Deep oscillation marks - Slab defects

Deep oscillation marks - Slab defects

Figure 5: Close-up of a representative periodic bloom surface defect exhibiting deep oscillation marks, depressions and glaciation.

Defect name: Deep oscillation marks
Record No.: 2175
Type of defect (Internal/Surface): Surface
Defect classification: Slab defects
Steel name: Steel
Steel composition in weight %: No data.
Note: The surfaces of continuously cast steel blooms exhibit a variety of surface features and defects, which were investigated to reveal the interactions at the meniscus between the steel shell and interfacial flux layers that caused them. One such defect formed at periodic intervals along the surface of first and second blooms in a sequence. It was characterised by gradually deepening oscillation marks, followed immediately by longitudinal striations or ‘glaciation marks’. In severe cases, deep depressions were clearly visible within the glaciated region. These defects were investigated through plant trials and both physical and mathematical modelling. The defects were found to exhibit a characteristic temperature history: temperature troughs that move down the mould at the casting speed. These defects may be monitored in much the same way as sticker breakouts, thereby allowing existing thermocouple based breakout detection systems to be modified to include a quality alarm.

The defect assumes a characteristic form with a commonly recurring sequence of features. Describing these features from right to left in Fig. 5, in the order in which they were created, the defect begins with a region of gradually deepening oscillation marks. This region extends over 5–10 cm of bloom length, where the oscillation mark depth increases from a standard depth of between 0.2 and 0.4 mm to a maximum depth of over 2 mm. This region then evolves into a 10–20 cm long region of longitudinal scrape marks or striations: ‘glaciation marks’. The glaciated regions are almost completely devoid of oscillation marks, and those that are visible are very faint. The glaciation marks are often interrupted by severe depressions of random orientation and shape, extending from 1 to 4 mm in depth. In severely affected blooms, every incidence of glaciation has these depressions, which intermittently traverse the entire perimeter of the bloom. Following the depressions, the glaciation region becomes most pronounced, particularly in the off-corner region of the narrow face. Following the last of the glaciation marks is a return to relatively uniform, regular oscillation marks of standard depth and average pitch. Reference: Not shown in this demonstration version.