JP2009013464A - Maraging steel for metal belt - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a maraging steel for a metal belt, which has such a composition as to reduce TiN that becomes a point of initiating a fatigue fracture in a high cycle region, enhances surface hardness by facilitating nitriding treatment, improves bending fatigue strength by increasing a compressive residual stress of a surface-nitrided layer, and has refined former austenite crystal grains so as to secure higher strength and ductility. <P>SOLUTION: The maraging steel for the metal belt having a high fatigue strength includes, by mass%, 0.01% or less of C, 0.1% or less of Si, 0.1% or less of Mn, 0.01% or less of P, 0.005% or less of S, 17.0 to 22.0% of Ni, 0.1 to 3.0% of Cr, 3.0 to 7.0% of Mo, more than 7.0% but 20.0% or less of Co, 0.1% or less of Ti, more than 0.15% but 2.5% or less of Al, 0.03% or less of N, 0.005% or less of O, 0.01% or less of B (excluding 0), 8.0 to 15.0% of Co/3+Mo+4Al, and the balance Fe with unavoidable impurities. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

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

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. Further, it is used by improving the fatigue strength by nitriding the surface to form a nitrided layer.
For a metal belt for a continuously variable transmission of an automobile engine, an improved alloy has been proposed for the purpose of solving a decrease in 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 Ti which forms a nonmetallic inclusion to 0.1% or less. Therefore, although advantageous in terms of the refinement of TiN, which is the starting point of fatigue fracture, there is a problem that nitriding treatment is difficult due to an alloy that simply suppresses the addition of elements that form non-metallic inclusions. .
Moreover, since the alloy disclosed in Japanese Patent Application Laid-Open No. 2001-240943 also reduces Ti, it is advantageous in terms of refinement of TiN, which is the starting point of fatigue fracture. However, since Co, which is one of the strengthening elements, is kept low, it is difficult to ensure high tensile strength. Further, Si and Mn are added in order to ensure the tensile strength, but there is a possibility that the toughness is lowered due to this.
Moreover, since the alloy disclosed in Japanese Patent Application Laid-Open No. 2002-167852 also reduces Ti, it is advantageous in terms of miniaturization of TiN that is the starting point of fatigue fracture. However, since C is actively added to increase the strength, carbides such as Cr and Mo precipitate, and this causes fatigue fracture to decrease, and the fatigue strength is reduced. The weldability required for machine parts may be reduced.
An object of the present invention is to have a composition that can reduce TiN that is the starting point of fatigue failure in a high cycle range, facilitate nitriding treatment to increase surface hardness, and increase the compressive residual stress of the surface nitrided layer. Thus, it is to provide a maraging steel for metal belts in which the prior austenite crystal grains are refined in order to improve the bending fatigue strength and ensure higher strength and ductility.

The inventor suppresses both Ti and N in order to reduce inclusion TiN, which is detrimental to improving fatigue strength, and reduces the strength decrease due to Ti decrease only in the amount of Co, Mo, Al and the value of Co / 3 + Mo + 4Al within an appropriate range It was found that it can be compensated by limiting to. 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 and Al in order to solve the difficulty of nitriding due to Ti reduction, By suppressing C to an impurity level, weldability is ensured 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: 0.1-3.0%, Mo: 3.0-7.0%, Co: Over 7.0% and 20.0% or less, Ti: 0.1 % Or less, Al: more than 0.15% and 2.5% or less, N: 0.03% or less, O: 0.005% or less, B: 0.01% or less (0 is not included), Co / 3 + Mo + 4Al : 8.0 to 15.0%, the balance being maraging steel for metal belts from Fe and inevitable impurities.
In the present invention, preferable composition ranges are C: 0.008% or less, Ni: more than 18.0% and 22.0% or less, Mo: more than 5.0% and 7.0% or less, and Ti: It is 0.05% or less, Co / 3 + Mo + 4Al: 10.0-15.0%. More preferably, it is more than Co: 12.0% and 20.0% or less by mass%.
In the present invention, in addition to the above-mentioned composition, it may further contain one or more of Ca: 0.01% or less and Mg: 0.005% or less by mass%.
In the present invention, the crystal grain size is ASTM No. Is a maraging steel for metal belts having a fine grain size of 10 or more.

  The maraging steel for metal belts of the present invention can reduce TiN, which is the starting point of fatigue failure, and can obtain high strength, high hardness of the surface after nitriding treatment, and large compressive residual stress. When used in a member that requires high fatigue strength, such as a power transmission metal belt used in a machine, it is expected to have a significant industrial effect, such as having a long fatigue life.

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 metal belts of the present invention, the reason why each chemical composition is specified in the following range is 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 a decrease in strength due to a decrease in Ti by refining the intermetallic compound precipitated during aging treatment or forming an intermetallic compound with Ni. However, since there is a possibility of lowering toughness, it is necessary to keep it low in the present invention in order to ensure toughness and ductility. 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, and thus can compensate for a decrease in strength due to a decrease in Ti. However, since there is a possibility of lowering toughness, it is necessary to keep it low in the present invention in order to ensure toughness and ductility. 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 cause segregation or formation of inclusions in the prior austenite grain boundaries, embrittle the maraging steel and reduce fatigue strength. Therefore, P is set to 0.01% or less, and 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 for N when nitriding, reduces the nitriding depth, increases the nitriding hardness, and increases the compressive residual stress on the nitriding surface, so is essential. However, if the amount is less than 0.1%, there is no effect. On the other hand, even if added over 3.0%, a further improvement effect is not seen, and the strength after aging is greatly reduced. It was set to 0.1 to 3.0%.
Ni needs to be at least 17.0% in order to stably form a low-C martensite structure, which is a base structure of maraging steel for metal belts. However, if it exceeds 22.0%, the austenite structure is stabilized and martensitic transformation is difficult to occur, so Ni was made 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. Is an important element contributing to aging precipitation strengthening by promoting precipitation of fine intermetallic compounds including Mo and Al. Therefore, it is necessary to add much Co in terms of strength and toughness. When Co is less than 7.0%, it is difficult to obtain sufficient strength with a metal belt maraging steel with reduced Si, Mn and Ti. On the other hand, when added over 20.0%, austenite is stabilized and a martensite structure is formed. Since it becomes difficult to obtain, the content is more than 7.0% and 20.0% or less. A preferable Co range is more than 12.0% and not more than 20.0%.
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. It is a harmful element. Therefore, Ti needs to be kept 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 0.1%, 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. Preferably it is 0.05% or less, and more preferably 0.01% or less.

Al is usually added in a small amount for deoxidation, but originally is an element that contributes to strengthening by forming an intermetallic compound with Ni during aging treatment. In the maraging steel for metal belts of the present invention in which Si, Mn and Ti are reduced, it is necessary to supplement the strength by adding Al. In addition, in order to obtain a good nitrided layer by facilitating nitriding in the maraging steel for metal belts with reduced Ti, addition of Al is necessary. When Al is less than 0.15%, sufficient strengthening action due to aging treatment cannot be obtained. On the other hand, when it exceeds 2.5%, a large amount of inclusions of AlN and Al ₂ O ₃ are formed to reduce the fatigue strength. Therefore, Al is made more than 0.15% and not more than 2.5% because a thin and stable oxide film is formed to inhibit the nitriding reaction.
Co, Mo, and Ti are all major strengthening elements in the maraging steel for metal belts, and Al is an element that contributes to aging strengthening of the maraging steel for metal belts. If Ti is kept low, it is necessary to compensate for the decrease in strength due to Ti by increasing the amount of addition of Co, Mo, and Al. However, the contribution of each element to the strengthening is not the same, and the strengthening by Co and Al is 1/3 and 4 times the strengthening by Mo, respectively.
Therefore, the strengthening by Co, Mo, and Al can be organized by Co / 3 + Mo + 4Al. If the value of Co / 3 + Mo + 4Al is less than 8.0% by mass%, the strength is not sufficient, while if it exceeds 15.0%, the strength becomes too high, and there is a possibility that the toughness is reduced. Therefore, Co / 3 + Mo + 4Al Was 8.0 to 15.0%. A preferable range of Co / 3 + Mo + 4Al 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 the maraging steel for metal belts 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 the maraging steel for metal belts containing almost no Ti, the amount of N mixed by ordinary vacuum melting has little adverse effect, so the content 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 solution treatment after cold working to contribute 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.
The maraging steel for metal belt of the present invention can produce an ingot by vacuum induction melting or by melting in a vacuum atmosphere such as vacuum arc remelting or electroslag remelting after vacuum induction melting. . However, it is technically difficult to completely eliminate non-metallic inclusions even when melting in a vacuum atmosphere is performed.
In the case of the present invention, since Al is positively added for the purpose of improving the strength, for example, there is a risk of formation of coarse and hard Al ₂ O ₃ inclusions exceeding 25 μm, or a risk of Al ₂ O ₃ being clustered. There is sex. Al ₂ O ₃ inclusions are hard and have a high melting point and, for example, hardly deform even during hot plastic working. For this reason, for example, there is a possibility of generating flaws on the roll during cold rolling to cause surface defects in the maraging steel for metal belts. Therefore, Al ₂ O ₃ inclusions are used as composite inclusions to reduce the hardness. It is better to lower the melting point. 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 that Ca: 0.01% or less, or also 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 metal belts of the present invention is made of prior austenite crystal grains (here, by solid 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. 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 metal belts of the present invention, the hardness, tensile strength, fatigue strength, impact toughness, etc. can be stably increased by refining the crystal grains. Effects such as the ability to reduce rough skin can be expected.

Since the maraging steel for metal belts of the present invention contains almost no Ti that forms a stable oxide film on the surface that may inhibit nitriding, ordinary gas nitriding, gas soft nitriding, sulfur nitriding, ion nitriding Various nitriding treatments such as salt bath nitriding can be easily performed. In addition, the absolute value of the compressive residual stress of the nitrided layer, which tends to decrease in the maraging steel for metal belts not containing Ti, is also nitrided by Cr and Al, which has the effect of 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 layer can be increased.
Further, when the maraging steel for metal belts adjusted within the chemical composition range defined in the present invention is subjected to nitriding treatment under suitable conditions on a steel strip formed into a strip shape of 0.5 mm or less, for example, nitrides A thin nitride layer having a thickness of about 20 to 40 μm can be formed on the surface without forming a large amount of metal, and a large compressive residual stress can be applied to the surface, so that improvement in fatigue characteristics can be expected.
A higher surface compressive residual stress is considered to be advantageous for fatigue characteristics, but the control can be achieved by appropriately adjusting the thickness of the nitrided layer.
The maraging steel for metal belts of the present invention is suitable for a metal belt for continuously variable transmissions of automobile engines because it has high tensile strength and high fatigue strength and is easily nitrided.

The following examples further illustrate the present invention.
The maraging steel for a metal belt of the present invention and the comparative steel were melted in a vacuum induction melting furnace to produce a 10 kg ingot, subjected to homogenization annealing, and then hot forged. Further, a steel strip having a thickness of about 0.3 mm was produced by hot rolling and cold rolling. Thereafter, a solid solution treatment was performed at 820 to 900 ° 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 maraging steel No. 1 for the metal belt of the present invention. 1-8, conventional steel and comparative steel No. 1 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 a maraging steel for metal belts which does not contain Ti and contains no added Cr or Al. In any of the maraging steels for metal belts, C was adjusted to a range of 0.01% or less to prevent deterioration of weldability. In addition, the steel No. of the present invention. 5, no. 6 added Mg. The Mg content is no. 5 is 10 ppm, no. 6 was 6 ppm.
Although not shown in the table, in the cross section of the above steel of the present invention and the comparative steel, observation and analysis of fine inclusions were performed using an electron microscope and an X-ray analyzer at 10 fields randomly at a magnification of 1000 times. Went. As a result, comparative steel No. No inclusions of TiN or Ti (C, N) exceeding 3 μm were observed in all the specimens except 21.
In addition, the steel No. 1 of the present invention subjected to the fatigue test described later. 1, No. 1 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.

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. Among the steels of the present invention, No. 1 and 6 and conventional steel No.1. 21, comparative steel no. Table 3 shows the fatigue test results after aging 22 at 480 ° C.
The fatigue test has various stress loading modes such as rotational bending, tensile compression, and torsion. Since the maraging steel strip of the present invention is a strip material, an evaluation method for applying bending stress is suitable. Therefore, in the repeated bending fatigue test, it is clear that the conventional maraging steel strip has high fatigue strength if it does not break when applied with such a high stress that it breaks. Therefore, when repeated bending stress was applied with an average stress of 537 MPa and a maximum stress of 1016 MPa, the number of repeated fractures was up to 10 ⁷ times.

From Table 2, the maraging steel No. 1 for the metal belt of the present invention. Nos. 1 to 8 each have a tensile strength after aging of 1800 MPa or more, and have sufficient strength as a metal belt application. Further, even after nitriding, it was found that the steel had high internal hardness, surface hardness and large surface compressive residual stress. It can be seen that even if compared with 21, it has the same or better characteristics.
Inventive steel No. In Nos. 1 to 8, the prior austenite grain size due to the effect of adding B is ASTM No. 10 or more fine grains are maintained.
On the other hand, comparative steel No. without addition of Ti and addition of Cr and Al. 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.
Further, from Table 3, the steel No. 1 of the present invention. Nos. 1 and 6 are fatigue test results after aging, and the number of repeated fractures exceeds 10 ⁷ times. It can be seen that it has fatigue properties better than 21. Further, as described above, the steel according to the present invention is superior in nitriding characteristics as compared with the conventional steel, so that further improvement in fatigue characteristics can be expected by nitriding treatment.
Thus, the maraging steel for metal belts of the present invention can have higher fatigue strength than conventional maraging steel.

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

Claims (5)

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% 0.0%, Cr: 0.1-3.0%, Mo: 3.0-7.0%, Co: over 7.0% and 20.0% or less, Ti: 0.1% or less, Al: More than 0.15% and 2.5% or less, N: 0.03% or less, O: 0.005% or less, B: 0.01% or less (0 is not included), Co / 3 + Mo + 4Al: 8.0 A maraging steel for a metal belt, characterized by 15.0%, the balance being Fe and inevitable impurities. C: 0.008% or less by mass%, Ni: more than 18.0% and 22.0% or less, Mo: more than 5.0% and 7.0% or less, Ti: 0.05% or less, Co / 3 + Mo + 4Al The maraging steel for metal belts according to claim 1, wherein the maraging steel is 10.0 to 15.0%. The maraging steel for metal belts according to claim 1 or 2, wherein Co in mass% is more than 12.0% and not more than 20.0%. The maraging steel for metal belts according to any one of claims 1 to 3, comprising one or more of Ca: 0.01% or less and Mg: 0.005% or less in mass%. The crystal grain size is ASTM No. The maraging steel for metal belts according to any one of claims 1 to 4, wherein the maraging steel has a fine grain size of 10 or more.