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.

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).