Development of steel continuous casting at the Nizhny Tagil steel-works

Steel continuous casting is one of fundamentals of modern steel industry. Specialists of Nizhny Tagil steel-works (NTMK) made an important contribution to the process perfection. The one-strand slab CCM of curvilinear type with radial mold and slab straightening by multipoint curve, designed and manufactured at Uralmashzavod, became a prototype for many machines, which were later manufactured at domestic and foreign steel-works in the end of 60th of the previous century. In the process of mastering of CCMs, which were put into operation within the programs of the plant modernization and transferring to BOF production with steel continuous casting, the CCMs were significantly modernized and technology of casting was perfected. To bring down the impact on the solidifying billet peel and decrease a billet swell, the scheme of the supporting rollers location was changed, which enabled to increase the length of the supported zone. A principally new scheme of billets cooling was implemented, which ensured a softer and uniform secondary cooling due to water-air “fog”. To create optimal conditions of axis zone forming during production of round billets of 430 mm diameter, a technology of protective heat screens application was used. The protective screens were installed in the end of ingot solidification zone. A large work was done on determination of optimal technological casting parameters – temperature and speed modes, types of slag forming mixtures, types of heat-insulating mixtures, methods of metal protection in the process of casting. As a result of the work done, at EVRAZ NTMK a complex of steel continuous casting was created, which enables to be flexible depending on the varying situation at the market and to produce continuously casted billets of various dimensions and wide range of steel grades.

Shrinkage Porosity Criterion and Its Application to A 5.5 Ton Steel Ingot

In order to predict the distribution of shrinkage porosity in steel ingot efficiently and accurately, a criterion R√L and a method to obtain its threshold value were proposed. The criterion R√L was derived based on the solidification characteristics of steel ingot and pressure gradient in the mushy zone, in which the physical properties, the thermal parameters, the structure of the mushy zone and the secondary dendrite arm spacing were all taken into consideration. The threshold value of the criterion R√L was obtained with combination of numerical simulation of ingot solidification and total solidification shrinkage rate. Prediction of the shrinkage porosity in a 5.5 ton ingot of 2Cr13 steel with criterion R√L>0.21 m · °C1/2 · s−3/2 agreed well with the results of experimental sectioning. Based on this criterion, optimization of the ingot was carried out by decreasing the height-to-diameter ratio and increasing the taper, which successfully eliminated the centreline porosity and further proved the applicability of this criterion.