Improving oxide cleanness on basis of MIDAS method

Abstract

The MIDAS method (Mannesmann inclusion detection by analysing surfboards) was established in 1987 and enabled steelmakers to detect less frequent macroscopic inclusion particles in large volumes of steel. The basic idea of MIDAS is the flat cross-rolling of a strand sample perpendicular to the casting direction. Improvements in secondary metallurgy and casting technology have been achieved, since it became possible to define macrocleanness in exact numbers. In this paper, MIDAS results are presented for various strand sections: billets, blooms, jumbo blooms, round strands, conventional slabs and thin slabs. The variation of cleanness during sequence casting, at the front portion and end portion of heats, at ladle change and during steady state casting is investigated. The effect of teeming disturbances and accidental casting events on macrocleanness is studied. Selection of clean material can be optimised by adjusting the cutting length and downgrading critical casting portions. Detected inclusion particles in ultrasonic tested 'surfboard' samples are subsequently identified by metallography and by SEM or EPMA. The exogenous sources of contaminations and the formation mechanisms of inclusion particles are elucidated. Fundamental aspects, such as flotation due to buoyancy forces, coagulation of liquid particles, agglomeration of solid particles due to turbulent fluid flow, and the emulsification of slag droplets from ladle, tundish or mould, are now more deeply understood and enable progressive steps towards clean steel practice. MIDAS results correlate with material behaviour and properties at manufacturing and end-use of long and flat steel products. Future objectives should stress the stabilisation of optimised steady state conditions to realise a higher consistency and reproducibility of properties. Revamping of older continuous casting machines with curved moulds and the introduction of vertical sections are most essential for clean steelmaking. To avoid subsurface inclusions at the midwidth of the strand, submerged entry nozzle design and fluid flow pattern in the mould and liquid pool of continuous cast slabs should be reconsidered.