Improving Steel and Steelmaking—an Ionic Liquid Database for Alloy Process Design
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The latest development of a thermodynamic database is demonstrated with application examples related to the steelmaking process and steel property predictions. The database, TCOX, has comprehensive descriptions of the solution phases using ionic models. More specifically, applications involving sulphur and oxygen, separately as well as combined, are presented and compared with relevant multi-component experimental information found in the literature. The over-all agreement is good.
KeywordsCALPHAD Ionic 2-sublattice liquid model Steelmaking Inclusions Sulphide Oxide
Customer demand on higher performance and increased competition on the global steel market make it even more desirable to produce better steels at a lower cost. An important factor to consider when developing new steels is the presence of inclusions . Their amount, type and size are of importance for the properties of the final product. Another important factor to consider is the steel’s corrosion resistance, as corrosion is very costly as it dramatically shortens the lifetime of steel products. Almost all iron ore is iron oxide minerals and among them hematite is the most common. During primary steelmaking, oxygen is used to lower the carbon content of the crude iron. In secondary steelmaking , slags which mostly consist of oxides are used to protect the steel and to absorb impurities. Oxide inclusions can have a detrimental impact on the mechanical properties of steel. Finally, oxygen is the main element responsible for corrosion of steels, when nature wants to reclaim the steel, rust, i.e. iron oxide, is formed. In addition to oxygen, sulphur is an important element for steelmaking and steel properties . Even though red-shortness, melting of iron sulphide at forging temperatures, rarely or never occurs today, sulphides are an important type of inclusion which often have a negative impact on mechanical properties. Manganese was early on used to tackle the problem of red-shortness and it is, for many reasons, a common alloying element today. Additionally, calcium is now commonly used as a sulphide former. Both MnS and CaS influence the mechanical properties of steel, particularly the impact toughness. Due to relatively high melting points of sulphides with chromium or nickel, red-shortness is not a problem in stainless steels. In addition, stainless steels are usually alloyed with other sulphide-forming elements, like calcium, manganese or titanium. The sulphides in stainless steels do not only impact the mechanical properties but also the corrosion resistance. More specifically, sulphides play a major role in pitting corrosion  which may be detrimental in high temperature applications.
It is essential to understand and describe the thermodynamic properties to be able to predict the influence of oxygen and sulphur during the steelmaking process and on the final steel properties. An efficient way to make equilibrium calculations is to use the CALPHAD-method . In particular, the compound energy formalism (CEF) [6, 7, 8] has proven useful for modelling solutions phases. The model for liquid phases within the formalism is the ionic two-sublattice liquid model that has been revised over the years since its introduction [9, 10]. This model is used in the TCOX database  in the Thermo-Calc package which contains a thermodynamic description involving many oxide systems. The COMPASS project, supported by the Swedish industry, had the ambition to improve the description in TCOX, mainly by adding sulphur to the database. In this paper, the present state of the TCOX database is demonstrated with calculations involving oxygen and sulphur, separately as well as combined. It should be emphasized that all calculations are done with TCOX alone as the ionic two-sublattice model is able to describe both the metallic liquid and the liquid slag—they merely have different compositions, i.e. are separated by a miscibility gap.
The latest update of the TCOX database includes the addition of 12 new elements: Ar, Co, Cu, F, Gd, La, Mo, Nb, P, S, V and W. Thus, the database now contains thermodynamic data for the following 24 elements: Al, Ar, C, Ca, Co, Cr, Cu, F, Fe, Gd, La, Mg, Mn, Mo, Nb, Ni, O, P, S, Si, V, W, Y and Zr. The intended application is for solid and liquid ionized materials, i.e. oxides, sulphides, fluorides and silicates, or a mixture of these. Applications could include development of ceramics, slags, refractories, metallurgical processing, ESR slags, material corrosion, thermal barrier coatings (TBC), yttria-stabilized-zirconia (YSZ), solid oxide fuel cell materials, sulphide formation and desulphurization, to name a few uses of the database. The scope of this paper will however limit itself to applications related to steels and steelmaking. Calculations related to steel and steelmaking are made and compared with experimental findings in studies found in various published papers. A large number of papers were found, however, only a few were found suitable to compare with calculations as the majority did not present data in a way that could be reproduced, e.g. too little information was provided or uncertainties regarding the boundary conditions were found.
Calculations Involving Oxygen
As mentioned in the ‘Introduction’, the TCOX database contains a thermodynamic description involving many oxide systems. The major alloying elements in steels, Fe, C, Cr, Ni and Mn, and the major slag components, Al2O3, CaO, MgO and SiO2, are included. In addition, some other important oxide-forming elements have been included in the database. In this part of the paper, three application examples related to oxygen during steelmaking are presented.
Calculations Involving Sulphur
As mentioned in the ‘Introduction’, one of the major aims with this paper is to demonstrate how the new sulphide description in TCOX can be used to make reliable calculations related to sulphur in steels. The ambition has been to compare calculations with experimental results. However, it must be mentioned that the number of suitable publications for this purpose is limited. Some binary or ternary systems have been investigated, but they were already used in the assessments of the subsystems and therefore cannot be used to validate the database. Other investigations are concerned with systems consisting of several elements not included in this description. In addition, reports on larger systems are usually concerned with process data and thus not equilibrium. Despite this, some useful results have been found in the literature.
Large efforts have been made to describe the thermodynamics of the Fe-Mn-Ca-Mg-S system, as presented in previous works [15, 16, 17, 18]. Other low order systems are also needed in order to be able to make calculations related to steels and steelmaking, in particular stainless steels where chromium and nickel are the dominating alloying elements. In a parallel work , the system Fe-Cr-Cu-Ni-S is assessed using compatible models for all phases.
Combining Oxygen and Sulfur
The final part of this paper discusses the combination of the existing TCOX database and the recently developed sulphur descriptions. The key system for such a combination is the Fe-Ca-O-S system which is presented in a separate work . As mentioned in that paper, the ionic two-sublattice liquid model seems to give very good extrapolations between oxide and sulphide systems. Moreover, the solubility of sulphur in solid oxide phases and oxygen in solid sulphide phases seem to be very low.
The extended TCOX thermodynamic database, now including sulphur, can be used to make calculations related to steelmaking involving both oxygen and sulphur. It should be emphasised that, although the database covers a large number of elements, not all subsystems are fully assessed. Nevertheless, the equilibrium calculations made by using the database are in good agreement with the reported experimental observations. This includes applications involving oxygen and sulphur separately as well as a combination of the two. The good agreement does not just demonstrate the usability of the thermodynamic database but also the essence of the CALPHAD method, that physically reasonable models and good descriptions of key lower order systems give reliable extrapolations into higher order systems.
The presented work was performed within the COMPASS project, which was funded by KTH Royal Institute of Technology, Outokumpu Stainless Foundation, Ovako, SSAB EMEA and Thermo-Calc Software. The project was also associated with the VINN Excellence Center Hero-m, financed by VINNOVA (Grant number 2012–02892), the Swedish Governmental Agency for Innovation Systems, Swedish industry, and KTH Royal Institute of Technology.
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