JPH05279831A - Method for controlling cooling of alloying furnace for hot-dip galvanizing - Google Patents

Abstract

PURPOSE:To control cooling conditions on the basis of the sheet temp. after cooling continuously detected by means of a turn roll and to obtain a galvannealed steel strip excellent in surface characteristics as well as in shape characteristics at the time of cooling a hot-dip galvanized steel strip after alloying treatment. CONSTITUTION:At the time of subjecting a plating layer, formed on the surface of a steel strip by immersing the steel strip into a hot-dip galvanizing bath, to alloying and to cooling by means of a succeeding alloying furnace for hot-dip galvanizing, the sheet temp. Tz of a galvannealed steel strip 15 is detected by means of a turn roll 10 disposed on the outlet side of a cooling zone 19 in the alloying furnace. The turn roll 10 is fitted with temp. detection block where plural thermocouples close to each other are embedded at respectively different depths. Cooling conditions in the cooling zone 19 are controlled so that the detected sheet temp. Tz becomes a value not higher than the zinc adhesion preventing temp. Tmax. Because the sheet temp. Tz can be detected with high precision, the galvannealed steel strip 15 can be cooled under proper conditions and a product excellent in shape characteristics and surface characteristics can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for highly accurately controlling cooling of an alloying furnace for hot dip galvanizing in order to produce high quality hot-dip galvanized steel strip.

[0002]

2. Description of the Related Art As a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet is known in which a galvanized layer is partially or wholly alloyed to form an Fe—Zn alloy layer. The galvanizing layer is alloyed by, for example, the equipment shown in FIG. The steel strip 1a which is a plating original plate is introduced into the hot dip galvanizing tank 2 after reduction treatment. The galvanized steel strip 1b goes around the sink roll 3 to form the galvanized bath 2
Then, the plating thickness control device 4 such as a wiper adjusts the plating thickness to a predetermined value.

Next, the plated steel strip 1b is sent to an alloying furnace 5 equipped with a heating zone 5a and a heat retaining zone 5b. The galvanized steel strip 1b heated by the heating zone 5a is maintained at the set temperature by the heat retaining zone 5b, and Fe is diffused from the base steel to the galvanized layer to form a uniform alloyed layer. Then, the semi-molten galvanized layer is cooled by the air cooling zone 5c and rapidly cooled by the water cooling zone 5d, and is fed from the alloying furnace 5 as the alloyed hot dip galvanized steel sheet 1c.

If the cooling in the water cooling zone 5d is insufficient, the viscosity of the galvanized layer on the surface of the alloyed hot-dip galvanized steel strip 1c sent from the alloying furnace 5 is still high, and the output of the alloying furnace 5 is increased. Zinc adheres to the surface of the turn roll 6 arranged on the side. The zinc adhering to the turn roll 6 is transferred to the subsequent galvannealed steel strip 1c and causes surface defects in the product. Also, the durability of the turn roll 6 itself is reduced. Conversely, when supercooling is performed in the cooling zone 5d, the alloyed hot-dip galvanized steel strip 1c is likely to be in a non-uniform cooling state in the width direction. As a result, cooling distortion becomes large and the product shape deteriorates.

[0005]

In order to prevent zinc from adhering to the turn roll 6 and obtain a product of excellent quality, the alloyed hot-dip galvanized steel strip 1c that contacts the turn roll 6 is obtained.
The plate temperature of the alloyed hot dip galvanized steel strip 1c is measured on the outlet side of the water cooling zone 5d so that the temperature of the sheet is lowered in advance to a temperature at which zinc does not adhere to the roll surface, and the water cooling zone is determined based on the measured value. It is necessary to control the cooling conditions in 5d.

An infrared radiation thermometer is used for measuring the plate temperature.
However, the emissivity of the surface of the hot-dip galvanized steel strip changes intricately depending on the plate temperature and the degree of alloying of the galvanized layer. Also,
The emissivity also varies depending on the gas near the surface of the steel strip. Therefore, it is difficult to detect the true plate temperature with an emissivity setting type infrared radiation thermometer which is usually used.

Therefore, in order to prevent zinc from adhering to the turn roll 6, a gas is jetted in the width direction from the inside of the turn roll 6 toward the surface of the alloyed hot-dip galvanized steel strip 1c and floated from the turn roll 6. A method of turning the galvannealed steel strip 1c in a non-contact state is adopted. It is also known to wind felt around the peripheral surface of the turn roll 6 to indirectly prevent zinc from adhering to the turn roll 6.

However, when the alloyed hot-dip galvanized steel strip 1c is turned while being floated, the shape of the galvanized steel strip 1b or the galvannealed galvanized steel strip 1c is stably passed inside the alloying furnace 5. In addition, a large gas pressure is required. Therefore, the size of the steel strip to be threaded is restricted, and the application target is limited to a relatively thin and narrow steel strip. Moreover, it becomes difficult to control the tension inside the alloying furnace 5.

On the other hand, when the turn roll 6 wound with felt is used, semi-molten zinc continuously adheres to the felt and penetrates into the felt. The zinc attached to the felt is deposited on the peripheral surface of the turn roll 6, and eventually the surface of the subsequent galvannealed steel strip 1c is flawed. Therefore, galvannealed steel strip 1
It is necessary to constantly inspect the surface condition of c, stop the line immediately when the occurrence of surface defects is detected, and replace the felt.

As described above, it is impossible to produce alloyed hot-dip galvanized steel strip having excellent surface quality under stable conditions by any of the methods.

The present invention has been devised in order to solve such a problem, and by incorporating a turn roll having a temperature detecting ability, the alloyed hot dip galvanized steel strip contacting the turn roll is provided. The purpose is to measure the temperature with high accuracy and to properly perform the cooling conditions in the water cooling zone.

[0012]

In order to achieve the object, the cooling control method of the present invention follows the plating layer formed on the surface of the steel strip by immersing the steel strip in a hot dip galvanizing bath. When alloying and cooling in an alloying furnace for hot dip galvanizing, a roll equipped with a temperature detection block that embeds multiple thermocouples that are close to each other at different depths is placed on the cooling zone side and detected by the roll. Based on the surface temperature of the galvanized steel strip, so that the surface temperature of the galvanized steel strip on the outlet side of the cooling zone is equal to or lower than the zinc adhesion prevention temperature, cooling control of the galvanized steel strip passing through the cooling zone may be performed. Characterize.

Based on the temperature information of the steel strip detected by the thermocouple, the cooling water injection pressure in the water cooling zone on the outlet side of the alloying furnace is controlled. Further, when a turn roll having a plurality of temperature detection blocks embedded in the width direction is used, the temperature distribution in the plate width direction can be accurately grasped. Therefore, it is possible to stabilize the shape of the galvannealed steel strip by controlling the cooling rate in the width direction of the water-cooled strip according to the temperature distribution in the strip width direction.

[0014]

[Operation] The inventors of the present invention accurately measure the plate temperature of the steel strip contacting the turn roll and control the cooling rate to prevent zinc adhesion to the turn roll and stabilize the shape of the steel strip. A method and a cooling control system were investigated.

When using a turn roll equipped with a temperature detection block having a plurality of thermocouples having different surface depths and as close to each other as possible in the longitudinal direction of the cylinder, alloyed molten zinc discharged from the alloying furnace is used. The plate temperature of the plated steel strip can be detected accurately and directly. In this respect, in the infrared radiation thermometer, the reflectance is greatly affected by the surface temperature, the degree of alloying, the gas in the vicinity of the surface, etc., and accurate temperature measurement cannot be performed.

The temperature information obtained by thermocouples embedded at different surface depths incorporates the effects of heat transfer from the galvannealed steel strip to the surface of the turn roll and heat conduction within the turn roll. .. Therefore, based on the temperature information inside the turn roll measured in reverse,
The plate temperature of the galvannealed steel strip can be calculated from the temperature gradient and its change with time. Unlike the measurement by the infrared radiation thermometer, the plate temperature thus obtained is a value that is not easily affected by a disturbance factor and has high reliability.

Therefore, when the cooling conditions in the water cooling zone are controlled based on this measured temperature, the sheet temperature of the galvannealed steel strip becomes exactly below the zinc adhesion preventing temperature, and the temperature distribution in the sheet width direction also. Can be made uniform. For example, if the plate temperature of the alloyed hot dip galvanized steel strip detected by the turn roll is below the predetermined zinc adhesion prevention temperature, but the temperature distribution in the strip width direction is not within the set range, each zone and strip of the water cooling strip The pressure of the water jet nozzles arranged in the width direction is selectively adjusted. Thereby, the cooling rate of the hot-dip galvanized steel strip is changed, and zinc adhesion to the turn roll is prevented. Further, it is possible to prevent the shape deformation of the alloyed hot-dip galvanized steel strip due to the non-uniform quenching by the cooling water.

The present invention will be specifically described below with reference to the drawings. The cooling control system according to the present invention includes a turn roll 10, a transmitter 11, a receiver 12, an A / D converter 13 and a computing device 14, as shown in FIG. The temperature information inside the turn roll 10 is obtained by the temperature detection mechanism shown in FIG. 3, FM-converted by the transmitter 11, transmitted, and received by the receiver 12. The received temperature information is converted into digital information by the A / D converter 13,
It is input to the arithmetic unit 14. 2 and 3, a wireless system using radio waves is adopted as a means for transmitting the temperature information obtained by the turn roll 10, but a slip ring system can also be used.

The computing device 14 computes the plate temperature T _z of the galvannealed steel strip 15 which comes into contact with the turn roll 10 on the basis of the inputted temperature information. The plate temperature T _z of the calculation result is the zinc adhesion prevention temperature T _{max in the} zinc adhesion determination device 16.
Compared to. When the plate temperature T _z is higher than the zinc adhesion prevention temperature T _max, it is determined that zinc adhesion to the turn roll 10 occurs. Then, in the cooling condition setting device 17, until the plate temperature T _{z in} the width direction of the alloyed hot-dip galvanized steel strip 15 becomes equal to or lower than the zinc adhesion preventing temperature T _max , cooling conditions such as cooling strengthening and a cooling pattern in the width direction are set. Calculate the control command to be adjusted. The control command is output to the spray pressure adjusting device 18.

The water cooling zone 19 of the alloying furnace is provided with a plurality of water jet nozzles 19a along the width direction of the traveling galvannealed steel strip 15. Each water injection nozzle 19a is connected to a cooling water supply source (not shown) via a pressure control valve 19b. Individual pressure control valve 1
A pressure control signal is input from the spray pressure adjusting device 18 to 9b. Therefore, cooling water having a pressure pattern controlled in the plate width direction is jetted onto the alloyed hot dip galvanized steel strip 15, and the alloyed hot dip galvanized steel strip 15 is cooled with a uniform temperature distribution in the plate width direction. ..

The temperature detecting mechanism 30 incorporated in the turn roll 10 has a plurality of thermocouples 31a, 3a as shown in FIG.
The temperature detection block 32 in which 1b is embedded is provided.
The detection terminal of the thermocouple 31a is arranged at a position shallower from the peripheral surface of the turn roll 10 than the detection terminal of the other thermocouple 31b. Further, the thermocouples 31a and 31b are provided at positions as close to each other as possible in the trunk length direction of the turn roll 10. These thermocouples 31a, 31b
When a plurality of temperature detection blocks 32 having the above are mounted on the circumferential surface of the turn roll 10 along the cylinder length direction, the temperature distribution in the plate width direction of the galvannealed steel strip 15 is detected.

The temperature detection block 32 is the turn roll 1
A plurality may be arranged also in the circumferential direction of 0. By arranging a plurality of pieces along the circumferential direction, the turn roll 10
Since the plate temperature T _z of the galvannealed steel strip 15 is detected at many measurement points during rotation, highly reliable measurement results can be obtained.

The temperature information detected by the thermocouples 31a and 31b is input to the transmitter 11 mounted on the side surface of the turn roll 10 via the compensating lead wire 33. Based on this temperature information, a predetermined pressure pattern of the cooling water is obtained by the cooling control system shown in FIG. 2, and the plate temperature T _{z of the} galvannealed steel strip 15 is uniformly distributed in the plate width direction.
Is reliably performed at a zinc adhesion prevention temperature T _maz or lower.

Next, a procedure for calculating the plate temperature T _z of the galvannealed steel strip 15 which comes into contact with the turn roll 10 based on the temperature information inside the turn roll 10 will be described. The depth from the peripheral surface of the turn roll 10 is X ₁ and X ₂ (X ₁
Consider the heat conduction with respect to two points <X ₂ ) and a point on the surface of the temperature detection block 32 that lies on the straight line. The surface temperature T _f of the turn roll 10 and the heat flux q on the roll surface of the turn roll 10 are represented by the following equations (1) and (2), respectively.

T _f = AT ₁ + BT ₂ + CT ₁ ′ + DT ₂ ′ (1) q = ET ₁ + FT ₂ + GT ₁ ′ + HT ₂ ′ (2) However, T ₁ ′ and T ₂ 'Is the _first derivative dt ₁ / dt and dt ₂ / dt of T ₁ and T ₂ with respect to time t, respectively.
Is. A to H are constants determined by the surface depths X ₁ and X ₂ and the material of the temperature detection block 32.

The plate temperature T _z of the galvannealed steel strip 15 when the turn roll 10 is in contact with the contact heat transfer coefficient between the galvannealed steel strip 15 and the turn roll 10 is a. _{When set to 1} , it is expressed by the following equation (3). T _z = T _f + q / a ₁ (3)

Therefore, based on (1) to (3),
Temperatures T ₁ and T ₂ at two points inside the turn roll 10
From this, the plate temperature T _z of the galvannealed steel strip 15 can be calculated. Note that three or more thermocouples may be embedded in one temperature detection block 32. In this case, since the number of measurement points inside the temperature detection block 32 is 3 or more, the plate temperature T _z of the galvannealed steel strip 15 is calculated according to the procedure described above for the temperature at any two points, and this calculation is performed plural times. Do it once. Thereby, highly accurate temperature information can be obtained.

The alloyed molten alloy obtained by the temperature detection mechanism 30.
Plate temperature T of lead-plated steel strip 15_z Is a zinc adhesion determination device 16
At zinc adhesion prevention temperature T_max Compared to. Plate temperature T_z Is
Lead adhesion prevention temperature T_max If it is lower than
continue. On the other hand, the judgment condition T _z <T_max If not satisfied
In this case, each water injection nozzle 19a arranged in the water cooling zone 19
Change the cooling water supply pressure to the pressure control valve 19b to a predetermined condition.
To change. And strengthen the cooling, plate temperature T_z Is the board width
Plate temperature T to be uniform in the direction_z Select the highest
Set cooling conditions such as forced cooling.

In FIGS. 2 and 3, the pressure control of the cooling water in the water cooling zone 19 is taken as the control target. However, the present invention is not limited to this, and it is also possible to cool the galvannealed steel strip 15 so as to have a predetermined temperature distribution by controlling the flow rate of the cooling water. Further, based on the plate temperature T _z of the alloyed hot-dip galvanized steel strip 15 which is also in contact with the turn roll 10, the air cooling strip 5 is
It is also possible to perform cooling control of c (see FIG. 1) or cooling control of both the air cooling zone 5c and the water cooling zone 19.

[0030]

EXAMPLE As the turn roll 10, the one equipped with a temperature detection block 32 in which thermocouples 31a and 31b were embedded at surface depths of 0.3 mm and 0.8 mm from the peripheral surface was used. The turn roll 10 is arranged on the exit side of the water cooling zone 19 of the alloying furnace, and the plate thickness is 0.4 mm and the plate width is 100 mm.
A 0 mm steel strip was passed at a line speed of 150 mm / min to produce an alloyed hot dip galvanized steel strip 15. In addition,
In order to prevent zinc from adhering to the roll surface of the turn roll 10, the plate temperature T _z of the galvannealed steel strip 15 at the time of contact with the turn roll 10 is 300 ° C.
It has been experimentally obtained that it needs to be below. That is, the zinc anti-adhesion temperature T _{ma x} is, 300 ° C.
Is set to.

According to the present invention, the plate temperature T _z of the galvannealed steel strip 15 which comes into contact with the turn roll 10 by the temperature detecting mechanism 30 mounted on the turn roll 10 is calculated as follows: When compared with the plate temperature directly measured by a thermocouple in contact with T _z , there was extremely high agreement as shown in FIG. 4 (a). On the other hand, the plate temperature T _z of the galvannealed steel strip 15 measured by an infrared radiation thermometer had large variations in the measured values as shown in FIG. 4 (b), and lacked reliability. .. Therefore, the plate temperature T _z obtained by the infrared radiation thermometer is unsuitable as a control factor used for cooling control.

FIG. 3 is a graph showing the time transition of the plate temperature T _z of the alloyed hot-dip galvanized steel strip 15 in contact with the turn roll 10 when cooling is controlled by the method of the present invention in comparison with the case where conventional cooling control is not performed. 5 shows. As a conventional method, a turn roll in which felt was wound around the roll peripheral surface was used.

As is apparent from FIG. 5, in the conventional method in which the cooling control is not performed, the plate temperature T _z of the galvannealed steel strip 15 is _{equal to} or higher than the zinc adhesion preventing temperature T _max, and it is wound on the roll surface. Zinc adhered and accumulated on the felt. Also, the plate temperature T _z of the galvannealed steel strip 15
Was not constant and fluctuated over a wide temperature range. Therefore, the obtained alloyed hot-dip galvanized steel strip 15 had uneven surface properties.

On the other hand, in the method of the present invention in which the turn roll 10 having the temperature detecting mechanism 30 is used for cooling control, the plate temperature T _z of the galvannealed steel strip 15 is set to the zinc adhesion preventing temperature 300. It could be stably suppressed to below ℃. Therefore, no zinc was observed on the roll surface of the turn roll 10. Further, the cooled alloyed hot-dip galvanized steel strip 15 has no temperature variation and the plate temperature T _z
The plate temperature T _z could be maintained in an extremely narrow temperature range just below the zinc adhesion preventing temperature T _max . Therefore, the water cooling zone 1
It was possible to stably perform the strip running in a state in which a predetermined tension was applied, without excessively increasing the injection pressure of the cooling water in No. 9.

According to the present invention, the plate shape of the alloyed hot-dip galvanized steel strip 15 controlled in cooling in the sheet width direction is compared with the plate shape of the alloyed hot-dip galvanized steel strip manufactured by the conventional method without cooling control. And shown in FIG. When the cooling control is not performed, the widthwise end portions of the alloyed hot-dip galvanized steel strip are likely to be cooled more rapidly than the central portion. As a result, the alloyed hot-dip galvanized steel strip has a medium-stretched shape in which it is excessively extended in the vicinity of the center in the sheet width direction, and the shape is deteriorated as shown in FIG. On the other hand, the alloyed hot-dip galvanized steel strip produced by the method of the present invention is uniformly cooled in the width direction of the sheet, and is thus a product having good shape characteristics.

[0036]

As described above, in the present invention, the turn roll equipped with the temperature detection block in which a plurality of thermocouples are embedded at different depths is brought into contact with the galvanized steel strip after the alloying treatment. By doing so, the plate temperature of the galvannealed steel strip is accurately and stably detected. The obtained temperature information is a highly accurate representation of the plate temperature of the galvannealed steel strip, which is free from the influence of disturbance and the like as compared with the measurement using an infrared radiation thermometer.

Therefore, based on the result of comparing the plate temperature of the galvannealed steel strip obtained from this temperature information with the zinc adhesion preventing temperature, while preventing zinc adhesion to the surface of the turn roll, In the cooling zone, the cooling rate of the galvannealed steel strip can be controlled with high accuracy. As a result, an alloyed hot-dip galvanized steel sheet having excellent surface properties and shape characteristics can be produced under stable operating conditions and at a high yield. Moreover, since the plate temperature of the galvannealed steel sheet can be controlled just below the zinc adhesion preventing temperature, the productivity of the continuous galvanizing line is improved.

[Brief description of drawings]

FIG. 1 Schematic diagram of continuous hot dip galvanizing line

FIG. 2 Cooling control system according to the present invention

FIG. 3 Temperature detection mechanism of a turn roll arranged on the water cooling zone outlet side

FIG. 4 Comparison of plate temperature measurement by the method of the present invention and an infrared radiation thermometer

FIG. 5: Comparison of plate temperature changes in the case where no cooling control is performed and when the cooling control is performed by the method of the present invention

FIG. 6 is a comparison of product shape characteristics when cooling control is not performed and when cooling control is performed by the method of the present invention.

[Explanation of symbols]

10 Turn Roll 15 Alloyed Hot Dip Galvanized Steel Strip 16 Zinc Adhesion Judgment Device 19 Water Cooling Zone of Alloying Furnace 19a Water Injection Nozzle 19b Pressure Control Valve 30 Temperature Detection Mechanism 31a, 31b Thermocouple 32 Temperature Detection Block

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Issei Nakamoto 1 Tsurumachi, Amagasaki City, Hyogo Prefecture Nisshin Steel Co., Ltd.

Claims (1)

[Claims] 1. When a steel strip is immersed in a hot dip galvanizing bath to alloy a cooling layer formed on the surface of the steel strip in a subsequent galvanizing alloying furnace and to be cooled, a plurality of steel strips that are close to each other are cooled. A roll equipped with a temperature detection block in which thermocouples are embedded at different depths is arranged on the cooling zone exit side, and based on the surface temperature of the plated steel strip detected by the roll, the plated steel at the cooling zone exit side A cooling control method for an alloying furnace for hot dip galvanizing, which comprises cooling and controlling the galvanized steel strip passing through the cooling zone such that the surface temperature of the strip is equal to or lower than the zinc adhesion preventing temperature.