Ca is added to improve fluidity, cleanliness and surface quality of steel. It also reduces gas content and prevents nozzle blockage during casting.
Inclusions were counted and mapped with EDS in 16MnCrS5 and 49MnVS3 steels after Mg-Ca treatment. The results showed that the single MnS inclusions tended to deform into long strips and were modified into complex inclusion with an oxide core.
Besides improving fluidity, Ca treatment also significantly modifies the composition, size and shape of inclusions. This modification is very important to reduce nozzle clogging and improve machinability, ductility, through thickness ductility and impact strength of the final steel product. In addition, the formation of globular Al2O3 and sulfide stringers can be prevented. Ca transforms inter dendritic sulfide galaxies into fine, type III inclusions which are less detrimental to the final steel products.
It is important to note that Ca is difficult to add to the molten steel due to its high reactivity and low melting point, and thus it has not been introduced as an effective modifier for inclusions until recently. Although Ca treatment can modify the inclusions, its modification efficiency is lower than that of Mg, and it may cause nozzle clogging. Moreover, its effect on the morphology change of inclusions is not well understood. Therefore, this research focuses on the modification of inclusions by Mg-Ca treatment.
During steelmaking, Ca reacts with inclusions and the oxygen present in the molten iron. This reaction decreases the gas content in the molten metal and prevents it from blowing during casting, which reduces surface oxidation and weld cracking. It also decreases the amount of sulfides in the finished product.
Adding calcium to the molten steel changes the composition, size and shape of inclusions. It transforms oxide inclusions into complex inclusions consisting of an oxide core and sulfide outer layer, eliminating the formation of elongated stringers that deform during hot working and reduce through-thickness ductility.
This change in inclusion morphology and composition is achieved by feeding Ca deep into the liquid steel using powder injection or cored wire. It is more effective than a traditional addition of CaSi lumps in the ladle, since calcium can be added via a core-spun steel sheath, which enables higher recovery of calcium in the liquid steel and lower consumption per ton of molten metal.
Since Ca is a strong sulfide forming element and its solubility in liquid steel is very low, it must be fed deep in the steel ladle. This is usually done by cored wire injection. This allows the Ca to react with inclusions, oxygen and sulfur at the steel melting temperature before it is diluted by slag. This modifies the morphology of oxide and sulphide inclusions and controls their composition, quantity and distribution in the steel.
The sulfur content can be significantly decreased by Ca treatment for Al killed steels. This is because solid Al2O3 inclusions are converted to liquid calcium aluminates which do not clog the continuous casting nozzles [10].
The S in the steel can be reduced to less than 0.05% by Ca treatment for rebar steels. This decrease in S reduces the risk of hot brittleness during forging and rolling operations. It also increases the ductility and toughness of the steel for improved welding and impact resistance.
Adding Ca to the steel melt results in a significant reduction of oxygen and sulphur content in addition to changing inclusion composition and shape. For example, in free-machining steels where hard Al2O3 inclusions cause excessive tool wear, calcium converts them into soft calcium aluminates or calcioaluminosilicates. It also modifies their shape so that they do not clog continuous casting nozzles.
The sulphide inclusions in high strength low alloy and high quality structural steel are also modified by the addition of Ca. The elongated inclusions are flattened by the Ca and are carried into the slag, which results in an increase of toughness and ductility.
However, a major challenge remains to make the process more efficient. The main problem is the vaporization of the added Ca at metal temperatures, as the boiling point of Ca is significantly higher than the temperature of the molten steel. EU-funded researchers have developed a method for slowing down the melting of the added calcium wire, which in turn lowers vaporization rates.
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