JP3037891B2 - High-strength case hardened steel that facilitates induction annealing of carburized part and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high fatigue strength (hereinafter referred to as high strength) skin which facilitates high-frequency annealing of a carburized portion suitable for manufacturing gears used in automobiles, construction vehicles, industrial machines, and the like. The present invention relates to a treatment method suitable for manufacturing gears and the like using case hardened steel and such case hardened steel.

[0002]

2. Description of the Related Art In automobiles, construction vehicles, industrial machines, etc., various types of gears and the like are used in each part. Many are used after carburizing, tempering or carbonitriding. The fatigue strength improvement of parts used is subjected to carburizing treatment, for example, Japanese Open Rights 4-8384
As disclosed in Japanese Patent Application Publication No. 8 (1994), in order to reduce grain boundary oxidation and incompletely quenched portions which cause a reduction in fatigue life, the amounts of Si, Mn, and Cr are reduced, and the hardenability of the carburized layer is reduced. To increase Ni, Mo and the like may be added.

[0003] Conventionally, automotive gears such as hypoid pinion gears are manufactured by carburizing these steels in order to obtain high fatigue strength. Therefore,
The threads in one part of the gear are similarly carburized and hardened. However, when the hardness of the threaded portion becomes hard, it causes delayed fracture. Conventionally, to prevent this, it is common practice to perform high-frequency annealing on the thread portion to reduce hardness in a short time, thereby preventing delayed fracture.

However, when a high-strength material is used as a material to increase the fatigue life, in the high-frequency annealing of the threaded portion, the influence of the alloy component (particularly, the Mo content) causes the conventional high-frequency annealing to be performed under the influence of the conventional high-frequency annealing. Such short-time heating does not reduce the hardness to the specified value. Further, even if the induction hardening condition is changed, it is difficult to find out the condition from the balance between the heat capacity by local heating and the heating time. Therefore, a method of suppressing an increase in hardness by performing a carburizing treatment on the thread portion before carburizing is operated. Since the carbon prevention treatment involves an operation of applying a carbon protection material to the thread portion of each gear, productivity is significantly reduced and costs are increased.

[0005]

SUMMARY OF THE INVENTION According to the present invention, in order to easily solve the above problems, the fatigue strength is increased by adjusting the chemical composition of steel, which is a raw material, and the steel can be easily formed even under conventional high-frequency annealing conditions. Another object of the present invention is to provide steel capable of lowering hardness.

[0006]

That is, in the present invention, C: 0.15 to 0.25%, Si: 0.1% or less, Mn: 0.30 to 1.
00% P: 0.030% or less, S: 0.005 to 0.020%, Ni: 0.30
2.00% Cr: 0.40-1.50%, Mo: 0.30-0.45%, Al: 0.01
5 ~0.030% N: 0.0100~0.0180%, O T: 0.0015% or less, or Nb to the above components: contain less than from 0.015 to 0.030% and the balance Fe and unavoidable impurity elements,
A high-strength case-hardened steel that has been subjected to high-frequency heating to facilitate the high-frequency annealing of a carburized portion having a hardness of Hv 420 or less and a high-strength skin in which a necessary portion to be annealed after carburizing the steel is induction-annealed at a temperature of 670 to 800 ° C. This is a method for producing hardened steel.

The reason for reaching the present invention is as follows.

Attention was first paid to Mo, which is said to increase tempering softening resistance due to the need to reduce annealing hardness by short-time high-frequency heating, and to change the content of the steel in various ways and to change other main components. After dissolving, forging and normalizing, a test piece for carburizing of 20 mmφ was prepared and subjected to a predetermined carburizing treatment to prepare a test piece for high-frequency annealing. Table 1 shows the component values of the test materials.

[0009]

[Table 1]

The test pieces were annealed by changing the surface temperature by changing the high-frequency heating time. After such annealing treatment, the relationship between the heating temperature and the hardness after annealing was determined by measuring the surface hardness. Representative results are shown in FIG. From FIG. 1, the temperature range of annealing in which the hardness is not more than the reference Hv420 without causing delayed fracture was determined. Table 2 shows the ranges of the steels tested and the temperature differences.

[0011]

[Table 2]

FIG. 2 shows the relationship between the Mo amount and the annealing temperature range from this table. From this result, it was found that there is a correlation between the Mo amount and the annealing temperature range. Therefore, it is considered that the wider the temperature range is, the easier the annealing becomes. Therefore, the minimum temperature range that can be controlled under industrial production conditions needs to be 60 ° C. or more. Therefore, the appropriate Mo amount should be 0.45% or less. Is confirmed. On the other hand, since high fatigue strength was required, the steel of the present invention was subjected to a rotational bending fatigue test using the test piece shown in FIG. 3 to confirm the fatigue strength. The results are shown in Table 2. FIG. 4 shows the relationship between the amount of Mo and the limit of rotational bending fatigue. Therefore, when a high bending strength with a fatigue limit of 900MP or more is required, the Mo content needs to be 0.3% or more. Therefore, the steel of the present invention is subjected to high-frequency annealing at a temperature of 670 to 800 ° C. so as to have a hardness Hv of 420 or less and a fatigue strength of 90.
It has been found that it has a characteristic of 0MP or more and further does not cause delayed fracture.

Next, the reasons for limiting the above composition of the present invention will be described.

C: C is 0.1 to obtain the required core hardness HRC 35 to 45 of the gear to be carburized and quenched.
Addition of 5% or more is required. However, a large amount of addition exceeding 0.25% cannot sufficiently introduce compressive residual stress on the surface after quenching, and lowers the impact value of the core. Therefore, the addition amount of C is set in the range of 0.15 to 0.25%.

Si: Si is an essential component and has an effect as a deoxidizing agent for molten steel. It is preferable to add Si in a large amount in order to lower the O _T value described later. In the carburized surface layer, it reacts with the atmospheric oxygen during the carburizing treatment and easily forms a grain boundary oxide layer. Therefore, the upper limit is set to 0.10% in order to reduce the grain boundary oxide layer (to a depth of 10 μm or less).

Mn: Mn requires at least 0.30% in order to secure hardenability. However, like Si, it reacts with oxygen in a carburizing atmosphere to form a grain boundary oxide layer. Therefore, the upper limit is set to 1.00% for the purpose of reducing this. Therefore, the addition amount of Mn is set to 0.30 to 1.00%.

Ni: Ni is an element that improves the toughness of the carburized layer and the core, and is required to be at least 0.3% or more in order to secure the toughness when the hardness of the carburized layer is increased. On the other hand, Ni is an element that promotes the formation of retained austenite, and excessive addition impairs economic efficiency. Therefore, the upper limit is set to 2.00%. Therefore, the addition amount of Ni is set in the range of 0.30 to 2.00%.

Cr: Like Mn, Cr is an element necessary for obtaining the required hardenability, and at the same time, is an element easily generated by oxygen in a carburizing atmosphere. Therefore, the lower limit is 0.40% and the upper limit is 1.50%. And

Mo: Mo is an element that improves the hardenability similarly to Ni and improves the toughness of the carburized layer and the core. In order to obtain a long bending fatigue life, it is necessary to add at least 0.3% or more. However, considering the induction annealing characteristics of the carburized material due to tempering softening resistance, the upper limit needs to be suppressed to 0.45%. Therefore, the addition amount of Mo is set in the range of 0.30 to 0.45%.

Al: Al combines with N to form AlN,
It is an element that has the effect of reducing the austenite grain size. Refinement of crystal grains has the effect of improving the toughness of the carburized layer and the core. Therefore, the lower limit is set to 0.015% in order to secure fine particles even at the carburizing temperature. On the other hand, a large amount of Al promotes the formation of Al ₂ O ₃ inclusions harmful to the fatigue strength and increases the quenching strain when the gear is used.
The upper limit was set to 0.03%.

N: N is an element required to combine with Al to form AlN to reduce the size of crystal grains, and the lower limit required for converting all contained Al to AlN is 0.1.
0100%, and excessive addition not only has no effect on miniaturization, but also lowers the workability in the cold state.
The upper limit was made 0.0180%.

Nb: Nb combines with C and N to form a carbonitride and, like AlN, is an element effective in refining the austenite crystal grain size. Contributes to the improvement of toughness. In particular, in gears and the like manufactured by cold forging, it is an essential element for preventing austenite crystal grains from becoming coarse during carburization. Therefore, the amount of addition is determined by the balance between Al and N. However, if the amount is small, no effect is obtained. Therefore, the lower limit is set to 0.015%.
Excessive addition forms and precipitates coarse carbonitrides and impairs the toughness of the carburized layer. Therefore, the upper limit is set to less than 0.030%.

P: P is an element which is apt to segregate at the crystal grain boundaries, and particularly has a large effect on the toughness of a high carbon steel such as a carburized layer.
The upper limit is set to 0.030% from the viewpoint of economy such as selection of raw materials.

S: S is mostly present as sulfide inclusions in steel, and when present in large amounts, it is an element that causes a reduction in fatigue strength. On the other hand, S is formed by cutting like a gear. In the parts to be machined, it is also an element that imparts machinability, so the lower limit is 0.00
5%, and the upper limit was 0.020%.

O _T : O _T exists as an oxide inclusion in steel and impairs the fatigue strength, so the upper limit was made 0.0015%.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to examples.

[0027]

EXAMPLE A hypoid pinion gear with a threaded portion was manufactured from the steel of the present invention indicated by the symbol f in Table 1, and after carburizing at 925 ° C. for 4 hours, followed by quenching and tempering, a heating temperature of 750 ° C. Induction annealing was performed. FIG. 5 shows the hardness results of the threaded portions after annealing. Each of the parts satisfies the hardness reference value Hv420 or less.
FIG. 6 shows a part of the results of a rotary bending fatigue test performed on the steel of the present invention. It can be seen that the steel of the present invention is superior in flexural fatigue life to SCM420H which is A of Comparative Steel 1.

[0028]

The steel of the present invention has a high performance in carburized parts such as hypoid pinion gears used in automobile construction vehicles and industrial machines, etc., which require high bending fatigue strength and long durability life, and parts having threaded parts. It also has the property of easily performing high frequency annealing of the threaded part while having fatigue strength. Therefore, by using the steel of the present invention, the gear parts can be reduced in size and weight under the current conditions without the need for a step of increasing the cost such as the carburizing treatment of the thread portion, and
High output is possible.

For these reasons, the effects of the present invention can contribute widely to the reduction of cost and the improvement of reliability in the industry using gears under severe conditions.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the heating temperature of a test material and the hardness after annealing.

2 is a graph showing the relationship between the temperature range of the Mo content and annealing.

FIG. 3 is a view showing a shape of a test piece for a rotating bending fatigue test.

FIG. 4 is a graph showing the relationship between the amount of Mo and the fatigue limit.

FIG. 5 is a graph showing the hardness of a screw portion after annealing.

FIG. 6 is a graph showing rotational bending test results of the steel of the present invention and a comparative steel.

────────────────────────────────────────────────── ─── Continued on front page (51) Int.Cl. ⁷ Identification code FI C22C 38/44 C22C 38/44 // C21D 1/32 C21D 1/32 (72) Inventor Kazuo Sakamoto 12 Nakamachi, Muroran-shi, Hokkaido Mitsubishi Inside Muroran Special Steel Co., Ltd. (72) Inventor Naohiko Sakamoto 5-33-8 Shiba, Minato-ku, Tokyo Inside Mitsubishi Motors Corporation (72) Youichi Yoichi Taniguchi 5-33 Shiba, Minato-ku, Tokyo No. 8 Inside Mitsubishi Motors Corporation (72) Kenji Kato 5-33 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Takashi Isoya 5-33 Shiba, Minato-ku, Tokyo No. 8 Inside Mitsubishi Motors Corporation (56) References JP-A-3-115542 (JP, A) JP-A-62-263653 (JP, A) JP-A-6-172867 (JP, A) (58) ) Surveyed fields (Int. Cl. ⁷ , DB name) C22C 38/00-38/44 C21D 1/32-1/42 C21D 6/00 C21D 9/32

Claims (4)

(57) [Claims] (1) C: 0.15 to 0.25%, Si: 0.1% or less, Mn: 0.30 to 1.
00% P: 0.030% or less, S: 0.005 to 0.020%, Ni: 0.30
2.00% Cr: 0.40-1.50%, Mo: 0.30-0.45%, Al: 0.01
5 ~0.030% N: 0.0100~0.0180%, O T: containing 0.0015% or less and the balance of Fe and unavoidable impurity elements,
A high-strength case-hardened steel that has a hardness of Hv420 or less by annealing by high-frequency heating and that facilitates high-frequency annealing of a carburized portion. 2. C: 0.15 to 0.25%, Si: 0.1% or less, Mn: 0.30 to 1.
00% P: 0.030% or less, S: 0.005 to 0.020%, Ni: 0.30
2.00% Cr: 0.40-1.50%, Mo: 0.30-0.45%, Al: 0.01
5 ~0.030% N: 0.0100~0.0180%, O T: containing 0.0015% or less after carburizing the balance Fe and steel unavoidable impurity elements, the annealing required portion 670-800
High strength characterized by high frequency annealing at a temperature of ℃
Method of manufacturing a Dohada hardened steel. C .: 0.15 to 0.25% by weight, Si: 0.1% or less, Mn: 0.30 to 1.
00% P: 0.030% or less, S: 0.005 to 0.020%, Ni: 0.30
2.00% Cr: 0.40-1.50%, Mo: 0.30-0.45%, Al: 0.01
5 to 0.030% N: 0.0100 to 0.0180%, O _T : 0.0015% or less, and N
b: 0.015 to less than 0.030%, the balance being Fe and unavoidable impurity elements,
A high-strength case-hardened steel that has a hardness of Hv420 or less by annealing by high-frequency heating and that facilitates high-frequency annealing of a carburized portion. 4. C .: 0.15 to 0.25% by weight, Si: 0.1% or less, Mn: 0.30 to 1.
00% P: 0.030% or less, S: 0.005 to 0.020%, Ni: 0.30
2.00% Cr: 0.40-1.50%, Mo: 0.30-0.45%, Al: 0.01
5 to 0.030% N: 0.0100 to 0.0180%, O _T : 0.0015% or less, and N
b: After carburizing a steel containing 0.015 to less than 0.030% with the balance being Fe and unavoidable impurity elements, the part required for annealing is 670 to 800.
High strength characterized by high frequency annealing at a temperature of ℃
Method of manufacturing a Dohada hardened steel.