KR20130088290A - Predicting method for quality of steel on continuous casting process - Google Patents

Abstract

PURPOSE: A method for predicting steel quality in a continuous casting process is provided to produce high quality of superlow carbon steel by facilitating quality prediction. CONSTITUTION: A method for predicting steel quality is as follows: a step of measuring a casting width, an amount of molten steel discharged from a dipping nozzle, a casting speed, and a superheat degree of the molten steel (S10); a step of calculating a hook index (X) using the casting width, the amount of the discharged molten steel, the casting speed, and the superheat degree of the molten steel (S20); a step of calculating a length and an inclination of a hook using the calculated hook index (S30); and a step of predicting the defect degree of the steel by checking the length and the inclination of the hook (S40). [Reference numerals] (AA) Start; (BB) Finish; (S10) Measure a casting width, an amount of molten steel discharged from a dipping nozzle, a casting speed, and a superheat degree; (S20) Calculate a hook index (X) using the obtained measurement values; (S30) Calculate a length and an inclination of a hook using the calculated hook index; (S40) Predict the defects by checking the length and the inclination of the hook

Description

Prediction of steel quality in continuous casting {PREDICTING METHOD FOR QUALITY OF STEEL ON CONTINUOUS CASTING PROCESS}

The present invention relates to a method for predicting the quality of steel during continuous casting, and more particularly, to a method for predicting the quality of steel during continuous casting, which predicts the quality of steel produced through the shape of a hook in a mold in the continuous casting process.

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 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 arts include Republic of Korea Patent No. 0524628 (Registration Date: October 21, 2005, Name: Method for predicting the hook characteristics of ultra-low carbon steel using the complete cooling index and the maximum mold acceleration).

The present invention is to provide a method for predicting the quality of steel during continuous casting that can easily predict the quality of the steel produced through the characteristics of the hook that can occur in the mold. In addition, to provide a method for predicting the quality of steel during continuous casting that can improve the quality of the steel produced effectively by controlling the hook characteristics.

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

In order to achieve the above object, the method of predicting the quality of steel during continuous casting of the present invention includes a casting width (A), a discharge amount (B) of molten steel discharged from an immersion nozzle, a casting speed (C), and a molten steel superheat degree (T). ) Measuring the casting width (A), the discharge amount (B), the casting speed (C) and the molten steel superheat degree (T), and calculating the hook index (X). Computing the hook length (L) and hook inclination (D) using the calculated hook index (X), and the degree of defects in the steel produced by checking the calculated hook length (L) and hook inclination (D) It may include the step of predicting.

Specifically, the hook index (X) may be calculated by the following equation (1).

Relationship 1

The hook length L may be calculated by the following relational expression 2 by introducing the hook index (X).

Relation 2

The hook inclination (D) may be calculated by the following relational expression 3 by introducing the hook index (X).

Relation 3

In relations 2 to 3, X is a hook index.

Hook inclination (D) may be defined as the angle between the wall surface of the mold and the generated hook.

Predicting the degree of defects,

It can be expected that there will be many pinhole defects in the semifinished product produced when the hook length L is long and the hook slope D is large.

As described above, the present invention can control the hook by adjusting factors influencing the characteristics of the hook that may occur in the mold. In addition, it is possible to easily predict the quality of the steel produced through the hook characteristics can prevent the degradation of the quality of the steel. In addition, since the quality can be easily predicted in continuous casting of the ultra low carbon steel having a high rate of hook generation, there is an effect of producing high quality ultra low carbon steel.

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.
Fig. 3 is a conceptual diagram showing the distribution form of the molten steel M in the mold and its adjacent portion in Fig. 2. Fig.
4 is a flowchart illustrating a method for predicting the quality of steel during continuous casting according to an embodiment of the present invention in order.
FIG. 5 is a view for explaining the formation of a solidification shell in a mold and the occurrence of pinhole defects by bubble collection during continuous casting according to the present invention. FIG.
FIG. 6 is a photograph of a pinhole defect in a slab produced by continuous casting according to the present invention.
7 is a schematic view illustrating molten steel flow and solidification shell formation in a mold during continuous casting according to the present invention.
Figure 8 is a photograph of the state of the hook structure formed in the mold during the continuous casting according to the present invention.
9 is a graph for explaining the calculation of the hook length in the embodiment of the present invention.
10 is a graph for explaining the calculation of the hook inclination of the 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 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 not only the lubrication of the mold 30 and the solidification shell 81 but also the prevention and oxidation of the molten metal in the mold 30, It also functions to absorb the emerging nonmetallic 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.

1, the apparatus including the support roll 60, the spraying means 65 and the pinch roll 70 is also referred to as a strand.

The shape of the molten steel M in the mold 30 and its adjacent portion will be described with reference to Fig. Fig. 3 is a conceptual diagram showing the distribution of the molten steel M in the mold 30 and its adjacent portion in Fig. 2. Fig.

Referring to FIG. 3, a pair of ejection openings 25a are formed on the end sides of the immersion nozzle 25 on the left and right sides in the drawing. The shapes of the mold 30 and the immersion nozzle 25 are assumed to be symmetrical with respect to the center line C, and thus only the left side is shown in this drawing.

The molten steel M discharged together with the argon (Ar) gas from the discharge port 25a draws a trajectory flowing in the upward direction A1 and downward direction A2 as indicated by arrows A1 and A2. do.

The powder layer 51 is formed on the upper part 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 a form in which the powder is supplied and a layer sintered by the heat of the molten steel M (sintered layer is formed closer to the unsolidified molten steel 82). Below the powder layer 51, a slag layer or a liquid fluidized layer 52 formed by melting powder by molten steel M is present. The liquid fluidized bed 52 maintains the temperature of the molten steel M in the mold 30 and blocks the penetration of foreign matter. A portion of the powder layer 51 solidifies at the wall surface of the mold 30 to form the lubrication layer 53. The lubrication layer 53 functions to lubricate the solidified shell 81 so as not to stick to the mold 30.

The thickness of the solidification shell 81 becomes thicker as it progresses along the casting direction. The portion of the solidification shell 81 located in the mold 30 is thin, and an oscillation mark 87 may be formed by oscillation of the mold 30. The solidification shell 81 is supported by the support roll 60, the thickness thereof is thickened by the spray means 65 for spraying water. The solidification shell 81 may be thickened and a bulging region 88 may be formed in which a portion protrudes convexly.

4 is a flowchart illustrating a method for predicting the quality of steel during continuous casting according to an embodiment of the present invention in order.

The present invention is to predict the quality of the steel produced by calculating the characteristics of the hook structure generated in the mold during the continuous casting process as a set value, first, the casting width (A), the discharge amount of the molten steel discharged from the immersion nozzle ( B), casting speed C, and molten steel superheat T are measured (S10).

Referring to FIG. 5, 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 formed 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. This leads to pinhole defects just below the semi-finished surface layer of the slab and the like finally produced, as shown in FIG.

At this time, the location where the bubbles or inclusions are collected is a hook tissue generated in the solidification shell. As shown in Fig. 7 and Fig. 8, the hook structure is an annular structure protruding sharply in the direction of the molten steel in the solidified shell formed on the side of the mold wall surface. 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 as described above develop into line defects in the process of forming the produced semifinished products into hot rolled and cold rolled coils, thereby degrading the quality of the final product. Therefore, slabs with pinhole defects must be processed, for example, by shaving 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.

To this end, in the present invention, the hook index (X) is calculated using the casting width (A), the discharge amount (B), the casting speed (C), and the superheat degree (T) that can be measured during continuous casting as described above (S20). .

In the present invention, the hook index (X) may be calculated by the following Equation 1.

<Relation 1>

As in Equation 1, the hook index X is proportional to the casting width A and is inversely proportional to the discharge amount B, the casting speed C, and the superheat degree T.

At this time, the casting width (A) is the same as the mold width used during casting. That is, it means a straight line distance between a pair of short sides facing each other among the square molds. In the present invention, the unit of the casting width (A) used when calculating the hook index (X) is mm (millimeter).

The discharge amount of molten steel (hereinafter referred to as 'discharge amount', B) means the amount of molten steel discharged from the immersion nozzle which is connected to the mold in the tundish and the molten steel contained in the tundish is moved into the mold. The unit is ton / min (minutes).

Molten steel superheat degree (hereinafter referred to as 'superheat degree', T) means the difference between the molten steel temperature and the theoretical solidification temperature in the tundish. In other words, the temperature is additionally corrected to the theoretical solidification temperature during continuous casting. In general, work is performed under superheat conditions of 20 to 30 degrees Celsius.

High superheat T means that the temperature of the molten steel supplied to the mold in the tundish is high. When a relatively high temperature molten steel is supplied into the mold, the hook formed first in the mold is prevented from growing into the molten steel, and thus the hook length L is reduced, so that the superheat degree T also affects the formation of the hook. . The superheat degree T may be calculated by measuring the difference between the measured temperature and the theoretical solidification temperature by measuring the molten steel temperature.

Casting speed (C) is the rate at which solidified molten steel exits the mold, in m / min (minutes).

By measuring each value, the hook index X can be represented by one value.

After calculating the hook index X as described above, the hook length L and the hook inclination D are calculated using the calculated hook index X (S30).

In addition, the hook length L and the hook inclination D calculated through the hook index X will be described in detail with reference to FIG. 8. The hook length (L) is the length of the straight line that forms the extension line in the horizontal direction from the created hook start point, and the extension line perpendicularly extends from the end point of the hook, i.e. the straight line distance between the start point and the end point, even if it is a hook of the curve. it means. In addition, the hook inclination (D) is an angle formed by the wall surface of the mold and the generated hook, as shown in Figure 8, the mold wall surface and the generated hook based on the direction in which the continuous casting is performed, that is, the moving direction of the molten steel Means angle.

In order to calculate the hook index (X), the factors that can affect the hook formation during continuous casting are selected by casting width (A), discharge amount (B), casting speed (C), and superheat degree (T). Observing the hook tissue in the produced slab while changing the conditions of these one by one, it was analyzed to derive the result value as shown in FIGS.

9 is measured by observing the hook length (L) in the slab by varying the hook index (X), that is, casting width (A), discharge amount (B), casting speed (C), superheat degree (T) This value is represented by a dot. Thus, a straight line was derived by regression analysis using a number of experimental values represented by dots. This straight line may be used as a formula for calculating the hook length (L), the hook length (L) calculation formula derived by the regression analysis in the present invention is shown in the following relation.

<Relation 2>

In relation 2, X means the hook index. As such, the hook length L is shown to decrease as the hook index X increases, and the hook index X varies depending on the influence of the factors shown in Equation 1.

10 is measured by observing the hook inclination (D) in the slab by changing the hook index (X), that is, casting width (A), discharge amount (B), casting speed (C), superheat degree (T) This value is represented by a dot. As described above, a straight line was derived by regression analysis using a plurality of experimental values represented by dots. This straight line may be used as a formula for calculating the hook inclination (D), and the hook inclination (D) calculation formula derived by the regression analysis in the present invention is as follows.

<Relationship 3>

In relation 3, X means the hook index. As such, the hook inclination D is shown to increase as the hook index X increases, and the hook index X varies depending on the influence of the factors shown in relation (1). Therefore, the hook length L can be lengthened or shortened by the casting width A, the discharge amount B, the casting speed C, and the superheat degree T necessary to calculate the hook index X. In addition, the hook inclination D may be smaller or larger.

The R ² values shown in Figures 9 to 10 is determined as a coefficient used for the regression analysis, measure the goodness of fit of the regression equation derived measure. The closer this value is to 1, the higher the relevance of the regression equation.In this regression analysis, when deriving the relation 2 for the hook length (L), the coefficient of determination is 0.752 and the relation 3 for the hook slope (D) is derived. It was found to have a high crystal coefficient value of 0.97. In other words, the fit of regression was high.

The degree of defects produced in the steel can be predicted by checking the hook length L and the hook slope D calculated as described above (S40).

That is, as the hook length L increases, the amount of argon bubbles or inclusions that may be caught on the hook increases, and thus, a large amount of pinhole defects in the slab may be generated. In addition, as the hook inclination D increases, the amount of argon bubbles or inclusions that may be caught on the hook increases, and thus, a large amount of pinhole defects in the resulting slab may be expected. When the degree of defect is predicted as described above, the casting width (A), casting speed (C), discharge amount (B), and superheat degree constituting the hook index (X) affecting the hook length (L) and the hook inclination (D). By adjusting (T), the continuous casting conditions can be changed to prevent pinhole defects in the slab.

As such, the present invention can control the hook by adjusting factors influencing the characteristics of the hook which may occur in the mold, and easily predict the quality of the steel produced by predicting the hook characteristic. In addition, in the case of the ultra low carbon steel having a relatively high generation rate of the hook compared to the low carbon steel, it is possible to easily predict the quality during continuous casting, thereby producing high quality ultra low carbon steel.

Such a method of predicting the quality of steel during continuous casting is not limited to the configuration and operation 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
30: Mold 40: Mold oscillator
50: powder feeder 51: powder layer
52: liquid fluidized bed 53: lubricating 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
87: oscillation trace 88: bulging zone
90: Cutter 91: Cutting point

Claims (6)

Measuring a casting width (A), a discharge amount (B) of molten steel discharged from the immersion nozzle, a casting speed (C), and a molten steel superheat degree (T);
Calculating a hook index (X) using the casting width (A), the discharge amount (B), the casting speed (C), and the molten steel superheat degree (T) measured above;
Calculating hook length (L) and hook slope (D) using the calculated hook index (X); And
Predicting the degree of defects produced in the steel by checking the calculated hook length (L) and the hook slope (D).
The method according to claim 1,
The hook index (X) is a quality prediction method of steel during continuous casting calculated by the following relation 1.
Relationship 1

The method according to claim 1,
The hook length (L) is a quality estimation method of steel during continuous casting is calculated by the following equation 2 by introducing the hook index (X).
Relation 2

X: Hook index
The method according to claim 1,
The hook slope (D) is a method of predicting the quality of steel during continuous casting is calculated by the following equation 3 by introducing the hook index (X).
Relationship 3

X: Hook index
The method of claim 4,
Hook slope (D) is a quality prediction method of the steel during continuous casting is defined by the angle between the wall surface of the mold and the generated hook.
The method according to claim 1,
Predicting the degree of defects,
A method of predicting the quality of steel during continuous casting, which predicts that there are many pinhole defects in the semifinished product produced when the hook length (L) is long and the hook slope (D) is large.

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