JPH1030632A - High fatigue strength crankshaft - Google Patents

Abstract

(57) [Problem] To provide a relatively inexpensive crankshaft having high fatigue strength. SOLUTION: C ≦ 0.01% by weight and Si ≦ 1% by weight
And 0.05% by weight ≦ Mn ≦ 0.5% by weight and P ≦ 0.
1% by weight, S ≦ 0.03% by weight, and 0.02% by weight ≦
sol. Al ≦ 0.1% by weight and 0.8% by weight ≦ Cu ≦
1.7% by weight and 0.02% by weight ≦ Ti ≦ 0.1% by weight
And a hot forged crankshaft using a steel material consisting of the balance Fe and unavoidable elements, and after solution treatment,
A soft nitriding treatment that also serves as an artificial aging treatment is performed. This crankshaft has a sufficiently deep surface hardened layer, and in its production, energy saving and production cost reduction are achieved by integrating artificial aging treatment and nitrocarburizing treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high fatigue strength crankshaft.

[0002]

2. Description of the Related Art Conventionally, a crankshaft for an internal combustion engine is made of a medium carbon steel material corresponding to JIS S45C to S50C, and a soft carbonitride material of a low and medium carbon steel material containing Al, Cr, V and the like. The thing using a steel material is known.
Each of the crankshafts is manufactured through hot forging, normalizing, machining, nitrocarburizing, and, if necessary, straightening.

[0003]

At present, the demand for higher output of an internal combustion engine requires that the crankshaft have a long integrated structure and high fatigue strength.

[0004] However, since the former using a medium carbon steel material has a good straightening property in the straightening process, the bending fatigue strength is relatively low even if a long integrated structure can be adopted. There's a problem. On the other hand, the latter using a steel material for nitrocarburizing has relatively high bending fatigue strength compared to the former, but the hardness of the surface and its vicinity becomes extremely high due to the nitrocarburizing treatment. It is extremely difficult to perform a straightening in
Therefore, a long integrated structure cannot be adopted.

[0005]

SUMMARY OF THE INVENTION The present invention has good hot forgeability and machinability, and after solution treatment,
It has a high fatigue strength and a long integrated structure that is made of a specific steel material that can perform nitrocarburizing treatment that also serves as artificial aging treatment and that can be bent after nitrocarburizing treatment. It is an object of the present invention to provide the crankshaft capable of adopting and improving dimensional accuracy.

[0006] In order to achieve the above object, according to the present invention,
C ≦ 0.01 wt%, Si ≦ 1 wt%, 0.05 wt% ≦ Mn ≦ 0.5 wt%, P ≦ 0.1 wt%, S
≦ 0.03% by weight and 0.02% by weight ≦ sol. Al ≦
0.1% by weight and 0.8% by weight ≦ Cu ≦ 1.7% by weight
And 0.02% by weight ≦ Ti ≦ 0.1% by weight and the balance Fe
A high fatigue strength crankshaft is provided, which is a hot forged crankshaft using a steel material made of an unavoidable element, which is subjected to a soft nitriding treatment also serving as an artificial aging treatment after a solution treatment.

[0007] Since the steel material having the above-mentioned composition has a ferrite single-phase structure on the metal structure, it has good hot forging workability substantially equivalent to mild steel and good workability of a crankshaft intermediate.

Further, since the steel material exhibits age hardening property with the supersaturated solid solution of Cu, it is possible to improve the mechanical strength of the crankshaft by subjecting the crankshaft intermediate after the solution treatment to artificial aging treatment. It is possible.

Further, by containing Ti under extremely low carbon, the steel material exhibits good nitrocarburizing properties at the artificial aging temperature of the steel material. That is, in this steel material, the artificial aging temperature and the nitrocarburizing treatment temperature substantially match.

[0010] Thus, the fatigue strength of the crankshaft can be sufficiently improved by subjecting the intermediate crankshaft to a soft nitriding treatment also serving as an artificial aging treatment. In addition, energy saving and reduction in manufacturing cost can be achieved by integrating both processes.

From the viewpoint of improving the fatigue strength, the depth d of the surface hardened layer (which means a fully nitrided layer; the same applies hereinafter) is d ≧ 0.6 mm.
It is desirable that However, the upper limit of the depth d is d = 1.0 mm. If d> 1.0 mm, the crankshaft becomes brittle.

Further, since the nitrocarburizing treatment is performed at a relatively low temperature, the distortion of the heat treatment of the crankshaft is small, and the hardness of the surface of the crankshaft and its vicinity is not so high after the nitrocarburizing treatment. Corrections can be made. Thus, it is possible to employ a long integrated structure in the crankshaft and improve the dimensional accuracy thereof.

[0013] In the steel material, the function and effect of each chemical component and the reason for limiting the content are as follows.

C: In order to secure high ductility and to make the surface hardened layer by nitrocarburizing treatment deeper by changing the metal structure of the steel material to a ferrite single phase structure, the C content is preferably as small as possible. When the C content is C> 0.01% by weight, the ductility of the steel material decreases, and the surface hardened layer becomes shallow.

Si: Si is an element for improving the strength of a steel material, and its content is adjusted according to the required strength. However, when the Si content is Si> 1% by weight, the ductility of the steel material is low, and the hot forgeability of the steel material is low.

Mn: Mn is an element for improving the strength of a steel material like Si, and its content is adjusted according to the required strength. However, when the Mn content is Mn> 0.5% by weight, the ductility of the steel material is low, so that the hot forgeability of the steel material is reduced. Flaws easily occur on the surface of the steel material.

P: P, like Si and Mn, is an element for improving the strength of steel and its content is adjusted according to the required strength. However, when the P content is P> 0.1% by weight, secondary working cracks occur.

S: The S content is preferably low in order to increase the ductility of the steel material. When the S content is S> 0.03% by weight, the ductility of the steel material is significantly reduced.

Al: Al has the effect of improving the nitrocarburizing property of steel. However, when the Al content is Al> 0.1% by weight, the hot forgeability and machinability of the steel material are reduced, while when Al <0.02% by weight, the effect of the addition is not obtained.

Cu: Cu imparts age hardening to steel as described above. However, when the Cu content is Cu> 1.7% by weight, the surface quality of the steel material is deteriorated. On the other hand, when Cu <0.8% by weight, there is no effect of the addition.

Ti: Ti imparts good nitrocarburizing properties to steel under extremely low carbon as described above. That is, Ti has an effect of forming a fine double nitride with Fe and deepening the surface hardened layer. However, if the Ti content is Ti> 0.1% by weight, the surface hardened layer becomes too deep and the steel material becomes brittle, while if Ti <0.02% by weight, there is no effect of the addition.

The steel material contains Ni as another additive element.
It may contain 15% by weight ≦ Ni ≦ 0.7% by weight.
Ni has the effect of improving the surface quality of the steel material and preventing hot brittleness.

Solution treatment temperature T₁Is 780 ℃ ≦ T₁≦ 1
050 ° C. and the processing time t₁Is t ₁≒ 0.5 hours
Each is set. This solution treatment is completed after hot forging.
It may be performed by quenching immediately after completion. In addition,
Solution treatment temperature T₁Is T₁<Saturated Cu at 780 ° C
To form a solid solution, while T₁> 1050 ° C
Causes coarsening of crystal grains, resulting in a decrease in strength and toughness.
Good.

The artificial aging temperature T ₂ of the steel is 550 ° C. ≦
T ₂ ≦ 600 ° C. However, if the artificial aging temperature T ₂ is T
₂ > At 600 ° C., overaging occurs and the internal hardness decreases, so that fatigue strength cannot be sufficiently improved.
At T ₂ <550 ° C., the artificial aging treatment and the nitrocarburizing treatment cannot be performed.

The processing time t_TwoIs 2 hours ≦ t_Two≤ 4 hours
It is preferred that Processing time t _Two<2 hours for surface
The depth d of the hardened layer is d <0.6 mm, while t_Two> 4
In time, the depth d exceeds the upper limit d = 1.0 mm.
You.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A crankshaft 1 shown in FIG.
Is used for a multi-cylinder internal combustion engine, in the illustrated example, an in-line four-cylinder internal combustion engine, and has a long integrated structure. The crank arm 2 at one end is also used as the gear 3.

Table 1 shows the composition of the round steel which is a constituent material of the crankshaft 1.

[0028]

[Table 1]

In the manufacture of the crankshaft 1, each process of hot forging, solution treatment, machining, soft nitriding and artificial straightening combined with artificial aging was used.

Hereinafter, each step will be described specifically.

A. Hot forging process of cutting out a 250 mm long steel slab from a 50 mm diameter round bar, heating the steel slab to 850 ° C., removing the scale of the steel slab, manufacturing a cast by a forging roll,
A crankshaft intermediate was obtained through a step of performing stamping including bending, rough stamping, and finishing by a high-speed forging press, a step of punching out by a crank press, and a step of shaping by a crank press.

B. Solution treatment Hot forging end temperature (solution treatment temperature T ₁ ) is 920
The crankshaft intermediate at ℃ was rapidly cooled, and the crankshaft intermediate was subjected to a solution treatment.

C. Machining The crankshaft intermediate was subjected to cutting and grinding to form a crank arm 2, a gear 3, a crank journal 4, a crank pin 5, a spline shaft 6, and the like.

D. Soft nitriding treatment also serving as artificial aging treatment A crankshaft 1 was obtained by subjecting a crankshaft intermediate to soft nitriding treatment also serving as artificial aging treatment. Processing conditions are
An NH ₃ gas atmosphere based on N ₂ gas, an artificial aging temperature T ₂ = 580 ° C., and a processing time t ₂ = 2 hours were respectively set. Another crankshaft 1 was obtained by setting the processing time t ₂ to t ₂ = 3 hours.

E. Curve straightening processing Both crankshafts 1 are long and integral and have a complicated shape, so during the nitrocarburizing treatment also serving as the artificial aging treatment, despite the relatively low treatment temperature T ₂ , Some bending occurs. Therefore, a bending straightening process was performed on both crankshafts 1 using a straightening machine. Here, the case where the processing time t _{2 in the} item D is t ₂ = 2 hours is referred to as Example 1, and the case where t ₂ = 3 hours is referred to as Example 2.

For comparison, medium carbon steel (JIS S48)
Using a round steel made of C) as a constituent material, the same hot forging as described above was performed to obtain a crankshaft intermediate. In this case, the heating temperature of the billet was set to 850 ° C. Next, the crankshaft intermediate is subjected to normalizing treatment at 880 ° C. for 2.5 hours, machining similar to that described above, nitrocarburizing treatment similar to that described above (however, the processing time t ₂ is t ₂ = 3 hours), and bending correction processing. Were sequentially applied to obtain a crankshaft. This is referred to as Comparative Example 1.

Further, steel for nitrocarburizing (0.2% by weight C-0.1
The same hot forging as described above was carried out using a round bar composed of weight% Al-1 weight Cr-balance Fe) to obtain a crankshaft intermediate. In this case, the heating temperature of the billet is 8
The temperature was set to 80 ° C. Next, the crankshaft intermediate was subjected to normalizing treatment at 900 ° C. for 2.5 hours, machining similar to the above, and nitrocarburizing treatment similar to the above (the treatment time t ₂ was t ₂ = 3 hours). I got a crankshaft. Although this crankshaft was bent, its correction was extremely difficult. This is referred to as Comparative Example 2.

FIG. 2 shows the relationship between the distance from the surface and the hardness HV0.2 for Examples 1 and 2 and Comparative Examples 1 and 2. As is clear from FIG. 2, Examples 1 and 2 have a higher depth d of the surface hardened layer than Comparative Examples 1 and 2, but the hardness at the surface and in the vicinity thereof is almost the same as Comparative Example 1. Therefore, it is significantly lower than that of Comparative Example 2. Based on this fact, Examples 1 and 2
It can be seen that the straightening process can be performed after soft nitriding.

Next, a bending fatigue test was performed on each of Examples 1 and 2 and Comparative Examples 1 and 2 by swinging to measure the bending fatigue strength at the fillet portion 7 of the crankpin 5.

FIG. 3 shows Examples 1 and 2 and Comparative Examples 1 and 2.
Stress repetition number N and stress amplitude σ _aThe relationship is shown below.
Table 2 shows the stress for Examples 1 and 2 and Comparative Examples 1 and 2.
Number of repetitions N = 10⁷Stress amplitude σ_aIs shown.

[0041]

[Table 2]

As is clear from FIG. 3 and Table 2, Examples 1 and 2 have higher flexural fatigue strength than Comparative Example 2 using nitrocarburizing steel.

[0043]

According to the present invention, it is possible to perform a nitrocarburizing treatment which has good hot forgeability and machinability, and after the solution treatment, also serves as an artificial aging treatment. It is possible to provide a crankshaft which has a high fatigue strength and is capable of adopting a long integrated structure and improving dimensional accuracy, each of which is made of a specific steel material that can be bent and straightened after processing. . In addition, by integrating the artificial aging treatment and the nitrocarburizing treatment, energy saving and reduction in manufacturing cost can be achieved, and a relatively inexpensive crankshaft can be provided.

[Brief description of the drawings]

FIG. 1 is a front view of a crankshaft.

FIG. 2 is a graph showing a relationship between a distance from a surface and a hardness HV0.2.

FIG. 3 is a graph showing a relationship between _a stress repetition number N and a stress amplitude σa.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Crankshaft 2 Crank arm 4 Crank journal 5 Crank pin

Claims (5)

[Claims] 1. C ≦ 0.01% by weight and Si ≦ 1% by weight
And 0.05% by weight ≦ Mn ≦ 0.5% by weight and P ≦ 0.
1% by weight, S ≦ 0.03% by weight, and 0.02% by weight ≦
sol. Al ≦ 0.1% by weight and 0.8% by weight ≦ Cu ≦
1.7% by weight and 0.02% by weight ≦ Ti ≦ 0.1% by weight
And a hot forged crankshaft using a steel material consisting of the balance Fe and unavoidable elements, and after solution treatment,
A high-fatigue-strength crankshaft characterized by being subjected to a nitrocarburizing treatment which also serves as an artificial aging treatment. 2. C ≦ 0.01% by weight and Si ≦ 1% by weight
And 0.05% by weight ≦ Mn ≦ 0.5% by weight and P ≦ 0.
1% by weight, S ≦ 0.03% by weight, and 0.02% by weight ≦
sol. Al ≦ 0.1% by weight and 0.8% by weight ≦ Cu ≦
1.7% by weight and 0.02% by weight ≦ Ti ≦ 0.1% by weight
0.15% by weight ≦ Ni ≦ 0.7% by weight and the balance Fe
A hot-forged crankshaft using a steel material comprising an unavoidable element, wherein the crankshaft is subjected to a soft nitriding treatment also serving as an artificial aging treatment after a solution treatment. 3. The artificial aging temperature T ₂ is 550 ° C. ≦ T ₂ ≦ 6.
The high fatigue strength crankshaft according to claim 1, wherein the temperature is 00 ° C. 4. 4. The high fatigue strength crankshaft according to claim 1, wherein the straightening treatment is performed after the soft nitriding treatment also serving as the artificial aging treatment. 5. The high fatigue strength crankshaft according to claim 1, wherein the crankshaft has a long integrated structure so as to be applied to a multi-cylinder internal combustion engine.