Inclusions in the Low Carbon Al-killed Steel Produced by a CSP Thin Slab Casting Process at Handan Steel

摘要

Since the start of the first CSP plant at Nuror Crawfordsville in 1989, there have been more than 50 CSP process lines in the world with a total annual tonnage of ～50 million tonnes. China now has 7 thin slab caster plants, and another three will start soon. The variety of steel grades and steel quality are also improved obviously. During the past 15 years, the research on the thin slab casting process mainly focuses on the market prospect of CSP products, techniques development such as the funnel mold, microstructure and mechanical properties of the hot strip, etc. There are few published reports investigating cleanliness and inclusions during the thin slab casting process. Inclusions, especially the large exogenous ones or clustered indigenous one, cause the surface defects on steel cold sheets. Like most CSP routes in China, the CSP route at Handan Steel is BOF based (BOF → LF → CSP), but not EAF based, which is different from most of the CSP routes in other countries. Handan Steel has started its CSP production since 1999, with current annual tonnage 2.4 million tones. The main parameters of the industrial trial are listed in Table 1. In the current paper, the main defects on Low Carbon Al-killed Steel (LCAK Steel) CSP hot strips produced at Handan Steel and their causes are investigated. Inclusions from ladle refining, to tundish, to thin slab caster slab are studied using microscope observation, Slime test, total oxygen and nitrogen analysis, EPMA and SEM detection, and slag composition measurement. BaO is added to ladle during BOF tapping. CeO{sub}2 (cerium oxide) is added into tundish shroud and La{sub}2O{sub}3 is mixed into tundish lining before trials. K{sub}2O and Na{sub}2O in the mold flux can be considered as its trace. Molten Steel and slag samples were taken at ladles during tapping and during LF refining process, at thin slab casting tundish and mold. Solid steel samples are also taken from several places of the slab. The effect of EMBr on steel cleanliness is investigated using industrial trial data and numerical simulation. Measures to further improve steel cleanliness are discussed. Samples of hot strips with typical defects are collected and analyzed. As comparison, the same grade steel samples produced by the traditional slab caster are also studied.