# Mechanistic Insights into Cooling-Rate-Governed Acicular Ferrite Transformation Kinetics and Strengthening-Toughening Synergy in EH36 Heavy Steel Plate

## TL;DR

This study explores how cooling rates affect the formation of acicular ferrite in heavy steel plates, improving strength and toughness.

## Contribution

The study identifies optimal cooling rates and nucleation mechanisms for acicular ferrite in heavy steel plates.

## Key findings

- Cooling rates of 3–7 °C/s promote acicular ferrite formation, with 5 °C/s yielding 74% volume fraction and an optimal aspect ratio of 5.97.
- TiOx-Al2O3·SiO2-MnO-MnS inclusions act as effective nucleation sites, with MnS reducing interfacial energy and promoting radial growth.
- Interlocking acicular ferrite increases microhardness by 14% through grain refinement and dislocation strengthening.

## Abstract

This study was focused on addressing the performance degradation in core microstructures of ultra-heavy steel plates (thickness ≥ 50 mm) caused by non-uniform cooling during thermo-mechanical controlled processing. Using microalloyed DH36 steel as the research subject, we systematically investigated the effects of cooling rate on the nucleation and growth of acicular ferrite and its consequent microstructure-property relationships through an integrated approach combining in situ observation via high-temperature laser scanning confocal microscopy with multiscale characterization techniques. Results demonstrate that the cooling rate significantly affects acicular ferrite formation, with the range of 3–7 °C/s being most conducive to acicular ferrite formation. At 5 °C/s, the acicular ferrite volume fraction reached a maximum of 74% with an optimal aspect ratio (5.97). Characterization confirmed that TiOx-Al2O3·SiO2-MnO-MnS complex inclusions act as effective nucleation sites for acicular ferrite, where the MnS outer layer plays a key role in reducing interfacial energy and promoting acicular ferrite radial growth. Furthermore, the interlocking acicular ferrite structure was shown to enhance microhardness by 14% (HV0.1 = 212.5) compared to conventional ferrite through grain refinement strengthening and dislocation strengthening (with a dislocation density of 2 × 108 dislocations/mm2). These results provide crucial theoretical insights and a practical processing window for strengthening-toughening control of heavy plate core microstructures, offering a viable pathway for improving the comprehensive performance of ultra-heavy plates.