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UBC Theses and Dissertations

Initial solidification phenomena in the continuous casting slab mould Takeuchi, Eiichi


In a study of initial solidification during the continuous casting of steel slabs, the formation of oscillation marks and their effect on the surface quality of the slabs have been examined by metallographical investigation of slab samples and by performing a set of mathematical analyses. The metallographic study of the oscillation marks has revealed that the adjacent subsurface structure may exhibit "hooks". The depth of oscillation marks exhibiting subsurface hooks is affected by the carbon content of the steel, while oscillation marks without adjacent hooks do not show the carbon dependence. Another important factor which affects the depth of oscillation marks is variation of the meniscus level. Quick upward movement of the meniscus level increases the depth of oscillation marks. The theoretical analysis of heat flow at the meniscus indicates that the meniscus may partially freeze within the period of a typical mould oscillation cycle. Lubrication theory has shown that a significant pressure can be generated in the flux channel by the reciprocating motion of the mould relative to the shell. The shape of the meniscus has been computed as a function of the pressure developed in the mould flux. This has demonstrated that the "contact" point of the meniscus with the mould wall moves out of phase with the mould displacement by π/2, and has a greater amplitude than the stroke of mould oscillation. Thus near the beginning of the positive strip period molten steel can overflow at the meniscus when a rigid hook-like shell exists, whilst the meniscus in the absence of a rigid shell, caused by high superheat and/or steel convection at the solidification front, is drawn toward the mould wall to form the oscillation marks without a subsurface hook. Consequently the effect of various casting variables on the depth of oscillation marks can be explained on theoretical grounds. Positive segregation of phosphorus has been observed at the bottom of the oscillation marks and has been classified mainly into two types. One type is observed at the end of the overflow region on the subsurface hook. A heat-flow model which takes into account the shape of the oscillation marks has revealed that this type of positive segregation is caused by local delay of solidification at the bottom of the oscillation marks. Another type of positive segregation has been found in a layer on the bottom of oscillation marks without subsurface hooks. This form of segregation cannot be explained by the heat flow model, but is likely due to a penetration mechanism in which the negative pressure in the flux channel generated during the upward motion of the mould draws out interdendritic liquid from the semi-solidified shell. Transverse cracks are found along the bottom of oscillation marks. The surface of the transverse cracks exhibits an interdendretic appearance in the vicinity of the slab surface, which implies that the cracks are hot tears initiated in the mould region. A heat-flow analysis predicts that deep oscillation marks cause nonuniformity of the shell in the mould, which was also observed in the metallographic investigation. According to the heat-flow analysis not only the depth but also the pitch of oscillation marks affects the shell profile. Therefore increasing the frequency of mould oscillation effectively reduces transverse cracks, by decreasing both the depth and the pitch of oscillation marks.

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