JP5746832B2 - Stainless steel screws - Google Patents

Description

  The present invention relates to a stainless steel screw having a threaded surface formed on a surface hardened layer of stainless steel and a method for manufacturing the same.

  In general, a technique of forming a hardened surface layer by permeating and diffusing carbon atoms on the surface of titanium or steel by performing a plasma carburizing process at a relatively low temperature of 350 ° C. or higher and lower than 700 ° C. is known. It is known that a hardened surface layer formed in an inclined manner so as not to form a clear boundary with an internal non-carburized portion is difficult to peel off and is durable (Patent Document 1).

  In addition, for steel for machine structure, the surface is hardened by carburizing treatment, nitriding treatment and quenching / tempering treatment at high temperature, and then barrel-polishing so that the surface roughness is Rmax 0.2 to 0.3 μm. It is known to improve the contact fatigue strength of the tooth surface of a gear (Patent Document 2).

  In addition, a nitrided hardened layer is formed on the surface of the austenitic stainless steel screw to improve tapping and drilling properties, and the nitrided hardened layer such as the head of the screw or the continuous neck under part is peeled off. However, it is known that the austenitic stainless steel material is exposed at portions other than the thread surface to improve the corrosion resistance (Patent Document 3).

JP 2001-152316 A JP-A-6-246548 JP-A-5-59530

  However, the above-described conventional technique has a problem that when a stainless steel screw is repeatedly fastened during assembly or maintenance, a screw seizure phenomenon (hereinafter simply referred to as “seizure”) is likely to occur. is there.

  Explaining in detail the state of occurrence of "seizure", for example, the tightening and loosening operations of the screw are repeated during assembly and maintenance, but in such cases the threaded surface where the "male thread" and "female thread" mesh. A large frictional force is repeatedly generated, and this frictional force causes the threaded surface to partially fuse and separate (so-called “galling” state). The “female screw” is fixed in a non-rotatable state and cannot be tightened or loosened.

  In general, the thread surface is easily worn and rough during screw tightening and loosening operations. In particular, stainless steel has a thermal conductivity of about 1/3 that of iron, and the thermal expansion coefficient is about 1.5 times that of iron. Therefore, the seizure phenomenon is extremely likely to occur.

  Also, stainless steel screws used in a vacuum or clean atmosphere, such as in vacuum equipment for semiconductor manufacturing equipment, in clean rooms, related equipment such as food and beverage manufacturing, and related equipment such as manufacturing cosmetics and medical products. Therefore, there is a need for a measure called “contamination” that suppresses the generation of minute dust, and for this reason, there is a need to use in a non-lubricated state without using lubricating oil.

  Incidentally, a plated or coated screw is not sufficient as a measure to suppress the generation of minute dust from the screw surface because a coating such as a plating layer is easily peeled off.

  Further, as described in Patent Document 2, it is effective to mechanically polish a gear for an automobile transmission in order to improve the contact fatigue strength of its friction surface. It is extremely difficult to uniformly barrel polish the thread surface by using it, and the barrel polishing has a lack of practicality since the dimensional accuracy of the screw thread is liable to be impaired.

  Furthermore, in the invention described in Patent Document 3, since the friction coefficient of the surface of the nitrided hardened layer is large and frictional heat is likely to be generated, there is a high possibility that seizure will occur when screws are tightened.

  Therefore, the problem of the present invention is to solve the above-described problems, and when the screw surface is repeatedly tightened or loosened in an environment where corrosion resistance is required using an austenitic stainless steel screw, An austenitic stainless steel screw that does not cause seizure and does not cause seizure, even if it is a dedicated screw that is used without lubrication in a vacuum or clean atmosphere. It is also possible to efficiently produce an austenitic stainless steel screw that can solve such problems.

  In order to solve the above-described problems, in the present invention, a surface hardened layer made of a carburized layer, a nitrided layer, or a carbonitrided layer is provided on the surface of an austenitic stainless steel screw, and at least the screw of the surface The surface was a stainless steel screw made of a polished surface obtained by electropolishing or chemically polishing the surface hardened layer.

  The stainless steel screw according to the present invention configured as described above has a slope in which a hardened surface layer composed of a carburized layer, a nitrided layer, or a carbonitrided layer does not have a clear boundary with a non-carburized or non-nitrided portion inside the steel material. Since the threaded surface is a polished surface obtained by electropolishing or chemically polishing the threaded surface made of such a surface hardened layer, such a threaded surface is hard and smooth. The surface is provided with wear resistance and sliding characteristics with a low coefficient of friction, that is, the thread surface is provided with wear resistance and peeling resistance, and further has low dust generation.

  In other words, repeated tightening and loosening operations using such screws do not cause threading or seizure on the screw surface, and are specially used screws that are used without lubrication, particularly in a vacuum or clean atmosphere. Then, for example, even in a high temperature environment of 200 ° C., an excellent stainless steel screw that does not cause seizure and does not generate dust is obtained.

  In addition, in order for the hardened surface of the threaded surface to be a polished surface having a more preferable hardness, a depth of 10 μm from the initial surface of the hardened layer made of a carburized layer, a nitrided layer, or a carbonitrided layer. A polished surface polished to a shallower depth is preferable.

  Such a surface hardened layer is a layer in which carbon or nitrogen is sufficiently enriched and diffused in stainless steel, and is a layer having a low coefficient of friction. The surface has a low coefficient of sliding property.

  In addition, it is preferable that the stainless steel screw be a dedicated screw that is used without lubrication in a vacuum or a clean atmosphere as an application that makes full use of characteristics that do not cause seizure and do not generate dust. As the stainless steel screw, an austenitic stainless steel screw can be adopted.

  In order to efficiently produce a stainless steel screw exhibiting advantageous effects as described above, plasma carburizing treatment or plasma nitriding treatment or plasma carburizing in an atmosphere of 350 ° C. or higher and lower than 700 ° C. is performed on the stainless steel screw. A method for producing a stainless steel screw comprising forming a surface hardened layer exceeding 30 μm by nitriding, and polishing the surface hardened layer from the initial surface to a depth of 10 μm or less by electrolytic polishing or chemical polishing to form a thread surface Is preferably adopted.

  When stainless steel, which is a material for stainless steel screws, is carburized at a high temperature exceeding the specified range, chromium in the stainless steel reacts with carbon to precipitate chromium carbide, which results in the surrounding chromium content. However, in this invention, by adopting plasma carburization in a low predetermined temperature range, carbon ions can be dissolved and diffused in stainless steel, and chromium carbide can be diffused. Without precipitation, the stainless steel's corrosion resistance is not compromised.

  The present invention provides a surface hardened layer made of a carburized layer, a nitrided layer, or a carbonitrided layer on the surface of a stainless steel screw such as austenitic, and at least the threaded surface of the surface is subjected to electropolishing or polishing the surface hardened layer. Since it is a stainless steel screw made of a chemically polished surface, the screw surface will not cause galling or seizure when the screw tightening or loosening operation is repeated. Alternatively, a dedicated screw used without lubrication in a clean atmosphere has an advantage of becoming a stainless steel screw that does not cause seizure and does not generate dust.

  In the manufacturing method of the present invention, a hardened surface layer is formed by a predetermined nitriding treatment or a plasma carburizing treatment at a predetermined temperature, and the hardened surface layer is polished to a depth of 10 μm or less from the initial surface by electrolytic polishing or chemical polishing. By forming the surface, there is an advantage that a stainless steel screw having an advantageous effect as described above can be efficiently manufactured.

Chart showing the relationship between hardness (HV) and depth from surface (μm) of stainless steel Chart showing the relationship between the axial force of various screws and various torques Chart showing the relationship between the axial force of various screws and various torques

  The stainless steel screw according to the present invention is provided with a hardened layer formed of a carburized layer, a nitrided layer, or a carbonitrided layer on the surface of the stainless steel screw, and at least the threaded surface of the surface is electropolished with the surface hardened layer. Alternatively, it is formed by a chemically polished surface.

  The stainless steel of the screw base material used in the present invention has a well-known chemical composition as a steel containing 12% or more of chromium, and a typical target material is JIS SUS304 (304L) of austenitic stainless steel. SUS305, SUS303, SUS309, SUS310, SUS316 (316L), Cu-containing XM-7, and the like.

  When plasma carburizing or plasma nitriding or plasma carbonitriding is performed using these as a base material, it is preferable that the surface is carefully ground after pre-treatment so that an austenite structure appears on the surface to be treated.

  Austenitic stainless steel is used as a steel material with excellent corrosion resistance in the chemical plant, food industry and many machine industries, but it has the characteristic of low hardness. A hard film having good adhesion to a soft base material can be formed by carburizing or nitriding as a surface treatment method for the material.

  In order to form a hardened surface layer on such stainless steel, in the atmosphere of carbonitriding gas containing nitrogen gas together with carburizing gas consisting of hydrocarbon gas or nitriding gas consisting of nitrogen gas or hydrocarbon gas, A cathode voltage is applied in a discharge state to a screw made of stainless steel such as an austenitic phase as a base material to form a surface hardened layer made of a plasma carburized layer, a plasma nitrided layer, or a plasma carbonitrided layer.

  The surface hardened layer may be a single layer, but after forming a carburized hardened layer consisting of a plasma carburized layer or a plasma carbonitrided layer, the plasma nitrided layer is formed on the workpiece surface in an atmosphere containing nitrogen gas. You can also

  In this way, a carburized hardened layer consisting of a plasma carburized layer or a plasma carbonitrided layer is first formed at a relatively low temperature on the austenitic stainless steel base material, and then plasma nitrided at a relatively low temperature from above the carburized hardened layer. A layer may be formed. In this case, carbon in the carburized hardened layer that has been formed earlier is driven to the deep part of the base material, and a plasma nitrided layer is formed on the surface side thereof. In addition, when the plasma nitrided layer is formed on the carburized hardened layer at a relatively low temperature, the thickness of the carburized hardened layer becomes thicker than the initial thickness.

  Incidentally, nitriding movement of the carburized layer has two types of trap sites and diffusion sites at the position of interstitial atoms in austenite. Nitrogen with a higher affinity than chromium traps from the carbon that previously occupied the trap site. Can be explained by the mechanism of intercepting the site. The presence of these two types of sites is also the reason why the concentration distribution is non-error function despite the diffusion that does not form a compound. The carbon expelled from the trap site diffuses through the diffusion site to the deeper part where the concentration of interstitial atoms is lower.

Plasma carburizing or plasma nitriding can be performed by the following operation.
First, a base material made of stainless steel is charged into the processing chamber and evacuated. Then, together with a dilution gas such as hydrogen and argon, the concentration of hydrocarbon gas such as methane is injected to about 1 to 20%, and the rest Is introduced into the processing chamber as a diluent gas such as argon (Ar), and a high DC voltage of about 3 A / cm ² is applied and held for about 12 hours.

  At this time, the introduced hydrocarbon gas or a mixed gas of nitrogen or an inert gas is also turned into plasma, and the potential is rapidly lowered near the cathode. For this reason, the carbon or nitrogen in the plasma is accelerated by the cathode fall in an ionized state, and is collided with the surface of the base material to be driven to form a carburized hardened layer and a nitrided layer.

As the above-described hydrocarbon gas, a methane congener represented by C _n H _{2n + 2} which is a gas at the carburizing temperature can be used without limitation. In particular, methane, ethane, propane, and butane that are gaseous at room temperature are preferable because they do not require vaporization equipment when used.

  The plasma carburizing, plasma carbonitriding or plasma nitriding treatment in the present invention is performed under the condition that the base material is heated to a predetermined temperature. That is, these treatments are performed at 350 ° C. or more and less than 700 ° C., preferably at a low temperature of 400 to 500 ° C., and the plasma nitriding treatment is preferably performed at 450 ° C. or less, more preferably 350 to 450 ° C. This is because, if plasma carburizing or plasma carbonitriding is performed below the predetermined range, a diffusion layer has a low diffusion rate, so that a cured layer having a sufficient thickness cannot be formed. This is because a bad layer is formed, which is not preferable.

  Next, the following treatment is performed so that at least the threaded surface is formed of a polished surface obtained by electrolytic polishing or chemical polishing of the surface hardened layer.

The chemical polishing is performed by degreasing treatment, immersion treatment in a polishing bath, and subsequent water washing treatment.
The polishing bath is composed of an appropriate blend of a component having sufficient solubility in stainless steel and a component having passivating ability such as an oxidizing agent, for example, 90 to 100% by volume of condensed phosphoric acid, 0 to 0 to sulfuric acid. An example of the polishing bath is 10% by volume.

  What is necessary is just to set immersion time for several seconds-several minutes on the temperature conditions of 150-200 degreeC with respect to such polishing bath, As preferable conditions in order to grind | polish from the initial surface of a surface hardening layer to 10 micrometer or less, It is preferable that the immersion time is 3 to 15 minutes at a temperature of 50 to 80 ° C.

  Electropolishing uses a sulfuric acid-based polishing bath, a stainless steel screw is connected to the anode in a glass-lasting electrolytic bath, a direct current voltage is applied with the large-area cathode facing the current, and the current density, temperature, and Adjust the time.

  As an example of the polishing bath for electropolishing, there may be mentioned one comprising phosphoric acid 40 to 45% by volume, sulfuric acid 35 to 40% by volume, chromic acid 5 to 7% by volume, and water (remainder).

Electrolytic polishing using such a polishing bath can be performed under the conditions of a current density of 40 to 70 A / dm ² , a temperature of 60 to 90 ° C., and a time of 2 to 15 minutes, but is shallower than 10 μm from the initial surface of the surface hardened layer. As the preferable conditions for polishing up to 30 mm, it is preferable that the current density is 40 to 70 A / dm ² , the temperature is 70 to 90 ° C., and the time is 3 to 10 minutes.

  When the surface hardened layer is electropolished or chemically polished in this way, large unevenness remains as it is like mechanical polishing, but the feature that micro unevenness is removed appears. Without being greatly scraped off, a passivating film is formed on the surface of the screw, and the hardened surface layer remaining at an appropriate thickness is inclined so as not to have a clear boundary with the non-carburized or non-nitrided portion inside the steel material. It diffuses with density, and the thread surface is difficult to generate fine particles by sliding. Thus, the stainless steel screw according to the present invention has wear resistance and sliding characteristics with a low coefficient of friction, and further has low dust generation.

[Example 1]
A threaded material having a hexagon socket head bolt was formed from a round bar of about 9 mm in diameter obtained by cutting austenitic stainless steel (SUS316) to a required length using a known press. This screw material was supplied to a well-known flat die rolling device, and thread rolling was performed to produce a screw (hexagon socket head bolt). Then, it heated to 500 degreeC equivalent to a carburizing process temperature, and performed the cleaning process using the nitrogen gas which mixed hydrogen gas.

  Then, an atmospheric gas composed of a mixed gas of methane gas (flow rate 0.5 L / min) as a carburizing gas and hydrogen gas (flow rate 3.0 L / min) as a dilution gas is introduced into the processing chamber, and the atmospheric gas temperature That is, the plasma carburizing process was performed under the conditions that the carburizing temperature was 500 ° C., the gas pressure was about 200 Pa, and the processing time was 12 hours.

  After the carburizing process was completed, the atmosphere gas was quickly exhausted, nitrogen gas was introduced into the processing chamber, and the screw material was forcibly cooled to room temperature.

For this screw, in order to investigate the depth of the carburized layer, the relationship between the depth (μm) from the surface and the hardness (Hv) was examined, and the result is shown in FIG.
As is clear from the results of FIG. 1, the surface hardening by plasma carburizing treatment, the formation of a hardened surface layer by permeation and diffusion of carbon is recognized from the surface to a depth of about 30 μm. Was observed to diffuse with a gradient density without a clear boundary.

Next, chemical polishing was performed under the following conditions.
Stainless steel having a threaded surface polished from the initial surface to a depth of 20 μm by chemical polishing for 2 minutes under a temperature condition of 180 ° C. using a polishing bath composed of 90% by volume condensed phosphoric acid and 10% by volume sulfuric acid. Steel screws were produced.

[ Reference example ]
In Example 1, a stainless steel screw was produced in exactly the same manner except that plasma nitriding was performed under the following conditions instead of plasma carburizing.
That is, in plasma nitriding, an atmosphere gas composed of a mixed gas of nitrogen gas (flow rate 1.0 L / min) and hydrogen gas (flow rate 3.0 L / min) as a dilution gas is introduced into the processing chamber, and the atmosphere gas temperature, That is, the plasma nitriding treatment was performed under the conditions that the nitriding temperature was 450 ° C., the gas pressure was about 200 Pa, and the treatment time was 12 hours.

  After completion of the treatment, the atmosphere gas was quickly exhausted, and nitrogen gas was introduced into the treatment chamber to forcibly cool the screw material to room temperature.

[Example 3]
In Example 1, a stainless steel screw was produced in the same manner except that the electrolytic polishing was performed under the following conditions instead of the chemical polishing.
As a polishing bath for electrolytic polishing, phosphoric acid 40% by volume, sulfuric acid 35% by volume, chromic acid 5% by volume, water 20% by volume, current density 60A / dm ² , temperature 75 ° C., time 10 minutes. I did it.

In order to polish from the initial surface of the surface hardened layer to 10 μm, conditions of current density of 50 A / dm ² , temperature of 80 ° C., and time of 5 minutes were adopted.

[Comparative Example 1]
A threaded material having a hexagon socket head bolt was formed from a round bar of about 9 mm in diameter obtained by cutting austenitic stainless steel (SUS316) to a required length using a known press. This screw material was supplied to a well-known flat die rolling device, and thread rolling was performed to produce a screw (hexagon socket head bolt).

[Comparative Example 2]
In Example 1, a hexagon socket head cap screw was manufactured in the same manner except that chemical polishing was not performed.

In order to investigate the characteristics of the stainless steel screws of Examples and Comparative Examples obtained as described above, the following tightening tests were performed.
[Tightening test]
Made by SDC: Using a screw performance tester (an improved version of the “Elastic / Plastic Area Fastening Screw Performance Testing Machine” designed by Dr. Kazuo Maruyama, Tokyo Institute of Technology), tightening 10 times by rotating a stainless steel screw. Then, the relationship between each torque (Nm) and axial force (kN) of the tightening torque, screw portion torque, and seating surface torque at that time was examined. FIG. 2 shows the results of Example 1, and FIG. Results are shown.

  As is apparent from the results of FIG. 3, the trial was repeated 10 times by bolt rotation. In Comparative Example 1 having no surface hardened layer, the initial tightening was normal, but as the number of tightening was repeated, The relationship between torque and axial force became unstable, and it was confirmed that a large change occurred in the threaded portion, seizure occurred by the eighth tightening, and the screw was broken.

  On the other hand, as is apparent from the results of FIG. 2, Example 1 having a threaded surface in which a hardened surface layer is formed of a polished surface has each torque (Nm) and axial force (kN) in 10 repetitions by bolt rotation. ) Was stable, and no seizure or galling phenomenon was observed.

In addition, in order to confirm whether the mechanical properties that can be used with respect to Examples 1 and 3 and Reference Examples 1 and 2 and Comparative Examples 1 and 2 that were separately prepared were used, the “mechanical machine for stainless steel corrosion resistant screw parts” of JIS B1054 When subjected to a tensile test of “proper properties”, Examples 1 and 3 and Reference Example had the same high tensile strength and the same elongation or fracture position as in Comparative Example 1, and formed and polished a surface hardened layer. It was confirmed that there was no decrease in strength due to.

Further, for Examples 1 and 3 and Reference Examples and Comparative Examples 1 and 2, female screws of the same size were formed on the SUS304 plate and tightened with a tightening torque of 10 N · m. The process of accommodating in this electric furnace and hold | maintaining at 200 degreeC atmosphere for 8 hours, after cooling to normal temperature after that, the process of loosening a screw was repeated 30 times.
As a result, in Examples 1 and 3 and the reference example , it was confirmed that no seizure phenomenon occurred, and it was confirmed that the screw after the test passed the screw accuracy test using a limit gauge.

  In this way, the stainless steel screw of the embodiment is different from the case where the surface hardened layer is formed by plating or painting, and there is no fear of peeling of the surface hardened layer. It can be used even in harsh indoor environments, and has the effect of preventing damage and failure of equipment due to seizure of screw parts, and it was clear that it is possible to extend the life of screws of the same type.

Claims (4)

A polished surface in which a surface hardened layer made of a carburized layer or a carbonitrided layer is provided on the surface of the stainless steel screw, and at least the threaded surface of the surface is electropolished or chemically polished from the initial surface to a depth of 10 μm or less. Stainless steel screw made of The stainless steel screw according to claim 1, wherein the stainless steel screw is a dedicated screw used without lubrication in a vacuum or a clean atmosphere. The stainless steel screw according to claim 1 or 2 , wherein the stainless steel screw is an austenitic stainless steel screw.   For a stainless steel screw, a surface hardened layer is formed by plasma carburizing treatment or plasma carbonitriding treatment in an atmosphere of 350 ° C. or higher and lower than 700 ° C., and this surface hardened layer is made shallower than 10 μm from the initial surface by electrolytic polishing or chemical polishing. A method for producing a stainless steel screw comprising polishing to form a threaded surface.

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