Effect of Zr Microalloying on Austenite Grain Size of Low-Carbon Steels

Abstract

The effect of microalloying addition of Zr on the characteristics of inclusions and prior austenite grain sizes following a quench heat treatment has been investigated for two custom-made steels. The average size of particles in the Zr-containing steel is found to be the same as the Zr-free steel (0.49 μm). However, the number of smaller particles in the Zr-containing steel is much higher than the Zr-free steel. The inclusions in the Zr-containing steel are composed of ZrO2-TiN-MnS, and inclusions in the Zr-free steel are consisted of TiOx-Ti(C,N). The average prior austenite grain size of the Zr-containing steel is consistently smaller than that of the Zr-free steel, due to a large number of fine oxide inclusions and Ti(C,N) precipitates, working to pin the austenite grain boundaries at temperatures up to 1673 K (1400 °C). The grain refinement mechanisms by inclusions through the addition of Zr are discussed via thermodynamic and kinetic calculations.

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Acknowledgments

Minghao Shi gratefully acknowledges the financial support from China Scholarship Council.

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Correspondence to Leijun Li.

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Manuscript submitted May 3, 2019.

Appendix

Appendix

Calculation of Interfacial Energy

The interfacial energy between particle and steel liquid is estimated from the relationship: \( \gamma_{\text{pl}} = \gamma_{\text{p}} + \gamma_{\text{l}} \cos \theta_{\text{pl}} \),[16] where \( \gamma_{\text{pl}} \)is the interfacial energy between particle and steel liquid; \( \gamma_{\text{l}} \)is the surface energy for steel liquid, 1910 mJ m−2[16]; \( \gamma_{{{\text{p(ZrO}}_{ 2} )}} \)is the surface energy for ZrO2 particle, 620 mJ m−2[17]; \( \gamma_{{{\text{p(TiO}}_{ 2} )}} \)is the surface energy for TiO2 particle, 1427 mJ m−2.[18] \( \theta_{\text{pl}} \) is the contact angle between particle and steel liquid, \( \theta_{{{\text{pl(ZrO}}_{ 2} {\text{ - FeO)}}}} \)is the contact angle between ZrO2 particle and steel liquid, 122 to 123 deg[19]; \( \theta_{{{\text{pl(TiO}}_{ 2} {\text{ - FeO)}}}} \) is the contact angle between TiO2 particle and steel liquid, 72 to 84 deg.[19] Consequently, the interfacial energy between ZrO2 particle and steel liquid (\( \gamma_{{{\text{pl(ZrO}}_{ 2} )}} \)) is 1335 to 1632 mJ m−2; the interfacial energy between TiO2 particle and steel liquid (\( \gamma_{{{\text{pl(TiO}}_{ 2} )}} \)) is 1626 to 2016 mJ m−2.

In the present study, it is assumed that the interfacial energy between ZrO2, TiO2 particle and steel liquid is 1335, 1626 mJ m−2, respectively, and was used for the calculation of radius of the critical nuclei and in the LSW equation.

Inputs for Equation 2 for Calculating the Growth Rate

\( C_{\text{m}} \) is the solute concentration in the steel liquid (0.01 pct for Zr, 0.015 pct for Ti); \( V_{\text{m}} \) is the molar volume of the particle (21.69 cm3 mol−1 for ZrO2, 23.49 cm3 mol−1 for TiO2); \( D_{\text{m}} \) is the diffusivity of the solute atoms (4.1 × 10−4 m2 s−1 for Zr, 3.6 × 10−4 m2 s−1 for Ti at 1873 K (1600 °C) steel liquid)[20]; \( r_{0} \) is the critical radius of particle; t is time. The temperature used in the present calculation shown in Figure A1 is 1873 K (1600 °C).

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Shi, M., Kannan, R., Zhang, J. et al. Effect of Zr Microalloying on Austenite Grain Size of Low-Carbon Steels. Metall Mater Trans B 50, 2574–2585 (2019). https://doi.org/10.1007/s11663-019-01701-1

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