Examination of structure-forming processes of HSLA steel with 0.025 % Nb was performed in relation with rolling of the heavy seamless tubes in the Big Mannesmann mill. Based on the dilatometry data, a DCCT diagram after deformation 0.35 at temperature of 900 °C was designed. Hardness HV30 of value 157 was determined for low cooling rate of 0.2 °C·s-1 and the structure was consisting mostly of ferrite and pearlite. For faster cooling with rate of 60 °C·s-1 the hardness was equal to 404 with entirely martensitic structure. Non-recrystallization temperature was determined by the rolling-cooling-quenching tests and metallography just above 850 °C. Finally, steel samples were subjected to temperature controlled rolling and cooling with the rate of 0.25°C·s-1 in the laboratory reversing mill with the working rolls’ diameter of 350 mm. Grain refinement as well as homogenization of the final microstructure was observed after lowering the finish rolling temperature in the interval from 990 to 850 °C. Greater effect of decreasing finish rolling temperatures was observed below 890 °C as a result of deceleration of the recrystallization kinetics due to the precipitation during the cooling phase and getting closer to non-recrystallization temperature. The smallest secondary grain size of 17 μm was achieved despite the initial coarse-grained structure (created by preheating at 1280 °C), low degree of material deformation and slow final cooling. Low-temperature finish rolling resulted in a significant increase in the roll forces – approximately by 50 % when comparing the results of experiments performed at temperatures of 990 °C and 850 °C.Keywords: HSLA steel, DCCT diagram, non-recrystallization temperature, finish rolling temperature, microstructure.
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