JP2002241845A - Method for cooling steel strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve productivity and profitability when cooling is applied to a steel strip by using a cooling roller in combination with a gas jet in a continuous annealing process. SOLUTION: In the method for cooling the steel strip where the steel strip transferred into a cooling zone of continuous annealing equipment is cooled by the blowing of cooling gas using the gas jet and then brought into contact with the cooling roller to undergo cooling, the forcing amount P at which the above cooling roller and the steel strip are brought into contact with each other is regulated to the optimum forcing amount PS at which the optimum heat-removal amount free from defects in the shape of the steel strip can be obtained and also the steel-strip temperature on the delivery side of the above cooling roller is subjected to feedback control by means of the above gas jet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for cooling a steel strip,
In particular, the present invention relates to a cooling method for a steel strip, which is suitable for rapid cooling of a steel strip by using a cooling roll and a gas jet together in a cooling zone of a continuous annealing facility.

[0002]

2. Description of the Related Art As a means for continuously and rapidly cooling a steel strip (steel plate) in a continuous annealing facility, a cooling method using a cooling roll for circulating a cooling medium therein has been generally used. For example, Japanese Patent Publication No. 58-47457 discloses a basic method for adjusting the temperature of a steel strip by changing the contact length of the steel strip with a cooling roll. In addition, Japanese Examined Patent Publication No. 63-14051 describes the dimensions of steel strip,
Substituting the physical property values, the temperature of the steel strip on the cooling roll entrance side, the steel strip tension, the refrigerant temperature, and the steel strip transport speed into the relational expressions obtained in advance, the winding angle of the steel strip to the cooling roll is calculated periodically. ,
A technique in which the winding angle is changed based on the value and the welding angle of the steel strip is controlled while the welding wire is passing through the cooling roll, thereby improving the precision of the cooling control of the steel strip is disclosed. .

However, the method of cooling with a cooling roll has a high cooling efficiency, and particularly when the temperature of the steel strip is high, a large temperature difference from the cooling roll causes a thermal deformation of the steel strip. easy. In particular, if cooling of a steel strip such as a high-tensile steel sheet requiring a high cooling rate is controlled only by the cooling roll, a shape defect occurs, and eventually buckling occurs. There is a limit to increasing the heat removal, that is, the cooling rate. Such a problem of the shape defect tends to occur particularly when the plate width is large.

Therefore, Japanese Patent Application Laid-Open No. 11-229043 discloses that
In combination with a gas jet cooling device (hereinafter simply referred to as a gas jet), when cooling with a gas jet first and then with a cooling roll, the average cooling rate of the steel strip by the cooling roll is lower than the average cooling rate of cooling by the gas jet. A method of rapidly cooling a steel strip by controlling the cooling rate based on a target cooling rate so as to increase the speed without impairing the material performance is disclosed.

[0005]

However, since the gas jet has a lower cooling efficiency than the cooling roll, the transport speed needs to be slowed down as the cooling amount by the gas jet cooling increases, so that the productivity is lowered. In addition to this, there is also a problem that the cost of cooling is higher in the gas jet, and thus the cost is low.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and a steel strip capable of improving productivity and improving economic efficiency by using a cooling roll and a gas jet together. It is an object of the present invention to provide a cooling method.

[0007]

According to the present invention, a steel strip conveyed to a cooling zone of a continuous annealing facility is cooled by spraying a cooling gas with a gas jet and then brought into contact with a cooling roll to cool the steel strip. In the method of cooling the strip, the cooling roll is pushed into the steel strip, and the pushing amount for changing the contact amount between the cooling roll and the steel strip is adjusted to an optimum pushing amount that can obtain a maximum heat removal amount that does not cause a shape defect in the steel strip. In addition, the problem is solved by feedback-controlling the steel strip temperature at the outlet side of the cooling roll by the gas jet.

That is, the present inventors have conducted various studies on a cooling method in a continuous annealing facility in order to improve productivity and economy of a steel strip requiring a high cooling rate such as a high-tensile steel sheet having high ductility. It has been found that there is an optimal pushing amount that gives the maximum heat removal amount without causing a shape defect in the pushing amount of the cooling roll related to the contact length between the cooling roll having a high cooling efficiency and the steel strip.

The present invention has been made based on the above findings. The optimum pushing amount of the cooling roll is determined in advance, the cooling roll is adjusted to the optimum pushing amount, and the temperature of the steel strip on the exit side of the cooling roll is adjusted. Since the feedback control is performed by the gas jet, the cooling amount by the cooling roll with high cooling efficiency can be increased, and the cooling amount by the gas jet can be reduced, so that the conveying speed of the steel strip can be increased and the productivity can be improved. Can be improved. In addition, since the cooling cost of the cooling roll is lower, the cost is excellent and the product cost can be significantly reduced.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows an outline of a cooling zone of a continuous annealing facility applied to one embodiment according to the present invention.

In the above-mentioned cooling zone, the steel strip S, which is heated to a predetermined temperature by a heating furnace and a soaking furnace (not shown) and then carried in through the entrance bridle roll 10, blows a cooling gas on both front and back surfaces thereof. After being sequentially cooled by a gas jet 12 arranged in each of four zones indicated by 1Z to 4Z and a pair of cooling rolls 14 arranged with a steel strip S interposed therebetween on the downstream side, the gas jet 12 is discharged. Bridle roll 1
After that, it is conveyed to equipment on the further downstream side.

The cooling zone is provided with a plurality of temperature measuring rolls 18 for measuring the temperature of the steel strip S in contact with the steel strip S while changing the conveying direction thereof. Radiation thermometers 20 for measuring the temperature of the passing steel strip S are provided on the entrance side and the exit side of the roll 14, respectively.

As shown in FIG. 2, one of the cooling rolls 14 has cooling water circulated therein in the direction of an arrow, and can forcibly and rapidly cool the steel strip S in contact therewith. At the same time, as shown by the double-headed arrow, it is possible to move forward and backward at a predetermined moving speed in a direction perpendicular to the surface of the steel strip S.

As shown in an enlarged view of the actual arrangement in FIG. 3, fixed rolls 22 are provided on the inlet and outlet sides of the pair of cooling rolls 14, respectively. By moving the steel strip S in the direction of the arrow perpendicular to the surface of the strip S and changing the pushing amount indicated by P in the drawing, the winding angle θ of the steel strip S, that is, the contact length (area) is changed, and the cooling capacity is reduced. It can be adjusted. In this example, when the pushing amount P = 0, the contact area becomes substantially zero, and the steel strip S is released.

In the present embodiment, after the steel strip S, which is continuously conveyed and heated to a predetermined temperature, is cooled by the gas jet 12 and then cooled by the cooling roll 14, the pushing amount P is reduced. The cooling temperature is adjusted to the optimum indentation amount P _S that gives the maximum heat removal amount that does not cause a shape defect, and the steel strip temperature on the exit side of the cooling roll 14 measured by the radiation thermometer 20 on the exit side is adjusted to the upstream side. The feedback control is performed by the gas jet 12 described above.

Here, in order to improve the usage rate of the cooling roll 14 having a high cooling efficiency and a low cost, a mechanism of the occurrence of a shape defect which has been studied diligently, and a model formula for providing an optimum indentation amount based on the analysis result are derived. Will be described.

When a cooling roll is used, it is known that when the cooling rate is too high, that is, when the cooling roll is pushed too much, a shape defect occurs. That is, according to the thermal buckling analysis by the finite element method, as shown in FIGS. 4A and 4B, when a sharp temperature gradient exists in the line direction, a large buckling occurs near the quenching start point A. The result that a waveform exists is obtained.

The reason will be described. FIG. 4C shows the thermal stress distribution in the quenching zone (starting portion) of the steel strip having a sheet width W.
As schematically shown in FIG. 2, in the quenching zone, shrinkage in the width direction occurs along with shrinkage in the line direction at all portions of the plate. In that case, only the displacement in the line direction due to the contraction in the line direction occurs at the width center, but at the width end, in addition to the displacement in the line direction, the displacement in the width center direction is larger than other parts, so the vicinity of the starting point Causes a large compressive stress in the width direction.

From the above, it is considered that the main cause of the shape defect generated by the chill roll is thermal stress generated by the temperature gradient in the strip (steel strip) in the line direction. From the above-mentioned mechanism of the shape failure, if the pushing amount of the cooling roll is gradually increased under the same conditions, the temperature gradient will increase, so that the compressive thermal stress generated at a certain pushing amount is critical. It is considered that the stress is reached and a strip shape defect occurs.

The main factor of the thermal stress is considered to be the plate width W together with the line-direction temperature gradient S in the strip due to the contact with the cooling roll. The compressive thermal stress is

(Equation 2) Can be represented by

The buckling of a flat plate is expressed by the following general formula:

(Equation 3) Is known to hold.

The occurrence of buckling (defective shape) is caused by σ
_X ≧ σ _B is a condition. Therefore,

(Equation 4) When this holds, buckling will occur.

From equation (3), it can be seen that the greater the temperature gradient, the wider the plate, and the thinner the plate, the higher the probability of buckling occurs.

The line-direction temperature gradient S and the indentation amount P are given by the following equations, respectively.

(Equation 5) Since it is known that there is a relationship, the following expression is obtained from the expression of the equal sign in the expression (3).

(Equation 6) Is obtained. Therefore, from this equation, the lower limit pushing amount P _B at which buckling may occur is:

(Equation 7) Given further from this equation, as shown the image in FIG. 5, the optimum depression depth P _S which gives the maximum heat removal amount (cooling rate) without shape defects occur, the following equation

(Equation 8) It can be estimated by The reason that such an optimum pushing amount exists is that if the pushing amount is larger than that, shape defects will occur, so that the contact area between the steel strip and the cooling roll decreases, and as a result, the heat removal amount decreases. It is thought that there is.

In order to verify this, an experiment was performed under the conditions of a specific plate thickness D, plate width W, and entry temperature _Tsi , and the results shown in FIG. 6 were obtained. As a result, the cooling rate (heat removal amount) is increased by pushing the cooling roll, and the point (A) at which the temperature gradient at which the compressive stress exceeds the critical stress that causes a shape defect is peaked, When it is pushed further, the heat removal decreases as shown at point (B). From FIG. 6, it is possible to confirm the existence of the optimum pushing amount.

In order to confirm the validity of the model formula (8) for calculating the optimum pushing amount, the line speed is changed when the sheet thickness, the sheet width, and the temperature on the cooling roll entrance side are constant. An experiment was conducted to determine how the optimum pushing amount was affected. Although not shown, it was confirmed that the experimental value of the optimum pushing amount also increased almost in proportion to the line speed.
Further, in order to see the effect of the sheet thickness on the optimum indentation amount, an experiment was performed with the sheet width, the line speed, and the cooling roll inlet side temperature kept constant, and a comparison with a model equation was performed. Similarly, a comparison between an experimental value and a model formula was performed to see the effect of the plate width, and the result in FIG. 8 was obtained.

From the above, it has been shown that the optimum indentation amount is almost proportional to the line speed, and similarly to the plate thickness and the plate width, and is also proportional to a constant power. It was confirmed that predetermined values were obtained as the constants A, B, and C in the equation (8). In addition, it was also confirmed that these constants were obtained as unique values according to the type of steel and equipment.

In this embodiment, as described above, based on the above findings, the sheet temperature T on the cooling roll entrance side actually measured by the radiation thermometer 20 on the entrance side of the cooling roll 14 shown in FIG.
_The optimal indentation amount P obtained by applying _si to the above equation (8)
In addition to adjusting the insertion amount of the cooling roll 14 to s, when the cooling roll delivery side sheet temperature measured by the delivery side radiation thermometer 20 deviates from the target temperature, the temperature deviation is set to 0. The feedback control is performed by the gas jet 12 located upstream.

Further, in the present embodiment, when a change point for changing the cooling condition for the steel strip S passes through the cooling roll 14, the change point specified by the change point detecting means (not shown) is tracked. The control for changing the pushing amount of the cooling roll 14 is performed. The case where the changed point is the steel strip position where the sheet thickness, the furnace speed (line speed), the inlet side sheet temperature, the strength, and the sheet width are changed will be described below.

As shown in FIG. 9A, in the case of a change point (weld point) where the plate thickness becomes thicker, the pushing amount is increased at the same time as the change point is passed, and the model thickness is changed with respect to the changed plate thickness. by adjusting to the optimum pressing amount P _S obtained from (8) it is controlled so that a change in the sheet temperature is as small as possible.
Conversely, in the case of a change that decreases as shown in FIG.
As shown in FIG. 7, the thinner the plate thickness, the more the optimum pushing amount P
_{Since S} becomes small, that is, shape defects easily occur,
The sheet temperature will change greatly, but give priority to the shape,
Before the changes are to pass through the cooling roll 14 (reach), control is performed to advance changed to the optimum depression depth P _S which is obtained for the thickness of the thinner.

As shown in FIG. 10 (A), in the case of a change point where the furnace speed becomes low, the pushing amount is reduced substantially in synchronization with the passage of the change point, and as shown in FIG. In the same manner, the control for increasing the pushing amount is performed in synchronization.

At this time, the change in the pushing amount of the cooling roll, that is, the moving speed V _P (m / s) of the cooling roll in the direction perpendicular to the surface of the steel strip is too fast or too slow with respect to the acceleration / deceleration rate of the furnace speed. If it is too long, the cooling amount by the cooling rolls becomes too large during the deceleration or acceleration of the furnace speed, causing buckling of the steel strip. A problem arises that cooling cannot be performed.

For this reason, the change speed of the furnace speed (change speed of the condition) when the furnace speed is changed, that is, the moving speed of the cooling roll (change speed of the optimal pushing amount) according to the acceleration / deceleration rate a (m / s ² ) ) Is preferably determined.

The optimum pushing amount P _s by the cooling roll is determined on the basis of the above-mentioned equation (8). Then, when accelerating or decelerating the furnace speed (line speed) V,
Differentiating equation (8) with time t gives:

(Equation 9) Becomes (9) where a dP _s / dt is the moving speed V _P, since dV / dt is acceleration factor a, based on the acceleration and deceleration rate a when the furnace speed changes, cooled with (9) roll it can be of determining the optimum moving speed V _P.

As shown in FIG. 11A, in the case of a change where the inlet side plate temperature becomes higher, the inlet side plate temperature T _{si is obtained} from the above equation (8).
Because more optimal push-in amount P _S decreases high, the passage of the changes at the same time (in synchronization), to reduce the pushing amount,
Feedback control is performed to increase the output (GJ output in the figure) of the gas jet 12 in order to adjust to the optimum pushing amount after the change and to suppress the rise of the outlet side temperature accompanying the adjustment.
Conventionally, the control for adjusting to the optimum pushing amount is not performed, and the pushing amount has been increased because it is necessary to cool down extra as the temperature rises.

As shown in FIG. 11B, the tensile strength (T
In the case of a change point that increases S), if the strength increases, the familiarity with the roll deteriorates. Therefore, it is necessary to change the relationship C in the above equation (8) to reduce the optimum pushing amount.
At this time, the pushing amount is changed to a value suitable for the strength after the change before passing the change point, and as a result, the rising sheet temperature is cooled by the feedback control by the gas jet 12.
Conventionally, the pushing amount has not been particularly changed.

As shown in FIG. 12, in the case of a change point where the plate width becomes large, as shown in FIG. 8, the change point passes because the larger the plate width is, the smaller the optimum pushing amount becomes. Previously, the changed sheet width W is changed to the optimum indentation amount obtained by applying the model formula (8), and the increase in the sheet temperature is feedback-controlled by the gas jet 12. Conventionally, if the width of the plate becomes wider, the amount of heat removal needs to be increased, so that the pushing amount is increased. Although illustration is omitted, in the case of a change point where the plate width becomes small, the optimum pushing amount becomes large, so that it can be changed to an appropriate value in synchronization with the passage.

According to the present embodiment described in detail above, the cooling roll may provide a large cooling rate necessary for producing a steel strip such as a high-strength steel sheet by continuous annealing.
Poor shape and buckling of the steel strip can be reliably prevented. Also, at this time, even when cooling a change point where the sheet thickness, the sheet width, and the conveying speed of the steel strip (furnace speed) change, the change point is tracked, and the pushing amount of the cooling roll is adjusted to the change point. , A steel strip (steel plate) having constantly stable physical properties can be obtained. Therefore, since the productivity of the steel strip can be improved and the economy can be improved, the product cost can be significantly reduced.

When the optimum pushing amount changes in accordance with a change in the condition of the steel strip S and the optimum pushing amount decreases, the condition change of the steel band reaches the cooling roll. It is preferable to change the pushing amount of the cooling roll to the optimum pushing amount for the steel strip after the condition change point before performing.

On the other hand, when the optimum pushing amount changes to the increasing side, after the condition change point of the steel strip reaches the cooling roll, the pushing amount of the cooling roll is changed to the steel band after the condition changing point. Is preferably changed to the above-mentioned optimum pushing amount.

Specifically, as the condition change of the steel strip S, for example, the thickness of the steel strip S at the connecting portion between the preceding steel strip and the succeeding steel strip,
When changing the sheet width and strength, the values change rapidly.
In some cases, such as when the inlet-side plate temperature changes abruptly, the value changes abruptly. In such a case, the plate thickness is changed from the thicker side to the thinner side, or the sheet width is changed from the smaller side to the larger side, or the strength is changed from the smaller side to the larger side. In the case of a sudden change from a low side to a high side, the optimum pushing amount changes to a small side. At this time, if the change in the pushing amount of the cooling roll is delayed, buckling occurs. Therefore, it is preferable to move the cooling roll to a position where the optimum pushing amount is reached before the change point passes through the cooling roll.

Conversely, changing from a thinner side to a thicker side,
When the plate width changes from the large side to the small side, or the strength changes from the large side to the small side, and furthermore, when the inlet side plate temperature rapidly changes from the high side to the low side, the optimum pushing amount is large. Change to the side. At this time, if the timing for changing the pushing amount of the cooling roll is too early, buckling occurs. Therefore, in this case, it is preferable to change the pushing amount after the condition change point of the steel strip passes through the cooling roll.

Even when the condition of the steel strip changes, if the change speed is small, the changing speed of the optimum pushing amount when changing the pushing amount of the cooling roll is changed to the changing speed of the condition of the steel strip. It is preferable to determine based on the change speed. In this case, even if the conditions of the steel strip change, the maximum heat removal by the cooling roll can be continuously obtained without disturbing the shape of the steel strip. For example, when the furnace speed is changed, a method of determining the moving speed of the cooling roll using the above-described equation (9) corresponds to this.

As described above, the present invention has been specifically described. However, the present invention is not limited to the above-described embodiment, and can be variously modified without departing from the gist thereof.

For example, the specific configuration of the cooling zone applicable to the present invention is not limited to the configuration shown in the above embodiment.
In particular, in the above-described embodiment, a case has been described in which the number of cooling rolls is one and the cooling roll on the upstream side is adjusted to the optimum pushing amount. However, the number may be one or three or more. At this time, it is sufficient that at least one cooling roll adjusts the pushing amount to the optimum pushing amount, and it is not always necessary to adjust the pushing amount. At least 1 for multiple cooling rolls
It is sufficient to adjust the amount to the optimum pushing amount, but it is more effective to increase the number of adjustments. In that case, it is better to determine the optimum pushing amount for each.

[0047]

As described above, according to the present invention,
In addition to using a cooling roll and a gas jet, the productivity can be improved and the product cost can be significantly reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an outline of a cooling zone applied to an embodiment according to the present invention.

FIG. 2 is a schematic perspective view showing characteristics of a cooling roll.

FIG. 3 is an explanatory diagram showing a pushing amount of a cooling roll.

FIG. 4 is an explanatory diagram showing thermal stress generated during rapid cooling of a steel strip.

FIG. 5 is a diagram showing the existence of an optimum pressing amount in an image.

FIG. 6 is a diagram showing actual measurement data of a relationship between an indentation amount and a heat removal amount.

FIG. 7 is a diagram showing actual measurement data of a relationship between an optimum indentation amount and a sheet thickness;

FIG. 8 is a diagram showing actual measurement data of a relationship between an optimum pushing amount and a sheet width;

FIG. 9 is a diagram showing characteristics of control with respect to a thickness change point;

FIG. 10 is a diagram showing characteristics of control with respect to a furnace speed change point.

FIG. 11 is a diagram showing characteristics of control for an inlet temperature change point and a tensile strength change point.

FIG. 12 is a diagram showing a characteristic of control with respect to a plate width change point.

[Explanation of symbols]

 S ... steel strip 10 ... entrance bridle roll 12 ... gas jet 14 ... cooling roll 16 ... exit bridle roll 18 ... temperature measuring roll 20 ... radiation thermometer 22 ... fixed roll

Claims (5)

[Claims] 1. A method for cooling a steel strip conveyed to a cooling zone of a continuous annealing facility, wherein the steel strip is cooled by spraying a cooling gas with a gas jet and then brought into contact with a cooling roll to cool the steel strip. Pressing the roll into the steel strip and adjusting the amount of contact between the cooling roll and the steel strip to adjust the pressing amount to the optimum amount that can obtain the maximum heat removal amount that does not cause shape defects in the steel strip, and A method for cooling a steel strip, wherein the steel strip temperature on the side is feedback-controlled by the gas jet. 2. The optimum indentation amount is substituted into a model equation obtained in advance by substituting the sheet thickness: D, the sheet width: W, the conveying speed: V, and the cooling roll entry side sheet temperature: T _si of the steel strip. The method for cooling a steel strip according to claim 1, wherein the calculation is performed. (3) When the model formula expresses an optimum pushing amount as P _S , The method for cooling a steel strip according to claim 2, wherein: 4. When the optimum pushing amount changes in accordance with a change in the condition of the steel strip, and the optimum pushing amount decreases, the condition changing point of the steel band reaches the cooling roll. Before, the pushing amount of the cooling roll is changed to the optimum pushing amount for the steel strip after the condition changing point, and when the optimum pushing amount changes to the increasing side, the condition changing point of the steel strip is changed to the cooling amount. 4. The method of cooling a steel strip according to claim 1, wherein after reaching the roll, the pushing amount of the cooling roll is changed to the optimum pushing amount for the steel strip after the condition change point. 5. The method according to claim 1, wherein when the optimum pushing amount changes in accordance with the change of the condition of the steel strip, the changing speed of the optimum pushing amount is determined based on the changing speed of the condition of the steel strip. The method for cooling a steel strip according to claim 1.