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Ultra-high strength structural steels with Ce (cerium) content of 0, 0.0367, 0.0559 and 0.0792 wt.% respectively are prepared to study the effect of Ce addition on inclusions and the toughness of ultra-high-strength steel. The results show that the 0.0367 wt.% Ce can refine the size of oxides or sulfides to about 1 μm and improve the inclusion morphology to spherical, resulting in improved toughness of L-Ce steel. The number of inclusions in the H-Ce steel increases sharply, mainly due to the precipitation of C-containing rare earth inclusions. The rare earth carbides are hard and brittle phases, with the size of about 1 μm and a spherical shape. In addition to the increase in the number and size of inclusions, the reason for the deterioration of the toughness of H-Ce steel is that the C-containing rare earth inclusions are easily broken during hot rolling act as stress concentrators under impact loading.

In order to control the grain size in thermomechanical processing, the grain growth behavior of hot extruded Mg–xAl–1Zn (x = 3, 6, 9) alloys and their relationship with second phase particles and solutes were investigated. The growth rate of AZ61 is greater than that of AZ31 and AZ91 at 300 °C, 350 °C, 400 °C, and 450 °C under isothermal annealing. The average grain growth exponents n of Mg–xAl–1Zn (x = 3, 6, 9) alloys were 2.26, 2.33, and 2.53 at 300–400 °C, respectively. The deviation from the theoretical value of 2 was attributed to the hindrance of grain boundary migration of Al-rich second phase particles and solute Al. Microscopic observations show that the grain size of the annealed samples is closely related to the shape, volume fraction, size, and distribution position of the second phase particles. Significantly, the pinning effect is stronger for lamellar and network-like second phase particles. In addition, the pinning effect of Al-rich second phase particles plays a more important role in grain refinement than the dragging of solute Al. The growth of abnormal grains in the microstructure is attributed to the high energy difference between the preferentially oriented <112¯0> grains and the surrounding grains, which drives the grain boundaries to overcome the same pinning force of the second phase particles.