PROCEDURE FOR FABRICATION OF HIGH STRENGTH COLD-ROLLED AND ANNEALED STEEL SHEETS AND SHEETS MANUFACTURED BY THIS PROCEDURE Russian patent published in 2011 - IPC C21D8/04 C21D9/48 C22C38/38 C22C38/34 

FIELD: metallurgy.

SUBSTANCE: for manufacture of cold rolled and annealed steel sheet of hardness over 1200 MPa there is melted steel of composition, wt %: 0,10 ≤ C ≤ 0.25, 1 ≤ Mn ≤ 3, Al ≤ 0.010, Si ≤ 2.990, S ≤ 0.015, P ≤ 0.1, N ≤ 0.008, also, 1 ≤ Si+Al ≤ 3, in case of necessity composition contains: 0,05 ≤ V ≤ 0.15, B ≤ 0.005, Mo ≤ 0.25, Cr ≤ 1.65, also, Cr+(3×Mo)≥0.3, Ti ≤ 0.040, also Ti/N ≥ 4, iron and unavoidable impurities produced at melting - the rest. A semi-finished product is cast of steel. This product is heated to temperature over 1150°C and hot rolled into a hot-rolled sheet. The sheet is wound, surface of the sheet is cleaned and the sheet is cold rolled with coefficient of reduction from 30 to 80 %. The cold rolled sheet is heated at rate V_c from 5 to 15°C/sec to temperature T₁ within the range from Ac₃ to Ac₃+20°C during time t₁ from 50 to 150 sec. Further, the said sheet is cooled at rate V_R1 exceeding 40°C/sec and less, than 100°C/sec to temperature T₂ within the range (from M_s-30°C to M_s+30°C), and conditioned at this temperature T₂ during time t₂ from 150 to 350 sec. It is further cooled at rate V_R2 less, than 30°C/sec to temperature of environment. Microstructure of the steel sheet contains from 15 to 90 % of bainite, while the rest part is martensite and residual austenite.

EFFECT: increased strength.

19 cl, 1 ex, 3 tbl, 2 dwg