JP4133842B2 - Stainless steel spring manufacturing method - Google Patents

Description

The present invention relates to a method for producing a stainless steel spring made of austenitic stainless steel, having excellent corrosion resistance and wear resistance, and also having non-magnetic characteristics.

  Austenitic stainless steel sheets and wires have excellent corrosion resistance and are used for various mechanical parts and equipment such as leaf springs, coil springs, power transmission wires, drive belts, chains, exterior cases, protective materials, and thin plate molded products. Has been.

  However, many of the above-mentioned austenitic stainless steel sheets and wires are different from ordinary carbon steel materials in the intermediate processing steps until the final strength of the material itself is finished, such as pressing, extrusion molding, punching, etc. Cold and warm processing represented by processing and the like are performed, and the material strength is improved by so-called work hardening. When using austenitic stainless steel as a spring material, the strength is insufficient as it is and the spring property is inferior, so it is used as a spring by performing cold working at a high working rate and strengthening it by work hardening. . In particular, when surface rigidity is required, wet chrome plating, Ni-P, etc., wet coating, PVD, CVD, etc., coating diffusion hardening treatment such as nitriding and carburizing, etc. Has been done.

Here, the following patent documents 1 and 2 are presented as what the applicant grasps as the prior art regarding the stainless steel spring.
JP 53-23834 A JP 11-1117045 A

  However, in the method of imparting strength by work hardening, since processing-induced martensite is generated by high-level processing, corrosion resistance is impaired and magnetism is obtained, so that magnetic disk devices, micro switches, relays, etc. This has been a problem when used in applications requiring non-magnetic properties. For this reason, as a spring material in a field where nonmagnetic properties are required, a Be—Cu nonferrous alloy that is expensive and has a concern about pollution and has a low strength has to be used.

  In addition, the strength (hardness) obtained by the method of improving the strength by work hardening is usually about Hv400 and is limited to about Hv550 at the maximum, so that there is a limit to the strength, that is, the wear resistance. On the other hand, in coating coating such as plating and CVD, there is a problem that peeling of the surface coating is likely to occur due to large bending deformation due to stress applied to the spring.

  Moreover, although what applied the nitriding process like patent document 1, and accompanied with precipitation of a carbide | carbonized_material like patent document 2, although the intensity | strength of a product improves, there exists a big problem that essential corrosion resistance falls. In addition, the nitriding treatment has disadvantages such as the formation of Cr nitride causes the product surface to swell, the surface roughness to deteriorate, or to become magnetized. In addition, since the formed nitride layer itself is not ductile at all, a crack is always generated during processing such as bending, which is a fatal defect as a spring.

The present invention has been made in view of such circumstances. By strengthening the surface layer portion of austenitic stainless steel without precipitating the chromium compound, the spring property is imparted, and excellent corrosion resistance and wear resistance / resistance are obtained. It has fatigue resistance, moreover an object to provide a manufacturing how stainless steel spring also having characteristics of non-magnetic.

In order to achieve the above object, a method for producing a stainless steel spring according to the present invention is the same as the above fluorination treatment, in which a spring material made of austenitic stainless steel is heated and held in a fluorine gas atmosphere to carry out fluorination treatment. Carburizing treatment is performed on the spring material at a time and / or thereafter to form a carbon solid solution layer in which chromium carbide is not substantially precipitated on the surface layer portion of the spring material, and the carburizing treatment is performed. The main point is to perform cold working on the spring material .

  The method for producing a stainless steel spring according to the present invention comprises performing a fluorination treatment by heating and holding a spring material made of austenitic stainless steel in a fluorinated gas atmosphere at the same time and / or after the fluorination treatment. Carburizing treatment is performed on the spring material. At this time, by the fluorination treatment, the surface of the austenitic stainless steel is activated and a fluoride film is formed on the surface, so that carbon easily enters. Then, by performing a carburizing process after the fluorination process, carbon enters and dissolves from the surface of the austenitic stainless steel. The intruded solid solution carbon diffuses from the surface of the base material toward the inside without forming chromium carbide particles, thereby forming a carbon solid solution layer on the spring material.

Since the carbon solid solution layer is in a state in which carbon atoms are dissolved without forming chromium carbide particles, the carbon concentration is increased, the lattice strain is increased, and the strength is improved. Therefore, the spring property can be imparted to the austenitic stainless steel by a process called carburization rather than work hardening as in the conventional case. And since the carbon solid solution layer is in a state in which carbon atoms are in solid solution without forming chromium carbide particles, the compound in which a solid solution of chromium atoms in the base material is maintained is not formed. For this reason, not only does the corrosion resistance of the austenitic stainless steel itself be impaired, but it also exhibits higher corrosion resistance. In addition, since the nonmagnetic properties of the austenitic stainless steel itself are maintained without being lost, it is possible to promote application to uses that hate magnetism. And it becomes possible to apply to devices requiring non-magnetic characteristics such as magnetic storage devices, microswitches, and relays, and the cost can be greatly reduced as compared with a spring made of a conventional Be-Cu alloy.
Moreover, since the cold work is performed on the spring material after performing the carburizing process, the carbon solid solution layer by carburization has high strength because carbon is dissolved without generating carbide or the like. At the same time, since a certain degree of ductility can be secured, the spring property can be further improved by adding work hardening by cold working in addition to strengthening in the vicinity of the surface layer portion by carburizing treatment. In addition, it is possible to perform molding by plastic deformation such as coiling or pressing after the hardening treatment by carburizing treatment. In this way, by molding after heat treatment, it becomes possible to obtain a spring product with extremely high accuracy while minimizing the influence of strain deformation due to heat treatment. In particular, it is advantageous in applications to fields where high accuracy is required, such as magnetic disk devices.

  In the method for producing a stainless steel spring of the present invention, when the spring material is previously cold-worked at a predetermined processing rate, the core portion by cold working is used to strengthen the vicinity of the surface layer portion by carburizing treatment. With the work hardening, the spring property can be further improved.

In the method for producing a stainless steel spring of the present invention, when the spring material is subjected to stress relief annealing after the cold work performed in advance , the stress relief annealing improves brittleness and stress corrosion cracking. In addition to improving resistance to heat, thermal deformation in subsequent fluorination treatment and carburization treatment is prevented. Therefore, in this case, it is preferable to perform dimensional correction after the stress removal annealing and before the fluorination treatment.

In the method for producing a stainless steel spring of the present invention, when stress relief annealing is performed after the cold working after the carburizing treatment, brittleness is improved by stress relief annealing and resistance to stress corrosion cracking is also improved.

  In the method for producing a stainless steel spring of the present invention, if the spring material has been subjected to a solution treatment in advance, carburization is performed on the spring material that has become completely non-magnetic by the solution treatment, and then cooled. The spring property can be imparted without inter-working, and is suitable for the field where non-magnetism is required. Moreover, the structure of the base material is normalized by the solution treatment, the carburization treatment is stabilized, the carbon solid solution layer is made uniform, the strength thereof is improved, and a stainless steel spring having stable spring characteristics is obtained.

  Next, the best mode for carrying out the present invention will be described in detail.

  The stainless steel spring manufacturing method of the present invention is performed mainly by carrying out the following steps.

  That is, a spring material made of austenitic stainless steel is heated and held in a fluorine gas atmosphere to perform fluorination treatment, and carburizing treatment is performed on the spring material at the same time and / or after the fluorination treatment. The carbon solid solution layer in which chromium carbide is not substantially precipitated is formed on the surface layer portion of the spring material.

  First, austenitic stainless steel, which is a base material to which the present invention is applied, will be described.

  Examples of the austenitic stainless steel include an austenitic stainless steel containing 50% by weight or more of iron, 12% by weight or more of chromium, and nickel. Specifically, 18-8 stainless steel materials such as SUS304, SUS316, and SUS303S, SUS310S and 309, which are austenitic stainless steels containing 25 wt% chromium and 20 wt% nickel, and a chromium content of 23 An austenite-ferrite two-phase stainless steel material containing 2% by weight of molybdenum and 2% by weight of molybdenum is exemplified. Further, heat-resistant steel cast steel such as SCH21 and SCH22 containing 19 to 22% by weight of nickel, 20 to 27% by weight of chromium and 0.25 to 0.45% by weight of carbon is also suitably used as the austenitic stainless steel of the present invention. It is done. Furthermore, SUH35 containing 20 to 22% by weight of chromium, 3.25 to 4.5% by weight of nickel, 8 to 10% by weight of manganese and 0.48 to 0.58% by weight of carbon, or 13.5% of chromium. A heat resistant steel such as SUH660 containing ˜16 wt%, nickel 24-24 wt% and molybdenum 1˜1.5 wt% can also be suitably used as the austenitic stainless steel of the present invention. Also, 17-7 stainless steel such as SUS301, 18-5-8Mn-N stainless steel such as SUS202 with a smaller amount of nickel, 17-4-6Mn-N stainless steel such as SUS201, 17 such as SUS301J, etc. So-called metastable austenite such as -8 series stainless steel, 17-7PH stainless steel such as SUS631 provided with precipitation hardening, PH15-7Mo stainless steel such as SUS632, 17-4PH stainless steel such as SUS630, etc. Stainless steel can also be suitably used as the austenitic stainless steel of the present invention.

  In this way, by using low carbon austenitic stainless steel containing nickel and chromium, it not only has excellent corrosion resistance but also high temperature strength and high temperature fatigue resistance, and it does not precipitate chromium compounds and maintains non-magnetism. In addition, a carbon solid solution layer is formed on the surface layer portion of austenitic stainless steel, and a nonmagnetic stainless steel spring having excellent wear resistance and corrosion resistance can be obtained. In addition, a non-magnetic austenitic stainless steel spring material that has a solid solution of carbon in the austenite of the base metal and substantially free of chromium carbides is precipitated over the entire spring material from the surface layer to the core. A phase is formed, and a nonmagnetic stainless steel spring having excellent wear resistance and corrosion resistance can be obtained.

  The austenitic stainless steel is formed into a plate having a predetermined thickness by rolling, or formed into a wire having a predetermined diameter by drawing to form a spring material.

  In the stainless steel spring manufacturing method of the present invention, as shown in FIG. 1A, the spring material is preliminarily subjected to predetermined cold working, and then subjected to fluorination treatment and carburization treatment. By doing in this way, work hardening of a core part is produced by cold working by a predetermined processing rate for reinforcement near the surface layer part by carburizing processing, and spring nature can be improved more.

  Further, in the method for manufacturing a stainless steel spring of the present invention, as shown in FIG. 1 (b), the spring material is preliminarily cold-worked at a predetermined processing rate, and thereafter stress-relief annealing is performed. Fluorination treatment and carburization treatment can be performed.

  By doing in this way, spring property can be improved more by accompanying the work hardening of the core part by cold work to reinforcement | strengthening of the surface layer part vicinity by a carburizing process. In addition, brittleness is improved by stress relief annealing, resistance to stress corrosion cracking is improved, and thermal deformation in subsequent fluorination treatment and carburization treatment is prevented. Therefore, in this case, it is preferable to perform dimensional correction after the stress removal annealing and before the fluorination treatment.

  As the treatment conditions for the stress relief annealing, suitable conditions can be used depending on the type of austenitic stainless steel used as a base material. For example, after annealing at a high temperature of about 600 to 900 ° C. for a predetermined time, it is gradually cooled. It is done by.

  Further, in the method for manufacturing a stainless steel spring of the present invention, as shown in FIG. 1 (c), the spring material after being subjected to the fluorination treatment and the carburization treatment is subjected to cold working at a predetermined working rate. Can be.

  By doing in this way, the carbon solid solution layer by carburization has high strength and can secure a certain degree of ductility because carbon is dissolved without generating carbides, etc. The spring property can be further improved by accompanying the work hardening of the core part by cold working in strengthening.

  In addition, it is possible to perform molding by plastic deformation such as coiling or pressing after the hardening treatment by carburizing treatment. In this way, by molding after heat treatment, it becomes possible to obtain a spring product with extremely high accuracy while minimizing the influence of strain deformation due to heat treatment. In particular, it is advantageous in applications to fields where high accuracy is required, such as magnetic disk devices.

  Furthermore, in the method for producing a stainless steel spring of the present invention, as shown in FIG. 1 (d), the spring material can be subjected to a solution treatment in advance, and then a fluorination treatment and a carburization treatment can be performed. .

  By doing in this way, it is possible to carburize the spring material that has become completely non-magnetic by the solution treatment, and to impart spring properties without performing cold work, in a field where non-magnetism is required. It will be appropriate. In addition, the carbides present in the base metal are solid-dissolved by the solution treatment, and the amount of carbon dissolved in the base material increases, and there is almost no fine carbide, and relatively large carbides are atomized to form a base. The material structure is normalized. As a result, the carburization treatment is stabilized, the carbon solid solution layer is made uniform, and the strength thereof is improved, and a stainless steel spring having stable spring characteristics is obtained.

  Moreover, since the internal distortion of the base material is removed by the solution treatment, thermal deformation and the like in the subsequent carburizing treatment and the like are reduced, the dimensional change of the product is small, and the deterioration of the surface roughness is also reduced.

  As conditions for the solution treatment, suitable conditions can be used depending on the type of austenitic stainless steel used as a base material, but the carbide is dissolved by heating at a temperature of 1000 ° C. or more for about 10 to several tens of minutes. It is carried out by allowing it to cool rapidly after allowing it to cool.

  Next, the fluorination treatment will be described in detail.

As the fluorine-based gas used for the fluorination treatment, a fluorine compound gas composed of NF ₃ , BF ₃ , CF ₄ , HF, SF ₆ , C ₂ F ₆ , WF ₆ , CHF ₃ , SiF ₄ , ClF ₃ or the like is used. can give. These may be used alone or in combination of two or more.

In addition to these gases, a fluorine-based gas containing fluorine (F) in the molecule can also be used as the fluorine-based gas of the present invention. Further, it is possible to use such a fluorine compound gas F ₂ gas and that generated by thermal decomposition at a thermal decomposition apparatus, as F ₂ gas is also the fluorine-based gas premade. Such a fluorine compound gas and F ₂ gas can be mixed and used in some cases.

Among these, NF ₃ is the most practical as the fluorine-based gas used in the present invention. This is because NF ₃ is gaseous at room temperature, has high chemical stability, and is easy to handle. Such NF ₃ gas is usually used in combination with N ₂ gas and diluted within a predetermined concentration range as described later.

The various fluorine-based gases exemplified above can be used alone, but are usually diluted with an inert gas such as N ₂ gas. The density | concentration of the fluorine-type gas itself in such a diluted gas is 10,000-100,000 ppm on a volume basis, for example, Preferably it is 20000-70000 ppm, More preferably, it is 30000-50000 ppm.

  The fluorination treatment using the fluorine-based gas as an atmosphere gas uses an atmosphere heating furnace such as a muffle furnace, which will be described later, and is charged with untreated austenitic stainless steel in the furnace, It is carried out by holding in a heated state under a gas atmosphere.

The heating and holding at this time is performed by holding the austenitic stainless steel itself at a temperature of, for example, 180 to 600 ° C, preferably 200 to 450 ° C. The holding time of the austenitic stainless steel in the fluorine gas atmosphere is usually set to 10 minutes to several hours. By heat-treating the austenitic stainless steel in such a fluorine-based gas atmosphere, the passive film containing Cr ₂ O ₃ formed on the surface of the austenitic stainless steel is changed to a fluoride film. The passive film has conventionally been considered not to be carburized. However, when the fluorination treatment is performed, the passive film changes to a fluoride film. This fluoride film facilitates the penetration of carbon atoms used for carburization compared to the passive film, and the surface of the austenitic stainless steel is considered to be a surface state that allows easy penetration of carbon atoms by the fluorination treatment. It is done.

  Next, carburizing treatment is performed on the austenitic stainless steel at the same time and / or after the fluorination treatment.

In the carburizing process, the austenitic stainless steel itself is heated to a carburizing temperature of 680 ° C. or less, and carburizing gas composed of CO + H ₂ or RX gas [CO 23 vol%, CO ₂ 1 vol%, H ₂ 31 vol%, Carburizing gas composed of H ₂ O 1 vol%, balance N ₂ ] + CO ₂ is used, and the inside of the furnace is set to a carburizing gas atmosphere. This carburizing gas atmosphere can be enriched with a carbon source gas such as propane gas, if necessary.

Thus, in the present invention, the carburizing process is performed in an extremely low temperature range as compared with a conventionally known carburizing process. In this case, the ratio of CO + H ₂ is preferably 2-50 vol% CO and 30-90 vol% H ₂ , and RX + CO ₂ is preferably 80-90 vol% RX and 3-7 vol% CO _2. . Moreover, CO + CO ₂ + H ₂ is also used as the gas used for carburizing. In this case, each of the ratios, CO5～55 _volume%, CO 2 1 to 3 _volume%, the proportion of H 2 50 to 95% by volume is preferred.

  As the heating temperature in the carburizing process, that is, the carburizing temperature, a temperature of 680 ° C. or lower, that is, 400 to 680 ° C. is preferable. When the carburizing temperature exceeds 680 ° C., the base material of the austenitic stainless steel itself is softened, or the carburized carbon atoms are combined with chromium dissolved in the base material to form chromium carbide. In addition to reducing the amount of chromium contained in itself, the corrosion resistance of the surface layer part is greatly reduced, the amount of carbon existing in a solid solution state penetrating into the carburized layer is reduced, the strength and corrosion resistance of the base material are reduced, This is because it becomes magnetic.

  For the same reason, a temperature range of 400 to 600 ° C. is more preferable as the carburizing treatment temperature, a temperature range of 400 to 550 ° C. is more preferable, and a temperature range of 450 to 500 ° C. is more preferable. In the present invention, by performing the fluorination treatment, carburizing treatment at such an extremely low temperature is possible, and carbon is intruded into the base material to form a solid solution without almost generating chromium carbide particles during the carburizing treatment, By increasing the lattice distortion, the surface layer of the base material is strengthened, and spring properties are imparted to the spring material.

By treating in this way, a carbon solid solution layer in which carbon diffuses and penetrates into the surface layer portion of the austenitic stainless steel is formed deeply and uniformly. This carbon diffusion layer is in a state in which a large amount of C atoms enter and dissolve in the austenite phase, which is the base phase, to cause lattice distortion, and the hardness is significantly improved as compared with the base material. . In addition, since the carbon atom penetrates and dissolves in the crystal lattice with little formation of chromium in the base material and carbides such as Cr ₇ C ₃ and Cr ₂₃ C ₆ , the carbon solid solution layer Is substantially free of chromium carbide particles and does not reduce the amount of chromium dissolved in the base material, so that the same level of corrosion resistance as the base material can be maintained.

  Further, the austenitic stainless steel subjected to the carburizing treatment as described above hardly deteriorates the surface roughness, and does not cause dimensional change or magnetism due to swelling. Therefore, the surface modification can be performed with relatively high accuracy with little reduction in surface roughness and dimensional change. Among the austenitic stainless steels, the stable austenitic stainless steel containing a large amount of nickel and the stable austenitic stainless steel containing molybdenum have better corrosion resistance of the carbon diffusion layer.

  The above fluorination treatment and carburization treatment can be performed in a metal muffle furnace 1 as shown in FIG. 2, for example. That is, in the muffle furnace 1, first, fluorination is performed, and carburization is performed at the same time as or after this fluorination.

  Moreover, it is preferable that the carburizing process is continued even after completion of the fluorination treatment. By doing so, more carbon atoms can be diffused and penetrated in a pure carburizing atmosphere to the spring material whose surface has been activated by fluorination treatment, and the surface strength can be increased or the hardening depth can be increased. This is because it is advantageous in increasing the size and effective for imparting springiness. In addition, by starting the carburizing process without waiting for the end of the fluorination process, the carbon can be diffused and penetrated while activating the surface by fluorination, and the surface strength can be increased or the hardening depth can be increased. This is advantageous when increasing the size. Further, the carburizing process can be started after the fluorination process is completed, the carburizing process may be started simultaneously with the start of the fluorination process, or the carburizing process is ended after the fluorination process is started. The intention is to start the carburizing process without waiting.

In FIG. 2, reference numeral 1 denotes a muffle furnace, which includes an outer shell 2, an inner container 4 having an inside formed in a processing chamber, and a heater 3 provided between the inner container 4 and the outer shell 2. . A gas introduction pipe 5 and an exhaust pipe 6 communicate with the inner container 4. The gas introduction pipe 5 communicates with a cylinder 15 filled with carburizing gas H ₂ and CO and a cylinder 16 filled with fluorination gas N ₂ + NF ₃ and CO ₂ . 17 is a flow meter, and 18 is a valve.

  An exhaust gas treatment device 14 and a vacuum pump 13 are connected to the exhaust pipe 6. Thus, the processing gas is introduced into the processing chamber in the inner container 4 and discharged. A fan 8 with a motor 7 for stirring the processing gas is provided in the processing chamber. Reference numeral 11 denotes a cage in which a spring material 10 made of austenitic stainless steel as a workpiece is inserted.

For example, a spring material 10 is placed in the muffle furnace 1, a cylinder 16 is connected to the flow path, a fluorine-based gas such as NF ₃ is introduced into the muffle furnace 1, and is subjected to fluorination treatment while being heated, and then exhausted. The gas is drawn from the pipe 6 by the action of the vacuum pump 13, detoxified in the exhaust gas treatment device 14, and discharged to the outside. Next, the cylinder 15 is connected to the flow path, and the carburizing gas described above is introduced into the muffle furnace 1 to perform the carburizing process, and then the gas is discharged to the outside via the exhaust pipe 6 and the exhaust gas processing device 14. To do. By this series of operations, fluorination treatment and carburization treatment are performed.

  By performing the fluorination treatment and the carburization treatment as described above, a carbon solid solution layer is formed on the surface layer portion of the austenitic stainless steel.

  The hardness of the carbon solid solution layer formed by the fluorination treatment and the carburization treatment is preferably set to Hv 570 or more, more preferably Hv 650 or more, Hv 700 or more, further Hv 800 or more or Hv 900 or more. If so, it is more preferable.

  By doing in this way, the carbon concentration of the carbon solid solution layer formed by the carburizing process is sufficiently high, particularly in the vicinity of the surface, and the strength is sufficiently improved by the lattice distortion, and excellent spring characteristics are imparted. In addition, since the spring characteristics of the product can be predicted to some extent by sampling and inspecting the intermediate product after carburizing treatment, the quality characteristic standard of the intermediate product is created, and fluorination treatment and carburizing treatment are performed again for those that do not meet it. It is possible to reduce the defective rate of the final product and improve the yield. In particular, by using the micro Vickers hardness or Knoop hardness measured from the surface of the base material as the hardness of the carbon solid solution layer, non-destructive product inspection can be performed and yield reduction can be reduced.

In the carburizing process, when the temperature of the carburizing process becomes high, particularly when the temperature exceeds 450 ° C., a phenomenon may occur that carbides such as Cr ₂₃ C ₆ are deposited even on the surface of the hardened layer, that is, the carbon diffusion layer. However, even in such a case, the carburized product is dipped in a strong acid such as HF-HNO ₃ , HCl-HNO ₃ and pickled to remove surface precipitates, and is corrosion resistant like a base material. And high surface hardness of Vickers hardness Hv800 or higher, further Hv850 or higher, and in some cases Hv900 or higher, can exhibit excellent spring characteristics. Further, the above pickling, may be removed _Fe 3 _{O 4} oxide such the (scale).

  Thus, the present invention is intended to include the case where a slight amount of precipitates and oxides on the surface are removed by pickling after carburizing. In addition, mechanical removal methods such as shot blasting and various types of dry and wet barrel polishing may be employed as long as they can remove precipitates and oxides on the surface. Such mechanical polishing such as shot blasting is performed not only for the purpose of removing carbides and oxides deposited on the surface, but also for the purpose of removing soot on the surface when sooting occurs in the carburizing process.

  The stainless steel spring manufactured in this way is made of nonmagnetic austenitic stainless steel, and has a concentration gradient in which the concentration of carbon solution decreases from the surface toward the inside. A carbon solid solution layer in which carbon is dissolved in the base material austenite is formed at least on the surface layer portion. Due to the intrusion and solid solution of carbon from the surface by carburizing treatment, the carbon solid solution portion of the base material is strengthened, and excellent spring properties, corrosion resistance, and wear resistance are imparted.

  For example, when the spring material is a plate having a relatively large plate thickness or a wire having a large wire diameter, the depth of the carbon solid solution layer is 5% or more of the plate thickness or the wire diameter from the surface of the spring material. Preferably it is formed to a depth. The ratio of the depth of the carbon solid solution layer to the plate thickness or wire diameter is more preferably 10% or more, and even more preferably 15% or more. The specific plate thickness in this case is preferably, for example, 1 mm or less as the plate thickness, and a more preferable upper limit is about 0.5 mm or less. Further, the specific wire diameter is preferably, for example, 1 mm or less, and a more preferable upper limit value is 0.5 mm or less. In this way, in a plate-like spring material having a relatively large plate thickness or a linear wire having a large wire diameter, carburizing treatment is carried out in a relatively short time by strengthening the surface layer portion with the carbon solid solution layer having a depth of 5% in the surface layer portion. A sufficient spring property is imparted.

  For example, when the spring material is a plate having a relatively small plate thickness or a wire having a small wire diameter, the depth of the carbon solid solution layer is 25% or more of the plate thickness or the wire diameter from the surface of the spring material. Preferably it is formed to a depth. The ratio of the depth of the carbon solid solution layer to the plate thickness or wire diameter is more preferably 30% or more, and even more preferably 40% or more. The specific plate thickness in this case is, for example, preferably 0.2 mm or less, more preferably 0.1 mm or less, and still more preferably about 0.05 mm or less. Moreover, the specific wire diameter is preferably 0.2 mm or less, more preferably 0.1 mm or less, and still more preferably about 0.05 mm or less. Thus, in a spring material having a relatively thin plate thickness or a linear wire material having a large wire diameter, a strong spring property is imparted by strengthening the surface layer portion with the carbon solid solution layer of the 25% depth of the surface layer portion. The

  In these cases, the carbon solid solution layer may be formed from the surface layer portion to the core portion, and the whole spring material may be made of a carbon solid solution phase. Thus, by making the whole spring raw material into a carbon solid solution phase, the whole spring raw material is strengthened as a carbon solid solution phase, and a strong spring property is exhibited.

  And since the chromium carbide is not substantially precipitated in the carbon solid solution layer or the carbon solid solution phase, the amount of chromium dissolved in the base material is not reduced, and the austenitic stainless steel which is the base material The same level of corrosion resistance can be maintained. Further, since the surface roughness hardly deteriorates and no dimensional change or magnetism occurs due to swelling, the surface modification can be relatively accurately performed with little reduction in surface roughness and dimensional change.

  As described above, according to the method for manufacturing a stainless steel spring, a spring material made of austenitic stainless steel is heated and held in a fluorine-based gas atmosphere to perform fluorination treatment. After that, carburizing treatment is performed on the spring material. At this time, by the fluorination treatment, the surface of the austenitic stainless steel is activated and a fluoride film is formed on the surface, so that carbon easily enters. Then, by performing a carburizing process after the fluorination process, carbon enters and dissolves from the surface of the austenitic stainless steel. Intruded solid solution carbon diffuses from the surface of the base material toward the inside without forming chromium carbide particles, and forms a carbon solid solution layer or a carbon solid solution phase in the spring material.

  The stainless steel spring obtained as described above has a carbon concentration of the carbon solid solution layer or the carbon solid solution phase because the carbon atoms are in a solid solution without forming chromium carbide particles. The lattice distortion increases and the strength increases. Therefore, the spring property can be imparted to the austenitic stainless steel by a process called carburization rather than work hardening as in the conventional case. And since the carbon solid solution layer or the carbon solid solution phase is in a state where carbon atoms are in solid solution without forming chromium carbide particles, the state in which chromium atoms that are solid solution in the base material are in solid solution is maintained. Therefore, the corrosion resistance of the austenitic stainless steel itself is not impaired, and the corrosion resistance higher than that is exhibited. In addition, since the nonmagnetic properties of the austenitic stainless steel itself are maintained without being lost, it is possible to promote application to uses that hate magnetism. And it becomes possible to apply to devices requiring non-magnetic characteristics such as magnetic storage devices, microswitches, and relays, and the cost can be greatly reduced as compared with a spring made of a conventional Be-Cu alloy.

  Next, examples will be described.

(1) Cross-sectional photographs of plate springs (Samples 1-10)
A stainless steel spring to which the present invention was applied was prepared. Fluorination treatment and carburization treatment were performed under the following treatment conditions.
[Processing conditions]
Base material: SUS316
Fluorination treatment: 10% by volume NF ₃ + balance N ₂ atmosphere
300 ° C. × 180 minutes Carburizing treatment: CO 50 volume% + H ₂ 10 volume% + N ₂ atmosphere
470 ° C x 4 to 26 hours

  Sample No. , Plate thickness, carburizing condition, depth of hardened layer (μm), depth ratio of carbon solid solution layer to plate thickness (%), cross-sectional area ratio (%) occupied by carbon solid solution layer, surface hardness (Hv), Table 1 below shows a list of figure numbers showing cross-sectional photographs.

  As can be seen from Table 1 and FIGS. 3 to 12, the examples show the surface hardness of Hv579 to 995, and the cross-sectional area ratio occupied by the carbon solid solution layer, that is, the hardened layer, is 25 to 100%. The scales in FIGS. 3, 4, 5, and 7 are the same as those shown in FIGS.

(2) Spring limit value of plate spring (sample 9)
The result of measuring the spring limit value using Sample 9 of Table 1 is shown below. It turns out that the spring limit value of the board | plate material of an Example is improving significantly rather than the thing of a comparative example. As a testing machine, an Akashi Seisakusho spring limit value testing machine (moment type testing machine) was used.

Spring limit value
Kgf / mm ² N / mm ²
Sample No. 9 (Example) 84.8 831
Untreated material (comparative example) 155.5 1524

(3) Bending test of plate spring (Sample 11)
A sample 11 was prepared by subjecting a 2 mm thick plate material to fluorination treatment and carburization treatment under the following treatment conditions.
[Processing conditions]
Base material: SUS316
Fluorination treatment: 10% by volume NF ₃ + balance N ₂ atmosphere
300 ° C. × 180 minutes Carburizing treatment: CO 50 volume% + H ₂ 10 volume% + N ₂ atmosphere
470 ° C x 26 hours

  The plate material of the sample 11 was subjected to a 180 ° bending test with a pressing piece at the tip 5R using the apparatus shown in FIG. The state of the carbon solid solution layer before the test is shown in photo (a), the state of the carbon solid solution layer on the pressing side after the test is shown in photo (b), and the state of the carbon solid solution layer on the back side after the test is shown in the photo ( c). As can be seen from the photographs (a), (b), and (c), there is no change in the state of the carbon solid solution layer even after bending, and no cracks or peeling are observed. Therefore, it can be seen that the carbon solid solution layer by carburization has high hardness and also has ductility, and can be processed after carburizing treatment.

(4) Cross-sectional photograph and cross-sectional hardness distribution of linear spring (sample 12)
Sample 12 was prepared by subjecting a 0.3 mm diameter wire to fluorination treatment and carburization treatment under the following treatment conditions.
[Processing conditions]
Base material: SUS316
Fluorination treatment: 10% by volume NF ₃ + balance N ₂ atmosphere
300 ° C. × 180 minutes Carburizing treatment: CO 50 volume% + H ₂ 10 volume% + N ₂ atmosphere
470 ° C x 18 hours

  FIG. 14 shows a cross-sectional photograph of the wire of Sample 12, and FIG. 15 shows the result of measuring the cross-sectional hardness distribution. As shown in FIGS. 14 and 15, the surface hardness is about Hv850 and the curing depth is about 20 μm, and it can be seen that the hardness gradually decreases from the surface toward the core. That is, it can be seen that there is a carbon concentration gradient in the depth direction from the surface.

(5) Tensile test of linear spring FIG. 16 shows the result of a tensile test performed on the sample 12 at a tensile speed of 5 mm / min and a distance between chucks of 70 mm. It can be seen that the untreated material as a comparative example has a tensile strength of about 1750 MPa, whereas the sample 12 as an example shows a tensile strength of 2000 MPa or more.

  With respect to a wire having a diameter of 0.3 mm similar to that of sample 12, a sample in which the hardened layer depth was changed by changing the carburizing conditions (carburizing time) was prepared. Examples are those having a cured layer depth of 15 μm (cured layer sectional area ratio 20%), 20 μm (cured layer sectional area ratio 23%), and 33 μm (cured layer sectional area ratio 39%). Three types were prepared, and an untreated material (hardened layer depth 0 μm) and a nitriding material were prepared as comparative examples.

  FIG. 17 shows the tensile test result of each sample. As can be seen from FIG. 17, in the examples, the higher the hardened layer depth, the higher the tensile strength, which was higher than that of the untreated material and the nitrided product, respectively.

(6) Repeated bending fatigue test of linear spring Sample 12 was subjected to repeated bending fatigue tests using the test apparatus shown in FIG. A wire sample connected endlessly was wound between a φ400 mm cylinder and a φ300 mm idler wheel giving a predetermined test load W by bending it to 90 ° with two φ60.3 mm test pulleys. Stress was repeatedly applied by rotating at a speed of 60 cpm and a speed of 120 times / minute.

Table 2 below shows the tensile stress, bending stress, total stress, and number of repetitions until rupture when the test load is changed in three stages for the untreated material as a comparative example and the examples. Further, FIG. 19 shows the number of repetitions until the untreated material and the breakage of the examples. Here, the total stress and the bending stress were calculated by the following equations, respectively. The elastic modulus is 135 GPa for the untreated material of the comparative example, 147 GPa for the example, the wire diameter is 0.3 mm, and the pulley diameter is 60.3 mm.
Total stress = Tensile stress + Bending stress Bending stress = Elastic modulus x Wire diameter / Pulley diameter

  As can be seen from Table 2 and FIG. 19, it can be seen that the fatigue strength of the example is dramatically improved as compared with the comparative example.

  As described above, the stainless steel spring obtained by the present invention exhibits excellent wear resistance, fatigue resistance, surface hardness, corrosion resistance, and spring properties. Moreover, when a solution treatment material is used, it will have a nonmagnetic characteristic. Therefore, it can be suitably used as a spring material in fields where non-magnetism is required, such as magnetic recording devices, connectors, relays, and switches. Moreover, the manufacturing method of the present invention and the metal product obtained thereby are not only used as springs, but also as high-strength materials such as power transmission wires, vibration damping materials, sink sinks, exterior cases such as cameras and notebook computers, etc. Can also be used.

  The stainless steel spring and the method for producing the same of the present invention can facilitate application to uses that dislike magnetism. And it becomes possible to apply to devices requiring non-magnetic characteristics such as magnetic storage devices, microswitches, and relays, and the cost can be greatly reduced as compared with a spring made of a conventional Be-Cu alloy.

It is a figure which shows the process of the manufacturing method of the stainless steel spring of this invention. It is a block diagram which shows the apparatus used for the manufacturing method of the stainless steel spring of this invention. It is a cross-sectional photograph of an Example (sample 1). It is a cross-sectional photograph of an Example (sample 2). It is a cross-sectional photograph of an Example (sample 3). It is a cross-sectional photograph of an Example (sample 4). It is a cross-sectional photograph of an Example (sample 5). It is a cross-sectional photograph of an Example (sample 6). It is a cross-sectional photograph of an Example (sample 7). It is a cross-sectional photograph of an Example (sample 8). It is a cross-sectional photograph of an Example (sample 9). It is a cross-sectional photograph of an Example (sample 10). It is a figure which shows the testing apparatus of the bending test of an Example (sample 11), and a cross-sectional photograph which shows a test result. It is a cross-sectional photograph of an Example (sample 12). It is a cross-sectional hardness distribution of an Example (sample 12). It is a figure which shows the measurement result of the tensile strength of an Example (sample 12). It is a diagram which shows the change of the tensile strength when changing the hardening layer depth of a wire spring. It is a figure which shows the repeated bending fatigue test method of a wire rod spring. It is a diagram which shows the repeated bending fatigue test result of a wire spring.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Muffle furnace 2 Outer shell 3 Heater 4 Inner container 5 Gas introduction pipe 6 Exhaust pipe 7 Motor 8 Fan 10 Spring material 11 Car 13 Vacuum pump 14 Exhaust gas treatment device 15 Cylinder 16 Cylinder 17 Flow meter 18 Valve

Claims (5)

A spring material made of austenitic stainless steel is heated and held in a fluorine-based gas atmosphere and subjected to fluorination treatment. Carburizing treatment is performed on the spring material at the same time and / or after the fluorination treatment. A carbon solid solution layer in which chromium carbide is not substantially precipitated is formed on the surface layer of the spring material, and the spring material after the carburizing treatment is subjected to cold working. Stainless steel spring manufacturing method.   The method of manufacturing a stainless steel spring according to claim 1, wherein the spring material is cold-worked at a predetermined processing rate in advance. The method of manufacturing a stainless steel spring according to claim 2, wherein the spring material is obtained by performing stress relief annealing after the cold working performed in advance . The manufacturing method of the stainless steel spring as described in any one of Claims 1-3 which performs stress relief annealing after the cold working after the said carburizing process . The method for producing a stainless steel spring according to any one of claims 1 to 4 , wherein the spring material is subjected to a solution treatment in advance.

Images