JP7626089B2 - Horizontal annealing furnace, method for controlling dew point thereof, and method for annealing metal strip - Google Patents

Description

The present invention relates to a horizontal annealing furnace that has an inlet for introducing humidified gas and anneals a metal strip while passing it horizontally, a method for controlling the dew point of the horizontal annealing furnace, and a method for annealing a metal strip.

In the method of annealing metal strips, the atmospheric gas in the annealing furnace is humidified and controlled to keep the dew point within an appropriate range in order to adjust the carbon, oxygen, and other components contained in the metal strip to obtain the desired quality.

For example, Patent Document 1 discloses that atmospheric gas humidified by an ultrasonic humidifier is sent only to the upper side of the metal strip to control the dew point. Patent Document 2 discloses that atmospheric gas humidified by an ultrasonic humidifier is supplied only from the lower side of the metal strip to each zone of the annealing furnace, and the dew point of each zone is controlled to be uniform. Patent Document 3 discloses that a pair of humidifiers supplies humidified gas from above and below the metal strip near the furnace outlet, thereby adjusting the dew point of the entire furnace along the flow of the atmospheric gas. Patent Document 4 discloses that water vapor is supplied from above and below the metal strip using a nozzle whose rotation angle can be changed, and the water vapor is mixed into the atmospheric gas using a partition plate that moves up and down, thereby making the dew point uniform.

Japanese Utility Model Application Publication No. 61-69262 Japanese Patent Application Publication No. 5-247529 Special Publication No. 2014-525986 JP 2005-264305 A

In a horizontal annealing furnace (hereafter simply referred to as "annealing furnace" or "furnace"), the interior is divided into upper and lower sections by the metal strip that is passed through it, and the atmospheric gas inside the furnace is difficult to move between the top and bottom through the gap between the metal strip and the furnace side wall when viewed in cross section, making it difficult for the atmospheric gas to mix through diffusion.

As a result, if humidified atmospheric gas is fed into the furnace from either the top or bottom of the metal strip, as described in the conventional patent documents 1 and 2 mentioned above, the dew points inside the furnace will be different between the top and bottom, causing a problem that the quality of the metal strip will be different between the front and back.

In order to solve the above problems, in Patent Documents 3 and 4, humidified gas is blown above and below the metal strip to keep the dew point of the atmospheric gas within a target range. In these methods, as shown in the examples described later, even if the dew point of the humidified gas blown in is the same, the dew points differ above and below the metal strip, making it difficult to obtain a uniform dew point in the furnace, and the quality of the front and back surfaces of the metal strip differs, resulting in a poor metal strip.

Furthermore, in Patent Document 4, water vapor is mixed into the atmospheric gas by using a partition plate that moves up and down, but this requires furnace length to install the movable partition wall, which creates the problem of high equipment costs.

The present invention has been made to solve the above problems, and aims to provide a method for uniformly maintaining the dew point within a horizontal annealing furnace equipped with a humidified gas inlet within a predetermined target range.

The inventors conducted extensive research to solve the above-mentioned problems, and as a result, they focused on the flow of humidified gas inside the furnace and discovered that uniform control of the dew point inside the furnace is possible by arranging the humidified gas inlet holes alternately up, down, left, and right.

The present invention was made based on these findings and further studies, and has the following configuration.
[1] A horizontal annealing furnace having an inlet for introducing humidified gas and annealing a metal strip while passing it horizontally,
The horizontal annealing furnace is characterized in that the humidified gas inlet holes are at least a pair of inlet holes arranged at relative positions that are symmetrical above and below the metal strip in a side view of the horizontal annealing furnace and that are point-symmetric with respect to the widthwise center of the metal strip in a cross-sectional view of the horizontal annealing furnace.
[2] The horizontal annealing furnace according to [1], characterized in that the introduction holes are provided in two or more pairs within the horizontal annealing furnace, and the introduction holes of each pair are arranged such that their relative positions in the up and down direction are different from the introduction holes of the pairs before and after them in a cross-sectional view of the horizontal annealing furnace.
[3] In the horizontal annealing furnace according to [1] or [2] above, the horizontal annealing furnace is characterized in that it is composed of a plurality of zones having different heat treatment conditions.
[4] A method for controlling a dew point in a horizontal annealing furnace according to any one of [1] to [3],
a dew point control method for a horizontal annealing furnace, comprising: measuring a dew point in the horizontal annealing furnace; and adjusting at least one of a flow rate of the humidified gas introduced from the inlet and an amount of water vapor to control the dew point within a predetermined range.
[5] The method for controlling a dew point in a horizontal annealing furnace according to [4], wherein the predetermined range of the dew point is −40 to 0° C.
[6] A method for annealing a metal strip, comprising: annealing the metal strip using the method for controlling a dew point in a horizontal annealing furnace according to [4] or [5] above.

According to the present invention, compared to the conventional technology, the dew point can be made uniform regardless of the region above or below the metal strip in the horizontal annealing furnace, and the dew point can be easily controlled to a predetermined target range. As a result, the annealing process of the metal strip (reactions such as decarburization) can be carried out efficiently as desired, which also has the effect of improving the productivity and yield of the metal strip.

1A and 1B are schematic diagrams showing an embodiment of a horizontal annealing furnace of the present invention (an example in which one zone has a pair of humidified gas inlet holes), in which (a) is a side view and (b) is a cross-sectional view (viewed along the line A-A). 1A is a schematic diagram showing an example of a conventional horizontal annealing furnace, and FIG. 1A is a schematic diagram showing another embodiment of the horizontal annealing furnace of the present invention (an example of two pairs of humidified gas inlet holes in one zone), in which (a) is a side view, (b) is a cross-sectional view (viewed along the CC arrows), and (c) is a top view (part of the furnace outlet side). FIG. 2 is a perspective view showing another embodiment of the horizontal annealing furnace of the present invention (an example in which one zone has two pairs of humidified gas inlet holes). FIG. 2 is a schematic side view showing another embodiment (example of two zones) of the horizontal annealing furnace of the present invention. 1 is a diagram showing changes in dew points above and below a metal strip at various positions in a furnace according to the present invention and a conventional example, (a) an example of the present invention, (b) a conventional example. 1A and 1B are schematic diagrams showing the distribution of temperature differences from a target dew point due to humidified gas in the present invention and the conventional example;

[Horizontal annealing furnace]
An embodiment of a horizontal annealing furnace according to the present invention will be described with reference to Figs.
As shown in FIG. 1(a), a horizontal annealing furnace 1 passes a metal strip 2 horizontally (strip passing direction S). Depending on the heat treatment conditions, some annealing furnaces are composed of a plurality of zones with different conditions.

The metal strip 2 is passed through from the furnace entry side to the furnace exit side, and the gas inside the furnace flows in the opposite direction to the passing direction of the metal strip 2.

Next, in order to adjust the dew point in the horizontal annealing furnace 1, a humidified gas W is supplied. This humidified gas W is supplied separately from the atmospheric gas F described below. For example, the atmospheric gas F is passed through a separately provided humidifier (not shown) to increase the amount of water vapor (water molecular weight) in the gas before supply. An example of the humidifier is a hollow fiber membrane humidifier. The supply amount (flow rate) of the humidified gas W is about 5 to 100 Nm ³ /h.

The atmospheric gas F is made of hydrogen and nitrogen and is supplied into the furnace from the outlet side, but may be supplied from various parts inside the furnace. The supply amount (flow rate) of the atmospheric gas F is about 50 to 500 Nm ³ /h.

[Humidified gas inlet]
As described above, in order to maintain the dew point in an annealing furnace uniformly within a predetermined range, which is an object of the present invention, it has been found that the flow of the humidified gas in the furnace is important. Based on this finding, it has been found that the dew point can be made uniform and good by specifying the arrangement of the humidified gas inlet (hereinafter also simply referred to as "inlet") as follows.

The explanation will be based on FIG. 1(b), which is a cross-sectional view (A-A arrow view) of the horizontal annealing furnace 1. In this embodiment, the pair of upper and lower inlet holes 3 and 4 are arranged in relative positions that are point symmetrical with respect to the center of the width direction of the metal strip 2. For example, the inlet hole 3 is arranged on the left side of the furnace, above the metal strip 2, and the inlet hole 4 is arranged on the right side of the furnace, below the metal strip 2, and these inlet holes 3 and 4 are arranged in the same position in the longitudinal direction of the furnace. Note that the inlet hole 3 may be on the lower side and the inlet hole 4 on the upper side. It is important that at least one pair of inlet holes 3 and 4 are arranged in relative positions that are point symmetrical with respect to the center of the width direction of the metal strip 2.

[Flow of humidified gas]
Next, an example of the flow of the humidified gas W will be described. In Fig. 1(b), the humidified gas W is fed into the furnace from the inlet holes 3 and 4. At this time, the humidified gas W fed from the inlet hole 3 passes above the metal strip 2, reaches the furnace wall on the opposite side to the inlet hole 3, and diffuses along the furnace wall to the lower side of the metal strip 2. On the other hand, the humidified gas W fed from the inlet hole 4 passes below the metal strip 2, reaches the furnace wall on the opposite side to the inlet hole 4, and diffuses along the furnace wall to the upper side of the metal strip 2.

In addition, the humidified gas W in each of the humidified gas inlet holes 3 and 4 forms a vortex-like flow, which promotes mixing of the atmospheric gas F and the humidified gas W.

[Comparison of the difference from the target dew point due to differences in humidified gas introduction]
Fig. 7 shows a simulation of the distribution of the difference between the target dew point and the humidified gas in the present invention and the conventional case. In the example of the present invention shown in Fig. 7(a), the humidified gas inlet holes are located at point symmetric positions in the vertical and width directions (horizontal), so the flow of the humidified gas circulates around the metal strip like a vortex, promoting the mixing of the atmospheric gas and the humidified gas, and the difference in the dew point distribution in the furnace is small and the humidity is almost the same. In contrast, in the conventional example shown in Fig. 7(b), the humidified gas is introduced from above and below the center of the width direction of the metal strip, and it can be seen that the dew point on the upper side of the metal strip, which is likely to become hot, is low, and the dew point on the lower side of the metal strip, which is likely to become cold, is high, resulting in a large difference in the dew point distribution in the furnace.

Here, an example of the conditions in the above simulation will be shown.
First, a horizontal annealing furnace with a height of 2 m, width of 2 m, and length of 30 m was simulated. The humidified gas inlet was installed at a position 4 m in the longitudinal direction from the outlet side of the furnace, and the direction of the humidified gas inlet was perpendicular to the furnace wall on which it was installed. Figure 7 of the simulation shows the difference between the target temperature and the cross section of the furnace 1 m from the humidified gas inlet toward the inlet side of the furnace. The flow rate of the humidified gas was 30 ^Nm3 /h in total for both pipes.

[Multiple pairs of inlets, multiple zone furnace]
In cases where a large amount of humidified gas needs to be introduced, such as when the target range of the dew point is narrow or the length of the annealing furnace is long, as shown in Figures 3 and 4, a pair of the inlet 3 on the upper left side of the furnace and the inlet 4 on the lower right side of the furnace is set as a first pair, and a pair of the inlet 3' on the upper right side of the furnace and the inlet 4' on the lower left side of the furnace, with the left and right arrangements reversed, may be installed in the longitudinal direction of the horizontal annealing furnace. Here, Figure 3 is a diagram showing an example of a horizontal annealing furnace having two pairs of inlet holes, (a) is a side view, (b) is a cross-sectional view (viewed in the direction of the arrow C-C), and (c) is a top view of a part of the furnace outlet side. Also, Figure 4 is a perspective view. In addition, when multiple pairs are installed, it is preferable to arrange the inlet holes of each pair so that the relative positions of the inlet holes of the front and rear pairs are reversed in the vertical direction. As a result, a large amount of humidified gas can be introduced into the horizontal annealing furnace, and the introduced humidified gas can be mixed more uniformly with the atmospheric gas.

Regarding the configuration of the horizontal annealing furnace, the above description is for the case where the furnace is configured as one zone, but there are also cases where the horizontal annealing furnace is configured as a multi-zone furnace. The multi-zone furnace has different heat treatment conditions for each zone. As an example, the horizontal annealing furnace in a continuous hot-dip galvanizing line is configured as a preheating zone, a non-oxidizing zone, a heating zone (radiant tube), a holding zone, a cooling zone, etc., and among these, atmosphere control, especially dew point control, is important in the non-oxidizing zone and the heating zone. In such a horizontal annealing furnace, optimal dew point control is possible by installing at least one pair of the pair of humidified gas introduction holes according to the present invention. Specifically, in the case of a two-zone furnace as shown in FIG. 5, the atmospheric gas F flows from the outlet side of the second zone (1b) to the inlet side of the first zone (1a) in the direction opposite to the sheet passing direction S, so that the pair of humidified gas introduction holes 3', 4', 3, 4 according to the present invention are installed near the outlet side of each of the first zone (1a) and the second zone (1b) from the inlet side of the furnace. This makes it possible to achieve a uniform dew point even in a horizontal annealing furnace with multiple zones.

[Dew point control and atmospheric gas adjustment method]
1(a) controls the flow rate and dew point of the humidified gas W fed through the humidified gas inlet holes 3 and 4 so that the dew points on the upper and lower sides of the metal strip 2 fall within a target range. Here, the control unit 9 controls at least one of the opening degree of the valve associated with the humidified gas inlet holes 3 and 4 and the air volume of the blower that supplies the humidified gas. The dew point of the atmospheric gas F fed into the furnace may also be adjusted by humidification.

The dew point sensors 7 and 8 are connected to the control unit 9, and are preferably installed above and below the metal strip 2 near the inlet side of the horizontal annealing furnace 1. Two or more dew point sensors 7 and 8 may also be installed.

As described above, the dew point control method for a horizontal annealing furnace according to the present invention measures the dew point in the horizontal annealing furnace and controls at least one of the flow rate of the humidified gas fed from the inlet and the amount of water vapor to keep the dew point within a predetermined range. Here, the flow rate of the humidified gas fed is about 5 to 100 ^Nm3 /h, and the amount of water vapor is about 50 to 500 ^Nm3 /h. The predetermined range of the target dew point is preferably -40 to 0°C.

[Method of annealing metal strip]
Furthermore, a method for annealing a metal strip according to the present invention is characterized in that the above-mentioned method for controlling a dew point in a horizontal annealing furnace is used to anneal a metal strip.

In order to demonstrate that the humidified gas is uniformly mixed into the atmospheric gas above and below the metal strip passing through the horizontal annealing furnace in the present invention and falls within the target range, a horizontal annealing furnace with a height of 2 m, a width of 2 m and a length of 30 m was used to pass a cold-rolled steel sheet with a thickness of 1 mm and a width of 1000 mm as the metal strip, and annealed at an average furnace temperature of 750° C. At that time, atmospheric gas (hydrogen + nitrogen gas) was flowed at 30 Nm ³ /h from the outlet side of the horizontal annealing furnace. The cold-rolled steel sheet used was a general SPCC steel sheet.

As an example of the present invention, humidified gas inlet holes were installed above and below the metal strip 4 m from the outlet of the horizontal annealing furnace 1 in the longitudinal direction. The humidified gas inlet holes were located ±0.2 m above and below the metal strip, and as shown in Figure 1(b), in a cross-sectional view of the horizontal annealing furnace 1, one humidified gas inlet hole 3 was located on the upper side of the left side of the horizontal annealing furnace 1, and the other humidified gas inlet hole 4 was located on the lower side of the right side of the horizontal annealing furnace 1.

Furthermore, for comparison, humidified gas was introduced from above and below the horizontal annealing furnace 10 through humidified gas inlet holes 5 and 6 shown in Fig. 2(a) and (b), as described in the conventional patent documents, at the top and bottom positions of the center of the width direction of the metal strip 4 m from the outlet side of the horizontal annealing furnace 10, and the tips of the humidified gas were positioned ±0.2 m above and below the metal strip. A cold-rolled steel sheet of the same size and material as the present invention example was annealed under the same atmospheric gas flow rate and the same annealing conditions.

In addition, the dew point was measured at multiple positions in the longitudinal direction of the furnace, above and below the center of the metal strip in the width direction.
6(a) and (b) show the results when a strip is threaded through a horizontal annealing furnace under steady conditions. The horizontal axis shows the position (m) in the furnace from the entry side (left) to the exit side (right), and the vertical axis shows the dew point at each position in the furnace. The upper and lower limits of the dew point targets are also shown in the figures.

Figure 6 (a) shows the results of the present invention. With the present invention, the dew point increases suddenly immediately after the humidified gas is introduced from the inlet holes 3 and 4 near the furnace outlet side, but the dew point is stable from inside the furnace to the inlet side, and is within the upper and lower limits of the target dew point value, which is remarkably good.

Figure 6 (b) shows the conventional results. Conventionally, after humidified gas is introduced through inlet holes 5 and 6 near the furnace outlet side, the dew point fluctuates greatly toward the furnace inlet side, and the dew point fluctuates greatly over approximately 1/3 of the furnace length from the furnace outlet side to the furnace inlet side, and it was found that the dew point was far outside the target range.

1. Horizontal annealing furnace (example of one zone in the present invention)
1a, 1b Horizontal annealing furnace (example of the present invention with two zones)
2 Metal strip 3, 4 Moisturized gas inlet (upper and lower sides of the first pair of metal strips in this example)
3', 4': Inlet holes for humidified gas (upper and lower sides of the second pair of metal strips in this example)
5, 6 Moisturized gas inlet holes (upper and lower sides in the center of the width direction of the metal strip in the conventional example)
7, 8 Dew point sensor (upper and lower sides of the metal strip width direction center)
9 Control unit 10 Horizontal annealing furnace (conventional example)
F: Atmospheric gas S: Strip passing direction W, W': Humidified gas (first pair, second pair)
(The arrow indicates the direction of flow.)

Claims (6)

A horizontal annealing furnace having a humidified gas inlet and annealing a metal strip while passing it horizontally,
The humidified gas introduction holes are at least a pair of introduction holes arranged at positions that are vertically symmetrical with respect to the metal strip in a side view of the horizontal annealing furnace and at relative positions that are point symmetrical with respect to a center portion in the width direction of the metal strip in a cross-sectional view of the horizontal annealing furnace ,
one of the pair of introduction holes is disposed above the metal strip and on one side surface of the horizontal annealing furnace ;
the other of the pair of introduction holes is disposed below the metal strip and on the opposite side of the horizontal annealing furnace;
a horizontal annealing furnace, characterized in that the inlet holes are arranged so that the humidified gas supplied from the one inlet hole and the other inlet hole reaches a furnace wall on a side surface opposite to the side surfaces on which the one inlet hole and the other inlet hole are respectively arranged . The horizontal annealing furnace according to claim 1, characterized in that the horizontal annealing furnace has two or more pairs of introduction holes, and the introduction holes of each pair are arranged so that their relative positions in the vertical direction differ from those of the introduction holes of the pairs before and after them in a cross-sectional view of the horizontal annealing furnace. The horizontal annealing furnace according to claim 1 or 2, characterized in that the horizontal annealing furnace is composed of multiple zones with different heat treatment conditions. The dew point control method for the horizontal annealing furnace,
4. The method for controlling a dew point in a horizontal annealing furnace according to claim 1, further comprising the steps of: measuring a dew point in the horizontal annealing furnace; and adjusting at least one of a flow rate of the humidified gas introduced through the inlet and an amount of water vapor to control the dew point within a predetermined range. The method for controlling the dew point of a horizontal annealing furnace according to claim 4, characterized in that the predetermined range of the dew point is -40 to 0°C. A method for annealing a metal strip, comprising annealing the metal strip using the dew point control method for a horizontal annealing furnace described in claim 4 or 5.

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