Inclusion Characteristics and Acicular Ferrite Formation in the Simulated Heat-Affected Zone of Ti-Zr-Killed Low-Carbon Steel

In this paper, high temperature hot rolling and weld simulations were performed in Al-killed and Ti-Zr-killed steels. The microstructure and impact toughness of the heat-affected zone (HAZ) of the two experimental steels were investigated, and the ferrite transformation behavior in HAZ was also observed. The characteristic inclusions in the Ti-Zr-killed steel were studied using electron probe microanalysis (EPMA) and electron backscatter diffraction (EBSD). The results suggested that the HAZ microstructure of the Al-killed steel consisted of coarse bainite ferrite (BF) and granular bainite (GB) packets. The HAZ microstructure of Ti-Zr-killed steel was significantly refined and mainly contained fine acicular ferrite (AF) and BF. The Ti-Zr-O + MnS inclusions were 0.3-2.1 mu m and could effectively promote the formation of AF after the weld simulations. MnS had a specific orientation relationship: {111}(MnS) parallel to{0001}(Ti-Zr-O), < 110 >(MnS)parallel to < 2<(1)over bar>(1) over bar0 >(Ti-Zr-O) with Ti-Zr-O inclusion, and its precipitation promoted the formation of the Mn-depleted zone (MDZ). AF formed around it. The formation of an interlocked AF structure in the HAZ of the Ti-Zr-killed steel obviously increased the proportion of high angle grain boundaries (HAGBs) and refined the average effective grain size of the HAZ microstructure. Consequently, the impact toughness of the Ti-Zr-killed steel had no deterioration, but the impact toughness of Al-killed steel had an obvious loss after the weld simulation with a heat input of 70 kJ/cm. The impact absorption energy of HAZ of Ti-Zr-killed steel was improved from 6 to 219 J compared to Al-killed steel.