JPH1017935A - Production of induction hardened parts - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce hardened parts excellent in quality characteristics by means of induction hardening. SOLUTION: A steel, having a composition consisting of, by mass, 0.5-0.75% C, 0.5-1.8% Si, 0.4-1.5% Mn, 0.019-0.05% Al, <=0.010% P, <=0.020% S, <=0.0015% O, 0.002-0.006% N, and the balance Fe with inevitable impurities, is used. A stock of this steel, in which the number and maximum size of oxide nonmetallic inclusions in the steel are <=2.5pieces/m<2> and <=19μm, respectively, is forged at a temp. in the range between (Ac3 point-100) and (Ac3 point + 200) deg.C at >=70% draft, cooled at (0.005 to 10) deg.C/s cooling rate after forging, and subjected to induction hardening and tempering treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a machine part manufactured by induction hardening and tempering using steel for machine structural use, and more particularly to an induction hardened part suitable for being applied to the manufacture of gears and excellent in economy. It relates to a manufacturing method.

[0002]

2. Description of the Related Art Gears used in automobiles and industrial machines are:
It is manufactured by using a carburizing alloy steel containing about 0.2% carbon, then forging → cutting → turning → gear cutting into a predetermined shape, then carburizing, quenching and tempering to give the necessary function as a gear. . Although such a carburizing process is a mainstream gear manufacturing process, the carburizing process requires several hours at a temperature of about 800 ° C to 950 ° C. Therefore, it is difficult to make the gear manufacturing process in-line, and there is a limit in improving the productivity, which has been a major obstacle in reducing the manufacturing cost.

[0003] In addition, carburization is generally carried out by gas carburizing method, but during gas carburizing, an abnormal surface layer is inevitably generated on the surface layer of the material to be treated, and this abnormal layer lowers fatigue strength and impact characteristics. Therefore, there is a limit in further improving the fatigue strength and impact characteristics. Furthermore, since the material to be processed is deformed by the heat treatment distortion generated during carburizing and quenching, strict control of the heat treatment conditions is essential. Assuming the adoption of carburizing process to overcome the above problems,
Developed high-strength carburizing steel intended to reduce Si, Mn, and Cr in steel materials, add Mo, Ni, etc. to reduce the abnormal surface layer generated during gas carburization and improve fatigue strength and impact properties. However, the use of a large amount of expensive alloy components leads to an increase in steel material costs and a deterioration in workability such as machinability, so that high strength can be achieved, but a problem arises in that manufacturing costs increase. was there.

On the other hand, it has been attempted to manufacture gears by induction hardening having a higher production efficiency than the carburizing and quenching process using alloy steels for machine structural use such as JIS standard SCM435 and S55C and carbon steel, which are more efficient than carburizing and quenching processes. It is difficult to apply to high-strength gears used for automobile transmissions and differentials, such as gears manufactured by carburizing process, because they are not originally determined in consideration of application to gears. The application was limited to gears of extremely low strength.

In order to solve these problems, for example, Japanese Patent Application Laid-Open No. Sho 60-169544 (parts for high-strength mechanical structures and a method for producing the same) regulates the composition of the components and applies them to the production of gears by an induction hardening process. The technology has been proposed and disclosed. However, according to studies by the inventors, this technique has a problem that the size of nonmetallic inclusions is large and the fatigue strength and rolling fatigue life required for gear steel cannot be said to be sufficient.

[0006]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and has a high frequency suitable for manufacturing gears and the like capable of securing equal or better quality characteristics as compared with the conventional carburizing process. It is an object of the invention to propose a method for manufacturing a hardened part.

[0007]

Means for Solving the Problems In order to achieve the above object, the inventors conducted various experiments and studies on the composition of steel materials and the like in order to secure the characteristics required for gears in the induction hardening process. The following knowledge was obtained.

Gears are required to have root strength, tooth surface strength and impact characteristics. Here, the root strength means the maximum stress at which the tooth portion is repeatedly subjected to stress and does not cause fatigue fracture from the root portion. Since the tooth root strength has a good correlation with the fatigue strength obtained by a fatigue test such as rotational bending, the chemical composition of steel materials was examined by a rotational bending fatigue test. As a result, the basic factors affecting fatigue strength are the hardness of the material, austenite grain size and non-metallic inclusions. As the material hardness decreases, the fatigue strength also decreases. In order to secure the hardness of this material to a value substantially equal to that of the carburized and quenched material by induction hardening, the amount of C needs to be about 0.5% or more. In order to ensure the hardness of the material, the addition of an alloy component is effective from the viewpoint of improving the hardenability, but it may be added in an appropriate amount according to the size of the gear.

Reducing the austenite grain size as well as the hardness of the material is also effective in improving fatigue strength. This is because fatigue cracks extend along the old austenite grain size, so making them finer increases resistance to fatigue crack propagation, and also segregates at grain boundaries such as P. This is because the concentration of the embrittlement component decreases due to the refinement of the austenite grains. Induction quenching is very effective for refining austenite because it is heating in a short period of time, but the addition of N, Al, etc., which forms precipitates that suppress the growth of austenite grains, is more effective. It is effective in improving fatigue strength by refining.

Next, in order to improve the fatigue strength,
It is important to secure the hardness of the material as described above, to reduce the austenite grains, and to reduce nonmetallic inclusions. That is, even if the hardness of the material can be ensured, if an oxide-based nonmetallic inclusion is present, fatigue fracture occurs from this portion, and the fatigue strength is reduced. In particular, hard non-metallic inclusions such as alumina are harmful, and therefore, reduction of O is essential. According to the examination results, O
It is necessary to reduce the amount to at least 0.0015% or less, but that alone is not sufficient, and the number and size of oxides must be limited to ensure the same or higher fatigue strength as conventional carburized materials Is important.

[0011] As described above, when non-metallic inclusions are present, fatigue fracture proceeds from the starting point, as described above.
As the non-metallic inclusions are larger, the degree of stress concentration generated in the inclusions becomes remarkable, and fatigue initial cracks are easily generated.
In addition, the generation of initial cracks is more remarkable as the non-metallic inclusions are larger and the degree of stress concentration is larger. Once a large initial crack is generated, the fatigue crack rapidly extends and leads to fatigue failure.

According to the examination results, it is important to prevent the presence of oxide-based nonmetallic inclusions having a size exceeding 19 μm in order to secure a fatigue strength equal to or higher than that of the conventional carburized and quenched material by induction hardening. I understood that. further,
As a result of examining the effect of the number of nonmetallic inclusions,
It has been found that even if the number is less than 19 μm, if the number exceeds 2.5 parts / mm ² , the same or higher fatigue strength as that of the conventional carburized and quenched material cannot be obtained. This is because when the non-metallic inclusions are small, the initial cracks generated from that part are small, but when they grow, they combine with the fatigue cracks generated from other non-metallic inclusions to become large fatigue cracks, and then the fatigue cracks rapidly This is because they grow and lead to fatigue failure in a short time. As described above, in order to secure the fatigue strength, it is essential to control not only the amount of O but also the number and size of the oxide-based nonmetallic inclusions.

Further, a method for reducing the amount and size of the oxide-based nonmetallic inclusion to the above range was studied. As a result, it was found that by limiting the amount of O in steel to 15 ppm or less, the amount of oxide-based nonmetallic inclusions can be reduced to the target 2.5 pieces / mm ² or less. Regulation alone is not enough. Therefore, as a result of various studies, the total cross-sectional reduction rate when rolling to the final steel material from the slab size during casting has a strong correlation with the nonmetallic inclusion size, and as the cross-sectional reduction rate increases, Inclusion size was found to decrease. This is because coarse nonmetallic inclusions are mechanically crushed by rolling. Then, in order to reduce the size to the target size of 19 μm or less, it was found that it was necessary to control the O content to 15 ppm or less and to reduce the area by 95% or more as the area reduction rate.

Next, fatigue damage called pitching occurs in the tooth surface due to repeated contact stress and friction. If this occurs, it becomes difficult for the gears to perform their normal functions, so that a tooth flank strength that can withstand them is required. This tooth surface strength has a good correlation with the rolling fatigue test, and can be evaluated by this test. By the way, relative slip occurs on the tooth surface of the gear, and this friction causes a remarkable temperature rise. Due to this temperature rise, the steel material is softened, and pitching occurs on the tooth surface. In order to suppress this, it is effective to add Si, Mo, V, Nb, etc., which increase the tempering softening resistance of the steel, and it is possible to increase the tooth surface strength by adding these. In addition, the rolling fatigue life is affected by the amount and size of the oxide-based nonmetallic inclusions in the same manner as the fatigue strength. However, the above-mentioned control of the O content and the reduction rate of the cross section when rolling from the slab to the final steel material are considered. It has been found that by controlling the amount and size of the nonmetallic inclusions, a rolling fatigue life equal to or more than that of the conventional carburized steel can be secured.

When an impact load is applied to the tooth root, if the impact characteristics of the steel material are low, the teeth are broken from the tooth root, and not only the gear but also the entire machine in which the gear is incorporated is damaged, which is difficult to recover. Up to Therefore, impact characteristics are extremely important characteristics. The amount of carbon has the largest effect on the impact characteristics, but the carbon concentration of the carburized part through the carburizing process is about 0.8%, while the hardness of the steel material is the same by induction hardening. The amount of C required to obtain
Since it is about 0.5 to 0.7%, it is advantageous from the viewpoint of securing impact characteristics. In addition to the factors affecting the impact characteristics, the austenite grain size during induction hardening and the impurity components such as P segregated at the grain boundaries also affect the fine particles of the austenite grains and the impurity components such as P. Is effective in improving the impact characteristics. However, when comparing only the unhardened part, the carbon content of the carburizing steel is as low as about 0.2%, while on the other hand, it is 0.5 to 0.7% for application to induction hardening.
Conventional carburized steel is more advantageous for the unhardened portion because it increases% C and C content.

Since the impact characteristics are determined by the effects of these factors when viewed as a whole gear, it is important to improve the impact characteristics of the non-hardened portion of the steel for induction hardening. Therefore, as a result of studying measures to improve the impact characteristics of the uncured portion, it was found that the impact characteristics of the entire gear can be further improved by specifying the forging temperature and the subsequent cooling rate in the forging process from a steel material to a gear. Was found.

In general, the improvement of the impact characteristics of a steel material is achieved by refining the microstructure of the steel. According to the above examination results, the forging temperature range is set to the range of Ac ₃ -100 ° C. to Ac ₃ + 200 ° C. It was found that setting the working rate at 70% and the subsequent cooling rate at 0.005 ° C./s or more was the most effective in refining the structure.

The present invention has been made based on the above findings, and the gist thereof is as follows.

C: 0.5 mass% or more, 0.75 mass% or less, Si: 0.5 mass% or more, 1.8 mass% or less, Mn: 0.4 ma
ss% or more, 1.5 mass% or less, Al: 0.019 mass% or more, 0.
05 mass% or less, P: 0.010 mass% or less, S: 0.020 mass
%, O: 0.0015 mass% or less and N: 0.002 mass%
As described above, 0.006 mass% or less is contained, and the balance is a composition of Fe and unavoidable impurities, and the number of oxide-based nonmetallic inclusions in the steel is 2.5 pieces / mm ² or less, and the maximum size is:
For a steel material of 19 μm or less, Ac ₃ points -100 ° C or more, Ac ₃ points +20
Heat to a temperature range of 0 ° C or less, and at that temperature range, process rate: 70
% Forging, and then more than 0.005 ℃ / s, 10 ℃ /
A method for manufacturing an induction hardened component, wherein the component is cooled in a cooling speed range of not more than s, and then subjected to induction hardening and tempering (first invention).

C: 0.5 mass% or more, 0.75 mass% or less
Bottom, Si: 0.5 mass% or more, 1.8 mass% or less, Mn: 0.4 ma
ss% or more, 1.5 mass% or less, Al: 0.019 mass% or more, 0.
05 mass% or less, P: 0.010 mass% or less, S: 0.020 mass
%, O: 0.0015 mass% or less and N: 0.002 mass%
Not less than 0.006 mass% and Ni: 0.1 mass%
1.0 mass% or less, Mo: 0.05 mass% or more, 0.50 mass
%, Ti: 0.005 mass% or more, 0.05 mass% or less and
B: Select from 0.0003 mass% or more and 0.005 mass or less
One or two or more, with the balance being Fe and
Unavoidable impurities and the oxide-based non-
Number of metal inclusions: 2.5 pieces / mm ^TwoBelow, the maximum size:
A steel material of 19 μm or less is_ThreePoint -100 ° C or higher, Ac_ThreePoint +20
Heat to a temperature range of 0 ° C or less, and at that temperature range, process rate: 70
% Forging, and then more than 0.005 ℃ / s, 10 ℃ /
Cooling in the cooling speed range below s, then induction hardening
Induction hardened parts characterized by performing a tempering process
(Second invention).

C: 0.5 mass% or more, 0.75 mass% or less, Si: 0.5 mass% or more, 1.8 mass% or less, Mn: 0.4 ma
ss% or more, 1.5 mass% or less, Al: 0.019 mass% or more, 0.
05 mass% or less, P: 0.010 mass% or less, S: 0.020 mass
%, O: 0.0015 mass% or less and N: 0.002 mass%
Not less than 0.006 mass% and V: 0.05 mass%
Not less than 0.5 mass% and Nb: not less than 0.01 mass%, 0.5
one or two selected from mass% or less, the balance being Fe and inevitable impurities, and the number of oxide-based nonmetallic inclusions in the steel is 2.5 / mm ² or less, maximum size: 19μm or less steel material is heated to a temperature range of Ac ₃ points -100 ° C or more and Ac ₃ points + 200 ° C or less,
After forging at a processing rate of 70% or more in that temperature range,
A method for producing an induction hardened part, comprising cooling at a cooling rate in the range of 0.005 ° C./s or more and 10 ° C./s or less, and then performing induction hardening and tempering (third invention).

C: 0.5 mass% or more, 0.75 mass% or less, Si: 0.5 mass% or more, 1.8 mass% or less, Mn: 0.4 ma
ss% or more, 1.5 mass% or less, Al: 0.019 mass% or more, 0.
05 mass% or less, P: 0.010 mass% or less, S: 0.020 mass
%, O: 0.0015 mass% or less and N: 0.002 mass%
Not less than 0.006 mass% and Ni: 0.1 mass%
1.0 mass% or less, Mo: 0.05 mass% or more, 0.50 mass
% Or less, Ti: 0.005 mass% or more, 0.05 mass% or less, and B: one or more kinds selected from 0.0003 mass% or more and 0.005 mass or less, and V: 0.05 mass% or more, 0.5 ma
ss% or less and Nb: one or two selected from 0.01 mass% or more and 0.5 mass% or less, with the balance being Fe
And becomes the composition of the unavoidable impurities, and an oxide-based nonmetallic inclusions thereof in the steel, the number: 2.5 / mm ² or less, the maximum size: 19 .mu.m the following steel, Ac ₃ point -100 ° C. or higher,
Ac ₃ points + heated to a temperature range of 200 ° C or less, forged at a working rate: 70% or more in that temperature range, and then cooled in a cooling rate range of 0.005 ° C / s or more and 10 ° C / s or less, Thereafter, a method of manufacturing an induction hardened component, which comprises performing an induction hardening and tempering process (a fourth invention).

[0023] The method for producing an induction hardened part according to any one of the first to fourth inventions, wherein the steel material has been subjected to a rolling process with a reduction rate of 95% or more in cross-section from the slab. ).

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limitation in carrying out the present invention will be described below. First, the reasons for limiting the component composition will be described. C: 0.5 to 0.75 mass% C is an essential component for obtaining the same surface hardness as conventional carburized steel by induction hardening, and is at least 0.5 ma.
It is necessary to contain ss% or more. But 0.75ma
If the content exceeds ss%, the impact characteristics and machinability required for the gear deteriorate. Therefore, its content is 0.5
mass% and 0.75mass% or less.

Si: 0.5 to 1.8 mass% Si is a component that improves the tempering softening resistance and improves the tooth surface strength. However, in order to secure the same tooth surface strength as the gears obtained by the conventional carburizing process, At least 0.5 mass
%, It is necessary to contain more than 1.8 mass%, but the solid solution hardening of the ferrite increases the hardness and lowers the machinability. Therefore, its content is 0.5
The content is set to not less than mass% and not more than 1.8 mass%, more preferably 0.5 to 1.0 mass%.

Mn: 0.4 to 1.5 mass% Mn is an essential component for improving the hardenability and securing the hardening depth during induction hardening.
If the content is less than 0.4 mass%, the effect is poor, and 1.5%
If it exceeds, the surface hardness is reduced rather by increasing the retained austenite after induction hardening, and the fatigue strength and rolling fatigue life are reduced. Therefore, the content is 0.4 mass% or more and 1.5 mass% or less, but a more preferable range is 0.7 to 1.3 mass%.

Al: 0.019 to 0.050 mass% Al is a component effective for deoxidation and useful for reducing oxygen, and combines with N to form AIN, which grows austenite grains during high frequency heating. The effect is poor when the content is less than 0.019 mass%, and the effect is saturated even if it exceeds 0.05 mass%. . Therefore, its content is 0.019 mass% or more, 0.05%
0 mass% or less.

P: 0.01 mass% or less P segregates at the grain boundaries of austenite and lowers the grain boundary strength, thereby lowering the tooth root strength.
At the same time, the impact characteristics are deteriorated, so that it is desirable to minimize the impact characteristics, and the content is allowed to be 0.01 mass%.

S: 0.020 mass% or less S forms MnS, which is a starting point of fatigue fracture, thereby lowering fatigue strength. On the other hand, MnS is also a component for improving machinability, so 0.020 mass%. Can be contained.

O: 0.0015 mass% or less O is preferably as small as possible. In order to control the amount and size of nonmetallic inclusions to target values or less, O which forms oxide nonmetallic inclusions such as alumina is preferred. Although it is necessary to reduce it, its content is set to 0.0015 mass% or less.

N: 0.002 to 0.006 mass% N combines with Al to form AIN. Since this improves the impact characteristics and fatigue strength by suppressing the growth of austenite grains during high frequency heating, it is positively added,
If the content is less than 0.002 mass%, the effect is small,
If it exceeds 6 mass%, the surface deformability of the slab increases during continuous casting by reducing the hot deformability. Therefore, the content is set to 0.002 mass% or more and 0.006 mass% or less.

In addition to the above component compositions, the present invention may contain Ni, Mo, Ti and B alone or in combination as the same components for improving fatigue strength, tooth surface strength and impact properties. These actions are as follows.

Ni: 0.1 to 1.0 mass% Ni is a component that improves hardenability and is effective not only in improving fatigue strength and tooth surface strength but also in improving impact characteristics. In particular, it may be used when the impact characteristics need to be improved. 0.1 mass% content
If the content is less than 1.0%, the effect is not sufficient. On the other hand, since Ni is an extremely expensive component, the content of more than 1.0 mass% increases the cost of the steel material, which is against the object of the present invention. Therefore, the content is preferably 0.1 mass% or more and 1.0 mass% or less, and a more preferable range is 0.6 to 0.9 mass%.

Mo: 0.05 to 0.50 mass% Mo is a component useful for improving hardenability and various properties, and is used for adjusting hardenability, and has a remarkable influence on the morphology of pearlite. Cementite forms a fragmented pearlite, which significantly improves machinability. In addition, since the tempering softening resistance is improved, the tooth surface strength can be improved, and further, the impurity component such as P segregating at the grain boundary is reduced, thereby improving the fatigue strength, the root strength and the impact characteristics. There is. Thus, in the present invention, since it is a suitable component, it is preferable to positively add it, but the content is 0.05 ma.
If it is less than ss%, the effect is poor, and if it exceeds 0.50 mass%, carbides which are difficult to dissolve in austenite by rapid and short-time heating such as induction hardening are formed. Therefore, the content is preferably 0.05% by mass or more and 0.50% by mass or less, and more preferably 0.10 to 0.30% by mass.

Ti: 0.005 to 0.05 mass% Ti is a component that is extremely easy to bond with N, and forms TiN which has the effect of reducing austenite grains during high-frequency heating. It has the effect of improving strength, tooth surface strength and impact characteristics. On the other hand, Ti is B
As described above, since it is easily bonded to N, in the case of a composite addition with B, the bonding between B and N is suppressed, and the hardenability of B is ensured (the hardenability of B is such that B exists alone in steel. In some cases). In order to exhibit these effects, a content of less than 0.005 mass% is not sufficient,
Exceeding mass% causes excessive precipitation of TiN, which becomes the starting point of fatigue fracture, and lowers fatigue strength and tooth surface strength. Therefore, its content is more than 0.005mass%, 0.05mass%
The following is preferred, but more preferably in the range of 0.01 to 0.025 mass%.

B: 0.0003 to 0.005 mass% Since B is a component that improves the hardenability by adding a small amount, other alloy components can be reduced. In addition, B segregates preferentially at the grain boundaries, and the concentration of P segregated at the grain boundaries is reduced, so that the fatigue strength, the root strength and the impact characteristics are remarkably improved. To achieve these effects
Although it is desirable to contain 0.0003 mass% or more, 0.005 mass%
Even if the content exceeds mass%, the effect is saturated. Therefore, its content is 0.0003 mass% or more, 0.005 mass%
The following is good, but the more preferable range is 0.0010 to 0.0030 mass
%.

Further, in the present invention, V and Nb having a precipitation strengthening action can be added alone or in combination. These actions are as follows. When the induction hardening process is performed, a quenching and tempering process is generally performed as a pre-heat treatment in order to secure the hardness of the central portion of the material to be processed. However, it is desirable to omit this heat treatment as much as possible to increase the cost. In order to omit the quenching and tempering as a pretreatment, it is important to increase the hardness of the material before induction hardening. For this purpose, the addition of V and Nb having a precipitation strengthening effect is effective. is there.

V: 0.05 to 0.5 mass% V has a very strong precipitation strengthening effect and is a component for improving the tempering softening resistance of steel materials, and is therefore extremely effective in improving the tooth surface strength. It is also effective to add quenching and tempering as a pre-heat treatment before induction quenching when it is necessary to omit it. If the content is less than 0.05 mass%, the effect is small, and if the content exceeds 0.5 mass%, the effect is saturated. Therefore, its content is 0.05mass%
At least 0.5 mass% is preferred.

Nb: 0.01 to 0.5 mass% Nb, like V, has an extremely strong precipitation strengthening effect and is a component for improving the resistance to tempering and softening of a steel material, so that Nb is extremely effective for improving the tooth surface strength. It is also effective to add quenching and tempering treatment as a pre-heat treatment before induction quenching when it is necessary to omit it. If the content is less than 0.01 mass%, the effect is small, and if the content exceeds 0.5 mass%, the effect is saturated. Therefore, the content is preferably 0.01 mass% or more and 0.5 mass% or less.

Next, in the present invention, the amount and size of the oxide-based nonmetallic inclusions are specified to be 2.5 pieces / mm ² or less and 19 μm or less, respectively, in order to secure the fatigue strength. As described above, the number of oxide-based nonmetallic inclusions is
2.5 / mm ² or less to the in the oxide exceeding this is present fatigue cracks generated from the respective non-metallic inclusions coalesce or are in rapid fatigue cracks developed fatigue fracture, and as a result the target fatigue This is because it becomes difficult to secure the strength. In addition, the size is specified to be 19 μm or less because the presence of non-metallic inclusions exceeding this size makes the initial cracks generated by these non-metallic inclusions easy and large, resulting in rapid growth of fatigue cracks and early fatigue. This is because destruction occurs.

The reason why the cross-sectional reduction rate is set to 95% or more in rolling from a slab to a steel material is to reduce the size of oxide-based nonmetallic inclusions to 19 μm or less, which is the target, and to reduce the cross-sectional reduction less than this. This is because the rate may not be smaller than the target size.

Next, the reasons for limiting the hot forging conditions will be described. Ac ₃ -100 ° C to Ac ₃ + 200 ° C as forging temperature
Is limited to a temperature of less than Ac ₃ -100 ° C.
Deformation resistance is high, is because forging becomes difficult, Ac ₃ +
If the temperature exceeds 200 ° C., the initial austenite grain size becomes large, and the recrystallization and grain growth of the austenite grains after processing occur very rapidly, and the structure transformed from the austenite is not sufficiently refined.

The reason why the working ratio is set to 70% or more is that if the working ratio is less than this, the austenite is not sufficiently refined and the microstructure of the transformed steel is not sufficiently refined. Further, the cooling rate is specified to be not less than 0.005 ° C./s and not more than 10 ° C./s because at a cooling rate lower than 0.005 ° C./s, the transformed structure becomes coarse and a sufficient refining effect cannot be obtained. If the cooling rate exceeds ° C / s, the machinability may be significantly reduced due to the formation of a martensite structure. Note that a more preferable cooling rate range is 0.05 ° C./s to 1.5 ° C./s.

[0044]

EXAMPLE A slab (cross-sectional size 200.times.225 mm) having a total of 23 component compositions of the applicable steel, comparative steel and conventional steel shown in Table 1 was cast by a converter-continuous casting process.

[0045]

[Table 1]

After that, each slab was rolled into a billet of 150 mm square through a breakdown step, and then rolled into a steel bar having a different size (changing the area reduction rate). Next, steel materials collected from each of these steel bars were forged under different conditions, and then heated at 850 ° C. for 30 minutes as a pre-heat treatment.
After quenching, a tempering treatment was performed at a temperature of 550 ° C. In addition,
For steel Nos. 11 and 12, some pre-heat treatment was omitted.

Subsequently, a rotary bending fatigue test piece and a rolling fatigue test piece were prepared from each test material, and a high frequency of 15 kHz was used for those using conforming steel and comparative steel (steel Nos. 1 to 21). After surface quenching with a quenching tester, 180
Tempering was performed at 2 ° C. × 2 hours. The impact test piece was subjected to the same induction hardening and tempering as described above, and a 2 mm 10R notch impact test piece was sampled from the vicinity of the steel material surface.

For conventional steels (Steel Nos. 22 and 23), 930 ° C. × 4 was used instead of induction hardening and tempering.
After carburizing and quenching for a time (carbon potential of 0.88), tempering was performed at 180 ° C. for 2 hours.

Using each of the test pieces thus obtained, a rotating bending fatigue test, a rolling fatigue test and an impact test were respectively performed.

Here, each test procedure is as follows.・ Fatigue test: At room temperature using an Ono-type rotary bending fatigue tester
The rotation was performed at 3600 rpm. Rolling fatigue test: A contact pressure of 3677 MPa was applied by pressing a 130 mmφ roller against a 25 mmφ test piece, and the life was evaluated by the number of stress repetitions until pitting occurred on the surface. -Impact test: The test was performed at a temperature of 20 ° C using a Charpy impact tester.

The rolling and forging conditions and test results are shown in Tables 2 to 5, respectively.

[0052]

[Table 2]

[0053]

[Table 3]

[0054]

[Table 4]

[0055]

[Table 5]

Here, Table 2 shows a suitable example of rolling and forging using the compliant steel of the present invention under the conditions suitable for the present invention, and Table 3 shows the forging conditions using the compliant steel and out of the limited range of the present invention. Comparative Example, Table 4 shows that the size of the oxide-based nonmetallic inclusions was out of the range of the present invention because the conforming steel was used and the rolling conditions were out of the range of the present invention. Comparative examples performed outside the range, and Table 5 summarizes comparative examples using the comparative steels and conventional steels whose component compositions deviate from the present invention, and conventional examples, respectively.

As apparent from these tables, the conforming examples of Sample Nos. 1 to 12 and 11 'to 12' in Table 2 correspond to Samples N in Table 5
o. Compared to the conventional examples (carburized and quenched materials) of 46 and 47, the impact value, fatigue strength and rolling fatigue life are extremely excellent.

Further, samples No. 13 to No. 13 of Table 3 were forged under the conditions outside the limited range of the present invention using the same steel as the applicable example.
In Comparative Example 24, the microstructure was not sufficiently refined because the forging temperature was high or the cooling rate after forging was low, and the impact value was inferior to that of the conforming example. However, as compared with the conventional example, although the fatigue strength and the rolling fatigue life are naturally excellent, the impact values are very equal or higher.

Further, using the same steel as the applicable example, the sample Nos. 25 to 36 in Table 4 in which the reduction ratio of the rolling section, the maximum size of the oxide-based nonmetallic inclusion, and the forging temperature are out of the limited range of the present invention.
In Comparative Example No., the microstructure was not sufficiently refined and the maximum size of the inclusions was large, so that the impact value, the fatigue strength, and the rolling fatigue life were inferior to the corresponding examples.

On the other hand, the comparative examples of Sample Nos. 37 to 45 in Table 5 in which the component composition of steel is out of the limited range of the present invention are as follows, for example:
Sample No. 37, which has a large amount of C, has a low impact value and a sample which has a small amount of C.
These comparative examples show that the fatigue strength and rolling fatigue life are inferior, while the rolling fatigue life is inferior in sample No. 39 with a small amount of Si and sample No. 45 with a large amount of O. ,
Either one of the fatigue strength and rolling fatigue life or overall is inferior to the conforming example.

From the above, it can be seen that the applicable example of the present invention is extremely excellent in the impact, fatigue and rolling fatigue characteristics required for the gear.

[0062]

According to the present invention, an induction hardened part is manufactured by limiting the composition of steel and the number and size of oxide-based nonmetallic inclusions and further defining forging conditions.
According to the present invention, induction hardening can be adopted in place of conventional carburizing and quenching in the manufacturing process of gears and the like, thereby reducing manufacturing costs and obtaining parts with extremely excellent quality characteristics.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location C22C 38/14 C22C 38/14

Claims (5)

[Claims] [Claim 1] C: 0.5 mass% or more, 0.75 mass% or less, Si: 0.5 mass% or more, 1.8 mass% or less, Mn: 0.4 mass% or more, 1.5 mass% or less, Al: 0.019 mass% or more, 0.05 mass %, P: 0.010 mass% or less, S: 0.020 mass% or less, O: 0.0015 mass% or less, and N: 0.002 mass% or more.
Containing 006 mass% or less, the balance being the Fe and unavoidable impurities, and an oxide-based nonmetallic inclusions thereof in the steel, the number: 2.5 / mm ² or less, the maximum size: 19 .mu.m or less of The steel is heated to a temperature range of Ac ₃ points -100 ° C or higher and Ac ₃ points + 200 ° C or lower, and forged at a working rate of 70% or higher in the temperature range, and then 0.005 ° C / s or higher and 10 ° C. A method for producing an induction hardened component, comprising: cooling in a cooling rate range of not more than / s and thereafter performing an induction hardening and tempering treatment. 2. C: 0.5 mass% or more, 0.75 mass% or less, Si: 0.5 mass% or more, 1.8 mass% or less, Mn: 0.4 mass% or more, 1.5 mass% or less, Al: 0.019 mass% or more, 0.05 mass %, P: 0.010 mass% or less, S: 0.020 mass% or less, O: 0.0015 mass% or less, and N: 0.002 mass% or more.
Including 006 mass% or less, Ni: 0.1 mass% or more, 1.0 mass% or less, Mo: 0.05 mass% or more, 0.50 mass% or less, Ti: 0.005 mass% or more, 0.05 mass% or less, and B: 0.00
One type selected from 03 mass% or more and 0.005 mass or less
Or contains two or more, the balance being Fe and inevitable impurities
Oxide and non-metallic inclusions in the steel
Items: 2.5 pieces / mm ^TwoBelow, maximum size: 19μm or less
Replace the lower steel with Ac_ThreePoint -100 ° C or higher, Ac_ThreePoint + 200 ° C or lower.
Heated and forged at a processing rate of 70% or more in that temperature range
After that, the cooling rate range from 0.005 ° C / s to 10 ° C / s
And then subject it to induction hardening and tempering.
And a method for producing an induction hardened part. 3. C: 0.5 mass% or more, 0.75 mass% or less, Si: 0.5 mass% or more, 1.8 mass% or less, Mn: 0.4 mass% or more, 1.5 mass% or less, Al: 0.019 mass% or more, 0.05 mass %, P: 0.010 mass% or less, S: 0.020 mass% or less, O: 0.0015 mass% or less, and N: 0.002 mass% or more.
Including 006 mass% or less, and V: 0.05 mass% or more, 0.5 mass% or less and Nb: 0.01 ma
One or two selected from ss% or more and 0.5 mass% or less, the balance being Fe and unavoidable impurities, and the number of oxide-based nonmetallic inclusions in the steel is : 2.5 / mm ² or less, the maximum size: 19 .mu.m the following steel, Ac ₃ point -100 ° C. or more, and heated to ₃ points +200 ° C. below the temperature range Ac, working ratio at that temperature range: 70% or more A method for manufacturing an induction hardened part, comprising: forging, cooling at a cooling rate in the range of 0.005 ° C./s or more and 10 ° C./s or less, and thereafter performing an induction hardening and tempering treatment. 4. C: 0.5 mass% or more, 0.75 mass% or less, Si: 0.5 mass% or more, 1.8 mass% or less, Mn: 0.4 mass% or more, 1.5 mass% or less, Al: 0.019 mass% or more, 0.05 mass %, P: 0.010 mass% or less, S: 0.020 mass% or less, O: 0.0015 mass% or less, and N: 0.002 mass% or more.
Including 006 mass% or less, Ni: 0.1 mass% or more, 1.0 mass% or less, Mo: 0.05 mass% or more, 0.50 mass% or less, Ti: 0.005 mass% or more, 0.05 mass% or less, and B: 0.00
One or more selected from among 03 mass% or more and 0.005 mass or less, and V: 0.05 mass% or more and 0.5 mass% or less and Nb: 0.01 ma
One or two selected from ss% or more and 0.5 mass% or less, the balance being Fe and inevitable impurities, and the oxide nonmetallic inclusions in the steel are:
Quantity: 2.5 / mm ² or less, the maximum size: a 19μm following steel, Ac ₃ point -100 ° C. or higher, then heated to a temperature range below Ac ₃ point +200 ° C., working ratio at that temperature range: 70% After forging, cooling in a cooling rate range of 0.005 ° C./s or more and 10 ° C./s or less, and then performing an induction quenching and tempering process. 5. The method for producing an induction hardened part according to claim 1, wherein the steel material has been subjected to a rolling process at a cross-sectional reduction rate of 95% or more from a slab.