What types of inclusions need to be modified?

Mainly   and   inclusions are modified.  inclusions are brittle and breaks on deformation. Similarly   inclusion in steel on deformation becomes stringer type.

Inclusion engineering

In aluminum killed steels, alumina inclusions are formed. Alumina inclusions are solid at steelmaking temperature. During continuous casting, alumina inclusions clog the nozzle and consequently steel flow rate to the mold is affected.   Calcium is used widely to modify inclusions in continuous casting of steel. The solubility of calcium in steel is 320 ppm. The density of calcium is 1.55   and is in vapour form at steelmaking temperature . Therefore suitable injection methods for calcium have to be developed.

One method is to inject calcium deep into the molten bath such that the ferrostatic pressure overcomes vapor pressure of calcium. In another method wire containing calcium components are injected at speeds of 80-300 m/min. Typically Ca-Si powder in which calcium content is 20-30% is used in the wire. Wire containing Ca-Si powder is injected deep into bath to avoid a) oxidation of Ca and b) to keep calcium in the liquid form.

Addition of calcium in steel containing oxygen and sulphur can form two phases: oxide and sulphide. Oxide phase consists of the compounds in  system. The different oxide compounds have different melting temperature as shown in the table.

       
It may be noted that has lowest melting point and it will remain in the liquid  at steelmaking temperature. The table also shows Ca/Al ratio in various   compounds. The  ratio is 1.27 in   compounds. This suggests that calcium/aluminum ratio has to be adjusted at 1.27 (theoretically) in order to obtain liquid product.

 The sulphide phase consists of CaS and MnS. The melting point of CaS is   and that of MnS   melt has some solubility for CaS.