JP5042785B2 - Short side taper control method for continuous casting mold. - Google Patents

Description

  The present invention relates to a taper control method for a short side mold in a continuous casting mold.

When the molten steel is continuously cast by the continuous casting equipment, the short side of the mold provided in the continuous casting equipment is inclined, that is, tapered toward the inside of the mold so that the surface in contact with the molten steel becomes lower. This is because, as the solidified shell moves downward from the meniscus portion in the mold, the slab width of the solidified shell is solidified and contracted, so that the distance between both short sides of the mold is reduced corresponding to the solidification contraction. Here, the taper amount a ₀ is defined as follows. The short side distance is the distance between the inner surfaces of both short sides.
a ₀ = (distance between short sides at the meniscus portion−distance between short sides at the lower end of the mold) / 2

  In continuous casting, it is important to adjust the taper amount appropriately. If the taper amount on the short side is too small, the contact between the solidified shell and the short side becomes insufficient, and the thickness of the solidified shell (hereinafter sometimes referred to simply as the shell) decreases due to poor heat removal. In some cases, the solidified shell becomes non-uniform, resulting in cracking or breakout due to slab bulging. On the other hand, when the taper amount on the short side is too large, the contact between the solidified shell and the short side becomes strong, and an excessive stress is applied to the solidified shell to cause breakage of the solidified shell and breakout accompanying the shell breakage. Alternatively, the life of the mold is reduced due to an increase in the frictional force between the solidified shell and the mold (for example, peeling of the plating covering the mold surface, wear of the copper plate).

  For this reason, for example, as proposed in Patent Document 1, the amount of solidification shrinkage caused by the volume shrinkage change according to the component, the amount of solidification shrinkage caused by the heat removal amount change according to the casting speed, the amount of solidification shrinkage according to the mold cooling condition There is a method for determining the taper of the short side of the mold in accordance with the determined amount of coagulation shrinkage.

  When controlling the taper amount of the short side according to the casting speed, as described in Patent Document 2, the control of decreasing the taper amount of the short side as the casting speed is increased has been adopted.

  However, in the method of Patent Document 1, the amount of solidification shrinkage on the short side of the molten steel is used as a control index as described above, but the position of about 50 mm from the corner of the long side of the slab (hereinafter simply referred to as “long side corner part”). ) May sometimes cause breakage of the slab or breakout due to this.

JP-A-2-247059 Japanese Patent Laid-Open No. 6-31418

  The present invention secures an appropriate shell thickness not only on the short side in the mold but also on the long side corner, thereby ensuring the quality of the slab surface and preventing breakouts. It is an object of the present invention to provide a taper control method.

The present invention has been made in order to solve the above-mentioned problems, and the means thereof is (1) a continuous casting process for continuously casting molten steel within a casting speed range of 1.0 to 3.0 m / min, In the method of controlling the taper amount of the short side constituting the continuous casting mold in the continuous casting process, the taper amount of the short side is controlled in a direction in which the taper amount of the short side is increased as the casting speed of the molten steel to be cast increases. In addition, the short side taper control method for the continuous casting mold is characterized in that the taper amount of the short side is controlled in the direction of increasing the taper amount of the short side as the superheat amount of the molten steel increases .
Here, the taper amount a ₀ on the short side is defined as follows. The short side distance is the distance between the inner surfaces of both short sides.
a ₀ = (distance between short sides at the meniscus portion−distance between short sides at the lower end of the mold) / 2
(2 ) The method for controlling a short side taper of a continuous casting mold as described in (1 ) above, wherein a mold having a short side taper of a straight shape or a multistage or curved shape is used.

  Superheat is a temperature difference between the molten steel temperature and the solidification temperature. It is preferable to use the temperature measurement result of the molten steel in the tundish as the molten steel temperature. It is preferable to use the liquidus temperature of the molten steel component as the solidification temperature.

  In addition, the straight shape indicates a shape set by one type of straight line with respect to the casting direction of the mold, and the multi-stage shape indicates a shape set by a combination of straight lines having different inclinations with respect to the casting direction of the mold. The curved surface shape indicates a shape set by a combination of curves having different curvatures with respect to the casting direction of the mold, a combination of straight lines and curves, or a combination of straight lines having different inclinations and different curvature curves.

  The present invention makes it possible to perform casting while suppressing slab cracking and breakout at the corner of the long side as well as the short side of the mold even if the casting speed and molten steel superheat fluctuate greatly, improving the product yield. In addition, productivity can be improved. Further, it is preferable to use a multi-stage or curved mold short side because cracking of a slab and breakout can be further suppressed.

  The present inventor, in order to ensure the shell thickness of the long side corner part as well as the short side in the mold during continuous slab casting, and cast without occurrence of slab cracking and breakout, Focusing on the importance of using an index, the present invention has been achieved. This will be described in detail below.

  First, the formation process of the solidified shell in the mold will be described with reference to FIG.

  FIG. 1 shows a shell deformation state of the long side corner obtained by solidification analysis calculation, (a) is a side cross section in the height direction of the mold short side, and (b) is a cross section of (a). A plan view at level A, (c) is a plan view at level B in (a). In the analytical calculation conditions, a straight mold short side is used.

  1) As shown in FIG. 1 (a), the molten steel 2 injected into the mold 1 starts to form a solidified shell 4 from the vicinity of the meniscus 3, and the thickness of the solidified shell 4 gradually increases toward the lower side of the mold 1. Become. In the process of forming the solidified shell 4, an air gap 5 starts to be generated between the mold 1 and the solidified shell 4.

  Thereafter, as shown in FIG. 1 (b), the solidified shell 4a on the long side 1a side is formed by a solidification shrinkage difference between the solidified shell 4a formed on the long side 1a side of the mold 1 and the solidified shell 4b formed on the short side 1b side. The slab corner 6 starts to rotate in the arrow Z direction by first contacting the mold 1 and bending moment acting on the solidified shell 4b on the short side 1b side by bulging acting in the arrow X direction.

  2) Then, at level B, which is further below the position in FIG. 1 (b), as shown in FIG. 1 (c), by rotating the slab corner 6 in the direction of arrow Z, the long side corner portion of the inner portion The solidified shell 4a (position of about 50 mm from the corner of the long side shell) is deformed into a concave shape toward the slab (on the short side of the opposite mold) and the air gap 5a at the long side corner is enlarged, and the inner surface of the long side 1a The contact with is further incomplete. As a result, the thickness in the circled portion of the solidified shell 4a on the long side tends to be thinner than other portions.

  3) In addition to this, when the casting speed Vc or the superheat ΔT of the molten steel (which is the temperature difference between the molten steel temperature and the solidification temperature, simply referred to as superheat) increases, as shown in FIG. The thickness of the entire solidified shell 4 is further reduced. Fig. 2 shows the shell thickness of the long side corner (the long side shell portion where the shell thickness is the thinnest at a position about 50 mm from the corner) at a position 400 mm deep from the meniscus, and the white band position of the actual slab section It is measured based on the actual measurement result. The results shown in FIG. 2 are due to the fact that when the casting speed Vc increases, the contact time between the mold 1 and the solidified shell 4 decreases, and when the superheat increases, the high temperature of the molten steel results in insufficient cooling and the shell is difficult to form. To do. By reducing the thickness of the solidified shell as a whole, the rigidity of the shell is lowered and the slab corner 6 starts to rotate from a position closer to the meniscus. Therefore, the deformation of the solidified shell 4a at the long side corner is promoted, and this portion is removed. Thermal failure is further aggravated, causing slab cracking and breakout.

  In order to prevent the deformation of the solidified shell 4a at the long side corner, the taper amount on the short side of the mold is adjusted according to the casting speed and the molten steel superheat.

  In addition, FIG. 3, FIG. 4, and FIG. 4 show the results of comparing the solidified minimum shell thickness at the corner of the long side at the lower end of the mold and the air gap amount by changing only the short side taper amount, only the casting speed, and only the molten steel superheat. Each is shown in FIG. Common casting conditions in this case were casting thickness: 250 mm, casting width: 1200 mm, and carbon concentration in molten steel: 0.15%. 3 and 5 show the casting speed: 1.3 m / min, FIGS. 3 and 4 show the superheat of the molten steel: 30 ° C., and FIGS. 4 and 5 show the short side taper amount: 6.0 mm.

  From FIG. 3, it can be seen that as the short side taper amount increases, the air gap amount at the long side corner portion decreases, and the solidified shell thickness at that position increases. FIG. 4 shows that the air gap amount in the long side corner portion increases as the casting speed increases. From FIG. 5, it can be seen that the higher the molten steel superheat, the greater the amount of air gap at the long side corner portion and the minimum long side corner portion shell thickness. In FIG. 4, the shell thickness at the long-side corner portion decreases rapidly as the casting speed increases. The main factor is that the elapsed time until reaching the lower end of the mold is shortened due to the increase in casting speed.

  In controlling the taper amount of the short side according to the casting speed, in view of the relationship between the short side taper amount and the air gap amount shown in FIG. 3 and the relationship between the casting speed and the air gap amount shown in FIG. The relationship between the casting speed and the short side taper amount should be determined so as to be constant.

  When controlling the taper amount of the short side according to the molten steel superheat, the relationship between the taper amount of the short side and the air gap amount or the shell thickness shown in FIG. 3, the relationship between the casting speed and the air gap amount or the shell thickness shown in FIG. In view of the above, the relationship between the casting speed and the short side taper amount may be determined so that the air gap amount is constant or the shell thickness is constant.

  Note that the taper amount control amount may be changed within a range of ± 10% with respect to the optimum short side taper amount control margin determined as described above.

  That is, the higher the casting speed or the higher the molten steel superheat, the greater the taper amount of the short side, and the inner side of the short side is pressed against the solidified shell 4b from the position closer to the meniscus, and the movement of the slab corner 6 The amount of rotation in the direction of the arrow Z is restrained by restraining, and deformation promotion of the solidified shell 4a at the long side corner is prevented.

  In addition, when the mold short side whose surface is processed into a multistage or curved shape is used, excessive contact between the solidified shell 4b and the mold short side inner surface 1b near the lower end of the mold is lower than that in the straight shape, This is preferable because the thickness of the solidified shell can be increased while suppressing wear on the inner surface of the mold.

  The relationship between the distance from the meniscus, the amount of taper, and the amount of solidification shrinkage when applying a straight mold having a length from the meniscus to the mold lower end of 800 mm and when applying a two-stage mold will be described below with reference to FIG. .

  Note that dotted lines 7a and 8a in FIG. 6 are values obtained by solidification analysis calculation, and indicate the amount of solidification shrinkage, which is one of the shell deformation states on the short side. Further, the straight mold has a taper as shown by a solid line 7, and the two-stage mold has a combination of two straight lines forming an angle of θ = 0.69 ° at a position 200 mm from the meniscus as shown by a solid line 8. It is an example.

  As can be seen from the solid line 7 in FIG. 6, when the straight mold is used, the amount of solidification shrinkage 7a of the shell is bent downward from about 350 mm from the meniscus and does not change linearly. The taper control amount on the short side for bringing the inner surface of the mold into contact with the solidified shrinkage shape tends to be slightly increased.

  On the other hand, in the case of the two-stage mold shown by the solid line 8, since the mold is stepped to approach the solidification shrinkage shape in advance, the short side taper control amount for bringing the inner surface of the mold into contact with the solidification shrinkage shape near the meniscus is set. It is estimated that the amount can be reduced as compared with the case of the straight mold of the solid line 7 and the contact with the solidified shell below the mold can be suppressed.

  Therefore, the inner shape of the short side has many bending points, that is, the more the number of steps, the more circular the final shape, the smaller the taper control amount of the short side is preferable. A two-stage or three-stage mold is most preferable because of complicated processing and delicate control.

  On the other hand, in the present invention, the casting speed is defined as a speed of 1.0 to 3.0 m / min or more because at the casting speed of less than 1.0 m / min, the solidified shell thickness is sufficiently secured, so This is because there is no risk of breakout. Further, the higher the casting speed, the more desirable for achieving the object of the present invention, and the productivity is also improved, so the upper limit is not particularly specified, but a speed of about 3.0 m / min is a general slab. This is the upper limit of the equipment capacity of the continuous casting machine.

It is desirable to set the short side taper amount necessary for securing the shell thickness continuously based on the relational expression composed of casting speed and molten steel superheat, but the taper change is controlled continuously by the mold cylinder. Since such a change is difficult when considering the equipment load, the relationship between the casting speed, the molten steel superheat, and the short side taper amount a ₀ is preferably set in a stepped manner as shown in FIG. However, there is no particular problem as a continuous setting method using a relational expression. The upper limit of the short side taper amount is determined by the value at which cracking occurs due to friction between the short side mold and the cast slab, and the upper limit is set by prior examination such as solidification analysis calculation and actual slab confirmation. On the other hand, since the solidified shell is not stable during unsteady casting such as at the start and end of casting, there is a risk of breakout if the taper is changed. In addition, if the taper amount is changed significantly even when it is not unsteady, there is a risk that the shell cannot withstand and breaks out. Therefore, for taper control, the taper is changed at a pitch of ± 0.5 mm during casting in a steady state.

  Hereinafter, the taper control method of the mold short side 1b will be described with reference to FIG.

When casting is started in the continuous casting process, the molten steel component values (C, Si, Mn, P, S, Ti of the alloy element) that are sample analysis values at the end of the processing in the secondary refining process (for example, vacuum degassing process) , Nb, etc...) Are input from the setting unit 100 to the solidification temperature calculation unit 101 to calculate the solidification temperature _TL of the molten steel being cast. It should be noted that the input value from the setting unit 100 is the molten steel sample analysis value in the TD from the time when the molten steel sample analysis value in the tundish (hereinafter referred to as TD) is found after the start of casting in the continuous casting process. Be changed.

  The calculation formula for the solidification temperature is described in various documents. In this example, the steel handbook / I foundation / p. The formula described in 205 was used.

Next, the molten steel temperature T measured in TD is inputted from the setting unit 102 to the molten steel superheat calculation unit 103 at a cycle of 1 second. The molten steel temperature T changes from time to time due to the natural temperature drop during casting and the steel output temperature variation for each pan. A difference ΔT between the molten steel temperature T and the solidification temperature _TL from the solidification temperature calculation unit 101, that is, a molten steel superheat ΔT is calculated and output to the taper amount calculation table selection unit 104. And this taper amount calculation table selection part 104 is a short side taper amount and casting speed Vc table (molten steel superheat, casting speed, for each molten steel superheat as shown in (a) to (c) of FIG. A table showing the relationship of the taper amount) is stored in advance, and the taper amount / casting speed table of interest is selected from the stored molten steel superheat ΔT, and the selected taper is selected. The amount / casting speed table is input to the taper amount calculation unit 106.

  The short side taper amount / casting speed Vc table for each molten steel superheat in FIG. 7 is the table in (a), where the molten steel superheat ΔT calculated by the molten steel superheat calculation unit 103 is 30 ° C. or less, and the molten steel superheat ΔT. If the temperature exceeds 30 ° C. to 40 ° C. or less, the table of (b) is selected, and if the molten steel superheat ΔT exceeds 40 ° C., the table of (c) is selected.

Further, the casting speed V ₀ during casting is inputted and sequentially stored in the casting speed storage unit 105.

When the selected taper amount / casting speed table is input, the taper amount calculation unit 106 inputs the current casting speed V ₀ at a predetermined period (for example, 1 second period) to the casting speed storage unit 105, and the taper amount / Compared with the casting speed Vc in the casting speed table, the current casting speed is more than 1.5 m / min to less than 2.0 m / min in the casting speed Vc range of the next step (for example, FIG. 7B). If it is within the range, it is determined whether or not it is in the range of more than 2.0 m / min to less than 2.5 m / min). The short side taper amount X is obtained and output to the short side drive amount calculation unit 107. On the other hand, if it is not within the casting speed range of the next step, there is no need for taper increase control, so the information for maintaining the taper amount (the same short side taper amount as the previous time) is calculated in the same way as described above. Output to the unit 107.

The short side drive amount calculation unit 107 obtains a movable taper amount ΔX which is a difference between the input taper amount X and the stored current taper amount a ₀ , and the obtained movable taper amount ΔX is a change allowable range set in advance. If it is within the change allowable range, the movable taper amount ΔX is transmitted to the cylinder drive unit 108 and the inputted taper amount X is stored. When the movable taper amount ΔX is inputted, the cylinder 9 is driven by the cylinder driving unit 108 to change the taper amount a ₀ of the short side mold 1b, and thereafter the casting speed V ₀ is increased to the casting speed Vc.

  By performing the short side taper control, casting can be performed while the shell thickness of the long side corner portion is secured.

  The case where the casting width in the continuous casting is constant and the distance from the meniscus to the lower end of the mold has been described above. When changing the casting width, the taper amount may be changed in proportion to the rate of change of the casting width. When changing the distance from the meniscus to the lower end of the mold, the taper amount may be changed in proportion to the rate of change of the distance from the meniscus to the lower end of the mold.

Shell deformation explanatory drawing in the long side corner part in the casting direction in a mold. Explanatory drawing which shows the relationship between casting speed, molten steel superheat, and a long side corner part shell thickness. Explanatory drawing which shows the relationship between short side taper amount, long side corner part shell thickness, and air gap amount. Explanatory drawing which shows the relationship between casting speed, long side corner part shell thickness, and the amount of air gaps. Explanatory drawing which shows the relationship between molten steel superheat, a long side corner part shell thickness, and the amount of air gaps. The figure which shows the relationship between the position in a casting_mold | template, and a solidification shrinkage | contraction amount in case a casting_mold | template has a straight shape and a two-stage shape. The figure which shows the relationship of the casting speed of the embodiment of this invention, molten steel superheat, and the amount of short side taper. The control block diagram of embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1; Mold 1a; Mold long side 1b; Mold short side 2; Molten steel 3; Meniscus 4; Solidified shell 4a; Solidified shell 4b on long side of mold Air gap of the part 6; Cast slab corner 7; Straight mold short side 7a; Shell solidification shrinkage shape when straight mold is applied 8; Two-stage mold short side 8a; Shell solidification shrinkage when two-stage mold is applied Shape 9; Cylinder a ₀ ; Short side taper amount

Claims (2)

In a continuous casting process in which molten steel is continuously cast at a casting speed in a range of 1.0 to 3.0 m / min, and the taper amount of the short side constituting the continuous casting mold of the continuous casting process is controlled. As the casting speed of the molten steel to be cast increases, the taper amount on the short side is controlled in a direction to increase the taper amount on the short side, and the taper amount on the short side increases in proportion to the superheat amount of the molten steel. A method for controlling a short side taper of a continuous casting mold, wherein the side taper amount is controlled .
Here, the taper amount a ₀ on the short side is defined as follows. The short side distance is the distance between the inner surfaces of both short sides.
a ₀ = (distance between short sides at the meniscus portion−distance between short sides at the lower end of the mold) / 2 2. The method for controlling a short side taper of a continuous casting mold according to claim 1, wherein a mold having a short side taper having a multistage or curved shape close to a straight shape or a solidification shrinkage shape of a slab is used.

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