JP6421634B2 - Manufacturing method of molten steel - Google Patents

Description

  The present invention is a method for producing molten steel by refining blast furnace hot metal using a converter, more specifically, after dephosphorizing the hot metal in the converter, leaving the hot metal and discharging slag. After the debris is decarburized and dephosphorized to produce molten steel, the slag after the treatment is left to produce the molten steel, and the remaining slag is used for dephosphorization of the hot metal in the next charge. It is related with the manufacturing method of the molten steel which repeats doing.

  Hot metal refining in a converter is generally carried out by charging the converter with blast furnace hot metal, adding flux mainly composed of quick lime, and dephosphorizing and decarburizing the hot metal by oxygen blowing to produce steel. It was. Subsequently, a steelmaking method that consolidates the refining functions over multiple processes into the converter, greatly reducing the loss of hot metal heat energy, and significantly reducing the expenses before and after the converter is disclosed in Patent Document 1, for example. It is disclosed.

In summary, when refining blast furnace hot metal to produce molten steel, scrap iron and hot metal are charged into the converter as the first step, flux is added as the second step, and slag basicity ( CaO / the SiO ₂₎ 1.0 to 2.0, the process temperature and吹酸at 1350 ° C. or less subjected to dephosphorization process, to discharge the slag generated in the second step as a third step, a fourth step By adding flux and blowing acid, decarburization and dephosphorization are performed up to the specified [C] and [P], and the steel is produced while leaving the slag generated in the fourth step as the fifth step, and the charcoal as the sixth step. The material is added to reduce slag (FeO), and the process returns to the first step. Thereafter, the amount of lime in the flux in the second step is set to zero or 25% or less of the amount added in the second step. It is a converter steelmaking method that repeats up to the sixth step.

  The reason why the sixth step is necessary in this method is to avoid bumping by the equation (i) at the first step of the next charge.

FeO + [C] = CO + [Fe] (i)
However, since the sixth step takes time, productivity decreases, and when the concentration of slag (FeO) decreases, the melting point of slag increases, so the dissolution rate of slag in the second step of the next charge decreases. Thus, there is a problem that the dephosphorization rate in the second step is lowered.

  In order to solve these problems, for example, in Patent Document 2, after solidifying the decarburized slag by charging the decarburized slag left in the furnace with a solidifying agent (light-burned dolomite or raw dolomite), A method of making an order is disclosed.

Japanese Patent No. 2558292 Japanese Patent No. 34868989

  In the method disclosed in Patent Document 1, since a large number of steps are performed in one furnace, the furnace body occupation time is long, and thus shortening the cycle time is indispensable for improving productivity. Therefore, the method described in Patent Document 2 is effective for shortening the solidification time of the decarburized slag left in the furnace. Moreover, in the method of patent document 1, scrap iron (scrap) is injected | thrown-in before charging the hot metal of the next charge. Since this scrap is a powerful cold material, it is easy to solidify the decarburized slag left in the furnace almost completely. However, at that time, a considerable part of scrap and decarburized slag adheres to the bottom of the converter furnace, and peels off from the furnace bottom at the early stage of the hot metal dephosphorization of the next charge (second step). It becomes difficult to surface. The same applies to the case where only a solidifying agent is added, as described in Patent Document 2.

Since the melting point of decarburized slag is usually as high as 1500 ° C. or higher, it does not melt at a temperature of 1400 ° C. or lower during hot metal dephosphorization. Then, the solidified decarburized slag is low in contact with SiO ₂ generated by the reaction ((ii), (iii)) of the top blown oxygen or iron oxide with [Si] in the hot metal in the second step. There is a problem in that a melting slag having a melting point cannot be formed and the dephosphorization reaction is difficult to proceed.

O ₂ + [Si] = SiO ₂ (ii)
2FeO + [Si] = SiO ₂ +2 [Fe] (iii)
The purpose of the present invention is to produce a molten steel by refining the blast furnace hot metal using a converter, a decrease in productivity when the invention described in Patent Document 1 is carried out, a decrease in dephosphorization rate, etc. It is to provide a way to solve the problem.

As a result of intensive studies in order to solve the above problems, the present inventor, in the invention described in Patent Document 1, in place of the FeO reduction operation in the sixth step, the slag composition and the type of solid agent to be added In addition, by adding a slag solidification operation using an agglomerate with a high concentration of Al ₂ O _3, which has been refrained from being used because it tends to induce slopping in the past, Knowing that the problem can be solved, the present invention has been completed. The present invention is listed below.

(1) A method for producing molten steel by refining blast furnace hot metal to produce molten steel, as a first step, charging scrap iron and molten iron into a converter and adjusting the amount of secondary raw materials to be charged The basicity (CaO mass / SiO ₂ mass) is 1.0-2.0, the treatment temperature is 1350 ° C. or less to perform dephosphorization treatment, and the slag produced in the first step is discharged as the second step. In the third step, the composition is mass% by addition of flux and blowing acid, CaO = 30-50%, Al ₂ O ₃ = 1-5%, T.W. _{Fe = 10~30%, SiO 2 =} 8~15%, a MgO = 6 to 12%, basicity: to produce a decarburization slag is CaO wt / SiO ₂ mass = 3.5 to 5.0 The decarburization and dephosphorization processes are performed, and the decarburization slag generated in the third step is left as the fourth step, and the steel is left out, and the particle size is 50 mm or less to the decarburization slag left in the furnace as the fifth step. The particle size is one or more solidifying agents selected from quick lime, limestone, light calcined dolomite, raw dolomite and cooled and decarburized slag having a ratio of 10 to 50 mm of 20% by mass or more. 10-50 mm and the composition is mass%, CaO = 30-50%, Al ₂ O ₃ = 10-30%, T.I. An ingot slag of Fe = 5-15% and SiO ₂ = 8-20% is added to solidify the decarburized slag, and then the process returns to the first step again, and the scrap iron and hot metal of the next charge are Charged to the converter after finishing the fifth step, dephosphorizing under the conditions defined as the first step, and repeatedly performing the subsequent fifth step in order to produce molten steel, Method.

  (2) In the fifth step, the mass of the solidifying agent added to the decarburized slag left in the furnace is 10 to 30% of the mass of the decarburized slag, and the mass of the ingot slag to be added is solidified. The method for producing molten steel according to the above (1), characterized in that the content is 10 to 100% of the mass of the agent.

In the present invention, the ingot slag is slag remaining in the ladle after the molten steel in the ladle has been supplied to a continuous casting machine or the like. Its composition is CaO = 30 to 50%, Al ₂ O ₃ = 10 to 30%, T. _{Fe = 5~15%, SiO 2 =} 8~20%, but is common. Some ingot slag is easy to be pulverized when the ingot slag is cooled, but in the present invention, one that is difficult to be pulverized (a large proportion of lumps) and is easy to handle is used.

Moreover, the charging basicity (CaO mass / SiO ₂ mass) adjusted in the first step is “CaO mass in the auxiliary raw material to be charged / {(Si mass in hot metal + Si mass in scrap) × 2”. .14 + SiO ₂ mass in the auxiliary raw material to be charged}, the decarburized slag remaining in the pre-charge and solidified, the added slag solidifying agent and the ingot slag are also charged auxiliary raw material Therefore, depending on the conditions in the previous charge and the conditions of the hot metal and scrap iron to be dephosphorized from now on, the auxiliary material newly charged in the first step is set to zero. It is possible to do.

According to the present invention, the low melting point ingot slag added to the decarburized slag surface of the converter bottom in the fifth step is melted at an early stage of the first step and separated from the converter bottom and floated. The decarburized slag left on the converter bottom becomes uneven. Then, hot metal that is flowing strongly agitated by the bottom blowing gas is likely floated by peeling off the uneven slag adhering to the furnace bottom, newly generated FeO, dissolved-air bearing on SiO ₂ or above in the first step Since the molten slag reacts promptly on the ingot-making slag and the hot metal bath to produce molten slag, the dephosphorization rate is improved and the dephosphorization rate after the first step is improved.

Moreover, since Al ₂ O ₃ in the ingot slag promotes forming of the dephosphorization slag generated in the first step, the slag discharge rate in the second step is improved. Then, since the amount of phosphoric acid carried over to the third step is reduced, the [P] concentration in the molten steel after the third step and the [P] concentration in the molten steel in the pan after the fourth step can be reduced.

By the way, conventionally, when the Al ₂ O ₃ concentration in the slag produced in the first step is high, there has been a problem of intense slopping in the first step. For example, when the pre-charged decarburized slag is poured into the furnace bottom and poured into the furnace bottom, the ingot slag is added and the first step (hot metal dephosphorization blowing) is performed. Since the slag produced is low in basicity due to slow peeling and flotation of the slag, and the ingot agglomerated slag dissolves early, the initial concentration of the molten slag in the furnace has a high alumina concentration. It was.

On the other hand, when the ingot slag containing 10 to 30% Al ₂ O ₃ is mixed with the slag solidifying agent and applied as in this method, the decarburized slag fixed to the furnace bottom is separated and floated relatively early. Therefore, the basicity of the dephosphorization slag produced in the first step is increased early. Further, since the dephosphorization slag amount produced by dissolution of the decarburization slag increases, alumina derived from ingot slag Usumari, decreases the concentration of Al ₂ O ₃ in the slag. As a result, the slapping behavior in the first step is considered to be at the same level as the conventional method that does not use ingot slag.

  The form for carrying out the present invention is as follows: scrap iron 50t and hot metal 290t (composition: [C] ≈4.4 mass%, [Si] ≈0.4 mass%, [P] ≈0.10 mass %) Is charged into the converter, and [C] after decarburization is about 0.10% by mass, and the [P] concentration of the molten steel in the ladle after steel removal is 0.019% by mass or less. This will be described using an example of manufacturing. Of course, the technical scope of the present invention is not limited to the conditions such as the amount of scrap iron and hot metal conditions exemplified below, and is applicable to all converter steelmaking methods for producing ordinary molten steel for ordinary blast furnace hot metal. Is possible.

In the following description, “%” relating to the concentration is used in the meaning of mass% unless otherwise specified.
As the first step, decarburized slag that has been charged and left in the previous charge so that the scrap iron and hot metal described above are charged into the converter and the basicity of the slag is 1.0 to 2.0, The auxiliary raw material is charged in consideration of the added slag solidifying agent and ingot slag. This is because a forming fluid slag having a moderate fluidity is formed to promote the dephosphorization reaction. The charging time of the auxiliary material into the furnace may be before or after charging the molten iron, but before 1 minute has elapsed since the supply of oxygen for blowing was started in consideration of the time required for the hatching. It is desirable to do. As this auxiliary material, limestone, light-burned dolomite, or the like may be used as appropriate in addition to general quick lime. Further, iron oxide is added for temperature adjustment, and dephosphorization blowing is performed at a processing temperature of 1350 ° C. or lower. Feed gas flow rate, the top-blown oxygen: 40000Nm ³ / h, bottom-blown oxygen: 3000 Nm ³ / h, bottom-blown LPG: a 200 Nm ³ / h approximately.
As the second step, about 80% of the slag generated in the first step is discharged.
As a third step, a basicity of decarburization slag (CaO mass / SiO ₂ mass) 3.5 to 5, in its composition by _{mass%, CaO = 30~50%, Al} 2 O 3 = 1~5 %, T.A. _{Fe = 10~30%, SiO 2 =} 8~15%, MgO = 6~12% become so quicklime, dolomitic (composition: CaO ≒ 60%, MgO ≒ 34%) was added and the temperature adjusted For this purpose, iron oxide is added, and decarburization blown (at the same time dephosphorization proceeds) to [C] ≈0.10%. The composition of this decarburized slag can be dephosphorized to the product level after decarburization blowing and can suppress the melting loss of the converter refractory as much as possible. Feed gas flow rate, the top-blown oxygen: 80000Nm ³ / h, bottom-blown oxygen: 3000 Nm ³ / h, bottom-blown LPG: a 200 Nm ³ / h approximately.
As the fourth step, steel is output with the decarburized slag generated in the third step left in the furnace.
As a fifth step, the furnace body is tilted to discharge a portion of the decarburized slag from the furnace port, leaving a predetermined amount of decarburized slag in the furnace. Along with one or more solidifying agents selected from charcoal slag, the composition is mass%, CaO = 30-50%, Al ₂ O ₃ = 10-30%, T.P. An agglomerated slag of Fe = 5-15% and SiO ₂ = 8-20% is added and solidified together with the residual slag in the furnace. Thereafter, returning to the first step again, the scrap iron 50t, the molten iron 290t, and the auxiliary materials of the next charge are charged into the converter after the fifth step and the dephosphorization process is performed.
Thereafter, the process is sequentially performed up to the fifth process, and then the first process to the fifth process are sequentially repeated.
The particle diameters of the solidifying agent and the ingot slag are, for the reasons described later, the solidifying agent is 50 mm or less and the ratio of 10 to 50 mm is 20% by mass or more, and the ingot slag is 10 to 50 mm. There is a need. The particle diameter of the slag that has not passed through the 10 mm square mesh is 10 mm or more, and the particle diameter of the slag that has passed through the 50 mm square mesh is 50 mm or less.
In addition, it is preferable that the addition time of the solidifying agent is within 10 minutes after the completion of steel production in the fourth step.

Moreover, the mass of the solidifying agent to be added is 10 to 30% of the mass of the decarburized slag left in the furnace, and the mass of the agglomerated slag to be added is 10 to 100% of the mass of the solidifying agent. preferable.
The mass of the decarburized slag remaining in the furnace can be estimated from the “furnace body tilt angle” when the converter is tilted and the slag is discharged from the furnace port. The correlation between the furnace body tilt angle and the residual slag amount in the furnace is obtained in advance by estimating the generated slag amount from the mass balance calculation and measuring the discharge slag amount for each furnace body tilt angle. Since decarburized slag has relatively high fluidity, there is a good correlation between the furnace body tilt angle and the amount of residual slag in the furnace.
Therefore, when carrying out the present invention, the relationship between the furnace body tilt angle and the amount of residual slag in the furnace is examined in advance, the amount of residual slag in the furnace is ascertained from the furnace body tilt angle, and the conditions for adding a solidifying agent and the like are determined. It is preferable to adjust. However, through repeated operations, even if the amount of residual slag in the furnace is grasped empirically and the conditions such as solidifying agent are determined, the requirements of the present invention can be satisfied and the effects of the present invention can be enjoyed. Can do.

  In order to investigate the addition conditions of the above-described solidifying agent and ingot slag, the charging basicity is set to 0.9 to 2.2, and the solidifying agent is added in an amount of 0 to 40% by mass with respect to the amount of residual slag in the furnace. The agglomerated slag is added in an amount of 0 to 120% by mass with respect to the solidifying agent, and other conditions are unified to the conditions exemplified in the embodiment for carrying out the present invention, and the dephosphorization results and the bumping phenomenon at the time of pouring are performed. The presence or absence of was investigated.

  In this investigation, the amount of residual decarburized slag in the furnace is obtained by tilting the converter, receiving the slag discharged from the furnace port into the pan, weighing it with the pan, and tilting the converter from the furnace port. The correspondence with the method estimated from the "furnace body tilt angle" when discharging slag was also confirmed. Since decarburized slag has a relatively high fluidity, it was confirmed that there is a good correlation between the furnace tilt angle and the amount of residual slag in the furnace.

  The survey results are shown in Table 1. The test was carried out continuously for 6 Ch under each condition, and the average values of 2 to 6 Ch are listed in Table 1.

  In addition, the evaluation criterion is “no” (no [P] in the pan after steel is 0.016 mass) when the steel is not bumped during pouring and [P] in the pan after steel is 0.019 mass% or less. % Or less, “◎”), otherwise “×”.

(1) Effect of ingot slag addition in the fifth step (Invention 1, 7, 8, 9, Comparative Example 1)
When the ingot slag was not added under the condition of adding the solidifying agent (Comparative Example 1), the decarburized slag adhered to the furnace bottom was difficult to peel and float early in the first step of the next charge. The dephosphorization rate in the process decreased, and the [P] concentration in the pan after the steelmaking became as high as 0.021%.

  On the other hand, when 10 to 120% of the ingot slag having a particle size of 10 to 50 mm is added to the solidifying agent in the fifth step (Invention 1, 7, 8, 9), the concentration of [P] in the pan after leaving steel Became 0.017% or less by adding ingot slag. However, when 120% of ingot-making slag having a particle size of 10-50 mm is added to the solidifying agent (Invention 9), the concentration of [P] in the pan after leaving the steel is higher than when other 10-100% is added. Therefore, it can be said that the addition ratio of the ingot slag to the solidifying agent is preferably 10 to 100%.

  This is considered as follows. Since the particle size of the ingot slag was moderately large, the ingot slag solidified before being uniformly dissolved into the residual decarburization slag in the furnace. And after the injection of the next charge, ingot slag melted and floated early. This is because the ingot slag has a low melting point and melts even at the hot metal temperature. On the other hand, the melting point of decarburized slag is as high as 1500 ° C. or higher and does not melt at the hot metal temperature.

When the ingot slag melts and floats as described above, the decarburized slag remaining on the converter bottom becomes uneven. Then, hot metal that is flowing strongly agitated by the bottom blowing gas is likely floated by peeling off the uneven slag adhering to the furnace bottom, newly generated FeO, dissolved-air bearing on SiO ₂ or above in the first step Since the molten slag was rapidly reacted on the molten ingot slag and the hot metal bath, the dephosphorization rate was improved and the dephosphorization rate after the first step was improved. However, when the amount of ingot slag added exceeds 100%, it is considered that the above-described mechanism has become difficult to function sufficiently.

(2) Influence of the basicity of charging in the first step (Inventions 2 and 3, Comparative Examples 2 and 3)
When the charging basicity in the first step is 1.0 to 2.0 (Inventions 2 and 3), a moderately fluid forming slag is formed and the dephosphorization reaction proceeds, and then the second A rejection rate of 80% was secured in the process. On the other hand, when the charge basicity was as low as 0.9 (Comparative Example 2), the dephosphorization ability of the slag was low, and the dephosphorization rate in the first step was low. For this reason, it is considered that the amount of [P] brought into the third process has increased, and the [P] concentration in the pan after steelmaking has increased to 0.021% by mass. And when charging basicity is as high as 2.2 (comparative example 3), since the melting | fusing point of slag is high and fluidity | liquidity is low and it did not form much, the rejection rate in a 2nd process falls to 60%. I have. For this reason, it is considered that the amount of [P] brought into the third process has increased, and the [P] concentration in the pan after steelmaking has increased to 0.022% by mass.

(3) Influence of solidifying agent addition (Invention 1, 4, 5, 6)
The particle diameter of the solidifying agent was 50 mm or less, and the ratio of 10 to 50 mm was 20% by mass or more of the entire solidifying agent. If the particle size is too large, it is difficult to hatch in the first step of the next charge. On the other hand, if the particle size is small, it is difficult to mix with the molten slag when the solidifying agent is added.

  When the amount of solidifying agent added is 10 to 30% of the residual decarburization slag in the furnace (Inventions 1 and 5), bumping does not occur at the time of pouring the next charge, and [P] in the pan after steel output is the target value 0.016 mass% or less was achieved.

  On the other hand, when the solidifying agent addition amount is lowered to 8% of the residual decarburization slag amount in the furnace (Invention 4), [P] in the pan after steel is 0.019% by mass and the target is 0.019% or less. The last minute was achieved. This is thought to be due to the fact that the decarburized slag adhered to the furnace bottom was difficult to peel and float early in the first step due to the small amount of solidification material added and the amount of added agglomerated slag being small. .

  In addition, when the amount of solidifying agent added is increased to 40% of the amount of residual decarburized slag in the furnace (Invention 6), the decarburized slag is strongly fixed to the furnace bottom. In addition, it was thought that the [P] concentration in the pan after the steel was achieved to 0.019% by mass and the target of 0.019% by mass or less because it was able to surface even slightly delayed during the first step.

(4) Effect of particle size of ingot slag (Invention 1, Comparative Examples 4 and 5)
If the particle size of the ingot slag is less than 10 mm (Comparative Example 4), it will be finely dissolved in the residual decarburization slag in the furnace, and the time when the decarburization slag peels off and rises from the furnace bottom in the first step of the next charge. It was not noticeably early. It can be considered that the unevenness of the slag adhered to the furnace bottom described above was small, so that the separation of the adhered slag by the molten iron flow was delayed. As a result, it is considered that the [P] concentration in the pan after the steelmaking could not reach the target of 0.020% by mass and 0.019% by mass or less.

  On the other hand, when the particle size of the ingot slag exceeds 50 mm (Comparative Example 5), although the ingot slag melts and floats as described above, the number of irregularities that can be made into the decarburized slag left at the bottom of the converter is too small. Even if the molten iron stirred and flowed strongly by the bottom blowing gas is used, the decarburization slag adhering to the bottom of the furnace does not peel and rise so fast, and the [P] concentration in the pan after the steel is 0.019 It is considered that the mass% and the target of 0.019 mass% or less could not be achieved slightly.

(5) Effect of adding only agglomerated slag without adding a solidifying agent (Comparative Example 6)
When only the ingot slag was added without the addition of a solidifying agent, bumping occurred at the time of pouring the next charge.
In this case, the ingot-making slag that is not in contact with the iron scrap was present in a molten state even after the iron scrap was added, so it is considered that bumping occurred during pouring.

[Example 1]
After performing the decarburization blown by satisfying the predetermined requirements according to the present invention, the furnace body is tilted to decarburize slag (composition: CaO = 30 to 50%, Al ₂ O ₃ = 1 to 5%). , T.Fe = 10-30%, SiO ₂ = 8-15%, MgO = 6-12%, basicity: CaO mass / SiO ₂ mass = 3.5-5.0) Limestone as a solidifying agent to decarburized slag left in the furnace at about 20 kg / t (particle size is 50 mm or less, and the ratio of particle size of 10 to 50 mm is 20% by mass or more of the whole limestone) Ingot slag having a diameter of 10 to 50 mm (composition: CaO about 45%, Al ₂ O ₃ about 20%, T.Fe about 10%, SiO ₂ about 10%, MgO about 10%) 1.0 kg / t (added) 10% by mass of solidifying agent and 50% by mass of ingot slag with respect to solidifying agent) It solidified the decarburization slag.

After that, after adding scrap iron 50t, injection of the next charge (290t (composition: [Si] about 0.4%, [P] about 0.10%)) is performed, and the charging basicity is 1.4. It was added and made as quicklime 4.7 kg / t, blowing dephosphorization blowing (above the target processing temperature of 1330 ° C. oxygen flow rate 40000Nm ³ / h, bottom-blown oxygen flow rate 3000 Nm ³ / h, bottom-blown LPG flow 200 Nm ³ / H) to remove the phosphorus.

About 80% of the slag produced in the first step is discharged as the second step, and the composition of the decarburized slag is CaO = 30 to 50%, Al ₂ O ₃ = 1 to 5%, T. _{Fe = 10~30%, SiO 2 =} 8~15%, MgO = 6~12%, basicity: CaO mass / SiO ₂ mass = 3.5-5.0 become as quicklime 5 kg / t, Light burned Dolomite (composition: CaO = 60%, MgO = 34%) was added at 3.5 kg / t, and decarburized and blown to [C] ≈0.10% (at the same time, dephosphorization proceeded). At this time, was a top-blown oxygen flow rate 80000Nm ³ / h, bottom-blown oxygen flow rate 3000 Nm ³ / h, bottom and blown LPG flow 200 Nm ³ / h.

  As the fourth step, the decarburized slag produced in the third step was left out in the furnace, and a molten steel having a [P] concentration of 0.014% after the steel was obtained.

As a fifth step, the furnace body is tilted to discharge a part of the decarburized slag from the furnace port, and to the decarburized slag left in the furnace about 20 kg / t as a solidifying agent, limestone (particle size is 50 mm or less, An agglomerated slag having a particle size of 10 to 50 mm together with 2 kg / t in which the ratio of the diameter of 10 to 50 mm is 20% by mass or more of the whole limestone (composition: CaO about 45%, Al ₂ O ₃ about 20%, T.Fe About 10%, SiO ₂ about 10%, MgO about 10%) Add 1.0kg / t (solidifying agent is 10% by mass for residual slag in the furnace, ingot slag is 50% by mass for solidifying agent) The decarburized slag was solidified.

  After that, returning to the first step again, after adding the scrap iron 50t, performing the next charge injection (290t (composition: [Si] about 0.4%, [P] about 0.10%)), The process up to the fifth step was repeated. The average value obtained by carrying out for 5 Ch continuously is shown in Invention 1 of Table 1. No bumping occurred during pouring, and the [P] concentration in the pan after steeling was as low as 0.014% by mass.

[Comparative Example 1]
After performing the decarburization blown by satisfying the predetermined requirements according to the present invention, the furnace body is tilted to decarburize slag (composition: CaO = 30 to 50%, Al ₂ O ₃ = 1 to 5%). , T.Fe = 10-30%, SiO ₂ = 8-15%, MgO = 6-12%, basicity: CaO mass / SiO ₂ mass = 3.5-5.0) Add 5kg / t of limestone as a solidifying agent to decarburized slag left in furnace about 20kg / t (particle size is 50mm or less and the ratio of particle size is 10-50mm is 20% by mass or more of the whole limestone) (The solidifying agent was 10% by mass with respect to the residual slag in the furnace) to solidify the decarburized slag.

After that, after adding 50t of scrap iron, injection of the next charge (290t (composition: [Si] about 0.4%, [P] about 0.10%) is performed, and the charging basicity is 1.4. so as to the addition of lime 5.0 kg / t, blowing dephosphorization blowing (above the target processing temperature of 1330 ° C. oxygen flow rate 40000Nm ³ / h, bottom-blown oxygen flow rate 3000 Nm ³ / h, bottom-blown LPG flow rate 200 Nm ³ / h) to remove phosphorus.

About 80% of the slag produced in the first step is discharged as the second step, and the composition of the decarburized slag is CaO = 30 to 50%, Al ₂ O ₃ = 1 to 5%, T. _{Fe = 10~30%, SiO 2 =} 8~15%, MgO = 6~12%, basicity: CaO mass / SiO ₂ mass = 3.5-5.0 become as quicklime 5 kg / t, Light burned Dolomite (composition: CaO = 60%, MgO = 34%) was added at 4 kg / t, and decarburized and blown to [C] ≈0.10% (at the same time, dephosphorization proceeded). At this time, was a top-blown oxygen flow rate 80000Nm ³ / h, bottom-blown oxygen flow rate 3000 Nm ³ / h, bottom and blown LPG flow 200 Nm ³ / h.

  As the fourth step, the decarburized slag generated in the third step is left in the furnace, and as the fifth step, the furnace body is tilted to discharge a part of the decarburized slag from the furnace port and enter the furnace. About 20 kg / t of remaining decarburized slag was added 2 kg / t as a solidifying agent (having a particle size of 50 mm or less and a ratio of particle size of 10 to 50 mm was 20% by mass or more of the entire limestone) (inside the furnace) The decarburized slag was solidified by adding 10% by mass of a solidifying agent to the residual slag.

  After that, returning to the first step again, after adding the scrap iron 50t, performing the next charge injection (290t (composition: [Si] about 0.4%, [P] about 0.10%)), The process up to the fifth step was repeated. The average value obtained by continuously performing for 5 Ch is shown in Comparative Example 1 of Table 1. No bumping occurred during pouring, and the [P] concentration in the pan after steeling was 0.021% by mass, not reaching the target value of 0.019% or less.

Claims (2)

A method for producing molten steel by refining blast furnace hot metal to produce molten steel,
In the first step, scrap iron and hot metal are charged into the converter, the amount of the auxiliary raw material to be charged is adjusted, and the slag charging basicity (CaO mass / SiO ₂ mass) is 1.0 to 2.0, treatment Perform dephosphorization by blowing acid at a temperature of 1350 ° C. or lower,
The slag generated in the first step is discharged as the second step,
As a third step, the composition is mass% with addition of flux and blowing acid, CaO = 30-50%, Al ₂ O ₃ = 1-5%, T.W. _{Fe = 10~30%, SiO 2 =} 8~15%, a MgO = 6 to 12%, basicity: to produce a decarburization slag is CaO wt / SiO ₂ mass = 3.5 to 5.0 To decarburize and dephosphorize,
Steel is left leaving the decarburized slag generated in the third step as the fourth step,
In the fifth step, the decarburized slag left in the furnace, quick lime, limestone, light calcined dolomite, raw dolomite and cooled and solidified decarburized having a particle size of 50 mm or less and a ratio of 10 to 50 mm of 20% by mass or more. Together with one or more solidifying agents selected from slag, the particle size is 10 to 50 mm and the composition is mass%, CaO = 30 to 50%, Al ₂ O ₃ = 10 to 30%, T.P. Ingot slag with Fe = 5-15% and SiO ₂ = 8-20% is added to solidify the decarburized slag,
After that, returning to the first step again, the scrap iron and hot metal of the next charge are charged into the converter after finishing the fifth step, and the dephosphorization process is performed under the conditions defined as the first step. A method for producing molten steel, comprising repeatedly performing up to five steps sequentially.   In the fifth step, the mass of the solidifying agent added to the decarburized slag left in the furnace is 10 to 30% of the mass of the decarburized slag, and the mass of the ingot slag to be added is the mass of the solidifying agent. The manufacturing method of the molten steel of Claim 1 characterized by the above-mentioned.