JPS63195210A - Production of low phosphorus steel - Google Patents

Abstract

PURPOSE:To reduce loss of Fe into slag and also to economically produce low phosphorus steel having good quality under little consumption of lime, by using the slag in a decarbonizing furnace together with fluor-spar having the specific ratio in a dephosphorizing furnace at the time of executing dephosphorized and decarbonized refining in molten iron by two sets of the top and bottom blowing converters. CONSTITUTION:In the converter 1 having an oxygen top blowing lance 6 and a bottom blowing nozzles 5, the molten iron 3, which is beforehand desulfurized, fine granule of the slag 4 from the decarbonizing furnace 2, which is formed as the same type converter in the next process, and lime and fluorspar are added to form the molten slag 4'. CaF2 content in the molten slag 4' is made to 14-20wt.% and by oxygen blowing from the bottom blowing nozzles 5, the molten iron 3 and the molten slag 4' are stirred and also by oxygen blowing from the top blowing lance 6, the molten iron 3 is refined to dephosphorize at <=1,400 deg.C. The low phosphorized molten iron 3 obtd. is charged in the decarbonizing furnace 2 and ore, lime, etc., are added to form the molten slag 4 and by oxygen blowing from the bottom blowing nozzles 5 and the top blowing lance 6, C in the molten iron 3 is oxidized to decarbonize and refined into molten steel and the generated slag is reused to the dephosphorizing furnace 1.

Description

[Detailed Description of the Invention] <Industrial Application Field> This invention performs high-efficiency dephosphorization while minimizing iron loss and minimizing the amount of quicklime used throughout the entire steelmaking process, producing high-quality steel at a low cost. This relates to a method of melting.

<Prior art and its problems> In recent years, quality requirements for various steel materials have become more sophisticated day by day, and in line with this, various innovations have been attempted in steel manufacturing methods and various new methods have been introduced.

Under these circumstances, there has recently been great hope 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, i) a method of performing preliminary dephosphorization by injecting a quicklime-based dephosphorizing agent or soda ash into the hot metal in a torpedo car, and ii) reblasting (spraying), which involves injecting quicklime-based flux into the hot metal in a ladle. iii) A method of performing preliminary dephosphorization by blasting hot metal with quicklime-based flux in a blast furnace casthouse trough.

However, although relatively low P content levels can be achieved using methods i) and ii) above,
Because dephosphorization relies on the "reaction that progresses during the floating process of the dephosphorizing agent (transition reactor reaction)," the efficiency of using the dephosphorizing flux is not always good, and the longer the treatment time, the more heat is removed during the treatment. is thick (there is a problem that the hot metal temperature decreases;
With method 2, the temperature of the hot metal after treatment can be kept higher than with the previous two methods, but because the dephosphorization treatment is performed on the hot metal immediately after being tapped from the blast furnace, the temperature of the dephosphorization treatment is too high. There is a disadvantage that the achieved P content level itself is worse than in methods i) and ii), and neither method is satisfactory.

Furthermore, when using quicklime etc. as hot metal dephosphorization flux, considering the amount of quicklime etc. used in the subsequent converter blowing, it is difficult to use any of the above methods. It was also pointed out that the required amount of slag forming agent (amount of quicklime, etc.) was not significantly reduced compared to the method of dephosphorizing using a converter alone.

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 process is used as a dephosphorizing agent in the upper process (i.e., a method in which converter slag is used as the main component of flux for hot metal dephosphorization, for example, The applicant had previously proposed “
Based on the basic research results of ``a method of reusing converter slag as hot metal dephosphorization flux in ex-furnace refining'', we also developed a method of ``reusing converter slag that has been proposed so far. The steel manufacturing method is
It is very difficult to stably secure efficient working conditions as out-of-furnace refining is also used, and the dephosphorization efficiency is not as high as expected, and it is especially difficult for mass production. We also focused on the problems in actual work, such as the need for exhaust gas dust collectors and dephosphorization slag removal equipment, which makes it difficult to take steps toward mass production of Product II steel.'' , proposed the following new steel manufacturing method (Patent Application No. 13251/1986)
No. 7).

In other words, 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 dephosphorization furnace 1 and the other as decarburization furnace 2.
Then, 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 I, and while bottom blowing gas is stirred by the stirring gas injection nozzle 5. After dephosphorizing the hot metal while keeping the temperature of the hot metal 3 in the dephosphorization furnace 1 below 1400°C by blowing oxygen gas upward from the lance 6, the obtained dephosphorized hot metal is decarburized in the decarburization furnace 2. This is method J. 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 using 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" that uses converter slag as hot metal dephosphorization flux, the amount of quicklime used in the entire steelmaking process is significantly reduced compared to the conventional method, making it possible to produce low-phosphorus steel with an extremely small amount of quicklime. Allows for blowing.

■ FeO in the converter slag will be 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 not reduced to Mn and remain in the slag as oxides, but in the method of this invention, the slag is Since it is reused as hot metal dephosphorization flux, the residual ore is effectively utilized, and it is useful for "reducing [Mn] loss or increasing [Mn]" in hot metal.

■ 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, Pr adhering to the furnace body
There is no concern about poor dephosphorization due to o.

In other words, the dephosphorization furnace has a high P20. slag, and low P20 in the decarburization furnace. Since only slag is attached, no dephosphorization failure occurs 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, there is no need to finely grind the dephosphorizing agent to near powder form as in the case of conventional hot metal dephosphorization methods, which can reduce costs accordingly. becomes.

■ If there is an idle converter, it can be used immediately as a dephosphorization furnace, and there is no need to prepare special equipment.

Furthermore, for example, in the case of a factory that operates 1/2 converter furnaces, one furnace can be used as a dephosphorization furnace, and the present invention can be implemented without capital investment by operating 272 converter furnaces. . If it becomes necessary to construct one of the bricks to extend the lifespan of the bricks, measures are taken to carry out conventional converter blowing using only one converter during this period and to prevent the use of idle furnaces. Refining is possible with great flexibility.

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 over conventional methods have been fully confirmed, there are still a number of A detailed analysis revealed that there were variations in iron loss from time to time, and that there was also some instability in the P level reached.

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 is observed in the above-mentioned "steel manufacturing method using two converter-type furnaces", although it is 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 Ca in the refining slag at that time.
The F2 ratio plays an extremely important role, and by adjusting the CaF ratio in the refined slag to a high level by adding fluorite, iron loss can be stably kept low. It has come to this.

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 CaFt in the slag.
Adding a refining agent containing fluorite in an amount of 14 to 20%, performing bottom-blown gas stirring and top-blowing oxygen gas to dephosphorize the hot metal while keeping the hot metal temperature below 1400 ° C.
The obtained dephosphorized hot metal is poured into a decarburization furnace to produce 7k per ton of hot metal.
By decarburizing and final dephosphorization by adding a refining agent that has an amount of quicklime of more than 1.5 g, it is possible to stably produce low-phosphorus steel with low iron loss and quicklime usage, and with good quality. It has the following characteristics.

Here, the amount of fluorite to be introduced into the dephosphorization furnace is determined by the amount of fluorite in the slag (:
The reason why the amount of CaFt in the dephosphorization slag is less than 14% is because sufficient slag formation cannot be ensured, so iron loss cannot be significantly suppressed. CaF in slag.

Even if the ratio is set to 20% or more, not only no further iron loss improvement effect can be obtained, but also an increase in cost due to increased fluorite consumption and significant erosion of the refractory.

Figure 1 is a graph showing the relationship between the CaF ratio in dephosphorization slag and iron content loss (including granular iron). It can be seen that this will lead to a rapid increase in losses.

The specific amount of fluorite for supplying the CaFz component is uncertain due to the amount of other slag components, but it is
A quantity of more than 13 kg/T (amount per 8 tons of pig iron), preferably more than 6 kg/T and up to 10 kg/T is suitable. What is shown in Figure 2 shows the relationship between the amount of fluorite required to maintain the CaFz ratio in the slag to achieve the desired dephosphorization with minimal iron loss and the Si content in the hot metal. However, in actual operation, the band (2"20[%S
If fluorite is added at a value that falls within the band between the line shown by ``il+2'' and the line shown by ``20[%St]+S'', the conditions specified in the present invention can be easily satisfied. good results can be obtained.

The refining agent (dephosphorization flux) used in the dephosphorization furnace is mainly composed of converter slag generated in the decarburization furnace, mixed with fluorite, but also contains iron oxide as a basic subcomponent. It is best to mix it as For example, converter slag: 40 to 80% by weight,
- Fluorite: 7 to 20% by weight, iron oxide = 20 to 60% by weight are recommended. 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, quicklime, dolomite, or limestone may be added in addition to the above, and manganese ore or ferromanganese ore may be added to improve the hot metal [Mn].
In addition to fluorite, Ca (J z +Na
tO・5totsNazCOx 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.

The amount of refining agent (lla) used in the dephosphorization furnace is determined based on the [P] level of the steel to be melted with the aim of stably ensuring dephosphorization of 60% or more, but it is usually 50% or more.
kg/ is fine.

Furthermore, 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. (When using a molten substance like this, it is sent to a dephosphorization furnace through a pot lined with a refractory), and one obtained in a decarburization furnace in consideration of ease of handling. It may be used after cooling and solidifying it and crushing it 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-formable, so even if the particle size is less than 100N, 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 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.

Now, in the method of this invention, the treatment temperature in the dephosphorization furnace is limited to 1400°C 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. This is because the total amount of Fe in the slag becomes low and the dephosphorization rate deteriorates.

However, if the temperature is too low, the loss of granular iron to the slag will increase, so the treatment temperature is preferably adjusted to 1250 to 1400°C. The treatment temperature is maintained by blowing oxygen gas from the top blowing lance or by 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 usually 2. ONm37min4 or less is sufficient, preferably 0.5 to 1. ONm2/m1n-T is effective. The above-mentioned "converter type furnace with both top and bottom blowing functions" is the "top and bottom blowing combined blowing converter" currently in use.
However, especially for dephosphorization furnaces, the refining conditions are milder than for decarburization furnaces, so 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. do not have.

The stirring gas blown from the bottom of the furnace is Ar, COt *
It may be any of Co, N, O, air, etc.

The amount of bottom gas in the dephosphorization furnace is 0.03~
0.2ONm310+1n-T is good. This is because if the bottom gas amount is less than 0.03Nm''/5in-T, the reaction will take a long time;
This is because even if it exceeds 1 in -T, not only will no further stirring effect be obtained, but there will also be a tendency to cause troubles such as increased tuyere melting loss.

When the dephosphorization treatment is performed under the above conditions, the desired dephosphorization can usually 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. If it is added, the CaO content contained in it is also converted): 7
It is preferable to secure at least kg/T. This is because the final dephosphorization progresses sufficiently in this process and [P] reaches 0.
．． This is because to obtain low phosphorus steel of 0.12% or less, it is necessary to secure slag of 10 kg/T or more, and to achieve this in normal operation, it is necessary to secure at least a quicklime input amount of near kg/T.

Figure 3 is a graph showing the relationship between CaO consumption in the decarburization furnace and the P content in molten steel at the end of blowing.
The figure also shows that the amount of quicklime input should be at least 7 kg/T (if light calcined dolomite is also added, the Ca contained in it should be
It is clear that it is preferable to secure a value that also converts O content.

In addition, during decarburization refining, manganese ore or ferromanganese ore can be added in addition to slag-forming agents mainly composed of 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 the progress of desulfurization is extremely slow in this method, but apart from this, when hot metal that has not been desulfurized in advance is used, the S content in the converter slag increases. This is because there is also a concern that the molten steel S content in the next charge may be increased. 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 hot metal temperature after treatment due to the conditions of the decarburization furnace, it may be advantageous to set the Si content of the hot metal to about 0.2%, so it should be determined according to the local conditions of the factory. It is.

By the way, depending on the factory, two cups of iron may be poured due to the crane capacity, but in this case, in order to simplify the process, it is recommended to process most of the hot metal in the dephosphorization furnace and perform additional iron in the decarburization furnace. It's a good idea.

Next, the present invention will be specifically explained using examples.

<Example> First, 250 tons of hot metal having the composition shown in the upper row of Table 1, which had been desulfurized in a KR (hot metal processing furnace), was poured into an upper and lower double blowing combined blowing converter used as a dephosphorization furnace. ,to this,
The converter slag generated in a similar type of decarburization furnace is cooled and solidified.
25kg/T crushed to a particle size of 00m111 or less,
Iron ore with similar particle size 8kg/T, quicklime 6kg/T
1 by adding T and 8 kg/T of fluorite in a mixed state.
Dephosphorization treatment was performed for 0 minutes. Ca in the slag at this time
The Fz percentage was 15%.

As mentioned above, the dephosphorization furnace and decarburization furnace used were both 250-ton top and bottom double blowing combined blowing converters capable of blowing gas from the bottom and stirring, and the operating conditions were as shown in Table 2. was adopted.

The dephosphorized pig iron thus obtained (the composition is shown in the middle row of Table 1) is first tapped into a pot and then poured into a decarburization furnace.
Main blowing was carried out using 910 kg/T of quicklime, 10 kg/T of lightly calcined dolomite, and 4 kg/T of silica sand as slag forming agents, which are used in normal converter operation. At this time, iron ore was added as a coolant at appropriate times so that the end point temperature (bell end temperature) was 1680°C.

The converter slag generated at this time was approximately 40 kg/T, and a series of operations were performed in which this, along with iron ore and fluorite, was added to the dephosphorization furnace again as a dephosphorizing agent raw material for the next charge to perform dephosphorization. repeated.

As a result, the sum of the amount of quicklime used and the amount of light burnt dolomite used in the entire steelmaking process was a small value of 26 kg/T, and the first
Molten steel with a P content of 0.010% by weight as shown in the lower part of the table was obtained. The amount of quicklime and light calcined dolomite used is about 1/3 of that used in ordinary 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 the drawing]

Figure 1 is a graph showing the relationship between the CaF ratio in slag and iron loss in a dephosphorization furnace, and Figure 2 is a graph showing the relationship between the SL content in hot metal and iron loss in order to perform the desired dephosphorization with less iron loss. Figure 3 is a graph showing the relationship between the required amount of fluorite input, 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 , 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 whose main component is converter slag, 5... Stirring gas blowing nozzle, 6... Lance.

Claims (1)

[Claims] 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 hot metal being injected into the dephosphorization furnace. A refining agent containing converter slag generated in the decarburization furnace as the main component and fluorite in an amount such that the CaF_2 ratio in the slag is 14 to 20% by weight is added to the slag, and while stirring the bottom blowing gas. Hot metal dephosphorization is performed while maintaining the hot metal temperature at 1400 ° C. or less by blowing oxygen gas over, and the obtained dephosphorized hot metal is poured into a decarburization furnace for decarburization and final dephosphorization. Method of manufacturing low phosphorus steel.