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 exact magnitude and distribution of the equivalent strain in the Al sheet processed during three ARB cycles using FEA were shown.•The evolution of the microstructure and hardness through thickness in the sheet processed by ARB is different depending on the roll diameter.•The effect of the stress–strain relations of the rolled sheets on the strain is very small in comparison with the effects of friction and roll diameter.•The method in order to know the magnitude of the strain in the multi-cycle ARB-processed sheet against various combinations of roll diameter and friction condition was shown.The accumulative roll-bonding (ARB) process, which is a severe plastic deformation process, was simulated using finite element analysis, including the influence of friction, stress–strain relations, and roll diameter. The complicated distributions of equivalent strain through the thickness of ARB-processed sheets were quantified. These quantitative strain analyses would be useful for analyzing the evolution of ultrafine-grained structures in the ARB process.

Bulk ultrafine-grained (UFG) low-carbon steel bars were produced by caliber rolling, and the impact and tensile properties were investigated. Initial samples with two different microstructures, ferrite-pearlite and martensite (or bainite), were prepared and then caliber rolling was conducted at 500 °C. The microstructures in the rolled bars consisted of an elongated UFG structure with a strong α-fiber texture. The rolled bar consisting of spheroidal cementite particles that distributed uniformly in the elongated ferrite matrix of transverse grain sizes 0.8 to 1.0 μm exhibited the best strength-ductility balance and impact properties. Although the yield strength in the rolled bar increased 2.4 times by grain refinement, the upper-shelf energy did not change, and its value was maintained from 100 °C to −40 °C. In the rolled bars, cracks during an impact test branched parallel to the longitudinal direction of the test samples as temperatures decreased. Delamination caused by such crack branching appeared, remarkably, near the ductile-to-brittle transition temperature (DBTT). The effect of delamination on the impact properties was associated with crack propagation on the basis of the microstructural features in the rolled bars. In conclusion, the strength-toughness balance is improved by refining crystal grains and controlling their shape and orientation; in addition, delamination effectively enhances the low-temperature toughness.

The each strain component and equivalent strain in rolled materials were quantified using finite element analysis (FEA) that takes the deformation history into account. FE simulations were carried out considering stress–strain relations that depend on strain rate and friction between rolls and sheet. Rolling condition of accumulative roll-bonding (ARB) where introduction of a very large shear strain in the surface layer of rolled sheet had been verified through the embedded-pin method was employed in FEA. The histories of total shear strain, total strain in rolling direction, and equivalent strain during rolling were studied, and the magnitude and distribution of each of them through sheet thickness after rolling were shown. The problems associated with the experimental determination from an embedded-pin method were clarified through the present analysis. The equivalent strain at the surface in the 1100 Al processed by one ARB cycle without lubricant corresponded to the equivalent strain in the material processed by five ARB cycles with lubricant. These quantitative strain analyses would be useful for analyzing the evolution of microstructures in the ARB process as well as the conventional rolling deformation under high friction conditions.

The three-dimensional finite element-analysis and electron backscattered diffraction analysis are used to study quantitative correlation between effective strain and deformed microstructure in low carbon steel bars fabricated through multi-pass warm caliber rolling. The strain imposed in the rolled bars has a distribution with maximum at around corners, and large strain of over 5.7 is introduced into corners when nominal reduction in area is 89%. The area of around corners introduced strain over 5.7 is filled with the ultrafine ferrite grains of below 680 nm. It is shown that microstructural parameters, the fraction of high angle grain boundaries (HAGBs) and the average misorientation, not only depend on strain imposed but also local reduction in area.