Horizontal annealing furnace, method for controlling dew point thereof, and method for annealing metal strip

Abstract

To provide a furnace and a method for controlling a dew point in a horizontal annealing furnace uniform within a predetermined target range while passing through a metal strip horizontally for annealing in a horizontal annealing furnace having a humidifying gas introduction hole.SOLUTION: A humidifying gas introduction holes 3 and 4 are at least a pair of introduction holes arranged at a position symmetrical vertically from the metal strip 2 in the side view of the horizontal annealing furnace 1 and at a relative position symmetrical to the center part in the width direction of the metal strip in the cross-sectional view of the horizontal annealing furnace, and the dew point in the horizontal annealing furnace is measured, and at least one of the input flow rate of the humidifying gas and the amount of water vapor from the introduction hole is controlled to make the dew point within a predetermined range.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a horizontal annealing furnace having a humidified gas inlet and for annealing a metal strip while passing it horizontally, a method for controlling a dew point in a horizontal annealing furnace, and a method for annealing a metal strip. [Background technology]

[0002] In a method for annealing a metal strip, in order to obtain a desired quality by adjusting the components such as carbon and oxygen contained in the metal strip, the atmospheric gas in the annealing furnace is humidified and controlled so that the dew point is within an appropriate range.

[0003] For example, Patent Document 1 discloses that atmospheric gas humidified by an ultrasonic humidifier is fed 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 fed only to the lower side of the metal strip for each zone of the annealing furnace, and the dew point of each zone is controlled to be uniform. Patent Document 3 discloses that humidified gas is fed from above and below the metal strip by a pair of humidifiers 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 fed from above and below the metal strip by a nozzle whose rotation angle can be changed, and the water vapor is mixed into the atmospheric gas by a partition plate that moves up and down to make the dew point uniform. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Utility Model Application Publication No. 61-69262 [Patent Document 2] Japanese Patent Application Publication No. 5-247529 [Patent Document 3] Special Publication No. 2014-525986 [Patent Document 4] JP 2005-264305 A Summary of the Invention [Problem to be solved by the invention]

[0005] In a horizontal annealing furnace (hereinafter simply referred to as an "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 upper and lower sections 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.

[0006] As a result, when humidified atmospheric gas is fed into the furnace from either the top or bottom of the metal strip, as described in the above-mentioned conventional Patent Documents 1 and 2, the dew points inside the furnace differ between the top and bottom, resulting in a problem that the quality of the metal strip differs between the front and back.

[0007] 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 problem that a good metal strip is not obtained.

[0008] 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 a furnace length to install the movable partition wall, which causes a problem of increased equipment costs.

[0009] Therefore, the present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a method for uniformly maintaining a dew point within a predetermined target range in a horizontal annealing furnace having a humidified gas inlet. [Means for solving the problem]

[0010] The inventors conducted intensive 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 by arranging the humidified gas inlet holes alternately up, down, left, and right, it is possible to uniformly control the dew point inside the furnace.

[0011] The present invention was made based on these findings and further studies, and has the following configuration. [1] A horizontal annealing furnace equipped with a humidified gas inlet 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 so that their relative positions in the up and down direction are different from those of 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. Effect of the Invention

[0012] According to the present invention, the dew point can be made uniform regardless of the upper and lower regions of the metal strip in the horizontal annealing furnace, and the dew point can be easily controlled within a predetermined target range, compared to the conventional technique. As a result, the annealing treatment (reactions such as decarburization) of the metal strip can be efficiently carried out as desired, and there is also an effect of improving the productivity and yield of the metal strip. [Brief description of the drawings]

[0013] [Figure 1] 1 is a schematic diagram showing one embodiment of a horizontal annealing furnace of the present invention (an example in which one zone has a pair of humidified gas inlet holes), (a) is a side view, and (b) is a cross-sectional view (viewed along the arrows AA). [Diagram 2] 1A and 1B are schematic diagrams showing an example of a conventional horizontal annealing furnace, in which (a) is a side view and (b) is a cross-sectional view (view taken along arrows B-B). [Diagram 3] 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), (a) is a side view, (b) is a cross-sectional view (viewed in the direction of arrow CC), and (c) is a top view (part of the furnace outlet side). [Figure 4] 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). [Diagram 5] FIG. 2 is a schematic side view showing another embodiment (example of two zones) of the horizontal annealing furnace of the present invention. [Figure 6] 1 is a diagram showing the change in dew point above and below the 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. [Figure 7] 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, (a) an example of the present invention, and (b) a conventional example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] [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, the furnace may be composed of multiple zones with different conditions.

[0015] The metal strip 2 is passed through the furnace from the entrance side to the exit side, and the gas in the furnace flows in the opposite direction to the passing direction of the metal strip 2.

[0016] 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, this 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. As the humidifier, a hollow fiber membrane humidifier or the like is used. The supply amount (flow rate) of the humidified gas W is 5 to 100 Nm 3 / h.

[0017] The atmospheric gas F is made of hydrogen and nitrogen and is supplied into the furnace from the furnace outlet side, but may also be supplied from various parts of the furnace. The supply amount (flow rate) of the atmospheric gas F is 50 to 500 Nm 3 / h.

[0018] [Humidified gas inlet] As described above, in order to maintain the dew point in an annealing furnace uniformly within a predetermined range, which is the object of the present invention, 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 maintained uniformly and satisfactorily by specifying the arrangement of the humidified gas inlet (hereinafter also simply referred to as "inlet") as follows.

[0019] Description will be given based on FIG. 1(b), which is a cross-sectional view (view along the arrow AA) of the horizontal annealing furnace 1. In this embodiment, a pair of upper and lower introduction holes 3 and 4 are arranged at relative positions that are point symmetrical with respect to the center of the width direction of the metal strip 2. For example, the introduction hole 3 is arranged on the left side of the furnace, above the metal strip 2, and the introduction hole 4 is arranged on the right side of the furnace, below the metal strip 2, and these introduction holes 3 and 4 are arranged at the same position in the longitudinal direction of the furnace. Note that the introduction hole 3 may be on the lower side and the introduction hole 4 on the upper side. It is important that at least a pair of introduction holes 3 and 4 are arranged at relative positions that are point symmetrical with respect to the center of the width direction of the metal strip 2.

[0020] [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 inlet holes 3 and 4. At this time, the humidified gas W fed from 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 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.

[0021] Moreover, the humidified gas W in each of the humidified gas inlet holes 3 and 4 forms a vortex-like flow, and this flow promotes mixing of the atmospheric gas F and the humidified gas W.

[0022] [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 method. In the example of the present invention shown in Fig. 7(a), the humidified gas inlet holes are located at point symmetric positions vertically and widthwise (horizontally), 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 it can be seen that 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 prone to high temperatures, is low, and the dew point on the lower side of the metal strip, which is prone to low temperatures, is high, resulting in a large difference in the dew point distribution in the furnace.

[0023] 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 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 in the target temperature at the cross section of the furnace 1 m from the humidified gas inlet toward the inlet of the furnace. The flow rate of the humidified gas was 30 Nm for both pipes. 3 / h.

[0024] [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 Figs. 3 and 4, a pair of an inlet 3 on the upper left side of the furnace and an inlet 4 on the lower right side of the furnace is set as a first pair, and a pair of an inlet 3' on the upper right side of the furnace and an inlet 4' on the lower left side of the furnace, with the left and right arrangements reversed, is set as a second pair. Here, Fig. 3 is a diagram showing an example of a horizontal annealing furnace having two pairs of inlet holes, where (a) is a side view, (b) is a cross-sectional view (viewed from the CC arrow), and (c) is a top view of a part of the furnace outlet side. Also, Fig. 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 up and down 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.

[0025] 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, the non-oxidizing zone and the heating zone are important for atmosphere control, particularly dew point control. In such a horizontal annealing furnace, the optimal dew point control can be achieved 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 having a multi-zone configuration.

[0026] [Dew point control and atmospheric gas adjustment methods] 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 above and below the metal strip 2 fall within a target range. Here, the control unit 9 controls at least one of the aperture 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.

[0027] The dew point sensors 7 and 8 are connected to a 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 be installed.

[0028] As described above, the method for controlling the dew point of 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 introduced from the introduction hole and the amount of water vapor to keep the dew point within a predetermined range. 3 / h, and the amount of water vapor is 50-500Nm 3 / h. The predetermined range of the target dew point is preferably -40 to 0°C.

[0029] [Metal strip annealing method] 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. EXAMPLES

[0030] In order to demonstrate that the humidified gas is uniformly mixed into the atmospheric gas both above and below the metal strip passing through the horizontal annealing furnace in the present invention, and that the mixture is within the target range, a cold-rolled steel sheet with a thickness of 1 mm and a width of 1000 mm was passed through a horizontal annealing furnace with a height of 2 m, a width of 2 m and a length of 30 m, and annealed at an average temperature of 750°C in the furnace. 3 The atmospheric gas (hydrogen + nitrogen gas) was flowed for 1000 sq. m. / h. The cold-rolled steel sheet used was a general SPCC steel sheet.

[0031] As an example of the present invention, humidified gas inlet holes were installed above and below the metal strip 4 m in the longitudinal direction from the outlet of the horizontal annealing furnace 1. The humidified gas inlet holes were located ±0.2 m above and below the metal strip, and as shown in FIG. 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.

[0032] 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 Figures 2(a) and (b) at the top and bottom of the center of the width direction of the metal strip 4 m from the outlet side of the horizontal annealing furnace 10, as described in the conventional patent documents, with the tips of the humidified gas being positioned ±0.2 m above and below the metal strip. A cold-rolled steel sheet of the same size and material as in the example of the present invention was annealed under the same atmospheric gas flow rate and the same annealing conditions.

[0033] 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. Figures 6(a) and (b) show the results when a strip was threaded through a horizontal annealing furnace under steady-state conditions. The horizontal axis shows the position (m) inside the furnace from the entry side (left) to the exit side (right), and the vertical axis shows the dew point at each position inside the furnace. The figures also show the upper and lower limits of the dew point, which are the targets.

[0034] Figure 6(a) shows the results of the present invention. In the present invention, the dew point increases suddenly immediately after the humidified gas is introduced from the inlet holes 3 and 4 near the outlet side of the furnace, but the dew point is stable from inside the furnace to the inlet side of the furnace and is within the upper and lower limits of the target dew point, which is extremely good.

[0035] Figure 6(b) shows the results for the conventional method. In the conventional method, after the humidified gas was introduced through the inlet holes 5 and 6 near the outlet side of the furnace, the dew point fluctuated significantly from the outlet side to the inlet side of the furnace for about 1 / 3 of the furnace length, and it was found that the dew point was far outside the target range. [Explanation of symbols]

[0036] 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 Humidified gas inlet holes (upper and lower sides of the first pair of metal strips in this example) 3', 4' Humidified gas inlet holes (upper and lower sides of the second pair of metal bands in this example) 5, 6 Humidified 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 Threading direction W, W' Humidified gas (first pair, second pair) (Arrows indicate the direction of flow)

Claims

1. 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 opposite to the side on which the one inlet hole and the other inlet hole are respectively arranged.

2. The horizontal annealing furnace according to claim 1, characterized in that the horizontal annealing furnace is provided with two or more pairs of introduction holes, and the introduction holes of each pair are arranged such that their relative positions in the up and down direction are different from those of the introduction holes of the front and rear pairs in a cross-sectional view of the horizontal annealing furnace.

3. 3. The horizontal annealing furnace according to claim 1, wherein the horizontal annealing furnace is composed of a plurality of zones each having a different heat treatment condition.

4. 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.

5. 5. The method for controlling a dew point in a horizontal annealing furnace according to claim 4, wherein the predetermined range of the dew point is −40 to 0° C.

6. 6. A method for annealing a metal strip, comprising: annealing the metal strip by using the method for controlling a dew point in a horizontal annealing furnace according to claim 4 or 5.

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