| The rapid cooling in continuous casting process produces large thermal stress in the slabs, due to steep thermal gradient between the interior and the exterior of the slabs and a mechanically induced stress caused by friction in the mold, roll pressure, etc. There is also an occurrence of tensile stress at the upper region and compressive stress at the lower region of the slabs at the straightening zone at around 700℃-900℃ in the case of vertical continuous casters. In the case of steels with poor hot ductility, the combination of the stresses mentioned above quite often results in the formation of cracks. Among various types of steels, it is known that steels containing strong carbide formers such as Nb are particularly susceptible to cracking during continuous casting. The present study is concerned with microstructural analyses of the Nb containing steel (A steel) and Nb+B containing steels (B steel) to reduce the transverse corner crack at the straightening zone, with emphasis on elucidating the mechanism of hot ductility. There are differences in the microstructures between the A steel and the B steel. The B steel has a smaller amount of ferrite films along the prior austenite grain boundaries than the A steel. Also, the B steel contains proeutectoid ferrite within the prior austenite grains and 5-㎛-sized coarse Fe_23(B, C)_6 along the prior austenite grain boundaries as well as within the prior austenite grains. The ductility trough is virtually non-existent in the B steel, although variation of values of reduction of area (RA) with temperature shows that the A steel has a trough of RA between 700℃ to 850℃. It is believed that the presence of intraganular ferrite improves the homogeneity of deformation and accordingly the hot ductility of the B steel. |
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