JP2008138292A - Maraging steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide new maraging steel in which the size of nonmetallic inclusions remaining in a maraging steel strip obtained by vacuum remelting is effectively reduced. <P>SOLUTION: In the method for manufacturing the Ti-containing maraging steel, a consumable electrode for vacuum remelting is produced and vacuum remelting is carried out using this consumable electrode and, further, the consumable electrode contains ≥5 ppm Ca. The maraging steel contains, at least, <15 ppm (excluding 0%) Ca, <10 ppm oxygen, <15 ppm nitrogen and ≤2.0% (excluding 0%) Ti. Moreover, based on the total number of oxide nonmetallic inclusions with a size of ≥10 μm in the structure, oxide nonmetallic inclusions with a size of ≥10 μm which contain ≥85 mass% of Al among metallic elements in the above oxide nonmetallic inclusions comprise <70%. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to maraging steel.

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.
This maraging steel usually contains appropriate amounts of Mo and Ti as strengthening elements, and by performing an aging treatment, intermetallic compounds such as Ni ₃ Mo, Ni ₃ Ti, and Fe ₂ Mo are precipitated. Steel that can obtain strength. A typical composition of this maraging steel containing Mo and Ti is 18% Ni-8% Co-5% Mo-0.45% Ti-0.1% Al-bal. Fe.

However, while maraging steel can obtain very high tensile strength, fatigue strength is not necessarily high. Non-metallic inclusions such as nitrides and carbonitrides such as TiN and TiCN are the biggest factors that degrade this fatigue strength. If these non-metallic inclusions grow greatly in steel, non-metallic inclusions From this point, fatigue failure will occur.
Therefore, a vacuum arc remelting (hereinafter referred to as VAR) method is generally used to reduce nonmetallic inclusions present in steel.

The maraging steel produced by this VAR method has the advantage that it is homogeneous (small component segregation) and the amount of non-metallic inclusions is reduced.
However, relatively large non-metallic inclusions such as TiN and TiCN nitrides and carbonitrides remain in the maraging steel produced by the VAR method, and the remaining large non-metallic inclusions are heated after VAR. It remains in the raw material after forging, heat treatment, hot rolling, and cold rolling, and causes fatigue failure starting from the remaining large non-metallic inclusions.
Various proposals have been made for this problem. For example, in Japanese Patent Laid-Open No. 2001-214212 (see Patent Document 1), a raw material for Ti-containing steel that does not contain TiN-based nonmetallic inclusions is melted in a vacuum induction furnace. There is a method for producing a Ti-containing steel in which a TiN nonmetallic inclusion is re-melted by a vacuum arc melting method using a Ti-containing steel material produced by casting as an electrode.

JP 2001-214212 A

The present inventors studied to further improve the cleanliness of maraging steel.
The above Japanese Patent Application Laid-Open No. 2001-214212 is characterized in that the TiN-based nitride can be made fine by using a raw material for Ti-containing steel that does not contain nitride-based non-metallic inclusions such as TiN and TiCN. Such quality control of the raw material itself is one means for reducing nitride-based nonmetallic inclusions, but there is a problem that a high-quality raw material is necessarily an expensive raw material and is expensive.
Moreover, since the occurrence of TiN-based non-metallic inclusions depends on the dissolution conditions and the like, the management of raw materials alone does not solve the problem sufficiently.

In fact, oxide-based nonmetallic inclusions have been confirmed in the maraging steel in addition to nitride-based nonmetallic inclusions. Although the number of oxide-based nonmetallic inclusions is small, a relatively large size, for example, a diameter exceeding 20 μm may be confirmed.
The presence of such large oxide-based non-metallic inclusions is likely to have an adverse effect on mechanical properties such as fatigue strength of the material as in the case of nitride-based non-metallic inclusions such as TiN.

As a technique for reducing non-metallic inclusions caused by gas components such as nitrides and oxides, there is a vacuum remelting method such as VAR. There was a limit to reducing the size of metal inclusions. Therefore, the development of a new breakthrough technology that is drastically effective in reducing the size and abundance of non-metallic inclusions in maraging steel is eagerly desired.
An object of the present invention is to provide a novel maraging steel that can dramatically reduce the size and abundance of non-metallic inclusions remaining in the maraging steel in view of the above problems.

  The present inventors explored the causal relationship between the generation behavior in the melting process, refining process, and remelting process of non-metallic inclusions due to the gas component in maraging steel and the elements present in the dissolved component, The present inventors have found the excellent reduction effect of non-metallic inclusions and refinement of Ca present in the consumable electrode used for vacuum remelting with respect to the non-metallic inclusions, and have reached the present invention.

  That is, the present invention is a maraging steel containing at least Ca: less than 15 ppm (not including 0), oxygen: less than 10 ppm, nitrogen: less than 15 ppm, and Ti: 2.0% or less (not including 0) in mass%. In addition, with respect to the total number of oxide-based nonmetallic inclusions of 10 μm or more in the structure, among the metal elements in the oxide-based nonmetallic inclusions, an oxide of 10 μm or more containing 85 mass% or more of Al. This is a maraging steel having a non-metallic inclusion (hereinafter referred to as alumina inclusion) of less than 70%.

Preferably, the above-described maraging steel is maraging steel in which the maximum length of oxide-based nonmetallic inclusions is 20 μm or less.
More preferably, the above-mentioned maraging steel is maraging steel in which the maximum length of nitride-based nonmetallic inclusions is 10 μm or less.
Further, the preferred chemical composition of the above-described maraging steel of the present invention is, in mass%, Ca: less than 15 ppm (excluding 0), oxygen: less than 10 ppm, nitrogen: less than 15 ppm, in addition to chemical composition in mass%, Mg: less than 15 ppm (excluding 0), C: 0.01% or less, Ni: 8.0-22.0%, Co: 5.0-20.0%, Mo: 2.0-9.0 %, Ti: 2.0% or less (0 is not included), Al: 1.7% or less, and the balance is maraging steel substantially composed of Fe.
The maraging steel of the present invention described above is a maraging steel that is a ribbon having a thickness of 0.5 mm or less.

The maraging steel of the present invention can reduce and reduce oxide-based nonmetallic inclusions, and can further reduce the size of nitride-based nonmetallic inclusions such as TiC and TiCN. Therefore, excellent fatigue strength can be realized.
The maraging steel ribbon of the present invention is optimal as a component for a continuously variable transmission of an automobile engine.

The maraging steel of the present invention is produced by adding a specific amount of Ca to a consumable electrode used for vacuum remelting such as VAR and vacuum ESR. It is presumed that the effect of reducing or miniaturizing non-metallic inclusions by adding Ca to these consumable electrodes is based on the following.
When an appropriate amount of Ca is added, the oxygen present during dissolution in the consumable electrode manufacturing process is combined with Ca, which has a higher affinity than Al, which is the origin of alumina, which is a typical non-metallic inclusion, and a CaO system mainly composed of CaO. Many non-metallic inclusions are produced. And since the aggregation property of this CaO type nonmetallic inclusion is weaker than that of alumina, extremely large oxide type nonmetallic inclusions are reduced in the electrode. In addition, as an actual form of oxide-based non-metallic inclusions, it is often an Al—Ca—O-based composite non-metallic inclusion of CaO and Al ₂ O ₃ .
Further, along with the formation of a large number of weakly cohesive CaO, nitrides and carbonitrides are generated in the consumable electrode by forming nitrides and carbonitrides using CaO as a nucleus.

When vacuum remelting is applied to such a consumable electrode, Ca, which is a volatile element, evaporates in a high temperature region, and the composite non-metallic inclusions of CaO and CaO—Al ₂ O ₃ are decomposed. Diffusion into the phase and liquid phase occurs. That is, the reduction of oxide is promoted by the decomposition of CaO. Although some oxygen diffuses into the liquid phase, there are not many oxide-based nonmetallic inclusions newly generated by this oxygen, and as a result, the oxide-based nonmetallic inclusions are fine.
On the other hand, since nitride-based non-metallic inclusions such as TiN and TiCN are finely present in the consumable electrode with CaO as the nucleus, the thermal decomposition of the nitride-based non-metallic inclusions is promoted during remelting, resulting in the nitride. Miniaturization of non-metallic inclusions is also achieved.

By the above estimated action, it is possible to provide a maraging steel in which nonmetallic inclusions are remarkably reduced and refined compared to the maraging steel obtained by the prior art.
In maraging steel, fine intermetallic compounds such as Ti and Al are formed, and elements contributing to strengthening by precipitation are required, while these elements form non-metallic inclusions. I had an inevitable problem.
The development of a manufacturing technique using Ca discovered by the present invention is a very effective breakthrough technique that can achieve both a reduction effect and a miniaturization effect for both nitride and oxide.
Note that the vacuum remelting means remelting while evacuating.

In addition, in the manufacturing method which manufactures the maraging steel of this invention, 5 ppm or more of Ca is contained in a consumable electrode. This is because if the Ca content is less than 5 ppm, the effects of reduction and refinement of nonmetallic inclusions due to addition do not appear significantly.
The upper limit of the desirable Ca concentration at the consumable electrode is 150 ppm or less in consideration of the toughness of the steel ingot or product after remelting, and the above effect is obtained if it is 5 to 150 ppm, so the upper limit is preferably 150 ppm.
However, the addition of Ca, which is highly volatile, is low in yield and is not economical, and Ca is violently evaporated by re-dissolving in vacuum, which not only harms the operation but also may deteriorate the steel ingot skin. A preferable upper limit is 100 ppm. A more preferred range is from 10 to 50 ppm.

Moreover, in the manufacturing method which manufactures the maraging steel of this invention, Mg can be added with Ca in the range of 1 ppm or more in a consumable electrode as what has the effect similar to Ca.
The reason why the consumable electrode contains 1 ppm or more of Mg is that if the Mg content is less than 1 ppm, the effect of further reduction and refinement of inclusions due to the addition of Mg hardly appears. 5 ppm or more is desirable.
The upper limit of the Mg concentration at the desired consumable electrode is 300 ppm or less in consideration of the toughness of the steel ingot or product after remelting, and the upper limit is 250 ppm because the above effect can be obtained more reliably if it is 5 to 250 ppm. good.
However, the addition of Mg, which is highly volatile, is low in yield and is not economical, and Mg evaporates violently by vacuum remelting, which not only harms the operation but also worsens the steel ingot skin. A preferable upper limit is 200 ppm. A more preferred range is from 10 to 150 ppm.

In the manufacturing method for manufacturing the maraging steel of the present invention, it is desirable to apply a vacuum induction melting method (hereinafter referred to as VIM) for manufacturing the consumable electrode. This is because the melting raw material in the crucible is melted in a vacuum, so that the increase in oxygen in the atmosphere, oxides in the steel due to the reaction between nitrogen and molten steel, carbonitrides can be avoided, oxygen and active Ca and This is because it is advantageous for stably adding Mg to molten steel, and has a function of removing oxygen and nitrogen inevitably mixed from raw materials.
In particular, in the case of maraging steel, since it contains active Ti, it is better to avoid contact between the molten steel and the atmosphere as much as possible, and a VIM that can produce a consumable electrode in an environment cut off from the atmosphere. Application is optimal.
In addition, if it is a melting | dissolving installation which has the function which can prevent the contamination of the molten steel with the same function, ie, air | atmosphere, and can add Ca and Mg, it can also be substituted for VIM.

  The vacuum remelting method includes the electron beam remelting method in addition to the vacuum arc remelting method, but the electron beam remelting method has a high running cost and the temperature of the surface of the molten steel irradiated with the electron beam under high vacuum. In some cases, the component elements are selectively evaporated and the component control is difficult. In addition, the vacuum electroslag remelting method can obtain the effect of adding Ca and Mg as in the vacuum arc remelting method. However, since the evaporation phenomenon of Ca and Mg is suppressed by slag, and the Ca addition effect is reduced, the vacuum arc remelting method that can realize the Ca and Mg addition effect in the present invention is preferable.

When the maraging steel produced by the above-described method is applied to a continuously variable transmission part of an automobile engine, a strip of 0.5 mm or less is formed by plastic working such as hot rolling or cold rolling.
Oxide-based non-metallic inclusions remaining in the steel ingot after vacuum remelting can be made finer by being crushed or stretched and further shredded by plastic working etc. Become. For example, CaO produced by the addition of Ca and CaO-based oxide produced at the time of vacuum remelting are divided and refined by hot or cold plastic working.
When Mg is added in combination, in addition to the CaO inclusions described above, MgO and Al—Mg—O-based (MgO · Al ₂ O ₃ -based) spinel-based inclusion agglomerates formed during vacuum remelting are used. The aggregate is also divided and refined by hot or cold plastic working.
By combining this plastic working, it is particularly suitable as a maraging steel ribbon for continuously variable transmission parts having high fatigue strength.

In order to make it more suitable for a maraging steel ribbon for continuously variable transmission parts, at least 5 at 1000 to 1300 ° C. in either or both of the steel ingot state after vacuum remelting and / or after hot working. It is preferable to apply a homogenization heat treatment that keeps for more than a time and reduces segregation of components.
When the homogenization heat treatment is performed, component segregation can be further reduced. When the homogenization heat treatment is performed at a high temperature for a long time, the segregation of components is reduced. However, when the holding temperature exceeds 1300 ° C., surface oxidation is excessively promoted. On the other hand, if the temperature is lower than 1000 ° C, the effect is low.

  Further, if the holding time of the homogenization heat treatment is shorter than 5 hours, the effect of homogenization is low. Therefore, the holding time is preferably at least 5 hours or more. When component segregation is linearly analyzed by EPMA, the maximum and minimum values of Ti and Mo are measured, and the ratio (maximum value / minimum value) is calculated to be within a range of 1.3 or less. it can.

As described above, nitride-based non-metallic inclusions can be reduced by adding appropriate amounts of Ca and Mg. The following method is effective for obtaining this effect more reliably.
(1) Increasing the solidification rate at the time of manufacturing the electrode ingot,
(2) reducing the nitrogen concentration of the electrode steel ingot,
(3) adjusting the size of the non-metallic inclusions of nitrides and carbonitrides present in the electrode to a maximum of 10 μm or less,
It is effective to apply the above manufacturing methods singly or in combination.

As described above, in the maraging steel obtained by applying the manufacturing method to be described, by vigorous addition of Ca, it becomes a characteristic oxide-based nonmetallic inclusion form that is not found in conventional maraging steel, and nitride and charcoal Nitride-based non-metallic inclusions such as nitride are also miniaturized.
Specifically, it is very small, for example, it cannot be easily found even by observation with an electron microscope, but there is a non-metallic inclusion of CaO alone or the total number of oxide-based non-metallic inclusions of 10 μm or more. On the other hand, alumina inclusions of 10 μm or more are less than 70%.

This is because alumina inclusions can be confirmed about 80% in the case where Ca is not actively added at the time of consumable electrode production, but when the production method of the present invention is applied, the total amount of oxide-based nonmetallic inclusions of 10 μm or more is confirmed. This is very characteristic in that alumina inclusions of 10 μm or more are less than 67%. A more preferred range is a range of less than 50% of alumina inclusions of 10 μm or more, and a further preferred range is less than 30%.
Note that the oxide-based non-metallic inclusions of 10 μm or more are the non-metallic inclusions having a size that may particularly affect the fatigue strength, and the non-metallic inclusions that are too small. This is because it is difficult to confirm the number accurately.

The alumina-based inclusions referred to in the present invention are gases constituting non-metallic inclusions when non-metallic inclusions in the structure are subjected to qualitative / quantitative analysis using, for example, EDX (energy dispersive X-ray analyzer). Among components, O (oxygen) peak is detected mainly, and among the detected elements other than O, non-metallic inclusions in which Al is 85 mass% or more are mentioned.
The CaO inclusions and the above-mentioned spinel-based nonmetallic inclusions are detected mainly from the O (oxygen) peak among the gas components constituting the nonmetallic inclusions, and other than O are detected. Among elements, Al is less than 85 mass%, and refers to non-metallic inclusions in which Ca is detected. In addition, in spinel inclusions detected when Mg is positively added, Al is less than 85 mass%. Yes, it refers to non-metallic inclusions in which Mg is detected.

In addition to the above-mentioned ratio of alumina inclusions to the total number of oxide-based nonmetallic inclusions, in the case where the production method of the present invention is applied, the adjustment of the Ca addition amount and the production conditions of the electrode steel ingot In addition to adjustment, by combining VIM, VAR, etc., the maximum length of the oxide-based nonmetallic inclusions can be 20 μm or less, and the maximum length of the nitride-based nonmetallic inclusions can be 10 μm or less.
When the maximum length of oxide-based nonmetallic inclusions is 20 μm or less, the risk of starting fatigue failure can be reduced, and it is particularly suitable as a maraging steel ribbon for continuously variable transmission parts having high fatigue strength. Become.

If the maximum length of nitride-based nonmetallic inclusions is also 10 μm or less, the risk of starting fatigue failure can be further reduced, and it is particularly suitable as a maraging steel ribbon for continuously variable transmission parts having high fatigue strength. Become. Note that the maximum length of the nitride-based non-metallic inclusions can be reduced to 8 μm or less by adjusting appropriate Ca addition, composite addition with Mg, the above-described electrode ingot manufacturing conditions, and the like.
Note that the maximum length referred to in the present invention is evaluated by the diameter of a circle circumscribing the nonmetallic inclusion when the nonmetallic inclusion is an oxide type, and the diameter of the circumscribed circle is the maximum of the nonmetallic inclusion. Is defined as the length of However, since the nitride-based nonmetallic inclusion has a rectangular shape, the long side a and the short side b are measured, and the diameter of a circle corresponding to the area a × b is set as the maximum length.

Next, the reason for limiting the composition range of the maraging steel of the present invention will be described. Unless otherwise specified, it is expressed as mass%.
First, the reasons for limitation of Ca, O (oxygen), N (nitrogen) and Ti, which are defined as essential, will be described.
Ca is essential and added during the production of the electrode in the present invention, and remains as an essential component even when maraging steel is obtained after vacuum remelting. However, if Ca remains at 15 ppm or more, excessive residual Ca is not preferable in terms of toughness as maraging steel as a product or maraging steel as a material for plastic working, and the vacuum remelting of the present invention is applied. Therefore, Ca should be reduced to less than 15 ppm. For this purpose, the upper limit of Ca in the consumable electrode is preferably controlled to 150 ppm or less as described above, and it is necessary to make it less than 15 ppm when the maraging steel is obtained after vacuum remelting.

O (oxygen) is limited to less than 10 ppm because it forms oxide-based nonmetallic inclusions. When O is contained in an amount of 10 ppm or more, the fatigue strength is remarkably lowered, so the content was made less than 10 ppm.
N (nitrogen) is limited to less than 15 ppm in order to form nitrides and carbonitride nonmetallic inclusions. When N is contained in an amount of 15 ppm or more, the fatigue strength is remarkably lowered, so the content was made less than 15 ppm.
Ti is an indispensable element that contributes to strengthening by forming and precipitating fine intermetallic compounds by aging treatment, but if its content exceeds 2.0%, ductility and toughness deteriorate. Therefore, the Ti content is set to 2.0% or less.

Next, in addition to the above chemical composition, the reasons for limiting the components defined as a preferred range will be described.
Mg may be added together with Ca at the time of electrode production in the present invention. When Mg is added, it remains slightly when it is made into maraging steel after vacuum remelting. However, if Mg remains at 15 ppm or more, excessive residual Mg is not preferable in terms of toughness as maraging steel as a product or maraging steel as a material for plastic working, and the vacuum remelting of the present invention is applied. Therefore, it is preferable to reduce Mg to less than 15 ppm. For this purpose, the upper limit of Mg in the consumable electrode is preferably controlled to 250 ppm or less as described above, and is preferably less than 15 ppm when the maraging steel is obtained after vacuum remelting.

Since C forms carbides and decreases the precipitation amount of intermetallic compounds to reduce fatigue strength, the upper limit of C is set to 0.01% or less in the present invention.
Ni is an indispensable element for forming a matrix structure with high toughness, but if it is less than 8.0%, the toughness deteriorates. On the other hand, if it exceeds 22%, austenite is stabilized and it becomes difficult to form a martensite structure. Therefore, Ni is set to 8.0 to 22.0%.

Co does not greatly affect the stability of the martensite structure that is the matrix, but strengthens the precipitation by reducing the solid solubility of Mo and promoting the precipitation of Mo by forming fine intermetallic compounds. However, if the content is less than 5.0%, a sufficient effect is not necessarily obtained, and if it exceeds 20.0%, embrittlement tends to occur, so the Co content is 5.0 to 20.0%.
Mo is an element that contributes to strengthening by forming a fine intermetallic compound by aging treatment and precipitating in the matrix. However, when its content is less than 2.0%, its effect is small, and 9.0 If the content exceeds 50%, it becomes easy to form coarse precipitates containing Fe and Mo as main elements which deteriorate ductility and toughness. Therefore, the Mo content is set to 2.0 to 9.0%.
Al not only contributes to strengthening by aging precipitation, but also has a deoxidizing action, but if it exceeds 1.7%, toughness deteriorates, so its content is 1.7% or less. did.

In the present invention, elements other than these specified elements are substantially Fe. However, for example, B is an element effective for refining crystal grains, so a range of 0.01% or less that does not deteriorate toughness. You may make it contain.
Moreover, the impurity element contained unavoidable is contained. Among these, Si and Mn promote precipitation of coarse intermetallic compounds that cause embrittlement, thereby reducing ductility and toughness, and forming non-metallic inclusions to reduce fatigue strength. .1% or less, preferably 0.05% or less, and P and S also cause grain boundary embrittlement or non-metallic inclusions to reduce fatigue strength. The following should be used.

Hereinafter, the present invention will be described in more detail as examples.
A consumable electrode for VAR melting having six different Ca contents as representative components of maraging steel was manufactured by VIM, and a consumable electrode in which Mg was added in combination was also manufactured by VIM. In addition, a consumable electrode manufactured by VIM under the condition of no Ca addition was also manufactured as a comparative material. Consumable electrodes having the same mold size and mold ratio were used.
In order to clarify the influence of the addition of Ca on nitrides and carbonitrides, consumable electrodes with nitrogen concentrations adjusted to 5 ppm and 10 ppm were manufactured and vacuum remelted.

In VIM, raw materials are carefully selected and vacuum refining is performed, and the size of titanium carbonitride nonmetallic inclusions such as TiCN and TiN, which have a detrimental effect on the fatigue properties of maraging steel, as well as oxide nonmetallic inclusions. It controlled to 10 micrometers or less.
In the control method, the mold ratio at the time of electrode production was 2.5, and the solidification rate was increased by blast cooling of the mold after casting. As the raw material, a raw material having a low nitrogen content such as a nitrogen content of 15 ppm was used.

In addition to the treatment for these carbonitrides, Ca was added by a NiCa alloy to produce an electrode for VAR ingot formation.
As for the addition of Ca, there is a method of directly adding Ca alloys such as Ni-Ca and Fe-Ca and metallic Ca to molten steel, but this time it is easy to handle and the adjustment of the Ca components makes Ni easy. Addition with -Ca alloy was performed.
As for the addition of Mg, there is a method of directly adding Mg alloys such as Ni-Mg and Fe-Mg and metal Mg to molten steel, but this time it is easy to handle and the Mg components can be easily adjusted. From this, addition with a Ni-Mg alloy was performed.

These VIM-made electrodes were remelted using VAR under the same conditions to produce steel ingots. The same VAR molds were used, the degree of vacuum was 1.3 Pa, and the input current was melted at 6.5 KA in the stationary part of the steel ingot.
Table 1 shows the chemical composition of the consumable electrode manufactured by VIM and the steel ingot obtained by vacuum remelting the electrode with VAR. The consumable electrode is indicated as “electrode”, and the post-VAR electrode is indicated as “steel ingot”.

The obtained steel ingot after VAR was soaked at 1250 ° C. for 20 hours, and then hot forged to obtain a hot forged product.
Next, hot rolling, solution treatment at 820 ° C. × 1 hour, cold rolling, solution treatment at 820 ° C. × 1 hour and aging treatment at 480 ° C. × 5 hours are performed on these materials, and the thickness is 0.5 mm. A maraging steel strip was produced.
First, no. 1-No. 100 g of a cross-section sample was collected from both ends of the maraging steel strip of No. 7, dissolved in a mixed acid solution or bromine-methanol solution, etc., filtered through a filter, and the oxide residue on the filter was observed with SEM. The composition and size of the system non-metallic inclusions were measured.
In measuring the sizes of these nonmetallic inclusions, the diameter of the circle circumscribing the nonmetallic inclusions was taken as the maximum length of the nonmetallic inclusions.

Furthermore, in order to evaluate nitrides and carbonitrides in detail, 10 g was collected, dissolved in a mixed acid solution or bromine-methanol solution, etc., and the filter area of the filter was reduced to increase the density of nitrides and carbonitrides. The maximum size was measured by observing 10,000 nitrides and carbonitrides with SEM.
Since nitride or the like has a rectangular shape, the long side a and the short side b were measured, and the diameter of a circle corresponding to the area a × b was set as the maximum length. Note that the oxide-based non-metallic inclusions had the maximum length of the non-metallic inclusions as the diameter of the circle circumscribing the non-metallic inclusions as described above.
The results are shown in Table 2.

From Table 2, when the value of the consumable electrode Ca exceeds 5 ppm, there is no oxide-based nonmetallic inclusion exceeding 20 μm in the maraging steel strip, and the size decreases as the electrode Ca content increases. I can see the trend.
The composition of oxide-based nonmetallic inclusions observed in this evaluation is mainly composed of CaO-based composite oxide and CaO according to the present invention, and alumina-based inclusions of 10 μm or more in Table 2 Most of the oxide-based nonmetallic inclusions other than the above were the above-mentioned spinel-based inclusions, MgO. The comparative examples mainly consisted of alumina inclusions.

Further, from Table 2, regarding the maximum size of nitride and the like, when the electrode nitrogen concentration is 5 ppm, the size of the nitride becomes 2 to 3 μm fine by adding Ca, and when the electrode nitrogen concentration is 10 ppm, the nitride and the like by adding Ca It can be seen that the size of is smaller by 3 to 4 μm.
In addition, the composition is almost No. In the case where Mg is added in combination with Ca in accordance with No. 1, No. 1 is added by adding Mg in combination. It can also be seen that the alumina-based inclusion ratio of 10 μm or more is reduced as compared with 1, and the maximum length of the nonmetallic inclusion is shortened.
In addition, No. The cross sections of 1 to 6 maraging steel strips were analyzed with EPMA for the maximum and minimum values of Ti and Mo, and the ratio (maximum value / minimum value) was calculated. Was 1.3 or less.

From the above results, the maraging steel of the present invention can reduce and reduce the number of oxide-based nonmetallic inclusions, and can further reduce the size of nitride-based nonmetallic inclusions such as TiC and TiCN. It can also be seen that it has excellent fatigue strength.
The maraging steel ribbon of the present invention is optimal as a component for a continuously variable transmission of an automobile engine.

Claims (5)

A maraging steel containing at least Ca: less than 15 ppm (excluding 0), oxygen: less than 10 ppm, nitrogen: less than 15 ppm, Ti: 2.0% or less (excluding 0) in mass%, 10 μm or more of oxide-based nonmetallic inclusions containing 85 mass% or more of Al among the metal elements in the oxide-based nonmetallic inclusions with respect to the total number of oxide-based nonmetallic inclusions of 10 μm or more Is a maraging steel characterized by being less than 70%. The maraging steel according to claim 1, wherein the maximum length of the oxide-based nonmetallic inclusion is 20 μm or less. The maraging steel according to claim 1 or 2, wherein the maximum length of nitride-based nonmetallic inclusions is 10 µm or less. The maraging steel according to any one of claims 1 to 3 is, in mass%, in addition to a chemical composition of Ca: less than 15 ppm (excluding 0), oxygen: less than 10 ppm, and nitrogen: less than 15 ppm. Mg: less than 15 ppm (excluding 0), C: 0.01% or less, Ni: 8.0-22.0%, Co: 5.0-20.0%, Mo: 2.0-9. Maraging steel characterized in that 0%, Ti: 2.0% or less (excluding 0), Al: 1.7% or less, and the balance being substantially made of Fe. The maraging steel according to any one of claims 1 to 4, wherein the maraging steel is a ribbon having a thickness of 0.5 mm or less.