INFLUENCE OF SOLIDIFICATION STRUCTURE AND CASTING SPEED ON THE CENTERPOROSITY OF DIFFERENT STEELS

The mechanism of centre solidification in billets and blooms and the a xial segregation resulting from this are deemed as adequately well-res earched. If the centre solidification is fully columnar, the axial mac rosegregation is significantly more pronounced and, at the same time, more irregular than for equiaxed crystallization. Seamless tubes, whic h are produced from continuously cast round material, do not have to h ave an equiaxed structure. The structure of the starting material is g enerally columnar. The present work sets out to examine the formation of shrinkage cavities in columnarly crystallized round strands continu ously cast in numerous steel grades. The centre porosity in fully-colu mnarly crystallized, continuously cast round material 177 mm in diamet er was determined by means of buoyancy measurements according to the A rchimedes' principle. Examined in this respect were 8 groups of steel grades extending from unalloyed structural steel through to creep-resi sting steel and bearing steels. The specific shrinkage cavity volume ( cm(3)/m) is subject to major statistical variations over the length of the strand. The mean values depend primarily on the steel grade and r ange from 3.5 cm(3)/m for unalloyed steels to, in extreme cases, 24.1 cm(3)/m for the 100 Cr 6 steel. If there are deviations from stationar y casting conditions, singular cavities as great as 45 cm(3)/m can for m. Generally, shrinkage cavities arise as a result of a constriction o f the liquid pool, and dendritic bridging (''mini-lingot'' model) occu rs. This mechanism is controlled by the permeability of the bridges an d the fluidity of the remaining molten material. In addition, the peri odicity and irregularity of the pool's constriction depend particularl y on the macroscopic solidification profile. If the strand is cooled c onventionally, thermal contraction causes a buildup of tangential and axial tensile stresses at the centre which, ill steels that are suscep tible to cracking, lead to the formation of spider cracks below solidu s at the lowest point of the liquid pool.