JP2807918B2 - Combustion control method for continuous annealing furnace - Google Patents

Description

The present invention relates to a combustion control method for a continuous annealing furnace, and more particularly to a dummy made of ordinary steel between a preceding stainless steel strip and a following stainless steel strip having different annealing conditions. The present invention relates to a combustion control method for annealing with a steel strip interposed therebetween, and is widely used for continuous annealing of a cold-rolled stainless steel strip.

[Conventional technology]

In a continuous annealing line for a cold rolled stainless steel strip, it is necessary to pass through a wide variety of steel strips of various types and sizes in order to normally meet the various demands of customers. on the other hand,
In order to reduce the manufacturing cost, it is desirable to continuously produce steel strips having the same or similar steel types and the difference in annealing conditions such as the thickness and width of the product sheet material.

Therefore, in the continuous annealing line, the succeeding steel strip is usually selected at the rear end of the preceding steel strip so that there is no significant difference in the annealing conditions such as heat pattern, annealing temperature, thickness, width, steel type and emissivity. And performing continuous annealing is most desirable for cost reduction.

However, due to the delivery date and the like, the above relationship cannot be maintained, and often it is necessary to combine subsequent steel strips having significantly different annealing conditions.

In such a case, as shown in FIG.
And a subsequent steel strip C, a dummy steel strip B made of ordinary steel having a length of several hundred meters is interposed, and the furnace temperature of the continuous annealing furnace is postponed during the passing of the dummy steel strip B. At present, after the steel strip C is adjusted to the annealing condition, the subsequent steel strip C is annealed.

On the other hand, the continuous annealing furnace 1 usually used is composed of zones such as a heating zone, a soaking zone, and a cooling zone as shown in FIG. 2, and the zone temperature of each zone is set via the thermocouple 2 and the temperature control device 3. Control. That is,
When raising the temperature of the furnace for each zone, for example, increasing the amount of combustion gas 4 in the case of a direct fire type, increasing the input of thermal energy, and conversely, when lowering the furnace temperature, inputting energy. If the amount is reduced, or if the cooling is further rapid, the furnace temperature is adjusted to the target temperature for each zone by an operation such as charging a refrigerant.

The purpose of such temperature control of each zone is to control the steel strip of the processing material to a temperature value that matches the optimal heat pattern.In general, the material temperature is measured directly and the heat pattern is adjusted to the desired temperature value. It is desirable to perform annealing by controlling the input amount of thermal energy so as to be as follows. However, the annealing temperature of the stainless steel strip is as high as 900 ° C or higher, and the heat absorption rate of the steel strip surface is extremely low, about 0.3 to 0.4.
At present, a temperature measuring device for industrially accurately measuring the surface temperature of a conveyed stainless steel strip has not yet been developed.

Therefore, in continuous annealing of a stainless steel strip, a large number of data between a normal furnace temperature and a sheet temperature are obtained experimentally, and the correlation between the two is known, and the continuous annealing is applied to industrial heat treatment. That is, in the case where annealing is to be performed on the treatment material according to the target heat pattern and sheet temperature, the method is a method of controlling the furnace temperature to obtain a heat pattern and a sheet temperature suitable for the characteristics of the treatment material. According to this method, the annealing conditions at the switching point of the preceding steel strip A and the succeeding steel strip C without a significant difference in the annealing conditions are changed. That is, the temperature of each zone of the annealing furnace is adjusted to the temperature for the succeeding steel strip C or the passing speed is changed. According to this method, the gas input amount is changed by the variation difference of the plate thickness, the plate width, and the line speed, and the furnace temperature is controlled so as not to change.

However, as described above, the dummy steel strip B interposed between the preceding steel strip A and the succeeding steel strip C, which have significantly different annealing conditions, is used repeatedly, and since a normal steel sheet is used, the surface of the dummy steel strip B is repeatedly used. It is oxidized, the thickness of the scale gradually increases, the color of the surface turns black, and the heat absorption rate tends to increase.

On the other hand, the relationship represented by the following equation (1) is established between the heat quantity Q absorbed by the steel strip and the temperature T _S.

Where Q: Absorbed heat (Kcal / hr) K: Constant φ _CG : Overall heat transfer coefficient (corresponding to heat absorption rate) T _F : Furnace temperature (° C) T _S : Steel strip temperature (° C) Q _L : Furnace Body heat dissipation (Kcal / hr) As is clear from the equation (1), the steel strip temperature T _S becomes smaller and the temperature does not rise as the heat absorption rate φ _CG becomes smaller even with a constant heat input. Steel strip temperature as the reverse to φ _CG is large becomes large temperature rises. Therefore, from the preceding steel strip A to the dummy steel strip B
In addition, at the connection between the dummy steel strip B and the succeeding steel strip C, control for correcting the above-mentioned difference in absorbed heat has been performed. Regardless, assuming a saturated state, φ _CG = 0.6, for example, as shown in FIG. That is, as described above, a dummy material having a large number of times of use and a large scale thickness of the surface and having a black surface, and a dummy material having a small number of times of use and having a surface gloss close to that of a product stainless steel strip are controlled by _φCG = 0.6. Was. Therefore, from the dummy steel strip B to the succeeding product steel strip C
As shown in FIG. 4, the T _S of the succeeding product steel strip C according to the equation (1) rises and suddenly rises from 850 ° C. to 950 ° C. as shown in FIG.

As for stainless steel, for example, SUS430 has a transformation point near 900 ° C, so when the temperature rises to near 930 ° C, it must be made a scrap that can be used as a product, and the yield rate decreases and the cost increases.

[Problems to be solved by the invention]

The object of the present invention is that, because annealing conditions are significantly different, when performing continuous annealing with a dummy steel strip interposed between a preceding steel strip and a following steel strip, conventionally, regardless of whether the dummy steel strip is new or old. Effective combustion control that can control the overall heat transfer coefficient to be constant and always stably perform annealing at an appropriate annealing temperature in view of the fact that there is an accident that excessively raises the annealing temperature of the succeeding product steel strip. It is to provide a method.

[Means for solving the problem]

The gist of the present invention is as follows. That is, (1) A dummy steel strip made of ordinary steel is interposed between a preceding stainless steel strip and a subsequent stainless steel strip having different annealing conditions, and the furnace temperature of the continuous annealing furnace is set during the passing of the dummy steel strip. When adjusting to the annealing conditions of the subsequent stainless steel strip, the combustion control of the continuous annealing furnace is characterized by controlling the heat input amount during the subsequent stainless steel strip annealing according to the number of times the dummy steel strip is passed. Method.

(2) In the continuous annealing furnace, the change in gas input amount at the time of transition from the passing of the dummy steel strip to the succeeding stainless steel strip was Δ
If Q (Nm ³ / hr), ΔQ (Nm ³ / hr) = K ₁ (N) × (F ₂ −F ₁ ) (1) where K ₁ (N): Threading of dummy steel strip Factor determined by the number of times F ₂ : Product of thickness, width, and line speed of succeeding stainless steel strip F ₁ : Product of thickness, width, and line speed of dummy steel strip expressed by formula (1) The method for controlling combustion in a continuous annealing furnace according to (1).

The details of the present invention will be described with reference to the accompanying drawings. A dummy steel strip B is interposed between a preceding steel strip A and a succeeding steel strip C whose annealing conditions are significantly different, and the annealing conditions are adjusted from B to C during the passing of the dummy steel sheet B, and the subsequent steel strip C is adjusted. In the method of controlling the furnace temperature most suitable for the above, a dummy steel strip B which is conventionally passed through
As shown in FIG. 3, the overall heat transfer coefficient (Kcal / m ² hr) is assumed to be constant, for example, when φ _CG = 0.6.
The present inventors have found that the overall heat transfer coefficient of the dummy steel strip B varies depending on the threading circuit. That is, it has been found that the surface gloss of the new dummy steel strip B is close to that of the product stainless steel strip and the heat absorption rate is extremely small. For example, the total heat transfer coefficient when this is passed through several times to several dozen times and almost saturated and becomes constant
In view of the occurrence of an accident in which excessive high-temperature annealing is performed by controlling the furnace temperature during passing of the succeeding steel strip C as 0.6, the present invention starts from, for example, 0.3 at first, as shown in FIG. It was found that the density gradually increased and reached 0.6, which was almost saturated with 10 passes.

Therefore, if ΔQ (Nm ³ / hr) is defined as the increase in the gas input amount when shifting from the dummy steel strip B to the succeeding product steel strip C, the following equation (2) is established.

^{ΔQ (Nm 3 / hr) =} K 1 (N) × (F C -F B) ......... (2) Here K ₁ (N): coefficient by the number of uses connexion determined dummy steel strip B F _C: The product of the thickness, width, and line speed of the succeeding product steel strip C, F _B , as in the product of the thickness, width, and line speed of the dummy steel strip B, according to the number of passes of the dummy steel strip B By controlling the amount of gas input finely, even when using a new dummy steel sheet, it is possible to prevent excessively high temperatures at the connecting end of the succeeding product steel strip C, and perform annealing suitable for the steel strip steel strip C. We were able to.

〔Example〕

At the time of transition from the preceding steel strip A of the austenitic stainless steel SUS304 to the succeeding steel strip C of the ferrite stainless steel SUS430, the steel sheet was annealed in a continuous annealing furnace with a new ordinary steel dummy steel strip C having two passes.

At this time, the dummy steel sheet B was shifted to the succeeding steel strip C with the overall heat transfer coefficient φ _CG = 0.6 according to the conventional method, and the dummy steel sheet B was shifted to the following steel strip C with φ _CG = 0.3 according to the present invention. Examples will be described in comparison.

(A) According to the conventional method, at the time of transition from the dummy steel sheet B to the succeeding steel strip C for the purpose of annealing at 900 ° C., φ _CG = 0.6 and the gas opening degree, that is, as shown in FIG. Dummy steel strip B where the gas calorie was input with the input gas amount / maximum gas amount) = 95%
As shown in FIG. 4 (A), the temperature decreased to 850 ° C., increased to 950 ° C. at the beginning of the transition to the succeeding steel strip C, and finally returned to the target temperature of 900 ° C. after a few seconds. Approximately 50 m of the part annealed in step 2 had a temperature exceeding the transformation point of 910 ° C., and this part had to be cut and discarded as a poorly annealed product. The length discarded as defective annealing is the following steel strip C
Approximately 3% of the total length.

(B) a dummy steel strip B as phi _CG = 0.3 in the case were cowpea to the present invention method new dummy steel strip B
Fig. 5 (B) shows the gas opening at the time of transfer from
As shown in FIG. 5, since the temperature was 40%, the temperature of the dummy steel strip B was 895 as shown in FIG.
° C, but the steel strip C immediately after the transition to the succeeding product steel strip C
Although the temperature temporarily showed 905 ° C., it reached the target annealing temperature of 900 ° C. after a few seconds, no loss of defective annealing occurred, and stable operation was performed at the target temperature of 900 ° C.

Overall heat transfer coefficient phi _CG new dummy material as shown in Figure 3, initially a 0.3Mcal / m ³ hr, about 10 times through saturation coefficient value which has been conventionally adopted in plate 0.6 kcal / m ^Since it was found that the time reached ² hr, the new dummy material B was then changed in φ _CG value according to the number of passes, and moved to the succeeding steel strip C.
When annealing was performed while controlling the gas opening (%), the yield of non-defective products could be improved by about 3% as compared with the conventional method.

〔The invention's effect〕

During continuous annealing where the annealing conditions of the preceding stainless steel strip and the subsequent stainless steel strip are significantly different, a dummy steel strip made of ordinary steel is interposed between the two to fit the following steel strip during the passing of the dummy steel strip. In the continuous annealing method in which the furnace temperature is adjusted, the overall heat transfer coefficient of the conventional dummy material is controlled to be constant at its saturation value and the gas opening during the passing of the succeeding steel strip C is controlled. While searching for the cause of the occurrence of defective products due to the inappropriate annealing temperature immediately after the transfer to the band C, the overall heat transfer coefficient due to the difference in the surface gloss of the dummy material changes according to the number of times of passing, It has been found that there is a function based on the above equation (2), and a method of adjusting the gas opening at the time of transition and controlling the amount of heat input has been adopted. Defective products due to excessive furnace temperature rise that occurred immediately after the shift Tondo is eliminated, it is possible to stabilize operation by proper furnace temperature by narrow control decided, it was possible to improve the good yield of about 3%.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing a situation in which a dummy steel strip B is interposed between a preceding steel strip A and a succeeding steel strip C to pass a steel sheet, and FIG. 2 is a relationship between a continuous annealing furnace and a continuous steel strip. FIG. 3 is a diagram showing a comparison between the conventional method and the method of the present invention in the relationship between the overall heat transfer coefficient (Kcal / m ² hr) of the dummy steel strip and the number of times of threading, and FIG. Figures (A) and (B) show the test results of the conventional method in the comparison test between the present invention and the conventional method.
Is a diagram showing the furnace temperature change over time in the continuous annealing furnace of the succeeding product steel strip, and (B) is the annealing gas opening percentage (input gas amount / maximum gas) corresponding to the time elapsed in (A). 5 (A) and 5 (B) are respectively diagrams in FIG. 4 (A),
(B) shows a test result according to the present invention similar to (B), (A) is a diagram showing a change in furnace temperature over time in a continuous annealing furnace of a succeeding product steel strip, (B) is a time of (A) FIG. 7 is a diagram showing the annealing gas opening% (input gas amount / maximum gas amount) corresponding to the passage of time. A: Leading steel strip, B: Dummy steel strip C: Leading steel strip 1: Continuous annealing furnace 2, Thermocouple 3: Temperature control device 4, Combustion gas (fuel)

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Takashi Sato 2-88 Wakihama Kaigandori, Chuo-ku, Kobe-shi, Hyogo Kawasaki Steel Corporation Hanshin Works (56) References JP-A-61-117229 (JP, A) JP-A-60-70128 (JP, A) JP-A-62-89820 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C21D 9/52-11/00 105

Claims (2)

(57) [Claims] A dummy steel strip made of ordinary steel is interposed between a preceding stainless steel strip and a succeeding stainless steel strip having different annealing conditions, and the furnace temperature of the continuous annealing furnace is adjusted during the passing of the dummy steel strip. When adjusting the annealing conditions for the continuous stainless steel strip, a method of controlling combustion in a continuous annealing furnace, comprising controlling the heat input during the subsequent stainless steel strip annealing according to the number of times the dummy steel strip is passed. . 2. If the change in gas input amount at the time of transition from the passing of the dummy steel strip to the subsequent stainless steel strip in the continuous annealing furnace is ΔQ (Nm ³ / hr), ΔQ (Nm ³ / hr) = K ₁ (N) × (F ₂ −F ₁ ) (1) where K ₁ (N) is a coefficient determined by the passing circuit of the dummy steel strip. F _{2 is} a plate of the following stainless steel strip. The product of the thickness, the sheet width, and the line speed F ₁ : the product of the sheet thickness, the sheet width, and the line speed of the dummy steel strip The combustion control method for a continuous annealing furnace according to claim 1, which is represented by the formula (1).