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The significant influence of the welding heat input on the microstructural evolution, precipitation behavior and resultant properties of the coarse grained heat affected zone (CGHAZ) in low carbon Nb-V-Ti-N quaternary microalloyed steel was comparatively studied on a Gleeble 3800 thermomechanical simulator. Metallographic investigations revealed that the dominant microstructure transformed from lath bainite (LB) at heat input of 50 kJ/cm to intragranular acicular ferrite (IAF) at 100 kJ/cm, to IAF and intragranular polygonal ferrite (IPF) at 150 kJ/cm and to IPF, IAF and grain boundary ferrite (GBF) at 200 kJ/cm. In addition, Ti- and Nb-enriched submicron-scale precipitates promoted IAF and IPF nucleation with an orientation relationship 01 1 − MC / / 1 − 11 Ferrite and effectively pinned grain boundaries; V-enriched nanoscale precipitates acted as obstacles for dislocation movement, consequently improving CGHAZ performance. Moreover, the impact absorbed energy at 20 °C and −20 °C first increased from 216 ± 4.3J and 58 ± 2.7J to 219 ± 3.7J and 64 ± 3.0J, respectively, and then decreased to 184 ± 7.5J and 45 ± 2.1J, respectively, when the heat input increased from 50 kJ/cm to 200 kJ/cm. The maximum value was obtained at heat input of 100 kJ/cm, which was primarily attributed to the largest amount of high angle grain boundaries originating from IAF. The microhardness slightly reduced as heat input increased from 50 kJ/cm to 100 kJ/cm and did not obviously decrease as heat input varied from 100 to 200 kJ/cm, which was directly related to the microstructure constituents and nanoscale precipitates. Furthermore, the optimal heat input to obtain a good combination of strength and toughness was 100 kJ/cm, and Nb-V-Ti-N microalloyed steel was suitable for high heat input welding. • The low carbon Nb-V-Ti-N microalloyed steel is suitable for high heat input welding. • The optimum welding heat input is 100 kJ/cm to obtain a strengthened and toughened CGHAZ. • The submicron Nb- and Ti-enriched particles promote intragranular ferrite nucleation and pin grain boundary. • The orientation relationship between nucleation site and the induced ferrite grain is 01 1 − MC / / 1 − 11 Ferrite. • The nanoscale V-enriched precipitates act as obstacles for dislocation movement. [ABSTRACT FROM AUTHOR]

The effect of welding heat input characterized by the cooling time taken from 800 °C to 500 °C (t 8/5) on the microstructural evolution, V(C,N) precipitation and resultant mechanical properties of the simulated CGHAZ in high-N V-alloyed steel was comparatively investigated using a Gleeble-1500D thermomechanical simulator. Metallographic analysis indicated that the dominant microstructure transformed from lath bainite to granular bainite when t 8/5 increased from 30 s to 90 s and then changed to intragranular ferrite at t 8/5 of 180 s. The nanoscale V(C,N) precipitates were coarsened as t 8/5 increased, coupled with a slightly increased number density. Furthermore, the orientation relationship [001] α // 1 1 ¯ 0 V(C,N) reduced the interfacial structural energy (MC/α) and enhanced the V(C,N) precipitation in the ferrite matrix. Moreover, the microhardness progressively decreased due to the combined effect of microconstituents, precipitates and grain boundaries. The impact toughness first decreased and then increased, and the optimal value was obtained at t 8/5 of 180 s. Furthermore, the formation of intragranular ferrite, especially acicular ferrite, and the increased fraction of high angle grain boundaries could completely remedy the detrimental effect caused by the increased content of grain boundary ferrite and the coarsened effective grain size and precipitates. In addition, the high nitrogen content of 240 ppm accelerated V(C,N) precipitation in the austenite region, and the submicron V(C,N) precipitates could promote the formation of intragranular ferrite by providing heterogeneous nucleation sites, in combination with the larger prior austenite grain size caused by higher welding heat input. Unlabelled Image • Microalloying of nitrogen (0.024 wt%) for V contained base metal was beneficial for simulated CGHAZ. • Acicular ferrite and high angle grain boundaries are more crucial in toughness than precipitates and effective grain size. • HRTEM observation showed that the orientation relationship [001] α // 1 1 ¯ 0 V(C,N) enhanced V(C,N) precipitation in the ferrite matrix. [ABSTRACT FROM AUTHOR]