The phase-field method is used as a powerful and versatile computational method to simulate the microstructural evolution taking place during solid-state phase transformations in iron and steel. This review presents the basic theory of the phase-field method and reviews recent advances in the phase-field modeling and simulation of solid-state phase transformations in iron and steel, with particular attention being paid to the modeling of the austenite-to-ferrite, pearlitic, bainitic, and martensitic transformations. This review elucidates that the phase-field method is a promising computational approach to investigate the microstructural evolutions (e.g., interface migration, solute diffusion, and stress/strain evolutions) that take place during the phase transformations. It also indicates that further improvements are required to enhance the predictive accuracy of the phase-field models developed to date. Finally, this review discusses the critical challenges and perspectives for the further improvement of the phase-field modeling of solid-state phase transformations in steel, i.e., the modeling of heterogeneous nucleation, the abnormal effect of the diffusion interface, and material parameter identification.

Research on alloy phase diagrams started in the middle of the 19th century and progressed into the laborious and time-consuming process of constructing phase diagrams through experiments and phenomenological calculations with thermodynamic analysis. More recently, phase diagram research has evolved into the computation of theoretical phase diagrams based on first-principles calculations and the development of new materials for high-throughput data-driven types of phase diagrams. This paper discusses the features and problems of each technique using a collection of recent papers, with the aim of describing future problems in this field.

Nickel slag is a kind of solid waste with a high yield and low utilization rate. However, there is a large amount of Fe in nickel slag, which mainly exists in the form of fayalite. In this study, nickel slag is used as raw material. The addition of CaO can destroy the network structure of fayalite, and the iron-rich phase can be oxidized to magnetite under oxidation condition. It is beneficial to the recovery of iron resources. The effect of basicity on structural reconstruction of molten slag and precipitation of magnetite is investigated. The results show that when the basicity is 0.38–1.50, the degree of polymerization of silicate structure decreases with the increase of basicity. When the temperature is 1450–1500°C, viscosity of slag decreases first and then increases with the increase of basicity. The viscosity is the lowest with the basicity of 0.90, and the granular magnetite begins to precipitate during the non-equilibrium solidification at 1455°C. The growth rate of the magnetite is 1.20 µm/s at 0–10 s, which is significantly higher than the magnetite growth rate of 0.16 µm/s at 10–22 s, and the grain size of the magnetite remains unchanged after 22 s.

In order to develop a new recycling process of steelmaking slag, reduction of (Fe_tO) and (P₂O₅) in steelmaking slag at high temperature have been investigated. In this work, 50 kg-scale experiments which simulated rotary kiln were conducted to investigate the separation behavior between slag and metal. Main results are as follows. (1) Molten iron was tapped out from the experimental furnace under the condition that more than 86% iron was reduced. (2) Weight of reduced iron did not affect the undefined ratio of phosphorus. (3) Common logarithm of phosphorus distribution ratio (logL_P), which was used as an index to explain the effect of slag composition, temperature and oxygen potential, correlates to the undefined phosphorus ratio. (4) In the experiments which simulated rotary kiln treatment, calculated phosphorus distribution ratio (L_P) was smaller than that from experimental results. It can be said that the phosphorus transfer into the metallic phase decreased because the smaller interface between slag phase and metallic phase was obtained due to the slag/metal separation.

Controlling the formation behavior of inclusions during solidification is an effective approach to improve the purity of Ni-based superalloys. In this study, the effects of the microsegregation on the formation behavior of inclusions in the continuous unidirectional solidification (CUS) process were investigated. Three types of inclusions were found in revert and the CUS solidified K418 alloy: nitrides, oxides and SiC, with the nitrides as the dominant inclusions. With the application of the CUS process, the precipitation and growth behaviors of inclusions were significantly suppressed by the larger cooling rate, refinement of microstructure, and a reduction of microsegregation. Compared to the revert K418 alloy, the removal rates of the inclusion index and contents of N and O were 93, 84.1, and 57.7%, respectively in the CUS solidified K418 alloy at the withdrawal velocity of 27 mm/min. Numerical simulations based on a coupled microstructure-microsegregation-nucleation-growth model were carried out in order to verify the experimental results of the formation behaviors of inclusions during solidification. This study provided a theoretical and practical approach to the recovery of the revert K418 alloy.

In this study, high-temperature confocal microscopy (HTCM) was used to perform in situ observations of the dissolution of iron ore particles in slag at different temperatures. Moreover, the shrinking core model (SCM) is used to explain the kinetic mechanism of the dissolution of iron ore particles. The area of the undissolved fraction of iron ore particles was used to analyze the dissolution rate of iron ore particles. The results show that the kinetic mechanism of dissolution of iron ore particles can be well explained by the SCM. The dissolution of iron ore particles is controlled by the diffusion of iron ore fractions in the boundary layer. The dissolution temperature, the concentration difference of iron ore fraction at the two ends of the boundary layer, and the initial particle size of iron ore particles affect the dissolution rate of iron ore particles. The dissolution rate constant was fitted by introducing a dissolution mechanism. The activation energy of iron ore dissolved in the CaO–SiO₂–MgO–Al₂O₃–FeO slag system is 679.13 kJ/mol. The equation for the dissolution rate constant of iron ore in HIsmelt slag is summarized.

The pre-reduction sintering process is an effective measure to fundamentally solve the detrimental circulating accumulation of phosphorus in iron- and steel-making procedures. The effect of the carbon mixing ratio on the mineral evolution, gasification dephosphorization and pre-reduction degree of sintering products was primarily investigated based on the phosphorus occurrence status in Bayan Obo iron ore concentrate. The obtained results indicated that iron oxide was dominantly transformed from magnetite into wüstite, and metallic iron was basically not detected. Regarding silicate slag, the precipitated dendritic phase accounted for a higher content of FeO and MgO and a lower content of Na₂O + K₂O and SiO₂ over the vitreous matrix phase. Moreover, as the carbon mixing ratio was increased from 5 to 25 wt%, the gasification dephosphorization and pre-reduction degree first increased from 11.3% and 30.7%, respectively, to 70.0% and 67.4%, respectively, due to the enhanced reducing atmosphere and then decreased to 45.0% and 59.0%, respectively, led by the increased content of liquid slag and the reduced permeability of the burden layer. Both maximum values were obtained at a 20 wt% carbon mixing ratio. Furthermore, the pre-reduction sintering products as raw materials for blast furnaces could not only largely reduce the phosphorus content in hot metal but could also dramatically decrease coke usage and carbon dioxide emissions, which could be highly beneficial to realize carbon peaking and carbon neutrality.

The pellet making method was studied for production of iron ore pellets with a carbon core. The pellets are to be fired in a sintering machine to produce sintered ores containing carbonaceous material. This new raw material for the blast furnace will have advantages such as high reducibility and low slag generation. In this study, pellet making tests were performed using laboratory and pilot scale disc pelletizers with 3 to 5 mm coke breeze, concentrated iron ores and additives in order to construct a method for producing pellets with a carbon core. Scaling-up of the method was considered with four discs with diameters of 0.6, 1.2, 2.0 and 3.5 m. As a result, it was found that there were two strategies for producing pellets with a carbon core at high yield: supplying a suitable level of water with a small droplet size, and spraying water in a specific area where the coke breeze flows on the surface of the contents in the disc. This study demonstrated that iron ore pellets with a carbon core can be obtained at a yield of more than 90% by using the disc pelletizer with the 3.5 m diameter. It was also estimated that a production capacity of 70 t/h can be expected with a 7.5 m diameter disc for commercial scale operation. This achievement of constructing a method for producing pellets with a carbon core was a breakthrough which overcame one of the technical hurdles for the development of sintered ores containing carbonaceous material.

Developing the innovative technology for efficient utilization of high-phosphate iron ore was the inevitable choice of resource strategy. In this paper, exploring a new method of gasification removal of phosphorus by iron ore sintering technology based on biomass. Combined with theoretical analysis and experimental research, the effects of different conditions (biochar type, addition proportion, basicity) on the reduction and gasification behavior of phosphorus were studied. The results showed that the addition of biochar instead of part of the coke powder could promote the reduction of apatite and increase the dephosphorization rate compared with the full coke breeze. Among different types of biochar fuels (fruit shell charcoal, straw charcoal, bamboo charcoal), bamboo carbon had the greatest effect on the dephosphorization rate. The optimal reduction temperature was 1100°C, substitution proportion of bamboo charcoal was 20% and basicity was 0.5 for the phosphorus gasification and removal, and the corresponding dephosphorization rate was 28.77%. Selective reduction of iron and phosphorus existed in the reduction process, iron oxide took precedence over the reduction of apatite, and part of phosphorus gas entered the metal iron to form iron phosphide, resulting in the decrease of dephosphorization rate.

In this work, the Planar Laser-Induced Fluorescence (PLIF) technique is applied to study the effect of the number of nozzles, their radial position and the gas flow rate on the thermal mixing during the secondary steel refining under non-isothermal conditions in a water model. In the physical model, three burners have been used to simulate the arc heating during the liquid metal processing.

The results suggest that an increase in the gas flow rate and injection nozzles located at the bottom of the model, in the same radial and angular positions as the heat sources affecting the liquid surface, improve the system’s heating efficiency.

The use of PLIF proved to be a relevant tool to analyze the heat transfer phenomena and increase the thermal performance of the ladle, opening the possibility of carrying out experiments in non-isothermal physical models of steelmaking reactors, which are currently scarce.

A three-dimensional (3D) model has been developed based on the Eulerian multiphase flow approach to investigate the fluid flow behavior and mixing efficiency in the multi-tuyere AOD process. The interphase forces, including drag force, lift force, virtual force, turbulent dispersion force, and wall lubrication force, were incorporated into this model. The model was used to simulate six-tuyere and seven-tuyere AOD processes. The phenomena of multi-jet penetration, bubble plume merging, 3D turbulent flow and mixing characteristics were considered. The results indicate that the bubble plume merging occurs in the upper part of the liquid bath, forming a typical plume cluster. The predicted penetration length for a single tuyere jet agrees well with the previous work. For the multi-jet system, the side jets penetrate deeper than the inside ones. The six-tuyere AOD has a good flow condition in the center of the liquid bath, while the seven-tuyere AOD has a better flow pattern in the sidewall region and the lower bath. Overall, the seven-tuyere AOD performs better in mixing efficiency than the six-tuyere AOD under the same gas flow rate. These findings increase the understanding of the AOD process, allowing further optimization of process parameters. This model can be further extended to incorporate the thermochemical reactions into the modeling of the AOD reactor.

Although there have been studies on models for predicting the mixing concentration of elements between old and new molten steel with different chemical components during continuous casting, there is limited information about the concentration of the mixed portion in the cross-sectional and the longitudinal directions of the intermixed bloom. In the present study, the concentration of the intermixed bloom generated by the different-grade continuous casting is measured in the cross-sectional and the longitudinal directions. A significant difference in the concentration of the mixed portion is noticed at different heights along the thickness direction of the intermixed bloom. Based on the measurement results, we propose a new mixing prediction model that can accurately predict the concentration of the mixed portion at the subsurface and the center in the intermixed bloom along the longitudinal direction. The present results show that each height which has the minimum and the maximum concentrations of the mixed portion in the cross-section varies along the longitudinal direction of the intermixed bloom. This implies that the maximum and the minimum concentrations of the mixed portion in the cross-section do not necessarily occur only at the surface and the center of the intermixed bloom. The newly proposed mixing prediction model can be employed to cut off the mixed portion by predicting the minimum and maximum concentrations of the mixed portion in the cross-section along the length direction of the intermixed bloom.

Estimation of changes in the composition and physical properties of mold flux during continuous casting was investigated for control of slab surface quality. In this study, 0.7 mass% Al steel and normal Al-killed steel were cast with three kinds of mold flux having Al₂O₃ contents of 1.3 to 6.0 mass% and different basicities. The results can be summarized as follows:

(1) The Al₂O₃ content of these mold fluxes increased to 30 mass% during continuous casting. The composition change of the mold flux can be reproduced by the Equilibrium Effective Reaction Zone Model (EERZM) by fitting parameters, referring to the casting results.

(2) The analysis by EERZM revealed that the viscosity of the mold flux and the throughput of the molten steel affect the rate of increase of the Al₂O₃ content in the mold flux.

(3) The change of the physical properties of the mold flux was estimated based on the changes in the flux composition. The change of the crystallization temperature and main crystal can be estimated by FactSage.

(4) Mold flux viscosity can be estimated by revising the modified Iida’s equation, which considers the effect of Al₂O₃ as an amphoteric oxide.

This research proposes an efficient strip steel surface defect classification model (ASNet) based on convolutional neural network (CNN), which can run in real time on commonly used serial computing platforms. We only used a very shallow CNN structure to extract features of the defect images, and an attention layer which makes the model ignore some irrelevant noise and obtain an effective description of the defects is designed. In addition, a nonlinear perceptron is added to the top of the model to recognize defects based on the extracted features. On the strip steel surface defect image dataset NEU-CLS, our model achieves an average classification accuracy of 99.9%, while the number of parameters of the model is only 0.041 M and the computational complexity of the model is 98.1 M FLOPs. It can meet the requirements of real-time operation and large-scale deployment on a common serial computing platform with high recognition accuracy.

A VUV-SPI-TOFMS system was developed for the continuous analysis of pyrolysis gases generated during coal carbonization and aromatic molecules were detected using high-resolution quantitative techniques. Although IR spectroscopy is suitable for detecting low-molecular-weight gases present in dry distillation gas, such as CH₄ and CO₂, it is not sensitive to aromatic hydrocarbon gases. On the other hand, the VUV-SPI-TOFMS technique is applicable for the quantification of low-concentration gases at the ppb concentration level, including aromatic hydrocarbons, generated during coal carbonization, which entails continuous heating from room temperature to 800°C. Benzene and toluene were predominantly detected at 540–590°C in the dry distillation gas of bituminous coal having relatively low oxygen concentration, whereas gases containing OH groups, such as phenol and cresol, are predominantly generated from sub-bituminous coal and lignite with high oxygen concentrations. Solid ¹³C NMR spectra obtained for each natural coal sample exhibited substantial proportions of oxygen bound to aromatic carbons (aromatic–O) in young coals. The temperature range at which cyclohexane was generated was found to be lower than that of aromatic molecules, indicating that coals releasing it may exhibit a structure that is more susceptible to thermal decomposition.

The bainitic transformation kinetics and carbon enrichment of austenite during isothermal holding at 723–923 K were investigated for an Fe-0.1mass%C-0.5mass%Si-2.0mass%Mn alloy. The transformation progressed rapidly until approximately 50 s, after which transformation stasis was observed at 823 K. The carbon concentration of austenite increased as the transformation proceeded, and showed an almost constant value during stasis. It reached approximately 0.45–0.50% at 823 K, which corresponds to the carbon concentration at the T₀’ composition with an additional strain energy of 100 J/mol associated with the transformation. After stasis, a slight increase in the ferrite or bainitic ferrite fraction was observed. The carbon concentration of austenite also increased and reached approximately 0.60%, clearly exceeding the carbon concentration at the T₀ composition. These results imply that at the first stage, the bainite transformation occurs and shows the incomplete transformation, following which at the second stage, diffusional ferrite transformation proceeds. The additional strain energy associated with the transformation calculated from the carbon concentration at stasis due to the incomplete bainite transformation tends to decrease as the holding temperature increases. This indicates that strain relaxation due to the transformation occurred at higher holding temperatures.

Coarse grained phase morphologies of iron-rich region of FeSi alloys at 1050 K are investigated by using first-principles phase field and special quasirandom structure methods without relying on any experimental or empirical information. From the free energy comparison, we find that, for the Si concentration less than 25 at%, a solid-solution-like homogeneous phase is most stable, although a random pattern in nm scale consisting of B2 Fe_4–xSi_x and D0₃ Fe₃Si phases may appear at 12.5 at% Si at somewhat lower temperatures. We make a conjecture that, around 12.5 at% Si, such a random pattern in nm scale is the origin of the zero magnetostriction and low magnetic anisotropy. This solves a long-standing problem of the experimentally observed zero magnetostriction at 6.5 wt% Si. On the other hand, for the Si concentration slightly larger than 25 at%, FeSi alloys prefer two-phase coexistence of the D0₃ Fe₃Si phase and the B2 FeSi phase. All these findings are in good accordance with the available experimental evidence.

The changes in the state of carbon in tempered martensite and retained austenite in carburized SAE4320 steel under the rolling contact fatigue (RCF) were investigated using atom probe tomography (APT). In the tempered martensite, the carbons in solid solution and in carbon cluster were readily transferred to the preexisting metastable (ε) carbide due to rolling contact, resulting in a localized change from tempered martensite to ferrite accompanied by the growth of carbides. This supports the recently proposed dislocation assisted carbon migration theory. On the other hand, retained austenite with uniformly distributed enriched solute carbon was partially transformed into the very fine deformation-induced martensite due to rolling contact. Furthermore, carbon seemed to be partitioned into retained austenite from the deformation-induced martensite during further rolling contact cycles. This is a new insight into the characteristics of deformation-induced martensite and retained austenite generated by rolling contact. The present study provides a plausible explanation to the phenomenon that the deformation-induced martensitic transformation improves the RCF life.

The relationship between hardness and solute carbon concentration estimated via electrical resistivity measurement was investigated in as-quenched and tempered martensitic steels containing carbon of 0.3–0.6 mass%. As a result of corelating the amount of hardening due to carbon in solid solution with the solute carbon concentration, by the calculation to subtract precipitation strengthening, grain refinement strengthening, dislocation strengthening, and softening due to retained austenite from the total strengthening, we derived an equation of solid solution strengthening, where the hardening increases proportionally to the 1/2 or 2/3 power of the solute carbon concentration. It was confirmed that the effects of the factors other than solid solution strengthening due to carbon on hardness are relatively small in tempered specimens when the tempering temperature is less than 673 K; therefore, the change in hardness in tempered martensitic steels can be mostly explained by solute carbon concentration regardless of carbon content.

Calcium looping (CaL) utilizing CaO-based adsorbent has been studied to reduce carbon dioxide emissions (CO₂). Synthesis of the CaO-based adsorbents from waste slags has captured the interest of the iron and steel industry, which is dealing with intensive amounts of waste slag produced. The drawback of using CaO-based adsorbents is their low regenerative ability during cyclic CO₂ adsorption. In this study, aiming to synthesize a CaO-based adsorbent with better cyclic stability, we used desiliconization slag as a raw material, which is produced during the steel purification process by minimizing the silicon concentration. We synthesized a CaO and mesoporous silica (CaO-MS) composite from desiliconization slag using P123 as an organic template and several organic acids, including formic acid (FA), acetic acid (AA), and citric acid (CA) as dissolution agents. The structure and performance of the adsorbents were investigated using X-ray diffraction analysis (XRD), N₂ adsorption-desorption, transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and thermogravimetric analysis (TG). Compared to the samples synthesized with other organic acids, the slag-derived adsorbent synthesized with acetic acid (DSslag-CaO-MS(AA)) displayed the optimum CO₂ adsorption capacity with 21.0 wt% per mass of adsorbent and the highest stability. The findings demonstrated that the mesoporous structure enhanced the CO₂ adsorption and acetic acid is the best dissolution agent in synthesizing CaO-MS adsorbent by separating the crystalline CaO phase and SiO₂ phase. Environmentally benign and economically viable CaO-based adsorbents synthesized from desiliconization slag can be used for CO₂ capture, particularly in the iron and steel industry.

Stainless steel dust is a type of high basicity iron containing solid waste from the iron and steel smelting industry. In this study, the carbon-containing composite briquettes were prepared by adding low basicity laterite nickel ore and stainless steel dust, and the valuable metals Fe, Cr, and Ni were recovered by carbothermal reduction. The results show that the addition of laterite nickel ore into CBSL (carbon containing composite briquette of stainless steel dust and laterite nickel ore) effectively improves the recovery of metal Fe, Cr and Ni and the self-pulverization rate of reduction slag. The reduction temperature is 1450°C, the reduction time is 30 min, the FC/O (molar ratio of fixed carbon to oxygen) is 1.0, and the laterite nickel ore addition is 6%, the recoveries of metal Fe, Cr and Ni are 92.1%, 90.1% and 91.6%, respectively, and the self-pulverization rate of the reduction slag reaches 91.5%. The Ca₂SiO₄ phase content of the reduced slag at high temperature is 71.8%, and the reduced slag system is located in the orthosilicate Ca₂SiO₄ region of the phase diagram, which effectively improves metal recovery, enables efficient separation of the reduced product and reduces energy consumption for subsequent processing.

Previous studies on the mathematical modelling of fluid flow-heat transfer phenomena in continuous casting, reported in this journal and elsewhere, are examined and, analyzed computationally. It is shown that mathematical model of continuous casting based on a constant, in lieu of a time varying inlet temperature, can produce different estimates of temperature field and solidified shell thickness in the mold. In contrast, thermal free convection phenomenon is found to be unimportant to the precise modelling of heat transfer in the mold. It is argued that a transient, rather than a steady state formulation, should be the preferred choice as the former ensures easy integration of additional process features as well as speedy convergence.