JP5413054B2 - Method and apparatus for determining surface maintenance of slab during continuous casting - Google Patents

Description

  The present invention relates to continuous casting of a molten metal, and more particularly to a method and apparatus for determining the surface maintenance of a slab manufactured using a vertical continuous casting machine.

In continuous casting of steel, a molten metal surface coating agent, so-called mold flux, is used for the purpose of lubricating the mold and slab, keeping the molten metal surface warm, absorbing non-metallic inclusions floating and separated from the molten metal, and the like.
However, when the fluctuation of the molten metal surface becomes large, this molten metal surface coating agent is trapped by the solidified shell and remains in the slab as it is, which may cause surface defects and internal defects of the product after rolling.

  In continuous casting of steel, argon gas is blown into the molten metal to prevent clogging of the immersion nozzle. The blown-in fine argon gas bubbles may accumulate and float with non-metallic inclusions immediately below the surface of the molten metal during the ascent. In such a case, if fluctuations in the molten metal surface become large, bubbles and non-metallic inclusions are trapped in the solidified shell and remain in the slab as they are, which may cause surface defects and internal defects of the product after rolling. .

As described above, the molten metal level fluctuation is closely related to the occurrence of surface defects and internal defects in the product after rolling. Therefore, when the molten metal surface fluctuation amount exceeds a certain reference value, it is necessary to care for the slab at the corresponding portion.
Regarding the setting of the critical value of the molten metal surface fluctuation characteristic value, which is a reference for cleaning the slab, it is common to empirically give a critical value using a general molten metal surface fluctuation amount as the characteristic value. In addition to such a general method, Non-Patent Document 1 proposes the idea of setting the sum of the melt flux layer thickness, the mold vibration stroke, and the molten metal meniscus radius of curvature as a critical value. Non-Patent Document 1 describes that a critical value is empirically given as a characteristic value of the product of the melt level change rate and the melt level fluctuation width in the mold.
Further, Patent Document 1 discloses a solidified shell thickness dm calculated from a time during which the molten metal descending speed Vm is greater than a cast slab descending speed Vc and a thickness at which the slab is scaled off in the hot rolling heating process. A method for setting a critical value is presented without relying on experience, in which a difference of do (dm−do> 0) or more is a maintenance condition.

Komai et al .: Iron and Steel, 70 (1984) No.1, p.81-88 JP2007-185675

However, when a maintenance critical value is provided based on the above-described fluctuation amount of the molten metal surface and the characteristic values described in Non-Patent Document 1, the critical value differs if the continuous casting machine is different even for the same steel type. There was a thing. In addition, product defects may not occur even under conditions where the molten metal surface fluctuation amount is large.
On the other hand, in Patent Document 1 that presents a care method that does not rely on experience, it is possible to reduce the oversight of a defective part of a slab, but there is a problem that the defective part often flows out or a maintenance load increases. It was.

  The present invention has been made to solve such problems, and an object of the present invention is to provide a slab care determination method and determination apparatus that can more accurately predict the degree of slab surface layer defects caused by molten metal surface fluctuations. It is said.

In order to solve the above problems, the inventor has eagerly studied the foreign matter capturing behavior of mold powder, bubbles, inclusions, etc. to the solidified shell due to molten metal surface fluctuation, and the oscillation mark (OSM) where the molten metal surface position is directly below the molten metal surface. It was discovered that the amount of trapped foreign matter increases abruptly when it becomes deeper than the position of the nail formed in the part. This phenomenon can be explained by considering the positional relationship between the tip of the solidified shell and the molten metal surface when the molten metal surface fluctuates, and the claws periodically formed on the solidified shell.
Hereinafter, this point will be described in detail.

2 and 3 are explanatory views for explaining this point in detail, and are explanatory views for explaining a mechanism in which foreign matters such as bubbles are trapped by the solidified shell in the mold.
When the vertical speed of the molten metal surface is Vm (the downward direction is positive) and the casting speed is Vc (constant), FIGS. 2 (a) and 3 (a) show the state where the molten metal surface is constant and does not fluctuate. In this example, Vm = 0 and Vm <Vc. FIG. 2B and FIG. 3B show a state where the molten metal surface is lowered, and shows a state where Vm> Vc. 2 (c) and 3 (c) show a state where the molten metal surface is rising, and shows a state where Vm <Vc.

In the case of a vertical mold vibration type continuous casting machine, an oscillation mark (hereinafter referred to as “OSM”) is formed in a ring shape in the mold circumferential direction on the surface of the slab below the tip of the solidified shell by one vertical vibration of the mold. 1) is formed. The theoretical pitch in the casting direction in which the OSM is formed can be calculated by the casting speed Vc / frequency f when the molten metal surface is stationary or the molten metal surface fluctuation is relatively small.
On the molten steel side where the OSM is formed, a so-called claw is formed in which the tip of the initial solidified shell is bent toward the molten steel side. Accordingly, the claws are also formed on the molten steel side at the pitch Vc / f similarly to the OSM (see FIGS. 2A and 3A).
The formed claw is deformed to the molten steel side due to the heat shrinkage caused by the temperature distribution in the solidified shell and the increase of the pressure in the molten powder between the mold and the solidified shell when the mold is lowered. Therefore, the nail | claw formed in the downward direction rather than the solidification shell front-end | tip part has bent more to the molten steel side.

  When the molten metal surface descends under such circumstances, the mechanism of trapping foreign substances such as bubbles at the initial solidified shell interface is the distance p from the initial solidified shell to the first claw portion (hereinafter, The case where it is smaller (FIG. 2) than the case where it is larger (FIG. 3) is different. Therefore, in the following, each mechanism will be described in each case of h <p (FIG. 2) and h ≧ p (FIG. 3).

In the case of h <p, the solidified shell tip formed in the meniscus portion is relatively immersed in the mold powder, but is not immersed in the nail portion (see FIG. 2B). Therefore, when the molten metal surface rises again, the mold powder is trapped inside the solidified shell, along with the bubbles and inclusions that cannot be lifted, and the bending to the molten steel side is relatively small. (See FIG. 2C).

If h ≧ p:
The tip of the solidified shell is immersed in the mold powder until it passes through the nail portion formed immediately below (see FIG. 3B). Therefore, when the molten metal surface rises again, the mold powder is added to the inside of the solidified shell tip, along with the bubbles and inclusions that cannot be lifted, which are present under the molten metal surface. A larger amount of bending toward the side is captured by the claw portion (see FIG. 3C).

  As can be seen from the above description, the amount of foreign matter trapped at the solidified shell interface is deeply related to the amount of molten metal descending h and the distance p from the initial solidified shell to the first claw, and h = p Suddenly changes at the boundary, and increases more rapidly when h ≧ p than when h <p.

The distance p can be calculated as Vc / f because it can be regarded as an OSM pitch when it suddenly occurs while the molten metal surface is stable.
As can be seen from the theoretical formula Vc / f, the OSM pitch is affected by the casting speed Vc and the mold frequency f. During the operation, the casting speed Vc is not necessarily constant, and the mold frequency f is generally changed depending on the casting speed. Therefore, the OSM pitch of the slab often changes during operation, and when the continuous casting machine changes, even if the same steel type is cast, Vc and f may be different. The OSM pitch of the slab cast in such a situation is different for each slab and for each continuous casting machine.

As can be seen from the above mechanism, the amount of foreign matter trapped at the solidified shell interface is determined by the relative relationship between the molten metal level fluctuation amount h and the distance p. Since the OSM pitch of the slab is different for each slab and for each continuous caster, the amount of foreign matter trapped at the solidified shell interface varies even if the molten metal surface fluctuation amount h is the same.
Therefore, it can be understood that even if an attempt is made to make a determination based on a certain critical value of the fluctuation level h of the molten metal surface, it cannot be applied if the operation conditions and the continuous casting machine are different.
Therefore, in the present invention, not a certain critical value of the melt level fluctuation amount h determined empirically, but the ratio h / p between the melt level fluctuation amount h and the distance p from the initial solidified shell to the first claw portion is used as a reference. I decided to judge.

If it is the above judgment criteria, even if the operation conditions and the continuous casting machine are different, they can be set basically in the same manner and become appropriate judgment criteria.
Theoretically, the amount of foreign matter trapped at the solidified shell interface changes with the ratio h / p = 1 of the molten metal level fluctuation amount h and the distance p from the initial solidified shell to the first claw. The slab maintenance standard is changed with p = 1 as a boundary, whereby the removal of defects can be effectively realized. For example, when h / p <1, it is possible to set a criterion that no care is performed or the amount of care is reduced, and when h / p ≧ 1, the reference is increased.

However, even when casting molten steel of the same composition, for example, when the molten steel temperature to be cast is extremely different, the mold powder is different depending on the casting speed, or the operating conditions after the hot rolling process are different, Differences in product defect rates may occur.
Therefore, in such a case, the slab maintenance reference may be set by setting h / p to a value around 1.0 (for example, 0.8 or 1.2).

Next, it was examined how to set the molten metal level fluctuation amount h and the value of the distance p from the initial solidified shell to the first nail portion.
As described above, the distance p can be regarded as the OSM pitch when it suddenly occurs while the molten metal surface is stable, and can be given as Vc / f.
In this case, the slab maintenance standard is changed with the ratio of the molten metal surface fluctuation amount h and the oscillation mark pitch (Vc / f), h / (Vc / f) = 1 as a boundary. Removal can be realized effectively. For example, when h / (Vc / f) <1, there can be a criterion that no care is performed or the amount of care is reduced, and when h / (Vc / f) ≧ 1, the reference is increased.

<Water level fluctuation amount h>
Normally, the molten metal level descent and the molten metal level rise occur in pairs, and there are many cases where the molten metal level descent phenomenon that causes a defect occurs instantaneously, so the molten metal level descent speed Vm can be considered to be greater than Vc. . Therefore, h may be given as the maximum value h _max of the conventional molten metal surface fluctuation range without distinguishing between the rising and falling of the molten metal surface.
Further, in the case where the rise and fall of the molten metal level can be distinguished by the molten metal surface position information analysis, h may be given as the maximum value hd _max of the molten metal surface lowered amount.

However, when the operation changes greatly or when the casting speed is high, if h / p is calculated based on the above-mentioned concept, it may not always be correctly evaluated. Therefore, in such a case, if the maximum value per slab of h ₍₂₎ calculated by the following equation (2) is given under the condition of Vm> Vc, h / p reflecting the actual phenomenon Can be calculated.

In the above description, as a method of giving the distance p, a method of obtaining h / p under a specific condition that the molten metal surface descends at every nail (OSM) formation timing is presented.
However, in actuality, there may be a case where the molten metal surface fluctuates regardless of the nail formation timing, so if p is given when the molten metal surface surface changes at a timing other than the time when the nail is formed every 1 / f time, it is actually It is possible to calculate h / p that more accurately reflects this phenomenon.
Therefore, in the following, the concept will be described for each case where h ≧ p and h <p.

In the case of h ≧ p FIG. 4 is a graph for explaining the trapping mechanism of bubbles and the like shown in FIG. 3 in more detail with the lapse of time. FIG. FIG. 4B is a graph showing temporal changes in the molten metal surface trajectory, the solidified shell tip trajectory, and the claw trajectory.
In FIG. 4A, the solid line indicates the solidified shell tip speed Vs, the fine broken line indicates the molten metal surface speed Vm, and the rough broken line indicates the casting speed Vc (constant). A portion where a plurality of lines overlap is indicated by a solid line.
In FIG. 4B, the solid line indicates the solidified shell tip locus, the fine broken line indicates the molten metal surface locus, and the rough broken line indicates the claw portion locus. In addition, regarding the shell tip trajectory and the molten metal surface trajectory, the overlapping portion is shown by a solid line.

  As shown in FIG. 4, the hot water surface starts at the point T (nail formation time, time to, hot water surface position Ho) and rises when the speed Vm is negative (downward positive). At 0, the uppermost position is reached, and the position starts to descend. Then, at point A (time ts, molten metal surface position Hs), the descending speed Vm becomes higher than the descending speed (= casting speed) Vc of the solidified shell tip. From the point T to the point A, the position of the molten metal surface and the position of the solidified shell tip coincide (see FIG. 4B). After point A, the molten metal surface descends and follows point B (time td, molten metal surface position Hd, dH / dt = Vc point) and takes a locus of point T-A-B-C-D.

On the other hand, the tip of the solidified shell is at the same position as the molten metal surface until point TA, but separated from the molten metal surface between AC and follows the path of dH / dt = Vc, along with the molten metal surface rising at point C. It reaches D point.
As a result, the nail formed by to descends by the distance Vc (td-to) by time td. The distance p from the shell tip to the nail at td is the sum of the difference between the hot water surface Hs and the nail formation position hot water surface Ho at point A and the descending distance Vc (ts-to) of the nail from point T to point A ( 4) It can be expressed by an equation.
In addition, the maximum depth h at which the molten metal surface descends below the shell tip can be expressed by equation (3). Therefore, finally, h / p is calculated using equations (3) and (4). it can.
h ₍₃₎ = Hd- [Vc (td-to) -p] (3)
p ₍₄₎ = (Hs-Ho) + Vc (ts-to) (4)

In the case of h <p FIG. 5 is a graph for explaining in detail the trapping mechanism of bubbles and the like shown in FIG. 2 along the passage of time. It is a graph which shows a change.
In the case of h <p, as shown in FIG. 5, h / p can be calculated based on the above equations (3) and (4) as in the case of h ≧ p.

Note that the time to when the claw is formed substantially corresponds to the time when the molten steel starts to overflow on the solidified shell tip where the claw is considered to be shaped, or the time when the solidified shell tip starts to bend toward the molten steel. The mold lowering speed VM may be set to a time point (start point of the negative strip time tn) when it becomes higher than the casting speed Vc.
The measurement position of the molten metal surface position H may be an intermediate position between the immersion nozzle and the short side at the center of the mold thickness as in the past. However, the present invention may be implemented based on H measured simultaneously at a plurality of locations, if necessary.
The present invention is based on the above knowledge, and specifically comprises the following configuration.

(1) The method for determining the surface maintenance of a slab during continuous casting according to the present invention is a method for determining whether or not the surface of a slab needs to be maintained based on molten steel surface position information in a continuous casting mold. ) Is determined to be unnecessary when the h / p value is smaller than a predetermined reference value, and is determined to be required for maintenance when the h / p value is greater than or equal to the predetermined reference value. is there.
h / p (1)
However, h: Hot water level fluctuation amount
p: Distance from the tip of the solidified shell to the nail

(2) In addition, in the above described (1), the value of h is the maximum value h _max of the molten metal surface fluctuation amount, and the value of p is an average oscillation mark pitch (Vc / f) mm).
However, casting speed when Vc: h is generated (mm / min)
f: Mold frequency when h is generated (cycle / min)

(3) In the above described (1), the value of h is the maximum value hd _max of the molten metal descending amount, and the value of p is an average oscillation mark pitch (Vc / f) mm).
However, casting speed when Vc: h is generated (mm / min)
f: Mold frequency when h is generated (cycle / min)

(4) In the above-mentioned (1), the value of h is given by h ₍₂₎ calculated by the following equation (2) under the condition of Vm> Vc, and the value of p is An average oscillation mark pitch (mm) calculated by Vc / f is used.

(5) Further, in the above (1), h and p in the above formula (1) are given by h ₍₃₎ and p ₍₄₎ obtained by the following formulas (3) and (4), respectively. It is characterized by.
h ₍₃₎ = Hd- [Vc (td-to) -p] (3)
p ₍₄₎ = (Hs-Ho) + Vc (ts-to) (4)
However,
Vc: Casting speed (mm / min)
ts: Time when the condition of Vm> Vc starts after the molten metal starts descending
td: Time when Vm = Vc again after ts
Hd: Molten surface position at time td (time when Vm = Vc again after ts) (mm)
Hs: Position of molten metal surface at time ts (Vm> Vc start time) (mm)
Ho: VM before time ts> Pod surface position at the start of Vc (mm)
to: VM before time ts> Vc start time (min)
VM: Mold lowering speed (mm / min)
Vm: Lowering speed of the molten metal surface (mm / min)

(6) The slab surface care determination device at the time of continuous casting according to the present invention is a device for determining the necessity of slab surface maintenance from the molten steel surface position information in the continuous casting mold,
A molten metal surface position detector for measuring the molten metal surface position, calculation means for inputting the molten metal surface position information of the molten metal surface position detector and predetermined casting conditions and performing a predetermined calculation, and inputting the calculation result of the calculating means And determining means for determining the necessity of maintenance, the calculating means calculates h _max / p based on the following expression (1), and the determining means includes h _max shown in the following expression (1): When the value of / p is smaller than a predetermined reference value, it is determined that maintenance is not necessary, and when the value of h _max / p is equal to or greater than a predetermined reference value, it is determined that maintenance is required .
h _max / p (1)
However, h _max : Maximum value of _molten metal level fluctuation (mm)
p: Average oscillation mark pitch calculated in Vc / f (mm)
Vc: Casting speed when h occurs (mm / min)
f: Mold frequency at the time of occurrence of h (cycle / min)

(7) Further, in the above-described (6), the calculating means calculates the equation (1) using a maximum value: hd _max of the molten metal level lowering amount instead of h _max. To do.

(8) Further, in the above (6), the calculating means calculates h _{(2) represented} by the following expression _(2), and uses h ₍₂₎ instead of h _max ( 1) It is characterized by calculating an expression.

(9) Further, in the above (6), h ₍₃₎ and p ₍₄₎ in the following formulas (3) and (4) are substituted for h _max and p in the formula (1), respectively. It is characterized by giving.
h ₍₃₎ = Hd- [Vc (td-to) -p] (3)
p ₍₄₎ = (Hs-Ho) + Vc (ts-to) (4)
However,
Vc: Casting speed (mm / min)
ts: Time when the condition of Vm> Vc starts
td: Time when Vm = Vc again after ts
Hd: Molten surface position at time td (time when Vm = Vc again after ts) (mm)
Hs: Position of molten metal surface at time ts (Vm> Vc start time) (mm)
Ho: VM before time ts> Pod surface position at the start of Vc (mm)
to: VM before time ts> Vc start time (min)
VM: Mold lowering speed (mm / min)
Vm: Lowering speed of the molten metal surface (mm / min)

  In the present invention, it is a method for determining the care standard of the slab surface from the molten steel surface position information in the continuous casting mold, the ratio of the molten metal surface fluctuation amount: h and the distance from the solidified shell tip to the claw: p Since the amount of slab care is determined based on h / p, it is a judgment standard that matches the phenomenon of foreign matter trapping in the mold, so even if the operating conditions and continuous casting machine are different, basically A common determination criterion can be used, and appropriate determination can be realized.

It is explanatory drawing of a determination apparatus for the care of the slab surface which concerns on one embodiment of this invention. It is a figure which shows typically the mode in a casting_mold | template, and is explanatory drawing explaining the mechanism by which a bubble etc. are capture | acquired by the solidification shell (when h <p). It is a figure which shows typically the mode in a casting_mold | template, and is explanatory drawing explaining the mechanism by which a bubble etc. are capture | acquired by the solidification shell (when h> = p). It is a graph for demonstrating in order to demonstrate the mechanism of capture | acquisition of the bubble etc. shown in FIG. 3 in detail along progress of time. It is a graph for demonstrating in order to demonstrate the capture | acquisition mechanism, such as a bubble shown in FIG. 2, in detail along progress of time. It is explanatory drawing explaining the effect of the Example of this invention. It is explanatory drawing explaining the effect of the Example of this invention. It is explanatory drawing explaining the result of the comparative example of this invention.

With reference to FIG. 1, a determination apparatus for cleaning the slab surface according to the present embodiment will be described.
First, the outline of the continuous casting apparatus used in the present embodiment will be described. The continuous casting apparatus 1 used in this embodiment includes a water-cooled mold 3 and a slab support roll 7 that is disposed below the mold 3 and supports the slab 5 drawn out of the mold 3. In the slab support roll 7, it is preferable to arrange a measuring roll 9 for measuring the casting length directly below the mold or at a predetermined position.
Moreover, the continuous casting apparatus 1 is provided with the control apparatus 10 which performs the setting and control of operation | movement of each apparatus corresponding to operation conditions.

The slab surface care determination device 11 according to the present embodiment includes a molten metal surface position detector 13 that continuously measures the molten metal surface level (the surface of the molten metal surface), information on the molten metal surface position detector 13, and a measurement roll 9. Information, the information of the control device 10 and the like are input, a calculation unit 15 that performs a predetermined calculation, a determination unit 17 that determines whether or not slab maintenance is necessary based on a calculation result of the calculation unit 15, and a determination criterion And a database 19 in which data is stored.
The position of the molten metal surface is preferably measured in the vicinity of the inner wall of the mold 3, preferably within 50 mm from the inner wall of the mold. As an example of the hot water surface position detector 13, for example, an eddy current type electromagnetic sensor, a γ ray sensor or the like is preferable.

The calculation means 15 calculates the descent speed Vm of the molten metal surface at predetermined time intervals based on the molten metal surface position information input from the molten metal surface position detector 13.
Further, the amount of descent of the slab 5 is measured by the measuring roll 9 in synchronization with the position measurement of the molten metal surface, and the information is input to the calculation means 15. And the calculating means 15 calculates the descent speed (casting speed) Vc of the slab 5 at the time corresponding to the calculated molten metal surface descent speed Vm based on the descent amount information of the slab 5.
Further, the calculation means 15 calculates the maximum value h _max of the molten _metal surface fluctuation amount per slab based on the molten metal surface position information from the molten metal surface position detector 13.
Further, the calculation means 15 calculates Vc / f when the maximum value of the molten metal surface fluctuation amount is generated based on the casting speed Vc and the mold frequency f input from the control device 10.
Further, h _max / (Vc / f) is calculated.

<Database>
The database 19 stores data indicating the relationship between h _max / (Vc / f) and slab care amount in actual operation performed in advance. An example of data is a reference value S that is a criterion for determining whether or not care is required if the value of h _max / (Vc / f) is S or more, and care is not required if the value is smaller than S. is there. Alternatively, when maintenance is required, data indicating a specific maintenance amount (mm) may be stored in association with the value of h _max / (Vc / f).

The determination means 17 compares the value of h _max / (Vc / f) calculated by the calculation means 15 with the determination reference value stored in the database 19 to determine the care amount.
For example, if h _max / (Vc / f) ≧ S from the reference value S stored in the database 19, maintenance is required, and if h _max / (Vc / f) <S, it is determined that maintenance is unnecessary.

  In the above example, the casting speed Vc is based on the information on the measuring roll 9, but a casting speed Vc obtained based on the information on the rotational speeds of the plurality of pinch rolls 19 may be used instead.

In the present embodiment configured as described above, during operation, the calculation means 15 calculates h _max / (Vc / f) by performing the above calculation, and the determination means 17 The necessity of maintenance is determined based on the calculated result.
As described above, in the present embodiment, since the necessity of maintenance is determined based on h _max / (Vc / f), it becomes a determination criterion that matches the phenomenon of foreign matter trapping in the mold, and hence the operating conditions. Even if the continuous casting machine is different, it can basically be set as a common judgment standard, which is an appropriate judgment standard.

[Embodiment 2]
In the first embodiment, an example of performing determination based on the maximum value h _max of the molten metal surface fluctuation, in the present embodiment, the molten metal surface lowering amount in place of the maximum value h _max of molten steel surface fluctuation The determination based on the maximum value hd _max is performed.
The database 19 of the present embodiment stores data indicating the relationship between hd _max / (Vc / f) and slab care amount in actual operation performed in advance.
Then, the calculation means 15 calculates the maximum value hd _max of the molten metal level lowering amount per slab based on the molten metal surface position information from the molten metal surface position detector 13. Further, the calculation means 15 calculates Vc / f when the maximum value of the molten metal level lowering amount is generated based on the casting speed Vc and the mold frequency f input from the control device 10, and further, hd _max / ( Vc / f) is calculated.
The determination unit 17 compares the value of hd _max / (Vc / f) calculated by the calculation unit 15 with the determination reference value stored in the database 19 to determine the care amount.

In the present embodiment, the maximum value hd _max of the molten metal level lowering amount is calculated, and further, the necessity of maintenance is determined based on hd _max / (Vc / f). Furthermore, it becomes a determination standard that matches the phenomenon of foreign matter trapping in the mold, and therefore, even if the operating conditions and the continuous casting machine are different, it can basically be set as a common determination standard, which is an appropriate determination standard.

[Embodiment 3]
In the second embodiment, the example in which the determination is performed based on the maximum value hd _max of the molten metal level lowering amount has been shown. However, in the present embodiment, in order to perform the determination based on the logic closer to the actual phenomenon, Instead of the maximum value hd _max of the descending amount, h ₍₂₎ calculated by the following equation (2) is given under the condition of Vm> Vc.

The calculation unit 15 performs the calculation based on the above equation (2). Further, the calculation means 15 presupposes the calculation of the equation (2), where Vm is the lowering speed of the molten metal surface (mm / min), Vc is h ₍₂₎ the casting speed when it is generated (mm / min), f : H ₍₂₎ Calculate the mold frequency (cycle / min) at the time of occurrence, ts: the time when the Vm> Vc condition starts after the molten metal surface starts descent, and td: the time when Vm = Vc again after ts.
A database 19 of this embodiment, showing the relationship between the (2) h based on h ₍₂₎ which is obtained by the formula _{(2) / (Vc / f} ) and slab care of in actual operation was performed previously Data is stored.

The determination means 17 compares the value of h ₍₂₎ / (Vc / f) calculated by the calculation means 15 with the determination reference value stored in the database 19 to determine the care amount.

In the present embodiment, h ₍₂₎ obtained by the equation ₍₂₎ is calculated, and the necessity of maintenance is determined based on h ₍₂₎ / (Vc / f). Therefore, even if the operation changes greatly or the casting speed is high, it is basically a common criterion. Can be an appropriate criterion.

[Embodiment 4]
In the third embodiment, the example in which the determination is performed based on h ₍₂₎ given by the above equation ₍₂₎ has been shown. However, in the present embodiment, in order to perform the determination based on the logic closer to the actual phenomenon. , the following equation (3), determined on the basis of (4) with h _(3), p ₍₄₎ a determined, was calculated h _(3), p ₍₄₎ ratio h ₍₃₎ of the / p ₍₄₎ Is to do.
h ₍₃₎ = Hd- [Vc (td-to) -p] (3)
p ₍₄₎ = (Hs-Ho) + Vc (ts-to) (4)
However,
Vc: Casting speed (mm / min)
ts: Time when the condition of Vm> Vc starts after the molten metal starts descending
td: Time when Vm = Vc again after ts
Hd: Molten surface position at time td (time when Vm = Vc again after ts) (mm)
Hs: Position of molten metal surface at time ts (Vm> Vc start time) (mm)
Ho: VM before time ts> Pod surface position at the start of Vc (mm)
to: VM before time ts> Vc start time (min)
VM: Mold lowering speed (mm / min)

The calculation means 15 performs the calculations of the above equations (3) and (4). Further, the calculation means 15 is premised on the calculation of the expressions (3) and (4), Vc: casting speed (mm / min), ts: the time when the condition of Vm> Vc starts after the molten metal surface starts to descend, td: time when Vm = Vc again after ts, Hd: hot water surface position (mm) at time td (time when Vm = Vc again after ts), Hs: hot water at time ts (Vm> Vc start time) Surface position (mm), Ho: VM at time ts before VM> Vc position at start of Vc (mm), to: VM before time ts> time at start of Vc (min), VM: Mold lowering speed (mm / min).
A database 19 of this embodiment, equation (3) in the actual operation was performed previously, (4) h which is obtained by the formula _(3), h ₍₃₎ based on p ₍₄₎ / p ₍₄₎ Data indicating the relationship with the slab care amount is stored.

  The determination means 17 compares the h / p value calculated by the calculation means 15 with the determination reference value stored in the database 19 to determine the care amount.

In the present embodiment, h ₍₃₎ and p ₍₄₎ obtained by equations ₍₃₎ and ₍₄₎ are calculated, and the necessity of maintenance is determined based on h ₍₃₎ / p _(4). As a result, it is a criterion that more accurately reflects the phenomenon of foreign matter trapping in the mold than in the case of the third embodiment. Therefore, even if the molten metal surface fluctuates regardless of the nail formation timing, Can be used as a common criterion, which is an appropriate criterion.

In order to confirm the effect of the present invention, the following implementation was performed.
Medium carbon steel slab (C: 0.08-0.13, Si: 0.2-0.4, Mn: 1.0-1.5, P: 0.015-0.025, S: 0.008-0.020, Al: 0.02-0.50, Ti: 0.08-0.020wt%, thickness 220mm and 250mm, width 1900-2100mm) were cast by different vertical bending type continuous casting machines, and rolled to product without maintenance.
During casting, the maximum value of h / p is calculated in units of casting slabs based on the molten metal surface position data measured every 0.1 second by the eddy current sensor at the center of the mold thickness, the center of the immersion nozzle and the short side, The relationship between h / p and the rate of occurrence of surface baldness (= number of bald coils / total number of investigated coils × 100) due to the powder of the product hot-rolled sheet (thickness 26 to 51 mm) was investigated.
For giving h / p, the four methods of Embodiments 1 to 4 were used, and the relationship with the rate of occurrence of surface hege caused by powder was investigated.

Operating conditions are casting speed Vc 1.0-2.2m / min, mold vibration waveform: sine waveform, slab thickness 220mm, 250mm, mold amplitude S = 7, 9mm, negative strip speed rate N [= (2Sf-Vc ) / Vc]: 0.22 and -0.20, and casting frequency was determined by determining the mold frequency f from the relationship Vc / f = (1 + N) / (2S). During casting, the mold powder has a slab thickness of 220mm and 250mm, respectively.
SiO2) 1.9, 1.5, crystallization temperature 1120, 1190 ° C, viscosity 0.5, 0.9P (1300 ° C) physical properties were used, and the tundish molten steel superheat was 20-35 ° C. Other conditions were also implemented with minimal changes.

The investigation results are shown in the graphs of FIGS. In the graph of FIG. 6, the vertical axis indicates the occurrence rate of shaving (%), and the horizontal axis indicates the maximum value per slab of h / p. FIG. 6 shows the relationship between h / p and the rate of occurrence of heges (%) by the four methods in Embodiments 1 to 4 for a slab thickness of 220 mm.
In FIG. 7, h / p is given by the method of the second embodiment, and the slab thickness is compared to 220 mm and 250 mm.
As shown in FIG. 6 and FIG. 7, by using the molten metal surface fluctuation characteristic value h / p based on the present embodiment, it is possible to get the hedging around h / p = 1, regardless of the difference in casting thickness. It can be seen that the incidence is changing rapidly.
From this, it was confirmed that according to the methods shown in the first to fourth embodiments, it can be a clear indicator of the rate of occurrence of hege.
On the other hand, in the graph of FIG. 8 in which the rate of occurrence of the bevel is arranged using the amount of fluctuation of the molten metal, the critical value of the amount of fluctuation of the molten metal surface where the rate of occurrence of the bevel increases rapidly varies depending on the casting thickness. As is clear from comparison with the above, the indexes of the first to fourth embodiments are excellent.

Next, actual determination was performed by the methods of the first to fourth embodiments, and the occurrence rate of lashes when the slab care was performed was also investigated.
In the first to fourth embodiments, it is determined that maintenance is necessary when h / p ≧ 1 as the determination formula, and the determination is based on the case where h ≧ 15 mm is required as the determination criterion based on the molten metal surface fluctuation amount h. . The product thickness was 2 mm (constant), and the product defect rates were compared. The results are shown in Table 1.

  As is apparent from Table 1, it can be seen that the defect occurrence rate is drastically reduced by using the maintenance criteria according to the present invention.

DESCRIPTION OF SYMBOLS 1 Continuous casting apparatus 3 Mold 5 Cast slab 7 Cast slab support roll 9 Measuring roll 10 Control apparatus 11 Slab care determination apparatus 13 Molten metal surface position detector 15 Calculation means 17 Determination means 19 Database

Claims (9)

A method for determining whether or not the slab surface needs to be maintained from molten steel surface position information in a continuous casting mold, and when the h / p value shown in the following formula (1) is smaller than a predetermined reference value, maintenance is not required. A method for determining the surface maintenance of a slab during continuous casting, characterized in that determination is made and it is determined that maintenance is required when the value of h / p is equal to or greater than a predetermined reference value .
h / p (1)
However, h: Hot water level fluctuation amount
p: Distance from the tip of the solidified shell to the nail 2. The continuous value according to claim 1, wherein the value of h is a maximum value h _max of the molten metal surface fluctuation amount, and the value of p is given as an average oscillation mark pitch (mm) calculated by Vc / f. A method for determining the surface maintenance of a slab during casting.
However, casting speed when Vc: h is generated (mm / min)
f: Mold frequency when h is generated (cycle / min) 2. The continuous value according to claim 1, wherein the value of h is set as a maximum value hd _max of the molten metal descending amount, and the value of p is given as an average oscillation mark pitch (mm) calculated by Vc / f. A method for determining the surface maintenance of a slab during casting.
However, casting speed when Vc: h is generated (mm / min)
f: Mold frequency when h is generated (cycle / min) The value of h is given by h ₍₂₎ calculated by the following equation (2) under the condition of Vm> Vc, and the value of p is an average oscillation mark pitch (Vc / f) The method for determining the surface maintenance of a slab during continuous casting according to claim 1, wherein:

The h and p in the formula (1) are given by h ₍₃₎ and p ₍₄₎ obtained by the following formulas (3) and (4), respectively. Surface care judgment method.
h ₍₃₎ = Hd- [Vc (td-to) -p] (3)
p ₍₄₎ = (Hs-Ho) + Vc (ts-to) (4)
However,
Vc: Casting speed (mm / min)
ts: Time when the condition of Vm> Vc starts after the molten metal starts descending
td: Time when Vm = Vc again after ts
Hd: Molten surface position at time td (time when Vm = Vc again after ts) (mm)
Hs: Position of molten metal surface at time ts (Vm> Vc start time) (mm)
Ho: VM before time ts> Pod surface position at the start of Vc (mm)
to: VM before time ts> Vc start time (min)
VM: Mold lowering speed (mm / min)
Vm: Lowering speed of the molten metal surface (mm / min) A device for determining whether or not the slab surface needs to be maintained from molten steel surface position information in a continuous casting mold,
A molten metal surface position detector for measuring the molten metal surface position, calculation means for inputting the molten metal surface position information of the molten metal surface position detector and predetermined casting conditions and performing a predetermined calculation, and inputting the calculation result of the calculating means And determining means for determining the necessity of maintenance, the calculating means calculates h _max / p based on the following expression (1), and the determining means includes h _max shown in the following expression (1): When the value of / p is smaller than a predetermined reference value, it is determined that maintenance is unnecessary, and when the value of h _max / p is equal to or higher than a predetermined reference value, it is determined that maintenance is required. A device for determining the surface of a piece.
h _max / p (1)
However, h _max : Maximum value of _molten metal level fluctuation (mm)
p: Average oscillation mark pitch calculated in Vc / f (mm)
Vc: Casting speed when h occurs (mm / min)
f: Mold frequency at the time of occurrence of h (cycle / min) 7. The surface treatment of the slab during continuous casting according to claim 6, wherein the calculating means calculates the formula (1) using a maximum value: hd _max of the molten metal level lowering amount instead of h _max. Judgment device. The said calculating means calculates h ₍₂₎ shown by the following (2) Formula, and calculates the said (1) formula using h ₍₂₎ instead of h _max. For determining the surface of a slab during continuous casting.

7. In continuous casting according to claim 6, wherein h ₍₃₎ and p ₍₄₎ in the following formulas (3) and (4) are given in place of h _max and p in the formula (1), respectively. Equipment for determining the surface maintenance of slabs.
h ₍₃₎ = Hd- [Vc (td-to) -p] (3)
p ₍₄₎ = (Hs-Ho) + Vc (ts-to) (4)
However,
Vc: Casting speed (mm / min)
ts: Time when the condition of Vm> Vc starts
td: Time when Vm = Vc again after ts
Hd: Molten surface position at time td (time when Vm = Vc again after ts) (mm)
Hs: Position of molten metal surface at time ts (Vm> Vc start time) (mm)
Ho: VM before time ts> Pod surface position at the start of Vc (mm)
to: VM before time ts> Vc start time (min)
VM: Mold lowering speed (mm / min)
Vm: Lowering speed of the molten metal surface (mm / min)

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