JP3510506B2 - Carburizing and carburizing steel - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to carburizing steel and carburized steel suitable for use in parts requiring fatigue strength and wear resistance such as gears, pulleys (sheaves) and rotary shafts,
In particular, the present invention relates to a carburizing steel and a carburized steel which are suitable for use in parts such as CVT shafts and pulleys (sheaves) that require large surface peeling properties and require a deep hardened layer.

[0002]

2. Description of the Related Art For example, in parts such as gears and rotary shafts provided for automobile drive systems, fatigue strength and abrasion resistance are required, and therefore strength on the surface of the parts is important. It is put in and further tempered as appropriate to harden the surface.

[0003]

By the way, the usual carburizing treatment is carried out at a temperature of about 920 ° C.
In parts such as shafts and pulleys (sheaves) of VT (continuously variable transmission), there is a great demand for the peeling property of the surface, and it is necessary to increase the depth of the hardened layer. In the case of a part having a large demand for the surface peeling property, in order to increase the depth of the hardened layer, it is necessary to lengthen the carburizing time at a normal carburizing temperature.

In order to shorten the carburizing time, it is considered that high temperature carburizing treatment in which the carburizing temperature is higher than usual is effective, but in the case of the conventional material (steel for carburizing), carburizing is performed at a high temperature of 1000 ° C. or more, for example. When the treatment is applied, there is a problem that the austenite crystal grains become large and the fatigue strength and the peeling property deteriorate. For example, the state of the crystal grains on the surface (location to be surveyed) of a CVT pulley manufactured by a conventional carburizing steel using a carburizing steel as shown in FIG. When the carburizing process is performed at about 920 ° C., the result is as shown in FIG.
When high-temperature carburization (about 1050 ° C) is performed,
It becomes like (c).

Incidentally, the austenite grain size is shown by the grain size number, which is "9" for carburizing at normal temperature (about 920 ° C) and "5" for high temperature carburizing (about 1050 ° C). It can be seen that in carburizing steel, crystal grains are significantly coarsened by high temperature carburization as compared with carburizing at normal temperature. In addition, as conventional carburizing steel, iron Fe,
Carbon C, silicon Si, manganese Mn, phosphorus P, sulfur S,
It contains nickel Ni, chromium Cr, molybdenum Mo, aluminum Al, niobium Nb, and nitrogen N,
A concrete example of the contents of these elements is shown in FIG.
There are carburizing steels contained in weight% like Nos. 1 to 7 of the comparative steels shown in FIG.

The present invention was devised in view of the above-mentioned problems, and the fatigue strength and the peeling characteristics are prevented from being deteriorated without causing the austenite crystal grains to grow large even when the high temperature carburization treatment is performed. It is intended to provide a carburizing steel capable of shortening the carburizing time while improving the strength and peeling characteristics, and also to provide a carburized steel using the carburizing steel. .

[0007]

Therefore, the carburizing steel of the present invention according to claim 1 has a weight percentage of 0.15 to 0.25.
% Carbon, 0.80% or less silicon, 1.50% or less manganese, 0.030% or less phosphorus, 0.005
.About.0.030% sulfur, 1.80% or less nickel, 1.50% or less chromium, 0.70% or less molybdenum, 0.030 to 0.070% aluminum, and .0. 030 to 0.060% niobium and 0.013
Both 0% or more of nitrogen, of 0.010 to 0.015%
It contains titanium and the balance is composed of iron and inevitable impurity elements. As a result, the austenite crystal grains do not become coarse even if the high temperature carburization process is performed.

The carburized steel of the present invention according to claim 2 is produced by carburizing the carburizing steel according to claim 1 at a carburizing temperature of 1000 ° C. or higher. Since the austenite crystal grains can be manufactured and the austenite crystal grains do not coarsen even when the high temperature carburizing treatment is performed, the steel material has good fatigue strength and peeling properties.

[0009]

[0010]

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. First, the carburizing steel according to the first embodiment of the present invention will be described. In this carburizing steel, each element is contained in iron Fe in the following weight%. That is, in weight%, 0.15 to 0.25% of carbon C, 0.80% or less of silicon Si, 1.50% or less of manganese Mn, 0.030% or less of phosphorus P, and 0 .0
05-0.030% sulfur S, 1.80% or less nickel Ni, 1.50% or less chromium Cr, 0.7
Molybdenum Mo of 0% or less, 0.030 to 0.070
% Aluminum Al, 0.030 to 0.060% niobium Nb, and 0.0130% or more nitrogen N. Other than these elements (the balance) is iron Fe, but an element that cannot be avoided (unavoidable impurity element) in the manufacturing stage will be mixed in in a minute amount.

Specifically, FIG. 1 shows steel Nos. 1 to 1 of the present invention.
Each element is contained in the weight% as shown as 5,8-10,12-16. Here, in this carburizing steel,
The reason for defining the content of each element as described above will be described. Note that the content rate is% by weight as described above. Further, although each of the steels shown in FIG. 1 contains copper Cu, this is not a purposely added one but an impurity level one (that is, an unavoidable impurity element).

First, carbon C is an essential component for steel, and it is necessary to add 0.15% or more in order to obtain the core hardness required for products when carburizing and quenching treatment is performed. . However, if carbon C is added so as to exceed 0.25, the compressive residual stress on the metal surface after quenching cannot be sufficiently introduced, and the impact value of the core is lowered (that is, the toughness is reduced). Decrease). Therefore, in order to avoid both of them, for carbon C, 0.15-
It is set to 0.25%.

Silicon Si is an essential component for steel, has an effect as a deoxidizing component of molten steel, and is preferably added in a large amount for the purpose of reducing the amount of oxygen in steel.
If a large amount of this silicon Si is added, a grain boundary oxide layer that reacts with atmospheric oxygen during the carburizing process and adversely affects the fatigue strength is formed in the carburized surface layer. Is 0.80% or less.

Manganese Mn is an element necessary for ensuring the hardenability, but it reacts with oxygen to form a grain boundary oxide layer which adversely affects the fatigue strength. Mn is set to 1.50% or less. Phosphorus P is an element that tends to segregate at the grain boundaries, and since it greatly affects the toughness of high-carbon steels such as carburizing steels, it is preferable that it be low in order to improve toughness, but phosphorus P is included. Not many raw materials, phosphorus P
Since it takes a cost to lower the value, phosphorus P is set to 0.030% or less in consideration of economy.

Most of sulfur S is present in the steel as sulfide inclusions, and this sulfur S is an element that causes fatigue strength deterioration when it is present in a large amount. Then it is also an element that gives machinability. Therefore, 0.030% is set as the upper limit from the viewpoint of not causing deterioration of fatigue strength, and 0.005% is set as the lower limit from the viewpoint of obtaining machinability.

Nickel Ni is an element necessary for improving the toughness, but it is an element that promotes the formation of retained austenite, and excessive addition impairs the economical efficiency. 80% or less. Chromium Cr is an element necessary for ensuring hardenability, but since it reacts with oxygen to form a grain boundary oxide layer that adversely affects fatigue strength, in order to avoid this,
Chromium Cr is set to 1.50% or less.

Molybdenum Mo is an element necessary for improving the toughness, but excessive addition impairs the economical efficiency, so molybdenum Mo is set to 0.70% or less. Aluminum Al is an element that combines with nitrogen N to form aluminum nitride (AlN) and has the function of refining the austenite grain size. Therefore, in the present carburizing steel, the lower limit of aluminum Al is set to 0.030% in order to refine the grain size. On the other hand, a large amount of aluminum Al promotes the formation of inclusions of aluminum oxide (Al ₂ O ₃ ) that are harmful to fatigue strength, and leads to a decrease in toughness.
It is 070%.

Niobium Nb is an element that is effective in reducing the grain size of austenite grains, similar to aluminum nitride (AlN), by forming carbonitrides by combining with carbon C and nitrogen N. Is an element that can contribute to improving the toughness of the carburized layer and the core through. Particularly, in the production of gears and the like, when carburizing is performed at a high temperature of about 1050 ° C., it is an essential element for preventing coarsening of austenite crystal grain size. The amount of this niobium Nb added is determined by the balance between the amount of aluminum Al that has the same effect and the amount of nitrogen N that bonds with niobium Nb in forming carbonitrides. The effect of refining the grain size becomes low. Therefore, the lower limit of niobium Nb is set to 0.030%. On the other hand, a large amount of niobium Nb
Is an excessive amount of carbonitride, leading to precipitation of carbonitride,
Since the toughness of the carburized layer is impaired, the upper limit of niobium Nb is set to 0.060%.

Nitrogen N is, as described above, aluminum A
It is an element necessary for refining the austenite grain size by forming a nitride by combining with 1 or niobium Nb. Therefore, the amount of nitrogen N is determined by the balance with the amounts of aluminum Al and niobium Nb added. However, due to the refinement of the crystal grain size, the nitrogen N content is 0.0130% or more.

Since the carburizing steel as the first embodiment of the present invention is configured as described above, when the carburizing treatment is performed using the present carburizing steel, for example, high temperature carburizing is performed at 1050 ° C. By carrying out the carburizing treatment for a short time, it becomes possible to obtain the carburized steel having a deep hardened layer and a fine austenite grain size, and therefore a high fatigue strength and a good peeling property.

Therefore, the present carburizing steel and carburized steel are parts such as gears, pulleys (sheaves) and rotary shafts that require fatigue strength and wear resistance, especially CVT shafts and pulleys (sheaves). In particular, it is effective when used for parts that require large surface peeling properties and require deeper hardened layers. For example, FIG. 2 is a schematic cross-sectional view showing a CVT 10 which is one of application examples of the main carburizing steel and the main carburized steel, and the primary sheave 21 is attached to the primary shaft 11.
The secondary shaft 12 is equipped with a secondary sheave 22, and both sheaves 21, so that the primary sheave 21 and the secondary sheave 22 are connected to each other.
The belt 31 is wound around the belt 22, and the gear ratio is adjusted by moving the movable sheave 21A of the primary sheave 21 and the movable sheave 22A of the secondary sheave 22 in the axial direction.

Since the primary shaft 11, the primary sheave 21, the secondary shaft 12, and the secondary sheave 22 are all required to have surface peeling properties, the main carburizing steel and the main carburized steel are applied. As a result, durability and peeling resistance can be greatly improved by carburizing for a short time, and product performance can be improved.

The characteristics of the carburized steel obtained by subjecting the carburizing steel of the first embodiment to a high temperature carburizing treatment at about 1050 ° C. will be described with reference to FIGS. 3 to 7 using experimental results. To do. 3 to 5 are diagrams showing grain size characteristics of carburized steel obtained by high temperature carburizing at about 1050 ° C. using the carburizing steel as shown in FIG. 1, and FIG. 3 shows the addition amount of aluminum Al. FIG. 4 shows the grain size characteristics with respect to the addition amount (wt%) of aluminum Al, FIG. 4 shows the grain size characteristics with respect to the addition amount (wt%) of niobium Nb, and FIG. The grain size characteristics with respect to the amount of N added (% by weight) are shown.

As shown in FIG. 3, aluminum among aluminum Al and niobium Nb (see ◯ mark) and aluminum Al, niobium Nb and titanium Ti (see ◯ mark) are aluminum. The addition amount of Al was set to 0.
It can be seen that the austenite grain size of about 03% or more is about 9 in terms of grain size number. Note that FIG.
In the above, only aluminum Al and no niobium Nb (see Δ) and aluminum Al and niobium Nb with a low addition amount of aluminum Al have austenite grain sizes of 2 to 3
It has stopped to some extent.

Further, as shown in FIG. 4, aluminum A
l and niobium Nb are added, aluminum Al is added by 0.03% or more, and niobium Nb is added by 0.
It can be seen that the austenite grain size is about 9 in terms of grain size number in the case of adding 0.3% or more (see the mark ●). In addition, in the case where the addition amount of aluminum Al is less than 0.03% (see □ mark), the austenite grain size is about 2 to 3 in terms of grain size number regardless of the addition amount of niobium Nb.

Further, as shown in FIG. 5, aluminum Al and niobium Nb are added and nitrogen N is 0.0
Among those added with 13% or more, those with an aluminum Al addition of 0.03% or more (see ▲) have the austenite grain size increased to about 9 in grain size number, but the aluminum Al addition is 0%. In the case of less than 0.03% (see Δ mark), the austenite crystal grain size is about 2 to 3 regardless of the addition amount of niobium Nb or nitrogen N.

From FIG. 5, aluminum Al
0.030 or more, niobium Nb 0.030 to 0.06
It can be seen that the austenite grain size becomes sufficiently fine by adding 0% and 0.0130% or more of nitrogen N. And, as shown in Table 1 below, the fatigue strength and peeling resistance of the present carburized steel (designated material) are greatly improved as compared with the conventional carburized steel (designated material). I understand.

[0029]

[Table 1] That is, FIG. 6 shows the test results regarding the rotating bending fatigue strength, and this fatigue test is performed with a predetermined shape as shown in FIG.
A test piece for fatigue test having a predetermined size was tested using a known rotary bending fatigue tester. As shown in FIG.
The conventional carburized steel (shown as a developed material with a triangle) has a rotating bending fatigue strength of about 700 MPa, whereas the carburized steel (shown as a developed material with a circle) has a rotating bending fatigue strength of about 850 MPa. You can see that it has improved.

Further, FIG. 7 shows the result of the peeling resistance test. This test is conducted in a predetermined shape as shown in FIG.
While applying a frictional load to a test piece for a pitching fatigue test having a predetermined size, a large roller and a small roller as shown in FIG. 7 are brought into sliding contact with a predetermined contact stress and a predetermined slip ratio,
This is a known test in which these rollers are rotated at a predetermined rotation speed to detect the number of tests (peeling occurrences) until peeling (wear and peeling) occurs.

As shown in FIG. 7, the conventional carburized steel (Δ
Peeling resistance (number of occurrences of peeling) of the developed material (marked with a mark) is about 2.8 × 10 ⁵ , whereas peeling resistance (peeling of the developed carburized steel) (peeling resistance) It can be seen that the number of occurrences has improved to about 5 × 10 ⁵ . Next, the carburizing steel according to the second embodiment of the present invention will be described. In this carburizing steel, iron Fe contains each element in the following weight%.

In other words, 0.15 to 0.25% by weight
Carbon C, 0.80% or less of silicon Si, and 1.50%
The following manganese Mn and 0.030% or less phosphorus P,
0.005-0.030% sulfur S, 1.80% or less nickel Ni, and 1.50% or less chromium Cr,
Molybdenum Mo of 0.70% or less and 0.030 to 0.
070% aluminum Al, 0.030-0.06
It contains 0.010 to 0.015% titanium Ti together with 0% niobium Nb and 0.0130% or more nitrogen N. Other than these elements (the balance) is iron Fe, but an element that cannot be avoided (unavoidable impurity element) in the manufacturing stage will be mixed in in a minute amount.

That is, the carburizing steel according to the second embodiment is 0.010 to the carburizing steel according to the first embodiment.
The composition is such that 0.015% titanium Ti is added. Specifically, FIG. 1 shows steel Nos. 6, 7, 11 of the present invention.
Each element is contained in the weight% as shown below. The reason for defining the content of each element is the same as that of the first embodiment except for the elements other than titanium Ti, and thus the description is omitted.

Titanium Ti combines with nitrogen N and carbon C to form a carbonitride [Ti (CN)], and aluminum A
Similar to 1 and niobium Nb, it is an element effective in refining the austenite grain size. As described above, titanium Ti is added because when the heating temperature (carburizing temperature) is assumed to be 1050 ° C. or higher or when a carburizing process with a slow heating rate is performed, the austenite grain size is likely to be coarse. Becomes 0.030-0.070% aluminum A
This is because it is considered that the austenite grain size cannot prevent coarsening only with 1 and 0.030 to 0.060% niobium Nb.

That is, titanium Ti is replaced by aluminum A
By adding in addition to 1 and niobium Nb, coarsening of the austenite grain size can be prevented by the combined action. The effect of preventing the coarsening of crystal grains at a high temperature by adding titanium Ti is small if the amount of titanium Ti added is small, and the effect is saturated if the amount of titanium Ti is large, so that the content of titanium Ti is 0.010 to 0. It is set to 015%.

Since the carburizing steel as the second embodiment of the present invention is configured as described above, when the carburizing treatment is performed using the present carburizing steel, for example, high temperature carburizing at about 1050 ° C. or higher is carried out. By carrying out the carburizing treatment, it becomes possible to obtain a carburized steel having a deep hardened layer and a fine austenite grain size, and therefore a high fatigue strength and a good peeling property, by carburizing for a short time. In particular, it is effective when used for a CVT shaft, a pulley (sheave), other gears, a pulley (sheave), and the like.

The carburizing steel and carburized steel of the present invention are not limited to the above-mentioned embodiment, and the temperature at the time of high temperature carburization is not limited to 1050 ° C but 1000 ° C. It is possible to adopt an appropriate temperature of about a certain degree or more. Further, depending on the carburizing temperature, it is more effective to apply the carburizing steel containing titanium Ti as in the second embodiment.

[0038]

As described in detail above, according to the carburizing steel of the present invention as set forth in claim 1, a carburized steel in which austenite crystal grains are not coarsened is obtained by a short-time carburizing treatment by high temperature carburizing treatment. Therefore, it becomes possible to obtain a steel material having excellent fatigue strength and peeling characteristics while shortening the carburizing time.

According to the carburized steel of the present invention as set forth in claim 2, a carburized steel in which austenite crystal grains are not coarsened can be obtained by the carburizing treatment for a short time by the high temperature carburizing treatment. Improves the peeling characteristics, especially when it is used for parts such as CVT shafts and pulleys (sheaves) that have large surface peeling characteristics and need to have a deep hardened layer. You will be able to

[Brief description of drawings]

FIG. 1 is a diagram showing a specific composition of a carburizing steel (invention steel) according to first and second embodiments of the present invention together with a specific composition of a conventional carburizing steel (comparative steel). is there.

FIG. 2 is a schematic cross-sectional view showing the structure of a CVT to which the carburizing steel and carburized steel according to the first embodiment of the present invention are applied.

FIG. 3 is a diagram showing the grain size characteristics of the carburized steel according to the first embodiment of the present invention.

FIG. 4 is a diagram showing grain size characteristics of the carburized steel according to the first embodiment of the present invention.

FIG. 5 is a diagram showing a grain size characteristic of the carburized steel according to the first embodiment of the present invention.

FIG. 6 is a diagram showing the results of a rotary bending fatigue strength test of carburized steel according to the first embodiment of the present invention.

FIG. 7 is a diagram showing a result of a peeling resistance test of the carburized steel according to the first embodiment of the present invention.

FIG. 8 is a view for explaining the problem of the present invention, (a) is a schematic view showing a CVT part manufactured by carburizing treatment using a conventional carburizing steel, and (b) is a normal temperature of the part. Enlarged view showing the state of crystal grains when manufactured by carburizing treatment of (c)
FIG. 3 is an enlarged view showing a crystal grain state when such a part is manufactured by a high temperature carburizing process.

[Explanation of symbols]

10 CVT using carburizing steel 11 Carburized steel primary shaft 12 Carburized steel secondary shaft 21 Carburized steel primary sheave 22 Carburized steel secondary sheave

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Yuichi Yamada 5-3-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Satoshi Ando 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Inventor Kazuhiko Kato 5-3-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Inventor Tatsuo Fukuzumi 12 Nakamachi, Muroran, Hokkaido Mitsubishi Steel Muroran Special Steel Incorporated (72) Inventor Hidenori Hiromatsu 12 Nakamachi, Muroran-shi, Hokkaido Mitsubishi Steel Muroran Special Steel Co., Ltd. (56) Reference JP-A-4-371522 (JP, A) (58) Fields investigated (Int.Cl . ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. By weight%, 0.15-0.25% carbon C, 0.80% or less silicon Si, 1.50% or less manganese Mn, and 0.030% or less phosphorus P. And 0.0
05-0.030% sulfur S, 1.80% or less nickel Ni, 1.50% or less chromium Cr, 0.7
Molybdenum Mo of 0% or less, 0.030 to 0.070
% Aluminum Al of, and 0.030 to 0.060% niobium Nb, together with 0.0130% or more nitrogen N,
Steel for carburization, containing 0.010 to 0.015% titanium Ti , and the balance being composed of iron Fe and inevitable impurity elements. 2. The steel for carburizing according to claim 1,
A carburized steel produced by performing a carburizing treatment at a carburizing temperature of C or higher.