Low-phosphorus stainless steel base material using scrap steel for smelting and method for manufacturing the same

Abstract

The application discloses a preparation method of low-phosphorus stainless steel base material smelted by scrap steel, and steps include scrap steel pretreatment, intermediate frequency furnace smelting, oxidation dephosphorization, reduction refining and continuous casting. The preparation method of the low-phosphorus stainless steel base material smelted by scrap steel adopts steps such as scrap steel pretreatment, intermediate frequency furnace smelting, oxidation dephosphorization and reduction refining, realizes efficient dephosphorization of scrap steel through Na2CO3 injection and citric acid cleaning, and improves the dephosphorization efficiency of the intermediate frequency furnace by using a CaO-FeO-Ca2SiO4 slag system; subsequently, the CaO-Ca2SiO4-FeO-MnO slag system is used for oxidation dephosphorization in the converter stage, and the CaO-Ca2SiO4-Al2O3-CaF2 slag system and Si-Fe alloy are used for reduction refining, so that the phosphorus content of the molten steel is stably controlled to be less than or equal to 0.010%, and the chromium element yield reaches more than 95%, which is obviously improved compared with a traditional process.

Description

Technical Field

[0001] This invention relates to the field of stainless steel smelting technology, and in particular to a low-phosphorus stainless steel base material smelted from scrap steel and its preparation method. Background Technology

[0002] In the production process of smelting stainless steel using scrap steel, the control of phosphorus has always been a key technical challenge. Phosphorus in stainless steel usually exists in the form of phosphates (such as Fe3P, Ca3(PO4)2, etc.), which are difficult to remove deeply during the smelting process. As a result, the phosphorus content of the finished steel is generally higher than 0.02%, which not only reduces the corrosion resistance of the material, but also seriously affects its processing performance.

[0003] Currently, scrap steel smelting processes mainly employ traditional methods such as single-slag, double-slag, and hot metal pretreatment. However, these processes generally suffer from the problem of balancing dephosphorization efficiency and chromium recovery rate. Specifically, efficient dephosphorization requires highly oxidizing slag (FeO > 15%) and strongly alkaline conditions (CaO / SiO2 > 3.0), while maintaining a low oxygen potential environment (FeO < 10%) is necessary to reduce chromium oxidation to Cr2O3 in order to ensure a high chromium recovery rate. The traditional single-slag method struggles to reconcile this contradiction, resulting in dephosphorization efficiency typically only 65–75% and chromium recovery rate less than 90%, severely hindering further optimization and promotion of the process.

[0004] Meanwhile, existing processes also suffer from significant chromium loss in converter slag and limited effectiveness in scrap steel pretreatment. Since chromium is inevitably oxidized during dephosphorization and enters the converter slag, the Cr2O3 content in the slag often reaches 8-12%, resulting in a severe waste of chromium resources (loss rate >5%) and significantly increasing the cost of subsequent slag treatment. Furthermore, conventional scrap steel pretreatment technologies primarily rely on physical sorting methods, which are ineffective at removing chemically bound phosphorus compounds (such as phosphates). Although hot metal pretreatment dephosphorization technology is relatively mature, it is only applicable to blast furnace hot metal and cannot be directly used in scrap steel smelting. As for vacuum refining processes, which can significantly reduce phosphorus content, their large-scale application is difficult to achieve economically feasible due to high equipment investment and operating costs. Summary of the Invention

[0005] The purpose of this invention is to propose a low-phosphorus stainless steel base material smelted from scrap steel and its preparation method, so as to solve the problem of difficulty in balancing dephosphorization efficiency and chromium recovery rate.

[0006] To achieve this objective, the present invention adopts the following technical solution: This invention provides a method for preparing low-phosphorus stainless steel base material from scrap steel smelting, comprising the following steps: Scrap steel pretreatment: Scrap steel is subjected to magnetic separation and eddy current separation to remove phosphorus-containing impurities, and then Na2CO3 solution is sprayed onto the surface of the scrap steel to obtain pretreated scrap steel; Induction furnace smelting: Pretreated scrap steel is added to an induction furnace and heated, and CaO-FeO-Ca2SiO4 slag system is injected for pre-dephosphorization smelting to obtain molten steel; Oxidative dephosphorization: The molten steel is transferred into a converter, oxidizing gas is introduced, and then CaO-Ca2SiO4-FeO-MnO slag system is injected to carry out oxidative dephosphorization. The temperature of oxidative dephosphorization is equal to or less than 1670℃. Reduction refining: CaO-Ca2SiO4-Al2O3-CaF2 slag system is injected into the molten steel after oxidation and dephosphorization for reduction refining, and Si-Fe alloy is added. The reduction refining temperature is equal to or less than 1750℃ to obtain low phosphorus molten steel. Continuous casting: The slag is recovered and the low-phosphorus molten steel is continuously cast to obtain stainless steel base billets, wherein the phosphorus content of the stainless steel base billets is ≤0.010%; In the intermediate frequency furnace smelting, oxidative dephosphorization and reduction refining steps, the P / (Si+P) atomic ratio is 0.085 to 0.398.

[0007] In the method for preparing low-phosphorus stainless steel base material from scrap steel, in the scrap steel pretreatment step, the concentration of the Na2CO3 solution is 10-15%, the blowing temperature is 600-800℃, and the treatment time is 10-35 min.

[0008] In the method for preparing low-phosphorus stainless steel base material from scrap steel, in the scrap steel pretreatment step, after Na2CO3 solution is sprayed onto the surface of the scrap steel, it is washed with water and ultrasonically dispersed, and then dried to obtain pretreated scrap steel; the water washing solution is a 0.5-1.0% citric acid solution.

[0009] In the method for preparing low-phosphorus stainless steel base material by smelting scrap steel, in the step of melting in an intermediate frequency furnace, the mass ratio of injected CaO:FeO:Ca2SiO4 is (65-71):(15-30):(5-15).

[0010] In the method for preparing low-phosphorus stainless steel base material using scrap steel smelting, in the intermediate frequency furnace smelting step, the slag basicity R is 2.8 to 3.2, the heating temperature is 1580 to 1630℃, and the pre-dephosphorization smelting time is 20 to 40 minutes.

[0011] In the method for preparing low-phosphorus stainless steel base material from scrap steel, a pulse-jet system is used to inject slag powder in the intermediate frequency furnace smelting, oxidative dephosphorization, and reduction refining steps. In the intermediate frequency furnace smelting step, CaO-FeO-Ca2SiO4 slag system and CO2 gas are alternately injected at a frequency of 0.3–0.6 Hz. In the oxidative dephosphorization step, CaO-Ca2SiO4-FeO-MnO slag system and CO2 gas are alternately injected at a frequency of 0.3–0.6 Hz. In the reduction refining step, CaO-Ca2SiO4-Al2O3-CaF2 slag system and CO2 gas are alternately injected at a frequency of 0.3–0.6 Hz, with a CO2 flow rate of 0.3–1.5 Nm³. 3 / min·t.

[0012] In the method for preparing low-phosphorus stainless steel base material from scrap steel, during the oxidative dephosphorization step, the oxidizing gas includes one of pure oxygen and an O2-Ar mixture; the mass ratio of O2 to Ar in the O2-Ar mixture is 3:1, and the gas supply intensity of the oxidizing gas is 0.8–1.2 Nm. 3 / min·t.

[0013] In the method for preparing low-phosphorus stainless steel base material by smelting scrap steel, in the step of oxidative dephosphorization, the mass ratio of CaO:Ca2SiO4:FeO:MnO is (54-56):(4-6):(34-36):(3-6); and the time for oxidative dephosphorization is 14-16 min.

[0014] In the method for preparing low-phosphorus stainless steel base material by smelting scrap steel, in the reduction refining step, the mass ratio of CaO:Ca2SiO4:Al2O3:CaF2 is (58-62):(4-6):(24-26):(8-10), in the Si-Fe alloy, the mass ratio of Si to Fe is (1-4):1, the mass ratio of the Si-Fe alloy to the molten steel after oxidative dephosphorization is 1:(300-500), and the reduction refining time is 10-20 min.

[0015] This invention provides a low-phosphorus stainless steel base material smelted from scrap steel, prepared using the above-described method for preparing low-phosphorus stainless steel base material smelted from scrap steel. The phosphorus content of the stainless steel base material billet is ≤0.01%, and the chromium recovery rate is ≥95%.

[0016] One of the technical solutions in this invention can have the following beneficial effects: The method for preparing low-phosphorus stainless steel base material from scrap steel employs the steps of scrap steel pretreatment, medium-frequency furnace smelting, oxidative dephosphorization, and reduction refining. High-efficiency dephosphorization of the scrap steel is achieved through Na2CO3 injection and citric acid cleaning, and the dephosphorization efficiency of the medium-frequency furnace is improved using a CaO-FeO-Ca2SiO4 slag system. Subsequently, in the converter stage, oxidative dephosphorization is carried out using a CaO-Ca2SiO4-FeO-MnO slag system, followed by reduction refining using a CaO-Ca2SiO4-Al2O3-CaF2 slag system and a Si-Fe alloy. This ensures that the phosphorus content of the molten steel is stably controlled at ≤0.010%, and the chromium recovery rate reaches over 95%, representing a significant improvement over traditional processes. Attached Figure Description

[0017] Figure 1 These are the SEM and EDS surface scan energy dispersive spectra from Example 3, where (a) is the SEM image; (b) is the EDS surface scan spectrum of Si; (c) is the EDS surface scan spectrum of Ca; (d) is the EDS surface scan spectrum of P; (e) is the EDS surface scan spectrum of Fe; (f) is the EDS surface scan spectrum of Mg; (g) is the EDS surface scan spectrum of Al; and (h) is the EDS surface scan spectrum of O. Detailed Implementation

[0018] The technical solution of the present invention will be further illustrated below through specific embodiments. To facilitate understanding of the present invention, a more comprehensive description is provided below. The present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of the present invention.

[0019] Where specific techniques or conditions are not specified in the examples, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0021] This invention provides a method for preparing low-phosphorus stainless steel base material from scrap steel smelting, comprising the following steps: Scrap steel pretreatment: Scrap steel is subjected to magnetic separation and eddy current separation to remove phosphorus-containing impurities, and then Na2CO3 solution is sprayed onto the surface of the scrap steel to obtain pretreated scrap steel; Induction furnace smelting: Pretreated scrap steel is added to an induction furnace and heated, and CaO-FeO-Ca2SiO4 slag system is injected for pre-dephosphorization smelting to obtain molten steel; Oxidative dephosphorization: The molten steel is transferred into a converter, oxidizing gas is introduced, and then CaO-Ca2SiO4-FeO-MnO slag system is injected to carry out oxidative dephosphorization. The temperature of oxidative dephosphorization is equal to or less than 1670℃. Reduction refining: CaO-Ca2SiO4-Al2O3-CaF2 slag system is injected into the molten steel after oxidation and dephosphorization for reduction refining, and Si-Fe alloy is added. The reduction refining temperature is equal to or less than 1750℃ to obtain low phosphorus molten steel. Continuous casting: The slag is recovered and the low-phosphorus molten steel is continuously cast to obtain stainless steel base billets, wherein the phosphorus content of the stainless steel base billets is ≤0.010%; In the intermediate frequency furnace smelting, oxidative dephosphorization and reduction refining steps, the P / (Si+P) atomic ratio is 0.085 to 0.398.

[0022] In traditional converter dephosphorization processes, Ca2SiO4 typically exists as a passive byproduct of the dephosphorization reaction. Studies have shown that in the CaO-SiO2-FeO-P2O5 slag system, the dephosphorization product C3P (3CaO·P2O5) can form a C2S-C3P solid solution with Ca2SiO4 present in the slag. However, in existing processes, Ca2SiO4 mainly depends on the combination of SiO2 and CaO during the reaction, and its formation amount, timing, and distribution are difficult to control precisely. It is an accompanying phenomenon of the dephosphorization reaction rather than an actively designed one. The fundamental reason why existing technologies cannot resolve the contradiction between dephosphorization efficiency and chromium recovery rate is that the formation of Ca2SiO4 in traditional processes is passive and uncontrollable, and its appearance often lags behind the main stage of the dephosphorization reaction. When Ca2SiO4 begins to form, a large amount of phosphorus has not yet been fixed in time, and the removed phosphorus is easily returned to the molten steel at high temperatures, resulting in phosphorus re-entry. At the same time, increasing alkalinity to promote Ca2SiO4 formation will lead to increased chromium oxidation loss, making it difficult to balance both goals.

[0023] In phosphorus-containing metallurgical slag systems, the high-temperature stable phase dicalcium silicate (2CaO·SiO2, abbreviated as C2S) can form a continuous solid solution (C2S-C3P solid solution) with the dephosphorization product tricalcium phosphate (3CaO·P2O5, abbreviated as C3P), thereby stabilizing and solidifying phosphorus atoms in the crystal lattice, significantly reducing the activity of phosphorus in the slag, which is the key to achieving deep dephosphorization and inhibiting phosphorus reversion.

[0024] Therefore, Ca2SiO4 and phosphorus can form a stable 2CaO·SiO2-3CaO·P2O5 solid solution, which forms slag and promotes the continuous positive reaction of dephosphorization, achieving deep dephosphorization and effectively inhibiting the re-dissolution of phosphorus under high-temperature conditions, commonly known as phosphorus reversion, thus ensuring the stability of the dephosphorization effect. Furthermore, by ensuring the sufficient and stable formation of the dicalcium silicate phase in the designed slag basicity (CaO / SiO2 ratio) and composition, the necessary structural basis for deep dephosphorization is provided. The preparation method does not require high-purity raw materials and can use industrial-grade dicalcium silicate pre-addition, achieving both high-efficiency dephosphorization and economic efficiency.

[0025] The method for preparing the low-phosphorus stainless steel base material transforms Ca2SiO4 from a traditional passive reaction product into an actively designed component. Ca2SiO4 is pre-added at each stage of the dephosphorization reaction, forming a synergistic composite slag system with other components. By upgrading Ca2SiO4 from a reaction product to an actively added slag system component and maintaining its stable presence at each stage, a three-in-one synergistic process system of "dephosphorization-solidification-reduction" is constructed. This system utilizes high oxygen potential to achieve deep dephosphorization, prevents phosphorus reversion through solid solution locking, and recovers chromium through reduction refining, ultimately achieving the dephosphorization target.

[0026] In the pretreatment of scrap steel, Na2CO3 solution is used for surface treatment of scrap steel because the solution is easy to adhere to, and the moisture is easy to vaporize and does not easily leak onto the scrap steel.

[0027] In the intermediate frequency furnace smelting process, a CaO-FeO-Ca2SiO4 slag system is used. As a prerequisite for the dephosphorization reaction, FeO provides a stable oxygen potential to the molten pool, continuously oxidizing [P] in the scrap steel to P2O5. The high concentration of free CaO can rapidly combine with the generated P2O5 to form stable 3CaO·P2O5 or 4CaO·P2O5 phases that enter the slag phase, driving the reaction forward and significantly increasing the phosphorus distribution ratio. At the same time, the Ca2SiO4 phase that is pre-existing or generated in the slag system can form a solid solution with (3CaO·P2O5), significantly reducing the activity of phosphorus in the slag and effectively preventing the "phosphorus return" phenomenon (i.e., phosphorus returning from the slag to the molten steel). This synergistic mechanism makes dephosphorization more thorough and stable, especially suitable for pre-treated scrap steel with large compositional fluctuations.

[0028] In the steps of medium-frequency furnace smelting, oxidative dephosphorization, and reduction refining, by controlling the slag composition, the molar ratio of Ca2SiO4 to phosphorus oxides (calculated as P2O5) in the slag is made such that it satisfies the stoichiometric ratio for forming a 2CaO·SiO2-3CaO·P2O5 (C2S-C3P) solid solution, thereby stabilizing the removed phosphorus in the slag phase and inhibiting phosphorus reversion. Preferably, by adjusting the slag composition, the P / (Si+P) atomic ratio is controlled within the range of 0.085 to 0.398 to ensure the effective formation of the solid solution.

[0029] The method for preparing low-phosphorus stainless steel base material from scrap steel employs the steps of scrap steel pretreatment, medium-frequency furnace smelting, oxidative dephosphorization, and reduction refining. High-efficiency dephosphorization of the scrap steel is achieved through Na2CO3 injection and citric acid cleaning, and the dephosphorization efficiency of the medium-frequency furnace is improved using a CaO-FeO-Ca2SiO4 slag system. Subsequently, in the converter stage, oxidative dephosphorization is carried out using a CaO-Ca2SiO4-FeO-MnO slag system, followed by reduction refining using a CaO-Ca2SiO4-Al2O3-CaF2 slag system and a Si-Fe alloy. This ensures that the phosphorus content of the molten steel is stably controlled at ≤0.010%, and the chromium recovery rate reaches over 95%, representing a significant improvement over traditional processes.

[0030] Specifically, in the scrap steel pretreatment step, the concentration of the Na2CO3 solution is 10-15%, the blowing temperature is 600-800℃, and the treatment time is 10-35 min.

[0031] Under high-temperature conditions, Na₂CO₃ is unstable and decomposes to produce more reactive sodium oxide and carbon dioxide, as shown in the following reaction: Na₂CO₃ → Na₂O + CO₂. Upon contact with dissolved phosphorus in scrap steel, the following reaction occurs: 2[P]+5FeO+3Na2CO3→2Na3PO4+5[Fe]+3CO2; When Na2CO3 comes into contact with independent phosphates, such as Fe3(PO4)2, the following reaction occurs: Fe3(PO4)2 + 3Na2CO3 → 2Na3PO4 + 3FeO + 3CO2, thereby removing decomposed phosphates from scrap steel.

[0032] Specifically, in the scrap steel pretreatment step, after the Na2CO3 solution is sprayed onto the surface of the scrap steel, it is washed with a water washing solution and ultrasonically dispersed, and then dried to obtain pretreated scrap steel; the water washing solution is a 0.5-1.0% citric acid solution.

[0033] By following the steps described above, residual sodium salts on the surface can be removed, preventing excessive sodium from entering the molten steel.

[0034] Specifically, in the intermediate frequency furnace smelting step, the mass ratio of the injected CaO:FeO:Ca2SiO4 is (65-71):(15-30):(5-15).

[0035] Using the above ratio, a highly efficient dephosphorization system was constructed. In this system, high-content CaO serves as the primary phosphorus fixation substrate, medium-content FeO acts as a continuous oxygen source, and medium-content Ca2SiO4 functions as both a stabilizer and flux. CaO, as the primary phosphorus fixation substrate, requires FeO to initiate the reaction and relies on Ca2SiO4 to lower the melting point of CaO and stabilize the reaction products. The effect of Ca2SiO4 is fully realized under the high alkalinity environment provided by high CaO content. The efficiency of FeO is maximized under conditions of high alkalinity and a sufficient reaction interface. Therefore, the above ratio enables a synergistic effect of CaO:FeO:Ca2SiO4.

[0036] Specifically, in the medium-frequency furnace smelting process, the slag basicity R is 2.8–3.2, the heating temperature is 1580–1630℃, and the pre-dephosphorization smelting time is 20–40 min.

[0037] Maintaining the slag basicity R between 2.8 and 3.2 enables highly efficient dephosphorization. When the slag basicity is below 2.5, there is insufficient free CaO, making effective phosphorus fixation impossible; when the slag basicity is above 3.5, the slag viscosity increases, fluidity deteriorates, and mass transfer is worsened. Precisely controlling the slag basicity between 2.8 and 3.2 achieves a balance between high phosphorus capacity and favorable reaction kinetics.

[0038] Dephosphorization is a strongly exothermic reaction. Low temperatures are conducive to thermodynamic equilibrium, but excessively low temperatures can lead to viscous slag and slow reaction. Setting the lower limit of the temperature range at 1580℃ ensures complete melting of the scrap steel and provides sufficient fluidity for the slag, offering favorable kinetic conditions for the reaction. Controlling the upper limit at 1630℃ can suppress the adverse effects of high temperatures on dephosphorization equilibrium, while reducing the oxidation and loss of iron and alloying elements, thus improving metal yield.

[0039] Specifically, in the intermediate frequency furnace smelting, oxidative dephosphorization, and reductive refining steps, a pulse-jet system is used to inject slag powder. In the intermediate frequency furnace smelting step, CaO-FeO-Ca2SiO4 slag and CO2 gas are injected alternately at a frequency of 0.3–0.6 Hz. In the oxidative dephosphorization step, CaO-Ca2SiO4-FeO-MnO slag and CO2 gas are injected alternately at a frequency of 0.3–0.6 Hz. In the reductive refining step, CaO-Ca2SiO4-Al2O3-CaF2 slag and CO2 gas are injected alternately at a frequency of 0.3–0.6 Hz, with a CO2 flow rate of 0.3–1.5 Nm³. 3 / min·t.

[0040] In the intermediate frequency furnace smelting, oxidative dephosphorization, and reduction refining steps, a pulse-jet injection system is used to inject slag powder. By alternately injecting slag powder and CO2 gas at a pulse frequency of 0.3–0.6 Hz, on the one hand, it can prevent the spray gun from clogging and improve the injection efficiency; on the other hand, CO2 reacts with carbon in the molten steel at high temperature to generate CO, and the volume expansion generates strong stirring, effectively expanding the reaction zone and renewing the slag-steel reaction interface, thereby accelerating the mass transfer process of the dephosphorization reaction.

[0041] Specifically, in the oxidative dephosphorization step, the oxidizing gas includes one of pure oxygen and an O2-Ar mixture; the mass ratio of O2 to Ar in the O2-Ar mixture is 3:1, and the gas supply intensity of the oxidizing gas is 0.8–1.2 Nm. 3 / min·t.

[0042] When using an O2-Ar mixed gas, an inert gas is introduced to improve the kinetic conditions of the reaction and protect alloying elements. On the one hand, Ar, as an inert gas, does not participate in the metallurgical reaction, and the bubbles it generates can effectively stir the molten pool and reduce the partial pressure of CO, thereby accelerating the decarburization reaction and improving the mass transfer conditions between slag and steel. On the other hand, the dilution effect of Ar can appropriately reduce the local oxygen potential, reducing the oxidation loss of chromium while ensuring phosphorus oxidation, effectively solving the problem of balancing dephosphorization efficiency and chromium recovery rate. At the same time, the mixed gas can avoid local overheating caused by pure oxygen reaction, which is conducive to controlling the temperature within the optimal dephosphorization range (≤1670℃) and preventing high-temperature phosphorus reversion. The gas supply intensity is limited to 0.8~1.2 Nm³ / min·t, which is a medium-to-high intensity gas supply, ensuring good kinetic conditions without causing excessive splashing or temperature drop due to excessive flow rate.

[0043] Specifically, in the oxidative dephosphorization step, the mass ratio of CaO:Ca2SiO4:FeO:MnO is (54-56):(4-6):(34-36):(3-6); the oxidative dephosphorization time is 14-16 min.

[0044] Using the above ratio, a high oxygen potential is rapidly established with the help of a high FeO content, preferentially oxidizing phosphorus before chromium is extensively oxidized, and promoting the rapid melting of lime to form liquid foam slag. Furthermore, the high CaO content provides strong phosphorus fixation capacity, pre-providing Ca2SiO4 to lock in the phosphorus phase, preventing phosphorus reversion, and achieving rapid and deep dephosphorization, thus creating conditions for slag removal.

[0045] In a specific embodiment of the present invention, this is achieved by adding 90% pure quicklime, 95% pure dicalcium silicate, 98% pure ferrous oxide and 99% pure manganese oxide in the above proportions.

[0046] Specifically, in the reduction refining step, the mass ratio of CaO:Ca2SiO4:Al2O3:CaF2 is (58-62):(4-6):(24-26):(8-10), the mass ratio of Si to Fe in the Si-Fe alloy is (1-4):1, the mass ratio of the Si-Fe alloy to the molten steel after oxidation and dephosphorization is 1:(300-500), and the reduction refining time is 10-20 min.

[0047] Using the above ratio, CaF2 can significantly reduce the viscosity and melting point of high-calcium slag, allowing the slag system to maintain excellent fluidity during the high-temperature reduction period (≤1750℃), thus providing sufficient kinetic conditions for deep dephosphorization. High Al2O3 content can absorb deoxidation products, helping to stabilize the slag system structure; the high CaO content works synergistically with Ca2SiO4, not only continuously promoting dephosphorization but also effectively stabilizing phosphorus products and preventing phosphorus reversion. Ultimately, this slag system achieves simultaneous high-efficiency reduction and extreme dephosphorization.

[0048] In a specific embodiment of the present invention, this is achieved by adding the above-mentioned proportions of 90% pure quicklime, 95% pure dicalcium silicate, 98% pure industrial alumina, and 85% pure fluorite.

[0049] Preferably, the scrap steel is ≤300mm in size, which ensures that it is completely melted during the medium-frequency furnace smelting stage, guaranteeing uniform steel composition; at the same time, it increases the bulk density to stabilize heating, reduce furnace lining impact, and provide good kinetic conditions for subsequent dephosphorization reaction.

[0050] The present invention also provides a low-phosphorus stainless steel base material smelted from scrap steel, which is prepared by the above-mentioned preparation method of low-phosphorus stainless steel base material smelted from scrap steel. The phosphorus content of the stainless steel base material billet is ≤0.01%, and the chromium recovery rate is ≥95%.

[0051] Chromium recovery rate refers to the percentage of the total amount of chromium that ultimately enters the molten steel during the steelmaking process, relative to the initial total amount of chromium added. It reflects the efficiency of the process in recovering the valuable alloying element chromium. The calculation formula is as follows:

[0052] Wherein, M_steel_molten is the total mass of the final molten steel (unit: kg or ton), [Cr]_endpoint is the mass percentage content of chromium in the final molten steel (%), M_chromium_raw_material is the total mass of all chromium-containing raw materials added (unit: kg or ton), and ω(Cr)_raw_material is the average mass percentage content of chromium in each chromium-containing raw material (%).

[0053] Example 1 A method for preparing low-phosphorus stainless steel base material from scrap steel smelting includes the following steps: Scrap steel pretreatment: Scrap steel ≤300mm in size is mechanically crushed, and then subjected to magnetic separation and eddy current separation to separate scrap steel with a Cr content of 18.5% and an Fe3P content >0.03%. The scrap steel is then placed in a pretreatment furnace, and Na2CO3 solution is injected at a pressure of 1.0MPa, a dosage of 10kg / ton of steel, and a temperature of 700℃ for 35 minutes to remove surface phosphates (Fe3P). After injection, the scrap steel is immersed in a 6wt.% citric acid solution and ultrasonically dispersed at a temperature of 70℃ and an ultrasonic power of 1.2kW. The liquid-to-solid ratio of the citric acid solution to the scrap steel is 3:1. After stirring for 20 minutes, the scrap steel is dried to obtain pretreated scrap steel. Induction furnace smelting: Pretreated scrap steel is added to the induction furnace and heated to 1580℃; simultaneously, a pulse-jet system is used to alternately inject CaO-FeO-Ca2SiO4 slag system and CO2 gas at a frequency of 0.5Hz. The pre-dephosphorization smelting time is 30 minutes, the slag basicity R is 3.0, the dephosphorization rate reaches 65%, and the phosphorus content of the molten steel is reduced to 0.011 wt.%; the mass ratio of CaO:FeO:Ca2SiO4 is 70:15:15; the amount of CaO-FeO-Ca2SiO4 slag system used is 1.8 kg / ton of steel, and the CO2 gas flow rate is 18 Nm³. 3 / h; Oxidative dephosphorization: Molten steel is transferred to a converter, and oxygen is introduced at an intensity of 1.0 Nm³. 3 The oxidation dephosphorization process was carried out by alternately injecting CaO-Ca2SiO4-FeO-MnO slag system and CO2 gas at a frequency of 0.5 Hz using a pulse jet system at a frequency of 1600 ℃. The ratio of CaO:Ca2SiO4:FeO:MnO was 55:5:35:5. The oxidation dephosphorization time was 15 min, and 60% high-phosphorus slag was discharged. Reduction refining: CaO-Ca2SiO4-FeO-MnO slag system and CO2 gas are alternately injected into the oxidized and dephosphorized steel using a pulse jet injection system at a frequency of 0.5Hz, and Si-Fe alloy is added. The reduction refining process is carried out for 10 minutes to obtain low-phosphorus steel. The reduction refining temperature is 1750℃. The mass ratio of CaO:Ca2SiO4:Al2O3:CaF2 is 60:5:25:10. The mass ratio of Si to Fe in the Si-Fe alloy is 1:1. The mass ratio of the Si-Fe alloy to the oxidized and dephosphorized steel is 1:400. Continuous casting: The slag is recycled and the low-phosphorus molten steel is continuously cast to obtain stainless steel base billets. In the intermediate frequency furnace smelting, oxidative dephosphorization and reduction refining steps, the P / (Si+P) atomic ratio is 0.398.

[0054] Testing revealed that the stainless steel base billet prepared in this embodiment contained 18.3% chromium, 0.0095 wt.% phosphorus, and had a chromium recovery rate of 96.2%.

[0055] Example 2 A method for preparing low-phosphorus stainless steel base material from scrap steel smelting includes the following steps: Scrap steel pretreatment: Scrap steel ≤300mm in size is mechanically crushed, and then subjected to magnetic separation and eddy current separation to separate scrap steel with a Cr content of 18.5% and P=0.040%. The scrap steel is then placed in a pretreatment furnace, and Na2CO3 solution is injected at a pressure of 1.0MPa, a dosage of 10kg / ton of steel, and a temperature of 700℃ for 35 minutes to remove surface phosphates (Fe3P). After injection, the scrap steel is immersed in a 6wt.% citric acid solution and ultrasonically dispersed at a temperature of 70℃ and an ultrasonic power of 1.2 kW. The liquid-to-solid ratio of the citric acid solution to the scrap steel is 3:1. After stirring for 20 minutes, the scrap steel is dried to obtain pretreated scrap steel. Induction furnace smelting: Pretreated scrap steel is added to the induction furnace and heated to 1600℃; simultaneously, a pulse-jet system is used to alternately inject CaO-FeO-Ca2SiO4 slag system and CO2 gas at a frequency of 0.5Hz. The pre-dephosphorization smelting time is 40 minutes, the slag basicity R is 3.0, the dephosphorization rate reaches 65%, and the phosphorus content of the molten steel is reduced to 0.013 wt.%; the composition ratio of CaO:FeO:Ca2SiO4 is 65:30:5; the amount of CaO-FeO-Ca2SiO4 slag system used is 1.8 kg / ton of steel, and the CO2 gas flow rate is 1.5 Nm³. 3 / h·t; Oxidative dephosphorization: Molten steel is transferred to a converter, and an O2-Ar mixed gas (O2:Ar=3:1) is introduced at an oxygen supply intensity of 1.0 Nm³. 3 The oxidation dephosphorization process was carried out by alternately injecting CaO-Ca2SiO4-FeO-MnO slag system and CO2 gas at a frequency of 0.5 Hz using a pulse jet system at a frequency of 1670 ℃. The ratio of CaO:Ca2SiO4:FeO:MnO was 55:5:35:5. The oxidation dephosphorization time was 15 min, and 60% high-phosphorus slag was discharged. Reduction refining: CaO-Ca2SiO4-FeO-MnO slag system and CO2 gas are alternately injected into the oxidized and dephosphorized steel using a pulse jet injection system at a frequency of 0.5Hz, and Si-Fe alloy is added. The reduction refining process is carried out for 15 minutes to obtain low-phosphorus steel. The reduction refining temperature is 1750℃. The mass ratio of CaO:Ca2SiO4:Al2O3:CaF2 is 60:5:25:10. The mass ratio of Si to Fe in the Si-Fe alloy is 4:1. The mass ratio of the Si-Fe alloy to the oxidized and dephosphorized steel is 1:400. Continuous casting: The slag is recovered and the low-phosphorus molten steel is continuously cast to obtain stainless steel base billets.

[0056] In the steps of medium-frequency furnace smelting, oxidative dephosphorization and reduction refining, the atomic ratio of P / (Si+P) is 0.085.

[0057] Testing revealed that the stainless steel base billet prepared in this embodiment contained 18.2% chromium, 0.009 wt.% phosphorus, and a chromium recovery rate of 97%.

[0058] Example 3 The slag was prepared using a mixture of 36.51CaO-27.56SiO2-19.99FeO-2.52Al2O3-8.41MgO-5.00P2O5. FeO was added as FeC2O4·2H2O, P2O5 as 3CaO·P2O5, and SiO2, Al2O3, and MgO were all analytically pure powders. Micron-sized CaO particles needed to be prepared separately using the following process: Analytically pure blocky calcium oxide was heated in a resistance furnace and held at 700℃ for 8 hours. After cooling in the furnace, it was removed and ground. Particles with an average size of 30 μm were obtained by sieving through a 500-mesh sieve. 100g of molten slag was placed in a magnesium oxide crucible fitted with a graphite crucible and heated in a muffle furnace while a protective Ar gas (3L / min) was introduced into the furnace. The temperature was raised to 1450℃ and held for 0.5h. After the slag was completely melted, 10g of Ca2SiO4 particles were added to the melt, and the melt was sheared and stirred using a conventional stirring device. After the Ca2SiO4 particles reacted with the slag for 600s, a slag sample was taken with an iron rod and argon gas was blown in for 5min for rapid cooling. After the slag sample cooled, it was mounted with resin, sanded and polished to obtain the slag sample.

[0059] Comparative Example 1 The preparation method of Comparative Example 1 is the same as that of Example 3, except that 100g of molten slag is replaced with 36.51CaO-27.56SiO2-19.99FeO-2.52Al2O3-8.41MgO-5.00P2O5 (wt.%). This slag system does not form a solid solution phase at 1450℃, and the liquid slag phase accounts for 99.7%.

[0060] The slag samples obtained in Example 3 and Comparative Example 1 were prepared into specimens, and their microstructures were analyzed using a Thermo Fisher Apreo C scanning electron microscope (SEM) and its accompanying EDAX ELECT SUPER energy dispersive spectroscopy (EDS). The results are shown in the figure. Figure 1 .

[0061] Figure 1 The images show the SEM and EDS surface scanning energy dispersive spectroscopy results of slag samples from the reaction of Ca2SiO4 particles with CaO-SiO2-FeO-Al2O3-MgO-P2O5 slag at 1450℃ for 600 s. The images show that the irregular light gray blocky regions mainly contain Ca, Si, P, and O elements, with a uniform distribution of P (showing a sharp drop compared to the surrounding area), indicating that the phase is a precipitated nCa2SiO4-Ca3P solid solution phase. Based on the elemental composition, the dark gray blocky regions are Ca2SiO4.

[0062] according to Figure 1 It is evident that a distinct P-enriched region was formed around the Ca2SiO4 particles, exhibiting a gradient distribution from the outside in, confirming the formation of the nCa2SiO4-C3P solid solution layer. This phenomenon directly demonstrates that the pre-added Ca2SiO4 can undergo a solid solution reaction with the dephosphorization products, stably fixing phosphorus atoms within the crystal lattice; and this solid solution process is continuous from the surface inwards, continuously capturing newly generated phosphorus throughout the entire smelting cycle. Simultaneously, the surrounding area is composed of both the nCa2SiO4-C3P solid solution phase and the MgO phase, verifying that the composite slag system of this invention can form a stable multiphase coexistence structure at high temperatures, indicating that the preparation method described above achieves deep dephosphorization (phosphorus content ≤ 0.010%) and effectively suppresses phosphorus reversion.

[0063] The technical principles of the present invention have been described above with reference to specific embodiments. These descriptions are merely for explaining the principles of the invention and should not be construed as limiting the scope of protection of the invention in any way. Based on this explanation, those skilled in the art can readily conceive of other specific embodiments of the invention without inventive effort, and these equivalent variations or substitutions are all included within the scope defined by the claims of this application.

Claims

1. A method for preparing low-phosphorus stainless steel base material from scrap steel smelting, characterized in that, Includes the following steps: Scrap steel pretreatment: Scrap steel is subjected to magnetic separation and eddy current separation to remove phosphorus-containing impurities, and then Na2CO3 solution is sprayed onto the surface of the scrap steel to obtain pretreated scrap steel; Induction furnace smelting: Pretreated scrap steel is added to an induction furnace and heated, and CaO-FeO-Ca2SiO4 slag system is injected for pre-dephosphorization smelting to obtain molten steel; Oxidative dephosphorization: The molten steel is transferred into a converter, oxidizing gas is introduced, and then CaO-Ca2SiO4-FeO-MnO slag system is injected to carry out oxidative dephosphorization. The temperature of oxidative dephosphorization is equal to or less than 1670℃. Reduction refining: A CaO-Ca2SiO4-Al2O3-CaF2 slag system is injected into the molten steel after oxidation and dephosphorization for reduction refining, and a Si-Fe alloy is added. The reduction refining temperature is equal to or less than [temperature value missing]. Low-phosphorus molten steel was obtained at 1750℃; Continuous casting: The slag is recovered and the low-phosphorus molten steel is continuously cast to obtain stainless steel base billets, wherein the phosphorus content of the stainless steel base billets is ≤0.010%; In the intermediate frequency furnace smelting, oxidative dephosphorization and reduction refining steps, the P / (Si+P) atomic ratio is 0.085 to 0.

398.

2. The method for preparing low-phosphorus stainless steel base material from scrap steel according to claim 1, characterized in that, In the scrap steel pretreatment step, the concentration of the Na2CO3 solution is 10-15 wt.%, the blowing temperature is 600-800℃, and the treatment time is 10-35 min.

3. The method for preparing low-phosphorus stainless steel base material from scrap steel according to claim 1, characterized in that, In the pretreatment step of scrap steel, after Na2CO3 solution is sprayed onto the surface of scrap steel, it is washed with water and ultrasonically dispersed, and then dried to obtain pretreated scrap steel. The washing solution is a 0.5-1.0% citric acid solution.

4. The method for preparing low-phosphorus stainless steel base material from scrap steel according to claim 1, characterized in that, In the intermediate frequency furnace smelting step, the mass ratio of the injected CaO:FeO:Ca2SiO4 is (65-71):(15-30):(5-15).

5. The method for preparing low-phosphorus stainless steel base material from scrap steel according to claim 1, characterized in that, In the medium-frequency furnace smelting process, the slag basicity R is 2.8–3.2, the heating temperature is 1580–1630℃, and the pre-dephosphorization smelting time is 20–40 min.

6. The method for preparing low-phosphorus stainless steel base material from scrap steel according to claim 1, characterized in that, In the intermediate frequency furnace smelting, oxidative dephosphorization, and reductive refining steps, a pulse-jet system is used to inject slag powder. In the intermediate frequency furnace smelting step, CaO-FeO-Ca2SiO4 slag and CO2 gas are injected alternately at a frequency of 0.3–0.6 Hz. In the oxidative dephosphorization step, CaO-Ca2SiO4-FeO-MnO slag and CO2 gas are injected alternately at a frequency of 0.3–0.6 Hz. In the reductive refining step, CaO-Ca2SiO4-Al2O3-CaF2 slag and CO2 gas are injected alternately at a frequency of 0.3–0.6 Hz, with a CO2 flow rate of 0.3–1.5 Nm³. 3 / min·t.

7. The method for preparing low-phosphorus stainless steel base material from scrap steel according to claim 1, characterized in that, In the oxidative dephosphorization step, the oxidizing gas includes one of pure oxygen and an O2-Ar mixture; the mass ratio of O2 to Ar in the O2-Ar mixture is 3:1, and the supply intensity of the oxidizing gas is 0.8–1.2 Nm. 3 / min·t.

8. The method for preparing low-phosphorus stainless steel base material from scrap steel according to claim 1, characterized in that, In the oxidative dephosphorization step, the mass ratio of CaO:Ca2SiO4:FeO:MnO is (54-56):(4-6):(34-36):(3-6); the oxidative dephosphorization time is 14-16 min.

9. The method for preparing low-phosphorus stainless steel base material from scrap steel according to claim 1, characterized in that, In the reduction refining step, the mass ratio of CaO:Ca2SiO4:Al2O3:CaF2 is (58-62):(4-6):(24-26):(8-10), the mass ratio of Si to Fe in the Si-Fe alloy is (1-4):1, the mass ratio of the Si-Fe alloy to the molten steel after oxidation and dephosphorization is 1:(300-500), and the reduction refining time is 10-20 min.

10. A low-phosphorus stainless steel base material smelted from scrap steel, characterized in that, The low-phosphorus stainless steel base material is prepared using the method for preparing low-phosphorus stainless steel base material from scrap steel according to any one of claims 1 to 9, wherein the phosphorus content of the stainless steel base material billet is ≤0.01% and the chromium recovery rate is ≥95%.

Images