Mechanism of solidification

Molds are water cooled. Killed steel solidifies in the ingot form as follows:

  i)  Metal near the mould walls and bottom is chilled by the cold surfaces and a thin shell or skin is               formed on the ingot surface. This surface has a fine equiaxed grains and the skin. The formation of       skin results in decrease in rate of solidification.

 ii)  Due to expansion of mould through the heat transferred from the solidifying steel  and contraction of       solidified skin an air gap forms between the mould and the skin.Th is results in decrease in the heat       transfer rate, because air gap has a high thermal resistance to heat flow

 iii)  The solidification front perpendicular to the mold faces moves inwards and towards the centre as a        result columnar grains form next to the chill surface. The columnar crystals rarely extend to the         centre of the mould.

iv) The central portion of the ingot solidifies as equi-axed grains of bigger size due to slow rate of        solidification.

The above zones of solidification depend on the evolution of CO gas due to carbon and oxygen reaction. In semi killed steels, not all oxygen removed from steel.  Oxygen content of steel is very low. The necessary super saturation level of carbon and oxygen reaches towards the end of solidification. As a result the central zone of the equi- axed crystal is disturbed by way of formation of blow holes in the top middle potion of the ingot.

Solidification of rimming steels is controlled by evolution of CO during solidification. Rimming steels are not killed. The gas is evolved at the solid/liquid interface which stirs the molten steel during solidification. Stirring circulates molten steel which brings hot metal to the surface and solidification of steel at top is delayed. Columnar grain formation is prevented due to a more uniform temperature at interior of an ingot. This gives rise to rimming ingots in which gas is entrapped mechanically as blow holes.