Transverse cracks are located at the trough of the vibration marks on the inner arc surface of the slab, and are usually hidden and invisible. The pickling inspection indicated that the crack depth could reach 7mm and the width was 0.2mm. The cracks are located in the ferrite network zone, which is just the primary austenite grain boundary. And there are the precipitation of fine particles (such as AlN) on the grain boundaries. Especially C—Mn—Nb(V) steel is more sensitive to cracks.
Causes of transverse cracks:
1) Too deep vibration marks are the origin of transverse cracks.
2) The content of A1 and Nb in the steel increases, which promotes the precipitation of particles (A1N) at the grain boundaries and induces transverse cracks.
3) The slab is straightened at a brittle temperature of 900-700°C.
4) The secondary cooling is too strong.
Measures to prevent transverse cracks:
1) The use of high frequency (200-400 times/min) and small vibration amplitude (2-4mm) in the crystallizer is an effective way to reduce the depth of vibration marks.
2) The secondary cooling zone adopts stable weak cooling, so that the surface temperature of the billet during straightening is greater than 900 °C.
3) The liquid level of the crystallizer is stable, and mold slag with good lubricating performance and low viscosity is used.
4) Clean surface cracks with flame.