Yang Cui

Morphology, size and distribution of MnS inclusions in non-quenched and tempered steel during heat treatment

This article reports the morphology, size, and distribution evolution of MnS inclusions in non-quenched and tempered steel during heat treatment. The variation of single large-sized MnS inclusions at high temperature was observed in situ using a confocal scanning laser microscope (CSLM). The slender MnS inclusions first changed to pearl-like strings. These small-sized pearls subsequently coalesced and became closer together as the temperature increased. Large-sized MnS inclusions in non-quenched and tempered steel samples with different thermal histories were investigated with respect to the evolution of their morphology, size, and distribution. After 30 min of ovulation at 1573 K, the percentage of MnS inclusions larger than 3 µm decreased from 50.5% to 3.0%. After a 3 h soaking period, Ostwald ripening occurred. Most MnS inclusions moved from the grain boundaries to the interior. The present study demonstrates that heat treatment is an effective method of changing the morphology, size, and distribution of MnS inclusions, especially large-sized ones.

Formation and evolution of macro inclusions in IF steels during continuous casting

The evolution of macro inclusions during continuous casting was investigated by large-area inclusion characterization using ASPEX and analysis of nozzle blockage deposits. Six kinds of inclusions over 5u2005μm were observed in samples taken from the tundish and the slabs: single alumina particles, alumina dendrites, refractory-related alumina, alumina associated with bubbles, alumina clusters and Al–Ti complex oxides. By examining the morphology of the nozzle blockage deposit, it was concluded that the refractory-related alumina in the slabs came from the decarburization layer washed away by the steel stream. Some of the alumina clusters that came from the nozzle blockage deposit dislodged by the steel flow, were formed by the agglomeration and sintering of 5–20u2005μm inclusions that were carried over from the tundish. Two kinds of Al–Ti oxides were found in the tundish, and their evolution mechanisms during the casting process were proposed.

Flow pattern control in continuous slab casting moulds: physical modelling and plant trials

Full-scale water modelling studies combined with nail-dipping industrial trials at Shougang Jingtang (SGJT) Works were carried out in order to investigate the parameters that influence flow patterns in the continuous slab casting mould. The effects of casting speed/mould width combinations, argon bubbling flow rates, submerged entry nozzle (SEN) immersion depths and SEN geometries (port angle and bottom shape) on the flow patterns were examined. A CMI factor defined as casting speed/mould width ratio index is put forward to describe the combined effect of these parameters on the fluid flow patterns in the mould. The results show that the CMI and argon injection rates are the dominant factors that determine the flow pattern. On the basis of the results from this study, critical argon flow rates for different casting combinations have been evaluated. These findings provide operating guidelines for generating optimal flow patterns based on double roll flow with appropriate surface velocities in continuous slab casting moulds. On the basis of these considerations, slab quality at SGJT Works was substantially improved and the percentage of flow pattern-related sliver defects in cold rolled products was decreased from 4.5 to 2.3%.

Inclusion behaviour in aluminium-killed steel during continuous casting

ABSTRACT The inclusion behaviour during continuous casting were investigated in ultra low carbon (ULC) and high strength low alloy (HSLA) steels. From tundish to cast slabs, the total oxygen (T.O) content decreased from 19 to 12u2005ppm, 12 to 11u2005ppm in ULC steel and HSLA steel, respectively. The number density of inclusions in ULC decreased by about 40%, while it kept almost constant in HSLA steel during casting. A simple calculation on the T.O removal agreed well with the measured value, which showed that T.O removal was much larger through mould flux absorption than nozzle attachment. The T.O removal in ULC steel is substantially higher than that of HSLA steel due to the fact that alumina particles in ULC steel tend to agglomerate easier than calcium aluminates in HSLA steel, which facilitates inclusion removal to the mould powder.