Structure and Properties of High-Strength Low-Alloy Martensitic Steels with an Overequilibrium Nitrogen Content

Abstract—The structure and mechanical properties of rods of low-alloy martensitic steels with an overequilibrium nitrogen content, which are melted in a vacuum induction furnace with subsequent electroslag remelting under nitrogen pressure and are manufactured by free forging, are studied. The distribution of alloying elements and the hardness along the rod length is found to be uniform. The α → γ transformation temperature on hearting, Ac1 = 662°C and Ac3 = 838°C, of the steels are determined by differential scanning calorimetry. The structure of these steels contains a small amount of disperse particles of manganese sulfides and vanadium nitrides. Quenching and low tempering of the steels are shown to lead to a good combination of a high strength (σu = 2150–2170 MPa, σ0.2 = 1450–1480 MPa), ductility (δ = 12–14%, ψ = 35–40%), and impact toughness (KCU = 0.6–0.7 MJ/m2) as a result of formation of a fine-grained structure consisting of 85–87% lath martensite and 13–15% retained austenite, thin layers of which are placed between martensite laths.