Boichenko S. Improvement of quality of flakesusceptible continuously cast steel grades on the basis of perfection of hydrodynamical processes in a tundish

The thesis aims to analysis and generalization of theoretical and experimental researches of author and industrial experience of some domestic and foreign plants, concerning continuous casting of flakesusceptible high-strength steel grades. By results of the ultrasonic control of continuously cast products it was found, that its rejection is caused by presence of flakes, fragile roundish zones near nonmetallic inclusions, bubbles, pores, the delayed cracks, corrosion of stresses. According to Tsapfe's and Sims's decohesive model and Bichem's theory of hydrogen increase in local plasticity interrelation of defects of flakesusceptible steel grades and the content of hydrogen more than 2 ppm in them is found. In the thesis the key basis of development of continuous casting technology on CCM flakesusceptible steel grades was inevitable increase of the hydrogen content in tundish in them, despite of preliminary vacuum treatment, due to hydrogen occlusion by metal on the first 5 melts of the continuous series, that arrived from accumulated in pores moisture and hydrogen, capillaries of a working layer restored lining, particles of slag mixtures added for whole series and as a result of water steam adsorptions in slag phase and in surface of nonmetallic inclusions which are immersed by streams of a liquid downwards. Theoretical bases and practical actions on prevention of brucite Mg(OH)2 contact, included gunning mass, with liquid steel were developed. Basic elements of new continuous casting technology are alternate flow rate of argon that blows through porous blocks of a special design in the bottom of the CCM tundish, and maintenance of a renewed cover on metal highly basic slag, that at industrial approbation has prevented rejection of metal products owing to stable decrease of hydrogen concentration in high-strength steel grades up to 1,4-1,7 ppm. Economic benefit of introduction can be 46,4 UAH on 1 tone metal products. Novelty and industrial utility of the developed technology are protected by Patents of Ukraine №90886 " Спосіб рафінування флокеночутливої рідкої сталі" і №90970 " Спосіб безперервної розливки флокеночутливих сталей " with a priority from 10.06.2014.