JP3282714B2 - Cooling method for hot steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for cooling a hot-rolled hot steel sheet, and more particularly to a method for cooling a thick steel sheet.

[0002]

2. Description of the Related Art Generally, hot-rolled hot steel sheets are
Cooling unevenness tends to occur during cooling due to differences in temperature distribution, plate shape, and surface state immediately after rolling. In particular, in the cooling of a thick steel plate, if cooling unevenness occurs, an operation trouble such as a failure in passing the plate due to deformation of the steel plate is likely to occur, and after cooling, deformation of the steel plate, residual stress, and variation in the material are likely to occur. Further, when the steel sheet is deformed, a refining step by a press or a straightening machine is required later, so that the production cost is increased. Therefore, in order to avoid these problems, various so-called uniform cooling methods have been conventionally proposed.

[0003] In cooling a thick steel plate, while the steel plate is constrained and passed by a conveying roll also serving as a table roller for conveying the steel plate and a constraining roll forming a pair with the conveying roll, cooling water is continuously supplied from above and below the steel plate. The online cooling method to inject water
Commonly used. In the cooling of the steel sheet, from the viewpoint of increasing productivity, it is desirable to lengthen the cooling device and cool it while transporting it at a high speed, for example, as in a run-out cooling device that is a cooling device for hot-rolled steel sheets. In the case of cooling, such a method cannot be adopted because a large amount of cooling water is used and since the plate is thick, it is necessary to perform cooling for a long time.

[0004] As a method for cooling a thick steel plate, for example, Japanese Patent Application Laid-Open No. 61-153235 discloses a device for cooling a thick steel plate by passing it through two cooling devices online. 2 from nozzle
Cool with a water amount of at least m ³ / min-m until the surface temperature becomes 600 ° C. or less, and then, with a second cooling device partitioned by a draining roll, use a spray nozzle to supply 0.7 m ³ / min-m or more of cooling water. There has been proposed a cooling method for performing cooling by injecting a gas.

[0005]

However, even in the method described in JP-A-61-153235, it is difficult to provide uniform and uniform cooling. Furthermore, this method requires enormous amounts of cooling water supply equipment and piping equipment, and the operation costs are enormous. In addition, since the cooling equipment becomes large, the cooling zone cannot be lengthened, and there is a limit in productivity.

The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a method for cooling a thick steel plate that can perform uniform cooling with a small amount of cooling water.

[0007]

Means for solving the above-mentioned problems are as follows. A high-temperature steel sheet is cooled on-line by injecting cooling water into the high-temperature steel sheet while passing the high-temperature steel sheet between a plurality of pairs of restraining rolls. A method for cooling a high-temperature steel sheet, characterized by injecting cooling water into the high-temperature steel sheet in such an amount that the maximum temperature of the cooling water injected into the high-temperature steel sheet is 60 ° C. or less, and performing cooling. (Claim 1).

(Circumstances leading to the invention) In order to provide uniform cooling with a small amount of cooling water, the inventors repeated cooling tests of steel sheets under various cooling conditions, and examined the cooling process.
Cooling water amount: 0.2 to 3.5 m ³ / min-m, steel sheet temperature at the start of cooling: 400 to 900 ° C. (steel sheet temperature at the end of cooling is 30 to
800 ° C.), the length of the cooling zone: 0.5 to 3 m, and the temperature of the supplied cooling water: 5 to 40 ° C. As a result of performing a cooling test, the temperature of the cooling water supplied from the nozzle and in contact with the steel plate becomes lower. It has been found that when the temperature exceeds 60 ° C., uneven cooling occurs.

This relationship also applies to laminar nozzles and spray nozzles other than the slit jet nozzle within the range of the above cooling conditions. For example, from the slit nozzle
When cooling water of 20 ° C of 1.0m ³ / min-m is injected in the longitudinal direction of the steel sheet having a steel sheet surface temperature of 800 ° C, about 1
m, the cooling water temperature exceeded 60 ° C., and as a result, cooling unevenness occurred due to uneven cooling on the downstream side.

FIG. 1 is a diagram showing the relationship between the maximum temperature of the cooling water during cooling and the difference between the maximum temperature and the minimum temperature in the steel sheet after cooling. As can be seen from FIG. 1, when the maximum temperature of the cooling water is 60 ° C. or less, the difference between the maximum temperature and the minimum temperature in the steel sheet remains within 30 ° C., but the maximum temperature of the cooling water is 60 ° C. When the temperature exceeds ℃, the difference between the maximum temperature and the minimum temperature in the steel sheet sharply increases.

This phenomenon is understood as follows. That is, while the cooling water flows in the longitudinal direction above and below the steel sheet, the temperature of the cooling water increases due to the heat taken from the steel sheet. When the temperature of the cooling water exceeds a certain temperature, the cooling mode is changed from nucleate boiling cooling to partial film boiling cooling immediately, resulting in uneven cooling and uneven cooling. From FIG.
To keep the maximum / minimum temperature difference within the steel sheet within 30 ° C in order to keep the variation of the steel sheet strength which affects the material of the steel sheet within the product specification, the allowable maximum temperature of the cooling water during cooling the steel sheet is within 60 ° C. It is understood that it is necessary to From the above, in the present invention, the allowable maximum temperature of the cooling water is limited to 60 ° C. or less.

As shown in FIG. 1, when the maximum temperature of the cooling water exceeds 80 ° C., the cooling mode completely shifts to film boiling cooling, so that the temperature difference in the steel sheet tends to be small. Since the cooling of the steel plate is insufficient in the region, it is not suitable for cooling the thick steel plate.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIG. 2 is a schematic diagram of a cooling device used in this embodiment. This cooling device is a cooling device that cools the steel sheet 1 immediately after rolling between 21 sets of the upper constraining rolls 2 and the lower constraining rolls 3 while online, and the pitch between the rolls is 1.0 m. The total length is 20 m. The lower constraining roll 3 also serves as a transport roll and is of a fixed type, while the upper constraining roll 2 can be moved up and down and the gap can be controlled at a pitch of 0.5 mm.

Further, when the steel sheet 1 having a thickness equal to or larger than the set gap passes through the upper constraining roll 2,
The structure is such that a restraining force is applied to the steel plate 1 by the hydraulic cylinder 7. This restraining force is applied to suppress the cooling warpage generated during cooling, and 1 to 5 t is applied to each roll.
Up to can be added.

Above and below the steel sheet on the inlet and outlet sides of the cooling device, a non-contact radiation thermometer 8 capable of measuring the surface temperature in the width direction of the steel plate is installed, and continuously monitors the temperature of the steel sheet.

On the upper surface side between the rolls, the slit nozzle 4 moves 1.8 m ³ / min.
m of water is flowing along the plate. On the other hand, 100m on the lower side
Water is injected from a circular nozzle 5 provided submerged in water at m pitches, and cooling is performed by a liquid flow generated by the accompanying flow. The cooling water amount was 1.8 m ³ / min-m. (In this embodiment, since the length of the cooling zone covered by one slit nozzle is 1 m, which is equal to the constrained roll pitch, the water volume density is 1.8 m ³ / min-m ^2. ) The water temperature was 20 ° C.

Cooling water is supplied to a slit nozzle 4 on the upper surface and a circular nozzle 5 on the lower surface of each zone through a flow control valve 6 capable of controlling the flow.

The temperature of the cooling water was measured by the following method. With respect to the upper surface cooling water, a thermocouple for measuring the temperature of the upper surface cooling water was installed downstream of each upper constraining roll 2, and the temperature was measured. As for the lower surface cooling water, a thermocouple for measuring the temperature of the lower surface cooling water was installed in a water tank of the lower surface cooling water, and the temperature was measured.

The cooling device having the above-described structure has a plate width of 3
The rolled high-temperature steel plate having a length of 20 m, a length of 20 m and a thickness of 40 mm was cooled at a conveying speed of 50 mpm. The gap between the rolls was set to a thickness of -1.5 mm, that is, 38.5 mm.

At this time, when the temperature of the cooling water for cooling the upper surface was measured with a thermocouple provided downstream of each upper roll, the cooling water temperature was 52 ° C. at the maximum. Also, the temperature in the water tank was 52 ° C. at the maximum for the lower surface cooling.

The surface distribution of the temperature immediately before the cooling and 20 seconds after the cooling was measured with a surface thermometer. The temperature in the width direction and the length direction in the plate was 835 ° C. to 850 ° C. before the cooling. On the other hand, after cooling, the temperature was 505 ° C. to 520 ° C., and the entire surface was uniformly cooled without expanding the temperature unevenness due to the cooling. There was no C warpage in the sheet width direction.

(Embodiment 2) In this embodiment, in the cooling device described in Embodiment 1, the flow rate of the cooling water jetted from the slit nozzle 4 on the upper surface and the circular tube nozzle 5 on the lower surface is set to 60 ° C. This is an example in which cooling is performed while adjusting so as not to exceed. As in the first embodiment, the temperature of the cooling water is a thermocouple provided on the downstream side of each upper constraining roll 2 on the upper surface side, and a thermocouple provided in a water tank of the lower surface cooling water on the lower surface side. The flow rate was adjusted using the flow rate control valve 6 so that these temperatures did not exceed 60 ° C.

In this cooling device, a plate width of 4.5 m and a length of 2 m
5m, 60mm thick hot rolled steel plate at a speed of 15mpm
And cooled. Immediately before and after cooling 20
The surface distribution of the temperature after seconds was measured with a surface thermometer. In this state, the temperature in the plate width direction and the plate longitudinal direction in the plate is 790 ° C to 820 ° C.
On the other hand, after cooling, the temperature was 490 ° C. to 520 ° C., and the temperature unevenness due to the cooling did not increase, and substantially uniform cooling was performed over the entire surface. At this time, the amount of cooling water is 1.6
It has decreased from m ³ / min-m to 0.4 m ³ / min-m. That is, the required cooling water amount decreases as the cooling proceeds and the steel sheet temperature decreases. In addition, for this plate, C
There was no warpage.

(Embodiment 3) The cooling device used in this embodiment is a cooling device having the same roll arrangement as the cooling device used in Embodiment 1. That is, this is a cooling device that performs on-line cooling while conveying the rolled steel sheet 1 between the 21 sets of upper and lower constraining rolls 2 and 3, and the pitch between the rolls is
1.0 m. Spray nozzles are provided on the upper surface side and the lower surface side between each roll at a pitch of 300 mm in the width direction and 300 mm in the longitudinal direction, and cooling is performed by injecting cooling water from these spray nozzles.

The adjustment of the flow rate of the upper and lower spray nozzles was performed in the following procedure. The heat flux from a steel sheet at a certain water density during spray cooling can be calculated using the equation (3) in Table (1) of “Forced Cooling of Steel” edited by the Iron and Steel Institute of Japan, logα = 2.358 + 0.663logW−0.00147θ. _S … (1) where α: heat transfer coefficient Kcal / m ² -hr- ℃ W: water density L / min-m ² θ _S : surface temperature ℃

At this time, the heat flux q transmitted from the steel sheet to the cooling water is: q = α (θ _S −θ _W ) (2) where q: heat flux Kcal / m ² -hr

The rise in temperature [Delta] [theta] _w is _{Δθ w = q / (60WρC p} ) ... (3) where C _p: specific heat of the coolant = 1.0 Kcal / Kg- ℃ ρ: Density of coolant = 1.0 Kg / L

Accordingly, the cooling water temperature rises to θ _wout below. θ _wout = θ _win + Δθ _w (4) where θ _win : supplied cooling water temperature ℃

FIG. 3 shows the case where the surface temperature is 600 ° C. and 700 ° C., and the cooling water arrival temperature θ _wout obtained from the equation (4) when the water density W is changed from 100 to 1400 L / min-m ^2. Is shown. The supplied cooling water temperature θ _win is constant at 30 ° C. Thus, if the surface temperature is 600 ° C. and 700 ° C., and the W is 500 and 800 L / min-m ² respectively, the cooling water temperature becomes 60
It can be easily determined that cooling is possible in a state of not more than ° C.

FIG. 4 is a diagram showing the relationship between the minimum water density at which the cooling water arrival temperature θ _wout is 60 ° C. and the steel sheet surface temperature determined in the same manner. The cooling water amount of each zone was determined based on FIG. 4, and cooling was performed under the cooling conditions.
That is, the steel sheet surface temperature in each zone was obtained by calculating the temperature history of the steel sheet surface during cooling, and the water density required for the surface temperature was determined from FIG.

This cooling device has a plate width of 4.5 m, a length of 25 m,
The rolled high-temperature steel sheet having a thickness of 20 mm was cooled at a speed of 75 mpm. The surface distribution of the temperature immediately before cooling and 20 seconds after cooling was measured with a surface thermometer. In this state, the temperature in the sheet width direction and the sheet longitudinal direction in the sheet was 490 ° C to 520 ° C after cooling, whereas the difference between the highest temperature and the lowest temperature was 790 ° C to 820 ° C before cooling. Almost the entire surface was uniformly cooled without increasing the temperature unevenness due to cooling. Also, there was no C warpage in the plate width direction for this plate.

In this embodiment, the method of determining the water density of the spray nozzle has been described. However, other cooling methods such as mist cooling, circular pipe laminar cooling, flat laminar cooling, jet cooling, It is possible to determine the required water density for pipe fountain cooling and the like. In many cases, cooling characteristics such as (1) to (4) are not obtained for those cooling methods. However, if the cooling characteristics are grasped by separate experiments, the maximum temperature of the cooling water can be reduced to 60 ° C. It is possible to determine in advance the water density required to suppress the water content.

Further, in this embodiment, the temperature of the supplied cooling water was 30 ° C.
Since the temperature may be 20 ° C., in this case, uniform cooling is possible with a smaller cooling water density. Conversely, when the temperature of the cooling water is about 40 ° C. as in summer, it is necessary to increase the density of the cooling water. As described above, according to the cooling method of the present invention, it is possible to appropriately and economically adjust the water density of the cooling water in accordance with the surrounding conditions (seasonal changes in the cooling water temperature and water temperature changes during the day and night). .

[0035]

As is apparent from the above description, the present invention relates to a hot steel sheet which is cooled on-line by injecting cooling water into the hot steel sheet while passing the hot steel sheet between a plurality of pairs of restraining rolls. A method for cooling a high-temperature steel sheet, characterized in that cooling is performed by injecting cooling water into the high-temperature steel sheet in such an amount that the maximum temperature of cooling water injected into the high-temperature steel sheet is 60 ° C. or less. Therefore, the following effects are obtained. (1) Uniform cooling becomes possible, and it is possible to prevent non-uniformity of material due to cooling unevenness and generation of distortion due to heat after cooling. (2) There is no defect in the shape of the material, and the cost for refining can be reduced. (3) Passing trouble due to defective shape of the plate during passage of the cooling device is reduced, and the operation rate of the equipment is increased. (4) Variations in the material of the steel sheet due to insufficient cooling of the material or overcooling are reduced, and a homogeneous product is obtained. Further, the yield loss due to the material slippage is reduced, and the generation rate of scraps is reduced. (5) The amount of cooling water can be adjusted according to the seasonal change of the cooling water temperature and the change of day and night without impairing the uniform cooling of the steel plate cooling temperature, which is economical.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a temperature of cooling water after cooling and a difference between a maximum temperature and a minimum temperature in a steel sheet after cooling.

FIG. 2 is a schematic diagram of a cooling device used in the first embodiment.

Fig. 3 When the water density W was changed from 100 to 1400 L / minm ² when the steel sheet surface temperature was 600 ° C and 700 ° C.
It is a figure which shows the cooling water arrival temperature (theta) _{wout calculated |} required from Formula (4).

FIG. 4 is a diagram showing the relationship between the steel sheet surface temperature and the minimum water density when the cooling water arrival temperature θ _wout in spray cooling is 60 ° C.

[Explanation of symbols]

 1-Upper constraining roll 2-Lower constraining roll 3-Steel plate 4-Slit nozzle 5-Circular nozzle 6-Flow control valve 7-Cylinder 8-Radiation thermometer

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-61-153235 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B21B 45/02 320

Claims (1)

(57) [Claims] 1. A method for cooling a hot steel sheet by injecting cooling water into the hot steel sheet while cooling the hot steel sheet while passing the hot steel sheet between a plurality of pairs of constraining rolls. Cooling the high-temperature steel sheet by injecting cooling water into the high-temperature steel sheet in such an amount that the maximum temperature of the steel sheet is 60 ° C. or less.