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It is still a big challenge to obtain excellent low-temperature toughness for bulk steel materials. Delamination is an effective method to improve low-temperature toughness. In the present study, delamination toughening in a low carbon microalloyed steel plate with elongated and ultrafine-grained microstructure rolled in the dual-phase region has been investigated in detail. When toughness was measured along normal direction, the steel plate had a high upper shelf energy and no delamination occurred in the upper shelf region. A large delaminated crack parallel to rolling plane started to appear and changed the propagation path of main crack when testing temperature was lower than −60°C. We find this kind of delamination induces a second upper shelf in the Charpy transition-temperature curve. The second upper shelf, reaching up to 300 J in the temperature range of −60°C to −140°C, results in excellent low-temperature toughness for the steel plate, and the ductile-brittle transition temperature is lowered to −157°C. The developed steel plate also has high low-temperature toughness measured along transverse direction due to delamination. The effect factors on upper shelf energy, delamination mechanism and delamination toughening are discussed.

The strength and toughness of 1000 MPa grade steel plate for hydropower station treated by different reheated quenching temperatures were investigated. With the increasing of reheated quenching temperature, the yield strength and tensile strength increase sharply, whereas the value of impact toughness decreases slowly. The lath martensite with high density dislocations enhances dislocation strengthening. On the contrary, the acicular or block ferrite (soft phase) produced by intercritical quenching reduces the phase transformation strengthening. Moreover, the ferrite has a low solubility of interstitial carbon due to the body‐centered‐cubic lattice structure. The bar‐shaped precipitates occur during the isothermal holding at the intercritical temperature and it will reduce the precipitation strengthening. The ferrite phase and high misorientation boundaries are the main factors that contribute to the toughening of the experimental steel. The lower the reheated quenching temperature is, the higher proportion of ferrite and high misorientation boundaries becomes. Considering the requirements for mechanical properties of 1000 MPa grade steel plate for hydropower station, the optimal temperature of reheated quenching is ∼920 °C.

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Fe-B-P submicrometre particles were synthesised by a novel chemical method. The size of the Fe-B-P particles (0?1-1 mm) is easily controlled by tuning the NaBH 4 concentration and its dropping speed. The saturation magnetisation (M s ) of Fe-B-P submicrometre particles is .130 emu g 21 , whereas the coercivity is ,10 Oe. The permeability of the composites made of Fe-B-P particles (y0?3 mm) and NiZn ferrite nanoparticles (10 nm) is ,25 when the volume percentage of NiZn ferrite is 30-50%. The resonance frequencies are all ,1 GHz for pure Fe-B-P particles and the composites.