Method for determining LF refining slag amount of ultra-low sulfur pipeline steel

Abstract

The invention belongs to the technical field of ferrous metallurgy, particularly relates to a method for determining the LF refining slag amount of ultralow-sulfur pipeline steel, provides a molten iron-free pretreatment-single slag method ultralow-sulfur pipeline steel production process, and establishes an optimal refining slag system of the ultralow-sulfur pipeline steel. According to the method, the sulfur capacity of a refining slag system is calculated through a KTH model, the sulfur distribution ratio of the refining slag is further obtained by combining current molten steel components, the theoretical adding amount of the refining slag is accurately obtained according to the requirements of the sulfur distribution ratio, the molten steel amount, the LF refining station entering sulfur content and the LF refining station exiting sulfur content, and a refining slag amount rapid calculation model based on LF station entering sulfur content classification is provided. And when the sulfur content of the entering molten steel is less than or equal to 0.030 wt%, the optimal desulfurization efficiency of the LF is ensured based on the LF slag system optimization and accurate slag quantity control technology, stable production of the ultra-low sulfur steel can be realized under the condition of no molten iron pretreatment, and the sulfur content of a finished product is controlled within 10ppm.

Description

Technical Field

[0001] The present invention belongs to the technical field of iron and steel metallurgy, and particularly relates to a method for determining the LF refining slag amount of an ultra-low sulfur pipeline steel. Background Art

[0002] With the increasing demand for oil and gas resources, countries around the world are committed to the exploration and development of oil and gas fields in remote areas and offshore oil and gas fields. As the most economical and safe transportation method for such oil and gas resources, oil and gas pipeline transportation has become the top choice for oil and gas transportation methods. Extreme environments and long-distance transportation have put forward more stringent requirements for the performance of pipeline steel, and it is necessary to control the S content at an extremely low level. This is because when the S content in the steel is too high, long strip-shaped MnS inclusions are likely to be generated, and hydrogen sulfide in oil and gas has a certain corrosion ability. The generated hydrogen atoms will enter the steel along the precipitates and inclusions, and under the action of stress, the hydrogen atoms will combine to form hydrogen gas, resulting in too high local hydrogen pressure, and microscopic cracks will be generated in the hydrogen precipitation area. Therefore, acid-resistant pipeline steel requires the S content in the steel to reach a relatively low level.

[0003] At present, there are mainly the following production process modes for ultra-low sulfur pipeline steel in the steelmaking production process: The first method is hot metal desulfurization pretreatment - converter blowing - LF refining - RH (VD) vacuum refining - continuous casting. This method can control S to below 0.002 wt%, but it will increase the smelting cycle and smelting cost. Especially for hot metal with a low S content, transferring the desulfurization task to the LF process is an effective strategy; the second method is the LF double-slag deep desulfurization process. The sulfur capacity of the secondary slag system is high, and the desulfurization efficiency is significantly improved. However, this method has a long refining time and higher requirements for production rhythm control; the third method uses the CaO-CaF2 refining slag system for desulfurization. Ultra-high alkalinity desulfurization is carried out while adding CaF2 to ensure the fluidity of the slag. However, the large use of CaF2 will cause environmental pollution and endanger the health of workers. These processes still have room for improvement and there are some problems that cannot be avoided under the existing technology:

[0004] (1) Adopting the production process without hot metal pretreatment puts forward higher requirements for the S content of hot metal and the desulfurization ability of the LF process. When the S content of hot metal exceeds a certain critical value, the LF single-slag method cannot effectively meet the low sulfur control target. Under high sulfur conditions, the added desulfurizer will quickly reach a saturated state and cannot further effectively desulfurize.

[0005] (2) In order to ensure the LF refining desulfurization efficiency, it is necessary to increase the alkalinity of the slag. However, when the alkalinity is too high, the melting point of the slag system increases, the refining slag is prone to caking, the fluidity of the molten steel becomes poor, the effect of the slag on adsorbing and removing alumina inclusions becomes poor, and the cleanliness level of the molten steel will be significantly reduced, reducing the market competitiveness of the product.

[0006] (3) Due to the different initial conditions of molten steel, in order to remove S in the molten steel to the target value and avoid problems with unqualified S composition, enterprises will all adopt the process route of large slag volume. The slag consumption for desulfurization is large, and it is difficult to accurately control the slag volume, resulting in an increase in the cost per ton of steel. At the same time, under the condition of large slag volume, the erosion of the refractory of the ladle is aggravated, and the repair frequency of the ladle increases, both of which will increase the steelmaking cost.

[0007] (4) Although some researchers have proposed different slag-making systems for slag-making desulfurization according to the S content in the incoming molten steel during LF refining, the S content range in the grading system is relatively large, and the slag volume control still has great volatility. The slag-making system is more cumbersome, and the operation complexity of the LF refining process is significantly improved, increasing the labor intensity of on-site workers. Summary of the Invention

[0008] To solve the problems existing in the prior art, the main object of the present invention is to propose a method for determining the slag volume of LF refining of ultra-low sulfur pipeline steel.

[0009] According to one aspect of the present invention, the present invention provides the following technical solutions:

[0010] A method for determining the slag volume of LF refining of ultra-low sulfur pipeline steel, which optimizes the slag system composition through Factsage thermodynamic software, determines that CaO / SiO2 is in the range of 5-7.5 and CaO / Al2O3 is in the range of 1.5-2.0, ensuring that the slag system has the characteristics of high alkalinity and low melting point, and improving the desulfurization efficiency and inclusion adsorption capacity. Combining with the KTH sulfur capacity model, calculate the sulfur capacity and sulfur distribution ratio of the optimal slag system composition, and accurately calculate the theoretical addition amount of LF refining slag. Further, according to the different sulfur contents (w1) of the incoming molten steel in LF refining, the molten steel is divided into three categories: low sulfur (w1≤0.010wt%), medium sulfur (0.010wt%<w1≤0.020wt%), and high sulfur (0.020wt%<w1≤0.030wt%), and the theoretical refining slag volume calculation equations are respectively fitted, and the goodness of fit R 2 All reach above 0.994. This method significantly improves the hit rate of the end sulfur content by accurately controlling the slag volume, reduces the addition of redundant slag materials, reduces the steelmaking cost, and improves the production efficiency. The experimental results show that this method can stably control the sulfur content of the molten steel below 10 ppm, and is applicable to the production of ultra-low sulfur pipeline steel from molten steel with an incoming sulfur content ≤0.030wt% smelted from hot metal without hot metal pretreatment process.

[0011] The beneficial effects of the present invention are as follows:

[0012] The present invention provides a method for determining the LF refining slag amount of ultra-low sulfur pipeline steel, provides a production process for ultra-low sulfur pipeline steel without hot metal pretreatment - single slag method, and establishes an optimal refining slag system for ultra-low sulfur pipeline steel. The sulfur capacity of the refining slag system is calculated by the KTH model, and further combined with the current molten steel composition to obtain the sulfur distribution ratio of the refining slag. According to the sulfur distribution ratio, the molten steel amount, the sulfur content at the inlet of LF refining and the sulfur content requirement at the outlet of LF refining, the theoretical addition amount of the refining slag is accurately obtained. A rapid calculation model for the refining slag amount based on the classification of the sulfur content at the inlet of LF is proposed. When the sulfur content of the molten steel at the inlet is ≤ 0.030 wt%, based on the LF slag system optimization and accurate slag amount control technology, the optimal desulfurization efficiency of LF is ensured, and the stable production of ultra-low sulfur steel can be achieved without hot metal pretreatment, and the finished product sulfur content is controlled within 10 ppm. Detailed implementation manners

[0013] The technical solutions in the embodiments will be described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0014] According to one aspect of the present invention, the present invention provides the following technical solutions:

[0015] A method for determining the LF refining slag amount of ultra-low sulfur pipeline steel, which determines the LF refining slag system for ultra-low sulfur pipeline steel; calculates the sulfur capacity of the refining slag system, and further combines the current molten steel composition to obtain the sulfur distribution ratio of the refining slag system. According to the sulfur distribution ratio, the molten steel amount, the sulfur content at the inlet of LF refining and the sulfur content requirement at the outlet of LF refining, the LF refining slag amount is obtained.

[0016] Preferably, there is an optimal balance point between the desulfurization efficiency and fluidity of the refining slag system. Finding this balance point is crucial for improving the desulfurization effect of molten steel and optimizing the refining process. First, the present invention uses the Factsage software to calculate the activities and viscosities of the components of the refining slag system, and obtains that when CaO / Al2O3 in the refining slag system is 1.5 - 2.0, CaO / SiO2 is 5 - 7.5, and MgO is about 4wt%, the area of the low melting point region of the high alkalinity refining slag system can be ensured to be the largest. It is calculated that when the CaO / SiO2 in the slag is in the range of 5 - 7.5, the fluidity of the refining slag can be maintained well and the adsorption and removal ability of alumina inclusions is relatively strong. When the CaO / Al2O3 in the slag is in the range of 1.5 - 2.0, the slag viscosity can be effectively reduced and the mass transfer coefficient in the slag can be increased. The obtained composition of the refining slag system is: CaO / SiO2 is 5 - 7.5, CaO / Al2O3 is 1.5 - 2.0, CaO is 49 - 59wt%, SiO2 is 6 - 10wt%, Al2O3 is 29 - 37wt%, MgO is 3 - 5wt%, and (FeO + MnO) < 1wt%.

[0017] Preferably, a calculation model for the addition amount of the refining slag is calculated according to the sulfur content of the molten steel entering the station and the target sulfur content. Based on the high alkalinity and low melting point refining slag system, the optimal refining slag system composition region block is obtained in the isothermal phase diagram of the refining slag system to ensure good slag melting point, slag fluidity and the adsorption ability for alumina inclusions. To obtain the slag system composition corresponding to the highest sulfur capacity value in this region block, the sulfur capacity values under various refining slag system compositions are calculated using the KTH model and plotted in the isothermal phase diagram of the refining slag system to obtain the isosulfur capacity phase diagram of the refining slag system. Combining the optimal refining slag system region block with the isosulfur capacity phase diagram, the refining slag system composition with the strongest desulfurization ability in this region block is obtained, the sulfur capacity of the refining slag under this composition is calculated, and the sulfur distribution ratio of the refining slag is further obtained in combination with the molten steel composition; then, according to the sulfur distribution ratio, the molten steel amount, the sulfur content of the molten steel entering the LF refining station and the sulfur content requirement of the molten steel leaving the LF refining station, the theoretical addition amount of the refining slag is accurately obtained. At the same time, a large amount of production data is used to calculate the theoretical slag amount of the LF refining and linear fitting is carried out with the help of software to obtain the relationship between the theoretical slag amount of the LF refining and the change of the sulfur content of the molten steel entering the LF station. Based on the sulfur contents of the steel and slag measured on site, the sulfur distribution ratio in the actual production process is obtained, the sulfur distribution ratio calculated theoretically is corrected, and the accuracy and effectiveness of the calculated slag amount are verified through actual production. This method can quickly remove the sulfur content in the molten steel to the target value by accurately adding the refining slag amount, avoiding the instability of the slag amount added by manual experience, and avoiding the increase in steelmaking costs caused by excessive addition of the refining slag amount, replacing the cumbersome multi-pass slag-making system, thereby reducing the labor of on-site workers.

[0018] Preferably, by combining the optimal refined slag system region block with the equal sulfur capacity phase diagram, the composition of the refined slag system with the strongest desulfurization ability in this region block is: CaO is 55.2 wt%, SiO2 is 7.6 wt%, Al2O3 is 33.2 wt%, and MgO is 4 wt%. The sulfur capacity Cs of this refined slag system composition is calculated through the KTH model, where ΔG θ is the standard Gibbs free energy of the slag-gas equilibrium reaction equation , T is 1873 K, and ξ is the interaction coefficient of this slag system; the alumina activity a Al2O3 is calculated according to the composition of this refined slag system, and through the Al-O equilibrium reaction in molten steel, combined with the Al content and Al activity coefficient f Al in molten steel, the oxygen activity a [O] is calculated. Through the oxygen activity a [O] , S activity coefficient f S and the sulfur capacity Cs of the best sulfur capacity slag system composition, the sulfur distribution ratio Ls of the best sulfur capacity slag system composition is calculated. The specific calculation process is as follows:

[0019]

[0020] Preferably, the amount of refined slag is calculated through the sulfur distribution ratio. The sulfur content of the molten steel entering the LF refining station is w1, wt%; the sulfur content of the molten steel leaving the LF refining station is w2, wt%; the weight of the molten steel is m1, t; the amount of refined slag is m2, kg; the sulfur in the molten steel and the slag should satisfy the steel-slag equilibrium, and the calculation method for the amount of refined slag m2 is:

[0021]

[0022] Preferably, based on the above refined slag system composition, combined with the weight of the molten steel and the composition of the molten steel entering the LF in the actual production data, the amount of refined slag is calculated. In order to further improve the prediction accuracy, according to the different sulfur content w1 of the molten steel entering the LF refining station, the molten steel is divided into the following categories for hierarchical control:

[0023] Low-sulfur molten steel: w1 ≤ 0.010 wt%

[0024] Medium-sulfur molten steel: 0.010 wt% < w1 ≤ 0.020 wt%

[0025] High-sulfur molten steel: 0.020 wt% < w1 ≤ 0.030 wt%

[0026] According to the classification of the sulfur content w1 of the molten steel entering the LF refining station, linear fitting is performed on low-sulfur molten steel, medium-sulfur molten steel, and high-sulfur molten steel respectively to obtain three corresponding calculation equations for the amount of refined slag, that is, the regression equation of the amount of refined slag m (kg / t) per ton of molten steel in LF refining and the sulfur content w (ppm) of the molten steel entering the LF refining station. The following is the classification fitting result:

[0027] Low-sulfur molten steel (w1 ≤ 0.010 wt%), fitting R 2 = 0.995

[0028] m 低 = (0.08492w1 - 0.77468) (6)

[0029] Medium-sulfur molten steel (0.010 wt% < w1 ≤ 0.020 wt%), fitting R 2 = 0.997

[0030] m 中 = (0.09045w1 - 0.81262) (7)

[0031] High-sulfur molten steel (0.020 wt% < w1 ≤ 0.030 wt%), fitting R 2 = 0.994

[0032] m 高 = (0.09845w1 - 0.87548) (8)

[0033] By classifying the molten steel entering the LF refining station according to sulfur content into low-sulfur, medium-sulfur and high-sulfur categories and performing linear fitting respectively, three high-precision calculation equations for the refining slag amount are obtained. These equations can accurately predict the required refining slag amount according to the different initial sulfur contents of the molten steel, thereby optimizing the desulfurization effect and ensuring the cleanliness of the molten steel.

[0034] The technical solution of the present invention will be further described below in conjunction with specific embodiments.

[0035] Example 1

[0036] The chemical composition (mass percentage) of the molten steel entering the LF refining furnace for ultra-low sulfur pipeline steel is as follows: C: 0.05109%, Si: 0.08738%, Mn: 1.03981%, Al: 0.04472%, S: 0.02138%. The weight of the molten steel is 139.9 t. Substituting the S content of 213.8 ppm at the LF furnace inlet into the formula for slag amount per ton of steel in LF refining, the slag amount per ton of steel is obtained as 20.17 kg / t. The slag materials are configured according to the optimal refining slag system composition (mass percentage) calculated by Factage thermodynamic software: CaO: 55.2%, MgO: 4.0%, Al2O3: 33.2%, SiO2: 7.6%. The chemical composition (mass percentage) of the white slag formed in LF refining is: CaO: 55.314%, MgO: 3.703%, Al2O3: 33.005%, SiO2: 7.348%, MnO: 0.085%, FeO: 0.545%, which conforms to the slag system range in the low melting point area calculated by Factage thermodynamic software. The chemical composition (mass percentage) of the molten steel at the LF refining furnace outlet is: C: 0.09963%, Si: 0.22436%, Mn: 1.23714%, Al: 0.06138%, S: 0.00085%, meeting the requirements for the molten steel at the refining furnace outlet, with S content ≤ 10 ppm.

[0037] Example 2

[0038] The chemical composition (mass percentage) of the molten steel entering the LF refining furnace for ultra-low sulfur pipeline steel is as follows: C: 0.09062%, Si: 0.08129%, Mn: 0.99664%, Al: 0.06347%, S: 0.01648%. The weight of the molten steel is 139.3 t. Substituting the S content of 164.8 ppm at the LF furnace inlet into the formula for slag amount per ton of steel in LF refining, the slag amount per ton of steel is obtained as 14.09 kg / t. The slag materials are configured according to the optimal refining slag system composition (mass percentage) calculated by Factage thermodynamic software: CaO: 55.2%, MgO: 4.0%, Al2O3: 33.2%, SiO2: 7.6%. The chemical composition (mass percentage) of the white slag formed in LF refining is: CaO: 54.637%, MgO: 4.153%, Al2O3: 33.244%, SiO2: 7.169%, MnO: 0.190%, FeO: 0.607%, which conforms to the slag system range in the low melting point area calculated by Factage thermodynamic software. The chemical composition (mass percentage) of the molten steel at the LF refining furnace outlet is: C: 0.12821%, Si: 0.21119%, Mn: 1.07314%, Al: 0.06053%, S: 0.00093%, meeting the requirements for the molten steel at the refining furnace outlet, with S content ≤ 10 ppm.

[0039] Example 3

[0040] The chemical composition (mass percentage) of the molten steel entering the LF refining furnace for ultra-low sulfur pipeline steel is as follows: C: 0.06943%, Si: 0.07436%, Mn: 1.13458%, Al: 0.04375%, S: 0.01926%. The weight of the molten steel is 139.8 t. Substituting the S content of 92.9 ppm when entering the LF furnace into the formula for the slag amount per ton of steel in LF refining, the slag amount per ton of steel is obtained as 7.11 kg / t. The slag materials are configured according to the optimal refining slag system composition (mass percentage) calculated by Factage thermodynamic software: CaO: 55.2%, MgO: 4.0%, Al2O3: 33.2%, SiO2: 7.6%. The chemical composition (mass percentage) of the white slag formed in LF refining is: CaO: 55.108%, MgO: 3.974%, Al2O3: 33.115%, SiO2: 7.215%, MnO: 0.109%, FeO: 0.479%, which meets the range of the slag system in the low melting point area calculated by Factage thermodynamic software. The chemical composition (mass percentage) of the molten steel leaving the LF refining furnace is: C: 0.09146%, Si: 0.23548%, Mn: 1.23718%, Al: 0.05278%, S: 0.00089%, meeting the requirements for leaving the refining furnace, with the S content ≤ 10 ppm.

[0041] The above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention accordingly. Any equivalent structural transformation made using the content of the specification of the present invention under the inventive concept of the present invention, or any direct / indirect application in other related technical fields is included in the patent protection scope of the present invention.

Claims

1. A method for determining the amount of LF refined slag of ultra-low sulfur pipeline steel, characterized in that: Determine the LF refining slag system for ultra-low sulfur pipeline steel; calculate the sulfur capacity of the refining slag system, and further combine the current molten steel composition to obtain the sulfur distribution ratio of the refining slag system; obtain the LF refining slag amount based on the sulfur distribution ratio, molten steel volume, LF refining inlet sulfur content and LF refining outlet sulfur content requirements.

2. The method for determining the amount of ultra-low sulfur pipeline steel LF refined slag according to claim 1, characterized in that: The composition of the refined slag system is: CaO / SiO2 is 5-7.5, CaO / Al2O3 is 1.5-2.0, CaO is 49-59wt%, SiO2 is 6-10wt%, Al2O3 is 29-37wt%, MgO is 3-5wt%, and (FeO+MnO) is less than 1wt%.

3. The method for determining the amount of ultra-low sulfur pipeline steel LF refined slag according to claim 2, characterized in that: The calculation model of the amount of refined slag added is calculated according to the sulfur content of the molten steel entering the station and the target sulfur content, and the optimal refined slag system composition area block is obtained in the above-mentioned refined slag system isothermal phase diagram. In order to obtain the slag system composition corresponding to the position with the highest sulfur capacity value in the area block, the KTH model is used to calculate the sulfur capacity values ​​under various refined slag system compositions, and the values ​​are plotted in the above-mentioned refined slag system isothermal phase diagram to obtain an isosulfur capacity phase diagram. The refined slag system composition with the strongest desulfurization ability in the area block is obtained by combining the optimal refined slag system composition area block with the isosulfur capacity phase diagram, and the sulfur capacity of the refined slag under the composition is calculated, and the refined slag sulfur distribution ratio is further obtained in combination with the molten steel composition; then the theoretical amount of refined slag added is accurately obtained according to the sulfur distribution ratio, molten steel volume, LF refining inlet sulfur content and LF refining outlet sulfur content.

4. The method for determining the amount of ultra-low sulfur pipeline steel LF refined slag according to claim 3, characterized in that: Combining the optimal refined slag system regional block with the isosulfur capacity phase diagram, the refined slag system composition with the strongest desulfurization ability in this regional block is: CaO is 55.2wt%, SiO2 is 7.6wt%, Al2O3 is 33.2wt%, and MgO is 4wt%.

5. The method for determining the amount of ultra-low sulfur pipeline steel LF refined slag according to claim 3, characterized in that: The sulfur capacity Cs of the refined slag components is calculated by the KTH model, where ΔG θ The slag gas equilibrium reaction equation is The standard Gibbs free energy is 1873K, ξ is the interaction coefficient of the slag system; the activity of alumina is calculated based on the composition of the refined slag system. Al2O3 And through the Al-O equilibrium reaction in molten steel, combined with the Al content in molten steel and the Al activity coefficient f Al Calculate oxygen activity a [O] , through oxygen activity a [O] , S activity coefficient f S The sulfur distribution ratio Ls of the slag system components with the best sulfur capacity is calculated by using the sulfur capacity Cs of the refined slag system components. The specific calculation process is as follows:

6. The method for determining the amount of LF refined slag of ultra-low sulfur pipeline steel according to claim 3, characterized in that: The amount of refined slag is calculated by the sulfur distribution ratio. The sulfur content of the molten steel entering the LF refining station is w1, wt%; the sulfur content of the molten steel leaving the LF refining station is w2, wt%; the weight of the molten steel is m1, t; the amount of refined slag is m2, kg; the sulfur in the molten steel and the slag should satisfy the balance of the slag, and the calculation method for the amount of refined slag m2 is:

7. The method for determining the amount of LF refined slag of ultra-low sulfur pipeline steel according to claim 3, characterized in that: Based on the above-mentioned refined slag system composition, combined with the molten steel weight in the actual production data and the molten steel composition of the LF refining station, the refined slag amount is calculated; according to the different sulfur content w1 in the molten steel of the LF refining station, the molten steel is divided into the following categories for graded control: Low sulfur molten steel: w1≤0.010wt% Medium sulfur molten steel: 0.010wt% <w1≤0.020wt% High sulfur molten steel: 0.020wt% <w1≤0.030wt% According to the classification of sulfur content w1 in molten steel entering LF refining station, linear fitting is performed on low-sulfur molten steel, medium-sulfur molten steel and high-sulfur molten steel respectively, and three corresponding calculation equations of refined slag amount are obtained, namely, the regression equation of LF refined slag amount m and LF refining station sulfur content w1; the following is the classification fitting result: Low sulfur molten steel refining slag volume m 低 =(0.08492w1-0.77468) (6) Medium sulfur steel molten refining slag volume m 中 =(0.09045w1-0.81262) (7) High sulfur molten steel refining slag volume m 高 =(0.09845w1-0.87548) (8) Wherein, the unit of LF refining slag amount m is kg / t, and the unit of LF refining station inlet sulfur content w1 is ppm.

Images