JPH0377246B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a method of highly efficient dephosphorization while minimizing the amount of quicklime used throughout the entire steelmaking process, and producing high-quality steel at a low cost. <Prior art and its problems> In recent years, quality requirements for various steel materials have become more sophisticated day by day, and with this, various innovations have been attempted in steel manufacturing methods and various new methods have been introduced. Under these circumstances, there have recently been great expectations for the development of a means to stably melt low-phosphorus steel at even lower costs, and much research is being carried out toward its realization. By the way, regarding the minimization of the total cost of steel manufacturing and the stable production of low phosphorous steel, the following preliminary dephosphorization method of hot metal has been proposed, and some of it has been put into practical use. Namely, () a method of performing preliminary dephosphorization by injecting a quicklime-based dephosphorizing agent or soda ash into hot metal in a torpedo car; () a method of injecting quicklime-based cracks into hot metal in a ladle or blasting (blasting); () A method of performing preliminary dephosphorization by blasting hot metal with quicklime-based cracks in a blast furnace casthouse trough. However, although relatively low P content levels can be achieved by the methods () and () above, dephosphorization is a "reaction that proceeds during the floating process of the dephosphorizing agent (transitary reactor reaction)". However, in the method (2) above, there is a problem that the utilization efficiency of the dephosphorizing flux is not necessarily good, and the longer the treatment time, the more heat is removed during the treatment, which lowers the hot metal temperature. The hot metal temperature after treatment can be kept higher than the previous two methods, but since the dephosphorization treatment is applied to the hot metal immediately after being tapped from the blast furnace, the dephosphorization treatment temperature is high and the P content level reached is low. There is a disadvantage that the method itself is worse than the above-mentioned method () or (2), and neither method is satisfactory. Furthermore, when using quicklime itself as flux for hot metal dephosphorization, considering the amount of quicklime used in the subsequent converter blowing (decarburization refining), it is difficult to use either of the above methods. Also “a method of omitting the preliminary dephosphorization step and performing dephosphorization only in a converter”
It was also pointed out that the required amount of slag forming agent (amount of quicklime, etc.) was not significantly reduced compared to the above. With these technical considerations in mind, we aim to create a method for mass-producing high-quality steel that can minimize the amount of slag-forming agent used, which greatly affects steelmaking costs, and does not require any new equipment. The results of the basic study conducted by the present inventors regarding the "required amount of slag-forming agents such as quicklime, which play an important role in the melting of high-quality steel with low phosphorus content," are based on the following: The amount of slag-forming agent required can be minimized when using "slag-metal countercurrent refining," which brings slag and metal into countercurrent contact, but in reality, it is almost impossible to fully realize countercurrent refining. Currently, the most labor-intensive steelmaking method that has the potential to reduce the amount of slag used is to divide the dephosphorization process into two stages and transfer the slag generated in the lower process to the upper process. A method of using converter slag as a dephosphorizing agent for hot metal dephosphorization (that is, a method of using converter slag as the main component of flux for hot metal dephosphorization, for example, a method of using converter slag as a dephosphorizing agent for hot metal dephosphorization; This led to a strong recognition that this is a method of reusing furnace slag as hot metal dephosphorization flux in ex-furnace refining. However, the steelmaking methods that have been proposed so far by reusing converter slag sometimes involve out-of-furnace refining, which makes it extremely difficult to stably maintain efficient working conditions. Efficiency is not as high as expected, and mass production requires a special exhaust gas dust collector and dephosphorization slag removal equipment, making it difficult to use as a method for mass production of high-quality steel. It was all I could do. Therefore, the inventors of the present invention fully understood the advantages of the steel manufacturing method by reusing converter slag, and decided to develop a total This is what we found when searching for a means to efficiently and stably implement the above-mentioned "steel manufacturing method including a two-stage dephosphorization process" that uses a small amount of slag-forming agent.(a) In the dephosphorization process of hot metal, From the viewpoint of efficiency, it is better to keep the processing temperature as low as possible, but if the temperature is too low, it will cause problems in the next process, and there will be problems such as increased loss of granulated iron to the slag after processing. The temperature is
The best temperature is around 1300-1350℃. but,
In actual work, the addition of the dephosphorizing agent itself is a major factor in lowering the processing temperature, so it is extremely difficult to maintain the above-mentioned somewhat low temperature. Therefore, the processing temperature can be maintained stably and easily. (b) In order to fully utilize the dephosphorizing ability of the flux and increase the dephosphorization efficiency, it is necessary to adjust the treatment temperature as described above, as well as to lack sufficient stirring to achieve a dephosphorization equilibrium state. I can't do it, but
In order to achieve sufficient and efficient stirring in a short time for highly efficient dephosphorization of high-temperature hot metal,
Gas agitation using gas blown from the bottom of the processing container is most preferred. (c) In addition, in order to perform efficient dephosphorization, the processing vessel must have sufficient free board (distance from the hot water surface to the top of the vessel) for slag forming.
is necessary. (d) In order to reduce erosion of the refractories of the processing vessel due to slag and increase the efficiency of dephosphorization work, it is preferable to use a basic lining. (e) In order to increase the efficiency of dephosphorization in a steel manufacturing process that includes a two-stage dephosphorization process, the efficiency of removing slag from the processing container cannot be ignored, and it is essential to use a processing container that allows easy removal of slag. thing. (f) In order to mass-produce high-quality steel with good workability, sufficient exhaust gas treatment equipment (dust collector) is necessary. (g) Considering these conditions, a converter-type furnace is ideal as a hot metal dephosphorization treatment vessel, especially a combined blowing converter with a top and bottom blowing function that allows stirring gas to be introduced from the bottom of the furnace. When this is used to carry out the above-mentioned "steel manufacturing method including two-stage dephosphorization process", sufficiently efficient dephosphorization can be achieved even if the amount of slag forming agent used throughout the gold steel manufacturing process is extremely small. Being able to mass-produce high-quality steel with high efficiency. Based on the fact that
The other side is used as a decarburization furnace, and a refining agent mainly composed of converter slag generated in the decarburization furnace is added to the hot metal injected into the dephosphorization furnace, and bottom-blown gas is stirred by a stirring gas injection nozzle. After dephosphorizing the hot metal while keeping the temperature of the hot metal in the dephosphorization furnace below 1400℃ by blowing oxygen gas upward from a lance, the obtained dephosphorized hot metal is decarburized and finish dephosphorized in a decarburization furnace. He previously proposed a steelmaking method that allows the production of steel at a normal phosphorus level or low-phosphorus steel with good workability and at low cost even with an extremely small amount of slag-forming agent (patent application 1983- No. 132517). The method previously proposed by the present inventors focuses on reducing the amount of flux such as quicklime used as much as possible, and it is certainly not an extremely effective method in that respect. It was hot. However, as a result of continued studies by the present inventors assuming various conditions through actual operation, it was found that ``Although the previously proposed method has a remarkable effect of reducing flux consumption, this method does not work well in decarburization furnaces. As it is assumed that final dephosphorization will be performed, there is a possibility that P will be off at the end of the decarburization furnace, and steel cannot be tapped until the amount of P is confirmed.It is impossible to completely avoid factors that inhibit high-efficiency steelmaking. ” It became clear. <Means for Solving the Problems> From the above-mentioned viewpoint, the present inventors proposed the method of decarburizing while reusing converter slag using two converters.・Steel manufacturing method that involves dephosphorization
We aim to omit the "confirmation of P amount at the end point of decarburization" required for decarburization, and to produce high-quality steel at a lower cost and with higher productivity. We conducted further research with the conviction that in order to be able to tap steel as it is, it is important to reduce the molten metal [P] in the dephosphorization furnace to a level close to the product [P] level.'' The following new knowledge was obtained. In other words, (a) Reducing the hot metal [P] to near the product [P] level in the dephosphorization furnace means that there is no need for substantial deP in the decarburization furnace; I would say that slagless refining is fine,
A sufficient amount of slag is necessary in the decarburization furnace to reduce the problems of humour loss and spitting loss. (b) However, in small quantities for the purpose of preventing humus loss and spitting loss, the continuous furnace slag (which essentially contains only a very small amount of P) from the decarburization furnace is returned to the dephosphorization furnace. Even if it is used as the main component of the dephosphorization flux, continuous furnace slag alone will not be sufficient as a CaO source, and [P] in the dephosphorization furnace will never drop to near the product [P] level. (c) Therefore, a method of using quicklime itself instead of the converter slag in the dephosphorization furnace comes to mind. 50 mm) as the main component of the dephosphorizing agent, it is not possible to stably obtain a high slag formation rate. (d) Of course, it is possible to inject a powdered quicklime-based dephosphorizing agent, but in this case, a special lance is required for blowing, and complicated control means are required, so this is not a good idea. That cannot be said. (e) However, if a small amount of the converter slag, which has a low melting point of approximately 1400°C, generated in the decarburization furnace is returned to the dephosphorization furnace, and quicklime is added to compensate for the insufficient CaO, As long as the amount of quicklime is adjusted, the mixed quicklime will be sufficiently turned into slag along with the converter slag with a low melting point, and no particular inconvenience will occur (in this case, the converter slag will be converted into slag of quicklime). It is assumed that it acts as an accelerator. This invention has been made based on the above-mentioned findings, and as shown in FIG.
This is a steelmaking method in which hot metal is refined using the other side as a decarburization furnace 2, in which converter slag 4 generated in the decarburization furnace 2 and quicklime are added to the hot metal 3 injected into the dephosphorization furnace 1 as a refining agent 4'. The hot metal is dephosphorized while maintaining the temperature of the hot metal 3 in the dephosphorization furnace 1 at 1450°C or less by blowing oxygen gas upward from the lance 6 while performing bottom-blown gas agitation using the stirring gas blowing nozzle 5. By carrying out the first step of decarburizing the dephosphorized hot metal, and the second step of pouring the obtained dephosphorized hot metal into the decarburization furnace 2 and decarburizing it by adding a normal slag-forming agent, an extremely small amount of slag-forming agent can be produced. Moreover, the present invention is characterized in that steel with a normal phosphorus level or low phosphorus steel can be produced with good workability and at a low cost without requiring the work of confirming the amount of [P] at the end point of decarburization. Here, the reason why the treatment temperature in the dephosphorization furnace is adjusted to 1450℃ or less is that if the hot metal treatment temperature is higher than this, decarburization will proceed and the total amount of Fe in the slag will decrease, resulting in a worsening of the dephosphorization rate. Because it does. However, if the temperature is too low, the loss of granulated iron to the slag will increase, so the treatment temperature is preferably 1200-1450℃.
It is best to adjust the temperature to 1250-1370℃. The treatment temperature is maintained by a combination of blowing oxygen gas from the top blowing lance or blowing oxygen gas from the bottom tuyere. In other words, the oxygen gas injection in the dephosphorization furnace is performed to ensure the dephosphorization treatment temperature. Therefore, the top blowing oxygen lance here may be a normal converter lance, but it may also be a new small flow rate lance for dephosphorization. The amount of oxygen gas used is determined by the temperature and silicon content of the hot metal before treatment, the temperature of the converter slag, the warmth of the dephosphorization furnace, the target temperature of the hot metal to be treated, etc., but is approximately 20Nm <sup>3</sup> /t. Below is fine, usually 5~
10Nm <sup>3</sup> /t is effective. Incidentally, the amount of decarburization at this time is 1% or less, about 0.5%. ``Converter type furnace with both upper and lower blowing functions''
The currently used "top-bottom blowing combined blowing converter" is the most preferable, but especially for dephosphorization furnaces,
Since the refining conditions are milder than in a decarburization furnace, the furnace itself can be made even smaller, so even if a new one is built specifically for dephosphorization, there will not be much of an impact on the cost. Refining agent used in dephosphorization furnace (dephosphorization flux)
The main components are converter slag and quicklime generated in a decarburizing furnace, but in order to obtain better results, it is better to adjust the ratio of quicklime added to the converter slag. This is because, although the slag-forming properties of quicklime can be improved if it is added together with the converter slag, if the proportion of quicklime is too high, there is still a concern that sufficient slag-formation will not be achieved at relatively low temperatures. Therefore, in this case, if the ratio of quicklime added is adjusted so that the value of F = converter slag amount / converter slag amount + quicklime amount x 100 is kept at 40% or more, in other words, the converter added to the dephosphorization furnace is If the amount of furnace slag is 40% or more of [the amount of converter slag + the amount of quicklime], the mixed quicklime will also become slag as well as the converter slag with a low melting point, which may cause special inconvenience. is preferable because it completely eliminates In addition, Figure 2 is a graph showing the relationship between [the ratio of converter slag to the total amount of converter slag and quicklime used] and [slag conversion rate], and this graph also shows that the amount of converter slag is It is clear that a sufficient slag formation rate can be ensured if it is 40% or more. By the way, as a refining agent used in a dephosphorization furnace, it is preferable to mix iron oxide and fluorite as basic subcomponents in addition to the above-mentioned converter slag and quicklime. For example, a recommended blending ratio is converter slag + quicklime: 40 to 60% by weight, iron oxide: 10 to 60% by weight, and fluorite: 5 to 25% by weight. Of course, it is not limited to this, but in order to turn converter slag and quicklime into slag to make dephosphorization slag with a low melting point, and to increase the oxidizing power of slag so that dephosphorization can proceed easily, it is possible to use iron oxide in combination. is extremely heavy. Also,
Fluorite is commonly used as a solvent, but CaCl,
NaO.SiO ₂ , Na ₂ CO ₃ and the like may be used alone or in combination with fluorite. The smaller the particle size of these dephosphorization flux raw materials other than the converter slag is, the more preferable it is from the viewpoint of slag formation, but there is no problem as long as it is of a generally used size. Refining agent used in dephosphorization furnace (dephosphorization flux)
The amount of P is determined by the [P] level of the steel to be melted, but it is usually around 50 kg/t. However, in the case of making low-phosphorus steel, the hot metal [P] in the dephosphorization furnace must also be reduced to, for example, 0.010% or less, and the CaO content is 15Kg/t or more (preferably 20Kg/t or less).
Kg/t or more) is required. However, even in this case, as will be described later, it is necessary to add more slag forming agent than necessary to the decarburization furnace so that the F value in the above formula becomes 40% or more. Now, as the converter slag, which is the main component of the refining agent used in the dephosphorization furnace, the molten slag generated in the decarburization furnace is preferable from the viewpoint of thermoeconomics and the ability of the dephosphorization flux to form slag. (If the molten material is used in this way, it is poured into the dephosphorization furnace through a pot lined with refractory material.) Considering ease of handling, etc., the material obtained in the decarburization furnace is used. It may be used after being cooled and solidified and crushed into granules or chunks (in addition,
Also at this time, from a thermal standpoint, the higher the slag temperature, the better.) However, in this case, the smaller the particle size is, the better in order to improve the ability to form slag in the dephosphorization furnace, but converter slag is inherently highly slag-forming, so the particle size is less than 50 mm. However, it does not cause any particular delay in slag formation, and it can be used even if it is larger than this. It should be noted that the timing of the converter slag to be used is preferably that of the previous charge, but it goes without saying that it may also be that which came out of the decarburizing furnace before that or that which was generated in the decarburizing furnace of another factory. Stirring gases blown in from the bottom of the furnace include Ar, CO ₂ ,
It may be any of CO, N ₂ , O ₂ , air, etc. The degree of agitation of the bottom gas in the dephosphorization furnace is the same as in normal double blowing combined blowing (0.03 to 0.2N
m ³ /t), but it is of course possible to increase the amount even more with the aim of improving the dephosphorization rate. When dephosphorization is performed under the above conditions, usually
The desired dephosphorization can be completed within 20 minutes, but it is also possible to complete the desired dephosphorization in about 10 minutes. Now, in order to dephosphorize to the product [P] level in a dephosphorization furnace, the CaO content must be 10 per ton of hot metal.
Kg, preferably 15 Kg. However, in this case, there is almost no need for dephosphorization in the decarburization furnace, so it is virtually unnecessary to add quicklime or dolomite for the purpose of dephosphorization. The amount of converter slag generated will disappear. Therefore, under such operations, it is necessary to use only quicklime-based dephosphorization flux in the dephosphorization furnace, but quicklime, which has a particle size of usually 30 to 50 mm, can be heated up to 10% by stirring at the bottom of the furnace and by blowing a small amount of oxygen over the top.
It is not possible to stably turn it into slag within a short processing time of ~15 minutes, making it unviable as a steelmaking system. However, in a decarburizing furnace, it is necessary to add sludge-forming agents such as quicklime or dolomite, which have virtually no relation to dephosphorization, in order to reduce the occurrence of humus loss and spitting. Of course, phosphorus progresses). but,
For slag forming agents used for this purpose, for example, if the amount of added quicklime or dolomite is 3 kg per ton of hot metal, the amount of converter slag generated will be approximately 6 kg (CaO in converter slag is usually approximately 50%), return this to the dephosphorization furnace and use the quicklime as a dephosphorizing agent by combining 12 kg/t of oxidation heat and fluorite. F = Converter slag amount / converter slag amount + quicklime amount x The value of 100 becomes 33%, and slag formation does not progress sufficiently. Therefore, in the present invention, it is important to add more slag-forming agents such as quicklime and dolomite than necessary in the decarburization furnace to produce more converter slag. For example, if the amount of slag forming agent is set to 4 kg/t or more so that the amount of converter slag generated in the decarburization furnace is 8 kg/t or more, the amount of added quicklime in the dephosphorization furnace can be, for example, 11 kg or less. Therefore, when the F value is 42% or more, the slag formation of the dephosphorizing agent is sufficiently improved. As described above, in the method of this invention, it may be necessary to add more slag forming agent than necessary to the decarburization furnace, but the converter slag at this time can be effectively used again as a dephosphorizing agent in the dephosphorizing furnace. Therefore, if you look at it as a whole, it is not wasted. Blowing in a decarburizing furnace is basically the same as blowing ordinary hot metal that has been dephosphorized outside the furnace. In addition to the slag-forming agent mainly composed of dolomite and fluorite, manganese ore and ferromanganese ore can also be added. (It is also good to add carbon-containing substances such as coke if necessary). In this way, MnO or MnO ₂ is reduced with [C] or carbonaceous material, [Mn] increases, and manganese alloy iron can be saved. In this case, since the converter slag containing a large amount of unreduced (MnO) is used again as a dephosphorizing agent in the dephosphorization furnace, there is also the advantage that [Mn] loss during dephosphorization can be reduced. It goes without saying that by returning the converter slag generated in the decarburization furnace to the dephosphorization furnace, valuable components such as FeO and granulated iron in the converter slag can be effectively utilized. That's true. By the way, when implementing the steel manufacturing method according to the present invention, it is preferable to perform a preliminary desulfurization treatment on the applied hot metal if possible. The reason for this is that in this steelmaking method, desulfurization progresses well when the slag basicity (CaO/SiO ₂ ) is 2.5 or higher, but when the basicity is lower than this, desulfurization progresses extremely slowly. . On the other hand, when hot metal that has not been desulfurized in advance is used and desulfurization does not proceed in the dephosphorization furnace, the S content in the converter slag increases, and the S content of the molten iron in the next charge increases. This is because there is also concern that it may increase. Note that the preliminary desulfurization may be performed by any of the commonly used hot metal desulfurization methods.
However, if hot metal desulfurization equipment is not available, after dephosphorization is performed in a dephosphorization furnace, 3 to 10 kg/t of Na ₂ CO ₃ is added to the furnace while leaving the dephosphorized slag in the furnace. is possible. At this time
As a method of adding Na ₂ CO ₃ , a simple overhead injection method may be used, but it is preferable to inject Na ₂ CO ₃ powder into the hot metal after dephosphorization through a bottom stirring gas nozzle on an Ar gas or N ₂ gas carrier. . Furthermore, the lower the Si content of the raw material hot metal used in this method, the better. This is because as the Si content in the hot metal increases, the basicity of the slag in the dephosphorization furnace decreases, the dephosphorization ability decreases, and the total amount of quicklime etc. used increases. It is a good idea to adjust the Si content of the hot metal to 0.3% or less, preferably 0.2% or less. However, even if [Si] is high, there is basically no problem as long as the amount of quicklime etc. used in the entire system is increased. Next, for the sake of explanation, the main effects obtained by this invention will be listed separately. <Effects of the invention> Since the converter slag is reused as a hot metal dephosphorization flux, steel can be manufactured using a smaller amount of slag forming agent, which is close to the amount used in the decarburization furnace, compared to the conventional method in which the converter slag is discarded. can be done. FeO in the converter slag is effectively used, improving the recovery rate of granulated iron and metal. Generally, when manganese ore or ferromanganese ore is used in a decarburization furnace, about half of these ores are Mn.
However, in the method of this invention, the slag is reused as hot metal dephosphorization flux, so the residual ore is effectively utilized, and the "[Mn] loss" in the hot metal is reduced. It is useful for “reducing ” or “increasing [Mn]”. Since the furnace used is a converter-type furnace, for example, even in the case of a dephosphorization furnace, only the dephosphorized pig iron can be tapped from the tapping port into the ladle, and then the slag in the furnace can be discharged to the boiling pot. Slag removal is easier than in the dephosphorization method. Since the furnace used is a converter-type furnace with both upper and lower blowing functions, the hot metal can be strongly stirred and the treatment can be carried out in a short period of time, resulting in less heat loss and less heat compared to other dephosphorization treatment methods. It is extremely advantageous economically.
In particular, when melting converter slag is used, its sensible heat content makes it more thermoeconomically advantageous. The slag generated in the dephosphorization furnace used in the method of this invention has a P ₂ O ₅ content of 4 to 10%, so it can be used as fertilizer, and since there is no free lime, it can be used as a roadbed material. It is also possible to make effective use of Since two furnaces are used, there is no concern about poor dephosphorization due to P ₂ O ₅ adhering to the furnace body.
In other words, high P ₂ O ₅ slag is deposited in the dephosphorization furnace, and only low P ₂ O ₅ slag is deposited in the decarburization furnace, so that dephosphorization failure or P recovery in the decarburization furnace does not occur. Moreover, when using melted converter slag,
Since the molten converter slag is added to the dephosphorization furnace after charging the hot metal, there is no need to worry about sudden explosive reactions. Since dephosphorization is performed while stirring the bottom-blown gas,
Unlike the conventional hot metal dephosphorization method, there is no need to finely grind the dephosphorizing agent to near powder form, making it possible to reduce costs accordingly. If there is an idle converter, it can be used immediately as a dephosphorization furnace, and there is no need to prepare special equipment. For example, in a factory that operates 1/2 converter units, one furnace can be used as a dephosphorization furnace, and 2/2 converter units can be operated.
This invention can be implemented without any capital investment in the form of a base operation. If it becomes necessary to construct one of the bricks to extend its lifespan,
During this period, conventional converter blowing can be carried out using only one converter to avoid idle furnaces, allowing for highly flexible refining. By the way, depending on the factory, two cups of iron may be poured due to crane capacity, but in this case, in order to simplify processing, most of the hot metal is processed in the dephosphorization furnace, and additional iron is poured in the decarburization furnace. It is a good idea to Next, the present invention will be specifically explained using examples. <Examples> Example 1 First, 160 tons of hot metal having the composition shown in the upper row of Table 1, which had been desiliconized in a blast furnace cast bed and desulfurized in a torpedo, was used as a dephosphorization furnace in a double blowing combined blowing furnace. The iron is poured into a converter, and the converter slag generated in a similar type of decarburization furnace is cooled and solidified and crushed to a particle size of 10 mm or less, 15 kg/t, quicklime with a particle size of 30 to 50 mm. 8Kg/t, iron ore with particle size similar to converter slag
Dephosphorization was carried out for 13 minutes by adding 25 kg/t of fluorite and 5 kg/t of fluorite in a mixed state. The dephosphorization furnace and decarburization furnace used are both capable of stirring by blowing gas from the bottom of the furnace, as mentioned above.
This was a 160 ton upper and lower double blowing combined blowing converter, and the operating conditions shown in Table 2 were adopted in all of the following examples. The dephosphorized pig iron thus obtained (component composition is shown in the middle row of Table 1) is tapped into a ladle and then poured into a decarburizing furnace.
Decarburization blowing was carried out using dolomite 3 kg/t and fluorite 2 kg/t as slag forming agents. At this time, iron ore was added as a coolant at appropriate times so that the end point temperature (blowing end temperature) was 1635°C. The converter slag generated at this time was 15 kg/t.

【table】

[Table] This was added as a dephosphorizing agent together with iron ore and fluorite to the next charge dephosphorizing furnace together with quicklime, iron ore, and fluorite, and a series of operations were repeated to perform dephosphorization. In this case, the F value calculated by the above formula is 65%, and the slag agent of the dephosphorization agent is as high as 92%, and as a result, the [P] after dephosphorization is also 0.015%, which is the product [P] level. We were able to reduce it to below. At this time, the amount used in the entire steelmaking process [quicklime amount + dolomite amount] was 15 kg/t, which was 1/3 to 1/2 of the conventional one-shot blowing in a converter furnace. The values shown in the lower row of Table 1 indicate the composition of the molten steel after decarburization. Example 2 160 tons of hot metal having the composition shown in the upper row of Table 3, which had been desiliconized in a blast furnace casthouse, was poured into an upper and lower double blowing combined blowing converter used as a dephosphorization furnace, and on top of this, After pouring 16 kg/t of molten converter slag generated in a similar type of decarburization furnace into a pot lined with refractories, the particle size is further increased to 30 to 50 mm.
of quicklime 9Kg/t, iron ore 26Kg/t and fluorite 5
Kg/t was added and dephosphorization was carried out for 13 minutes under the conditions shown in Table 2 in the same manner as in Example 1. Thereafter, 5 kg/t of Na ₂ CO ₃ was injected into the dephosphorized hot metal through a furnace bottom stirring gas nozzle for desulfurization treatment. Next, the obtained dephosphorized pig iron (component composition is shown in the middle row of Table 3) is tapped into a ladle and then poured into a decarburizing furnace.
Kg/t, light calcined dolomite 3Kg/t, fluorite 2
kg/t and 2 kg/t of silica as a slag forming agent, and decarburization blowing was carried out. And at this time, iron-
Manganese ore (total Fe: 22%, total Mn: 42%) 8
Kg/t was added, and iron ore as a coolant was added at appropriate times so that the end point temperature (blowing end temperature) was 1630°C. The molten converter slag generated at this time was 16 kg/t, and this was added to the dephosphorization furnace together with quicklime, iron ore, and fluorite as a dephosphorizing agent for the next charge to perform dephosphorization. repeated. The F value at this time was 64%, the dephosphorization agent slag formation rate was as high as 97%, and the [P] after dephosphorization was also 0.011.

[Table] % and Product [P] It was possible to reduce the amount to below the level. Of course, the confirmation work for [P] was not carried out at the time of steel extraction in the decarburization furnace. In addition, [amount of quicklime + amount of dolomite] was substantially used in the entire steelmaking process.
[Mn] was confirmed to be as low as 16 kg/t, and it was also possible to increase [Mn] by 0.20%. Furthermore, as can be seen from Table 3, since the molten converter slag was used in the dephosphorization furnace, the temperature after the hot metal dephosphorization treatment was also slightly more thermally advantageous than in Example 1. Note that the values shown in the lower row of Table 3 indicate the composition of molten steel after decarburization treatment. Example 3 A top and bottom double blowing combined blowing converter using 160 tons of hot metal having the composition shown in the upper row of Table 4, which was desiliconized in the blast furnace casthouse trough and then desulfurized in the torpedo, as a dephosphorization furnace. In addition to this, converter slag generated in a similar type of decarburization furnace was cooled and solidified and crushed into particles with a particle size of 10 mm or less at 17 kg/t, a particle size of 30 to 50 mm.
12 kg/t of quicklime, 30 kg/t of iron ore having the same particle size as the converter slag, and 10 kg/t of fluorite were added in a mixed state under the conditions shown in Table 2 in the same manner as in Example 1. 16
Dephosphorization was performed for a minute. Next, the obtained dephosphorized pig iron (component composition is shown in the middle row of Table 4) was poured into a decarburizing furnace, and 7.5 kg/t of quicklime, 1 kg/t of light calcined dolomite, and Decarburization was carried out using 2 kg/t of fluorite and 2 kg/t of silica as slag forming agents. At this time, scale as a coolant was added at appropriate times so that the end point temperature (blowing end temperature) was 1670°C. The converter slag generated at this time was 17 kg/t, and this was added to the dephosphorization furnace as a dephosphorizing agent for the next charge, along with quicklime, iron ore, and fluorite, and a series of operations were repeated to perform dephosphorization. Ta. The F value at this time was 58%, the dephosphorizing agent had a high slag formation rate of 93%, and the [P] after the dephosphorization treatment was also low at 0.005%. Note that the values shown in the lower row of Table 4 indicate the composition of molten steel after decarburization treatment. As explained above, according to the present invention, the amount of slag forming agent required throughout the entire steelmaking process can be kept relatively low, and furthermore, there is no need for "confirmation of the amount of P at the end point of decarburization", and good quality can be achieved. This makes it possible to manufacture high-quality steel with high productivity and at low cost, paving the way to further expand the fields of use of high-quality steel, and bringing about extremely useful effects industrially.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing an overview of the steel manufacturing method according to the present invention, and Fig. 2 shows [ratio of converter slag to the total amount of converter slag and quicklime used] and [slag conversion rate] It is a graph showing the relationship between In the drawings, 1... dephosphorization furnace, 2... decarburization furnace,
3... Hot metal, 4... Converter slag, 4'... Dephosphorization slag whose main components are converter slag and quicklime, 5... Stirring gas blowing nozzle, 6... Lance.

Claims (1)

[Scope of Claims] 1. A steelmaking method in which hot metal is refined by using one of two converter-type furnaces having upper and lower blowing functions as a dephosphorization furnace and the other as a decarburization furnace, the method comprising: Converter slag and quicklime generated in the decarburization furnace are added to the hot metal injected into the phosphor furnace as main components of the refining agent, and oxygen gas is blown upward while stirring the bottom blowing gas to lower the temperature of the hot metal.
It consists of a first step of dephosphorizing hot metal while keeping it at 1450℃ or less, and a second step of pouring the obtained dephosphorized hot metal into a decarburization furnace and adding a normal slag-forming agent to decarburize it. A steel manufacturing method characterized by the following. 2. Claim 1, in which the amount of converter slag added to the dephosphorization furnace is 40% or more of the total amount of converter slag and quicklime.
Steel manufacturing method described in section. 3. The steelmaking method according to claim 1 or 2, wherein the converter slag generated in the decarburization furnace is added in a molten state to the hot metal in the dephosphorization furnace. 4. The steelmaking method according to claim 1 or 2, wherein the converter slag generated in the decarburization furnace is once cooled and solidified and then added to the hot metal in the dephosphorization furnace. 5. The steelmaking method according to any one of claims 1 to 4, wherein the hot metal to be treated is subjected to a preliminary desiliconization treatment to reduce Si to 0.30% by weight or less.