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

Abstract

The purpose of the present invention is to manufacture a zinc-plated steel sheet that is free of pickups and that has a stable quality. In this method for heating a steel plate, a steel plate passes through a direct-fired heating furnace having an oxidization band in which operation is performed with an air ratio of 1 or higher, and passes through a reduction band in which operation is performed with an air ratio of less than 1. While the steel plate is passing through at least the reduction band, the front surface side and the back surface side of the steel plate are subjected to heating by fire blasted from one or more slit burners disposed extending along the width direction of the steel plate.

Description

[Technical field]

[0001] The present invention relates to a method for heating a steel sheet, a method for producing a plated steel sheet, a direct-fired heating furnace, and a continuous hot-dip galvanizing facility using a direct-fired heating furnace. [Background technology]

[0002] To increase the 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 the cost of adding it is lower than other elements and that it can increase the strength of steel without impairing its ductility. Therefore, Si-containing steel is promising for use in high-tensile steel sheets. However, when a large amount of Si is contained in steel, the following problems occur.

[0003] 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 is concentrated 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, resulting in bare areas. Furthermore, if Si is concentrated on the surface, even if zinc plating is adhered, it causes a significant delay in alloying during the alloying process after hot-dip galvanizing, thereby reducing productivity.

[0004] To address this issue, a well-known method is to heat the steel sheet in an oxidation zone equipped with a direct flame burner, form an oxide film on the steel sheet surface, then reduce part of the oxide film (surface layer) on the steel sheet surface in a reduction zone to form reduced Fe, and then further reduce the oxide film in the subsequent reduction annealing zone to improve wettability with zinc. In particular, if the oxide film formed in the reduction zone is not sufficiently reduced, oxide scale adheres to the rolls inside the furnace, causing scratches on the steel sheet (a phenomenon known as pick-up), so a method has been published to keep the capacity of the reduction zone uniform.

[0005] For example, in a direct fired furnace (DFF) or a non-oxidizing furnace (NOF), a technique for preventing pickup by adjusting the atmospheric gas concentration in the oxidation zone, reduction zone, and reduction annealing zone has been described.

[0006] For example, Patent Document 1 discloses a technique of performing an oxidation treatment, followed by reduction annealing, followed by hot dip plating. Specifically, in the oxidation treatment, in the first stage, heating is performed at a temperature of 400°C to 750°C in an atmosphere with an O2 concentration of 1000 volume ppm or more and an H2O concentration of 1000 volume ppm or more. Then, in the second stage, heating is performed at a temperature of 600°C to 850°C in an atmosphere with an O2 concentration of less than 1000 volume ppm and an H2O concentration of 1000 volume ppm or more. However, for example, in the case of a burner with a circular shape of the conventionally used burner nozzle outlet described in Patent Documents 2 and 3, even if the burners are distributed in a staggered arrangement, the thickness of the reduced Fe on the surface layer cannot be uniformly controlled, and pick-up occurs. On the other hand, Patent Document 4 proposes a method of using a slit burner with a burner nozzle outlet shape parallel to the steel sheet width direction in the oxidation zone of a horizontal furnace in order to uniformize the sheet width direction. However, even if a slit burner is installed only in the oxidation zone and in the oxidation furnace behind the non-oxidation furnace, the atmosphere of the oxidation furnace will flow into the non-oxidation furnace in a horizontal furnace, causing unevenness in the strip 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. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6323628 [Patent Document 2] Japanese Patent Application Publication No. 62-29820 [Patent Document 3] Japanese Patent Application Publication No. 9-59753 [Patent Document 4] Patent No. 3889019 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made in view of the above problems, and has an object to produce a galvanized steel sheet of stable quality free from pick-up by a relatively easy method suitable for practical use. [Means for solving the problem]

[0009] The present invention, which has been made to solve the above problems, has the following configuration. [1] A method for heating a steel plate, in which the front and back sides of a steel plate 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 are heated by flames sprayed from one or more slit burners extending in the width direction of the steel plate, at least while the steel plate passes through the reduction 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 draws in combustion exhaust gas from an exhaust port installed below the slit burner. [3] The air ratio in the oxidation zone is 1.00 or more and less than 1.50; The air ratio of the reduction zone is 0.70 or more and less than 1.00. The method for heating a steel plate according to [1] or [2], wherein the heating temperature is controlled to the above. [4] 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 the above [1] to [3]; and further subjecting the cold-rolled steel sheet to a plating treatment. [5] The method for producing a plated steel sheet according to [4], wherein the plating treatment is any one of electrolytic galvanizing treatment, hot-dip galvanizing treatment, and alloyed hot-dip galvanizing treatment. [6] 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 reduction zone, The apparatus includes one or more slit burners extending in the width direction of the steel plate, which spray flames toward the steel plate passing through the oxidation zone and the reduction zone, on each of the front side and the back side of the steel plate. Direct-fired heating furnace. [7] A direct-fired heating furnace as described in [6], 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. [8] The air ratio in the oxidation zone is greater than or equal to 1.00 and less than 1.50; The air ratio of the reduction zone is controlled to be greater than or equal to 0.70 and less than 1.00. [6] or [7]. A direct-fired heating furnace. [9] A continuous hot-dip galvanizing facility comprising the direct-fired heating furnace according to any one of [6] to [8].

[10] The continuous hot-dip galvanizing system according to [9], further comprising an alloying facility for alloying the hot-dip galvanized coating. Effect of the Invention

[0010] According to the present invention, it is possible to obtain an excellent galvanized steel sheet which suppresses uncoated areas and has a beautiful surface appearance without pick-up. The present invention is particularly effective when a high-Si-containing steel sheet, which is particularly difficult to galvanize, is used as the base material, and is useful as a method for improving the plating quality in the production of high-Si-containing galvanized steel sheet. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 shows one embodiment of a direct-fired heating furnace arranged in a continuous hot-dip galvanizing apparatus of the present invention, in which (a) is a vertical sectional view of the direct-fired heating furnace and (b) 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. [Diagram 2] FIG. 1 is a diagram showing an example of continuous hot-dip galvanizing equipment according to the present invention. [Diagram 3] 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. [Figure 4] FIG. 1 is a diagram showing an example of the configuration of a direct-fired heating furnace of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Direct-fired heating furnaces, which use a direct flame 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, the temperature of the steel sheet can be controlled, and at the same time, when hot-dip galvanizing high-tensile steels, such as high-Si steels, the atmosphere of the direct flame burner can be controlled to be oxidizing. This ensures an appropriate oxide film (Fe-based oxide) on the steel sheet surface, and then by controlling the atmosphere downstream of the direct flame burner to be reducing, part of the surface layer of the Fe-based oxide is reduced by reduced Fe, and further reduced Fe is formed in the subsequent reduction annealing zone, preventing pick-up.

[0013] However, in the case of conventional burners with a circular burner nozzle outlet, even if the burners are distributed in a staggered arrangement, the thickness of the reduced Fe cannot be controlled uniformly in the steel sheet traveling direction / width direction, and pick-up occurs.

[0014] In the present invention, therefore, a method was devised in which a slit burner is used in the reduction zone to control the thickness of the reduced Fe uniformly in the traveling direction and width direction of the steel sheet.

[0015] Hereinafter, a direct-fired heating furnace and a steel sheet heating method arranged in a continuous hot-dip galvanizing facility according to an embodiment of the present invention will be described with reference to the drawings.

[0016] FIG 1 shows an embodiment of a direct-fired heating furnace (DFF) arranged in the annealing equipment of a continuous hot-dip galvanizing equipment according to an embodiment of the present invention. Here, the type of annealing equipment 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 becomes possible to thread the sheet at high speed without expanding the scale of the equipment 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 refers to 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 the oxidation zone of the DFF, 1-2 is the 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 plate (including the steel plate), 4 is a radiation thermometer, 5 is a flame, 6 is an exhaust port, L is the length of the steel plate S heating area by the burner group 14 in the reduction zone from the most upstream burner in the steel plate movement direction 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. Although not shown, a control device is provided for controlling the air ratio in the oxidation zone and the reduction zone.

[0017] <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.

[0018] The direct-fired heating furnace 1 of the present invention is intended to be a heating furnace introduced into the heating zone 21 of a continuous hot-dip galvanizing line. Here, the example of FIG. 1 will be described in detail. The slit burners are arranged facing the steel sheet surface. Also, to accommodate the width of the steel sheet, the flame injection port is divided into four 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 injection structure of the slit burner and the width of the steel sheet. Meanwhile, the circular burners are arranged in a dispersed manner facing the steel sheet surface.

[0019] The direct-fired heating furnace 1 is composed of an oxidation zone 1-1 and a reduction zone 1-2. The oxidation zone 1-1 is composed of three burner groups (zones) 11 to 13 in the steel sheet traveling direction, and circular burners are used for the burner groups 11 to 13 in the oxidation zone. Their flame nozzles are indicated by the symbol 3 in the figure. The reduction zone has only one zone, the burner group 14 in the reduction zone, and a slit burner is used. The flame nozzle of the slit burner is indicated by the symbol 2 in the figure. The circular burners of the burner groups 11, 12, and 13 in the oxidation zone 1-1 and the slit burners of the burner group 14 in the reduction zone can be independently controlled in terms of combustion rate and air ratio for each burner group. The circular burners of the burner groups 11 to 13 in the oxidation zone and the slit burners of the burner group 14 in the reduction zone are combusted under conditions where the combustion rate is equal to or higher than a predetermined threshold value.

[0020] There is no limit to the number of burners included in each of the burner groups. It is practical to divide the entire DFF into 2 to 5 parts and control each part as a group.

[0021] For example, as in the facility shown in FIG. 4, the slit burners may be provided not only in the reduction zone 1-2 but also in both the oxidation zone 1-1 and the reduction zone 1-2.

[0022] The slit burners are disposed facing the steel sheet surface in the width direction of the steel sheet S passing through the reduction zone 1-2. In order to heat the steel sheet S uniformly and without unevenness in the width direction, the slit burners are disposed extending in the width direction of the steel sheet so that the flame 5 is sprayed over the entire width of the steel sheet S. In order to accommodate the manufacture of steel sheets S of various widths, the flame spray amount can be controlled for each of the regions divided into four in the width direction. Here, the region is divided into four, but 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 sheet.

[0023] An annealing furnace (RT furnace), a cooling zone, a hot-dip galvanizing equipment, an alloying treatment equipment, etc. are arranged downstream of the direct-fired heating furnace. The RT furnace, the cooling zone, the hot-dip galvanizing equipment, the alloying treatment equipment, etc. are not particularly limited and may be those that are usually employed. A preheating furnace may be arranged upstream of the direct-fired heating furnace.

[0024] <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.

[0025] The slit burner refers to a burner having a rectangular burner flame outlet whose opening length in the width direction of the steel sheet S is longer than the opening length 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, when 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, a burner that injects a slit-shaped flame 5, such as having a flame outlet in a long and narrow rectangular (slit) shape, is collectively called a "slit burner". Therefore, there are no particular limitations on the internal structure or the outlet. Furthermore, the flame outlet 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.

[0026] Although only one slit burner is effective in the traveling direction of the steel sheet S, more efficient reduction can be achieved by arranging several of them in tandem. When arranging them in tandem, there are no limitations on the spacing between them, but spacing them apart by about 3B to 10B makes it more difficult for the flames 5 to interfere with each other or for temperature unevenness to occur.

[0027] In addition, the flame injection nozzles 2 associated with the slit burners may be arranged in a shifted manner in the direction of travel of the steel sheet S on the front and back sides of the steel sheet, i.e., 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 a wider area 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, the heating temperature may differ between the front and back sides. In a vertical furnace, the burners are arranged in the vertical direction, so the flame 5 becomes unstable due to interference with the flame 5 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 5 and combustion gas can be alleviated by staggering the burners, but in the case of a slit burner, there is no break in the flame 5 in the width direction, so the influence of interference from the downstream side becomes stronger. Therefore, in the section where the slit burners are installed, a slit-shaped exhaust port 6 may be provided below the slit burners in order to release the flow of the combustion exhaust gas from the downstream side. It is preferable to suck the combustion exhaust gas from the exhaust port 6 installed below the slit burners. If the equipment length and heating capacity satisfy the required performance, the exhaust port 6 may be provided for each installed slit burner. Also, as shown in Figure 4, it is possible to obtain sufficient effects by providing an exhaust port at the connection part of each burner group. 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 the reaction products 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.

[0028] Regardless of whether it is the oxidation zone 1-1 or the reduction zone 1-2, the combustion rate of the burner 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. When the burner is burned at the maximum combustion load, the combustion rate is 100%. In the present invention, the combustion rate of the burner is not particularly limited, but it is preferable to set it to the following threshold value or higher because the burner cannot obtain a stable combustion state when the combustion load is low. 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 the maximum combustion load. The threshold value of the combustion rate varies somewhat depending on the burner structure, etc., but can be easily determined by performing a combustion test, etc. Usually, the threshold value is about 30%.

[0029] <Air ratio in oxidation zone and reduction zone> The burner groups 11 to 13 in the oxidation zone 1-1 can freely select whether to start or stop combustion for each burner group. When burning, it is preferable to set the combustion rate to a predetermined set value or more, and it is necessary to operate the oxidation zone 1-1 with an air ratio of 1 or more in order to stably oxidize the steel sheet surface. It is preferable to operate the oxidation zone 1-1 with an air ratio of 1.00 or more. It is more preferable to operate the oxidation zone 1-1 with an air ratio of 1.05 or more, and even more preferable to operate it with 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 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 even more preferable to operate it with an air ratio of 1.30 or less. The air ratio is a 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.

[0030] In addition, the slit burners of the burner group 14 in the reduction zone 1-2 must be operated with an air ratio of less than 1, and preferably with an air ratio of 0.70 or more and less than 1.00, so that the combustion rate can be controlled. 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, while if it is 1.00 or more, the oxygen concentration in the combustion gas will increase and the steel sheet will be oxidized. The presence of reduced Fe on the steel sheet surface layer prevents the adhesion of oxides to the rolls when the steel sheet S coming out of the direct-fired heating furnace comes into contact with the rolls in the RT furnace, and defects (pickups) caused by the adhesion of oxides can be prevented. For this reason, the air ratio is preferably 0.70 or more. The air ratio is more preferably 0.75 or more, and even more preferably 0.80 or more. The air ratio is less than 1, preferably 0.95 or less, and more preferably 0.90 or less.

[0031] The number of burner groups to be combusted is determined taking into consideration the heating load, amount of oxide formed, etc., for various steel sheets S passing through, and the burner groups to be combusted are set to values ​​within the above ranges for the air ratio and combustion rate, thereby reducing the sheet temperature fluctuation in the traveling direction of the steel sheet S for various steel sheets S, and for example, a sufficient amount of Fe oxide required for internally oxidizing Si can be stably generated in the traveling direction of the steel sheet S. The reduction in sheet temperature fluctuation 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 reducing zone 1-2, and also contributes to preventing insufficient reduction of Fe oxide in the RT furnace, contributing to the internal oxidation of Si, and contributing to suppressing oxide adhesion to the rolls of the RT furnace.

[0032] In this embodiment, the burner group 11-13 in the oxidation zone 1-1 are oxidation burners operated at an air ratio of 1.00 or more, and the burner group 14 in the reduction zone 1-2 are reduction burners operated at an air ratio of less than 1.00, so that the heating area by the burner group 11-13 in the DFF oxidation zone 1-1 is the oxidation zone, and the heating area by the burner group 14 in the DFF reduction zone 1-2 is the reduction zone.

[0033] If the length of the reduction zone 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 reduction zone is long, a surface concentrated layer of Si, etc. is formed on the steel sheet surface during the subsequent reduction annealing, which impairs plating properties. Therefore, it is preferable to set the length of the reduction zone as follows.

[0034] 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 in consideration of uniformity in the width direction. More preferably, it is 500 mm or more, and most preferably, it is 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 amount ΔTrd in the reduction zone becomes large, so it becomes necessary to reduce the temperature rise amount ΔTox in the oxidation zone. Since such a long reduction zone is disadvantageous in ensuring the amount of oxidation, it is desirable to set it to 10 m or less. More preferably, it is 5 m or less, and even more preferably, it is 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 S is the length ("L" in FIG. 1) of the steel sheet S heating region by the burner group from the burner located at the most upstream to the burner located at the most downstream in the traveling direction of the steel sheet S of the burner group 14 in the reduction zone 1-2.

[0035] It is preferable that the oxidation zone length is long enough to ensure the required amount of internal oxidation, but since the amount of oxidation varies depending on the type of steel being passed, temperature history, passing speed, and size of the steel, it is necessary to set the zone length so that the required amount of oxidation can be ensured even under the most difficult production conditions for oxidation.

[0036] In the present invention, the steel sheet S is oxidized and then reduced in a direct-fired heating furnace 1. In particular, the amount of oxidation formed in the oxidation zone needs to be precisely controlled in the traveling direction / width direction of the steel sheet S. In order to control the amount of oxidation to an appropriate amount for various steel types, temperature histories, traveling speeds, and sizes of steel sheets passing through, it is preferable 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 S, and to make it possible to independently control the combustion rate and air ratio for each group. When determining 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.

[0037] On the other hand, in the reduction zone, the intended effect of the present invention can be obtained even if the burners are controlled as one group. Therefore, in the present invention, the burners arranged facing the steel sheet surface in the oxidation zone 1-1 may be divided into two or more burner groups in the traveling direction of the steel sheet S, each of which can independently control the combustion rate and air ratio.

[0038] The thickness of the Fe-based oxide film formed in the oxidation zone 1-1 (oxidation zone 1-1 burner group 11-13) varies depending on the Si content and sheet thickness of the steel sheet to be treated, but is preferably 100-500 nm. That is, if the thickness is less than 100 nm, the function as a barrier layer that prevents 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, even if the thickness exceeds 500 nm, the function as a barrier layer is almost unchanged, and there are also disadvantages such as a longer heating time in the oxidation zone 1-1 and an increased amount of fuel to be used, so 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.

[0039] The thickness of the reduced Fe formed in the reduction zone 1-2 (reduction zone 1-2 burner group 14) varies depending on the Si content and sheet thickness of the steel sheet to be treated, but is preferably 1 to 30 nm. That is, if the thickness is less than 1 nm, the pick-up prevention effect may be insufficient, so the thickness of the reduced Fe is preferably 1 nm or more. The thickness of the reduced Fe is more preferably 5 nm or more, and further preferably 10 nm or more. On the other hand, if the thickness exceeds 30 nm, the reduced Fe becomes excessive, and a surface concentrated layer of Si or the like is formed on the surface layer of the steel sheet during the subsequent reduction annealing, thereby impairing the galvanizing property. Therefore, the thickness of the reduced Fe is preferably 30 nm or less. The thickness of the reduced Fe is more preferably 25 nm or less, and further preferably 20 nm or less.

[0040] The thickness of the Fe-based oxide film and reduced Fe can be estimated relatively easily by monitoring the strip temperature at the inlet and outlet of the direct-fired heating furnace 1 and correcting for the steel type, strip thickness, line speed, air ratio in the oxidation zone 1-1 / reduction zone 1-2, and combustion rate in the oxidation zone 1-1 / reduction zone 1-2. By adjusting mainly the combustion rate in the oxidation zone 1-1 / reduction zone 1-2 based on this value, stable oxidation and reduction conditions can be determined and ensured, and as a result, steel strip free of uncoated defects can be obtained.

[0041] The steel sheet 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 galvanizing, or further alloyed as necessary. After reduction annealing, a conventional method may be used. The plating method is not particularly limited, and electrogalvanizing may be performed instead of hot-dip galvanizing.

[0042] After an appropriate amount of Fe-based oxide is formed in the direct-fired heating furnace 1, the surface layer is reduced to form reduced Fe, and in the subsequent reduction annealing process, all of the Fe-based oxide is reduced, Si is internally oxidized, and oxide adhesion to the rolls is prevented. Therefore, no indentations due to roll pickup, no surface concentration of Si, and no plating defects due to insufficient reduction of Fe-based oxide occur.

[0043] 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, and specifically, the present invention is suitable for manufacturing a hot-dip galvanized steel sheet with a high Si content of 0.1 to 3.0 mass%. EXAMPLES

[0044] In a CGL equipped with a direct-fired heating furnace (DFF) 1, a DFF consisting of four burner groups 11-14 was used as the heating burners. The three burner groups 11-13 on the upstream side of the steel strip S in the traveling direction were placed in the oxidation zone 1-1, and the final burner group 14 was placed in the reduction zone 1-2. Furthermore, tests were conducted in the oxidation zone 1-1, with the air-fuel ratio and combustion rate controlled individually for each burner group, and with the burner groups 11-13 in the oxidation zone all controlled under the same conditions. Figure 1 shows an example of the burner arrangement. In Figure 1, a flame nozzle 3 associated with a circular burner is placed in the burner groups 11-13 in the oxidation zone, and a flame nozzle 2 associated with a slit burner is placed in the burner group 14 in the reduction zone. The burner type was changed for each burner group depending on the conditions, and tests were conducted. A gas with the composition shown in Table 1 was used as the fuel gas for the burners. The length of each burner group ("L" in Figure 1) was 3 m, and the slit gap B was 20 mm.

[0045] The steel composition of the steel strip S used in the test is shown in Table 2.

[0046] Other test conditions were: sheet thickness 1.0 mm, sheet width 1000 mm, DFF inlet average sheet temperature 200°C, DFF outlet average temperature 650°C, annealing temperature in the RT furnace 850°C, plating bath temperature 463°C, plating Al concentration 0.135%, and alloying temperature 550°C. Three levels of steel strip S speed (LS) were considered: 60 mpm, 90 mpm, and 120 mpm.

[0047] The evaluation was carried out on the quality deviation in the direction of travel and width of the plating, as well as on the defects (pick-up) caused by overoxidation and the appearance of the plating. In each test, the evaluations A and B were passed, and C was failed.

[0048] The method was the same as that described in the following Patent Document 5. The pick-up defects caused by peroxidation were measured at 1 mm on the surface of randomly selected steel sheets. 2 The field of view was inspected using an optical surface defect meter. The 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 pickup, here called loki defects. [Patent Document 5] Patent No. 6607339 A (good): 1m 2 0 per piece (no low-key defects) B (almost good): 1m 2 1-2 per piece (minor roughness defects are seen here and there) C(inferior):1m 2 3 or more per piece (with low-key defects)

[0049] The plating appearance was evaluated by measuring the variation of the Fe concentration (index of alloying rate) in the plating on the steel sheet surface after alloying treatment from the target value. 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 Patent Document 6 below, which is calculated from the change in the diffraction peak angle of the alloy phase that constitutes the plating layer by X-ray diffraction method. [Patent Document 6] Patent No. 5962615 A (Good): Less than ±0.5% (no uneven plating or alloying) B (almost good): Less than ±1% (slightly unplated or / and slight alloying unevenness) C (deterioration): ±1% or more (clear unplated areas and / or clear alloying unevenness) Grades A and B are passing grades, and C is failing grade.

[0050] Furthermore, the quality in the travel direction and width direction was evaluated by selecting three locations in the travel direction of the head, center, and tail of steel strip S, taking 1000mm-long samples in the width direction, and evaluating the roughness and plating appearance at the center of each width. The width direction was evaluated as follows based on the roughness and plating appearance evaluation results at five points in the width direction center, 1 / 4-width and 3 / 4-width locations, and both ends of a width x 1000mm sample taken from the center of steel strip S. ◎: Only A in both the evaluation of roughing and plating under the same conditions ○: Both the lacquering and plating properties under the same conditions are rated A or B. △: Only B was rated for roughing or plating under the same conditions. ×: Evaluation of roughness or plating property under the same conditions includes C In the present invention, a product that passes the test is one that has no loki defects or plating appearance that is rated C, and is rated as ◎, ○, or △ in the width direction and running direction. The test was judged as pass (○) if it was △ or higher in both the width direction and running direction, and fail (×) if it included ×.

[0051] Conditions No. 1 to 7 were produced under the condition that the conveying speed of the steel strip S was 60 mpm. Condition 1 is a conventional type (comparative example) in which a circular burner is used in the reduction zone burner group 14. The reducing power was not stable in the width direction and the traveling direction, and there were occasional low-ki defects due to overoxidation, resulting in failure. Condition 2 is an example of the present invention in which a slit burner is used in the reduction zone burner group 14 instead of a circular burner. The burner flame was uniform in the width direction, and reduced Fe was obtained uniformly in the width direction and the traveling direction, so a steel strip S of stable quality was obtained without low-ki defects due to overoxidation. Condition 3 uses a slit burner, but the air ratio of the oxidation zone is 0.90, and the plating appearance is poor due to insufficient oxidizing power. Condition 4 is a case in which the air ratio of the oxidation zone is excessive compared to condition 3. The surface quality is more stable than condition 3 and falls within the acceptable range. Condition 5 uses a slit burner like conditions 2 and 3, but the air ratio of the reduction zone is high at 1.00, so there were occasional low-ki defects. Condition 6 is an example in which the air ratio of the reduction zone is lower than condition 5. Compared to condition 5, the number of surface defects was reduced and fell within the acceptable range. Condition 7 is an example in which the air ratio in the reduction zone was adjusted and the combustion rate in the reduction zone was also reduced. The number of surface defects was reduced more than in condition 5 and fell within the acceptable range.

[0052] Conditions 8 and 9 are examples where the conveying speed of the steel strip S was 90 mpm. Condition 8 applied a circular burner to the reduction zone burner group 14, and like condition 1, the quality was poor and it was rejected. On the other hand, condition 9 is an example where a slit burner was applied. As a result, the surface quality was improved compared to condition 8 and it was accepted.

[0053] Conditions 10, 11, and 12 are examples where the conveying speed of steel strip S was 120 mpm. Condition 10 used a circular burner, just like conditions 1 and 8, and the surface quality was unacceptable. Condition 11 used a slit burner, and just like condition 9, the surface quality was improved and stable, passing. Condition 12 is an example where slit burners were used in both the oxidation and reduction zones, resulting in high and stable surface quality.

[0054] Condition 13 is an example where slit burners were applied to the heating zone of a horizontal furnace. Of the four burner groups that make up the heating zone, the most downstream burner group 14 was operated as a reduction zone with an air ratio of less than 1. Because it was a horizontal furnace, the production efficiency was lower than other examples, but the defects remained low and were within the acceptable range. However, because it was a horizontal furnace, the atmosphere separation between the heating zone and the soaking zone was insufficient, and the uniformity in the width direction and traveling direction decreased.

[0055] Condition 14 is an example in which an exhaust port was installed upstream of the burner group using slit burners, in contrast to condition 2. By installing the exhaust port, interference between flames was prevented, and uniformity in the width direction and travel direction was further improved, resulting in good and stable surface quality.

[0056] From the above results, it was confirmed that the surface quality can be improved by introducing a slit burner into the reduction zone, and furthermore, better surface quality can be obtained by optimizing the control method and combustion conditions.

[0057] [Table 1]

[0058] [Table 2]

[0059] [Table 3] [Explanation of symbols]

[0060] 1 Direct Fired Furnace (DFF) 1-1 Oxidation Zone 1-2 Reduction Zone 2 Flame nozzle attached to the slit burner 3. Flame nozzle attached to circular burner 4 Radiation thermometer 5. Flame 6 Exhaust port S steel plate (steel strip) L is the length of the steel strip heating area by the burner group 14 from the most upstream burner to the most downstream burner in the traveling direction of the steel strip 11 Oxidation zone burner group 12 Oxidation zone burner group 13 Oxidation Zone Burners 14 Burners in reduction zone B Slit Gap 20 Pre-tropical zone 21 Heating Zone (Direct Heating) 22. Equal temperature 23 Cooling Zone 24 Cooling Zone 25 Plating bath (zinc pot) 26 Alloying Zone 27 Cooling Zone

Claims

1. 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 being passed through the direct-fired heating furnace are heated by flames sprayed from one or more slit burners extending in the width direction of the steel sheet at least while the steel sheet is passing through the reduction zone; The method for heating a steel plate, wherein the combustion rate of the slit burner is 30% or more.

2. 2. The method for heating a steel plate according to claim 1, wherein the direct-fired heating furnace transports the steel plate in a vertical direction and draws in combustion exhaust gas from an exhaust port provided below the slit burner.

3. The air ratio of the oxidation zone is 1.00 or more and less than 1.50; The air ratio of the reduction zone is 0.70 or more and less than 1.

00. The method for heating a steel plate according to claim 1, wherein the heating temperature is controlled to be equal to or higher than the heating temperature.

4. The air ratio of the oxidation zone is 1.00 or more and less than 1.50; The air ratio of the reduction zone is 0.70 or more and less than 1.

00. The method for heating a steel plate according to claim 2, wherein the heating temperature is controlled to be equal to or higher than the heating temperature.

5. 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 4; and further subjecting the cold-rolled steel sheet to a plating treatment.

6. The method for producing a plated steel sheet according to claim 5 , wherein the plating treatment is any one of electrolytic galvanizing treatment, hot-dip galvanizing treatment, and alloyed hot-dip galvanizing treatment.

7. 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 reduction 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 combustion rate of the slit burner is 30% or more.

8. 8. The direct-fired heating furnace according to claim 7, wherein the steel plate is transported in a vertical direction, and combustion exhaust gas is sucked 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 air ratio of the reduction zone is controlled to be 0.70 or more and less than 1.

00.

8. The direct-fired heating furnace according to claim 7.

10. The air ratio of the oxidation zone is 1.00 or more and less than 1.50; The air ratio of the reduction zone is controlled to be 0.70 or more and less than 1.

00.

9. The direct-fired heating furnace according to claim 8.

11. A continuous hot-dip galvanizing facility comprising the direct-fired heating furnace according to any one of claims 7 to 10.

12. The continuous hot-dip galvanizing facility according to claim 11, further comprising an alloying facility for alloying the hot-dip galvanized material.

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