JPS6315331B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a continuous annealing method for steel strips, and in particular, the technology described in this specification is a roll crown control method for suppressing meandering and heat buckling of hearth rolls in a vertical continuous annealing furnace. The following is a proposal on how to smoothly operate a continuous annealing furnace.

Prior Art Generally, continuous annealing of a steel strip imparts predetermined material properties by passing the steel strip under appropriate temperature conditions, threading speed, and tension. In the case of a continuous annealing furnace, particularly a vertical furnace, a large number of hearth rolls are installed in the upper and lower parts of the furnace, and the steel strip is passed through these hearth rolls. FIG. 1 shows an example of a typical vertical continuous annealing furnace. In the figure, 1 is a steel strip, 2 is a heating zone, 3 is a soaking zone, 4 is a slow cooling zone, 5 is a rapid cooling zone, and 6 is a hearth roll. While the steel strip 1 is conveyed from the heating zone 3 to the quenching zone 5 in the directions of arrows A and B, it comes into contact with a large number of hearth rolls 6 and is simultaneously maintained at a required temperature and speed and heat treated.

However, when a steel strip is heat treated in such a vertical continuous annealing furnace, the steel strip may shift relative to the line center depending on the shape of the steel strip, the tension balance in the furnace, or the temperature conditions. , so-called lateral displacement may occur. When this lateral shift becomes large, the steel strip meanderes, and furthermore, problems arise in that the edge portion of the steel strip is damaged or broken due to contact with the furnace wall.

Therefore, in order to prevent such lateral displacement, a hearth roll 6 with a crown as shown in FIG. 2 is generally used. Such a crowned hearth roll generates a force F _c acting toward the center in the width direction of the steel strip 1 as shown by the arrow, thereby correcting the meandering.

However, if this self-centering force F _c is too large, heat buckles will occur in the steel strip. This is because the steel strip buckles in the width direction near the center due to the self-centering force F _c directed from both sides of the steel strip toward the center, and this buckled portion is crushed by being wrapped around the hearth roll, causing plastic deformation. This is a defect caused by This phenomenon is particularly common in heating zones.

For this reason, proposals have been made to adjust the crown amount of the hearth roll itself, which has both an appropriate meandering correction ability and a heat buckling prevention ability. For example, Japanese Patent Application Publication No. 177930 (1982), Japanese Patent Application Publication No. 177930 (1983),
In the technology disclosed in No. 10546, the hearth roll is heated and
It is proposed that the roll crown be adjusted by cooling, and the method disclosed in Utility Model Application Publication No. 172359/1988 proposes that a bending device is provided to adjust the roll crown. However, in order to apply these known techniques to actual operations, a crown measuring device and a crown controlling device must be installed for each roll, which increases costs and requires delicate mutual adjustment between rolls. In reality, many problems remained.

Purpose of the Invention The purpose of the present invention is to provide a continuous annealing method that reliably suppresses heat buckling and meandering caused by excess or deficiency of crown amount, which cannot be achieved by conventional techniques. The gist of the invention is as stated in the claims at the beginning of this specification.

Structure of the Invention The hearth roll installed in a vertical continuous annealing furnace is normally low at the center where it contacts the steel strip and high at the ends. Figure 3 is an example of measuring the temperature distribution in the axial direction of a hearth roll.
The temperature in the center is approximately 60°C lower than in the edges.
A thermal crown occurs on the hearth roll due to the difference in thermal expansion that occurs with temperature as seen in this temperature distribution.

In short, the roll crown that affects the meandering of the steel strip and the occurrence of heat buckles must be considered in addition to the initial crown originally given to the roll itself, as well as the thermal crown mentioned above. Regarding crown control to suppress or prevent heat crown, there is no effective method other than adjusting the heat crown described above. Note that the total crown ΔD can be expressed as follows.

ΔD=ΔD _i +ΔD _t ... ≒ΔD _i +α・d・ΔT ...′ ≒ΔD _i +α・d・(T _RC - T _RE ) ...″ Here, ΔD _i : Initial crown ΔD _t : Thermal crown α: Coefficient of thermal expansion d: Plate thickness ΔT≡T _RC ―T _RE T _RC : Temperature at center of hearth roll T _RE : Temperature at end of hearth roll What actually influences the fluctuation of total crown ΔD is the above equation. Thermal crown inside ΔD _t
If the thermal crown is perfectly controlled, the meandering and heat buckling can be suppressed or prevented. For example, in the first half of the heating zone 2 of the annealing furnace, the temperature of the steel strip 1 is low, so the temperature T _RE at the end of the roll is much higher than the temperature T _RC at the center, and as a result, the total crown ΔD shown in the equation The self-centering force is weakened and meandering is more likely to occur. Therefore, the above-mentioned problem can be avoided by controlling the thermal crown ΔD _t .

In short, during annealing, the total crown ΔD, especially the thermal crown ΔD _t , may be controlled within the range of the upper limit crown ΔD _H where heat buckles do not occur and the lower limit crown ΔD _L where meandering does not occur.

ΔD _L ΔDΔD _H ……This means that the temperature difference ΔT between the roll end and the center, which causes thermal crown, is T _L ΔT=T _RC ―T _RE T _H …… T _L ……lower limit temperature (meandering The problem can be solved by setting the upper limit temperature (crown amount for suppressing heat buckling) T _H ....

As a result, in thermal crown control, the present invention develops a thermal crown setting model formula for the steel strip temperature, the hearth roll center and both end temperatures using the furnace temperature, heat treatment rate, and plate thickness as parameters, based on actual measurements. A continuous annealing method for a steel strip, characterized in that the temperature in the furnace and the heat treatment rate are determined while successively correcting the values to fall within the range of the upper limit crown amount for suppressing heat buckling and the lower limit crown amount for preventing meandering, and the heat treatment is performed continuously. It is.

FIG. 4 is a flowchart of the method of the present invention. First, the dimensions and settings (target) of the steel strip to be heat treated.
The steel strip temperature is determined. Here, the predicted steel strip temperature
T _S is, T _S ≒ a ₁ + T _G + a ₂・V・B + a ₃ ... Here, T _G : Furnace temperature V: Heat treatment rate B: Plate thickness a ₁ ~ a ₂ : Coefficient Also, setting (target) Hearth roll end temperature T _RE
And regarding the hearth roll center temperature T _RC , T _RE ≒b ₁・T _G +b ₂・V・B+b ₃ ... T _RC ≒C ₁・T _G +c ₂・V・B+C ₃ ... However, b ₁ ~b ₃ and c ₁ ~c ₃ can be expressed by a model formula consisting of coefficients.

Next, measure the temperature T _G ′ of the steel strip and find the actual value
It is determined whether T _G ′ is within the allowable range of the set steel strip temperature using the formula.

T _SL T _S ′T _SH Where, T _SL : Lower limit temperature (Meandering prevention crown amount) T _SH : Upper limit temperature (Heat buckling suppression crown amount) T _S ′ : Set steel strip temperature And, if the judgment result formula is satisfied. If not, the model formula is corrected by changing the coefficients a ₁ , a ₂ , and a ₃ of the formula, and the furnace temperature T _G and heat treatment rate V are reset and annealing is performed.

As a result of the determination, if the formula is satisfied, then the temperatures at the ends and center of the hearth roll are actually measured, and it is determined whether these measured temperatures satisfy the above formula,
If it is satisfied, the operation will continue, but if it is not satisfied, the coefficients b ₁ , b ₂ and c ₁ , c ₂ , c ₃ of the equation will be changed.
and the modified model formula,
Then, the furnace temperature T _G and speed V are set again and the heat treatment is continued.

It is best to modify each of the above coefficients by learning control. For example, if the coefficient to be modified is a, based on the following formula, a _o+1 = αa _o + (1-α) a _o-1 ... 0<α<1 By adopting the method explained above, the difference between the predicted steel strip temperature T _S ′ and the predicted temperatures T _RE and T _RC at the end and center portions of the hearth roll and these actual values is reduced by learning control. Therefore, by controlling the furnace temperature T _G and the processing speed V, it is possible to indirectly control the roll crown without impairing predetermined material properties.

Next, an example of the apparatus according to the present invention will be explained based on FIG. Reference numeral 1 denotes a steel strip, and 6 a hearth roll. The hearth roll has thermometers embedded in at least two locations, one at the end and the center (the thermometer is not shown). 7 is an annealing specification generator, 8 is a computing unit, and 9 is a furnace condition setting device. Further, 10 is a steel strip temperature determination device, 11 is a hearth roll temperature determination device, 1
2 is a learning device. And 13 is a steel strip thermometer,
14 is an output device. Based on the steel strip dimensions and set (target) steel strip temperature inputted from the annealing specification generator 7, the arithmetic unit 8 calculates the heat treatment rate V and the furnace temperature T _G using the formulas ~ and outputs them to the setting device 9. Based on this, the continuous annealing operation will be performed, but on the other hand, a setting completion signal is sent to the steel strip temperature determination device 10.
Output to. Next, the steel strip temperature determination device 10 takes in the steel strip temperature from the thermometer 13 and performs determination using a method. If satisfied, a signal is output to the hearth roll temperature determiner 11. If it is not satisfied, the signal is output to the learning device 12, where the learning device 12 corrects the coefficients a ₁ to _{a 3} of the equation based on the equation, sends the correction result to the arithmetic unit 8, and repeats the above procedure. become. On the other hand, if the formula is satisfied, the hearth roll temperature determiner 11 that receives the signal takes in the measured temperatures at the ends and center of the hearth roll, and performs a determination based on the formula. If the conditions are satisfied, the operation continues without changing the furnace settings; however, if the conditions are not satisfied,
The learning device 12 corrects the coefficients b ₁ , b ₂ , b ₃ and c ₁ , c ₂ , c ₃ of the equation. Next, the result is sent to the arithmetic unit 8, and the above procedure is repeated again.

Note that the output device 14 is, for example, an alarm device such as a buzzer, and may be configured to sound when the roll temperature does not satisfy the formula.

Example In continuous annealing, a model equation related to heat crown with "furnace temperature T _G , processing speed V, and plate thickness" as parameters, and an example in which operation was performed while modifying the equation by learning control will be shown.

First, FIG. 6 shows the relationship between the furnace temperature T _G and the hearth roll end temperature T _RE , and gives an example of the model equation. Further, FIG. 7 shows the relationship between the roll center temperature T _RE and the product of the heat treatment rate (V) and the plate thickness (B), and gives an example of the above model formula. Furthermore, FIG. 8 shows the relationship between the furnace temperature T _G and the product of the heat treatment rate (V) and the plate thickness (B), and is a specific example of the operation of the above model formula.

Figure 9 shows the relationship between the temperature difference ΔT=T _RC -T _RE and meandering and heat buckles.If tension fluctuations are not considered, the temperature difference ΔT is approximately -30ΔT.
Smooth continuous annealing is possible if the temperature is controlled within a temperature range of 30°C.

Effects of the Invention According to the present invention, the above-mentioned model formula (~)
By continuing operations while sequentially correcting the problems, for example, yield deterioration due to heat buckling can be reduced to 1.5%, and uneven breakage due to meandering can be reduced to 2.4%.
The occurrence frequency can be reduced from 0.5 times/month to 0.5 times/month, making it possible to operate the so-called continuous annealing furnace smoothly.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of a vertical continuous annealing furnace, Fig. 2 is a front view of a hearth roll, Fig. 3 is an axial temperature distribution diagram of the hearth roll, and Fig. 4 is a flowchart of the method of the present invention. , FIG. 5 is a block diagram for explaining the method of the present invention, FIG. 6 is a graph showing the relationship between the temperature inside the furnace and the temperature at the end of the roll, and FIG. Figure 8 is a graph showing the relationship between core temperature, Figure 8 is a graph showing the relationship between furnace temperature and the product of speed and plate thickness, and Figure 9 is a graph showing the relationship between temperature difference, meandering, and heat buckle generation. be. 1... Steel strip, 2... Heating zone, 3... Soaking zone, 4
... slow cooling zone, 5 ... rapid cooling zone, 6 ... hearth roll, 7 ... annealing specification generator, 8 ... computing unit, 9 ...
... Furnace condition setting device, 10 ... Steel strip temperature determination device, 11
...Hearth roll temperature judger, 12...Learner,
13... Steel strip thermometer, 14... Output device.

Claims (1)

[Claims] 1. In a method of continuously annealing a steel strip in a vertical continuous annealing furnace under the roll crown control of a hearth roll, the temperature of the steel strip, the central part and both ends of the hearth roll, using the furnace temperature, heat treatment rate, and plate thickness as parameters. The temperature inside the furnace and the heat treatment rate are determined while sequentially modifying the thermal crown setting model formula for each temperature so that the measured values fall within the range of the upper limit crown amount for heat buckling suppression and the lower limit crown amount for meandering prevention. A continuous annealing method for steel strip, characterized by heat treating the steel strip.