Steel cleanliness depends greatly on the size distribution, morphology & composition of Non-Metallic Inclusions (NMI), in steel.
There are many sources of inclusions like de-oxidation, re-oxidation slag entrapment, chemical reactions and other exogenous inclusions. Steel cleanliness depends greatly on the size distribution, morphology & composition of Non-Metallic Inclusions (NMI), in steel. The inclusion size distribution is particularly important because of large macro inclusions, which are most harmful to steel’s mechanical properties.
One kg of Low Carbon Aluminum Killed (LCAK) steel may have about 107-109 inclusions, 400 inclusions of size 80-130 µm, about 10 inclusions of size 130-200 µm and less than one inclusion of size 200–270 µm. Making clean steel involves not only controlling mean inclusion content in steel, but also avoiding inclusions of larger than the critical size, which is harmful for the product.
Classification of Non Metallic Inclusions
1. Indigenous Inclusions
Indigenous inclusions are deoxidation products or precipitated inclusions during cooling and solidification of steel.
Deoxidation Product: Alumina (Al2O3) inclusions in LCAK and Silica (SiO2) inclusions in silicon killed steel are generated by reaction between dissolved oxygen and added aluminum & silicon. Alumina inclusions are dendrite like, when formed in high oxygen environment.
Precipitated Product: Precipitated inclusions are formed during cooling and solidification of steel. During cooling, the concentration of dissolved oxygen, nitrogen & sulphur in the liquid steel increases, while solubility of elements decreases. Thus, inclusions such as alumina, silica & sulphide are precipitated. Sulphides forms interdendritical inclusions during solidification and after nucleate on oxides already present in the liquid steel. These inclusions are normally small (<10NM).
2. Exogenous Inclusion
These inclusions arise primarily from the incidental chemical reactions such as re-oxidation and mechanical interaction of liquid steel with its surroundings (slag formation & erosion of lining refractory). In machining they produce chatter, causing pits and gouges on the surface of machined sections frequent breakage as well as excessive tool wear.
Characteristic of Exogenous Inclusions
- Large size Inclusion from refractory erosion are generally larger those from slag etrainment
- Compound composition are multiphase and they’re caused due to
a) the reaction between molten steel & SiO2, FeO & MnO in the slag and lining refractory.
b) As exogenous inclusion are of large size, they may trap de-oxidation inclusions such as Al2O3 on their surface
c) Exogenous inclusions act as heterogeneous nucleus site for precipitation of new iclusions during their motion in molten steel
d) Slag re-oxidation inclusion may react with refractory linings or it can be further dissolved into steel
e) Small number compared with small inclusions
f) Sporadic distribution in the steel and not well dispersed as small inclusions
Air is the most common source of re-oxidation. Molten steel in the tundish mixes with air from top surface during initial pouring due to strong turbulence. Air is sucked into molten steel at joints between ladle & tundish and between tundish & mould. During this kind of oxidation, de-oxidation of elements like Al, Ca, Si, & other’s take place. These oxidized elements further develop into nonmetallic inclusions.
Parameters Affecting Steel Cleanliness
- Flotation of inclusion
- Method of Calcium addition
- Method of Aluminum addition
- Slag control: Steel making slag has three main goals
- Preventing atmospheric re-oxidation
- Thermal insulation
- Take up of non-metallic inclusion
Various slag like synthetic ladle slag, tundish slag and mould slag can affect steel cleanliness.
Steel cleanliness is an important factor of steel quality and the demand for cleaner steel increases every year. The so-called clean steel generally is the steel in which the content of impurity elements, such as phosphorus, sulphur, total oxygen, nitrogen, hydrogen (including carbon sometimes) and inclusions are very low. Improvement on steel cleanliness has therefore become subject of importance in the development of ferrous metallurgical technology.