KR101466202B1 - Controlling method for surface quality of slab - Google Patents

Abstract

The present invention relates to a slab surface quality control method, in which a measuring wire made of a different metal material is inserted into a powder, a sintered layer, and a liquid layer formed on the surface of molten steel injected into a mold of a continuous casting process, A step of measuring the thickness of the powder, the sintered layer and the liquid phase layer, and the thickness of the powder, the sintered layer and the liquid phase layer measured through the measurement wire are substituted into the relational expression, Wherein the step of predicting the incidence of extreme low carbon steel pinholes in the slab includes adjusting the powder and sintered layer to a predetermined range or more to reduce the predicted pinhole incidence rate, Layer thickness to reduce the incidence of extremely low carbon steel pinholes in the slab It includes.

Description

TECHNICAL FIELD [0001] The present invention relates to a slab surface quality control method,

The present invention relates to a slab surface quality control method, and more particularly, to a slab surface quality control method capable of reducing the occurrence rate of defects such as pinholes or hooks in slab manufacturing, by controlling the thicknesses of the powder, sintered layer, will be.

In general, a continuous casting machine is a machine which is produced in a steel making furnace, receives molten steel transferred to a ladle by a tundish, and supplies it to a mold for a continuous casting machine to produce a cast steel having a predetermined size. The continuous casting machine includes a ladle for storing molten steel, a casting mold for casting a tundish and molten steel introduced from the tundish into a cast piece having a predetermined shape, and a plurality of casting pieces connected to the mold Of pinch rolls.

In other words, the molten steel introduced in the ladle and the tundish is formed into a cast steel having a predetermined width, thickness and shape in the mold and is conveyed through the pinch roll, and the cast steel conveyed through the pinch roll is cut by a cutter, Or a semi-finished product such as a slab, a bloom, or a billet.

During the continuous casting process, the molten steel injected into the mold from the tundish is cooled while passing through the mold, so that a hook structure in the molten steel that solidifies during the cooling process can be generated. Such a hook structure acts as a defect factor in the produced steel, which may cause degradation of steel quality.

Related Prior Art Korean Patent Laid-Open No. 10-2005-0002223 (published on Jan. 7, 2005, entitled "Prediction of Hook Characteristics of Extremely Low Carbon Steel Using the Warming Index and Mold Maximum Movement Acceleration)".

More particularly, the present invention relates to a slab surface quality control method capable of reducing the occurrence rate of defects such as pinholes or hooks in the manufacture of slabs by controlling the thicknesses of the powder, sintered layer and liquid phase layer.

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

According to another aspect of the present invention, there is provided a slab surface quality control method comprising: inserting a measuring wire made of a different metal material into a powder, a sintered layer, and a liquid layer formed on a surface of molten steel injected into a mold of a continuous casting process, The thickness of the powder, the sintered layer and the liquid phase layer is measured through the length of the slurry, the thickness of the powder, the sintered layer and the liquid phase layer measured through the measuring wire is substituted into the relational expression, The method comprising the steps of: predicting the incidence of extreme low carbon steel pinholes; predicting the incidence of extreme low carbon steel pinholes in the slab; adjusting the powder and sintered layer to above a predetermined range to reduce the predicted pinhole incidence rate; The thickness of the slab is controlled to be equal to or less than the set thickness of the liquid layer to reduce the incidence of extremely low carbon steel pinholes in the slab And the step of including pohamreul,
The relational expression
The pinhole generation rate (%) of the ultra-low carbon steel = - 0.2784 x (powder + sintered layer) / liquid phase layer +1.1493,
The predetermined range thickness of the powder and sintered layer is 40 mm or more, and the predetermined range thickness of the liquid phase layer may be 15 mm or less.

Specifically, in the step of measuring the thicknesses of the powder, the sintered layer and the liquid phase layer, the measuring wire inserted into each layer may be Al, Cu and Fe.

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As described above, according to the present invention, the thickness of a powder, a sintered layer and a liquid phase layer formed on a surface of a molten steel injected into a mold through a measuring wire formed of a metal having a different melting point is measured, And the thickness of the liquid phase layer is controlled, it is possible to reduce the incidence of defects such as pinholes or hooks in the manufacture of slabs.

FIG. 1 is a conceptual diagram for explaining a continuous casting machine around a flow of molten steel M related to an embodiment of the present invention.
Fig. 2 is a conceptual diagram showing a distribution pattern of the molten steel M in a mold and its vicinity adjacent to the present invention.
3 is a flowchart illustrating a slab surface quality control method according to an embodiment of the present invention.
4 is a view illustrating a method of inserting a metal material wire of a slab surface quality control method according to the present invention into a powder layer to measure the thickness.
5 is a graph showing the rate of occurrence of extremely low carbon steel pinholes according to the slab surface quality control method according to the present invention.
6 is a graph showing the incidence of extremely low carbon steel pinholes according to the slab surface quality control method according to the present invention.

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.

FIG. 1 is a conceptual view showing a continuous casting machine according to an embodiment of the present invention, focusing on the flow of a molten steel M. FIG.

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, or billets that are primarily rolled materials and long products of simple cross-section, such as square, rectangular, or circular.

The continuous casting machine includes a ladle 10 and a tundish 20, a mold 30, secondary cooling bands 60 and 65, a pinch roll (not shown), and a cutter (not shown) .

A tundish 20 is a container for receiving molten metal from a ladle 10 and supplying molten metal to a mold 30. The ladles 10 are provided in pairs to alternately receive molten steel and supply the molten steel 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.

Mold 30 is typically made of water-cooled copper and allows the molten steel taken to be first 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, mainly the walls, of the mold 30 can be rotated to be away from or close to each other to have a certain level of taper.

The mold 30 has a function of forming a solidified shell or a solidified shell 81 so that the casting struc- ture pulled out from the mold maintains a constant shape and a molten metal which is not yet solidified does not flow 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 to prevent the molten steel from adhering to the wall surface of the mold, and in order to prevent friction between the mold 30 and the solidification shell 81 during oscillation, A lubricant such as a powder is used. The powder is added to the molten metal in the mold 30 to become slag, and the lubricant of the mold 30 and the solidification shell as well as the prevention of the oxidation and nitriding of the molten metal in the mold 30 and the non-metallic inclusion It also carries out the function of absorption.

The secondary cooling bands 60 and 65 further cool the molten steel that has been 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. Most of the solidification of the cast steel is accomplished by the secondary cooling.

The pulling device employs a multi-drive type or the like that uses several pairs of pinch rolls (not shown) so as to pull out the casting slides without slipping. The pinch roll (not shown) pulls the solidified tip portion of the molten steel in the casting direction, thereby allowing molten steel passing through the mold 30 to continuously move in the casting direction.

The continuously produced musical piece is cut to a predetermined size by a predetermined cutter (not shown).

That is, 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 so as to be immersed in the molten steel in the tundish 20 so that the molten steel M is not exposed to the air and oxidized and nitrided.

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, discharge speed and 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.

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 by the uncooled molten steel M being wrapped in the solidifying shell 81 by the way that the peripheral portion first coagulates.

The non-solidified molten steel M moves together with the solidifying shell 81 in the casting direction as the pinch roll (not shown) pulls the tip end portion 83 of the fully-solidified cast slab 80. The non-solidified molten steel (M) 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 (M) 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 solidified shell 81 as a whole. 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.

Referring to FIG. 2, a pair of discharge ports 25a are formed on the end sides of the immersion nozzle 25 on the left and right sides in the drawing. It is assumed that the shapes of the mold 30 and the immersion nozzle 25 are symmetrical with respect to the center line C, and only the left side is shown in the drawing.

The molten steel M discharged along with the argon (Ar) gas at the discharge port 25a draws a locus that flows in the upward direction A1 and the downward direction A2 as indicated by arrows A1 and A2 do.

A powder layer 51 is formed on the upper portion of the mold 30 by a powder supplied from a powder feeder (not shown). The powder layer 51 may include a layer existing in the form in which the powder is supplied and a layer sintered by heat of the molten steel M (the sintered layer is formed closer to the non-solidified molten steel M). A slag layer or a liquid flowing layer 52 formed by melting the powder by the molten steel M is present below the powder layer 51. The liquid flowing layer (52) maintains the temperature of the molten steel (M) in the mold (30) and blocks the penetration of foreign matter. A part of the powder layer 51 solidifies at the wall surface of the mold 30 to form the lubricant layer 53. [ The lubricating layer 53 functions to lubricate the solidifying shell 81 so that it does not adhere to the mold 30.

The thickness of the solidifying shell 81 becomes thicker along the casting direction. The portion of the solidifying shell 81 located in the mold 30 is thin and an oscillation mark 87 is formed according to the oscillation of the mold 30. [ The solidifying shell 81 is supported by the support roll 60 and thickened by the spraying means 65 for spraying water. The solidifying shell 81 is thickened, and a bulging region 88 is formed in which a portion protrudes convexly.

During the above process, when the molten steel M is discharged from the mold 30 to manufacture the slab, the temperature of the molten steel M is maintained at the surface of the mold 30, and a part thereof flows into the wall surface of the mold 30, Defects such as pinholes or hooks may be generated in the slab produced according to the thickness of the powder layer. In order to prevent this, in the present invention, the thickness of the powder, the sintered layer and the liquid phase layer constituting the powder layer is measured and the thickness is controlled to reduce the occurrence of defects such as pinholes and hooks.

3 is a flowchart illustrating a method of controlling slab surface quality according to an embodiment of the present invention. The present invention relates to a method for manufacturing a slab, which comprises the steps of measuring the thickness of a powder, a sintered layer and a liquid phase layer formed on a surface of a mold, and substituting the thicknesses of a powder, a sintered layer and a liquid phase layer, Predicting the incidence, and adjusting the thickness of the powder, the sintered layer and the liquid phase layer to above or below a predetermined range.

In the continuous casting process, a powder, a sintered layer and a liquid phase layer are formed on the surface of the mold made of slab by discharging the molten steel to prevent the temperature of the molten steel from dropping.

As shown in FIG. 4, the temperature of each layer of the powder, the sintered layer and the liquid phase layer is differently formed. By inserting measurement wires of metal which are not dissolved at the temperature of each layer into each layer, The thickness of the sintered layer and the liquid phase layer is measured (S10).

That is, the powder layer is formed at a temperature of 200 to 600 ° C., the sintered layer is formed at a temperature of 650 to 1000 ° C., and the liquid phase layer is formed at a temperature of 1100 to 1200 ° C. as a layer adjacent to the molten steel.

Using these temperature characteristics, Cu does not dissolve at the temperature of the sintered layer but dissolves at the temperature of the sintered layer. However, Cu does not dissolve at the temperature of the liquid phase but dissolves at the temperature of the liquid phase. The measuring wire is formed of dissolving Fe material and inserted into each layer to measure the thickness of each layer.

When the thicknesses of the powder, the sintered layer and the liquid phase layer are measured through the measurement wire, the measured thickness is substituted into the relational expression to predict the rate of occurrence of extremely low carbon steel pinholes of the slab manufactured through the mold (S20).

At this time,

The pinhole generation rate (%) of the ultra-low carbon steel = - 0.2784 x (powder + sintered layer) / liquid layer +1.1493.

Through the above relation, the generation rate of extreme low carbon steel pinholes in the slab is predicted to adjust the powder and sintered layer to a predetermined range or more, and the thickness of the liquid phase layer is controlled to be less than the set liquid phase layer thickness to reduce the incidence of extremely low carbon steel pinholes in the slab (S30).

At this time, the predetermined range thickness of the powder and sintered layer is 40 mm or more, and the set range thickness of the liquid phase layer is set to 15 mm or less.

Hereinafter, the present invention will be described with reference to an embodiment.

The powder layer formed on the surface of the mold composed of the powder, the sintered layer and the liquid phase layer is different in temperature from each layer due to the distance from the molten steel in the mold.

Using this temperature difference, a measuring wire formed so as to have a different metal material is inserted into each layer of the powder layer, thereby measuring the thickness of each layer.

In the method of measuring the thickness of the powder, sintered layer and liquid layer through the measuring wire, a measuring wire of Al, Cu, Fe material is inserted into each layer, the inserted measuring wire is taken out, The thickness of each layer is measured.

By adjusting the thickness of each layer to the range set through the thickness of each layer thus measured, it is possible to reduce defects such as hooks and pinholes that may occur during slab manufacturing.

When adjusting the thickness of the powder, the sintered layer and the liquid phase, the results are shown in Table 1 below.

At this time, the precondition for implementation is as follows.

- Slab thickness: 200 ~ 300mm

- Slab width: 1000 ~ 2000mm

- Discharge volume: 3.5 ~ 4.0t / min

- immersion nozzle angle: downward 25 degrees

- immersion nozzle shape: round

- Nozzle Depth Depth: 100 ~ 200mm

 (Based on depth from the bath surface to the top of the discharge port)

- Powder properties: Basicity 0.5 ~ 1.5

                Viscosity 2 ~ 5poise

. 5 and FIG. 6 show the relationship between the extremely low carbon steel pinhole incidence rate.

division Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Raw powder + sintered layer / liquid phase layer 2.0 2.3 2.0 1.8 2.3 3.4 Ultra Low Carbon Steel Pinhole Occurrence Rate (%) 58.7 43.4 56.6 64.4 59.6 16.3 Average thickness of raw powder + sintered layer (mm) 41 40 31 31 37 47 Average thickness of liquid phase (mm) 23 21 20 20 19 15

As described above, in the case of Example 1, the average thickness of the powder and the sintered layer was 41 mm, the average thickness of the liquid phase layer was 23 mm, the value obtained by dividing the thickness of the powder plus sintered layer by the thickness of the liquid phase was 2.0 It can be seen that the incidence of extremely low carbon steel pinholes is 58.7%.

In the case of Example 2, when the average thickness of the powder and the sintered layer was 40 mm, the average thickness of the liquid phase layer was 21 mm, and the value obtained by dividing the thickness of the powder + sintered layer by the thickness of the liquid phase layer was 2.3, Is 43.4%.

In the case of Example 3, when the average thickness of the powder and sintered layer was 31 mm, the average thickness of the liquid phase layer was 20 mm, and the value obtained by dividing the thickness of the powder plus sintered layer by the thickness of the liquid phase layer was 2.0, Is 56.6%.

In the case of Example 4, when the average thickness of the powder and sintered layer was 31 mm, the average thickness of the liquid phase layer was 20 mm, and the value obtained by dividing the thickness of the powder plus sintered layer by the thickness of the liquid phase layer was 1.8, Is 64.4%.

In the case of Example 5, when the average thickness of the powder and sintered layer was 37 mm, the average thickness of the liquid phase layer was 19 mm, and the value obtained by dividing the thickness of the powder plus sintered layer by the thickness of the liquid phase layer was 2.3, Is 59.6%.

In Example 6, when the average thickness of the powder and sintered layer was 47 mm, the average thickness of the liquid phase layer was 15 mm, and the value obtained by dividing the thickness of the powder + sintered layer by the thickness of the liquid phase layer was 3.4, Is 16.3%.

It can be seen that the average thickness of the powder and the sintered layer is 40 mm or more and the average thickness of the liquid layer is 15 mm or less, the rate of occurrence of pinholes and hooks is considerably reduced.

According to the present invention thus constituted, the thickness of the powder, sintered layer and liquid phase layer constituting the powder layer formed on the surface of the molten steel injected into the mold through the measuring wire formed of a metal having a different melting point is measured, Since the thickness of the layer is controlled, there is an advantage that the occurrence rate of defects such as pinholes or hooks can be reduced in the manufacture of slabs.

The slab surface quality control method as described above is not limited to the configuration and operation of the embodiments described above. The embodiments may be configured so that all or some of the embodiments may be selectively combined so that various modifications may be made.

10: Ladle 20: Tundish
30: Mold 51: Powder layer
60: Support roll 65: Spray
80: Casting Casting 81: Solidification Shell
82: non-solidified molten steel 83:
85: Solidification completion point 91: Cutting point

Claims (4)

A measuring wire made of a different metal material is inserted into the powder, the sintered layer and the liquid phase layer formed on the surface of molten steel injected into the mold of the continuous casting process, and the thickness of the powder, the sintered layer and the liquid phase layer is measured ;
Substituting the thicknesses of the powder, the sintered layer and the liquid phase layer, measured through the measuring wire, into a relational expression to predict the rate of occurrence of extremely low carbon steel pinholes in the slab produced through the mold; And
The powder and the sintered layer are adjusted to a predetermined range or more and the thickness of the liquid phase layer is controlled to be less than or equal to the set liquid phase layer thickness in order to reduce the predicted pinhole incidence rate by predicting the rate of occurrence of the extremely low carbon steel pinholes of the slab, Reducing the incidence of extreme low carbon steel pinholes in the slab,
The relational expression
The pinhole generation rate (%) of the ultra-low carbon steel = - 0.2784 x (powder + sintered layer) / liquid phase layer +1.1493,
Wherein the predetermined range thickness of the powder and the sintered layer is 40 mm or more and the set range thickness of the liquid phase layer is 15 mm or less.
The method according to claim 1,
Wherein the measuring wire inserted in each layer in the step of measuring the thickness of the powder, sintered layer and liquid layer is Al, Cu and Fe.
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