Influence of Composition on Defects in Continuously Cast Products

 Influence of Composition on Defects in Continuously Cast Products

Of the many types of defect in continuously cast product, only transverse surface cracks are known to be strongly influenced by the microalloying elements V, Nb and Ti. Some of the other types of surface defect, such as longitudinal surface cracks, are influenced by composition, particularly C (0.07-0.18% being prone to longitudinal cracking) S, P, and Mn/S ratio, increased P and S, and decreased Mn/S ratio leading to increased cracking. Internal crack formation is also influenced by composition, and again C, S and P are particularly important.

Cracking is much more likely to occur if regions of low ductility are present.It is possible to identify 4 distinct regions of low ductility under test conditions relevant to continuous casting. These four regions are illustrated on a schematic hot ductility curve, 

Region I - Embrittlement by Incipient Melting

Region IIa - Embrittlement by Second Phase Particles - (Mn,Fe)S

Region IIb- Embrittlement by Second Phase Particles - Nb(CN), AlN, V(CN)

Region III - Embrittlement by Transformation

Region I occurs at high temperatures, typically 20-50°C below the mean solidus temperature. Fracture surfaces are characterized by inter-dendritic failure and the presence of particles such as MnS. This region of low ductility is associated with incipient melting at interdendritic and grain boundaries, and is important in the formation of many types of defect in continuously cast products, such as longitudinal surface cracking. The segregation of elements such as S to inter-dendritic regions during solidification is important to this type of

failure. This region of low ductility may be responsible for the initiation phase of transverse surface cracks, as small subsurface cracks have been observed associated with oscillation marks.The oscillation marks themselves are regions in which high degrees of segregation of elements such as S, P and Mn can occur. Heat transfer to the mould in the vicinity of the oscillation mark is also reduced, which will tend to keep temperatures high, and within the brittle zone.

 

Region II occurs over the approximate temperature range 1200-900°C, depending on composition and test conditions, and fracture surfaces are typically along austenite grain boundaries, and sometimes show the presence of second phase particles, with ductile dimples around these second phase particles. These low ductility regions are associated with precipitates - (Mn,Fe)S for Region IIa and Nb(CN), V(CN), Ti(CN) and AlN for Region IIb. The distinction between regions IIa and IIb is determined by the stability of the different particle types. Type IIa low ductility is only apparent at quite high strain rates; at lower strain rates, or when there is an extended hold prior to testing, ductility is good. On the other hand, Type IIb ductility loss is worse as strain rate decreases. Type IIa ductility loss is strongly dependent on composition, particularly Mn/S ratio. It has also been suggested that IIa ductility loss is due to the precipitation of liquid FeS particles, and reduction of grain boundary decohesion due to S segregation.Transverse cracking is usually associated with high strength microalloyed steels, with high Mn contents, and therefore high Mn/S ratios. The strain rates during the processing of continuously cast slabs are also too slow for Type IIa ductility loss to occur, and this suggests that type IIa ductility loss is not responsible for transverse crack formation. Type IIb ductility loss is initiated by austenite grain boundary sliding, which encourages crack formation at grain boundaries and the presence of second phase particles such as Nb(CN), V(CN) or AlN. These particles have two major roles; they can delay the onset of recrystallisation, and they can reduce the strain required for fracture. The high temperature end of this ductility trough is believed to be associated with the onset of recrystallisation. If recrystallisation can occur prior to failure, any developing grain boundary cracks become isolated, and further propagation is not possible. It is well known that the microlloying elements Nb and V can delay recrystallisation, either in solution or as precipitates, and this retardation of recrystallisation is believed to be responsible for extending the Type IIb ductility trough to higher temperatures. However, in this respect, V is much less effective than Nb in delaying recrystallistion. The presence of microalloy precipitates can also reduce the strain to fracture by a number of possible mechanisms: precipitate free zones are often observed adjacent to austenite grain boundaries, and this may lead to strain concentration at the grain boundary; the particles (or groups of particles) at the grain boundaries may act as crack initiation sites; or general matrix precipitation can lead to an increase in strength, and an overall reduction in ductility. The proposed mechanism for low ductility failures in the presence of Nb and V carbonitrides.

Region III occurs over the approximate temperature range 900-600°C, depending on composition, and if Type II low ductility is present, these two ductility troughs can merge together. Fracture surfaces are characterised by intergranular failures, and the facets of the individual grains are often associated with void formation around second phase particles. It is believed that this region of low ductility is associated with the austenite to ferrite transformation. On cooling below the transformation temperature, ferrite formation

commences at austenite grain boundaries, leading to the formation of films of ferrite around the austenite grains. At temperatures within the transformation range, ferrite is softer than austenite and so when deformation commences, strain is concentrated within the ferrite at grain boundaries, and the processes of ductile failure, i.e. void nucleation at second phase particles, and the growth of these voids, continues within the ferrite film.Thus on a microscopic scale, fracture can be described as ductile, but overall the failure is brittle. The mechanism for this type of fracture is illustrated . The high temperature end of the ductility trough is associated with the start of transformation, and is thus determined by composition and processing conditions. There appears to be a good relationship between the temperature at ductility starts to fall and the Ar3 temperature, the transformation temperature measured during cooling. It has also been suggested that the temperature at which ductility starts to fall is very close to the equilibrium transformation temperature Ae3, rather than the Ar3, as the deformation process accelerates the transformation kinetics. Ductility recovers at lower temperatures because the volume fraction of ferrite is higher, and the strain distribution between austenite and ferrite becomes more uniform. At lower temperatures, the strength differential between austenite and ferrite is also less, which will again contribute to a more uniform distribution of strain between austenite and ferrite. For ductility to recover completely, it appears that approximately 50% of the austenite must have transformed to ferrite, Microalloying elements can influence the position of this type of ductility trough through their influence on transformation temperature. For example, the presence of Nb in solution prior to transformation is known to reduce transformation temperatures, and has also been shown to reduce the temperature at which the type III ductility trough occurs, In this respect V also has a lesser effect, as quite large additions of V are required to depress the transformation temperature significantly. Microalloys can also deepen this type of ductility trough when they are present in the form of precipitates. The precipitates may act as nucleation sites for voids within the thin ferrite films, or reduce the ductility of the thin ferrite films by retarding recovery processes.