Fig. 1 Schematic diagram of isothermal heat treatment process

Fig. 2 Optical micrographs obtained from dilatometer specimens isothermal holding at a-c 700 °C, d-f 650 °C, and g-i 600 °C for 10, 20, and 60 min, respectively; with plots in displaying variation in the ferrite volume fraction and the grain size versus g isothermal temperature for 10 min, k isothermal time at 650 °C

Fig. 3 a TTT curves of experimental steel simulated by JMatpro simulation, relationship of precipitated phase b corresponding composition of these phases c-e with temperatures

Fig. 4 TEM micrographs showing precipitates with cuboid shape a, b, c spherical shape; the corresponding EDS of precipitate d, e in b, c, respectively

Fig. 5 Schematic of TEM observation about the interphase precipitation carbides (s is the perpendicular sheet spacing; ω is the average intercarbide spacing in the sheet plane; ω 0 is the mean projected value of the intercarbide spacing in sheet)

Fig. 6 a Morphology of row-like sheet plane of the interphase precipitation and the inset is the corresponding diffraction pattern of the ferrite with the zone axis [100] α ; b the morphology of the random dispersion of carbides obtained by tilting the TEM thin foil to the zone axis [210] α and the inset is the corresponding SADP; the random distribution carbides with the zone axis of [110] α and [100] α in the specimen isothermally treated at 700 °C for 60 min

Fig. 7 Morphology of random distributed nanometer-sized carbides in the zone axes of [100] α in specimens isothermally treated at a 600, b 650, c 700 °C for 10 min

Fig. 8 Schematic diagram shows formation of interphase precipitation thickening of polygonal ferrite by ledge migration

Fig. 9 a Representative HRTEM micrograph of nanometer-sized carbide and b plots showing average size and number density of the carbides as a function of isothermal holding temperature

Fig. 10 TEM micrographs of the specimen isothermally treated at 650 °C for a 10, b 20, c 60 min at d 600 °C for 20 min

Fig. 11 Size distribution histograms of nanoscale precipitates in the specimens with different isothermal holding times of a 10, b 20, c 60 min at 650 °C, d plots showing variation of precipitation size and fraction as a function of isothermal holding time

Fig. 12 a Typical indentation morphology by nanoindenter, hardness distribution of the specimens isothermally treated at b 600 °C, c 650 °C for 20 min

Fig. 13 a TEM image of interphase precipitation, b HRTEM image of the interphase precipitation and the corresponding FFT diffractograms of nanocarbides obtained from the specimen isothermally treated at 700 °C for 60 min

Fig. 14 Schematic diagram showing the determination method of intercarbide spacing in slip plane