Mould Powder Requirements for High-speed Casting

Abstract

Mould powders play an important role in the stability of the continuous casting process of steel. The main functions of mould slag (i.e. molten powder) are to provide sufficient lubrication and to control the heat transfer between the developing steel shell and the mould. Sufficient lubrication requires an undisturbed melting of mould powders and uniform infiltration of mould slag. Based on the casting practice in IJmuiden, it is found that these demands become even more important for the applied high casting speeds in thin slab casting at 5 to 6 m/min. At Corus RD&T, mould powders were characterised by X-ray diffraction and subsequent fUlly quantitative Rietveld analysis. Additionally, the melting of mould powders has been studied in-situ using high-temperature X-ray diffraction, to gain crucial knowledge about melting relations. Slag rims obtained from the thin slab caster mould were characterised using extended microscopic techniques in order to describe the mechanisms of rim formation and growth. Finally, slag films obtained after casting were characterised. As a result, not only the melting process of mould powder, but also the mechanism of formation and growth of slag rims is much better understood. This knowledge will be applied to define the demands on the composition and properties of mould powder for even higher casting speeds.

Heat Transfer in Funnel-mould Casting: Effect of Plate Thickness

“…Slag film fragments previously taken from the mould exit of the DSP caster ranged from 50 to 500 mm in thickness 15) and many of the fragments were found to be split into thinner mould-side and shell-side pieces. An average thickness of 200 mm was used as a first approximation in calculating the velocity ratio with Eq.…”

Estimation of Time–Temperature-Transformation Diagrams of Mold Powder Slags from Thermo-analysis of Non-isothermal Crystallization

“…of Eq. [12] is constant. The volume fraction of crystals at the peak maximum, x m , can be obtained from DSC curves through the following expression,…”

“…[8,10,11] The slag film is rather thin, with thicknesses between 0.25 and 4 mm, [11] or even finer (0.05 and 0.5 mm). [12] Although the teachings of the industrial trials are of great relevance, the intense pressure on productivity makes highly desirable the availability of laboratory methods that can provide a good measure of in-mold powder behavior. [25] The glass and crystalline fractions of in-mold samples are often characterized by differential thermal analysis, DTA, comparing the areas under the exothermic peaks for these samples against those of pure glass samples of the same slag.…”

“…[11,12] These samples vary from highly crystalline to fully glassy and usually exhibit a crystalline layer on the mold wall side. [6] However, the opposite was found in slag for peritectic grade steels.…”

“…The slag film is made of a liquid and a solid layer, and the latter may have both glassy and crystalline phases accommodated in different and complex fashions or can be fully glassy or crystalline. [5][6][7][8][9][10][11][12][13] Heat is transferred through the mold strand gap by conduction and radiation through the liquid and solid slag layers and air gap which act as a series of thermal resistances. Heat transfer is dominated by radiation and conduction in parallel through the liquid slag layer and by conduction through the glassy-and-crystalline solid layers.…”