Method for improving castability by controlling calcium in low-aluminum fractionally deoxidized 20Mn2 steel

Abstract

The application discloses a method for improving castability by low-aluminum staged deoxidization and calcium control of 20Mn2 steel, and belongs to the technical field of steel metallurgy. The method comprises the following steps: adding carbonaceous pre-deoxidization materials before tapping to perform aluminum-free pre-deoxidization; adding silicon-manganese alloy in the middle and later stages of tapping to perform main deoxidization and manganese alloying; adding aluminum deoxidizer in the later stage of LF refining to perform micro-aluminum final deoxidization, and controlling acid-soluble aluminum and total oxygen; according to a CaS nodule risk index IC, selecting low-calcium variable treatment, micro-calcium treatment or a calcium-free route; after final deoxidization or calcium treatment, performing weak argon stirring within a limited time window; and finally, performing full-protective casting. The application reduces the generation of Al2O3 inclusions from the source by staged deoxidization, effectively avoids CaS type nozzle clogging by staged calcium control, controls total oxygen in the steel and stopper opening fluctuation, improves continuous casting stability, and is suitable for continuous casting production of 20Mn2 steel round billets for oil casings.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, and in particular to a method for improving the castability of 20Mn2 steel through low-aluminum graded deoxidation and calcium control. Background Technology

[0002] 20Mn2 steel is one of the important steel grades for billet steel used in oil casing, typically characterized by medium to low carbon content, relatively high manganese content, and certain sulfur content control requirements. In round billet continuous casting production, this type of steel is easily affected by inclusion morphology, deoxidation process, and deposition behavior on the inner wall of the nozzle. When there are many high-melting-point Al2O3 inclusions, CaO-Al2O3-CaS composite inclusions, or sulfur-containing composite inclusions in the molten steel, these inclusions tend to adhere and grow on the inner wall of the submerged entry nozzle, causing nozzle nodules, stopper rod opening fluctuations, tundish liquid level fluctuations, and limited casting speed. In severe cases, it can even lead to interruption of continuous casting.

[0003] In current production, to reduce the oxygen content of molten steel, aluminum-based deoxidizers are often used for strong deoxidation during tapping or LF refining, followed by calcium treatment to transform Al2O3 inclusions into calcium aluminate inclusions. This approach is effective for some aluminum-killed steels, but it presents two contradictions when used for 20Mn2 oil casing steel: First, the oxygen activity of molten steel is high in the early stages of tapping, and adding a large amount of aluminum too early will form numerous, small, and diffusely distributed Al2O3 inclusions, which are difficult to completely transform and float even with subsequent calcium treatment; second, the sulfur content in 20Mn2 steel usually needs to balance the requirements of cutting, rolling, and billet quality. When the amount of calcium treatment is too high or the timing of calcium addition is unreasonable, CaS or CaO-Al2O3-CaS composite inclusions are easily formed, which will aggravate nozzle adhesion.

[0004] Therefore, relying solely on strong aluminum deoxidation followed by calcium treatment is insufficient to reliably solve the castability problem of 20Mn2 steel in continuous casting. Reducing the amount of aluminum may result in insufficient deoxidation, high total oxygen content, and inadequate flotation of inclusions; while blindly increasing the amount of calcium treatment may cause Al2O3-type nodules to transform into CaS-type nodules. Summary of the Invention

[0005] The purpose of this invention is to provide a method for improving the castability of 20Mn2 steel through low-aluminum graded deoxidation and calcium control. Without significantly changing the existing converter, LF and continuous casting equipment conditions, the method unifies the design of aluminum-free pre-deoxidation before tapping, silicon-manganese main deoxidation, micro-aluminum final deoxidation, low-calcium or calcium-free graded selection, and the castability retention time after final deoxidation. This reduces the formation of Al2O3 inclusions from the source and avoids CaS-type nozzle nodules at the end.

[0006] To achieve the above objectives, this invention provides a method for improving the castability of 20Mn2 steel through low-aluminum graded deoxidation and calcium control. The 20Mn2 steel is a billet steel for oil casing, and its chemical composition by mass percentage is: C 0.17%~0.24%, Mn 1.40%~1.85%, Si 0.10%~0.35%, P≤0.020%, S 0.005%~0.018%, Cr≤0.30%, Ni≤0.25%, Cu≤0.25%, with the balance being Fe and unavoidable impurities. The process control procedure includes the following steps: S1. Aluminum-free pre-deoxidation before steel tapping: Before the steel tapping volume reaches 30% to 50% of the total steel tapping volume of the ladle, aluminum-based deoxidizers are not added, but carbonaceous pre-deoxidation materials are added for aluminum-free pre-deoxidation. S2. Silicon-manganese main deoxidation in the middle and later stages of steel tapping: When the steel tapping volume reaches 40% to 70% of the total steel tapping volume of the ladle, silicon-manganese alloy is added for main deoxidation and manganese alloying; slag blocking operation is adopted during the steel tapping process; S3, LF furnace micro-aluminum final deoxidation: Aluminum-based deoxidizer is added in the later stage of LF refining for final deoxidation, so that the acid-soluble aluminum in the steel is controlled at 0.006%~0.016% and the total oxygen is controlled at 10~25ppm; S4. Graded calcium control treatment: Select low-calcium modification treatment, micro-calcium treatment or calcium-free route according to the sulfur content, total oxygen content and acid-soluble aluminum content in the steel. When using low-calcium denaturation treatment, the total calcium in the steel after calcium treatment should be controlled to be 6ppm < total calcium ≤ 12ppm; when using micro-calcium treatment, the total calcium in the steel after calcium treatment should be controlled to be 3ppm < total calcium ≤ 6ppm; when using a calcium-free route, calcium lines are not actively fed in, and the total calcium in the steel should not exceed 3ppm. S5. Castability maintenance: After final deoxidation or calcium treatment, control the time when molten steel enters the tundish and perform weak argon stirring. S6. Continuous casting protective casting: Full protective casting is adopted, and the superheat of round billet continuous casting is controlled at 15~35℃. During the continuous casting process, the change of stopper opening and the fluctuation of liquid level in tundish are used as monitoring indicators.

[0007] Preferably, in S1, the carbonaceous pre-deoxidizing material includes a low-nitrogen carbon raiser, graphite carbon powder, silicon carbide, and a low-ash composite carbonaceous material; the amount of carbonaceous pre-deoxidizing material added is 0.20~1.20 kg / t steel; the carbonaceous pre-deoxidizing material has the following requirements: fixed carbon ≥90%, ash ≤5%, S ≤0.05%, and N ≤0.15%.

[0008] Preferably, in S2, the silicon-manganese alloy is a low-aluminum silicon-manganese alloy, and the silicon-manganese alloy composition meets the following requirements: Al≤0.50%, P≤0.20%, S≤0.05%; the total amount of silicon-manganese alloy added is 10~28kg / t of steel; the slag blocking operation adopts slag blocking cone, sliding plate slag blocking or pneumatic slag blocking, and the slag layer thickness of the steel ladle does not exceed 5cm.

[0009] Preferably, in S3, the basicity R of the LF refining slag is controlled at 2.8~4.5, the mass fraction of (FeO+MnO) in the slag is controlled at less than 1.5%, the Al2O3 in the slag is controlled at 18%~32%, and the MgO is controlled at 5%~10%; the amount of aluminum-based deoxidizer added is 0.05~0.35 kg / t steel.

[0010] Preferably, in S3, an Al2O3 source term index IA is introduced for process control. The control expression is IA = [Als] × [TO], where [Als] is the mass percentage of acid-soluble aluminum and [TO] is the total oxygen content in ppm. IA is controlled between 0.08 and 0.32. When IA exceeds 0.35, the addition of aluminum-based deoxidizer is stopped.

[0011] Preferably, in S4, the CaS nodule risk index IC is introduced for graded calcium control judgment, and the expression for graded calcium control judgment is IC=[Ca]. aim ×[S] / [TO], where [Ca] aim The total calcium value is the reference target value preset according to the conventional calcium treatment scheme before the grading judgment, rather than the actual measured total calcium value after grading treatment, and the unit is ppm; [S] is the sulfur content in steel, and the unit is ppm; [TO] is the total oxygen content, and the unit is ppm. The classification is determined in order of priority based on high risk: when S > 0.014% or IC > 100 in the steel, the calcium-free route is adopted; After excluding the calcium-free route, when the S content in the steel is 0.010%~0.014%, or when the S content in the steel is <0.010% and 60 < IC ≤ 100, micro-calcium treatment or reducing the target calcium value should be adopted. After excluding calcium-free routes and micro-calcium treatments, when S < 0.010% and IC ≤ 60 in steel, low-calcium modification treatment is adopted.

[0012] Preferably, in S4, when the conditions for calcium-free route and micro-calcium treatment are not met, when S in steel is less than 0.010%, TO is less than 20 ppm, Al2O3 source term index IA is within the target range and IC is less than 60, low-calcium denaturation treatment is adopted, and the total calcium in steel after calcium treatment is controlled to be 6 ppm < total calcium ≤ 12 ppm. After excluding the calcium-free route, when the S in the steel is 0.010%~0.014%, or when the S in the steel is <0.010% and 60 < IC ≤ 100, micro-calcium treatment is adopted. After calcium treatment, the total calcium in the steel is controlled to be 3ppm < total calcium ≤ 6ppm. When the S content in the steel is greater than 0.014% or the IC content is greater than 100, a calcium-free route is adopted, and the total calcium content in the steel is controlled to be no higher than 3 ppm. The calcium-free route ensures castability through front-end aluminum-free pre-deoxidation, silicon-manganese main deoxidation, micro-aluminum final deoxidation, and inclusion flotation control, avoiding the deposition of CaS or CaO-Al2O3-CaS composite inclusions due to excessive calcium.

[0013] Preferably, in S5, when low-calcium denaturation treatment or micro-calcium treatment is used, the time from the end of calcium treatment to the molten steel entering the tundish is controlled to be 8~18 minutes; this time window is used to take into account the effect of calcium treatment, effective calcium decay, secondary oxidation and temperature drop. When using a calcium-free route, the time from the end of micro-aluminum final deoxidation to the entry of molten steel into the tundish is controlled to be 10-22 minutes; this time window is used to complete the flotation and separation of inclusions and temperature homogenization. The bottom-blowing argon flow rate of the weak argon stirring is controlled at 0.20~0.60 NL / (min·t) per unit of molten steel.

[0014] Preferably, in S6, the fluctuation of the stopper opening during the stable casting stage is controlled within ±5%, and the fluctuation of the liquid level in the tundish is controlled within ±10mm.

[0015] Therefore, the present invention employs the above-mentioned method for improving the castability of 20Mn2 steel through low-aluminum graded deoxidation and calcium control, which has the following beneficial effects: 1) No aluminum-based deoxidizer is added before tapping. The oxygen activity of the molten steel is reduced by aluminum-free carbon-oxygen pre-deoxidation and silicon-manganese main deoxidation, so that aluminum is transformed from the main deoxidizer to the final deoxidation fine-tuning agent, reducing the fine and dispersed Al2O3 inclusions formed in the early stage of tapping and reducing the load of subsequent calcium treatment.

[0016] 2) Based on the sulfur, total oxygen, acid-soluble aluminum and calcium content to be controlled in 20Mn2 steel, the graded judgment is made. Low-sulfur and low-oxygen furnaces adopt low-calcium or micro-calcium treatment, while furnaces with high sulfur or high CaS risk adopt the calcium-free route to avoid Al2O3 type nodules from being converted into CaS type nodules.

[0017] 3) At the same time, limit the window for maintaining castability after final deoxidation or calcium treatment, so that the inclusions have enough time to complete denaturation, collision growth and floating separation, while avoiding secondary oxidation and effective calcium decay due to excessive time.

[0018] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0019] Figure 1 This is the overall process flow diagram of the low-aluminum graded deoxidation, graded calcium control and fully protected continuous casting of 20Mn2 steel according to the present invention; Figure 2This is a comparison chart of the stopper opening fluctuation, tundish liquid level fluctuation, and number of stable continuous casting furnaces in each embodiment and comparative example. Figure 3 This is a comparison chart of the Al2O3 inclusion number density, CaS enriched composite inclusion ratio, and sprue nodulation index of each embodiment and comparative example. Detailed Implementation

[0020] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments. Unless otherwise defined, the technical or scientific terms used in this invention should be understood in their ordinary sense by those skilled in the art. The described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. The process parameters can be appropriately adjusted within the scope of the above technical solution according to the actual production equipment, ladle tonnage, and continuous casting section conditions.

[0021] The overall process flow is as follows: Figure 1 As shown, the general process is as follows: primary furnace smelting, aluminum-free pre-deoxidation before tapping, silicon-manganese main deoxidation and manganese alloying in the middle and later stages of tapping, low-oxidation refining in the LF furnace, micro-aluminum final deoxidation, low-calcium or calcium-free graded control, weak stirring and time window maintenance after final deoxidation or calcium treatment, and continuous casting of round billets with full protection. The primary furnace can be a converter or an electric arc furnace; for large-scale, long-process production, a converter is preferred, while for short-process production, an electric arc furnace can be used.

[0022] This process moves castability control forward to the tapping and LF stages. Before tapping, aluminum-free pre-deoxidation reduces free oxygen, preventing the formation of large amounts of dispersed Al2O3 under high oxygen conditions. In the middle and later stages of tapping, silicon-manganese alloying simultaneously completes primary deoxidation and manganese alloying, reducing the subsequent final aluminum deoxidation load. In the later stages of LF, a low-calcium or calcium-free route is selected based on the actual oxygen, sulfur, and aluminum levels in the molten steel, avoiding treating all heats as strongly calcium-treated heats.

[0023] Example 1: Low-calcium denaturation treatment was used in low-sulfur and low-oxygen furnaces.

[0024] This embodiment uses a 120t converter → LF furnace → round billet continuous casting process to produce 20Mn2 oil casing billet steel. The target chemical composition of 20Mn2 steel, by mass percentage, is: C 0.20%, Mn 1.62%, Si 0.24%, P≤0.018%, S≤0.010%, with the balance being Fe and unavoidable impurities.

[0025] The converter's final carbon concentration was 0.08%, the final phosphorus concentration was 0.013%, the tapping temperature was 1635℃, and the final oxygen concentration was approximately 520ppm.

[0026] S1. Aluminum-free pre-deoxidation before tapping: No aluminum-based deoxidizer is added before tapping. When the tapping volume reaches 15%~35% of the total ladle output, 0.50 kg / t of low-nitrogen carbonaceous material is added for aluminum-free pre-deoxidation. This utilizes the carbon-oxygen reaction to reduce the oxygen activity of the molten steel in the initial tapping stage, allowing some dissolved oxygen to be discharged as CO. The carbonaceous pre-deoxidizing material used has a fixed carbon content ≥92%, ash content ≤4%, S ≤0.03%, and N ≤0.10%.

[0027] The aluminum-free pre-deoxidation process before tapping does not aim to significantly increase carbon content. Instead, it utilizes the carbon-oxygen reaction to reduce oxygen activity in the early stages of tapping and delays the addition of aluminum until the oxygen activity in the molten steel has already decreased. Through this step, aluminum is transformed from a traditional primary deoxidizer into a fine-tuning agent for final deoxidation, reducing the amount of Al2O3 inclusions formed at the source.

[0028] S2. Mid-to-late stage silicon-manganese main deoxidation: When the steel output reaches 40%~70% of the total ladle output, approximately 18 kg / t of low-aluminum silicon-manganese alloy is added for main deoxidation and manganese alloying. The low-aluminum silicon-manganese alloy contains Al≤0.50%, P≤0.15%, and S≤0.03%. A sliding plate is used to block slag during tapping, resulting in a slag layer thickness of approximately 3 cm in the ladle.

[0029] In this invention, the silicon-manganese alloy is not merely an alloying material, but simultaneously serves as both the manganese alloying agent for 20Mn2 steel and the primary deoxidizer. Through silicon-manganese primary deoxidation, the deoxidation products preferentially form low-aluminum composite inclusions (MnO-SiO2-Al2O3) or composite oxides easily absorbed by the refining slag, thus reducing the subsequent final aluminum deoxidation load.

[0030] S3, LF Furnace Micro-aluminum Final Deoxidation: After the LF furnace arrives at the station, it is powered on and heated to cover the molten steel, creating a low-oxidizing reducing slag. The basicity R of the LF refining slag is controlled at 3.5, the mass fraction of (FeO+MnO) in the slag is reduced to about 0.9%, Al2O3 in the slag is controlled at 22%~28%, and MgO is controlled at 6%~8%. In the later stage of LF, a small amount of aluminum wire is fed in for final deoxidation, with the amount of aluminum wire added being about 0.12 kg / t of steel, so that the acid-dissolved aluminum is controlled at 0.011% and the total oxygen is about 17 ppm.

[0031] In this method, the main tasks of the LF furnace are heating, generating low-oxidizing reducing slag, adjusting composition, adsorbing and flotating inclusions, and performing final deoxidation with micro-aluminum. Upon arrival at the station, the LF furnace should be powered on and heated as soon as possible, and the molten steel should be covered to avoid prolonged exposure. In the later stages of LF refining, depending on the total oxygen and acid-soluble aluminum content in the steel, a small amount of aluminum wire, aluminum granules, or a low-aluminum composite final deoxidizer is used for final deoxidation.

[0032] S4. Graded calcium control treatment: In this batch of steel, S is 0.007%, TO is 17 ppm, acid-soluble aluminum is 0.011%, and the Al2O3 source term index IA = [Als] × [TO] = 0.011 × 17 ≈ 0.187, which is within the target range of 0.08~0.32. CaS nodule risk index IC = [Ca aim Based on the calculated total calcium content of 8 ppm (×[S] / [TO]), IC = 8 × 70 / 17 ≈ 32.9, which is less than 60, indicating a low-risk heat cycle. Therefore, a low-calcium modification treatment is adopted. After calcium treatment, the total calcium in the steel is approximately 8 ppm. IA reflects the source term intensity for the formation of Al2O3 inclusions in the molten steel, while IC reflects the risk of calcium reacting with sulfur to form CaS under the simulated calcium treatment conditions and participating in nozzle deposition.

[0033] S5. Maintaining Castability: After calcium treatment, perform weak argon stirring. The bottom-blowing argon flow rate is controlled at 0.30 NL / (min·t) per unit of molten steel, ensuring the molten steel is not exposed or slag is not trapped. The time from the end of calcium treatment to the molten steel entering the tundish is controlled to be 12 minutes.

[0034] S6. Continuous Casting Protective Casting: Full protective casting is adopted. Long nozzles are used for protection from the ladle to the tundish, and argon gas is introduced through the seal. The molten steel in the tundish is continuously covered with a covering agent. Submerged entry nozzles are used from the tundish to the crystallizer. The superheat of the round billet continuous casting is controlled at 22~26℃. During the stable casting stage, the stopper rod opening fluctuation remains within ±3%, and the tundish liquid level fluctuation is within ±5mm. There were no forced speed reductions or casting interruptions due to nozzle clogging.

[0035] Example 2: High sulfur furnace batches were treated with micro-calcium.

[0036] This embodiment uses the same 120t converter → LF furnace → round billet continuous casting process as in Embodiment 1 to produce 20Mn2 oil casing billet steel. The final carbon content in the converter is 0.09%, the tapping temperature is 1640℃, and the final oxygen content is approximately 560ppm.

[0037] S1. Aluminum-free pre-deoxidation before tapping: No aluminum-based deoxidizer is added before tapping. Instead, 0.60 kg / t of carbonaceous material is added for aluminum-free pre-deoxidation. The carbonaceous material has a fixed carbon content of ≥90%, ash content of ≤5%, S content of ≤0.04%, and N content of ≤0.12%.

[0038] S2. Mid-to-late stage silicon-manganese main deoxidation: When the steel output reaches 40%~70% of the total steel output from the ladle, approximately 20 kg / t of low-aluminum silicon-manganese alloy is added to complete the main deoxidation and manganese alloying. A slag-blocking cone is used during the tapping process, and the slag layer thickness in the ladle is approximately 4 cm.

[0039] S3, LF furnace micro-aluminum final deoxidation: The basicity R of the LF refining slag is controlled at 3.0~4.0, the mass fraction of (FeO+MnO) in the slag is controlled below 1.0%, Al2O3 in the slag is controlled at 20%~30%, and MgO is controlled at 5%~9%. Aluminum wire is added later in the LF process for final deoxidation, with acid-soluble aluminum controlled at 0.010% and total oxygen at 16ppm.

[0040] S4. Graded Calcium Control Treatment: The sulfur content (S) in this furnace is 0.013%, total oxygen is 16 ppm, acid-soluble aluminum is 0.010%, and IA = 0.010 × 16 = 0.160. Before grading, a target total calcium value of 8 ppm was preset according to the conventional calcium treatment scheme, with IC = 8 × 130 / 16 = 65.0. Since the sulfur content in the steel is in the range of 0.010% to 0.014% and IC ≤ 100, micro-calcium treatment is adopted to control the measured total calcium at approximately 5 ppm.

[0041] S5. Maintaining Castability: After the micro-calcium treatment is completed, perform weak argon stirring. Control the time from the end of the micro-calcium treatment to the molten steel entering the tundish to be 15 minutes. Control the bottom blowing argon flow rate at 0.25~0.40 NL / (min·t).

[0042] S6. Continuous casting protective pouring: The superheat is controlled at 20~25℃, the stopper rod opening changes smoothly, and no obvious fluctuations occur during the pouring process.

[0043] Example 3: Microcalcium treatment was used for critical furnace batches with sulfur content.

[0044] This embodiment uses a 150t converter → LF furnace → round billet continuous casting process to produce 20Mn2 steel. The final carbon content at the converter is 0.07%, and the final oxygen content is approximately 600ppm.

[0045] S1. Pre-deoxidation without aluminum before tapping: Add 0.45 kg / t of low-ash carbonaceous material, without adding aluminum-based deoxidizer. The carbonaceous material has a fixed carbon content of ≥91% and an ash content of ≤3.5%.

[0046] S2. Silicon-manganese main deoxidation in the middle and later stages of steel tapping: Add about 19 kg / t of low-aluminum silicon-manganese alloy and use pneumatic slag blocking during the tapping process.

[0047] S3, LF furnace micro-aluminum final deoxidation: The basicity R of the LF refining slag is controlled at 3.2~3.8, the (FeO+MnO) in the slag is controlled below 1.0%, the Al2O3 in the slag is controlled at 22%~30%, and the MgO is controlled at 5%~8%. The acid-soluble aluminum in the later stage of LF is controlled at 0.012%, and the total oxygen is 19ppm.

[0048] S4. Graded Calcium Control Treatment: In this batch, sulfur (S) was 0.010%, total oxygen was 19 ppm, acid-soluble aluminum was 0.012%, and IA = 0.012 × 19 ≈ 0.228. Before grading, a target total calcium value of 8 ppm was preset, with IC = 8 × 100 / 19 ≈ 42.1. Since S reached the micro-calcium threshold of 0.010% and IC ≤ 100, micro-calcium treatment was adopted to control the measured total calcium at around 5 ppm.

[0049] S5. Maintaining Castability: After the microcalcium treatment is completed, the mixture is stirred with weak argon for 10 minutes and then introduced into the intermediate package. The bottom blowing argon flow rate is controlled at 0.30~0.50 NL / (min·t).

[0050] S6. Continuous casting protective pouring: superheating degree is 20~24℃, stopper rod opening fluctuation is less than ±4%. Inclusion detection shows that the number of Al2O3 inclusions has decreased, and no large number of CaS shell-type composite inclusions have formed.

[0051] Example 4: Adjust the amount of aluminum added according to the Al2O3 source term index.

[0052] This embodiment uses the same 120t converter → LF furnace → round billet continuous casting process as in Embodiment 1 to produce 20Mn2 steel. The final carbon content in the converter is 0.10%, and the final oxygen content is approximately 580ppm.

[0053] S1. Aluminum-free pre-deoxidation before steel tapping: Add 0.55 kg / t of carbonaceous pre-deoxidation material to perform aluminum-free pre-deoxidation.

[0054] S2, silicon-manganese main deoxidation in the middle and later stages of steel tapping: add about 17 kg / t of low-aluminum silicon-manganese alloy.

[0055] S3, LF Furnace Micro-aluminum Final Deoxidation: The total oxygen content in the mid-stage of LF furnace was high, approximately 24 ppm. If aluminum wire were added as usual, the acid-soluble aluminum content might exceed 0.020%, and the Al2O3 source term index IA would increase. On-site, the total oxygen content was first reduced to below 20 ppm by improving the slag surface diffusion deoxidation capacity, maintaining a low (FeO+MnO) refining slag content, and using weak argon stirring. Then, only a small amount of aluminum wire was added to stabilize the acid-soluble aluminum content at 0.013%. At this point, the total oxygen content was 18 ppm, and IA = 0.013 × 18 ≈ 0.234. The basicity R of the LF refining slag was controlled at 3.2, and the (FeO+MnO) mass fraction in the slag was controlled below 0.8%.

[0056] S4. Graded calcium control treatment: The S in this batch is 0.008%, the total oxygen is 18ppm, the acid-soluble aluminum is 0.013%, and IA=0.013×18≈0.234. Before the graded judgment, the preset reference target value of total calcium is 8ppm, and IC=8×80 / 18≈35.6. Low calcium denaturation treatment is adopted, and the actual measured total calcium after treatment is about 7ppm.

[0057] S5. Maintaining castability: After calcium treatment, stir with weak argon for 13 min, and control the bottom blowing argon flow rate at 0.35 NL / (min·t).

[0058] S6. Continuous casting protective pouring: Superheat was controlled at 21~25℃, and no obvious nozzle nodules appeared. This example illustrates that controlling the amount of aluminum added by the Al2O3 source term index IA can avoid the increase of Al2O3 inclusion source term caused by blindly adding aluminum.

[0059] Example 5: Graded avoidance control after VD treatment.

[0060] This embodiment uses a 120t converter → LF furnace → VD → round billet continuous casting process to produce 20Mn2 oil casing billet steel.

[0061] S1. Aluminum-free pre-deoxidation before tapping: 0.50 kg / t of low-nitrogen carbon raiser is added to the steel before tapping for aluminum-free pre-deoxidation, without adding aluminum-based deoxidizers.

[0062] S2. Silicon-manganese main deoxidation in the middle and later stages of steel tapping: When the steel tapping amount reaches 40%~70% of the total steel tapping amount in the ladle, add about 18 kg / t of low-aluminum silicon-manganese alloy.

[0063] S3, LF furnace micro-aluminum final deoxidation: The LF stage is carried out with low oxidizing refining according to the present invention. Before leaving the LF station, the acid-soluble aluminum is 0.009%, the total oxygen is 21ppm, and the S is 0.009%. The basicity R of the LF refining slag is controlled at 3.0~3.5, and the mass fraction of (FeO+MnO) in the slag is controlled below 1.0%.

[0064] The VD treatment primarily focuses on degassing and inclusion flotation, reducing total oxygen to 16 ppm and acid-soluble aluminum to 0.008%. After VD degassing, no further strong aluminum deoxidation is performed; only a small amount of aluminum wire is adjusted based on the total oxygen content to keep acid-soluble aluminum controlled at 0.010%.

[0065] S4. Graded Calcium Control Treatment: In this batch, S was 0.009%, TO was 16 ppm, acid-soluble aluminum was 0.010%, and IA = 0.010 × 16 ≈ 0.160. Before grading, the preset reference target value for total calcium was 8 ppm, and IC = 8 × 90 / 16 = 45.0. Since S < 0.010% and IC ≤ 60, a low-calcium denaturation treatment was adopted, and the actual measured total calcium after treatment was 7 ppm.

[0066] S5. Maintaining pourability: The time from the end of calcium treatment to entering the intermediate ladle is 11 min, and the weak argon stirring intensity is controlled at 0.25 NL / (min·t).

[0067] S6. Continuous casting protective pouring: Superheat is controlled at 20~23℃, stopper rod opening is stable, and the surface quality of the billet is good. This embodiment illustrates that the method of the present invention is well compatible with the VD vacuum degassing process. After VD treatment, there is no need to change the graded calcium control strategy. The core remains low-aluminum graded deoxidation and CaS nodule risk graded avoidance.

[0068] Comparative Example 1: Calcium treatment after conventional strong aluminum deoxidation Using a similar steel grade and continuous casting section as in Example 1, but with a larger amount of aluminum particles added for strong deoxidation before tapping, the acid-soluble aluminum content reached 0.026% in the later stage of LF (sulfuric acid treatment), and the total oxygen was approximately 18 ppm. IA = 0.026 × 18 ≈ 0.468, exceeding the control limit of 0.35. Conventional calcium treatment was then applied, resulting in a total calcium content of approximately 14 ppm in the steel. Although the total oxygen content was lower under this process, the steel contained a large number of fine Al2O3 inclusions, some of which failed to fully transform or float. During continuous casting, the stopper opening gradually increased, and significant alumina deposition was visible on the inner wall of the nozzle in the later stage of casting. The number of consecutive heats was 4, far lower than the 9 heats in Example 1. This comparative example illustrates that strong aluminum deoxidation before tapping increases the Al2O3 source term index IA, which is detrimental to the castability of 20Mn2 steel.

[0069] Comparative Example 2: After low-aluminum graded deoxidation, a fixed high-calcium treatment was still used.

[0070] The process employed an aluminum-free pre-deoxidation route, followed by ferrosilicon-manganese primary deoxidation and micro-aluminum final deoxidation. In the later stages of LF (sulfur-free leaching), the acid-soluble aluminum content was 0.011%, and the total oxygen content was 16 ppm, but the sulfur content in this heat was 0.014%. No CaS nodulation risk assessment was performed on-site; calcium treatment was still carried out with a fixed high calcium content, resulting in a total calcium content of 16 ppm in the steel after treatment. Calculations showed that the IC (indices of calcium concentration) was 16 × 140 / 16 = 140, exceeding 100, which falls within the applicable range of the calcium-free route. Forcibly adopting a high calcium content significantly increased the risk of CaS formation. During continuous casting, the initial pouring stage was relatively stable, but the stopper opening continuously increased in the middle and later stages, leading to a high proportion of CaS and CaO-Al2O3-CaS composite inclusions in the deposits on the inner wall of the nozzle. This comparative example illustrates that after low-aluminum graded deoxidation, a fixed high calcium content treatment cannot be blindly applied; calcium control must be based on the CaS nodulation risk index IC and sulfur content for graded assessment.

[0071] Comparative Example 3: The time to enter the intermediate package was not controlled after low calcium treatment.

[0072] Using a low-calcium treatment route similar to Example 1, the acid-soluble aluminum content was 0.011%, total oxygen was 17 ppm, and sulfur was 0.008%. The target total calcium value before grading and the actual measured total calcium after treatment were both approximately 8 ppm, with IC = 8 × 80 / 17 ≈ 37.6. However, the casting entered the tundish approximately 3 minutes after the calcium treatment, failing to meet the 8-18 minute castability retention time window requirement. Continuous casting records showed that the stopper opening fluctuation peak-to-peak value was approximately 13%, and the maximum absolute fluctuation of the tundish liquid level was approximately 12 mm. This comparative example illustrates that low-calcium or micro-calcium treatment must be combined with a reasonable castability retention time window.

[0073] Comparative Example 4: Simply reducing the amount of aluminum used without reducing the total oxygen content.

[0074] In one heat, the aluminum addition was significantly reduced to decrease Al2O3 inclusions. However, a low-oxidizing refining slag was not sufficiently established during the LF stage, with (FeO+MnO) content in the slag remaining at approximately 2.5%. The total oxygen content at the LF stage was still above 30 ppm, and the IA (analytical inert gas) index failed to meet control requirements. Secondary oxidation and oxide inclusions were significant in the molten steel during continuous casting, and nozzle fluctuations remained pronounced. This comparative example illustrates that simply reducing aluminum addition cannot replace low-oxidizing refining and inclusion flotation control; it is necessary to synergistically control acid-soluble aluminum and total oxygen within the target range.

[0075] Comparative Example 5: For furnaces with high sulfur content, conventional calcium treatment was still used.

[0076] In a certain heat of LF (Leaf Processing) steel, the acid-soluble aluminum content was 0.010%, total oxygen was 17 ppm, and sulfur was 0.016%. Calcium treatment was performed on-site based on conventional experience, resulting in a total calcium content of 12 ppm in the steel. Calculations showed that IC = 12 × 160 / 17 ≈ 112.9, exceeding 100. The sulfur content in this heat exceeded 0.014%, falling within the applicable range for the calcium-free route. During casting, the stopper opening fluctuated gradually, and numerous CaS-rich areas were found in the deposits on the inner wall of the nozzle. This comparative example illustrates that when the sulfur content of 20Mn2 steel is high, calcium treatment may shift from improving Al2O3 modification to inducing CaS-type nodules, necessitating graded control using low-calcium, micro-calcium, or calcium-free methods.

[0077] To more clearly demonstrate the effects of the technical solution of the present invention, the key process parameters and continuous casting effects of each embodiment and comparative example are listed in Table 1.

[0078] Table 1. Comparison of key parameters and continuous casting effects between each embodiment and the comparative example.

[0079] Als represents the mass percentage of acid-soluble aluminum; TO, Ca, and S are all expressed in ppm. The Ca reference value is the total calcium target preset according to the conventional calcium treatment scheme before grading judgment, and the Ca measured value is the value detected after grading treatment. Figure 2The fluctuation of the tundish stopper opening is statistically analyzed based on the peak and peak values ​​during the stable pouring stage, while the fluctuation of the tundish liquid level is statistically analyzed based on the maximum absolute deviation relative to the set liquid level.

[0080] Depend on Figure 2 It can be seen that, compared with the comparative examples, the peak-to-peak value of the stopper opening fluctuation in each embodiment decreased from 13%~16% to 5%~7%, the maximum absolute fluctuation of the liquid level in the tundish decreased from 11~13mm to 4~6mm, and the number of continuous casting heats increased from 4~5 heats to 8~9 heats. This indicates that low-aluminum graded deoxidation, graded calcium control, and the time window for maintaining castability are beneficial to improving the stability of continuous casting.

[0081] Depend on Figure 3 It can be seen that the Al2O3 inclusion density, CaS enriched composite inclusion ratio, and sprue nodulation index of each embodiment are generally lower than those of the corresponding embodiments.

[0082] Therefore, this invention employs the aforementioned method for improving the castability of 20Mn2 steel through low-aluminum graded deoxidation and calcium control. By using a graded deoxidation mode of aluminum-free pre-deoxidation, silicon-manganese primary deoxidation, and micro-aluminum final deoxidation, aluminum is transformed from the primary deoxidizer into a final deoxidizer fine-tuning agent, reducing the amount of Al2O3 inclusions generated at the source. Graded calcium control is achieved by introducing the CaS nodulation risk index IC, using low-calcium / micro-calcium / calcium-free routes based on the sulfur content in the steel, effectively avoiding CaS-type nozzle nodulation. Simultaneously, a limited castability maintenance time window is defined, and weak argon stirring is used to ensure sufficient flotation of inclusions. The total oxygen in the steel is stably controlled at 10~25ppm, and the stopper opening fluctuation is controlled within ±5%, significantly improving continuous casting stability and demonstrating good industrial application value.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for improving the castability of 20Mn2 steel through low-aluminum graded deoxidation and calcium control, characterized in that, The 20Mn2 steel is a billet steel for oil casing, and its chemical composition by mass percentage is: C 0.17%~0.24%, Mn 1.40%~1.85%, Si 0.10%~0.35%, P≤0.020%, S 0.005%~0.018%, Cr≤0.30%, Ni≤0.25%, Cu≤0.25%, with the balance being Fe and unavoidable impurities; The process control procedure includes the following steps: S1. Aluminum-free pre-deoxidation before steel tapping: Before the steel tapping volume reaches 30% to 50% of the total steel tapping volume of the ladle, aluminum-based deoxidizers are not added, but carbonaceous pre-deoxidation materials are added for aluminum-free pre-deoxidation. S2. Silicon-manganese main deoxidation in the middle and later stages of steel tapping: When the steel tapping volume reaches 40% to 70% of the total steel tapping volume of the ladle, silicon-manganese alloy is added for main deoxidation and manganese alloying; slag blocking operation is adopted during the steel tapping process; S3, LF furnace micro-aluminum final deoxidation: Aluminum-based deoxidizer is added in the later stage of LF refining for final deoxidation, so that the acid-soluble aluminum in the steel is controlled at 0.006%~0.016% and the total oxygen is controlled at 10~25ppm; S4. Graded calcium control treatment: Select low-calcium modification treatment, micro-calcium treatment or calcium-free route according to the sulfur content, total oxygen content and acid-soluble aluminum content in the steel. When using low-calcium denaturation treatment, the total calcium in the steel after calcium treatment should be controlled at 6ppm < total calcium ≤ 12ppm; when using micro-calcium treatment, the total calcium in the steel after calcium treatment should be controlled at 3ppm < total calcium ≤ 6ppm; when using a calcium-free route, calcium lines are not actively fed in, and the total calcium in the steel should not exceed 3ppm. S5. Castability maintenance: After final deoxidation or calcium treatment, control the time when molten steel enters the tundish and perform weak argon stirring. S6. Continuous casting protective casting: Full protective casting is adopted, and the superheat of round billet continuous casting is controlled at 15~35℃. During the continuous casting process, the change of stopper opening and the fluctuation of liquid level in tundish are used as monitoring indicators.

2. The method for improving the castability of 20Mn2 steel through low-aluminum graded deoxidation and calcium control according to claim 1, characterized in that, In S1, the carbonaceous pre-deoxidizing material includes a low-nitrogen carbon raiser, graphite carbon powder, silicon carbide, and a low-ash composite carbonaceous material; the amount of carbonaceous pre-deoxidizing material added is 0.20~1.20 kg / t steel; the carbonaceous pre-deoxidizing material index requirements are: fixed carbon ≥90%, ash content ≤5%, S ≤0.05%, N ≤0.15%.

3. The method for improving the castability of 20Mn2 steel through low-aluminum graded deoxidation and calcium control according to claim 1, characterized in that, In S2, the silicon-manganese alloy is a low-aluminum silicon-manganese alloy, and the silicon-manganese alloy composition meets the requirements of Al≤0.50%, P≤0.20%, and S≤0.05%; the total amount of silicon-manganese alloy added is 10~28kg / t of steel; the slag blocking operation adopts slag blocking cone, sliding plate slag blocking or pneumatic slag blocking, and the slag layer thickness of the steel ladle does not exceed 5cm.

4. The method for improving the castability of 20Mn2 steel through low-aluminum graded deoxidation and calcium control according to claim 1, characterized in that, In S3, the basicity R of the LF refining slag is controlled at 2.8~4.5, the mass fraction of (FeO+MnO) in the slag is controlled below 1.5%, the Al2O3 in the slag is controlled at 18%~32%, and the MgO is controlled at 5%~10%; the amount of aluminum-based deoxidizer added is 0.05~0.35 kg / t steel.

5. The method for improving the castability of 20Mn2 steel through low-aluminum graded deoxidation and calcium control according to claim 1, characterized in that, In S3, the Al₂O₃ source term index IA is introduced for process control, and the control expression is IA=[Als]×[T.O], where [Als] is the mass percentage of acid-soluble aluminum, [T.O] is the total oxygen content with the unit of ppm; IA is controlled at 0.08~0.32, and the addition of aluminum-based deoxidizer is stopped when IA exceeds 0.

35.

6. The method for improving the castability of 20Mn2 steel through low-aluminum graded deoxidation and calcium control according to claim 1, characterized in that, In S4, the CaS nodule risk index IC is introduced for graded calcium control assessment. The expression for graded calcium control assessment is IC = [Ca]. aim ×[S] / [TO], where [Ca] aim [S] represents the reference total calcium target value preset according to the conventional calcium treatment scheme before grading judgment, in ppm; [S] represents the sulfur content in steel, in ppm; [TO] represents the total oxygen content, in ppm. The hierarchical judgment is carried out in the order of high risk priority: when S in steel >0.014% or IC>100, the calcium-free route is adopted; After excluding the calcium-free route, when S in steel is 0.010%~0.014%, or S in steel <0.010% and 60<IC≤100, micro-calcium treatment is adopted or the target calcium value is reduced; After excluding the calcium-free route and the micro-calcium treatment, when S in steel <0.010% and IC≤60, low-calcium modification treatment is adopted.

7. The method for improving the castability of 20Mn2 steel through low-aluminum graded deoxidation and calcium control according to claim 6, characterized in that, In S4, when the conditions for the calcium-free route and the micro-calcium treatment are not met, if S in steel <0.010%, T.O≤20ppm, the Al₂O₃ source term index IA is within the target range and IC≤60, the low-calcium modification treatment is adopted, and the total calcium in the steel after calcium treatment is controlled to 6ppm < total calcium ≤12ppm; After excluding the calcium-free route, when S in steel is 0.010%~0.014%, or S in steel <0.010% and 60<IC≤100, the micro-calcium treatment is adopted, and the total calcium in the steel after calcium treatment is controlled to 3ppm < total calcium ≤6ppm; When S in steel >0.014% or IC>100, the calcium-free route is adopted, and the total calcium in the steel is controlled not to exceed 3ppm.

8. The method for improving the castability of 20Mn2 steel by low-aluminum graded deoxidation and calcium control according to claim 1, characterized in that, In S5, when the low-calcium modification treatment or the micro-calcium treatment is adopted, the time from the end of calcium treatment to the entry of molten steel into the tundish is controlled to 8~18min; this time window is used to balance the calcium treatment effect, effective calcium attenuation, reoxidation and temperature drop; When the calcium-free route is adopted, the time from the end of final deoxidation with micro-aluminum to the entry of molten steel into the tundish is controlled to 10~22min; this time window is used to complete the floating separation of inclusions and temperature homogenization; The bottom argon blowing flow rate for the weak argon stirring is controlled to 0.20~0.60NL / (min·t) per unit mass of steel.

9. A method for improving the castability of 20Mn2 steel through low-aluminum graded deoxidation and calcium control according to claim 1, characterized in that, In S6, the stopper rod opening fluctuation in the stable casting stage is controlled within ±5%, and the liquid level fluctuation of the tundish is controlled within ±10mm.

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