The thermomechanical treatment of low-alloy steel plate (thickness up to 100 mm; strength category S420–S460) used in the manufacture of welded structures for civil engineering, bridge building, wind power, and other applications is complicated by the need to ensure excellent mechanical properties distributed uniformly over the thickness. The temperature conditions in the thermomechanical treatment of thick rolled plate (thickness more than 80 mm) in the γ and γ + α regions are considered. Test results for thick plate manufactured on the 4200 mill at NLMK DanSteel by thermomechanical and normalization rolling are presented. The microstructure and impact strength are studied over the plate thickness, as a function of the initial slab thickness and the final plate thickness.