Microalloy precipitation in hot charged slabs

This research was directed toward understanding niobium precipitation in hot charged slabs relative to alloy additions and position within the slab. Steels containing low, medium and high concentrations of niobium were tested at multiple slab locations. Preliminary results from a vanadium-containing steel confirmed that the quenching method used for sample collection during CSP processing was adequate. Replicate testing also confirmed that the electrochemical extraction method was reproducible. Three steels with different niobium additions were used to determine the extent of niobium precipitation prior to entry into the hot rolling mill relative to position within a slab (which affects thermal history) and alloy content. The areas of interest included the chill zone (i.e., slab surface), the columnar region and the centerline of the slab. The slab surface/edge results were associated with greater variability and increased precipitation in the high- and medium-niobium alloys. In the high-niobium alloy, some niobium dissolution occurred during equalization in the tunnel furnace subsequent to casting. This behavior was consistent with the lower solubility associated with higher niobium levels and lower temperature. The three alloys demonstrated that the absolute amount of niobium in precipitated form was greatest in the high-niobium steel, while the relative amount was apparently greatest in the lowniobium steel. The columnar region that comprises the bulk volume of the material demonstrated less variability between samples in comparison to the edge location. Especially for the high-niobium alloy, the amount of alloy precipitated on the slab surface was greater than observed at the other sampling locations. The amount of precipitation in the centerline region was slightly greater compared to the columnar region.