Zotov D. Increase of Properties of Microalloyed with Nitride Formative Elements Steels

Subject of inquiry is structural state and properties of low C-Al-Ti-N and low C-Cr-Mn-Si-Al-Ti-N steels Thesis objective is receiving of improved complex of properties for application in civil engineering of Al,Ti,N microalloyed low carbon steels due to formation of bainitic structure. Research methods - dilatometry; LM, SEM, high-temperature microscopy; X-ray diffractometer. Theoretical and practical results: CCT diagrams have been built for Al,Ti,N microalloyed low carbon steels. Significant predominance of acicular ferrite over packet bainite in structure of low C-Al-Ti-N steels after austenitizing temperature higher than 1000 С has been shown. Heating of low C-Al-Ti-N steel till melting leads to partial Ti(C,N) dissociation on 33 - 36%. Cementite particles segregate on acicular ferrite plates during tempering. Quenching after 1000 С and tempering 550…600 С allows low C-Al-Ti-N steel to correspond to 440YTS strength class. Novelty of the research - mechanism of structure formation has been set for low C-Al-Ti-N steels on ground of the CCT diagrams (received first); it has been proved that austenitising temperature influences drastically on morphology of bainite of low C-Al-Ti-N steels (developed further); it has been proved by means of in-situ observation that Ti(C,N) don't dissolve completely at the temperature of melting of C-Al-Ti-N steel (developed further); the concept of the shear mechanism of acicular ferrite transformation has been evolved due to ferric carbides precipitation after tempering of acicular ferrite in low C-Al-Ti-N steel (developed further). Implementation extent: Possibility of application in civil engineering has been proved for flat product with acicular ferrite structure, this is confirmed by the act of results implementation from "Factory Master-Profi Ukraine" Ltd dated 22.08.2012. Range of application: production of high strength rolled product for civil engineering.