Micro-structuring micro-alloyed steels via non-metallic precipitate formation

Steel, the most used material in the world by value, is essential to virtually all industrial sectors: automotive, construction, mechanical engineering, shipbuilding, household appliance, consumer electronics, road, bridge and rail construction. Nearly 10 Mt of steel was manufactured in UK in 2012 and the UK steel industry has evolved to become one the leanest and most efficient of any manufacturing industry. The UK steel industry also has a long tradition of sustainable energy and raw-material management and minimization of carbon foot printing. The energy cost and CO2 output per ton of steel has been reduced by nearly 30% during the last two decades. There are nevertheless many opportunities for further improvement, with huge potential energy savings, but there are several key challenges that need to be overcome. Full cycle analysis studies demonstrate that further reductions in energy consumption and CO2 production of 25-30% can be made, in the current manufacturing of steel. A potential path for such savings lies in processes that allow for-hot charge after casting the product to a near-net (final) shape) directly into the rolling stage. This eliminates costly reheating before hot-rolling and it reduces the amount of rolling needed. It requires however, that the quality of the cast product is significantly better than what it is today in terms of bulk and surface properties since there is less chance to alter it after it is cast. We propose that this can be achieved by engineering the non-metallic particles, which are generally thought to be defects, such that they promote finer crystal grains in the cast metal. The research we aim to carry out provides the fundamental knowledge required to design such a process and we intend to develop in-situ characterization methods that enable scientists and engineers to study the specifics of how steel solidifies, rather then deducing what has happened from samples at room temperature.

The successful outcome of the proposed research would have a significant impact on the way engineers and materials scientists understand the development and control of alloy microstructure. Direct control of the solidification and post-solidification structures through the engineering of non-metallic particles will enable a manufacturing process which is superior in terms of both product quality and process control.

The near-term benefits would be mostly academic, but as envisaged, the primary long-term impact would be a paradigm shift in the way industry develops new process that require less energy, more yield and higher quality. The proper control of solidification microstructure, grain growth and austenite decomposition in the casting process would result in a finer microstructure and less surface defects.

This research is at present unique, both in concept and scope. Since no other group in the world is currently undertaking in-situ characterization of non-metallic particles and their impact on controlling the matrix microstructure, the timing of the research is critical for the UK steel industry, which stands to gain a lead in this field as an outcome. The availability of the facilities at Diamond and WMG for fundamental real-time experiments, followed by ductility tests on selected alloys, will validate and extend the usability of the theoretical and numerical models developed in parallel. The project team is multi-disciplinary, as befits a project that combines expertise on physical metallurgy of solidification and phase transition with expertise on chemical metallurgy on reaction kinetics of non-metallic precipitates.

The proposers have good ongoing collaboration with a number of manufacturing companies (Tata Steel, JLR, Tata Motors, Alcan, Alcoa, POSCO) and have been involved in several knowledge transfer projects and networks (incl. KTP and TSB schemes). These links show readiness to collaborate with industry for future exploitation of the knowledge, new processing techniques, or alloy compositions developed during the programme. Analytically, we identify the following likely contributions:

Academic Impact
-Documentation of the conditions in the mushy zone (T, time, solid fraction, phase fraction) when non-metallic precipitates form, and details of their evolution.
-Elucidation of how the precipitates alter the matrix steel microstructure
-A new dimension in the control of alloy solidification
-The opening of a new area of research for the understanding of processes in the mushy zone
-Determination of interaction (steric and chemical) between non-metallic precipitates and the steel microstructure
-Contribution to current DTCs at WMG and at UoM
-Training of young researchers at the University of Warwick and Manchester in high-temperature experimental methods, image analysis and multi-physics methods during the project

Economic & Societal Impact
-UK lead on a new control mechanism for alloy microstructure manipulation
-Yield improvement in current manufacturing lines in UK plagued by yield loss due to surface quality
-Development of future energy efficient steel manufacturing lines that enable continuous-near net shape casting and direct-hot charging, reducing CO2 release during production