JP5633466B2 - Continuously variable transmission belt and manufacturing method thereof - Google Patents

Description

  The present invention relates to a continuously variable transmission belt and a manufacturing method thereof, and more particularly to a continuously variable transmission belt subjected to carburizing / quenching treatment and nitriding treatment and a manufacturing method thereof.

  In recent years, automobiles have been strongly demanded to reduce fuel consumption from the viewpoint of environmental problems, and belt-type continuously variable transmissions (hereinafter referred to as “CVT”) that can contribute to reducing fuel consumption are often used as transmissions. In the CVT, a plurality of thin metal belts (stepless transmission belts) are stacked to form a belt, and the belt is assembled to a friction member called an element and used.

  The metal belt rotates to transmit power by being assembled to the element. For this reason, tension and repeated bending stress act on the metal belt during rotation, and friction occurs with the element. Accordingly, the metal belt is required to have various properties such as tensile strength and fatigue strength, and is currently made of maraging steel having excellent tensile strength and fatigue strength.

  As maraging steel, there exists what was described in the following patent document 1, for example. In Patent Document 1 below, first, a solution treatment is performed on maraging steel at an appropriate temperature of 825 to 960 degrees. An aging treatment is performed at 490 degrees, and then gas nitriding is performed at 450 to 470 degrees. As a result, in this maraging steel, the internal hardness becomes 500 HV or more by age hardening, the surface hardness becomes 800 HV or more by nitriding, and a large compressive residual stress of about 1200 MPa is generated on the surface. Thus, a metal belt excellent in tensile strength and fatigue strength is produced using maraging steel.

JP 2001-240944 A

  By the way, maraging steel has obtained high tensile strength because the alloying element (Ni, Co, Mo, etc.) which it contains forms an intermetallic compound by age hardening, and this intermetallic compound precipitates. Regarding fatigue strength, the contained Ti increases the compressive residual stress generated on the surface by nitriding, and greatly contributes to the improvement of fatigue strength. However, since maraging steel contains a lot of expensive alloy elements, there is a problem that the metal belt made of maraging steel is expensive. Therefore, it is desired to manufacture a metal belt at low cost using ordinary steel instead of maraging steel.

  Here, when manufacturing a metal belt using a normal steel material, in order to obtain a big tensile strength, it is possible to quench the carbon steel. This is because when tensile strength is obtained by quenching, it is not necessary to add many alloy elements and there is a cost merit. However, in the case of carbon steel, a large tensile strength can be obtained, but a large fatigue strength, that is, a large compressive residual stress cannot be generated on the surface. This is because, in the case of carbon steel containing a large amount of C, unlike maraging steel that does not contain a lot of C, it cannot contain a large amount of Ti in a solid solution state, and nitriding does not cause a large compressive residual stress on the surface. is there.

  That is, in a steel material such as carbon steel, even if Ti is added, C and Ti are combined in the process of steelmaking (dissolution). As a result, inclusions of titanium carbide (TiC) are formed, and C and Ti do not exist (are unlikely to exist) as separate solid solutions. Therefore, even if a large amount of Ti is added, since Ti does not exist as a solid solution due to the combination of C and Ti, the compressive residual stress generated on the surface by nitriding cannot be increased, and the fatigue strength of carbon steel is increased. Can not do it. Further, if Ti is contained more than C, C does not exist in a solid solution state due to the combination of C and Ti, so that the effect of increasing the tensile strength due to the quenching process does not occur. In short, when a metal belt is manufactured using carbon steel, the tensile strength cannot be increased and the fatigue strength cannot be increased.

  The present invention has been made to solve the above-described problems, and a continuously variable transmission belt manufacturing method capable of manufacturing a continuously variable transmission belt having high tensile strength and fatigue strength at low cost, and tensile strength. It is another object of the present invention to provide a continuously variable transmission belt that has a high fatigue strength and is inexpensive.

In order to achieve the above-described problems, a method for manufacturing a continuously variable transmission belt according to the present invention has the following configuration.
(1) A ring member forming step for forming a ring member in which C is 0.05% or less, Ti is 0.1% or more, and 1.0% or less in terms of mass%, and the ring member A carburizing / quenching process for performing a carburizing process and a quenching process, and a nitriding process for performing a nitriding process on the carburized / quenched ring member.
(2) In the method of manufacturing a continuously variable transmission belt described in (1), the carburizing / quenching step is characterized by performing a diffusion treatment after the carburizing treatment and before the quenching treatment. To do. Here, the “diffusion process” is a process in which the distribution of C content in the ring member is made uniform by heating and holding for a predetermined time at the transformation temperature of the ring member.
(3) In the method of manufacturing a continuously variable transmission belt described in (1) or (2), in the ring member formed in the ring member forming step, Ti is 0.3% or more by mass%. And 0.7% or less.
(4) In the method of manufacturing a continuously variable transmission belt according to any one of (1) to (3), in the ring member formed in the ring member forming step, C is 0.001% by mass%. It is characterized by the following.
(5) In the method of manufacturing a continuously variable transmission belt according to any one of (1) to (4), in the ring member forming step, a metal material containing Ti is formed, and the metal material has a thickness of 0. forming a thin plate of .42Mm, a tubular sheet formed by rolling a pre-Symbol thin plate, to form a thin pipe is welded to the circumferential end portion of the tubular sheet, the ring member by cutting the thin pipe It is characterized by forming.

In order to achieve the above-described problems, the continuously variable transmission belt of the present invention has the following configuration.
(6) A continuously variable transmission belt manufactured by subjecting a ring member containing Ti to carburizing / quenching treatment and nitriding treatment, wherein the belt is mass% and C is 0.2% or more. And 0.8% or less, and Ti is 0.1% or more and 1.0% or less, and the surface contains 10% or more of Ti contained in the ring member by mass%. Present as titanium nitride, in the central portion in the thickness direction from the surface, 10% or more of Ti contained in the ring member is present as Ti in a solid solution state, and the thickness from the surface is present. The Vickers hardness at the center in the vertical direction is 400 HV or more and 600 HV or less.
(7) The continuously variable transmission belt described in (6) is characterized in that, in mass%, Ti is 0.3% or more and 0.7% or less.
(8) The continuously variable transmission belt described in (6) or (7) is characterized in that C is 0.3% or more and 0.5% or less in mass%.

The operation and effect of the above-described stepless transmission belt manufacturing method will be described.
(1) According to the method of manufacturing a continuously variable transmission belt of the present invention, first, the ring member formed in the ring member forming step has a very low C content, whereas the Ti content is low. The amount is relatively large. For this reason, in this ring member, the deposit of titanium carbide (TiC) is hardly produced | generated. And, by the carburizing process, a large amount of C enters the ring member, and a large amount of C and Ti that are in a solid solution state are present separately. Next, due to the quenching process, C is confined in supersaturation and becomes a martensite state, and the hardness of the ring member is significantly increased. Thus, the ring member has a sufficiently high tensile strength. Thereafter, Ti, which is appropriately present in a solid solution state by nitriding, generates fine precipitates and causes a large compressive residual stress on the surface. Thus, the continuously variable transmission belt has sufficiently high fatigue strength. And this continuously variable transmission belt is not manufactured from the maraging steel in which many expensive alloy elements are contained, but is manufactured from the ring member comprised with the normal steel material. Therefore, according to this manufacturing method, a continuously variable transmission belt having high tensile strength and fatigue strength can be manufactured at low cost.
(2) In this case, the diffusion treatment can eliminate the difference in the C content between the front surface (back surface) of the ring member and the central portion in the thickness direction, and by quenching treatment after the diffusion treatment. A ring member (stepless transmission belt) with stable quality can be manufactured. Further, since the diffusion treatment is performed after the carburizing treatment, the carburizing treatment can be performed at a low temperature, and the treatment can be performed more precisely and in a short time. Further, the carburizing process can be quickly completed in a state where C does not enter the ring member, and the content of C is made uniform by subsequent diffusion processing, thereby shortening the processing time in the carburizing / quenching process. it can.
(3) In this case, Ti in a solid solution state can particularly increase the compressive residual stress on the surface of the continuously variable transmission belt by nitriding, and can sufficiently increase the fatigue strength.
(4) In this case, adverse effects caused by the formation of titanium carbide inclusions can be sufficiently suppressed.
(5) In this case, since the welded ring member is subjected to carburizing and quenching treatment, the C content can be arbitrarily determined, and the continuously variable transmission belt having extremely high tensile strength. Can be manufactured. That is, since the content of C is very small before welding, cracks do not occur in the welded portion even when welding. And since it carburizes and quenches after welding, C content can be made larger than 0.4% and tensile strength can be made very large. Therefore, according to this manufacturing method, even a continuously variable transmission belt that requires extremely high tensile strength can be manufactured.

The operation and effect of the above-described continuously variable transmission belt will be described.
(6) According to the continuously variable transmission belt of the present invention, since C is 0.2% to 0.8%, the continuously variable transmission belt satisfies the required tensile strength range. . Further, since Ti is 0.1% to 1.0%, the continuously variable transmission belt satisfies the required fatigue strength range. In addition, 10% or more of Ti contained on the surface exists as titanium nitride, and 10% or more of Ti contained in the central portion in the thickness direction exists as Ti in a solid solution state, This continuously variable transmission belt is appropriately subjected to nitriding treatment. Furthermore, since the Vickers hardness is 400 HV to 600 HV at the central portion in the thickness direction, it satisfies the range of internal hardness required for a continuously variable transmission belt. The continuously variable transmission belt is not manufactured from maraging steel containing a lot of expensive alloy elements, but manufactured from ordinary steel. Therefore, this continuously variable transmission belt has high tensile strength and fatigue strength and is inexpensive.
(7) In this case, Ti, which is in a solid solution state, increases the compressive residual stress on the surface of the continuously variable transmission belt by nitriding, so that the fatigue belt has a sufficiently large fatigue strength. .
(8) In this case, the tensile hardness is in the optimum range required for the continuously variable transmission belt.

(A) It is explanatory drawing which showed the external appearance shape of the metal belt. (B) It is the elements on larger scale of a metal belt. (C) It is sectional drawing which showed the state in which a metal belt is used. It is the flowchart which showed the process for manufacturing a metal belt. It is explanatory drawing of the state by which a thin plate is rounded. It is explanatory drawing of the welded thin wall pipe. It is explanatory drawing of the cut | disconnected ring member. It is explanatory drawing of the rolled ring member. (A) It is a fragmentary sectional view of a ring member. (B) It is explanatory drawing which showed content of C of the ring member by which the carburizing process was carried out. It is explanatory drawing of the ring member by which distortion correction and circumference adjustment were carried out. It is explanatory drawing which showed the relationship between the depth from the surface of the ring member by which the nitriding process was carried out, and Vickers hardness. (A) It is the table | surface which showed the compressive residual stress in the surface of the metal belt of this embodiment, and the compressive residual stress in the surface of the metal belt as a comparative product. (B) It is the graph which compared the fatigue life of the metal belt of this embodiment, the metal belt which consists of maraging steel, and the metal belt as a comparative product. It is explanatory drawing which showed the relationship between the depth from the surface and the density | concentration of each component in the metal belt of this embodiment. It is explanatory drawing which showed the relationship between the depth from the surface and the density | concentration of each component in the metal belt as a defect. (A) It is the graph which showed the relationship between the depth from the surface, and N density | concentration in the case where content of Ti is 0.5% and 1.0%. (B) It is the table | surface which showed the compressive residual stress in the surface in the case where content of Ti is 0.5%, and the case where it is 1.0%. (A) It is explanatory drawing which showed content of C of the ring member before carrying out a diffusion process. (B) It is explanatory drawing which showed content of C of the ring member after carrying out a diffusion process.

  A continuously variable transmission belt and a manufacturing method thereof according to the present invention will be described below with reference to the drawings. FIG. 1 (A) is an explanatory view showing the appearance of the metal belt 1, FIG. 1 (B) is a partially enlarged view of the metal belt 1, and FIG. 1 (C) is used by the metal belt 1. It is sectional drawing which showed the state performed.

  The metal belt 1 is used as a power transmission belt in a belt type continuously variable transmission, and corresponds to a continuously variable transmission belt of the present invention. As shown in FIG. 1A, the metal belt 1 is a thin plate-like ring, and as shown in FIG. 1B, a curved shape in which the cross-sectional shape is convex in the radially outward direction, a so-called crowning R shape. It has become. As shown in FIG. 1C, a plurality of metal belts 1 are overlapped to form a pair of belts BT, and the pair of belts BT are assembled to the element 2 to transmit power transmission belts. Is functioning as The crowning R shape is provided to stabilize the overlapped state of the metal belt 1 when the belt BT is inserted into the recess 2a of the element 2 and assembled as shown in FIG. Yes.

  In the present embodiment, the metal belt 1 described above is manufactured through the respective steps shown in FIG. FIG. 2 is a flowchart showing a process for manufacturing the metal belt 1. As shown in FIG. 2, in the material forming process, a metal material that forms the material of the metal belt 1 is formed (step 10). That is, by melting of a VIM melting furnace (vacuum induction melting apparatus), by mass%, C is 0.001% or less, Ti is 0.3% or more and 0.7% or less, Si is 0.5% or less, Mn is 0.8% or less, Cr is 1.0% or more and 6.0% or less, Mo is 0.5% or more and 1.5% A metal material which is the following and the remainder is Fe and inevitable impurities is formed. The chemical composition of the metal material is accurately measured by chemical analysis using, for example, ICP-MS (inductively coupled plasma mass spectrometer).

C: 0.001% or less, Ti: 0.3% to 0.7%,
The reason for this will be described in detail later.
Mn: 0.8% or less,
Mn is an optional element and does not need to be contained, but can be contained in a small amount because it is an effective element as a deoxidizer during melting. If the Mn content is too large, the ductility decreases and cold rolling becomes difficult, so the upper limit is set to 0.8%.

Cr: 1.0% to 6.0%,
Cr is effective for improving the internal hardness by a quenching process, which will be described later, and is also effective for improving the hardness near the surface by a nitriding process, which will be described later. In order to obtain these effects, the lower limit of the Cr content is set to 1.0%. On the other hand, when the content of Cr is too large, a large amount of Cr nitride is generated by the nitriding treatment described later, and the nitrogen concentration distribution becomes discontinuous. As a result, the compressive residual stress is lowered and the fatigue strength is lowered. For this reason, the upper limit of the content of Cr is set to 6.0%.

Mo: 0.5% to 1.5%
Mo is an element effective for improving strength and toughness without impairing ductility. In order to obtain the effect, the lower limit value of the Mo content is set to 0.5%. On the other hand, even if the Mo content is excessive, the effect is saturated and the cost is increased, so the upper limit of the Mo content is set to 1.5%.
The inevitable impurities described above are, for example, S, P, N, O, Al, and the like.

  Here, it returns to the flowchart of FIG. After the material forming process, in the tubular forming process, a thin sheet of metal material is rolled to form a tubular sheet (step 11). That is, a thick plate is formed from the above-described metal material by hot forging, and a thin plate (steel strip) is formed from the thick plate by cold rolling. FIG. 3 is an explanatory diagram of a state in which the thin plate 3 is rolled. As shown in FIG. 3, the thin plate 3 is rolled to form the tubular thin plate 4.

  Next, in the welding process, the thin pipe 5 is formed by joining the circumferential ends of the tubular thin plate 4 by laser welding (step 12). FIG. 4 is an explanatory view of the welded thin pipe 5. In this welding process, since the content of C in the tubular thin plate 4 is an extremely small value of 0.001% or less, no crack is generated in the welded portion. In addition, when the content of C in the tubular thin plate 4 exceeds 0.4%, the welded portion is cracked. After the welding process, since the welded portion of the thin pipe 5 is hardened, an annealing process is performed to make the thickness of the thin pipe 5 uniform.

  Subsequently, in the ring-shaped cutting process, the thin pipe 5 is cut into a ring using a dedicated ring cutting machine, and the ring member 6 is formed as shown in FIG. 5 (step 13). FIG. 5 is an explanatory view of the cut ring member 6. The ring member 6 has a thickness of about 0.42 mm, a diameter of about 100 mm, and a width of about 10 mm. After the ring-shaped cutting step, barrel polishing is performed using a barrel polishing machine in order to remove burrs generated on the ring member 6.

  Thereafter, in the rolling process, as shown in FIG. 6, the ring member 6A is formed by rolling using a rolling mill so that the diameter of the ring member 6 is increased (step 14). FIG. 6 is an explanatory diagram of the rolled ring member 6. Here, the rolled ring member 6 is referred to as a ring member 6A. In the ring member 6A, the thickness d (see FIG. 7A) is about 0.18 mm. This ring member 6A corresponds to the ring member formed by the ring member forming step according to the present invention. Further, in this ring member 6A, as in the case of the metal member described above, in mass%, C is 0.001% or less, Ti is 0.3% or more and 0.7% or less.

As shown in FIG. 2, in the carburizing / quenching process, the ring member 6A is subjected to a carburizing process and a quenching process. First, the carburizing process will be described. In the carburizing process, the ring member 6A is heat-treated in a mixed gas atmosphere of CO gas, H ₂ gas, and N ₂ gas. Specifically, the processing temperature is about 850 to 950 degrees, and the holding time is about 30 minutes to 1 hour. Here, FIG. 7A is a partial cross-sectional view of the ring member 6A, and FIG. 7B is an explanatory view showing the C content of the carburized ring member 6A. In this gas carburizing process, as shown in FIG. 7B, the C content of the ring member 6A is 0.3% to 0.4% from the front surface (back surface) to the central portion in the thickness direction. Thus, the mixed gas atmosphere is adjusted.

  Next, the effect of the carburizing process will be described. In the ring member 6A before the carburizing treatment is performed, the C content is 0.001% or less. Thus, even if it quenches with respect to 6 A of ring members with very little content of C, tensile strength cannot be enlarged. For this reason, the tensile strength can be increased by the subsequent quenching process by increasing the C content by the carburizing process.

  Here, it is conceivable that the content of C is made larger than 0.05%, for example, in the process of steelmaking (dissolution) in the material forming step without performing the carburizing treatment. However, in this case, C and Ti in a solid solution state combine to form titanium carbide (TiC) precipitates, and C and Ti no longer exist in a solid solution state. It becomes difficult.) In addition, titanium carbide precipitates may grow and become large inclusions. Thus, even if the C content is increased during the steelmaking process, the effect of increasing the tensile strength due to the quenching treatment cannot be produced by the formed titanium carbide precipitates. For this reason, by performing the carburizing process, a precipitate of titanium carbide is not formed and C in a solid solution state is present, and the tensile strength can be increased by the quenching process. In the carburizing treatment, since the temperature is not high until the steel is melted as in steelmaking (dissolution), the amount of titanium carbide formed is small, and C in a solid solution state can be contained. Here, the inclusions are the ones in which the precipitates are formed into large lumps and greatly reduce the strength. For this reason, in a product, destruction starts from the inclusion.

  Subsequently, the quenching process will be described. In the quenching process, the ring member 6A in the austenite state is air-cooled. In addition, you may quench with water, oil, etc. As a result, C is trapped in supersaturation and enters a martensite state, and the hardness of the ring member 6A is significantly increased. At this time, even in the central portion of the ring member 6A in the thickness direction, baking occurs and the Vickers hardness increases to about 550 HV. Thus, the ring member 6A has a sufficiently high tensile strength. The ring member 6A subjected to the carburizing / quenching process is referred to as a ring member 6B.

  Here, it returns to the flowchart of FIG. After the carburizing / quenching process, in the distortion correction / peripheral length adjusting process, the distortion generated by the quenching process is corrected and the perimeter is adjusted (step 16). FIG. 8 is an explanatory diagram of the ring member 6B that has been subjected to distortion correction and circumference adjustment. As shown in FIG. 8, the ring member 6 </ b> B is spanned between the pair of rollers 7 and 7, and rotates in a state where tension is applied in a direction in which the space between the rollers 7 and 7 expands. Thus, the distortion is corrected and the circumference is adjusted. The ring member 6B that has been subjected to distortion correction and circumferential length adjustment is referred to as a ring member 6C.

Then, as shown in FIG. 2, in the nitriding process, the nitriding process is performed on the ring member 6C (step 17). In the nitriding treatment, the ring member 6C is heat-treated in a mixed gas atmosphere of NH ₃ gas and N ₂ gas. Specifically, the processing temperature is about 400 to 500 degrees, and the holding time is about 30 minutes to 1 hour. Here, FIG. 9 is an explanatory view showing the relationship between the depth from the surface of the nitrided ring member 6C and the Vickers hardness. In this gas nitriding treatment, as shown in FIG. 9, the Vickers hardness of the surface is 850 HV or more, and the Vickers hardness of the central portion in the thickness direction (the portion having a depth of 90 μm from the surface) is about 550 HV. Thus, the mixed gas atmosphere is adjusted. The ring member 6C thus nitrided is the metal belt 1 shown in FIGS. 1 (A), (B), and (C).

  Next, the effect of nitriding will be described. In the ring member 6C before being subjected to the nitriding treatment, there is a relatively large amount of Ti that is in a solid solution state. This is because, in the carburizing process, contained C and Ti in a solid solution are combined to form titanium carbide, but not all become titanium carbide. For this reason, the nitridation of Ti which is in a solid solution state can increase the compressive residual stress generated on the surface of the metal belt 1 and increase the fatigue strength of the metal belt 1. In other words, as a result of diligent research, the inventor can increase the compressive residual stress generated on the surface of the metal belt 1 by nitriding in a state where Ti that is in a solid solution is appropriately present. It was found that the fatigue strength can be increased.

  Therefore, a fatigue test was performed to compare the fatigue life (fatigue strength) of the metal belt 1 of the present embodiment, the metal belt 1X made of maraging steel, and the metal belt 1Y as a comparative product. In the fatigue test, the metal belts 1, 1X, 1Y are rotated by rotating a plurality of rollers while applying a predetermined tension to the metal belts 1, 1X, 1Y between the plurality of rollers using a dedicated fatigue testing machine. Bending stress is repeatedly applied to. And it evaluated by the frequency | count of repeated bending until metal belt 1,1X, 1Y fractures | ruptures. The metal belt 1Y as a comparative product is mass%, C is 0.35% or less, Si is 0.2%, Mn is 0.8%, Cr is 1.0%, It is manufactured through each step shown in FIG. 2 (carburization treatment may be omitted) using a metal material in which Mo is 1.0% or less and the remainder is Fe and inevitable impurities. . That is, unlike the metal belt 1, the metal belt 1Y is manufactured from a metal material that does not contain Ti.

  Here, FIG. 10A is a table showing the compressive residual stress Ps on the surface of the metal belt 1 of the present embodiment and the compressive residual stress Ps on the surface of the metal belt 1Y as a comparative product. FIG. 10B is a graph comparing the fatigue life of the metal belt 1 of the present embodiment, the metal belt 1X made of maraging steel, and the metal belt 1Y as a comparative product. As shown in FIG. 10A, in the metal belt 1, the compressive residual stress Ps generated on the surface is larger than that in the metal belt 1Y. Further, as shown in FIG. 10B, the fatigue life of the metal belt 1 is approximately the same as the fatigue life of the metal belt 1X, whereas the fatigue life of the metal belt 1Y is 1 to 1X. This is about one-tenth of the fatigue life.

  For this reason, it can be seen from FIGS. 10 (A) and 10 (B) that when Ti that is present as a solid solution is nitrided, the compressive residual stress Ps generated on the surface increases and the fatigue strength increases. Further, it can be seen that even the metal belt 1 made of ordinary steel can have the same fatigue strength as the metal belt 1X made of maraging steel containing a lot of expensive alloy elements. Therefore, the metal belt 1 of the present embodiment is inexpensive and has high fatigue strength.

  Next, the concentration of each component in the metal belt 1 will be described. FIG. 11 is an explanatory diagram showing the relationship between the depth from the surface of the metal belt 1 and the concentration of each component in the metal belt 1. As shown in FIG. 11, the concentration of N or the like in a solid solution state decreases as the depth from the surface increases. This is because the greater the depth from the surface, the smaller the penetration of N by nitriding. Note that N or the like in a solid solution state includes a nitride with an element other than Ti in addition to N in a solid solution state.

  In addition, as shown in FIG. 11, the contained Ti exists in three forms of titanium nitride (TiN), solid solution Ti, and titanium carbide (TiC). A part of titanium nitride and titanium carbide are dissolved in the matrix (Fe) structure mainly as fine precipitates. Thus, in particular, the distribution of titanium nitride, which is a fine precipitate, greatly contributes to the distribution of the Vickers hardness of the nitride layer of the metal belt 1 (the portion from the surface to a depth of about 30 μm). As a result, as shown in FIG. 9, the surface has a Vickers hardness of 900 HV or higher.

  As shown in FIG. 11, most of the contained C is supersaturated in the matrix (Fe) structure by quenching, and is in a martensite state. This supersaturated solid solution C distribution greatly contributes to the distribution of Vickers hardness from the portion having a depth of 30 μm to the central portion in the thickness direction (portion having a depth of 90 μm). As a result, as shown in FIG. 9, the Vickers hardness from the portion having a depth of 30 μm to the central portion in the thickness direction (portion having a depth of 90 μm) is 550 HV to 600 HV. In addition, since the content of C was originally 0.001% or less in the metal material, C contained as shown in FIG. 11 is contained by carburizing treatment. Thus, even with a metal belt 1 made of ordinary steel (metal material) with a very low C content, the metal belt 1 should have the Vickers hardness shown in FIG. 9 by carburizing / quenching and nitriding. Can do. Thus, the metal belt 1 of this embodiment has a high tensile strength.

  Here, with respect to the metal belt 1Z (defective product) manufactured through the respective steps shown in FIG. 2 (carburizing / quenching treatment and nitriding treatment) using a metal material having a high C content, FIG. 12 is used. explain. Here, the metal material forming the metal belt 1Z has a higher C content than the metal material forming the metal belt 1 of the present embodiment, for example, the C content is more than 0.3%. Let it be big. FIG. 12 is an explanatory diagram showing the relationship between the depth from the surface and the concentration of each component in the metal belt 1Z.

  As shown in FIG. 12, in the metal belt 1Z, a large amount of titanium carbide (TiC) precipitates are present. This is because C and Ti, which are in a solid solution state, are combined in the process of steelmaking (dissolution) for producing a metal material. Thereby, in hardening process, since C which is a solid solution state does not exist, Vickers hardness cannot be enlarged. That is, as shown in FIG. 12, there is little C that is supersaturated and dissolved in the matrix (Fe) structure. For this reason, this metal belt 1Z is a defective product having a Vickers hardness of about 200 HV from a portion having a depth of 30 μm from the surface to the center portion in the thickness direction and not satisfying the strength as a metal belt. Further, in this metal belt 1Z, since Ti which is in a solid solution state hardly exists in the nitriding treatment, as shown in FIG. 12, titanium nitride is hardly formed. Therefore, the metal belt 1Z has a small Vickers hardness near the surface and a small fatigue strength.

  Next, the C content in the metal material forming the metal belt 1 of the present embodiment will be described. The C content of the metal material needs to be 0.05% or less by mass. This is because, as described above, C and Ti, which are in a solid solution state, combine in the process of steelmaking (dissolution) to make a metal material, and Ti in the solid solution state is reduced, thereby obtaining the effect of nitriding treatment. It is because it becomes impossible. That is, when the C content is greater than 0.05%, titanium carbide (TiC) grows due to C combined with Ti, thereby reducing fatigue strength. Therefore, the lower the C content of the metal material, the better. In this way, the inventor has found that the adverse effect caused by the formation of titanium carbide inclusions can be sufficiently suppressed when the C content of the metal material is 0.001% or less, and is particularly preferable. It was.

  Subsequently, the Ti content in the metal material forming the metal belt 1 of the present embodiment will be described. The Ti content of the metal material needs to be 0.1% or more and 1.0% or less by mass. The reason why the lower limit of the Ti content is 0.1% is that when the Ti content is less than 0.1%, the effect of nitriding Ti in a solid solution state cannot be obtained. That is, the fatigue strength is small like the metal belt 1Y shown in FIG.

  On the other hand, the reason why the upper limit of the Ti content of the metal material is 1.0% will be described with reference to FIG. FIG. 13A is a graph showing the relationship between the depth from the surface of the metal belt and the N concentration when the Ti content is 0.5% and 1.0%. FIG. 13B is a table showing the compressive residual stress Ps on the surface of the metal belt when the Ti content is 0.5% and when it is 1.0%.

  As shown in FIG. 13A, when the Ti content is 0.5%, the N concentration distribution gradient in the vicinity of the surface of the metal belt is steep, whereas the Ti content is low. When it is 1.0%, the N concentration distribution gradient in the vicinity of the surface of the metal belt is gentle. The reason for the difference in the N concentration distribution gradient is that the greater the Ti content, the higher the probability that N entering from the surface will encounter Ti in the nitriding process. That is, as the Ti content increases, N entering from the surface reacts with Ti and enters deeper, and the N concentration distribution gradient becomes gentler. Further, there is a relationship that the compressive residual stress Ps is less likely to occur on the surface of the metal belt as the N concentration distribution gradient becomes gentler.

  Thus, as shown in FIG. 13B, when the Ti content is 0.5%, the compressive residual stress Ps on the surface of the metal belt is about 1200 MPs, and the metal belt has sufficient fatigue strength. It will be big. However, when the Ti content is 1.0%, the compressive residual stress Ps on the surface of the metal belt is about 900 MPa, and the metal belt has a low fatigue strength. For this reason, the Ti content of the metal material needs to be 1.0% or less. And as a range which the inventor can enlarge especially the compressive residual stress Ps of the surface of a metal belt by nitriding, it is preferable that content of Ti is 0.3% or more and 0.7% or less. I found out.

  Next, the metal belt 1 manufactured as described above is characterized by having the following chemical components and hardness. That is, in this metal belt 1, C is 0.2% or more and 0.8% or less in terms of mass%, and Ti is 0.1% or more and 1.0% or less. Further, 10% or more of Ti contained in the ring member 6A (metal material) is present on the surface of the metal belt 1 as titanium nitride (TiN). Further, in the central portion in the thickness direction from the surface of the metal belt 1, that is, in the portion where the depth from the surface of the metal belt 1 is 90 μm, 10% or more of Ti contained in the ring member 6A by mass%. However, it exists not as titanium carbide or titanium nitride but as Ti in a solid solution state. The chemical component of the metal belt 1 described above is measured using, for example, EPMA (electronic microanalyzer) and SEM (scanning electron microscope). Furthermore, the Vickers hardness is 400 HV or more and 600 HV or less from the surface of the metal belt 1 in the center in the thickness direction. The range of the above chemical components and hardness will be described.

C: 0.2% to 0.8%
The C content and the Vickers hardness are in a proportional relationship, and in order to satisfy the minimum Vickers hardness required for the metal belt 1, the lower limit value of the C content is 0.2%. On the other hand, since C enters up to 0.8% by carburizing and quenching treatment, the upper and lower values of the C content are set to 0.8%. Here, the inventor satisfies the range of optimum Vickers hardness (tensile strength) required for the metal belt 1 when the C content is 0.3% or more and 0.5% or less. I found.

Ti: 0.1% to 1.0%
When the Ti content is less than 0.1%, the effect of Ti nitriding as described above is not obtained, so the lower limit of the Ti content is set to 0.1%. Further, when the Ti content is greater than 1.0%, the fatigue strength is small as described above, so the upper limit of the Ti content is 1.0%. Here, as described above, the inventor considered that the compressive residual stress Ps on the surface of the metal belt 1, that is, the range in which the fatigue strength can be increased, the Ti content is 0.3% or more and 0.7% or less. It has been found that it is preferable.

The surface of the metal belt 1: 10% or more of Ti exists as titanium nitride,
If less than 10% of the Ti contained in the ring member 6A (metal material) is formed with titanium nitride on the surface of the metal belt, the effect of nitriding Ti in the solid solution state as described above is obtained. It will not be done. That is, the nitriding treatment is not properly performed. Therefore, the condition is that 10% or more of Ti exists as titanium nitride on the surface of the metal belt 1.

The central portion of the metal belt 1 in the thickness direction: 10% or more of Ti exists as Ti in a solid solution state,
If less than 10% of Ti is present in a solid solution state in the central portion in the thickness direction from the surface of the metal belt, that is, in the portion where the depth from the surface is 90 μm in the metal belt 1, the solid solution as described above is present. The effect of nitriding Ti in the molten state is not obtained. That is, the nitriding treatment is not properly performed. Or the nitriding process is performed to the center part of the metal belt 1, and N which penetrate | invades from the surface reacts with Ti and penetrates deeply. At this time, the N concentration distribution gradient is gentle, the compressive residual stress Ps generated on the surface of the metal belt is small, and the metal belt 1 has low fatigue strength. Therefore, the condition is that 10% or more of Ti exists in the central portion of the metal belt 1 as Ti in a solid solution state.

Center portion of metal belt 1 in thickness direction: Vickers hardness is 400HV to 600HV
When the Vickers hardness is smaller than 400 HV, the Vickers hardness required as the minimum internal hardness of the metal belt 1 is not satisfied. Therefore, the lower limit value of Vickers hardness is set to 400 HV. On the other hand, when the Vickers hardness is larger than 600 HV, the difference between the surface hardness and the internal hardness of the metal belt becomes small, and the metal belt 1 has low toughness. In this case, the progress of cracks generated when repeated bending stress is applied is not suppressed, and the fatigue strength required for the metal belt 1 is not satisfied. Therefore, the upper limit value of Vickers hardness is set to 600 HV.

The effect of this embodiment is demonstrated.
According to the manufacturing method of the continuously variable transmission belt of this embodiment, first, the ring member 6A formed in the ring member forming step has a very low C content, whereas the Ti content is very small. There are relatively many. For this reason, in this ring member 6A, the deposit of titanium carbide (TiC) is hardly produced | generated. Then, by the carburizing process, a large amount of C enters the ring member 6A, and a large amount of C and Ti that are in a solid solution state exist separately. Next, due to the quenching process, C is confined in supersaturation and becomes a martensite state, and the hardness of the ring member 6B is significantly increased. Thus, the ring member 6B has a sufficiently high tensile strength. Thereafter, Ti, which is appropriately present in a solid solution state by nitriding, generates fine precipitates and causes a large compressive residual stress on the surface. Thus, the metal belt 1 has a sufficiently high fatigue strength. And this metal belt 1 is not manufactured from the maraging steel in which many expensive alloy elements are contained, but is manufactured from the ring member 6A comprised with normal steel materials. Therefore, according to this manufacturing method, the metal belt 1 having high tensile strength and fatigue strength can be manufactured at low cost.

  Moreover, according to the manufacturing method of the continuously variable transmission belt of this embodiment, carburizing and quenching processing is performed on the welded ring member 6A after the welding process. For this reason, the C content can be arbitrarily determined, and the metal belt 1 having an extremely large tensile strength can be manufactured. That is, since there is a proportional relationship between the C content and the tensile strength, it is necessary to increase the C content in order to produce a metal belt having a large tensile strength. However, in the past, if the C content was made larger than 0.4% in the steelmaking (melting) process in order to obtain a large tensile strength, there was a problem that cracks occurred in the welded part in the welding process. On the other hand, according to the manufacturing method of this embodiment, since the content of C is extremely small before the welding process, no cracks are generated in the welded part in the welding process. And after a welding process, tensile strength can be made very large by making content of C larger than 0.4% in a carburizing and hardening process. Therefore, according to this manufacturing method, even the metal belt 1 that requires extremely high tensile strength can be manufactured.

  Further, according to the metal belt 1 (stepless transmission belt) of this embodiment, since C is 0.2% to 0.8%, the metal belt 1 satisfies the required tensile strength range. It has become. Further, since Ti is 0.1% to 1.0%, the metal belt 1 satisfies the required fatigue strength range. Further, 10% or more of contained Ti exists as titanium nitride on the surface, and 10% or more of contained Ti exists as Ti in a solid solution state in the central portion in the thickness direction. Therefore, the metal belt 1 is appropriately subjected to nitriding treatment. Furthermore, since the Vickers hardness is 400 HV to 600 HV at the central portion in the thickness direction, the internal hardness range required for the metal belt 1 is satisfied. The metal belt 1 is not manufactured from maraging steel containing a lot of expensive alloy elements, but is manufactured from ordinary steel. Therefore, the metal belt 1 has high tensile strength and fatigue strength and is inexpensive.

Next, a second embodiment will be described.
In the second embodiment, in the carburizing / quenching step, the diffusion treatment is performed on the ring member 6A (see FIG. 6) after the carburizing process and before the quenching process. In the carburizing process, the ring member 6A is heat-treated in a mixed gas atmosphere of CO gas, H ₂ gas, and N ₂ gas. Specifically, the processing temperature is about 600 to 900 degrees, and the holding time is about 5 to 30 minutes. Subsequently, in the diffusion treatment, the ring member 6A is heated and held in a mixed gas atmosphere of H ₂ gas and N ₂ gas. Specifically, the processing temperature is about 850 to 950 degrees, and the holding time is about 30 minutes to 1 hour.

  Here, the diffusion process is a process of heating and holding for a predetermined time at the transformation temperature of the ring member 6A to make the distribution of the C content of the ring member 6A uniform. Thereafter, in the quenching treatment, the ring member 6A in the austenite state is cooled to be in the martensite state. When performing the carburizing process, the diffusion process, and the quenching (cooling) process, the temperature and the mixed gas atmosphere may be adjusted using a single furnace. This is to reduce energy waste due to cooling and heating.

The effect of 2nd Embodiment is demonstrated using FIG.
FIG. 14A is an explanatory view showing the C content of the ring member 6A before the diffusion treatment. On the other hand, FIG. 14B is an explanatory view showing the C content of the ring member 6A after the diffusion treatment. As shown in FIG. 14A, before the diffusion treatment, that is, immediately after the carburization treatment, there is a large difference in the C content between the front surface (back surface) and the central portion in the thickness direction. Here, if the quenching process is performed on the ring member 6A in this state, only the C content on the front surface (back surface) is large, so that only the front surface (back surface) becomes too hard, The ring member 6B is easily broken in the distortion correction / peripheral length adjusting step (see FIG. 8). Furthermore, in this case, since only the C content in the central portion in the thickness direction is small, the internal hardness is lowered and the strength as the metal belt 1 is not satisfied. Therefore, as shown in FIG. 14B, the diffusion process can eliminate the difference in C content between the front surface (back surface) and the central portion in the thickness direction, and the above-described problem occurs. Can be prevented. That is, the ring member 6B (metal belt 1) with stable quality can be manufactured by the quenching process after the diffusion process.

  Moreover, according to this 2nd Embodiment, control of mixed gas atmosphere becomes easy in a carburizing process. That is, in the first embodiment, as shown in FIG. 7B in the carburizing process, the C content is 0.3% to 0.4% from the front surface (back surface) to the center in the thickness direction. Therefore, it is necessary to precisely control the mixed gas atmosphere. On the other hand, according to the second embodiment, since the diffusion treatment is performed, in the carburization treatment, the C content on the front surface (back surface) is larger than 0.4% as shown in FIG. Further, the C content in the central portion in the thickness direction may be smaller than 0.3%. Thereby, since the carburizing process can be performed at a low temperature, it is easy to control, and the process can be performed more precisely and in a short time.

  Moreover, according to this 2nd Embodiment, the process time in a carburizing and hardening process can be shortened compared with 1st Embodiment. That is, in the first embodiment, it takes a lot of time (30 minutes to 1 hour) to precisely control the mixed gas atmosphere as described above in the carburizing process. On the other hand, according to the second embodiment, it is easy to control the mixed gas atmosphere in the carburizing process, and a short time is required (5 to 30 minutes). In the diffusion treatment, since the ring member 6B is thin (thickness is about 0.18 mm), the time for making the C content distribution uniform is relatively short. Thus, in the carburizing / quenching process of the second embodiment, the carburizing process is quickly terminated in a state where C does not enter the ring member 6B, and the content of C is made uniform by the subsequent diffusion process. The processing time in the carburizing / quenching process can be shortened. Other functions and effects of the second embodiment are the same as the functions and effects of the first embodiment, and a description thereof will be omitted.

As described above, in the continuously variable transmission belt and the manufacturing method thereof according to the present invention, the present invention is not limited to this, and various modifications can be made without departing from the scope of the invention.
For example, in the second embodiment, when the carburizing process, the diffusion process, and the quenching (cooling) process are performed, the temperature and the mixed gas atmosphere are adjusted using one furnace. However, using two furnaces, the ring member 6A is cooled after being carburized in the first furnace, reheated in the second furnace, and then diffused and quenched into the ring member 6B (cooling process). ) May be applied.
Moreover, in each embodiment, the ring member 6A in which the mass%, C is 0.05% or less, Ti is 0.1% or more and 1.0% or less is used as the material forming step and the tube forming step. The ring member forming step for forming the ring member 6A is not limited to the above-described steps, and can be appropriately changed. It is.

1 Metal belt 2 Element 3 Thin plate 4 Tubular thin plate 5 Thin pipe 6, 6A, 6B, 6C Ring member

Claims (8)

A ring member forming step of forming a ring member in which, by mass%, C is 0.05% or less, Ti is 0.1% or more and 1.0% or less;
Carburizing / quenching step for carburizing and quenching the ring member;
And a nitriding step of nitriding the carburized / quenched ring member. A method for producing a continuously variable transmission belt. The method of manufacturing a continuously variable transmission belt according to claim 1,
In the carburizing / quenching process, a diffusion process is performed after the carburizing process and before the quenching process. In the method for manufacturing a continuously variable transmission belt according to claim 1 or 2,
A ring member formed in the ring member forming step, wherein, in mass%, Ti is 0.3% or more and 0.7% or less, and the method of manufacturing a continuously variable transmission belt. The method of manufacturing a continuously variable transmission belt according to any one of claims 1 to 3,
A ring member formed in the ring member forming step, wherein C is 0.001% or less by mass%, and the method for manufacturing a continuously variable transmission belt. In the manufacturing method of the continuously variable transmission belt according to any one of claims 1 to 4,
In the ring member forming step,
Forming a metal material containing Ti,
Forming the metal material into a thin plate having a thickness of 0.42 mm;
The tubular sheet is formed by rolling a pre-Symbol thin plate,
Welding the circumferential end of the tubular thin plate to form a thin pipe,
A method of manufacturing a continuously variable transmission belt, wherein the thin pipe is cut to form the ring member. A continuously variable transmission belt manufactured by subjecting a ring member containing Ti to carburizing / quenching treatment and nitriding treatment,
In mass%, C is 0.2% or more and 0.8% or less, and Ti is 0.1% or more and 1.0% or less,
On the surface, 10% or more of Ti contained in the ring member by mass% is present as titanium nitride,
At the center in the thickness direction from the surface, 10% or more of Ti contained in the ring member is present as Ti in a solid solution state in mass%,
A continuously variable transmission belt, wherein a Vickers hardness at a central portion in a thickness direction from the surface is 400 HV or more and 600 HV or less. The continuously variable transmission belt according to claim 6,
A continuously variable transmission belt characterized in that, by mass%, Ti is 0.3% or more and 0.7% or less. In the continuously variable transmission belt according to claim 6 or 7,
A continuously variable transmission belt characterized in that C is 0.3% or more and 0.5% or less in terms of mass%.

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