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<p>The present study investigates the effects of hot-rolling followed by quenching and tempering on microstructural evolution, mechanical properties, and their corresponding micro-mechanisms in a low-carbon Nb + V-added microalloyed steel. Hot-rolling + quenching (HRQ) of the steel resulted in an ultimate tensile strength of 1404&#xa0;MPa and yield strength of 1108&#xa0;MPa with 14.4 pct ductility, possibly the highest strength-ductility combination attained till now in this steel grade. The higher strength-ductility combination is attributed to the fine lath martensitic structure consisting of carbon atoms, dislocations, and spherical-shaped Nb-rich precipitates. Analysis of strengthening contributions showed that the ultra-high yield strength of the HRQ sample is mainly governed by dislocation hardening (679&#xa0;MPa) and martensite size (287&#xa0;MPa) strengthening. During tempering, the cementite’s shape changed from lenticular (at 300–400&#xa0;°C tempering temperature) to spherical (at 500–600&#xa0;°C), reducing its interfacial energy with the matrix through spheroidization. The specimen subjected to tempering at 400&#xa0;°C resulted in a lower ductility (13.2 pct from 14.4 pct in the HRQ sample), which is attributed to temper martensite embrittlement phenomena as de-cohesion occurs at the matrix/cementite interface during tensile deformation. Conversely, at 500&#xa0;°C tempering temperature, ductility and impact-toughness of the specimen increased to 18 pct and 41 ± 1&#xa0;J, respectively, as resistance against fracture provided by nano-size spherical carbide particles. The crystallography analysis depicts that the variant pairs with lower interfacial energy, i.e., V1–V2 (i.e., twinned variant pairs, 60° [111]<sub>γ</sub>), and V1–V3/V5, do not substantially change during tempering, indicating their thermal stability. Hence, these variant pairs contribute to retaining the higher strength-ductility combination after the tempering treatment of the HRQ specimen.</p>

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Synergistic Effects of Hot Rolling and Heat Treatments on Microstructural Characteristics and Achieving Ultra-high Strength in a V-Nb Microalloyed Steel

  • 期刊论文

  • 发表日期：

    2025年7月25日

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  • Pravendra Pratap Singh,
  • R. Murugesh Kumar,
  • Suhrit Mula

摘要

The present study investigates the effects of hot-rolling followed by quenching and tempering on microstructural evolution, mechanical properties, and their corresponding micro-mechanisms in a low-carbon Nb + V-added microalloyed steel. Hot-rolling + quenching (HRQ) of the steel resulted in an ultimate tensile strength of 1404 MPa and yield strength of 1108 MPa with 14.4 pct ductility, possibly the highest strength-ductility combination attained till now in this steel grade. The higher strength-ductility combination is attributed to the fine lath martensitic structure consisting of carbon atoms, dislocations, and spherical-shaped Nb-rich precipitates. Analysis of strengthening contributions showed that the ultra-high yield strength of the HRQ sample is mainly governed by dislocation hardening (679 MPa) and martensite size (287 MPa) strengthening. During tempering, the cementite’s shape changed from lenticular (at 300–400 °C tempering temperature) to spherical (at 500–600 °C), reducing its interfacial energy with the matrix through spheroidization. The specimen subjected to tempering at 400 °C resulted in a lower ductility (13.2 pct from 14.4 pct in the HRQ sample), which is attributed to temper martensite embrittlement phenomena as de-cohesion occurs at the matrix/cementite interface during tensile deformation. Conversely, at 500 °C tempering temperature, ductility and impact-toughness of the specimen increased to 18 pct and 41 ± 1 J, respectively, as resistance against fracture provided by nano-size spherical carbide particles. The crystallography analysis depicts that the variant pairs with lower interfacial energy, i.e., V1–V2 (i.e., twinned variant pairs, 60° [111]γ), and V1–V3/V5, do not substantially change during tempering, indicating their thermal stability. Hence, these variant pairs contribute to retaining the higher strength-ductility combination after the tempering treatment of the HRQ specimen.