JP3421265B2 - Metastable austenitic stainless steel sheet for continuously variable transmission belt and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength metastable austenitic stainless steel sheet for a continuously variable transmission belt metal ring, which is required to have high strength, fatigue characteristics and wear resistance,
And a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, 18Ni maraging steel has been used as a metal ring material for a continuously variable transmission belt. The steel is
It is almost a single phase of martensite in the quenched state, and the hardness is increased by the aging treatment, and the nitriding treatment is further applied to ensure the improvement of wear resistance and fatigue characteristics.

[0003]

The structure of 18Ni maraging steel is substantially a martensite single phase, and this martensite itself is poor in toughness and ductility. Therefore, the presence of inclusions in the steel tends to deteriorate the fatigue properties. In particular, since 18Ni maraging steel uses Ti as an age-hardening element, Ti-based inclusions are likely to be formed, which causes a problem of deterioration of fatigue properties. When this steel is applied to applications where fatigue characteristics are important, such as continuously variable transmission belts, impurities in the steel must be thoroughly reduced so that inclusions do not occur. It takes a lot of labor for manufacturing, such as selection of, the high vacuum melting and the introduction of secondary refining. As a result, the productivity is lower than that of general-purpose steel materials and the manufacturing cost is significantly high.

The present invention exhibits higher fatigue properties than the conventional 18Ni maraging steel, and the reliability that stable high fatigue properties can be obtained without taking a special step for reducing inclusions. It is an object of the present invention to provide a high-strength steel plate for a continuously variable transmission belt having high durability.

[0005]

In order to achieve the above object, the invention of claim 1 is such that the mass% is C: 0.15% or less, S
i: 1.0 to 4.0%, Mn: 5.0% or less, Ni: 4.0 to 10.0%, C
r: 12.0 to 18.0%, Cu: 0 to 3.5% (including no additives), M
o: 1.0 to 5.0%, including N: 0.15% or less, C + N ≧ 0.10%,
Si + Mo ≧ 3.5% and Md (N) = 580−520C−2Si−16Mn−16Cr−23Ni−300N−10M
Oxidation-induced multi-phase structure of martensite + austenite containing these elements so that the value of Md (N) defined by o is 20 to 100, and the balance is Fe and unavoidable impurities. And has a nitride layer on the surface,
It is a metastable austenitic stainless steel plate for continuously variable transmission belts with excellent fatigue characteristics.

The invention of claim 2 defines the point of the invention of claim 1 that the average amount of work-induced martensite in the steel sheet is 30 to 80% by volume. Here, the "average amount of work-induced martensite in the steel sheet" means an average of all the amount of work-induced martensite in the steel sheet. The invention of claim 3 provides the invention according to claim 1, wherein the surface hardness of the steel sheet is Hv 700 or more. here,
The “surface hardness of the steel sheet” is the hardness of the surface layer portion of the steel sheet, and is specifically a value specified by measurement using a Vickers hardness meter with a load of 300 g.

According to a fourth aspect of the invention, in the invention of the first aspect, the average amount of work-induced martensite in the steel sheet is 30 to 80% by volume, and the amount of work-induced martensite in the surface layer portion from the surface to a depth of 10 μm. Is 60% by volume or more and the surface hardness is Hv 700 or more.
Here, "the average amount of induced martensite in the steel sheet" has the same meaning as in claim 2, and "the surface hardness of the steel sheet" has the same meaning as in claim 3. In the invention of claim 5, in the invention of claim 1, the average amount of work-induced martensite in the steel sheet is 30 to 80% by volume, and the amount of work-induced martensite in the surface layer portion from the surface to a depth of 2 μm is 80 volume. % And the surface hardness is Hv 700 or more. Here, "the average amount of induced martensite in the steel sheet" has the same meaning as in claim 2, and "the surface hardness of the steel sheet" has the same meaning as in claim 3.

A sixth aspect of the present invention relates to a method for producing the metastable austenitic stainless steel sheet for a continuously variable transmission belt according to the first to third aspects, containing 30 to 80% by volume of work-induced martensite. The cold-rolled steel sheet is characterized by being subjected to an aging nitriding treatment. Here, the "aging nitriding treatment" is a heat treatment for simultaneously performing the aging treatment and the nitriding treatment. According to a seventh aspect of the present invention, in the manufacturing method according to the sixth aspect, the cold-worked steel sheet to be subjected to the aging nitriding treatment is welded to the end faces of the solution-treated steel sheet to form a ring-shaped endless belt. It is defined as the one that has been subjected to 30 to 80% by volume to generate processing-induced martensite. Here, "ring rolling" refers to endless metal belts.
This is a rolling method in which a drum is rolled around a drum or a pulley while being rotated in a state where tension is applied, and cold rolling is performed by a rolling roll, which is generally used in the manufacture of steel belts and the like.

The invention of claim 8 relates to a method for producing a metastable austenitic stainless steel plate for a continuously variable transmission belt according to claim 4, wherein the end faces of the solution treated steel plates are welded to each other to form a ring shape. Endless belt of which the average martensite content in the steel sheet is 30 to 80% by volume and the martensite content in the surface layer from the surface to a depth of 10 μm is 60% by volume or more by ring rolling. It is characterized in that the work-induced martensite is generated in and then subjected to age nitriding treatment.

A ninth aspect of the present invention relates to a method for producing the metastable austenitic stainless steel sheet for a continuously variable transmission belt according to the fourth aspect, wherein the end faces of the solution treated steel sheets are welded to each other to form a ring shape. The endless belt of No. 3 was subjected to ring rolling, and then either barrel polishing, shot peening or shot blasting was applied to obtain an average martensite content of 30
It is characterized in that the work-induced martensite is generated so that the amount of martensite in the surface layer portion from the surface to the depth of 10 μm is 60% by volume or more, and then the aging nitriding treatment is performed.

A tenth aspect of the present invention relates to a method for producing the metastable austenitic stainless steel sheet for a continuously variable transmission belt according to the fifth aspect, wherein the end surfaces of the solution treated steel sheets are welded to each other to form a ring shape. Endless belt of which the average martensite amount in the steel sheet is 30 to 80% by volume and the martensite amount in the surface layer from the surface to a depth of 2 μm is 80% by volume or more by ring rolling. It is characterized in that the work-induced martensite is generated in and then subjected to age nitriding treatment.

The invention of claim 11 is the invention of claims 6 to 11, which specifies that the aging nitriding treatment is performed by a gas nitriding method in which a steel sheet is heated for 30 minutes or more in a nitriding atmosphere at 300 to 650 ° C. Is. The invention of claim 12 relates to claim 6
In the inventions Nos. 11 to 11, the aging nitriding treatment is performed by a gas sulphidizing nitriding method in which a steel sheet is heated for 20 minutes or more in a gas of 300 to 650 ° C in which H ₂ S is mixed with a gas whose basic component is ammonia gas. It has been prescribed. The invention of claim 13 is
In the inventions according to claims 6 to 11, plasma nitriding is performed by aging nitriding treatment in which a steel sheet is heated at 300 to 650 ° C for 20 minutes or more by plasma generated between the steel sheet and the furnace wall in a depressurized gas containing nitrogen gas as a basic component. It defines the points to be done by law. According to a fourteenth aspect of the present invention, in the inventions of the sixth to eleventh aspects, the steel material is subjected to age nitriding treatment in a salt bath of 300 to 650 ° C. containing NaCN, KCN, NaCNO and one or more of KCNO as a basic component. The point is defined by a salt bath nitriding method in which the steel is immersed for 20 minutes or more.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have conducted research on producing a highly reliable high strength stainless steel plate that can stably obtain high fatigue characteristics as a material for a continuously variable transmission belt that can endure a severe use environment. Overlaid. As a result, such a steel sheet is not a so-called "martensitic stainless steel" with a single phase of martensite, but a "metastable austenitic stainless steel" that can be controlled to a multiphase structure of work-induced martensite + austenite and that can be age hardened. It has been found that However, simply changing the steel type to such a system cannot be directly applied to the application of the continuously variable transmission belt. It requires some ingenuity. Hereinafter, matters specifying the present invention will be described.

C is an austenite forming element and is extremely effective in suppressing δ ferrite generated at high temperature and strengthening the martensite phase induced by cold working. However,
Since the steel targeted by the present invention has a high Si content, the solid solution limit of C is lowered. If a large amount of C is contained, coarse Cr carbides will precipitate during aging treatment, which will cause deterioration of intergranular corrosion resistance and fatigue properties. Therefore, the content of C is 0.15 mass% or less (not including 0%). Restricted to. A particularly preferable range of C content is 0.05 to 0.10% by mass.

Si is usually used for the purpose of deoxidation, and in that case, a content of 1.0 mass% or less is sufficient as seen in the examples of work-hardening stainless steels such as SUS310 and SUS304.
However, in the present invention, the effect of containing a relatively large amount of Si and significantly promoting the formation of martensite by cold working is utilized. Further, Si hardens the martensite phase induced by working, and also forms a solid solution in the austenite phase to harden it, improving the strength after cold working. Furthermore, in the aging treatment, the age hardening ability is improved by the interaction with Cu. In order to fully exert the effects of Si and obtain the characteristics suitable for the continuously variable transmission belt, Si of 1.0 mass% or more is required. However, when a large amount of Si exceeding 4.0% by mass is contained, high temperature cracking tends to be induced, which causes various problems in manufacturing. A particularly preferable Si content range is more than 1.0 to 3.5 mass%.

In the present invention, Mn can be contained in a relatively wide content range for the purpose of adjusting the stability of the austenite phase. Its content is determined by the balance with other elements, but if the Mn content becomes too high, martensite is less likely to be induced in cold rolling, so Mn
Is preferably contained in a range of 5.0% by mass or less (not including 0%). Particularly preferable Mn content range is 0.1 to 4.5% by mass.
Is.

Ni is an essential element for obtaining an austenite phase at high temperature and room temperature. However, in the present invention, it is desirable that the metal structure in the annealed state is an austenite single phase, and furthermore, an appropriate amount of martensite must be induced by cold working. When the Ni content is less than 4.0% by mass, a large amount of δ ferrite phase is generated at high temperature, and martensite is generated in the cooling process up to room temperature, and there is a high possibility that the austenite phase cannot exist as a single phase in the annealed state. . On the other hand, when the Ni content exceeds 10.0 mass%, martensite is less likely to be induced by cold working. Therefore, the Ni content is set to 4.0 to 10.0 mass%. The particularly preferable Ni content range is 5.0 to 9.5 mass%.

Cr is an essential element for ensuring corrosion resistance. 12.0 mass% or more of Cr is required to provide the desired corrosion resistance to the continuously variable transmission belt. However, Cr is also a ferrite-forming element, so if it is too much, a large amount of δ-ferrite phase will be formed at high temperatures. When increasing the Cr content, it is necessary to add austenite forming elements (C, N, Ni, Mn, Cu, etc.) in large amounts in order to suppress the δ ferrite phase, but excessive addition of these elements is not recommended. This results in stabilization of austenite at room temperature and insufficient formation of work-induced martensite, which makes it impossible to obtain sufficiently high strength after aging treatment. In order to achieve both the corrosion resistance and strength desired for the continuously variable transmission belt, it is preferable to limit the upper limit of the Cr content to 18% by mass. Particularly preferable Cr content range is 12.0 to
It is 16.5% by mass.

Since Cu is an element which improves the age hardening ability by interaction with Si, it is desirable to add Cu positively in the present invention. However, since excessive addition of Cu deteriorates hot workability and causes cracking of the steel sheet, it is preferable to add Cu in an amount of 3.5 mass% or less. A particularly preferable range of Cu content is 1.0 to 3.0 mass%.

Mo has the effect of improving the corrosion resistance and finely dispersing the carbonitride in the aging treatment. Further, in the present invention, it is advantageous to increase the aging temperature in order to reduce excessive rolling strain which adversely affects the fatigue properties.
Is a very effective element in the present invention because it suppresses the rapid release of strain during high temperature aging. Furthermore, since Mo forms an aging precipitate that contributes to the strength, addition of Mo makes it possible to prevent the strength from decreasing even if the aging treatment is performed at a considerably high temperature range. In order to sufficiently exert these effects, the present invention secures a Mo content of 1.0 mass% or more. However, addition of a large amount of Mo promotes the formation of the δ-ferrite phase at high temperature and also increases the deformation resistance at high temperature, which causes deterioration of hot workability. Therefore, it is preferable to set it in the range of 5.0 mass% or less. A particularly preferable range of Mo content is 1.0 to 4.5 mass%.

N is an austenite-forming element and an extremely effective element for hardening the austenite phase and the martensite phase. However, a large amount of addition causes blow holes during casting, so 0.15 mass% or less (not including 0%) is included.

C and N have the same hardening effect as each other, and in order to fully exert their effects, the total amount of C + N must be 0.10 mass% or more.

In the present invention, a Mo-based precipitate is formed by the aging treatment, but the addition of Si causes the Mo-based precipitate to be finely and uniformly dispersed. In order to sufficiently obtain the effect of such a composite addition of Si and Mo, it is preferable that the total amount of Si + Mo is 3.5% by mass or more.

In the present invention, as means for improving the fatigue characteristics of the continuously variable transmission belt, martensite-induced transformation in the belt manufacturing stage (eg, during ring rolling) and use as a product (continuously variable transmission belt). Utilizes the martensite-induced transformation in time. Therefore, strain applied in cold working (eg ring rolling) after solution treatment,
It is important that martensite is optimally formed with respect to local strain when used as a continuously variable transmission belt. Therefore, as a result of various studies, the steel composition was specified such that the Md (N) value (stability of austenite for working) defined by the following equation was in the range of 20 to 100. Md (N) = 580−520C−2Si−16Mn−16Cr−23Ni−300N−10M
o

In the steel composition in which Md (N) is less than 20, unless the cold working such as ring rolling is carried out at a low temperature which is industrially very difficult in the belt manufacturing stage, the martensite phase which contributes to the strength is sufficient. Cannot be generated. Further, when used as a continuously variable transmission belt, "austenite → martensite transformation" that effectively works for improving fatigue characteristics does not sufficiently occur. Furthermore, since the austenite is stable, the amount of martensite in the surface layer of the steel sheet does not exceed 80% by volume, and it is difficult to stabilize it to 60% by volume or more. Surface nitriding does not proceed sufficiently during the treatment, and as a result, it is not possible to expect dramatic improvements in wear resistance and fatigue properties. On the other hand, with a steel composition in which Md (N) exceeds 100, martensite is generated too early due to “deformation” when used as a continuously variable transmission belt, which is not effective for improving fatigue properties. It will be effective. For this reason, in the present invention, the Md (N) value is 20 to 100.
It is necessary to use steel whose composition has been adjusted to fall within the range.

Steel having the above chemical composition is melted, hot-rolled or cold-rolled, and then solution heat-treated to obtain a steel sheet whose metal structure is adjusted to a metastable austenite phase. . In the present invention, the solution-treated steel sheet is subjected to cold working such as ring rolling to introduce working strain. At that time, a part of the metastable austenite phase is transformed into the martensite phase. The amount of martensite produced at this stage is important for maintaining the balance between the “strength” and “toughness / fatigue properties” of the steel sheet after the aging nitriding treatment in the subsequent step. That is, the strength of the aged material increases as the cold working degree increases, but the toughness decreases significantly. In particular, excessive working also causes deterioration of fatigue properties. From the viewpoint of strength, it is desirable that the average amount of martensite in the steel sheet is 30% by volume or more in order to increase the strength of the material itself and further increase the nuclear sites of precipitates formed by the aging nitriding treatment. On the other hand, in terms of toughness and fatigue properties, untransformed austenite is also required, and it is desirable to keep the average martensite content in the steel sheet to 80 vol% or less.

In the present invention, in order to increase the strength of the surface of the product and improve the fatigue characteristics, a nitriding treatment that also serves as an aging treatment is performed. For nitriding, the martensitic structure is more advantageous than the austenitic structure because the diffusion of nitrogen is significantly faster. However, the average amount of martensite in steel should be limited to 80% by volume or less as described above. Therefore, in the present invention, when generating work-induced martensite in cold working such as ring rolling, while maintaining the average martensite amount of the entire steel sheet at 30 to 80% by volume, martensite with a high volume ratio only in the steel sheet surface layer portion. A method of forming a site phase is adopted.

Considering the uniformity of the nitrided layer on the entire surface of the steel sheet, it is desirable that the amount of martensite in the surface layer portion from the surface to the depth of 10 μm is 60% by volume or more, and especially from the surface to the depth of 2 μm. It is more desirable that the amount of martensite in the surface layer is 80% by volume or more. Further, when such a steel sheet is subjected to an aging nitriding treatment, one having a surface hardness of Hv 700 or more is stably obtained, and ideal wear resistance as a continuously variable transmission belt is imparted.

The average amount of martensite in the entire steel sheet is 30-80.
As a cold-working method of maintaining a volume% and forming a high volume ratio martensite phase only on the surface layer of the steel sheet, the end faces of solution-treated steel sheets are welded to each other to form a ring-shaped endless belt, which is then ring-rolled. The method of applying can be suitably applied. After ring rolling, barrel polishing,
By performing either shot peening treatment or shot blasting treatment, the amount of martensite in the steel sheet surface layer portion can be controlled more effectively.

The aging nitriding treatment is a "gas nitriding method" or a "gas soft nitriding method" which is a general nitriding treatment of steel materials with respect to a steel sheet which has undergone cold working and whose metallographic structure has been adjusted as described above. In addition to the above, it is possible to apply the "gas-sulfur-nitriding method", the "plasma-nitriding method", and the "salt bath nitriding method". Further, the “ion nitriding method”, the “salt bath carbonitriding method”, and the “salt bath carbonitriding method” can also be applied.

In the gas nitriding method, a gas containing ammonia gas as a main component can be used. For example, ammonia gas alone or RX gas (endothermic conversion gas;
Carbon monoxide + hydrogen + nitrogen), NX gas (a metamorphic gas obtained by completely burning butane or the like; nitrogen is the main component), propane, butane, or a mixture of (CO ₂ + CO) mixed gas and the like can be mentioned. In the gas nitrocarburizing method, ammonia gas contains CO ₂ + N ₂ + H.
Gas mixed with ₂ S can be used. In the plasma nitriding method,
N ₂ + H ₂ mixed gas can be used. In the salt bath nitriding method, NaCN,
A molten salt containing one or more of KCN, NaCNO and KCNO as a basic component and one or two of Na ₂ CO ₃ and K ₂ CO ₃ added thereto can be used.

In order to effectively perform the nitriding treatment that also serves as the aging treatment in the metastable austenitic stainless steel having the composition range specified in the present invention, the aging nitriding treatment temperature is 300 to 650.
It is desirable to set the temperature in the range of ° C. If the temperature is lower than 300 ° C, nitriding does not proceed sufficiently and strength increase due to aging cannot be expected. 650 ° C
When it exceeds, a part of the martensite phase formed on the surface of the steel material undergoes reverse transformation to an austenite phase, resulting in strength reduction and stagnation of nitriding reaction. Also, the heating time of the aging nitriding treatment
If it is less than 20 minutes, there is a high possibility that sufficient nitriding cannot be achieved.
The desirable heating time for the aging nitriding treatment is 20 minutes to 5 hours, more preferably 30 minutes to 5 hours.

[0033]

[Example] [Example 1] Table 1 shows the chemical component values and Md (N) values of the test materials. In Table 1, C1 to C6 are steels having a chemical composition within the specified range of the present invention (invention target steels), and D1 to D6 are other steels (comparative steels). Of these, D5 and D6 are the conventional 18
Ni maraging steel.

[0034]

[Table 1]

All the steels were melted in a vacuum melting furnace, and after forging → hot rolling → intermediate annealing → cold rolling, solution heat treatment of “1050 ° C. × 1 minute holding → water cooling” was performed, and thereafter, A steel belt is welded into a ring-shaped belt, which can be rolled at various rolling ratios.
Ring rolling was performed to 0.18 mm. In this ring rolled material,
50 in a nitriding atmosphere of 50% ammonia gas + 50% NX gas
Aged nitriding treatment was performed using a gas nitriding method of heating at 0 ° C for 3 hours. For each test material, the average martensite amount of the entire sample after ring rolling, the average martensite amount at the surface layer of 2 μm, and the surface hardness, tensile strength, and bending-tensile fatigue properties of the aged nitride material were investigated.

The amount of martensite in the entire sample was calculated from the ratio by utilizing the fact that the amount of martensite and the amount of saturation magnetization are proportional to each other by obtaining the saturation magnetization, which is a magnetic property, using a vibrating sample magnetometer. The amount of martensite in the surface layer portion was specified by X-ray diffraction. The surface hardness was measured with a Vickers hardness meter with a load of 300 g. The tensile strength was determined by using the No. 13B test piece specified in JIS Z 2201 and the tensile test method specified in JIS Z 2241. Bending-tensile fatigue properties were measured by using a test piece with a parallel length of 100 mm and a width of 3 mm with a diameter of 37.5 mm.
The sample was ruptured as it was reciprocated at a speed of 500 rpm (the number of repetitive (up and down) movements by the pulley was 500 times / min) by alternately pulling it with a belt that was hung on a drive pulley at both ends. It evaluated by the number of cycles. The maximum stress of the tension applied to the parallel part of the sample is 1450.
N / mm ² and the minimum stress were set to 50 N / mm ² . The bending-tensile fatigue test was carried out up to 1000 × 10 ⁴ cycles, and those that did not break were evaluated as “1000 × 10 ⁴ cycles or more”. The results of these tests are shown in Table 2.

[0037]

[Table 2]

As can be seen from the results in Table 2, the materials using the steels of the invention all had a surface hardness of Hv 700 or more, a tensile strength of 1900 N / mm ² or more, up to fracture in the bending-tensile fatigue test. The number of amplitudes is 700 × 10 ⁴ or more, which is extremely excellent in strength and fatigue characteristics, and has characteristics suitable for a continuously variable transmission belt. On the other hand, the material using comparative steel has a low amplitude frequency of less than 700 × 10 ⁴ before breaking, which is not sufficient as a continuously variable transmission belt.

As described above, Md (N) is an index of the stability of the austenite phase against processing, and if this value is small, it means that the processing-induced martensite is difficult to be formed during processing, and if it is large, it is easily formed. To do. C1 to C9 of the steels of the invention all have Md (N) values of 20 to 100, and austenite → martensite transformation occurs during deformation in the fatigue test. And, it can be said that the appropriate easiness thereof governs the fatigue characteristics of the material. For example, Md
Comparative steel D1 (N) is less than 20, fatigue properties are low because austenite → martensite transformation does not occur during deformation in the fatigue test, conversely, in Comparative steel D2 Md (N) exceeds 100,
Since the martensite is formed quickly, the fatigue property is also low.

The low tensile strengths of comparative steels D3 and D4 are due to
It is considered that D3 has a small Si content and thus little contribution of strengthening by aging, and D4 has a small Mo content, and therefore, softening started early during heating in the aging nitriding treatment. Since the comparative steels D5 and D6, which are conventional materials, have a substantially martensitic single-phase structure in the quenching treatment, they are liable to fracture at the time of local deformation in the fatigue test, and as a result, their fatigue properties are inferior to those of the present invention. ing.

[Example 2] Using the test materials shown in Table 1, ring rolling was performed to a plate thickness of 0.18 mm in the same process as in Example.
The ring-rolled material was subjected to an aging nitriding treatment by using gas nitrocarburizing, plasma nitriding, or salt bath nitriding. The aging nitriding treatment using the gas nitrocarburizing method was performed by heating in a mixed gas of 80% by volume of ammonia, 0.1% by volume of H ₂ S and the balance of CO ₂ at 560 ° C for 3 hours. Aged nitriding using plasma nitriding method is N ₂ 80% by volume + H ₂ 20% by volume
The sample was put in a closed furnace in which 5 Torr of the mixed gas was injected, a DC voltage of about 1000 V was applied to the furnace wall to generate plasma, and the sample was brought into contact with the plasma and heated at 540 ° C. for 3 hours. Age nitriding using the salt bath nitriding method is NaCN
40 mass% + Na ₂ CO ₃ 40 mass% as the main component and the balance is (NaK) ₄
The sample was immersed in a salt bath of Fe (CN) ₆ at 520 ° C. for 1 hour.

For each test material, the average martensite content of the entire sample after ring rolling was measured in the same manner as in Example 1,
The average martensite amount in the surface layer of 10 μm and the surface hardness, tensile strength and bending-tensile fatigue properties of the aged nitride material were investigated. The results of these tests are shown in Table 3.

[0043]

[Table 3]

The results of Table 3 in which the aging nitriding treatment was carried out by using various nitriding methods other than the gas nitriding method were the same as those in the case of Table 2 by the gas nitriding method. That is, in the steel to be invented, various nitriding methods can be applied to impart characteristics suitable for a continuously variable transmission belt having extremely excellent strength and fatigue characteristics.

[Embodiment 3] With respect to the influence of the temperature and time of the aging nitriding treatment, an example in the case of applying the gas nitriding method or the salt bath nitriding method will be shown. In the example by the gas nitriding method, C5 shown in Table 1 was used, and after subjecting this solution-treated steel sheet to ring rolling of 58%, aging nitriding was performed at various temperatures and times in the nitriding atmosphere of the same gas composition as in Example 1. Treated. In addition, in the example by the salt bath nitriding method, C4 in Table 1 was used and the solution treated steel sheet was
% Ring rolling, and then shot peening treatment (pressure 1 kgf / mm ² , bead diameter 0.25 to 0.60 mm) for about 1 min, the sample was subjected to various salt baths of the same composition as in Example 2 at various temperatures. And was aged for aged nitriding treatment.
The surface hardness, tensile strength, and bending-tensile fatigue properties of these samples subjected to age nitriding were examined. The results are shown in Table 4.

[0046]

[Table 4]

Whatever nitriding method is applied, those subjected to the aging nitriding treatment under the conditions specified in the present invention have sufficient nitriding and a surface hardness of Hv 700 or more. Furthermore, the tensile strength of 2000 N / mm ² or more and the number of amplitudes up to fatigue rupture 900 × 10 ⁴ or more were satisfied at the same time, and the characteristics as a continuously variable transmission belt were not insufficient. On the other hand, in the comparative example, due to insufficient strength, the number of amplitudes before fatigue fracture is 600 × 10 ⁴ or less.

Example 4 An experiment was also conducted in which the maximum stress value applied in the bending-tensile fatigue test was changed. By using three types of pulleys with diameters of 35.0 mm, 37.5 mm, and 40.0 mm for the test piece, the maximum stress value is 15 each.
Three levels of 50N / mm ² , 1450N / mm ² and 1380N / mm ² were set. However, the minimum stress value was set to 50 N / mm ² . Figure 1 shows C4, D2, and D5 in Table 1, and Figure 2 shows the same.
It is the graph which plotted the relationship between the maximum stress applied in a bending-tensile fatigue test and the number of times of amplitude to fracture about C5, D3, and D6, respectively. The samples in FIG. 1 are all prepared by the gas nitriding method, and the samples in FIG. 2 are C5 in the salt bath nitriding method and D3.
Is the gas sulphidic nitriding method, and D6 is the salt bath nitriding method. In FIG. 1, a conceptual diagram showing the bending-tensile fatigue test method and a conceptual diagram showing its stress application cycle are also shown. From FIGS. 1 and 2, it can be seen that the steel using the present invention exhibits significantly higher fatigue properties than the steel using the comparative steel at each maximum stress level.

[0049]

As described above, according to the present invention, in an austenitic stainless steel whose austenite stability is appropriately adjusted, the surface by martensite transformation / work hardening, age hardening, and nitriding during cold working such as ring rolling. By continuously extracting the effects of hardening and martensitic transformation when using a continuously variable transmission belt, a continuously variable transmission with high strength and fatigue characteristics not possible with conventional martensitic stainless steels. We have made it possible to provide steel sheets for belts.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the maximum stress applied in a bending-tensile fatigue test and the number of amplitudes until breakage, a conceptual diagram showing a bending-tensile fatigue test method, and a conceptual diagram showing its stress application cycle.

FIG. 2 is a graph showing the relationship between the maximum stress applied in a bending-tensile fatigue test and the number of times of amplitude until breakage.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI F16G 5/16 F16G 5/16 B (72) Inventor Naoto Hiramatsu 4976 Nomura Minamimachi, Shinnanyo, Yamaguchi Prefecture Nisshin Steel Co., Ltd. Technical Research In-house (56) Reference JP-A-7-300654 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (14)

(57) [Claims] 1. In mass%, C: 0.15% or less, Si: 1.0 to 4.
0%, Mn: 5.0% or less, Ni: 4.0 to 10.0%, Cr: 12.0 to 18.
0%, Cu: 0-3.5% (including no additives), Mo: 1.0-5.0
%, N: Including 0.15% or less, C + N ≧ 0.10%, Si + Mo ≧ 3.5
% And Md (N) = 580−520C−2Si−16Mn−16Cr−23Ni−300N−10M
Oxidation-induced multi-phase structure of martensite + austenite containing these elements so that the value of Md (N) defined by o is 20 to 100, and the balance is Fe and unavoidable impurities. And has a nitride layer on the surface,
Metastable austenitic stainless steel sheet for continuously variable transmission belts with excellent fatigue characteristics. 2. The metastable austenitic stainless steel sheet for a continuously variable transmission belt according to claim 1, wherein the average amount of processing-induced martensite in the steel sheet is 30 to 80% by volume. 3. The metastable austenitic stainless steel sheet for a continuously variable transmission belt according to claim 1, wherein the surface hardness of the steel sheet is Hv 700 or more. 4. The average work-induced martensite amount in a steel sheet is 30 to 80% by volume, the work-induced martensite amount in the surface layer portion from the surface to a depth of 10 μm is 60% by volume or more, and the surface hardness is Is Hv 700 or more, The metastable austenitic stainless steel sheet for a continuously variable transmission belt according to claim 1. 5. The average work-induced martensite amount in the steel sheet is 30 to 80% by volume, the work-induced martensite amount in the surface layer portion from the surface to a depth of 2 μm is 80% by volume or more, and the surface hardness is Is Hv 700 or more, The metastable austenitic stainless steel sheet for a continuously variable transmission belt according to claim 1. 6. In mass%, C: 0.15% or less, Si: 1.0 to 4.
0%, Mn: 5.0% or less, Ni: 4.0 to 10.0%, Cr: 12.0 to 18.
0%, Cu: 0-3.5% (including no additives), Mo: 1.0-5.0
%, N: Including 0.15% or less, C + N ≧ 0.10%, Si + Mo ≧ 3.5
% And Md (N) = 580−520C−2Si−16Mn−16Cr−23Ni−300N−10M
Contains these elements so that the value of Md (N) defined by o becomes 20 to 100, the balance consists of Fe and unavoidable impurities, and contains 30 to 80% by volume of processing-induced martensite. The method for producing a metastable austenitic stainless steel sheet for a continuously variable transmission belt according to claim 1, wherein the cold-worked steel sheet is subjected to an aging nitriding treatment. 7. A cold-worked steel sheet is formed by welding the end faces of solution-treated steel sheet to form a ring-shaped endless belt, which is ring-rolled to form 30-80% by volume of work-induced martensite. The manufacturing method according to claim 6, which is a product. 8. In mass%, C: 0.15% or less, Si: 1.0 to 4.
0%, Mn: 5.0% or less, Ni: 4.0 to 10.0%, Cr: 12.0 to 18.
0%, Cu: 0-3.5% (including no additives), Mo: 1.0-5.0
%, N: Including 0.15% or less, C + N ≧ 0.10%, Si + Mo ≧ 3.5
% And Md (N) = 580−520C−2Si−16Mn−16Cr−23Ni−300N−10M
The Md (N) value defined by o contains these elements so that the value is from 20 to 100, and the balance of the solution-treated steel sheets consisting of Fe and inevitable impurities is welded to each other to form a ring shape. The endless belt is ring-rolled so that the average martensite content in the steel sheet is 30 to 80% by volume and the martensite content in the surface layer from the surface to the depth of 10 μm is 60% by volume or more. The method for producing a metastable austenitic stainless steel sheet for a continuously variable transmission belt according to claim 4, wherein work-induced martensite is generated and then an aging nitriding treatment is performed. 9. In mass%, C: 0.15% or less, Si: 1.0 to 4.
0%, Mn: 5.0% or less, Ni: 4.0 to 10.0%, Cr: 12.0 to 18.
0%, Cu: 0-3.5% (including no additives), Mo: 1.0-5.0
%, N: Including 0.15% or less, C + N ≧ 0.10%, Si + Mo ≧ 3.5
% And Md (N) = 580−520C−2Si−16Mn−16Cr−23Ni−300N−10M
The Md (N) value defined by o contains these elements so that the value is from 20 to 100, and the balance of the solution-treated steel sheets consisting of Fe and inevitable impurities is welded to each other to form a ring shape. The endless belt is subjected to ring rolling, which is then subjected to barrel polishing, shot peening treatment, or shot blasting treatment so that the average amount of martensite in the steel sheet becomes 30 to 80% by volume, and the surface To the depth of 10 μm, the martensite content is 60
The method for producing a metastable austenitic stainless steel sheet for a continuously variable transmission belt according to claim 4, wherein the work-induced martensite is generated so as to have a volume% or more, and then an age nitriding treatment is performed. 10. Mass%, C: 0.15% or less, Si: 1.0 to
4.0%, Mn: 5.0% or less, Ni: 4.0 to 10.0%, Cr: 12.0-1
8.0%, Cu: 0-3.5% (including no additives), Mo: 1.0-5.0
%, N: Including 0.15% or less, C + N ≧ 0.10%, Si + Mo ≧ 3.5
% And Md (N) = 580−520C−2Si−16Mn−16Cr−23Ni−300N−10M
The Md (N) value defined by o contains these elements so that the value is from 20 to 100, and the balance of the solution-treated steel sheets consisting of Fe and inevitable impurities is welded to each other to form a ring shape. The endless belt is subjected to ring rolling so that the average martensite amount in the steel sheet becomes 30 to 80% by volume, and the martensite amount in the surface layer portion from the surface to the depth of 2 μm becomes 80% by volume or more. The method for producing a metastable austenitic stainless steel sheet for a continuously variable transmission belt according to claim 5, wherein work-induced martensite is generated and then an age nitriding treatment is performed. 11. The manufacturing method according to claim 6, wherein the aging nitriding treatment is performed by a gas nitriding method in which a steel sheet is heated in a nitriding atmosphere at 300 to 650 ° C. for 30 minutes or more. 12. The aging nitriding treatment is carried out by a gas nitrocarburizing method in which a steel sheet is heated for 20 minutes or more in a gas of 300 to 650 ° C. in which H ₂ S is mixed with a gas containing ammonia gas as a basic component. The manufacturing method of 6-10. 13. The aging nitriding treatment is performed by a plasma nitriding method in which a steel sheet is heated at 300 to 650 ° C. for 20 minutes or more by plasma generated between a steel sheet and a furnace wall in a depressurized gas containing nitrogen gas as a basic component, The manufacturing method according to claim 6. 14. The aging nitriding treatment, which comprises one or more of NaCN, KCN, NaCNO and KCNO as a basic component, and is 300 to 6
The production method according to claim 6, which is carried out by a salt bath nitriding method in which a steel material is immersed in a salt bath at 50 ° C. for 20 minutes or more.