JP2008088540A - Maraging steel having high fatigue strength and maraging steel strip using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a maraging steel having high fatigue strength in which surface hardness is increased by reducing TiN being the starting point of fatigue fracture in a high cycle region and facilitating nitriding treatment, also, bending fatigue strength is improved by increasing the compressive residual stress of a surface nitrided layer, and old austenite crystal grains are refined for securing high strength and ductility, and which has satisfactory nitriding properties and weldability. <P>SOLUTION: The maraging steel having high fatigue strength has a composition comprising, by mass, ≤0.01% C, ≤0.1% Si, ≤0.1% Mn, ≤0.01% P, ≤0.005% S, 17.0 to 22.0% Ni, 0.1 to 3.0% Cr, 3.0 to 7.0% Mo, >7.0 to 20.0% Co, ≤0.1% Ti, >0.15 to 2.5% Al, ≤0.03% N, ≤0.005% O, ≤0.01% (not including zero) B, 8.0 to 15.0% Co/3+Mo+4Al, and the balance Fe with inevitable impurities. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to maraging steel 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, and the like. It relates to a 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. It is an object of the present invention to provide a maraging steel having high fatigue strength with good nitriding properties and weldability, in which prior austenite crystal grains are refined in order to improve and secure 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 having high fatigue strength composed of 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. The maraging steel is 10 or more fine grains.
Furthermore, the present invention is a maraging steel strip in which a nitrided layer is formed on the surface of the maraging steel described above and a compressive residual stress is applied to the surface.

  The maraging steel of the present invention can reduce TiN that 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 for a member that requires high fatigue strength, such as a ring product of a power transmission belt to be used, 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 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 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 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 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. When Co is less than 7.0%, it is difficult to obtain sufficient strength with maraging steel with reduced Si, Mn, and Ti, while when added over 20.0%, austenite is stabilized and it is difficult to obtain a martensite structure. Therefore, the content is more than 7.0% and not more than 20.0%. 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. 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 0.1%, 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 of the present invention in which Si, Mn, and Ti are reduced, it is necessary to supplement the strength by adding Al. Moreover, addition of Al is also necessary in order to obtain a good nitrided layer by facilitating nitriding in maraging steel with reduced Ti. 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 maraging steel, and Al is an element that contributes 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 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 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, 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 of the present invention can produce an ingot by vacuum induction melting or 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, high melting point, it is not possible to deform almost for example, even hot during plastic working, therefore, causes surface defects of maraging steel by generating scratches for example, a roll of cold rolling There is a possibility. 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 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 of the present invention is obtained by subjecting prior austenite crystal grains (here, maraging) to a solution treatment at an appropriate temperature according to the composition, for example, about 780 to 1000 ° C., after cold working of 10% or more. In the case of steel, crystal grains refer to prior austenite grains) according to ASTM No. The particle size can be reduced to 10 or more.
In the maraging steel of the present invention, it is possible to stably increase the hardness, tensile strength, fatigue strength, impact toughness, etc. by refining the crystal grains, and the surface roughness of the steel strip is slight. The effect of being able to be made can be expected.

Since the maraging steel 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, salt bath Various nitriding treatments such as nitriding can be easily performed. In addition, the absolute value of the compressive residual stress of the nitrided layer, which tends to decrease in maraging steel not containing Ti, is also improved by compressing the nitride layer by Cr and Al, which has the effect of increasing the hardness of the nitrided layer and the absolute value of the compressive residual stress of the nitrided layer. The absolute value of the residual stress can be increased.
In addition, maraging steel adjusted within the chemical composition range defined in the present invention described above is nitrided under appropriate conditions to a steel strip formed into a strip shape so that it can be applied to a continuously variable transmission part of an automobile engine, for example. When the treatment is performed, a thin nitride layer of about 20 to 40 μm can be formed on the surface without forming almost any nitride, a large compressive residual stress can be applied to the surface, and sufficient fatigue strength can be obtained. At this time, in order to obtain a 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 of the present invention has a high tensile strength and a high fatigue strength, and is easily nitrided. Therefore, the maraging steel is suitable for a ring product of a continuously variable transmission for an automobile engine.

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 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 the steel No. 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 maraging steel which does not contain Ti and which does not contain Cr or Al. In all maraging steels, 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.
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 inventive steel No. 5, 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. Nos. 1 to 8 each have a tensile strength after aging of 1800 MPa or more, and have sufficient strength as maraging steel. 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. Also,
Since B was not included, the crystal grain was slightly 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.

  Since the maraging steel of the present invention can obtain high strength, high hardness of the surface after nitriding treatment, and large compressive residual stress, it is like a ring product of a power transmission belt used in a continuously variable transmission for automobiles. It can be applied to members that require high tensile strength and high fatigue strength.

Claims (6)

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 having high fatigue strength, 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 having high fatigue strength according to claim 1, which is 10.0 to 15.0%. The maraging steel having high fatigue strength according to claim 1 or 2, characterized in that Co in mass% is more than 12.0% and not more than 20.0%. The maraging steel having high fatigue strength according to any one of claims 1 to 3, characterized by containing at least one of Ca: 0.01% or less and Mg: 0.005% or less in mass%. The crystal grain size is ASTM No. The maraging steel having high fatigue strength according to any one of claims 1 to 4, wherein the maraging steel has a fine grain size of 10 or more. A maraging steel strip, wherein a nitrided layer is formed on the surface of the maraging steel according to any one of claims 1 to 5 and compressive residual stress is applied to the surface.