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   P   u    b    l    i   s    h   e    d    b   y    M   a   n   e   y    P   u    b    l    i   s    h    i   n   g    (   c    )    I    O    M     C   o   m   m   u   n    i   c   a    t    i   o   n   s    L    t    d Investigation of strand surface defects using mould instrumentation and modelling B. G. Thomas 1 , M. S. Jenkins 2 *  and R. B. Mahapatra 3 The surfaces of continuously cast steel blooms exhibit a variety of surface features and defects, which were investigated to reveal the interactions at the meniscus between the steel shell and interfacial flux layers that caused them. One such defect formed at periodic intervals along the surface of first and second blooms in a sequence. It was characterised by gradually deepening oscillation marks, followed immediately by longitudinal striations or ‘glaciation marks’. In severe cases, deep depressions were clearly visible within the glaciated region. These defects were investigated through plant trials and both physical and mathematical modelling. The defects were found to exhibit a characteristic temperature history: temperature troughs that move down the mould at the casting speed. These defects may be monitored in much the same way as sticker breakouts, thereby allowing existing thermocouple based breakout detection systems to be modified to include a quality alarm. This study attributes these defects to high amplitude, low frequency, mould level fluctuations. A mechanism is proposed which ascribes the generation of these defects to the interaction of the meniscus with the slag rim at peaks in the mould level cycle. Installing an improved mould level control system eliminated the defects. Keywords: Continuous casting, Billet casting, Casting defects, Instrumentation, Modelling  I&S/1821 Introduction The surfac e condit ion of the str and is an imp ort ant aspect of continuous cast steel quality and is controlled mainly by the initial stages of solidication in the mould. Und etecte d sur fac e def ect s can cause seri ous qua lit y problems that may persist into the nal steel product, and in severe cas es, may cause breakouts during the continuous casting process. To avoid these problems, it is useful to understand how each different type of defect for ms, so that its pr e se n ce may be det ected by inexpensive monitoring of process sensor signals rather than full inspection of the nal product. Thi s study was und ert ake n to inv est iga te sur fac e defects in continuous cast steel, based on blooms cast at the Rod and Bar Products Division of BHP Steel, Ne wc ast le, Aust ral ia. This lar ge fo ur strand cas te r produced 6306400 mm blooms that were hot charged to a reheating furnace and then hot rolled into a variety of steel products, inclu ding semi- nished 89–158 mm square billets. The billets occasionally suffered a range of surfa ce defec ts, needing expensi ve, time consumin g surfa ce grind ing, and in severe cases, even downg radin g or re je ct ion. Th e re cond itioning yard wa s of te n a bottleneck in the process, and the cost associated with removing these defects was about $3 500 000 per year. Surfa ce quali ty probl ems persis ted despite chang es in rolling practice, demonstrating that at least some of the defects originated in the mould. After introducing a variety of different surface defects, this pape r fo cus es on a parti cul ar surfa ce def ect of periodic surface depressions. A plant trial was conducted to simultaneously monitor mould variables (mould level and thermocouple temperatures) and examine the asso- ciated bloom and billet surfaces. Metallurgical investiga- tions studied the sub-surface structure of bloom samples thro ugh dep ressions and osci lla tio n mark s. Wit h the assista nce of mathematica l and physica l modell ing, the ndings were interpreted to understand how the defects ar ise . It was envi sio ne d that by li nking eve nt s in the mould instrumentation data to specic bloom and billet surf ace fea tur es, a means of pred ict ing surf ace qual ity problem s could be devised. Finally , changes were made to min imi se the occurrence of the se defe cts and imp rove both bloom and billet quality. It is hoped that insights gained from this study will help to understand and avoid future surface defects. Surface defects Surface features on the strand reveal a great deal about events at the meniscus during their formation. To save energy via hot charging, however, careful inspection of the cooled bloom surface rarely occurred. This section presents some exampl e bloom surface imp erf ection s observed in this study. The inspection revealed a wide variety of features including oscillation mark problems 1 Unive rsity of Illino is at Urba na-Ch ampai gn, Mecha nical & Industrial Engineering, Urbana, IL, USA 2 Monash University, Chemical Engineering, Clayton, Vic., Australia 3 BHP Steel, Port Kembla, NSW, Australia *Corresponding author, e-mail [email protected] 2004 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 19 August 2003; accepted 1 September 2004

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