*3.1. Original Austenite Grains*

According to the measured phase transition temperature, it was found that the AC3 temperature of the experimental steel increased after adding rare-earth elements. To further verify this point, the two groups of experimental steels were subjected to original austenite corrosion to observe the changes in the original austenite grains. Figure 2 shows the original austenite metallographic structure and grain size distribution. It can be seen in panels (a) and (b) that the austenite grains of the experimental steel with rare-earth Ce were smaller. The increase in AC3 temperature by 17 ◦C indicates that the austenite grains nucleated first in the austenitizing process of the experimental steel without RE elements, while the austenite grains nucleated later in the experimental steel with RE elements. Therefore, after the quenching treatment, the original austenite grains in the experimental steel containing rare earth were smaller [11]. The average grain sizes of the original austenite of the two groups of experimental steels were measured using Image-Pro Plus software to be 3.3 μm and 2.5 μm, respectively. The grain size distribution is shown in panels (c) and (d). The grain size of rare-earth experimental steel was small, where the grains between 2 and 4 μm accounted for about 43.91% and the grains less than 2 μm accounted for about 40.92%. The grain size of experimental steel without rare earth was mainly concentrated in 2~4 μm (about 56.23%). It can be seen that with the addition of trace rare-earth elements the austenite grains in the experimental steel were refined and the microstructure became more uniform.

**Figure 2.** Original austenite microstructure and grain size distribution of two groups of experimental steels: (**a**,**c**) 0 RE and (**b**,**d**) 9 ppm RE.
