JPH11156422A - Device and method for cooling thick plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the cooling device and the cooling method thereof by which the uniformity of a plate temperature can be secured to obtain the thick plate with a uniform mechanical characteristic. SOLUTION: For a time from the cooling start of the thick plate to a point of time of a cooling completion, in the range of a time from 0.2 second or over to below 1 second strong cooling is performed under a condition of thermal conductivity h1 >=8000 (W/m<2> .K). After that, at a point of time when 1 second or over has (have) elapsed from a point of time of the above cooling start, when the plate thickness of the above thick plate is expressed by (t), weak cooling is performed under a condition of thermal conductivity h2 >=(350000/t)-2000 (W/m<2> .K).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a steel plate (4 mm or more) for forcibly cooling a steel plate after finishing rolling in a hot rolling process in order to obtain a steel plate having high strength and high toughness. And a method of cooling the same.

[0002]

2. Description of the Related Art Generally, in order to obtain a thick steel plate having high strength and high toughness, a steel plate is forcibly cooled after finishing rolling in a hot rolling process.

[0003] In this case, as shown in Fig. 10, the steel plate cooling device is installed on a transfer table behind a finishing mill. The thick steel plate that has been finish-rolled by the finish rolling mill 1 is transported to the steel plate cooling device 3 installed on the transport table 2. In the steel plate cooling device 3, cooling water is supplied from the water supply pump 4 to the steel plate cooling spray header 5, and the supplied cooling water is supplied through the steel plate cooling spray header 5 at a predetermined passing speed to the thick steel plate being passed. Are projected onto the upper and lower surfaces of the steel plate to cool the thick steel plate.

[0004] The amount of water supplied to the steel plate cooling spray header 5 can be appropriately adjusted by a flow control valve 6 or the like provided in the piping.

[0005] In the method of cooling a steel sheet using the above-described apparatus, temperature control during cooling is extremely important in order to obtain a steel sheet having uniform mechanical properties. 63-1156
No. 10 discloses a method of obtaining a heat transfer coefficient of a steel sheet from cooling start and end temperatures during pre-cooling and an actually measured value of a sheet passing speed, and setting a temperature to a predetermined value at the time of subsequent cooling from the heat transfer coefficient. ing.

Japanese Patent Application Laid-Open No. Sho 63-144814 discloses that a heat transfer equation is solved from a change in heat transfer coefficient due to a temperature change of a sheet passing speed and a surface temperature of a metal plate, and a cooling water injection amount is calculated from the heat transfer equation. In addition, a method of determining a sheet passing speed to bring a steel sheet to a predetermined temperature is disclosed. In addition, Japanese Unexamined Patent Publication
JP-A-330155 discloses a method of controlling the heat transfer coefficient by controlling the scale thickness to control the cooling rate and / or the cooling stop temperature of the steel sheet.

On the other hand, JP-B-7-47166 discloses that
Ar ₃ points-heat transfer coefficient 2500Kcal near 500 ℃
/ M ² · h · ° C. (about 2150 W / m ² · K) or more, and heat-transfer coefficient is 150 to 2 at 500 to 200 ° C.
A method of weak cooling under the condition of 000 Kcal / m ² · h · ° C. is disclosed.

[0008]

However, especially in the case of hot rolling of one slab such as a thick steel plate, the temperature of the steel plate varies from time to time, and the positional temperature is also affected by the difference in the rolling shape. Due to the difference, and also due to the difference in scale thickness, the heat transfer coefficient changes significantly.

Accordingly, Japanese Patent Application Laid-Open No. 63-115610 and Japanese Patent Application Laid-Open No. 63-115610 are performed using the estimated value of the heat transfer coefficient.
According to the methods described in JP-A-144814 and JP-A-6-330155, it is industrially extremely difficult to control the temperature so that the average in the longitudinal direction, the width direction, and the upper and lower surfaces of the steel sheet are uniform. Difficult,
Temperature variability cannot be avoided, causing mechanical characteristics and shape defects.

On the other hand, in a water-cooled, as the boiling curve shown in FIG. 2, the heat transfer rate to the steel sheet surface temperature, the high temperature side (from about 600 ° C. or higher, T _A temperature above) the low and low-temperature side (about 600 ° C. It is known that the temperature rises sharply at a temperature below T _A ).

However, in the method described in Japanese Patent Publication No. 7-47166, in order to cool the steel sheet so as to exhibit a target heat transfer coefficient in response to a change in the steel sheet temperature, the temperature of the steel sheet must be uniform at the preceding stage. It is necessary to ensure the nature. However, to do this, as mentioned above,
It is premised that the rapid change of the heat transfer coefficient near 600 ° C. is accurately predicted. For this reason, it is necessary to perform high-precision control. In practice, however, the high-precision cooling control is difficult. Therefore, at present, it is difficult to make the mechanical properties of the steel sheet uniform with the temperature equalization industrially.

Accordingly, a main object of the present invention is to provide a cooling apparatus and a cooling method thereof capable of ensuring uniformity of the temperature of a steel sheet in order to obtain a thick steel sheet having uniform mechanical properties.

[0013]

According to a first aspect of the present invention, there is provided a steel plate cooling apparatus for cooling a thick steel plate provided behind a hot rolling mill for a thick steel plate.
(1) The time from the start of cooling of the steel plate to the end of cooling is within 0.2 to less than 1 second, under the condition of heat transfer coefficient h ₁ ≧ 8000 (W / m ² · K). And (2) at a point of time equal to or longer than one second from the start of the cooling, where t is the thickness of the thick steel plate, and the heat transfer coefficient h ₂ ≧ (350,000 / t) −2000 (W / m ²・
And a subsequent weak cooling zone for weak cooling under condition K) is provided in order.

According to a second aspect of the present invention, there is provided a cooling method for cooling a thick steel plate provided behind a hot rolling mill for a thick steel plate,
(1) The time from the start of cooling to the end of cooling of the steel plate is within 0.2 to less than 1 second under the condition of heat transfer coefficient h ₁ ≧ 8000 (W / m ² · K). Cool down,
(2) Thereafter, at the time when one second or more has elapsed from the cooling start time, the heat transfer coefficient h ₂ ≧ g, where t is the thickness of the thick steel plate.
(350,000 / t) -2000 (W / m ² · K) This is a method for cooling a thick steel plate, characterized in that it is weakly cooled.

According to a third aspect of the present invention, there is provided a cooling method for cooling a thick steel plate provided behind a hot rolling mill for a thick steel plate,
(1) The time from the start of cooling to the end of cooling of the steel plate is within 0.2 to less than 1 second under the condition of heat transfer coefficient h ₁ ≧ 8000 (W / m ² · K). The step of cooling is repeated a plurality of times over time. (2) Thereafter, at the point of time when one second or more has elapsed from the start of the cooling, the thickness of the thick steel plate is t, and the heat transfer coefficient h ₂ ≧ (350,000 / t) )
This is a method for cooling a thick steel plate, characterized by weak cooling under the condition of -2000 (W / m ² · K).

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in more detail with reference to embodiments shown in the drawings.

FIG. 1 shows an example of equipment including a cooling device according to the present invention. A steel plate finish-rolled through a finishing mill 1 is transferred to a transfer table 2 downstream of the rolling mill 1.
Is cooled in the steel plate cooling device 3 installed for the cooling device.

In the present invention, the steel plate cooling device 3 is provided separately in a first-stage strong cooling zone 31 and a second-stage weak cooling zone 32 having the ability to cool the surface temperature of the thick steel plate to 300 ° C. in a short time. You.

Here, the pre-stage strong cooling zone 31 has a time from the start of cooling of the steel plate to the end of cooling of 0.2%.
Heat transfer coefficient h ₁ ≧ 80 within a time of not less than one second and less than one second
Strong cooling is performed under the condition of 00 (W / m ² · K).

Further, at the time when one second or more has elapsed from the time when the cooling was started, a heat transfer coefficient h ₂ ≧ (350,000 / t) −20, where t is the thickness of the thick steel plate,
The cooling is weak under the condition of 00 (W / m ² · K).

In order to achieve this condition, as shown in FIG. 1, an in-line thermometer 5 is installed on the exit side of the front-stage strong cooling zone 31 in the steel plate cooling device 3 and on the entrance side of the rear-stage weak cooling zone. On the basis of the thick steel plate surface temperature signal from the in-line thermometer 5, the controller 10 can perform field-back temperature control for controlling the flow rate of the water supply pump 4. Further, a steel sheet conveying speedometer 7 is provided at an appropriate position of the conveying table 2, the speed of the steel sheet is controlled by the control device 10, and the residence time of the thick steel plate in the steel sheet cooling device 3 is controlled, thereby controlling the temperature of the thick steel plate. It is desirable to do.

In general, when the temperature of the heat transfer surface is increased to a temperature equal to or higher than the saturation temperature of the liquid, heat transfer accompanied by evaporation, that is, boiling heat transfer occurs on the heat transfer surface. When the surface temperature of the thick steel plate is plotted on the horizontal axis and the heat transfer coefficient is plotted on the vertical axis, the change along the boiling characteristic curve shown in FIG. 2 is shown. When the surface temperature of the steel sheet is higher than T _A, film boiling, when between T _{A and} T _B , transition boiling, and T
If it is lower than _B , it is nucleate boiling.

In the case of the water cooling system, since heat is taken from the steel sheet surface, the temperature of the steel sheet surface decreases. However, FIG.
As shown in (2), when the steel sheet surface temperature is higher than T _A , the change in the steel sheet surface temperature is almost constant because the heat transfer coefficient is almost constant. However, in the case between T _{A and} T _B , the boiling characteristic curve has a negative slope. Therefore, when the surface temperature starts to decrease, the steel sheet surface temperature reaches T _B (B
Keep falling until the point is reached). This means that if the time required to reach point A differs depending on the steel sheet position, the temperature difference between the steel sheet positions will be amplified. Therefore, before the steel sheet enters the cooling device 3, if there is a temperature difference depending on the position of the steel plate, or if there is a difference in scale thickness of the steel plate that governs the cooling ability of the cooling device and irregularities on the surface of the steel plate, T _A 〜 Doing cooling between T _B (a-B points), the temperature difference between the water-cooled to amplify.

On the other hand, the boiling characteristic curve has a positive slope between T _{B and} T _C (points _{B to C} ), and the lower the temperature, the lower the heat transfer coefficient h becomes on the lower temperature side than T _B. Even if there is the above-mentioned temperature difference factor before entering the cooling device 3, the temperature difference factor tends to be eliminated, and T _{B to} T _B
The temperature difference in the steel sheet between _C decreases.

Therefore, a first subject of the present invention is to cool the steel sheet surface temperature in the vicinity of T _B (near point B), for example, between points B and C before cooling, to eliminate the temperature difference (factor). Thus, uniform cooling is performed. However, according to the present invention, once, as long as the steel sheet surface temperature will be cooled to near T _B, then, even if the recuperation in the water cooling in the steel sheet occurs, T _A ~T _B (A~B
Since the boiling characteristic curve shows a negative slope between the points (a) and ( _b) , the temperature tends to return to the temperature T _B , so that the temperature difference due to the position of the steel sheet is extremely small.

Further, a second subject of the present invention is that when the surface temperature of the steel sheet is cooled to around T _B (around point B), a stable heat transfer coefficient can always be obtained near point B. The transmission rate can be controlled only by the residence time in the preceding stage of the cooling device for the steel sheet, so that, for example, compared to controlling the temperature by controlling the amount of water cooling,
Stable temperature control becomes possible.

Next, the conditions according to the present invention will be described more specifically. FIG. 4 shows that the surface temperature T _O (° C.) of the semi-infinite solid becomes the surface temperature T _S after a time τ seconds.
(C) shows the temperature gradient in a semi-infinite solid. Here, x is the depth (mm) in the thickness direction in the solid, δ
(Τ) is the temperature penetration depth (mm) after a certain time τ seconds. However, if α is a temperature conductivity, the following equations (1) to
(3) is established.

[0028]

(Equation 1)

[0029]

(Equation 2)

[0030]

(Equation 3)

Solving equations (1) to (3) gives

[0032]

(Equation 4)

Is derived, and from equation (4),

[0034]

(Equation 5)

## EQU1 ## On the other hand, h is the heat transfer coefficient (W / m ²
Assuming that K) and λ are thermal conductivities, equation (6) is established from the heat balance.

[0036]

(Equation 6)

From equation (5) and equation (6), equation (7) is obtained. T _W is the temperature (° C.) of the cooling water.

[0038]

(Equation 7)

This equation (7) shows the necessary theoretical boundary condition of the heat transfer coefficient h ₁ in the strong cooling. Therefore, under the condition that the heat transfer coefficient h ₁ is larger than the value on the right side of the equation (7), And good strong cooling can be performed.

FIG. 6 shows the change of the temperature distribution over time when the steel sheet is cooled from the upper and lower surfaces. That is, cooling (water cooling)
At the start (τ = 0), those entire steel sheet is constant temperature, now showing the temperature gradient in the steel plate by a cooling of a steel plate upper and lower surfaces, if the point of tau ₃ after a predetermined time has elapsed, the cooling Temperature penetration depth δ (τ) is 1/1 of plate thickness
Reach 2. As long as the cooling after the time τ ₃ is performed in a temperature range near T _S = 300 ° C. according to the present invention, the heat removal and the heat conduction during the cooling of the steel sheet are balanced, and the temperature gradient in the steel sheet becomes constant thereafter. .

This means, conversely, that the cooling temperature penetration depth δ
This means that the heat transfer coefficient h from when (τ) reaches に of the plate thickness is constant. Therefore, in equation (4), δ = t / 2 can be satisfied, and equation (8) is given. This equation (8) gives the condition of weak cooling.

[0042]

(Equation 8)

By substituting equation (8) into equation (7),
Equation (9) is given.

[0044]

(Equation 9)

Therefore, under the condition that the heat transfer coefficient h ₂ is larger than the value on the right side of the equation (9), good weak cooling can be performed.

[Embodiment] Next, the effect of the present invention will be clarified by an embodiment, and the above-mentioned conditions and the like as seen from the results of experiments will be considered.

FIG. 7 shows an outline of the equipment in the embodiment. This equipment is basically the same as that shown in FIG. 1, except that a front-stage strong cooling zone 31 and a rear-stage weak cooling zone 32
In order to prevent the influence of the former-stage strong cooling on the latter-stage weak cooling, a constraining roller 9 is provided, which rotates while contacting the upper and lower surfaces of the steel plate M to prevent the movement of the cooling water on the surface of the steel plate M. In the embodiment, the first-stage strong cooling zone nozzle 31a
Slit nozzle (vertical respectively water 15 m ³ / min) as
However, a spray nozzle (water amount of 1 m ³ / m ² · minute for each of upper and lower portions) was used as the latter weak cooling zone nozzle 32a. In addition,
The effective injection length of the first-stage strong cooling zone nozzle 31a (in other words, the effective length of the first-stage strong cooling zone) is 0.1 m. Any of these nozzle types such as a slit jet nozzle, a misting jet nozzle, a slit nozzle, and a spray nozzle can be used.

Under such equipment, the thickness t is about 40 ° C., about 800 ° C. after the finish rolling carried at a carrying speed of 25 m / min.
Was subjected to strong cooling and weak cooling with cooling water having a temperature of 30 ° C.

Under these conditions, the strong cooling time τ is 0.1
m ÷ 24 m / min = 0.25 seconds, and when this τ value is substituted into equation (4), δ ≒ 0.008 m. Therefore,
When the temperature distribution with respect to the depth in the thickness direction of the steel sheet shown in FIG. 4 is approximated to be a straight line, Expression (10) is obtained.

[0050]

(Equation 10)

In this equation (10), T _S = 300 ° C., T _W
= 30 Substituting ° C. and T _O = 800 ° C., formula (1
1).

[0052]

[Equation 11]

Since λ in the equation (11) is 35 W / m · K, substituting δ = 0.008 m gives the following heat transfer coefficient in strong cooling. ha = 1.85 × 35 ÷ 0.008 ≒ 8094 (W / m ² · K) (A) On the other hand, at the time of weak cooling, t =
0.04 m, T _O = 800 ° C., T _S = 300 ° C., T _W =
Substituting 30 ° C. and λ = 35 W / m · K gives the following heat transfer coefficient in weak cooling. hb = 259 / t = 259 / 0.04 = 6475 (W / m ² · K) (B) The steel sheet obtained under such conditions has a good plate shape and excellent mechanical properties. It was.

<Second Example> In the second example, the thickness t is 5
The 8 mm thick steel plate was cooled at a conveying speed of 10 m / min. In this case, if the same calculation is performed,
Since τ = 0.6 seconds and δ = 0.012 m, the heat transfer coefficient in the strong cooling is as follows. hc = 1.85 × 35 ÷ 0.012 ≒ 5396 (W / m ² · K) (C) The heat transfer coefficient in weak cooling is as follows. hd = 259 / t = 259 / 0.058 = 4465 (W / m ² · K) (D) Also in this second example, the obtained steel sheet has a good sheet shape and good mechanical properties. Was also excellent.

(Summary) The temperature at the point B in FIG. 2 is found to be T _S = 300 ° C. and T _S = 300 ° C. since it has been found that the steel sheet surface temperature T _B ≒ 300 ° C. in the relation between the thick steel sheet and water.
_W = 30 ° C., and T _O
= 800 ° C., and thermal conductivity α = 2 × 1 as a physical property
0 ⁻⁵ , and thermal conductivity λ = 35 W / m · K, respectively, according to equation (7)
, And considering the type of the target plate thickness, the solid line graph shown in FIG. 5 is obtained as the optimum heat transfer coefficient boundary condition in the strong cooling. From this graph, τ = 1
It can be seen that the heat transfer coefficient h ₁ ≒ 8000 (W / m ² · K).

On the other hand, the boundary condition of the heat transfer coefficient in weak cooling is theoretically given by the above equation (8) or (9), and the heat transfer coefficient h ₂ is determined by the sheet thickness t (mm). It is strongly correlated and can be represented on the graph of FIG. Incidentally, under many experiments, the heat transfer coefficient h ₂ in the case of a good plate shape and excellent mechanical properties was 6475 W / m ² · K at a plate thickness t = 40 mm, Thickness t = 50mm
At the time of 5180 W / m ² · K and the thickness t = 60 mm 43
16 / m ² · K.

Further, when a disturbance element is included, the boundary condition of the heat transfer coefficient h ₂ in a suitable weak cooling is a graph shown in FIG. 3, which can be approximated by the following equation.

Heat transfer coefficient h ₂ = (350,000 / t) -2
000 (W / m ² · K) Incidentally, the graph shown in FIG. 5 shows that T _S = 300 ° C. during the strong cooling performed by the preceding strong cooling zone in the present invention.
A heat transfer coefficient h ₁ needed to, during weak cooling performed by the subsequent weak cooling zone shows the heat transfer coefficient h ₂ required for maintaining the surface temperature achieved by the strong cooling For example, in the case of t = 40 mm, in the graph of FIG. 5, strong cooling is performed until the solid line portion and the broken line portion corresponding to t = 40 mm intersect from the start of cooling of the steel plate, and thereafter, the cooling is performed by weak cooling. , T ₀ = 300 ° C., the entire steel sheet can be uniformly cooled.

In the present invention, the point of time τ of the strong cooling is set to 0.
The time within 2 seconds or more and less than 1 second is defined by the relationship between τ and the cooling start temperature T ₀ and the relationship between τ and the plate thickness t.
Although theoretically considered to be limited by equations (8) and (9), from a practical point of view, the lower limit time of τ is limited to 0.2 seconds or more. The upper limit time of τ (less than 1 second) was similarly determined from a practical viewpoint.

The starting point of the weak cooling can be theoretically expressed by the following equation, as can be understood by modifying equation (8). ^{τ ≧ (t 2/4)} × (1 / 12α) Up to this point, in theory larger right-hand side of equation (9), in reality, ^{8000 (W / m 2 · K} ) or more heat transfer coefficient it is necessary to cool with a h _1.

FIG. 8 shows the results obtained by examining the amount of warpage of steel sheets of various thicknesses when the steel sheet conveying speed is changed. According to this result, if an appropriate conveyance speed is selected according to the thickness of the steel plate, it means that the warpage of the steel plate can be reliably prevented.

In the above calculation example, the cooling start temperature is set to 8
Since it is assumed that the temperature is set to 00 ° C., for example, if the cooling start temperature is set to 700 ° C., it will be apparent that the value will be different.

In the case of strong cooling, for example, if the cooling start temperature is 700 ° C., the surface temperature of the steel sheet is 300 ° C.
It may not be able to be below ℃. In this case, according to the third aspect of the invention, the heat transfer coefficient h ₁ ≧ 8000 (W / m ²
The step of strongly cooling under the condition (K) can be repeated several times over time. This is schematically shown in FIG.
τ ₁ <1 and τ ₂ <1. In this case, in terms of equipment, a plurality of pre-stage strong cooling zones are divided and installed, and a post-stage weak cooling zone is provided downstream thereafter.

[0064]

As described above, according to the present invention, it is possible to obtain a thick steel plate having uniform mechanical properties and a good plate shape.

[Brief description of the drawings]

FIG. 1 is a schematic view of a cooling system for a thick steel plate according to the present invention.

FIG. 2 is a boiling characteristic curve.

FIG. 3 is a graph showing the relationship between the heat transfer coefficient of a steel sheet and the thickness of the steel sheet during weak cooling according to the present invention.

FIG. 4 is a graph showing a temperature gradient in a semi-infinite solid.

FIG. 5: During steel sheet cooling, the steel sheet surface temperature is 300 ° C.
5 is a graph showing a relationship between a heat transfer coefficient required to be held at a temperature, an elapsed time from the start of cooling, and a thickness of a steel sheet.

FIG. 6 is a diagram showing a temperature distribution change in a cooling process.

FIG. 7 is a schematic diagram of a steel plate cooling device in an embodiment.

FIG. 8 is a graph showing a relationship between a conveying speed of a steel sheet and an amount of warpage of the steel sheet.

FIG. 9 is a schematic explanatory view of another example of the present invention.

FIG. 10 is a schematic view of a conventional steel plate cooling device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Finish rolling mill, 2 ... Convey table, 3 ... Steel plate cooling device, 4 ... Water supply pump, 5 ... In-line thermometer, 7 ... Steel plate conveyance speedometer, 10 ... Control device, 31 ... Preceding strong cooling zone, 32 ... Stage Weak cooling zone.

Claims (3)

[Claims] 1. A steel plate cooling device provided for cooling a thick steel plate provided behind a hot rolling mill for a thick steel plate, wherein: (1) a time from the start of cooling of the thick steel plate to the end of cooling is 0.2 seconds or more; Within a time period of less than 1 second, a strong cooling zone in the former stage for performing strong cooling under the condition of heat transfer coefficient h ₁ ≧ 8000 (W / m ² · K); ,
The heat transfer coefficient h ₂ ≧ (350
000 / t) -2000 (W / m ² · K). A cooling device for a thick steel plate, comprising: 2. A cooling method for cooling a thick steel plate provided behind a hot rolling mill for a thick steel plate, wherein: (1) the time from the start of cooling of the thick steel plate to the end of cooling is 0.2 seconds or more; Within a time period of less than a ^second , the steel sheet is strongly cooled under the condition of a heat transfer coefficient h ₁ ≧ 8000 (W / m ² · K). The heat transfer coefficient h ₂ ≧
(350,000 / t) -2000 (W / m < ² > K). 3. A cooling method for cooling a thick steel plate provided behind a hot rolling mill for a thick steel plate, wherein: (1) the time from the start of cooling of the thick steel plate to the end of cooling is 0.2 seconds or more; Within a time period of less than a ^second , the step of strongly cooling under the condition of heat transfer coefficient h ₁ ≧ 8000 (W / m ² · K) is repeated a plurality of times over time. (2) Then, 1 second from the cooling start point At the time when the above has elapsed, the thickness of the thick steel plate is t, and the heat transfer coefficient h ₂ ≧
(350,000 / t) -2000 (W / m < ² > K).