Mechanism and control of hydrogen induced breakout in MCCR thin slab continuous casting with high casting speed
ZHANG Congcong1, LUO Yanzhao1, WANG Shengdong2, JI Chenxi1, DONG Wenliang1, WANG Hao2
1. Research Institute of Technology, Shougang Group Co., Ltd., Beijing 100043, China; 2. Steel Rolling Department, Shougang Jingtang United Iron and Steel Co., Ltd., Tangshan 063200, Hebei, China
Abstract：In order to study the causes and evolution process of breakout in SPA-H steel for MCCR production line, the morphology of the breakout shell was observed using scanning electron microscopy and metallographic microscopy. It was found that there were a large number of pinhole defects and cracks on the inner surface of the breakout shell, and a large number of micro-pores at the edge of cracks. The study believed that the high hydrogen content of molten steel was the main reason for the breakout. The changes in hydrogen solubility during solidification of molten steel with initial hydrogen mass fraction of 0.000 5%, 0.001 0% and 0.002 0% were calculated using FactSage software. It was determined that when the initial hydrogen mass fraction of the molten steel was controlled below 0.000 5%, pinhole defects on the surface of the shell could be avoided. The effects of hydrogen mass fraction in molten steel of tundish at 0.000 17% and 0.000 67% on the crystallization and heat transfer properties of mold powder were studied through industrial experiments and XRD phase analysis. The results show that when the hydrogen content in molten steel is high, the slag rim increases, the strength of lance spar precipitation peak increases by about 80%, and the temperature for the first three rows of thermocouples in the "H" column at the wide face center decreases by about 15 ℃, 12 ℃ and 18 ℃ respectively. The formation mechanism of hydrogen induced steel breakout is determined, when the hydrogen content in the molten steel is too high, hydrogen bubbles will generate inside the shell and escape from the inner surface of shell, forming pinhole defects on the inner surface of the shell, weakening the shell strength. In addition, the precipitated hydrogen bubbles increase the crystallization performance of the mold powder, leading to the thinning of the shell and further decrease in strength. Under the static pressure of molten steel and the friction force of mold, pinhole defects are torn to form cracks, leading to steel breakout. By controlling the materials, refractory, and refining operations to control the hydrogen mass fraction of steel below 0.000 55% can prevent hydrogen induced breakout. The research result can provide reference for other similar production lines to avoid such breakout accidents.
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2023, Vol. 33 
