PROCESS FOR PRODUCING A STEEL SHEET, WITH MICROLAMINATED MICROSTRUCTURE AND STEEL SHEET WITH MICROLAMINATED MICROSTRUCTURE

摘要

The invention relates to a process for producing a steel sheet and to a sheet metal made of weldable slightly alloyed steel, with very high resistivity and with excellent hardness at cryogenic temperature, in the base plate and in the heat affected zone (HAZ) when welded, having a tensile strength traction higher than 830 MPa (120 ksi) and a microlaminated microstructure containing austenite pellicular layers and plates of finely granulated martensite and/or lower bainite, which is obtained by: heating a steel slab containing iron and specified gravimetric percentages of some or all additives: carbon, manganese, nickel, nitrogen, copper, chrome, molybdenum, silicon, niobium, vanadium, titanum, aliminium and boron, followed by reducing the slab to form a sheet metal, by one or more passes through a temperature range to obtain recrystallized austenite; finishing the sheet metal rolling in one or more passes through a temperature range, under the temperature of austenite recrystallization and over the Ar3 conversion temperature; annealing the rolled sheet up to an adequate quenching stopping temperature (QST); quenching stopping and either maintaining the sheet metal, for a while, substantially isothermal to QST, or gradually cooling the sheet, before air-cooling, or air cooling at the room temperature.