JP6460265B2 - Converter blowing method - Google Patents

Description

  The present invention is a top-bottom converter, which removes phosphorus with a low Si concentration with a small amount of slag and performs a series of blowing from the hot metal without discharging the slag produced by the dephosphorization. The present invention relates to a converter blowing method for melting steel.

  In recent years, the demand for steel having a high C concentration and a low P concentration is increasing, and it is desired to mass-produce molten steel having such a composition using a converter. However, even if the molten iron is oxidized by the top-blown oxygen gas to produce FeO in the slag, if the C concentration in the molten iron is high, FeO is likely to be reduced by the C in the molten iron. Thereby, during the period when the C concentration in the molten iron during blowing is high, the FeO concentration in the slag changes at a low value, and the dephosphorization ability of the slag becomes low. Therefore, the P concentration in the molten iron becomes high during that period. For the above reasons, it is difficult to lower the P concentration in the molten steel when the C concentration in the molten steel is high.

  Therefore, since the C concentration is as high as 3.5% by mass or more and the temperature is as low as 1400 ° C. or less, the hot metal is dephosphorized in the hot metal stage which is thermodynamically advantageous for the dephosphorization reaction, A hot metal dephosphorization method for separating and removing high slag has been proposed. When this method is used, a molten steel having a high C concentration and a low P concentration can be easily produced simply by decarburizing the dephosphorized hot metal to a desired C concentration. As a method with the highest productivity among them, Patent Document 1 discloses a method of using two converters, dephosphorizing the hot metal in one converter, and then decarburizing and blowing in the other converter. Yes.

  However, in recent years, it has become difficult to use fluorite (calcium fluoride), which has been used as a quickening agent for quicklime, which is a major dephosphorizing agent, in order to reduce the environmental burden. Although the melting point of quicklime is very high at about 2300 ° C., it has become difficult to obtain high dephosphorization utilization efficiency by rapid hatching when hot metal dephosphorization is performed at a relatively low temperature. In addition, if two converters are used for dephosphorization and decarburization, the manufacturing cost increases. Therefore, it is possible to manufacture molten steel having a high C concentration and a low P concentration in a series of blowing processes from the hot metal stage. desired.

Moreover, in order to reduce the environmental load, reduction of the amount of slag generation is also calculated | required. In order to increase the dephosphorization utilization efficiency of quicklime, it is effective to increase the basicity (CaO / SiO ₂ mass ratio) of the slag. Therefore, in order to increase the basicity of slag using a small amount of quicklime, the Si concentration in the hot metal may be lowered. Therefore, when oxygen gas is blown up to the hot metal in order to perform hot metal dephosphorization treatment, Si in the hot metal is oxidized and SiO ₂ is generated. Therefore, the higher the Si concentration in the hot metal, the higher the SiO ₂ that is taken into the slag. The amount will increase. Therefore, in order to increase the basicity of slag with a small amount of slag, it is necessary to lower the Si concentration in the hot metal.

As a method for performing hot metal dephosphorization with a small amount of quicklime without using fluorite, a method of spraying a mixed powder of CaO powder, Al ₂ O ₃ powder, and Fe ₂ O ₃ powder together with oxygen gas onto hot metal is disclosed in Patent Document 2. Is disclosed. In this method, CaO powder, which is a main dephosphorizing agent, is sprayed onto a hot spot having a high temperature of 2000 ° C. or higher where the top blown oxygen and hot metal come into contact, and CaO having a high melting point is hatched relatively quickly. At this time, the hatching of CaO is further accelerated by mixing CaO and Al ₂ O ₃ that generates a low melting point compound. Also, by mixing CaO and a low melting point compound, and also mixing Fe ₂ O ₃ that can improve the oxidizing power of slag, the fluidity of the slag generated at the fire point is ensured and the dephosphorization ability of the slag is also improved. It is said that hot metal dephosphorization can be realized with extremely high efficiency.

  However, the slag produced by this method focuses only on the viewpoint of improving the dephosphorization efficiency at the hot metal stage, and as a series of blowing from the hot metal without discharging the slag produced by the dephosphorization, the decarburization blow is performed as it is. It is unclear whether molten steel with a high C concentration and a low P concentration can be produced even if smelting is continued.

  On the other hand, Patent Document 3 discloses a method for producing a molten steel having a low C concentration of 0.20 to 0.25% by mass by blowing CaO powder together with top blowing oxygen gas into a hot metal in a converter. Is disclosed. However, in this method, since iron oxide is not blown up together with oxygen gas, the FeO concentration of the CaO—FeO-based melt in the slag generated at the fire point is low and the temperature of the fire point is high. Dephosphorization utilization efficiency as high as the method described in 1) cannot be obtained. Moreover, it is unclear whether the P concentration can be sufficiently reduced even when the C concentration is difficult to dephosphorize is 0.3 mass% or more.

  Patent Document 4 discloses a high carbon molten steel smelting method in which lime powder is blown for 70% or more of the blowing time. However, in this method, it is necessary to reduce the P concentration of the hot metal to 0.05% by mass or less in advance by a dephosphorization process that is separately performed, and the dephosphorization process and the decarburization process cannot be performed in a series of blowing processes. .

Japanese Patent Publication No.55-30042 Japanese Patent No. 3525766 JP 2005-139511 A JP 2002-275520 A

  The present invention has been made in view of the above-described problems, and an object thereof is to provide a converter blowing method for producing molten steel having a high C concentration and a low P concentration without interrupting blowing from the hot metal stage with a small amount of slag. And

The gist of the present invention is as follows.
(1) A hot metal having a Si concentration of 0.15% by mass or less and a P concentration of 0.09% by mass or more is charged into the top-bottom blown converter, and is a mixed powder of CaO powder and Fe ₂ O ₃ powder. The mass ratio of CaO content (CaO mass + CaCO ₃ mass × 0.56) to Fe ₂ O ₃ content (Fe ₂ O ₃ mass + FeO mass) is 9: 1 to 6: 4, and CaO, CaCO ₃ , FeO, and Fe ₂ O _{3 in} a total amount of 95% by mass or more are mixed with oxygen gas at a flow rate of 1.5 to 3.0 Nm ³ / min / molten ton. The amount of CaO content in the powder is 10-15 kg / molten ton, the supply rate of CaO is 1.5-3.0 kg / min / molten ton, and the C concentration in the molten iron is 2.5% by mass. While this is the case, the top blowing of the mixed powder is terminated, and the top blowing of the mixed powder is completed. Converter blowing method but the C concentration continues to blow over the oxygen gas even after the termination, characterized in that to produce the molten steel above 0.3 mass%.
(2) The converter blowing method according to (1), wherein a maximum particle size of the mixed powder is 0.15 mm or less.
(3) The converter blowing method according to (1) or (2), wherein during the top blowing, CO ₂ gas is further blown at a bottom of 0.05 to 0.3 Nm ³ / min / molten metal ton. .

  According to the present invention, molten steel having a high C concentration and a low P concentration can be produced with high efficiency without interrupting blowing from hot metal having a P concentration of 0.09% by mass or more with a small amount of slag.

  In order to produce a high-efficiency molten steel with a high C concentration and a low P concentration from ordinary hot metal (P concentration of 0.09 to 0.15% by mass), the present inventors have added CaO-containing powder together with oxygen gas. As a result of diligent study on the blowing method to blow, the following method was conceived. The molten steel having a high C concentration refers to a molten steel having a C concentration of 0.3 to 1.0 mass%, and the molten steel having a low P concentration refers to a molten steel having a P concentration of 0.02 mass% or less. To do.

In the converter blowing method according to the present invention, molten iron having a Si concentration of 0.15% by mass or less (P concentration of 0.09% by mass or more) is charged into an upper bottom blowing converter, and CaO powder and Fe ₂ are added. It is a mixed powder of O ₃ powder, the mass ratio of CaO component and Fe ₂ O _{3 component} is 9: 1 to 6: 4, and the total of CaO, CaCO ₃ , FeO, and Fe ₂ O ₃ is 95 mass % Of the mixed powder which is not less than 1.5% with oxygen gas at a flow rate of 1.5 to 3.0 Nm ³ / min / molten ton. / Feeding ton, and the feeding rate of the CaO component is 1.5 to 3.0 kg / min / toning ton, and the top blowing of the mixed powder is completed while the C concentration in the molten iron is 2.5% by mass or more. The oxygen gas is blown up even after the powder mixture is blown up. A molten steel having a C concentration of 0.3% by mass or more is continuously produced without interruption. During the treatment, an upper-bottom blowing converter having a bottom blowing function is used to stir the molten steel and promote dephosphorization. For the bottom blowing, CO gas or CO is introduced from the tuyere installed at the bottom of the furnace. It is preferable to blow _two gases at 0.05 to 0.3 Nm ³ / min / molten metal ton.

The CaO powder is mainly powdered lime, limestone or the like, and is mainly CaO or CaCO ₃ . On the other hand, Fe ₂ O ₃ powder is mainly powdered iron ore and mainly composed of Fe ₂ O ₃ , but may contain FeO or crystal water. In addition, when the CaO powder is a mixed powder of quicklime and limestone, the CaO content is calculated by (CaO mass + CaCO ₃ mass × 0.56). In addition, Fe ₂ O ₃ minutes is regarded as Fe ₂ O ₃ minutes including FeO.

According to this method, a mixed powder of CaO powder and Fe ₂ O ₃ powder is blown up onto the hot metal bath surface together with O ₂ gas, and high basicity slag is generated at a fire point of 2000 ° C. or higher. As a result, a slag having a high basicity and a high FeO concentration is formed in the hot spot region, and reacts with P in the molten iron, so that a very high dephosphorization efficiency can be realized. Even if the basicity is high, if the FeO concentration is high, the melting point of the slag becomes relatively low, so that fluidity is ensured and the dephosphorization reaction proceeds very quickly. In addition, the maximum particle size of the mixed powder is preferably 0.15 mm or less in consideration of the conveyance with O ₂ gas and the ease of slag formation at the fire point.

The activity of P in molten iron increases as the C concentration in molten iron increases. That is, the higher the C concentration in the molten iron, the easier the dephosphorization reaction proceeds. Therefore, when the relationship between the C concentration in the molten iron and the dephosphorization rate at the time of completion of blowing the mixed powder of CaO powder and Fe ₂ O ₃ powder was investigated, the C concentration in the molten iron was 2.5% by mass or more. the mixed powder of CaO powder and Fe ₂ O ₃ powder were blown upward together with O ₂ gas while there, then, when the O ₂ gas alone was blown on to the decarburization blowing, readily low high P concentration C concentration It has been found that molten steel can be produced.

Simultaneously with the dephosphorization reaction, FeO in the slag is reduced by C in the molten iron, and as the FeO concentration in the slag decreases, the high melting point 2CaO · SiO ₂ -3CaO · P ₂ O ₅ solid solution or 3CaO in the molten slag · _P 2 _{O 5} solid phase crystallizes out. Then, after most of the P ₂ O ₅ in the slag crystallizes as the solid solution, the remaining FeO in the liquid phase is further reduced, the melting point of the liquid phase slag rises, and the liquid phase slag solidifies.

In addition, after the C concentration in the molten iron is 2.5% by mass or more, the mixed powder of CaO powder and Fe ₂ O ₃ powder is blown up to increase the dephosphorization rate, and then only O ₂ gas is blown up. During decarburization and blowing, the temperature of the molten iron and slag continues to rise. Thus, among the CaO powder and Fe ₂ O ₃ powder slag solidified above produced by blown onto the mixed powder, high partial concentration of FeO is melted, is reduced by C in the molten iron. On the other hand, as the temperature rises during blowing, part of 2CaO · SiO ₂ -3CaO · P ₂ O ₅ solid solution or 3CaO · P ₂ O ₅ solid phase in slag also dissolves in the molten part where FeO concentration in slag is high After that, it is reduced by C in the molten iron to produce reverse phosphorus. _However, since extremely high _{2CaO · SiO 2 -3CaO · P 2} O 5 the melting point of the solid solution or 3CaO _· P 2 _{O 5} solid phases of about 1750 ° C. or higher, recovery phosphorus rate was also found extremely slow.

Next, the experimental results for verifying each condition in the blowing process will be described.
First, desiliconization treatment was performed on the hot metal having a P concentration of about 0.1% by mass discharged from the blast furnace so that the Si concentration in the hot metal was 0.01 to 0.15% by mass or less. After that, blowing was performed using an upper bottom blowing converter. In the blowing process, first, the desiliconized molten iron is charged into an upper bottom blowing converter, and then, as a temperature adjusting agent, Fe ₂ O _{3 is} used so that the molten steel temperature at the end of blowing is 1670 ± 5 ° C. Was appropriately added. Next, only CaO powder having a particle size of 0.15 mm or less, or either or both of Fe ₂ O ₃ powder and Al ₂ O ₃ powder in CaO powder was mixed with O ₂ gas and top-blown.

(Step 1)
First, an object of the present invention is to provide a converter blowing method for producing molten steel having a high C concentration and a low P concentration without interrupting blowing from the hot metal stage with a small amount of slag. In view of the above, C concentration and P concentration at the end of blowing are unified to C ≦ 0.85-0.99 mass% and P ≦ 0.020 mass% (molten steel temperature is 1670 ± 5 ° C.), CaO powder and C concentration in the molten iron in the end of blowing on the mixed powder of _Fe 2 _{O 3} powder as a range of 2.5 to 3.6 wt%, oxygen top blowing a mixed powder of CaO powder and _Fe 2 _{O 3} powder The steel was blown up while the C concentration in the molten iron together with the gas satisfied a predetermined concentration condition, and then only the O ₂ gas was continuously blown up to produce molten steel.
At this time, “the influence of the Si concentration in the hot metal, the influence of the composition of the mixed powder of the CaO powder and the Fe ₂ O ₃ powder, the influence of the flow rate of the top blowing oxygen gas, the amount of top blowing of the CaO content in the mixed powder during the top blowing. And the influence of the supply rate of the CaO content ”.
At this time, unless otherwise specified, the Si concentration in the hot metal: 0.05% by mass, the mass ratio of CaO content and Fe ₂ O ₃ content in the mixed powder of CaO powder and Fe ₂ O ₃ powder is 4: 1, mixing The CaO content in the powder was unified to 10 to 15 kg / molten ton, 1.5 to 3.0 kg / min / molten ton, and O ₂ gas flow rate 2.0 to 2.5 Nm ³ / min / molten ton. In addition, the C concentration in the molten iron at the time when the mixed powder is blown up is the relationship between the supply amount of the oxygen source (oxygen gas, iron oxide, etc.) and the amount of oxygen necessary for oxidizing the hot metal components (carbon, silicon) ( However, the oxygen utilization efficiency expected from operational experience is taken into account).

Table 1 shows the blowing conditions and results. In addition, the ratio of the top-blown mixed powder shown in Table 1 indicates the ratio when impurities are removed, and in the mixed powder, the total of CaO content + Fe ₂ O ₃ minutes + Al ₂ O ₃ is 95% by mass or more. Shall.

(1) Influence of Si concentration in hot metal In Examples 1 to 3 of the present invention, hot metal whose Si concentration was reduced to 0.01 to 0.15 mass% by desiliconization treatment was charged into an upper-bottom blowing converter, and Fe 6.3 kg of iron ore containing 96% by mass of ₂ O ₃ / ton ton was added on top. The flow rate of 2.5 Nm ³ / min / hot metal minute CaO with _{O 2} gas ton _Fe 2 _{O 3} minutes and 4 in the weight ratio: 1 and the mixed powder (CaO in the powder mixture is 15 kg / hot metal ton) Was blown up. At this time, the top blowing speed of the CaO content in the mixed powder was set to 2.5 kg / min / molten ton, and decarburization blowing was performed at the same oxygen flow rate even after finishing the top blowing of the mixed powder. As a result, it was confirmed that the Si concentration in the molten iron was 0.15% or less and the P concentration in the molten steel after the treatment could be set to a target value of 0.020% by mass or less.

  Since the mixed powder melts at the hot spot and contributes to the dephosphorization reaction, Si in the hot metal is not so necessary for the hatching of CaO. Therefore, it is considered that the lower the Si concentration in the hot metal, the higher the average basicity of the slag produced, and the lower the P concentration in the molten steel after treatment.

(2) Influence of top blown O ₂ flow rate In Invention Examples 4 to 6, molten iron whose Si concentration was lowered to 0.05% by mass by desiliconization treatment was charged into the top bottom blown converter, and Fe ₂ O ₃ was added. Iron ore 6.3 to 7.5 kg / molten iron ton containing 96% by mass was added on top. And the mixed powder (CaO content in the mixed powder is 4: 1) by mass ratio of CaO content and Fe ₂ O ₃ content together with O ₂ gas with a flow rate of 1.5 to 3.0 Nm ³ / min / molten ton. 10-15 kg / molten ton) was blown up. At this time, the top blowing speed of the CaO content in the mixed powder was 1.5 to 2.5 kg / min / molten ton, and decarburization blowing was performed at the same oxygen flow rate even after finishing the mixed powder top blowing. . As a result, the flow rate of the top blown O ₂ gas is 1.5 to 3.0 Nm ³ / min / molten iron ton, and the P concentration in the molten steel after treatment can be set to a target value of 0.020% by mass or less. I was able to confirm.

Note that if the flow rate of the top blown O ₂ gas is less than 1.5 Nm ³ / min / molten iron ton, the blowing time of the converter increases and the productivity decreases, so the lower limit of the O ₂ gas flow rate is 1.5 Nm. ³ / min / molten ton. On the other hand, if the flow rate of O ₂ gas exceeds 3.0 Nm ³ / min / molten ton, there is a possibility that the FeO concentration in the slag increases excessively and slopping occurs in the initial stage of blowing. Therefore, the upper limit of the flow rate of O ₂ gas is set to 3.0 Nm ³ / min / molten metal ton.

(3-1) Influence of composition of mixed powder (ratio of CaO content in mixed powder of CaO powder and Fe ₂ O ₃ powder)
In Invention Examples 7 to 9 and Comparative Examples 1 to 4, the hot metal whose Si concentration was reduced to 0.05% by mass by desiliconization treatment was charged into an upper bottom blowing converter, and 96% by mass of Fe ₂ O ₃ was contained. Iron ore 0 to 8.3 kg / molten iron ton was added on top. Then, the flow rate together with _{O 2} gas 2.5 Nm ³ / min / hot metal ton, only CaO powder 15 kg / molten pig iron ton or 20 kg / molten pig iron ton or CaO content of the powder mixture, is a 15～20Kg / hot metal ton, CaO content The mixed powder in which the ratio of Fe ₂ O ₃ minutes is changed is top blown at a CaO powder or CaO content in the mixed powder at an upper blowing rate of 2.0 to 2.5 kg / min / molten ton, and the CaO powder or Even after finishing the top blowing of the mixed powder, decarburization blowing was performed at the same oxygen flow rate. As a result, when the mass ratio of the CaO content and the Fe ₂ O ₃ content in the mixed powder is 9: 1 to 6: 4, the P concentration in the molten steel after the treatment is 0.020% by mass which is a target value. I was able to:

When only CaO powder was blown up as in Comparative Examples 1 and 2, the amount of CaO content was 15-20 kg / molten ton and the CaO content was sprayed 2.0-2.5 kg / min / Even if it changed with hot metal ton, P concentration in the molten steel after a process was set to 0.030-0.033 mass%, and the target value (P concentration <= 0.020 mass%) was not able to be achieved. When only CaO powder is used, since the FeO concentration is low in the CaO-FeO melt in the slag generated at the fire point, the melting point becomes relatively high, and therefore the fluidity becomes relatively low. Furthermore, the temperature of the CaO—FeO melt in the slag also becomes higher than when the Fe ₂ O ₃ powder is mixed and blown up. For the above reasons, it is considered that the dephosphorization reaction does not proceed easily, and the dephosphorization rate has become low even when the same CaO basic unit is blown up. For the same reason, even when the Fe ₂ O ₃ concentration in the mixed powder is as low as 5% by mass as in Comparative Example 3, it is considered that the P concentration in the molten steel after the treatment did not reach the target value.

On the other hand, when the Fe ₂ O ₃ concentration in the mixed powder was increased to 45% by mass as in Comparative Example 4, the P concentration in the molten steel after the treatment did not reach the target value. In this case, the FeO concentration in the CaO-FeO melt in the slag produced at the fire point is excessively increased, the CaO concentration is decreased, and the dephosphorization ability of the CaO-FeO melt is decreased. This is thought to be the cause. Further, it is considered that the dephosphorization reaction efficiency has been lowered because the hot spot cooling by Fe ₂ O ₃ is too strong, the temperature of the CaO—FeO melt is excessively lowered and the fluidity is lowered. From the above experimental results, the mass ratio of CaO content to Fe ₂ O ₃ content in the mixed powder is 9: 1 to 6: 4.

(3-2) Influence of the composition of the mixed powder (CaO + Fe ₂ O ₃ + Al ₂ O ₃ mixed powder)
In Comparative Example 5, when the mixed powder of CaO, Fe ₂ O ₃ and Al ₂ O ₃ was used and blown under the conditions shown in Table 1, the P concentration in the molten steel after the treatment did not reach the target. In this case, the CaO—FeO—Al ₂ O ₃ melt produced at the fire point in the slag has a low melting point and very high fluidity. Therefore, the dephosphorization reaction proceeds very efficiently while the mixed powder is being blown up. However, when the temperature of the molten iron rises during the subsequent decarburization blowing, it is considered that the slag produced when the mixed powder was blown up began to melt and rebounded significantly.

(4) Influence of CaO amount in mixed powder In Invention Examples 10 and 11 and Comparative Example 6, hot metal having a Si concentration lowered to 0.05 mass% by desiliconization treatment was charged into an upper bottom blowing converter, Iron ore 6.3 to 8.0 kg / molten iron ton containing 96% by mass of Fe ₂ O ₃ was added on top. The flow rate and 2.5~3.5Nm ³ / min / hot metal minute CaO with _{O 2} gas ton _Fe 2 _{O 3} minutes at a mass ratio of 4: 1 CaO mixed powder (mixed powder in which the 8 Up to 15 kg / molten ton), with the top blowing rate of CaO in the mixed powder being 1.5 to 2.5 kg / min / molten ton, and even after finishing the mixed powder top blowing, at the same oxygen flow rate. Decarburized and blown. As a result, when the CaO content in the mixed powder was 10 to 15 kg / molten ton, the P concentration in the molten steel after the treatment could be set to the target value of 0.020% by mass or less.

However, when the CaO content in the mixed powder was reduced to 8 kg / molten ton as in Comparative Example 6, the P concentration in the molten steel after treatment did not reach the target value. Most of (P ₂ O ₅ ) incorporated in the slag is present in the slag as a 2CaO · SiO ₂ -3CaO · P ₂ O ₅ solid solution or a 3CaO · P ₂ O ₅ solid phase. That is, it was confirmed that a certain amount of CaO was required in the slag. Therefore, the lower limit of the CaO amount is 10 kg / molten ton. From the viewpoint of the above experimental results and the environmental load and the cost of quicklime, the CaO amount in the mixed powder is 10 to 15 kg / molten ton.

(5) Effect of top blowing speed of CaO content in mixed powder In Invention Examples 12 to 14 and Comparative Examples 7 to 8, hot metal having a Si concentration reduced to 0.05 mass% by desiliconization treatment The furnace was charged with 6.3 to 7.5 kg of iron ore containing 96% by mass of Fe ₂ O ₃ / molten metal ton. And the mixed powder (CaO in the mixed powder is 15 kg) in which the flow rate is 2.0 to 2.5 Nm ³ / min / O ₂ gas of molten iron and the CaO content and the Fe ₂ O ₃ content are 4: 1 by mass ratio. / Molten metal ton) is decarburized at the same oxygen flow rate even after finishing the top spraying of the mixed powder with a top spraying speed of CaO in the mixed powder of 1.3 to 3.5 kg / min / molten ton. Blowed. As a result, if the top blowing speed of the CaO content in the mixed powder is 1.5 to 3.0 kg / min / molten ton, the P concentration in the molten steel after the treatment is set to a target value of 0.020% by mass or less. We were able to.

  However, when the top blowing speed of the CaO content in the mixed powder was lowered to 1.3 kg / min / molten ton as in Comparative Example 7, the P concentration in the molten steel after the treatment did not reach the target value. In this case, it is considered that the CaO concentration in the CaO—FeO melt produced at the fire point in the slag is excessively lowered, and the dephosphorization ability of the CaO—FeO melt is lowered. On the other hand, when the top blowing speed of the CaO content in the mixed powder is increased to 3.5 kg / min / molten ton as in Comparative Example 8, the CaO concentration in the CaO-FeO melt generated at the fire point in the slag is increased. It is considered that the melting point of the CaO—FeO melt is increased, the fluidity is lowered, and the dephosphorization reaction rate is lowered. For the above reasons, the top blowing speed of the CaO component in the mixed powder is 1.5 to 3.0 kg / min / molten iron ton.

(Step 2)
Next, based on what was confirmed in Step 1 above, hot metal having a Si concentration of 0.15 mass% or less and a P concentration of 0.09 mass% or more was charged into the top-bottom blown converter, and CaO A mixed powder of powder and Fe ₂ O ₃ powder and having a mass ratio of 9: 1 to 6: 4 of CaO and Fe ₂ O ₃ is flow rate of 1.5 to 3.0 Nm ³ / min. / Blowing together with oxygen gas of hot metal ton, and during top blowing, the top blowing amount of CaO content in the mixed powder is 10 to 15 kg / molten ton, and the supply rate of CaO content is 1.5 to 3.0 kg / Min / ton Ton
The effect of ending the top blowing of the mixed powder while the C concentration in the molten iron is equal to or higher than the predetermined concentration, and continuing the top blowing of the oxygen gas after the top blowing of the mixed powder is completed. It was confirmed whether or not it was possible to stably manufacture molten steel having a concentration of 0.3% by mass or more and a P concentration of 0.020% or less. In addition, similarly to the blowing of Step 1, in order to adjust the molten steel temperature after the treatment, 3.6 to 7.5 kg of iron ore containing 96 mass% of Fe ₂ O ₃ / molten iron ton was added on top.

(6) Influence of C concentration in molten steel at the time of finishing the top blowing of mixed powder In this investigation, in addition to the above-mentioned common conditions, the C concentration in the molten steel after treatment was set to 0.80 to 1.00. Standardized to mass%. In Examples 4, 6, 12, 13, 17, and 18 of the present invention, which were the objects of this investigation, the P concentration after treatment was able to achieve 0.020% by mass or less.

  On the other hand, when the C concentration in the molten iron at the time of finishing the top blowing of the mixed powder as in Comparative Examples 10 and 11 is reduced to 2.1% by mass or 2.4% by mass, the P concentration in the molten steel after the treatment is increased. The target value was not reached. Since the activity of P is higher as the C concentration in the molten iron is higher, the dephosphorization reaction is more likely to proceed. For this reason, as described above, it is considered that the dephosphorization rate is improved by terminating the top blowing of the mixed powder while the C concentration in the molten iron is 2.5% by mass or more. For the above reason, the top blowing of the mixed powder is terminated while the C concentration in the molten iron is 2.5% by mass or more. In addition, since decarburization proceeds simultaneously at the time of dephosphorization, the upper limit of the C concentration of the molten iron when the top blowing of the mixed powder is finished is substantially about 3.6% by mass.

(7) Effect of C concentration in molten steel after treatment In this investigation, in addition to the above-mentioned common conditions, the C concentration in the molten steel at the point of time when the top blowing of the mixed powder is finished is 3.0 mass% or 3 Standardized to 1% by mass. In Examples 15 to 18 of the present invention, the hot metal having a Si concentration lowered to 0.05% by mass by desiliconization treatment was charged into an upper bottom blowing converter, and Fe ₂ O ₃ was 96% by mass. The iron ore containing 6.3 kg / molten iron ton was added on top. The flow rate of 2.5 Nm ³ / min / hot metal minute CaO with _{O 2} gas ton _Fe 2 _{O 3} minutes and 4 in the weight ratio: 1 and the mixed powder (CaO in the powder mixture is 15 kg / hot metal ton) Was blown up at a CaO content in the mixed powder of 2.5 kg / min / molten ton, and decarburized and blown at the same oxygen flow rate even after the mixed powder was blown up. As a result, when the C concentration in the molten steel after treatment is in the range of 0.3 to 1.0 mass%, the P concentration in the molten steel after treatment can be set to a target value of 0.020 mass% or less. I understood.

As described above, since the temperature of the molten iron and slag continues to rise rapidly after finishing the top blowing of the mixed powder, in the solidified slag generated when the mixed powder of CaO powder and Fe ₂ O ₃ powder was being blown up The part with a high FeO concentration is melted and reduced by C in the molten iron. On the other hand, as the temperature rises during blowing, part of the 2CaO · SiO ₂ -3CaO · P ₂ O ₅ solid solution in the slag also dissolves in the portion where the FeO concentration is high, and then the solid solution is reduced by C in the molten iron. This causes recovery. However, the melting point of 2CaO.SiO ₂ -3CaO.P ₂ O ₅ solid solution is as high as about 1750 ° C. or higher, and if the C concentration is 0.3% by mass or more, the reversion rate is extremely slow. It is thought that the concentration was achieved.

  In addition, as above-mentioned, after finishing the top blowing of mixed powder, although there is a possibility of returning, it is hard to think that dephosphorization will advance notably. Therefore, even if the C concentration in the molten iron is 1.0% by mass or more, the P concentration at the time after the end of the mixed powder top blowing is considered to be lower than the P concentration when the C concentration is 1.0% by mass.

  Further, in the experiment, it is assumed that a molten steel having a C concentration of 0.3 to 1.0% by mass is produced as a high C concentration, but the effect is particularly effective in a molten steel having a C concentration of 0.8 to 1.0% by mass or more. Is more preferable because it becomes more prominent.

On the other hand, as in Comparative Example 9, when blowing was continued until the C concentration in the treated molten steel reached 0.2% by mass, the P concentration in the treated molten steel did not reach the target value. When the C concentration in the molten steel after the treatment is lowered to 0.2% by mass, the molten iron is remarkably oxidized by the oxygen gas blown up, and around the solidified slag generated when the mixed powder was blown up. A molten slag having a high FeO concentration is generated, and the dissolution rate of the solidified slag is increased. Then, P ₂ O ₅ that has melted out of the solidified slag is reduced by C in the molten steel, thereby causing recovery. As a result, it is considered that the P concentration in the molten steel after treatment was 0.025% by mass, and the target value could not be achieved.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example)
The hot metal in which the Si concentration in the hot metal was reduced to 0.05% by mass by desiliconization was charged into the top-bottom blowing converter, and the iron ore containing 96% by mass of Fe ₂ O ₃ was increased to 6.3 kg / molten ton. Added. The flow rate of 2.5 Nm ³ / min / hot metal minute CaO with _{O 2} gas ton _Fe 2 _{O 3} minutes and 4 in the weight ratio: 1 and the mixed powder (CaO in the powder mixture is 15 kg / hot metal ton) Was blown off at the same oxygen flow rate even after finishing the top blowing of the mixed powder. The C concentration in the molten iron at the end of the top blowing of the mixed powder was 3.1% by mass. During the treatment, CO ₂ gas was continuously blown from the tuyere installed at the bottom of the furnace at 0.1 Nm ³ / min / molten metal ton.

  As a result, the molten steel after treatment was 1670 ° C., and in the molten steel, the C concentration was 0.98 mass% and the P concentration was 0.015 mass%, achieving the target P concentration of 0.020 mass% or less.

(Comparative example)
The hot metal in which the Si concentration in the hot metal was reduced to 0.05% by mass by desiliconization was charged into the top-bottom blowing converter, and the iron ore containing 96% by mass of Fe ₂ O ₃ was added at 8.3 kg / molten ton. Added. The flow rate of 2.5 Nm ³ / min / hot metal minute CaO with _{O 2} gas ton _Fe 2 _{O 3} minutes and 4 in the weight ratio: 1 and the mixed powder (CaO in the powder mixture is 20 kg / hot metal ton) Was blown up at a rate of 1.5 kg / min / molten ton of CaO in the mixed powder, and decarburized and blown at the same oxygen flow rate even after the mixed powder was blown up. The C concentration in the molten iron at the end of the top blowing of the mixed powder was 2.2% by mass. During the treatment, CO ₂ gas was continuously blown from the tuyere installed at the bottom of the furnace at 0.1 Nm ³ / min / molten metal ton.

  As a result, the P concentration in the molten steel after treatment was 0.023% by mass, and the target P concentration of 0.020% by mass or less could not be achieved.

  According to the present invention, a high C concentration and a low P concentration molten steel can be produced with high efficiency without interrupting blowing from hot metal having a P concentration of 0.09 mass% or more with a small amount of slag. Industrial value is great.

Claims (3)

A hot metal having a Si concentration of 0.15% by mass or less and a P concentration of 0.09% by mass or more is charged into the top-bottom blown converter, and is a mixed powder of CaO powder and Fe ₂ O ₃ powder. Minute (CaO mass + CaCO ₃ mass × 0.56) and Fe ₂ O ₃ min (Fe ₂ O ₃ mass + FeO mass) are 9: 1-6: 4, and CaO, CaCO ₃ , FeO, And Fe ₂ O ₃ is 95% by mass or more, and the mixed powder is blown up with oxygen gas at a flow rate of 1.5 to 3.0 Nm ³ / min / molten ton. The amount of CO in the molten iron is 2.5% by mass or more, assuming that the top blowing amount of CaO is 10 to 15 kg / molten ton and the supply rate of CaO is 1.5 to 3.0 kg / min / molten ton. In the meantime, the top blowing of the mixed powder is finished, and the top blowing of the mixed powder is finished. Converter blowing method C concentration continues to blow over the oxygen gas even after is characterized in that to produce the molten steel above 0.3 mass%.   2. The converter blowing method according to claim 1, wherein the maximum particle size of the mixed powder is 0.15 mm or less. The converter blowing method according to claim 1 or 2, wherein during the top blowing, CO gas or CO ₂ gas is further blown at a bottom of 0.05 to 0.3 Nm ³ / min / molten ton.