Study On Formation Mechanism Of Initial Defects In Continuous Casting Slab Based On Analysis Of Mechanical Behavior

As the existence of the complicated solidification conditions,initial solidifying shell that has the low strength and ductility in continuously oscillating mold is influenced significantly.The induced initial defects would be expanded and propagated in the secondary cooling,straightening,rolling and other subsequent handling process and evolved into serious defects that are able to restrict the development of steel products.In this study,two mechanical models of meniscus shell and affiliated primary dendrite based on the principle of materials mechanics were developed for describing the change of stress state during mold oscillation.In combination to the casting parameters in plant and high temperature mechanical properties of materials,the formation mechanisms of initial crack and subsurface solidification defects in casting slab were proposed.The main conclusions are obtained as follows:(1)Simply supported beam of initial solidifying shell in meniscus was established for analyzing the stress state of shell and found that the mechanical stress on meniscus shell induced by mold oscillation influencing significantly.The bending stress on initial solidifying shell would reach to-152.5 kPa during positive strip time,which can reach to 117.5 kPa when the mold oscillating in negative strip period.The shell would be broken so as to induce initial crack formation due to the critical fracture strength of meniscus shell is 113.3 kPa only.(2)In the process of continuous casting low-carbon steel,the initial surface crack would be formed in the distance from 1.9 mm to 5.6 mm below free surface of steel when the mold oscillation velocity exceeds 1.6 m·min-1 during positive strip time,while the subsurface crack would be formed in the distance from 3.1 mm to 4.3 mm below free surface of steel when the mold oscillation velocity oversteps 6 m·min-1 during negative strip time.(3)The initial surface crack in solidifying shell can be reduced by increasing oscillation factor,decreasing oscillation amplitude and frequency properly.The initial subsurface crack can be reduced by decreasing oscillation factor,amplitude and frequency.(4)The maximum bending stress on primary dendrite in meniscus solidification front would reach to 28.7-73 MPa.The dendrite is likely to be broken at the bottom during growing process.The growth of solidification structure in certain part is slowed and the advancing speed of solidification front is slower than the surrounding area in which primary dendrite is normal growth.Then,the unevenness solidification in meniscus is induced.(5)The bending deformation of initial solidifying shell would result in the regular change of primary dendrite growth and inter-dendritic bridging.An approximate enclosed space is formed to interdict the contact with the outside liquid steel temporarily.The inter-dendritic solute concentration,shrinkage,surface blowhole and inclusion would be produced in the solidification structure in positive strip time.