METHOD FOR PRODUCTION OF COLD-ROLLED HIGH-STRENGTH ROLLED LOW-ALLOY STEEL SHEET Russian patent published in 2020 - IPC C21D8/02 C22C38/12 C23C2/06 

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, particularly, to production of cold-rolled sheets from high-strength low-alloyed steels used in automotive industry. Steel is melted with following composition, wt. %: C 0.05–0.07, Mn 0.35–0.60, Al 0.03–0.06, N not more than 0.007, Nb 0.025–0.035, Fe and unavoidable impurities are the rest, steel is poured. Hot rolling is performed to obtain hot-rolled strips and their cooling with water, at that hot rolling is completed at temperature in range of 830–870 °C. Hot-rolled strips are rolled into rolls at temperature of 550–600 °C, and then cold rolling is performed to produce cold-rolled stock, recrystallization annealing with application of zinc coating and tempering. Recrystallization annealing with application of zinc coating is performed in continuous zinc plating. Temperature of the recrystallization annealing and the temperature band at the outlet from the cooling section after galvanizing bath set depending on the desired strength class is numerically equal to the required minimum yield strength of 300 N/mm², 340 N/mm² and 380 N/mm² according to the dependency: T_ann. = (−0.875 K_mys + 1062.5) ± 15 and T_tzb. = (−0.75 K_mys + 485) ± 20, where T_ann. is temperature of recrystallization annealing, °C, T_tzb. - temperature of the strip at the outlet of the cooling section downstream of the zinc coating bath, °C, K_mys - a dimensionless parameter, which is numerically equal to the required minimum yield strength of 300 N/mm², 340 N/mm² and 380 N/mm², −0.875; 1062.5; −0.75; 485 are empirical coefficients. After recrystallization annealing, the cold-rolled stock before the inlet into the temperature equalizing chamber located in front of the zinc plating bath is accelerated to the accelerated cooling end temperature at a rate which ensures carbon retention in the solid solution and its participation in hardening during aging at the end temperature of accelerated cooling in range of 560–600 °C for rolled metal with minimum yield point value of 300 N/mm² or at the end of the accelerated cooling in range of 510–550 °C for rolled with a minimum yield strength of 340 N/mm² and 380 N/mm².

EFFECT: higher ductility, as well as expansion of technological capabilities of the method due to production of rolled different strength grades from steel of identical chemical composition.

1 cl, 2 tbl