Abstract: Metallurgical defects initiate rejections in ultra sonic inspection were carefully characterized in present study, with emphasis on the origins of macro-sized calcium magnesia aluminates. It was found that large calcium magnesia aluminates could be categorized into two types. The first type was featured by huge sizes of several millimeters, as a layer of scatter oxide particles of calcium magnesia aluminates, spinel, calcium aluminates with certain amounts of SiO₂, and some CaS particles, resulting in steel plate delamination. The second kind of calcium magnesia aluminates were with sizes about several hundred microns, and appeared in rolling direction as string shaped B-type inclusions. Based on the correlated information of rolled plates and as-cast slabs, it was found that rolled plates contained the first kind of calcium magnesia aluminates were casted during the transition of ladles. As a result, it could be reasonably inferred that the first type of calcium magnesia aluminates were exogenous inclusions originated from entrapped teeming ladle slag, which agglomerated with endogenous spinel, CaS and calcium magnesia aluminate inclusions to form a huge inclusion cluster. During hot rolling, the agglomerated huge inclusion clusters were crushed into a layer of densely scatter oxide inclusion particles with sizes of millimeter scale. The first type of inclusions can be reduced and avoided by strictly controlling the ladle residue during casting and accurately marking non steady state slabs.The second type of inclusions should be endogenous ones formed during refining process, which was with estimated maximum sizes about 200-300 μm in as-cast slab and elongated into B-type inclusions in rolled plates because of chemical compositions locating in low melting point region in the ternary system of CaO-MgO-Al₂O₃. Hence, the latter type of inclusions can be decreased or avoided by refining process optimization. 

Key words: macro inclusion, calcium magnesia aluminate, ultra sonic inspection, steel plate