JPS63121644A - Heating furnace - Google Patents

Abstract

PURPOSE:To permit always adequate heating treatment as against a wide change of a line speed by providing a zone having functions for heating and holding and zone having functions for holding and cooling between a heating zone and cooling zone of a heating furnace. CONSTITUTION:A steel strip 8 is passed through an annealing furnace 60 to a galvanizing bath 6 and a galvanized layer of a prescribed plating deposition is formed thereon by a device 7 for adjusting the plating deposition; thereafter, the strip is passed through the heating furnace 5 where the strip is subjected to a heating treatment to alloy the iron and zinc of the plating layer. Said heating furnace 5 is constituted of the 1st zone 1 having the function for heating, the 2nd zone 2 having the functions for heating and holding, the 3rd zone having the functions for holding and cold retaining and the 4th zone having the function for cooling. The furnace is so constituted that these four functions and furnace length can be varied. The prescribed heating treatment meeting the line speed of the annealing furnace 60 operating on the rate-determined speed is thus executed. The ideal alloying speed cycle as against the wide change of the line speed is thereby embodied.

Description

Detailed Description of the Invention <Industrial Application Field> The present invention provides Fe-Zn in the wrapping layer by heat treatment.
The present invention relates to a continuous furnace that performs so-called alloying heat treatment for growing intermetallic compounds, and particularly to a heating furnace that can always perform appropriate heat treatment even if the line speed varies widely.

<Prior art and its problems> In order to impart properties such as paint film adhesion, post-painting corrosion resistance, and spot weldability to galvanized steel sheets, heat treatment is performed after galvanizing to add Fe in the plating layer. A so-called alloyed galvanized steel sheet in which a -Zn-based intermetallic compound is grown requires heat treatment to promote alloying.

Alloyed galvanized steel sheets are manufactured on continuous hot-dip galvanizing lines (C, G, L), most of which have heating furnaces.

For example, as shown in FIG. After that, it is transported into a heating furnace for alloying.

In this way, annealing in the continuous annealing furnace 60 and alloying heat treatment in the heating furnace 5 are performed continuously on the same line.

For this reason, the line speeds were the same, resulting in the following problems.

That is, the mechanical properties of a steel sheet material are mostly determined by the annealing conditions (heating temperature, heating time, cooling rate, etc.).

Usually, the thickness and type of steel sheet vary, and the required level of mechanical properties also varies, so the annealing conditions are also changed as appropriate. However, since the furnace length of the annealing furnace is fixed and there are limits to heating capacity (upper limit of heating temperature) and cooling capacity, in most cases the annealing conditions must be changed by changing the line speed. ing.

Furthermore, annealing furnaces are designed on this premise.

Therefore, the line speed in the production line for alloyed galvanized steel sheets is rate-determining the annealing process, and if the line speed of the annealing process is changed, the line speed of the alloying heat treatment must also be changed.

However, conventional heating furnaces have a fixed furnace length and are composed of a heating zone, a high-temperature heating zone, and atmospheric gas from outside the furnace to be sucked in and mixed to control the temperature. Since the line usually consists of a zone whose purpose is only for heat retention and a zone whose purpose is only forcible cooling, the tolerance range for line speed is extremely narrow, and this can lead to deterioration of the quality characteristics of the plated layer. This was an important cause of the problem.

For example, as seen in JP-A No. 59-229493 "Title of the Invention: Galvanealed Steel Sheet and Method for Producing the Same", an alloyed galvanized steel sheet with excellent quality characteristics of the coating layer, particularly excellent coating layer workability, is manufactured. In order to increase the heating rate 2
Alloying heat treatment with a heating temperature of 675°C or more and 780°C or less, and a cooling rate of 20°C/second or more is essential.

That is, heat treatment as shown in FIG. 2 is essential. In Figure 2, if the required heat retention time is constant (e.g. 8 seconds), the total time required for alloying heat treatment is determined by the regulations regarding heating rate and cooling rate, and if it is longer or shorter than this time, In either case, the quality characteristics of the plating layer will deteriorate.

Therefore, since the furnace length of the heating furnace is constant, if the line speed is fast or slow, the heating rate should be increased by 25
The alloying heat treatment must be performed in a manner that deteriorates the quality characteristics of the plating layer, such as by making the heating temperature slower than 0.degree. C./second, or by making the heating holding time shorter than 8 seconds, or conversely, making it longer.

The same applies to general alloyed galvanized steel sheets. In other words, general alloyed galvanized steel sheet has 5
Alloying heat treatment is performed at a heating temperature of 00 to 600°C, but the inventors' research has shown that even if the degree of alloying is appropriate, alloying heating at a relatively low temperature close to 500°C for a relatively long time is necessary. The alloyed galvanized steel sheet obtained by the treatment has been found to have excellent quality properties. However, for example, as the line speed increases, alloying heat treatment must be performed at a relatively high temperature and for a relatively short time in order to obtain an appropriate degree of alloying, which inevitably leads to deterioration of quality characteristics. That is, even with general alloyed galvanized steel sheets, there is a problem that the quality characteristics deteriorate when used in a conventional heating furnace. On the other hand, the line speed may be changed depending on the function of the heating furnace.

Normally, the line speed is often set to a low speed when it is necessary to allow sufficient time to maintain a particularly high temperature in the heating furnace, but in this case, countermeasures such as lowering the annealing temperature are required. Frequently increasing or decreasing the annealing temperature inevitably causes disadvantages in the mechanical properties of the steel sheet or the energy consumption rate until the transition to the target temperature is completed.

<Objective of the Invention> The object of the present invention is to be able to respond to a wide range of changes in line speed, to perform suitable alloying heat treatment even when the line speed changes over a wide range, and to improve the quality of steel sheet materials. The object of the present invention is to provide a heating furnace that enables the production of alloyed galvanized steel sheets with excellent mechanical properties and excellent coating layer quality characteristics.

<Structure of the invention> As mentioned above, conventional heating furnaces have three
Due to this structure, it was not possible to cope with the inevitable change in line speed, and an alloying heat treatment was performed that was not necessarily suitable.

Therefore, we conducted research to develop a heating furnace that can flexibly respond to wide variations in line speed and that can perform suitable alloying heat treatment.

As a result, it can respond to a wide range of line speed changes,
A heating furnace capable of suitable alloying heat treatment has been invented.

That is, the present invention provides a heating furnace comprising a zone having a heating function, a zone having a heating and heat retention function, a zone having a heat retention and cooling function, and a zone having a cooling function. I will provide a.

The present invention will be explained in detail below.

FIG. 1 shows a heating furnace of the present invention. As shown in the figure, the heating furnace of the present invention has (1) a zone having a heating function (hereinafter referred to as the first zone) 1, and (2) a zone having two functions of heating and heat retention (hereinafter referred to as the second zone). )2, (3),
It is characterized by comprising a zone (hereinafter referred to as the third zone) 3 having two functions of heat retention and cooling, and further (4) a zone 4 having the function of cooling (hereinafter referred to as the fourth zone) 4.

In the present invention, the temperature raising heating may be performed by electrical induction heating, or heating by a general burner,
The heating method does not matter. In short, it is sufficient as long as it is possible to heat up the steel plate at a suitable rate. Heat retention is to maintain the temperature of the heated steel plate at a constant level or at the temperature after 1) η, so for example, the combustion generated gas from the burner for temperature rise is mixed with low-temperature air from outside the furnace. And the mixing ratio! ! It is also possible to maintain the target temperature by setting one section. Further, the temperature can be maintained by heating with a burner with a small amount of heat or by using electric induction heating in combination, if necessary.

Cooling is performed not only in the case of alloying heat treatment where it is essential to rapidly cool down the steel sheet temperature, but also for cooling when it is necessary to lower the steel sheet temperature in the subsequent process following alloying heat treatment. However, this can be done by blowing a gas such as air or nitrogen gas onto the surface of the steel plate at a relatively high speed. Further, a cooling method may be selected depending on the required cooling rate, such as roll cooling in which a steel plate is brought into contact with the roll surface, or cooling by blowing an aqueous solution into the gaseous body.

(1) Structure of the second zone To provide both heating and heat retention functions, for example, electric induction heating is provided. In order to actively raise the temperature, a high voltage may be applied, and in the case of heat retention, a relatively low voltage sufficient to maintain the target temperature may be applied. Further, two objectives can be satisfied by arranging a large number of burners with a high calorific value and adjusting the combustion amount of the burners depending on whether the temperature is to be actively raised or whether the high temperature is to be maintained.

(2) Structure of the third zone In order to combine the two functions of heat retention and cooling, the high-temperature combustion gas generated during the heating mentioned above is attracted and mixed with the air sucked in from outside the furnace. By adjusting the mixing ratio, two purposes can be met. In other words, in the case of high temperature maintenance, the target temperature is achieved by relatively lowering the mixing ratio of air, and in the case of rapid cooling, the supply of combustion gas is cut off or the mixing ratio of combustion gas is lowered. By spraying this evenly onto the surface of the steel plate, the target cooling rate can be achieved.

Note that nitrogen gas can also be used instead of air. In this case, it is known that oxidation of the steel sheet surface is relatively small and the surface of the steel sheet becomes beautiful. It is also possible to arrange several units of electric induction heating devices and gas cooling devices alternately in the line flow direction to serve the two purposes. Furthermore, it can also be handled by arranging several units each of an electric induction heating device and a gas-water mixing spray device.

As described above, it is possible to provide both functions in both cases of heating and keeping warm, and keeping warm and cooling.

In addition, each zone of the heating furnace of the present invention may be subdivided into zones and partition walls may be provided, thereby making it possible to clarify the functions within the zone and achieve the target alloying heat cycle. can be reproduced more faithfully. Furthermore, it is not necessary to completely connect each zone; for example, an appropriate gap may be provided between the third zone 3 and the fourth zone 4 for the purpose of heat insulation or for discharging the atmospheric gas of the third zone 3. In short, they only need to be functionally connected.

Furthermore, the heating furnace of the present invention may be a general vertical furnace as shown in FIG. 1, or may be a horizontal furnace as shown in FIG. In the case of a horizontal furnace, sag may occur due to the weight of the steel plate (pass line fluctuates), so it is recommended to increase the tension of the steel plate in the furnace or use a non-contact type hearth roll (floating roll 9). preferable.

Further, the heating furnace of the present invention is not limited to use only for alloying heat treatment. Even if it is widely used in annealing furnaces, it can accommodate a wide range of line speed changes and can perform suitable heat treatment.

In order to always obtain an appropriate heat cycle in response to changes in line speed using a heating furnace with the above configuration, it is necessary to shift the longitudinal position of the furnace where each function of each zone works in accordance with the line speed. The ideal alloying beat cycle is obtained by:

Changes to each zone's function and furnace length can be made automatically or manually.

A preferred embodiment of the heating furnace of the present invention shown in FIG. 5 will be described in detail below, but the present invention is not limited to the following embodiment.

The furnace body of the heating furnace 5 includes a first zone 1, a second zone 2, and a third zone.
It consists of zone 3 and fourth zone 4.

The first zone 1 has a furnace entry side 11 where the steel strip 8 is carried in, and the fourth zone 4 has a furnace exit side 12 from which the steel strip 8 is carried out.

A coolant 13 is usually provided downstream of the heating furnace 5 in the line direction, and the steel plate 8 alloyed by the heating furnace 5 is cooled and sent to the next process.

The first zone 1 is a zone having a heating function, and is composed of a hollow national body housing 25 that is horizontally elongated in the line direction, and a steel strip 8 slit through the central axis of the housing 25 moves in the line direction.

The gas burner 14 is attached to a steel strip 8 on the side wall of the housing 25.
A plurality of them are provided at symmetrical positions.

Further, an outlet 15 for combustion gas in the furnace is provided on the downstream side wall of the first zone 1, and a gas pipe 2 is connected from the outlet 15 to a mixing tank 20 fixed at an appropriate position outside the heating furnace 5.
It is communicated with 3. A suction fan 16 is provided near the outlet 15 of the gas pipe 23 to suck the combustion gas in the first zone 1 into the mixing tank 20 .

A flow rate adjustment valve 19 is provided in the gas pipe 23 near the mixing tank 20, and the amount of combustion gas sucked into the mixing tank 20 can be adjusted. An exhaust port 18 is provided in the gas pipe 23 between the suction fan 16 and the flow rate adjustment valve 19 via the flow rate adjustment valve 17, and the exhaust port 18 allows excess combustion gas to be exhausted.

The second zone 2 is a zone that has two functions of heating and heat retention, and consists of a housing 25 similar to the first zone 1.
The first zones 1 are separated by a partition wall 26, and the partition wall 26 is provided with an opening through which the steel strip 8 passes.

A high-frequency heating device 21 having a high-frequency induction heating coil (not shown) is fixed in the housing 25 of the second zone 2 at a symmetrical position in the line direction of the steel strip 8 .

7jS3 Zone 3 is a zone that has two functions of heat retention and cooling, is composed of a housing 25 similar to the first zone 1, and is provided downstream of the second zone 2.

In the housing 25 of the third zone 3, the housing 25
A plurality of gas jetters 31 are provided symmetrically with respect to the line direction near the surface of the steel strip 8 passing through the central axis of the steel strip 8 .

The gas jetter 31 is connected to the housing 2 through the gas pipe 27.
It communicates with the mixing tank 20 through an opening 24 provided in the side wall of the mixing tank 5 . A blower 34 is installed in the gas pipe 27 near the mixing tank 20, and blows the mixed gas in the mixing tank 20 through the flow ff131 regulating valve 33.
The gas is ejected from the gas jetter 31 onto the surface of the copper strip 8 through the gas jetter 31.

A separate gas pipe 29 is connected to the mixing tank 20, and an air suction [135] provided in the gas pipe 29 is connected to the mixing tank 20.
Similarly, the blower 36 provided in the gas pipe 29 allows
Air is sucked through the flow rate adjustment valve 37.

In the mixing tank 20, from the above-mentioned first zone 1,
The suctioned combustion gas and the air suctioned from the air suction port 35 are mixed in an appropriate ratio to form a heat medium at a desired temperature, and this mixed gas is ejected from the gas jetter 31 to properly heat the steel strip 8. keep it warm or cooled to a certain temperature.

In addition, an auxiliary burner unit 32 is separately provided at an appropriate position within the housing of the third zone 3, and if necessary, the auxiliary burner unit 32 is operated to raise the atmospheric temperature within the third zone 3. It's good to do that.

The fourth zone 4 is a zone having a cooling function, is composed of a housing 25 similar to the third zone 3 described above, and is provided downstream of the third zone 3.

In the housing 25 of the fourth zone 4, the housing 25
A gas cooling device 41 is provided near the surface of the steel strip 8 passing through the central axis of the steel strip 8 at a symmetrical position with respect to the line direction.

The gas cooling device 41 is connected to a cooling gas cylinder (not shown) provided outside the heating furnace 5, and has a function of jetting cooling gas onto the surface of the copper strip 8 to cool the steel strip 8 to a predetermined temperature.

<Action of the invention> The operation of the present invention will be explained in detail.

FIG. 2 shows an example of a beat cycle for alloying heat treatment. The following explanation is for line speeds of 50 to 100 m/
A case will be described in which an alloyed galvanized steel sheet with excellent coating layer quality characteristics is produced by faithfully reproducing this beat cycle even if the beat cycle changes within a range of 1000 to 3000 min.

When the line speeds are significantly different, for example, 50 m/min and 100 m/min, in order to realize the beat cycle shown in Figure 2 at a line speed of 50 l/min to 100 m/min, the steel plate temperature in the longitudinal direction of the heating furnace is as follows. It must be as shown in the one-dot chain line (line speed 50 m/min) and the solid line (line speed 100 m/min) shown in FIG. That is,
In the case of a conventional heating furnace with a line speed of 10 On+/min, if the furnace length and function of the heating furnace are determined so that the alloying heat treatment is completed in the heating furnace, the heating function of the heating furnace will be 50 m/min. The alloying heat treatment is completed in the zone having the heat retention function and the zone having the heat retention function.

In other words, a satisfactory alloying heat cycle can be obtained at a line speed of 100 m/min, but at 50 m/min, heating and heat retention must be completed before reaching the heat retention zone, and rapid cooling must be performed in the heat retention zone. Because it doesn't happen,
This does not result in the sufficient alloying beat cycle of FIG.

In the heating furnace of the present invention, the furnace body has a first zone (a zone with a heating function), a second zone (a zone with two functions of heating and heat retention), and a third zone (a zone with two functions of heat retention and cooling). zone)
, and a fourth zone (a zone with a cooling function).

When the line speed is 100 m/min, the first zone 1 and the second zone 2 are used for the heating process as shown in Figure 3.
The third zone 3 is used for the heat retention process, and the fourth zone 4 is used for the cooling process, thereby realizing the heat cycle shown by the solid line in FIG. 3.

When the line speed is 50 m/min, the first zone 1 is used for the heating process, the second zone 2 is used for the warming process, and the third zone is used for the heating process.
A part of zone 3 is used for the cooling process, and the remaining parts of the heating furnace (part of third zone 3 and fourth zone 4) are kept at room temperature. In this way, the heat cycle shown by the dashed line in FIG. 3 is realized.

If you divide the horizontal axis showing the furnace length in Figure 3 by each line speed, you will get the time on the horizontal axis in Figure 2, and the dotted line in Figure 3 and 1
The heat cycle indicated by the dotted chain line is the same beat cycle as shown in FIG.

When the line speed is between 50 and 100 m/min, the furnace length of each function in each zone is changed in the middle of the above.

<Example> The present invention will be specifically explained below using Examples.

Example (1) In a laboratory, a heating furnace of the present invention (1/10 scale of the commercial line) shown in FIG. 5 was installed on a 1710-scale continuous hot-dip galvanizing test line of the commercial line.

A slit copper strip (thickness 0.5 mm, width 5
5fllI11), followed by the second coating.
Alloying heat treatment of the alloying beat cycle shown in the figure was performed. The results were scaled up and shown in FIG. 6 as a time-strip temperature relationship curve.

To measure the temperature of the steel strip, use a thermocouple (C, A line) on the steel strip.
This method involves welding and sending out long lead wires according to the line speed.

In addition, regarding the workability of the plating layer of the alloyed galvanized steel sheet manufactured with the heating furnace of the present invention at each line speed, the conventional heating furnace: heating zone 1250 mm, heat retention zone 133
An alloyed galvanized steel sheet produced using a cooling zone of 0 mm and a cooling zone of 125°+nm was investigated as a comparison material. The results are shown in Table 1.

The method for investigating the workability of the plating layer is 90 with the test surface inside.
After bending the tape several times, the cellophane tape was peeled off and the amount of peeling powder of the plating layer adhering to the tape was evaluated against a limit sample.

The limit sample was prepared as follows.

Limit sample 1: No peeling powder adhesion or only a slight amount of adhesion Limit sample 2: Small adhesion limit sample 3: Medium adhesion limit sample 4; Large adhesion limit sample 5; Adhesion pole <Large amount 1 From Table 6 It can be seen that the alloying heat cycle shown in FIG. 2 can be faithfully reproduced at each line speed. Furthermore, from Table 1, it can be seen that the alloyed galvanized steel sheets produced at each line speed have significantly higher workability of the coating layer than the conventional alloyed galvanized steel sheets.

As described above, by using the heating furnace of the present invention, it is possible to flexibly realize the target alloying beat cycle even with a wide range of line speed changes, and as a result, excellent plating layer quality characteristics can be achieved. We were able to produce alloyed galvanized steel sheets.

<Effects of the Invention> The heating furnace of the present invention has zones having four functions,
Furthermore, since each function and furnace length can be made variable, it is possible to realize the ideal alloying heat cycle over a wide range of line speed changes.

As a result, an alloyed galvanized steel sheet with excellent coating layer quality characteristics can be manufactured.

In addition, since the heating furnace function can accommodate a wide range of line speeds, disadvantages in mechanical properties and energy consumption of steel sheet materials can be eliminated.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating the invention in detail. FIG. 2 is a diagram showing an example of an alloying beat cycle that is essential for quality characteristics of the plating layer. FIG. 3 is a diagram illustrating the flexible adaptability to line speed brought about by the function of the heating furnace of the present invention. FIG. 4 is a diagram showing another embodiment of the heating furnace of the present invention. FIG. 5 is a diagram illustrating details of the heating furnace of the present invention used in Example (1). FIG. 6 is a time-temperature curve of the steel plate during alloying heat treatment with varying line speeds in Example (1). Explanation of symbols 1...1st zone, 2...2nd zone, 3...3rd
Zone, 4-4th zone, 5... Heating furnace, 6.
- Zinc plating bath, 7.-- Plating adhesion amount adjustment device, 8.- Steel strip, 9- Floating roll, 10
- Guide roll, 11 - Furnace entry side, 12 - Furnace exit side, 13 - Coolant, 14 - Gas burner, tS - Outlet, 16 - Suction fan, 17.1
9,33.37-first-class 11 adjustment valve, 18--exhaust port, 20--mixing tank, 21-high-frequency induction heating device, 23.27.29--gas piping, 24--opening, 25-- Housing, 26--Partition wall, 31--Gas jetter, 32--Auxiliary burner unit, 34.36--Blower, 35--Air intake port, 41--Gas cooling device, 60--Annealing Furnace FIG, 2 hours (control)

Claims (1)

[Claims] (1) A heating furnace comprising a zone having a heating function, a zone having a heating and heat retention function, a zone having a heat retention and cooling function, and a zone having a cooling function.