JP3551573B2 - Steel for carburized gear with excellent gear cutting - Google Patents

Description

【００３３】
表２に実験結果を示している。１〜１０が本発明鋼の試験結果であり、１１〜１８が比較鋼の試験結果である。尚、１１はＪＩＳ鋼のＳＣｒ４２０の結果を示している。
【００３４】
【表２】

【００３５】
表２の結果において、本発明鋼は何れもＳＣｒ４２０に比べ、２〜９倍の良好な歯切り性を有するとともに、２〜４倍の高い衝撃値が得られている。また疲労強度の低下もなく、ＳＣｒ４２０と同等以上の疲れ限度を確保している。
【００３６】
一方、Ｃ量が請求範囲以下である比較鋼１２は歯切り性と衝撃特性は良好であるものの、疲れ限度がＳＣｒ４２０に比べ低い値となっている。逆にＣ量の高い１３やＭｎ量の高い１５は焼ならし硬さが高く、歯切り性が低下している。
Ｓｉ量の高い１４は、浸炭処理時に生成する粒界酸化層の悪影響により、疲労強度，衝撃特性共に低下している。
【００３７】
ＴｉとＮの関係式の値が−０．０１以下である１６は、Ｂの焼入れ性向上効果が十分に発揮されず、浸炭後の内部硬さが低いものとなり、その結果疲労強度の低下を招いている。
また同様にＤ_Ｉ値が１．５インチ未満の１８も低い内部硬さにより低い疲れ限度を示している。逆にＤ_Ｉ値が４インチを超える高い値を有する１７は焼ならし硬さが非常に高く、本試験条件では寿命評価が不可能なほど低い工具寿命であった。
【００３８】
【発明の効果】
以上のように本発明によれば、浸炭歯車用鋼に良好な歯切り性と耐衝撃特性を、疲労強度を低下させることなく付与することができ、これにより歯車部品の製造コストの削減や歯車部品の小型化を推進できるなど産業上大きな効果を得ることができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a carburizing or carbonitriding steel capable of obtaining a gear part having good gear cutting performance and excellent impact resistance.
[0002]
Problems to be solved by the prior art and the invention
Conventionally, case hardening steels represented by JIS steels SCr420 and SCM420 have been used as gear steels used for gears, shafts and the like as mechanical structural parts.
However, with increasing demands for higher strength gears, various carburized gear steels with improved fatigue strength and impact strength have been proposed. These steels for gears mainly improve the strength by increasing the toughness-improving elements such as Ni and Mo, but on the other hand, the increased amounts of these elements deteriorate the machinability such as machinability and become a problem. I have.
[0003]
Among the machinability, in particular, the gear cutting is affected by the hardness of the material, and it is known that a hard and sticky bainite structure is precipitated and the tool life is extremely reduced when the hardness of the material becomes 90HRB or more. I have.
[0004]
As a method of improving machinability by alloy design, addition of free-cutting elements such as Pb and Bi is generally performed for the purpose of reducing cutting resistance. However, in the region where the hardness of the material is 90 HRB or less, the effect of the free-cutting element works effectively, but when the hardness is 90 HRB or more, the degree of improvement of the tool life by the addition of the free-cutting element becomes small, and a desired tool life cannot be obtained. A problem has arisen.
[0005]
The present invention has been made in view of such circumstances, and an object of the present invention is to provide a carburized gear steel having excellent gear cutting performance and excellent impact strength.
[0006]
[Means for Solving the Problems]
The invention of the present application has been made to solve such problems, and the gist of the invention is that the composition of the steel for carburized gears is C: 0.08 to 0.35% by weight%, Si: ≦ 0.5%, Mn: ≤ 1.5%, P: ≤ 0.03%, S: ≤ 0.03%, O: ≤ 0.0020%, and s-B: 0.0003-0. 0050%, Nb: 0.005 to 0.20%, Ti: 0.005 to 0.10%, N: ≦ 0.020%, s-Al: ≦ 0.05%, 0.02 ≧ Ti-3 .6N ≧ -0.01, a composition comprising the balance substantially Fe, and the ideal critical diameter _{D I} value in hardenability test is in that no such that 1.5 to 4 inches.
[0007]
Here, in the present invention, one or more of Cr, Ni, Mo, and V are further added to Cr: ≦ 5.0%, Ni: ≦ 3.0%, Mo: ≦ 1.0%, and V: It can be contained in an amount of ≦ 1.0%.
[0008]
Further, one or more of Pb, Bi, Te, Ca, and Se may be used by adding Pb: ≦ 0.4%, Bi: ≦ 0.4%, Te: 0.001 to 0.05%, Ca: 0.0005 to 0.0030%, Se: 0.003 to 0.05%.
[0009]
[Action]
The present invention adjusts the alloy composition, secures a hardness of 85 HRB or less after hot forging or after normalizing, obtains good gear-cutting properties, and uses conventional steel after carburizing, quenching and tempering. It is characterized by utilizing the effect of improving the hardenability of B in order to obtain a good strength equal to or higher than that of B. The details will be specifically described below.
[0010]
The hardness of the case hardened steel after normalizing is, of course, determined by the structure after normalizing. However, quantitative conversion from texture to hardness is not easy, and there is another difficulty in reflecting it in alloy design, and a simple method has been desired.
As a result of intensive studies with these circumstances as background, it was found that by using the results of the hardenability test as an index, it is possible to estimate the hardness after normalization and reflect it in alloy design. Was.
[0011]
The quenchability test is intended to grasp properties after quenching, such as the critical diameter after quenching, and the effects of various alloying elements including C, Si, and Mn have been clarified by conventional studies. The results of this hardenability test are widely applied to the case of carburizing and quenching and tempering of gears, and are used as one of the quality standards of steel products.
[0012]
However, when using the results of the quenchability test for carburized gears, for example, taking the results of the Jominy-type one-end quenching method as an example, most often define the hardness at a distance from the quenched end of 5 mm to 15 mm, The fact is that the values after that are not given much importance.
This is determined from the size and gear shape of the carburized gear when optimizing the root hardness and core hardness from the viewpoint of ensuring the strength of the gear. And as mentioned earlier, it is controlled as an important factor of steel performance from the viewpoint of reduction in variation in product strength, and a steel manufacturing process utilizing the correlation between hardenability characteristics and alloy composition has been established. I have.
[0013]
On the other hand, the cooling in the normalizing treatment of the case hardening steel is generally air cooling, so-called air cooling, but the effect of various alloying elements on the normalizing hardness is almost clear similarly to the hardenability.
Focusing on this point, the results of investigation on the basis of the results of normalizing the hardness and hardenability test, normalizing the hardness of hardened steel that is used in the carburized gear and Jominy formula end hardenability test results in D _I value And found that a good correlation was obtained.
[0014]
Doing these findings normalizing alloy design for the hardness and low value after based, although certainly lower normalizing hardness Tokuraeru, also the hardness of the near J ₁₅ from J ₅ Similarly, the strength becomes low, and the strength after quenching and tempering after carburizing cannot be secured.
As means for solving this problem, the present invention focused on the effect of improving the hardenability of B.
[0015]
Features of hardenability improving effect of B is different from the other alloying elements, the distance 20mm from the quenching end: Although hardenability until J ₂₀ improves, to a point such as the effect is hardly seen in the subsequent distance is there.
[0016]
By combining this with alloy design to regulate the addition of B and D _I values, it becomes possible to obtain a gear cutting of a gear strength both in steel for good carburization gear. That is, by reducing the amount of other alloy elements by the effect of improving the hardenability of B, the normalization hardness can be reduced, and the gear cutting performance can be dramatically improved compared to conventional steel. Was.
Hereinafter, the reasons for limiting each alloy element will be described in detail.
[0017]
C: 0.08 to 0.35%
C is an element essential for securing the strength of steel, and its content determines the core hardness after carburizing and tempering. Therefore, in the present invention, the lower limit of the C content is set to 0.08%, and the hardness of the core is secured.
However, if the content is too large, the hardness after hot forging or normalizing increases, thereby reducing machinability and causing adverse effects such as lowering impact strength after carburizing and tempering. Therefore, the upper limit of the C content is set to 0.35%.
[0018]
Si: ≦ 0.5%
Si is a ferrite reinforced element, and when added in a large amount, machinability deteriorates with an increase in hardness. Further, since it promotes grain boundary oxidation at the time of carburization and easily becomes a starting point of fracture, its upper limit is made 0.5%.
[0019]
Mn: ≦ 1.5%
Mn is added in order to enhance the hot workability of the steel and secure the hardenability. However, if added excessively, it becomes difficult to soften and anneal the material, and also deteriorates machinability and cold forgeability, so the upper limit is made 1.5%.
[0020]
P: ≦ 0.03%
Since P segregates at austenite grain boundaries and lowers toughness, it is desirable that the content is as small as possible. However, since the toughness is improved by the effect of suppressing the grain boundary embrittlement of B described later, the upper limit is set to 0.03. %.
[0021]
S: ≦ 0.03%
S forms non-metallic inclusions of MnS in steel to improve machinability, but impairs lateral toughness, so the upper limit is made 0.03%.
[0022]
O: ≦ 0.0020%
O produces alumina which is hard and becomes a starting point of fatigue fracture, so that it is reduced as much as possible.
[0023]
s-B: 0.0003-0.0050%
B is an important element in the present invention, and has an effect of improving hardenability, suppressing P segregation at grain boundaries, and improving toughness as described above. If the content is less than 0.0003%, the effect is small. If the content is more than 0.0050%, the effect is saturated and red hot embrittlement occurs.
[0024]
Nb: 0.005 to 0.20%
Nb is an element for refining austenite crystal grains, but its effect is small when its content is less than 0.005%, and when it exceeds 0.20%, its effect is not only saturated but also a large crystallized substance. And deteriorates the manufacturability.
[0025]
Ti: 0.005 to 0.10%
N: ≦ 0.020%
0.02 ≧ Ti−3.6N ≧ −0.01
N is an element that combines with B in steel to form BN and impairs the effect of improving the hardenability of B. In order to prevent this, Ti having an affinity for N that is stronger than B is added. In order to ensure that the effect is exerted, the value of Ti-3.6N (%) needs to be -0.01 or more.
On the other hand, if Ti and N are added unnecessarily, not only does the productivity of the steel material decrease, but also the strength of the gear decreases due to the generation of huge TiN. Therefore, the upper limit of each of Ti and N is regulated, and the value of Ti-3.6N is set to 0.02% or less.
[0026]
s-Al: ≦ 0.05%
Al is an element necessary as a deoxidizing agent, but when added in a large amount, a large alumina inclusion serving as a starting point of fatigue fracture is generated and the strength is reduced, so the upper limit is made 0.05%.
[0027]
D _I value: 1.5 to 4 inches machinability, particularly in order to ensure the toothed resistance is preferably less than or equal to the hardness 85HRB. Result of various research, there is a good correlation between the hardness and D _I values after the normalizing in hardening steel, it is necessary to not more than 4 inches and D _I value in order to ensure the following 85HRB There was found.
On the other hand, if D _I value is less than 1.5 inches, it can not be secured hardening depth after carburizing or carbonitriding, fatigue strength decreases.
[0028]
Cr: ≦ 5.0%
Ni: ≦ 3.0%
Mo: ≦ 1.0%
V: ≦ 1.0%
Each of these elements is an element effective for improving the hardenability of steel, refining the crystal grains, and improving the toughness of the carburized portion and the inside. Therefore, these are 5.0% or less, 3.0% or less, 1.0% or less, and 1% or less, respectively. You may add individually or in combination within the range of 0.0% or less. However, even if added beyond the above range, the effect of improving toughness is not only saturated, but also it becomes difficult to secure hardness of 230 HRB or less.
[0029]
Pb: ≦ 0.4%
Bi: ≦ 0.4%
Te: 0.001 to 0.05%
Ca: 0.0005 to 0.0030%
Se: 0.003 to 0.05%
These elements have the function of improving machinability and suppressing the generation of burrs at the time of cutting, and the effects are exhibited by setting the above ranges. However, if each element is added too much, the productivity is deteriorated.
[0030]
【Example】
Ono-type rotary bending fatigue test pieces having a parallel portion of 8 mm and Charpy impact test pieces having a notch diameter of 10 mmR were prepared from a hot-rolled steel material having the chemical composition shown in Table 1 after normalizing treatment. In the normalizing process, the sample was kept at 925 ° C. for 1 hour and air-cooled.
The Ono-type rotating bending fatigue test piece and the Charpy impact test piece were subjected to carburizing quenching and tempering treatment, and subjected to a test at room temperature. The carburizing quenching and tempering treatment conditions were as follows: the target value of the surface carbon concentration was 0.8%, the carburizing treatment was performed at 910 ° C. for 2 hours, the diffusion treatment was performed for 1 hour, and the temperature was maintained at 830 ° C. and then 80 ° C. Quenched in oil. Tempering was carried out at 160 ° C. for 2 hours and air-cooled.
[0031]
As an evaluation of the machinability, a gear hobbing test was performed on the normalized material using a carbide hob, and the tool life was compared. The tool life was defined as a life at which crater wear became 50 μm. The cutting conditions were as follows: cutting speed: 148 m / min, feed: 4 mm / rpm, cutting depth: 5.4 mm.
[0032]
[Table 1]

[0033]
Table 2 shows the experimental results. 1 to 10 are the test results of the steel of the present invention, and 11 to 18 are the test results of the comparative steel. In addition, 11 has shown the result of SCr420 of JIS steel.
[0034]
[Table 2]

[0035]
In the results shown in Table 2, the steels of the present invention each have 2 to 9 times better gear cutting performance than SCr420, and also have 2 to 4 times higher impact value. Also, there is no decrease in fatigue strength, and a fatigue limit equal to or higher than that of SCr420 is secured.
[0036]
On the other hand, the comparative steel 12 having a C content equal to or less than the claimed range has good gear cutting performance and impact characteristics, but has a lower fatigue limit than SCr420. Conversely, 13 having a high C content and 15 having a high Mn content have a high normalizing hardness and a low gear cutting property.
In the case of 14 having a high Si content, both the fatigue strength and the impact characteristics are reduced due to the adverse effect of the grain boundary oxide layer generated during the carburizing treatment.
[0037]
When the value of the relational expression between Ti and N is -0.01 or less, the effect of improving the hardenability of B is not sufficiently exhibited, and the internal hardness after carburization becomes low, resulting in a decrease in fatigue strength. Inviting.
Similarly D _I value indicates a low fatigue limit by 18 is low internal hardness of less than 1.5 inches. D _I value conversely is very high 17 normalizing hardness with a high value of more than 4 inches in this test conditions were low tool life as impossible life evaluation.
[0038]
【The invention's effect】
As described above, according to the present invention, good gear cutting performance and impact resistance can be imparted to a carburized gear steel without reducing fatigue strength, thereby reducing the manufacturing cost of gear parts and reducing gears. Industrially significant effects can be obtained such as promotion of miniaturization of parts.

Claims (3)

C: 0.08 to 0.35% by weight%
Si: ≦ 0.5%
Mn: ≦ 1.5%
P: ≦ 0.03%
S: ≦ 0.03%
O: ≦ 0.0020%
And s-B: 0.0003 to 0.0050%
Nb: 0.005 to 0.20%
Ti: 0.005 to 0.10%
N: ≦ 0.020%
s-Al: ≦ 0.05%
0.02 ≧ Ti−3.6N ≧ −0.01
The balance substantially Fe, and toothed excellent in carburizing gear steel, characterized in that the ideal critical diameter D _I value in hardenability test is 1.5 to 4 inches. 2. The method according to claim 1, wherein one or more of Cr, Ni, Mo, and V are Cr: ≦ 5.0%.
Ni: ≦ 3.0%
Mo: ≦ 1.0%
V: ≦ 1.0%
Carburized gear steel with excellent gear cutting characteristics, characterized in that it is contained in an amount of: 3. The method according to claim 1, wherein one or more of Pb, Bi, Te, Ca, and Se are Pb: ≦ 0.4%.
Bi: ≦ 0.4%
Te: 0.001 to 0.05%
Ca: 0.0005 to 0.0030%
Se: 0.003 to 0.05%
Carburized gear steel with excellent gear cutting characteristics, characterized in that it is contained in an amount of: