Influence of primary cooling conditions and austenite conditioning on the hot ductility of simulated continuous cast peritectic steels

Abstract

Surface transverse cracking is still one of the main problems in the continuous casting of steel. The cooling rate at the corners of the slab and strand is usually the highest. Therefore, depending on the cooling regime, the initial temperature drop (primary cooling to the Tmin values) in the corner regions can result in temperatures that fall into the low-temperature range of the austenite region or the α+γ transformation zone. This can cause ferrite formation or promote the precipitation of non-metallic inclusion particles at the grain boundaries and in ferrite due to the lower solubility of these particles in ferrite than in austenite. The objective of this study was to simulate the effect of the initial austenite conditioning, the extent of primary cooling, the magnitude of the temperature rebound and the unbending temperature on the ductility properties of a plain carbon peritectic steel grade under conditions resembling the commercial continuous casting process. The austenite grain conditioning was studied using two methods, the 1350 °C treatment and the simulated in-situ melting conditionings. Both of these conditionings were utilised to accomplish the initial austenite grain sizes similar to the as-cast microstructure in the magnitudes of ± 500 μm to ± 1000 μm. Bähr DIL805 Dilatometer equipment was used to simulate the heat treatments which allowed the study of the initial austenite grain size distributions. The Gleeble 1500D thermomechanical simulator was used to study the hot ductility behavior of the plain carbon peritectic steel grade. During the hot ductility test, the tensile specimens are usually solution treated at high temperatures, followed by cooling to the unbending temperatures and then fractured isothermally. However, in this study, instead of cooling the specimens directly to the unbending temperatures after the austenite treatment, the specimens were subjected to simulated primary cooling, followed by temperature rebound (i.e. ΔTr) of either 200 °C or 300 °C as well as a simulated secondary slow cooling process (at a cooling rate of 0.1 °C/s) and then isothermally deformed to fracture in the temperature range of 630–1060 °C. In both cases of the austenite conditioning, the ductility was observed to be high when the hot deformation specimens were subjected to Tmin (830 °C), this temperature being the minimum temperature reached after primary cooling and was very close to the equilibrium austenite start transformation temperature, 840 °C. In both cases of Tmin values closer to the equilibrium austenite start transformation temperature, the coarse-grained (± 500 μm) specimens showed better ductility results, compared to the abnormally large grained (±1000 μm) specimens. This was attributed to the differences in the microstructure such as the initial austenite grain sizes, the segregation effects and high fraction of non-metallic inclusion particles at the austenite grain boundaries. The influence of the magnitude of the rebound steps (i.e. ΔTr) was also studied. The result showed that for the specimens subjected to the Tmin (830 °C), ductility increased as the ΔTr increased from 200 °C to 300 °C. Moreover, with the rebound step of 300 °C ductility values increased further with an increase in the unbending temperatures (TU) and this was observed for the specimens heated to 1350 °C. In contrast to this observation for the specimens treated at 1350 °C, small ΔTr (200 °C) showed better hot ductility values than large ΔTr (300 °C) for the specimens molten in-situ condition and this was observed in the unbending temperature range of 830-940 °C. However, the hot ductility values of these specimens were observed to increase with an increase in unbending temperature range of 980-1040 °C. In both cases of the austenite conditionings, the relatively good ductility results were attributed to the beneficial effect of Tmin values. These temperatures were 10 °C and 30 °C below the equilibrium austenite start transformation temperature, Ae3 for the specimens treated at 1350 °C and molten in-situ conditions, respectively. After quenching the specimens from these temperatures (Tmin), no grain boundary films of ferrite were observed. Due to the absence of ferrite, a smaller density of inclusion particles at the grain boundaries was expected. Furthermore, the effect of Tmax values (e.g. 1030 °C and 1130 °C) and high unbending temperatures (830-1060 °C and 830-960 °C) were also thought to have contributed towards good ductility results. The hot ductility values only decreased when the unbending temperatures fell below the Ar3S (~788 °C) temperature and this was observed for both austenite conditionings.