JPH0441621A - Continuous heat treatment for steel strip - Google Patents

Abstract

PURPOSE:To improve thermal efficiency in continuous heat treatment and to reduce equipment costs by burning unburnt gas in a combustion exhaust gas from a direct-fire heating furnace by blowing preheated combustion air from an auxiliary burner into a preheating furnace in a position lying to the outlet side of the preheating furnace. CONSTITUTION:A fuel (coke-oven gas) is subjected to incomplete combustion at 0.8 air ratio by means of a nonoxidizing combustion burner in a direct-fire heating furnace 3 to apply direct-fire heating to a steel strip S, and a combustion exhaust gas (a) generated in the direct-fire heating furnace 3 is allowed to flow into a preheating furnace 2. The unburnt gas in the combustion exhaust gas (a) is burned by means of combustion air blown from a burner 11 in the position lying to the outlet side of the preheating furnace 2, and the steel strip S is preheated by means of a gaseous mixture (b) of the combustion exhaust gas (a) from direct-fire heating furnace 3 and the combustion exhaust gas from the unburnt gas. By this method, the oxidation of the steel strip S can be prevented, and the high quality steel strip S can be obtained.

Description

[Detailed description of the invention] [Industrial application field] This invention relates to a continuous heat treatment method for steel strip.

The present invention is utilized in continuous annealing lines for cold-rolled deep-drawn steel sheets, stainless steel sheets, heat treatment lines for electrical steel sheets, hot-dip plating lines including continuous heat treatment, and the like.

[Prior Art] Some continuous heat treatment equipment for steel strips is equipped with a direct heating furnace instead of an indirect heating furnace using a radiant tube. A direct-fire heating furnace is equipped with a group of burners directed toward the surface of a running steel strip, and directly heats the steel strip with combustion flame from the burners. Also, a non-oxidizing combustion burner is used as a burner. The non-oxidizing combustion burner directly heats the steel strip with a combustion flame, prevents the formation of oxide scale, and further reduces the amount of scale by receding the oxide scale that has already been generated.

The combustion exhaust gas generated in the direct-fired heating furnace is led to a preheating furnace connected to the inlet side of the direct-fired heating furnace. The steel strip is preheated by combustion exhaust gas in the preheating furnace to reduce heat loss due to the exhaust gas. In addition, in a direct-fired heating furnace equipped with the above-mentioned non-oxidizing combustion burner, fuel is combusted at an air ratio of 0.7 to 0.95, so the combustion exhaust gas from the direct-fired heating furnace contains large amounts of carbon monoxide, hydrogen, etc. Contains unburned matter. Discharging combustion exhaust gas containing unburned matter directly outside the furnace will result in increased heat loss, so conventionally the unburned matter is combusted before being discharged outside the furnace, and the combustion heat is used to preheat the steel strip. There is.

As a technique for burning the unburned content in the combustion waste in this manner, there is, for example, the technique disclosed in Japanese Patent Publication No. 53-39848.

In the continuous annealing furnace disclosed in Japanese Patent Publication No. 53-39848, a preheating furnace is equipped with an air nozzle and an afterburning chamber. A large amount of unburned gas contained in the combustion exhaust gas from the direct-fired furnace is completely combusted with excess air in the afterburning chamber and then supplied to the preheating furnace.

[Problem B to be Solved by the Invention] In continuous heat treatment equipment, the burner load of the direct-fired heating furnace may be reduced depending on the operating conditions. In such a case,
The temperature of the combustion exhaust gas from a direct-fired heating furnace may drop to below the ignition temperature of unburned gas. If unburned matter is not combusted, heat loss will increase. Further, oxygen in the air for burning unburned matter remains, and the remaining oxygen rapidly oxidizes the steel strip in the preheating furnace. When the steel strip is oxidized, there is a problem in that the oxidized scale adheres to the hearth roll, causing defects in the plate, or causing a decrease in surface adhesion.

SUMMARY OF THE INVENTION Therefore, the present invention aims to provide a method for continuous heat treatment of steel strips, in which unburned components contained in combustion exhaust gas from a direct-fired heating furnace can be reliably combusted in a preheating furnace.

[Means for Solving the Problems] The method for continuous heat treatment of a steel strip of the present invention includes the following steps: In this method, combustion air is blown into the furnace from an auxiliary burner near the exit side of the preheating furnace to burn unburned content in the combustion exhaust gas from the direct-fired heating furnace. Infuse.

In order to effectively utilize the heat generated by the combustion of unburned components in the combustion exhaust gas for preheating the steel strip, the unburned components are combusted near the exit side of the preheating furnace. Although the method of preheating the combustion air is not particularly limited, a method of preheating using exhaust gas from a preheating furnace can improve the thermal efficiency of continuous heat treatment and reduce the cost of equipment. The preheating temperature is approximately 300°C or higher, preferably 400°C or higher. It is desirable that the combustion air be blown in parallel to the steel strip surface. Blowing combustion air parallel to the steel strip surface prevents the steel strip from being oxidized by the combustion air, and also prevents the steel strip surface from being directly heated by the combustion flame from the auxiliary burner. Direct heating may cause the steel strip to be preheated to a higher temperature than necessary. The burner combusts unburned substances by burning the combustion exhaust gas at an excess air ratio, but in order to prevent the burner from extinguishing and to reliably maintain combustion, a small amount of fuel is supplied to the burner. Coke oven gas, light oil, etc. are used as fuel to be supplied to the burner. The amount of air supplied is the amount necessary for combustion of the unburned gas produced in the direct-fired heating furnace.

The direct-fired heating furnace may be vertical or horizontal.

[Operation: The unburned components contained in the combustion exhaust gas from the direct-fired heating furnace are combusted by the combustion air blown through the burner. Even if the temperature of the combustion exhaust gas from the direct-fired heating furnace drops, the combustion air is preheated, so the ignition temperature of the unburned components is maintained.
There is no extinguishing.

[Example 1] FIG. 1 is a schematic diagram of a hot-dip plating facility equipped with a continuous heat treatment facility for carrying out the method of the present invention.

As shown in FIG. 1, the continuous heat-treated film @i consists of a preheating furnace 2, a direct heating furnace 3, a soaking furnace 4, a slow cooling furnace 5, and a rapid cooling furnace 6. Hot-dip plating equipment 8 is provided following continuous heat treatment equipment 1 . The direct-fired heating furnace 3 is provided with a group of non-oxidizing combustion burners (not shown) or in the direct-fired heating zone 3a. The burner 11 is connected to a preheating furnace 2 as shown in FIG.
They are placed on both sides of the steel strip S near the exit side of the steel strip S, and are attached to the furnace side wall. An air preheater 13 is arranged on the inlet side of the preheating furnace 2. The outlet side of the air preheater 13 is connected to the burner 11 via a pipe 15.

In the direct-fired heating furnace 3, the fuel (coke oven gas) is incompletely combusted by the non-oxidizing combustion burner at an air ratio of 0.8, and the steel strip S is directly heated. Since the direct-fired heating furnace 3 has a draft directed toward the preheating furnace 2, the combustion exhaust gas a generated in the direct-fired heating furnace 3 flows into the preheating furnace 2. Near the exit side of the preheating furnace 2, unburned components in the combustion exhaust gas a are combusted by combustion air blown from the burner 11. The steel strip S is preheated by the mixed gas b of the combustion exhaust gas a and the unburned combustion exhaust gas from the direct-fired heating furnace 3. The mixed gas b is guided to the air preheater 13, heats combustion air, and then is discharged from a chimney (not shown).

FIG. 3 is a diagram showing an example of the relationship between preheated air temperature and residual oxygen concentration. As is clear from FIG. 3, when the preheating temperature is about 400° C. or higher, the residual oxygen concentration becomes 0%.

[Effect of the invention] According to this invention, since the combustion air is preheated,
Even if the temperature of the combustion exhaust gas from the direct-fired heating furnace drops, the ignition temperature of unburned gas is maintained and the fire will not be extinguished. As a result, it is possible to improve the thermal efficiency of the continuous heat treatment equipment.

In addition, oxidation of the steel strip due to residual oxygen in the air for combustion of unburned matter can be prevented, and a high-quality steel strip can be obtained.

[Brief explanation of drawings]

Figure 1 shows an example of equipment for carrying out the method of the present invention, and is a schematic diagram of a hot-dip plating equipment equipped with continuous heat treatment equipment, and Figure 2 shows the preheating furnace and direct-fired heating furnace of Figure 1. The drawings and FIG. 3 are diagrams showing an example of the relationship between preheated air temperature and residual oxygen concentration. 1... Continuous heat treatment equipment, 2... Preheating furnace, 3... Direct fire heating furnace, 4... Soaking furnace, 5... Slow cooling furnace, 6...
Quenching furnace, 8...Hot-dip plating equipment, 11--Auxiliary burner, 13--Recubator, a...Combustion exhaust gas from the direct-fired heating furnace, b...Combustion exhaust gas from the direct-fired heating furnace and a mixed gas of unburned combustion exhaust gas, S-... steel strip.

Claims (1)

[Claims] 1. When continuously heat treating steel strips using heat treatment equipment equipped with a preheating furnace and a direct-fired heating furnace connected to the outlet side of the preheating furnace, combustion air is blown into the furnace at a position near the outlet side of the preheating furnace. A continuous heat treatment method for steel strip, which is a method for burning unburned content in combustion exhaust gas from a direct-fired heating furnace, characterized in that the combustion air is preheated and blown into the furnace.