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Low carbon ultrafine grained dual phase steels Sangem, Vishnu Charan

Abstract

Ultrafine grained (UFG) (i.e. grain size of the order 1 µm) dual phase (DP) steels have been investigated using a thermo-mechanical process designed to obtain fine ferrite-carbide aggregates prior to intercritical annealing. The effects of carbon content, alloying additions, initial starting structures and processing parameters like heating and cooling rate on the mechanical properties have been quantified. Experimental work was conducted on two plain low carbon steels (0.06 and 0.12 wt pct C) and two low carbon (0.06 wt pct) micro-alloyed steels, one with Mo (0.15 wt pct) and the other with Nb (0.06 wt pct) addition, respectively. Fine ferrite-carbide aggregates were obtained only for the plain carbon steels and not for the micro-alloyed steels; hence no further studies are carried out for the micro-alloyed steels. A Gleeble 3500 thermomechanical simulator is used for the intercritical annealing operation to obtain UFG DP steels. The tensile test results showed that a critical carbon (about 0.1 wt pct) content may be necessary for obtaining a good balance of strength and ductility. The lower carbon steels showed better true strain to fracture behavior, but fall behind in terms of strength and ductility compared to the higher carbon steel. Higher heating rates are necessary for achieving fine microstructures as well as better mechanical properties, whereas lower heating rates helped in obtaining good strength-elongation (engineering tensile strength × uniform elongation) balance. For the sake of comparison martensite starting structures are also used to obtain UFG DP steels. There is no effect of heating rate on the strength values, but higher heating rates resulted in better elongation as well as fracture strain values. Overall, the UFG DP steels fabricated by using martensite starting structures showed better strength-elongation balance, moreover the number of processing steps are considerably reduced.

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