JPH0433844B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> This invention uses two upper and lower double blowing composite blowing furnaces to melt high-quality steel at a low cost and with good workability without running out of heat source during the process. The present invention relates to a steelmaking method that allows for the production of steel, and also allows for the omission of hot metal desulfurization preliminary treatment. <Background technology> In recent years, various studies 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 "reaction that progresses during the floating process of the dephosphorizing agent (transitary reaction)".
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 greater the ripening during treatment and the lowering of 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. Based on the above situation, the present applicant first used two converter-type furnaces with both upper and lower blowing functions as schematically shown in FIG. , the other is decarburization furnace 2
Then, a refining agent whose main component is the converter slag 4 generated in the decarburization 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. While blowing oxygen gas upward from lance 6, the temperature of hot metal 3 in dephosphorization furnace 1 is raised 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-mentioned invention previously proposed by the present applicant is based on the concept that ``the required amount of slag-forming agent during the gold and steel making process is determined by the slag-forming agent, which brings the slag and metal into contact with each other in a countercurrent manner.''
Countercurrent refining of metals is the most effective method, but in reality, it is almost impossible to fully realize countercurrent refining. Based on the inventor's understanding, the only possible method for producing steel that could be used is 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 work stability, dephosphorization efficiency, equipment cost, etc., the following findings were made in the research aimed at resolving the problems ( A) to (F), 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 may cause problems in the next process. This 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 efficient stirring in a short time that is sufficient 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 cannot be ignored, and it is essential to use a processing container that allows easy removal of slag. For mass production, sufficient exhaust gas treatment equipment (dust collector) is required. (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 throughout the gold steel manufacturing process even if the amount of slag forming agent used is extremely small.
High-quality steel can be mass-produced with high efficiency. It was completed based on. The method previously proposed by the applicant is
It was an extremely advantageous steel manufacturing method in that low phosphorus steel could be stably produced at a low cost with the amount of slag used being kept to a minimum, and high quality steel could be provided at low cost. However, as a result of subsequent studies by the present inventors through actual operations, it was found that in the method proposed earlier, the dephosphorization process is carried out in a composite blowing converter having both upper and lower blowing functions, and therefore blowing oxygen is unavoidable. It has been pointed out that there is a concern that the consumption of [C] in hot metal by gas and dephosphorizing agents will increase, and that the heat source for the next decarburization furnace refining will tend to be insufficient, which will hinder decarburization refining. Summer. Furthermore, based on the understanding that ``refining in a dephosphorization furnace is a dephosphorization process,'' the slag composition and oxygen potential of the dephosphorization furnace were often set to conditions that would prevent desulfurization from proceeding. It is an important condition to desulfurize the hot metal beforehand outside the furnace, and the resulting temperature drop and cost loss are disadvantages that cannot be overlooked. <Means for Solving the Problems> Therefore, the inventors of the present invention devised a system for converting molten pig iron 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. A method of melting high-quality steel at a low cost with stable workability while taking advantage of the previously proposed steelmaking method of refining and avoiding the problems mentioned above. As we continued our research to discover this, we came to the following new findings. (a) When refining in a dephosphorization furnace, if lump or powdered carbon is added at the same time as charging the refining agent, some of the carbon will dissolve into the hot metal and become a heat source in the decarburization process. At the same time, in the dephosphorization furnace, top-blown oxygen selectively burns the carbonaceous material floating on the surface of the hot metal, which reduces the oxidation loss of [C] in the hot metal. Dephosphorization refining can be carried out while ensuring a sufficient [C] concentration in hot metal, which is the heat source in the process. (b) In this case, as in the case of "addition of carbonaceous material in a torpedo car or hot metal pot" described in JP-A-57-57811 and JP-A-60-75506, It was speculated that unless the material is pulverized and blown using a special device (such as an injector), the [C] in the hot metal cannot be uniformly and efficiently concentrated, leading to higher costs. When using a smelting furnace, a strong stirring effect is activated by bottom blowing during refining, and for this reason, sufficient results can be achieved even if lump or granular carbonaceous material is input without processing such as pulverization. , ``A normal additive injection method that does not involve high costs'' using an existing additive injection device can be applied as is to the injection of carbonaceous material. (c) In this case, there was also a concern that the slag would be reduced by the added carbonaceous material, resulting in poor dephosphorization, but since the [C] concentration in the hot metal would increase due to the addition of the carbonaceous material, ε <sub>P</sub> <sup>C</sup> = 12.8 The activity coefficient expressed by the formula increases, and the frequency of contact between the dephosphorizing slag and metal increases due to the stirring action of the bottom blowing gas, so this does not lead to the extreme deterioration of dephosphorization as feared. thing. (d) Furthermore, with the addition of carbonaceous materials as described above, there is a concern that the sulfur concentration in hot metal will increase due to the sulfur content contained in the carbonaceous materials. In the aforementioned steelmaking method using a composite blowing furnace, if the slag composition in the dephosphorization furnace is made into a reducing atmosphere by adding carbon, sufficient desulfurization can proceed simultaneously with dephosphorization. This not only makes it possible to avoid the adverse effects of S pick-up from carbonaceous materials, but also makes it acceptable to omit desulfurization preliminary treatment of hot metal. 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 mainly composed of converter slag 4 generated in the decarburization furnace 2 is added to the hot metal. A carbon material is added to compensate for the carbon in the hot metal that is reduced by being burned by the top-blown oxygen, and while the bottom-blown gas is stirred by the stirring gas injection nozzle 5, oxygen gas is blown upward from the lance 6 to form the hot metal. By including the dephosphorization process and the process of refining the obtained dephosphorized hot metal in the decarburization furnace 2, high-quality steel can be produced at a low cost with good workability without running out of heat source. It is characterized by the fact that it can be manufactured easily. Here, as the above-mentioned "converter type furnace with both top and bottom blowing functions", the currently used "top and bottom blowing combined blowing converter" is most preferable, but especially for the dephosphorization furnace, the refining conditions are Since it is milder than a charcoal furnace, the furnace itself can be made even smaller, so even if a new furnace is built specifically for dephosphorization, there will not be much of an impact on the cost. Note that the top-blowing oxygen lance in the dephosphorization furnace 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 here depends on the temperature of the hot metal before treatment, the silicon content,
It is determined by the temperature of the converter slag, the degree of warmth of the dephosphorization furnace, the target temperature of hot metal to be treated, etc., but from 0.6 to
When the flow rate is adjusted to about 1.0Nm ³ /min・t, the selective combustion of the input carbon material floating on the surface of the hot metal becomes stable, and it is possible to effectively suppress the oxidation reduction of [C] in the hot metal. Therefore, the amount of oxygen gas blown at this point should be adjusted within this range if possible. The refining agent (solvent) in the dephosphorization furnace is one whose main component is converter slag generated in the decarburization furnace (in addition to converter slag, ordinary substances such as quicklime, fluorite, manganese ore, iron ore, etc.) are used. Additives are added as appropriate) are used, but
For example, the recommended composition is converter slag: 40-80% by weight, iron oxide: 20-60% by weight, and fluorite: 0-20% by weight. In this case, if the slag basicity is adjusted to 2.4 to 3.0 and the S distribution ratio between slag and metal is set to 30 or more, desulfurization will proceed sufficiently at the same time as dephosphorization, resulting in deterioration of hot metal quality due to S pick-up from the added carbon material. This is extremely desirable because it not only makes it possible to suppress the amount of heat generated, but also makes it possible to omit desulfurization pretreatment of the hot metal. Fig. 2 is a graph showing the relationship between slag basicity and S distribution, and from this Fig. 2 it can be seen that compared to using a slag with low basicity that focuses only on dephosphorization, It is clear that the higher the basicity of the slag used in the dephosphorization furnace, the higher the desulfurization rate. Considering this fact and the balance between increased cost due to the increase in solvent and decreased yield of tapped water, it is most preferable to adjust the slag basicity to 2.4 to 3.0. Note that even if no carbonaceous material is added, the same degree of desulfurization effect can be obtained by adjusting the basicity to this level. The amount of refining agent (solvent) used in the dephosphorization furnace is determined by the [P] level of the steel to be melted, but is usually about 30 to 60 kg/t. 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 from the previous charge, but it goes without saying that it may also be from a previous decarburization furnace or from another improved decarburization furnace. On the other hand, the type of carbonaceous material added together with the refining agent in the present invention is not particularly limited, and may be in any form such as lumps, granules, or powders, so cheap coal, coke, etc. can be applied as is, and does not require special addition equipment for injection operation. Further, the amount of carbon material to be added is not particularly specified, and it is sufficient to add about 5 to 15 kg per ton of hot metal, for example. Note that it is desirable to suppress the hot metal treatment temperature in the dephosphorization furnace to 1400°C or less (approximately 1250 to 1350°C).
This is because if the hot metal treatment temperature is higher than this, decarburization will only progress, the amount of oxidizing agent in the slag will decrease, and there is a concern that the dephosphorization rate will deteriorate. However, 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 will increase.'' The treatment temperature is maintained by burning the added carbonaceous material by blowing oxygen gas from a lance. 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. By the way, if a method is adopted in which iron ore used as a dephosphorizing agent is introduced into the final stage of blowing in a dephosphorizing furnace and during rinsing (stirring only by bottom blowing after blowing), it is possible to "lower the slag temperature" and Due to the effects of “increasing the oxygen potential in the slag at the final stage of blowing”, P
Since the distribution can be maintained in a favorable state, it is compatible with effects such as an increase in the activity coefficient due to an increase in the [C] concentration in hot metal due to the addition of carbonaceous materials, and an increase in the frequency of contact between slag and metal due to strong stirring. In addition, the P level after processing can be kept low more stably. 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. 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
Desired dephosphorization or dephosphorization and desulfurization can be completed within 20 minutes. 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. As mentioned above, the present invention is capable of desulfurization and desulfurization with a high concentration of [C], which will serve as the heat source in the next process, by simply adding extremely inexpensive carbonaceous material while utilizing the strong stirring of the composite blowing furnace. Phosphorus hot metal can be stably produced without picking up S from carbonaceous materials, and furthermore, without requiring any prior desulfurization treatment (in fact, if refining is carried out without adding carbonaceous materials, approximately As much as 0.5% of [C] in the hot metal is oxidized and lost), but by subjecting it to decarburization treatment in the next step, a composite blowing furnace, the amount of slag forming agent used is reduced and sufficient thermal margin is maintained. The following steps, as shown in FIG. 3, can be cited as examples of recommended processing steps. That is, the first step: hot metal after being tapped from the blast furnace is poured into a dephosphorization furnace. 2nd step: At the same time as charging the converter slag used as a dephosphorizing agent, carbon material is charged using a normal additive charging chute. Third step: Considering the amount of hot metal [Si] and the amount of converter slag, a predetermined solvent is added so that the basicity is 2.4 to 3.0. Fourth step: In order to prevent dephosphorization from deteriorating, iron ore is added at the end of blowing and during rinsing to adjust the temperature. Fifth step: The dephosphorized pig iron discharged from the dephosphorization furnace is poured into the decarburization furnace and refined. 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 in 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" under the conditions shown in Table 2. We carried out dephosphorization blowing.

【table】

【table】

[Table] (Note) * indicates that the conditions are outside the conditions specified in the present invention.

[Table] The blowing results are also shown in Table 2. As is clear from the blowing results in the dephosphorization furnace shown in Table 2, the method of the present invention, in which carbonaceous material is added together with the refining agent for refining, has no adverse effect on dephosphorization; It can be confirmed that the [C] concentration increase rate and desulfurization rate after treatment are far superior to the comparative method in which no addition was made. Here, focusing on desulfurization, in the method of the present invention, sufficient desulfurization proceeds through the above-mentioned refining, so there is no need for preliminary desulfurization treatment, but in the comparative method, the desulfurization effect is small, and therefore desulfurization is performed by pretreatment. I understand what is needed. The temperature after treatment in this dephosphorization furnace is determined by the “thermal margin in the next step of decarburization furnace refining” and the “stability of dephosphorization”.
The temperature was adjusted using iron ore or Mn ore with the aim of achieving both of these conditions, and aiming for a temperature of 1300℃. Next, the hot metal that had been refined in the dephosphorization furnace was poured into a ``250-ton upper and lower double blowing combined refining converter used as a decarburizer'' and decarburized blowing was carried out under the conditions shown in Table 3. Ivy. The tapping temperature of the molten steel obtained in this way is
It is also shown in the table. As a result, if hot metal is treated according to the conditions specified in the present invention, the amount of quicklime consumed in the gold and steel making process is extremely small, and the [S] concentration can be reduced without the need for desulfurization of the hot metal by pretreatment. It was confirmed that phosphor steel can be stably produced with sufficient heat margin. <Summary of Effects> As explained above, the present invention has industrially useful effects such as making it possible to melt high-quality steel efficiently and at low cost with good workability. 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 slag basicity and S distribution in dephosphorization furnace refining. FIG. 3 is an explanatory diagram of an example of a processing step according to the present invention. FIG. 4 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)

[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. After being injected into the dephosphorization furnace, a refining agent mainly composed of converter slag generated in the decarburization furnace and a carbon material for compensating for the carbon in the hot metal that is burned by top-blown oxygen and reduced. The method is characterized by comprising a step of dephosphorizing the hot metal by adding and top-blowing oxygen gas while stirring the bottom-blown gas, and a step of refining the obtained dephosphorized hot metal in a decarburization furnace. steel making method. 2. The steelmaking method according to claim 1, wherein the hot metal dephosphorization step is carried out under a slag composition in which desulfurization simultaneously proceeds in a reducing atmosphere with added carbon.