JPH1162943A - Soft nitrided as rolled or normalized crankshaft and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a soft intrided as rolled or normalized crankshaft to have excellent characteristics even when soft intrided as rolled or normalizing is applied without thermal refining and to provide manufacture thereof. SOLUTION: An as rolled or normalized crankshaft is manufactured of a steel consisting of 0.25-0.35 wt.% C, 0.05-1.50 wt.% Si, 0.80-1.20 wt.% MN, 0.005-0.03 wt.% Ti, 0.005-0.01 wt.% Al, 0.010-0.030 wt.% N, 0.10 or less wt.% S, and 0.0030 or less wt.% Ca, a rest consisting of Fe or inevitable impurities, and 0.03 wt.% P as impurities, 0.15 wt.% or less, and 0.02 wt.% V, and is soft- nitriding-treated. A material steel may further contain 0.05-0.20 wt.% Pb to improve machinability. The crankshaft is naturally cold-heat-dissipated after forging of a material. Thereafter, by applying a machining work and soft- nitriding treatment without effecting thermal treatment, manufacturing is executed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a steel sheet having high fatigue strength and excellent bending straightening even if it is subjected to nitrocarburizing treatment without tempering such as "quenching and tempering" and "normalizing" after forging. The present invention relates to a nitrocarburized non-heat treated crankshaft made of a steel having heat resistance, and a method of manufacturing the crankshaft.

[0002]

2. Description of the Related Art Crankshafts for automobiles and the like that require high fatigue strength are often subjected to surface treatment such as induction hardening and soft nitriding after forging and machining. Soft nitriding is slightly inferior to induction hardening in terms of improvement in fatigue strength, but is extremely excellent in that a hard compound layer is formed on the surface and improves seizure resistance and galling resistance. Such crankshafts (hereinafter referred to as "nitrocarburized crankshafts") are also widely used.

FIG. 1 shows a method for manufacturing a soft-nitrided crankshaft using a conventional tempered steel and a steel used as a material of a crankshaft of the present invention described below (hereinafter, referred to as "the present invention steel" for convenience). FIG. Here, (a) shows the steps up to the product when the conventional steel is used and (b) uses the steel of the present invention as a raw material.

In recent years, as shown in FIG. 1 (b), there is a lot of so-called "non-heat treatment" in which a heat treatment is omitted and a product is produced as forged in order to reduce costs and shorten production lead time. The same applies to nitrocarburized crankshafts. However, there is a performance that is degraded by omitting the tempering process, and therefore, there are parts that cannot be non-tempered.

First, after forging, a part subjected to a nitrocarburizing treatment without performing a tempering treatment (hereinafter referred to as a "non-tempered nitrocarburized steel part")
) Is lower than that of a part obtained by subjecting steel of the same composition to forging, tempering and then nitrocarburizing (hereinafter referred to as “tempered nitrocarburized steel part”).

Secondly, large cracks occur in the non-heat treated nitrocarburized steel parts when straightening after nitrocarburizing. Deformation caused by the nitrocarburizing treatment is corrected by applying bending deformation in the opposite direction. However, the critical strain amount at which cracks occur in non-heat treated nitrocarburized steel parts due to the bending (hereinafter referred to as “bending straightening strain amount”) Is smaller than that of tempered nitrocarburized steel. In general, the smaller the amount of strain that can be bent, the lower the fatigue limit of the part when the part is used in an automobile.

[0007] As described above, the non-heat-treated nitrocarburized steel part has a smaller amount of strain that can be straightened than the tempered nitrocarburized steel part. Cannot be used.

[0008] The non-heat treated steel is heated to 1100 ° C or higher,
Since the forging is completed at a temperature of 00 ° C. or more and the steel is left to cool, its structure is composed of a thin net-like ferrite along a giant old austenite grain boundary and pearlite of the rest. In contrast, the structure of tempered steel is transformed from fine austenite, (a) a mixed structure of fine ferrite and pearlite (in case of normalization), or (b) a martensite consisting of extremely fine lath and carbide. Bainite (in the case of quenching and tempering). The ferrite volume fraction of the non-heat treated steel is smaller than that of the normalized steel. This reflects that the hardenability is higher by the larger austenite grain size of the non-heat treated steel, and the ferrite transformation is suppressed accordingly.

Attempts have been made to simultaneously improve the fatigue limit and bend straightening of non-heat-treated nitrocarburized steel parts, but there is no example in which the object has been sufficiently achieved. For example,
An invention has been made in which, by adding a precipitation hardening element at a high concentration, a high fatigue limit can be obtained without forging treatment or nitrocarburizing treatment while forging. Such inventions are disclosed in JP-A-7-102340 and JP-A-4-193393.

Japanese Patent Application Laid-Open No. 8-144018 discloses an invention in which only the hardness after nitriding is considered. The steels of these inventions are all steels containing a high concentration of vanadium (V), which is a strong precipitation hardening element, and are expensive.
Further, when seizure resistance or the like becomes a problem, these high-V steels must be subjected to nitrocarburizing treatment, but the bending straightening properties of the high-V steel after nitrocarburizing are extremely poor.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for reducing the fatigue of the R portion of a fillet where stress and strain are concentrated due to repeated bending or when straightening is performed even if nitrocarburizing treatment is performed without tempering treatment. To provide a nitrocarburized non-refined crankshaft having a high limit and a crack generated at the time of straightening that is small enough to cause no practical problem, or a high strain limit at which a crack is generated, and a method of manufacturing the same. It is in.

Specifically, a typical J that has been subjected to normalizing
Provided is a crankshaft having a performance higher than that of a soft-nitrided tempered crankshaft made of IS S48C steel, that is, a performance that simultaneously satisfies a high fatigue limit and bending straightening property (bend straightening strain), and a method of manufacturing the same. With the goal.

[0013]

The gist of the present invention is as follows.
(1) The crankshaft according to (2) and the method according to (3).

(1) By weight%, C: 0.25 to 0.35%, Si: 0.
05-1.50%, Mn: 0.80-1.20%, Ti: 0.005-0.03%,
Al: 0.0005 to 0.01%, N: 0.010 to 0.030%, S: 0.10
% Or less, Ca: 0.0030 or less, the balance consists of Fe and unavoidable impurities, P as an impurity is 0.03% or less, Cr
A non-heat-treated crankshaft manufactured from steel with 0.15% or less and V of 0.02% or less and subjected to nitrocarburizing treatment.

(2) In addition to the above component (1), 0.05
Non-heat treated crankshaft manufactured from steel containing ~ 0.20% Pb and nitrocarburized.

(3) Forging a steel having the chemical composition described in (1) or (2) above into a crankshaft, allowing it to cool naturally, and thereafter subjecting it to machining and soft nitriding without heat treatment. A method for producing a soft-nitrided non-refined crankshaft, comprising:

The nitrocarburizing treatment in the method (3) is preferably performed under the following conditions.

Nitrogen gas atmosphere: RX gas: ammonia = 0.8 to
1.2 atmosphere Nitriding temperature ・ ・ ・ 570-600 ℃ Nitriding time ・ ・ ・ ・ 60-120 min Cooling after nitriding ・ ・ ・ Oil cooling

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In general, a nitrided layer formed by a nitriding treatment comprises a compound layer on the outermost surface and a diffusion layer thereunder. The starting point at which fatigue fracture occurs in a non-heat-treated nitrocarburized steel part is in the diffusion layer or at the boundary between the diffusion layer and the base material, and a crack that poses a problem in bending correction is a crack in the diffusion layer.
That is, it is the properties of the diffusion layer that govern fatigue fracture and cracking during bending correction. Therefore, in the following description, “surface” means the surface side of the diffusion layer excluding the compound layer.

The cause of the low fatigue limit of the conventional non-heat treated nitrocarburized steel parts is considered as follows.

In a non-heat-treated nitrocarburized steel part, a tensile stress remains near the boundary between the diffusion layer and the base material.

In order to improve the fatigue limit, it is necessary to reduce the tensile residual stress or more desirably to the compressive residual stress.

In the case of a non-heat treated nitrocarburized steel part, even if the material is a steel containing no precipitation hardening element, the hardness is significantly increased on the surface and decreases steeply toward the inside. For this,
It is presumed that a high compressive residual stress is generated on the surface, but a tensile residual stress is generated near the boundary, which is balanced with it.

In non-heat-treated nitrocarburized steel parts, the fact that the hardness of only the surface is extremely high and the depth of the hardened layer is small means that in non-heat-treated nitrocarburized steel parts, nitrogen that has entered from the outside hardly enters the interior. Means staying in In order to reduce the high tensile residual stress at the boundary part serving as the crack initiation point, it is necessary to diffuse the nitrogen atoms into the inside to make the hardness gradient gentle and to distribute the hardness deeply.

The diffusion rate of nitrogen is high in ferrite and extremely low in pearlite because diffusion is inhibited by layered cementite. In non-heat treated steels, nitrogen can only diffuse through the ferrite due to the thin concentration of ferrite at the former austenite grain boundaries.
On the other hand, in the microstructure subjected to the normalizing treatment, fine ferrite is distributed not only in the grain boundaries but also in the entire microstructure, and thus a diffusion path exists throughout the microstructure. For this reason, it is presumed that the tempered steel can have a gentle hardness distribution from the surface to the inside when the nitriding treatment is performed. Further, the coarse structure of the non-heat treated steel itself is also a factor that lowers the fatigue limit.

Next, the occurrence and size of cracks due to bending correction will be described.

The higher the surface hardness of steel, the easier it is for cracks to be formed during bending correction, and the longer the crack length. However, the crack length is not uniquely determined by the surface hardness alone. In addition, the crack propagates with pearlite grains as one unit. Therefore, the smaller the pearlite grains, the smaller the tendency.

Summarizing the above facts and conjectures,
It can be concluded that (a) the structure of the non-heat treated steel itself makes it difficult to solve the above problems, and (b) it is not preferable to use an element that increases the surface hardness after nitriding.

In order to confirm a specific method for improving the structure and the like, the present inventors conducted the following experiment.

Using a medium carbon steel containing 0.30% of C as a basic composition and 24 steels having various contents of various elements as materials, a test piece 1 simulating a crankshaft shown in FIG. A fatigue test and a bending test after the treatment were performed.
Table 1 shows the chemical compositions of the 24 types of base steel.

Steel No. X1 in the uppermost column of Table 1 is a basic composition. Correspondingly, steels with steel No. X2 and below are C, Mn, Cr,
It is a steel having a composition in which the contents of Ti, N and V are varied in order to know the effects. The raw material steel bars produced by these laboratories were heated to 1200 ° C., hot forged, naturally cooled, and processed into the test specimens shown in FIG.
Gas nitrocarburizing treatment (in an atmosphere of RX gas: NH ₃ = 1: 1)
After holding at 585 ° C. for 1.5 hours, oil cooling was performed. In addition,
For comparison, S commonly used for crankshafts
After forging 48C steel, normalizing treatment (reheating to 860 ° C and 15
After holding for one minute, the same test was performed after performing the same nitriding treatment.

[0032]

[Table 1]

The fatigue test was carried out in a room temperature atmosphere at a repetition rate of 5 Hz by three-point bending supporting the end of the journal portion 2 of the test piece and the center of the pin portion 3 with load control swing.
The stress amplitude at which the number of fracture cycles was 10 ⁷ was defined as the fatigue limit.

Here, the stress is a stress (measured and calculated by a strain gauge having a length of 1 mm) at a pin portion fillet R (indicated by reference numeral 4 in FIG. 2) where a fatigue crack occurs. on the other hand,
Bending straightness was evaluated by a static bending test using the same specimen. Attach the strain gauge to the same place where the strain gauge was attached at the time of the fatigue test, apply bending in the room temperature atmosphere, consider the breakage of the strain gauge as the occurrence of cracks,
The amount of strain at that time was defined as the amount of strain that can be corrected. Since the amount of bendable strain was large, the test was performed using four test pieces for each steel type, and the average value was evaluated.

FIG. 3 shows the effects of C, Mn, and Cr on the fatigue limit and the amount of strain that can be corrected (FIG. 3 (a)) and Ti,
The effects of N and V (FIG. 3 (b)) are shown. (a) At the left end of the figure, the fatigue limit and the amount of strain that can be bent are shown as a guide for the S48C standardized material.

From the above test results and the observation of the structure, it was confirmed that the following measures were effective in achieving the above object.

Suppression of austenite grain growth during forging heating by trace amount of Ti Uniform distribution of ferrite over intragranular grain boundaries by reducing C content Ensuring solid solution nitrogen amount by optimizing Ti content V Nitriding by limiting V and Cr Suppression of Subsequent Surface Hardness The present invention has been made by combining the above-described basic knowledge with detailed studies on the effects of each alloy component and impurities and the conditions of soft nitriding treatment.

I. Regarding the nitrocarburized non-heat treated crankshaft of the present invention Steel used as the material of the nitrocarburized crankshaft of the present invention (steel of the present invention)
The reason for limiting the action of each constituent element and the content of each element is as follows (all percentages of the component contents are% by weight).

C: 0.25% to 0.35% C is an element effective for securing the tensile strength, and therefore, a content of 0.25% or more is necessary. As shown in FIG. 3, the fatigue limit is low at less than 0.25%. But 0.
If the content exceeds 35%, ferrite is less likely to be generated from within the grains, and the structure of the non-heat treated steel cannot be close to that of the heat treated steel. As a result, FIG.
The upper limit is 0.35% because the bending straightenability decreases as shown in
The following is assumed.

Si: 0.05-1.50% Si is required as a deoxidizing agent at the time of melting, and at least 0.05% is required to obtain its effect. However, excessive Si content promotes decarburization during forging, so the upper limit of the content is
1.50%.

Mn: 0.80 to 1.20% Mn is an element effective for improving the fatigue limit as shown in FIG. To ensure the effect, set the lower limit
Must be 0.80%. However, excessive addition increases the volume ratio of pearlite, so that the bending straightening property decreases. Therefore, the appropriate range of the Mn content is 0.80 to 1.20%.

Ti: 0.005 to 0.03% A trace amount of Ti makes the ferrite pearlite structure finer by suppressing the growth of austenite grains during heating prior to forging. As a result, the structure of the non-heat treated steel can be made close to that of the normal steel, and the crack generated at the time of straightening can be reduced. In a component system having a high ferrite volume fraction, the contribution to bending straightness is small due to the refinement, but even in the composition range of the steel of the present invention, in a high C-high Mn component system, addition of Ti contributes to improvement in bending straightness. To obtain this effect, 0.005% or more is required. On the other hand, the content of Ti is 0.03
%, The solute N in the steel decreases and the fatigue limit decreases as shown in FIG. Therefore, the proper range of Ti content is
0.005 to 0.03%.

Al: 0.0005 to 0.01% Al is also required as a deoxidizing agent at the time of melting, and at least 0.0005% is required to obtain the effect. However, when Al is excessive, hard inclusions increase, and both durability and machinability of steel decrease. Therefore, the content of Al should be suppressed to 0.01%.

N: 0.010 to 0.030% As is clear from FIG. 3, N is an element effective for improving the fatigue limit. To achieve this effect, 0.010% or more is required. However, even if the content exceeds 0.030%, the effect is saturated and the bending straightening property decreases,
0.010 to 0.030%.

S: 0.10% or less S does not have to be positively added. That is, the content may be in the range of inevitable impurities. However, since S has an effect of improving machinability, it may be added positively. To obtain the effect, the content is desirably 0.04% or more. However, if S exceeds 0.10%, defects occur in the continuously cast slab, so the upper limit is 0.10%.

Ca: 0.0030% or less Ca, like S, need not be positively added. Therefore, its content may be in the range of unavoidable impurities. However, Ca is a component that can be positively added when improving machinability. Ca is effective in improving machinability
Since it is 0.0003% or more, it is desirable to secure a higher content when adding. On the other hand, Ca is 0.0030%
If it exceeds 300, the inclusion of large inclusions in the steel is inevitable.
Therefore, even when Ca is added, its content is 0.0030%
Should stop by.

One of the material steels for the crankshaft of the present invention comprises, in addition to the above components, the balance consisting of Fe and unavoidable impurities. The other one, which emphasizes particularly good machinability, is a steel containing the following Pb in addition to the above components.

Pb: 0.05 to 0.20% The effect of improving the non-machining property by adding Pb is as follows.
It becomes remarkable when it is more than%. However, when Pb is excessive, inclusions in the steel increase and the fatigue limit is remarkably reduced. Therefore, when Pb is added, its content is 0.03-0.20%
Should be. If Pb is further added after positively adding S and Ca described above, the effect of improving machinability is significantly increased due to the combined effect of these elements.

The steel of the present invention also has a great feature in that P, Cr and V described below are suppressed as impurities.

P: not more than 0.03% P decreases the impact value and fracture toughness value of steel.
It is desirable to have as little as possible. However, if the content is very small, the effect is small, and the refining cost increases to reduce P to an extremely low level. Therefore, the allowable upper limit is set to 0.03%.

Cr: 0.15% or less As shown in FIG. 3, Cr is effective in improving the fatigue limit, but deteriorates the bending straightness. This is because Cr forms nitrides by nitriding to increase the hardness.

Therefore, in the steel of the present invention, Cr is not actively added, and the upper limit thereof is suppressed to 0.15% as an impurity. 0.15
%, The harmful effects are relatively small, and refining costs increase significantly if the Cr content is extremely low.
The contents up to are determined to be acceptable. Below the upper limit, the smaller the better, the better.

V: not more than 0.02% As is clear from FIG. 3, V also increases the surface hardness after nitriding similarly to Cr, and significantly lowers the straightness. Therefore, V is not added beyond mixing as unavoidable impurities. It must be 0.020% or less as inevitable impurities.

II. Manufacturing method of soft-nitrided non-heat treated crankshaft of the present invention The soft-nitrided non-heat treated crankshaft of the present invention can be manufactured by the following method.

That is, the material having the above composition (the steel of the present invention) is heated and forged to obtain a desired shape. The heating temperature at this time is preferably as low as possible, but a low temperature forging requires a large pressing capacity.
The standard is set to 00 ° C, and the range is determined in the range of 1150 to 1250 ° C depending on the pressing ability. After forging, allow to cool naturally due to manufacturing cost
(Air cooling). However, there is no problem if forced air cooling is performed by air blowing or the like in order to shorten the manufacturing time.

After the desired shape is prepared, a soft nitriding treatment is performed without performing a tempering treatment such as normalizing or quenching and tempering that is generally performed. The conditions of soft nitriding are RX gas:
Ammonia = 0.8-1.2 atmosphere, temperature 570-600
℃ for 60 to 120 minutes, then oil-cooled directly. Here, the gas composition ratio, the environmental temperature and the time are determined as described above in order to obtain a proper compound layer for improving seizure resistance and a diffusion layer having a sufficient depth.

[0057]

Example 1 Table 2 is a list showing the chemical compositions of 10 kinds of steels of the present invention, 13 kinds of comparative steels, and 2 kinds of S48C equivalent steels subjected to the tests. After smelting 150 kg of each of these steels in an air melting furnace, the steels were heated to 1200 ° C., hot forged into crank-shaped test pieces having the shape and dimensions shown in FIG. 2 and allowed to cool. Thereafter, a slight machining was performed, and a soft nitriding treatment was performed.

In the gas nitrocarburizing, the gas ratio is set to RX gas: NH ₃ =
1: 1 and the specimen was heated to 585 ° C in that atmosphere,
After holding for 90 minutes, the mixture was oil-cooled in oil at 150 ° C. Each nitrided specimen was used for each test as it was.

The fatigue test was as described above, and the stress amplitude at which the number of repetitive fractures was 10 ⁷ was defined as the fatigue limit. On the other hand, bending straightness was also evaluated by the same method as described above. Tool life tests were also performed on all steels for non-machinability. The machinability was evaluated by a relative comparison based on a temper-treated steel (No. Z25 in Table 2) obtained by adding 0.05% of Pb to S48C steel.

[0060]

[Table 2]

Table 3 shows the results of each of the fatigue, bending, and non-machining tests. As is clear from the table, the nitrocarburized non-heat treated crankshaft of the present invention (example of the present invention) has a target value (No. Z24) in both the fatigue limit and the amount of strain that can be corrected.
Of nitrocarburized tempered crankshaft made of S48C steel. That is, a fatigue limit of 60 kgf / mm ² and a bendable strain of 3.5% have been achieved. On the other hand, none of the comparative examples simultaneously achieve the target fatigue limit and the amount of strain that can be corrected.

The machinability shown in Table 3 was obtained by adding Pb to S48C.
Those having a tool life equal to or greater than that of No. Z25 were marked as good, and those with a shorter tool life were marked x. It can be seen that among the examples of the present invention, those to which Pb was added had good machinability while satisfying the fatigue limit and the bending straightening property at the same time.

[0063]

[Table 3]

FIG. 4 shows the results of examining the effects of the temperature and the treatment time of the nitrocarburizing treatment on the fatigue limit using test specimens made of the steel X1 shown in Table 1 as a raw material. The gas composition for the nitriding treatment is as described above. As shown in the figure, in order to obtain the target fatigue limit (the fatigue limit of the nitrocarburized tempered crankshaft made of S48C), the processing temperature must be 570 ° C. or more and the processing time must be 60 minutes or more. The effect is almost saturated at a processing temperature of 600 ° C. or more and a processing time of 120 minutes.

[0065]

The soft-nitrided non-heat treated crankshaft of the present invention has a conventional temper due to the effect of improving the microstructure of the material steel even if it is subjected to a soft nitriding process after hot forging without tempering. Excellent fatigue limit and bending straightness equal to or higher than that of the treated nitrocarburized crankshaft can be secured. Since the method of the present invention for manufacturing the crankshaft does not require the step of the refining treatment, it has a significant effect of reducing costs and manufacturing time.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 (a) is a diagram showing a process for manufacturing a conventional nitrocarburized crankshaft (tempered material), and FIG. 1 (b) is a diagram illustrating a process for manufacturing a nitrocarburized crankshaft (non-tempered material) of the present invention. It is.

FIG. 2 is a drawing showing a shape of a test body simulating a crankshaft subjected to a fatigue test and a bending test, wherein (a) is a front view and (b) is a side view.

FIG. 3 is a drawing showing the effects of C, Mn, Cr, V, Ti, and N on the fatigue limit and the amount of strain that can be corrected.

FIG. 4 is a diagram showing the influence of temperature and treatment time of nitrocarburizing treatment on the fatigue limit and the amount of strain that can be bent.

Claims (4)

[Claims] C .: 0.25 to 0.35% by weight, Si: 0.05 to
1.50%, Mn: 0.80 to 1.20%, Ti: 0.005 to 0.03%, Al:
0.0005 to 0.01%, N: 0.010 to 0.030%, S: 0.10% or less, Ca: 0.0030 or less, the balance consists of Fe and unavoidable impurities, P as an impurity is 0.03% or less, and Cr is 0.
Non-heat treated crankshaft manufactured from steel with 15% or less and V of 0.02% or less and subjected to soft nitriding. 2. C .: 0.25-0.35% by weight, Si: 0.05-% by weight
1.50%, Mn: 0.80 to 1.20%, Ti: 0.005 to 0.03%, Al:
0.0005 to 0.01%, N: 0.010 to 0.030%, S: 0.10% or less, Ca: 0.0030 or less, and Pb: 0.05 to 0.20%, the balance being Fe and unavoidable impurities, and 0.03% of P as impurities. Hereinafter, a non-heat treated crankshaft manufactured from steel having a Cr content of 0.15% or less and a V of 0.02% or less and subjected to nitrocarburizing treatment. 3. A steel according to claim 1 or 2, wherein the steel is forged into a crankshaft, allowed to cool naturally, and thereafter subjected to machining and nitrocarburizing without heat treatment. A method for manufacturing a nitrided non-heat treated crankshaft. 4. The nitrocarburizing treatment is performed using RX gas: ammonia = 0.8.
4. The method for producing a soft-nitrided non-heat treated crankshaft according to claim 3, wherein the method is performed by heating at 570 to 600 [deg.] C. for 60 to 120 minutes in an atmosphere of to 1.2 and then oil cooling.