JP4613698B2 - Steel strip and strip - Google Patents

Description

The present invention relates to a steel for a ribbon, and provides a steel exhibiting excellent fatigue strength when used as a material for various metal belts.

For example, as a material for a metal belt constituting a CVT (continuously variable transmission), maraging steel added with Ti is usually used. If the example of a specific alloy composition is given, C: 0.03% or less, Ni: 12.0-25.0%, Co: 8.0-12.5%, Mo: 3.5-4.8 %, Ti: 0.2 to 1.7% and Al: 0.1 to 0.3%, and the balance is a maraging steel made of Fe and inevitable impurities.

This type of maraging steel is not only very strong and has excellent toughness, but also has good machinability, workability and weldability, so it is suitable for various applications, but contains Ti. Therefore, the fatigue properties including fatigue strength are slightly insufficient, and particularly when processed into a thin plate having a thickness of 0.5 mm or less, there is a disadvantage that the fatigue properties are deteriorated.

In order to compensate for the above-mentioned drawbacks of existing maraging steel, the applicant has developed and disclosed maraging steel to which a relatively large amount of C is added (Patent Document 1). The steel is based on the phenomenon that C combines with Cr and Mo by aging to form carbides, and uses a mechanism in which carbides precipitate and cause secondary hardening. A large amount of “0.3% or less” of C is added to the C content of “0.03% or less”. In the Example of the said patent document 1, 0.16-0.22% of C is added.
JP-A-2002-167652

The inventors have examined the influence of C content on the physical properties in the region of C content exceeding 0.03% of conventional maraging steel and reaching the upper limit of 0.3% proposed by Patent Document 1. The following knowledge was obtained.
・ As shown in the upper graph of Fig. 1, the relationship between C content and tensile strength is higher than that of conventional products until it reaches 0.3% beyond the conventional low C region. Be done. (The value indicated by the broken line in the upper part of FIG. 1 is the strength level of conventional maraging steel.)
・ However, with regard to fatigue properties, when the C content exceeds the conventional one, the fatigue limit improves to a certain extent as shown in the lower graph of FIG. When it exceeds, it starts to decrease and reaches 0.23%, which is below the level of conventional materials (shown by a broken line in the lower part of FIG. 1). It is understood that the fatigue characteristics equal to or higher than those of conventional materials can be obtained up to 0.23% because the strength of the base metal itself is helped.
-Fatigue properties depend on the depth of the nitrogen diffusion layer formed on the surface of the maraging steel product by nitriding treatment. The depth of this layer depends on the C content as shown in the middle graph of FIG. When it exceeds 0.15%, it decreases. This result corresponds to the change in fatigue strength in the lower graph.

Based on the above findings, the inventors have used 0.05% or more, exceeding the upper limit of 0.03% of conventional maraging steel, as a guideline for designing an alloy of maraging steel with improved fatigue properties. Thus, it was found that the C content in the region of 0.15% or less that the invention of Patent Document 1 did not try should be selected. In order to increase the fatigue strength of maraging steel, it is necessary to harden the surface of the material, but it is necessary to diffuse nitrogen at an appropriate concentration deep inside the material. That is, it is important to realize a compressive residual stress state by nitrogen diffusion in a pattern close to the gradient of the bending stress distribution applied in the thickness direction of the product ribbon.

The object of the present invention is to combine the above-mentioned findings of the inventors with various matters known in the technical field of the art, while ensuring the characteristics of maraging steel with high strength and toughness, while maintaining fatigue strength. The aim is to provide a strip steel which is superior in properties to the known ones. It is also included in the object of the present invention to use the maraging steel to provide a steel belt having higher durability than conventional products.

The steel sheet for ribbon according to the present invention that achieves the above-mentioned object is, by weight%, C: 0.05 to 0.15%, Ni: 10.0 to 18.0%, Co: 5.0 to 30.0. %, Mo: 1.0 to 5.0%, Al: 0.5 to 1.3%, and Cr: 1.0 to 3.0%, Ti: 0.10% or less, S: 0 0.0003% or less, P: 0.03% or less, N: 0.03% or less, and O: 0.03% or less, the bending fatigue characteristics having an alloy composition with the balance being Fe and inevitable impurities It is an excellent steel strip. The ribbon according to the present invention is a ribbon used for applications in which repeated bending stress is applied by forming a steel for a ribbon having the above alloy composition into a ribbon shape and subjecting the surface to nitriding treatment.

The steel for a strip according to the present invention exhibits excellent bending fatigue characteristics when a product obtained by nitriding a product processed into a strip is subjected to repeated bending stress application. The tensile strength and toughness of this product are not inferior to those of conventional maraging steel. Therefore, the ribbon manufactured with the steel of the present invention exhibits high durability when used for a CVT belt in a power transmission system of an automobile.

In addition to the above-described alloy components, the steel for a strip of the present invention can have any one or two or more of optional additive components belonging to the following groups.
1) One or more of B: 0.0001 to 0.010%, Ca: 0.0001 to 0.010%, and Mg: 0.0001 to 0.010%,
2) Zr: 0.005 to 0.04%,
3) V: 0.01 to 1.0%, Nb and / or Ta (when combined, in total amount): 0.01 to 1.0%, W: 0.01 to 1.0% and Cu : 0.01 to 1.0% of one or more, and 4) REM: 0.005 to 0.020%

The reason why the alloy composition of the steel strip for use in the present invention is determined as described above will be described first for the essential alloy component and then for the optional additive component.

C: 0.05 to 0.15%
As explained above, in the conventional maraging steel, C combines with Ti to form TiC, which is limited to a low content of 0.03% or less as it reduces strength and toughness. . However, in the present invention, C is incorporated for the purpose of causing secondary hardening by utilizing the fact that it combines with Cr and Mo by aging and precipitates as these carbides. Add a large amount. However, if C exceeds 0.15%, a sufficient nitrided layer depth cannot be obtained during nitriding, and the pattern of destruction shifts from the internal origin type to the surface origin type, and is affected by surface scratches and the external environment. As a result, fatigue strength decreases. From the balance between the strength of the base metal and the nitriding characteristics, with increasing C content, bending fatigue strength equal to or higher than that of ordinary maraging steel can be obtained up to 0.23%, but excellent bending fatigue. A range of 0.05 to 0.15% at which strength characteristics are stably obtained was selected.

Ni: 10.0-18.0%
Ni makes the matrix an austenitic structure, which is cooled to a room temperature from the solution heat treatment temperature to room temperature, thereby improving the strength and toughness and increasing the ductility / toughness transition temperature. At the same time, Mo and Al form an intermetallic compound by aging, which precipitates and causes secondary hardening. Such an effect brought about by Ni is made possible by addition of 10.0% or more, but if added over 18.0%, austenite remains even after the above cooling, and the whole is martensite. Can not be.

Co: 5.0-30.0%
Co increases age-hardening characteristics to improve strength, and also raises the martensite transformation temperature to facilitate transformation to martensite, so is added for the purpose. In order to obtain these effects, it is necessary to contain 5.0% or more. However, if it exceeds 20.0%, the additive effect tends to be saturated, so 30.0% is made the upper limit. A preferable addition amount is 8.0 to 20.0%.

Mo: 1.0-5.0%
Mo not only combines with Ni by aging to precipitate the intermetallic compound Ni ₃ Mo, but also bonds to C in the same way as Cr to precipitate carbides, and these precipitates cause secondary hardening. This effect is recognized when 1.0% or more is added, and if it exceeds 5.0%, ductility and toughness are lowered. Therefore, the addition amount is selected within the above range.

Al: 0.5 to 1.3%
Al is a deoxidizer at the time of melting, and binds to Ni by aging to precipitate an intermetallic compound (NiAl or the like), and brings about an effect of improving strength by secondary hardening. In order to obtain this effect, 0.5% or more of Al is added. However, since addition exceeding 1.3% impairs ductility and toughness, the addition amount is set within this range.

Cr: 1.0-3.0%
As described above with respect to Mo, Cr precipitates carbides to cause secondary hardening, and dissolves in the matrix to enhance corrosion resistance. The amount of Cr added for these purposes must be at least 1.0%. Addition of a large amount adversely affects ductility and toughness, so the upper limit is 3.0%.

Ti: 0.10% or less Ti combines with C and N to form Ti-based non-metallic inclusions and deteriorates fatigue characteristics such as fatigue strength. Therefore, the amount is limited to 0.10%.

S: 0.003% or less S is an element that enhances machinability, but MnS that brings about machinability is not preferable for toughness and fatigue strength, so the content is limited to 0.003% or less. .

P: 0.03% or less P is a component that is negative for toughness and fatigue strength. The content must be 0.03% or less.

N: 0.03% or less N combines with Ti as described above to form Ti-based non-metallic inclusions, which lowers fatigue properties, so the content is made 0.03% or less. suppress.

O: 0.03% or less O combines with Al to form oxide-based non-metallic inclusions. Since they are undesirable for fatigue properties, the O content is reduced to 0.03% or less.

One, two or more of B, Ca and Mg (in the case of combined use, the total amount): 0.0001 to 0.010%
B, Ca, and Mg are effective elements for improving fatigue strength, and are effective elements for improving hot workability. This effect appears at a content of 0.0001% or more, but excessive addition causes a low melting point boride to precipitate at the grain boundaries or to form an oxide, thereby reducing the cleanliness of the steel. In addition to lowering hot workability and cold workability, it also causes a decrease in fatigue strength, so it is necessary to limit the amount to 0.010% or less.

Zr: 0.005 to 0.040%
The significance of adding Zr is to increase the fatigue strength by suppressing the formation of TiC. This effect can be obtained by addition of 0.005% or more. However, if excessive addition is performed, segregation increases and the toughness is impaired. Therefore, the addition amount should be selected within the range of 0.040% without any problem. It is.

V: 0.01 to 1.0%, Nb and / or Ta (when combined use, in total amount): 0.01 to 1.0%, W: 0.01 to 1.0% and Cu: 0 .01-1.0% of one or more,
V, Nb, Ta, W and Cu are preferably added for that purpose because they dissolve in the matrix and improve the fatigue strength. The effects become clear only when the addition amount is 0.01% or more. Addition of a large amount exceeding 1.0% lowers the ductility and toughness, so the respective contents are selected within the range of 0.01 to 1.0%.

REM: 0.005-0.020%
REM increases the adhesion of the oxide film and improves the corrosion resistance at high temperatures. Therefore, depending on the usage environment, it is recommended to add REM. This effect is recognized at a content of 0.005% or more. However, addition over 0.020% has a detrimental effect of locally lowering the melting point and impairing hot workability. To do.

In the production of the thin plate material of the present invention, (1) a raw material with few impurities is melted in a vacuum induction furnace and cast into an ingot, or (2) a melting raw material such as scrap and alloy elements is an eruc arc furnace. The resulting molten steel is transferred to a ladle refining furnace equipped with a heating device and a vacuum device, vacuum refined, and cast into an ingot. The ingot thus obtained is remelted by the electroslag remelting method or the vacuum arc remelting method and solidified to form a remelted ingot.

The remelted ingot is soaked by heating to a temperature in the range of about 1000 to 1300 ° C. for about 6 to 72 hours, air cooled, and then processed into a desired product shape by hot working or cold working. After being given the final product shape, it is heated to a temperature of 800-1000 ° C. for about 1-60 minutes for solid solution treatment, air-cooled, and then polished to remove the surface scale and homogenize the surface quality. Weigh the scale and end. Subsequently, in order to obtain a predetermined material strength, it is heated in air at 100 to 600 ° C. for 1 to 5 hours in a vacuum, in an inert atmosphere or in a weakly reducing atmosphere, and is then air-cooled at 100 to 400 ° C. in air. Oxidation treatment is performed for ˜60 minutes, and finally, the product is obtained by gas cooling with an inert gas every 1 to 5 hours in a nitriding atmosphere at 100 to 600 ° C.

According to the method (2) described above, melting and refining of scrap by an arc furnace, vacuum refining by a ladle refining furnace, and vacuum arc remelting were performed to obtain an ingot having a diameter of 510 mm. The alloy composition is as shown in Table 1. These ingots are homogenized by soaking by heating at 1150 to 1200 ° C. for 24 hours to form a plate material having a thickness of 4 mm by hot forging and hot rolling, and subsequently a thin plate material having a thickness of 0.4 mm by cold rolling. I made it. The thin plate material was subjected to the following heat treatment.
1) Solid solution heated to 900 ° C for 60 minutes-Air cooling 2) Heating to 480 ° C for 4 hours-Air cooling 3) Oxidation treatment heating to 250 ° C for 60 minutes in the atmosphere 4) Heating to 450 ° C for 60 minutes in a nitriding atmosphere-Not used Aging nitriding with gas cooling with active gas.

The following test was performed on the final product of the sheet material.
[Cleanliness]
The area ratio (%) of inclusions was measured in accordance with the microscope test method for nonmetallic inclusions in steel defined in JIS G0555. The test piece was manufactured by stacking five sheets of 0.4 mm × 5 mm × 10 mm strips and embedding them in a resin, followed by mirror polishing.
[Tensile test]
The tensile strength (MPa) was measured according to the metal tensile test method defined in JIS Z2241. The test piece was a No. 5 test piece according to JIS Z2201.
[T / 2 hardness]
The test was performed according to the Vickers hardness test method defined in JIS Z2241. The test piece was the same as that prepared for the above cleanliness test, and was measured at a load of 0.5 N. The measurement site is a position (T / 2) that is 1/2 the sample thickness from the surface, placed on the cross section. The measured value was an average value of 10 points.

[Nitrogen diffusion layer depth]
For the same test piece prepared for the above cleanliness test, the nitrogen concentration distribution is measured by EPMA, and the graph shown in FIG. 2 is drawn, where the hatched portion, that is, the nitrogen concentration is measured from the surface. The depth until saturation at the lowest value was taken as the nitrided layer depth (μm).
[Bending fatigue properties]
Investigated according to the general rules for fatigue testing of metal materials as defined in JIS Z2273. Specifically, as shown in FIG. 3, a strip-shaped test piece having dimensions of 0.4 mm × 10 mm × 100 mm is subjected to a swing stress of 1000 to 1300 N / mm ² and an excitation speed of 1000 rpm. The test piece was repeatedly bent and deformed, and the number of vibration (deformation) repetitions until breaking was measured. Fatigue strength (N / mm ² ) was defined as the strength at which the number of repetitions until breakage was 10 ⁷ times or more by changing the maximum value of the vertical stress applied to the base of the test piece.

The test results are shown in Table 2.

In Examples A to L according to the present invention, the amount of C is appropriate (0.05 to 0.15%), so the tensile strength is high and the depth of the nitrogen diffusion layer is sufficient, resulting in high fatigue strength. Has been realized. Further, in Example F to which B and Ca were added, the fatigue strength was further improved as compared with Example B having the same amount of C. In Examples G to L, by adding Zr, V, Nb, Ta, W, Cu, or the like, the tensile strength is improved as compared to Examples A to E in which they are not added, and the fatigue strength is further increased. Has also been improved.

On the other hand, in Comparative Examples M to O, the amount of C is insufficient (less than 0.05%), so that the nitrogen diffusion layer has a sufficient depth, but the tensile strength of the material is low and the fatigue strength is low. Is an unsatisfactory value. In Comparative Examples P to V, since the amount of C was excessive (exceeding 0.15%), the nitrogen diffusion layer tended to become shallow, and the fatigue strength remained at a low level. Particularly in Comparative Examples T to V, the tensile strength of the material is also reduced, which spurs the decrease in fatigue strength. Comparative Example W is an alloy composition corresponding to conventional maraging steel, which has a relatively high tensile strength and a sufficient nitrogen diffusion layer depth, but a low fatigue strength. This is due to the use of Ti for strengthening the material. That is, rupture occurs with low stress starting from TiN inclusions produced during production.

Both are graphs showing the relationship between various properties and C content when the C content is changed in maraging steel, where the upper graph shows the relationship between tensile strength and C content, and the middle graph shows the nitrogen diffusion layer. The relationship between the depth and the C content, and the lower graph show the relationship between the fatigue limit and the C content, respectively. The conceptual diagram which shows the definition of the "depth" of a nitrogen diffusion layer. The conceptual diagram which shows the conditions of the bending fatigue test performed in the Example of this invention.

Claims (6)

% By weight, C: 0.05 to 0.15%, Ni: 10.0 to 18.0%, Co: 5.0 to 30.0%, Mo: 1.0 to 5.0%, Al: 0.5 to 1.3% and Cr: 1.0 to 3.0%, Ti: 0.10% or less, S: 0.003% or less, P: 0.03% or less, N: 0.03% or less and O: 0.03% or less, and the steel for a ribbon having excellent bending fatigue characteristics having an alloy composition consisting of Fe and unavoidable impurities as a balance is used as a material. A thin ribbon that is formed into a band shape and subjected to nitriding treatment on the surface, and is used for applications where repeated bending stress is applied . In addition to the alloy components defined in claim 1, one or two of B: 0.0001 to 0.010%, Ca: 0.0001 to 0.010% and Mg: 0.0001 to 0.010% 2. The ribbon according to claim 1, wherein the ribbon steel having an alloy composition containing the above is used as a material . The ribbon according to claim 1 or 2, wherein a steel for a ribbon having an alloy composition containing Zr: 0.005 to 0.040% in addition to the alloy components defined in claim 1 or 2 is used as a material . In addition to the alloy components defined in any one of claims 1 to 3, V: 0.01 to 1.0%, Nb and / or Ta (in combination, in total amount): 0.01 to 1. A steel for a ribbon having an alloy composition containing one or more of 0%, W: 0.01 to 1.0% and Cu: 0.01 to 1.0% is used as a material. One or three ribbons . Any of Claims 1 to 4 which uses as a material the steel for strips which has the alloy composition containing REM: 0.005-0.020% in addition to the alloy component specified in any one of Claims 1 to 4. That ribbon . A ring for CVT formed by processing the ribbon according to claim 1 having a thickness of 0.5 mm or less.

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