Determination of Niobium precipitations in micro alloyed steels

Micro-alloyed High Strength Low Alloy steels (HSLA-steels) are widely used in civil construction, automobile and line pipe applications. They contain small quantities of alloying elements between 0,01 – 0,1 wt%, such as Niobium, Titanium, Vanadium, Aluminium and Nitrogen. These steels rely on thermo-mechanical rolling, a technique that simultaneously provides high strength, excellent toughness and ductility by grain refinement. Due to their low carbon content, HSLA-steels also show good welding and cold forming properties. During the rolling and cooling process, carbon precipitations up to 100 nm are formed. Depending on the hot rolling parameters like temperature, deformation rate and degree, interpasstime and cooling conditions, precipitation formation is very complex and determines changes in material properties. The most important effect of Niobium in HSLA-steels is the retardation of austenite (γ-Fe) recrystallization which provides more nuclei for the γ/α transformation and thus a finer grain size. Besides Niobium’s role in solid solution by delaying all diffusion controlled processes, its tendency to form carbides provides the dominant effect [1, 2]. The main problem in analysing Niobium carbides in HSLA steels is the low volume content of these precipitations. Therefore, transmission electron microscopy (TEM) was almost exclusively used in order to characterize the precipitations. The disadvantage of this method lies in the extensive sample preparation and the limited sample volume. High-energy synchrotron radiation, e.g.at BW5 (E~100 keV) provides a high photon flux, an excellent angular resolution in combination with low absorption. These features allow investigations of very large irradiated sample volumes (~10 mm3). The small amount of precipitates cause very low intensities of the interesting carbides in comparison with those of the ferrite ( -Fe) (Fig.1a). However, using a special secondary slit system to hide the strong iron reflections precipitates can be detected clearly (Fig.1b) [3]. Experiments with high-energy beam-line P02.1 providing even higher photon flux and higher angular resolving power can be performed without any secondary slit system.