Master of Engineering Materials
School of Mechanical, Materials and Mechatronic Engineering
Wang, Zhizhang, The effect of cooling rate and coiling temperature on the niobium retention in ultrathin castrip steel, Master of Engineering Materials thesis, School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, 2016. http://ro.uow.edu.au/theses/4867
The main objective of this study is to investigate the effect of the cooling rate from 1 to 40°C/s and the coiling temperatures of 500 and 675°C on the niobium retention in ultra-thin CASTRIP steel.
The samples were solution treated at 1150 and 1280°C for 5 mins and cooled to 900°C at the cooling rate of 50°C/s. Then the samples were cooled from 900°C to the coiling temperatures of 675 and 500°C at 1, 5 or 40°C/s. The cooling rate from the coiling temperatures to 300°C was at 0.17°C/s. The samples were age hardened at 700°C for 60s. All the experiments were carried out on a theta dilatometer and dilatation curves obtained from the dilatometer were used to analyse the γ-α phase transformation of the samples in the temperature ranges of 900 to 500°C. The microstructures of the samples in the various cooling conditions were characterized to determine the influence of the cooling rates and coiling temperatures on the γ-α phase transformations. An INDENTEC Vickers hardness tester and a LECO M-400-H1 Micro Vickers hardness testing machine were used to measure the Vickers hardness of the samples and Micro Vickers hardness of the individual constitutes. The hardness results were used to assess how the cooling rates and coiling temperatures influenced the niobium retention in the samples.
It was found as expected that at the coiling temperature of 500°C, more Nb atoms were retained in the solid solution for the sample cooled at the cooling rate of 40°C/s compared to the samples cooled at 1 and 5°C/s However, at the coiling temperature of 675°C, more Nb atoms were retained in the solid solution for the sample cooled at 1°C/s compared to the samples cooled at 5 and 40°C/s. This may be due to the fact that the coiling temperature of 675°C is well within the γ→α transformation temperature range and the cooling rate below the coiling temperature is one magnitude slower than the cooling rates above the coiling temperature. The retention of Nb depends on the γ→α transformation temperature range of the samples and most of the γ→α transformation occurred above the coiling temperature of 675°C for the sample cooled at 1°C/s and the most of the γ→α transformation occurred at or below the coiling temperature. Consequently more Nb will be retained and the larger age hardening effect was obtained in the samples cooled at 1 °C/s. The reheating temperature also has an effect on the retention of the Nb in the samples. The higher the reheating temperature the lower Nb retention and lower age hardening effect in the samples. This may be explained by the fact that the higher reheating temperature resulted in the larger austenite grain size and the lower γ→α transformation temperatures. The austenite grain size of CASTTRIP steel is more likely larger than the austenite sizes used in this investigation. The retention of Nb in CASTRIP steel is likely smaller than the one obtained in this investigation.