The effect of high welding heat inputs in the range of 50–200 kJ/cm on the microstructural evolution, MX (M = Ti, Nb and V; X = N and C) precipitation and mechanical properties was investigated in the coarse-grained heat-affected zone (CGHAZ) of a high-Nb (0.10 wt.%) structural steel. The results showed that the primary microconstituents varied from lath bainite (LB) to intragranular acicular ferrite (IAF) + intragranular polygonal ferrite (IPF), and the most content of IAF was acquired at 100 kJ/cm. Moreover, the submicron Ti- and Nb-rich MX precipitates not only pinned prior austenite grain boundaries but also facilitated IAF and IPF nucleation with the Kurdjumov–Sachs orientation relationship of \([01\mathop 1\limits^{ - } ]_{{{\text{MX}}}} //[1\mathop 1\limits^{ - } \mathop 1\limits^{ - } ]_{{{\text{Ferrite}}}}\) ; the nanoscale V-rich MX precipitates hindered dislocation movement and followed the Baker–Nutting orientation relationship of \([001]_{{{\text{MX}}}} //[001]_{{{\text{Ferrite}}}}\) with ferrite matrix, synergistically strengthening and toughening the CGHAZ. In addition, the −20 °C impact absorbed energy firstly elevated from 93 ± 5.2 J at 50 kJ/cm to 131 ± 5.4 J at 100 kJ/cm and finally decreased to 59 ± 3.0 J at 200 kJ/cm, being related to the IAF content, while the microhardness decreased from 312 ± 26.1 to 269 ± 12.9 HV0.1, because of the coarsened microstructure and the decreased content of LB and martensite. Compared to the CGHAZ properties with 0.05 wt.% Nb, a higher Nb content produced better low-temperature toughness, as more solid dissolved Nb atoms and precipitated Nb-rich MX particles in austenite limited prior austenite grain growth and promoted IAF formation. Furthermore, the welding process at 100 kJ/cm was most applicable for the high-Nb steel.
