Converter refining method

Abstract

To provide a converter refining method capable of stably producing low-phosphorus steel with P density of 0.01 mass% or less.SOLUTION: A converter refining method for carrying out decarburization blowing by spraying oxygen to molten iron from a top-blowing lance comprises the steps of: setting charge basicity before starting the decarburization blowing to be equal to or more than 4.2; and spraying a total of 0.4 to 4.0 kg / t-steel of powders including at least 80 mass% of CaO together with the oxygen during a period from a time when 80% or more of the total amount of the oxygen to be spray has been sprayed to a time when at least 95% or more of the total amount of the oxygen to be spray has been sprayed during the decarburization blowing.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a converter refining method for producing low phosphorus steel. [Background technology]

[0002] Conventionally, the processes for refining molten iron using converters have been developed as follows: a process in which dephosphorization and decarburization are performed consecutively in one converter without discarding the slag (intermediate discarding) (hereinafter referred to as the first process); a process in which the molten iron is dephosphorized in a first converter, and then the molten iron tapped from the first converter is charged into a second converter and decarburized in the second converter (hereinafter referred to as the second process); and a process in which dephosphorization is performed in one converter, the slag generated by the dephosphorization is discarded (intermediate discarding), and then decarburization is continued in the same converter (hereinafter referred to as the third process).

[0003] In the first process, since there is no intermediate slag discharge, heat loss during refining is small and productivity is high, but since it is necessary to increase the slag basicity throughout the entire blowing process, the amount of CaO source, which is an auxiliary material, is large, which is disadvantageous in terms of cost.In addition, when melting low phosphorus steel, it is necessary to increase the amount of slag, but the amount of slag decreases due to slopping, making it difficult to melt low phosphorus steel as intended.

[0004] The second process has a high refining capacity and is easier to produce low-phosphorus steel than the first process, but requires two converters, which increases the equipment cost and increases the heat loss caused by diffusion, which tends to reduce the melting capacity of iron ore and scrap. The third process can shorten the total blowing time, reduce the amount of flux required for dephosphorization, and reduce the heat loss during refining, compared to the second process. However, in the third process, it is difficult to stably control the amount of intermediate slag discharge, and for example, it may be difficult to reduce the P concentration in the molten steel after refining is completed to a low concentration.

[0005] In addition, in recent years, there has been a strong demand for producing low-phosphorus steel with an even lower P concentration. For this purpose, the dephosphorization blowing performed before the decarburization blowing is insufficient, and it is necessary to further reduce the P concentration by supplying a CaO source in the decarburization blowing.

[0006] In converter blowing, refining agents such as quicklime (CaO) and lightly burnt dolomite (CaO·MgO) are added to ensure stable dephosphorization. However, CaO and MgO have melting points of over 2000°C by themselves, which prevents the refining agents from dissolving and inhibits the dephosphorization reaction. Therefore, a technique has been proposed in which part or all of the CaO source is made into powdered CaO, and it is blown into the molten iron in the converter using a carrier gas from a top blowing lance.

[0007] The features of this technology are as follows. Powdered CaO is blown from a top-blowing lance together with oxygen onto a hot spot (over 2000°C) at the hot spot, where the top-blowing oxygen reacts with the molten pig iron to produce a high-temperature FeO-based melt, and the powdered CaO reaches this FeO-based melt to produce a CaO-FeO-based melt. The top-blowing powdered CaO melts quickly and has a high dephosphorization ability, so the dephosphorization reaction can proceed with high efficiency. It is also believed that by blowing powdered CaO onto the molten pig iron, not only the permanent reaction that occurs between the slag on the molten pig iron surface and the bulk metal can be achieved, but also the transitory reaction in which the powdered CaO dephosphorizes the molten pig iron while floating up in the bath can be achieved. As described above, spraying powdered CaO from a top-blowing lance promotes the dephosphorization reaction, and various studies are being conducted to optimize the conditions for spraying powdered CaO.

[0008] Patent Document 1 discloses a method of adding quicklime for 90-100% of the entire blowing period in order to promote dissolution of the CaO source to reduce phosphorus in the first process. Patent Document 2 discloses a method of blowing a CaO source in the early and late stages of blowing in order to promote dephosphorization in the first process.

[0009] Patent Document 3 discloses a method for blowing the entire amount or a part of a CaO source onto the molten iron through a lance nozzle while controlling the dynamic pressure of the gas jet ejected from the lance nozzle onto the molten iron surface in the decarburization blowing of the first and second processes. Patent Document 4 discloses a method for blowing the CaO source only at the beginning of blowing in the decarburization blowing of the third process. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] JP 2016-79434 A [Patent Document 2] JP 2015-92018 A [Patent Document 3] International Publication No. 2013 / 094634 [Patent Document 4] JP 2020-105562 A Summary of the Invention [Problem to be solved by the invention]

[0011] However, the method described in Patent Document 1 limits the timing of additional addition of the CaO source, making it difficult to produce low-phosphorus steel with a P concentration of 0.01 mass% or less. The method described in Patent Document 2 also has a risk of rephosphorization from the middle to the end of blowing, making it difficult to produce low-phosphorus steel with a P concentration of 0.01 mass% or less.

[0012] In addition, the method described in Patent Document 3 is prone to differences in P concentration after decarburization blowing due to the amount of slag carried over after preliminary treatment, etc., and is therefore unable to stably produce low-phosphorus steel with a P concentration of 0.01 mass% or less. Furthermore, the method described in Patent Document 4 is prone to rephosphorization from the middle to the end of blowing, making it difficult to produce low-phosphorus steel with a P concentration of 0.01 mass% or less.

[0013] In view of the above-mentioned problems, an object of the present invention is to provide a converter refining method capable of stably producing low-phosphorus steel having a P concentration of 0.01 mass % or less. [Means for solving the problem]

[0014] The inventors of the present invention considered that if CaO powder is injected from the top lance in the early stage of decarburization blowing, phosphorus will return in the later stage of decarburization blowing, and low phosphorus steel cannot be stably produced. On the assumption that CaO powder is injected in the later stage of decarburization blowing, more detailed blowing conditions were studied intensively. Noting that the formation of limescale is promoted by blowing CaO powder together with oxygen from the top lance to a hot point at high temperatures (over 2000°C), more detailed blowing conditions were studied intensively. From calculations using FactSage8.2 (registered trademark), the equilibrium composition of the liquid phase of CaO-SiO2-FeO-MgO-MnO-P2O5-Al2O3 slag saturated with CaO at 1650°C, which corresponds to the final stage of decarburization blowing, is about 4.5 in actual basicity ((mass%FeO)≒25), and thermodynamically, it is considered that CaO will not form slag even if the basicity of the charge is increased any more. Therefore, the inventors considered that if CaO powder is blown from the top blowing lance, the hot point of over 2000°C will be reached, which will promote the formation of CaO slag, and the actual basicity can be increased to above 4.5, thereby promoting dephosphorization. However, after the blowing of CaO powder, the CaO supplied to the bulk slag crystallizes at a temperature lower than the hot point, which reduces the actual basicity and makes rephosphorization more likely to proceed. Therefore, the inventors considered that the progress of rephosphorization could be suppressed by delaying the end timing of the blowing of CaO powder as long as possible.

[0015] Furthermore, the inventors conducted extensive research into the type of powder to be blown from the top blowing lance. They discovered that by using a CaO-SiO2 mixed powder as the powder type, the dephosphorization product at the hot spot is not a CaO-P2O5-based compound such as 3CaO·P2O5, but a 2CaO·SiO2-3CaO·P2O5 solid solution, and that the 2CaO·SiO2 suppresses contact between the molten iron and 3CaO·P2O5, slowing down the rephosphorization rate. From the above research, they discovered the optimal conditions for blowing powder in the latter half of the decarburization blow, leading to the invention.

[0016] The present invention is as follows. [1] A converter refining method for decarburization by blowing oxygen onto molten iron from a top blowing lance, comprising the steps of: A converter refining method characterized in that the charging basicity before the start of the decarburization blowing is set to 4.2 or more, and in the decarburization blowing, a powder containing 80% by mass or more of CaO is blown together with the oxygen in a total amount of 0.4 to 4.0 kg / t-steel from the time when 80% or more of the total amount of oxygen to be blown has been blown until at least 95% of the total amount of oxygen has been blown. [2] The converter refining method according to the above [1], characterized in that the powder contains 95 mass % or more of CaO. [3] The converter refining method according to the above [1], characterized in that the powder contains CaO and SiO2 in total 95 mass% or more, and the proportion of SiO2 is more than 0 mass% and 17 mass% or less. Effect of the Invention

[0017] According to the present invention, it is possible to provide a converter refining method capable of stably producing low-phosphorus steel having a P concentration of 0.01 mass % or less. [Brief description of the drawings]

[0018] [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 PREFERRED EMBODIMENTS

[0019] Hereinafter, a converter refining method according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram for explaining 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 a refractory material. Oxygen gas is supplied to the top-blowing lance 2 from a gas supply path (not shown) connected to the upper end side, and a jet 3 of oxygen gas is sprayed toward the slag 4 and molten iron 5 from at least two nozzle holes formed at the lower end at a position of height H.

[0020] At the bottom of the furnace body 11, one or more bottom-blowing tuyeres 6 are provided, and agitation gas is blown into the molten iron 5 in the furnace body 11 from the bottom-blowing tuyeres. The number of bottom-blowing tuyeres is not particularly limited, but is usually one or more and five or less. The types of gas used are roughly divided into oxygen gas and inert gas, and examples of inert gas include nitrogen gas and argon gas. The shapes of the bottom-blowing tuyeres include a single-tube nozzle, a double-tube nozzle, a collecting tube nozzle, a porous nozzle, and the like. When the type of gas used is oxygen, O2 gas can be blown in from the inner tube of the tuyeres of the double-tube nozzle, and cooling gases such as CO2, N2, and LPG can be blown in from the gap between the inner tube and the outer tube.

[0021] Next, detailed conditions of 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 generated in 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 blowing lance 2 toward the slag 4 and molten iron 5 to start the decarburization blowing. In this embodiment, powder containing CaO is also blown together with oxygen gas at the end of the decarburization blowing.

[0022] First, the charge basicity before the start of decarburization blowing will be explained. In decarburization blowing, the temperature and carbon concentration are high and low, which are thermodynamically unfavorable for dephosphorization, so rephosphorization from the dephosphorization slag to the molten iron is likely to occur. Therefore, in order to suppress rephosphorization from the dephosphorization slag to the molten iron in decarburization blowing, it is necessary to add auxiliary materials containing CaO before the start of decarburization blowing to increase the charge basicity of the slag. Here, the 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, the actual basicity refers to the ratio of CaO to SiO2 calculated by excluding the undissolved CaO from the total CaO.

[0023] The advantage of increasing the basicity in the first half of the decarburization blowing is that it not only suppresses rephosphorization but also prevents slopping in the early stage of the decarburization blowing. Therefore, the charge basicity (C / S) before the start of the decarburization blowing is set to 4.2 or more. Note that the actual basicity of the slag in the 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 the decarburization blowing is preferably set to 4.8 or less. Here, the charge basicity (C / S) before the start of the decarburization blowing can be calculated by the following formula (1). Charging basicity (C / S)={C1×(1-α / 100)+C2} / {S1×(1-α / 100)} ···(1)

[0024] In formula (1), C1 represents the amount of CaO (kg / ton-steel) in the dephosphorization blowing, C2 represents the amount of CaO (kg / ton-steel) of the auxiliary material added before the decarburization blowing, S1 represents the amount of SiO2 (kg / ton-steel) in the dephosphorization blowing, and α represents the intermediate slag removal rate (%).

[0025] Next, the period during which the powder containing CaO is blown will be described. In the converter refining method of this embodiment, the powder is blown from the time when 80% or more of the total amount of oxygen blown in the decarburization blowing is blown until at least 95% of the total amount of oxygen is blown. If the powder containing CaO is blown before the time when 80% of the total amount of oxygen is blown, the amount of powder required may increase excessively, which may be disadvantageous in terms of cost, or the amount of powder supplied per unit time may decrease, which may reduce the dephosphorization effect. In addition, if the start of the powder blowing is too late, it is necessary to complete the blowing of the amount of powder described later in a short time. In other words, the powder blowing is started at a timing when it is possible to blow all of the amount of powder described later.

[0026] Furthermore, if the blowing of the powder containing CaO is terminated before 95% of the total oxygen amount has been blown, rephosphorization occurs at the end of the decarburization blowing, making it impossible to produce the desired low-phosphorus steel. The powder containing CaO blown from the top blowing lance has an average particle size of 500 μm or less, and the components contained in the powder will be described later.

[0027] Next, the amount of powder containing CaO to be sprayed will be explained. The larger the amount of powder sprayed, the more the CaO will turn into slag at the hot point, and the higher the actual basicity of the slag will be, but the higher the cost will be. In addition, the more powder is sprayed during the period from when the total oxygen amount reaches 80% or more to when blowing is stopped, the more the powder supply rate must be increased, but the powder supply rate is limited by the upper limit of the equipment. In light of the above, the total amount of powder containing CaO to be sprayed is set to 0.4 to 4.0 kg / t-steel. If the total amount of powder containing CaO to be sprayed is less than 0.4 kg / t-steel, the dephosphorization effect will be insufficient. In addition, if the total amount of powder containing CaO to be sprayed exceeds 4.0 kg / t-steel, the powder supply rate is limited depending on the equipment, and the spraying period must be excessively increased.

[0028] Next, the type of powder sprayed from the top blowing lance will be described. In order to promote the formation of CaO into slag at the hot spot and to increase the actual basicity to more than 4.5 to promote the dephosphorization reaction, the powder to be sprayed contains 80% by mass or more of CaO. This is because if the proportion of CaO is less than 80% by mass, the dephosphorization effect is not sufficient. Preferably, the proportion of CaO is 95% by mass or more.

[0029] In addition, the dephosphorization effect can be obtained even when the powder to be sprayed is CaO powder consisting almost entirely of CaO. However, as mentioned above, after spraying the powder containing CaO, the CaO supplied to the bulk slag will crystallize because its temperature is lower than that of the fire point, and this may cause a decrease in actual basicity and rephosphorization. Therefore, instead of CaO powder, a CaO-SiO2 mixed powder containing CaO and SiO2 in total of 95 mass% or more may be used as the powder to be sprayed. The CaO-SiO2 mixed powder may be a mixture of CaO powder and SiO2 powder, or a mixture of CaO and SiO2, which is melted, solidified, and then pulverized. In this case, the dephosphorization product at the fire point will be a 2CaO·SiO2-3CaO·P2O5 solid solution, rather than a CaO-P2O5 compound such as 3CaO·P2O5, and the 2CaO·SiO2 will suppress the contact between the molten iron and 3CaO·P2O5, thereby further suppressing rephosphorization.

[0030] When using CaO-SiO2 mixed powder, the CaO content is set to 80 mass% or more as mentioned above. On the other hand, the basicity of the slag before decarburization blowing is set to 4.2 or more, so the actual basicity must not be lowered by blowing. If the SiO2 content in the powder is about 17 mass%, the basicity of the CaO-SiO2 mixed powder will be about 4.8, so it is preferable that the SiO2 content in the powder is more than 0 mass% and 17 mass% or less.

[0031] In this embodiment, in order to promote the dephosphorization reaction, the actual basicity is increased by spraying powder containing CaO, but the amount of powder containing CaO used increases accordingly, leading to increased costs. Therefore, it is preferable to obtain the dephosphorization effect while minimizing the amount of powder used. In the intermediate slag removal performed before the decarburization blowing, the amount of SiO2 in the dephosphorization slag decreases as the intermediate slag removal rate increases, so that the actual basicity can be increased with a relatively small amount of powder. If the intermediate slag removal is performed with an intermediate slag removal rate α of more than 60%, dephosphorization can be promoted with a relatively small amount of powder and the amount of slag discharged can be reduced.

[0032] Next, other conditions in the decarburization blowing will be described. The flow rate of the oxygen jet in the 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). The nozzle outlet diameter is not particularly limited, but the smaller the nozzle outlet diameter, the higher the powder velocity at the bath surface. From the above-mentioned range of the number of nozzle holes and the range of the oxygen jet flow rate, the nozzle outlet diameter is preferably 40 to 80 mm.

[0033] In addition, in this embodiment, as shown in FIG. 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.

[0034] In addition, this embodiment has been described on the premise of a process (third process) in which dephosphorization blowing is performed in one converter, the slag generated by the dephosphorization is intermediately removed, and then decarburization blowing is continued in the same converter. However, this embodiment may also be applied to a process (second process) in which dephosphorization blowing of molten pig iron is performed in a first converter, the molten pig iron tapped from the first converter is then charged into a second converter, and decarburization blowing is performed in the second converter. It is also possible to apply this to a process (first process) in which dephosphorization blowing and decarburization blowing are performed consecutively in one converter without discarding slag (intermediate slag discarding). However, since dephosphorization blowing and decarburization blowing are performed consecutively, the decarburization blowing is considered to start from the stage of 40% of the total blowing period, and powdered CaO is blown from the point at which 87% or more of the total amount of oxygen blown in the decarburization blowing has been blown until at least 96% of the oxygen has been blown. EXAMPLES

[0035] Next, an embodiment of the present invention will be described, but the conditions in the embodiment are merely an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0036] First, a converter with a furnace radius of R = 5500 mm was prepared as a refining furnace as shown in Fig. 1, and a total of 300 t of molten pig iron and scrap with a P concentration of 0.13 mass% and a Si concentration of about 0.60 mass% were charged. Flux and other materials were then added, and dephosphorization blowing was performed by blowing oxygen jets from a top lance with a nozzle hole number n of 6, an outlet diameter of each nozzle of 70 mm, and a nozzle inclination angle θ of 23°, and the generated slag was intermediately discarded. In this case, the P concentration in the molten pig iron after dephosphorization blowing was about 0.065 mass%, and the intermediate slag discarding rate was changed depending on the heat.

[0037] Next, decarburization was performed by blowing an oxygen jet from the top lance, and from the middle of the decarburization blow, CaO powder or CaO-SiO2 mixed powder was blown together with the oxygen jet. In the decarburization blow, the basicity of the charge before the start of the decarburization blow, the powder blowing amount, the powder blowing timing, and the weight ratio of CaO and SiO2 in the powder were changed depending on the heat. The flow rate of the oxygen jet in the decarburization blow was 157 Nm 3 The molten steel temperature at the end of blowing was 1660°C, and the C concentration in the molten steel at the end of blowing was 0.035 mass%.

[0038] In the experiment, the case where the P concentration in the molten steel at the time of blowing out was 0.010 mass% or less was evaluated as ○, and the case where the P concentration was higher than 0.010 mass% was evaluated as ×. In addition, after powder spraying, slag was sampled from the hot point, and the slag composition was analyzed to determine the actual basicity after powder spraying. The experimental results are shown in Table 1.

[0039] [Table 1]

[0040] The underlines in Table 1 indicate that the results are outside the scope of the present invention. In Examples 1 to 12, the charge basicity before the start of decarburization blowing, the powder spraying amount, the timing of the start and end of powder spraying, and the weight ratio of CaO in the powder all satisfied the specified conditions, so the P concentration in the molten steel at the end of blowing was 0.01 mass% or less. On the other hand, in Comparative Example 1, the timing of the start and end of powder spraying was too early, so rephosphorization occurred at the end of decarburization blowing after the end of powder spraying, and the P concentration in the molten steel at the end of blowing did not reach 0.01 mass% or less. In Comparative Example 2, the amount of powder sprayed was too small, so the actual basicity after powder spraying could not be increased, and the P concentration in the molten steel at the end of blowing did not reach 0.01 mass% or less. In Comparative Examples 3 and 4, the weight ratio of CaO in the powder was too low, so the actual basicity after powder spraying could not be increased, and the P concentration in the molten steel at the end of blowing did not reach 0.01 mass% or less. In Comparative Example 5, the timing of ending powder blowing was too early, so rephosphorization occurred at the end of decarburization blowing after the end of powder blowing, and the P concentration in the molten steel at the end of blowing did not reach 0.01 mass% or less. In Comparative Examples 6 and 7, the charging basicity before the start of decarburization blowing was outside the specified conditions, so in both cases the P concentration in the molten steel at the end of blowing did not reach 0.01 mass% or less. [Explanation of symbols]

[0041] 1. Smelting furnace 2 Top blowing lance 3. Jet 4. Slug 5. Molten Iron 6 bottom-blown tuyere 7 Still bath surface 11 Furnace body 12 Opening

Claims

1. A converter refining method for decarburization by blowing oxygen onto molten iron from a top blowing lance, comprising the steps of: a charging basicity of 4.2 or more before the start of the decarburization blowing is set, and in the decarburization blowing, a powder containing 80% by mass or more of CaO is blown together with the oxygen in a total amount of 0.4 to 4.0 kg / t-steel from a point in time when 80% or more of a total amount of oxygen to be blown has been blown until at least 95% of the total amount of oxygen has been blown.

2. 2. The converter refining method according to claim 1, wherein the powder contains 95 mass % or more of CaO.

3. The powder is CaO and SiO 2 and SiO 2 The converter refining method according to claim 1, characterized in that the ratio of is more than 0 mass% and 17 mass% or less.

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