Rare Earth Elements and their Application in Ironmaking and Steelmaking
Rare Earth Elements and their Application in Ironmaking and Steelmaking
Rare earth elements (REEs) are widely used in ironmaking and steelmaking because of their remarkable properties, such as the ability to induce a refined microstructure and modify the morphology of the inclusions. REEs are added into iron and steel to meet specific requirements, and they play an important role in improving the quality of the resulting material. Technologies related to the addition of REEs into steels are also being used to develop the new generation of steels.
The chemical properties and states of REEs have been known for a long time, but their applications were not studied extensively. During the period of World War II, it was found that the addition of REEs to steel could considerably improve its properties, since then, REEs have been widely used in the production of steel.
REEs consist of a group of elements which is composed of 15 elements that range in atomic number from 57 (lanthanum) to 71 (lutetium) on the periodic table of elements, and are officially referred to as the ‘lanthanoids’, although they are normally referred to as the ‘lanthanides’. The REE promethium (Pm, atomic number 61) is unstable in nature. Yttrium (Y, atomic number 39) is normally regarded as an REE because of its chemical and physical similarities and affinities with the lanthanoids. Scandium (Sc, atomic number 21) is chemically similar to, and thus hence included sometimes with the REEs.
REEs have similar electronic shell structures, where electron numbers range from 0 to 14 in the 4f transitional level, and the outer electrons consist of one electron in the ‘d’ orbital and two electrons in the ‘s’ orbital. REEs have atomic diameters ranging from 1.641 angstroms to 2.042 angstroms, and are slightly more electronegative than alkali earth metals, and so can lose electrons and easily become positive ions. REEs are hence chemically active in liquid steel.
REEs have very strong chemical activity because of their unique electronic structures. The filling and spatial extent of the outer electron (5𝑑5d and 6𝑠6s) shells, which are most important in chemical bonding, are essentially unchanged across the entire rare earth series. What varies from element to element is the number of electrons in the inner ‘f’ shell. Since the electro-negativities of the atoms are nearly identical, a compound which incorporates a given rare earth can easily incorporate one of the others as a substitute.
Traditionally, the REEs are divided into two groups on the basis of atomic weight namely (i) the light REEs which comprise lanthanum (La) to gadolinium(Gd) (atomic numbers 57 to 64), and (ii) the heavy REEs comprise terbium (Tb) to lutetium (Lu) (atomic numbers 65 to 71). Some authorities include europium and gadolinium within the group of heavy REEs. Yttrium, although light (atomic number 39), is included with the heavy REE group because of its similar chemical and physical properties.
Cerium (Ce) is positively trivalent and is the second lanthanide element belonging to the light REEs located in Group IIIB, the sixth period of the periodic table. The electronic structure of Ce makes it a strong reducing agent. At room temperature, Ce can react with oxygen (O2) and under some conditions it can react with non-metallic elements. Physical properties of Ce are shown in Tab 1.
|Tab 1 Physical properties of cerium|
|Melting point||deg C||798|
|Boiling point||deg C||3,426|
Ce, La, Nd (neodymium), and Pr (praseodymium), normally in the form of a mixed oxide are known as misch metal (Fig 1). Misch metal is used in steelmaking to remove impurities, as well as in the production of special steels. These REEs, along with yttrium (Y), individually or in combination, are also used in various special alloys of chromium (Cr), magnesium (Mg), molybdenum (Mo), tungsten (W), vanadium (V), and zirconium (Zr).
Fig 1 Rare earth elements of misch metal
The history of the REEs began in 1788 in Sweden. The discovery of the rare earths themselves occurred over nearly 155 years from 1788 to 1941. This was followed with the issue of separating them from one another for scientific study or industrial application. This has been one of the most challenging tasks of rare earth technology. While the attempts in separating the rare earths began with the work of Mosander during the period 1839 to 1841, much of the effort directed to the separation of various rare earths occurred from 1891 to 1940. During this period, from the available mixed and separated compound intermediates, many rare earth alloys and metals were prepared and commercial applications were developed for mixed or roughly separated rare earths. The following two decades, 1940 to 1960, were the most productive in terms of effective process development. Beginning in the 1960s, much progress was made in the large scale production of purer rare earths, in the identification of newer properties, and in their use in a variety of important commercial applications. The usable forms of rare earths encompass naturally occurring oxide mixtures, and products synthesized from them, high purity individual metals, alloys, and compounds. Fig 2 shows some oxides of the rare earth elements.
Fig 2 Oxides of rare earth elements
Rare earths have major applications as metallurgical alloys. The oldest of these alloys is misch metal which is an alloy consisting only of Ce, La, Nd, and Pr, normally in the form of mixed oxides, with the individual REE present in the same proportion in which they naturally occur in bastnasite or monazite. Misch metal is the form in which the REEs were introduced as constituents in numerous alloys for a variety of applications
With respect to use as an addition to steel, the most important REEs are lanthanum and cerium. Starting in the late 1960s, additions of REEs to steel became a widely accepted practice. The practice of adding rare earths to steel was reviewed by Wauby in 1978. A few kilograms of misch metal were added to each metric ton of special steel used in the manufacture of an Alaska oil pipeline since misch metal improved the physical properties of steel under arctic conditions. Because of its use in the Alaska pipeline steel, a major increase in demand occurred for misch metal in the period 1971 to 1978. In spite of the fact that the major consumption of REEs is in the iron and steel industry, the annual tonnage of the steel which was treated with La had indeed been very small. The use of rare earths in the form of rare earth silicides or misch metal in steels grew explosively in the 1970s and peaked around 1975.
The use of REEs as either additions of pure lanthanum or of pure cerium or as misch metal to improve the mechanical properties of steels began around 1970 and was common from around 1970 until around 1980. The use of rare earths has much declined since the early 1980s as their use seems incompatible with the continuous casting process of liquid steel. From the point of view of steelmaking the outstanding features of the REEs are that they form extremely stable oxides, sulphides and oxy-sulphides. When REEs are added to steel, they combine both with oxygen (O2) and sulphur (S) forming inclusions which are oxides, sulphides and oxy-sulphides.
In the early 1990s, the application of iron and steel needed these materials to have improved strength at room temperature under static loading. The improvement in the strength was being achieved during that time at the expense of the toughness. For meeting the needs of the automotive industry, the development of next-generation steel started with the aim of improving both the strength and the toughness. Use of REEs has provided great potential to improve the strength and the toughness properties of steel.
REEs also react with S and O2 to form products with high melting points, and can thus reduce the harm to the properties of steel due to these impurities. They can be used to reduce the quantity of O2 and S to an extremely low level. REEs can dissolve in iron to form an alloy solution. The dissolved REEs exist and function in steel, acting as defects based on their difference from iron atoms. Such properties mean that REEs can be used as cleaning agents and inclusion modifiers, making them promising elements to promote the strength and toughness of new generation steels. As an example, maraging steel can be successfully produced by using La.
Due to environmental concerns, the large energy consumption associated with producing steel, and the great demand for special steels with better performance, it has become necessary to develop advanced technologies for the steel production, in order to improve the material properties, reduce the cost and cut pollution. In this context, inclusion engineering, especially technologies concerning REE modified steel, can be used to create advanced steel with high toughness and strength at lower cost and energy consumption.
REEs additions, either as misch metal or rare earth silicide, are used in several ladle treatment processes in secondary steelmaking. In ironmaking, REEs provide the basis for the graphite morphology in cast irons. In steelmaking, REEs additions are made to aluminum (Al) killed steels for desulphurization and the control of inclusion composition and morphology. Rare earth oxides can also be used in the desulphurization of medium calorific value gaseous fuels and stack gases. Ce-S-O and La-S-O phase diagrams are normally used to determine the role of the REEs in the external processing of iron and steel and gaseous desulphurization.
The REEs played a leading part in the discovery and commercialization of nodular iron. Nodular iron has properties similar to mild steel and is essentially a ductile cast iron. Nodular iron results when the graphite flakes in cast iron are converted to nodules or spheroids. In the 1940s it was discovered that spheroidal graphite could be routinely produced in the laboratory in irons containing 0.02 % Ce. The REEs clean the metal of elements which prohibit spherical graphite growth, and the compounds they then form provide heterogeneous substrates for graphite nucleation. Its good physical and foundry properties have made nodular iron an attractive engineering material, particularly in the automotive industry. The ductile iron market is quite sizeable for the REEs.
During the production of nodular iron, REEs are added as misch metal or mixed rare earth silicides and not as pure REEs, mainly due to cost considerations. Mg has emerged as a competitor for rare earths for graphite nodularization in the cast iron, threatening the continued use of misch metal for this purpose. Additions to the extent of around 0.2 % serve to spheroidize graphite, to counteract the effects of deleterious impurities, and to enhance the fluidity of the liquid iron. The acceptance of misch metal over magnesium, even though the former costs more, is due to the fact that magnesium volatilizes from the melt, making it difficult to control the quality of the ductile iron, while the rare earths do not volatilize and thus the quality of the final product is easier to control. The addition is normally made as a ferro-misch metal silicide or ferro-cerium silicide.
The addition of rare earths to steels is continually gaining acceptance. The extent of addition in this particular area of application amounts to around 0.2 %. The addition of REEs helps in controlling the S concentration and hence brings about improvements mainly in the workability and the transverse impact values. The major use of the REEs in the steel production has been as additives to sheet, plate, and pipeline steels. Sulphidic forms of the REEs constitute the most stable sulphides known and only calcium sulphide has a comparable free energy of formation. Additions are normally made in the form of a ferro-misch metal (or cerium) silicide.
The harmful effect of S on the mechanical properties of freshly continuously cast steel is well known. Iron sulphides form and concentrate at the boundaries between the grains of steel formed on solidification. Such steels are very brittle and fracture on working. The addition of REEs in steel not only forms rare earth oxides, but also rare earth sulphides and nitrides. The rapid and complete desulphurization which occurs in this process can lower the amount of S to an extremely low level. I addition, the rare earth sulphides which form can modify the compositions and shapes of the sulphide inclusions, thus improving plasticity of the inclusion during hot working, as well as the mechanical properties of the steel. The rare earth nitrides which form when REEs are added to the steel improve not only the mechanical properties but also the corrosion properties of the steel. The diffusion of REEs in steel can produce finer nitrides, better microstructures, and higher micro-hardness, which are needed to improve its corrosion resistance.
Addition of REEs to steel causes the S content to be captured in the form of very stable compounds such as RE2S3 or RE2S2O. These compounds tend to form globular or spherical inclusions which do not concentrate at the grain boundaries, thus greatly enhance the ductility. The sulphides and oxy-sulphides are very stable at steelmaking temperatures and, unlike other sulphides such as those of manganese (Mn), they neither deform nor elongate under the steel processing conditions. As the concentration of REEs is increased, manganese sulphides (MnS) inclusions are displaced by sulphides and oxy-sulphides of the REEs. The stability of granular rare earth sulphides reduces the detrimental effects of elongated MnS inclusions on toughness.
Besides improvements in the toughness characteristics of the high strength low alloy (HSLA) steels, additions of REEs also improve fatigue, creep, and several other mechanical properties of the steels. The REEs react quite efficiently with hydrogen (H2) in steel and also lower the H2 diffusion coefficient.
REEs are added to steel as misch metal, rare earth silicides (RE content 30 %), and alloys such as Fe–Si–10 RE, and Mg–Fe–Si–0.1 to 0.2 RE. The effect of REEs in steels, in whichever form they are introduced, is the same. Though the quantity of misch metal normally added to iron and steels is around 0.1 % to 0.2 %, still such small additions do lead to considerable beneficial effects in both nodular iron and steels.
In the early 1960s, researchers at General Electric discovered that stainless steel containing both Al and Y possessed exceptional high temperature corrosion resistance. Beginning then and until around 1975, these alloys were produced for limited application in the nuclear industry. The alloy, known as ’fecralloy’ to denote the presence of Fe, Cr, Al, and Y in it, has since been widely adopted for the fabrication of furnace heating elements and has been considered as a replacement for ceramic substrates in emission control catalysts for the automobile industry. La is also used in high temperature iron based alloys. An alloy with 200 ppm (parts per million) La combines oxidation resistance to 1,100 deg C with good ductility and ease of fabrication.
The deoxidizing properties of Ce and La are considerably higher than that of Al, which is used as the strongest deoxidizing agent in conventional (structural) steels. Ce and La have a considerably higher affinity for O2 than other common deoxidizing elements, such as Mn and Si (silicon). These two most frequently used REEs normally form very stable oxides, Ce2O3 and La2O3, respectively. REEs are also characterized by a high affinity for S and form sulphides CeS, Ce2S3, and La2S3, or oxy-sulphides (RE)2O2S, which is reflected in their use by a reduction in the proportion of MnS and a better degree of desulphurization of steel. The advantage of using REEs is the even distribution of S in the steel even during slow cooling, which is due to the high melting point of the majority of the REE oxides and sulphides. The melting point of these inclusions is higher than the solidification temperature of the steel, which leads to the inclusions being precipitated earlier and being located inside the steel grain. The average length of sulphide inclusions decreases sharply with increasing Ce content in steel, upto a content of at least around 0.06 % Ce. Then, it remains almost constant.
Ce also forms other stable compounds in steel, namely carbides and nitrides. Knowledge of the thermodynamic properties of liquid and solid phases at high pressures and high temperatures and the thermo-chemical evolution and thermodynamic behaviour of oxide inclusions in specific steel compositions is necessary for understanding melting phase relations in complex geological systems. The elements of the REEs have a very high ability to deoxidize and desulphurize steel (their effect is definitely stronger than the addition of Mg), which results in minimizing the quantity of O2 and S in the steel.
High reactivity of REEs causes the formation of oxides and sulphides, the presence of which can additionally modify the composition and morphology of magnesium oxides and sulphides. REEs refine the continuous cast structure of the steel, reduce the zone of columnar dendrites, and improve the quality of the macro-structure of cast steel (slab, bloom or billet), which also has a positive effect on the mechanical properties of steel products.
When using REEs, the micro-segregations of alloying elements, especially Si, S, and P (phosphorus) are limited. Also, the presence of Ce in steel has a favourable effect on its recrystallization. Alloying with REE also increases fatigue strength. Fatigue cracks are reduced due to the formation of sulphide inclusions of a favourable nature. The quantity of REEs needed to achieve the optimum content in the steel, especially in terms of mechanical properties, depends on the O2 and S contents in the steel before their alloying.
For ensuring high utilization of Ce when alloying into the liquid steel, it is desirable to inject the filled profile with misch metal (typically around 50 % Ce, 25 % La with smaller amounts of Nd and Pr) after vacuum degassing, i.e., after reducing the content of O2, S, and inclusions in the steel. However, sufficient time is to be provided for the formation of non-metallic inclusions (REE oxides and sulphides, which are formed due to the high affinity of REEs for O2 and S) after alloying with misch metal into liquid steel.
REE additions to steel have been the subject of different studies with several different objectives. First, there have been studies on the use of REEs to desulphurize steel. It is difficult to imagine that REEs are successful in desulphurizing steel since the densities of the oxides, sulphides, and oxy-sulphides of the REEs are around the same as the density of the liquid steel.
REEs additions have been used for shape control of inclusions, that is, the REEs additions un the appropriate quantities result in the formation of equi-axed inclusions and elimination of inclusion stringers. This shape control has the result of eliminating differences in the Charpy impact energy when the Charpy samples are of the longitudinal orientation and the transverse orientation. In addition, REEs additions can lead to an increase in the upper shelf toughness and this effect is due to the inclusions being larger and more widely spaced when REEs are added than when the S is transformed as the particles of MnS or CrS (chromium sulphide).
Temper embrittlement of steels is associated with the segregation of impurities such as S, P, Sn (tin), As (arsenic), and Sb (antimony) to prior austenite grain boundaries. The segregation of these impurities to the prior austenite grain boundaries promotes fracture along prior austenite grain boundaries and results in an increase in the ductile-to-brittle transition temperature. REEs additions reduce the severity of temper embrittlement by combining with O2 and Sn, as well as getting P, Sn, Sb, and As to form inclusions containing these elements.
Function of Ce in steel – Ce is used in the improving the steel cleanliness, modifying the morphology of inclusions, and micro-alloying of steels. It has the ability to improve the cleanliness of steel, as, for example, it can deoxidize and desulphurize steel and prevent harm due to H2, P, As, Sn, and Pb (lead). Ce not only cleans the liquid steel but also refine the microstructures of continuous cast steel. This is because Ce can modify the properties, distribution, and shapes of inclusions. The dissolving of Ce in the crystal lattice of iron results in lattice distortion which improves the toughness of the resulting steel. Ce can also segregate on the grain boundaries and hence overcome the weakness due to the presence of other elements.
Improving steel cleanliness with REEs – The cleanliness (deoxidization, desulfurization, and removal of elements with low melting points) of steel by REEs relies on their reactions with O2, S, Pb, As, Sn, and Sb, which can easily form non-metallic compounds with high melting temperatures. Steel cleanliness is achieved when these non-metallic compounds float to the upper slag layer and hence the quantities of impurities in the resulting steel can be reduced. Based on the Gibbs free energy of RE compounds, when the O2 content is sufficiently low, REEs combine with S first and then remove it.
Modification of Inclusions -REEs can affect the structure, morphology, and distribution of inclusions and impurities and hence eliminate the defects in steel. The properties of steel are greatly improved when its grains are refined by REEs, and the products from deoxidization and desulphurization are modified by the addition of REEs to the liquid steel. Products with high melting points easily cluster and float, improving the inclusion distribution. Inclusions with high melting temperatures are randomly distributed around the grain boundary when a small quantity of REE is added. Complete desulphurization can be achieved if the ratio of REE to S is precisely controlled. Modification can be achieved when the REE/S ratio is higher than 3. Compounds of REEs and S can replace MnS, fully eliminating elongated MnS inclusions. RE compounds, which look like small spheres or spindles evenly distributed in steel, do not deform during the continuous casting. Since REE inclusions and steel have similar thermal expansion coefficients, the fatigue strength of steel is considerably improved with the addition of REE. This is because stress concentrations are avoided during the continuous casting.
Micro-alloying – In micro-alloying, the microstructure and texture are influenced by solid dissolution and the reaction of the solid phase, and hence these can be controlled to improve the properties of steel. The criteria for judging a micro-alloy are based on the element state, dissolution, and the quantity dissolved in steel. For metallic materials, solid dissolution means the atoms exist in substrate matrices and there are random defects. For REEs, small quantities dissolve in steel rather than form a solid solution according to the Hume-Rothery principle, and the atomic diameters of REEs are 0.5 times larger than those of iron atoms. The diameters of REEs are altered by polarization between metallic and non-metallic atoms.
For example, when the degree of ionization of a La atom reaches 60 %, the atomic covalent diameter of La is reduced to 1.277 angstroms from 1.877 angstrom, which is 5.5 % times larger than that of an iron atom (1.210 angstroms). The atoms of REE form a substitutional solid solution in the crystal by occupying the lattice section points using a vacancy diffusion mechanism. The tested solubility of REEs in steel is of the order of around 0,000001 ppm to 0.00001 ppm based on the electrolysis of rare earth inclusions and plasma mass spectrometry, using physical and chemical methods and internal friction peaks. Tiny quantities of an REE dissolved in steel can distort the iron crystal lattice and improve the strength of the steel. REEs tend to segregate at grain boundaries and eliminate the local weaknesses due to S and P atoms in steel, improving the strength of grain boundaries and shock resistance.
Grain refinement – Solid particles of rare earth compounds act as heterogeneous nucleation sites and can segregate at the interface of crystalline structures, hindering cell growth. Thermodynamic conditions are hence needed to refine the steel grains with the addition of REEs. The effects of adding REEs at various quantities on macroscopic and microscopic crystal structures have been studied. One study has found that the distance between the crystalline structures is decreased considerably and that the solidification of an inter-dendritic liquid film with low melting temperature is promoted with the addition of REEs. The functions of REEs in high S steel, in which rare earth compounds act as the core of non-spontaneous crystallization, are grain refinement and the promotion of the equi-axed grain rate. The effects of REEs on the crystal structure of low S steel are reflected in the thinning space of the dendrite arms. Study also shows that the heterogeneous nucleation sites mainly composed of Ce2O3, which are formed after the addition of REEs in liquid steel due to their high melting point, can have the effects on grain size of ultra-low C (carbon) steel. The yield strength of the steel is considerably improved and the cast grain size is considerably reduced due to the increasing number of nucleation sites of the solid and liquid phases.
Influence of REEs on the microstructures of steel – In plain C steel, REE atoms exist in cementite as replacements of iron atoms rather than as carbides. REE atoms tend to segregate at the interface of ferrite and cementite due to their large radius and high aberration energy. Hence, RE atoms are mainly distributed at the interface of cementite alloys and grain boundaries. The grain sizes of austenite decrease considerably with the addition of a greater amount of REEs. The austenite grain size can be controlled to around 10 micrometers when the quantity of REEs is more than 50 ppm. However, the austenite grain size does not change considerably with further increases in the quantity of REE.
Grain boundaries are the preferred nucleation sites for the precipitated phase. The existence of dissolved atoms and the higher rate of atomic diffusion at grain boundaries seen with volume diffusion or lattice diffusion contribute to nucleation and grain growth for the precipitated phase. The segregation, diffusion, and precipitation of REE atoms at grain boundaries can greatly affect the properties of steel. A limited quantity of REEs can improve the preservative ability of steel, while an excess quantity of REEs can deteriorate this. It has been reported that steel with 21 ppm of REEs has the optimal properties of hardness and inclusion modification.
A number of studies have been done to investigate the effects of REEs additions on the degree of impurity segregation to prior austenite grain boundaries and on the severity of temper embrittlement. It is the ability for the REEs to get these impurities as inclusion particles which suggests that REEs additions can be useful from the standpoint of reducing susceptibility of steel to H2 embrittlement. If rare earth additions are used then they eliminate the majority of the segregating and embrittling impurities to reduce the inter-granular fracture when H2 is present in the steel. The importance of the use of the REEs is not that they eliminate H2 embrittlement or even reduce the severity of the H2 embrittlement but that inter-granular fracture seems to be eliminated and this eliminates one weak link in the microstructure. Further, additions of appropriate quantity of REEs like Ce considerably reduce the susceptibility of steel to hydrogen-induced cracking.
In a study in which the effect of misch metal on the behaviour of inclusions in the liquid stainless steel was studied, it has been noticed that during the use of the elements Ce, La, Al, and O2, clogging of the immersion nozzles occurs with oxides of these elements during the continuous casting of steel. There was an increased tendency for large inclusions to form, resulting in clusters when Al was used. While analyzing this theoretically, it was confirmed that the capillary interaction is strongly influenced by the particle shape, size, and surface tension of inclusions in the liquid.
In an another study on nozzle clogging mechanisms during casting of REE treated stainless steels weighing of 350 kg, the experimental results showed differences between fast and slow clogging rates. Steel containing mainly small single inclusions clogged faster than steel containing mainly large inclusion clusters. The reason was believed to be that the small inclusions could stick to the nozzle wall at narrow passages where the steel flow velocity was high, while the larger ones could not. The source of the small inclusions was believed to be reoxidation.
The addition of REEs to steel also affects the corrosion resistance of steel. The REE addition reduces the surface electro-chemical activity and the adsorption tendency of chloride ion on the metal surface. The addition of REEs has a considerable effect on both the meta-stable and stable pitting processes – the dissolution of inclusions induced the electro-chemical activities of pitting corrosion and further inhibits the propagation of local corrosion.
In a study done to know the effect of Ce on the corrosion resistance and mechanical properties of the structural quality steel sheet in a seawater environment (3.5 % sodium chloride solution), Ce was added to the steel during production. The experimental results had shown that the corrosion resistance of the structural quality steel containing cerium was higher compared to that of structural steel, which did not contain Ce. Mechanical properties were also enhanced such as tensile strength increased by 6 %, and yield strength by 8 %. At a Ce content of 0.009 % and at a test temperature of 0 deg C, the notch strength of the steel is increased by 9 %.
The effect of different Ce contents on the micro-structure and morphology of inclusions in structural steel has been studied. A sample with an optimal quantity of 0.0235 % Ce and an S/O2 ratio of around 7 contained predominantly cerium oxides, oxy-sulphides, and only a small amount of sulphides. By increasing the quantity of cerium and decreasing the S/O2 ratio, an increased quantity of oxy-sulphide inclusions was noticed. Inclusions of 4 micrometers to 7 micrometers in size served as heterogeneous nucleation sites for intra-granular acicular ferrite formation.