Method for controlling slag composition and method for producing steelmaking slag using the same.

Abstract

To provide a slag composition control method and a steel-making slag production method using the same, capable of avoiding generation of a large amount of slag to be reprocessed regardless of the fluctuation of surrounding environment without construction of large-scale equipment and a large amount of operation cost.SOLUTION: A slag composition control method according to the present invention includes calculating, before and / or during blowing, an amount of auxiliary raw material to be charged such that a free lime index value indicating the free lime content of slag falls within a preset target range, and charging the auxiliary raw material into a converter before tapping according to the calculated amount of auxiliary raw material to be charged. As a preferred example, when it is necessary to additionally charge lime during the blowing in addition to the charging of the auxiliary raw material, lime is additionally charged by operator action. The calculation C / S is calculated for the basicity (C / S) in the slag, a silica charging amount that satisfies calculation C / S≤target C / S is calculated, and a silica-rich auxiliary raw material is charged into the converter during the blowing according to the calculated silica charging amount.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a method for controlling the slag composition for controlling free CaO (hereinafter referred to as "f-CaO") contained in steelmaking slag, and a method for producing steelmaking slag using the same.

Background Art

[0002] Steelmaking slag is dense and hard, and can support a large load when compacted, so it is used as a roadbed material. However, a part of calcium oxide (CaO) used during refining remains unreacted in the steelmaking slag, and this CaO undergoes a hydration reaction with moisture such as rainwater or seawater and expands in volume. Therefore, when using steelmaking slag as a roadbed material (JIS A 5015: Crusher-run steel slag), it is necessary to perform an appropriate aging treatment to suppress volume expansion. If the aging is insufficient, after several years, ridge-shaped unevenness may occur according to the expansion of the roadbed, or a pop-out may occur where it locally swells into a mountain shape and breaks through the asphalt, hindering the passage of vehicles and pedestrians.

[0003] JIS A 5015, which stipulates steel slag for road use, shows the minimum aging period for each product type and the upper limit of expansibility as a roadbed material. Common roadbed products of steelmaking slag are HMS-25, MS-25, CS-40, etc., and the expansibility is required to be 1.0% or less in terms of the water immersion expansion rate. However, sometimes due to variations within the product, although the measured samples meet the standards, expansion may occur on the road surface several years after laying. Therefore, it is also commonly done to composite a low-expansion material with the steelmaking slag to suppress variations in product expansibility, or to significantly lower the expansion rate from the specified value to ensure safety.

[0004] In the steelmaking process, a large amount of lime is added as a by-raw material to remove phosphorus, silicon, and other elements contained in the molten iron. As a result, undissolved lime and lime that crystallizes during cooling remain in the steelmaking slag as free CaO (also called free lime, hereafter referred to as "f-CaO"). This f-CaO has the property of expanding in volume by about twice its original size when it undergoes a hydration reaction to become Ca(OH)2. Therefore, when slag containing a large amount of f-CaO comes into contact with water, it expands and collapses due to the hydration of f-CaO. For this reason, the management of f-CaO is important.

[0005] Conventional slag quality control methods involve measuring the water immersion expansion coefficient to determine expansion. If the estimated amount of f-CaO, which is the cause of this expansion, exceeds a standard value, a method of modification is known in which the slag is rapidly cooled and solidified under conditions that cause oxidation in the molten state (see, for example, Patent Document 1). Similarly, a method of modifying steelmaking slag is known in which the estimated amount of f-CaO is calculated from the slag composition, and oxygen gas-containing gas is blown into the molten steelmaking slag (see, for example, Patent Document 2). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2015-189601 [Patent Document 2] Japanese Patent Publication No. 2017-141148 [Overview of the project] [Problems that the invention aims to solve]

[0007] As described above, when reusing steelmaking slag as a by-product, it is necessary to address its expansion and collapse properties, and reducing the amount of f-CaO contained in the slag is an effective solution. However, current atmospheric aging methods have problems such as uneven slag modification levels within the yard due to temperature fluctuations and uneven penetration during watering, as well as long aging times and the need for a large yard. Steam aging methods also have many challenges from the standpoint of steam equipment costs, running costs, and processing capacity, and furthermore, the aforementioned Patent Documents 1 and 2 do not contain any descriptions of methods for reducing f-CaO, so the development of new slag modification technologies to replace these is desired. In addition, it is desirable that f-CaO be reduced fundamentally before the slag is produced as a by-product.

[0008] In conventional technology, methods for modifying by-product slag by predicting the risk of expansion based on its composition are disclosed in Patent Documents 1 and 2. However, these methods are post-processing steps after slag is produced as a by-product of the steelmaking process, and since the risk of expansion is measured based on the actual values ​​of the slag composition in the final process, if a problem occurs, a large amount of slag needs to be reprocessed. For this reason, it is desirable that the slag composition adjustment and modification be performed in the steelmaking process. Furthermore, when predicting the slag composition in the steelmaking process, it can be predicted from the weight and composition of the raw materials brought into the furnace. By calculating the estimated f-CaO (referred to as the free lime content index value) from this predicted composition and comparing it with a non-expansion threshold indicating the risk of slag expansion, it is possible to determine the risk of expansion. However, in the steelmaking process, slag outflow such as eruption occurs during processing, making it difficult to perform blowing while satisfying the target free lime content index value.

[0009] The object of the present invention is to propose a method for controlling the composition of slag and a method for producing steelmaking slag using the same, which can avoid generating large amounts of slag that need to be reprocessed, regardless of fluctuations in the surrounding environment and without the need for the construction of large-scale facilities or high operating costs. [Means for solving the problem]

[0010] The present invention's method for controlling the slag composition involves calculating the amount of auxiliary materials to be added before and / or during smelting so that the free lime content index value of the slag falls within a predetermined target range, and then adding the auxiliary materials to the converter according to the calculated amount until tapping. Here, "the amount of auxiliary materials to be added within a predetermined target range" is, for example, the amount of auxiliary materials to be added so that the calculated free lime content index value is below a predetermined non-expansion threshold indicating a concern for expansion, but is not limited to this and can be determined arbitrarily.

[0011] Furthermore, in the slag composition control method according to the present invention configured as described above, (1) The calculated amount of auxiliary raw materials to be added is displayed on the display means, and the auxiliary raw materials are to be added according to the display. (2) The free calcium index value shall be expressed by the following formula (1): Free lime content index value = [T-CaO]-(1.87×[SiO2]+0.70×α×[T.Fe]+1.10×[Al2O3]+1.18×[P2O5])...(1) Here, [ ] represents the content (mass%) of the compound or element in the brackets, where T-CaO is total CaO and T.Fe is total Fe. Also, α is a coefficient obtained from the actual ratio of T.Fe value to Fe2O3 value in the slag. (3) If it is necessary to add lime in addition to the auxiliary raw materials during blowing, the operator shall add the lime by operator action, calculate the calculated C / S for the basicity (C / S) in the slag, calculate the amount of silica to be added so that the calculated C / S ≤ the target C / S for slag quality, and add silica-rich auxiliary raw materials to the converter during blowing according to the calculated amount of silica to be added. (4) The calculated amount of silica to be added is displayed on the display means, and silica-rich auxiliary materials are added in accordance with the display. (5) Add the silica-rich auxiliary material in the amount of silica specified above to modify the slag before tapping after the smelting is complete. This is considered to be a more preferable solution.

[0012] Furthermore, the steelmaking slag manufacturing method of the present invention involves calculating the amount of auxiliary raw materials to be added before and / or during smelting so that the free lime content index value, which indicates the amount of free lime in the slag, falls within a predetermined target range, and then adding auxiliary raw materials to the converter up to the time of tapping according to the calculated amount of auxiliary raw materials to control the slag composition, thereby obtaining steelmaking slag. [Effects of the Invention]

[0013] According to the slag composition control method of the present invention, the expansion potential of the slag can be predicted by a free lime content index value, which indicates the amount of free lime in the slag. By presenting this free lime content index value during the steelmaking process and instructing the amount of auxiliary materials to be added so that the predicted free lime content index value is below the threshold for expansion concerns, it is possible to reduce the generation of slag that may expand in the future and require reprocessing. This prevents increased costs due to steam aging reprocessing of substandard slag and extended processing times. [Brief explanation of the drawing]

[0014] [Figure 1] This diagram illustrates one aspect of the generation and treatment method of oxidized slag produced during the refining process of a converter. [Figure 2] This is a flowchart illustrating one aspect of the method for controlling the slag composition of the present invention. [Figure 3] This is a flowchart illustrating a preferred embodiment of the slag composition control method of the present invention. [Modes for carrying out the invention]

[0015] The following detailed description illustrates many specific details by illustrating embodiments of the present invention to provide a complete understanding of the invention. However, it is clear that one or more embodiments can be carried out without such specific detail descriptions. Also, for the sake of brevity, the drawings show well-known structures and devices in schematic form.

[0016] <Outline of the method for controlling slag composition according to the present invention> First, an overview of an embodiment of the present invention will be described. In the refining process during steel manufacturing, before blowing, in addition to the purpose of dephosphorizing the steel, the input amount of auxiliary materials is calculated by a computer so that the free lime amount in the slag falls within the target range, and the input amount of auxiliary materials is instructed before blowing. Also, during blowing, lime may be additionally input by spraying or the like, or may be continuously input rather than in batches such as lime injection. In such cases, it is difficult to keep the free lime amount in the slag within the target range only by the input of auxiliary materials based on the calculation before blowing.

[0017] In one embodiment of the present invention, considering such cases, not only before blowing but also during blowing, the free lime content index value is calculated based on the actual amount of auxiliary materials charged into the converter and checked so as not to exceed. Thereby, the generation of a large amount of slag that needs to be reprocessed can be avoided.

[0018] <Regarding the steel manufacturing method for implementing the slag composition control method of the present invention> In the steel manufacturing method according to this embodiment, hot metal discharged from a blast furnace is charged into a converter and subjected to oxidative refining treatment to produce molten steel. Hereinafter, hot metal and molten steel are also collectively referred to as molten iron. The hot metal to be subjected to oxidative refining treatment may be previously subjected to hot metal pretreatment such as desiliconization treatment, dephosphorization treatment, and desulfurization treatment in other refining facilities.

[0019] In this embodiment, as shown in FIG. 1, first, after charging molten iron, which is hot metal, into the converter, an inert gas is blown into the molten iron from a plurality of bottom blowing tuyeres, and oxygen gas is injected into the molten iron from an upper blowing lance, thereby performing a refining process step of oxidative refining treatment. The oxidative refining treatment is a treatment that adds an oxygen source to the hot metal and oxidizes and removes impurity components such as carbon and phosphorus in the hot metal. In this embodiment, by the oxidative refining treatment, a decarburization reaction in which at least carbon in the molten iron is removed and a dephosphorization reaction in which phosphorus in the molten iron is removed proceed. Hereinafter, the addition of oxygen gas (oxygen source) to the molten iron by the injection of oxygen gas from the upper blowing lance is also referred to as blowing.

[0020] In the refining process, the decarburization reaction proceeds, oxidizing and removing carbon from the molten iron, producing molten steel with a low carbon concentration. Furthermore, in the refining process, auxiliary materials such as slag-forming agents are introduced into the converter to promote the dephosphorization reaction. At this time, multiple types of slag-forming agents with different component compositions are added in amounts corresponding to the target slag composition. The amount of auxiliary materials such as slag-forming agents is predetermined according to various blowing conditions, including the composition and temperature of the molten iron before oxidation refining, the target composition and target temperature of the molten iron after oxidation refining, the efficiency of the refining reaction, and the free lime content in the slag after oxidation refining. The determined amount is then added at the beginning of the oxidation refining process. In the slag composition, the ratio of the CaO concentration (mass%) to the SiO2 concentration (mass%) ((%CaO) / (%SiO2)) is called the basicity. Furthermore, the amount of auxiliary materials to be added is calculated in advance before the oxidation refining process based on the mass balance, including the composition of the molten iron and the planned amount of oxygen to be added. However, additional auxiliary materials may be added during the refining process to suppress slag and granular iron blasting, depending on the blasting situation. In the refining process, auxiliary materials are added, followed by blowing, and the oxidation refining process is completed when the composition and temperature of the molten iron reach the target.

[0021] First, the amount of oxides contained in the main raw materials (metals recovered from scrap and slag) charged into the converter before the oxidation refining process is calculated. Furthermore, the amount of auxiliary raw materials added during the oxidation refining process, the amount of oxides generated by the oxidation of components in molten iron by the oxygen source, the amount of slag carried over from the previous oxidation refining process, and the amount of elution from the refractory bricks lining the converter are calculated, including some estimations as needed. Based on these calculated values, the amount of slag in the converter and the calculated slag composition are derived. The estimated basicity of the slag calculated from this calculated slag composition is also called the calculated basicity. From this calculated slag composition, the free lime content index value in the slag according to the embodiment is calculated.

[0022] After the oxidation refining process, the converter is tilted to discharge the molten iron in a tapping process. The discharged molten iron is collected in a ladle (not shown) located below the converter and sent to the next process. After the tapping process, slag remains in the converter. The slag remaining in the converter is then discharged downwards from the furnace opening as the converter is tilted in the opposite direction from the tapping process.

[0023] The discharged slag is collected in a slag receiving pot located below the converter, and then discharged and cooled in a slag storage area. It is also sorted into good and bad grades as needed.

[0024] Subsequently, after crushing and magnetic separation, an aging treatment is performed to convert the free lime in the slag into calcium hydroxide. The aged oxidized slag is then subjected to a water immersion expansion test according to the JIS A 5015 test method to determine whether it is suitable for shipment as roadbed material. Good quality products are shipped as roadbed material.

[0025] <Regarding one embodiment of the method for controlling the slag composition of the present invention> The invention according to this embodiment reduces the generation of slag that may require reprocessing by presenting a free lime content index value during the steelmaking process and instructing the amount of auxiliary material to be added so that the predicted free lime content index value is below the non-expansion threshold, which indicates a concern for expansion. In this embodiment, the free lime content index value and the non-expansion threshold are described as being designed so that the product is good when the free lime content index value is below its non-expansion threshold, but the design of the free lime content index value and the non-expansion threshold is not limited to this relationship. For example, it may be designed so that the product is good when the free lime content index value is above the non-expansion threshold (or when it is below or above it). This design depends on the characteristics of the free lime content index value, etc., and the relationship is not limited.

[0026] Figure 2 shows an example of the operation flow for implementing the present invention. After scrap and molten iron are charged into the converter, blowing is started. At this time, the amount and composition of the auxiliary raw materials initially charged are used to calculate the free lime content index. One example of the free lime content index is given by the following equation (1). In equation (1), first, the amount of CaO contained as a compound in the minerals contained in the slag is estimated from the amount of oxides such as SiO2, Al2O3, Fe2O3, and P2O5 in the slag, based on the ratio of oxides to CaO in each mineral. Then, the amount of CaO contained as a compound in the minerals is subtracted from the amount of CaO in the slag composition, and the remainder is often used as the amount of free lime.

[0027] Free lime content index value = [T-CaO]-(1.87×[SiO2]+0.70×α×[T.Fe]+1.10×[Al2O3]+1.18×[P2O5])...(1) Here, [ ] represents the content (mass%) of the compound or element in the brackets, where T-CaO is total CaO and T.Fe is total Fe. Also, α is a coefficient obtained from the actual ratio of T.Fe value to Fe2O3 value in the slag.

[0028] If the free lime content index value exceeds the non-expansion threshold, the amount of auxiliary material to be added to bring it below the non-expansion threshold is instructed. During smelting, molten steel is sampled by inserting a device called a sublance, which has a molten steel sensor attached to its tip to measure temperature, oxygen concentration, and carbon content, into the furnace. Until the measurement by the sublance performed before the end of smelting (hereinafter referred to as the endpoint S / L), there is continuous addition of CaO sources and additional CaO action due to the effects of ejection, so the free lime content index value and the non-expansion threshold are repeatedly checked sequentially in accordance with these changes in the amount of auxiliary material added (steps 1 to 5). In step 5, it is preferable to instruct the addition of auxiliary material by displaying the calculated amount of auxiliary material on a display device, and to add the auxiliary material according to the displayed amount.

[0029] At the completion of the final S / L (Step 6), the obtained final temperature and final oxygen values ​​are reflected in the prediction calculation of T.Fe, etc., to improve the accuracy of the estimated T.Fe and P concentrations. The free lime content index value is recalculated from this information (Step 7), and the non-expansion threshold is determined (Step 8). If the non-expansion threshold is exceeded, guidance is provided to calculate the auxiliary materials that will satisfy the non-expansion threshold and to introduce them into the converter before tapping (Step 9). Then, the auxiliary materials are introduced according to the guidance (Step 10), and tapping and slag discharge are performed (Step 11). By implementing threshold management of the free lime content index value using this guidance, it is possible to reduce the generation of slag that may require reprocessing. If the recalculated free lime content index value does not exceed the non-expansion threshold, tapping and slag discharge are performed as is (Step 11). In step 10, it is preferable to indicate the amount of auxiliary raw materials to be added by displaying the amount calculated in step 9 on a display device such as a screen, and to add the auxiliary raw materials according to the displayed amount.

[0030] In a preferred embodiment of the slag composition control method of the present invention, in addition to the slag composition control method of the embodiment described above, the following control method is implemented. That is, if it is necessary to add lime in addition to the auxiliary raw materials during blowing, the lime is added by operator action, the calculated C / S for the basicity (C / S) in the slag is calculated, the amount of silica to be added that satisfies calculated C / S ≤ target C / S is calculated, and silica-rich auxiliary raw materials are added to the converter during blowing according to the calculated amount of silica to be added. Here, calculated C / S is a value calculated from the amount of auxiliary raw materials and molten iron components added and their component ratios (%) of CaO and SiO2. The target C / S is a target value that takes into account dephosphorization and the risk of slag expansion. Furthermore, in this embodiment, it is preferable to use silica as the silica-rich auxiliary raw material.

[0031] Furthermore, regarding the basicity (C / S) of the slag after lime addition, the amount of silica added that satisfies the calculated C / S ≤ target C / S is displayed on a display device or other display means, and it is preferable to add silica-rich auxiliary materials according to the displayed amount of silica. In addition, it is preferable to add the instructed amount of silica-rich auxiliary materials before tapping after the completion of smelting to modify the slag.

[0032] Figure 3 shows an example of an operational flow for implementing the preferred embodiment described above. The operational flow shown in Figure 3 is performed in addition to the operational flow shown in Figure 2 as needed (in the example in Figure 3, when the amount of slag decreases due to eruption). From the start of blowing (step 21), if a decrease in the amount of slag is expected due to, for example, eruption (step 22), lime (CaO) is added by operator action to recover the decrease (step 23). The calculated C / S is calculated for the slag to which lime has been added (step 24), and the amount of silica (SiO2) to be added that satisfies calculated C / S ≤ target C / S is displayed from the calculated C / S and a predetermined target C / S (step 25). Thereafter, silica-rich auxiliary materials are added according to the displayed amount of silica to be added (step 26).

[0033] As described above, according to one embodiment of the present invention, a function is provided to calculate the free lime content index value based on the actual amount of auxiliary materials fed into the converter and to check that it does not exceed the limit. As a result, according to one embodiment of the present invention, it is possible to avoid generating a large amount of slag that needs to be reprocessed, regardless of fluctuations in the surrounding environment and without the construction of large-scale facilities or high operating costs.

[0034] In an embodiment of the present invention relating to the method for producing steelmaking slag, the slag in which the amount of free lime in the slag is controlled using the slag composition control method described above is used as steelmaking slag. For example, in an embodiment relating to the method for producing steelmaking slag of the present invention, the amount of auxiliary raw materials to be added so that the free lime content index value, which indicates the amount of free lime in the slag, falls within a predetermined target range is calculated before and / or during smelting, and the auxiliary raw materials are added to the converter according to the calculated amount of auxiliary raw materials to control the slag composition, thereby using the resulting slag as steelmaking slag. This makes it possible to produce steelmaking slag suitable for applications such as roadbed material. [Examples]

[0035] <Example 1> Table 1 below shows examples of one embodiment of the present invention. In these examples, the target ranges are 6.0% or less for f-CaO and 0.5% or less for water immersion expansion. In Examples 1 to 4, it can be seen that the f-CaO and water immersion expansion are low because auxiliary materials were added to adjust the free calcium content index value to the target range. On the other hand, in the comparative examples, since auxiliary materials were not added to adjust the free calcium content index value to the target range, the f-CaO and water immersion expansion are high, and it can be seen that there is a risk of increased costs due to steam aging reprocessing of non-good slag and extension of processing time.

[0036] [Table 1]

[0037] <Example 2> Table 2 below shows preferred examples of one embodiment of the present invention. Similar to Example 1, the target ranges are 6.0% or less for f-CaO and 0.5% or less for water immersion expansion. This preferred example is a case in which lime was added by ejection during the blowing process. In Examples 11 to 14, it can be seen that the f-CaO and water immersion expansion are low because auxiliary materials were added to adjust the free lime content index value to the target range. On the other hand, in Comparative Example 11, since auxiliary materials were not added to adjust the free lime content index value to the target range, the f-CaO and water immersion expansion are high, and it can be seen that there is a risk of increased costs due to steam aging reprocessing of non-good slag and extension of processing time.

[0038] [Table 2]

Claims

1. The amount of auxiliary raw materials to be added before and / or during smelting is calculated so that the free lime content index value, which indicates the amount of free lime in the slag, falls within a predetermined target range. The auxiliary raw materials are then added to the converter according to the calculated amount until tapping. A method for controlling the composition of slag, comprising: adding lime through operator action in addition to the aforementioned auxiliary raw material input during blowing; calculating the basicity (C / S) of the slag; calculating the amount of silica to input that satisfies the calculated C / S ≤ target C / S for slag quality; and adding silica-rich auxiliary raw materials to the converter during blowing according to the calculated amount of silica to input.

2. The method for controlling the slag composition according to claim 1, wherein the calculated amount of auxiliary raw materials to be added is displayed on a display means, and the auxiliary raw materials are added according to the display.

3. The free lime content index value is expressed by the following formula (1), the method for controlling the slag composition according to claim 1: Free lime index value = [T - CaO] - (1.87 × [SiO]) ２ ]+0.70×α×[T-Fe]+1.10×[Al ２ O ３ ] + 1.18 × [P ２ O ５ ])・・・(1) Here, [ ] represents the content (mass%) of the compound or element in the brackets, where T-CaO is total CaO and T-Fe is total Fe. Also, α is the T-Fe value in the slag and Fe ２ O ３ This coefficient is derived from the ratio of actual values.

4. The method for controlling the slag composition according to claim 1, wherein the calculated amount of silica to be added is displayed on a display means, and silica-rich auxiliary materials are added according to the display.

5. A method for producing steelmaking slag, comprising: calculating the amount of auxiliary raw materials to be added before and / or during smelting so that the free lime content index value, which indicates the amount of free lime in the slag, falls within a predetermined target range; adding auxiliary raw materials to the converter according to the calculated amount until tapping steel; adding lime during smelting in addition to the aforementioned auxiliary raw materials through operator action; calculating the calculated C / S for the basicity (C / S) in the slag; calculating the amount of silica to be added so that the calculated C / S ≤ target C / S for slag quality; and adding silica-rich auxiliary raw materials to the converter during smelting according to the calculated amount of silica to control the slag composition to produce steelmaking slag.

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