Converter refining method

Abstract

To provide a converter refining method where secondary combustion heat is efficiently applied to molten iron while suppressing the erosion of refractories without using fluorite.SOLUTION: In a converter refining method, the basicity of charged slag is adjusted to 4.2 or more, and oxygen is blown from a top-blowing lance to start decarburization blowing. While oxygen is blown at a ratio of 10-20% of the total amount of oxygen blown in the decarburization blowing, a powder of SiO2 is blown together with oxygen from the top-blowing lance toward a molten iron surface of 0.7-1.7 kg / t-steel to foam the slag.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a converter refining method. [Background technology]

[0002] Recently, in order to realize a carbon-neutral society, it is essential to develop a process that can use large amounts of scrap and direct hydrogen reduced iron in the steelmaking industry. In order to reduce carbon dioxide emissions in converters as well, there is a need to develop technology to increase the proportion of scrap as the main raw material.

[0003] When increasing the scrap ratio, it is important to secure a heat source, and one way to improve the thermal tolerance of a converter is to use an auxiliary heat source. Using large amounts of auxiliary heat sources such as earth graphite or ferrosilicon can increase the thermal tolerance of a converter, but using large amounts of these increases the decarburization time and the amount of slag discharged. Therefore, effective use of secondary combustion heat as another auxiliary heat source has been attracting attention.

[0004] Various studies have been conducted on methods that utilize the heat of secondary combustion (see Non-Patent Documents 1 and 2), and it is known that the heat of secondary combustion can be increased by increasing the contact area between the CO gas generated by the decarburization reaction and oxygen. However, since much of the heat of secondary combustion generated in the upper part of the furnace is transferred to the refractory, it is necessary to efficiently transfer the heat of secondary combustion to the molten iron. Furthermore, if too much heat is transferred to the refractory, the load on the refractory increases, which may cause the refractory to melt.

[0005] On the other hand, Non-Patent Document 3 reports that foamed slag is a medium for transferring heat from secondary combustion to molten iron. Therefore, it has been proposed to reduce the rate of heat transferred to the refractory and increase the rate of heat transferred to the molten iron by foaming the slag. Patent Document 1 discloses a method for efficiently transferring heat from secondary combustion to molten iron by controlling the foaming height within a predetermined range, utilizing the foaming promotion effect of adding fluorite and the foaming suppression effect of adding graphite. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 5-43924 [Non-patent literature]

[0007] [Non-Patent Document 1] Okamura, S., Nakajima, H., Marukawa, Y., Anesaki, M., Tosaki, Y., Mori, A., Katoki, K., Ichihara, K.: "Development of Heat Compensation Technology for Combined Converters," Iron and Steel, 71(1985), pp. 1787-1794. [Non-patent document 2] Shinichi Nishioka, Hideo Nakamura, Kenji Takahashi, Yoshihiko Kawai, Shunichi Sugiyama: "High Secondary Combustion Rate and High Heat Transfer Efficiency Technology in Strongly Stirred Iron Bath Furnaces," Iron and Steel, 76(1990), pp. 2019-2024. [Non-patent document 3] K. Ito and RJ Fruehan: "Study on the Foaming of CaO-SiO2-FeO Slag: Part 1. Foaming Parameters and Experimental Results", Metall. Trans. B, 20B (1989), p509-514. [Non-patent document 4] Kiyoshi Segawa: Iron Metallurgical Reaction Engineering, Nikkan Kogyo Shimbun (1969), pp.52-55. Summary of the Invention [Problem to be solved by the invention]

[0008] However, the method described in Patent Document 1 uses fluorite, which has limitations on its use, and therefore there is a need for a method for controlling slag foaming without using fluorite.

[0009] In view of the above-mentioned problems, an object of the present invention is to provide a converter refining method that does not use fluorite and efficiently transfers the heat of secondary combustion to molten iron while suppressing the melting damage of refractories. [Means for solving the problem]

[0010] The inventors focused on the finding that the rate of FeO formation in the slag increases as the slag basicity decreases. The FeO in the slag reacts with the carbon in the molten iron, generating fine CO gas, which promotes slag foaming. On the other hand, if the basicity is too low, refractories may be damaged or the slag may rephosphorize. Therefore, the inventors discovered the optimal conditions for slag foaming: when an oxygen jet is blown toward the molten iron surface, SiO powder is also blown toward the hot spot to lower the slag basicity near the hot spot, generating FeO.

[0011] The present invention is as follows. [1] A converter refining method for decarburization blowing by blowing oxygen onto molten iron from a top lance, A converter refining method characterized in that the decarburization blowing is started with the slag charging basicity set to 4.2 or more, and while blowing oxygen at a rate of 10 to 20% of the total amount of oxygen blown in the decarburization blowing, SiO2 powder is blown together with the oxygen at a rate of 0.7 to 1.7 kg / t-steel. [Effects of the Invention]

[0012] According to the present invention, the heat of secondary combustion can be efficiently transferred to the molten iron while suppressing the melting damage of the refractory without using fluorite. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 2 is a diagram for explaining the structure of a refining furnace used in an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] A converter refining method according to an embodiment of the present invention will be described below with reference to the drawings. In this embodiment, slag is foamed to efficiently transfer the heat of secondary combustion to the molten iron. Here, the heat of secondary combustion is the combustion heat generated when CO gas reacts with oxygen to generate CO gas. By efficiently transferring the heat of secondary combustion to the molten iron, the proportion of scrap used as the main raw material can be increased.

[0015] First, we will explain the mechanism of secondary combustion in converters. In the converter process, the oxygen jet injected from the lance entrains surrounding gases, such as CO gas produced in the decarburization reaction shown in equation (1) below, and the secondary combustion reaction shown in equation (2) below mainly occurs at the outer edge of the oxygen jet. CO2 that does not reach the ignition point and CO produced in the decarburization reaction rise and are discharged outside the system, but some is entrained in the oxygen jet, generating secondary combustion heat again. Comparing the reaction heat of the decarburization reaction (equation (1)) and the secondary combustion reaction (equation (2)), the secondary combustion heat generated by the secondary combustion reaction is greater. Therefore, generating a large amount of secondary combustion heat is important in order to secure a heat source. C+1 / 2O2(g)=CO(g)+9.2MJ / (kg-C) (1) CO(g)+1 / 2O2(g)=CO2(g)+23.6MJ / (kg-C) (2)

[0016] For example, by blowing an oxygen jet toward the molten iron surface using a soft blower or by using a lance with sub-holes around the main hole or on the side wall of the lance, the contact area between the oxygen jet and the CO gas can be increased, and the secondary combustion rate, expressed by the following equation (3), can be increased. Secondary combustion rate (%) = CO2 / (CO+CO2) (3) Here, CO and CO2 in formula (3) each represent a component (Vol%) in the exhaust gas.

[0017] Furthermore, in order to efficiently transfer the generated heat of secondary combustion to the molten iron, it is necessary to form the slag. By forming the slag, the heat of secondary combustion is transferred to the foamed slag in contact with the jet, and the sensible heat of the exhaust gas taken out of the furnace is also easily transferred to the foamed slag, and this heat is transferred to the molten iron via the foamed slag. In the following, in this embodiment, the optimal conditions for generating a large amount of heat of secondary combustion and efficiently transferring the heat to the molten iron will be described.

[0018] FIG. 1 is a diagram illustrating the structure of a refining furnace used in this embodiment. As shown in FIG. 1, the refining furnace 1 has a furnace body 11 and a top-blowing lance 2. The top-blowing lance 2 is a lance that can be raised and lowered in the vertical direction (up and down direction in FIG. 1). The furnace body 11 is a refining vessel with an opening 12 formed at the top, and the inside is covered with refractory material. Oxygen gas is supplied to the top-blowing lance 2 from a first supply path (not shown) connected to the upper end side, and an oxygen jet 3 is sprayed toward the slag 4 and molten iron 5. At a hot spot 8 where the oxygen jet 3 comes into contact with the molten iron 5, oxygen reacts with C in the molten iron to generate CO gas.

[0019] In order to increase the contact area between the oxygen jet 3 and the CO gas and thereby increase the secondary combustion rate, at least two nozzle holes are provided at the bottom end of the top-blowing lance 2, and the oxygen jet 3 is injected from these nozzle holes. Furthermore, in this embodiment, a second supply path (not shown), which is different from the first supply path for supplying oxygen gas, is connected to the top end of the top-blowing lance 2, and SiO powder 7 is supplied from the second supply path. In order to foam the slag 4, the SiO powder 7 is supplied to the top-blowing lance 2 together with the oxygen gas during a portion of the decarburization blowing, and the oxygen jet 3 containing the SiO powder 7 is injected toward the slag 4 and molten iron 5. Note that the second supply path for supplying the SiO powder 7 may not be directly connected to the top-blowing lance 2, but may merge with the first supply path.

[0020] A plurality of bottom-blowing tuyeres 6 are provided at the bottom of the furnace body 11, and agitation gas is injected into the molten iron 5 in the furnace body 11 through these bottom-blowing tuyeres. The number of bottom-blowing tuyeres is not particularly limited, but is typically between one and five. The gases used can be broadly divided into oxygen gas and inert gases, and examples of inert gases include nitrogen gas and argon gas. The shape of the bottom-blowing tuyeres can be a single-tube nozzle, a double-tube nozzle, a collecting pipe nozzle, a porous nozzle, or the like. When oxygen is used as the gas type, O2 gas can be injected through the inner tube of the tuyeres of the double-tube nozzle, and a cooling gas such as CO2, N2, or LPG can be injected through the gap between the inner and outer tubes.

[0021] In the example shown in Figure 1, SiO2 powder 7 is sprayed together with oxygen gas from the top lance 2, but the SiO2 powder may be sprayed using a lance other than the top lance 2. In this case, the SiO2 powder is sprayed so that it joins the oxygen jet from the top lance 2. In addition, the carrier gas for the lance spraying the SiO2 powder in this case does not have to be limited to oxygen, and may be argon or nitrogen gas.

[0022] In addition, in this embodiment, as shown in Figure 1, an example has been described in which the top-blowing lance 2 is installed on the central axis of the furnace body 11, but in terms of dimensional accuracy, the position of the top-blowing lance 2 may be shifted from the central axis.

[0023] Next, detailed conditions for the decarburization blowing in this embodiment will be described. First, dephosphorization blowing is performed in the refining furnace 1, and after completion of the dephosphorization blowing, the furnace body is tilted to perform intermediate slag removal of the dephosphorization slag produced during the dephosphorization blowing. After the intermediate slag removal, with some of the dephosphorization slag remaining, auxiliary materials containing CaO are added to adjust the charging basicity, and oxygen gas is blown from the top lance 2 toward the slag 4 and molten iron 5 to start the decarburization blowing. In this embodiment, SiO2 powder is also blown together with the oxygen gas during a portion of the decarburization blowing.

[0024] First, we will explain the charge basicity before the start of decarburization blowing. During decarburization blowing, the high temperature and low carbon concentration region are thermodynamically unfavorable for dephosphorization, making rephosphorization from the dephosphorization slag into the molten iron more likely. Therefore, to prevent rephosphorization from the dephosphorization slag into the molten iron during decarburization blowing, it is necessary to increase the charge basicity of the slag by adding auxiliary materials containing CaO before the start of decarburization blowing. Here, charge basicity refers to the ratio of CaO to SiO2 in the slag calculated from the slag carried over from the previous process and the auxiliary materials added before the start of decarburization blowing. In contrast, actual basicity refers to the ratio of CaO to SiO2 calculated from the total CaO excluding the unreacted CaO.

[0025] The advantage of increasing the charge basicity before decarburization blowing is that it not only suppresses rephosphorization but also prevents refractory corrosion. Therefore, the charge basicity (C / S) before the start of decarburization blowing is set to 4.2 or higher. Note that the actual basicity of the slag in decarburization blowing saturates at about 4.5, so even if the charge basicity is increased too much by adding a large amount of auxiliary materials containing CaO, it will only produce a large amount of undissolved CaO, which is disadvantageous in terms of cost. Therefore, the charge basicity (C / S) before the start of decarburization blowing is preferably set to 4.8 or lower. Here, the charge basicity (C / S) before the start of decarburization blowing can be calculated using the following equation (4): Charging basicity (C / S)={C1×(1-α / 100)+C2} / {S1×(1-α / 100)} (4)

[0026] In equation (4), C1 represents the CaO equivalent amount (kg / ton-steel) in the dephosphorization blowing, C2 represents the CaO equivalent amount (kg / ton-steel) of the auxiliary materials added before the decarburization blowing, S1 represents the SiO2 equivalent amount (kg / ton-steel) in the dephosphorization blowing, and α represents the intermediate slag removal rate (%).

[0027] Next, the conditions for spraying SiO2 powder will be described. In the converter refining method of this embodiment, SiO2-containing powder is also sprayed during a portion of the period during which oxygen gas is sprayed during decarburization blowing. The amount of SiO2 powder sprayed is set to 0.7 to 1.7 kg / t-steel. If the amount of SiO2 powder used is less than 0.7 kg / t-steel, the actual basicity is not sufficiently reduced, resulting in insufficient FeO production in the slag, which in turn prevents sufficient slag foaming and prevents the rate of secondary combustion heat transfer to the molten iron from increasing. On the other hand, if the amount of SiO2 powder used exceeds 1.7 kg / t-steel, the actual basicity is excessively reduced, resulting in refractory damage and rephosphorization. As described above, since the charging basicity is set to 4.2 or more, the actual basicity of the slag after powder spraying can be ensured to be 3.5 or more within the above-mentioned range of SiO2 powder use, and refractory damage can be prevented.

[0028] The above-mentioned amount of SiO2 powder is sprayed while 10 to 20% of the total amount of oxygen sprayed during decarburization blowing is being blown. If the blowing period is shorter than 10%, the time during which the slag is foamed is short, and the secondary combustion heat cannot be efficiently transferred to the molten iron. On the other hand, if the blowing period is longer than 20%, the amount of SiO2 powder used per unit time decreases, and the sprayed powder is blown up by the generated gas in the furnace, reducing the powder landing efficiency.

[0029] On the other hand, there are no particular restrictions on the timing for starting the blowing of SiO2 powder, but because the decarburization reaction and secondary combustion reaction are exothermic reactions, it is preferable to start the blowing in the first half of the decarburization blowing when the molten iron temperature is relatively low. Also, from the viewpoint of promoting slag foaming, it is preferable to not blow the SiO2 powder intermittently after starting the blowing of SiO2 powder, but to blow the SiO2 powder continuously until all of the above-mentioned amount of SiO2 powder has been blown.

[0030] In dephosphorization blowing, SiO2 powder may be sprayed under similar conditions, but the molten iron temperature is lower than in decarburization blowing, and the actual basicity is lower, so the slag tends to foam even without SiO2 powder spraying. In contrast, in decarburization blowing, the slag hardly foams unless SiO2 powder is sprayed, so the foaming promotion effect can be maximized.

[0031] Next, other conditions for decarburization blowing will be explained. The flow rate of the oxygen jet in decarburization blowing is not particularly limited, but in order to shorten the cycle time, the flow rate of the oxygen jet is set to 100 to 200 Nm 3 / h·ton-steel range is preferable. There are no particular restrictions on the nozzle outlet diameter, but the smaller the nozzle outlet diameter, the higher the powder velocity at the bath surface. Given the range of the number of nozzle holes and the range of the oxygen jet flow rate mentioned above, the nozzle outlet diameter is preferably 40 to 80 mm.

[0032] Furthermore, this embodiment has been described assuming a process in which dephosphorization blowing is performed in one converter, the slag generated by dephosphorization is intermediately drained, and then decarburization blowing is subsequently performed in the same converter. However, this embodiment may also be applied to a process in which dephosphorization blowing of molten pig iron is performed in a first converter, and then the molten pig iron tapped from the first converter is charged into a second converter, where decarburization blowing is performed in the second converter. It may also be applied to a process in which dephosphorization blowing and decarburization blowing are performed consecutively in one converter without draining the slag (intermediate draining). However, from the viewpoint of converter thermal tolerance, a process in which refining is performed in the same furnace is preferable to a process in which the furnaces are changed. [Example]

[0033] Next, examples of the present invention will be described, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0034] First, a converter with a radius of 5500 mm was prepared as a refining furnace, as shown in Figure 1. A total of 300 t of hot metal and scrap with a P concentration of approximately 0.13% by mass and Si concentration of approximately 0.60% by mass was charged. Flux and other additives were then added, and dephosphorization blowing was performed using oxygen jets from a top-blowing lance with six nozzle holes (n), each with an outlet diameter of 70 mm, and a nozzle inclination angle θ of 23°. The resulting slag was removed intermediately. The P concentration in the hot metal after dephosphorization was approximately 0.065% by mass, and the intermediate slag removal rate varied depending on the heat. Lime was then added before decarburization blowing to achieve the charge basicity shown in Table 1.

[0035] Next, decarburization blowing was performed by blowing an oxygen jet from the top lance, and from the middle of the decarburization blowing, SiO2 powder was also blown together with the oxygen jet. In the decarburization blowing, the powder blowing amount and powder blowing period were changed depending on the sample, as shown in Table 1. The flow rate of the oxygen jet in the decarburization blowing was 157 Nm 3 The carbon concentration in the molten iron at the end of blowing was set to 0.035 mass%.

[0036] While the SiO2 powder was being sprayed, the slag forming height was measured using a μ-wave level meter, and the forming ratio was calculated. The forming ratio was calculated as "maximum thickness of the formed slag" divided by "slag thickness when not forming." Furthermore, the increase in the heat transfer ratio was calculated by calculating the heat balance of the reaction in the furnace using actual measured values. Specifically, the increase in the heat transfer ratio was calculated using the following equation (5) using the thermal analysis method described in Non-Patent Document 4. H2=H1·γ hp +(2.2ΔC+5.57×PCR×α·ΔC+7.46ΔSi+1.68ΔMn+8.4ΔP)×10 6 gamma pig +9.0×10 3 (%FeO) γ slag -320×10 3 γ slag -900×10 3 γ scale -1.23×10 6 γore -1.1×10 6 ΔC-1000ΔT (5) H2: Heat content of molten steel after blowing (kcal / ton-steel) H1: Heat content of molten steel before blowing (kcal / ton-steel) ΔX: Component X in molten steel (ton / ton-steel) PCR: Secondary combustion rate (-) α: Heat transfer rate (-) gamma hp :Hot metal ratio(-) gamma pig : Total amount of hot metal and mold iron used (ton / ton-steel) gamma slag : Slag volume (ton / ton-steel) gamma scale : Scale amount (ton / ton-steel) gamma ore : Iron ore consumption (ton / ton-steel) ΔT: Blowing time (min)

[0037] Here, the secondary combustion rate PCR in equation (5) was calculated from the exhaust gas composition. The second term on the right side represents the heat of combustion of molten steel components, the third term represents the heat of slag formation, the fourth term represents the sensible heat of slag, the fifth term represents the heat of scale decomposition, the sixth term represents the heat of iron ore decomposition, the seventh term represents the sensible heat of furnace top gas, and the eighth term represents the heat dissipated from the furnace body. The temperatures before and after the start of blowing were measured, and the heat transfer ratio α was calculated by satisfying equation (5). Furthermore, the heat transfer ratio without SiO2 powder spraying was calculated in advance, and the increase in the heat transfer ratio compared to that case was calculated as the increase in the heat transfer ratio.

[0038] In addition, the basicity of the slag was measured after the end of blowing, and if the actual basicity after spraying was lower than 3.5, it was considered that there was a tendency for the refractory to dissolve and was evaluated as ×. Furthermore, if the increase in the heat transfer rate was 5% or more, it was evaluated as ○, and if it was less than 5%, it was evaluated as ×. An overall evaluation was then made taking these effects into consideration. Table 1 shows the experimental results.

[0039] [Table 1]

[0040] The underlined values ​​in Table 1 indicate values ​​outside the scope of the present invention. In Examples 1 to 5, the charge basicity before decarburization blowing, the powder spraying rate, and the powder spraying period satisfied the above-mentioned conditions, so no tendency for refractory melting was observed and the heat transfer rate increased significantly. On the other hand, in Comparative Example 1, the powder spraying rate was too low, so the slag could not be sufficiently foamed, and the increase in the heat transfer rate was less than 5%. In Comparative Example 2, the powder spraying rate was excessive, so the actual basicity after spraying was 3.3, and so a tendency for refractory melting was observed.

[0041] In Comparative Example 3, the powder spraying period was too short, so foaming did not progress sufficiently, and the increase in the heat transfer rate was less than 5%. In Comparative Example 4, the spraying period was too long, so the amount of powder per unit time was small and the powder landing efficiency was too low, so foaming did not progress sufficiently and the increase in the heat transfer rate was less than 5%. In Comparative Example 5, the charge basicity before decarburization blowing was too low, so spraying the powder caused even more excessive foaming of the slag, and the increase in the heat transfer rate was large, but the actual basicity after spraying was 2.7, so a tendency for refractory melting was observed. [Explanation of symbols]

[0042] 1. Smelting furnace 2 Top-blowing lance 3 Jet 4. Slug 5. Molten Iron 6 bottom-blown tuyere 7 Powder 8 fire point 11 Furnace body 12 Opening

Claims

[Claim 1] A converter refining method for decarburization blowing by blowing oxygen onto molten iron from a top lance, The decarburization blowing is started by setting the slag basicity at 4.2 or more, and while blowing oxygen at a rate of 10 to 20% of the total amount of oxygen blown in the decarburization blowing, SiO 2 A converter refining method characterized by spraying the powder of the above formula (1) at a rate of 0.7 to 1.7 kg / t-steel.

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