Effect of Slab Subsurface Microstructure Evolution on Transverse Cracking of Microalloyed Steel during Continuous Casting

As to the continuous casting process of low carbon microalloyed steel, subsurface microstructure evolution plays an important role in the slab surface cracking. In order to study the effect of the slab subsurface microstructure evolution on the transverse cracking, three different secondary cooling patterns （i. e. , mild cooling, strong cooling and controlled cooling） were performed in the corresponding slab curved continuous caster. Based on the metallo- graphic results, three transformation regions were found to be formed with the evolution of microstructures at different depths in the slab subsurface. The three regions are strong cooling transformation （SCT） region, double phase transformation （DPT） region and mild cooling transformation （MCT） region, respectively. Meanwhile, it was also found that the crack index used for evaluating slab surface cracking susceptibility was decreased when the range of the DPT region was increased. This can be explained by the fact that the double phase transformation （austenite-ferrite-austenite） occurred resulting from thermal cycling in DPT region, which resulted in promoting the refinement of prior austenite grains and inhibiting the precipitation of film-like ferrite and chain-like precipitates. Under the con- trolled cooling pattern, the widely-distributed DPT region was formed in the range of 3.5--8.0 mm to the slab surface. And compared with other cooling patterns, the cracking susceptibility is lowest with a crack index of 0.4.

Evolution of Surface Transversal Crack in Intermediate Slabs during Hot Rolling Deformation

In the continuous casting of steels, surface transversal cracks are often found. These defects may severely influence the final quality of the products. The evolution of transversal V-shaped cracks with different depth on the surface of a continuously cast steel slab during hot rolling was studied. The artificial V-shaped cracks were made on the surface. The rolling process parameters from an industrial rolling mill have been used as a reference. The speci- mens of rolled workpiece in intermediate slabs were obtained after different rolling passes. The morphology of sur- face crack and microstructure evolution in the rolling process were investigated by optical microscopy. The results show that the depth of surface transversal crack gradually decreased with the increase of rolling passes. The grain size of ferrite and pearlite on the sample surface also gradually reduced. The microstructures around cracks with the dif ferent depth are almost identical, without direct correlation with the initial crack depth.