Zushu Li, Mo Ji, Prof. Claire Davis, Dr Richard Thackray, Will Robertson
Increasing usage of steel scrap in either BF-BOF or EAF routes is an inevitable trend for the steel industry because of its significant benefits in reducing CO2 emissions and energy consumption.
However, one of the challenges of using steel scrap is the residual elements present, such as Sn and Cu, which are known to cause issues such as hot shortness as well as segregation during steel processing.
These residual elements present in steel are known to affect both thermomechanical processing control and final product quality. The mechanisms of residual elements reducing hot ductility in certain steel grades and their solute retardation effects on metallurgical transformations are not yet fully understood.
In Task 3, the impact of residual elements on the hot rolling process of free-cutting steels and rail steels will be evaluated to improve the understanding of the role of residual elements in steel processing.
Surface quality, productivity and weldability are key research areas. The outcomes of this research will provide a foundation for increasing the steel scrap in steel production processes and developing more efficient and environmentally friendly methods for the production of high-quality steels.
Residual elements
The SUSTAIN project Task 3, being carried out at the Universities of Warwick and Sheffield, aims to promote the increased usage of UK steel scrap either in BF-BOF route or the shift from BF-BOF to EAF route at low cost and reduced environmental impact.
This is achieved by studying the impact of residual elements (inherited from steel scrap) during thermomechanical processing in steels. Research has been conducted on free-cutting steels and rail steels from Liberty Steel and British Steel respectively, and the ingots with varying levels of Cu have been cast through VIM. High-temperature in-situ tensile tests and hot bending tests have been performed to understand the cracking mechanism of free-cutting steels during reheating and deformation. The task is also developing machinability test capability in-house at WMG.
Since the start of SUSTAIN Task 3, a deeper understanding has been gained of the effect of Cu concentration on the hot ductility of free-cutting steels. It has been found that 0.8 wt.% Cu results in a significant decrease in hot ductility even without oxidation (hot shortness). The focus of rail steel grades research has shifted towards weldability, and the scope of experiments has been expanded to include the solute drag parameter for static recrystallisation, which will broaden the understanding of the effect of residual elements on TMCR processes.
Key findings
One of the key findings of the research on free-cutting steels with 0.8 wt.% Cu is that these grades exhibit poor hot ductility and hot shortness behaviour.
This is due to the tendency of Cu to segregate along the inclusions and matrix interface, leading to enrichment at these locations.
However, a solution has been developed to address this issue. By undergoing a reheating process at a temperature above 1100 °C for more than 30 minutes, the Cu enrichment can be removed from the inclusions, effectively restoring the hot ductility of the steel products.
This solution has been found to be effective in improving the performance of free-cutting steels with high Cu content.
Industrial impact
The impact of residual elements on the processability and product qualities of both free-cutting and rail steel grades determined in this task will provide the guidance on the tolerance of residual elements in steels with current processing parameters and process modifications to increasing the tolerance of residual elements. This will be a methodology for optimising scrap utilisation for the UK steel industry, which will be shared with our industrial partners for implementation. It is expected to establish guidelines for determining economically feasible limits of residual elements.
The barrier to implementing this research outcome is mainly associated with the costs as well as the technical feasibility. It might require process modification and modelling for our industrial partners.
Next steps
Future scientific research activities will include an in-house machinability study, welding simulation using the Gleeble system at WMG, TEM/STEM characterization, high-temperature CLSM in-situ tensile tests, and DIC.
Additionally, the researchers aim to collaborate with industrial partners to translate the experimental data and findings into practical applications in the industry.
The effect of electrification
The goal of this task is to encourage the utilisation of steel scrap as the primary source of iron materials for steelmaking, supporting the shift to the EAF route (i.e. the planned electrification of steelmaking). So this research is a technical enabler to the electrification of steelmaking.
Through this research, a deeper understanding of residual element behaviour during the reheating and hot-rolling process will be achieved. The precise control of thermomechanical processing to increase the tolerance of residual elements, and improve productivity and final steel quality, will be achieved through understanding the behaviours of residual elements from steel scraps.
