JPH01215930A - Method for continuously annealing steel sheet - Google Patents

Abstract

PURPOSE:To prevent the thickness of an oxide film on a steel sheet from being increased after heating by using a prescribed atmosphere as the atmosphere in a direct fire type heating furnace for heating the steel sheet and by regulating the temp. of the outlet of the furnace to a prescribed value to form a region in which no oxidation reaction takes place or a reducing region. CONSTITUTION:A reducing atmosphere consisting of a reducing component such as H2 or CO and a very small amt. of O2 is used as the atmosphere in a direct fire type heating furnace provided with a direct fire type burner. A beltlike steel sheet is continuously transferred in the atmosphere and heated to a prescribed temp. The temp. of the steel sheet at the outlet of the furnace is regulated to <=400 deg.C to form a region in which no oxidation reaction takes place or to >=800 deg.C to form a reducing region. The steel sheet is then soaked and cooled in accordance with a prescribed pattern.

Description

[Detailed description of the invention] [Industrial field of application] The great invention is a continuously conveyed cold rolled thin steel plate (hereinafter referred to as
Regarding the continuous annealing method in which a thin steel plate (referred to as a thin steel plate) is heated to a predetermined temperature, then soaked and cooled along a predetermined annealing pattern, it is possible to avoid the formation of an oxide film when heating the thin steel plate using a direct flame method. This invention relates to a continuous annealing method.

[Conventional technology]

Conventionally, when heating thin steel sheets in a continuous annealing furnace, an indirect heating method using radiant heat from a radiant tube has been adopted. However, in recent years, from the viewpoint of energy saving, a method has been adopted in which the thin steel plate is directly heated with a direct burner.

By the way, the direct-fired burner used in the above-mentioned direct-fired heating furnace burns fuel at a temperature below the stoichiometric air ratio, and achieves zero temperature at this high temperature.
□It is configured to heat a thin steel plate using a jet of low-concentration non-oxidizing gas, flame, and radiation from burner tiles. However, since a trace amount (approximately 40 ppm) of oxygen remains in the flame from the above-mentioned direct-fired burner, the vicinity of the thin steel plate is considered to be a weakly oxidizing atmosphere, resulting in the problem of easy formation of an oxide film. There is.

Therefore, in order to suppress the deposition of oxide film due to the above-mentioned direct-fired burner, there has been proposed, for example, in Japanese Patent Application Laid-Open No. 55-97432. This detects the surface temperature of a thin copper plate in a direct-fired heating furnace and the amount of oxygen near the thin steel plate, and detects that the detected surface temperature and oxygen amount exceed the upper limit of the allowable oxide film thickness on the surface of the steel plate. The air ratio and fuel flow rate of each burner are controlled to prevent this. That is, the temperature of the thin steel plate is determined in advance.

The relationship between the oxygen concentration in the atmosphere and the oxide film thickness on the surface of the thin steel sheet is determined, and burner combustion conditions and the thin steel sheet temperature are determined so that the detected oxide film thickness becomes an oxide film thickness that can be reduced in the soaking zone following this heating furnace. In this method, the air ratio of the burner and the heating furnace outlet temperature of the TittiEr plate are controlled based on the set conditions.

[Problem that the invention seeks to solve]

However, with the conventional method of controlling the air ratio and fuel flow rate by detecting the oxygen concentration and the temperature of the thin steel sheet, it is difficult to sufficiently heat the thin steel sheet while preventing the formation of an oxide film. That is, according to the relationship between the outlet temperature and the oxide film thickness (see Figure 6) and the relationship between the 02 concentration and the oxide film thickness (see Figure 7) described in the above publication, if the outlet temperature is raised, ,
The oxide film thickness becomes significantly thicker, and therefore, in order to prevent this, the 0□ concentration must be extremely reduced. Therefore, in the end, with this conventional method that focuses only on the thin steel sheet temperature and O2 concentration, there is a limit to how high the heating furnace outlet temperature can be while keeping the oxide film thin.

An object of the present invention is to provide a continuous annealing method for a thin steel sheet that can secure a necessary heating furnace exit temperature while suppressing the formation of an oxide film on the thin steel sheet.

[Means for solving problems]

In order to achieve the above object, the inventors of the present invention have conducted intensive research and found that the formation of an oxide film by direct heating cannot be determined solely by the oxygen concentration in the atmosphere and the temperature of the thin steel sheet. We focused on the fact that the atmospheric gas components in the direct-fired heating furnace and the heating rate of the thin steel sheet in the heating furnace also have an effect.

In particular, even when combustion is carried out at a low air ratio using a direct-fired burner, about 40 PPm of free oxygen always exists in the unburned state in the combustion gas, and in the past, the thickness of the oxide film formed in this state was simply It was thought that the thickness of the thin steel sheet increases as the temperature rises, but it was discovered that the thickness actually peaks in a certain temperature range, depending on the balance between oxidizing and reducing components in the atmosphere and the relationship with the sheet temperature. Ta. Furthermore, it has been found that in the temperature range where the oxide film thickness peaks, the thickness of the oxide film produced can be adjusted by changing the heating temperature.

Therefore, the specific invention of the present application provides a continuous annealing method for a TR steel plate, in which a thin steel plate is continuously conveyed and heated to a predetermined temperature in a direct-fired heating furnace, and then heat-treated along a predetermined annealing pattern. The atmosphere of the direct-fired heating furnace is, for example, CO
Hydrogen by using G, LDG gas as fuel and reducing the air ratio to 0.8 or less.

It is characterized by controlling the reducing atmosphere to include a trace amount of oxygen in reducing components such as carbon monoxide, and controlling the outlet temperature of the direct-fired heating furnace of the FifA plate to 400°C or lower or 800°C or higher. There is.

The reason why the outlet temperature was set below 400°C or above 800°C is because, as described later, in direct flame heating in a reducing atmosphere, the oxide film thickness is significantly thicker in the temperature range of 400 to 800°C. However, the temperature is below 400℃.

This is because it has been found that at 800° C. or higher, the oxide film thickness can be made approximately the same as the oxide film thickness on the original plate before heating (at room temperature).

Further, the related invention of the present application provides the continuous annealing method for thin steel sheets, in which the atmosphere in the direct-fired heating furnace is made into a reducing atmosphere containing a trace amount of oxygen in addition to reducing components such as hydrogen and carbon monoxide; The heating rate of the thin steel sheet in the direct-fired heating furnace is set to 50°C/sec or more at least between 400 and 600°C.

Here, the heating temperature between 400 and 600℃ is 5
The reason why the above temperature range is set at 0°C/sec or higher is that, as will be clear from the explanation below, the above temperature range is a region where oxidation is easy. This is because the growth of the oxide film can be prevented by shortening as much as possible. Note that the rate of temperature increase outside the above-mentioned temperature range has almost no effect on the growth of the oxide film, and is not particularly limited, but it goes without saying that the faster the rate, the shorter the processing time can be. However, in reality, there are problems with the burner arrangement method, heating capacity, etc., and if the speed is faster than necessary, the equipment cost will be excessive, so it is usually better to slow it down.

Next, an experiment in which the above-mentioned numerical values were found in the specific invention of the present application will be explained.

In this experiment, COG gas and LDG gas were used as fuel for the direct-fired burner, and by setting the air ratio to 0.8, the combustion gas atmosphere was changed to about 40 PPm of reducing components such as hydrogen and carbon monoxide. A reducing atmosphere containing a trace amount of oxygen was created, and changes in the oxide film thickness due to changes in the exit temperature of the Fti plate were investigated.

Figure 1 shows the experimental results. This figure is '1ilti
It is a characteristic diagram showing the relationship between the temperature of the @ plate and the oxide film thickness, in which the curve a is the characteristic of COG gas and the curve vAb is the characteristic of LDG gas. Note that the oxide film thickness of the original plate before heating is about 20 people. What is clear from Fig. 1 is that when heating in the above-mentioned reducing atmosphere, the heating temperature of the thin steel plate is 400°C.
Until the temperature reaches 600°C, no oxidation occurs, forming an oxidation unreacted region (■), and once this region I is exceeded, an oxide film begins to form, and an oxidized growth region (■) is formed with a peak temperature of about 600°C. There is.

When the heating temperature exceeds 600°C, the reduction action begins to work, and at about 800°C, an oxidation decline region ■ is formed, and this 8
When the temperature exceeds 00.degree. C., the oxide film thickness decreases to approximately the thickness before heating (20 layers), and a reduced region (2) is formed.

There is.

That is, it has been found from the above experimental results that the oxide film thickness can be reduced to 20 Å or less if the exit temperature of the thin steel sheet is in the oxidation unreacted region I or the reduced region (2) in a reducing atmosphere. Therefore, in order to prevent the formation of an oxide film, the atmosphere in the direct-fired heating furnace should be controlled to be a reducing atmosphere, and the outlet temperature of the direct-fired heating furnace for the thin steel sheet should be set to 400°C or less, or to 800°C. All you have to do is set it as above. In addition, when the outlet temperature of the thin steel plate is set to 800° C. or higher, it is sufficient to transport the thin steel plate as it is to a soaking zone and then cool it. Further, when the temperature is set to 400° C. or lower, the above-mentioned direct-fired heating furnace may be used as a preheating zone, and after heating to a predetermined temperature in the next step, soaking and cooling may be performed. Furthermore, the temperature control of the thin steel plate may be adjusted by the combustion amount of the burner and the conveyance speed of the vR steel plate.

Next, an experiment in which the above numerical values were found in the related invention of the present application will be explained.

In this experiment, similar to the experiment in the above-mentioned specific invention, the combustion gas atmosphere from the direct-fired burner was made into a reducing atmosphere consisting of reducing components such as hydrogen and carbon monoxide, and a trace amount of oxygen, and the direct-fired heating furnace was used. The surface properties of the oxide film were investigated by changing the heating rate of the passing "A" steel sheet.The outlet temperature of the heating furnace for the thin steel sheet was set to be 800° C. or higher.

Figure 2 shows the results. This figure is a characteristic diagram showing the relationship between heating time and thin steel plate temperature in a direct-fired heating furnace.
In the figure, C is a temperature increase rate of 50°C/sec, and d is 20°C/sec.
sec characteristics, and the circle mark indicates the oxide film thickness before heating (2
0), the mark X indicates the case where the oxide film thickness is greater than this. What has become clear from Figure 2 is that when heated in a reducing atmosphere, the temperature increase rate of the thin steel sheet is 20℃8e.
When the temperature exceeds 400°C, an oxide film starts to grow, and even if the exit temperature exceeds 800°C, the oxide film is thicker than the original plate.
×. On the other hand, in the case of 50°C/sec, although the oxide film becomes thicker around 500°C and 700°C,
When the temperature exceeds 800°C, the temperature becomes similar to that of the original plate, and it becomes O.

This is considered to be because the oxidized material was reduced to the same oxide film thickness as before heating. That is, it was found from the above experimental results that the temperature increase rate of the thin steel plate was increased to at least 400%.
By increasing the heating rate to 50° C./sec or more between 600° C. and 600° C., the oxide film thickness can be reduced to the same level as that of the original plate.

[Effect]

According to the continuous annealing method for Fi steel sheets according to the specific invention of the present application, the inside of the direct-fired heating furnace is made into a reducing atmosphere containing a trace amount of oxygen as a reducing component, and the outlet temperature of the direct-fired heating furnace for the NfA sheet is adjusted. Since it is below 400℃ or north of 800℃, 40
If the temperature is below 0℃, the oxidation remains unreacted, so
Furthermore, when the temperature is 800° C. or higher, the oxidized film is in the reduction region where what has been oxidized is reduced again, so it is possible to maintain the same oxide film thickness as before heating. Moreover, the above-mentioned reducing atmosphere can be realized by using a hydrocarbon fuel such as CO'G or LDG gas as the fuel and by setting the air ratio of the direct-fired burner to 0.8 or less.

Further, according to the related invention of the present application, the reducing atmosphere is set, the outlet temperature of the direct-fired heating furnace is set to 800°C or higher, and the temperature increase rate in the heating furnace is set to 50°C between 400 and 600°C. ℃/sec or more, the time in the oxide film formation temperature region is minimized, and therefore the formation of oxide film is suppressed, and the carbonated thin steel sheet is reduced again, and as a result, the temperature is lower than that before heating. It is possible to maintain the same oxide film thickness. Also in this case, similar to the above-mentioned specific invention, it can be realized by adjusting only the selection of fuel, the air ratio of the burner, and the thermal power.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

First, a continuous annealing furnace for realizing the continuous annealing method of the present invention is shown in FIGS. 4 and 5.

In the figure, 1 is a continuous annealing furnace, which has an entrance looper 2. Direct-fire heating furnace 31 Indirect heating soaking zone 4.5. Gas jet cooling zone6. and an overaging treatment zone (not shown),
It consists of a rapid cooling zone and an outlet looper connected to each other. Further, a large number of conveyor rolls 7 are installed above and below the input side louver 2 and each of the bands 3 to 6 in the furnace width direction, and the thin steel plate W is wound around each of the rolls 7 in sequence. moreover,
A large number of radiant cup-type direct-fired burners 8 are inserted into the opposite side walls 3a of the above-mentioned direct-fired heating furnace 3 along the conveyance direction of the thin steel sheet W, and each burner 8 has an M It is disposed opposite to the steel plate W and close to the acid 3 steel plate W so that the flame directly hits the acid 3 steel plate W. Also,
Each burner 8 has a combustion air supply pipe 8a and a fuel supply pipe 8.
b is connected. As a result, the thin steel plate W is
The heat treatment is performed according to a predetermined annealing pattern while sequentially passing through the annealing pattern.

Next, an embodiment of a continuous annealing method for a thin copper plate W according to the specific invention of the present application will be described.

In this embodiment, COG is used as fuel for the direct-fired burner 8.
Alternatively, LDG gas is employed, and this fuel is combusted at a stoichiometric air ratio or lower, for example, 0.8. As a result, a reducing atmosphere gas containing reducing components such as hydrogen and carbon monoxide and a trace amount (about 40 PPm) of oxygen is generated, and the flame is
The steel plate W is heated while being surrounded by flame. In this case, the combustion amount of the burner 8 is set based on the conveyance speed of the thin steel sheet W so that the outlet temperature A of the direct-fired heating furnace 3 of the thin steel sheet W is 400° C. or lower or 800° C. or higher. . Of course, in this case, the conveyance speed of the thin steel plate W may be set based on the combustion amount of the burner 8. This results in
The thin steel sheet W is heated to a predetermined temperature in the direct-fired heating furnace 3, then transported to the soaking zone 4.5, and then cooled.

Here, the outlet temperature A of the heating furnace 3 of the thin steel sheet W is set to 800.
When the temperature is set at .degree. C. or above, the material may be transported in this state to a soaking zone 4.5, soaked under predetermined annealing conditions (for example, 600.degree. C.), and then subjected to cooling treatment. On the other hand, when the outlet temperature A of the thin steel sheet W is set to 400° C. or lower, the temperature of the soaking zone 4 may be set to a temperature that allows reheating according to the annealing conditions.

As described above, according to the continuous annealing method of this embodiment, the combustion gas from the direct-fired burner 8 disposed in the heating furnace 3 is heated to contain reducing components such as H2 and Co. The atmosphere is reduced to a reducing atmosphere containing a trace amount of 02, and the outlet temperature A of the heating furnace 3 for the thin tllr plate W is set to 400°C or less, or 800°C or less.
Since the temperature was set at 00° C. or higher, the thickness of the oxide film on the surface of the thin steel sheet W before heating can be maintained at about 20, and the thin steel sheet W with good surface properties can be obtained.

That is, as shown in Fig. 1, when the temperature is set at 400°C or lower, an oxide film is hardly generated on the surface of the thin steel sheet W, and when the temperature is set at 800°C or higher, an Since the membrane is also reduced again, the above-mentioned good surface quality is finally obtained. If the temperature is set to 800°C or higher, as shown in Figure 1, it is necessary to pass through the 400-600°C region (2) where oxidation is likely to occur. It is preferable to raise the temperature in a short period of time, and a heating rate of 50° C./sec or more is desirable.

In addition, in the method of this embodiment, COG and LDG gases are used as fuel for the direct-fired burner 8, and the air ratio of the burner 8 is set to 0.
8 or less, and by simply setting the combustion amount or the conveyance speed of the thin steel plate W to the predetermined exit temperature A, the above-mentioned reducing atmosphere can be achieved, the formation of an oxide film can be suppressed, and the equipment cost increases. Nor. Furthermore, the annealing method of this example can be easily applied by adding a direct-fired heating furnace to an existing continuous annealing furnace.

Here, if the air ratio is lowered, the calorific value of the fuel cannot be used effectively, and the fuel consumption rate decreases. On the other hand, in this embodiment, the flame from the direct-fired burner 8 is applied directly to the thin steel plate W, and the thin steel plate W is heated while being surrounded by the flame, so that the heating efficiency can be improved accordingly.

By the way, in the conventional method, if the flame is directly applied to the thin steel plate,
It was thought that the remaining 0□ in the flame would easily cause oxidation, so the burner was placed parallel to the thin steel plate or placed far away from it. On the other hand, experiments conducted by the present inventors have revealed that in a reducing atmosphere, oxidation hardly progresses even when directly exposed to flame, so the above configuration was adopted.

Next, an embodiment of a continuous annealing method for a thin steel plate W according to a related invention of the present application will be described.

In this example, the same reducing atmosphere as in the above example was used, and the temperature increase pattern of the thin steel plate W as shown in FIG. 3 was set. That is, the temperature increase rate from point 0 of 400°C to point B of 600°C is set to be 50°C/sec, and the rate of temperature increase from the inlet temperature point (room temperature) of the direct-fired heating furnace 3 to the above 0 point is set to be 50°C/sec. From the temperature increase rate and the above point B to the heating furnace 3
The rate of temperature increase to the outlet temperature point A (800° C. or higher) is set lower than the above 50° C./sec. In this case, the above D-
The temperature increase rate at point C and point B-A may be the same, or may be set at 50° C./sec at points 0 to 8 above. and,
As a method for realizing the above temperature increase pattern, since the conveyance speed of the thin steel plate W is constant at any point in the heating furnace 3, the heating power of the burner 8 at the portion corresponding to the above C-B point may be increased. It is also conceivable to increase the number of burners 8 provided. The reason why the temperature increase rate at points D-C and points B-A was set slower than that at point C-B at 50°C 8e is because the temperature range has almost no effect on the formation of oxide film thickness. This is to avoid an increase in combustion cost due to increasing the number of burners 8 or increasing the thermal power. Thereby, the thin steel plate W is heated to a predetermined temperature in the direct-fired heating furnace 3, and then soaked and cooled along a predetermined annealing pattern.

As described above, according to the annealing method of this embodiment, the inside of the direct-fired heating furnace 3 was made into a reducing atmosphere, and the temperature increase rate of the thin steel sheet W was set at 50°C/sec or more between 400 and 600°C. Therefore, the time in the temperature range where oxidation is likely to occur is shortened as much as possible, and therefore the formation of an oxide film is suppressed to that extent. In addition, since the outlet temperature A was set to 800°C or higher, the once-oxidized steel sheet W is reduced again, and the oxide film thickness can be reduced to about 20 layers before heating, and the same effect as in the above example can be obtained. It will be done.

In addition, although the said Example demonstrated the case where it applied to the continuous annealing furnace as an example, the continuous annealing method of this invention can also be applied to continuous galvanizing equipment.

〔Effect of the invention〕

As described above, according to the continuous annealing method for thin steel sheets according to the present invention, the atmosphere of the direct-fired heating furnace is reduced to a reducing atmosphere, and in the specific invention, the outlet temperature of the direct-fired heating furnace for 811A plate is set to 400°C. or below or above 800°C, and in the related invention, the temperature increase rate in a direct-fired heating furnace for thin steel sheets is set at 400°C to 600°C.
During 00°C, the rate should be 10°C/sec or more, and the outlet temperature should be 8°C.
00℃ or higher, it has the effect of making the oxide film thickness of the thin steel sheet the same as before heating, and this can be achieved simply by controlling the air ratio of the direct burner and the temperature at the outlet of the heating furnace for the thin steel sheet or the heating rate. Because it can be done, there is no increase in equipment costs.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram showing the relationship between thin steel plate temperature and oxide film thickness to explain the process of establishing the specific invention of the present application, and Figure 2 is a characteristic diagram showing the relationship between heating time and thin steel plate to explain the process of establishing the related invention. A characteristic diagram showing the relationship with temperature, FIG. 3 is a diagram showing a temperature increase pattern for explaining an embodiment of the related invention, and FIGS. 4 and 5 are diagrams for carrying out the embodiment method of the present invention, respectively. A schematic configuration diagram showing a continuous annealing furnace, and FIGS. 6 and 7 are characteristic diagrams for explaining conventional problems, respectively. In the figure, 1 is a continuous annealing furnace, 3 is a direct-fired heating furnace, and 4 is a continuous annealing furnace.
5 is the soaking zone, 6 is the gas jet cooling zone (cooling), 8 is the direct burner, A is the outlet temperature, and W is 8! It is an I steel plate. Patent applicant Kobe Steel Co., Ltd. Patent attorney Tsutomu Shimoichi. 2 Ippu W 4 礪 and G mother tongue @ 9 ← Tsutomuji Jikan 3

Claims (2)

[Claims] (1) After continuously conveying a strip-shaped thin steel plate and heating it to a predetermined temperature in a direct-fired heating furnace equipped with multiple direct-fired burners, it is soaked and cooled according to a predetermined annealing pattern. In the continuous annealing method for thin steel sheets, the atmosphere of the direct-fired heating furnace is controlled to be a reducing atmosphere containing reducing components such as hydrogen and carbon monoxide, and a trace amount of oxygen, and A continuous annealing method for thin steel sheets, characterized in that the outlet temperature of a direct-fired heating furnace is set to 400°C or lower or 800°C or higher. (2) After continuously conveying a strip-shaped thin steel plate and heating it to a predetermined temperature in a direct-fired heating furnace equipped with multiple direct-fired burners, it is soaked and cooled according to a predetermined annealing pattern. In the continuous annealing method for thin steel sheets, the atmosphere of the direct-fired heating furnace is controlled to be a reducing atmosphere containing reducing components such as hydrogen and carbon monoxide, and a trace amount of oxygen, and Continuous annealing of a thin steel sheet, characterized in that the outlet temperature of a direct-fired heating furnace is 800°C or higher, and the temperature increase rate in the heating furnace is 50°C/sec or higher between 400 and 600°C. Method.