Growth of Solidified Shell Just below the Meniscus in Continuous Casting Mold

In the high speed continuous casting of hypo-peritectic steel slabs, growth of solidified shell just below the meniscus in the mold was researched. Rate of cooling and solidification was estimated on the basis of experimental results of thickness profiles of solidified shell, heat flux in the mold and dendrite arm spacing in the solidified structure beneath the surface of the slabs cast at 3-5 m/min. As a result, it was found that there is a delaying period of solidification growth at the beginning, till the shell grows up to about I mm thick. After that, it grows in linear relation to square root of solidification time. Dealing with the thickness profiles as a function of solidification time and solidification rate was obtained by differential. Furthermore, cooling rate of the shell was estimated with this solidification rate and heat flux in the mold. This cooling rate showed good agreement with that estimated from dendrite secondary arm spacing of the slabs. Estimated cooling rates were compared with the slabs by conventional continuous caster at 1.1-1.6 m/min and the influence of casting speed was discussed, It resulted in that cooling rate increases with casting speed and the difference by casting speed begins to be remarkable in the period of 0.1-1s for solidification time, when uneven solidification just begins to occur. This agreement was considered to be one of the reasons why uneven solidification or longitudinal surface cracking of slabs tends to occur with increase of casting speed.