Open AccessArticle

Analysis of Longitudinal Cracking and Mold Flux Optimization in High-Speed Continuous Casting of Hyper-Peritectic Steel Thin Slabs

College of Metallurgy and Energy, North China University of Science and Technology, Tangshan 063210, China

School of Material Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China

Author to whom correspondence should be addressed.

Metals 2024, 14(8), 909; https://doi.org/10.3390/met14080909

Submission received: 9 July 2024 / Revised: 6 August 2024 / Accepted: 9 August 2024 / Published: 11 August 2024

(This article belongs to the Special Issue Green and Intelligent Steelmaking Technologies with Low Carbon Emissions)

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Figure 1
(a) Phase diagram of solidification; (b) solidification two-phase zone transition diagram. "> Figure 2
(a) The surface temperatures of blank shells at different casting speeds; (b) the location of solid phase transformation on the surface of thin slab shells at different casting speeds. "> Figure 3
Thickness of thin slab shell at different casting speeds. "> Figure 4
Longitudinal crack morphology of hyper-peritectic steel continuous casting thin slab. "> Figure 5
Schematic diagram of SHTT-II crystallization temperature tester. "> Figure 6
Schematic diagram of viscosity experimental setup. "> Figure 7
Temperature control curve of melting point and melting rate tester. "> Figure 8
(a) Mold heat flow density; (b) liquid slag layer thickness and mold flux consumption statistics. "> Figure 9
(a) The overall surface condition of the casting thin slab after using the B-type mold flux; (b) local conditions of the surface of the casting thin slab after acid washing treatment. ">

Versions Notes

Abstract

Longitudinal crack defects are a frequent occurrence on the surface of thin slabs during the high-speed continuous casting process. Therefore, this study undertakes a detailed analysis of the solidification characteristics of hyper-peritectic steel thin slabs. By establishing a three-dimensional heat transfer numerical model of the thin slab, the formation mechanism of longitudinal cracks caused by uneven growth of the initial shell is determined. Based on the mechanism of longitudinal crack formation, by adjusting the performance parameters of the mold flux, the contradiction between the heat transfer control and lubrication improvement of the mold flux is fully coordinated, further reducing the incidence of longitudinal cracks on the surface of the casting thin slab. The results show that, using the optimized mold flux, the basicity increases from 1.60 to 1.68, the F^- mass fraction increases from 10.67% to 11.22%, the Na₂O mass fraction increases from 4.35% to 5.28%, the Li₂O mass fraction increases from 0.68% to 0.75%, and the carbon mass fraction reduces from 10.86% to 10.47%. The crystallization performance and rheological properties of the mold flux significantly improve, reducing the heat transfer performance while ensuring the lubrication ability of the molten slag. After optimizing the mold flux, a surface detection system was used to statistically analyze the longitudinal cracks on the surface of the casting thin slab. The proportion of longitudinal cracks (crack length/steel coil length, where each coil produced is about 32 m long) on the surface of the thin slab decreases from 0.056% to 0.031%, and the surface quality of the thin slab significantly improves.

Keywords:

hyper-peritectic steel; high-speed casting; surface longitudinal crack; mold flux; heat transfer performance

1. Introduction

High-speed continuous casting technology for thin slabs is at the core of modern steel technology development, and it is vital for achieving a low carbon, green, and highly efficient steel production process [1]. In the iron and steel industries, such a manufacturing technology can help us to meet carbon neutral strategic objectives. High-speed continuous casting technology has the advantages of a short production process, low investment costs for construction, low equipment consumption and labor costs, and production efficiency, and has been rapidly developed in recent years both domestically and internationally [2,3].

However, compared with conventional casting speeds (3.8 m/min~4.5 m/min), high-speed casting (4.5 m/min~5.5 m/min) can present problems by increasing longitudinal cracks on the surface of thin slabs, raising the chance of sticker alarms. Such problems have become evident, especially in the pouring of medium-carbon steel, and limit the stable production of high-quality thin slabs [4,5]. Therefore, in the design of the steel composition, the carbon content should be moderately increased to avoid the steel reaching the peritectic point and becoming hyper-peritectic [6]. However, this effect is not obvious in continuous casting production, requiring further research on steel crack sensitivity.

Previous crack susceptibility studies mainly focused on hypoeutectic and eutectic steel. Li et al. [7] concluded that the carbon content of hypoeutectic steel has a fundamental effect on plasticity, which leads to cracking. Yao et al. [8] found that the transformation of the δ phase of the body-centered cubic lattice to the γ phase of the face-centered cubic lattice resulting from the inclusion reaction would produce a large volume contraction, directly leading to the occurrence of longitudinal cracks. Furthermore, research has concluded that the early onset of the phase transition contraction in the peristaltic reaction and the uneven contraction generate longitudinal cracks [9,10,11]. The crack sensitivity of hyper-peritectic steel has been studied less extensively because the shrinkage of hyper-peritectic steel due to the peristaltic reaction is mainly volume shrinkage and the crack sensitivity is lower [12,13]. However, in the production process of high-speed and even ultra-high-speed (5.5 m/min~6.0 m/min) continuous casting of hyper-peritectic steel thin slabs, the surface of the thin slab is still subject to longitudinal cracks. This is mainly related to the continuous casting process and the mismatch of the corresponding mold flux.

To reduce the risk of longitudinal cracks in thin slabs of hyper-peritectic steel during high-speed continuous casting, it is necessary to analyze their formation mechanism with a finite element analysis method and to optimize the existing mold flux. Bai et al. [14] conducted numerical simulation research on the solidification process of slab continuous casting under conventional casting speeds using a finite element analysis method; Paweł et al. [15] also used a finite element model to study the effect of the surface quality of the mold on the temperature during continuous casting, indicating that the use of a finite element analysis method to investigate the solidification and heat transfer process of the casting thin slab during continuous casting is relatively mature.

Therefore, this study analyzes the effect of the peristaltic reaction on the contraction of a thin slab shell, based on the iron and carbon equilibrium phase diagram. Furthermore, the temperature distribution of the thin slab shell in the mold along with the thickness change and the corresponding height of the phase change are investigated by a finite element analysis method to systematically and fully explain the causes of longitudinal cracks. The longitudinal crack phenomenon on the surface of hyper-peritectic steel thin slabs is analyzed to determine the main problems in the plant’s existing mold flux. According to the performance requirements of the mold flux for the high-speed continuous casting of thin slabs, the composition and performance indicators of the existing mold flux are optimized. The research results provide technical support for the smooth and efficient production of continuous casting and the realization of a low-carbon and green steel process.

2. Analysis of Longitudinal Cracking in Thin Slabs of Hyper-Peritectic Steel

2.1. Solidification Characteristics of Hyper-Peritectic Steel

The composition of hyper-peritectic steel thin slabs is shown in Table 1. To analyze the solidification characteristics, the “Phase Diagram” module in FactSage 8.2 thermodynamic software was used to calculate the equilibrium phase diagram of the steel. FS steel was selected as the database, and the results are shown in Figure 1a. The liquidus temperature of the steel was approximately 1525 °C and the solid phase line temperature was approximately 1475 °C. When the carbon content varied between 0.16% and 0.20%, with a reduction in temperature, the solidification mode of hyper-peritectic steel was L → L + δ → L + γ + δ → L + γ → γ. Although the carbon content of the steel avoided the encapsulation point and was classed as hyper-peritectic steel, encapsulation still occurred in this solidification mode. There was a greater liquid phase than δ phase prior to the reaction, with some liquid remaining after the end of the inclusion reaction, which crystallized into austenite during the subsequent cooling process. Ferrite belongs to the body-centered cubic and austenite to the face-centered cubic. Austenite has a greater density and smaller volume than ferrite, meaning that the peristaltic reaction resulted in volume shrinkage.

Figure 1b shows the solidification two-phase zone transition of carbon steel grades in thin slabs. From Figure 1a,b, it can be seen that when the carbon content changed from 0.16% to 0.20%, the peristaltic reaction occurred in the solid phase rate range of 0.80 to 0.95. The dendrites were connected and had a degree of strength when the solid phase rate was relatively high, before the peristaltic reaction occurred, and the generated stress caused the dendrites to move. Therefore, in the meniscus area, the incipient thin slab shell was very thin and tensile, driving thin slab shell movement. The surface movement of the thin slab shell caused an uneven distribution of the air gap and uneven growth of the thin slab shell, which eventually lead to the generation of longitudinal cracks on the surface. The solidification front of the two-phase region of the peristaltic reaction had an adverse effect of pulling on the thin slab shell.

2.2. Influence of the High-Speed Continuous Casting Process on Longitudinal Cracking on the Surface of Thin Slabs

During the continuous casting process, there is a large temperature gradient (about 450 °C) on the surface of the thin slab inside the mold. To further analyze the mechanism of longitudinal crack defects on the surface of hyper-peritectic steel thin slabs in high-speed continuous casting, a three-dimensional numerical model was established to represent the heat transfer of a thin slab in a mold using ANSYS17.0 finite element analysis software. The steady-state temperature field of the thin slab in the mold was analyzed during the casting of SS400 steel (w(C) = 0.18%), with the effective length of the mold being 1100 mm and a casting cross-section of 1517 mm × 72 mm. Based on this heat transfer model, it was possible to calculate the surface temperature and shell thickness of the thin slab at different locations in the mold when pouring hyper-peritectic steel.

The solid model used a thin slab produced by an FTSC (Flexible Thin Slab Casting) continuous casting mold (Tangshan Iron and Steel Group Co., Ltd, Tangshan, China). Due to the symmetry of the thin slab, only a quarter was used for the geometric model and a three-dimensional heat transfer model was established [16,17]. ANSYS 17.0 software was used to perform grid division, map the model surface into grids, and produce the mapping mesh for model surface delineation. In addition, boundary conditions were set for the finite element model of the casting thin slab:

(1): The top of the model, which was the liquid level of the mold, adopted the first boundary condition, where the node temperature was equal to the pouring temperature of the steel liquid.
(2): The thin slab model had a symmetrical structure with wide and narrow center planes and adopted adiabatic boundary conditions.
(3): When the continuous casting thin slab in the mold cooled and solidified, a conduction heat transfer boundary condition was applied to its surface to transform the heat transfer resistance into an equivalent thermal resistance. The equivalent thermal resistance was then transformed into thermal conductivity.

The physical parameters of the molten steel used in the simulation calculation process are shown in Table 2.

Figure 2a shows the simulation results of the surface temperature of the center shell of the thin slab under different casting speeds. Figure 2b shows the simulated locations of solid phase transformation at the surface of the blank shell for different casting speeds.

Figure 2a shows that with an increase in the casting speed, the surface temperature of the thin slab gradually increased, with a greater increase observed in the lower part of the mold. This indicates that the higher the casting speed, the higher the thin slab shell surface temperature. After the surface layer of the cast thin slab was transformed to γ, the near-surface region of the thin slab shell sub-skin underwent a δ → γ phase transition contraction with downward movement of the thin slab shell. Under the effect of the above persistent contraction, the closeness of fit between the thin slab shell and the mold deteriorated. This created differences in the degree of cooling, increasing thin slab shell thickness inhomogeneity and causing longitudinal crack defects to form.

The higher casting speed led to a further extension of the phase change shrinkage region. From Figure 2b, with an increase in casting speed, the location of the solid phase transformation on the surface of the thin slab shell moved downward, specifically from 72 and 75 mm at 5.0 and 5.5 m/min, respectively, to 80 mm at 6.0 m/min. Therefore, the higher the casting speed, the longer the region of action, because in the longer region of the mold, the solid phase transformation was present and resulted in thin slab shell movement. As the solid phase transformation region expanded, the deformation range of the thin slab shell also increased, inducing larger longitudinal cracks.

In addition, as shown in Figure 2b, the phase change reaction started in the upper part of the mold, close to the meniscus, where the static pressure of the molten steel was low. A combination of the mechanical stress due to the funnel shape of the thin slab mold, friction between the blank shell and the mold, solidification shrinkage stress, and thermal stress caused by the temperature difference inside and outside the mold further increased the stresses on the thin slab shells. In the stress concentration of the mold, the static pressure of the steel was unable to prevent the thin slab shell phase change contraction. This pressed the steel to the back to the wall of the mold, creating an air gap between the thin slab shell and the mold, which reduced heat transfer and slowed the growth of the thin slab shell [18]. Variability in the degree of thin slab shell contraction presents a complex, random distribution, resulting in the thin slab shell exhibiting uneven thickness growth at the macro level, making it more prone to cracking.

Figure 3 shows the simulation results of the thickness change rule of high-speed thin slabs under different casting speeds. At a distance of 0.6 m between the center of the mold and the liquid steel surface, for every 0.5 m/min increase in the casting speed, there was an average reduction of 0.4 mm in the blank shell thickness. In the lower part of the mold, at 1.0 m from the liquid steel surface, for every 0.5 m/min increase in the casting speed, there was an average reduction of 0.6 mm in the blank shell thickness. The results show that increasing the casting speed results in a thinner blank shell, which was more evident the closer the shell was to the lower part of the mold. The thinner the shell, the greater the support pressure between the shell and the mold. Therefore, it is necessary to improve the lubrication performance of the mold flux.

To summarize, surface longitudinal cracks were evident in the high-speed casting process of hyper-peritectic steel thin slabs. The cooling effect of the mold on the incipient thin slab shell should be reduced to promote the uniform solidification of the shell, which can help to reduce the longitudinal crack defects on the surface of the cast thin slab [19]. Therefore, in the design and development of the mold flux, the upper part of the mold (meniscus area) must appropriately increase the thermal resistance of the slag film. This encourages earlier and faster precipitation of crystals, controlling the heat transfer and promoting the uniform growth of primary thin slab shells, limiting longitudinal crack generation. Furthermore, higher casting speeds and larger specific surface areas result in low slag consumption, thin liquid slag film (only 1/3 of the ordinary slab thickness), and increased friction between the copper wall and thin slab shells. This reduces heat transfer performance to ensure the lubrication of the lower and middle parts of the mold, achieving low heat transfer and a high lubrication performance of the mold flux. Recently, this has become a key area of focus in the field of mold flux research.

3. Mold Flux Improvement Measures and Inspection Methods

3.1. Production Situation before Optimization

The equipment sequence for producing hyper-peritectic steel was as follows: Iron Pretreatment → Converter → LF (Ladle Furnace) Refining → Straight Arc Type Thin Slab Continuous Casting Machine (Tangshan Iron and Steel Group Co., Ltd, Hebei, China). The plant ladle capacity was 150 t and the continuous casting machine tundish capacity was 50 t. A funnel-type continuous casting mold was used, with a mold outlet thin slab thickness of 72 mm and the work of the pull speed was 4~6 m/min. Table 3 shows the hyper-peritectic steel casting parameters.

Before increasing the test speed, the casting speed was controlled at 3.8~4.5 m/min, the original mold flux use was relatively normal, and the liquid slag layer had a thickness of 5~7 mm. The bonding phenomenon rarely occurred and thin slab surface longitudinal cracks were not evident. However, at the beginning of the speed test stage, the liquid slag layer was thin with an average thickness of 2~4 mm, sticker alarms sometimes occurred, and the casting speed fluctuated significantly. This seriously restricted production efficiency and increased the manufacturing cost. Furthermore, a large number of longitudinal cracks were observed on the surface of the cast thin slab (the proportion of longitudinal cracks is around 0.058% and the morphology of some longitudinal cracks is shown in Figure 4), leading to a decline in the surface quality and a judgment reduction.

3.2. Mold Flux-Specific Improvement Measures

Due to the strong susceptibility of the thin slab surface of hyper-peritectic steel to longitudinal cracks under the high-speed continuous casting, it is necessary to attenuate the heat transfer capacity of the mold flux film in the upper part of the mold (δ → γ transition region), to reduce the heat flow between the copper plate/thin slab shell and promote uniform growth of the thin slab shell. Improving the crystallization properties of the mold flux increases the proportion of crystalline slag film, and the use of microporous interfaces within the crystallization body weakens the lattice vibration to reduce thin slab heat transfer through the mold flux film to the mold copper plate. In addition, due to the high-quality requirements of thin slabs, the viscosity and melting properties of the mold flux should also be adjusted to avoid the occurrence of bonding phenomena. At the same time, it is also necessary to coordinate the lubricating properties of the mold flux to ensure that the cast slab has a good surface quality. The specific improvement measures are as follows:

(1): Improve the crystallization rate and crystallization temperature of the mold flux, thus increasing the slag film thermal resistance and reducing heat transfer from the mold [20,21].
(2): Moderately reduce the viscosity of the mold flux to ensure a reasonable consumption and reduce the friction between the thin slab shell and the mold.
(3): Increase the thickness of the liquid slag layer and reduce the slag circle, to offset the fluctuations in the liquid level and ensure the presence of liquid slag at the rising edge of the mold [22].
(4): To avoid decline in the lubrication ability due to the enhanced mold flux crystallization ability, the melting and breaking temperatures of the mold flux should be moderately reduced to ensure that the lower part of the liquid slag film exists in the lower part of the mold, reducing friction of the thin slab cast inside the mold [23]. This ensures the quality of the surface avoids adhesion or an accidental breakout of the steel.
(5): Maintain the stability of the mold flux liquid slag performance.

3.3. Testing Method for Physical and Chemical Properties of the Mold Flux

The following test methods were applied:

(1): The crystallization temperature, incubation time, and critical cooling rate of the mold flux were determined using the SHTT-II melting crystallization temperature tester. The schematic diagram of the device is shown in Figure 5.
(2): The viscosity of the mold flux was determined using a Brookfield rotating viscometer, heated to 1300 °C in a MoSi₂ high-temperature furnace. The schematic diagram of the device is shown in Figure 6.
(3): The melting temperature and speed of the mold flux were determined using an automatic slag melting point and melting speed tester. The melting speed was the time required for the slag samples to be completely melted at a constant temperature of 1350 °C. The temperature control curve is shown in Figure 7.
(4): The thickness of the liquid slag layer was measured by inserting a steel plate above the liquid surface of the mold. The measurement site was located at a distance of half the width of the thin slab from the narrow copper plate.

4. Comparison of Mold Flux Effects

4.1. Mold Flux before Optimization

The chemical composition of the A-type mold flux used for casting hyper-peritectic steel before optimization is shown in Table 4. The basicity of this mold flux (

ω (C a O) / {ω (S i O}_{2}

)) was 1.60, and the mass fractions of Na₂O, Li₂O, and F⁻ were 14.35%, 0.68%, and 0.87%, respectively. The compositional indexes are closely related to the crystallization ability of the mold flux, and Li₂O and F^- can significantly reduce the viscosity of the mold flux. Therefore, it is important to carefully consider the design of the mold flux.

Table 5 lists the test results of the viscosity, melting, and crystallization properties of the A-type mold flux. The main basis for measuring the ability of the mold flux to control heat transfer is the crystallization performance, including three parameters: crystallization temperature, incubation time, and critical cooling rate. From the test results, the A-type mold flux crystallization temperature was relatively low (1240 °C), and the incubation time was long (120 s), indicating difficulty in controlling the heat transfer. The mold flux critical cooling rate is 20 °C/s, which means that the precipitation of crystals needs to be realized at a slow cooling rate. Therefore, it is concluded that the crystallization properties of an A-type mold flux do not meet the requirements of high-speed continuous casting of hyper-peritectic steel thin slabs, which requires adjustment and optimization.

For hyper-peritectic steel, due to the limited shrinkage in the solidification process, it is generally believed that there is only a small possibility of steel breakout. However, if the thin slab shell lubrication is poor, it may increase the friction between the thin slab shell and the mold, increasing the thin slab shell transverse contraction resistance and exacerbating longitudinal cracking. Bonding breakouts can also be triggered if the friction is too great. To improve the crystallization ability, the mold flux basicity should be adjusted. There is a contradiction between the high crystallization ability of the mold flux and the lubrication performance [24,25]. With an increase in basicity, extra attention should be paid to the lubricating effect of the mold flux, adjusting its relevant physical and chemical performance parameters to suit the casting process.

As seen in Table 5, the break temperature of an A-type mold flux is 1152 °C. According to Figure 2a, at a casting speed of 5.5 m/min, the surface temperature of the steel thin slab at the outlet of the mold was 1130 °C, which was lower than the mold flux break temperature. The break temperature of the mold flux is relatively high, limiting the liquid slag film on the surface of the thin slab in the lower half of the mold, which is highly unfavorable for ensuring the lubrication of the thin slab shell. The high melting temperature of an A-type mold flux of 1095 °C limits the timely formation of a certain thickness of the liquid slag layer on the surface of the steel, with a slow melting rate of 28 s. This may cause an uneven distribution of liquid slag film between the shell and the mold, resulting in uneven heat transfer and solidification of the shell, and exacerbating the development of surface cracks on the thin slab. In addition, the mold flux viscosity of this model was 0.14 Pa s, which can moderately reduce the mold flux viscosity, improve the slag fluidity and consumption, and reduce friction.

Therefore, the main problems with an A-type mold flux are the poor crystallization performance and insufficient ability to control heat transfer. To solve the problem of longitudinal cracks on the surface of continuous casting thin slabs, the focus should be on improving the crystallization ability of the mold flux. In addition, the mold flux rheological properties and process conditions do not align, so improving the crystallization ability may further deteriorate the lubrication effect of the thin slab shell. To avoid poor lubrication, the slag viscosity should be moderately reduced. Meanwhile, attention should be paid to reducing the melting temperature of the mold flux during the optimization process to improve the lubrication ability.

4.2. Optimized Mold Flux

The chemical composition of a B-type mold flux (optimized mold flux) is shown in Table 6. A comparison of Table 4 and Table 6 shows that, compared with the A-type slag, the basicity of the B-type slag increased from 1.60 to 1.68. Increasing the basicity can increase the crystallization temperature of the mold flux and promote crystallization at higher temperatures. The mass fraction of Na₂O increased from 4.35% to 5.28%. Na₂O belongs to the network component modifier, which can destroy the structure of the silicate network and reduce the resistance to ionic transfer in the mold flux. It can reduce the melting point, viscosity, and the crystallization energy barrier of the mold flux. This encourages the slag to crystallize to achieve the purpose of controlling the heat transfer. However, slag crystallization should not be too high (generally < 10%), otherwise it will affect the lubrication [26,27]. The mass fraction of Li₂O increased from 0.68% to 0.75%. As a high-performance mesh breaking material, Li₂O can reduce viscosity, broaden the melting range, improve the fluidity of the mold flux, and accelerate crystallization in the meniscus area of the mold, thereby increasing thermal resistance and reducing the occurrence of longitudinal cracks. It also slows down the crystallization in the area below the meniscus, thereby improving the lubrication performance of the slag film in the lower part of the mold and reducing the occurrence of adhesion [28].

The mass fraction of F- increased from 10.67% to 11.22%. CaF₂ promoted the precipitation of 3CaO·2SiO₂·CaF₂, especially when the F- mass fraction increased from 10% to 12%, due to its strong ability to reduce the viscosity of the mold flux. CaF₂ also promoted the migration of ions in the molten slag, increased the crystallization temperature of the mold flux, and strengthened the precipitation of crystals in the mold flux. The increase in CaF₂ content also helped to reduce the melting point of the mold flux and improve the lubricating properties [29]. The carbon mass fraction decreased from 10.86% to 10.47%. Carbon has the main function of regulating the melting rate in the mold flux, which can form skeleton particles and effectively segregate the mold flux base material, thus achieving the purpose of controlling the melting rate. In addition, increasing the CaO and F- content can further destroy the Si-O tetrahedral network structure, reduce the deformation resistance of the molecular chain, and lower the viscosity of the mold flux.

The physicochemical properties of the B-type mold flux are shown in Table 7. The crystallization properties of the mold flux changed considerably after the adjustment of the composition. The crystallization temperature and incubation time were adjusted from 1240 °C and 120 s to 1276 °C and 56 s, respectively, making the mold flux easier to crystallize. An increase in the critical cooling rate from 20 °C/s to 72 °C/s helped the mold flux in the thin slab shell and mold temperature gradient between the meniscus and the region below to crystallize as soon as possible. This alleviated the uneven growth of the thin slab shell caused by the stress–strain, to improve the longitudinal cracking defects on the surface of the thin slab.

As the basicity of the mold flux was increased, to coordinate the lubrication performance and avoid poor lubrication caused by a high proportion of crystals, the melting temperature of the mold flux was decreased from 1095 °C to 1050 °C by adjusting the content of Na₂O, Li₂O, and CaF_2. This increased the thickness of the liquid slag layer, which improved the pressure of the mold flux inflow channel and promoted uniform inflow of the mold flux into the mold, achieving uniform growth of the thin slab shell and alleviating longitudinal cracks [30]. Furthermore, reducing the carbon content increased the melting speed from 28 s to 22 s. The adjusted mold flux viscosity was reduced from 0.14 Pa·s to 0.08 Pa·s. The above changes in performance are in line with the expected optimization of design requirements.

4.3. Actual Use Effect of Mold Flux before and after Optimization

A comparison of the effect of using mold flux before and after optimization is shown in Figure 8. Figure 8a shows the comparison of the two types of mold flux in the normal high-speed (5.5 m/min) casting as well as the density of heat flow in the mold under the cross-section. Figure 8b shows the statistical results of the corresponding thickness of the liquid slag layer and the consumption of the mold flux when using A-type and B-type mold fluxes, respectively. From Figure 8a, the heat flow on the wide side of the mold (fixed and loose side) was reduced by 50–60 kW/m² and that in the narrow side was reduced by 55–60 kW/m² after the addition of the improved B-type mold flux. The results indicate that the newly developed B-type mold flux has an increased thermal resistance and reduced heat transfer capacity, which effectively controls the heat transfer and reduces the density of the heat flow in the mold, thus limiting the formation of longitudinal cracks.

From Figure 8b, it can be seen that the thickness of the liquid slag layer of the B-type mold flux significantly improved at approximately 6~8 mm, whilst that of the original A-type mold flux was approximately 4~6 mm. Furthermore, the B-type mold flux increased consumption from 0.26 kg/t to 0.29 kg/t. Combined with the liquid slag layer thickness improvement, this indicates that the new B-type mold flux has a stronger lubrication effect and can meet the requirements of high-speed casting.

Figure 9a shows the overall condition of the wide surface of the casting thin slab after using the B-type mold flux. Figure 9b shows the local situation after acid washing treatment on the surface of the casting thin slab. The surface of the washed thin slab is smooth, free of slag, and the distribution of vibration marks is uniform. Compared with Figure 4, there are no obvious longitudinal cracks. This is because this surface of the B-type mold flux has a strong heat transfer control ability and a good lubrication performance, promoting the uniform growth of the primary thin slab shell and significantly reducing the incidence of surface longitudinal cracks.

Using the B-type mold flux for high-speed casting mass production of 10 coils, according to the surface inspection system statistics, the system can accurately measure the length of all longitudinal cracks appearing on the surface of the thin slabs during continuous casting and rolling processes through multi-angle cameras around the thin slabs. Then, this length is divided by the total length of the steel coil produced during the measurement time, and the proportion of longitudinal cracks is finally obtained, with an error controlled within ±0.005%. The specific results are shown in Table 8; it shows that the longitudinal cracks and other cracking defects were reduced compared with the original A-type mold flux. The degree of cracking has been reduced, showing that the new B-type mold flux can better control the longitudinal surface cracks on the casting thin slab and improve the surface quality, effectively ensuring smooth production.

5. Conclusions

The main findings of this study were as follows:

(1): Through numerical simulation calculations using FactSage thermodynamic software and ANSYS finite element software, it was found that at a casting speed of 5.0~6.0 m/min, the solidification mode of hyper-peritectic steel in the thin slab mold was determined by δ → γ. The volume shrinkage caused by phase transformation resulted in a solid phase rate of up to 0.80~0.95. Furthermore, for every increase in casting speed of 0.5 m/min, the solid phase transformation position on the surface of the thin slab extended downwards by 4 mm and the average thickness of the thin slab decreased by 0.5 mm. In addition, factors such as air gaps and poor heat transfer in the initial solidification area resulted in the uneven growth of the primary thin slab, ultimately leading to longitudinal cracks on the surface.
(2): To solve this, it was necessary to reduce the mold flux heat transfer performance and to ensure the lubrication of the lower part of the mold. Therefore, the basicity was increased from 1.60 to 1.68, the F^- mass fraction was increased from 10.67% to 11.22%, the Na₂O mass fraction was increased from 4.35% to 5.28%, the Li₂O mass fraction was increased from 0.68% to 0.75%, and the carbon mass fraction was reduced from 10.86% to 10.47%. The crystallization performance of the mold flux was significantly improved, and the rheological properties were optimized, so that the lubricating ability of the mold flux was ensured while controlling the heat transfer.
(3): After optimization and adjustment, the use of a new mold flux reduced the mold heat flow density, improved the mold flux liquid slag layer thickness and slag consumption, reduced the proportion of longitudinal cracks on the surface of the thin slab from 0.056% to 0.031%, and also significantly reduced the depth and length of longitudinal cracks. No sticker alarm or steel breakout accident occurred during the production process, and the expected effect was achieved.

Overall, this research achievement provides technical support for the smooth and efficient production of continuous casting and the realization of a low-carbon and green steel process. The next step will be to further optimize the anti-slag entrapment performance of the mold flux, in order to achieve the most ideal usage effect.

Author Contributions

Conceptualization, Z.Y. and L.Z.; methodology, Z.Y.; software, K.Z.; validation, X.W. and K.Z.; formal analysis, X.W.; investigation, K.Z.; resources, X.W.; writing—original draft preparation, Z.Y.; writing—review and editing, L.Z.; supervision, K.Z.; funding acquisition, Z.Y., L.Z. and X.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 52374335 (X.W.); Natural Science Foundation of Hebei Province, grant number E2022208019 (L.Z.); Hebei Province Higher Education Science and Technology Research Project, grant number BJK2024057 (Z.Y.); and Tangshan Municipal Science and Technology Plan Project, grant number 22130229H (Z.Y.).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

The authors gratefully acknowledge the support by Hebei High Quality Steel Continuous Casting Technology Innovation Center and the assistance of engineer Penghui Gong from Tangshan Iron and Steel Group Co., Ltd.

Conflicts of Interest

The authors declare no conflicts of interest.

References

Wang, S.Z.; Gao, Z.H.; Wu, G.L.; Mao, X.P. Development status and prospects of thin slab continuous casting and rolling technology. Chin. J. Mech. Eng. 2022, 44, 534–545. [Google Scholar]
Jayakrishna, P.; Ananda, S.; Chakraborty, S.; Ganguly, S.; Talukdar, P. Interfacial heat flux estimation in a funnel-shaped mould and analysis of solidification characteristics in thin slab continuous casting. J. Heat Trans. 2021, 143, 122401–122412. [Google Scholar] [CrossRef]
Jiao, D.G.; Feng, H.J.; Feng, Y.P.; Xie, J. Progress and development direction of thin plate continuous casting and rolling technology under new situation. Foundry Tech. 2019, 9, 9111–9914. [Google Scholar]
Zhu, L.G.; Yuan, Z.P.; Xiao, P.C.; Wang, X.J.; Yin, K.; Zhang, J. Research and optimization of mold flux for high speed continuous casting of low carbon steel thin slab. Iron Steel 2020, 55, 65–73+102. [Google Scholar]
Li, L.P.; Wang, X.H.; Deng, X.X.; Ji, C.X. Process and quality control during high speed casting of low carbon conventional slab. J. Iron Steel Res. 2015, 22, 1–9. [Google Scholar] [CrossRef]
Gao, X.; Li, H.X.; Han, L.; Santillana, B.; Ruvalcaba, D.; Zhuang, L.Z. Constitutive modeling and activation energy maps for a continuously cast hyperperitectic steel. Metall. Mater. Trans. A 2018, 49, 4633–4648. [Google Scholar] [CrossRef]
Li, S.S.; Zhang, L.F.; Yang, X.G.; Li, M. Research on control of transverse cracks at the corner of continuous casting billets of hypo-peritectic steel. Steelmak 2016, 32, 67–72. [Google Scholar]
Yao, C.G.; Yuan, S.Q.; Chen, L.; Wang, Z.J. Research and process optimization of corner cracks in Q235D continuous casting blooms of peritectic steel. Hot Working Tech. 2011, 40, 53–56. [Google Scholar]
Li, Q.H.; Lan, P.; Wang, H.J.; Ai, H.Z.; Chen, D.L.; Wang, H.D. Formation and control of the surface defect in hypo-peritectic steel during continuous casting: A review. Int. J. Min. Met. Mater. 2023, 30, 2281–2296. [Google Scholar] [CrossRef]
Furumai, K.; Aramaki, N.; Oikawa, K. Influence of heat flux different between wide and narrow face in continuous casting mould on unevenness of hypo-peritectic steel solidification at off-corner. Ironmak. Steelmak. 2022, 49, 1–15. [Google Scholar] [CrossRef]
Konishi, J.; Militzer, M.; Samarasekera, I.V.; Brimacombe, J.K. Modeling the formation of longitudinal facial cracks during continuous casting of hypoperitectic steel. Metall. Mater. Trans. B 2002, 33, 413–423. [Google Scholar] [CrossRef]
Zhang, X.M.; Wang, H.F.; Cheng, N.L. Study on the peritectic reaction of steel and the sensitivity of casting billet cracks. Contin. Cast. 2018, 43, 47–50. [Google Scholar]
Wang, J.H.; Pan, S.Y.; Li, Y.S.; Shen, X.P.; Zhang, Q.Y.; Jia, D. A phase-field study of the peritectic reaction mechanisms in Fe-Ni alloys. Comp. Mater. Sci. 2023, 230, 112491–112501. [Google Scholar] [CrossRef]
Bai, L.; Wang, B.; Zhong, H.G.; Ni, J.; Zhai, Q.J.; Zhang, J.Y. Experimental and numerical simulations of the solidification process in continuous casting of slab. Metals 2016, 6, 53–64. [Google Scholar] [CrossRef]
Paweł, K.; Paweł, S.; Grzegorz, K.; Szymon, K.; Krystian, F. External surface quality of the graphite crystallizer as a factor influencing the temperature of the continuous casting process of ETP grade copper. Materials 2021, 14, 6309–6321. [Google Scholar] [CrossRef] [PubMed]
Wang, M. Simulation Research on Solidification Heat Transfer of Thin Slab Continuous Casting at High Casting Speed. Master’s Thesis, Shenyang University of Technology, Shenyang, China, 12 June 2021. [Google Scholar]
Liu, Z.X.; Yang, Y.S.; Xiao, P.C.; Zhu, L.G.; Zhu, R. Deformation behavior of billet shell in ultra-high speed thin slab continuous casting mold. Steelmak 2023, 39, 58–65. [Google Scholar]
Li, Q.P.; Wen, G.H.; Chen, F.H.; Tang, P.; Hou, Z.B.; Mo, X.Y. Irregular initial solidification by mold thermal monitoring in the continuous casting of steels: A review. Int. J. Min. Met. Mater. 2023, 31, 1–13. [Google Scholar] [CrossRef]
Gu, S.T.; Sun, M.F.; Wang, B.; Zhang, J.Y. Simulation and experimental study of fluid flow and solidification behavior in thin slabs continuous casting process under secondary electromagnetic stirring. Steel Res. Int. 2023, 95, 2300398–2300413. [Google Scholar] [CrossRef]
Wang, Z.; Tang, P.; Mi, X.X.; Hu, Q.; Lu, Y.F.; Wen, G.H. Effect of w(CaF₂) on crystallization properties of CaO-SiO₂-Al₂O₃ based mold fluxes. Iron Steel 2018, 53, 38–44. [Google Scholar]
Anisimov, K.N.; Longinov, A.M.; Gusev, M.P. Influence of mold flux on the thermal processes in the mold. Steel Trans. 2016, 46, 589–594. [Google Scholar] [CrossRef]
Wu, J. Research on Lubrication Characteristics of Mold Flux for Continuous Casting Mold. Master’s Thesis, Yanshan University, Qinhuangdao, China, 15 June 2022. [Google Scholar]
Zhu, L.L.; He, S.P.; Mao, J.H. Series planning of special thick plate continuous casting mold flux. Iron Steel 2016, 51, 44–48. [Google Scholar]
Weng, J.J. Research and application of high basicity and high lubricity continuous casting mold flux. Steelmak 2016, 32, 52–54. [Google Scholar]
Yang, J. Lubrication Characteristics and Heat Transfer Behaviors in Continuous Casting Mold for High Mn High Al Steel. Ph.D. Thesis, Northeastern University, Shenyang, China, 14 June 2018. [Google Scholar]
Yuan, Z.P.; Zhu, L.G. Effect of Li₂O on rheological properties, structure, and crystallization of mould flux for high-speed thin-slab continuous casting. Ironmak. Steelmak. 2022, 49, 226–237. [Google Scholar] [CrossRef]
Gao, J.X.; Wen, G.H.; Sun, Q.H.; Tang, P. The influence of Na₂O on the solidification and crystallization behavior of CaO-SiO₂-Al₂O₃-based mold flux. Metall. Mater. Trans. B 2015, 46, 1850–1859. [Google Scholar] [CrossRef]
Zhang, J.H.; Qi, J.; Liu, C.J.; Jiang, M.F. Effect of Li₂O on the crystallization properties of CaO-Al₂O₃ based mold flux. J. Iron Steel Res. 2022, 34, 1211–1218. [Google Scholar]
Wang, X.J.; Fan, Y.P.; Zhu, L.G.; Wang, L.J.; Wu, B.B.; Tian, K. Influence of CaF₂ content in the continuous casting mold flux on the devitrification properties. Iron Steel Van. Tit. 2017, 38, 106–112. [Google Scholar]
Wang, S.S.; Wang, W.H.; Chen, S.J.; Sun, M.W.; Zhang, X.B.; He, S.P. Effect of basicity on flow, crystallization and structure of high TiO₂-containing mold slags. China Met. 2024, 34, 100–109. [Google Scholar]

Figure 1. (a) Phase diagram of solidification; (b) solidification two-phase zone transition diagram.

Figure 2. (a) The surface temperatures of blank shells at different casting speeds; (b) the location of solid phase transformation on the surface of thin slab shells at different casting speeds.

Figure 3. Thickness of thin slab shell at different casting speeds.

Figure 4. Longitudinal crack morphology of hyper-peritectic steel continuous casting thin slab.

Figure 5. Schematic diagram of SHTT-II crystallization temperature tester.

Figure 6. Schematic diagram of viscosity experimental setup.

Figure 7. Temperature control curve of melting point and melting rate tester.

Figure 8. (a) Mold heat flow density; (b) liquid slag layer thickness and mold flux consumption statistics.

Figure 9. (a) The overall surface condition of the casting thin slab after using the B-type mold flux; (b) local conditions of the surface of the casting thin slab after acid washing treatment.

Table 1. Composition of carbon steel in thin slabs (w_B)%.

C	Si	Mn	P	S	Cr	Fe
0.18	0.18	0.30	0.01	0.01	0.05	99.27

Table 2. Physical parameters of molten steel used in the simulation.

Phase	Parameters	Value
Molten steel	Density/kg·m⁻³	7100, T = 1525 °C; 7800, T = 1475 °C
	Viscosity/kg·m⁻¹·s⁻¹	0.0068
	Specific Heat/J·kg⁻¹·°C⁻¹	720
	Thermal Conductivity/W·m⁻¹·°C⁻¹	13.86 + 0.01113 × T
	Latent heat/J·kg⁻¹	268,400

Table 3. Casting parameters of hyper-peritectic steel.

Casting Temperature (°C)	Casting Speed (m·min⁻¹)	Mold Taper (mm)	Mold Outlet Size (mm)
1544	5.5	7.5	1517 × 72

Table 4. Chemical composition of existing hyper-peritectic steel mold flux (wt.%).

Type	SiO₂	MgO	Al₂O₃	CaO	Na₂O	K₂O	Li₂O	F^-	C
A	22.46	6.11	7.17	35.95	4.35	1.75	0.68	10.67	10.86

Table 5. Typical hyper-peritectic steel mold flux physical and chemical performance parameters.

Viscosity (Pa·s)	Break Temperature (°C)	Melting Temperature (°C)	Melting Range (°C)	Melting Rate (s)	Crystallization Temperature (°C)	Incubation Time (s)	Critical Cooling Rate (°C·s⁻¹)
0.14	1152	1095	46	28	1240	120	20

Table 6. Chemical composition of optimized mold flux (wt.%).

Type	SiO₂	MgO	Al₂O₃	CaO	Na₂O	K₂O	Li₂O	F^-	C
B	21.35	5.69	7.78	35.83	5.28	1.63	0.75	11.22	10.47

Table 7. Improvement parameters of physical and chemical properties of the mold flux for hyper-peritectic steel.

Viscosity (Pa·s)	Break Temperature (°C)	Melting Temperature (°C)	Melting Range (°C)	Melting Rate (s)	Crystallization Temperature (°C)	Incubation Time (s)	Critical Cooling Rate (°C·s⁻¹)
0.08	1110	1050	68	22	1276	56	72

Table 8. Cracking degree statistics.

Mold Flux	Casting Speed m/min	Degree of Cracking (Crack Length/Steel Coil Length)
A	5.5	0.056%
B	5.5	0.031%

Note: “Degree of cracking” comes from the statistical method used in actual production, which is based on the results obtained from the surface inspection system.

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Yuan, Z.; Zhu, L.; Wang, X.; Zhang, K. Analysis of Longitudinal Cracking and Mold Flux Optimization in High-Speed Continuous Casting of Hyper-Peritectic Steel Thin Slabs. Metals 2024, 14, 909. https://doi.org/10.3390/met14080909

AMA Style

Yuan Z, Zhu L, Wang X, Zhang K. Analysis of Longitudinal Cracking and Mold Flux Optimization in High-Speed Continuous Casting of Hyper-Peritectic Steel Thin Slabs. Metals. 2024; 14(8):909. https://doi.org/10.3390/met14080909

Chicago/Turabian Style

Yuan, Zhipeng, Liguang Zhu, Xingjuan Wang, and Kaixuan Zhang. 2024. "Analysis of Longitudinal Cracking and Mold Flux Optimization in High-Speed Continuous Casting of Hyper-Peritectic Steel Thin Slabs" Metals 14, no. 8: 909. https://doi.org/10.3390/met14080909

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