Steel is considered superior when compared to other metals when intrinsic properties like strength, ductility, toughness, and durability are taken into account. Ductility includes deep drawability, cold formability and low-temperature toughness while durability is against wear, fatigue and hydrogen-induced cracking. But these properties are significantly reduced by the presence of large-sized inclusions. Clean steel contains few evenly distributed and small-sized inclusions. Restricting the size and the numbers are the key to retaining the properties and alleviate problems related to processing and productivity.
Non-metallic inclusions include oxides, sulfides, nitrides, carbides, and their compounds. They precipitate during the solidification of the steel melt. These have to be removed by employing proper ladle refining techniques, tundish technology, employing proper mold powder and machine design.
Indigenous and Exogenous Origin of inclusions
Indigenous inclusions form in the steel due to the reaction between the dissolved oxygen and the deoxidizing element like Al, Si, etc. whereas exogenous inclusions are formed due to the reoxidisation of refined melt when the melt comes in contact with slag, air or due to entrapment of reoxidised product, slag and refractory.
Micro and Macro inclusions
Inclusions less than or about 50 micrometers are considered to be micro inclusions and sizes greater than 50 micrometers are considered to be macro inclusions.
The emerging demand for the stringent quality of steel has decreased the permissible critical size of the inclusion with critical size for ball bearing and steel cord being restricted to 15 and 30 micrometre in order not to encounter defects in the final product.
Exogenous Inclusions:
The origins for the formation of exogenous inclusions| are listed as follows:
· Reoxidation of the refined metal when it comes in contact with air due to air ingress between the ladle shroud and the collector nozzle.
· Re-oxidation in tundish during ladle change over and unshrouded casting.
· Re-oxidation due to reaction with ladle slag, tundish flux (containing iron oxide, manganese oxide , silica and calcium oxide).
· Slag getting entrained into the tundish from ladle due to vortexing at the time of ladle closure due to mistake by the observer or failure of the slag detecting device.
· Emulsification of tundish slag and entrainment into the melt due to turbulent impinging stream and turbulent surface level due to un-submerged ladle opening during ladle change, assisted ladle open, unshrouded casting and less shroud submergence into the melt.
· Entrainment of tundish slag into the metal due to vortex formation at the time of heat delay where bath level falls considerably (during ladle change) or during draining of the tundish at the time of machine closure.
· Entrainment of tundish lining refractory and fragments of SEN and ladle shroud glaze due to erosion.
· Entrainment of Alumina clusters getting dislodged from the SEN due to turbulent kinetic energy of the moving metal stream.
· Entrainment of mold slag due to turbulent meniscus level.
Mostly exogenous inclusions are the origin for the formation of macro inclusions and their removal becomes difficult owing to their formation at the later stage. The composition can be complicated consisting of agglomerates of mold slag, tundish slag, ladle slag or reoxidised metal. The slime method is one of the method employed to quantify and identify the origin of macro inclusions. Macro Inclusions formed can have many compositions ranging from: Al203 or MgO-Al203, BOF slag of CaO-MgO-SiO2-Feo system, Ladle slag of Cao-Al2O3-SiO2 system, tundish Slag of Cao-Al2O3-SiO2 system or Mold flux of Cao-Al2O3-Sio2-NaF.
Indigenous Inclusions:
These are formed as a result of the deoxidation reaction between steel melt and deoxidizer (Al, Si, Mn, Si-Mn, Fe-Mn, Fe-Si etc) added to the melt. During the addition of these deoxidizer, significant supersaturation takes place at the periphery of these metals or alloys and nucleation and growth of this product ultimately forms liquid inclusions, irregular solid inclusions or clusters of solid. The nucleation process is determined by surface tension on the boundary of the inclusions and gets promoted further by the low wetting angle between the nucleus and the substrate inclusion.
Growth of iron oxide–alumina inclusions take place initially but later dominated by nucleation or growth of alumina particles on the reduced iron-aluminates. The growth is supported by turbulent mass transfer of Al and O towards the Al2O3 nuclei. The nucleation, growth, collision, and agglomeration is further supported by enhanced bulk metal flow and local eddy flow. The floatation of these inclusions is governed by the number density of Al2O3 inclusions, the concentration of Al and O and the turbulent dissipation rate of stirring energy of the melt. Inclusions with higher surface energy have a higher chance to merge when they collide. The resultant agglomeration of alumina particle gives rise to large size dendritic plate, plate shape or irregular shaped particles.
© Metal World Insight
