JP7622869B2 - Steel sheet heating method, coated steel sheet manufacturing method, direct-fired heating furnace and continuous hot-dip galvanizing equipment - Google Patents

Description

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

To increase the tensile strength of steel plates, solid solution strengthening elements such as Si, Mn, P, and Al are often added. In particular, Si has the advantage that the cost of adding it is lower than other elements and that it can increase the strength of the steel without impairing its ductility. For this reason, Si-containing steel is promising as a high-tensile steel plate. However, if a large amount of Si is contained in steel, the following problems arise.

High-tensile steel sheets are annealed in a reducing atmosphere at temperatures between 600 and 900°C immediately prior to plating processes such as hot-dip galvanizing. Since Si is an element that is more easily oxidized than Fe, Si concentrates on the steel sheet surface during this process. As a result, Si oxides are formed on the steel sheet surface, which significantly deteriorates wettability with zinc and causes bare areas. Furthermore, if Si concentrates on the surface, even if zinc plating adheres, it causes a significant delay in alloying during the alloying process after hot-dip galvanizing, reducing productivity.

As a countermeasure against this, Patent Document 1 proposes a pre-plating method in which an Fe-based plating is applied to the steel sheet (original sheet) before plating by electroplating. Alternatively, Patent Documents 2 and 3 propose an oxidation-reduction method in which the steel sheet 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 reducing furnace.

However, if the former pre-plating method is to be adopted, it would be necessary to install electroplating equipment on the inlet side of the annealing furnace in a continuous hot-dip galvanizing line, which would be difficult to implement considering the space and capital investment costs.

In addition, the latter oxidation-reduction method can be applied to conventional non-oxidation furnace (NOF) or direct-fired furnace (DFF) hot-dip plating lines by adjusting the combustion atmosphere.

However, for example, in the case of burners having a circular burner nozzle outlet shape as described in Patent Documents 4 and 5, even if the burners are dispersed in a staggered arrangement, the thickness of the Fe-based oxide film during oxidation to ensure good plating properties 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 a horizontal furnace, with the shape of the burner nozzle outlet parallel to the width direction of the steel plate, to achieve uniformity in the plate width direction. However, even if a slit burner is installed in the oxidation furnace behind the non-oxidation furnace, the oxidation furnace atmosphere flows into the non-oxidation furnace in the horizontal furnace, causing uneven plate temperature and non-uniformity in the Fe-based oxide film, and the effect of making the flame uniform in the width direction by the slit burner cannot be obtained.

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

However, with conventional pre-plating methods, even if the atmosphere inside the furnace during combustion is adjusted, the thickness of the oxide film remains non-uniform with the conventional burner nozzle shape and arrangement. Furthermore, even if a slit burner is used, when a horizontal furnace consisting of a non-oxidizing furnace, an oxidizing furnace, and a reducing furnace is used in combination, the Fe-based oxide film is formed non-uniformly, and the non-uniformity cannot be eliminated even by using a slit burner in the oxidation furnace.

The present invention has been made in consideration of the above problems, and aims to produce zinc-plated steel sheets of stable quality without bare spots using 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] A method for heating a steel sheet, comprising heating a front side and a back side of a steel sheet passing through a direct-fire 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, with flames sprayed from one or more slit burners at least while the steel sheet passes through the oxidation zone.
[2] The method for heating a steel plate according to [1], wherein the direct-fired heating furnace transports the steel plate in a vertical direction and sucks in 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 sheet according to [1] or [2], wherein an air ratio in the reducing zone is controlled to be 0.70 or more and less than 1.00.
[4] In the range where the temperature of the steel sheet passing through the oxidation zone is 400 ° C. or more,
Heating a steel plate with at least one or more of the slit burners installed;
The method for heating a steel sheet according to any one of [1] to [3].
[5] A method for producing a plated steel sheet, comprising: heat-treating a cold-rolled steel sheet by the heating method according to any one of [1] to [3]; and further subjecting the cold-rolled steel sheet to a plating treatment.
[6] The method for producing a plated steel sheet according to [5], wherein the plating treatment is any one of electrolytic galvanizing treatment, hot-dip galvanizing treatment, and alloyed hot-dip galvanizing treatment.
[7] A catalytic converter having 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 ratios of the oxidation zone and the reduction zone,
At least a portion of the oxidation zone,
a direct-fired heating furnace including one or more slit burners extending in the width direction of the steel plate, on each of the front side and the back side of the steel plate, for spraying a flame toward the steel plate passing through the oxidation zone and the reduction zone.
[8] The direct-fired heating furnace according to [7], wherein the steel plate is transported in a vertical direction and combustion exhaust gas is sucked in through an exhaust port provided 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 of the reduction zone is controlled to be 0.70 or more and less than 1.00.
[10] The slit burner is
In the range where the temperature of the steel sheet 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 direct-fired heating furnace is installed.
[11] The direct-fired heating furnace according to any one of [7] to [9], wherein the oxidation zone includes a burner group having two or more slit burners capable of independently controlling the air ratio and the combustion rate.
[12] A continuous hot-dip galvanizing facility equipped with the direct-fired heating furnace according to any one of [7] to [9].
[13] The continuous hot-dip galvanizing facility according to [12], further comprising an alloying facility for alloying the hot-dip galvanized steel.

According to the present invention, an excellent galvanized steel sheet having a beautiful surface appearance without uncoated areas can be obtained. The present invention is particularly effective when the base material is a high-Si-containing steel sheet, which is particularly difficult to galvanize, and is useful as a method for improving the plating quality in the manufacture of high-Si-containing galvanized steel sheets.

1A and 1B show an embodiment of a direct-fired heating furnace arranged in a continuous hot-dip galvanizing apparatus of the present invention, in which FIG. 1A is a longitudinal sectional view of the direct-fired heating furnace, and FIG. 1B is a front view of an example of the arrangement of direct-fired burners arranged on the wall surface of the direct-fired heating furnace. 1 is a schematic diagram of a continuous hot-dip galvanizing facility according to one embodiment of the present invention. FIG. 2 is an explanatory diagram showing an image of an actual combustion heating state of a steel plate by a slit burner of the present invention. FIG. 1 is a diagram showing an example of the configuration of a direct-fired heating furnace of the present invention.

Direct-fired heating furnaces, which use a direct-fire burner to heat steel sheets, have the advantage of being able to heat steel sheets to a specified temperature at low cost due to their high thermal efficiency. In direct-fired heating furnaces, it is necessary to control the temperature of the steel sheet and, when hot-dip galvanizing high-tensile steels such as high-Si steels, to ensure an appropriate oxide film (Fe-based oxides) on the steel sheet surface by controlling the direct-fire burner atmosphere to be oxidizing. After ensuring an appropriate amount of Fe-based oxides, reduction annealing is performed to internally oxidize the Si, improving the galvanizability of high-Si steel.

However, in the case of conventionally used burners with a circular burner nozzle outlet, even if the burners are distributed in a staggered arrangement, the thickness of the Fe-based oxide coating required to ensure good plating properties cannot be uniformly controlled in the steel sheet travel direction/width direction, resulting in unplated defects.

Therefore, in this invention, a slit burner is used instead of a circular burner, and a method is devised to uniformly control the thickness of the Fe-based oxide coating in both the travel and width directions.

Below, we will explain the direct-fired heating furnace and steel sheet heating method installed in the continuous hot-dip galvanizing equipment of an embodiment of the present invention, with reference to the drawings.

FIG. 1 shows an embodiment of a direct-fired furnace (DFF) arranged in an annealing facility of a continuous hot-dip galvanizing facility according to an embodiment of the present invention. Here, the annealing facility is preferably a vertical furnace, that is, by transporting the steel sheet in the vertical direction (including transporting while turning back and forth in the vertical direction), it is possible to thread the steel sheet at high speed without expanding the facility scale in the horizontal direction. In addition, there is an advantage that the atmospheres of the heating zone and the soaking zone can be easily separated. Transporting in the vertical direction means transporting in the vertical direction.
In Fig. 1, (a) is a vertical cross-sectional view of a direct-fired heating furnace, and (b) is a front view of an example of the arrangement of direct-fired burners arranged on the wall of the direct-fired heating furnace. In Fig. 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 nozzle attached to a slit burner, 3 is a flame nozzle attached to a circular burner, S is a steel sheet (including a steel strip), 4 is a radiation thermometer, 5 is a flame, 6 is an exhaust port, L is a length of a steel sheet S heating region by a burner group from the most upstream burner in the steel strip S movement direction of the burner group 14 in the reduction zone to the most downstream burner, 11 is a burner group in the oxidation zone, 12 is a burner group in the oxidation zone, 13 is a burner group in the oxidation zone, and 14 is a burner group in the reduction zone. In addition, a control device is provided that controls the air ratio of the oxidation zone and the reduction zone, although not shown.

<Direct-fired heating furnace installed in continuous hot-dip galvanizing equipment>
An example of a continuous hot-dip galvanizing facility is shown in Fig. 2. From the inlet side of the facility, a preheating zone 20, a heating zone 21, a soaking zone 22, cooling zones 23 and 24, a galvanizing bath (zinc pot) 25, and an alloying zone 26 as required 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 the continuous hot-dip galvanizing facility, the steel sheet to be heated may be in the form of a steel strip (coil) rather than a cut sheet. The steel sheet is not particularly limited, but a cold-rolled steel sheet is often used.

The direct-fired heating furnace 1 of the present invention is intended to be a heating furnace that is installed in the heating zone 21 of a continuous hot-dip galvanizing facility.

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 direction of steel sheet travel. In this example, a circular burner is installed in the burner group 11 in the oxidation zone, and its flame nozzle is indicated by 3 in the figure, and slit burners are installed in the burner groups 12 and 13 in the same (oxidation zone), and their flame nozzles are indicated by reference number 2 in the figure. The reduction zone has only one zone, burner group (burner group in the reduction zone) 14, and a circular burner is installed. The flame nozzle of the circular burner is indicated by reference number 3 in the figure. The burner groups 11, 12, and 13 in the oxidation zone 1-1 can independently control the burner combustion rate and air ratio 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.

There is no limit to the number of burners in each burner group. It is practical to divide the entire direct-fired heating furnace into 2 to 5 parts and control each part as a group.

In addition, for example, as in the equipment shown in Figure 4, slit burners may be installed not only in the oxidation zone, but also in both the oxidation zone 1-1 and the reduction zone 1-2.

The slit burners are positioned opposite the steel plate surface in the width direction of the steel plate S passing through the oxidation zone 1-1. In order to heat the steel plate S evenly and uniformly in the width direction, the slit burners are positioned to extend in the width direction of the steel plate so that the flame 5 is sprayed across the entire width of the steel plate S. In order to accommodate the production of steel plates S of various widths, the flame spray amount can be controlled for each of the four divided regions in the width direction. Although it is divided into four here, the number of divisions is not limited, and division may not be necessary depending on the flame spray structure of the slit burner and the width of the steel plate. On the other hand, the circular burners are positioned in a dispersed manner opposite the steel plate surface.

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

A slit burner refers to a burner having a rectangular flame injection port in which the length of the opening in the width direction of the steel sheet S is longer than the length of the opening in the direction in which the steel sheet S travels (also called the slit gap B), and the detailed dimensions are not particularly limited. As a guideline, if the length of the opening in the direction in which the steel sheet S travels, i.e., the short side, is B, the opening length in the width direction, i.e., the long side, is about 2B to 200B. In the present invention, burners that inject slit-shaped flames, such as those having a long and narrow rectangular (slit) flame injection port, are collectively referred to as "slit burners". Therefore, there are no particular limitations on the internal structure or injection port. Furthermore, the flame injection port can be divided in the width direction to control the injection width of the flame 5, and by utilizing the above, it is possible to adjust the injection width of the flame 5 according to the width of the target steel sheet.

Although a single slit burner in the direction of steel plate travel is effective, oxidization can be carried out more efficiently by arranging several in tandem. There are no restrictions on the spacing when arranging in tandem, but spacing of about 3B to 10B reduces the likelihood of interference between the flames 5 or temperature unevenness.

Furthermore, since most of the Fe-based oxide film is formed in the range where the sheet temperature is 400°C or higher, it is preferable to apply at least one or more slit burners to the oxidation zone 1-1, particularly in the area where the sheet temperature is in the above range. It is even more preferable to apply the slit burner to the range of 450°C or higher. On the other hand, because the amount of oxidation increases rapidly at high temperatures such as above 650°C, it is preferable to apply the slit burner to the area where the sheet temperature is 650°C or lower. More preferably, it is 600°C or lower, and most preferably, it is 550°C or lower. In order to heat the steel sheet S with the slit burner when it reaches the suitable temperature range, the sheet temperature can be estimated in advance based on the steel type, sheet thickness, sheet width, line speed, air ratio, combustion rate, etc. It is also possible to install radiation thermometers at several points in the sheet passing direction in the oxidation zone 1-1 and actually measure the sheet temperature.

For the above reasons, it is advisable to apply a slit burner on the downstream side in the sheet passing direction of the oxidation zone 1-1, 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 regular maintenance is required as the slit may become clogged with foreign matter such as burner tile fragments or the slit may become deformed due to the high temperature of the flame 5. Therefore, a conventional circular burner may be placed on the upstream side of the oxidation zone 1-1, where the sheet temperature is low, and a slit burner on the downstream side. Even when a circular burner is used on the upstream side, it is preferable to use a direct flame heating method in which the flame impinges vertically on the steel sheet from the standpoint of heating efficiency.

In addition, the arrangement of the flame injection ports 2 associated with the slit burners may be shifted in the direction of travel of the steel sheet S on the front and back sides of the steel sheet S, that is, offset. By offsetting, it is possible to prevent the flames 5 protruding from the ends of the steel sheet S from interfering with each other. Therefore, it is possible to heat the steel sheet S more uniformly than when there is no offset. The offset amount is approximately in the range of B to 3B. If the offset amount is too large, there is a risk that the heating temperature will change on the front and back sides. In a vertical furnace, the burners are arranged in the vertical direction, so the flame becomes unstable due to interference with the flame and combustion gas injected by the burner on the downstream side (lower side of the furnace), and the temperature uniformity and stability in the width and length directions of the steel sheet are reduced. In the case of a circular burner, interference of the flame and combustion gas can be alleviated by staggering the burners, but with a slit burner, there is no break in the flame in the width direction, so interference from the downstream side is unavoidable. 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 providing a slit-shaped exhaust port makes it easier to reduce interference between the flame and combustion gas.
It is preferable that at least one pair of exhaust ports is installed on the front and back of each connecting portion of each zone. It is preferable that the exhaust port is installed below the slit burner, and the combustion exhaust gas is sucked from the exhaust port. Specifically, the combustion exhaust gas refers to a high-temperature gas generated by the reaction of fuel and air, and is a gas composed mainly of reaction products such as carbon dioxide and water vapor, and nitrogen contained in the air, as well as unreacted surplus fuel components, oxygen, and trace components such as intermediate products of the reaction.
If the equipment length and heating capacity satisfy the required performance, an exhaust port may be installed between the individual slit burners that make up each zone.

Regardless of whether it is the oxidation zone 1-1 or the reduction zone 1-2, the burner combustion rate is the amount of fuel gas actually introduced into the burner divided by the amount of fuel gas in the burner at maximum combustion load, and the combustion rate is 100% when the burner is burned at maximum combustion load. In the present invention, the burner combustion rate is not particularly limited, but since a stable combustion state cannot be obtained when the burner combustion load is low, it is preferable to set it to the following threshold value or higher. 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 at which a stable combustion state can be ensured to the amount of fuel gas at maximum combustion load. The threshold value of the combustion rate varies somewhat depending on the burner structure, etc., but can be easily determined by conducting a combustion test, etc. Normally, the threshold value is about 30%.

<Air ratio in oxidation zone and reduction zone>
Burner groups 11 to 13 in oxidation zone 1-1 can freely select whether to start or stop combustion for each burner group. During combustion, it is preferable to set the combustion rate to a predetermined set value or higher, and it is necessary to operate oxidation zone 1-1 with an air ratio of 1 or higher in order to stably oxidize the steel sheet surface. It is preferable to operate oxidation zone 1-1 with an air ratio of 1.00 or higher. It is more preferable to operate oxidation zone 1-1 with an air ratio of 1.05 or higher, and most preferable to operate oxidation zone 1-1 with an air ratio of 1.10 or higher.
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 oxidation zone 1-1 with an air ratio of less than 1.50. It is more preferable to operate the oxidation zone 1-1 with an air ratio of 1.40 or less, and most preferable to operate the oxidation zone 1-1 with an air ratio of 1.30 or less. The air ratio is the value obtained by dividing the amount of air actually introduced into the burner by the amount of air required for complete combustion of the fuel gas.

In addition, 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, allowing control of the combustion rate. By burning with the burner group 14 in the reduction zone 1-2 at an air ratio of 0.70 or more and less than 1.00, the Fe oxides formed on the steel sheet surface can be reduced and reduced Fe can be formed on the surface layer. Specifically, if the air ratio is less than 0.70, the fuel consumption rate will deteriorate and the steel sheet 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, and most preferably 0.80 or more. On the other hand, if the air ratio is 1.00 or more, the oxygen concentration in the combustion gas will be high and the steel sheet will be oxidized. The presence of reduced Fe in the surface layer of the steel sheet prevents oxides from adhering to the rolls in the RT furnace (annealing furnace) when the steel sheet S coming out of the direct-fired heating furnace 1 comes into contact with the rolls, and thus prevents defects (pick-ups) caused by oxide adhesion. Therefore, the air ratio is preferably less than 1.00. More preferably, the air ratio is 0.95 or less, and most preferably 0.90 or less.

The number of burner groups to be burned is determined for various steel sheets S passing through, taking into consideration the heating load, amount of oxidation formed, etc. For the burner groups to be burned, the air ratio and combustion rate are set to values within the above ranges to reduce sheet temperature fluctuations in the traveling direction of the steel sheet S for various steel sheets S. As a result, a sufficient amount of Fe oxide, for example, required for internally oxidizing Si, can be stably generated in the traveling direction of the steel sheet S. The reduction in sheet temperature fluctuations in the traveling direction of the steel sheet S also contributes to stabilizing the oxide reduction action in the burner group 14 of the subsequent reduction zone 1-2. The reduction in the above-mentioned sheet temperature fluctuations also contributes to preventing insufficient reduction of Fe oxide in the RT furnace, internal oxidation of Si, and suppressing oxide adhesion to the rolls of the RT furnace.

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

If the length of the reducing atmosphere is short, an Fe oxide film remains on the surface, resulting in insufficient pick-up prevention. On the other hand, if the length of the reducing atmosphere is long, a surface concentrated layer of Si and other elements will form on the surface of the steel sheet during the subsequent reduction annealing, hindering plating properties.

<Length of reduction zone>
The length of the burner group 14 in the reduction zone 1-2 in the traveling direction of the steel sheet S (reduction zone length) is preferably 150 mm or more, and more preferably 300 mm or more when the uniformity in the width direction is also taken into consideration. It is even more preferably 500 mm or more, and most preferably 1000 mm or more. There is no particular upper limit to the length of the reduction zone, but if it is too long, the temperature rise ΔTrd in the reduction zone becomes large, and it becomes necessary to reduce the temperature rise ΔTox in the oxidation zone. For this reason, a reduction zone that is too long is disadvantageous in ensuring the amount of oxidation, so it is desirable for it to be 10 m or less. It is more preferably 5 m or less, and even more preferably 3 m or less. This is also advantageous in terms of cost. The length of the burner group 14 in the reduction zone 1-2 in the traveling direction of the steel sheet is the length ("L" in FIG. 1) of the heating region of the steel sheet S by the burner group from the flame nozzle 3 associated with the circular burner located at the most upstream position in the traveling direction of the steel sheet of the burner group 14 in the reduction zone 1-2 to the flame nozzle 3 associated with the circular burner located at the most downstream position. Note that even when a slit burner is applied to the burner group 14 in the reduction zone 1-2, the reduction zone length is preferably as described above.

<Length of oxidation zone>
The length of the burner group 11 to 13 in the oxidation zone 1-1 in the traveling direction of the steel sheet (oxidation zone length) should be long enough to ensure the required amount of internal oxidation. However, since the amount of oxidation varies depending on the type of steel being passed, temperature history, passing speed, and size of the steel sheet, it is necessary to ensure a zone length that can ensure the required amount of oxidation even under the most difficult production conditions for oxidation.

In the present invention, the steel sheet S is oxidized and then reduced in a direct-fired heating furnace 1. The amount of oxidation formed in the oxidation zone must be precisely controlled in the traveling direction/width direction of the steel sheet S. To control the amount of oxidation appropriately for the various steel types, temperature histories, traveling speeds, and sizes of steel sheets that are passed through, it is necessary to divide the burners arranged opposite the surface of the steel sheet S into at least two groups in the traveling direction of the steel sheet, and to make it possible to independently control the combustion rate and air ratio for each group. When deciding on the burner groups, it is better to separate the slit burners and circular burners into separate groups and control them separately, rather than mixing them in one group.

In the reduction zone, the intended effect of the present invention can be achieved even if the burners are controlled as one group. Therefore, in the present invention, the burners arranged opposite the surface of the steel sheet S in the oxidation zone 1-1 can be divided into two or more burner groups in the traveling direction of the steel sheet S, whose combustion rate and air ratio can be independently controlled.

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 steel sheet S to be treated, but is preferably 100 to 500 nm. If the thickness is less than 100 nm, the function as a barrier layer that prevents Si from diffusing and concentrating on 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 remains almost unchanged, and there are also disadvantages such as the heating time of the oxidation zone 1-1 becoming longer and the fuel used increasing. 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 Fe-based oxide film can be estimated relatively easily by monitoring the strip temperature entering and leaving the direct-fired heating furnace 1 and correcting for the steel type, strip thickness, line speed, air ratio in the oxidation zone 1-1, and combustion rate in the oxidation zone 1-1. By adjusting mainly the combustion rate in the oxidation zone 1-1 based on this value, stable oxidation conditions can be determined and ensured, thereby making it possible to obtain steel strip S without uncoated defects.

The steel sheet S oxidized/reduced in the direct-fired heating furnace 1 is subsequently reduction annealed in an RT furnace, cooled, and then immersed in a hot-dip galvanizing bath for hot-dip galvanization, or further alloyed as necessary. After reduction annealing, conventional methods may be used. The plating method is not particularly limited, and electrogalvanization may be performed instead of hot-dip galvanization.

After an appropriate amount of Fe-based oxides is formed in the direct-fired heating furnace 1, the surface layer is reduced and reduced Fe is present, so in the subsequent reduction annealing process, the Fe-based oxides are reduced and the Si is internally oxidized, and oxide adhesion to the rolls is prevented. Therefore, there are no indentations caused by roll pickup, no surface concentration of Si, and no 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 it contains a large amount of metal elements such as Si that are more easily oxidized than Fe, but specifically, the present invention is particularly effective in the manufacture of hot-dip galvanized steel sheet with a high Si content that contains 0.1 to 3.0 mass% Si.

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

In a CGL equipped with a direct-fired heating furnace (DFF) 1, the heating burners are composed of four burner groups (11-14). The three burner groups (11-13) on the upstream side of the steel strip S's traveling direction are the oxidation zone 1-1, and the final burner group (14) is the reduction zone 1-2. Furthermore, the oxidation zone 1-1 was tested in two cases: (A) the air ratio and combustion rate were individually controlled for each burner group, and (B) the burner groups 11-13 in the oxidation zone were collectively controlled under the same conditions. The air ratio and combustion rate in the reduction zone are controlled separately from the oxidation zone. Figure 1 shows an example of the burner arrangement. In Figure 1, the burner group 11 in the oxidation zone, the burner group 14 in the reduction zone, and the flame nozzle 2 associated with the slit burner are arranged. The burner type was changed for each burner group depending on the conditions. The fuel gas used for the burners had the composition shown in Table 1. The length of each burner group ("L" in Figure 1) was 3 m, and the slit gap B was 20 mm.

The steel composition of the steel strip S used in the test is shown in Table 2.

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

The evaluation included pick-up defects caused by overoxidation, and the appearance of the plating was evaluated for quality deviations in the direction of travel and width of the steel sheet, as well as temperature deviations in the direction of travel of the steel sheet. Grades A and B were pass, while C was fail.

In the present invention, the same method as that described in the following Patent Document 7 was used to determine the loki defects (pickups) caused by overoxidation. A field of view of 1 ^m2 on the surface of the steel plate of randomly selected steel strips S was inspected using an optical surface defect meter. The above surface defect meter was capable of detecting defects with a diameter of 0.5 mm or more, which were judged to be dent defects caused by contact with the pick-up, here loki defects.
[Patent Document 7] Japanese Patent No. 6607339 A (Good): 0 defects per ¹ m2 (no loki defects)
B (almost good): 1 to 2 defects per ¹ m2 (minor roughness defects are seen here and there)
C (Poor): 3 or more defects per 1 ^m2 (Loaki defects)

The plating appearance was evaluated by measuring the variation of the Fe concentration (alloying rate) in the plating from the target value on the steel sheet surface after the alloying treatment. The smaller the variation of the Fe concentration in the plating from the target value, the better the plating appearance is judged to be. The Fe concentration was measured by the same method as that described in the following Patent Document 8, which is calculated from the change in the diffraction peak angle of the alloy phase constituting the plating layer by X-ray diffraction method.
[Patent Document 8] Japanese Patent No. 5962615 A (Good): Less than ±0.5% (no bare spots or alloying unevenness)
B (almost good): Less than ±1% (slightly unplated or/and slight alloying unevenness)
C (poor): ±1% or more (clear bare spots and/or clear alloying unevenness)
Grades A and B are passing grades, and C is failing grade.

Furthermore, the qualities in the traveling direction and width direction were evaluated by selecting three locations in the traveling direction of the front end, center, and tail end of the steel strip S, taking 1000 mm long samples in the width direction, and evaluating the roughness and plating appearance of each of the central portions of the width. The width direction was evaluated as follows based on the roughness and plating appearance evaluation results of five points, the central portion in the width direction, 1/4 width and 3/4 width locations, and both ends, in a sample of width x 1000 mm taken from the center of the steel strip S.
◎: Only A for both ro-ki and plating properties under the same conditions ○: A or B for both ro-ki and plating properties under the same conditions △: Only B for ro-ki or plating properties under the same conditions ×: C for ro-ki or plating properties under the same conditions In the present invention, a product that passes the test is one that has no ro-ki defects or plating appearance with a rating of C, and is rated ◎, ○, or △ in the width direction and running direction. A rating of △ or higher in both the width direction and running direction was considered to be pass (○), and a rating of × was considered to be fail (×).

Conditions No. 1 to 8 and 15 were produced at a conveying speed of steel strip S of 60 mpm.

Condition 1 is a comparative example in which only circular burners were used. While using circular burners, the combustion rate was less than 30% and the burner combustion state was unstable. In addition, burner groups 11 to 13 in the oxidation zone were controlled collectively, resulting in large quality variations in the width and travel directions.

Condition 2 is when slit burners were applied to burner groups 11 to 13 instead of the circular burners used in condition 1. The burner combustion state was unstable, just like in condition 1, but the application of slit burners improved both the number of loki defects and the plating appearance, 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 burner groups 11 to 13 were controlled collectively in conditions 1 and 2. This makes it possible to operate only the necessary combustion burners (in this case, only burner group 13). However, because the operated burner group 13 in condition 3 was a circular burner, uneven combustion was likely to occur and the surface quality tended to be inferior to that in condition 1.

Condition 4 is an example of controlling each burner group as in condition 3, but the burner shape was changed from circular to slit burners. This improved the surface quality and made the quality in the width and travel directions more uniform.

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

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

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

Condition 8 is an example where the air ratio in the reduction zone is lower than 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 of production performed at a steel strip S conveying speed of 90 mpm, and in both cases the air ratio and combustion rate were controlled for each burner group.

Condition 9 uses both slit burners and circular burners in the oxidation zone. However, since the slit burners are not used in the range where the steel plate temperature reaches 400°C, although the surface quality was within the acceptable range, there were some areas where it was inferior.

In condition 10, the burners used in the oxidation zone consisted only of slit burners, and the surface quality was better than that in condition 9.

Conditions 11 to 13 are examples produced at a conveying speed of steel strip S of 120 mpm. In all cases, a slit burner was introduced into the oxidation zone.

Condition 11 does not control each burner group individually but instead uses centralized control, and some circular burners are used, but as with condition 2, the introduction of slit burners means that the surface quality is within the acceptable range.

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

Condition 13 is an example where a slit burner was used in the reduction zone as well as the oxidation zone. This further improved the surface quality.

Condition 14 is an example where a slit burner was introduced into a horizontal annealing furnace and used as an oxidation zone. Because it was a horizontal furnace, the conveying speed was slower than in the invention example, at 30 mpm. A slit burner was applied in the oxidation zone, and the overoxidation defects and plating appearance were within the acceptable range, but since the mechanism was not capable of conveying the steel sheet in the vertical direction and there was no exhaust port, exhaust air partially flowed into the non-oxidation furnace upstream, causing minor non-plating and alloying unevenness in the width and length directions.

Condition 15 was an example in which operation was carried out under the same conditions as condition 2, but the exhaust ports installed between each zone were closed (equivalent to no exhaust ports) and combustion gases were not sucked in. As a result, the flames interfered with each other, reducing stability and increasing defects compared to condition 2, but the surface quality was within the acceptable range.

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

1. Direct-fired heating furnace (DFF)
1-1 Oxidation zone 1-2 Reduction zone 2 Flame nozzle attached to slit burner 3 Flame nozzle attached to circular burner 4 Radiation thermometer 5 Flame 6 Exhaust port S Steel sheet L Length of steel sheet heating area by burner group 14 from the most upstream burner to the most downstream burner in the steel strip movement direction of burner group 14 11 Burner group in oxidation zone 12 Burner group in oxidation zone 13 Burner group in oxidation zone 14 Burner group B in reduction zone Slit gap 20 Preheating 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 (11)

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, the front side and the back side of the steel sheet passing through the oxidation zone are heated by flames sprayed from one or more slit burners at least while passing through the oxidation zone ;
The direct-fired heating furnace conveys the steel plate in a vertical direction and draws in 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 sheet according to claim 1, wherein an air ratio in the reducing zone is controlled to be 0.70 or more and less than 1.00. In the range where the temperature of the steel sheet passing through the oxidation zone is 400°C or higher,
Heating a steel plate with at least one or more of the slit burners installed;
The method for heating a steel sheet according to claim 1 or 2 . A method for producing a plated steel sheet, comprising the steps of: heating a cold-rolled steel sheet by the heating method according to claim 1 or 2 ; and then plating the cold-rolled steel sheet. The method for producing a plated steel sheet according to claim 4 , wherein the plating treatment is any one of electrolytic galvanizing treatment, hot-dip galvanizing treatment, and alloyed hot-dip galvanizing treatment. The system includes 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 ratios of the oxidation zone and the reduction zone,
At least a portion of the oxidation zone,
one or more slit burners extending in the width direction of the steel sheet are provided on each of the front side and the back side of the steel sheet , the slit burners injecting flames toward the steel sheet passing through the oxidation zone and the reduction zone;
A direct-fired heating furnace in which the steel plate is transported in a vertical direction and combustion exhaust gas is sucked in 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;
7. The direct-fired heating furnace according to claim 6 , wherein the air ratio of the reduction zone is controlled to be 0.70 or more and less than 1.00. The slit burner is
In the range where the temperature of the steel sheet passing through the oxidation zone is 400° C. or higher,
8. The direct-fired heating furnace according to claim 6 or 7, wherein at least one direct-fired heating furnace is installed. 8. The direct-fired heating furnace according to claim 6 , wherein the oxidation zone includes a burner group having two or more slit burners capable of independently controlling the air ratio and the combustion rate. A continuous hot-dip galvanizing facility comprising the direct-fired heating furnace according to claim 6 or 7 . The continuous hot-dip galvanizing facility according to claim 10 , further comprising an alloying facility for alloying the hot-dip galvanized material.

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