Influence of Mold Flux on Initial Solidification of Hypo-Peritectic Steel in a Continuous Casting Mold

Abstract

Continuous casting of hypo-peritectic steel was conducted with a pilot slab caster. Such experimental data as local heat flux, thickness of solidified shell or mold flux film, and dendrite primary arm spacing were obtained. On the basis of these experimental results, influence of mold flux on initial solidification in the mold was discussed.With mild cooling by crystallization of mold flux, local heat flux and solidification rate decreased in the mold. The changes in them quantitatively correspond to each other. Dendrite primary arm spacing increased with the mild cooling. Relationship between the arm spacing and cooling rate was established and cooling rate on quite initial stage of solidification was stimated. Cooling rate at 1 mm thickness of solidified shell was estimated as about 10 000-17 000 K/min and changed by mold flux. Unevenness of the solidified shell thickness becomes remarkable when the shell grows to be 1 mm thick. Relation between the unevenness and the cooling rate was discussed, and critical cooling rate against the uneven solidification was observed around 17 000 K/min. Thermal resistance of mold flux film was also evaluated and it was clarified that thermal resistance in the film is larger than that by air gap, and Crystallization in the film contributes to increase of both resistances. It is also considered that increase of casting speed makes air gap thinner, so reduction of radiation by crystallization of mold flux becomes more important in high speed continuous casting.

Structure and Properties of Slags Used in the Continuous Casting of Steel: Part 1 Conventional Mould Powders

“…104) Values of R int are shown in Table 2 and it can be seen that R int values tend to be (i) scattered and (ii) larger in simulation experiments [98][99][100][101][102][103][104] than in pilot plant tests. 96,97) The latter finding was attributed to the effect of ferro-static pressure pushing on the shell and reducing R int . 96,97) Given the complexity of the heat transfer process and the difficulties in simulating, accurately, the heat transfer across the slag film, it is suggested that results of the pilot plant trials be adopted until proved wrong.…”

“…96,97) The latter finding was attributed to the effect of ferro-static pressure pushing on the shell and reducing R int . 96,97) Given the complexity of the heat transfer process and the difficulties in simulating, accurately, the heat transfer across the slag film, it is suggested that results of the pilot plant trials be adopted until proved wrong.…”

“…There is still a debate about whether R int or the reflection of IR radiation is the dominant mechanism in reducing q hor ; however, the latter mechanism was found to be dominant in the only pilot trials carried out to derive R int . 96,97) The initial slag film is probably glassy but crystallises over time and f cryst probably attains a steady state. Primary crystallisation consists in the precipitation of cuspidine (C 3 S 2 Fl) on the shell side, 123,124) and involves the inward diffusion of CaF 2 , CaO and SiO 2 and the outward diffusion of Na 2 O and Al 2 O 3 .…”

Structure and Properties of Slags Used in the Continuous Casting of Steel: Part 2 Specialist Mould Powders

“…The values of viscosity and T br 37,38) are, in general, consistent with values derived from empirical rules but there is a tendency for T br values to be slightly higher than the calculated values, presumably to ensure that the slag film provides a satisfactory slag film. Pilot plant trials 72,73) have shown that the interfacial thermal resistance, R int , is lower than the values measured in simulation experiments which has been attributed to the effect of the ferrostatic pressure on the shell in the plant trials. 72,73) Attempts have been made to increase the melting rate and the slag pool depth by reducing the %C in the powder but frequently this is carried out by adding the carbon in the form of carbon black (with low particle size) which tends to negate any advantages with reducing the %C.…”

“…Pilot plant trials 72,73) have shown that the interfacial thermal resistance, R int , is lower than the values measured in simulation experiments which has been attributed to the effect of the ferrostatic pressure on the shell in the plant trials. 72,73) Attempts have been made to increase the melting rate and the slag pool depth by reducing the %C in the powder but frequently this is carried out by adding the carbon in the form of carbon black (with low particle size) which tends to negate any advantages with reducing the %C. The reduction of the %C can also lead to freezing of the steel meniscus (since carbon combustion leads to reduction in the thermal Best to keep slag rim (C/A) = 0.9-2.0 Crystn C/A<1.5 -LA+ CaF2 C/A >1.5 (C3-xNx)A; Cryst tend min at C/A =1.5 [68]; C3S2Fl followed by CB or C9B3Fl [66] Plant trial Large slag rim + poor lubrication [27] High melting phases promote large slag rims [27] Small slag rims (C/A) = 0.9 to 2].…”