Innovations in Thermo-mechanical Processing to Develop High Strength Steel Plates

The thermo-mechanical controlled processing (TMCP) with accelerated cooling (ACC) has been used to manufacture steel plates with high yield strength and high impact toughness with the following chemistry: 0.08% C; 1.56% Mn; 0.043% Nb; 0.016% Ti; 0.17% Cr. Innovative TMCP regimes were employed with minimum amounts of micro-alloying elements. viz., Nb and Ti to enhance yield strength with high impact toughness. The phenomenon of precipitation hardening and grain refinement was used to improve yield strength and high Charpy Impact energy with enhancement of toughness values. High yield strength (510–550 MPa) and Charpy Impact energy for toughness (189–230 J at 0 °C and 185–226 J at −20 °C) were obtained. It was noted that high cumulative reductions in last three finishing passes (≥ 65%) played a critical role in a further increase in yield strength and Charpy Impact energy (248–262 at 0 °C, 242–254 at − 20 °C). The grain refinement of ferrite, as revealed in optical micrographs, due to very high reduction in finishing passes particularly at low finishing temperature, was favourable for additional improvement in yield strength and toughness values. EBSD maps showed more prominently that plates having more than 65% cumulative reductions in last three finishing passes developed deformation induced ferrite.