JP3428282B2 - Gear steel for induction hardening and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material for gears manufactured by induction hardening and a method of manufacturing the steel material, and is intended to improve machinability and fatigue strength without increasing the cost. is there.

[0002]

2. Description of the Related Art Gears used in automobiles and industrial machines are 0.
Manufactured by using an alloy steel for carburizing containing about 2 mass% of carbon to forge → cutting → turning → gear cutting into a predetermined shape, and then carburizing and tempering to give the necessary function as a gear. ing. Manufacturing by such a carburizing process is a mainstream process for gear manufacturing, but carburizing requires processing at a temperature of about 800 to 950 ° C for several hours, so it can be inlined in the gear manufacturing process. Therefore, there is a limit to improving productivity and thus reducing manufacturing cost.

Further, although carburizing is generally carried out by a gas carburizing method, an abnormal surface layer is inevitably formed on the surface of the material to be treated during gas carburizing, and this abnormal layer deteriorates fatigue strength and impact characteristics. Therefore, improvement in fatigue strength and impact properties is limited. Furthermore, because the heat treatment strain generated during carburizing and quenching causes deformation of the material to be treated,
It also left a problem where strict control of heat treatment conditions was required.

In order to overcome the above-mentioned problems, on the premise of the carburizing process, Si, Mn, Cr in the steel material is reduced to reduce Mo,
A high-strength carburizing steel was developed with the intention of improving the fatigue strength and impact characteristics by reducing the abnormal surface layer generated during gas carburization by adding Ni, etc., but this steel contains a large amount of expensive alloy elements. In addition to increasing the cost of the steel material, the problem remains that the machinability such as machinability deteriorates.

[0005] Further, by using alloys for machine structures such as JIS standards SCM435 and S55C and carbon steel, it has been attempted to manufacture gears by induction hardening, which has a higher production efficiency than the carburizing and hardening process, but these steel types are originally Since the chemical composition was not determined in consideration of application to gears, it is difficult to apply to high-strength gears used for automobile transmissions and differentials, such as gears manufactured by a carburizing process, and therefore relatively low. It is only applicable to high strength gears.

In order to solve these problems, for example, Japanese Patent Application Laid-Open No. 60-169544 discloses that the chemical composition is regulated within a specific range so that it can be applied to the production of gears by an induction hardening process. Steel materials have been proposed. However, according to the study by the inventors, the chemical composition disclosed in the above publication has an extremely low machinability as compared with the conventional carburizing steel, and therefore the cutting step which is an essential process in the manufacturing process. There was a limit to the improvement of productivity by changing the process from carburizing and induction hardening to induction hardening.

[0007]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned conventional problems, and it goes without saying that it is excellent in machinability and also has various characteristics required as a gear. The purpose of the present invention is to propose, together with its advantageous manufacturing method, a mechanical structural steel suitable for manufacturing gears by induction hardening, which can secure characteristics equal to or better than those of gears manufactured by a conventional carburizing process. And

[0008]

Means for Solving the Problems In order to achieve the above object, the inventors of the present invention have made an in-depth study on the chemical composition of a steel material for ensuring the characteristics required for gears in an induction hardening process. It was As a result, the following findings have been obtained. Gears are required to have root strength, tooth surface strength and impact characteristics. Here, the root strength is
It means the maximum stress at which the tooth part is repeatedly stressed and does not cause fatigue failure from the root part. By the way, this root strength is
Since there is a good correlation with the fatigue strength of fatigue tests such as rotary bending, the inventors examined the chemical composition of steel materials by rotary bending fatigue tests.

By the way, the basic factors that affect the fatigue strength are the hardness of the material and the non-metallic inclusions, and when the material hardness decreases, the fatigue strength also decreases. In order to secure the hardness almost equal to that of the carburized and quenched material by induction hardening, the amount of C needs to be about 0.5 mass% or more. In addition, in order to improve the fatigue strength, it is effective to make the austenite grain size finer. The reason for this is that fatigue cracks grow along the former austenite grain size, so by refining them, resistance to fatigue crack propagation increases, and segregation at grain boundaries such as P segregates. This is because the concentration of the element that makes it brittle decreases due to the fine graining.

Since induction hardening is a rapid heat treatment for a short period of time, it is extremely effective for austenite grain refinement. However, when N, Al or the like which forms a precipitate for suppressing the growth of austenite grains is added, Since the particles are made finer, the fatigue strength is further improved. Further, in order to obtain a predetermined material hardness, it is necessary to add an alloying element from the viewpoint of ensuring hardenability, but this may be added in an appropriate amount according to the size of the gear.

Further, in order to improve fatigue strength,
Ensuring material hardness is not enough, and it is necessary to reduce non-metallic inclusions as well. That is, even if the material hardness can be secured, if non-metallic inclusions are present, fatigue fracture will occur from this portion, and the fatigue strength will be significantly reduced. In particular, hard non-metallic inclusions such as alumina are harmful. For this purpose, it is essential to reduce O, but according to the study by the inventor, it is essential to set it to 0.0015 mass% or less in order to secure the fatigue strength comparable to that of the carburized material.

Further, fatigue damage called pitching occurs on the tooth surface due to repeated contact stress. When this occurs, it is difficult for the gear to perform its normal function, so tooth surface strength is required. This 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 friction occurs on the tooth surface portion, and this friction causes a significant temperature rise. However, due to this temperature rise, the steel material softens and pitching occurs. In order to suppress this, it is effective to add Si, Mo, V, Nb and the like, which increase the temper softening resistance, to the steel, and the addition of these can increase the tooth surface strength.

Further, when an impact load is applied to the tooth root, if the impact property of the steel material is low, the tooth is broken from the tooth root portion, and it is difficult to recover not only the gear but also the entire machine in which the gear is incorporated. Will be damaged. Therefore, the impact characteristics are extremely important characteristics. C is the most significant factor affecting impact properties, but the C concentration in the carburized part after the carburizing process is about 0.8 mass%, while the equivalent steel hardness by induction hardening Since C required to obtain C is about 0.5 to 0.7 mass%, it is advantageous from the viewpoint of securing impact characteristics. However, not only the factors that affect the impact properties but also the austenite grain size during induction hardening and the impurity elements such as P segregated at the grain boundaries also affect the grain size of γ grains and P grain size.
Reduction of impurity elements such as is also effective in improving impact characteristics.

Further, merely securing the characteristics required for the gear as described above is not sufficient as a method for manufacturing the gear by induction hardening, and as described above, the machinability, especially the machinability is secured. is important. Regarding this machinability, in the case of carburizing process, low C steel is used, so it has relatively high machinability before carburizing and quenching, but on the other hand, in case of induction hardening process. Since it is indispensable to increase the carbon content compared to carburized steel, it is extremely disadvantageous in terms of ensuring machinability.

Then, the present inventors have studied the various factors affecting the machinability of high C steel and, as a result, have obtained the following findings. That is, in the steel having a C content of 0.5 mass% or more, when the free-cutting element is constant, the factor that most affects machinability is its microstructure. It turned out to affect. In the case of high C steel, the microstructure has a ferrite-pearlite structure, but as ferrite increases, machinability improves. This is because not only the hardness of the steel material decreases as the amount of ferrite increases, but also the increase in ferrite increases the ferrite / pearlite interface, which is the cracking part during cutting, thus improving machinability. .

The form of pearlite also has an extremely large effect. That is, in the case of a structure in which pearlite lamella is well developed in a layered manner, the ductility of the pearlite part is high, so the cracked part during cutting is limited to the ferrite / pearlite interface, but such lamellar is developed. In the case of a non-structure, cracks will 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, resulting in a dramatic improvement in machinability. Of. In order to form such undeveloped pearlite, it is important to select and optimize the alloying elements in the steel, and especially to reduce Mn and Cr that lower the transformation point and promote the stratification of lamella. It is effective. Further, the addition of Mo suppresses the stratification of the lamella and forms the structure in which the cementite is divided, so that it is effective in improving the machinability.

Based on the above-mentioned examination results, the inventors have
As an induction hardened steel material, the steel material previously disclosed in Japanese Patent Application No. 7-125101 was developed.
It is relatively low at 0.015 mass% or less. As is well known, it is extremely difficult to reduce P in the steelmaking process.
A pretreatment for reducing P in the hot metal stage is disadvantageous.
However, performing such a pretreatment leads to an increase in cost and is insufficient from the viewpoint of gear manufacturing at the target low cost.

Therefore, the inventors have made further studies,
The conditions under which desired characteristics were obtained even when the P content was high were examined. Now, as described above, P is an element that segregates in the austenite grain boundaries and reduces the grain boundary strength, thereby reducing the impact characteristics and the fatigue strength. That is, the grain boundary segregation amount, not the addition amount, is important for these characteristics. Here, the amount of grain boundary segregation varies depending on the austenite grain size even if the addition amount is the same, and it is possible to reduce the amount of grain boundary segregation per unit volume by refining.

Then, the inventors next investigated the relationship between the grain boundary segregation amount of P and the austenite grain size. As a result, when the amount of P is more than 0.015 to 0.03 mass%, by reducing the austenite grain size to 16 μm or less, it is possible to reduce the amount of grain boundary segregation to the same level as when the amount of P is 0.015 mass% or less. found. Further, in order to reduce the austenite grain size to 16 μm or less, Al: 0.019 to
It was also found that increasing the amount of N is most effective in the range of 0.05%, and that the amount needs to exceed 150 ppm.

Further, the inventors have also examined the methods for producing those steels and have obtained the following findings. MnS formed in steel stretches in the rolling direction along with hot rolling, but the degree thereof varies depending on hot rolling conditions. When MnS is elongated, the fatigue strength of the sample taken from the direction perpendicular to this extending direction is extremely reduced. The reason for this is that the elongated MnS becomes the starting point of fatigue cracks. In an actual gear, teeth are often formed at right angles to the extension direction of MnS, so the extension of MnS may reduce the fatigue strength of the actual gear. Further, it is well known that the shape of MnS also affects machinability, and that elongation of MnS deteriorates machinability.

Therefore, in order to improve machinability and fatigue strength, it is necessary to suppress elongation of MnS during hot rolling. In order to suppress the elongation of MnS, it is known to add Ca to form spherical CaS, but the addition of Ca also causes the formation of coarse Ca-based non-metallic inclusions. As a result, the rolling fatigue life is shortened.

Then, the inventors have studied the heating temperature and the hot rolling conditions at the time of hot rolling that affect the shape of MnS, and have obtained the following findings. That is, when the heating temperature at the time of hot rolling is increased, a part of MnS is dissolved as a solid solution, and the grain size of MnS becomes smaller than that at the slab stage. When this material is hot rolled, MnS has a lower elongation than it does at lower temperatures due to grain size reduction. Further, since MnS once solid-dissolved reprecipitates relatively finely during rolling, the average degree of elongation of MnS of the steel material is remarkably suppressed as compared with the case of low temperature heating. Further, as a result of studying the shape of MnS before heating and rolling, it was found that the more MnS was elongated, the larger the elongation by subsequent rolling was.

With respect to the hot rolling conditions, it was found that the temperature range in which MnS stretches most is 900 to 1000 ° C., and the degree of stretching decreases in a region higher or lower than this temperature range. Therefore, while raising the heating temperature,
If you avoid the rolling temperature range of 900-1000 ℃,
It is possible to suppress the elongation of MnS and, by extension, improve fatigue characteristics and machinability.

The present invention was developed based on the above findings, and its gist structure is as follows. 1) C: 0.5 to 0.75 mass%, Si: 0.5 to 1.8 mass%, Mn: 0.
1-0.4mass%, P: 0.015 over-0.03mass%, S: 0.020ma
ss% or less, Al: 0.019 to 0.05 mass%, O: 0.0015 mass%
Hereinafter, N: 0.015 contains super ～0.02Mass%, remainder Ri Do the Fe and unavoidable impurities, and fatigue strength of the C-direction
A steel material for gears for induction hardening, which is characterized by excellent hardness (first invention).

2) C: 0.5 to 0.75 mass%, Si: 0.5 to 1.8 ma
ss%, Mn: 0.1-0.4mass%, P: 0.015 over-0.03mass
%, S: 0.020 mass% or less, Al: 0.019 to 0.05 mass%,
O: 0.0015 mass% or less, N: more than 0.015 to 0.02 mass%, Mo: 0.05 to 0.5 mass%, B: 0.0003 to 0.005
mass%, Ti: 0.005-0.05mass%, Ni: 0.1-1.0mass%
Contain at least one selected from among, balance Ri Do the Fe and unavoidable impurities, and fatigue strength of the C-direction
Induction hardening steel for the gear, characterized in Rukoto good time (second invention).

3) C: 0.5 to 0.75 mass%, Si: 0.5 to 1.8 ma
ss%, Mn: 0.1-0.4mass%, P: 0.015 over-0.03mass
%, S: 0.020 mass% or less, Al: 0.019 to 0.05 mass%,
O: 0.0015 mass% or less, N: more than 0.015 to 0.02 mass%, V: 0.05 to 0.5 mass%, Nb: 0.01 to 0.5 mass%
%, Containing at least one of the
And Ri Do the composition of the unavoidable impurities, and fatigue C direction
Steel for the induction hardening gears excellent wherein Rukoto strength (third invention).

4) C: 0.5 to 0.75 mass%, Si: 0.5 to 1.8 ma
ss%, Mn: 0.1-0.4mass%, P: 0.015 over-0.03mass
%, S: 0.020 mass% or less, Al: 0.019 to 0.05 mass%,
O: 0.0015 mass% or less, N: more than 0.015 to 0.02 mass%, Mo: 0.05 to 0.5 mass%, B: 0.0003 to 0.005
mass%, Ti: 0.005-0.05mass%, Ni: 0.1-1.0mass%
At least one selected from the above, and V: 0.05-
0.5 mass%, Nb: contains at least one selected from among 0.01～0.5Mass%, remainder Ri Do the Fe and unavoidable impurities, and characterized by Rukoto excellent fatigue strength of the C-direction high-frequency Steel material for hardening gears (4th invention).

5) C: 0.5 to 0.75 mass%, Si: 0.5 to 1.8 mass%, Mn: 0.1 to 0.4 mass%, P: more than 0.015 to 0.03 mass%, S: 0.020 mass% or less, Al: 0.019 to 0.05 mass%, O: 0.0015 mass% or less, N: More than 0.015 to 0.02 mass% When a steel slab having a composition containing more than 0.015 mass% is hot rolled, it is heated to a temperature of 1100 to 1250 ℃ and 1000 ℃ or more. A method for manufacturing a steel material for gears for induction hardening, which comprises terminating hot rolling at the temperature (5th invention).

6) C: 0.5 to 0.75 mass%, Si: 0.5 to 1.8 mass%, Mn: 0.1 to 0.4 mass%, P: more than 0.015 to 0.03 mass%, S: 0.020 mass% or less, Al: 0.019 to 0.05 mass of O, 0.0015 mass% or less, N: more than 0.015 to 0.02 mass% When a cast slab having a composition containing 2% hot rolling is made into a steel material, after heating to a temperature of 1100 to 1250 ° C, 1000
At the temperature above ℃, finish the first stage hot rolling,
A method for manufacturing a steel material for gears for induction hardening, which comprises heating to a temperature of ˜1150 ° C. and then finishing the second stage hot rolling at a temperature of 1000 ° C. or higher (sixth invention).

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons why the composition of steel in the present invention is limited to the above range will be described below. C: 0.5 to 0.75 mass% C is an essential component for obtaining the same surface hardness as conventional carburized steel by induction hardening, and is at least 0.5 ma.
It is necessary to add more than ss%. However, 0.75mass
%, The impact properties and machinability required for gears will deteriorate, so it is recommended to add up to 0.75 mass%.

Si: 0.5 to 1.8 mass% Si is an element effective for improving the temper softening resistance and, in turn, the tooth surface strength. In order to secure the tooth surface strength comparable to that of a gear by the conventional carburizing process. Is at least 0.5
It is necessary to add more than mass%, but if added in excess of 1.8 mass%, the solid solution hardening of ferrite increases the hardness and lowers the machinability, so it was limited to the range of 0.5 to 1.8 mass%.

Mn: 0.1 to 0.4 mass% Mn is an essential element because it improves the hardenability and increases the depth of the hardened layer during induction hardening to improve the fatigue strength and the tooth surface strength. The transformation temperature of pearlite is lowered and the layering of pearlite is promoted, which deteriorates machinability. In order to secure hardenability, it is necessary to add 0.1 mass% or more, but if added in excess of 0.4 mass%, machinability deteriorates, so the upper limit is made 0.4 mass%.

P: more than 0.015 to 0.030 mass% P segregates at the austenite grain boundaries and lowers the grain boundary strength to lower not only the root strength but also the impact characteristics. Although it is desirable to reduce the amount, if it is reduced to 0.015 mass% or less, an increase in the manufacturing cost is unavoidable, which is contrary to the object of the present invention of reducing the manufacturing cost.
It was decided to allow the inclusion of P in excess of ss%. However, even if the P content is more than 0.015 mass%, 0.030ma
Up to ss%, austenite grain size is 16μm
By controlling as follows, the grain boundary segregation amount of P is
Since it is almost the same as the case of 0.015 mass% or less and P can be made harmless, the amount of P is limited to the range of more than 0.015 to 0.030 mass%.

S: 0.020 mass% or less S forms MnS, which acts as the starting point of fatigue fracture and reduces fatigue strength. On the other hand, MnS is also an element that improves machinability, so 0.020 mass% It will be added below.

Al: 0.019 to 0.05 mass% Al is not only useful for lowering oxygen by deoxidation, but also combines with N to form AlN, which suppresses the growth of austenite grains during high frequency heating. It is positively added because it contributes effectively to the improvement of impact characteristics and tooth root fatigue strength, but if it is added in an amount of less than 0.019 mass%, its effect is poor, while if it is added in an amount of more than 0.05 mass%, its effect is saturated. Therefore, the range was limited to 0.019 to 0.05 mass%.

O: 0.0015 mass% or less O forms non-metallic inclusions such as alumina, which serves as a starting point of fatigue fracture and lowers tooth root strength, and also reduces tooth surface strength, so that it is reduced as much as possible. Desirable, but 0.
Up to mass% is allowed.

N: more than 0.015 to 0.020 mass% N combines with Al to form AlN, which suppresses the growth of austenite during high frequency heating to reduce the grain boundary segregation amount of P, impact characteristics and Although it is positively added to improve the fatigue strength, addition of 0.015 mass% or less has a small effect of securing the austenite grain size of 16 μm or less, which is the object of the present invention, while addition of more than 0.020 mass% causes blowholes. Since casting defects such as, etc. increase remarkably, it was limited to the range of more than 0.015 to 0.020 mass%.

Although the essential components have been described above, in the present invention, in addition to the above essential components, M is used as a hardenability improving component.
At least one selected from o, B, Ti and Ni can be appropriately added. The preferred contents of these components are as follows. Mo: 0.05 to 0.5 mass% Mo is an element useful for improving the hardenability and is used for adjusting the hardenability. At the same time, the addition of Mo significantly affects the structural morphology of pearlite, forming cementite-separated pearlite. 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 has the effect of improving the root strength and impact properties by reducing the impurity elements such as P segregated at the grain boundaries, and is a useful element for this purpose, so it is positively added.
However, if the addition amount is less than 0.05 mass%, its effect is poor, while if it exceeds 0.5 mass%, carbides that are difficult to dissolve in austenite are formed by rapid rapid heating such as induction hardening. , 0.05 to 0.5
It is necessary to add it in the range of mass%.

B: 0.0003 to 0.005 mass% B is an element that improves the hardenability by adding a trace amount, so that other alloying elements can be reduced. Further, B segregates preferentially at the grain boundaries, and has the effect of reducing the concentration of P segregated at the grain boundaries and improving the root strength and impact characteristics. To obtain such effect, 0.0003mass%
It is necessary to add the above, but the effect is saturated even if added in excess of 0.005 mass%, so the amount of B is 0.0003 to 0.005.
It was limited to the mass% range.

The effect of improving the hardenability of Ti: 0.005 to 0.05 mass% B is remarkable when B is present alone. On the other hand, B is an element that easily bonds with N. The effective effect disappears. Since the effect of improving the hardenability of B can be sufficiently exhibited by adding Ti which is more likely to be bonded to N than B, Ti is used in such a case. However, the content is 0.005
If the content is less than mass%, the effect is small. On the other hand, if the content exceeds 0.05 mass%, a large amount of TiN is formed. As a result, this becomes the starting point of fatigue fracture and reduces the root strength and tooth surface strength. It was supposed to be added in the range of ~ 0.05 mass%. Further, since TiN has an action of making the austenite grain size fine during high frequency heating, only the addition of Ti alone has the action of improving tooth surface strength and fatigue strength. Also in this case, the addition amount is preferably in the range of 0.005 to 0.05 mass%.

Ni: 0.1 to 1.0 mass% Ni has the effect of improving not only hardenability but also impact properties by the addition thereof, so it is necessary to adjust the hardenability or to improve the impact properties. It is used when However, addition of less than 0.1 mass% is poor in its addition effect, while Ni is an extremely expensive element, so addition of more than 1.0 mass% increases the cost of the steel material, which is against the purpose of the present invention. , 0.1 to 1.0
It was added in the range of mass%.

Further, in the present invention, at least one selected from V and Nb can be added. These actions are as follows. When an induction hardening process is performed, it is common to perform a quenching and tempering process as a pre-heat treatment in order to secure the hardness of the central portion of the untreated material. However, it is desirable to omit this heat treatment because it increases costs. QT as pretreatment
In order to omit the above, it is necessary to increase the hardness of the material before induction hardening, but for that purpose, addition of V and Nb having a precipitation strengthening effect is effective.

V: 0.05 to 0.5 mass% V is an element having an extremely strong precipitation strengthening effect, so V is added when it is necessary to omit the quenching and tempering treatment as a preheat treatment before induction hardening, but 0.05 mass% If the addition amount is less than 0.5 mass%, the effect is small, while if the addition amount exceeds 0.5 mass%, the effect reaches saturation, so 0.05 to 0.5 mass%
Was added in the range. Further, the addition of V is also effective in improving the temper softening resistance of the steel material, and is therefore extremely useful in improving the tooth surface strength.

Nb: 0.01 to 0.5 mass% Nb is an element having an extremely strong precipitation strengthening action, so Nb is added when it is necessary to omit the quenching and tempering treatment as a preheat treatment before induction hardening. If the addition amount is less than 0.5 mass%, the effect is small, while if the addition amount exceeds 0.5 mass%, the effect reaches saturation, so 0.01 to 0.5 mass%
Was added in the range. Also, Nb, like V, effectively contributes to the improvement of temper softening resistance of steel,
It is also useful as an element for improving tooth surface strength.

Next, the manufacturing method of the present invention will be described. In the fifth aspect of the invention, the heating temperature is limited to the range of 1100 to 1250 ° C. because the MnS does not form a solid solution at all when the heating temperature is less than 1100 ° C. Therefore, it is impossible to suppress the elongation due to rolling,
On the other hand, if the temperature exceeds 1250 ° C, the grain boundaries are partially melted and the hot deformability is lowered, and hot rolling itself becomes difficult.
Further, the hot rolling temperature is set to 1000 ° C. or higher because, as described above, the elongation of MnS is remarkable at a temperature lower than this.

Next, in the sixth invention, the hot rolling of the first stage is performed by heating temperature: 1100 to 1250 ° C., hot rolling end temperature: 1000 ° C.
The above is the same as in the case of the fifth invention described above. Further, in the second stage hot rolling, since the MnS is refined by the first stage hot rolling, the heating temperature is 1150 ° C.
Can be lowered to: However, when the heating temperature is lower than 1050 ° C, it becomes difficult to maintain the subsequent hot rolling temperature at 1000 ° C or higher, so the heating temperature is 1050 to 11 ° C.
It should be 50 ℃. Even in this second stage hot rolling, it is the same as in the first stage that the hot rolling temperature must be 1000 ° C. or higher.

[0047]

Example 1 Various steels having the chemical compositions shown in Tables 1 and 2 were made into blooms of 560 × 400 mm by a converter-continuous casting process.
Then, hot rolling was performed under the conditions shown in Tables 3, 4, and 1
After forming a 50 mm square billet, it was rolled into a 50 mmφ steel bar.
This 50 mmφ steel bar was hot forged into a 30 mmφ steel bar. This 30 mmφ steel bar was quenched at 845 ° C for 30 min and then tempered at 550 ° C. Using these as raw materials, 8 mmφ smooth rotary bending fatigue test pieces and 27 mmφ rolling fatigue test pieces were prepared, surface-quenched by a 15 kHz induction hardening tester, and then tempered at 180 ° C. for 1 h. Further, the same induction hardening treatment and tempering treatment were applied to the 30 mmφ quenched and tempered material, and an impact test piece having a 2 mm 10 R notch was produced from the vicinity of this surface. Further, the converter-melting in a continuous casting process, rolled to 50mmφ through the same process as above,
After that, using the SCM420 steel that was hot forged to 30 mmφ, the same test pieces as above were prepared, and these were subjected to carburizing treatment of 930 ° C, 4 h (carbon potential: 0.88) → quenching, and then 1
It was tempered at 80 ° C for 2 hours.

The results of examining the impact value, fatigue strength, rolling fatigue life and machinability of each sample thus obtained are
It is also shown in Tables 3, 4 and 5. The impact test was conducted at + 20 ° C using a Charpy impact tester. The fatigue test was carried out at room temperature using an Ono-type rotary bending fatigue tester at 3600 rpm.
Was carried out at a speed of. For rolling fatigue test, test piece is 130mmφ
The contact stress of 3677MPa was applied by pressing the roller of No. 3, and the life was evaluated by the time until the pitting occurred on the surface. In addition, in the as-hot-forged state, the carbide tool P10
A cutting test was performed using a cutting tool under the conditions of a cut depth of 2 mm, a feed length of 0.25 mm / rev, and a cutting speed of 200 m / min. The machinability was evaluated by the cutting time until the flank wear reaches 0.2 mm. The hardened layer after induction hardening and carburizing was corroded by an aqueous picric acid solution to reveal the former austenite grain size, and the grain size was measured.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

[Table 3]

[0052]

[Table 4]

[0053]

[Table 5]

In Tables 3, 4 and 5, Nos. 1 to 12 are inventive examples, and particularly Nos. 11 'and 12' are Nos. 11 and 12 when induction hardening is performed without quenching and tempering. Is. In Nos. 13 to 24, the steel compositions satisfy the range of the present invention, but the hot rolling conditions are out of the proper range. No.25
〜36, when N amount is below the proper range,
Is the case where the amount of P is below the proper range. No.49 ~ 55
Of C, Si, Mn, P, S, Al and O are outside the scope of the present invention. No. 56 is the conventional example SCM420
It is steel.

The invention examples of Nos. 1 to 12 'are superior to the conventional examples in impact properties, fatigue strength, rolling fatigue life and machinability. On the other hand, the comparative examples of Nos. 13 to 24 are inferior in fatigue strength in the C direction and machinability as compared with the invention examples. In particular, the fatigue strength in the C direction is inferior to that of the conventional example, but this means that it is difficult to secure the same properties as those of the conventional steel unless the hot rolling conditions satisfy the proper range of the present invention. Shows. Nos. 25 to 36, which have a low N content, are inferior to the conventional examples in any of impact properties, fatigue strength, rolling fatigue life and machinability. In this respect, Nos. 37 to 48 having a small P content have values comparable to those of the invention examples in all characteristics, but as described above, there is a disadvantage that the cost is higher than that of the present invention. No. 49 to 55 in which any one of the component compositions deviates from the proper range of the present invention, one of the characteristics is inferior to the conventional example.

Example 2 In the same manner as in Example 1, various steels having chemical compositions shown in Tables 1 and 2 were subjected to 560 × 40 by a converter-continuous casting process.
It was a 0 mm bloom. Then, hot rolling was performed under the conditions shown in Tables 6, 7, and 8 to make a 150 mm square billet, and then 50
It was rolled into a steel bar of mmφ. This 50 mmφ steel bar was hot forged into a 30 mmφ steel bar. This 30mmφ steel bar is 845 ℃, 30min
After quenching, was tempered at 550 ° C. A fatigue test piece, a rolling fatigue test piece and an impact test piece were produced in the same manner as in Example 1 using these as raw materials.

The results of examining the impact value, fatigue strength, rolling fatigue life and machinability of each of the samples thus obtained are
It is also shown in Tables 6, 7, and 8.

[0058]

[Table 6]

[0059]

[Table 7]

[0060]

[Table 8]

Nos. 1 to 12 are examples of the invention, but all the characteristics are higher than those of the conventional example, and by applying the present invention,
It can be seen that the same properties as carburized steel can be obtained by induction hardening. Nos. 13 to 24 are when the first stage hot rolling conditions are out of the appropriate range of the present invention, and Nos. 25 to 36 are
In the case where the second-stage hot rolling conditions are out of the proper range of the present invention, the fatigue strength in the C direction is remarkably reduced as compared with the invention examples, and the value is lower than the conventional example. Further, the machinability is also lower than that of the inventive examples, and it is understood that the machinability can be improved by applying the hot rolling conditions of the present invention. Nos. 37 to 48 are cases in which the N content is less than the lower limit of the present invention, but one of the characteristics is inferior to the conventional example. Nos. 49 to 60 are cases where the P content is lower than that of the present invention, but the hot rolling conditions of the second stage are out of the proper range of the present invention. Compared with the invention, the fatigue strength in the C direction is low and the machinability is poor, and if the hot rolling conditions of the present invention are not applied, even if the P content is lowered, the same or better characteristics as those of the conventional example are secured. It turns out to be difficult. Nos. 61 to 67 are cases where any of the component compositions deviates from the proper range of the present invention, but any of the characteristics is inferior to the conventional example.

[0062]

As described above, according to the present invention, it is possible to obtain a steel material for gears which exhibits the same characteristics as or better than those of conventional carburized steel by induction hardening without significantly reducing the P content. Therefore, it is possible to manufacture a gear with low cost and high productivity.

─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-62-196323 (JP, A) JP-A-7-118791 (JP, A) JP-A-8-53714 (JP, A) JP-A-6- 93374 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/00

Claims (6)

(57) [Claims] 1. C: 0.5 to 0.75 mass%, Si: 0.5 to 1.8 mass
%, Mn: 0.1 to 0.4 mass%, P: more than 0.015 to 0.03 mass%,
S: 0.020 mass% or less, Al: 0.019 to 0.05 mass%, O: 0.
0015mass% or less, containing N: more than 0.015 to 0.02mass%,
Remainder Ri Do the Fe and unavoidable impurities, and C direction
Steel material for gears for induction hardening, which is characterized by excellent fatigue strength . 2. C: 0.5 to 0.75 mass%, Si: 0.5 to 1.8 mass
%, Mn: 0.1 to 0.4 mass%, P: more than 0.015 to 0.03 mass%,
S: 0.020 mass% or less, Al: 0.019 to 0.05 mass%, O: 0.
0015mass% or less, containing N: more than 0.015 to 0.02mass%,
Furthermore, Mo: 0.05-0.5mass%, B: 0.0003-0.005mass
%, Ti: 0.005~0.05mass%, Ni : and at least one selected from among 0.1～1.0Mass%, remainder Ri Do the Fe and unavoidable impurities, and the fatigue strength of the C-direction
Steel for induction hardening of gears characterized by excellent Rukoto. 3. C: 0.5 to 0.75 mass%, Si: 0.5 to 1.8 mass
%, Mn: 0.1 to 0.4 mass%, P: more than 0.015 to 0.03 mass%,
S: 0.020 mass% or less, Al: 0.019 to 0.05 mass%, O: 0.
0015mass% or less, containing N: more than 0.015 to 0.02mass%,
Furthermore V: 0.05~0.5mass%, Nb: contains at least one selected from among 0.01～0.5Mass%, remainder Ri Do the Fe and unavoidable impurities, and the fatigue strength of the C-direction
Steel for induction hardening of gears characterized by excellent Rukoto. 4. C: 0.5 to 0.75 mass%, Si: 0.5 to 1.8 mass
%, Mn: 0.1 to 0.4 mass%, P: more than 0.015 to 0.03 mass%,
S: 0.020 mass% or less, Al: 0.019 to 0.05 mass%, O: 0.
0015mass% or less, containing N: more than 0.015 to 0.02mass%,
Furthermore, Mo: 0.05-0.5mass%, B: 0.0003-0.005mass
%, Ti: 0.005 to 0.05 mass%, Ni: 0.1 to 1.0 mass%, and V: 0.05 to 0.5 m
ass%, Nb: contains at least one selected from among 0.01～0.5Mass%, the remainder is characterized by Rukoto excellent fatigue strength of Do Ri and C direction to Fe and unavoidable impurities induction hardened Required gear steel. 5. C: 0.5 to 0.75 mass%, Si: 0.5 to 1.8 mass
%, Mn: 0.1 to 0.4 mass%, P: more than 0.015 to 0.03 mass%,
S: 0.020 mass% or less, Al: 0.019 to 0.05 mass%, O: 0.
When a cast slab having a composition containing less than mass% and N: more than 0.015 to 0.02 mass% is formed into a steel material by hot rolling, 11
A method for producing a steel material for gears for induction hardening, which comprises heating to a temperature of 00 to 1250 ° C and then finishing hot rolling at a temperature of 1000 ° C or higher. 6. C: 0.5 to 0.75 mass%, Si: 0.5 to 1.8 mass
%, Mn: 0.1 to 0.4 mass%, P: more than 0.015 to 0.03 mass%,
S: 0.020 mass% or less, Al: 0.019 to 0.05 mass%, O: 0.
When a slab having a composition containing less than 0.01 mass% and N: more than 0.015 to 0.02 mass% is formed into a steel material by hot rolling in two stages, after heating to a temperature of 1100 to 1250 ° C., a temperature of 1000 ° C. or more is applied. For gears for induction hardening, characterized by terminating the first stage hot rolling, further heating to a temperature of 1050 to 1150 ° C, and then terminating the second stage hot rolling at a temperature of 1000 ° C or higher. Steel material manufacturing method.