**1. Introduction**

In recent years, the third generation of automobile steel, represented by medium manganese steel (Mn content of 3–10%), has gradually become the main research hotspot at home and abroad due to its excellent high-strength and high-plasticity mechanical properties [1]. The design of alloy composition is an important factor affecting the final properties of materials and has been extensively studied [2] As a strategic resource in China, rare earth (RE) is widely used in the field of functional materials, such as high-performance permanent magnet materials and luminescent materials, but is used less in metal structural materials [3].

Some scholars have studied the effects of rare-earth elements on steel, such as Guo Feng et al. [4], who found that rare earth can improve the morphology of inclusions, purify grain boundaries, improve the strength of grain boundaries, reduce the possibility of crack propagation through defects, and improve impact toughness. Zhao et al. [5] found that the addition of rare earth increased the AC1 and AC3 phase transition temperatures of low-carbon microalloyed high-strength steel, reduced the critical cooling rate of the pearlite transformation, increased the incubation period, and made it easier to obtain a bainite structure. Liu Chengjun et al. [6] pointed out that rare earth can refine the austenite grain boundary to improve impact toughness. QU et al. [7] studied the second phase in rare-earth HSLA steel and found that the average size of precipitates was refined by about 15 nm after adding rare earth, indicating that rare earth can promote the precipitation of fine carbon and nitride, which is beneficial for improving the strength and toughness of materials.

**Citation:** Zhao, Q.; Dong, R.; Lu, Y.; Yang, Y.; Wang, Y.; Yang, X. Effect of Trace Rare-Earth Element Ce on the Microstructure and Properties of Cold-Rolled Medium Manganese Steel. *Metals* **2023**, *13*, 116. https:// doi.org/10.3390/met13010116

Academic Editors: Andrey Belyakov, Andrea Di Schino and Claudio Testani

Received: 22 November 2022 Revised: 2 January 2023 Accepted: 4 January 2023 Published: 6 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

At present, the effect of rare-earth elements on medium manganese steel is not clear, and the role of rare earth in steel is not clear [8–10]. How to reasonably and effectively use rare-earth elements in steel remains to be further studied.

The selection of rare-earth content is also critical to the study of the microstructure and properties of steel. Considering the internal quality of the material, adding a large amount of rare-earth elements produces large inclusions in the structure, which affects the plasticity and performance of the material and deteriorates the forming performance of automobile steel. From the perspective of production practice, automobile steel is mostly plate, and a large number of rare-earth elements are added. During the casting process, large particle inclusions block the casting nozzle and cause casting accidents. This study selected trace rare-earth elements for testing and research.

In this paper, cold-rolled medium manganese experimental steels with 9 ppm rareearth Ce and without rare earth were selected for comparative study. The two groups of experimental steel were subjected to the ART annealing treatment, and their microstructures, properties, and textures were analyzed to explore the effect of the trace rare-earth element Ce on the microstructure and properties of cold-rolled medium manganese steel, which provided a theoretical basis for the research and development of new third-generation rare-earth automotive steel, which is very economically significant for the development of the rare-earth steel and automotive fields.

#### **2. Experimental Materials and Methods**

The experimental steel was melted and cast by a laboratory vacuum induction furnace, and the ingot size was 100 mm × 100 mm × 300 mm. Then, a 1 mm thick experimental steel plate was obtained by rolling. The chemical composition of the test steel is shown in Table 1. The phase transformation temperature of the experimental steel was measured by a NETZSCH STA449 F3 comprehensive thermal analyzer at a rate of 20 ◦C/min to 950 ◦C under argon protection. The AC1 and AC3 values of the experimental steel without rare earth were 596 ◦C and 734 ◦C, respectively. The AC1 and AC3 values of the rare-earth experimental steel were 598 ◦C and 751 ◦C, respectively. The heat treatment process was conducted as shown in Figure 1. The experimental steel plate was quenched at 800 ◦C for 5 min and then annealed at 625 ◦C, 645 ◦C, or 665 ◦C for 15 min.

**Table 1.** Chemical composition of test steel (%).


The original austenite corrosion was carried out on the quenched experimental steel, and the supersaturated picric acid solution was used as the corrosive agent. The metallographic structure after corrosion was observed, and the original austenite grain size distribution was determined using Image Pro Plus software [11]. The small <sup>10</sup> × <sup>10</sup> × 1 mm<sup>3</sup> pieces were cut from the annealed steel plate as a microstructure observation sample. The samples were polished and corroded with 4% nitric acid alcohol. The microstructures were observed using a field emission scanning electron microscope. The tensile specimens were prepared according to the GB/T 228.1-2010 standard. The length direction was parallel to the rolling direction. The original gauge was calculated by the proportional gauge (*L*<sup>0</sup> <sup>=</sup> 5.65√*S*0), and the tensile test was carried out on a universal tensile testing machine. Two groups of experimental steels were tested using an X-ray diffractometer (XRD, X Pert PRO MPD). The sample size was 10 mm × 15 mm. The (200), (220), (311) crystal plane diffraction peaks of the FCC phase and the (200) and (211) crystal plane diffraction peaks

of the BCC phase were measured, and the reverse-transformation austenite content was calculated using the following formula:

**Figure 1.** Heat treatment process of test steel.

In the formula, *G* represents the lattice parameter correlation value of the FCC phase and BCC phase [12], and the detailed correspondence is shown in Table 2. *Vi* is the volume fraction of reversed austenite corresponding to different *G* values, *Iγ* is the integral intensity of the (200), (220), and (311) crystal plane diffraction peaks of the FCC phase, and *Iα* is the integral intensity of the (200) and (211) crystal plane diffraction peaks of the BCC phase. The texture of the samples was also measured using an X-ray diffractometer. The ODF cross section was characterized using the Roe method, and the content of each texture component was calculated using the texture analysis software ResMat-TexTools.

**Table 2.** G values corresponding to different crystal face indices.

