A fundamental understanding of clinking during the cooling and reheating of as-cast steel slabs

Abstract

Catastrophic slab fractures during the steel making process cause significant material losses and process interruptions. For steel manufacturers at Tata Steel’s Port Talbot plant it is of upmost importance to understand what alloys are sensitive to fracture and what the key factors are that drive the fracture events. One type of slab fracture is that which occurs between the continuous caster and the rolling mill. At this stage in the steel making process, slabs have solidified and are transported via a slab yard to the hot mill. Slab fractures are noted to occur both upon cooling in the slab yard, and upon reheating (known as charging) in the rolling mill. The slab fractures are often preceded by a process known as: clinking. This is an audible cracking that occurs on cooldown steel manufacturers on-plant observe with alloys sensitive to slab fracture. Research on these catastrophic slab fractures focusses primarily on transverse cracks that form in the continuous caster. However, there has been little research on understanding the propagation of cracks to cause slab breakages and the relationship to clinking and the unique as-cast microstructure. The as-cast microstructure shows three distinct regions: (i) a chill zone containing small grains, (ii) a columnar grain region (iii) an equiaxed region. Whilst the chill zone and equiaxed regions had good strength, the columnar grain region was significantly weak. In addition, there were many significant pre-existing cracks that had arrested within extracted specimens from this region. EBSD mapping showed the inherent cracks had propagated along the {100} planes, showing the fracture is transgranular cleavage failure. Macro tensile thermal stresses up to 250 MPa form in the casting direction during cooling and reheating. These are the main driver of cracking which always occurs transverse to the direction of casting. Clinking was found to be the propagation of small cleavage cracks that occurs at the upper shelf of the transition temperature. Low temperature fracture toughness tests revealed a lower bound KIc = 28 MPa√m. It is believed that upon cooling clinking occurs creating sharp defects that under large tensile thermal stresses, propagate to break the slab. Several plant recommendations are proposed in order to reduce the frequency of clinking and catastrophic slab fracture. Composition and processing parameters should be altered to maximise the size of the equiaxed region. Scheduling in the plant should be done to encourage slow cooling, to above the DBTT and upper shelf to avoid clinking. A minimum slab temperature of 350°C is proposed.