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ABSTRACTSS400 steel with 13 ppm magnesium was prepared to study the effects of supercooling degree on the formation behaviour of acicular ferrite (AF) and microstructures of Mg-containing low-carbon steel. The inclusions characterised using an automated inclusion analyzer, ASPEX, were mostly MnS, MgAl2O4and MgO with sizes of 1–2 μm. The growth behaviour of AF during the cooling process for austenite transformation to ferrite and continuous cooling temperature diagram was investigated using in situobservation with high-temperature confocal laser scanning microscopy. The initial formation temperature of AF decreased with increasing cooling rate. The temperature of AF transformation was in a narrow range of around 100–150 K. The probability of AF nucleation from inclusions and the average AF lath growth rate increased with increasing cooling rate due to the larger driving force and thermal strain energy around the inclusions during the cooling processes.