Sulfur partition by process stages of metallurgical production of JSC EVRAZ NTMK

The current level of development of the industry requires the manufacture of steel with high purity in relation to detrimental impurities. Sulfur is one of such impurities that significantly reduces the service properties, the content of which is strictly regulated in the finished product. Deep desulfurization may be achieved through development of cross-cutting technology of metal production as a single process including agglomeration and blast-furnace process and steelmaking. Sulfur partition by metallurgical process stages via conversion from raw materials of the blast-furnace process to steel continuously cast blank is studied in this work. It is shown that removal of sulfur from the half-finished product at the desulfurization plant will not provide the required sulfur content (less than 0.005 %) in the steel ingot. It is determined that the activity plan comprising treatment of the half-finished product at the desulfurization plant, smelting in the converter, treatment of steel at the ladle furnace unit and then in the vacuum vessel is required to obtain the final content of sulfur of less than 0.005%.


Introduction
Modern consumer of metallurgical products imposes increasingly demanding requirements to the produced steel quality. Sulfur is subjected to the strictest limitations on content in metal [1]. Significant number of theoretical and practical works is dedicated to deep desulfurization of metal in order to obtain low sulfur content in the finished products. This direction has special actuality for production of high-strength steels (pressure vessels, oil and gas pipes of large diameter [2][3][4]), sheet steels for deep drawing [5,6], reinforcement steels [7,8], as well as mass-produced steels [9,10]. Steels with the regulated low sulfur content are required for aviation, power and transport engineering, construction and other industrial fields [11][12][13].
Normally increase of requirements to steel properties outstrips development of processing methods aimed at improvement of metal purity. As a result, it is necessary to search for combinations of efficient methods of refining for all types of process stages of metallurgical production. Currently, not a single type of metallurgical raw materials is used without preliminary preparation, which allows to carry out measures for sulfur removal at the stage of preparing the charge materials for blast-furnace smelting [14,15]. Behavior of sulfur during agglomeration of iron-ore materials is well-studied, therefore, in case of correct process organization, a high sulfur removal rate will be achieved ensuring structural and other characteristics of agglomerated materials. Major part of work for removal of sulfur from metal is carried out in the blast-furnace process and during cast iron treatment afterwards. Continuous mode of desulfurization implemented in the blast furnace is the most efficient among all periodic processes offered alternatively. Cast iron quality becomes particularly important for the stage of converter operation where the possibilities of sulfur removal are limited [16][17][18][19].
Steel treatment at ladle furnace unit is an integral part of production of high-quality metal [16]; the technology provides induction of new high-basic free-running slag. Final concentration of sulfur in steel reduces together with increase of slag layer and its main basicity. Duration of argon blowing of the melt, increase of blowing rate and steel temperature result in reduction of sulfur concentration. Desulfurization conditions will deteriorate in case of increase of initial sulfur concentration and slag oxidation [17].
Special place of sulfur among other impurities included in steels is related to its impact on mechanical properties. Increase of sulfur content induces red shortness, decreases impact hardness, corrosion resistance, electric properties, and deterioration of plastic properties results in mass faulty production during stamping of deep-drawing hollow parts of sheet steel [1,11,18].

Theoretical data
Sulfur has unlimited solubility in the liquid iron (up to 38 % if its content in FeS is 36.5 %) and limited solubility in the solid iron: in γ-Fe at 1365 о С ~0.055%, and in α-Fe at 900 о С ≤0.015% and at room temperature ~0.0001 % [1]. At transition of metal from liquid state to solid state, solubility of sulfur in it sharply decreases and continues to decrease at cooling of solid metal from crystallization temperature to room temperature. That is, hardening and cooling of metal with high sulfur content will lead to eutectic and release of FeS iron sulfide from the solid solution.
Fe-FeS eutectic has a low melting point (985 o C) compared to the temperature of the solidus of steel. In the presence of oxygen due to the formation of oxysulphides, eutectic crystallization occurs at even lower temperature. Presence of liquid state along grain boundaries in combination with contraction stresses of ingot and deformations of forging, rolling, etc. will lead to formation of cracks. Such phenomenon has been termed the 'red shortness'. Sulfur may also have a negative influence on impact hardness of steel at low temperatures (less than -30 о С), embrittling metal. High sulfur content leads to sharp anisotropy of properties in the finished rolled products ( Figure 1) [1]. Anisotropy of mechanical characteristics which depend on the mutual orientation of sulphide inclusions (orientation of sample relating to direction of rolling) is mostly demonstrated at plastic properties of metal, for example, relative elongation, relative reduction and especially impact hardness.
However, the Figure 1 shows that if sulfur content is less than 0.005 %, impact hardness of steel in transverse samples is significantly lower than in transverse ones, i.e. metal according to its properties becomes homogeneous at full volume.
Consequently, sulfur content of the metal, which does not influence the homogeneity of its properties is taken as not more than 0.005 %. However, in order to guarantee the absence of influence, its content must be even less and must not exceed 0.0001 % -the limit of dissolution of this element in iron at standard conditions [1].
Finished steel in some quantity contains all impurities that are part not only of metallic melting stock, but also of the slag and gas phases and lining, so the sulfur content in the final product depends on both the technology of production and the raw materials used. Conversion in metallurgy usually means the processing of material in which the chemical or phase composition of the product significantly changes its physical and mechanical properties. In order to achieve the required results in removing sulfur from the metal, it is necessary to analyze the main sources of sulfur input to the melt,

Research results
The classical steel production scheme includes the following stages: 1. Obtaining of ore concentrate (ore production, sinter and pellets production. 2. Blast-furnace process (cast iron production). 3. External cast iron desulfurization. 4. Basic oxygen furnace process, steel production. 5. Steel processing in ladle furnace unit (LFU). 6. Metal processing at decreased pressure units.

Steel teeming in continuous-casting machines (CCMs).
At each stage, the sulfur content in metal will change, depending on the physical and chemical processes in the melting facility.
Depending on the chemical composition of cast iron, several special technological options are used to convert it into steel. JSC EVRAZ NTMK implemented vanadium conversion -processing of vanadium cast iron into steel and vanadium slag. The classic scheme is distinguished by the presence of a duplex process -a two-stage conversion. The first stage consists in extracting vanadium from cast iron and converting it as an oxide phase into slag. The result of this stage is the production of halffinished product (de-vanadium cast iron) and vanadium slag. After vanadium extraction, the molten metal with carbon content of 2.6-3.5 % goes to the converter for processing to obtain steel with a given chemical composition -this is the second stage. Between two stages of conversion, the halffinished product can be delivered to the desulfurization plant. Change of sulfur content in metal in stages of metallurgical conversion is given in Table 1.
Graphically the change of sulfur content and the degree of its change for conversion of production are presented in Figures 2 and 3.
The data presented in the figures show that the sulfur content after treatment at the desulfurization plant is 0.0068%, which is not sufficient to eliminate the effect of detrimental impurities on the service properties of the products. The minimum sulfur content in metal reaches the value of 0.0039% only with the complete metal treatment complex in the extra-furnace steel processing units -the ladlefurnace unit and the circulation vacuum vessel.
In order to study the sulfur partition (LS) between metal and slag in the LFU, let us compose a system of equations (1)-(3).  (2) λ = (3) whereλ -mass ratio of slag and metal.
Using equations 1-3, let us define the sulfur partition coefficient between metal and slag LS: Let us apply the formula 4 and the data of Table 1 for calculation of average values of the sulfur partition coefficient between metal and slag. Let us compare the calculated value LS=47.0 with the data of sources [1] (LS = 50-100) and [20] (LS ≈50-250). Despite the 'satisfactory' final sulfur content in metal (less than 0.005%), the steel manufacture technology including such stages as the treatment of the half-finished product at the desulphurization plant and the treatment of steel in the LFU and vacuum vessel, has reserves for the removal of sulfur. By adjusting the composition of the LFU slag, it is possible to increase their desulphurization capacity.

Conclusion
Thus, the current level of development of the industry requires the manufacture of steel with high purity in relation to detrimental impurities. Sulfur is one of the impurities that significantly reduces the service properties and the content of which in the finished product is strictly regulated. Deep desulfurization may be achieved through development of cross-cutting technology of metal production as a single process including agglomeration and blast-furnace process and steelmaking. Sulfur partition by metallurgical process stages via conversion from raw materials of the blast-furnace process to steel continuously cast blank is studied in this work. It is shown that removal of sulfur from the half-finished product at the desulfurization plant will not provide the required sulfur content (less than 0.005 %) in the steel ingot. It was determined that the activity plan comprising treatment of the half-finished product at the desulfurization plant, smelting in the converter, treatment of steel at the ladle furnace unit and then in the vacuum vessel is required to obtain the final content of sulfur of less than 0.005%. According to the study of sulfur partition coefficient between metal and slug, despite the low final sulfur content in metal, this technology has reserves related to desulfurization achieved by correction of LFU.