JPH01147011A - Steelmaking method - Google Patents

Abstract

PURPOSE:To produce high-quality molten steel at a low cost by, at the end of dephosphorization process in which the oxidizing slag of the decarburizing furnace and scrap steel are added to a dephosphorizing furnace, further iron ore is sent to the decarburizing furnace when molten iron is refined into steel in the dephosphorizing furnace and decarburizing furnace of top- and bottom-blown converter type. CONSTITUTION:The predesiliconized and desulfurized molten iron 3 is charged into the dephosphorizing furnace 1 of converter type having a top-blowing oxygen lance 6 and a bottom- blowing tuyere 5, the oxidizing molten slag 4 generated in the dephosphorizing furnace 2 of the same type at the succeeding stage is added, lightweight scrap steel having <=30mm width and <=15mm thickness is also added, the molten iron is heated to <=1400 deg.C which is an appropriate temp. for dephosphorization, oxygen is blown in from the lance 6, an inert gas is blown in from the bottom-blowing tuyere 5 at the rate of 0.07Nm<3>/min per ton of molten iron 3, and the molten iron 3 and the molten slag 4 are sufficiently agitated. Iron ore or a solid oxidizing agent such as scales is finally added to carry out sufficient dephosphorization reaction. The dephosphorized molten iron is charged into the decarburizing furnace 2, oxygen is blown in from the lance 6, and inert gas is injected from the tuyere 5, and the molten iron is decarburized into molten steel. The consumption of a slag forming agent is reduced, the consumption of inexpensive scrap steel is increased, and hence molten steel is produced at a low cost.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention uses two upper and lower double blowing composite blowing furnaces to improve quality with high flexibility in the use of scrap and with a small amount of slag-forming agent. The present invention relates to a method for producing high-quality steel at low cost and with high efficiency.

<Background technology> In recent years, various studies have been conducted with the aim of developing a means to stably melt low-phosphorus steel at even lower costs. Regarding the stable melting of low phosphorus steel and low phosphorus steel, the following preliminary dephosphorization method of hot metal is used: (a) A method in which preliminary dephosphorization is performed by injecting quicklime-based flux or soda ash into hot metal in a torpedo.

(b) A method in which preliminary dephosphorization is performed by injecting or blasting quicklime-based flux into the hot metal in the ladle.

(A method in which preliminary dephosphorization is performed by blasting quicklime-based flux to the hot metal in the C1 blast furnace casthouse gutter.

have been proposed, and some have even been put into practical use.

However, in the methods (a) and (b) above, the dephosphorization is related to the "reaction that progresses during the floating process of the dephosphorizing agent (transitary reactor reaction)", so the efficiency of using the dephosphorizing flux is not necessarily good. There is also the problem that the longer the treatment time, the greater the heat removal during the treatment and the lowering of the hot metal temperature. It has been pointed out that since the dephosphorization process is performed on hot metal, the temperature of the dephosphorization treatment is as high as about 1400°C, and therefore it is difficult to achieve a sufficiently satisfactory level of P content.

Furthermore, when using quicklime etc. as a hot metal dephosphorization flux, considering the amount of quicklime etc. used in the subsequent converter blowing, both of the above methods are effective in eliminating the preliminary dephosphorization step. It could not be said that the effect of reducing the required amount of slag forming agent (amount of quicklime, etc.) was not so remarkable compared to the method of dephosphorizing using only a converter.

Therefore, in light 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. one is a phosphorification furnace (1), the other is a decarburization furnace (2), and the dephosphorization furnace (11
A refining agent whose main component is the converter slag (4) generated in the decarburization furnace (2) is added to the hot metal (3) injected into the furnace, and the bottom blowing gas is stirred by the stirring gas injection nozzle (5). While dephosphorization is being carried out, oxygen gas is blown upward from the lance (6) and the dephosphorization furnace (
After dephosphorizing the hot metal (1) while keeping the temperature of the hot metal (3) below 1400°C, the obtained dephosphorized hot metal is transferred to the decarburization furnace (2).
A special patent application entitled "A steelmaking method that enables the production of steel at a normal phosphorus level or low phosphorus steel with good workability and at low cost with an extremely small amount of slag-forming agent through decarburization and final dephosphorization." It was proposed as No. 61-132517.

The above invention previously proposed by the present applicant is “
The amount of slag forming agent required throughout the entire steelmaking process is minimized when slag and metal are brought into contact with each other in a countercurrent manner, called "slag-metal countercurrent refining." This is almost impossible to achieve, and currently the only steelmaking method that has the potential to reduce the amount of slag used with the least amount of labor is to divide the dephosphorization process into two stages, and remove the slag that occurs in the lower process. The inventor recognized that there was no other way to use the slag produced by the dephosphorization process as a dephosphorizing agent in the upstream process. The following findings (A.
) ~ (F), that is, (^) 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 cause problems in the next process. The temperature should be set at 1300 to 135°C to prevent the problem of increasing granular iron loss to the slag after treatment.
A temperature of about 0°C is best.

However, 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 lower temperature. The processing temperature can be maintained stably and easily.

(B) In order to fully utilize the dephosphorizing ability of the flux and increase the dephosphorizing efficiency, it is necessary to adjust the treatment temperature as described above, as well as to lack sufficient stirring to achieve a dephosphorizing equilibrium state. However, in order to achieve satisfactory and efficient stirring in a short time for high-efficiency dephosphorization of high-temperature hot metal, gas stirring using gas blown from the bottom of the processing vessel is most preferable.

(C) In addition, in order to perform an efficient dephosphorization process, the processing vessel must have sufficient freeboard (distance from the scene to the top of the vessel) for slag forming.

(D) In order to reduce erosion of the processing vessel refractories due to slag and increase dephosphorization work efficiency, it is preferable to use a basic lining.

(E) In order to increase the efficiency of dephosphorization in a steelmaking process that includes a two-stage dephosphorization process, the efficiency of removing slag from the processing container cannot be ignored, and it is essential to use a processing container that allows easy removal of slag. .

(F) In order to mass-produce high-quality steel with good workability, sufficient exhaust gas treatment equipment (dust collector) is necessary.

(G) Considering these conditions, a converter-type furnace is ideal as a hot metal dephosphorization treatment vessel, especially a combined blowing converter that has both upper and lower blowing functions that can introduce stirring gas from the bottom of the furnace. When this is used to carry out the above-mentioned "steel manufacturing method including two-stage dephosphorization process", sufficiently efficient dephosphorization can be achieved even if the amount of slag forming agent used throughout the entire steel manufacturing process 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 can stably produce low-phosphorus steel with low-cost operations that minimize the amount of slag-forming agent used, and is effective in providing high-quality steel at low cost. This was an extremely advantageous steel manufacturing method.

On the other hand, due to the recent stabilization of steel demand, a large amount of scrap as a raw material for steelmaking has become available, and the price has become extremely advantageous. Attempts to reduce it are also noticeable.

For this reason, the present inventors, when implementing the "previously proposed method" using the two converters mentioned above,
We considered using scrap as part of the steelmaking raw materials fed into the dephosphorization furnace to further reduce steelmaking costs.

However, in the method proposed earlier, a sufficient amount of iron ore, which is a solid oxygen refining agent, is blended with the aim of avoiding poor dephosphorization during hot metal dephosphorization and blowing, and from a thermal balance point of view it is difficult to scrap. We had no choice but to conclude that we could not afford to charge.

In other words, when scrap is used as part of the raw material in upper and lower double blowing combined converter refining, the bottom blowing gas flow is restricted until the scrap melting is complete, resulting in insufficient stirring of the hot metal and desorption. Phosphorus deficiency occurs. Therefore, the usual practice is to temporarily stop blowing and tilt the converter in order to promote scrap melting, but this is not a desirable method as it causes a large operational loss.

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 scrap melting condition.

Means for Solving the Problems> The present inventors have developed a system for 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 a high scrap usage rate and good workability. [The width and thickness of the scrap melted in the dephosphorization furnace was regulated to below a certain value, and sufficient bottom-blown gas agitation was carried out, and some of the solid oxygen refining agents such as iron ore and scale were removed.] If dephosphorization furnace refining is carried out at the end of blowing, even if the hot metal temperature is relatively low, the melting of scrap through carbon diffusion will be completed quickly without the need for tilting 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 ``As shown in FIG. , in a steelmaking method in which hot metal is refined using the other side as a decarburization furnace (2),
After injecting hot metal into the dephosphorization furnace (1), it is mixed with a refining agent whose main component is converter slag (4) generated in the decarburization furnace (2) and a width of 30 fl or less and a thickness of 151 m. Add the following scraps and flow it from the stirring gas blowing nozzle (5).
: 0.07N n? /m1n- or more, while performing bottom-blown gas stirring, blowing oxygen gas upward from the lance (6) to maintain the hot metal temperature below 1400°C, and blowing is carried out at the final stage of this blowing. The hot metal dephosphorization process involves adding a solid oxygen refining agent to
By including the refining process in step 2), a high proportion of scrap is used, and the consumption of slag-forming agent is reduced.
It is characterized by the ability to manufacture high-quality steel at low cost.

Here, the reason why the hot metal treatment temperature in the dephosphorization furnace is adjusted to 1400°C or less is that if the hot metal temperature is higher than this, decarburization will progress and T and Fel in the slag will decrease, resulting in a worsening of the dephosphorization rate. Because it does. However, it is also necessary to take into consideration 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 depends on the temperature of the hot metal before treatment, the silicon content, the temperature of the converter slag, and the warmth of the dephosphorization furnace.

It is determined by the target hot metal temperature, etc., but approximately 2
．． ON n? /m1n- may be below, usually 0.5~
1, ON rrr/m1nt is sufficient.

The reason for numerically limiting the width and thickness of the scrap to be input as described above is that the melting of scrap in the present invention "lowers the melting point due to carbon diffusion from the hot metal into the scrap."
Because the axial force of the scrap (exceeding 30N or the thickness exceeds 151 kg), even if the bottom blowing gas agitation is increased, the scrap melting process will not be possible within the specified blowing time without tilting the converter. This is because the process is not completed.

Furthermore, the bottom blowing gas flow rate at this time was set to 0.07N rr? /
The reason min・ was set to be more than 0.07N r
If it is less than rl'/min Ht, there is a concern that the hot metal will not be sufficiently stirred and the scrap will not be completely melted without tilting the converter, and the solid oxygen refining agent added for dephosphorization may This is because poor dephosphorization occurs due to decreased contact with 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.'' It can also be seen from FIG. 2 that when performing scrap melting, it is preferable to set the bottom blowing gas flow rate to 0.07 Nm/m1n- or more.

Note that even if the amount of solid oxygen refining agent (iron ore, etc.) added is small (but the bottom blowing gas flow rate is 0.07 N m/m1n- or more, the desired dephosphorization rate can be maintained due to the stirring power). This is because strengthening allows sufficient contact between the refining agent and the hot metal.

As the stirring gas blown from the bottom of the furnace, A r r CO
z +Co, N2. It may be either O□, air, or the like.

Furthermore, the term "solid oxygen refining agent" as used herein refers to a refining agent (dephosphorizing agent) containing iron oxide such as iron ore or scale. 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 in stages.

Figure 3 is a graph showing the relationship between the timing of adding solid oxygen refining agent (iron ore) and the dephosphorization rate in dephosphorization furnace refining.
It can also be seen from FIG. 3 that the dephosphorization rate of hot metal is significantly improved by setting the charging timing to the final stage of blowing.

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, 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 be no significant impact in terms of cost. There isn't.

The refining agent in the dephosphorization furnace is mainly composed of converter slag generated in the decarburization furnace, but in addition to this converter slag, the above-mentioned iron oxide (
A solid oxygen refining agent) is used as a basic subcomponent, and fluorite is also preferably included. For example, converter slag: 40 to 80% by weight.

Iron oxide = 20-60% by weight.

A blending composition of 20 to 20% by weight of fluorite is recommended. 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 refining 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,
Additionally, quicklime, dolomite or limestone may be blended, and manganese ore or ferromanganese ore may be blended to improve the hot metal [Mn].

Fluorite is commonly used as a solvent, but Na, O.310g1
CallJz, NazCo3, etc. may be used alone, or they may be used in combination with fluorite.

The amount of refining agent used in the dephosphorization furnace is the [P] of the steel to be melted.
Although it is determined by the level, it is usually around 30 to 60 kg/.

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. (
When using a molten substance like this, it is poured into 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 being once cooled and solidified, and then crushed into granules or chunks.

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 since converter slag is inherently highly slag-forming, the particle size is less than 100 mm. However, it does not cause any particular inconvenience, and it can be used even if it is larger than this.

The timing of the converter slag to be used is preferably one that was charged last time, but it is better to use one that has been taken out of the decarburization furnace before that.
Needless to say, it is also possible to use the material generated in other improved decarburization furnaces.

When dephosphorization is carried out under the above conditions, "dissolution of scrap to a ratio of about 5.0% to hot metal" and "desired dephosphorization or dephosphorization and desulfurization" are usually completed within 20 minutes. can do.

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 (either desulfurized hot metal or undesulfurized hot metal may be used) after being tapped from 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: To prevent dephosphorization from worsening, temperature is adjusted by using iron ore at the end of blowing (including rinsing).

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 uses two converter-type furnaces with both upper and lower blowing functions, one of which is a dephosphorization furnace and the other a decarburization furnace, to reduce the consumption of slag forming agent and produce hot metal. When refining, the strong stirring of the composite blowing furnace is used to make it possible to use scrap raw materials in the dephosphorization furnace refining without adversely affecting the dephosphorization rate.

In other words, as mentioned above, the low-temperature scrap melting mechanism of the present invention is explained with reference to FIG.

That is, when scrap is put into hot metal, the carbon concentration at the interface between the scrap (steel) and the hot metal increases due to the diffusion of [C] in the hot metal, and the liquid phase at that part increases. Melting occurs when the wire temperature drops to the ambient hot metal temperature. This phenomenon progresses and scrap melting at low temperature is completed. According to experiments, the speed of scrap melting in the composite blowing furnace is 0.75 mm/min.
It has been confirmed that scrap with dimensions of width: 3ON or less and thickness: 15wm or less will be completely melted within the dephosphorization blowing time.

However, when scrap is fed into a dephosphorization furnace for melting, the amount of iron ore (solid oxygen) fed as a dephosphorizing agent (refining agent) must be reduced in order to prevent the hot metal temperature from dropping. There was a concern that the dephosphorization rate would decrease, but if the frequency of contact between slag and metal was increased by increasing the flow rate of bottom blowing gas (more than 0.07 N m/m1n-) and vigorously stirring the hot metal, this could be reduced. The dephosphorization rate does not decrease, and in addition to this measure to ensure the dephosphorization rate, if iron ore (solid oxygen) is used in the final stage of blowing, the temperature of the overlaid slag will decrease and the slag will increase. The increased oxygen potential further improved the dephosphorization rate, making it possible to dissolve scrap into hot metal without deteriorating the dephosphorization rate.

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 with the dimensions shown in Table 2 was input and dephosphorization blowing was performed 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 when blowing was carried out under the conditions stipulated in the present invention, the scrap was not melted, leaving no residue. While there was no dephosphorization failure, the “comparative example” had large scrap size and low bottom blowing gas flow rate.
It can be seen that melting of scrap results in residue and poor dephosphorization.

Then, the hot metal that had been refined in the dephosphorization furnace as specified in the present invention was poured into a "250 ton upper and lower double blowing combined blowing converter used as a decarburization furnace" to perform decarburization blowing. However, we were able to obtain a sufficiently satisfactory low phosphorus steel.

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. This brings about extremely useful effects industrially, such as making it possible to perform melting efficiently.

[Brief explanation of the drawing]

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 melting 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 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 with upper and lower blowing functions as a dephosphorization furnace and the other as a decarburization furnace, and the hot metal is injected into the dephosphorization furnace. After that, 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. ^2/
Hot metal dephosphorization is performed by blowing while keeping the hot metal temperature below 1400℃ by top blowing oxygen gas while bottom blowing gas stirring is performed at min・t or more, and adding a solid oxygen refining agent at the end of this blowing. and a step of refining the obtained dephosphorized hot metal in a decarburization furnace.