As discussed previously, the average grain size of the three
steels gradually decreases with increasing Ti content. Nevertheless, both the tensile strength and elongation sharply decrease,
which is contradictory with the Hall–Petch formula as the typical
grain refining strengthening theory. Based on the previous analysis
of precipitates inSection 3.2, the TiN particles with a dystectic
point, high hardness and chemical stability at high temperature
exhibit strong anti-deformability during the tensile test, which
causes local stress concentration in the vicinity of TiN particles.
When the stress concentration reaches a critical intensity with an
increasing load, the cracks arising from stress concentration rapidly emerge and expand at a high speed and the specimens
fracture immediately without bearing larger deformation. It is
without doubt that, the increases in size and number of the TiN
clusters greatly intensify the local stress concentration of the
testing specimens, leading to the higher probability of cracks occurring caused by stress concentration