A concept of control of processes of vanadium, niobium and titanium carbonitrides forming by consecutive alloying

Abstract

Base on laboratory and industrial experiments, as well as subsequent studies of the microstructure of steel samples alloyed with a certain sequence by vanadium, niobium, and titanium, a concept including compliance with the order of those microalloying elements introducing into steel developed and justified. According to the concept, the sequence of introduction is determined by the difference in the degree of their thermodynamic affinity to the nitrogen and carbon dissolved in the steel.Investigations of the microstructure of experimental microalloyed samples by an optical microscope, with a magnification x250, showed the most significant grain refinement with a consecutive additive – first vanadium with niobium and after 10 minutes of holding – titanium.The efficiency of the developed alloying method for the advanced formation of vanadium and niobium carbonitrides was evaluated by studies with the Mira3 Tescan electron scanning microscope having an X-ray energy dispersive microanalysis system X-Act (Oxford Instruments). When studying the compositions and the form of carbonitrides discovered in steel samples alloyed with a different sequence of additives, it was established, that during simultaneous additive of titanium, vanadium and niobium into steel, titanium carbonitrides account for a majority, while vanadium and niobium carbonitrides are not actually formed or are represented by single inclusions. Conversely, in steel samples alloyed with a consecutive additive to steel, first vanadium with niobium and later titanium, carbonitride of vanadium and niobium inclusions prevail. In this case, titanium carbonitrides are represented only by single and fine inclusions. Thus, first introducing of vanadium and niobium, allows them to react fully with stoichiometrically insufficient concentrations of nitrogen and carbon, ahead of the formation of titanium nitrides.Based on the results of the research in JSC “ArcelorMittal Temirtau”, using the developed concept of consecutive alloying by carbonitride-forming elements, the technology of 09G2FB grade steel production with a ferrite-bainite structure developed and implemented, fully meeting API Spec 5L requirements for steel of strength category X80.