JP2000265241A - Non-heat treated steel for induction contour hardening gear - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-heat treated steel for induction contour hardening gear, capable of providing homogeneous hardened layer structure even if a hardening process by ultrashort-time heating, that is, an induction contour hardening process is adopted and capable of providing a gear product having high rolling contact fatigue strength, at the time of manufacturing a gear by using the non-heattreated steel as a material and applying hot forging and successive machining to it. SOLUTION: The steel has a composition consisting of, by weight, 0.45-0.8% C, 1.0-2.0% Si, 0.1-1.5% Mn, 0.05-1.0% Cr, 0.05-0.3% V, 0.015-0.05%; sol.Al, and the balance Fe with impurities. This steel satisfies the condition of (hardenability index)=1.2-1.4×C(%)-0.28×Mn(%)-0.49×Cr(%)<=0.3, and the tempering hardness of the hardened material at 300 deg.C is regulated to >=HV600. This steel can further contain either or both of the following optional additive elements: (I) 0.0005-0.005% B and 0.005-0.05% Ti; and (II) <=0.2% S and/or <=0.1% Te.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-heat-treated steel for high-frequency contour hardened gears, and more particularly, to a gear shape formed by hot forging and, if necessary, machining, followed by induction hardening. The present invention relates to a steel that has been subjected to a surface hardening treatment to be used as a gear product without being tempered. In the present invention, the term "gear" has a wide meaning as exemplified below.

[0002]

2. Description of the Related Art For example, a transmission gear of an automobile engine, a rolling element for a continuously variable transmission, and other mechanical parts having a broad gear shape are manufactured without quenching and tempering after hot forging. In this case, a non-heat treated steel having an alloy composition obtained by adding a small amount of V to medium carbon steel is preferably used. In the case of a part requiring high contact fatigue strength among such forged products of non-heat treated steel, it is general to harden the surface by induction hardening after hot forging and machining.

However, a conventional non-heat treated steel obtained by adding a trace amount to a medium carbon steel has a high proeutectoid ferrite area ratio and requires a sufficiently long heating time to obtain a homogeneous induction hardened structure. .

On the other hand, according to the high-frequency contour quenching established in recent years, only the surface layer of a part can be quenched by heating for a very short time of 0.1 to 1.0 second. The quenched material treated by this technique is characterized by high strength because a high compressive residual stress is applied to the surface layer. However,
Conventional non-heat treated steel requires long-time heating, so high-frequency contour quenching, which is characterized by short-time heating, cannot provide a uniform hardened layer and can only provide low-strength products. There are many. Using tempered steel, quenching after hot forging
If tempering is performed, a uniform hardened layer can be obtained by high frequency contour quenching, but the cost advantage of the non-heat treated steel cannot be enjoyed.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its object is to provide:
After hot forging, if necessary, machine processing to obtain the desired part shape, high-frequency contour quenching of ultra-short time heating without quenching and tempering, to obtain a homogeneous hardened layer structure, high rolling An object of the present invention is to provide a non-heat treated steel for high frequency contour hardened gears that can provide a gear product having contact fatigue strength.

[0006]

The non-heat-treated steel for high-frequency contour hardened gears of the present invention which achieves the above object has a basic alloy composition of C: 0.45 to 0.8% by weight. , Si:
1.0 to 2.0%, Mn: 0.1 to 1.5%, Cr:
0.05-1.0%, V: 0.05-0.3% and so
l-Al: 0.015 to 0.05% is contained in a ratio satisfying the following hardenability index condition, and hardenability index = 1.2-1.4 × C (%) − 0.28 × Mn
(%) − 0.49 × Cr (%) ≦ 0.3 The balance consists of Fe and impurities.
The tempering hardness at 00 ° C. is HV600 or more.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The non-heat-treated steel for high-frequency contour hardened gears of the present invention is characterized in that, in addition to the above-mentioned alloy composition comprising essential components,
Further, one or both of the following optional components of Groups I and II can be contained. (I) B: 0.0005 to 0.005% and Ti:
0.005 to 0.05% (II) S: 0.2% or less and / or Te: 0.1
%Less than.

It is a matter of course that alloys containing these optional components must also satisfy the above-mentioned hardenability index conditions and ensure the lower limit of the tempered hardness of the material after quenching. .

Regarding the alloy composition, the action of the essential components and optional additives and the reasons for limiting the composition range will be described below.

C: 0.45 to 0.8% C is an important element for obtaining the strength of steel. It secures the surface hardness after induction hardening, and has a static strength, a bending fatigue strength and a rolling contact. In order to improve the fatigue strength, the carbon content of the normal carbon steel S40C or S45C is higher than that of the normal carbon steel S40C or S45C by 0.45%.
% Or more must be added. On the other hand, the C content is 0.80%
If the eutectoid point is exceeded, on the contrary, the surface hardness is lowered and the strength cannot be improved, and furthermore, proeutectoid cementite is formed and the toughness is impaired. Therefore, the upper limit is set to 0.8%.

Si: 1.0-2.0% In addition to adding Si as a deoxidizing agent at the time of melting, 300 ° C.
It has the function of increasing the tempering softening resistance at the following temperatures and increasing the rolling contact fatigue strength. Gears are often exposed to a certain temperature for a long time depending on use conditions, and the degree of temper softening resistance at low temperatures is an important problem. In view of these effects, 1.0% or more of Si is added. If the amount is too large, the machinability, hot workability and induction hardenability are reduced. Therefore, the addition is limited to 2.0%.

Mn: 0.1-1.5% Mn also acts as a deoxidizing agent during melting. Further, it is an element that enhances the induction hardening property, and in order to expect this effect, addition of 0.1% or more is required. If added in excess,
Since bainite is formed after hot forging and the machinability is significantly reduced, the upper limit is set to 1.5%.

Cr: 0.05 to 1.0% Cr is an element that improves the induction hardenability like Mn, but the addition of a large amount also causes the formation of bainite and increases the hardness of the material. Machinability and workability are impaired. The lower limit was set to 0.05% at which the effect was recognized, and the upper limit was set to 1.0% at which no adverse effect of bainite formation occurred.

V: 0.05% to 0.3% V forms carbonitrides during air forging after hot forging, and has the effect of finely precipitating and enhancing the strength. V is an essential element for a tempered steel that must develop strength without tempering. The gear should also have a high core strength, for which the presence of V helps. To achieve this effect, 0.05
% Or more of V is required. If added in a large amount, the hardness becomes too high, and it is economically disadvantageous.
It is advisable to add up to.

Sol-Al: 0.015 to 0.05% Al is an element that acts as a deoxidizing agent during melting.
The production of aluminum-killed steel requires an addition of 0.015% or more. However, excessive addition causes an increase in inclusions,
Since the toughness and the fatigue strength are reduced, the amount of addition is 0.
Choose up to 05%.

The function of the optional additive element and the reason for limiting the composition range are as follows. B: 0.0005% to 0.005% B is useful for enhancing the hardenability and obtaining a stable hardened layer depth, and effectively preventing the hardenability from changing with the amounts of Mn and Cr. Suppress. B to 0.0
If 005% or more is added, this effect can be obtained stably. Since the effect is reduced even if it is added in excess, the addition should be limited to the upper limit of 0.005%.

Ti: 0.005 to 0.05%, Ti combines with N in steel to form a TiN compound, and BN
Since the formation of the compound is suppressed, B has a function of supporting the effect of improving the hardenability of the steel from the aspect. Therefore, when B is added, an appropriate amount of Ti is usually added together. However, since the presence of a large amount of Ti causes a decrease in toughness and fatigue strength, the upper limit is set to 0.05% in order to avoid this. The relationship with N is preferably Ti / N ≧ 3.4.

S: 0.2% or less Te: 0.1% or less These are elements for improving machinability, and are desirably added when manufacturing a gear that includes machining. Therefore, S and Te are added alone or in combination within the range of 0.2% or 0.1%, respectively. The upper limit is determined from the viewpoint that addition of these elements does not cause a decrease in mechanical properties of the product.

Condition of hardenability index: Hardenability index = 1.2-
1.4 x C (%)-0.28 x Mn (%)-0.49 x Cr
(%) ≦ 0.3 When the hardenability index defined by this equation exceeds 0.3,
Since the amount of proeutectoid ferrite after hot forging becomes large and a uniform hardened layer cannot be expected by heating for an extremely short time, the strength after induction hardening becomes insufficient. Tempering hardness of the quenched material at 300 ° C .: HV6
00 or more

Since steel that does not satisfy this condition has low tempering softening resistance, when the temperature of the rolling surface of the gear increases during rolling contact, the steel is tempered and the hardness decreases, leading to breakage in a short time. There is a risk. Steels having an HV of 600 or more have a small decrease in the hardness of the rolling surface and exhibit high rolling contact fatigue strength.

[0021]

EXAMPLE Steel having the alloy composition shown in Table 1 was melted in a high frequency induction furnace and cast into a 150 kg ingot. Although not listed in Table 1, each steel is an impurity usually contained in ordinary steel, P: 0.03% or less, Cu: 0.35 or less, N
i: 0.2% or less, N: 0.03% or less, and O: 0.
003%.

Table 1 No. C Si Mn Cr V sol-Al B Ti Other Examples 1 0.46 1.25 1.20 0.15 0.15 0.028---2 0.55 1.30 1.20 0.15 0.14 0.028----3 0.75 1.25 1.21 0.14 0.15 0.021--- 4 0.60 1.05 1.20 0.15 0.13 0.023---5 0.55 1.61 0.60 0.34 0.15 0.020---6 0.55 1.83 0.25 0.81 0.15 0.024---7 0.55 1.52 1.20 0.15 0.28 0.025---8 0.54 1.92 1.20 0.08 0.15 0.027--S : 0.10 9 0.55 1.31 1.20 0.15 0.05 0.021 − − S: 0.05 Te: 0.001 10 0.55 1.54 1.21 0.14 0.15 0.025 0.0015 0.025 −11 0.54 1.53 1.20 0.16 0.15 0.045 0.0013 0.023 S: 0.10 12 0.54 1.66 1.20 0.15 0.15 0.026 0.0015 0.024 S: 0.05 Te: 0.001 Comparative example A 0.40 1.21 1.20 0.10 0.15 0.024 − − − B 0.85 1.20 1.20 0.10 0.14 0.028 − − − C 0.55 0.60 1.20 0.10 0.15 0.026 − − − D 0.55 2.51 1.20 0.10 0.15 0.022 − − − E 0.55 1.21 0.25 1.20 0.15 0.028---F 0.54 1.20 1.20 0.10 0.40 0.022 − − G 0.55 1.30 1.20 0.15 0.14 0.028 − − − H 0.55 1.21 1.20 0.10 0.15 0.005 − − − I 0.56 1.19 1.20 0.10 0.15 0.063 − − − J 0.46 1.24 0.70 0.10 0.16 0.024 − − − K 0.55 1.18 1.20 0.10 0.16 0.024 − − S: 0.26 Te: 0.05 L 0.55 1.21 1.20 0.10 0.15 0.024 0.0015 0.100 − M 0.55 1.23 1.20 0.11 0.14 0.025 0.0015 0.024 S: 0.27 Te: 0.05

A roller pitting test was performed to evaluate the rolling contact fatigue strength. The above ingot was hot forged at 1200 ° C. to form a round bar having a diameter of 32 mm, and the round bars were cooled to room temperature while being spaced apart from each other so as not to thermally affect each other. A cylindrical test piece with a diameter of 26 mm was cut out from each round bar after cooling, and high-frequency contour quenching was performed under the following conditions. Frequency: 150 KHz Method: Fixed quench Power: 600 KW Tempering: None Surface A quenched layer having an effective hardened layer depth of about 1 mm (distance from the test piece surface at which a hardness of 500 HV or more was obtained) was formed. The roller pitting test was performed under the following conditions: Counterpart material: SCM418 carburized disk (130 m in diameter)
m, surroundings are crowned at 150 r) Surface pressure: 2.94 GPa Revolution: 1500 rpm Slip ratio: -40% Oil temperature: 80 ° C. Evaluation: Time until pitting occurs on the surface of the test piece.

In order to evaluate the machinability, a turning test was performed. The 150 kg ingot described above was forged into a round bar having a diameter of 90 mm, and a non-tempering forging simulation was performed by a normalizing process in which the ingot was kept at 1100 ° C. for 1 hour. Thereafter, it was processed into a cylindrical test piece having a diameter of 86.4 mm until normalizing, and cut under the following conditions: Tool: Carbide P10 Cutting speed: 200 m / min. Sending: 0.2 mm Cutting: 2 mm Cutting Oil: None Life judgment: Rolling contact fatigue test of VB = 0.2 mm or more and machinability test results
Table 2 shows the values of the hardenability index and the temper hardness.

Table 2 No. Hardenability Index Tempering Hardness Rolling Contact Machinability (HV) Fatigue Strength Example 2 Reference Example 1 0.15 600 5.20 1.32 2 0.02 662 ≧ 10 1.00 3 -0.26 783 ≧ 10 0.59 4 -0.05 668 9.40 0.93 5 0.10 693 ≧ 10 1.09 6 -0.04 716 ≧ 10 0.92 7 0.02 684 ≧ 10 0.65 8 0.05 718 ≧ 10 1.89 9 0.02 663 ≧ 10 2.84 10 0.02 686 ≧ 10 0.91 11 0.03 679 ≧ 10 1.94 12 0.03 692 ≧ 10 2.42 Comparative Example A 0.26 558 0.93 1.68 B -0.38 841 5.72 0.39 C 0.04 591 2.43 1.18 D 0.04 785 ≧ 10 0.51 E-0. 18 656 ≧ 10 0.43 F −0.23 653 ≧ 10 0.47 G 0.06 645 ≧ 10 0.32 H 0.04 653 6.17 1.02 I 0.03 657 5.15 0.99 J 0.31 599 2.11 1.72 K 0.04 650 0.87 2.99 L 0.04 653 0.76 1.02 M 0.04 655 0.79 3.04

The steels of Examples 1 to 12 satisfy all the requirements of the present invention and have excellent fatigue strength and machinability. In the embodiment in which the free-cutting element is added, the machinability is particularly improved.

On the other hand, each of the steels of the comparative examples has some disadvantages as follows. First, since the steel A has a low C content, the quenching hardness of the hardened layer is low and the strength is inferior.
On the other hand, since steel B has too much C content, proeutectoid cementite is formed, which lowers the strength. Moreover, the hardness after forging is high and the machinability is poor. Since the C steel has a low Si content, the tempering hardness does not reach 600 HV and the strength is low.

Each steel of D, E, F and G is S
Since the contents of i, Mn, Cr and V are too high, the hardness of the material after hot forging is too high, and the machinability is severely deteriorated.

Since the H steel has too low a sol-Al content, crystal grains after hot forging are coarsened, and as a result, the strength is reduced. Conversely, the I steel having a too high sol-Al content also has a low strength, because an excessive amount of Al nitride was generated.

Although the amount of each alloy component in the J steel is within the range, since the hardenability index is too large, a uniform hardened layer structure cannot be obtained by high-frequency contour quenching in which heating is performed in a very short time, so that the strength is high. Is low. The machinability of K steel is remarkably improved due to the excessive addition of the free-cutting elements S and Te, but the strength is insufficient. The same applies to M steel.

In the L steel, the Ti content is too large and a large amount of Ti carbonitride is generated, which causes the strength to decrease as inclusions.

[0032]

The non-heat treated steel for high frequency contour hardened gears of the present invention can be obtained by appropriately selecting the alloy composition, keeping the hardenability index below a specific value, and securing a constant temper hardness. It is a non-heat treated steel that does not require quenching and tempering after hot forging, and can obtain a homogeneous hardened layer structure by heating for a very short time. Thus, it is possible to manufacture a gear having high strength. The steel according to a preferred embodiment to which an appropriate amount of free-cutting elements is added has, in addition to the above-mentioned advantages, an advantage that the machinability is good and machining which is substantially indispensable for the manufacture of gears is easy.

Claims (4)

[Claims] 1. C: 0.45 to 0.8% by weight, S
i: 1.0 to 2.0%, Mn: 0.1 to 1.5%, C
r: 0.05 to 1.0%, V: 0.05 to 0.3% and sol-Al: 0.015 to 0.05% in a ratio satisfying the following hardenability index conditions. Hardenability index = 1.2-1.4 x C (%)-0.28 x Mn
(%) − 0.49 × Cr (%) ≦ 0.3 The balance consists of Fe and impurities.
A non-heat treated steel for high-frequency contour hardened gears, wherein the tempering hardness at 00 ° C is HV600 or more. 2. C: 0.45 to 0.8% by weight, S
i: 1.0 to 2.0%, Mn: 0.1 to 1.5%, C
r: 0.05 to 1.0%, V: 0.05 to 0.3% and sol-Al: 0.015 to 0.05% in a ratio satisfying the following hardenability index conditions. Hardenability index = 1.2-1.4 x C (%)-0.28 x Mn
(%) − 0.49 × Cr (%) ≦ 0.3 Further, B: 0.0005 to 0.005% and Ti:
Non-heat treated high frequency contour hardened gear characterized by containing 0.005 to 0.05%, the balance being Fe and impurities, and the tempered hardness at 300 ° C of the quenched material is HV600 or more. steel. 3. C: 0.45 to 0.8% by weight, S
i: 1.0 to 2.0%, Mn: 0.1 to 1.5%, C
r: 0.05 to 1.0%, V: 0.05 to 0.3% and sol-Al: 0.015 to 0.05% in a ratio satisfying the following hardenability index conditions. Hardenability index = 1.2-1.4 x C (%)-0.28 x Mn
(%) − 0.49 × Cr (%) ≦ 0.3 Further, S: 0.2% or less and / or Te: 0.
1% or less, the balance being Fe and impurities,
The tempered hardness of the quenched material at 300 ° C is HV6
Non-heat-treated steel for high-frequency contour hardened gears, characterized by being at least 00. 4. C: 0.45 to 0.8% by weight, S
i: 1.0 to 2.0%, Mn: 0.1 to 1.5%, C
r: 0.05 to 1.0%, V: 0.05 to 0.3% and sol-Al: 0.015 to 0.05% in a ratio satisfying the following hardenability index conditions. Hardenability index = 1.2-1.4 x C (%)-0.28 x Mn
(%) − 0.49 × Cr (%) ≦ 0.3 Further, B: 0.0005 to 0.005% and Ti:
0.005 to 0.05%, S: 0.2% or less and / or Te: 0.1% or less, the balance being Fe and impurities, 300 ° C. of the material after quenching
Non-heat treated steel for high-frequency contour hardened gears, wherein the tempered hardness is HV600 or more.