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<p>Understanding the microstructural evolution of medium-carbon low-alloy steels during continuous cooling is essential for optimizing heat treatment processes in AHSS-grade applications. This study investigates DIN 1.5025 steel (0.529C–1.67Si wt.%) using dilatometry, color metallography, and microhardness profiling across cooling rates from 0.1 to 100&#xa0;°C/s. Emphasis is placed on the correlation between optimized etching protocols and the cooling rate to enhance phase contrast and identification accuracy. The results reveal cooling-rate-dependent transformation sequences from ferrite–pearlite to bainite–martensite, with hardness increasing from ~190 to ~470 HV. Notably, martensite start temperature increased from 255 to 290&#xa0;°C with faster cooling, linked to reduced carbon enrichment. The effectiveness of specific tint etchants—Marshall’s reagent with 2% Nital versus Nital + Picral—was shown to vary with phase content. Compared to previous TTT-based findings for this steel, the continuous cooling transformation temperatures for pearlite (start: 672&#xa0;°C; finish: 610&#xa0;°C) were slightly lower, highlighting the kinetic suppression in non-isothermal conditions. These findings contribute practical guidance for process design and microstructural control in steels with AHSS potential.</p>

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Phase Interaction and Microstructural Progression in DIN 1.5025 Steel with Medium Carbon Content During Cooling: A Color Metallography Approach

  • 期刊论文

  • 发表日期：

    2026年2月10日

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  • Shima Pashangeh,
  • Seyyed Sadegh Ghasemi Banadkouki,
  • Fatemeh Mehrabi

摘要

Understanding the microstructural evolution of medium-carbon low-alloy steels during continuous cooling is essential for optimizing heat treatment processes in AHSS-grade applications. This study investigates DIN 1.5025 steel (0.529C–1.67Si wt.%) using dilatometry, color metallography, and microhardness profiling across cooling rates from 0.1 to 100 °C/s. Emphasis is placed on the correlation between optimized etching protocols and the cooling rate to enhance phase contrast and identification accuracy. The results reveal cooling-rate-dependent transformation sequences from ferrite–pearlite to bainite–martensite, with hardness increasing from ~190 to ~470 HV. Notably, martensite start temperature increased from 255 to 290 °C with faster cooling, linked to reduced carbon enrichment. The effectiveness of specific tint etchants—Marshall’s reagent with 2% Nital versus Nital + Picral—was shown to vary with phase content. Compared to previous TTT-based findings for this steel, the continuous cooling transformation temperatures for pearlite (start: 672 °C; finish: 610 °C) were slightly lower, highlighting the kinetic suppression in non-isothermal conditions. These findings contribute practical guidance for process design and microstructural control in steels with AHSS potential.