Abstract: Isothermal transformation curve (TTT curve) of 65Mn steel was obtained by quenching phase change machine. Based on this curve, the evolution of microstructure and mechanical properties at different coiling temperatures was studied, and the possible causes and control methods of flat coil were discussed. The results show that the typical microstructure of the 65Mn steel, namely proeutectoid ferrite and pearlite, is obtained between 600-750 ℃. The TTT curve presents a typical "C" shape, with a "nose tip" temperature of 550 ℃. The incubation period is relatively short at this temperature, only 0.25 s. Within the typical microstructure formation temperature range of the 65Mn steel, the incubation period gradually increases with the increase of temperature, and the content of proeutectoid proeutectoid ferrite increases, while the content of pearlite decreases. According to the actual production, the selected coiling temperature of the 65Mn steel is between 650-750 ℃, and the microstructure is proeutectoid ferrite and pearlite. With the coiling temperature decreases from 750 ℃ to 650 ℃, the proeutectoid ferrite content decreases from 19.4% to 4.1%, and the distribution gradually transitions from block distribution to network distribution. The pearlite lamellar spacing decreases from 277.0 nm to 178.0 nm, resulting in an increase in the yield strength of the 65Mn steel from 534 MPa to 637 MPa, an increase in ultimate tensile strength from 776 MPa to 899 MPa, an increase in hardness from 18.8 HRC to 24.3 HRC, and a decrease in percentage total extension at fracture from 28.9% to 19.4%. In actual production, the coiling temperature and the holding time before coil discharging are adjusted based on TTT curve and the evolution of microstructure and mechanical properties under different coiling temperatures can achieve synergistic regulation of microstructure, mechanical properties and flat coil defects.

Key words: 65Mn steel, TTT curve, coiling process, microstructure and properties, flat coil