*3.4. Mechanical Properties*

Figure 6 shows a comparison of the mechanical properties of the two groups of experimental steels after the ART annealing treatment. It can be seen in the figure that the yield strength and tensile strength of the two groups of experimental steels after annealing were very similar, but the mechanical properties of the experimental steel with the rare-earth element Ce were improved. The yield strength decreased with the increase in the annealing temperature, and the maximum value was 850 MPa at 625 ◦C. The tensile strength increased with the increase in the annealing temperature, and the maximum value was 1100 MPa at 665 ◦C. The elongation increased first and then decreased with the increase in the annealing temperature and reached a maximum at 645 ◦C. The elongation of the experimental steel without rare earth was 27.82%, while the elongation of the experimental steel with rare earth was 33.89%. The change rule of the product of strength and elongation was consistent with that of elongation, which increased first and then decreased. The maximum value is obtained when the annealing temperature reached 645 ◦C. At this time, the products of strength and elongation of the experimental steels without rare earth and with rare earth were 24.3 GPa·% and 28.47 GPa·%, respectively. It can be seen that the rare-earth element was beneficial for the improvement of the mechanical properties of the experimental steel, and 800 ◦C for 5 min and 645 ◦C for 15 min was the best heat treatment condition for the experimental steel.

**Figure 6.** Mechanical properties of two groups of experimental steels at different annealing temperatures: (**a**) yield strength, (**b**) tensile strength, (**c**) elongation, and (**d**) the product of strength and elongation.

The change in the product of strength and elongation was the same as that of the austenite content after annealing at different temperatures, which indicates that the austenite content is an important factor affecting the comprehensive mechanical properties of experimental steel. Combined with the analysis of the XRD results, the reason for this change in the experimental steel may be that at a lower annealing temperature there are carbides in the microstructure and the C element in the matrix does not diffuse on a large scale, that is, it is enriched in the reverse-phase-transformation austenite. At the same time, the volume fraction of austenite is low and the stability is strong at this temperature. The comprehensive effect of the second-phase strengthening of carbides and the solidsolution strengthening of the C element makes the tensile strength and yield strength of the experimental steel remain within a certain range. With the increase in annealing temperature, the carbides in the microstructure continue to dissolve until they disappear, and the second-phase strengthening effect in the alloy also decreases and disappears. When the temperature is high, the microstructure of the experimental steel is transformed into ferrite, austenite, and martensite multiphase structures. The volume fraction of reversed austenite decreases and becomes unstable, and the TRIP (transformation-induced plasticity, TRIP) effect is weakened accordingly. During the cooling process, the hard-phase martensite increases. The higher the temperature, the more the martensite is transformed by cooling, so the tensile strength increases and the elongation decreases. Some scholars have also put forward the same conclusion that increasing the temperature increases the austenite content, but the austenite stability is poor, which is due to the tendency for martensite transformation in high-temperature annealing and the coarsening of austenite, so it is partially transformed into martensite during cooling and the tensile strength increases [17].
