JP2017137532A - Low-phosphorus and low-sulfur steel manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a low-phosphorus and low-sulfur steel while recycling steel materials, the method being able to reduce a risk for a change of steel grades without increasing a capital investment and process time and without performing desulfurization in a secondary refining step.SOLUTION: This manufacturing method includes: a desulfurization step of desulfurizing a hot metal; a dephosphorization step of dephosphorizing the hot metal, which has undergone the desulfurization step, in a dephosphorization furnace; a decarburization step of decarburizing the hot metal, which has undergone the dephosphorization step, in a decarburization furnace; and a S analyzing step of analyzing a sulfur content [S] in the hot metal which has undergone the dephosphorization step and has not undergone the decarburization step. The dephosphorization step includes a step of charging steel materials having a large diameter in an amount of which [S] in the hot metal after dephosphorization does not exceed a predetermined control value, into the dephosphorization furnace by using a scrap chute. The decarburization step includes a step of loading the steel materials having a small diameter in an amount of which [S] in a molten steel after decarburization does not exceed a control value, to the decarburization furnace from a hopper above a furnace according to a value [S] acquired in the S analyzing step.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing low phosphorus, low sulfur (low P, low S) steel, and in particular, low phosphorus low sulfur steel using two converters and recycling metal having a high S (sulfur) content. It relates to a method of manufacturing.

  There is an increasing need for steel products having excellent toughness, and the production of low-phosphorus low-sulfur steels that meet such demands, that is, steels with reduced P content and S content are increasing. Typical compositions of low phosphorus low sulfur steel are [P] ≦ 0.015% and [S] ≦ 0.0030%. Here, when the chemical component is X, [X] represents the X content (mass%) of hot metal, molten steel, or steel.

  In producing such low-phosphorus low-sulfur steel, the amount of dephosphorization slag generated when dephosphorization (de-P) is performed and the amount of desulfurization slag generated when desulfurization (de-S) is increased. Yes. These slags contain bullion, and it is an issue to recycle such bullion, especially bullion with high S content contained in desulfurized slag.

As a typical method for producing low-phosphorus low-sulfur steel, there is a method of treating hot metal obtained in a blast furnace by the following steps (1) to (5).
(1) A step (2) of obtaining a low sulfur hot metal having a [S] of about 0.0010% by mechanically stirring the hot metal and flux discharged from the blast furnace by the KR method to desulfurize the hot metal. In the dephosphorization furnace, scrap is added as a raw material to the low-sulfur hot metal obtained in the step (1) above, and further, flux is added and oxygen is blown to desiliconize (desiliconize) and dephosphorize. (3) As a primary refining process, a flux is added to the low phosphorus low sulfur hot metal obtained in the above step (2) and blown with oxygen as primary refining. Step (4) for obtaining low phosphorus low sulfur molten steel with reduced C content by decarburization Low secondary phosphorous low sulfur molten steel obtained in the step (3) by secondary refining equipment as secondary refining In contrast, the process of vacuum degassing (for example, by the RH method) and fine adjustment of the components (5) In Zoki, the molten steel obtained in the step (4), coagulation and molding to step

  A converter is used as the dephosphorization furnace in the step (2) and the decarburization furnace in the step (3). In the converter, the scrap is loaded into the scrap chute and then charged into the converter by an overhead traveling crane. The flux is transported to a storage hopper (hereinafter referred to as “furnace hopper”), which is an auxiliary material charging facility disposed in the upper part of the converter, stored in the furnace hopper, and a predetermined amount is cut out when necessary.

  In the method for producing low-phosphorus low-sulfur steel described above, desulfurization slag is generated along with the reaction of desulfurizing the hot metal to the flux when performing desulfurization of the hot metal in the step (1). Since the desulfurized slag is separated from the hot metal and floats on the hot metal, it can be removed by mechanical operation. However, at that time, the desulfurized slag and hot metal cannot be completely separated, and a part of the hot metal is mixed into the desulfurized slag as a metal. Such bullion is preferably recycled.

  As a method for recycling the bullion, the desulfurized slag is cooled and then crushed by a crusher, and is sorted into slag and bullion using a magnetic separator, and the bullion (hereinafter referred to as “rabbit”). There is a method of charging the steel together with scrap into a dephosphorization furnace (converter). In this case, the selected slag is recycled as a raw material for the sintered ore.

  [S] of crude iron is equivalent to [S] of hot metal after desulfurization (low-sulfur hot metal), so if slag is not included, addition of rough rice will be performed regardless of the amount of hot metal used. [S] is not affected. However, in practice, it is unavoidable that desulfurization slag is mixed into rough slag, and desulfurization slag contains S at a higher content than rough slag. For this reason, when rough iron is charged into a dephosphorization furnace, the hot metal [S] may increase, and a low-phosphorus low-sulfur steel may not be obtained.

  When the hot metal [S] rises in the dephosphorization furnace, for example, it is necessary to additionally perform a desulfurization process in the secondary refining process to reduce the [S] of the molten steel to a desired level. In this case, the manufacturing cost increases. Further, when the treatment for reducing the [S] of the hot metal or molten steel is not performed after the dephosphorization, it is inevitably used (steel type transfer) as a steel type that is not a low-phosphorus low-sulfur steel. Since the unit price of steel, which is a product, decreases due to the transfer of the steel grade, economic loss occurs.

[S] increase rate A of hot metal due to the use of rough slag is expressed by the following formula (1) using the rough slag usage rate P, the desulfurization slag content Y of rough slag, and the S content (S) of desulfurization slag. Can be calculated. Here, when the chemical component is X, (X) represents the X content (mass%) of slag.
A (%) = P (%) × (Y (%) / 100) × ((S) / 100) (1)

  The usage rate P (%) of the rough slag is determined by the amount of the rough slag used (the amount of rough slag charged into the dephosphorization furnace; t / ch) and the amount of hot metal contained in the dephosphorization furnace (t / ch). Dividing by the total amount of scrap (t / ch) charged into the phosphorus furnace and multiplying by 100. Here, “ch” means charging (charging) of a raw material for one treatment in a furnace. Desulfurization slag content Y is obtained by dividing the amount (t) of desulfurization slag contained in rough slag by the amount (t) of rough slag (including desulfurization slag contained in rough slag).

  Usually, the generation amount of rough slag is about 0.5% with respect to the raw steel production of low phosphorus low-sulfur steel, and the desulfurization slag content Y of rough slag is about 10-20%. (S) is about 1.5 to 2.5%. Therefore, the rate of increase A of the molten steel [S] when the entire amount of the rough is used is 0.0015% on average, and is in the range of 0.0008 to 0.0025%.

  Since the desulfurization slag content Y of rough sand and the S content (S) of desulfurization slag vary widely, it is difficult to reliably obtain samples having average Y and (S). In addition, since the amount of rough slag is large, so that an analytical value can be obtained that can be regarded as representative of the total amount of rough slag, a large amount of rough slag has a desulfurized slag content Y and a desulfurized slag S content ( It is practically impossible to analyze S). Therefore, it is difficult to grasp the desulfurization slag content Y of the rough debris and the S content (S) of the desulfurization slag with high accuracy before using the rough debris.

  When [S] after desulfurization of hot metal is 0.0010%, if [S] increase rate A of hot metal due to use of hot metal is 0.0015%, [S] of hot metal becomes 0.0025%, which is low. There is no problem in producing phosphorous low sulfur steel. However, when the hot metal [S] increase rate A due to the use of hot metal is 0.0025%, the hot metal [S] is 0.0035%. In this case, as described above, there is a problem that cost and process time increase due to additional desulfurization treatment, or economic loss due to transfer of the production steel type occurs.

  As a method for adding rough slag, a method of adding rough slag to the hot metal before desulfurization is known. For example, Patent Document 1 discloses a method of adding roughing to hot metal before desulfurization in a chaotic car (torpedo car). According to this method, since desulfurization is performed on hot metal after the introduction of crude iron, it is possible to substantially eliminate the rise in [S] of the hot metal.

  In addition, as a method for adding scrap, not only charging in a dephosphorization furnace but also a method of charging divided into a dephosphorization furnace and a decarburization furnace is known. For example, Patent Document 2 discloses a method of charging scrap using a scrap chute in both the dephosphorization furnace and the decarburization furnace, instead of charging the scrap only in the dephosphorization furnace. Yes.

  Using this technique, as with scrap, roughing is divided and charged into both the dephosphorization furnace and the decarburization furnace, and after analyzing [S] of the hot metal after dephosphorization, Depending on the analysis result, it is conceivable to use the scrap chute to charge the remaining rough slag in the decarburization furnace. Thereby, the risk that [S] of molten steel exceeds a control value can be significantly reduced.

Patent No. 4438562 Japanese Patent No. 5164312

  However, in order to carry out the method of Patent Document 1, it is necessary to provide roughing equipment and the like.

  In addition, regarding the method of Patent Document 2, after the dephosphorization, [S] of the hot metal is analyzed, the amount of roughing in the scrap chute is adjusted, and when the roughing is charged into the converter, the raw material to the converter The charging is delayed and the process time is significantly increased.

  Specifically, although it depends on the factory layout and operating conditions, it generally takes about 20 minutes to pour molten iron into the decarburization furnace after the molten iron is removed. In this case, after analyzing [S] of the dephosphorized hot metal (pig iron), the time required for adjusting the amount of rough in the scrap chute and charging the rough into the decarburization furnace is 20 If it is within minutes, the process time will not increase. However, in practice, this time exceeds 20 minutes, so the process time increases.

  Therefore, the present invention is a method for producing low phosphorus low-sulfur steel while recycling rough waste, without increasing capital investment and process time, and without desulfurization in the secondary refining process. It aims at providing the manufacturing method which can reduce the risk of transfer.

The gist of the present invention is the following method for producing low-phosphorus low-sulfur steel.
Classification process of classifying rough slag with a sieving machine and sorting it into small-diameter rough that can be fed from the furnace hopper and large-diameter rough including larger diameter than the small-diameter rough When,
A desulfurization process for desulfurizing the hot metal,
A dephosphorization step of dephosphorizing the hot metal after the desulfurization step in a dephosphorization furnace;
A decarburization step of decarburizing the hot metal after the dephosphorization step in a decarburization furnace;
An S analysis step of analyzing the sulfur content [S] of the hot metal after passing through the dephosphorization step and before performing the decarburization step,
In the dephosphorization step, the large-diameter roughing is charged into the dephosphorization furnace using a scrap chute in an amount that does not exceed a predetermined control value of the hot metal [S] after dephosphorization. Including steps,
In the decarburization step, depending on the value of [S] obtained in the S analysis step, [S] of the molten steel after decarburization is an amount in a range that does not exceed the control value, and the small diameter rough rust is removed. , Including a step of feeding the decarburization furnace from the furnace hopper,
Manufacturing method of low phosphorus low sulfur steel.

  The production method of the present invention includes a desulfurization step and a dephosphorization step. In the dephosphorization step and the decarburization step, rough iron is charged (introduced) in such an amount that [S] of the hot metal or molten steel that has passed through these steps does not exceed a predetermined control value. For this reason, since the molten steel obtained through the decarburization process tends to be sufficiently reduced in P and S, it is possible to produce a low phosphorus low-sulfurized steel without desulfurization in the secondary refining process High nature. Therefore, the risk of having to transfer the steel type can be reduced.

  In addition, in the decarburization process, roughing (small-diameter roughing) can be put into a decarburization furnace using a hopper, so it is not necessary to make capital investment such as providing a scrap chute used for the decarburization furnace. Absent.

  Furthermore, when charging roughing with a scrap chute in a decarburization furnace, it is necessary to complete the charging of roughing before starting the hot metal charging. When a hopper is used, it is possible to cut out a desired amount of rough from the furnace hopper during oxygen blowing and put it into the decarburization furnace. Thereby, after the analysis result of hot metal [S] after the dephosphorization step is obtained, the cycle time in the case of charging roughing into the decarburization furnace is compared with the case of not charging roughing in the decarburization furnace. Does not increase substantially.

FIG. 1 is a graph showing the relationship between the rate of use of crude iron in a dephosphorization furnace and the rate of increase in [S] of hot metal after dephosphorization.

  Embodiments of the present invention will be described below. In the following description, “%” for the content is mass%.

  The manufacturing method of this embodiment includes a classification step, a desulfurization step, a dephosphorization step, a decarburization step, and an S analysis step. A desulfurization process and a dephosphorization process are implemented as a hot metal pretreatment process. A decarburization process is implemented as a primary smelting process. After the decarburization step, a secondary refining step and a continuous casting step are sequentially performed.

<Classification process>
In this process, rough slag is classified with a sieving machine to make a small-diameter rough that can be fed from the furnace hopper and a large-diameter rough that includes a rough larger than this small-diameter rough. Sort out. Aragon is slag (desulfurization slag) generated in the desulfurization process, cooled, crushed by a crusher, and selected from the slag using a magnetic separator.

  The size of the rough shore is about 700 mm in maximum diameter, although it depends on the desulfurization slag crushing method and the magnetic separation method. The scrap chute can be charged with a raw material having a maximum diameter of about 1000 mm, whereas the furnace hopper can be charged only with a raw material having a maximum diameter of about 50 to 100 mm. Therefore, the size of the small-diameter rough is adjusted to about 40 to 60 mm by classification according to the specifications of each facility such as the furnace hopper of the converter.

<Desulfurization process>
In this step, the hot metal desulfurization is performed by mechanically stirring the hot metal and flux discharged from the blast furnace by the KR method, for example. Thereby, a low sulfur hot metal having [S] of about 0.0010% (about 0.0004 to 0.0012%) is obtained.

<Dephosphorization process>
In this process, scrap is added as a raw material to the low-sulfur hot metal after the desulfurization process in a dephosphorization furnace, and further, flux is added and oxygen is blown to perform dephosphorization, thereby reducing the low-phosphorus low-sulfur hot metal. Get. In this step, hot metal desiliconization may be performed simultaneously. The dephosphorization furnace shall have a scrap chute.

  The dephosphorization step includes a step of charging the large-diameter rough obtained in the classification step into a dephosphorization furnace using a scrap chute. The amount of large-diameter rough soot to be charged is set so that the [S] of the hot metal after dephosphorization does not exceed the control value even if variation is taken into consideration. The management value can be 0.0030%, for example.

<S analysis process>
In this step, the hot metal after the dephosphorization step and before the decarburization step is sampled, and [S] is analyzed by, for example, an infrared absorption method. The analysis result need not be known before the hot metal after the dephosphorization process is transferred from the dephosphorization furnace to the decarburization furnace, but at the latest, immediately after the start of oxygen blowing (blowing) in the decarburization process described later. Need to be known.

<Decarburization process>
In this process, as the primary refining, decarburization is performed by adding oxygen to the hot metal (low phosphorus low-sulfur hot metal) that has undergone the dephosphorization process in a decarburization furnace and blowing oxygen, and the C content is reduced. A phosphorous low sulfur molten steel is obtained. The decarburization furnace is assumed to have a furnace hopper. In the furnace hopper, the small-diameter rough obtained in the classification process is stored.

  This step includes a step of feeding the small-diameter rough raft obtained in the classification step into the decarburization furnace from the furnace hopper. Small diameter roughing may be introduced from above the hot metal while oxygen is blown. The amount of small-diameter rough to be introduced is set in a range in which the [S] of the molten steel after decarburization does not exceed the control value according to the value of [S] obtained in the S analysis step, and the input amount is 0 In some cases (when rough is not thrown in).

<Secondary refining process>
In this step, vacuum degassing (for example, by a known RH method) and fine adjustment of components are performed on the low phosphorus low-sulfur molten steel that has undergone the decarburization step in the secondary refining apparatus.

<Continuous casting process>
In this process, the molten steel that has undergone the secondary refining process is solidified and molded by a continuous casting machine equipped with a mold and a roll.

  The effect of the present invention was confirmed by conducting a test for producing low-phosphorus low-sulfur steel as a slab under the following production conditions A to I. The purpose of the test was to produce a low-phosphorus low-sulfur steel with [P] of 0.015% or less and [S] of 0.0030% or less. That is, these content rates are control values (standard upper limit values) of slabs obtained by manufacturing tests.

The heat of production conditions A to I is roughly divided into groups A to C, D to F, and G to I as described below.
Heat of production conditions A to C: The dephosphorization furnace is charged with the entire amount of rough slag using a scrap chute (in the decarburization furnace, the rough slag is not charged).
Heat of production conditions D to F: In each of the dephosphorization furnace and the decarburization furnace, rough slag is charged using a scrap chute. The amount of crude iron charged into the decarburization furnace is determined by the result of [S] analysis of the hot metal after dephosphorization. [S] Depending on the result of the analysis, no debris is charged into the decarburization furnace.
Heat of production conditions G to I: In a dephosphorization furnace, rough slag is charged using a scrap chute, and in a decarburization furnace, rough slag is charged using a furnace hopper. The amount of crude iron charged into the decarburization furnace is determined by the result of [S] analysis of the hot metal after dephosphorization. [S] Depending on the result of the analysis, no debris is charged into the decarburization furnace.

  The manufacturing method of the low phosphorus low sulfur steel according to the manufacturing conditions G to I is an example of the present invention, and the manufacturing method of the low phosphorus low sulfur steel according to the manufacturing methods A to F does not satisfy at least one of the requirements of the present invention. It is an example.

  The slag content of rough slag is the same in manufacturing conditions A, D, and G, the same in manufacturing conditions B, E, and H, and the same in manufacturing conditions C, F, and I. It was. The group of production conditions A, D, and G, the group of production conditions B, E, and H, and the group of production conditions C, F, and I were different from each other in the slag content.

A common procedure in this manufacturing test will be described.
First, 270 t of hot metal discharged from the blast furnace was subjected to desulfurization treatment by the KR method as a desulfurization step, and a low sulfur hot metal having [S] of 0.0010% was obtained. Subsequently, as a dephosphorization process, a dephosphorization process was performed in a dephosphorization furnace using the low-sulfur hot metal obtained in the desulfurization process and about 30 t of scrap as raw materials.

  A part of the obtained dephosphorized hot metal is sampled, and [S] of the pig iron solidified by the sampled dephosphorized hot metal is analyzed. For other dephosphorized hot metal, decarburization treatment is performed in a decarburizing furnace. Carried out. For the obtained molten steel, as a secondary refining process, vacuum degassing by the RH method and fine adjustment of components by adding a predetermined amount of a predetermined element, and then as a continuous casting process, a continuous casting machine The slab slab was obtained by solidification and molding.

  Arsenal was prepared by the following method before the desulfurization step. That is, the desulfurization slag generated by the desulfurization process performed separately was discharged to a slag tank, carried out of the factory, water-cooled for more than 24 hours, and then the slag tank was inverted and discharged. Then, the desulfurized slag was subjected to crushing and magnetic separation, so that it was separated into a magnetized product (rough) and a non-magnetized product (slag) of 15 mm or less.

  In the manufacturing conditions A to C, rough rust which is this magnetic deposit was used. In manufacturing conditions D to I, the following treatment was further used for roughing. That is, the rough crushed was passed through a sieve having a mesh opening of 50 mm, and classified into a small sized rough with a diameter of 50 mm or less and a large sized rough with a rough larger than 50 mm. The large-diameter rough was intended for charging into the dephosphorization furnace via a scrap chute. The small-diameter rough was for charging from the furnace hopper or scrap chute to the converter, which is a decarburization furnace.

  Table 1 shows manufacturing conditions and results.

  When the production conditions A to C, that is, the rough groin was thrown into the dephosphorization furnace only by the scrap chute, the following results were obtained. Although the production conditions A and B were both the same as the ratio of use of rough slag (the ratio of the amount of slag to be added to the amount of hot metal) of 0.5%, [S] after steel output was The production condition A was 0.0025%, and the production condition B was 0.0034%. Such a difference in [S] is due to the difference in the slag content of the used rough slag.

  In production condition B, [S] after steelmaking exceeded the control value, so desulfurization in secondary refining was necessary. In other words, the steel obtained under the production condition B has to be changed in steel type unless desulfurization is performed separately from desulfurization as a hot metal pretreatment such as desulfurization in the secondary smelting process. .

  Considering the result of manufacturing condition B, in order to prevent [S] after steelmaking from exceeding the control value, in manufacturing condition C, the rough crushed usage rate is 0.3%, and manufacturing conditions A and B Compared to less. As a result, [S] after steelmaking under the production condition C was as low as 0.0015%, and it was not possible to increase the use rate of rough slag. The average rough usage rate under production conditions A to C was 0.43%.

  In the production conditions D to F, rough debris is introduced using a scrap chute in a dephosphorization furnace, and in accordance with the result, the amount of rough debris introduced from the scrap chute of the decarburization furnace is determined in advance. Rough usage rate in the dephosphorization furnace was calculated so that [S] after phosphorus does not exceed 0.0030%.

  FIG. 1 is a diagram showing the relationship between the usage rate of rough iron and the [S] increase rate of hot metal after dephosphorization. As a result of performing the test several times for the same rough bar use rate, the [S] increase rate of the molten steel varied, so FIG. 1 shows the [S] increase rate of the molten steel for the same rough bar use rate. The maximum value and minimum value (corresponding to the upper and lower ends of the vertical line segment, respectively) and the average value (black circle) are shown. [S] The variation in the rate of increase is due to the difference in the slag content of the rough used.

  Table 2 shows the relationship between the roughage usage rate and the maximum value of the [S] increase rate of the molten steel. In calculating the rough usage rate in the dephosphorization furnace under the manufacturing conditions D to F, the relationship shown in Table 2 was used, and the rough usage rate was set to 0.4%.

  [S] of the obtained dephosphorization was analyzed, and according to the result, the amount of roughing to be introduced using a scrap chute in a decarburization furnace was determined.

  In production condition D, the hot metal [S] after dephosphorization was 0.0022%, which was 8 ppm lower than the upper limit of [S] of the product. 1%. In production condition E, [S] of the hot metal after dephosphorization was 0.0029%, which was only 1 ppm lower than the standard upper limit value of the product [S], so no debris was used in the decarburization furnace. . In production condition F, the hot metal [S] after dephosphorization was 0.0016%, which was 14 ppm lower than the upper limit of the [S] specification of the product. %. In any of the production conditions D to F, [S] after steelmaking was 0.0030% or less, so desulfurization was not performed in the secondary refining.

  Under the production conditions D to F, the usage rate of rough rust was 0.5% on average, which was larger than the production conditions A to C. In addition, [S] of the dephosphorizing hot metal was found before the start of blowing in the decarburization furnace, but in any of the production conditions D to F, it took time to load the roughing into the decarburization furnace, The cycle time in the decarburization furnace increased to 32 minutes compared with the case where rough groin was not charged (24 minutes). Here, with respect to the decarburization furnace, the cycle time is the time (minutes) from the start of charging the raw material to the decarburization furnace (converter) until the end of the discharge, and a maximum of 60 ch / day in the case of a 24-minute cycle. However, if it is a 32-minute cycle, the maximum is 45 ch / day.

  In the production conditions G to I, the roughing is introduced using a scrap chute in the dephosphorization furnace, and according to the result, the production conditions are determined in determining the amount of the roughing from the furnace hopper of the decarburization furnace. As with D to F, the rough usage rate in the dephosphorization furnace was calculated in advance so that [S] after dephosphorization did not exceed 0.0030%. The use rate of rough slag in the dephosphorization furnace was 0.4%. [S] of the dephosphorized hot metal was found before the start of blowing in the decarburization furnace.

  In production condition G, [S] of the hot metal after dephosphorization was 0.0022%, which was 8 ppm lower than the standard upper limit value of [S] of the product. 1%. In the production condition H, [S] of the hot metal after dephosphorization was 0.0029%, which was only 1 ppm lower than the standard upper limit value of the product [S], so no debris was used in the decarburization furnace. . In the production condition I, the hot metal [S] after dephosphorization was 0.0016%, which was 14 ppm lower than the upper limit of the [S] standard of the product. %. In any of the production conditions G to I, since [S] after steelmaking was 0.0030% or less, desulfurization was not performed in the secondary refining.

  Moreover, the cycle time of the decarburization furnace did not increase. The average usage rate of rough sand was 0.5%, and a recycling amount commensurate with the amount of rough sand generated was achieved.

Claims (1)

Classification process of classifying rough slag with a sieving machine and sorting it into small-diameter rough that can be fed from the furnace hopper and large-diameter rough including larger diameter than the small-diameter rough When,
A desulfurization process for desulfurizing the hot metal,
A dephosphorization step of dephosphorizing the hot metal after the desulfurization step in a dephosphorization furnace;
A decarburization step of decarburizing the hot metal after the dephosphorization step in a decarburization furnace;
An S analysis step of analyzing the sulfur content [S] of the hot metal after passing through the dephosphorization step and before performing the decarburization step,
In the dephosphorization step, the large-diameter roughing is charged into the dephosphorization furnace using a scrap chute in an amount that does not exceed a predetermined control value of the hot metal [S] after dephosphorization. Including steps,
In the decarburization step, depending on the value of [S] obtained in the S analysis step, [S] of the molten steel after decarburization is an amount in a range that does not exceed the control value, and the small diameter rough rust is removed. , Including a step of feeding the decarburization furnace from the furnace hopper,
Manufacturing method of low phosphorus low sulfur steel.

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