Utilization Optimization of Ferroalloys during Steelmaking
Utilization Optimization of Ferroalloys during Steelmaking
Ferroalloys are alloys of iron with a high percentage of one or more of other elements such as silicon (Si) and manganese (Mn) etc. They are brittle and unsuitable for direct use in fabricating of products of use. Ferroalloys are important raw materials for the steelmaking process. They are used mainly for deoxidation and alloying of steels.
Ferroalloys have lower melting ranges than the pure elements and have lower density hence can be incorporated more readily in the liquid steel than the pure elements. Ferroalloys are added to liquid steel to carry out the de-oxidation process of removal of excess oxygen (O2) from the liquid steel. They have high affinity for O2 and form oxides in the form of slag.
Ferroalloys are usually classified into two groups namely (i) bulk ferroalloys, and (ii) noble or special ferroalloys. Bulk ferroalloys consist of principal alloys namely ferro manganese (Fe-Mn), ferro chrome/charge chrome (Fe-Cr) and ferro silicon (Fe-Si). Noble ferroalloys are the vital inputs for the production of special and alloy steels. These ferroalloys are of high value and consumed in low volumes. They are namely ferro molybdenum (Fe-Mo), ferro vanadium (Fe-V), ferro tungsten (Fe-W), ferro titanium (Fe-Ti) and ferro niobium (Fe-Nb). Some of the alloys such as silico-manganese which contains iron only in very small percentage are also generally classified as bulk ferroalloys.
In good steelmaking practice, ferroalloys are not generally added to the melting furnace and around 60 % to 90 % of ferroalloy additions are made to the ladle during tapping of the liquid steel and balance is added during secondary steelmaking processes. However, there is a trend towards making more additions during secondary steelmaking processes or even at the tundish using specialty wire products.
During the addition of the ferroalloy to the liquid steel bath, it is necessary to immerse the ferroalloy into the liquid steel. Upon addition, a steel shell is frozen on the surface of the ferroalloy. Heat transferred from the liquid steel re-melts this shell back to the original ferroalloy surface. Convective heat transfer is a function of bath stirring and superheat and it governs shell melting.
Several physical and chemical properties of ferroalloys affect the dissolution of the ferroalloy in the liquid steel. Melting point is the primary factor for the rate of dissolution of the ferroalloys, while other important properties include density, thermal conductivity, specific heat, and enthalpy of mixing. Density determines whether the addition floats (ferrosilicon), sinks (ferromanganese) or get entrained within the liquid bath (ferrochromium). Thermal conductivity, along with specific heat and density, determines the thickness of the steel shell, which forms on the cold addition. Finally, a strong exothermic reaction between the ferroalloy and liquid steel (enthalpy of mixing) can substantially reduce the assimilation time (75% ferrosilicon).
Minimizing ferroalloy size improves dissolution rate, which is contrary to historical practice where large lumpy ferroalloys were employed to aid in penetrating the slag layer. Unfortunately, small size means more surface area on which to transport undesirable gases and moisture, plus, small alloy size increases dust losses and incurs handling difficulties. It has been determined that the optimum size for ferroalloys is between 3 mm and 20 mm. Wire and powder injection are both means of overcoming limitations imposed by fine alloy size.
Efficient steelmaking processes rely on the motion of the liquid steel (i) to dissolve the ferroalloys, (ii) to float the inclusions, and (iii) to eliminate chemical and temperature inhomogeneities. Natural forces can induce liquid steel motion, e.g. convection due to temperature gradient or energy of the falling stream during tap. Natural convection is relatively slow and the tap induced motion is time limited. In contrast, external forces, such as gas injection through a lance, plug, or tuyere can create significantly more intense motion. Gas injection via lances and porous plugs is the predominant method of stirring which is normally employed during the steelmaking process.
The aim of ideal deoxidation practices is to bind free O2 in liquid steel and remove oxide inclusions which are formed in deoxidation during tapping and increasing the efficiency of the deoxidant elements. The process of de-oxidation of the liquid steel is a peculiar phenomenon. The increase of the concentration of ferroalloys over a certain critical value results into re-oxidation of the liquid steel.
Normally three main deoxidation methods are used during tapping of the liquid steel. These are (i) rimming steel which means no killing of the liquid steel, (ii) semi-killed steel which means that the liquid steel is partially killed, and (iii) fully – killed steel. Rimming steels and semi-killed steels are not suitable for the continuous casting of the liquid steel and are primary aimed to produce Mn-Al (aluminum)-silicate inclusions which are deformable in hot rolling, and to avoid hard crystalline inclusions such as Al2O3 (alumina), and MgO·Al2O3 (magnesia -alumina).
Majority of present steelmaking practices aims at complete killing of the liquid steels. Under-killing of steel leads to the presence of O2 bubbles in the liquid steel bath which in turn leads to the presence of blow-holes in the cast product, and tundish stopper running etc. Over-killing of the liquid steel re-oxidizes the liquid steel bath and adds more cost to the steel. Optimization of deoxidization is necessary to achieve the minimum dissolved O2 in the liquid steel before the start of the casting.
Ferroalloys are added to liquid steels in the ladle for a variety of reasons such as (i) to deoxidize the liquid steel by reacting with O2 and forming oxides which are to be absorbed into the slag, (ii) to adjust the final chemical composition of the steel, and (iii) to achieve the specified mechanical properties in the steel by modifying inclusions present in the steel. They are a major cost contributing factor during the process of steelmaking. They can also be a constant source of production disturbances and unexpected process behaviour.
During the addition of ferroalloys to the liquid steel, pickup of other elements (for example carbon) also takes place. When adding ferroalloys, it is also important to be aware of, and if necessary to calculate the effect of other components of the ferroalloys on the overall steel composition. Pickup of C (carbon) can be critical in certain low C and ultra-low C steel grades. In such cases, it becomes necessary to use the more expensive low C or high purity ferroalloys.
Addition of ferroalloys is normally made as ladle additions. It can be made at the tapping, and at each of the secondary steelmaking units (such as argon rinsing station, ladle furnace, CAS-OB, or vacuum degassing unit). In general, bulk additions of ferroalloys are normally made during tapping in the ladle, with ‘trimming’ additions being made at subsequent stages.
It is important to be aware that additions of ferroalloy made to the ladle do not result in instantaneous changes to the steel composition, but take a finite time to dissolve. Hence, sufficient time (mixing time) is to be provided for ferroalloy additions to dissolve. The issues related to the mixing time which are important are (i) coarse particles dissolve at a slower rate in the liquid steel than powders, wires and fine particle additions, (ii) stirring the ladle (i.e. by argon bubbling) accelerates the dissolution process and is also essential for homogenizing the liquid steel composition, and (iii) mixing time increases as the temperature decreases.
One of the substantial costs during the steelmaking operation is the additions of the ferroalloys. Hence, it is essential to have a better understanding of the factors which determine alloy recovery and improve control of the final chemistry. Ferroalloy recovery along with the steelmaking practice needs a good control for the optimized utilization of the ferroalloys.
The precision which the modern steelmaking practice needs has resulted in more careful addition practice of ferroalloys to the liquid steel bath. Modern steelmaking practice needs repeatable and consistent results with high level of recoveries. No doubt, steelmaking practice plays a part in the final recovery rates as well as the degree of deoxidation of the heat, turbulence in the ladle, and a number of other factors. However, the physico-chemical properties of the ferroalloy are very important since it has a major effect on the recovery of the ferroalloy.
The recovery rate of the ferroalloy or simply the recovery of the ferroalloy is the amount of the element which actually increases in the liquid steel composition rather than being lost to the slag. Ferroalloy recovery is important not only from an economic standpoint but also from the standpoint of making high quality products with highly reproducible mechanical properties. Recovery of ferroalloys depends on a vast spectrum of parameters, summarized in three groups namely (i) type of additive elements (concentration of the additive elements and their chemical activity, in particular, affinity to O2, density, size and shape of particles, and concentration of impurities etc.), (ii) methods of addition (in the furnace, ladle, and special injection methods, such as wire injection etc.) , and (iii) steelmaking technology (type and size of steelmaking furnace, composition, condition, and quantity of slag, temperature of the heat, and time period of the heat).
The recovery of ferroalloys during the steelmaking operations is affected by several factors. Oxidation of the ferroalloy is generally the primary cause of poor or erratic recovery. The recovery of ferroalloy is a function of the dissolution rate of the alloy, density of the alloy, and dissolved O2 in the liquid steel (for nitride forming alloys, dissolved N2 is also to be included). The increase in the dissolved O2 in the steel lowers the recovery of the ferroalloy and decreases the dissolution rate of ferroalloys with melting points greater than steel.
The dissolution rate is the single most important property determining the ferroalloy recovery. The faster the ferroalloy go into solution, there are lesser chances exist for losses. Also the ferroalloy density is an important factor. The ideal density of ferroalloy is to be in the range of 6.2 grams per cubic centimeters (g/cc) to 7.6 g/cc. However, if the liquid steel is not deoxidized well, then the recovery can be low even if the ferroalloy has a high solution rate and an ideal density.
The transfer of ferroalloy additions from a solid to the liquid state can be considered as melting or dissolution. Melting occurs when heat is applied, while the dissolution takes place when the solid material comes in contact with a liquid at temperatures below the melting point of the solid. The dissolution process can be divided into two consecutive steps. The first step is the surface reaction in which the solid goes through a phase change to the liquid. The second step is the transporting of the resulting solute atoms from the interface into the bulk of the liquid steel by diffusion through a boundary layer. Either step can be rate controlling in the process of dissolution.
In addition to the value of the additive elements, ferroalloys contain iron and potentially chemical energy. The iron in the ferroalloy has a substantial value since it is well defined and available in a fast melting, lumpy shape. The chemical energy also has a potential value, but it can just as well be a problem and a cost. Ferroalloys can also contain inclusions and tramp elements. The tramp elements in ferroalloys can have special limitations due to their influence on oxide inclusions or other precipitates (nitrides, carbides). Ferroalloys can also contain small amounts of impurities such as sulphur (S), phosphorus (P), gases such as O2, nitrogen (N2), and hydrogen (H2), and moisture.
In general, in order to produce clean steel the liquid steel is to be deoxidized and the products of deoxidation are to be removed as well as the reoxidation by the slag is to be prevented effectively. During tapping of the liquid steel from the primary steelmaking furnace, Si/Mn/Al complex de-oxidation is carried out with ferroalloys to achieve the target low melting point soft oxide inclusions which get deformed during rolling and to avoid solid Al2O3 inclusions in Aluminum de-oxidation or by individual de-oxidation of de-oxidizers.
The universal equations for the reaction are defined as ‘x [M] + y[O] = MxOy’. In this equation M is the additive element and O is the dissolved oxygen in the liquid steel. The equilibrium constant for equation is dependent on the (i) free energy change for deoxidation reaction, (ii) dissolved elements in liquid steel, (iii) activity ‘a’ of the additive element (a[M] = fM * %[M] where fM is the activity coefficients of the additive element relative to the 1% standard state Fe, (iv) the activity coefficients of O2 relative to the 1% standard state Fe, and (v) activity of deoxidation product generally taken as 1 in solid state. For a steel composition at a specific temperature, the activity coefficients of the additive elements are constants and the equilibrium constant for each oxide forming elements can be calculated. The most common deoxidation reactions with their equilibrium constants and their values are given below.
Ferroalloys have a chilling effect on the liquid steel. The addition of ferroalloys results in a decrease in temperature of the liquid steel. The reduction of the temperature of the liquid steel depends upon the heat capacity and heat of solution of the various solutes. The one important exception is Al, which reacts exothermically with any O2 present (either dissolved in the steel, or injected through a lance) to heat the steel. Normally, deoxidation with Al is more efficient at lower temperatures. Also it is to be kept in mind that the liquid steel cools after deoxidation, the Al-O ‘solubility product’ (i.e. the equilibrium curve in Fig 1) also becomes lower. This means that Al and O continue to react, with the possibility of very fine Al2O3 particles forming. Unless these have time to float out, these will be trapped in the final product.
Fig 1 Al-O equilibrium curves at three different temperatures