The CCT diagrams of ultra low carbon bainitic steels and their impact toughness properties.

The CCT diagrams of ULCB_Ni steels; HN5MV, HN3MVCu having 5,1%Ni and 3,5% Ni and Cu bearing steels; HN3M1,5Cu, HSLA 100 have been determined. The reduced carbon concentration in steel, in order to prevent the formation of cemenite, allowed for using nickel, magnese, chromium and molybdenum to enhance hardenability and refinement of the bainitic microstructures by lowering Bs temperature. Coper and microadditions of vanadium and niobum are sucessfully used for precipitation strengthening of steel both in thermomechanically or heat treated conditions. Very good impact fracture toughness at low temperatures and highly yield strength properties of HN3MVCu and HN5MV steels allowed for fulfilment of the requirements for steel plates for pressure vessels and cryogenic applications.