JP2010110813A - Secondary cooling method and apparatus for continuously cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a secondary cooling method and a secondary cooling apparatus which prevent the occurrence of surface cracks and internal cracks of a slab by accurately controlling the surface temperature of the slab in the secondary cooling zone of a continuous casting machine, and consequently produce the slab of high quality. <P>SOLUTION: The secondary cooling method for a continuous cast slab is characterized in that, in a method of cooling a slab cast by a continuous casting machine in the secondary cooling zone at the lower part of a die, (i) the surface temperature in the width direction of the continuously cast slab is measured, and (ii), when the maximum value or the minimum value of the surface temperature exceeds an objective temperature range, the atomization water quantity W(L/min) of the cooling water is adjusted based on the temperature and atomization water quantity density (L/m<SP>2</SP>min) of the cooling water, and the cooling is performed in such a manner that the surface temperature lies in the objective temperature range. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a cooling method and a cooling device for cooling a slab cast with a continuous casting mold in a secondary cooling zone.

  In a continuous casting machine, a slab consisting of a solidified shell solidified in a mold and containing unsolidified molten steel inside is drawn out of the mold, and a cooling medium is applied to the slab surface in a curved secondary cooling zone following the mold. The slab is cooled by spraying, but cooling in the secondary cooling zone (hereinafter sometimes referred to as “secondary cooling”) greatly affects the quality of the slab.

  For example, when the slab is excessively cooled (strongly cooled), surface cracks are generated on the surface of the slab at the slab bending part and the slab bending return part of the secondary cooling zone of the continuous casting machine. On the other hand, when the slab is slowly cooled, an internal crack is generated inside the slab when the slab still having a relatively thin solidified shell and receiving a large molten steel static pressure passes through the upper part of the secondary cooling zone.

  In continuous casting, in the secondary cooling zone, appropriately controlling the surface temperature of the slab is important in producing a high-quality slab. Therefore, in order to keep the surface temperature or surface temperature distribution of the slab within a predetermined range, many secondary cooling methods and / or secondary cooling devices for controlling the amount of cooling water sprayed on the slab have been proposed ( References 1 to 13).

  For example, in Patent Document 1, the temperature distribution in the width direction of the slab surface is measured, and the spray water amount is adjusted so that the temperature distribution matches the target temperature, or the spray nozzle and the slab are adjusted. It has been proposed to change the distance between the surfaces. Patent Document 2 also proposes measuring the surface temperature of a slab and adjusting the amount of cooling water sprayed.

  Furthermore, in Patent Document 3, the amount of cooling water determined according to the casting speed of the slab is corrected based on the temperature of the secondary cooling water, and the cooling water for controlling the amount of cooling water sprayed on the steel sheet in the secondary cooling zone is disclosed. Control devices have been proposed.

  Since the cooling capacity of the cooling water in the secondary cooling zone varies depending on the temperature of the cooling water, in the cooling water control device proposed in Patent Document 3, the amount of cooling water sprayed in the secondary cooling zone is set to the temperature of the cooling water. It is estimated that the cooling capacity of the cooling water is kept constant by the correction coefficient based on the above.

  On the other hand, in the operation of a continuous casting machine, a cooling method for adjusting the amount of cooling water in the secondary cooling zone based on the casting speed and the steel type has also been proposed (see Patent Documents 5, 7, and 8).

  In secondary cooling, it is important to keep the cooling capacity constant, but there are many factors that affect the cooling capacity of the cooling water, and these factors may be related to each other, Actually, it is difficult to accurately control the surface temperature or surface temperature distribution of a slab and stably produce a high-quality slab.

JP 54-032130 A Japanese Patent Laid-Open No. 55-045581 JP 2000-271713 A JP 2001-096346 A JP 2001-138019 A JP 2001-191158 A JP 2002-086252 A JP 2002-248553 A JP 2004-058117 A JP 2005-052881 A JP 2006-315011 A JP 2007-061888 A JP 2007-330998 A

  By measuring the slab temperature and adjusting the spray water amount of the cooling water, it is possible to keep the slab temperature within a predetermined range and obtain a high quality slab. However, cracks occur on the surface and / or inside of the slab even by the conventional secondary cooling method.

  This is because the effect of the cooling water temperature (hereinafter sometimes referred to as “water temperature”) on the cooling capacity of the cooling water is not taken into account. This is because the control accuracy deteriorates.

  For example, if the water temperature drops in winter and the measured value of the slab surface temperature is below the target temperature range, the difference between the measured slab surface temperature and the target value is necessary to enter the target temperature. The cooling water adjustment allowance is calculated, and based on the result, the cooling water spray water amount is adjusted (in this case, the water amount is reduced), but if the effect of the water temperature on the cooling capacity is not taken into account, the calculation is performed. The cooling water adjustment allowance does not match the adjustment allowance required for cooling the actual machine, and depending on the water density and the slab surface temperature, the water adjustment allowance will be excessive or insufficient.

  As a result, the adjustment of the cooling water amount is repeated until the measured slab surface temperature falls within the target temperature range. If the resulting surface crack occurs and the amount of water reduction is too large, an internal crack due to insufficient cooling will occur.

  Therefore, the present invention is based on the premise that the surface temperature of the slab is not properly controlled in the conventional secondary cooling method, and the surface temperature of the slab is determined in the secondary cooling zone of the continuous casting machine. It is an object to produce a high-quality cast slab by properly controlling the above-mentioned to prevent the occurrence of surface cracks and internal cracks. And it aims at providing the secondary cooling method and apparatus which solve this subject.

  The present inventors investigated the reason why the surface temperature of the slab is not properly controlled in the conventional secondary cooling method.

As a result, the cooling capacity of the cooling water changes due to the temperature change of the cooling water.
(X) The change width of the cooling capacity varies depending on the amount of cooling water (water density), and
(Y) It has been found that (x) is not considered in the conventional secondary cooling method.

  Based on knowledge (x), the inventors of the present invention can prevent the occurrence of surface cracks and internal cracks by appropriately controlling the amount of cooling water sprayed on the slab. Based on the idea that it can be stably produced, the inventors have intensively researched and made the present invention. The gist of the present invention is as follows.

(1) In a method of cooling a slab cast by a continuous casting machine in a secondary cooling zone below the mold,
(I) measuring the surface temperature in the width direction of the continuous cast slab,
(Ii) When the maximum value or the minimum value of the surface temperature exceeds the target temperature range, the spray water amount W (L / min) of the cooling water is defined by the temperature of the cooling water and the following formula (1). A secondary cooling method for a continuous cast slab, wherein the cooling is performed based on a water density (L / m ² min) and cooling so that the surface temperature falls within a target temperature range.
Spray water density (L / m ² min) = cooling water spray water quantity W (L / min) / spray thickness (m) / spray nozzle pitch (m) (1)

  (2) The secondary cooling method for a continuous cast slab according to (1), wherein the surface temperature is measured at an upper portion of the curved portion of the secondary cooling zone.

(3) An apparatus for performing the secondary cooling method of the continuous cast slab according to (1) or (2),
(X) temperature measuring means for measuring the surface temperature in the width direction of the slab,
(Y) The surface temperature measured by the temperature measuring means is compared with the target temperature range, and when the maximum value or the minimum value of the surface temperature exceeds the target temperature, the spray of cooling water that keeps the surface temperature within the target temperature range A water amount calculation control device for calculating the water amount W (L / min) based on the temperature of the cooling water and the spray water density (L / m ² min) defined by the following equation (1);
(Z) A flow rate adjusting means for adjusting the spray water amount of the cooling water sprayed from the nozzle based on the spray water amount W calculated by the water amount calculation control device.
A secondary cooling device for continuously cast slab, comprising:
Spray water density (L / m ² min) = cooling water spray water quantity W (L / min) / spray thickness (m) / spray nozzle pitch (m) (1)

  (4) The secondary cooling device for a continuous cast slab according to (3), wherein the temperature measuring means is disposed above the curved portion of the secondary cooling zone.

  According to the present invention, the amount of water sprayed in the cooling water is adjusted according to the temperature of the cooling water and the water density, so that the accuracy of the slab surface temperature does not deteriorate even when the temperature of the cooling water changes.

  For example, if the temperature of the cooling water drops in winter and the measured value of the slab surface temperature is below the target temperature range, the difference between the measured slab surface temperature and the target value is set so that the target temperature is entered. The required cooling water adjustment allowance is calculated.

  In the present invention, since the influence of the water temperature and the water density on the cooling capacity is taken into account, the cooling phenomenon in the actual machine can be accurately estimated, and the calculated cooling water adjustment allowance coincides with the actually required cooling water adjustment allowance. Therefore, the water amount adjustment time is shortened, and the occurrence of surface cracks due to overcooling during water amount adjustment and internal cracks due to insufficient cooling is greatly reduced.

  Further, according to the present invention, since the surface temperature in the width direction of the slab is measured at the upper part of the curved portion of the secondary cooling zone of the continuous casting machine, even if the slab is rapidly cooled (even if it is supercooled) ), It is possible to immediately reduce the amount of cooling water sprayed water, to sufficiently secure the time required for the slab to reheat, and to relieve the slab.

  The inventors of the present invention "manufacturing high-quality slabs by appropriately controlling the surface temperature of the slab in the secondary cooling zone of the continuous casting machine to suppress the occurrence of surface cracks and internal cracks." First, we investigated the effect of cooling water temperature on the cooling rate.

  After heating a steel plate (test piece) embedded with a thermocouple at a depth of 4 mm from the surface to 1000 ° C or higher, spray the cooling water on the steel plate surface, and investigate the effect of cooling water temperature on the cooling rate. did.

  In FIG. 1, the cooling curve in case the temperature of a cooling water is 25 degreeC, 35 degreeC, and 45 degreeC is shown. FIG. 2 shows a relationship (heat transfer curve) between the surface temperature and the heat transfer coefficient obtained based on the cooling curve shown in FIG.

  It can be seen from FIG. 1 that the cooling rate is faster as the temperature of the cooling water is lower. Moreover, even if the surface temperature of a slab is the same from FIG. 2, when the temperature of cooling water is low, it turns out that a heat transfer coefficient is large.

  Further, the cooling curve shown in FIG. 1 and the heat transfer curve shown in FIG. 2 have an inflection point, and the inflection point shifts to a higher temperature side as the temperature of the cooling water is lower. (See the dotted box in the figure).

  The cooling water boils on the surface of the steel sheet and forms a film boiling state at the beginning of spraying, but transitions to a transition boiling state as the surface temperature decreases. The inflection point shown in the figure indicates a point in time when the boiling state on the surface of the steel material being cooled has shifted from the film boiling state to the transition boiling state.

  The film boiling state is a state in which a vapor film is formed on the steel plate surface. Since the cooling of the steel sheet proceeds as the cooling water takes heat through the vapor film, the heat transfer coefficient in the film boiling state is small.

  The transition boiling state is a state in which a portion where the vapor film is locally lost is generated on the steel material surface. The cooling of the steel sheet proceeds by the cooling water directly contacting the steel sheet and taking heat away at the portion where the vapor film disappears, so that the heat transfer coefficient in the transition boiling state is large.

  As is clear from FIG. 1, the inflection point shifts to the high temperature side and simultaneously shifts to the short time side. That is, the time for which the steam generated on the steel surface at the start of cooling disappears (changes to the transition boiling state) becomes shorter as the cooling water temperature is lower. In addition, this was confirmed by observing the steel plate surface visually.

  The transition of the inflection point to the high temperature side and the short time side is due to the fact that the evaporated water vapor is absorbed by the water film, and the amount of condensate changes depending on the water temperature and the vapor film thickness changes. Conceivable.

  That is, the lower the temperature of the cooling water sprayed on the steel sheet surface, the more water vapor is absorbed in the water film, and the more water is condensed, so that the vapor film disappears faster.

From the above, the following can be understood.
(I) Even if the surface temperature of the steel sheet is the same, the lower the temperature of the cooling water, the larger the heat transfer coefficient.
(Ii) When the temperature of the cooling water is lower, the transition point from the film boiling state to the transition boiling state (the inflection point in the figure; hereinafter, sometimes referred to as “transition point”) is the high temperature side and the short time side. Migrate to
That is, when the temperature of the cooling water changes, the thickness of the vapor film on the steel sheet surface changes.

  The thickness of the vapor film on the surface of the steel sheet is determined by an equilibrium relationship between the amount of steam generated and the amount of heat removed by the sprayed cooling water. When the spray water density density of the cooling water increases, the cooling water continuously contacts the steel material surface, so the amount of heat removal increases and the thickness of the vapor film on the steel plate surface decreases.

The spray water density (L / m ² min) is an index defined by the following equation (1).
Spray water density (L / m ² min) = cooling water spray water quantity W (L / min) / spray thickness (m) / spray nozzle pitch (m) (1)

  FIG. 8 schematically shows the technical meaning of the above equation (1). It can be seen from FIG. 8 that when the spray water density of the cooling water increases, the cooling water continuously comes into contact with the steel material surface, and the amount of heat removal increases.

  When the temperature of the cooling water changes, the phenomenon that the thickness of the vapor film on the surface of the steel sheet changes is a phenomenon similar to the above phenomenon that appears when the amount of water sprayed (water density) is changed.

Therefore, the present inventors consider that “the change width of the cooling capacity that changes due to the temperature change of the cooling water varies depending on the density of the spray water amount”, and the spray water amount (water density) of the coolant and the cooling water A cooling test was performed at different temperatures. The results are shown in FIGS. The change in the thickness of the vapor film is indicated by the change in transition temperature from the film boiling state to the transition boiling state. FIG. 4 shows the relationship between the heat transfer coefficient (kcal / m ² hr ° C.) and the coolant water density (L / m ² min).

  From FIG. 3, (a1) the temperature at which the film boiling state shifts to the transition boiling state (transition temperature) varies depending on the temperature of the cooling water and the amount of water sprayed (water density), that is, (a2) cooling. It can be seen that the boiling state on the surface of the steel material is different depending on the amount of water sprayed (water density).

  Moreover, since the boiling state changes according to the amount density and temperature of the cooling water under the condition where the surface temperature of the steel sheet is constant, it can be seen from FIG. 4 that the influence of the temperature of the cooling water changes according to the amount density of the cooling water. I understand.

  The above knowledge is important for accurately controlling the temperature of the steel sheet surface when cooling the steel sheet. If the present inventors introduce the above knowledge into cooling in the secondary cooling zone of a continuous casting machine, the occurrence of surface cracks and internal cracks can be prevented, and high quality slabs can be manufactured more stably. I thought it was possible.

  FIG. 5 shows an embodiment of the present invention in the secondary cooling zone of the continuous casting machine. The slab 2 from which the mold 1 has been drawn downward is guided by the support roll 3, passes through the secondary cooling zone through the bending portion 4, and is cooled while being bent back by the bending portion 5.

  A thermometer 6 is disposed above the curved portion of the secondary cooling zone of the continuous casting machine mold (between AA 'in the figure; hereinafter referred to as "curved portion AA'"). With the thermometer 6, the surface temperature in the width direction of the slab is measured, and the measured value is transmitted to the water amount calculation control device 9.

  The arrangement position of the thermometer 6 is not limited to a specific position as long as the surface temperature can be accurately measured at the upper part of the curved portion AA '.

  In order to stably produce high quality slabs, it is necessary to maintain the slab surface temperature above the embrittlement temperature and avoid surface cracks when passing through the bent part where distortion occurs in the slab. There is.

  The surface temperature of the slab is measured in the vicinity of the bent portion 5 and when the surface temperature falls below the embrittlement temperature, a method of reducing the amount of sprayed water is conceivable. In the end, surface bending occurs at the bent-back portion 5. Therefore, it is preferable to install the thermometer 6 on the upstream side of the bent back portion 5.

  Furthermore, when supercooling occurs, a sufficient recuperation time is ensured, and the internal crack has a relatively thin solidified shell thickness, and the slab where the molten steel static pressure is acting is a bent part ( Considering that it is likely to occur when passing through the upper part of the curved part), it is preferable that the thermometer 6 is attached to the upper part of the curved part AA ′ at a position where the surface temperature can be accurately measured. The thermometer is preferably a radiation thermometer in terms of durability and measurement accuracy.

In the water amount calculation control device 9, the maximum value or minimum value of the surface temperature transmitted from the thermometer 6 is compared with the set target temperature, and when the maximum value or minimum value exceeds the target temperature, the target value and measurement are performed. The required cooling water adjustment allowance W ₂ (L / min) is calculated from the difference in temperature between the values, and from the spray cooling water amount W ₀ (L / min) preset based on the casting speed and / or steel type, W ₂ (L / min) is subtracted to calculate the spray cooling water amount W (L / m ² min).

Next, the amount of cooling water sprayed from the spray nozzle 7 is changed from the spray cooling water amount W ₀ (L / min) set in advance based on the casting speed and / or steel type to the spray cooling water amount W (L / m ² min). Is sent to the flow rate adjustment valve 8.

By changing the amount of cooling water sprayed from the spray nozzle 7 from the spray cooling water amount W ₀ (L / min) preset based on the casting speed and / or steel type to the spray cooling water amount W, the width of the slab The surface temperature in the direction can be kept within the target temperature range.

  An appropriate number of spray nozzles are arranged throughout the secondary cooling zone, and are arranged in groups of spray nozzles so that the amount of spray cooling water can be adjusted for each specific range. The section for adjusting the amount of spray cooling water need not be specified in particular, but internal cracks and surface cracks often occur between the bent part 4 and the bent part 5 of the secondary cooling zone. It is preferable to adjust the amount of spray cooling water upstream of the part 5.

  Next, examples of the present invention will be described. The conditions of the examples are one example of conditions adopted for confirming the feasibility and effects of the present invention, and the present invention is limited to this one example of conditions. Is not to be done. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Example 1
The present inventors constructed a transient solidification analysis model that estimates the transition of the surface temperature of the slab and the growth transition of the thickness of the solidified shell, and based on the knowledge obtained from the test results, the temperature of the cooling water is The surface temperature of the slab when changed was estimated.

  The unsteady solidification analysis model used was a general method described in, for example, iron and steel, volume 60 (1974), page 1023.

  Using the following method, the transition of the surface temperature when the temperature of the cooling water was changed was estimated by matching the transition of the actual machine surface temperature with the model analysis value.

(1) In order to consider the influence of the cooling water temperature and the amount of spray water on the cooling capacity, a regression equation related to the cooling capacity obtained in the cooling test of the present inventors was incorporated into the unsteady solidification analysis model.
(2) A thermometer was attached to the actual machine, and the surface temperature of the slab was measured when the temperature of the cooling water changed.
(3) By multiplying the regression equation related to the cooling capacity by a coefficient, the surface temperature when the temperature of the cooling water changed and the model analysis value were matched.

  Note that the maximum target value and the minimum target value of the surface temperature of the slab at the upper part of the curved portion of the secondary cooling zone (here, in the range of meniscus to 10 m) are set to 900 ° C. and 700 ° C., respectively, and cooled. The effect on the surface temperature when the water temperature changed was estimated.

In addition, the installation conditions and casting conditions in an Example are as follows.
[Equipment conditions]
Captain of continuous casting machine: 30m
Secondary cooling zone cooling zone: 9 zones (cooling zone up to the upper part of the curve: 4 zones)
[Casting conditions]
Slab thickness: 250mm
Slab width: 1250mm
Casting speed: 1.2 mpm
Spray water volume per cooling section: Table 1

  FIG. 6 shows the maximum value and the minimum value of the surface temperature of the slab at the upper part of the curved portion of the secondary cooling zone (here, the position of meniscus to 10 m). From FIG. 6, the surface temperature of the slab changes according to the temperature of the cooling water, and when the temperature of the cooling water exceeds 38 ° C., the surface temperature of the slab is outside the target temperature range (above 900 ° C.). I understand that Similarly, when the temperature of the cooling water is less than 30 ° C., it can be seen that the surface temperature of the slab is outside the target temperature range (less than 700 ° C.).

  Therefore, when the temperature of the cooling water: Tw> 38 ° C. or Tw <30 ° C., it is necessary to increase or decrease the amount of cooling water sprayed to return the slab temperature to the target temperature range.

  FIG. 7 shows the maximum value and the minimum value of the surface temperature of the slab at the upper part of the curved portion when the amount of spray water is increased or decreased according to the temperature of the cooling water. It can be seen that the surface temperature of the slab is within the target temperature range regardless of the coolant temperature.

  As described above, according to the present invention, cracks generated on the surface and / or inside of the slab due to fluctuations in the temperature of the cooling water can be prevented, and a high-quality slab can be stably produced. Even if the slab cools down rapidly, the amount of spray water of cooling water can be reduced immediately to ensure sufficient time for the slab to reheat and to rescue the slab. Can do.

  Therefore, the present invention has great applicability in the steel industry using continuous casting technology as a pillar.

It is a figure which shows the cooling curve of the steel materials in case the temperature of a cooling water is 25 degreeC, 35 degreeC, and 45 degreeC. It is a figure which shows the relationship between the surface temperature (degreeC) calculated | required based on the cooling curve shown in FIG. 1, and a heat transfer coefficient ((alpha)). It is a figure which shows the relationship between water quantity density (L / m < ² > min) and transition temperature (degreeC). It is a figure which shows the relationship between a heat transfer coefficient (kcal / m < ² > hr (degreeC)) and the amount density (L / m < ² > min) of cooling water. It is a figure which shows the aspect of the secondary cooling zone of a continuous casting machine. It is a figure which shows the maximum calculated value and the minimum calculated value of the surface temperature of the slab in the curved part upper part (here meniscus-10m position) of a secondary cooling zone. It is a figure which shows the maximum value and minimum value of the surface temperature of the slab in the upper part of a curved part at the time of increasing / decreasing the amount of spray water according to the temperature of a cooling water. It is a figure which shows typically the technical meaning of spray water quantity density (L / m < ² > min).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Mold 2 Cast slab 3 Support roll 4 Bending part 5 Bending part 6 Thermometer 7 Nozzle 8 Flow control valve 9 Water quantity calculation control device AA 'curved part W ₀ Cooling water quantity

Claims (4)

In the method of cooling the slab cast by the continuous casting machine in the secondary cooling zone at the bottom of the mold,
(I) measuring the surface temperature in the width direction of the continuous cast slab,
(Ii) When the maximum value or the minimum value of the surface temperature exceeds the target temperature range, the spray water amount W (L / min) of the cooling water is defined by the temperature of the cooling water and the following formula (1). A secondary cooling method for a continuous cast slab, wherein the cooling is performed based on a water density (L / m ² min) and cooling so that the surface temperature falls within a target temperature range.
Spray water density (L / m ² min) = cooling water spray water quantity W (L / min) / spray thickness (m) / spray nozzle pitch (m) (1)   The method for secondary cooling of a continuous cast slab according to claim 1, wherein the surface temperature is measured at an upper portion of a curved portion of the secondary cooling zone. An apparatus for carrying out the secondary cooling method for continuously cast slabs according to claim 1 or 2,
(X) temperature measuring means for measuring the surface temperature in the width direction of the slab,
(Y) The surface temperature measured by the temperature measuring means is compared with the target temperature range, and when the maximum value or the minimum value of the surface temperature exceeds the target temperature, the spray of cooling water that keeps the surface temperature within the target temperature range A water amount calculation control device for calculating the water amount W (L / min) based on the temperature of the cooling water and the spray water density (L / m ² min) defined by the following equation (1);
(Z) A flow rate adjusting means for adjusting the spray water amount of the cooling water sprayed from the nozzle based on the spray water amount W calculated by the water amount calculation control device.
A secondary cooling device for continuously cast slab, comprising:
Spray water density (L / m ² min) = cooling water spray water quantity W (L / min) / spray thickness (m) / spray nozzle pitch (m) (1)   The secondary cooling device for a continuous cast slab according to claim 3, wherein the temperature measuring means is disposed above the curved portion of the secondary cooling zone.

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