Strength-toughness balance of low-alloy steel by fail-safe design

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Microstructural design for improving the strength-toughness balance was studied in low-alloy steel. High-strength medium-carbon steels with an ultrafine elongated grain (UFEG) structure with an average transverse grain size of 0.3 µm and an ultrafine equiaxed grain (UFG) structure with a grain size of 0.7 µm were fabricated by multi-pass caliber rolling at a warm working temperature and subsequent annealing. For comparison, conventionally quenched and tempered steel with a martensitic structure and a 480 MPa-class low-carbon steel with a ferrite (grain size 20 µm)-pearlite structure were also prepared. A quasi-static three-point bending test was conducted at a temperature range from 200°C to -196°C. The steels, except for the UFEG steel, exhibited a typical energy transition curve, in which the fracture energy decreases with decreasing temperature. In the UFEG steel, the fracture energy increased with the occurrence of delaminating cracks as the temperature decreased from 200°C, reached a maximum at ambient temperature, and then decreased. In other words, the steel showed inverse temperature dependence of the toughness. As a result, the strength-toughness balance of the UFEG steel was excellent compared with that of all other steels. For stronger, tougher steel, it is important to design a heterogeneous microstructure rather than a homogeneous microstructure.

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