JPH0369972B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Field of Application) The present invention relates to a direct flame reduction heating method for steel strip. (Prior Art) As a method of heating a steel strip, a direct flame heating method is known in which the steel strip is heated by injecting high-temperature combustion gas from a burner onto the steel strip. The advantage of direct heating is that the heating rate is significantly faster than indirect heating using radiant tubes. By the way, in annealing equipment such as continuous annealing equipment for steel strips and continuous hot-dip galvanizing equipment (hereinafter referred to as CGL), if it were possible to heat the steel strip in a completely non-oxidizing state by direct flame heating, it would be possible to heat the steel strip in a completely non-oxidized state. Subsequent post-processing is simplified, providing many advantages. In other words, there is no need for a reducing atmosphere gas in a soaking furnace using a radiant tube, and there is no need to pick up rolls in a heating furnace or the like. In addition, the Sendzimir CGL line eliminates the need for a reduction furnace, and continuous annealing equipment eliminates the need for pickling. (Problems to be Solved by the Invention) However, with the conventionally commonly used direct flame heating method, it has not been possible to heat a steel strip completely without oxidation at a high heating rate. In other words, according to the oxidation-reduction theory between steel and combustion gases in an equilibrium state, as shown in Figure 7, in order to heat steel from 20°C to 700°C in a reduced state, CO/CO ₂ > The gas composition must be 1.5, H ₂ /H ₂ O > 4.0, and to obtain a combustion product gas with such a composition, combustion must be performed at an air ratio of 0.5 or less. Therefore, there are problems such as a large amount of unburned gas being generated and a marked deterioration in heating efficiency, and the fact is that conventional furnaces called non-oxidizing furnaces (NOF) are accompanied by some oxidation. On the other hand, the American company SelaS proposes a method of direct reduction heating of a steel strip at an air ratio of about 0.9 using a premix burner called a radiant cup burner (US Pat. No. 3,320,085). This method is thought to utilize a reduction reaction under non-equilibrium conditions during the combustion process, as described in JP-A-60-121230. However, premix burners involve the risk of backfire, require a flashback prevention device, and have other problems, such as the inability to recover sensible heat from exhaust gas through air preheating. In response to these, a method has been proposed in which direct flame reduction heating is performed using a diffusion burner by rapidly mixing fuel and air using a swirling air flow, as seen in Japanese Patent Publication No. 62-21051. There is. With this method, there is no risk of backfire, and it is also possible to recover sensible heat from the exhaust gas by preheating the air. However, in diffusion combustion due to swirling flow, the flame becomes extremely short, and the range suitable for reductive heating exists only in a very narrow area immediately adjacent to the burner, and vibrations of the steel strip cause it to deviate from the reduction range, causing the steel strip to oxidize. In extreme cases, the burner and the steel strip may come into contact. To summarize the above, in order to efficiently heat the steel strip in a direct flame reduction state, it is necessary to use the combustion process in a non-equilibrium state, that is, to use the reduction reaction in the flame, and to use the fuel gas in a state where there is a slight air shortage. It is necessary to mix the fuel and air sufficiently so that no unburned oxygen remains in the flame. The above-mentioned U.S. Patent No. 3,320,085 and Japanese Patent Publication No. 62-21051 have proposed burners that meet these conditions, but there remain problems such as the risk of backfire and the narrow range suitable for reductive heating. There is. The present invention was made in view of the above-mentioned circumstances, and its purpose is to provide a method for reductively heating a steel strip stably and over a wide range using a burner that does not require premixing of air and fuel gas. That is. (Means for Solving the Problems) The gist of the present invention is that when heating a steel strip, a double pipe with 5 or more holes is provided per 100 cm ² of bottom area at the bottom of the burner tile, and the inner pipe is filled with fuel gas. (or air), flow air (or fuel gas) into the outer tube parallel to the burner center axis, adjust the air ratio (ratio to the theoretical air amount) to 0.7 to 0.9, and then connect the steel strip and burner tile. distance from the bottom
The purpose is to adjust the heating to 100 to 400 mm. The technical points of the direct flame reduction heating of the present invention are as follows. That is, it is necessary to generate a flame having a reducing ability over a wide range, and to sufficiently mix fuel gas and air so that no unburned oxygen remains in the flame. After conducting various experiments as a means to achieve the above conditions using a diffusion burner, we discovered that a countless number of diffusion flames can be gathered together and used as a single flame. A burner designed based on the above idea is shown in FIGS. 1 and 2. In FIG. 1, a burner tile bottom 1 is provided with five or more double-pipe type openings 2 per 100 cm ² of bottom area. The double-pipe vent hole 2 is composed of an outer pipe 3 and an inner pipe 4, and air flows through the outer pipe 3 and fuel gas flows through the inner pipe 4 parallel to the burner center axis.
Alternatively, the same effect can be obtained when fuel gas flows through the outer tube 3 and air flows through the inner tube 4 parallel to the burner center axis. (Function) Next, the function of the burner according to the present invention will be described based on the results of basic experiments. That is, the reduction region was investigated by heating experimental pieces that had been previously oxidized in the atmosphere while varying the number of holes, air ratio, and distance from the bottom of the burner tile. Table 1 shows the results of changing the number of leaf holes. For example, if the number of leaf holes is set to 4 per ^100cm2 of the bottom area of the burner tile, the diameter of each leaf hole becomes larger, and the diameter of each leaf hole increases. Since the interval also becomes large, an oxidizing portion is generated in a part of the reduced region, and as a result, unevenness between the oxidized portion and the reduced portion occurs on the steel strip, which is not preferable. A good number of leaf holes is 5 or more per ^100cm2 of burner tile area. Next, Table 2 shows the results of changing the air ratio. If the air ratio is less than 0.7, soot will precipitate and the surface of the test piece will turn black. The flame temperature itself also decreases, and the heating rate decreases. On the other hand, at an air ratio of 1.0, there are almost no unburned components such as CO and _H2 in the combustion gas width, so if, for example, there is insufficient mixing locally, the residual oxygen will directly oxidize the steel material. . According to experimental results, an air ratio in the range of 0.7 to 0.9 is suitable. Also,
Table 3 shows the results of varying the distance from the bottom of the burner tile. A distance of less than 100 mm from the bottom of the burner tile corresponds to a region where fuel gas and air do not completely diffuse and burn, and rapid oxidation due to residual oxygen is observed. On the other hand, if it is more than 400mm from the bottom of the burner tile,
No reduction reaction occurs under non-equilibrium conditions during the fuel process, and oxidation occurs with CO ₂ and H ₂ O according to the conventional oxidation-reduction equilibrium reaction. The experimental results show that 100~ from the bottom of the burner tile.
A range of 400mm is preferred.

【table】

【table】

[Table] (Example) Next, the results of heating a steel plate with this burner will be explained. Combustion conditions: Fuel; Coke oven gas-air ratio; 0.8 Combustion amount; 50000 kcal/Hr Test burner: (1) Burner of the present invention (2) Conventional air swirl burner (see Figure 3) Rated combustion amount, burner diameter Both test results are the same: Figure 5 shows the results of heating a test piece that had been atmospherically oxidized in the burner of the present invention to 800°C and cooling it with nitrogen gas. Furthermore, FIG. 6 shows the results of a similar test conducted using a conventional rotary burner. In FIGS. 5 and 6, the horizontal axis is the distance from the bottom of the burner tile, and the vertical axis is the air ratio. After heating a test piece that has been oxidized in the atmosphere in advance, the oxide film is reduced in the reduced region, giving it a metallic luster. Similarly, in the oxidized region, an oxide film grows and appears black. In the conventional rotary burner, as shown in Fig. 6,
The flame is short, so the reduction area is narrow and exists approximately 100mm from the bottom of the burner tile. On the other hand, in the burner of the present invention, as shown in FIG. 5, the flame is long and the reduction region exists within a range of 150 to 400 mm from the bottom of the burner tile. (Effects of the Invention) As explained above, the present invention provides a reduction area over a wide range, and therefore, even if the distance between the burner and the steel strip changes due to vibration of the steel strip, etc.
The steel strip can be heated rapidly without oxidizing. There is also no risk of backfire, and sensible heat recovery from exhaust gas is possible through air preheating. In the Examples, the results were described using a cylindrical burner, but similar effects can be obtained using an elliptical or rectangular cylindrical burner.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of the burner of the present invention, FIG. 2 is a cross-sectional view taken along the - line in FIG. 1, FIG. 3 is a cross-sectional view of a conventional rotary burner, and FIG.
5 is a diagram of a heating experiment using the burner of the present invention, FIG. 6 is a diagram of a heating experiment using a conventional burner, and FIG. 7 is a diagram of the oxidation-reduction equilibrium of steel. 1: Bottom of burner tile, 2: Double pipe discharge hole, 3: Outer pipe, 4: Inner pipe.

Claims (1)

[Claims] 1. When heating the steel strip, at the bottom of the burner tile, 2 or more holes with 5 or more holes per ^100cm2 of the bottom area.
A heavy pipe is installed, and the inner pipe is filled with fuel gas (or air).
, flow air (or fuel gas) into the outer tube parallel to the burner center axis, adjust the air ratio (ratio to the theoretical air amount) to 0.7 to 0.9, and adjust the distance between the steel strip and the bottom of the burner tile. A direct-fire reduction heating method for steel strip, which is characterized by adjusting and heating the steel strip to 100 to 400 mm.