JP7172080B2 - Maraging steel for metal belts - Google Patents

Description

TECHNICAL FIELD The present invention relates to maraging steel for metal belts used, for example, in continuously variable transmissions for automobiles.

Maraging steel has a very high tensile strength of around 2000 MPa, so for example, it is processed into a steel strip of 0.5 mm or less and used for metal endless belts of continuously variable transmissions for automobiles, which require high strength. It is Its typical composition includes, by weight percent, 18% Ni-8% Co-5% Mo-0.45% Ti-0.1% Al-bal. There is Fe. However, while the maraging steel described above has a very high tensile strength, it is not necessarily high in terms of fatigue strength. TiN inclusions are one of the biggest factors that degrade the fatigue strength of maraging steel. Since the TiN inclusions are likely to be large in size and have a cubic shape, fatigue fracture starting from the inclusions is likely to occur. Therefore, maraging steels to which Ti is not added have been proposed. Ti contained in maraging steel for metal endless belts to be nitrided is an important element that not only improves the strength of the matrix of the alloy, but also increases the strength of the nitrided layer. Without this Ti addition, the strength of both the matrix and the nitride layer would be reduced. Therefore, in a maraging steel to which Ti is not added, it is necessary to reinforce the matrix and the nitrided layer by using an element that replaces Ti.

As a method of strengthening the base and the nitride layer, there is a method of increasing the content of Co or Mo that contributes to strengthening the base and then using Cr or Al to strengthen the nitride layer. This method is disclosed, for example, in International Publication WO2009/008071 Pamphlet (Patent Document 1) filed by the applicant of the present application. This proposal is for maraging steel for metal endless belts used as members of automobile continuously variable transmissions. Even in maraging steel to which Ti is not added, high tensile strength and internal hardness can be obtained after aging treatment and after gas nitrocarburizing. This indicates that the

International publication WO2009/008071 pamphlet

The maraging steel shown in Patent Document 1 described above is advantageous in that high strength can be obtained even in a maraging steel to which Ti is not added. The standard has changed greatly, and there was a problem that it was difficult to obtain stable mechanical properties in terms of manufacturability. If the internal hardness of maraging steel for metal belts is too high, it will break prematurely due to internal defects in the material. If the change is large, it becomes a big problem in putting it into practical use.
An object of the present invention is to provide a maraging steel for metal belts with excellent manufacturability and mechanical properties by optimizing the strength and hardness levels resulting from the alloy composition of the maraging steel for metal belts. .

The inventor of the present invention found that stable mechanical properties can be obtained by controlling the amounts of Cr, Mo, Co, and Al to be added in an appropriate range in response to a large change in the strength level due to the alloy composition. reached.
That is, in the present invention, in mass%, C: 0.01% or less, Ni: 18.0 to 20.0%, Cr: 0.8 to 1.2%, Mo: 4.5 to 6.0%, Co: 9.0 to 14.0%, Al: 0.8 to 1.7%, and the balance being Fe and unavoidable impurities, this is a maraging steel for metal belts.
Preferably, the maraging steel is a maraging steel for metal belts having a Vickers hardness of 550 HV or more when subjected to solution treatment at 900° C. for 1 hour and aging treatment at 480° C. for 1 hour. be.
Preferably, the maraging steel is a maraging steel for metal belts having a Vickers hardness of 600 HV or more when solution treatment at 900° C. for 1 hour and aging treatment at 480° C. for 3 hours are performed. be.

The maraging steel for metal belts of the present invention has excellent manufacturability by optimizing the strength and hardness levels due to the alloy composition, and it can be used like endless metal belts for continuously variable transmissions for automobiles. It is expected to have industrial effects such as obtaining stable mechanical properties when used for members that require strength and manufacturability.

As described above, an important feature of the present invention is the optimization of the alloy composition necessary to obtain stable manufacturability and mechanical properties.
In the maraging steel for metal belts of the present invention, the reasons for defining each chemical composition within the following ranges are as follows. In addition, it describes as the mass % unless there is a description in particular.
<C: 0.01% or less>
C (carbon) forms carbides with Mo and Cr to reduce the amount of intermetallic compounds and nitrides to be precipitated, thereby lowering the strength, so it must be kept low. Further, aggressive addition of C increases the risk of deterioration in weldability required for, for example, continuously variable transmission parts. For these reasons, C is set to 0.01% or less. Preferably, it is 0.008% or less. On the other hand, the addition of a small amount of C forms fine carbides and contributes to strength improvement, so 0.001% or more is preferable.

<Ni: 18.0 to 20.0%>
At least 18.0% of Ni is required in order to stably form a low-C martensite structure, which is the base structure of maraging steel for metal belts. However, if the Ni content exceeds 20.0%, the austenitic structure is stabilized and the martensitic transformation is difficult to occur. The preferred upper limit of Ni is over 18.5% and the preferred lower limit is 19.5%.
<Cr: 0.8 to 1.2%>
Cr is an element that has a strong affinity with N when nitriding is performed, makes the nitriding depth shallow, increases the nitriding hardness, and increases the compressive residual stress of the nitriding surface, so it is essential to be added. However, if it is less than 0.8%, the effect is small, and on the other hand, if it exceeds 1.2%, the nitriding hardness becomes too high, and surface scratches, etc., which are inevitably formed in the manufacturing process of the metal belt, occur. Cr was set to 0.8 to 1.2% because the susceptibility to defects increases and the fatigue strength decreases. A more preferable lower limit of Cr is 0.9%, and a preferable upper limit thereof is 1.1%.

<Mo: 4.5 to 6.0%>
Mo is an important element that forms fine intermetallic compounds such as Ni ₃ Mo during aging treatment and contributes to precipitation strengthening. Also, Mo is an effective element for increasing surface hardness and compressive residual stress due to nitriding. For this purpose, when Mo is less than 4.5%, the tensile strength becomes insufficient due to a decrease in the amount of precipitation of metal compounds, while when it is more than 6.0%, coarse intermetallic compounds containing Fe and Mo as main elements are formed. Mo is set to 4.5 to 6.0% in order to facilitate the formation of , and to strengthen the austenite phase formed by the solution treatment and inhibit the martensite transformation in the cooling process. A preferred lower limit for Mo is greater than 4.5% and a preferred upper limit is 5.8%.
<Co: 9.0 to 14.0%>
Co increases the solid solubility of Mo, which is an element that forms aging precipitates, at the solution treatment temperature without significantly affecting the stability of the martensitic structure of the matrix, and increases the solid solubility of Mo in the aging precipitation temperature range. Mo is an important element that promotes the precipitation of fine intermetallic compounds containing Mo by reducing Therefore, it is necessary to add a large amount of Co from the standpoint of strength and toughness. When the Co content is less than 9.0%, it is difficult to obtain sufficient strength in the maraging steel for metal belts with reduced Ti content, while when the Co content exceeds 14.0%, the austenite is stabilized, making it difficult to obtain a martensitic structure. Therefore, it is set to 9.0% or more and 14.0% or less. The preferred lower limit of Co is over 10.0% and the preferred upper limit is 13.5%.

<Al: 0.8 to 1.7%>
Al is usually added in a small amount for deoxidization, but originally it is an element that contributes to strengthening by forming NiAl together with Ni during aging treatment. In the maraging steel for metal belts of the present invention to which Ti is not added, it is necessary to supplement the strength by adding Al. In addition, addition of Al is also necessary in order to facilitate the nitriding treatment and obtain a good nitrided layer in the maraging steel for metal belts to which Ti is not added. If Al is less than 0.8%, a sufficient strengthening effect due to aging treatment cannot be obtained. Since Al forms an oxide film and inhibits the nitriding reaction, Al is set to 0.8% or more and 1.7% or less. A preferable lower limit of Al is 0.9%, and a preferable upper limit of Al is 1.6%.
<Remainder: Fe and impurities>
The remainder is substantially Fe, but contains impurities that are unavoidably mixed in the manufacturing process. Although it is preferable that the content of impurities is small, it does not matter if the content is within the following range.
P≤0.05%, S≤0.05%, Zr≤0.01%, Ca≤0.01%, Mg≤0.005%, N≤0.05%

Next, the Vickers hardness after heat treatment defined in the present invention will be explained.
When the maraging steel for metal belts of the present invention is used as a member of a continuously variable transmission for automobiles, it is used after refining heat treatment such as solution treatment and aging treatment and surface treatment such as nitriding treatment. The solution treatment at 900° C. for 1 hour dissolves the alloying elements necessary for the precipitation of intermetallic compounds and nitrides into the matrix austenite, and the aging treatment precipitates intermetallic compounds such as Ni ₃ Mo and NiAl. The purpose is to
In the present invention, the aging time at 480° C. is 1 hour and 3 hours, and the Vickers hardness after the aging treatment is 550 HV or more and 600 HV or more, respectively. Aging treatment at 480°C for 1 hour is a consideration of manufacturability, and is specified to obtain a hardness level of 550 HV or more, which is required for metal continuously variable belts for continuously variable transmissions for automobiles, in a short period of time. is. If the hardness is lower than 550 HV, for example, when a tensile stress acts on the metal belt, it easily undergoes plastic deformation and cannot be used as a machine part. On the other hand, the aging treatment at 480°C for 3 hours is intended to increase the strength of the metal belt. It can be expected to contribute to higher torque and displacement of stepped transmissions. On the other hand, when it is lower than 600 HV, there is a risk of plastic deformation when, for example, a metal belt is subjected to high stress associated with high displacement or high torque.

By using the maraging steel for metal belts of the present invention described above, the strength and hardness levels due to the alloy composition can be optimized, and the maraging steel for metal belts with excellent manufacturability and mechanical properties can be obtained. can be done.
A steel strip composed of the maraging steel of the present invention is mainly used as a metal endless belt used in continuously variable transmissions for automobiles, and therefore has improved mechanical properties such as tensile strength and fatigue strength. It is ideal as a ring material for power transmission belts.

The following examples further illustrate the invention.
A 200 kg steel ingot was produced by vacuum melting, forged, hot rolled, and repeatedly annealed and cold rolled to produce a maraging steel strip for a metal belt with a width of 180 mm and a thickness of 0.42 mm. After that, solution treatment was performed at 900° C. for 1 hour, and aging treatment was further performed at 480° C. for 1 hour and 3 hours. The chemical composition of this example is shown in Table 1.
Table 2 shows the internal hardness of each sample after aging. The hardness was measured using a Vickers hardness tester under a load of 500 gf.

From Table 2, maraging steel No. 1 for metal belts of the present invention. Both Nos. 1 and 2 had a Vickers hardness of 550 HV or more after aging treatment at 480° C. for 1 hour. The Vickers hardness after aging at 480° C. for 3 hours is 580 HV or more, and has sufficient internal hardness for metal belt applications in automotive transmissions. Above all, No. For maraging steels 1 and 2, a hardness of 600 HV or higher was obtained.
On the other hand, No. of the comparative example with a high Mo addition amount of 6.4%. No. 10 has a low Vickers hardness of 472 HV after aging treatment at 480° C. for 3 hours. Since the amount of Mo added is high, Mo stabilizes the austenite phase during solution treatment and suppresses martensite transformation. presumably decreased. Comparative example No. 1 with a low Co addition amount of 8%. In No. 11, the Vickers hardness after aging treatment is as low as 543 HV after 1 hour and 574 HV after 3 hours. It is considered that this is due to the decrease in the amount of Ni ₃ Mo precipitated due to the decrease in the amount of Co added.

The maraging steel for metal belts of the present invention can be used for metal belts used under severe conditions, so it has high tensile strength such as power transmission metal belts used for continuously variable transmissions for automobiles. , can be applied to members that require high fatigue strength.

Claims (2)

% by mass, C: 0.01% or less, Ni: 18.0 to 20.0%, Cr: 0.8 to 1.2%, Mo: 4.5 to 6.0%, Co: 10.0 Over to 14.0 % or less, Al: 0.8 to 1.7%, the balance being Fe and unavoidable impurities, solution treatment at 900 ° C. for 1 hour and aging treatment at 480 ° C. for 3 hours. A maraging steel for a metal belt, characterized by having a Vickers hardness of 600 HV or more when crushed. 2. The maraging steel according to claim 1, wherein the maraging steel has a Vickers hardness of 550 HV or more when solution treatment at 900° C. for 1 hour and aging treatment at 480° C. for 1 hour are performed. maraging steel for metal belts.