ｃｒ］≦０の場合（Ｃｒの添加量に対するＣｏ、Ｎｉ、又はＣｕの添加量が不足する場合；図３中の×印）、ねじり疲労強度はＣｒの添加量が増大するにつれて急速に低下する。これに対して［Ｃｏ］＋２．１×［Ｎｉ］＋２．８×［Ｃｕ］−２．４［Ｃｒ］＞０の場合（図３中の○印）、Ｃｒ添加量が増大してもねじり疲労強度の低下が抑制されている。
【００５３】
また図４より明らかなように、Ｃｒ含有量が増大するにつれて、耐ピッチング性も向上する。そこで、Ｃｒ含有量を０．５質量％以上程度で区切り、耐ピッチング性に優れた試験片において、式［Ｃｏ］＋２．１×［Ｎｉ］＋２．８×［Ｃｕ］−２．４［Ｃｒ］とねじり疲労強度との関係を整理した。結果を図５に示す。図５より明らかなように、Ｃｒ含有量を０．５質量％以上とし、［Ｃｏ］＋２．１×［Ｎｉ］＋２．８×［Ｃｕ］−２．４［Ｃｒ］＞０とすれば、耐ピッチング性とねじり疲労強度とを両立できる。
【００５４】
なお実験例１４は、１０万回疲労強度に優れているものの、Ｃ含有量が少ないため、強度（静的強度）の点で他のＣ含有量が多い実験例の方が優れている。実験例１５は、１０万回疲労強度に優れているものの、Ｃ含有量が多いため、被削性の点で他のＣ含有量が少ない実験例の方が優れている。
【００５５】
【発明の効果】
本発明の鋼によれば、Ｃｒの添加量が増大するにつれて、Ｃｏ、Ｎｉ、Ｃｕなども増量添加しているため、浸炭処理した際に浸炭層中のＣｒ炭化物の生成量を抑制することができ、浸炭部材のねじり疲労強度を向上させることができる。
【図面の簡単な説明】
【図１】図１はＣｒ含有量と鋼材特性との関係を概念的に示すグラフである。
【図２】図２はねじり疲労試験で使用する試験片の概略図である。
【図３】図３は実施例で試験した鋼材について、Ｃｒ含有量とねじり疲労強度（ＭＰａ）との関係を示すグラフである。
【図４】図４は実施例で試験した鋼材について、Ｃｒ含有量とローラーピッチング寿命との関係を示すグラフである。
【図５】図５は計算式［Ｃｏ］＋２．１×［Ｎｉ］＋２．８［Ｃｕ］−２．４［Ｃｒ］の値とねじり疲労強度との関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a carburizing steel excellent in torsional fatigue characteristics useful for manufacturing a shaft-like member used for automobile parts, building machine parts, industrial machine parts and the like.
[0002]
[Prior art]
In automobiles, construction machines, industrial machines, etc., the output of motive power has been increased. For example, in automobiles, engine output has increased. As these outputs increase, improvement in torsional fatigue strength of shaft-like members (shafts and the like) used for power transmission in the automobiles and the like has been demanded.
[0003]
As the shaft member, a member whose surface strength is increased by induction hardening is often used. Induction hardening is generally used because it can be quenched with relatively simple equipment and has high productivity. For this reason, many inventions related to induction hardening have been carried out, and many inventions in which torsional fatigue strength is increased in induction hardening steels. For example, in Japanese Patent No. 2774118, Cr and Ni are elements useful for improving torsional fatigue strength by induction hardening, and Cr is contained in an amount of 1.0 wt% or more and Ni is contained in an amount of 0.10 wt% or more. In addition, it is described that high torsional fatigue strength can be obtained by setting Ni + 0.70Cr to 0.250 wt% or more.
[0004]
However, induction hardening steel generally has a problem in machinability because of the large amount of C. In addition, induction hardening is a method in which only the surface of the member is heated by high frequency, and thus it is difficult to harden a member having a complicated shape. Therefore, for example, when manufacturing a member having a hole or a groove, it is difficult to use induction hardening.
[0005]
In the case where the surface of a member having a complicated shape such as having a hole or a groove is hardened, the number of cases in which carburizing and quenching is performed has increased in recent years. In carburizing and quenching, even a member having a complicated shape can easily harden the surface. However, while various inventions for improving torsional fatigue strength have been made for induction hardening, few inventions have been found for improving torsional fatigue strength for carburizing.
[0006]
[Problems to be solved by the invention]
The present invention has been made paying attention to the above-described circumstances, and an object thereof is to improve torsional fatigue characteristics in carburizing steel.
[0007]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present inventors have conducted detailed research on torsional fatigue fracture of a carburized and quenched shaft-like member, and as a result, have been regarded as important in improving the bending fatigue characteristics of conventional carburized members. It was discovered that the amount of carbide in the carburized layer that had not been found had an extremely large effect in terms of torsional fatigue characteristics. Hereinafter, this point will be described in detail with reference to FIG.
[0008]
That is, as shown in FIG. 1, in the carburizing steel, the hardenability of the carburized layer is improved as the Cr content is increased, and the surface hardness is improved. As a result, the pitting resistance and wear resistance are improved (FIG. 1). (See the solid line). This is because Cr has a characteristic of solidifying the cementite in the steel and hardening the steel. Therefore, it can be said that the larger the amount of Cr added, the better from the viewpoint of pitting resistance and wear resistance.
[0009]
However, according to the study by the present inventors, it was found that the torsional fatigue characteristics of the carburized member decrease as the Cr content increases, unlike the induction-hardened member (see the thin broken line in FIG. 1). As a result of further studies, it was found that the torsional fatigue characteristics decline as the amount of Cr carbide in the carburized layer increases, not the amount of Cr itself. In other words, Cr alone is an element that easily forms carbides. Normally, when the addition amount is increased, Cr and carbon are combined in a carburized portion outermost layer having a high carbon concentration, and a large amount of Cr carbide is generated (see FIG. 1). (See dashed line). In torsional fatigue, it was thought that this increased Cr carbide promotes fatigue crack growth and reduces fatigue strength. Therefore, it was found that even if the amount of Cr added is increased, the torsional fatigue strength is improved as compared with the conventional case (see the thick line in FIG. 1) if the amount of carbide generated is suppressed (see the thick dashed line in FIG. 1). (See dashed line). In summary, the inventors have found that in carburizing steels where it is desirable to increase the amount of Cr added, it is important to suppress the formation of Cr carbides in order to improve torsional fatigue properties. It is.
[0010]
As a result of further intensive studies to suppress the amount of Cr carbide produced, as the amount of Cr increases, the amount of Co, Ni, or Cu increases, and carbide (particularly, a film at the grain boundary). The present inventors have found that the formation of carbides precipitated in a shape can be suppressed and the torsional fatigue strength can be improved, and the present invention has been completed.
[0011]
That is, the carburizing steel excellent in torsional fatigue characteristics of the present invention that can achieve the above-mentioned object has a carbon content of 0.1 to 0.3% by mass, and in the carburizing steel containing Cr, Co, It has a gist in that at least one selected from Ni and Cu is contained within a range satisfying the following formula (1).
[0012]
[Co] + 2.1 × [Ni] + 2.8 × [Cu] −2.4 × [Cr]> 0 (1)
[In the formula, [Co], [Ni], [Cu], or [Cr] are Co content (% by mass), Ni content (% by mass), and Cu content (% by mass) in steel, respectively. Or Cr content (% by mass)]
The preferred contents of Co, Ni, Cu and Cr are Co: 7% by mass or less (including 0%), Ni: 4.5% by mass or less (including 0%), Cu: 4% by mass or less (0 %), Cr: 0.5 to 2% by mass (however, Co, Ni and Cu are not simultaneously 0%)
The carburizing steel further includes Mo: 0.45% by mass or less (excluding 0%), B: 0.003% by mass or less (not including 0%), Ti: 0.1% by mass or less (0% Nb: 0.1% by mass or less (not including 0%), Al: 0.1% by mass or less (not including 0%), and the like.
[0013]
The carburizing steel is usually further Si: 0.5% by mass or less (not including 0%), Mn: 2% by mass or less (not including 0%), N: 0.05% by mass or less (0% P: 0.03% by mass or less (not including 0%), S: 0.03% by mass or less (not including 0%).
[0014]
DETAILED DESCRIPTION OF THE INVENTION
The carburizing steel of the present invention has a carbon content of 0.1 to 0.3% by mass and contains Cr. The reason why the carbon content is 0.1% by mass or more is that the strength of the obtained member is insufficient when the carbon content is low. The carbon content is preferably 0.10% by mass or more, and more preferably 0.15% by mass or more. In addition, the upper limit of the carbon content is set to 0.3% by mass because the carbon concentration on the surface can be increased in order to carburize in the manufacturing process of the member, so that machinability is maintained at the stage of the steel material. This is because it is desirable to suppress the carbon concentration. The upper limit of the carbon content is desirably 0.25% by mass.
[0015]
The content of Cr is not particularly limited, but considering that it is a carburizing steel, the higher the content, the better the wear resistance and pitting resistance of the resulting member. The Cr content is, for example, 0.5% by mass or more, preferably 0.6% by mass or more, and more preferably 0.7% by mass or more. In addition, when there is too much Cr content, even if it takes the Cr carbide | carbonized_material suppression measure mentioned later, a carbide | carbonized_material cannot fully be suppressed and the torsional fatigue strength of a member may fall. Therefore, the Cr content is desirably suppressed to, for example, 3% by mass or less, preferably 2% by mass or less, more preferably 1.5% by mass or less, and particularly 1.2% by mass or less.
[0016]
In the present invention, elements such as Co, Ni, and Cu are also added to the carburizing steel containing Cr as described above. The greater the Cr content, the more easily Cr carbides are generated during carburizing, and the torsional fatigue strength is likely to be reduced.In the present invention, depending on the amount of Cr added, Co, Ni, Cu, etc. Since the addition amount of the element is also increased, the formation of Cr carbide can be suppressed and the torsional fatigue strength can be prevented from being lowered. Co, Ni, Cu, and the like are elements that suppress the formation of carbides, and the addition of these elements can also suppress the formation of Cr carbides. The addition amount of Co, Ni, and Cu can be appropriately determined according to the carbide suppressing ability of these elements and the addition amount of Cr. For example, the following formula (1), preferably the following formula (2), and more preferably the following formula (2) Addition is made within a range satisfying the formula (3).
[0017]
[Co] + 2.1 × [Ni] + 2.8 × [Cu] −2.4 × [Cr]> 0 (1)
[Co] + 2.1 × [Ni] + 2.8 × [Cu] −2.4 × [Cr]> 1 (2)
[Co] + 2.1 × [Ni] + 2.8 × [Cu] −2.4 × [Cr]> 2 (3)
[In the formula, [Co], [Ni], [Cu], or [Cr] are Co content (% by mass), Ni content (% by mass), and Cu content (% by mass) in steel, respectively. Or Cr content (% by mass)]
The larger the value on the left side of the above formulas (1) to (3), the more the amount of Cr carbide in the carburized layer after carburizing can be suppressed, and the torsional fatigue strength of the carburized member can be increased. The Co, Ni, and Cu may be added alone or in combination of two or more.
[0018]
The amount of Co added is not particularly limited as long as the above formulas (1) to (3) are satisfied. For example, 7% by mass or less (including 0%), preferably 6% by mass or less, and more preferably 5% by mass or less. In particular, it is desirable to suppress to 3% by mass or less (for example, 1.5% by mass or less). Even if Co is added too much, the effect is saturated as compared with the added amount, and further, since it is an expensive element, the manufacturing cost is improved.
[0019]
Further, the amount of Ni added is not particularly limited as long as the above formulas (1) to (3) are satisfied. For example, it is 4.5% by mass or less (including 0%), preferably 3.5% by mass or less, more preferably It is desirable to suppress to 2.5% by mass or less, particularly 1.7% by mass or less. Even if Ni is added too much, the effect is saturated as compared with the added amount.
[0020]
The amount of Cu added is not particularly limited as long as the above formulas (1) to (3) are satisfied. For example, 4% by mass or less, preferably 3% by mass or less, more preferably 2% by mass or less, particularly 1.5% by mass. It is desirable to suppress to the following. Even if Cu is added excessively, the effect is saturated as compared with the added amount, and further, the recyclability of the product is lowered.
[0021]
The carburizing steel is usually Si: 0.5% by mass or less (not including 0%), Mn: 2% by mass or less (not including 0%), and N: 0.05% by mass or less (0%) Is not contained), P: 0.03% by mass or less (not including 0%), S: 0.03% by mass or less (not including 0%) is often suppressed, The balance may be Fe and inevitable impurities, and may contain various additive components as necessary. Hereinafter, the reasons for limiting the above components will be described.
[0022]
Si: 0.5% by mass or less (excluding 0%)
Si invites the formation of a grain boundary oxide layer and may reduce fatigue strength, so it is desirable not to contain Si more than necessary. The Si content is, for example, 0.5% by mass or less, preferably 0.35% by mass or less, and more preferably 0.15% by mass or less. Since Si is useful as a deoxidizing element, it may be actively added. In the case of positively adding, the Si content is, for example, 0.05% by mass or more, and preferably about 0.10% by mass or more.
[0023]
Mn: 2% by mass or less (excluding 0%)
If the content of Mn is too large, depending on the steelmaking conditions, wrinkles and center segregation may be induced and the quality of the steel material may be deteriorated. Therefore, the Mn content is, for example, 2% by mass or less, preferably 1.5% by mass or less, and more preferably 1.2% by mass or less. Note that Mn is useful for deoxidation of steel materials and is effective in increasing the depth of the carburized hardened layer because it is a hardenability improving element, and may be actively added. In the case of positive addition, the Mn content is, for example, 0.5% by mass or more, preferably 0.6% by mass or more, and more preferably 0.8% by mass or more.
[0024]
N: 0.05% by mass or less (excluding 0%)
If N is excessive, nonmetallic inclusions are formed and the toughness is reduced. Accordingly, the N content is, for example, 0.05% by mass or less, preferably 0.03% by mass or less, and more preferably 0.02% by mass or less. In addition, when steel contains Al and Ti, if N exists, it has the effect | action which prevents the coarsening of a crystal grain by producing | generating AlN and TiN, and improves pitting resistance. In this case, the N content is, for example, 0.005% by mass or more, preferably 0.01% by mass or more, and more preferably 0.015% by mass or more.
[0025]
P: 0.03 mass% or less (excluding 0%)
Since P segregates at the grain boundaries and degrades the fatigue strength of the steel material, it is desirable to suppress it as much as possible. For example, the P content is preferably 0.03% by mass or less, preferably 0.02% by mass or less, and more preferably 0.015% by mass or less.
[0026]
S: 0.03 mass% or less (excluding 0%)
S binds to Mn in the steel to form MnS inclusions, and depending on the part shape, may cause a decrease in fatigue strength, so it is desirable to suppress as much as possible. For example, the S content is desirably 0.03% by mass or less, preferably 0.025% by mass or less, and more preferably 0.02% by mass or less.
[0027]
The carburizing steel further contains at least one selected from Mo: 0.45% by mass or less (not including 0%) and B: 0.003% by mass or less (not including 0%). It is desirable.
[0028]
Mo is useful for improving the hardenability of the carburized layer, and can reduce the carburized abnormal layer and improve the fatigue strength. The Mo content is, for example, 0.1% by mass or more, preferably 0.2% by mass or more, and more preferably 0.25% by mass or more. On the other hand, if Mo is added excessively, the cost is increased, and the hardness of the rolled material is increased, so that machinability and workability are reduced. Therefore, the Mo content is, for example, 0.45 mass% or less, preferably 0.4 mass% or less, and more preferably 0.3 mass% or less.
[0029]
B is also useful for improving the hardenability of the carburized layer. The content of B is, for example, 0.0005% by mass or more, preferably 0.001% by mass or more. On the other hand, even if a large amount of B is added, the effect of improving the hardenability is saturated and only the cost is increased. Accordingly, the B content is, for example, 0.003% by mass or less, preferably 0.002% by mass or less, and more preferably 0.0015% by mass or less.
[0030]
In the carburizing steel of the present invention, Ti: 0.1% by mass or less (not including 0%), Nb: 0.1% by mass or less (not including 0%), and Al: 0.1% by mass It is desirable to contain at least one selected from the following (excluding 0%). Ti is useful for fixing N in steel and refining γ grains. Nb forms fine carbides and is useful for refining γ grains. Al, like Ti, is useful for fixing N and making γ grains finer. In addition, the preferable addition amount of these components is as follows.
[0031]
Ti: Preferably 0.01 mass% or more (more preferably 0.02 mass% or more), 0.1 mass% or less (more preferably 0.05 mass% or less)
Nb: Preferably 0.01 mass% or more (more preferably 0.02 mass% or more), 0.1 mass% or less (more preferably 0.05 mass% or less)
Al: Preferably 0.01 mass% or more (more preferably 0.02 mass% or more), 0.1 mass% or less (more preferably 0.05 mass% or less)
The shape of the carburizing steel is not particularly limited, but it is desirable that the carburizing steel be linear or rod-shaped in consideration of processing into a shaft-shaped member.
[0032]
The carburizing steel can be processed into a carburized member (such as a shaft-shaped member) by performing carburizing after performing various conventional treatments such as rolling, wire drawing, forging, heat treatment, and cutting as required. .
[0033]
The carburizing conditions are not particularly limited, and carburizing may be performed under general-purpose conditions. The carburizing temperature can be appropriately selected from the range of, for example, about 750 to 1100 ° C, preferably about 800 to 1000 ° C, and more preferably about 850 to 950 ° C. Further, the carbon potential at the time of carburization (equilibrium carbon concentration in the atmosphere) is about 0.5 to 1.6% by mass, preferably about 0.6 to 1.4% by mass, and more preferably 0.7 to 1.0%. It can select suitably from the range of about mass%. In the present invention, even when carburizing under such general-purpose conditions, the steel contains a predetermined amount of Co, Ni, and / or Cu according to the amount of Cr added, so the amount of Cr carbide produced is suppressed. As a result, the torsional fatigue strength of the carburized member is increased.
[0034]
The carburizing conditions are performed under conditions that can suppress the formation of carbides as long as the hardness of the surface of the member can be maintained at a predetermined level or higher by carburization, that is, as long as the carbon concentration of the carburized portion can be set to a predetermined level or higher. Also good. For example, the carbon potential of the carburizing atmosphere may be suppressed to about 0.5 to 0.8 mass%, preferably about 0.5 to 0.7 mass%.
[0035]
The carbon concentration of the carburized portion surface of the carburized member obtained by the present invention is, for example, about 0.6 to 1.2% by mass, preferably about 0.7 to 1.0% by mass, and more preferably 0.8 to 0%. About 9% by mass.
[0036]
Since the carburized member is excellent in torsional fatigue strength, for example, as a shaft-shaped member, preferably a shaft-shaped member used for transmitting power in automobile parts, building machine parts, industrial machine parts, etc. Very useful.
[0037]
The torsional fatigue strength of the carburized member is, for example, 100,000 times by cutting out a dumbbell-shaped test piece shown in FIG. 2 and performing a one-way descending test at a repetition rate of 5 Hz in a load torque range of 70 to 120 kgf · m. It can be evaluated by obtaining a stress (100,000 times fatigue strength) that can withstand a fatigue test. The torsional fatigue strength (100,000 times fatigue strength) of the carburized member obtained in the present invention is, for example, 680 MPa or more (preferably 700 MPa or more, more preferably 720 MPa or more, particularly 740 MPa or more), and usually 1000 MPa or less.
[0038]
According to the present invention, the Cr content can be increased while maintaining a high torsional fatigue strength. Therefore, when using a high Cr steel material, the pitching resistance of the shaft-like member can be improved. Specifically, the pitting resistance can be measured by a two-roller test method shown in the following 1) to 4).
[0039]
1) A cylindrical specimen (however, a semi-columnar projection having a radius of 12 mm is provided so as to make one round in the circumferential direction of the side wall of the cylindrical specimen in the same manner as the method of manufacturing the shaft member. The diameter of the test piece including the part = 70 cm is manufactured, and the counterpart cylindrical member (however, a rectangular parallelepiped protrusion with a height of 12 mm and a width of 10 mm is provided so that the side wall of the cylindrical test piece makes one round in the circumferential direction) The diameter of the test piece including the protrusions is 70 cm (made of JIS SUJ2 steel), and the test pieces are arranged in parallel so as to contact each other at the protrusions.
[0040]
2) The cylindrical test piece and the counterpart cylindrical member are pressed at a surface pressure of 5000 MPa, and the cylindrical test piece is rotated at a rotation speed of 1910 rpm while supplying gear oil for automatic transmission at a flow rate of 2 L / min to the sliding part (contact part). And the other cylindrical member is rotated at a rotation speed of 1364 rpm (sliding speed; 2.0 m / s).
[0041]
3) Measure the number of rotations of the cylindrical test piece until pitching occurs on the sliding surface and vibration abnormality occurs.
[0042]
4) The tests shown in the above 1) to 3) are performed using a plurality of cylindrical test pieces, and the rotation speed at which vibration abnormality occurs in 10% of the test pieces is determined (L10 life).
[0043]
According to the present invention, the L10 life required by the test can be, for example, 5 × 10 ⁷ times or more, preferably 7 × 10 ⁷ times or more, more preferably 9 × 10 ⁷ times or more.
[0044]
【Example】
EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.
[0045]
Experimental Examples 1 to 23
In a vacuum induction melting furnace (VIF furnace), 150 kg of steel having the chemical composition shown in Table 1 (the balance is Fe and unavoidable impurities) is melted and made into a round bar steel having a diameter of 50 mm by hot forging, followed by solution treatment. (1300 ° C. × 1 hour) and heat conditioning (900 ° C. × 2 hours) were performed. This round steel bar was cut and cut into a test piece shape (dumbbell shape) shown in FIG.
[0046]
The dumbbell-shaped steel material is subjected to a first step (temperature 900 ° C., 150 minutes, carbon potential: 0.95 mass%), a second step (temperature 840 ° C., 30 minutes, carbon potential: 0.85 mass%), and After carburizing and quenching comprising the third step (water cooling), tempering was performed at a temperature of 180 ° C. for 2 hours, and surface polishing was performed from the outermost surface to a depth of 0.1 mm (± 0.025 mm).
[0047]
The obtained dumbbell-shaped carburized member is set in a torsional fatigue testing machine, and a one-way descending test at a repetition rate of 5 Hz is performed in a load torque range of 70 to 120 kgf · m, and a stress (100,000 to withstand 100,000 fatigue tests). Fatigue strength) was determined.
[0048]
Further, in the same manner as the dumbbell-shaped carburized member, a cylindrical carburized member (provided that a semi-columnar projecting portion having a radius of 12 mm makes one round in the circumferential direction of the side wall portion of the cylindrical test piece.) The diameter of the test piece including 70 mm was manufactured. Using this cylindrical carburized member, the above-mentioned pitching resistance test (two-roller test) was performed, and the pitching resistance (L10 life) was measured.
[0049]
The results are shown in Table 2, FIG. 3 and FIG. In FIG. 3, the circles correspond to the experimental results of [Co] + 2.1 × [Ni] + 2.8 × [Cu] −2.4 [Cr]> 0, and the crosses indicate [Co] +2. 1 × [Ni] + 2.8 × [Cu] −2.4 [Cr] ≦ 0.
[0050]
[Table 1]

[0051]
[Table 2]

[0052]
As apparent from Table 2 and FIG. 3, in the case of [Co] + 2.1 × [Ni] + 2.8 × [Cu] −2.4 [Cr] ≦ 0 (Co, Ni with respect to the added amount of Cr, or When the addition amount of Cu is insufficient; x in FIG. 3), the torsional fatigue strength decreases rapidly as the addition amount of Cr increases. On the other hand, in the case of [Co] + 2.1 × [Ni] + 2.8 × [Cu] -2.4 [Cr]> 0 (marked with a circle in FIG. 3), the torsion occurs even when the Cr addition amount increases. A decrease in fatigue strength is suppressed.
[0053]
As is clear from FIG. 4, as the Cr content increases, the pitting resistance also improves. Therefore, in a test piece excellent in pitting resistance by dividing the Cr content by about 0.5% by mass or more, the formula [Co] + 2.1 × [Ni] + 2.8 × [Cu] -2.4 [Cr And the torsional fatigue strength. The results are shown in FIG. As is clear from FIG. 5, if the Cr content is 0.5 mass% or more and [Co] + 2.1 × [Ni] + 2.8 × [Cu] −2.4 [Cr]> 0, Both pitting resistance and torsional fatigue strength can be achieved.
[0054]
In addition, although Experimental Example 14 is excellent in 100,000 times fatigue strength, since the C content is small, the experimental example having a large other C content is superior in terms of strength (static strength). Although the experimental example 15 is excellent in 100,000 times fatigue strength, since the C content is large, the experimental example with less other C content is superior in terms of machinability.
[0055]
【The invention's effect】
According to the steel of the present invention, as the added amount of Cr increases, Co, Ni, Cu, and the like are also added in an increased amount, so that the amount of Cr carbide generated in the carburized layer can be suppressed when carburizing treatment is performed. The torsional fatigue strength of the carburized member can be improved.
[Brief description of the drawings]
FIG. 1 is a graph conceptually showing the relationship between Cr content and steel material properties.
FIG. 2 is a schematic view of a test piece used in a torsional fatigue test.
FIG. 3 is a graph showing the relationship between Cr content and torsional fatigue strength (MPa) for the steel materials tested in the examples.
FIG. 4 is a graph showing the relationship between Cr content and roller pitting life for steel materials tested in Examples.
FIG. 5 is a graph showing the relationship between the value of the calculation formula [Co] + 2.1 × [Ni] +2.8 [Cu] -2.4 [Cr] and torsional fatigue strength.

Claims (3)

Carbon content is 0.1 to 0.3% by mass, Cr content is 0.5 to 2% by mass, Co content is 7% by mass or less (including 0%), Ni content The amount is 1.7% by mass or less (including 0%), and the Cu content is 4% by mass or less (including 0%) (however, Co, Ni and Cu are not simultaneously 0%). Further, Si: 0.5% by mass or less (not including 0%), Mn: 2% by mass or less (not including 0%), Al: 0.1% by mass or less (not including 0%), and N: 0.05% by mass or less (not including 0%), P: 0.03% by mass or less (not including 0%), S: 0.03% by mass or less (not including 0%) and the balance is Fe and inevitable impurities, under following formula (1) excellent carburizing steel torsional fatigue characteristics characterized and Turkey to satisfy.
[Co] + 2.1 × [Ni] + 2.8 × [Cu] −2.4 × [Cr]> 0 (1)
[In the formula, [Co], [Ni], [Cu], or [Cr] are Co content (% by mass), Ni content (% by mass), and Cu content (% by mass) in steel, respectively. Or Cr content (% by mass)] Further, Mo: 0.45 wt% or less (not including 0%), and B: carburizing according to claim 1 containing at least one selected from 0.003 wt% or less (not including 0%) Steel. Further Ti: 0.1 wt% or less (not including 0%), and Nb: 0.1 wt% or less (not including 0%) or al selected at least one containing claim 1 or 2 Carburizing steel.

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