JPH0437133B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> This invention relates to a method for producing high-quality steel at a low cost by performing highly efficient dephosphorization while minimizing iron loss and minimizing the amount of quicklime used throughout the entire steelmaking process. be. <Prior art and its problems> In recent years, quality requirements for various steel materials have become more sophisticated day by day, and along 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. In other words, preliminary dephosphorization is carried out by injecting quicklime-based dephosphorizing agent or soda ash into the hot metal in the torpedo car, and by injecting or blasting (spraying) quicklime-based flux into the hot metal in the ladle. A method of performing preliminary dephosphorization, a method of performing preliminary dephosphorization by blast staining hot metal with quicklime-based flux in a blast furnace casthouse trough. However, although it is possible to achieve a relatively low P content level using the methods described in () and () above, dephosphorization is limited to "reactions that proceed during the floating process of the dephosphorizing agent (transitary reactor reaction)". However, the method described above has the problem that the utilization efficiency of dephosphorization flux is not necessarily good, and the longer the treatment time, the more heat is removed during treatment, which lowers the hot metal temperature. The hot metal temperature can be kept higher than the previous two methods, but since the dephosphorization treatment is performed on the hot metal immediately after being tapped from the blast furnace, the dephosphorization treatment temperature is too high and the P content level reached is the same. (mentioned above)
Both methods were unsatisfactory because they were worse than the methods (and). Furthermore, when using quicklime etc. as flux for hot metal dephosphorization, considering the amount of quicklime etc. used in the subsequent converter blowing, neither of the methods described above It was also pointed out that the required amount of slag-forming agent (the amount of quicklime, etc.) was not significantly reduced compared to the method of dephosphorizing using only a converter, omitting the process. Therefore, we have developed a method that can minimize the amount of slag-forming agent used, which greatly affects steelmaking costs, and that can efficiently produce high-quality steel without requiring any new equipment. The applicant strongly recognized the need for ``slag-metal countercurrent refining,'' in which slag and metal come into contact with each other in a countercurrent manner, and the amount of slag forming agent required throughout the entire steelmaking process is minimal. However, in reality, it is almost impossible to fully realize countercurrent refining, and the only steelmaking method that has the potential to reduce the amount of slag-forming agent with the least effort is currently A method in which the phosphorization process is divided into two stages and the slag generated in the lower stage is used as a dephosphorizing agent in the upper stage (i.e., a method in which converter slag is used as the main component of the flux for hot metal dephosphorization, for example, This is the basic idea of ``method of reusing converter slag as hot metal dephosphorization flux in outside-furnace refining,'' which the present applicant previously proposed in Japanese Patent Publication No. 55-30042. Based on the research results, he added, ``The steelmaking methods that have been proposed so far by reusing converter slag also involve out-of-furnace refining, making it extremely difficult to stably secure efficient working conditions.'' Moreover, the dephosphorization efficiency is not as high as expected, and mass production requires a special exhaust gas dust collector and dephosphorization slag removal equipment, making it a step forward as a means of mass production of high-quality steel. After paying attention to the problem in actual work, such as "It's something people hesitate to do," they proposed the following new steel manufacturing method (Patent application 1986-
No. 132517). That is, as shown in Figure 4, ``Two converter-type furnaces with both upper and lower blowing functions are used, and one of them is used as the dephosphorization furnace 1.
The other side is a decarburization furnace 2, and a refining agent mainly composed of converter slag 4 generated in the decarburization furnace 2 is added to the hot metal 3 injected into the dephosphorization furnace 1.
The temperature of the hot metal 3 in the dephosphorization furnace 1 is controlled by blowing oxygen gas upward from the lance 6 while stirring the bottom-blown gas.
This is a method in which hot metal is dephosphorized while being kept at 1400°C or lower, and then the obtained dephosphorized hot metal is decarburized in a decarburization furnace 2. According to this method, it has become possible to manufacture steel with 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. The main advantages of the above method are specifically listed below. Because it is a "two-stage countercurrent refining" method that uses converter slag as the hot metal dephosphorization flux, the amount of quicklime used in the entire steelmaking process is significantly reduced compared to conventional methods.
It is possible to blow low phosphorus steel with an extremely small amount of quicklime. 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 used, 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 into the slag ladle. Removal of slag is easier than in other dephosphorization methods. Since the furnace used is a converter-type furnace with both upper and lower blowing functions, the hot metal can be stirred strongly and the treatment can be carried out in a short time.Therefore, the amount of heat removed is small, compared to other dephosphorization treatment methods. It is extremely advantageous in terms of thermoeconomics. In particular, when melting converter slag is used, it becomes more thermoeconomically advantageous due to its sensible heat. The slag generated in the dephosphorization furnace has little free lime (free CaO is less than 1%), so it can be effectively used as a roadbed material. 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, since high P ₂ O ₅ slag adheres to the dephosphorization furnace and only low P ₂ O ₅ slag adheres to the decarburization furnace, dephosphorization defects do not occur in the decarburization furnace. Moreover, when molten converter slag is used, the molten converter slag is charged into the dephosphorization furnace after the hot metal is charged, so there is no fear that a sudden explosive reaction will occur. 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. In addition, for example, in the case of a factory that operates 1/2 converter furnaces, one furnace can be used as a dephosphorization furnace, and this invention can be implemented without capital investment by operating 2/2 converters. It is. If it became necessary to build one of the bricks to extend the lifespan of the bricks, measures were taken to carry out conventional converter blowing using only one converter during this period to avoid leaving idle furnaces. Flexible refining is possible. As mentioned above, the "steel manufacturing method using two converter-type furnaces" proposed earlier by the applicant has many advantages, and is particularly effective as a means of producing low phosphorus steel. However, after further examination through many actual operations, it was found that
``Although the advantages of this method compared to conventional methods have been fully confirmed, a detailed analysis of numerous operational results shows that there are variations in iron loss from time to time, and the P level reached is somewhat It became clear that the instability of the <Means for Solving the Problems> Therefore, the present inventors have developed a method to suppress the iron content loss and the variation in the attained P level, which are observed in the above-mentioned "steel manufacturing method using two converter-type furnaces", although they are slight. As a result of conducting research to further stabilize its operability, it was found that ``variations in iron loss mainly occur during dephosphorization refining in a dephosphorization furnace, and in particular, the _CaF2 ratio in the refining slag at that time is extremely important.'' They found that by adding fluorite and adjusting the proportion of _CaF2 in the refined slag to a high level, iron loss can be stably kept low. This invention was made based on the above knowledge, and involves refining hot metal by using one of the two converter type furnaces having both upper and lower blowing functions as a dephosphorization furnace and the other as a decarburization furnace. In this process, the hot metal injected into the dephosphorization furnace contains converter slag generated in the decarburization furnace as a main component, and also contains fluorite in an amount such that the _CaF2 ratio in the slag is 14 to 20%. Add the refining agent and blow oxygen gas upward while stirring the bottom blowing gas to bring the temperature of the hot metal to 1400.
Hot metal dephosphorization is carried out while keeping the temperature below ℃, and the obtained dephosphorized hot metal is poured into a decarburization furnace, and a refining agent that ensures an amount of quicklime of 7 kg or more per ton of hot metal is added to perform decarburization and final dephosphorization. As a result, it is possible to stably produce low phosphorus steel with low iron loss and quicklime usage, and with good quality. Here, the reason why we limited the amount of fluorite to be fed into the dephosphorization furnace to "the amount that makes the _CaF2 ratio in the slag 14 to 20%" is that it is sufficient if the _CaF2 ratio in the dephosphorization slag is less than 14%. Iron loss cannot be significantly suppressed because proper slag formation cannot be ensured, and on the other hand, even if the _CaF2 ratio in the slag is 20% or more, not only will no further iron loss improvement effect be obtained, but fluorite consumption will increase. This results in increased costs and significant erosion of refractories. Figure 1 is a graph showing the relationship between the _CaF2 ratio in dephosphorization slag and iron loss (including granular iron).
It can also be seen from FIG. 1 that when the CaF ₂ ratio in the slag is less than 14%, iron content loss increases rapidly. Although the specific amount of fluorite for supplying _{the two} CaF components is uncertain due to the amount of other slag components, it is 2 to 13 Kg/T (amount per ton of hot metal), preferably 6 Kg/T. It is preferable to set the amount from more than 10 kg/T to 10 kg/T. What is shown in Figure 2 is the relationship between the ``amount of fluorite input necessary to secure the <sub>CaF2</sub> ratio in the slag to perform the desired dephosphorization with minimal iron loss'' and the ``Si content in the hot metal.'' However, in actual operation, the bands shown in FIG. If you add fluorite at a value that falls within the band between
The conditions defined by the present invention can be easily satisfied and good results can be obtained. Refining agent used in dephosphorization furnace (dephosphorization flux)
The main component is converter slag generated in a decarburization furnace, and fluorite is added to this, but iron oxide is also preferably added as a basic subcomponent. For example, it is recommended to use a blending ratio of converter slag: 40 to 80% by weight, fluorite: 7 to 20% by weight, and iron oxide: 20 to 60% by weight. Of course, it is not limited to this, but in order to turn converter slag into slag and turn it into dephosphorization slag with a low melting point, or to increase the oxidizing power of slag so that dephosphorization can proceed easily, it is extremely important to use iron oxide in combination. is important. In addition to the above, quicklime, dolomite, or limestone may be additionally blended, and manganese ore or ferromanganese ore may be blended to improve hot metal [Mn].
In addition to fluorite, CaCl ₂ and Na ₂ are also used as solvents.
O.SiO ₂ , Na ₂ CO ₃ or the like may be added. The smaller the particle size of the dephosphorization flux raw material 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 based on the [P] level of the steel to be melted, aiming at an amount that can stably ensure dephosphorization of 60% or more, but it is usually about 59 kg/t. In addition, as the converter slag, which is the main component of the refining agent used in the dephosphorization furnace, molten slag generated in the decarburization furnace is preferable from the viewpoint of thermoeconomics and slag formation of the dephosphorization flux. (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 temperature of the slag, the better. ) However, in this case, the smaller the particle size is, the better in order to improve the slag formation in the dephosphorization furnace, but since the converter slag is inherently highly slag formation, the particle size is less than 100mm. However, it does not cause any particular inconvenience, 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 from the previous charge, but it goes without saying that it may also be one that came out of the decarburization furnace before that or one that was generated in a decarburization furnace at another factory. Now, in the method of this invention, the treatment temperature in the dephosphorization furnace is limited to 1400℃ or less, but the reason for adjusting the temperature in this way is that if the hot metal treatment temperature is higher than this, only decarburization will proceed. All in the slag
This is because the amount of Fe decreases and the dephosphorization rate deteriorates.
However, if the temperature is too low, the loss of granulated iron to the slag will increase, so it is better to adjust the treatment temperature to 1250-1400℃.Maintaining such a treatment temperature requires blowing oxygen gas from a top blowing lance. Alternatively, this can be carried out in combination with oxygen gas injection 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 depends on the hot metal temperature and silicon content before treatment, the temperature of the converter slag,
It is determined by the temperature of the dephosphorization furnace, the target temperature of hot metal, etc., but usually 2.0Nm ³ /min・T
The following level is sufficient, preferably 0.5 to 1.0Nm ³ /
min·T is effective. As the above-mentioned "converter type furnace with both upper and lower blowing functions", the currently used "top and bottom blowing combined blowing converter" is most preferable.
In particular, with regard to dephosphorization furnaces, since the refining conditions are milder than in decarburization furnaces, the furnace itself can be made even smaller, so even if a new furnace is built specifically for dephosphorization, there is not much of an impact on the cost. The 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 amount of bottom gas in the dephosphorization furnace is 0.03~
0.20Nm ³ /min·T is good. This is because if the bottom gas amount is less than 0.03Nm ³ /min・T, the reaction will take a long time, while if it exceeds 0.20Nm ³ /min・T, no further stirring effect will be obtained. This is because there is a tendency to cause troubles such as increased tuyere melting loss. When dephosphorization is performed under the above conditions, usually
The desired dephosphorization can be completed within 20 minutes. Blowing in a decarburizing furnace is basically the same as blowing ordinary hot metal that has been dephosphorized outside the furnace. included in the investment.
CaO content is also converted): It is preferable to secure 7 kg/T or more. This is because in this process, in order to sufficiently advance the final dephosphorization and obtain low phosphorus steel with [P] of 0.012% or less, it is necessary to secure slag of 10 kg/T or more, and this cannot be achieved in normal operation. This is because it is necessary to ensure at least an input amount of quicklime: 7 kg/T. Figure 3 is a graph showing the relationship between CaO consumption in the decarburization furnace and P content in molten steel at the end of blowing. If light calcined dolomite is also added, it is clear that it is preferable to secure a value calculated by converting the CaO content contained therein. In addition, during decarburization refining, manganese ore or ferromanganese ore can be added in addition to slag-forming agents, mainly quicklime and dolomite, for the purpose of increasing the Mn content of molten steel at the end point. By the way, when carrying out the method according to the present invention, it is preferable to perform a preliminary desulfurization treatment on the applied hot metal if possible. The first reason is that desulfurization progresses extremely slowly in this method;
Apart from this, if hot metal that has not been desulfurized in advance is used, the S content in the converter slag will increase, and there is also a concern that the S content of the molten steel in the next charge will increase. Note that the preliminary desulfurization may be performed by any of the commonly used hot metal desulfurization methods. 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. Therefore, it is a good idea to adjust the Si content of hot metal to 0.4% or less, preferably 0.3% or less. In addition, if it is desired to raise the temperature of the hot metal after treatment due to the conditions of the decarburization furnace, it may be advantageous to set the Si content of the hot metal to around 0.2%, so it should be determined based on the local conditions of the factory. It is. By the way, depending on the factory, the crane capacity may be 2.
In some cases, cup pouring is performed, but in this case, most of the hot metal is processed in the dephosphorization furnace to simplify processing.
It is advisable to perform re-irritation in a decarburizing furnace. Next, the present invention will be specifically explained using examples. <Example> First, the first
250 tons of hot metal with the composition shown in the upper row of the table was poured into a double blowing combined blowing converter used as a dephosphorization furnace, and the converter slag generated in a similar type of decarburization furnace was cooled.・25Kg/T of solidified and crushed to a particle size of 100mm or less, iron ore with a similar particle size8
Kg/T, quicklime 6Kg/T, and fluorite 8Kg/T were added in a mixed state and dephosphorization was carried out for 10 minutes. The _CaF2 ratio in the slag at this time was 15%.

【table】

[Table] The dephosphorization furnace and decarburization furnace used are both capable of stirring by blowing gas from the bottom of the furnace, as mentioned above.
It was a 250-ton upper and lower double blowing combined blowing converter, and the operating conditions shown in Table 2 were adopted. 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 to produce quicklime used in normal converter operation.
10Kg/T and light calcined dolomite 10Kg/T and silica sand 4Kg/
Main blowing was carried out using T as a slag forming agent. At this time, iron ore was added as a coolant at appropriate times so that the end point temperature (blowing end temperature) was 1680°C. The converter slag generated at this time was approximately 40 kg/T.
This was added to the dephosphorization furnace together with iron ore and fluorite as a dephosphorizing agent raw material for the next charge, and a series of operations were repeated in which dephosphorization was carried out. As a result, the sum of the amount of quicklime used and the amount of light calcined dolomite used in the entire steelmaking process is a small value of 26Kg/T, and the P amount in the steel is 0.010 as shown in the lower row of Table 1.
The amount of quicklime and light calcined dolomite used to obtain molten steel of 1/3% by weight is approximately 1/3 of that used in normal low phosphorus steel melting. <Summary of Effects> As explained above, according to the present invention, it is possible to produce high-quality low-phosphorous steel with stable and low iron loss while keeping the amount of slag forming agent low throughout the steelmaking process. This makes it possible to manufacture high-quality steel, and brings about extremely useful effects industrially, such as reducing the manufacturing cost of high-quality steel and paving the way to further expand its fields of use.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the _CaF2 ratio in slag and iron loss in the dephosphorization furnace, and Figure 2 is a graph showing the relationship between the Si content in hot metal and the iron loss required to perform the desired dephosphorization with less iron loss. Figure 3 is a graph showing the relationship between CaO consumption in the decarburization furnace and P content in molten steel at the end of blowing, and Figure 4 is a graph showing the relationship between the amount of fluorite input and Figure 4. It is a schematic explanatory diagram showing an outline of a steel manufacturing method using two converters. In the drawings, 1... dephosphorization furnace, 2... decarburization furnace,
3... Hot metal, 4... Converter slag, 4'... Dephosphorization slag containing converter slag as a main component, 5... Stirring gas blowing nozzle, 6... Lance.

Claims (1)

[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, in which hot metal is injected into the dephosphorization furnace. The main component of the hot metal is converter slag generated in the decarburization furnace, and the _CaF2 ratio in the slag is 14~14.
Adding a refining agent containing fluorite in an amount of 20% by weight,
Hot metal dephosphorization is performed while bottom blowing gas is stirred and oxygen gas is blown upward to maintain the hot metal temperature below 1400℃, and the obtained dephosphorized hot metal is poured into a decarburization furnace for decarburization and final dephosphorization. A method for producing low phosphorus steel, characterized by: