The Future Role of Ferroalloys in Iron and Steel
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The Future Role of Ferroalloys in Iron and Steel 1 2 3 0ystein Grong , Par J0nsson and Ole-Svein Klevan 1) Department of Materials Technology and Electroch~mistry, NTNU, N-7491 Trondheim, Norway. 2) Department of Metallurgy, KTH, S-100 44 Stockholm, Sweden. 3) Elkem ASA, c/o Elkem Thamshavn, PO Box 10, N-7301 Orkanger, Norway ABSTRACT terms of stricter quality specifications for In the present paper the future role of greater uniformity and higher purity. Thus, ferroalloys in iron and steel has been the future role of ferroalloys bemust viewed reviewed, with particular emphasis on how on the background of the needs of the iron non-metallic inclusions that form during and steelmaker because the requests for deoxidation affect the resulting alloy modifications or possibly new grades micro structure evolution during clearly derive from these. solidification and in the solid state by a process of heterogeneous nucleation. The present paper summarises the progress Traditionally, ferroalloys have been made in developing inoculants and grain regarded as bulk products where the refiners for iron and steel. The ideas overriding concern has been to meet the presented below derive from a broad interest customers' specifications and demands in in ferrous materials ranging from ferroalloy terms of composition, size and costs. More production, iron and steel making to recently, the important role of ferroalloys in fabrication of shaped castings and slabs for iron and steel manufacturing has been fully further working to standard stocks and recognised; particularly their ability to acts finished products by means of as inoculants and grain refiners through a thermomechanical processing and welding l- modification of the inclusion chemistry and 11. The essence of the problem is either to crystal structure. This, in tum, has brought create or introduce submicroscopic non about new ideas about ferroalloy product metallic inclusions in the liquid steel that development and collaboration between can act as heterogeneous nucleation sites for industry and academia on a multilateral different types of microstructures during basis. In the paper, results are presented solidification and subsequent solid state from on-going research projects, where both transformations (e.g. graphite, ferrite or the potential successand of the approach are austenite ), without compromising the demonstrated. Some principal guidelines for resulting mechanical properties. In the new ferroalloy product developments are authors opinion this represents a golden presented towards the end. opportunity for the innovative ferroalloy producer to modify the existing qualities to 1. INTRODUCTION ensure that the overall performance is more Many of the different elements to be added in accordance with the future demand for in iron and steelmaking are supplied through inoculants and grain refiners in shaped ferroalloys. Process developments and castings and wrought steel products. product quality shifts in ferrous metallurgy rely much on improved compositional 2. REFININGGRAIN OF STEELS control, in particular the content of The demand for higher performance 1 impurities and minor alloying elements . materials with optimum combination of This, in tum, has an impact on ferroalloys in properties is steadily becoming more 56~ 14 15 critical. Since the grain size in steel controls and 50µm ' , which makes such alloys the resulting mechanical properties, the unsuitable for grain refining of steel. desired property profile can only be obtained by the development of a properly adjusted microstructure. 12 . At present, no grain refiners (or inoculants) are commercially available for steels, as opposed to cast iron and aluminium alloys where such remedies are widely used to control the microstructure and thus the resulting mechanical properties 4 13 of the final products ' . This represents a new and interesting marked segment for the ferroalloy industry, but the success relies heavily on the chances of controlling the inclusion chemical composition and size distribution during full-scale steel production by a late addition of a specially designed grain refiner. ------------~ 2. 1 Characteristics of Candidate 0.9% ,- Ferroalloys • DT (3.4-6.7 µm) D DM (6.7-1 3.4 µm) ferroalloys have been 0.6% Traditionally, 0 DH ( 13.4-26.9 µm) regarded as bulk products where the • DP ( >26.9 µm) overriding concern has been to meet the 0.3% customers' specifications with respect to composition, size and costs. A B c D E F 2.1.1 Alloy cleanliness Ferromanganese grade It is well established that both FeCr and Fig. 2 Size distribution of non-metallic inclusions found in some commercial grades of FeMn, produced by means of conventional 15 . From Sjoqvist . casting methods, contain an intrinsic LC and MC ferromanganese distribution of oxides and sulphides, the 2.1.2 Dissolution behaviour former group being the most important one a properly sized ferroalloy containing 14, 15 . p1gure· 1 shows examples of complex When a given distribution of inclusions is added to MnS and MnO-SiOi-MnS inclusions liquid steel, the inclusions will be commonly found in commercial MC FeMn. 14 transferred from the ferroalloy to the steel • These systems are characterised by a high 15 . inclusions will eventually loose their oxygen solubility in the liquid state (up to The 14 identity, but their ability to survive in the about 0.5 % 0 by weight or higher) . The liquid steel depends on the time of addition inclusions form naturally both prior to and of the ferroalloy, the dissolution rate of the during the casting operation. owing to ferroalloy and the deoxidation practice reactions between 0 and S and Cr Si and 11 prior to the addition • By Mn present in the alloys. However,' applied be~ause considering and exploring these the cooling rate associated with characteristics, it is possible to control the conventional sand mould casting is low, the dissolution and mixing behaviour of the resulting size distribution of the Cr20 3, Si02, in the liquid steel prior to MnO or MnS oxide and sulphide inclusions ferroalloy well as avoid excessive is rather coarse; as shown in Fig. 2. solidification as coarsening. Typically, the size of the inclusions in inclusion commercial FeCr and FeMn is between 5 ;563 2.1.3 Inclusion control form by a process of homogeneous Controlled laboratory experiments have nucleation during cooling, where the number 8 3 shown that the additions of a strong oxide density may exceed 10 particles per mm . and sulphide former such as Ce to a liquid This is readily achieved in steel weld metals, ferrous alloy will result in the formation of as shown in Fig. 4, owing to the high 16 3 Ce20 3 and CeS . The initial size of the cooling rates involved • A similar inclusion inclusions obtained with this conventional distribution can also be obtained in alloying technique is between 1 and 1.5 µm, ferroalfoys by the· choice of an appropriate but coarsening of the inclusion population casting technique. will occur gradually with time, as indicated in Fig. 3. 2.1.4 Potential for new product 3 .---- ----- - -----,3,000 developments Based on the circumstantial evidence 2.5 2.500 'E ~ . E presented above it is possible to point out a i 2 \ 2,000 .. Ci) \ i:. direction for modification of the existing E , - 'in ~ 1.5 .... 1500, ~ ferroalloy qualities, "O where the overall c:: m 1 1,000 ~ performance is more in accordance with the ::;; E ::J future demand for 0.5 500 z grain refiners in shaped .. castings and wrought steel products. It is 10 20 30 desirable that these alloys contain a high Holding time, min number density of finely dispersed oxides Fig. 3 Coarsening behaviour of cerium-based and/or sulphides with the ability to nucleate inclusions (Ce20 3) in liquid iron melts at 20 austenite or delta ferrite at very small 1600°C. Data . from Gou and Suito undercoolings. Both REM (Ce and La) oxides and sulphides as well as Zr and Ti Therefore, unless the melt is immediately oxides are known to promote the formation quenched after the Ce addition the 11 20 of ferrite or austenite in steels - . In view inclusions will grow larger and eventually of the world-wide steel production of about become detrimental to the steel mechanical 800 million tonnes per year, the potential properties. On the other hand, if a marked for such special treatment alloys supersaturated liquid 1s containing the reactive enormous. elements in solution is quenched from a high 2.2 Grain Size Control in Steels A deep understanding of steel 30,----.-----.----,.-----, microstructures has enabled the steelmaker Low Al (0.018 wt%) to exploit systematically the property Low Ti (0.005 wt%) . t potential of the soft iron. A major objective ~ 20 has been to increase strength without giving r away too much of toughness, with good weldability retained, and all this at minimum costs. In practice, these contradictory r requirements can only be met by designing 2.0 Particia diameter (µ.m} ~ steels with a properly balanced 3 21 22 microstructure ' ' . Fig. 4 Size distribution of non-metallic inclusions commonly found in steel weld metals. 3 2.2.1 r After Grong . As-cast microstructures As-cast steels are prime examples of temperature, a fine distribution of materials where the properties achieved submicroscopic inclusions will inevitably depend upon the characteristics of the solidification microstructure. In general, a coarse columnar grain structure will 56L • inevitably evolve upon solidification if 2.2.2 Grain refinement by inoculation potent heterogeneous nucleation sites ahead Inclusions are known to play an of the solidifying front are absent. In the important · role in development of the presence of effective seed crystals, fine steel solidification microstructure and equiaxed grains form directly in the melt, as substantial grain refining has been shown schematically in Fig. 5. observed in a number of systems, Columnar Equiaxed including2,3, 12, 16-24: Grains Grains • Aluminium-titanium deoxidised low alloy steels due to nucleation of delta ferrite at titanium oxide/nitride inclusions.