JP2000008141A - Non-heat treated soft-nitrided steel forged parts and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce non-heat treated soft-nitrided steel forged parts excellent in fatigue strength and bending straightenability after nitriding treatment. SOLUTION: These non-heat treated soft-nitrided steel forged parts have a chemical compsn. contg., by weight, 0.02 to 0.30% C, 1.0 to 2.0% Mn, <=0.10% P, 0 to 0.15% Cr, 0 to 0.01% sol.Al, <=0.02% Ti, 0.010 to 0.030% N and 0 to 0.02% V with inevitable impurity elements. For improving the machinability thereof, one or more kinds of elements among 0.04 to 0.10% S, 0.0003 to 0.0030% Ca and 0.05 to 0.20% Pb may be incorporated therein. This steel is forged, is, if required, subjected to machining as non-heat treated, is thereafter subjected to nitriding treatment and is, if required, subjected to bending straightening.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a non-heat treated soft nitrided steel forged part having high fatigue strength and excellent bending straightening property, which is not subjected to a tempering treatment such as quenching and tempering or normalizing after forging. It relates to the manufacturing method. The present invention is particularly suitable for a crankshaft used for an automobile engine or the like.

[0002]

2. Description of the Related Art Forged steel parts requiring high fatigue strength,
For example, a crankshaft for an automobile engine is often subjected to a surface strengthening process such as induction hardening or soft nitriding after forging and machining. Soft nitriding is slightly inferior to induction hardening in terms of improvement in fatigue strength, but forms a hard compound layer on the surface and is extremely excellent in seizure resistance and galling resistance. Therefore, soft nitriding is often used. The nitrocarburizing treatment is one in which the nitrided layer does not become brittle and has appropriate softness and toughness. In the following description, the nitrocarburizing treatment is simply referred to as nitriding treatment.

FIG. 1 is a process chart showing a method for producing a soft-nitrided forged part of a tempered steel and a non-tempered steel. FIG. 1 (a) shows a case of a tempered steel, and FIG. This is the case for high quality steel. As shown in Fig. 5 (a), in the case of tempered steel, the billet is left to cool (not shown) after hot forging, and the metal structure is adjusted by tempering treatment. Then, the billet is an as-forged structure. Next, a nitriding treatment is performed. However, as shown in the figure, in the case of a forged part that requires machining, such as a crankshaft, the surface is hardened after nitriding and machining is difficult, so machining is performed before nitriding. In some cases, the forged part after the nitriding treatment may be bent, and the product is subjected to appropriate bending correction as necessary.

In recent years, non-heat treatment shown in FIG. 1 (b) has been studied for many nitrocarburized forged parts for cost reduction and production lead time reduction, and the same applies to crankshafts. However, some performance may be degraded by omitting the temper treatment, and therefore, there are some nitrocarburized forged parts that cannot be non-heat treated.

One of the performance degradations is the problem of fatigue strength. The fatigue limit of parts that have been subjected to nitriding without tempering after forging (hereinafter referred to as non-tempered nitrocarburized steel) is the parts that have been subjected to tempering after tempering after forging steel of the same composition. (Hereinafter referred to as tempered nitrocarburized steel).

[0006] Second, there is a problem of bend straightening. The nitriding process may deform the forged part, which must be straightened. A large crack may be generated during bending straightening, and the limit amount of strain at which cracking occurs (hereinafter referred to as the amount of strain that can be straightened) is greater for non-tempered nitrocarburized steel than for tempered nitrocarburized steel. Is also small.

Generally, the smaller the amount of strain that can be bent, the lower the fatigue limit of the part. In particular, in the manufacture of crankshafts, bending correction is often performed after nitriding treatment, so that it is difficult to apply non-heat-treated nitrocarburized steel.

Heretofore, an invention has been made to obtain a high fatigue limit in a forged state without adding a tempering treatment or a nitriding treatment by adding a precipitation hardening element in a high concentration.

For example, Japanese Patent Application Laid-Open No. 8-144018 discloses a steel material in which appropriate amounts of Cr, V, and Al are added to aim at surface hardening by nitrocarburizing treatment and to prevent a decrease in the strength of a core material due to nitrocarburizing. I have.

Japanese Patent Application Laid-Open No. 7-102340 discloses M
n, S, Ti, V are added in appropriate amounts, and MnS, TiN, VN
Discloses a method in which the fatigue strength is ensured by complex precipitation of, the amount of ferrite + pearlite structure after forging is controlled, and the yield strength is improved by further performing aging treatment.

[0011] Japanese Patent Application Laid-Open No. 193931/1992 discloses C,
There is disclosed a material which can be used as forged skin by adding an appropriate amount of V, suppressing the addition of Si, preventing decarburization of the ferrite phase and preventing a decrease in hardness.

[0012] These are steels containing a high concentration of vanadium (V), which is a strong precipitation hardening element, and are expensive. If seizure resistance or the like becomes a problem, these high-V steels must be soft-nitrided.
At this time, the bending straightness of the high-V steel after the nitriding treatment is extremely poor as shown in the examples described later. Further, these publications do not attempt to simultaneously improve the fatigue limit and the bending straightness.

[0013]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a steel sheet without forging after tempering, without any cracks occurring at the time of bending correction after nitriding treatment, or a high strain amount at which cracks occur. Another object of the present invention is to provide a forged steel part having a high fatigue limit and a method of manufacturing the same. Specifically, the performance of a typical S48C steel soft-nitrided tempered crankshaft that has been subjected to normalizing treatment,
That is, it is to simultaneously achieve the fatigue limit and the bending straightness (the amount of strain that can be bent).

[0014]

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 where fatigue fracture occurs in non-heat treated nitrocarburized steel is within the diffusion layer or at the boundary between the diffusion layer and the base metal,
A crack that is a problem in straightening is a crack in the diffusion layer. Both show that the properties of the diffusion layer determine the performance. In the following description, “surface” refers to the surface side of the diffusion layer excluding the compound layer.

(A) The reason why the fatigue limit of the non-heat treated nitrocarburized steel is low is as follows. In the vicinity of the boundary between the diffusion layer and the base material, high tensile stress remains in the non-heat-treated nitrocarburized steel. This tensile residual stress becomes the starting point of fatigue fracture.

In the case of non-tempered nitrocarburized steel, even if the steel does not contain any precipitation hardening element, the hardness is significantly increased on the surface and decreases steeply toward the inside. For this reason, a high compressive residual stress is generated on the surface, but it is presumed that a tensile residual stress is generated near the boundary of the base material, which is balanced with it.

It is presumed that the reason why the tensile residual stress increases near the boundary of the base material is that the hardness of only the surface is extremely high and the depth of the hardened layer is small. This means that nitrogen entering from the outside hardly enters the inside and stays on the surface. From the viewpoint of the diffusion behavior of nitrogen during nitriding, the cause is considered as follows.

Non-heat treated steel is heated to 1100 ° C.
Since the forging is completed at a temperature of 00 ° C. or more and the mixture is left to cool as it is, the structure thereof is composed of a thin net-like ferrite along a giant old austenite grain boundary and the remaining pearlite. On the other hand, the structure of the tempered steel is a mixed structure of fine ferrite and pearlite (in the case of normalization) transformed from fine austenite, or martensite composed of extremely fine lath structure and carbide. Or bainite structure (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 is because the hardenability is large due to the large austenite grain size of the non-heat treated steel, and the ferrite transformation is suppressed.

When nitriding, the diffusion rate of nitrogen is high in ferrite and extremely low in pearlite because the diffusion is inhibited by layered cementite. In the non-heat-treated steel, since the ferrite is thinly concentrated in a net shape along the old austenite grain boundary, diffusion of nitrogen into the inside can be performed only through the ferrite. Therefore, nitrogen that has entered from the outside hardly enters the inside. This is considered to be a cause of the tensile stress remaining near the boundary of the base metal part in the non-heat treated steel.

On the other hand, in the structure subjected to the tempering treatment (normalizing treatment), fine ferrite is distributed not only in the grain boundaries but also in the whole structure, so that a diffusion path exists throughout the structure. For this reason, it is presumed that a mild hardness distribution can be formed from the surface to the inside of the tempered steel when the nitriding treatment is performed.

(B) The cause of the decrease in the amount of strain that can be bent in non-tempered nitrocarburized steel is as follows. The higher the surface hardness of steel, the easier it is for cracks to occur during straightening. However, the crack length cannot be uniquely determined only by the surface hardness.

As described above, the structure of the non-heat treated steel is a mixed grain in which the ferrite transformation is suppressed. The crack generated at the time of bending correction propagates as a unit of pearlite grains and stops at ferrite at the grain boundary, so that once a crack occurs, the crack length increases.

On the other hand, in the tempered steel, since fine ferrite grains are distributed throughout the structure, it is considered that even if a crack is generated by bending correction, the crack does not progress much.

Summarizing the above, the problems of the nitriding treatment of the non-heat treated steel are as follows: (a) the small depth of the hardened layer causes the deterioration of the fatigue strength; and (b) the surface hardness after the nitriding treatment. When it is raised, it deteriorates the bend straightening ability, and it is concentrated.

Therefore, in order to reduce the high tensile residual stress at the boundary which becomes the crack initiation point in the non-heat-treated nitrocarburized steel, the base metal should have a structure in which nitrogen atoms are easily diffused and the hardness gradient should be gentle. It is necessary that the surface is not excessively hardened by nitriding.

Therefore, the present inventors conducted the following experiment in order to specifically confirm a method for improving a tissue or the like.

C: 0.05% C, Mn: 1.4%, N:
Based on a low-medium carbon steel having 0.017% and Ti: 0.008% as a basic composition, 22 steels with various contents of various elements were melted to prepare a raw steel bar.

FIG. 2 is a schematic view showing a test piece obtained by forging the above-mentioned material steel bar to simulate a crankshaft and machined.

Table 1 shows a list of the above 22 types of steel. Steel X1 in the uppermost column of Table 1 is the basic composition. On the other hand, steels of steel X2 or less are C, Mn, P, Cr, Ti, N,
It is a steel with a composition for knowing the effect of V.

[0030]

[Table 1]

The raw steel bars produced by these laboratory melts were used for 120
Heated to 0 ° C., hot forged, air-cooled, processed into test pieces without tempering, and gas-nitrided (RX gas: NH
After maintaining at 570 ° C. for 3 hours in an atmosphere of ₃ = 1: 1, oil cooling was performed. Here, the RX gas is CO, N ₂ , H
_It is an endothermic gas with carburizing properties consisting of ₂ .

After forging S48C steel which is generally used as a tempered steel for crankshafts for comparison, normalizing treatment (8
After reheating to 60 ° C. and holding for 15 minutes, the mixture was air-cooled), processed into the same test piece, and subjected to the same nitriding treatment. A fatigue test and a bending test were performed on the test pieces after the nitriding treatment.

The fatigue test was carried out at room temperature at a repetition rate of 5 Hz by a load control swing with a three-point bending supporting the end of the journal and the center of the pin shown in FIG. the stress amplitude of 10 ⁷ times was defined as the fatigue limit. The stress here is the stress at the pin fillet R where a fatigue crack occurs, and is converted from a value measured by a strain gauge having a length of 1 mm.

On the other hand, the bending straightness was evaluated by a static bending test using the same test piece. Attach the strain gauge to the same place where the strain gauge was attached at the time of the fatigue test, apply bending at room temperature, consider the disconnection of the strain gauge as the occurrence of a crack, and calculate the amount of strain at that time to be able to correct the bending. And Since the amount of strain that can be bent is large, 1
Four tests were performed for each steel type, and the average was evaluated.

FIG. 3 is a graph showing the influence of C, Mn, P, Cr, Ti, N and V on the fatigue limit and the amount of strain that can be bent. In the same figure, the fatigue limit and the amount of strain that can be bent and corrected for the normalized material of the S48C steel are shown as a guide.

From these results and the observation of the structure, it was found that the following was necessary in order to improve the fatigue strength and the bending straightness.

A small amount of Ti suppresses austenite grain growth during forging heating. By reducing the C content, the ferrite is uniformly distributed over the intragranular grain boundaries. Ti and A
By suppressing the l content, the amount of dissolved nitrogen is secured and the fatigue limit is improved. By limiting V, Cr and Al, excessive surface hardness after nitriding is suppressed. The fatigue limit is improved by increasing the contents of P, Mn, and N.

The present invention has been completed based on the above findings and has the following (1) to (3).

(1) Chemical composition in weight%, C: 0.02 to
0.30%, Mn: 1.0 to 2.0%, P: 0.10%
Hereinafter, Cr: 0 to 0.15%, sol. Al: 0-0.
01%, Ti: 0.02% or less, N: 0.010-0.
030%, V: 0 to 0.02%, and an unavoidable impurity element is contained.

(2) The chemical composition of the forged non-heat treated nitrocarburized steel according to the above item (1) further includes S: 0.04 to 0.10.
%, Ca: 0.0003 to 0.0030%, Pb: 0.
A non-heat treated nitrocarburized steel forged part containing one or more elements of from 0.05 to 0.20%.

(3) Chemical composition in weight%, C: 0.02 to
0.30%, Mn: 1.0 to 2.0%, P: 0.10%
Hereinafter, Cr: 0 to 0.15%, sol. Al: 0-0.
01%, Ti: 0.02% or less, N: 0.010-0.
030%, V: 0 to 0.02%, and after hot forging a steel slab containing an unavoidable impurity element, it is allowed to cool and then subjected to a soft nitriding treatment without a tempering treatment. To produce a non-heat treated soft nitrided steel forged part.

[0042]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The action of each constituent element of the present invention and the reason for limiting the concentration of each element are as follows.

C: 0.02 to 0.30%, and when C is excessive, the volume ratio of pearlite increases, and as a result, the bending correctability deteriorates. Also, the fatigue limit tends to decrease. Therefore, the upper limit is set to 0.30%. However, C as a solid solution strengthening element for ferrite is important for securing fatigue strength, and for that purpose, it is necessary to add a solid solution limit at room temperature or more in view of safety. Therefore, the lower limit of the addition amount is set to 0.02%. The preferred range is 0.05 to 0.30
%.

Mn: 1.0 to 2.0%, Mn is an element effective for improving the fatigue limit. Therefore, the lower limit must be 1.0%. However, if it is added in excess of 2.0%, the volume percentage of pearlite is increased, and the bending straightening property is reduced. Therefore, the Mn content is 1.0
To 2.0%. The preferred range is 1.0-1.6%.

P: 0.10% or less, P is an element effective for improving the fatigue limit. However, at the same time, the impact value and the fracture toughness value are reduced, and the bending correctability is deteriorated. Therefore, the addition amount should be determined in consideration of the balance between the two. However, if contained in a large amount, the bending straightening property is greatly deteriorated in spite of the improvement of the fatigue limit, so the upper limit was made 0.10% or less. Preferably it is 0.08% or less. The lower limit is preferably set to 0.01% in view of the cost for removing P and the like.

Cr: 0 to 0.15%, desirably does not contain Cr. This is because when Cr is contained, nitrides are generated by nitriding treatment to increase the hardness of the surface and deteriorate the bending straightness. However, reducing the content to 0.15% or less greatly increases the refining cost. Therefore, the content up to 0.15% is allowed. Preferably 0.07
% Or less.

Sol. Al: 0 to 0.01%, Al is an element generally effective as a deoxidizing agent for steel. However, in forged parts, even at a normal level of content,
Has an undesired effect of lowering the fatigue limit by fixing nitrogen as AlN to reduce solid solution nitrogen and hardening the surface by nitriding treatment to deteriorate bending straightness. Therefore, sol. It is desirable that Al be minimized as an impurity. Even if Al is added for deoxidation, its content should be kept to a minimum. Therefore, sol. The content as Al was set to 0.01% or less. Preferably it is 0.005% or less.

Ti: 0.02% or less, a trace amount of Ti makes the ferrite pearlite structure fine by suppressing the growth of austenite grains during heating before 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. However, as the amount of addition increases, the amount of solute N in steel decreases and the fatigue limit decreases. Therefore, the upper limit is set to 0.02%. In addition, in a component system having a high ferrite volume ratio, the contribution to bending correctability by miniaturization is small, so that addition of Ti is not necessarily required. A preferred range is 0.01% or less.

N: 0.010-0.030%, N is an element effective for improving the fatigue limit. To obtain this effect, 0.010% or more is required. However, 0.03
If the content exceeds 0%, the effect is saturated and the bend straightening property is reduced. Therefore, the content is set to 0.010 to 0.030%. The preferred range is 0.012 to 0.020%.

V: 0 to 0.02%, V is not added beyond mixing as unavoidable impurities. Inevitable impurities must be made 0.02% or less. If the content exceeds 0.02%, the surface hardness after the nitriding treatment increases, and the bend straightening property is significantly reduced. Preferably 0.0
1% or less.

Further, when machinability is required, it is desirable to intentionally add one or more of the following three elements.

S: 0.04 to 0.10%. Since S is effective in improving machinability, 0.04% or more is necessary.
The reason for setting the content to 0.10% or less is that if it exceeds this value, a defect occurs in the continuously cast slab. The preferred range is 0.04 to
0.07%.

Ca: 0.0003-0.0030%, C
Since a is effective for improving machinability, 0.0003% or more is necessary. However, if the content exceeds 0.0030%, the inclusion of large inclusions cannot be avoided.
0.0030%. The preferred range is 0.0010 to 0.00.
0030%.

Pb: 0.05 to 0.20%, Pb is effective for improving machinability, so 0.05% or more is required. However, if it is contained excessively, the amount of inclusions increases and the fatigue limit is remarkably reduced. Therefore, the content is set to 0.05 to 0.20%. The preferred range is 0.05-0.16%.

It should be noted that Si as a common element of steel is necessary as a deoxidizing material and is not particularly limited, but is preferably in the usual range of 0.05 to 0.60.

Next, a method for manufacturing a forged part of a non-heat treated soft nitrided steel according to the present invention will be described. The steel slab having the chemical composition 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. However, since the pressing ability is required accordingly, as a general condition, 1 is required.
200 ° C as standard, 1150 to 1400 depending on press capacity
Determine in the range of 1250 ° C. After forging, it is allowed to cool (air cooling) in terms of manufacturing cost. However, there is no problem even if forced air cooling by air blowing or the like is performed to slightly shorten the manufacturing time.

After the shape is formed by forging and, if necessary, machining, it is not necessary to perform a tempering treatment such as normalizing or quenching and tempering.

Next, a nitriding treatment is performed. Nitriding conditions are, for example, in a gaseous atmosphere of RX gas: ammonia = 1: 0.8 to 1.2, an ambient temperature of 570 to 600 ° C., and a time of 60 minutes to 2 hours.
Nitriding is performed for 40 minutes, and then oil cooling is performed directly. The gas composition ratio, ambient temperature and time are determined as described above in order to obtain a proper compound layer for seizure resistance and a diffusion layer having a sufficient depth. There is no particular difference from the shaft manufacturing method. The nitriding method may be any method other than gas soft nitriding, as long as it can be performed in a short time. For example, salt bath nitriding or ion nitriding may be performed. After the nitriding treatment, the product is bent and straightened as necessary, such as when bending or strain is large.

[0059]

EXAMPLES Table 2 shows a list of chemical compositions of 10 types of steel according to the present invention (Example of the present invention) and 16 types of steel for comparison (Comparative Example).

[0060]

[Table 2]

After smelting 50 kg of these steels in an air melting furnace, they were heated to 1200 ° C., hot forged into a rough shape of a test piece simulating a crankshaft shown in FIG. 2, and then allowed to cool. After that, some machining and gas nitriding were performed.
In the gas nitriding, the test piece was heated to 570 ° C. in an atmosphere having a gas ratio of RX: NH ₃ = 1: 1 and held for 180 minutes.
Oil cooled in oil at ℃. Each nitrided specimen was used for each test as it was.

The fatigue test was carried out in the same manner as described above, and the stress amplitude at which the number of repetition of breaking was 10 ⁷ was defined as the fatigue limit.

On the other hand, the bending straightness was evaluated by the same method as described above. As for machinability, all steels were tested for tool life. Table 3 shows the results of these tests on fatigue, bending and machinability.

[0064]

[Table 3]

As shown in the table, the steel of the present invention achieved the target values (equivalent to tempered nitrocarburized steel forged parts made of S48C steel) in both the fatigue limit and the amount of strain that can be corrected. ing. On the other hand, none of the steels of the comparative examples simultaneously achieve the target values of the fatigue limit and the amount of strain that can be corrected.

The results of the machinability in Table 3 show that Pb was added to S48C steel.
Was subjected to a tempering treatment on steel to which 0.05% was added. A tool having a tool life equal to or longer than this is regarded as good and is marked with a circle. P of the steel of the present invention example
It can be seen that the one with the addition of b has good machinability while simultaneously satisfying the fatigue limit and bending straightness.

[0067]

According to the forged non-heat-treated nitrocarburized steel component of the present invention, the structure of the material steel is improved even if the nitrocarburized nitro-nitride treatment is performed without performing the temper treatment after hot forging. Excellent fatigue limit and bend straightening property equal to or better than that of conventional tempered nitrocarburized steel forged parts can be secured. Since the refining process can be omitted, a large cost reduction can be achieved.

[Brief description of the drawings]

FIG. 1 is a process chart showing a method for producing a soft-nitrided forged part of a tempered steel and a non-heat-treated steel, and FIG.
(b) is the case of non-heat treated steel.

FIG. 2 is a schematic diagram showing a test piece for a fatigue test and a bending correction test simulating a crankshaft.

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

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C23C 8/38 C23C 8/38

Claims (3)

[Claims] 1. The chemical composition in% by weight, C: 0.02 to
0.30%, Mn: 1.0 to 2.0%, P: 0.10%
Hereinafter, Cr: 0 to 0.15%, sol. Al: 0-0.
01%, Ti: 0.02% or less, N: 0.010-0.
030%, V: 0 to 0.02%, and an unavoidable impurity element is contained. 2. The chemical composition of the forged non-heat treated nitrocarburized steel according to claim 1, further comprising: S: 0.04 to 0.10%, C
a: 0.0003-0.0030%, Pb: 0.05-
A forged non-heat-treated nitrocarburized steel component containing at least one element of 0.20%. 3. The chemical composition in weight%, C: 0.02
0.30%, Mn: 1.0 to 2.0%, P: 0.10%
Hereinafter, Cr: 0 to 0.15%, sol. Al: 0-0.
01%, Ti: 0.02% or less, N: 0.010-0.
030%, V: 0 to 0.02%, and after hot forging a steel slab containing an unavoidable impurity element, it is allowed to cool and then subjected to a soft nitriding treatment without a tempering treatment. To produce a non-heat treated soft nitrided steel forged part.