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| Longauerová M., Fedorová M., Longauer S., Kadlec J., Bořuta J., Fujda M., Janák G., Vojtko M. |
| HETEROGENEITIES OF CHEMICAL COMPOSITON IN CAST PRODUCTS |
Abstract
The aim of this paper was to evaluate heterogeneities of chemical composition in continuous-cast products. Two types of continuous cast steels IF steel and 34CrMo4 steel were analyzed in the near-surface and also the central parts of continuous-cast semis.
Shallow oscillation wrinkles occurred on the IF steel slab surface, arranged almost in parallel. Other types of defects, for example cracks, did not occur. On the other hand, 34CrMo4 steel blooms had internal defects of varying intensity.
Results of notch toughness tests producing forced fractures in the surface zone of IF steel slabs demonstrated the heterogeneity of brittle-fracture properties in this zone. Transcrystalline dimple fractures dominated the whole width of the slab except in one small area near the slab’s right edge, where samples were brittle, which also confirmed very low values of KCV. Fractographic analysis of fractures in this embrittled area showed that transgranular cleavage facets predominated in the presence of intergranular facets on the fracture surfaces. In the case of forced fractures which were ductile, their fracture surfaces showed dimple morphology. The presence of intergranular facets suggests weakening of grain boundaries by segregation of impurities, which can negatively influence the quality of rolled products, mainly in the local embrittlement area.
The concentration profiles of elements in slab cross-section confirm the enrichment of the slab surface zone with the impurities Sn, As, P, N and also carbon, even though it is very clean IF steel. The segregation of these elements to the original austenite grain boundaries causes decrease in their cohesion strength and embrittlement in the given area.
Metallographic analysis of samples taken from round blooms of 34CrMo4 steel revealed microstructural heterogeneity resulting from chemical heterogeneity. This was confirmed by the structural morphology, which was acicular with the presence of a white network which had two times higher content of Cr, Mn and S compared with the matrix. These places are areas with martensitic or bainitic structure with microhardness ranging between 728 - 564 HV0,02. Central defects in the shape of big cavities and also small interdendritic cavities and inclusions occurred in these places as a result of local segregation.
Dendritic morphology was confirmed on the fracture surfaces of samples from the vicinity of these defects, and both fractures and metallographic samples from this cavity zone revealed an increased proportion of inclusions based on sulphides, which attests to the increased centreline segregation of impurities. EDX analysis of each bloom cut-out confirmed increased presence of inclusions of MnS and particles rich in Al and Mg, sometimes in combination with Ca. Individual blooms are cast under varying conditions of continuous casting, that is in terms of reheating and casting rate, which influences their central porosity and segregation. Low casting rates also limit the bulging of the solidifying shell by increasing its toughness. This demonstrates that in order to eliminate non-homogeneity and reduce structural heterogeneity in the analyzed blooms of 34CrMO4 steel, it is important not only to maintain low reheating but also necessary to optimalize casting rates towards slower speeds.
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| Keywords: Slabs|surface segregation|structure heterogeneities|centreline noncompactibilities|round blooms|central segregation| |
| No 1 (2007), p. 17-25 |
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