As a core piece of equipment in a continuous casting production line, the heat treatment process of the continuous casting roller table directly determines its wear resistance, thermal crack resistance, and service life. During operation, the continuous casting roller table must withstand the combined effects of friction from high-temperature cast billets, rapid heating and cooling by cooling water, and mechanical loads. Therefore, the heat treatment process must focus on improving the overall performance of the material, addressing key issues such as balancing hardness and toughness, controlling residual stress, and ensuring microstructure uniformity.
Common materials for continuous casting roller tables are high-chromium cast iron and alloy steel. While these materials possess high hardness potential, their as-cast microstructure exhibits severe segregation, with carbides distributed in a coarse network, resulting in insufficient toughness. Before heat treatment, diffusion annealing is necessary to eliminate compositional inhomogeneity. For example, the rolls are heated above the critical temperature and held for an extended period to allow the carbides to dissolve and reprecipitate, forming a fine, dispersed distribution. During this stage, the heating rate must be strictly controlled to prevent cracking due to thermal stress. Segmented holding is also employed to ensure uniform temperature throughout the roll body.
The quenching process is a crucial factor determining the performance of the continuous casting roller table. Traditional integral quenching often leads to excessive hardness differences between the roll body and the roll neck, causing stress concentration. Modern processes often employ differential temperature quenching, using induction heating to rapidly raise the surface of the roll body to the quenching temperature while keeping the roll neck at a low temperature, combined with high-pressure water jet quenching to achieve selective hardening. For example, the surface hardness of the roll body can reach HRC55 or higher after quenching, while the roll neck maintains HRC35-40, meeting both wear resistance requirements and ensuring assembly strength. The selection of the quenching medium must consider both cooling capacity and roll table deformation control; oil cooling is suitable for complex roll table structures, while water-based polymer solutions are used for thick-walled parts.
Tempering is crucial for eliminating quenching stress and stabilizing the microstructure. The martensitic microstructure of a continuous casting roller table after quenching is brittle and requires tempering above the martensitic transformation temperature. High-temperature tempering can obtain a tempered sorbite microstructure, which combines high strength and good toughness. Tempering temperature needs to be adjusted according to the roller table material. For high-chromium cast iron roller tables, the tempering temperature is typically controlled at 500-550℃, with the holding time extended to 1.2-1.5 times the quenching holding time to ensure complete decomposition of residual austenite. Air cooling after tempering avoids second-type temper brittleness and improves the roller table's impact resistance.
Surface strengthening technology is an important supplement to improving the performance of continuous casting roller tables. Laser cladding can prepare a metal-ceramic composite coating on the roller table surface, achieving a hardness of HRC60 or higher, significantly improving wear resistance. The cladding layer must form a good metallurgical bond with the substrate to prevent peeling during service. Induction hardening uses electromagnetic induction to heat the roller table surface, achieving rapid hardening, suitable for local repairs or pretreatment of new roller tables. This process requires precise control of heating depth and cooling rate to prevent surface cracking.
Residual stress control is crucial throughout the entire heat treatment process. Continuous casting roller tables have significant residual stress after welding or machining, which needs to be eliminated through stress-relief annealing. Annealing temperatures are typically set below Ac1, employing slow heating and prolonged holding to allow for full stress release. For large casting roller tables, vibration-assisted annealing can be used, inducing residual stress relaxation through mechanical vibration and shortening the processing cycle.
Quality inspection is crucial for a closed-loop heat treatment process. Continuous casting roller tables require ultrasonic testing to detect internal defects and magnetic particle testing to inspect for surface cracks. Hardness testing necessitates sampling at multiple points along the roller's axial and circumferential directions to ensure uniform hardness meets standards. Metallographic analysis verifies carbide morphology and distribution, ensuring the requirement for fine and dispersed carbides is met.
