JP7471946B2 - Manufacturing method of titanium ingot - Google Patents

Description

This invention relates to a method for producing titanium-based ingots by melting a raw material containing titanium sponge and casting the resulting molten metal.

When manufacturing titanium-based ingots containing titanium, for example, an electron beam melting furnace may be used to melt the raw materials and cast them. Titanium-based ingots can be cylindrical or prismatic, with a polygonal cross section perpendicular to the longitudinal direction. Prismatic titanium-based ingots in particular include slabs with a rectangular cross section that can be subjected to hot rolling.

One example of this type of technology is described in Patent Document 1. Patent Document 1 aims to "provide a technology for supplying powdered alloy raw materials and granular metal raw materials to an electron beam melting furnace with good yield and uniformity," and proposes "a method for producing metal ingots using an electron beam melting furnace, which is characterized by mixing lump oxide and granular metal, and supplying the mixture to the electron beam melting furnace as the melting raw materials."

As described in Patent Document 1, the raw material melted when producing titanium-based ingots may contain titanium sponge obtained by the Kroll process or the like. In the Kroll process, titanium tetrachloride is reduced with metallic magnesium to produce a titanium sponge mass. At this time, magnesium chloride is produced as a by-product of the reduction reaction. The titanium sponge mass is crushed to obtain the above-mentioned titanium sponge in granular form or the like. Depending on the part of the titanium sponge mass from which it was extracted, the titanium sponge may have a relatively high chlorine content due to the magnesium chloride, etc.

International Publication No. 2008/078402

However, when titanium sponge with a relatively high chlorine content is used as a melting material to cast a titanium-based ingot, pores may be formed inside the surface layer of the titanium-based ingot. When a titanium-based ingot is subjected to rolling or other processing, cutting may be performed beforehand, and if internal pores appear on the surface as a result of this cutting, hot rolling cannot be performed as is. To perform hot rolling, further cutting is required until the pores that appear on the surface disappear, which is a heavy workload. Also, if there are too many internal pores, the ingot may not be usable as a product.
For this reason, titanium sponge, which has a relatively high chlorine content, has not been used in the manufacture of titanium-based ingots.

The object of this invention is to provide a method for producing titanium-based ingots that can suppress the occurrence of internal pores in the surface layer of the titanium-based ingot, even when using titanium sponge with a relatively high chlorine content as the melting raw material.

After extensive research, the inventors came to the following conclusion as to why numerous internal pores occur on the surface of titanium-based ingots when sponge titanium with a relatively high chlorine content is used as the melting raw material.

If the chlorine contained in the titanium sponge is derived from magnesium chloride, it is assumed that such sponge also contains water due to the hygroscopicity of magnesium chloride. Among the components that make up water, hydrogen dissolves in the molten metal when the raw material is melted, and can become gas in the molten metal due to a decrease in solubility caused by cooling during subsequent casting. During casting, the temperature of the molten metal decreases as the metal solidifies, and gas is likely to be generated at a position that is somewhat distant from the molten metal surface in the depth direction. In addition, since the molten metal is cooled from the side close to the inner surface of the molten metal in the casting mold, the time from gas generation to solidification of the molten metal is thought to be shorter in the area that will become the surface layer of the ingot. Conventionally, the surface temperature of the molten metal in the casting mold was relatively low, so the molten metal solidified at a temperature below its freezing point while the above-mentioned gas in the molten metal was trying to escape from the surface on the inner surface side of the mold, which is thought to have been the cause of pores in the surface layer of titanium-based ingots. Furthermore, since the cooling rate is relatively slow near the center of the molten metal, away from the inner surface of the mold, it is believed that even if gas is generated near the center, the gas can escape from the surface of the molten metal.

In response to this, it has been found that if the surface temperature of the molten metal in the mold is raised above a predetermined temperature, the molten metal will not solidify until the above-mentioned gases in the molten metal have sufficiently escaped from the inner surface of the mold, and as a result, it is possible to suppress the occurrence of internal pores in the surface layer of the titanium-based ingot. However, this invention is not limited to the theory described above.

The method for producing a titanium-based ingot of this invention includes a melting step of melting a raw material containing a titanium sponge with a high chlorine content of 0.15% by mass or more, and the proportion of the amount of the high chlorine content titanium sponge in the total amount of titanium sponge is 4% or more by mass, to obtain a molten metal, and a casting step of pouring the molten metal into a mold and setting the liquid surface temperature Ts (°C) of the molten metal in the mold to Ts ≧ Tm + 105°C in the peripheral region near the inner surface of the mold, where Tm (°C) is the melting point of the material that constitutes the molten metal.

The peripheral region is preferably an area between the position of the mold inner surface and a position 20 mm to 40 mm away from the mold inner surface along a normal line drawn on the mold inner surface.

In the casting step, it is preferable to cast a titanium-based ingot by continuously withdrawing the molten metal from the bottom side of the mold while solidifying the molten metal.
In this case, it is preferable that the withdrawal speed from the mold in the casting step is 2.5 ton/hr or less.

The titanium-based ingot manufacturing method of this invention can produce titanium-based ingots that are, for example, prismatic or cylindrical.

The method for producing titanium-based ingots of this invention makes it possible to suppress the occurrence of internal pores on the surface layer of titanium-based ingots, even when sponge titanium with a relatively high chlorine content is used as the melting raw material.

FIG. 1 is a cross-sectional view showing an example of an electron beam melting furnace that can be used in a method for producing a titanium-based ingot according to one embodiment of the present invention. FIG. 2 is a plan view of a mold provided in the electron beam melting furnace of FIG. 1 . FIG. 11 is a plan view showing another example of a mold in an electron beam melting furnace. FIG. 11 is a plan view showing still another example of a mold in an electron beam melting furnace.

Hereinafter, an embodiment of the present invention will be described in detail.
In the manufacturing method of titanium-based ingot according to one embodiment of the present invention, an electron beam melting furnace 1 equipped with a hearth 2, a mold 3, an electron gun 4, etc., as exemplified in Fig. 1, can be used. This embodiment includes a melting step in which a molten raw material Ms containing titanium sponge is melted in the hearth 2 by irradiation of an electron beam from the electron gun 4 to obtain a molten metal Mm, and a casting step in which the molten metal Mm is poured into a mold 3 and cooled and solidified in the mold 3. The molten raw material Ms includes a high chlorine content titanium sponge having a chlorine content of 0.15 mass% or more as the titanium sponge, and the proportion of the amount of the high chlorine content titanium sponge in the total amount of the titanium sponge is 4% or more by mass. In the casting step, the melting point Tm (°C) of the material constituting the molten metal Mm and the liquid surface temperature Ts (°C) of the molten metal Mm in the peripheral region near the mold inner surface 3a in the mold 3 are adjusted so that the relationship Ts ≧ Tm + 105°C is satisfied.

In this embodiment, the details will be described later, but by increasing the liquid surface temperature Ts of the peripheral region near the mold inner surface 3a of the liquid surface Sm of the molten metal Mm in the mold 3 in the casting process as described above, it is possible to produce a titanium-based ingot in which the occurrence of internal pores in the surface layer is suppressed, even when using a molten raw material Ms that contains a certain amount of high-chlorine content titanium sponge. This makes it possible to effectively utilize the high-chlorine content titanium sponge and suppress a decrease in the yield of titanium-based ingot production.

(Titanium ingot)
To manufacture a titanium-based ingot, for example, a molten raw material Ms containing at least titanium sponge and, if necessary, alloying elements, etc. is melted in an electron beam melting furnace 1, and the resulting molten metal Mm is poured into a mold 3 for casting. The molten metal Mm is cooled and solidified in the mold 3 to become a titanium-based ingot having a predetermined shape corresponding to the shape of the mold 3. The titanium-based ingot may be cylindrical or prismatic, such as a slab having a polygonal cross section perpendicular to the longitudinal direction.

The titanium-based ingot can be, for example, a titanium ingot that is substantially made of titanium, or a titanium alloy that contains titanium and alloying components. For example, titanium can be titanium that is equivalent to JIS standards types 1 to 4. For example, titanium alloys are alloys of titanium and metals (alloying elements) such as Fe, Sn, Cr, Al, V, Mn, Zr, and Mo. Specific examples include Ti-6-4 (Ti-6Al-4V), Ti-5Al-1Fe, Ti-5Al-2.5Sn, Ti-8-1-1 (Ti-8Al-1Mo-1V), Ti-6-2-4-2 (Ti-6Al-2Sn-4Zr-2Mo-0.1Si), Ti-6-6-2 (Ti-6Al-6V-2Sn-0.7Fe-0.7Cu), Ti-6-2-4-6 ( Ti-6Al-2Sn-4Zr-6Mo), SP700 (Ti-4.5Al-3V-2Fe-2Mo), Ti-17 (Ti-5Al-2Sn-2Zr-4Mo-4Cr), β-CEZ (Ti-5Al-2Sn-4Zr-4Mo-2Cr-1Fe), TIMETAL555 ("TIMETAL" is a registered trademark), Ti-5553 (Ti-5Al-5Mo-5V-3Cr-0.5Fe), TIMETAL21S (Ti-15Mo-2.7Nb-3Al-0.2Si), TIMETAL Examples include LCB (Ti-4.5Fe-6.8Mo-1.5Al), 10-2-3 (Ti-10V-2Fe-3Al), Beta C (Ti-3Al-8V-6Cr-4Mo-4Cr), Ti-8823 (Ti-8Mo-8V-2Fe-3Al), 15-3 (Ti-15V-3Cr-3Al-3Sn), BetaIII (Ti-11.5Mo-6Zr-4.5Sn), Ti-13V-11Cr-3Al, etc. In the above list, the numbers in front of each alloying element indicate the content (mass%). For example, "Ti-6Al-4V" refers to a titanium alloy containing 6 mass% Al and 4 mass% V.

A cylindrical titanium-based ingot may have a diameter of 100 mm to 1500 mm and a length of 9000 mm or less, for example. A titanium-based ingot as a prismatic slab or the like may have a width of 700 mm to 1600 mm, typically 1100 mm to 1350 mm, a thickness of 220 mm to 260 mm, and a length of 9000 mm or less, for example. However, the dimensions of the titanium-based ingot are not limited to these.
The magnesium content in the titanium-based ingot may be less than 10 ppm by mass due to sufficient reduction in the hearth during the melting process described below.

(Melting raw materials)
The melting raw material Ms used in the production of titanium-based ingots is in the form of briquettes, rods, powders and/or particles, etc., and contains at least titanium sponge. For example, when a briquette is produced using titanium sponge as a raw material and the briquette is used as the melting raw material Ms, the melting raw material Ms contains a processed product of titanium sponge, and therefore contains titanium sponge. The melting raw material Ms may contain titanium sponge in its original form, or may contain a processed product such as a briquette obtained by processing titanium sponge.

As this titanium sponge, one obtained by the Kroll process can be suitably used. More specifically, titanium tetrachloride such as purified titanium tetrachloride is dropped onto molten magnesium metal, and the titanium tetrachloride is reduced with the magnesium metal to produce a titanium sponge mass. In this process, magnesium chloride is also produced as a by-product. The titanium sponge mass is then crushed to produce titanium sponge in the form of granules of a specified size. To obtain the above-mentioned purified titanium tetrachloride, for example, a reducing agent such as coke is reacted with titanium oxide in titanium ore and chlorine gas at a high temperature of about 1000°C, and the crude titanium tetrachloride produced is distilled.

The quality of the titanium sponge mass produced in the reduction process varies depending on the location. For example, the bottom side of the titanium sponge mass tends to have a relatively high magnesium chloride content. The titanium sponge obtained from the bottom side of the titanium sponge mass may have a relatively large amount of residual magnesium chloride. For this reason, the titanium sponge may contain a relatively large amount of chlorine, such as 0.15 mass% or more, for example, 0.20 mass% to 1.50 mass%, due to the residual magnesium chloride by-product produced by the Kroll process. A titanium sponge with a chlorine content of 0.15 mass% or more is referred to here as a high chlorine content titanium sponge. On the other hand, a titanium sponge with a chlorine content of less than 0.15 mass% is referred to as a low chlorine content titanium sponge. As mentioned above, the chlorine content in the titanium sponge may vary depending on the location where the titanium sponge mass is collected. The chlorine content of the titanium sponge can be determined by silver nitrate titration.

The melting raw material Ms contains high chlorine content titanium sponge. More specifically, the melting raw material Ms contains high chlorine content titanium sponge and low chlorine content titanium sponge, and the proportion of the amount of high chlorine content titanium sponge in the total amount of titanium sponge is 4% or more by mass. Furthermore, the melting raw material Ms can be such that the proportion of the amount of high chlorine content titanium sponge in the total amount of titanium sponge is 4-10% by mass, or 6%-10% by mass. The total amount of titanium sponge is the sum of the amount of high chlorine content titanium sponge and the amount of low chlorine content titanium sponge. If the melting raw material Ms contains a processed product such as the above-mentioned briquette, the proportion referred to here is determined from the amount of titanium sponge used in the production of the processed product.

When a titanium-based ingot is produced by melting and casting using a melting raw material Ms containing a high-chlorine content titanium sponge, a large number of internal pores may occur in the surface layer of the titanium-based ingot. Note that internal pores tend to occur easily in the surface layer of the titanium-based ingot, and are less likely to occur in the inner side than the surface layer. In contrast, in this embodiment, even if a melting raw material Ms containing a relatively large amount of high-chlorine content titanium sponge is used as described above, the generation of internal pores in the surface layer of the titanium-based ingot can be suppressed by performing the casting process described below. This makes it possible to effectively utilize high-chlorine content titanium sponge, which has not been used in the production of titanium-based ingots until now. In addition, by limiting the amount of high-chlorine content titanium sponge to a certain amount among the total sponge titanium contained in the melting raw material Ms, the amount of gas generated during casting can be suppressed.

When producing a titanium-based ingot made of titanium, the melting raw material Ms may contain only high-chlorine content titanium sponge and low-chlorine content titanium sponge. Alternatively, when producing a titanium-based ingot made of a titanium alloy, the melting raw material Ms may contain, in addition to high-chlorine content titanium sponge and low-chlorine content titanium sponge, raw materials containing alloying elements of the titanium alloy in amounts according to the content of the alloying elements, etc.

(Dissolving process)
In the melting step, the raw material Ms is melted to obtain the molten metal Mm using an electron beam melting furnace (so-called EB furnace) or the like. Although a plasma melting furnace for carrying out a plasma arc melting method or other melting furnaces may be used, it is preferable to use an electron beam melting furnace, which is superior in terms of removing impurities in the raw material Ms and adjusting the composition of the titanium-based ingot.

The electron beam melting furnace 1 in the illustrated example includes a hearth 2 for melting the molten raw material Ms, a feeder 5 for feeding the molten raw material Ms into the hearth 2, a mold 3 for cooling and solidifying the molten metal Mm poured from the hearth 2, and one or more electron guns 4 (two in this case) for irradiating the molten raw material Ms or the molten metal Mm in the hearth 2 or the mold with an electron beam. The number of electron guns 4 can be changed as appropriate in consideration of the configuration of the electron beam melting furnace 1.

The molten raw material Ms fed from the feeder 5 into the hearth 2 is irradiated with an electron beam from the electron gun 4 to become molten metal Mm. The molten metal Mm is poured from the hearth 2 into the mold 3 while being irradiated with an electron beam from the electron gun 4 as necessary to maintain the molten state.

Although not shown, the hearth 2 may include a melting hearth for melting the raw material, a refining hearth for refining the molten metal by precipitating or evaporating impurities in the molten metal, and a pouring hearth for distributing the refined molten metal as necessary into one or more molds before pouring it. The hearth 2 may be a cold hearth such as a water-cooled copper hearth. In this case, by melting and pouring the molten metal into the mold 3 with a skull formed on the inner surface of the hearth 2, the amount of low-density inclusions can be reduced and the variation in the composition of the titanium-based ingot can be suppressed. In many cases, the melting process and the casting process described below are performed in a reduced pressure atmosphere such as a vacuum inside the electron beam melting furnace 1.

When the molten raw material Ms is melted in the hearth 2, hydrogen, a component of water, that is contained in the molten raw material Ms by being adsorbed to the magnesium chloride in the molten raw material Ms, is thought to dissolve into the molten metal Mm obtained by dissolving the molten raw material Ms. In the hearth 2, the molten metal Mm is maintained at a relatively high temperature, so the amount of hydrogen that can dissolve is relatively large, and it is presumed that the hydrogen in the molten metal Mm is brought into the mold 3 in a dissolved state when the molten metal Mm is poured into the mold 3. Note that oxygen, a component of water, can be appropriately reduced in the hearth 2 without dissolving into the molten metal Mm.

(Casting process)
After the melting step, the molten metal Mm is poured into the mold 3, and a casting step is performed to solidify the molten metal Mm. Although the electron beam melting furnace 1 in FIG. 1 is equipped with one mold 3, the number of molds is not particularly limited, and an electron beam melting furnace equipped with two or more molds may be used. When there are multiple molds, the molten metal may be poured into all of the molds, or may be poured into some of the molds. In the mold 3 such as a water-cooled copper mold, the molten metal Mm gradually solidifies from the side of the surrounding mold inner surface 3a by cooling with the flow of a cooling medium such as water through a flow path provided in the peripheral wall. This allows a titanium-based ingot to be obtained.

At this time, it is believed that the solubility of hydrogen and the like dissolved in the molten metal Mm decreases due to cooling in the mold 3, etc., and the hydrogen and the like are released from the molten metal Mm and become gas. Here, when the temperature (also called "liquid surface temperature") of the molten metal Mm at the liquid surface Sm of the molten metal Mm in the mold 3 is low, the molten metal Mm is rapidly cooled and solidified near the mold inner surface 3a of the molten metal Mm in the mold 3, so it is presumed that the above-mentioned gas generated in a deep position away from the liquid surface Sm is sealed in the solidifying molten metal Mm before being discharged from the liquid surface Sm. Note that, since the cooling of the molten metal Mm is slower near the center in the radial direction away from the mold inner surface 3a than near the mold inner surface 3a, the gas generated there is likely to be discharged from the liquid surface Sm to the outside before the molten metal Mm solidifies. It is believed that the gas sealed in the molten metal Mm in the middle of solidifying near the mold inner surface 3a in this way forms internal pores in the surface layer of the titanium-based ingot.

In contrast, in this embodiment, in the casting process, for example, the electron gun 4 irradiates an electron beam into the mold 3 from an opening on the upper side of the mold 3, thereby raising the liquid surface temperature Ts of the molten metal Mm that flows from the hearth 2 into the mold 3 in the peripheral region near the mold inner surface 3a to a predetermined high temperature. Specifically, the liquid surface temperature Ts of the molten metal Mm in the peripheral region near the mold inner surface 3a and the melting point Tm of the material (metal or alloy) that constitutes the molten metal Mm are set to satisfy the relationship Ts ≧ Tm + 105°C. For example, if the material that constitutes the molten metal Mm is titanium, the melting point of titanium is 1668°C, so the liquid surface temperature Ts is set to 1773°C or higher. In this case, it is considered that the molten state of the molten metal Mm is easily maintained even in the vicinity of the mold inner surface 3a until the gas escapes from the liquid surface Sm of the molten metal Mm. Regardless of whether this mechanism is correct or not, this embodiment effectively suppresses the occurrence of internal pores in the surface layer of the titanium-based ingot that is formed by solidifying the molten metal Mm during the casting process.

The liquid level Sm of the molten metal Mm in the mold 3 is set to the above temperature Ts in the peripheral region between the position of the mold inner surface 3a and a position a predetermined distance D inward from the mold inner surface 3a along a normal line erected on the mold inner surface 3a, as viewed in plan of the molten metal Mm stored in the mold 3. For example, in the case of a mold 3 having an annular upper opening as shown in the plan view of Figure 2, the liquid level Sm of the molten metal Mm in the mold 3 is set to the above peripheral region in the annular region between the position of the mold inner surface 3a and a position (shown by the dashed line in the figure) a distance D inward from the mold inner surface 3a in the direction of the normal line to the mold inner surface 3a (radial direction). In the case of a mold 13 having an upper opening in the shape of a rectangular ring as shown in FIG. 3, the liquid level Sm of the molten metal Mm in the mold 13 is defined as a rectangular ring-shaped region between the position of the mold inner surface 13a and a position (indicated by a broken line in the figure) that is a distance D away from the mold inner surface 13a inward in the direction of the normal to the mold inner surface 13a. When a slab is produced as a titanium-based ingot, the liquid level Sm of the molten metal Mm in the mold 13 is substantially rectangular. In the case of a mold 23 having an upper opening in the shape of an ellipse ring as shown in FIG. 4, the liquid level Sm of the molten metal Mm in the mold 23 is defined as a ring-shaped region between the position of the mold inner surface 23a and a position (indicated by a broken line in the figure) that is a distance D away from the mold inner surface 23a inward in the direction of the normal to the mold inner surface 23a. The temperature of the liquid level Sm in the peripheral region may be the above-mentioned temperature Ts. This is because the solidification mode of the molten metal Mm present near the mold inner surface has a significant effect on the occurrence of internal pores in the surface layer of the titanium-based ingot. Preferably, the predetermined distance D between the position of the mold inner surface and a position away from the mold inner surface toward the center (in the example of FIG. 2, the radial distance of the peripheral region) may be within a range of 20 mm to 40 mm, and may further be within a range of 20 mm to 30 mm. Note that here, the specific configuration etc. of the mold 3 shown in FIG. 2 will be mainly described as a representative example, but they can also be applied to the molds 13 and 23 shown in FIGS. 3 and 4, and other molds not shown. There is no particular limit to the shape or structure of the mold.

The liquid surface temperature Ts of the peripheral region of the molten metal Mm in the mold preferably satisfies Tm + 105°C ≦ Ts ≦ Tm + 170°C. By not setting the liquid surface temperature Ts too high, it is possible to prevent breakouts, ensure a good casting surface, and reduce costs (electricity costs and maintenance costs). In the production of titanium-based ingots, it may be preferable for the liquid surface temperature Ts of the peripheral region of the molten metal Mm in the mold to be 1780°C to 1830°C.

The temperature of the central region, which is the region other than the peripheral region at the liquid surface Sm of the molten metal Mm in the mold, may be, for example, Tm+25°C or higher and Tm+170°C or lower, or even Tm+25°C or higher and Tm+105°C or lower. If the liquid surface temperature Ts of the peripheral region of the molten metal Mm in the mold 3 is set to 1780°C to 1830°C, the temperature of the central region may be, for example, 1700°C to 1830°C, or even 1700°C to 1780°C. By keeping the temperature of the central region at a relatively low level, costs (electricity costs and maintenance costs) can be reduced.

The surface temperature of the molten metal Mm in the mold can be measured, for example, by a radiation thermometer that is placed above the opening of the mold and is capable of measuring temperature from thermal radiation.
The melting point Tm of the material constituting the molten metal Mm can be confirmed by a thermal arrest method. The thermal arrest method is widely known as a method mainly used for measuring the melting point of high melting point materials. The specific measurement method is as follows. First, a sample made of the material constituting the molten metal Mm is placed in a heat-resistant capsule, and the heat-resistant capsule is placed in a heating furnace. Then, in the heating furnace, the sample is heated while the temperature of the sample in the heat-resistant capsule is measured. At this time, the change in the heat curve is read when the rate of temperature rise stagnates or decreases due to the latent heat associated with the melting of the sample. This makes it possible to determine the melting point of the material constituting the sample.

While various types of molds can be used, the molds 3, 13, and 23 shown in the figures are also open at the bottom, and the molten metal Mm can be pulled out downward from the bottom side as shown by the arrow in Figure 1 while it is solidifying. With such molds, at least the portion of the molten metal Mm that is in contact with the mold inner surface 3a at the bottom side solidifies, and continuous casting can be performed in which the solidified portion is continuously pulled out. This makes it possible to obtain a long titanium-based ingot.

In this case, the withdrawal speed of the solidified portion Pc of the molten metal Mm from within the mold is preferably 2.5 tons/hr or less, and particularly 1.0 to 2.0 tons/hr. The withdrawal speed is the weight of the ingot withdrawn from the mold per hour. By slowing down the withdrawal speed of the solidified portion Pc to a certain extent in this way, the above-mentioned gases can be sufficiently removed before the molten metal Mm solidifies, so that the occurrence of internal pores in the surface layer of the titanium-based ingot can be further suppressed. On the other hand, by not slowing down the withdrawal speed of the solidified portion Pc too much, the production capacity of the equipment can be ensured.

In addition, by forming the cross-sectional shape of the mold in a direction perpendicular to the longitudinal direction into a circular shape, a rectangular shape, or other polygonal shape, it is possible to produce a titanium-based ingot in a cylindrical shape or a prismatic shape having a polygonal cross-section perpendicular to the longitudinal direction. In this embodiment, it is also possible to produce a slab having a rectangular cross-section perpendicular to the longitudinal direction as the titanium-based ingot.
In the embodiment described above, for example, titanium-based ingots with a weight of about 5 tons to 30 tons per ingot can be produced.

Next, the method for producing titanium-based ingots according to the present invention was experimentally carried out, and its effects were confirmed, which will be described below. However, the description here is merely for illustrative purposes and is not intended to be limiting.

A high-chlorine content titanium sponge (high Cl titanium sponge) with a chlorine content of 0.20% by mass to 1.20% by mass and a low-chlorine content titanium sponge (low Cl titanium sponge) with a chlorine content of 0.12% by mass or less were prepared. In Examples 1 to 6 and Comparative Example 1, these titanium sponges were mixed in a predetermined ratio as shown in Table 1 and used as the melting raw material. The mixture was melted and cast in an electron beam melting furnace as shown in FIG. 1 to produce a titanium-based ingot made of titanium or titanium alloy with a weight of 10 tons. In the Reference Example, the high-chlorine content titanium sponge was not mixed, and the sponge titanium of the melting raw material was made of only low-chlorine content titanium sponge, and a titanium-based ingot was produced in the same manner. In Examples 1 to 6, Comparative Example 1, and Reference Example, an electron beam was irradiated to the liquid surface of the molten metal poured into the mold, and the temperature of the peripheral region of the molten metal (the region where the distance D in FIG. 2 is 20 mm) was adjusted to the temperature shown in Table 1. The withdrawal speed when the solidified part of the molten metal was withdrawn from the mold is also shown in Table 1. In addition, in all of Examples 1 to 6, Comparative Example 1, and Reference Example, the temperature of the central region of the molten metal in the mold was approximately the same as the temperature Ts of the peripheral region.

The surface of each titanium-based ingot was subjected to cutting processing to a depth of 5 mm, and internal pores in the surface layer were confirmed. The results are also shown in Table 1. The internal pores were evaluated by the number of those visually confirmed, using the location where scratches (holes) appeared from the inside during cutting processing as the criterion for judgment.
From Table 1, it can be seen that in Examples 1 to 6, the liquid surface temperature of the peripheral region of the molten metal in the mold was relatively high during casting, and thus the number of internal pores was sufficiently reduced to 10 or less, compared to Comparative Example 1, in which the liquid surface temperature was low.

The magnesium content of each titanium-based ingot was measured by ICP atomic emission spectrometry, and the results shown in Table 1 were obtained. The detection limit for this measurement was 10 ppm by mass. This shows that the magnesium content of each titanium-based ingot was sufficiently low. From this, it is presumed that even if a relatively large amount of high-chlorine content titanium sponge, which is thought to have a high chlorine content due to residual magnesium chloride, is used as the melting raw material, magnesium is successfully reduced during melting. It can also be seen from Table 1 that even if the magnesium content of a titanium-based ingot is low, there may be a large number of internal pores, as in Comparative Example 1.

Furthermore, a test was conducted under the same conditions as in Example 1, except that the temperature of the central region of the molten metal in the mold was set to 1750°C, and a test was conducted under the same conditions as in Example 1, except that the distance D from the inner surface of the mold was set to 40 mm and the temperature of the central region of the molten metal in the mold was set to 1750°C. As a result, the number of internal pores was 1 per piece in both tests.

The above suggests that the manufacturing method for titanium-based ingots of this invention can suppress the occurrence of internal pores in the surface layer of titanium-based ingots, even when sponge titanium with a relatively high chlorine content is used as the melting raw material.

Reference Signs List 1 Electron beam melting furnace 2 Hearth 3 Mold 3a Mold inner surface 4 Electron gun 5 Feeder Ms Melted raw material Mm Molten metal Sm Liquid surface of molten metal in mold Pc Solidified part of molten metal D Distance from mold inner surface

Claims (4)

a melting step of melting a melting raw material containing a titanium sponge having a high chlorine content of 0.15 mass% or more, the proportion of the amount of the high chlorine content titanium sponge in the total amount of the titanium sponge being 4 mass% or more, to obtain a molten metal;
a casting process in which the molten metal is poured into a mold, and when the melting point of a material constituting the molten metal is Tm (°C), the liquid surface temperature Ts (°C) of the molten metal in the mold is set to Ts≧Tm+105°C in a peripheral region near the inner surface of the mold,
a peripheral region between the position of the mold inner surface and a position spaced 20 mm inward from the mold inner surface along a normal line drawn on the mold inner surface . 2. The method for producing a titanium-based ingot according to claim 1 , wherein in the casting step, the molten metal is continuously pulled out from a bottom side of a mold while being solidified to cast a titanium-based ingot. 3. The method for producing a titanium-based ingot according to claim 2 , wherein in the casting step, a withdrawal speed from within the mold is set to 2.5 tons/hr or less. The method for producing a titanium-based ingot according to any one of claims 1 to 3 , wherein a rectangular or cylindrical titanium-based ingot is produced.

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