JPH0437135B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> This invention uses two upper and lower double blowing composite blowing furnaces to produce high-quality steel at a low cost with high flexibility in the use of scrap and the use of a small amount of slag-forming agent. This invention relates to a method for high-efficiency melting. <Background technology> In recent years, various researches have been conducted with the aim of developing means to stably melt low-phosphorus steel at even lower costs. The following methods of preliminary dephosphorization of hot metal are used for the stable melting of low phosphorus steels: (b) Preliminary dephosphorization by injecting or blasting quicklime-based flux onto hot metal in a ladle; (c) Blasting quicklime-based flux onto hot metal in a blast furnace cast bed trough. A method of performing preliminary dephosphorization using phosphorescence has been proposed, and some of it has been put into practical use. However, in methods (a) and (b) above, dephosphorization is a “transitary reaction” that progresses during the floating process of the dephosphorizing agent.
Because the dephosphorization flux is not always efficiently used because it relies on the ``reactor reaction'', there is also the problem that the longer the treatment time, the more heat is removed during the treatment, which lowers the hot metal temperature. In method c), the dephosphorization treatment is performed on hot metal immediately after being tapped from the blast furnace, so the dephosphorization treatment temperature is as high as approximately 1400℃, and therefore it is difficult to reach a sufficiently satisfactory level of P content. Furthermore, when using quicklime etc. as flux for hot metal dephosphorization, considering the amount of quicklime etc. used in the subsequent converter blowing, both of the above methods Compared to the "method of omitting the preliminary dephosphorization step and performing dephosphorization only in a converter," the effect of reducing the amount of required slag forming agent (amount of quicklime, etc.) could not be said to be that remarkable. In view of the above situation, the applicant first decided to use two converter-type furnaces with both upper and lower blowing functions as schematically shown in FIG. 1. The other side is a 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 1, and bottom-blown gas is stirred by the stirring gas injection nozzle 5. At the same time, oxygen gas is blown upward from lance 6 to raise the temperature of hot metal 3 in dephosphorization furnace 1 to 1400℃.
After dephosphorizing the hot metal while maintaining the following conditions, the obtained dephosphorized hot metal is decarburized and final dephosphorized in the decarburization furnace 2 to produce steel with a normal phosphorus level or A patent application was filed in 1983 for a steel manufacturing method that enables low-phosphorus steel to be produced with good workability and at low cost.
- Proposed as No. 132517. The above invention previously proposed by the present applicant states that ``the required amount of slag forming agent throughout the entire steelmaking process is determined by ``slag-metal countercurrent refining'' in which slag and metal are brought into contact with each other in a countercurrent manner. However, in practice, it is almost impossible to fully realize countercurrent refining, and it can be cited as the steelmaking method that is currently the least labor intensive and has the potential to reduce the amount of slag-forming agent used. The inventor realized that there is no other way than to divide the dephosphorization process into two stages and use the slag generated in the lower process as a dephosphorizing agent in the upper process. Regarding the "steel manufacturing method by reusing converter slag," which was expected to have disadvantages in terms of phosphorus efficiency or equipment cost, the following findings (A) to (F) were made in research aimed at resolving the problems. That is, (A) In the dephosphorization treatment of hot metal, it is better to keep the treatment temperature as low as possible from the viewpoint of dephosphorization efficiency, but if the temperature becomes too low, it will not only cause problems in the next process, but also cause problems in the slag after treatment. The problem arises that the loss of granular iron increases, so the temperature is
A temperature of about 1300 to 1350°C is best. 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. (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. Adequate agitation to achieve equilibrium is essential, 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 injected from the bottom of the processing vessel is most preferable. distance)
(D) In order to reduce erosion of the processing vessel refractories caused by slag and increase dephosphorization work efficiency, it is preferable to use a basic lining. (E) Steel manufacturing including a two-step dephosphorization process In order to increase the efficiency of dephosphorization in the method, the efficiency of removing slag from the processing container can be ignored, and it is essential to use a processing container that allows easy removal of slag. Sufficient exhaust gas treatment equipment (dust collector) is required for mass production. (G) Taking these conditions into consideration, a converter-type furnace is recommended as the hot metal dephosphorization treatment vessel, and stirring gas is introduced from the bottom of the furnace. A composite blowing converter with both upper and lower blowing functions is ideal, and if this is used to carry out the above-mentioned "steel manufacturing method including a two-stage dephosphorization process," the slag-forming agent will be removed throughout the entire steel manufacturing process. It was completed based on the following principles: Even if the amount used is extremely small, sufficient dephosphorization can be performed and high-quality steel can be mass-produced with high efficiency. The method previously proposed by the applicant is
It was possible to stably produce low-phosphorus steel at a low cost by minimizing the amount of slag-forming agent used, and it was an extremely advantageous steel-making method for providing high-quality steel at a low cost. On the other hand, due to the recent stabilization of steel demand, large quantities of scrap as a raw material for steelmaking have become available.
As it has become extremely advantageous in terms of price, there are also prominent attempts to reduce steelmaking costs by increasing the proportion of steelmaking raw materials that are made up of steelmaking raw materials. For this reason, the present inventors decided to use scrap as part of the steelmaking raw material fed into the dephosphorization furnace when implementing the "previously proposed method" using the two converters mentioned above. We considered further reducing steel manufacturing costs by this method. However, in the method proposed earlier, a sufficient amount of iron ore, which is a solid oxygen refining agent, is mixed with the aim of avoiding dephosphorization failure during hot metal dephosphorization blowing, and from the viewpoint of heat balance, it is difficult to scrap. I had no choice but to conclude that there was no room for charging. That is, when scrap is used as part of the raw material in double-blown double converter refining, the flow of bottom-blown gas is restricted until the scrap is completely melted, resulting in insufficient stirring of the hot metal and desorption. Phosphorus deficiency occurs. Therefore, the usual practice was to temporarily stop blowing and tilt the converter in order to promote scrap melting, but this was not a desirable method due to large operational losses. . Furthermore, in the previously proposed method, the hot metal temperature was kept low (below 1400°C) in order to ensure the dephosphorization rate, which was even more unsatisfactory as a melting condition for scrap. <Means for Solving the Problems> The present inventors have developed a method of refining hot metal by using one of two converter type furnaces having both upper and lower blowing functions as a dephosphorization furnace and the other as a decarburization furnace. While taking advantage of the advantages of the "previously proposed steelmaking method," we are conducting research to solve the above-mentioned problems and stably produce high-quality steel with good workability even with a high scrap usage rate. ``The width and thickness of the scrap melted in the dephosphorization furnace must be regulated to below a certain value, and sufficient bottom-blown gas agitation must be carried out to eliminate some of the solid oxygen refining agents such as iron ore and scale.'' If dephosphorization furnace refining is carried out with the charging time at the end of blowing, even if the hot metal temperature is relatively low, the dissolution of scrap by carbon diffusion will be completed quickly without the need to tilt the converter, resulting in high New knowledge was obtained that it is possible to obtain the desired low phosphorus pig iron at a low dephosphorization rate. This invention was made based on the above knowledge, and it is said that ``As shown in Fig. 1, one of the two converter-type furnaces having both upper and lower blowing functions is dephosphorizing furnace 1, and the other is In a steelmaking method in which hot metal is refined using a decarburization furnace 2, after the hot metal is injected into the dephosphorization furnace 1, a refining agent containing converter slag 4 generated in the decarburization furnace 2 as a main component is added to the hot metal. Scrap with a width of 30 mm or less and a thickness of 15 mm or less is added, and while performing bottom-blown gas stirring by blowing gas from the stirring gas blowing nozzle 5 at a flow rate of 0.07 Nm ³ /min・t or more, the lance 6
The hot metal dephosphorization process involves blowing oxygen gas upward to maintain the hot metal temperature below 1400℃, adding a solid oxygen refining agent at the end of this blowing process, and decarburizing the resulting dephosphorized hot metal. By including the process of refining in furnace 2, a high proportion of scrap is used,
It is characterized by the fact that it consumes less slag making agent and can produce high-quality steel at a low cost. Here, the reason why the hot metal treatment temperature in the dephosphorization furnace is adjusted to 1400℃ or less is that if the hot metal temperature is higher than this, decarburization will proceed and the amount of T.Fe in the slag will decrease, and the dephosphorization rate will decrease. This is because it will get worse. but,
It is also necessary to take into account the fact that "if the temperature is too low, the loss of granular iron to the slag increases", so it is preferable to adjust the temperature to about 1250 to 1400°C. Note that such processing temperature is maintained by blowing oxygen gas from the top blowing lance or by using this in combination with blowing oxygen gas from the bottom tuyere. In other words, it is no exaggeration to say that the oxygen gas injection in the dephosphorization furnace is carried out 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 blown in is determined by the temperature of the hot metal before treatment, the silicon content, 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 2.0Nm ³ / It should be less than min・t, usually 0.5~
1.0Nm ³ /min·t is sufficient. The reason for numerically limiting the width and thickness of scrap to be input as described above is that the melting of scrap in the present invention is performed by "lowering the melting point due to carbon diffusion from hot metal into scrap", so the width and thickness of scrap is This is because if the thickness exceeds 30 mm or the thickness exceeds 15 mm, scrap melting will not be completed within the specified blowing time without tilting the converter, even if the bottom blowing gas agitation is increased. Furthermore, the bottom blowing gas flow rate at this time was set to 0.07Nm ³ /
The value set as min・t or more is 0.07Nm ³ /
If it is less than min・t, the stirring of hot metal will be insufficient,
This is because there is a concern that the scrap will not be completely melted without tilting the converter, and dephosphorization failure will occur due to the reduced contact frequency between the solid oxygen refining agent added for dephosphorization and the hot metal. Figure 2 is a graph showing the relationship between the bottom blowing gas flow rate and the phosphor removal rate in the case of ``with scrap melting'' and ``without scrap melting.'' From Figure 2, when performing scrap melting, the bottom blowing gas flow rate must be adjusted.
It can be seen that it is preferable to set it to 0.07Nm ³ /min·t or more. In addition, even if the amount of solid oxygen refining agent (iron ore, etc.) added is small, the bottom blowing gas flow rate can be reduced to 0.07Nm ³ /
The reason why the desired dephosphorization rate is maintained by setting it to min·t or more is that sufficient contact between the refining agent and the hot metal can be achieved by strengthening the stirring force. Stirring gases blown from the bottom of the furnace include Ar, CO ₂ ,
It may be any of CO, N ₂ , O ₂ , air, etc. Also, the "solid oxygen refining agent" mentioned here is a refining agent (dephosphorizing agent) containing iron oxide such as iron ore and scale.
refers to The reason why we decided to add the additional solid oxygen refining agent at the end of the dephosphorization blowing process is because if we add the solid oxygen refining agent in the middle of the dephosphorization blowing process as usual, it would be difficult to add the solid oxygen refining agent at the end of the dephosphorization blowing process. This is because there is a possibility that the dephosphorization effect will not be fully exerted due to the influence. On the other hand, if it is added at the final stage of blowing, the P distribution can be maintained in a favorable state due to the effects of "lowering the temperature of the overlying slag" and "increasing the oxygen potential in the slag at the final stage of blowing". Dephosphorization is complete. The "final stage of dephosphorization blowing" refers to the period from 3 minutes before the end of dephosphorization blowing to 1 minute after the end (during rinsing), and the solid oxygen refining agent is added several times during this period. It is best to add it in divided doses. Figure 3 is a graph showing the relationship between the time of input of solid oxygen refining agent (iron ore) and the dephosphorization rate in dephosphorization furnace refining, and from this figure, the time of input is considered to be the final stage of blowing. It can be seen that this significantly improves the dephosphorization rate of hot metal. Figure 4 is a graph showing the change trend of the dephosphorization rate according to the amount of solid oxygen refining agent (iron ore) input. It has been clearly shown that the results are improved. By the way, 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, but especially for dephosphorization furnaces, the refining conditions are Since it is milder than a 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. The main component of the refining agent in the dephosphorization furnace is the converter slag generated in the decarburization furnace, but in addition to this converter slag, the aforementioned iron oxide (solid oxygen refining agent) is used as a basic subcomponent. It is also preferable to include fluorite. For example, it is recommended that the composition be comprised of converter slag: 40 to 80% by weight, iron oxide: 20 to 60% by weight, and fluorite: 0 to 20% by weight. Of course, it is not limited to this, but it is possible to convert converter slag into slag to produce dephosphorization slag with a low melting point, or to increase the oxidizing power of slag so that dephosphorization can proceed easily. The concomitant use of these drugs is extremely important. However, in the present invention, as described above, a part of the solid oxygen testing agent (iron oxide content refining agent such as iron ore and scale) is added at the final stage of dephosphorization blowing. In addition to the above-mentioned refining agents in the dephosphorization furnace, quicklime, dolomite, or limestone may be additionally blended, or manganese ore or ferromanganese ore may be blended to improve the hot metal [Mn]. It's okay. Fluorite is commonly used as a solvent, but Na ₂ O,
SiO ₂ , CaCl ₂ , Na ₂ CO ₃ and the like may be used alone or in combination with fluorite. The amount of refining agent used in the dephosphorization furnace is determined by the [P] level of the steel to be melted, but is usually between 30 and 60.
Kg/t is sufficient. By the way, 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 once cooled and solidified and then crushed into granules or chunks. However, in this case, the smaller the particle size is, the better it is 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 100 mm.
Even if it is smaller than this, no particular inconvenience will occur, and even if it is larger than this, it can be used. The timing of the converter slag to be used is preferably one that has been previously charged, but it goes without saying that it may also be one that has been taken out of the decarburization furnace before that, or one that was generated in another improved decarburization furnace. When dephosphorization is performed under the above conditions, usually
Within 20 minutes, ``dissolution of scrap up to about 5.0% in proportion to hot metal'' and ``desired dephosphorization or dephosphorization and desulfurization'' can be completed. Blowing in a decarburizing furnace is basically the same as blowing ordinary hot metal that has been dephosphorized and desulfurized outside the furnace. For this purpose, manganese ore or ferromanganese ore can also be added in addition to slag-forming agents mainly composed of quicklime and dolomite. By the way, FIG. 5 diagrammatically shows a preferred example of the steel manufacturing process according to the present invention, which will be described below. First step: Hot metal (which may be either desulfurized hot metal or undesulfurized hot metal) after being tapped in the blast furnace is poured into a dephosphorization furnace. 2nd step: Charge the converter slag to be used as a dephosphorizing agent, and also charge the scrap. Third step: Taking into consideration the amount of hot metal [Si] and the amount of converter slag, a solvent is introduced to achieve a predetermined basicity and blowing is performed. Fourth step: In order to prevent dephosphorization from worsening, iron ore is dispensed at the final stage of blowing (including rinsing) to adjust the temperature. Fifth step: The dephosphorized pig iron discharged from the dephosphorization furnace is poured into the decarburization furnace and refined. <Function> As described above, the present invention is characterized in that one of the two converter type furnaces having both upper and lower blowing functions is used as a dephosphorization furnace.
The other side is used as a decarburization furnace to refine hot metal with less consumption of slag-forming agent, and the strong stirring of the combined blowing furnace is used.
This makes it possible to use scrap raw materials in dephosphorization furnace refining without adversely affecting the dephosphorization rate. In other words, as mentioned above, the mechanism of dissolving scrap at low temperature in the present invention is explained in FIG. In other words, when scrap is thrown into hot metal,
As shown in Figure 5, due to the diffusion of [C] in the hot metal, the carbon concentration at the interface between the scrap (steel) and the hot metal increased, and the liquidus temperature at this area decreased to the temperature of the surrounding hot metal. Lysis occurs at this point.
As this phenomenon progresses, scrap dissolution at low temperature is completed. According to experiments, the speed of scrap melting in a composite blowing furnace is approximately 0.75 mm/min, and if the dimensions are within 30 mm in width and 15 mm in thickness, it will be completely melted within the dephosphorization blowing time. This has been confirmed. However, when scrap is put into the dephosphorization furnace for melting, iron ore (solid oxygen) is used as a dephosphorizing agent (refining agent) to prevent the temperature of the hot metal from dropping.
The amount had to be reduced, and there were concerns that the dephosphorization rate would decrease, but the bottom blowing gas flow rate was set to a sufficient level (0.07Nm ³ /min・
t or more), if the frequency of contact between slag and metal is increased by vigorously stirring the hot metal, the dephosphorization rate will not decrease so much, and in addition to this measure to ensure the dephosphorization rate, When iron ore (solid oxygen) is added to the slag, the temperature of the overlying slag decreases and the oxygen potential in the slag increases, further improving the dephosphorization rate. This made it possible to dissolve scrap into hot metal without any problems. Next, the present invention will be explained in more detail through Examples and in comparison with Comparative Examples. <Example> First, blast furnace hot metal having a composition within the range shown in Table 1 was poured into a "250 ton upper and lower double blowing combined blowing converter used as a dephosphorization furnace", and with the exception of three cases, Scrap having the dimensions shown in Table 2 was input and dephosphorization blowing was carried out under the conditions shown in Table 3. The results of this blowing are also shown in Table 3. As is clear from the blowing results in the dephosphorization furnace shown in Table 3, it goes without saying that no unmelted scrap was left when blowing was carried out under the conditions specified in the present invention. It can be seen that while no phosphorus defects occurred, in the "comparative example" in which the size of the scrap was large and the bottom blowing gas flow rate was small, undissolved scraps and dephosphorization defects occurred. Then, the hot metal that has been refined in the dephosphorization furnace as specified in the present invention is used as a “decarburization furnace.”
When iron was poured into a 250-ton upper and lower double blowing combined blowing converter and decarburization blowing was carried out, it was found that a satisfactorily low phosphorous steel was produced.

【table】

【table】

[Table] (Note) * indicates that the conditions are outside the conditions specified in the present invention.
was able to obtain. As a result, it was confirmed that when hot metal is treated according to the conditions specified in the present invention, the amount of quicklime consumed in the entire steelmaking process is extremely small, and because of the addition of scrap, low-phosphorus steel can be stably produced at a low hot metal rate. Ta. <Summary of Effects> As explained above, according to the present invention, high-quality steel can be produced at low cost by reducing the amount of slag-forming agent consumed and the proportion of blast furnace pig iron used, and without causing dephosphorization defects. Efficient melting becomes possible, etc.
Industrially extremely useful effects are brought about.

[Brief explanation of drawings]

FIG. 1 is a conceptual diagram of the process of the present invention. FIG. 2 is a graph showing the relationship between the bottom blowing gas flow rate and the dephosphorization rate. FIG. 3 is a graph showing the relationship between the addition timing of the solid oxygen refining agent (iron ore) and the dephosphorization rate. FIG. 4 is a graph showing a change in dephosphorization rate depending on the amount of solid oxygen refining agent (iron ore) added. FIG. 5 is an explanatory diagram of an example of a processing step according to the present invention. FIG. 6 is a diagram schematically explaining the scrap dissolution mechanism. FIG. 7 is a conceptual diagram of the process related to the steel manufacturing method proposed earlier. In the drawings, 1... dephosphorization furnace, 2... decarburization furnace,
3...Hot metal, 4...Converter slag, 4'...Dephosphorization slag mainly composed of converter slag, 5...Stirring gas injection 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, and in which the hot metal is introduced into the dephosphorization furnace. After injecting, a refining agent mainly composed of converter slag generated in the decarburization furnace and lightweight scrap with a width of 30 mm or less and a thickness of 15 mm or less were added, and the blowing gas flow rate was 0.07 Nm ³ / A hot metal dephosphorization process in which blowing is performed while bottom blowing gas is stirred at min・t or more while oxygen gas is blown upward to maintain the hot metal temperature below 1400℃, and a solid oxygen refining agent is added at the end of this blowing. and a step of refining the obtained dephosphorized hot metal in a decarburization furnace.