JPH10219392A - Parts for induction hardening and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain parts for induction hardening excellent in machinability and furthermore capable of securing characteristics as parts such as a gear or the like by specifying the compsn. of C, Si, Mn, P, S, Al, O, N and Fe, oxide nonmetallic inclusions and induction hardenability. SOLUTION: A steel contg., by weight, 0.5 to 0.75% C, 0.5 to 1.8% Si, 0.4 to 1.5% Mn, 0.010 to 0.020% P, <=0.020% S, 0.019 to 0.05% Al, <=0.0015% O and 0.006 to 0.015% N, furthermore contg., at need, prescribed amounts of Mo, B, Ti, Ni, V and Nb, and the balance Fe with inevitable impurities is heated at 1100 to 1250 deg.C and is subjected to hot rolling at >=95% reduction of area, and the rolling is finished at >=1000 deg.C. In this way, the parts for induction hardening composed of the steel in which the number of oxide nonmetallic inclusions is regulated to <=2.5/mm, also, the maximum size thereof is regulated to <=19μm, and moreover, the grain size of austenite at the time of induction hardening is regulated to <=16μm and capable of obtaining characteristics equal to or above those by the conventional carburizing process can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an induction hardened part and a method of manufacturing the same, and more particularly, to a gear and other parts which are conventionally manufactured by subjecting carbon steel to surface treatment such as carburizing and nitriding. You can do it.

[0002]

2. Description of the Related Art Conventionally, gears used in automobiles and industrial machines are processed into a predetermined shape by sequentially forging, cutting, turning, and gear-cutting a carburizing alloy steel containing about 0.2% of carbon. After that, it is manufactured by a method of performing a carburizing, quenching and tempering treatment to secure a necessary function as a gear. Manufacturing by such a carburizing process is the mainstream of the conventional gear manufacturing process.
Since processing at a temperature of about 50 ° C. for several hours is necessary, it is difficult to incorporate it into a gear manufacturing line, and there is a limit in improving productivity. As a result, there has been a natural limitation in reducing the manufacturing cost.

In general, carburizing is generally performed by a gas carburizing method. However, during gas carburizing, an abnormal surface layer is inevitably generated on the surface layer of the material to be treated, and this abnormal layer causes fatigue strength and impact characteristics. Therefore, there is a limit to the improvement in fatigue strength and impact properties. In addition, since the material to be processed is deformed by heat treatment distortion generated during carburizing and quenching, strict control of heat treatment conditions is required.

[0004] In order to overcome the problems associated with the conventional carburizing, quenching and tempering processes, the amounts of Si, Mn, and Cr in steel are reduced and M
The addition of o, Ni, etc. has reduced the surface abnormal layer generated during gas carburization, and has led to the development of high-strength carburizing steel intended to improve fatigue strength and impact characteristics. However, even in that case, there is a problem that the use of a large amount of expensive alloying elements causes an increase in steel material cost and deteriorates machinability such as machinability, so that high strength can be achieved but manufacturing cost increases. . In addition, JIS standard SCM43
Production of gears by induction hardening, which has a higher production efficiency than the carburizing and quenching process, has been attempted using alloy steels for machine structural use such as No. 5 and S55C and carbon steel, but these steels are originally used for gears. Because it is not a chemical composition determined in consideration of application, it is difficult to apply it to high-strength gears used in automobile transmissions and differentials like gears manufactured by carburizing process, and only relatively low-strength gears Application only.

[0005]

In order to solve the conventional problems in producing such high-strength parts, for example, Japanese Patent Application Laid-Open No. 60-169544 discloses that the chemical composition of steel is restricted to a specific range. Thus, there is disclosed a technology that enables high-strength gears to be manufactured by an induction hardening process.

However, according to the study of the present inventors, according to the technique disclosed in Japanese Patent Application Laid-Open No. 60-169544, the size of nonmetallic inclusions present in steel is large, and the size of non-metallic inclusions in steel is large. There is an unsolved problem that required fatigue strength and rolling fatigue life cannot be secured.

[0007] Further, since the content of P is limited to 0.01% by weight or less, there is another unsolved problem that the steel making cost is increased. Further, in the chemical composition disclosed in the above publication, the machinability is extremely reduced as compared with the conventional carburizing steel, so that the production efficiency in the cutting process, which is an essential process in the manufacturing process, is low, and carburizing and quenching are performed. Yet another unsolved problem is that there is a limit to improving productivity by changing the process to induction hardening.

The present invention has been made in view of such unresolved problems of the prior art, and is excellent in machinability and at the same time, compared with parts such as gears manufactured by a conventional carburizing process. It is an object of the present invention to provide a component for induction hardening and a method for manufacturing the same, which can secure the same or better characteristics.

[0009]

Means for Solving the Problems In order to achieve the above object, the present inventors have studied the chemical composition of a steel material for securing the characteristics required for a gear in an induction hardening process, and have studied the following. Such knowledge was obtained.

That is, gears are required to have root strength, tooth surface strength and impact characteristics. 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 rotary bending, the present inventors examined the chemical composition of steel by a rotary bending fatigue test.

The basic factors affecting fatigue strength are:
Material hardness and non-metallic inclusions. As the material hardness decreases, the fatigue strength also decreases. In order to secure the same hardness as that of the carburized and quenched material by induction hardening, the carbon content (C content) of about 0.5% by weight or more is required.
is necessary.

In order to improve the fatigue strength, it is effective to make the austenite grain size finer. The reason is that fatigue cracks extend along the prior austenite grain size, so making them finer increases resistance to fatigue crack propagation and segregates at grain boundaries to make them brittle. This is because the concentration of an element such as P to be reduced is reduced by the fine graining. In order to reduce the grain size of austenite, induction quenching, which is a process of heating in a short time, is very effective. Further, the addition of N, Al, or the like, which forms precipitates that suppress the growth of austenite grains, further promotes grain refinement, and is effective in improving fatigue strength.

Further, in order to obtain the material hardness, it is necessary to add an alloy element from the viewpoint of securing hardenability. These alloying elements may be added in appropriate amounts according to the size of the gear.

Further, in order to improve the fatigue strength,
It is not enough to secure the material hardness as described above, and it is necessary to reduce nonmetallic inclusions. That is, even if the material hardness can be ensured, if oxide-based nonmetallic inclusions are present, fatigue fracture occurs from this portion, and the fatigue strength is extremely reduced. In particular, hard nonmetallic inclusions such as alumina are harmful, and for this purpose, a reduction in the oxygen content (O content) is essential. According to the study of the present inventors, it is necessary at least to make the O content 0.0015% by weight or less, but this alone is not sufficient.

Further, as a result of investigations by the present inventors, it has become clear that it is necessary to limit the number and size of oxides in order to secure the same fatigue strength as that of the conventional carburized material. As described above, the presence of non-metallic inclusions causes fatigue fracture to proceed from this point, but as the non-metallic inclusions are larger, the degree of stress concentration at the inclusions becomes more pronounced and the initial cracks in fatigue Easily occur.

Further, the initial cracks are more remarkable as the nonmetallic inclusions are larger and the degree of stress concentration is larger. Once a large initial crack has occurred, the fatigue crack will quickly propagate and lead to fatigue failure. According to the study of the present inventor, in order to secure a fatigue strength higher than that of the conventional carburized and quenched material, it is required that
It has been found that it is necessary that no oxide-based nonmetallic inclusions having a size exceeding m exist.

Further, as a result of examining the effect of the number of non-metallic inclusions, it was found that even if the number of non-metallic inclusions was 19 μm or less, if the number exceeded 2.5 / mm ² , the conventional carburizing and quenching It was found that the same level of fatigue strength as the material could not 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 rapidly the fatigue cracks Is to grow and lead to fatigue failure in a short time.

As described above, in order to ensure 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. Furthermore, the present inventors have studied a method for reducing the amount and size of the oxide-based nonmetallic inclusion to the above-mentioned range. As a result, it was found that by limiting the amount of O in the steel to 15 ppm or less, the amount of oxide-based nonmetallic inclusions can be reduced to the target 2.5 or less / mm ² or less. A stipulation of the quantity alone is not sufficient. The present inventors have found that the cross-sectional reduction rate when finally rolling to steel material from the slab size during casting has a strong correlation with the non-metallic inclusion size, and as the cross-sectional reduction rate increases, the non-metallic inclusions The size was found to decrease. This is because coarse non-metallic inclusions are mechanically crushed by rolling. as a result,
In order to achieve the target size of 19 μm or less, the O amount should be 1
For steel controlled to 5 ppm or less, the area reduction rate is 95%.
It was found that a reduction of at least% was required.

On the other hand, fatigue damage called pitching occurs on the tooth surface of the gear due to repeated contact stress. When this occurs, it is difficult for the gears to perform their normal functions, so tooth surface strength is required.

The tooth surface strength has a good correlation with the rolling fatigue test, and can be evaluated by this test. However, in the case of a gear, relative slip occurs on the tooth surface portion, and the friction causes a significant temperature rise. Due to this temperature rise, the steel material is softened and pitting occurs. In order to suppress this, it is effective to add Si, Mo, V, Nb, etc., which increase the tempering softening resistance of steel.
These additions can increase the tooth surface strength.

The rolling fatigue life is influenced by the amount and size of the oxide-based nonmetallic inclusions as in the case of the fatigue strength. It was found that if the amount and size of the nonmetallic inclusions were controlled by controlling the cross-sectional reduction rate, the same rolling fatigue life as that of the conventional carburized steel was 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. To receive. Therefore, impact characteristics are extremely important characteristics.

As a factor affecting the impact characteristics, the amount of C has the largest effect. However, while the C concentration in the portion carburized through the carburizing process is about 0.8% by weight, the C concentration required to obtain the same steel hardness by induction hardening is 0.5%. About 0.7% by weight,
Induction hardening is advantageous from the viewpoint of ensuring the impact characteristics. However, not only are the factors affecting the impact characteristics, but also the austenite grain size during induction quenching and the impurity elements such as P segregated at the grain boundaries also affect the γ grain size reduction and P etc. Reduction of impurity elements is also effective in improving impact characteristics.

As described above, since P segregates at the grain boundary to embrittle the grain boundary, reducing P is effective for improving the tooth root strength and impact characteristics. However, reducing P leads to an increase in cost in the steel making process. As a means of lowering the amount of grain boundary segregation P without reducing P to increase the grain boundary strength, it is conceivable to refine the grains to increase the grain boundary area and reduce the relative amount of segregation P. . As a result of examination on this point, it was found that when the P content is up to 0.020%, sufficient tooth root strength and impact characteristics can be obtained if the austenite particle size during induction hardening is 16 μm or less. .

The above-described measures only for securing the characteristics required for a gear are not sufficient for manufacturing gears by induction hardening, and it is important to ensure workability, especially machinability. In this regard, in the case of the carburizing process,
Since low-C steel is used, it has relatively high machinability before carburizing and quenching, but on the other hand, in the case of induction hardening process, higher C than carburizing steel is essential, This is extremely disadvantageous in terms of ensuring machinability. The present inventors have studied various factors affecting the machinability of the high-C steel, and as a result, have obtained the following knowledge. That is, in the steel of 0.5% C or more, when the free-machining element is fixed, the factor that most affects the machinability is its microstructure. In particular, it was found that the amount of ferrite and the form of pearlite had the most significant effects. That is, in the case of a high-C steel, the microstructure has a ferrite-pearlite structure, but the machinability improves with an increase in ferrite. This is because the hardness of the steel material decreases due to an increase in the amount of ferrite, and the ferrite / pearlite interface, which is a portion where cracks occur during cutting, increases due to an increase in ferrite, thereby improving machinability. It is. Also, the form of pearlite has a very large effect. That is, in the case of a structure in which the pearlite lamellar is well-developed in a layered manner, the ductility of the pearlite portion is high, and the portion where cracks are generated during cutting is limited to the ferrite / pearlite interface. In a part that undergoes deformation during cutting, cracks are easily generated from the cementite / ferrite interface in pearlite in addition to the ferrite / pearlite interface, thereby greatly improving machinability. In order to form such undeveloped pearlite, it is necessary to select and optimize alloying elements in steel, and the reduction of Mn and Cr, which lowers the transformation point and promotes lamellar layering, is extremely effective. It is a target. Further, the addition of Mo is effective in improving the machinability because it suppresses lamellar layering and forms a cementite-separated structure.

Furthermore, the present inventors have studied methods for producing such steels, and have obtained the following findings. That is, MnS formed in steel elongates in the rolling direction along with hot rolling, but the extent varies depending on hot rolling conditions. Mn
As a result of the elongation of S, the fatigue strength measured by a fatigue test piece taken from a direction perpendicular to the elongation direction is extremely reduced. This is because the elongated MnS becomes the starting point of the fatigue crack. By the way, in an actual gear, MnS
Are often formed at right angles to the extension direction of MnS
Elongation may reduce the fatigue strength of the actual gear.

It is well known that the shape of MnS also affects the machinability, and that elongation of MnS deteriorates machinability. Therefore, it is necessary to suppress the elongation of MnS during hot rolling in order to further improve machinability and fatigue strength.

There is known a method for suppressing the elongation of MnS by adding Ca to form CaS to suppress the elongation. Addition of Ca forms coarse Ca-based oxide-based nonmetallic inclusions. As a result, there is a problem that the rolling fatigue life is reduced. Therefore, the inventors of the present application have studied the heating temperature and hot rolling conditions during hot rolling that affect the shape of MnS. As a result, the following findings were obtained.

As the heating temperature during hot rolling is increased, M
nS is partially dissolved, and the particle size of MnS is smaller than that in the slab stage. When this is hot-rolled, MnS
Is smaller in elongation than in the case of heating at a lower temperature. Further, since once dissolved MnS precipitates relatively finely in the middle of rolling, the average degree of elongation of MnS of the steel material is suppressed as compared with the case of low temperature heating. As a result of examining the shape of MnS before the hot rolling, it was found that the MnS having a more elongated shape had a greater elongation by subsequent rolling.

Considering the effect of the rolling conditions, the largest MnS
The region where elongation extends is a temperature region of 900 to 1000 ° C., and it has been found that the degree of elongation is small in regions higher and lower than this temperature. Therefore, the heating temperature is increased and the rolling temperature range is 900 to 1000 ° C.
It has been found that, by avoiding the period, the elongation of MnS can be remarkably improved, and the fatigue strength and machinability can be improved.

The present invention has been made based on the above findings, and the gist thereof is as follows.
That is, the invention relating to the component for induction hardening of the present invention has a weight ratio of C: 0.5 to 0.75% and Si: 0.5 to 1.0.
8%, Mn: 0.4-1.5%, P: 0.010-0.
020%, S: 0.020% or less, Al: 0.019-
0.05%, O: 0.0015% or less, N: 0.006
０．０１0.015%, and optionally Mo:
0.05-0.5%, B: 0.0003-0.005
%, Ti: 0.005 to 0.05%, Ni: 0.1 to
1.0% and V: 0.05 to 0.5%, Nb: 0.0
1 to 0.5% of at least one, the balance being Fe and inevitable impurities, the number of oxide-based nonmetallic inclusions is 2.5 / mm ² or less, and the maximum size is 19 μm or less. In addition, it is characterized by being made of a steel material having an austenite grain size of 16 μm or less during induction hardening.

The gist of the invention according to the production method of the present invention is that C: 0.5 to 0.75% by weight, Si:
0.5-1.8%, Mn: 0.4-1.5%, P: 0.
010-0.020%, S: 0.020% or less, Al:
0.019-0.05%, O: 0.0015% or less,
N: 0.006 to 0.015%, and if necessary, Mo: 0.05 to 0.5%, B: 0.0003
-0.005%, Ti: 0.005-0.05%, N
i: 0.1-1.0% and V: 0.05-0.5%,
Nb: contains at least one of 0.01 to 0.5%,
In addition to the balance of Fe and unavoidable impurities, the number of oxide-based nonmetallic inclusions is 2.5 / mm ² or less and the maximum size is 19 μm or less, and the austenite grain size during induction hardening is 16 μm or less. Is hot rolled from a slab, the slab is 1100 to 125
A method for producing a component for induction hardening, characterized in that the component is heated to a temperature range of 0 ° C. and rolling is completed in a temperature range of 1000 ° C. or higher. Here, when the above-mentioned steel material is hot-rolled from a slab, the slab is heated to a temperature range of 1100 to 1250 ° C., and the first-stage rolling is completed in a temperature range of 1000 ° C. or more, and further, 1050 to 1150 And the second stage of rolling is completed in a temperature range of 1000 ° C. or higher.

[0033] Further, when the above-mentioned steel material is hot-rolled from the slab, rolling is performed so that the cross-sectional reduction rate becomes 95% or more.

[0034]

Embodiments of the present invention will be described below. First, the reasons for limiting the components of the steel material used in the present invention will be described.

[C: 0.5 to 0.75%] C is an essential component for obtaining a surface hardness of the same degree as that of conventional carburized steel by induction hardening, and at least 0.5% or more is added. is required. However, if added in excess of 0.75%, the impact characteristics and machinability required for the gears deteriorate, so that
Add up to 0.75%.

[Si: 0.5 to 1.8%] Si is an element for improving tempering softening resistance. Although the tooth surface strength is improved by this, at least 0.5% or more is necessary in order to secure the same tooth surface strength as the gear by the conventional carburizing process. However, if it is added in excess of 1.8%, the hardness increases due to solid solution hardening of the ferrite and the machinability is reduced. Therefore, the addition is made 1.8% or less. [Mn: 0.4-1.5%] Mn improves hardenability,
It is an essential component for securing the hardening depth during induction hardening and is positively added. However, if it is less than 0.4%, the effect is poor. On the other hand, if added in excess of 1.5%, the residual austenite after induction hardening is increased, which in turn lowers the surface hardness and lowers the fatigue strength and rolling fatigue life. I do.

[P: 0.010-0.020%] P segregates at the grain boundaries of austenite and lowers the grain boundary strength, thereby not only lowering the tooth root strength but also lowering the impact characteristics. It is desirable to lower it as much as possible, but it is acceptable up to 0.020%. If the content is less than 0.010%, it leads to an increase in steel making cost.
It is not practical to make it below%. Also, 0.020
% Or less, sufficient tooth root strength and impact characteristics can be obtained by setting the austenite particle size to 16 μm or less, so that 0.010 to 0.020% is allowable.

[S: 0.020% or less] S forms MnS, and this serves as a starting point of fatigue fracture, thereby lowering the fatigue strength. On the other hand, MnS is also an element which improves machinability, so that it forms 0%. Additions of 0.020% or less are acceptable.

[Al: 0.019-0.05%] A1 is an element effective for deoxidation, is an element useful for reducing oxygen, and combines with N to form AlN. Suppresses growth of austenite grains during high frequency heating. Accordingly, the impact characteristics and the tooth root fatigue strength are improved, so that the addition is positive. However, if the addition is less than 0.019%, the effect is small, and if the addition exceeds 0.05%, the effect is saturated. Therefore, it is added 0.019 to 0.05%.

[N: 0.006 to 0.015%] N is A
AlN and TiN are formed by combining with l and Ti. This is positively added because it suppresses the growth of austenite during high-frequency heating, thereby improving impact characteristics and fatigue strength. However, when added less than 0.006%, the austenite grain size during high-frequency heating is reduced according to the present invention. Target 16μm
0.006% because the grain boundary embrittlement due to P segregating at the grain boundary occurs and the impact properties and fatigue strength are reduced.
The above addition is made. However, if added in excess of 0.015%, the hot deformability is reduced and the surface defects of the slab are significantly increased during continuous casting, so the addition is made 0.015% or less.

In the present invention, in addition to the above chemical composition, Mo: 0.05 to 0.5%, B: 0.000%
3 to 0.005%, Ti: 0.005 to 0.05%, N
i: One or more of 0.1 to 1.0% can be contained.

The action of these elements and the reasons for limitation are as follows. [Mo: 0.05 to 0.5%] Mo is an element useful for improving hardenability and is used for adjusting hardenability. The addition of Mo at the same time has a significant effect on the morphology of the pearlite, forming cementite with a fragmented pearlite.
As a result, the machinability is significantly improved. Further, Mo improves the resistance to tempering and softening, so that the tooth surface strength can also be improved. Further, Mo has an effect of improving the tooth root strength and impact characteristics by reducing impurity elements such as P segregated at the grain boundary. In the present invention, Mo is a suitable element, so it is positively added. When the addition is less than 0.05%, the effect is small. On the other hand, when the addition is more than 0.5%, carbides which are difficult to dissolve in austenite by rapid and short-time heating such as induction quenching are formed.
It is added in the range of 5 to 0.5%.

[B: 0.0003 to 0.005%] Since B is an element which improves hardenability by adding a small amount, other alloying elements can be reduced. B is an element that segregates preferentially at the grain boundaries, reduces the concentration of P segregated at the grain boundaries, and significantly improves tooth root strength and impact characteristics. For this purpose, 0.0003% or more must be added, but if it exceeds 0.005%, the effect is saturated, so the addition is made 0.005% or less.

[Ti: 0.005 to 0.05%] The effect of improving the hardenability of B is remarkable when B is present alone, but B is an element that easily bonds to N. In such a case, the above-described advantageous effects disappear. Since the effect of improving the hardenability of B can be sufficiently exerted by adding Ti which easily bonds to N more than B, Ti may be used in such a case. However, the effect is small when the addition is less than 0.005%. On the other hand, if added in excess of 0.05%, a large amount of TiN will be formed, and this will be the starting point of fatigue failure, lowering the tooth root strength and tooth surface strength.

Since TiN has the effect of reducing the austenite grain size during high-frequency heating, the addition of Ti alone alone has the effect of improving tooth surface strength and fatigue strength. Also in this case, the addition amount of Ti is 0.005 to 0.5.
A range of 05% is preferred.

[Ni: 0.1-1.0%] Ni is not only an element which improves hardenability by its addition, but also an element which improves impact characteristics. Alternatively, it may be used when the impact characteristics need to be improved. However, if the addition is less than 0.1%, the effect is small, so the addition is 0.1% or more. On the other hand, Ni is an extremely expensive element, and if added in excess of 1.0%, the cost of the steel material rises, which is contrary to the purpose of the present invention, so that the addition is less than 1.0%.

In the present invention, V, Nb
One or more of the above. The effects of these elements will be described.

When an 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, since this heat treatment increases the cost, it is desirable to omit this as much as possible. In order to omit quenching as pretreatment, it is necessary to increase the material hardness before induction hardening. For this purpose, V and Nb having a precipitation strengthening action
Is effective.

The reasons for limiting the added amounts of V and Nb are as follows. [V: 0.05-0.5%] V is an element having a very strong precipitation strengthening effect, and is added when it is necessary to omit the quenching and tempering treatment as a pre-heat treatment before induction hardening. , Less than 0.05%, the effect is small, while adding more than 0.5% saturates the effect.

Further, V is an element for improving the tempering softening resistance of the steel material, and is therefore very effective for improving the tooth surface strength. [Nb: 0.01 to 0.5%] Since Nb is an element having an extremely strong precipitation strengthening effect, it is added when it is necessary to omit the quenching and tempering treatment as a pre-heat treatment before induction hardening. , Less than 0.01%, the effect is small,
On the other hand, if the content exceeds 0.5%, the effect is saturated, so the content is set to 0.01 to 0.5%. In addition, since Nb is an element that improves the tempering softening resistance of a steel material, it is also extremely effective in improving the tooth surface strength.

In the present invention, in order to secure the fatigue strength, the amount (number) and size of the oxide-based nonmetallic inclusions are not more than 2.5 / mm ² and 1 respectively.
It is specified to be 9 μm or less. If the number of oxide-based nonmetallic inclusions exceeds this number, the fatigue cracks generated from each nonmetallic inclusion will coalesce, and the fatigue cracks will grow rapidly, leading to fatigue fracture, thereby securing the target fatigue strength. This is because it becomes difficult to do so. In addition, if oxide-based nonmetallic inclusions having a size exceeding 19 μm are present, the initial cracks generated from the nonmetallic inclusions become large, and as a result, fatigue cracks rapidly develop and early fatigue fracture occurs. is there.

[O: 0.0015% or less] In order to control the amount and size of the above-mentioned oxide-based nonmetallic inclusions to below the above-mentioned target values, oxide-based nonmetallic inclusions such as alumina are required. It is necessary to reduce the amount of O to be formed. For this purpose, it is necessary to reduce the amount of O to at least 0.0015% or less. Therefore, in the present invention, the oxygen content in steel is limited to O: 0.0015% or less.

In the present invention, the austenite grain size during induction hardening is specified to be 16 μm or less. The reason is as follows. The finer the austenite grain size, the lower the concentration of P that segregates at the austenite grain boundaries and causes grain boundary embrittlement. When the P content is 0.020% or less and the austenite particle size is larger than 16 μm, the grain boundary concentration of P increases and the grain boundaries become brittle, so that sufficient root strength and impact properties cannot be obtained. Conversely, when the thickness is 16 μm or less, embrittlement of the grain boundary is considerably reduced, and this effect is combined with the effect of grain refinement, whereby sufficient root strength and impact characteristics can be obtained. For this reason, the austenite grain size during induction hardening is specified to be 16 μm or less. Furthermore, in the present invention, the cross-sectional reduction rate at the time of rolling from a slab to a steel material is 95% or more. This is also for the purpose of reducing the size of the oxide-based nonmetallic inclusion to 19 μm or less, and the target of the size of the oxide-based nonmetallic inclusion cannot be achieved at a cross-sectional reduction rate of less than 95%, resulting in early fatigue fracture. Is caused.

Next, the reason for limiting the manufacturing conditions of the induction hardened part of the present invention will be described. In the present invention, M
In order to suppress the elongation of nS, the heating temperature and the rolling conditions during hot rolling are specified. In the production of the induction hardening part of the present invention, the heating temperature of the hot rolling is set to 1100 to 1250 ° C. At a temperature lower than 1100 ° C., MnS does not form a solid solution at all and is rolled as coarse MnS at the time of casting. This is because extension is remarkable. Further, the reason why the temperature is set to 1250 ° C. or less is that if the temperature is higher than this temperature, the grain boundary is partially melted and the hot deformability is reduced, so that hot rolling becomes difficult. In addition, the reason why the rolling temperature is set to 1000 ° C. or higher is that the MnS elongation becomes remarkable in a temperature range below the rolling temperature.

In the manufacturing method of the present invention, two
In some cases, the final shape is formed by multiple rollings. In this case, the hot rolling temperature conditions are also specified, but the rolling temperature of the first stage is the same as described above. Regarding the second-stage rolling temperature condition, since the MnS is refined by the first-stage rolling, the heating temperature may be reduced to a temperature at which MnS does not form a solid solution. However, when the hot rolling temperature is lower than 1000 ° C., the MnS elongation occurs.
It is necessary to roll at a temperature of 00 ° C. or higher. That is, the reason why the second-stage heating temperature is set to 1050 is that it is difficult to maintain the hot rolling temperature at 1000 ° C. or higher at a lower temperature, and the upper limit is set to 1250 ° C. If the temperature exceeds this, the hot deformability decreases and hot rolling becomes difficult.

Hereinafter, the present invention will be described with reference to examples. (Example 1) In this example, the relationship between the chemical composition of the material steel of the component for induction hardening, the austenite grain size, the state of nonmetallic inclusions, and the like and the component characteristics was examined.

Steel having the chemical composition shown in Table 1 was produced by the converter-continuous casting process. No. in the table. Is the test No. And steel No. And also serves as.

[0058]

[Table 1]

The size of the piece at the time of casting is 200 × 225 mm.
Met. This slab is subjected to 150
After being rolled into a mm square billet, it was rolled into a bar at a predetermined cross-sectional reduction rate (described in Table 2 below). This is 845 ° C x
After quenching for 30 minutes, tempering treatment was performed at 550 ° C. Using these as raw materials, a rotating bending fatigue test piece having a diameter of 8 mm and a rolling fatigue test piece having a diameter of 27 mm were prepared.
Surface quenching was performed by a 15 kHz induction hardening tester, followed by tempering at 180 ° C. × 1 h. The same quenching and tempering material was subjected to the same induction quenching and quenching and tempering treatments to produce impact test pieces of 2 mm 0R notch from near the surface.

Further, it is melted in a converter-continuous casting process,
Test pieces similar to the above were prepared using SCM420 steel rolled at the cross-sectional reduction rate shown in Table 2 through the same process as above, and these were subjected to 930 ° C. × 4 h (carbon potential 0.
88) → Carburizing treatment of quenching was performed, and tempering was performed at 180 ° C. for 2 hours.

Table 2 shows details such as the cross-sectional reduction rate during rolling and the number and size of nonmetallic inclusions.

[0062]

[Table 2]

Using these samples, an impact test, a rotating bending fatigue test and a rolling fatigue test were carried out. The impact test was performed using a Charpy impact tester at + 20 ° C.

The fatigue test was carried out at room temperature at a speed of 3600 rpm using an Ono-type rotary bending fatigue tester. In the rolling fatigue test, a roller having a diameter of 130 mm was pressed against a test piece to give a contact stress of 3677 MPa, and the life was evaluated by the time until pitting occurred on the surface.

Table 3 shows the test results.

[0066]

[Table 3]

No. 1 to No. 12 is an example of the present invention.
No. Reference numeral 29 denotes a conventional example, which is commonly used as carburized steel.
IS SCr420 equivalent steel.

No. Reference numeral 30 is also a conventional example and is a high-strength carburized steel obtained by improving JIS steel. No. 13 to 15 are comparative examples,
This is the case where the cross-sectional reduction rate during rolling is out of the range of the present invention, and as a result, the size of the oxide-based nonmetallic inclusion is out of the range of the present invention. Although it has better characteristics than the above-mentioned SCr420 steel, the fatigue strength and the rolling fatigue life are deteriorated as compared with the examples of the present invention.

No. Comparative Examples 16 to 18 are cases where the austenite particle size is out of the range of the present invention, and the impact value and the fatigue strength are considerably deteriorated as compared with the inventive examples. N
o. 19-No. Comparative Example 28 has any of the chemical compositions outside the scope of the present invention.

That is, No. No. 19 is the case where the N content is below the range of the present invention, and as a result of the coarse austenite grain size, the impact value and the fatigue strength are considerably lower than those of the present invention. They are inferior even compared to 30 high strength carburized steels.

No. No. 20 is a case where the C content exceeds the upper limit of the present invention, and the impact value is extremely lower than that of the conventional steel. No. No. 21 is a case where the C content is less than the range of the present invention, and as a result of the low surface hardness, the fatigue strength and the rolling fatigue life are significantly reduced, and the properties are inferior to SCr420.

No. 22 is a case where the amount of Si is below the range of the present invention, the rolling fatigue life is extremely reduced,
The properties are lower than SCr420. No. 23 is
In this case, the Mn content exceeds the range of the present invention, and the fatigue strength and the rolling fatigue life are reduced.

No. No. 24 is the case where the P content exceeds the upper limit of the present invention, and the impact value and the fatigue strength are significantly reduced. N
o. 25 is a case where the amount of S exceeds the upper limit of the present invention,
The fatigue strength has decreased.

No. In No. 26, the Al content was lower than that of the present invention. As a result, the amount of AlN produced decreased, the austenite grain size became coarse, and the impact properties and fatigue strength were reduced. In particular, the impact value is lower than SCr420.

No. No. 27 is the case where the amount of Ti exceeds the upper limit of the present invention, and the fatigue strength and the transfer fatigue life are remarkably reduced. No. 28 is the case where the O content exceeds the upper limit of the present invention, and as a result, the number of oxide-based nonmetallic inclusions is large,
Fatigue strength and rolling fatigue life are extremely reduced. Its value is inferior to SCr420.

That is, in the case of a steel outside the scope of the present invention, any of the various properties are lower than those of SCr420 or high-strength carburizing steel, whereas in the case of the steel of the present invention, Is also superior to that of the conventional carburized steel SC420, and is almost equal to or higher than that of the high-strength carburizing steel.

(Example 2) In this example, in addition to the material conditions such as the chemical composition of the material steel, the austenite grain size, and the nonmetallic inclusions of the material for the induction hardening part, the manufacturing conditions such as the hot rolling temperature and the like were determined. The relationship with the component characteristics was examined.

Steel having the composition shown in Table 4 was melted into a 560 × 400 mm bloom by a converter-continuous casting process.
Table 5 shows details such as the reduction ratio of the cross-section of rolling and the number and size of nonmetallic inclusions.

[0079]

[Table 4]

[0080]

[Table 5]

In the table, No. Nos. 31 to 57 are sample steel Nos. It is. This bloom was rolled into a steel bar having a diameter of 100 mm under the hot rolling conditions shown in Table 6 below. From this material, the diameter 30 degrees from the direction perpendicular to the rolling direction and from the rolling direction.
A 850 mm x 30 min quenching treatment was performed on the material, and then a 550 C tempering treatment was performed. Using these as raw materials, rolling-bending fatigue test specimens having a diameter of 8 mm and rolling fatigue specimens having a diameter of 27 mm were prepared, and 15 kHz.
Was subjected to surface quenching with a high-frequency induction tester, and then tempered at 180 ° C. × 1 h.

Further, the same induction hardening treatment and quenching and tempering treatment were applied to the quenched and tempered material having a diameter of 30 mm, and an impact test piece having a notch of 2 mm and 10R was prepared from the vicinity of the surface.
On the other hand, the melted in the converter-continuous casting process, after rolling into a steel bar of 90 mm in diameter, and then preparing a test piece similar to the above using SCr420 steel prepared by cutting a material having a diameter of 30 mm from the steel bar, These were subjected to 930 ° C. × 4 h (carbon potential 0.88) → carburizing treatment of quenching,
Tempering at 0 ° C. × 2 h was performed.

Using these samples, an impact test, a rotating bending fatigue test and a rolling fatigue test were carried out under the same conditions as in Example 1. In addition, in the state of hot rolling, using a cemented carbide tool P10, cut 2 mm, feed 0.25 mm
A cutting test was performed under the conditions of / rev and a cutting speed of 200 / min. The machinability was evaluated by the cutting time (tool life) until the flank wear reached 0.2 mm.

In the test No. of the second embodiment. 2-1 to 2-
Tables 6 and 7 show the results of the 39 tests.

[0085]

[Table 6]

[0086]

[Table 7]

In the table, the steel No. 31 to 57 are steel N of Table 4
o. It corresponds to. Test No. 2-1 to 2-12 are examples of the present invention. Test No. 2-13 to 2-24 are cases where the chemical composition falls within the range of the present invention, but the hot rolling conditions are outside the range of the present invention.

Test No. 2-25 to 2-37, the chemical composition is out of the range of the present invention or the cross-sectional reduction rate during rolling is out of the range of the present invention, and as a result, the size of the oxide-based nonmetallic inclusions is smaller than that of the present invention. This is the case when it is out of the range.

Test No. Test pieces Nos. 2-38 and 39 are conventional examples. 2-38 is JIS SCr420 equivalent steel that is frequently used as carburized steel. Test No.
39 is a high-strength carburized steel obtained by improving JIS steel.

Test No. 2-1 to No. The inventive examples 2-12 are superior in all of the impact characteristics, the fatigue strength, the rolling fatigue life and the machinability as compared with the carburized steel SCr420.

On the other hand, Test No. In Comparative Examples 2-13 to 2-24, the fatigue strength in the C direction is lower than that of the present invention, and is lower than SCr420 of the conventional steel. This fact indicates that if the hot rolling conditions do not satisfy the present invention, SCr
This indicates that it is difficult to secure characteristics equivalent to 420.

Test No. 2-25 to 2-37 are cases where the chemical composition or the definition of oxide-based non-metallic inclusions are out of the range of the present invention, and include any one of impact characteristics, fatigue strength, rolling fatigue life and machinability. Or several characteristics are S
Inferior to Cr420.

That is, according to the present embodiment, in the case of steel outside the scope of the present invention, one of the various characteristics is SCr
420 or a lower value than the high-strength carburizing steel, whereas in the case of the steel of the present invention, any of the characteristics is superior to the conventional carburizing steel SCr420, and furthermore, almost equal to or higher than the high-strength carburizing steel It is clear that it is a value.

(Embodiment 3) In this embodiment, the relationship between the characteristics of parts for the induction hardening and the characteristics of the parts in the case where the first and second steps are subjected to two-step rolling is examined. It was done.

Steels having the chemical compositions shown in Table 4 (steel Nos. 31 to 31)
57) is converted to 560 × 40 by the converter / continuous casting process.
It was melted in a 0 mm bloom. This bloom was rolled into a 150 mm square billet under the hot rolling conditions shown in Table 7,
Further, it was rolled into a steel bar having a diameter of 50 mm by hot rolling.
From this material, a material having a diameter of 30 mm from the direction perpendicular to the rolling direction and from the rolling direction was prepared, and this was 845 ° C. × 30 mi.
After n quenching, tempering treatment was performed at 550 ° C. Using these as raw materials, a rotating bending fatigue test piece having a diameter of 8 mm and a rolling fatigue test piece having a diameter of 27 mm were prepared, and a 15 kHz
z was subjected to surface quenching by an induction hardening tester, followed by tempering at 180 ° C. × 1 h. In addition, the same induction hardening treatment and quenching and tempering treatment were applied to a quenched and tempered material having a diameter of 30 mm, and an impact test piece having a 2 mm notR notch was produced from the vicinity of the surface.

Further, S manufactured by the same process as described above
Test pieces similar to the above were prepared using Cr420, and these were subjected to 930 ° C. × 4 h (carbon potential 0.88) →
After carburizing treatment of quenching, tempering at 180 ° C. × 2 h was performed.

Using these samples, an impact test, a rotating bending fatigue test and a rolling fatigue test were carried out under the same conditions as in Example 1. In addition, in the state of hot rolling, using a cemented carbide tool P10, cut 2 mm, feed 0.25 mm
A cutting test was performed under the conditions of / rev and a cutting speed of 200 / min. The machinability was evaluated by the cutting time (tool life) until the flank wear reached 0.2 mm.

In the test No. of the third embodiment. 3-1 to 3-
Tables 8 and 9 show the results of the 51 tests.

[0099]

[Table 8]

[0100]

[Table 9]

Test No. in the table 3-1 to 3-12 are examples of the present invention. Both properties are higher than SCr420,
Some examples have properties equal to or higher than high-strength carburized steel. It is clear that if the two-stage hot rolling of the present invention is applied, it is possible to obtain characteristics equal to or higher than that of carburized steel by induction hardening.

Test No. 3-13 to 3-24 are cases where the conditions of the first hot rolling are out of the range of the present invention. Also,
Test No. 3-25 to 3-36 are cases where the conditions of the second-stage hot rolling are out of the range of the present invention. In each case, the fatigue strength in the C direction was significantly reduced as compared with the examples of the present invention.
In some cases, the value is lower than that of Cr420. In addition, the machinability is also reduced compared to the present invention example,
It is understood that the machinability can be improved by applying the hot rolling conditions of the present invention.

Test No. 3-37 to 3-39 specify oxide-based nonmetallic inclusions. 3-40-3
-49 is a case where the chemical composition is out of the range of the present invention. It can be seen that one or more of the characteristics are significantly reduced as compared with the example of the present invention.

As described above, by applying the present invention, the gear manufacturing process can be changed to induction hardening, which is more productive than carburized steel, which contributes to reduction of gear manufacturing costs. Is big.

[0105]

According to the present invention, the chemical composition of steel, the number and size of oxide-based nonmetallic inclusions are regulated, and the hot forging conditions in the secondary working process are regulated. High-productivity induction hardening can be applied to mechanical parts such as high-strength gears manufactured by the process, and as a result, it is possible to easily mass-produce parts having the same or better characteristics as carburized products. .

Claims (7)

[Claims] 1. C: 0.5 to 0.75% by weight, S
i: 0.5 to 1.8%, Mn: 0.4 to 1.5%, P:
0.010-0.020%, S: 0.020% or less, A
1: 0.019 to 0.05%, O: 0.0015% or less, N: 0.006 to 0.015%, the balance is composed of Fe and unavoidable impurities, and the number of oxide-based nonmetallic inclusions is 2.5 / mm ² or less and the maximum size is 19 μm
In addition to the following, a component for induction hardening characterized by being made of a steel material having an austenite grain size of 16 μm or less during induction hardening. 2. The steel material further comprises, by weight ratio, Mo: 0.05-0.5% and B: 0.0003-
0.005%, Ti: 0.005 to 0.05%, Ni:
The component for induction hardening according to claim 1, wherein the component contains at least one of 0.1 to 1.0%. 3. The composition according to claim 1, wherein said steel material further comprises, by weight, V: 0.05 to 0.5% and Nb: 0.01 to 0.5.
2. The component for induction hardening according to claim 1, wherein the component contains at least one of the following components. 4. The steel material further comprises, by weight ratio, Mo: 0.05-0.5%, B: 0.0003-
0.005%, Ti: 0.005 to 0.05%, Ni:
0.1-1.0% or more and V: 0.05-0.5
% And Nb: at least one of 0.01 to 0.5%. 5. When the steel material according to claim 1 is hot-rolled from a slab, the slab is heated to a temperature range of 1100 to 1250 ° C. and 1000 ° C.
A method for producing a component for induction hardening, wherein rolling is completed in the above temperature range. 6. When hot rolling the steel material according to any one of claims 1 to 4 from a slab, the slab is heated to a temperature range of 1100 to 1250 ° C. and is heated to a temperature range of 1000 ° C. or more. The first stage of rolling is completed and an additional 1050
A method for producing a component for induction hardening, comprising heating to a temperature range of about 1150 ° C. and finishing the second-stage rolling in a temperature range of 1000 ° C. or more. 7. When hot rolling a steel material from the slab,
The method for producing a component for induction hardening according to claim 5 or 6, wherein rolling is performed so that a cross-sectional reduction rate becomes 95% or more.