JP7436839B2 - Melting method for ultra-low phosphorus steel - Google Patents

Description

The present invention relates to a method for producing ultra-low phosphor steel.

In order to reduce the cost of melting steel, it is necessary to reduce the amount of slag materials such as quicklime (CaO) in the refining process that performs dephosphorization and decarburization. Hot metal pretreatment using the same converter-type vessel can reduce the amount of slag material, but the slag remains in the decarburization process and returns to molten steel, so the amount of slag material can be reduced. The challenge is to simultaneously produce low phosphorous steel. For this reason, there is a demand for efficient dephosphorization in refining converter-type vessels.

Regarding the above-mentioned efficient dephosphorization, a method is known in which both hot metal preliminary treatment (dephosphorization treatment) and decarburization treatment are performed in a converter type vessel. This method uses blowing acid for the purpose of hot metal pretreatment such as dephosphorization, slag removal after the hot metal pretreatment (intermediate slag), and blowing acid for the purpose of decarburization after the intermediate slag. This is the way to do it. Hereinafter, such a method of blowing acid twice may be referred to as a "two-blow method."

Moreover, in recent years, a technique has been implemented in which an intermediate slag removal step is further provided in the hot metal pretreatment, and the hot metal pretreatment is performed twice. This method includes a first hot metal pretreatment step involving blowing acid for the purpose of dephosphorization, a first intermediate slag removal step, a second hot metal pretreatment step involving blowing acid for the purpose of dephosphorization, etc. The process involves five stages: a second intermediate slag removal process, and a decarburization process involving blowing acid. By going through such a process, it becomes possible to melt ultra-low phosphorus steel. Hereinafter, a method of blowing acid three times as described above may be referred to as a "three-blow method." As an example of a method for producing ultra-low phosphorous steel using the three-blow method as described above, there are methods disclosed in Patent Document 1 and Patent Document 2 below, for example.

In Patent Document 1, acid blowing is carried out three times and slag removal is carried out twice in between, and quicklime is added during the second acid blowing to bring the basicity of the slag within a predetermined range. It is disclosed that ultra-low phosphorus steel can be produced by melting.

In addition, Patent Document 2 also discloses that slag removal is performed multiple times with three times of acid blowing, and furthermore, slag from decarburization and refining (i.e., slag obtained after the third time of acid blowing) is It is disclosed that it is used in the desiliconization treatment of the next charge (a treatment in which the first acid blowing is performed), and that a slag-forming material such as quicklime is added during the second acid blowing.

Japanese Patent Application Publication No. 2011-144415 Japanese Patent Application Publication No. 2018-188730

However, in the methods of Patent Document 1 and Patent Document 2, quicklime is added as a slag material during the second acid blowing, so the amount of slag material such as quicklime used is reduced in order to further reduce costs. It is hoped that

As a result of intensive study by the present inventor regarding the above desire, it was found that it was not possible to achieve the desired phosphorus content by simply reducing the amount of quicklime used during the second acid blowing, and an extremely low phosphorus steel was realized. It became clear that further improvements were necessary.

The present invention has been made in view of the above problems, and an object of the present invention is to reduce the amount of slag material used while making it possible to more stably produce ultra-low phosphorus steel. Another object of the present invention is to provide a method for producing ultra-low phosphorus steel.

In order to solve the above problems, the present inventor has conducted intensive studies and found that it is important to control the amount of phosphorus in the entire system including molten steel and slag, rather than focusing only on the phosphorus concentration in molten steel. Based on this knowledge, we have completed the present invention.
The gist of the present invention, which was completed based on the above findings, is as follows.

(1) A method for producing ultra-low phosphorous steel in which hot metal preliminary treatment and decarburization treatment are performed using a converter-type vessel, the method comprising: a first dephosphorization step in which hot metal is dephosphorized; a first intermediate slag step carried out after the phosphorization step; a second dephosphorization step in which the molten metal is dephosphorized again after the first intermediate slag step; and a second dephosphorization step carried out after the second dephosphorization step. a second intermediate slag removal step carried out; and a decarburization treatment step of adding a CaO source to the molten metal after the second intermediate slag removal step to perform decarburization treatment, In the dephosphorization step, a CaO source is added to the hot metal so that the basicity of the slag after the first dephosphorization step is within the range of 0.8 to 1.3, and the amount of oxygen outside the desiliconization is While supplying oxygen so that the temperature is within the range of 4.0 to 10.0 Nm ³ /ton, the end point temperature of the hot metal is set within the range of 1300 to 1380°C, and in the first intermediate slag removal step, the slag is The slag is discharged outside the converter type container, and 30 to 50% by mass of the slag amount before being discharged remains in the converter type container, and in the second dephosphorization step, the second dephosphorization step is completed. Add a CaO source of 0 to 5 kg/ton in terms of CaO so that the basicity of the subsequent slag is within the range of 0.8 to 1.3, and within the range of 2.0 to 4.0 Nm ³ /ton. While supplying oxygen with an amount of blown acid, the end point temperature of the molten metal is set within the range of 1300 to 1380°C, and in the second intermediate slag step, 30 to 60% by mass of the slag before slag is A method for producing ultra-low phosphorus steel that drains the slag out of a furnace-type container.
(2) The method for producing ultra-low phosphorus steel according to (1), wherein the Si content of the hot metal is within the range of 0.1 to 1.2% by mass.

As explained above, according to the present invention, it is possible to more stably produce ultra-low phosphorus steel while reducing the amount of slag material used.

FIG. 2 is a schematic diagram for explaining findings obtained by the present inventor. FIG. 2 is a schematic diagram for explaining findings obtained by the present inventor. FIG. 1 is an explanatory diagram schematically showing the flow of a method for producing ultra-low phosphorus steel according to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Note that, in this specification and the drawings, components having substantially the same functional configurations are designated by the same reference numerals and redundant explanation will be omitted.

(Concerning the contents of the study by the inventor)
In the following, before explaining the method for producing ultra-low phosphorus steel according to the embodiment of the present invention, the contents of the study conducted by the present inventor to solve the above problems will first be explained.

In order to solve the above-mentioned problems, the present inventor first studied the techniques disclosed in the above-mentioned patent documents. First, the method based on the 3-blow method disclosed in Patent Document 2 distinguishes between the desiliconization treatment during the first acid blowing and the dephosphorization treatment during the second acid blowing as hot metal pretreatment. The dephosphorization treatment is carried out separately, and the dephosphorization treatment is carried out only once. Therefore, in the method disclosed in Patent Document 2, it has been found that there is a limit to the level of phosphorus reduction when the purpose is to produce extremely low phosphorus steel with a phosphorus content of 100 ppm or less.

On the other hand, in the method based on the three-blow method disclosed in Patent Document 1, the intermediate slag is sandwiched once and the dephosphorizing and blowing acid treatment is performed twice, and then the intermediate slag and the decarburizing and blowing acid treatment are performed. Therefore, it is possible to reduce the amount of phosphorus accordingly. In this method, a CaO source of 5 to 10 kg/ton in terms of CaO is input during the second dephosphorization process, but according to the knowledge of the present inventor, slag materials such as quicklime are input during the dephosphorization process. There was still room for reduction in quantity.

In addition, in the method disclosed in Patent Document 1, there may be variations in the phosphorus concentration after the dephosphorization process is completed, so it is necessary to add an excessive amount of slag material such as quicklime during the decarburization process. However, there was room for reduction in the amount of slag material input.

Therefore, the inventor of the present invention reduced the amount of slag material used during the second dephosphorization treatment by reducing the amount of CaO source input to 5 kg/ton or less, including the case where no CaO source was used as slag material. In order to reduce the amount of phosphorus, we worked on a study aimed at more stably producing ultra-low phosphorus steel with a phosphorus content of 100 ppm or less.

(Regarding knowledge obtained by the inventor)
<First finding>
In Patent Document 1, by introducing a CaO source during the second dephosphorization treatment, the concentration of P ₂ O ₅ in the slag obtained after the second dephosphorization treatment is lower than that obtained after the first dephosphorization treatment. It is mentioned that since the P ₂ O ₅ concentration in the slag can be lowered, rephosphorization during decarburization can be suppressed, and extremely low phosphorus steel can be produced.

However, as a result of various studies conducted by the present inventor, the phosphorus (P) contained in the slag (residual slag after the second slag treatment) brought into the converter type container during decarburization treatment is generally It was discovered that all of the materials were once reconstituted in the molten metal. Based on this knowledge, the present inventor came up with the idea of reducing the total amount of phosphorus in the slag rather than the phosphorus concentration in the slag brought into the decarburization process. In other words, even if the phosphorus concentration in the slag brought into the decarburization process is low, if the total amount of phosphorus contained in the slag is large, the amount of phosphorus returned to the molten metal will be large, which is disadvantageous to the production of ultra-low phosphorus steel. We realized that this would not only have a negative effect on the environment, but would also be difficult to reduce the amount of slag material used.

Figure 1 shows the inside of the system (in other words, the inside of the converter type vessel) in each process of hot metal pretreatment in the 3-blow method based on the above idea, assuming that the mass ratio of P contained in the hot metal to be treated is 100. This diagram schematically shows the change in phosphorus content. Note that Figure 1 is only a schematic representation, and the values shown in the figure accurately reflect the amount of phosphorus in each step of the ultra-low phosphorus steel melting method described in detail below. Do not mean.

In the first dephosphorization process, some of the phosphorus that is present in the molten metal before the start of the dephosphorization process is distributed to the slag, and at the end of the first dephosphorization process, it becomes slag as shown in Figure 1, for example. It exists in the molten metal and the slag in a large proportion.

By performing the first intermediate slag treatment, a portion of the slag is discharged outside the system, so the total amount of phosphorus in the system is lower than at the start of the first dephosphorization process. .

In the second dephosphorization process, as in the first dephosphorization process, the phosphorus in the molten metal is distributed to the slag, and at the end of the second dephosphorization process, the phosphorus is distributed in the proportion shown in Figure 1, for example. , both in the molten metal and in the slag. By performing the second intermediate dephosphorization process, some of the slag is discharged outside the system, so the total amount of phosphorus in the system is lower than at the end of the first dephosphorization process. .

According to the idea obtained by the present inventor, the total amount of phosphorus present in the system can be reduced by discharging the phosphorus in the system to the outside of the system through multiple dephosphorization steps and intermediate scavenging steps. to reduce the total amount of phosphorus present in the system at the beginning of the decarburization process (ie, the total amount of phosphorus contained in the molten metal and slag). This makes it possible to reduce the total amount of phosphorus that is once returned to the molten metal during decarburization, making it possible to produce ultra-low phosphorus steel.

FIG. 2 schematically shows the change in phosphorus concentration during the decarburization process, with the horizontal axis representing the acid blowing time during the decarburization process and the vertical axis representing the phosphorus concentration [mass %] in the molten metal. In FIG. 2, the mass percentage of the total amount of phosphorus in the molten metal at the start of the decarburization process is expressed as 100% by mass.

In addition, in Figure 2, what is shown as "before improvement" indicates the change in phosphorus concentration in the molten metal when the total amount of phosphorus in the converter type vessel is high, and what is shown as "after improvement" The figure shows the change in phosphorus concentration in the molten metal when the total amount of phosphorus in the converter-type vessel is low.

As decarburization progresses, return phosphorus is generated from the slag as shown in Figure 2, and the phosphorus concentration in the molten metal becomes lower than that at the start of the decarburization process (in other words, at the end of the second dephosphorization process). will also be of high rank. At this time, as shown in Figure 2, the higher the total amount of phosphorus in the converter-type vessel, the faster the rate of increase in the amount of reversion phosphorus, and the higher the amount of reversion into the final molten metal. . On the other hand, the lower the total amount of phosphorus in the converter type vessel, the slower the rate of increase in the amount of reversion, and the lower the amount of reversion into the final molten metal.

Moreover, as shown in FIG. 2, at the end of the decarburization process, the dephosphorization reaction proceeds again, and the phosphorus concentration in the molten metal decreases.

Since the amount of phosphorus in the molten metal changes as shown in FIG. It can be seen that it is important to pay attention to the total amount of phosphorus present in the converter type vessel.

<Second finding>
In both the 2-blow method and the 3-blow method, the intermediate slag removal process involves discharging the slag from the container by tilting the converter-type container at a constant angular velocity, and then tilting the container when the molten metal starts to be discharged. Stopping prevents the discharge of molten metal. In such an intermediate slag removal process, the proportion of slag remaining inside the converter-type vessel depends on the slag forming situation, and generally, the more slag forming progresses, the more the slag discharge amount can be increased (in other words, the more slag forming progresses, the more the slag discharge amount can be increased. For example, it is known that the amount of slag remaining in a converter type vessel can be reduced.

As a result of focusing on the above-mentioned viewpoints, the present inventors have realized that by stably increasing the forming of slag before the first intermediate slag removal process and before the second intermediate slag removal process, each The inventors have come up with the idea that it is possible to suppress variations in the amount of slag remaining in the converter-type vessel after the intermediate slag treatment, and as a result, it is possible to suppress variations in the total amount of phosphorus contained in both the molten metal and the slag.

The variation in the amount of slag remaining after the first intermediate slag treatment causes the variation in the amount of slag remaining after the second intermediate slag treatment, and the variation in the amount of slag remaining after the second intermediate slag treatment This causes variations in the total amount of phosphorus (especially the total amount of phosphorus in the slag) during charcoal treatment. As a result, during the decarburization process, it becomes necessary to add a large amount of quicklime or the like as a CaO source.

Therefore, by ensuring stable slag forming before each intermediate slag removal process, it is possible to stabilize the total amount of phosphorus during decarburization at a low level, further reducing the amount of slag forming material used. It becomes possible to achieve this goal. Furthermore, by stabilizing the forming of the slag, it is possible to suppress variations in the total amount of phosphorus between charges, and as a result, it is possible to further improve operational stability.

As a result of further studies based on the two findings described above, the present inventor has completed a method for producing ultra-low phosphorous steel according to an embodiment of the present invention, which will be described below.

(About the melting method of ultra-low phosphorus steel)
Below, with reference to FIG. 3, the method for producing ultra-low phosphorus steel according to the present embodiment, which was completed based on the above two findings, will be described in detail. FIG. 3 is an explanatory diagram schematically showing an example of the flow of the method for producing ultra-low phosphorus steel according to the present embodiment.

The method for producing ultra-low phosphorous steel according to this embodiment is to perform preliminary treatment and decarburization of hot metal using a converter-type vessel, such as a top-blown converter or a top-bottom blowing converter. This is a method for producing ultra-low phosphorus steel with a content of 100 ppm or less. As schematically shown in FIG. 3, this method for producing ultra-low phosphorous steel includes a first dephosphorization step, a first intermediate slag removal step, a second dephosphorization step, and a second intermediate slag removal step. and a decarburization step, and after the decarburization step is completed, a tapping step is performed to tap the obtained ultra-low phosphorus steel.

In the following, we will use a top-blown converter as an example of a converter-type vessel, and focus on the case of melting ultra-low phosphorus steel by supplying oxygen from a top-blowing lance inserted in the top-blowing converter. Each step constituting the method for producing ultra-low phosphorus steel according to the embodiment will be described in detail. Note that even when oxygen is supplied by a different blowing acid method using a converter-type container other than a top-blown converter, the method for producing ultra-low phosphorus steel according to this embodiment can be applied. be.

<First dephosphorization step>
The first dephosphorization step is a step in which hot metal charged into a converter-type vessel (for example, a top-blowing converter as shown in FIG. 3) is subjected to dephosphorization treatment. More specifically, in the first dephosphorization step, the basicity of the slag after the first dephosphorization step is within the range of 0.8 to 1.3 with respect to the hot metal in the converter type vessel. While adding a CaO source and supplying oxygen so that the amount of oxygen outside the desiliconization is within the range of 4.0 to 10.0 Nm ³ /ton, the end point temperature of the hot metal is controlled to be within the range of 1300 to 1380°C.

Here, it is preferable that the silicon (Si) content of the hot metal charged into the converter-type vessel represented by the top-blowing converter 10 is within the range of 0.1 to 1.2% by mass. By setting the Si content of the charged hot metal within the range of 0.1 to 1.2% by mass, the effects of the ultra-low phosphorus steel melting method according to the present embodiment can be more reliably expressed. becomes possible.

By setting the basicity of the slag after the first dephosphorization step (=CaO content in the slag [mass%]/ _SiO2 content in the slag [mass%]) to 0.8 or more, the first It becomes possible to ensure the progress level of dephosphorization of the molten metal while ensuring the amount of slag forming in the dephosphorization process. The basicity of the slag after the first dephosphorization step is preferably 0.9 or more. On the other hand, by setting the basicity of the slag after the first dephosphorization step to 1.3 or less, the amount of slag foaming in the first dephosphorization step can be stably ensured (i.e., the variation in the amount of slag forming can be suppressed). ) becomes possible. The basicity of the slag after the first dephosphorization step is preferably 1.2 or less.

In addition, when the Si content in the hot metal is within the above range of 0.1 to 1.2 mass%, the basicity of the slag at the end of the first dephosphorization step is set to 0.8 to 1.3. In order to achieve this, the required amount of CaO to be added is approximately 2 to 31 kg/ton per ton of hot metal (molten metal).

As mentioned above, the Si content in hot metal varies, for example, from about 0.1 to 1.2% by mass. For example, when oxygen is supplied to hot metal from a top blowing lance as shown in Fig. 3, the oxidation reaction (desiliconization reaction) of Si contained in the hot metal proceeds first, and after the desiliconization reaction is almost completed, , the dephosphorization reaction begins to proceed. Therefore, if the amount of blown acid is simply specified, the amount of oxygen contributing to the dephosphorization reaction will vary due to the variation in the Si content as described above. Therefore, in the method for producing ultra-low phosphorus steel according to the present embodiment, the amount of oxygen other than that used for the desiliconization reaction is defined as the amount of oxygen other than desiliconization.

Here, the Si content of the hot metal used can be determined prior to the first dephosphorization step by a known measurement method such as ICP (Inductivity Coupled Plasma) emission spectrometry. Based on the Si content of the obtained hot metal, the amount of oxygen required for the desiliconization reaction (desiliconization oxygen amount) can be calculated stoichiometrically. Therefore, the value obtained by subtracting the above desiliconizing oxygen amount from the oxygen amount (blowing acid amount) actually supplied to the hot metal becomes the non-siliconizing oxygen amount corresponding to the supplied oxygen amount (blowing acid amount).

In addition, by adding the calculated amount of oxygen for desiliconization and the control target value of the amount of oxygen outside of desilication, it is possible to determine the amount of oxygen (amount of blown acid) to be supplied to the hot metal in the first dephosphorization step. can.

In the first dephosphorization step according to the present embodiment, the amount of oxygen outside of desiliconization (the amount of oxygen outside of desiliconization per ton of hot metal) as described above is within the range of 4.0 to 10.0 Nm ³ /ton. Then, oxygen is supplied to the hot metal. By setting the amount of extrasilicic oxygen to 4.0 Nm ³ /ton or more and achieving the basicity of the slag as described above, it is possible to stably secure the amount of slag foaming. The amount of extra-silicic oxygen is preferably 4.5 Nm ³ /ton or more, more preferably 5.0 Nm ³ /ton or more. On the other hand, if the amount of extra-silicic oxygen exceeds 10.0 Nm ³ /ton, the amount of slag forming may increase rapidly and slag may blow out from the converter type container, so such blowing out of slag can be prevented. In order to do this, it becomes necessary to use a foaming sedative, which results in variations in the amount of slag foaming. Therefore, in order to suppress variations in the amount of slag foaming, the amount of oxygen outside the desiliconization is set to 10.0 Nm ³ /ton or less. The amount of extra-silicic oxygen is preferably 9.5 Nm ³ /ton or less, more preferably 9.0 Nm ³ /ton or less.

As mentioned above, by setting both the basicity of the slag after the first dephosphorization step and the amount of oxygen outside the desiliconization within a relatively narrow range, the first dephosphorization step according to the present embodiment , it is possible to achieve a stable slag forming amount. As a result, the amount of slag forming generated for each charge is stabilized, and the range of variation thereof can be made narrower than before. As a result, when a predetermined slag residual rate is set as a control target in the next step, the first intermediate slag removal step, it is possible to suppress variations in the amount of residual slag. For example, in the first intermediate slag removal process of the next process, when the control target is to set the slag residual rate to an average of 40% by mass, for example, the slag residual rate may vary between 32 and 48% by mass. Instead of an average of 40% by mass as a result, it can be set as an average of 40% by mass as a result of variations between 36 and 44% by mass. Thus, the basicity of the slag after the first dephosphorization step and the amount of oxygen outside the desiliconization process are important factors contributing to stabilizing the amount of slag foaming.

Further, in the first dephosphorization step, the end point temperature of the hot metal is within the range of 1300 to 1380°C. By setting the end point temperature of the hot metal to 1300° C. or higher, it is possible to ensure that the progress level of the dephosphorization reaction of the hot metal is in an appropriate state. In order to further advance the dephosphorization reaction, the end point temperature of the hot metal is preferably 1320°C or higher, more preferably 1330°C or higher. On the other hand, by setting the end point temperature of hot metal to 1380° C. or lower, it becomes possible to advance the dephosphorization reaction while suppressing a decrease in dephosphorization efficiency. From the viewpoint of suppressing the temperature increase of the molten metal in the second dephosphorization step in the latter stage to reduce the efficiency of the dephosphorization reaction and prevent the generation of rephosphorization, the end point temperature of the hot metal is preferably 1360 ° C. or less, More preferably, the temperature is 1350°C or less.

In addition, in the first dephosphorization step, if the ranges of the slag basicity, the amount of oxygen outside the desiliconization, and the end point temperature of the hot metal are within the above ranges, the oxygen supply time (oxygen blowing time) is particularly specified. However, in order to more stably achieve the desired slag foaming amount, the oxygen supply time (acid blowing time) is preferably within the range of 1 to 6 minutes.

<First intermediate slag removal process>
The first intermediate slag removal step is a step carried out after the first dephosphorization step, in which slag is removed from the converter type container and 30 to 50% by mass of the slag before removal is removed from the converter. This is the process of leaving it in the mold container.

In the method for producing ultra-low phosphor steel according to the present embodiment, in order to make the input amount of the CaO source zero or a small amount (more specifically, 5 kg/ton or less) in the second dephosphorization step, dephosphorization is performed. The slag that functions as a material is left in an amount of 30% by mass or more of the amount of slag before being discharged. That is, in the first intermediate slag removal step according to the present embodiment, the residual rate of slag in the converter type container is set to 30% by mass or more. When the residual rate of slag is less than 30% by mass, the amount of residual slag that functions as a dephosphorizing agent is insufficient and it becomes necessary to add a CaO source, reducing the amount of quicklime used as a slag forming material. Can not do it. The residual rate of slag is preferably 32% by mass or more, more preferably 34% by mass or more.

On the other hand, by allowing slag to remain in the converter type container in an amount of 50% by mass or less of the amount of slag before slag discharge (i.e., keeping the residual rate of slag to 50% by mass or less), the amount of slag contained in the slag can be reduced. It becomes possible to reduce the total amount of phosphorus, and it is possible to realize a reduction in the amount of return phosphorus in the subsequent decarburization process. The residual amount of slag is preferably 45% by mass or less, more preferably 40% by mass or less.

Here, the above-mentioned slag residual rate (or intermediate slag removal rate which is a value obtained by subtracting the slag residual rate from 100%) can be calculated using the method described in JP-A-2018-115350, for example. .

In addition, when discharging slag from a converter-type container, the slag discharge start time, the slag discharge time from the start time, and the inclination speed (angular velocity) during the slag discharge time are A common slag removal method is to control the slag residual rate within a desired range by keeping each constant. In this embodiment, in the first dephosphorization step mentioned above, a stable slag forming amount with little variation is ensured. Therefore, by employing, for example, the above-described slag removal method, it is possible to further stabilize the amount of slag left in the converter type container.

<Second dephosphorization process>
The second dephosphorization step is a step performed after the first intermediate slag removal step, and is a step in which the molten metal in the converter type container is subjected to dephosphorization treatment again. More specifically, in the second dephosphorization step, the CaO source is used in an amount of 0 to 5 kg in terms of CaO so that the basicity of the slag after the second dephosphorization step is within the range of 0.8 to 1.3. /ton, and while supplying oxygen at an amount of blown acid in the range of 2.0 to 4.0 Nm ³ /ton, the end point temperature of the molten metal is controlled to be in the range of 1300 to 1380°C.

By setting the basicity of the slag after the second dephosphorization step to 0.8 or more, it is possible to ensure the level of molten metal dephosphorization progress while ensuring the amount of slag forming in the second dephosphorization step. Become. The basicity of the slag after the second dephosphorization step is preferably 0.9 or more. On the other hand, by setting the basicity of the slag after the second dephosphorization step to 1.3 or less, the amount of slag foaming in the second dephosphorization step can be stably ensured (i.e., the variation in the amount of slag forming can be suppressed). ), it is also possible to stabilize the slag removal rate in the second intermediate slag removal step. The basicity of the slag after the second dephosphorization step is preferably 1.2 or less.

In addition, in the method for producing ultra-low phosphorous steel according to the present embodiment, the residual rate of slag is within the range of 30 to 50% by mass in the preceding first intermediate dephosphorization step, thereby eliminating the slag necessary for the second dephosphorization step. A large portion of the amount of dephosphorizing material (ie, slag) is reserved in advance. Therefore, in the second dephosphorization step according to the present embodiment, the amount of CaO source added as a slag material can be set to 0 kg/ton or more and 5 kg/ton or less in terms of CaO.

Further, in the second dephosphorization step according to the present embodiment, the amount of oxygen (that is, the amount of blown acid) supplied to the molten metal is within the range of 2.0 to 4.0 Nm ³ /ton. By setting the basicity of the slag after the second dephosphorization step within the above range and setting the blowing acid amount to 2.0 Nm ³ /ton or more, the desired amount of dephosphorization can be achieved to obtain ultra-low phosphorus steel. can be realized. The amount of blown acid is preferably 2.2 Nm ³ /ton or more, more preferably 2.4 Nm ³ /ton or more. On the other hand, if the amount of blown acid exceeds 4.0 Nm ³ /ton, the amount of slag foaming may increase rapidly and slag may blow out from the converter type container. It becomes necessary to use a foaming sedative for this purpose, which results in variations in the amount of slag foaming. Therefore, in order to suppress variations in the amount of slag foaming, the amount of blown acid is set to 4.0 Nm ³ /ton or less. The amount of blown acid is preferably 3.8 Nm ³ /ton or less, more preferably 3.6 Nm ³ /ton or less.

As mentioned above, by setting both the basicity of the slag after the second dephosphorization process and the amount of blown acid within a relatively narrow range, the second dephosphorization process according to the present embodiment can be stabilized. It is possible to achieve a high slag forming amount. As a result, the amount of slag forming generated for each charge is stabilized, and the range of variation thereof can be made narrower than before. As a result, when a predetermined slag removal rate is set as a control target in the next second intermediate slag removal step, it is possible to suppress variations in the amount of removed slag. For example, in the second intermediate slag removal step of the next step, when the control target is an average slag removal rate of 40% by mass, for example, the slag removal rate may vary between 32 and 48% by weight. Instead of an average of 40% by mass resulting from a variation in the content between 36 and 44% by mass, an average of 40% by mass can be used. Thus, the basicity of the slag after the second dephosphorization step and the amount of acid blowing are important factors that contribute to stabilizing the amount of slag foaming.

Further, in the second dephosphorization step, the end point temperature of the molten metal is within the range of 1300 to 1380°C. By setting the end point temperature of the molten metal to 1300° C. or higher, it is possible to ensure that the progress level of the dephosphorization reaction of the molten metal is in an appropriate state. In order to further advance the dephosphorization reaction, the end point temperature of the molten metal is preferably 1320°C or higher, more preferably 1330°C or higher. On the other hand, by setting the end point temperature of the molten metal to 1380° C. or lower, it becomes possible to advance the dephosphorization reaction while suppressing a decrease in dephosphorization efficiency and generation of rephosphorization. The end point temperature of the molten metal is preferably 1360°C or lower, more preferably 1350°C or lower.

In addition, in the second dephosphorization step, as long as the slag basicity, the amount of blown acid, and the end point temperature of the molten metal are within the above ranges, the oxygen supply time (the blown acid time) is not particularly specified. However, in order to more stably achieve the desired amount of slag foaming, the oxygen supply time (oxygen blowing time) is preferably within the range of 0.3 to 3.0 minutes.

<Second intermediate slag removal process>
The second intermediate slag removal step is a step carried out after the second dephosphorization step, and is a step in which 30 to 60% by mass of the slag before slag is removed to the outside of the converter type container.

In the method for producing ultra-low phosphorus steel according to the present embodiment, since the dephosphorization reaction proceeds even in the subsequent decarburization process, a CaO source is added depending on the desired level of phosphorus content in the molten steel. That is required. The CaO source added in the later decarburization process needs to be turned into slag, but since the newly added CaO source alone generally has a high melting point, the slag produced in the second dephosphorization process is put into a converter-type vessel. By leaving a predetermined amount remaining, it is possible to promote slag formation of the added CaO source in the subsequent decarburization step.

Furthermore, in the subsequent decarburization step, the surface of the molten metal except for the ignition point of the blown acid needs to be shielded with slag, so it is also important to secure slag as a shield for the surface of the molten metal.

From the above two viewpoints, in the second intermediate slag according to the present embodiment, 60% by mass or less of the slag before slag is slaged out of the converter type container (in other words, slag is At least 40% by mass of the slag before slag remains in the converter type vessel). This makes it possible to reduce the total amount of phosphorus contained in the slag, making it possible to reduce the amount of return phosphorus during the decarburization step, which will be described later. The slag removal rate in the second intermediate removal step is preferably 55% by mass or less, more preferably 50% by mass or less.

On the other hand, if the amount of slag discharged outside the converter-type container is less than 30% by mass of the slag before slag, the total amount of phosphorus contained in the slag remaining in the converter-type container after the slag is is insufficiently reduced, and the amount of reversion in the next step of decarburization increases. Therefore, the amount of slag discharged outside the converter type container should be 30% by mass or more of the slag before slag is discharged (in other words, 70% by mass or less of the slag before slag is discharged from the converter type container). remain inside). The slag removal rate in the second intermediate removal step is preferably 35% by mass or more, more preferably 40% by mass or more.

Note that the above-mentioned slag removal rate can be calculated using the method described in JP-A-2018-115350, for example, as in the case of the first intermediate removal step. Also, regarding the control method of the slag removal rate, it is possible to employ the same sludge removal method as in the first intermediate slag removal step. In this embodiment, in the first dephosphorization step and the second dephosphorization step mentioned above, a stable slag forming amount with little variation is ensured. Therefore, by employing the above-described slag removal method, it is possible to further stabilize the amount of slag removed to the outside of the converter type container.

<Decarburization process>
The decarburization process is a process in which a CaO source is added to the molten metal after the second intermediate slag removal process to perform decarburization treatment. The decarburization treatment method employed in the decarburization step is not particularly limited, and various known decarburization treatment methods can be employed.

In the method for producing ultra-low phosphorus steel according to the present embodiment, the total amount of phosphorus in the slag is reduced through the first dephosphorization step to the second intermediate slag removal step, and the amount of slag brought into the decarburization step is also reduced. There is little variation. As a result, the amount of reversion during decarburization is stabilized, and it becomes possible to more easily produce ultra-low phosphorus steel with a phosphorus content of 100 ppm or less.

Further, as described above, since there is little variation in the amount of slag, the amount of CaO source added as needed to the required phosphorus concentration of ultra-low phosphorus steel can be kept at a generally constant level. As a result, it becomes possible to suppress the input of excessive CaO source, and it becomes possible to reduce the amount of slag material input.

The method for producing ultra-low phosphorous steel according to the present embodiment has been described in detail above with reference to FIG. 3.

Hereinafter, the method for producing ultra-low phosphorus steel according to the present invention will be specifically described with reference to Examples. The example shown below is only one example of the method for producing ultra-low phosphorus steel according to the present invention, and the method for producing ultra-low phosphorus steel according to the present invention is not limited to the following example.

As an example of a converter-type vessel, a top-bottom blowing converter with a capacity of 350 tons is used, and after charging the hot metal in the range of 300 tons to 350 tons, the first dephosphorization step and the first dephosphorization step are performed. The processing was performed in the following order: 1 intermediate slag removal process, 2nd dephosphorization process, 2nd intermediate slag removal process, and decarburization process. As a result, molten steel was produced with the phosphorus concentration of the molten steel obtained after treatment set as a target value of 0.0010% by mass (10 ppm) or less.

The contents of the main components of the hot metal used are as follows, with the remainder being Fe and impurities.
C: 3.0 to 5.0 (mass%)
Si: 10 to 100 (×10 ^-2 % by mass)
Mn: 5 to 10 (×10 ⁻² % by mass)
P: 50 to 200 (×10 ^-3 % by mass)
S: 1 to 20 (×10 ^-3 % by mass)

On the premise that all Si in the hot metal is oxidized, set the target value of basicity of slag as shown in Table 1 below, and select the CaO source (quicklime, limestone, recycled slag) to be added in the first dephosphorization step. The amount of addition (CaO equivalent value) was determined.

In addition, when blowing oxygen from the top blowing lance, calculate the amount of oxygen for desiliconization from the Si content in the hot metal, and adjust the amount of blown acid so that the amount of oxygen for desiliconization is as shown in Table 1 below. The amount of cold material added was adjusted so that the hot metal end point temperature was determined and shown in Table 1 below. The acid blowing time was about 1 to 6 minutes. In addition, during the treatment of the first dephosphorization step, bottom-blowing stirring using CO ₂ was also implemented. Note that oxygen can also be used in combination with bottom-blown stirring. Since the amount of blown acid in the commonly used bottom blown method is extremely small compared to the amount of blown acid in the top blown method, the effect of the blown acid in the present invention may be based on the top blown oxygen.

In the first intermediate slag removal step, the target value of the slag residual rate was set at 35 to 45% (however, in Table 1 below, in Comparative Example No. 7, the target value was set at 10 to 20%, and in Comparative Example No. 8, the target value was set at 35 to 45%. ), the tilting speed (angular velocity) of the top-bottom blowing converter was kept constant, and the tilting was stopped at a predetermined angle before the start of discharging the molten metal. In addition, in the example of the present invention, the deviation is approximately ±5 from the target value, so if the target value is 35 to 45%, the actual value is 30 to 50%. Some of the comparative examples were under conditions where the slag residual rate varied, and there were cases where the actual value exceeded +5 and/or fell below -5 with respect to the target value listed in Table 1.

In addition, the actual slag residual rate was determined by calculating the slag discharge rate using the method described in JP-A-2018-115350, and subtracting the obtained slag discharge rate from 100%.

In the second dephosphorization step, the amount of blown acid shown in Table 1 below was set as the target value, and the amount of cold material added was adjusted so as to achieve the molten metal end point temperature shown in Table 1 below. In addition, the CaO source (quicklime, limestone, recycled slag) is in the range of 0 to 5 kg/ton in terms of CaO, and silica stone is sometimes used in combination to adjust the basicity of the slag. The acid blowing time was about 1 to 6 minutes. Note that during the second dephosphorization step, bottom-blowing stirring using CO ₂ was also performed. Note that oxygen can also be used in combination with bottom-blown stirring. Since the amount of blown acid in the commonly used bottom blown method is extremely small compared to the amount of blown acid in the top blown method, the effect of the blown acid in the present invention may be based on the top blown oxygen.

In the second intermediate slag removal step, the target value of the slag removal rate is 35 to 55% (however, in Table 1 below, the target value is 10 to 20% in Comparative Example No. 15, and the target value is 10 to 20% in Comparative Example No. 16. (The target value was 75 to 85%.) The tilting speed (angular velocity) of the top-bottom blowing converter was kept constant, and the tilting was stopped at a predetermined angle before the start of discharging the molten metal. In addition, in the example of the present invention, the fluctuation range is approximately ±5 with respect to the target value, so when the target value is 35 to 55%, the actual value is 30 to 60%. Some of the comparative examples were under conditions where the slag removal rate varied, and there were cases where the actual value exceeded +5 and/or fell below -5 with respect to the target value listed in Table 1. .

In addition, the actual slag removal rate was calculated using the method described in JP 2018-115350A.

In the decarburization process, decarburization was performed by top-blowing oxygen. The amount of blown acid was 30 to 60 Nm ³ /ton. Other detailed conditions were determined according to known and commonly used methods. Furthermore, based on past experience in operations for producing ultra-low phosphorus steel, the amount of CaO source to be added was determined so that the molten steel phosphorus concentration would be 0.0010% by mass or less.

The operation was carried out for half a year in accordance with the method explained above, and the operation results of the operation corresponding to each operation condition shown in Table 1 below were summarized. Each No. As a result, they are operating multiple charges. In Table 1 below, the target slag residual rate in the first intermediate slag removal process and the target slag removal rate in the second intermediate slag removal process are numerical ranges with upper and lower limits. , Table 1 Each No. In the multiple charge operation, the operating conditions are the upper and lower target values and the intermediate value of 1 or more, which is a total of 3 or more.

<Evaluation method>
[Variations in slag residual rate and discharge rate]
The variation between charges in the slag residual rate in the first intermediate slag removal process and the slag discharge rate in the second intermediate slag removal process is within ±5 with respect to the target value of the residual rate (slag removal rate). If the actual value of the residual rate (slag removal rate) is achieved, the score will be "A", and if the actual value is more than +5 and/or less than -5 compared to the target value, the score will be "B". did.

[Amount of CaO source added in all processes]
The total amount of CaO sources added in each of the first dephosphorization step, second dephosphorization step, and decarburization step was calculated. Using the total amount added in Comparative Example 1 in Table 1 below as the standard amount, the case where the amount was reduced by 30% or more from the standard amount is given a rating of "A," and the case where the amount used is equal to or greater than the standard amount is given a rating of "A." B”. If the total amount added is less than the standard amount but the reduction amount is less than 30%, it is judged to be equivalent to the standard amount and given a rating of "B".

The obtained results are summarized in Table 1 below.

In Comparative Example 1, a large amount of foaming tended to occur in the first dephosphorization step, and a foaming inhibitor had to be regularly used to prevent slopping, and as a result, a sufficient amount of slag foaming could not be secured. Therefore, the slag residual rate varied in the first intermediate slag removal process. Further, due to the variation in the slag residual rate, the slag removal rate in the second intermediate slag removal process also varied. As a result, it was necessary to increase the CaO source in the decarburization process.

In Comparative Example 2, forming was insufficient in the first dephosphorization step, and a sufficient amount of slag forming could not be secured, and as a result, as in Comparative Example 1, in the first intermediate slag removal step and the second intermediate slag removal step. , variations occurred in the slag residual rate and the slag removal rate, and the amount of CaO source added in all steps was the same as in Comparative Example 1.

In Comparative Example 3 and Comparative Example 4, the dephosphorization reaction did not proceed in the first dephosphorization step, and the total amount of phosphorus in the system increased in the decarburization step, so it was necessary to increase the CaO source in the decarburization step. The amount of CaO source added in all steps was larger than that in Comparative Example 1.

In Comparative Example 5, since the amount of extra-desiliconizing oxygen was insufficient in the first dephosphorization step, a sufficient amount of slag forming could not be ensured, and the slag residual rate varied in the first intermediate slag removal step. Further, due to the variation in the slag residual rate, the slag removal rate in the second intermediate slag removal process also varied. As a result, the amount of CaO source had to be increased in the decarburization process, and the amount of CaO source added in the entire process was greater than that in Comparative Example 1.

In Comparative Example 6, excessive slag foaming occurred at the end of the first dephosphorization process because the amount of oxygen outside the desiliconization process became too large in the first dephosphorization process, and a foaming sedative was used. As a result, the slag residual rate varied in the first intermediate slag removal step, and the amount of CaO source added in all steps was larger than in Comparative Example 1.

In Comparative Example 7, the target value of the slag residual rate in the first intermediate dephosphorization process is too low, and the actual value of the slag residual rate is about 5 to 25%, but less than 30%, so the dephosphorization process is carried out in the second intermediate dephosphorization process. This caused a shortage of phosphor material (ie, slag), and the dephosphorization reaction in the second dephosphorization step did not proceed sufficiently. As a result, the amount of CaO source had to be increased in the decarburization process, and the amount of CaO source added in the entire process was the same as in Comparative Example 1.

In Comparative Example 8, the target value of the slag residual rate in the first intermediate slag removal process was too high, and the actual value of the slag residual rate was about 55 to 85%, which exceeded 50%, resulting in a system failure in the decarburization process. Since the total amount of phosphorus in the sample increased, the amount of CaO source had to be increased in the decarburization process, and the amount of CaO source added in the entire process was larger than in Comparative Example 1.

In Comparative Example 9, a large amount of foaming tended to occur in the second dephosphorization step, and a foaming inhibitor had to be regularly used to prevent slopping, and as a result, a sufficient amount of slag foaming could not be secured. Therefore, the slag removal rate in the second intermediate removal step also varied. As a result, the amount of CaO source had to be increased in the decarburization process, and the amount of CaO source added in the entire process was the same as in Comparative Example 1.

In Comparative Example 10, since a stable slag forming amount could not be ensured in the second dephosphorization step, the slag removal rate in the second intermediate slag removal step also varied. As a result, the amount of CaO source had to be increased in the decarburization process, and the amount of CaO source added in the entire process was greater than that in Comparative Example 1.

In Comparative Example 11, the dephosphorization reaction did not proceed sufficiently because the amount of blown acid was insufficient in the second dephosphorization step. As a result, the amount of CaO source had to be increased in the decarburization process, and the amount of CaO source added in the entire process was greater than that in Comparative Example 1.

In Comparative Example 12, the amount of blown acid was too large in the second dephosphorization step, so excessive slag foaming occurred at the end of the second dephosphorization step, and a foaming sedative was used. As a result, variations occurred in the slag removal rate in the second intermediate waste removal step, and the amount of CaO source added in all steps was greater than in Comparative Example 1.

In Comparative Example 13 and Comparative Example 14, the dephosphorization reaction did not proceed in the second dephosphorization step, and the total amount of phosphorus in the system increased in the decarburization step, so it was necessary to increase the CaO source in the decarburization step. The amount of CaO source added in all steps was larger than that in Comparative Example 1.

In Comparative Example 15, the target value of the slag removal rate in the second intermediate removal process is too low, and the actual value of the slag removal rate is about 5 to 25%, which is less than 30%, so as a result, the decarburization process Since the total amount of phosphorus in the system increased in the decarburization process, the CaO source had to be increased in the decarburization process, and the amount of CaO source added in the entire process was larger than in Comparative Example 1.

In Comparative Example 16, the target value of the slag removal rate in the second intermediate slag removal step is too high, and the actual value of the slag removal rate is about 70 to 90%, which is over 60%, so there is too little slag, and the slag removal rate is too low. Since it was difficult to expect slag formation to proceed in the charcoal process, the amount of CaO source had to be increased, and the amount of CaO source added in all processes was larger than in Comparative Example 1.

On the other hand, in the example corresponding to the example of the present invention, a stable slag forming amount is achieved in the first dephosphorization step and the second dephosphorization step, so that the slag residual rate in the first intermediate slag removal step and It was possible to suppress variations in the slag removal rate in the second intermediate slag removal process, and it was also possible to reduce the amount of CaO source added in the entire process by 30% or more compared to Comparative Example 1.

Although preferred embodiments of the present invention have been described above in detail with reference to the accompanying drawings, the present invention is not limited to such examples. It is clear that a person with ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea stated in the claims. It is understood that these naturally fall within the technical scope of the present invention.

Claims (2)

A method for producing ultra-low phosphorous steel using a converter type vessel to perform hot metal pretreatment and decarburization treatment, the method comprising:
A first dephosphorization step for dephosphorizing hot metal;
a first intermediate slag removal step carried out after the first dephosphorization step;
a second dephosphorization step in which the molten metal is subjected to dephosphorization treatment again after the first intermediate slag drainage step;
a second intermediate slag removal step carried out after the second dephosphorization step;
A decarburization treatment step of adding a CaO source to the molten metal after the second intermediate slag removal step to perform decarburization treatment;
has
In the first dephosphorization step, a CaO source is added to the hot metal so that the basicity of the slag after the first dephosphorization step is within the range of 0.8 to 1.3, and desiliconization is performed. While supplying oxygen so that the amount of external oxygen is within the range of 4.0 to 10.0 Nm ^/ ton, the end point temperature of the hot metal is within the range of 1300 to 1380 ° C.,
In the first intermediate slag step, slag is slaged outside the converter-type container, and 30 to 50% by mass of the slag amount before slag remains in the converter-type container;
In the second dephosphorization step, a CaO source is added in an amount of 0 to 5 kg/ton in terms of CaO so that the basicity of the slag after the second dephosphorization step is within the range of 0.8 to 1.3. and while supplying oxygen at an amount of blown acid in the range of 2.0 to 4.0 Nm ³ /ton, the end point temperature of the molten metal is in the range of 1300 to 1380 ° C.,
In the second intermediate slag step, 30 to 60% by mass of the slag before slag is slaged out of the converter type container. The method for producing ultra-low phosphorus steel according to claim 1, wherein the Si content of the hot metal is within a range of 0.1 to 1.2% by mass.

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