KR20030049809A - soft cooling method of slab corner region in continuous casting process - Google Patents

Abstract

PURPOSE: A method for soft cooling corner part of cast slab in second cooling zone of continuous casting process is provided to reduce defects of corner and cross sectional parts of the slab by installing soft cooling unit at the short side part between foot roll and bender and at the short side part between casting bow and straightening part. CONSTITUTION: The method for soft cooling corner part of cast slab in second cooling zone of continuous casting process is characterized in that generation of defects on corner part and cross sectional part of cast slab(26) is reduced by installing upper and lower soft cooling units formed of insulation board(25) at the short side part of the section between foot roll and bender that is a section from the underneath of mold of continuous caster to a bending stress generation spot, and at the short side part of the section between the lower part of casting bow and straightening part that is a correctional stress generating section respectively, thereby lowering cooling speed of the cast slab.

Description

Soft cooling method of slab corner region in continuous casting process in secondary cooling zone of continuous casting process

The present invention relates to a slow cooling method of the slab corner portion in the secondary cooling zone of the continuous casting process, and more specifically, to the continuous casting machine consisting of insulation boards before the B / D section and C / B ~ S / T section It relates to a method of raising the surface temperature of the cast using a cooling device, and using the upper and lower slow cooling device at the same time.

In general, the continuous casting process, as can be seen in FIG. 1, is performed by passing molten steel into a water-cooled copper mold 1 from tundish, through a player segment consisting of many rolls 2 after the first solidification layer is formed. The solidification proceeds to the secondary by the cooling water 4 injected from the nozzle 3 of the liver, and the solidified slab is cut to a predetermined length by the cutter 5 at the segment end.

As shown in FIG. 1, the segment of such a player is from segment 1 to segment 9 downward from mold 1. Segment 1 (foot roll: F / R, 6) is an integral part of the mold (1), and segment 2 (bender: B / D, 7) is the part where the slab is subjected to bending stress in the vertical part and its lower part. , Segments 3 to 5 (casting bow: C / B, 8) correspond to the bent slab, and segment 7 (straightening: S / T, 9) is the portion where the straightening stress is applied. . The final solidification of the unrolled cast piece corresponds to segments 8 to 10 (horizontal part: H / Z part 10).

However, the surface of the produced cast slab occurs in a face shape in various forms as shown in FIG. Among them, the corner burst 11 and the short edge 12 are high incidence. The corner burst 11 usually occurs at the corners at both ends of the upper surface 13 and the lower surface 14 of the cast steel, and the short edge 12 is biased to the upper surface 13 and the lower surface 14 in the cross section.

Normally cast steel receives bending stress in the B / D part and corrective stress in the S / T part, and tensile stress is applied to the lower surface 14 and the upper surface 13 of the cast steel, respectively. At this time, when the high temperature ductility of the cast slab decreases, cast slabs 11 and 12 occur.

In FIG. 3, the high temperature ductility of the cast steel shows a brittle section in which ductility falls between 900 and 700 degrees. Therefore, in the B / D and S / T sections where the surface tension force of the slab acts, the slab must control the surface temperature of the slab so as to avoid brittle sections. However, when the secondary cooling is completely cooled, the cooling effect of the roll 2 is reduced, resulting in deterioration of the roll 2, and thus there is a limit.

Conventionally, in order to solve this problem, there is an induction heating method (Japanese Patent Publication No. 62-31987) which heats the short side with an induction coil in the coil short side in the rolling process. Since the use of electrical energy is uneconomical, the performance of the playing process has not been reported. In the process of casting, there is a method in which the short side insulation cover is installed in the H / Z part for hot direct rolling, and then the cooling of the slab is directly cooled and sent directly to the hot rolling mill (US patent 4658882). Therefore, there is no effect in reducing defects of cast steel. In addition, there is a method of insulated by installing a plate that reflects the heat radiated to the corner of the slab to collect the corner (Japanese Patent Publication No. 61-201721), but the actual process consisting of compact rolls and high temperature and high humidity conditions This is not applicable because there is no installation space and the heat collecting plate may corrode.

The present invention has been made to solve the above problems, and the section before the bending stress generation (F / R ~ B / D) and directly under the mold (1) to reduce the temperature of the corner portion of the cast iron in the secondary cooling table and Continuous casting process to reduce the occurrence of defects in the corners and cross-sections of the cast steel by installing a slow cooling device at the short side of the calibrated stress generator (C / B bottom ~ S / T) to reduce the cooling speed of the cast steel It is an object of the present invention to provide a method of fully cooling a slab corner portion in a secondary cooling table of the present invention.

1 is a schematic view of a continuous casting process according to the prior art.

Figure 2 is a schematic diagram showing the surface transverse defects in the playing cast.

3 is a graph showing the results of high temperature ductility measurement of cast steel;

4 is a diagram illustrating a thermal insulation measurement test of the caster corner portion.

5 is a graph showing the insulation of the slab corner portion different in the shape of the insulation board and the distance between the board and the slab surface.

Figure 6 is a graph showing the insulation of the slab corner portion different in the thickness of the insulation board and the distance between the board and the slab surface.

7 is a schematic view of the upper slow cooling device.

8 is a schematic view of the lower slow cooling device.

Fig. 9 is a graph showing an example of thermal analysis results of cast pieces in a player.

10-11 is a graph showing the results of the thermal analysis of the cast in the player.

12 is a graph showing a corner burst occurrence rate according to the combination method of the upper and lower slow cooling device.

13 is a graph showing a comparison of plant life according to the shape and structure of a slow cooling device.

* Brief description of the major reference numerals

1. mold, 2. roll,

3. nozzle, 4. coolant,

5. cutting machine, 6. segment 1,

7. segment 2, 8. segment 3 ~ 5,

9. Segment 7, 10.Segments 8 ~ 10,

11. corner burst, 12. short edge burst,

13. The upper face of the cast, 14. If the cast is

15. electric furnace, 16. cast steel,

17-19. K type thermocouple, 20 ~ 22. Fully cooled boards of type A, B and C,

23. Mold short side, 24. Side support roll,

25. insulation board, 26. cast steel,

27.Link Pin

Hereinafter, the present invention will be described in more detail with reference to the drawings.

In order to achieve the above object, in order to achieve the above object, the surface temperature of the cast slab is increased by using a slow cooling device composed of insulation boards before and after the B / D section and the C / B ~ S / T section in the continuous casting machine. It provides a method of using a slow cooling device at the same time.

In addition, in the present invention, the slow cooling device is fixed and moved to the mold short side, both the upper and lower slow cooling device is composed of a plurality of insulation board is composed of several parts, each element insulation board is connected to the link pin, the insulation board is only on the slab short side As the shape of the insulation board is installed, the distance between the slab and the insulation board is within 30mm from the surface of the slab, the thickness of the insulation board layer is more than 40mm, and the insulation board is provided with the insulation board installed only at the short sides.

The present invention will be described in detail with reference to the following examples.

Example 1

In FIG. 4, the predetermined slabs cut to 20 mm, 20 mm, and 25 mm in height were heated at 1200 degrees for 2 hours in an inert atmosphere electric furnace 15 to uniform the temperature inside and outside of the slab 16. . K-type thermocouples 17, 18 and 19 were attached to the central portion and both ends of the upper surface of the cast steel. With the heated slab 16 removed from the electric furnace 15, the A, B, and C-type slow cooling boards 20, 21, and 22 shown in the drawing are installed only on the upper left side of the slab, and the other upper right side remains intact. The temperature (17-19) of each part was measured, cooling.

The amount of heat dissipated in the cast is represented by the following heat transfer equation.

Q = h (T _a -T _s )

Where h is the heat transfer coefficient, Ta is the ambient temperature near the surface of the cast, and Ts is the surface temperature of the cast.

Even if the initial temperature is the same, the heat transfer proceeds in both directions, so the heat dissipation per unit volume is large. Therefore, the amount of surface temperature drop at the corners is large. When the insulation board is applied, the Ta becomes higher and the heat transfer rate (Q) decreases, so that the surface temperature decreases. Therefore, the surface temperature of the cast steel can be maintained in the high temperature brittle range.

In order to optimize the shape of the insulation board and the distance between the board and the slab surface, the heat transfer rate was measured according to the distance between the insulation board and the slab surface while varying the shape of the insulation board.

In FIG. 5, the faster the heat transfer rate, the lower the slab temperature. Therefore, the difference between the slab center and corner temperatures (= DT) increases. DT is large in order of type C> B> A, and the distance increases drastically above 30mm. Therefore, it can be seen that the distance should be within 30mm. Moreover, the shape was favorable in order of C type> B type> A type.

In order to examine the heat insulation layers 19 to 21 made of ceramic wool, as shown in FIG. 6, the DT was decreased as the heat insulation layers 20 to 22 were added. When the thickness was more than 40mm, there was little DT reduction effect.

Example 2

Numerical analysis was performed to determine the slow cooling insulation section in the actual performance process. The heat flux (Q _m ) in the mold was calculated by the following equation (1), and the heat flux (Q _2nd ) of the secondary cooling zone was used by the equation (2). The heat transfer coefficients (h) used in equation (2) are shown in equation (3), respectively. The heat transfer coefficient obtained in Example 1 was used for the application area when the insulation board was applied at the corner part and was calculated by the finite difference method.

Q _m = 63-5.3√tm ------------- (1)

Q _2nd = h (T _a -T _w ) ------------- (2)

h = CW ^b ------------- (3)

Where tm is the molten steel retention time in the mold (sec), T _{w is the} secondary cooling water temperature (C), C is the constant, and W is the specific water quantity (l / m ² min).

The slow cooling section is divided into upper and lower slow cooling devices. The upper device is a structure that is fixed to the mold as shown in Figure 7 is moved at the same time when moving the short-side copper plate by changing the width, the lower device is a structure installed in the C / B lower and S / T as shown in FIG.

FIG. 9 is an example of thermal analysis performed on the B-type of FIG. 4 as a thermal analysis result of the cast steel. Unlike the central surface b of the cast steel, the short side surface c and the corner portion d have a large temperature decrease as they pass through the player. The temperature increase is 100 degrees at the B / D outlet and S / T inlet when both upper and lower cooling units are applied.

In summary, in comparison with the conventional case in FIG. 10, the B / D exit temperature is 100, 0, and 100 degrees when only the upper slow cooling device is applied, the lower warm cooling device is applied, and both are respectively applied. S / T entrance temperatures were 10, 70 and 100 degrees, respectively. Therefore, when the upper and lower slow cooling devices are all applied, the most excellent effect is applied, and when only the upper slow cooling device is applied, even though the temperature is high in the B / D part, it can be seen that most temperatures are lowered before the S / T.

In FIG. 11, when the upper and lower slow cooling devices are applied, the temperature is higher in the order of C> B> A when comparing the B / D exit side and the S / T entrance side temperature by the shape of the insulation board.

Example 3

The upper and lower slow cooling devices were actually manufactured as shown in FIGS. 7 and 8. The upper slow cooling device is installed on the side support roll 24 at the lower side of the mold (1) short side 23 in FIG. 7 and the steel reinforcement insulation board 25 in the section corresponding to the vertical part of the caster short side side. The shape hangs on. 7 shows the lower surface of the player and the insulation board 25 in the absence of the cast steel, and the second side of the player and the insulation board 25 viewed from the short side in the presence of the cast steel. The third picture is a cross-sectional view of the player as seen from the top of the player. At the bottom of the B / D part, the cast steel 26 was installed to secure a high temperature over a brittle temperature section when subjected to bending stress. The slow cooling device was tested by making two types of integrated and detachable link pins 27.

The lower slow cooling device was actually manufactured as shown in FIG. 8. The first figure is a plan view looking down from the top of the player with the cast steel and the insulation board 27 is installed on the short side. The second picture is the insulation board and the picture from the short side. Two types of insulation boards of one type of cooler and one type of separate type were separated and tested.

In the actual playing process, three cases were tested using the upper, lower and upper and lower slow cooling units as shown in O 12. As a result, when calculating the existing corner burst generation rate to 100, when only the upper slow cooling device is used, the corner burst generation rate on the upper surface side of the cast steel is higher than the lower surface side. This is because the surface temperature is high in the B / D portion where the bending stress of the cast steel due to the upper gentle cooling is reduced corner burst generation. In the case of using the lower slow cooling device, it is considered that the occurrence of corner burst is reduced due to the increase of the surface temperature by the lower slow cooling device in the S / T part where the stress of the cast steel is generated. However, when both upper and lower slow cooling devices were used, the upper and lower side corner bursts decreased. Therefore, it is preferable to use both upper and lower slow cooling devices.

When the slow cooling device was tested using an integrated type and a separate type, when the life of the type B device is calculated as 100 in FIG. 13, the lifespan is shorter in the order of A type and C type. Also, the detachable type has a much longer life than the integral type. In the case of the integral type, the steel reinforcing board of the insulation board is deformed and damaged by the heat of the cast steel, or it is damaged by the impact when the cast steel or the dummy bar is charged, thereby shortening the service life.

As described above, according to the present invention, the section before the bending stress generation (F / R ~ B / D) and the correction stress generated under the mold (1) so as to reduce the temperature of the corner portion of the slab in the secondary cooling table A slow cooling device is provided at the short side of the portion (C / B lower portion ~ S / T) to reduce the cooling rate of the cast steel, thereby reducing the occurrence of defects in the corner portion and the cross section of the cast steel.

Claims (4)

Section of F / R (foor roll) and B / D (Bender) sections before bending stress occurs directly under mold in continuous casting machine, and S / T section under C / B (casting bow) section of correction stress generation section. 2, the continuous casting process to reduce the occurrence of defects in the corners and cross sections of the cast steel by lowering the cooling rate of the cast steel by installing upper and lower slow cooling devices each consisting of insulation boards at the short sides of the straightening section. Slow cooling method of caster corner part in car cooling stand. According to claim 1, In the upper slow cooling device is fixed to the mold to be moved at the same time when moving the short side copper plate by changing the width, and installed in succession to the side support roll of the lower side of the mold at the same time, each of the plurality of insulation board Of the continuous casting process to reduce the occurrence of defects in the corners and cross-sections of the cast pieces, so that the insulating boards can be separated by being connected with link pins, and the insulating boards are installed only at the short sides of the cast pieces. Slow cooling method of the corner of cast steel in the secondary cooling stand. According to claim 1, In the lower slow cooling device is installed on the short side of the section of the S / T (straightening) in the lower portion of the C / B (casting bow) that is the calibration stress generation section, respectively, the insulating board is Each part consists of a plurality of parts, and each insulation board is connected to the link piece to be separated, and the insulation board is installed only on the short side of the cast, continuous casting to reduce the occurrence of defects in the corners and cross-section of the cast Slow cooling method of caster corner part in secondary cooling zone of process. The method according to any one of claims 1 to 3, The distance between the insulation board in the cast steel is within 30mm from the surface of the cast steel, the thickness of the insulation board layer is 40mm or more cast slab in the secondary cooling stage of the continuous casting process to reduce the occurrence of defects in the corners and cross-section of the cast steel Slow cooling method of corner part.