AOD dephosphorization process for Cr-Mn-based stainless steel

Abstract

This invention discloses an AOD dephosphorization process for Cr-Mn stainless steel, belonging to the field of stainless steel metallurgical refining technology. Addressing the technical challenges of dephosphorization difficulties, low dephosphorization rates, severe oxidation loss of chromium-manganese alloys, and poor smelting economics in existing Cr-Mn austenitic stainless steel smelting processes, this invention precisely selects the end of the AOD desiliconization stage as a dedicated oxidative dephosphorization process window. Based on the slag ion-molecule coexistence theory (IMCT), a thermodynamic prediction model for phosphorus distribution ratio is constructed. A multi-component composite dephosphorization slag system adapted to the Cr-Mn steel system is designed, along with precise slag removal pretreatment and quantitative control technology of slag components, achieving synergistic control of efficient dephosphorization and low chromium oxidation loss. By quantitatively controlling the main mass ratios of Na2O, Cr2O3, CaO, SiO2, MgO, and CaF2 in the dephosphorization slag, combined with an 80% quantitative slag removal pretreatment process, this invention can stably achieve a steel melt dephosphorization rate ≥70%, while controlling chromium oxidation loss to within 2.5%, resulting in low dephosphorization costs. This process requires no modification to existing AOD and LF industrial production lines, has strong equipment adaptability and good process repeatability, and can significantly reduce the content of phosphorus, oxygen and sulfur impurities in steel, greatly improve the purity and corrosion resistance of 201 type Cr-Mn series stainless steel, effectively solve the industry technical bottleneck that traditional processes cannot simultaneously achieve dephosphorization and chromium retention, and is suitable for large-scale industrial production and international technology transfer applications.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel metallurgical refining technology, specifically involving an efficient AOD dephosphorization and chromium retention process for ultra-pure smelting of low-nickel Cr-Mn austenitic stainless steel, which is suitable for industrial AOD ladle refining mass production scenarios. Background Technology

[0002] Cr-Mn austenitic stainless steel, with its significant advantages of low nickel and low cost, is a core economical alternative to high-nickel stainless steels such as 304, and is widely used in building decoration, kitchen and bathroom hardware, rail transportation, and light industrial manufacturing. However, the high manganese and low chromium and nickel composition of this steel leads to significant process difficulties and performance shortcomings in its smelting process.

[0003] Phosphorus is a typical harmful impurity element in stainless steel. It readily segregates at grain boundaries during the solidification process of molten steel, expanding the solid-liquid two-phase space and forming a brittle phosphide enrichment layer. This induces defects such as cold brittleness, intergranular corrosion, and welding cracks in the steel, severely degrading the mechanical and corrosion resistance properties of stainless steel. Dephosphorization is extremely difficult in almost all series of stainless steel because: chromium (Cr) in molten stainless steel binds to oxygen significantly better than phosphorus (P), and chromium is preferentially oxidized and burned off during oxidation; the high-melting-point Cr₂O₃ generated by oxidation significantly increases slag viscosity, worsening the slag-steel mass transfer kinetics; simultaneously, the matrix chromium reduces the activity coefficient of phosphorus, further inhibiting the forward dephosphorization reaction, creating a technical contradiction of "difficult dephosphorization, even more difficult chromium retention."

[0004] Existing industrial dephosphorization processes for stainless steel suffer from significant technical deficiencies: traditional single-CaO-based alkaline slag systems cannot effectively improve phosphorus oxidation selectivity, resulting in low dephosphorization efficiency; reduction dephosphorization processes require extremely low oxygen partial pressure and a high-reducing atmosphere in the furnace, making industrial implementation difficult and costly, and easily generating toxic phosphorus-containing waste gas, posing high environmental risks; conventional oxidation dephosphorization processes generally achieve dephosphorization rates below 20%, accompanied by severe chromium oxidation losses, significantly reducing yield and production economics. Furthermore, existing technologies have not matched the optimal dephosphorization process window to the AOD smelting characteristics of Cr-Mn stainless steel, and the mismatch in process matching further exacerbates the contradiction between dephosphorization and chromium preservation, severely restricting the high-quality, low-cost mass production of ultra-pure Cr-Mn stainless steel.

[0005] To address the aforementioned technological bottlenecks in the industry, there is an urgent need to develop a new AOD dephosphorization process that features precise process matching, high dephosphorization efficiency, low chromium loss, controllable cost, and suitability for industrial mass production. Summary of the Invention

[0006] Purpose of the invention The purpose of this invention is to overcome the shortcomings of existing Cr-Mn stainless steel smelting processes, such as low dephosphorization efficiency, large chromium-manganese alloy burn-off, poor process stability, and high production costs. This invention provides an AOD dephosphorization process for Cr-Mn stainless steel, which achieves synergistic control of efficient dephosphorization and low chromium loss through optimal process window positioning, quantitative slag removal pretreatment, and precise proportioning of multi-component composite slag system. This process balances product purity, performance indicators, and industrial economics, and is suitable for the industrial production needs of high-quality stainless steel both domestically and internationally. Technical solution

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: A process for AOD dephosphorization of Cr-Mn stainless steel includes the following steps: S1. Optimal Dephosphorization Window Positioning: Combining the thermodynamic and kinetic characteristics of the entire AOD process for Cr-Mn stainless steel, the end of the AOD desiliconization stage is determined to be the optimal process window for oxidative dephosphorization. The main components of the molten steel at this stage are: 3.0~3.5wt%C, 0.001~0.02wt%Si, 0.40~0.65wt%Mn, 10.0~14.0wt%Cr, 0.9~3.0wt%Ni, 0.02~0.06wt%P, and 0.03~0.06wt%TO. The furnace oxygen potential and slag state are most well matched, which can suppress the non-selective oxidation of chromium-manganese alloys to the greatest extent.

[0008] S2. Quantitative slag removal pretreatment: Before the dephosphorization reaction is started, the original desiliconization slag in the AOD furnace is quantitatively removed, and the slag removal rate is strictly controlled to be greater than 70% to completely reduce the interference of residual old slag on the dephosphorization system and create a stable reaction environment for the new composite dephosphorization slag.

[0009] S3. Quantity and Precise Configuration of Multi-component Composite Dephosphorization Slag: Based on thermodynamic calculations using the Ion-Molecular Coexistence Theory (IMCT) and industrial experimental verification, the quantity of the dedicated composite dephosphorization slag is configured as follows: slag-to-steel ratio of 0.01~0.1. The mass percentages of each major molten slag component are: 45.0~56.0 wt.% Na2O, 10~15 wt.% CaO, 10.0~15.0 wt.% SiO2, 2~20 wt.% MnO, 0.9~2.0 wt.% FeO, 2.0~25.0 wt.% Cr2O3, 1.0~3.0 wt.% MgO, and 1.0~3.0 wt.% CaF2. Among them, Na2O can decompose the polysilicate network structure, reduce the high-temperature viscosity of slag, and improve the mass transfer efficiency and phosphorus distribution ratio at the slag-steel interface; MnO and Cr2O3 can precisely control the oxidation sequence of the system, improve the oxidation selectivity of phosphorus, and inhibit the oxidation and burn-off of matrix chromium and manganese; saturated MgO can effectively protect the AOD furnace lining and avoid high-temperature alkaline slag erosion; CaF2, as a flux, stabilizes and ensures the high-temperature fluidity of the slag, ensuring that the dephosphorization reaction proceeds fully. The slag materials are all compounded from conventional industrial raw materials, which are highly versatile and cost-controllable.

[0010] S4. Temperature of efficient dephosphorization reaction: The smelting temperature in the AOD furnace is stably maintained at 1380~1460℃ to ensure that the dephosphorization reaction at the slag-steel interface continues in a positive direction, stably achieving a dephosphorization rate of ≥70%, while strictly controlling the chromium oxidation loss of molten steel to within 2.5%.

[0011] S5 and LF Precision Refining Optimization: After dephosphorization, the molten steel is sent to the LF refining process. An optimized deoxidation and desulfurization slag system is adopted, and the mass ratio of CaO / SiO2 in the slag system is controlled to be 1.5-3.0, the MgO content is 2.0-8.0 wt.%, and the CaF2 content is 3-35.0 wt.%. This can reduce the oxygen content in the steel to below 15 ppm and the sulfur content to below 10 ppm, thereby obtaining Cr-Mn stainless steel with high purity and high corrosion resistance. Beneficial effects

[0012] Compared with the prior art, the present invention has the following significant technical advantages and economic benefits: First, this invention innovatively locks the end of AOD desilication as the optimal dephosphorization window, and, in conjunction with a quantitative 70% slag removal pretreatment process, solves the core contradiction of traditional processes that cannot simultaneously achieve dephosphorization and chromium retention from the source of the process. The process logic is scientific and highly adaptable.

[0013] Secondly, the use of Na2O-Cr2O3 synergistic compound slag system significantly improves the oxidation priority of phosphorus, with a stable dephosphorization rate of ≥70% and chromium oxidation loss of ≤2.5%. While ensuring efficient dephosphorization, it greatly reduces the burn-off of precious alloys and improves the yield.

[0014] Third, this process is fully compatible with existing AOD-LF industrial production lines, requires no equipment modification, uses conventional industrial auxiliaries as raw materials, is simple to operate, has good repeatability, and is cost-controllable. The cost per ton of steel slag can be as low as US$25, and its economic advantages for large-scale production are significant.

[0015] Fourth, this process can achieve ultra-low oxygen and ultra-low sulfur purity control in molten steel. The Cr-Mn stainless steel produced has no obvious pitting defects and its corrosion resistance is close to that of commercial 304 stainless steel, which greatly improves product quality and market added value.

[0016] Fifth, the use of an oxidative dephosphorization system avoids the problem of toxic waste gas emissions from traditional reduction dephosphorization processes. It is green and environmentally friendly, with high process stability, and meets the environmental protection and high-quality production standards of the metallurgical industry at home and abroad. Detailed Implementation

[0017] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Example

[0018] A process for AOD dephosphorization of Cr-Mn stainless steel includes the following steps: The desilication stage at the end of the AOD smelting process for 201 type Cr-Mn stainless steel was selected as the dephosphorization process. At this time, the composition of the molten steel was 3.34% C, 0.01% Si, 0.55% Mn, 12.48% Cr, 0.99% Ni, 0.0422% P, and 0.037% TO. The slag removal rate in the furnace was controlled at 80%. A composite dephosphorization slag was prepared, with the main components being: 53.3 wt.% Na2O, 13.1 wt.% CaO, 1.0% wt. FeO, 11.9 wt.% SiO2, 14.0 wt.% Cr2O3, 2.6 wt.% MnO, 1.7 wt.% MgO, and 1.3 wt.% MgO. wt.% CaF2, with trace amounts of other components; furnace temperature 1450℃, after the slag-steel dephosphorization reaction is completed, the phosphorus content in the steel is reduced to 0.0121%; the subsequent LF refining uses an optimized slag system with the main components CaO / SiO2=1.8, MgO 2.0 wt.%, and CaF2 10.0 wt.% to complete the deoxidation and desulfurization refining.

[0019] The test results of this example are as follows: dephosphorization rate 71.3%, chromium oxidation loss 2.1%, oxygen content in the steel after LF refining 12.6ppm, sulfur content 8.3ppm, no pitting defects in the 24h acid immersion test, and slag cost of 30 US dollars per ton of steel. Example

[0020] A process for AOD dephosphorization of Cr-Mn stainless steel includes the following steps: The desilication stage at the end of the AOD smelting process for 201 type Cr-Mn stainless steel was selected as the dephosphorization process. At this time, the composition of the molten steel was 3.44% C, 0.02% Si, 0.50% Mn, 12.11% Cr, 0.95% Ni, 0.044% P, and 0.039% TO. The slag removal rate in the furnace was controlled at 80%. The main components of the composite dephosphorization slag were: 48.3 wt.% Na2O, 11.5 wt.% CaO, 1.2% wt. FeO, 10.9 wt.% SiO2, 22.0 wt.% Cr2O3, 2.4 wt.% MnO, 1.57 wt.% MgO, and 1.13 wt.% MgO. wt.% CaF2, with trace amounts of other components; furnace temperature 1440℃, after the slag-steel dephosphorization reaction is completed, the phosphorus content in the steel is reduced to 0.0105%; the main components of the subsequent LF refining slag are an optimized slag system of CaO / SiO2=2.0, MgO 3.0 wt.%, and CaF2 12.0 wt.% to complete the deoxidation, desulfurization and refining.

[0021] The test results of this example are as follows: dephosphorization rate 76.1%, chromium oxidation loss 2.0%, oxygen content in steel after LF refining 13.4ppm, sulfur content 7.1ppm, no pitting defects in 24h acid immersion test, and slag cost of 28 US dollars per ton of steel, which has excellent production economy. Example

[0022] A process for AOD dephosphorization of Cr-Mn stainless steel includes the following steps: The desilication stage at the end of the AOD smelting process for 201 type Cr-Mn stainless steel was selected as the dephosphorization process. At this time, the composition of the molten steel was 3.11 wt.% C, 0.01 wt.% Si, 0.44 wt.% Mn, 11.91 wt.% Cr, 0.94 wt.% Ni, 0.042 wt.% P, and 0.042 wt.% TO. The slag removal rate in the furnace was controlled at 78%. The main components of the composite dephosphorization slag were: 50.7 wt.% Na2O, 12.0 wt.% CaO, 1.3 wt.% FeO, 11.2 wt.% SiO2, 2.8 wt.% Cr2O3, 18.0 wt.% MnO, and 1.35 wt.% TiO2. The slag contained 1.19 wt.% MgO and 1.19 wt.% CaF2, with trace amounts of other components. After the dephosphorization reaction of the slag and steel was completed at a furnace temperature of 1430℃, the phosphorus content in the steel was reduced to 0.0125%. The subsequent LF refining used an optimized slag system with CaO / SiO2=1.9, MgO 4.0 wt.% and CaF2 11.0 wt.% as the main components to complete the deoxidation and desulfurization refining.

[0023] The test results of this example are as follows: dephosphorization rate 70.2%, chromium oxidation loss 2.2%, oxygen content in the steel after LF refining 15.0ppm, sulfur content 9.1ppm, no pitting defects in the 24h acid immersion test, and slag cost of 26 US dollars per ton of steel, which has excellent production economy.

[0024] In the final stage of AOD desilication, only a single CaO dephosphorizing agent was added, and the remaining process parameters were completely consistent with those in Example 1. Test results showed that the dephosphorization rate was only 4.3%, the chromium oxidation loss was as high as 6.8%, there was almost no dephosphorization effect, and the alloy loss was serious, which could not meet the production index requirements of ultra-pure stainless steel.

[0025] The original desiliconization slag was retained, and the 80% slag removal pretreatment was omitted. The composite dephosphorization slag of this invention was directly added, and the rest of the process was the same as in Example 1. The original residual slag components seriously interfered with the dephosphorization reaction, and the final dephosphorization rate was only 21.6%, which could not meet the industrial ultrapure smelting standard. Implementation effect

[0026] Through comparative verification using multiple examples and embodiments, this invention, through precise process window positioning, quantitative slag removal pretreatment, and synergistic optimization of a multi-element slag system, can stably achieve efficient dephosphorization and low chromium loss production of Cr-Mn stainless steel, effectively solving the technical bottlenecks of traditional processes. The stainless steel prepared by this invention has extremely low impurity content, uniform microstructure, and excellent corrosion resistance, combining process stability and production economy, making it suitable for large-scale industrial mass production and international technology promotion. Attached Figure Description

[0027] none Implementation

[0028] The optimal industrial-scale implementation parameters of this invention are as follows: selecting the final stage of AOD desilication process window, a slag removal rate of 80%, and a dephosphorization slag composition of 48.3 wt.% Na₂O, 12.1% CaO, 11.9% SiO₂, 22.0 wt.% Cr₂O₃, 2.4 wt.% MnO, 1.57 wt.% MgO, and 1.13 wt.% CaF₂, with trace amounts of other components; a furnace temperature of 1440℃, and matching an LF refining slag system with CaO / SiO₂ = 1.8. This parameter combination can achieve a dephosphorization rate ≥76.1% and a chromium loss ≤2.2%, achieving an optimal balance between product performance, production cost, and process stability, making it suitable for large-scale industrial continuous mass production.

[0029] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made to the present invention within the inventive concept and principles of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A process for AOD dephosphorization of Cr-Mn stainless steel, characterized in that, Includes the following steps: S1. Process window selection: The final stage of the desiliconization process in the AOD smelting of Cr-Mn stainless steel is selected as the dedicated reaction process for oxidative dephosphorization of molten steel. The main components of the molten steel are: 3.0~3.5 wt %C, 0.001~0.02 wt %Si, 0.40~0.65 wt %Mn, 10.0~14.0 wt %Cr, 0.9~3.0 wt %Ni, 0.02~0.06 wt %P, and 0.03~0.06 wt %TO. S2. Slag Pretreatment and Slag Removal: Slag is removed from the furnace after desiliconization in the AOD furnace, and the overall slag removal rate is controlled to be greater than 70%. S3. Precise Configuration of Composite Dephosphorizing Slag: Composite slag material is added to the remaining slag in the furnace, with a slag-to-steel mass ratio of 0.01~0.

1. The mass percentage of each major component of the dephosphorizing slag is quantitatively controlled as follows: 45.0~56.0 wt.% Na2O, 10~15 wt.% CaO, 10.0~15.0 wt.% SiO2, 2~20 wt.% MnO, 0.9~2.0 wt.% FeO, 2.0~25.0 wt.% Cr2O3, 1.0~3.0 wt.% MgO, 1.0~3.0 wt.% CaF2, with the balance being conventional impurities from smelting. Among these components, Na2O is the main dephosphorizing component, MgO inhibits high-temperature furnace lining erosion, MnO and Cr2O3 respectively inhibit the oxidation loss of Mn and Cr, and CaF2 acts as a flux to ensure the high-temperature fluidity of the slag. S4. Dephosphorization reaction: Maintain the smelting temperature in the AOD furnace at 1380~1460℃ to carry out the slag-steel interface coupled dephosphorization reaction, so as to achieve efficient dephosphorization of molten steel and low chromium oxidation loss smelting. S5. Subsequent refining and control: After the dephosphorization process is completed, the molten steel is transferred to the LF refining process to complete deoxidation, desulfurization and precise fine-tuning of composition, and to prepare ultra-pure Cr-Mn stainless steel.

2. The AOD dephosphorization process for Cr-Mn stainless steel according to claim 1, characterized in that, The composite slag material described in step S3 is prepared by compounding commonly used industrial raw materials, including chromite, magnesium spheres, fluorite and soda ash. No special chemical raw materials are required. The raw materials are readily available and inexpensive, making it suitable for large-scale industrial continuous production.

3. The AOD dephosphorization process for Cr-Mn stainless steel according to claim 1, characterized in that, The composition range of the dephosphorization slag system described in step S3 is obtained by precise thermodynamic calculations of the slag component activity, phosphorus distribution ratio, and multi-component reactions between slag and steel in the CaO-Na2O-MgO-CaF2-FeO-MnO-SiO2-Cr2O3-Fe2O3 multi-component slag system based on the IMCT ion-molecule coexistence theory.

4. The AOD dephosphorization process for Cr-Mn stainless steel according to claim 1, characterized in that, In step S4, the chromium oxidation loss of the molten steel is ≤2.5%, and the overall dephosphorization rate of the molten steel is ≥70%.

5. The AOD dephosphorization process for Cr-Mn stainless steel according to claim 1, characterized in that, The 70% slag removal pretreatment described in step S2 is a necessary pre-process for efficient dephosphorization. Incomplete removal of the original desiliconized slag will inhibit the oxidation priority of phosphorus and significantly reduce the dephosphorization efficiency.

6. The AOD dephosphorization process for Cr-Mn stainless steel according to claim 1, characterized in that, A single alkaline oxide CaO slag system cannot achieve efficient dephosphorization of Cr-Mn stainless steel. It is necessary to synergistically combine two alkaline oxides, Na2O and CaO, with Cr2O3 and MnO components to improve the selectivity of phosphorus oxidation and achieve integrated synergistic control of dephosphorization and chromium retention.

7. The AOD dephosphorization process for Cr-Mn stainless steel according to claim 1, characterized in that, This process is suitable for the industrial smelting of Cr-Mn austenitic stainless steel. With the optimization of the LF refining slag system, the total oxygen content in the steel can be controlled below 15ppm and the sulfur content below 10ppm, which significantly improves the corrosion resistance and microstructure uniformity of stainless steel.

8. The AOD dephosphorization process for Cr-Mn stainless steel according to claim 1, characterized in that, The cost per ton of steel from the dephosphorized slag can be controlled within the range of US$25 to US$45, and production can be adapted to the fluctuations in industrial raw material prices.