JP6347405B2 - Method for producing maraging steel - Google Patents

Description

  The present invention relates to a method for producing 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 provide 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.
In recent years, this maraging steel has been used for continuously variable transmission belts of automobiles. The continuously variable transmission belt is required to have high strength and high fatigue strength.

By the way, maraging steel contains Mo which tends to cause component segregation during melting and solidification. When component segregation increases, there is a risk of variation in mechanical characteristics within the continuously variable transmission belt. In some cases, Ti that forms non-metallic inclusions (hereinafter referred to as “inclusions”) of nitrides or carbonitrides such as TiN or TiCN that deteriorate fatigue strength may be contained.
As invention which implement | achieves both prevention of such component segregation and inclusion refinement | miniaturization, for example, Unexamined-Japanese-Patent No. 2001-64755 (patent document 1) has the circumference corresponded to the circumference of the steel ingot top part. The diameter of the equivalent circle is D1, the diameter of the equivalent circle having a circumference corresponding to the circumference of the steel ingot bottom is D2, the height of the steel ingot is H, and the circumference corresponding to the circumference of the steel ingot at the H / 2 position is When the diameter of the equivalent circle is D and the long side length and short side length of the steel ingot at the H / 2 position are W1 and W2, respectively, the taper Tp = (D1−D2) × 100 / H is 5.0. ~ 25.0%, high diameter ratio Rh = H / D is 1.0 to 3.0, flat ratio B = W1 / W2 is 1.5 or less, C: 0.01% or less, Ni: 8 to 19%, Co: 8 to 20%, Mo: 2 to 9%, Ti: 0.1 to 2%, Al: 0.15% or less, N: 0. Casting a molten steel containing 03% or less, O: 0.0015% or less and the balance substantially having a chemical component of Fe, the size of inclusions is 30 μm or less, the Ti component segregation ratio and the Mo component segregation ratio are There has been proposed a method for producing maraging steel having excellent fatigue characteristics of 1.3 or less.

JP 2001-64755 A

The method proposed in Patent Document 1 described above is to produce maraging steel for machine structure having excellent productivity with ordinary melting equipment without performing vacuum arc remelting (hereinafter referred to as VAR). However, in the maraging steel which does not perform VAR, component segregation is easy and there is a problem in intervening articles, and it is not applied to a continuously variable transmission belt of an automobile.
The objective of this invention is providing the manufacturing method of the maraging steel which can suppress a component segregation more reliably.

The present invention has been made in view of the above-described problems.
That is, the present invention
A primary melting step to obtain a consumable electrode of maraging steel;
Remelting step of obtaining a steel ingot by performing vacuum arc remelting using the consumable electrode;
In the manufacturing method of maraging steel containing
During the remelting step, He gas is introduced between the mold and the steel ingot at a pressure of 0.9 kPa or more and less than 1.9 kPa,
It is a manufacturing method of maraging steel which makes the depth of a molten steel pool 170mm or less.

  According to the present invention, component segregation can be more reliably suppressed.

It is a schematic diagram which shows an example of the structure of VAR which introduce | transduces He gas of this invention. It is a schematic diagram which shows an example of the outline | summary of the electromagnetic stirring apparatus of VAR which introduce | transduces He gas of this invention.

The present invention is described in detail below.
As described above, VAR is applied in the present invention. Therefore, first, a consumable electrode for VAR is manufactured as primary melting.
VIM (vacuum induction melting) is suitable for the production of a consumable electrode of extremely low C steel such as maraging steel. If VIM is applied, it is possible to avoid the increase of oxygen in the atmosphere, oxides in the steel due to the reaction between nitrogen and molten steel, nitrogen carbides, and stable addition of oxygen and active alloy elements to the molten steel. This is because it is advantageous in that it has a function of removing oxygen and nitrogen inevitably mixed from raw materials, and is optimal for the production of consumable electrodes.

In the primary melting of the present invention, it is desirable to obtain a consumable electrode of maraging steel having Mg oxide. This is because, when maraging steel contains Ti, Ti-based inclusions are formed. This Ti inclusion is easy to crystallize with an oxide mainly composed of MgO as a nucleus, and thus can be in the form of a Ti inclusion / MgO complex. Furthermore, Ti inclusions can be present in the consumable electrode in a finely dispersed form. Thereby, Ti-based inclusions in the consumable electrode obtained in the primary melting step can be finely dispersed.
In order to make the oxide in the remelting consumable electrode mainly composed of Mg oxide, the amount of Mg added at the time of primary vacuum melting is preferably 10 to 200 ppm.
Further, when performing VAR using the consumable electrode, the vacuum degree is set to a reduced pressure atmosphere as much as possible to promote Mg evaporation from the molten steel surface during remelting.
The disappearance of the MgO portion constituting a part of the Ti-based inclusion-MgO complex causes the Ti-based inclusions to be finely decomposed and promotes thermal decomposition to completely melt the Ti-based inclusions in the molten steel. it can. That is, if the Ti-based inclusions can be completely melted, the size of the Ti-based inclusions depends on the growth during solidification in VAR. Therefore, the effect of introducing the He gas of the present invention can be sufficiently exerted.

Next, in the present invention, a remelting step is performed to obtain a steel ingot by performing vacuum arc remelting using the consumable electrode. During the remelting step, it is necessary to introduce He gas at a pressure of 0.9 kPa or more and less than 1.9 kPa between the mold and the steel ingot so that the depth of the molten steel pool is 170 mm or less.
By the way, VAR can increase the solidification rate of the steel ingot after remelting by using a water-cooled copper crucible, and can prevent component segregation and solidification defects. On the other hand, the solidified steel ingot shrinks to create a gap with the copper crucible (mold), and the heat removal effect decreases. When the heat removal effect is reduced, the solidification rate is reduced, and the effect of suppressing component segregation is also reduced. Therefore, in the present invention, the gap between the contracted steel ingot and the copper crucible is filled with He gas. The filled He gas has a thermal conductivity of approximately 0.144 W / mK, and the He gas acts as a medium for heat transfer and exerts an effect on heat removal from the steel ingot.
In addition, in maraging steel containing Ti, Ti-based inclusions formed in the steel have a high melting point, so that some of them remain undissolved when the consumable electrode is remelted and exist as a solid in the molten steel pool. To do. And it grows when a molten steel pool solidifies and it becomes a steel ingot. If the cooling rate can be increased, the growth time of the Ti-based inclusions is shortened, so that the Ti-based inclusions can be miniaturized. However, even if the speed at which the consumable electrode is melted is changed in the conventional VAR, it is difficult to greatly change the cooling rate during solidification if the diameter of the steel ingot is the same. This is because, in VAR, after the steel ingot solidifies and shrinks and a gap is formed between the steel ingot and the water-cooled copper mold, the conduction heat transfer is cut off, and the gap generated between the steel ingot and the copper crucible is a reduced pressure atmosphere. This is because convective heat transfer hardly occurs, and heat is extracted only by radiant heat transfer.

Although it is difficult to greatly change the cooling rate during solidification even if the rate at which the consumable electrode is melted in the VAR with the same steel ingot diameter as described above, it is difficult to change the cooling rate during solidification. It is possible to reduce the depth. The reason for this is that the amount of molten metal supplied from the consumable electrode decreases, that is, the hot water rising speed decreases. When the hot water rising speed decreases, the solidifying pool speed increases, so the molten steel pool depth decreases. However, if the dissolution rate is slowed down, the time for dissolving the consumable electrode becomes longer and the productivity deteriorates, and the power consumption rate also deteriorates. Therefore, it is not desirable to slow down the consumable electrode too slowly. Therefore, filling the He gas and reducing the depth of the molten steel pool without changing the melting rate is a very advantageous method for improving productivity.
In addition, it is effective for suppression of a component segregation to make the molten steel pool depth shallow in VAR. When the time to pass through the liquidus line and the solidus line is long, the time until solidification becomes long, so that component segregation is likely to occur. When the depth of the molten steel pool becomes shallower, the time for passing through the liquidus and solidus becomes shorter, so that component segregation can be effectively suppressed. In particular, the top part of the steel ingot in VAR is a part corresponding to the feeder of the ordinary steel ingot, and the time until solidification is longer than other parts, so when the molten steel pool depth becomes smaller, the component segregation at the top part of the steel ingot , And the number of parts that are discarded as out of specification can be reduced to improve the yield.
The same effect as He gas can be obtained with Ar gas and nitrogen gas, but Ar gas has low thermal conductivity, and nitrogen gas promotes the formation of nitride inclusions. Therefore, He gas is used in the present invention.
He gas filling is performed only in the gap between the contracted steel ingot and the copper crucible. For example, the steel ingot near the molten steel pool and the copper crucible are in contact, but when the He gas reaches the area where the consumable electrode is melted beyond the contact state, the arc becomes unstable and inclusions increase. There is a risk of causing.

As described above, in the present invention, He gas is used as the gas introduced into the gap between the steel ingot and the mold. Since the He gas does not chemically react with the molten steel and the steel ingot, there is no risk of forming new inclusions, and the risk of an explosion accident due to a chemical reaction can be avoided. Further, when He gas is used, He gas containing an impurity gas with such a degree that the chemical reaction between the molten steel and the steel ingot can be ignored can be used. The ratio is preferably 99.9% by volume or more.
FIG. 1 is a schematic diagram showing an example of the structure of a VAR in which He gas of the present invention is introduced. The introduction pressure of He gas can be controlled by measuring the pressure in the pipe for sending the gas from the gas cylinder to the water-cooled copper mold 4 with the pressure measuring device 6 and installing the pressure control valve 7. Basically, by increasing the He gas pressure, the heat capacity per unit volume of the gas can be increased and the effect of convective heat transfer can be enhanced, but the He gas pressure between the mold and the steel ingot is less than 0.9 kPa. The effect of convective heat transfer is low, and the effect of increasing the cooling rate is poor. Moreover, the pressure of He gas also affects the depth of the molten steel pool, and if it is less than 0.9 kPa, the pool depth increases and inclusions tend to grow, and component segregation tends to increase. Therefore, the lower limit of the He gas pressure is 0.9 kPa. A preferable lower limit of the pressure of He gas is 1.2 kPa. In addition, since VAR always operates in a reduced-pressure atmosphere, even if the pressure of the rare gas introduced into the gap between the steel ingot and the mold is increased, it is exhausted by the vacuum pump. Not only is it difficult to increase the pressure, it is also difficult to stabilize the gas pressure. Therefore, the upper limit of the pressure of He gas is set to 1.9 kPa. A preferable upper limit of the pressure of He gas is 1.6 kPa.

  Moreover, in this invention, the depth of a molten steel pool can be made shallow by 10 mm or more compared with the case where it does not fill with He gas by adjustment of He gas pressure mentioned above. If the molten steel pool becomes excessively deep, the time until solidification becomes longer and inclusions tend to grow, and the component segregation tends to increase. According to the present invention, the depth of the molten steel pool can be 170 mm or less. According to the study of the present inventor, the aforementioned He gas pressure is required to be 0.9 kPa or more in order to reduce the depth of the molten steel pool by 10 mm or more. In order to reduce the pool depth, it is better to increase the pressure of the He gas to be filled. However, even if the convective heat transfer effect between the steel ingot and the mold is enhanced, the heat removal of the steel ingot itself is hindered. Therefore, there is a limit to the effect of reducing the depth of the molten steel pool by introducing He gas. As described above, even if the pressure of the He gas is excessively high, it becomes difficult to enhance the convective heat transfer effect, so the practical lower limit of the depth is about 120 mm at most.

Moreover, the manufacturing method of maraging steel prescribed | regulated by this invention is especially effective with respect to a steel ingot diameter of 300-800 mm. The reason is that as the steel ingot diameter increases, the influence of the thermal resistance of the steel ingot itself becomes larger than the influence of convective heat transfer between the steel ingot and the mold, and the cooling rate of the steel ingot depends on the steel ingot diameter. The tendency of the steel ingot having a smaller thermal conductivity becomes stronger, and the effect of improving the cooling rate of the steel ingot becomes remarkable when the steel ingot diameter is 300 mm or more. On the other hand, if it exceeds 800mm, noble gas is introduced to enhance the convective heat transfer effect between the steel ingot and the mold, and heat removal is hindered by the thermal resistance of the steel ingot itself, improving the cooling rate to the center of the steel ingot The effect to make becomes easy to become small. Therefore, the steel ingot diameter is preferably set to 300 to 800 mm.
As described above, the manufacturing method according to the present invention described above enables heat removal by convective heat transfer between the steel ingot and the mold by introducing He gas from the gas introduction nozzle into the gap between the steel ingot and the mold. The cooling rate inside is increased and the depth of the molten steel pool is reduced. As a result, it is possible not only to maximize the component segregation preventing effect of VAR but also to suppress the growth of inclusions during VAR.

By the way, some maraging steels do not contain Ti, but even Ti-added maraging steels are sufficient if, for example, Mo that easily segregates is actively added. The component segregation preventing effect of the invention can be obtained. Of course, when the production method of the present invention is applied to maraging steel containing Ti in addition to Mo, both the suppression effect and the inclusion refinement effect can be obtained, which is particularly effective. A preferred specific maraging steel composition is as follows. In addition, content is described as mass%.
Ti is an element that contributes to strengthening by forming and precipitating fine intermetallic compounds by aging treatment. Therefore, it can be added with 3.0% as the upper limit. In the case of maraging steel to which Ti is positively added, a preferable lower limit for obtaining the effect of Ti is 0.2%.
O (oxygen) is an element that forms oxide inclusions. It is desirable to reduce the amount of oxygen that becomes oxide inclusions. Therefore, O should be limited to less than 0.001%.
N (nitrogen) is an element that forms nitrides and carbonitride inclusions. In the present invention, nitride inclusions can be miniaturized, but it is desirable to reduce the amount of nitrogen that becomes nitride inclusions. Therefore, N should be limited to less than 0.0015%.
C (carbon) forms carbides and carbonitrides, reduces the precipitation amount of intermetallic compounds and lowers fatigue strength, so C is preferably 0.01% or less.

Ni is an indispensable element for forming a tough matrix structure. However, if it is less than 8%, the toughness deteriorates. On the other hand, if it exceeds 22%, austenite becomes stable and it becomes difficult to form a martensite structure. Therefore, Ni is preferably 8 to 22%.
Co does not greatly affect the stability of the matrix martensite structure, but strengthens precipitation by reducing the solid solubility of Mo and promoting the precipitation of Mo by forming fine intermetallic compounds. Is an element that contributes to However, if the content is less than 3%, sufficient effects are not necessarily obtained, and if the content exceeds 20%, embrittlement tends to occur, so the Co content is preferably 3 to 20%.
Mo is an element that contributes to strengthening by forming a fine intermetallic compound by aging treatment and precipitating it in the matrix. However, if the content is less than 2%, the effect is small, and if it exceeds 9%, coarse precipitates that deteriorate ductility and toughness are easily formed, so the Mo content is reduced to 2 to 9%. Good.
Al not only contributes to strengthening by aging precipitation, but also has a deoxidizing action, so it is better to contain 0.01% or more, but if it exceeds 1.7%, toughness will deteriorate. Therefore, the content is preferably 1.7% or less.
Other than the above elements, Fe may be substantially used. For example, B is an element effective for refining crystal grains, and therefore may be contained in a range of 0.01% or less to the extent that toughness does not deteriorate. Good.
Inevitable impurity elements are contained.

Example 1
The present invention will be described in detail as Example 1. Five consumable electrodes for redissolving were produced in the primary melting step. Table 1 shows the He gas pressure conditions when each of the five was VAR. Four of the five remelting electrodes 1 introduced He gas having a He ratio of 99.9% by volume or more between the steel ingot 3 and the water-cooled copper mold 4 when remelted by VAR. Examples of the present invention are as follows. 1, 2, 3 and Comparative Examples It was set to 11. The remaining one did not introduce He gas between the steel ingot 3 and the water-cooled copper mold 4 when the remelting electrode was VARed. This comparative example is No. It was set to 12. Although the He gas pressure was attempted to be 1.9 kPa or higher, the pressure was not stable in a reduced-pressure atmosphere. Therefore, in the examples, the He gas pressure was set to 1.60 kPa. The diameter of the steel ingot of the inventive example and the comparative example was 500 mm. The dissolution conditions for current, voltage, and dissolution rate other than the He gas introduction pressure were the same.

For cooling with He gas, a remelting electrode 1 was installed using a vacuum arc remelting furnace shown in FIG. During melting, He gas is introduced into the gap between the steel ingot 3 and the mold 4 from the gas introduction nozzle 5 installed at the lower part of the water-cooled copper mold 4. The pressure in the pipe for sending gas from the He gas cylinder to the mold 4 was measured by the pressure measuring device 6, and the He gas pressure set by installing the pressure control valve 7 was always controlled to be constant. The introduced He gas was filled in the gap between the steel ingot 3 and the water-cooled copper mold 4 to remove heat from the steel ingot 3, and the gas leaked from the gap was finally discharged to the outside by a vacuum pump (not shown).
During the dissolution, the piping valve 8 installed in the piping was opened, and after confirming that the set He gas pressure was controlled, the dissolution of the electrode for re-dissolution was continued. Invention Example No. The He gas pressure in the pipe set in 1. was 1.60 kPa, and Example No. of the present invention. 2 was 1.33 kPa, Inventive Example 3 was 0.93 kPa, and Comparative Example 11 was 0.53 kPa. After the dissolution of the electrode was finished, the pipe valve 8 installed in the pipe was closed, and the set value of the pressure control device was set to zero. Invention Example No. 1, 2, 3 and Comparative Example No. Table 2 shows the compositions of the remelting electrodes 11 and 12.

In order to make it easy to discriminate the shape of the molten steel pool, when all of the five consumable electrodes were melted and the weight of the steel ingot reached 2100 kg, a magnetic field was applied to generate electromagnetic stirring in the molten steel pool. The dendritic tip of the solidification interface is cut by electromagnetic stirring, and equiaxed crystallization is promoted along the solidification interface. FIG. 2 shows an outline of the electromagnetic stirring device.
A magnetic field application coil 10 is wound around a SUS water-cooled jacket 9 containing the water-cooled copper mold 4, and an external magnetic field is applied by passing an electric current through the coil 10. As a result, a Lorentz force is generated and stirred in the molten steel pool. As a magnetic field application pattern, positive and negative were reversed at a cycle of 40 s with a magnetic flux density of 20 × 10 ⁻⁴ T. The time of electromagnetic stirring at the location where the steel ingot weight was 2100 kg was 5 min. Equiaxial crystallization is promoted by reversing the sign of the magnetic flux density at a constant period. After completion of electromagnetic stirring, dissolution was stopped after 30 min. After the dissolution was stopped, the steel ingot was taken out after 60 min.

Next, in order to discriminate the depth of the molten steel pool from the top of the maraging steel ingot remelted by VAR, the ingot was cut along the center line, and a longitudinal section slice sample was collected. The cut surface of the longitudinal section slice sample was polished and macro-corroded to confirm the molten steel pool shape.
A solution obtained by diluting industrial hydrochloric acid in an equal volume of water was used as the corrosive liquid. From the corroded vertical slice, the distance from the molten steel pool shape to the bottom when the electromagnetic stirring occurred was measured as the molten pool depth. The depth of the molten steel pool here is that at the 2100 kg weight position from the bottom of the steel ingot where electromagnetic stirring was generated. Table 3 shows the depths of the molten steel pools of Invention Examples 1, 2, 3 and Comparative Examples 1, 2.

From Table 3, the present invention example No. in which He gas was introduced. In 1, 2, 3, and Comparative Example 11, the depth of the molten steel pool was shallower than that of Comparative Example 12 in which He gas was not introduced. However, the depth of the molten steel pool of Comparative Example 11 was only about 5 mm shallower than that of Comparative Example 12, and the effect of reducing the depth of the molten steel pool by introducing He gas was small. Even if the molten steel is melted under the same conditions, the depth of the molten steel pool may fluctuate by several millimeters. Therefore, in Comparative Example 11, it could not be clearly said that there was a sufficient effect by introducing He gas.
On the other hand, Invention Example No. In 1, 2 and 3, the depth of the molten steel pool was shallower by 10 mm or more than that of Comparative Example 12. It was confirmed that the effect of reducing the molten steel pool was great when the He gas introduction pressure was 0.9 kPa or more. Furthermore, the depth of the molten steel pool became shallower as the gas introduction pressure was increased up to 1.6 kPa. In particular, when the pressure was increased from 0.9 kPa to 1.33 kPa, the reduction width of the molten steel pool depth increased with respect to the increase width of the gas pressure.
Therefore, the He gas introduction pressure needs to be 0.9 kPa or more in order to make the molten steel pool depth remarkably shallower than those of Invention Examples 1, 2, and 3. Furthermore, it can be seen that increasing the pressure from 0.9 kPa to 1.33 kPa has a greater effect of reducing the depth of the molten steel pool, so that the He gas introduction pressure should desirably be 1.2 kPa or higher.

The depth of the molten steel pool is determined by the balance between the amount of heat input from the consumable electrode 1 and the amount of heat removed from the molten steel pool 2 and the steel ingot 3 to the copper mold 4. Invention Example No. Since the arc heat input amounts of 1, 2, 3 and Comparative Examples 11 and 12 are almost the same, when the He gas is introduced, the depth of the molten steel pool becomes shallow. It can be said that the amount of heat removed to the mold 4 has increased. That is, when He gas is not introduced, the gap between the steel ingot 3 and the copper mold 4 is a reduced pressure atmosphere, so the amount of heat removal is small due to convective heat transfer, but the He gas is introduced into the gap and becomes a heat medium. It is thought that the amount of heat removed by convective heat transfer increased. It is considered that the heat capacity per unit volume of the He gas existing in the gap between the steel ingot 3 and the copper mold 4 is increased by setting the He gas introduction pressure to a certain amount or more.
Therefore, when the pressure of He gas is set to 0.9 kPa or more, the heat removal amount of the convection heat transfer increases, and the molten steel pool depth becomes remarkably shallow. However, the inside of the VAR furnace is in a reduced pressure atmosphere by a vacuum pump, and if the He gas introduction pressure is increased, it leaks from the contact portion between the steel ingot 3 and the copper mold 4 and is exhausted. is there. In Example 1, since gas pressure was not stabilized when it exceeded 1.9 kPa, it was judged that the operation of the pressure exceeding 1.9 kPa was difficult.

(Example 2)
Three consumable electrodes for redissolving were produced in the primary dissolving step. Table 4 shows the He gas pressure conditions when each of the three was subjected to VAR. Two of the three remelting electrodes 1 introduced He gas having a He ratio of 99.9% by volume or more between the steel ingot 3 and the water-cooled copper mold 4 when remelted by VAR. This invention example is No. 4 and 5. For the remaining one, no He gas was introduced between the steel ingot 3 and the water-cooled copper mold 4 when the remelting electrode was remelted by vacuum arc. This comparative example is No. It was set to 13. The diameter of the steel ingot of the present invention example and the comparative example was 800 mm. The dissolution conditions for current, voltage, and dissolution rate other than the He gas introduction pressure were the same.

  The same apparatus as in Example 1 was used for cooling with He gas. Invention Example No. The He gas pressure in the pipe set in No. 4 is 1.33 kPa, and the present invention example No. 5 was 1.20 kPa. After the dissolution of the electrode was finished, the pipe valve 8 installed in the pipe was closed, and the set value of the pressure control device was set to zero. Invention Example No. 4, 5 and Comparative Example No. Table 5 shows the composition of 13 redissolving electrodes.

  In order to make it easy to distinguish the molten steel pool shape, electromagnetic stirring was performed as in Example 1. In all three, when the consumable electrode was melted and the weight of the steel ingot reached 6800 kg, a magnetic field was applied to generate electromagnetic stirring in the molten steel pool. The diameter of the steel ingot used in Example 2 was 800 mm, and it was necessary to dissolve the steel ingot weight more than the steel ingot with a diameter of 500 mm before the molten steel pool depth was stabilized. The magnetic field application pattern is the same as in the first embodiment.

  Next, in order to discriminate the depth of the molten steel pool from the top of the maraging steel ingot remelted by VAR, the ingot was cut along the center line and a longitudinal section slice was collected. The corrosion method for the longitudinal slice and the method for measuring the depth of the molten steel pool are the same as in Example 1. The depth of the molten steel pool here is that at a weight position of 6800 kg from the bottom of the steel ingot where electromagnetic stirring is generated. Table 6 shows the depths of the molten steel pools of Invention Examples 4 and 5 and Comparative Example 13.

  From Table 6, the present invention example No. in which He gas was introduced. No. In Nos. 4 and 5, the molten steel pool depth was shallower by 50 mm or more than Comparative Example 13 in which no He gas was introduced. In the case of a steel ingot diameter of 800 mm, it was confirmed that the effect of reducing the molten steel pool was great when the He gas introduction pressure was 1.2 kPa or more. Furthermore, if the gas introduction pressure was increased from 1.2 kPa to 1.3 kPa, the molten steel pool depth became shallower. From the above, it was confirmed that the molten steel pool depth becomes shallow by introducing He gas into the gap between the steel ingot 3 and the copper mold 4 even in the case where the steel ingot diameter is 800 mm larger than Example 1.

  From the above, when the maraging steel is produced by vacuum arc remelting, the depth of the molten steel pool can be reduced by introducing He gas into the gap between the steel ingot and the mold. In this case, by selecting an appropriate gas introduction pressure, it is possible to increase the cost effectiveness in a manufacturing process using expensive He gas. As a result, it is possible to stabilize the quality and characteristics of maraging steel products by suppressing component segregation in maraging steel and making inclusions finer.

1 Consumable electrode for remelting 2 Molten steel pool 3 Steel ingot 4 Water-cooled copper mold 5 Gas introduction nozzle 6 Pressure measuring instrument 7 Pressure control valve 8 Piping valve 9 Water-cooled jacket 10 made of SUS Magnetic field coil

Claims (1)

A primary melting step to obtain a consumable electrode of maraging steel;
Remelting step of obtaining a steel ingot by performing vacuum arc remelting using the consumable electrode;
In the manufacturing method of maraging steel containing
During the remelting step, He gas is introduced between the mold and the steel ingot at a pressure of 0.9 kPa or more and less than 1.9 kPa,
The depth of a molten steel pool shall be 170 mm or less. The manufacturing method of the maraging steel characterized by the above-mentioned.