JPH08224648A - Method for restraining surface defect in continuous casting - Google Patents

Abstract

PURPOSE: To obtain a method for restraining surface defect in a continuous casting, in which estimation accuracy of the surface quality of a cast slab is stably kept by obtaining the depth in the range having steep temp. gradient just below a meniscus and the molten steel temp. in the stable temp. range at the lower part thereof. CONSTITUTION: A temp. measuring probe 2 embedded with sheath thermocouples 1 at a prescribed pitch is dipped into the molten steel 5 in a mold 4 during casting for a prescribed time with an elevating device 3 and the molten steel temperatures are measured several times simultaneously at 5-10 points in the depth direction at the initial stage, middle stage and end stage of the casting. The measured temp. data are inputted into a process computer and the depth (d) of the range of the steep temp. gradient and the molten steel temp. TM in the stable temp. range are instantaneously obtd. When the depth (d) of the range of the steep temp. gradient in the mold 4 and the molten steel temp. TM in the stable temp. range are out of the safety range of the allowable surface quality of the cast slab, at least one condition among the charging quantity of powder, the impressed current for electromagnetic stirring and the depth of an immersion nozzle 6 is varied, and therefor, the surface quality of a cast slab is stably kept, and the man-hour for surface cleaning can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for suppressing surface defects in continuous casting of steel for maintaining stable surface quality of the slab.

[0002]

2. Description of the Related Art In continuous casting of steel, when a phenomenon such as a solidified shell produced in the initial solidification process in the mold sticks to the inner surface of the mold or inclusions are caught in the solidified shell, the solidified shell is directly under the mold. At this point, the solidified shell breaks and a breakout occurs in which molten steel flows out. In addition, when the powder used as the lubricant flows in unevenly in the mold, various defects occur on the surface of the solidified shell. For example, such a casting defect requires a long time to recover when a breakout occurs, which significantly lowers productivity.
In addition, when a surface defect occurs, the surface quality of the final product is impaired, and therefore the surface of the semi-finished product which is an intermediate stage is often cared for and ground. At this time, it is common to inspect the surface texture of semi-finished products and, if there is a defect, to clean the surface, but since the inspection must be performed for all products, lead time is extended and semi-finished products are inventoried. It will be.

In particular, recently, the continuous casting speed has been increased and the continuous casting and the rolling process have been directly connected. However, the occurrence of the casting defects has been a major obstacle to the implementation thereof. . Therefore, many techniques have been proposed in the past for early prediction or detection of the casting defect. For example, a method of embedding a thermocouple in the wall surface of the mold and predicting when the temperature detected by the thermocouple once rises and then falls below the average temperature in the normal state as a breakout (Japanese Patent Laid-Open No. 57-152356). ), A method of detecting temperatures on respective surfaces of the mold, comparing them with each other, and detecting a prior phenomenon of breakout occurrence by using the temperature difference as an index (JP-A-55-84259), embedded in the mold. A method for detecting a phenomenon in which a large inclusion is entrained in the surface of the solidified shell because the temperature detected by the thermocouple has dropped sharply below the average temperature (Japanese Patent Laid-Open No. 57-1).
No. 15960), a method of detecting an abnormality of a solidified shell by obtaining a time change rate from the temperature detected by a thermocouple embedded in a mold and comparing the time change rate with a value in a predetermined range. Japanese Unexamined Patent Publication No. 57-115962) has been proposed.

However, in any of the casting defect detection methods based on the above-mentioned conventional techniques, the absolute value of the temperature detected by the temperature detecting end such as a thermocouple is used as it is, and the absolute value of the temperature detected at one location in the casting direction is used. Based on the value, the average temperature in the steady state or the temperature of the opposite wall surface is compared, or the rate of increase or the rate of decrease thereof is compared with a predetermined target range. However, when the casting defect occurs, the temperature rises and falls or their variation per unit time greatly varies depending on the type of casting defect and the situation at that time, and in the extreme case, it is the same casting defect. However, the temperature change pattern is usually large and varies. Therefore, the feature recognition of the temperature change pattern when a casting defect occurs becomes complicated, and it cannot be expected that the casting defect can be accurately detected.

As a method of eliminating the above-mentioned drawbacks of the prior art, in a method of detecting a casting defect from a temperature transition pattern by a thermocouple embedded in a mold, a time-series temperature detection value at the time of occurrence of a casting defect in the past is Fourier-transformed. , A coefficient between casting defects is set from the correlation between each term coefficient and the casting defect occurrence status, and then each term coefficient is obtained by Fourier transforming the temperature detection value actually measured during continuous casting. A method of judging that an abnormality has occurred when the coefficient is within the coefficient between the occurrences of the casting defects (JP-A-61-200453),
The mold temperature and the slab surface temperature immediately below the mold are measured with time at a plurality of points in the width direction, and the temperature measurement values are used, when the mold temperature is lower than the first set value, or the mold temperature and the When the difference between the mold temperature measured before the mold temperature and the mold temperature is larger than the second set value, the slab surface temperature at the mold temperature measurement width direction position is locally higher than other parts, and the temperature difference is When the slab surface temperature is higher than the fourth setting value, or a part of the surface temperature is higher than the other part, and the temperature difference is higher than the fifth setting value. There has been proposed a method (Japanese Unexamined Patent Publication No. 1-210160) for predicting the occurrence of vertical cracking flaws or vertical cracking breakouts when the mold temperatures are all lower than the sixth set value.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The method disclosed in Japanese Patent No. 0453 discloses detecting casting defects from a temperature transition pattern by a thermocouple embedded in a mold, and is intended to detect casting defects such as breakouts and surface defects due to non-uniform inflow of powder. Since it does not quantitatively measure and estimate the initial solidification phenomenon in the
Although the breakout can be detected, it has a drawback that defective slabs are overlooked, or slabs with good quality are judged as poor quality, and unnecessary surface maintenance is performed. Further, the method disclosed in Japanese Unexamined Patent Publication No. 1-210160 targets vertical cracks or breaks that occur on the surface of the slab, and it is impossible to detect surface defects such as oscillation marks and pinholes. It is possible.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a method of suppressing surface defects in continuous casting which can stably maintain the estimation accuracy of the surface quality of the cast pieces in continuous casting.

[0008]

[Means for Solving the Problems] The inventors of the present invention have conducted extensive studies to improve the accuracy of estimation of the surface quality of a slab to achieve the above object. As a result, the molten steel temperature distribution in the depth direction in the mold is stable at ± 2 ° C. in the deep layer away from the meniscus, as shown in FIG. 6, but there is a sharp temperature gradient region just below the meniscus. Exists. _Assuming that the thickness of the region where this temperature gradient is abrupt is d (mm) and the molten steel temperature in the temperature stable region of the deep layer is T _M (° C), the surface quality of the slab is determined by the initial solidification state (meniscus) in the mold. It is known that it is greatly affected by solidification in a region of about 5 mm immediately below, but the correlation between the thickness d of the region where the temperature gradient is abrupt and the molten steel temperature T _M in the temperature stable region and the slab surface quality is investigated. As a result, it was as shown in FIG. 7.

As shown in FIG. 7, the higher the molten steel temperature T _M in the temperature stable region and the smaller the thickness d in the region where the temperature gradient is abrupt, the better the surface quality of the slab tends to be. This is because the molten steel temperature T _M in the temperature stable region is increased to promote the slag formation of the mold powder that should flow into the gap between the cast piece and the mold, and to obtain a uniform initial solidification due to a uniform inflow. It is considered that, since the thickness d of the region where the temperature gradient is abrupt becomes small, the temperature immediately below the meniscus becomes stable at a high level, and uniform growth of the initial solidified shell can be obtained. On the other hand, regardless of the molten steel temperature T _M in the temperature stable region, when the thickness d of the region where the temperature gradient is abrupt is 2 mm or less, the surface quality defect of the slab increases.
The small thickness d of 2 mm or less in the region where the temperature gradient is abruptly means that the molten steel flow velocity immediately below the meniscus is high. It is presumed that the surface quality is poor due to the occurrence. Therefore, the surface quality of the cast slab is controlled by controlling the thickness d of the region having a sharp temperature gradient immediately below the meniscus and the molten steel temperature T _M in the temperature stable region of the deep layer to shift to the surface quality stable region of the cast slab. The present invention has been achieved by investigating that the above can be stably maintained.

That is, according to the first invention of the present application, in continuous casting of molten steel, the temperature of the molten steel in the mold is 2 to 3 m in the depth direction.
Simultaneously measuring 5 to 10 points at m intervals, grasping the temperature distribution and the absolute temperature to determine the depth d of the region with a steep temperature gradient directly below the meniscus, and the molten steel temperature T _M in the temperature stable region therebelow. determined, the molten steel temperature T _M of the depth d and the temperature stability range of sharp regions of the temperature gradient, and the molten steel temperature T _M and slab surface quality of the depth d and the temperature stability range of rapid areas of temperature gradients Suppression of surface defects in continuous casting characterized by changing at least one of powder input amount, electromagnetic stirring applied current and immersion nozzle depth in order to shift to a safe area of slab surface quality determined from the relationship Is the way.

In the second invention of the present application, in continuous casting of molten steel, the molten steel temperature in the mold is simultaneously measured at 5 to 10 points in the depth direction, and the temperature distribution and absolute temperature are grasped to determine the temperature immediately below the meniscus. The depth d of the steep gradient region is obtained, and the molten steel temperature T _M in the temperature stable region below is obtained,
The molten steel temperature T _M of the depth d and the temperature stability range of sharp regions of the temperature gradient, the relationship between the molten steel temperature T _M and slab surface quality of the depth d and the temperature stability range of rapid areas of temperature gradients At least one of the powder input amount, electromagnetic stirring, and immersion nozzle depth was changed in order to shift to the required safety area of the surface quality of the cast piece. It is a method for suppressing surface defects in continuous casting, which is characterized by passing through steps.

[0012]

In the present invention, the depth d of the region where the temperature gradient is abrupt in the mold and the molten steel temperature T _{M in} the temperature stable region are measured as shown in FIG.
As shown in, the temperature measuring probe 2 in which the sheath thermocouple 1 is embedded in the temperature measuring paper tube at a pitch of 2 to 3 mm is immersed in the molten steel 5 in the mold 4 for about 1 minute during casting by the elevating device 3, and in the initial stage of casting, Measured several times during the middle and final stages, input the temperature measurement data of each sheath thermocouple 1 of the temperature measurement probe 2 to the process computer (not shown), and instantly the depth d of the steep temperature gradient region and the molten steel in the temperature stable region. The temperature T _M is obtained. In this case, the temperature measuring probe 2 is, as shown in FIG.
At least the distance W from the wall surface is 15 to 20 mm toward the center so that the wall surface of the mold 4 is not affected, and the immersion depth H is
Is about 20 mm, and the distance L from the immersion nozzle 6 is 300 to
Install at 400 mm. In addition, 7 is a mold powder.

When the measured depth d of the region where the temperature gradient is abrupt and the molten steel temperature T _{M in the} temperature stable region are outside the safe region of the slab surface quality, the safe region of the slab surface quality is measured. It is possible to improve the surface quality of the cast slab by shifting to. The relationship between the mold powder powder layer thickness and the depth d of the region where the temperature gradient in the mold is abrupt is as shown in FIG. 2, and a large amount of powder is injected into the mold to increase the mold powder powder layer thickness. By strengthening the heat retention of the molten steel surface, the depth d of the region where the temperature gradient is abrupt in the mold can be reduced. The relationship between the current (A) applied to the electromagnetic stirring device in the mold (M-EMS) and the depth d of the region where the temperature gradient in the mold is steep is as shown in FIG. The flow of current just below the surface of the molten steel is promoted by the application of a current to the molten steel, and the depth d of the region in the mold where the temperature gradient is abrupt becomes small. Furthermore, the relationship between the immersion nozzle depth and the molten steel temperature T _{M in the} temperature stable region and the molten steel temperature in the tundish is as shown in FIG. 4, and the deeper the immersion nozzle depth, the lower the molten steel temperature T _{M in the} temperature stable region. It is considered that this is because the discharge hole of the immersion nozzle becomes far from the molten steel surface.

As described above, the depth d of the region where the temperature gradient is steep in the mold can be controlled by changing the amount of the powder charged into the mold and / or the current applied to the electromagnetic stirrer. The molten steel temperature T _{M in the} stable region can be controlled by changing the immersion nozzle depth. Therefore, the depth d of the region where the temperature gradient is abruptly measured in FIG. 1 or the molten steel temperature T in the temperature stable region is measured.
_{When M} is out of the safe area of the slab surface quality, the depth d of the area where the temperature gradient is sharp can be changed by changing the amount of powder input into the mold and / or the current applied to the electromagnetic stirrer. Further, by changing the depth of the immersion nozzle, the molten steel temperature T _M in the temperature stable region can be put in the safe region of the surface quality of the slab.

In the first invention of the present application, the molten steel temperature in the mold is measured simultaneously at 5 to 10 points at intervals of 2 to 3 mm in the depth direction, and the temperature distribution and absolute temperature are grasped to determine the temperature gradient immediately below the meniscus. The depth d of the abrupt region is obtained, the molten steel temperature T _M in the lower temperature stable region is obtained, and the depth d of the abrupt region of the temperature gradient and the molten steel temperature T _M of the temperature stable region are calculated as follows. In order to shift to the safe area of the slab surface quality, which is obtained from the relationship between the depth d of the abrupt area, the molten steel temperature T _{M in the} temperature stable area, and the slab surface quality, the powder input amount,
By changing at least one of the applied current of the electromagnetic stirring and the depth of the immersion nozzle, the surface quality of the slab can be stably maintained, and the man-hour for surface maintenance can be significantly reduced.

Further, in the second invention of the present application, the molten steel temperature in the mold is simultaneously measured at 5 to 10 points at intervals of 2 to 3 mm in the depth direction, the temperature distribution and absolute temperature are grasped, and the temperature immediately below the meniscus is measured. The depth d of the steep gradient region is obtained, and the molten steel temperature T _M in the temperature stable region below is obtained,
The molten steel temperature T _M of the depth d and the temperature stability range of sharp regions of the temperature gradient, the relationship between the molten steel temperature T _M and slab surface quality of the depth d and the temperature stability range of rapid areas of temperature gradients At least one of the powder input amount, the applied current of electromagnetic stirring, and the immersion nozzle depth was changed in order to move to the safe area of the required slab surface quality. By passing through the surface care step, the surface quality of the slab can be stably maintained, the number of surface care steps can be significantly reduced, and the occurrence of surface defects in the semi-finished product or the final product can be prevented.

[0017]

[Example] Stainless steel S melted in a converter and a vacuum furnace
Using molten steel of US304, a vertical slab continuous casting machine equipped with an electromagnetic stirrer, a thickness of 206 mm, and a width of 1000 to 1
As shown in Table 1, when continuously casting a CaO—SiO ₂ powder on a 250 mm slab at a casting speed of 0.5 to 0.9 m / min, as shown in Table 1, the applied current to the electromagnetic stirrer and the mold powder powder Casting was performed with the layer thickness and the immersion nozzle depth being constant, and the depth d of the region where the temperature gradient was abrupt in the mold and the molten steel temperature T _M in the temperature stable region were measured at 2 mm intervals, and the temperature gradient in the mold was abrupt. Of the surface area of the slab that could not be cast in the safe area of the slab surface quality obtained from the relationship between the depth d of various areas and the molten steel temperature T _{M in the} stable temperature area and the surface quality of the slab. The depth d of the steep temperature gradient region and the molten steel temperature T _M in the temperature stable region are set to 2 m.
In order to make a transition to a safe region of the slab surface quality, which is obtained from the relationship between the depth d of the region where the temperature gradient is abrupt in the mold and the molten steel temperature T _{M in the} temperature stable region and the slab surface quality, measured at m intervals. As shown in Table 1, at least one of the powder charging amount, the applied current of electromagnetic stirring, and the immersion nozzle depth was changed, and as a result, the slab that could not be cast in the safe area of the slab surface quality was surface-treated. For each of the methods of the present invention, the slab care ratio and the defect occurrence rate of the final product were investigated. The result is shown in FIG. 5 with one week average as one plot.

[0018]

[Table 1]

As shown in FIG. 5, by applying the method of the present invention, the accuracy of estimation of the surface quality of the slab is improved, and with the minimum necessary surface care of the slab, the surface defects of the product are reduced to about half or less of those of the conventional method. I was able to suppress it to a low level.

[0020]

As described above, according to the method of the present invention, it is possible to stably maintain the estimation accuracy of the surface quality of the slab in continuous casting, and the surface defects of the product can be significantly reduced by the minimum required slab surface care. It can be reduced.

[Brief description of drawings]

FIG. 1 is an explanatory view of a measurement method of a depth d of a region having a sharp temperature gradient in a mold and a molten steel temperature T _{M in} a temperature stable region,
(A) is a side sectional view, and (B) is a plan view.

FIG. 2 is a graph showing the relationship between the mold powder powder layer thickness and the depth d of a region where the temperature gradient in the mold is sharp.

FIG. 3 is a graph showing the relationship between the current applied to an electromagnetic stirrer (M-EMS) and the depth d of a region having a sharp temperature gradient in the mold.

FIG. 4 is a graph showing the relationship between the molten steel temperature in the tundish, the immersion nozzle depth, and the molten steel temperature T _{M in the} temperature stable region.

FIG. 5 is a graph showing a relationship between a slab care ratio and a product defect occurrence rate in Examples.

FIG. 6 is a state explanatory view of a molten steel temperature in a mold, a measurement depth, a depth d in a region where a temperature gradient is abrupt and a molten steel temperature T _{M in} a temperature stable region.

FIG. 7 is a graph showing the relationship between the molten steel temperature T _M in the temperature stable region in the mold, the depth d of the region where the temperature gradient is sharp, and the surface quality of the slab.

[Explanation of symbols]

 1 Sheath Thermocouple 2 Temperature Measuring Probe 3 Lifting Device 4 Mold 5 Molten Steel 6 Immersion Nozzle 7 Mold Powder

Claims (2)

[Claims] 1. In continuous casting of molten steel, the molten steel temperature in the mold is simultaneously measured at 5 to 10 points in the depth direction, and the temperature distribution and absolute temperature are grasped to determine the depth of a region with a steep temperature gradient immediately below the meniscus. The molten steel temperature T _M in the temperature stable region below the temperature d is obtained, and the depth d of the region where the temperature gradient is abrupt and the molten steel temperature T _M in the temperature stable region are calculated as the depth of the region where the temperature gradient is abrupt. At least one of the powder input amount, electromagnetic stirring, and immersion nozzle depth in order to shift to a safe area of the slab surface quality obtained from the relationship between the surface d, the molten steel temperature T _{M in the} temperature stable range, and the slab surface quality. A method for suppressing surface defects in continuous casting, characterized in that: 2. In continuous casting of molten steel, the molten steel temperature in the mold is simultaneously measured at 5 to 10 points in the depth direction, and the temperature distribution and absolute temperature are grasped to determine the depth of a steep temperature gradient region immediately below the meniscus. The molten steel temperature T _M in the temperature stable region below the temperature d is obtained, and the depth d of the region where the temperature gradient is abrupt and the molten steel temperature T _M in the temperature stable region are calculated as the depth of the region where the temperature gradient is abrupt. At least one of the powder input amount, electromagnetic stirring, and immersion nozzle depth in order to shift to a safe area of the slab surface quality obtained from the relationship between the surface d, the molten steel temperature T _{M in the} temperature stable range, and the slab surface quality. The method for suppressing surface defects in continuous casting is characterized in that the slab that cannot be cast in the quality and safety area as a result is subjected to a surface maintenance process.