Metal borides such as ${\mathrm{MgB}}_2$ distinguish themselves from cuprate, iron-based, and hydride superconductors by integrating structural stability, high hardness, and simple stoichiometry, making them exceptionally suited for practical applications. Here, we predict two hard superconducting phases of ${\mathrm{ZrB}}_6$ ($P6$/mmm and $\mathit{Cm}$) that are dynamically stable at ambient pressure. Sharing a characteristic three-boron-layer sandwich structure, these phases nevertheless exhibit strikingly distinct superconducting behaviors. The $P6$/mmm phase achieves a high $T_c$ of 34 K, the highest among transition metal borides, which is enabled by strong electron-phonon coupling from synergistic flat bands and high-frequency boron phonons. In contrast, the $\mathit{Cm}$ phase exhibits a much lower $T_c$ of 5.8 K due to the absence of such electronic features. Notably, both phases also possess high Vickers hardness values of 33.6 and 39.8 GPa, respectively. Our results not only identify ${\mathrm{ZrB}}_6$ as a promising platform for hard superconductors but also provide a design strategy toward multifunctional materials for high-field magnet and fusion energy technologies.

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• Electron-phonon coupling
• Fermi surface
• Flat bands
• Hardness
• Superconductivity
• Borides
• First-principles calculations