JP3503289B2 - Steel for induction hardening - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material for induction hardening which is used as a material for parts to be induction hardened, and in particular, a steel material for induction hardening which is suitable for use as a material for gears. Regarding

[0002]

Gears used in automobiles and industrial machines are
Usually, carburizing alloy steel containing about 0.2% carbon is used as a material, and this material is subjected to forging, turning, gear cutting, etc. to form a gear with a predetermined shape, and then carburizing and tempering treatment. Is manufactured by securing the necessary function as a gear. Gear manufacturing methods that undergo such a carburizing process have become the mainstream of gear manufacturing methods.
However, since the carburizing process requires heat treatment for several hours at a temperature in the range of 800 ° C. to 950 ° C., it is difficult to inline the carburizing process in the gear manufacturing line. Therefore, there is a limit to the improvement of gear productivity, and as a result, there is a limit to the reduction of manufacturing cost.

As the carburizing method, a gas carburizing method is usually adopted. However, during the gas carburizing, an abnormal surface layer is inevitably formed on the surface layer of the material to be treated, and the abnormal surface layer is treated. It reduces the fatigue strength and impact properties of the material. Therefore, there is a limit to the improvement of fatigue strength and impact characteristics in gas carburizing.
Further, heat treatment strain is generated during carburizing and quenching, and the material to be treated is deformed, so that strict control of heat treatment conditions is required.

In order to overcome the above problems, Si,
High strength carburizing steel that reduces the Mn and Cr content and uses Mo, Ni and other steel materials to reduce the abnormal surface layer that occurs during gas carburization and to improve the fatigue strength and impact characteristics of the material to be treated. Is being developed. However, since this high-strength carburizing steel contains a large amount of expensive alloy elements such as Mo and Ni, it causes an increase in steel material cost and deteriorates machinability such as machinability. As a result, although the strength of the material to be treated can be increased, there is a problem that the manufacturing cost is increased.

In addition, JIS standards SCM435 and S55C
It has been attempted to manufacture gears by induction hardening, which has a higher production efficiency than the carburizing and quenching process, using alloy steel for machine structural use and carbon steel. However, steels such as SCM435 and S55C are not originally chemical compositions determined in consideration of application to gears. Therefore, it is difficult to apply gears manufactured by induction hardening using these steels as high-strength gears used for automobile transmissions and differentials, and for relatively low-strength gears. It remains.

As a steel for solving these problems, for example, Japanese Unexamined Patent Publication (Kokai) No. 60-169544 discloses a steel which can be applied to gear manufacturing by an induction hardening process by limiting the chemical composition to a specific range. Has been done.

[0007]

However, according to the studies by the present inventors, the machinability of the steel disclosed in the above publication is extremely lower than that of the conventional carburizing steel. Therefore, the production efficiency in the cutting process, which is an essential process in the manufacturing process, is low, and there is a limit to the improvement in productivity by changing the process from carburizing and induction hardening to induction hardening.

In view of the above circumstances, the present invention has an object to provide a steel material for induction hardening which is excellent in machinability and can secure characteristics equal to or better than those of, for example, a gear manufactured through a carburizing process. And

[0009]

Means for Solving the Problems The induction hardening steel material of the present invention for achieving the above object is C: 0.5 mass%.
Above 0.75 mass%, Si: 0.5 mass%
Or more 1.8 mass% or less, Mn: 0.1 mass% or more and 0.4 mass% or less, P: 0.015 mass% or less, S: 0.020 mass% or less, Al: 0.019
mass% or more and 0.05 mass% or less, O: 0.00
15 mass% or less, N: 0.003 mass% or more and 0.015 mass% or less, and the balance is Fe and inevitable impurities.

Here, in the above-mentioned induction hardening steel material of the present invention, Mo: 0.05 mass% or more and 0.5 mass% or more.
Hereinafter, B: 0.0003 mass% or more and 0.005 ma
ss% or less, Ti: 0.005 mass% or more 0.05
mass% or less, Ni: 0.1 mass% or more, 1.0 m
It is preferable to contain at least one element out of four elements consisting of not more than ass%.

Further, the above-mentioned steel material for induction hardening of the present invention contains two elements of V: 0.05 mass% or more and 0.5 mass% or less and Nb: 0.01 mass% or more and 0.5 mass% or less. It is preferable to contain one or more of these elements. In addition, in the above-described steel material for induction hardening of the present invention, Mo: 0.05 mass% or more and 0.5 ma
ss% or less, B: 0.0003 mass% or more and 0.00
One of four elements consisting of 5 mass% or less, Ti: 0.005 mass% or more and 0.05 mass% or less, and Ni: 0.1 mass% or more and 1.0 mass% or less.
Contains at least one element, and V: 0.05 mass
% Or more and 0.5 mass% or less, Nb: 0.01 mass
% Or more and 0.5 mass% or less, it is preferable to contain at least one element out of two elements consisting of.

Further, it is preferable that the number of contained oxide-based non-metallic inclusions is 2.5 or less / mm ² and the maximum size thereof is 19 μm or less. here,
As a method for checking the number, a steel material is rolled into a steel bar having a diameter of 30 to 65 mm, and a sample is taken so that a surface perpendicular to the rolling direction becomes a surface to be inspected. At this time, the surface to be inspected of the sample must be sampled so that the center portion and the surface portion of the steel bar enter. This sample was ground, and 300 mm ² centered on the 1/4 diameter part with an optical microscope having a magnification of 400 times or more.
The method is performed by observing the above region and counting the number of non-metallic inclusions such as oxides.

In addition, the method for examining the size is the same as the above-described method for examining the number, the sampling method, and the test area, but the major axis and minor axis are measured when observing non-metallic oxide inclusions. The area of the metal inclusion is calculated as an ellipse, the diameter of a circle having the same size as the area is calculated, and this diameter is used as the size of the non-metal inclusion.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION In order to achieve the above object, the present inventors have studied the chemical composition of a steel material capable of ensuring the characteristics required for gears in an induction hardening process, and obtained the following findings. It was Gears are required to have root strength, tooth surface strength, and impact characteristics.

First, the root strength will be described. The root strength means the maximum stress that does not cause fatigue fracture from the root even if the tooth is repeatedly subjected to stress. It is known that this root strength has a good correlation with the fatigue strength of a fatigue test such as rotary bending, and the present inventors examined the chemical composition of the steel material by a rotary bending fatigue test.

The basic factors affecting fatigue strength are
It is the hardness of the material and non-metallic inclusions. When the material hardness decreases, the fatigue strength also decreases. In order to secure the hardness almost the same as the hardness of the carburized and quenched material by induction hardening, it is necessary to set the hardness to about 0.
C of about 5 mass% or more is required. Further, in order to improve the fatigue strength, it is effective not only to increase the material hardness but also to reduce the austenite grain size. There are two reasons for this, and one is that fatigue cracks grow along the prior austenite grain size, and therefore, by making the austenite grain size finer, resistance to fatigue crack propagation increases. The other is that the concentration of an element such as P that segregates at the grain boundaries and embrittles the grain boundaries decreases due to grain refinement. Induction hardening is a treatment for rapidly heating a material to be treated in a short time, and thus is extremely effective for austenite grain refinement. Furthermore, by adding N, Al, or the like, which forms a precipitate that suppresses the growth of austenite grains, to the steel material, the austenite grains become finer and the fatigue strength improves. Further, from the viewpoint of ensuring hardenability in order to obtain the material hardness, it is necessary to add an alloy element, which may be added in an appropriate amount according to the size of the gear.

In order to improve the fatigue strength, it is not enough to secure the material hardness as described above, and it is also necessary to reduce the non-metallic inclusions. The reason for this is that if non-metallic inclusions are present even though the material hardness can be ensured, fatigue fracture occurs from the portion where the non-metallic inclusions exist, and therefore the fatigue strength is extremely reduced.
In particular, hard nonmetallic inclusions such as alumina are harmful, and reduction of O (oxygen element) is essential to reduce such nonmetallic inclusions. According to the study by the present inventor, it was found that setting O to 0.0015 mass% or less is indispensable for securing the fatigue strength comparable to that of the carburized material.

Further, as a result of the study by the present inventors, in order to secure the fatigue strength exceeding the fatigue strength of the conventional carburized material by induction hardening, oxide-based non-metallic inclusions in the induction hardening steel material. It has been found necessary to limit the number and size of the. If the oxide-based nonmetallic inclusions are present, as described above, the fatigue fracture progresses starting from the oxide-based nonmetallic inclusions. As the oxide-based non-metallic inclusions are larger, the degree of concentration of stress generated in the inclusions becomes more remarkable, and fatigue early cracking easily occurs. The initial fatigue crack is more remarkable as the oxide-based nonmetallic inclusions are larger and the stress concentration is higher. Therefore, once a large initial crack has occurred, the fatigue crack rapidly propagates and leads to fatigue failure. According to the study by the present inventor, in order to secure the fatigue strength significantly exceeding the fatigue strength of the conventional carburized and hardened material by induction hardening, there is no oxide-based nonmetallic inclusion having a size of more than 19 μm. Turned out to be necessary. Furthermore, as a result of studying the influence of the number of oxide-based nonmetallic inclusions, even if the size of the oxide-based nonmetallic inclusions is 19 μm or less, if the number exceeds 2.5 / mm ² ,
It was found that it is difficult to obtain fatigue strength that greatly exceeds the fatigue strength of conventional carburized and quenched materials. The reason for this is that when the oxide nonmetallic inclusions are small, the fatigue initial cracks generated from the oxide nonmetallic inclusions are small, but when this fatigue initial crack grows, the surrounding oxide nonmetallic inclusions grow. This is because it merges with the generated fatigue crack to form a large fatigue crack, and then the fatigue crack grows rapidly, leading to fatigue failure in a short time. As described above, in order to secure even better fatigue strength, it is essential to control not only the amount of O but also the number and size of oxide-based nonmetallic inclusions.

Next, the tooth surface strength will be described. Fatigue damage called pitching occurs on the tooth surface due to repeated contact stress. When pitting occurs, it is difficult for the gear to perform its normal function, and therefore tooth surface strength is required. This tooth surface strength has a good correlation with the rolling fatigue test, and the tooth surface strength can be evaluated by this test.
However, when the gears are meshed with each other and rotate, relative slippage occurs in the tooth flanks, causing friction, and this friction causes the temperature of the tooth flanks to rise significantly. Due to this temperature rise, the tooth surface portion is softened, and pitching easily occurs. In order to suppress this softening, S that increases the temper softening resistance of the steel is added.
The addition of i, Mo, V, Nb, etc. is effective, and the addition of these elements can suppress the softening of the tooth surface portion and increase the tooth surface strength.

Next, the impact characteristics will be described. When an impact load is applied to the tooth surface, if the impact characteristics of the steel material are low, the teeth are cut off from the root portion, and not only the gear but also the entire machine in which the gear is incorporated is difficult to recover. Therefore, the impact characteristics are extremely important characteristics. As the factor that affects the impact characteristics, the influence of the amount of C is the largest. The C concentration of the portion carburized through the carburizing process is about 0.
Although it is about 8 mass%, the amount of C required to obtain equivalent hardness by induction hardening is 0.5 to 0.7 ma.
It is about ss%. Therefore, from the viewpoint of securing the impact characteristics, induction hardening is more advantageous than carburizing and hardening.
Further, not only the amount of C but also the austenite grain size at the time of induction hardening and the impurity elements such as P segregated at the grain boundaries are factors affecting the impact characteristics. Reduction of impurity elements such as P and P is also effective from the viewpoint of improving impact characteristics.

By the way, it is not enough to manufacture gears by induction hardening if only the above-mentioned characteristics required for gears are secured, and it is important to secure machinability, especially machinability. Is. The reason for this is that in the case of carburizing process, low C steel is used as the material, so the material has a relatively high machinability before carburizing and quenching, while in the case of induction hardening process. This is because a material having a C concentration higher than that of carburized steel is used as the material. Therefore, the material used for induction hardening is extremely disadvantageous in terms of ensuring machinability as compared with the material used for carburizing and quenching.

Therefore, the present inventors have studied the various factors affecting the machinability of high C steel, and have obtained the following findings. In steels with 0.5 mass% C or more,
When the content of free-cutting elements is constant, the factor that most affects machinability is the microstructure of steel. In particular, experiments have shown that the amount of ferrite and the morphology of pearlite have the most significant effect. The results of this experiment are shown in FIG.

This experiment was conducted using a cemented carbide tool P10. In the case of high C steel, the microstructure is a ferrite-pearlite structure, but as shown in FIG. 1, the machinability improves as the amount of ferrite increases. The reason for this is that the hardness of the steel material decreases as the amount of ferrite increases, and that the ferrite / pearlite interface, which is the crack generation part during cutting, increases because the amount of ferrite increases. . Further, as shown in FIG. 1, the form of pearlite also has a great influence on machinability. That is, in the case where the pearlite lamella has a well-developed structure in a layered manner, the ductility of the pearlite part is high, and the cracked part during cutting is limited to the ferrite / pearlite interface. on the other hand,
In the case of a structure in which pearlite lamella is not developed, cracks easily occur not only at the ferrite / pearlite interface but also at the cementite / ferrite interface in pearlite at the portion that undergoes deformation during cutting, which results in the development of pearlite lamella. Machinability is dramatically improved in an unstructured structure. In order to form such an undeveloped pearlite, it is necessary to select and optimize the alloying elements in the steel, and the reduction of Mn and Cr that lowers the transformation point and promotes the layering of the pearlite lamella is extremely reduced. It is effective. Further, the addition of Mo suppresses the layering of pearlite lamella and forms a structure in which cementite is divided, which is effective in improving machinability.

The reasons for limiting the elements contained in the steel material for induction hardening of the present invention will be described below. C: C is an essential component for obtaining the same surface hardness as conventional carburized steel even if induction hardening is performed, and it is necessary to add at least 0.5 mass% or more. But,
If added in excess of 0.75mass%, the impact properties and machinability required for gears will deteriorate, so 0.75mass
s% or less is added.

Si: Si is an element that improves the temper softening resistance and improves the tooth surface strength, but at least 0.5 mass% or more is required to secure the same tooth surface strength as that of the gear by the conventional carburizing process. Is required. On the other hand, if added in excess of 1.8 mass%, the solid solution hardening of the ferrite will increase the hardness and reduce the machinability, so the content should be 1.8 mass% or less.

Mn: Mn is an essential component for improving the hardenability and ensuring the hardening depth during induction hardening, and is positively added, but if it is less than 0.1 mass%, its effect is poor. , 0.4 mass% is added, the layering of pearlite is promoted and the machinability is deteriorated.
It is added at a mass% or less. P: P segregates at the grain boundaries of austenite and not only lowers the root strength by lowering the grain boundary strength, but also lowers the impact properties, so it is desirable to reduce the content as much as possible. Allowed up to 015 mass%.

S: S forms MnS, and this MnS becomes the starting point of fatigue fracture and reduces the fatigue strength. On the other hand, M
Since nS is also an element that improves machinability, it is 0.020.
Addition of less than mass% is allowed. O: O forms non-metallic inclusions such as alumina, and these non-metallic inclusions become the starting point of fatigue fracture and reduce the root strength. In addition, non-metallic inclusions also lower the tooth surface strength and should be reduced as much as possible, but 0.0015 mass% is allowed.

Al: Al is an element effective for deoxidation and a useful element for reducing oxygen. Further, it combines with N to form AlN, and this AlN suppresses the growth of austenite grains during high frequency heating, so that the impact characteristics and the root fatigue strength are improved. For this reason, it is added actively,
If the addition amount is less than 0.019 mass%, the effect is small, and if the addition amount is more than 0.05 mass%, the effect is saturated, so 0.019 mass% or more and 0.05 mass% or more.
s% or less is added.

N: N combines with Al to form AlN,
This AlN suppresses the growth of austenite grains during high frequency heating, and thus improves impact properties and fatigue strength. Therefore, it is positively added, but its effect is small if it is added less than 0.003 mass%, and if it is added more than 0.015 mass%, the hot deformability is lowered and the surface defects of the slab are remarkably increased during continuous casting. 0.003m because it increases
The amount of addition is not less than ass% and not less than 0.015 mass%.

In addition to the above chemical composition, in the steel for induction hardening of the present invention, Mo is contained in an amount of 0.05 mass% or more and 0.1.
5 mass% or less, B is 0.0003 mass% or more and 0.005 mass% or less, and Ti is 0.005 mass%
% To 0.05 mass% and Ni to 0.1 mass
% Or more and 1.0 mass% or less is preferable. The reason for this is that these elements have the following effects.

Mo: An element useful for improving the hardenability and used for adjusting the hardenability. Addition of Mo significantly affects the morphology of pearlite, and addition of Mo forms pearlite in which cementite is fragmented. As a result, machinability is significantly improved. Further, Mo improves the temper softening resistance, and therefore the tooth surface strength can also be improved. Further, Mo reduces the impurity elements such as P segregated at the grain boundaries, and therefore has the effect of improving the root strength and impact characteristics. Since Mo is a suitable element for this purpose, it is positively added. However, if less than 0.05 mass% is added, the effect is small, and if more than 0.5 mass% is added, carbides that are difficult to dissolve in austenite are formed by rapid rapid heating such as induction hardening. The addition is 0.5 mass% or less.

B: B is an element that improves the hardenability by adding a trace amount, so that other alloying elements can be reduced. B is an element that segregates preferentially at grain boundaries and reduces the concentration of P segregated at grain boundaries, thereby significantly improving the root strength and impact characteristics. 0.0003m for this
It is necessary to add more than ass%, but 0.005mass
Even if added over s%, its effect will be saturated, so 0.0
The amount added is not more than 05 mass%.

The effect of improving the hardenability of Ti: B is remarkable when B is present alone. On the other hand, B is an element that easily bonds with N. In this case, the above-mentioned preferable B is preferable. The effect disappears. The effect of improving the hardenability of B can be sufficiently exhibited by adding Ti, which is more easily bonded to N than B. Therefore, Ti may be used in such a case. If the addition amount is less than 0.005 mass%, the effect is small, and if the addition amount exceeds 0.05 mass%, Ti
Since a large amount of N is formed and this TiN becomes the starting point of fatigue fracture and reduces the root strength and tooth surface strength, it is 0.05 m.
The addition is less than ass%.

Further, since TiN has a function of refining austenite grains during high frequency heating, addition of Ti alone has a function of improving tooth surface strength and fatigue strength.
Also in this case, the addition amount is preferably in the range of 0.005 mass% or more and 0.05 mass% or less. Ni: addition of Ni not only improves hardenability, but also
Since the impact properties are improved, it may be used when adjusting the hardenability or when the impact properties need to be improved.
However, if the addition amount is less than 0.1 mass%, the effect is small, so the addition amount is set to 0.1 mass% or more. On the other hand, N
Since i is an extremely expensive element, the addition of more than 1.0 mass% increases the cost of the steel material, which is against the purpose of the present invention, so i is added at 1.0 mass% or less.

Further, in the steel material for induction hardening of the present invention, it is preferable that V is contained in an amount of 0.05 mass% or more and 0.5 mass% or less and Nb is contained in an amount of 0.01 mass% or more and 0.5 mass% or less. The reason for this is that these elements have the following effects. When an induction hardening process is performed, it is common to perform a quenching and tempering process as a pretreatment in order to secure the hardness of the central portion of the material to be processed. However, this heat treatment increases costs, so
It is desirable to omit this heat treatment as much as possible. In order to omit the quenching and tempering as a pretreatment, it is necessary to increase the material hardness before induction hardening, but for that purpose, addition of V or Nb having a precipitation strengthening effect is effective. .

V: V is an element having an extremely strong precipitation strengthening action, so it is added when it is necessary to omit the quenching and tempering treatment as a pretreatment before induction hardening. However, 0.
If the addition amount is less than 05 mass%, the effect is small, and if the addition amount is more than 0.5 mass%, the effect is saturated. Therefore, the addition amount is 0.05 mass% or more and 0.5 mass% or less. Further, since the addition of V improves the temper softening resistance of the steel material, V is also extremely effective in improving the tooth surface strength.

Nb: Nb is an element having an extremely strong precipitation strengthening action, so it is added when it is necessary to omit the quenching and tempering treatment as a preheat treatment before induction hardening. However, if the addition amount is less than 0.01 mass%, the effect is small, and if the addition amount is more than 0.5 mass%, the effect is saturated.
% Or less. Further, addition of Nb improves the temper softening resistance of the steel material, so Nb is also extremely effective in improving the tooth surface strength.

In the present invention, the amount and size of oxide-based nonmetallic inclusions are specified to be 2.5 pieces / mm ² or less and 19 μm or less, respectively, in order to ensure the fatigue strength. Number of oxide-based non-metallic inclusions is 2.5 /
The reason why the size is less than or equal to mm ² is that if there are more oxides than this number, the fatigue cracks generated from the respective non-metallic inclusions propagate and fatigue fracture occurs, and as a result, the target fatigue strength is secured. Because it becomes difficult. The reason for defining the size as 19 μm or less is that when non-metallic inclusions exceeding this size are present, the initial cracks generated from these non-metallic inclusions increase, and as a result, fatigue cracks rapidly progress and This is because fatigue failure occurs in the.

In order to control the amount and size of the above-mentioned oxide-based non-metallic inclusions to a target value or less, it is necessary to reduce O that forms oxide-based non-metallic inclusions such as alumina. To this end, at least 0.0015 mass%
It is necessary to reduce O below. Therefore, in the present invention, as described above, the amount of O is 0.0015mass.
The content is limited to s% or less.

[0040]

EXAMPLE A first example of the steel material for induction hardening of the present invention will be described below together with a comparative example. Steels having chemical compositions shown in Table 1 (excluding No. 21) were melted by a converter-continuous casting process.

[0041]

[Table 1]

A slab having the chemical composition shown in Table 1 was rolled into a 150 mm square billet through a breakdown process and then rolled into a steel bar having a diameter of 50 mm. This steel bar having a diameter of 50 mm was hot forged into a steel bar having a diameter of 30 mm. This diameter 3
A 0 mm steel bar was held at a temperature of 845 ° C. for 30 minutes for quenching and then tempered at 550 ° C. Using a steel bar that has been subjected to this quenching and tempering treatment as a raw material, a rotary bending fatigue test piece with a diameter of 8 mm and a rolling fatigue test piece with a diameter of 27 mm were prepared, and the prepared test piece was surface-treated by an induction hardening tester at 15 kHz. After quenching, tempering treatment was performed at 180 ° C. for 1 hour. In addition, the above-mentioned material was subjected to the same induction hardening and tempering treatment as above, and 2 mm10 from the surface vicinity
R-notch impact test pieces were prepared.

Further, in Table 1, No. SCr 21
420 standard steel is melted by converter-continuous casting process,
A steel bar having a diameter of 50 mm was rolled through a process similar to the above, and then hot forged into a steel bar having a diameter of 30 mm. A test piece similar to the one described above was produced using this steel bar, and this test piece was subjected to carburizing treatment (carbon potential 0.88) at 930 ° C. for 4 hours, followed by quenching after carburizing and further 180 ° C.
After tempering for 2 hours, No. 2 in Table 2 described later. There were 21 test pieces.

The impact test was conducted by using a Charpy impact tester under the condition of + 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. The rolling fatigue test uses a test piece with a diameter of 130 mm.
The roller was pressed to give a contact stress of 3677 MPa, and the life was evaluated by the time until pitting occurred on the surface.

Further, in the as-hot-forged state, the cemented carbide tool P1
Using No. 0, a cutting test was performed under the conditions of a cut of 2 mm, a feed of 0.25 mm / rotation, and a cutting speed of 200 mm / min. The machinability was evaluated by the cutting time until the flank wear reaches 0.2 mm. The results are shown in Table 2. No. of Table 1 And No. 2 in Table 2. Corresponds to.

[0046]

[Table 2]

Steel No. 1 to No. Up to 12 are steel materials for induction hardening according to the present invention. 21 is a conventional carburized steel. No. 11 ', No. 12 'is also an example of the present invention. This is an example in which quenching and tempering before induction hardening are omitted using the steels 11 and 12. No. 13 is Si
Is a comparative example below the range of the present invention, and the rolling fatigue life is extremely reduced.

No. No. 14 is a comparative example in which Mn exceeds the range of the present invention, and the machinability is significantly deteriorated. No. 15
Is a comparative example in which P exceeds the upper limit of the present invention, the impact value and fatigue strength are reduced, and No. Its properties are inferior to those of No. 21 conventional steel. No. No. 16 is a comparative example in which S exceeds the upper limit of the present invention, and the fatigue strength is No. It is inferior to 21 conventional steels.

No. No. 17 is a comparative example in which Al is lower than that of the present invention. As a result, O is increased and fatigue strength and rolling contact fatigue life are both extremely decreased. It is below the value of 21 conventional steel. No. No. 18 is a comparative example in which C exceeds the upper limit of the present invention, and the impact value and machinability are No. It is much lower than the conventional steel of No. 21.

No. No. 19 is a comparative example in which N falls below the range of the present invention, and the impact value is No. It is inferior to 21 conventional steels. No. No. 20 is a comparative example in which C is less than the present invention, and the fatigue strength and rolling fatigue life are No. Two
1 Inferior to conventional steel. As described above, in the comparative example of the steel outside the scope of the present invention, any one of the characteristics is N
o. 21 is lower than the conventional steel,
In the case of the steel of the present invention, all properties have No. The value is equal to or more than that of the conventional steel of No. 21. As a result, by using the steel material for induction hardening of the present invention, for example, it is possible to change the manufacturing process of gears to induction hardening, which has higher productivity than conventional carburizing and quenching. It can greatly contribute to cost reduction.

Next, a second embodiment of the present invention will be described. Here, the results of various experiments are shown using a steel material in which not only the chemical composition but also the amount and size of oxide-based nonmetallic inclusions are limited. Steels having chemical compositions shown in Tables 3 and 4 (except No. 54 in Table 4) were melted by a converter-continuous casting process.

[0052]

[Table 3]

[0053]

[Table 4]

The slab size at the time of casting is No. 34 to N
o. Up to 45 is 300 x 400 mm, others are 5
It was 60 × 400 mm. After rolling this slab into a 150 mm square billet through a breakdown process,
Rolled to 0 mm steel bar. This steel bar having a diameter of 50 mm was hot cast into a steel bar having a diameter of 30 mm. This diameter is 30mm
Of the steel bar was held at a temperature of 845 ° C. for 30 minutes for quenching, and then tempered at 550 ° C. Using the steel bar that has been subjected to this quenching and tempering treatment as a material, a rotating bending fatigue test piece with a diameter of 8 mm and a rolling fatigue test piece with a diameter of 27 mm were prepared, and the prepared test piece was surface-quenched by an induction hardening tester at 15 kHz. Then, tempering treatment was performed at 180 ° C. for 1 hour. In addition, the above-mentioned steel bar with a diameter of 30 mm was subjected to the same induction hardening and tempering treatment as above, and 2 m from the vicinity of this surface.
A m10R notch impact test was made.

In Table 4, No. SCr 54
Steel corresponding to the standard of 420 was melted by a converter-continuous casting process, rolled into a steel bar with a diameter of 50 mm through the same process as described above, and then hot cast into a steel bar with a diameter of 30 mm.
Using the steel bars, test pieces similar to the above were prepared, and these test pieces were subjected to carburizing treatment (carbon potential 0.88%) at 930 ° C. for 4 hours, then quenched after carburizing to 180 ° C.
And tempered for 2 hours.

The impact test was conducted by using a Charpy impact tester under the condition of + 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. The rolling fatigue test uses a test piece with a diameter of 130 mm.
The roller was pressed to give a contact stress of 3677 MPa, and the life was evaluated by the time until pitting occurred on the surface.

Further, in the as-hot-forged state, the cemented carbide tool P1
Using No. 0, a cutting test was performed under the conditions of a cut of 2 mm, a feed of 0.25 mm / rotation, and a cutting speed of 200 mm / min. The machinability was evaluated by the cutting time until the flank wear reaches 0.2 mm. The results are shown in Table 5.

[0058]

[Table 5]

Steel No. 22 to No. No. 33 to the fifth invention, No. 34 to No. Up to 45, the first to fourth invention examples,
No. 46 to 53 are comparative examples, No. 54 is steel equivalent to JIS SCr420 which is often used as carburized steel. In addition, No. 55 is a high strength carburized steel which is an improved version of JIS steel. No. 34 to No. Up to 45, the chemical composition other than the number or size of the oxide-based nonmetallic inclusions is No. 45. 22
33 to 33 are the same as the first to fourth invention examples. JIS
It has almost the same characteristics as SCr420 steel,
It is inferior to the fifth invention in terms of fatigue strength and rolling fatigue life.

No. No. 46 is a comparative example in which Si is below the range of the present invention, and the rolling fatigue life is extremely reduced.
No. No. 47 is a comparative example in which Mn exceeds the range of the present invention, and the machinability is significantly deteriorated. No. No. 48 is a comparative example in which P exceeds the upper limit of the present invention, the impact value and the fatigue strength are remarkably lowered, and its characteristics are inferior to those of SCr420.

No. No. 49 is a comparative example in which S exceeds the upper limit of the present invention, and the fatigue strength is inferior to that of SCr420. No. No. 50 is a comparative example in which Al is lower than that of the present invention, and as a result, O is increased, and both fatigue strength and rolling contact fatigue life are extremely reduced, which is below SCr420.

No. No. 51 is a comparative example in which C exceeds the upper limit of the present invention, and the impact value and machinability are extremely lower than those of the conventional steel. No. No. 52 is a comparative example in which N is below the range of the present invention, and the impact value is inferior to that of SCr420. No. No. 53 is a comparative example in which C is below the range of the present invention, and the fatigue strength and rolling fatigue life are SCr.
Inferior to 420.

That is, in the case of steel outside the scope of the present invention, one of the various characteristics has a value lower than that of SCr420, whereas in the case of the steel of the present invention, all the characteristics are the same as those of conventional carburizing. It is superior to the steel SCr420, and in the case of the fifth invention steel, it is possible to secure the fatigue strength, rolling fatigue life, and machinability that are substantially equal to or higher than those of the high-strength carburizing steel. In addition, No. As in No. 28, impact characteristics can be improved by adding Ni. By using the steel material for induction hardening of the present invention, it is possible to change the gear manufacturing process to induction hardening, which has higher productivity than carburized steel,
It greatly contributes to the reduction of gear manufacturing costs.

[0064]

As described above, the steel material for induction hardening of the present invention has excellent machinability, and a gear using the steel material for induction hardening of the present invention is a gear manufactured through a carburizing process. It is possible to secure characteristics equal to or higher than the characteristics of.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a ferrite area ratio and a tool life.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinichi Amano 1-chome, Mizushima Kawasaki-dori, Kurashiki City (no street address) Kawasaki Steel Co., Ltd. Mizushima Steel Works (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (5)

(57) [Claims] 1. C: 0.5 mass% or more and 0.75 ma
ss% or less, Si: 0.5 mass% or more and 1.8 mass% or less, Mn: 0.1 mass% or more and 0.4 mass% or less, P: 0.015 mass% or less, S: 0.020 mass% or less, Al: 0. 019 mass% or more and 0.05 mass% or less, O: 0.0015 mass% or less, N: 0.003 mass% or more and 0.015 mass% or less, and the balance is Fe and inevitable impurities Required steel material. 2. Mo: 0.05 mass% or more and 0.5 mass% or less, B: 0.0003 mass% or more and 0.005 mass%
Hereinafter, it is characterized by containing at least one element out of four elements consisting of Ti: 0.005 mass% or more and 0.05 mass% or less and Ni: 0.1 mass% or more and 1.0 mass% or less. The steel material for induction hardening according to claim 1. 3. V: 0.05 mass% or more and 0.5 mass% or less, Nb: 0.01 mass% or more and 0.5 mass% or less,
The steel material for induction hardening according to claim 1, containing at least one element out of the two elements consisting of. 4. Mo: 0.05 mass% or more and 0.5 mass% or less, B: 0.0003 mass% or more and 0.005 mass%
Hereinafter, one or more elements out of four elements of Ti: 0.005 mass% or more and 0.05 mass% or less and Ni: 0.1 mass% or more and 1.0 mass% or less are contained, and V: 0.05 mass% or more and 0.5 mass% or less, Nb: 0.01 mass% or more and 0.5 mass% or less,
The steel material for induction hardening according to claim 1, containing at least one element out of the two elements consisting of. 5. An oxide-based nonmetallic inclusion is contained, the number of said oxide-based nonmetallic inclusion is 2.5 pieces / mm ² or less, and the maximum size is 19 μm or less. The steel material for induction hardening according to claim 1, 2, 3, or 4.