Effect of Cerium on Inclusions, Microstructure, and Mechanical Properties of Sulfur-Based Free-Cutting Steel
摘要
The current research investigates how rare earth cerium affects the inclusions, microstructure, and mechanical properties of sulfur‑based free‑cutting steel. Y45Mn steels with cerium contents ranging from 0 to 652 ppm (0, 137, 226, and 652 ppm) were produced by adding Ce–Fe alloy in a vacuum induction furnace at 1550 °C; the ingots were then forged at 1200 °C and air-cooled. The role of cerium content on inclusion evolution, microstructural alterations, and mechanical performance was systematically analyzed. In comparison with the 0 ppm Ce reference steel, cerium additions at 137, 226, and 652 ppm significantly modify the inclusions. The chain‑like and clustered MnS inclusions located at grain boundaries are converted into ellipsoidal rare‑earth‑containing composite inclusions, evolving in the sequence: MnS → CeAlO3–MnS → Ce2O2S–MnS → CeS–MnS. These rare earth composite inclusions not only induce substantial changes in the number density, size, and distribution features of the inclusions in steel but also promote ferrite nucleation and growth, leading to an increase in ferrite content from 4.6 to 6.5 pct as the Ce content rises from 0 to 652 ppm. In addition, when the Ce content increases from 0 to 652 ppm, the geometrically necessary dislocation (GND) density increases from 1.92 × 1014 to 2.78 × 1014/m2. Cerium addition significantly raises impact energy and inhibits crack propagation at room temperature. The impact energy measures 13.0, 28.06, 43.5, and 30.33 J for Ce contents of 0, 137, 226, and 652 ppm, respectively, reaching a maximum of 43.5 J at 226 ppm—an enhancement of about 235 pct compared to the cerium-free steel (13.0 J). Nevertheless, excessive cerium (652 ppm) increases the amount of large‑sized CeS–MnS composite inclusions, which consequently results in degraded room-temperature impact toughness and room-temperature tensile strength.