KR20220018556A - Method and apparatus for secondary cooling of continuous casting slabs - Google Patents

Abstract

A method and apparatus for secondary cooling of continuously cast slabs capable of securing the surface properties of slabs without impairing productivity and requiring no significant addition of energy costs are obtained.
In the secondary cooling method of the continuous casting slab in which cooling water is sprayed to the slab 5 in the secondary cooling zone 7 and cooled, and solidification of the slab 5 is completed in the section up to the end of the horizontal band 17 In this case, in the section on the upstream side in the casting direction of the horizontal band 17, a strong water cooling section in which the sprayed cooling water cools the cast slab 5 by spraying cooling water under the condition that the sprayed cooling water is in a state of nuclei boiling at all positions in the width direction of the surface of the cast slab. In addition, by making the section downstream in the casting direction from the strong water cooling section and up to the end of the horizontal band 17 as a non-water cooling section where the injection of cooling water is stopped, after the strong water cooling section, the horizontal band 17 The surface temperature of the slab at the end of the horizontal bar 17 is set to a predetermined range while increasing the surface temperature of the slab in the casting direction over the ends of the slab.

Description

Method and apparatus for secondary cooling of continuous casting slabs

The present invention relates to a method and apparatus for secondary cooling of continuously cast slabs.

The manufacturing method of a general continuous casting slab is demonstrated based on FIGS. 3 and 4 taking the vertical bending type continuous casting equipment as an example.

The molten steel poured into the mold 3 from the tundish (not shown) is cooled primarily in the mold 3 and becomes a flat slab 5 that forms a solidified shell, and the vertical bar 9 is lowered on the flat plate. It proceeds to the curvature (13). And in the bent portion 11 on the inlet side of the curvature 13, the slab 5 is bent while being guided by a plurality of rolls (not shown) so as to maintain a constant radius of curvature.

After that, in the straightening section 15, bending is returned (corrected) while increasing the radius of curvature sequentially, and when the straightening section 15 is exited, the slab 5 becomes flat again and proceeds to the horizontal table 17 . After solidification is completed on the horizontal stand 17, the slab 5 is cut to a predetermined length by a gas cutter 23 installed on the outlet side of the continuous casting machine.

The gas cutter 23 moves in the casting direction in synchronization with the conveyance speed of the slab 5, and moves a torch to the width direction at the same time. And cutting oxygen is sprayed while heating the slab 5 with the preheating flame of a torch, and the slab 5 is melted and cut by the oxidation heat of oxygen and steel.

When the casting speed is too fast or the slab temperature is too low, the cutting pitch of the gas cutter 23 and the casting speed cannot be synchronized, resulting in problems such as restriction of the casting speed and poor cutting. Therefore, setting of a casting speed suitable for cutting ability and temperature control of the slab 5 become important. And the cast slab 5 cut|disconnected by the gas cutter 23 is conveyed to the slab refinery factory which is a next process, or a rolling mill.

After exiting the mold (3), the cast steel (5) is sprayed with water (water hydraulic spray or water-air mixed mist spray) to complete solidification from the vertical stand (9) to the horizontal bar (17) to the center. Secondary cooling using

Usually, in secondary cooling, a large flow rate of water is sprayed from the vertical stand 9 just below the mold 3, and the cooling rate of the cast slab 5 is increased (in this specification, the cooling rate of the cast slab is increased This is called "strong cooling"), and the strength of the solidified shell is ensured. Conversely, after the curved zone 13, cooling is weakened, and the surface temperature of the cast steel 5 is raised (reheated) by heat conduction from an internal high-temperature part. And in the straightening part 15, the surface temperature is adjusted so that it may become more than the embrittlement temperature range, and generation|occurrence|production of the transverse crack of the slab 5 is avoided.

The cast steel 5 that has passed through the straightening unit 15 is completely solidified to the center during cooling on the horizontal table 17 . When the solidification rate is slower than the casting rate, the solidification completion position does not enter the machine of the continuous casting machine, and molten steel flows out from the end surface at the time of gas cutting, causing great damage such as equipment destruction and operation stoppage. Conversely, when the completion of solidification is too early, not only the cooling water after completion of solidification is wasted, but also the temperature drop of the cast steel 5 is large, and cutting becomes difficult as mentioned above. Accordingly, the setting of the cooling conditions on the horizontal stand 17 greatly affects the productivity and the securing of manufacturing stability.

4 is a graph showing the results of numerical analysis that reproduces the temperature history of the slab 5 in the conventional general continuous casting method, the vertical axis is the temperature, and the horizontal axis is the distance from the meniscus (molten steel surface in the mold). represents

In the upper part of the graph, the code|symbol of the area|region after the casting_mold|template 3 shown in FIG. 3 and the area|region corresponding to it is described.

In addition, in the graph, the solid line is the center of the surface width of the cast steel, the broken line is the corner portion (corner portion) of the cast steel, and the dashed-dotted line is the temperature history at the center of the cross section of the cast steel. Moreover, in a graph, the minimum temperature which can be cut|disconnected is shown with a broken line, and if it is a temperature range (refer an arrow) higher than this, it has shown that it is a cutable temperature. In addition, in the graph, the solidification completion position is set to A, and the base end of the continuous casting machine is shown as B.

As indicated by the temperature history at the center of the surface width of the cast slab, the shell thickness is increased by strong cooling with a large flow rate of water spray in the vertical stand 9 from just below the mold 3 . From the bending section 11 and the curved band 13 that follow, the cooling rate is slowed and the heat is restored from the inside of the slab, so that the surface temperature of the slab at the time of passing through the straightening section 15 is controlled to be higher than the embrittlement temperature range 25. As a result, the slab 5 with good surface properties can be obtained.

And also in the horizontal stand 17, cooling is continued, and when solidification of the center part of a slab is completed at the point A, the temperature drop of the center part of a slab becomes large. And in the point B, it passes through the base end of a continuous casting machine, it is cut|disconnected by the gas cutter 23 to predetermined length, and it is conveyed to the next process. In this example, since the solidification completion position is sufficiently upstream from the base end of the continuous casting machine, and the slab corner temperature is also sufficiently higher than the cuttable temperature, cutting can be performed without any problem.

As a problem in the manufacturing process of a slab as mentioned above, surface defects, such as a longitudinal crack and a transverse crack, are mentioned. Among them, transverse cracking is characterized in that it occurs near the corner of the upper surface of the cast steel in a facility including bending correction, such as a continuous casting machine of a curved type and a vertical bending type. When passing through the straightening section, if the surface layer temperature of the slab is in the embrittlement (III region embrittlement) region of the steel spanning the γ low temperature region to the γ/α transformation temperature region, transverse cracks will occur due to the tensile stress of the surface generated at the time of straightening. As a method of preventing this transverse cracking, for example, non-patent document 1 describes that cracking can be prevented by gently cooling the secondary cooling of the cast steel and avoiding the embrittlement zone to the high temperature side at the time of calibration.

Moreover, in patent document 1, the technique of preventing surface cracks by reducing or stopping the cooling water amount of secondary cooling in the correction|amendment part, that is, in the vicinity of a horizontal band entrance to a final correction point, and reheating the slab surface layer is disclosed.

However, in the method of avoiding the embrittlement temperature to the high temperature side, the average temperature of the slab cross-section at the exit side of the straightening part rises. As a result, since the completion of solidification of the central part of the slab becomes slow, in order to complete solidification in the machine of the continuous casting machine, the length extension and casting speed of the continuous casting machine are limited, and productivity may be impaired.

On the other hand, in order to make the coagulation|solidification complete position enter into a plane, the technique of cooling by providing an adjustment cooling apparatus in the horizontal stand downstream of a correction|amendment part is disclosed by patent document 2.

However, in Patent Document 2, there is no specific mention of cooling conditions. Therefore, depending on the cooling conditions, there is a possibility that significant temperature unevenness may occur in the width direction of the surface, and there is a risk of surface cracking (longitudinal crack) due to thermal stress resulting from the temperature unevenness on the surface of the slab, and solidification is completed in the width direction There is a risk of internal quality non-uniformity due to non-uniform positioning.

On the other hand, patent document 3 discloses the technique of suppressing the cooling nonuniformity in secondary cooling. According to this, it is said that cooling can be stabilized by maintaining the boiling state of water within the collision range of the water spray at the film boiling state at the front end of the cooling zone and the nuclear boiling state at the rear end.

In general, if the cooling conditions are made constant in the width direction, heat generation from the side surface is also applied to the corner portions of the slab, so that the cooling rate is increased compared to the central portion of the width of the slab. Further, when cooling is started in the film boiling state, a phenomenon of transition to the nucleus boiling state is observed when the temperature of the surface to be cooled decreases. Therefore, when trying to maintain a film-boiling state like patent document 3, the corner part of a slab with a rapid temperature fall will transition to a nucleus boiling state first, and the temperature will fall more rapidly. Such a sudden temperature difference causes surface cracking of the cast steel due to thermal stress. Moreover, there exists a problem that the temperature fall of the slab corner part causes the fall of a cutting property, and an increase in cutting time in the gas cutter on the exit side of a continuous casting machine. Specific examination is not made in patent document 3 with respect to such a problem, and the method of the temperature control at the outlet side of a continuous casting machine is not clarified.

On the other hand, Patent Document 4 discloses a technique for preheating and cutting a slab corner for the purpose of securing the cutting property on the gas cutter side. However, in the case of strong cooling by nuclei boiling as described above, the temperature drop of the cast steel is large, and it is necessary to take a longer preheating time than usual. In addition, when the casting speed is increased according to the thickness of the cast steel or the type of steel, the speed of gas cutting is not sufficient to limit the casting speed, or it is necessary to input more energy for preheating.

Japanese Patent No. 4690995 Japanese Laid-Open Patent Publication No. 62-064462 Japanese Patent No. 6079387 Japanese Patent No. 2605329

Ogibayashi et al.: "Mechanical Behavior in Continuous Casting", Steel Foundation Joint Study Group of the Iron and Steel Association, l985, p184

As mentioned above, the secondary cooling conditions which do not impair productivity, and do not require the addition of large energy cost, ensuring surface properties have not been clarified.

In view of the above problems, the present invention provides a method and apparatus for secondary cooling of a continuously cast slab, which can ensure the surface properties of the slab without impairing productivity and without requiring addition of large energy costs. is aimed at

(1) The secondary cooling method of the continuous casting slab according to the present invention is a secondary cooling zone of a continuous casting machine configured in the order of a vertical band, a bending section, a curved band, a straightening section, and a horizontal band from the upstream side in the casting direction, As a method of cooling by spraying cooling water on the cast, and completing solidification of the cast in the section up to the end of the horizontal band, the cooling water sprayed in the section on the upstream side in the casting direction among the horizontal bar is the width of the surface of the cast A strong water cooling section in which the cooling water is sprayed under the condition that the slab is in a state of nuclei boiling at all positions in the direction, and a section downstream in the casting direction from the strong water cooling section and up to the end of the horizontal band, the cooling water By setting as a non-water cooling section in which the injection of It is characterized in that the range of

(2) Further, in the secondary cooling method of the continuous cast slab described in (1) above, the horizontal band is divided into n (n: integer, 3 ≤ n) sections in the casting direction, ni to nth Let the section of (i: integer, 0 ≤ i < n-1) as the non-water cooling section, and the 1st to ni-1th section as the strong water cooling section,

The water density per unit time of the cooling water in the 1st to jth section (j: integer, 1 ≤ j < ni-1) among the precipitation cooling sections of the 1st to ni-1th section, j + It is characterized in that it is larger than the water density per unit time of the cooling water in the 1st to ni-1th section.

(3) Further, in the secondary cooling method of the continuously cast slab according to the above (2), in the strong water cooling section of the 1st to ni-1th section, the 1st to jth (j: integer, 1≤j) The water density of the cooling water in the section <ni-1) is 500 L/(m2·min) (provided that min is a minute in time unit) or more and 2000 L/(m2·min) or less, j+1 It is characterized in that the water density of the cooling water in the to ni-1 th section is 50 L/(m 2 ·min) or more and less than 500 L/(m 2 ·min).

(4) Further, in the secondary cooling method of the continuous cast slab according to any one of (1) to (3) above, the surface temperature of the slab at the end of the horizontal band is the lowest temperature in the slab width direction It is characterized in that it is set to 350 ° C. or higher at the position showing.

(5) The secondary cooling device of the continuous casting slab according to the present invention sprays cooling water to the slab in the secondary cooling zone of the continuous casting machine configured in the order of a vertical band, a curved band, and a horizontal band from the upstream side in the casting direction, As to complete the solidification of the slab in the section to the end of the horizontal band, the horizontal band is divided into n (n: integer, 3 ≤ n) sections in the casting direction, and the section of the horizontal band is A plurality of spray nozzles arranged and formed in each, and a water supply means and a water supply control device capable of controlling the injection and stopping of the cooling water from the plurality of spray nozzles, and the water density per unit time of the cooling water for each section, The water supply control device is, in the 1st to ni-1th (i: integer, 0 ≤ i < n-1) section from the upstream side of the casting direction, the sprayed cooling water at all positions in the width direction of the surface of the cast slab The cooling water is sprayed from the spray nozzle so as to become a strong water cooling section in a nuclear boiling state, and in the ni to n-th (i: integer, 0 ≤ i < n-1) section, from the spray nozzle to a non-water cooling section It is characterized in that the injection of the coolant is stopped.

(6) In addition, in the secondary cooling device for the continuous casting slab according to (5), the water supply control device includes: 1 to j-th (j) : integer, so that the water density per unit time of the cooling water in the interval of 1 ≤ j < ni-1) is greater than the water density per unit time of the cooling water in the interval of j + 1 to ni-1) , to control the injection of the cooling water from the spray nozzle.

(7) Moreover, in the secondary cooling apparatus for the continuous casting slab according to the above (6), the water supply control device includes: 1 to j-th (j: integer) among the 1-th to ni-1 strong water cooling sections , 1 ≤ j < ni-1), the water density of the cooling water is 500 L/(m 2 min) (provided that min is a minute in time unit) or more and 2000 L/(m 2 min) or less , j + 1 to ni-1 th section, so that the water density of the cooling water is 50 L/(m 2 ·min) or more and less than 500 L/(m 2 ·min) of the cooling water from the spray nozzle. It is characterized in that the injection is controlled.

In the present invention, the section on the upstream side in the casting direction in the horizontal band is a strong water cooling section in which the sprayed cooling water is sprayed to cool the slab under the condition that the injected cooling water is in a state of nuclei boiling at all positions in the width direction of the surface of the slab. In addition, the section downstream in the casting direction from the strong water cooling section and up to the end of the horizontal band is a non-water cooling section where the injection of cooling water is stopped, so that after the strong water cooling section, across the end of the horizontal band, the casting direction While increasing the surface temperature of the slab with a slab, the surface temperature of the slab at the end of the horizontal bar was set to a predetermined range, so it does not impair productivity and does not require the addition of large energy costs, and the surface properties of the slab are can be obtained

BRIEF DESCRIPTION OF THE DRAWINGS It is explanatory drawing explaining the outline|summary of the continuous casting facility in one Embodiment of this invention.
It is a graph which shows the temperature history of the slab of the continuous casting method in one Embodiment of this invention.
It is explanatory drawing explaining the outline|summary of the conventional general continuous casting installation.
Figure 4 is a graph showing the temperature history of the slab of the conventional general continuous casting method.

The continuous casting machine used for the secondary cooling method of the continuous casting slab which concerns on this embodiment is outlined based on FIG.

The continuous casting machine 1, as shown in FIG. 1, supports the molten steel poured into the mold 3 from a tundish (not shown) with rolls (not shown), and a cooling spray (not shown) formed between the rolls. It is an apparatus which draws out as a slab 5 while secondary cooling by a slab.

As shown in Fig. 1, the secondary cooling table 7 for secondary cooling the cast steel 5 is a vertical stand 9, a bending section 11, a curved section 13, a straightening section 15, and a horizontal section. It is divided into the base|base 17, and the secondary cooling method of this invention has a characteristic mainly in the cooling method of the cast steel 5 in the horizontal stand 17. As shown in FIG.

The secondary cooling zone 7 of the continuous casting machine 1 is divided into n (n: integer, 3 ≤ n) sections on the horizontal table 17, and ON/OFF and cooling of cooling water in each section A strong cooling facility 21 provided with water supply means capable of controlling the water quantity and a water supply control device 19 is provided.

Although the number of n is set in advance according to facilities, which section of the n sections is made into a strong water cooling section or a non-cooling section can be appropriately set by the water supply control device 19 .

On the horizontal platform 17, although depending on the scale of the facility, rolls close to 100 are arranged and formed at predetermined intervals in the casting direction, and spray nozzles for spraying cooling water are arranged between the rolls, and between the rolls A plurality of spray nozzles are arranged in the slab width direction.

The strong cooling installation 21 of this embodiment sets the spray nozzle installed between the several rolls in the casting direction (for example, between 10 rolls) into one group, and divides the horizontal board 17 into n sections. is divided into

Accordingly, in each section, a plurality of spray nozzles are grouped so as to spray a large flow rate of cooling water in order to quickly stabilize the boiling state of the cooling water to the nuclear boiling state.

Moreover, in each section, for example, the nozzle used and the piping can be switched so that it can respond not only to a large flow rate condition but to a low flow rate condition.

The spray nozzle used here is not limited to the hydraulic water spray, as long as the water density per unit time to be described later can be realized, and a water-air mixed mist spray nozzle or the like may be used.

The secondary cooling method of the continuous casting slab of this embodiment, the slab 5 being cast with the above-mentioned continuous casting machine 1, the vertical stand 9, the bending part 11, the curved stand 13, In the secondary cooling table 7 having the straightening section 15 and the horizontal bar 17, cooling water is sprayed on the cast slab 5 to cool, and the solidification of the cast slab 5 is completed in the section up to the end of the horizontal bar. In this case, the section on the upstream side in the casting direction in the horizontal band 17 is a strong water cooling section in which the sprayed cooling water is sprayed with cooling water under the condition that the slab is in a nuclear boiling state on the surface of the cast slab to cool the cast slab 5. In addition, the section from the strong water cooling section to the end of the horizontal band on the downstream side in the casting direction is a non-water cooling section in which injection of cooling water is stopped.

Then, after the strong water cooling section, the surface temperature of the slab at the end of the horizontal band is set to a predetermined range while increasing the surface temperature of the slab in the casting direction over the end of the horizontal band.

The result of numerical analysis which reproduces the temperature history of the surface of the slab manufactured using the above continuous casting machine 1 is shown in FIG. In Fig. 2, the temperature history of the center of the surface width of the cast steel, the corner portions (corners), and the center of the cross section of the cast steel is indicated by a solid line, a broken line, and a dashed-dotted line, respectively, and the lowest temperature that can be cut is indicated by a thin line. In addition, in FIG. 2, the solidification completion position is made into A', and the continuous casting machine base end is shown as B. In FIG. 2, the coagulation|solidification completion position A in the conventional example shown in FIG. 4 is also shown.

Cooling from immediately below the mold 3 to passing through the straightening unit 15 is performed in the same manner as in the prior art, and the surface temperature of the cast steel 5 in the straightening unit 15 is adjusted to the embrittlement temperature range 25 ) to the higher temperature side.

On the other hand, when entering the horizontal stage 17 and starting cooling with the strong cooling facility 21, the horizontal stage 17 on the downstream side in the casting direction after the water spray installed between the first rolls entering the horizontal stage 17 , the nuclei boiling state is realized uniformly in the width direction by a large flow rate of water spray. As a result, it can be seen that the temperature of the center of the width of the slab and the corner of the slab is reduced to a temperature close to the water temperature at the same time and is stabilized.

After that, the strong cooling is continued to maintain the nuclei boiling state, and after solidification is completed at the point A', the internal temperature starts to decrease. After internal coagulation is completed or even before coagulation is completed, it is not necessary to cool after the temperature is sufficiently lowered and coagulation is surely completed to the base end. Therefore, spraying is stopped in the i+1 area|region of n-i-th - n-th (i: integer, 0 ≤ i < n-1), and after point C, the surface of the cast steel is reheated. As a result, in the point B, the temperature of the slab corner part became more than the cut-out possible temperature, and it was able to cut without a problem.

In general, temperature control with respect to the fluctuation of the casting speed of the cast slab 5 is often performed by changing the flow rate of the cooling water, but as in the present invention, strong cooling is performed from the viewpoint of cooling stabilization, and the temperature is near room temperature. In the case of cooling, it is not possible to control the flow rate from the viewpoint of maintaining nuclei boiling. Therefore, as described above, it is necessary to control the cooling end temperature by adjusting the water cooling time by stopping cooling in some cooling sections.

When the present invention is applied, by performing strong cooling on the horizontal stand 17, when the casting speed is the same as in the prior art, the solidification completion position A' is higher than the position A in the case of applying the prior art to the continuous casting machine 1 Because it moves to the side, it is possible to speed up the casting speed compared to the conventional conditions. At this time, as the casting speed increases, the time passing through the cooling zone decreases, thereby shortening the cooling time. Then, solidification can be reliably completed in the continuous casting machine 1 by reducing the number of non-water cooling sections i+1 where cooling is stopped and extending the length of the cooling zone which is cooling.

On the other hand, the casting speed will fall conversely at the time of casting start and the time of completion|finish. In this case, control can be performed so that the number of non-water cooling sections i+1 is enlarged and the temperature of the whole slab 5 does not fall and a slab corner part does not fall below the cut-out possible temperature.

Regarding the spraying conditions (quantity density per unit time) of the cooling water in the present invention, the nuclei boiling rapidly over the entire width irrespective of variations in casting speed, manufacturing conditions such as steel type, or facility conditions such as spray arrangement intervals. As a result of examining the conditions for realizing the Here, the water density per unit time is a value obtained by dividing the water quantity (L/min) of the cooling water in the cooling section by the area (m2) of the cooling section.

Below this density of water per unit time, when the high-temperature slab 5 is cooled, it does not stably reach the nuclei boiling state, and the temperature drop is large (slab corners, etc.) and the temperature drop is small (slab width center, etc.) ), the timing of nucleation boiling differs greatly, causing a significant temperature difference in the width direction.

In addition, depending on the equipment layout and steel type, the cooling water of the water spray greatly recuperates in areas where the cooling water is not directly sprayed (just under and in the vicinity of the guide roll, etc.) can be the cause of And it originates in such a temperature difference, the cast steel 5 will deform|transform, and will cause defects, such as a crack.

On the other hand, when nuclei boiling is realized, the dependence of the cooling capacity on the water density per unit time becomes small because cooling by boiling becomes dominant. Therefore, at a water density per unit time greater than 2000 L/(m2·min), a significant improvement in cooling capacity cannot be expected, and the total amount of cooling water used becomes excessive, which increases facility investment in water treatment facilities. It is appropriate that the water density per unit time in the section is in the range of 500 L/(m 2 ·min) or more and 2000 L/(m 2 ·min) or less.

When the slab 5 enters the above-mentioned strong water cooling section and the surface temperature of the slab decreases due to nucleation boiling, it is possible to stably maintain the nuclei boiling state even without a large flow rate of 500 L/(m 2 min) or more. .

Therefore, if there is a restriction on the total amount of cooling water that can be used in the entire continuous casting machine 1, per unit time of the 1st to jth (j: integer, 1 ≤ j ≤ ni-1) section of the strong water cooling section is a large flow area of 500 L/(m2·min) or more, and the remaining j + 1st to ni-1th sections need only have enough water density per unit time to maintain nuclei boiling, so 50 It can be set as the low-flow area in which the water quantity was suppressed to L/(m 2 ·min) or more and less than 500 L/(m 2 ·min). At this time, the number of sections j in the high flow rate region at the front end may be arbitrarily set according to manufacturing conditions such as steel type or cast steel thickness.

Moreover, as a result of examining the temperature range which can ensure the cutting property in the gas cutter on the exit side of a continuous casting machine, it turned out that it is necessary to control the temperature of the slab corner part immediately before a cutter to 350 degreeC or more. Therefore, it is preferable that the surface temperature of the slab at the end of the horizontal band 17 be 350 degreeC or more at the position which shows the lowest temperature in the slab width direction.

As described above, in this embodiment, the secondary cooling zone 7 of the horizontal band 17 is divided by the strong cooling facility 21 into a plurality of sections, and a strong water cooling section in which the core boiling state is maintained and cooled. And, a non-cooling section in which the injection of cooling water is stopped is formed on the downstream side in the casting direction of the strong water cooling section, and the range of this section can be changed according to conditions such as casting speed, so it does not cause large temperature unevenness on the surface Without it, the temperature at the end of casting can be controlled.

Thereby, it becomes possible to cast at high speed while maintaining the surface properties of the cast slab 5 at high quality, and even when the casting conditions change, the slab 5 can be cut without any problem, and the cast slab 5 can be stably cut with high quality. ) can be manufactured while maintaining high productivity.

In addition, the non-cooling section formed on the downstream side in the casting direction of the strong water cooling section is a section in which the injection of cooling water is stopped because the cast steel is not actively cooled, for example, the remaining liquid in the pipe flows to the surface of the cast steel. Even when cooling water is applied to the surface of the cast without intention of cooling, such as a state in which a small amount of water is supplied to prevent clogging of the spray nozzle, the cooling water for active cooling of the cast steel as described above It goes without saying that if the injection is stopped, it is included in the non-cooling section.

In addition, in the non-cooling section, not only the injection of cooling water is stopped, but also, by using auxiliary means such as a thermal insulation cover or an edge heater, the temperature of the corner part of the slab where the surface temperature of the slab tends to decrease may be maintained and raised. .

If the water density per unit time cannot be achieved for some reason, such as a facility abnormality due to water leakage from the pipe, and the slab enters the strong water cooling section, and does not quickly reach the nuclei boiling state, the boiling state It is necessary to achieve and maintain the nuclear boiling state reliably by increasing the quantity while monitoring the

When the cooling water in contact with the slab surface boils, it vaporizes to become water vapor, and steam (water smoke) in which this water vapor condensed in the air can be observed. Here, in the state of nuclei boiling, the cooling water in contact with the surface of the cast steel foams violently, a large amount of water vapor is generated, and the amount of water smoke generated increases. On the other hand, in the film boiling state, the foaming of the boiling cooling water is small, and the amount of water vapor and water smoke generated also decreases. Then, a camera is installed in each section, and the generation amount of a water stream is monitored by observation with the naked eye or measurement with a transmittance meter. In advance, it is determined whether the nuclear boiling state is achieved in a predetermined section by obtaining the threshold value of the amount of occurrence of the suspension that distinguishes the nuclear boiling and the film boiling by experiment, and confirming whether the amount of occurrence of the suspension exceeds the threshold value. Check it. Then, when the nuclear boiling state is not achieved, it is adjusted so as to increase the amount of cooling water. Thereby, it is possible to reliably achieve and maintain the nuclei boiling state.

Example

Since the slab 5 was manufactured using the continuous casting machine 1 (FIG. 1) which is embodiment mentioned above, and the effect of this invention was confirmed, it demonstrates below.

In the present Example, the horizontal platform 17 was divided into 12 sections (n = 12), and the presence or absence of injection and the injection flow rate were controlled for each section. Moreover, the length of the continuous casting machine 1 is 45 m, and the thermometer which measures the temperature distribution of the surface of a cast steel, and the gas cutter 23 are provided in the base end.

The slab (5) is manufactured by changing manufacturing conditions such as the water density per horizontal unit time (L/(m2·min)), the casting speed, and the slab thickness, and the temperature non-uniformity during cooling or the estimated solidification completion position in the casting machine , the temperature of the corner part of the slab at the time of cutting, and the surface properties after casting were evaluated.

The manufacturing conditions and evaluation are shown in Table 1 below. In the table, those within the scope of the example of the present invention are referred to as Examples 1 to 7, and those outside the scope of the invention are referred to as Comparative Examples 1 to 8.

In addition, estimation of the solidification completion position is performed in advance by numerical analysis, and in some comparative examples, as a result of preliminary examination, it was judged that there is a risk that the solidification completion position does not enter the continuous casting machine 1, so it is not actually manufactured. there is also

Hereinafter, the results of Table 1 will be considered for each related comparative example and example.

<Comparative Examples 1 and 2, Examples 1 and 2>

In Comparative Examples 1 and 2 and Examples 1 and 2, slabs 5 having a thickness of 235 mm were manufactured by applying the prior art and the technique of the present invention, respectively.

Comparative Example 1 is an example manufactured under the same cooling conditions as in the prior art (water density per unit time of 10 L/(m 2 ·min), no cooling stop area). In this example, since film boiling was always stably maintained on the surface, temperature non-uniformity did not occur, and no problem was confirmed by inspection of the surface state of the slab after manufacturing. Moreover, the temperature of the slab corner part at the time of cutting was 580 degreeC, and there was no obstacle in cutting.

However, the casting speed was limited to a maximum of 1.0 mpm in order to bring the solidified position into the cabin (estimated 36 m position).

Therefore, in Comparative Example 2, a case in which the casting speed was increased to 2.5 mpm was examined for productivity improvement. On this condition, since the calculated result that the estimated solidification completion position was outside the room became a result, actual manufacture was not performed. As described above, even with the prior art, the cast slab 5 with good surface properties can be manufactured, but the casting speed is limited.

In contrast, in Example 1, by applying the technology of the present invention, strong cooling is performed by setting the water density per unit time to 500 L/(m 2 ·min) in the 1st to 9th sections, and the 10th to 12th sections The surface temperature was adjusted by recuperating by stopping the cooling water. At this time, casting was performed by increasing the casting speed to 2.5 mpm. As a result, the nuclei boiling state was uniformly reached in the width direction by strong cooling, and temperature nonuniformity did not arise. In addition, since the estimated solidification completion position was 38 m and had fully entered into the machine, it manufactured. As a result, although the temperature of the corner part of the slab at the time of cutting|disconnection fell with respect to the comparative example 1 at 420 degreeC, it entered into a cutable area|region, and was able to cut|disconnect without a problem. Moreover, when the surface state of the slab was inspected after production, no cracks were observed, and the slab 5 with good surface properties could be manufactured without trouble and with high efficiency.

In Example 2, applying the technology of the present invention, strong cooling is performed by setting the water density per unit time to 2000 L/(m 2 ·min) in the 1st to 10th areas, and the areas where the cooling water is stopped are 11 to 12 It was set as the second section. At this time, the casting speed could be further increased to 3.5 mpm, and there was no problem in cutting or surface properties, and the high-quality cast slab 5 could be manufactured with high efficiency.

<Comparative Examples 3 and 4>

Comparative Examples 3 and 4 are results of changing the cooling conditions of the strong water cooling section with reference to the conditions of Example 1. In Comparative Example 3, strong cooling was performed by setting the water density per unit time to 500 L/(m 2 ·min) in all sections without forming a cooling stop region. At this time, there was no temperature nonuniformity by cooling, and the solidification completion position also entered the cabin. However, since the time during which the strong cooling was performed was long, and the heat was not sufficiently reheated at the base end, the temperature of the corner portion of the slab at the time of cutting fell to 320°C. As a result, since the cutting took time and there was a fear that the cutting could not be completed within the movable range of the gas cutting machine 23, it was necessary to urgently reduce the casting speed. Moreover, since the casting speed changed greatly, the problem that the surface quality and internal quality of the slab 5 which were being cast at that time fell.

In Comparative Example 4, the water density per unit time in the 1st to 10th sections was 400 L/(m 2 ·min), and the cooling water was stopped in the 11th to 12th sections. As a result, at this flow rate, the core boiling state was not stably reached at the width position of a part of the cast steel in the strong water cooling section, and the nucleus boiling first occurred at the corner part of the cast steel where the temperature drop was large, resulting in a significant temperature difference in the width direction. For this reason, there arises a problem that cracks on the surface of the cast slab or internal cracks occur, and the quality of the slab 5 deteriorates.

<Examples 3 and 4, Comparative Examples 5 and 6>

In Examples 3 and 4 and Comparative Examples 5 and 6, with respect to Example 1, only the first section of the strong water cooling section was a large flow rate region, and the flow rates in the second and subsequent sections were reduced.

In Example 3, the water density per unit time in the first large flow rate section is 500 L/(m2·min), and the water density per unit time in the second to eleventh sections is 50 L/(m2·min) ), and the cooling water was stopped in the 12th section. At this time, in the cooling of the first section of the strong water cooling section, the nuclear boiling state was reached, and the nuclear boiling state was maintained without recuperation in the subsequent section. As a result, the cooling nonuniformity of the width direction did not arise. Moreover, the coagulation|solidification completion position also entered the plane at 43 m. The slab corner temperature at the time of cutting was 430 degreeC, and it was able to cut without a problem. Further, when the surface state of the cast slab was inspected after production, no cracks were observed and a cast slab 5 having good surface properties could be produced.

Further, in Example 4, the water density per unit time in the strong water cooling section was 2000 L/(m2·min) in the first section, 1000 L/(m2·min) in the second section, and 500 L/(m2·min) in the third section. (m2·min), 100 L/(m2·min) for the 4th to 5th, and 50 L/(m2·min) for the 6th to 10th. In the 11th to 12th sections, the cooling water was stopped. At this time, the nuclear boiling state was reached by cooling in the first section of the strong water cooling section, and the nuclear boiling state was maintained without recuperation in the subsequent section. As a result, the cooling nonuniformity of the width direction did not arise. In addition, the coagulation completion position was also in the plane at 40 m. The temperature of the corner part of the cast steel at the time of cutting was 370 degreeC, and it was able to cut without a problem. Further, when the surface state of the cast slab was inspected after production, no cracks were observed and a cast slab 5 having good surface properties could be produced.

On the other hand, in Comparative Example 5, the water density per unit time in the low flow rate region in the second half of the strong water cooling section was 40 L/(m 2 ·min). As a result, nuclei boiling could not be maintained at the center of the width of the slab with large recuperation, and the temperature increased, and significant temperature nonuniformity occurred in the width direction. Although the solidification completion position entered the machine, the slab deformed due to the temperature nonuniformity in the width direction, and cracks arose on the surface.

Further, in Comparative Example 6, the water density per unit time in the large flow area in the first half of the strong water cooling section was 400 L/(m 2 ·min). As a result, the core boiling state could not be realized quickly at the stage where the cast steel 5 entered the strong water cooling section, and the nuclear boiling state and the film boiling state were mixed in the width direction. Therefore, the nonuniformity of the surface temperature was large, and surface cracks occurred, and as a result of which cooling became nonuniform, the solidification completion position became nonuniform|heterogenous, and internal quality fell.

<Example 5>

Example 5 is an example of a case in which the casting speed had to be greatly reduced at the time of casting start or end, etc. with respect to Example 1. At this time, the casting speed is lowered to 2.0 mpm, and since the time for performing strong cooling is prolonged, the non-water cooling section is expanded to the 8th to 12th times. As a result, cooling nonuniformity did not generate|occur|produce, and the solidification completion position was 35 m, and the temperature of the slab corner part at the time of cutting was also able to enter the range which can cut at 460 degreeC. Moreover, when the surface state of the slab was inspected after production, no cracks were observed, and the slab (5) with good surface properties could be manufactured without any problem even when the casting speed was significantly changed.

<Comparative Examples 7 and 8, Examples 6 and 7>

Comparative Examples 7 and 6, and Comparative Examples 8 and 7 are results when the slab thickness is changed to 260 mm and 200 mm, respectively. Comparative Examples 7 and 8 are cases in which the slab thickness is changed to 260 mm and 200 mm under the cooling conditions of the prior art in the same manner as in Comparative Example 1.

In Comparative Example 7, the slab thickness was 260 mm, and since the temperature drop was reduced by increasing the slab thickness compared to Comparative Example 1, the casting speed was reduced to 0.8 mpm, so that the solidification completion position was entered into the cabin. In Comparative Example 8, the slab thickness was 200 mm, and the casting speed was increased to 2.0 mpm in order to avoid unnecessary temperature drop after completion of solidification of the central part due to the slab thickness being reduced compared to Comparative Example 1.

In contrast, Example 6 is a case where the slab thickness is 260 mm, and since the slab thickness is increased compared to Example 1, the temperature drop is small, so the casting speed is the same and the strong water cooling section is extended to the 1st to 11th. The distribution of water density per unit time in the precipitation cooling section was the same as in Example 1. As a result, cooling nonuniformity did not generate|occur|produce, and the solidification completion position was 42 m, and the temperature of the slab corner part at the time of cutting was also able to enter into the range which can cut at 440 degreeC. In addition, when the surface state of the slab was inspected after production, no cracks were observed, and a cast slab 5 with good surface properties could be produced without problems while maintaining a high casting speed even when the casting thickness was increased.

Example 7 is a case where the slab thickness is 200 mm, and since the slab thickness is thinner than in Example 1, the temperature drop becomes large, so the casting speed was increased to 3.0 mpm. The distribution of water density per unit time in the strong water cooling section was the same as in Example 1, and the non-water cooling section was expanded to the 9th to 12th times. As a result, cooling nonuniformity did not generate|occur|produce, and the solidification completion position was 37 m, and the temperature of the slab corner part at the time of cutting was also able to enter into the range which can cut at 430 degreeC. Further, when the surface state of the slab was inspected after production, no cracks were observed, the casting speed was not significantly reduced even when the casting thickness was reduced, and the cast slab 5 with good surface properties could be manufactured without any problems.

In this way, by applying the technique of the present invention, even when the slab thickness is changed, there is no need to change the casting speed as large as in the prior art, and a high-quality slab 5 can be stably manufactured with high efficiency.

As described above, in the section on the upstream side of the casting direction in the horizontal platform 17, the cooling water is sprayed under the condition that the injected cooling water is in a nuclear boiling state at all positions in the width direction of the surface of the cast steel to cool the cast steel 5 In the strong water cooling section, the section on the downstream side in the casting direction from the strong water cooling section and up to the end of the horizontal band is a non-water cooling section in which the injection of cooling water is stopped, thereby limiting the casting speed even when casting conditions change It was demonstrated that it can be manufactured while maintaining the slab 5 at a temperature easy to cut without requiring the addition of large energy costs for heating.

1: continuous casting machine
3: mold
5: Cast
7: Secondary cooling zone
9: vertical bar
11: bending part
13 : curve
15: correction department
17 : level
19: water supply control device
21: steel cooling equipment
23 : gas cutter
25: embrittlement temperature range

Claims (7)

From the upstream side in the casting direction, in the secondary cooling zone of the continuous casting machine consisting of a vertical band, a bending section, a curved band, a straightening section, and a horizontal band in the order of cooling water by spraying cooling water on the slab, in the section to the end of the horizontal band As a secondary cooling method of a continuous cast slab to complete the solidification of the slab,
The section on the upstream side of the casting direction among the horizontal bands is a strong water cooling section in which the sprayed cooling water is sprayed under the condition that the coolant is in a nuclear boiling state at all positions in the width direction of the surface of the cast steel to cool the cast steel, In addition, by making the section up to the end of the horizontal band on the downstream side in the casting direction from the strong water cooling section as a non-water cooling section where the injection of the cooling water is stopped, after the strong water cooling section, across the end of the horizontal band, in the casting direction A secondary cooling method for a continuously cast slab, wherein the surface temperature of the slab at the end of the horizontal band is set within a predetermined range while the surface temperature of the slab is raised. The method of claim 1,
The horizontal band is divided into n (n: integer, 3 ≤ n) sections in the casting direction, and the ni to n-th section (i: integer, 0 ≤ i < n-1) is used as the non-water cooling section, , let the 1st to ni-1th section be the strong water cooling section,
The water density per unit time of the cooling water in the 1st to jth section (j: integer, 1 ≤ j < ni-1) among the precipitation cooling sections of the 1st to ni-1th section, j + Secondary cooling method of continuous casting slab, characterized in that it is made larger than the water density per unit time of cooling water in the 1st to ni-1th section. 3. The method of claim 2,
The water density of the cooling water in the 1st to jth (j: integer, 1 ≤ j < ni-1) section among the precipitation cooling sections of the 1st to ni-1th sections is 500 L/(m2) min) or more and 2000 L/(m2·min) (however, min is a minute in time unit) or less, the water density of the cooling water in the section j+1 to ni-1 is 50 L/(m2) ·min) or more and less than 500 L/(m 2 ·min), the secondary cooling method of the continuous casting slab. 4. The method according to any one of claims 1 to 3,
The secondary cooling method of a continuously cast slab, wherein the surface temperature of the slab at the end of the horizontal band is set to 350° C. or higher at a position showing the lowest temperature in the slab width direction. From the upstream side of the casting direction, in the secondary cooling zone of a continuous casting machine consisting of a vertical band, a curved band, and a horizontal band, cooling water is sprayed on the slab to cool, and solidification of the slab is completed in the section up to the end of the horizontal band As a secondary cooling device for continuous casting slabs,
The horizontal band is divided into n (n: integer, 3 ≤ n) sections in the casting direction,
A plurality of spray nozzles disposed and formed in each of the sections of the horizontal stand, and a water supply means capable of controlling the spraying and stopping of the cooling water from the plurality of spray nozzles, and the water density per unit time of the cooling water for each section, and having a water supply control device;
The water supply control device is, in the 1st to ni-1th (i: integer, 0 ≤ i < n-1) section from the upstream side of the casting direction, the sprayed cooling water at all positions in the width direction of the surface of the cast slab The cooling water is sprayed from the spray nozzle so as to become a strong water cooling section that is in a nuclear boiling state, and in the ni to n-th (i: integer, 0 ≤ i < n-1) section, from the spray nozzle to a non-water cooling section Secondary cooling device for continuous casting slab, characterized in that to stop the injection of the cooling water. 6. The method of claim 5,
The water supply control device is configured to control the amount of the cooling water per unit time in the 1st to jth section (j: integer, 1 ≤ j < ni-1) among the strong water cooling sections of the 1st to ni-1th sections. 2 of a continuous casting slab characterized in that the injection of the cooling water from the spray nozzle is controlled so that the water density is greater than the water density per unit time of the cooling water in the j+1 to ni-1th section car cooling system. 7. The method of claim 6,
The water supply control device is configured such that the water density of the cooling water in the 1st to jth (j: integer, 1 ≤ j < ni-1) section among the 1st to ni-1th strong water cooling section is 500 L/(m2·min) or more and 2000 L/(m2·min) (however, min is a minute in time unit) or less, the water density of the cooling water in the interval j+1 to ni-1 is 50 A secondary cooling device for continuous casting slab, wherein the injection of the cooling water from the spray nozzle is controlled so as to be not less than L/(m 2 ·min) and less than 500 L/(m 2 ·min).

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