LAUSR

Clean steel is the main objective during steel making process and non-metallic inclusions should be eliminated as completely as possible1) because they can act as nucleation sites of cleavage cracks.2) During thermal cycling of high heat input welding, austenite grain size of heat affected zone (HAZ) coarsens and brittle microstructure such as martensite/austenite (M/A), upper bainite (UB) and ferrite side plate (FSP) are formed, which can deteriorate the impact toughness. To address the problem, the concept of “oxide metallurgy” was proposed.3–5) Fine inclusions (0.2–3 μm) are controlled and dispersed in steel which provide heterogeneous nucleation site for acicular ferrite (AF), or pin the movement of austenite grains, enhancing the toughness.6,7) In order to realize optimum efficiency of oxide metallurgy, the complete process of steel making needs to be carefully controlled,3) including deoxidation conditions (deoxidizing elements, temperature, etc.), solidification condition (solidification velocity, cooling velocity, etc.) and condition of heat treatment after solidification. In fact, various inclusions have been reported to have an ability to induce nucleation of AF, such as titanium oxide,1,8,9) TiN,10) MnS,11) and Ti–M–oxide (M = Mg, Ca, Zr, etc.).7,9,12–14) In addition, Lee et al. suggested that when oxygen content changed from 0.005% to 0.013% in Ti-bearing low carbon steel, the percentage of inclusions and areal density Evolution of Inclusions and Associated Microstructure in Ti–Mg Oxide Metallurgy Steel