JP3314233B2 - Surface nitriding method of steel sheet molded product and steel sheet molded product - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for nitriding the surface of a molded product obtained by molding a steel sheet, and a molded steel sheet product subjected to a nitriding treatment.

[0002]

2. Description of the Related Art Generally, automatic transmission (hereinafter, referred to as "AT") parts of an automobile are known.
Flange components to be connected to various clutches are manufactured by pressing a thin steel plate or the like because it is necessary to easily process a complicated shape. Here, due to the recent demand for higher output power of the engine or compact AT unit, the load on the portion of the flange component that is connected to the clutch plate tends to increase. Improvement in strength (for example, fatigue strength) is required. However, since high formability is required in order to manufacture a complicated shape by press working, it is difficult to press-work a high-strength steel sheet to manufacture a flange part. Therefore, after forming a steel plate with formability suitable for press working into a flange part,
A method of hardening the surface of the flange component by performing a soft nitriding treatment or plating is employed.

[0003]

However, there are the following problems in the method of nitrocarburizing or hardening the surface of the formed flange part by plating.

First, in the method of nitrocarburizing the surface of a flange component, since the thickness of the flange component is not always uniform, nitrogen may diffuse to the core (total nitriding) in a portion where the thickness is small. is there. For this reason, the abrasion resistance of the flange component is ensured by the nitriding treatment, but the fatigue strength and the impact strength (toughness) at the portion where the plate thickness is small are rather lowered. Especially, JIS SPHD
In general steel sheets such as these, since a nitrogen trap element such as Ti or Al is not added, a decrease in fatigue strength or the like is likely to occur.

On the other hand, in the method of plating the surface of the flange component, a hard coating or a lubricating coating is only formed on the surface of the flange component, so that the wear resistance is secured.
The strength is substantially the same as when no plating is performed. Further, the plating method has a disadvantage that it is disadvantageous in cost as compared with the nitrocarburizing method.

For this reason, when it is desired to achieve both wear resistance and strength of a thin steel plate flange component having a small thickness, for example, a portion having a thickness of 4 mm or less, a small thickness portion, That is, a method of performing a nitriding treatment by applying masking to a portion not desired to be nitrided is adopted.
However, in this method, not only a masking step is required, but also a masking jig is required, a complicated operation is required, and productivity is extremely poor.

The present invention was conceived in view of the above circumstances, and is intended to improve both the wear resistance and strength of a steel sheet molded product in a cost-effective manner and with good productivity. The task is to provide a technology that can do

[0008]

DISCLOSURE OF THE INVENTION In order to solve the above problems, the present invention employs the following technical means.

That is, according to a first aspect of the present invention, there is provided a method for nitriding a steel sheet molded product. This nitriding method is a method of nitriding the surface of a molded product having a non-uniform overall thickness obtained by molding a steel sheet. The nitrogen diffusion layer depth and the mechanical strength of the steel sheet with respect to the steel sheet having a uniform cross-section The relationship between the maximum diffusion layer depth at which a predetermined mechanical strength is secured for each plate thickness is determined as the critical diffusion layer depth at that plate thickness, and the plate thickness is determined. The relationship between the change of the critical diffusion layer depth and the change of the critical diffusion layer depth is obtained. From this correlation, the critical diffusion layer depth for the minimum thickness portion of the molded product is obtained, and the depth at which nitrogen is to be diffused into the molded product. Is characterized in that the entire molded article is uniformly nitrided so as to be not more than the critical diffusion layer depth with respect to the minimum thickness portion of the molded article.

The critical diffusion layer depth in the present invention is defined as the maximum diffusion layer depth that ensures a predetermined mechanical strength for each plate thickness. More specifically, the critical diffusion layer depth is set to prevent the mechanical strength from being lowered when nitriding (total nitriding) occurs over the entire plate thickness. Is smaller than the depth of the diffusion layer (1/2 of the plate thickness) at which the occurrence of the diffusion occurs. In addition, since the nitriding treatment enhances the abrasion resistance and strength of a steel sheet molded product, the critical diffusion layer depth secures a predetermined strength (uniquely determined because it varies depending on the use of the molded product). Must be something. In other words, the critical diffusion layer depth can be said to be the maximum diffusion layer depth allowed in a range where a predetermined strength can be ensured and within a range of less than １ ／ of the plate thickness.

In the above nitriding method, the depth of the diffusion layer to be subjected to the nitriding treatment on the steel sheet molded product is set to be equal to or less than the critical diffusion layer depth of the minimum thickness portion in the steel sheet molded product. Therefore, in the above-mentioned nitriding method, there is no possibility that the steel sheet is fully nitrided at a portion where the thickness is small, or the steel sheet is nitrided more than the depth of the critical diffusion layer, thereby lowering the strength.
Of course, in other parts, the nitrogen is diffused only to a depth smaller than the depth of the critical diffusion layer in that part, so that the strength is increased, and the strength is improved as a whole. That is, in the present invention, the most characteristic feature of the steel sheet molded article having a non-uniform thickness is to uniformly nitride the whole on the basis of the depth of the critical diffusion layer at the minimum thickness where the nitriding conditions are most severe. This is because, if a predetermined strength is secured in the minimum thickness portion, a portion having a larger plate thickness naturally has a predetermined strength characteristic.

Further, in the above-mentioned nitriding method, it is not necessary to mask the thin portion so that the thin portion is not completely nitrided. For this reason, there is no need to facilitate a masking jig or perform a masking operation in order to avoid a decrease in strength of a portion having a small thickness, which is advantageous in terms of cost.

In addition, in the above-mentioned nitriding method, since the entire steel sheet including the small thickness part of the steel sheet is subjected to the nitriding treatment, it goes without saying that the entire steel sheet is provided with wear resistance.

Here, the nitriding method includes a salt bath nitriding method,
A gas nitriding method, an ion nitriding method, a nitrocarburizing method, etc. may be mentioned, but the nitrocarburizing method is preferably adopted in consideration of both abrasion resistance and fatigue strength.

In this soft nitriding method, a steel sheet molded product is heat-treated for several hours in an atmosphere of about 500 to 600 ° C. containing a nitrogen compound such as ammonia and a carbon compound such as carbon monoxide, for example, for several hours. This is a method of diffusing nitrogen and carbon from the surface. in this way,
A carbonitride is formed on the surface of the molded product, so that the molded product has improved wear resistance and fatigue strength.

As the atmosphere gas, a gas containing an inert gas (a rare gas or a nitrogen gas) in addition to a nitrogen compound or a carbon compound is preferably employed. This is because the diffusion rate of nitrogen is relatively high, so that nitrogen diffuses to a relatively deep portion and the strength is reduced.

Note that the term molded article on the present side refers to all flat-plated articles having no uniform cross section. That is, a molded product is formed by bending a steel sheet into a predetermined shape having a concave portion or a convex portion by pressing or the like (for example, A
Not only T-flange parts), but also steel plate-like parts with concave and convex portions formed in place and not having a uniform cross section are included. Further, as the steel sheet, not only a mild steel sheet but also a high strength steel sheet, a steel sheet for nitriding, and the like are suitably adopted.

In a preferred embodiment of the present invention, at least one of static strength (such as tensile strength, bending strength or compressive strength), impact strength and fatigue strength is measured as the strength, and the strength is measured for each sheet thickness. The diffusion layer depth at which one of the measured intensities can be maintained at or above a predetermined value is defined as the critical diffusion layer depth.

That is, as the intensity, an arbitrary intensity that does not want the intensity to decrease may be selected, and the critical diffusion layer depth is determined based on this intensity. Of course, the reference strength may be one or two or more. However, since the flange component of the AT unit to which the present invention is suitably applied is repeatedly subjected to a load, in this case, the critical diffusion layer depth is increased. As the strength for determining the strength, fatigue strength or the like is suitably adopted. Here, the fatigue strength is a concept including a fatigue limit.

In a preferred embodiment of the present invention, a depth 0.05 mm smaller than the critical diffusion layer depth relative to the minimum thickness of the molded article is set as the depth at which nitrogen is to be diffused.

Normally, the diffusion depth of nitrogen does not exactly coincide with each other at various places on the object to be nitrided, and there is some variation. The variation varies depending on the composition of the object to be nitrided, the composition and temperature of the nitriding atmosphere, and the like, but it has been confirmed by the present inventors that the degree of the variation is about ± 0.05 mm.

Therefore, if the depth at which nitrogen is to be diffused is set to be 0.05 mm smaller than the critical diffusion layer depth in consideration of the variation in the nitrogen diffusion depth, the nitrogen can reduce the critical diffusion layer depth. It will not spread beyond
There is no decrease in strength.

According to a second aspect of the present invention, there is provided another method of nitriding a steel sheet molded product. This nitriding method is a method of nitriding the surface of a steel sheet molded article having a uniform thickness obtained by shaping a steel sheet. The relationship with the mechanical strength is examined for each of various plate thicknesses, and the maximum diffusion layer depth at which a predetermined mechanical strength is secured for each plate thickness,
The depth of the critical diffusion layer at the plate thickness is determined, and the correlation between the change in the thickness of the plate and the change in the depth of the critical diffusion layer is determined. From this correlation, the depth of the critical diffusion layer of the molded article to be nitrided is determined. Is characterized in that the entire molded article is uniformly nitrided so that the depth at which nitrogen is to be diffused into the molded article is equal to or less than the critical diffusion layer depth corresponding to the thickness of the molded article.

In the present aspect, the idea of previously examining the correlation between the change in the plate thickness and the change in the critical diffusion layer depth, and setting the depth of the diffusion layer to be formed in the molded article from this correlation, has been proposed. It is also applied to steel plate molded products with a uniform thickness.

According to a third aspect of the present invention, there is further provided a molded article of a steel sheet subjected to a nitriding treatment. The molded product of the steel plate is a molded product of a steel plate, and the surface thereof is subjected to nitriding by any one of the nitriding methods described in the first and second aspects of the present invention. I have.

Since the surface of such a molded article is subjected to the nitriding treatment by the nitriding method described in the first or second aspect of the present invention, the abrasion resistance and the wear resistance of the entire molded article are improved. The effect is obtained that the mechanical properties are enhanced and the operation for nitriding becomes easy.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a molded article nitridated by the nitriding method of the present invention will be briefly described with reference to FIG.

FIG. 1 shows a main part of an AT flange part (a flange part connected to a clutch plate) as a molded article 1. This molded article 1 is subjected to a nitriding treatment on the entire surface. Thus, the diffusion layer 2 is formed uniformly. That is, the depth t of the nitrogen diffusion layer 2 is the same at various points of the molded article 1 regardless of the thickness. As described above, in the molded article 1, the hardness of the entire surface of the molded article 1 is increased by performing the nitriding treatment, and the wear resistance is improved.

The depth at which nitrogen is to be diffused into the molded article 1, that is, the depth t of the diffusion layer, is determined as follows. That is, first, the relationship between the diffusion layer depth of nitrogen and the mechanical strength of the steel sheet for a steel sheet having a uniform cross section is examined for each of various sheet thicknesses, and the maximum mechanical strength at which a predetermined mechanical strength is ensured for each sheet thickness is obtained. The depth of the diffusion layer is obtained as the critical diffusion layer depth at the plate thickness, and the correlation between the change in the plate thickness and the change in the critical diffusion layer depth is obtained. From this correlation, the critical diffusion layer depth corresponding to the thickness T of the minimum thickness portion 10 in the molded article 1 is obtained, and the depth at which nitrogen is to be diffused into the molded article 1 is determined by the minimum thickness of the molded article 1. The depth is set to be equal to or less than the critical diffusion layer depth for the portion 10.

As the strength, for example, fatigue strength (fatigue limit) or the like is suitably adopted. The depth t at which nitrogen is to be diffused is, for example, 0.05 mm larger than the critical diffusion layer depth of the minimum thickness portion 10 of the molded article 1. Small depth.

In such a nitriding method, since the diffusion layer depth t is set with reference to the minimum thickness portion 10 of the molded article 1 having the strictest conditions in terms of strength, the strength of the minimum thickness portion 10 is equal to or less than a predetermined value. And never. Needless to say, the strength does not naturally become lower than the predetermined value even in a portion other than the minimum thickness portion 10. Therefore, the molded article 1 that has been nitridated by the above-mentioned nitriding method is improved in strength by clearing a desired standard set in strength.

Next, examples of the present invention will be described together with comparative examples, but it goes without saying that the technical scope of the present invention is not limited to the following examples.

Prior to carrying out the examples and comparative examples, the relationship between the change in the thickness of a steel sheet having a uniform cross section and the change in the depth of the critical diffusion layer was examined. The result is shown in FIG. Note that JIS SPHD, which is generally used as an AT flange part, was used as the steel sheet, and a gas soft nitriding method was used as the nitriding method.

(Measurement of Depth of Critical Diffusion Layer) In addition to having the shape shown in FIG.
Specimens 3 were prepared in seven different thicknesses (0.6 mm,
0.8mm, 1.0mm 1.2mm 1.6mm
(2.4 mm and 3.2 mm), and the fatigue strength of each steel plate before the nitrocarburizing treatment was measured. The fatigue strength test was performed according to JIS Z 227.
5 (uniform four-point bending fatigue test). on the other hand,
Each steel sheet (specimen 3) was subjected to a soft nitriding treatment under the conditions shown in Table 1, and a plurality of steel sheets (specimen 3) having different nitrogen diffusion depths in 0.05 mm units were obtained. Each steel plate (test piece 3) was prepared for each and the fatigue strength was measured in the same manner. The results are shown in Table 2. From this result, the maximum diffusion layer depth at which a predetermined mechanical strength (fatigue strength shown as a reference value in Table 2) is ensured for each sheet thickness is determined by the sheet thickness. Defined as the critical diffusion layer depth at
As shown in FIG.

(Gas Nitriding Method) Each steel sheet was subjected to nitrocarburizing treatment in an atmosphere containing RX (mixed gas of butane and nitric oxide), NH ₃ and N ₂ . The temperature of the atmosphere gas,
The composition and the processing time are shown in Table 1. That is, the conditions of the nitriding treatment were determined by the depth at which nitrogen was to be diffused irrespective of the thickness of the steel sheet.

Example 1 First, an AT flange part (a flange part connected to a clutch plate) was formed by press-forming a steel sheet having a thickness of 3.2 mm. Next, the composition ratio of RX, NH ₃ and N ₂ was 11: 7: 18 with respect to the AT flange part.
Heat treatment at 570 ° C. for 1.5 hours in an atmosphere of 0.2 mm, which is the critical diffusion layer depth of a steel plate having a thickness of 0.6 mm.
(Refer to FIG. 2) 0.05 mm deeper (0.15
mm). A single-part fatigue test and an actual unit operation endurance test were performed on the AT flange parts thus nitrided. Table 3 shows the results. The fatigue strength test was performed using a hydraulic servo fatigue tester. In addition, the actual unit operation endurance test was carried out in a state where the parts were actually incorporated in the AT unit, and the
35 ° C., the clutch plate is connected to the AT flange part by applying a torque of 686 N · m,
The amount of wear after 10,000 rotations was measured.

EXAMPLE 2 An AT flange part having a minimum thickness of 1.6 mm was formed by press-forming a steel sheet having a thickness of 3.2 mm, and a composition ratio of RX, NH ₃ and N ₂ was 9: 7. : 1
In the atmosphere of No. 6, heat treatment was performed at 570 ° C. for 2 hours, and the critical diffusion layer depth of a 1.6 mm thick steel plate was 0.4 mm.
(See FIG. 2)
mm). A fatigue test and a real unit operation durability test were performed on the steel sheet thus nitrided in the same manner as in Example 1. Table 3 shows the results.

Example 3 An AT flange part whose entire thickness was made uniform by press-forming a steel plate having a thickness of 3.2 mm was manufactured by using RX,
Heat treatment was performed at 570 ° C. for 2.5 hours in an atmosphere in which the composition ratio of NH ₃ and N ₂ was 10:10:15 to obtain a thickness of 3.2.
Nitrogen was diffused to a depth (0.40 mm) smaller by 0.05 mm than 0.45 mm, which is the critical diffusion layer depth of a steel plate of 2 mm. A fatigue test and an actual unit operation durability test were performed on the steel sheet thus nitrided in the same manner as in Example 1. Table 3 shows the results.

[0039] The same AT flange component as used in Comparative Example 1 Example 1 was carried out to fatigue strength tests and beauty actual unit actuating durability test in the same manner as in Example 1 without nitriding. Table 3 shows the results.

COMPARATIVE EXAMPLE 2 The same AT flange part as used in Example 1 was subjected to 570 in an atmosphere in which the composition ratio of RX and NH ₃ was 1: 1.
A heat treatment was performed at 3 ° C. for 3 hours for total nitriding, and a fatigue strength test and an actual unit operation durability test were performed in the same manner as in Example 1. Table 3 shows the results.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

[0044]

As is clear from Table 3, the molded product (AT flange part) provided by the nitriding method of the present invention.
Has a higher fatigue strength (fatigue limit) than the reference value, and has a higher strength, and is excellent in abrasion resistance. This means
When nitriding of a steel plate-shaped molded product with a non-uniform overall thickness due to the formation of concave portions or convex portions in the right place, or a bent molded product by performing press working, etc. By setting the depth below the critical diffusion layer depth of the minimum thickness part in the product to the depth where nitrogen is to be diffused, it is possible to obtain a molded product that clears the standard value and has excellent wear resistance. I have.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a main part showing an example (AT flange part) of a molded article nitridated by a nitriding method of the present invention.

FIG. 2 is a graph showing a correlation between the thickness of a steel sheet and the depth of a critical diffusion layer.

FIG. 3 is a diagram for explaining a test piece.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-9-225557 (JP, A) JP-A-9-279295 (JP, A) JP-A-7-179985 (JP, A) JP-A-7-79 90541 (JP, A) JP-A-5-65592 (JP, A) JP-A-9-257041 (JP, A) JP-A-11-335808 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 8/00-8/80

Claims (5)

(57) [Claims] 1. A method for nitriding a surface of a steel sheet molded article having a non-uniform overall thickness obtained by forming a steel sheet, comprising: a nitrogen diffusion layer depth for a steel sheet having a uniform cross section; The relationship with the mechanical strength is examined for each of various plate thicknesses,
The maximum diffusion layer depth at which the specified mechanical strength is secured for each plate thickness is determined as the critical diffusion layer depth at that plate thickness, and the correlation between the plate thickness change and the critical diffusion layer depth change is determined. From this correlation, the critical diffusion layer depth for the minimum thickness part of the molded article is determined, and the depth at which nitrogen is to be diffused into the molded article is determined by the critical diffusion layer depth for the minimum thickness part of the molded article. A method for nitriding a surface of a steel sheet molded product, wherein the entire product is uniformly nitrided as described below. 2. The mechanical strength is measured for at least one of tensile strength, bending strength, compressive strength, impact strength and fatigue strength, and the mechanical strength measured for each sheet thickness is maintained at a predetermined value or more. The surface nitriding method according to claim 1, wherein the maximum possible diffusion layer depth is defined as a critical diffusion layer depth. 3. The surface nitriding method according to claim 1, wherein a depth 0.05 mm smaller than a critical diffusion layer depth relative to a minimum thickness portion of the molded product is set as a depth at which nitrogen is to be diffused. 4. A method for nitriding a surface of a steel sheet molded article having a uniform thickness obtained by forming a steel sheet, comprising: a nitrogen diffusion layer depth for a steel sheet having a uniform cross section; The relationship with the mechanical strength is examined for each of various plate thicknesses,
The maximum diffusion layer depth at which the specified mechanical strength is secured for each plate thickness is determined as the critical diffusion layer depth at that plate thickness, and the correlation between the plate thickness change and the critical diffusion layer depth change is determined. The critical diffusion layer depth of the molded article to be nitrided is determined from this correlation, and the depth at which nitrogen is to be diffused into the molded article is determined by the depth of the critical diffusion layer according to the thickness of the molded article. A method for nitriding a surface of a steel sheet molded product, wherein the entire product is uniformly nitrided as described below. 5. A molded product of a steel sheet, the surface of which is subjected to a nitriding treatment by the nitriding method according to any one of claims 1 to 4.