To improve the delayed fracture strength of ultra high-strength low alloy TRIP-aided steels with bainitic ferrite matrix (TBF steels), the effects of aluminum content on hydrogen absorption behavior and delayed fracture properties of 0.2%C-0.2-1.5%Si-1.5%Mn TBF steel were investigated. When aluminum was added to the TBF steel, the diffusible hydrogen increased. It was expected that the hydrogen was charged not only in retained austenite films but also on lath boundary. Delayed fracture strength of TBF steels containing aluminum were significantly increased, compared with conventional TBF steel. This was mainly caused by (1) suppression of the stress-assisted martensite transformation resulting from the stabilized or carbon-enriched retained austenite, (2) hydrogen trapping to refined interlath retained austenite films and lath boundary, and (3) relaxation of localized stress concentration by TRIP effect of the retained austenite.
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