Hydrogen embrittlement properties were investigated for 1.8-GPa-class ultra-high strength low-alloy steels by means of slow-strain-rate test of the pre-hydrogen-charged notched specimens, accelerated atmospheric corrosion test, and thermal desorption spectrometry. A Mo-bearing steel with a chemical composition of Fe-0.4C-2Si-1Cr-1Mo (mass%) was quenched and tempered at 773 K for 1 h and then deformed by multi-pass caliber rolling with a cumulative rolling reduction of 76% at 773 K to create an ultrafine elongated grain structure with a strong <110>//rolling direction fiber texture. The warm tempformed (TF) sample was subsequently annealed for 1 h to clarify the hydrogen trapping effect of nanoscale carbides relative to additive Mo. When the TF sample was annealed at 843 K (TFA sample), the hydrogen absorption capacity was enhanced significantly through the formation of nanoscale Mo-rich precipitates in the matrix of ultrafine elongated grains. A high potential for hydrogen embrittlement resistance in an atmospheric corrosion environment was demonstrated in both the TF and TFA samples with an ultra-high tensile strength of 1.8 GPa. The TF and TFA samples were much less susceptible to hydrogen embrittlement as compared to the tempered martensitic samples at an ultra-high tensile strength of 1.8 GPa. The hydrogen trapping states and the high resistance to hydrogen embrittlement in the TF and TFA samples are discussed in association with the anisotropic, ultrafine grained structures with the nanoscale Mo-rich precipitates.

The effect of phosphorus (P) on delamination toughening was examined for 0.4%C–1%Cr–0.7%Mn–0.2%Mo steels (mass%) comprised of ultrafine elongated grain (UFEG) structures with strong <110>//rolling direction (RD) fiber textures. The UFEG structures evolved through the plastic deformation of tempered martensitic structures by multi-pass caliber rolling at a temperature of 773 K (warm tempforming, WTF). The addition of P, up to 0.093% (mass%), had little influence on the evolution of the UFEG structure and the strength of the steels. Although the tensile ductility and upper-shelf energy showed a slight tendency to decrease as the P concentration increased from 0.001% to 0.093%, the delamination perpendicular to the notch orientation of the impact specimens was pronounced over a wider temperature range. As a result of delamination, the 0.093% P-doped steel exhibited a significant inverse temperature dependence of toughness at temperatures from 250 K to 350 K. The delamination toughening was dominated by the UFEG structure, and further assisted by the phosphorus segregation. It was considered that the formation of distinct P segregation bands, which presented a structure consisting of brittle and ductile layers, may be especially effective in accelerating delamination and improving toughness in the P-doped steel with an UFEG structure.

A deformation of tempered martensite, namely tempforming at an elevated temperature, was applied to a medium-carbon low-alloy steel. This thermomechanical treatment led to an evolution of an ultrafine fibrous grain structure with a strong 〈1 1 0〉∥RD fiber deformation texture and to a remarkable improvement in Charpy impact strength (uE = 165 J) at a tensile strength of 1.5 GPa.

Hydrogen induced delayed fracture was investigated in 0.6% O steel with an average ferrite grain size of 0.3 μm and oxide particles of 10 nm. The ultrafine grained steel exhibited markedly high resistance to hydrogen embrittlement at the tensile strength of 1300 MPa by hydrogen trapping effect related to the nanosize oxide particles.