| Popis: |
The aim of this work was to analyze the influence of technology on the morphology of fractures and Charpy impact toughness in the TiNb microalloyed steel slab surface zone. The slab was made by continuous casting using different cooling rates in the secondary cooling zone (2 cooling rates were selected for testing) and 2 slab pulling rates 0.5 m/min and 0.8 m/min. It turned out that, with a higher slab pulling rate for both cooling rates applied, the impact toughness was generally lower than that with the slow pulling rate. Microstructure analyses showed the composition of the surface zone was formed by ferrite and pearlite. Coarser ferrite was seen in the surface zone with the higher slab pulling rate and higher cooling rate in the secondary cooling zone. The surface zone microstructure was polyedric for the lower cooling rate and sporadically nonpolyedric with needle-like, or acicular ferrite for faster cooling. Brittle fracture test pieces showed fracture surfaces with transcrystalline cleavage facets (TCF) regardless of the applied cooling rate. With lower cooling rates, smooth facets of intercrystalline decohesion (FID) were identified too, but at less than 0.1%. With faster cooling they showed up in a few isolated cases only. The occurrence of dimpled transcrystalline ductile fractures (DTDF) was generally low. It was confirmed that the morphology of forced fractures was influenced by the cooling rate via the produced microstructure. The embrittlement of the tested samples was assisted by clusters and single particles. They were identified using EDX as based on Al, or combined with Ti, Nb nitrides, or carbide and sulphide eutectics, or inclusions ordered in rows in the ferrite network. Since the occurrence of intercrystalline fractures was low with faster cooling and high slab pulling rate, distinctive suppression of segregation can be assumed for this technology, if compared to slow cooling and low slab pulling rate. |
