JPH09241798A - Steel stock for gear for induction hardening and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel stock for gear for induction hardening, excellent in fatigue strength, impact characteristic, and machinability in spite of its high P content. SOLUTION: This steel stock has a composition consisting of, by mass, 0.5-0.75% C, 0.5-1.8% Si, 0.1-0.4% Mn, >0.015-0.03% P, <=0.020% S, 0.019-0.05% Al, <=0.0015% O, >0.015-0.02% N, and the balance Fe with inevitable impurities.

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 the mainstream gear manufacturing process, but carburizing requires a treatment at a temperature of about 800 to 950 ° C for several hours, so it can be inlined into the gear manufacturing process. It is difficult, and therefore, there is a natural limit to improving the productivity and the reduction of the manufacturing cost.

[0003] Carburization is generally carried out by gas carburization, but during gas carburization, an abnormal surface layer is inevitably formed on the surface of the material to be treated, and this abnormal layer lowers fatigue strength and impact characteristics. Therefore, the improvement of the fatigue strength and the impact characteristics is limited. Furthermore, since the material to be processed is deformed by the heat treatment distortion generated during carburizing and quenching,
Problems still remain where strict control of heat treatment conditions is required.

[0004] In order to overcome the above-mentioned problems, Si, Mn and Cr in steel materials are reduced and Mo,
High-strength carburizing steel was developed by adding Ni etc. to reduce the abnormal surface layer generated during gas carburizing and improve fatigue strength and impact properties.However, this steel contains a large amount of expensive alloying elements. In addition to this, there is a problem in that not only the cost of steel material increases but also the workability such as machinability deteriorates.

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

As a solution to these problems, for example, Japanese Patent Application Laid-Open No. Sho 60-169544 discloses that the chemical composition is restricted to a specific range so that it can be applied to gear manufacturing by an induction hardening process. Steel has 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 present inventors have conducted a thorough study on the chemical composition of a steel material for securing the characteristics required for a gear in an induction hardening process. 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 portion is repeatedly subjected to stress and does not cause fatigue fracture from the root portion. By the way, this root strength is
Since there is a good correlation with the fatigue strength obtained by a fatigue test such as rotary bending, the inventors studied the chemical composition of a steel material by a rotary bending fatigue test.

The basic factors affecting the fatigue strength are the hardness of the material and nonmetallic inclusions, and the lower the hardness of the material, the lower the fatigue strength. Here, 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 the fatigue cracks extend along the prior austenite grain size, so that by making them smaller, the resistance to fatigue crack propagation increases, and segregation at the grain boundaries such as P occurs. This is because the concentration of the element that makes this brittle decreases due to finer grains.

[0010] Induction quenching is a rapid and short-time heat treatment, so it is extremely effective in reducing the size of austenite. However, when N, Al, etc., which form precipitates that suppress the growth of austenite grains, are added. Since the grain size is further reduced, the fatigue strength is further improved. Further, in order to obtain a predetermined material hardness, it is necessary to add an alloy 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 the fatigue strength,
It is not enough to ensure the hardness of the material, and it is necessary to reduce nonmetallic inclusions as well. That is, even if the material hardness can be ensured, the presence of non-metallic inclusions causes fatigue fracture from this portion, and the fatigue strength is significantly reduced. In particular, hard non-metallic inclusions such as alumina are harmful. For this purpose, the reduction of O is indispensable, but according to the study of the inventor, from the viewpoint of securing the same fatigue strength as that of the carburized material, it is essential that the content be 0.0015 mass% or less.

Further, fatigue damage called pitching occurs in the tooth surface due to repeated contact stress. When this occurs, it is difficult for the gear to perform its normal function, and thus the 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, a relative slip occurs on the tooth surface portion, so that the friction causes a remarkable temperature rise, but the temperature rise softens the steel material and causes pitting. To suppress this, it is effective to add Si, Mo, V, Nb, etc., which increase the tempering softening resistance to the steel, and it is possible to increase the tooth surface strength by adding these.

Further, when an impact load is applied to the tooth root, if the impact characteristic of the steel material is low, the teeth are broken from the root and the gears and the entire machine in which the gears are incorporated are difficult to recover. Will be damaged. Therefore, the impact characteristics are extremely important characteristics. The effect of C is the largest factor affecting the impact characteristics, but the C concentration in the part carburized through the carburizing process is about 0.8 mass%, while the hardness of the steel material is the same by induction hardening. Since C required to obtain C is about 0.5 to 0.7 mass%, it is advantageous from the viewpoint of ensuring impact characteristics. However, not only are the factors influencing the impact characteristics, but also the austenite grain size during induction hardening and impurity elements such as P segregated at the grain boundaries, so that the refinement of γ grains and the
The reduction of impurity elements such as is effective in improving impact characteristics.

[0014] Further, the measures for ensuring the characteristics required as a gear as described above are not sufficient as a method of manufacturing a gear by induction hardening, and as described above, the workability, especially the machinability is ensured. is important. Regarding this machinability, in the case of the carburizing process, low C steel is used, so that it has a relatively high machinability before the carburizing and quenching, whereas, in the case of the induction hardening process, However, since higher C is indispensable than carburized steel, it is extremely disadvantageous in terms of ensuring machinability.

[0015] The inventors of the present invention have studied various factors affecting the machinability of the high-C steel, and have obtained the following findings. That is, in steels with C of 0.5 mass% or more, when the free-cutting element is fixed, the factor that most affects the machinability is its microstructure. In particular, the amount of ferrite and the form of pearlite have the most significant effects. It was found to have an effect. In the case of a high-C steel, the microstructure has a ferrite-pearlite structure, but as the ferrite increases, the machinability improves. This is because, in addition to the decrease in the hardness of the steel material due to the increase in the amount of ferrite, the machinability is improved because the ferrite / pearlite interface, which is the portion where cracks occur during cutting, increases due to the increase in ferrite. .

[0016] The form of pearlite also 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, since the pearlite portion has high ductility, the portion where cracks occur during cutting is limited to the ferrite / pearlite interface, but such a lamellar has developed. In the case of a non-textured structure, cracks are easily generated from the cementite / ferrite interface in pearlite in addition to the ferrite / pearlite interface in the portion that undergoes deformation during cutting, so that machinability is dramatically improved. It is. In order to form such undeveloped pearlite, it is important to select and optimize the alloying elements in the steel, and particularly to reduce Mn and Cr, which lower the transformation point and promote lamellar layering. It is effective. Further, the addition of Mo is effective in improving machinability because it suppresses lamellar layering and forms a cementite-separated structure.

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 elongates in the rolling direction along with hot rolling, but the extent varies depending on the hot rolling conditions. When MnS elongates, the fatigue strength of a sample taken from a direction perpendicular to the direction of elongation extremely decreases. 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 direction in which MnS extends, so that the extension of MnS may reduce the fatigue strength of the actual gear. 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, a method of forming spherical CaS by adding Ca is known, but the addition of Ca simultaneously causes the formation of coarse Ca-based oxide nonmetallic inclusions. As a result, the rolling fatigue life is reduced.

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, MnS is partially dissolved as a result, and the particle size of MnS is smaller than in the slab stage. When this material is hot rolled, the elongation of MnS is lower than that of heating at lower temperatures due to the reduction in grain size. Further, since once dissolved MnS reprecipitates relatively finely during rolling, the average elongation of MnS of the steel material is significantly suppressed as compared with the case of low temperature heating. Furthermore, as a result of examining the shape of MnS before the hot rolling, it was found that the longer the MnS was, the larger the elongation by the 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 increasing the heating temperature,
By avoiding the temperature range of 900 to 1000 ° C as the rolling temperature range,
It is possible to suppress the elongation of MnS, and consequently to expect improvement in 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 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%, A steel material for gears for induction hardening (first invention), characterized by containing O: 0.0015 mass% or less, N: more than 0.015 mass% to 0.02 mass%, and the balance being Fe and inevitable impurities.

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: 0.0015 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 to 0.05 mass%, Ni: 0.1 A steel material for gears for induction hardening (second invention), characterized in that it contains at least one selected from the range of 1.0 mass% and the balance is Fe and inevitable impurities.

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: 0.0015 mass% or less, N: more than 0.015 to 0.02 mass%, and at least one selected from V: 0.05 to 0.5 mass% and Nb: 0.01 to 0.5 mass%, and the balance Is a steel material for gears for induction hardening (third invention), which has a composition of Fe and inevitable impurities.

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: 0.0015 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 to 0.05 mass%, Ni: 0.1 To at least 1.0 mass% and at least one selected from V: 0.05 to 0.5 mass% and Nb: 0.01 to 0.5 mass%, with the balance being Fe and inevitable impurities. A steel material for gears for induction hardening, which is characterized in that (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.
It is necessary to add ss% or more. However, 0.75mass
%, The impact characteristics and machinability required for the gear deteriorate, so the content should be 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
The addition of more than 1.8 mass% is necessary, but if it exceeds 1.8 mass%, the hardness increases due to solid solution hardening of ferrite and the machinability decreases, so the content 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 manufacturing cost is unavoidable, which is contrary to the object of the present invention of reducing 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 P amount 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. 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 0015 mass% is acceptable.

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 added as appropriate. 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. The addition of Mo at the same time has a significant effect on the morphology of the pearlite, forming cementite fragmented pearlite. As a result, the machinability is significantly improved. Further, Mo improves the tempering softening resistance, 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 boundaries, and is a useful element for this purpose.
However, if the addition amount is less than 0.05 mass%, the effect of the addition is poor, while if it exceeds 0.5 mass%, carbides which are difficult to dissolve in austenite by rapid and short-time heating such as induction hardening are formed. , 0.05-0.5
It must be added 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. In addition, B preferentially segregates at the grain boundaries, has the effect of reducing the concentration of P segregating at the grain boundaries, and improving the root strength and impact characteristics. To obtain such effect, 0.0003mass%
Although the above addition is necessary, even if it exceeds 0.005 mass%, the effect is saturated, so the B content is 0.0003 to 0.005 mass%.
It was limited to the range of mass%.

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 exerted by adding Ti that easily bonds to N more than B, Ti is used in such a case. However, the content is 0.005
When the content is less than 0.05 mass%, the effect is small. On the other hand, when the content exceeds 0.05 mass%, a large amount of TiN is formed, which becomes a starting point of fatigue fracture and lowers the tooth root strength and tooth surface strength. It was added in the range of -0.05 mass%. Further, since TiN has an 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 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, if the addition is less than 0.1 mass%, the effect of the addition is poor. On the other hand, since Ni is an extremely expensive element, the addition of more than 1.0 mass% increases the cost of the steel material, which is contrary to the object of the present invention. , 0.1-1.0
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 as much as possible to increase the cost. QT as preprocessing
It is necessary to increase the hardness of the material before induction hardening in order to omit it, but for that purpose, the 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 described above. Further, since the addition of V is also effective for improving the tempering softening resistance of the steel material, it is extremely useful for 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 described above. 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, when the temperature exceeds 1250 ° C., the grain boundaries are partially melted, the hot deformability is reduced, and the hot rolling itself becomes difficult.
Further, the reason why the hot rolling temperature is set to 1000 ° C. or higher is that, as described above, the MnS elongation 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 aspect described above. In the second-stage hot rolling, since the MnS is refined by the first-stage hot rolling, the heating temperature is set to 1150 ° C.
Can be lowered to: However, if 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
Must be 50 ° C. In the second stage hot rolling, the hot rolling temperature needs to be set to 1000 ° C. or higher, as in the first stage.

[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. The 30 mmφ steel bar was quenched at 845 ° C for 30 minutes 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 performed 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 under the conditions of cutting depth: 2 mm, feed: 0.25 mm / rev, and cutting speed: 200 m / min. The machinability was evaluated by the cutting time until the flank wear reached 0.2 mm. In addition, the hardened layer after induction hardening and carburizing hardening was corroded by picric acid aqueous solution to reveal the prior 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.
The bloom was 0 mm. 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. Using these as raw materials, a fatigue test piece, a rolling fatigue test piece and an impact test piece were produced in the same manner as in Example 1.

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 turns out that the characteristics equivalent to carburized steel can be obtained by induction hardening. Nos. 13 to 24 are when the first stage hot rolling conditions are out of the proper 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 invention 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.

Claims (6)

[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: 0.0015 mass% or less, N: more than 0.015 to 0.02 mass%, the balance being Fe and unavoidable impurities composition, a steel material for gears for induction hardening. 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: 0.0015 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 to 0.05 mass%, Ni: 0.1 A steel material for gears for induction hardening, characterized in that it contains at least one selected from ~ 1.0 mass% and the balance is Fe and inevitable impurities. 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: 0.0015 mass% or less, N: more than 0.015 to 0.02 mass%, and at least one selected from V: 0.05 to 0.5 mass% and Nb: 0.01 to 0.5 mass%, and the balance Is a steel material for gears for induction hardening, which has a composition of Fe and inevitable impurities. 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: 0.0015 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 to 0.05 mass%, Ni: 0.1 To at least 1.0 mass%, and at least one selected from V: 0.05 to 0.5 mass% and Nb: 0.01 to 0.5 mass%, with the balance being Fe and inevitable impurities. A steel material for gears for induction hardening, which is characterized in that 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 slab containing a composition is made into steel by hot rolling, after heating to a temperature of 1100 to 1250 ° C, 1000 ° C or more A method for manufacturing a steel material for gears for induction hardening, which comprises terminating hot rolling at the temperature of. 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: 0.0015 mass% or less, N: More than 0.015 to 0.02 mass% When a slab containing a composition is made into a steel material by two-stage hot rolling, 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.