JPH0673732B2 - Continuous casting method for steel - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a continuous casting method for steel, and more specifically, to thermocouples provided at symmetrical positions with respect to a dipping nozzle arranged in a mold copper plate and a mold copper plate. The molten steel drift in the mold is detected based on the temperature difference detected by the thermocouple buried in the depth direction and the heat flux difference obtained by the temperature difference of the latter thermocouple, and this drift is detected by the copper plate of the mold. The present invention relates to a continuous casting method for steel in which the quality of a slab is stabilized by controlling the fluctuation of the molten metal surface by controlling the movement of the short side of the steel.

2. Description of the Related Art The continuous casting method for steel is capable of omitting the steps of soaking and decomposing from the steps of melting, ingot, cauterizing, decomposing, heating and rolling, which are conventional manufacturing processes of rolled steel, It has been widely adopted as an alternative to the conventional rolling steel manufacturing method because it can increase the yield.

In continuous casting of steel, non-metallic inclusions are contained in molten steel, and these non-metallic inclusions are brought into the inside of the slab by the injection flow of the molten steel flow, and most of them float above the molten metal surface. However, the remaining part is captured as it is in the slab.

It is known that the amount of non-metallic inclusions trapped varies greatly depending on the molten steel flow in the cast during casting, and the molten steel flow discharged from the immersion nozzle should be large and deep over a wide range. Tends to increase.

Therefore, in the slab continuous casting machine, the dipping nozzle is arranged in the center of the mold copper plate, its discharge hole is directed to the short side of the mold copper plate, and the molten steel flow discharged from the discharge hole flows while flowing through the stored molten steel. The velocity is reduced and it becomes a reversal flow due to the collision with the short side wall surface of the mold copper plate, one reversal flow is an upward flow toward the molten metal surface side, and the other is a downward flow direction toward the downside. The ascending flow does not involve the flux on the surface of the molten metal, and the descending flow does not reach deeply into the slab, so that casting quality is improved.

However, when fluctuations occur in the molten steel flow that descends the immersion nozzle due to the throttle opening of the sliding nozzle of the immersion nozzle, the casting speed, or when non-metallic inclusions such as alumina adhere to the inner wall of the immersion nozzle. The flow of molten steel from any one of the discharge holes becomes strong, and so-called non-uniform flow occurs. When this uneven flow occurs, as a result of the molten steel flow in the mold, the side that has generated a strong flow has a stronger upflow or downflow, resulting in flux entrainment or deterioration of internal quality due to the downflow reaching deep inside the cast piece. Occurs.

Also, if the cause of drift in casting is clogging of the discharge nozzle discharge hole, there is no countermeasure, and if the degree is almost blocked, replace the immersion nozzle or stop casting. However, there are problems such as a decrease in yield and a decrease in productivity.

Conventionally, as a method for measuring the occurrence of molten steel drift in the mold, (1) "Method for measuring casting flow during continuous casting" disclosed in JP-A-59-104512 (2) JP-A-55-149017. "Method of measuring behavior of molten steel surface in mold" disclosed in the publication,
(3) Only the "method of detecting molten steel drift in a mold during continuous casting" disclosed in JP-A-62-197255 is proposed.

(1) is a method of inserting a pressure receiving body into the molten metal in the mold, detecting the pressure of the molten metal fluid by this pressure receiving body, and grasping the state of the pouring flow. The detection sensor is supported by a follow-up drive unit that is driven to follow while maintaining a constant distance between the molten metal surface and the molten metal surface detection sensor, detects vibration of the molten metal surface, and detects entrainment of flux and the like. ) Is a method of detecting a drift by detecting the amount of rising of the molten steel induced by the collision with the short side accompanying the flow from the immersion nozzle.

However, in the technique (1), it is necessary that the pressure receiving body to be used has durability such that it can be immersed in the molten metal during casting, and it is extremely difficult to carry out because the molten steel has a high temperature. However, there is a problem that floating inclusions are attached to the immersed pressure receiving body, and the inside of the slab is also contaminated due to separation of the inclusions.

Further, since the technique of (2) simply detects the vibration of the molten steel surface in the mold, it detects outside of problems such as the influence of the inert gas blown to prevent the adhesion of alumina to the inner wall of the immersion nozzle and the level of the molten metal. The driving method of the sensor becomes a problem in implementation, and there is a problem that a highly reliable means in a high temperature environment is required.

Further, in the technique (3), since the amount of rising of the molten steel differs depending on the difference in the casting width of the mold, there is a problem in that the accuracy of detecting the drift is low.

In short, as described above, in the conventional example, a method of detecting a drift in the mold during continuous casting has been proposed, but all of these are difficult to carry out, or the drift detection accuracy is low. . Further, many surface cracks of the slab obtained by the conventional continuous casting method occur, the product quality is inferior, and
It has low yield and low productivity. For this reason, there has been no proposal at all for a method for accurately detecting a drift in the mold during continuous casting and a method for continuous casting of steel.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention The present invention proposes a continuous casting method for steel, which aims to solve these problems.

That is, when the molten steel drifts during casting, the molten steel discharge amount to both short side surfaces of the mold copper plate with the immersion nozzle as a boundary is different. For this reason, heat supply to the meniscus portion is insufficient near the short side on the side where the amount of molten steel discharged is small, and deckles (non-sedimentation lumps) are likely to occur, which may lead to trapping or trapping powder in the slab. On the other hand, on the side where the amount of molten steel discharged is large, the fluctuation of the meniscus surface becomes large, and the inflow of powder in the slab tends to become uneven. The surface of the slab tends to become rough. Also, when the drift becomes large, the side with a large amount of molten steel discharged causes solidification delay on the short side or remelting of the solidified shell, leading to breakout.

It is an object of the present invention to provide a continuous casting method for steel that solves these problems.

Means for Solving the Problem and Its Action That is, the method of the present invention, the molten steel discharge holes are arranged in the center of the mold with the immersion nozzles respectively formed toward the short side of the mold, and the immersion nozzle is the boundary. The mold is divided into left and right parts, and at least two thermocouples are installed in the long and short sides of the mold copper plate on each of the left and right parts so that they are symmetrical under the molten steel meniscus. While detecting the temperature difference, at least two thermocouples are embedded in the depth direction of the long side and the short side, and the heat flux difference obtained from the temperature difference detected between these thermocouples is obtained to obtain the temperature difference or When the heat flux difference becomes larger than a certain value, both short sides are moved toward the part corresponding to the larger one to reduce the temperature difference or heat flux difference between the left and right parts.

Therefore, the structure and operation of these means will be described more specifically as follows.

First, by discharging molten steel from the discharge hole at the tip of the central immersion nozzle to the short side of the mold, when pouring the molten steel in the continuous casting mold, if drift occurs, a single boiling phenomenon occurs in the molten steel, A difference in temperature and a difference in heat flux between the copper plates occur in the copper mold plates on the left and right sides of the immersion nozzle.

Accurately measure such temperature difference or heat flux difference,
When uneven flow occurs, by adjusting the mold width of the mold to adjust, it is possible to suppress uneven flow and fluctuation of the molten metal surface.If this uneven flow occurs due to clogging of the discharge nozzle of the immersion nozzle, casting can be performed without replacing the immersion nozzle, Suppression of fluctuations in the molten metal surface can also be achieved with a simple device or operation.

Further, the present invention will be described in detail with reference to the drawings.
It is as follows.

1 is a perspective view showing a thermocouple embedded state in the copper plate of the mold, FIG. 2 is a developed view of the copper plate of FIG. 1, and FIG.
The drawing is a cross-sectional view taken along the line AA of FIG. 2, FIG. 4 is a layout drawing of one embodiment of an apparatus used for carrying out the method of the present invention, and FIG. FIG. 6 is an explanatory diagram when the short side is moved by the method of the present invention when uneven flow occurs, and FIG. 6 is a graph showing the occurrence of surface cracks in a cast piece cast by the method of the present invention in comparison with the conventional method.

Reference numeral 1 is the long side surface of the mold copper plate, 2 is the short side surface of the mold copper plate, 3
Is a thermocouple, 4 is a dipping nozzle, 5 is a meniscus, and
6 is a discharge flow, 7 is a temperature converter, 8 is a heat flux calculator, 9 is a heat flux difference monitoring device, 10 is a temperature difference monitoring device, 11 is an alarm device (voice announcement), 12 is a mold width changing device, 13 Indicates a nozzle deposit, x indicates a thermocouple embedding position, and a and b indicate the distance from the surface of the copper mold plate.

First, as shown in FIG. 1, FIG. 2 and FIG. 3, an immersion nozzle 4 is arranged in the center of the mold, and the tip of this immersion nozzle 4 faces the mold short sides 2 and 2 and Discharge holes are formed. In a rectangular parallelepiped mold composed of copper plates having long sides 1 and short sides 2 of a copper plate for a mold, a dipping nozzle 4 at the center thereof
A plurality of thermocouples 3 are embedded in the long side 1 and the short side 2 in each of the left and right portions with the boundary as a boundary.

That is, within the long sides 1 and 1 and the short sides 2 and 2, at a depth of 5 mm to 20 mm from the inner surface thereof, and moreover, in the vertical and horizontal directions of the long sides 1 and the short sides 2,
A plurality of thermocouples 3 are arranged symmetrically with respect to the immersion nozzle 4, for example, at the positions indicated by crosses in FIGS. 1 and 2.
To bury.

Then, the temperature of these thermocouples 3 is continuously monitored during casting, and at the same time, the temperature of the copper plates 1 and 2 is reduced by 2
Points, eg amm and bmm from the surface of copper plates 1 and 2 in Fig. 3
Heat flux Q due to the temperature difference (ΔT) of the temperature of the thermocouple at the position
For monitoring.

The heat flux Q is calculated by the following equation.

Q = λΔT / d where λ is the thermal conductivity Therefore, regarding the temperature and heat flux of each thermocouple 3 that is continuously monitored in this way, the left and right parts separated by the immersion nozzle 4 as a boundary, that is, both short sides The left and right parts divided into one and one direction are compared to grasp the magnitude of the molten steel discharge amount from the immersion nozzle 4 on the left side and the right side. At this time, it is judged that the molten steel discharge amount is large on the side where the copper plate temperature is high and the heat flux is large.

It should be noted that the comparison of the temperature and the heat flux is performed with the thermocouple 3 which is symmetrically arranged at each of the left and right parts with the immersion nozzle 4 as a boundary. For example, in the thermocouple 3 of the long side face 1, for example, comparison is made between the thermocouples 3 of c and d in FIG. 1, and the temperature of each thermocouple 3 is measured between the opposing short side faces 2 of the left side and the right side. Total and compare.

As a result, when the temperature difference or the heat flux difference of the thermocouple 3 exceeds a certain limit value (a value that differs depending on the steel type and the casting speed), for example, the nozzle deposits are separated in the left and right parts separated by the immersion nozzle 4 in the mold. It can be seen that the uneven flow 14 (see FIG. 5) to one side is generated by the generation of 13 (see FIG. 5).

Further, as shown in FIG. 5, both short sides 2 and 2 of the mold are configured to be movable along the flow direction of the long sides 1 and 1 by, for example, cylinders 21 and 21. Therefore, as shown in FIG. 5, when the drift 14 is generated, both short sides 2,
2 is moved in the same direction in the direction of the arrow at the same speed (the direction of drift 14). When moved in this way, the temperature difference or heat flux difference becomes less than or equal to the limit value.

When the short sides 2 and 2 are moved and controlled in this manner, the discharge amount per unit volume of molten steel on both the left and right sides of the immersion nozzle 4 in the mold becomes substantially equal, and the solidification delay on the short side is prevented. And the meniscus surface fluctuation due to drift 14
15. It is possible to prevent the generation of unprecipitated lumps.

As described above, when casting by the method of the present invention,
According to the device shown in the figure, continuous casting can be performed while automatically monitoring.

That is, as described above, the thermocouples 3 are arranged in the long sides 1 and the short sides 2 of each copper plate of the mold, and the thermoelectromotive force from these thermocouples 3 is converted into temperature by the temperature converter 7. The temperature difference between the symmetrical thermocouples of these temperatures is converted into a heat flux heat flux by the heat flux calculator 8. The heat flux is monitored by the heat flux difference monitoring device 9 in comparison with a predetermined limit value.

On the other hand, the temperature from the temperature converter 7 enters a temperature difference monitoring device 10 as a temperature difference between symmetrical thermocouples, and is compared with a predetermined limit value to be monitored.

As a result, when either of them exceeds the respective limit value and indicates an abnormality, an alarm is issued by the alarm device 11, and a signal is immediately sent to the mold width changing device 12 to adjust the mold width.

Therefore, by using such a device, the temperature and heat flux of each thermocouple can be constantly monitored during casting, and in this case, the temperature difference between the thermocouples located symmetrically with respect to the immersion nozzle 4 is used. And the heat flux difference can be constantly monitored. When the temperature difference or heat flux difference exceeds a certain limit value (constant value), the alarm device 11 issues an alarm and the mold width changing device 12 is instructed to change the mold width, resulting in a drift. The short sides 2 and 2 on both the left and right sides are simultaneously moved in the same direction at the same speed, and when the temperature difference or the heat flux difference becomes less than a certain threshold value, the mold width change is finished.

If the above operations are performed as necessary, a slab of stable quality can always be obtained.

Example An experiment was conducted with a slab continuous casting machine using the apparatus shown in FIG. As shown in Fig. 1, the thermocouple in the copper plate of the mold is 200 mm pitch in the width direction of the long side, 200 mm and 300 mm from the top of the mold in the height direction, 5 mm and 15 mm from the mold surface in the depth direction,
The short side of the mold is 200mm from the top of the mold in the center height direction.
And 300 mm, and 5 mm and 15 mm in the depth direction. During casting, the temperature difference between the thermocouples on both short side surfaces was monitored, and the temperature difference was about 50 ° C at the time when boiling on the molten metal surface in the mold (due to uneven flow) occurred. At this point, both short sides were simultaneously moved at the same speed for 10 mm in the direction in which the surface of the molten metal was boiling (the direction in which it drifted). As a result, the temperature difference on the short side was reduced to about 30 ° C, and the visible boiling on the surface of the molten metal was also calmed down.

Steel was continuously cast as described above, and the obtained slab was visually inspected for surface cracking. As a result, as shown in FIG. It can be seen that the occurrence of surface cracks such as corner cracks is decreasing.

Incidentally, FIG. 6 shows the surface crack index of the product of the present invention when the surface crack of the conventional product is 1.

<Effects of the Invention> As described above, according to the method of the present invention, the mold is divided into the left and right with the immersion nozzle as a boundary, and the temperature difference is obtained between the thermocouples symmetrically arranged at each of the left and right parts, and The heat flux difference is calculated from the temperature difference between the thermocouples provided in the depth direction from the surface of the side or the short side, and when the temperature difference or the heat flux difference exceeds a certain limit value, the opposite short sides are By moving in a direction that exceeds the limit value, the uneven flow in each of the left and right parts separated by the immersion nozzle is softened, and the temperature difference or heat flux difference is reduced to a certain value or less.

Therefore, the eccentricity, which could not be detected with high precision in the conventional continuous casting, can be detected promptly, and the countermeasure against this eccentric flow can be taken promptly.

Therefore, fluctuations in the molten metal surface due to uneven flow can be suppressed, and slab surface cracks such as vertical cracks, corner cracks, and corner cracks in the slab can be significantly reduced, and high quality products can be efficiently obtained.

Further, delay of solidification on the short side due to uneven flow or breakout due to remelting of the solidified shell can be suppressed, and stable high-speed casting can be performed.

[Brief description of drawings]

FIG. 1 is a perspective view showing a thermocouple embedded state in a mold copper plate, FIG. 2 is a developed view of the copper plate of FIG. 1, and FIG. 3 is an arrow A of FIG.
FIG. 4 is a cross-sectional view taken along the line A, FIG. 4 is a layout view of one embodiment of an apparatus used for carrying out the method of the present invention, and FIG. 5 is a method of the present invention when a drift occurs in the apparatus shown in FIG. FIG. 6 is an explanatory view when the short side is moved, and FIG. 6 is a graph showing a situation of occurrence of surface cracking of a slab cast by the method of the present invention in comparison with the conventional method. Reference numeral 1 ... Long side of mold copper plate 2 ... Short side of mold copper plate 3 ... Thermocouple 4 ... Immersion nozzle 5 ... Meniscus (metal surface) 6 ... Discharge flow 7 ... Temperature converter 8 ... … Heat flux calculator 9 …… Heat flux difference monitor 10 …… Temperature difference monitor 11 …… Alarm (voice announcement) 12 …… Mold width changer 13 …… Nozzle deposit × …… Thermocouple embedding position a , B …… Distance from the surface of the copper mold plate

Claims (1)

[Claims] 1. Immersion nozzles each having a molten steel discharge hole formed toward the short side of the mold are arranged at the center of the mold, and the mold is divided into left and right parts with the immersion nozzle as a boundary. At least two thermocouples are provided in the long side and the short side of the copper plate so as to be symmetrical under the molten steel meniscus, and a temperature difference is detected between the thermocouples that are symmetrical at the left and right parts, while the long side and the short side are detected. By embedding at least two thermocouples in the depth direction of the side, the heat flux difference obtained from the temperature difference detected between these thermocouples is obtained, and the temperature difference or the heat flux difference exceeds a certain value. When it becomes large, both short sides are moved in the direction of the part corresponding to the larger one to reduce the temperature difference or heat flux difference between the left and right parts, and the continuous casting method for steel.