Method for melting high-carbon steel

Abstract

This invention provides a method for melting high-carbon steel materials that can sufficiently reduce the phosphorus content of the steel by employing a process with three steps: dephosphorization, intermediate slag removal, and decarburization, using a single top-bottom blown converter. [Solution] When melting high-carbon steel using one top-bottom blow converter, the process includes a first step of charging molten iron and a CaO source into the converter and performing dephosphorization blowing, a second step of tilting the converter after the first step to remove slag while leaving molten iron in the furnace, and a third step of performing decarburization blowing thereafter. A method for melting high-carbon steel, characterized in that, in the third step, at the point when 40-60% of the total amount of oxygen blown from above in the third step has been supplied, a solid iron oxide source containing a total of 75% by mass or more of FeO and Fe2O3, and 40% by mass or more of FeO, is added at a rate of 5.0 kg / t-steel or more.

Description

[Technical Field]

[0001] The present invention relates to a method for melting high-carbon steel, and more particularly to a method for melting high-carbon steel using a single top-bottom blow converter. [Background technology]

[0002] In steel refining using a top-blown converter, dephosphorization and decarburization primarily occur within the converter. Typical processes include: a process in which dephosphorization and decarburization reactions proceed in a single refining cycle using one top-blown converter; a process in which dephosphorization (first stage), intermediate slag removal (second stage), and decarburization (third stage) are performed using one top-blown converter; and a process in which dephosphorization is performed using one top-blown converter, the molten metal after dephosphorization is removed, and decarburization is performed using another top-blown converter. Various methods for improving the dephosphorization capacity in converter refining have been disclosed.

[0003] In the refining of low-carbon steel using a top-blown converter, as the carbon concentration in the molten steel decreases towards the end of the refining process, the FeO concentration in the slag increases, maintaining the dephosphorization capacity and reducing the phosphorus concentration in the molten steel. On the other hand, in the case of high-carbon steel, the carbon concentration in the molten steel remains high towards the end of the refining process, so the FeO concentration in the slag does not increase sufficiently, making it difficult to adequately maintain the dephosphorization capacity of the slag.

[0004] As a process for carrying out dephosphorization and decarburization reactions in a single refining step using a converter, the inventions described in Patent Documents 1 and 2 below are known.

[0005] Patent Document 1 discloses a method for producing medium-to-high carbon steel by using a converter, adding quicklime, a slag accelerator, and a dephosphorization flux mainly composed of iron oxide, and then performing oxygen blowing. During blowing, the slag accelerator and / or iron oxide are added, and the flow rate of the top-blowing acid is increased to promote the dephosphorization reaction. After the start of blowing, when 10-50% of the total blowing time has elapsed, the slag accelerator and / or iron oxide are added.

[0006] Patent Document 2 discloses a converter steelmaking method that can reliably produce low-nitrogen steel. When decarburizing and blowing using a converter, the increase in nitrogen concentration of the molten steel is suppressed by promoting slag forming at the end of blowing when the carbon concentration of the molten steel is 0.3% by mass or less. The substance that promotes slag forming is a titanium oxide-containing substance, an alumina-containing substance, or an iron oxide-containing substance, and the oxide-containing substance is scale or iron ore.

[0007] Patent Document 3 discloses a method for producing high-carbon, ultra-low-phosphorus steel using two converter vessels: a molten iron dephosphorization furnace and a decarburization refining furnace. In the molten iron dephosphorization furnace, a lime-based dephosphorizing agent is used as the dephosphorizer, the amount of FeO generated in the furnace is estimated based on the amount of accumulated oxygen determined by calculation, and the top-blowing acid supply conditions are adjusted according to the estimated amount of FeO to reduce the phosphorus concentration after treatment to 0.015 mass% or less. Then, after slag removal, the dephosphorized iron is charged into the decarburization refining furnace and decarburized by blowing in the decarburization refining furnace to the desired carbon concentration.

[0008] The MURC method (Multi-Refining Converter) is known as a method for discharging slag after dephosphorization without removing the molten metal from the furnace (for example, Non-Patent Document 1). In this MURC method, during the dephosphorization blowing in the first step, the slag is formed to a height suitable for discharge from the converter mouth. A portion of the slag, which has a high phosphorus concentration, is then discharged from the furnace mouth by tilting the converter (intermediate discharge in the second step). New refining material is then added to the converter, which contains molten iron with a low phosphorus concentration, to perform further dephosphorization and decarburization (decarburization blowing in the third step).

[0009] Patent Document 4 discloses a refining process called Process A, in which a converter is used to perform decarbonization blowing (first step), followed by intermediate slag removal in the second step, and then decarburization blowing (decarbonization treatment) in the third step. In the decarbonization treatment of Process A, slag-forming materials (quicklime, light sludge, raw sludge, silica) and iron ore are fed from a hopper on top of the furnace, and gaseous oxygen is blown upwards to perform decarbonization blowing.

[0010] The solid iron oxide source to be added to the converter during converter blowing is added into the furnace from the converter opening, similar to other auxiliary materials. Alternatively, iron oxide powder can be used as the solid iron oxide source, and the iron oxide powder can be supplied to the molten iron surface along with the conveying gas from the top blowing lance.

[0011] Patent Document 5 discloses a method for supplying a solid oxygen source from an upper-blowing oxygen lance when dephosphorizing molten iron while blowing a gaseous oxygen source from the upper-blowing oxygen lance of a converter. The upper-blowing oxygen lance is provided with a main gas supply pipe for supplying the gaseous oxygen source and a pipe for supplying the solid oxygen source together with the transport gas. The dust used as the solid oxygen source had an FeO content of 31.40%.

[0012] Patent Document 6 relates to a method for dephosphorizing molten iron to be carried out prior to the decarburization process in a converter. In this method, a first lance and a second lance are inserted horizontally at a distance apart as an upper blowing lance for refining. Gaseous oxygen is supplied from the first lance, and a solid oxygen source is supplied from the second lance along with a transport gas. The solid oxygen source is an iron oxide source such as sintered ore, mill scale, or iron ore. [Prior art documents] [Patent Documents]

[0013] [Patent Document 1] Japanese Patent Publication No. 2000-008113 [Patent Document 2] Japanese Patent Publication No. 2004-277830 [Patent Document 3] Japanese Patent Publication No. 2006-206930 [Patent Document 4] Japanese Patent Publication No. 2005-206922 [Patent Document 5] Japanese Patent Publication No. 2016-151051 [Patent Document 6] Japanese Patent Publication No. 2017-101293 [Non-patent literature]

[0014]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0015] The present invention is directed to a method for melting high-carbon steel materials using a top-bottom blown converter. Here, high-carbon steel materials mean that the product carbon concentration is 0.5% by mass or more. High-carbon steel materials such as bars and wire rods are refined and tapped in a converter with a certain amount of C remaining in the molten steel without blowing down C in the hot metal. Therefore, as described above, it is extremely difficult to reduce P in high-carbon steel materials compared to low-carbon steel grades that are melted by blowing down C in the hot metal in a converter.

[0016] When melting high-carbon steel materials using one top-bottom blown converter, the present invention adopts a process having a first step of performing dephosphorization blowing, a second step of intermediate slag removal, and a third step of performing decarburization blowing, and aims to provide a method for melting high-carbon steel materials that can sufficiently achieve low phosphorus in steel.

Means for Solving the Problems

[0017] That is, the gist of the present invention is as follows. [1] When melting high-carbon steel materials using one top-bottom blown converter, a first step of charging hot metal and a CaO source into the converter and performing dephosphorization blowing, and a second step of tilting the converter after the first step and performing intermediate slag removal while leaving hot metal in the furnace, and further having a third step of performing decarburization blowing after that, In the third step, at the time when oxygen corresponding to 40 to 60% of the total amount of gaseous oxygen blown from above in the third step is supplied, a solid iron oxide source containing 75% by mass or more in total of FeO and Fe2O3 and 40% by mass or more of FeO is added at 5.0 kg / t-steel or more. A method for melting high-carbon steel materials, characterized by this.​[2] In the third step, an iron oxide powder is used as the solid iron oxide source to be added, and the iron oxide powder is supplied to the molten iron surface together with the carrier gas from the top blowing lance. The method for melting a high-carbon steel material according to [1]. [3] After the first step and the second step, and before the third step, the first step and the second step are further carried out one or more times. The method for melting a high-carbon steel material according to [1] or [2].

Effect of the Invention

[0018] When melting a high-carbon steel material using a single top-bottom blowing converter, the present invention has a first step of dephosphorization blowing, a second step of intermediate slag removal, and a third step of decarburization blowing. In the third step, by adding an appropriate amount of a solid iron oxide source with appropriate components at an appropriate time, sufficient dephosphorization of the steel can be achieved.

Brief Description of the Drawings

[0019] [Figure 1] It is a diagram showing a converter process, where (A1)(A2) are the first step, (B) is the second step, and (C) is the third step. [Figure 2] It is a diagram showing a converter process, where (A1)(A2) are the first step for the first time, (B) is the second step for the first time, (C) is the first step for the second time, (D) is the second step for the second time, and (E) is the third step. [Figure 3] It is a diagram showing the transition of the slag (FeO) concentration (A) and the transition of the molten iron [P] concentration (B) with respect to the transition of the molten iron [C] concentration in the third step.

Embodiments for Carrying out the Invention

[0020] 《Explanation of the Premise Conditions of the Present Invention》 As described above, when melting a high-carbon steel material using a single top-bottom blowing converter, the present invention employs a process having a first step of dephosphorization blowing, a second step of intermediate slag removal, and further having a third step of decarburization blowing after that.

[0021] As shown in Figure 1, the top-bottom blow converter has a converter 1, a top-blowing lance 2, and a bottom-blowing tuyere 3. Figure 1(A1) shows the initial stage of the first process (dephosphorization blowing), where molten iron 11 is contained in the converter, a CaO source 15 is added, oxygen gas 13 is supplied from the top-blowing lance 2, and bottom-blowing gas 14 is supplied from the bottom-blowing tuyere 3. Figure 1(A2) shows the end of the first process, where slag 12 has formed up to near the furnace opening 4. Figure 1(B) shows the second process (intermediate slag removal), where the converter 1 is tilted to remove the slag 12 from the converter into the slag pot 6. Figure 1(C) shows the third process (decarburization blowing), where a solid iron oxide source 16 is added from the furnace opening 4. After the third process is completed, the converter is tilted to the opposite side from the slag removal point, and the molten steel is tapped into a ladle (not shown) via the tapping port 5, completing the refining process using the converter.

[0022] Figure 3 shows the typical relationship between slag (FeO) (Figure 3(A)) and molten iron [P] (Figure 3(B)) and molten iron [C] during decarburization blowing (third step). The dashed line in Figure 3 shows the progression in a conventional example. The horizontal axis in Figures 3(A) and 3(B) represents molten iron [C] during decarburization blowing, and the [C] concentration decreases sequentially as decarburization blowing progresses.

[0023] As shown by the dashed line in Figure 3(A), the slag (FeO) concentration is high at the start of the decarburization blowing process in the third stage (point B1). The slag with a high (FeO) concentration generated in the dephosphorization blowing process in the first stage is not completely removed during the intermediate slag removal in the second stage and remains. This is the reason why the slag (FeO) is high at the beginning of the third stage.

[0024] In decarburization blowing (third stage), the top-blowing oxygen jet is a harder blow compared to dephosphorization blowing (first stage), making it difficult for FeO to be generated at the heating point. Therefore, although the slag (FeO) concentration is high at the start of decarburization blowing (point B1 in Figure 3(A)), the FeO in the slag is reduced by C in the molten iron during decarburization blowing, and the slag (FeO) decreases from the middle to the end of decarburization (dashed line in Figure 3(A)). As a result, the phosphorus removal capacity of the slag decreases significantly, and phosphorus is regenerated from P2O5 in the dephosphorization slag carried over from dephosphorization blowing, causing a large increase in [P] in the molten iron (dashed line in Figure 3(B)).

[0025] In the decarburization blowing of low-carbon steel, the blowing is carried down to the low-carbon region at the end of the decarburization blowing process, causing the decarburization reaction by top-blowing oxygen to stagnate and generating a large amount of FeO (point B4 in Figure 3(A)). In the case of low-carbon steel types where [C] is blown down in molten iron, sufficient dephosphorization is possible because there is a large amount of (FeO) in the stop slag (point B4 in Figure 3(B)).

[0026] On the other hand, in high-carbon steel grades, it is necessary to leave a certain amount of [C] in the molten iron at the end of decarburization blowing, and decarburization blowing is terminated at point B3 in Figure 3. At this point, it is not possible to sufficiently increase the amount of FeO (blowout) (point B3 in Figure 3(A)), and the steel must be tapped in a state where dephosphorization is insufficient (point B3 in Figure 3(B)).

[0027] Description of the present invention To address this challenge, we discovered that by adding an appropriate amount of a solid iron oxide source with an appropriate composition at the appropriate timing of decarburization blowing (point A2 in Figures 3(A) and 3(B)), it is possible to maintain a high level of slag (FeO) from the middle of decarburization onward, and to maintain a high level of slag P removal ability at the point when high coal content blowing is stopped. The solid line in Figure 3 shows the concept when the present invention is applied. The solid iron oxide source 16 is added at point A2 in Figure 3. Subsequently, as is clear from comparing the dashed line (conventional example) and the solid line (example of the present invention) in Figure 3(A), at the same [C] concentration, the slag (FeO) concentration increases in the example of the present invention, while the molten iron [P] concentration continues to decrease without increasing. Then, at point A3, where the molten iron [C] concentration is high, the slag (FeO) concentration is high, making it possible to reduce the molten iron [P] concentration to a sufficiently low level.

[0028] The following describes preferred conditions for the addition of the solid iron oxide source 16 in the third step of the present invention.

[0029] Timing of adding solid iron oxide source In the third stage of decarburization blowing, it is necessary to maintain a high slag (FeO) concentration during the mid-to-late stages of decarburization, when slag (FeO) levels are low. The timing of adding the solid iron oxide source is calculated using equation (1) below. If the timing of adding the solid iron oxide source in equation (1) is less than 40%, that is, if the timing of adding the solid iron oxide source is too early, less than 40% of the total gaseous oxygen blown into the third stage, the added iron oxide will dissolve prematurely, making it impossible to maintain a high level of (FeO) from the mid-stage of decarburization onward. On the other hand, if the timing of adding the solid iron oxide source in equation (1) is more than 60%, that is, if the timing of adding the solid iron oxide source is too late, exceeding 60% of the total gaseous oxygen blown into the third stage, the time from addition to stopping high-carbon blowing is short, and a sufficient phosphorus removal improvement effect cannot be obtained. Timing of addition of solid iron oxide source (%) = 100 × Amount of top-blown gaseous oxygen supplied before adding the solid iron oxide source / Total amount of gaseous oxygen supplied in the top-blown refining process of the third step (1)

[0030] Amount of solid iron oxide source added To promote phosphorus removal, it is necessary to increase the amount of slag (FeO) during the peak and final stages of decarburization. If the amount of solid iron oxide source added during the preferred addition period is less than 5.0 kg / t-steel, the absolute amount of iron oxide is insufficient to adequately increase the amount of slag (FeO).

[0031] Components of a solid iron oxide source Since it is a solid iron oxide source for raising the level of slag (FeO), it must contain at least 75% by mass of FeO and Fe2O3 in total. On the other hand, the melting point of elemental FeO is 1377°C, and the melting point of elemental Fe2O3 is 1566°C, so even though they are both iron oxides, a higher proportion of FeO is advantageous in terms of the solubility of the iron oxide source. Here, iron ore, a common solid iron oxide source, has a slow dissolution rate because the majority of its components are Fe2O3, and therefore the present invention cannot fully demonstrate its effects. Accordingly, in order to obtain the effects of the present invention, the solid iron oxide source added at the above preferred addition time must be a solid iron oxide source containing 40% by mass or more of FeO, such as scale or OG dust. The remaining components of the solid iron oxide source are expected to be CaO, SiO2, MgO, MnO, Al2O3, M.Fe, etc. The Fe2O3 and FeO content in the solid iron oxide source can be evaluated by chemical analysis.

[0032] The solid iron oxide source is selected from auxiliary materials commonly used as a solid oxygen source in converter blowing, such as iron ore, sintered powder, scale, and dust, and in actual operation, materials with a particle size of approximately 5 to 30 mm are generally used. When these are added in lumps, it takes a certain amount of time for them to dissolve completely, so iron oxide can be continuously supplied to the slag over a wide blowing period during the dissolution process. Furthermore, it has been shown that the effects of the present invention can be even more pronounced by taking measures such as using a solid iron oxide source with a fast dissolution rate, specifically scale or OG dust containing a large amount of FeO, or continuously supplying iron oxide as powder from the lance.

[0033] Furthermore, in the third step, the solid iron oxide source may be added at a time other than the preferred addition time described above. In that case, the components of the solid iron oxide source do not need to be limited to the component range of the solid iron oxide source added at the preferred addition time described above.

[0034] Method for adding a solid iron oxide source The solid iron oxide source to be added in the third step may be added into the furnace from the converter opening during the decarburization and blowing process in the third step, just like the other auxiliary materials. More preferably, iron oxide powder can be used as the solid iron oxide source to be added, and the iron oxide powder can be supplied to the molten iron surface along with the conveying gas from the top blowing lance.

[0035] It has been found that supplying fine iron oxide powder at a constant rate from an upward-blowing lance provides a higher dephosphorization improvement effect than adding a solid iron oxide source in chunks. This is because fine iron oxide powder dissolves much faster than chunks, allowing for a rapid increase in slag (FeO). While there is no specific requirement for the particle size of the iron oxide powder, it is desirable that it be under 1 mm. Furthermore, when blowing iron oxide powder from the existing oxygen nozzle of the main lance, if it contains FeO or granular iron, blowing it together with oxygen carries the risk of ignition or explosion. Therefore, it is desirable to supply it together with an inert gas such as N2 or Ar from a sub-lance or a sub-nozzle of the main lance.

[0036] The following describes the repetition of the first process (dephosphorization blowing) and the second process (intermediate slag removal) before moving on to the third process (decarburization blowing).

[0037] As shown in Figure 1, the present invention allows for immediate transition to the third step (decarburization blowing) after performing the first step (dephosphorization blowing) and the second step (intermediate slag removal).

[0038] The present invention may further involve performing the first and second steps one or more times after the first and second steps and before the third step, as shown in Figure 2.

[0039] Figure 2(A1) shows the initial stage of the first step (dephosphorization blowing) of the first cycle, where molten iron 11 is contained in the converter, CaO source 15 is added, oxygen gas 13 is supplied from the top blowing lance 2, and bottom blowing gas 14 is supplied from the bottom blowing tuyeres 3. Figure 2(A2) shows the final stage of the first step of the first cycle, where slag 12 has formed up to near the furnace opening 4. Figure 2(B) shows the second step of the first cycle (intermediate slag removal), where the converter 1 is tilted to remove the slag 12 from the converter into the slag removal pan 6. Figure 2(C) shows the second step of the first cycle (dephosphorization blowing), where slag 12 has formed up to near the furnace opening 4. Figure 2(D) shows the second step of the second cycle (intermediate slag removal), where the converter 1 is tilted to remove the slag 12 from the converter into the slag removal pan 6. Figure 2(E) shows the third step (decarburization blowing), in which the solid iron oxide source 16 is added from the furnace opening 4. After the completion of the third step, the converter is tilted to the opposite side from the slag discharge point, and the molten steel is tapped into a ladle (not shown) via the tapping opening 5, completing the refining process using the converter.

[0040] To reduce the phosphorus (P) content after decarburization blowing, it is necessary to reduce the amount of phosphorus carried over from de-P blowing. By performing de-P blowing and intermediate slag removal two or three times, the amount of P carried over to decarburization blowing can be significantly reduced. In addition, by performing intermediate slag removal twice and increasing the total amount of slag removed, the amount of slag in decarburization blowing can also be reduced, thus improving the effect of increasing slag (FeO) per solid iron oxide source. [Examples]

[0041] The present invention was implemented using a top-bottom blowing converter with a molten iron capacity of 280t. The components of the charged molten iron were: charged molten iron [C]: 4.0~4.4 mass%, charged molten iron [Si]: 0.3~0.7 mass%, charged molten iron [Mn]: 0.2~0.3 mass%, and charged molten iron [P]: 0.10~0.13 mass%.

[0042] Dephosphorization blowing (first step) uses an acid supply rate of 25,000 Nm. 3 / h, oxygen supply amount: 10.0~12.0Nm 3 The rate is / t, and the decarburization blowing (third step) is acid supply rate: 25000 Nm 3 / h, oxygen supply amount: 24.0~28.0Nm3 / t.

[0043] The temperature after treatment was 1300 - 1350 °C for dephosphorization blowing (the first step) and 1630 - 1660 °C for decarburization blowing (the third step).

[0044] The manufacturing conditions and results are shown in Table 1. In Table Ⅰ, values and items outside the scope of the present invention are underlined.

[0045]

Table 1

[0046] In Table 1, Nos. 1 - 7, 11 - 17 are examples where the number of blowing times in the first step and the number of slag removal times in the second step are 1. The first and second steps are carried out once, and then the third step is proceeded. Hereinafter, it is called "slag removal once". Nos. 8 - 10, 18 are examples where the above - mentioned number is 2. The first and second steps are repeated twice, and then the third step is proceeded. Hereinafter, it is called "slag removal twice".

[0047] The intermediate slag removal rate (the second step) shown in Table 1 was calculated using the mass balance formula of equation (2).

Number

[0048] The actual results of the intermediate slag removal rate were as shown in Table 1, 55 - 65% for slag removal once and 70 - 80% for slag removal twice.

[0049] The addition conditions of the solid iron oxide source in the third step are as shown in Table 1.

[0050] The components (FeO + Fe2O3 concentration, FeO concentration) of the solid iron oxide source used in the examples are shown in Table 1. Scale and iron ore were used as raw materials for the solid iron oxide source, and the components of the solid iron oxide source were adjusted by adjusting the mixing ratio of the raw materials.

[0051] This section describes the shape and method of adding the solid iron oxide source. When the solid iron oxide source was added in chunks, it was added from the furnace bunker in a particle size of 5 to 30 mm. On the other hand, when the solid iron oxide source was added in powder form, the solid iron oxide source was crushed to less than 1 mm, and the iron oxide was blown upwards as powder from the powder addition sub-lance. Oxygen was supplied upwards along with inert gas from the powder addition sub-lance, not from the main lance which blows oxygen upwards.

[0052] The timing of adding the solid iron oxide source is calculated using equation (1) above and is shown in Table 1 as the timing corresponding to the total amount of oxygen blown upward in the third step. The amount of solid iron oxide source added is also shown in Table 1.

[0053] Under the above conditions, the third step of decarburization blowing was performed, and since the target was the melting of high-carbon steel grades, the third step was completed with a blowdown [C] in the range of 0.5 to 0.9 mass%. The actual blowdown [P] results are shown in Table 1.

[0054] The method for confirming the effects in the examples is as follows:

[0055] The phosphorus removal rate (%) was calculated using the following formula (3). Dephosphorization rate = 100 × (Hot metal before 1st process [P] - Molten steel after 3rd process [P]) / Hot metal before 1st process [P]...(3) (3) When the dephosphorization rate of formula (3) was 85% or higher, it was determined that the effect of the invention was achieved, and when it was 90% or higher, it was determined that the effect of the invention was even more significant.

[0056] Examples 1 (No. 1-4) in Table 1 use a solid iron oxide source and satisfy the conditions of claim 1, achieving the effects of the invention (dephosphorization rate: 85-90%).

[0057] Examples 2 (No. 5-7) in Table 1 involve blowing a solid iron oxide source as a powder, satisfying the conditions of claim 2, and demonstrating particularly remarkable effects of the invention (dephosphorization rate ≥ 90%).

[0058] Examples 3 (No. 8-10) in Table 1 involve performing dephosphorization and blowing in the first step and slag removal in the second step twice, satisfying the conditions of claim 3 and demonstrating particularly remarkable effects of the invention (dephosphorization rate ≥ 90%).

[0059] The comparative examples (Nos. 11-18) in Table 1 are comparative examples. In samples No. 11 and 12, a solid iron oxide source was not added in the third step, resulting in poor dephosphorization during the middle to final stages of decarburization. In No. 13, the FeO+Fe2O3 concentration in the solid iron oxide source was low, resulting in a slow dissolution rate of the iron oxide source itself and a small effect on increasing the amount of slag (FeO). In No. 14, the FeO concentration in the solid iron oxide source was low, and the dissolution rate of the iron oxide source itself was slow, resulting in a small effect in increasing the amount of slag (FeO).

[0060] In No. 15, the solid iron oxide source was introduced too early, and therefore did not contribute to promoting dephosphorization in the middle and later stages of decarburization. In No. 16, the timing of adding the solid iron oxide source was too late, resulting in a short time between the addition and the cessation of high-carbon coal blowing, which prevented sufficient improvement in phosphorus removal.

[0061] In experiment No. 17, the amount of solid iron oxide source added was insufficient, resulting in a small effect in increasing slag (FeO).

[0062] In No. 18, dephosphorization and smelting were performed twice in the first process and twice in the second process, but because a solid iron oxide source was not added in the third process, dephosphorization was insufficient at the end of the decarburization and smelting stage. [Explanation of Symbols]

[0063] 1 Converter 2. Upward-blowing lance 3 Bottom-blown tuyere 4 Hearth 5 Steel tapping port 6. Drain pan 11 Molten iron 12 slags 13. Oxygen gas 14 Bottom-blowing gas 15 CaO source 16. Solid iron oxide source

Claims

1. When melting high-carbon steel using a single top-bottom blow converter, the process includes a first step of charging molten iron and a CaO source into the converter and performing dephosphorization blowing, a second step of tilting the converter after the first step to remove slag while leaving molten iron in the furnace, and a third step of performing decarburization blowing thereafter. In the third step, when 40-60% of the total amount of oxygen blown upward in the third step has been supplied, FeO and Fe ２ O ３ A method for melting high-carbon steel, characterized by adding 5.0 kg / t-steel or more of a solid iron oxide source containing a total of 75% by mass or more of and 40% by mass or more of FeO.

2. The method for melting high-carbon steel according to claim 1, characterized in that, in the third step, iron oxide powder is used as the solid iron oxide source to be added, and the iron oxide powder is supplied to the molten iron surface together with the conveying gas from an upper blowing lance.

3. A method for melting high-carbon steel according to claim 1 or 2, characterized in that the first and second steps are performed one or more times after the first and second steps and before the third step.

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