- Reasons for grain refinement:
After grain refinement in steel, there is an increase in the number of grain boundaries, which have irregular atomic arrangements, higher impurity and defect concentrations, and higher energy levels. This impedes the movement of dislocations, thus hindering plastic deformation and achieving high strength.
- Reasons for good plasticity and toughness:
The finer the grain size, the greater the number of grains within a given volume. Consequently, under the same amount of plastic deformation, the deformation is dispersed among a larger number of grains, resulting in more uniform deformation. Additionally, each grain contains fewer accumulated dislocations, reducing the chances of cracking caused by stress concentration. This allows the material to withstand larger amounts of deformation before fracture, exhibiting higher plasticity. In fine-grained metals, crack initiation (due to reduced stress concentration) is less likely, and crack propagation is hindered by the tortuous grain boundaries. As a result, more energy is absorbed during the fracture process, leading to higher toughness.