Abstract

In order to produce Mn-series low carbon steels using alloying elements with low cost and following the conventional production line without controlled cooling facilities and not need- ing special heat-treatment, Mn-series low carbon air cooling grain boundary allotriomorphic ferrite (FGBA)/granular bainite (BG) steels, alloying with Si-Mn-Cr, have been developed. The effect of 0.02%Nb on the phase transformation and strength-toughness of a FGBA/BG steel have been inves- tigated by using Gleeble-1500D machine and hot rolling test in this paper. Two specimens with and without Nb addition were deformed by 21% at 850 and then air cooling to room temperature. For Nb free steel, the average size of FGBA is 25 µm in length and 10 µm in width. The average size of intragranular ferrite is about 6—8 µm. For FGBA/BG steel with 0.02%Nb, the average size of FGBA is 10 µm in length and 4 µm in width. The average size of intragranular ferrite is less than 4 µm. Compared with Nb free steel, the size of bainitic ferrite and martensite/austenite (M-A) island de- crease and their corresponding volume fractions increase in the FGBA/BG steel with 0.02%Nb. The experimental results indicate that the addition of 0.02%Nb increases the hardenability of the FGBA/BG steel, suppresses the transformation of ! �, refines the size of grain boundary allotriomorphic ferrite, promotes the granular bainitic transformation, lowers the bainitic start temperature (Bs), and refines the granular bainite including its bainitic ferrite and M-A islands. It is suggested that the effects of

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