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The phases, microstructures, and their formation mechanisms of cast zirconium-containing 304 stainless steel (304SS-Zr) alloys with excellent mechanical properties were investigated. Optical microscopy, scanning electron microscopy, transmission electron microscopy, and statistical analysis were employed to further understand the structure-property relationships. The in situ three-phase composite microstructure in 304SS-Zr alloys consisting of micron-sized (MS) austenite, MS intermetallic, and nanocrystalline (NC) ferrite was obtained by simple vacuum melting route. As zirconium (Zr) content increased, the grain sizes of austenite and ferrite became significantly refined. The multiple heterogeneous structures in 304SS-Zr alloys produced the super-high strength which also had higher strain hardening than conventional 304SS. For expanding the nanostructure design ideas, in this work, all the phases were identified and the mechanisms related to phases and microstructure evolution were analyzed from both the kinetic and thermodynamic aspects. For the development of high-performance 304SS-Zr alloys, the mechanical behaviors and strain-hardening mechanisms were also discussed.