Mechanism of the Influence of Microadditions of Niobium on Microstructure and Properties of Thick-Sheet Low-Alloy Pipe Steel

The properties of thick-sheet steels of the 0.09% C – 1.5% Mn – 0.035% Si – (0.04 – 0.09)% Nb system (steel type 09G2B) with low (up to 0.10%) additions of niobium are analyzed. The main mechanisms of the influence of microalloying on the microstructure and mechanical properties of the steels are considered, i.e., suppression of the growth of austenite grains in heating for rolling, deceleration of the processes of recrystallization of hot-deformed austenite, refinement of grains, and precipitation hardening with particles of Nb(C, N) carbonitride in cooling after the rolling. The conditions for microalloying with niobium raising the strength properties and lowering the cold shortness threshold or, on the contrary, affecting positively the impact toughness without changing the strength are determined.