JPH07258716A - Method for extending service life of iron shell for converter - Google Patents

Abstract

PURPOSE:To provide an extending method of the service life of an iron shell for converter. CONSTITUTION:In the furnace body of the converter provided with the iron shell having forcedly cooling function and permanent bricks and wearing bricks in the inner surface of the iron shell, a jointing gap between the wearing bricks at the time of the construction of the furnace is made to be 0.6-1.0mm and the max. temp. of the iron shell at the center part of the furnace body during the one service life of the furnace is controlled to 300-400 deg.C to extend the service life of the iron shell for converter. By this method, the developments of heat deformation and fatigue crack of the iron shell are restrained and the service life of the iron shell can be extended.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of preventing thermal deformation and fatigue cracking in the central portion of a converter iron shell and extending the life of the iron shell.

[0002]

2. Description of the Related Art The life of a converter furnace body is deeply related to the improvement of the productivity of steel, and it is necessary to extend the life of the converter body in order to reduce the manufacturing cost.

The life of the furnace body is determined in part by the life of the brick. Although the brick itself has a short life, the life of the brick rarely determines the life of the brick because the brick is regularly replaced before the entire furnace, including the iron shell, becomes unusable. That is, when the wear bricks on the inner surface (operating surface) of the furnace are worn or dropped and the permanent (hereinafter referred to as “perm”) bricks are exposed, the operation is interrupted and the brick transshipment work is generally performed. It is rare to continue the operation until the perm bricks are significantly worn and removed, and the steel skin is damaged. One furnace bill refers to the period from this brick transshipment to the next brick transshipment.

On the other hand, the life of the iron shell is generally much longer than that of bricks, and even minor damage can be temporarily repaired. However, when damage such as deformation and fatigue cracking of the iron shell is significant, it is no longer possible to continue operating the furnace, and the life of the furnace body is determined by the life of the iron shell.

Conventionally, as a thick steel plate for a steel shell of a converter furnace body,
JIS G3103 SB42 (steel for boiler), JIS G3106 SM50 (steel for welded structure), etc. were mainly used, but in recent years, high temperature tapping has been performed in converter operation, and mag carbon (MgO ・ C)
Due to factors such as the use of materials with high thermal conductivity such as bricks as lining materials, the usage conditions for the iron shell have become strict, so the above conventional steels cannot be adequately dealt with, and at the most important parts of the furnace. Deformation and cracks occur in the iron skin of a certain central part of the furnace body, which causes problems such as a decrease in productivity and the suspension of operations.

Therefore, in order to prevent the deformation of the iron shell by improving the high temperature strength and creep strength, as a thick steel plate for the iron shell of the steelmaking converter, a high strength steel (WES 3005
-1977, PMS steel) has been invented
(See Japanese Patent Laid-Open No. 2-61034).

However, the operating conditions of converters have become more severe in recent years, and the use ratio of MgO.C bricks has increased. The maximum temperature of the iron shell in the case where the brick fell off and only the perm brick remained) is over 500-600 ℃.

In order to prevent thermal deformation of the furnace shell that is used at such a high temperature, it is not enough to increase the strength of the steel shell material, and the furnace shell is forced by water cooling or air cooling. Attempts have been made to prevent thermal deformation of the iron shell by cooling and suppressing the iron shell temperature. For example,
Japanese Patent No. 61-174311 discloses a cooling method capable of forcibly air-cooling the entire abdomen of a steelmaking converter furnace body from the outside.

The conventional knowledge and problems regarding the deformation or crack of the iron shell in terms of cooling of the iron shell and thermal stress or physical stress applied to the iron shell are as follows.

When the iron shell is not subjected to forced cooling, the yield strength and tensile strength of the iron shell material in the iron shell temperature range at the end of the brick life are low, so that the iron shell is likely to be thermally deformed. On the other hand, when the iron shell is subjected to forced cooling, the iron shell temperature is suppressed, and the iron shell material is used in a high temperature range in which the yield strength and the tensile strength are high, so that the thermal deformation of the iron shell is also suppressed.

The main stresses that occur in the iron shell are the stress due to the weight of the iron shell and the molten steel, as well as the thermal stress caused by the temperature rise of the iron shell and the temperature distribution (temperature gradient) in the iron shell. There is a stress caused by thermal expansion of a brick installed on the inner surface side of the iron skin.

Since the wear brick is in contact with molten steel and slag having a temperature of about 1600 ° C. or higher, the temperature on the inner surface (operating surface) side of the brick is always 1400 ° C. or higher. In this way, the bricks in the furnace become extremely hot during operation and thermally expand toward the outer surface of the furnace. This amount of thermal expansion is partly absorbed by the joints between the bricks, but especially the amount of thermal expansion that is not absorbed if the joint allowance between the wear bricks is too small is bound by the iron shell. That is, the thermal expansion of the brick is restrained by the iron shell, so that the iron shell receives a force to push it outward.

If the iron shell is not subjected to forced cooling, the temperature of the iron shell becomes high, so that the amount of thermal expansion of the iron shell itself toward the outer surface of the furnace is large. For this reason, the constraint rate of the thermal expansion of the brick by the iron shell is small, and the iron shell stress due to the thermal expansion of the brick is small.
On the other hand, when the iron shell is forcibly cooled, the temperature of the iron shell is suppressed, so that the thermal expansion amount of the iron shell itself is small. Therefore, the iron shell strongly restrains the thermal expansion of the brick, and a large stress is applied to the iron shell.

In the steelmaking converter, for the purpose of renewing worn bricks (reloading bricks), inspecting and repairing the furnace body, the furnace body is periodically cooled (when operation is stopped) and reheated (restarted operation). Time). The iron skin stress caused by the thermal expansion of bricks is repeatedly applied every time such operation is stopped and restarted. Therefore, although it is possible to prevent damage due to thermal deformation of the iron shell by forced cooling of the iron shell, fatigue stress occurs in the central iron shell due to repeated load of stress due to brick thermal expansion, which shortens the life of the iron shell. There are dots.

In addition, when the iron shell is excessively forcibly cooled, a remarkable temperature gradient is generated between the forced cooling portion and the non-cooling portion (the portion where the forced cooling is not applied) of the iron shell, and especially that portion is the furnace. When hitting a flange portion installed in the furnace body for the purpose of supporting the body or strengthening the furnace body, cracks occur at the welded portion of the flange portion, which causes a problem of shortening the life of the iron shell.

Since the temperature of the brick surface in contact with the molten steel is almost determined by the molten steel temperature, if the iron shell is excessively forcibly cooled, the temperature gradient in the radial direction in the brick becomes large,
Thermal stress is generated inside the brick, causing brick separation (spalling)
・ Detachment becomes remarkable.

[0017]

As described above, in the conventional furnace body iron shell, thermal cycle stress and strain are generated in the iron shell due to the thermal expansion of the brick and the contraction of the iron shell due to forced cooling. The biggest problem is that if the steel skin is forcibly cooled excessively, the steel skin will crack.

The present invention has been made to solve the above problems. An object of the present invention is to provide a method of suppressing the thermal deformation of a converter iron shell and preventing the occurrence of iron skin fatigue cracks caused by thermal expansion and excessive forced cooling of bricks, and extending the iron shell life. To do.

[0019]

The gist of the present invention resides in the following method of extending the life of a steel shell.

In a converter furnace body equipped with an iron shell having a forced cooling function and a permanent brick and a wear brick on the inner surface of the iron shell, the joint allowance between the wear bricks during the furnace construction is 0.6 to 1.0 m.
m, and the maximum temperature of the steel shell in the center of the furnace during one furnace is 300 ~
A method for extending the life of converter iron shells, which is characterized by controlling at 400 ℃.

The present invention is characterized in that the furnace construction conditions (joint allowance of bricks) and the temperature control conditions of the central furnace shell for reducing the heat cycle stress and strain of the steel shell are combined.

The definition of the iron shell of the central part of the furnace body here will be described.

FIG. 1 is a partial vertical cross-sectional view showing a structural example of a steelmaking converter iron shell. In this case, the iron shell is composed of the furnace mouth flange 1, the throttle portion 2, the furnace belly portion (straight body portion) 3, the furnace bottom portion 4, and the flange portion 5 for fixing to the trunnion, and the furnace body central portion. The iron skin is a part of the furnace belly 3.

[0024]

In the method of the present invention, the reason why the conditions are limited as described above will be described.

In order to suppress the thermal deformation of the iron shell due to the rise of the iron shell temperature and to suppress the occurrence of fatigue cracks due to the repeated loading of the iron shell stress due to the thermal expansion of bricks,
It is necessary to satisfy the condition of.

The range of the forced cooling and the appropriate range of the maximum temperature of the steel shell during one furnace cost are determined so that the excessive forced cooling of the steel shell is not performed.

[0027] By setting a proper joint allowance between wear bricks,
By absorbing the thermal expansion of bricks in the brick joint cost, the constraint of the thermal expansion of bricks due to the iron shell is reduced.

The maximum temperature of the iron shell is controlled by forced cooling from the outside of the iron shell. As the device, a large number of air nozzles are provided around the furnace body central part iron shell, and air is jetted from the nozzle and blown directly to the central part iron shell, or piping is provided in the furnace body central part iron shell, It is preferable that cooling water or steam be passed through the inside.

By carrying out forced cooling, the maximum temperature of the central part steel shell during one furnace cost is controlled within the range of 300 to 400 ° C.

For measuring the temperature, it is desirable that a thermocouple be spot-welded on the surface of the steel shell in the central portion of the central steel shell, in the vicinity of the flange, and in the vicinity of the boundary with the furnace bottom to continuously measure the temperature during operation. A radiation thermometer may be provided instead of the thermocouple, and the temperature of the entire central skin may be appropriately measured.

During operation, the maximum temperature of the central steel skin is 400
When the temperature exceeds ℃, the strength of the iron shell material is lowered, a sufficient life prolonging effect cannot be obtained, and the thermal deformation of the iron shell becomes large. On the other hand, the lower limit of the maximum temperature of the central steel shell was set to 300 ° C because if the central steel shell is excessively cooled, the central steel shell is the forced cooling section and the furnace bottom steel shell is the uncooled section. When there is a significant temperature gradient between the
This is because cracks occur at this weld and the life of the steel shell is shortened.

In order to suppress the iron shell temperature to a low temperature of less than 300 ° C., it is necessary to improve the performance of the forced cooling device, including increasing the capacity and the number of cooling nozzles. Furthermore, in this case, in order to suppress the occurrence of cracks due to the temperature gradient between the forced cooling section and the non-cooled section, it is necessary to cool the furnace bottom as well. It is also disadvantageous from a viewpoint.

If the joint allowance between the wear bricks at the time of furnace construction is less than 0.6 mm, the steel skin will be subjected to stress due to thermal expansion of the bricks, so that a sufficient life extension effect for cracking of the steel skin cannot be obtained. On the other hand, when the joint cost of the brick is increased, the stress generated in the brick or the iron shell caused by the thermal expansion of the brick is reduced, and the crack initiation life is extended. However, if the joint cost exceeds 1.0 mm, the life prolonging effect is saturated. Further, if the joint allowance is excessively increased, molten steel may enter the gaps between the wear bricks, and there is a risk that the bricks may be melted or spalled. Considering these points, the upper limit of the joint cost between the wear bricks is set to 1.0 mm.

Range of joint cost between this wear brick 0.6
~ 1.0mm is a necessary condition for both new furnace construction and brick transshipment.

[0035]

[Examples] The following examinations were conducted for a 300-ton steel-making converter having a steel skin (material: JIS G3106 SM50) structure shown in FIG.

The influence of the iron skin temperature and the brick joint cost on the fatigue crack initiation life of the iron skin was quantitatively investigated using the FEM analysis results and the creep fatigue characteristics of the iron skin material at high temperature. Here, an elasto-plastic creep analysis was performed using a three-dimensional analysis model as the FEM analysis model.

In the analysis, the thermal cycle of starting / stopping (heating / cooling) of the converter accompanying the periodical repair of the furnace body such as transshipment of bricks was considered, and the changes in the stress and strain of the iron shell under this thermal cycle were taken into consideration. I asked.

Further, using the stress and strain values evaluated by this FEM analysis and the creep fatigue characteristics of the iron skin material, the crack initiation life of the iron skin was evaluated.

The material of the wear brick was MgO.C type, the material of the perm brick was fired magnesia type, and the joints between the perm bricks were treated as empty joints. The temperature of the molten steel was 1700 ° C. Table 1 shows the main conditions of the above examination and the evaluation results.

[0040]

[Table 1]

FIG. 2 is a diagram showing the relationship between the fatigue crack initiation life of the converter iron shell, the maximum temperature of the central iron shell, and the joint allowance between the wear bricks when bricks are stacked. Here, the temperature of the central shell is shown as a typical temperature at the end of the brick life, which is the maximum temperature when the wear brick is worn and only the perm brick remains.

As for the crack initiation life in the central steel shell, the maximum temperature of the central steel shell is 600 ° C. and the brick joint cost is 0.3 m as the condition when the central steel shell is not subjected to forced cooling.
The standard value (= 1) was used for m, and the life under other conditions was shown as a ratio to this standard value.

From the results shown in FIG. 1 and Table 1, it is clear that the relationship between the fatigue crack initiation life, the maximum temperature of the central shell and the brick joint cost has the following characteristics.

When the brick joint cost is small, the effect of extending the fatigue crack initiation life is small even if the central shell temperature is suppressed by forced cooling.

The lower the temperature of the central skin is, the greater the effect of extending the life of the fatigue cracking of the steel due to the increase in the joint cost of bricks is.

The larger the brick joint cost is, the greater the effect of extending the fatigue crack initiation life of the central steel skin by forced cooling is.

Since the high temperature strength of the material for the steel shell drops sharply in the high temperature range from around 400 ° C, the maximum temperature of the steel shell is 400 ° C.
In the case of a high temperature exceeding 10 ° C., even if the fatigue crack initiation life is extended, the thermal deformation of the iron shell becomes large, and the iron shell life is determined by the thermal deformation.

When the maximum temperature of the iron shell is less than 300 ° C., the temperature difference between the forced cooling portion and the non-cooling portion becomes large, and fatigue cracks occur at this boundary portion. Since the crack generation life at this time is caused by the temperature gradient in the iron shell, even if the joint cost is increased and the stress due to the thermal expansion of the brick is reduced, the life cannot be extended.

As shown in FIG. 1 and Table 1, the maximum temperature of the central steel shell is 300 to 400 ° C. and the brick joint cost is 0.6 to 1.
When the thickness is 0 mm, not only the fatigue crack initiation life of the central steel skin can be extended, but also thermal deformation of the steel skin and fatigue crack initiation from the welded portion can be suppressed.

[0050]

EFFECTS OF THE INVENTION According to the method of the present invention, the maximum temperature of the central shell is suppressed to a proper range by forced cooling, and the joint margin set value between the wear bricks during furnace construction is optimized. The following effects can be obtained.

Prevents thermal deformation of the central skin.

It is possible to prevent the central portion of the iron shell that has been subjected to forced cooling from excessively restraining the thermal expansion of the brick in the furnace, and suppress the occurrence of fatigue cracking of the iron shell due to the thermal expansion of the brick.

Fatigue cracking is prevented from occurring at the boundary between the cooled and uncooled parts of the iron shell due to overcooling of the iron shell.

Therefore, the life of the iron shell can be extended, which can contribute to stable operation of the furnace and improvement of productivity.

[Brief description of drawings]

FIG. 1 is a partial vertical cross-sectional view showing a structural example of a steelmaking converter iron shell.

FIG. 2 is a diagram showing the relationship between the crack initiation life, the maximum temperature of the skin and the joint cost of the brick in the steel skin of the central part of the steelmaking converter.

[Explanation of symbols]

1: Furnace mouth flange, 2: Throttling part, 3: Furnace belly part (central part steel skin), 4: Furnace bottom part, 5: Flange part

Claims (1)

[Claims] 1. A converter furnace body having an iron skin having a forced cooling function and permanent bricks and wear bricks on the inner surface of the iron skin, and the joint allowance between the wear bricks at the time of furnace construction is 0.6 to 1.0 m.
m, and the maximum temperature of the steel shell in the center of the furnace during one furnace is 300 ~
A method for extending the life of converter iron shells, which is characterized by controlling at 400 ℃.