JPH0368931B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a direct-fired heating furnace in continuous annealing equipment for steel strips. [Prior art and its problems] In a conventional continuous annealing furnace, an indirect heating method using a radiant tube is generally adopted. However, while this heating method does not cause the problem of steel strip oxidation that occurs with general direct flame heating methods,
There are disadvantages such as low heating capacity, which increases the equipment burden compared to direct flame methods. In contrast to such indirect heating methods, a method called a non-oxidizing direct flame heating method is known. This method suppresses oxidation of the steel strip by reducing the air ratio in each combustion zone as the steel strip temperature rises. However, strictly speaking, this method is not non-oxidizing, but produces an oxide film, and therefore, it is necessary to reduce the oxide film in a reducing atmosphere furnace (soaking furnace) subsequent to the non-oxidizing furnace. However, in such a reducing atmosphere furnace, although iron oxide is reduced, Si, Al,
There is a problem that oxides of alloying elements such as Ti are hardly reduced, which deteriorates the quality of the final product surface. [Means for Solving the Problem] The present inventor has made repeated studies in view of such conventional problems, and as a result, it has been found that a non-equilibrium region exists in the flame, that is, a combustion intermediate product exists and unreacted. A heating burner that can form an oxygen-free region is extremely effective for non-oxidizing heating of steel strip, and by placing it in a heating furnace under specified conditions, it is possible to heat the steel strip in a reducing state throughout the course of passing through the heating furnace. I found it. That is, the present invention provides heating burners that can form a non-equilibrium region in the flame that contains combustion intermediate products (intermediate ions, radicals, etc.) and does not contain unreacted oxygen. A plurality of them are arranged in the entire effective heating range of the heating furnace so that they collide with the surface of the steel strip at right angles and in the non-equilibrium region. In the present invention, a heating burner is used in which a non-equilibrium region, that is, a region where combustion intermediate products are present and no unreacted oxygen (oxygen ions, oxygen radicals) is present, can be formed in the flame. In such a heating burner, the region containing CO ₂ , H ₂ O, N ₂ , H ₂ , CO, etc. in the flame, that is, the quasi-equilibrium region, is oxidizing, whereas the non-equilibrium region is reducing, By colliding this flame with the steel strip in the non-equilibrium region, the steel strip can be heated without oxidizing it. FIGS. 2 and 3 show an example of such a heating burner, in which a plurality of combustion air discharge holes 2 are provided on the inner wall 6 of a cylindrical burner tile 1 at intervals in the circumferential direction, and A fuel gas discharge hole 3 is provided in the inner center of the burner, and the combustion air discharge hole 2 and the fuel gas discharge hole 3 have the following configuration. (a) An angle θ of 60° or less is attached to the air supply direction of the air discharge hole 2 with respect to the tangent to the inner circumference of the burner tile. (b) The distance N between the fuel gas discharge hole 3 and the air discharge hole 2 in the burner axial direction is expressed as (-) when the fuel gas discharge hole is closer to the burner tile outlet than the air discharge hole.
If the opposite is (+), -0.1D to +0.25D
(D: burner inner diameter). (c) The distance L from the air discharge hole 2 to the burner tile outlet 5 is set to 0.6D to 3D. The heating burner configured in this way has an air ratio of
By using it at 1.0 or less, a non-equilibrium region is formed within a predetermined range in the flame. That is, in such a heating burner, rapid combustion is achieved by the swirling flow of combustion air from the air discharge hole 2 and the fuel gas discharged from the center of the burner, and the combustion continues over a predetermined range outside the burner outlet. A region containing a large amount of intermediate products and no unreacted free oxygen, that is, a non-equilibrium region is formed. Fig. 4 shows an example of measurement using an ion detection probe of the non-equilibrium region in the flame formed by such a heating burner. This means that a large amount of combustion intermediate products are present. According to this, a non-equilibrium region is formed over a predetermined range outside the burner outlet, and the outside thereof becomes a quasi-equilibrium region containing CO ₂ , H ₂ O, N _{2 ,} etc. that have almost completed their reactions. There is. Figure 5 shows the reductive heating characteristics of such a heating burner, that is, the limit temperature at which it can be heated without oxidation (the limit temperature for a thin plate of ordinary steel), and shows the temperature at which a steel strip can be heated in the air ratio range of 0.85 to 0.95. about
It has been shown that it can be heated up to 900℃. Further, in the present invention, in addition to the heating burner described above, for example, a so-called radiant cup burner can be used. In order to perform a rapid combustion reaction, this burner rapidly burns a pre-mixed gas mixture of air and fuel gas in the hemispherical recess of the burner tile, raising the temperature of the inner surface of the burner tile and heating mainly through radiant heat transfer. It has the characteristic that a high heat flux can be obtained in a region where the temperature of the heated object is high. By performing combustion in this burner at an air ratio of 1.0 or less, a non-equilibrium region is formed in the flame. However, since this radiant burner uses a premixing method for combustion air and fuel gas, it is not possible to preheat the combustion air, and since the air cannot be preheated in this way, non-oxidation heating is limited to about 750℃.
There are drawbacks such as inability to apply this method in cases where heating in a higher temperature range is required. On this point, the second
With the heating burner shown in the figure, non-oxidizing heating is possible up to around 900℃ because preheated air can be used, and since the flame temperature can be raised by using preheated air in this way, compared to a radiant burner, it is possible to heat up to about 900℃ without oxidation. The reduction action itself by the reaction product can also be effectively improved. In the present invention, a plurality of such heating burners are arranged in the entire effective heating range of the heating furnace so that the flame of each burner collides with the surface of the steel strip substantially at right angles to the steel strip and in its non-equilibrium region. FIG. 1 shows an example of this, in which the heating burner A is arranged in a one-pass type continuous annealing heating furnace 8 over its entire effective heating range, that is, over the entire area that requires heating. That is, the heating burners A are arranged on both sides of the steel strip S at intervals in the longitudinal direction of the heating furnace. Each heating burner A has its flame approximately perpendicular to the steel strip S.
Moreover, it is arranged so as to collide with the surface of the steel strip in the non-equilibrium region. The heating burners used in conventional direct-fired heating furnaces are not formed in such a way that the non-equilibrium region is clearly distinguished from other regions, as in the above-mentioned burner, and therefore the flame does not directly strike the steel strip. When a collision occurs, the surface of the steel strip is significantly oxidized, so the burner is arranged so that the flame is formed parallel to the width direction of the steel strip. In contrast, the purpose of the present invention is to heat the steel strip using a non-equilibrium region formed in the longitudinal middle of the burner flame. The burners are arranged so that they collide. In such a direct-fired heating furnace of the present invention, the steel strip to be threaded is heated without being oxidized from beginning to end by the heating burners arranged in the entire effective heating range. The structure of the heating burner shown in FIGS. 2 and 3 will be specifically explained. In the figure, 7 is a fuel gas nozzle projecting from the inner end wall 4 of the burner tile, and in this embodiment, fuel gas discharge holes 3 are formed at intervals in the circumferential direction of the fuel gas nozzle 7. In such a heating burner, the reason why the air discharge hole 2 has an air supply angle θ is to create a swirling flow in the combustion air within the burner tile.
This swirling flow forms a negative pressure region inside the burner, and this negative pressure promotes combustion by recirculating the gas, thereby creating an appropriate non-equilibrium region. This air supply angle θ is the maximum
By setting the angle to 60°, preferably 20 to 40°, stable swirling properties of the airflow can be obtained. When the distance N between the fuel gas discharge hole 3, the air discharge hole 2, and the burner axial direction is on the (-) side, the gas temperature is high and the combustion intermediate products are also highly distributed over a wide range, but on the other hand, the free O ₂ (unreacted O ₂ ) tends to be distributed long in the axial direction. In order to appropriately form the non-equilibrium region targeted by the present invention, it is necessary to minimize the remaining distance of this unreacted O ₂ in the burner axial direction, and its limit is -0.1D. If N is on the (+) side, a proper non-equilibrium region will be formed, but if it becomes too large, the inner end wall of the burner tile will be heated to over 1400°C, which is undesirable, and will cause problems in protecting the SiC on the inner end wall of the burner tile. +0.25D is the limit. Figure 6 shows the burner axial distance from the burner outlet, gas temperature in the burner tile, O ₂ concentration, and This study investigated each relationship with ionic strength, and the results show that when N is on the (-) side, there is a large residual distance L ₀ of unreacted O ₂ in the axial direction. has been done. FIG. 7 shows the relationship between the burner axial distance N between the fuel gas hole and the air discharge hole and the axial remaining distance L ₀ of free _O2 . ) side, L ₀ increases rapidly, and therefore -0.1D is the limit on the (-) side. On the other hand, FIG. 8 shows the relationship between the burner axial distance from the burner outlet, O ₂ concentration, ion strength, and gas temperature when N is +0.1D. According to Figures 7 and 8, if N is on the (+) side, there is no problem with the O ₂ concentration, and an appropriate non-equilibrium region is formed at a distance of 0.5D or more from the burner outlet. has been done. However, when N is increased to the (+) side, the burner tile inner end wall 4 is heated, so as shown in the relationship graph between the distance N and the temperature Tb of the burner tile inner end wall 4 in FIG. Tb is 1400 at +0.25D
℃ or higher, and therefore, considering that the material of the inner end wall is SiC, it is preferable to set the temperature to +0.25D or lower in view of the heat resistance limit. From the above, regarding the burner center axis distance N between the combustion gas discharge hole and the air discharge hole, -
The range is preferably 0.1D to 0.25D. The distance L from the air discharge hole 2 to the burner tile outlet 5 has a close relationship with the formation range of the non-equilibrium region. That is, if L exceeds 3D, the non-equilibrium region will be formed only in the portion immediately after the burner tile exit, which is not preferable. On the other hand, if L is less than 0.6D, the flame becomes a petal-shaped flame immediately after the exit of the burner tile, and an appropriate non-equilibrium region cannot be stably obtained on the burner central axis. Therefore, it is preferable to set L in the range of 0.6D to 3.0D. When continuously heating a thin steel plate, unless the distance between the burner tile outlet 5 and the steel plate is at least a certain distance (usually about 100 mm or more), there is a risk that the steel plate will come into contact with the burner during threading. Therefore, it is preferable that the non-equilibrium region in the flame be formed in as wide a range as possible, including the steel strip passing position located at a predetermined distance from the burner outlet side. Figure 10 shows the distance L
and the distance L _R from the burner outlet to the end of the non-equilibrium region (the end on the anti-burner side, e.g. point A in Figure 8)
This study investigated the relationship between According to this, when L exceeds 3D, the non-equilibrium region is formed only immediately after the exit of the burner tile, and is hardly formed in front of it. As L becomes smaller, the range of non-equilibrium region formation expands, but as L becomes smaller,
In the region (X) of less than 0.6D, the flame becomes a petal-shaped radial flame immediately after the burner tile exit, and an appropriate non-equilibrium region is not stably formed on the burner axis. From the above, it is desirable that the distance L from the air discharge hole 2 to the burner tile outlet 5 is in the range of 0.6D to 3.0D. In addition, in the structure of the heating burner as described above, if the swirling flow of the air discharged from the combustion air discharge hole 2 is too strong, the temperature distribution of the combustion gas on the burner outlet side in the burner radial direction becomes uneven, As a result,
In some cases, it becomes difficult to form a stable and wide nonequilibrium region. In such cases, in order to reduce the air swirling flow and make the temperature distribution uniform,
The combustion gas discharge hole 3 is formed into a swirling flow, in which the injection direction is non-perpendicular to the tangent to the combustion nozzle outer periphery, and the combustion gas flow is in the opposite direction to the air flow from the combustion air discharge hole 2. A structure that forms a swirling flow that collides with the swirling flow from the opposite direction, or a structure in which the combustion gas discharge hole 3 is formed so that the injection direction is in the direction of the burner axis or in a direction inclined with respect to the direction of the burner axis. structure,
Furthermore, a structure that gives the air discharge hole 2 an inclination angle (torsion angle) toward the burner opening direction with respect to the radial direction of the burner tile may be employed alone or in combination. In addition, in order to expand the heating area by the burner, at least the inner wall of the burner tile 1 on the side where the combustion air discharge hole is formed is provided with a widening angle such that the inner diameter of the burner increases toward the tip opening side;
Furthermore, in order to facilitate the formation of the air discharge hole 2,
A structure may also be adopted in which a swirling flow path for combustion air is provided in the wall of the cylindrical burner tile along the circumferential direction of the burner, and a plurality of combustion air discharge holes are provided to communicate the swirling flow path with the inside of the burner. be able to. [Example] Table 1 shows that 0.8% Si-containing steel was produced using the continuous annealing furnace (test furnace) of the present invention as shown in FIG. A cold-rolled steel strip (thickness: 1.6 mm) was heat-treated (heated to 700°C, soaked for 1 minute), then subjected to chemical conversion treatment, and its surface treatment characteristics were evaluated.

[Table] According to this, oxidation of steel strips subjected to continuous annealing using the heating furnace of the present invention is suppressed to a lower level than that of steel strips made by combining a conventional non-oxidizing heating furnace and a reduction furnace. It can be seen that the adhesion of the chemical conversion film was also ensured well. [Effects of the Invention] According to the present invention described above, it is possible to heat the steel strip without oxidizing it from beginning to end, and sometimes Si,
Since it is not necessary to oxidize alloying elements that form stable oxides such as Al and Ti, it is possible to obtain products with excellent surface quality even for steel strips containing these types of alloying elements.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view showing an embodiment of the heating furnace of the present invention. 2 and 3 show an example of a heating burner to be applied to the heating furnace of the present invention.
The figure is a longitudinal sectional view, and FIG. 3 is a sectional view taken along the line -- in FIG. FIG. 4 shows an example of measurement of the non-equilibrium region forming range in the heating burner shown in FIGS. 2 and 3. FIG. FIG. 5 similarly shows the reduction heating characteristics of the heating burner. Figures 6 to 10 show the characteristics of the heating burner shown in Figures 2 and 3, and Figure 6 shows the distance N between the fuel gas discharge hole and the air discharge hole in the burner axial direction.
The relationship between the distance from the burner outlet, gas temperature, and O ₂ concentration ionic strength when set to 0.25D. Figure 7 shows the relationship between the distance N between the fuel gas discharge hole and the air discharge hole in the burner axial direction and the unreacted O ₂ in the burner. Axial remaining distance L ₀
Figure 8 shows the relationship between the distance L from the burner outlet and gas temperature, O ₂ concentration, and ion strength when the distance N is +0.1D. Figure 9 shows the relationship between the fuel gas discharge hole and the air discharge hole. Figure 10 shows the relationship between the distance N and the burner tile rear wall temperature Tb, and the relationship between the distance L from the air discharge hole to the burner outlet and the distance L _R to the end of the non-equilibrium region. In the figure, 8 indicates a heating furnace, and A indicates a heating burner.

Claims (1)

[Claims] 1 A heating burner capable of forming a non-equilibrium region in the flame containing combustion intermediate products and no unreacted oxygen is installed so that the flame of each burner is approximately perpendicular to the steel strip, and the steel strip is located in the non-equilibrium region. A direct-fired heating furnace used in continuous annealing equipment for steel strips, in which multiple units are arranged in the entire effective heating range of the heating furnace so as to collide with the surface.