The 0.2 wt%C steels with the addition of 2 wt%Si and 0–4 wt%Mn were prepared by hot rolling followed by subsequent annealing for normalization. The materials were subjected to friction stir welding (FSW) conducted above A3 temperature. Microstructures and tensile properties using small tensile specimens were investigated for both base materials and stir zones. The base materials with the addition of over 2 wt% Mn showed a relatively good balance of strength and ductility when Si content was 2 wt% due to the transformation induced plasticity effect of retained austenite. The tensile properties of stir zones were substantially enhanced by FSW, and the stir zones of the 0.2C–2Si–3Mn (wt%) and 0.2C–2Si–4Mn joints exhibited the high tensile strength of approximately 1700 MPa and the high elongation of about 30%, in particular excellent local elongation of 25%. This is due to the refinement of packet and block sizes in the lath martensite induced by the grain refinement of the prior austenite grains by FSW.
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