METHOD FOR MANUFACTURING COLD-ROLLED TWO-PHASE FERRITE-MARTENSITE STEEL MICRO-ALLOYED WITH NIOBIUM Russian patent published in 2017 - IPC C21D8/04 C21D9/46 C22C38/58 

FIELD: metallurgy.

SUBSTANCE: method includes steel melting containing, wt %: C 0.09-0.14; Si 0.05-0.40; Mn 1.7-2.3; Cr 0.20-0.40; Mo 0.10-0.40; Al 0.02-0.08; Nb 0.01-0.04; P is not more than 0.02; S not more than 0.02; Fe and unavoidable impurities, hot rolling at start temperature from 1075 to 1250°C and finish at 800-890°C, coiling of sheet to roll at temperature not below 600°C, cold rolling with total reduction of 45-70% for thickness of 0.9-1.5 mm and heat treatment in continuous-action unit by heating to annealing temperature 720-780°C, soaking, slow cooling to temperatures below A_r1, accelerated cooling to 270-400°C and ageing at said temperature. The sheet is moved in the unit at a speed provided that: V_smr= [(T_an-680°C/k-10m/min]÷[(T_an-680°C/k +10 m/min], where V_smr- strip movement rate in the unit, m/min, k = 1×min×°C/m, T_an - annealing temperature, °C, and the coiling temperature is set under the condition: T≥ct(690 -2000×k×Nb %), where t_ct is coiling temperature,°C, Nb is content of niobium, wt %, k=1×°C/%.

EFFECT: provision of required level of BH-effect and capacity to crush the hole with preservation of mechanical properties, with strength class of 780 mpa of ferrite-martensite steel.

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