JP2003342645A - In-line annealing furnace for continuous hot-dip galvanizing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an in-line annealing furnace for continuous hot-dip galvanizing which can easily and securely control oxidation-reduction of a steel sheet and can perform hot-dip galvanizing on the high tensile steel sheet without generating any non-plated part, through an in-line annealing process employing indirect heating. <P>SOLUTION: In the in-line annealing furnace for indirect heating-type continuous hot-dip galvanizing, an oxidative atmosphere zone retaining a negative pressure relative to the surrounding area is established inside the furnace, and a reducing zone is established next to the oxidative atmosphere zone. The oxidative atmosphere zone is preferably established within a zone wherein the steel sheet temperature reaches 300-850°C and equipped with an outside-air suction blower and an exhaust blower. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot dip galvanizing facility for continuously hot-dip galvanizing a steel strip by heating / annealing it by an in-line method, and particularly to a high-strength steel sheet containing a large amount of plating inhibiting components such as Si and Mn. The present invention relates to an in-line annealing furnace for continuous hot dip galvanizing suitable for continuous hot dip galvanizing.

[0002]

2. Description of the Related Art In general, when a steel sheet (steel strip) is continuously galvanized, an in-line annealing method is adopted to recrystallize the structure of a cold-rolled steel sheet to soften it, and to apply molten zinc to the surface of the steel sheet. It provides the necessary conditions for plating. As a heating furnace for performing this annealing, a so-called all-radiant heating furnace based on an indirect heating method has been widely adopted in recent years.

In this all-radiant indirect heating furnace, the cold-rolled steel sheet is sufficiently degreased and cleaned by, for example, an electrolytic degreasing method, and then the atmosphere of iron (Fe) in a non-oxidizing or reducing region, For example, 3 to 1 in capacity ratio
Annealed with HN gas containing 0% hydrogen and balance nitrogen. Therefore, in principle, the surface of the steel sheet is neither oxidized nor reduced.

In the case of a normal mild steel plate, it is possible to obtain a good plating layer without any particular problem even if it is annealed by such a method and passed through a plating bath. However, for example, when hot-dip galvanizing a high-strength steel sheet used for an outer panel of an automobile by this method, Si, Mn or
Due to the alloy components such as P, non-plating may occur in which plating is not completely performed. It has been clarified that this non-plating is caused by the above alloy components being concentrated and oxidized on the surface of the steel sheet during annealing, and thus the wettability between the steel sheet surface and the molten zinc in the plating bath is reduced. As a countermeasure, the surface of the steel sheet is slightly oxidized in the first half of annealing,
After that, a means for reducing the oxide layer to form a fresh iron (Fe) layer on the surface and plating is performed (see, for example, JP-A-34210).

[0005]

However, the indirect heating furnace of the all-radiant system is maintained in the HN gas atmosphere having almost the same composition from the entrance side to the exit side of the furnace, and it has the above-mentioned characteristics. As described in Kaihei 7-34210, it is difficult to oxidize iron (Fe) on the surface of the steel sheet in the annealing furnace (pre-tropical zone) and then perform reduction annealing because of the problem of adjusting the atmosphere, which is very difficult in terms of equipment. Accompany. Further, it is difficult to adjust the degree of oxidation according to the material of the steel sheet, and there is a problem that the non-plating of the high-strength steel sheet cannot be completely solved.

According to the present invention, when the above-mentioned conventional in-line annealing method by indirect heating is adopted, oxidation of a steel sheet is performed.
An object of the present invention is to propose an in-line annealing furnace for continuous hot-dip galvanizing, which enables reliable and easy reduction control and enables hot-dip galvanizing of high-strength steel sheets without the occurrence of unplated portions.

[0007]

Means for Solving the Problems The inventor of the present invention indirectly heats a means for exposing a fresh iron (Fe) layer when oxidizing a steel plate surface during annealing and then reducing and plating the steel plate surface. Method, so-called all-radiant type heating furnace is examined, the heating zone is provided with an oxidizing atmosphere zone to create an atmosphere necessary for oxidation by sucking air,
The present invention has been completed based on the finding that the intended purpose can be reliably achieved by providing a reducing zone subsequently and preventing the atmospheres in both zones from mixing.

According to the present invention, in an in-line annealing furnace for continuous hot-dip galvanizing by an indirect heating system, an oxidizing atmosphere zone which is maintained at a negative pressure with respect to the surroundings is provided in the annealing furnace, and the oxidizing atmosphere zone is followed by the oxidizing atmosphere zone. A reduction zone is provided.

In the oxidizing atmosphere zone, the steel plate temperature is 300 to 850.
It is preferable to provide it in a zone where the temperature becomes 0 ° C., and the oxidizing atmosphere zone is preferably constructed by providing an outside air suction blower and a suction blower.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be specifically described below with reference to the drawings. FIG. 1 is a conceptual diagram showing an overall configuration of an in-line annealing furnace 11 for continuous hot-dip galvanizing to which the present invention is applied, and FIG. 2 is a sectional view taken along the line AA.
The steel strip (material) S is heated to a predetermined annealing temperature by the heating zone 12, and cooled to the hot dip galvanizing temperature in the cooling zone 13,
A hot-dip galvanized steel sheet (product) G is obtained by applying a predetermined thickness of plating in the hot-dip galvanizing tank 14. The heating zone 12 means a heating zone in a broad sense including a so-called temperature rising zone, a heating zone, and a soaking zone.

In the main annealing furnace 11, a radiant tube is attached to the wall surface for heating, and the steel sheet is heated by radiant heat generated from the radiant tube. HN gas is used as the annealing atmosphere unless otherwise specified, and the dew point of the furnace gas is set to -20 to -40 ° C. Therefore, as a whole, annealing is performed in the atmosphere of the reduction region.

In the in-line annealing furnace 11 for continuous hot-dip galvanizing of the present invention, the heating zone 12 is provided with an oxidizing atmosphere zone 21 in order to oxidize and then reduce the surface of the steel sheet as necessary during the annealing process, and then the reducing zone. 15 are provided. As described above, the heating zone 12 is a so-called temperature rising zone,
It means a heating zone in a broad sense including a heating zone and a soaking zone, and the reduction zone 15 can be provided in any of these zones.

As shown in FIG. 2, the oxidizing atmosphere zone 21 is provided with an outside air suction blower 22 and an exhaust blower 23. The oxidizing atmosphere zone 21 is adjusted by adjusting the outputs of these and the control valve 24. The internal pressure can be adjusted to a negative pressure with respect to the pressure before and after the zone and the atmospheric pressure. As a result, outside air (air) can be introduced into the zones before and after the oxidizing atmosphere zone 21 without leaking the oxidizing gas.

As shown in FIG. 3, the concrete structure is such that a refractory brick chamber 25 is arranged so as to surround a steel strip.
Seal rolls 26A and 26B, for example, are arranged at the inlet and outlet of the steel strip as means for improving the airtightness of the oxidizing atmosphere zone 21. The chamber 25 is provided with an outside air intake port and an exhaust port (not shown), to which an outside air suction blower 22 and an exhaust blower 23 are connected. In addition,
The chamber 25 can also be made of steel.

On the other hand, as shown in FIG. 1, the reduction zone 15 is located in the latter stage of the oxidizing atmosphere zone and is responsible for heating and soaking in the reduction and annealing steps of the surface of the steel strip.

Annealing of a steel sheet using the in-line annealing furnace 11 for continuous hot-dip galvanizing is as follows. First, when a high-strength cold-rolled steel sheet having a high alloy content such as Si or Mn is introduced into the line, the signal is input to the process computer together with the steel sheet component, command line speed (passing speed), and annealing temperature. In the process computer, the oxidation and reduction conditions of the steel sheet are selected from a predetermined table, and the outputs of the outside air suction blower 22 and the exhaust blower 23 and the opening degree of the control valve 24, which are also attached to the oxidizing atmosphere zone 21, are selected. To issue a control signal. Upon receiving the control signal, the outside air suction blower 22, the exhaust blower 23, and the control valve 24 operate.

These conditions can be obtained in advance by analyzing the above-mentioned steel sheet composition, operating conditions, and mutual relationships of product properties. For example, FIG. 4 shows a steel plate component Mn: 2mas.
High-strength cold-rolled steel sheet containing s%, the temperature of the oxidizing atmosphere zone 700
It is a graph which shows the relationship between the oxide film thickness produced | generated in an oxidizing atmosphere zone, and a non-plating score, when it plate | plates by making it (degree C). From this, it can be seen that the steel plate having this composition has a non-plating rating of 1 or less when an oxide film having a thickness of 1 μm or more is formed. The standard of non-plating score is shown in Table 2.

Further, FIG. 5 shows that the thickness of the oxide film and the oxidative atmosphere generated in the oxidative atmosphere zone when the same steel sheet was heated up to 700 ° C. at a rate of 25 ° C./s and retained in the oxidative atmosphere zone for 2 s. It is a graph showing the relationship with the oxygen concentration in the zone. In this case, if the oxygen concentration in the oxidizing atmosphere zone is 1.0 vol.% Or more, the required 1 μm
It can be seen that the oxide film having the above thickness can be obtained.

The appropriate thickness of the oxide film and the oxygen concentration in the oxidizing atmosphere zone for obtaining it are different depending on the steel sheet composition and the operating conditions of the annealing furnace (passing speed, temperature rising speed, etc.). However, these conditions can be clarified by analyzing the accumulated operation results. For example, steel components (especially Mn and Si
It is advisable to determine the operating conditions by a process computer so that the plate temperature in the oxidizing atmosphere zone and the residence time are appropriate from the content), plate thickness, plate passing speed, furnace temperature, etc., and control the operation accordingly. Needless to say, the thickness of the oxide film to be formed is set to a thickness that is completed by the reduction reaction in the subsequent reduction zone, and is a thickness that can finally form a fresh iron layer sufficient to prevent non-plating.

The steel strip temperature when passing through the oxidizing atmosphere zone is preferably 300 to 850 ° C. If the temperature is too low, it takes too much time to form an oxide film with an appropriate thickness, and the plate passing speed must be reduced, which is not economical. On the other hand, if the temperature exceeds 850 ° C, the product tends to be excessively softened, and it is costly because additional energy is input.

In this way, the steel sheet on the surface of which the oxide film having the appropriate thickness is formed is then transferred to the reduction zone, and the oxide film is reduced. The reducing condition is determined by the hydrogen concentration, the dew point, etc., and these are determined in advance in consideration of the thickness of the oxide film, the temperature of the steel sheet, the sheet passing speed and the like. The smaller the difference in hydrogen concentration between the heating zone and the cooling zone, the easier the operation.

The pressure in the reduction zone is preferably maintained at a positive pressure with respect to the oxidizing atmosphere zone. As a result, invasion of oxidizing gas such as air from the oxidizing atmosphere zone is completely prevented, and an annealed plate having a fresh iron layer on the surface and having excellent plating adhesion can be obtained.

The steel sheet (steel strip) whose surface layer has been oxidized and then reduced is then passed through a cooling zone and cooled to a temperature required for plating, and then placed in a plating tank according to normal operating conditions. The product is dipped. The atmosphere of the cooling zone may be the atmosphere of the reduction zone as it is, but may be a normal HN gas atmosphere. At this time,
In the cooling zone, a quenching process necessary for obtaining a high-strength steel sheet and an alloying treatment after plating can be performed as necessary.

The case where the high-strength steel sheet having a high alloy content is plated by using the in-line annealing furnace for continuous hot dip galvanizing according to the present invention has been described above. The present invention is not limited to the ordinary mild steel sheet (C: 0.03 mass). %, In addition to those containing a small amount of elements such as Si, Mn, and P), the plating adhesion can be further improved. However, since ordinary mild steel sheets have sufficient plating properties even if they are annealed with simple HN gas, the gas used in the in-line annealing furnace is switched to HN gas during the passage to simplify the operation. It is economical to do.

[0025]

[Example] Using an in-line annealing furnace for continuous hot-dip galvanizing having the specifications shown in Table 1, a mild steel strip (thickness: 1.0 mm, width: 1000 mm) containing 0.01 mass% of Mn was passed at a speed of 60 m / min. After stripping, high-strength steel strip containing 1.0 mass% Mn (thickness: 1.
0 mm, width: 1000 mm) was passed at a speed of 60 m / min. When passing these mild steel strips and high-strength steel strips, enter the composition of the strip to be stripped into the process computer and select the reducing conditions from the predetermined condition table to determine the oxygen concentration in the oxidizing atmosphere strip. Calculated and controlled.

[0026]

[Table 1]

FIG. 6 shows the operation result. As a result of applying the present invention, the O ₂ concentration of the oxidizing atmosphere zone is 1 v
As a result of proper oxide film formation and reduction in the reduction zone, the non-plating score became substantially 0.
On the other hand, when the present invention was not applied and annealing was simply performed with HN gas, the formation of a fresh iron layer was not performed and the non-plating score was 4 to 5. The non-plating score is shown in Table 2 including the cases described above.

[0028]

[Table 2]

[0029]

INDUSTRIAL APPLICABILITY The present invention makes it possible to reliably control the oxidation-reduction of a steel sheet while adopting an in-line annealing method by indirect heating, and to generate hot dip galvanized non-plated portions on a high-strength steel sheet. Can be done without.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing the overall configuration of an in-line annealing furnace for continuous hot-dip galvanizing to which the present invention is applied.

FIG. 2 is a sectional view taken along line AA of FIG.

FIG. 3 is a conceptual diagram showing a structure of an oxidizing atmosphere zone.

FIG. 4 is a graph showing a relationship between an oxide film thickness generated in an oxidizing atmosphere zone and a non-plating score.

FIG. 5 is a graph showing the relationship between the thickness of an oxide film generated in the oxidizing atmosphere zone and the oxygen concentration in the oxidizing atmosphere zone.

FIG. 6 is a graph showing the effect of preventing the occurrence of non-plating when the present invention is carried out.

[Explanation of symbols]

11: In-line annealing furnace 12: Heating zone 13: Cooling zone 14: Hot dip galvanizing tank 15: Reduction zone 16: Radiant tube 21: Oxidizing atmosphere zone 22: Outside air suction valve 23: Blower 24: valve 25: Chamber 26A, 26B: Seal roll S: Cold rolled steel sheet (material) G: Hot-dip galvanized steel strip (product)

Continued front page    F term (reference) 4K027 AA02 AA05 AC12 AD01 AD25                       AD26                 4K043 AA01 CA02 DA05 EA06 FA03                       FA07 FA09 FA12 GA01 GA02                       GA03 HA04

Claims (3)

[Claims] 1. In an in-line annealing furnace for continuous hot-dip galvanizing using an indirect heating system, an oxidizing atmosphere zone maintained at a negative pressure with respect to the surroundings is provided in the annealing furnace,
An in-line annealing furnace for continuous hot-dip galvanizing, wherein a reducing zone is provided following the oxidizing atmosphere zone. 2. The oxidizing atmosphere zone has a steel plate temperature of 300 to 850 ° C.
It is provided in the band which becomes.
In-line annealing furnace for continuous hot-dip galvanizing described. 3. The in-line annealing furnace for continuous hot dip galvanizing according to claim 1, wherein an outside air suction blower and a suction blower are provided in the oxidizing atmosphere zone.