JPH09217126A - Method for preventing superannealing of steel strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily determine treating velocity and prevent overannealing by making it clear that the whole thermal conduction coefficient changes according to the velocity of a steel strip and incorporating it into a heat transfer model, at the time of reducing treating velocity from stationary state in a continuous annealing furnace. SOLUTION: A steel strip of D(mm) sheet thickness is continuously introduced into a soaking zone in a continuous annealing furnace of Tf ( deg.C) furnace temp. under the conditions of T1 ( deg.C) sheet temp. at the inlet of the soaking zone and V0 (m/min) treating velocity, and, from stationary continuous treatment at T2 ( deg.C) sheet temp. at the outlet of the soaking zone, the treating velocity of the steel strip is reduced. At that time, the treating velocity of the steel strip is regulated to a treating velocity V(m/min) satisfying inequality V>=V0 (a+b.V0 +c.D+d.T1 +e.Tf +f.T2 . In the inequality, the symbols (a) to (f) are constants fixed by the structure of a furnace, the material of a steel strip, and heat treatment conditions and are previously determined by the experiments with respect to the annealing furnace to be used. Further, this method is particularly suitable for use at the time when the measurement of the sheet temp. at the inlet and/or outlet of the soaking zone in the continuous annealing furnace is performed by using a temp. measuring roll.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for annealing a steel strip in a continuous annealing furnace, and more particularly to a method for preventing overannealing.

[0002]

2. Description of the Related Art In continuous annealing equipment (CAL) for heat-treating cold-rolled thin sheets such as automobile materials and tinplate materials, the maximum attainable temperature and soaking time of steel strip are strictly controlled in order to impart the required characteristics to the material. doing. For this purpose, for example:
To measure the temperature of the steel strip, a temperature-measuring roll is installed in the continuous annealing furnace, and a radiation thermometer is installed on the exit side of the soaking zone.

The temperature measuring roll usually has a temperature sensor embedded near the surface of the hearth roll and measures the temperature of the steel strip in contact with the roll. However, in the temperature measuring roll, since the temperature sensor does not directly contact the steel strip, there is a problem that it is difficult to follow a rapid change of the steel strip temperature, that is, the responsiveness is poor. Further, in general, the radiation thermometer is installed so that the temperature can be measured at a position where the steel strip is wound around the hearth roll and apart from the heating zone in order to prevent temperature measurement error due to vibration of the steel strip. The radiation thermometer itself can follow the rapid temperature change of the object to be measured, but when the installation position is far from the location where the rapid temperature change occurs, there remains a problem that the responsiveness deteriorates. Was there.

Even when the operation of the continuous annealing furnace is controlled by the above-mentioned temperature sensor having poor response, no particular problem occurs in the steady state. But unsteady state,
For example, there is a problem that the plate temperature of the steel strip becomes higher than the set temperature when the transport speed is changed, particularly when it is reduced or when there is no other choice but to suddenly decelerate due to equipment trouble. In such a situation, measurement with a temperature-measuring roll requires some time until the temperature sensor senses it, and with a radiation thermometer installed at a distant position, the steel strip does not move until the steel strip arrives at that position. It cannot be detected that the strip plate temperature has dropped and a rapid temperature change has occurred.

In particular, recently, there is a tendency to anneal at a high temperature in order to improve the productivity of CAL, and in a non-steady state, if the plate temperature of the steel strip rises, a situation in which the transformation point is exceeded easily occurs. In the case of cold-rolled thin plate, the mechanical properties deteriorate significantly when it is annealed beyond the transformation point. For this reason, in the operation managed by the temperature sensor having poor response, there is a possibility that the operation may not be recognized as abnormal. Further, even if the transportation speed is reduced, the temperature of the steel strip is not correctly recognized, so that there remains a problem that it is not possible to determine how far the deceleration can be performed.

[0006]

SUMMARY OF THE INVENTION In the continuous annealing furnace of the present invention, when the processing speed is reduced from the steady state, the processing speed capable of preventing over-annealing can be easily determined.
An object of the present invention is to provide a method for preventing over-annealing in continuous annealing of steel strip.

[0007]

Means for Solving the Problems The present inventor clarified that the overall heat transfer coefficient changes depending on the steel strip speed, as a result of detailed examination and analysis of the dynamic behavior of the steel strip temperature in the unsteady state, By incorporating this into a heat transfer model, it was found that it is possible to set the processing speed V from the measured value in the steady state so that the steel strip temperature at the time of deceleration does not exceed the upper limit temperature, and the present invention is realized. Configured.

That is, according to the present invention, in a continuous annealing furnace, a steel strip having a plate thickness D is continuously charged in a soaking zone at a furnace temperature T _f at a sheet temperature T ₁ at a soaking zone inlet and a processing speed V ₀ , In decelerating the processing speed of the steel strip from the steady continuous processing with the plate temperature T ₂ at the soaking zone outlet, the processing speed of the steel strip is calculated by the following formula V ≧ V ₀ (a + b · V ₀ + c · D + d · T ₁ + e · T _f
It is a method for preventing overannealing in continuous annealing of a steel strip, which is characterized in that a processing speed V satisfying + f · T ₂ ) is set. Here, V: processing speed after deceleration (m / min), V ₀ : steady-state processing speed (m / min), D: plate thickness (mm), T ₁ : plate temperature at the uniform temperature inlet (° C.) ), T ₂ : plate temperature at the soaking zone (° C), T _f : soaking zone furnace temperature (° C), a, b, c, d,
e, f: constants. Furthermore, the present invention is particularly suitable when the plate temperature at the soaking zone inlet and / or outlet of the continuous annealing furnace is measured with a temperature measuring roll.

[0009]

FIG. 1 shows an example of a continuous annealing furnace to which the method of the present invention can be suitably applied. The steel strip 7 is conveyed in the direction of the arrow in the figure and loaded into the continuous annealing furnace 8. The continuous annealing furnace 8 is divided into a heating zone 1, a soaking zone 2 and a cooling zone 3, and a steel strip 7 is supported and conveyed by a hearth roll 4. In the soaking zone 2, temperature measuring rolls 5 are installed at the entrance and the exit. A radiation thermometer 6 is installed on the exit side of the soaking zone 2. The temperature of the steel strip is measured by these temperature measuring rolls 5, 5
And the radiation thermometer 6 measures. Then, the steel strip is cooled to room temperature in the cooling zone 3 and wound up. In the steady state, the plate temperature of the steel strip is constantly measured by the temperature measuring rolls 5 and 5. The strip temperature T ₁ (° C.) of the steel strip at the soaking zone inlet and the strip temperature T of the steel strip at the soaking zone outlet measured in this way
₂ (° C), and further, the steel strip temperature when the processing speed is reduced from the steel strip thickness D (mm), the processing speed V ₀ (m / min), and the soaking temperature T _f (° C) However, the processing speed V (m / min) is set so as not to exceed the upper limit temperature of the steel strip temperature.

The processing speed V is set so as to satisfy the following expression (1). V ≧ V ₀ (a + bV ₀ + cD + dT ₁ + eT _f + fT ₂ ) ... (1) where V ₀ is the steady-state processing speed (m / min), D
Is the plate thickness of the steel strip (mm), T ₁ is the strip temperature of the strip at the soaking zone (° C), T ₂ is the strip temperature of the strip at the soaking zone (° C), and T _f is the furnace temperature of the heating zone. (℃), a, b, c,
d, e, and f are constants. Constants a, b, c, d, e, f
Is a constant that is determined by the furnace structure, the material of the steel strip, and the heat treatment conditions, and is determined experimentally for the annealing furnace used in advance. Insert the measured steady-state value into equation (1),
Calculate V. If the decelerated processing speed is equal to or higher than the calculated V value, even if the processing speed is decelerated from the steady state, the plate temperature of the steel strip does not exceed the upper limit value determined by the material of the steel strip.

Further, the method of the present invention prevents the deterioration of the material due to over-annealing, and also makes it possible to detect the occurrence of over-annealing even when the processing speed is reduced to a value not satisfying the formula (1). It is advantageous in terms of guarantee. In addition, although the case where the plate temperature of the steel strip at the soaking zone inlet and the outlet is measured by the temperature measuring roll has been described, the temperature is not necessarily limited to the temperature measuring roll. It can be used regardless of the response speed as long as it can accurately measure the plate temperature of the steel strip in the steady state.
A radiation thermometer may be used.

[0012]

EXAMPLE A continuous annealing furnace shown in FIG. 1 was used. In this continuous annealing furnace, in order to measure the heat history of the steel strip, temperature-measuring rolls 5 and 5 are installed on the inlet side and the outlet side of the soaking zone to measure the plate temperature, and at a position 11 m downstream of the soaking zone outlet. A radiation thermometer 6 is installed at. In this continuous annealing furnace, a steel strip (low carbon steel) having a plate thickness of 0.6 mm was subjected to a steady continuous treatment at a soaking temperature of 880 ° C. at a treatment speed of 180 m / min. From this steady continuous processing, it was necessary to reduce the processing speed due to equipment trouble. At this time, the measured steady state V ₀ (= 180 m / min), T ₁ (= 820
° C.), T ₂ (= 880 ° C.), was calculated from the T _f (= 945 ° C.) and D (= 0.6 mm) using (1), the deceleration possible processing speed V from equation (1). Note that the constants of the equation (1) are a = −1110.97, b = −0.18346, c
= -21.710, d = 0.031816, e = 0.5
3999 and f = 0.76725. Equation (1) is V
= 0.55V ₀ (93.6 m / min), and therefore, as an example, the processing speed is V = 94 m / min which satisfies the condition (1), and as a comparative example, the formula (1) is satisfied. The processing speed was reduced to V = 90 m / min. As described above, the steel strips processed at the reduced processing speed were subjected to the heat treatment, the microstructure observation such as the crystal grain size, and the measurement of the mechanical properties. As an example,
When the processing speed was reduced to V = 94 m / min, there was no change in crystal grain size or microstructure, no abnormal mechanical properties were observed, and it was not observed that the steel strip was annealed beyond the transformation point. It was On the other hand, as a comparative example, the processing speed V = 90 m
In the steel strip decelerated to / min, the crystal grain size became coarse, and it could be confirmed that the steel strip was annealed beyond the transformation point.

FIG. 2A shows the change in the temperature of the steel strip after the processing speed is reduced in the comparative example as a measurement result by the radiation thermometer 6 installed at the position shown in FIG.
Further, the change in the steel strip temperature (calculated value) at the soaking zone outlet after decelerating the processing speed for this comparative example is shown in FIG.
(B). Although the actual strip temperature of the steel strip rises above the transformation point, the strip temperature of the steel strip measured by the radiation thermometer 6 shows almost no change. Therefore, if temperature control is performed at a temperature measured far away from such a soaking zone, even if overannealing actually occurs, overannealing will occur as long as it is controlled by this thermometer. You're not doing it. The presence or absence of over-annealing cannot be detected sensitively. As a result, when it is shipped as a product, it causes problems such as rough skin (orange peel) during processing.

[0014]

According to the present invention, even when a thermometer having a high response such as a radiation thermometer cannot be installed on the outlet side of the soaking zone, the processing speed of the continuous annealing furnace is reduced without overannealing. The quality of the steel strip can be guaranteed reliably and reliably.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing an arrangement of equipment of a continuous annealing furnace.

FIG. 2 is a graph showing a change in steel strip temperature after decelerating the processing speed in the comparative example, showing (a) the measurement result of the steel strip temperature by the radiation thermometer 6 and (b) the soaking zone outlet. The change of steel strip plate temperature (calculated value) is shown.

[Explanation of symbols]

 1 heating zone 2 soaking zone 3 cooling zone 4 hearth roll 5 temperature measuring roll 6 radiation thermometer 7 steel strip 8 continuous annealing furnace

Claims (2)

[Claims] 1. In a continuous annealing furnace, a steel strip having a plate thickness D is continuously charged in a soaking zone at a furnace temperature T _f at a sheet temperature T ₁ at a soaking zone inlet and a processing speed V _{0, and} at a soaking zone outlet. When the processing speed of the steel strip is decelerated from the steady continuous processing with the plate temperature T _{2 of} No. 2, the processing speed of the steel strip is set to the processing speed V satisfying the following formula (1). Of preventing over-annealing in continuous annealing. Note V ≧ V ₀ (a + b · V ₀ + c · D + d · T ₁ + e · T _f + f · T ₂ ) ... (1) where V: processing speed after deceleration (m / min) V ₀ : steady state Processing speed (m / min) D: Plate thickness (mm) T ₁ : Plate temperature at soaking zone (° C) T ₂ : Plate temperature at soaking zone (° C) T _f : Temperature of soaking zone (° C) ° C) a, b, c, d, e, f: constants 2. The method for preventing over-annealing in continuous annealing of a steel strip according to claim 1, wherein the sheet temperature at the inlet and / or outlet of the soaking zone of the continuous annealing furnace is measured with a temperature measuring roll.