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Abstract
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Partial quenching below the martensite start temperature accelerates the subsequent bainite transformation in
nanostructured bainitic steels. However, it is essential to clarify how the microstructural variations affect the
resultant mechanical properties in these materials. This article aims to study the strength-ductility combinations
in nanobain after introducing partial martensite within the microstructure. Two series of heat treatments were
designed after preliminary dilatometry tests. First set of samples were directly transformed to bainite at 250 and
300 ◦C and for the second set, bainite transformation has been implemented after partial quenching at 150, 140
and 120 ◦C to obtain different amounts of pre-existing martensite. Mechanical properties have been described
according to the microstructural evolutions by optical, field-emission gun scanning electron microscopy and XRD
tests. It has been shown that pre-existing martensite changed the amount of high carbon retained austenite and
increased both carbon content in solid solution and dislocation density in austenite. Interfacial autocatalytic
transformation of bainite approved during which bainite started to form from pre-existing martensite/austenite
interfaces. Microstructural changes gave rise to strength promotion and reduction of the elongation and impact
energy even if still an acceptable balance of strength-elongation-impact energy could be obtained.
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