Study On A Kind Ultra-Low-Carbon Bainitic Steel Without Nb, Ti And Other Metal Compounds Forming Elements

Ultra-low carbon bainite steel is one of the most popular steels in internationalengineering field. As generally reported, the technological process of its production is tedious,and hard to control. Also the energy consumption and cost is too high. This restricts theengineering application of it. This article focuses both on the composition and the solid-statephase transformation of it. We arm at researching the phase change in directly cooling andmiddle temperature isothermal process of a kind ultra low carbon steel which do not includemetal compounds form elements, such as Nb, Ti. So we can provide the theoretical basis toproduce a kind of ultra low carbon steel which has better toughness under lower energyconsumption.The specimen’s phase transition point features for this experimental steel（0.05%C-1.7%Mn-0.35%Ni-0.3%Si-0.25%Mo） is researched by XRD, the microstructureobservation, impact toughness and hardness test. The state of microstructure, the hardness andphase composition under the isothermal450℃and directly cooling after high temperaturedeformation also are researched.The experiment result shows that high temperature stays process after high temperaturedeformation affect the experimental steel’s medium temperature transformation point.Although the CCT curve still has the characteristic of two areas, it moves to the top leftcompared with the samples which do not go through high temperature recovery process.Experimental steel has nearly same micro-structure under different cooling rate. It’smainly made up of both lath bunchy bainite and granular bainite. When cool rate improved,their relative content changes obviously, and the formation of granular bainite will beinhibited, the development of lath bunchy bainite will be promoted. The high temperaturestays process can improve the content of lath bunchy bainite through the formation ofdislocation cells, and it works prominently when the cooling rate is faster than5℃/S.After high temperature deformation, when the experimental steel cooling to450℃at15℃/S, and goes through the isothermal treatment process, it has nearly same micro-structure.It’s mainly made up of lath bunchy bainite and retained austenite. The high temperature staysprocess can separate parent grain through the formation of dislocation cells, and it canimprove the content of lath bunchy bainite. This works prominently when the isothermal timeis shorter than30min.When the isothermal time is longer, whether the specimens go throughthe high temperature stays or not, there is no difference between their micro-structure.As the result of impact toughness test and hardness test shows, under different condition,the formation of thin and much lath bunchy bainite can make the experimental steel has agood toughness and hardness.