A floor-panel structure for a vehicle includes a front body subassembly, a front floor structure, including a central longitudinal tunnel, two lateral longitudinal beams, connected to the floor structure and the body subassembly, and two intermediate longitudinal beams, which connect the floor structure to the body subassembly and extend in areas between the lateral longitudinal beams and central longitudinal tunnel. The lateral longitudinal beams each include a sheet-metal profile element having an open cross section, a front portion reinforced for withstanding relatively high axial loads, and a remaining portion that is more ductile than said front portion so as to be more liable to collapse. Each lateral longitudinal beam is constituted by a single sheet element of steel, and said reinforced front portion is constituted by a hardened portion obtained by subjecting only the front portion to a thermal treatment using high-frequency induction heating.