Characterisation and numerical analysis of 304L austenitic stainless steel wires under tensile and cold rolling deformation

The phenomenon of strain-induced martensite in 304 L stainless steel is well known to increase the mechanical strength of the rolled samples. The martensite being harder than the austenite, this transformation is also responsible for a pronounced damage by wear of the rolls and dies. The present study focuses on the rolling of wire, particularly for complex shapes like flat wires with rounded edges, intending to get a deeper understanding of martensite formation during the rolling of 304 L under industrial conditions. In addition to tensile tests performed until rupture over a temperature range of 20 °C to 200 °C and strain rates ranging from 10−5 to 10−1 s−1, interrupted tensile tests were particularly undertaken to investigate the variation of the volume fraction of martensite with the plastic strain. In parallel rolling experiments on a laboratory rolling mill were conducted with thickness reduction ratios of 20%, 40%, and 60%. Microstructural analyses using Electron BackScatter Diffraction (EBSD) revealed an inhomogeneous distribution of martensite, with a particularly larger value of the volume fraction of martensite in the centre of the rolled samples in comparison to their surfaces. Numerical simulations provided insights into the temperature distribution through the thickness of the rolled samples, explaining the lower martensite volume fraction at the surface and highlighting the significant impact of triaxiality on martensite formation.