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<p>In this study, multiple gradient microstructures such as austenite volume fraction, grain size and dislocation density were introduced into medium manganese steel by friction stir processing combined with inter-critical annealing. Uniaxial tensile tests showed that the steel exhibited excellent strength-ductility synergy after friction stir processing and inter-critical annealing at 800&#xa0;°C for 10&#xa0;min, that is, the yield strength increased from 570 to 605.5&#xa0;MPa, the tensile strength increased significantly from 914.9 to 1041&#xa0;MPa, and the total elongation increased from 48.7 to 50.1%. Microstructure analysis showed that during the tensile deformation process, the hardness difference induced by the gradient microstructure of the gradient medium manganese steel enhances the mechanical incompatibility between different regions, thereby activating significant heterogeneous deformation-induced strengthening and hardening. At the early stage of deformation, the strain distribution associated with the gradient microstructure effectively inhibits the austenite transformation in the stir zone, favoring its stability under large strains. Simultaneously, it promoted austenite transformation in the heat-affected zone and, in synergy with heterogeneous deformation-induced hardening, led to simultaneous improvements in both strength and ductility. Among these mechanisms, heterogeneous deformation-induced strengthening is considered the primary contributor to the observed increase in yield strength. This study offers new insights into the design of gradient microstructures and the fundamental mechanisms behind strength–ductility synergy in medium manganese steel.</p>

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Simultaneous improvement of strength and ductility in medium-Mn steel through gradient microstructure design via friction stir processing and inter-critical annealing

  • 期刊论文

  • 发表日期：

    2026年3月5日

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  • Wangnan Zuo,
  • Xinyue Liu,
  • Yonggang Yang,
  • Zexuan Su,
  • Qi Zhang,
  • Yu Zhang,
  • Yanxin Wu,
  • Wei Chang,
  • Zhenli Mi

摘要

In this study, multiple gradient microstructures such as austenite volume fraction, grain size and dislocation density were introduced into medium manganese steel by friction stir processing combined with inter-critical annealing. Uniaxial tensile tests showed that the steel exhibited excellent strength-ductility synergy after friction stir processing and inter-critical annealing at 800 °C for 10 min, that is, the yield strength increased from 570 to 605.5 MPa, the tensile strength increased significantly from 914.9 to 1041 MPa, and the total elongation increased from 48.7 to 50.1%. Microstructure analysis showed that during the tensile deformation process, the hardness difference induced by the gradient microstructure of the gradient medium manganese steel enhances the mechanical incompatibility between different regions, thereby activating significant heterogeneous deformation-induced strengthening and hardening. At the early stage of deformation, the strain distribution associated with the gradient microstructure effectively inhibits the austenite transformation in the stir zone, favoring its stability under large strains. Simultaneously, it promoted austenite transformation in the heat-affected zone and, in synergy with heterogeneous deformation-induced hardening, led to simultaneous improvements in both strength and ductility. Among these mechanisms, heterogeneous deformation-induced strengthening is considered the primary contributor to the observed increase in yield strength. This study offers new insights into the design of gradient microstructures and the fundamental mechanisms behind strength–ductility synergy in medium manganese steel.