KR100993859B1 - Method for reducing pinch roll pressed defect in twin roll strip caster - Google Patents

Abstract

The present invention relates to a pinch roll indentation defect reduction method of a twin-roll thin plate casting apparatus, comprising: forming a cast directly from molten steel by two casting rolls, and transferring the formed cast to a rolling mill through a loop fit of an inert atmosphere And rolling the transferred slabs to cast the thin plates, wherein the oxide scale is pressed into the pinch rolls by controlling the atmospheric oxygen concentration of the loop fit to be maintained in a range of 2 to 3%. A pinch roll indentation defect reduction method for reducing indentation defects generated is provided.

Twin Roll Strip Caster, Loop Pit, Cast, Oxidation Scale, Pinch Roll

Description

TECHNICAL FOR REDUCING PINCH ROLL PRESSED DEFECT IN TWIN ROLL STRIP CASTER}

The present invention relates to a twin roll thin plate casting apparatus, and more particularly, to a method for reducing the indentation defect of the pinch roll by controlling the loop fit atmosphere oxygen concentration of the twin roll thin plate casting apparatus to an appropriate level.

In the twin-roll type sheet casting process, molten steel is supplied to the space formed by a pair of casting rolls and a pair of edge dams to be cooled by a molten steel supply nozzle to maintain a constant height of the molten steel pool and face the casting rolls. It is a method of continuously producing a thin plate directly from molten steel by rotating in order to solidify the shell formed on the surface of the casting roll is coalesced at the proximity between the casting rolls to form a cast.

1 is a schematic view showing a twin roll thin plate casting apparatus. Referring to FIG. 1, the twin roll type sheet casting process will be described. First, molten steel stored in the ladle 14 is supplied to the tundish 3 through the stopper 15, and the molten steel supplied to the tundish 3 is immersed again. The nozzle 4 is supplied to the sump formed by the two casting rolls 1 and the edge dams 7 on both sides of the casting rolls. The molten steel supplied to the sump is solidified while being pressed between two casting rolls 1 rotating in opposite directions and cast into the cast piece 8. The load cell 2 provided on one side of the casting roll serves to detect the pressing force between the casting rolls according to the roll repulsion force (RSF) when pressed by the casting roll 1. The cast slab 8 is rolled by the rolling roll 10 to be made into a thin plate of the final thickness according to the demand of the demand, and finally wound by a coiler 13 into a coil. A loop fit 15 is provided between the casting roll 1 and the rolling roll 10 to seal the slab 8 from the outside and move in an inert atmosphere. A pinch roll 9 is installed at the front end of the rolling roll 10. The pinch roll 9 adjusts the length of the loop formed by the cast piece exiting the casting roll, and the rolling by the rolling roll 10 is stable. It plays the role of making it happen.

On the other hand, in recent years, there is an increasing demand for high quality steel sheet, and accordingly, studies on minimizing defects occurring on the surface of the steel sheet have been continuously made. The surface defects of the thin plate cast by the twin-roll thin plate casting process are caused by the mixing of scum, which is an oxidative inclusion present in the molten steel pool, into the solidification shell, or the rolling becomes unstable due to variations in the rolling force caused by the casting roll. Can be caused by

FIG. 2 is a diagram showing a distribution tendency of pinch roll indentation defects and actual indentation defects. Referring to FIG. 2, it can be seen that surface defects are concentrated at about 300 mm from both edges of the thin plate cast by the twin-roll thin plate casting process. That is, surface defects occur at a low frequency in the center portion and the edge portion of the cast steel, but occur mainly at a point corresponding to about 300 mm from both edges of the thin plate. A point corresponding to about 300 mm from both edges of the thin plate is a quarter part in which the pinch roll is in contact with the slab, and the surface defect may be considered to be caused by a process in which the slab is in contact with the pinch roll. Such defects occur not only in the upper surface of the sheet, but also in the lower surface, and are not observed during casting, but are characterized by visual observation after annealing and pickling.

Such surface defects act as a factor of degrading the quality of the product and lowering the merchandise, but conventionally, there is a lack of preparation for a method for preventing the occurrence of such surface defects.

The present invention was created to solve the above-mentioned problems of the prior art, and has been improved to minimize the occurrence of pinch roll indentation defects by optimizing the atmospheric oxygen concentration of the loop fit to form an oxide scale having an appropriate thickness. It is an object of the present invention to provide a method for reducing indentation flaws of a twin roll sheet metal casting device.

In order to solve the above problems, the pinch roll indentation defect reduction method of the twin-roll thin plate casting apparatus of the present invention comprises the steps of directly forming molten steel from the molten steel by two casting rolls, and forming the cast steel into a rolling mill through a loop fit of inert atmosphere. In a twin roll sheet metal casting process comprising a step of transporting and rolling the transported cast piece to cast a thin plate, the oxidation scale is controlled by maintaining the atmospheric oxygen concentration of the loop fit within the range of 2 to 3% It is characterized in that the press-in defect generated by press-fitting in the.

At this time, the atmosphere oxygen concentration of the roof fit may be controlled by adjusting the amount of inert gas introduced into the roof fit through the gas supply nozzle and the amount of the inert gas discharged out of the roof fit through the gas discharge vent.

According to the pinch roll indentation defect reduction method of the twin-roll thin plate casting apparatus of the present invention,

By controlling the oxygen concentration of the roof fit to be maintained in the range of 2 to 3%, an oxide scale having an appropriate thickness is formed on the surface of the slab, thereby preventing occurrence of indentation defects caused by the oxide scale being pressed into the pinch roll. Will be. Therefore, it is possible to cast high quality thin plates, and increase the error rate of the product while casting the thin plates stably.

Hereinafter, with reference to the accompanying drawings will be described in detail the pinch roll indentation defect reduction method of the twin-roll thin plate casting apparatus of the present invention.

The inventors were able to find a mechanism for the cause of the occurrence of the surface defect in the process of seeking a method for minimizing the surface defect occurring about 300 mm from both edges of the sheet. First, the present inventors thought that the occurrence of the surface defects would be affected by any casting variable, and thus the correlation between the casting variables, which is assumed to be related to the occurrence of the surface defects of the cast steel, was investigated. FIG. 3 is a diagram showing casting parameters related to pinch roll indentation defects and shows casting parameters such as coiling speed, pinch roll pressing force, cast steel temperature, and loop fit oxygen concentration. As a result of analysis of correlation among surface defects among these casting variables, most of the surface defects occurred mainly in the early stage and the middle of casting, and when the defects occurred, the other factors did not change much, but the oxygen concentration of the loop fit (Loop pit Oxygen) showed a change. That is, as shown in Figure 3 it can be seen that the surface defect occurred in the section of the oxygen concentration of the loop fit is changed from 10% to 2%. This tendency is similar in several casting processes at the time of indentation defects, and no surface defects occurred in the post-middle section without further oxidation concentration change. On the other hand, when the initial oxygen concentration of the loop fit started at about 2% and maintained the oxygen concentration without significant change until the end of the casting, surface defects did not occur even in the initial casting period as well as the section after the middle casting.

4 is a schematic view for explaining the cause of the pinch roll indentation defect.

Referring to FIG. 4, the mechanism for the occurrence of surface defects will be described. The surface defects are influenced by the atmospheric oxygen concentration of the loop fit. Specifically, the surface defects generated by reacting with the oxygen of the loop fit (A I think it is due to).

That is, when the atmospheric oxidation concentration of the roof fit is high, the thickness of the oxidation scale (A) generated on the surface of the cast steel increases, and the bond strength between the oxide scale and the base material is weakened by the increase in thickness. As the binding force of the oxide scale and the base material is weakened, the oxide scale A is picked up by the pinch roll 9, and the oxide scale that has been picked up by the pinch roll falls back to the cast steel. Oxidation scale (B) dropped to the cast steel is generated by a rolling defect (pressing defect) while being pressed down by the rolling roll (IRM). The indentation defect is mainly generated in the quarter portion (about 300 mm from the edge of the slab) where the pinch roll is in contact with the slab, and is continuously formed in the casting direction C to remain as a surface defect of the thin plate. In the middle and the middle of the casting, the oxygen concentration of the roof fit is lowered, so that the thickness of the oxide scale is reduced, and the bonding force between the oxide scale and the base material is increased. Due to the increase in the bonding force, the oxide scale is not picked up by the pinch roll and no surface defects are generated.

The inventors have found that the oxygen concentration of the loop fit is directly related to the occurrence of the pinch roll indentation defect, which is a surface defect, and the effect of the oxygen concentration of the loop fit on the frequency of occurrence of the pinch roll indentation defect. The following experiment was conducted to see if it affects.

A thin roll of 3.5mm thickness was cast by the twin roll sheet casting process, and the pinch roll was changed by changing the oxygen concentration of the loop fit to 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 4%, and 5%. The incidence of indentation defects was measured and the experimental results are shown in Table 1.

Table 1

Indentation defects occurred at an atmosphere oxygen concentration of 4% and 5% of the roof fit, but no indentation defects occurred at an oxygen concentration of 0.5%, 1%, 2%, 2.5%, and 3%. Therefore, in order to minimize the occurrence of indentation defect, it is preferable to set the oxygen concentration of the loop fit to 3% or less.

On the other hand, as a result of evaluating the surface quality of the thin plate cast under each oxygen concentration condition, when the oxygen concentration of the loop fit is 0.5%, 1%, the surface quality was insufficient. The reason for this is that when the oxygen concentration of the loop fit is 1% or less, the slab width uniformity of the thickness of the oxide scale formed on the surface of the slab is lowered. That is, when the oxygen concentration of the loop fit is 2% or more, the thickness of the oxide scale is increased as compared with the case where the oxygen concentration is 1% or less, and an oxide scale having a uniform thickness of a predetermined level or more is formed in the width direction of the cast steel. If less than%, the oxidation scale may not be formed more than a certain level as a whole over the width direction of the cast. In addition, when the oxygen concentration of the loop fit is 1% or less, the thickness of the oxide scale is relatively thin, and thus is greatly affected by the thickness variation of the oxide scale. Therefore, when the oxygen concentration of the loop fit is 1% or less, the rolling force applied to the slab during rolling by the rolling roll may not be constant due to the thickness variation in the slag width direction. In addition, since the oxide scale of the surface of the slab is hardly formed, there may be an exposed part of the base material. The exposed base material and the rolling roll are in direct contact with the base material, which causes the surface material to fall off or become damaged, resulting in surface defects. Therefore, it is desirable to control the oxygen concentration of the loop fit to be maintained at 2% or more.

In summary, it can be seen that the atmosphere oxygen concentration of the loop fit should be controlled to be maintained in the range of 2 to 3%. If the atmospheric oxygen concentration of the roof fit exceeds 3%, indentation defects may occur due to the oxidative scale indentation of the pinch roll. If the atmospheric oxygen concentration of the roof fit is less than 2%, surface defects may occur during the rolling process. Because it happens.

Controlling the atmospheric oxygen concentration of the roof fit may be controlled by adjusting the amount of inert gas introduced into the loop fit through the gas supply nozzle 16 and the amount of inert gas discharged out of the loop fit through the gas discharge vent 17. Can be. At this time, the amount of inert gas supplied or discharged is determined according to the size of the loop fit and the degree of closure. Nitrogen or argon may be used as the inert gas. When the amount of the inert gas supplied is relatively larger than the amount of the inert gas discharged, a positive pressure is formed inside the loop fit, and the oxygen concentration inside is reduced. On the contrary, when the amount of inert gas supplied is relatively less than the amount of inert gas discharged, the atmospheric oxygen concentration of the loop fit increases.

According to the pinch roll indentation defect reduction method of the present invention as described above by controlling the atmosphere oxygen concentration of the loop fit to 2% or more, 3% or less by adjusting so as to produce an oxide scale of the appropriate thickness. By preventing occurrence, it is possible to improve the quality of the sheet, and to increase the error rate of sheet casting and to ensure the stability of the casting.

1 is a schematic view showing a twin-roll thin plate casting apparatus.

Figure 2 is a view showing the distribution tendency of the pinch roll indentation defects and the actual indentation defects generated.

3 is a view showing casting parameters associated with pinch roll indentation failure.

4 is a schematic view for explaining the cause of the pinch roll indentation defect.

♧ description of symbols for the main parts of the drawing

1 casting roll 2 load cell 3 tundish

4: immersion nozzle 5: meniscus shield 6: brush roll

7: edge dam 8: cast 9: pinch roll

DESCRIPTION OF SYMBOLS 10 Rolling roll 11 Water-cooling apparatus 12 Coiler pinch roll

13: coiler 14: ladle 15: loop fit

16 gas supply nozzle 17 gas exhaust vent

Claims (2)

Forming a slab directly from the molten steel by two casting rolls, transferring the formed slabs to the rolling mill through a loop fit of an inert atmosphere, and rolling the transferred slabs to cast a thin plate. In the casting process, Method of reducing the pinch roll indentation flaw of the twin-roll thin plate casting apparatus characterized in that the control to maintain the atmosphere oxygen concentration of the loop fit within the range of 2 to 3% to reduce the indentation defect generated by the oxidation scale is pressed into the pinch roll. The atmospheric oxygen concentration of the loop fit is A pinch roll indentation defect reduction method of a twin roll sheet metal casting apparatus, characterized in that by controlling the amount of inert gas flowing into the loop fit through the gas supply nozzle and the amount of inert gas discharged out of the loop fit through the gas discharge vent.

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