A method for controlling carbon increase in molten steel based on slag system modification

Abstract

This invention discloses a method for controlling carbon increase in molten steel based on slag system modification, comprising the following steps: S1, smelting endpoint control: controlling the carbon content, phosphorus and sulfur content, and tapping temperature of molten steel at the endpoint of converter or electric furnace smelting; S2, tapping alloying and initial carbon increase: adding a multi-element nitride alloy and a first carbon increaser to the ladle during tapping; S3, argon blowing and stirring: performing a first stage of high-flow-rate argon blowing and stirring; S4, argon blowing station fine-tuning: transporting molten steel to the argon blowing station for sampling and analysis, and supplementing with a second carbon increaser; S5, composition homogenization and soft blowing: sequentially performing a second stage of medium-flow-rate argon blowing and stirring, and a third stage of low-flow-rate soft blowing; wherein the mass percentage of manganese in the multi-element nitride alloy is ≥5%. The advantage of this invention is that by combining optimized alloy composition, segmented carbon increase, and gradient stirring, it achieves precise control of a narrow composition window in molten steel.

Description

Technical Field

[0001] This invention relates to the field of continuous casting production technology in iron and steel metallurgy, and in particular to a method for controlling carbon increase in molten steel based on slag system modification. Background Technology

[0002] In the production of high-strength steel bars for construction (such as HRB400E and HRB500E), precise control of the carbon (C) content in the molten steel is crucial to ensuring that its mechanical properties (such as yield strength and tensile strength) meet national standards (such as GB1499.2). To reduce the consumption of expensive microalloying elements (such as V and Nb), steel mills typically strive to stably control the carbon content within a narrow process window (e.g., 0.23%~0.25%) close to the standard upper limit (e.g., 0.25%), in order to achieve an optimal balance between cost and performance.

[0003] However, a prominent contradiction exists in existing technologies: to further enhance performance and save costs, nitrogen enrichment is often achieved by adding multi-element nitride alloys (mainly composed of silicon nitride, etc.). However, the silicon (Si) content in these alloys, up to approximately 45%, generates a large amount of high-melting-point silicon dioxide (SiO2, melting point 1713℃) in the high-temperature oxidizing atmosphere of molten steel treatment. This SiO2 forms a viscous, thick layer of foamy slag on the surface of the molten steel. When a carburizing agent (especially a low-cost but unstable yield common carburizing agent, such as calcined coal carburizing agent) is subsequently added to the molten steel, the carburizing agent particles are easily encapsulated and blocked by this foamy slag layer, preventing carbon from effectively penetrating the molten steel. This results in significant fluctuations and generally low yields of the carburizing agent. This makes final carbon content fine-tuning at the argon blowing station extremely difficult, often leading to finished product carbon content exceeding the control range, causing performance defects or increased alloy costs, becoming a technical bottleneck restricting the stable and efficient production of high-quality construction steel reinforcement.

[0004] In the prior art, Chinese patent application CN202010028733.1 discloses a method for adding alloys in stages, but it does not propose a solution to the specific mechanism of "the surface foam slag caused by the nitride alloy hindering carbonization". The difficulty in carbonization is due to insufficient stirring power or delayed detection, and it is often solved by increasing the stirring power or replacing expensive carbonizers. This not only consumes a lot of energy, but also fails to address the root cause of "deterioration of the physical properties of the slag phase". Summary of the Invention

[0005] The purpose of this invention is to provide a method for controlling the carbonization of molten steel based on slag system modification. By combining optimized alloy composition, segmented carbonization, and gradient stirring, a narrow composition window for molten steel can be precisely controlled.

[0006] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0007] A method for controlling carbon increase in molten steel based on slag system modification includes the following steps: S1, Smelting endpoint control: Controlling the carbon content, phosphorus and sulfur content, and tapping temperature of molten steel at the endpoint of converter or electric furnace smelting; S2, Alloying and Initial Carbon Addition: Before the molten steel is tapped, a multi-element nitride alloy is added to the bottom of the ladle, and the first carbon additive is added during the tapping process. S3, Argon blowing and stirring: Perform the first stage of high-flow argon blowing and stirring to promote alloy melting and mixing; S4, Argon blowing station fine-tuning: The molten steel is transported to the argon blowing station for sampling and analysis, and a second carburizing agent is added as a supplement; S5, composition homogenization and soft blowing: the second stage of medium-flow argon blowing and the third stage of low-flow soft blowing are carried out sequentially. In the multi-element nitride alloy, the mass percentage of manganese is ≥5%, which is used to work synergistically with silicon in the alloy to form a low-melting-point liquid phase slag on the surface of molten steel.

[0008] Preferably, in step S1, the carbon content is controlled to be 0.04%~0.06%, the phosphorus content to be ≤0.045%, the sulfur content to be ≤0.045%, and the tapping temperature to be 1620~1650℃.

[0009] Preferably, in step S2, the mass percentage of each component in the multi-element nitride alloy is: C≤5%, N≥25%, Si+Mn≥45%, and Mn content≥5%, P≤0.15%, S≤0.015%, with the balance being Fe and unavoidable impurities. The mass ratio of Si to Mn is controlled between (4~8):1, and the amount of multi-element nitride alloy added is 0.60~0.65 kg / t.

[0010] The alloy can also contain 0.03~0.10% rare earth element RE, which can be used to further reduce the viscosity of the slag and purify the molten steel.

[0011] Preferably, in step S2, the first carbon raiser is a calcined coal carbon raiser, and the amount of carbon added to the first carbon raiser is 80% to 90% of the target carbon content value of the molten steel.

[0012] Preferably, in step S3, the flow rate of the high-flow-rate argon blowing stirring is 40~50 NL / min·t, and the stirring time is 2~5 minutes, so as to promote alloy melting and initial mixing.

[0013] Preferably, in step S4, the second carbonizing agent is a petroleum pitch coke carbonizing agent or carbonizing ball, and the amount of carbon added to the second carbonizing agent is the difference between the target carbon content value of the molten steel and the carbon content value of the molten steel sample analysis.

[0014] Preferably, in step S5, the argon flow rate for stirring in the second stage is 35~45 NL / min·t, and the stirring time is 4~8 minutes to ensure that the second carburizing agent is completely dissolved and the composition of the molten steel is highly uniform.

[0015] Preferably, in step S5, the argon flow rate of the third stage low-flow soft blowing is 30~40 NL / min·t, and the soft blowing treatment lasts for 2~4 minutes.

[0016] Preferably, in step S5, the target carbon content of the molten steel is 0.23% to 0.25%, and the fluctuation of the target carbon content is controlled within ±0.02% to promote the floating of small inclusions, stabilize the temperature and flow field of the molten steel, and create conditions for subsequent casting processes.

[0017] In summary, the present invention has the following beneficial effects: 1. By forcibly adding no less than 5% manganese (Mn) to a multi-element nitride alloy, the manganese oxide (MnO) generated by its oxidation in molten steel reacts with the high-melting-point SiO2 generated by the oxidation of silicon nitride to form a low-melting-point manganese silicate (such as 2MnO·SiO2). This substance is liquid at the steel treatment temperature, which can effectively "digest" and reduce the viscous foam slag on the surface of molten steel, fundamentally improving the steel-slag interface conditions and removing physical obstacles to the efficient absorption of carburizing agents.

[0018] 2. This invention proposes a two-stage carbonization mode of "low-cost coarse adjustment of carbonizing agent + high-efficiency carbonizing agent fine adjustment". In the tapping stage, strong stirring is used to complete most (80%~90%) of the carbonization task at low cost; at the argon blowing station, based on accurate detection, a small amount of high-efficiency carbonizing agent with stable yield is used to complete the final fine adjustment. This strategy achieves the optimization of carbonization cost without sacrificing control accuracy.

[0019] 3. This invention designs a three-stage gradient argon blowing process of "high-medium-low". The high flow rate is used for initial alloy melting and mixing; the medium flow rate is used for composition homogenization during the fine-tuning stage of the argon blowing station; and the low flow rate is used for pure molten steel and stable state. This system provides optimal kinetic conditions for the carbonization process, ensuring high carbon yield and uniform distribution.

[0020] 4. This invention integrates the above technical solutions into an organic whole. Optimizing the alloy is the "fundamental solution," making precise carbon addition possible; segmented carbon addition is the "efficiency enhancement," achieving a win-win situation of cost and precision on the basis of addressing the fundamental solution; gradient argon blowing is the "escort," ensuring that the effects of the first two steps are maximized. The three work together to achieve the significant effect of stabilizing the carbon content fluctuation of the finished steel product within ±0.02%, increasing the hit rate of the carbon content target range (such as 0.23%~0.25%) to over 99%, and increasing the product mechanical property qualification rate to over 99.5%. Detailed Implementation

[0021] The specific embodiments of the present invention will be further described below. These embodiments do not constitute a limitation on the present invention.

[0022] Example 1 (Taking the production of HRB400E steel bars in a 120-ton converter with a target carbon content of 0.24% as an example) 1. Smelting: The final control of converter smelting is as follows: carbon content 0.05%, phosphorus content 0.032%, sulfur content 0.030%, and tapping temperature 1625℃.

[0023] 2. Tapping and Alloying: Before tapping, the optimized multi-element nitride alloy (composition: C: 4.2%, N: 28.5%, Si: 38%, Mn: 8%, P≤0.10%, S≤0.010%, balance Fe and impurities) is added to the bottom of the ladle at 0.62 kg / t. The ladle is opened to the tapping position, and bottom blowing argon is started at a flow rate of 45 NL / min·t. When about 1 / 3 of the steel has been tapped, ordinary calcined coal carbonizer is added. The amount added is calculated to increase the carbon content of the molten steel from the final carbon content to 85% of the target value of 0.24% (i.e., about 0.20%). Argon blowing and stirring at this stage last for about 3 minutes.

[0024] 3. Argon blowing station fine-tuning: Molten steel is transported to the argon blowing station, allowed to stand for 1 minute, and then sampled. Rapid analysis shows that the carbon content is 0.198%. Based on this result, petroleum pitch coke carbonizer is added, with the amount added being the amount of carbon required to accurately reach a carbon content of 0.24%. Subsequently, the argon blowing flow rate is adjusted to 40 NL / min·t, and the mixture is stirred for 5 minutes.

[0025] 4. Soft blowing: Reduce the argon flow rate to 35 NL / min·t and perform soft blowing treatment for 3 minutes. After the treatment, take a sample and measure the carbon content of the final product to be 0.241%.

[0026] Example 2 (Taking the production of HRB500E steel bars in a 100-ton electric furnace with a target carbon content of 0.23% as an example) 1. Smelting: The final control of electric furnace smelting is as follows: carbon content ≤0.10%, phosphorus content ≤0.030%, sulfur content ≤0.030%, and tapping temperature 1635℃.

[0027] 2. Tapping and Alloying: During tapping, the optimized multi-element nitride alloy (composition: C: 4.5%, N: 26.8%, Si: 40%, Mn: 6%, P≤0.12%, S≤0.012%, balance Fe) is added to the steel stream at a rate of 0.65 kg / t. The bottom-blown argon flow rate is 42 NL / min·t. Ordinary calcined coal carbonizer is added, with the addition amount calculated to increase the carbon content to 0.23% (i.e., 0.1955%), and the mixture is stirred for 2.5 minutes.

[0028] 3. Argon blowing station fine-tuning: The first analysis showed a carbon content of 0.195%. Add carbon-raising balls (a highly efficient carbon-raising agent) for precise replenishment. Blow argon at a flow rate of 38 NL / min·t and stir for 6 minutes.

[0029] 4. Soft blow: 32 NL / min·t, soft blow for 2.5 minutes. The final sample carbon content was 0.232%.

[0030] Comparative Example 1 (using conventional processes) Except for replacing the multi-element nitride alloy with a conventional high-silicon nitride alloy (Mn content <4%), and adding all the carbon raisers (using ordinary calcined coal carbon raisers) at once during steel tapping based on the target carbon content of 0.24%, the other process parameters are the same as in Example 1.

[0031] Results: The carbon content of the first sample from the argon blowing station was only 0.18%. After adding the carbon raiser, the carbon content of the final sample was 0.228%. However, process control was difficult, there was a lot of foam slag on the surface of the molten steel, the carbon raiser yield was unstable, and the carbon content fluctuated between batches by more than ±0.03%.

[0032] Comparative Example 2 (using an optimized alloy, but without segmented carbon addition) The same optimized alloy as in Example 1 was used, but all the carburizing agents (using petroleum pitch coke carburizing agents) were added at once during tapping.

[0033] Results: Although the final carbon content can reach 0.239%, the cost per ton of steel alloy is significantly higher than the segmented carbonization strategy in Example 1 due to the use of expensive and efficient carbon additives throughout the process.

[0034] Comparative Example 3 (Alloy without Mn control group) A conventional high-silicon nitride alloy (Mn content <4%) was used, and the remaining process parameters were the same as in Example 1.

[0035] Results: The initial carbon content at the argon blowing station was only 0.185%, far below the lower limit of the target range. Despite multiple additions, the final carbon content still fluctuated significantly. This demonstrates that Mn ≥ 5% is a necessary condition for slag system modification; the lack of this characteristic will lead to a deterioration of carbon addition kinetics, making precise carbon blending difficult.

[0036] Comparative Example 4 (Control group with Fe-Mn alloy added directly to molten steel) Using conventional high-silicon nitride alloys (Mn content <4%), the remaining process parameters are the same as in Example 1, and the same 0.65 kg / t Fe-Mn alloy is added when the steel is tapped from the converter.

[0037] Results: The carbon content of the first sample from the argon blowing station was only 0.175%, far below the lower limit of the target range. Despite multiple additions, the final carbon content still fluctuated significantly. This demonstrates that "directly adding Mn to molten steel" has limited effect on improving slag viscosity, while "adding Mn to multi-element nitriding alloys" has a significant effect. The lack of this feature will result in poor carbon addition dynamics and make it difficult to achieve precise carbon proportioning.

[0038] Comparison of effects: One hundred furnaces were produced using the method of the present invention (processes of Example 1 and Example 2), and compared with one hundred furnaces produced using the old process (similar to Comparative Examples 1, 2, 3, and 4). The results are as follows:

[0039] Furthermore, the technical solution of the present invention differs from that of Comparative Example 4, which involves directly adding Mn to molten steel, in the following ways: 1. The formation location and kinetic conditions of oxidation products (1) Direct addition of Mn to molten steel: Mn is usually added to molten steel in a metallic state, such as Fe-Mn alloy or silicon-manganese alloy. At this time, the oxygen content in the molten steel is extremely low (because the steelmaking process is to remove oxygen). Mn mainly plays an alloying role in molten steel, and there is a lack of sufficient oxidants such as FeO or MnO to rapidly and massively oxidize it into MnO.

[0040] Even if a small amount of Mn is oxidized, the resulting MnO will quickly disperse in the molten steel and is unlikely to accumulate in the slag phase to a concentration sufficient to alter the rheological properties of the entire slag system.

[0041] (2) Adding Mn to multi-element nitriding alloys: The alloy itself contains a high proportion of Si and N. In the oxidizing / weakly oxidizing atmosphere at the tapping point, the Si in the alloy will be preferentially and violently oxidized to generate a large amount of SiO2. At the same time, the Mn in the alloy is also oxidized to generate MnO. The key point is that MnO and SiO2 are generated simultaneously in the same small region (near the melting point of the alloy).

[0042] This "in-situ generated" MnO can immediately come into contact with and react with the surrounding high concentration of SiO2 to form a low-melting-point manganese silicate liquid phase. When Mn≥5%, the generated MnO reacts with SiO2 to form low-melting-point manganese silicate (such as 2MnO·SiO2).

[0043] This newly generated compound is liquid at the steel treatment temperature (1620~1650°C), which can effectively "wet" and disperse the originally hard SiO2 particles, thereby transforming the entire slag phase from a viscous solid foam slag into a liquid slag with good flowability.

[0044] 2. Synergistic effect (1) Adding Mn directly to molten steel: It only has a single deoxidation or alloying function and does not form a synergy with the high silicon environment.

[0045] (2) Adding Mn to multi-element nitriding alloys: Mn is not simply a deoxidizer here, but a slag conditioner, specifically used to deal with the negative effects (high melting point SiO2) brought about by high silicon nitriding alloys.

[0046] This invention successfully solves the industry problem of inaccurate carbon content control in the nitrogen-enhancing process of multi-element nitriding alloys by using a three-in-one system process of "optimizing alloy composition, segmented carbon increase, and gradient argon blowing". While ensuring or even improving product quality, it achieves better cost control and has significant industrial application value and economic benefits.

[0047] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art can make various modifications or equivalent substitutions to the present invention within the scope of its essence and protection. Such modifications or equivalent substitutions should also be considered to fall within the protection scope of the present invention.

Claims

1. A molten steel carbon addition control method based on slag system modification, characterized by, Includes the following steps: S1, Smelting endpoint control: Controlling the carbon content, phosphorus and sulfur content, and tapping temperature of molten steel at the endpoint of converter or electric furnace smelting; S2, Alloying and Initial Carbon Addition: Before the molten steel is tapped, a multi-element nitride alloy is added to the bottom of the ladle, and the first carbon additive is added during the tapping process. S3, Argon blowing and stirring: Perform the first stage of high-flow argon blowing and stirring; S4, Argon blowing station fine-tuning: The molten steel is transported to the argon blowing station for sampling and analysis, and a second carburizing agent is added as a supplement; S5, composition homogenization and soft blowing: the second stage of medium-flow argon blowing and the third stage of low-flow soft blowing are carried out sequentially. In multi-element nitride alloys, the mass percentage of manganese is ≥5%.

2. The method for controlling carbon increase in molten steel based on slag system modification according to claim 1, characterized in that: In step S1, the carbon content is controlled to be 0.04%~0.06%, the phosphorus content to be ≤0.045%, the sulfur content to be ≤0.045%, and the tapping temperature to be 1620~1650℃.

3. The method for controlling carbon increase in molten steel based on slag system modification according to claim 1, characterized in that: In step S2, the mass percentage of each component in the multi-element nitride alloy is as follows: C≤5%, N≥25%, Si+Mn≥45%, with Mn content≥5%, P≤0.15%, S≤0.015%, and the balance being Fe and unavoidable impurities. The mass ratio of Si to Mn is controlled between (4~8):1, and the amount of multi-element nitride alloy added is 0.60~0.65 kg / t.

4. The method for controlling carbon increase in molten steel based on slag system modification according to claim 1, characterized in that: In step S2, the first carbon raiser is a calcined coal carbon raiser, and the amount of carbon added to the first carbon raiser is 80% to 90% of the target carbon content value of the molten steel.

5. The method for controlling carbon increase in molten steel based on slag system modification according to claim 1, characterized in that: In step S3, the flow rate of the high-flow-rate argon blowing stirring is 40~50 NL / min·t, and the stirring time is 2~5 minutes.

6. The method for controlling carbon increase in molten steel based on slag system modification according to claim 1, characterized in that: In step S4, the second carbonizing agent is a petroleum pitch coke carbonizing agent or carbonizing ball, and the amount of carbon added to the second carbonizing agent is the difference between the target carbon content value of the molten steel and the carbon content value of the molten steel sample analysis.

7. The method for controlling carbon increase in molten steel based on slag system modification according to claim 1, characterized in that: In step S5, during the second stage, the argon flow rate for stirring is 35-45 NL / min·t, and the stirring time is 4-8 minutes.

8. The method for controlling carbon increase in molten steel based on slag system modification according to claim 1, characterized in that: In step S5, the argon flow rate of the third stage low-flow soft blowing is 30~40 NL / min·t, and the soft blowing treatment lasts for 2~4 minutes.

9. The method for controlling carbon increase in molten steel based on slag system modification according to claim 1, characterized in that: In step S5, the target carbon content of the molten steel is 0.23% to 0.25%, and the fluctuation of the target carbon content is controlled within ±0.02%.

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