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Background of branched crack formation in surface zone of continuously-cast Ti-Nb microalloyed steel slab

Bekeč P., Longauerová M., Vojtko M., Milkovič O., Kadlec J., Tréfa G., Grimplini G.

Journal of Achievements in Materials and Manufacturing Engineering

2014

Vol. 67, nr 2

58--64

Purpose: 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. Design/methodology/approach: Samples were taken from the individual cut-outs for further analyzes. Macroscopic analysis was used to evaluate the surface quality of the analysed slabs using a Leica Wild M3Z macroscope. The microstructure was observed with an OLYMPUS VANOX-T light microscope. For the evaluation of the concentration profile of the selected elements in planes parallel to the slab surface, wavelength dispersive X-ray analysis (WDX) was used with a Camebax-MICROanalyzer. Findings: 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. Wavelength-dispersive X-ray analysis of a sample with a branched crack showed chemical heterogeneity of harmful elements, mainly S and As. Research limitations/implications: Precipitation of microalloying elements in connection with the cementite can lead to higher probability of surface crack formation, as also confirmed in this study. Originality/value: of this work is background of branched crack formation in surface zone of continuously-cast Ti-Nb microalloyed steel slab.