Abstract
It has been known that microstructures in steel welding heat affected zones can be improved by inclusion-induced intragranular ferrite nucleation. In this paper, the influence of inclusions on microstructure in steel base metal under thermo-mechanical conditions was further researched. Hot deformation and controlled cooling process were simulated with Ti–Zr deoxidized low carbon steels. Microstructural characteristics and transformation behaviors were analyzed. Results showed that inclusions were mainly (Ti, Zr)-rich oxide with MnS precipitation on the surface and they were effective for acicular ferrite nucleation in the coarse-grained heat affected zone. Acicular ferrite could still form in hot deformed and recrystallized austenite grains, but its volume fraction reduced with the decrease in deformation temperature. Accelerated cooling above 600°C and addition of boron significantly promoted acicular ferrite structure and the hardness value also increased. Hot rolling simulation indicated that for the experimental Ti–Zr–B steel, microstructure could not be further improved by applying conventional controlled rolling deformation. Instead, fine-grained acicular ferrite structure and high hardness value were obtained through high temperature deformation and controlled cooling process, during which (Ti, Zr)-oxide inclusions contributed to the intragranular acicular ferrite formation.
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