Influence of plastic deformation conditions on structure evolution in Nb–Ti microalloyed steel

To achieve the optimum strength of Nb–Ti microalloyed steel, TMT was applied by varying the reheating temperature, the percentage reduction in the recrystallization region, the percentage reduction in the non-recrystallization region and the coiling temperature. Specimens were reheated between 900 and 1250 °C for 25 min to select the appropriate reheating temperature of 1150 °C. Then they were deformed at 1150 °C in the range of 30–50%, subsequently were deformed at 860 °C in the range of 40–70%, then cooled down to a temperature between 520 and 670 °C and finally were furnace cooled over about 24 h to ambient temperature. The prior austenite grain size, the recrystallized austenite grain size, the effective nucleation area (Sv), and the ferrite grain size were determined by optical microscopy. The mechanical properties of strips with different coiling ferrite–pearlite structure were determined by the tensile test. It was found that there is an abnormal grain growth of austenite grains in the reheating stage at 1050 °C. The increase of percentage reduction in the non-recrystallization region from 40 to 70% results in increase of Sv due to the greater amount of elongated austenite grains and deformation bands present in the interior, of the grains consequently very fine ferrite structures were formed. Decrease of the coiling temperature from 670 to 520 °C resulted in refining the ferrite grain size, thereby improving the tensile strength and yield-strength. Coiling at 550 °C results in an additional yield-strength increase due to the precipitation-strengthening effect.