Destruction of the Dendritic Bridge in Soft Reduction of Continuous-Cast Slab

In order to improve the properties of structural steels, the crystalline structure and chemical composition of the continuous-cast billet must be more isotropic. A promising approach is soft reduction of the billet by a special (dynamic) section in the roller track of the continuous-casting machine Industrial tests of soft reduction in the continuous casting of steel slabs on a vertical machine at OAO Severstal were described in Soft reduction increased the proportion of melts with an axial chemical-inhomogeneity score no greater than 1 from 68.9 to 95.8%, according to At the International Conference on Continuous Steel Casting (Madrid, 1987), the use of soft reduction in continuous casting of round and square bar billet was reported Soft reduction reduced the axial porosity of the cast billet by a factor of around 1.5-3, according to Two main factors are responsible for the axial chemical inhomogeneity in continuous-cast slabs, according to 2) swelling of the shell of the solidifying billet due to the ferrostatic pressure of the melt in the nonsolidifying part, with a sufficiently large distance between the supporting rollers. We now consider the theoretical principles underlying the reduction in axial chemical inhomogeneity of continuous-cast slabs by soft reduction, taking account of both the factors noted in We begin with the assumption that the reduced axial inhomogeneity in the billet is due to destruction of the dendritic bridges that are formed by the growing crystals (columnar or equiaxial crystals) and hinder the melt's access to the shrinkage zones in the central part of the solidifying billets. This is not an original suggestion. Under the influence of soft reduction, "the core is strengthened, the liquational material is driven out, and the dendrites are broken down, with improvement in axial structure of the ingot," according to In the present work, we attempt quantitative estimation of the probability that the bridges preventing melt access to the shrinkage cavities (pores) in the axial zone of the billet will disintegrate under the action of soft reduction. In production tests, it is found that soft reduction is most effective when the liquid-phase content S within the two-phase zone of the solidifying billet is 20-80%. When the content of solid phase Ψ = 1 -S is below the lower limit Ψ 1 = 0.2, the solidifying metal in the twophase zone consists of a suspension of crystals in melt, and the application of external forces leads to motion of the suspension as a whole within the liquid core (the nonsolidifying part of the billet), without significant improvement in the cast structure. On reaching the critical quantity of solid phase Ψ cr = 0.2, the crystals suspended in the melt, together with the oriented (columnar) dendrites, form a dendritic skeleton (filled with melt) capable of resisting the applied external force. The application of compressive force to the shell of the cast billet by the supporting rollers may fracture the dendritic skeleton; this does not always improve the cast structure. Evidently, when the solid-phase content in the two-phase zone exceeds the upper limit Ψ 2 = 0.8, the resistance of the dendritic skeleton to the externa

IL-6 and IL-8 secretion by human glioma cells proliferating after Gamma-knife irradiation

One of the modern methods of treating patients with primary and recurrent brain tumors is radiosurgical irradiation using Gamma Knife, which allows therapeutic doses to be delivered to tumors not exceeding 2.5 cm in diameter in 1–2 sessions. Tumor cells on the periphery of this tissue volume that receive lower radiation doses can resume proliferation and serve as a source of recurrence. The increase of radiation dose may cause necroses formation and a worsening prognosis. The properties of glioblastoma cells that survive and resume proliferation long after stereotactic irradiation are still poorly known. The aim of the work was to evaluate the expression of IL-6 and IL-8 by glioblastoma A172, R1, T2, and T98G cell lines that resumed proliferation after sublethal Gamma Knife irradiation. Cells were irradiated once at doses ranging from 6 to 16 Gy, and then cultured for 40 days. Cell number was counted weekly; lethal and sublethal irradiation doses for each glioblastoma cell line were determined. In cultures descendant from proliferation of single most resistant cells, the level of IL-6 and IL-8 secretion after 96 hours cultivation (ng/1000 cells) was determined by ELISA. The cells of all four glioblastoma lines secreted IL-6 and IL-8 into culture medium. The highest production of cytokines, never before demonstrated for glioblastomas, was discovered in R1 cells. Glioblastoma T2 also had high interleukin production levels. In contrast to these lines, glioblastoma A172 (highly sensitive to the action of cytostatic drugs and radiation) secreted IL-6 at 30 times lower level than R1 cells. Glioblastoma T98G (highly resistant to the action of cytostatic drugs and radiation) also exhibited low interleukins production level. R1, T2, and T98G glioblastoma cells that resumed proliferation after irradiation had increased secretion of IL-6 and, to a lesser extent, IL-8. The dependence of cytokine production increase on irradiation dose for these cells was not linear. In contrast, A172 cells reduced IL-6 and IL-8 secretion under irradiation. The multidirectional changes in IL-6 and IL-8 production by cells of different glioblastoma lines were long-term and persisted for more than a month. The presented results cast doubt on the possibility to use IL-6 and IL-8 production by glioblastoma cells as potential biomarkers for early diagnosis, therapy monitoring as well as prognostic markers of the disease course