JP2006265664A - Method for producing belt for continuously variable transmission - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a belt for continuously variable transmission, having reasonable strength and fatigue characteristics with a producing process of a reduced processing load. <P>SOLUTION: In the method for producing the belt for non-stage transmission, a ringed steel sheet which has the chemical composition composed of, by mass, ≤0.15% C, ≤3.0% Si, ≤1.0% Mn, ≤0.06% P, ≤0.01% S, 0.05-7.0% Ni, 8.0-18.0% Cr, ≤0.10% N, 0-0.05% Ti, 0-3.0% Cu, 0-0.015% B and the balance Fe with inevitable impurities and has the metallic structure of 0-10 vol% retained austenitic phase, 0-50 vol% ferritic phase, and martensitic phase in a remaining matrix part, in the base material and the welded part, and forms as the ringed state with the welding, is subjected to a nitriding treatment at 300-600°C for 1-120min with one among a salt-bath nitriding method, a gas-sulfurizing nitriding method and a plasma nitriding method. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method of manufacturing a continuously variable transmission belt comprising an endless metal belt ring-shaped by welding.

  Conventionally, 18Ni maraging steel has been used as a metal ring material used for a continuously variable transmission belt. This material is substantially martensite single phase in the quenched state, and can increase the hardness by aging treatment and further impart the wear resistance and fatigue characteristics of the material by hardening the surface layer by nitriding treatment. However, 18Ni maraging steel is very expensive because it contains 10% or more of Co, and there is a problem in recyclability. In addition, since the hardness increases due to the heat history during ring welding, it is difficult to control the plate thickness and the circumference when subjected to ring rolling as it is. For this reason, it is the present condition that the annealing process is put after ring welding, and the process load is also large.

  The following Patent Documents 1 and 2 describe a material using a metastable austenitic stainless steel as a continuously variable transmission belt material having improved fatigue characteristics as compared with 18Ni maraging steel. This realizes extremely high fatigue properties by generating work-induced martensite during ring rolling and further applying an aging nitriding treatment.

JP 2000-63998 A JP 2002-53936 A

  However, since the metastable austenitic steels of Patent Documents 1 and 2 are basically an austenite single-phase structure after solution treatment, in order to ensure a sufficient amount of work-induced martensite, the ring rolling process is performed. A heavy load is applied. In particular, since the welded portion has an austenite single-phase structure with less strain, it is necessary to increase the load in ring rolling in order to obtain a structure state with almost no difference between the base metal portion and the welded portion. In addition, since many austenite phases exist after ring rolling, a long time is required for the nitriding treatment. According to Patent Document 1, aging nitriding treatment is said to be required for 20 minutes or more. However, as a result of subsequent investigations by the inventors, it has been found that nitriding is not always stably achieved in a short time of about 20 minutes.

On the other hand, due to advances in design technology for continuously variable transmissions, the use of ring materials that do not require as high fatigue characteristics as the materials of Patent Documents 1 and 2 is increasing. The materials of Patent Documents 1 and 2 are not necessarily reasonable in terms of cost and characteristics.
The present invention uses a material excellent in recyclability without adding a special element such as Co, and is easy to implement and has a tensile strength of 1350 N / mm ² or more and a fatigue limit of 650 N / mm ² or more in a process with low process load. The aim is to provide a method for producing a continuously variable transmission belt that can stably impart fatigue characteristics and has an excellent cost / performance balance.

  As a result of detailed studies by the inventors, in order to achieve the above-described object, the metal structure of the material is changed to a martensite-based structure state in which the retained austenite phase is reduced to 10% by volume or less, It has been found that it is extremely effective to use the nitriding treatment of any one of “bath nitriding method”, “gas nitrosulphurizing method” and “plasma nitriding method”. And in order to obtain such a structure before nitriding, use a solution-treated steel sheet in which the retained austenite phase is previously reduced to 25% by volume or less as a material to be subjected to ring rolling, and appropriately set the ring rolling rate. It became clear that control was effective.

That is, in the present invention, by mass%, C: 0.15% or less, Si: 3.0% or less, Mn: 1.0% or less, P: 0.06% or less, S: 0.01% or less, Ni: 0.05-7.0%, Cr: 8.0-18.0%, N: 0.10% or less, Ti: 0-0.05%, Cu: 0-3.0%, Mo: 3.0% or less, B: 0 to 0.015%, preferably 0.001 to 0.015%, having a chemical composition comprising the balance Fe and unavoidable impurities. 10% by volume, ferrite phase: 0-50% by volume, ring-shaped steel sheet (endless metal belt) welded into a ring shape with the remaining matrix having a martensitic phase metal structure, the following “salt bath nitriding” Method of manufacturing a continuously variable transmission belt that is subjected to nitriding treatment by any one of “method”, “gas nitrosulphurizing method”, and “plasma nitriding method” Is provided.
[Salt bath nitriding method] In the molten steel at 300 to 600 ° C. containing at least one of NaCN, KCN, NaCNO and KCNO as a basic component and containing at least one of Na ₂ CO ₃ and K ₂ CO ₃ , the ring-shaped steel plate Is immersed for 1 to 120 minutes.
[Gas nitrosulphurizing method] The ring-shaped steel sheet is exposed for 1 to 120 minutes in a gas atmosphere of 300 to 600 ° C in which ammonia gas is a basic component and H ₂ S is mixed.
[Plasma nitriding method] The ring-shaped steel sheet is heated at 300 to 600 ° C. for 1 to 120 minutes by plasma generated between the ring-shaped steel sheet and the electrode in a reduced pressure gas containing nitrogen gas as a basic component.
In the continuously variable transmission belt, the ferrite phase is preferably 50% by volume or less.

As the chemical composition, those whose components are adjusted so that the D value defined by the following formula (1) is 80 or more are particularly suitable.
D = -1667C-28Si-33Mn-61Ni-42Cr-1667N-30Cu-42Mo + 1311 (1)

  Here, Ti, Cu, Mo, and B are arbitrarily added elements, and 0% of the lower limit thereof is a case that is below the measurement limit in a normal elemental analysis method performed in the steel making process. Ring rolling is a rolling method in which a ring-shaped endless metal belt is cold-rolled by a rolling roll while being rotated in a state where tension is applied to two drums or pulleys. The value of the content of each element expressed in mass% is substituted for the element symbol in formula (1).

As a more specific production method, it has the above-mentioned chemical composition, the solution phase is 0 to 50% by volume, preferably the residual austenite phase is 0 to 25% by volume, and the remaining matrix has a martensitic phase metal structure. Welding the processed steel sheet into a ring,
Ring rolling in a rolling rate range of 3 to 60%, and adjusting the residual austenite phase to 0 to 10% by volume for both the base metal and the welded portion;
A step of applying a mechanical surface treatment for applying a compressive stress to the surface such as barrel polishing, shot peening treatment, shot blast treatment, etc., if necessary,
A step of performing nitriding treatment by any of the above-mentioned “salt bath nitriding method”, “gas nitrosulphur nitriding method”, and “plasma nitriding method”;
A method of manufacturing a continuously variable transmission belt having the following is provided.

  According to the present invention, as compared with the case of using conventional 18Ni maraging steel, circumferential length control and plate thickness control are easily performed during ring rolling, and the obtained continuously variable transmission belt is excellent in recyclability. Also, compared to the case of using metastable austenitic stainless steel, the process load of ring rolling and nitriding treatment can be greatly reduced, and a continuously variable transmission belt that has almost no difference in characteristics between the base metal and the welded part is easy. Can be manufactured. Furthermore, according to the present invention, excellent strength and fatigue characteristics can be stably imparted. Therefore, the present invention provides a continuously variable transmission belt excellent in balance between cost and performance, and can contribute to the spread of continuously variable transmissions.

In the present invention, as described above, the tensile strength of 1350 N / mm ² or more and the fatigue limit are finally obtained by applying a nitriding treatment after forming a martensite-based microstructure in which the residual austenite phase is 10% by volume or less. Excellent strength and fatigue characteristics of 650 N / mm ² or more are stably imparted to the continuously variable transmission belt. For this purpose, a manufacturing process in which the chemical composition of steel and the metal structure of intermediate products are strictly controlled is required. Hereinafter, matters for specifying the present invention will be described.

[Chemical composition]
Hereinafter, “%” of the component elements means “% by mass” unless otherwise specified.
C: 0.15% or less C is an austenite-forming element and is extremely effective for strengthening the martensite phase. However, as the C content increases, Cr carbides precipitated at the grain boundaries during the cooling process after the solution treatment or during the nitriding process increase, which causes a decrease in intergranular corrosion resistance and fatigue characteristics. This can be avoided to some extent by optimizing the heat treatment conditions and the like, but considering the industrial limit, the upper limit of the C content is limited to 0.15%. The lower limit of the C content is not particularly limited, but is preferably 0.005% or more, and more preferably 0.01% or more.

Si: 3.0% or less Si is usually added for the purpose of deoxidation, but strengthens the martensite phase by solid solution, and contributes to high strength especially after ring rolling. It also has the effect of improving age hardening ability by strain aging. However, when the Si content is increased, hot cracking is likely to be induced, causing manufacturing problems, and the ferrite phase content is increased, leading to a decrease in strength. For this reason, Si content is restrict | limited to 3.0% or less.

Mn: 1.0% or less Mn is an austenite-forming element and contributes to strength improvement by increasing the martensite phase ratio after solution treatment. However, when a large amount of Mn is contained, the amount of retained austenite after solution treatment is increased, which causes a decrease in strength. Moreover, it may combine with S and precipitate in the form of MnS, which may adversely affect fatigue properties. For this reason, the Mn content is limited to 1.0% or less. A particularly preferable Mn content is 0.2 to 0.8%.

P: 0.06% or less P is an element having a large solid solution strengthening ability, but may combine with Ti or the like to form a phosphide and lower fatigue characteristics in a high strength region. Therefore, the lower the P content, the better. In the present invention, the P content is limited to 0.06% or less.

S: not more than 0.01% S is an element which is not preferable in terms of occurrence of edge breakage in hot rolling. Moreover, it couple | bonds with Mn, Ti, etc., forms a precipitate, and exerts a bad influence on a fatigue characteristic. Therefore, the lower the S content, the better. In the present invention, the S content is limited to 0.01% or less.

Ni: 0.05-7.0%
Ni, like Mn, exhibits the effect of increasing the martensite phase ratio after solution treatment and improving the strength. In order to exhibit this effect sufficiently, it is desirable that Ni content is 0.05% or more. On the other hand, a large amount of Ni contained increases the amount of retained austenite after the solution treatment and causes a decrease in strength. For this reason, the Ni content is specified to be 0.05 to 7.0%.

Cr: 8.0-18.0%
Cr contributes to solid solution strengthening and is important for imparting corrosion resistance. For continuously variable transmission applications, it is desirable to ensure a Cr content of at least 8.0%. In order to obtain corrosion resistance as stainless steel, a Cr content of 11.0% or more, preferably 13.0% or more is secured. However, if the Cr content is too large, it becomes difficult to obtain a metal structure mainly composed of martensite, and there is a possibility that sufficient strength as a continuously variable transmission belt cannot be secured. This harmful effect becomes noticeable when the Cr content exceeds 18.0%. Therefore, the upper limit of the Cr content needs to be specified to 18.0%.

N: 0.10% or less N, like C, is an austenite-forming element and contributes to strengthening of the martensite phase. However, if N is contained in a large amount, the amount of retained austenite after solution treatment increases, which causes a decrease in strength. In addition, non-metallic inclusions with Ti are likely to be generated, and the fatigue characteristics may be reduced. For this reason, the N content is limited to 0.10% or less. A preferable range of the N content is 0.005 to 0.07%.

Ti: 0.05% or less Ti has a precipitation hardening action, but in the present invention, it is not necessary to use the action of Ti. Rather, it has been found that when Ti is contained, Ti nitride increases, and the fatigue characteristics of the continuously variable transmission belt may be impaired. In the present invention, from the viewpoint of stably imparting excellent fatigue properties, Ti is not added or the content is limited to 0.05% or less.

Cu: 3.0% or less Cu contributes to age hardening. It is effective to set the Cu content to 0.5% or more. However, excessive addition of Cu leads to a decrease in hot workability. Therefore, when adding Cu, it is necessary to carry out within a range of 3.0% or less.

Mo: 3.0% or less Mo has an effect of finely dispersing carbide during aging treatment, and also has an effect of improving the corrosion resistance as stainless steel when the Cr content is large. Furthermore, Mo has an effect of suppressing rapid release of strain when aged at a high temperature, and can contribute to a high temperature and a short time of nitriding treatment also serving as aging. However, even if a large amount of Mo is added, these effects are saturated and the raw material cost increases, which is not rational. Therefore, when adding Mo, it is desirable to carry out in the range of 3.0% or less. A particularly preferable Mo content range is 0.5 to 2.0%.

B: 0.015% or less B forms fine precipitates with N to suppress grain coarsening during final annealing, and bonds at the grain boundaries of δ ferrite phase and austenite phase in the hot rolling temperature range It has the effect of improving the hot workability by increasing the force, and is particularly effective in preventing hot-ear breakage. Addition of 0.001% or more of B is particularly effective for sufficiently exhibiting such an action. However, excessive addition leads to the formation of a low-melting boride and conversely deteriorates hot workability. For this reason, when adding B, it is necessary to carry out in the range of 0.015% or less.

  In addition, Ca, REM (rare earth element), Y, and Mg may be included for the purpose of improving hot workability. In this case, the content is preferably 0.05% or less so as not to impair the fatigue characteristics and other material characteristics required for the continuously variable transmission belt.

D value: 80 or more It is desirable to adjust the content of each component element so that the D value defined by the following formula (1) is 80 or more.
D = -1667C-28Si-33Mn-61Ni-42Cr-1667N-30Cu-42Mo + 1311 (1)
The D value is an index having a good correlation with the amount of retained austenite in a steel plate (solution treated steel plate) that has been solution treated and water cooled. In the present invention, as described later, it is desirable to prepare a solution-treated steel sheet having a residual austenite phase of 25% by volume or less. By adjusting the components so that the D value is 80 or more, the amount of retained austenite of the solution-treated steel sheet can be controlled to 25% by volume or less.

[Solution treatment]
For melting the steel having the above-mentioned chemical composition by a normal method, for example, after passing through the process of “hot rolling → intermediate annealing → cold rolling”, it is subjected to solution treatment, and used for ring rolling described later. Get the material steel plate. The plate thickness is determined in consideration of the plate thickness of the continuously variable transmission belt, which is the final product, and the ring rolling rate. The solution treatment can be performed by a general method, for example, under the conditions of “850 to 1100 ° C. × 0 to 60 minutes holding → water cooling”. By using appropriately adjusted steel, the amount of retained austenite of the solution-treated steel sheet can be controlled to 25% by volume or less. If the amount of retained austenite exceeds 25% by volume at the stage of the solution-treated steel sheet, a great process load is required to obtain a desired structure in the subsequent ring rolling, or a desired structure is obtained. It becomes impossible.

On the other hand, when there are many ferrite phases in a steel plate, the strength is insufficient. As a result of various studies, in order to stably obtain a tensile strength of 1350 N / mm ² or more as a continuously variable transmission belt, it is extremely effective that the ferrite content in the belt is 50 volume% or less. Since the amount of ferrite is not substantially changed by ring rolling or nitriding, it is preferable to keep the amount of ferrite at 50 volume% or less at the stage of the solution-treated steel sheet. That is, it is preferable to prepare a solution-treated steel sheet having a ferrite phase: 0 to 50% by volume, a retained austenite phase: 0 to 25% by volume, and the remaining matrix having a martensitic metal structure. Here, the matrix (steel substrate) refers to a ferrite phase, an austenite phase, and a martensite phase. The metal structure may contain precipitates and inclusions in addition to the matrix.

〔welding〕
This solution-treated steel sheet is cut into a certain length, and both ends in the longitudinal direction are joined together by welding to form an endless metal belt.

[Ring rolling]
Next, the endless metal belt is subjected to ring rolling. That is, the endless metal belt is cold-rolled with a rolling roll while being rotated in a state where tension is applied to two drums or pulleys. In ring rolling, i) to adjust the product to a predetermined sheet thickness, ii) to adjust the residual austenite phase to a metal structure of 10% by volume or less, iii) to impart rolling strain to improve strength and fatigue characteristics The main purpose.

  In the present invention, the adjustment of the metal structure of ii) is extremely important for stably imparting excellent strength and fatigue characteristics while reducing the process load. In order to stably form a deep nitrided layer in the subsequent nitriding treatment, it is necessary to reduce the residual austenite phase as much as possible. That is, the austenite phase has a slower diffusion of N than the ferrite phase and martensite phase. When subjected to nitriding with a large amount of residual austenite phase, the surface layer is not sufficiently nitrided due to the slow diffusion of N, and the variation in the thickness of the nitrided layer increases, so that the fatigue characteristics tend to vary. As a result of various studies, it has been clarified that such a nitriding defect is remarkably improved when the residual austenite phase is reduced to 10% by volume or less.

  However, the ring rolling rate needs to be in the range of 3 to 60%. If it is less than 3%, it is difficult to obtain uniform characteristics over the entire belt circumference. If it exceeds 60%, excessive working strain is introduced, and the fatigue characteristics are rapidly deteriorated.

  In the steel composition having the chemical composition defined in the present invention, when a solution-treated steel sheet in which the amount of retained austenite is adjusted to 25% by volume or less is used as a material, the base material is in a range of a ring rolling rate of 3 to 60%. In both the welded portion and the welded portion, the amount of retained austenite phase can be made 10% by volume or less. Part or all of the retained austenite phase is transformed into a work-induced martensite phase by ring rolling, and the retained austenite phase ratio can be reduced. The amount of retained austenite in the nitriding treatment may be 0%. When the amount of retained austenite is already 0 to 10% by volume at the stage of the solution-treated steel sheet, there is no problem because the amount of retained austenite is further reduced or 0% by ring rolling. The preferable metal structure after the ring rolling is a structure in which the base metal and the welded part have a retained austenite phase: 0 to 10% by volume, a ferrite phase: 0 to 50% by volume, and the remaining matrix is a martensite phase.

[Mechanical surface treatment]
After the ring rolling, a mechanical surface treatment for applying a compressive stress to the surface of the ring-shaped steel sheet can be performed as necessary. This treatment is more advantageous in stably improving the fatigue characteristics of the final product. Specific means include barrel polishing, shot peening treatment, and shot blasting treatment. Only one of these may be adopted, or “composite processing” in which two or more of them are sequentially applied may be employed.

〔Nitriding treatment〕
The ring-shaped steel plate that has undergone ring rolling or further mechanical surface treatment is subjected to nitriding treatment by exposing it to a nitriding environment of 300 to 600 ° C. for 1 to 120 minutes. By performing the nitriding treatment under these conditions, it can also serve as an aging treatment, and the strength can be improved together with the fatigue characteristics.
As the nitriding environment, any one of “salt bath nitriding method”, “gas nitrosulphur nitriding method”, and “plasma nitriding method” can be used.

In the case of the salt bath nitriding method, a salt bath in which at least one of NaCN, KCN, NaCNO and KCNO is a basic component and at least one of Na ₂ CO ₃ and K ₂ CO ₃ is added thereto is used. The content of one or more of Na ₂ CO ₃ and K ₂ CO ₃ is desirably about 2 to 50% by mass in total. The salt bath is maintained at 300 to 600 ° C., and the ring-shaped steel plate is immersed in the salt bath for 1 to 120 minutes.

In the case of the gas nitrosulphurizing method, H ₂ S is mixed with a gas containing ammonia gas as a basic component, and a mixed gas containing CO ₂ and N ₂ can be used as necessary. This mixed gas is kept at 300 to 600 ° C., and the ring-shaped steel sheet is exposed to the gas atmosphere for 1 to 120 minutes.

In the case of the plasma nitriding method, nitrogen gas is a basic component, and if necessary, a ring-shaped steel plate is installed in a reduced-pressure gas atmosphere containing H _2, and by the plasma generated between the ring-shaped steel plate and the electrode, The said ring-shaped steel plate is heated at 300-600 degreeC for 1-120 minutes. It is also efficient to use the furnace wall of a metal hermetic container as an electrode for generating plasma. That is, the ring-shaped steel plate may be heated by plasma generated between the ring-shaped steel plate and the furnace wall.

  In any of these nitriding methods, it is important to heat the ring-shaped steel plate to 300 to 600 ° C. If it is less than 300 ° C., the diffusion rate of N is slow, and a sufficient nitriding depth cannot be obtained. On the other hand, when the temperature exceeds 600 ° C., the strain applied by martensite tempering and ring rolling occurs, so that the strength level of the base material and the welded portion decreases, and accordingly, sufficient fatigue characteristics as a continuously variable transmission belt. Cannot be obtained. The nitriding time is preferably 1 to 120 minutes in any nitriding method. The ring-shaped steel sheet used for this nitriding treatment has a low amount of residual austenite with a slow N diffusion of 0 to 10% by volume, and the remaining matrix is composed of a martensite phase, or a martensite phase and a ferrite phase. A sufficient nitriding depth can be obtained by nitriding. However, if it is less than 1 minute, it is difficult to stably obtain a sufficient nitriding depth. Usually, it is preferable to secure a nitriding time of about 10 to 100 minutes from the viewpoint of product quality stability and economic efficiency.

The continuously variable transmission belt made by the above manufacturing method has excellent strength and fatigue characteristics. In particular, a continuously variable transmission belt having a rational characteristic with a plate thickness of 0.03 to 0.5 mm, a width of 3 to 50 mm, a tensile strength of 1350 N / mm ² or more, and a fatigue limit of 650 N / mm ² or more is provided. It can be provided as an excellent balance between cost and performance.

Steel with the composition shown in Table 1 is melted in a vacuum melting furnace, forged, hot-rolled, intermediate-annealed, cold-rolled, held at 1020 ° C. for 1 minute, and then subjected to a solution treatment that is water-cooled to endless metal A raw steel plate (hereinafter referred to as “solution-treated steel plate”) for use as a belt was produced. The thickness of the solution-treated steel sheet was set according to the ring rolling rate to be carried out so that all the thicknesses after ring rolling were equal to 0.18 mm. A belt having a width of 20 mm was cut out from each solution-treated steel sheet, and both ends in the longitudinal direction were joined together by TIG welding to form an endless metal belt. This was put on two pulleys and rotated with tension applied, and ring rolling was performed at various rolling rates to obtain a ring-shaped steel plate (hereinafter referred to as “ring-rolled steel plate”) having a thickness of 0.18 mm. . Subsequently, except for some, one or more mechanical surface treatments of barrel polishing, shot blasting, and shot peening were performed. Thereafter, nitriding treatment was performed at various temperatures and times in a salt bath composed of 80% by mass NaCl, 15% by mass NaCNO and 5% by mass Na ₂ CO ₃ . Table 2 shows the manufacturing conditions.

  A test piece was cut out from the endless metal belt after the nitriding treatment, and a tensile test and a fatigue test were performed. The tensile test was performed according to JIS Z2241 using a No. 13B test piece defined in JIS Z2201. In the fatigue test, using a test piece having a parallel part of 100 mm in length and 8 mm in width, a double-bending bending fatigue test was performed under the conditions of a rotational speed of 600 rpm and a stress ratio (maximum stress / minimum stress) = − 1. The fatigue limit was measured accordingly. Moreover, the amount of retained austenite and the amount of ferrite were investigated about the solution-treated steel plate and the ring-rolled steel plate by X-ray diffraction and optical microscope observation. The remaining matrix excluding the retained austenite phase and ferrite phase was a martensite phase. Table 3 shows the results.

As can be seen from Table 3, the examples of the present invention had a residual austenite phase of 10% by volume or less in both the base metal and the welded portion after ring rolling, and were endless metal belts after nitriding (corresponding to products of continuously variable transmission belts) ), The strength and fatigue characteristics with a tensile strength of 1350 N / mm ² or more and a fatigue limit of 650 N / mm ² or more were cleared with a margin. In particular, a tensile strength of 1400 N / mm ² or more, a process with a low process load, which is easy to implement in a commercial production line, with a ring rolling rate of 3 to 60%, a nitriding temperature of 300 to 600 ° C., and a nitriding time of 10 to 100 minutes, fatigue. It was confirmed that excellent strength / fatigue properties of 700 N / mm ² or more were stably obtained.

  On the other hand, Comparative Example Q1 was inferior in strength and fatigue characteristics because the nitriding temperature was too high. In Q2, the nitriding temperature was too low, and in Q3, the nitriding time was too short, so that nitriding was insufficient and the fatigue limit was low. Q4 had an excessively high rolling strain because the ring rolling rate was too high, and the fatigue limit was low. In Q5, although the amount of retained austenite after solution treatment was relatively large, the ring rolling rate was too low, so that the amount of retained austenite could not be adjusted to 10% by volume or less by ring rolling. Insufficient fatigue properties. Q6 is a steel having a low D value of less than 80. The amount of retained austenite after solution treatment exceeded 25% by volume, and the amount of retained austenite after ring rolling exceeded 10% by volume. As a result, nitriding was insufficient and the fatigue characteristics were inferior.

  For steel A1 shown in Table 1, an endless metal belt having a width of 20 mm was produced by the same process as in Example 1. This was put on two pulleys and rotated in a state where tension was applied, and ring rolling was performed at a rolling rate of 46% to obtain a ring-shaped steel plate having a plate thickness of 0.18 mm. Subsequently, except for some, one or more mechanical surface treatments of barrel polishing, shot blasting, and shot peening were performed. Thereafter, nitriding treatment was performed at 480 ° C. for 60 minutes by various methods described in Table 4. With respect to the endless metal belt after nitriding, the tensile strength and fatigue limit were examined in the same manner as in Example 1. The results are shown in Table 4. The amount of retained austenite and the amount of ferrite after the solution treatment and after ring rolling are the same as P1 shown in Table 3.

  As can be seen from Table 4, a continuously variable transmission belt having excellent strength and fatigue characteristics can be manufactured by using any of the salt bath nitriding method, gas nitrosulphurizing method and plasma nitriding method.

Claims (9)

In mass%, C: 0.15% or less, Si: 3.0% or less, Mn: 1.0% or less, P: 0.06% or less, S: 0.01% or less, Ni: 0.05- 7.0%, Cr: 8.0 to 18.0%, N: 0.10% or less, Ti: 0 to 0.05%, Cu: 0 to 3.0%, Mo: 3.0% or less, B: 0 to 0.015%, balance Fe and chemical composition consisting of unavoidable impurities, both base metal and weld zone, residual austenite phase: 0 to 10% by volume, ferrite phase: 0 to 50% by volume, balance Nitriding of a ring-shaped steel sheet with a matrix having a martensitic phase metallized by welding using one of the following "salt bath nitriding method", "gas nitrosulphurizing method", or "plasma nitriding method" A process for manufacturing a continuously variable transmission belt.
[Salt bath nitriding method] In the molten steel at 300 to 600 ° C. containing at least one of NaCN, KCN, NaCNO and KCNO as a basic component and containing at least one of Na ₂ CO ₃ and K ₂ CO ₃ , the ring-shaped steel plate Is immersed for 1 to 120 minutes.
[Gas nitrosulphurizing method] The ring-shaped steel sheet is exposed for 1 to 120 minutes in a gas atmosphere of 300 to 600 ° C in which ammonia gas is a basic component and H ₂ S is mixed.
[Plasma nitriding method] The ring-shaped steel sheet is heated at 300 to 600 ° C. for 1 to 120 minutes by plasma generated between the ring-shaped steel sheet and the electrode in a reduced pressure gas containing nitrogen gas as a basic component.   The method for manufacturing a continuously variable transmission belt according to claim 1, wherein the ring-shaped steel plate is formed into a ring shape by welding and then cold-rolled in a range of 3 to 60%.   2. The ring-shaped steel sheet according to claim 1, wherein the ring-shaped steel sheet is formed into a ring shape by welding and then cold-rolled in a rolling rate range of 3 to 60%, and thereafter subjected to mechanical surface treatment for imparting compressive stress to the surface. A process for producing the continuously variable transmission belt as described.   The ring-shaped steel sheet is formed into a ring shape by welding and then cold-rolled in a rolling rate range of 3 to 60%, and then subjected to at least one of barrel polishing, shot peening treatment and shot blasting treatment. Item 12. A method for manufacturing a continuously variable transmission belt according to Item 1. In mass%, C: 0.15% or less, Si: 3.0% or less, Mn: 1.0% or less, P: 0.06% or less, S: 0.01% or less, Ni: 0.05- 7.0%, Cr: 8.0 to 18.0%, N: 0.10% or less, Ti: 0 to 0.05%, Cu: 0 to 3.0%, Mo: 3.0% or less, B: a step of welding a solution-treated steel plate having a chemical composition of 0 to 0.015%, the balance Fe and unavoidable impurities and having a ferrite phase of 0 to 50% by volume to form a ring,
Ring rolling in a rolling rate range of 3 to 60%, and adjusting the residual austenite phase to 0 to 10% by volume for both the base metal and the welded portion;
A step of performing nitriding treatment by any of the following "salt bath nitriding method", "gas nitrosulphur nitriding method", "plasma nitriding method",
Method for manufacturing continuously variable transmission belt having
[Salt bath nitriding method] In the molten steel at 300 to 600 ° C. containing at least one of NaCN, KCN, NaCNO and KCNO as a basic component and containing at least one of Na ₂ CO ₃ and K ₂ CO ₃ , the ring-shaped steel plate Is immersed for 1 to 120 minutes.
[Gas nitrosulphurizing method] The ring-shaped steel sheet is exposed for 1 to 120 minutes in a gas atmosphere of 300 to 600 ° C in which ammonia gas is a basic component and H ₂ S is mixed.
[Plasma nitriding method] The ring-shaped steel sheet is heated at 300 to 600 ° C. for 1 to 120 minutes by plasma generated between the ring-shaped steel sheet and the electrode in a reduced pressure gas containing nitrogen gas as a basic component. In mass%, C: 0.15% or less, Si: 3.0% or less, Mn: 1.0% or less, P: 0.06% or less, S: 0.01% or less, Ni: 0.05- 7.0%, Cr: 8.0 to 18.0%, N: 0.10% or less, Ti: 0 to 0.05%, Cu: 0 to 3.0%, Mo: 3.0% or less, B: a metal having a chemical composition consisting of 0 to 0.015%, the balance Fe and unavoidable impurities, residual austenite phase: 0 to 25% by volume, ferrite phase: 0 to 50% by volume, and the balance matrix being a martensite phase Welding a solution-treated steel sheet having a structure to form a ring;
Ring rolling in a rolling rate range of 3 to 60%, and adjusting the residual austenite phase to 0 to 10% by volume for both the base metal and the welded portion;
A step of performing nitriding treatment by any of the following "salt bath nitriding method", "gas nitrosulphur nitriding method", "plasma nitriding method",
Method for manufacturing continuously variable transmission belt having
[Salt bath nitriding method] In the molten steel at 300 to 600 ° C. containing at least one of NaCN, KCN, NaCNO and KCNO as a basic component and containing at least one of Na ₂ CO ₃ and K ₂ CO ₃ , the ring-shaped steel plate Is immersed for 1 to 120 minutes.
[Gas nitrosulphurizing method] The ring-shaped steel sheet is exposed for 1 to 120 minutes in a gas atmosphere of 300 to 600 ° C in which ammonia gas is a basic component and H ₂ S is mixed.
[Plasma nitriding method] The ring-shaped steel sheet is heated at 300 to 600 ° C. for 1 to 120 minutes by plasma generated between the ring-shaped steel sheet and the electrode in a reduced pressure gas containing nitrogen gas as a basic component. Between ring rolling and nitriding treatment,
A process of performing at least one of barrel polishing, shot peening and shot blasting;
A process for producing a continuously variable transmission belt according to claim 5 or 6.   The process for producing a continuously variable transmission belt according to claim 1, wherein the chemical composition has a B content of 0.001 to 0.015%. The method for manufacturing a continuously variable transmission belt according to claim 1, wherein the chemical composition has a D value defined by the following formula (1) of 80 or more.
D = -1667C-28Si-33Mn-61Ni-42Cr-1667N-30Cu-42Mo + 1311 (1)