A method for smelting 70-90ppm high-nitrogen weather-resistant container plate SPA-H in an electric arc furnace

Abstract

The application discloses a smelting method of an electric arc furnace for smelting 70-90ppm high-nitrogen weather-resistant container plate SPA-H, and belongs to the technical field of metallurgical steelmaking. A special furnace charge structure is used, which is composed of 75%-80% high-quality heavy scrap steel and 20%-25% high-carbon pig iron, and the whole process is submerged-arc smelting with foamed slag, and argon is blown at the bottom in sections. The nitrogen content in the molten steel tapped from the electric arc furnace is accurately controlled to be 62-72ppm. The nitrogen content is accurately controlled through slag-remaining and slag-embedding tapping and LF refining, and the whole process is protected pouring without bare molten steel. The nitrogen content of the final product is stably controlled to be 70-90ppm. The process does not need VD vacuum refining, shortens the smelting cycle by more than 10 minutes, and significantly reduces the cost per ton of steel. The finished product forms a synergistic corrosion-resistant system with Cu, P and Cr, and the salt spray corrosion rate is reduced by 15% compared with conventional low-nitrogen SPA-H steel. The matching Als / N is 2.2-2.8, the aging embrittlement is avoided, the excellent stamping and welding performance are combined, and the existing short-process production line batch production is adapted.

Description

Technical Field

[0001] This invention relates to the field of metallurgical steelmaking technology, and in particular to a smelting method for 70-90ppm high-nitrogen weather-resistant container steel plate SPA-H using an electric arc furnace. It is especially suitable for electric arc furnace production lines that use scrap steel as the main raw material, and precisely controls the nitrogen content in the steel to be stable in the range of 70-90ppm. By utilizing nitrogen element in conjunction with Cu, P and Cr alloying elements, the atmospheric corrosion resistance of the steel is significantly improved. Background Technology

[0002] SPA-H is currently the most widely used weather-resistant container steel. It relies on a Cu, P, and Cr composite alloy system to form a dense rust layer, resisting corrosion from marine and industrial atmospheres. It is widely used in the side panels, top panels, and bottom panels of shipping containers. Conventional electric arc furnace short-process production of SPA-H steel has two major drawbacks: First, the nitrogen content in the steel produced by conventional smelting processes is uncontrollable. Under uncontrolled nitrogen processes, the nitrogen content of the finished product is generally higher than 100 ppm, easily forming coarse AlN inclusions, deteriorating the steel's stamping performance, and causing cracking during thin-plate stamping. After adopting the VD vacuum denitrification process, the nitrogen content is reduced to 40-60 ppm, but this low nitrogen content cannot fully utilize the corrosion-resistant benefits of nitrogen. Second, existing SPA-H steel relies solely on Cu-P-Cr to achieve weather resistance. In the high-salt-spray environment of the ocean, the passivation film on the steel surface is insufficiently dense, resulting in a high corrosion rate and limiting the service life of the container.

[0003] Studies have shown that an appropriate amount of dissolved nitrogen (70-90 ppm) can refine the pearlite structure of SPA-H steel matrix, improve the stability of the passivation film, and produce a synergistic corrosion resistance effect with Cu and P elements in the steel, thereby reducing the anodic dissolution rate of the steel. Compared with conventional low-nitrogen SPA-H steel, the atmospheric corrosion rate can be reduced by 12%-18%. However, nitrogen content below 70 ppm has no effect on improving corrosion resistance, while content above 90 ppm exacerbates the aging brittleness of the steel and deteriorates its welding and forming performance.

[0004] Existing steelmaking processes struggle to stably control nitrogen levels within a narrow window of 70-90 ppm in a short-process, vacuum-free electric arc furnace refining operation, while simultaneously maintaining steel formability and corrosion resistance. Therefore, developing a low-cost, industrially scalable electric arc furnace process for smelting SPA-H steel that precisely controls nitrogen and enhances corrosion resistance has significant practical value for production. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for smelting 70-90ppm high-nitrogen weather-resistant container panels SPA-H using an electric arc furnace. This method eliminates the need for a VD vacuum refining process and utilizes a dual-process of electric arc furnace and LF refining to precisely target the nitrogen content of the finished product to 70-90ppm. It refines the metallographic structure using dissolved nitrogen and enhances corrosion resistance in conjunction with alloying elements, while avoiding AlN inclusions and aging embrittlement caused by high nitrogen content. This ensures that the stamping and welding performance of the container panels meets the required standards.

[0006] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for smelting 70-90ppm high-nitrogen weather-resistant container steel plate SPA-H in an electric arc furnace, comprising the following steps: (1) The raw materials fed into the furnace are 75%-80% high-quality heavy scrap steel and 20%-25% high-carbon pig iron by mass ratio; 13-15 kg / t steel of carbon raiser, 56-59 kg / t steel of lime, and 11-13 kg / t steel of dolomite are added. (2) The raw materials are smelted by submerged arc with foam slag throughout the process. The furnace door and the charging port are sealed. The electric arc furnace is blown with high-purity argon gas throughout the process. The argon gas flow rate is 90-110 NL / min during the melting period and is increased to 160-190 NL / min after melting. (3) When the carbon content of the raw material being smelted is controlled at 0.05%-0.08%, the steel is tapped at a temperature of 1635-1650℃, the oxygen content is 450-550ppm, and the nitrogen content of the steel tapped from the electric arc furnace is controlled at 62-72ppm. (5) The slag retention tapping process is adopted, retaining 1 / 3 of the foam slag in the furnace to isolate the air. The steel flow is buried with slag throughout the tapping process. Cu, Cr and Ni alloys are added along with the flow during the tapping process to complete the basic alloy composition adjustment. The ladle is bottom-blown argon stirring throughout the process, with a stirring flow rate of 110-130 NL / min. (6) After tapping, let the ladle stand for 2 minutes; (7) The ladle inlet temperature is 1560-1580℃, the inlet nitrogen content is 62-72ppm, LF refining produces high-basicity white slag, the slag basicity is controlled at 2.3-2.7, and the FeO in the slag is ≤0.8%; the nitrogen content in the inlet is adjusted according to two operating conditions: (8) Cast the steel ladle in step (7), wherein the steel ladle-tundish is protected by a long nozzle argon seal, the tundish is covered and sealed, the immersion nozzle is sealed as a whole, the crystallizer is covered with protective slag throughout, and the nitrogen increase during the casting process is controlled to be ≤3ppm; the continuous casting speed is constant at 1.3-1.4m / min, the tundish bottom blows 35-45NL / min of trace argon gas, and the nitrogen content of the final finished steel plate is stably controlled at 70-90ppm.

[0007] Preferably, in step (3), silicon-manganese alloy is added for pre-deoxidation in the early stage of steel tapping, and aluminum wire is fed in for final deoxidation when the steel volume reaches 1 / 2 of the molten steel. Preferably, the two operating condition controls in step (7) are specifically as follows: (1) When the nitrogen content at the station is 62-67 ppm, manganese ferronitride with a nitrogen content of ≥5% is used as a solid nitrogen supplement agent and added to the surface of the molten steel in batches at a uniform speed. For every 1 kg / t of manganese ferronitride added, the nitrogen content of the molten steel increases by 9 ppm. After the nitrogen supplementation is completed, argon gas with a medium flow rate of 190-210 NL / min is used to stir for 5-6 min. (2) When the nitrogen inlet is 68-72 ppm, use a small flow rate of argon gas at 80-100 NL / min for heating and stirring. Only homogenize the composition and temperature, and do not perform denitrification or nitrogen replenishment operations.

[0008] Preferably, in step (7), before the end of refining, the nitrogen content of the two samples is determined and controlled to be 75-85ppm. In the later stage of refining, a soft blowing argon flow rate of 60-70NL / min is used for soft blowing for ≥6min. The outlet temperature is controlled to be 1610-1625℃ and the Als / N mass ratio is controlled to be maintained at 2.2-2.8.

[0009] Preferably, in step (1), in order to avoid the abnormal increase of nitrogen in molten steel caused by thin scrap steel, thin scrap steel and rusted iron sheets are strictly prohibited from entering the furnace.

[0010] The present invention also provides a finished SPA-H steel product prepared according to a smelting method, with the following chemical composition (mass fraction): C: 0.06%-0.10%, Si: 0.25%-0.40%, Mn: 0.40%-0.55%, P: 0.08%-0.12%, S≤0.010%, Cu: 0.25%-0.35%, Cr: 0.30%-0.40%, Ni: 0.04%-0.09%, Als: 0.020%-0.030%, N: 70-90ppm, and the balance being Fe and unavoidable impurities.

[0011] Compared with the prior art, the present invention has the following beneficial effects: Significantly improves the corrosion resistance of steel: This invention stably controls the nitrogen content of SPA-H steel at 70-90ppm, and the dissolved nitrogen refines the matrix grains, forming a synergistic corrosion-resistant system with Cu, P and Cr. The density of the rust layer on the steel surface is greatly improved, and the corrosion rate in marine salt spray environment is reduced by about 15% compared with conventional low nitrogen SPA-H steel, effectively extending the service life of containers. Precise nitrogen content control and strong process stability: No VD vacuum refining equipment is required, shortening the smelting cycle by more than 10 minutes and reducing the smelting cost per ton of steel; nitrogen is controlled in stages throughout the process, and the nitrogen content of the finished product fluctuates by ≤±5ppm, stably falling within the target range of 70-90ppm, with no problems of excessive or low nitrogen content; Balancing corrosion resistance and steel performance: By precisely matching the ratio of acid-soluble aluminum to nitrogen content, the fine and dispersed precipitation of AlN is controlled, avoiding coarse aluminum nitride inclusions. The stamping and welding performance of the steel is consistent with that of conventional SPA-H steel, without stamping cracks or aging embrittlement defects. Adaptable to existing electric arc furnace short-process production lines: No need to modify the main steelmaking equipment, only optimize the furnace charge structure, bottom blowing gas system and LF refining nitrogen adjustment process, and it can be directly industrialized for mass production, with strong process adaptability. Detailed Implementation

[0012] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention. It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the present invention.

[0013] Furthermore, regarding the numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, are also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0014] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar to or equivalent to those described herein may be used in the implementation or testing of this invention.

[0015] The terms “comprising,” “including,” “having,” “containing,” etc., used in this invention are all open-ended terms, meaning that they include but are not limited to.

[0016] The chemical composition (mass fraction) of the SPA-H steel product obtained by smelting in this invention is as follows: C: 0.06%-0.10%, Si: 0.25%-0.40%, Mn: 0.40%-0.55%, P: 0.08%-0.12%, S≤0.010%, Cu: 0.25%-0.35%, Cr: 0.30%-0.40%, Ni: 0.04%-0.09%, Als: 0.020%-0.030%, N: 70-90ppm, with the balance being Fe and unavoidable impurities; Core principle: Controlling the appropriate dissolved nitrogen content in steel refines ferrite grains and promotes the formation of a continuous and dense nitrogen-rich composite passivation rust layer on the steel surface, inhibiting the penetration of corrosive media; at the same time, matching the appropriate acid-soluble aluminum content controls the precipitation morphology of AlN and avoids nitrided inclusions from harming the mechanical properties of the steel.

[0017] Overall process flow: Electric arc furnace (EAF) primary refining → steel alloying → LF refining and precise nitrogen adjustment → continuous casting with full protection, eliminating the VD vacuum denitrification process throughout. Specific steps are as follows: Step 1: Initial smelting in an electric arc furnace, establishing the basic nitrogen content of molten steel. Furnace charge ratio: The raw materials charged into the furnace are proportioned by weight as follows: 75%-80% high-quality heavy scrap steel, 20%-25% high-carbon pig iron. Light and thin scrap steel and rusted iron sheets are strictly prohibited from being charged into the furnace (light and thin scrap steel can easily cause abnormal nitrogen increase in molten steel); add 13-15 kg / t steel of carbon raiser, 56-59 kg / t steel of lime, and 11-13 kg / t steel of dolomite. Nitrogen control during melting: Foam slag submerged arc smelting is used throughout the process, and the furnace door and charging port are sealed to prevent uncontrolled nitrogen increase caused by arc ionization of air; the electric arc furnace is bottom-blown with high-purity argon throughout the process, with an argon flow rate of 90-110 NL / min during the melting period and increased to 160-190 NL / min after melting and cleaning, relying on the synergistic removal of primary free nitrogen in the molten steel by carbon-oxygen boiling bubbles and argon bubbles. Electric arc furnace endpoint control: The carbon content at the endpoint of electric arc furnace tapping is controlled at 0.05%-0.08%, the tapping temperature is 1635-1650℃, and the oxygen content at the endpoint is 450-550ppm; nitrogen is determined before tapping, and the nitrogen content of the electric arc furnace tapping is controlled at 62-72ppm, which reserves a range for precise nitrogen adjustment in LF refining.

[0018] Step 2: Deoxidation and pre-alloying of steel after tapping from the ladle, with strict control over secondary nitrogen addition during tapping. The steelmaking process with slag retention retains 1 / 3 of the foamy slag from the furnace to isolate it from the air. The steel stream is buried in slag throughout the tapping process to prevent the exposed steel stream from absorbing nitrogen. Deoxidation sequence: Add silicon-manganese alloy for pre-deoxidation in the early stage of tapping, and feed aluminum wire for final deoxidation when the steel volume reaches 1 / 2 of the total volume, so as to avoid strong deoxidation in the early stage causing nitrogen absorption in the molten steel; During the tapping process, Cu, Cr, and Ni alloys are added along with the flow, and the basic alloy composition is adjusted simultaneously. The ladle is bottom-blown with argon throughout the process, with a stirring flow rate of 110-130 NL / min to ensure uniform composition. After tapping, the ladle is left to stand for 2 minutes.

[0019] Step 3: Precise nitrogen adjustment in LF refining to lock in the target nitrogen range (core process) The ladle inlet temperature is 1560-1580℃, and the nitrogen content is 62-72ppm. LF refining produces high-basicity white slag, with slag basicity controlled at 2.3-2.7 and FeO in the slag ≤0.8%. This suppresses disordered nitrogen absorption at the steel-slag interface, and the nitrogen content is precisely controlled under two operating conditions: Operating Condition 1: Nitrogen at the inlet is 62-67 ppm, requiring nitrogen replenishment: Use ferromanganese nitride (nitrogen content ≥5%) as a solid nitrogen replenisher, adding it to the molten steel surface in batches at a uniform rate. Add the amount of ferromanganese nitride according to the nitrogen content gap. For every 1 kg / t of ferromanganese nitride added, the nitrogen content of the molten steel increases by 9 ppm. After nitrogen replenishment, stir with medium-flow argon gas at 190-210 NL / min for 5-6 min to ensure uniform solid solution of nitrogen. High-flow argon blowing for nitrogen removal is prohibited. Operating Condition 2: Inlet nitrogen 68-72ppm, no nitrogen replenishment required: Use a small flow of argon gas 80-100NL / min for gentle stirring, only to homogenize the composition and temperature, without performing denitrification or nitrogen replenishment operations; Refining endpoint control: Before the end of refining, double-sample nitrogen determination is performed to accurately control the nitrogen content at the outlet to 75-85ppm; during the later stage of refining, the soft blowing argon flow rate is 60-70NL / min, and the soft blowing time is ≥6min to remove inclusions while avoiding nitrogen content fluctuations; the outlet temperature is controlled at 1610-1625℃, and the acid-dissolved aluminum is simultaneously fine-tuned to the target range, and the Als / N mass ratio is controlled to maintain 2.2-2.8 to refine the AlN precipitation size and eliminate aging brittleness.

[0020] Step 4: Continuous casting with full protection during pouring to prevent nitrogen content from exceeding the standard. The ladle-tundish system is protected by argon sealing of the long nozzle, the tundish is sealed with a cover, the immersion nozzle is completely sealed, and the crystallizer is covered with protective slag throughout the process, so that there is no exposed molten steel throughout the entire process, eliminating secondary nitrogen absorption during the casting process and controlling the nitrogen increase during the casting process to ≤3ppm. A constant continuous casting speed of 1.3-1.4 m / min stabilizes the liquid level in the crystallizer and reduces surface fluctuations; a small amount of argon gas is blown into the tundish bottom at 35-45 NL / min to uniformly heat the molten steel without altering its nitrogen content; the nitrogen content of the final finished steel plate is stably controlled at 70-90 ppm. Example

[0021] Industrial furnace tests were conducted according to the process of this invention. The furnace charge ratio was 78% heavy scrap steel and 22% high carbon pig iron. The nitrogen content of the steel produced by the electric arc furnace was 66 ppm. 0.7 kg / t of manganese nitride was added during LF refining, and the nitrogen content of the steel produced by the LF station was 81 ppm. Nitrogen was added by 2 ppm during the continuous casting process, and the nitrogen content of the finished steel plate was 83 ppm.

[0022] Finished product inspection: Corrosion test showed a corrosion rate of 0.0215 mm / a, while the corrosion rate of conventional low-nitrogen SPA-H steel was 0.25 mm / a, resulting in a 16% improvement in corrosion resistance; the steel's stamping and mechanical properties fully meet the requirements of container panel standards. Example

[0023] The nitrogen content of the steel produced by the electric arc furnace is 71 ppm. No nitrogen supplementation is required for LF refining. The nitrogen content of the steel after gentle stirring is 78 ppm. Nitrogen is added by 2 ppm in continuous casting, and the nitrogen content of the finished product is 80 ppm. The corrosion rate is 0.0215 mm / a, and the mechanical and forming properties are qualified.

[0024] Comparative Example 1 In electric arc furnace smelting of SPA-H steel, the molten steel undergoes VD vacuum denitrification, resulting in a finished product with a nitrogen content of 52 ppm and a corrosion rate of 0.0248 mm / a. Compared to SPA-H with a nitrogen content of approximately 75 ppm, the strength decreases by 15-20 MPa, requiring the addition of silicon-manganese alloy, which increases costs. Without vacuum and controlled nitrogen processes, the finished product has a nitrogen content of 108 ppm, and the steel exhibits slight aging brittleness, resulting in a 7% decrease in stamping pass rate.

[0025] Comparative Example 2 Traditional converter smelting of SPA-H steel, after LF refining, results in a finished product with a nitrogen content of 45ppm and a corrosion rate of 0.0249 mm / a. Compared with SPA-H with a nitrogen content of about 75ppm, the strength decreases by 15-20MPa and by about 18-23MPa, respectively. This requires the addition of silicon-manganese alloy, which increases costs.

[0026] Table 1 compares the performance of SPA-H with different N contents, under the premise of strictly controlling the Al / N mass ratio to maintain at 2.2-2.8 and reducing free N content. SPA-H with N content of 70-90 ppm balances strength and corrosion resistance, exhibiting significant economic advantages; lower N content results in better processing performance but requires increased alloy cost to improve lightness; higher N content increases free N, impairing corrosion resistance and processing performance, and N content >100 ppm actually decreases strength due to porosity and grain boundary embrittlement.

[0027] Table 1 Comparison Table

[0028] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for smelting 70-90ppm high-nitrogen weather-resistant container steel plate SPA-H in an electric arc furnace, characterized in that, The steps include the following: (1) The raw materials fed into the furnace are 75%-80% high-quality heavy scrap steel and 20%-25% high-carbon pig iron by mass ratio; 13-15 kg / t steel of carbon raiser, 56-59 kg / t steel of lime, and 11-13 kg / t steel of dolomite are added. (2) The raw materials are smelted by submerged arc with foam slag throughout the process. The furnace door and the charging port are sealed. The electric arc furnace is blown with high-purity argon gas throughout the process. The argon gas flow rate is 90-110 NL / min during the melting period and is increased to 160-190 NL / min after melting. (3) When the carbon content of the raw material being smelted is controlled at 0.05%-0.08%, the steel is tapped at a temperature of 1635-1650℃, the oxygen content is 450-550ppm, and the nitrogen content of the steel tapped from the electric arc furnace is controlled at 62-72ppm. (5) The slag retention tapping process is adopted, retaining 1 / 3 of the foam slag in the furnace to isolate the air. The steel flow is buried with slag throughout the tapping process. Cu, Cr and Ni alloys are added along with the flow during the tapping process to complete the basic alloy composition adjustment. The ladle is bottom-blown argon stirring throughout the process, with a stirring flow rate of 110-130 NL / min. (6) After tapping, let the ladle stand for 2 minutes; (7) The ladle inlet temperature is 1560-1580℃, the inlet nitrogen content is 62-72ppm, LF refining produces high-basicity white slag, the slag basicity is controlled at 2.3-2.7, and the FeO in the slag is ≤0.8%; the nitrogen content in the inlet is adjusted according to two operating conditions: (8) Cast the steel ladle in step (7), wherein the steel ladle-tundish is protected by a long nozzle argon seal, the tundish is covered and sealed, the immersion nozzle is sealed as a whole, the crystallizer is covered with protective slag throughout, and the nitrogen increase during the casting process is controlled to be ≤3ppm; the continuous casting speed is constant at 1.3-1.4m / min, the tundish bottom blows 35-45NL / min of trace argon gas, and the nitrogen content of the final finished steel plate is stably controlled at 70-90ppm.

2. The smelting method according to claim 1, characterized in that, In step (3), silicon-manganese alloy is added for pre-deoxidation in the early stage of steel tapping, and aluminum wire is fed in for final deoxidation when the steel tapping reaches 1 / 2 of the molten steel volume.

3. The smelting method according to claim 1, characterized in that, The two operating condition controls in step (7) are specifically as follows: (1) When the nitrogen content at the station is 62-67 ppm, manganese ferronitride with a nitrogen content of ≥5% is used as a solid nitrogen supplement agent and added to the surface of the molten steel in batches at a uniform speed. For every 1 kg / t of manganese ferronitride added, the nitrogen content of the molten steel increases by 9 ppm. After the nitrogen supplementation is completed, argon gas with a medium flow rate of 190-210 NL / min is used to stir for 5-6 min. (2) When the nitrogen inlet is 68-72 ppm, use a small flow rate of argon gas at 80-100 NL / min for heating and stirring. Only homogenize the composition and temperature, and do not perform denitrification or nitrogen replenishment operations.

4. The smelting method according to claim 1, characterized in that, In step (7), before the end of refining, the nitrogen content of the two samples is determined and controlled to be 75-85ppm. In the later stage of refining, a soft blowing argon flow rate of 60-70NL / min is used for soft blowing for ≥6min. The outlet temperature is controlled to be 1610-1625℃ and the Als / N mass ratio is controlled to be maintained at 2.2-2.

8.

5. The smelting method according to claim 1, characterized in that, In step (1), to avoid the abnormal increase of nitrogen in molten steel caused by thin scrap steel, it is strictly forbidden to put thin scrap steel and rusted iron sheets into the furnace.

6. A finished SPA-H steel product prepared by the smelting method according to any one of claims 1-5, characterized in that, Chemical composition (mass fraction): C: 0.06%-0.10%, Si: 0.25%-0.40%, Mn: 0.40%-0.55%, P: 0.08%-0.12%, S≤0.010%, Cu: 0.25%-0.35%, Cr: 0.30%-0.40%, Ni: 0.04%-0.09%, Als: 0.020%-0.030%, N: 70-90ppm, balance being Fe and unavoidable impurities.

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