Untersuchungen zur Oberflächenrissbildung beim Stranggießen von Stahl

In the present work, the susceptibility to surface-crack formation under nearly continuous casting conditions is investigated for a 0.17% C steel, the further developed and used method being the “In-Situ Material Characterisation – Bending” (IMC-B) test. The principle of the ex-periment is explained in detail. In combination with simulations (ABAQUS and IDS 2.0) and additional experimental methods (confocal microscopy and oxidation experiments), the influence of deformation parameters, temperature, cooling processes and precipitation be-haviour on the formation of surface cracks can be determined. The results show that damage is caused by a combination of several factors. The mecha-nisms can be described separately. The critical temperature range is 850°C to 1000°C. A high strain rate has a positive effect, which can be attributed, amongst others, to the time dependence of the crack formation. Selective high temperature grain boundary oxidation massively deteriorates the surface ductility in the critical temperature range. The cause is a formation of notches on the austenite grain boundaries which are responsible for stress con-centration during tensile loads. An increased amount of AlN precipitations can be the trigger for a significantly higher number of cracks at 900°C and 850°C. Contrary to popular belief, a negative influence of the γ/α transformation cannot be observed. A critical strain is defined for each test, which expresses a value for the investigated conditions, leading to first cracks in the sample surface. It represents a reference value, which can be transferred as a cracking criterion in the continuous casting process.