The study focuses on controlling the properties of steels produced by selective laser melting through a scientifically grounded selection of heat treatment parameters accounting for porosity, grain size, and residual stresses. The aim of the work is to establish the relationship between annealing temperature and duration, the evolution of microstructure, and the complex of mechanical properties, as well as to propose technological solutions for bringing additive products to the level of requirements of critical industries. The object of investigation is austenitic steel grade 316L printed on an industrial selective laser melting installation using optimized scanning parameters and a protective gas environment. The samples underwent vacuum heat treatment at temperatures ranging from 650 to 1150 degrees with a one-hour hold, after which measurements of density and porosity, microstructural analysis using scanning electron microscopy and electron backscatter diffraction, X-ray determination of residual stresses, as well as tensile tests and microhardness measurements, were performed. It has been shown that low-temperature annealing around 650 degrees, while practically not altering the ultrafine-grained structure, provides a sharp reduction of tensile residual stresses by almost an order of magnitude with only a slight decrease in strength, making such a regime an effective means of stabilizing component dimensions. High-temperature regimes in the range of 850–1150 degrees initiate recrystallization and growth of equiaxed grains accompanied by a noticeable decrease in porosity and a transition from a rigid, high-strength but brittle state to a ductile and tough one, with elongation exceeding half of the original sample length. Simultaneously, the expected decrease in yield strength and microhardness is observed, confirming the dominant role of the grain boundary strengthening mechanism in additive steel after annealing. The comparison of structural and mechanical parameters makes it possible to identify several operational windows—from the state with maximum strength and relieved stresses to the state with enhanced fracture toughness and fatigue durability. The obtained results demonstrate that a rational selection of the heat treatment regime transforms the microstructure of laser-manufactured steel into a controllable resource and provides the technologist with a tool for purposeful design of properties according to the requirements of a specific product component.