FIELD: metallurgy.
SUBSTANCE: to provide ultimate tensile strength of at least 980 MPa, relative elongation of at least 16%, hole expansion factor of more than 20 %, method for production of sheet steel involves the following consecutive stages: obtaining cold-rolled sheet steel containing, in wt %: 0.15 ≤ C ≤ 0.23; 1.4 ≤ Mn ≤ 2.6; 0.6 ≤ Si ≤ 1.5; 0.02 ≤ Al ≤ 1.0; at that 1.0 ≤ Si + Al ≤ 2.0; 0 ≤ Nb ≤ 0.035; 0 ≤ Mo ≤ 0.3; 0 ≤ Cr ≤ 0.3; Fe and unavoidable impurities—the rest, annealing of sheet steel at annealing temperature TA, which is within range from Ac1 to Ac3, so as to obtain structure, containing at least 40 % of austenite and at least 40 % intercritical ferrite, tempering sheet from temperature of at least 600 °C, at cooling rate, making at least 20 °S/s, up to quenching temperature QT, concluded within 180 °C to 260 °C, sheet heating up to PT tempering temperature in range from 375 °C to 470 °C and maintaining the sheet at a given tempering temperature of PT for a time Pt in range of 25 s to 440 s, wherein time Pt will be included in range from 100 s to 440 s in case of tempering temperature of PT, ranging from 375 °C to 400 °C, and will be confined within 25 s to 150 s in case of tempering temperature of PT, enclosed within 450 °C to 470 °C, sheet cooling to room temperature, wherein sheet steel has a final microstructure, consisting of parts of area from: at least 11 % of tempered martensite, from 10 % to 20 % of residual austenite, from 40 % to 60 % of ferrite, at most 6 % of fresh martensite, at most 18 % of bainite.
EFFECT: proposed method of producing high-strength sheet steel characterized by improved ductility and moldability, and produced sheet steel.
35 cl, 5 tbl, 1 dwg
