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

Description

[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. Si, in particular, has the advantage of being less expensive to add than other elements and being able to increase the strength of steel without impairing its ductility. Therefore, Si-containing steel is promising for use as high-strength steel sheets. However, the following problems arise when large amounts of Si are added to steel.

[0003] High-tensile steel sheets are annealed in a reducing atmosphere at temperatures between 600 and 900°C immediately prior to hot-dip galvanizing and other coating processes. Because Si is more easily oxidized than Fe, it concentrates on the steel sheet surface during this process. As a result, Si oxides form on the steel sheet surface, significantly reducing wettability with zinc and resulting in bare areas. Furthermore, even if zinc is deposited on the surface, the concentration of Si significantly delays the alloying process after hot-dip galvanizing, resulting in reduced productivity.

[0004] To address this issue, Patent Document 1 proposes a pre-plating method in which an Fe-based plating is applied to a steel sheet (base sheet) before plating by electroplating. Alternatively, Patent Documents 2 and 3 propose an oxidation-reduction method in which a steel sheet is heated in an oxidizing atmosphere to form an Fe-based oxide film on the surface, and then annealed and plated in a reducing furnace.

[0005] However, if the former pre-plating method is to be adopted, it is necessary to install an electroplating facility on the inlet side of the annealing furnace in a continuous hot-dip galvanizing facility, which is difficult to implement in consideration of space and capital investment costs.

[0006] The latter oxidation-reduction method can be applied to conventional hot-dip plating lines using the non-oxidizing furnace (NOF) method or the direct-fired furnace (DFF) method by adjusting the combustion atmosphere.

[0007] However, in the case of conventional burners having circular burner nozzle outlets as described in Patent Documents 4 and 5, for example, even if the burners are arranged in a staggered pattern, it is not possible to uniformly control the thickness of the Fe-based oxide film during oxidation to ensure good plating properties, resulting in unplated defects.

[0008] On the other hand, Patent Document 6 proposes a method of using a slit burner in a horizontal furnace, in which the shape of the burner nozzle outlet is parallel to the width direction of the steel plate, in order 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 in the horizontal furnace flows into the non-oxidation furnace, causing uneven plate temperatures and non-uniform Fe-based oxide films, and therefore the effect of making the flame uniform in the width direction by the slit burner cannot be obtained. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-144264 [Patent Document 2] Japanese Patent Application Publication No. 4-202630 [Patent Document 3] Japanese Patent Application Publication No. 7-34210 [Patent Document 4] Japanese Patent Application Publication No. 62-29820 [Patent Document 5] Japanese Patent Application Publication No. 9-59753 [Patent Document 6] Patent No. 3889019 Summary of the Invention [Problem to be solved by the invention]

[0010] However, with conventional pre-plating methods, even if the atmosphere inside the furnace during combustion is adjusted, the thickness of the oxide film remains uneven with the conventional burner nozzle shape and arrangement. Furthermore, even if a slit burner is used, the Fe-based oxide film is formed unevenly when a horizontal furnace consisting of a non-oxidizing furnace, an oxidizing furnace, and a reducing furnace is used in combination, and the unevenness cannot be eliminated even when a slit burner is used in the oxidation furnace.

[0011] The present invention has been made in view of the above problems, and has as its object to produce a zinc-plated steel sheet of stable quality without bare spots by a relatively easy method suitable for practical use. [Means for solving the problem]

[0012] 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 the 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 with flames injected from one or more slit burners at least while the steel plate 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 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 temperature is controlled to the temperature above 1000°C. [4] In the range where the temperature of the steel plate passing through the oxidation zone is 400 ° C or higher, The steel plate is heated by at least one or more of the slit burners. The method for heating a steel plate according to any one of [1] to [3]. [5] A method for producing a plated steel sheet, comprising: heat-treating a cold-rolled steel sheet by the heating method according to any one of [1] to [3]; and then plating the cold-rolled steel sheet. [6] The method for producing a plated steel sheet according to [5], wherein the plating treatment is any one of electrogalvanizing 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 having one or more slit burners extending in the width direction of the steel plate on each of the front and back sides of the steel plate, which spray flames toward the steel plate as it passes through the oxidation zone and the reduction zone. [8] A direct-fired heating furnace according to [7], 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. [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. [7] or [8]. A direct-fired heating furnace.

[10] The slit burner The temperature of the steel sheet passing through the oxidation zone is 400°C or higher. At least one direct-fired heating furnace according to any one of [7] to [9] 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 that can independently control the air ratio and 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 coating. [Effects of the Invention]

[0013] According to the present invention, an excellent galvanized steel sheet having a beautiful surface appearance without bare areas can be obtained. The present invention is particularly effective when a high-Si-content steel sheet, which is particularly difficult to galvanize, is used as the base material, and is useful as a method for improving the coating quality in the production of high-Si-content galvanized steel sheet. [Brief explanation of the drawings]

[0014] [Figure 1] 1A and 1B show an embodiment of a direct-fired heating furnace arranged in a continuous hot-dip galvanizing apparatus according to the present invention, in which FIG. 1A is a longitudinal cross-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. [Figure 2] 1 is a schematic diagram of a continuous hot-dip galvanizing facility according to one embodiment of the present invention. [Figure 3] FIG. 1 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] 1 is a diagram showing an example of the configuration of a direct-fired heating furnace of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] Direct-fired heating furnaces, which use direct-fired burners to heat steel sheets, are characterized by their high thermal efficiency, allowing them to heat steel sheets to a specified temperature at low cost. In direct-fired heating furnaces, when hot-dip galvanizing high-tensile steels, such as high-silicon steels, it is necessary to control the temperature of the steel sheet while also ensuring an appropriate oxide film (Fe-based oxides) on the steel sheet surface by controlling the direct-fired burner atmosphere to be oxidizing. After ensuring the appropriate amount of Fe-based oxides, reduction annealing is performed to internally oxidize the silicon, improving the galvanizability of high-silicon steels.

[0016] However, in the case of conventional burners with circular burner nozzle outlets, even if the burners are arranged in a staggered pattern, it is not possible to control the thickness of the Fe-based oxide film, which is necessary to ensure good plating performance, uniformly in the steel sheet travel direction / width direction, resulting in unplated defects.

[0017] Therefore, in the present invention, a slit burner is used instead of a circular burner, and a method is devised to control the thickness of the Fe-based oxide film uniformly in the traveling direction and width direction.

[0018] Hereinafter, a direct-fired heating furnace and a steel sheet heating method to be installed in a continuous hot-dip galvanizing facility according to an embodiment of the present invention will be described with reference to the drawings.

[0019] FIG. 1 shows one embodiment of a direct-fired furnace (DFF) to be installed in the annealing equipment of a continuous hot-dip galvanizing line according to an embodiment of the present invention. Here, the annealing equipment is preferably a vertical furnace, which means that steel sheets are transported in the vertical direction (including transporting them while turning back and forth in the vertical direction), allowing for high-speed threading without expanding the scale of the equipment in the horizontal direction. Another advantage is that the atmospheres in the heating zone and the soaking zone can be easily separated. "Transporting in the vertical direction" refers to transporting in the vertical direction. In Figure 1, (a) is a longitudinal 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 on the wall of the direct-fired heating furnace. In Figure 1, 1 denotes a direct-fired heating furnace (DFF), 1-1 denotes the oxidation zone of the DFF, 1-2 denotes the reduction zone of the DFF, 2 denotes a flame nozzle attached to a slit burner, 3 denotes a flame nozzle attached to a circular burner, S denotes the steel plate (including the steel plate), 4 denotes a radiation thermometer, 5 denotes a flame, 6 denotes an exhaust port, L denotes the length of the heating area of ​​the steel plate S by the burner group 14 in the reduction zone from the most upstream burner in the direction of steel plate S movement to the most downstream burner, 11 denotes the burner group in the oxidation zone, 12 denotes the burner group in the oxidation zone, 13 denotes the burner group in the oxidation zone, and 14 denotes the burner group in the reduction zone. Also, although not shown, a controller is provided to control the air ratio in the oxidation zone and the reduction zone.

[0020] <Direct-fired heating furnace installed in continuous hot-dip galvanizing equipment> FIG. 2 shows an example of continuous hot-dip galvanizing equipment. From the inlet side of the equipment, it is equipped with a preheating zone 20, a heating zone 21, a soaking zone 22, cooling zones 23 and 24, a galvanizing bath (zinc pot) 25, and, if necessary, an alloying zone 26. A cooling zone 27 may be provided after the alloying zone 26. When the heating furnace of the present invention is thus applied to part of the continuous hot-dip galvanizing equipment, 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 cold-rolled steel sheet is often used.

[0021] 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.

[0022] In this embodiment, it is also possible to independently control multiple slit burners. 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. 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. Slit burners are installed in the burner groups 12 and 13 (oxidation zone), and their flame nozzles are indicated by 2 in the figure. The reduction zone has only one zone, burner group 14 (burner group in the reduction zone), and is equipped with a circular burner. The flame nozzle of the circular burner is indicated by 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. The burner groups 11 to 13 in the oxidation zone combust under conditions that result in a combustion rate above a predetermined threshold.

[0023] There is no limit to the number of burners included 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.

[0024] Also, as in the equipment shown in FIG. 4, for example, slit burners may be provided not only in the oxidation zone but also in both the oxidation zone 1-1 and the reduction zone 1-2.

[0025] The slit burners are arranged facing the steel sheet surface in the width direction of the steel sheet S passing through the oxidation zone 1-1. In order to heat the steel sheet S evenly and uniformly in the width direction, the slit burners are arranged extending in the width direction of the steel sheet so that the flame 5 is sprayed across the entire width of the steel sheet S. In addition, to accommodate the production of steel sheets S of various widths, the flame spray amount can be controlled for each of the four divided regions in the width direction. Although divided into four regions 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. On the other hand, the circular burners are arranged in a dispersed manner facing the steel sheet surface.

[0026] <Slit burner> FIG. 3 is an explanatory diagram showing an image of the actual combustion and 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 diagram.

[0027] A slit burner refers to a burner having a rectangular burner 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 of travel of the steel sheet S (also referred to as the slit gap B), and its detailed dimensions are not particularly limited. As a rough guide, if the length of the opening in the direction of travel of the steel sheet S, i.e., the short side, is B, the length of the opening in the width direction, i.e., the long side, is approximately 2B to 200B. In the present invention, burners that inject a slit-shaped flame, 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 this, it is possible to adjust the injection width of the flame 5 according to the width of the target steel sheet.

[0028] Although a single slit burner in the direction of steel sheet travel is effective, arranging several in tandem allows for more efficient oxidation. There are no restrictions on the spacing between tandem burners, but spacing of approximately 3 to 10 Angles will prevent interference between the flames 5 and temperature unevenness.

[0029] Furthermore, since most Fe-based oxide films are formed in the strip temperature range of 400°C or higher, it is preferable to apply at least one slit burner to the oxidation zone 1-1, particularly in the area where the strip temperature is within this range. It is even more preferable to apply a slit burner to the area of ​​450°C or higher. On the other hand, because the amount of oxidation increases rapidly at temperatures above 650°C, it is preferable to apply a slit burner to the area where the strip temperature is 650°C or lower. It is even more preferable to apply a slit burner to the area where the strip temperature is 600°C or lower, and most preferably to apply a slit burner to the area where the strip temperature is 550°C or lower. To heat the steel sheet S with a slit burner when it reaches the appropriate temperature range, the strip temperature can be predicted and determined by estimating the strip temperature in advance based on the steel grade, thickness, width, line speed, air ratio, combustion rate, etc. It is also possible to install radiation thermometers at several locations in the strip running direction in the oxidation zone 1-1 and measure the strip temperature.

[0030] For the reasons stated above, it is advisable to use a slit burner on the downstream side in the strip running direction of the oxidation zone 1-1, where the strip temperature is high. Slit burners can be used for 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 to prevent blockage by foreign matter such as burner tile fragments, or deformation of the slit due to the high temperature of the flame 5. Therefore, it is also possible to place a conventional circular burner on the upstream side of the oxidation zone 1-1, where the strip 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-fire heating method in which the flame impinges perpendicularly on the steel strip, from the perspective of heating efficiency.

[0031] In addition, the flame nozzles 2 attached to the slit burners may be arranged offset in the direction of travel of the steel sheet S on the front and back sides. By offsetting, the flames 5 extending from the edges of the steel sheet S can be prevented from interfering with each other. This allows for more uniform heating of the steel sheet S than without offsetting. The offset amount is approximately B to 3B. If the offset is too large, the heating temperature may vary between the front and back sides. In vertical furnaces, burners are arranged vertically, so the flame becomes unstable due to interference from the flame and combustion gas emitted by the burner downstream (lower part of the furnace), reducing the temperature uniformity and stability across the width and length of the steel sheet. In the case of circular burners, interference from the flame and combustion gas can be reduced by staggering the burners, but with slit burners, interference from the downstream side is unavoidable due to the lack of flame gaps in the width direction. Therefore, in the present invention, it is preferable to provide a slit-shaped exhaust port in the section where the slit burner is installed, and providing a slit-shaped exhaust port makes it easier to reduce interference between the flame and combustion gas. It is preferable to install at least one pair of exhaust ports on the front and back of each zone connection. The exhaust ports are preferably installed below the slit burner, and combustion exhaust gas is sucked through the exhaust ports. Specifically, combustion exhaust gas refers to high-temperature gas generated by the reaction of fuel and air, and is composed mainly of reaction products such as carbon dioxide and water vapor, and nitrogen contained in the air, as well as trace components such as unreacted excess fuel components and oxygen, and 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.

[0032] Regardless of whether it is the oxidation zone 1-1 or the reduction zone 1-2, the burner combustion rate is the value obtained by dividing the amount of fuel gas actually introduced into the burner by the amount of fuel gas at the maximum combustion load, and a combustion rate of 100% is obtained when the burner is burning at the maximum combustion load. In the present invention, the burner combustion rate is not particularly limited, but because a stable combustion state cannot be obtained when the burner combustion load is low, it is preferable to set the combustion rate at or above the following threshold. The predetermined combustion rate threshold 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 combustion rate threshold varies somewhat depending on the burner structure, etc., but can be easily determined by conducting combustion tests, etc. Typically, the threshold is about 30%.

[0033] <Air ratio in oxidation zone and reduction zone> The burner groups 11 to 13 in the oxidation zone 1-1 can be freely switched on or off for each burner group. When burning, it is preferable to set the combustion rate to a predetermined value or higher, and to stably oxidize the steel sheet surface, it is necessary to operate the oxidation zone 1-1 at an air ratio of 1 or higher. It is preferable to operate the oxidation zone 1-1 at an air ratio of 1.00 or higher. It is more preferable to operate the oxidation zone 1-1 at an air ratio of 1.05 or higher, and most preferably at an air ratio of 1.10 or higher. To prevent excessive oxide film formation, nitrogen oxide generation, and flame blowout, it is preferable to operate the oxidation zone 1-1 at an air ratio of less than 1.50. It is more preferable to operate the oxidation zone 1-1 at an air ratio of 1.40 or less, and most preferable to operate it at an air ratio of 1.30 or less. The air ratio is the amount of air actually introduced into the burner divided by the amount of air required for complete combustion of the fuel gas.

[0034] Furthermore, the circular burners of the burner group 14 in the reducing zone 1-2 must have an air ratio of less than 1.00, and preferably operate at an air ratio of 0.70 or more but less than 1.00, allowing for control of the combustion rate. By burning at an air ratio of 0.70 or more but less than 1.00 in the burner group 14 in the reducing zone 1-2, Fe oxides formed on the steel sheet surface can be reduced and reduced Fe can be formed in the surface layer. Specifically, an air ratio of less than 0.70 results in a deterioration in fuel consumption rate and contamination of the steel sheet 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, an air ratio of 1.00 or more increases the oxygen concentration in the combustion gas, causing oxidation of the steel sheet. The presence of reduced Fe in the surface layer of the steel sheet prevents oxide adhesion to the rolls in the RT furnace (annealing furnace) when the steel sheet S exits the direct-fired heating furnace 1 and comes into contact with the rolls. This prevents defects (pickup) caused by oxide adhesion. Therefore, the air ratio is preferably less than 1.00, more preferably 0.95 or less, and most preferably 0.90 or less.

[0035] 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. By setting the air ratio and combustion rate of the burner groups to values ​​within the above ranges, the temperature fluctuation in the direction of travel of the steel sheet S is reduced for various steel sheets S. As a result, a sufficient amount of Fe oxide, for example, required for internal oxidation of Si, can be stably produced in the direction of travel of the steel sheet S. The reduction in temperature fluctuation in the direction of travel of the steel sheet S also contributes to stabilizing the oxide reduction action in the burner group 14 in the subsequent reducing zone 1-2. Furthermore, the reduction in temperature fluctuation prevents insufficient reduction of Fe oxide in the RT furnace, contributes to the internal oxidation of Si, and contributes to suppressing oxide adhesion to the rolls of the RT furnace.

[0036] 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 the heating area by the burner group 11 to 13 in the oxidation zone 1-1 is the oxidation zone, and the heating area by the burner group 14 in the reduction zone 1-2 is the reduction zone.

[0037] If the reducing atmosphere is too short, an Fe oxide film remains on the surface, resulting in insufficient pick-up prevention.On the other hand, if the reducing atmosphere is too long, a surface enriched layer of Si and other elements forms on the steel sheet surface during the subsequent reduction annealing, impairing galvanizability.

[0038] <Length of reduction zone> The length of the burner group 14 in the reduction zone 1-2 in the direction of travel of the steel sheet S (reduction zone length) is preferably 150 mm or more, and more preferably 300 mm or more, taking into account uniformity in the width direction. 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, making it necessary to reduce the temperature rise ΔTox in the oxidation zone. Therefore, since a reduction zone that is too long is disadvantageous in ensuring the amount of oxidation, 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 direction of travel of the steel sheet is the length ("L" in Figure 1) of the heating zone of the steel sheet S by the burner group from the flame nozzle 3 associated with the circular burner located most upstream in the direction of travel 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 most downstream. Even when slit burners are used in the burner group 14 of the reduction zone 1-2, the reduction zone length is preferably set as described above.

[0039] <Length of oxidation zone> The length of the burner group 11-13 in the oxidation zone 1-1 in the direction of steel strip travel (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 through, temperature history, passing speed, and steel strip size, 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.

[0040] 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 and width direction of the steel sheet S. To appropriately control the amount of oxidation for various steel types, temperature histories, traveling speeds, and sizes of steel sheets, the burners arranged opposite the surface of the steel sheet S must be divided into at least two groups in the traveling direction of the steel sheet, with the combustion rate and air ratio of each group being independently controllable. When determining the burner groups, it is better to separate slit burners and circular burners and control them separately, rather than mixing them in the same group.

[0041] 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 opposite the surface of the steel sheet S 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.

[0042] 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, but is preferably 100 to 500 nm. A thickness of less than 100 nm may result in an insufficient barrier layer function to prevent Si diffusion and enrichment on the surface. Therefore, the thickness of the Fe-based oxide film is preferably 100 nm or more. A thickness of 150 nm or more is more preferable, and 200 nm or more is even more preferable. On the other hand, a thickness exceeding 500 nm results in little change in the barrier layer function, and also has the disadvantages of longer heating times in the oxidation zone 1-1 and increased fuel consumption. Therefore, the thickness of the Fe-based oxide film is preferably 500 nm or less. A thickness of 450 nm or less is more preferable, and 400 nm or less is even more preferable.

[0043] The thickness of the Fe-based oxide film 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, 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 enabling the production of steel sheet S free of uncoated defects.

[0044] 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 galvanizing, or further alloyed as necessary. Conventional methods may be used after reduction annealing. The plating method is not particularly limited, and electrogalvanizing may be performed instead of hot-dip galvanizing.

[0045] After an appropriate amount of Fe-based oxides is formed in the direct-fired heating furnace 1, the surface layer is reduced, leaving reduced Fe. In the subsequent reduction annealing step, the Fe-based oxides are reduced, the Si is internally oxidized, and oxide adhesion to the rolls is prevented. This prevents indentations caused by roll pickup, surface concentration of Si, and coating defects caused by insufficient reduction of Fe-based oxides.

[0046] 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 particularly effective in manufacturing hot-dip galvanized steel sheets with a high Si content that contain 0.1 to 3.0 mass% Si.

[0047] An annealing furnace (RT furnace), a cooling zone, a hot-dip galvanizing facility, an alloying treatment facility, etc. are arranged downstream of the direct-fired heating furnace 1. The annealing furnace, cooling zone, hot-dip galvanizing facility, alloying treatment facility, etc. are not particularly limited and may be any commonly employed facility. A preheating furnace may also be arranged upstream of the direct-fired heating furnace 1. [Example]

[0048] The CGL equipped with a direct-fired furnace (DFF) 1 was used. The DFF 1 consisted of four burner groups (11-14). The three burner groups (11-13) upstream of the steel strip S travel direction were designated as the oxidation zone 1-1, and the final burner group (14) was designated as the reduction zone 1-2. Furthermore, tests were conducted in two cases: (A) the air ratio and combustion rate of each burner group in the oxidation zone 1-1 were individually controlled, and (B) the air ratio and combustion rate of the burner groups 11-13 in the oxidation zone were controlled collectively under the same conditions. The air ratio and combustion rate of the reduction zone were controlled separately from those of the oxidation zone. Figure 1 shows an example of the burner arrangement. Figure 1 shows the burner group 11 in the oxidation zone and the burner group 14 in the reduction zone, each equipped with a flame nozzle 3 attached to a circular burner, and the burner groups 12 and 13 in the oxidation zone, each equipped with a flame nozzle 2 attached to a slit burner. 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.

[0049] The steel composition of the steel strip S used in the test is shown in Table 2.

[0050] 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, coating bath temperature 463°C, coating Al concentration 0.135%, alloying temperature 550°C. Three strip speeds (LS) were considered: 60 mpm, 90 mpm, and 120 mpm. The burner was used at a combustion rate of 30% or more.

[0051] The evaluation consisted of evaluation of lo-ki defects (pickup) caused by peroxidation, quality deviation in the direction of travel and width of the steel sheet for the plating appearance, and temperature deviation in the direction of travel of the steel sheet. A rating of A or B is pass, and C is fail.

[0052] In the present invention, the same method as that described in Patent Document 7 was used to determine the number of pick-up defects (pick-ups) due to peroxidation. 2The field of view was inspected using an optical surface defect meter. The surface defect meter was able to detect 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 7] 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 found here and there) C (inferior): 1m 2 3 or more per piece (with low-key defects)

[0053] The coating appearance was evaluated by measuring the variation of the Fe concentration (alloying rate) in the coating relative to the target value on the steel sheet surface after alloying treatment. The smaller the variation of the Fe concentration in the coating relative to the target value, the better the coating appearance is judged to be. The Fe concentration was measured using the same method as described in Patent Document 8 below, which is calculated from the change in the diffraction peak angle of the alloy phase that constitutes the coating layer using X-ray diffraction. [Patent Document 8] Patent No. 5962615 A (Good): Less than ±0.5% (no unplated or alloyed unevenness) B (almost good): Less than ±1% (slight unplated areas and / or slight alloying irregularities) C (poor): ±1% or more (clear unplated or / and clear alloying unevenness) A grade of A or B is a pass, and a grade of C is a fail.

[0054] Furthermore, the quality in the traveling direction and width direction was evaluated by selecting three locations in the traveling direction of the head, center, and tail of steel strip S and taking 1000mm long samples in the width direction, and evaluating the roughness and plating appearance at the center of each width.In addition, the width direction was evaluated as follows based on the results of evaluation of roughness and plating appearance at five points in the width direction, the center, 1 / 4 width and 3 / 4 width locations, and both ends, for a width x 1000mm sample taken from the center of steel strip S. ◎: Only A is rated for both the lacquering and plating properties under the same conditions. ○: Both the evaluation of the lacquer and plating properties under the same conditions are A or B. △: Only B was rated for lacquering or plating under the same conditions. ×: Evaluation of the burnishing or plating property under the same conditions includes C In the present invention, a product is deemed to pass if it has no roughness defects or plating appearance with a rating of C, and is rated as ◎, ○, or △ in the width direction and running direction. The rating was passed (○) if it was △ or higher in both the width direction and running direction, and failed (×) if it included ×.

[0055] Conditions Nos. 1 to 8 and 15 were produced under the condition that the conveying speed of the steel strip S was 60 mpm.

[0056] 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. Furthermore, burner groups 11 to 13 in the oxidation zone were controlled collectively, resulting in large variations in quality in the width and travel directions.

[0057] Condition 2 is a case where 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 by applying the slit burners, both the loki defects and the plating appearance improved, and the quality variation in both the width direction and the traveling direction was slightly improved.

[0058] Condition 3 is a case where the air ratio and combustion rate can be controlled for each burner group, whereas in conditions 1 and 2, burner groups 11 to 13 were controlled collectively. This makes it possible to operate only the necessary combustion burners (in this case, only burner group 13). However, in condition 3, because the burner group 13 that was operated was a circular burner, combustion unevenness was likely to occur, and the surface quality tended to be inferior to that of condition 1.

[0059] 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.

[0060] 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, although better uniformity in the width direction and travel direction than condition 3 was obtained, many defects appeared and the product was rejected.

[0061] Condition 6 is an example in which the air ratio of burner group 13 was 1.65, which was excessive compared to condition 5. The degree of the defect was reduced and fell within the acceptable range.

[0062] Condition 7 is an example in which the air ratio in the reduction zone of the burner group 14 is 1.00, which is higher than that of condition 4. In this case, as with condition 5, the uniformity was relatively good, but there were many defects and it was rejected.

[0063] Condition 8 is an example in which the air ratio in the reduction zone is lower than that of condition 7. The degree of defects was reduced compared to condition 7 and fell within the acceptable range.

[0064] Conditions 9 and 10 are examples of production performed under conditions where the conveying speed of the steel strip S was 90 mpm, and in both cases the air ratio and combustion rate were controlled for each burner group.

[0065] Condition 9 uses both slit burners and circular burners in the oxidation zone. However, because 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.

[0066] In condition 10, the burners used in the oxidation zone consisted of only slit burners, and the surface quality was better than that of condition 9.

[0067] Conditions 11 to 13 are examples produced under the condition that the conveying speed of the steel strip S was 120 mpm. In all conditions, a slit burner was introduced into the oxidation zone.

[0068] Condition 11 does not control each burner group but controls them all at once, and some circular burners are used, but as with condition 2, the introduction of slit burners ensures that the surface quality is within the acceptable range.

[0069] 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.

[0070] Condition 13 is an example in which a slit burner was used in both the oxidation and reduction zones, which further improved the surface quality.

[0071] Condition 14 is an example in which a slit burner was installed in 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 used in the oxidation zone, and the peroxidation defects and coating appearance were within acceptable limits. However, because the system was not equipped with a mechanism for conveying the steel sheet vertically and no exhaust port was provided, some exhaust air flowed into the upstream non-oxidation furnace, resulting in minor uncoated areas and uneven alloying in the width and length directions.

[0072] 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.

[0073] From the above examples, it was confirmed that the surface quality can be improved by introducing a slit burner into the oxidation zone, and that even better surface quality can be obtained by optimizing the control method and combustion conditions.

[0074] [Table 1]

[0075] [Table 2]

[0076] [Table 3] [Explanation of symbols]

[0077] 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 L is the length of the steel plate heating area by the burner group 14 from the most upstream burner to the most downstream burner in the steel strip movement direction. 11 Oxidation zone burner group 12 Oxidation zone burner group 13 Oxidation zone burner group 14 Burner group in reduction zone B Slit gap 20 Pre-tropical zone 21 Heating zone (direct flame 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 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

7. The direct-fired heating furnace according to claim 6.

8. 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.

9. 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.

10. A continuous hot-dip galvanizing facility comprising the direct-fired heating furnace according to claim 6 or 7.

11. 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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