Branched cracks are undesirable and dangerous, and under the oxidized surface of a continuously-cast slab they are mostly difficult to identify. They can cause the formation of defects in thick sheet or on thin strip. This work deals with the study of the background to the formation of branched cracks in a continuously-cast slab, at pulling rate 0.43 m.min-1. The results of the work show that branched cracks extended to a depth of 10 mm below the slab surface and occurred mostly below oscillation marks. Cracks were mostly present in the edge parts of the slab, where they were also deeper in comparison with locations in the middle of the slab width. Microstructural analysis confirmed heterogeneity of ferrite grain sizes in the slab surface skin. The microstructure was formed predominantly of polyhedral ferritic-pearlitic grains, and on the ferritic grain boundaries the presence of tertiary cementite was observed. In the area around the cracks mainly non-equilibrium microstructure of acicular character was observed. A typical feature of these cracks after their opening is intercrystalline fracture. Wavelength-dispersive X-ray analysis of a sample with a branched crack showed chemical heterogeneity of harmful elements, mainly S and As. Large amounts of these elements point to their significant segregation. The formation of branched cracks is conditioned not only by segregation of impurities, but possibly also by the presence of undesirable brittle cementite networks. The presence of cementite was confirmed not only by microstructural analysis, but also diffraction of hard X-ray radiation. Precipitation of microalloying elements in connection with the cementite can lead to higher probability of surface crack formation, as also confirmed in this study.Keywords: slab, branched cracks, oscillation marks, segregation, precipitation

Current global trends in the development of new materials and technologies, as well as the increasing demands for performance and lifetime of individual production units in steel casting brought several key innovative features, including the method of preparing tundish working lining and conditions for its direct utilization in the casting process. Since the worldwide introduction of dry tundish lining and cold start casting concept, there were several Research projects related to this technology from the refractory point of view, however, only limited findings were presented about its influence on liquid steel and overall casting process. Besides the initial measurements and evaluation of hydrogen pickup during casting of the first and second heat on tundish, this research work was focused on investigation of influence of mentioned technology on cleanliness of tin steel grades, especially the presence of spinel type inclusions (Al 2 O 3 x MgO) and its direct comparison with regularly used wet tundish working lining practice.

The final quality of flat products is directly related to surface and internal quality of continuously cast slabs. This aspect is very important especially in case of IF and deep-drawing tin grades, in which even the smallest defects can cause problem in the final processing of the material, especially during forming by stamping, when the material fails. This work is devoted to the analysis of the surface defect of star crack type. The presence of defect was visually detected after scarfing process during removal of surface layer on the small radius side of the slab. This type of defect occurs particularly in cases, when steel is contaminated by copper in the process of continuous slab casting. In view of the copper melting point (1083°C) and the surface temperature of the cast slab shell at the mold exit (about 1200°C) in the process of steel cooling at continuous casting, Cu penetrates along the boundaries of primary austenitic grain, or separate particles are precipitated at the grain boundaries. Initial cracks are formed in the area of primary cooling in the mold. Thermal and mechanical stresses acting in the surface area of slab shell when the slab passes thorough the rollers of secondary cooling cause gradual expansion of crack defects along boundaries of grains contaminated by copper.

The paper analyzes the marginal cut-outs from two slabs of Ti-Nb microalloyed steel. The first slab was transitional with initial pulling rate 0.5 m.min -1 and final pulling rate 0.8 m.min -1 . The second slab was cast at real pulling rate 1.03 m.min -1 . The V-notch Charpy impact test showed that at low pulling rate 0.5 m.min -1 , the values of impact toughness (KCV) were higher than those at high slab pulling rates. When the pulling rate was 0.5 m.min -1 , mixed fractures were obtained, occasionally ductile, but mostly brittle. On the other hand pulling at rates 0.8 m.min -1 and 1.03 m.min -1 resulted only in brittle fractures with low KCV. Comparison of the mean ferrite grain size d ST , showed that the most fine-grained structure was achieved at a low pulling rate 0.5 m.min -1 , and the coarsest structure at pulling rate 0.8 m.min -1 . Furthermore, it was shown that, at high slab-pulling rates 0.8 m.min -1 and 1.03 m.min -1 , a greater amount of tertiary cementite was found on the boundaries of ferritic grains, compared with the lowest rate of slab pulling 0.5 m.min -1 . At the highest rate of slab pulling 1.03 m.min -1 , in addition to tertiary cementite, presence particles were observed here and there on the boundaries of ferritic grains, mostly oval shaped. On the fracture surfaces of brittle samples transcrystalline cleavage facets (TCF) dominated. In small amounts smooth facet intercrystalline decohesion (FID) was also identified with max. 0.2% , and also ductile transcrystalline failure with dimples (DDTF), usually present only in the form of narrow dimple bridges between cleavage facets. Observation of failures using scanning electron microscope (SEM) revealed the presence of either clusters of inclusions or individual particles, and EDX analysis confirmed the presence of Al, S, O, Mn, i.e. most probably oxides of aluminum and manganese sulphides. Aluminum can come from poorlycleaned melt contaminated by fumes from deoxidation or from mould powders. There were also complex particles based on (C, Ti, Mn, O, Al, S, Nb, Zr, Ca) observed mainly on the TCF. Moreover, the impact of heterogeneous distribution of particles on embrittlement cannot be excluded -carbides, nitrides, and complex carbonitrides based on (Ti, Nb) (C, N).