Abstract: The strengthening and toughening mechanism of nonquenched and tempered bainitic SG steel with tempered at 350 °C was studied by low frequency mechanical spectrum apparatus, scanning electronic microscope, transmission electronic microscope and JMatPro software. By the Snoek and SKK peaks in the internal friction depended on temperature and microstructure analysis, the distribution and precipitation behavior of carbon atoms in the bainitic ferrite and decomposition law of islandlike matensite/austenite were discussed. The results show that the intensity of Snoek in SG steel is very low and bainitic ferrite contains a few solid solution content of carbon atoms; and the yield strength of SG steel with tempered at 350 °C for 2 h is the highest, which is due to the precipitation strengthening from Cr7C3 and dislocation strengthening.