Determination of high temperature surface crack formation criteria in continuous casting and thin slab casting

This ECSC project aimed at developing strategies to eliminate surface cracking, both in continuous slab casting and thin slab casting. The work combined laboratory studies, mathematical modelling and industrial trials in a multinational collaboration between MPI, Dillinger Huttenwerke, Rautaruukki Oy, Thyssen Krupp Stahl and IRSID. First step of the research was an investigation of surface quality on industrial products. The observation of surface defects at Rautaruukki and Dillingen revealed similar features : most cracks lay in the vicinity of oscillation marks, perpendicularly to casting direction. After etching, the cracks appeared to follow former austenite grain boundaries and to be intragranular concerning delta-ferrite structure, which meant that they had propagated in solid state. Dillingen observed also some segregated cracks lying along the primary delta-ferrite grain boundaries and formed during solidification. The surface defects observed by TKS on the thin slab caster were quite different from the ones identified at Dillingen and Rautaruukki, since they had a longitudinal orientation and were attributed to the special shape of the mould. The oscillation mark geometric characteristics were investigated thanks to metallographic analysis and systematic measurements on as cast slabs. Three main types of oscillation marks were identified at Dillingen : hook, depression and folded depression types, the depression marks being the most frequent ones. Measurements of oscillation mark depth and spacing depending on casting parameters were carried out at TKS and Rautaruukki. A preliminary comparative study of measurement techniques used at Rautaruukki, TKS (laser sensors) and IRSID (mechanical sensor) had shown that the values of oscillation mark depth obtained with the 3 techniques were dispersed, mainly because of the data analysis method. Therefore one should not focus on absolute values but study the tendencies of oscillation mark depth variations depending on process parameters. Dillingen studied the influence of carbon content on the depth of oscillation marks. The oscillation marks appeared to he the deepest in the following ranges : ultra low carbon range, because the solid shell in the mould is all the easier to deform as the carbon content is low. near the peritectic point (i.e. about 700 to 1200 ppm of carbon), because the phase transformation from δ-ferrite to austenite deteriorates the mark formation. The influence of casting speed on oscillation mark depth was debatable, depending on the oscillation strategy. At Dillingen, where the negative strip time rises when casting speed is reduced, the depth of oscillation marks deteriorated at low casting speed. The contrary was observed at Rautarrukki on peritectic steels. Rautaruukki observed also that higher frequency, lower stroke, lower negative strip time tended to decrease oscillation mark depth. It was confirmed by TKS on conventional caster that increasing oscillation frequency reduced oscillation mark depth and improved their regularity. On thin slab caster, oscillation marks were very shollow, because time for formation of oscillation marks is lower than for conventional casting since higher casting speeds and higher oscillation frequencies are used.