Borysenko A. Development of the structural heredity theory in carbon steel for energy efficient production of rolled products from continuously cast billets

It was established that in austenite, there are two main types of small-angle intercrystalline boundaries of dislocation structure after crystallization: 1 – primary, formed during crystallization as a result of coupling of dendrite branches ; 2 – secondary, arising after crystallization as a result of plastic deformation by 1–2% under the action of shrinkage stresses. Secondary intercrystalline boundaries are formed both at the points of conjugation of dendrites and the ones crossing them in different directions with the formation of a 6–15 degrees misorientation angle between grains. The formation of grain boundaries of austenite grains in cast steels occurs by dislocation mechanisms at a decreasing temperature and increasing internal stresses of approximately 1–4 MPa by means of opening slip bands, interaction of slip bands and interaction of twins. A theoretical substantiation is given to the relationship between the carbon content, the formation of the austenite dislocation structure and its mechanical properties at the temperatures of grain boundary formation in cast steel with the value of the stacking-fault energy of iron atoms. The microchemical heterogeneity of steel caused by dendritic segregation determines the heterogeneity of the rheological properties of austenite, which affects hereditarily the uniformity of its high-temperature plastic deformation and thereby the shape, size and uniformity of the formation of the austenite grain structure and ferrite-pearlite structure in the continuously cast billet. The larger size of dendrites in large-size continuously cast billets corresponds to the larger size of the austenite grains. Using EBSD method analysis it is shown that after crystallization austenite undergoes crystallographically regulated plastic deformation in accordance with sliding and twinning mechanisms. Normalization of cast steel is accompanied by additional deformation of austenite according to the mechanism of grain boundary slip. The formation of the lamellar structure of pearlite occurs through the simultaneous formation and advancement of a group of ferrite plates deep into the austenite grains due to the polymorphic gamma→alpha transformation of iron along the slip bands of dislocations. The concept has been developed to describe the existence of the hierarchy of levels and the hereditary relationship of dislocation processes of austenite structure formation and ferrite-pearlite structure formation during its transformation, depending on the chemical composition, temperature and the magnitude of internal stresses in cast, hot-deformed and heat-treated steel. The substantiation is given and the economic feasibility is demonstrated for the use of on casting and rolling units of rolled of cast billets is given, the section size of which is as close as possible to the size of the wire rod section.