Slag composition control method and steel-making slag production method 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 composition of steelmaking slag in order to control the amount of free CaO (hereinafter referred to as "f-CaO") contained in steelmaking slag, and a method for producing steelmaking slag using the same. [Background technology]

[0002] Steelmaking slag is used as a roadbed material because it is dense and hard and can support a large load when compacted. However, some of the calcium oxide (CaO) used during refining remains unreacted in steelmaking slag, and this CaO undergoes a hydration reaction with moisture such as rainwater or seawater, causing volume expansion. For this reason, when steelmaking slag is used as a roadbed material (JIS A 5015: Crushed steel slag), it is necessary to carry out an appropriate aging process to suppress volume expansion. If aging is insufficient, after several years the roadbed will expand, causing ridge-like unevenness or pop-outs that rise locally into mountain-like shapes and break through the asphalt, obstructing the passage of vehicles and pedestrians.

[0003] JIS A 5015, which specifies steel slag for road use, specifies the minimum aging period for each product type and the upper limit of expansion as a roadbed material. HMS-25, MS-25, CS-40, etc. are the most common steel slag roadbed products, and their expansion rate when immersed in water must be 1.0% or less. However, sometimes, due to variations within the product, even if the measured sample meets the standard, it may cause expansion on the road surface several years after installation. For this reason, it is common to combine steel slag with low-expansion materials to suppress the variation in product expansion, or to reduce the expansion rate significantly below the standard to ensure safety.

[0004] In the steelmaking process, a large amount of lime is added as an auxiliary material to remove phosphorus, silicon, and other substances contained in 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 becomes Ca(OH)2 through hydration and has the property of expanding in volume by approximately twice its original size. 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, it is important to manage f-CaO.

[0005] In a conventional method for controlling slag quality, the expansion rate upon immersion in water is measured to measure the expansion property, but if the estimated value of the amount of f-CaO that causes this is equal to or exceeds a reference value, a method is known in which the slag is rapidly solidified under conditions that cause oxidation in the molten state to modify the slag (see, for example, Patent Document 1). Similarly, a method is known in which the estimated value of the amount of f-CaO is calculated from the slag composition, and an oxygen-containing gas is blown into the molten steelmaking slag to modify the slag (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2015-189601 A [Patent Document 2] JP 2017-141148 A Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, when steelmaking slag is reused as a by-product, it is necessary to solve the problem of expansion and collapse, and reducing the f-CaO contained in the slag is an effective countermeasure. However, the current air aging method has problems such as the modification level of the slag in the yard being easily inhomogeneous due to temperature fluctuations and uneven penetration when water is sprayed, and furthermore, it has problems such as a long aging treatment time and the need for a large yard. In addition, the steam aging method has many issues in terms of steam equipment costs, running costs, and processing capacity, and furthermore, the aforementioned Patent Documents 1 and 2 do not describe a method for reducing f-CaO, so the development of a new slag modification technology to replace these is desired. In addition, it is desirable to fundamentally reduce f-CaO before slag is by-produced.

[0008] In the prior art, there are methods disclosed in Patent Documents 1 and 2 for predicting the risk of expansion of by-produced slag from its components and modifying it. However, these methods are post-processing of slag produced as a by-product from the steelmaking process, and the risk of expansion is measured from the actual measured values ​​of the slag components in the final process, so if a problem occurs, a large amount of slag needs to be reprocessed. For this reason, it is desirable to adjust the composition and modify the slag in the steelmaking process. In addition, when predicting the slag components in the steelmaking process, they can be predicted from the weight and components of the raw materials brought into the furnace. The expansion risk can be determined by calculating the estimated f-CaO (referred to as the free lime content index value) from this predicted composition and comparing it with the non-expansion threshold value that indicates the risk of slag expansion. However, in the steelmaking process, slag outflow such as eruptions occurs during processing, making it difficult to perform blowing while meeting the target free lime content index value.

[0009] The object of the present invention is to propose a method for controlling slag composition, which can avoid the generation of 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 large operating costs, and a method for producing steelmaking slag using the same. [Means for solving the problem]

[0010] In the method for controlling the slag composition of the present invention, the amount of auxiliary raw materials charged that brings the free lime content index value of the slag into a preset target range is calculated before and / or during blowing, and the auxiliary raw materials are charged into the converter before tapping according to the calculated amount of auxiliary raw materials. Here, the "amount of auxiliary raw materials charged that brings the calculated free lime content index value into a preset target range" is, for example, the amount of auxiliary raw materials charged that brings the calculated free lime content index value into a non-expansion threshold value that indicates a concern of expansion that is set in advance, and is not limited thereto and can be determined arbitrarily.

[0011] In the method for controlling the slag composition according to the present invention configured as described above, (1) The calculated amount of auxiliary raw material to be input is displayed on a display means, and the auxiliary raw material is input in accordance with the display; (2) The free lime index value is expressed by the following formula (1): Free lime index value = [T-CaO]-(1.87×[SiO2]+0.70×α×[T.Fe]+1.10×[Al2O3]+1.18×[P2O5])...(1) Here, [ ] is the content (mass%) of the compound or element in the brackets, T-CaO is the total CaO, T.Fe is the total Fe, and α is a coefficient calculated from the actual ratio of the T.Fe value and the Fe2O3 value in the slag. (3) When it is necessary to add lime during blowing in addition to the addition of the auxiliary raw materials, add 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 is equal to or less than 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 stone to be added is displayed on a display means, and the silica stone-rich auxiliary raw material is added in accordance with the display. (5) Adding the silica-rich auxiliary material in the amount of silica specified above before tapping after the end of blowing to modify the slag; is considered to be a more preferable solution.

[0012] In addition, in the method for producing steelmaking slag of the present invention, the amount of auxiliary raw materials to be charged, which will bring the free lime content index value, indicating the amount of free lime in the slag, into a preset target range, is calculated before and / or during blowing, and the auxiliary raw materials are charged into the converter in accordance with the calculated amount of auxiliary raw materials to be charged until steel tapping, to produce steelmaking slag whose slag composition has been controlled. Effect of the Invention

[0013] According to the method for controlling the slag composition of the present invention, the expansiveness of the slag can be predicted by the free lime index value, which indicates the amount of free lime in the slag. By presenting this free lime index value during the steelmaking process and indicating the amount of auxiliary raw material to be added so that the predicted value of the free lime index value is equal to or less than the threshold value for the expansion concern, it is possible to reduce the generation of slag that has a future expansion concern and requires reprocessing. This makes it possible to prevent the reprocessing of non-conforming slag by steam aging or the increase in costs due to the extended processing time. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram for explaining one embodiment of the generation and treatment method of oxidized slag generated in the refining process of a converter. [Diagram 2] 1 is a flowchart for explaining one embodiment of a method for controlling a slag composition of the present invention. [Diagram 3] 1 is a flowchart for explaining a preferred embodiment of a method for controlling a slag composition of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] In the following detailed description, numerous specific details are set forth to illustrate embodiments of the present invention in order to provide a thorough understanding of the present invention. However, it will be apparent that one or more embodiments may be practiced without such specific details. Also, for the sake of simplicity, the drawings may show diagrammatic representations of well-known structures and devices.

[0016] <Outline of the method for controlling slag composition according to the present invention> First, an outline of one embodiment of the present invention will be described. In the present invention, in the refining process in the production of steel, in addition to the purpose of dephosphorization of steel before blowing, the amount of auxiliary materials to be charged is calculated by a computer, and the amount of auxiliary materials to be charged is instructed before blowing so that the amount of free lime in the slag falls within a target range. In addition, there are cases where lime is added by blowing or the like during blowing, or where lime is continuously charged instead of in batches, such as by lime blasting, and in such cases, it is difficult to keep the amount of free lime in the slag within the target range only by charging the auxiliary materials based on the calculation before blowing.

[0017] In one embodiment of the present invention, taking such a case into consideration, the free lime index value is calculated based on the actual amount of auxiliary materials charged into the converter not only before but also during blowing, and checked to ensure that it does not exceed the free lime index value, thereby preventing the generation of a large amount of slag that needs to be reprocessed.

[0018] <Steel manufacturing method implementing the slag composition control method of the present invention> In the steel manufacturing method according to the present embodiment, molten pig iron tapped from a blast furnace is placed in a converter and oxidized to produce molten steel. In the following, molten pig iron and molten steel are collectively referred to as molten iron. The molten pig iron to be oxidized may be subjected to a molten pig iron pretreatment such as desiliconization, dephosphorization, and desulfurization in another refining facility.

[0019] In this embodiment, as shown in FIG. 1, first, molten iron, which is molten pig iron, is charged into a converter, and then 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 a top-blowing lance, thereby performing an oxidation refining process. The oxidation refining process is a process in which an oxygen source is added to the molten iron, and impurity components such as carbon and phosphorus in the molten iron are oxidized and removed. In this embodiment, the oxidation refining process at least causes a decarburization reaction in which carbon is removed from the molten iron, and a dephosphorization reaction in which phosphorus is removed from the molten iron. In the following, the addition of oxygen gas (oxygen source) to the molten iron by injection of oxygen gas from the top-blowing lance is also referred to as blowing.

[0020] In the refining process, the carbon in the molten iron is oxidized and removed by the decarburization reaction, and molten steel with a low carbon concentration is produced. In the refining process, auxiliary materials such as slag-forming agents are added into the converter to promote the dephosphorization reaction. At this time, multiple types of slag-forming agents with different composition are added in amounts according to the target slag composition. The auxiliary materials such as slag-forming agents are determined in advance according to various blowing conditions such as the composition and temperature of the molten iron before the oxidation refining process, the target composition and target temperature of the molten iron after the oxidation refining process, the efficiency of the refining reaction, and the free lime content in the slag after the oxidation refining process. The determined input amount is then input at the beginning of the oxidation refining process. In the composition of the slag, the ratio of the CaO concentration (mass%) to the SiO2 concentration (mass%) ((%CaO) / (%SiO2)) is called the basicity. The amount of auxiliary materials to be added is calculated before the oxidation refining process based on the mass balance of the molten iron components, the planned amount of oxygen source to be added, etc., but additional auxiliary materials may be added to suppress the blowing up of slag and granular iron during the refining process. In the refining process, auxiliary materials are added and the blowing process is performed, and the oxidation refining process is completed when the molten iron components and temperature reach the target values.

[0021] First, the amount of oxide contained in the main raw material (scrap and ingots recovered from slag) charged into the converter before the oxidation refining process is calculated. In addition, the amount of auxiliary raw materials charged during the oxidation refining process, the amount of oxide generated by the oxidation of the components in the molten iron by the oxygen source, the amount of slag carried over from the previous oxidation refining process, the amount of refractory bricks lining the converter, etc. are calculated, including some estimates as necessary. Then, 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, a free lime 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 from the converter in the 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 downward from the throat by tilting the converter in the opposite direction to the tapping process.

[0023] The discharged slag is collected in a slag receiving ladle located below the converter, and then discharged to a slag pile where it is cooled. If necessary, it is also separated into good and bad slag for management.

[0024] After that, after crushing and magnetic separation, the slag undergoes aging treatment to convert the free lime in the slag into lime hydroxide. The oxidized slag that has undergone the aging treatment undergoes a water immersion expansion test according to the JIS A 5015 test method to determine whether it can be shipped as roadbed material, and non-defective products are shipped as roadbed material.

[0025] <One embodiment of the method for controlling 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 index value during the steelmaking process and instructing the amount of auxiliary raw material input that makes the predicted value of the free lime index value equal to or less than the non-expansion threshold value that indicates the expansion concern. In this embodiment, the free lime index value and the non-expansion threshold value are designed so that the product is good when the free lime index value is equal to or less than the non-expansion threshold value, but the design of the free lime index value and the non-expansion threshold value is not limited to this relationship. For example, the product may be designed so that the product is good when the free lime index value is equal to or more than the non-expansion threshold value (or less than or exceeding it). This design depends on the characteristics of the free lime index value, and the relationship is not limited.

[0026] FIG. 2 shows an example of an operation flow for carrying out the present invention. After scrap and molten iron are charged into the converter, blowing is started. At this time, the free lime index value is calculated by referring to the amount and composition of the auxiliary materials charged at the beginning. The free lime index value can be expressed as the following formula (1) as an example. In formula (1), the amount of CaO contained as a compound in the minerals contained in the slag is first estimated from the abundance ratio of oxides in each mineral and CaO based on the amount of oxides such as SiO2, Al2O3, Fe2O3, and P2O5 in the slag. 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 calculated as the amount of free lime, and a value calculated based on this is often used.

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

[0028] If the free lime index value exceeds the non-expansion threshold, the amount of auxiliary materials to be charged that satisfies the non-expansion threshold or less is instructed. During blowing, a device called a sublance, which has a molten steel sensor attached to its tip to measure temperature, oxygen concentration, and carbon content, is inserted into the furnace to sample the molten steel. Until the measurement using the sublance performed before the end of blowing (hereinafter referred to as the end point S / L), there is continuous charging of CaO source and CaO addition action due to the blowing effect, so the free lime index value and the non-expansion threshold are repeatedly checked in accordance with such changes in the amount of auxiliary materials charged (steps 1 to 5). It is preferable that the charging of auxiliary materials in step 5 is instructed by displaying the calculated amount of auxiliary materials to be charged on a display or other display means, and the auxiliary materials are charged according to the displayed amount of auxiliary materials.

[0029] At the time when the end point S / L is completed (step 6), the obtained end point temperature and end point oxygen values ​​are reflected in the prediction calculation of T.Fe, etc., to improve the accuracy of the estimated values ​​of T.Fe and P concentration. From this information, the free lime index value is recalculated (step 7), and the non-expansion threshold is judged (step 8). If it exceeds the non-expansion threshold, the auxiliary materials that satisfy the non-expansion threshold or less are calculated, and guidance is given to input them into the converter before tapping (step 9). Then, the auxiliary materials are input according to the guidance (step 10), and steel is tapped and slag is removed (step 11). By implementing threshold management of the free lime index value according to this guidance, it has become possible to reduce the generation of slag that may require reprocessing. If the recalculated free lime index value does not exceed the non-expansion threshold, steel is tapped and slag is removed as is (step 11). In addition, it is preferable that the amount of the secondary raw material calculated in step 9 is displayed on a display means such as a display device and the secondary raw material is added according to the displayed amount of the secondary raw material.

[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 above embodiment, the following control method is performed. That is, when lime needs to be added in addition to the auxiliary raw materials during blowing, lime is added by an operator action, a calculated C / S is calculated for the basicity (C / S) in the slag, the amount of silica stone to be added that satisfies the relationship of 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 stone. Here, the calculated C / S is a numerical value calculated from the amounts of auxiliary raw materials and molten iron components added and their CaO and SiO2 component ratios (%). 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 stone as the silica-rich auxiliary raw material.

[0031] In addition, the amount of silica stone to be charged that satisfies the calculated C / S≦target C / S for the basicity (C / S) of the slag after lime addition is preferably displayed on a display means such as a display, and the silica-rich auxiliary raw material is charged according to the displayed amount of silica stone. In addition, the silica-rich auxiliary raw material in the indicated amount is preferably charged before tapping after the end of blowing to modify the slag.

[0032] FIG. 3 shows an example of an operation flow for carrying out the above-mentioned preferred embodiment. The operation flow shown in FIG. 3 is performed in addition to the operation flow shown in FIG. 2 as necessary (in the example of FIG. 3, when the amount of slag is reduced due to the occurrence of an eruption). When the amount of slag is expected to decrease due to, for example, the occurrence of an eruption from the start of blowing (step 21) (step 22), in order to recover the decrease, lime (CaO) is added by an operator action (step 23). A calculated C / S is calculated for the slag to which lime has been added (step 24), and the amount of silica (SiO2) added that satisfies the calculated C / S≦target C / S is displayed based on the calculated calculated C / S and a predetermined target C / S (step 25). After that, a silica-rich auxiliary material is added according to the displayed amount of silica added (step 26).

[0033] As described above, according to one embodiment of the present invention, a function is provided for calculating the free lime index value based on the actual amount of auxiliary materials charged into the converter and checking to ensure that the free lime index value does not exceed the calculated value. As a result, according to one embodiment of the present invention, it is possible to avoid the generation of 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 large operating costs.

[0034] In an embodiment of the method for producing steelmaking slag of the present invention, the steelmaking slag is slag in which the amount of free lime in the slag is controlled using the above-mentioned method for controlling the slag composition. For example, in an embodiment of the method for producing steelmaking slag of the present invention, the amount of auxiliary raw materials charged that brings the free lime content index value, which indicates the amount of free lime in the slag, into a preset target range may be calculated before and / or during blowing, and the auxiliary raw materials may be charged into the converter according to the calculated amount of auxiliary raw materials charged before tapping to control the slag composition, to produce 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 according to 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 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 the comparative example, no auxiliary materials were added to adjust the free lime content index value to the target range, so 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-defective slag and extended processing time.

[0036] [Table 1]

[0037] <Example 2> Table 2 below shows preferred examples according to one embodiment of the present invention. As in 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 where lime was added by injection during blowing. In Examples 11 to 14, it can be seen that f-CaO and water immersion expansion are low because auxiliary materials were added to adjust the free lime index value to the target range. On the other hand, in Comparative Example 11, auxiliary materials were not added to adjust the free lime index value to the target range, so f-CaO and water immersion expansion are high, and it can be seen that there is a risk of cost increase due to steam aging reprocessing of non-defective slag and extension of processing time.

[0038] [Table 2]

Claims

1. A method for controlling slag composition, comprising calculating amounts of auxiliary materials to be charged before and / or during blowing so that a free lime content index value, which indicates the amount of free lime in the slag, falls within a preset target range, and charging the auxiliary materials into a converter in accordance with the calculated amounts of auxiliary materials to be charged before tapping.

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

3. The method for controlling the slag composition according to claim 1, wherein the free lime index value is expressed by the following formula (1): Free lime content index = [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, [ ] is the content (mass%) of the compound or element in the brackets, T-CaO is the total CaO, T.Fe is the total Fe, and α is the ratio of the T.Fe value in the slag to the Fe value. ２ O ３ This is a coefficient calculated from the actual ratio of the value.

4. 2. The method for controlling a slag composition according to claim 1, wherein, when it is necessary to additionally add lime during blowing in addition to the addition of the auxiliary raw materials, the method additionally adds lime by an operator action, calculates a calculated C / S for a basicity (C / S) in the slag, calculates an amount of silica stone to be added that satisfies the calculated C / S≦a target C / S for slag quality, and adds a silica-rich auxiliary raw material to the converter during blowing in accordance with the calculated amount of silica stone to be added.

5. 5. The method for controlling a slag composition according to claim 4, wherein the calculated amount of silica stone to be charged is displayed on a display means, and a silica-rich auxiliary material is charged in accordance with the display.

6. 6. The method for controlling a slag composition according to claim 5, wherein the silica-rich auxiliary material is added in the instructed amount of silica to be added before tapping after completion of blowing to modify the slag.

7. A method for producing steelmaking slag, comprising calculating amounts of auxiliary raw materials to be charged before and / or during blowing so that a free lime content index value, which indicates the amount of free lime in the slag, falls within a preset target range, and feeding the auxiliary raw materials into a converter before steel tapping in accordance with the calculated amounts of auxiliary raw materials to produce slag with a controlled slag composition, thereby producing steelmaking slag.

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