Use of an automatic optical inspection system for the detection of defects arising from continuous casting processes, based on Machine Learning algorithms, for the analysis of incidence of oscillation marks on AISI 316L and 316LI austenitic stainless-steel slabs

Nowadays, continuous casting is the technology for the solidification of steel that is prevailing throughout the world. It has established itself thanks to the high productivity, the metallurgical high-quality of the products and the elevated degree of automation that distinguishes it from other processes. However, defects arising from continuous casting can compromise the structural integrity of the semi-finished products as well as affect the quality of the final product. For this reason, automatic slab inspection systems have been developed in recent years to monitor the surface of the semi-finished casting products, in order to limit further processing of products with critical and dangerous defects. By exploiting the potential of the inspection system installed on one of the casting lines of Acciai Speciali Terni S.p.A., it was possible to evaluate the impact of oscillation marks on AISI 316L and 316LI austenitic stainless-steel slabs. The aim of this research is to correlate marks distribution and their depth to the main process parameters. The analysis showed that the casting speed is a critical parameter that proportionally affects the oscillation marks distribution along the slab. In addition, the analysis revealed a lower lubrication of the edges, as oscillation marks increase in terms of frequency and depth in this position, especially on the bottom side, because of the weight of the semi-finished product itself. The preferential occurrence of this defect on one side of the slabs is associated with a slight misalignment of the casting machine. The different nitrogen content of the two steel grades analysed contributes to modify the casting powder lubricating behaviour on such edges. The results show that the AISI 316L stainless-steel transition slabs (i.e., those at the beginning and at the end of the sequence) are far more affected by the presence of edge oscillation marks than AISI 316LI. The trend is reversed for the central slabs of the sequence, for which the greater availability of nitrogen in AISI 316L leads to an increase of the kinematic viscosity of casting powder, resulting in improved lubrication.