*3.2. Microstructural Evolution*

Two groups of experimental steels were annealed by ART, and the microstructure after annealing was observed. The results are shown in Figure 3. It can be seen from the diagram that the microstructure after annealing was mainly composed of ferrite and retained austenite (or new martensite), in which the protruding structure was austenite or new martensite and the concave structure was ferrite [13]. Panels (a) and (d) show the microstructure at 625 ◦C. A large number of white granular carbides were dispersed in the microstructure. Panels (b) and (e) show the microstructure at 645 ◦C. It can be seen that there were still many white granular carbides on the ferrite [14], while there were almost no white granular carbides on the protruding austenite structure. This was because the annealing temperature was low at this time, the carbides had not been completely dissolved during the growth of austenite, and these carbides were attached to the ferrite. Panels (c) and (f) show the microstructure of the experimental steel at 665 ◦C. At this time, the carbides had completely disappeared, and the microstructure was composed of ferrite, austenite, and a small amount of new martensite. The austenite grains in the structure grew, and there were two forms, which were the lath and block distributions in the structure. This was also because the austenite grains grew with the increase in temperature, and some larger austenite was transformed into martensite during cooling due to the lower stability of carbon and manganese per unit volume.

**Figure 3.** Microstructure of two groups of experimental steels at different annealing temperatures: (**a**,**d**) 625 ◦C, (**b**,**e**) 645 ◦C, (**c**,**f**) 665 ◦C; (**a**–**c**) 0 RE, (**d**–**f**) 9 ppm RE.

Through the comparative analysis of the microstructure of the two groups of experimental steels under different annealing processes, it was found that the grain size of the experimental steel with trace rare-earth elements was smaller. There were two reasons for this. On one hand, rare-earth elements themselves have the effect of grain refinement. The radius of rare-earth atoms is about 1.5 times that of Fe atoms, which can only dissolve into defects in the crystal but cannot dissolve into austenite to form a solid solution. Because there are many defects at the grain boundary, it is easy to accumulate rare-earth atoms near the grain boundary, which hinders the diffusion of atoms, thereby inhibiting the growth of austenite grains and achieving the effect of grain refinement [15]. On the other hand, rare-earth elements have the effect of deoxidation and desulfurization, which can produce fine and stable rare-earth oxides, rare-earth sulfides, or oxygen sulfides. These rare-earth compounds can be used as heterogeneous nuclei to provide excellent conditions for the refinement of crystallization.

## *3.3. Residual Austenite Content*

The experimental steel was tested using XRD after different heat treatment processes, and the results are shown in Figure 4. In the figure, γ represents the FCC phase, α represents the BCC phase, and the volume fraction of austenite was calculated according to the integral strength of the austenite and ferrite diffraction peaks. As can be seen in the figure, with the increase in the annealing temperature, the austenite diffraction peak was enhanced. When the annealing temperature was 645 ◦C, the austenite content was the highest. When the temperature continued to increase, the diffraction peak of the FCC phase obviously decreased, while that of the BCC phase increased, indicating that the austenite content decreased with the increase in the annealing temperature, and new martensite was formed in the matrix structure. The FCC phase diffraction peak of the experimental steel containing rare earth was significantly higher than that of the experimental steel without rare earth. It can be seen that the addition of trace rare-earth elements increased the volume fraction of the retained austenite in the steel.

**Figure 4.** XRD patterns of the experimental steel at different annealing temperatures: (**a**) 0 RE, (**b**) 9 ppm RE.

Figure 5 shows the calculated volume fractions of the retained austenite of the two groups of experimental steels after different annealing processes. It can be seen in the figure that with the increase in temperature, the content of retained austenite increased first and then decreased. This was due to the unstable growth of austenite grains in the microstructure at higher annealing temperatures; the insufficient distribution of C and Mn elements, leading to the low contents of C and Mn elements per unit volume of austenite; and the transformation tendency of martensite to increase during high-temperature annealing, so some austenite transforms into martensite during the cooling process [16]. When the experimental steel was annealed at 645 ◦C, the austenite content was the largest, and the residual austenite contents of the two experimental steels were 21.1% and 22.8%, respectively. In comparison, it is found that adding trace rare-earth elements to steel was beneficial for retaining more austenite at room temperature.

**Figure 5.** Residual austenite contents of the experimental steels at different annealing temperatures.
