JP5046363B2 - Maraging steel for power transmission belt with high fatigue strength and maraging steel strip for power transmission belt using the same - Google Patents

Description

The present invention relates to a power transmission belt having high fatigue strength suitable for use in a member requiring high fatigue strength, such as a ring product of a power transmission belt used in a continuously variable transmission for automobiles. it relates use maraging steel and power transmission belts for maraging steel strip using the same.

Since maraging steel has a very high tensile strength of around 2000 MPa, members that require high strength, such as rocket parts, centrifuge parts, aircraft parts, automobile engine continuously variable transmission parts, It is used for various applications such as molds. Its typical composition includes 18% Ni-8% Co-5% Mo-0.4% Ti-0.1% Al-bal. Fe.
And the maraging steel contains appropriate amounts of Co, Mo and Ti as strengthening elements, and by performing an aging treatment, an intermetallic compound such as Ni ₃ Mo, Ni ₃ Ti and Fe ₂ Mo is precipitated. Steel that can provide strength. Also, especially in steel strips used for parts for continuously variable transmissions of automobile engines, fatigue strength in the high cycle region is an important required characteristic, so it exists inside maraging steel having high strength. It is necessary to make non-metallic inclusions such as TiN as fine as possible, and a nitrided layer is formed on the surface to form a nitride layer to improve fatigue strength.
In the field of continuously variable transmissions for automobile engines, improved alloys have been proposed for the purpose of solving the deterioration of fatigue strength starting from non-metallic inclusions. (For example, refer to Patent Documents 1, 2, and 3.)
Japanese translation of PCT publication No. 2004-514056 JP 2001-240943 A Japanese Patent Laid-Open No. 2002-167652

The alloy disclosed in the above-mentioned Japanese translations of PCT publication No. 2004-514056 has reduced the Ti forming non-metallic inclusions to 0.1% or less, so in terms of the refinement of TiN that becomes the starting point of fatigue fracture Although advantageous, there is a problem that it is difficult to perform nitriding because the alloy simply suppresses the addition of elements that form non-metallic inclusions.
Further, since the alloy disclosed in Japanese Patent Application Laid-Open No. 2001-240943 also reduces Ti, it is advantageous in terms of miniaturization of TiN, which is the starting point of fatigue fracture, but Co, which is one of strengthening elements, is added. Since the tensile strength is kept low, it is difficult to ensure high tensile strength, and Si and Mn are added to ensure tensile strength. For this reason, the toughness may be reduced.
Further, since the alloy disclosed in Japanese Patent Application Laid-Open No. 2002-167652 also reduces Ti, it is advantageous in terms of refining TiN, which is the starting point of fatigue fracture, but C is actively added to increase strength. As a result, carbides such as Cr and Mo are precipitated, and this causes fatigue failure, and the fatigue strength is reduced. Also, the positively added C reduces the weldability required for the transmission parts without permission. there's a possibility that.
The object of the present invention is to reduce TiN, which is the starting point of fatigue failure in a high cycle region, to facilitate nitriding treatment to increase surface hardness, and to increase the compressive residual stress of the surface nitrided layer to increase bending fatigue strength. Improved marine aging steel for power transmission belts with high fatigue strength with good nitriding properties and weldability, with refined prior austenite grains to ensure higher strength and ductility, and power transmission using the same It is to provide a maraging steel strip for an industrial belt .

The inventor suppresses both Ti and N in order to reduce inclusion TiN, which is detrimental to improving fatigue strength, reduces the strength decrease due to Ti decrease, and increases the amount of Co and Mo and the value of Co / 3 + Mo within an appropriate range. We found that it can be compensated by limiting.
Moreover, in order to improve tensile strength, ductility, and fatigue strength, it has been found that crystal grain refinement is effective, but this can be solved by adding a small amount of B.
Furthermore, the present inventor has found that the absolute value of the surface compressive residual stress due to nitriding can be increased by adding an appropriate amount of Cr in order to solve the difficulty of nitriding due to Ti reduction, and C is The weldability is ensured by suppressing the impurity level, and the present invention has been achieved.

That is, in the present invention, C: 0.01% or less, Si: 0.1% or less, Mn: 0.1% or less, P: 0.01% or less, S: 0.005% or less, Ni: 17.0~22.0%, Cr: 0.1~4.0%, Mo: 3.0~7.0%, Co: 1 2. More than 0% and 20.0% or less, Ti: 0.05% or less, Al: less than 0.1%, Al + Ti: 0.1% or less, N: 0.03% or less, O: 0.005% or less, B: 0.01% or less (excluding 0), Co / 3 + Mo: 8.0 to 15.0%, the balance being maraging steel for power transmission belts having high fatigue strength composed of Fe and inevitable impurities There is .
In the present invention, in addition to the above-described composition, it may further contain one or more of Ca: 0.01% or less and Mg: 0.005% or less by mass%.
Preferably, the grain size is ASTM No. Is a maraging steel for power transmission belts having a fine particle size of 10 or more. The maraging steel strip for power transmission belts is characterized in that a nitride layer is formed on the surface of these maraging steels for power transmission belts and compressive residual stress is applied to the surface.

The maraging steel for power transmission belts of the present invention can reduce TiN, which is the starting point of fatigue fracture, and can obtain high strength, high hardness of the surface after nitriding treatment, and large compressive residual stress. When used in parts that require high fatigue strength, such as ring products for power transmission belts used in step transmissions, etc., it has a significant industrial effect, such as having a long fatigue life. There is expected.

The present invention has been made based on the above-described novel findings, and the action of each element in the present invention will be described below.
In the maraging steel for power transmission belts of the present invention, the reasons why each chemical composition is specified in the following range are as follows. Unless otherwise specified, the mass% is indicated.
C forms a carbide with Mo and decreases the strength of the intermetallic compound to be precipitated, so it is necessary to keep C low. Further, when C is positively added, for example, there is a high risk that the weldability required for a transmission component without permission is reduced. For these reasons, C is set to 0.01% or less. Preferably, it is 0.008% or less.
Si is an element that can compensate for the strength reduction due to Ti reduction by refining the intermetallic compound that precipitates during aging treatment or forming an intermetallic compound with Ni, but may reduce toughness. Therefore, in order to secure toughness and ductility, it is necessary to keep it low in the present invention. If added over 0.1%, the toughness and ductility are lowered, so Si was made 0.1% or less. A preferred range for ensuring toughness and ductility more reliably is 0.05% or less.
Mn is an element that forms an intermetallic compound together with Ni during the aging treatment and contributes to age hardening. Therefore, Mn is an element that can compensate for a decrease in strength due to a decrease in Ti, but may reduce toughness. Therefore, in order to ensure toughness and ductility, it is necessary to keep it low in the present invention. If added over 0.1%, the toughness and ductility deteriorate, so Mn was made 0.1% or less. A preferred range for ensuring toughness and ductility more reliably is 0.05% or less.

P and S are harmful elements that segregate and form inclusions in the prior austenite grain boundaries, embrittle the maraging steel and reduce fatigue strength. Therefore, P is 0.01%. Hereinafter, S is set to 0.005% or less. Preferably, P is 0.005% or less, and S is 0.004% or less.
Cr is an element that has a strong affinity with N when nitriding, reduces the nitriding depth, increases the nitriding hardness, and increases the compressive residual stress on the nitriding surface, and thus is essential. However, if the amount is less than 0.1%, there is no effect. On the other hand, even if added over 4.0%, a further improvement effect is not seen, and the strength after aging is greatly reduced. Was 0.1 to 4.0%. A preferable lower limit of Cr is 0.2%.
Ni stably forms a low-C martensite structure, which is a base structure of maraging steel, so at least 17.0% is necessary, but if it exceeds 22.0%, the austenite structure is stabilized and martensite is formed. Since it is difficult to cause transformation, Ni is set to 17.0 to 22.0%. The preferable range of Ni is more than 18.0% and 22.0% or less.
Mo is an important element that contributes to precipitation strengthening by forming fine intermetallic compounds such as Ni ₃ Mo and Fe ₂ Mo during aging treatment. Mo is an effective element for increasing the surface hardness and compressive residual stress due to nitriding. For this purpose, if the Mo content is less than 3.0%, the tensile strength is insufficient. On the other hand, if it exceeds 7.0%, it becomes easy to form a coarse intermetallic compound containing Fe and Mo as main elements. Mo was 3.0 to 7.0%. The preferable range of Mo is more than 5.0% and 7.0% or less.

Co increases the solid solubility of aging precipitate-forming elements such as Mo and Al at the solution treatment temperature without greatly affecting the stability of the martensitic structure of the matrix, so that Mo and Al in the aging precipitation temperature range are increased. This is an important element that contributes to the strengthening of aging precipitation by promoting the precipitation of fine intermetallic compounds including Mo and Al by reducing the solid solubility of Ni, and it is necessary to add more in terms of strength and toughness. is there. Co is 1 2 . If it is less than 0%, it is difficult to obtain sufficient strength with maraging steel with reduced Si, Mn, Ti, and Al. On the other hand, if added over 20.0%, austenite is stabilized, so it is difficult to obtain a martensite structure. Therefore, 1 2 . It exceeded 0% and was made into 20.0% or less .
Ti is originally one of the important strengthening elements in maraging steel, but at the same time, it forms inclusions TiN or Ti (C, N) to reduce fatigue strength particularly in the ultra-high cycle region. Since it is also a harmful element, it is necessary to keep it low as an impurity when emphasizing fatigue strength.
Further, Ti easily forms a thin and stable oxide film on the surface. When this oxide film is formed, the nitriding reaction is inhibited, so that it is difficult to obtain a sufficient compressive residual stress on the nitrided surface. In order to easily perform nitriding and to increase the compressive residual stress on the surface after nitriding, Ti is a harmful impurity element and needs to be kept low.
If Ti exceeds 0.05%, a sufficient effect for reducing TiN or Ti (C, N) cannot be obtained, and a stable oxide film can be easily formed on the surface. It was as follows. Desirably 0.01% or less is good.

Al is usually added in a small amount for deoxidation, but since it is an element that contributes to strengthening by forming an intermetallic compound with Ni during aging treatment, the strength can be improved by adding Al. However, on the other hand, there is a possibility that nonmetallic inclusions are formed in combination with oxygen and nitrogen, and the fatigue strength is lowered. When the fatigue strength is important, it is necessary to keep it low. When Al is added in an amount of 0.1% or more, AlN and Al ₂ O ₃ inclusions are formed and the fatigue strength is lowered, so Al was made less than 0.1%. A preferable range of Al is 0.05% or less.
Further, since both Al and Ti are elements that form non-metallic inclusions, it is effective for improving the fatigue strength to keep the total amount of Al + Ti low, so Al + Ti is made 0.1% or less. A preferable range of Al + Ti is 0.07% or less.
Co, Mo, and Ti are both major strengthening elements in maraging steel and are elements that contribute to aging strengthening of maraging steel. If Ti is kept low, it is necessary to compensate for the decrease in strength due to Ti by increasing the amount of Co and Mo added. However, the contribution of each element to the strengthening is not the same, and the strengthening by Co is 1/3 times the strengthening by Mo.
Therefore, the reinforcement by Co and Mo can be organized by Co / 3 + Mo. If the value of Co / 3 + Mo is less than 8.0% by mass%, the strength is not sufficient. On the other hand, if it exceeds 15.0%, the strength becomes too high and the toughness may be lowered. Was 8.0 to 15.0%. A preferable range of Co / 3 + Mo is 10.0 to 15.0%.

N is an impurity element that combines with Ti to form inclusions of TiN or Ti (C, N), and lowers fatigue strength particularly in the ultra-high cycle region. In maraging steel containing Ti, it is necessary to keep N significantly low in order to prevent the formation of coarse TiN or Ti (C, N). However, in maraging steel containing almost no Ti, N has a small adverse effect even if mixed in by ordinary vacuum melting, so it was made 0.03% or less. Desirably, 0.01% or less is good. More preferably, 0.005% or less is good.
O is an impurity element that forms oxide inclusions and lowers toughness and fatigue strength, so is limited to 0.005% or less. Desirably, it is 0.003% or less.
B is an element that has the effect of refining the prior austenite crystal grains when subjected to the solid solution treatment after cold working and contributing to strengthening and suppressing surface roughness, and is essential. When B is more than 0.01%, the toughness is lowered. Therefore, B is set to 0.01% or less (not including 0%). Desirably, 0.005% or less (excluding 0%) is good. A preferable lower limit of B that can refine the prior austenite crystal grains more reliably is 0.0002%, and a more preferable lower limit is 0.0003%.

In the present invention, one or more of Ca: 0.01% or less and Mg: 0.005% or less can be contained. Power transmission belts for the maraging steel of the present invention, ing a cause of fatigue fracture surface defects is the starting point.
The maraging steel for power transmission belts of the present invention is manufactured by vacuum induction melting or vacuum induction melting followed by melting in a vacuum atmosphere such as vacuum arc remelting or electroslag remelting. Can do. The alloy of the present invention has an element range in which non-metallic inclusions are not formed as much as possible, but it is technically difficult to completely eliminate non-metallic inclusions even when melting in a vacuum atmosphere. It is.
Among them, for example, coarse and hard Al ₂ O ₃ inclusions exceeding 25 μm may be formed, or Al ₂ O ₃ may be clustered. Al ₂ O ₃ inclusions are hard and have a high melting point and, for example, hardly deform even during hot plastic working. Therefore, for example, wrinkles may be generated in the roll during cold rolling to cause surface defects in the maraging steel. Therefore, it is preferable to reduce the hardness or lower the melting point by using Al ₂ O ₃ inclusions as composite inclusions. At the same time, it is preferable to prevent inclusion defects by adding an element capable of preventing clustering.
Examples of elements effective for making Al ₂ O ₃ inclusions complex inclusions include Si, Mn, Ca, and Mg. In the present invention, Si and Mn are added as elements that lower toughness and ductility. To regulate. Therefore, it is preferable that Al ₂ O ₃ inclusions be combined inclusions by adding one or more of Ca and Mg other than Si and Mn. Ca and Mg also have an effect of preventing clustering of Al ₂ O ₃ inclusions. Therefore, in this invention, it was supposed to contain 1 or more types of Ca: 0.01% or less and Mg: 0.005% or less.
In order to reliably obtain the effects of Ca and Mg, the lower limit is preferably 0.001% for Ca and 0.0001% for Mg.
As mentioned above, it is set as Fe and an unavoidable impurity except the element demonstrated.
The following elements may be added for the purpose of deoxidation, desulfurization, etc. within the following ranges.
Zr ≦ 0.01%

The maraging steel for power transmission belts of the present invention is made of prior austenite crystal grains by performing a solution treatment at an appropriate temperature according to the composition, for example, a temperature of about 780 to 1000 ° C., after cold working of 10% or more. Here, in the case of maraging steel, crystal grains refer to prior austenite crystal grains). The particle size can be reduced to 10 or more. In the maraging steel for power transmission belts of the present invention, by making the crystal grains fine, the hardness, tensile strength, fatigue strength, impact toughness, etc. can be stably increased, Can be expected to have the effect of reducing surface roughness.

In addition, the maraging steel for power transmission belts of the present invention contains almost no Ti that forms a stable oxide film on the surface that may inhibit nitriding, so that ordinary gas nitriding, gas soft nitriding, sulfurization Various nitriding treatments such as nitriding, ion nitriding and salt bath nitriding can be easily performed. Further, since Cr is effective in increasing the nitriding hardness and the absolute value of the compressive residual stress of the nitrided layer, the absolute value of the compressive residual stress of the nitrided layer, which tends to decrease in maraging steel not containing Ti, can be increased.
Further, the maraging steel adjusted in the chemical composition range defined in the present invention described above is formed into a belt shape so that it can be applied to, for example, a power transmission belt of a continuously variable transmission for an automobile engine. When nitriding the maraging steel strip for power transmission belts under appropriate conditions, a thin nitride layer of about 20 to 40 μm can be formed on the surface with almost no nitride, giving a large compressive residual stress to the surface. And sufficient fatigue strength can be obtained. In order to obtain sufficiently high fatigue strength, the surface compressive residual stress after nitriding is preferably 1050 MPa or more, more preferably 1100 MPa or more.
In addition, although the one where the surface compressive residual stress is higher is preferable, the control is possible by adjusting the thickness of a nitrided layer suitably.
The maraging steel for power transmission belts of the present invention has high tensile strength, high fatigue strength, and is easily nitrided, so that it is suitable for ring products for continuously variable transmission parts of automobile engines.

The following examples further illustrate the present invention.
The steel of the present invention and the comparative steel were melted in a vacuum induction melting furnace, a 10 kg ingot was produced, homogenized annealing was performed, and then hot forging was performed. Further, a steel strip having a thickness of about 0.3 mm was produced by hot rolling and cold rolling. Thereafter, a solution treatment was performed at 820 ° C., an aging treatment was further performed at 490 ° C., and then gas soft nitriding was performed at 450 to 470 ° C. under a condition that the nitriding depth was 20 to 40 μm.
Table 1 shows the steel No. of the present invention. 1-5, Reference Example No. 6. Conventional steel and comparative steel No. 6 The chemical composition of 21-22 is shown. Here, comparative steel No. No. 21 is a conventional steel containing Ti and comparative steel No. 21. 22 is maraging steel which does not contain Ti and which does not contain Cr or Al. Any of maraging steel be adjusted C in the range of 0.01% or less, to prevent the deterioration of weldability. In addition, the steel No. of the present invention. 5 and Reference Example No. 6 added Mg. The Mg content is no. 5 is 7 ppm, no. 6 was 12 ppm.
Table 2 shows the prior austenite grain size, tensile strength, internal hardness after nitriding treatment, surface hardness, and residual stress on the surface after nitriding treatment after aging each sample. Here, as for the sign of the residual stress in Table 2, + indicates tension and-indicates compression, and all are compressive residual stresses.
Although not shown in the table, in the cross-sections of the steel of the present invention and the comparative steel, the microscopic inclusions were observed and analyzed using an electron microscope and an X-ray analyzer. It was confirmed that the amount of inclusions of TiN and Ti (C, N) was extremely small in all the test pieces except for No. 21.
In addition, the steel No. of the present invention. 5 and Reference Example No. For No. 6, cross-sectional observation was performed with an electron microscope at 1000 magnifications with 10 fields of view, but Al ₂ O ₃ inclusions could not be observed.

From Table 2, steel of the present invention No. 1-5 and Reference Example No. No. 6 has a tensile strength after aging of 1700 MPa or more, and has sufficient strength as maraging steel. Further, it has been found that even after nitriding, it has high internal hardness, surface hardness and large surface compressive residual stress, and conventional steel No. 1 which is a maraging steel containing Ti. It can be seen that even if compared with 21, it has the same or better characteristics.
In addition, the steel No. of the present invention. 1-5 and Reference Example No. No. 6 has the prior austenite grain size of ASTM No. 10 or more fine grains are maintained.
On the other hand, comparative steel No. with no addition of Ti and no addition of Cr. No. 22 has lower tensile strength after aging, internal hardness after nitriding, surface hardness, and surface compressive residual stress than the steel of the present invention. Moreover, since B was not included, the crystal grain was somewhat coarse.
As described above, the steel according to the present invention has much less TiN inclusions than the conventional maraging steel, and has high tensile strength and good nitriding characteristics, so that high fatigue strength can be expected.

The maraging steel for power transmission belt of the present invention can obtain high strength, high hardness of the surface after nitriding treatment, and large compressive residual stress. Therefore, the power transmission belt used for continuously variable transmissions for automobiles, etc. It can be applied to members that require high tensile strength and high fatigue strength such as ring products.

Claims (4)

C: 0.01% or less, Si: 0.1% or less, Mn: 0.1% or less, P: 0.01% or less, S: 0.005% or less, Ni: 17.0-22 in mass% .0%, Cr: 0.1~4.0%, Mo: 3.0~7.0%, Co: 1 2. More than 0% and 20.0% or less, Ti: 0.05% or less, Al: less than 0.1%, Al + Ti: 0.1% or less, N: 0.03% or less, O: 0.005% or less, B: 0.01% or less (excluding 0), Co / 3 + Mo: 8.0 to 15.0%, the balance being made of Fe and inevitable impurities, and a power transmission belt having high fatigue strength For maraging steel. Ca mass%: 0.01% or less, Mg: power transmission belt maraging steel having high fatigue strength according to claim 1, characterized in that it contains one or more than 0.005% of. The crystal grain size is ASTM No. The maraging steel for power transmission belts having high fatigue strength according to claim 1 or 2 , wherein the fine-grained steel has 10 or more fine grains. According nitride layer on the surface of the power transmission belt maraging steel according to any one of claim 1 to 3 is formed, and power transmission belts for maraging steel, characterized in the compressive residual stress that was applied to the surface band.