PSI - Issue 13

Guy Khosla et al. / Procedia Structural Integrity 13 (2018) 1447–1452 Guy Khosla/ Structural Integrity Procedia 00 (2018) 000–000

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Clinking is occurs due to the large longitudinal residual stresses that develop in the ingot skin after casting (Thomas, Brimacombe, and Samarasekera 1986). These can originate from thermal, mechanical and transformation stresses that develop during continuous casting and the subsequent cooling (Samoilovich et al. 2008). In addition, it is well documented that casting can produce defects (Brimacombe and Sorimachi 1977). These defects, specifically transverse cracks, act as crack initiators for clinking. Currently slabs are ‘hot charged’ i.e. after casting the slabs are put into an insulated box to ensure they do not drop below a designated temperature before they enter the reheat furnaces. This reduces the peak longitudinal stresses and is effective at preventing clinking, however this is expensive to implement and can cause delays with the process. Furthermore, it is used as a conservative measure to ensure clinking does not occur in the reheat furnaces though not all alloys may need this remedial treatment.

a

b

Fracture initiation point

Particular steel alloys are more susceptible to clinking than others. The influence of microstructure and alloying elements on clinking is debated amongst authors. Kumowicz (Kumowicz and Al 1966) argues that clinking is largely dictated by the influence of alloying elements on the thermal conductivity, and therefore the thermal stresses developed. Some producers use The Kaltenhauser Ferrite Factor (FF), calculated through Equation (1) (B.J.Ginn and Gooch 1998), as a measure of clinking susceptibility in ferritic steels (Wai, Robinson, and Cortie 1992). A lower FF indicates that more second phase martensite or austenite is formed which is believed to provide resistance to crack propagation (Wai, Robinson, and Cortie 1992). �� � ��� � ���� � ���� � ���� � ���� � ���� � ���� � ���� � ����� � �� (1) Transverse casting cracks are continuous casting defects which form in the transverse direction during casting. They appear close to the surface. Transverse cracks are different to clink cracks in that clink cracks refer to the loud, fast, audible and catastrophic fracture that occurs upon cooldown or reheat after casting, whereas transverse cracks occur in the high temperature low ductility regions upon phase transformation during the casting operation. It is, however, believed that transverse cracks act as the initiator for the catastrophic clinking fracture. Transverse cracks form due to dramatic drops in ductility experienced at high temperatures called ‘ductility troughs’. Cracks occur when the straightening operation occurs during temperatures where hot ductility troughs occur, typically between 700 ° C 900 ° C (B Mintz et al. 2000). The straightening operation occurs as the caster takes molten metal in vertically, which is bent as the molten metal solidifies to become straight slabs. This imposes tensile strains on the surface. Ductility troughs are strongly influenced by composition and strain rate. The minimum reduction of area required for processing is around 40% (Barrie Mintz 1999), however it generally agreed to avoid the straightening operation in continuous casters at temperatures at the ductility troughs. At strain rates experienced in continuous casting between � � �� �� and ��� � �� �� �� (B. Mintz, Yue, and Jona 1991) deformation induced ferrite (DIF) can form on the γ boundaries. The ferrite is softer than the austenite and strain concentrates in this region. Second phase particles such as Nb, V or AlN then act as sites for cracks to form at the grain boundaries (Crowther 2001). Compositions of steels are generally controlled carefully such that impurity levels do not rise above those acceptable for cracking to occur. However, during the casting of steels, segregation can occur where particular elements do not distribute evenly throughout the slab. For example, centerline segregation is known to be a problem in slabs where impurities such as sulphur that can cause cracking segregate towards the centre of the slab (Fujda 2005). Fig. 1. (a) Clinking observed in the reheat furnaces prior to hot rolling (b) Clinking on a slab that has cooled down after casting in the slab yard, showing the fracture initiation point (Jones 2017)