Brief analysis of the causes and control of slab microcracks

Micro-cracks in cast slabs are important factors affecting the subsequent processing of cast slabs and the quality of final steel products. At the micro-cracks on the surface of the steel plate, there is a decarburized layer caused by internal oxidation and oxidation spots near the surface, indicating that the micro-cracks originate from the surface micro-cracks of the cast slab. After flame cleaning and hot acid etching at the corresponding positions of the cast slab, the depth of the micro-cracks revealed is about 4-5mm. Metallographic analysis shows that the membranous ferrite precipitated at the austenite grain boundaries is the main cause of crack generation, and the cracks exhibit the morphological characteristic of intergranular fracture. No inclusion aggregation was found, ruling out the possibilities of crack formation caused by the scraping damage of the mold copper plate and the residual harmful elements in the molten steel Thermodynamic calculations for the steel grade S355 were performed using the Thermocalc software. Considering the actual production conditions, the straightening temperature of the cast slab should be increased to 950 °C to avoid the temperature range of the third brittle zone of this steel grade. By optimizing the secondary cooling process of continuous casting, controlling the stacking cooling time and temperature, etc., the re-grading rate of surface micro-cracks of medium-carbon Nb-containing micro-alloyed steel plates has decreased from the original 1.31% to 0.45%.