2016 年 82 巻 844 号 p. 16-00246
The criterion of micro-crack initiation on stronger, tougher steel, i.e., fail-safe steel, with an ultrafine elongated grain (UFEG) structure was studied through a combinations of numerical simulation and experimental observations and measurements. The test sample was machined from the rolled bar with 0° and 90° rotation along the rolling direction (RD). The static three-point bending test was conducted in a temperature range from 200 °C to -196 °C. The stresses near the initial notch were analyzed by three-dimensional finite element method. The conventionally quenched and tempered steel with a martensitic structure fractured brittlely with a peak bending loading. On the other hand, the developed steel did not fully fracture due to a lamella fracture even if load drops occurred during the bending test. The load drops are attributed to micro-cracks with normal to the loading direction (LD) or with an angle of 45° to the LD that occurred from near the initial notch root. When the crack orientation, i.e., LD is parallel to the RD, the developed steel showed a catastrophic fracture behavior. In the developed steel, the brittle fracture stress normal to the RD, σF⊥RD, was 3.2 GPa at temperature over -100 °C and 2.6 GPa at temperature below -170 °C. The σF of weak direction in the developed steel was 0.7 times lower than σF(QT) of the conventionally steel. When the effective grain size on fracture is assumed to be packet in the martensitic structure, the brittle fracture stress parallel to the RD, σF//RD, was estimated to be 6.2 GPa at temperature over -100 °C and 5.0 GPa at temperature below -170 °C. These values are 1.4 times as compared with the σF(QT), 1.9 times higher than the σF⊥RD regardless of the test temperatures. The anisotropic brittle fracture stress in the fail-safe steel is attributed to the microstructural features, and we would be able to control a fracture of the steel by designing microstructures.
