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.

Dual phase (DP), interstitial free (IF) and advanced high strength low alloy steel (HSLA)sheets have been successfully used for different components of car body. DP and HSLA are used ascrash resistant and IF as cover or “skin” of car body. The development of new vehicles nowadays isbeing driven by the need to simultaneously reduce mass and increase of passenger and pedestriansafety as well as costs saving through cold forming instead of hot forming. Limited publishedinformation is available on the changes in microstructure of these steel grades at different highstrain rates [1-3].This paper deals with changes of mechanical properties, microstructure and fractography of threesteel grades, which were tested at quasi static (10-3 s-1) and high strain rate (3000 s-1). Themicrostructures were characterized in terms of ferrite grain size, aspect ratio of ferrite andelongation of constituent phases. Deformed and undeformed specimens were compared to assess thechanges in the microstructure. The fracture appearance analysis indicates that the fracture patternunder high strain rates is mainly ductile, regardless of steel grades. The microstructure changessignificantly during the deformation process under both quasi-static and dynamic tension in allinvestigated steels. The plastic deformation in ferrite dominates in this process.

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

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This paper deals with the optimalization of tempering temperature of 9CrNB steel in Železiarne Podbrezová Steelworks, where hot-rolled tubes were produced with dimensions of 88.9 x 12.51 mm. Austenitising at 1070°C/12m/hr was carried out after rolling, and samples were subsequently tempered at 790°C, 760°C and 720°C/4m/hr. The results of testing the mechanical properties show that only tempering at 790°C fulfilled all of the mechanical properties requirements (Rp0,2, Rm, A5, HBW, KV2). The mechanical properties of grade P92 were used for comparison with 9CrNB mechanical properties, according to the relevant standard of STN EN 10216-2+A2. Yield strength requirements (Rp0,2) were also fulfilled in the temperature range from 100 to 600 °C. Microstructural analysis showed that tempering at 720°C, and also at 760°C does not lead to the complete tempering of martensite microstructure. We observed segregation of secondary phases at the grain boundary, but cementite films between individual laths did not coagulate to form carbide phases. By tempering at 790°C the intensity of formation of carbide phases, coagulation and growth of carbide phases is very high and leads to disintegration of laths. Despite satisfactory results, theoretical studies with respect to the selected chemical composition of 9CrNB steel show that to achieve sufficient dissolution of carbide or nitride phases (especially BN), it is necessary to use high temperature austenitization up to about 1200°C, followed by tempering below Ac1.