Results of studying the effect of a calcium modifier of 10KhSND low-alloy steel on its physicomechanical characteristics are presented. The rolled sheets of commercially produced shipbuilding steel, wherein before teeming during melting a ferroalumocalcium alloy was added, were used for investigation. The corrosion rate of examined steel in a model aggressive medium, as well as its fatigue resistance in salt water in torsion, axial loading, and bending tests against the calcium concentration, was elucidated. The effect of calcium concentrations on the austenite grain sizes and dispersity of nonmetallic inclusions, as well as the fracture toughness characteristics (critical stress intensity factor and crack opening) at different temperatures was examined. The microstructure of fracture facets of shipbuilding steel specimens of long-term service on brittle fracture along pearlite grains and ductile fracture along ferrite ones for as-received steel was revealed. Nonmetallic inclusions in specimen fractures of shipbuilding steel at different calcium concentrations were studied. Their quantity grows considerably with its service terms, which exerts negative influence on the corrosion and mechanical properties of the metal. The calcium alloying of steel is shown to contribute to its corrosion resistance and fracture toughness on long-term service in seawater. The recommendations are given as regards optimal modifier concentrations in the shipbuilding metal, which is consistent with minimum austenite grain sizes and dispersity of nonmetallic inclusions.