The α′-martensite formation in the corrosion-resistant 12Kh18N10T steel unirradiated and irradiated by neutrons (to a dose of 5 × 10¹⁸ cm⁻²) is studied by magnetometric measurements during its static deformation in the course of repeated unloading-loading cycles. Kinetic curves are plotted, and the parameters of the variation in the α′-martensite content in deformed samples are determined for the following two types of measurements: under loading (dynamic curve of α′-martensite accumulation) and unloading (static curve). The nature of the complex change in the magnetization of the deformed steel samples during unloading, which manifests itself in detecting the so-called elastic martensite, is discussed.