JPH07138655A - Method for giving crown to hearth roll for steel strip continuous annealing furnace - Google Patents

Abstract

PURPOSE:To effectively prevent the development of meandering and heat buckling of a steel strip in a furnace by predicting thermal expansion in the radial direction of a hearth roll in the furnace over the whole length in the axial direction and giving a crown deducting the thermal expansion rate from a desired crown at the time of manufacturing the roll. CONSTITUTION:At the time of manufacturing the hearth roll, the crown calculated with the equation I is given to the roll. Further, the temp. distribution in the axial direction of the hearth roll in the furnace at the time of operating is predicted with the equation II. By using this method, designing the crown of the hearth roll, even in the developing condition of the thermal crown during hot-operation, the suitable crown can be obtd., and the yield and the quality of the product in a continuous annealing furnace can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for applying a crown to a hearth roll for a continuous annealing furnace for continuously heat treating a steel strip (strip) after cold rolling.

[0002]

2. Description of the Related Art In a continuous annealing furnace, a hearth roll is installed to stably convey the strip. The outer diameter of the hearth roll is changed along the axial direction of the hearth roll for the purpose of preventing the strip from meandering in the width direction during passing.

That is, as shown in FIG. 2, a convex crown 7 having an outer diameter that is large at the center of the roll and becomes smaller toward the end of the roll is provided, and a tension is applied to the strip 6 to generate a meandering correction force 8. It can be activated and the strip 6 can be centered automatically. When the convex crown 7 is too small, the meandering correction force 8 acting on the strip 6 is insufficient, causing meandering, causing the strip 6 to come off the hearth roll 5 and collide with the furnace wall, which causes a serious problem. On the contrary, when the convex crown 7 is applied excessively, a problem called a heat buckle occurs in which the strip 6 buckles in the width direction due to the excessive meandering correction force 8 and becomes a flaw. That is, an appropriate amount of crown design is required so that these two problems do not occur.

The roll crown that is mechanically applied in the cold state in advance is called an initial crown 9 (see FIG. 3). However, in the heating furnace, the end of the roll is heated by the heat from the furnace, The strip 6 removes heat from the central portion, and the roll shape changes due to the difference in thermal expansion. This is called a thermal crown 10 (see FIG. 3). That is, in the furnace, the hearth roll 5 is provided with “initial crown + thermal crown” (hereinafter simply referred to as “crown”). If the initial crown is not properly designed, in a severe case, it may become a concave crown as shown in FIG. 3, and conversely a force 11 that promotes meandering may be generated. Therefore, it is necessary to design an initial crown in consideration of the change of the crown due to the difference in thermal expansion and apply it to the roll.

For this reason, in the past, Japanese Patent Laid-Open No. 4-259330 has been used.
Predict the thermal crown at the plate edge,
The method of evaluating and making the sum of the amount of initial crowns and the amount of thermal crowns within a certain range is used.

[0006]

However, even if only the crown amount of a part of the roll such as the plate end portion is focused as in this method, the temperature changes rapidly in the axial direction between the plate end portion and the roll end portion. It is difficult to say that it is possible to solve the problem because there is a high possibility that it will become a concave crown in this part.

An object of the present invention is to provide a crown applying method capable of effectively preventing meandering and heat buckling by evaluating and predicting the thermal crown of the entire roll and designing a hearth roll. .

[0008]

The present invention comprises the following three inventions. (1) The thermal expansion in the radial direction of the hearth roll in the continuous annealing furnace is predicted over the entire length in the axial direction, and the amount of the crown obtained by subtracting the thermal expansion amount from the desired crown is applied at the time of manufacturing the hearth roll. Method of applying crown to hearth roll for continuous annealing furnace of steel strip.

(2) To the hearth roll for continuous annealing furnace of steel strip according to item (1), characterized in that a crown calculated by the following specified formula is added at the time of manufacturing the roll of the hearth roll for continuous annealing furnace. How to give crown. Yi _(x) = Y _(x) −α (r−T _(x) ) × R Yi _(x) : Crown amount to be applied to the roll by cold machining in advance Y _(x) : Formed in the furnace Amount of crown to be x: Axial coordinate from roll center α: Coefficient of thermal expansion r: Temperature at roll center T _(x) : Roll shell temperature at point x R: Roll radius

(3) The temperature distribution in the axial direction of the hearth roll in the furnace is predicted by using the formula specified below. Crown application method.

[0011]

[Equation 2]

[0012]

[Operation] If the axial temperature distribution of the hearth roll in the continuous annealing furnace is assumed to be a one-dimensional axial steady state, the heat balance equation becomes

[Equation 3] Can be described as This equation can be converted into a difference equation to calculate the temperature distribution T _(x) at an arbitrary point in the hearth roll axis direction by a computer.

On the other hand, if the temperature distribution is unsteady, the heat balance equation is expressed by the following equation.

[Equation 4]

If this boundary equation is converted into a differential equation and the hearth roll center is equal to the steel strip temperature on the roll, and the temperature distribution of minute increments in the roll axis direction is calculated moment by moment, an arbitrary time will elapse. The temperature distribution T _(x) in the axial direction of the hearth roll at any position can be obtained. Here, if the cumulative time of calculation is made sufficiently large (about 20 minutes), almost the same result as the formula was obtained.

Further, since the restraining force of the roll shell in the radial direction is sufficiently small compared to the rigidity of the roll and can be ignored, the amount of thermal expansion in the radial direction of the roll shell at an arbitrary position relative to the center of the roll, that is, Thermal crown amount Yt
_(x) can be expressed by the following equation in which the relative temperature distribution from the center is multiplied by the coefficient of thermal expansion α. Yt _(x) = α (r−T _(x) ) × D / 2 r: Temperature at the center of roll D: Roll diameter

Therefore, if the roll crown desired to be formed on the roll in the furnace is Y _(x) , the initial crown Yi _(x)
Is designed so that Yi _(x) = Y _(x) −Yt _(x) , a desired crown can be obtained over the entire roll in a state where a thermal crown is formed in the furnace.

As for the formula for calculating the temperature distribution in the roll axial direction, there is a method of analytically calculating in addition to the method of numerically obtaining a solution as in the present invention. When the temperature distribution is calculated analytically, it is necessary to linearly approximate the differential equation that describes the heat transfer state. On the contrary, when calculating numerically, linear approximation is not necessary, so more accurate calculation of heat transfer state is possible, but the numerical value calculated by another computer in advance is newly added to the CNC controller of the machine tool to calculate the trajectory of the cutting tool. It is necessary to go through the somewhat complicated procedure of inputting as coordinates.

However, when there is a large difference between the temperature of the steel sheet and the furnace temperature, such as in the case of a roll installed near the inlet side of the heating zone of the continuous annealing furnace, the absolute value of the thermal crown formed is large, and the error in the analytical solution also increases. Easy to grow. Therefore, when the temperature difference between the steel plate and the atmosphere in the furnace is large, it is advantageous to improve the accuracy by numerically and more accurately evaluating the thermal crown as shown in the present invention.

Here, the equation is an analysis of the thermal crown in the steady state, and the equation describes the thermal crown in the non-steady state. Although the formula is simpler and quicker to calculate, it is economical, but since the formula can analyze the thermal crown in an unsteady state such as when changing the plate size or changing the strip running speed, it is more It is useful when designing hearth roll crowns for continuous annealing furnaces for various types and small lots.

[0020]

Example A heat-resistant cast steel hearth roll having a diameter D of 800 mm, a body length L of 2300 mm, and a shell thickness of 25 mm was installed in the front stage of a heating furnace. At this time, the target crown had a flat portion length of 550 mm and the crown amount at the roll end portion was 1 mm.
Typical strip width of this continuous annealing equipment is 120
When the plate of this size was annealed at 820 ° C., the furnace temperature near the roll was 850 ° C., and the average plate temperature near the roll was 300 ° C.

First, the equations described in the specification were numerically analyzed by the difference method to calculate the temperature distribution in the roll axis direction. The boundary condition of the difference method is that the roll center temperature is equal to the steel plate temperature and the temperature gradient is zero. The result is shown by symbol 1 in FIG.
This result is in good agreement with the result of temperature measurement with an embedded thermocouple installed in the roll. The amount of thermal crown at that time is shown by symbol 2 in FIG.

Next, the amount of crown to be preliminarily applied to the roll in the cold was calculated using the formula described in the specification and applied to the hearth roll. The added crown is shown by symbol 4 in FIG.

Then, this roll is installed in the annealing furnace,
As a result of operating and measuring under substantially the same conditions as above, it was possible to obtain a crown that substantially matches the desired crown shown by symbol 3 in FIG. 1 over the entire hearth roll. At that time, the meandering amount of the steel plate was reduced to about half or less of the conventional amount, and as a result, the processing speed could be increased.

[0024]

EFFECTS OF THE INVENTION By designing a hearth roll crown in a continuous annealing furnace according to the present invention, an appropriate crown can be obtained even in the state where a thermal crown is generated during hot operation, and buckling or meandering of a steel sheet is prevented. It can be effectively prevented. As a result, the product yield and quality of the steel sheet in the continuous annealing furnace can be improved.

[Brief description of drawings]

1 is an embodiment of the present invention, the horizontal axis is the axial distance from the roll center, the upper row shows the axial roll temperature distribution, and the lower row shows the thermal crown, the target crown, and the initial crown, respectively.

FIG. 2 shows a perspective schematic view of a hearth roll.

FIG. 3 is a conceptual diagram of a roll having a concave crown generated by a thermal crown.

[Explanation of symbols]

 1 roll temperature distribution 2 thermal crown 3 target crown 4 initial crown 5 hearth roll 6 strip 7 crown (amount) 8 meandering correction force 9 initial crown 10 thermal crown 11 force that promotes meandering

Claims (3)

[Claims] 1. A method of predicting the radial thermal expansion of a hearth roll in a continuous annealing furnace over the entire axial length and applying a crown in an amount obtained by subtracting the thermal expansion from a desired crown at the time of manufacturing the hearth roll. A method of applying a crown to a hearth roll for a continuous annealing furnace, which is a characteristic steel strip. 2. A crown for a hearth roll for a continuous annealing furnace according to claim 1, wherein a crown calculated by the formula specified below is applied during roll manufacturing of the hearth roll for a continuous annealing furnace. Granting method. Yi _(x) = Y _(x) −α (r−T _(x) ) × R Yi _(x) : Crown amount to be applied to the roll by cold machining in advance Y _(x) : Formed in the furnace Amount of crown to be x: Axial coordinate from roll center α: Coefficient of thermal expansion r: Temperature at roll center T _(x) : Roll shell temperature at point x R: Roll radius 3. The method for applying a crown to a hearth roll for a continuous annealing furnace of a steel strip according to claim 2, wherein the temperature distribution in the axial direction of the hearth roll in the furnace is predicted by using an equation specified below. . [Equation 1]