JPH093601A - Nitriding steel and its production - Google Patents

Abstract

PURPOSE: To provide a steel material having excellent fatigue limit and length of crack resulting from straightening, equal to or higher than those of heat treated steel, even if nitriding is applied without performing refining treatment. CONSTITUTION: The nitriding steel has a composition containing, by weight, 0.30-0.40% C, 0.05-0.40% Si, 0.20-0.60% Mn, <=0.08% P, 0.02-0.10% S, <=0.10% Cr, <=0.005% sol.Al, 0.005-0.013% Ti, 0.0003-0.0030% Ca, <=0.20% Pb, and 0.010-0.030% N. A steel product can be produced by applying nitriding treatment to this steel while omitting refining treatment. By this method, nitriding treatment can be applied to a non-heat treated steel composed of this steel. By using this steel, manufacturing costs, e.g. for automobile crankshaft can be remarkably reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nitriding steel having a high fatigue limit and an excellent bending straightening property even if it is subjected to nitriding treatment without being tempered. INDUSTRIAL APPLICABILITY The steel of the present invention is suitable, for example, as a material for an automobile crankshaft that is subjected to nitriding.

[0002]

2. Description of the Related Art Conventionally, automobile parts such as crankshafts, connecting rods or knuckles are hot forged into a desired shape with steel slabs such as carbon steel for machine structure, and then tempered for the purpose of improving fatigue limit. A manufacturing method of performing treatment (normalizing treatment or quenching and tempering treatment) has been adopted.

FIG. 1 is a drawing comparing the conventional method of manufacturing a heat-treated steel and the method of manufacturing a product of the present invention described later. Here, (a) shows a process up to a conventional steel product, and (b) shows a process up to the product of the present invention steel. In either case, nitriding is performed when seizure resistance, galling resistance, or even higher fatigue limit is required.

In recent years, as shown in FIG. 1 (b), so-called non-heat treated steel is adopted for many automobile parts, in which the heat treatment is omitted for commercialization without forging to reduce the cost. Is being considered. However, there is a performance that is deteriorated by omitting the refining process, and for this reason, there are some parts that cannot be non-refined.

First of all, the fatigue limit of a part that has been subjected to nitriding treatment without forging treatment after forging (hereinafter referred to as "non-tempered nitrided steel") has a fatigue limit after forging a steel of the same composition. It is lower than that of the part that has been subjected to the nitriding treatment (hereinafter referred to as heat-treated nitrided steel). Secondly, large cracks are generated during straightening after nitriding. The deformation caused by the nitriding treatment is corrected by the reverse bending deformation, and the cracks generated in the non-heat treated nitrided steel due to the bending are larger than those in the heat treated nitrided steel. In general, the larger the crack, the lower the fatigue limit of the part when the part is incorporated in an automobile and used (hereinafter, the crack generated during nitriding and straightening will not cause a problem in practical use. In case of (0.10mm or less),
Bending straightness is good). Since the crack length of non-heat treated nitrided steel generally exceeds 0.10 mm, it cannot be used for a crankshaft or the like for straightening bending when the strain after nitriding is large.

[0006] Non-heat treated steel is heated to 1100 ° C or higher and then heated to 10
Since the forging is completed at a temperature of 00 ° C. or higher and the material is left to cool, the structure is composed of thin net-like ferrite along the huge former austenite grain boundary and the remaining part of pearlite. In comparison, the structure of heat-treated steel is composed of (a) a mixed structure of fine ferrite and pearlite (in the case of normalization) transformed from fine austenite, or (b) extremely fine lath and carbide. It is either martensite or bainite (in the case of quenching and tempering).

Further, the volume fraction of ferrite of non-heat treated steel is smaller than that of normalized steel. This is because the hardenability is large due to the large austenite grain size of the non-heat treated steel,
This reflects that the ferrite transformation is suppressed to that extent.

Although attempts have been made so far to simultaneously improve the fatigue limit and bend straightening property of non-heat treated nitrided steel, there is no example that achieves it. An invention has been made in which, by adding a precipitation hardening element in a high concentration, a high fatigue limit can be obtained as-forged without performing heat treatment or nitriding treatment. JP-A-64-68245 and JP-A-4.
No. 19391 gazette is such an invention. All of these are steels containing a high concentration of vanadium (V), which is a strong precipitation hardening element, and are expensive. If seizure resistance is a problem, these high V
The steel must be nitrided, but at this time, the straightening property of the high V steel after the nitriding is extremely poor.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a high fatigue limit after nitriding treatment without heat treatment.
Another object of the present invention is to provide a steel in which cracks generated during straightening are reduced to such an extent that they do not become a problem in practice. Specifically, it is intended to simultaneously achieve the performance of S48C or more subjected to the normalizing treatment, the fatigue limit of 38 kgf / mm ² or more, and the crack length of 0.10 mm or less by bending straightening.

[0010]

Generally, the nitride layer formed by the nitriding treatment is composed of a compound layer on the outermost surface and a diffusion layer thereunder. The origin of fatigue fracture in non-heat treated nitrided steel is the boundary between the diffusion layer and the base metal, and the crack that is a problem in straightening is the crack in the diffusion layer. Both show that the properties of the diffusion layer determine the performance. Therefore,
In the following description, the term “surface” refers to the surface side of the diffusion layer excluding the compound layer.

(1) The reason why the fatigue limit of non-heat treated nitrided steel is low is as follows.

In the non-heat treated nitrided steel, 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 this tensile residual stress or, preferably, compressive residual stress. In non-heat treated nitriding steel, even if the steel does not contain a precipitation hardening element, the hardness becomes extremely high on the surface and decreases sharply toward the inside. For this reason, although high compressive residual stress is generated on the surface, it is presumed that tensile residual stress in equilibrium with it occurs near the boundary. The fact that the hardness of only the surface of the non-heat treated nitrided steel is extremely high means that in the non-heat treated nitrided steel, it is difficult for the nitrogen entered from the outside to enter the inside and stays on the surface. In order to reduce the high tensile residual stress at the boundary, it is necessary to diffuse the nitrogen atoms into the interior to make the hardness gradient gentle and distribute the hardness deep.

The diffusion rate of nitrogen is high in ferrite and extremely low in pearlite because the diffusion is impeded by the layered cementite. In non-heat treated steel, since ferrite is thinly concentrated in the former austenite grain boundary, diffusion of nitrogen into the inside can be done only through the ferrite.
On the other hand, in the structure subjected to the normalizing treatment, since the fine ferrite is distributed not only in the grain boundaries but in the entire structure, there is a diffusion path 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.

The fact that the structure of non-heat treated steel is rough is also a factor that lowers the fatigue limit.

(2) The occurrence and size of cracks due to straightening are governed by the following.

The higher the surface hardness of the steel, the more
It easily cracks and increases the crack length. However, the crack length cannot be uniquely determined only by the surface hardness.

The crack propagates with pearlite grains as one unit. Therefore, smaller pearlite grains tend to be smaller.

Summarizing the above facts and speculations,
It is summarized that (a) the structure of non-heat treated steel itself makes it difficult to achieve this problem, and (b) the use of an element that increases the surface hardness after nitriding is not preferable.

Therefore, in order to specifically confirm the method of improving the structure and the like, the present inventors conducted the following experiments. With a basic composition of medium carbon steel having 0.38% C,
The fatigue limit and bending test after the nitriding treatment were performed on 28 steel types with different contents of various elements. Table 1 is a list of 28 types of steels that have been subjected to these tests. Steel X1 in the uppermost column of Table 1 has a basic composition. On the other hand, steels of steel X2 and below are steels having a composition for knowing the influence of C, Si, Mn, P, Cr and the like. 1250 of these laboratory melted steels
It was heated to ℃, forged into a round bar with a diameter of 30 mm during hot work, and the Ono-type rotary bending fatigue test piece and 3 were used without tempering.
It was processed into a point-bend (φ20 mm round bar) test piece and subjected to gas soft nitriding treatment (N ₂ : NH ₃ = 1: 1 in an atmosphere of 3 at 570 ° C.).
After holding for a time, it was subjected to oil cooling). For comparison, after forging S48C steel used as a steel material for automobile parts, normalizing treatment (reheating to 860 ° C. and holding for 15 minutes and air cooling) was performed, and the same nitriding treatment was performed, and then the same test was performed. went.

[0020]

[Table 1]

The fatigue test was repeated at room temperature in the atmosphere at a repetition rate of 5
It was carried out at 0 Hz, and the stress amplitude at which the number of repeated fractures was 10 ⁷ was defined as the fatigue limit. On the other hand, the bend straightening property was evaluated by a 3-point bending test. A load is applied at a strain rate of about 100 μ / sec at the site of maximum strain in room temperature atmosphere, and when the amount of strain reaches 1.5%, the load is removed, and then a cross section of the test piece is cut out and the crack length is cut. Was measured.

FIG. 2 is a drawing showing the effects of C, Si, Mn and P on the fatigue limit and the crack length due to three-point bending. In this figure, the fatigue limit (3
8 kgf / mm ² ) and crack length due to bending (0.1
0 mm) is shown as a guide.

FIG. 3 is a drawing showing the effects of Al, Cr, N, Ti and V on the fatigue limit and crack length.

From these results and the observation of the structure, the following improved clue can be obtained.

Suppression of austenite grain growth during forging heating by a small amount of Ti Uniform distribution of grain boundaries of ferrite by reduction of C and Mn contents Improvement of fatigue limit by suppression of Al and Ti contents (amount of solute nitrogen) Suppression of surface hardness after nitriding by restriction of V, Cr, Al, etc. Improvement of fatigue strength by addition of P that has little influence on surface hardness Here, the present invention is based on these results. The gist is the steel having the composition described in (1) and a method for producing a product obtained by subjecting the steel to nitriding treatment.

(1) C: 0.30 to 0.40 in% by weight
%, Si: 0.05 to 0.40%, Mn: 0.20
0.60%, P: 0.08% or less, S: 0.02 to 0.
10%, Cr: 0.10% or less, solAl: 0.005
% Or less, Ti: 0.005 to 0.013%, Ca: 0.
A nitriding steel containing 0003 to 0.0030%, Pb: 0.20% or less and N: 0.010 to 0.030%, and the balance being Fe and inevitable impurity elements.

(2) After hot forging the steel having the composition according to claim 1, nitriding is performed without tempering to form a nitride layer on the surface of the steel. A method for producing nitriding steel, which is characterized.

[0028]

The action of each constituent element of the present invention and the reason for limiting the concentration of each element are as follows.

C: 0.30 to 0.40% C is an element effective for ensuring the tensile strength, and for this purpose, the content of 0.30% or more is necessary. However, if the concentration exceeds 0.40% and becomes excessively large, ferrite is less likely to be generated from within the grains, and the structure of the non-heat treated steel cannot be brought close to that of the heat treated steel, so it is set to 0.40% or less.

Si: 0.05 to 0.40% Si is essential as a deoxidizing element during melting, and 0.05%
The above is necessary. However, an excessive content exceeding 0.40% promotes surface decarburization during forging and causes a decrease in the fatigue limit, so the content is made 0.05 to 0.40%.

Mn: 0.20 to 0.60% When Mn is suppressed to a low level, ferrite is generated from within the grains and the structure of the non-heat treated steel can be brought close to that of the normal. Therefore, the upper limit must be 0.60%. However, due to deoxidation during refining, sulfur (S)
The content of 0.20% or more is necessary in order to prevent the deterioration of high temperature ductility due to. Therefore, the Mn content is 0.20
~ 0.60%.

P: 0.08% or less P may not be intentionally added. However, since it has the effect of improving the fatigue limit without increasing the crack length due to bending of non-heat treated nitrided steel, 0.02 to 0.02 is added when added.
It is desirable to set the content to 0.08%. 0.02%
The reason for this is that a remarkable effect cannot be obtained unless the content is further increased, and the content is made 0.08% or less because the toughness is significantly deteriorated when the content is exceeded.

S: 0.02 to 0.10% S improves the machinability, so S is made 0.02 to 0.10%. The content of 0.02% or more is because the machinability is not sufficient if the content is not more than 0.02%, and the content of 0.10% or less is because a defect occurs in the continuous casting slab if it is exceeded.

Cr: 0.10% or less It is desirable not to contain Cr. When it is contained, a nitriding treatment produces a nitride to increase the hardness, thereby deteriorating the bend straightening property. However, the refining cost increases significantly if the content is reduced to 0.10% or less.
Content up to 10% is allowed.

Al: 0.005% or less Al is an element effective as a deoxidizing agent. However, even if the content is at a normal level, the fixed limit of solute nitrogen is fixed as AlN and the fatigue limit is lowered due to the decrease of the solute nitrogen content.
It is desirable to reduce as much as possible from the two points of hardening the surface by nitriding treatment and deteriorating the straightening property. Al as sol Al contains 0 and is limited to 0.005% or less as a means for stopping the minimum amount after having a deoxidizing function.

Ti: 0.005 to 0.013% A trace amount of Ti suppresses the growth of austenite grains at the time of heating before forging, thereby making the ferrite-pearlite structure fine. As a result, the structure of the non-heat treated steel can be brought close to that of the normal, and the cracks that occur during bending correction can be reduced. For that purpose, 0.005% or more is necessary, but if it exceeds 0.013%, the solid solution N in the steel is reduced and the fatigue limit is lowered. Therefore, it is set to 0.005 to 0.013%.

Ca: 0.0003 to 0.0030% Since Ca is effective in improving machinability, 0.0003%
The above is necessary. However, if it exceeds 0.0030%, the inclusion of large inclusions cannot be avoided, so 0.0003-
It is set to 0.0030%.

Pb: 0.20% or less Pb is extremely effective in improving machinability, but if it is contained in excess, inclusions increase and the fatigue limit remarkably decreases.
Then, when machinability is not important, it is not necessary to add intentionally. 0.03 only when machinability is required
It is preferably contained in the range of 0.20%. 0.0
This is because if it is not contained in an amount of 3% or more, a large improvement in machinability cannot be obtained, and if it exceeds 0.20%, the fatigue limit remarkably decreases.

N: 0.010 to 0.030% N is an element effective for improving the fatigue limit. To obtain this effect, 0.010% or more is necessary. However, even if the content exceeds 0.030%, the effect is saturated, so the content is made 0.010 to 0.030%.

V and Nb: V as impurities are not added as much as they are mixed as unavoidable impurities. Even inevitable impurities must be less than 0.010%. 0.
This is because if it is contained in an amount of 010% or more, the surface hardness after nitriding treatment increases and the bend straightening property deteriorates. Nb also has the same effect, so it is not added. Even as inevitable impurities,
It must be less than 0.010%.

Next, a method for producing a product of nitriding the steel of the present invention will be described.

The steel of the present invention is heated and forged to obtain a desired shape. The forging process is not particularly limited, and a commonly used method may be used. As shown in FIG. 1, after forging, mechanical processing such as cutting may be performed if necessary. After adjusting to a desired shape, nitriding treatment is performed without performing heat treatment such as normalizing or quenching and tempering. As the nitriding treatment, all nitriding methods such as gas soft nitriding, ion nitriding or tufftride treatment can be used. This is because the structure in which the diffusion path of nitrogen atoms is secured, the strengthening by solid solution nitrogen and phosphorus, and the restriction of the precipitation hardening element are effective regardless of the nitriding method.

[0043]

EXAMPLES Table 2 is a list showing the chemical compositions of eight types of the present invention steels and 15 types of comparative steels. 50k of these steels
After melting in an atmospheric melting furnace, it was heated to 1250 ° C., hot forged into a φ30 mm round bar in the range of 900 ° C. or higher, and allowed to cool. Ono type rotary bending fatigue test pieces (φ20 mm) for fatigue test and φ20 mm × 400 mm round rod for bending test were sampled from the round bar and subjected to gas nitrocarburizing. The gas nitrocarburizing is carried out in the gas ratio N ₂ : NH ₃ = 1:
The test piece was heated to 570 ° C. in the atmosphere of No. 1 and held for 3 hours, and then cooled in oil at 150 ° C. Each of the nitrided test bodies was directly subjected to each test. The fatigue test was carried out in a room temperature atmosphere at a repetition rate of 50 Hz, and the number of fracture repetitions was 10 ⁷
The stress amplitude is defined as the fatigue limit. On the other hand, the bend straightening property was evaluated by a three-point bending test of a φ20 mm × 400 mm round bar test body. A load is applied at a strain rate of about 100 μ / sec at the site of maximum strain in room temperature atmosphere, and when the strain amount reaches 1.5%, the load is unloaded, the cross section of the test piece is cut out, and the The crack length was measured. With respect to machinability, tool life tests were conducted on all steels.

[0044]

[Table 2]

Table 3 shows the results of these fatigue, 3-point bending and machinability tests. As is clear from the table, the steels of the present invention have target values for both the fatigue limit and the crack length due to bending (the fatigue limit 38 kgf / mm ² and the crack length due to bending of a nitrided tempered forged product made from S48C). 0.1
0 mm) has been achieved. On the other hand, no comparative steel achieves the fatigue limit of the target value and the crack length due to bending at the same time. The results of machinability in Table 3 show that S48C steel has P
A steel to which 0.05% of b was added was subjected to a tempering treatment, which was used as a standard. A tool with a tool life equivalent to or higher than that is considered good and marked with a circle. It can be seen that among the steels of the present invention, the ones to which Pb is added have good machinability while simultaneously satisfying the fatigue limit and bend straightening property.

[0046]

[Table 3]

[0047]

EFFECTS OF THE INVENTION Even when the steel of the present invention is subjected to a nitriding treatment without being subjected to a tempering treatment, it is possible to secure an excellent fatigue limit and bend straightening property equal to or higher than those of the tempered steel due to the effect of improving the structure. By using the steel of the present invention for an automobile crankshaft or the like which is subjected to nitriding treatment, the refining treatment can be omitted and a large cost can be omitted.

[Brief description of drawings]

FIG. 1 is a drawing comparing product manufacturing methods applied to a conventional steel (heat treated steel) and a steel of the present invention (non-heat treated steel).
(A) shows the process until the product of the conventional steel (heat treated steel) and (b) the steel of the present invention (non-heat treated steel).

FIG. 2 is a drawing showing the effects of C, Si, Mn, and P on the fatigue limit and the crack length due to three-point bending.

FIG. 3 is a drawing showing the effects of Al, Cr, N, Ti, and V on the fatigue limit and the crack length due to three-point bending.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toru Kato 4-533 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries Co., Ltd. No. 1 Sumitomo Metal Industry Co., Ltd. Kokura Works (72) Inventor Koji Watari 4-533 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industry Co., Ltd.

Claims (2)

[Claims] 1. C .: 0.30 to 0.40% by weight, S:
i: 0.05 to 0.40%, Mn: 0.20 to 0.60
%, P: 0.08% or less, S: 0.02 to 0.10%,
Cr: 0.10% or less, solAl: 0.005% or less,
Ti: 0.005-0.013%, Ca: 0.0003
~ 0.0030%, Pb: 0.20% or less and N:
A nitriding steel containing 0.010 to 0.030% and the balance being Fe and unavoidable impurity elements. 2. A steel having the composition according to claim 1 is hot forged, and then nitrided without tempering to form a nitride layer on the surface of the steel. And a method for producing steel for nitriding.