Transactions of the Iron and Steel Institute of Japan
Online ISSN : 1881-1183
Print ISSN : 0021-1583
Macro- and Semi-macroscopic Features of the Centerline Segregation in CC Slabs and Their Effect on Product Quality
Osamu HAIDAHidenari KITAOKAYasuhiro HABUSetsuo KAKIHARAHajime BADAShoji SHIRAISHI
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1984 Volume 24 Issue 11 Pages 891-898

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Abstract

The effect of Electro Magnetic Stirring (EMS) and the soft reduction or the reduction of slab thickness by 0.8mm/m at near the crater end on the centerline segregation in CC slabs has been revealed by macroscopic and semi-macroscopic investigations of slabs and plates. The degree of the centerline segregation has been evaluated in two methods; a macroscopic one by use of sulfur prints and a semi-macroscopic one by use of an EPMA Macro Analyzer.
It has been found that the centerline segregation is composed of interconnected semi-macro segregates the size of which is 300μm-3mm, being medium between the inter-dendritic micro segregation and the macro one such as the centerline, V and A segregations in ingots. In the slab cast with the soft reduction, the centerline segregation is narrow in thickness (TM=1.6mm) but high in segregation ratio (Pmax/P0=2.4), because the mutually interconnected segregates are distributed densely in the narrow thickness. Quite contrary to this, the centerline segregation in the slab cast with EMS is wide in thickness (TM=3.2mm) but low in segregation ratio (Pmax/P0=1.7) because the isolated globular semi-macro segregates are sparsely distributed in the wide thickness. Owing to these characteristics of the semi-macro segregates, the degree of the macroscopic centerline segregation visually measured from the sulfur prints is substantially improved by the application of EMS or the soft reduction. It has been estimated, however, by use of the measured values of TM and Pmax/P0 that the volume of the solute enriched liquid transported due to bulging to form the centerline segregation is not significantly influenced by EMS or the soft reduction.
It has been revealed that the degree of the semi-macro segregation or the segregation ratio in the semi-macro segregates increases a little by EMS and substantially increases by the soft reduction. This phenomenon has been qualitatively explained by the effect of EMS and the soft reduction to decrease the volume of liquid steel which forms the segregates after mixed with the solute enriched liquid transported due to the bulging.
The frequency of the occurrence of the Hydrogen Induced Cracking (HIC) in plate has been correlated not to the degree of the macro segregation but to the degree of the semi-macro segregation. It has been also found that the less degree of the semi-macro segregation and, therefore, the improvement of the anti-HIC property of plates is performed by lowering the bulk concentration of phosphorus from 0.010 to 0.005% and the heat treatment of slab at 1250°C for 16hr.

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