The charateristics of nonmetaliic inclusions such as composition, size, and number are informed to affect formation of the interagranular acicular ferrite(IAF) in an austenite grain greatly. Fine inclusions of Ti oxide, Ti nitride, Mn sulfide, and Ti oxide compexes have been reported to be especially effective for IAF nucleation[1-4]. There are mostly found to be Ti oxide complexes which are mainly composed of Ti-Mn-O-MnS-(TiN) in CC slab produced from BOF steel plant. The number, size, and composition are different from slab thickness. Its size is fine and its number is large in the slab surface but be coarse and of small number in the slab center. Ideally, nonmetaalic inclusions of number of 15,286ea/mm~3 whose size are 1~5 mu m is necessary in order to drive IAF nucleation in an austenite of 50 mu m diameter. It is very hard to meet the requirements of size and number through thickness of the CC slab of Ti oxide steel. In steel melt, oxides need not to be likely to agglomerate each other and need to keep stable solid phase. Therefore, to keep large number of fine inclusions in a CC slab, Mg deoxidizer was used to Ti oxide steel as Mg is reported to be strong deoxidizer and that its oxides have contact angle near 90 deg where they are not likely to agglomerate each other. Mg-Ti-(AI) oxide steel was produced from BOF-CC-Plate rolling process. In laboratory experiment nonmetaliic inclusions of steel ingots with various Mg and Al contents were identified by SEM and EDS, and phase changes with Mg ans Al addition was studied. Charateristics of nonmetaliic inclusions in the CC slabs such as size, number, and composition were invetigated through slab thickness with each heat by ESAA method which was developed by POSCO as one of the extraction methods by electroysis. It was compared with that of Ti oxide steel produced from BOF-CC-Plate rolling process. HAZ toughness at weld joint of the Mg-T i-(AI) oxide steel plates was also invetigated.
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