JPH07138635A - Non-oxidizing heating method of steel - Google Patents

Abstract

PURPOSE:To provide a non-oxidizing heating method of a steel by a direct firing heating system which does not impinge flame directly on the steel. CONSTITUTION:In a combusion system which does not impinge the flame of each burner 8, 9 directly on the steel 7 by burning plural burners 8, 9 arranged in a furnace at <=1.0 of air ratio to each, fuel gas flow rate of the each burner 8, 9 is uniformized and air flow rate is distributed so that the air ratio becomes alternately high and low in the width direction of the furnace and/or in the longitudinal direction of the furnace and the flames having <1.0 air ratio are filled in the furnace and heated. By this method, and productivity and the quantity of the treated steel plate are improved by enlarging the reduction area at the treating temp.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-oxidizing heating method for steel products by a direct flame heating system in which a flame does not directly collide with steel products.

[0002]

2. Description of the Related Art As a non-oxidizing heating technology for steel materials, there are an indirect heating method and an open flame heating method. The former indirect heating method is
For example, as shown in the radiant tube heating furnace 21 of the horizontal continuous annealing furnace 19 in FIG. 10, the atmosphere in the furnace is (N ₂ + H ₂ ).
Since the non-oxidizing atmosphere of the gas is used, the non-oxidizing heating property is extremely good, but due to indirect heating, as shown in the comparison of the heating rate of FIG. 12 and the comparison of the responsiveness when changing the furnace temperature of FIG. There is a drawback that the heating capacity is smaller than that of direct flame heating. On the other hand, the latter open flame heating method has a large heating capacity. In this direct-fired heating system, for example, as shown in the direct-fired non-oxidizing heating furnace 20 of the horizontal continuous annealing furnace 19 in FIG. 10, a large number of burners 22 are horizontally installed inwardly on the side wall of the furnace to directly burn the flame. As shown in the direct fire non-oxidizing furnace type that does not collide with the steel material and the direct fire heating zone 24 of the vertical continuous annealing furnace 23 of FIG. 11, a large number of burners are provided on the heating zone wall toward both sides of the thin steel sheet 26 to be heated. 25 are provided, and there are two types of type in which a flame is directly collided with a steel material.

FIG. 2 shows the relationship between the combustion gas of the coke oven and the redox of the steel material. From this figure, there is a slight change depending on the temperature condition, but the air ratio m is about 0.45.
It can be seen that the atmospheric gas does not become reducible unless it is lowered to below. Therefore, if the air ratio is 1.0 or less (if the air ratio is reduced too much, the amount of fuel used increases significantly and soot generation becomes a problem. In many cases, even if operated), in a direct fire non-oxidizing furnace that does not directly impinge on the combustion area of the flame, the steel material is actually slightly oxidized.

On the other hand, in the heating method in which the flame is directly collided with the object to be heated and the air ratio is 1.0, the steel material can be heated to a temperature of more than 800 ° C. without oxidation or reduction. . This is because, unlike the combustion completed gas, reducing active species are present in the flame in which combustion is progressing. However, since this type of heating method requires that the burner and the object to be heated be placed close to each other, it can be used in vertical furnaces for thin steel sheets, but it is not necessary in horizontal furnaces for thin steel sheets, slabs, billets, and steel pipes. Oxidative heating is difficult to apply.

The present invention provides a non-oxidizing heating method which can be applied to a field where it is difficult to apply the conventional direct flame reduction heating method. The following is an example of the conventional direct flame reduction heating method. .

That is, in the direct flame of the continuous annealing furnace,
In the previous stage of heating, the region where the combustion reaction of the combustion gas burned with an air ratio of 1.0 ended is applied to the surface of the steel strip to heat it, and in the latter stage of heating, the combustion gas burned with an air ratio of 1.0 or less. The method of heating by applying a region containing no unreacted oxygen and having intermediate ions in the middle of the reaction to the surface of the steel strip (Japanese Patent Application Laid-Open No. 6-58242).
3-5456) or a region surrounding the material to be heated is burned at an air ratio of less than 1.0, and the unburned components generated by the burning of this region are burned outside the region (Japanese Patent Laid-Open No. Hei 10 (1999)). 3-170614).

However, in the former heating method disclosed in Japanese Patent Laid-Open No. 63-5456, the combustion conditions such as the heating rate of the steel strip, the line speed and the like, and the combustion conditions such as the air ratio due to the furnace pressure change. Due to this, it is impossible to control the combustion conditions at a constant level, so the steel strip and flame (burner)
The distance to and needs to be adjusted frequently. Further, the latter combustion method described in JP-A-3-170614 is
This is a method in which both edges of the steel strip are mainly subjected to non-oxidative heating by direct flame. The object to be heated is heated in a flame region of an air ratio of less than 1.0 (primary combustion air ratio of 0.45), and then the vicinity of flame Since the air required for the unburned components is blown from the upper and lower sides through the slit-shaped port, the control of the blowing position and the amount of blown air becomes complicated.

[0008]

As described above, in the conventional non-oxidizing heating method of the direct flame heating method for steel materials, the combustion conditions cannot be controlled to be constant, so that the steel strip and the flame (burner) are combined. It is necessary to adjust the distance frequently, and since the combustion air ratio is changed between the front stage and the rear stage during heating, there are various problems such as complicated control of the blown air amount.

The present invention solves the problems found in the above-mentioned conventional methods, and can be applied even to the fields which were difficult to apply by the conventional non-oxidizing heating method of the direct flame heating system. A heating method is provided.

[0010]

In order to achieve the above object, the inventors have conducted an experiment in a small experimental furnace to reduce the air ratio by using a burner that burns coke oven gas.
At that time, the burner used had good compatibility with the fuel gas and air. As a result, as shown in Fig. 7, when the air ratio m is as large as 0.9, the flame boundary can be clearly identified (Fig. A), but when the air ratio m is reduced to 0.7, the flame becomes large (Fig. B) Further, when the air ratio m is further reduced to 0.55, the flame fills the entire furnace (Fig. C). Therefore, it is possible to fill the entire furnace with flames by reducing the air ratio during combustion. The air ratio with which the flame fills the furnace varies depending on the burner, and in a burner in which the fuel gas and air are mixed slowly, a filled flame can be obtained even with an air ratio of 0.85. As described above, when the flame is filled in the furnace, reducing active species are generated along with the combustion reaction, so that the oxidation of the steel material can be effectively prevented. If the air ratio is too low, the heating efficiency is significantly deteriorated. To prevent this, it is necessary to form a flame with a high air ratio and raise the average air ratio. The present invention has been completed based on the above findings.

That is, the non-oxidizing heating method of the steel material of the present invention is a combustion method in which a plurality of burners arranged in the furnace are burned at an air ratio of 1.0 or less and the flame of each burner is not directly collided with the steel material. In, the fuel gas flow rate of each burner is made equal, and the air flow rate is distributed so that the air ratio becomes higher and lower alternately in the width direction of the furnace and / or the longitudinal direction of the furnace, and the air ratio is less than 1.0 in the furnace. Fill with the flame and heat.

[0012]

FIG. 1 shows a box-type heating furnace having a fireproof wall structure of an experimental apparatus for carrying out the present invention, and a steel material 3 (steel type: SS40).
0, size: thickness 1.0 mm x width 150 mm x length 55
(0 mm) is provided with a cooling box 2 through which the center of the furnace is passed vertically and the atmosphere is a non-oxidizing atmosphere of (N ₂ + H ₂ ) gas, and combustion flame is directly applied to the steel material 3 inside the furnace. Four burners are arranged so as not to hit. Then, the burners, ... Are made to have an equal fuel flow rate and are burned at a flow rate corresponding to an air ratio of m0.7. At this time, the air distribution is set to the same amount for the burner and the burner, and the air ratio between the burner + and the burner + is set to 3: 7 to 4: 6.

In the above, even in the combustion state of the burner with a large air distribution, combustion is performed at an air ratio of 1.0 or less. on the other hand,
The burner with a small air distribution is an extremely low air ratio combustion aiming at slow combustion due to excessive fuel, and produces H radicals having a strong reducing property and prevents the oxidation of steel materials.

When coke oven gas (COG) is used as fuel, the theoretical air amount Aov is 4.72 Nm ³ / hr.
Is. COG = 1Nm for each burner
When burning ³ / hr, when air ratio m = 0.7,
The total air amount is 13.22 Nm ³ / hr according to the following formula 1.

[0015]

Equation 1 Total (~) air volume = 13.22Nm ³ / hr

When the total air amount is 13.22 Nm ³ / hr and the air ratio of the burner + and the burner + is 3: 7, the air amount of the burner + with a small air distribution is 13.22 Nm ³ / hr. × 3/10 is 1.98 Nm ³ / hr per unit, and the air volume of the burner + with a large air distribution is 13.22 Nm ³ / hr × 7/1
At 0, it is 4.63 Nm ³ / hr per line. That is, the air ratio m with a small air distribution is 1.98 Nm ³ / hr.
/4.72 Nm ³ /hr=0.42, and the air ratio m with a large air distribution is 4.63 Nm ³ /hr/4.72 Nm ³ / hr.
= 0.98, and the burner with a large air distribution also has an air ratio of m <1.0, and combustion and heating are performed under non-oxidizing conditions.

If the air ratio of the burner + and the burner + is set to 4: 6, the burner with a small air amount,
Has an air ratio m of 2.65 (Nm ³ /hr)/4.72.
(Nm ³ /hr)=0.56, the air ratio m of the burner with a large air amount is 3.97 (Nm ³ / hr) / 4.
72 (Nm ³ /hr)=0.84, and when m <1.0, combustion and heating are performed under non-oxidizing conditions.

[0018]

【Example】

Example 1 As shown in FIGS. 3 and 4, in a continuous annealing furnace composed of a pre-tropical zone 4, a heating / soaking zone 5 and a cooling zone 6, a plain steel sheet was heat-treated under the following conditions. In the preheat zone 4, the unburned exhaust gas 10 produced in the heating and soaking zone 5 is blown through the duct, and the auxiliary burner 11 is used to support the air ratio m = 1.0. In the heating and soaking zone 5, the supply amount of the fuel coke oven gas is made the same with the burner 8 having a small air amount and the burner 9 having a large air amount, and the air ratio m in the total exhaust gas is 0.7, The burner 8 having a small amount of air and the burner 9 having a large amount of air are burned in a staggered arrangement of 3: 7 or 4: 6. In the cooling zone, a non-oxidizing gas such as nitrogen gas is sprayed from the upper and lower surfaces of the steel plate 7 to cool it. As shown in FIG. 8, the burner has a small-diameter fuel injection pipe 14
A burner having a good mixing property, which is composed of a double pipe of a large-diameter air injection pipe 15 and

The steel plate to be treated has a thickness of 1.2 mm and a width of 900 m.
m steel strip at a line speed of 60 m / min. The heat pattern at this time is shown in FIG. In addition, the results of examining the non-oxidizing heating range (operations with a surface oxide film thickness of 100 Å or less) in this heat treatment are shown together with the results of conventional non-oxidizing heating under the same conditions for all burners (total air ratio is constant). As shown in FIG. FIG. A shows the results obtained by carrying out the present invention. In both cases where the air ratio of the low air ratio burner and the high air ratio burner is 3: 7 and 4: 6, it is possible to perform non-oxidative heating up to 800 ° C. On the other hand, the conventional method was possible up to 700 ° C. as shown in FIG. From this result, the non-oxidizing heating range is about 10 by the practice of this invention.
It can be seen that the temperature has been expanded by 0 ° C.

Example 2 Instead of the burner having a good mixing property of the heating and soaking zone 5 in the continuous annealing furnace shown in FIG. 3 used in Example 1, a small diameter fuel injection pipe 16 shown in FIG. Primary air injection pipe 17
The same ordinary steel material as in Example 1 was heat-treated by using a burner which is of a two-stage combustion type in which a secondary air injection pipe 18 having a large diameter is combined and which can delay the mixing of fuel and air. In this case, unlike the first embodiment, the air ratio of each burner can be the same. Although the air ratio of this burner was 0.85, the flame could be filled in the furnace, and the same effect as in Example 1 was obtained.

[0021]

EFFECTS OF THE INVENTION According to the non-oxidizing heating method for steel materials of the present invention, the production of scale can be reduced, and the productivity and the quality of the processed steel sheet are improved by expanding the reduction region at the processing temperature. As a result, the installation of the indirect heating zone, which is required in the conventional non-oxidizing heating furnace, becomes unnecessary, the installation space of the furnace can be reduced, and the construction cost can be reduced.

[Brief description of drawings]

FIG. 1 is an experimental apparatus for carrying out a non-oxidizing heating method for a steel material according to the present invention, in which A is a plan view and B is a front view.

FIG. 2 is a graph showing the oxidation and reduction regions of iron in the coke oven gas combustion gas.

FIG. 3 is a vertical cross-sectional front view showing an example of a continuous annealing furnace for carrying out the non-oxidizing heating method for a steel material of the present invention.

4 is a vertical sectional side view of the continuous annealing furnace of FIG.

5 is a graph showing a heat pattern when a steel material is heated using the continuous annealing furnace of FIG. 3 according to Example 1. FIG.

FIG. 6 is a graph showing a comparison of reduction ranges, where A is a graph showing the relationship between the air amount ratio (low air ratio burner / high air ratio burner) and the material temperature in the case of implementing the present invention, and B is the conventional graph. It is a graph which shows the case by implementation of a method by the relationship between an air ratio and material temperature.

FIG. 7 is an explanatory diagram showing a change in flame in a combustion furnace due to a difference in air ratio, where A is an air ratio m = 0.9, B is an air ratio m = 0.7, and C is an air ratio. This is the case when m = 0.55.

FIG. 8 is a vertical cross-sectional view showing an example of a burner having good mixability used in the practice of the present invention.

FIG. 9 is a vertical cross-sectional view showing an example of a burner that can delay mixing of fuel and air used in the practice of the present invention.

FIG. 10 is an explanatory diagram showing an example of a horizontal continuous annealing furnace.

FIG. 11 is an explanatory diagram showing an example of a vertical continuous annealing furnace.

FIG. 12 is a graph showing a comparison of heating rates of direct flame heating and indirect heating (radiant tube heating).

FIG. 13 is a graph showing the responsiveness when changing the furnace temperature between direct fire heating and indirect heating (radiant tube heating).

[Explanation of symbols]

 1 Box Type Heating Furnace 2 Cooling Box 3 Steel Material 4 Pre-Tropical Zone 5 Heating / Soaking Zone 6 Cooling Zone 7 Steel Plate 8 Burner with Small Air Volume 9 Burner with Large Air Volume 10 Unburned Exhaust Gas 11 Combustion Burner 12 Non-oxidizing Gas 13 Roller 14 Fuel injection pipe 15 Air injection pipe 16 Fuel injection pipe 17 Primary air injection pipe 18 Secondary air injection pipe 19 Direct fire non-oxidizing heating furnace 20 Radiant tube heating furnace 21 Direct heating heating zone 22 Radiant tube heating zone 23, 24 Burner, , Burner

Claims (1)

[Claims] 1. A combustion system in which a plurality of burners arranged in a furnace are burned at an air ratio of 1.0 or less, and the flame of each burner is not directly collided with the steel material, the fuel gas flow rate of each burner is equalized. And a steel material characterized in that the air flow rate is distributed so that the air ratio becomes alternately high and low in the width direction of the furnace and / or in the longitudinal direction of the furnace, and a flame having an air ratio of less than 1.0 is filled in the furnace. Non-oxidizing heating method.