Steel smelting method

Abstract

Provided is a steel melting method in which the following steps are continuously carried out: a first dephosphorization step in which a first dephosphorization treatment is carried out on a molten iron having a Si concentration of 0.30-0.50 mass%, said molten iron being charged into a converter-type container and diluted with a cold iron source; a first intermediate deslagging step; a discharge step for discharging a portion of the slag in the converter-type container after the first dephosphorization step; a second dephosphorization step in which, after the first intermediate deslagging step, an iron oxide source is added in an amount so as to lower the temperature of the molten iron in the converter-type vessel by 15-45 DEG C inclusive, and the mass ratio (CaO / SiO2) of the CaO concentration to the SiO2 concentration in the slag in the converter-type vessel is set to 0.8-less than 1.5, thereby performing a second dephosphorization treatment of the molten iron; a second intermediate slag discharge step in which, after the second dephosphorization step, part of the slag in the converter-type container is discharged; and a decarburization step in which, after the second intermediate slagging step, the molten iron is subjected to a decarburization treatment in the converter-type vessel.

Description

Technical Field

[0001] The present invention relates to a method for smelting steel using a top-bottom blown converter for refining steel. Background Art

[0002] For phosphorus (P) in steel, it sometimes becomes a factor deteriorating the properties of steel, so it is mostly removed by oxidative refining in a torpedo car (TPC, torpedo car) outside the converter as an iron melt pretreatment or in a converter called primary refining during the refining stage. Phosphorus is contained in iron ore. In recent years, since high-grade iron ore is being depleted and the proportion of low-quality iron ore is increasing, the phosphorus concentration in the iron melt tapped from the blast furnace has a tendency to increase. In addition, silicon (Si) is inevitably contained in the iron melt tapped from the blast furnace and is mostly removed in the same way as phosphorus outside the converter as an iron melt pretreatment or in the converter (primary refining), and then added in secondary refining as needed. In primary refining, carbon (C) in the iron melt is oxidized and removed, and at the same time, the temperature rises with oxidation. After the blowing in the converter is completed, an iron melt with low Si, low P, and low C can be obtained.

[0003] In order to remove phosphorus from the iron melt by iron melt pretreatment and primary refining, the phosphorus in the iron melt is oxidized and removed into the slag in the form of 2CaO·SiO 2 -3P 2 O 5 ·FeO. Therefore, CaO and SiO 2 are contained in the slag. Since oxidative refining is carried out, the slag basically becomes a CaO-SiO 2 -FeO system. At this time, the higher the mass ratio of CaO to SiO 2 (hereinafter referred to as C / S), the higher the dephosphorization ability of the slag. In addition, the lower the temperature of the iron melt, the more favorable it is for dephosphorization. In order to increase C / S, a CaO source such as quicklime is added. However, if the Si concentration in the iron melt is high, the amount of SiO 2 generated also increases. Therefore, in addition to the increase in the amount of CaO required to adjust C / S, the amount of slag after refining also increases. Therefore, desiliconization treatment is mostly carried out in advance using TPC and then dephosphorization blowing is carried out using a converter.

[0004] As a treatment using a converter, in Patent Document 1, a method was developed in which the same converter is used for desiliconization and dephosphorization blowing, a part of the slag is discharged, decarburization blowing is carried out, and the slag after decarburization blowing is reused for desiliconization and dephosphorization treatment of the next charge. At this time, in order to smelt low-phosphorus steel, it is effective to increase the dephosphorization ability by increasing C / S of the slag during blowing. However, when C / S is increased, the amount of slag increases, so the treatment cost becomes a problem.

[0005] In Patent Document 1, a converter steelmaking method is disclosed. In the dephosphorization process of using the same converter for dephosphorization and decarburization, the C / S of the slag is set to be 1.0 or more and 2.0 or less, and the treatment temperature is set to be 1350 °C or less. In addition, in Patent Document 2, a method for dephosphorizing hot metal is disclosed, which is characterized in that in the dephosphorization process of using the same converter for dephosphorization and decarburization, the C / S of the slag is set to be 0.8 or more and 1.2 or less, and the treatment end temperature is 1300 °C or less. In this way, if the temperature of the hot metal during the dephosphorization treatment can be lowered, even with a low C / S, effective dephosphorization can be achieved, which is also obvious thermodynamically.

[0006] In addition, it is known that T.Fe in the slag also affects the dephosphorization ability of the slag. In Patent Document 3, a converter steelmaking method is disclosed, which is characterized in that dephosphorization refining is carried out by adjusting T.Fe in the treated slag to be 10% or more and 30% or less. This is because: according to Patent Document 3, if the T.Fe concentration in the slag becomes lower than 10%, the phosphorus distribution ratio (phosphorus concentration in the slag / phosphorus concentration in the metal) decreases due to insufficient oxygen potential, and if it exceeds 30%, the basic components in the slag are diluted and the phosphorus distribution ratio decreases.

[0007] After transferring phosphorus to the slag in the dephosphorization process, the slag is discharged out of the system (hereinafter referred to as intermediate slag discharge). In the case of using this method, the phosphorus concentration after decarburization blowing in the next process becomes dependent on the amount of phosphorus in the hot metal and the slag at the start of decarburization blowing (hereinafter referred to as the carried-over phosphorus amount). However, in the case of using the same converter to treat dephosphorization and decarburization, it is difficult to completely discharge the slag. Therefore, compared with the method of transferring the hot metal from a dephosphorization-only furnace to a decarburization-only furnace for smelting, there is a problem that is disadvantageous for the smelting of low-phosphorus steel.

[0008] To address this problem, in Patent Document 4, a converter refining method is disclosed, which reduces the carried-over phosphorus amount during decarburization blowing by repeating dephosphorization blowing and intermediate slag discharge twice using the same converter. In this method, by adding a flux in the second dephosphorization blowing, the C / S at the end of the second dephosphorization blowing is set to be 1.5 or more and 2.0 or less, and the molten metal temperature is set to be 1400 °C or less, thereby promoting the dephosphorization reaction.

[0009] In addition, Patent Document 5 discloses a converter steelmaking method in which, when using one converter to continuously process desiliconization, intermediate slag removal, dephosphorization, intermediate slag removal, and decarburization, the slag generated in the dephosphorization or decarburization process is reused in the next process. In this method, by adding a cold iron source after intermediate slag removal, a condition favorable for dephosphorization is created, thereby promoting the dephosphorization reaction. At this time, the C / S of the dephosphorization slag is 1.5 or more and 3.5 or less, and the dephosphorization ability of the slag is improved by increasing the C / S.

[0010] In this way, in order to effectively smelt low-phosphorus steel using one converter, in addition to using slag with a high C / S of the slag and performing low-temperature treatment, a treatment in which dephosphorization blowing and intermediate slag removal are repeated twice is introduced, thereby reducing the amount of phosphorus introduced.

[0011] Prior Art Documents

[0012] Patent Documents

[0013] Patent Document 1: Japanese Patent Laid-Open No. 05-140627

[0014] Patent Document 2: Japanese Patent Laid-Open No. 05-247512

[0015] Patent Document 3: Japanese Patent Laid-Open No. 07-070626

[0016] Patent Document 4: Japanese Patent Laid-Open No. 2011-144415

[0017] Patent Document 5: Japanese Patent Laid-Open No. 2018-188730

[0018] Non-Patent Documents

[0019] Non-Patent Document 1: Iron Metallurgy Reaction Engineering by Kiyoshi Segawa, pp. 52-60. Summary of the Invention

[0020] Problems to be Solved by the Invention

[0021] With the deterioration of iron ore raw materials in recent years, the phosphorus concentration in hot metal has a tendency to increase year by year. Due to the increase in the phosphorus concentration in hot metal at the initial stage of converter blowing, it has gradually become difficult to smelt low-phosphorus steel that could be smelted by the above method in the past by the same method.

[0022] The object of the present invention is a method for smelting steel that uses the same converter-type container to perform hot metal pretreatment and decarburization treatment. The hot metal pretreatment includes desiliconization treatment and dephosphorization treatment of hot metal. The desiliconization treatment of hot metal can be carried out only by a converter, or it can also be a treatment for removing a part of the silicon in the hot metal using TPC or a hot metal ladle, etc. for the hot metal before charging into the converter. The dephosphorization treatment is carried out in the converter.

[0023] The object is a method for smelting low-phosphorus steel in which the following steps are continuously carried out in a converter: a first dephosphorization step of dephosphorizing hot metal; a first intermediate slag discharging step of discharging a part of the slag in the converter; a second dephosphorization step of dephosphorizing hot metal again, a second intermediate slag discharging step of discharging a part of the slag in the converter; and a decarburization step of decarburizing hot metal. In the first dephosphorization blow (first dephosphorization step), first, a desiliconization reaction is carried out, and then, a dephosphorization reaction is carried out.

[0024] In the case of applying a method of reducing the amount of phosphorus introduced by repeating dephosphorization blowing and intermediate slag discharging twice using one converter, for the timing of the second dephosphorization blow (second dephosphorization step), the hot metal and slag compositions and temperature at the start of blowing are different compared to those at the time of the first dephosphorization blow (first dephosphorization step). That is, at the moment when the first dephosphorization blow starts, the slag-forming agent represented by the added CaO source has not melted, the Si concentration and carbon concentration in the hot metal are high, and the hot metal temperature is also low. On the other hand, at the end stage of the first dephosphorization blow, most of the Si is oxidized to become SiO 2 , and the added slag-forming agent is mostly dissolved. In addition, the oxidation of Si in the hot metal ends and decarburization starts. Along with the decarburization reaction in which carbon in the hot metal reacts with FeO in the slag at this time, the slag foams, and the intermediate slag discharging is effectively carried out. Therefore, at the moment when the second dephosphorization blow starts, the hot metal temperature rises due to the oxidation of Si and carbon in the hot metal. In addition, since foaming is the reaction of FeO in the slag with carbon in the hot metal, the T.Fe concentration also decreases, which is a situation unfavorable for dephosphorization in the second dephosphorization blow.

[0025] Generally, in the situation before the first dephosphorization blow, according to the hot metal temperature, a cold iron source, an iron ore or sinter powder and other FeO-containing auxiliary raw materials, and a MgO-containing auxiliary raw material such as dolomite are mostly added. In addition, in order to remove P 2 O 5 from the slag in the previous smelting when smelting low-phosphorus steel, slag is mostly formed by quicklime or limestone without heat-reusing the slag, and the amount of slag is small at the moment when the first dephosphorization blow starts. In contrast, at the moment when the second dephosphorization blow starts, the auxiliary raw materials are added in the situation where the slag has already been formed by the first dephosphorization blow.

[0026] In either the first dephosphorization blow or the second dephosphorization blow, the slag foams as the blow progresses. If blowing is continued in this state, the foaming slag will overflow from the converter. Thus, in either the first dephosphorization blow or the second dephosphorization blow, the end period of blowing occurs just before the slag overflows. Even if the dephosphorization ability of the remaining slag remains, the dephosphorization blow will end.

[0027] The second dephosphorization blow is more likely to foam compared to the first dephosphorization blow, resulting in a shorter blow time. Therefore, if a cold iron source is added during the second dephosphorization blow, due to the short blow time, melting residue is generated and becomes a hindrance factor during intermediate slag removal. In addition, when an iron oxide source is added to adjust the T.Fe concentration and temperature during the second dephosphorization blow, it promotes the reaction of FeO with carbon in the molten iron, advancing the foaming time, and thus a problem arises that the blow time cannot be sufficiently ensured.

[0028] To address such problems, in Patent Documents 4 and 5, a CaO source is added as a flux during the second dephosphorization blow to increase the C / S of the slag and promote dephosphorization. However, from the viewpoints of cost and slag treatment, it is preferable not to add a CaO source during the second dephosphorization blow. In addition, even when the slag is formed with a specified C / S, the less the Si content in the main raw materials charged, the less the slag volume can be reduced.

[0029] Here, the "Si concentration before blow" is defined. The Si concentration before blow is the Si concentration (mass%) obtained by diluting the Si in the molten iron before charging into the converter with cold iron sources such as scrap iron charged into the converter together with the molten iron, and can be calculated by Equation (1). The part described as the "Si concentration before blow" refers to the Si concentration diluted with cold iron sources such as scrap iron. When referring to the Si concentration before dilution, it is called the "Si concentration before charging into the converter" in the molten iron. When charging iron ingots into the converter, the iron ingots are also included in the molten iron volume.

[0030] Si concentration before blow (mass%) = Si concentration before charging into the converter (mass%) × Molten iron volume (ton) / (Molten iron volume (ton) + Cold iron sources other than added pig iron (ton)) (1)

[0031] Therefore, from the viewpoint of suppressing the slag volume, a dephosphorization blow method under the conditions of a low Si concentration (Si concentration before blow) before the start of the first dephosphorization blow and a low C / S condition, that is, a condition where the slag volume becomes smaller, was studied. Experiments were conducted by changing the C / S condition, and the results showed that under high C / S conditions, dephosphorization proceeds corresponding to the increase in the oxygen supply amount other than the oxygen consumed by desiliconization (hereinafter referred to as oxygen outside desiliconization), but under low C / S conditions, even if the oxygen outside desiliconization is increased, dephosphorization sometimes stagnates.

[0032] The object of the present invention is to provide a steel melting method having an efficient dephosphorization blowing method. When continuously performing two dephosphorization blowings (the first dephosphorization blowing - the first intermediate slag discharge - the second dephosphorization blowing) using the same converter-type vessel, the "Si concentration before blowing" is minimized as much as possible. In the second dephosphorization blowing, a low C / S slag is used without adding a CaO source. Under these conditions, the dephosphorization reaction during the second dephosphorization blowing does not stagnate, and dephosphorization proceeds with an increase in external oxygen for desiliconization.

[0033] Means for solving the problem

[0034] The inventors of the present invention investigated by making various changes to the molten iron, slag composition, and temperature during blowing, and studied the above-mentioned dephosphorization stagnation factors. As a result, it was recognized that dephosphorization stagnation occurs in the following situation: Under the condition of a low Si concentration (Si concentration before blowing) before the first dephosphorization step, since the amount of slag is small under the same C / S condition, if the amount of FeO reduction accompanying the decarburization reaction is the same, the rate of decrease in the T.Fe concentration in the slag after the first dephosphorization step is large. In addition, the temperature of the molten iron in the second dephosphorization step is high. Dephosphorization stagnation occurs under the condition where all of the conditions of a low Si concentration (Si concentration before blowing) in the molten iron, low C / S, and high molten iron temperature are satisfied, which is a hitherto undiscovered recognition.

[0035] In view of the above recognition, the inventors of the present invention conducted an in-depth study on the generation mechanism of foaming, which greatly affects the dephosphorization behavior and the ensuring of the treatment time during dephosphorization blowing. As a result, it was recognized that by reducing the temperature of the molten iron under the condition of a low Si concentration (Si concentration before blowing) in the molten iron and appropriately adjusting the T.Fe concentration of the slag, it is possible to avoid dephosphorization stagnation even under low C / S conditions. It was recognized that as a specific method thereof, by adding an iron oxide source into the converter, a part of the added iron oxide source is reduced by carbon in the molten iron to lower the temperature of the molten iron, and the remaining iron oxide source can adjust the T.Fe concentration in the slag. Moreover, under the condition of a small amount of slag, it is possible to ensure the blowing time without early occurrence at the moment of foaming generation.

[0036] The specific means of the present invention are as follows.

[0037] A method for smelting steel, which continuously performs the following steps: First dephosphorization step: performing a first dephosphorization treatment on the molten iron charged into a converter-type vessel and having a Si concentration of 0.30% by mass or more and 0.50% by mass or less after being diluted by a cold iron source; First intermediate slag discharging step: after the first dephosphorization step, discharging a part of the slag in the converter-type vessel; Second dephosphorization step: after the first intermediate slag discharging step, adding an amount of iron oxide source that reduces the temperature of the molten iron in the converter-type vessel by 15°C or more and 45°C or less, and setting the mass ratio (CaO / SiO 2 concentration (CaO / SiO 2 ) to 0.8 or more and less than 1.5 to perform the second dephosphorization treatment on the molten iron; Second intermediate slag discharging step: after the second dephosphorization step, discharging a part of the slag in the converter-type vessel; and Decarburization step: after the second intermediate slag discharging step, performing a decarburization treatment on the molten iron in the converter-type vessel.

[0038] By applying the present invention, even when the Si concentration before blowing is low and the C / S is low, it is possible to avoid the stagnation of dephosphorization in the second dephosphorization step, and reduce the amount of phosphorus introduced into the decarburization blowing. As a result, it is possible to reduce the phosphorus concentration in the molten steel after decarburization blowing, and achieve low phosphorus content in the finished product and reduction of melting cost such as reduction of auxiliary materials during decarburization blowing. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 is a diagram showing the processing steps of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0040] 1. Definition of terms in the present invention

[0041] As Figure 1 shown in, the converter-type vessel 1 is a refining vessel used for the purpose of reducing the concentrations of Si, P, C, etc. in the molten iron, and having a top-blown lance 7 capable of supplying oxygen from the furnace mouth 2 at the upper part of the molten iron and a bottom-blown tuyere 3 for blowing a stirring gas from the bottom of the vessel.

[0042] The molten iron pretreatment refers to an oxidative refining treatment mainly for the purpose of reducing the silicon concentration and phosphorus concentration in the molten iron, such as blowing oxygen ( = blowing) on the molten steel or blowing oxygen through an immersion lance. In the molten iron pretreatment of the present invention, the desiliconization treatment is performed in one or both of outside and inside the converter furnace, and the dephosphorization treatment is performed inside the converter furnace. In addition, the decarburization treatment refers to an oxidative refining treatment for the purpose of reducing the carbon concentration in the molten iron. In the present invention, the oxidative refining treatment is performed in the same converter-type vessel to reduce the Si and P concentrations in the molten iron (molten iron pretreatment) and reduce the C concentration (decarburization treatment).

[0043] The so-called low-phosphorus steel refers to molten steel with a low phosphorus concentration after pretreatment and decarburization treatment of molten iron. There are also cases where low-phosphorus steel is smelted through pretreatment of molten iron. However, in the present invention, the pretreatment of molten iron carried out outside the converter-type vessel is limited to pre-desiliconization, and a converter-type vessel is used to reduce the concentrations of Si, P, and C in the molten iron. As an example, it refers to steel with a finished product phosphorus concentration of 0.020% or less.

[0044] The so-called dephosphorization process refers to a treatment in which Si and P in the molten iron are oxidized and removed by bottom-blowing from the bottom of the converter-type vessel and top-blowing oxygen from the upper part of the molten iron. In the present invention, the dephosphorization treatment is carried out twice. In the first dephosphorization process, as a pre-stage of the dephosphorization reaction, a desiliconization reaction occurs. In addition, in the second dephosphorization process, a dephosphorization reaction mainly occurs.

[0045] The so-called intermediate slag removal process refers to a treatment in which, after the top-blowing of oxygen accompanying the dephosphorization process is completed, the converter is tilted and a part of the slag in the converter is discharged from the furnace mouth 2 to the outside of the converter. In the present invention, since the dephosphorization treatment is carried out twice, the intermediate slag removal is also carried out twice in total after each dephosphorization treatment. The so-called slag removal rate is the value obtained by dividing the amount of slag discharged to the outside of the converter through the intermediate slag removal process by the amount of slag before slag removal. For example, when the amount of slag before slag removal is 10 tons, if the amount of slag discharged to the outside of the converter is 4 tons and the amount of slag remaining in the converter is 6 tons, the slag removal rate becomes 40%. The amount of slag before slag removal can be assumed to be such that all the Si in the molten iron becomes SiO 2 , and is calculated from the total value of the addition amount of auxiliary raw materials and the amount of FeO in the slag. If the top-blowing oxygen supply conditions are the same, there is not much change, so the FeO concentration in the slag at this time can be calculated based on the actual operation. The amount of slag discharged to the outside of the converter is obtained by weighing the discharged slag.

[0046] The so-called decarburization process refers to a treatment in which carbon and phosphorus are oxidized and removed by bottom-blowing from the bottom of the converter-type vessel and top-blowing oxygen from the upper part of the molten iron. Although the same vessel is used, since the top-blowing oxygen flow rate, C / S and amount of the slag, and the temperature at the end of the blowing during decarburization are different from those in the dephosphorization process, generally, in terms of the end of the dephosphorization process and the end of the decarburization process, the phosphorus concentration at the end of the decarburization process is lower.

[0047] The Si concentration before blowing is the Si concentration (mass %) obtained by diluting the Si in the hot metal before charging into the converter with cold iron sources such as scrap iron charged into the converter together with the hot metal, and can be calculated by formula (1). The part recorded as the Si concentration before blowing refers to the Si concentration after dilution with scrap iron, etc. When referring to the Si concentration before dilution, it is called the Si concentration in the hot metal before charging into the converter. When ingots are charged into the converter, the ingots are also included in the amount of hot metal. Hereinafter, unless otherwise specified, the hot metal in the converter refers to the hot metal charged and mixed with cold iron sources existing in the converter.

[0048] Si concentration before blowing (mass %) = Si concentration in the hot metal before charging into the converter (mass %) × amount of hot metal (ton) / (amount of hot metal (ton) + added cold iron source (ton)) (1)

[0049] The so-called C / S is the mass ratio of the CaO concentration to the SiO 2 concentration in the slag (CaO / SiO 2 ). The C / S (C / S P1 ) at the end of the first dephosphorization process is set by dividing the total amount of the CaO component in the auxiliary raw materials added as slag formers in the first dephosphorization process by the total amount of SiO 2 components in the auxiliary raw materials and the Si in the hot metal assuming that all of them are oxidized to become SiO 2 , and the value can be calculated by formula (2). The amounts of CaO and SiO 2 in the auxiliary raw materials can be calculated by analyzing the CaO and SiO 2 content rates (%) in the auxiliary raw materials in advance and multiplying by the addition amount. 2

[0050] C / S P1 = CaO (kg) in the auxiliary raw materials added in the first dephosphorization process / (SiO 2 (kg) in the auxiliary raw materials added in the first dephosphorization process + SiO 2 (kg) generated by the oxidation of Si in the hot metal) (2)

[0051] The C / S in the slag when performing the second dephosphorization process P2 can be calculated by formula (3) considering the amounts of CaO and SiO 2 obtained by multiplying the numerator and denominator of formula (2) by the first intermediate slag discharge rate and the CaO and SiO 2 contained in the iron oxide source added in the second dephosphorization process after the first intermediate slag discharge process. In the present invention, since no CaO source is added in the second dephosphorization process, there is no term for the CaO source added in the second dephosphorization process in the following formula (3).

[0052] C / S P2 = (CaO (kg) in the auxiliary raw materials added in the first dephosphorization process × (1 - (the first intermediate slag discharge rate (%) / 100)) + CaO (kg) in the iron oxide source added in the second dephosphorization process) / {(SiO 2 (kg) in the auxiliary raw materials added in the first dephosphorization process + SiO 2 (kg) generated by the oxidation of Si in the hot metal) × (1 - (the first intermediate slag discharge rate (%) / 100)) + SiO 2 (kg) in the iron oxide source added in the second dephosphorization process} (3)

[0053] The so-called iron oxide source refers to auxiliary raw materials in which the T.Fe concentration (the mass concentration of iron in iron oxide (FeO and Fe 2 O 3 ) excluding metallic iron) in the constituent components of the auxiliary raw materials excluding volatile components exceeds 50%. Specifically, it corresponds to auxiliary raw materials such as iron ore and sintered ore. As the components contained in the iron oxide source, in addition to iron oxide, it mainly contains CaO and SiO 2 . It should be noted that in order to adjust the C / S during the second dephosphorization blowing, as detailed in the second dephosphorization process described later, it is only necessary to adjust the C / S during the first dephosphorization blowing according to the ratio of CaO and SiO 2 contained in the iron oxide source and the first intermediate slag discharge rate D P1 .

[0054] 2. Melting step

[0055] The melting step of the low-phosphorus steel using a converter-type vessel in the present invention is as follows.

[0056] The hot metal that has been pre-desiliconized by an iron liquid pretreatment device such as TPC is charged into the converter. At this time, for the purpose of adjusting the hot metal temperature to about 1300°C and adjusting the Si concentration before blowing, or for the purpose of achieving the HMR (Hot Metal Ratio) determined by the production plan in the integrated ironmaking process, cold iron sources such as scrap iron are charged into the converter as needed.

[0057] Regarding the Si concentration before blowing, an iron liquid sample can be collected at the end of the iron liquid pretreatment in the previous stage such as TPC, and calculated by formula (1) based on the Si concentration before charging into the converter analyzed during the period before transporting the iron liquid to the converter and the amount of cold iron source added for temperature adjustment.

[0058] The Si concentration before blowing, which is the object of the present invention, is in the range of 0.3% by mass or more and 0.5% by mass or less. When the Si concentration before blowing is higher than 0.5% by mass, the amount of slag generated by the oxidation of Si during blowing increases. When the amount of slag is large, even if the dephosphorization ability of the slag is insufficient, due to the low P 2 O 5 concentration in the slag, dephosphorization of the molten steel is carried out, but this leads to an increase in the slag treatment cost and makes foaming likely to occur at an early stage. In addition, when the Si concentration before blowing is lower than 0.3% by mass, the amount of slag is insufficient, dephosphorization becomes insufficient, and in addition, intermediate slag removal becomes difficult. In order to set the Si concentration before blowing to 0.5% by mass or less, pre-desiliconization of hot metal outside the converter can be applied. In order to set the Si concentration before blowing to 0.3% by mass or more, it can be dealt with by reducing the amount of desiliconization in the pre-desiliconization of hot metal outside the converter or by reducing the amount of scrap iron charged into the converter.

[0059] "First Dephosphorization Process"

[0060] In the first dephosphorization process, as slag formers, a CaO source such as quicklime and limestone, an iron oxide source such as iron ore and sintered ore, and a MgO source such as dolomite are added to form slag. Auxiliary raw materials such as the CaO source, the iron oxide source, and the MgO source are added from above through the top charging hopper of the converter before the start of blowing. At this time, recycled slag products can also be added as needed, but as the slag at this time, recycled slag products after reduction refining (secondary refining) with a small P 2 O 5 content are preferably used. Regarding the addition amount of the CaO source added in the first dephosphorization process, it will be described in detail in the subsequent second dephosphorization process. Specifically, it is adjusted so that the C / S P2 of the slag in the second dephosphorization process becomes 0.8 or more and less than 1.5.

[0061] After adding the slag formers, while blowing a stirring gas from the bottom blowing tuyere 3 at the bottom of the converter, the top blowing lance 7 is lowered from the upper part of the converter, and a blowing treatment of blowing oxygen, that is, the first dephosphorization process, is carried out. During blowing, Si burns to generate SiO 2 , promoting slag formation and starting the oxidation of phosphorus. At a stage where dephosphorization has proceeded to a certain extent, the carbon in the hot metal reacts with FeO in the slag, and the slag foams. Blowing is stopped before the slag 12 flows out of the furnace mouth 2.

[0062] "First Intermediate Slag Removal Process"

[0063] After stopping the blowing, the top blowing lance 7 is quickly withdrawn from the converter, and the furnace body is tilted to perform the first intermediate slag discharging in which the foaming slag 12 is discharged from the converter mouth 2 into the slag discharging tank 5. At this time, if necessary, the slag 12 is sampled during the intermediate slag discharging to confirm the C / S P1 . In the present invention, since the addition of the CaO source in the second dephosphorization step is not performed, the slag remaining in the converter after the first intermediate slag discharging becomes the dephosphorization flux in the second dephosphorization step. In addition, the slag discharging rate in the first intermediate slag discharging step becomes necessary in terms of regulating the addition amount of the CaO source added in the first dephosphorization step. In the first intermediate slag discharging, by stopping the slag discharging at the stage when the molten iron starts to flow out of the converter mouth, a slag discharging rate of about 40% can be achieved.

[0064] "Second Dephosphorization Step"

[0065] After the first intermediate slag discharging, when the furnace body is raised to the vertical state, metal sampling and temperature measurement are performed as needed. Then, an iron oxide source is added to the molten iron.

[0066] After the first intermediate slag discharging is completed, the iron oxide source added in the second dephosphorization step is set to an amount such that the temperature drop ΔT calculated by the formula (4) is in the range of 15°C or more and 45°C or less, and the C / S of the slag in the second dephosphorization step described below is 0.8 or more and less than 1.5. The iron oxide source is added together before the start of the blowing treatment in the second dephosphorization step. The formula (4) is a method calculated by the method described in Non-Patent Document 1. According to Non-Patent Document 1, T 0 is 18.8°C (when 10 kg / ton of scrap iron is added, the temperature of the molten iron drops by 18.8°C), but since this value changes according to the amount of molten iron (ton / ch) in each treatment, in the present invention, it is calculated according to the formula (5) obtained from experience. In addition, the multiple iron oxide source brands are represented by the subscript i respectively. The cooling capacity SC' i of the iron oxide source brand i is a value representing the ratio of the iron oxide source addition amount required to lower the temperature of the molten iron by the same ΔT with scrap iron as the reference. In the formula (4), "Σ i " means the sum of all brands of iron oxide sources added. In the present invention, the SC' of each brand added as the iron oxide source can be calculated as 3.3 in the case of iron ore and 2.8 in the case of sintered ore. In addition, when adding brands other than iron ore and sintered ore as the iron oxide source, it is only necessary to add after calculating the SC' of the added brand in advance.

[0067] When the temperature drop ΔT caused by the addition of the iron oxide source is less than 15°C, the cooling effect of the molten iron accompanying the addition of the iron oxide source is insufficient, and it is impossible to prevent the stagnation of dephosphorization. On the other hand, when the temperature drop ΔT exceeds 45°C and is added, the melting time during the second dephosphorization process is insufficient, resulting in melting residue. In addition, the heat source in the decarburization process gradually becomes insufficient. Therefore, the temperature drop ΔT must be 45°C or less.

[0068] ΔT = -0.1×T 0 ×Σ i (SC’ i ×M i ) (4)

[0069] ΔT: Temperature change range (°C)

[0070] T 0 : Molten iron temperature drop (°C) when adding 10 kg / ton of scrap iron

[0071] SC’ i : Cooling capacity of iron oxide source brand i (-)

[0072] M i : Addition rate of iron oxide source brand i (kg / ton)

[0073] T 0 = -0.0304×W + 29.45 (5)

[0074] W: Molten iron volume (ton / ch)

[0075] After that, in the second dephosphorization process, the top-blowing lance 7 is lowered from the furnace mouth 2 at the upper part of the converter in the same manner as in the first dephosphorization process, and the blowing treatment of blowing oxygen, that is, the second dephosphorization treatment, is carried out. In the second dephosphorization process, since blowing is carried out in the existing slag, the slag foams in a short time compared with the first dephosphorization process, and the blowing is stopped before the slag flows out of the furnace.

[0076] The C / S in the slag during the second dephosphorization treatment in the second dephosphorization process P2 Must be 0.8 or more and less than 1.5. In the case of C / S P2 Less than 0.8, since the dephosphorization ability of the slag is insufficient, it is difficult to smelt low-phosphorus steel. On the other hand, in the case of C / S P2 Is 1.5 or more, although the dephosphorization ability is satisfied, as the added CaO source increases, the treatment cost due to slag treatment and the like increases, and there is a tendency for the slag dischargeability to decrease. Therefore, the C / S in the slag P2 Must be 0.8 or more and less than 1.5.

[0077] In the present invention, no CaO source is added in the second dephosphorization step. Therefore, as described above, the C / S ratio in the slag during the second dephosphorization step is P2 It can be calculated by the above formula (3). As shown in formula (3), C / S P2 SiO produced by oxidation of Si in the molten iron 2 amount, the amount of CaO in the auxiliary material added in the first dephosphorization step, the amount of SiO in the auxiliary material added in the first dephosphorization step 2 amount, the first intermediate slag discharge rate, the amount of CaO in the iron oxide source added in the second dephosphorization step, and the amount of SiO 2 Therefore, the first intermediate slag removal rate is predicted based on past performance, and the amount of iron oxide source added in the second dephosphorization step is predetermined in advance so that the temperature drop range ΔT is within the range of 15°C or more and 45°C or less. The C / S ratio calculated by formula (3) is calculated. P2 The amount of CaO in the auxiliary material added in the first dephosphorization step may be specified so as to be 0.8 or more and less than 1.5.

[0078] 《Second intermediate slag removal process》

[0079] After blowing is stopped, the top-blowing lance 7 is quickly pulled out of the converter, and the furnace body is tilted to perform the second intermediate slag discharge to discharge the foamed slag from the converter furnace port 2 into the slag discharge pot 5. At this time, the slag is sampled during the intermediate slag discharge to check the C / S as needed.

[0080] 《Decarbonization process》

[0081] After the second intermediate slag discharge, metal sampling and temperature measurement are performed as needed when the furnace body is raised to a vertical state. After that, auxiliary raw materials represented by CaO-based slag-making agents are added, and the lance is lowered from the upper part of the converter to perform blowing treatment with oxygen, i.e., the decarburization process. In the decarburization process, the temperature of the molten steel rises mainly through the combustion of carbon, and dephosphorization in the molten iron is also performed. Blowing is continued until the temperature and carbon concentration of the molten steel are within the specified range.

[0082] Steelmaking process

[0083] After blowing is completed, the converter is tilted so that the molten steel can be tapped from the tapping hole 4 into the ladle 6 .

[0084] 3. How to confirm the effect

[0085] The effects of the invention are confirmed by analyzing metal and slag samples collected at the moments after the first and second dephosphorization processes and comparing the amount of phosphorus introduced in the decarburization process calculated by mass balance. When the C / S and the amount of slag during decarburization blowing are set to certain conditions, the phosphorus concentration in the molten steel at the end of decarburization blowing depends on the amount of phosphorus in the system at the start of decarburization blowing, that is, the amount of introduced phosphorus. Therefore, the amount of introduced phosphorus is used as an index.

[0086] Examples

[0087] The molten iron tapped from the blast furnace is subjected to molten iron pretreatment using TPC. After adjusting the Si concentration in the molten iron (Si concentration before charging into the converter), it is transferred to the converter. Using one converter-type vessel 1, Figure 1 The first dephosphorization process, the first intermediate slag removal process, the second dephosphorization process, the second intermediate slag removal process, and the decarburization treatment process are continuously carried out in the process shown in

[0088] The phosphorus concentration in the molten iron before the first dephosphorization process varies slightly depending on the amount of scrap iron added, but in this example, it is 0.13 mass% or more and 0.14 mass% or less and approximately the same concentration. In addition, the temperature of the molten iron before the first dephosphorization process after adding cold iron sources such as scrap iron is in the range of 1300°C to 1310°C, and the amount of molten iron after melting and mixing the cold iron sources is 420 t.

[0089] In Table 1, inventive examples and comparative examples are shown. Nos. 1 to 11 in Table 1 are examples of the present invention that meet the conditions shown in the claims, and Nos. 12 to 18 are comparative examples. In Table 1, values that deviate from the scope of the present invention are underlined.

[0090]

[0091] The Si concentration before blowing in Table 1 is the Si concentration calculated by formula (1) considering the addition of cold iron sources such as scrap iron in the converter after desiliconization using TPC.

[0092] In the first dephosphorization process, bottom blowing is carried out from the bottom blowing tuyeres 3 of the converter-type vessel 1 (hereinafter simply referred to as "converter") containing the molten iron 11 and the slag 12, and top blowing is carried out by inserting the top blowing lance 7 into the furnace from the furnace mouth 2. On the basis of assuming the slag discharge rate in the first intermediate slag removal process and assuming the amount of iron oxide source added in the second dephosphorization process as shown below, according to the C / S after the second dephosphorization process calculated by formula (3) P2In the manner of becoming a specified value, the addition amount of the auxiliary raw material containing CaO in the first dephosphorization step is specified and added. Oxygen is blown from above through the top-blown lance 7, and an inert gas is blown in from the bottom tuyeres 3 at the bottom of the converter to stir the molten iron 11 and the slag 12. The blowing speed of oxygen and the bottom-blowing conditions are all the same. Desiliconization and dephosphorization are carried out, and oxygen supply is stopped before the slag in the converter foams and overflows from the furnace mouth 2 at the upper part of the converter. After pulling out the top-blown lance 7, the furnace body is immediately tilted to discharge the foamed slag 12 from the furnace mouth 2 into the slag pot 5 (the first intermediate slag-discharging step). The slag discharging is carried out until just before the molten iron 11 is about to be discharged from the furnace mouth 2 of the converter. The foaming and sedimentation conditions of the slag 12 in the converter are affected by the C / S of the slag 12 P1 and are affected by the C / S of the slag 12 P1 Under the condition that the C / S is 1.3 or less, the slag-discharging rate is 40%, but under the condition that the C / S P1 exceeds 1.3, the slag-discharging rate is 35%. The slag-discharging rate in the first intermediate slag-discharging step required for the calculation in the above formula (3) can be assumed based on such past operation results.

[0093] When calculating the actual slag-discharging rate D P1 (%) in the first intermediate slag-discharging step and the actual slag-discharging rate D P2 (%) in the second intermediate slag-discharging step, the amount of slag discharged is obtained by weighing the amount of slag discharged into the slag pot. In addition, the amount of slag before slag discharging is assumed to be calculated from the addition amount of the auxiliary raw material as all the Si in the molten iron becomes SiO 2 through oxidative refining (the first dephosphorization step).

[0094] In addition, in the second dephosphorization step, an iron oxide source is added as an auxiliary raw material in such a manner that the temperature change range calculated by formula (4) becomes the temperature drop amount caused by the addition of iron oxide in Table 2. Here, T calculated by formula (5) 0 is 16.7 °C, and the cooling capacity of each iron oxide source is used with the above values. In addition, the addition amount of this iron oxide source is calculated before the first dephosphorization step and is used for the calculation of the above formula (3).

[0095] After adding the iron oxide source before blowing in the second dephosphorization step, oxygen is blown from above through the top-blown lance 7, and an inert gas is blown in from the bottom tuyeres 3 at the bottom of the converter to stir the molten iron 11 and the slag 12. The blowing speed of oxygen and the bottom-blowing conditions are all the same. The dephosphorization reaction is carried out, and oxygen supply is stopped before the slag in the converter foams and overflows from the furnace mouth 2 at the upper part of the converter. After pulling out the top-blown lance 7, the furnace body is tilted to discharge the foamed slag 12 into the slag pot 5 (the second intermediate slag-discharging step).

[0096] After that, according to the C / S after the decarburization step CAfter adding the auxiliary raw materials in such a way as to become a specified value, oxygen is blown from above through the top-blown lance 7 (decarbonization process), and in the tapping process after the decarbonization process, the molten steel is tapped from the tapping hole 4 into the ladle 6.

[0097] P in the hot metal in Table 1 P1 (%) and P in the hot metal P2 (%) are respectively indicators of the phosphorus content in the hot metal after the end of the first dephosphorization treatment and after the end of the second dephosphorization. It is the total amount of phosphorus in the hot metal of No. 12 at the stage when the first slag removal process ends and the phosphorus in the slag (system P P1 ) normalized to 100%. No. 12 is a comparative example where no iron oxide source was added at the start of the second dephosphorization process and dephosphorization stagnation occurred during the second dephosphorization process, and it serves as a benchmark when judging the effects of the present invention.

[0098] The phosphorus input amount is the value obtained by adding P in the hot metal after the second dephosphorization process P2 and the phosphorus in the slag after the second intermediate slag removal process. In addition, under each condition, with the benchmark condition of No. 12 as the reference (= 100%), the treatment cost including auxiliary raw materials and slag treatment is calculated. A treatment cost of 120% or less is set as ○, and a treatment cost exceeding 120% is set as ×, as shown in Table 1.

[0099] In the present invention, the condition where the phosphorus input amount in the decarbonization process is 74% or less and the treatment cost is 120% or less is judged to have the effects of the invention.

[0100] No. 1 to 11 are invention examples, all of which satisfy the conditions of the present invention. The phosphorus input amount is reduced to 74% or less, and the treatment cost is also 80% - 115% of No. 12, which is the benchmark, without an increase.

[0101] No. 12 to 18 are comparative examples.

[0102] For No. 12 and No. 17, the Si concentration before blowing is 0.5% and the set C / S P1 is set to 1.1 or 1.4. However, no iron oxide source was added in the second dephosphorization process, and the phosphorus input amount was not reduced to 74% or less.

[0103] For No. 13, the Si concentration before blowing is 0.5% and the set C / S P1 is set to 1.1. An iron oxide source was added in the second dephosphorization process in such a way that the temperature drops by 10°C, but the phosphorus input amount was not reduced to 74% or less.

[0104] No.14 is the condition where an iron oxide source is added in the second dephosphorization process in such a way that the temperature drops to 50°C. However, under this condition, melting residue of auxiliary materials occurred after the second dephosphorization process, so it is set outside the scope of the present invention.

[0105] No.15 is the condition where the Si concentration before blowing is 0.6% and the set C / S in the first dephosphorization process P1 is set to 1.1, and an iron oxide source is added before the second dephosphorization process in such a way that the temperature drops to 20°C. However, under this condition, the amount of slag in the first dephosphorization process is large, foaming starts early, and the amount of phosphorus carried in is not reduced to less than 74%.

[0106] No.16 is the condition where the Si concentration before blowing is 0.25% and the set C / S in the first dephosphorization process P1 is set to 1.1, and an iron oxide source is added before the second dephosphorization process in such a way that the temperature drops to 20°C. However, the amount of slag is small. Although a large amount of oxygen can be supplied before the start of foaming, since the amount of phosphorus that the slag can absorb is insufficient, the amount of phosphorus carried in is not reduced to less than 74%.

[0107] No.18 is the condition where the Si concentration before blowing is 0.5% and the set C / S in the first dephosphorization process P1 is set to 1.5. However, due to the high C / S of the slag P1 dephosphorization is carried out. Even if an iron oxide source is not added before the second dephosphorization process, the amount of phosphorus carried in is reduced to less than 74%. However, in order to adjust the C / S P1 to 1.5, the addition amount of auxiliary materials and the amount of slag generated are large, and the processing cost increases to 125% of No.12 used as a reference.

[0108] From the above, it is known that by adding an iron oxide source in the second dephosphorization process within the scope of the present invention, low-phosphorus steel can be smelted.

[0109] Explanation of reference numerals

[0110] 1: Converter-type vessel, 2: Furnace mouth, 3: Bottom blowing tuyere, 4: Tapping hole, 5: Slag discharge tank, 6: Ladle, 7: Top blowing lance, 11: Hot metal, 12: Slag.

Claims

1. A method for smelting steel, which continuously performs the following steps: First dephosphorization step: performing a first dephosphorization treatment on the molten iron charged into a converter-type container and having a Si concentration of 0.30 mass% or more and 0.50 mass% or less diluted by a cold iron source; First intermediate slag discharge step: after the first dephosphorization step, discharging a part of the slag in the converter-type container; Second dephosphorization step: After the first intermediate slag removal step, an iron oxide source in an amount that reduces the temperature of the molten iron in the converter-type vessel by 15 °C or more and 45 °C or less is added, and the mass ratio CaO / SiO of the CaO concentration to the SiO concentration in the slag in the converter-type vessel is set to 0.8 or more and less than 1.5, thereby performing the second dephosphorization treatment of the molten iron; 2 concentration, CaO / SiO 2 is set to 0.8 or more and less than 1.5; Second intermediate slag discharge step: after the second dephosphorization step, discharging a part of the slag in the converter-type container; and Decarburization step: after the second intermediate slag discharge step, performing a decarburization treatment on the molten iron in the converter-type container.

2. The method for smelting steel according to claim 1, wherein, in the second dephosphorization step, an iron oxide source in an amount that reduces the temperature of the molten iron in the converter-type container by 15°C or more and 25°C or less is added.

3. The method for smelting steel according to claim 1, wherein, in the second dephosphorization step, the iron oxide source is added together before the start of blowing.

4. The method for smelting steel according to any one of claims 1 to 3, wherein, the iron oxide source contains iron ore or sinter.

5. The method for smelting steel according to claim 1, wherein, Set the mass ratio CaO / SiO in the second dephosphorization process 2 to be not less than 1.0 and not more than 1.

2.

6. The method for smelting steel according to claim 1 or claim 5, wherein, Based on the mass ratio CaO / SiO in the second dephosphorization step 2 to specify the amount of CaO in the auxiliary raw materials added in the first dephosphorization step.

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