POLYGRADIENT CRYSTALLIZATION - A NEW WAY OF HIGH-QUALITY CAST METAL PRODUCTS MANUFACTURING

摘要

Problems connected with formation of structural and chemical inhomogeneity in the form of dendritic liquation during solidification process have been considered. The positive influence of high-speed crystallization methods on the structure homogeneity of the cast product has been marked. At the same time the high-speed crystallization has a restricted industrial application, for example in case of metallic powder or amorphous film production. Such limitations are bound up with inherent thermal and structure stresses accumulation as the solidification speed increased, what inevitably caused the crystallizing product cracking.rnA new way of effective reduction or full internal stresses elimination in solidifying cast product has been offered by authors. While casting the main point of the method comes to organization of such method during cooling when temperature decrease occurs not steadily but "cooling-warming" pulses alternate each other in time and intensity. Pulses alternation takes place in the phase of temperature interval transformation. During "cooling" pulses a solidified shell thickness growing occurs and during "warming" stage (reheating by special device or from the heat of ingot core zone) — relaxation of an internal stress appears.rnSuch a solidification technique may be carried out in various casting technological schemes. But in the authors' opinion the best results can been reached using continuous casting methods. The quality of a solidified billet is provided by optimization of casting parameters, the main ratio of which is the period of cooling and warming by number and duration of pulses. The task has been solved by mathematical simulation applying methods of finite elements. Adequacy of the model was tested by pilot laboratory plants and actual industrial machines for continuous casting (CCM): For the latter Serp i Molot" Plant, Moscow, Russia single-strand of the Danielli Italian Company and "Nizhniy Tagil Iron and Steel Works", Nizhniy Tagil, Russia multi-strand of the VAI Austrian Company were applied.rnA new technology has been tested successfully on various steels of different types and purposes with a melt weight from 15 kg up to 160 tons. Positive results have been achieved on low-carbon steels (0.5 - 0.8%C), stainless steels (ferritic and austenitic), carburized chrome-nickel (20H2N4A), graphitized, etc.rnA complex metallographic analysis has been carried out, structure features has been studied, an estimation of an intracrystalline inhomogeneity level of steel with a various composition has been made. The structure and mechanical properties data for cast steels and after hot working are given. It is remarkable that a high quality of metal, which has been reached during pulse crystallization, remains inherently after billets plastic deformation. In the first place it concerns high level of structure and chemical homogeneity of the metal.