KR20130120872A - Control method for casting of ultra low carbon steel - Google Patents

Abstract

The present invention relates to a casting control method of ultra low carbon steel which controls the quality of ultra low carbon steel by controlling the discharge amount of molten steel depending on the sulfur content in the molten steel, and provides: a step for determining a target pin hole index and the width of casting; a step for controlling the sulfur content of the molten steel through a second tempering process and for calculating the actual sulfur content of the molten metal; a step for calculating the discharge amount of required molten steel according to the target pin hole index, the width of a product, and the sulfur content of the molten steel; and a step for starting casting by judging whether the discharge amount of the required molten steel compared with the predetermined reference discharge amount is over the reference discharge amount or not. [Reference numerals] (AA) Start;(BB) End;(S10) Determining a target pin hole index and the width of casting;(S20) Controlling the sulfur content of the molten steel through a second tempering process;(S30) Caculating the actual sulfur content of the molten metal;(S40) Calculating the discharge amount of required molten steel according to the target pin hole index, the width of a product, and the sulfur content of the molten steel;(S50) Required discharge amount >the predetermined reference discharge amount?;(S51) [Re-processing time + ladle transporting time] > current remaining time for casing the ladle?;(S60) Producing the predetermined ultra low carbon steel;(S61) Changing the kind of steel for production

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a control method for casting ultra-

More particularly, the present invention relates to an extremely low carbon steel casting control method for controlling an extremely low carbon steel quality by controlling a molten steel discharge amount according to sulfur content in molten steel.

In general, a continuous casting machine is a facility for producing cast steel of a certain size by receiving a molten steel produced in a steelmaking furnace and transferred to a ladle (Tundish) and then supplied to a continuous casting machine mold.

 The continuous casting machine includes a ladle for storing molten steel, a casting mold for initially cooling the molten steel introduced from the tundish and the tundish into a strand having a predetermined shape, and a plurality of strands connected to the mold to move the strand formed in the mold Of pinch rolls.

In other words, the molten steel introduced from the ladle and the tundish is formed into a strand having a predetermined width, thickness and shape in the mold and is conveyed through the pinch roll, and the strand conveyed through the pinch roll is cut by a cutter, Such as a slab, a bloom, or a billet having a slab, such as a steel plate,

Related prior art is Korean Patent Publication No. 2005-21961 (published on Mar. 03, 2005, entitled " Method of manufacturing ultra-low carbon steel slab).

The present invention is to provide an extremely low carbon steel casting control method for producing steel by calculating a discharge amount suitable for a steel using a target pinhole index, a casting width, and a sulfur content of molten steel actually measured.

The present invention is intended to provide an extremely low carbon steel casting control method for controlling the steel quality which controls the sulfur content secondarily in accordance with the residual casting time of the granite.

The technical problems to be achieved by the present invention are not limited to the technical problems mentioned above.

According to another aspect of the present invention, there is provided an ultra low carbon steel casting control method comprising the steps of: determining a target pinhole index and a casting width; Controlling the sulfur content of the molten steel by secondary refining and measuring the actual sulfur content of the molten steel; Calculating a required discharge amount of molten steel based on the target pinhole index, product width, and sulfur content of molten steel; And a step of comparing the required discharge amount of molten steel with the reference discharge amount to determine whether the calculated amount is equal to or greater than a preset reference discharge amount, and commencing casting.

Specifically, if the required discharge amount of the molten steel calculated above is compared with the reference discharge amount and is equal to or less than the reference discharge amount, it is determined whether the sum of the sulfur content of the molten steel is greater than the residual casting time, And converting the steel grade into a steel grade by predicting the index.

Further, if the sum of the reprocessing time and the ladle transport time is less than the residual ladle casting time, the step of controlling the sulfur content of the molten steel may be further controlled.

Further, the necessary discharge amount [T] of the molten steel can be determined by the following relational expression.

Relation

Required discharge amount [T] = (PDI-a0 [S] + a2W + a3) / a1

The pinhole defect index (PDI) is the target pinhole index, W is the casting width (m), [S] is the sulfur content of molten steel, a0 is 474.9, a1 is 7.44, a2 is 7.9 and a3 is 14.1.

Also, the reference discharge amount may vary depending on the casting width.

As described above, the present invention has an effect of minimizing the incidence of defects by controlling the pinholes of the slab to a desired level by predicting the required discharge amount of molten steel without measuring the performance of the pinhole index.

1 is a side view showing a continuous casting machine according to an embodiment of the present invention.
Fig. 2 is a conceptual diagram for explaining the continuous casting machine of Fig. 1 centered on the flow of molten steel M. Fig.
3 is a photograph of a pinhole defect in a slab produced by continuous casting.
4 is a flowchart illustrating an extremely low carbon steel casting control method according to an embodiment of the present invention.
5 is a graph showing the pinhole generation index according to the casting width.
FIG. 6 is a graph showing the pinhole generation index according to the discharge amount.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same components are denoted by the same reference symbols whenever possible. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

1 is a side view showing a continuous casting machine according to an embodiment of the present invention.

Continuous casting is a casting process in which a molten metal is continuously cast into a bottomless mold while continuously drawing a steel ingot or steel ingot. Continuous casting is used to manufacture slabs, blooms and billets, which are mainly rolled materials, and long products of simple cross-section such as square, rectangle, and circle.

The shape of a continuous casting machine is classified into a vertical type and a vertical bending type. In Figs. 1 and 2, a vertical bending-like shape is illustrated.

1, the continuous casting machine may include a ladle 10 and a tundish 20, a mold 30, a secondary cooling stand 60 and 65, a pinch roll 70, and a cutter 90 have.

The tundish 20 is a container that receives the molten metal from the ladle 10 and supplies the molten metal to the mold 30. Ladle 10 is provided in a pair, alternately receives molten steel to supply to the tundish 20. In the tundish 20, the supply rate of the molten metal flowing into the mold 30 is controlled, the molten metal is distributed to each mold 30, the molten metal is stored, and the slag and the nonmetallic inclusions are separated.

The mold 30 is typically made of water-cooled copper and allows the molten steel to be primary cooled. The mold 30 has a pair of structurally opposed faces open to form a hollow portion for receiving molten steel. In the case of manufacturing the slab, the mold 30 includes a pair of barriers and a pair of end walls connecting the barriers. Here, the end wall has a smaller area than the barrier. The walls of the mold 30, mainly short walls, may be rotated away from or close to each other to have a certain level of taper. This taper is set to compensate for the shrinkage due to the solidification of the molten steel M in the mold 30. The degree of solidification of the molten steel (M) will vary depending on the carbon content, the type of powder (steel cold Vs slow cooling), casting speed and the like depending on the steel type.

The mold 30 is formed such that a solidified shell or a solidified shell 81 is formed so as to maintain the shape of the casting pluck pulled out from the mold 30 and to prevent the molten metal from being hardly outflowed from flowing out. . The water-cooling structure includes a method using a copper tube, a method of water-cooling the copper block, and a method of assembling a copper tube having a water-cooling groove.

The mold 30 is oscillated by the oscillator 40 to prevent the molten steel from sticking to the wall of the mold. Lubricant is used to reduce friction between the mold 30 and the solidification shell 81 and prevent burning during oscillation. As the lubricant, there is oil to be sprayed and powder added to the molten metal surface in the mold 30. The powder is added to the molten metal in the mold 30 to become slag so that the lubricating of the mold 30 and the solidifying shell 81 as well as the prevention and nitriding of the molten metal in the mold 30 and the warming, It also functions to absorb non-metallic inclusions. A powder feeder 50 is installed to feed the powder into the mold 30. The portion of the powder feeder 50 for discharging the powder is directed to the inlet of the mold 30.

The secondary cooling zones 60 and 65 further cool the molten steel primarily cooled in the mold 30. The primary cooled molten steel is directly cooled by the spraying means 65 for spraying water while being maintained by the support roll 60 so that the coagulation angle is not deformed. The solidification of the cast steel is mostly made by the secondary cooling.

The drawing device adopts a multidrive method using a pair of pinch rolls 70 and the like so as to pull out the cast pieces without slipping. The pinch roll 70 pulls the solidified tip of the molten steel in the casting direction, thereby allowing the molten steel passing through the mold 30 to continuously move in the casting direction. The cutter 90 is formed so as to cut a continuously produced musical instrument into a predetermined size. As the cutter 90, a gas torch or an oil pressure shearing machine may be employed.

Fig. 2 is a conceptual diagram for explaining the continuous casting machine of Fig. 1 centered on the flow of molten steel M. Fig. Referring to this figure, the molten steel M flows into the tundish 20 while being accommodated in the ladle 10. For this flow, the ladle 10 is provided with a shroud nozzle 15 extending toward the tundish 20. The shroud nozzle 15 extends into the molten steel in the tundish 20 so that the molten steel M is not exposed to the air to be oxidized and nitrided. The case where the molten steel M is exposed to air due to breakage of the shroud nozzle 15 or the like is referred to as open casting.

The molten steel M in the tundish 20 is caused to flow into the mold 30 by the submerged entry nozzle 25 extending into the mold 30. The immersion nozzle 25 is disposed at the center of the mold 30 so that the flow of the molten steel M discharged from both the discharge ports of the immersion nozzle 25 can be made symmetrical. The start, the discharge speed and the interruption of the discharge of the molten steel M through the immersion nozzle 25 are determined by a stopper 21 provided on the tundish 20 in correspondence with the immersion nozzle 25. Specifically, the stopper 21 can be vertically moved along the same line as that of the immersion nozzle 25 so as to open and close the inlet of the immersion nozzle 25. The control of the flow of the molten steel M through the immersion nozzle 25 may use a slide gate method different from the stopper method. The slide gate controls the discharge flow rate of the molten steel (M) through the immersion nozzle (25) while the plate material slides horizontally in the tundish (20).

Molten steel (M) in the mold (30) starts to solidify from a portion in contact with the wall surface of the mold (30). This is because the periphery of the molten steel M is liable to lose heat by the water-cooled mold 30. The rear portion along the casting direction of the cast slab 80 is formed into a shape in which the non-solidified molten steel 82 is wrapped in the solidifying shell 81 by the method in which the peripheral portion first coagulates.

As the pinch roll 70 (FIG. 1) pulls the tip portion 83 of the completely cast solid cast piece 80, the unsolidified molten steel 82 moves together with the solidified shell 81 in the casting direction. The non-solidified molten steel (82) is cooled by the spraying means (65) for spraying the cooling water in the up-shifting process. This causes the thickness of the non-solidified molten steel (82) to gradually decrease in the cast steel (80). When the cast steel 80 reaches one point 85, the cast steel 80 is filled with the solidification shell 81 of the entire thickness. The solidification casting 80 having been solidified is cut to a predetermined size at the cutting point 91 and is divided into a slab P such as a slab or the like.

Such slabs are particularly susceptible to pinhole defects as shown in Fig. 3 if the solidification temperature is high and the oscillation marks are deep, especially in case of ultra low carbon steels. In case of using as slabs for automobiles, pinhole defects cause fatal defects in the final product.

The present invention will be described with reference to FIG. 4 for controlling the extremely low carbon steel quality in the above process.

The casting control method of the ultra-low carbon steel of the present invention can be applied to the case where the electromagnetic stirring device in the mold is not used.

First, a target pinhole index and a casting width according to the set steel grade are determined (S10). In the embodiment of the present invention, it is preferable to set an extremely low carbon steel containing 0.29 to 0.036% by weight of [Ti] and 0.09 to 0.12% by weight of [Mn] in the total components.

Subsequently, the sulfur content [S] of the molten steel is controlled by secondary refining (S20). At this time, the secondary refining may include an LF (Ladle Furnace) and a vacuum degassing (RH) process. At this time,

Subsequently, the actual sulfur content (actual [S]) of the molten steel whose sulfur content is controlled is measured (S30). At this time, the actual sulfur content can be analyzed by sampling the controlled molten steel.

Subsequently, the discharge amount [T] of the molten steel is calculated from the target pinhole index, the casting width, and the sulfur content of the molten steel (S40)

At this time, the necessary discharge amount of molten steel is calculated by the following relational expression 1.

Relationship 1

Required discharge amount = (PDI - a0 · [S] + a2 · W + a3) / a1

In this case, the PDI (Pinhole Defect Index) is the target pinhole index, W is the casting width (m), [S] is the sulfur content of the filler, a0 is 474.9, a1 is 7.44, a2 is 7.9, and a3 is 14.1.

Here, the required discharge amount can also be referred to as required [T].

Subsequently, the discharge amount of molten steel calculated above is compared with the reference discharge amount, and it is determined whether the reference discharge amount is equal to or greater than the reference discharge amount (S50). At this time, the reference discharge amount (reference [T]) is variable according to the casting width, which can be expressed as shown in Table 1.

Table 1

As described above, it can be seen that the reference discharge amount varies depending on each casting width, which corresponds to the lower limit of the discharge amount. That is, when casting is performed below the reference discharge amount, casting is affected.

Subsequently, when the required discharge amount of the molten steel is equal to or greater than the reference discharge amount, casting is started at the set extremely low carbon steel (S60). If the calculated discharge amount of the molten steel is less than the reference discharge amount, (S51). If the sum is greater than the remaining casting time (S51), the steel product is switched to produce a larger steel product (S61).

The reason why the required discharge rate of molten steel is less than the standard discharge amount is that the quality of the steel is lowered due to the fact that the pinhole index of the actual molten steel does not reach the target pinhole index due to a large amount of S contained in the molten steel. .

The reason why the sum of the [S] reconditioning time of the molten steel and the ladle transport time is compared with the residual casting time of the ladle is that the ladle casting time must be made consecutively to prevent the casting from being stopped, If the sulfur content of the molten steel is re-controlled and the casting of the ladle is stopped, the cost of producing the steel is higher than the cost of production.

For example, if the residual casting time is 30 minutes, and the [S] re-control time is 20 minutes and the transport time is 5 minutes, re-controlling the sulfur content of the molten steel is preferable for controlling the steel quality , If the residual casting time of the granite is 25 minutes, [S] the reconditioning time is 20 minutes, and the transport time is 5 minutes, it is preferable to convert the steel grade to produce.

Here, the residual casting time of the floors can be found by the following relational expression (2).

Relation 2

It is also clear that the residual casting time of the ladle, the [S] re-control time, and the transportation time may vary depending on the equipment.

As described above, when the casting width and the target pinhole index are set and the content of sulfur in the actual molten steel is measured, the required molten steel discharge amount can be calculated by the relational expression 1. The required molten steel discharge amount can be compared with the standard discharge amount Can be determined. Table 2 summarizes them.

Table 2

Here, as shown in Table 2, when the required discharge amount according to the set casting width, the target pinhole index, and the actual sulfur content [S] of the molten steel is larger than the reference discharge amount, the first, second, fourth, fifth If casting is possible but the required discharge amount is smaller than the reference discharge amount, the casting can not be performed with the set steel type in case of No. 3 or No. 6, and the casting is produced by converting the steel type, or the secondary refining is performed to lower the sulfur content Make it possible to cast with steel.

FIG. 5 is a graph showing the relationship between the sulfur and the pinhole generation index when the discharge amount is 3.1 according to the casting width. As the casting width is larger, the influence of the pinhole generation index due to sulfur in the molten steel is small. The effect of sulfur content on the index of pinholes is shown by the content of sulfur in molten steel.

6 is a graph showing the relationship between the sulfur content of the molten steel and the pinhole generation index when the casting width is 1600 mm according to the discharge amount. As the discharge amount is smaller, the influence of the pinhole generation index depending on the molten steel content is smaller, The greater the effect of the index of pinhole occurrence according to the content of molten steel. That is, even if the content of sulfur in the molten steel is large, the amount of molten steel discharged is influenced by the discharged amount of molten steel. Therefore, in the embodiment of the present invention, the amount of discharged molten steel is controlled to control the pinhole generation index.

Here, the amount of discharge is small, which means that the discharge speed is slow because of a small amount of discharged molten steel per minute, and the discharge amount is the same as the discharge speed is fast because of a large amount of molten steel discharged per minute.

Therefore, although the required sulfur content varies depending on the slab width and the target pinhole index, it is possible to produce a steel in the target pinhole index without excessively lowering the sulfur content. By controlling the sulfur content to produce steel, the cost savings are excellent.

Also, since the pinhole index can be predicted before the actual pinhole index test, the error of the quality of the steel can be reduced.

The ultra-low carbon steel casting control method as described above is not limited to the construction and the operation manner of the embodiments described above. The above embodiments may be configured such that various modifications may be made by selectively combining all or part of the embodiments.

10: Ladle 15: Shroud nozzle
20: Tundish 25: Immersion Nozzle
25a: Discharge port 30: Mold
51: Powder layer 52: liquid flowing layer
53: Lubrication layer 60: Support roll
65: Spray 80: Play piece
81: Solidification shell 82: Non-solidified molten steel
83: tip 87: oscillation mark
88: bulging region 90: cutter
91: Cutting point

Claims (5)

A target pinhole index, and a casting width;
Controlling the sulfur content of the molten steel by secondary refining and measuring the actual sulfur content of the molten steel;
Calculating a required discharge amount of molten steel based on the target pinhole index, product width, and sulfur content of molten steel; And
And comparing the required discharge amount of the molten steel calculated above with a preset reference discharge amount to determine whether the discharge amount is equal to or greater than the reference discharge amount, and starting casting.
The method according to claim 1,
When the required discharge amount of the molten steel calculated above is less than the standard discharge amount, it is determined whether or not the sulfur content of the molten steel is greater than the current ladle remaining casting time if the sum of the re-control time and ladle transportation time is larger than the current ladle casting time. Production method by converting the steel grade in anticipation of the ultra-low carbon steel casting control method.
The method according to claim 2,
And re-controlling the sulfur content of the molten steel when the sum of the reprocessing time and the ladle transportation time is less than the current ladle remaining casting time.
The method according to claim 1,
Wherein the required discharge amount [T] of the molten steel is determined by the following relational expression.
Relation
Required discharge amount [T] = (PDI-a0 [S] + a2W + a3) / a1
At this time, the PDI (Pinhole Defect Index) is the target pinhole index, W is the casting width (m), [S] is the sulfur content of the molten steel, a0 is 474.9, a1 is 7.44, a2 is 7.9, and a3 is 14.1.
The method according to claim 1,
Wherein the reference discharge amount is varied according to a casting width.

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