In this study samples from a metastable austenitic steel AISI 301 are tested in tension at room temperature in the presence of a magnetic emissions detection system. This steel is known for presenting strain-induced martensitic transformations (γ → ϵ and γ →α'). It was possible to follow the transformation to α' martensite induced by deformation thanks to the detection of magnetic pulses, similar to Magnetic Barkhausen Noise (MBN), but more intense, synchronized with the advance of the plastic deformation. The results indicate that the α' phase begins after the elastic part of the tensile test. The issue of magnetic pulses during the martensitic transformation, a phenomenon called SME (Magnetic Spontaneous Emission), had previously been described during cooling at cryogenic temperatures of metastable austenite. The results were discussed in light of results for Magnetic Barhausen Noise (MBN), X-ray diffraction and microstructures obtained before and after the tests.