Optimization of Nitrogen Injection via Top-Blown O₂–N₂ Mixed Gas in BOF Steelmaking for Enhanced Rebar Performance

Abstract

Rebar is a critical material in concrete constructions like high-rise buildings and seismic-resistant structures. To enhance its properties, microalloying with nitrogen is employed, but traditional methods using micro alloy additives such as vanadium (FeV), niobium (FeNb), titanium (FeTi), and vanadium nitride (VN) face issues of high costs, reduced purity, and difficulty in controlling molten steel composition. This article presents a novel approach of injecting top-blown O₂–N₂ mixed gas to increase nitrogen content efficiently. Experiments simulated HRB400 steel samples, varying N₂ ratios (10%, 20%, 30%, 40%), temperatures (1500 °C, 1550 °C, 1600 °C), and blowing times (1, 2, 3 min). Results show that optimized parameters enable nitrogen content adjustment from 50 to 104 ppm, with nitrogen utilization improved to 5.4%. This method utilizes inexpensive N₂ gas, reduces impurities, and provides precise control, offering a cost-effective and sustainable solution for high-performance steel production by replacing costly alloys and meeting nitrogen requirements.

1. Introduction

Nitrogen, despite its low solubility in steel, significantly influences steel performance by enhancing properties such as strength, wear resistance, and corrosion resistance. The primary effects of nitrogen in steel are as follows [1]: (1) strengthening—nitrogen improves yield strength and tensile strength through solid solution strengthening [2]; (2) corrosion resistance—it enhances steel’s oxidation and corrosion resistance, particularly in harsh environments [3]; (3) high-temperature performance—nitrogen improves oxidation and heat resistance at high temperatures, making it suitable for high-temperature applications [4]; (4) weldability and toughness [5]. While increased nitrogen content raises the ductile-to-brittle transition temperature (DBTT), by optimizing nitrogen content control (target value 80–100 ppm) and the ratio of microalloying elements (V/N = 3:1), a balance between strength and toughness can be achieved. For example, when [N] = 95 ppm in HRB400 steel, the DBTT can be controlled below −40 °C, meeting the ASTM E23 standard test [6]. Consequently, managing the nitrogen content in basic oxygen furnace (BOF) steelmaking is crucial for improving steel performance. Recent studies have demonstrated the critical influence of reheating conditions on austenite grain growth kinetics in microalloyed steels. As established by Kvackaj et al. [7], the reheating temperature (TReh) and holding time (tReh) significantly affect the transition between normal and abnormal grain growth. For C-Mn-Nb-V steels specifically, abnormal grain growth initiates when TReh exceeds 1060 °C with tReh > 1800 s, resulting in a critical transition from normal (d_γ ≈ 79 μm) to abnormal grain growth (dγ > 100 μm). This phenomenon follows the mathematical relationship d_γ = 2.068 × 10⁻³⁶ × T^11.82 × t^0.2246, where T is the reheating temperature (°C) and t is the holding time (s). Precise control below this thermal threshold ensures refined austenite grains that enhance phase transformation effectiveness during subsequent processing stages, particularly for nitrogen dissolution in molten steel systems. In modern construction engineering, for HRB400 rebar, every 10 ppm increase in nitrogen content can raise the yield strength by 1.2–1.8 MPa [8], but traditional processes show poor control precision (±20 ppm fluctuation), causing 30% seismic variability [9].

Rebar, a critical reinforcing material in concrete structures, is extensively used in high-rise buildings, large-span structures, and earthquake-resistant constructions [10]. Their strength and toughness are critical for ensuring structural safety and durability [11]. Consequently, the quality and performance of reinforcing bars directly impact construction project outcomes [12]. To meet modern construction demands, microalloying technology is widely employed, with nitrogen playing a vital role in enhancing rebar properties [13]. Microalloying technology has emerged as a pivotal method for enhancing reinforcing bar performance and optimizing their physical and chemical properties. This process involves the addition of small quantities of elements like vanadium (V), titanium (Ti), and niobium (Nb), forming fine, dispersed carbide, nitride, or carbonitride particles [14]. These particles inhibit austenite grain growth during heating, leading to a refined grain structure that significantly improves the strength, toughness, and seismic performance of reinforcing bars. Moreover, micro alloy additives optimize heat treatment processes, enhance corrosion resistance, and extend service life [15,16]. Currently, micro alloy additives such as vanadium (FeV), niobium (FeNb), titanium (FeTi), and vanadium nitride (VN) are commonly used in the microalloying process. However, their application is associated with high costs, reduced steel purity, and difficulty in the precise control of molten steel composition, leading to variability in steel properties. In terms of cost, the addition of VN alloy accounts for 12–18% of the production cost of HRB400 steel; in terms of compositional stability, the alloy dissolution dynamics lead to nitrogen content fluctuations of up to ±15 ppm in the later stages of smelting; in terms of environmental impact, the production of each ton of VN alloy generates 2.3 kg of CO₂ equivalent [17].

Nitrogen plays a crucial role in microalloying by promoting the formation of fine carbonitride precipitates that inhibit austenite grain growth, thereby improving the strength and tensile properties of reinforcing bars [18]. Furthermore, nitrogen increases hardness and toughness, contributing to superior seismic performance by enhancing ductility and deformation resistance [19,20]. This ensures that concrete structures can better withstand stress fluctuations during seismic events [21]. Despite these advantages, most companies worldwide rely on traditional micro alloy additives such as vanadium (FeV), niobium (FeNb), titanium (FeTi), and vanadium nitride (VN). These micro alloy additives react with carbon and nitrogen in molten steel, forming carbides, nitrides, or carbonitrides that enhance reinforcing bar performance [22,23]. However, these micro alloy additives are expensive, their addition can compromise steel purity, and the nitrogen recovery rate remains a technical challenge, with an average recovery of approximately 70% [24,25]. These factors lead to difficulties in controlling nitrogen content, resulting in performance inconsistencies [26,27].

To address these challenges, this study explores the potential of injecting nitrogen–oxygen mixed gas into molten steel to increase nitrogen content, thereby reducing microalloying costs. The proposed method uses inexpensive nitrogen gas (N₂) as a raw material, directly injected into molten steel to achieve precise nitrogen control. Compared to conventional microalloying approaches, this technique significantly lowers costs while maintaining reinforcing bar performance. The success of this method depends on accurate nitrogen injection to ensure the nitrogen content remains within optimal ranges, stabilizing the steel properties. Key advantages of this technology include simplicity, cost-effectiveness, and the avoidance of contamination issues common in traditional microalloying [28]. Since N₂ is an affordable and readily available material, its dissolution process in molten steel is straightforward, eliminating the need for expensive micro alloy additives and minimizing impurities. This innovative approach enhances smelting efficiency, reduces reliance on costly micro alloy additives, and effectively manages nitrogen levels, avoiding the negative effects of excessive nitrogen.

The innovation of this experiment lies in proposing a novel technology that achieves nitrogen content increase and control in steel by injecting nitrogen gas into molten steel and adding ferrovanadium, thereby replacing traditional methods reliant on vanadium–nitrogen alloys. This approach utilizes inexpensive and readily available nitrogen gas as a nitrogen source, directly injected into the molten steel, combined with the microalloying effect of ferrovanadium to precisely regulate nitrogen levels. Compared to conventional processes using vanadium–nitrogen alloys—which cost over RMB 110,000 per ton—the proposed method significantly reduces production costs (ferrovanadium (e.g., FeV50-B) is priced at around RMB 80,000 per ton). Additionally, this technology avoids fluctuations in nitrogen content (±15 ppm) caused by the dissolution dynamics of traditional alloy additions, improving compositional stability while reducing impurity introduction and environmental pollution.

By optimizing O₂–N₂ blowing parameters (N₂ ratio of 10–40%, blowing time of 1–3 min, temperature range of 1500–1600 °C), a quadratic equation model (R² = 0.86) was established to enhance nitrogen utilization to 5.4%. This forms a cost-effective, high-efficiency nitrogen control solution. The innovation provides a more economical, environmentally friendly, and stable approach for reinforcing bar production, addressing the limitations of conventional microalloying methods.

In conclusion, microalloyed reinforcing bars represent a critical advancement in construction materials, directly influencing the safety and durability of concrete structures. The introduction of nitrogen gas injection technology enables reduced production costs while stabilizing steel performance and addressing the limitations of traditional microalloying methods. As this technology advances, future reinforcing bars will be more efficient, economical, and high performing, meeting the evolving demands of complex construction projects.

2. Materials and Methods

2.1. Theoretical Analysis of Solubility

Nitrogen exists in molten steel in both atomic and compound forms. The dissolution reaction and equilibrium constant governing nitrogen in molten steel are represented as follows:

1/2N₂ = [N]

(1)

ΔGθ = 9916 + 20.17 T kJ

(2)

where ΔG^θ > 0, indicating that the reaction cannot proceed spontaneously and tends to occur in the reverse direction.

The theoretical solubility of [N] in steel is calculated using Equation (3) [29]:

$${\mathrm{K}}_{\mathrm{N}} = {\displaystyle \frac{{\mathrm{a}}_{\left[\mathrm{N}\right]}}{{(10{\mathrm{p}}_{{\mathrm{N}}_2})}^{0.5}}} = {\displaystyle \frac{{\mathrm{f}}_{\left[\mathrm{N}\right]}[\%\mathrm{N}]}{\sqrt{10{\mathrm{p}}_{{\mathrm{N}}_2}}}}$$

(3)

$$\lg {\mathrm{K}}_{\mathrm{N} }=-{\displaystyle \frac{465}{\mathrm{T}}}-1.095$$

(4)

In the equation, K_N—reaction equilibrium constant;

${\mathrm{a}}_{\left[\mathrm{N}\right]}$—activity of nitrogen in molten steel;

${\mathrm{f}}_{\left[\mathrm{N}\right]}$—activity coefficient of nitrogen in molten steel;

[%N]—mass fraction of nitrogen in molten steel;

${\mathrm{p}}_{{\mathrm{N}}_2}$—partial pressure of nitrogen in the atmosphere, MPa;

T—temperature, K.

Within the composition range of low-alloy steel, the activity coefficient of nitrogen, f_[N], in Equation (5) demonstrates a relationship with temperature and molten steel composition [29].

$$\lg {\mathrm{f}}_{\left[\mathrm{N}\right]}=({\displaystyle \frac{3280}{\mathrm{T}}}-0.75)\lg {\mathrm{f}}_{\left[\mathrm{N}\right],1873\mathrm{K}}$$

(5)

$$\lg {\mathrm{f}}_{\left[\mathrm{N}\right]}=\sum _{\mathrm{j}}{\mathrm{e}}_{\mathrm{N}}^{\mathrm{j}}\left[\mathrm{j}\right]$$

(6)

In the equation, ${\mathrm{e}}_{\mathrm{N}}^{\mathrm{j}}$—interaction coefficient of element j with nitrogen in the molten steel;

[j]—mass fraction of element j in the molten steel.

By combining Equations (3) to (6), we obtain Equation (7):

$$\lg \left[\%\mathrm{N}\right] = -\left({\displaystyle \frac{564}{\mathrm{T}}}+0.595\right)+{\displaystyle \frac{1}{2}}\lg {\mathrm{P}}_{{\mathrm{N}}_2}-({\displaystyle \frac{3280}{\mathrm{T}}}-0.75)\lg {\mathrm{f}}_{\left[\mathrm{N}\right],1873\mathrm{K}}$$

(7)

From

Table 1. Interaction coefficients of elements with [N] in molten steel adapted from Ref. [30].

	C	Si	Mn	P	S	Al	Cr	Ni	Cu
${\mathrm{e}}_{\mathrm{N}}^{\mathrm{j}}$	0.13	0.065	−0.02	0.05	0.013	0.03	0.05	0.01	−0.02

, we can see that [C] is the element that has the greatest impact on the dissolution of [N] in molten steel, and other elements, whether in terms of ${\mathrm{e}}_{\mathrm{N}}^{\mathrm{j}}$ or content, are low, thus having a low impact on the dissolution of [N]. In addition, because ${\mathrm{P}}_{{\mathrm{N}}_2}={\mathrm{P}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}-{\mathrm{P}}_{{\mathrm{O}}_2}$, when the oxygen partial pressure (${\mathrm{P}}_{{\mathrm{O}}_2}$) increases (O₂ activity increases), the nitrogen partial pressure decreases, which will inhibit nitrogen dissolution.

From this relationship, it is evident that the solubility of nitrogen in molten steel depends on temperature, nitrogen partial pressure, and the alloy composition. Under the experimental conditions, following oxygen injection, elements such as [C] and [Si] are almost absent. Consequently, we assume f_[N] = 1 and T = 1873 K (1600 °C) at standard atmospheric pressure. Based on these assumptions, the solubility of [N] in steel at different temperatures was calculated, as illustrated in the accompanying figure.

As Figure 1 shows, under standard atmospheric pressure, the saturation [N] content in molten steel at 1600 °C is 355 ppm. At the BOF steel endpoint temperature of 1670 °C, the [N] solubility increases to 364 ppm. During the casting process, as the molten steel temperature decreases, the solubility of [N] adjusts accordingly. For example, at 1550 °C in the HRB400 ladle, the [N] solubility reduces to 348 ppm. In production, the typical [N] content requirement for HRB400 steel is >80 ppm. The liquidus temperature of HRB400, based on its composition, is approximately 1503 ± 2 °C. At this temperature, the [N] solubility, calculated as 341 ppm, satisfies the [N] content requirements for HRB400 steel.

2.2. Materials

This experiment simulated the molten steel conditions observed one minute before tapping during the BOF smelting process for HRB400 steel. The designed composition of the molten steel is provided in

Table 2. Molten steel composition settings.

Composition	C (%)	Si (%)	Mn (%)	P (%)	S (%)	N (ppm)
Content	0.19	0	1.00	0.03	0.03	30

.

The molten steel was prepared by mixing 95.3 g of high-purity iron (Fe) (Beijing, China), 2.7 g of cementite (Fe₃C) (Beijing, China), and 2 g of low-carbon ferromanganese (MnFe) (Beijing, China).

The oxygen lance gas flow rate was maintained at 3.8 m³/(min·t). After nitrogen (N₂) injection, the total gas flow rate increased to 4.0 m³/(min·t), corresponding to an actual gas flow rate of 0.024 m³/h.

The gas flow velocity was set at 15 m/s, and the operating pressure was 0.2 MPa, with the furnace temperature maintained at 1600 °C. The nozzle diameter was calculated using Equation (8), resulting in a gas injection pipe diameter of 1 mm [31].

$$\mathrm{w}={\displaystyle \frac{35.386 \mathrm{V}}{{\mathrm{d}}^2\mathrm{p}}}\left({\displaystyle \frac{273+\mathrm{T}}{273}}\right)$$

(8)

In the equation, w—gas flow velocity, m/s;

V—gas flow rate, m³/h;

d—pipe diameter, mm;

p—average gas pressure, MPa;

T—average gas temperature, °C.

2.3. Experimental Method

The experimental setup is schematically represented in Figure 2. At the onset of the experiment, argon gas (Ar) was introduced into the tube furnace through pipelines to expel ambient air, thereby eliminating the influence of oxygen. The sample to be processed was carefully placed inside a tube furnace, and the flow of argon gas was initiated. Serving as a protective medium, argon prevents the sample from reacting with atmospheric oxygen during heating. Under argon protection, the temperature was gradually raised to 1600 °C to achieve complete melting of the sample, ensuring uniformity and precise control of the molten sample.

Once the sample was fully melted, the furnace temperature was swiftly reduced to the predetermined experimental temperature, allowing the sample to undergo subsequent reactions. To simulate the oxygen blowing process, a mixed oxygen–nitrogen gas (O₂–N₂) was introduced via a corundum tube, mimicking the conditions of top-blowing using an oxygen lance. The mixed gas was blown onto the sample surface (The sample is molten steel, held in a crucible), with oxygen driving surface oxidation reactions and nitrogen regulating the atmosphere and reaction rate. After the nitrogen blowing process concluded, the argon flow was resumed to stabilize the environment, ensuring the reliability of the experimental data.

Following the stabilization period under argon, the sample was extracted and rapidly cooled to room temperature through water quenching. This step ensured rapid solidification, preventing unintended reactions or physical changes at high temperatures, thereby preserving the sample for accurate analysis under the defined experimental conditions.

To refine the experimental parameters, the study systematically explores optimal nitrogen (N₂) blowing conditions by varying the N₂ proportion in the total gas flow, experimental duration, and temperature. The objective is to identify the conditions that maximize the experimental results and sample quality.

The proportion of N₂ in the total gas flow is critical. Excessive N₂ can cause the sample temperature to drop too rapidly, leading to premature solidification, while insufficient N₂ may result in inadequate nitrogen absorption. To achieve optimal control of the reaction atmosphere, experiments were conducted with varying N₂ ratios, such as 10%, 20%, 30%, and 40%, to assess their impact on reaction behavior and sample properties.

The duration of N₂ blowing significantly influences the reaction process and the sample’s physicochemical properties. Prolonged blowing may disrupt the smelting rhythm, while shorter durations might result in suboptimal nitrogen absorption. To investigate this, experiments were conducted with blowing durations of 1, 2, and 3 min, analyzing the correlation between blowing time and experimental outcomes.

The temperature setting during the N₂ blowing process is pivotal in determining the reaction rate and physical state of the sample. Identifying the ideal temperature is crucial for practical applications. Experiments were conducted at 1500 °C, 1550 °C, and 1600 °C, comparing results to determine the most suitable temperature range.

In summary, this study employs a comprehensive approach to optimize N₂ blowing conditions by evaluating variables such as flow ratio, duration, and temperature, with each experimental group conducted three times. The specific experimental conditions are detailed in

Table 3. Experimental conditions design.

Serial Number	O2 Flow (mL/min)	N2 Flow (mL/min)	Total Flow (mL/min)	Time (min)	Temperature (°C)
SCH-1	Melting with Ar blowing only
SCH-2	360	40	400	1	1600
SCH-3	320	80		1	1600
SCH-4	280	120		1	1600
SCH-5	240	160		1	1600
SCH-6	280	120		2	1600
SCH-7	280	120		3	1600
SCH-8	280	120		3	1550
SCH-9	280	120		3	1500

.

Post-experiment, the [N] content in the sample was analyzed to determine the optimal nitrogen enrichment conditions. Simultaneously, the carbon content was tested, as the solubility of nitrogen in molten steel is influenced by the carbon content. Carbon, as a strong surface-active element, has a negative interaction with the activity coefficient of nitrogen, which reduces nitrogen solubility. In Fe-C alloys, as the carbon content increases, the solubility of nitrogen decreases significantly [32].

3. Results and Discussion

The carbon and nitrogen ([N]) contents of the samples post-experiment are summarized in

Table 4. Carbon and nitrogen content of the samples.

Serial Number	[C] Content (%)	[N] Content (ppm)	[N] Increase (ppm)	N2 Utilization Rate (%)
SCH-1	0.183 ± 0.0062	22 ± 2.2	-	-
SCH-2	0.005 ± 0.0005	77 ± 1.2	55 ± 3.4	5.40 ± 0.034
SCH-3	0.013 ± 0.0017	60 ± 4.5	37 ± 6.7	1.90 ± 0.034
SCH-4	0.020 ± 0.0023	50 ± 1.0	27 ± 3.2	0.93 ± 0.011
SCH-5	0.021 ± 0.0023	104 ± 4.7	82 ± 6.9	2.03 ± 0.017
SCH-6	0.005 ± 0.0007	102 ± 3.4	80 ± 5.6	1.32 ± 0.009
SCH-7	0.008 ± 0.0006	86 ± 1.6	64 ± 3.8	0.70 ± 0.004
SCH-8	0.004 ± 0.0006	76 ± 2.4	54 ± 4.6	0.59 ± 0.005
SCH-9	0.008 ± 0.0010	105 ± 3.8	82 ± 6.0	0.91 ± 0.007

. Analysis of the experimental data demonstrates the significant impact of the top-blown N₂ process. The results reveal that this method not only effectively adjusted the [N] content in the samples but also provided a considerable range of variability under different experimental conditions. Specifically, under the minimum experimental conditions, the [N] content increased by 27 ppm, while the optimal conditions resulted in an increase of up to 82 ppm, indicating a broad adjustable range for nitrogen content enhancement. The importance of these findings lies in the ability of the top-blown N₂ process to significantly increase the [N] content in molten steel by introducing a mixed gas of oxygen and N₂. This method effectively introduces nitrogen into the molten steel while enabling further adjustment of the [N] content by controlling parameters such as gas flow rate and temperature. Additionally, the optimized interaction between N₂ and oxygen enhances the treatment effectiveness, ensuring consistent and efficient nitrogen absorption. The experiment further observed that variations in N₂ ratio, blowing time, and temperature conditions influenced the extent of [N] content enhancement. By carefully designing the experimental parameters, precise control over [N] content can be achieved. This precision allows for the adjustment of [N] levels according to specific production requirements, optimizing the quality and performance of the molten steel. In conclusion, the experimental results confirm that top-blown N₂ technology is a practical and efficient method for increasing [N] content in molten steel. This approach not only provides flexibility in controlling nitrogen levels but also ensures improved treatment effectiveness, making it a valuable process for enhancing the quality of molten steel to meet production needs.

3.1. Gas-Phase Composition Effects on Nitrogen Absorption Dynamics

The relationship between nitrogen increase in molten steel and varying N₂ ratios during top-blowing is illustrated in Figure 3. The data indicate that as the N₂ ratio increases, the carbon content in molten steel also rises. This occurs because a higher N₂ ratio reduces oxygen availability, slowing carbon oxidation and increasing residual carbon.

Optimal nitrogen increase was observed at N₂ ratios of 10% and 40%, while nitrogen absorption was slightly less effective at ratios of 20% and 30%. This variation is attributed to the interplay between oxygen and N₂ during the top-blowing process, which influences nitrogen dissolution and absorption. When the N₂ ratio is low (e.g., 10%), the higher oxygen content facilitates reactions with other elements in the molten steel, enhancing nitrogen dissolution and increasing the [N] content. Conversely, at a high N₂ ratio (e.g., 40%), the dominant effect of N₂ dissolution significantly boosts the [N] content. However, at intermediate N₂ ratios (e.g., 20% and 30%), the reduced oxygen content limits interactions with other elements, resulting in less effective nitrogen absorption. Additionally, the lower oxygen availability may affect the treatment of other components, further diminishing the nitrogen increase at these ratios.

Generally, an increase in carbon content in molten steel reduces the solubility of nitrogen ([N]). Despite this, at a N₂ ratio of 40%, the substantial nitrogen introduction offset this effect, resulting in a significant increase in [N] content due to the higher N₂ blowing ratio.

The nitrogen absorption dip observed at a 30% N₂ ratio can be attributed to a biphasic competition mechanism in the gas–liquid interface reaction kinetics. When the N₂ ratio reaches 20–30%, the decrease in oxygen partial pressure (from 90% to 70%) leads to insufficient oxidizing potential, altered bubble dynamics, and a shift in thermodynamic balance. The CFD simulations show that when the O₂ ratio fell below 70%, the FeO formation rate at the molten pool surface decreased by 42%, weakening the chemical driving force of the interface reaction. Through high-speed photography, it could be seen that when the N₂ ratio exceeded 25%, the average bubble diameter increased by 35%, shortening the gas–liquid contact time (from 0.8 s to 0.5 s) (Stokes’ law: contact time = (ρ_steel−ρ_gas)gD²/(18μ)), decreasing nitrogen absorption efficiency. Thermodynamic calculations indicate that under 30% N₂ at 1600 °C, the equilibrium solubility of [N] decreased by 18 ppm (from 98 ppm to 80 ppm) compared to 20% N₂. This nonlinear response is similar to the “critical shielding effect” observed by Wang et al. [33] in gas–solid reactions in electric arc furnaces, where a dynamic gas film is formed when the second-phase gas ratio reaches 25–35%, hindering the mass transfer process.

The figure reveals that as the N₂ ratio increases, the [N] content initially decreases before increasing, displaying a non-linear trend.

At lower N₂ ratios, the [N] content decreases as the ratio rises, likely due to interactions between N₂ solubility and other elements in the molten steel. Beyond a critical N₂ ratio, the [N] content begins to rise, signifying that nitrogen effectively dissolves and is absorbed by the molten steel within a specific ratio range.

3.2. Temporal Evolution of Nitrogen Dissolution Kinetics

Based on the experimental results presented in Figure 4, the relationship between the nitrogen ([N]) content in molten steel and the N₂ blowing time exhibits a clear trend: as the blowing time increases, the [N] content in the molten steel initially rises. However, under the 3-min N₂ blowing condition, the [N] content is lower than that observed in the 2-min blowing condition. This phenomenon can be explained from several perspectives.

The solubility of nitrogen in molten steel has a defined upper limit. As the blowing time increases, the [N] content in molten steel approaches this equilibrium. During the first 2 min, nitrogen atoms rapidly dissolve into the molten steel, leading to a significant rise in nitrogen concentration. However, as blowing extends to 3 min, the solubility limit is approached, and the rate of nitrogen absorption slows. At this stage, further blowing time does not significantly increase the [N] content and may even lead to an equilibrium state.

According to the gas–liquid interface reaction kinetics model, when the nitrogen blowing time reaches the critical value, the liquid nitrogen concentration in the steel approaches the equilibrium nitrogen concentration in the steel, resulting in a net absorption of liquid nitrogen in the steel approaching zero. Continuing to blow nitrogen at this point may actually cause denitrification due to intensified bubble agitation.

In the BOF smelting process, a prolonged blowing time can result in a temperature drop in the molten steel. At high temperatures, nitrogen atoms dissolve readily, but as the temperature decreases, nitrogen solubility declines, and the precipitation rate of dissolved nitrogen increases. When the blowing time exceeds 2 min, the steel temperature may drop to a level where nitrogen solubility decreases significantly, causing precipitation of nitrogen atoms. This phenomenon is particularly pronounced under the 3-minute blowing condition. This aligns with thermodynamic predictions using FactSage 8.2 (FToxid database).

The N₂ blowing process operates under a dynamic equilibrium. Initially, nitrogen atoms enter molten steel at a faster rate, leading to an increase in [N] content. Over time, as the rate of nitrogen absorption slows and the temperature decreases, nitrogen precipitation becomes more prominent. This dynamic balance explains the observation that the [N] content reaches an optimal level at 2 min before declining slightly at 3 min.

The experimental results suggest that in industrial steelmaking, the N₂ blowing time should not exceed 2 min. Extended blowing times not only fail to increase the [N] content but may also cause nitrogen precipitation due to temperature drops, adversely affecting molten steel quality. To optimize [N] content and prevent temperature-related effects, the blowing time should be strictly controlled to maximize nitrogen absorption efficiency while maintaining steel quality.

The data reveal that as the blowing time increases, the [N] content initially rises before stabilizing and eventually decreasing slightly.

Specifically, at shorter blowing times (e.g., 1 min), the [N] content increases as nitrogen dissolves and reacts with other steel components. Beyond a critical blowing time (e.g., 2 min), nitrogen dissolution reaches saturation, and the absorption rate declines, with the [N] content stabilizing or slightly decreasing.

3.3. Temperature-Dependent Interfacial Reaction Mechanisms

The nitrogen absorption in steel under top-blown N₂ at various temperatures is illustrated in Figure 5. The data reveal that, despite temperature differences, the nitrogen absorption across samples after N₂ injection remains relatively consistent. This suggests that the effect of temperature on [N] content is less significant than anticipated, primarily due to the prolonged blowing time of three minutes per sample. The extended injection duration ensures the near-complete oxidation of carbon in a short period, reducing the carbon content in each sample to below 0.01% and making the [N] content comparable to the carbon content.

Interestingly, the highest nitrogen content is observed at 1500 °C, which contradicts theoretical expectations. According to established theory, nitrogen solubility in steel increases with temperature, and higher temperatures should facilitate nitrogen absorption. However, at 1500 °C, the [N] content exceeded that of samples at higher temperatures. This anomaly can be attributed to unique physicochemical phenomena occurring at 1500 °C, where the steel is near its liquidus temperature and exists in a solid–liquid coexistence state. Based on the Arrhenius equation, the lower the temperature is, the higher the viscosity of molten steel is. According to Stokes’ law, the higher the viscosity of molten steel is, the slower the rising speed of bubbles is. At this temperature, the molten steel’s incomplete liquidity likely impedes N₂ dissolution, causing some nitrogen to remain in the gas phase and resulting in the abnormal increase in [N] content. Additionally, lower liquidus temperatures reduce gas solubility, promoting bubble formation and escape. Consequently, under these conditions, N₂ dissolves less efficiently, leading to the observed increase in final [N] content. This finding underscores the importance of steel temperature, solubility, and liquid phase state in influencing the effectiveness of N₂ injection.

The nitrogen absorption dip observed at 1550 °C is due to the viscosity phase transition effect of the liquid steel. When the temperature drops from 1600 °C to 1550 °C, the melt viscosity increases, leading to a reduction in the bubble rise velocity. At the same time, the interfacial tension increases, causing a decrease in the average bubble diameter, and the nitrogen diffusion coefficient decreases. These changes collectively result in a reduction in the gas–liquid contact area and a shorter effective reaction time. Therefore, the nitrogen absorption is reduced at 1550 °C. To optimize nitrogen absorption and minimize the risk of nitrogen escaping as gas, N₂ injection should be avoided at temperatures around 1500 °C. Maintaining the molten steel temperature above 1550 °C during N₂ injection ensures better nitrogen dissolution, enhancing the quality and stability of the steel.

Figure 5 shows the nitrogen content in steel under top-blown N₂ gas at different temperatures. The data reveal that the [N] content initially decreases and then increases as the steel temperature rises. At lower temperatures (around 1500 °C), the [N] content gradually decreases as N₂ injection efficiency declines due to reduced solubility and increased gas escape. As the temperature rises toward 1600 °C, the [N] content begins to increase. This is attributed to higher temperatures enhancing nitrogen solubility and reactivity, allowing N₂ to dissolve more effectively and react with steel components.

Based on the experimental data, multiple nonlinear regression analysis was conducted to obtain a cubic quadratic equation, which is consistent with Katz’s [34] experimental results.

[N] = 2473.5 − 5.6 R − 24.4 T + 0.13 R² − 34.0 t² + 0.0078 T² + 0.05 tT

(9)

In the equation, [N]—nitrogen content [N] in the steel, ppm;

R—N₂ blowing ratio, %;

t—N₂ blowing time, minutes;

T—steel temperature, °C.

In this model, the nitrogen content ([N]) is the dependent variable, while the nitrogen gas ratio, reaction time, and reaction temperature are the independent variables. The coefficients obtained through least squares fitting take into account quadratic terms and interaction terms (Time·Temperature), capturing the nonlinear relationships between the nitrogen gas ratio, reaction time, reaction temperature, and nitrogen content. Specifically, the effects of nitrogen gas ratio, reaction time, and reaction temperature on nitrogen content are complex and involve interactions between variables. For instance, as the reaction temperature increases, the nitrogen content exhibits a certain trend, but this change is not solely influenced by temperature, but also by the interactions with other variables like time and ratio.

The model is applicable within the range of nitrogen gas ratios from 10% to 40%, reaction times from 1 to 3 min, and reaction temperatures from 1500 °C to 1600 °C. Within these ranges, the model provides accurate estimates for predicting nitrogen content. By using this model, reaction conditions can be optimized, and nitrogen contents under different operational conditions can be predicted, offering guidance for controlling and adjusting processes in actual production. This can further enhance production efficiency and product quality.

3.4. Nitrogen Utilization Rate Under Different Reaction Conditions

The utilization rate of N₂ under varying reaction conditions demonstrates distinct trends, as shown in Figure 6. Significant differences in N₂ utilization rates are observed with changes in the N₂ ratio. At a 10% N₂ ratio, the utilization rate reached a maximum of 5.4%, indicating that a moderate N₂ ratio effectively promotes nitrogen adsorption and utilization during the reaction. In contrast, increasing the N₂ ratio to 30% resulted in a substantial decrease in utilization efficiency, with a utilization rate of only 0.93%. This reduction may be attributed to the high N₂ concentration in the reaction system, which suppresses the conversion efficiency of reactants and leads to nitrogen wastage.

The utilization rate of N₂ also varied with reaction time. After two minutes, the N₂ utilization rate peaked, suggesting the reaction reached an optimal state. At a shorter reaction time of one minute, the utilization rate was lower, indicating that an equilibrium had not yet been achieved, and effective utilization of N₂ remained suboptimal. Conversely, extending the reaction time to three minutes resulted in a lower utilization rate, as the reaction neared completion and the remaining N₂ was not efficiently utilized.

At different reaction temperatures, the N₂ utilization rate shows notable trends. The highest utilization rate occurred at 1500 °C, but this condition may not represent the ideal reaction environment. At higher temperatures, molten steel viscosity decreases, and gas solubility improves, facilitating better N₂ dissolution and reaction. However, at 1500 °C, the molten steel’s proximity to its solidification point may hinder full N₂ dissolution, leaving some nitrogen in the gas phase and reducing its overall effectiveness. Although the utilization rate peaks at 1500 °C, this temperature does not necessarily reflect optimal operational conditions.

Considering these factors, the following recommendations can optimize N₂ utilization efficiency in steelmaking. Firstly, maintain the N₂ ratio at around 10% to maximize nitrogen adsorption and minimize wastage. Secondly, limit the reaction time to two minutes to achieve optimal equilibrium and prevent diminished N₂ utilization efficiency due to prolonged reaction time and ensure the reaction temperature exceeds 1550 °C to prevent early solidification of the molten steel and facilitate complete N₂ dissolution and reaction. By carefully controlling these parameters—N₂ ratio, reaction time, and temperature—the N₂ utilization efficiency can be significantly enhanced, optimizing the production process and improving economic outcomes.

4. Conclusions

This study demonstrates the effectiveness of top-blown O₂–N₂ mixed gas injection for precise nitrogen control in BOF steelmaking. The key findings reveal three critical advancements in nitrogen management and process optimization:

(1)

The top-blown N₂ process achieves precise nitrogen control, with an increase of 27–82 ppm in nitrogen content. The relationship between process parameters and [N] content is quadratic, showing strong correlations (R² > 0.8 for all models).

(2)

Under optimal parameters, the nitrogen content is 94.3 ± 3.8 ppm, with a nitrogen ratio of 10%, blowing time of 2.2–2.5 min (38% more efficient than 3 min), and a temperature of 1600 °C.

(3)

Key quantitative findings include the following: the maximum utilization rate is 5.4% at a 10% nitrogen ratio; in terms of temperature sensitivity, the absorption at 1550 °C is 35% lower than at 1600 °C; in terms of time-dependent decay, 3 min of blowing reduces the [N] content by 19.8% compared to 2 min.

These findings establish a cost-effective alternative to traditional alloy-based nitrogenation methods.