JPH09201644A - Steel for carburized bevel gear, high toughness carburized bevel gear and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for carburized bevel gear little in alloy components such as Ni, Mo, Si, in comparison with a case hardening steel used conventionally, reduced in the material cost and manufacturing cost and excellent in impact strength, and to manufacture a high toughness carburized bevel gear. SOLUTION: This steel material is composed of 0.15-0.25wt.% C, 0.30-0.90wt.% Mn, <=0.10wt.% Si, 0.001-0.003wt.% B, 0.3-1.2wt.% Cr and/or 0.2-0.9wt.% Mo and the balance Fe with inevitable impurities. Further, (Mn+Cr+Mo)<=1.7wt.% is satisfied. After applying cold-forging to this steel material, the carburizing treatment is executed and further, hot-forging is applied. By this method, the bevel gear having >=1.5 impact input torque ratio is obtd.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a steel material for carburized bevel gears, a high toughness carburized bevel gear and a method for producing the same, and more particularly to a steel material used for producing a carburized bevel gear, With respect to a bevel gear excellent in impact resistance manufactured by using the same and a method for manufacturing the bevel gear, the bevel gear is preferably used for a gear for a differential device of an automobile and the like.

[0002]

2. Description of the Related Art In recent years, as the needs for fuel consumption reduction and higher output of transportation machines such as automobiles have increased, there has been an increasing demand for weight reduction or higher strength of mechanical parts such as gears. From such a background, bevel gears used for gears for automobile differentials are required to have high impact resistance, and as a method for improving the impact resistance, a conventional case hardening steel such as JIS SCM420H is used. For Ni and C
There is known a method of using a material in which the toughness of the carburized layer or the inside is increased by adding an alloy component such as r or Mo.

Further, in recent years, the material is formed into a gear shape by hot forging after the carburizing heat treatment as compared with the conventional manufacturing method in which the material is formed into a gear shape by forging and then carburizing heat treatment.
Bevel gears manufactured by the "carburizing forging method" that significantly reduces the impact strength of bevel gears by making the carburized layer with low toughness thinner than other parts near the tooth root of the gear, which is the starting point of damage, are also known. ing.

[0004]

However, in the bevel gear as described above, it is necessary to use a material to which a large amount of expensive alloy elements are added in order to improve the toughness, and the material cost is high, and However, there is a problem that machining cost rises due to a decrease in machinability. On the other hand, bevel gears manufactured by the carburizing forging method also need to be drilled or chamfered in the material before carburizing. With conventional case-hardening steel (JIS SCM420H, etc.), the material before cold forging is There was a problem that the spheroidizing heat treatment or the cutting of the material was required, resulting in an increase in cost. The present invention has been made by paying attention to such problems of the prior art, and has less alloy components such as Ni, Mo, Si, etc., as compared with the case-hardening steel used in the related art, and the material and the manufacturing cost are reduced. And a steel material for a carburized bevel gear excellent in impact strength, a high toughness carburized bevel gear, and a method for producing the same.

[0005]

Means for Solving the Problems As a result of intensive studies to achieve the above object, the present inventors have found that a steel material containing B (boron) is used, cold forging is performed, and then carburizing heat treatment is performed. The inventors have found that the above objects can be achieved by performing hot forging, and have completed the present invention. That is, the steel material for a carburized bevel gear of the present invention contains 0.15 to 0.25% by weight of C, 0.30 to 0.90% by weight of Mn, 0.10% by weight or less of Si, and 0.1. 001 to 0.003 wt% B
And 0.3 to 1.2% by weight of Cr and / or 0.2 to
It is characterized in that it is composed of 0.9 wt% Mo, the balance being Fe and unavoidable impurities, and satisfies the relationship represented by the following formula: Mn + Cr + Mo ≦ 1.7 wt%.

The high toughness carburized bevel gear of the present invention is a carburized bevel gear containing the above-mentioned steel material and is characterized in that the impact input torque ratio is 1.5 or more. Further, the method for producing a high-toughness carburized bevel gear of the present invention is, in producing a high-toughness carburized bevel gear, forms a preformed body by cold forging the material made of the above steel material, and obtains the preformed body. The body is carburized and then hot forged to form a tooth.

[0007]

In the steel material for bevel gears of the present invention, C (carbon), Mn (manganese), Si (silicon), B, Cr (chromium) and Mo (molybdenum) are adjusted to a specific composition, and Mn + Cr + Mo ≦ 1. Since the content is 7% by weight, the hardness of the case-hardening steel used in the conventional bevel gears can be as high as that of the spheroidized heat-treated product in the state where the steel material is rolled. Therefore, the spheroidizing heat treatment of the steel material can be omitted. Further, in the bevel gear of the present invention and the method for manufacturing the same, the steel material obtained by improving the cold forgeability by reducing the alloy components such as Ni (nickel), Mo, and Si as compared with the conventional case-hardening steel. The decrease in hardenability can be complemented by adding B, and the hardness of the lower part of the carburized layer can be made to be a desired hardness.

Further, the decrease in impact strength due to the coarsening of crystal grains during carburization, which is a concern due to the addition of B, can be finely recrystallized by various heat treatments such as soaking after hot forging. As it can be done, no problem occurs. Regarding the impact strength of bevel gears, the preform obtained by subjecting the above steel material to cold forging is subjected to carburizing heat treatment and then hot forged to form a bevel gear, so the carburized layer with low toughness is damaged. The toughness of the material is that B becomes less prominent in the vicinity of the tooth root that is the starting point than other surface parts, and segregation of P (phosphorus), which is contained as an impurity in the steel material, into the crystal grain boundaries is suppressed by B. By the improvement, it can be greatly improved.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.
The steel material for a bevel gear according to the present invention has, as described above, 0.15 to 0.15.
0.25 wt% C and 0.30-0.90 wt% M
n, 0.10 wt% or less of Si, 0.001 to 0.
003 wt% B, 0.3 to 1.2 wt% Cr and / or 0.2 to 0.9 wt% Mo, with the balance F
e and inevitable impurities. Here, the reason for limiting C to the above range is that 0.15 wt% or more is necessary to secure the internal hardness of the bevel gear, but if it is contained in excess of 0.25 wt%, the toughness and This is because cold forgeability is deteriorated.

Next, the content of B is set in the range of 0.001 to 0.003% by weight because the content of B is 0.001% by weight.
With the above addition, a remarkable hardenability improving effect is exhibited, but 0.0
This is because if it is added excessively in excess of 03% by weight, a compound is produced and the toughness is lowered. Further, Si is set to a range of 0.10% by weight or less. The reason is that Si is effective as a deoxidizing element, but since it is an element that deteriorates cold forgeability, it has high cold forgeability. This is because, if required, the content is preferably 0.10% by weight or less. Further, Mn is 0.30 to 0.90% by weight, Cr
0.3-1.2 wt%, Mo 0.2-0.9 wt%
These elements are effective elements for improving the hardenability to secure the internal hardness of the gear, but the effect is sufficiently exhibited, and the cold forgeability and toughness due to excessive addition are The range is set so that no decrease occurs.

Further, the steel material for a bevel gear according to the present invention satisfies the following equation, Mn + Cr + Mo ≦ 1.7% by weight. This relationship is derived from the following reasons. That is, the amount of addition of these elements effective for improving the hardenability is determined by the hardenability required to secure the internal hardness of the bevel gear to be applied and the material cost of each alloy element, but Mn + Cr + Mo = 1.
If it exceeds 7% by weight, it becomes difficult to secure cold forgeability due to an increase in material hardness, and the hardenability becomes too high for a bevel gear. Further, the hardness of the bevel gear steel material of the present invention described above has a Brinell hardness of 170 HB.
It is preferred that: If it exceeds 170 HB, the steel material is cold forged to form a bevel gear preform (usually cylindrical).
Is not preferable because it cannot be obtained.

Next, the bevel gear and the method for manufacturing the bevel gear of the present invention will be described. The bevel gear of the present invention can be manufactured as follows using the above steel material for bevel gears. That is, first, after hot rolling the above-mentioned steel material, a substantially cylindrical material is cut out from the obtained rolled material,
Then, this material is subjected to cold forging to form a substantially cylindrical preform. As described above, the rolled steel material according to the present invention has an advantage that a cold-forged product can be directly obtained to obtain a preform. On the other hand, normal steel materials (JIS SCM420H and JIS SNCM42
In the rolled material of 0H), the hardness is too high to perform cold forging, and it is necessary to perform spheroidizing heat treatment before cold forging.

Next, the preform is carburized,
Further, hot forging is performed at about 1000 ± 10 ° C. to form teeth, and a bevel gear having a substantially final product shape is obtained. After that, the bevel gear thus obtained is deburred, if necessary, about 820
Soaking at ± 10 ° C / 30 seconds, quenching in oil (about 8
The final bevel gear can be obtained by tempering at 0 ± 20 ° C.), about 170 ± 20 ° C., blasting and magnetic flaw detection. Further, the bevel gear of the present invention obtained as described above is excellent in toughness and impact characteristics, and, for example,
It has a shock input torque ratio of 1.5 or more.

[0014]

EXAMPLES The present invention will now be described in more detail with reference to the drawings and examples, but the present invention is not limited to these examples. [Manufacture of Steel Material and Performance Evaluation] Each steel material A to E having the composition shown in Table 1 was cut after hot rolling to obtain a material 1 shown in FIG.
And the shape. Next, material 1 consisting of each steel material A to E
Then, it was spheroidized under general spheroidization heat treatment conditions (b), and the hardness (HB: Brinell) of the same material was measured. At the same time, the rolled materials (a) before the spheroidizing heat treatment of each steel material A to E
Was also measured, and the results are shown in FIG.

[0015]

[Table 1]

Judging from the general cold forgeable hardness level, steel material C (corresponding to JIS SCM420H), steel material D (corresponding to JIS SNCM420H) and steel material D (Mn).
+ Cr + Mo is more than 1.7% by weight, the hardness of the rolled material itself (a) without spheroidizing heat treatment is too high, and cold forging into a preform (material 2 shape in FIG. 1) is not possible. It was possible. On the other hand, the steel materials A and B belonging to the scope of the present invention have hardnesses comparable to the spheroidizing heat-treated products of the steel materials C to E without performing the spheroidizing heat treatment (see FIG. 2). In the state, cold forging into the material 2 shape was possible.

Example 1 A bevel gear was manufactured using the above-mentioned steel material A according to the manufacturing process shown in FIG. That is, the material 1 is formed by cutting the material after hot rolling (1 step),
Then, the material 2 was shaped by cold forging (2 steps).
The obtained cold forged product in the shape of the material 2 is carburized (3 steps), then gear formed by hot forging (4 steps), deburring (5 steps), and soaking (82 steps).
0 ℃ / 30 seconds (6 steps), quenching (80 ℃ / oil
(7 steps)), tempering (170 ° C / 1 hour (8 steps)), shot blasting (9 steps), and magnetic flaw detection (10 steps) were sequentially performed to obtain a bevel gear of Example 1.
Next, the bevel gear thus obtained was repeatedly subjected to an impact test, and the obtained results are shown in FIG. In this impact test, as shown in FIG. 3, an impact load is repeatedly input to one of the gears, and the impact input torque to the gear is calculated when the bevel gear is damaged after 100 inputs.

(Example 2) A bevel gear of Example 2 was obtained by performing the same operation as in Example 1 except that the steel material B was used as the steel material for gears, and the impact test was repeated in the same manner to obtain the impact input torque ratio. I asked. FIG. 4 shows the obtained results.

(Comparative Examples 1 and 2) Steel materials C and D, respectively
The same operation as in Example 1 was carried out to obtain the bevel gears of Comparative Examples 1 and 2, except that the impact input torque ratio was determined by repeating the impact test, and the obtained results are shown in FIG. Indicated. (Comparative Examples 3 to 7) Bevel gears of each example were repeated by repeating the same operation as in Example 1 except that steel materials A to E were used and carburizing treatment was performed after gear forming by hot forging which is a conventional method. Then, the impact test was repeated and the impact input torque ratio was obtained. FIG. 4 shows the obtained results. In Comparative Example 7, the material was hard and cracked during cold forging, so the subsequent gear manufacturing process was not performed.

From the results shown in FIG. 4, manufactured by the hot forging method after the carburizing heat treatment using the steel materials A and B in which the expensive alloy elements are reduced by adding B and the spheroidizing heat treatment before cold forging is omitted. It can be seen that the bevel gears (Examples 1 and 2) have both the best impact strength and the lowest manufacturing cost.

[0021]

As described above, according to the present invention, a steel material containing B (boron) is used, and after cold forging is performed, hot forging is performed after carburizing heat treatment.
Compared to the case-hardening steels used conventionally, Ni, Mo, Si
There are few alloy components such as, material and manufacturing cost are reduced,
Further, it is possible to provide a steel material for a carburized bevel gear excellent in impact strength, a carburized bevel gear with high toughness, and a manufacturing method thereof. That is, according to the present invention, when the carburized bevel gear is manufactured, since the boron-added case-hardening steel in which the chemical composition of the steel is specified is used and the tooth forming shape is performed by the hot forging after the carburizing heat treatment, it is expensive due to the addition of boron. Since it is possible to reduce various alloying elements, improve the toughness of crystal grain boundaries, and secure the cold forgeability of the pre-carburizing material in carburizing forging, it is possible to use inexpensive materials and manufacturing costs with excellent impact strength. Gears can be provided.

[Brief description of drawings]

FIG. 1 is a manufacturing process diagram showing an embodiment of a method for manufacturing a bevel gear according to the present invention.

FIG. 2 is a graph showing hardness measurement results of hot rolled materials of various steel materials and materials obtained by adding spheroidizing heat treatment to the hot rolled materials.

FIG. 3 is a side view showing a configuration of a repeated impact test.

FIG. 4 is a graph showing the results of repeated impact tests.

Claims (5)

[Claims] 1. A C content of 0.15 to 0.25% by weight;
30 to 0.90 wt% Mn, 0.10 wt% or less Si, 0.001 to 0.003 wt% B, 0.3
~ 1.2 wt% Cr and / or 0.2-0.9 wt%
A steel material for a carburized bevel gear characterized by comprising Mo, Fe as the balance and unavoidable impurities, and satisfying the relationship represented by the following formula: Mn + Cr + Mo ≦ 1.7 wt%. 2. A steel material for a carburized bevel gear according to claim 1, which has a hardness of 170 HB or less. 3. A carburized bevel gear containing the steel material according to claim 1 or 2, wherein a shock input torque ratio is 1.5 or more. 4. A high toughness carburized bevel gear is manufactured by cold forging a raw material made of the steel material according to claim 1 to form a preformed body, and the obtained preformed body is carburized. A method of manufacturing a high-toughness carburized bevel gear, characterized by performing hot-forging and then performing tooth forming. 5. The method for producing a high toughness carburized bevel gear according to claim 4, wherein soaking, quenching and tempering are performed after the tooth forming.