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<p>This study reports the formation of a mixed gradient microstructure in low-carbon microalloyed steel rods via controlled cooling. The mechanism of formation of the mixed gradient microstructure and its effect on the mechanical behavior of low-carbon microalloyed steel at room temperature were analyzed via microstructure characterization, finite element analysis, thermodynamic calculation, and hardness and uniaxial tensile tests. The results showed that a mixed gradient structure with a gradual decrease in the martensite and bainite mixture and a gradual increase in ferrite was formed at a depth of ~ 5&#xa0;mm (<i>R</i> = 12&#xa0;mm) from the steel rod surface. The mixed gradient structure comprised dislocated martensite and Fe<sub>3</sub>C nanoparticles, indicating that the steel bars were self-tempered during the controlled cooling process. The strength, hardness, and yield ratio of the mixed gradient structure decreased rapidly with an increase in the volume fraction of ferrite on increasing the distance from the surface to the core. Specifically, the tensile strength reduces from 935 to 706&#xa0;MPa, and the hardness decreases from 290 HV to 197 HV. The change rule of uniform elongation is opposite to the change rule of strength. The yield strength of the mixed gradient structure was different from the average yield strength value of the independent uniform structure but was closer to the strength estimated using the Hall–Petch relation. The mixed gradient structure exhibited a work-hardening rate peak and a high work-hardening retention capacity, resulting in better uniform plastic deformation of the attached martensite structure than the independent martensite structure. The mixed gradient microstructure of engineering component-level samples and its influence on mechanical behavior are presented in detail. The relevant results can regulate the mechanical behavior of low-carbon microalloyed steel components and provide ideas for the study of the microstructure performance of different metals with a mixed gradient structure.</p>

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Microstructure and Mechanical Behavior of Low-Carbon Microalloyed Steel with a Mixed Gradient Structure

  • 期刊论文

  • 发表日期：

    2025年9月18日

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  • Yang Guiling,
  • Jia Zhiyao,
  • Ren Yunfei,
  • Zheng Chengsi,
  • Zhang Mingya,
  • Sun Ji,
  • Liu Dongming

摘要

This study reports the formation of a mixed gradient microstructure in low-carbon microalloyed steel rods via controlled cooling. The mechanism of formation of the mixed gradient microstructure and its effect on the mechanical behavior of low-carbon microalloyed steel at room temperature were analyzed via microstructure characterization, finite element analysis, thermodynamic calculation, and hardness and uniaxial tensile tests. The results showed that a mixed gradient structure with a gradual decrease in the martensite and bainite mixture and a gradual increase in ferrite was formed at a depth of ~ 5 mm (R = 12 mm) from the steel rod surface. The mixed gradient structure comprised dislocated martensite and Fe3C nanoparticles, indicating that the steel bars were self-tempered during the controlled cooling process. The strength, hardness, and yield ratio of the mixed gradient structure decreased rapidly with an increase in the volume fraction of ferrite on increasing the distance from the surface to the core. Specifically, the tensile strength reduces from 935 to 706 MPa, and the hardness decreases from 290 HV to 197 HV. The change rule of uniform elongation is opposite to the change rule of strength. The yield strength of the mixed gradient structure was different from the average yield strength value of the independent uniform structure but was closer to the strength estimated using the Hall–Petch relation. The mixed gradient structure exhibited a work-hardening rate peak and a high work-hardening retention capacity, resulting in better uniform plastic deformation of the attached martensite structure than the independent martensite structure. The mixed gradient microstructure of engineering component-level samples and its influence on mechanical behavior are presented in detail. The relevant results can regulate the mechanical behavior of low-carbon microalloyed steel components and provide ideas for the study of the microstructure performance of different metals with a mixed gradient structure.