An Ultra-low Carbon, Thermomechanically Controlled Processed Microalloyed Steel: Microstructure and Mechanical Properties

Abstract

In the current study, a novel ultra-low carbon, high-molybdenum-bearing microalloyed steel has been thermomechanically processed. Transformation of this steel during continuous cooling has been assessed. Variation in the microstructure and mechanical properties at different finish rolling temperatures has been studied. The average grain size, misorientation of grain boundary, and distribution of ferrite grains have been analyzed by using electron backscatter diffraction. The lower yield strength (251 to 377 MPa) with moderate tensile strength (406 to 506 MPa) along with high ductility (30 to 47 pct) has been achieved in the selected range of finish rolling temperatures. Superior impact toughness value in the range of 153 to 162 J is obtained in the subsize specimen even at subzero temperatures (233 K [−40 °C]), which is attributed to fine average ferrite grain size. The acicular ferrite dominated microstructure obtained at the 1023 K (750 °C) finish rolling temperature is the most attractive microstructure for pipeline applications due to its excellent combination of strength and toughness.