2022 Volume 108 Issue 5 Pages 295-305
Novel oxide particles composed of Ti, rare-earth metal and Zr was found by the present authors to promote the formation of acicular ferrite (AF), but their effect on toughness has not yet been clarified. Therefore, influence of AF formation on toughness of low carbon steel has been investigated using Ti-Rare earth metal-Zr-killed (TRZ) steel and Al-killed (AL) steels in this study. Dominant microstructure of the AL steel is bainite forming block/packet structure elongated from prior austenite grain boundaries. In contrast, a large amount of AF which grows from intragranular inclusions is formed in the TRZ steel. Toughness of the TRZ steel is better than the AL steel. In addition, at higher austenitizing temperature, AF formation is promoted in the TRZ steel, leading to lowering ductile-brittle transition temperature. Cracks in Charpy impact specimens propagate along {001}α plane and deflect at Bain unit boundary across which {001}α planes are largely misoriented, which means the effective grain size for crack propagation is Bain unit size. Bain unit size is smaller in the TRZ steel than in the AL steel since bainite structure is subdivided by pre-formed AFs belonging to different Bain groups from bainite. Martensite-austenite constituent (MA) in AF is less-elongated in comparison to those in bainite and thus the amount of elongated MA in the TRZ steel is less than in the AL steel. These results clarify that AF formation in the TRZ steel improves toughness due to the refinement of Bain unit size and reduction of the amount of elongated MA.
