FREQUENTLY ASKED QUESTIONS
What is the specific weight of FeNb?
The specific weight of FeNb is 8.1g/cm3, this makes it heavier than liquid steel (which is estimated at just over 7g/cm3) which means the addition of FeNb is trouble-free as it will sink towards the bottom of the ladle when added.
What is the melting range of FeNb?
FeNb has a melting range of between 1580oC to 1630oC which marks the solidus to liquidus phases. This is higher than the melting point of steel (approximately between 1425oC and 1540oC) which means that the FeNb goes into solution in the liquid steel (i.e. it does not melt). There is also no exothermic reaction with the liquid steel (unlike for example with high grade ferro-silicon or aluminium).
How long does FeNb take to go into solution?
The time taken for FeNb to go into solution is dependent upon a number of factors. Firstly, it is a time dependant process and will also depend on the FeNb lump size used, the temperature of the liquid steel, whether there is any bubbling (e.g. using argon) and the nature of it etc. Typically, the dissolution speed will be around 20mg/cm2 per second at 1,600oC, but will naturally be lower as the temperature decreases. It is important to note that the mechanism of dissolution is not so straight forward, as there is an initial delay due to the cold surface area of the lump and how the liquid steel reacts with the initial lump surface. For a lump size of 20mm, without any bubbling, should take 90-100 seconds whereas a 40mm lump size will take nearer 280 seconds. If some stirring is applied through bubbling, then the time will be reduced.
Where and when are additions made?
Typically, additions are made during the ladle treatment or during tapping of the liquid steel into the ladle itself. As FeNb (as with some other ferro-alloys) has an affinity for oxygen at high temperatures then it is added after ferro-silicon, aluminium and ferro-manganese have been alloyed. If additions are made into the ladle, then it is important that a bare/open spot or hole is created in the slag layer thereby exposing the liquid steel and the addition is made through this area thereby avoiding coating of the lump material with the slag layer.
What sort of recovery can I expect?
The level of recovery will depend upon several parameters including how and when the additions are made, the time-temperature of the addition, whether any argon stirring is applied etc. Also, the pre-addition handling of the material will also play a role in determining the recovery levels. Although FeNb lumps are relatively robust, if the material is handled roughly and lumps knock/rub together then smaller particles and/or fines and dust will be created which will lead to some very small losses. Therefore the material should always be handled with a little care to avoid any excess creation of fines/dust. Typical recovery rates for large scale producers will be between 92-98% (nearer 100% if cored wire injection is used), however, lower recoveries are possible when smaller ladles are used or the temperature drops are too severe.
What does the term microalloying mean?
Microalloying is a term that is used to describe the deliberate addition of very small amounts of elements, usually individually below 0.1wt% to improve the steel properties. This is different from trace elements, which are usually undesirable. For steel products microalloying is used to describe the addition of the elements of Niobium (or Columbium as it is still sometimes referred to in North America), Vanadium and Titanium either singularly or in combination. It is important to differentiate that unlike normal alloying elements which predominately affect the matrix of the steel, microalloying elements nearly always influence the microstructure via the precipitation of a second phase besides a solute drag effect.
What is meant by the solubility product of NbC, NbN and NbCN?
The solubility product is an equation which best describes the relationship between the microalloying element, the interstitial element (i.e. carbon and/or nitrogen) being dissolve in austenite or ferrite phase as a compound (or precipitate) at a given temperature. The solubility products are based on equilibrium conditions and the temperature dependency is expressed by an Arrhenius relationship, however, typically they are expressed in a shortened form with constants for a given system and a logarithm base 10 relationship. In brief, the solubility product calculates the temperature at which the given precipitate system will go into solution in the austenite or ferrite phase. For Nb, it will form a complex precipitate of Nb(CN) or niobium-carbo-nitride. This is because niobium carbide (NbC) shows extensive mutual solubility with niobium nitride (NbN). Studies of the literature will show a number of different solubility products as these will have been developed by separate researchers, and for a given system there will be separate products for austenite and ferrite phases. Therefore, the solubility products enable us to calculate at what reheat or soaking temperature will all the Nb enter into solution prior to hot rolling or forging etc.
How does the carbon and nitrogen content affect the solubility product?
As shown from the solubility product, the temperature at which all of the Nb will enter into solution will depend on the level of interstitial elements of carbon and nitrogen available in the steel. The higher the level of available interstitial, the higher will be the affective dissolution temperature. Carbon in particular dominates the relationship, therefore lowering of the carbon content (as well as nitrogen) will allow a greater amount of Nb to enter into solution where it can be utilised for grain refinement and precipitation strengthening. Lowering of the carbon will also help improve other properties such as the weldability.
What is the solute drag effect on retarding recrystallisation?
As with all other elements, microalloy elements in solid solution in steel retard all diffusion controlled processes. This retardation is stronger with bigger difference in the atom size of any specific element compared of the iron atom. Out of the three common microalloys of Nb, V and Ti, niobium is the largest atom being 15.6% larger than the iron atom (and is more than twice the size difference compared to vanadium) and therefore is the most effective. This larger size difference means that the niobium exerts a large drag force on the deformed austenite grain boundary which is trying to move during the process of recrsytallisation (which is when a high-energy deformed structure is being replaced by a defect free low energy structure by grain boundary migration). Therefore the presence of solute niobium is observed to retard (or delay) the recrystallisation process in steel. It is one of the two mechanisms which retards the process of recrystallisation, the other more powerful mechanism is that of Zenner pinning of the austenite grain boundary by precipitates.
