KR101443584B1 - Method for scarfing slab of low carbon steel - Google Patents

Abstract

The present invention relates to a slab scarfing method for ultra low carbon steel which varies the degree of scouring of slabs according to the sulfur content of molten steel, comprising the steps of measuring the sulfur content in the molten steel of the tundish, Calculating a capping depth, and scarring the slab cast by the molten steel to a scarping depth calculated in the furnace.

Description

[0001] METHOD FOR SCARFING SLAB OF LOW CARBON STEEL [0002]

The present invention relates to a slab scarfing method for extremely low carbon steels, and more particularly, to a slab scarfing method for ultra low carbon steels which varies the degree of scarping of slabs according to the sulfur content of molten steel.

Furnaces for steelmaking include blast furnaces and electric furnaces. The blast furnace is also called a furnace, which is a furnace used to make pig iron from stones. In the electric furnace, steel scrap and coal are charged into the furnace, and oxygen of high purity is blown from one side to oxidize and burn carbon, manganese, silicon, phosphorus and sulfur contained in the molten steel. At this time, the oxide is slagged and removed by lime, and deoxidation and deoxidation are performed in parallel, so that a steel having a low content of phosphorus and oxygen is produced.

The electric arc furnishes a high current to the electrode rod to generate high-temperature arc heat, thereby dissolving the charged scrap. The molten steel thus dissolved is fed to a continuous casting machine through a refining process.

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 casting mold for forming a tundish and a molten steel that is guided in the tundish first to form a cast slab having a predetermined shape, and a casting member connected to the mold, A plurality of pinch rolls and the like. The molten steel introduced from the ladle and the tundish is formed into a cast slab having a predetermined width, thickness and shape in the mold and is transported through the pinch roll. The slab transported through the pinch roll is cut by a cutter to have a predetermined shape Slabs, blooms, billets, and the like.

Slabs, blooms, billets, and the like produced in the continuous casting process are sandwiched between two rotating rolls to form an elongated shape.

A rolling mill is a device for forming a plate material, a bar material or the like through plastic deformation of a material while passing the material between rotating rolls at room temperature or high temperature. A conveyance guide is provided between the rolling mill and the rolling mill to feed the rolling mill to the rolling mill in the next step.

The slabs, such as slabs, are rolled and then produced as hot rolled coils.

Related prior art is Korean Patent Publication No. 2011-0011030 (published on Feb. 2, 2011, Scabbing device of slab and its method).

The present invention provides a slab scarfing method of an ultra-low carbon steel which scrapes a sample of molten steel of a tundish to measure the content of sulfur and sets a scarping depth of a slab generated at a corresponding time according to the content of the measured sulfur, .

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

According to an aspect of the present invention, there is provided an ultra low carbon steel slab scarfing method comprising the steps of: measuring sulfur content in molten steel of a tundish; Calculating a scarping depth according to the sulfur content measured above; And scouring the slab cast by the molten steel to the scarping depth calculated above.

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

[Relational expression]

Here, A and B are coefficients derived by a regression equation, where A is a value between 0.023 and 0.28, and B is a value between 2.1 and 2.5.

In addition, when the sulfur content is 60 ppm or more, the scarping depth can be at least 4.0 mm or more.

As described above,

The present invention has an effect of obtaining a surface of a slab free from surface defects by scouring to an effective depth according to the degree of occurrence of bubble defects on the surface of an extremely low carbon steel slab, and has an effect of increasing the recovery rate of the slab.

1 is a side view showing a continuous casting machine according to an embodiment of the present invention.
Fig. 2 is a conceptual diagram for explaining the continuous casting machine of Fig. 1 centered on the flow of molten steel M. Fig.
3 is a flowchart illustrating a slab scarping method of an ultra-low carbon steel according to an embodiment of the present invention.
4 is a graph showing the relationship between sulfur and scarping depth in a slab scarfing method of an extremely low carbon steel 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 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.

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. 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 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 surface of the mold. 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. The powder is added to the molten metal in the mold 30 to become slag so that the lubricating of the mold 30 and the solidifying shell 81 as well as the prevention and nitriding of the molten metal in the mold 30 and the warming, It also functions to absorb non-metallic inclusions. A powder feeder 50 is installed to feed the powder into the mold 30. The portion of the powder feeder 50 for discharging the powder is directed to the inlet of the mold 30.

The secondary cooling 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 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.

The non-solidified molten steel 82 moves together with the solidifying shell 81 in the casting direction as the pinch roll 70 (Fig. 1) 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.

Such slabs are particularly susceptible to fatal defects in the final product due to bubble defects when they are used as automotive exterior materials, especially when the ultra low carbon steel has a high coagulation temperature and a deep oscillation mark resulting in bubble defects such as pinhole defects do.

However, in order to remove such bubble defects, the surface is spun to a predetermined depth, which varies depending on the type of steel, but is set to about 4 mm for ultra low carbon steels.

In this case, the distribution of bubble defects on the surface of the slab may be reduced, but by scarring the surface irrespective of the bubble defect distribution, an excessive scarping operation can be achieved even in the case of bubble defect distribution.

Therefore, it is necessary to determine the scarping depth according to the main factors influencing the bubble defects and perform the scarping operation efficiently.

FIG. 3 is a flowchart illustrating a method of scribing a very low carbon steel slab according to an embodiment of the present invention. The scarping method is as follows.

First, the sulfur content in the molten steel of the tundish is measured (S10). At this time, the molten steel is sampled and the sulfur content can be known through the sampled molten steel.

Next, the scarping depth is calculated according to the sulfur content measured in the above (S20). At this time, the relationship between the sulfur content and the scarping depth can be seen from FIG. The higher the sulfur content, the deeper the depth to scarcify. That is, the sulfur content in the molten steel can deepen the scarping depth to reduce bubble defects.

Here, it is preferable that the present invention is applied to a slab corresponding to a measured sulfur content in the molten steel of the tundish.

Also, as shown in Fig. 4, a regression equation as shown in the following relational expression 1 can be derived through a graph showing the relationship between the content of sulfur and the scarping depth.

Relationship 1

In this case, A and B are coefficients derived by a regression formula, A is a value between 0.023 and 0.28, B is a value between 2.1 and 2.5, more preferably, A is 0.025 and B is 2.316.

R ² is the coefficient of determination used in the regression analysis, which indicates the measure of fitness of the derived regression equation. The closer the value to 1, the higher the fitness of the regression equation.

That is, it can be seen that the regression equation according to the relational expression 1 has a high fitness.

Finally, the slab is scarcapped to the calculated scarping depth (S30). At this time, the moving speed of the slab may vary depending on the scarping depth. In general, because of the use of fixed scarping devices for scarping, the speed of the slabs moved between the scarping devices is controlled to adjust the scarping depth.

Table 1 below shows the bubble defect index according to the sulfur content and the scarping depth. For extremely low carbon steel, the bubble defect index should be 3 or less.

Table 1

Considering the bubble defect index, Table 1 shows that, in Example 1 and Example 2, in which sulfur content is 60 ppm, 3.6 mm is scarcely scratched, and in Example 2, scarring is 4.0 mm, It can be seen that the state of slab in Example 1 is 3.9 and the state of slab is poor in Example 1, and that in Example 2, the state of slab is good because the number of defective defects is 3.

As is also the case with Examples 3 to 5, it can be seen that the slab state is good when the scoop depth is varied according to the content of the impregnation.

Of course, deepening the scarping depth may lower the bubble defect index and result in a good slab state. However, if the depth of scarfing becomes deeper, the efficiency of the slab becomes lower. Therefore, a proper bubble defect index The branching slab is a more efficient slab.

Therefore, the present invention has an effect of obtaining a surface of a slab free from surface defects by scouring to an effective depth according to the degree of occurrence of bubble defects on the surface of an extremely low carbon steel slab, and has an effect of increasing the recovery rate of the slab.

The slab scarping method of the ultra-low carbon steel 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
70: pinch roll 80: performance cast
81: Solidification shell 82: Non-solidified molten steel
91: Cutting point

Claims (3)

Measuring sulfur content in the molten steel of the tundish;
Calculating a scarping depth according to the sulfur content measured above; And
Scouring the slab cast by the molten steel to the scarping depth calculated above,
Wherein the scarping depth is calculated by the following relationship: < EMI ID = 1.0 >
[Relational expression]

Here, A and B are coefficients derived by a regression formula, A is a value between 0.023 and 0.28, and B is a value between 2.1 and 2.5.
delete The method according to claim 1,
Wherein the scouring depth is at least 4.0 mm or more when the sulfur content is at least 60 ppm.

Images