Shchudro A. Features of formation of structure and properties of a zone of thermal influence of welded connections from microalloyed building steels

The object of research - the processes of structure formation during hot deformation of steels 09G2S and 10HSND and subsequent welding on the mechanical properties of sheet metal and products for building and bridge structures. The purpose of the dissertation is to develop a rational mode of controlled rolling of high-strength steels for construction and bridge construction 09G2S and 10HSND and the formation of structures in the area of thermal impact of welded joints of these steels, providing a level of mechanical properties above the base metal. Research methods: light microscopy, scanning electron microscopy, static mechanical tests, mathematical modeling. Theoretical and practical results: the practical significance of the work is to reduce the percentage of defects in mechanical properties and reduce the risk of stratification of welded I-beams for products of responsible construction and bridge construction and aimed at expanding the scope of low-carbon microalloy steels of domestic production. structure and properties of the base metal and the zone of thermal influence.For steels 10HSND and 09G2S controlled rolling according to the rational regime was proposed (the regime is based on the preservation of the dislocation structure of austenite and ferrite in the intercritical temperature range and subsequent regulated cooling). Scientific novelty: the implementation of deformation in the finishing stand for steel 09G2S and 10HSND was further developed, which opens additional backup opportunities to increase and stabilize the mechanical properties of sheet metal after welding by correlation, single-factor, multifactor dispersion and regression analysis; For the first time for steel 10HSND and 09G2S the rational mode of the controlled rolling which is directed on preservation of a dislocation substructure of austenite and ferrite in the temperature range between 850… 715 ° C is offered. This is done by saturating the dislocation subboundaries in austenite and ferrite, blocking recrystallization not only in the intercritical temperature range, but also with subsequent accelerated cooling; improved welding mode of steel 09G2S and 10HSND, so in the thermal zone instead of ferritic-martensitic structure, obtained a section with dual ferrito-bainitic structure, which has high strength compared to the base metal, and reduces the risk of brittle fracture in this area; for the first time for welded steel products 10HSND mechanical properties were obtained at the yield strength of 680… 690 MPa, which is realized by maintaining a stable dislocation substructure of austenite and ferrite during fabrication and welding (this is realized by forming fine ferritic-perlite structure, increasing mechanical properties in the area with spheroidized perlite and reduce the risk of brittle fracture in the area of formation of dual ferrito-bainite structure). Degree of implementation: The results of the dissertation are introduced into the educational process of the Department of Materials Science and Materials Processing in teaching the following disciplines: "Theory of processes of structure and properties of structural materials" and "Modern methods of materials research" and in Dnepropetrovsk Ministry of Internal Affairs of Ukraine in the course of research of destroyed relief marks and masked welds, as well as for assignment and confirmation of qualification of forensic expert of Dnipropetrovsk NDEKTs MIA in expert specialties 8.9 "Research of metals and alloys" and 8.17 "Research of destruction of metal products" . Scope: production of thick-sheet rolled metal from low-carbon microalloyed steels for construction and bridge construction.