Preparation method and product of 2.4gpa grade ultra-low phosphorus ultra-high strength steel based on process collaborative control

Abstract

The application discloses a preparation method of a 2.4GPa-grade ultra-low-phosphorus ultra-high-strength steel based on process synergistic control and an article, and the method comprises the following steps: KR molten iron pretreatment, converter smelting, LF refining, RH vacuum treatment and continuous casting, so as to construct a whole-process synergistic phosphorus content control system, and the system comprises a unique process combination of the following three stages: (1) a special dephosphorizing agent is adopted in the KR process to realize deep dephosphorization of the molten iron; (2) a 'high alkalinity, low temperature and strong stirring' system is adopted in the converter process to realize efficient dephosphorization and inhibit phosphorus reversion; and (3) a'solid phosphorus slag' with high alkalinity and moderate FeO content is innovatively adopted in the LF process to change passive prevention of phosphorus reversion into active stabilization of solid phosphorus. Through the synergistic effect of the three stages, the dephosphorization limit of a single station is broken, the technical problem that the phosphorus content of the 2.4GPa-grade steel is difficult to be stably controlled to be equal to or less than 0.003% is solved, and a strong guarantee is provided for production of high-toughness ultra-high-strength steel.

Description

Technical Field

[0001] This application belongs to the field of steelmaking technology in iron and steel metallurgy, specifically relating to a method for preparing 2.4GPa grade ultra-low phosphorus ultra-high strength steel and its products based on process collaborative control. Background Technology

[0002] With the increasing demands for lightweighting and passive safety in automobiles, cold-rolled ultra-high-strength steel with a tensile strength of 2400 MPa (2.4 GPa) has become a core material for manufacturing critical automotive safety components such as A / B pillars, sill beams, and crash beams. While achieving extremely high strength, this type of steel suffers from a significant decrease in plasticity, toughness, and resistance to hydrogen-induced delayed cracking, placing extremely stringent requirements on the control of residual impurities. Phosphorus, in particular, tends to segregate at austenite grain boundaries, significantly reducing the material's low-temperature impact toughness and resistance to delayed fracture, becoming a key factor restricting the stable engineering application of 2.4 GPa-grade ultra-low phosphorus ultra-high-strength steel. To ensure service safety, the industry typically requires the phosphorus content in the steel to be controlled below 0.003%.

[0003] Currently, the industrial production of this type of ultra-high strength steel generally adopts a long process of "KR hot metal pretreatment—BOF smelting—LF refining—continuous casting". However, under the existing technology system, achieving a stable phosphorus content below 0.003% faces significant challenges. In this process, phosphorus control mainly faces the following technical bottlenecks: 1. Heavy and fluctuating dephosphorization burden: The dephosphorization task is almost entirely placed on converter smelting. However, the high temperature and low oxidation conditions in the later stage of converter smelting easily lead to phosphorus reversion. The final phosphorus content is greatly affected by the iron conditions and operating level, and fluctuates drastically (usually between 0.008% and 0.020%), making it difficult to stably provide ultra-low phosphorus primary steel for the refining process.

[0004] 2. Risk of phosphorus reversion in refining process: LF furnace refining requires the production of reducing slag for desulfurization and deoxidation. This process will reduce (FeO) and (P2O5) brought into the ladle by the converter slag, causing phosphorus to re-enter the molten steel from the slag, resulting in phosphorus reversion and rendering the converter's dephosphorization effect useless.

[0005] 3. Lack of a systematic solution: Existing technologies mostly focus on optimizing individual processes, such as improving converter dephosphorization models or preventing phosphorus reversion in LF furnaces, but lack a systematic and coordinated phosphorus content control scheme from molten iron to continuous casting. The disconnect between dephosphorization and phosphorus retention functions in each process makes it difficult for the phosphorus content of the final product to consistently exceed the 0.003% threshold, thus restricting further improvement in the performance of 2.4GPa grade ultra-high strength steel.

[0006] Therefore, there is an urgent need to develop a whole-process collaborative preparation method to meet the needs of large-scale production of 2.4GPa grade ultra-low phosphorus ultra-high strength steel. Summary of the Invention

[0007] In view of this, this application provides a method for preparing 2.4GPa grade ultra-low phosphorus ultra-high strength steel based on process collaborative control and the product thereof. This preparation method provides a solid metallurgical foundation for high-purity steel, enabling 2.4GPa grade ultra-low phosphorus ultra-high strength steel to have good low-temperature toughness and service reliability, high strength, good resistance to delayed cracking, and impact energy of over 30J at -40℃.

[0008] In a first aspect, embodiments of this application provide a method for preparing 2.4 GPa grade ultra-low phosphorus ultra-high strength steel, the method comprising: Molten iron is subjected to KR molten iron pretreatment to obtain pretreated molten steel. In this process, a composite dephosphorizing agent is injected into the molten iron at an intensity of 6-20 kg / ton of molten iron to ensure that the phosphorus content in the pretreated molten steel is ≤0.030%. The pretreated molten steel is smelted in a converter to obtain converter molten steel, wherein the slag basicity R in the converter smelting is controlled to be 3.8-4.8, and the tapping temperature of the converter molten steel is 1580-1610℃. The molten steel from the converter is refined in an LF furnace to obtain refined molten steel. A phosphorus-fixing slag with a basicity of 5.0-7.5 is used, and the FeO content in the phosphorus-fixing slag is 5.0%-9.0%. The refined molten steel is subjected to RH vacuum refining and continuous casting to obtain 2.4 GPa grade ultra-low phosphorus ultra-high strength steel. The ultra-low phosphorus ultra-high strength steel comprises the following components by mass percentage: C: 0.4-0.5%, Si: 0.5-0.6%, Mn: 0.4-0.8%, P≤0.003%, S≤0.002%, Als: 0.02-0.06%, Cr: 0.5-0.8%, Mo: 0.2-0.3%, Nb: 0.02-0.04%, Ni: 0.8-1.0%, N≤0.003%, B: 0.001-0.003%, with the balance being iron and other unavoidable impurities.

[0009] In some embodiments, the composite dephosphorizing agent comprises iron oxide scale, lime and fluorite in a mass ratio of (40-50):(45-55):(5-8).

[0010] In some embodiments, the spraying time of the composite dephosphorizing agent is 8-15 minutes.

[0011] In some embodiments, when tapping steel from the converter, the bottom blowing argon flow rate of the ladle is controlled to be 60-150 Nm³ / h.

[0012] In some embodiments, in the step of smelting the pretreated molten steel in a converter to obtain converter molten steel, a double-barrier technology of front and rear slag blocking is adopted when tapping the steel to ensure that the amount of slag discharged is ≤3kg / ton of steel.

[0013] In some embodiments, in the step of refining the converter steel in an LF furnace to obtain refined steel, in the early stage of LF furnace treatment, argon blowing and stirring are performed for 3-6 minutes at an argon flow rate of 400-800 NL / min, followed by the addition of a strong deoxidizer to promote the phosphorus fixation reaction.

[0014] In some embodiments, the step of subjecting the refined molten steel to RH vacuum refining and continuous casting to obtain 2.4 GPa grade ultra-low phosphorus ultra-high strength steel includes: The refined molten steel is subjected to RH vacuum refining and continuous casting to obtain a billet. The cast billet was further processed to obtain ultra-low phosphorus and ultra-high strength steel with a strength of 2.4 GPa.

[0015] Secondly, embodiments of this application provide an ultra-low phosphorus ultra-high strength steel of the 2.4 GPa level, which is prepared by the preparation method of the first aspect.

[0016] In some embodiments, the mechanical properties of the 2.4GPa grade ultra-low phosphorus ultra-high strength steel meet the following requirements: tensile strength ≥2400MPa, yield strength ≥1800MPa, elongation of 8%~10%, and impact energy at -40℃ ≥30J.

[0017] Secondly, embodiments of this application provide a steel product, which is obtained by processing the ultra-low phosphorus and ultra-high strength steel of the first aspect.

[0018] This application has at least the following beneficial effects: According to the method of this application embodiment, in the KR stage, the thermodynamic advantages of low temperature and high carbon in molten iron are utilized to achieve efficient "coarse removal" of phosphorus by injecting 6-20 kg / ton of composite dephosphorizing agent into the molten iron, reducing phosphorus to ≤0.030%. Then, in the converter stage, by precisely controlling the slag basicity (R=3.8~4.8) and the tapping temperature (1580~1610℃), phosphorus reversion is effectively prevented while achieving deep dephosphorization, further reducing phosphorus to 0.008~0.012%. Finally, in the LF refining stage, a high basicity (R=5.0~7.5) phosphorus-fixing slag with FeO content strictly controlled at 5.0%~9.0% is innovatively adopted. Under a reducing atmosphere, residual phosphorus is "targeted" to be captured by the slag phase, breaking through the cognitive limitation that traditional LF cannot dephosphorize, and finally the phosphorus in the steel is stably controlled at ≤0.003%. Each stage is not an isolated operation, but rather the preceding process creates clean conditions for the following process, and the following process precisely reinforces and blocks impurities in the preceding process, forming a closed-loop synergistic mechanism of "efficient dephosphorization - prevention of phosphorus reversion - deep phosphorus fixation". This breaks through the dephosphorization limit of a single station. The three processes of KR hot metal pretreatment, converter smelting and LF refining have built a highly efficient and stable whole-process phosphorus control system, which provides a solid metallurgical foundation for the preparation of 2.4GPa grade ultra-low phosphorus ultra-high strength steel with high purity (P≤0.003%, S≤0.002%, N≤0.003%). This gives the 2.4GPa grade ultra-low phosphorus ultra-high strength steel good low-temperature toughness and service reliability, high strength, good resistance to delayed cracking, and an impact energy of over 30J at -40℃. Attached Figure Description

[0019] To more clearly illustrate the technical solutions of the implementation regulations of this application, the drawings used in the embodiments of this application will be briefly introduced below. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 An appearance diagram of the ultra-low phosphorus ultra-high strength steel according to an embodiment of this application is shown.

[0021] Figure 2 A micrograph of the metallographic structure of ultra-low phosphorus ultra-high strength steel according to an embodiment of this application is shown. Detailed Implementation

[0022] To make the purpose, technical solution, and beneficial technical effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the implementation details described in this specification are merely for illustrative purposes and are not intended to limit the scope of this application.

[0023] For simplicity, this application only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form a range not explicitly stated; and any lower limit can be combined with other lower limits to form a range not explicitly stated, just as any upper limit can be combined with any other upper limit to form a range not explicitly stated. Furthermore, although not explicitly stated, every point or individual value between the endpoints of the range is included within that range. Therefore, each point or individual value can be used as its own lower or upper limit and combined with any other point or individual value or with other lower or upper limits to form a range not explicitly stated.

[0024] In the description of this application, it should be noted that, unless otherwise stated, "above" and "below" include the stated number, and "multiple" in "one or more" means two or more.

[0025] The foregoing description of this application is not intended to describe every disclosed implementation or method. Instead, the following description provides more specific examples of exemplary embodiments. Throughout the application, guidance is provided through a series of embodiments, which can be used in various combinations. The examples listed are representative only and should not be construed as exhaustive.

[0026] In a first aspect, embodiments of this application provide a method for preparing 2.4GPa grade ultra-low phosphorus ultra-high strength steel, the method comprising: S100~S400.

[0027] S100. The molten iron is subjected to KR molten iron pretreatment to obtain pretreated molten steel. A composite dephosphorizing agent is injected into the molten iron at an intensity of 6-20 kg / ton of molten iron to ensure that the phosphorus content in the pretreated molten steel is ≤0.030%. Optionally, the injection intensity of the composite dephosphorizing agent can be any value or a range of combinations thereof from 6 kg / ton molten iron, 7 kg / ton molten iron, 8 kg / ton molten iron, 9 kg / ton molten iron, 10 kg / ton molten iron, 11 kg / ton molten iron, 12 kg / ton molten iron, 13 kg / ton molten iron, 14 kg / ton molten iron, 15 kg / ton molten iron, 16 kg / ton molten iron, 17 kg / ton molten iron, 18 kg / ton molten iron, 19 kg / ton molten iron, and 20 kg / ton molten iron.

[0028] For example, the phosphorus content in pretreated molten steel can be 0.010%, 0.015%, 0.020%, 0.025%, 0.030%, etc.

[0029] KR hot metal pretreatment is a desulfurization and dephosphorization pretreatment technology that involves injecting powdered agents into molten hot metal with mechanical stirring. Utilizing the thermodynamic advantages of the low-temperature (~1300℃) and high-carbon environment of molten hot metal, phosphorus is rapidly transferred from the metallic phase to the high-basicity oxide slag under strong stirring. Simultaneously, the addition of a composite dephosphorizing agent significantly reduces the phosphorus load entering the furnace, preventing subsequent converters from being forced to increase the final temperature or slag volume due to high phosphorus levels. This lays the foundation for ultra-low phosphorus production and significantly reduces the pressure on the converter and LF (Laminated Fluid Furnace).

[0030] S200. The pretreated molten steel is smelted in a converter to obtain converter molten steel, wherein the slag basicity R in the converter smelting is controlled to be 3.8-4.8, and the tapping temperature of the converter molten steel is 1580-1610℃. Optionally, the slag basicity R in converter smelting can be any value or a range of combinations thereof from 3.80, 3.85, 3.90, 3.95, 4.00, 4.05, 4.10, 4.15, 4.20, 4.25, 4.30, 4.35, 4.40, 4.45, 4.50, 4.55, 4.60, 4.65, 4.70, 4.75, and 4.80.

[0031] The slag basicity R can be calculated using R=CaO / SiO2, reflecting the basicity of the slag. Appropriate control of the tapping temperature of the converter steel can prevent excessively high temperatures from weakening dephosphorization or excessively low temperatures from causing poor fluidity. Slag with the aforementioned basicity promotes the formation of stable calcium phosphate (4CaO·P2O5), while moderate FeO provides oxidation potential. Temperature control ensures a balance between dephosphorization kinetics and steel purity, preventing a decrease in phosphorus distribution due to high temperatures and avoiding slag viscosity and difficulty in flotation due to low temperatures. Simultaneously, through precise control of basicity, temperature, and oxidizing properties, the dual goals of "deep dephosphorization + prevention of phosphorus reversion" are achieved, providing clean steel for the LF process.

[0032] Optionally, the tapping temperature of the molten steel in the converter can be any value or a range of combinations thereof from 1580℃, 1581℃, 1582℃, 1583℃, 1584℃, 1585℃, 1586℃, 1587℃, 1588℃, 1589℃, 1590℃, 1591℃, 1592℃, 1593℃, 1594℃, 1595℃, 1596℃, 1597℃, 1598℃, 1599℃, 1600℃, 1601℃, 1602℃, 1603℃, 1604℃, 1605℃, 1606℃, 1607℃, 1608℃, 1609℃, and 1610℃.

[0033] S300. The molten steel from the converter is refined in an LF furnace to obtain refined molten steel, using a phosphorus-fixing slag with a basicity of 5.0-7.5, and ensuring that the FeO content in the phosphorus-fixing slag is 5.0%-9.0%. Optionally, the alkalinity of the phosphorus fixation slag can be 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, etc.

[0034] Optionally, the FeO content in the phosphorus fixation slag can be 5.0%, 5.5%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%, 8.5%, 9.0%, etc.

[0035] Phosphorus-fixing slag is a specially formulated high-CaO slag system, characterized by its FeO content controlled between 5.0% and 9.0%. This slag system can form stable compounds such as calcium iron phosphate n(3CaO·P2O5)·2CaO·FeO, which firmly fixes the phosphorus (P) that may be introduced from upstream processes into the slag, transforming the LF station from a "phosphorus return risk point" into a "phosphorus content stabilizer," thus effectively fixing the P content.

[0036] With R=5.0%~7.5% and FeO=5.0%~9.0%, ultra-low phosphorus ultimate control can be achieved, eliminating the risk of grain boundary segregation and significantly improving impact toughness at -40℃. The above-mentioned high basicity design directly relies on the fact that the phosphorus has been reduced to a low level in the first two processes; otherwise, the slag capacity would be insufficient, reflecting gradient purification and improving the cleanliness of molten steel.

[0037] S400. The refined molten steel is subjected to RH vacuum refining and continuous casting to obtain 2.4 GPa grade ultra-low phosphorus ultra-high strength steel. The ultra-low phosphorus ultra-high strength steel comprises the following components by mass percentage: C: 0.4-0.5%, Si: 0.5-0.6%, Mn: 0.4-0.8%, P≤0.003%, S≤0.002%, Als: 0.02-0.06%, Cr: 0.5-0.8%, Mo: 0.2-0.3%, Nb: 0.02-0.04%, Ni: 0.8-1.0%, N≤0.003%, B: 0.001-0.003%, with the balance being iron and other unavoidable impurities.

[0038] In some embodiments, the composite dephosphorizing agent comprises iron oxide scale, lime and fluorite in a mass ratio of (40-50):(45-55):(5-8).

[0039] In some embodiments, the spraying time of the composite dephosphorizing agent is 8-15 minutes.

[0040] The timing of injecting the composite dephosphorizing agent ensures that the dephosphorizing agent achieves strong mixing and a large contact area under optimal kinetic conditions, which is conducive to the participation of the composite dephosphorizing agent in the reaction and avoids the efficiency reduction caused by the later slag layer covering. It is also conducive to shortening the treatment cycle and reducing the temperature drop of molten steel. Together with the injection intensity and ratio, it constitutes the "three elements" of KR dephosphorization, ensuring high efficiency, low consumption, and low temperature drop, and providing the converter with raw materials with stable composition and suitable temperature.

[0041] In some embodiments, when tapping steel from the converter, the bottom blowing argon flow rate of the ladle is controlled to be 60-150 Nm³ / h.

[0042] Argon blowing at the above flow rate and time can promote mixing.

[0043] In some embodiments, in the step of smelting the pretreated molten steel in a converter to obtain converter molten steel, a double-barrier technology of front and rear slag blocking is adopted when tapping the steel to ensure that the amount of slag discharged is ≤3kg / ton of steel.

[0044] The dual-barrier technology for slag blocking at the front and rear during steel tapping refers to an advanced technology that uses two devices with different principles and positions during the steel tapping process in a phased and relay-like manner to prevent slag from flowing into the ladle with the molten steel, thereby ensuring the purity of the molten steel.

[0045] During tapping, a front baffle (slide plate slag block) and a rear baffle (electromagnetic / pneumatic slag block plug) are used to maximally prevent high-phosphorus slag from entering the ladle, effectively blocking the phosphorus return path. This means that P2O5 in the slag is easily reduced and dissolved back into the molten steel by [Al] or [C] in the reducing ladle, preventing a rebound in phosphorus content before LF refining and ensuring the effectiveness of the LF phosphorus-fixing slag. If the slag block fails, all previous dephosphorization efforts will be partially wasted.

[0046] In some embodiments, in the step of refining the converter steel in an LF furnace to obtain refined steel, argon blowing and stirring are first performed for 3-6 minutes at an argon flow rate of 400-800 NL / min, during which no strong deoxidizer is added to promote the phosphorus fixation reaction.

[0047] Argon blowing at the aforementioned flow rate enhances mass transfer, ensuring a complete phosphorus fixation reaction. Deoxidation reduces FeO fluctuations, maintaining FeO within the 5%–9% window, providing the necessary oxidation potential without excessive oxygenation. This improves phosphorus removal efficiency while controlling TO (total oxygen) ≤15ppm, reducing Al2O3 inclusions and improving cleanliness. Creating the kinetic and thermodynamic conditions for the phosphorus fixation slag to function effectively is a key operation for the success of LF phosphorus fixation.

[0048] In some embodiments, the refined steel is subjected to RH vacuum refining and continuous casting to obtain 2.4 GPa grade ultra-low phosphorus ultra-high strength steel, comprising: The refined molten steel is subjected to RH vacuum refining and continuous casting to obtain a billet. The cast billet was further processed to obtain ultra-low phosphorus and ultra-high strength steel with a strength of 2.4 GPa.

[0049] RH vacuum refining further purifies molten steel (removing gas and inclusions), and with precise composition (C, Cr, Mo, B, etc.), achieves ultra-high strength at the 2.4 GPa level. Ultra-low P+S+N ensures high toughness, while microalloying elements (Nb, B) inhibit grain growth, and B improves hardenability, ultimately resulting in ultra-low phosphorus ultra-high strength steel at the 2.4 GPa level with a balance of high strength and high toughness.

[0050] Secondly, embodiments of this application provide an ultra-low phosphorus ultra-high strength steel of the 2.4 GPa level, which is prepared by the method of the first aspect.

[0051] In some embodiments, the mechanical properties of the 2.4GPa grade ultra-low phosphorus ultra-high strength steel meet the following requirements: tensile strength ≥2400MPa, yield strength ≥1800MPa, elongation of 8%~10%, and impact energy at -40℃ ≥30J.

[0052] Secondly, embodiments of this application provide a steel product, which is manufactured by processing ultra-low phosphorus and ultra-high strength steel as described in the first aspect.

[0053] Example The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on weight, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0054] Example 1 Steel grade: CC9880A0.

[0055] Objective: To produce a 2.4 GPa grade low-phosphorus ultra-high strength steel with the following composition by mass percentage: C: 0.4-0.5%, Si: 0.5-0.6%, Mn: 0.4-0.8%, P≤0.003%, S≤0.002%, Als: 0.02-0.06%, Cr: 0.5-0.8%, Mo: 0.2-0.3%, Nb: 0.02-0.04%, Ni: 0.8-1.0%, N≤0.003%, B: 0.001-0.003%, with the balance being iron and other unavoidable impurities.

[0056] This embodiment provides a method for preparing 2.4 GPa grade ultra-low phosphorus ultra-high strength steel, including: Step 1: KR molten iron pretreatment: Initial molten iron composition: C: 4.2%, Si: 0.35%, P: 0.095%, S: 0.030%, with the balance being iron and other unavoidable impurities.

[0057] Add a composite dephosphorizing agent with the following composition: iron oxide scale: lime: fluorite = 45:50:5.

[0058] Operation: The composite dephosphorizing agent was injected at an intensity of 12 kg / ton of molten iron for 12 minutes, supplemented by top-blown oxygen. Result: The composition of the treated molten iron was P: 0.022%, S: 0.003%.

[0059] Step 2: BOF converter smelting: KR-treated molten iron is charged. Process control: High basicity operation is adopted, with a final slag basicity R=4.2. Bottom blowing stirring is strengthened, and the argon blowing flow rate is 80 Nm³ / h.

[0060] Steel tapping control: The tapping temperature is 1601℃. A dual-barrier technology with front and rear slag barriers is used during tapping, and the estimated slag discharge is 2.7 kg / t.

[0061] Endpoint components: C: 0.05%, P: 0.005%.

[0062] Step 3: LF furnace refining: Slag formation process: Add pre-melted phosphorus-fixing slag material, control the basicity of refining slag to 5.8, and fine-tune by adding a small amount of iron ore, so that FeO in the final slag is 7.5%.

[0063] Process control: The molten steel obtained from converter smelting is transferred to the LF furnace. For the first 4 minutes before the molten steel enters the treatment area, it is stirred with argon gas at 450 NL / min without adding aluminum blocks or ferrosilicon. Then, the power is supplied to raise the temperature. When the temperature reaches above 1580℃, aluminum granules are added for final deoxidation, slag formation, and alloying.

[0064] Results: At the end of the LF treatment, the molten steel temperature was 1625℃ and the phosphorus content was 0.003% (a decrease compared to the converter endpoint, indicating a phosphorus fixation effect).

[0065] Step 4: RH Vacuum Refining Processing time: 23 minutes; high vacuum (≤67Pa) maintained for 15 minutes.

[0066] After treatment, [H] = 1.2 ppm, [N] = 23 ppm.

[0067] Final phosphorus content: 0.0026%.

[0068] Step 5: Continuous casting: The superheat of the tundish is 22℃, and the pulling speed is 1.1m / min.

[0069] The cast billet is of good quality and has no surface defects.

[0070] Final product performance: After cold rolling and continuous annealing, 2.4GPa grade ultra-low phosphorus ultra-high strength steel was obtained. The steel plate has a tensile strength of 2450MPa, a yield strength of 2085MPa, an elongation after fracture of 9.0%, and an impact energy of 38J at -40℃.

[0071] Example 2 Steel grade: CC9880A0.

[0072] Objective: To produce a 2.4 GPa grade cold-rolled ultra-high strength steel with the following composition by mass percentage: C: 0.4-0.5%, Si: 0.5-0.6%, Mn: 0.4-0.8%, P≤0.003%, S≤0.002%, Als: 0.02-0.06%, Cr: 0.5-0.8%, Mo: 0.2-0.3%, Nb: 0.02-0.04%, Ni: 0.8-1.0%, N≤0.003%, B: 0.001-0.003%, with the balance being iron and other unavoidable impurities.

[0073] Step 1: KR molten iron pretreatment: Initial molten iron composition: C 4.5%, Si 0.3%, P 0.105%, S 0.045%.

[0074] Composite dephosphorizing agent ratio: iron oxide scale: lime: fluorite = 45:50:5.

[0075] Operation: Injection intensity 15 kg / ton of molten iron, injection time 14 minutes, supplemented by top-blown oxygen.

[0076] Results: The composition of the treated molten iron was P 0.026% and S 0.004%.

[0077] Step Two: BOF Converter Smelting The molten iron after KR treatment is loaded.

[0078] Process control: High basicity operation is adopted, with a final residue basicity R=4.5. Bottom blowing stirring is strengthened, and the argon blowing flow rate is 110 Nm³ / h.

[0079] Steel tapping control: tapping temperature 1595℃. Strict slag control, with an estimated slag discharge rate of 2.4 kg / t.

[0080] Endpoint components: C: 0.04%, P: 0.0058%.

[0081] Step 3: LF furnace refining: Slag formation process: Add pre-melted phosphorus-fixing slag material, control the basicity of refining slag to 6.2, and fine-tune by adding a small amount of iron ore, so that FeO in the final slag is 8.0%.

[0082] Process control: The molten steel obtained from the converter smelting is transferred to the LF furnace. For the first 4 minutes before the molten steel enters the treatment position, it is stirred with argon gas at 620 NL / min without adding aluminum blocks or ferrosilicon. Then, the power is supplied to raise the temperature. When the temperature reaches above 1580℃, aluminum granules are added for final deoxidation, slag formation, and alloying.

[0083] Results: After the LF treatment, the molten steel temperature was 1618℃ and the phosphorus content was 0.0028% (a decrease compared to the converter endpoint, indicating a phosphorus fixation effect).

[0084] Step 4: RH Vacuum Refining Processing time: 20 minutes; high vacuum (≤67Pa) maintained for 12 minutes.

[0085] After treatment, [H] = 1.3 ppm, [N] = 25 ppm.

[0086] Final phosphorus content: 0.0023%.

[0087] Step 5: Continuous casting: The tundish was superheated to 18°C ​​and the stretching speed was 1.2 m / min.

[0088] The cast billet is of good quality and has no surface defects.

[0089] Final product performance: After cold rolling and continuous annealing, the steel plate has a tensile strength of 2435 MPa, a yield strength of 2120 MPa, an elongation after fracture of 9.2%, and an impact energy of 35 J at -40℃.

[0090] Example 3 The difference between this embodiment and Embodiment 1 is that the spraying intensity of the composite dephosphorizing agent is 8 kg / ton of molten iron, so that the phosphorus content in the pretreated molten steel is ≤0.030%.

[0091] Example 4 The difference between this embodiment and Embodiment 1 is that the pretreated molten steel is smelted in a converter to obtain converter molten steel, wherein the slag basicity R in the converter smelting is controlled to be 3.8, and the tapping temperature of the converter molten steel is 1590℃.

[0092] Example 5 The difference between this embodiment and embodiment 1 is that the molten steel from the converter is refined in an LF furnace to obtain refined molten steel, and a phosphorus-fixing slag with an alkalinity of 5.5 is used, and the FeO content in the phosphorus-fixing slag is 5.6%.

[0093] Example 6 The difference between this embodiment and Embodiment 1 is that in the step of refining the converter steel in an LF furnace to obtain refined steel, argon blowing and stirring are performed for 3 minutes at a flow rate of 420 NL / min to promote the phosphorus fixation reaction.

[0094] Comparative Example 1 Steel grade: GC9013A5, its composition by mass percentage is: C: 0.2%-0.3%, Si: 0.3%-0.5%, Mn: 0.8%-1.0%, P≤0.003%, S≤0.002%, Als: 0.02%-0.06%, Cr: 0.3%-0.5%, Mo: 0.4%-0.5%, Nb: 0.02%-0.04%, Ni: 0.8%-1.0%, N≤0.003%, B: 0.001%-0.003%, with the balance being iron and other unavoidable impurities.

[0095] Step 1: KR molten iron pretreatment: Desulfurization was performed only. The initial molten iron had a phosphorus content of 0.090%. A lime-based desulfurizing agent was used, and after treatment, the sulfur content in the molten iron was 0.003%, but the phosphorus content remained at 0.089%.

[0096] Step Two: BOF Converter Smelting The dephosphorization load was heavy, the endpoint temperature was high (1625℃), and the endpoint phosphorus content was 0.010%.

[0097] Step 3: LF furnace refining: The process employs traditional techniques to produce highly reducing white slag (FeO<1.0%), with a reducing atmosphere throughout.

[0098] After deoxidation, tests revealed that the phosphorus content had rebounded to 0.012% (significant phosphorus rebound), and the phosphorus content did not meet the requirements. The molten steel was then returned to the furnace or refining into other steel grades.

[0099] Conclusion: It is difficult to achieve the limit control of phosphorus content in molten steel using traditional processes. The converter dephosphorization load is heavy, and the phosphorus composition cannot meet the requirements of the finished product.

[0100] Comparative Example 2 Steel grade: GC9013A5, its composition by mass percentage is: C: 0.2%-0.3%, Si: 0.3%-0.5%, Mn: 0.8%-1.0%, P≤0.003%, S≤0.002%, Als: 0.02%-0.06%, Cr: 0.3%-0.5%, Mo: 0.4%-0.5%, Nb: 0.02%-0.04%, Ni: 0.8%-1.0%, N≤0.003%, B: 0.001%-0.003%, with the balance being iron and other unavoidable impurities.

[0101] Step 1: KR molten iron pretreatment: The KR process is the same as in Example 1, but the molten iron P is reduced to 0.025%.

[0102] Step Two: BOF Converter Smelting The BOF process is the same as in Example 1, with an endpoint P=0.005%.

[0103] Step 3: LF furnace refining: The LF process uses the traditional white slag process (FeO<1.0%), and does not use the phosphorus-fixing slag of this invention.

[0104] Results: The phosphorus content recovered to 0.0055% after LF treatment. The final product had a phosphorus content of 0.0058% and an impact energy of 26J at -40℃.

[0105] Conclusion: Even with good dephosphorization in the preceding processes, the phosphorus content cannot reach ultra-low levels and exhibits poor toughness without the "phosphorus fixation and stabilization" effect of LF. This demonstrates the indispensability of LF phosphorus fixation slag in the process of this invention.

[0106] Comparative Example 3 The difference between this comparative example and Comparative Example 1 is that: the KR hot metal pretreatment step one of Comparative Example 1 is used, with a P content of 0.028%; and the BOF converter smelting step two and LF furnace refining step three of Example 1 are used. All other aspects are the same as in Example 1.

[0107] Comparative Example 4 The difference between this comparative example and Example 1 is that this comparative example uses step two of the BOF converter smelting process from Comparative Example 1, and step three of the LF furnace refining process from step one of the KR molten iron pretreatment process from Example 1. All other aspects are the same as in Example 1.

[0108] Test section The 2.4 GPa grade ultra-low phosphorus ultra-high strength steels prepared in the examples and comparative examples were tested.

[0109] 1. Mechanical property testing of 2.4GPa grade ultra-low phosphorus ultra-high strength steel: According to GB / T228.1-2010 "Metallic materials, tensile testing - Part 1: Room temperature test method", the high-strength steel plates prepared in Examples 1-5 were used to prepare samples. Tensile tests were conducted using a German Zwick tensile testing machine with a load range of 50 to 1500 kN and a displacement speed of 2 mm / min. The tensile strength, yield strength, elongation, and other test data of the materials were obtained by computer-generated graphs. The test results are shown in Table 1.

[0110] Table 1

[0111] The performance parameters measured above are taken as average values.

[0112] In summary, the comparison between the embodiments and comparative examples fully demonstrates that the embodiments of this application provide a method for preparing 2.4GPa grade ultra-low phosphorus ultra-high strength steel through multi-stage synergistic control of KR, BOF, LF, etc. This method solves the technical problem of the difficulty in stably controlling the phosphorus content of ultra-high strength steel to ≤0.003wt%, and takes into account the low-temperature toughness and service reliability, high strength, good resistance to delayed cracking, and impact energy of up to 30J at -40℃ of 2.4GPa grade ultra-low phosphorus ultra-high strength steel, thus realizing the industrialized and stable production of existing steel mill processes.

[0113] The above are merely specific embodiments of this application, but the scope of protection of this application is not limited thereto. Any skillful means or substitutions should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for preparing 2.4 GPa grade ultra-low phosphorus ultra-high strength steel based on process collaborative control, characterized in that, The method includes: Molten iron is subjected to KR molten iron pretreatment to obtain pretreated molten steel. In this process, a composite dephosphorizing agent is injected into the molten iron at an intensity of 6-20 kg / ton of molten iron to ensure that the phosphorus content in the pretreated molten steel is ≤0.030%. The pretreated molten steel is smelted in a converter to obtain converter molten steel, wherein the slag basicity R in the converter smelting is controlled to be 3.8~4.8, and the tapping temperature of the converter molten steel is 1580~1610℃. The molten steel from the converter is refined in an LF furnace to obtain refined molten steel. A phosphorus-fixing slag with a basicity of 5.0 to 7.5 is used, and the FeO content in the phosphorus-fixing slag is 5.0% to 9.0%. The refined molten steel is subjected to RH vacuum refining and continuous casting to obtain 2.4 GPa grade ultra-low phosphorus ultra-high strength steel. The ultra-low phosphorus ultra-high strength steel comprises the following components by mass percentage: C: 0.4-0.5%, Si: 0.5-0.6%, Mn: 0.4-0.8%, P≤0.003%, S≤0.002%, Als: 0.02-0.06%, Cr: 0.5-0.8%, Mo: 0.2-0.3%, Nb: 0.02-0.04%, Ni: 0.8-1.0%, N≤0.003%, B: 0.001-0.003%, with the balance being iron and other unavoidable impurities.

2. The method according to claim 1, characterized in that, The composite dephosphorizing agent comprises iron oxide scale, lime and fluorite, with a mass ratio of (40-50):(45-55):(5-8).

3. The method according to claim 1, characterized in that, The spraying time of the composite dephosphorizing agent is 8-15 minutes.

4. The method according to any one of claims 1 to 3, characterized in that, When tapping steel from the converter, the bottom blowing argon gas flow rate of the ladle should be controlled at 60-150 Nm³ / h.

5. The method according to any one of claims 1 to 3, characterized in that, In the step of smelting the pretreated molten steel in a converter to obtain converter steel, a double-barrier technology of front and rear slag blocking is adopted when tapping the steel to ensure that the amount of slag discharged is ≤3kg / ton of steel.

6. The method according to any one of claims 1 to 3, characterized in that, In the step of refining the converter steel in an LF furnace to obtain refined steel, in the early stage of LF furnace treatment, argon blowing and stirring at a flow rate of 400-800 NL / min is performed for 3-6 minutes to promote the phosphorus fixation reaction.

7. The method according to any one of claims 1 to 3, characterized in that, The refined steel is subjected to RH vacuum refining and continuous casting to obtain 2.4 GPa grade ultra-low phosphorus ultra-high strength steel, comprising: The refined molten steel is subjected to RH vacuum refining and continuous casting to obtain a billet. The cast billet was further processed to obtain ultra-low phosphorus and ultra-high strength steel with a strength of 2.4 GPa.

8. A 2.4 GPa grade ultra-low phosphorus ultra-high strength steel, characterized in that, It is prepared by the method described in any one of claims 1 to 7.

9. The mechanical properties of the 2.4GPa grade ultra-low phosphorus ultra-high strength steel meet the following requirements: tensile strength ≥2400MPa, yield strength ≥1800MPa, elongation ≥8%~10%, and impact energy ≥30J at -40℃.

10. A steel product, characterized in that, It is obtained by processing the ultra-low phosphorus ultra-high strength steel according to claim 8 or 9.

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