TWI637062B - Method for manufacturing molten steel - Google Patents

Abstract

In a state in which a part of the slag generated by the previous feeding is left in the decarburization furnace, the slag which has solidified the slag generated by the conventional decarburization blowing is put into the decarburization furnace in the current feeding. Next, molten iron having a phosphorus concentration of less than 0.060% by mass is charged into the above-described decarburization furnace and subjected to decarburization blowing. Further, when the concentration of P ₂ O ₅ in the slag generated by the decarburization blowing in the current charging is below a predetermined value, a part of the slag is left in the decarburization furnace as the slag for the next feeding.

Description

Method for manufacturing molten steel

FIELD OF THE INVENTION The present invention relates to a method for producing a molten steel which is particularly suitable for use in reducing the amount of flux added during decarburization blowing.

BACKGROUND OF THE INVENTION Conventionally, in order to reduce the phosphorus concentration below the required product specification, the molten iron is blown in a decarburization furnace to produce a molten steel, and a flux is introduced from an input chute provided in the converter. The flux introduced into the molten iron melts on the surface of the molten iron at a high temperature to form a slag mainly composed of CaO and SiO ₂ . Further, during the blowing, pure oxygen is blown into the molten iron through the main lance, and the phosphorus in the molten iron becomes phosphorus oxide, and forms a solid solution with CaO in the slag, and is removed into the slag. Thereby, the concentration of phosphorus in the molten iron is lowered. The amount of the flux to be charged is determined by the molten iron component after the molten iron preparation, the phosphorus concentration of the product specification, and the target temperature. However, in recent years, the high performance of steel products and the strictness of user specifications have been progressing. The phosphorus concentration is further lowered, and the amount of the flux tends to increase.

Therefore, in order to reduce the cost in the steel making step, there is a slag recovery method as a method of reducing the amount of newly added flux. The slag recovery method has two methods of cold recovery and heat recovery. In the following description, the flux refers to a slag material introduced from outside the slag recovery system, and is made to be free of recovered slag.

The cold recovery is a technique widely used in countercurrent refining such as the SRP method, and Patent Document 1 and Patent Document 2 disclose a technique for cold recovery. Generally, in the cold recovery, the slag discharged from the decarburization furnace to the slag bucket (hereinafter referred to as the decarburization slag) is transported to the steelmaking plant after being subjected to magnetic separation, pulverization, sieving, etc. after cooling. . Then, the removed decarburization slag is recovered as a cold slag into the dephosphorization furnace.

On the other hand, heat recovery is mainly a technique used in the MURC method, and is disclosed, for example, in Patent Documents 3 and 4. In heat recovery, the characteristics of dephosphorization blowing and decarburization blowing are alternately carried out in the same converter. After the decarburization and tapping, the hot furnace slag remains in the furnace, and the waste is put into the next feeding to solidify the hot slag. To the extent that molten iron can be injected, the molten iron and dephosphorization blown for the next feeding. As described above, the slag treatment outside the step can be omitted, and the slag can be transferred to the dephosphorization blow of the next feed.

As described above, in either method, the high alkalinity decarburization slag is recovered to a dephosphorization furnace having a lower alkalinity than the decarburization furnace, but the slag may be recovered in the decarburization furnace. Thereby, the amount of newly added flux in the dephosphorization furnace or the decarburization furnace can be reduced.

Further, Patent Document 5 discloses a technique in which the decarburization slag is separately classified according to the concentration of P ₂ O ₅ when slag is discharged in the decarburization furnace, and the most suitable P ₂ O ₅ concentration decarburization is determined depending on the steel grade. The slag is cold recovered or heat recovered, but it is not disclosed that a cold recovery method is used when solidifying the heat recovered slag. Further, as in Patent Document 5, when the decarburization slag is cold-recovered or heat-recovered only in accordance with the P ₂ O ₅ concentration at the time of slag discharge, if the molten iron having a high phosphorus concentration is charged into the decarburization furnace, P ₂ O in the slag _{5 The} concentration often becomes very high, so there is also the problem that heat recovery cannot be performed at all.

In addition, when cold storage is carried out, since the powdery portion of the cold slag is likely to contain water, the reactivity is large when it is put into a converter, and there is a concern that the exhaust pipe or the dust collecting filter cloth has a large burden. Therefore, it is difficult to recover a large amount of cold recovery in a decarburization furnace.

On the other hand, in the decarburization furnace, the operation of not using waste is performed. Therefore, when the heat recovery is performed in the decarburization furnace, since the hot slag is not sufficiently solidified, it is necessary to use a flux as a coolant separately, and there is a problem that a sufficient amount of heat recovery cannot be performed when the coolant is not used.

Further, the slag recovery contains a concern that dephosphorization is caused by the concentration of P ₂ O _{5 in the} slag. In the cold recovery, since the component analysis of the decarburization slag can be performed, the method described in Patent Document 5 can be used, and the slag can be selected depending on the concentration of P ₂ O ₅ and depending on the steel grade. However, since the decarburization slag is continuously recovered in the heat recovery, the component analysis of the decarburization slag cannot be performed, and the degree of concentration of P ₂ O ₅ is not clear. Therefore, only a sufficient margin can be left to concentrate P ₂ O ₅ to the extent that dephosphorization can be avoided, and therefore there is a problem that a sufficient amount of heat recovery cannot be performed.

CITATION LIST Patent Literature Patent Literature 1: Japanese Patent Laid-Open No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. Patent No. 5,671,801 Patent Document 5: Japanese Patent No. 3829696

Non-Patent Literature Non-Patent Document 1: Materials and Processes Vol.8 (1995), p.183

Disclosure of the Invention Problems to be Solved by the Invention The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for producing a molten steel which can sufficiently reduce the amount of flux newly introduced in decarburization and blowing.

Means for Solving the Problems The present invention is as follows. (1) A method for producing a molten steel, characterized in that in a state in which a part of the slag produced by the previous feeding is left in the decarburization furnace, the conventional decarburization is blown in the current feeding. The slag solidified by the generated slag is placed in the decarburization furnace, and then molten iron having a phosphorus concentration of less than 0.060% by mass is charged into the decarburization furnace to perform decarburization blowing. (2) The method for producing a molten steel according to the above (1), wherein a part of the slag is left in the decarburization when a concentration of P ₂ O ₅ in the slag generated by the decarburization blowing in the current charging is below a predetermined value The furnace is used as the slag used for the next feeding. (3) The method for producing a molten steel according to the above (1) or (2), wherein in the current charging, the solidified slag is introduced into the decarburization furnace and a flux is introduced, and the amount of the flux is based on the former The concentration of P ₂ O ₅ in the slag produced by the secondary feed is determined. (4) The method for producing a molten steel according to any one of the above (1) to (3) wherein the solidified slag has a particle diameter of 50 mm or less.

Advantageous Effects of Invention According to the present invention, it is possible to provide a method for producing a molten steel which can sufficiently reduce the amount of a flux newly charged in decarburization blowing.

Embodiments for Carrying Out the Invention Hereinafter, embodiments of the present invention will be described in detail. Hereinafter, the symbol [] attached to the element symbol indicates the concentration of the element in the molten iron or molten steel, and the symbol () added to the element symbol or compound indicates the concentration of the element or compound in the slag.

In the present invention, the slag generated when the molten iron is decarburized in the decarburization furnace is used for cold recovery and heat recovery, and the cold slag generated by cold recovery and the hot slag generated by heat recovery are used for decarburization blowing. Refining to reduce [P] in the molten steel; the molten iron is a molten iron which is dephosphorized by a molten iron preparation treatment other than a converter or a dephosphorization treatment of a dephosphorization furnace.

In the cold recovery, after the molten steel is tapped by the decarburization furnace, the decarburization furnace slag is discharged into the slag bucket, and after the cooling, the decarburization slag of the discharged slag is pulverized and sieved to a predetermined particle size to obtain Cold furnace slag. Then, in a few days after the addition, the cold slag is put into the decarburization furnace by the input chute provided on the decarburization furnace. In this embodiment, cold slag which is used in a decarburization furnace instead of a dephosphorization furnace is produced.

In the heat recovery, after the molten steel is tapped by the decarburization furnace, the slag is not discharged, and the high-temperature decarburization slag (hot furnace slag) remains in the decarburization furnace, and a flux having a high cooling effect is introduced to make it Cured. Then, the molten iron for the next feeding is injected and transferred to the decarburization blowing.

In the present embodiment, first, cold slag is generated in advance by cold recovery. Then, the hot slag generated by the previous feeding is directly left in the decarburization furnace, and in the current feeding, the cold slag is put in the presence of the hot slag, and when the slag amount is insufficient, the flux is further injected, and then loaded. Into the molten iron for decarburization and blowing. Moreover, it is advisable to predict (P ₂ O ₅ ) in the slag at the time of the current feeding, and determine whether the decarburization slag generated in the current feeding can be recycled to the next feeding. When it is judged that it is recyclable, a part of the decarburization slag is left in the decarburization furnace as the hot slag for the next feeding. The operation procedure will be described in detail below.

First, a part of the decarburization slag which is produced by the decarburization blowing of the previous feed is left in the decarburization furnace in advance. The decarburization slag is determined to be recyclable slag by the determination of the recovery. The details of the determination of the recycling will be described later. Next, in the current feeding, the molten iron, the cold slag, and the flux are put into a decarburization furnace, and decarburization blowing is performed.

The [P] of the molten iron charged in the decarburization furnace was set to be less than 0.060% by mass. The reason why [P] in the molten iron is less than 0.060% by mass is that if [P] in the molten iron is 0.060% by mass or more, it is necessary to increase the amount of dephosphorization and (P ₂ O ₅ ) in the slag. It will become high and become unsuitable as a slag that is recycled to the decarburization furnace. Further, the [P] of the molten iron charged in the decarburization furnace is preferably 0.035 mass% or less. The [P] in the molten iron of the blast furnace is generally more than about 0.100% by mass, but the method of making the [P] in the molten iron less than 0.060% by mass is not particularly limited. For example, the [P] in the molten iron can be made less than 0.060% by mass by dephosphorization treatment using a dephosphorization furnace, and can also be easily obtained in the molten iron by applying a well-known molten iron preparation processing technique [ P] is less than 0.060% by mass.

In the decarburization blowing, the required phosphorus distribution ratio is determined depending on the [P] of the target molten steel and [P] after the dephosphorization treatment, and the necessary slag amount is calculated. Further, in the decarburization blowing, the alkalinity is 3.6 to 3.8, and from the viewpoint of protecting the refractory in the melting furnace, (MgO) in the slag is 6 to 9% by mass. Based on these conditions, the amount of flux input can be determined.

Here, the phosphorus distribution ratio is a value obtained by dividing the phosphorus concentration in the slag by the phosphorus concentration in the molten steel ((P)/[P]), and the required phosphorus distribution ratio is in the range of about 30 to 60, and when the target is melted For [P] hours of steel, the necessary phosphorus partition rate is set to a larger value. Since the amount of molten steel of tapping can be predicted in advance, the amount of necessary slag can be calculated as long as the phosphorus partition ratio can be set.

On the other hand, it has been reported that the phosphorus partition ratio is related to various slag components and end temperature in the decarburization furnace (see, for example, Non-Patent Document 1). Therefore, a combination of a plurality of fluxes is determined to satisfy the necessary slag amount, alkalinity, (MgO) in the slag, and the above correlation. However, the method of determining the combination of the flux is not unique, but is determined by considering the operating experience in each furnace or the easiness of flux scheduling, and local conditions such as cost. Examples of the fluxing agent include quicklime, dolomite, vermiculite, peridotite, and light burnt dolomite. If it is quicklime, the particle size should be 3~10mm. If it is light burnt dolomite, the particle size should be 5~30mm.

In addition, the cold slag is previously charged into the decarburization furnace during the period from the previous feeding and the current feeding to the start of the molten iron. The reason why the cold slag is put in during this period is to solidify the hot slag generated in the previous feeding, and also to prevent the boiling reaction caused by the input of the cold slag. Further, if cold slag is introduced during this period, the hot slag can be used to remove moisture from the cold slag. Here, when the cold slag is generated, it is preferable to sieve the particle size to 50 mm or less. The reason why the particle size of the cold slag is 50 mm or less is that the funnel is easily stored in the melting furnace, and it is easy to put the cold slag into the decarburizing furnace from the funnel, and it can be smoothly slag in the decarburizing furnace. On the other hand, if the particle size of the cold slag exceeds 50 mm, there are problems associated with handling such as handling and cutting out to the funnel, and there is a possibility that heat transfer and slag formation are required in the decarburization furnace for a long time. Further, the scale may be used together with the cold slag or the flux as a cold material for temperature adjustment.

As described above, the molten iron, the cold slag and the flux are put into the decarburization furnace, and the main lance is blown with oxygen to start decarburization and blowing. Then, when the predetermined [C] is reached, the molten steel is tapped to the barrel by the decarburization furnace. The decarburization slag is directly left in the decarburization furnace as the hot slag. At this time, it is also possible to determine whether or not heat recovery is performed by (P ₂ O ₅ ) in the decarburization slag.

When heat recovery is performed, (P ₂ O ₅ ) in the slag is concentrated compared to the case where dephosphorization is performed by the slag generated by the newly-added flux. If (P ₂ O ₅ ) is excessively concentrated, the dephosphorization ability of the slag is lowered. Therefore, it is also possible to predict (P ₂ O ₅ ) in the slag in advance and determine whether the decarburization slag produced in the current charging can be recovered to the next feeding. (P ₂ O ₅ ) in the slag is predicted by a mass balance formula represented by the following formulas (1) to (3). In the slag (P ₂ O ₅ ) (% by mass) = slag (P ₂ O ₅ ) (kg / t) slag amount (kg / t) × 100 ‧ ‧ (1) slag (P ₂ O ₅ ) (kg/t) = residual slag (P ₂ O ₅ ) (kg/t) + cold slag (P ₂ O ₅ ) (kg/t) + current feed slag (P ₂ O ₅ ) (kg/t ) + previous feeding hot slag (P ₂ O ₅ ) (kg / t) ‧ ‧ (2) slag amount kg / t) = residual slag amount (kg / t) + cold slag amount (kg / t) + Current feeding slag amount (kg/t) + previous feeding hot slag amount (kg/t) ‧‧‧(3)

Here, the "residual slag" refers to the slag accompanying the molten iron from the previous step, and the amount thereof can be grasped by the mass balance investigation result of the slag component. The term "current feed slag" refers to slag from a flux.

The term "pre-feeding hot slag" refers to hot slag which is not completely discharged from the decarburization slag which is formed in the previous feeding before the current feeding, and which is recovered by remaining in the furnace. The converter is ultimately determined by tilting the angle and is therefore manageable. Although it is difficult to directly analyze the composition of the sample in the previous charging furnace slag (P ₂ O ₅ ) (kg/t), the slag (P ₂ O ₅ ) has been calculated by using the formula (1) in the previous feeding. %), so you can master.

As described above, when (P ₂ O ₅ ) (% by mass) of the slag is less than or equal to a predetermined value, a part of the decarburization slag is drained and heat is recovered, and (P) in the slag is calculated in the next feeding. The phosphorus partition rate is calculated to determine the amount of flux. At this time, cold recovery can also be performed by using the decarburization slag of the discharged slag. Further, if heat recovery is not performed, all the decarburization slag is discharged, and only the cold slag and the flux are used for the decarburization blowing in the next feeding. Further, it is also possible to carry out decarburization blowing using only a flux in the next feeding.

Further, in order to generate the cold slag, when the slag (P ₂ O ₅ ) (% by mass) is less than or equal to a predetermined value, heat recovery may be performed without partial heat recovery.

According to the above, the amount of recovery of the decarburization slag can be expanded in the decarburization furnace, and the newly added flux amount can be greatly reduced.

EXAMPLES Hereinafter, the examples of the present invention will be described, but the conditions in the examples are merely examples of conditions used to confirm the applicability and effects of the present invention, and the present invention is not limited by the conditions. The present invention can adopt various conditions as long as the object of the present invention can be achieved without departing from the gist of the present invention.

In order to confirm the effects of the present invention, Examples 1 to 4 are operations for decarburizing and blowing molten iron having a phosphorus concentration of less than 0.060% by mass in a decarburization furnace. The molten iron used is decalcified iron which is dephosphorized by molten iron of 280 to 290 tons, and the molten iron component contains: [C]=3.3 to 3.8% by mass, [Si]≦0.01% by mass, [Mn]=0.10~ 0.30% by mass, and [P]=0.010% by mass or more and less than 0.060% by mass.

The decarburization blowing is carried out for a steel grade in which [P] = 0.018% is the upper limit of the product specification. On the other hand, it is considered that the [P] ≦ 0.015% in the molten steel tapped from the decarburization furnace is set to a better range after the secondary refining. The decarburization slag is in the range of 30 to 60 in the formula of the water to be described in Non-Patent Document 1, and uses a flux such as quicklime, dolomite, vermiculite, peridotite, cold slag, and heat. The slag is used for decarburization blowing.

First, for the cold slag, the decarburization furnace slag is discharged and cooled in another feed, and then magnetic separation is carried out, and after being pulverized by a pulverizer, it is sieved by a sieve having a pore diameter of 25 mm, and the sieve granules are returned again. The pulverizer is pulverized and sieved again. Then, the sieved granules are transported as cold slag to the steelmaking plant. The average size of the cold slag received at the steelmaking plant is 10 mm.

In this feeding, a predetermined amount of the cold slag produced in accordance with the above is put into the decarburization furnace through the input chute from the furnace funnel of the converter. The effect of the present invention is evaluated by using two factors of the amount of reduction of the flux and [P] in the steel tapped from the decarburization furnace. However, the ratio of implementation described below refers to the ratio of the feed to be recycled relative to all feeds. In the case where the feed which has not been subjected to the recovery is judged as "NO" by the determination of whether or not the charge is recovered, the feed which has not been carried out due to other operations is also included.

(Example 1 (Inventive Example 1)) First, the slag of the decarburization furnace slag which was previously fed is not completely discharged, and about 15 kg/t is left as a hot slag in the decarburization furnace in advance, and the granules are as described above. The adjusted decarburization furnace slag is used as the cold slag, and 10 kg/t is input from the input chute provided on the decarburization furnace. In addition, quicklime, dolomite, vermiculite, and peridotite are used as a flux together with the cold slag, and a scale of about 7 kg/t is put into the cold material for temperature adjustment. After the cold slag and the flux are introduced, the dephosphorized iron of 280 to 290 tons is placed in a decarburization furnace to perform decarburization blowing. Further, in the hot slag and the cold slag, (CaO) = 42% by mass and (SiO ₂ ) = 11% by mass. At this stage, the flux equivalent to the following is reduced. CaO: (hot furnace slag + cold furnace slag) (kg / t) × 0.42 SiO ₂ : (hot furnace slag + cold furnace slag) (kg / t) × 0.11

Further, in the pre-decarburization blowing or the decarburization blowing of the current feed, the concentration of (P ₂ O ₅ ) in the slag of the current feed is calculated by the above formulas (1) to (3). Then, after the decarburization and blowing, the molten steel is tapped, and when the (P ₂ O ₅ ) (% by mass) of the slag is calculated to be 2.5% by mass or more, the complete slag removal of the decarburization slag is carried out, and when the slag is in the slag (P _{When 2} O ₅ ) (% by mass) is less than 2.5% by mass, the slag is not completely discharged, and a small amount of slag is discharged from the decarburization slag so that the hot slag is about 15 kg/t. However, when the slag is completely discharged, the cold slag and the flux are used in the next feeding, or only the flux is used for the decarburization blowing.

Then, repeat the above operation exactly 200 times. As a result, the heat recovery implementation ratio (= heat recovery execution feed times ÷ total feed times) was 61%, and the cold recovery implementation ratio (= cold recovery implementation feed times ÷ total feed times) was 98%.

(Example 2 (Inventive Example 2)) First, the slag of the decarburization furnace slag which was previously fed is not completely discharged, and about 15 kg/t is left as a hot slag in the decarburization furnace in advance, and the granules are granulated as described above. The adjusted decarburization furnace slag is used as the cold slag, and 10 kg/t is input from the input chute provided on the decarburization furnace. In addition, quicklime, dolomite, vermiculite, and peridotite are used as a flux together with the cold slag, and a scale of about 7 kg/t is put into the cold material for temperature adjustment. After the cold slag and the flux are introduced, the dephosphorized iron of 280 to 290 tons is placed in a decarburization furnace to perform decarburization blowing. Further, in the hot slag and the cold slag, (CaO) = 42% by mass and (SiO ₂ ) = 11% by mass. At this stage, the flux equivalent to the following is reduced. CaO: (hot furnace slag + cold furnace slag) (kg / t) × 0.42 SiO ₂ : (hot furnace slag + cold furnace slag) (kg / t) × 0.11

Moreover, after the decarburization and blowing, the molten steel is tapped, and it is not considered whether the heat recovery to the next feeding can be carried out, and a small amount of the slag is directly discharged from the decarburization slag so that the hot slag is about 15 kg/t, and the heat is continuously applied. Recycling. Then, repeat the above operation exactly 200 times. As a result, the heat recovery implementation ratio was 82%, and the cold recovery implementation ratio was 98%.

(Example 3 (Conventional Example 1)) First, the slag of the decarburization furnace slag which was previously fed is not completely discharged, and about 15 kg/t is left as a hot slag in the decarburization furnace, and the quicklime and the white cloud are put in. Stone, vermiculite and peridotite are used as fluxing agents, and a scale of about 7 kg/t is used as a cold material for temperature adjustment. Also, no cold slag was introduced. After the flux is charged, the dephosphorized iron of 280 to 290 tons is placed in a decarburization furnace to perform decarburization blowing. Further, in the hot slag, (CaO) = 42% by mass and (SiO ₂ ) = 11% by mass. At this stage, the flux equivalent to the following is reduced. CaO: hot furnace slag (kg/t) × 0.42 SiO ₂ : hot furnace slag (kg/t) × 0.11

Moreover, after the decarburization and blowing, the molten steel is tapped, and it is not considered whether the heat recovery to the next feeding can be carried out, and a small amount of the slag is directly discharged from the decarburization slag so that the hot slag is about 15 kg/t, and the heat is continuously applied. Recycling. Then, repeat the above operation exactly 200 times. As a result, the heat recovery implementation ratio was 72%. Although the heat recovery implementation ratio is lower than that of Example 2, this is because it takes time to cure the hot slag without performing cold recovery. If there is no margin in the short period of the feeding period, the decarburization slag will be completely Drainage.

(Example 4 (Conventional Example 2)) After the heat removal is performed and the decarburization slag is completely discharged in the previous feeding, the dephosphorized iron of 280 to 290 tons is placed in a decarburization furnace to perform decarburization blowing. Refining. Then, in order to promote the melting, 10 kg/t of the above-mentioned cold slag is charged within 5 minutes from the start of the blowing, and quicklime, dolomite, vermiculite, and peridotite are introduced as a flux, and a scale of about 7 kg/t is put as a temperature. Adjust the cold material used. Further, in the cold slag, (CaO) = 42% by mass and (SiO ₂ ) = 11% by mass. At this stage, the flux equivalent to the following is reduced. CaO: cold slag (kg/t) × 0.42 SiO ₂ : cold slag (kg/t) × 0.11

Then, after decarburization and blowing, the molten steel is tapped, and the complete slag removal of the decarburization slag is carried out. Repeat the above operation for exactly 200 feedings. As a result, the cold recovery implementation ratio was 65%, and the cold recovery implementation ratio was lower than that of Examples 1 and 2. If the heat recovery is not carried out and the cold slag is introduced before the injection of the molten iron, a large bump reaction occurs due to the powder and moisture adhering to the converter slag when the molten iron is injected. Therefore, in Example 4, the cold slag was put into the converter 5 minutes after the start of the blowing. Although a bumping reaction occurs in the case of inputting cold slag in this case, the reaction is small compared to the case before the injection of molten iron. However, in the case of transporting cold slag containing water in a rainy day, the cold slag contains a lot of water and the reaction is remarkable, so that it is difficult to use it stably, and it is understood that the recovery amount cannot be ensured only by cold recovery.

(Example 5 (Inventive Example 3)) Example 5 is an operation of decarburizing and blowing molten iron having a phosphorus concentration of 0.035 mass% or less in a decarburization furnace. The molten iron used is decalcified iron which is dephosphorized by molten iron of 280 to 290 tons, and the molten iron component contains: [C]=3.3 to 3.8% by mass, [Si]≦0.01% by mass, [Mn]=0.10~ 0.30% by mass and [P]=0.010 to 0.035 mass%.

First, the slag of the decarburization furnace slag that was previously fed is not completely discharged, and about 22 kg/t is left as a hot slag in the decarburization furnace, and the slag of the decarburization furnace granules adjusted by the above-mentioned particle size is used as the cold slag. 11 kg/t was charged from the input chute provided on the decarburization furnace. Further, stainless steel of about 7 kg/t was put into the cold material for temperature adjustment without introducing a flux. After the cold slag is charged, the dephosphorized iron of 280 to 290 tons is placed in a decarburization furnace to perform decarburization blowing. Further, in the hot slag and the cold slag, (CaO) = 42% by mass and (SiO ₂ ) = 11% by mass. At this stage, the flux equivalent to the following is reduced. CaO: (hot furnace slag + cold furnace slag) (kg / t) × 0.42 SiO ₂ : (hot furnace slag + cold furnace slag) (kg / t) × 0.11

Further, in the pre-decarburization blowing or the decarburization blowing of the current feed, the concentration of (P ₂ O ₅ ) in the slag of the current feed is calculated by the above formulas (1) to (3). Then, after the decarburization and blowing, the molten steel is tapped, and when the (P ₂ O ₅ ) (% by mass) of the slag is calculated to be 2.5% by mass or more, the complete slag removal of the decarburization slag is carried out, and when the slag is in the slag (P _{When 2} O ₅ ) (% by mass) is less than 2.5% by mass, the slag is not completely discharged, and a small amount of slag is discharged from the decarburization slag so that the hot slag is about 20 kg/t. However, when the slag is completely discharged, the cold slag and the flux are used in the next feeding, or only the flux is used for the decarburization blowing.

Then, repeat the above operation exactly 50 times. As a result, the heat recovery implementation ratio was 100%, and the cold recovery implementation ratio was 97%.

(Example 6 (Conventional Example 3)) Example 6 is an operation of decarburizing and blowing molten iron having a phosphorus concentration of 0.100% by mass or less in a decarburization furnace. The molten iron used is decalcified iron which is dephosphorized by molten iron of 280 to 290 tons, and the molten iron component contains: [C]=3.3 to 3.8% by mass, [Si]≦0.01% by mass, [Mn]=0.10~ 0.30% by mass, and [P]=0.060 to 0.100% by mass.

First, the slag of the decarburization furnace slag of the previous feed is not completely discharged, and about 15 kg/t is left as a hot slag in the decarburization furnace, and the slag of the decarburization furnace granule adjusted by the above-mentioned particle size is used as the cold slag. 10 kg/t was charged from the input chute provided on the decarburization furnace. In addition, quicklime, dolomite, vermiculite, and peridotite are used as a flux together with the cold slag, and a scale of about 7 kg/t is put into the cold material for temperature adjustment. After the cold slag and the flux are introduced, the dephosphorized iron of 280 to 290 tons is placed in a decarburization furnace to perform decarburization blowing. Further, in the hot slag and the cold slag, (CaO) = 42% by mass and (SiO ₂ ) = 11% by mass. At this stage, the flux equivalent to the following is reduced. CaO: (hot furnace slag + cold furnace slag) (kg / t) × 0.42 SiO ₂ : (hot furnace slag + cold furnace slag) (kg / t) × 0.11

Further, in the pre-decarburization blowing or the decarburization blowing of the current feed, the concentration of (P ₂ O ₅ ) in the slag of the current feed is calculated by the above formulas (1) to (3). Then, after the decarburization and blowing, the molten steel is tapped, and when the (P ₂ O ₅ ) (% by mass) of the slag is calculated to be 2.5% by mass or more, the complete slag removal of the decarburization slag is carried out, and when the slag is in the slag (P _{When 2} O ₅ ) (% by mass) is less than 2.5% by mass, the slag is not completely discharged, and a small amount of slag is discharged from the decarburization slag so that the hot slag is about 15 kg/t. However, when the slag is completely discharged, the cold slag and the flux are used in the next feeding, or only the flux is used for the decarburization blowing.

Then, repeat the above operation exactly 50 times. As a result, the heat recovery implementation ratio was 20%, and the cold recovery implementation ratio was 40%.

(Comparative Example) After decarburization slag was completely discharged in the previous feeding without heat recovery, 280 to 290 tons of the above-mentioned dephosphorized iron was placed in a decarburization furnace to carry out decarburization blowing. Then, lime, dolomite, vermiculite, and peridotite are used as a flux, and a scale of about 7 kg/t is put as a cold material for temperature adjustment. After decarburization and blowing, the molten steel is tapped and the complete slag removal of the decarburization slag is carried out. Repeat the above operation for exactly 200 feedings.

(Experimental Results) Fig. 1 is a view showing the details of the composition of the decarburization slags in Examples 1 to 4 and Comparative Examples. Further, as shown in Fig. 1, the decarburization slag is composed of hot slag, cold slag, flux (current feed slag), and residual slag. In either case, the total amount is about 38 kg/t, and the residual slag is 5.3 kg/t. Therefore, the amount of recovered slag (hot slag and cold slag) can be evaluated as the reduced flux.

Further, in Fig. 1, the values of the hot slag and the cold slag are values obtained by multiplying the input amount of each example by the implementation ratio. Examples 1 and 2 in which cold recovery and heat recovery are used in combination are examples of the present invention, and the newly added flux amount can be further reduced. In particular, in Example 2 in which heat recovery was carried out regardless of (P ₂ O ₅ ) (% by mass) in the slag, the new fluxing amount was the lowest. The amount of flux reduction showed a maximum of 22.1 kg/t of Example 2, followed by 19.0 kg/t of Example 1. Further, Examples 3 and 4 of the conventional examples showed a result that the amount of reduction of the new flux was small.

Fig. 2 is a view showing [P] in the molten steel discharged from the decarburization furnace in Examples 1 to 4 and Comparative Examples. In Examples 1 to 4 and Comparative Examples, the value of [P] = 0.018 mass%, which is less than the upper limit of the product specifications in all the feeds, was used.

In the comparative example, since the slag was not recovered and all of the flux was used, [P] in the molten steel was at a low level. Example 1 is equivalent to Comparative Example 1, and [P] in the molten steel is at a low position. This is because the reuse of the molten slag can increase the slagability more than the newly added flux, and is further set to use the hot slag only when the slag (P ₂ O ₅ ) (% by mass) is less than 2.5% by mass. And ensure the dephosphorization ability.

In the second embodiment, it is not determined whether or not the hot slag can be recovered. Therefore, although the amount of flux reduction is large, it is a result of a large difference in [P] in the molten steel. Further, in a total of 200 additions, although the ratio of 2.0% is outside the range of the preferred range, that is, [P] ≦ 0.015 mass%, since it is still less than the upper limit of the product specification [P] = 0.018 mass%, Therefore, it can be confirmed that there is no problem in the product.

In Example 3, it was not judged whether or not heat recovery was possible, and therefore, the result of [P] in the molten steel was large. Further, in a total of 200 additions, although the ratio of 1.0% exceeded the range of the preferred range, i.e., [P] ≦ 0.015 mass%, it was still less than the upper limit of the product specification [P] = 0.018%. In Example 4, only the cold recovery was carried out to ensure the dephosphorization ability, so that the amount of flux reduction was small, but [P] in the molten steel was lower than that of Examples 2 and 3.

[P] in the molten steel of Example 5 was lower than Example 1. This is because the decarburization treatment is performed only with the molten iron having a phosphorus concentration of 0.035 mass% or less, whereby (P ₂ O ₅ ) (% by mass) in the hot slag is kept at a lower position. In the embodiment 6, the molten iron having a phosphorus concentration of 0.065 mass% or more is decarburized, whereby a new flux is increased to ensure the dephosphorization ability, so [P] in the molten steel is at a low position. Moreover, since there is a judgment as to whether or not heat recovery is possible, the difference is not large.

Fig. 3 is a graph showing the relationship between (P ₂ O ₅ ) in the hot slag and [P] in the molten steel after tapping. In order to clarify the relationship, data on the effects of the blowing conditions ([P], slag amount, actual alkalinity, and end point [C]) in the molten iron after dephosphorization were extracted. As the (P ₂ O ₅ ) in the hot slag increases, [P] in the molten steel after tapping also tends to increase. Therefore, in Examples 2 and 3, the stagger of [P] in the molten steel after tapping was caused by the concentration of (P ₂ O ₅ ) in the hot slag.

4 is a view showing the details of the composition of the decarburization slag in Examples 1, 5, and 6, and shows a decarburization slag in a molten iron having a phosphorus concentration of 0.035 mass% or less, less than 0.060 mass%, and 0.100 mass% or less. The impact of the composition. Further, as shown in Fig. 4, the decarburization slag is composed of hot slag, cold slag, flux (current feed slag), and residual slag. In either case, the total amount is about 38 kg/t. Therefore, the amount of recovered slag (hot slag and cold slag) can be evaluated as the reduced flux.

In Fig. 4, the values of the hot slag and the cold slag are values obtained by multiplying the input amount of each embodiment by the implementation ratio. Example 5, which uses cold recovery and heat recovery as described above, is an example of the present invention, which allows the newly-injected flux dose to be zero. On the other hand, in Example 6, the newly-introduced flux amount was the largest, and the amount of the flux could not be reduced. This is because the amount of (P ₂ O ₅ ) which is predicted to be high in the decarburization slag is large, and the implementation ratio of heat recovery is drastically lowered.

According to the above results, in the examples of the present invention, that is, in the first, second, and fifth embodiments, the heat recovery and the cold recovery are appropriately combined, the hot slag is solidified by the cold slag, and the moisture in the cold slag is removed by the hot slag. It can alleviate the problems of both parties. As a result, the amount of recovery can be increased, so that the newly-added flux can be reduced, and the effect of improving the slagability of the slag can be utilized to stably reduce the molten steel [P].

Industrial Applicability According to the present invention, it is possible to provide a method for producing a molten steel which can sufficiently reduce the amount of flux newly added in decarburization and blowing, and which has a very high industrial value.

Fig. 1 is a view showing the details of the composition of the decarburization slags in Examples 1 to 4 and Comparative Examples. Fig. 2 is a view showing [P] in a molten steel obtained by a decarburization furnace in Examples 1 to 6 and a comparative example. Fig. 3 is a graph showing the relationship between (P ₂ O ₅ ) in the hot slag and [P] in the molten steel after tapping. Fig. 4 is a view showing the details of the composition of the decarburization slags in Examples 1, 5, and 6.

Claims (5)

A method for producing a molten steel, characterized in that in a state in which a part of the decarburization slag generated by the previous feeding is left as a hot slag in a decarburization furnace, in the current feeding, the conventional removal is performed The cold slag in which the decarburization slag generated by the carbon blowing is solidified is put into the decarburization furnace, and then molten iron having a phosphorus concentration of less than 0.060% by mass is charged into the decarburization furnace to perform decarburization blowing. The method for producing a molten steel according to claim 1, wherein a part of the decarburization slag is left in the decarburization when a concentration of P ₂ O ₅ in the decarburization slag generated by the decarburization smelting in the current feed is below a predetermined value. The furnace is used as the hot slag for the next feeding. The method for producing a molten steel according to claim 1 or 2, wherein in the current feeding, the solidified cold slag is put into the decarburizing furnace and a flux is introduced, and the amount of the flux is generated according to the previous feeding. The concentration of P ₂ O ₅ in the decarburization slag is determined. The method for producing a molten steel according to claim 1 or 2, wherein the solidified cold slag has a particle diameter of 50 mm or less. The method for producing a molten steel according to claim 3, wherein the solidified cold slag has a particle diameter of 50 mm or less.