Microstructural refinement by the formation of acicular ferrite on Ti–Mg oxide inclusion in low-carbon steel

Abstract

In this paper, the mechanical properties and microstructure of hot-rolled low carbon steel containing Ti–Mg oxide inclusions as well as the effect of austenite grain size on acicular ferrite (AF) nucleation and the nucleation mechanism of AF were studied. The results indicated that the dominant inclusions in Ti–Mg deoxidized steel comprised Ti–Mg–Mn–O·MnS, which effectively promoted the nucleation of AF; thus, the microstructure after hot rolling consisted of fine AF and bainite (B). The variety of tensile property of Ti–Mg deoxidized steel was not obvious but the low-temperature impact toughness was significantly improved from 4 J to 126 J at −40 °C compared with Al deoxidized steel. Increasing the austenitizing temperature contributed to the nucleation of AF, and large amounts of fine AF formed in Ti–Mg deoxidized steel when the austenitizing temperature was 1250 °C with an average austenite grain size of ~92 μm. Two kinds of nucleation mechanisms of AF were elucidated: a Nishiyama-Wasserman (N–W) orientation relationship of (100)ferrite‖(110)oxide, [011]ferrite‖[-111]oxide between AF and Ti–Mg–Mn–O oxide, which caused low lattice disregistry, and a Mn-depleted zone (MDZ) around Ti–Mg–Mn–O·MnS inclusions. In order to investigate the reason for the formation of the MDZ, focused ion beam (FIB) was used to prepare the transmission electron microscope (TEM) sample containing a Ti–Mg–Mn–O·MnS inclusion, the specific orientation relationship of (-11-1)MnS‖(11–1)oxide, [-112]MnS‖[011]oxide between MnS and the Ti–Mg–Mn–O oxide was examined, and the atomic image of the Ti–Mg–Mn–O oxide was observed. The precipitation of MnS on the surface of Ti–Mg–Mn–O oxide followed by the absorption of Mn atoms by Ti–Mg–Mn–O oxide resulted in the formation of a MDZ around Ti–Mg–Mn–O·MnS inclusions.