Mechanism and control of hydrogen induced abnormal sticky behavior in slab casting mould

Abstract

Abnormal sticky behavior in continuous casting of steel refers to an event when in an instrumented mould, temperatures of all layers of thermocouple drop to a near single temperature level and thus doesn't help in identifying any phenomena happening in the mould. No signal is generated to capture any sticker or any other abnormal event occurring in the mould. In this situation the risk of sticker breakouts increases significantly. The knocks on effect are casting delays and slab downgrades. This study was carried out by collecting top slag samples and mould slag films from the mould for both normal and abnormal heats. Optical microscopy and SEM images revealed presence of pores in the slag film complemented with large crystals in abnormal heats. Porous structure in the slag film creates high thermal resistance to the heat flow from the slab to the mould, which retards the solidification of the slab. Also crystalline structure of slag film hinder heat transfer between solidifying shell and water cooled mould and lead to abnormal sticky behavior. Measurement of diffusible hydrogen content of liquid steel in tundish was measured to find out if hydrogen is accelerating the slag crystallization. It was found out that, in case of abnormal heats hydrogen content was 30-40% higher than the normal heats. The primary and secondary steelmaking parameters were looked into and it was found out that the choice of material added and addition pattern along with treatment time has major contribution in increasing hydrogen content of steel.