Secondary cooling control based on solidification characteristics of non-quenched and tempered steel

Abstract

Due to the high crack sensitivity of non-quenched and tempered steel and the difficulty of accurate control of secondary cooling, surface cracks of the continuous casting strand occur frequently. A secondary cooling control method based on the solidification characteristics of non-quenched and tempered steel was proposed. For the solidification characteristics, the effect of the cooling rate on the secondary phase precipitation was studied using a confocal microscope and field emission scanning electron microscopy (FESEM), and the phase transformation mechanism of proeutectoid ferrite was clarified. Results show that the second-phase particles start to precipitate at 1086 °C and reach a peak at ~912 °C. When the cooling rate ranges from 0.1 to 5 °C·s−1, the size and volume fraction of the second-phase particles decrease with the increase of the cooling rate, and the second-phase particles transition from a chain-like distribution at the grain boundaries to a uniform distribution in the matrix. Increasing the cooling rate is helpful to weaken the pinning effect of the precipitates and strengthen the microstructure of the bloom surface. As for the secondary cooling optimization, a heat transfer and solidification model considering a transverse water distribution was established, and a secondary cooling optimization method based on the solidification characteristics of non-quenched and tempered steel was proposed. Strong cooling is performed after the strand leaves the mold to meet the requirements of a reasonable cooling rate and temperature range for controlling the precipitation of particles. Industrial trials confirm the feasibility of the technical solution. In addition, the study shows that reducing the spray distance can improve the transverse non-uniformity of secondary cooling water. In this study, the influence of the secondary cooling water amount and spray distance on the crack sensitivity of non-quenched and tempered steel was comprehensively considered, and the secondary cooling process was optimized by studying the “longitudinal‒transverse” solidification cooling. The proposed optimization scheme contributes to the improvement of surface and subsurface cracks of continuous casting bloom.

Keywords

• continuous casting
• non-quenched and tempered steel
• solidification characteristics
• carbonitride precipitate
• cooling rate
• secondary cooling optimization