DOI 10.1515/jmsp-2012-0022 J. Manuf. Sci. Prod. 2013; 13(1–2): 91 – 102 Jeferson L. Klug*, René Hagemann, Nestor C. Heck, Antônio C. F. Vilela, Hans P. Heller and Piotr R. Scheller Solidification Behaviour of Slags: The Single Hot Thermocouple Technique Abstract: With the Single Hot Thermocouple Technique the solidification behaviour of metallurgical slags has been studied, building TTT and CCT diagrams, with in situ observation for melting and solidification under various thermal conditions. Slags in the system CSTNA were studied, analysing the kinetics of crystals growth. Following findings are reported: (i) for the slag CS (%CaO/%SiO 2 = 0.7) no crystal is observed during contin- uous cooling even to low cooling rates, but on the other hand intense crystallisation is obtained when increas- ing the temperature after reaching a particular range of lower temperatures; (ii) it was observed that the longer the duration of the superheating is, the longer the incu- bation times for a slag CST (41 %CaO, 29 %SiO 2 , 30 %TiO 2 ) becomes; (iii) the addition of Na 2 O into the CST slag system shortens intensely the crystals incubation times at TTT diagrams to the range of seconds. Keywords: Single Hot Thermocouple Technique, slags, fluorine-free mould powders, slab casting PACS ® (2010). 81.10.Fq *Corresponding author: Jeferson L. Klug: Universidade Federal do Rio Grande do Sul, Brazil; Technische Universität Bergakademie Freiberg, Germany. E-mail: klug@ufrgs.br Nestor C. Heck, Antônio C. F. Vilela: Universidade Federal do Rio Grande do Sul, Brazil René Hagemann, Hans P. Heller, Piotr R. Scheller: Technische Universität Bergakademie Freiberg, Germany 1 Introduction To control the phenomena in the mould during continu- ous casting of steel is essential to the success of the pro- cess. The mould slag i.e. the mould powder which was melted in the mould is one of the most important slags in the melting shop. The choice of the mould powder is a dif- ficult task because mould powders have several functions. Due to the complex requirements in the industrial process the choice is based on trials in the caster [1–3]. Commercial mould powders contain fluorine, typi- cally as calcium fluoride (CaF 2 ). However, there is a trend to eliminate fluorine from slags which are used during production of steel. The main problem related to the de- velopment of fluorine-free mould powders for slab casting is how to control the heat transfer between steel shell and mould. The crystallisation of cuspidine (3CaO.2SiO 2 .CaF 2 ) from F-bearing mould fluxes is thought to be the most effective way for heat transfer control. Although the mech- anism of heat transfer control via the crystallisation of cuspidine has not yet been determined, two ideas have been proposed. An idea is that radiation heat flux is de- creased by scattering at the boundary between the crystal- line and the liquid layers; another idea is that the total heat flux is reduced by the large thermal resistance of the air gap formed as a result of the solidification shrinkage of the solidified slag layer. In any case, the crystallisation of cuspidine has a great effect on heat transfer control [4]. Several papers have been reporting possibilities to replace the phase cuspidine regarding the heat transfer control function, by the formation of different crystals. Some papers indicate that TiO 2 -bearing raw materials can be used by formation of TiO 2 crystals [4–6]. And besides, there are other works with reports on the solidification behaviour of mould fluxes without fluorine or with lower fluorine content, considering different crystals, like Na 2 O.CaO.3SiO 2 [7]; Na 2 O.2CaO.3SiO 2 [8]; CaSiO 3 , Ca 3 Si 2 O 7 and Ca 2 SiO 4 [9]; CaB 2 SiO 7 and CaAl 14 B 2 (SiO 4 ) 8 [10]. The Single Hot Thermocouple Technique (SHTT) can be used to build TTT diagrams. With this technique the sample is put on a thermocouple tip, heated and cooled at the desired rates. One feature of the technique is to allow in situ observation of melting and solidification under various thermal conditions. Due to the low mass of the system (sample and thermocouple) high heating and cooling rates (>3000 °C/min) can be easily obtained. Besides SHTT, using the hot thermocouple technique another experimental mode is also possible: the Double Hot Thermocouple Technique (DHTT). With the DHTT the solidification behaviour can be observed in situ applying temperature gradients, simulating the conditions during continuous casting. In this mode the slag sample is placed between the tips of two thermocouples. Both sides can be controlled independently and mould-like temperature gradients or heat peaks can be applied [11]. Authenticated | klug@ufrgs.br author's copy Download Date | 4/20/13 1:54 AM