| The ultra-fine acicular ferrite/bainite microstructures were experimentally obtained through proper chemical composition design and advanced thermomechanical processing schedules in pilot reversing mill. At the same time, the formation mechanism of ultra-fine microstructures and its effects on the mechanical properties were analyzed by optical microscopy and TEM technique. The experimental results indicate that: for the tested Mn-Mo-Nb-B ultra-low carbon microalloyed steel, the size of bainitic packets less than 1 mm and the average thickness of pancakes (flatted prior austenitic grains) less than 5 mm were produced when the compression ratio reaches 7 in non-recrystallization region. The excellent properties, that is, tensile strength Rm>1000 MPa, the percentage elongation (%EL) A >14 % and charpy notch impact AKV >120 J (-30 °C) have been achieved. The high strength accompanying better toughness of the ultra-fine tested steel is correlated with the changes in substructures, i.e. the thickness of pancakes, the width of bainitic packets and denser dislocations pinned by second particles. |
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