JP6747142B2 - Secondary cooling method and secondary cooling device for continuous casting - Google Patents

Description

The present invention relates to a secondary cooling method and a secondary cooling device when performing continuous casting of a slab with a continuous casting machine.

In continuous casting in the steel industry, spray-type cooling has been widely used as a method for secondary cooling of a slab. In this secondary cooling method, a spray nozzle is arranged between support rolls that convey the slab, and cooling water is sprayed onto the surface of the slab to cool it.

However, in the case of the spray method, when water is sprayed on a high temperature slab, the water is scattered and the sprayed water is not used efficiently, so that the cooling capacity is limited. Therefore, in the future, in order to increase the casting speed and improve the productivity, it is necessary to significantly increase the amount of water supply or extend the length of the continuous casting machine to increase the secondary cooling section. That is, the current continuous casting machine cannot cope with this, and in order to increase the speed of continuous casting, it is desired to significantly improve the heat transfer coefficient in the secondary cooling.

Conventionally, in order to reduce the temperature unevenness in the secondary cooling and uniformly cool, for example, Patent Document 1 discloses a secondary cooling method in which the slab surface temperature is held in the region of film boiling for cooling, and It is described that a perforated plate is disposed in the nozzle to eject cooling water.

Further, as a method of improving the cooling capacity of the secondary cooling, for example, Patent Document 2 discloses a cooling grid facility using a wear plate.

Further, Patent Document 3 discloses a secondary cooling method for a continuous cast product which cools the cast product by utilizing a water film flow to enhance the cooling capacity.

Japanese Patent No. 5146006 Japanese Patent No. 4453562 JP, 2002-086253, A

However, in the case of Patent Document 1, since the cooling water is sprayed from a plurality of ejection holes arranged in the longitudinal direction of the slab, interference between the cooling water and accompanying cooling water easily occur, and uniform cooling cannot be performed. .. Further, in Patent Document 1, cooling is performed in the region of film boiling so as not to cause overcooling, and a significant improvement in cooling capacity cannot be expected.

In addition, in the case of Patent Document 2, since the wear plate is in contact with the slab, a flaw is generated on the surface of the slab, which causes a problem in quality.

Further, in the case of Patent Document 3, the water film forming plate that moves continuously in the direction opposite to the drawing direction of the slab, for example, in the gap between the water film forming plate driven by using a caterpillar and the slab, A secondary cooling method of continuous casting is disclosed in which water is supplied from a provided water supply port to form a water film flow having a thickness of 0.1 to 2.5 mm, but from a plurality of water supply ports arranged in the longitudinal direction. Since the cooling water is supplied, the cooling water is likely to interfere with each other and the cooling water is likely to stay there, and uniform cooling cannot be performed.

Therefore, the present invention improves the cooling capacity of the secondary cooling in the continuous casting machine, significantly increases the amount of water, without extending the length of the continuous casting machine, it is possible to correspond to the speeding up of the casting speed, continuous An object of the present invention is to provide a secondary cooling method for casting and a secondary cooling device.

MEANS TO SOLVE THE PROBLEM In order to solve the said subject, this invention is the secondary cooling method of continuous casting, WHEREIN: The support which conveys the slab in the said slab surface temperature makes the range of 600 to 900 degreeC a cooling range, and the said slab. A cooling device is provided in a gap between the rolls, and the cooling device is installed in a coolant guide plate substantially parallel to the slab with a gap for forming a coolant flow path between the slab and the surface of the slab. And a refrigerant pipe for supplying the refrigerant to the gap, and the gap is set to be more than 0.5 mm and d ₀ mm or less satisfying the following formula (1), and the refrigerant supplied to the gap is non-boiling Alternatively, there is provided a secondary cooling method for continuous casting, which comprises contacting the slab in the nucleate boiling region to cool the slab. Note that non-boiling or nucleate boiling refers to a state in which at least a part of the refrigerant is in a liquid state and contacts the surface of the slab to cool the slab.
d ₀ =3.838×10 ⁻⁴ ×W ^0.48 ×(T+273) ^0.80 (1)
here,
d ₀ (mm): Flow path gap distance at which water film cooling and spray cooling have the same heat transfer coefficient
W (L/min.m ² ): Water quantity density
T (°C): Evaluation temperature
The water film cooling is cooling using the cooling device of the present invention, and the spray cooling is cooling using a refrigerant that is sprayed.

The coolant is supplied to the gap through a supply port formed in the coolant guide plate, the supply port is a plurality of holes arranged in a row in the width direction of the slab, or the width of the slab It may be a slit whose direction is the longitudinal direction.

Further, the present invention is a cooling device for performing secondary cooling in continuous casting, wherein the steel slab surface temperature is in the cooling range of 600° C. to 900° C., and the support rolls for conveying the slab in the cooling range are supported. And a coolant guide plate that is installed substantially parallel to the slab with a gap for forming a coolant flow path between the slab and the surface of the slab, and supplies the coolant to the gap. And a gap of more than 0.5 mm and satisfying the following formula (1), d ₀ mm or less, and the refrigerant supplied to the gap has the non-boiling or nucleate boiling region. There is provided a secondary cooling device for continuous casting, which is characterized in that the slab is cooled by contacting with.
d ₀ =3.838×10 ⁻⁴ ×W ^0.48 ×(T+273) ^0.80 (1)
here,
d ₀ (mm): Flow path gap distance W (L/min.m ² ) at which the heat transfer coefficient of water film cooling is equal to that of spray cooling: Water amount density T (° C.): Evaluation temperature

A supply port for supplying the refrigerant to the gap is formed in the coolant guide plate, and the supply port is a plurality of holes arranged in a row in the width direction of the slab, or the width direction of the slab is long. It may be a slit having a direction.

According to the present invention, since the cooling efficiency of the secondary cooling of continuous casting can be significantly improved, it is possible to cope with the increase in casting speed without increasing the amount of cooling water.

It is a side view which shows the outline of the continuous casting machine concerning embodiment of this invention. It is sectional drawing which shows the outline of the experimental apparatus which tests the cooling capacity of spray cooling. It is the graph which showed the heat transfer coefficient of the spray cooling measured by the experimental apparatus of FIG. 2 with respect to the water amount density when the slab temperature is 900 degreeC and 600 degreeC. It is sectional drawing which shows the outline of the experimental apparatus which tests the cooling capacity of water film cooling. It is a figure explaining the change of the state of the water which contacts a cast in water film cooling. When the slab temperature is 900° C., the heat transfer coefficient of water film cooling measured by the experimental apparatus of FIG. 4 is compared with the heat transfer coefficient of spray cooling measured by the experimental apparatus of FIG. Is a graph shown for. When the slab temperature is 600° C., the heat transfer coefficient of water film cooling measured by the experimental apparatus of FIG. 4 is compared with the heat transfer coefficient of spray cooling measured by the experimental apparatus of FIG. Is a graph shown for. It is a side view which shows a part of continuous casting machine provided with the cooling device concerning embodiment of this invention. It is the figure which looked at FIG. 8 by facing the surface of a cast piece. It is a side view which shows a part of continuous casting machine provided with the cooling device concerning other embodiment of this invention. It is the figure which looked at FIG. 10 by facing the surface of a cast piece.

Hereinafter, embodiments of the present invention will be described. FIG. 1 is an explanatory diagram showing an outline of the configuration of a continuous casting machine 1 according to this embodiment.

As shown in FIG. 1, the continuous casting machine 1 includes a tundish 2 for temporarily storing molten steel, an immersion nozzle 4 for injecting molten steel into a mold 3 from the bottom of the tundish 2, and a cast piece H withdrawn from the mold 3. It is provided with a slab passage 5 to be passed and a pair of roll groups 6 and 7 which are arranged to face each other with the slab passage 5 interposed therebetween.

The pair of roll groups 6 and 7 are provided on both surfaces of the slab passage 5 so as to guide the slab H in the casting direction D ₁ along the slab passage 5. The inner peripheral roll group 6 has a plurality of support rolls 10 for guiding the inner peripheral side of the cast slab H in the cast slab passage 5. The respective support rolls 10 are arranged in a line along the casting direction D ₁ such that the central axis thereof is oriented in the width direction of the cast slab H. The roll group 7 on the outer peripheral side has a plurality of support rolls 11 that guide the outer peripheral side of the cast slab H in the cast slab passage 5. The respective support rolls 11 are arranged in a line along the casting direction D ₁ so that their central axes are oriented in the width direction of the cast slab H.

The molten steel in the tundish 2 is poured from the upper side of the mold 3 through the immersion nozzle 4 and is primarily cooled in the mold 3 to form a solidified shell on the contact surface with the mold 3. Further, the cast slab H having the solidified shell as the outer shell and the unsolidified molten steel inside is continuously sandwiched between the supporting rolls 10 and 11 below the mold 3 while being cooled by the secondary cooling water. Then, a cast slab H that has been completely solidified to the center is produced.

FIG. 2 shows an experimental apparatus for measuring the cooling capacity of the spray nozzle 15 which is generally used in the current continuous casting machine. Cooling water was sprayed onto the surface of the steel slab 16 from above the central portion of the steel slab 16 preheated to a temperature equal to or higher than a predetermined evaluation temperature to cool the steel slab 16. The temperature change of the steel slab 16 during cooling was measured, and the heat transfer coefficient on the surface of the steel slab was obtained using the measurement result. At this time, the temperature transition of the portion of the surface of the steel billet where the spray jet 17 of the cooling water from the spray nozzle 15 does not directly collide is also measured, and the spray jet 17 of the cooling water discharged from the spray nozzle 15 shows the steel billet. A value obtained by averaging over a range of a rectangle whose inscribed circle is an ellipse formed by colliding with the surface was calculated as a heat transfer coefficient when the spray nozzle 15 was used. The temperature of the steel piece 16 was measured by embedding a thermocouple at a position 2 mm inside from the cooling surface of the steel piece 16 in the thickness direction.

Table 1 shows the measured values of the heat transfer coefficient when the evaluation temperature was 900°C. Further, FIG. 3 is a plot of the heat transfer coefficient with respect to the water amount density. Here, the water amount density is obtained by dividing the amount of cooling water jetted from the spray nozzle by the rectangular area on the steel slab.

It can be seen from Table 1 and FIG. 3 that the heat transfer coefficient improves as the water density increases.

Further, Table 2 shows the measured values of the heat transfer coefficient when the evaluation temperature is 600° C., and it can be seen that even at the evaluation temperature of 600° C., the heat transfer coefficient improves as the water amount density increases. The measured values at 600° C. are also shown in FIG.

Next, the cooling effect of water film cooling was tested. FIG. 4 schematically shows a model device 21 for testing the cooling capacity of water film cooling. Refrigerant guide plates 23 were provided at appropriate intervals from the surface of the steel piece 22, and water was supplied from the water supply nozzle 24 toward the gap 25 between the steel piece 22 and the refrigerant guide plate 23. The gap 25 serves as a flow path for water, a water film is formed on the surface of the steel piece 22, and the steel piece 22 is cooled. The temperature of the steel piece 22 was measured depending on the distance from the water supply nozzle 24 in the direction of water flow (X direction), and the cooling capacity was investigated. The temperature of the steel piece 22 was measured by embedding a thermocouple at a position 1.5 mm inside from the cooling surface of the steel piece 22 in the thickness direction (Z direction).

Tables 3 and 4 show the measured values of the heat transfer coefficient by water film cooling when the evaluation temperatures were 900°C and 600°C, respectively. In the water film cooling experiment, the area in which the water film was formed on the surface of the steel slab was taken as the evaluation target area. From Tables 3 and 4, it was found that increasing the water content density improves the heat transfer coefficient, and also for the same water content density, the heat transfer coefficient can be improved by reducing the flow path gap interval. It is considered that this is because the flow velocity of the water film flowing between the steel slab and the refrigerant guide plate was increased when the flow path gap interval was reduced, and the formation of the vapor film on the surface of the steel slab was suppressed. The water amount density is defined as the amount of cooling water supplied from the water supply port for forming the water film flow divided by the area of the steel slab.

When water film cooling is performed, it is conceivable that the cooling capacity for the cast slab H varies greatly depending on the state of the water contacting the cast slab H. That is, as shown in FIG. 5, the water comes into contact with the hot slab H at the water supply point, and in sequence, non-boiling (section A), nucleate boiling (section B), film boiling (section C), anhydrous (section D). ) State. The cooling effect is large in the sections A to B, and the cooling effect decreases from the section C in which the film boiling state occurs. When cooling the water film, a high heat transfer coefficient can be exhibited by keeping the cooling water in contact with the slab H in a non-boiling or nucleate boiling state.

As shown in Tables 3 and 4, it is impossible to cool the steel piece at the level where the coolant guide plate and the steel piece are brought close to each other until the passage gap is 0.5 mm, and the heat transfer coefficient is measured. I couldn't. It is presumed that this is because the flow path of the cooling water was blocked by the scale generated on the surface of the steel slab due to cooling and the bending of the steel slab generated by the cooling.

Further, as shown in Tables 3 and 4, the maximum value of the water amount density in the water film cooling experiment was 1000 L/min. m ² . This is because the maximum supply capacity of the cooling water pump in the existing continuous casting machine is 1000 L/min. This is because it is about m ² and when trying to supply more cooling water than this, a new cooling water pump needs to be installed, resulting in an excessive amount of capital investment, which is not realistic.

The distance between the support rolls in continuous casting is about 200 mm to 250 mm, and when a coolant guide plate for cooling the water film is installed between the support rolls, the length of the cooling guide plate is about 200 mm at the maximum. As will be described later, it was assumed that water was supplied from the central portion of the refrigerant guide plate, and half of the supplied cooling water flows upward (to the mold side) and the remaining half flows downward. Therefore, in this test, the length of the water film flow was set to 100 mm.

FIG. 6 is a plot of the heat transfer coefficient due to water film cooling when the evaluation temperature is 900° C., with the water amount density plotted on the horizontal axis. The numerical value indicated on the shoulder of each plot indicates the flow path gap under each condition. Further, the solid line shown in FIG. 6 is the measured value of the heat transfer coefficient by spray cooling performed at the evaluation temperature of 900° C. From FIG. 6, for example, water density 600 L/min. It can be seen from the plot of m ^{2 that} a very large heat transfer coefficient can be obtained when the flow path gap spacing is 0.6 mm. From this, it can be seen that the heat transfer coefficient decreases as the flow path gap distance increases, and the heat transfer coefficient becomes the same as that of spray cooling when the flow path gap distance is 2.3 mm. That is, at this point, the same heat transfer coefficient is obtained by both spray cooling and water film cooling. Furthermore, at the level where the flow path gap spacing was increased to 4.0 mm, the measured heat transfer coefficient was lower than the spray cooling value, and even if water film cooling was introduced, the heat transfer coefficient was lower than that of spray cooling. Is not improving. This is because when the flow path gap spacing is increased, the flow velocity of the water film flowing between the steel piece and the refrigerant guide plate decreases, and the effect of cooling the water film, which is the effect of suppressing the formation of a vapor film on the surface of the steel piece, disappears. it is conceivable that. From the above, the evaluation temperature was 900° C., the water amount density was 600 L/min. In the case of m ² , it can be read that the flow channel gap interval for water film cooling should be set in the range of 0.6 mm to 2.3 mm.

Similar to FIG. 6, FIG. 7 is a graph in which the heat transfer coefficient due to water film cooling at the evaluation temperature of 600° C. is plotted on the horizontal axis of the water amount density. By performing the same examination with respect to the other plots in FIG. 6 and the respective plots in FIG. 7, it is possible to read the flow channel gap spacing of water film cooling that is effective under each condition.

In other words, when replacing the part where the slab is secondarily cooled by the spray nozzle with water film cooling, there is an upper limit to the water film thickness that can be set in order to obtain a larger heat transfer coefficient than spray cooling. I understood.

Further, under both conditions of the evaluation temperature of 900° C. and 600° C., it is estimated from the value of the measured heat transfer coefficient that the cooling water in the water film is a non-boiling liquid that has a larger heat transfer coefficient than the spray cooling. Or it is thought that it is in the state of nucleate boiling.

The above-mentioned examination was conducted on the experimental results of all the levels shown in Table 3 and Table 4, and the level of the condition that the heat transfer coefficient of the water film cooling was equal to or higher than the heat transfer coefficient of the spray cooling was marked with "○", and the water film cooling was carried out. The heat transfer coefficient of is smaller than that of spray cooling or the level at which cooling is not possible with water film cooling is marked with "x" in the table as a condition for predominant water film cooling.

The inventors analyzed the measurement results of the heat transfer coefficients of water film cooling and spray cooling at different evaluation temperatures and water density. As a result, we have derived a mathematical formula for obtaining the flow path gap interval. Here, the flow path gap interval at which the heat transfer coefficients of water film cooling and spray cooling are equal is defined as d ₀ (mm), the water amount density is W (L/min.m ² ), and the evaluation temperature is defined as T (° C.). Then, d ₀ can be expressed by the following equation (1).
d ₀ =3.838×10 ⁻⁴ ×W ^0.48 ×(T+273) ^0.80 ...Equation (1)
here,
d ₀ (mm): Flow path gap distance W (L/min.m ² ) at which the heat transfer coefficient of water film cooling is equal to that of spray cooling: Water amount density T (° C.): Evaluation temperature

In deriving the above equation (1), the flow path gap distance d _{0 at} which the heat transfer coefficients of water film cooling and spray cooling are equal to each other, the water amount density W and the power of the evaluation temperature T are expressed by the following equation (2). It is assumed that it is calculated by the product of The water amount density W is divided by a length of 100 mm in which water flows, and the evaluation temperature T is divided by an absolute temperature and then divided by 873K (600°C). Then, the coefficient A and the multipliers B and C were obtained from the measurement results shown in Tables 3 and 4, and the above formula (1) was derived. The inventors have confirmed that the above equation (1) holds in the evaluation temperature range of 600°C to 900°C.
d ₀ =A×(W/100) ^B ×{(T+273)/873} ^C (2)

(Embodiment 1)
8 and 9 show an embodiment of the present invention and show a part of a continuous casting machine 1 including a cooling device 31 for cooling a water film. As shown in FIGS. 8 and 9, a cooling device 31 is provided in the gap between the support rolls 10 that convey the cast slab H. The cooling device 31 has a coolant guide plate 32 having the width direction of the cast slab H as a longitudinal direction and a water supply pipe 33 as a coolant pipe, and is supported by a support mechanism (not shown). The coolant guide plate 32 is installed substantially parallel to the surface of the cast slab H with a gap 34 for forming a flow path of cooling water between the surface of the cast slab H and the surface of the cast slab H being adjusted. Is installed as. Further, the coolant guide plate 32 is provided with a water supply port 35 which is a supply port into the gap 34 of the cooling water supplied from the water supply pipe 33. The water supply port 35 may be, for example, as shown in FIG. 9, a plurality of round holes of about φ5 mm arranged in a row in the width direction of the slab H, or one slit having the width direction of the slab H as the longitudinal direction. Alternatively, a plurality of slits arranged in a row in the width direction may be used.

The water supplied from the water supply pipe 33 to the gap 34 becomes a water film flow in the gap 34, cools the surface of the cast slab H, flows in the gap 34, and then is discharged from the end of the refrigerant guide plate 32. ..

(Embodiment 2)
Next, another embodiment of the present invention will be described. 10 and 11 show a part of a continuous casting machine 1 including a cooling device 31 according to another embodiment. The cooling device 31 has a refrigerant guide plate 32 and a water supply pipe 33, as in the above embodiment. In the cooling device 31, water is supplied from the central portion of the refrigerant guide plate 32 via the water supply pipe 33, and half of the supplied cooling water flows upward (to the mold side) and the remaining half flows downward. In the illustrated example, the cooling device 31 is provided in the vertical portion or the curved portion of the continuous casting machine 1, but as shown in FIGS. 8 and 9, the cooling device 31 is provided in the horizontal portion. May be.

Although the preferred embodiment of the present invention has been described above, the present invention is not limited to this example. It is obvious to those skilled in the art that various modifications or modifications can be conceived within the scope of the technical idea described in the claims, and of course, the technical scope of the present invention is also applicable to them. Be understood to belong to.

INDUSTRIAL APPLICABILITY The present invention can be applied to a method and apparatus for cooling a cast slab or a steel plate with a refrigerant.

DESCRIPTION OF SYMBOLS 1 Continuous casting machine 2 Tundish 3 Mold 4 Immersion nozzle 5 Cast piece passage 6, 7 Roll group 10, 11 Support roll 15 Spray nozzle 16 Steel piece 17 Cooling water spray jet 21 Model device 22 Steel piece 23 Refrigerant guide plate 24 Water supply Nozzle 25 Gap 31 Cooling device 32 Refrigerant guide plate 33 Water supply pipe 34 Gap 35 Water supply port H Cast slab

Claims (4)

A secondary cooling method for continuous casting,
The range in which the surface temperature of the billet is 600°C to 900°C is the cooling range,
Providing a cooling device in the gap between the support rolls that convey the slab in the cooling range ,
The cooling device supplies a coolant to the coolant guide plate, which is installed substantially parallel to the cast slab with a gap for forming a flow path of the coolant between the slab and the surface of the cast slab. Equipped with a refrigerant pipe,
The gap is 0.5 mm or less and d ₀ mm or less that satisfies the following formula (1) ,
A secondary cooling method for continuous casting, characterized in that the cooling medium supplied to the gap contacts the slab in a non-boiling or nucleate boiling region to cool the slab.
d ₀ =3.838×10 ⁻⁴ ×W ^0.48 ×(T+273) ^0.80 (1)
here,
d ₀ (mm): Flow path gap distance at which water film cooling and spray cooling have the same heat transfer coefficient
W (L/min.m ² ): Water quantity density
T (°C): Evaluation temperature The refrigerant is supplied to the gap through a supply port formed in the refrigerant guide plate,
2. The continuous casting according to claim 1, wherein the supply port is a plurality of holes arranged in one row in the width direction of the slab, or slits having the width direction of the slab as a longitudinal direction. Secondary cooling method. A cooling device for performing secondary cooling of continuous casting,
The range in which the surface temperature of the billet is 600°C to 900°C is the cooling range,
It is provided in the gap between the support rolls that convey the slab in the cooling range, and is installed substantially parallel to the slab with a gap for forming a flow path of a refrigerant between the slab and the surface of the slab. A refrigerant guide plate and a refrigerant pipe for supplying the refrigerant to the gap,
The gap is set to more than 0.5 mm and not more than d ₀ mm that satisfies the following formula (1), and the refrigerant supplied to the gap contacts the slab in a non-boiling or nucleate boiling region and the casting is performed. Secondary cooling device for continuous casting, characterized by cooling pieces.
d ₀ =3.838×10 ⁻⁴ ×W ^0.48 ×(T+273) ^0.80 (1)
here,
d ₀ (mm): Flow path gap distance W (L/min.m ² ) at which the heat transfer coefficient of water film cooling is equal to that of spray cooling: Water amount density T (° C.): Evaluation temperature In the refrigerant guide plate, a supply port for supplying the refrigerant to the gap is formed,
The continuous casting according to claim 3 , wherein the supply port is a plurality of holes arranged in a row in the width direction of the slab, or slits having the width direction of the slab as a longitudinal direction. Secondary cooling system.

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