Characterisation of microstructure and mechanical properties in two different nanostructured bainitic steels

Abstract

Recently, valuable combinations of mechanical properties with strength of 1.9 GPa accompanied by very decent ductility of 19 % and toughness of 31 J, have been achieved in a set of nanostructured bainitic steels. However, it is necessary to elucidate the significance of various microstructural features responsible of that extraordinary mechanical response in more detail.Thus, using two steels, with different Mn, Ni and V contents, and changing the nanostructured bainite isothermal transformation temperatures (200-300 ºC), has led to a plethora of subtle and essential microstructural variations, necessary to explain how the mechanical response of nanostructured bainite is attained.

Infusion of hydrogen into nanostructured bainitic steel

“…the is estimated concentration of carbon trapped at defects. The microstructure consists essentially of a mixture of two phases, thin plates of bainitic ferrite and carbon enriched regions of retained austenite with morphologies of films and micro-blocks as described by other authors [30,31]. Figure 2 illustrates the microstructure obtained from TEM for the samples isothermally transformed, where the lighter phase is ferrite and the darker is retained austenite.…”

Effect of Austenitising Temperature on Mechanical Properties of Nanostructured Bainitic Steel

“…This phenomenon is attributed to high-density dislocations in bainitic ferrite and a large amount of retained austenite that has higher work hardening capacity induced by dislocation multiplication and strain-induced martensite. Continuous yielding was observed in the other two nanostructured bainitic steels [ 18 ]. The mechanical properties—namely, UTS, yield strength (YS, 0.2% proof stress), total elongation, hardness, and impact toughness—are given in Table 1 .…”

“…This characteristic suggests that the major microstructural contributor to strength is the fine sub-unit size, rather than the sheaf or austenite grain size. Avishan et al [ 18 ] also discussed the good linear relation between strength (YS and UTS) and the ratio of the volume fraction to the thickness of the bainitic ferrite, such that both YS and UTS increase with the aforementioned ratio. Lowering isothermal transformation temperature not only reduces bainitic lath thickness and increases the bainitic ferrite fraction, but also increases the dislocation density and C content in bainitic ferrite.…”

Ultrahigh Charpy impact toughness (~450J) achieved in high strength ferrite/martensite laminated steels

“… ( a ) Impact toughness (RT) vs. tensile strength, indicating remarkable enhanced toughness at the same strength level in comparison with other homogeneous low alloy steels; ( b ) Impact toughness (−40 °C) vs. tensile strength; ( c ) Impact toughness (RT) vs. tensile area reduction; ( d ) Description of the symbols. The referenced data from literature include low carbon steels 24 25 , DP steels 20 21 26 27 , pipeline steels 28 29 30 31 32 33 , HSLA steels 2 4 6 34 35 36 37 38 , Bainitic steels 39 40 41 42 43 , laminated steel composites 13 15 18 , and our TG steels 44 . …”