Abstract: Take 30CrMnSiA steel as comparison, phase transition process of the G50 ultra-high strength steel with typical chemical composition when heated and cooled near Ac₁ was studied, meanwhile the relationship between annealing hardness and annealing specification was analyzed. The results show that the present G50 steel annealed at the temperature slightly below the Ac₁ measured by means of the expansion method forms austenite and consequently cooling forms untempered martensite which leads to the hardness rise, and the original 680 ℃ annealing temperature is no longer applicable, resulting in the phenomenon of the longer the annealing time, the higher the hardness. Verification experiment of multiple batches shows that although some austenite is formed by annealing at 660 ℃ for 6 h, more softening degree of the original normalized martensite can effectively reduce the hardness of the G50 steel forgings, and it has universal applicability.

Key words: ultra-high strength steel, annealing, microstructure transformation