Effect of Microstructure of Surface Layer in Steel Vessel Used for Sponge Titanium Manufacturing on Fe Elution

To make clear factors affecting Fe elution into molten Mg during sponge titanium production process and reasons why Fe elution from the steel vessel in the first use is high while it considerably reduces in the second use in the actual mill, immersion experiments into molten Mg at 950oC were conducted using specially designed specimens simulating the first and the second uses of the vessel, and microstructure change of the steel during the use was investigated in detail. TiFe, TiFe2 and Tidiffused steel layers formed on the surface of the steel specimen after the treatment similar to the first use of the vessel. The outermost portion of Ti-diffused steel contained 3mass% Ti, which should have transferred to the α phase at this temperature. Fe elution from TiFe and TiFe2 was almost zero, while Fe elution from Fe3mass%Ti was 1/4 of that from the unused steel. By taking the information on the phases appeared on the surface of the actual vessel into account, Fe elution from the vessel at the second use was estimated as 1/16 of that at the first use, which is well consistent with the value in the actual production mill.


Introduction
Fe, one of the impurities contained in titanium, strongly influences mechanical properties as well as corrosion resistance, and it is quite important to properly control Fe content. Fe contamination in titanium is mainly caused by Fe elution from a steel vessel into molten Mg in reduction process [1][2][3] , in which TiCl 4 reacts with Mg and forms sponge titanium. According to the phase diagram of Mg-Fe system shown in Fig.1 [4] , solubility of Fe in molten Mg strongly depends on temperature, and it is higher at higher temperature. For example, solubility of Fe at 700, 800 and 900ºC is 0.03, 0.08% and 0.17%, respectively.
In the reduction process in an actual production mill, Fe content in sponge titanium, which is transmitted from a steel vessel through molten Mg, is high when the vessel is used for the first time. However, there is a tendency that Fe content decreases to 1/10 -1/20 at an opportunity of the repeated use twice or more times. It suggests that the nature of the inner surface of the steel vessel varies by contacting with newly precipitated sponge titanium and it may contribute to reduce Fe elution into molten Mg.
In this study, to clarify the details of this phenomenon, specially designed experiments were conducted, in which the circumstances where the inner surface of the vessel is actually exposed were reproduced in the laboratory cells, and microstructures of the surface layers of the steel specimens exposed to the simulated circumstances were closely analyzed. After that, an effect of microstructure changes of the steel on Fe elution into Mg was discussed based on the obtained experimental results and the information on the actual mill.

Materials Used
A plain steel (JIS SS400) plate was used in this study, which is the same material as that used for the vessels in actual sponge titanium manufacturing process. The plate was subdivided into 20x20mm cut pieces, and the following treatments were employed to simulate the first and the second uses of the vessel.
Specimen A : To simulate the first use of the vessel, the cut pieces were used without employing any additional treatment.
Specimen B : To simulate the second use of the vessel, specimens having the same nature as the inner surface of the vessel after the first use (before the second use) were prepared. At first, Ti powders of 10 to 45 μm in size were placed on Specimen A and heat treated in vacuum at 1000ºC for 10h, which is similar to the heat pattern in the actual vacuum separation process in which residual MgCl 2 is separated from sponge titanium. An appearance of Specimen B is shown in Fig.2. Microstructures of Specimen B were closely investigated with SEM and EPMA.
TiFe, TiFe 2 , Fe-3mass%Ti As explained later in the "Results", TiFe, TiFe 2 and Ti-diffused steel layers were recognized on the surface of Specimen B.
Maximum Ti concentration in the Ti-diffused layer was 3 mass%. To examine effects of each substance on Fe elution into molten Mg, TiFe, TiFe 2 and Fe containing 3 mass%Ti (Fe-3mass%Ti) were prepared by arc melting. Brittle TiFe and TiFe 2 were easily crushed with a hammer and blocks of around 10mm in size were obtained. From the ingot of Fe-3mass%Ti, 20x20mm cut pieces were taken.

Immersion experiment into molten Mg
To grasp Fe elution into molten Mg, specimens described in 2.1 were immersed into molten Mg and eluted Fe content was measured. It is noted that Ti powders as well as some brittle intermetallics formed on the steel surface of Specimen B was removed before the immersion by a scraper to correctly simulate the second use of the steel vessel, which is termed as Specimen B′.
Before the immersion, ceramic coating was conducted on the surfaces except 10x10mm area for Specimen A, Specimen B′ and Fe-3%Ti, which were 20x20mm in size. As the ceramic is quite stable in molten Mg and acts as a barrier of Fe elution, Fe elution from 10x10mm area of each specimen was correctly evaluated. An appearance of ceramic coated Specimen A is shown in Fig.3. The blocks of TiFe and TiFe 2 were directly immersed into molten Mg. The apparatus used for the immersion experiments is schematically shown in Fig.4. Several crucibles made of magnesia were placed in the furnace and one specimen was put in one crucible. 200g of Mg was inserted on the specimen in each crucible and heated in Ar to 800ºC. Once temperature reached 800ºC, molten Mg was stirred and then temperature was raised to 950ºC. A crucible was taken out one by one after prescribed time elapsed, and cooled down to room temperature. Fe content in

Microstructure after reaction with Ti (Specimen B)
Fig.5 is a backscattered electron image of the cross section of Specimen B. In the steel, three layers having different contrast can be seen, which are designated as "a", "b" and "steel". In Ti powders coated on the steel, two phases having different contrast can be seen. They are considered as the α phase and the Fe concentrated β phase. Fig.6 shows Ti (a) and Fe (b) distributions detected by EPMA near the steel/Ti powder interface. In Fig.6(a), it is clearly shown that Ti diffused into the steel for more than 200 μm, and Fe concentration was discontinuously distributed near the steel/Ti powder interface. It is considered that the layer "a" is TiFe and "b" is "TiFe 2 ".     As shown in Fig.8 (a), Fe elution from Specimen B′, which simulates the second use of the steel vessel, is drastically reduced compared to Specimen A, which simulates the first use. This is consistent with the actual situation.

Immersion experiment into molten Mg
Although the surface was cleaned in Specimen B′ before the immersion, the surface should be covered with remnant of TiFe and/or TiFe 2 and the newly emerged steel surface on which 3%Ti is contained as shown in Fig.7. It is clearly shown that those substances had high resistance against Fe elution in Fig.8 (b)

Microstructure change of steel vessel by first use
As described in the "Results", three layers (TiFe, TiFe 2 and Ti-diffused Fe) formed on the steel surface by contacting with Ti powders, which simulate newly precipitated sponge titanium. According to the Ti-Fe binary phase diagram [5,6] shown in Fig.9, they are the three phases in the high Fe content region and it is reasonable to think that those phases formed by interdiffusion of Fe and Ti. Here, it should be noted that the outermost surface of the Ti-diffused steel contains 3mass% Ti, which is the solubility limit of Ti in the ferrite α phase at around 1000ºC.
Steel is basically composed of the α phase at room temperature and the α phase transfers to the austenite γ phase at 950ºC, operating temperature of the vessel, as shown in Fig.9 and suggested by Nagesh et al [7,8] . However, most portion of the Ti-diffused Fe should be composed of the α phase at this temperature as shown in Fig.9.
High resistance against Fe elution into Mg of Fe-3mass%Ti is probably caused by the formation of the α phase by Ti inward diffusion at operating temperature of the steel vessel.

Comparison of Fe elution to actual production vessel
As described in the "Introduction", Fe content in sponge titanium is high when the vessel is used for the first time. This is consistent with the experimental result for Specimen A (Fig.8 (a)). In addition, Fe content is reduced at an opportunity of the repeated use of the vessel. This tendency is also consistent with the experimental result for Specimen B´ (Fig.8(a)). However, in the actual production mill, Fe elution reduces to 1/10-1/20 at the second use, while the experimental results indicates Fe elution from Specimen B´ simulating the second use of the vessel is 1/4 of that from Specimen A simulating the first use. The difference between the experiment and the actual mill is probably brought by the difference of the amount of the remnant on the steel surface. In the actual vessel, 3/4 of the surface area of the vessel is covered with the intermetallics, TiFe and TiFe 2 , which remain on the surface without removed during the process of taking out sponge titanium from the vessel, while they are almost completely removed by a scraper in Specimen B´ because of the tiny specimen size. In fact, Fe elution of Specimen B´, which is covered with the Ti-diffused Fe, is almost the same as that of Fe-3mass%Ti, which is 1/4 of that in Specimen A.
Based on the experimental results and the above discussion, why Fe elution reduces to 1/10-1/20 in the actual mill at the second use of the vessel is discussed next using Fig.10, in which microstructure change of the surface region in the steel vessel during the first and second uses is schematically depicted.
The vessel made of unused steel is heated to around 950ºC of operation temperature in the actual reduction process. At that time, the α phase transfers to the γ phase.
The Ti-diffused layer forms as well as TiFe and TiFe 2 intermetallics. Here, outer portion in which high amount of Ti is contained transfers to the α phase again.
After the first operation, a lump of sponge titanium is taken out from the vessel. At that time, 1/4 of the area of the inner surface was effectively peeled out and brittle intermetallics are removed. It means that 3/4 of the area is still covered with intermetallics and Fe-3mass%Ti appears on 1/4 of the area of the total surface.
Fe elution from the steel vessel at the second use is estimated as 1/16 of that at the first use by the following calculation : Fe elution at the second use