KR101377484B1 - Method for estimating carbon-increasing of molten steel - Google Patents

Abstract

여기서, a는 0.25 내지 0.3 사이의 계수값이고, b는 6.0 내지 7.0 사이의 상수값임.In the present invention, the carbon pick-up amount (Cp; ppm) introduced into the molten steel from the powder layer of the mold during continuous casting is carbon of molten steel calculated by the following relation using the thickness (Lt; mm) of the liquid layer of the mold powder layer. Provided is a pickup amount prediction method.
Relation

Where a is a coefficient value between 0.25 and 0.3, and b is a constant value between 6.0 and 7.0.

Description

Prediction method of carbon pick-up of molten steel {METHOD FOR ESTIMATING CARBON-INCREASING OF MOLTEN STEEL}

The present invention relates to carbon pickup amount (Cp) prediction of molten steel, and more particularly, to a carbon pickup amount prediction method of molten steel for predicting and coping with the carbon pickup amount (Cp) of molten steel in a mold during a casting process. will be.

The continuous casting machine is a machine that is produced in the steel making furnace, receives the molten steel transferred to the ladle by the tundish, and supplies it to the mold for the continuous casting machine to produce the cast steel of a certain size.

The continuous casting machine includes a ladle for storing molten steel, a playing mold for cooling the tundish and the molten steel discharged from the tundish for the first time to form a casting cast having a predetermined shape, and the casting cast formed in the mold connected to the mold. It includes a plurality of pinch rolls.

In other words, the molten steel tapping out of the ladle and tundish is formed as a cast piece having a predetermined width, thickness and shape in a mold and is transferred through a pinch roll, and the cast piece transferred through the pinch roll is cut by a cutter. It is made of slabs (Slab), Bloom (Bloom), Billet (Billet) and the like having a predetermined shape.

A related prior art is Korean Patent Registration No. 10-0113104 (registered on March 14, 1997, name: mold flux for high carbon steels to reduce the constraint breakout of a slab machine).

The present invention is to provide a method for predicting the amount of carbon pick-up of molten steel for predicting in advance the degree of carbon pick-up amount (Cp) of the molten steel using the thickness of the mold powder liquid layer on the top of the molten steel in the mold.

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

In the method of estimating the carbon pick-up amount of molten steel of the present invention for realizing the above object, the carbon pick-up amount (Cp; ppm) introduced into the molten steel from the powder layer of the mold during continuous casting is the thickness of the liquid layer of the mold powder layer. It can be calculated by the following relationship using (Lt; mm).

Relation

Here, a may be a coefficient value between 0.25 and 0.3, and b may be a constant value between 6.0 and 7.0.

Specifically, by comparing the calculated carbon pick-up amount (Cp) and the set reference value, when the carbon pick-up amount (Cp) exceeds the reference value may be to reduce the casting speed of the molten steel.

The reference value may be 3.7 ppm.

The slab cast through the mold may have a thickness of 210 to 230 mm and a casting width of 1000 to 1300 mm.

The depth of the immersion nozzle deposited on the mold is 100 to 200mm, and the downward angle of the immersion nozzle may satisfy 25 °.

According to the present invention as described above, by predicting and coping with the carbon pick-up amount (Cp) degree of the molten steel in the mold during continuous casting in advance it is possible to improve the quality of the produced cast.

In addition, it is possible to reduce the cost by reducing the supply amount of the mold powder supplied to suppress the carbon pick-up amount (Cp) of the molten steel.

1 is a conceptual diagram showing a continuous casting machine according to an embodiment of the present invention mainly on the flow of molten steel.
Figure 2 is a cross-sectional view showing the state of the molten steel and mold powder layer in the mold of the continuous casting machine according to the present invention.
3 is a graph illustrating a carbon concentration gradient of the mold powder layer according to the present invention.
4 is a graph showing the measured height of the molten steel in the mold according to the present invention.
5 is a flowchart illustrating a method for predicting carbon pick-up amount of molten steel of the present invention.
6 is a graph showing the relationship between the thickness of the liquid layer and the carbon pick-up amount Cp of the short side portion according to the exemplary embodiment of the present invention.
7 is a graph showing the relationship between the casting speed of molten steel and the thickness of a mold powder layer 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 diagram showing a continuous casting machine according to an embodiment of the present invention mainly on the flow of molten steel.

Continuous casting is a casting method in which molten metal is continuously cast (P) or steel ingot while solidifying in a mold without mold (Mold) (30). 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 Fig. 1, a vertical bending type is illustrated.

1, a continuous casting machine may include a ladle 10 and a tundish 20, a mold 30, a secondary cooling stand, and a pinch roll 70. [

A tundish 20 is a container for receiving molten metal from a ladle 10 and supplying molten metal to a mold 30. 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, mainly the walls, of the mold 30 may be rotated to be 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) varies depending on the carbon content according to the steel type, the kind of the powder (strong cold type Vs and cold type), the casting speed and the like.

The mold 30 has a strong solidification angle or solidified shell 81 so that the cast liquor 80 extracted from the mold 30 maintains its shape and does not leak molten metal which is still less solidified. It serves to form. 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 surface of the mold 30. A lubricant is used to reduce the friction between the mold 30 and the solidification shell 81 during oscillation and to prevent burning. As the lubricant, there is oil to be sprayed and powder added to the molten metal surface in the mold 30. 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 zone further cools 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. Most of the solidification of the cast slab 80 is achieved by the secondary cooling.

The pulling device employs a multi-drive type or the like which uses several pairs of pinch rolls 70 so that the casting piece 80 can be pulled out without slipping. The pinch roll 70 pulls the solidified tip portion 83 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 continuous casting machine configured as described above allows the molten steel M accommodated in the ladle 10 to flow into the tundish 20. 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 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.

On the other hand, the playing mold of the present invention is the playing mold 30 in a condition that the mold 30 width 1000 to 1300 mm, the immersion depth of the immersion nozzle 25 100 to 200 mm, the downward angle of the immersion nozzle 25 25 °, such a The slab cast through the mold has a thickness of 210 to 230 mm and a casting width of 1000 to 1300 mm.

The non-solidified molten steel 82 moves together with the solidification shell 81 in the casting direction as the pinch rolls (Figs. 1 and 70) pull the distal end portion 83 of the fully cast solidification casting 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 slab 80 reaches one point, the cast slab 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.

On the other hand, during continuous casting, the mold powder is continuously supplied to the upper end of the molten steel in the mold 30 to prevent reoxidation of the molten steel, to help the molten steel behaviour, and to reduce the heat loss of the molten steel. At this time, the mold powder forms a mold powder layer 100 separated into four layers as shown in FIG.

Such, the mold powder layer 100 is divided into respective layers divided into the following four characteristics, specifically, the first top layer is supplied into the mold 30 and then unmelted and unreacted raw powder layer 101, Second, it is divided into heterogeneous sintered layer 103 by heat of molten steel in the middle, third carbon enrichment layer 105 in which high concentration of carbon is accumulated, and fourth liquid layer 107 in contact with molten steel. .

In addition, each layer of the mold powder layer 100 has a carbon concentration gradient according to each layer as shown in FIG. That is, based on the carbon concentration of the raw powder layer 101, the carbon concentration in the non-uniform sintered layer 103 is slightly lower, while the carbon in the non-uniform sintered layer 103 and the liquid layer 107 in the carbon enriched layer 105 Is in contact with the carbon concentration is very high, and then the carbon concentration tends to gradually decrease from the liquid layer 107 to the molten steel.

For this reason, when the hot water surface of the molten steel is unstable during continuous casting, when the nonuniform sintered layer 103 and the carbon enrichment layer 105 contact the molten steel, the carbon of the nonuniform sintered layer 103 and the carbon enrichment layer 105 flows into the molten steel. This is a direct cause of raising the carbon concentration of the molten steel being played. In addition, when the carbon concentration in the molten steel is increased in this way, the ductility of the molten steel is lowered, thereby lowering the molding strength of the plate manufactured using the same.

Therefore, in order to prevent the non-uniform sintered layer 103 and the carbon enrichment layer 105 from being in contact with molten steel, it is necessary to secure the thickness of the liquid layer 107 positioned therebetween to a certain depth.

On the other hand, the molten steel in the mold 30 shows a certain trend along the flow of the molten steel conveyed through the immersion nozzle (SEN) 25 connected from the tundish 20 as shown in FIG. Specifically, the molten steel in the mold 30 is somewhat stable at the center of the immersion nozzle 25 and the short side inner wall of the mold 30, while the immersion nozzle 25 is surrounded by the inner side of the immersion nozzle 25 and the mold 30. As the side surface rises toward the side, among the short side inner side side surface of the mold 30 is not only high, but also very unstable as the rising and falling widths are severe depending on the flow of molten steel. As described above in the description of FIG. 3, when the heterogeneous sintered layer 103 and the carbon enrichment layer 105 contact the molten steel, the carbon of the nonuniform sintered layer 103 and the carbon enrichment layer 105 may be molten steel. It is a direct cause of raising the carbon concentration of molten steel which is playing.

Accordingly, in the present invention, as shown in the flowchart of FIG. 5, the carbon pickup amount Cp of the molten steel is estimated through the thickness of the liquid layer 107 of the mold powder layer 100 of the inner side wall portion of the mold 30. At this time, the thickness of the liquid layer 107 is good to measure the thickness of the inner portion 50 to 100mm in the immersion nozzle 25 direction from the inner side of the mold 30 short side.

On the other hand, the method of measuring the thickness of the liquid layer 107 has a number of known techniques, for example, by dipping aluminum (Al) wire, copper (Cu) wire, and steel (Steel) wire of the same length by the same depth. After melting and cutting the length of the cut can be known. More specifically, since the temperature of molten steel is closer to molten steel, the melting point of aluminum, which is the melting point near 660 ° C, is the boundary between the raw powder layer 101 and the heterogeneous sintered layer 103, and the melting point near 1085 ° C. It can be seen that the part where the phosphorus copper is melted at the boundary between the non-uniform sintered layer 103 and the liquid layer 107 and the melting point around 1535 ° C is the boundary between the liquid layer 107 and the molten steel. The thickness 105 is not so large that the temperature difference between the upper and lower portions of the carbon enrichment layer 105 cannot be measured in this way, and it is regarded as being between the non-uniform sintered layer 103 and the liquid layer 107. That is, the thickness of the liquid layer 107 is determined by measuring the difference between the lengths of the cut steel and copper.

Specifically, the method of predicting carbon pick-up amount of molten steel of the present invention is a method of predicting carbon pick-up amount (Cp; ppm) introduced into molten steel from a powder layer of a mold during continuous casting, and the carbon pick-up amount (Cp) is a value of a mold powder layer. It can be calculated by the following relation 1 using the thickness (Lt; mm) of the liquid layer.

Relationship 1

Here, a may be a coefficient value between 0.25 and 0.3, and b may be a constant value between 6.0 and 7.0.

As shown in the graph of FIG. 6, the thickness of the liquid layer 107 and the carbon pick-up amount Cp in the molten steel are generally different in terms of the thickness of the liquid layer 107 and the carbon pick-up amount Cp in the molten steel. It can be seen that there is an approximately inverse relationship in this same state.

At this time, the dot is actually measured data of carbon pick-up amount (Cp) of the liquid layer 107, the thickness in Fig. 6, the solid line as a predictive model fitting the measured data to a linear, the actual and predictive model rather consistent (R ² = 0.8198). In addition, it can be seen from the graph that a is 0.287 and b is 6.57.

That is, it may be preferable that the carbon pick-up amount Cp is calculated by the following Equation 2.

Relation 2

After calculating the carbon pick-up amount Cp of the molten steel, the calculated carbon pick-up amount Cp of the molten steel is compared with the reference value, and the casting speed of the molten steel decreases when the carbon pick-up amount Cp exceeds the reference value. Can be controlled. This is because, as shown in the graph of FIG. 7, when the casting speed of the molten steel is decreased, the molten steel stays in the mold, thereby increasing the thickness of the liquid layer by sufficiently melting the mold flux layer on the top of the molten steel. Here, the reference value is 3.7 ppm, which is a carbon pickup amount (Cp) in the range generally allowed in the steelmaking process.

Specifically, when the carbon pick-up amount (Cp) of the molten steel is 3.7 ppm or less, the casting speed is maintained because the carbon pick-up amount (Cp) of the ultra-low carbon steel is within the allowable range. However, when the carbon pick-up amount (Cp) of the molten steel is 3.7 ppm or more, the carbon pick-up amount (Cp) is exceeded, so that the casting speed is lowered to further control the thickness of the liquid layer 107. In this case, when the casting speed is lowered and the time for which the mold powder layer 100 at the top of the molten steel is maintained in the mold becomes longer, the mold powder is melted by heat to form more liquid layer 107.

Through the present invention, the liquid layer thickness can be controlled to be formed to about 10mm or more, and in general, during the playing process, the thickness of the liquid layer 107 is controlled to be in the range of 10 to 20 mm.

According to the present invention as described above, by predicting and coping with the carbon pick-up amount (Cp) degree of the molten steel in the mold during continuous casting in advance it is possible to improve the quality of the produced cast.

In addition, it is possible to reduce the cost by reducing the supply amount of the mold powder supplied to suppress the carbon pick-up amount (Cp) of the molten steel.

The carbon pick-up amount prediction method of the molten steel 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 21: stopper
25: immersion nozzle 30: mold
40: Mold oscillator 50: Powder feeder
60: Support roll 65: Spray means
70: pinch roll 80: performance cast
81: Solidification shell 82: Non-solidified molten steel
83: tip portion 85: solidified point
91: cutting point 100: mold powder layer
101: raw powder layer 103: nonuniform sintered layer
105: carbon-enriched layer 107: liquid-phase layer
110: glassy flux 120: crystalline flux
130: Slag figure M: Molten steel

Claims (5)

The amount of carbon pickup (Cp; ppm) flowing into the molten steel from the powder layer of the mold during continuous casting is
The method of predicting the carbon pickup amount of molten steel calculated by the following relation using the thickness (Lt; mm) of the liquid layer of the mold powder layer.
Relation

Where a is a coefficient value between 0.25 and 0.3, and b is a constant value between 6.0 and 7.0.
The method according to claim 1,
Comparing the calculated carbon pick-up amount Cp with a predetermined reference value, and if the carbon pick-up amount Cp exceeds the reference value, the casting speed of the molten steel is reduced.
The method according to claim 2,
The reference value is a carbon pickup amount prediction method of molten steel of 3.7 ppm. The method according to claim 1,
A method of predicting carbon pick-up amount of molten steel of which the thickness of the slab cast through the mold is 210 to 230 mm and the casting width is 1000 to 1300 mm.
The method according to claim 1,
And a depth of immersion nozzle deposited on the mold is 100 to 200 mm, and the downward angle of the immersion nozzle satisfies 25 °.

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