METHOD OF MAKING HIGH-STRENGTH STEEL SHEET, HAVING IMPROVED STRENGTH, MOLDABILITY, AND OBTAINED SHEET Russian patent published in 2019 - IPC C21D8/02 C21D9/46 C22C38/02 C22C38/04 C22C38/06 C22C38/12 C22C38/14 

FIELD: metallurgy.

SUBSTANCE: steel sheet of the following chemical composition, wt%: 0.13≤C≤0.22; 1.2≤Si≤1.8; 1.8≤Mn≤2.2; 0.10≤Mo≤0.20; Nb≤0.05; Ti≤0.05; Al≤0.5; Fe and unavoidable impurities making up the rest are annealed at temperature higher than 865 °C, but below 1,000 °C for a time greater than 30 s and quenched by cooling to temperature of 310–375 °C at cooling rate of at least 30 °C/s. A structure consisting of austenite and at least 50 % of martensite is prepared. Then the sheet is heated to 370–470 °C, held at this temperature for 50–150 s and cooled to room temperature. Structure of steel includes 3–15 % of residual austenite and 85–97 % of sum of martensite and bainite without ferrite, average size of austenite grain makes 5 mcm or less, average size of grain or blocks of martensite and bainite makes 10 mcm or less.

EFFECT: high-strength steel sheet with improved strength and formability, breaking strength not less than 850 MPa, ultimate tensile strength of not less than 1,180 MPa, total elongation of not less than 13 % and opening distribution coefficient of not less than 30 %.

9 cl, 2 dwg, 1 tbl, 5 ex