Ferro-Niobium Market Trend and Alloying Techniques Dr Jitendra Patel 1,2 Director - International Metallurgy Ltd- UK CBMM Technology Suisse S.A. - Switzerland BACKGROUND Niobium does not occur in nature as a pure metal and is largely found in carbonatite deposits. The world s largest carbonatite deposit with exceptionally large reserves of rich niobium (pyrochlore) ore is located in the Araxá district of Minas Gerais, Brazil. This deposit is managed and operated by Companhia Brasileira de Metalurgia e Mineração (CBMM) and contains an average of 2.5% niobiumpentoxide (Nb2O5). The ore body itself is circular in shape, approximately 4.5km in diameter and one of the characteristic features of the mine is a wide weathered mantle, 1.8km in diameter, which is highly mineralised. The Araxá deposit having reserves estimated at over 800 million tonnes is unique. By comparison, other deposits, even the largest have significantly smaller reserves and ore grades. The maximum pyrochlore content is found in the central weathered mantle contains up to 5% Nb2O5 and the economic value of the Araxá carbonatites is determined not only by the immense reserves and exceptional ore quality, but also by its favourable location and the scope for opencast mining. Figure 1 shows some pictures of the CBMM production facilities and mine at Araxá. The production of ferro-niobium (FeNb) at the Araxá complex has been on-going since 1965, during which time CBMM has continuously developed and introduced modern technologies permitting the ethical and environmental conscious production of niobium-based products. The primary consumers of FeNb continue to be the steel industry. Having first established its effectiveness in generating high-strength and high-toughness properties for the offshore and pipeline industries, it has progressed with time and technology to play a key role in the development of modern high-strength IF, Dual Phase, TRIP, Complex Phase and stainless steels alike. Sustained technical development in niobium metallurgy continues to demonstrate its cost-effectiveness to both steelmaker and the end user for both flat and long products. Having established itself within the industry, over the last few decades CBMM has extended its technical and process development activities to bring the use of niobium into new market areas and new steel products. The company has an extensive development programme ranging from the development of high purity niobium metal for particle accelerators, the use of Nb-master alloys for the aerospace industry to the development of Nbpowder, foil and wire for electronic capacitors and the new oxides for the chemical industry.
THE GROWTH of FeNb USAGE Over the last twenty years the application of niobium in steel has achieved a compound annual growth rate (CAGR) of approximately 9% and can be split into three stages of growth. From the early years through to mid- 1990s the majority of FeNb was being used in the production of high strength low alloy steels (HSLA) destined for large diameter oil and gas pipeline transmission systems. Small microadditions (0.020 to 0.10%Nb) allowed high strengths and importantly, lighter pipe wall thicknesses to be used coupled with significant improvements in material performance demanded by the market. Although a degree of FeNb was being used in the automotive and structural sector it was not until the mid-1990s that the growth in automotive high strength steels gained momentum. This was primarily due to the need to lightweight vehicles but also improve crash safety performance. This period was accompanied by a growth in high strength structural steels as the world economy, largely due to developing nations, started to grow. With taller buildings, bridges spanning ever greater lengths and more energy generating infrastructure, the benefits that lower carbon, high strength, weldable steels could bring was self evident. Aligned to this period was also the installation of new state-of-the-art steel making and rolling mills around the world, further boosting the development of new advanced high strength steels and creating greater penetration of FeNb into the wider market. Over the last seven years a CAGR of 12% has been registered despite the market turbulence experienced in 2009, and against a CARG of only 5% for the global steel industry. This impressive and sustained growth rate has been due to widespread market acceptance of the value addition that small additions of niobium can bring to steel and the end product. This is demonstrable from Figure 2 which shows how the average intensity of niobium that is to be found per tonne of steel produced worldwide has grown from 12 grams/ tonne to nearer 50 grams/tonne from 1990 to 2010 respectively. However, if the intensity figure is examined according to an individual product, such as large diameter gas pipeline, as much as 1,000 grams/tonne of steel has been widely applied (for API X80 and X70 linepipe containing 0.10%Nb). For high strength automotive and structural steels, a level of 400-500 grams/tonne and 200-500 grams/ tonne are typical. Furthermore, within the steel community, some plants have an overall average FeNb usage of over 200 grams/tonne reflecting higher strength, value added products that are produced and sold. PROPERTIES OF FERRO-NIOBIUM From the binary iron-niobium (Fe-Nb) phase diagram a 66%Nb ferro-alloy contains a number of phases as shown in Figure 3. The matrix consists of the intermetallic μ-phase (Fe21Nb19) which contains approximately 60 mass percent Nb. This μ-phase forms the bulk of the ferro-alloy and because it is brittle in nature it facilitates the crushing of the cast ingots of Fe- Nb from its production into smaller pieces. As the embedded niobiumrich islands are more than 95 mass percent containing Nb and the melting point of these phases are different, this results in a melting range of the ferroalloy from around 1,550 C to more than 1,800 C. Therefore, FeNb is considered a class-ii ferro-alloy that does not melt when added to liquid iron but rather goes into solution [1-3]. This is an important distinction to highlight, and does mean that FeNb can be readily used during the steelmaking process. Furthermore, there is no exothermic reaction between the ferro-niobium and liquid iron. There is a sequence of kinetic steps that determines the total dissolution time of a class-ii ferro-alloy (schematically shown in Figure 4). As the FeNb lump material is at room temperature before entering the melt (A), a steel shell forms and solidifies around it after entering the melt (B). Next, inside this steel shell the FeNb heats up before the steel shell re-melts (C) and finally dissolution of FeNb occurs (D-E). In actual fact the shell formation slightly delays the dissolution process, which itself is a diffusion processes between solid FeNb and liquid iron. As is to be expected, a finer FeNb lump size results in a larger total surface area in relation to the volume mass and therefore results in a faster dissolution process to the liquid steel melt. Smaller lumps also have a lower heat capacity and thereby reduce the formation of the shell. As FeNb has a higher specific weight than liquid iron it will tend to sink to the bottom of the ladle. Therefore, the FeNb lump size should not be too large in size as this will help the mixing-process. However, with finer lump sizes any abrasion contact between the lumps during material handling can lead to the generation of additional fines. Although these finer particles will dissolve quicker in the steel melt, there is a greater likelihood that such particles can become entrapped in the ladle slag if the addition isn t made properly. The bath temperature controls the mass flux during the free dissolution period, as shown in Figure 5. By agitating the bath through argon bubbling the melt the mass flux is increased. The free dissolution period (tdiss) can be calculated using the density of FeNb (ρ=8.1 g/cm3), the mass flux (v) and the lump diameter (d) as follows: t diss = d ρ / 2 v (1) The shell period, which is in the order of only tens of seconds in the steel, has to be added to the result of equation 1 to obtain the total mixing time. The total mixing time of FeNb in liquid steel is typically below 300 seconds. 2
FeNb ALLOYING IN LIQUID STEEL The affinity of niobium to oxygen is much lower than that of other microalloying elements and even that of manganese (as shown in Figure 6). Therefore, FeNb can be added to both semi and fully-killed steels and in general it is one of the last alloy additions to be made after bulk alloying of other elements. Recovery rates of 95% and above are typical, which also allows for greater accuracy in achieving the aim Nb content in the final melt. FeNb is available in a range of lump sizes (10-30mm and 5-50mm in 1 tonne bags or 250kg drums), additions via a ladle hopper or manual additions through packing the FeNb into small sacks and then into the ladle can be made. Additionally, FeNb fines (<2mm) in cans have also been developed to add specific amounts such as 10kg (the fines act in a similar way as cored wire additions). Crucially, the addition should be made above a bare spot that has been created in the slag layer as to avoid entrapment of FeNb in the slag. When combined with argon gas stirring through bottom bubbling, an excellent mix is attained (as highlighted in Figure 7). Furthermore, the relationship between the FeNb particle (lump) size and the time for complete dissolution with respect to whether the bath is static or agitated is highlighted in Figure 8. Figure 1. Views of the CBMM FeNb production facilities and mine in Araxá, Brazil For ladles that have only one active porous plug the alloy addition should be made in the main melt circulation plane, preferably above the plug [4]. This plane is defined by the ladle axis and the eccentric plug position. Increasing the stir gas flow rate results in an acceleration of the mixing-process and a shorter total mixing time. Figure 9 represents the mixing behaviour of FeNb with regard to lump size and stir gas flow rate. With cored wire injection feeding, the feeding location and plug position have only minor influence on the required mixing-time as the time for dissolution is very short. From an economical point of view, alloying by 3
Figure 1. Views of the CBMM FeNb production facilities and mine in Araxá, Brazil cored wire is only attractive when the recovery of bulk FeNb is well below 90% as a cored wire feeding station will be required. Finally, additions of FeNb can also be made via the CAS process results in very high recovery since no slag layer covers the liquid bath that could lead to trapping of alloying agent. The argon atmosphere also prevents any loss due to oxidation. In this respect very precise alloying is possible by the CAS process.
FINAL REMARKS As world steel demand continues to grow, higher strength value added products are becoming the steel of choice and the average intensity of niobium use is expected to grow further. CBMM has a long established Niobium Technology Programme that provides expert technical support to its customers and the aligned supply chain. The programme has a successfully worked in collaboration with steel producers to develop new grades for applications where niobium can increase the value of the product and add value to the supply chain. The programme seeks to create value in the final product through the correct metallurgical use of niobium. It is never added as a substitute or as an alternative to other alloys, but where a competitive advantage can be made. The efforts of these initiatives and resulting evolution of the FeNb process has not only resulted in a stable supply of FeNb for the world s steel industry, but has enabled niobium to be introduced to a wide range of new product applications such as Nb-powder for electronic capacitors, oxides for catalysis reactions and thin film applications to name a few. The continuous search for new applications for niobium, the use of modern technologies and consistent improvements of methods, remain fundamental to CBMM s philosophy; innovate, respect, compete. Figure 2. Intensity of niobium usage in steel in grams / tonne from 1990-2010 (Source: CRU and CBMM) Figure 3. Scanning Electron Micrograph of a 66%Nb containing ferro-alloy REFERENCES 1. P Sismanis et al., The dissolution of Nb, B and Zr Ferroalloys in Liquid Steel and Liquid Iron, I&SMJournal, July 1989, p.39 2. S Argyropoulos, On the Recovery and Solution Rate of Ferroalloys, I&SM Journal, May 1990, p.77 3. S Argyropoulos and P Sismanis, The Mass Transfer Kinetics of Niobium Solution into Liquid Steel, Metallurgical Transactions B, Vol. 22B, August 1991, p.417. Figure 4. Schematic representation of the steps determining the dissolution of FeNb 4. K Marx, et al., Advanced Strategies for Alloying Processes in Steelmaking Ladles, Proc. of 5th European Oxygen Steelmaking Conference, Aachen 2006, p.312
Figure 5. Free reaction enthalpies for the formation of various oxides Figure 7. Commercial alloying of FeNb in the ladle with Ar-bubbling Figure 6. Experimentally determined mass flux for FeNb in iron and steel melt [1] Figure 8. Dissolution time of FeNb with varying lump sizes and also with and without agitation Figure 9. Mixing behaviour of FeNb in a large ladle with argon stirring [4]