KR101435122B1 - Controlling method for surface quality of ultra low carbon steel slab - Google Patents

Abstract

The present invention relates to a method for controlling the surface quality of a very low carbon steel slab for controlling the molten steel component in order to minimize surface defects that can occur in the slab during the production of ultra low carbon steel. In a molten steel casting an ultra low carbon steel slab, (S), and calculating a hook depth in the slab based on the sulfur content as a parameter, comparing the calculated hook depth with a reference value, and comparing the hook depth And controlling the content of the sulfur component to produce a slab when the slab is out of the reference value.

Description

TECHNICAL FIELD [0001] The present invention relates to a surface quality control method for ultra slim carbon steel slabs,

The present invention relates to a method for controlling surface quality of ultra-low carbon steel slabs, and more particularly to a surface quality control method of ultra low carbon steel slabs for controlling the molten steel composition in order to minimize surface defects that may occur in slabs .

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)".

The present invention provides a method for controlling the surface quality of a very low carbon steel slab capable of minimizing the surface defects of the slabs produced through the control of the molten steel component in the production of ultra low carbon steel.

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

In order to achieve the above object, the present invention provides a method of controlling surface quality of an ultra low carbon steel slab, comprising the steps of: measuring a sulfur component content in a molten steel in a molten steel casting an ultra low carbon steel slab; Calculating a hook depth in the slab by using the content as a variable, comparing the calculated hook depth with a reference value, and controlling the content of the sulfur component when the hook depth is out of the reference value to manufacture the slab Step < / RTI >

Specifically, the hook depth can be calculated by the following relational expression.

Relation

Hook depth (mm) = Aln (X) + B

(Where 0.9? A? 0.99, 5.7? B? 6.4 and X is the sulfur content (wt%) in the molten steel)

The reference value may be set in the range of 0.5 to 1.5 mm.

In the step of producing the slab, the sulfur component can be controlled in the range of more than 0 to 0.008 wt%.

INDUSTRIAL APPLICABILITY As described above, the present invention minimizes the surface defects of slabs produced by controlling the composition of molten steel in the production of ultra-low carbon steel, thereby making it possible to produce ultra-low carbon steel slabs of high quality.

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 method of controlling the surface quality of a very low carbon steel slab according to an embodiment of the present invention.
4 is a view for explaining the formation of a solidification shell in a mold and the occurrence of pinhole defects due to bubble collection during continuous casting according to the present invention.
5 is a photograph of a pinhole defect in a slab produced by continuous casting according to the present invention.
6 is a schematic view illustrating molten steel flow and solidification shell formation in a mold during continuous casting according to the present invention.
7 is a photograph showing the shape of the hook structure formed in the mold and the definition of each part in the continuous casting according to the present invention.
8 is a view for explaining a method for calculating the hook depth according to the present invention.
9 is a graph for explaining a method of calculating an optimum sulfur component-containing region according to an embodiment of 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.

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

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 may include a ladle 10 and a tundish 20, a mold 30, secondary cooling bands 60 and 65, a pinch roll 70, and a cutter 90, as 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 to become slag and functions not only to lubricate the mold 30 and the solidifying shell but also to prevent oxidation and nitriding of the molten metal in the mold and to maintain the temperature and to absorb nonmetallic inclusions floating on the surface of the molten metal do.

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 in which a plurality of pinch rolls (70) are used so as to pull out the casting slides without slipping. The pinch roll 70 pulls the solidified leading end portion of the molten steel in the casting direction so that molten steel passing through the mold 30 can be continuously moved 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 submerged in the molten steel in the tundish 20 so that the molten steel M is not exposed to the air and oxidized and nitrided.

Molten steel M in the tundish 20 flows into the mold by a submerged entry nozzle 25 extending into the mold. 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 starts to solidify from the portion contacting 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.

The non-solidified molten steel 82 moves together with the solidifying shell 81 in the casting direction as the pinch roll 70 pulls the tip end portion 83 of the fully-solidified cast slab 80. 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 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 the powder supplied from the powder feeder 50 (see FIG. 1). The powder layer 51 may include a layer existing in the form in which the powder is supplied and a layer sintered by the heat of the molten steel M (the sintered layer is formed closer to the non-solidified molten steel 82). 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.

3 is a flowchart illustrating a method of controlling the surface quality of a very low carbon steel slab according to an embodiment of the present invention. The present invention minimizes hook defects that may occur on the surface of slabs by controlling the composition of molten steel during the manufacture of ultra-low carbon steel slabs. First, the sulfur content (S) The sulfur content in each of the samples is measured (S10).

Referring to FIG. 4, the molten steel injected during the continuous casting gradually solidifies while passing through the mold, and after the continuous casting is completed, the molten steel is molded into a semi-finished product such as a slab. At this time, in the mold, the solidified shell is formed by solidifying from the molten steel which is in contact with the side mold near the mold wall surface. In addition, the immersion nozzle emits argon gas together with molten steel. When bubbles of argon gas or molten inclusions introduced into the mold are collected in the solidifying shell, the collected bubbles and inclusions remain. As a result, as shown in FIG. 5, pinhole defects are formed immediately below the semi-finished product surface layer such as a slab finally produced.

At this time, the position where bubbles or inclusions are collected is a hook structure generated in the solidifying shell. As shown in Fig. 6 and Fig. 7, the hook structure is an annular structure protruding sharply in the direction of the molten steel in the solidification shell formed on the mold wall surface side. The hook generally refers to the interface between the solidified shell formed during the period when the mold speed is higher than the moving speed of the molten steel and the solidified shell formed during the period when the mold speed is slower than the molten steel moving speed.

As described above, these hooks provide a condition for trapping bubbles and inclusions of argon gas. Therefore, the degree of trapping of bubbles and inclusions may vary depending on the length of the hook structure and the angle of the hook structure, and thus the degree of pinhole defects may change.

The pinhole defects generated in this way lead to the formation of line defects in the process of forming the semi-finished products into hot-rolled and cold-rolled coils, thereby deteriorating the quality of the final product. Therefore, slabs in which pinhole defects are generated must be subjected to a scraping process such as scraping the surface to a certain depth. Therefore, in order to minimize the occurrence of defects, it is necessary to anticipate the occurrence of defects in the continuous casting process and prepare for them.

Particularly, in the present invention, the sulfur content in the slab for ultra low carbon steel is calculated to control the sulfur content of the molten steel so that the hook depth is within the set range. For this purpose, sulfur in the ultra low carbon steel slab Determine the ingredient content. Thereafter, a sample to be subjected to a tissue observation is prepared and taken out from each slab, and a hook structure produced in the slab is observed using a method capable of observation of a hook structure such as a scanning electron microscope.

The observed hook depth is calculated using the sulfur content in each slab as a variable (S20).

In the present invention, the hook depth (HD) is calculated by the following equation (1), where X is the sulfur content (wt%) in the molten steel.

Relationship 1

Hook Depth (HD) = Aln (x) + B

The value of the constant in the relational expression 1 can be determined to satisfy 0.9? A? 0.99, 5.7? B?

Further, the hook depth may be a value calculated by the hook length (L), which is the total hook length, and the hook inclination (D), which is the angle between the hook structure and the slab surface.

7, the hook depth indicating the straight line distance from the surface of the solidifying shell in the slab to the end of the hook is determined by the hook length L, which is the total length of the hook, and the hook length, which is the angle formed by the hook structure and the slab surface Can be calculated through the following equation (2) through the inclination (D).

Relation 2

As shown in relation 2, the calculation of the hook depth (HD) is performed by using an equation of drawing a virtual triangle along the surface of the solidifying shell and the shape of the hook and obtaining the hook inclination as shown in FIG. In more detail, when a virtual right triangle is drawn along the shape of a hook as shown in FIG. 8, and a hook inclination is obtained, the following relation 3 is derived.

Relation 3

In order to define the hook depth HD as a predetermined index proportional to the hook inclination D and the hook length L through the hook depth through the hook depth as in the above-described relational expression 2, in the present invention, 2 introduces the concept of hook depth (HD). That is, the hook depth (HD) is a constant derived from the concept of the relationship (3), which is a number indicating the straight line distance (mm) from the solidifying shell surface to the hook end.

The hook depth HD calculated by this method is compared with a reference value (S30). In the present invention, it is preferable that the hook depth capable of minimizing the surface defect in the slab is set to 0.5 to 1.5 mm. The thus set reference value is compared with the calculated hook depth.

If the calculated hook depth deviates from the reference value, the hook depth is controlled so as to be within the reference value while controlling the sulfur content in the slab manufacturing process (S40).

FIG. 9 shows the change of the hook depth according to the sulfur content in the ultra low carbon steel slab. FIG. 9 shows a sulfur-containing region in which the depth of a hook is reduced, which is capable of generating defects when a very low carbon steel slab is manufactured. Referring to this, in the present invention, the optimum sulfur content Region is in the range of more than 0 to 0.008 wt% (0 to 80 ppm).

Specifically, FIG. 9 shows the result of regression analysis of various types of slabs having different sulfur content contents in the production of ultra-low carbon steel and using the sulfur content in the slab as a variable, , And the hook depth in the range of more than 0 to 0.008 wt% is within the range of 0.5 to 1.5 mm which is the reference value.

 The sulfur content in the molten steel is controlled in the preparation of the ultra-low carbon steel according to the calculated optimum sulfur content-containing region, and the molten steel thus produced is continuously cast to produce a slab (S40). Specifically, in the present invention, molten steel is controlled by controlling the sulfur content to be in the range of more than 0 to 0.008% by weight, which is the optimum sulfur content-containing region capable of preventing surface defects of the ultra-low carbon steel slab, . When the sulfur content in the ultra-low carbon steel slab is more than 0.008% by weight, there is a problem in that the final product is subjected to rolling and processing after the slab is manufactured, thereby causing brittleness in the product.

 As described above, the present invention reduces the hook depth that can be generated on the surface of slabs produced through control of the sulfur content in molten steel in the production of ultra low carbon steel, thereby minimizing the surface defects of the slabs that can occur through the hooks, .

The method for controlling the surface quality of the ultra-low carbon steel slabs as described above is not limited to the construction 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
70: pinch roll 80: performance cast
81: Solidification shell 82: Non-solidified molten steel
83: tip portion 85: solidified point
90: Cutter 91: Cutting point

Claims (4)

Measuring the sulfur (S) component content in molten steel in molten steel casting ultra-low carbon steel slabs;
Calculating a hook depth in the slab using the sulfur content as a variable;
Comparing the calculated hook depth with a reference value; And
And controlling the content of the sulfur component to manufacture the slab if the hook depth is out of the reference value,
Wherein said hook depth is calculated by the following relationship: < EMI ID = 1.0 >
Relation
Hook depth (mm) = Aln (X) + B
(Where 0.9? A? 0.99, 5.7? B? 6.4 and X is the sulfur content (wt%) in the molten steel)
delete The method according to claim 1,
Wherein the reference value is set in the range of 0.5 to 1.5 mm.
The method according to claim 1,
In the step of producing the slab,
Wherein the sulfur content is controlled in the range of more than 0 to 0.008 wt%.

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