Effects of V, Nb, Si, Mn, Mo on Microstructural Evolution and Strength–Toughness Balance of P20 Plastic Mold Steel

With the continuous development of plastic products toward larger dimensions, higher precision, and extended service life, plastic mold steels are required to simultaneously possess superior wear resistance, strength, and toughness. The effects of five alloying elements (V, Nb, Si, Mo, and Mn) on the microstructural evolution and mechanical properties of P20 plastic mold steel were systematically investigated after air-cooling from 860 °C, followed by tempering at 525 °C, and the underlying strengthening and toughening mechanisms were elucidated. The results revealed that, in the 0.2 V steel, approximately 62.6% of V existed in the form of fine VC carbides after austenitization at 860 °C, effectively inhibiting austenite grain coarsening. The remaining dissolved V atoms subsequently precipitated as nanoscale V–Mo-rich MC-type carbides during tempering, with an average size of less than 50 nm. This precipitation strengthening contributed an estimated strengthening increment of approximately 760 MPa, corresponding to a measured tensile strength increase of 326 MPa relative to the P20. In contrast, in the 0.1 Nb specimen, solubility calculations indicate that over 99% of Nb remains in undissolved NbC particles; TEM observations show these particles range from coarse 1–3 μm to finer 100–200 nm in size. The contribution of coarse NbC particles to material strength improvement is limited. The addition of Mo promoted the formation of abundant nanoscale MoC-type carbides (2–10 nm), which also exhibit a notable precipitation strengthening effect. Meanwhile, Si mainly contributed to solid-solution strengthening, whereas Mn enhanced the strength through solid-solution strengthening and grain refinement. Charpy impact tests demonstrated that, despite the remarkable strengthening induced by nanoscale carbide precipitation in the 0.2 V steel (tensile strength: 1237 MPa), the impact toughness deteriorated severely, dropping to 21 J. This severe toughness loss is proposed to be associated with local stress concentration around the fine carbides, which promotes secondary crack propagation. Similarly, coarse micrometer-sized NbC particles acted as detrimental sites for crack initiation and impaired impact toughness. Comparative analysis indicated that the steels containing 0.75 wt.% Si, 0.7 wt.% Mo, and 1.5 wt.% Mn achieved a favorable balance between strength, ductility, and toughness. In particular, the 0.7 Mo steel exhibited the most outstanding combination of mechanical properties, attaining a tensile strength of 1207 MPa and an impact energy of 136 J.