Fig. 1. Phase transformation fractions, determined by dilatometric measurements, as a function of temperature during continuous cooling, in the X65 pipeline steel specimens austenitized at the different temperatures from 850 °C to 1000 °C [28].

Fig. 2. Optical micrographs of X65 pipeline steel specimens after continuous cooling, with the different austenitizing temperatures as: (a) 850 °C, (b) 900 °C, (c) 950 °C and (d) 1000 °C [28].

Fig. 3. Optical micrographs of the Nb-bearing HSLA steel specimens after quenching (with a rate of 13 °C/s), with different austenitizing temperatures as: (a) 1050 °C, (b) 1200 °C [45].

Fig. 4. The phase transformation fraction, f, as fitted by the analytical phase transformation model as a function of the transformation temperature of HSLA steel samples subjected to various cooling rates: (a) 700 °C/min, (b) 900 °C/min and (c) 1100 °C/min [49].

Fig. 5. The phase fractions of acicular ferrite and martensite as a function of temperature for specimens with different cooling rates (martensite transformation is indicated by circles) [57].

Fig. 6. Volume fraction of acicular ferrite, as fitted by the displacive phase transformation model as a function of the isothermal time for the different isothermal temperatures in a HSLA steel [61].

Fig. 7. Schematic illustration of intercritical heat treatment.

Fig. 8. Optical micrographs of the specimens after (a) directly slow cooling (10 °C/min)[47], (b) directly medium cooling (500 °C/min)[57], (c) directly fast cooling/quenching (6000 °C/min) [57], and (d) intercritical normalizing treatment [69] (P: pearlite; F: polygonal ferrite; LB: lower bainite; M: martensite; AF: acicular ferrite).

Fig. 9. Scanning microscope photographs of the samples after (a) quenching +intercritical quenching [73], (b) quenching +intercritical normalizing [73], and (c) normalizing + intercritical normalizing[66] (WC: water cooling, AC: air cooling).

Fig. 10. Optical micrograph of the specimen after (a) step quenching +tempering [74] and (b) step normalizing +tempering [69].

Fig. 11. Effect of austenitizing temperatures on (a) impact energy and (b) yield tensile ratio of the samples after directly quenching +tempering (QT), step quenching +tempering (SQT) and intercritical quenching +tempering (IQT) [74].