The aim of this work was to observe the relationship between hot ductility and morphology, distribution and size of particles in TiNb IF steel after hot torsion testing at the critical temperatures of deformation with low as well as maximum values of plasticity. Transmission electron microscopy showed that the particles at all temperatures of deformation with minimum number of turns to failure e.g. 1132°C (33.72 rev.), 946°C (6.24 rev.), 637°C (5.54 rev.) as well as with maximum value of plasticity at 844°C (1726 rev.) were of globular, cuboid or elliptical shape. EDX analysis revealed that there were different types of particles such as carbides, sulfides, and carbonitrides of Ti and Nb, Al and Si oxides, Mn sulfides, and phosphides. Quantitative evaluation of particle size in the carbon extracted replicas showed 20% of total number of particles with size 2r = 30-39 nm and an average linear dimension = 42 nm at the deformation temperature of 1132°C. There were 28% of the particles with size 2r = 20-29nm and = 41nm at the temperature of 946°C while at the temperature of 637°C there were 29% of particles with size 2r = 20-29 and 26% with size 2r=10-19 nm and = 41 nm. In the case of maximum plasticity (1726 rev.) at 844°C, the presence of large particles was confirmed with = 105 nm size and 9% distribution in three size categories of 20-29, 30-39, 90-99 nm.
The aim of the work was to evaluate the hot ductility loss in TiNb stabilized IF steel directly from the continuously-cast slab using hot torsion testing (plastometry) in the temperature range 600–1250 °C according to the basic programme, and also after temperature cycling. A good match of the temperature dependences of number of turns to failure (Nf) and intensity of deformation Se was confirmed. In both cases, the existence of three temperature areas with decrease in plasticity to a minimum was confirmed. The two-stage temperature cycling according to the CT1150 and CT900 programmes mostly resulted in a decrease in plasticity compared to the basic programme. The most significant effect of cycling was related to the CT900 programme below the maximum plasticity in the base programme at 850 °C. A less pronounced decrease was observed for CT1150 cycling below the maximum plasticity in the base program at 1050 °C. In the case of CT1150 cycling, more complex particles were observed at the fractures compared with the basic programme, namely carbonitrides of Ti and Nb in combination with oxisulfides respectively, then Ti nitrides with oxisulfides or oxides and, in addition, complex (Fe,Nb)P4, (Ti,Nb)3S4 type particles. Their mean size determined statistically using TEM was much finer, only 20 nm versus 42 nm in the basic programme. Similarly, CT900 cycling revealed finer particles with an average size of 37 nm compared to 105 nm in the basic programme. The observed particles were Al oxides, Ti(N,C) and (Ti,Nb)2S, in contrast to the particles probably of TiFe and FeMnS in the basic programme. The decrease in plasticity corresponded to the finer particles, newly created in the temperature cycling.
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.
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