EFFECT OF EAF AND ESR TECHNOLOGIES ON THE YIELD OF ALLOYING ELEMENTS IN TOOL STEELS

Transcription

1 EFFECT OF EAF AND ESR TECHNOLOGIES ON THE YIELD OF ALLOYING ELEMENTS IN TOOL STEELS T. Mattar, H.S.R. El-Faramawy, A. Fathy, M. Eissa, K.A. El-Fawakhry Steel Metallurgy & Ferroalloys Dept. Central Metallurgical Research and Development Institute (CMRDI) P.O. Box 87 Helwan, Cairo Egypt Abstract Keywords: Alloying elements in steel are greatly affected by the melting and refining technology. In this study the effect of electric arc furnace (EAF) smelting and electro-slag refining (ESR) of selected three grades of tool steels (cold work, hot work and high-speed steels) on the yield of alloying elements is studied. The effect of EAF technology on the yield of alloying elements was studied by melting these graded of tool steels under both carbide and white reducing slag producing consumable electrodes. The produced consumable electrodes were electro-slag remelted under three different CaF 2 based fluxes. The effect of physical properties and chemical composition of used EAF reducing slag and ESR used fluxes on the yield of alloying elements were studied. This study showed that refining of tool steels under carbide slag in EAF reduces the losses of alloying elements in ESR process. In ESR process, the much strong oxidizable elements such as Si and V are oxidized with atmospheric oxygen. The oxidation process increases by increasing the remelting rate. Elements with lower affinity to oxygen, such as Mn and Cr, are oxidized by atmospheric oxygen and diffused oxygen through the molten slag during the molten droplet transfer depending mainly on the physical properties of the used flux. The highest losses of such elements were detected by remelting under flux with the lowest viscosity and highest interfacial tension. These oxidation processes are accompanied by increasing the concentration of Mo and W. Electric arc furnace, electro-slag refining, tool steels, alloying elements, yield, slag, flux, physical properties. 411

2 412 6TH INTERNATIONAL TOOLING CONFERENCE INTRODUCTION Tool steels are usually made in small capacity electric arc furnaces (EAF) and refined either in vacuum or by remelting using electro-slag remelting (ESR) technique. The refining of tool steels melt is usually accomplished through application of reducing slags in conjunction with alloy deoxidation. The use of reducing slags is an important technique for controlling the oxygen content of the melt to low levels and at the same time maximizes alloy recovery as well as minimizes the amount of deoxidation products formed. The reducing slag serves primarily as a blanket between the furnace atmosphere and the metal bath, to retard the transfer of oxygen from the air to the bath to the maximum extent possible. Under such conditions, the carbon drop is arrested and ferroalloys of oxidizable elements such as chromium, silicon and manganese can be added to the heat at nearly 100 percent recovery. In ESR, according to Kato (1985) [1], the processes from melting of consumable electrode to solidification are: forming a thin layer of molten iron on the surface of the tip of electrode forming a droplet at the tip of electrode, dropping of the droplet into the fused slag layer forming metal pool then solidifying. Consequently the possible reaction sites or interfaces in ESR are: the electrodeatmosphere, the slag-atmosphere, the electrode tip-slag, the droplet-slag, the metal pool-slag and the metal pool-ingot. Depending upon their composition, tool steels can retain high values of strength and wear resistance to an appreciable depth in the metal. Highcarbon high-chromium steels as reported by Wills (1935) [2] have high wear resistance imported by the numerous hard chromium carbides combined with nondeforming qualities to make these steels very useful for dies. These steels are of ledeburitic type and contain high additions of chromium (5-18 per cent). Their structure as shown by Geller (1978) [3], greatly improves the wear resistance in working hard materials at moderate dynamic loads. Also Hiraoka (1992) [4] concluded that high resistance to thermal shocks and the increase of creep strength and rupture life can be obtained with a high content of chromium (above 10%).

3 Effect of EAF and ESR Technologies on the Yield of Alloying Elements in Tool Steels413 Low alloy semi-thermostable tool steels are used in hot forming dies operating at high dynamic loads. These steels retain an elevated toughness (more than 4 kg.f.m/cm 2 ) both in the longitudinal and lateral directions. This characteristic only is met by limited content of carbide-forming elements. That the carbide phases in them is cementite with small amounts of M 23 C 6 carbide (Geller 1978 [3]). Complex alloying is often resorted in order to improve the carbide distribution and toughness in large sections. The hardness of steel grade (AISI L6, GOST 5XHM, or DIN 56NiCrMoV7) in the core of a block mm is only 1 or 2 HRC numbers lower than on the surface. Which is associated with the high stability of supercooled austenite as concluded by Geller (1978) [3]. High-speed steels are high alloyed high carbon steels with a complex pattern of carbides. It has been shown by Ghomashchi (1985) [5] that these carbides result from an eutectic reaction and segregate with a variety of morphologies, dependent upon the composition of steel and its cooling rate during solidification. It was reported by Ghomashchi (1985) [4] that the eutectic carbides in the (AISI M2 grade) high speed steel are segregated into three chemically different groups, i.e. MC, M 2 C, and M 6 C, where M represents the metallic element and C is carbon. Also it was established that the MC is vanadium rich carbide while the others contain high percentages of tungsten. The M 2 C carbide, however, contains a low percentage of iron. Investigation employing X-ray analysis of extracted carbides by Kuo (1993) [6] has shown that the complex carbides in high speed steel actually consists of several phases. In addition, an intermetallic compound corresponding to Fe 3 W 2 or Fe 3 Mo may also appear as an excess phase in steels having insufficient carbon to satisfy the total number of tungsten, molybdenum and vanadium atoms present. EXPERIMENTAL With the objective of this paper, the effect of EAF and ESR processes on the yield of alloying elements in tool steels was studied. A set of experimental melts were designed, where six 100 kg, melts of three grades of tool steels(cold work tool steel (D3), hot work tool steel (L6) and high speed steel (M2)) were carried out under white and carbide slags in EAF. The effect of EAF slag composition on the yield of alloying elements is studied. Metal samples were collected during this period for chemical analysis. Once the carbon fell below the required analysis and the phosphorus was sufficiently

4 414 6TH INTERNATIONAL TOOLING CONFERENCE low, the oxidized slag was completely removed to prevent reversion of phosphorus to the metal when adding the reducing slag. The composition of these slags is given in Table 1. Table 1. Reducing Slag White Carbide Composition of used EAF reducing slags. Unit wt. % wt. Kg wt.% wt. Kg Lime CaO Fluorspar CaF Ferrosilicon Fe-Si Coke The molten steel produced from every heat was cast into refractory moulds, to yield ingots of 1200 mm long with 60 and 100 mm diameter. The used ingot moulds were clean and without damaged surfaces. Slides were cut from the bottom of metal ingots for the chemical analysis. Furthermore, in order to investigate the effect of physical and chemical properties of used flux in ESR on the yield of alloying elements of these grades of steel, each grade was electro-slag remelted under three different flux compositions, Table 2. The ESR process is a special refining and remelting to make the sound ingot of less impurity with good quality, by making the best use of the physical and chemical properties of slag. This process has gradually been proven to be excellent as an ingot making technique of high-grade steels. The electrical parameters of ESR furnace used in this study are given in Table 3. The rate of cooling water is 7 m 3 /hr. Each one of six heats of steel produced in EAF was electro-slag remelted under three different flux compositions. The steel samples taken during the course of melting and refining processes in EAF and ESR were subjected to chemical analysis. Carbon and sulphur were determined volumetrically by ignition method. Manganese, silicon and phosphorus were determined by X-ray analysis. The final produced steel after EAF and ESR processes were chemically analyzed by the same methods and in addition, they were subjected to complete chemical analysis by spectrographic analysis using Rang-Ailger E983 Polyvag spectrometer.

5 Effect of EAF and ESR Technologies on the Yield of Alloying Elements in Tool Steels415 Table 2. ESR Melt No. Chemical composition and physical properties of ESR fluxes used Flux Flux Composition, wt % CaO/ No. CaF 2 CaO Al 2O 3 Al 2O 3 Density gr/cm 3 Surface Tension mn/m Interfacial Tension mn/m Viscosity Poise Electrical Conductivity Ω 1 cm Table 3. Electrical data of used remelting process Main current Melting current Melting volt Melting power 5.3 ka ka V 45 kw RESULTS AND DISCUSSION To study the effect of electric-arc furnace (EAF) refining slag; on the yield of alloying elements of consumed electrode in electro-slag remelting (ESR), two groups of three tool steel grades were melted in pilot-plant electric arc. Two kinds of slag were used in the EAF; white or carbide for melting of cold work D3, hot work L6 and high speed M2 tool steels. The produced ingots were used as consumable electrodes in ESR. In electro-slag refining process, three types of fluxes were used, Table 2. These fluxes have approximately the same density and different viscosity, interfacial tension, basicity and CaO content. The chemical composition of consumable electrodes produced by EAF and produced steel ingots after ESR (remelted under the different investigated slags) are given in Table 4. Figures 1, 2, 3(a) and 3(b) illustrate the effect of flux composition used in ESR process and the type of refined

6 416 6TH INTERNATIONAL TOOLING CONFERENCE Table 4. Chemical composition of produced tool steels at different refining processes Steel Grade Code No. EAF Slag ESR Chemical Composition, wt % Flux C Si Mn Cr Ni V Mo W Cold Work Tool Steel (D3) Hot Work Tool Steel (L6) High Speed Steel (M2) 3 white white white white carbide carbide carbide carbide white white white white carbide carbide carbide carbide white white white white carbide carbide carbide carbide Figure 1. steel. Effect of EAF slag and ESR flux compositions on Si, Cr and Mn losses in D3

7 Effect of EAF and ESR Technologies on the Yield of Alloying Elements in Tool Steels417 Figure 2. steel. Effect of EAF slag and ESR flux compositions on Si, Cr and Mn losses in L6 slag in EAF on the yield of Si, V, Mn, Cr, Mo and W. In case of refining under white reducing slag in EAF, the decrement in alloying element during ESR such as Si, V, Mn and Cr is slightly more than that for steel refined under carbide reducing slag. This could be attributed to the oxygen content in molten steel. Oxygen content in the molten steel refined under carbide slag is much lower than that refined under white slag. So, it was expected that the yield of alloying elements of consumed electrode refined under carbide slag would be higher than that refined under white slag. On the other hand, an increment by ESR in Mo and W is obtained for steels refined under either carbide or white slag. The increment in such elements could be attributed to the losses of Si, V, Mn and Cr, which consequently lead to increase the concentration of Mo and W in the produced ingots. Unfortunately, published data are scarce on the nature and intensity of oxidation of alloying elements during ESR process. One could describe the oxidation behaviour of alloying elements during the electro-slag refining process as follows: i as the temperature of slag bath rises above the melting point of the metal, droplets melt off the tip of the electrode and fall through the slag, ii as the temperature of fallen droplet is fairly high, the most oxidizable elements will be oxidized. The affinity towards oxygen at the remelting temperature of Cr, Mn, V and Si is increased, respectively. In other

8 418 6TH INTERNATIONAL TOOLING CONFERENCE (a) (b) Figure 3. Effect of EAF slag and ESR flux compositions on (a) Cr, Mn, V and Si losses in M2 steel and (b) Mo and W increement in M2 steel. words, at the molten temperature, silicon and vanadium will be oxidized faster than Mn or Cr. iii increasing the wetting of fallen droplet with slag, i.e. decreasing the interfacial tension, represents a protective layer against diffusion of oxygen towards metal droplet with the result of decreasing the oxidation rate and hence increasing the yield of alloying elements and/or, iv inhibition of the diffusion of oxygen towards metal droplet by increasing the slag viscosity results in increasing the yield of alloying elements. So,

9 Effect of EAF and ESR Technologies on the Yield of Alloying Elements in Tool Steels419 one could expect that using flux No. 2 with the lowest viscosity and the highest interfacial tension (i.e. increasing the diffusion of oxygen towards metal droplet and decreasing the chance of formation a protective slag layer on the molten droplet) would lead to increase the losses of alloying elements. In contrary to this expectation, the obtained results clearly show that the yield of vanadim and silicon of consumed electrode remelted under flux No. 2 is the highest among the three in used fluxes. So, it could be concluded that the yield of such elements of consumed electrode in ESR is completely independent of the type of flux used or its physical properties. On the other hand, the yield of Mn and Cr, which have lower affinity to oxygen than Si and V, depends mainly on the physical properties of used flux. Flux No. 2 gave the highest losses of Mn and Cr. So, one could assume that the oxidation of alloying elements of consumed electrode is taken place directly by oxygen in atmosphere as the droplets melt off the tip of the electrode and/or by diffused oxygen through the molten flux. Elements with high affinity toward oxygen, will be oxidized with atmospheric oxygen during the fallen of metal droplet off the tip of consumed electrode. Consequently the rate of its oxidation depends mainly on the surface area exposed to atmospheric oxygen. In other words, the oxidation rate of such elements depends on the remelting rate, i.e. the number of droplets per unit time. Increasing the remelting rate leads to decrease the yield of such elements. (a) (b) Figure 4. loss. Effect of remelting rate in ESR process on the (a) Silicon loss and (b) Chromium

10 420 6TH INTERNATIONAL TOOLING CONFERENCE (a) (b) Figure 5. Vanadium. Effect of remelting rate in ESR process on the (a) Manganese loss and (b) Figures 4 and 5 show a linear relationship between the rate of remelting and the yield of Si, V, Mn and Cr. The major part of oxidation process for elements with lower affinity to oxygen is taken place during the molten droplet transfer through molten slag by diffusion of oxygen. Consequently, the oxidation rate of such elements depends mainly on the physical properties of used flux. In other words, highest losses of Mn and Cr will be obtained in ingot produced by remelting under flux with lowest viscosity and highest interfacial tension, i.e. flux No. 2. The obtained results confirm this assumption. Figures 6 and 7 illustrate maximum chromium losses in ingot produced by remelting under slag with the lowest viscosity and the highest interfacial tension. This effect is more pronounced in steels with lower carbon content. As the carbon content in the produced steel increases, the yield of chromium increases, which can be attributed to the lower oxygen content of the melt with the higher carbon content. The same phenomenon is observed in the oxidation of manganese. Remelting of steels with low silicon content (i.e. high oxygen content) under slag with low viscosity (i.e. high rate of diffusion of oxygen towards metal droplet) enhances the oxidation process by diffused oxygen through the molten slag with the result of high manganese losses, Fig. 8.

11 Effect of EAF and ESR Technologies on the Yield of Alloying Elements in Tool Steels421 Figure 6. Effect of ESR flux viscosity on Chromium loss. Figure 7. Effect of ESR flux interfacial tension on Chromium loss. SUMMARY AND CONCLUSIONS Correlation and interpretation of obtained data were made and the following results were concluded: Refining of tool steels under carbide slag in EAF reduces the losses of alloying elements in ESR process. In ESR process, the much strong oxidizable elements with high affinity towards oxygen, such as Si and V, are oxidized with atmospheric

12 422 6TH INTERNATIONAL TOOLING CONFERENCE Figure 8. Effect of Silicon content on Manganese loss in ESR steel. oxygen during the fallen of metal droplet off the tip of the consumed electrode. The oxidation process increases by increasing the remelting rate. Elements with lower affinity to oxygen, such as Mn and Cr, are oxidized by atmospheric oxygen and diffused oxygen through the molten slag during the molten droplet transfer. The major part of this oxidation process occurs according to the second mechanism and depends mainly on the physical properties of the used flux. The highest losses of such elements are detected by remelting under flux with the lowest viscosity and highest interfacial tension. The oxidation of oxidizable elements is accompanied by increasing the concentration of non-oxidizable elements, such as Mo and W, in the refined ingots. REFERENCES [1] M. KATO, "Survey on Electro-slag Remelting", Nagoya International Training Center, Nagoya, Japan, (1985), pp [2] W. H. WILLS, "Practical Observations on High-carbon High-chromium Tool Steels", Trans. ASM, 23, (1935), p [3] Yu. GELLER, Tool Steels; Mir Publishers, Moscow, (1978), p. 659.

13 Effect of EAF and ESR Technologies on the Yield of Alloying Elements in Tool Steels423 [4] H. HIRAOKA, Y. KATAOKA, K. YUDA, K. TANIGUCHI, M. SASADA and I. HISH- INUMA, "Application of a High Chromium Steel to Roughing Work Rolls for Hot Strip Mills", ISIJ International, Vol. 32 (1992), No. 11, pp [5] M. R. GHOMASHCHI, "The Morphology of Eutectic Carbides in M2-grade High-speed Steel", Metallurgical Transactions, Vol. 16A, Dec. (1985), pp [6] K. KUO, "Carbides in Chromium, Molybdenum and Tungsten Steels", J. Iron Steel Inst., 173, (1993), p. 363.