JP7568922B2 - Refining Method - Google Patents

Description

The present invention relates to a refining method.

The refining process of molten pig iron in a converter-type vessel, such as a converter, involves charging blast furnace molten pig iron into a converter-type vessel, adding a flux mainly consisting of quicklime, and blowing oxygen (acid) into the molten pig iron to desiliconize (Si), dephosphorize (P), and decarburize (C) the molten iron to produce steel. In recent years, in order to produce high-purity steel such as low-P steel, and to reduce the amount of steelmaking slag generated as a by-product of converter refining, a converter-type vessel refining method (hereinafter abbreviated as converter refining method) that separates the desiliconization and dephosphorization processes from the decarbonization process has been used.

Among the converter refining methods mentioned above, the most widely used method is the MURC method, in which after de-Si and de-P treatments (hereinafter abbreviated as "de-Si/P treatment"), the molten iron after treatment is not discharged outside the converter, but the slag is discharged from the throat by tilting the converter (hereinafter referred to as intermediate slag discharge), and then the molten iron in the converter is continuously de-C treated.

The MURC process is characterized by the fact that the blowing process is divided into two processes, de-Si/P treatment and de-C treatment, with intermediate slag removal in between, making it possible to optimize the slag composition and blowing conditions separately for each of the de-Si/P treatment and de-C treatment. This makes it possible to produce high-purity steel and reduce the amount of slag generated, compared to the converter refining method (so-called one-blow blowing), in which de-Si treatment, de-P treatment, and de-C treatment are performed all at once.

In addition, with the MURC process, the molten metal is not dumped outside the furnace during intermediate slag dumping, but the furnace body is tilted to dump the slag, making it possible to quickly separate the molten metal and slag after processing. This contributes to improved productivity compared to the converter refining process, in which the molten metal is dumped outside the furnace once after the Si/P de-processing process, then recharged into the converter to perform the C de-processing process.

In the MURC process, the slag composition and the acid blowing conditions are controlled during the de-Si/P treatment to perform intermediate slag removal, and the slag is expanded by CO and _CO2 generated by the reaction between the charged oxygen and the carbon in the molten iron, utilizing a phenomenon known as foaming. In the MURC process, the slag is appropriately expanded by foaming, and then the converter is tilted, making it possible to discharge only the slag outside the furnace.

However, if excessive foaming occurs during the Si/P removal process, it can cause slopping, in which slag spills out of the furnace throat, resulting in reduced productivity.

For this reason, various techniques have been disclosed in the past, such as a technique for measuring the forming height (also called the forming level) in a converter to prevent slopping, and a method for desiliconization treatment up to a forming height at which intermediate slag removal is easy.

For example, the following Patent Document 1 discloses a microwave measuring device for measuring the forming height in a furnace and a measurement method using such a measuring device.

In addition, the following Patent Document 2 discloses a molten iron pretreatment method in which desiliconization and dephosphorization of molten iron are carried out continuously using a single converter-type refining furnace with a slag removal process in between, in which the desiliconization process is terminated at a specified height using a microwave level meter, thereby preventing the sudden outflow of slag (slopping) during the slag removal process after the desiliconization process, and quickly removing the target specified amount of desiliconized slag from the furnace in a short period of time.

In addition, the following Patent Document 3 discloses a technique for refining molten pig iron using a single converter-type refining furnace to continuously perform a primary blowing process and a secondary blowing process with a slag removal process in between, and uses the height of foaming in the primary refining process to measure the height of foaming with a microwave level meter for processing. More specifically, the following Patent Document 3 discloses a method in which blowing is performed until the foaming height ratio (the ratio of the height of the slag to the freeboard in the converter) is 0.6 or more, foaming suppressant is added when the foaming height is between 0.6 and less than 0.8, foaming suppressant is added to control the foaming height, and the primary refining process is terminated when the height ratio is between 0.5 and 0.7, thereby suppressing the sudden outflow of slag in the primary blowing process and enabling the targeted amount of slag to be quickly discharged out of the furnace in a short time in the slag removal process after the primary blowing process.

In addition, the following Patent Document 4 discloses a technology for refining molten pig iron using a single converter-type refining furnace to continuously perform a primary blowing process and a secondary blowing process with a slag removal process in between, in which the primary blowing process is terminated when the slag height ratio in the furnace is 0.3 or more and less than 0.6, and then bottom blown gas is maintained for 1 minute to 5 minutes, and then slag removal is started when the slag height ratio is 0.3 or more and less than 0.6, thereby preventing the removed slag from foaming in the slag pot, and thereby quickly removing the desired amount of slag from the furnace in a short period of time.

As such, technologies that have been disclosed include those for monitoring the forming state inside the furnace, those for suppressing sudden slag outflows during refining by utilizing the forming state inside the furnace, those for controlling forming, and those for quickly discharging slag during intermediate slag discharge.

JP 2018-87364 A International Publication No. 2014/115526 JP 2016-29212 A JP 2017-133060 A

However, although the above-mentioned Patent Document 1 discloses a method for monitoring the forming state inside the furnace, it does not disclose any technology for avoiding slopping or keeping the amount of slag discharge constant.

Furthermore, the above Patent Documents 2 to 4 disclose techniques for preventing slopping and quickly discharging slag. However, as a result of verification by the present inventors, it was found that when the techniques disclosed in Patent Documents 2 to 4 are used to properly form slag when there is little slag in the furnace, it becomes difficult to sufficiently discharging the slag during intermediate slag discharging. This causes problems such as a reduction in the effectiveness of the flux supplied to the molten metal during the refining process after intermediate slag discharging.

Furthermore, as a result of the inventor's investigation, it became clear that in the techniques of Patent Documents 2 to 4, the amount of slag remaining in the furnace after intermediate slag removal varies depending on the initial molten iron composition and amount of slag in the furnace. When such a phenomenon occurs, the variation in the amount of slag causes problems in the refining process after intermediate slag removal. For example, when the refining process after intermediate slag removal is a decarburization process, the amount of rephosphorization during decarburization varies, leading to a situation where excessive use of CaO to suppress the variation in the amount of rephosphorization cannot be suppressed.

As such, even if the technologies disclosed in Patent Documents 2 to 4 above were used, there was still room for improvement in terms of steadily discharging a large amount of slag during intermediate draining of the slag generated after hot metal pretreatment (desiliconization and dephosphorization) in a converter.

The present invention has been made in consideration of the above problems, and the object of the present invention is to provide a refining method that can stably discharge a large amount of slag when performing intermediate slag removal of the slag generated after molten iron pretreatment in a converter-type vessel.

In order to solve the above problems, the present inventors conducted further studies and discovered that it was necessary to focus on the foaming behavior of slag in a converter-type vessel, leading to the completion of the present invention described below.
The gist of the present invention, which was completed based on these findings, is as follows.

[1] A refining method in a top-blowing refining process for refining steel using a converter-type vessel, comprising: a first step of charging molten pig iron into the converter-type vessel; a second step of adding a lime-based flux into the converter-type vessel and blowing acid therein to subject the molten pig iron to a de-Si treatment and a de-P treatment and form slag; a third step of performing intermediate slag disposal in which the slag is discharged from a throat of the converter-type vessel by tilting the converter-type vessel; and a fourth step of adding flux into the converter-type vessel after the intermediate slag disposal and performing a top-blowing refining process, wherein the Si concentration of the molten pig iron at the start of the second step is 0.6 mass% or less, and the slag at the time when the acid blowing in the second step is stopped has an _Al2O3 concentration of 0.5 _mass % or more and 5.0 mass% or less, a CaO concentration (unit: mass%) of 0.5 mass% or more and a SiO2 concentration (unit: mass%) of 0.5 mass% or more and a CaO ... a basicity divided by the concentration (unit: mass%) of ₂ is set to 0.8 or more and 1.5 or less, and in the second step, a height of the foaming is measured, and the height of the foaming at the time when the acid blowing before the intermediate slag discharge is stopped satisfies the following formula (1):
0.6-0.3 x (Si concentration of molten iron at the start of the second process (unit: mass%)) ≦ foaming height ratio at the time of stopping acid blowing ... formula (1)
Here, in the above formula (1), the foaming height ratio is the ratio of the measured foaming height to the height of the in-furnace freeboard in the converter-type vessel.
[2] A refining method in a top-blowing refining process for refining steel using a converter-type vessel, comprising: a first step of charging molten pig iron into the converter-type vessel; a second step of adding a lime-based flux into the converter-type vessel and blowing acid therein to subject the molten pig iron to a desiliconization treatment and a dephosphorization treatment, thereby foaming slag; a third step of performing intermediate slag discharge by tilting the converter-type vessel to discharge the slag from the throat of the converter-type vessel; and a fourth step of adding flux into the converter- type vessel after the intermediate slag discharge, thereby performing a top-blowing refining process, wherein the slag at the time when the acid blowing in the second step is stopped has an Al ₂ O ₃ concentration of 0.5 mass% or more and a CaO concentration (unit: mass%) of 0.5 mass% or more and a SiO a basicity divided by the _{concentration} (unit: mass%) of 0.8 to 1.5, and in the second step, a height of the foaming is measured, and the height of the foaming at the time when the acid blowing is stopped before the intermediate slag discharge satisfies the following formula (2):
0.6-0.3 x (Si concentration of molten iron at the start of the second process (unit: mass%)) ≦ foaming height ratio at the time of stopping acid blowing ≦ 0.8-0.3 x (Si concentration of molten iron at the start of the second process (unit: mass%)) ... formula (2)
Here, in the above formula (2), the foaming height ratio is the ratio of the measured foaming height to the height of the in-furnace freeboard in the converter-type vessel.
[3] The refining method according to [1] or [2], wherein a microwave level meter is used to measure the height of the forming in the second step.
[4] The refining method according to any one of [1] to [3], wherein the fourth step is a step of performing decarburization refining, and a part of the molten slag generated after the decarburization refining is left in the furnace, and the left molten slag is used as a part or all of the lime-based flux in the second step of the next charge.
[5] The refining method according to any one of [1] to [3], wherein the fourth step is a step of performing decarburization refining, and the solidified slag obtained by cooling and solidifying the molten slag generated after the decarburization refining is used as part or all of the lime-based flux in the second step of another charge.

As described above, according to the present invention, when performing intermediate draining of the slag generated after the molten iron pretreatment in a converter-type vessel, it is possible to stably discharge a large amount of slag. Furthermore, by being able to stably discharge a large amount of slag during intermediate draining, it is possible to suppress variation in the amount of slag remaining in the converter.

FIG. 2 is an explanatory diagram showing a schematic flow of a refining method in a top-blowing refining process that is the focus of an embodiment of the present invention. FIG. 2 is a schematic diagram for explaining the tilting angle of a converter-type vessel. FIG. 1 is a graph showing the relationship between the molten iron Si concentration and the tilting angle at the start of intermediate slag removal. FIG. 2 is an explanatory diagram showing a schematic diagram of a microwave level meter used in the embodiment. FIG. 2 is an explanatory diagram showing a schematic diagram of a microwave level meter used in the embodiment.

A preferred embodiment of the present invention will be described in detail below with reference to the attached drawings.

(Studies and findings by the inventors)
Before describing the refining method according to the embodiment of the present invention, the study by the inventors and the findings that led to the conception of the refining method according to the embodiment will be specifically described with reference to FIGS. 1 to 3.
Fig. 1 is an explanatory diagram showing a flow of a refining method in a top-blowing refining process that is the focus of this embodiment. Fig. 2 is a schematic diagram for explaining the tilting angle of a converter-type vessel. Fig. 3 is a graph showing the relationship between the molten iron Si concentration and the tilting angle at the start of intermediate slag removal.

The refining method in the top-blowing refining process, which is the focus of the present embodiment described in detail below, includes the following steps as shown in FIG. 1: a first step of charging molten iron into a converter-type vessel; a second step of subjecting the molten iron to desiliconization and dephosphorization; a third step of performing intermediate slag removal by tilting the converter-type vessel to remove slag from the throat of the converter-type vessel; and a fourth step of performing top-blowing refining. As shown in FIG. 1, the molten metal after the top-blowing refining process in step 4 is tapped out of the furnace from the tapping hole of the converter-type vessel (step 5).

<First step>
The first step is a step of charging molten iron into a converter-type vessel as shown in Fig. 1. Here, Fig. 1 focuses on a case where a top-blown converter is used as an example of a converter-type vessel and refining is performed by supplying oxygen from a top-blowing lance inserted into the top-blown converter, but the same can be said about a case where a converter-type vessel other than a top-blown converter is used and oxygen is supplied by a different acid blowing method, as will be described below.

In the first step, molten iron is charged from a container such as a ladle into the converter-type vessel, which contains molten slag that was left in the furnace in the previous charge and solidified slag that was cooled and solidified from such molten slag. By having such slag inside the converter-type vessel, it is possible to reduce the amount of lime-based flux used in the subsequent second step, and it is possible to suppress flux costs. In addition, the amount of molten iron charged into the converter-type vessel is not particularly specified, and is set appropriately depending on the size of the converter-type vessel used, etc.

The chemical composition of the molten iron used in the first step is not particularly limited, but generally, the molten iron has a chemical composition that contains 0.3 to 1.2 mass% Si, 0.08 to 0.18 mass% P, and the remainder Fe and impurities.

<Second step>
The second step is a step of subjecting the molten iron in the furnace to desiliconization and dephosphorization. The desiliconization and dephosphorization are carried out by adding various lime-based fluxes (slag forming agents) to the converter-type vessel furnace and blowing (blowing) oxygen-containing gas (e.g., oxygen gas, oxidizing gas such as air, etc.) from the top blowing lance. In the second step, the slag generated by the acid blowing is foamed. The lime-based flux used here is not particularly specified, but various CaO sources such as quicklime, limestone, and various recycled slags including decarburized slag are used. The lime-based flux may be added before the start of acid blowing or during acid blowing, as long as a predetermined refining purpose is achieved.

Here, performing desiliconization and dephosphorization on the molten pig iron using acid blowing means supplying oxygen to the molten pig iron to oxidize the Si in the molten pig iron to SiO ₂ , and converting the Si in the molten pig iron into slag (i.e., desiliconization). In addition, acid blowing for the purpose of desiliconization inevitably causes the removal of P in the molten pig iron (dephosphorization). Therefore, if acid blowing is used for the purpose of dephosphorization, desiliconization is also inevitably caused. In addition, dephosphorization using acid blowing is generally performed as a molten pig iron preliminary treatment. By such acid blowing, substances having the same oxidation properties as Si and P are oxidized and removed from the molten pig iron. For example, according to the commonly known Ellingham diagram, elements that are often treated as impurities, such as Mg, Li, Ti, Mn, Cr, Zn, and Sn, can be removed from the molten pig iron.

The amount of acid blown from the top blowing lance is not specifically regulated, but is set appropriately depending on the chemical composition and volume of the molten iron to be treated.

The degree of slag foaming (more specifically, the height of foaming) generated in the second process can be measured using a known measuring device such as a microwave level meter. Based on the measurement results using such a measuring device, the extent of the foaming height of the slag can be ascertained.

It is useful to express such forming height as a percentage of a certain reference height. In what follows, the height of the in-furnace freeboard in a converter-type vessel is used as the reference, and the ratio of the slag forming height to this in-furnace freeboard height is treated as the forming height ratio.

More specifically, with the converter-type vessel standing upright, the molten metal bath surface (the top surface of the molten metal) is taken as the reference position, and the vertical height to the throat of the converter-type vessel is taken as the in-furnace freeboard height. Then, the ratio of the slag height (the position of the top surface of the formed slag) from the measured reference position to the in-furnace freeboard height is calculated to obtain the forming height ratio.

<Third step>
The third step is a step of performing intermediate slag removal by tilting the converter-type vessel to remove the slag from the throat of the converter-type vessel. Here, the tilting angle of the converter-type vessel is defined as the angle between the center line of the furnace when the converter-type vessel is upright (standing vertically) (the center line when the furnace is down in FIG. 2) and the center line of the furnace when the converter-type vessel is tilted (the center line when tilted in FIG. 2). This tilting angle is defined as zero degrees when the converter-type vessel is upright, and increases as the converter-type vessel is tilted, until the tilting angle reaches 90° when the throat is facing horizontally.

As shown in FIG. 2, inside the converter-type vessel at the end of the second process, the molten metal that has been subjected to the desiliconization and dephosphorization processes is located at the bottom of the furnace, and above that, the liquefied slag is present, and above the liquefied slag, the foamed slag is present. When the converter-type vessel is tilted to a certain tilting angle, the foamed slag is discharged from the furnace mouth to the outside of the system. Hereinafter, the tilting angle at which the foamed slag starts to be discharged from the furnace mouth is treated as the tilting angle at the start of intermediate slag discharge. Note that liquefied slag has high viscosity, so it is difficult to discharge it from the furnace mouth even if the converter-type vessel is tilted. Only by foaming the slag to reduce its viscosity can the slag in the furnace be discharged to the outside of the system.

<Fourth step>
The fourth step is a step of adding flux into the converter-type vessel after the intermediate slag removal and performing a top-blowing refining process. The top-blowing refining process performed in the fourth step is not particularly limited, and may be a decarbonization process or a dephosphorization process for the purpose of producing low-phosphorus steel, for example, a molten steel with a phosphorus concentration of 100 ppm or less. By the fourth step, the molten metal to be treated is further subjected to a top-blowing refining process, and a molten metal (molten steel) in a desired state can be obtained.

<Contents of study by the present inventor>
In the top blowing refining process carried out in the above-mentioned manner, the present inventors have conducted extensive research in order to solve the problems that arise during intermediate slag removal as mentioned above.
The present inventors have conducted extensive research into the causes of insufficient slag removal volume and variations in the amount of slag remaining in a converter-type vessel when intermediately draining the slag generated after molten iron pretreatment (de-Si treatment and de-P treatment) in a converter, and have discovered that attention should be focused on the foaming behavior of the slag.

As explained above, for intermediate slag removal, it is important that the slag is properly formed, and the tilt angle at the start of intermediate slag removal (hereinafter simply referred to as the intermediate slag removal start angle) depends on the height of the slag formation. Therefore, it was presumed that if the height of the slag formation in the converter-type vessel varies for each treatment charge, it will cause variations in the amount of slag remaining in the furnace for each treatment charge, and also cause a shortage of slag discharge. Based on this idea, the inventor carried out operations with a constant slag formation height, but was unable to suppress the variation in the slag formation height. Therefore, the inventor conducted a detailed investigation into the relationship between the actual state of slag formation in the above-mentioned second process and the intermediate slag removal start angle in the third process (intermediate slag removal process).

As a result, it became clear that even when operation was performed with the forming height kept constant, the angle at which intermediate slag removal began varied. Furthermore, it was found that the tilt angle at the start of intermediate slag removal correlated with the Si concentration of the molten iron (before the start of molten iron pretreatment). An example of such a survey result is shown below with reference to Figure 3.

In Fig. 3, the horizontal axis represents the Si concentration (mass%) in the molten iron (before the start of molten iron pretreatment) and the vertical axis represents the intermediate slag drainage start angle (degrees). In the investigation shown in Fig. 3, the foaming height at the time when the acid blowing in the second process was stopped was controlled to be constant at a foaming height ratio of 0.6 to 0.7, and _{the Al2O3} concentration in the slag at the time when the acid blowing was stopped was in the range of 0.1 to 0.3 mass%.

As shown in Figure 3, even if the forming height ratio is controlled to be constant at 0.6 to 0.7, the intermediate slag discharge start angle fluctuates in the range of approximately 55° to 73°. In addition, when the molten iron Si concentration is plotted on the horizontal axis, it can be seen that the intermediate slag discharge start angle can be plotted with a strong correlation to the molten iron Si concentration.

For example, when the molten iron Si concentration is a low value of 0.6 mass% or less, although there is some variation in the plot, it can be seen that intermediate slag removal cannot begin unless the converter-type vessel is tilted to approximately 65° or more. It is believed that when the acid blowing is stopped, gas is released from the bubbles in the foamed slag, and the height of the foaming begins to decrease. In other words, it is presumed that when the molten iron Si concentration is a low value of 0.6 mass% or less, the foaming of the slag subsides significantly. Based on the above findings suggested by Figure 3, the inventors have come to the conclusion that the rate at which the height of the foaming decreases changes depending on the molten iron Si concentration.

When focusing on the case where the molten iron Si concentration is low (0.6 mass% or less), even if tilting of the converter-type vessel is started after the acid blowing is stopped in the second process, the rate at which the foaming height decreases is relatively fast, so slag discharge does not start until the converter-type vessel is tilted to approximately 65° or more. Furthermore, since the slag whose foaming height has decreased becomes liquefied slag (high-density slag from which gas has been removed) and is in a highly viscous state, it is not as easy to discharge as foamed slag, even if the tilting angle is increased, and it is presumed that a large amount of liquefied slag will remain in the furnace when the discharge of the molten metal begins (when intermediate slag discharge is completed).

Fluctuation in the molten iron Si concentration for each treatment charge can occur relatively frequently, and judging from past operational results, the molten iron Si concentration can vary, for example, between 0.1 and 1.2 mass% (frequently between 0.3 and 1.2 mass%). As a result, it was inferred that variation occurs in the rate of foam height reduction for each treatment charge, and as a result, the amount of slag remaining in the converter-type vessel varies for each treatment charge.

Based on the above findings, the present inventor came up with the idea of reducing the rate at which the foaming height of the slag decreases and weakening the dependency of the rate at which the foaming height decreases on the Si concentration of the molten iron. He discovered that the Al ₂ O ₃ concentration in the slag, which has not been given much attention in the past, can be considered as an operational factor that can realize such a situation.

Furthermore, as a result of further investigation based on the above findings, the inventors came up with a method that focuses on the height of slag foaming and enables stable discharge of large amounts of slag during intermediate slag discharge in the processing of multiple charges, including cases where the molten iron Si concentration is low at 0.6 mass% or less and high at over 0.6 mass%.

Based on the above findings, in the first embodiment of the present invention shown below, a refining method capable of stably discharging a large amount of slag when intermediately discharging slag when the molten iron Si concentration is low, at 0.6 mass% or less, is described. In addition, in the second embodiment of the present invention shown below, a refining method capable of stably discharging a large amount of slag when intermediately discharging slag, regardless of the molten iron Si concentration, is described.

(First embodiment)
In the first embodiment described below, a refining method in which the molten iron Si concentration (molten iron Si concentration at the start of the second step) is 0.6 mass% or less will be described in detail. The lower limit of the Si concentration of the molten iron is not particularly specified, but is usually about 0.1 mass%.

The refining method according to the first embodiment of the present invention is a refining method in a top-blowing refining process in which steel is refined using a converter-type vessel. As shown in FIG. 1, the refining method includes a first step of charging molten pig iron into a converter-type vessel, a second step of adding a lime-based flux into the converter-type vessel and blowing acid therein to de-Si and de-P the molten pig iron and form slag, a third step of performing intermediate slag dumping by tilting the converter-type vessel to dump the slag from the throat of the converter-type vessel, and a fourth step of adding flux into the converter-type vessel after the intermediate slag dumping and performing top-blowing refining.

As explained above, when the Si concentration of the molten iron (molten iron before the acid blowing treatment in the second process) was 0.6 mass% or less, the foaming height reduction rate was high, and there was a tendency for a large amount of slag to remain inside the converter-type vessel even after intermediate slag removal.

However, in the refining method according to the present embodiment, as described in detail below, _{the Al2O3} concentration and the basicity obtained by dividing the CaO concentration (unit: mass %) by the _SiO2 concentration (unit: mass %) of the slag at the end of the second step are set within predetermined ranges, and the foaming height in the second step is controlled to satisfy specific conditions, so that a large amount of slag can be stably discharged during intermediate slag discharge.

<Chemical composition of slag when acid blowing in the second step is stopped>
In the blowing method according to the present embodiment, the Al ₂ O ₃ concentration of the slag at the time when the acid blowing in the second step is stopped is set to be in the range of 0.5 mass % or more and 5.0 mass % or less, and the basicity is set to be in the range of 0.8 or more and 1.5 or less.

[Basicity]
In the second step of the refining method according to the present embodiment, foaming of the slag by blowing acid is an important factor. Here, in order to generate a desired amount of foaming of the slag with a normally applied amount of blowing acid, in the refining method according to the present embodiment, the basicity of the slag, which is the value obtained by dividing the CaO concentration (unit: mass%) by the _SiO2 concentration (unit: mass%), is set to a range of 0.8 to 1.5. If the basicity of the slag is less than 0.8, the foamed slag tends to cause slopping (the foamed slag overflowing from the converter-type vessel), which is not preferable. The basicity of the slag at the end of the second step is preferably 0.9 or more, more preferably 1.0 or more.

On the other hand, if the slag basicity exceeds 1.5, the slag will not be sufficiently slag-formed, making it difficult for foaming to occur, which is not preferred. The slag basicity is preferably 1.4 or less, and more preferably 1.3 or less.

[ _{Al2O3 concentration} ]
According to the findings of the present inventors, the foaming that occurs in the second step subsides after the acid blowing is stopped, and the foaming subsides within the time from the stop of the acid blowing to the start of intermediate slag removal (usually 0.5 to 10 minutes). During this time (approximately 10 minutes), the height of the foam decreases. If the decrease in the height of the foam is not approximately constant, as mentioned above, the intermediate slag discharge start angle will not be constant and will vary. This causes variation in the amount of slag remaining in the furnace.

According to the verification by the present inventors as mentioned above, it has been found that the rate of decrease in foam height tends to be affected by the Al ₂ O ₃ concentration of the slag, the concentration of which has not been focused on in the past.

Specifically, when the Al ₂ O ₃ concentration is less than 0.5 mass%, Al ₂ O ₃ in the slag may be unevenly distributed. As a result, depending on the position of the slag in the furnace where the foaming is performed, there are places where the foaming height reduction rate is high, while other places have a low foaming height reduction rate, which leads to variation in the foaming height reduction rate. This causes the foaming height reduction rate for each treatment charge to become inconsistent. From this perspective, the Al ₂ O ₃ concentration in the slag at the end of the second process is set to 0.5 mass% or more. The Al ₂ O ₃ concentration in the slag at the end of the second process is preferably 1.0 mass% or more. By setting the Al ₂ O ₃ concentration to 1.0 mass% or more, it is possible to more significantly suppress the variation in the foaming height reduction rate.

As a result of the verification by the inventor, it is possible to further reduce the rate of decrease in the forming height with an increase in the _{Al2O3 concentration} . Here, at an _Al2O3 concentration of about _4.0 mass%, the effect of suppressing the variation in the rate of decrease in the forming height as described above begins to saturate, and at an _Al2O3 concentration of 5.0 mass%, the increase in the effect is no longer noticeable. From this viewpoint, the _Al2O3 concentration in the slag at the end of the second step _is set to _5.0 mass% or less.

The upper limit of the Al ₂ O ₃ concentration that does not inhibit the dephosphorization action is about 6 mass %. Generally, the Al ₂ O ₃ concentration in slag is controlled by adding a slag former, but from the viewpoint of cost reduction, it is desirable that the Al ₂ O ₃ concentration is low.

As described above, by setting the Al ₂ O ₃ concentration in the slag at the end of the second process to be 0.5 mass% or more and 5.0 mass% or less, it is possible to reduce the rate of decrease in the foaming height, and it is possible to achieve the desired foaming amount with the amount of acid blowing that is normally applied.

In addition to the above components, the slag often contains iron oxide at a concentration of about 10 to 40% in terms of Fe concentration, and may contain other components as well. However, in order to obtain the effects of the refining method according to this embodiment, the above range of components is sufficient, and no specific concentrations are specified.

The contents of components such as Al ₂ O ₃ , CaO, and SiO ₂ in the slag can be determined by known measurement methods such as ICP (Inductivity Coupled Plasma) emission spectroscopy.

<Height of slag foam when acid blowing is stopped in the second step>
According to the findings of the present inventors, as explained above, the rate of decrease in the foaming height is related to the Si concentration in the molten iron in addition to the Al ₂ O ₃ concentration.

In the second step, the silicon in the molten iron is almost entirely oxidized by the acid blowing, generating _SiO2 according to the silicon concentration, which becomes part of the slag. The slag generated during acid blowing includes _SiO2 formed by oxidation of silicon, as well as iron oxide, manganese oxide, and phosphoric acid generated by the acid blowing, and the total amount of these components is about 70% of the slag after the acid blowing is stopped. The CaO source, which is a refining material, is usually determined according to the silicon concentration of the molten iron, and the amount of slag in the furnace is generally determined according to the silicon concentration of the molten iron.

Looking at Figure 3 from the above perspective, it can be seen that even if the slag foaming height is the same, the rate at which the foaming height decreases varies depending on the amount of slag in the furnace (corresponding to the horizontal axis). Note that in Figure 3, the basicity is in the range of 0.8 to 1.5, the CaO source is 100% converted to CaO, and quicklime (powdered CaO) mixed with 0 to 70 mass% converter decarburization slag is used, and no effect on the intermediate slag discharge start angle due to the origin of CaO could be confirmed.

As mentioned earlier, it has been found that when the molten iron Si concentration is 0.6 mass% or less, the rate of decrease in the forming height is so high that a large amount of slag remains in the furnace after intermediate slag removal.

Therefore, as is the focus of this embodiment, when the molten iron Si concentration is 0.6 mass% or less, a higher foaming height is achieved in advance after the acid blowing refining in the second process, assuming that the rate at which the foaming height decreases is high, and then the intermediate slag removal in the third process is performed. This allows a large amount of foaming slag to be removed from the system before it liquefies, making it possible to reduce the amount of slag remaining in the furnace.

As such, in the second step, it is important to keep track of the slug height and determine the degree of forming. Therefore, in the second step of this embodiment, the forming height is measured.

[Forming height measurement]
Here, in the second step according to the present embodiment, it is preferable to use a microwave level meter as shown in Fig. 4A and Fig. 4B in order to measure the forming height. This makes it possible to measure the forming height in a non-contact manner, and it is possible to further improve the convenience of operation.

The device configuration and usage of the microwave level meter are briefly described below. The device configuration shown in Fig. 4A and Fig. 4B is merely an example, and the microwave level meter used in this embodiment is not limited to the one shown in Fig. 4A and Fig. 4B.

The microwave level meter is a device that uses microwaves to measure the height of the slag surface inside a converter-type vessel. As shown in FIG. 4A, the microwave level meter has an antenna unit and a device body that controls the antenna unit. As shown in FIG. 4B, the antenna unit has a transmitting antenna that irradiates microwaves into the furnace and a receiving antenna that receives microwaves from inside the furnace. In addition, a lens unit is provided at the tip of each of the transmitting antenna and the receiving antenna to increase the antenna gain (increase the signal strength), and a heat insulating plate is provided on the furnace body side of the lens unit to prevent heat from inside the furnace. As shown in FIG. 4A and FIG. 4B, the antenna unit of the microwave level meter is attached to an opening for inserting a sublance, which is provided in the exhaust hood above the furnace mouth.

Using such a microwave level meter, microwaves with a frequency exceeding 10 GHz and not exceeding 90 GHz are transmitted from a transmitting antenna into the furnace of a converter-type vessel, and the microwaves reflected from the formed slag surface and received by a receiving antenna are measured, thereby determining the time from transmission to reception of the microwaves. Half of the distance traveled by the microwaves during the determined time corresponds to the distance to the slag surface. From this relationship, the height of the slag forming can be determined.

By using such a microwave level meter, the forming height can be determined quickly even when hot, which can lead to a reduction in the amount of residual slag and suppression of variation in the amount of residual slag.

[Conditions that forming height must satisfy]
Based on the above findings, the inventors conducted a detailed investigation into the correlation between the molten iron Si concentration, the slag foaming height ratio, and the amount of slag remaining in the furnace at the time of intermediate slag draining, and were able to determine the boundary line, on a coordinate plane defined by the molten iron Si concentration and the slag foaming height ratio, between a region where the amount of slag remaining in the furnace at the time of intermediate slag draining is preferable and a region where the amount of slag remaining in the furnace at the time of intermediate slag draining is unpreferable.

More specifically, operations are carried out while changing the Si concentration of the molten iron, and for each operation, the slag foaming height ratio is identified and it is determined whether the amount of slag remaining in the furnace during intermediate slag removal is in a desirable state. The results obtained are plotted on a coordinate plane defined by the molten iron Si concentration and the slag foaming height ratio, using markers corresponding to desirable states and markers corresponding to undesirable states. Using the plot obtained, a straight line representing the boundary between the two regions can be determined by the linear least squares method, and an equation representing the boundary line can be obtained.

As described above, the inventor was able to obtain the following formula (1) which represents the boundary line between the region where the amount of slag remaining in the furnace during intermediate slag draining is favorable and the region where the amount of slag remaining in the furnace during intermediate slag draining is unfavorable.

0.6-0.3 x (Si concentration of molten iron at the start of the second process (unit: mass%)) ≦ foaming height ratio at the time of stopping acid blowing ... formula (1)

In other words, in the refining method according to this embodiment, by controlling the foaming height ratio of the slag when the acid blowing is stopped in the second step so as to satisfy the above formula (1), it becomes possible to stably discharge a large amount of slag during intermediate slag discharge. Furthermore, by being able to stably discharge a large amount of slag during intermediate slag discharge, it becomes possible to suppress variation in the amount of slag remaining in the converter.

From the viewpoint of reducing the amount of slag remaining in the converter-type vessel, it is not necessary to set an upper limit for the foaming height ratio when the acid blowing is stopped. However, since the occurrence of slopping can be suppressed to enable smooth refining operations, it is preferable that the foaming height ratio when the acid blowing is stopped be 1.0 or less.

<End point temperature of molten iron in the second process>
In the second step, the end point temperature of the molten iron is preferably set within a range of 1280 to 1380°C. By setting the end point temperature of the molten iron to 1280°C or higher, it is possible to ensure that the progress levels of the desiliconization reaction and the dephosphorization reaction of the molten iron are in an appropriate state. In order to further advance these reactions, the end point temperature of the molten iron is preferably set to 1300°C or higher, and more preferably set to 1310°C or higher. On the other hand, by setting the end point temperature of the molten iron to 1380°C or lower, it is possible to advance these reactions while suppressing the decrease in the desiliconization efficiency and the dephosphorization efficiency. The end point temperature of the molten iron is preferably set to 1360°C or lower, and more preferably set to 1350°C or lower.

In the second step, as long as the slag basicity, Al ₂ O ₃ concentration, and foaming height ratio satisfy the above conditions, the oxygen supply time (acid blowing time) is not particularly specified. However, in order to more stably achieve the desired amount of slag foaming, it is preferable that the oxygen supply time (acid blowing time) is within the range of 1 to 6 minutes.

The second step in the refining method according to this embodiment has been described in detail above.

<Other processes>
In the refining method according to this embodiment, the first and third steps are not particularly limited, and each step may be carried out in the manner previously described with reference to FIG.

In the fourth step, flux is added to the converter-type vessel after intermediate slag removal in the same manner as previously described with reference to Figure 1, and top-blowing refining processes such as decarbonization and dephosphorization can be performed.

Here, the end temperature of the molten steel in the fourth step is not particularly specified and may be set appropriately depending on the target treatment, but for example, in the case of deP treatment, it is preferable to set it in the range of 1300 to 1380°C. By setting the end temperature of the molten steel to 1300°C or higher, it is possible to ensure that the progress level of the deP reaction of the molten steel is at an appropriate state. In order to further advance the deP reaction, the end temperature of the molten steel is preferably 1320°C or higher, and more preferably 1330°C or higher. On the other hand, by setting the end temperature of the molten steel to 1380°C or lower, it is possible to advance these reactions while suppressing a decrease in the deP efficiency. The end temperature of the molten steel is preferably 1360°C or lower, and more preferably 1350°C or lower.

In addition, the end point temperature of the molten steel in the fourth step is preferably in the range of 1560 to 1750°C, for example, in the case of decarbonization. By setting the end point temperature of the molten steel at 1560°C or higher, it is possible to ensure that the progress level of the decarbonization reaction of the molten steel is appropriate. In order to further advance the decarbonization reaction, the end point temperature of the molten steel is preferably 1580°C or higher, and more preferably 1600°C or higher. On the other hand, by setting the end point temperature of the molten steel at 1750°C or lower, even if work that causes a decrease in the molten steel temperature, such as an alloy addition process subsequent to the decarbonization process, is performed, the molten steel temperature can be prevented from decreasing after being tapped from the converter, and an excessive increase in the molten steel temperature can be avoided.

In the fourth step, the oxygen supply time (acid blowing time) is not particularly specified, but it is preferable that the oxygen supply time (acid blowing time) is within the range of 1 to 6 minutes for dephosphorization treatment and within the range of 7 to 25 minutes for decarbonization treatment.

When decarbonizing the molten steel as the fourth step, some or all of the molten slag generated during the decarbonization process may be left in the converter-type vessel and used as a lime-based flux in the second step of the next charge. By adopting this method of using molten slag, it is possible to reduce the flux cost in the second step.

In addition, when decarbonizing molten steel as the fourth step, some or all of the slag generated and discharged during the decarbonization process may be cooled, solidified, and recovered, and the resulting solidified slag may be used as a lime-based flux in the second step of another charge. By adopting such a method of using solidified slag, it is possible to reduce the flux cost in the second step.

The refining method according to the first embodiment of the present invention has been described in detail above.

As described above, the refining method according to the first embodiment of the present invention makes it possible to discharge a large amount of slag during intermediate slag disposal after molten iron pretreatment (de-Si treatment and de-P treatment) in a converter.

As a result, for example, if the process after intermediate slag removal (i.e., the fourth step) is a decarbonization process, it is possible to reduce the amount of rephosphorization during the decarbonization process. Also, if the process after intermediate slag removal is a dephosphorization process, since there is only a small amount of slag remaining in the furnace, it is possible to reduce the amount of quicklime introduced into the furnace in the fourth step and achieve the dephosphorization effect.

In addition, by using this refining method, it is possible to reduce the average amount of slag remaining when multiple charges are processed.

Second Embodiment
In the first embodiment described above, a refining method for eliminating a large amount of slag remaining after intermediate slag removal when the molten iron Si concentration is low, ie, 0.6 mass% or less, has been described. By using this refining method, it is possible to reduce the average amount of slag remaining when multiple charges are processed.

When multiple charge processing is considered, the amount of residual slag may vary even though it is a low value, which may lead to variations in the refining process after intermediate slag removal, such as variations in the amount of rephosphorization (in the case of decarbonization processing) and variations in the amount of dephosphorization (in the case of dephosphorization processing).

Therefore, in the second embodiment shown below, a refining method capable of suppressing the variation in the amount of residual slag between multiple charges, regardless of the molten iron Si concentration, will be described in detail. The lower limit of the Si concentration of the molten iron is not particularly specified, but may be about 0.1 mass%, and is usually about 0.3 mass%. Similarly, the upper limit of the Si concentration of the molten iron is not particularly specified, but is usually about 1.2 mass%.

The refining method according to the second embodiment of the present invention is a refining method in a top-blowing refining process in which steel is refined using a converter-type vessel. As shown in FIG. 1, this refining method includes a first step of charging molten pig iron into a converter-type vessel, a second step of adding a lime-based flux into the converter-type vessel and blowing acid therein to de-Si and de-P the molten pig iron and form slag, a third step of performing intermediate slag dumping by tilting the converter-type vessel to dump the slag from the throat of the converter-type vessel, and a fourth step of adding flux into the converter-type vessel after intermediate slag dumping to perform top-blowing refining.

<Chemical composition of slag at the end of the second step>
In the blowing method according to this embodiment, as in the first embodiment, the _Al2O3 concentration of the slag at the end of the _second step is set to a range of 0.5 mass% or more and 5.0 mass% or less, and the basicity is set to a range of 0.8 or more and 1.5 or less.

The reasons for setting the slag basicity and Al ₂ O ₃ concentration within the above ranges and the effects obtained by setting them within such ranges are similar to those in the first embodiment, so detailed explanations will be omitted below.

<Height of slag foam when acid blowing is stopped in the second step>
In the multi-charge process of this embodiment, the amount of slag remaining in the furnace varies in each charge, which causes the rate of reduction of the forming height to vary, and one of the main causes of such variation is the molten pig iron Si concentration in each charge. Therefore, by setting the forming height to a value range corresponding to the molten pig iron Si concentration, it is possible to suppress the variation in the amount of slag remaining in the furnace in each charge due to the variation in the molten pig iron Si concentration.

More specifically, in the refining method according to this embodiment, when the molten iron Si concentration is high and the rate at which the foaming height decreases is slow, the foaming height is set relatively low and the starting position of the foaming height decrease is set low. On the other hand, when the molten iron Si concentration is low and the rate at which the foaming height decreases is fast, the foaming height is set relatively high and the starting position of the foaming height decrease is set high, as in the first embodiment. This makes it possible to suppress variation in the amount of slag remaining in the furnace at the end of intermediate slag removal.

Based on this policy, the inventors conducted research to identify an equation that represents the boundary between the region in which the amount of slag remaining in the furnace during intermediate slag removal is favorable and the region in which the amount of slag remaining in the furnace during intermediate slag removal is unfavorable, as in the first embodiment described above. As a result, the equation that represents the boundary was identified, and the condition expressed by the following equation (2) was obtained.

0.6-0.3 x (Si concentration of molten iron at the start of the second process (unit: mass%)) ≦ foaming height ratio at the time of stopping acid blowing ≦ 0.8-0.3 x (Si concentration of molten iron at the start of the second process (unit: mass%)) ... formula (2)

The above formula (2) specifies the upper and lower limits of the foaming height ratio. For example, when the molten iron Si concentration is low, a higher foaming height ratio range is set according to the rate of decrease of the foaming height, which indicates a fast value. On the other hand, when the molten iron Si concentration is high, a lower foaming height ratio range is set according to the rate of decrease of the foaming height, which indicates a slow value. In the refining method according to this embodiment, including the effect of slowing down the rate of decrease of the foaming height due to the chemical composition of the slag, even if there is variation in the molten iron Si concentration, the variation in the foaming state (foaming ratio) of the slag in the furnace at the end of intermediate slag removal is largely suppressed. As a result, the refining method according to this embodiment makes it possible to suppress the variation in the amount of slag remaining in the furnace.

In other words, in the refining method according to this embodiment, by controlling the foaming height ratio of the slag when the acid blowing is stopped in the second step so as to satisfy the above formula (2), even if the molten iron Si concentration fluctuates, the variation in the amount of slag remaining in the furnace is reduced, and it becomes possible to stably discharge a large amount of slag when the slag is intermediately drained. Furthermore, by being able to stably discharge a large amount of slag during intermediate draining, it becomes possible to suppress the variation in the amount of slag remaining in the converter. In addition, by controlling so as to satisfy the above formula (2), it becomes possible to suppress the variation in the amount of slag remaining between multiple charges.

In the refining method according to this embodiment, it is preferable to use a microwave level meter to measure the forming height, as in the first embodiment. The device configuration and method of use of the microwave level meter, as well as the effects obtained thereby, are the same as in the first embodiment, so detailed explanations will be omitted below.

<Other conditions in the second step>
In the second step, other conditions such as the end temperature of the molten iron and the acid blowing time are also similar to those in the first embodiment, and therefore detailed description thereof will be omitted below.

The second step in the refining method according to this embodiment has been described in detail above.

<Other processes>
In the refining method according to this embodiment, the first and third steps are not particularly limited, and each step may be carried out in the manner previously described with reference to FIG.

In the fourth step, flux is added to the converter-type vessel after intermediate slag removal in the same manner as previously described with reference to Figure 1, and top-blowing refining processes such as decarbonization and dephosphorization can be performed.

Here, other conditions in the fourth step, such as the end temperature of the molten steel and the acid blowing time, are the same as those in the first embodiment, so detailed explanations will be omitted below.

In addition, when decarbonizing molten steel as the fourth step, some or all of the molten slag generated in the decarbonizing process may be left in the converter-type vessel and used as a lime-based flux in the second step of the next charge, or some or all of the slag generated and discharged in the decarbonizing process may be cooled, solidified, and recovered, and the resulting solidified slag may be used as a lime-based flux in the second step of another charge. By adopting such a method of using slag, it is possible to reduce the flux cost in the second step.

The refining method according to the second embodiment of the present invention has been described in detail above.

As described above, the refining method according to the second embodiment of the present invention makes it possible to discharge a large amount of slag when intermediately draining the slag generated after the molten iron preliminary treatment (de-Si treatment and de-P treatment) in a converter-type vessel, regardless of the level of the Si concentration of the molten iron in the furnace, and also makes it possible to suppress variation in the amount of slag remaining in the furnace after intermediate draining.

As a result, for example, when the process after intermediate slag removal (i.e., the fourth step) is a de-C process, it becomes possible to suppress the variation in the amount of re-phosphorization during the de-C process for each charge. Also, when the process after intermediate slag removal is a de-P process, it becomes possible to stabilize the phosphorus concentration in the molten metal after the de-P process, making it possible to stably produce low-phosphorus steel with a P content of, for example, 100 ppm or less.

The refining method according to the present invention will be specifically described below with reference to examples. Note that the example shown below is merely one example of the refining method according to the present invention, and the refining method according to the present invention is not limited to the following example.

As an example of a converter-type vessel, a top-and-bottom blown converter with a capacity of 350 tons was used. Multiple double-tube tuyeres were installed at the bottom of this converter. Molten pig iron was charged into the converter in an amount ranging from 300 to 330 tons (first step). The composition of the molten pig iron was as shown in Table 1 below, with the remainder being Fe and impurities.

After the hot metal was charged, the top blowing lance arranged above the furnace throat was lowered to blow oxygen-containing gas such as oxygen gas or air into the hot metal. At the same time, as auxiliary materials, iron oxide sources such as sintered ore, iron ore, and dust, limestone and quicklime as refining agents, lime raw materials such as slag discharged and recovered in the decarbonization process, Al ₂ O ₃ raw materials such as agglomerate slag and Al ₂ O ₃ powder, and dolomite as a refractory protection material were added from the top of the converter. At this time, in order to increase the reaction rate, oxygen and air as oxidizing gases were blown from the inner tube at the bottom of the converter, and LPG, nitrogen, and CO ₂ as cooling gases were blown from the outer tube, and the hot metal and molten steel in the converter were constantly stirred.

As a result, in the de-Si/P treatment, which is an example of the second step, the de-Si reaction proceeded in the furnace by blowing in oxygen, forming _SiO2 , which further reacted with the added CaO-based solvent agent to form slag. In addition, toward the end of the de-Si reaction, the de-P reaction started. At this time, a portion of the blown oxygen reacted with C in the molten iron to generate CO and _CO2 bubbles, and these bubbles caused the foaming of the slag to proceed at the end of the de-Si/P treatment.

The slag composition in the second step was adjusted to have a basicity between 0.8 and 1.5 and _{an Al2O3} concentration of 0.5% or more and 5% or less by adjusting the amount of added _Al2O3 sources such as lime raw material and agglomerate slag. The specific _values shown in Tables 1 and 2 below were measured by ICP atomic emission spectrometry.

In the second step, the height of the slag foaming was constantly measured using a microwave level meter installed in the sublance hole located at the top of the converter. When the state of the slag foaming satisfied the above formula (1) (in the case of Table 1 below) or formula (2) (in the case of Table 2 below), blowing was stopped and the top blowing lance was raised. After that, intermediate slag draining was performed by tilting the converter to discharge the slag from the throat (third step). The slag was discharged into a slag pot installed below the converter. At this time, the angle at which the slag started to flow out after tilting the converter was recorded as the intermediate slag draining start angle. In addition, the furnace body was returned to the vertical position just before the molten pig iron in the furnace flowed out, and intermediate slag draining was completed.

After intermediate slag removal was completed, the top blowing lance was lowered again to blow in oxygen-containing gas, and at the same time, quicklime and limestone, which are refining agents, were added to increase the slag basicity to 3.0 or higher, and decarbonization was carried out (fourth step). After decarbonization, the converter was tilted and the molten steel was removed from the converter's tapping hole into a ladle, completing the refining process.

<Method for measuring the amount of slag remaining in the furnace>
The amount of slag in the furnace was calculated based on the amount of flux added, the Si concentration of the molten iron, and the slag composition. The amount of slag discharged was obtained using a weighing scale installed in the slag ladle, and the amount of slag remaining in the furnace was calculated by calculating the difference between the calculated total slag amount and the value obtained from the weighing scale.

<Evaluation criteria in Table 1, which is a test example corresponding to the first embodiment>
In Table 1, which is a test example corresponding to the first embodiment, a rating of "A" was given when the amount of slag remaining in the furnace was less than 26 kg/t, and a rating of "B" was given when the amount of slag remaining in the furnace was 26 kg/t or more.

The reason why the threshold value of 26 kg/t was adopted as the judgment standard is as follows. In this invention, the acid blowing is performed in a state where blast furnace slag may be mixed in and where the Si in the molten iron is oxidized, so there is a tendency for the amount of slag before intermediate slag removal to be large, and the amount of slag remaining in the furnace to be large. Therefore, a slightly larger value of 26 kg/t was adopted as the threshold value.

<Evaluation criteria in Table 2, which is a test example corresponding to the second embodiment>
In Table 2, which is a test example corresponding to the second embodiment, 20 charges of operation were carried out within a specific range of hot metal composition, the amount of residual slag was measured, and the minimum and maximum values of the amount of residual slag were evaluated. The difference between the maximum and minimum values was then taken as a representative value of the variation. When the obtained variation was 10 kg/t or less, a rating of "A" was given, and when the variation exceeded 10 kg/t, a rating of "B" was given.

The results obtained are summarized in Tables 1 and 2 below.
In Table 2 below, the notation "xy" indicates that the value in the corresponding column was within the range of "x or more and y or less."

As is clear from Table 1 above, for Nos. 1 to 7, which are within the scope of the present invention, the amount of slag remaining in the furnace was less than 26 kg/t.

In addition, when the Al ₂ O ₃ concentration of the slag composition similar to that of No. 5 (wherein the Al ₂ O ₃ concentration was 5.0 mass% and the residual slag was 20 kg/t) was increased to 6.0 mass%, the residual slag became 20 kg/t, similar to No. 5. As is clear from these results, the effect of increasing the Al ₂ O ₃ concentration was not obtained.

On the other hand, in Nos. 8 to 10, where acid blowing was stopped when the foaming height ratio was below the lower limit of formula (1), the foaming height decreased significantly between the end of acid blowing and the start of intermediate slag removal, and an excessive amount of slag remained in the furnace.

In No. 11, where the basicity was below the lower limit of 0.8, excessive slopping occurred during blowing, and the planned blowing could not be carried out. In No. 12, where the basicity exceeded the upper limit of 1.5, the slag could not be foamed due to poor slag formation of the added flux, and the slag could not be removed even when intermediate slag removal was performed.

No. 13, which has an alumina concentration below the lower limit of 0.5% by mass, had a variation in the rate at which foaming decreased in the furnace, with many areas where the rate of foaming decreased quickly, resulting in a decrease in the amount of intermediate slag discharged and an increase in the amount of slag remaining in the furnace.

Furthermore, when operations were carried out within the ranges shown in Table 2 above, regardless of the Si concentration range, as long as the slag composition and forming height ratio were within the ranges specified in the second embodiment, the difference between the maximum and minimum values was 10 kg/t or less, and the variation in the amount of slag remaining in the furnace after intermediate slag removal could be suppressed.

On the other hand, if the operation was stopped when the forming height ratio exceeded the upper limit value of formula (2), excessive slag was discharged during intermediate slag discharge, and the difference between the maximum and minimum amounts of slag remaining in the furnace exceeded 10 kg/t, resulting in large variations.

In addition, in both Tables 1 and 2, the amount of rephosphorization and the amount of quicklime added during decarburization were able to obtain the effects of the present invention according to the amount of residual slag and the degree of variation in the amount of residual slag.

Although the preferred embodiment of the present invention has been described in detail above with reference to the accompanying drawings, the present invention is not limited to such an example. It is clear that a person having ordinary knowledge in the technical field to which the present invention pertains can conceive of various modified or altered examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally belong to the technical scope of the present invention.

Claims (5)

A refining method in a top blowing refining process for refining steel using a converter-type vessel, comprising the steps of:
A first step of charging molten iron into the converter-type vessel;
a second step of adding a lime-based flux into the converter-type vessel and blowing acid therein to desiliconize and dephosphorize the molten iron and form slag;
A third step of performing intermediate slag draining by tilting the converter-type vessel to drain the slag from a throat of the converter-type vessel;
A fourth step of adding flux into the converter-type vessel after intermediate slag removal and performing top blowing refining;
Including,
The Si concentration of the molten iron at the start of the second step is 0.6 mass% or less,
With respect to the slag at the time of stopping the acid blowing in the second step, the Al ₂ O ₃ concentration is 0.5 mass% or more and 5.0 mass% or less, and the basicity obtained by dividing the CaO concentration (unit: mass%) by the SiO ₂ concentration (unit: mass%) is 0.8 or more and 1.5 or less;
In the second step, a height of the foaming is measured, and the height of the foaming when the acid blowing is stopped before the intermediate slag removal satisfies the following formula (1):

0.6-0.3 x (Si concentration of molten iron at the start of the second process (unit: mass%)) ≦ foaming height ratio at the time of stopping acid blowing ... formula (1)

Here, in the above formula (1), the foaming height ratio is the ratio of the measured foaming height to the height of the in-furnace freeboard in the converter-type vessel.
A refining method in a top blowing refining process for refining steel using a converter-type vessel, comprising the steps of:
A first step of charging molten iron into the converter-type vessel;
a second step of adding a lime-based flux into the converter-type vessel and blowing acid therein to desiliconize and dephosphorize the molten iron and form slag;
A third step of performing intermediate slag draining by tilting the converter- type vessel to drain the slag from a throat of the converter-type vessel;
A fourth step of adding flux into the converter-type vessel after intermediate slag removal and performing top blowing refining;
Including,
With respect to the slag at the time of stopping the acid blowing in the second step, the Al ₂ O ₃ concentration is 0.5 mass% or more and 5.0 mass% or less, and the basicity obtained by dividing the CaO concentration (unit: mass%) by the SiO ₂ concentration (unit: mass%) is 0.8 or more and 1.5 or less;
In the second step, a height of the foaming is measured, and the height of the foaming when the acid blowing is stopped before the intermediate slag removal satisfies the following formula (2):

0.6-0.3 x (Si concentration of molten iron at the start of the second process (unit: mass%)) ≦ foaming height ratio at the time of stopping acid blowing ≦ 0.8-0.3 x (Si concentration of molten iron at the start of the second process (unit: mass%)) ... formula (2)

Here, in the above formula (2), the foaming height ratio is the ratio of the measured foaming height to the height of the in-furnace freeboard in the converter-type vessel.
The refining method according to claim 1 or 2, in which a microwave level meter is used to measure the height of the forming in the second step. The fourth step is a step of performing decarburization refining,
The refining method according to any one of claims 1 to 3, wherein the molten slag generated after the decarburization refining is partially left in the furnace, and the left molten slag is used as a part or the whole of the lime-based flux in the second step of the next charge. The fourth step is a step of performing decarburization refining,
The refining method according to any one of claims 1 to 3, wherein the solidified slag obtained by cooling and solidifying the molten slag generated after the decarburization refining is used as a part or all of the lime-based flux in the second step of another charge.

Images