Using the results from testing industrial batches of 23 mm steel heavy plates after thermomechanical rolling and subsequent post-weld heat treatment, the patterns of fatigue crack formation in the fracture specimens during CTOD (Crack Tip Opening Displacement) testing for fracture toughness are investigated. Visual, microstructural, and fractographic studies of the nature of fracture formation and the surface of the secondary separations have been conducted. The probable causes of the manifestation of the potential “pop-in” effect on the load–displacement diagrams of the notch opening displacement are described, as well as its potentially negative impact on the interpretation of test results.

The quality of heavy plates produced from low-carbon steel is directly linked to the structural characteristics inherited from the initial continuously cast thick slabs. This study explores how different casting technologies affect the morphology and distribution of allotriomorphic ferrite along prior austenite grain boundaries (PAGBs) within these slabs. Using quantitative microstructural analysis based on advanced computer vision techniques (OpenCV), the research identifies significant variations in ferrite boundary thickness and volume fraction associated with different casting methods. These microstructural differences strongly correlate with variations in Charpy V-notch impact energy (KVZ20) and susceptibility to microcrack formation during subsequent rolling processes. The results obtained allow us to evaluate the influence of the cast structure on the formation of the initial structural characteristics of the material, especially on the formation of microcracks of the slab microstructure and their propagation during further processing.