Abstract
Continuous casting of low carbon low silicon steel processed though Basic Oxygen Furnace (BOF) followed by Online Purging station (OLP) and Ladle Furnace (LF) inherently suffers from the problem of clogging of submerged entry nozzles (SEN) during casting due to gradual alumina deposition at the inner surface of the refractory wall. This restricts the incoming flow of liquid steel from tundish to mould through SEN and thus limits the productivity of the caster. On the other hand, excess alumina deposition and their periodic dislodgement from the nozzle refractory surface gives rise to undue stopper rod movement and melt level fluctuations in the mould. All these abnormalities lead to decreased production of good quality slabs. In the present work, influence of ladle addition practice on the castability of typical low carbon low silicon steel has been investigated for a conventional thick slab caster. For identification of inclusions characteristics at different stages of processing, liquid steel samples were collected using the lollipop and a specially designed large sampler during casting. Also, the clog deposits in the used SEN and the natures of inclusions in the clog deposits as well as in the liquid steel were identified by SEM–EDS analysis. Subsequently, to measure the process the different plant trials were carried out for several casting sequences using different deoxidation practices. The upstream processing parameter and corresponding casting data with nozzle clogging index (NCI) for a complete casting sequence were examined for evaluating their casting performances. Addition practice of deoxidant and lime during the ladle treatment has been found to have an important effect of the liquid steel casting. It has been clearly demonstrated the faulty addition practice can lead to severe nozzle clogging during casting. Though a series of plant trials the best addition practice has been determined and implemented in the regular production in the plant for ensuring the smooth production of the given grade of steel. For the quantification of steel cleanliness, total oxygen content (T [O]) of liquid steel, was measured from the samples collected after each ladle treatment stage. T [O] of liquid steel was found in the range of 60−120 ppm at OLP and 30−55 ppm at tundish of low carbon low silicon grade of steel.
Highlights
Aluminium killed steel inherently suffers from the problem of nozzle clogging during continuous casting [1, 2]
Slab casting of low carbon and low silicon aluminium killed steel has been quite problematic in comparison to other grades
These grades suffered from undue stopper rod movement and higher mould level fluctuation during casting, leading to significant downgrading of cast slabs
Summary
Aluminium killed steel inherently suffers from the problem of nozzle clogging during continuous casting [1, 2]. Anil Kumar et al.: Influence of Ladle Addition Practice on the Castability of Typical Low Carbon Low Silicon Steel inclusions [5, 6] Such steels are commonly produce through three process routes viz., (a) direct route where liquid steel is sent directly for the casting just after primary deoxidation and argon rinsing (ARS) treatment in the ladle during tapping from the vessel, (b) in the LF route where liquid steel is further treated in the ladle furnace after ARS before casting, and (c) RH route where liquid steel is first homogenised and partially deoxidised after the vessel tapping and ARS subjected to treatment in the RH degasser before casting. Plant trials were conducted with modified liquid steel deoxidation practices and improvement in casability was evaluated on the basis of the observed inclusion characteristics and corresponding NCI values of various heats in the casting sequence
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