JPH0261009A - Continuous annealing furnace for steel strip - Google Patents

Abstract

PURPOSE:To eliminate the development of thermal deformation and pickup flaw in a steel strip and to enable high speed annealing by setting tension developing devices at inlet and outlet sides of a furnace and arranging a guide device for passing the steel strip under non-contacting with gas injection at a turning part. CONSTITUTION:In the vertical furnace parallel setting preheating zone 1, heating zone 2, soaking zone 3 and cooling zone 4, deflector rolls 5 are set between the preheating zone 1 and the heating zone 2. The turning part is arranged between the soaking zone 3 and the cooling zone 4 and gas injection toward radius direction is executed to the steel strip with the guide device 6. Tension bridle rolls 8a, 8b are arranged at the inlet side of the preheating zone l and the outlet side of the cooling zone 4 to give the steel strip the tension. Th. guide device 6 is composed of plural divided chambers 13 connected to each piping 12 from a circulating fan 11 and the gas sent with circulating fan 11 is injected to the steel strip passing the turning part 7 from nozzle hole 14 formed in each divided chamber 13.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a continuous annealing furnace for steel strips, and particularly aims to prevent thermal deformation and pick-up flaws in steel strips while increasing the speed of annealing. It is something to do.

(Prior Art) A steel strip, such as a stainless steel strip, after cold rolling is subjected to an annealing treatment. The annealing furnace used here is typically a direct-fired continuous annealing furnace as described in Japanese Patent Publication No. 50-28048.

That is, as shown in FIG. 7, the annealing furnace 21 has a preheating zone 22,
It is composed of a heating zone 23, a soaking zone 24, and a cooling zone 25, and a hearth roll 26 is provided on the exit side and inside the annealing furnace 21.
A plurality of steel strips S are installed, and a direct-fired burner 27 as a heating means and a cooling header 28 as a cooling means are installed above and below the steel strip S to be threaded. Then, the steel strip S is moved in the direction of the arrow while catenary supported by the hearth roll 26, and is subjected to a predetermined heat treatment by the heating/cooling means described above.

However, the steel strip S is oxidized in the furnace, and oxide scale is generated on its surface. This oxidized scale adheres to and grows on the hearth roll and causes scratches, so-called pick-up scratches, on the steel strip.

These pick-up flaws are more likely to occur as the steel strip is conveyed at a higher speed, so the hearth roll must be replaced more frequently, which reduces productivity.

In addition, in this type of furnace, measures are taken to prevent pick-up scratches, such as applying asbestos rolls (internally water-cooled) to the hearth rolls, and reducing the number of rolls as much as possible and supporting the steel strip in a catenary. As long as hearth rolls are used, pick-up scratches cannot be avoided, and since the hearth rolls are internally water-cooled, there is a disadvantage that heat loss is large and temperature unevenness occurs in the steel strip.

To solve these problems, Japanese Unexamined Patent Application Publication No. 1986-
There is one described in Japanese Patent No. 163219. That is, as shown in FIG. 8, it has been proposed to support the steel strip S in a non-contact manner using a floating support device (floater) 29 instead of the hearth roll. The floater 29 supports the steel strip by blowing gas from its nozzle onto the steel strip using a blower 30 and using the pressure generated at that time.

Further, the furnace shown in FIG. 9 is a one-bath oven type furnace in which the hearth roll is excluded from the furnace.

Furthermore 1. The furnace of the type shown in Fig. 10 has heating sections for preheating, heating, and soaking zones and a cooling zone vertically arranged side by side, and the steel strip S is made into a free loop between deflation crawls 31 and rolled from inside the furnace. are excluded.

(Problems to be Solved by the Invention) In the direct-fired continuous annealing furnace for steel strips described above, pick-up scratches do not occur at all because there are no hearth rolls in the furnace, but the following problems remain. . First, in the furnace shown in Fig. 8, multiple floaters must be installed, which increases the equipment cost, and since the weight of the steel strip is supported by the ejected gas, a large-capacity blower is required. Running costs will also be very high. Furthermore, there is a problem that the flying height is not sufficient and it cannot be applied to thick objects.

Furthermore, since the furnace shown in Figure 9 has a vertical configuration from the pre-cooling zone to the cooling zone, the furnace height is extremely high, increasing construction costs. It is difficult to increase processing power.

In the furnace shown in FIG. 10, the steel strip is threaded in a free loop in the furnace, that is, no tension is applied to the steel strip, and it hangs down in a loop due to its own weight. Therefore, the steel strip during threading is very unstable and prone to vibrations, etc., and this phenomenon is especially noticeable when the conveyance speed of the steel strip is increased (the higher the conveyance speed of the steel strip is, the more noticeable it becomes, causing problems such as breakage of the steel strip and poor heat treatment. In other words, there is a limit to the conveyance speed, and in reality the conveyance speed is at most 2
0 m/min, and no improvement in production capacity could be expected.

Furthermore, in each of the furnaces shown in FIGS. 9 and 10, there is a problem in that a deformation phenomenon of the steel strip, so-called canoeing, occurs.

Canoeing is a type of shape defect in which a steel strip warps in the width direction of the steel strip.The cause of this is not yet clear, but it is thought to be as follows: In other words, in the soaking zone, thermal strain occurs in the strip width direction due to thermal expansion, while in the cooling zone, thermal strain occurs due to thermal contraction. It is thought that a large relative thermal strain occurs, resulting in warping in the width direction.

This canoeing hardly occurs in the furnaces shown in FIGS. 7 and 8 above. This is thought to be because even if canoeing occurs, the bending is corrected by the hearth roll or steel strip flotation device. In the furnace shown in Figure 9, canoeing occurs because there is no in-furnace straightening means.
In addition, in the furnace shown in Figure 10, there is a bending section at the bottom of the furnace, but at this lowest point, the steel strip's own weight and tension are nil, and there is almost no straightening effect, so force noing occurs, and the conveying speed also increases. As the temperature rises, the temporal fluctuations between the soaking zone and the cooling zone become more pronounced, and canoeing becomes more pronounced.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a continuous annealing furnace for steel strips that is free from thermal deformation of steel strips and generation of pick-up flaws, and is capable of high-speed annealing.

(Means for Solving the Problems) The present invention provides a vertical furnace with at least two baths, which is equipped with a turning section for changing the direction of steel strip threading, and a tension generator is installed on the entrance and exit sides of the furnace. This is a continuous steel strip annealing furnace in which a guide device is installed in the turning section to guide the non-contact threading of the steel strip by gas injection. It is advantageously suitable for a furnace in which a heating zone is followed by a soaking zone, the rear half of the furnace is configured as a cooling zone, and a preheating zone is provided on the entry side of the heating zone, through which the steel strip is passed from bottom to top.

In addition, during implementation, a deflector roll is installed inside or outside the furnace between the preheating zone and the heating zone to change the direction of steel strip threading from top to bottom, and a guide device is installed to prevent the ejected gas from being exposed to the atmosphere or from the preheating zone. Preferably, combustion exhaust gas is used.

(Function) The continuous annealing furnace for steel strip according to the present invention eliminates the hearth roll from inside the furnace, and realizes non-contact threading by gas injection at the turning part of the steel strip, so that the steel strip surface is Even if oxide scale is formed, no pick-up flaws will occur.

In addition, tension generators are installed on the inlet and outlet sides of the furnace, and a non-contact guide device is installed inside the furnace, so a predetermined tension can be applied to the steel strip. It is possible to achieve stable annealing treatment without meandering even when heat treatment is performed at high speed.

In addition, the non-contact guide device is like a conventional floater,
Since there is no need to support the steel strip's own weight, the amount of gas to be ejected, gas pressure, and electric power are also much lower.

Furthermore, since the steel strip can be bent and straightened at the turning point between the soaking zone and the cooling zone, canoeing can be avoided.

As mentioned above, the continuous annealing furnace according to the present invention is effective in heat treatment at high speeds and high temperatures, and is therefore particularly suitable for continuous annealing of stainless steel strips.

(Example) A continuous annealing furnace according to the present invention will be specifically described below with reference to FIG.

The illustrated example is a direct-fired continuous annealing furnace, and is a three-pass vertical annealing furnace in which a preheating zone 1, a series of heating zones 2, a soaking zone 3, and a cooling zone 4 are arranged in parallel.

A deflector roll 5 is installed between the preheating zone 1 and the heating zone 2 to guide the threading direction of the rope S from the top to the bottom after leaving the preheating zone 1. In between, a guide device 6 injects gas in the radial direction to the steel strip S.
A turning section 7 is provided to force the steel strip S to pass in a U-shape, and tension bride rolls 8a and 8b are installed on the entrance side of the preheating zone 1 and the exit side of the cooling zone 4 to apply tension to the steel strip S. It becomes.

That is, the first pass is a preheating zone 1, the second pass is a heating zone 2 and a soaking zone 3, and the third pass is a cooling zone 4 with the turning section 7 as a boundary.

Note that the preheating zone 1 may be omitted, and heating may be performed using a radiant tube.

The steel strip S is first passed through the pre-heating zone 1 from bottom to top in the direction of the arrow, and when it exits the pre-heating zone 1, the steel strip S is guided downward by the deflector roll 5, and then passed through the heating zone 2 and the equalizing zone. The sheet is passed through the tropical zone 3 from top to bottom, then the direction is changed at the turning section 7 at the hearth bottom, and the sheet is passed through the cooling zone 4 again from bottom to top.

Note that 9 is a burner, 10 is a cooling header, and 11 is a circulation fan for sucking gas ejected from the guide device 6 and ejecting it from the guide device 6 again.

As shown in FIG. 2, the guide device 6 consists of a plurality of compartments 13 to which piping 12 from a circulation fan 11 is connected, and the air is sent by the circulation fan 11 from a nozzle hole 14 formed in each compartment 13. The gas is ejected toward the steel strip S passing through the turning section 7.

The gas ejected by the guide device 6 is set to a pressure corresponding to the tension applied to the steel strip S.

The steel strip S is a tension pry roll 8a.

8b and the guide device 6 to apply a predetermined tension to achieve tight threading, and while moving through the furnace, preheating, heating, and soaking are performed in the front half of the furnace, bordering on the turning section 6. The material is rotated 180 degrees while being pressed downward by the gas ejected from the guide device 6, and then cooled to a temperature of 100 to 150'C or less before exiting from the furnace.

Since the temperature of the steel strip S outside the furnace is sufficiently low, there is almost no chance that oxide scale will adhere to or grow on the roll and generate pick-up flaws.

In the case of stainless steel strips, as shown in Fig. 3, if the steel strip temperature exceeds 350'C, scratches will increase rapidly when the steel strip comes into contact with the deflation crawl, so the temperature of the steel strip as it passes through the deflation crawl increases. It is preferable that the temperature is 350"C or less. Also, since the steel strip leaving the soaking zone 3 has reached a temperature of 900 to 1200"C and has already been subjected to soaking treatment, the gas ejected from the guide device 6 A cooling effect may be provided by using room temperature air, or a low concentration of 02 (2-5%
) The combustion exhaust gas may be guided and ejected from the guide device 6 (see the chain line in FIG. 1), and it is also possible to attach a cooler midway through the pipe 12 to the guide device 6.

Next, continuous annealing of a stainless steel strip using the continuous annealing furnace shown in FIG. 1 will be described.

FIG. 4 shows the results of annealing a 5US304 cold-rolled steel strip having a thickness of 1 mm and a width of 1000 mm at a soaking temperature of 1100° C. and a soaking time of 20 seconds. For comparison, the results of a similar annealing process using the conventional furnace shown in FIG. 7 are also shown in FIG. 4. The figure shows the conveyance speed on the horizontal axis and the flaw occurrence rate on the vertical axis.
The black circles show the results when using the conventional furnace, and the white circles show the results when using the furnace according to the present invention. From the figure, it can be seen that when the furnace according to the present invention is used, the number of pick-up flaws is significantly reduced. In addition, in conventional furnaces, the threading speed is 20 to 30 m.
/min, and the hearth roll had to be replaced frequently, but in the furnace according to the present invention, the threading speed can be reduced to 1/min.
There was no problem even when the heat treatment was performed at 20 m/min. Although some scratches occurred in the furnace according to the present invention, none of them were pick-up scratches.

Furthermore, regarding the occurrence of canoeing during similar continuous annealing, FIG. 5 shows the results of a comparison with the conventional furnace shown in FIGS. 9 and 10.

From the same figure, it can be seen that when the furnace according to the present invention is used, the amount of canoeing generated is significantly reduced. This is the effect of tensile bending straightening by bending the steel strip under tension at the turning point between the outlet side of the soaking zone and the inlet side of the cooling side.

In addition, in similar continuous annealing, two cases were used: one was using combustion exhaust gas from the pre-preparation zone (ambient oxygen concentration 2-5%, temperature approximately 200'C) as the ejected gas from the guide device, and the other was air at room temperature. The results of the comparison are shown in FIG. From the figure, it can be seen that when exhaust gas in the preheating zone is used, the scale thickness on the steel plate surface is reduced to about 273 when using air.

Therefore, the pickling process in the next step could be shortened, and the yield was improved by reducing scale loss.

(Effects of the invention) According to this invention, since the hearth roll in the furnace is eliminated, pick-up scratches can be avoided, and between the heating zone and the soaking zone, the steel strip is pressed from above instead of using a steel strip flotation device. A gas injection type guide device is installed, and a tension generator is installed on the exit side of the furnace to apply tension to the steel strip, so even when processed at high speed, there is no canoeing or meandering. It is possible to greatly improve productivity and reduce running costs.

[Brief Description of the Drawings] Fig. 1 is a schematic diagram of a continuous annealing furnace for steel strip according to the present invention, Fig. 2 is a schematic diagram of a guide device, and Fig. 3 shows the relationship between steel strip temperature and flaw generation rate. Figure 4 is a graph showing the relationship between sheet threading speed and flaw occurrence rate, Figure 5 is a graph showing the relationship between sheet threading speed and amount of canoeing, and Figure 6 is a graph showing the relationship between soaking temperature and scale thickness. Figures 7 to 10 are schematic diagrams of a conventional continuous annealing furnace. 1...Pre-preparation zone 2...Heating zone 3...Soaking zone 4...Cooling zone 5...Deflation crawl 6...Guide device 7...Turning section 8a, 8b...Tension Pride roll 9...
・Burner 10...Cooling header 11...
・Circulation fan 12... Piping 13... Compartment chamber 14... Nozzle hole Figure 2 Figure 3

Claims (1)

[Claims] 1. An at least two-pass vertical furnace equipped with a turning section that changes the direction of steel strip threading, wherein a tension generator is installed on the entrance and exit sides of the furnace, and the turning section is provided with a tension generator. A continuous annealing furnace for steel strips equipped with a guide device that guides the non-contact threading of steel strips using gas injection. 2. The continuous annealing furnace for steel strip according to claim 1, wherein the first half of the furnace is configured as a heating zone and then a soaking zone in order from the top, with the turning section as a boundary, and the second half of the furnace is configured as a cooling zone. 3. The continuous annealing furnace for steel strip according to claim 2, further comprising a preheating zone provided on the entry side of the heating zone, through which the steel strip is passed from bottom to top. 4. The continuous annealing furnace for steel strip according to claim 3, further comprising a deflector roll provided between the pre-heating zone and the heating zone to change the direction in which the steel strip passes from top to bottom. 5. The continuous annealing furnace for steel strip according to claim 4, wherein a deflector roll is provided outside the furnace. 6. The continuous annealing furnace for steel strip according to claim 1 or 2, wherein the guide device uses atmospheric air as the ejected gas. 7. The continuous annealing furnace for steel strip according to claim 3, wherein the guide device uses combustion exhaust gas from the preheating zone as the ejected gas.