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Double-twist torsion testing to assess microstructural evolution in microalloyed steels
Ballard, Trevor J.
Ballard, Trevor J.
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2022
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A double-twist torsion test was developed to determine the non recrystallization temperature (Tnr) and assess changes in recrystallization behavior at typical rolling temperatures between 1200 and 750 ˚C. The double-twist torsion test combines advantages of double-hit compression and multi-step hot torsion testing into a single test: pairs of isothermal deformation passes are applied at progressively lower temperatures to a single specimen, and changes in austenite recrystallization are inferred from calculations of fractional softening while still accounting for effects of multiple deformation and recrystallization events. The double-twist torsion test was applied to a 0.06C-0.06Nb-0.01Ti-0.005N (wt pct) nominal base composition and to five additional microalloyed steels with varying Nb, Ti, V, C, and N contents using a Gleeble® 3500. All alloys contained Nb additions between 0.03 and 0.06 wt pct.
Partial softening occurred over an approximate 100 ˚C temperature range suggesting a similar range of temperatures over which partial recrystallization occurred and in which rolling should be avoided to prevent a non-uniform grain structure. Tnr determined with established laboratory techniques was consistently situated within the partial softening regime, implying partial recrystallization near Tnr. Increasing the Nb content resulted in lower softening values within the partial softening regime, indicating an increased barrier to recrystallization. Altering the Ti, V, C, and N contents had modest effects on fractional softening and microstructure for the compositions and testing parameters investigated. Strain accumulation throughout the partial softening regime was examined using light optical microscopy and measurements of prior austenite grain aspect ratio where higher aspect ratios indicated greater accumulated strain. The fraction of grains with high aspect ratios increased throughout the partial softening regime, correlating well with the decrease in softening at the same temperatures. Thus, the trends observed in the fractional softening calculations followed the observed microstructural changes.
Precipitation state was studied using transmission electron microscopy (TEM). Scanning TEM and energy dispersive X-ray spectroscopy showed that Nb precipitation occurred at high temperatures as a cap forming on a generally Ti-rich core. Extensive fine-scale precipitation occurred at temperatures below 1000 ˚C. Increasing the Nb content resulted in greater numbers of both capped and fine scale precipitates. Changes in precipitation state throughout the double-twist test were characterized relative to the corresponding microstructure and mechanical data.
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