During the continuous casting of microalloyed high-strength steels, surface cracks readily occurs due to coarsening austenite grains and segregation of precipitates along grain boundary, with Nb-containing grades exhibiting an especially narrow processing window. Traditional process optimization methods often lacks precision, while the Surface Structure Control (SSC) technique provides a promising alternative. However, its potential has not yet been fully realized, primarily due to limited understanding of the underlying mechanisms and the narrow range of feasible cooling adjustments. This study systematically investigates how SSC improves hot ductility and proposes a practical strategy to stably enhance the ductility of continuously cast slabs under industrial conditions. Four simulation schemes were designed: one conventional cooling process and three SSC processes. One SSC scheme employed cyclic phase transformation with a minimum cooling temperature of 600 °C to refine grains, while two others—targeted at 750 °C and 650 °C—were designed to better represent industrial production conditions. Results indicate that under the conventional cooling, AH36 steel exhibits a ductility trough at 750 °C (RA: 24.66 pct), attributed to coarse austenite grains (200 to 300 μm) and the formation of film-like ferrite induced by the chain carbonitride precipitation. The cyclic phase transformation refined grains to 30 to 50 μm; however, incomplete elimination of ferrite films caused localized coarse grains (> 150 μm) resulted in pronounced ductility fluctuations at 850 °C (RA: 45.24 to 73.79 pct) and implementation challenges. Strong cooling to 750 °C followed by reheating increased RA to 53.21 pct by suppressing carbonitride precipitation. But coarse grains persisted and film-like ferrite reprecipitated during slow cooling, leaving residual brittleness near 750 °C. In contrast, strong cooling to 650 °C, which induced a limited amount of non-diffusional transformation, effectively prevented film-like ferrite formation during subsequent slow cooling, despite the coarse austenite matrix, leading to a significantly improved RA of 64.60 pct. It is proposed that cyclic phase transformation generates carbon-enriched zones with dense carbides near grain boundaries, influencing the nucleation behavior of proeutectoid ferrite during later transformations. This work elucidates a novel ferrite control mechanism and, together with the inhibitory effect of rapid cooling on precipitation, establishes a feasible SSC strategy capable of stably improving hot ductility across a broad temperature range in industrial continuous casting.

Mechanistic Insights into Surface Structure Control for Improving Hot Ductility in Continuous Casting

期刊论文
发表日期：

2026年4月24日

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Pu Wang,
Jian Wu,
Haijie Wang

摘要

During the continuous casting of microalloyed high-strength steels, surface cracks readily occurs due to coarsening austenite grains and segregation of precipitates along grain boundary, with Nb-containing grades exhibiting an especially narrow processing window. Traditional process optimization methods often lacks precision, while the Surface Structure Control (SSC) technique provides a promising alternative. However, its potential has not yet been fully realized, primarily due to limited understanding of the underlying mechanisms and the narrow range of feasible cooling adjustments. This study systematically investigates how SSC improves hot ductility and proposes a practical strategy to stably enhance the ductility of continuously cast slabs under industrial conditions. Four simulation schemes were designed: one conventional cooling process and three SSC processes. One SSC scheme employed cyclic phase transformation with a minimum cooling temperature of 600 °C to refine grains, while two others—targeted at 750 °C and 650 °C—were designed to better represent industrial production conditions. Results indicate that under the conventional cooling, AH36 steel exhibits a ductility trough at 750 °C (RA: 24.66 pct), attributed to coarse austenite grains (200 to 300 μm) and the formation of film-like ferrite induced by the chain carbonitride precipitation. The cyclic phase transformation refined grains to 30 to 50 μm; however, incomplete elimination of ferrite films caused localized coarse grains (> 150 μm) resulted in pronounced ductility fluctuations at 850 °C (RA: 45.24 to 73.79 pct) and implementation challenges. Strong cooling to 750 °C followed by reheating increased RA to 53.21 pct by suppressing carbonitride precipitation. But coarse grains persisted and film-like ferrite reprecipitated during slow cooling, leaving residual brittleness near 750 °C. In contrast, strong cooling to 650 °C, which induced a limited amount of non-diffusional transformation, effectively prevented film-like ferrite formation during subsequent slow cooling, despite the coarse austenite matrix, leading to a significantly improved RA of 64.60 pct. It is proposed that cyclic phase transformation generates carbon-enriched zones with dense carbides near grain boundaries, influencing the nucleation behavior of proeutectoid ferrite during later transformations. This work elucidates a novel ferrite control mechanism and, together with the inhibitory effect of rapid cooling on precipitation, establishes a feasible SSC strategy capable of stably improving hot ductility across a broad temperature range in industrial continuous casting.