Producing robust recrystallization models which can assist metallic microstructural design requires effectively understanding recrystallization nucleation. When the nucleation of static recrystallization (SRX) occurs at deformed grain boundaries, strain-induced boundary migration (bulging) is generally accepted as the nucleation mechanism. However, the present study challenges that view, showing, for a Ni-30%Fe alloy, that nucleation at deformed grain boundaries is not solely determined by bulging: results indicate that the number of bulges developed in the deformed microstructure is over four times larger than the number of SRX grains. On the other hand, SRX nucleation is shown to occur only when the low-angle boundary (LAB) between a pre-existing bulge and its parent grain transforms into a high-angle boundary (HAB). Based on this, a novel nucleation criterion is proposed, which may apply to SRX irrespective of the nucleation site (and to dynamic/metadynamic recrystallization): nucleation occurs whenever the misorientation of the LAB surrounding a bulge reaches the minimum HAB misorientation (e.g., 15°). Besides, correlation exists between the dislocation density accumulated around the various triple junction and grain boundary types in the microstructure, and their nucleation efficiency. This has been attributed to the higher fraction of relatively large initial subgrain misorientations measured for higher boundary dislocation density.

Microalloyed low-carbon steels strengthened by vanadium carbide (VC) nanoprecipitates are receiving increasing attention, particularly in the automotive industry. A clear understanding of the nanoprecipitate chemistry is essential for optimizing the alloy composition and processing routes, thereby enhancing the mechanical properties of such advanced steels. The chemical evolution of VC precipitates, especially regarding the incorporation of iron into the nanoprecipitates, remains uncertain. Here, a model vanadium-microalloyed low-carbon steel is studied by atomic-resolution scanning transmission electron microscopy (STEM) techniques. The steel contains nanoscale VC precipitates formed either as interphase precipitates (IP) at the austenite/ferrite interface during the austenite-to-ferrite phase transformation, or as randomly distributed precipitates (RP) in the ferrite matrix during bainite tempering. The first-time observation of carbon sublattice atoms in VC is achieved using integrated differential phase-contrast STEM (iDPC-STEM). Non-equilibrium compositions are identified under both precipitation mechanisms, with no correlation between precipitate size and associated elemental contents. Most interphase VC nanoprecipitates contain higher amounts of not only iron but also manganese compared to random VC nanoprecipitates. Complementary ex-situ small-angle neutron scattering (SANS) analysis and solute-drag effect (SDE) modeling support the co-segregation of iron and manganese into the precipitates. Manganese typically appears to form a core–shell-like structure within VC. Experimental evidence is presented for the SDE-assisted formation of manganese-rich–core (fibrous) interphase VC precipitates, and a mechanism is proposed for iron–manganese co-enrichment in random VC precipitates. This study offers new insights into future strategies to tune nanoprecipitate chemistry in microalloyed steels.

Successful implementation of third generation advanced high strength steels (3rd gen AHSS) can be accelerated by developing steels that can be heat treated in existing industrial lines. Here, we develop new carbide free bainitic (CFB) steels in which bainite formation is accelerated by a 0.2 volume fraction of prior martensite and thus can be realized in 5 min, making them suitable for manufacturing in modern continuous annealing lines for bare steel strips. The resulting microstructure consists of bainitic ferrite, tempered martensite, and retained austenite. Carbon and silicon had the most pronounced effect on the mechanical properties among the studied alloying elements (manganese, niobium, chromium, and molybdenum) because of their influence on the fraction and stability of retained austenite. Our proposed treatment, which we call bainite accelerated by martensite (BAM), showed higher strength and lower global formability than traditional CFB without prior martensite (also called TRIP-assisted bainitic ferrite, TBF) and quenched and partitioned (Q&P) steels. Five of the designed steels showed tensile strength higher than 1370 MPa, a total elongation higher than 8%, and hole expansion capacity higher than 30%, and thus meet the requirements for the strongest commercial grades of complex phase steels with improved formability. This work broadens the possibilities of using existing industrial lines for manufacturing novel 3rd gen AHSS.

The formation of nanoscale vanadium carbide (VC) precipitates is reported in steels subjected to two different thermal treatments. The thermal treatments lead to either interphase precipitation (IP) or random precipitation (RP). Small-angle neutron scattering measurements coupled with transmission electron microscopy analysis are performed to determine the VC precipitate volume fraction and size distribution. It is seen that the samples exhibiting IP show a higher number density of VC precipitates compared to those undergoing RP. Moreover, a broader size distribution of the precipitate radii is observed in the samples with RP, where lens-shaped nanoscale VC precipitates are found predominantly at grain boundaries (GBs) and sub-grain boundaries (SGBs), with smaller precipitates dispersed within the matrix. It is seen that the addition of carbon and vanadium does not increase the VC precipitate number density when the mechanism of precipitation is IP, whereas an increase in the VC precipitate number density with carbon and vanadium addition is seen in case of RP.

While experiments show that refining the prior austenite grain size can either accelerate or decelerate bainite formation in steels, kinetic models based on the successive nucleation of bainitic ferrite subunits can only predict an acceleration. In this work we develop a physically-based model for bainite kinetics assuming a displacive growth mechanism which is able to reproduce both faster and slower bainite formation kinetics induced by austenite grain refinement. A theoretical analysis of the model and comparison against published experimental data show that slower kinetics for smaller grains is favored as the difference between the activation energy for grain boundary and autocatalytic nucleation of bainite increases, and as the austenite grain refinement results in finer bainite sub-units. We also theoretically analyze the density of initially present potential nucleation sites for bainite and show that the values of density used in other published bainite nucleation models are mostly underestimated. After using physically consistent values for the density of potential nucleation sites, we were able to calculate the apparent lengthening rate of bainite sheaves which were in line with experimentally measured lengthening rates.

In the present study, the nucleation of static recrystallization (SRX) in austenite after hot deformation is experimentally analyzed using a Ni-30 pct Fe model alloy. In agreement with the predictions by current models, nucleation rate exhibits a strong peak, early during SRX. Whereas such an early peak is explained by current models by the saturation of nucleation sites, this condition is far from reached, even after the peak declines. In addition, triple-junction and grain-boundary sites are shown to make a quantitatively similar contribution to nucleation. However, for a given boundary between deformed grains, nucleation predominantly starts at one of the triple junctions. Triple-junction nucleation initiates by strain-induced boundary migration of the nucleus (bulging) along one of the boundaries at the junction. Annealing twin boundaries contribute negligibly to nucleation through their grain-boundary sites. By contrast, their junctions with the boundaries of the parent grains do play a relevant role. The earlier nucleation at the triple junctions is attributed to the higher dislocation density observed around them, and the energy of the boundary consumed by the bulge. Both the maximum and average number of nuclei formed per boundary between deformed grains increase with increasing boundary length.

Modelling dislocation glide over the initial part of a stress–strain curve of metals received little attention up to now. However, dislocation glide is essential to ones understanding of the fundamental relationship between inelastic deformation and the evolution of the dislocation network structure. Therefore, we present a model of dislocation-driven deformation under static loading conditions. We reproduce repeated cyclic uniaxial tensile tests on Interstitial-Free and Low-Alloy steels. The elastic mechanical behaviour is described by isotropic linear elasticity, pre-yield anelastic mechanical behaviour by a dislocation bow-out model with dissipation, and the post-yield evolution of dislocation network structure by a statistical storage model. We hypothesise that when the local anelastic compliance is lower than the global plastic compliance, deformation is mechanically recoverable, and vice versa. This hypothesis is corroborated with the classical Taylor relation. We report the relation between stable and unstable dislocation glide using this prototypical modelling framework. We find four structural variables, that are based on dislocation physics, to describe the stress–strain curve: total dislocation density, average dislocation segment length, dislocation junction formation rate, and average dislocation junction length. Firstly, we quantify the dislocation network evolution during uniaxial monotonic loading, and verify work-hardening by dislocation junction formation and a Taylor-type equation for flow. Finally, we present a semi-empirical relation for the evolution of the dislocation network structure. Which allows us to: refine the physical interpretation of the Taylor relationship, and rationalise experimental observations on apparent modulus degradation by thermomechanical processing. Both these findings circumvent the limitations of current, physics-based hardening models.

A three-dimensional computational fluid dynamics (CFD) model for the HIsarna off-gas system is set up and validated by real plant data. In the model detailed reaction mechanism and kinetic data for post-combustion of CO-H2 mixture and carbon particles are incorporated. The results are presented and discussed in another study (Part 1) by the same authors. In the present paper, the focus will be on geometry modification of the off-gas system and the effects on the operating parameters. The effect of this modification on heat loss, temperature profile, carbon conversion and gaseous phase composition across the off-gas system is investigated. It is shown that the modified geometry leads to a higher heat loss through the reflux chamber walls which can change the temperature profile and consequently species composition. The modified geometry also offers possibility of higher CO-H2 mixture and carbon particles conversion rate and reduce unwanted emission from the reflux chamber.

Galvanized steel scrap flow and injection into the HIsarna reactor are investigated using discrete element method (DEM). The scrap particle is fed into the reactor through an inclined chute and hits the slag surface where the zinc content is evaporated and solid particles melt. A DEM model is setup and validated using experimental data obtained from the exact plant-scale chute geometry and scrap particles. Using the DEM model, the effect of chute inclination, injection elevation, injection mode (batch and continuous), batch size, and flowrate on particle distribution and exerted pressure on the slag surface are investigated. It is found that continuous mode of injection is the most suitable method to increase the spread of particles and also to reduce the exerted pressure on the slag surface. Placing dent-like obstacles at the tip of the chute significantly increases the impact area, especially for batchwise injection, thus reducing force and pressure on the slag surface that minimizes the risk of liquid splash. Larger particle impact area is also beneficial to obtain higher zinc evaporation rate from particle surface and also to minimize the slag surface temperature disturbance.

HIsarna reactor is characterized by a high raw materials versatility and is therefore attractive for processing secondary iron sources. Among the materials that can be recycled through HIsarna, zinc-bearing material has drawn a special attention. Based on the plant data, once dust-containing Zinc was injected into the main reactor, a final collected dust with a zinc content of 16% was achieved which opened up possibilities of higher enrichment for direct reuse in Zn smelting as a secondary source and an alternative for Zn ore (the primary source). However Zn vapor can react with iron oxide to form zinc ferrite (ZnFe₂O₄), which is an undesired product. Hence, the main focus of this study is to minimize the formation of ZnFe₂O using thermodynamic (FactSage) and computational fluid dynamic tools. After detecting regions with high potential of ZnFe₂O₄ formation, proper geometrical and operational modifications of the off-gas system is proposed to minimize the formation of zinc ferrite.

For all industrial applications, predicting system characteristics and behavior plays a vital role before constructing costly and complex multi-physic systems. Correct and reliable predictions become even more important once the aim is to go from small- to large-scale processes to establish an industrial demonstrations. In this study, a CFD-based scale-up of HIsarna off-gas system based on the Eulerian–Lagrangian approach is investigated and detailed step in scale-up procedure is discussed. A three-dimensional CFD model is developed and validated based on the available pilot scale data and used to design and scale up the post-combustion chamber (also known as reflux chamber). Detailed kinetics for volumetric and gas–solid reactions are incorporated in validated CFD model with a special attention to the wall boundary condition and modeling. The effect of reflux chamber geometry, oxygen injection ports, oxygen injection flowrate, isolation wall thickness, and inlet flue gas composition on different system characteristics such as heat loss through the wall, CO–H₂–carbon mixture conversion, flue gas, and wall temperature are investigated. The aim of the scaled up geometry, like pilot scale, is to achieve full combustion of unwanted species inside the reflux chamber to assure zero emissions from the off-gas system. Compared to the pilot scale, the scaled up reflux chamber is capable of handling and removing higher amount of unwanted species coming from the main reactor and therefore lower CO–H₂ and carbon particle emissions, mainly due to a larger size which provides larger volume and residence time for volumetric and gas–solid reaction to proceed.

HIsarna is a novel ironmaking process with great raw materials versatility that is attractive for various secondary resources. Among the materials that can be recycled, there is steel scrap which is fed to the furnace bath through an inclined chute. The velocity distribution of the scrap particles along the chute affects the particles’ distribution on the liquid slag and, thereupon, the efficient operation of the reactor. In this study, the flow of steel scrap particles along an inclined chute with the same dimensions as those of the actual chute of the HIsarna plant is investigated experimentally and numerically. The simulations are validated using chute tip velocity and mass fractions collected at the different compartments of a sampling device. Translational and angular velocity distributions along and across the chute are reported, and the effect of different parameters are investigated. The impact of the shape of the particles on the simulation process is found to be negligible. The angular velocity distribution in cross-sections of the chute exhibited a V-shaped orientation, whereas the translational velocity displayed similar values across the cross-sections. Moreover, translational velocity appeared to increase with increasing inclination angles, whereas angular velocity increased with decreasing batch size.

Within the steelmaking industry, a large amount of zinc-bearing waste is produced which cannot be effectively treated through integrated steel mills. Concurrently, zinc smelters generate waste residues containing significant amounts of iron and zinc which are stored or landfilled. The zinc concentration of iron and steelmaking residues inhibits its recycling to the blast furnace but is insufficient to be sent directly to the zinc producers. Consequently, a means of up-concentration is required. The pilot HIsarna ironmaking furnace has shown potential for processing secondary iron-bearing resources. Furthermore, zinc can be concentrated in the off-gas flue dust, providing an enriched input for zinc smelters. The potential recyclability of blast furnace (BF) and basic oxygen furnace (BOF) dust and ‘goethite’ residue from the zinc industry has been studied. The input materials have been comprehensively characterized and their reduction–vaporization behavior, has been investigated. Individual samples were tested at temperatures of up to 1300 °C. Here, it was shown that minimal reduction of iron and volatilization of zinc occurred in the goethite and BOF samples. Conversely, even at 1000 °C, the BF dust showed complete reduction of iron and removal of zinc within 30 min. This was due to its high carbon content (40 wt%) which can act as a reductant. Consequently, mixtures of BOF dust and goethite with BF dust were studied. It has been shown that mixtures of 30:70 BF dust to goethite and 20:80 BF dust to BOF dust are suitable for recovering zinc to the gas phase and fully reducing the contained iron. Graphical Abstract: [Figure not available: see fulltext.]

The HIsarna process is a new and breakthrough smelting reduction process for hot metal (liquid iron) production from iron ores and coal directly fed into the reactor. The flue gas from the main reactor enters the off-gas system containing small amounts of H₂, CO and carbon particles which need to be removed before further treatment by post combustion oxygen injection. A three-dimensional Computational Fluid Dynamics (CFD) simulation of the HIsarna off-gas system is performed and validated using a detailed reaction mechanism and kinetic data for post-combustion of a CO–H₂ mixture and carbon particles. Using the validated model, a series of simulations were performed to investigate the effect of water quenching and post combustion oxygen injection. It was found that water quenching can significantly reduce the off-gas temperature. It is also possible to reduce oxygen injection during operations where inlet CO content of the off-gas system is low.

Intricate knowledge of dislocation networks in metals has proven paramount in understanding the constitutive behaviour of these materials but current experimental methods yield limited information on the characteristics of these networks. Recently, the isotropic anelastic response of metals has been used to investigate complex dislocation networks through the well-known phenomenon that the observed elastic constants are influenced by dislocations. Considering the dependence of the behaviour of a Frank-Read (FR) source on its initial dislocation character and using discerning characteristics of dislocations, i.e. Burgers vector, line sense and slip system, the present paper takes dislocation character, crystal structure and dislocation network geometry into account and obtains the anisotropic mechanical response for a generic Poisson's ratio. In this work, the tensile test tangent moduli and yield points are presented for spatially uniform and nonuniform dislocation distributions across slip systems. First, the reversible shear strain of the FR source is derived as a function of initial dislocation character. The area swept by a mobile and initially straight dislocation segment pinned at both ends is given as an explicit function of the line stress. Secondly, the anisotropic anelastic strain contribution of FR sources to the total pre- and at-yield strain in single crystallites is calculated. For a given normal stress and superposition of the principal infinitesimal linear elastic lattice strain and anelastic dislocation strain, the tangent moduli are presented. The moduli and the inception of plastic flow have a notable dependence on initial dislocation character, spatial dislocation distribution and loading direction.

The circular economy demands waste utilization for the production of high-value products, and this requires the development of novel processing routes. In this study, rare earth (La and Ce) oxides were completely (>99%) recovered from polishing waste by a combined novel reductive acid leaching and alkali treatment process. About 70% of rare earths were dissolved during the first leaching step. The undissolved rare earth compounds are converted to oxides/hydroxides by alkali treatment and dissolved in the acid solution – the 2nd leaching step – for the complete recovery of rare earths. The recovered rare earth oxides were used for producing in-situ high-value Al-La-Ce alloys with fused salt electrolysis. Mechanical properties of our Al-La-Ce alloys are similar to the known high temperature Al-Ce alloys. This development of new alloys by our novel process helps in utilization of both overproduced primary La and Ce oxides as well as La and Ce recovered from polishing waste.

In-situ Neutron Diffraction and Small-Angle Neutron Scattering (SANS) are employed for the first time simultaneously in order to reveal the interaction between the austenite to ferrite phase transformation and the precipitation kinetics during isothermal annealing at 650 and at 700 °C in three steels with different vanadium (V) and carbon (C) concentrations. Austenite-to-ferrite phase transformation is observed in all three steels at both temperatures. The phase transformation is completed during a 10 h annealing treatment in all cases. The phase transformation is faster at 650 than at 700 °C for all alloys. Additions of vanadium and carbon to the steel composition cause a retardation of the phase transformation. The effect of each element is explained through its contribution to the Gibbs free energy dissipation. The austenite-to-ferrite phase transformation is found to initiate the vanadium carbide precipitation. Larger and fewer precipitates are detected at 700 than at 650 °C in all three steels, and a larger number density of precipitates is detected in the steel with higher concentrations of vanadium and carbon. After 10 h of annealing, the precipitated phase does not reach the equilibrium fraction as calculated by ThermoCalc. The external magnetic field applied during the experiments, necessary for the SANS measurements, causes a delay in the onset and time evolution of the austenite-to-ferrite phase transformation and consequently on the precipitation kinetics.