JP2007077483A - Converter steelmaking - Google Patents

Abstract

PROBLEM TO BE SOLVED: To minimize the total cost in a steel making method in which preliminary treatment and decarburization are continuously performed in a converter.
SOLUTION: The first step of charging hot metal, hot metal and scrap, or hot metal and scrap and pig iron as main raw materials, the second step of removing Si and P, and tilting the converter. And a third step for discharging the slag generated in the second step, a fourth step for erecting the furnace, supplying oxygen from the top blowing lance and performing de-C, a fifth step for removing the generated molten steel, In the converter steelmaking method in which the slag after de-C refining produced in the fifth step is left in the furnace, the process returns to the first step, and the fifth step is repeated from the first step. When slag is discharged, the time T 1 from the start of slag discharge to the end of slag discharge and the time T ₁ to reach the intermediate angle of the converter tilt angle from the slag discharge start to the end of slag are as follows: Converter that satisfies the equation and the slag rejection rate is 40-60% To control the motion.
1.5 ≦ T / T ₁
[Selection] Figure 1

Description

  The present invention relates to a converter steelmaking method for reusing hot slag after de-C refining.

  The refining of hot metal in the conventional converter is a method in which blast furnace hot metal is charged into the converter, the flux is mainly composed of quick lime, and the hot metal is de-P, de-C, and steel is made by oxygen blowing. It was general. After that, refining functions over multiple processes are concentrated in the converter, effectively using the energy of the hot metal, greatly reducing energy loss, and increasing fixed costs (equipment costs, labor costs) before and after the converter. For example, Patent Document 1 discloses a method that enables easy reduction.

  In the method disclosed in Patent Document 1, hot metal is charged into a converter as a first step, and flux addition and oxygen blowing are performed as a second step to perform de-Si / de-P refining and contain a predetermined amount of P. As a third step, the converter is tilted to discharge the slag generated in the second step, and then, as the fourth step, in the same converter, with flux addition and oxygen blowing, C is removed to a predetermined C content, and as the fifth step, the slag produced in the fourth step is left in the converter, and then returned to the first step, up to the fifth step. It is a method that is repeatedly performed.

  In Patent Document 1, in order to improve the yield of Mn ore, it is effective to make the rejection rate as close as possible to 100%, but from the viewpoint of reducing the flux for dephosphorization, the rejection rate exceeds 60%. And since the reduction rate of quicklime basic unit becomes small, it is disclosed that the rejection rate 60% is the minimum required rejection rate.

  On the other hand, in Patent Document 2, in order to improve the slag removal rate in the third step, a plurality of tuyere provided from the furnace bottom side to the furnace port side of the furnace belly are used, It is disclosed that gas is blown into the furnace while increasing the gas flow rate toward the tuyere on the furnace port side, and the slag is moved to the furnace port side to be discharged.

JP-A-7-70626 Japanese Patent Laid-Open No. 5-140627

  In the method disclosed in Patent Document 1 and Patent Document 2, since the de-P and de-C processes are performed continuously using the same converter, the energy of the hot metal is effectively utilized and the energy loss is greatly reduced. At the same time, fixed costs (equipment costs and labor costs) before and after the converter can be significantly reduced. If the amount of slag discharged in the third process is small, the P removed in the slag in the second process is the fourth. Since it returns to the molten steel again in the process, it is necessary to remove P again in the fourth process, and the amount of flux such as quicklime must be increased, leading to an increase in cost. Although attempts have been made to increase the amount as much as possible, it has been found that simply increasing the slag discharge rate not only destabilizes the operation but also does not reduce the cost.

  That is, it is clear that the basic unit of the auxiliary material decreases as the rejection rate increases, but on the other hand, liquid slag floating on the hot metal due to the structure of the converter is discharged from the furnace port. Therefore, the converter needs to be tilted, and as the residual slag in the converter decreases (that is, the removal rate increases), the furnace is tilted to discharge without discharging the molten iron in the furnace. Not only does it take more time than necessary to reduce the speed, but also the converter cycle time (the time required from the first step to the fifth step of steelmaking) is extended by the amount that the discharge time becomes longer. It has been found that the longer the steel receiving time, the shorter the life of the refractory, and at the same time, the production volume is reduced, resulting in the deterioration of utility costs such as electric power.

  Then, this invention aims at making the total cost on operation minimum by making the slag removal in a 3rd process appropriate.

  The gist of the converter steelmaking method of the present invention made to solve the above problems is as follows.

(1) In the converter, the first step of charging hot metal, hot metal and scrap, or hot metal and scrap and pig iron as main raw materials, the removal step of removing Si and de-P, tilting the converter, In the third step of discharging the slag generated in the process, in the fourth step of raising the furnace upright, supplying oxygen from the top blowing lance and performing de-C, the fifth step of removing the generated molten steel, and the fifth step In the converter steelmaking method in which the slag after de-C refining produced in this way is left in the furnace, the process returns to the first step, and the fifth step is repeated from the first step, and the slag is discharged in the third step. In this case, the time T from the start of slag discharge (start of slag outflow from the furnace port) to the end of slag discharge and the intermediate angle from the start of slag discharge from the furnace port to the converter tilt angle at the end of discharge time T ₁ to reach satisfies the following formula, and slag Haikasuritsu is 40% to 60% BOF steelmaking method characterized by controlling the converter tilting so.
1.5 ≦ T / T ₁

  (2) In the converter steelmaking method described in (1) above, the rejection rate in the third step is defined in advance based on the auxiliary raw material cost, the converter refractory cost, and the utility cost to be input in the converter. The converter steelmaking process, characterized by

  According to the present invention, in the actual operation, in particular, in the cross section where the productivity of the converter process is required, by setting the intermediate rejection time in a certain region, the rejection rate is set in a certain region. It is possible to optimize the total cost including the raw material cost, refractory cost and utility cost.

  In the converter, hot metal, hot metal and scrap, or the first step of charging hot metal and scrap and pig iron as main raw materials, the removal process of removing Si and P, and tilting the converter, generated in the second step Generated in the third step of discharging the slag, the fourth step of raising the furnace upright, supplying oxygen from the top blowing lance to de-C, the fifth step of removing the generated molten steel, and the fifth step In the converter steelmaking method in which the slag after de-C refining is returned to the first step after remaining in the furnace and the fifth step is repeatedly performed from the first step, conventionally, focusing on only the auxiliary material, We have implemented operations to increase the rejection rate in the third process as much as possible.

  However, in order to improve the rejection rate in the third process, it is necessary to lengthen the rejection time. As a result, the productivity of the converter is reduced, and therefore the utility cost for converter operation is reduced. Has been found to get worse. Furthermore, by increasing the discharge time in the third step, the time in which the hot metal and molten steel are in the furnace becomes longer, the effect on the refractory in the converter becomes larger, and as a result, the refractory cost also deteriorates. Conceivable.

  When considering such secondary effects, it is not necessary to increase the rejection rate by spending time in the third step of the rejection operation. Rather, the rejection rate should be within the appropriate range. It has been found that it is rather advantageous to specify and not make the elimination time unnecessarily long.

  In the third step, the converter is generally tilted after enclosing gas in the slag and forming the slag so that the in-furnace slag can easily flow out. However, it is necessary to remove the slag in the furnace while the slag once formed is calming down.

  However, according to Tachikawa et al. (Iron and Steel: 1974, A19), the volume change of the formed slag is known to be determined by the balance between the generation and disappearance of bubbles in the slag. To establish.

Where V _s is the forming slag volume (m ³ ), V _{s, ∞} is the non-forming slag volume (m ³ ), Q _gas is the gas generation rate (m ³ / s), and τ is the bubble life (= foaming index). ) (S), t represents time.

  Equation (2) is derived from Equation (1), and the bulk specific gravity is inversely proportional to the forming slug volume.

Here, V _{s, i} represents the initial forming slag volume (m ³ ), W _s represents the slag mass (t), and ρ _s represents the slag density (t / m ³ ).

  From the equation (2), as time t elapses, the volume of the formed slag in the furnace becomes smaller and the slag height cannot be secured, that is, it becomes difficult to remove the slag without discharging the molten iron. Recognize. In other words, even if it takes too much time, a significant improvement in the rejection rate cannot be expected.

  On the other hand, when the converter is tilted as soon as possible before the slag in the furnace is calmed down and the slag is to be discharged, the molten iron is dragged by the shearing force when the slag flows as shown in FIG. Since it flows out, it is necessary to obtain the molten iron outflow limit in consideration of the balance between the shearing force acting on the tip of the molten iron and gravity (= buoyancy).

  The shear force when slag flows out is expressed by equation (4), gravity is expressed by equation (5), and the balance at the time of molten iron outflow limit is expressed by equation (6).

Here, τ is the shearing force (Pa), μ _s is the slag viscosity (Pa · s), u is the slag flow velocity (m / s), L is the distance between the molten iron surface and the furnace port (m), and s is the molten iron uplift height. (M), B represents gravity (Pa), ρ _m represents molten iron density (kg / m ³ ), ρ _s represents slag density (kg / m ³ ), and g represents gravitational acceleration (m ² / s).

  From equations (4) to (6), the quadratic equation (7) relating to s (= molten iron uplift height) is derived, but the condition under which molten iron does not flow out is that s has a real solution. 8) It is expressed by the formula.

  That is, it is understood that the value of L at which the molten iron begins to flow out increases as the outflow speed u during slag discharge increases.

  Therefore, at the initial stage of the discharge with a large distance L between the molten iron and the furnace port, even if the flow velocity u at the time of slag outflow is large, the shearing force τ applied to the molten iron is suppressed and the outflow of the molten iron hardly occurs. On the contrary, since the distance L between the molten iron and the furnace port becomes small at the final stage of the waste disposal, it is necessary to reduce the flow velocity u at the time of slag outflow.

  Since the flow velocity u at the time of slag outflow increases with the increase in the tilting speed of the converter, by adjusting the tilting speed of the converter according to the discharge stage, rapid discharge while suppressing the outflow of molten iron Is possible.

  As described so far, regarding the waste in the third step, it is assumed that it does not take a long time but enters the optimum operation region by not being too short. Therefore, considering both of these, we have found out that the operation is limited to the optimum region based on the index.

The total drainage time from the start of slag discharge of the converter (start of slag outflow from the furnace port) to the end of slag discharge, which is a factor affecting the calm of the former formed slag, is T On the other hand, even if the flow velocity at the time of slag outflow is large, the shearing force applied to the molten iron is suppressed, so that the molten iron becomes difficult to flow out.The converter tilting intermediate angle θ from the slag discharge start to the end of discharge from the furnace port _1, and "the time to reach the Haikasu start to the middle angle theta ₁ T _1" when the investigated the influence of molten iron flowing out during BOF tilting by the ratio of both "T / T _1".

Here, the specific theta _1, tilt angle theta ₀ slag Haikasu start angle in a state in which the converter is upstanding from the furnace outlet as a reference (slug flow starting from throat), slag Haikasu ends (discharge When the maximum tilt angle during dredging is θ ₂
θ ₁ = (θ ₀ + θ ₂ ) / 2
It is represented by

FIG. 2 shows the relationship with molten iron yield when T ₁ = 0.3 to 5.3 and T = 1.1 to 6.4 are changed and the value of T / T ₁ is varied. The survey results are shown. As shown in FIG. 2, the molten iron yield tends to decrease as T / T ₁ decreases. However, when T / T ₁ is 1.5 or more, the molten iron yield is almost 100%, and the influence of molten iron outflow is almost eliminated. I understood it.

That is, it has been found that by adjusting so as to satisfy the following formula (9), it is possible to prevent harmful effects caused by long-term excretion and bullion outflows caused by short-term exhausting.
1.5 ≦ T / T ₁ (9)

When T / T ₁ is less than 1.5, as shown in FIG. 2, the intermediate point (tilt angle is θ ₁ ) from rejection start (tilt angle θ ₀ ) to reject complete (tilt angle θ ₂ ). ) Will be larger than the tilting speed from the midpoint to the completion of evacuation, and the shearing force at the time of the slag outflow will increase, so it is considered that the molten iron flows out by the slag. .

Preferably, T / T ₁ is 13 or less. There are two cases where T / T ₁ exceeds 13: T is an extremely large value, and conversely, T ₁ is an extremely small value.

  In the former case, the molten iron can be discharged without flowing out, but by increasing the time T too much, as shown in the equation (1), the in-furnace slag is calmed down. In addition to increasing T, productivity may decrease, and conversely, the rejection rate may be decreased.

On the other hand, in the latter case where T ₁ is extremely small, increasing the converter tilting speed increases the slag outflow speed, and although the time T ₁ until the initial slag outflow becomes extremely short, the slag outflow speed Therefore, it is easy to cause molten iron outflow due to shearing force, and the molten iron yield may be lowered.

On the other hand, even if T / T ₁ satisfies the expression (9), the total cost cannot be minimized. That is, when the time T from the start of slag discharge (start of slag outflow from the furnace opening) to the end of slag discharge becomes longer, the slag formed as described above is settled in the converter and is difficult to be discharged. Not only that, even if the iron yield is improved, productivity is expected to deteriorate.

  Therefore, in order to improve the productivity by shortening the discharge time, while reducing the utility cost for operating the auxiliary materials such as auxiliary materials, converter refractory, and electric power, the appropriate discharge rate The scope was examined.

Evaluation was performed by using a converter with a molten steel amount of 370 ton and controlling the tilting of the converter so that T / T ₁ = 2.2 was constant in the third step. In cost evaluation, auxiliary materials (CaO) cost that fluctuates depending on the rejection rate in the third step, utilities such as electric power calculated from productivity that varies with the rejection time that increases with the improvement of the rejection rate, and converter fire resistance A comprehensive evaluation was made on the basic unit of goods.

  FIG. 3A shows a result in which the horizontal axis is the rejection time T of the third step and the vertical axis is the rejection rate.

  As shown in FIG. 3 (a), if the evacuation time is lengthened, the evacuation rate is improved, the auxiliary raw material basic unit is reduced, and the process proceeds in a cheaper direction.

  On the other hand, if the removal time is shortened, the removal rate will be reduced, but the overall operation cycle of the converter (from hot metal charging in the first process to steel output in the fifth process) will be shortened. , Improve productivity. Therefore, in this case, it is possible to reduce the basic unit of utilities such as electric power for converter operation.

  Furthermore, if the discharge time is shortened, the in-furnace time in the hot metal / molten steel converter can be shortened, and the refractory unit in the converter can be improved.

  FIG. 3B shows the result of arranging the total of these costs as a total cost together with the relationship with the exclusion time T for each of the three main items. As shown in FIG. 3 (b), as the total cost, when the rejection rate of the third step is 40 to 60% (exclusion time is about 2.0 minutes to 4.0 minutes), the total cost is It is considered to be a small and optimal area.

  Note that even if the unit price of utilities such as secondary raw material unit price, refractory unit price, and electric power fluctuates, the absolute value of the total cost shown in FIG. In the case of ˜60%, it was confirmed that the total cost was not small.

Here, in order to realize the tilt pattern having the above rejection rate of 40 to 60%, the values of the tilt angle θ _{0 at} which the slag starts to flow out of the furnace port and the maximum tilt angle θ ₂ at the time of discharge are Although it is necessary, an approximate value can be predicted for θ ₀ from the refractory profile in the furnace from the furnace volume during tilting, the amount of molten iron and the amount of slag in the furnace, and an accurate value. Is possible by simply grasping the actual value of the tilt angle at which the slag flows out of the furnace port.

On the other hand, for θ ₂ , it is possible to predict the tilt angle at which the molten iron flows out when tilted quasi-statically from the furnace volume during tilting and the amount of molten iron in the furnace calculated from the refractory profile in the furnace However, since the tilt angle at which the molten iron actually flows out is influenced by the immediately preceding tilt speed, it is necessary to change the tilt speed in advance to determine θ ₂ in advance.

Furthermore, θ ₀ and θ ₂ change from time to time depending on the shape of the furnace, such as the refractory condition of the refractories in the furnace, and the amount of molten iron or slag charged. Therefore, θ ₀ and θ ₂ can be predicted from these conditions. This is more desirable.

  In actual tilting, it is realistic to set a tilting pattern in advance so as to satisfy the equation (2) and perform tilting along that pattern. Further, the tilt pattern control may be a control that switches the tilt speed stepwise or a control that switches the tilt speed continuously.

  Examples using an actual converter will be described below.

  The test was conducted in a 370 t converter. After charging scrap and hot metal in the furnace, preliminarily dephosphorizing the hot metal by adding a fossilizing agent such as quick lime and silica stone according to the amount of Si in the hot metal so that a predetermined basicity and slag amount are obtained. Processed. After dephosphorization, the converter was turned over and slag was discharged from the furnace port.

  Here, the rejection rate is the proportion of slag that has been excluded from the slag produced during the preliminary dephosphorization treatment, and was determined by mass balance calculation and slag weighing based on the slag composition.

Table 1 shows test levels and test results. The converter tilting speed pattern was controlled so that T / T ₁ = 1.2 to 13.3. In Table 1, as already described, the cost is defined as 100 with the value when the elimination time is 8 minutes being defined as 100 and dimensionless.

Levels 1 to 7 are examples of the present invention in which the tilt pattern or rejection rate is within the scope of the present invention, and excellent results were obtained in both the molten iron yield and cost (unit consumption). In Level 7, T / T ₁ was 15, and the rejection rate was 60% or less, but the molten steel yield was slightly lower and the rejection time was slightly longer, about 4 minutes. In addition, the total cost was a little worse, but it was acceptable.

  On the other hand, levels 8 to 14 are comparative examples in which the tilt pattern or the rejection rate is outside the scope of the present invention.

In level 8, T / T ₁ was 1.6. However, since the rejection time T was short and the rejection rate was less than 40%, the cost of the auxiliary raw materials deteriorated and the total cost deteriorated. It was.

In Level 9, as in Level 7, T / T ₁ was 1.6, but because T was lengthened, the rejection rate exceeded 60%. As a result, the utility and refractory costs due to the long time of molten iron in the furnace deteriorated, and as a result, the total cost deteriorated.

At level 10, T / T ₁ was 2.9, but as a result of T being small and the rejection rate being below the lower limit of 40%, the cost of the auxiliary material was worse than that of the present invention example, and the total cost was also low. It got worse.

In level 11, T / T ₁ was 2.9 as in level 10, but the value of T was large, exceeding 60% of the upper limit of the rejection rate. As a result of prolonged furnace time, utility and refractory costs deteriorated, resulting in a deterioration in total costs.

Level 12 is the result of increasing the converter tilt speed in the second half of the rejection, shortening the time (T−T ₁ ), and setting it to 1.2, which is smaller than the lower limit of T / T _1. -Despite the fact that the furnace opening distance has become shorter, the molten iron yield has deteriorated to 97% due to the increase in the outflow speed at the time of discharge.

Level 13 is also set to 1.2, which is smaller than the lower limit of T / T ₁ , and further, T is reduced and the rejection rate is lower than the lower limit of 40%. have done. Furthermore, the molten iron yield has deteriorated to 97%.

Level 14 is also set to 1.2, which is smaller than the lower limit of T / T ₁ , and as a result of increasing T and the rejection rate exceeding the upper limit of 60%, the time in which the molten iron is in the furnace becomes longer, utilities and refractories The cost has deteriorated and the total cost has also deteriorated. Furthermore, the molten iron yield has deteriorated to 97%.

It is a figure which shows notionally the outflow model of the metal part in the converter during waste. Is a diagram showing the proper range of the value of T / T ₁ of the present invention. It is a figure which shows the various ranges corresponding to the appropriate range and the rejection rate of the rejection rate in this invention. (A) shows a rejection rate, (b) shows cost.

Claims (2)

The first step of charging the converter with hot metal, hot metal and scrap, or hot metal, scrap and pig iron as main raw materials,
Removal process for removing Si and removing P,
A third step of tilting the converter and rejecting the slag generated in the second step;
A fourth step of erecting the furnace and supplying oxygen from the top blowing lance to perform de-C,
A fifth step of producing the molten steel,
In the converter steelmaking method of returning to the first step after leaving the slag after de-C refining generated in the fifth step in the furnace, repeatedly performing the fifth step from the first step,
When slag is discharged in the third step, the time T from the start of slag discharge (start of slag outflow from the furnace port) to the end of slag discharge, and from the start of slag discharge to the end of slag from the furnace port time T ₁ that reaches an intermediate angle of the converter tilt angle satisfy the following expression, and, BOF steelmaking method characterized by controlling the converter tilt such slag Haikasuritsu is 40% to 60% .
1.5 ≦ T / T ₁   In the converter steelmaking method according to claim 1, operation is performed in advance by prescribing the rejection rate in the third step based on the auxiliary material cost, converter refractory cost and utility cost to be input in the converter. Converter steelmaking process characterized by

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