SUPERHIGH STRENGTH DOUBLE-PHASE STEEL WITH HIGH IMPACT VISCOSITY AT CRYOGENIC TEMPERATURE Russian patent published in 2003 - IPC

FIELD: manufacture of sheets of superhigh strength weldable low-alloy double-phase steel. SUBSTANCE: steel has microstructure containing ferrite phase and second phase, mainly lamellar martensite and lower bainite. Sheet is made by heating slab of steel containing iron and next elements: carbon, manganese, nickel, nitrogen, chrome, molybdenum, silicon, niobium, vanadium, titanium, aluminum and boron. Slab is heated and reduced for making sheet at one or several passes in temperature range of austenite recrystallization. Steel sheet is reduced in addition at one or several passes of hot rolling in temperature range lower than that of austenite recrystallization but higher than temperature of phase conversion Ar3. Sheet is subjected to finish rolling at temperature range between temperature values of phase conversion Ar3 and Ar1. Then sheet is quenched until temperature of quenching termination. Such steel has ultimate tension strength more than 830 MPa. EFFECT: high impact viscosity at cryogenic temperature of main steel sheet and zones of thermal influence at welding. 22 cl, 4 dwg, 1 tbl