Effect of Nb on inclusions and phase transformation in simulated high heat input coarse-grain HAZ of Nb/Ti low carbon microalloyed steel

• Published in: Materials characterization 2022-07, Vol.189, p.111966, Article 111966
• Main Authors: Yang, Yulong, Jia, Xiao, Ma, Yaxin, Wang, Ping, Zhu, Fuxian
• Format: Article
• Language: English
• Subjects: Acicular ferrite; CGHAZ; High heat input welding; Inclusions; Nb/Ti microalloyed steel
• ISSN: 1044-5803; 1873-4189
• DOI: 10.1016/j.matchar.2022.111966

In this paper, two groups of Nb/Ti low carbon microalloyed steels (Nb-bearing and Nb-free) were obtained through laboratorial methods. The evolution and effect of Nb/Ti-containing complex inclusions were characterised and analysed after being subjected to high heat input thermal simulation of 100 kJ/cm. The results showed that the prior austenite grain size in the coarse grain heat-affected zone (CGHAZ) was not affected by 0.045 wt% Nb addition. However, the acicular ferrite nucleation, induced by effective micron-level inclusions, was inhibited. Nb tended to converge towards the interfaces between the micron-level inclusions and matrix, which had a negative effect on the nucleation potential of acicular ferrite. The re-precipitation behaviour of nano-level inclusions satisfied the dynamic conditions during the high heat input thermal cycle, under which the dislocation movement was inhibited due to the pinning effect of re-precipitation particles. Besides, the size, morphology and composition of nano-level precipitates were changed by Nb (0.045 wt%) addition. In general, the evolution and effect of inclusions in simulated high heat input CGHAZ provide theoretical references for the study of welding performance in Nb/Ti low carbon microalloyed steels. •Nb segregation at the interface between micron inclusion and matrix affects the nucleation of acicular ferrite.•The acicular ferrite nucleation induced by effective inclusions is inhibited by a certain amount of Nb.•The size, morphology and even composition of the nano-level inclusions in CGHAZ are changed by 0.045wt%Nb addition.