Understanding effects of steel casting cooling rate ranges on segregation spatial distributions and properties

Introduction

The ever-increasing demand for reducing energy consumption in steel making means that using near-net shape casting is desirable along with the use of higher scrap content. Different casting options, such as belt casting, thick/thin slab and strip casting, give varying cooling rates (CR) and as-cast thicknesses. The cooling rates typically range from 0.6 °C/s for conventional thick slab continuous casting, to 500 °C/s for twin roll strip casting with as-cast thicknesses from around 1 mm to 250 mm. The role of cooling rate and composition on micro-segregation, considering Mn and Cu, has been studied in this project.

Progress to Date

A number of steels, based on a DP800 composition but with varying Mn and Cu content, have been cast using a wedge mould (Figure 1) and 30mm wide rectangular mould. Secondary dendrite arm spacings (SDAS) have been measured for the rectangular mould through thickness. Utilising known relationship between CR and SDAS for this material () a COMSOL model was created, with boundary conditions defined by fitting with experimental results. Using boundary conditions, the wedge mould was designed providing greater range of CR that are appropriate for range of casting techniques - thick slab to belt casting (0.6 to 5.8°C/s for regions of columnar solidification). Relationship between measured SDAS and predicted CR for the wedge mould was determined (Figure 2).

An EDX grid was used to measure spatial compositional variation (segregation ratio, SR) at 1mm from cast surface at different positions, corresponding to different CR. Thermocalc was used to predict SR for the same CR conditions (Figure 3). The Mn SR was defined as, local 95th/5th percentile value. Clear trend in SR with CR was seen, which relates to a combination of partitioning during solidification followed by back diffusion post solidification (where the diffusion distance is related to the SDAS).

Next Steps

The effect of cooling rate on the solidification structure (SDAS) and segregation behaviour when residual elements (i.e. Cu) will be investigated in the next stage of the work. Similar partitioning behaviour for Mn and Cu is predicted by Thermocalc, but different back diffusion rates are expected. The influence of the residuals on the SDAS and then the SR for both Mn and Cu at different cooling rates will be explored.

Green hydrogen based direct reduced iron (DRI) for steel manufacturing

Introduction

Steel production through conventional methods such as blast furnaces (BF) and basic oxygen furnaces (BOF) significantly contributes to global CO₂ emissions, accounting for approximately 8% of the total emissions worldwide. To align with climate targets and reduce environmental harm, the steel industry is shifting toward sustainable practices. A promising decarbonisation strategy involves "green steel" production, which utilises hydrogen-based direct reduced iron (H-DRI) processed in electric arc furnaces (EAF). This research explores the optimisation of H2DRI melting behaviour through numerical modelling and experimental approaches, aiming to enhance the efficiency and scalability of eco-friendly steel manufacturing and accelerate the industry’s transition to net-zero emissions.

Progress to Date

This study initiates an exploration of steel production methodologies, with a concentrated focus on Electric Arc Furnace (EAF) and operations. The investigation analysed the melting behaviour of steel feedstocks scrap, direct reduced iron (DRI), and hydrogen-based direct reduced iron (H-DRI)—to systematically evaluate knowledge gaps and implementation challenges in integrating H-DRI into steel feedstocks. Building on these insights, a robust thesis framework was formulated, encompassing defined research questions, a methodological roadmap, and a rotational experimental strategy to balance empirical work with analytical rigor.

Preliminary 2D model numerical simulation was executed in commercial CFD COMSOL software to gaining a comprehensive understanding of the melting of and identifying difference between melting behaviour of H2DRI and other materials.

A comprehensive melting experiment was conducted using a High Temperature Confocal Laser Scanning Microscope (HT-CLSM) to investigate the melting behaviour of scrap, C-DRI and H-DRI to

• capture real-time observations of the melting phenomena at a microscopic level

• understand the complex phase transformations kinetics during the process

• evaluate key parameters influencing the melting behaviour, including reduction degree, carbon concentration and porosity

• validate simulation results through experimental data

Next Steps

• Utilise the department's facilities to prepare the sample through direct reduction process, employing pure hydrogen as the reducing agent.

• Analyse the microstructure of the H-DRI sample before and after reduction using advanced imaging techniques (e.g., SEM, optical microscopy) to quantify porosity, pore diameter, and morphological changes

• Develop a computational fluid dynamics (CFD) 3D model using COMSOL Multiphysics to simulate melting process of H-DRI under controlled conditions.

• ·Validate the simulation results against experimental data to assess the accuracy of the model and refine understanding of how microstructure-driven properties (porosity, degree of reduction and reduction temperature) influence melting kinetics