Effect of Zr on undissolved phases and carbide precipitation in Ti microalloyed low-carbon steel

Abstract

The undissolved phases and carbide precipitation in Ti and Ti–Zr microalloyed low-carbon steels were investigated by scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectrometry. At 1225 °C, the replacement of Ti by Zr formed Zr2CS and (Zr, Ti)N (the Ti/Zr atomic ratio is 0.11) and reduced the consumption of Ti. At 925 °C, it was identified that TiC phases were precipitated at first and Zr was incorporated into the TiC lattice in the subsequent precipitation process, which promoted the precipitation of titanium carbide. The calculation of the interaction coefficient between Ti, C, N and Zr showed that Zr reduced the activity of Ti and C and increased the activity of N in the iron matrix. Therefore, with the addition of Zr, the solubility of Ti was increased, and the consumption of Ti was reduced at high temperature in Ti microalloyed low-carbon steel. The thermodynamic calculation of carbide precipitation transformation showed that the replacement of Ti by Zr increased the nucleation driving force and the nucleation rate of titanium carbide, while the critical core size and the critical nuclear energy were reduced. As the holding time was extended, the Zr/Ti atomic ratio increased and the size of the precipitates also increased. When the Zr/Ti atomic ratio reached a certain level, the size of the precipitates did not increase with further increase in atomic ratio. When the Zr/Ti atomic ratio in (Ti, Zr)C was 0.05–0.17, (Ti, Zr)C was the most stable carbide and the easiest to nucleate at 925 °C. There was more of the (Ti, Zr)C phase than TiC at 925 °C after 50 and 100 s, and the time to complete the coarsening behavior of (Ti, Zr)C was shorter than that of TiC.