STEEL CONTINUOUS-CASTING MACHINE AND STEEL CONTINUOUS-CASTING METHOD

Abstract

A steel continuous-casting machine including: a cooling apparatus that is configured to cool cast steel with water, wherein: the cooling apparatus includes a plurality of cooling-water-discharging nozzles configured to be arranged in a width direction of the cast steel, and the plurality of cooling-water-discharging nozzles are configured to be arranged such that spray discharge surfaces of adjacent cooling-water-discharging nozzles of the plurality of cooling-water-discharging nozzles that are adjacent to each other in the width direction of the cast steel do not overlap.

Claims

1. A steel continuous-casting machine comprising: a cooling apparatus that is configured to cool cast steel with water, wherein: the cooling apparatus includes a plurality of cooling-water-discharging nozzles configured to be arranged in a width direction of the cast steel, and the plurality of cooling-water-discharging nozzles are configured to be arranged such that spray discharge surfaces of adjacent cooling-water-discharging nozzles of the plurality of cooling-water-discharging nozzles that are adjacent to each other in the width direction of the cast steel do not overlap.

2. The steel continuous-casting machine according to claim 1, wherein: the spray discharge surfaces of the adjacent cooling-water-discharging nozzles are rectangular or elliptical, and each cooling-water-discharging nozzle of the plurality of cooling-water-discharging nozzles is disposed to satisfy Inequality (1) below: $\begin{array}{cc}\frac{L}{\sqrt{2}}>L\sin {}_1>t_1& \left(1\right)\end{array}$ where L is an arrangement interval (m) of the plurality of cooling-water-discharging nozzles, .sub.1 is an angle () of a long-side direction or a major-axis direction of a spray discharge surface of the spray discharge surfaces relative to the width direction, and t.sub.1 is a length (m) of a short side or a minor axis of the spray discharge surface of the spray discharge surfaces.

3. The steel continuous-casting machine according to claim 1, wherein: the spray discharge surfaces of the adjacent cooling-water-discharging nozzles are square, and each cooling-water-discharging nozzle of the plurality of cooling-water-discharging nozzles is disposed to satisfy Inequality (2) below: $\begin{array}{cc}L\sin {}_2>t& \left(2\right)\end{array}$ where L is an arrangement interval (m) of the plurality of cooling-water-discharging nozzles, .sub.2 is an angle () of a direction of one side of a spray discharge surface of the spray discharge surfaces relative to the width direction, the one side of the spray discharge surface of the spray discharge surfaces being one side of sides of the spray discharge surface that is closest to an adjacent spray discharge surface of the spray discharge surfaces, and t is a length (m) of the one side.

4. The steel continuous-casting machine according to claim 1, wherein: the spray discharge surfaces of the adjacent cooling-water-discharging nozzles are circular, and each cooling-water-discharging nozzle of the plurality of cooling-water-discharging nozzles is disposed to satisfy Inequality (3) below: $\begin{array}{cc}L>D& \left(3\right)\end{array}$ wherein L is an arrangement interval (m) of the plurality of cooling-water-discharging nozzles and D is a diameter (m) of a spray discharge surface of the spray discharge surfaces.

5. The steel continuous-casting machine according to claim 2, wherein an aspect ratio of a spray discharge surface of the spray discharge surfaces is 100 or less.

6. The steel continuous-casting machine according to claim 1, wherein a surface-layer cooling rate of the cast steel in the cooling apparatus is in a range of 0.3 C./sec or more and 100 C./sec or less.

7. The steel continuous-casting machine according to claim 1, further comprising a processor that is configured to control an amount of cooling water discharged from the plurality of cooling-water-discharging nozzles and a transportation speed of the cast steel.

8. The steel continuous-casting machine according to claim 6, further comprising a processor that is configured to control an amount of cooling water discharged from the plurality of cooling-water-discharging nozzles and a transportation speed of the cast steel.

9. A steel continuous-casting method comprising: cooling cast steel with water, wherein: the cast steel is cooled by discharging cooling water from a plurality of cooling-water-discharging nozzles arranged in a width direction of the cast steel, and plurality of cooling-water-discharging nozzles are arranged such that spray discharge surfaces of adjacent cooling-water-discharging nozzles of the plurality of cooling-water-discharging nozzles adjacent to each other in the width direction of the cast steel do not overlap.

10. The steel continuous-casting method according to claim 9, wherein: the spray discharge surfaces of the adjacent cooling-water-discharging nozzles are rectangular or elliptical, and each cooling-water-discharging nozzle of the plurality of cooling-water-discharging nozzles is disposed to satisfy Inequality (1) below: $\begin{array}{cc}\frac{L}{\sqrt{2}}>L\sin {}_1>t_1& \left(1\right)\end{array}$ where L is an arrangement interval (m) of the plurality of cooling-water-discharging nozzles, .sub.1 is an angle () of a long-side direction or a major-axis direction of a spray discharge surface of the spray discharge surfaces relative to the width direction, and t.sub.1 is a length (m) of a short side or a minor axis of the spray discharge surface of the spray discharge surfaces.

11. The steel continuous-casting method according to claim 9, wherein: the spray discharge surfaces of the adjacent cooling-water-discharging nozzles are square, and each cooling-water-discharging nozzle of the plurality of cooling-water-discharging nozzles is disposed to satisfy Inequality (2) below: $\begin{array}{cc}L\sin {}_2>t& \left(2\right)\end{array}$ where L is an arrangement interval (m) of the plurality of cooling-water-discharging nozzles, .sub.2 is an angle () of a direction of one side of a spray discharge surface of the spray discharge surfaces relative to the width direction, the one side of the spray discharge surface of the spray discharge surfaces being one side of sides of the spray discharge surface that is closest to an adjacent spray discharge surface of the spray discharges surfaces, and t is a length (m) of the one side.

12. The steel continuous-casting method according to claim 9, wherein: the spray discharge surfaces of the adjacent cooling-water-discharging nozzles are circular, and each cooling-water-discharging nozzle of the plurality of cooling-water-discharging nozzles is disposed to satisfy Inequality (3) below: $\begin{array}{cc}L>D& \left(3\right)\end{array}$ wherein L is an arrangement interval (m) of the plurality of cooling-water-discharging nozzles and D is a diameter (m) of a spray discharge surface of the spray discharge surfaces.

13. The steel continuous-casting method according to claim 10, wherein an aspect ratio of a spray discharge surface of the spray discharge surfaces is 100 or less.

14. The steel continuous-casting method according to claim 9, wherein a surface-layer cooling rate of the cast steel is in a range of 0.3 C./sec or more and 100 C./sec or less.

15. The steel continuous-casting method according to claim 9, wherein at least one of a surface-layer cooling rate of the cast steel and a temperature drop of the cast steel is controlled by controlling an amount of cooling water discharged from the plurality of cooling-water-discharging nozzles and a transportation speed of the cast steel.

16. The steel continuous-casting method according to claim 14, wherein at least one of a surface-layer cooling rate of the cast steel and a temperature drop of the cast steel is controlled by controlling an amount of cooling water discharged from the plurality of cooling-water-discharging nozzles and a transportation speed of the cast steel.

17. The steel continuous-casting machine according to claim 2, wherein a surface-layer cooling rate of the cast steel in the cooling apparatus is in a range of 0.3 C./sec or more and 100 C./sec or less.

18. The steel continuous-casting machine according to claim 2, further comprising a processor that is configured to control an amount of cooling water discharged from the plurality of cooling-water-discharging nozzles and a transportation speed of the cast steel.

19. The steel continuous-casting method according to claim 10, wherein a surface-layer cooling rate of the cast steel is in a range of 0.3 C./sec or more and 100 C./sec or less.

20. The steel continuous-casting method according to claim 10, wherein at least one of a surface-layer cooling rate of the cast steel and a temperature drop of the cast steel is controlled by controlling an amount of cooling water discharged from the plurality of cooling-water-discharging nozzles and a transportation speed of the cast steel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG. 1 is a graph showing the relationship between the surface temperature of cast steel and the boiling state in a water cooling process for the cast steel.

[0039] FIG. 2 is a schematic sectional view of a steel continuous-casting machine according to an embodiment of the present disclosure.

[0040] FIG. 3 illustrates spray discharge surfaces of cooling water discharged from cooling-water-discharging nozzles on cast steel S.

[0041] FIG. 4 is a schematic diagram illustrating an example of the structure of a control device 10.

[0042] FIG. 5 is a schematic partial sectional view of a steel continuous-casting machine including a water purging roll.

[0043] FIG. 6 is a schematic partial sectional view of a steel continuous-casting machine including a purge nozzle.

[0044] FIG. 7 is a schematic partial sectional view of a steel continuous-casting machine including a water purging roll and a purge nozzle.

[0045] FIG. 8 is a schematic sectional view illustrating another example of a steel continuous-casting machine according to the present embodiment.

[0046] FIG. 9 is a schematic sectional view illustrating another example of a steel continuous-casting machine according to the present embodiment.

[0047] FIG. 10 is a schematic sectional view illustrating another example of a steel continuous-casting machine according to the present embodiment.

DETAILED DESCRIPTION

[0048] The present disclosure will now be described by means of embodiments of the present disclosure. The embodiments described below illustrate devices and methods for embodying the technical idea of the present disclosure. In the technical idea of the present disclosure, materials, shapes, structures, arrangements, and the like of constituent parts are not limited to those in the embodiments described below. The drawings are schematic, and therefore it is to be noted that the relationships, ratios, and the like between the thicknesses and the planar dimensions are different from the actual ones. The drawings may also differ from each other in dimensional relationships and ratios.

[0049] FIG. 2 is a schematic sectional view of a steel continuous-casting machine according to an embodiment of the present disclosure. The main components of a steel continuous-casting machine 1 according to the embodiment of the present disclosure include a mold 2 that cools molten steel poured from a tundish (not illustrated) and form an outer shell shape of cast steel; a cooling apparatus 3 that cools the cast steel extracted from the mold; a thermometer 4 that measures a temperature of the cast steel at the exit side of the cooling apparatus 3; and a control device 10 that controls the operation of the cooling apparatus 3. In the following description, a long-side surface of the cast steel at the upper right in FIG. 2 is referred to as a first surface, a short-side surface at the near side as a second surface, a long-side surface at the lower left side as a third surface, and a short-side surface at the far side as a fourth surface.

[Mold 2]

[0050] The molten steel is produced in a refining apparatus at a location separate from the steel continuous-casting machine 1, and is poured into the mold 2. The molten steel poured into the mold 2 is cooled by the mold 2 so that the molten steel solidifies from the contact surface between the molten steel and the mold 2 toward an inner layer and that the outer shell shape is formed. In the following description, the molten steel whose outer shell shape is formed, including completely solidified steel, is referred to as cast steel S. The cast steel S extracted from the mold 2 is cooled by the cooling apparatus 3 while being supported and transported by cast-steel support rolls 5 provided at the exit side of the mold 2. A commonly known mold may be used as the mold 2.

[Cooling Apparatus 3]

[0051] The cooling apparatus 3 includes a water cooling device 31 that cools the cast steel S with water under predetermined cooling conditions. The water cooling device 31 includes the cast-steel support rolls 5 that support and transport the cast steel S, and cooling-water-discharging nozzles 32. The cast-steel support rolls 5 on the first-surface side and the third-surface side of the cast steel S form pairs and are arranged with equal intervals in the casting direction. At positions between the cast-steel support rolls 5 adjacent to each other in the casting direction, two or more cooling-water-discharging nozzles 32a on the first-surface side and two or more cooling-water-discharging nozzles 32c on the third-surface side form pairs across the cast steel S and are arranged at a predetermined interval in the casting direction. The cooling-water-discharging nozzles 32 discharge cooling water W toward the cast steel S. The cast steel S is fed into the cooling apparatus 3, so that the cast steel S is transported in the casting direction while being cooled by the cooling water W discharged from the cooling-water-discharging nozzles 32 in the secondary cooling process. In the following description, a cooling section in which one pair of cast-steel support rolls 5 in the casting direction serves as a unit is referred to as a cooling zone, and cooling sections will be counted in units of zones. Although a total of 19 zones are provided as the cooling zones in FIG. 2, the number of zones is not limited to this, and may be greater or smaller than 19.

[0052] The steel continuous-casting machine 1 illustrated in FIG. 2 is a vertical-bending continuous-casting machine, and is characterized in that the cooling apparatus 3 is constituted by a vertical section 6 in which the cast steel S extracted from the mold 2 and orientated vertically is cooled with water; a curved section 7 in which the cast steel S is cooled with water while being curved by the cast-steel support rolls 5; and a horizontal section 8 in which the cast steel S is cooled with water after being curved to extend horizontally by the cast-steel support rolls 5. Although the vertical-bending continuous-casting machine will be described as an example in the present embodiment, the continuous-casting machine is not limited to a vertical-bending continuous-casting machine, and may be a vertical continuous-casting machine including only the vertical section 6 in the cooling apparatus 3, a curved continuous-casting machine including only the curved section 7 and the horizontal section 8, or a horizontal continuous-casting machine including only the horizontal section 8.

[0053] Operation parameters of the water cooling device 31 include the amount of cooling water W (amount of cooling water) and the amount of compressed air discharged from the cooling-water-discharging nozzles 32 and the transportation speed at which the cast steel S is transported. As the amount of cooling water increases, the cooling rate and the temperature drop of the cast steel S increase. As the transportation speed of the cast steel S decreases, the temperature drop of the cast steel S increases. Therefore, at least one of the surface-layer cooling rate and the temperature drop of the cast steel S can be controlled by controlling at least one of the transportation speed of the cast steel S and the amount of cooling water, and the cast steel S having the desired material quality can be produced. In addition, the cooling capacity and the surface distribution of the cooling water W can be adjusted by adding the compressed air to the cooling water W discharged from the cooling-water-discharging nozzles 32. The desired properties, deterioration of the machine over time, and small changes in the nozzle arrangement may also be controlled under appropriate conditions by adjusting the amount of compressed air.

[0054] The balance between the amounts of cooling water in the cooling zones may be changed as an operation parameter of the water cooling device 31 (for example, the amount of cooling water may be increased in upstream cooling zones and reduced in downstream cooling zones). Thus, the cooling rate can be controlled for each temperature range of the cast steel S. In addition, the number of cooling zones in which the cooling water is discharged may be changed. The temperature drop of the cast steel S can be controlled while the cooling rate is maintained constant by changing the number of cooling zones that are used.

[0055] Furthermore, the ratio between the amount of cooling water W discharged from the cooling-water-discharging nozzles 32a on the first-surface side and the amount of cooling water W discharged from the cooling-water-discharging nozzles 32c on the third-surface side can be changed. Accordingly, shape defects due to the difference in the temperature drop between the first surface and the third surface of the cast steel S can be controlled. In addition, the amount of cooling water can be changed in accordance with the composition of the cast steel S. This is because the thermal conductivity of the cast steel S changes in accordance with the composition of the cast steel S, and the cooling state changes accordingly. When the amount of cooling water is changed, the amount of compressed air and the transportation speed of the cast steel S may also be changed. The cooling capacity and the surface distribution of the cooling water W may be controlled by changing the amount of compressed air, and the temperature history of the cast steel S may be controlled by changing the transportation speed of the cast steel S. Thus, the cooling conditions can be finely adjusted, thereby improving the quality of the cast steel.

[0056] The operation parameters of the water cooling device 31 may be changed as the casting proceeds. In particular, the leading and trailing end portions of the cast steel S are additionally cooled from the leading and trailing end faces and therefore tend to become nonstationary portions. Therefore, the operation parameters of the water cooling device 31 may be finely adjusted for these portions, so that high quality can be ensured over the entire length and width and the yield in the nonstationary portions can be increased.

[0057] The operation parameters of the water cooling device 31 may also be finely adjusted for the stationary portion excluding the leading and trailing end portions as the casting proceeds. When, for example, the properties of the stationary portion vary in the longitudinal direction due to the composition segregation of the cast steel S, the cast steel having a uniform quality over the entire length can be obtained by finely adjusting the operation parameters of the water cooling device 31.

[0058] The cooling rate is preferably variable in accordance with the material, operating conditions, and the machine status. When the surface-layer cooling rate exceeds 100 C./sec, the surface layer is often transformed into martensite, causing defects, such as cracks, on the surface of the cast steel. Therefore, the surface-layer cooling rate is preferably less than or equal to 100 C./sec. When the surface-layer cooling rate is less than 0.3 C./sec, the cooling rate is substantially equal to that in the case where the cast steel is allowed to be naturally cooled, and the production efficiency is reduced. In addition, as the cooling rate decreases, the segregation in the cast steel is increased, and the quality of the cast steel is degraded. Therefore, the surface-layer cooling rate is preferably greater than or equal to 0.3 C./sec.

[0059] As is clear from the graph illustrated in FIG. 1, the film boiling state eventually transitions to the nucleate boiling state. Therefore, discharging of the cooling water W toward the cast steel S is to be stopped while the temperatures of the front and back surfaces of the cast steel S are 500 C. or higher, preferably 600 C. or higher. When the temperatures of the front and back surfaces of the cast steel S are sufficiently increased due to internal recuperation after the cooling water W is discharged, the cooling water W may be discharged again. When the cooling rate is increased by discharging the cooling water W again, the production efficiency can be increased and the quality of the cast steel can be improved.

[0060] Nozzles capable of uniformly discharging cooling water at a predetermined flow rate are preferably used as the cooling-water-discharging nozzles 32. Although spray nozzles are used as the cooling-water-discharging nozzles 32 in the present embodiment, the cooling-water-discharging nozzles 32 are not limited to spray nozzles, and may be slit-type nozzles, multi-hole jet nozzles, mist nozzles, or fog nozzles. The cooling-water-discharging nozzles 32 may be either one-fluid nozzles that discharge only liquid (generally water) or two-fluid nozzles that discharge mixed fluid containing liquid (generally water) and gas (generally air).

[0061] Nozzles that discharge only air does not cause a transition of the boiling state, but may be disposed at positions near or slightly displaced from the cooling-water-discharging nozzles 32. These nozzles may discharge air to improve drainage or for water purging. In addition, the cooling-water-discharging nozzles 32 are preferably capable of varying the amount of cooling water and the amount of compressed air in accordance with the desired cooling rate.

[0062] When the cooling water W discharged from the cooling-water-discharging nozzles 32 comes into contact with the cast steel S such that collision of the sprays of the cooling water W discharged from the adjacent cooling-water-discharging nozzles 32 occurs on the cast steel S, horizontal momentum changes to vertical momentum at the collision point, and the downward component of the vertical flow breaks the vapor film and causes a local disruption of the film boiling state. When the spray has a rectangular spray discharge surface, the horizontal component of the moment of the cooling water W discharged in the long-side direction is greater than that of the cooling water W discharged in the short-side direction. Therefore, collision of the sprays of the cooling water W discharged from the adjacent cooling-water-discharging nozzles 32 in the long-side direction needs to be avoided.

[0063] FIG. 3 illustrates spray discharge surfaces on the cast steel S toward which the cooling water is discharged from the cooling-water-discharging nozzles. FIG. 3(a) illustrates rectangular spray discharge surfaces. Assume that the cooling-water-discharging nozzles 32 are such that the spray discharge surfaces on the cast steel S have a rectangular shape with short sides having a length of t.sub.1 (m) and long sides having a length of t.sub.2 (m). Due to the geometric relationship, to avoid the collision of the spray discharge surfaces in the long-side direction, the cooling-water-discharging nozzles 32 need to be placed such that the spray discharge surfaces of the cooling-water-discharging nozzles 32 adjacent to each other in the width direction of the cast steel S do not overlap. More specifically, the cooling-water-discharging nozzles 32 are arranged such that the arrangement interval of the cooling-water-discharging nozzles 32 adjacent to each other in the width direction of the cast steel S, the angle of the long-side direction relative to the width direction of the cast steel, and the length of the short sides of the spray discharge surfaces are in the ranges that satisfy Inequality (1) below. When the cooling-water-discharging nozzles 32 are arranged such that Inequality (1) below is satisfied, collision in the long-side direction of the sprays of the cooling water W discharged from the cooling-water-discharging nozzles 32 adjacent to each other in the width direction of the cast steel S can be suppressed, so that the disruption of the film boiling state can be suppressed. This also applies to the cooling-water-discharging nozzles whose spray discharge surfaces on the cast steel S are elliptical. In such a case, in Inequality (1) below, the long-side direction may be changed to the major-axis direction, and the length of the short sides may be changed to the length of the minor axis.

[00007]$\begin{array}{cc}\left[\mathrm{Math}.3\right]& \\ \frac{L}{\sqrt{2}}>L\sin {}_1>t_1& \left(1\right)\end{array}$

[0064] In Inequality (1) above, L is the arrangement interval (m) of the cooling-water-discharging nozzles, .sub.1 is the angle () of the long-side direction of the spray discharge surface relative to the width direction of the cast steel, and t.sub.1 is the length (m) of the short sides of the spray discharge surfaces.

[0065] When the spray discharge surfaces on the cast steel S are rectangular or elliptical, the aspect ratio of the discharge surfaces is preferably 100 or less. When sprays having the spray discharge surfaces with a low aspect ratio are used, the spray discharge surfaces can be broadened, and the local concentration of the cooling water can be suppressed, so that the film boiling state can be maintained longer. The aspect ratio of the spray discharge surfaces is more preferably 50 or less, and still more preferably 30 or less.

[0066] When nozzles having the long sides of the same length are used to discharge the same amount of water, the nozzles preferably have a low aspect ratio and a long minor-axis dimension. When the nozzles have a low aspect ratio and short sides that are long, the area of the spray discharge surfaces is increased. Therefore, the water flow density is reduced and the local concentration of the cooling water can be suppressed. When the aspect ratio is too low, the amount of water that flows in the major-axis direction of the sprays is reduced, and the cooling water W easily remains on the cast steel S. Therefore, the aspect ratio is preferably two or more, more preferably 5 or more, and still more preferably 10 or more.

[0067] The angle .sub.1 of the long-side direction is preferably less than 45 so that the cooling water W discharged toward the cast steel S can be quickly removed to the outside of the cast steel. The angle .sub.1 of the long-side direction of 45 or more is not preferable because when the velocity of the cooling water W in the long-side direction is resolved into the transport direction of the cast steel and the width direction of the cast steel, the component in the transport direction is greater than the component in the width direction of the cast steel. More preferably, the angle .sub.1 of the long-side direction is 30 or less.

[0068] FIG. 3(b) illustrates square spray discharge surfaces. When the spray discharge surfaces on the cast steel S are square, collision of the sprays of cooling water can be suppressed by arranging the cooling-water-discharging nozzles 32 such that the arrangement interval of the cooling-water-discharging nozzles 32 adjacent to each other in the width direction of the cast steel S, the angle of the direction of one side of each spray discharge surface relative to the width direction of the cast steel, and the length of one side of each spray discharge surface are in the ranges that satisfy Inequality (2) below.

[00008]$\begin{array}{cc}L\sin {}_2>t& \left(2\right)\end{array}$

[0069] In Inequality (2) above, L is the arrangement interval (m) of the cooling-water-discharging nozzles 32, .sub.2 is the angle () of the direction of one side of each spray discharge surface relative to the width direction of the cast steel, and t is the length (m) of one side of each spray discharge surface. Here, one side of each spray discharge surface is one of the sides of the spray discharge surface that is closest to an adjacent spray discharge surface. More specifically, in a spray discharge surface 20 illustrated in FIG. 3(b), the direction of one side is the direction of a side 21 that is closest to an adjacent spray discharge surface 22. In the spray discharge surface 22, the direction of one side is the direction of one of sides 23 and 24 that are closest to adjacent spray discharge surfaces 20 and 25, respectively.

[0070] FIG. 3(c) illustrates circular spray discharge surfaces. When the spray discharge surfaces on the cast steel S are circular, collision of the sprays of cooling water can be suppressed by arranging the cooling-water-discharging nozzles 32 such that the arrangement interval of the cooling-water-discharging nozzles 32 adjacent to each other in the width direction of the cast steel S and the diameter of the spray discharge surfaces are in the ranges that satisfy Inequality (3) below.

[00009]$\begin{array}{cc}L>D& \left(3\right)\end{array}$

[0071] In Inequality (3) above, L is the arrangement interval (m) of the cooling-water-discharging nozzles 32, and D is the diameter (m) of the spray discharge surfaces.

[0072] The level of local concentration of the cooling water can be evaluated by the water flow density defined as flow rate/discharge area. Since the water flow density of the discharged cooling water and the boiling transition temperature are positively correlated, the film boiling state can be maintained longer by reducing the water flow density. With regard to the local water flow density of the cooling water in the steel continuous-casting machine 1, to stabilize the film boiling state, the water flow density of the cooling water is preferably 1000 L/(m.sup.2min) or less, more preferably 800 L/(m.sup.2min) or less, and still more preferably 600 L/(m.sup.2min) or less.

[Thermometer 4]

[0073] The thermometer 4 may be a device that scans the cast steel S in the width direction to measure the surface temperature of the cast steel S, or be one or more devices arranged in the width direction of the cast steel S to measure the surface temperature of the cast steel S. It can be checked whether the cast steel S is cooled as expected by measuring the surface temperature of the cast steel S cooled by the cooling apparatus 3 by using the thermometer 4.

[0074] Referring to FIG. 2 again, although the thermometer 4 is disposed at the exit side of the cooling apparatus 3 in FIG. 2, the thermometer 4 may be disposed in the cooling apparatus 3 when the temperature of the steel sheet that has been cooled by the water cooling device 31 can be measured. In such a case, multiple thermometers 4 may be arranged next to each other in the transport direction of the cast steel S to measure the temperature of the cast steel S in each cooling zone.

[0075] In addition, the thermometer 4 may be disposed at the entry side of the cooling apparatus 3 and the entry side of the mold 2 to measure the initial temperature of the cast steel S and the temperature of the molten steel that is poured. This is because the accuracy of the calculation of the cooling rate can be increased by additionally taking into account the result of the temperature measurement at the entry side of the cooling apparatus 3. One or more thermometers 4 may be disposed in the cooling apparatus 3 to measure the temperature of the cast steel S during water cooling. When the result of the temperature measurement in the cooling apparatus 3 is additionally taken into account, the accuracy of the calculation of the cooling rate can be increased and the time history of the cooling rate can be obtained.

[0076] The temperature information of the molten steel and the result of the temperature measurement of the cast steel S may be used in heat transfer calculation or heat transfer simulation to calculate the cooling rate during water cooling and check whether the cast steel S is being cooled as expected. In addition, the temperature distribution along the surfaces of the cast steel S during or after water cooling may be measured to check whether the cast steel S is uniformly cooled. Furthermore, the temperature distribution along the surfaces of the cast steel S before water cooling may be measured to check whether the temperature distribution along the surfaces of the cast steel S that enters the cooling apparatus 3 is uniform. The operation parameters of the cooling apparatus 3 and the operating conditions of the steel continuous-casting machine 1 may be changed based on the above-described calculation results.

[Control Device 10]

[0077] The control device 10 will now be described. FIG. 4 is a schematic diagram illustrating an example of the structure of the control device 10. The control device 10 is an information processing device, such as a personal computer. The control device 10 receives, from the host computer 11, the molten steel temperature, the size information, such as the thickness, of the cast steel S, and information regarding the desired range of the amount of cooling required to achieve the desired material quality and the desired range of the cooling rate. The control device 10 calculates operating conditions of the steel continuous-casting machine 1 for achieving the desired amount of cooling and the desired cooling rate, and determines the operation parameters of the devices.

[0078] The control device 10 includes the control unit 12 and the storage unit 13. The control unit 12 is, for example, a CPU that executes programs read from the storage unit 13 to cause the control unit 12 to function as the computing unit 14 and the output unit 15. The storage unit 13 is, for example, an information recording medium, such as a re-recordable flash memory, a hard disk that is built-in or connected with a data communication terminal, or a memory card, and a read/write device for the information recording medium. The storage unit 13 stores programs for causing the control unit 12 to execute the functions and data used by the programs.

[0079] The computing unit 14 performs a heat transfer calculation based on an internal model to determine the number of cooling zones to be used, the amount of cooling water, the amount of compressed air, and the transportation speed of the cast steel S for achieving the desired amount of cooling and the desired cooling rate set as the cooling conditions. The thus determined command values of the amount of cooling water, the amount of compressed air, and the transportation speed of the cast steel S are output from the output unit 15 to the water cooling device 31. Based on the command values of the amount of cooling water, the amount of compressed air, and the transportation speed of the cast steel S, the water cooling device 31 generates commands regarding the number of cooling water pumps to be operated and operating pressures thereof, the number of air compressors to be operated and operating pressures thereof, the number of headers disposed upstream of the cooling-water-discharging nozzles 32, the opening degree of the flow control valve, and the rotational speeds of the cast-steel support rolls 5, and determines the operating conditions of the water cooling device 31.

[0080] One or more of the number of cooling zones to be used, the amount of cooling water, the amount of compressed air, and the transportation speed of the cast steel S may be determined in advance by using a table based on information such as the composition and size information of the cast steel S and the desired material quality, and transmitted to the water cooling device 31 as the commands. Preferably, adjustment parameters are provided to allow the number of cooling zones to be used, the amount of cooling water, the amount of compressed air, and the transportation speed of the cast steel S to be varied in accordance with changes in conditions during the operation.

[Steel Continuous-Casting Method]

[0081] A steel continuous-casting method using the steel continuous-casting machine 1 illustrated in FIG. 2 will now be described. First, molten steel produced in a refining apparatus at a location separate from the steel continuous-casting machine 1 is poured into the mold 2 through a tundish (not illustrated). The poured molten steel is cooled by the mold 2 and solidifies from the contact surface between the molten steel and the mold 2 toward the inner layer, so that the outer shell shape of the cast steel S is formed.

[0082] The cast steel S extracted from the mold 2 is cooled by the cooling apparatus 3 while being supported and transported by the cast-steel support rolls 5 disposed at the exit side of the mold 2. In a cooling step, the number of zones to be used, the amount of cooling water, the amount of compressed air, and the transportation speed are calculated and set by the control device 10 in accordance with the size of the cast steel S and the desired properties of the cast steel S. In the present embodiment, it is assumed that the cast steel S is cooled by discharging water and air in all the zones illustrated in FIG. 2.

[0083] A predetermined amount of cooling water W and a predetermined amount of compressed air A are discharged from 19 pairs of cooling-water-discharging nozzles 32, and the cast-steel support rolls 5 are rotated at a predetermined speed. These parameters are set by the control device 10 so that desired cast steel properties can be obtained, and are transmitted to the cooling-water-discharging nozzles 32 and the cast-steel support rolls 5. The cast steel S having the desired properties can be produced by cooling the cast steel S with the cooling apparatus 3. After the cooling step, the cast steel S is subjected to a subsequent process.

[0084] Although an embodiment of the present disclosure is described above, the present disclosure is not limited to this, and various alterations and modifications are possible. FIG. 5 is a schematic partial sectional view of a steel continuous-casting machine including a water purging roll. As illustrated in FIG. 5, a water purging roll 33 may be disposed at the exit side of the water cooling device 31 of the steel continuous-casting machine 1 to purge the cooling water W remaining on the cast steel S. Thus, the cast steel S can be prevented from being cooled in a partial or entire area by the cooling water W remaining on the cast steel S such that the desired amount of cooling cannot be obtained and that the desired properties cannot be obtained as a result in a partial or entire area.

[0085] To achieve good purging performance, the pressing force that presses the water purging roll 33 against the cast steel S is preferably 4 tons or more. The pressing force that presses the water purging roll 33 against the cast steel S is more preferably 6 tons or more, and still more preferably 8 tons or more. When the pressing force that presses the water purging roll 33 against the cast steel S is excessively increased, the water purging roll 33 is bent by elastic deformation, and a gap is formed between the cast steel S and the water purging roll 33, resulting in degradation of the purging performance. Therefore, the pressing force that presses the water purging roll 33 against the cast steel S is preferably 20 tons or less.

[0086] A mechanism that applies the pressing force to the water purging roll 33 may be a spring-type mechanism, such as a spring, or a pneumatic or hydraulic mechanism capable of applying a constant pressing force. To adjust the bending of the water purging roll 33, the mechanism is preferably capable of maintaining the pressing force constant, and is more preferably capable of changing the pressing force in the longitudinal direction of the cast steel S.

[0087] The cast-steel support rolls 5 may serve as water purging rolls. In such a case, the pressing force applied by the cast-steel support rolls 5 is not limited to the above-described ranges. This is because the quality of the cast steel may be improved by subjecting the cast steel S to rolling reduction by the cast-steel support rolls 5.

[0088] FIG. 6 is a schematic partial sectional view of a steel continuous-casting machine including a purge nozzle. As illustrated in FIG. 6, a purge nozzle 34 may be disposed instead of the water purging roll 33, and a water purging jet 35 is emitted to purge the cooling water W remaining on the cast steel S. The water purging jet 35 may be liquid, gas, or fluid of gas-liquid mixture. When liquid is used as the water purging jet 35, the portion receiving the liquid may be cooled and the temperature deviation along the surface of the cast steel S may be increased. Therefore, gas is preferably used as the water purging jet 35. To reduce production costs, more preferably, air is used as the water purging jet 35.

[0089] FIG. 7 is a schematic partial sectional view of a steel continuous-casting machine including a water purging roll and a purge nozzle. As illustrated in FIG. 7, the water purging roll 33 and the purge nozzle 34 may both be used. In addition, one or both of the water purging roll 33 and the purge nozzle 34 may be disposed at the entry side of the water cooling device 31 to purge the cooling water W that leaks from the water cooling device 31. In this case, a reduction in the temperature of the cast steel S that enters the cooling apparatus 3 can be suppressed, and the cooling water W can be prevented from entering another device (for example, the mold 2) disposed upstream of or around the cooling apparatus 3.

[0090] In addition, one or both of the water purging roll 33 and the purge nozzle 34 may be disposed not only at the entry and exit sides of the water cooling device 31 but also at the entry and exit sides of each cooling zone to divide the cooling zones from each other. When the amount of water discharged differs between the cooling zones, the temperature history of the steel sheet can be determined by dividing the zones with different amounts of cooling water from each other.

[0091] It is not necessary that all of the cooling-water-discharging nozzles 32 included in the cooling apparatus 3 satisfy Inequality (1), (2), or (3) above as long as two or more cooling-water-discharging nozzles 32 adjacent to each other in the width direction of the cast steel satisfy Inequality (1), (2), or (3) above. In such a case, compared to a steel continuous-casting machine in which none of the cooling-water-discharging nozzles 32 satisfy Inequality (1), (2), or (3) above, the collision of the sprays of cooling water can be further suppressed, so that the disruption of the film boiling state can be further suppressed.

[0092] FIG. 8 is a schematic sectional view illustrating another example of a steel continuous-casting machine according to the present embodiment. Referring to FIG. 8, a steel continuous-casting machine 40 includes cooling-water-discharging nozzles 36 capable of discharging an amount of cooling water that changes the boiling state to the nucleate boiling state at both the entry and exit sides of the water cooling device 31. The cooling-water-discharging nozzles 36 are used to cool the cast steel S depending on the desired properties of the cast steel S. Thus, the cooling-water-discharging nozzles 36 that discharge an amount of cooling water that changes the boiling state to the nucleate boiling state may be disposed at the entry and exit sides of the water cooling device 31. Although the cooling-water-discharging nozzles 36 are disposed at both the entry and exit sides of the water cooling device 31 in FIG. 8, the cooling-water-discharging nozzles 36 are not limited to this, and may be provided at one of the entry and exit sides. In addition, although the cooling-water-discharging nozzles 36 are provided for three zones at the entry side and three zones at the exit side of the water cooling device 31 in FIG. 8, the number of cooling zones for which the cooling-water-discharging nozzles 36 are provided is not limited to this, and may be other than three.

[0093] FIG. 9 is a schematic sectional view illustrating another example of a steel continuous-casting machine according to the present embodiment. Referring to FIG. 9, a steel continuous-casting machine 50 includes the cooling-water-discharging nozzles 32 and cooling-water-discharging nozzles 36 capable of discharging an amount of cooling water that changes the boiling state to the nucleate boiling state in the same cooling zones. Thus, the cooling-water-discharging nozzles 32 and the cooling-water-discharging nozzles 36 may be disposed in the same cooling zones. When cooling by the cooling-water-discharging nozzles 32 and cooling by the cooling-water-discharging nozzles 36 are combined, cooling with various temperature histories can be achieved. Although the cooling-water-discharging nozzles 32 and the cooling-water-discharging nozzles 36 are disposed in two zones in FIG. 9, the number of zones is not limited to this, and may be other than 2.

[0094] FIG. 10 is a schematic sectional view illustrating another example of a steel continuous-casting machine according to the present embodiment. Referring to FIG. 10, a steel continuous-casting machine 60 includes cooling-water-discharging nozzles 37 that discharges only an amount of water that changes the boiling state to the nucleate boiling state in place of some of the cooling-water-discharging nozzles of the water cooling device 31. Thus, the cooling-water-discharging nozzles 37 may be provided in place of some of the cooling-water-discharging nozzles of the water cooling device 31. This is because when a portion excluding the cooling-water-discharging nozzles 37 is regarded as the water cooling device 31 and collision of the sprays of cooling water is suppressed in this portion, the effect that the disruption of the film boiling state is suppressed can be obtained. The cooling-water-discharging nozzles 37 capable of discharging an amount of water that changes the boiling state to the nucleate boiling state may be one-fluid nozzles that discharge only air, one-fluid nozzles that discharge only water, or two-fluid nozzles that discharge mixed fluid containing water and air.

[0095] The cast steel S extracted from the mold 2 is generally transported while being subjected not only to cooling but also to rolling reduction by the cast-steel support rolls 5. This is because the internal segregation can be reduced and the quality of the cast steel can be improved by performing rolling reduction on the cast steel S. Therefore, the steel continuous-casting method according to the present embodiment and a commonly known cast-steel rolling reduction technology may both be applied. In such a case, the quality of the cast steel that is produced can be further improved. Preferably, the operation parameters of the water cooling device 31 and the operation parameters regarding the commonly known cast-steel rolling reduction technology are both satisfied.

[0096] In the steel continuous-casting machine 1 according to the present embodiment, the cast-steel support rolls 5 are disposed on the first and third surfaces of the cast steel S. However, the cast-steel support rolls 5 are not limited to this, and may be disposed on the second and fourth surfaces. When the second and fourth surfaces are supported and subjected to rolling reduction by the rolls, the expansion in the width direction resulting from the rolling reduction by the cast-steel support rolls 5 on the first and third surfaces can be suppressed.

EXAMPLES

[0097] An example in which the steel continuous-casting machine 1 illustrated in FIG. 2 was used to cool the continuously cast steel S with the cooling apparatus 3 and manufacture a slab as a rolling material will now be described. In the steel continuous-casting machine 1, the cooling apparatus 3 was disposed downstream of the mold 2. The cooling apparatus 3 included 19 pairs of cooling-water-discharging nozzles 32 constituting the water cooling device 31 and 20 pairs of cast-steel support rolls 5. The cooling-water-discharging nozzles 32 were structured such that rectangular spray nozzles, square spray nozzles, and circular spray nozzles were removably attachable and replaced in accordance with the casting conditions.

[0098] The thermometer 4 was disposed at a position 5 m downstream from the exit of the cooling apparatus 3 to measure the surface-layer temperature distribution of the cast steel S in the width direction after the cast steel S passed through the cooling apparatus 3. In the temperature distribution of the cast steel S in the width direction measured by the thermometer 4, a value obtained by subtracting the minimum value from the maximum value was evaluated as the temperature deviation in the cast steel S. The temperature deviation of less than 50 C. was evaluated as acceptable. In addition, a slab produced by cutting the cast steel S was subjected to a subsequent process of follow-up inspection for small cracks on the surface of the slab. In addition, the number of segregated grains at the center of the cast steel in the thickness direction was counted. In addition, the slab was subjected to hot rolling and cold rolling, and the resulting steel strip was subjected to a subsequent process of follow-up inspection for checking whether or not defects were found on the steel strip after cold rolling. In addition, a heat transfer simulation was performed based on the result of the temperature measurement by the thermometer 4, and the surface-layer cooling rate of the cast steel S was calculated. Table 1 shows the casting conditions and the evaluation results of the cast steel S according to Example. In Table 1, the angle is .sub.1 for a rectangular shape and .sub.2 for a square shape. In addition, the length is t.sub.1 for a rectangular shape, t for a square shape, and the diameter D for a circular shape.

TABLE-US-00001 TABLE 1 Surface Defects Nozzle Aspect Interval Water Flow Cooling Temperature After Discharge Ratio Angle L Length Density Rate Deviation Rolling Surface [] [] [mm] [mm] [L/(m.sup.2 .Math. min)] [ C./sec] [ C.] Disclosure Rectangular 2 30 350 150 120 4.0 37 No Example 1 Disclosure Rectangular 30 30 350 30 12 No Example 2 Disclosure Rectangular 120 30 350 10 25 No Example 3 Disclosure Rectangular 30 40 350 30 39 No Example 4 Disclosure Square 1 30 350 150 40 No Example 5 Disclosure Circular 1 30 350 150 42 No Example 6 Disclosure Rectangular 30 30 350 30 500 179.0 18 No Example 7 Disclosure Square 1 30 350 150 45 No Example 8 Disclosure Circular 1 30 350 150 43 No Example 9 Disclosure Rectangular 30 30 350 30 10 0.2 8 No Example 10 Disclosure Square 1 30 350 150 32 No Example 11 Disclosure Circular 1 30 350 150 34 No Example 12 Comparative Rectangular 30 5 200 30 120 4.0 90 Yes Example 1 Comparative Rectangular 30 60 200 30 218 Yes Example 2 Comparative Square 1 30 200 250 86 Yes Example 3 Comparative Circular 1 30 200 250 92 Yes Example 4

[0099] For each of Disclosure Examples 1 to 6, the cast steel was evaluated as acceptable. A slab with high quality over the entire width and length was obtained, and no defects were found on the steel strip after cold rolling; the resulting product was shippable. In Disclosure Example 4, no defects were found on the steel strip after cold rolling, and the temperature deviation was less than 50 C. However, the temperature deviation was greater than that for Disclosure Example 2. This may be because since the discharge angle .sub.1 was increased, the velocity component of the spray water in the transport direction was increased, and the cooling water discharged toward the cast steel was not quickly removed to the outside of the cast steel, resulting in the occurrence of local subcooling portions. In addition, under the same conditions, the temperature deviation in the width direction was smaller when the rectangular spray nozzles were used than when the circular or square spray nozzles were used. This may be because when the rectangular spray nozzles were used, the cooling water W discharged in the major-axis direction at a high flow rate was quickly removed to the outside of the cast steel S in the width direction.

[0100] When rectangular spray nozzles were used, the temperature deviation was smallest when the spray nozzles having the spray discharge surfaces with an aspect ratio of 30 were used. When the aspect ratio is low, the effect of removing the cooling water W is reduced. When the aspect ratio is high, the local concentration of the cooling water W occurs, and the temperature deviation increases. This result shows that the aspect ratio of the spray discharge surfaces has an optimum value.

[0101] Disclosure Examples 7 to 9 are examples in which casting was performed using rectangular, square, and circular spray nozzles and in which the water flow density was increased. In Disclosure Examples 7 to 9, the temperature deviation was less than 50 C. and was acceptable, but several small cracks were found on the surface of the slab. The formation of small cracks may be because the cooling rate was excessively high and the surface layer of the cast steel S transformed into martensite. However, since the temperature deviation was acceptable for Disclosure Examples 7 to 9, no defects were found on the steel strip after cold rolling, and the resulting product was shippable.

[0102] Disclosure Examples 10 to 12 are examples in which casting was performed using rectangular, square, and circular spray nozzles and in which the water flow density was reduced. In Disclosure Examples 10 to 12, the temperature deviation was less than 50 C. and was acceptable, but the number of segregated grains at the center of the cast steel in the thickness direction increased. The increase in the number of segregated grains at the center of the cast steel in the thickness direction may be because the cooling rate was excessively low and the temperature gradient in the cast steel S was reduced. However, since the temperature deviation was acceptable for Disclosure Examples 10 to 12, no defects were found on the steel strip after cold rolling, and the resulting product was shippable.

[0103] Comparative Example 1 is an example in which casting was performed using flat spray nozzles, and the discharge angle was reduced such that Inequality (1) above was not satisfied. In Comparative Example 1, the temperature deviation was 90 C. The quality of the produced slab was not uniform over the entire width, and defects were found on the steel strip after rolling. Therefore, the steel strip produced from this slab was not shippable. This may be because the sprays of the cooling water W discharged from adjacent nozzles interfered with each other, causing a disruption of the film boiling state and an increase in the cooling capacity in that region.

[0104] Comparative Example 2 is an example in which casting was performed using rectangular spray nozzles, and the discharge angle was increased such that Inequality (1) above was not satisfied. In Comparative Example 2, the temperature deviation was 218 C. The quality of the produced slab was not uniform over the entire width, and defects were found on the steel strip after rolling. Therefore, the steel strip produced from this slab was not shippable. This may be because the cooling water W on the cast steel S was not removed to the outside of the cast steel S in the width direction and remained on the cast steel S, causing a local transition from the film boiling state to the nucleate boiling state.

[0105] Comparative Examples 3 and 4 are examples in which square or circular spray nozzles were used and in which the nozzle arrangement interval was reduced such that Inequality (2) or (3) above was not satisfied. In Comparative Examples 3 and 4, the temperature deviation was 86 C. and 92 C., respectively. The quality of the produced slab was not uniform over the entire width, and defects were found on the steel strip after cold rolling. Therefore, the steel strip produced from this slab was not shippable. This may be because the sprays of the cooling water W discharged from adjacent spray nozzles interfered with each other, causing an increase in the cooling capacity in that region.

REFERENCE SIGNS LIST

[0106] 1 steel continuous-casting machine [0107] 2 mold [0108] 3 cooling apparatus [0109] 4 thermometer [0110] 5 cast-steel support roll [0111] 6 vertical section [0112] 7 curved section [0113] 8 horizontal section [0114] 10 control device [0115] 11 host computer [0116] 12 control unit [0117] 13 storage unit [0118] 14 computing unit [0119] 15 output unit [0120] 20 spray discharge surface [0121] 21 side [0122] 22 spray discharge surface [0123] 23 side [0124] 24 side [0125] 25 spray discharge surface [0126] 31 water cooling device [0127] 32 cooling-water-discharging nozzle [0128] 32a cooling-water-discharging nozzle [0129] 32c cooling-water-discharging nozzle [0130] 33 water purging roll [0131] 34 purge nozzle [0132] 35 water purging jet [0133] 36 cooling-water-discharging nozzle [0134] 37 cooling-water-discharging nozzle [0135] 40 steel continuous-casting machine [0136] 50 steel continuous-casting machine [0137] 60 steel continuous-casting machine [0138] S cast steel [0139] W cooling water