On the determining role of acicular ferrite in V-N microalloyed steel in increasing strength-toughness combination

• The microstructure was ferrite and pearlite at finish cooling temperature of 660 °C.
• Ferrite and fine pearlite were obtained at finish cooling temperature of 630 °C.
• Acicular ferrite with fine plates was formed at finish cooling temperature of 570 °C.
• There are high densities of high misorientation grain boundaries in acicular ferrite.
• Acicular ferrite played a determining role in enhancing strength and toughness.

Controlled rolling followed by accelerating cooling was adopted during industrial processing of a low-C V-N microalloyed steel. Furthermore, the effect of finish cooling temperature on the microstructure and mechanical properties was studied. The microstructure consisted of polygonal ferrite and coarse pearlite at finish cooling temperature of 660 °C, while fine pearlite was obtained instead of coarse pearlite at finish cooling temperature of 630 °C. With further decrease in finish cooling temperature to 570 °C, the microstructure comprised of fine polygonal ferrite, acicular ferrite, and a small amount of fine pearlite, such that high yield strength of 625 MPa and excellent impact toughness of 149 J at − 60 °C was obtained. Acicular ferrite characterized by fine non-parallel ferrite platelets and high degree of misorientation played a determining role in enhancing strength and toughness.