Enhancing impact toughness of Q-P-T plain steels via multiscale design: Resolving the strength-toughness balance

The impact toughness of steel is crucial for structural applications under dynamic loading. Current steel development trends emphasize enhancing both strength and toughness in cost-effective plain steels, resolving the inherent strength-toughness balance. Here, we propose a novel multiscale design to significantly improve the impact toughness of quenching-partitioning-tempering plain steel. By leveraging the dislocation across martensite/austenite interface effect, we optimize the retained austenite content to approximately 10 %, evidently reduce brittle strain-induced twinned martensite while maintaining exceptional ductility. Furthermore, surface mechanical attrition treatment induces a residual compressive stress in sample surface accompanying with microstructure gradient and improving energy absorption during impact. This strategy achieves a fourfold increase in impact toughness, resolving the strength-toughness balance. The approach not only demonstrates the potential of multiscale design on compressive stress gradient accompanying microstructure gradient to optimize steel performance but also provides a scalable, cost-effective solution for high-performance materials in dynamic loading application. © 2025 Acta Materialia Inc.