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The precipitation and re-austenitization behavior of α-ferrite during thermal rebound process of casting slab are crucial to the austenite grain size distribution. Accurately obtaining austenite grain size distribution plays an important guiding role for avoiding the formation of slab cracks and effectively improving the quality of steel. In this work, a quantitative model to calculate α-ferrite precipitation at different temperatures has been constructed by employing high-temperature expansion test and peak area method. On this basis, a prediction model for the austenite grain size distribution of casting slab in the thermal rebound process has been theoretically derived with consideration of the α-ferrite precipitation and re-austenitization. Then, the accuracy of the quantitative model for α-ferrite precipitation and the prediction model for austenite grain size distribution have been evaluated by high-temperature quenching experiments. The results show that the maximum error between the calculation values of the prediction model and the experimental values is only 3.67 %, which can reliably predict the austenite grain size distribution of slab under different phase fractions of precipitated α-ferrite and initial austenite grain sizes. When the phase fraction of precipitated α-ferrite reaches about 70 %, the average grain size after re-austenitization comes to nearly 50 % of the initial size, which achieves the best refining effect. The reduction in initial austenite grain size is beneficial to the refinement of austenite grain after thermal rebound. Reasonable control of thermal rebound temperature and multiple cycles of phase transformation are effective means of obtaining austenite microstructure with smaller average grain size and more uniform distribution.

Due to the complex cross-section and scale effects, surface and internal defects are prone to occur in the ultra-large-section beam blank. In this study, a coupled model of steel liquid flow and heat transfer within a 1300 × 510 × 140 mm cross-section beam blank mould was established. The results indicate that single-port SEN reduced the flow activity on the meniscus, and the three-ports SEN led to significant high temperature zones and re-melting of the solidified shell in the flange tip and web. The four-ports nozzle not only optimizes the internal flow behavior of the mould but also enhances the uniformity of temperature and solidification. The maximum circumferential difference in shell thickness under the four-ports SEN was only 8.1 mm, compared to 13.2 mm and 13.8 mm under the single-port and three-ports SEN, respectively.

Ni–Co–B coating was electrodeposited in sulfate bath with dimethylamine borane (DMAB) as reducing agent. The experiment explored the effect of DMAB concentration (CDMAB, 1–10 g/L), current density (Jk, 2–8 A/dm2), bath temperature (Tbath, 30–60 °C) on the coating morphology and hardness, and determined the best parameters: CDMAB = 8 g/L, Tbath = 60 °C, Jk = 4 A/dm2. The average hardness of the crack-free coating is 818 HV, and the B content is about 2 wt%. In order to develop a stress-relieving heat treatment scheme, the effects of heat treatment temperatures (100–500 °C) and holding times (1.5 h and 3 h) on the hardness of Ni–Co–B coating were studied. The highest hardness (1130 HV) is obtained at 350 °C for 1.5 h. The heat-treated Ni–Co–B coating was characterized by XRD, SEM and EPMA, and the second phase Ni3B was detected. In addition, the wear resistance and wear mechanism of the coating at room temperature and high temperature (300 °C) were studied. The results show that the wear volume of Ni–Co–B coating is reduced by more than 55% compared with Ni–Co coating. In general, Ni–Co–B coating can be used as an alternative to Ni–Co wear-resistant coating.

The hydrogen concentration in steel is directly related to the hydrogen embrittlement (HE) sensitivity of the steel. This study combined electrochemical hydrogen charging, the slow strain rate test (SSRT), and hydrogen permeation experiments to investigate the variation in the hydrogen concentration in pipeline steel with the electrochemical hydrogen-charging time. The influence of the hydrogen concentration in steel on the mechanical properties of X80 pipeline steel was obtained, and ultimately, a quantitative relationship between the hydrogen concentration in steel and the hydrogen embrittlement sensitivity was established. The results show that the hydrogen concentration in the steel gradually increased with the time of hydrogen charging, and the quantitative relationship formula can be given as CH = 5.35 − 4.2 exp (−0.26t); the HE index of X80 steel increased with the hydrogen concentration. Additionally, once the hydrogen concentration in steel reaches 5.08 × 10−6 mol/cm3, even the slightest alteration in the hydrogen content will precipitate a dramatic decrease in plasticity. The quantitative relationship formula between the hydrogen concentration and the HE index (FH) in X80 steel can be given as FH=0.029 exp (1.5CH) − 0.029. When the hydrogen concentration in steel is at a maximum, the FH of X80 steel reaches 88.6%. This study provides a reference for analyzing the quantitative relationship between the hydrogen concentration and the HE index in steel after electrochemical hydrogen charging.

The introduction of solute atoms has a critical effect on the mechanical properties of nickel-cobalt alloys. In the present study, we first prepare the Ni3Co alloy coating by electrodeposition and perform structural characterization. The results show that the alloy has a single-phase FCC structure with no other distinct characteristic peaks. On this basis, we established the L12-Ni3Co crystal model, and detailed analysis of the mechanism of varying nonmetallic atoms (C, B, N, P) on the elastic properties of the alloy by first-principle calculation. The results of mechanical and thermal performance parameters such as elastic modulus, hardness and Debye temperature show that compared with the other three elements, the strengthening effect of N atom is the best, especially when in the interstitial site, the strengthening effect of smaller non-metallic atoms is more significant. Moreover, the doped systems are both elastic and sound velocity anisotropic, and the strength of the alloy is approximately positively correlated with the degree of anisotropy. The analysis of the electronic structure shows that the introduction of interstitial atoms leads to a transformation of the local bonding properties, and the strengthening effect can be attributed to the strong covalent interaction between them and the Ni or Co atoms.

Exploring the mechanism of the α-ferrite precipitation process on high-temperature properties plays an important guiding role in avoiding slab cracks and effectively regulating quality. In this work, in situ observation of the α-ferrite sustained precipitation behavior for peritectic steel during the austenitic phase transition process has been investigated using high-temperature confocal scanning laser microscopy. Meanwhile, the high-temperature evolution of the phase fractions during the phase transition process was quantitatively analyzed based on the high-temperature expansion experiment using the peak separation method. Furthermore, the high-temperature properties variations of the casting slab during the α-ferrite sustained precipitation process were investigated with the Gleeble thermomechanical simulator. The results show that the film-like ferrite precipitated along the austenite grain boundaries at the initial stage of phase transition, then needle-like ferrite initiates rapid precipitation on film-like ferrite when the average thickness reaches 15~20 μm. Hot ductility reached a minimum at the ferrite phase fraction fα = 10~15%, while high-temperature properties returned to a higher level after fα > 40~45%. The appearance of a considerable amount of needle-like ferrite and grain refinement effectively improves the high-temperature properties with the α-ferrite precipitation process advances.

Accurate prediction of the crack sensitive temperature region and phase fractions variation of slabs during continuous cooling is an important guide to avoid cracks and effectively control the quality. Based on finite number of measurements, at different cooling rates of the continuous casting process, a prediction model for characteristic temperatures of austenite decomposition, the variation of phase fractions with temperature, the crack sensitive temperature regions, and the final microstructural compositions of casting slabs at different cooling rates has been established and evaluated the accuracy. The results show that austenite decomposition temperature range moves toward the low temperature region as cooling rate increases, and the independent peak of ferrite transition become weaker. The characteristic temperatures of austenite decomposition can be quantitatively calculated by TC(CR) = A−exp(B + C/CR) at different cooling rates, which the maximum relative error for experimental steels is −2.2%. The ferrite and pearlite phase fractions increases with decreasing temperature during continuous casting cooling, which means that the ability of the billet to resist deformation and external force changes. Meanwhile, the final ferrite content of slabs for Steel B and Steel C at different cooling rates are 83.24620−exp(2.59364–13.72283/CR) and 85.07143−exp(1.71320–15.82244/CR), respectively. The crack sensitive temperature region Ae3 ∼ Tα40%(CR) calculated by the prediction model is in good agreement with the low ductility zone measured by experiment. Moreover, the critical temperatures Tα40%(CR) of the crack sensitive temperature regions are 890.35731−exp(2.99719–20.67781/CR), 745.87462−exp(4.83056–44.18511/CR) and 729.46168−exp(2.96621–12.21949/CR) for three experimental steels under different cooling rates.

The oxidation kinetics, oxide layers composition and thicknesses of X70 steel, Y55 steel and 304 stainless steel are investigated. The results show that the oxidation rate of X70 steel increases rapidly at 1000 °C, while that of Y55 steel and 304 stainless steel increases rapidly at 1100 °C. The oxidation kinetic coefficients of X70 steel, Y55 steel and 304 stainless steel are exp (10.7), exp (11.58) and exp (30.15), respectively, and the apparent activation energy of oxide layer are 196879.7 J/mol, 207608.1 J/mol and 469284.3 J/mol, respectively. The cross-sectional morphology and element distribution of oxide layer are obtained. With the oxidation time increases from 30min to 80 min at 1100 °C, the oxide layer thickness of X70 steel and Y55 steel increases by 1242.33 μm and 965.59 μm respectively when the weight of steels increases by 1 mg/mm2, and the oxide layer of Y55 steel is denser than that of X70 steel. A variable temperature oxidation mathematical model of the slab surface is established to predict the oxide layer thickness of slab surface.

Mold structure and cooling parameters are significant factors that affect the heat transfer capacity of high-speed continuous casting molds of billets. Therefore, a three-dimensional fluid flow and heat transfer model of a 160 mm × 160 mm billet mold was established, and its accuracy was verified. Thereby, the characteristics of heat transfer and influences of mold structure and cooling parameters on heat transfer in the high-speed continuous casting billet mold region were revealed. It was found that extending the effective length of a mold is the most valuable method to improve its heat transfer capability and achieve high-speed continuous casting. The total heat and the shell thickness at a mold outlet increased by 19% and 9.21% on average with every 100 mm extension. Enlarging the fillet radius could enhance the uniformity of heat transfer in the mold. Considering the loss of material, the optimal fillet radius of the mold was determined to be R = 10 mm.

In continuous casting, it is common to close single or multiple submerged nozzles of multi-strands tundish to adapt to production rhythm due to insufficient liquid steel or equipment failure. However, the closure of the nozzle will change the flow field in the tundish and further affect the removal efficiency of inclusions in the tundish. For this reason, based on numerical simulation, the flow behavior of liquid steel and the removal of inclusion in tundish with different nozzle closed were studied, and the optimal nozzle closing scheme was obtained, which provided a basis for the selection of nozzle closing in tundish. At the same time, the gas curtain is set in the tundish to alleviate the negative effects such as the increase of dead zone caused by closing nozzle. The results show that the removal rate of inclusions with sizes 10, 30, 50, 70, and 90 μm change from 12.4%, 39.1%, 74.2%, 93.3%, and 95.6% to 14.7%, 36.4%, 76.4%, 85.3%, and 93.8%, respectively. The volume of the tundish dead zone is increased after closure of nozzle, the dead zone of the tundish is improved when the gas is installed, and the dead zone volume was reduced from 14.8% and 16.4% to 13.9 and 14.1%.

The accurate prediction of mechanical properties and phase microstructures is of great significance to the design and development of the ultra-high strength hot stamping steel (UHSHSS). Based on the prediction of JMatPro and the experimental measurement, an accurate modified prediction model of JMatPro for the tensile strength of UHSHSS was proposed in the study, considering the influence of austenitizing temperature. At the same time, the prediction model of phase transition in quenching process of UHSHSS was established. The tensile strength and phase transition models of UHSHSS under different conditions were studied using the prediction models. Results showed that the tensile strength of UHSHSS increased with the increase of C content and decreased with the increase of Si content. For every 0.01 wt% increase in C content, the tensile strength increased by around 30 MPa. For the hot stamping process used in this study, the carbon content of hot stamping steel should be more than 0.35 wt% to achieve 2000 MPa in tensile strength. The volume fraction of the residual austenite in UHSHSS increased with the increase of C and Si content. During the quenching process, the phase transition in UHSHSS was the fastest between 4 s and 8 s, and the average decomposition rate of the austenite reached 17.8% per second. The average volume fraction of residual austenite and martensite in UHSHSS was 3.3% and 96.7%, respectively, when the quenching die temperature was 150 °C. With the increase of the die temperature, the volume fraction of residual austenite in steel increased.

The thicknesses of mold flux film and air gap are significant factors that affect the high-efficiency heat transfer, the strand lubrication and mold taper design of billet ultra-high speed continuous casting mold. Therefore, this paper established the three-dimensional fluid flow, heat transfer and solidification model, interfacial heat transfer model and two-dimensional stress-strain model to conduct multiphysics modeling. Thereby the thickness distributions of liquid slag, solid slag and air gap in the ultra-high speed billet continuous casting mold were obtained, and analyzing the effects of melting temperature of mold flux and mold taper. The results indicate that the thicknesses of liquid slag and solid slag increase and decrease respectively along the casting direction, and air gap mainly concentrates near the mold corner. The maximum thicknesses of liquid slag, air gap, and solid slag at the mold outlet are respectively 0.18 mm at the center of the strand surface (x = 0 mm), 0.28 mm at the strand corner (x = 80 mm) and 0.67 mm at x = 74 mm. The lower melting temperature of mold flux, the greater the liquid slag thicknesses and ascend from 0.14 to 0.18 mm, and conversely the maximum air gap thicknesses descend from 0.31 to 0.28 mm and existing ranges also get smaller, which is more favorable for the strand lubrication. To eliminate the air gap, the appropriate linear mold taper is 0.45% m−1 at the 6.5 m/min in casting speed.

Ultra-high casting speed is an important development feature for efficient continuous casting of billet. A mathematical model which addresses the high flow rate of molten steel in the billet mold is presented, coupling slag flow with heat flux and solidification. The model is then applied to investigate the correlation between transient steel flow, slag infiltration and heat transfer in the mold-shell gap during ultra-high casting speed. The results show that the meniscus level experiences a hump near the mold wall and a valley near the nozzle as a result of the recirculating flow in the upper mold. The liquid slag around the mold wall infiltrates preferentially into the gap to be consumed quickly, whereas those near the SEN flows to the mold with a low speed. Severe fluctuations in meniscus level will disturb the slag infiltration, which in turn causes the formation of uneven slag films, leading to intensified heat flux fluctuations in the mold-shell gap, especially in the area of 20−60 mm from the mold corner.

The lubricating property of mold slag is significant to determine the surface quality of strands. To investigate the friction property of mold slag in the mold, the friction coefficient of the crystalline slag and glassy slag at high temperatures was measured through using a friction and wear testing machine. An edible oil was selected to simulate the lubrication behavior of liquid slag. The results show that during raising temperature from 25 °C, the friction coefficient of slag will increase due to the surface softening. The surface sintering will obviously reduce the friction coefficient of crystalline slag at around 800 °C. The friction coefficient of glassy slag is smaller than that of crystalline slag. The equivalent friction coefficient of liquid slag is below 0.2. The measured friction coefficient can be applied in mathematical models of friction.

A modified coupling model, considering microsegregation, precipitation and growth of inclusion, and variable solute partition coefficient (ki), was established to investigate the precipitation and coarsening of TiN inclusion for the 0.053 wt% Ti-microalloyed steel. The influence of TiN inclusion on the steel impact toughness was revealed. Results show that the effect of phase composition on both the kTi and kN is greater than the effect of temperature, and the kTi and kN are in the range from 0.20 to 0.30 and from 0.28 to 0.45 during solidification, respectively. The change of the ki in different crystalline phases significantly affects the microsegregation and TiN precipitation. The TiN precipitation is promoted by segregation and the TiN starts to precipitate around 1754 K (corresponding to fs = 0.82) in the mushy zone. The predicted size of TiN inclusion is between 7.57 and 16.68 μm at a conventional cooling rate (1–5 K/s), which is consistent with the measured results in the slab. The square or triangle TiN inclusions with large size are harmful to the impact toughness. Many TiN inclusions can be found at the junction of the cleavage cracks. To control the TiN size and ensure the impact toughness, the cooling rate for the Ti-0.053 wt% steel solidification should be enhanced and greater than 10 K/s. Furthermore, reducing the Ti content to 0.03 wt% is a more effective way to control the TiN size.

The solute partition coefficient (ki), characterizing the solute redistribution on the solid-liquid interface, is an important parameter for the study of segregation. Based on thermodynamic equilibrium, the effects of composition fluctuation, phase transition, MnS precipitation, and temperature on the ki for the solidification of 1215 high-sulfur steel were investigated by orthogonal design. The results show that there is only L→δ phase transition for 0.05–0.07 wt% C steel solidification, while there are L→δ and L+δ→γ (peritectic reaction) phase transition in turn for the solidification of steel with 0.07–0.09 wt% C. On the L/δ interface, as the temperature decreases, the kSi and kP increase, the kMn decreases, while the kC and kS both decrease first and then increase because of MnS precipitation. In the L+δ+γ coexistent phase, the kiL/δ+γ is comprehensively determined by kiL/δ and kiL/γ. Moreover, the kiL/δ+γ for each solute is closer to the kiL/δ comparing to kiL/γ, because the mass fraction of γ phase (mγ) in the solidified phase (consisting of δ+γ) is relatively small. The peritectic reaction has small effect on the kiL/δ and kiL/γ, but has significant effect on the mγ, leading to the discrepancy in kiL/δ+γ for the steels with different C contents. The orthogonal analysis indicates that the composition fluctuation in 1215 steel has small effect on the ki. The Mn and S contents have the greatest effect, followed by C content. Si and P contents have the least effect. The quantitative ki in terms of temperature, phase composition, and solute content were fitted. Keywords: Solute partition coefficient, Thermodynamic, Orthogonal design, High-sulfur steel, Solidification, Segregation

The interface bonding strength is one of the most important properties of coatings, which is significantly affected by the interface microstructure. By electron backscatter diffraction technology, it was determined that the grain orientation relationship of coating and matrix at interface is dominated by Cu 〈101〉/NiCo 〈101〉, some of Cu 〈101〉/NiCo 〈101〉 grains contain a certain twist angle, range of 0 to 63.5°. On this basis, using the first principles calculation to predict the Cr, Os, and Ir doping on the interface bonding strength and the tensile failure mechanism of NiCo coating. Heat of segregation shows that the doping elements have no tendency to diffuse to the matrix and interface. Work of separation reveals that the Cu(101)/NiCo(101) interface has good bonding strength (3.758 eV), stronger than Cu (2.203 eV), weaker than NiCo (4.860 eV) and the Cr doping at Co2 site and Os doping at Ni3 site can improve the interface bonding strength. The ideal tensile test reveals that Cr and Os doping can increase the max tensile strength of NiCo coating (along the 〈101〉 direction), and Cr doping increased by 34%. The analysis of atomic position and bond length shows that the tensile failure occurs inside the Cu atom layers. By the electron properties, the strengthening effect is attributed to the enhanced covalent properties between the Cr-Ni or Co atoms. The electron location function and charge density map intuitively reflect the origin of crack of interface structure in the stretching process. In summary, the Cr and Os doping not only strengthen the tensile strength of NiCo coating itself, but also improve the interface bonding strength, which is worthy of further study.

Clarifying the influence of Nb and V microalloying on the ultra-high strength hot stamping steel (UHSHSS) and exploring appropriate process parameters are the basis for effectively regulating properties of the final product. In this study, the effects of different austenitizing temperatures and holding times on the phase transitions, grain sizes and mechanical properties of 22MnB5NbV with Nb and V alloyed are studied by using JMatPro thermodynamic calculations and experiments. By comparing with 22MnB5 without Nb and V alloyed, the effects of Nb and V elements on quenching microstructures, grain sizes and mechanical properties of UHSHSS are analyzed. The suitable austenitizing process parameters of 22MnB5NbV have been obtained. The results show that the grain size of Nb-V-alloyed UHSHSS grows with the increase in the austenitizing temperature and holding time. The 22MnB5NbV steel can be completely austenitized while the austenitizing temperatures ≥870 °C and holding time ≥3 min. Combined with the actual production process, the best austenitizing temperature and holding time are 930 °C and 3 min. Under these conditions, the average grain size is 7.7 μm, the tensile strength, elongation and strength-ductility product are 1570.8 MPa, 6.6% and 10.4 GPa·%, respectively. With the addition of Nb and V elements, the nanoscale precipitates lead to the refinement of the quenched structure and grain size, and the comprehensive properties of UHSHSS have been effectively promoted, in which the elongation and strong-plastic products are increased by ~0.6% and ~1.2 GPa·%, respectively.

As an effective flux, CaF2 is beneficial in improving the fluidity of slag in the steel-making process, which is crucial for dephosphorization. To reveal the existence form and functional mechanism of CaF2 in phosphosilicate systems, the microstructures and transport properties of CaO-SiO2-CaF2-P2O5 quaternary slag systems are investigated by molecular dynamics simulations (MD) combined with experiments. The results demonstrate that the Si-O coordination number does not vary significantly with the increasing CaF2 content, but the P-O coordination number dramatically decreases. CaF2 has a minor effect on the single [SiO4] but makes the structure of the silicate system simple. On the contrary, F− ions could reduce the stability of P-O bonds and promoted the transformation of [PO4] to [PO3F], which is beneficial for making the P element-enriched phosphate network structure more aggregated. However, the introduction of CaF2 does not alter the tetrahedral character of the original fundamental structural unit. In addition, the results of the investigation of the transport properties show that the self-diffusion coefficients of each ion are positively correlated with CaF2 content and arranged in the order of F− > Ca2+ > O2− ≈ P5+ > Si4+. Due to CaF2 reducing the degree of polymerization of the whole melts, the viscosity decreases from 0.39 to 0.13 Pa·s as the CaF2 content increases from 0% to 20%. Moreover, the viscosity of the melt shows an excellent linear dependence on the structural parameters.

A reasonable flow field in the continuous casting mold is beneficial to produce high quality billets, and the design of the nozzle parameters of the mold is key to regulating the flow behavior of molten steel. Through combining the numerical simulation and physical experiments and taking SEN immersion depth and inner diameter as indicators, the flow behavior of molten steel in the mold during high-speed casting of a 160 mm × 160 mm billet was investigated in detail, and the nozzle parameters were optimized. The results demonstrate that, compared with the inner diameter of the nozzle, the immersion depth has a significant influence on the impact depth of molten steel. On the premise of ensuring that the velocity distribution of molten steel on the surface of the mold is uniform and the impact range inside is appropriate, the inlet immersion depth after optimization is 100–120 mm and the inner diameter is 40 mm. The corresponding impact depth is 605–665 mm, and the maximum velocity of molten steel on the mold surface is between 0.04 and 0.045 m/s. Additionally, the results of the physical experiment and numerical simulation reveal that the optimized nozzle parameters can adapt well to the continuous casting process with a high casting speed.

The flow behavior in the mold has a considerable influence on the final product quality of the strand. In this paper, the variation of flow field, level fluctuation, and liquid slag distribution was analyzed by a hydraulic modeling experiment of a high-speed billet continuous casting mold with and without consideration of hydrostatic pressure and a solidified shell. The results indicate that a mold with hydrostatic pressure and a solidified shell possesses an impact depth shallower by 10–30 mm, level fluctuation greater by 3–15%, and more active liquid slag layer at different casting speeds than a mold without them. Moreover, the results of the hydraulic modeling with hydrostatic pressure and a solidified shell agree well with those of the numerical simulation. Therefore, the mold flow behavior modeled with hydrostatic pressure and a solidified shell is closer to the actual behavior than that obtained by models without them. The method in this paper contributes to improving the accuracy of the hydraulic modeling experiment and establishing a foundation for further study of continuous casting.

High casting speed coincides with the development trend of billet continuous casting, which significantly changes the casting characteristics. A mathematical model of the billet mold, which includes multiphase fluid flow, transient heat transfer, and solidification during ultra-high speed of the casting process was developed. The model is first applied to investigate the flow field of molten steel in the mold, studying the influence of steel flow upon the melting and flowing behavior of mold flux. The temperature and velocity distributions of the flux pool that formed above the molten steel surface are described. A parametric study on the melting temperature and viscosity of mold flux on liquid flux thickness and flow velocity is then carried out. Finally, the model is used to derive the relationship between interfacial tension and level fluctuations. The predictions provide an improved understanding of the melting and flowing behavior of mold flux in the billet mold and give the guidance for the design and optimization of mold flux for ultra-high speed of billet casting.

Elucidating the evolution law of the elastic properties of the matrix phase is of great significance for the control of steel properties and quality during continuous casting and subsequent heat treatment. In this paper, thermal expansion experiments and ab initio calculations are used to study the elastic properties of the interstitial free (IF) steel matrix phase in different magnetic states and crystal structures. The results show that the bulk modulus B and the tetragonal shear elastic constant C’ for the entire temperature range decrease with increasing temperature, but C44 is the opposite. While from paramagnetic (PM) to ferromagnetic (FM) state, C’(C44) have changed ~188% (~27%), B increases by ~55% during the crystal structure change (fcc→bcc). With the FM to PM state, the Zener anisotropy parameter increases sharply, and Young’s modulus decreases significantly in the [001] direction; the maximum difference is ~76 GPa. The evolution rate of average Young’s modulus in single bcc-phase FM (fcc-phase PM) range reaches ~5.5(~5.6) × 10−2 GPa K−1. The research provides an effective method for ab initio calculation of the elastic properties of interstitial free and ultra-low carbon steels at high temperature, also furnishing a basis for the application of ab initio calculations to the high temperature performance of steel materials.

Hot stamping process is widely used in the manufacture of the high strength automotive steel, mainly including the stamping and quenching process of the hot-formed steel. In the hot stamping process, the steel is heated above the critical austenitizing temperature, and then it is rapidly stamped in the mold and the quenching phase transition occurs at the same time. The quenching operation in the hot stamping process has a significant influence on the phase transition and mechanical properties of the hot-stamping steel. A proper quenching technique is quite important to control the microstructure and properties of an ultra-high strength hot-stamping steel. In this paper, considering the factors of the austenitizing temperature, the austenitizing time and the cooling rate, a coupled model on the thermal homogenization and phase transition from austenite to martensite in quenching process was established for production of ultra-high strength hot-stamping steel. The temperature variation, the austenite decomposition and martensite formation during quenching process was simulated. At the same time, the microstructure and the properties of the ultra-high strength hot-stamping steel after quenching at different austenitizing temperature were experimental studied. The results show that under the conditions of low cooling rate, the final quenching microstructure of the ultra-high strength hot-stamping steel includes martensite, residual austenite, bainite and ferrite. With the increase of the cooling rate, bainite and ferrite gradually disappear. While austenitizing at 930 °C, the tensile strength, yield strength, elongation and strength-ductility product of the hot-stamping steel are 1770.1 MPa, 1128.2 MPa, 6.72% and 11.9 GPa%, respectively.

To clarify the characteristics of slag infiltration and heat transfer behaviors in the meniscus region during the casting of high Mn−high Al steel, a mathematical model of a continuous casting mold that couples fluid flow with heat transfer, and solidification is developed. The model is based on the change in slag composition and properties caused by the steel/slag reaction. The formation and evolution of the meniscus profile and slag films for different mold fluxes during mold oscillation are described. The results show that the rapid growth of the slag rim with a high Al2O3 content approaches and deforms the meniscus so that a series of casting problems such as slag infiltration blocking, large fluctuations in heat flux, and even meniscus breaking occur in the continuous casting process. Predictions are in good agreement with plant measurements. These findings provide an improved understanding of the complex phenomena occurring in the meniscus region and give new insights into the evaluation and optimization of mold flux properties for high Mn−high Al steel casting.

Slab continuous casting internal cracking is a common quality defect in the production process. The ability to predict the quality of each continuous casting product and assess whether it is suitable for hot delivery or needs to be cleaned down will greatly increase the rolled product rate and reduce the scrap rate and production management cost. According to the quality defects of internal cracks during slab continuous casting and based on the solidification and heat transfer simulations, stress and strain calculations and theoretical analysis of metallurgical processes related to continuous casting combined with an abnormal casting event expert system, the internal crack generation index of the slice unit is used to predict the crack occurrence rating of each sized slab. Moreover, the internal crack prediction model for the slab is successfully developed and applied in a domestic steel mill. The accuracy of the model prediction reached 86.85%. This method achieved the organic combination of theoretical analysis and an expert system and provides an important theoretical tool for the prediction of crack quality defects in slab continuous casting; the method can be applied in slab continuous casting production.

Electric-furnace smelting has become the dominant process for the production of the titanium slag from ilmenite in China. The crystallization behaviors of anosovite and silicate crystals in the high CaO and MgO titanium slag were studied to insure smooth operation of the smelting process and the efficient separation of titanium slag and metallic iron. The crystallization behaviors were studied by a mathematical model established in this work. Results show that the crystallization order of anosovite and silicate crystals in high CaO and MgO titanium slag during cooling is: Al2TiO5 > Ti3O5 > MgTi2O5 > MgSiO3 > CaSiO3 > FeTi2O5 > Mn2SiO4 > Fe2SiO4. Al2TiO5 and Ti3O5 have higher crystallization priority and should be responsible for the sharp increase in viscosity of titanium slag during cooling. The total crystallization rates of anosovite and silicate crystals are mainly controlled by Al2TiO5 and MgSiO3, respectively. The mass ratio of Ti2O3/ΣTiO2 has a prominent influence on the total crystallization rate of anosovite crystals while the mass ratio of MgO/FeO has a slight influence on the total crystallization rate of anosovite crystals.

A combined hybrid 3-D/2-D simulation model was developed to investigate the flow and solidification phenomena in turbulent flow and laminar flow regions during slab continuous casting (CC). The 3-D coupling model and 2-D slicing model were applied to the turbulent flow and laminar flow regions, respectively. In the simulation model, the uneven distribution of cooling water in the width direction of the strand was taken into account according to the nozzle collocation of secondary cooling zones. The results from the 3-D turbulent flow region show that the impact effect of the molten steel jet on the formation of a solidification shell is significant. The impact point is 457 mm below the meniscus, and the plug flow is formed 2442 mm below the meniscus. In the laminar flow region, grid independence tests indicate that the grids with a cell size of 10 × 10 mm2 are sufficient in simulations to attain the precise temperature distribution and solidification profile. The liquid core of the strand is not entirely uniform, and the solidification profile agrees well with the integrated distribution of cooling water in secondary cooling zones. The final solidification points are at a position of 400–500 mm in the width direction and are 17.66 m away from the meniscus.

The solidification path of peritectic steel Q345 was calculated and compared with in-situ observations to investigate the effect of phase transition on the equilibrium partition coefficient. Subsequently, a thermodynamic model for calculating the equilibrium partition coefficient was established and thermodynamic calculations were performed under different phase configurations. Results indicate that L (liquid phase) + δ, L + δ + γ, and L + δ phases coexist in sequence during the solidification of peritectic steel Q345. The phase constitution of the mushy zone evidently affects the evolution of the equilibrium partition coefficient of solutes. The temperature dependence of the equilibrium partition coefficient was quantified through the regression analyses of C, Si, Mn, P, and S solutes under different phase configurations. The average equilibrium partition coefficients of Mn, Si, P, C, and S are 0.696, 0.615, 0.273, 0.2, and 0.033, respectively, thereby indicating the strongest segregation tendency for S and the weakest for Mn.

The flow transport of a 420 × 320 × 90 mm beam blank continuous casting mold that used open-stream pouring combined with submerged refractory funnels was studied. By considering the dynamic similarity, geometric similarity, and air entrapment quantity similarity, a full-size water model was established. Meanwhile, the 3D mathematical models that included three phases were applied. Through the combination of the water model and the mathematical model, the distribution and morphology of the phases in the mold were investigated. The results indicate that bubbles existed in the molten steel due to entrapment and the flow pattern was different from that of the full protection-poured mold. Furthermore, the effects of funnel immersion depth and funnel diameter on the bubbles’ impact depth, funnel’s inside wall shear stress, and overall area of the air/steel interface were discussed. The results provide useful information for the industrial continuous casting process.