The influence of weld thermal simulation on ICGC HAZ microstructure and mechanical properties of Cu containing Nb-Ti-microalloyed steel has been investigated. Low heat input of 0.7 kJ/mm (simulated fast cooling of Delta t (8/5) = 5 s) and high heat input of 4.5 kJ/mm (simulated slow cooling of Delta t (8/5) = 61 s) were used to generate double-pass thermal cycles with peak temperatures of 1350 and 800 A degrees C, respectively. The microstructure after high heat input mainly consisted of polygonal and quasi-polygonal ferrite (QF) grains with certain amount of acicular ferrite, whereas, after the low heat input, microstructure mainly consisted of lath or elongated bainite-ferrite, QF and M-A constituents. The size of ferrite grains decreased and volume of M/A constituents increased with fast cooling rate. The precipitation characteristics were found to be similar in both cooling rates. However, the precipitation of Cu-related phases was promoted by slow cooling rate. By fast cooling rate, the investigated steel exhibited an increase in hardness from 187(HV) to 197(HV). Consequently higher yield strength with considerable loss in the (-10 A degrees C) CTOD fracture toughness (delta(fast cooling) = 0.86 mm and delta(slow cooling) = 1.12 mm) were demonstrated.