WO2024014372A1 - Method for heating steel plate, method for producing plated steel plate, direct-fired heating furnace, and continuous hot-dip galvanizing equipment - Google Patents

Abstract

The purpose of the present invention is to produce a galvanized steel plate that is free from unplating and has stable quality. A method for heating a steel plate, the method including heating the obverse and reverse sides of a steel plate passing through a direct-fired heating furnace that has an oxidation zone operated at an air ratio of 1 or greater and a reduction zone operated at an air ratio of less than 1 by using flames that are injected from one or more slit burners while the steel plate passes at least through the oxidation zone.

Description

Heating methods for steel sheets, methods for manufacturing galvanized steel sheets, direct-fired heating furnaces, and continuous hot-dip galvanizing equipment

The present invention relates to a method for heating a steel sheet, a method for manufacturing a plated steel sheet, a direct-fired heating furnace, and continuous hot-dip galvanizing equipment using a direct-fired heating furnace.

To increase the tensile strength of steel sheets, solid solution strengthening elements such as Si, Mn, P, and Al are often added. In particular, Si has the advantage that its addition cost is low compared to other elements, and it can increase the strength of steel without impairing its ductility. Therefore, Si-containing steel is promising as a high-strength steel plate. However, when a large amount of Si is contained in steel, the following problems occur.

High-strength steel sheets are annealed at a temperature range of 600 to 900°C in a reducing atmosphere in a process immediately before a galvanizing process such as hot-dip galvanizing. Since Si is an element that is more easily oxidized than Fe, Si is concentrated on the surface of the steel sheet at this time. As a result, Si oxide is formed on the surface of the steel sheet, and this Si oxide significantly deteriorates the wettability with zinc, resulting in non-plating. Furthermore, if Si is concentrated on the surface, even if zinc plating is attached, there will be a significant delay in alloying in the alloying process after hot-dip galvanizing, and productivity will deteriorate.

Conventionally, as a countermeasure for this, Patent Document 1 proposes a pre-plating method in which Fe-based plating is performed on a steel plate (original plate) before plating using an electroplating method. Alternatively, Patent Documents 2 and 3 propose an oxidation-reduction method in which a steel plate is heated in an oxidizing atmosphere in advance to form an Fe-based oxide film on the surface, and then annealed and plated in a reduction furnace.

However, when trying to adopt the former pre-plating method, it is necessary to install electroplating equipment on the entry side of the annealing furnace in continuous hot-dip galvanizing equipment, which is difficult to implement considering space and equipment investment costs. be.

Furthermore, the latter oxidation-reduction method can be applied by adjusting the combustion atmosphere in a conventional non-oxidation furnace (NOF) method or direct fire furnace (DFF) method hot-dip plating line.

However, for example, in the case of conventionally used burners with a circular burner nozzle outlet shape as described in Patent Documents 4 and 5, even if the burners are arranged in a staggered manner, it is difficult to ensure good plating performance. The thickness of the Fe-based oxide film during oxidation cannot be uniformly controlled, resulting in non-plating defects.

On the other hand, Patent Document 6 proposes a method of using a slit burner in which the shape of the burner nozzle outlet is parallel to the width direction of the steel sheet in a horizontal furnace for uniformity in the width direction of the steel sheet. However, even if a slit burner is installed in the oxidation furnace behind the non-oxidation furnace, in a horizontal furnace, the oxidation furnace atmosphere flows into the non-oxidation furnace, causing uneven plate temperature, and the Fe-based oxide film becomes uneven, causing the slit burner to flow into the non-oxidation furnace. The burner does not have the effect of making the flame uniform in the width direction.

Japanese Patent Application Publication No. 2008-144264 Japanese Patent Application Publication No. 4-202630 Japanese Patent Application Publication No. 7-34210 Japanese Unexamined Patent Publication No. 62-29820 Japanese Patent Application Publication No. 9-59753 Patent No. 3889019

However, in the conventional pre-plating method, even if the atmosphere in the furnace during combustion is adjusted, the thickness of the oxide film is uneven due to the conventional burner nozzle shape and arrangement. In addition, even if a slit burner is used, if a horizontal furnace that is followed by a non-oxidizing furnace, an oxidizing furnace, and a reducing furnace is used in combination, the Fe-based oxide film will be formed non-uniformly, and even if a slit burner is used in an oxidizing furnace, non-uniformity will occur. Not resolved.

The present invention has been made in view of the above problems, and aims to produce galvanized steel sheets of stable quality without any unplating by a relatively easy method suitable for practical use.

The present invention, which has been made to solve the above problems, has the following configuration.
[1] The front and back sides of a steel plate passing through a direct-fired heating furnace having an oxidation zone operated at an air ratio of 1 or more and a reduction zone operated at an air ratio of less than 1, passing through at least the oxidation zone. A method of heating a steel plate by heating it with a flame sprayed from one or more slit burners during the heating process.
[2] The method for heating a steel plate according to [1], wherein the direct-fired heating furnace conveys the steel plate in the vertical direction and sucks combustion exhaust gas from an exhaust port installed below the slit burner. .
[3] The air ratio of the oxidation zone is 1.00 or more and less than 1.50,
The method for heating a steel plate according to [1] or [2], wherein the air ratio in the reduction zone is controlled to 0.70 or more and less than 1.00.
[4] In the range where the temperature of the steel plate passing through the oxidation zone is 400 ° C. or higher,
heating a steel plate with at least one or more installed slit burners;
The method for heating a steel plate according to any one of [1] to [3].
[5] A method for producing a plated steel sheet, comprising heating a cold rolled steel sheet by the heating method described in any one of [1] to [3], and further subjecting the cold rolled steel sheet to a plating treatment.
[6] The method for manufacturing a plated steel sheet according to [5], wherein the plating treatment uses any one of electrogalvanizing treatment, hot-dip galvanizing treatment, and alloying hot-dip galvanizing treatment.
[7] An oxidation zone operated at an air ratio of 1 or more, a reduction zone operated at an air ratio of less than 1, and a control device capable of controlling the air ratio of the oxidation zone and the reduction zone,
At least a part of the oxidation zone,
Direct-fire type, comprising one or more slit burners on the front side and the back side of the steel plate, each extending in the width direction of the steel plate and injecting a flame toward the steel plate passing through the oxidation zone and the reduction zone. heating furnace.
[8] The direct-fired heating furnace according to [7], wherein the steel plate is conveyed in the vertical direction and combustion exhaust gas is sucked through an exhaust port installed below the slit burner.
[9] The air ratio of the oxidation zone is 1.00 or more and less than 1.50,
The direct-fired heating furnace according to [7] or [8], wherein the air ratio in the reduction zone is controlled to be 0.70 or more and less than 1.00.
[10] The slit burner comprises:
In a range where the temperature of the steel plate passing through the oxidation zone is 400 ° C. or higher,
The direct-fired heating furnace according to any one of [7] to [9], wherein at least one or more direct-fired heating furnaces are installed.
[11] The oxidation zone includes a burner group having two or more slit burners that can independently control the air ratio and combustion rate, according to any one of [7] to [9]. Direct-fired heating furnace.
[12] Continuous hot-dip galvanizing equipment equipped with the direct-fired heating furnace according to any one of [7] to [9].
[13] The continuous hot-dip galvanizing equipment according to [12], further comprising an alloying equipment for alloying hot-dip galvanizing.

According to the present invention, an excellent galvanized steel sheet having a beautiful surface appearance with no unplating can be obtained. The present invention is particularly effective when the base material is a high-Si content steel sheet, which is particularly difficult to galvanize, and is useful as a method for improving the plating quality in the production of high-Si content galvanized steel sheets.

One embodiment of the direct-fired heating furnace arranged in the continuous hot-dip galvanizing apparatus of the present invention is shown, (a) is a longitudinal cross-sectional view of the direct-fired heating furnace, and (b) is arranged on the wall surface of the direct-fired heating furnace. FIG. 3 is a front view of an example of the arrangement of each open fire burner. BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram of continuous hot-dip galvanizing equipment which is one embodiment of the present invention. FIG. 2 is an explanatory diagram showing an image of an actual state of combustion and heating of a steel plate by the slit burner of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows an example of the structure of the direct-fired heating furnace of this invention.

A direct-fired heating furnace that heats a steel plate using a direct-fired burner has a high thermal efficiency, so it has the characteristic of being able to heat a steel plate to a predetermined temperature at low cost. Direct-fired heating furnaces control the temperature of the steel plate, and at the same time, when hot-dipping high-strength steel, such as high-Si steel, the atmosphere of the direct-fired burner is controlled to be oxidizing, so that the surface of the steel plate is It is necessary to ensure an appropriate oxide film (Fe-based oxide). After securing an appropriate amount of Fe-based oxide, reduction annealing is performed to internally oxidize Si, thereby improving the plating properties of high-Si steel.

However, in the case of conventionally used burners with a circular burner nozzle outlet shape, even if the burners are distributed in a staggered arrangement, the thickness of the Fe-based oxide film to ensure good plating properties cannot be increased as the steel plate progresses. Uniform control in the direction/width direction is not possible, resulting in unplated defects.

Therefore, in the present invention, a method has been devised in which a slit burner is used instead of a circular burner to uniformly control the thickness of the Fe-based oxide film in the traveling direction/width direction.

Hereinafter, a direct-fired heating furnace disposed in a continuous hot-dip galvanizing facility and a method of heating a steel plate according to an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows an embodiment of a direct-fired heating furnace (DFF) arranged in an annealing facility of a continuous hot-dip galvanizing facility according to an embodiment of the present invention. Here, it is preferable that the type of annealing equipment be a vertical furnace, in other words, by transporting the steel plate in the vertical direction (including transporting it while turning it up and down in the vertical direction), it is possible to avoid expanding the scale of the equipment in the horizontal direction. , it becomes possible to thread the sheet at high speed. Another advantage is that it is easy to separate the atmosphere in the heating zone and the soaking zone. Conveying in the vertical direction refers to conveying in the vertical direction.
In FIG. 1, (a) is a longitudinal cross-sectional view of a direct-fired heating furnace, and (b) is a front view of an arrangement example of direct-fired burners arranged on a wall surface of the direct-fired heating furnace. In Figure 1, 1 is a direct-fired heating furnace (DFF), 1-1 is an oxidation zone of the DFF, 1-2 is a reduction zone of the DFF, 2 is a flame injection port attached to a slit burner, and 3 is attached to a circular burner. S is the steel plate (including the steel strip), 4 is the radiation thermometer, 5 is the flame, 6 is the exhaust port, L is the burner located most upstream in the direction of movement of the steel strip S of the burner group 14 in the reduction zone. The length of the steel plate S heating area by the burner group up to the most downstream burner, 11 is the burner group in the oxidation zone, 12 is the burner group in the oxidation zone, 13 is the burner group in the oxidation zone, and 14 is the burner group in the reduction zone. be. Further, although not shown, a control device is provided to control the air ratio in the oxidation zone and the reduction zone.

<Direct-fired heating furnace installed in continuous hot-dip galvanizing equipment>
Figure 2 shows an example of continuous hot-dip galvanizing equipment. From the entrance side of the equipment, a preheating zone 20, a heating zone 21, a soaking zone 22, cooling zones 23 and 24, a plating bath (zinc pot) 25, and an alloying zone 26 as necessary are provided. A cooling zone 27 may be provided after the alloying zone 26. In this way, when the heating furnace of the present invention is applied to a part of continuous hot-dip galvanizing equipment, the steel plate to be heated may be in the form of a steel strip (coil) rather than a cut plate. The steel plate is not particularly limited, but cold-rolled steel plates are often used.

The direct-fired heating furnace 1 of the present invention is assumed to be a heating furnace introduced into the heating zone 21 in continuous hot-dip galvanizing equipment.

In this embodiment, it is also possible to control multiple slit burners independently. The direct-fired heating furnace 1 is composed of an oxidation zone 1-1 and a reduction zone 1-2, and the oxidation zone 1-1 is composed of three burner groups (zones), 11 to 13, in the steel plate advancing direction. There is. In this example, a circular burner is installed in the burner group 11 in the oxidation zone, and its flame injection port is indicated by 3 in the figure, and slit burners are installed in the burner group 12 and 13 (in the oxidation zone). 2, the flame injection port is indicated by the reference numeral 2. The reduction zone had only one zone of 14 burner groups (burner groups in the reduction zone), and circular burners were installed. The flame injection port of the circular burner is designated by numeral 3 in the figure. The combustion rate and air ratio of the burner groups 11, 12, and 13 in the oxidation zone 1-1 can be controlled independently for each burner group. The burner groups 11 to 13 in the oxidation zone burn under conditions where the combustion rate is equal to or higher than a predetermined threshold value.

The number of burners included in each burner group is not limited. It is practical to divide the entire direct-fired heating furnace into 2 to 5 parts and control each part as a group.

Furthermore, as in the equipment shown in FIG. 4, for example, slit burners may be provided not only in the oxidation zone but also in both the oxidation zone 1-1 and the reduction zone 1-2.

Note that the slit burner is arranged opposite to the steel plate surface in the width direction of the steel plate S passing through the oxidation zone 1-1. Further, in order to uniformly heat the steel plate S in the width direction, a slit burner is arranged to extend in the width direction of the steel plate so that the flame 5 is injected over the entire width of the steel plate S. Furthermore, in order to accommodate the manufacture of steel plates S of various widths, the amount of flame injection can be controlled for each of the four regions divided in the width direction. Although it is divided into four here, the number of divisions is not limited, and depending on the flame injection structure of the slit burner and the width of the steel plate, division may not be necessary. On the other hand, the circular burners are distributed and arranged opposite to the steel plate surface.

<Slit burner>
FIG. 3 is an explanatory diagram showing an image of the actual state of combustion and heating of a steel plate by the slit burner of the present invention, and the slit burner will be explained below based on the contents described therein.

The slit burner has a rectangular burner flame injection port in which the length of the opening in the width direction of the steel plate S is longer than the length of the opening in the direction in which the steel plate S advances (also referred to as slit gap B). The detailed dimensions are not particularly limited. As a guide, if the length of the opening in the direction in which the steel plate S moves, ie, the short side, is B, then the length of the opening in the width direction, ie, the long side, is approximately 2B to 200B. In the present invention, burners that inject a slit-shaped flame, such as those having elongated rectangular (slit) flame injection ports, are collectively referred to as "slit burners." Therefore, there are no particular limitations on the internal structure or injection port. Further, the flame injection port can control the injection width of the flame 5 by dividing the injection port in the width direction, and by using the above, the injection width of the flame 5 can be adjusted according to the width of the target steel plate. is possible.

Although it is effective to use only one slit burner in the steel plate traveling direction, oxidation can be carried out more efficiently by arranging several slit burners in tandem. When arranging them in tandem, the spacing between them is not limited, but if the spacing is about 3B to 10B, interference between the flames 5 and temperature unevenness will be less likely to occur.

Furthermore, since most of the Fe-based oxide film is generated in the range where the plate temperature is 400°C or higher, at least one slit burner is installed in the oxidation zone 1-1, especially in the area where the plate temperature is in the above range. It is good to apply. Further, it is more preferable to use a slit burner in a temperature range of 450° C. or higher. On the other hand, since the amount of oxidation increases rapidly at high temperatures exceeding 650°C, it is preferable to use a slit burner where the plate temperature is 650°C or lower. The temperature is more preferably 600°C or lower, and most preferably 550°C or lower. In order to heat the steel plate S with a slit burner when it reaches a suitable temperature range, the plate temperature can be determined by determining the steel type, plate thickness, plate width, line speed, and air ratio. , it is possible to estimate it in advance based on the combustion rate, etc. It is also possible to actually measure the plate temperature by installing radiation thermometers at several locations in the oxidation zone 1-1 in the sheet passing direction.

From the above, it is preferable to apply the slit burner on the downstream side of the oxidation zone 1-1 in the sheet passing direction, where the sheet temperature is high. Slit burners may be applied to all burners in the oxidation zone 1-1, but the slit gap B at the slit burner outlet is narrower than that of a circular burner, and it may be clogged with foreign matter such as pieces of burner tiles, or the flame 5 may be at a high temperature. Since regular maintenance is required to avoid deformation of the slits, a conventional circular burner may be placed upstream of the oxidation zone 1-1, where the plate temperature is low, and a slit burner may be placed downstream. Even when a circular burner is used on the upstream side, from the viewpoint of heating efficiency, it is desirable to use a direct heating method in which the flame collides with the steel plate perpendicularly.

Further, the arrangement of the flame injection ports 2 associated with the slit burner may be arranged so as to be shifted in the traveling direction of the steel plate S on the front and back sides of the steel plate S, that is, may be offset. By offsetting, it is possible to prevent the flames 5 protruding from the ends of the steel plate S from interfering with each other. Therefore, it is possible to heat the steel plate S more uniformly than in the case without offset. As a guideline, the offset amount is in the range of about B to 3B. If the amount of offset is too large, there is a risk that the heating temperature will differ between the front and back surfaces. In a vertical furnace, the burners are arranged vertically, so the flame is unstable due to the interference of the flame and combustion gas injected by the burner on the downstream side (lower part of the furnace), and the temperature is not uniform across the width and length of the steel plate. performance and stability will decrease. In the case of circular burners, interference of flame and combustion gas can be alleviated by arranging them in a staggered manner, but with slit burners, interference from the downstream side is unavoidable because there is no cut in the flame in the width direction. Therefore, in the present invention, it is preferable to provide a slit-shaped exhaust port in the section where the slit burner is installed, and by providing the slit-shaped exhaust port, interference of flame and combustion gas can be easily alleviated.
Preferably, at least one set of exhaust ports is installed at the connection portion of each zone, one on the front and the other on the back. The exhaust port is preferably installed below the slit burner, and the combustion exhaust gas is sucked through the exhaust port. Specifically, combustion exhaust gas refers to the high-temperature gas produced by the reaction between fuel and air, and contains mainly carbon dioxide and water vapor, which are reaction products, and nitrogen contained in the air, as well as unreacted surplus fuel components. It is a gas composed of trace components such as gas, oxygen, and intermediate products of reactions.
If the equipment length and heating capacity satisfy the required performance, an exhaust port may be installed between the individual slit burners forming each zone.

Regardless of oxidation zone 1-1 or reduction zone 1-2, the burner combustion rate is the value obtained by dividing the amount of fuel gas actually introduced into the burner by the amount of fuel gas in the burner at the maximum combustion load. The combustion rate is 100% when the fuel is burned at the maximum combustion load. In the present invention, the combustion rate of the burner is not particularly limited, but if the combustion load of the burner becomes low, a stable combustion state cannot be obtained, so it is preferably set to the following threshold value or more. The predetermined threshold value of the combustion rate is the ratio of the amount of fuel gas at the lower limit of the combustion load that can ensure a stable combustion state to the amount of fuel gas at the maximum combustion load. The combustion rate threshold varies somewhat depending on the burner structure, etc., but can be easily determined by conducting a combustion test. Usually, the threshold value will be about 30%.

<Air ratio of oxidation zone and reduction zone>
For the burner groups 11 to 13 in the oxidation zone 1-1, combustion or combustion stop can be freely selected for each burner group. When burning, it is preferable to set the combustion rate to a predetermined setting value or more, and in order to stably oxidize the steel plate surface, it is necessary to operate the oxidation zone 1-1 at an air ratio of 1 or more. It is preferable to operate the oxidation zone 1-1 at an air ratio of 1.00 or more. It is more preferable to operate the oxidation zone 1-1 at an air ratio of 1.05 or more, and most preferably to operate at an air ratio of 1.10 or more.
In order to prevent the formation of an excessive oxide film, the generation of nitrogen oxides, and the blowing out of the flame, it is preferable to operate the air ratio in the oxidation zone 1-1 at less than 1.50. It is more preferable to operate the oxidation zone 1-1 at an air ratio of 1.40 or less, most preferably 1.30 or less. The air ratio is the amount of air actually introduced into the burner divided by the amount of air required to completely burn the fuel gas.

Further, the circular burners of the burner group 14 in the reduction zone 1-2 must have an air ratio of less than 1, and are preferably operated with an air ratio of 0.70 or more and less than 1.00, thereby controlling the combustion rate. is also possible. By burning in the burner group 14 of reduction zone 1-2 at an air ratio of 0.70 or more and less than 1.00, Fe oxides generated on the surface of the steel plate are reduced and reduced Fe is generated on the surface layer. be able to. Specifically, if the air ratio is less than 0.70, the fuel consumption rate will deteriorate and the steel plate will be contaminated by soot, so the air ratio is preferably 0.70 or more. More preferably, the air ratio is 0.75 or more, most preferably 0.80 or more. On the other hand, if it is 1.00 or more, the oxygen concentration in the combustion gas will be high and the steel plate will be oxidized. The presence of reduced Fe in the surface layer of the steel plate prevents oxides from adhering to the roll when the steel plate S leaving the direct-fired heating furnace 1 comes into contact with the roll in the RT furnace (annealing furnace). , defects (pickup) caused by oxide adhesion can be prevented. Therefore, the air ratio is preferably less than 1.00. More preferably, the air ratio is 0.95 or less, most preferably 0.90 or less.

For the various steel sheets S to be passed, the number of burner groups to be burned is determined by considering the heating load, amount of oxidation formed, etc. Regarding the burner group for combustion, by setting the air ratio and combustion rate to values within the above ranges, plate temperature fluctuations in the traveling direction of the steel plates S are reduced for various steel plates S. As a result, a sufficient amount of Fe oxide necessary for internally oxidizing Si, for example, can be stably generated in the direction of movement of the steel plate S. Reducing plate temperature fluctuations in the traveling direction of the steel plate S also contributes to stabilizing the oxide reduction action in the burner group 14 of the subsequent reduction zone 1-2. Further, the reduction in the plate temperature fluctuation contributes to prevention of insufficient reduction of Fe oxide in the RT furnace, internal oxidation of Si, and also contributes to suppressing oxide adhesion to the rolls of the RT furnace.

In this embodiment, the burner groups 11 to 13 in the oxidation zone 1-1 are oxidation burners, the burner group 14 in the reduction zone 1-2 is a reduction burner, and the heating area by the burner groups 11 to 13 in the oxidation zone 1-1 is The area heated by the burner group 14 in the oxidation zone and the reduction zone 1-2 becomes the reduction zone.

If the length of the reducing atmosphere is short, an Fe oxide film will remain on the surface layer and the pickup prevention effect will be insufficient. On the other hand, if the length of the reducing atmosphere is long, a surface enriched layer of Si or the like will be formed on the surface layer of the steel sheet during subsequent reduction annealing, which will impede plating properties.

<Length of reduction band>
The length in the traveling direction of the steel plate S of the burner group 14 of the reduction zone 1-2 (reduction zone length) is preferably 150 mm or more, and more preferably 300 mm or more when uniformity in the width direction is also considered. More preferably, it is 500 mm or more, and most preferably 1000 mm or more. Although the upper limit of the length of the reduction zone is not particularly defined, if it is too long, the amount of temperature increase ΔTrd in the reduction zone will increase, so it will be necessary to reduce the amount of temperature increase ΔTox in the oxidation zone. For this reason, a reduction zone that is too long is disadvantageous in securing the amount of oxidation, so it is desirable that the reduction zone be 10 m or less. The length is more preferably 5 m or less, and even more preferably 3 m or less. Furthermore, this is advantageous in terms of cost. The length in the traveling direction of the steel plate of the burner group 14 in the reduction zone 1-2 is from the flame injection port 3 attached to the circular burner located most upstream in the traveling direction of the steel plate in the burner group 14 in the reduction zone 1-2 to the most downstream length. This is the length ("L" in FIG. 1) of the heating area of the steel plate S by the burner group up to the flame injection port 3 attached to a certain circular burner. Note that even when a slit burner is applied to the burner group 14 of the reduction zone 1-2, it is preferable that the reduction zone length is set as above.

<Length of oxidation zone>
The length of the steel plates of the burner groups 11 to 13 in the oxidation zone 1-1 in the advancing direction (oxidation zone length) should be long enough to ensure the necessary amount of internal oxidation. However, the amount of oxidation changes depending on the type of steel being threaded, temperature history, threading speed, and steel sheet size, so it is important to ensure a zone length that can secure the required amount of oxidation even under the least oxidizing production conditions. is necessary.

In the present invention, the steel plate S is oxidized and then reduced in the direct-fired heating furnace 1. Among these, the amount of oxidation formed in the oxidation zone needs to be precisely controlled in the traveling direction/width direction of the steel plate S. In order to control the amount of oxidation to an appropriate amount for various steel types, temperature histories, threading speeds, and sizes of steel sheets to be threaded, a burner placed facing the surface of the steel sheet S is It is necessary to divide the fuel into at least two groups and to be able to independently control the combustion rate and air ratio for each group. When deciding on a burner group, it is better not to mix slit burners and circular burners in one group, but to separate them into separate groups and control them separately.

Even if the burners in the reduction zone are controlled as one group, the intended effects of the present invention can be obtained. Therefore, in the present invention, the burners arranged facing the surface of the steel plate S in the oxidation zone 1-1 are divided into two or more burner groups in the traveling direction of the steel plate S, whose combustion rate and air ratio can be independently controlled. Bye.

The thickness of the Fe-based oxide film formed in the oxidation zone 1-1 varies depending on the Si content and thickness of the target steel sheet S, but is preferably 100 to 500 nm. If the thickness is less than 100 nm, the function as a barrier layer for preventing diffusion and concentration of Si to the surface may be insufficient, so the thickness of the Fe-based oxide film is preferably 100 nm or more. The thickness of the Fe-based oxide film is more preferably 150 nm or more, and even more preferably 200 nm or more. On the other hand, if the thickness exceeds 500 nm, the function as a barrier layer will hardly change, and the heating time of the oxidation zone 1-1 will become longer, and the amount of fuel used will also increase. Therefore, the thickness of the Fe-based oxide film is preferably 500 nm or less. The thickness of the Fe-based oxide film is more preferably 450 nm or less, and even more preferably 400 nm or less.

The thickness of the above-mentioned Fe-based oxide film is determined by monitoring the plate temperature at the entrance and exit of the direct-fired heating furnace 1, and by determining the steel type, plate thickness, line speed, air ratio in the oxidation zone 1-1, and combustion rate in the oxidation zone 1-1. By correcting it, it can be estimated relatively easily. By mainly adjusting the combustion rate of the oxidation zone 1-1 based on this value, stable oxidation conditions can be determined and ensured, thereby making it possible to obtain a steel plate S free of unplated defects.

The steel sheet S oxidized/reduced in the direct-fired heating furnace 1 is subsequently reductively annealed in an RT furnace, cooled, and further immersed in a hot-dip galvanizing bath to be hot-dip galvanized, or further subjected to alloying treatment if necessary. be done. After reduction annealing, conventional methods may be used. The plating method is not particularly limited, and electrogalvanizing may be used instead of hot-dip galvanizing.

After an appropriate amount of Fe-based oxide is formed in the direct-fired heating furnace 1, the surface layer is reduced and reduced Fe is present, so in the next reduction annealing step, the Fe-based oxide is The reduction causes internal oxidation of Si, and also prevents oxides from adhering to the roll. Therefore, there are no indentations caused by roll pickup, surface layer concentration of Si, and plating defects caused by insufficient reduction of Fe-based oxides.

The hot-dip galvanized steel sheet to be manufactured by the present invention is effective when containing a large amount of metal elements such as Si that are more easily oxidized than Fe, but specifically contains 0.1 to 3.0 mass% of Si. It is particularly effective in producing high-Si-containing hot-dip galvanized steel sheets.

An annealing furnace (RT furnace), a cooling zone, hot-dip plating equipment, alloying treatment equipment, etc. are arranged downstream of the direct-fired heating furnace 1. The annealing furnace, cooling zone, hot-dip plating equipment, alloying treatment equipment, etc. are not particularly limited, and any commonly used equipment may be used. A preheating furnace may be arranged upstream of the direct-fired heating furnace 1.

In a CGL equipped with a direct-fired heating furnace (DFF) 1, the DFF 1 consisting of four burner groups (11 to 14) is used as the heating burner, and the three burner groups ( 11 to 13) are the oxidation zone 1-1, the final burner group (14) is the reduction zone 1-2, and the oxidation zone 1-1 is the case where the air ratio and combustion rate are individually controlled for each burner group ( The tests were conducted separately for cases A) and case (B), where burner groups 11 to 13 in the oxidation zone are collectively controlled under the same conditions. Note that the air ratio and combustion rate in the reduction zone are controlled separately from those in the oxidation zone. Figure 1 shows an example of burner arrangement. Figure 1 shows the burner group 11 in the oxidation zone, the flame injection port 3 attached to the circular burner in the burner group 14 in the reduction zone, the burner group 12 in the oxidation zone, the flame injection port 2 attached to the slit burner in the burner group 13 in the oxidation zone. are placed. The burner type was changed for each burner group depending on the conditions and the test was conducted. A gas having the composition shown in Table 1 was used as the fuel gas for the burner. The length of each burner group ("L" in FIG. 1) was 3 m, and the slit gap B was 20 mm.

Table 2 shows the steel composition of the steel strip S used in the test.

Other test conditions were: plate thickness 1.0 mm, plate width 1000 mm, DFF1 inlet average plate temperature 200 °C, DFF1 outlet average temperature 650 °C, annealing temperature (RT furnace) 850 °C, plating bath temperature 463 °C, plating The Al concentration was 0.135% and the alloying temperature was 550°C. Three levels of steel strip S speed (LS) were examined: 60 mpm, 90 mpm, and 120 mpm. The burner was used at a combustion rate of 30% or more.

Evaluations included low-ki defects (pickups) caused by peroxidation, plating appearance, quality deviations in the steel plate advancing direction and width direction, and temperature deviations in the steel plate advancing direction. Evaluations A and B are passed, and evaluation C is failed.

In the present invention, it was determined using the same method as described in Patent Document 7 below. For low-ki defects (pick-ups) caused by peroxidation, a 1 m ² field of view of the surface of the steel plate in the randomly sampled steel strip S was inspected using an optical surface defect meter. The surface defect meter described above can detect flaws with a diameter of 0.5 mm or more, and these were determined to be dent defects caused by contact with the pickup, here as low-ki defects.
[Patent Document 7] Patent No. 6607339 A (good): 0 pieces per 1 m ² (no low kick defects)
B (almost good): 1 to 2 pieces per ¹ m2 (slight low-ki defects are seen here and there)
C (poor): 3 or more pieces per 1 ^m2 (with low kick defects)

The appearance of the plating was evaluated by measuring the dispersion of the Fe concentration (alloying ratio) in the plating with respect to the target value on the surface of the steel plate after the alloying treatment. It is determined that the smaller the variation in the Fe concentration in the plating with respect to the target value, the better the appearance of the plating. Note that the Fe concentration was measured by the same method as described in Patent Document 8 below, which is calculated from the change in the diffraction peak angle of the alloy phase constituting the plating layer using the X-ray diffraction method.
[Patent Document 8] Patent No. 5962615 A (good): less than ±0.5% (no unplated and alloyed unevenness)
B (almost good): less than ±1% (slight unplating and/or slight alloying unevenness)
C (poor): ±1% or more (clear unplatedness and/or clear alloying unevenness)
Evaluations A and B are passed, and evaluation C is failed.

Furthermore, the quality in the traveling direction and the width direction was determined by selecting three locations in the traveling direction of the steel strip S at the tip, center, and tail end, and taking samples with a length of 1000 mm in the width direction. This was done based on the evaluation results of the lower part of the central part and the appearance of the plating. In addition, in the width direction, in a sample of width x 1000 mm taken from the center of the steel strip S, the low marks at 5 points at the center, 1/4 width, 3/4 width, and both ends, and the plating appearance. Based on the evaluation results, the evaluation was made as follows.
◎: Both the low kick and plating properties are evaluated as A or B under the same conditions ○: The low kick and plating properties are both evaluated as A or B under the same conditions △: Low kick or plated under the same conditions Items with a plating property evaluation of only B ×: Items with a low-ki or plating property evaluation of C under the same conditions Items that pass the test in this invention have no low-ki defects or plating appearance that is rated C. In addition, judgments of ◎, ◎, and △ were obtained in the width direction and the traveling direction. The judgment was made as a pass (〇) if the score was △ or more in both the width direction and the direction of travel, and as a fail (x) if the score was x.

Condition No. Samples Nos. 1 to 8 and 15 were manufactured under the condition that the conveyance speed of the steel strip S was 60 mpm.

Condition 1 is a comparative example using only a circular burner. Although a circular burner was used, the combustion rate was less than 30%, and the combustion state of the burner was unstable. In addition, the burner groups 11 to 13 in the oxidation zone are collectively controlled, and there are large variations in quality in the width direction and the traveling direction.

Condition 2 is a case where slit burners are applied to the burner groups 11 to 13 in contrast to the circular burners of Condition 1. The combustion state of the burner was unstable as in Condition 1, but by applying the slit burner, both the low-kick defects and the plating appearance were improved, and the quality variation in both the width direction and the traveling direction was slightly improved.

Condition 3 is a case where the air ratio and combustion rate can be controlled for each burner group, whereas in conditions 1 and 2 the burner groups 11 to 13 are controlled all at once. Thereby, only the necessary combustion burners (in this case, only the burner group 13) can be operated. However, since the burner group 13 operated under Condition 3 was a circular burner, uneven combustion was likely to occur and the surface quality tended to be inferior to that under Condition 1.

Condition 4 is an example in which control is performed for each burner group as in Condition 3, but the burner shape is changed from circular to slit burner. This improved the surface quality and made the quality more uniform across the width and in the direction of travel.

Condition 5 is an example in which the slit burner was used under the same control as Condition 4, but the air ratio of the burner group 13 decreased to 0.90. In this example, although more uniformity in the width direction and the traveling direction was obtained than under condition 3, many defects appeared and the product was rejected.

Condition 6 is an example in which the air ratio of burner group 13 was 1.65, which was excessive compared to condition 5. The degree of defects was reduced and fell within the acceptable range.

Condition 7 is an example in which the air ratio of the reduction zone of the burner group 14 is higher than that of condition 4 at 1.00. In this case as well, like Condition 5, the uniformity was relatively good, but there were many defects and it was rejected.

Condition 8 is an example in which the air ratio in the reduction zone is lower than that in condition 7. The degree of defects was reduced compared to Condition 7 and fell within the acceptable range.

Conditions 9 and 10 are examples in which the steel strip S was produced at a conveyance speed of 90 mpm, and in both cases, the air ratio and combustion rate were controlled for each burner group.

Condition 9 uses both a slit burner and a circular burner in the oxidation zone. However, since the slit burner was not applied in the range where the steel plate temperature reached 400°C, although the surface quality was within the acceptable range, there were some areas where it was poor.

In Condition 10, the burner used in the oxidation zone consisted of only a slit burner, and the surface quality was superior to Condition 9.

Conditions 11 to 13 are examples in which the steel strip S was manufactured at a conveying speed of 120 mpm. A slit burner was introduced into the oxidation zone under all conditions.

Condition 11 does not perform control for each burner group, but performs collective control, and although some circular burners are used, the surface quality is within the acceptable range due to the introduction of slit burners, similar to condition 2.

Condition 12 is an example of the invention in which the surface quality was better than Condition 11 by controlling each burner group.

Condition 13 is an example in which a slit burner is used in the reduction zone in addition to the oxidation zone. This further improved the surface quality.

Condition 14 is an example in which a slit burner was introduced into the horizontal annealing furnace and operated as an oxidation zone. Because it was a horizontal furnace, the conveyance speed was lower than that of the invention example, and the production was performed at 30 mpm. A slit burner was used in the oxidation zone, and the peroxide defects and plating appearance were within the acceptable range, but the upstream Exhaust gas partially flowed into the non-oxidizing furnace, causing slight unplating and alloying unevenness in the width and length directions.

Condition 15 is an example in which the operation was carried out under the same conditions as Condition 2, but the exhaust ports installed between each zone were closed (equivalent to a state where there is no exhaust port) and combustion gas was not sucked. As a result, although the stability decreased due to interference between flames and the number of defects increased compared to Condition 2, the surface quality was within the acceptable range.

From the above examples, it has been confirmed that introducing a slit burner into the oxidation zone improves the surface quality, and furthermore, by optimizing the control method and combustion conditions, better surface quality can be obtained.

1 Direct-fired heating furnace (DFF)
1-1 Oxidation zone 1-2 Reduction zone 2 Flame injection port attached to slit burner 3 Flame injection port attached to circular burner 4 Radiation thermometer 5 Flame 6 Exhaust port S Steel plate L Most upstream in the steel strip movement direction of burner group 14 Length of the steel plate heating area by the burner group from the burner in the downstream to the most downstream burner 11 Burner group in the oxidation zone 12 Burner group in the oxidation zone 13 Burner group in the oxidation zone 14 Burner group B in the reduction zone Slit gap 20 Preparation zone 21 Heating zone (direct flame heating)
22 Soaking zone 23 Cooling zone 24 Cooling zone 25 Plating bath (zinc pot)
26 Alloying zone 27 Cooling zone

Claims (13)

The front and back sides of a steel plate passing through a direct-fired heating furnace having an oxidation zone operated at an air ratio of 1 or more and a reduction zone operated at an air ratio of less than 1, at least while passing through the oxidation zone. A method of heating steel plates using flames emitted from one or more slit burners. The method for heating a steel plate according to claim 1, wherein the direct-fired heating furnace conveys the steel plate in the vertical direction and sucks combustion exhaust gas from an exhaust port installed below the slit burner. The air ratio of the oxidation zone is 1.00 or more and less than 1.50,
The method for heating a steel plate according to claim 1 or 2, wherein the air ratio in the reduction zone is controlled to be 0.70 or more and less than 1.00. In a range where the temperature of the steel plate passing through the oxidation zone is 400 ° C. or higher,
heating a steel plate with at least one or more installed slit burners;
The method for heating a steel plate according to any one of claims 1 to 3. A method for producing a plated steel sheet, comprising heating a cold-rolled steel sheet by the heating method according to any one of claims 1 to 3, and further subjecting the cold-rolled steel sheet to a plating treatment. The method for manufacturing a plated steel sheet according to claim 5, wherein the plating treatment uses any one of electrogalvanizing treatment, hot-dip galvanizing treatment, and alloying hot-dip galvanizing treatment. An oxidation zone operated at an air ratio of 1 or more, a reduction zone operated at an air ratio of less than 1, and a control device capable of controlling the air ratio of the oxidation zone and the reduction zone,
At least a part of the oxidation zone,
Direct-fire type, comprising one or more slit burners on the front side and the back side of the steel plate, each extending in the width direction of the steel plate and injecting a flame toward the steel plate passing through the oxidation zone and the reduction zone. heating furnace. The direct-fired heating furnace according to claim 7, wherein the steel plate is conveyed in the vertical direction and combustion exhaust gas is sucked through an exhaust port installed below the slit burner. The air ratio of the oxidation zone is 1.00 or more and less than 1.50,
The direct-fired heating furnace according to claim 7 or 8, wherein the air ratio in the reduction zone is controlled to be 0.70 or more and less than 1.00. The slit burner is
In a range where the temperature of the steel plate passing through the oxidation zone is 400 ° C. or higher,
The direct-fired heating furnace according to any one of claims 7 to 9, wherein at least one or more direct-fired heating furnaces are installed. The direct-fired heating furnace according to any one of claims 7 to 9, wherein the oxidation zone includes a burner group having two or more slit burners whose air ratio and combustion rate can be independently controlled. . Continuous hot-dip galvanizing equipment comprising the direct-fired heating furnace according to any one of claims 7 to 9. The continuous hot-dip galvanizing equipment according to claim 12, further comprising alloying equipment for alloying hot-dip galvanizing.

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