Ironmaking & Steelmaking: Processes,Products and Applications

A unique in blow sampling system has been applied to a blowing converter to retrieve simultaneously representative bulk metal bath and slag/metal emulsion samples from seven specified positions and every 2 min from start of blow. Full sample datasets from 20 heats have been grouped according to differences in the bulk bath phosphorus removal profiles and analysed with respect to relative refining ability of the slag/metal emulsion and the bulk metal bath. The complexity of the thermokinetic relationships behind the removal of carbon and the transfer of silicon, phosphorus, manganese and sulphur between the metal and slag is highlighted and the metal circulation rate in the emulsion is derived.

The characterisation of steel microstructures is an important tool for metallurgists as mechanical properties are controlled by microstructural parameters such as grain size, phase balance and precipitates. The majority of microstructural characterisation tools are indirect (inference from measurement of temperature), destructive (optical metallography, SEM and X-ray) or require small samples and laboratory equipment (dilatometry and DSC). This paper describes a multifrequency electromagnetic sensor that has been designed to detect changes in the relative permeability and resistivity of steel, which can be related directly to changes in microstructure. COMSOL multiphysics modelling software, considering the sensor design, sample geometry and sample microstructure, has been used to relate the measured sensor signal to changes in the steel microstructure. Examples presented in this paper are where the sensor has been used to monitor phase transformation (austenite to ferrite) below the Curie temperature (∼770°C) during online processing of steel; detection of decarburisation, both online and offline; and evaluation of the effect of long term thermal exposure on alloyed steels (changes in martensitic/bainitic microstructures).

The combined influence of prior plastic strain in austenite and applied stress in the course of the transformation on the crystallographic texture of bainitic steel was studied. The experimental data were obtained using electron back scatter diffraction and analysed using two methodologically different approaches. It was concluded that the plastic deformation of parent austenite grains could suppress variant selection in bainite despite maintaining an externally applied load throughout the bainitic transformation.

Based on the careful analysis of the heat transfer mechanism of the cooling process of high temperature sinter, a one-dimensional unsteady state mathematical model for the gas–solid heat transfer process of high temperature sinter was established according to the energy conservation. The mathematical model was verified by the use of industrial practical data, and the results showed that it was correct and reliable. Lastly, the model was used to investigate the effect of operation parameters (such as the trolley moving speed and the cooling air velocity) on the cooling process of the annular cooler, and the proposals were put forward for optimising operations of the annular cooler.

A combined first principle and experimental study of the microstructural characteristics of oxide scales developed on type 430 stainless steel during hot rolling is presented. The oxide layer structures have been investigated by means of SEM, XPS and GDS. The oxide scales were found to have a multilayer structure with Si enrichment at the oxide/matrix interface and were identified as (Fe,Cr)₂O₃/(Fe,Cr)₃O₄/Cr₂O₃, FeO and Si rich region/Fe–Cr stainless steel from the outer to the inner layer. An atomistic model of the Fe–Cr/FeO interface has been generated through first principle methods based on density functional theory. Structural and electronic properties are compared to available experimental data and studied as they evolve across the Fe–Cr/FeO and Fe–Cr (Si)/FeO interface.

Much of the current research through national and international, academic and industrial collaborations targets some of the great complexity exhibited by microstructural evolution in multicomponent systems such as steels. Such work clearly has an important role in the understanding and development of alloys, but it must be remembered that in many cases, simple approaches give sufficient answers and indeed are desirable for incorporation into macromodels where the progress of the microstructural evolution routine is interrogated a large number of times. Simple approaches for solidification and subsolidus homogenisation are presented for this purpose and validated against established models and experiment.

This paper tracks the progress in research regarding the use of twinning induced plasticity (TWIP) steel in the automobile industry. The chemical composition of TWIP steel ensures that it has stable austenite and proper stacking fault energy at room temperature, allowing the main deformation mechanism (twinning) to work. The effects of alloying elements on the microstructure and deformation mechanism of TWIP steel are explained in detail, and their properties deformed under static and dynamic conditions are examined. The TWIP steel deformed at a low strain rate shows higher total elongation and strength. When the TWIP steel deforms under dynamic strain conditions, the stress, microhardness and the work hardening rate, all increase along with the increase in strain and the strain rate. The twin characteristics of TWIP steels deformed at various strain rates vary also and the twins generated under a high strain rate exhibit thinner widths and smaller interspaces compared with those formed under a low strain rate. It has also been observed that multisystem twins are able to generate and develop together. The mechanisms of toughening and strengthening in TWIP steels are noted. Finally, some potential application fields have been found for the promising material.

A mathematical model based on an inverse heat transfer calculation was built to determine the heat flux between the mould and slab based on the measured mould temperatures. With K–ϵ turbulence model, a mathematical model of three-dimensional heat transfer and solidification of molten steel in continuous slab casting mould is developed. Solidification has been taken into consideration, and flow in the mushy zone is modelled according to Darcy’s law as is the case of flow in the porous media. The heat flux prescribed on the boundaries is obtained in the inverse heat conduction calculation; thus, the effect of heat transfer in the mould has been taken into consideration. Results show that the calculated values of mould temperature coincide with the measured ones. Results also reveal that the temperature distribution and shell thickness are affected by the fluid flow and heat transfer of slab which is governed by the heat flux on the mould/slab interface.

A software to simulate the solidification, heat transfer and water flowrate distribution in slab continuous casting was developed by establishing a mathematical model for the heat transfer and solidification in medium thickness slab casting. This model was validated by pin shooting and surface temperature measurement experiments. A reasonable target surface control temperature was found by testing the high temperature mechanical properties of Nb bearing ship plate steel, and then the water flowrate of each loop of the secondary cooling zone was determined by the software. The influence of uneven secondary cooling in the slab width direction on the quality of the slab was also investigated, which provided data for the optimisation of the secondary cooling of the slab caster. On the basis of the above research, an optimisation scheme for a secondary cooling system was proposed. Experimental results showed that the quality of the slab was significantly improved after optimisation. The centreline macrosegregation was reduced, and the ratio of equiaxed grains was increased by 3·18%. In addition, the transverse cracking of the slab was almost eliminated.