Dephosphorization method

Abstract

To provide a dephosphorization method having excellent dephosphorization efficiency.SOLUTION: A dephosphorization method according to the present disclosure includes: a preparation step (S1) of preparing a dephosphorization vessel (2) containing a molten iron (3); and a blowing step (S2) of introducing a dephosphorization agent into the molten iron (3) contained in the dephosphorization vessel (2) while blowing oxygen into the molten iron. The dephosphorization agent comprises: one or more selected from CaO, CaCO3, and Ca(OH)2; and Al2O3, and further satisfies equation (1):0.24≤Al2O3 / (α+Al2O3)≤0.43. In the blowing step, when the blowing of oxygen is completed, the temperature of the molten iron (3) is 1250 to 1280°C. In the equation (1), Al2O3 is substituted with the Al2O3 content in the dephosphorization agent in mass%, and α in the equation (1) is defined by the equation (2):α=CaO+56.1 / 100.1×CaCO3+56.1 / 74.1×Ca(OH)2.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a method for dephosphorizing hot metal. [Background technology]

[0002] In the preliminary treatment of molten pig iron, desulfurization is performed to reduce the sulfur concentration of the molten pig iron, and dephosphorization is performed to reduce the phosphorus concentration of the molten pig iron. This is followed by decarburization to reduce the carbon concentration of the molten pig iron after the dephosphorization process. The molten pig iron that has undergone the decarburization process is tapped off as molten steel and undergoes secondary refining.

[0003] In recent years, there has been a demand for high-purity molten steel with reduced impurity concentrations. In this case, it is preferable to reduce the sulfur and phosphorus concentrations in the pretreatment process of the molten iron. In this context, there has been a demand for a dephosphorization method with improved dephosphorization efficiency.

[0004] So far, dephosphorization methods with improved dephosphorization efficiency have been proposed. Specifically, JP 2010-138443 A (Patent Document 1), JP 2017-171975 A (Patent Document 2), and WO 2018 / 135351 A (Patent Document 3) propose dephosphorization methods with improved dephosphorization efficiency.

[0005] The dephosphorization method disclosed in Patent Document 1 involves adding an oxygen source, a CaO-based dephosphorization agent, and a substance containing a non-oxide-based silicon compound and / or carbon to molten iron contained in a reaction vessel, and carrying out a dephosphorization treatment on the molten iron. Patent Document 1 discloses that this dephosphorization method can increase the thermal margin of the molten iron and furthermore, can carry out the dephosphorization treatment efficiently.

[0006] The dephosphorization method disclosed in Patent Document 2 uses CaO and Fe 2 O 3 The main components are SiO 2 The concentration is 2 to 10 mass%, and (SiO 2In this method, a dephosphorization agent is used that is made of calcium ferrite, in which the ratio of (CaO) / (CaO) satisfies a range of 0.05 to 0.3, with the remainder being unavoidable impurities. Patent Document 2 discloses that this dephosphorization method makes it possible to efficiently produce low-phosphorus steel even with short-time blowing.

[0007] The dephosphorization method disclosed in Patent Document 3 is carried out by adding a dephosphorization agent to molten iron. The dephosphorization agent has a pore size of 0.1 to 2.0 μm, the sum of all pore volumes of which is 0.1 mL / g or more, and is R-CO 2 The dephosphorization method includes 1 mass % or more of a lime-based dephosphorization agent. Patent Document 3 discloses that this dephosphorization method can improve the dephosphorization efficiency. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2010-138443 A [Patent Document 2] JP 2017-171975 A [Patent Document 3] International Publication No. 2018 / 135351 Summary of the Invention [Problem to be solved by the invention]

[0009] Patent Documents 1 to 3 all propose dephosphorization methods with improved dephosphorization efficiency. On the other hand, the dephosphorization efficiency may be improved by techniques other than those disclosed in Patent Documents 1 to 3.

[0010] An object of the present disclosure is to provide a dephosphorization process having excellent dephosphorization efficiency. [Means for solving the problem]

[0011] The dephosphorization method according to the present disclosure comprises: A method for dephosphorizing molten iron, comprising the steps of: a preparation step of preparing a dephosphorization vessel containing the molten iron; a blowing step of introducing a dephosphorization agent while blowing oxygen into the molten iron contained in the dephosphorization vessel, The dephosphorization agent is CaO, CaCO 3 , and Ca(OH) 2 and one or more selected from the group consisting of Al 2 O 3 and further satisfying formula (1), In the blowing step, the temperature of the molten iron is 1250 to 1280°C when the blowing of oxygen is completed. 0.24≦Al 2 O 3 / (α+Al 2 O 3 )≦0.43 (1) Here, Al in formula (1) 2 O 3 The Al content in the dephosphorization agent is 2 O 3 The content is substituted in units of mass %, and α in formula (1) is defined by formula (2). α = CaO + 56.1 / 100.1 × CaCO 3 +56.1 / 74.1×Ca(OH) 2 (2) In the formula (2), the CaO content in the dephosphorization agent is substituted in mass%, and CaCO 3 The CaCO 3 The content is entered in units of mass%, Ca(OH) 2 The Ca(OH) in the dephosphorization agent 2 The content is entered in units of mass%. Effect of the Invention

[0012] The dephosphorization method according to the present disclosure has excellent dephosphorization efficiency. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a flow diagram showing the steps of the molten iron dephosphorization method according to the present embodiment. [Diagram 2]FIG. 2 is a schematic diagram showing an example of the blowing process in which oxygen is blown into molten iron contained in a dephosphorization vessel. [Diagram 3] FIG. 3 is a schematic diagram showing another example of the blowing process in which oxygen is blown into molten iron contained in a dephosphorization vessel. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The present inventors have conducted various investigations into a dephosphorization method having excellent dephosphorization efficiency, and have obtained the following findings.

[0015] A method for dephosphorizing molten pig iron using calcium oxide (CaO) has been known. In the method for dephosphorizing molten pig iron using CaO, a dephosphorizing agent is introduced into a dephosphorization vessel containing the molten pig iron, and CaO is contained in the slag. In addition, a blowing process is performed in which oxygen is blown into the molten pig iron. In the blowing process, phosphorus (P) in the molten pig iron is oxidized and then fixed as CaO in the slag. In this way, the P concentration in the molten pig iron is reduced. Specifically, the dephosphorization reaction of molten pig iron using CaO proceeds according to the following chemical reaction formula:

[0016]

number

[0017] Referring to formulas (A) and (B), first, phosphorus [P] in the molten iron is oxidized by iron oxide (FeO) in the slag. The phosphorus oxide (P 2 O 5 ) is fixed to calcium oxide (CaO) in the slag, forming a stable compound (P 2 O 5 ·CaO) is formed in the slag. As a result, the phosphorus concentration in the molten iron can be reduced.

[0018] The dephosphorization agent for making the slag contain CaO may contain CaO or a CaO source other than CaO. In this specification, calcium oxide (CaO), calcium carbonate (CaCO 3 ), and calcium hydroxide (Ca(OH) 2 ) are also collectively called "CaO source". Since the inside of the dephosphorization vessel containing the molten iron is at high temperature, CaCO 3 When CaCO is included, 3 is decarbonated to form CaO. Similarly, Ca(OH) 2 If Ca(OH) is included, 2 is dehydrated and CaO is formed.

[0019] Here, both of the above chemical reaction formulas (A) and (B) proceed in the liquid phase. On the other hand, CaO has a melting point of over 2000°C, and there is a concern that it may not melt sufficiently at the temperature of the molten pig iron. Here, the ratio of CaO that melts and becomes part of the slag to the CaO introduced by the CaO source is called the "CaO slag ratio." The higher the CaO slag ratio, the more CaO contributes to the reaction, and the more efficiently the dephosphorization reaction proceeds. From this, it can be considered that the higher the temperature of the molten pig iron in the dephosphorization vessel, the higher the CaO slag ratio and the more efficiently the dephosphorization reaction proceeds.

[0020] Here, it is known that the distribution ratio of P in molten iron and P in slag can be expressed by the following equation, called the 'Ogawa equation'.

[0021]

number

[0022] With reference to formulas (C) and (D), the distribution ratio of the P concentration is affected by various factors. With reference to formula (C), the higher the molten iron temperature T, the smaller the P concentration ratio in the slag. In other words, in order to increase the dephosphorization efficiency, it is effective to lower the molten iron temperature T.

[0023] However, as mentioned above, if the hot metal temperature T is decreased, there is a concern that the CaO slag conversion rate will decrease and the dephosphorization efficiency will decrease. Therefore, the inventors of the present invention have investigated a method for increasing the CaO slag conversion rate while maintaining the hot metal temperature T low. As a result, it was found that the dephosphorization agent contains not only a CaO source but also Al. 2 O 3 It was discovered that by adding Al to the dephosphorization agent to lower the melting point of CaO, the CaO slag conversion rate can be increased while maintaining the molten iron temperature T low. 2 O 3If it contains CaO-Al, which has a lower melting point than CaO, 2 O 3 As a result, the CaO slag conversion rate can be increased. On the other hand, referring to formula (D), the Al content in the dephosphorization agent is 2 O 3 If the ratio is increased, the term (%X) in formula (D) increases. As a result, the CaO concentration (CaO activity) in the slag that contributes to the chemical reaction decreases, and there is a concern that the P concentration ratio in the slag will decrease.

[0024] In other words, simply lowering the hot metal temperature T may increase the dephosphorization efficiency, but it may also decrease the dephosphorization efficiency due to a decrease in the CaO slag conversion rate. Also, simply increasing the hot metal temperature T may increase the CaO slag conversion rate, but the P concentration ratio in the slag may decrease, resulting in a decrease in dephosphorization efficiency. Similarly, when Al is added to the dephosphorization agent, 2 O 3 If the dephosphorization agent contains Al, the CaO slag conversion rate will increase, and the dephosphorization efficiency will increase. 2 O 3 If the ratio is simply increased, there is a concern that the CaO activity will decrease and the P concentration ratio in the slag will become smaller.

[0025] Therefore, the inventors have investigated the Al content of the dephosphorization agent. 2 O 3 In this study, we focused on the ratio of P in the slag and the molten iron temperature T to improve the dephosphorization efficiency. Specifically, we investigated the relationship between the molten iron temperature T at which the P concentration ratio in the slag can be increased and the Al content in the dephosphorization agent at which a high CaO slag conversion rate can be obtained. 2 O 3 As a result of detailed investigations by the present inventors, it was found that the Al ratio of the dephosphorization agent is 2 O 3 It was revealed that if the ratio satisfies the following formula (1), the CaO slag conversion rate can be maintained even if the molten iron temperature T at the end of blowing is reduced to 1250-1280°C. 0.24≦Al 2 O 3 / (α+Al 2 O 3 )≦0.43 (1) Here, Al in formula (1) 2 O 3 The Al content in the dephosphorization agent is 2 O 3 The content is substituted in units of mass %, and α in formula (1) is defined by formula (2). α = CaO + 56.1 / 100.1 × CaCO 3 +56.1 / 74.1×Ca(OH) 2 (2) In formula (2), the CaO content in the dephosphorization agent is substituted in mass%, and CaCO 3 In the dephosphorization agent, CaCO 3 The content is entered in units of mass%, Ca(OH) 2 The Ca(OH) in the dephosphorization agent 2 The content is entered in units of mass%.

[0026] Here, Fn1=Al 2 O 3 / (α+Al 2 O 3 Fn1 is the amount of CaO source and Al in the dephosphorization agent. 2 O 3 Al relative to the total amount 2 O 3 It means the ratio of. If Fn1 is too low, the CaO slag conversion rate cannot be increased sufficiently, and the dephosphorization efficiency cannot be increased sufficiently. On the other hand, if Fn1 is too high, the decrease in CaO activity becomes apparent, and the P concentration ratio in the slag becomes smaller. In this case, the dephosphorization efficiency cannot be increased sufficiently. Therefore, in this embodiment, Fn1 is set to 0.24 to 0.43. In this case, even if the molten pig iron temperature T at the end of blowing is lowered to 1250 to 1280°C, the CaO slag conversion rate can be increased sufficiently. As a result, the dephosphorization method according to this embodiment can achieve both a low molten pig iron temperature T and a high CaO slag conversion rate, and has excellent dephosphorization efficiency.

[0027] The gist of the dephosphorization method according to the present embodiment, which was completed based on the above findings, is as follows.

[0028] [1] A method for dephosphorizing molten iron, comprising the steps of: a preparation step of preparing a dephosphorization vessel containing the molten iron; a blowing step of introducing a dephosphorization agent while blowing oxygen into the molten iron contained in the dephosphorization vessel, The dephosphorization agent is CaO, CaCO 3 , and Ca(OH) 2 and one or more selected from the group consisting of Al 2 O 3 and further satisfying formula (1), In the blowing step, when the blowing of oxygen is completed, the temperature of the molten iron is 1250 to 1280°C. Dephosphorization method. 0.24≦Al 2 O 3 / (α+Al 2 O 3 )≦0.43 (1) Here, Al in formula (1) 2 O 3 The Al content in the dephosphorization agent is 2 O 3 The content is substituted in units of mass %, and α in formula (1) is defined by formula (2). α = CaO + 56.1 / 100.1 × CaCO 3 +56.1 / 74.1×Ca(OH) 2 (2) In the formula (2), the CaO content in the dephosphorization agent is substituted in mass%, and CaCO 3 The CaCO 3 The content is entered in units of mass%, Ca(OH) 2 The Ca(OH) in the dephosphorization agent 2 The content is entered in units of mass%.

[0029] [2] The dephosphorization method according to [1], The dephosphorization agent is CaCO 3 Contains Dephosphorization method.

[0030] [3] The dephosphorization method according to [1] or [2], In the preparation step, iron oxide is introduced into the dephosphorization vessel. Dephosphorization method.

[0031] The dephosphorization method according to the present embodiment will be described in detail below.

[0032] [Dephosphorization method] Fig. 1 is a flow diagram showing steps of a molten iron dephosphorization method according to the present embodiment. With reference to Fig. 1, the dephosphorization method according to the present embodiment includes a preparation step (S1) and a blowing step (S2). Each step will be described below.

[0033] [Preparation process (S1)] In the preparation step (S1), a dephosphorization vessel containing molten iron is prepared. In the dephosphorization method according to the present embodiment, the dephosphorization vessel is not particularly limited, and a well-known vessel can be used. The dephosphorization vessel may be, for example, a converter, a torpedo car, or a ladle. Note that when the blowing step (S2) described later is performed in a converter, the dephosphorization vessel is a converter.

[0034] [Molt metal] In the dephosphorization method according to the present embodiment, the molten pig iron to be used is not particularly limited. The molten pig iron may have a chemical composition corresponding to carbon steel, a chemical composition corresponding to stainless steel, or a chemical composition corresponding to Ni-based alloy. When the molten pig iron has a chemical composition corresponding to stainless steel, the stainless steel may be, for example, martensitic stainless steel, ferritic stainless steel, ferritic-martensite duplex stainless steel, ferritic-austenite duplex stainless steel, or austenitic stainless steel.

[0035] The molten pig iron on which the dephosphorization method according to the present embodiment is carried out may be subjected to a desulfurization treatment to reduce or remove sulfur (S) in the molten pig iron. The desulfurization reaction is more likely to proceed at high temperatures than the dephosphorization reaction. Moreover, the dephosphorization method according to the present embodiment is carried out at a low temperature as described above. Therefore, it is preferable that the molten pig iron on which the dephosphorization method according to the present embodiment is carried out is subjected to a desulfurization treatment in advance.

[0036] [Blowing process (S2)] In the blowing step (S2), a dephosphorization agent is introduced while blowing oxygen into the molten pig iron contained in the dephosphorization vessel. FIG. 2 is a schematic diagram showing an example of the blowing step (S2) in which oxygen is blown into the molten pig iron 3 contained in the dephosphorization vessel 2. FIG. 2 shows the blowing step (S2) when the dephosphorization vessel 2 is a converter. Referring to FIG. 2, the dephosphorization apparatus 1 includes the dephosphorization vessel 2. The dephosphorization vessel 2 has an opening at an upper portion. The dephosphorization vessel 2 further contains molten pig iron 3 therein, and slag 4 is contained above the molten pig iron 3.

[0037] Referring further to Fig. 2, the dephosphorization apparatus 1 further includes a lance 5 capable of blowing oxygen into the molten pig iron 3 from above the dephosphorization vessel 2. The gas blown from the lance 5 only needs to contain oxygen, and may contain an inert gas other than oxygen. The method of blowing oxygen from the lance 5 is not particularly limited, and may be a well-known method. For example, a gas containing oxygen may be jet-injected from the lance 5 to form a slope on the bath surface of the molten pig iron 3 as shown in Fig. 2.

[0038] Further referring to Fig. 2, the blowing process (S2) includes a region (hot spot) 6 where oxygen blown from a lance 5 is absorbed by the molten pig iron 3. At the hot spot 6, iron (Fe) in the molten pig iron 3 is oxidized to form iron oxide (FeO). Since the oxidation reaction of Fe is an exothermic reaction, the temperature at the hot spot 6 becomes higher than that at other regions of the molten pig iron 3.

[0039] [Dephosphorization agent] In the blowing step (S2), a dephosphorizing agent is introduced into the molten pig iron 3. There is no particular limitation on the method of introducing the dephosphorizing agent. For example, the dephosphorizing agent may be introduced through the lance 5, or through a dispenser installed on the lance 5. When the dephosphorizing agent is introduced into the molten pig iron 3 from the vicinity of the lance 5, the dephosphorizing agent is efficiently introduced to the hot point 6 having a high temperature, and CaO formed from the dephosphorizing agent is more likely to be turned into slag. In addition, the dephosphorizing agent is preferably in the form of a powder. If the dephosphorizing agent is in the form of a fine powder, CaO formed from the dephosphorizing agent is more likely to be turned into slag.

[0040] The dephosphorization agent according to the present embodiment is a CaO source and Al 2 O 3 As described above, in this specification, the term "CaO source" refers to CaO, CaCO 3 , and Ca(OH) 2 In other words, dephosphorization agents are CaO, CaCO 3 , and Ca(OH) 2 and one or more selected from the group consisting of Al 2 O 3 Each of the CaO sources supplies CaO to the slag 4 when introduced into the molten iron 3. 3 When CaCO is included, 3 is decarbonated to form CaO. Similarly, Ca(OH) 2 If Ca(OH) is included, 2 is dehydrated and CaO is formed.

[0041] Here, the dephosphorization agent is CaCO 3 In this case, since the decarbonation reaction is an endothermic reaction, the hot metal temperature T drops locally in the area near where the dephosphorization agent is introduced. As a result, in the area where the hot metal temperature T drops, the phosphorus equilibrium between the slag 4 and the hot metal 3 changes to increase the phosphorus on the slag 4 side, further increasing the dephosphorization efficiency. Therefore, the dephosphorization agent should preferably contain CaCO 3 More preferably, the CaO source contained in the dephosphorization agent is CaCO 3and impurities.

[0042] The dephosphorization agent according to the present embodiment is a CaO source and Al 2 O 3 The Al contained in the dephosphorization agent 2 O 3 is not particularly limited, Al 2 O 3 In other words, materials with high Al content can be used as dephosphorization agents. 2 O 3 If we could supply pure Al, 2 O 3 Materials other than Al may be used. 2 O 3 For example, bauxite may be used as the 2 O 3 Slag with a high CaO content may be used. 2 O 3 Each of these powders is preferably in the form of a powder, and a mixture of these powders is preferably used as the dephosphorization agent.

[0043] The dephosphorization agent according to the present embodiment is CaO, CaCO 3 , and Ca(OH) 2 and one or more selected from the group consisting of Al 2 O 3 and further satisfying formula (1). 0.24≦Al 2 O 3 / (α+Al 2 O 3 )≦0.43 (1) Here, Al in formula (1) 2 O 3 The Al content in the dephosphorization agent is 2 O 3 The content is substituted in units of mass %, and α in formula (1) is defined by formula (2). α = CaO + 56.1 / 100.1 × CaCO 3 +56.1 / 74.1×Ca(OH) 2 (2) The CaO in formula (2) is the CaO content in the dephosphorization agent, CaCO 3The CaCO 3 The content is Ca(OH) 2 The Ca(OH) in the dephosphorization agent 2 The contents are entered in units of mass%.

[0044] Fn1 (=Al 2 O 3 / (α+Al 2 O 3 )) is the CaO source and Al in the dephosphorization agent. 2 O 3 Al relative to the total amount 2 O 3 In other words, Fn1 is an index of the CaO slag conversion rate. If Fn1 is too low, the CaO slag conversion rate cannot be increased sufficiently, and the dephosphorization efficiency cannot be increased sufficiently. On the other hand, if Fn1 is too high, the decrease in CaO activity becomes apparent, and the P concentration ratio in the slag decreases instead. In this case, the dephosphorization efficiency cannot be increased sufficiently.

[0045] Therefore, in this embodiment, Fn1 is set to 0.24 to 0.43. In this case, even if the molten iron temperature T at the end of blowing is lowered to 1250 to 1280°C, the CaO slag conversion rate can be sufficiently increased. As a result, the dephosphorization method according to this embodiment can achieve both a low molten iron temperature T and a high CaO slag conversion rate, and has excellent dephosphorization efficiency. The lower limit of Fn1 is preferably 0.25, more preferably 0.27, more preferably 0.30, and even more preferably 0.32. The upper limit of Fn1 is preferably 0.42, more preferably 0.40, and even more preferably 0.39.

[0046] Referring to formula (2), α = CaO + 56.1 / 100.1 × CaCO 3 +56.1 / 74.1×Ca(OH) 2 is a formula for converting the content of the CaO source into the formula weight of CaO. Here, CaO, CaCO 3 , and Ca(OH) 2 If it is not included, "0" is assigned.

[0047] As described above, the dephosphorization agent according to the present embodiment contains a CaO source and Al 2 O 3 In other words, the dephosphorization agent according to the present embodiment contains a CaO source and Al 2 O 3 The dephosphorization agent according to the present embodiment may contain a CaO source and an Al source. 2 O 3 The impurities in the dephosphorization agent are the CaO source and Al 2 O 3 The dephosphorization agent according to the present embodiment is preferably a mixture of a CaO source and an Al source. 2 O 3 The total content of the dephosphorization agent is 85 mass % or more.

[0048] In this embodiment, the CaO source and Al in the dephosphorization agent 2 O 3 The Ca and Al contents in 0.1 g of the dephosphorization agent can be obtained by the following method. Specifically, 0.1 g of the dephosphorization agent and lithium borate powder are mixed and stirred in an electric furnace. The stirred mixture is dissolved in a 10% hydrochloric acid solution to obtain a solution. The obtained solution is subjected to elemental analysis by ICP atomic emission spectrometry to obtain the Ca and Al contents in 0.1 g of the dephosphorization agent. Furthermore, the Ca and Al are separated into CaO and Al in the dephosphorization agent. 2 O 3 Assuming that the CaO and Al in the dephosphorization agent are present as CaO and Al, the CaO and Al in the dephosphorization agent are calculated from the Ca and Al contents obtained. 2 O 3 The content is calculated in mass%.

[0049] As described above, in the blowing step (S2), the dephosphorization agent is introduced while blowing oxygen. That is, the introduction of the dephosphorization agent into the molten pig iron 3 and the blowing of oxygen are carried out simultaneously. However, the introduction of the dephosphorization agent into the molten pig iron 3 may be carried out continuously or intermittently. That is, there may be a time in the blowing step (S2) when the dephosphorization agent is not introduced and oxygen is blown.

[0050] [Hot metal temperature T] In the blowing step (S2), when the blowing of oxygen is completed, the temperature of the molten pig iron 3 is 1250 to 1280°C. Here, the temperature of the molten pig iron 3 at the end of the blowing of oxygen in the blowing step (S2) is also referred to as the "molten pig iron temperature T". If the molten pig iron temperature T is too high, as shown in formula (C), the P concentration ratio in the slag 4 becomes small, and the dephosphorization efficiency decreases. On the other hand, if the molten pig iron temperature T is too low, there is a concern that the CaO slag conversion rate decreases, and the dephosphorization efficiency decreases. Therefore, in this embodiment, the molten pig iron temperature T is set to 1250 to 1280°C.

[0051] In the blowing step (S2) according to this embodiment, by setting Fn1 to 0.24 to 0.43, the CaO slag conversion rate of the dephosphorization agent can be maintained high even if the molten iron temperature T is lowered to 1250 to 1280° C. Specifically, in this embodiment, the CaO slag conversion rate of the dephosphorization agent in the blowing step (S2) is 85% or more.

[0052] In this embodiment, the temperature of the molten pig iron 3 at the start of the blowing step (S2) is not particularly limited, but when the molten pig iron temperature T is 1250 to 1280°C, the temperature of the molten pig iron 3 at the start of the blowing step (S2) is, for example, 1300 to 1350°C. The temperature of the molten pig iron 3 at the start of the blowing step (S2) and the temperature of the molten pig iron 3 at the end of the oxygen blowing in the blowing step (S2) (molten pig iron temperature T) can be measured by inserting a thermocouple into the molten pig iron 3 in the dephosphorization vessel 2. Specifically, by inserting a thermocouple at a position at a depth of 20 cm or more from the liquid surface of the molten pig iron 3, the temperature of the molten pig iron 3 can be stably measured.

[0053] [Other configurations] The dephosphorization method according to this embodiment includes the above-mentioned preparation step (S1) and blowing step (S2). The molten iron 3 that has been subjected to the dephosphorization method according to this embodiment may be subjected to decarburization treatment using the same dephosphorization vessel 2, or may be transferred to a different vessel and then subjected to decarburization treatment. When the decarburization treatment is performed using the same dephosphorization vessel 2, it is preferable to discharge the slag 4 from the dephosphorization vessel 2. The dephosphorization method according to this embodiment may have other configurations described below.

[0054] Fig. 3 is a schematic diagram showing another example of the blowing step (S2) in which oxygen is blown into the molten pig iron 3 contained in the dephosphorization vessel 2. Referring to Fig. 3, the bottom surface of the dephosphorization vessel 2 may be provided with a bottom blowing tuyeres 10. In this case, in addition to the blowing of oxygen from the lance 5 arranged above the molten pig iron 3, oxygen can also be blown from the bottom blowing tuyeres 10 arranged below the molten pig iron 3. In this case, stirring of the molten pig iron 3 is promoted, and the dephosphorization efficiency is further improved. In this case, the amount of iron oxide in the molten pig iron 3 is further increased, and the dephosphorization efficiency is further improved.

[0055] 3, iron oxide (FeO) 20 is immersed in the molten pig iron 3 contained inside the dephosphorization vessel 2. When iron oxide 20 is immersed in the molten pig iron 3, the P concentration ratio in the slag 4 increases, as shown in formula (C) above, and the dephosphorization efficiency is further improved. When iron oxide 20 is immersed in the molten pig iron 3, it is preferable to introduce iron oxide 20 into the dephosphorization vessel 2 in the preparation step (S1).

[0056] In the blowing step (S2) according to the present embodiment, the basicity of the slag 4 is preferably 1.2 to 2.2. The basicity of the slag 4 is determined by the content of SiO 2 It means the ratio (mass %) of CaO to P. Within this range of basicity, the higher the basicity, the greater the distribution ratio of P in the molten iron 3 to P in the slag. More preferably, the basicity of the slag 4 is 1.5 to 2.1.

[0057] In the above description, the dephosphorization vessel 2 is a converter. However, as mentioned above, the dephosphorization vessel 2 is not limited to a converter. In the dephosphorization method according to this embodiment, the dephosphorization vessel 2 may be a torpedo car or a ladle. Even in these cases, the dephosphorization method according to this embodiment has excellent dephosphorization efficiency since the preparation step (S1) and the blowing step (S2) satisfy the conditions of this embodiment. EXAMPLES

[0058] The effect of the dephosphorization according to this embodiment will be described more specifically below with reference to examples. Note that the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effect of the dephosphorization method according to this embodiment. Therefore, the dephosphorization method according to this embodiment is not limited to the examples described below.

[0059] Approximately 250 tons of molten pig iron containing 4.5% C, 0.40% Si, and 0.145% P were prepared. The molten pig iron was placed in a converter-type refining vessel. The phosphorus concentration P0 (mass%) of the molten pig iron was measured before the molten pig iron was placed in the converter-type refining vessel. The phosphorus concentration was measured by performing elemental analysis by X-ray fluorescence analysis on a sample obtained by taking a few grams of the molten pig iron and solidifying it. Furthermore, up to 12 tons of iron oxide was immersed in the molten pig iron placed in the vessel.

[0060] The molten iron of each test number was placed in a vessel and oxygen was blown from a lance to perform blowing. During blowing, the dephosphorization agent was introduced from a dispenser attached to the lance to the hot point of the molten iron. For each test number, the Fn1 (=Al 2 O 3 / (α+Al 2 O 3 )) are as shown in Table 1. In Test Nos. 1 and 2, CaCO was used as a dephosphorization agent. 3 In tests 3 and 4, CaCO was used as a dephosphorization agent. 3 and Al 2 O 3A mixed powder consisting of CaO and impurities was used. In this example, the charging basicity of the slag was 1.6. The charging basicity of the slag is the ratio of CaO introduced from the dephosphorization agent and SiO in the slag. 2 The mass percentage of the component (A) is calculated based on the mass percentage of the component (A).

[0061] [Table 1]

[0062] The molten iron temperature at the end of blowing was measured for each test number. The molten iron temperature at the end of blowing was measured by inserting a thermocouple at a depth of 20 cm or more below the molten iron surface. The molten iron temperature at the end of blowing for each test number is shown in the "Molt metal temperature T (℃)" column in Table 1.

[0063] Using the above method, the molten pig iron of each test number was subjected to dephosphorization treatment. The phosphorus concentration P1 (mass%) of the molten pig iron of each test number that had been subjected to dephosphorization treatment was measured. The phosphorus concentration P1 was measured in the same manner as the phosphorus concentration P0. The dephosphorization rate (%) was calculated using the obtained phosphorus concentration P0 (mass%) before the dephosphorization treatment and the phosphorus concentration P1 (mass%) after the dephosphorization treatment and the following formula (I). The obtained dephosphorization rate (%) is shown in Table 1. Dephosphorization rate (%) = 100 × (P0-P1) / P0 (I)

[0064] Test No. 1 is the dephosphorization agent containing Al 2 O 3 As a result, although the molten iron temperature at the end of blowing reached 1250 to 1280°C, the dephosphorization rate could not be increased sufficiently.

[0065] Test No. 2 is the dephosphorization agent containing Al 2 O 3 In addition, the temperature of the molten iron at the end of blowing exceeded 1280°C. As a result, the dephosphorization rate could not be increased sufficiently.

[0066] In test number 3, the molten iron temperature at the end of blowing exceeded 1280°C. As a result, the Al in the dephosphorization agent 2 O3 Although the ratio Fn1 satisfied the range of 0.24 to 0.43, the dephosphorization rate could not be increased sufficiently.

[0067] Test No. 4 is the Al content in the dephosphorization agent. 2 O 3 The ratio Fn1 was in the range of 0.24 to 0.43, and the molten iron temperature at the end of blowing was in the range of 1250 to 1280° C. As a result, the dephosphorization rate was increased compared to Test Nos. 1 to 3, and excellent dephosphorization efficiency was achieved. EXAMPLES

[0068] Before the molten iron was placed in the converter-type refining vessel, the phosphorus concentration P0 (mass%) of the molten iron was measured in advance. The method for measuring the phosphorus concentration was the same as in Example 1. Furthermore, 15 tons or more of iron oxide was immersed in the molten iron placed in the vessel.

[0069] The molten iron of each test number was placed in a vessel and oxygen was blown from a lance to perform blowing. During blowing, the dephosphorization agent was introduced from a dispenser attached to the lance to the hot point of the molten iron. For each test number, the Fn1 (=Al 2 O 3 / (α+Al 2 O 3 )) are as shown in Table 2. In each test number, CaCO was used as a dephosphorization agent. 3 and Al 2 O 3 In this embodiment, the slag basicity was 2.0.

[0070] [Table 2]

[0071] For each test number, the molten iron temperature at the end of blowing was measured. The molten iron temperature at the end of blowing was measured in the same manner as in Example 1. The molten iron temperature at the end of blowing for each test number is shown in the "molten iron temperature T (°C)" column in Table 2.

[0072] Using the above method, the molten pig iron of each test number was subjected to dephosphorization. The phosphorus concentration P1 (mass%) of the molten pig iron of each test number that had been subjected to dephosphorization was measured. As described above, the phosphorus concentration was determined in the same manner as in Example 1. The dephosphorization rate (%) was determined using the obtained phosphorus concentration P0 (mass%) before the dephosphorization treatment and the phosphorus concentration P1 (mass%) after the dephosphorization treatment, and the following formula (I). The obtained dephosphorization rate (%) is shown in Table 2. Dephosphorization rate (%) = 100 × (P0-P1) / P0 (I)

[0073] In test number 5, the molten iron temperature at the end of blowing exceeded 1280°C. As a result, the Al in the dephosphorization agent 2 O 3 Although the ratio Fn1 satisfied the range of 0.24 to 0.43, the dephosphorization rate could not be increased sufficiently.

[0074] Test numbers 6 to 9 are the Al content in the dephosphorization agent. 2 O 3 The ratio Fn1 was in the range of 0.24 to 0.43, and the molten iron temperature at the end of blowing was in the range of 1250 to 1280° C. As a result, the dephosphorization rate was increased compared to Test No. 5, and excellent dephosphorization efficiency was achieved.

[0075] Test numbers 7 to 9 further show that the Al content of the dephosphorization agent is 2 O 3 The ratio Fn1 was 0.27 or more. As a result, compared with Test No. 6, the dephosphorization rate could be further increased, and the dephosphorization efficiency was superior.

[0076] Test Nos. 8 and 9 further examined the Al content of the dephosphorization agent. 2 O 3 The ratio Fn1 was 0.32 or more. As a result, compared with Test No. 7, the dephosphorization rate could be further increased, and further excellent dephosphorization efficiency was obtained.

[0077] Test No. 8 further examined the Al content of the dephosphorization agent. 2 O 3The ratio Fn1 satisfied the condition of 0.40 or less. As a result, compared with Test No. 9, the dephosphorization rate could be further increased, and extremely excellent dephosphorization efficiency was obtained.

[0078] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments without departing from the spirit of the present disclosure. [Explanation of symbols]

[0079] 1. Dephosphorization equipment 2. Dephosphorization vessel 3. Molten iron 4. Slug 5. Lance 6 fire point 10 Bottom-blown tuyere 20 Iron Oxide

Claims

1. A method for dephosphorizing molten iron, comprising the steps of: a preparation step of preparing a dephosphorization vessel containing the molten iron; a blowing step of introducing a dephosphorization agent while blowing oxygen into the molten iron contained in the dephosphorization vessel, The dephosphorization agent is CaO, CaCO ３ , and Ca(OH) ２ and one or more selected from the group consisting of Al ２ O ３ and further satisfying formula (1), In the blowing step, when the blowing of oxygen is completed, the temperature of the molten iron is 1250 to 1280 ° C. Dephosphorization method. 0.24≦Al ２ O ３ / (a+Al ２ O ３ )≦0.43 (1) Here, Al in formula (1) ２ O ３ The Al content in the dephosphorization agent is ２ O ３ The content is substituted in units of mass %, and α in formula (1) is defined by formula (2). α=CaO+56.1 / 100.1×CaCO ３ +56.1 / 74.1×1(OH) ２ (2) In formula (2), CaO is substituted with the CaO content in the dephosphorization agent in mass%, and CaCO ３ The CaCO ３ The content is substituted in mass% and Ca(OH) ２ The Ca(OH) in the dephosphorization agent ２ The content is entered in units of mass%.

2. The dephosphorization method according to claim 1, The dephosphorization agent is CaCO ３ Contains Dephosphorization method.

3. The dephosphorization method according to claim 1 or 2, In the preparation step, iron oxide is introduced into the dephosphorization vessel. Dephosphorization method.

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