JP3211627B2 - Steel for nitriding and method for producing the same - Google Patents

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 excellent bending straightness even when a nitriding treatment is performed without a tempering treatment. The steel of the present invention is suitable, for example, as a material for an automotive crankshaft to be subjected to nitriding treatment.

[0002]

2. Description of the Related Art Conventionally, automotive parts such as a crankshaft, a connecting rod or a knuckle are hot forged from a steel slab such as carbon steel for a machine structure into a desired shape, and thereafter are subjected to tempering to improve the fatigue limit. A manufacturing method of performing a treatment (normalizing treatment or quenching and tempering treatment) has been adopted.

FIG. 1 is a drawing comparing a conventional tempered steel and a product manufacturing method applied to the steel of the present invention described later. Here, (a) shows the process up to the product of the conventional steel, and (b) shows the process up to the product of the steel of the present invention. In each case, a nitriding treatment is applied if seizure resistance, galling resistance, or a higher fatigue limit is required.

In recent years, as shown in FIG. 1 (b), the use of so-called non-heat treated steel, which is provided for commercialization as it is forged by omitting the tempering treatment for cost reduction, has been applied to many automobile parts. Are being considered. However, there is a performance that is deteriorated by omitting the tempering process, and therefore, there are parts that cannot be non-tempered.

First, the fatigue limit of a part that has been subjected to nitriding without tempering after forging (hereinafter referred to as non-heat-treated nitrided steel) is determined by subjecting a steel of the same composition to forging after tempering. It is lower than that of a part that has been subjected to a nitriding treatment (hereinafter referred to as tempered nitrided steel). Secondly, large cracks are generated when straightening after nitriding. The deformation caused by the nitriding treatment is corrected by the bending deformation in the opposite direction, but the crack generated in the non-heat-treated nitrided steel by the bending is larger than that of the heat-treated nitrided steel. In general, the larger the crack, the lower the fatigue limit of the part when the part is used in an automobile. (0.10mm or less)
Bending straightening is good). Since non-heat-treated nitrided steel usually has a crack length greatly exceeding 0.10 mm, it cannot be used for a crankshaft or the like that performs bending straightening when strain after nitriding is large.

[0006] Non-heat treated steel is heated to 1100 °
Since the forging is completed at a temperature of 00 ° C. or more and the steel is left to be cooled, its structure is composed of a thin net-like ferrite along a huge old austenite grain boundary and the remaining pearlite. The structure of the tempered steel, on the other hand, consists of (a) a mixed structure of fine ferrite and pearlite (in case of normalization) transformed from fine austenite or (b) a very fine lath and carbide. Either martensite or bainite (in the case of quenching and tempering).

[0007] 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 because the austenite grain size of the non-heat treated steel is large,
This reflects that the ferrite transformation is suppressed.

Attempts have been made to simultaneously improve the fatigue limit and the bending straightness of non-heat-treated nitrided steel, but none has been achieved. There has been an invention in which a high concentration of a precipitation hardening element is added to obtain a high fatigue limit without forging treatment or nitriding treatment as it is forged. JP-A-64-684245 and JP-A-Hei.
Japanese Patent Publication No. -19391 discloses such an invention. These are steels containing a high concentration of vanadium (V), which is a strong precipitation hardening element, and are expensive. If the seizure resistance becomes a problem, these high V
The steel must be subjected to a nitriding treatment. At this time, the bending straightness of the high-V steel after the nitriding treatment is extremely poor.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a high fatigue limit after a nitriding treatment without performing a tempering treatment.
Another object of the present invention is to provide a steel in which a crack generated during straightening is reduced to a level that does not actually cause a problem. Specifically, it is to simultaneously achieve the performance of S48C or more subjected to 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]

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 the non-heat-treated nitrided steel is at the boundary between the diffusion layer and the base material, and the 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. Therefore,
In the following description, the surface indicates the surface side of the diffusion layer excluding the compound layer.

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

In the vicinity of the boundary between the diffusion layer and the base material, tensile stress remains in the non-heat-treated nitrided steel. In order to improve the fatigue limit, it is necessary to reduce the tensile residual stress or desirably to the compressive residual stress. In a non-heat-treated nitrided steel, even if the steel does not contain a precipitation hardening element, the hardness is significantly increased on the surface and decreases steeply toward the inside. For this reason, although a high compressive residual stress is generated on the surface, it is presumed that a tensile residual stress balanced with the compressive residual stress is generated 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, nitrogen entering from the outside hardly enters 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 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 a mild hardness distribution can be formed from the surface to the inside of the tempered steel when the nitriding treatment is performed.

[0014] The coarse structure of the non-heat treated steel itself is also a factor that lowers the fatigue limit.

(2) The occurrence and size of cracks caused by bending are governed by the following.

[0016] The higher the surface hardness of the steel, the more the
Cracks easily occur, increasing the crack length. However, the crack length cannot be uniquely determined only by the surface hardness.

The crack propagates with the pearlite grains as one unit. Therefore, the smaller the pearlite particles, the smaller the tendency.

Summarizing the above facts and conjectures,
(A) The structure itself of the non-heat treated steel itself makes it difficult to achieve the object, and (b) the use of elements that increase the surface hardness after nitriding is undesirable.

Therefore, in order to specifically confirm a method for improving a tissue or the like, the present inventors conducted an experiment described below. Medium carbon steel with 0.38% C as the basic composition,
Fatigue limits and nitriding tests after nitriding were performed on 28 steel grades with different contents of various elements. Table 1 is a list of 28 types of steels subjected to those tests. Steel X1 in the uppermost column of Table 1 is the basic composition. On the other hand, steels of steel X2 or less are steels having compositions for knowing the influence of C, Si, Mn, P, Cr and the like. 1250 of these laboratory melted steels
C., hot forging into a round bar of φ30 mm, and Ono-type rotating bending fatigue test piece and 3
It is processed into a point bending (φ20 mm round bar) test piece and subjected to a gas soft nitriding treatment (570 ° C. in an atmosphere of N ₂ : NH ₃ = 1: 1).
After holding for a time, oil cooling was performed. For comparison, after forging S48C steel used as a steel material for automobile parts, normalization treatment (reheating to 860 ° C., holding for 15 minutes, and air cooling) was performed, and after performing the same nitriding treatment, the same test was performed. went.

[0020]

[Table 1]

The fatigue test was performed at a repetition rate of 5 in a room temperature atmosphere.
The test was performed at 0 Hz, and the stress amplitude at which the number of repetition of fracture was 10 ⁷ was defined as the fatigue limit. On the other hand, bending straightness was evaluated by a three-point bending test. A load was applied at a strain rate of about 100 μ / sec at the site where the strain was maximum in the room temperature atmosphere, and after the strain amount reached 1.5%, the specimen was unloaded. Was measured.

FIG. 2 is a drawing showing the influence 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 the 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 the crack length.

From these results and the observation of the structure, the following clues for improvement could be obtained.

Suppression of austenite grain growth during forging heating by trace amount of Ti Uniform distribution of ferrite over intragranular grain boundaries by reduction of C and Mn contents Improvement of fatigue limit by suppression of Al and Ti contents (solid nitrogen content) Suppression of surface hardness after nitriding by restriction of V, Cr, Al, etc. Improvement of fatigue strength by addition of P which has little effect on surface hardness Here, the present invention will be described based on these results. And a method for producing a product obtained by subjecting the steel to a nitriding treatment.

(1) By weight%, C: 0.30 to 0.40
%, Si: 0.05-0.40%, Mn: 0.20-0
0.60%, P: 0.08% or less, S: 0.02-0.
10%, Cr: 0.10% or less, sol . Al: 0.0
0.05% or less, Ti: 0.005 to 0.013%, Ca:
0.0003-0.0030%, Pb: 0.20% or less and N: 0.010-0.030%, the balance being F
Ri Do from e and inevitable impurity elements, as impurities
V, Nb is nitriding steel characterized by der Rukoto less than 0.010% none.

(2) After the steel having the composition according to claim 1 is hot forged, a nitriding treatment is performed without performing a tempering treatment to form a nitrided layer on the surface of the steel. A method for producing a nitriding steel.

[0028]

The effects of the constituent elements of the present invention and the reasons for limiting the concentration of each element are as follows.

C: 0.30 to 0.40% C is an element effective for securing the tensile strength. For that purpose, the content of 0.30% or more is necessary. However, if the concentration is more than 0.40% and the concentration is excessive, 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.

Si: 0.05 to 0.40% Si is indispensable as a deoxidizing element at the time of melting, and 0.05%
The above is necessary. However, an excessive content exceeding 0.40% promotes surface decarburization during forging and causes a reduction in fatigue limit, so the content is set to 0.05 to 0.40%.

Mn: 0.20 to 0.60% By suppressing Mn to a low level, ferrite is generated from within the grains, and the structure of the non-heat-treated steel can be made closer to that of a normal steel. Therefore, the upper limit must be set to 0.60%. However, due to deoxidation during refining, sulfur (S)
In order to prevent a decrease in ductility at high temperature caused by the above, a content of 0.20% or more is necessary. Therefore, the Mn content is 0.20
０．６0.60%.

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 the non-heat treated nitrided steel, the addition of 0.02 to
Desirably, the content is 0.08%. 0.02%
The reason for the above is that a remarkable effect cannot be obtained unless it is more than this, and the reason for being 0.08% or less is that if it exceeds this, the toughness is significantly deteriorated.

S: 0.02 to 0.10% Since S improves the machinability, S is set to 0.02 to 0.10%. The reason why the content is made 0.02% or more is that the machinability is not sufficient if the content is not further contained, and the content is made not more than 0.10% because the continuous cast slab causes a defect.

Cr: 0.10% or less It is desirable not to contain Cr. When it is contained, a nitride is generated by the nitriding treatment to increase the hardness, thereby deteriorating the bending straightness. However, reducing the content to 0.10% or less greatly increases the refining cost.
Content up to 10% is acceptable.

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 amount of dissolved nitrogen is fixed as AlN, and the lowering of the amount of dissolved nitrogen lowers the fatigue limit.
It is desirable to lower as much as possible from the two points of hardening the surface by nitriding treatment and deteriorating the bending straightness. As a means to stop the minimum amount after having a function of deoxidation, sol . A
Al as 1 includes 0 and is limited to 0.005% or less.

Ti: 0.005 to 0.013% A very small amount of Ti refines the ferrite pearlite structure by suppressing austenite grain growth 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. For that purpose, 0.005% or more is necessary. However, if it exceeds 0.013%, solid solution N in steel is reduced, and the fatigue limit is lowered. Therefore, the content is made 0.005 to 0.013%.

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

Pb: 0.20% or less Pb is extremely effective in improving machinability, but if it is contained excessively, inclusions increase and the fatigue limit is significantly reduced.
Then, when machinability is not important, it is not necessary to add intentionally. 0.03 only when machinability is required
It is preferable to contain it in the range of up to 0.20%. 0.0
If the content is not more than 3%, a large improvement in machinability cannot be obtained, and if it exceeds 0.20%, the fatigue limit is significantly reduced.

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, the effect is saturated even if the content exceeds 0.030%, so that the content is set to 0.010 to 0.030%.

V and Nb: V as impurities are not added beyond mixing as unavoidable impurities. Inevitable impurities must be less than 0.010%. 0.
If the content is 010% or more, the surface hardness after the nitriding treatment increases, and the bending straightening property is deteriorated. Since Nb also exerts a similar effect, it is not added. Even as inevitable impurities,
Must be less than 0.010%.

Next, a method for producing a product in which the steel of the present invention is subjected to nitriding treatment will be described.

The steel of the present invention is heated and forged to obtain a desired shape. There is no particular limitation on the forging process, and a commonly used method may be used. As shown in FIG. 1, after forging, machining such as cutting may be performed as necessary. After adjusting to a desired shape, a nitriding treatment is performed without performing a tempering treatment such as normalizing or quenching and tempering. As the nitriding treatment, any nitriding method such as gas nitrocarburizing, ion nitriding or tuftride treatment can be used. This is because the structure in which the diffusion path of the nitrogen atoms is secured, the strengthening by solid-solution nitrogen or phosphorus, or the limitation of the precipitation hardening element are effective regardless of the method of the nitriding treatment.

[0043]

EXAMPLES Table 2 is a table showing the chemical compositions of eight kinds of steels of the present invention and fifteen kinds of comparative steels. 50k of these steels
g After being melted in a melting furnace in the atmosphere, it was heated to 1250 ° C., hot forged into a φ30 mm round bar in a range of 900 ° C. or more, and allowed to cool. An Ono-type rotary bending fatigue test piece (φ20 mm) for a fatigue test and a φ20 mm × 400 mm round rod for a bending test were collected from the round bar, and subjected to gas soft nitriding. Gas nitrocarburizing has a gas ratio of 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 nitrided specimen was used for each test as it was. Fatigue tests were performed at a repetition rate 50Hz at room temperature in the atmosphere, breaking the repetition number is ¹⁰⁷
Was defined as the fatigue limit. On the other hand, the bending straightness was evaluated by a three-point bending test of a φ20 mm × 400 mm round bar specimen. A load is applied at a strain rate of about 100 μ / sec at a site where the strain is maximum in a room-temperature atmosphere, and when the strain amount reaches 1.5%, the load is unloaded. The crack length was measured. As for machinability, all steels were tested for tool life.

[0044]

[Table 2]

Table 3 shows the results of these tests on fatigue, three-point bending and machinability. As is clear from the table, the steel of the present invention has a target value in both the fatigue limit and the crack length due to bending (the fatigue limit of a nitrided tempered forging made from S48C of 38 kgf / mm ² and the crack length due to bending). 0.1
0 mm). On the other hand, none of the comparative steels achieve the target fatigue limit and the crack length due to bending simultaneously. The machinability results in Table 3 show that P
Based on a temper-treated steel to which 0.05% b was added. A tool having a tool life equal to or longer than that is regarded as good and is marked with a circle. It can be seen that the steel of the present invention to which Pb is added has good machinability while simultaneously satisfying the fatigue limit and bending straightness.

[0046]

[Table 3]

[0047]

The steel of the present invention, even when subjected to a nitriding treatment without a tempering treatment, can secure an excellent fatigue limit and bend straightening property equal to or higher than that of the tempered steel for a structure improving effect. By using the steel of the present invention for an automotive crankshaft or the like to be subjected to nitriding treatment, the tempering treatment can be omitted, and a large cost can be saved.

[Brief description of the drawings]

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

FIG. 2 is a drawing showing the influence 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 fatigue limit and crack length due to three-point bending.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Mitsuo Uno 1st Konomi-cho, Kokurakita-ku, Kitakyushu-shi, Fukuoka Prefecture Sumitomo Metal Industries Co., Ltd. Kokura Works (72) Inventor Koji Watari 4-chome Kitahama, Chuo-ku, Osaka-shi, Osaka No. 5-33 Sumitomo Metal Industries, Ltd. (56) References JP-A-7-90363 (JP, A) JP-A-7-90364 (JP, A) JP-A-1-177338 (JP, A) 63-216950 (JP, A) JP-A-63-137147 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C23C 8/28

Claims (2)

(57) [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, sol . Al: 0.005% or less, Ti: 0.005 to 0.013%, Ca: 0.00
03~0.0030%, Pb: 0.20% or less and N: the balance containing from 0.010 to 0.030% is Ri Do Fe and unavoidable impurity elements, V as impurities, Nb
There Both nitriding steel characterized by der Rukoto less than 0.010%. 2. A steel having the composition according to claim 1 is subjected to hot forging, and then subjected to nitriding without heat treatment to form a nitrided layer on the surface of the steel. A method for producing a steel for nitriding.