One of our plants make peritectic grade structural plates ( Carbon 0.15-0.17 %, Mn 0.8-1.1 %, Si - 0.2 - 0.3%, Al-0.03-0.04%).
Recently we are noticing appearance of surface defects of the nature as shown in photograph along the edge of the plates (25-40mm inside the edge). Sometimes the surface defect appears on both the longitudinal edge or on only one side. Most of the time, whenever we have noticed cracks, it occurs almost along the complete length of plate.
Trying to unravel the mystery why this may be happening.
Pertectic grades are prone to developing cracks at oscillation marks. The volume change associated with the peritectic reaction results in uneven surface solidification in the mould and can lead to the formation of thinner shell walls that can lose contact with the mould wall. This results in a localised delay in cooling (in the thinner section) due to a reduction of heat extraction via the mould wall. The delay in cooling promotes an even coarser austenite structure, which is more susceptible to intergranular cracking.
During rolling, any slab edge/corner defects can be shifted to the top edge of the plate
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