STUDY ON LOW TEMPERATURE TOUGHNESS IMPROVEMENT OF WELDING COARSE GRAIN ZONE OF HULL STEELS BY Ti TREATMENT

The welding thermal simulation experiments were carried out on Ti/Al treated steels with different heat inputs by using Gleeble 1500D. The oxide inclusions and microstructures in the two base metals and simulated coarse grain heat affected zones (CGHAZ) were studied by using OM, SEM and EDS. At a higher heat input of 50 kJ/cm the low temperature Charpy impact energy in the CGHAZ of Ti treated steel (TTS) is higher than that of Al treated steel (ATS), and the impact energy of TTS reaches 60 J when the welding heat input was 75 kJ/cm. The dispersed Ti oxide inclusion with good high temperature stability in TTS can promote the nucleation of intragranular ferrite, and the welding heat input of 75 kJ/cm has only little effect on the morphology, composite and size of the inclusion. Whereas the Ti-rich nitride precipitated at the core of Al2O3 dissolved at high temperature, which can't promote acicular ferrite nucleation. The higher low temperature toughness of TTS is related to the formation of intragranular acicular ferrite in the CGHAZ. When t8/5 ranged from 40 s to 100 s, the volume fraction of M-A islands in TTS can be reduced and the ferrite matrix microstructure can be refined by formation of fine intragranular acicular ferrite nucleated at Ti oxide inclusion, and the hardness in the simulated CGHAZ in TTS is lower than that of ATS at the same t8/5 value. Simultaneously, the austenite grain growth became abnormal and the second phase particles, TiN and Nb(C,N), were coarsened in ATS, resulting in bigger austenite grain size than that in TTS.