Investigation On Strength-Toughness Mechanism Of G50 Ultra-high Strength Steel

In this paper, the mechanical properties of G50 ultra-high Strength steel smelted by VIM +VAR and electric arc furnace +VOD +VAR have been analyzed ,and the systematic research emphases have been put on the mechanical properties, microstructure, morphology of MnS inclusions, strengthening and toughening mechanism through every kinds of effective analytical methods. Its purpose are to theoretically explain the strengthening and toughening mechanism of G50 ultra-high Strength steel, seek potential path for improving comprehensive properties and decreasing the cost, provide reference for researching and developing new UHS. The study indicates that decrease of strength caused by development of austenite and decarburization is counteracted to a certain extent by increase ofε-carbide in martensite with raising hardening heat that lead to little effect. In the meantime, impact toughness and hardness is well related with microstructure as austenitizing time increases, because the content of retained austenite becomes more and more. And tensile properties are nearly expose to microstructure regardless of the morphology and size distribution of inclusion on G50 steel. Studying the morphology and size distribution of inclusion on G50 steel by SEM, EDAX, which were smelted by different metallurgy processes. Results have indicated that composite inclusions are slightly smaller and their aspect ratios are distinctly larger, it will very well lead to an increase of its mean spacing. Especially, the much more fine inclusions also produce the more corresponding densely distributed area. Size distribution and morphology in G50 ultra-high Strength steel smelted by VIM +VAR were obviously improved due to the higher calcium content of composite MnS inclusions lead to hot deforming more difficult. The gas content of G50 steel smelted by EAF+VOD+VAR metallurgy process is equivalent to the steel smelted by VIM+VAR, however the former Sulphur content drop below 0.001% by high alkalinity slag during VOD process in ladle so that volume fraction of sulphide inclusion greatly fall and it could obviously improve cleanliness and toughness. As composite inclusions are deformed severely with a decrease in the calcium content, it will lead to an increase of aspect ratio. Impact toughness and fracture toughness of G50 steel smelted by electric arc furnace +VOD +VAR have been improved obviously, as its aspect ratio is larger than G50 ultra-high Strength steel smelted by VIM +VAR due to the higher calcium content of composite MnS inclusions in the former in favor of the toughness improvement and isotropy. Martensite matrix appears quasi-cleavage crack gradually, and effect of MnS inclusions on impact toughness and isotropy weaken foot by foot with testing temperature falling.