KR20110000392A - Method of controlling casting speed in continuous casting - Google Patents

Abstract

PURPOSE: A casting speed control method for continuous casting is provided to improve the recovery rate of molten metal by preventing the bursting of a slab. CONSTITUTION: A casting speed control method for continuous casting is as follows. An initial slab initially drawn out of a continuous cast mold is accelerated. The initial slab accelerated during a first acceleration period is transferred at uniform speed. The initial slab transferred at the uniform speed is accelerated again and transferred. The first acceleration period is a period from the starting of continuous casting until the slab is drawn out to the length of the initial slab. The duration of the uniform speed period is 1 to 3 minutes after the first acceleration period.

Description

METHOD OF CONTROLLING CASTING SPEED IN CONTINUOUS CASTING}

The present invention relates to a method for controlling the casting speed of continuous casting, and more particularly, to control the feed speed of the cast during continuous casting to prevent cracking and cracking of the cast surface.

In general, the continuous casting apparatus is a device for manufacturing molten steel dissolved in a steelmaking furnace such as an electric furnace into slabs (Slab) or Bloom, Billet (Billet) and the like used during hot rolling.

In general, thin slabs are manufactured by CSP (Compact Strip Production, CSP) continuous casting method.

In CSP continuous casting, the dummy bar is raised before the start of continuous casting, and the dummy bar head is stopped at a lower position of the continuous casting mold at a predetermined position to couple the dummy head's rust head into the continuous casting mold, and the mold copper plate is moved to move the dummy bar and the continuous casting. After minimizing the spacing between the molds, the molten steel is prepared by completely sealing between the continuous casting mold and the dummy bar with a paper wire or ceramic material.

The rust head serves as an initial coolant and a connection between the initial solidified molten steel and the dummy bar head.

  At the start of continuous casting of CSP, when molten steel is injected from the ladle into the tundish, when the molten steel in the tundish becomes more than a predetermined amount, the molten steel is injected into the continuous casting mold from the tundish by opening the exit of the tundish.

The injected molten steel is solidified in close contact with the coolant and the dummy bar head, the dummy bar is moved downward by the pinch roll of the cast steel carrier, and the solidified initial solidified cast steel is also moved downward to start continuous casting. The dummy bar and the slab are separated by the movement of the bending guide roll at the bending guide roll position of the slab feeder.

The separated dummy bar is moved downward, and the separated cast piece is moved along the bending guide roll and continuous casting is performed.

Once the continuous casting is started, the casting speed starts to increase from the time when the dummy bar moves to the lower side and varies depending on the target casting speed by steel type and width, but in the continuous continuous casting, the casting speed is continuously increased.

In the continuous casting and as the casting speed increases, the amount of secondary cooling water also increases according to the set value.

The secondary coolant injected to the cast when the cast is transported along the cast tray increases with the casting speed as set, but the secondary coolant spray is controlled by a PID-controlled valve. If the value is continuously changed, a deviation between the set value and the actual value occurs, which does not always coincide with the set value according to the casting speed value.

  The secondary cooling pattern is preset through experiments and practical tests, and is usually set to best suit the surface quality and the internal quality.

However, if the casting speed is continuously changed, the target injection quantity is continuously changed, which shows the difference between the actual value and the set value.

In addition, the deviation value shows a difference for each cooling region that is a basic unit of the secondary cooling pattern.

In particular, in the early stages of casting in which the casting speed is rapidly changed, deviations occur more frequently than when the casting speed is constant.

 If the injection amount set by the cooling pattern and the actual injection amount show a deviation, there is a problem that a cooling nonuniformity occurs and a surface crack or break out phenomenon occurs, and the surface of the cast steel is excessively large due to the large amount of injection of cooling water. When it is cooled, surface cracks occur, and when the injection amount is insufficient, solidification of the cast steel is insufficient, causing the cast out to break out.

In particular, in the case of CSP continuous casting, which is a type of continuous slab casting, the casting speed is usually 4 m / min or higher, so the initial circumferential speed change is much larger than that of general performance, and thus the variation in the amount of injection of the cooling water is large. The failure rate of the slab is increased due to surface cracks or break outs due to the variation in the injection amount, and accidents frequently occur during the continuous casting operation.

It is an object of the present invention to provide a casting speed control method of continuous casting to control the casting speed during continuous casting to prevent the solidification unevenness of the initial cast is drawn and cut in the continuous casting mold for the first time.

This object of the present invention includes a first acceleration section in which the initial cast is initially drawn in the continuous casting mold during the continuous casting;

A constant velocity section in which the initial slab accelerated to the first acceleration section is conveyed at constant speed at a constant speed;

It is solved by providing a casting speed control method of continuous casting in which the casting speed is controlled to have a second acceleration section for accelerating and transferring the initial cast at constant velocity.

The first acceleration section is a section until the slab is drawn by the length of the initial slab first sheared after the start of continuous casting.

The constant velocity section is a section maintained for 1 minute or more and less than 3 minutes after the first acceleration section.

The first acceleration section, the constant velocity section, and the second acceleration section are applied to a continuous casting method of continuously casting slabs having a thin thickness of 50 to 80 mm at a casting speed of 4.0 to 6.0 m / min.

The first acceleration section is to accelerate the casting speed to reach 3.5 ~ 4.0m / min,

The constant velocity section is maintained at 1 minute 45 seconds or more and less than 2 minutes 15 seconds at a constant speed accelerated to the first acceleration section.

The second acceleration section accelerates at a casting speed within the range of 4.0 to 5.0 m / min after the constant velocity section.

The present invention improves the quality of the initial cast steel which is first drawn in the continuous casting mold and cut by the cutting machine by controlling the casting speed, lowering the defective rate, as well as preventing the cast bursting phenomenon to increase the production yield and stable continuous casting operation It has the effect of enabling it.

In addition, the present invention is to reduce the shear amount of the front end of the initial cast to increase the output of the cast, thereby increasing the recovery rate of molten steel.

When described in detail with reference to the accompanying drawings a preferred embodiment of the present invention.

1 is a schematic view showing a conveying table of a general continuous casting, a vertical conveying portion, an arc conveying portion, a horizontal conveying portion, an example in which a shear for cutting a cast piece drawn in a continuous casting mold to a predetermined length is provided with a horizontal conveying portion Indicates.

Figure 2 is a graph showing the casting speed of the present invention, showing a state in which the casting speed is changed based on the time after the start of the drawing of the first cast to be drawn in the continuous casting mold.

Although not shown, a typical continuous casting apparatus includes a tundish for receiving and storing molten steel from a ladle;

A continuous casting mold (10) for cooling the molten steel out of the tundish to form a cast steel (20) having a predetermined shape;

Cast steel conveying table for cooling while conveying the cast steel 20 discharged from the continuous casting mold (10);

It includes a cooling water injection device for spraying the cooling water to the cast steel 20 is transferred to the cast steel transport.

As shown in FIG. 1, the cast steel conveying table includes a vertical conveying part 1 for vertically conveying the slab 20 from the lower portion of the continuous casting mold 10;

An arc transfer unit (2) connected to the vertical transfer unit (1) to transfer the slab (20) vertically transferred to a circular arc of a predetermined radius;

A horizontal conveying part 3 connected to the circular conveying part 2 to horizontally convey the slab 20 conveyed by an arc of a predetermined radius;

It comprises a shearing machine (4) for shearing the slab (20) to the horizontal conveying portion (3) to a predetermined length.

In addition, the vertical conveying unit 1, the circular conveying unit 2, and the horizontal conveying unit 3 of the cast steel conveying group are a plurality of guide rolls and a driving motor for guiding the conveying of the cast steel 20 discharged from the continuous casting mold 10 It includes a pinch roll (1a) rotated by (not shown) to control the feed speed of the cast steel (20).

The pinch roll 1a accelerates the feed speed of the cast piece 20 entering the circular feed part 2, that is, the casting speed, so that the cast piece 20 smoothly enters and feeds into the circular feed part 2.

In the lower part of the pinch roll 1a, a bending roll 1b for detaching the dummy bar 30 from the slab 20 is provided at the end side of the vertical transfer part 1, and is first drawn out from the mold 10 for continuous casting. The dummy bar 30 for guiding the transfer of the cast steel 20 is separated, and the separated cast steel 20 is smoothly guided to the circular transfer part 2.

On the other hand, when the molten steel is a predetermined amount or more in the tundish by operating the stopper to inject molten steel into the continuous casting mold 10 in the tundish, the molten steel injected into the continuous casting mold 10 is a continuous casting mold (10) The primary surface is solidified in the state of being in close contact with the coolant and the dummy bar 30 head in the inside and is formed as a cast steel (20).

The primary surface solidified slab 20 is drawn out from the continuous casting mold 10 by the vertical movement of the dummy bar 30, that is, continuous casting is first started while being discharged, and vertically lowered along the vertical transfer part 1. While being separated from the dummy bar 30, the circular transfer unit 2, the horizontal transfer unit 3 is sequentially transferred along the shearing process to a predetermined length in the horizontal transfer unit (3).

At this time, the initial slab first sheared by the shearing machine 4 after the start of continuous casting is rapidly accelerated while being transported along the vertical conveying unit 1 and the arc conveying unit 2, and then sharply accelerated and then sheared in an accelerated state. .

As described above, the initial slab first sheared by the shearing machine 4 after the start of continuous casting rapidly changes the casting speed, and thus predicts the optimum casting temperature according to the casting speed so that the amount of cooling water that is injected while being fed along the casting tray is preliminary. Even if it is set to, the secondary coolant injection amount shows the deviation (set value-actual value) by the PID-controlled valve (Proportional-Integral-Derivative controller), which causes the solidification layer unevenness of the initial cast and surface cracks. And Break Out phenomenon occurs.

In addition, since cracks generated on the surface of the initial slab are propagated along the grain boundary until stabilization, not only the initial slab but also the next slabs (the second slab and the third slab that are sheared in turn after the initial slab) are affected.

As described above, the occurrence of surface cracks due to variation in the amount of cooling water injection is particularly severe when a slab having a thickness of 50 to 80 mm is continuously cast by a CSP (Compact Strip Production, CSP) continuous casting method.

The above-mentioned CSP continuous casting method casts slabs having a thin thickness of 50 to 80 mm at a casting speed of 4.0 to 6.0 m / min, and thus thinner thin slabs are manufactured at a higher casting speed than a general continuous casting speed, so that the amount of cooling water Surface cracking and cast rupture due to the deviation will occur more frequently.

The present conditions of the occurrence of surface cracks in the second and third slabs sequentially sheared after the initial cast and the initial cast by the CPS continuous casting method are as shown in Table 1 below.

division Early cast 2nd cast 3rd cast Surface crack incidence multiples compared to conventional materials 25 times 11 times 4.5x

Table 1 is a table comparing the incidence rate of the surface cracks of the first cast during the continuous casting, that is, the first cast and the second cast, the third cast and the subsequent cast is sheared after the initial cast.

In order to solve the above problems, as shown in FIG. 2, the casting speed of the continuous casting is first drawn out from the continuous casting mold 10, that is, a first acceleration in which the initial slab discharged by the dummy bar 30 is accelerated. Section A;

A constant velocity section B in which the initial slab accelerated to the first acceleration section A is conveyed at constant speed at a constant speed;

It is to control to have a second acceleration section (C) to accelerate and feed the initial cast again the constant velocity.

The first acceleration section (A) is a section until the cast is drawn out, i.e., discharged by the length of the initial cast is sheared after the start of continuous casting, the cast 20 is first drawn in the continuous casting mold 10 It is to increase the casting speed until discharged by the first shearing length.

The following embodiment is an embodiment of the present invention applied to the CSP continuous casting method for casting a slab having a thin thickness of 50 ~ 80mm at a casting speed of 4.0 ~ 6.0m / min, CSP continuous casting method to apply the present invention This is illustrated as the most efficient reason, and it can be found that it can be applied to the continuous casting method for producing various castings.

In the CSP continuous casting method, the first acceleration section A is accelerated so that the casting speed reaches 3.5 to 4.0 m / min, and the constant velocity section B is at a constant speed within the range of 3.5 to 4.0 m / min. It is a section maintained for 1 minute or more and less than 3 minutes, More preferably, it is maintained for 1 minute 45 seconds or more and less than 2 minutes 15 seconds.

This has a problem of lowering the productivity of the cast steel 20 when the constant velocity section B is kept longer, and the stress due to the deviation (set value-actual value) of the amount of cooling water when the constant velocity section B is kept shorter. Unevenness cannot be prevented.

In addition, the second acceleration section (C) is to accelerate the casting speed in the range of 4.0 to 5.0 m / min by gradually increasing the casting speed after the constant velocity section (B).

The casting speed is stably maintained within the range of 4.0 to 5.0 m / min after the above-mentioned second acceleration section C to produce a slab which is sheared sequentially to the shearer 4 after the initial slab, the second slab, and the third slab. Will be done.

As described above, when the casting speed is controlled in the first acceleration section (A), the constant velocity section (B), and the second constant velocity section (B), the second and third slabs are sequentially sheared after the initial and initial slabs. Crack occurrence status is as confirmed in Table 2 below.

division Early cast 2nd cast 3rd cast Surface crack incidence multiples compared to conventional materials 4.25 times 1.76 times 0.225 times

As shown in Table 2, the crack incidence rate of the initial cast steel has a reduction rate of about 83% compared to Table 1, a reduction rate of about 84% for the second cast steel, and a reduction rate of about 95% for the third cast steel. You can see that this will be lowered.

In other words, in the case of the initial cast, the main speed changes rapidly, and if the main speed changes rapidly, even if the amount of cooling water is controlled by a PID control (Proportional-Integral-Derivative controller), it takes a certain amount of time to stabilize. After the deviation stabilization time, set the pattern to increase the main speed in the range where the main speed increase speed is kept constant by the PID control (Proportional-Integral-Derivative controller). It is to prevent the coagulation unevenness caused by (Setting Value-Actual Value).

More specifically, the casting speed is the main speed for a time during which the amount of cooling water deviation (set value-actual value) is kept below a certain level after the first acceleration section A in which the casting speed is increased until the length of shearing the initial slab is drawn out. By maintaining a constant velocity section (B) to maintain the, and increasing the casting speed within the range where the cooling water deviation is a constant or less after the constant velocity section (B) is a method of maintaining the cooling water amount deviation to be a constant or less.

By applying this method, it is possible to improve the solidified layer unevenness of the initial slab caused by the variation in the amount of cooling water, and to prevent the surface cracks and the bursting of the slab of the initial slab.

As described above, the crack of the initial cast is prevented, thereby reducing the shear amount of the tip of the initial cast, which is sheared by the surface crack of the initial cast, thereby increasing the recovery rate of the molten steel.

The present invention is not limited to the above-described embodiments, and various changes can be made without departing from the gist of the present invention, which is understood to be included in the configuration of the present invention.

1 is a schematic view showing a conveyer of a typical continuous casting

2 is a graph showing the casting speed of the present invention

* Description of the major symbols in the drawings *

1: vertical feed unit 2: circular feed unit

3: horizontal feed unit 4: shearing machine

10: mold for continuous casting 20: cast

Claims (6)

A first acceleration section in which the initial slab initially drawn in the continuous casting mold is accelerated during continuous casting; A constant velocity section in which the initial slab accelerated to the first acceleration section is conveyed at constant speed at a constant speed; The casting speed control method of the continuous casting, characterized in that the casting speed is controlled to have a second acceleration section to accelerate and feed the initial cast again the constant velocity. The method according to claim 1, The first acceleration section is a casting speed control method of the continuous casting, characterized in that the section until the cast is drawn by the length of the initial cast is sheared after the continuous casting start. The method according to claim 1 or 2, The constant velocity section is a casting speed control method of the continuous casting, characterized in that the section is maintained for more than 1 minute and less than 3 minutes after the first acceleration section. The method according to claim 1, The first acceleration section, the constant velocity section, the second acceleration section of the continuous casting method characterized in that it is applied to a continuous casting method of continuously casting a slab having a thin thickness of 50 ~ 80mm at a casting speed of 4.0 ~ 6.0m / min Casting speed control method. The method according to claim 4, The first acceleration section is to accelerate the casting speed to reach 3.5 ~ 4.0m / min, The constant velocity section is a casting speed control method of a continuous casting, characterized in that maintained at 1 minute 45 seconds or more and less than 2 minutes 15 seconds at a constant speed accelerated to the first acceleration section. The method according to claim 5, And the second acceleration section accelerates at a casting speed within the range of 4.0 to 5.0 m / min after the constant velocity section.

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