The effect of cooling rate and coiling temperature on the niobium retention in Ultra-Thin Cast Strip steel

This laboratory study utilised a dilatometer to simulate the run-out table cooling rate and the coiling temperature to investigate the effect of the cooling rate and simulated coiling conditions on the age hardening response of a niobium microalloyed Ultra-thin Cast Strip (UCS®) steel, produced by the CASTRIP® Process. Three cooling rates of 1, 5 and 40°C/s, covering very slow (1°C/s) to typical run-out table cooling rates (40°C/s), down to two coiling temperatures of 500 and 675°C were used. Dilatation curves were used to determine the temperature range over which the γ-α phase transformations occurred and the final microstructures were characterized using an optical microscope equipped with an image analysis software. The subsequent age hardening response, which previous studies have shown, results from the retention of Nb in solid solution, was assessed by the hardness changes after a post heat treatment at 700°C for 60s. A range of age hardening responses were obtained, depending on cooling rates and cooling stop (coiling) temperatures, which indicate a different degree of Nb retention. At the same cooling rate, the lower coiling temperature of 500°C resulted in higher Nb retention compared to the higher coiling temperature of 675°C. As the coiling temperature of 675°C was within the austenite to ferrite transformation range, the simulated slow cooling of the coil impacted the precipitation behaviour of Nb rendering the interpretation more complex and this will be discussed in this paper. For the 500°C simulated coiling temperature, the higher cooling rate resulted in a higher age hardening increment thus more Nb retention.