Electroslag Refining of CrNiMoWMnV Ultrahigh-Strength Steel

Increasing demands for ultrahigh-strength steels in commercial as well as mil-itary applications have raised interest in finding alternatives to the high-cost high-alloyed steel and super-alloys currently used, e.g. the use of economic low-alloy compositions processed via low-cost air induction melting and electroslag refining (ESR). In this work the yield of alloying elements and the removal of the impurities nitrogen, sulphur and phosphorus as a result of electroslag refining (ESR) in a newly developed CrNiMoWMnV ultrahigh-strength steel (UHSS) have been studied in relation to their activities in the molten metal pool. Six experimental heats of CrNiMoWMnV UHSS with different chemical compositions were designed, melted in an induction furnace (IF) and refined using ESR. This was followed by hot forging of the ingots at 1100˚C to 950˚C. ESR using a CaF 2 -CaO-Al 2 O 3 slag system led to a high yield in Cr, Ni, Mo, W, Mn and V, while the yield of Si is low. The desulphurization of all six UHSS grades was pronounced with most of the sulphur removed either to the slag or by gas reactions. The degree of dephosphorization was only 5% irrespective of the steel composition. On the other hand, denitrification (removal of nitrogen) was achieved. It ranged from 8% to 63% depending on the steel composition. The yield of the alloying elements and removal of impurities from the steel during ESR depends on the chemical and physical properties of the ESR slag and the activity of the elements in the molten state, taking into account elemental interactions.


Introduction
There is no generally accepted definition of what constitutes ultrahigh-strength in structural steels as it depends on the field of application. According to Philip [1] [2] it implies a yield strength exceeding 200 ksi, i.e. 1380 MPa, but more recently the term has been applied to steels with a yield strength of about 900 MPa, and above [3]. Common to all ultrahigh-strength steels is the need to have a low level of impurities, such as sulfur, phosphorous and non-metallic inclusions (NMI), as well as freedom from defects like voids and cracks, and homogeneous compositions within narrow tolerances [4].
The electroslag refining (ESR) process is one of the most important secondary refining processes owing to its relatively low production cost, low capital investment and ability to produce high quality steel with desired chemistry and homogenous structure. It is also found that the ductility, impact toughness and transition characteristics and corrosion resistance are remarkably improved due to the pronounced effect of ESR on the removal of NMI. ESR has also been shown to enhance strength as a result of the grain refinement resulting from the addition of some inoculants during the ESR process [4] [5] [6].
In the ESR process, the slag plays different roles: as a heating source, protection of the melt from oxidation and facilitation of chemical refining which has the main powerful effect on the yield of alloying elements, besides desulphurization and removal of exogenous non-metallic inclusions [7] [8].
It is important to use reducing slags to control the oxygen content of the melt to low levels, which leads to high yields of the alloying elements in the final product.
Sulphur enhances the machinability of steel but has a detrimental effect on the properties of most other steel properties such as ductility, impact toughness and weldability, so, in the case of structural steels especially those with ultrahigh-strength, it must be reduced to a minimum. In ESR, two reactions result in the removal of sulphur from the steel; those at the slag/metal and slag/gas interfaces as given in Equations (1) As indicated in Equation (1), sulphur transfer from the molten metal to the molten slag depends on the amount and activity of oxygen ions in the metal and slag. Decreasing the activity of oxygen in the metal and increasing it in the slag enhances the desulphurization process. Hayashi et al. [11] gave another mechanism for the desulphurization process in which the removal of sulphur increases by dissolution with an O − ion thus supplying oxides in slag as follows: Vaish et al. [4] and Eissa et al. [12] stated that most sulphur transfer occurs from the slag to the atmosphere, but that depends mainly on the sulphur transfer Depending on its location in the steel, nitrogen can be either beneficial or detrimental to mechanical properties. In the form of nitrides like VN nitrogen can enhance grain refinement giving higher ductility and toughness [17] [18] [19].
[20] Nitrogen can also exist in interstitial solid solution like carbon but with a higher solubility than carbon. As nitrogen can diffuse easily in the iron matrix at low temperatures, this reduces the cold forging performance. Nitrogen may also lead to the formation of porosity and blowholes [20] [21] [22].
Nitrogen dissolves in the molten slag either in the form of free nitride (N 3− ) or as an incorporated nitride (N − ) depending on the slag composition. In a basic slag system, nitrogen reacts with free oxygen (O 2− ) as a free nitride as given in Equation (5). However, in an acidic slag system of low basicity the nitrogen reacts with non-bridging oxygen (O − ) as an incorporated nitride as given by Eq- Removal of nitrogen in the ESR process may take place by two means: physical removal of nitride inclusions by entrainment in the slag or by dissolution depending on the capacity of the ESR slag, the carbon content of the system, and the presence or absence of strong nitride formers, according to one of the following reactions at the molten metal-slag interface: Lower activity of oxygen in the metal and higher in the slag leads to an enhancement of denitrification.
During the electroslag refining process the following steps take place. Increasing the oxygen activity, oxidation rate and decreasing the reduction rate lead to high losses of alloying elements or formation of inclusions [28]. R: alloying element with high affinity for oxygen such as Si, Al or Ti The carbon content of the steel can be increased during electroslag refining as a result of carbon in the molten slag, which increases during the pre-fusion process or from additives by means of carbide ions oxidized to carbon.
High oxygen activity in the molten slag leads to oxidation of a high percent of Si in the consumable electrode with the formation of silica as shown in Equation (9). Then SiO 2 reacts with CaO to form the very stable calcium silicate in the slag produced, thereby lowering the silica activity and the yield of Si in the steel produced [28].
Mattar [29] showed that slag consisting of 70% CaF 2 , 15% CaO and 15% Al 2 O 3 (a 70-15-15 slag) not only has a high desulphurizing power but also a remarkable effect on the number density, size and distribution of nonmetallic inclusions in the steel matrix.
This work aims at studying the effect of the ESR process on the yield of valuable alloying elements, together with S and P removal and the possibility of N removal for CrNiMoWMnV low alloyed potential ultrahigh-strength steels (UHSS) when using the above 70-15-15 slag system. There are only a limited number of studies of the effect of primary chemical composition on the final chemical composition of the produced ingot [30]. Therefore the aim of this work is to study the relationships between the activities of alloying elements and impurities in the molten metal and their effects on the yields of alloying elements and the removal of impurities.

Experimental Work
Six experimental heats of steel with different chemical compositions were designed to satisfy the targeted properties. Low to medium C content was used to improve weldability, Mn, Si and Ni were used to stabilize the austenite phase and enhance toughness, Cr was added to enhance strength and hardenability, Mo to enhance hardenability, W to enhance strength and wear resistance, and V to increase toughness. The heats were made in the Steel Technology Department, Central Metallurgical Research and Development Institute (CMRDI), Egypt by melting the scrap, ferroalloys and nickel alloy shown in Table 1 in an air induction furnace lined with spinel. The chemical composition of the scrap was analyzed using Optical Emission Spectrometry (SPECTRO analytical instrument) while ferroalloys and nickel alloy were analyzed using a Philips PW 1410 X-Ray Spectrometer with PW 1390 channel control After the chemical composition and the temperature of the molten steel were adjusted, the molten metal was tapped at 1560˚C -1580˚C into a steel mould 250 mm long and 70 mm in diameter. Samples, 10 mm thickness and 70 mm in diameter, were cut from the bottom of the steel ingots after discarding 20 mm from the bottom and used for determination the chemical composition of the steels. The surfaces of all the ingots were cleaned prior to further processing.
M. Ali et al. After reheating to 1100˚C and holding for one hour, half of the steel ingots were forged into steel bars with cross sections 28 × 30 mm to produce a consumable electrode for the ESR process. The forging start temperature was about 1100˚C while the finish temperature was adjusted to be 950˚C.
Fused synthetic slag was prepared giving stable slag components with a minimum tendency to moisture pick up in order to minimize the hydrogen content of the steel produced. The slag composition by weight was about 70% CaF 2 , 15% Al 2 O 3 and 15% CaO. It was produced in a 10-kg pilot plant using submerged electric arc furnace. The furnace was operated at an average of 480 A and 35 V.
The furnace wall and bottom were rammed with a thick magnesite layer and another layer of carbon paste. After completing the melting process, the fused slag was cast into metallic moulds of 100 mm inner diameter and 250 mm height. The product was left to cool to room temperature in the mold and then crushed to an appropriate size (<20 mm) for use in the ESR refining process.
Representative samples of the slag were subjected to chemical analysis using XRF.
At the start of ESR process, a starting disc 28 mm × 10 mm × 30 mm was cut from the base material of the consumable electrode and put on the water cooled copper base plate to protect it.
The ESR process was initiated with an exothermic capsule composed of 54 g Al powder and 160 g of mill scale Fe 2 O 3 and a small amount of Mg. Such a composition gives the stoichiometric composition needed for the exothermic reaction, which gives the heat for starting the process.  Table 2. The refined steel ingots were subjected to the same forging process that was used on the base ingots.
X-ray Fluorescence (XRF) analysis was made using a Philips PW 1410 X-Ray Spectrometer with PW 1390 channel control to determine the percentage of all chemical constituents in the synthetic fused slags before and after ESR. A LECO CS444 (C.S-Mat 650) was used to determine the C content in slag before and after the ESR Process. Optical Emission Spectrometry (SPECTRO analytical instrument) was used to determine the chemical composition of the steel ingots after different stages. About 1 g of small chips of steel and 1 g of slag powder were analyzed for N using an ELTRA ONH 2000 and the Kjeldahl method [31] respectively. Oxygen content was analyzed at the company Outokumpu in Finland using LECO combustion analysis for a sample cut from the forged bar.

Effect of ESR Process on the Yield of Alloying Elements
The chemical composition of the fused slag and the six grades of UHSS steel before and after electroslag refining is given in Table 3 and Table 4 respectively.     The following steps are pertinent to the oxidation of alloying elements in ESR: As the temperature of the slag pool rises above the melting point of the metal, droplets melt off the tip of the consumable electrode and fall through the slag pool. As the temperature of the falling droplet is fairly high, the oxidation of alloying elements can take place depending on their affinity to oxygen. For elements with a very high affinity to oxygen, the oxidation reaction may be take place with the atmospheric oxygen and its oxidation rate depend mainly on the surface area exposure to the atmospheric oxygen so by increasing the melting rate the oxidation reaction will be increased. The major part of the oxidation process for the elements with lower oxygen affinity take place during the transfer of metal droplet through the molten slag by diffusion of oxygen, which depends on the physical properties of the slag. Increasing wetting of the falling droplets with the slag, i.e., decreasing interfacial tension, leads to the formation of a protective layer that reduces the diffusion of oxygen towards the metal droplet and the oxidation rate and hence increasing the yield of the alloying element. Increasing slag viscosity leads to a reduction of oxygen transport towards the metal droplets.
The direction of the equation depends on the oxygen potential which given by either the activity of oxidizing anions or activity of reducing cations and the activity of Mn in the melt pool.
From the chemical composition of the six grades of UHSS before and after ESR, the effect of the refining process on the stability of the alloying elements C, Cr, Ni, Mo, W, Mn, Si and V in steels under investigation was studied, as shown in Figure 2. It is observed that electroslag refining using synthetic fused 70-15-15 slag is remarkably effective in attaining high yields of the alloying elements; viz., Cr, Ni, Mo, W, Mn and V. This may be attributed to the inhibition of oxygen transfer from atmospheric air to the molten metal due to the high viscosity and lower interfacial tension of the slag, which provides a protective layer against oxygen attack on the molten metal droplets. Also, the addition of Al deoxidant reduces the oxygen activity and the oxidation kinetics of the alloying elements in the slag. Mo is increased as a percentage in steels C, D, E and F due to its extremely low interaction coefficient with oxygen (see Table 5) and its less negative free energy of oxidation combined with the fact that the impurities S, N and P and other alloying elements like Si, Mn, V and Cr are removed. These results are in good agreement with those obtained by Burak et al. [33] who concluded that the losses of Cr, Mo and W alloying elements do not exceed 10% and also those obtained by Mattar et al. [29] who concluded that the increment in the non oxidizable element like Mo and W in the produced ingot could be attributed to the losses oxidizable elements such as Si, V, Mn and Cr. The net change in the elements or losses are calculated according to the following expression:   Figure 3 illustrates the change, % of alloying elements Cr, Ni, Mo, W, Mn, Si and V in relation to the free energy of their oxidation reaction. It is clear that, while there is a tendency for the free energy of oxidation to affect the alloy losses during ESR, other factors are acting too. Account must also be taken of the reac-  (13 -19).
tion kinetics between the slag and metal and the initial chemical composition of the electrode as well.
Since all the parameters of the ESR process such as the starting slag composition, melting rate, and power are fixed, the only variable is the initial chemical composition of the steel electrode. During ESR the molten steel contains many elements i.e. Cr, Ni, Mo, W, Mn, V, Si, C, Al, Cu, S, P and N, and the combined effect of all these elements must be considered. Therefore, the variation in the yield of the alloying elements was studied by considering the interaction parameters between all the elements in the molten state for the initial chemical composition. The activity coefficient of each alloying element was calculated using the WLE-formalism as shown in the following equation: [34] [ ] activity coefficient for each element in this study are given in Table 5. Activities and activity coefficients for all elements in the molten state are given in Table 6.
Subsequently, SiO 2 reacts with CaO to form very stable calcium silicate in the slag which decreases the silica activity and the yield of Si in the alloy. As shown in Figure 2, Si losses during ESR varied from 9% to 51% in the six UHSS. This result is in agreement with the observation of Bandyopadhyay et al. [36], who concluded that the losses of silicon content during ESR was found to be in the range 0% -55%, and the results of Burak et.al. [33], who using various synthetic slag compositions which composed mainly of CaF 2 -Al 2 O 3 -CaO ternary systems with traces of FeO, MnO, SiO 2 and MgO, reported that the losses of Si up to 60% in ESR.
In the case of Si, there seems to be a tendency for the losses to decrease somewhat as the activity of Si increases, see Figure 5. This may be due to the lower percent of oxygen resulting from addition of Al as by increasing the Al pick up in the produced ingot the losses of oxygen are increased while the losses of Si are decreased which clearly illustrated in Figure 6.  There is an increase in the Al content of all steel ingots in the range (0.016% -0.051%, see Table 4) due to the addition of Al as a deoxidant during ESR. The mechanism of Al pick up is not clear. As Al is added to reduce the FeO content in the slag. Any excess Al will form a layer floating on the slag surface as the density of molten Al is lower than the density of the slag. This layer will be oxidized to Al 2 O 3 which implies that it is difficult for the Al added to directly enter the metal. However, the presence of elements like Si may reduce some alumina at the slag/metal interface lead to an increase of the Al content in the produced ingot as shown in Figure 7 and according to the reaction in Equation (23). The high scattering shown in Figure 7 and the lower value of Al content at Si activity 1.12 and 1.11, may be results from increasing the Al % and its activity in the slag which leads reaction (23) goes backward.
The pre-fusion process increases the carbon content of the slag partly due to the layer of carbon paste on the furnace wall and bottom and partly due to the graphite electrode used in the submerged electric arc furnace. This leads to a slight increment of C content of the steel ingots and a corresponding decrease in the carbon content of the slag during ESR as can be seen in Table 7.

Effect of ESR Process on the P, S and N Contents
Desulphurization depends on the activity of sulphur in the steel consumable electrode. As shown in Figure 8, ESR with the 70:15:15 slag has a good desulphurizing power (from 21% -46%) for all the UHSS studied. These results are in agreement with those of Bandyopadhyay et al. [36], who concluded that there is a decrease in the concentration of sulphur in the metal during ESR reaching up to 30% -50%. These carry S from the molten metal droplet to the slag as CaS, MnS, etc. By increasing the activity of S in the molten steel pool, the desulphurization process is enhanced as shown in Figure 9.
Also, the activity of sulphur is based on the activity of oxygen as increasing the activity of oxygen in the molten steel leads to a decrease in the activity of sulphur and consequently decreases the desulphurization degree. This can be illustrated by the following:  To determine the mechanism of the desulphurization process, the S contents of the different steels and their related slags after the electroslag refining process were determined and used to calculate the percentages of S removed to the slag and to the atmosphere. The results are shown in Table 8. It was observed that the main mechanism for the desulphurization process, accounting for about 97% -99% of the sulphur removal, was removed as gas by metal-gas and slag-gas interactions. These results are in good agreement with those of Mattar et al. [8] who concluded that about 80% of sulphur removal is occurs by metal-gas and slag-gas reactions.
Dephosphorization Process In the steels under investigation, P removal was limited to 5%, slightly decreasing as the P activity increased, as shown in Figure   10. Thus, there is an inappreciable effect of ESR on the dephosphorization process. The removal of P from steel is based on the relatively high partial pressure and activity of oxygen to form phosphorus pentoxide which can then be transferred to the slag, but this condition cannot be achieved in the present case, so there is little effect of ESR on the removal of P from the investigated steels.
Denitrification process Many parameters influence the nitrogen content in steel ingot produced from ESR process, but the most important parameters are the chemical compositions of the slag and the consumable electrode. To help understand the denitrification phenomenon the nitrogen distribution between the metal and the slag were determined before and after ESR, see Table 4 and Table 9.
The removal of nitrogen from the steels ranged from 8% to 63%, which shows that denitrification can be obtained during ESR, see also    Many factors influence the removal of nitrogen such as slag viscosity; removal of nitride inclusions by physical removal, dissociation and flotation, or the reaction with the slag phases or carbon according to Equations (7) and (8). The presence of traces of TiO 2 can raise the solubility of N in the slag where the nitrogen will incorporate into titanate networks. The increasing content of Al 2 O 3 in the slag during the process leads to an increase in the slag viscosity and removal of N. However, all of these parameters are fixed or the same in this study because they are related to the single slag that was used in this study. However, we still see variations in the degree of denitrification obtained with different electrode compositions so the chemical composition of the steel is also an important factor. Chen [30] also concluded that alloying elements and impurities contained in the steel affect the denitrification process. surface [40]. Figure 11 shows that the degree of denitrification in the current study increases with increasing N activity in the molten metal.
Based on the nitrogen content of steel and slag before and after ESR, the sources of N in the slag can be calculated as shown in Table 10. These results show that a lower solubility of atmospheric N in the slag provides a higher protection for the molten steel against N pick-up. Figure 12 illustrates the presence of a strong correlation between denitrification and S activity of the investigated steels. This can be understood on the basis  Figure 11. Effect of activity of N on the denitrification, % process. of the effect of S on the kinetics of denitrification through its tendency to segregate to surfaces.

Conclusions
The effects of the activities of the alloying elements C, Cr, Ni, Mo, W, Mn and V and the impurities S, P, N and O on the yield of alloying elements and the degree of purification caused by electroslag remelting with a prefused CaF 2 based slag has been studied for six potential ultrahigh-strength steel compositions. They were melted in an induction furnace, refined using ESR together with a slag comprising 70% CaF 2 , 15% CaO 15% Al 2 O 3 , and then hot forged before final chemical analysis. The results showed the following.
1) Overall, the synthetic slag used in the refining process gave a high yield of the alloying elements apart from Si.
2) The yield of alloying elements not only depends on the slag composition and ESR parameters, but also on the starting chemical composition of the consumable electrode, or rather, the activities of the alloying elements as determined by the interaction coefficients between the elements.
3) Increasing the activity of the Cr, W, V, Mn and Ni led to a decrease in their yield in the final product.
4) The yield of Mo was even increased in some cases, due to its extremely low interaction coefficient with oxygen and its less negative free energy of oxidation combined with the fact that the impurities S, N and P and other alloying elements like Si, Mn, V and Cr are removed. 5) Due to the low interaction coefficient of Ni with O the yield of Ni is high in all the investigated steels.
6) The relatively high yield of manganese despite the strong negative interaction with O and S can be explained by the high activity coefficient of MnO ( MnO ) in the slag. 7) There is a high loss of silicon due to its highly negative free energy of oxidation. Also, when Si in the consumable electrode is oxidized to SiO 2 in the slag, reaction with CaO to form a stable calcium silicate lowers the activity of SiO 2 , which further reduces the concentration of Si in the final ingots. 8) Increasing the activity of S in the molten steel pool enhances the degree of desulphurization. About 97% -99% of the sulphur removed from the steel is removed as gas by metal-gas and slag-gas interactions. Also by ESR causes no pronounced dephosphorization. 9) Denitrification can be performed using ESR with 70-15-15 CaF 2 -CaO-Al 2 O 3 synthetic slag. The degree of denitrification depends not only on the slag properties but also on the chemical composition of the consumable electrode.
10) This study shows that not only the chemical composition of the slag but also the activity of the elements in the molten metal must be taken into account in any further studies related to the production of high quality steel using ESR or pressure electroslag remelting (PESR).