Method for heating steel sheet, method for manufacturing plated steel sheet, direct-fire heating furnace, and continuous hot-dip galvanizing facility

Abstract

The purpose of the present invention is to manufacture a galvanized steel sheet having stable quality without roll marks. A method for heating a steel sheet, said method being characterized in that: the front surface side and the rear surface side of the steel sheet passing through a direct-fire heating furnace having an oxidation zone in which operation is performed under the condition of an air ratio of 1 or more and a reduction zone in which operation is performed under the condition of an air ratio of less than 1; the steel sheet is heated by flames injected from one or more slit burners extending in the width direction of the steel sheet at least while passing through the reduction zone.

Description

Technical Field

[0001] The present invention relates to a method for heating a steel plate, a method for manufacturing a plated steel plate, a direct-fire type heating furnace, and continuous hot-dip galvanizing equipment using the direct-fire type heating furnace. Background Art

[0002] In order to increase the tensile strength of steel sheets, solid solution strengthening elements such as Si, Mn, P, and Al are often added. In particular, Si has the advantages of being low in addition cost compared to other elements and being able to increase the strength without damaging the ductility of steel. Therefore, Si-containing steel is expected to be a high-tensile steel sheet. However, when Si is contained in large amounts in steel, the following problems arise.

[0003] High-tensile steel sheets are annealed in a reducing atmosphere at a temperature range of 600 to 900°C in a process immediately before the plating process such as hot-dip galvanizing. Since Si is an easily oxidizable element compared to Fe, Si is concentrated on the surface of the steel sheet at this time. As a result, Si oxides are formed on the surface of the steel sheet, which significantly deteriorate the wettability with zinc and cause non-plating. In addition, when Si is concentrated on the surface, even if a zinc coating is attached, a significant alloying delay will occur in the alloying process after hot-dip galvanizing, and productivity will deteriorate.

[0004] In order to solve such a problem, the following method is known: a steel plate is heated in an oxidation zone provided with a direct-fire burner to form an oxide film on the surface of the steel plate, and then a part (surface layer) of the oxide film on the surface of the steel plate is reduced in a reduction zone to form reduced Fe, and the oxide film is further reduced in a subsequent reduction annealing zone, thereby improving wettability with zinc. In particular, the following method is disclosed: when the reduction of the oxide film formed in the reduction zone is insufficient, the oxide scale adheres to the rollers in the furnace, and an indentation (rolling mark) is generated on the steel plate, which causes a so-called rolling mark phenomenon, so that the capacity of the reduction zone is uniformly maintained.

[0005] For example, a technique for preventing roller marks by adjusting the atmospheric gas concentrations in an oxidation zone, a reduction zone, and a reduction annealing zone in a direct-fired furnace (DFF) and a non-oxidation furnace (NOF) is described.

[0006] For example, Patent Document 1 discloses a technique of performing an oxidation treatment, followed by reduction annealing, and then performing a hot-dip treatment. Specifically, the technique is as follows: in the above-mentioned oxidation treatment, in the front stage, heating is performed at a temperature of 400°C or more and 750°C or less 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 rear stage, heating is performed at a temperature of 600°C or more and 850°C or less in an atmosphere with an O2 concentration of less than 1000 volume ppm and an H2O concentration of 1000 volume ppm or more. However, in the case of a burner having a circular shape of a burner nozzle outlet as described in Patent Documents 2 and 3, even if the burners are dispersedly arranged in a staggered manner, the thickness of the reduced Fe in the surface layer cannot be uniformly controlled, resulting in the generation of roller marks. On the other hand, Patent Document 4 proposes a method of using a slit burner having a shape of a burner nozzle outlet parallel to the width direction of the steel plate in the oxidation zone of a horizontal furnace in order to make the plate width uniform. However, even if a slit burner is installed only in the oxidation zone and a slit burner is installed in the oxidation furnace behind the non-oxidation furnace, the oxidation furnace atmosphere in the horizontal furnace will flow into the non-oxidation furnace, resulting in uneven plate temperature and uneven Fe-based oxide film, making it impossible to use the slit burner to achieve the effect of making the flame uniform in the width direction.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent No. 6323628

[0010] Patent Document 2: Japanese Patent Application Laid-Open No. 62-29820

[0011] Patent Document 3: Japanese Patent Application Laid-Open No. 9-59753

[0012] Patent Document 4: Japanese Patent No. 3889019 Summary of the invention

[0013] Problems to be solved by the invention

[0014] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to manufacture a galvanized steel sheet with stable quality and without roller marks by a relatively easy method suitable for practical use.

[0015] Methods used to solve problems

[0016] The gist of the present invention made in order to solve the above-mentioned problems lies in the following configurations.

[0017] [1] A method for heating a steel plate, wherein the surface side and the back side of the steel plate passing through a direct-fired heating furnace having an oxidation zone operating at an air ratio of 1 or more and a reduction zone operating at an air ratio of less than 1 are heated by flames ejected from one or more slit burners extending in the width direction of the steel plate, at least during the period of passing through the reduction zone.

[0018] [2] The method for heating a steel plate according to [1], wherein the direct-fired heating furnace conveys the steel plate in a vertical direction and sucks combustion exhaust gas from an exhaust port provided on a lower side of the slot burner.

[0019] [3] The method for heating a steel plate according to [1] or [2], wherein the air ratio in the oxidation zone is controlled to be greater than 1.00 and less than 1.50, and the air ratio in the reduction zone is controlled to be greater than 0.70 and less than 1.00.

[0020] [4] A method for producing a plated steel sheet, comprising heating a cold-rolled steel sheet using the heating method described in any one of [1] to [3] above, and further subjecting the cold-rolled steel sheet to a plating treatment.

[0021] [5] The method for producing a plated steel sheet according to [4], wherein the plating treatment uses any one of electrogalvanizing, hot-dip galvanizing, and alloyed hot-dip galvannealing.

[0022] [6] A direct-fire heating furnace, comprising an oxidation zone operating at an air ratio of 1 or more, a reduction zone operating at an air ratio of less than 1, and a control device capable of controlling the air ratio between the oxidation zone and the reduction zone, wherein at least one slit burner extending in the width direction of the steel plate and spraying flames onto the steel plate passing through the oxidation zone and the reduction zone is disposed on the surface side and the back side of the steel plate, respectively, in at least a portion of the reduction zone.

[0023] [7] The direct-fired heating furnace according to [6], wherein the steel plate is conveyed in the up-down direction, and combustion exhaust gas is sucked from an exhaust port provided on a lower side of the slot burner.

[0024] [8] The direct-fired heating furnace according to [6] or [7], wherein the air ratio in the oxidation zone is controlled to be greater than 1.00 and less than 1.50, and the air ratio in the reduction zone is controlled to be greater than 0.70 and less than 1.00.

[0025] [9] A continuous hot-dip galvanizing facility comprising the direct-fired heating furnace according to any one of [6] to [8] above.

[0026]

[10] The continuous hot-dip galvanizing equipment according to [9], further comprising an alloying equipment for alloying the hot-dip galvanized layer.

[0027] Effects of the Invention

[0028] According to the present invention, an excellent galvanized steel sheet can be obtained which suppresses non-galvanizing and has a beautiful surface appearance without roller marks. The present invention is particularly effective when a high-Si content steel sheet, which is particularly difficult to galvanize, is used as a base material, and is useful as a method for improving the coating quality in the manufacture of high-Si content galvanized steel sheets. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The present invention shows an embodiment of a direct-fired heating furnace configured in a continuous hot-dip galvanizing device of the present invention, wherein (a) is a longitudinal sectional view of the direct-fired heating furnace, and (b) is a view showing an example of the configuration of direct-fired burners configured on the wall surface of the direct-fired heating furnace as viewed from the front.

[0030] Figure 2 This is a diagram showing an example of the continuous hot-dip galvanizing facility of the present invention.

[0031] Figure 3 This is an explanatory diagram showing an image of an actual combustion heating state of a steel plate using the slot burner of the present invention.

[0032] Figure 4 This is a diagram showing an example of the structure of the direct-fired heating furnace of the present invention. DETAILED DESCRIPTION

[0033] The direct-fire type heating furnace that uses a direct-fire burner to heat the steel plate has the characteristic of being able to heat the steel plate to a specified temperature at a low cost due to its high thermal efficiency. In the direct-fire type heating furnace, when hot-dip coating is performed on high-tensile steel represented by high-Si steel while controlling the temperature of the steel plate, the atmosphere of the direct-fire burner can be controlled to be oxidizing. Thus, after ensuring an appropriate oxide film (Fe-based oxide) on the surface of the steel plate, the atmosphere of the rear section of the direct-fire burner is controlled to be reducing, thereby reducing a portion of the surface layer of the Fe-based oxide using reduced Fe, and then further forming reduced Fe using the subsequent reduction annealing zone, thereby preventing roller marks.

[0034] However, in the case of burners with circular burner nozzle outlets, even if the burners are dispersedly arranged in a staggered manner, the thickness of reduced Fe cannot be uniformly controlled in the steel sheet running direction / width direction, resulting in roll marks.

[0035] Therefore, in the present invention, a method of applying a slot burner in the reduction zone to uniformly control the thickness of reduced Fe in the running direction and width direction of the steel plate was considered.

[0036] Hereinafter, a direct-fired heating furnace and a method for heating a steel sheet arranged in a continuous hot-dip galvanizing facility according to an embodiment of the present invention will be described with reference to the drawings.

[0037] Figure 1 An embodiment of a direct-fired heating furnace (DFF) configured in an annealing facility of a continuous hot-dip galvanizing facility according to an embodiment of the present invention is shown. Here, the annealing facility is preferably a vertical furnace, that is, by conveying the steel plate in the up-down direction (including conveying while turning back in the up-down direction), the plate can be passed at a high speed without expanding the scale of the facility in the horizontal direction. In addition, there is also an advantage of easily separating the atmosphere of the heating zone and the soaking zone. Conveying in the up-down direction means conveying in the vertical direction.

[0038] exist Figure 1 In the figure, (a) shows a longitudinal sectional view of a direct-fire type heating furnace, and (b) shows a front view of an example of the arrangement of each direct-fire burner arranged on the wall surface of the direct-fire type heating furnace. Figure 1 1 is a direct-fired heating furnace (DFF), 1-1 is an oxidation zone of the DFF, 1-2 is a reduction zone of the DFF, 2 is a flame jet port attached to a slit burner, 3 is a flame jet port attached to a circular burner, S is a steel plate (including a steel strip), 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 of ​​the burner group 14 in the reduction zone from the burner located most upstream in the steel strip moving direction to the burner located most downstream, 11 is a burner group in the oxidation zone, 12 is a burner group in the oxidation zone, 13 is a burner group in the oxidation zone, and 14 is a burner group in the reduction zone. In addition, although not shown in the figure, a control device for controlling the air ratio between the oxidation zone and the reduction zone is provided.

[0039] <Direct-fire heating furnace installed in continuous hot-dip galvanizing equipment>

[0040] Figure 2 An example of a continuous hot-dip galvanizing facility is shown in FIG. From the inlet side of the facility, there are a preheating zone 20, a heating zone 21, a soaking zone 22, cooling zones 23, 24, a plating bath (zinc pot) 25, and an alloying zone 26 as required. A cooling zone 27 may be provided after the alloying zone 26. Thus, when the heating furnace of the present application is applied to a part of a continuous hot-dip galvanizing facility, the steel plate to be heated may be in the form of a steel strip (coil) instead of a cut plate. The steel plate is not particularly limited, but cold-rolled steel plates are mostly used.

[0041] The direct-fired heating furnace 1 of the present invention is assumed to be a heating furnace introduced into the heating zone 21 even in a continuous hot-dip galvanizing facility. Figure 1The example is described in detail. The slit burner is arranged relative to the steel plate surface. In addition, in order to correspond to the width of the steel plate, the flame injection port is divided into four equal parts in the width direction. Although it is divided into four equal parts here, there is no limit to the number of divisions, and there are cases where division is not required depending on the flame injection structure of the slit burner and the width of the steel plate. On the other hand, the circular burner is dispersedly arranged relative to the steel plate surface.

[0042] The direct-fire heating furnace 1 is composed of an oxidation zone 1-1 and a reduction zone 1-2, wherein the oxidation zone 1-1 is composed of three burner groups (sections) 11 to 13 in the direction of travel of the steel plate, and circular burners are used in the burner groups 11 to 13 in the oxidation zone. Their flame jets are represented by symbol 3 in the figure. The reduction zone is provided with only one section, namely the burner group 14 in the reduction zone, and a slit burner is used. The flame jet of the slit burner is 2 in the figure. With respect to the circular burners of the burner groups 11, 12, and 13 in the oxidation zone of the oxidation zone 1-1 and the slit burner of the burner group 14 in the reduction zone, the combustion rate and air ratio can be independently controlled for each burner group. The circular burners of the burner groups 11 to 13 in the oxidation zone and the slit burner of the burner group 14 in the reduction zone burn under the condition that the combustion rate is a combustion rate above a predetermined threshold value.

[0043] There is no limit to the number of burners included in each of the above-mentioned burner groups. It is practical to divide the entire DFF into 2 to 5 parts and control each part as a group.

[0044] It should be noted that, for example, Figure 4 As shown in the apparatus, not only the reduction zone 1-2 is provided with the slot burner, but both the oxidation zone 1-1 and the reduction zone 1-2 are provided with the slot burner.

[0045] It should be noted that the slit burner is arranged opposite to the steel plate surface in the width direction of the steel plate S passing through the reduction zone 1-2. In addition, in order to uniformly heat the steel plate S in the width direction, the slit burner is arranged to extend in the width direction of the steel plate so that the flame 5 is sprayed on the entire width of the steel plate S. In addition, in order to correspond to the manufacture of steel plates S of various widths, the flame spraying amount can be controlled in each area after being divided into four equal parts in the width direction. Here, although the four equal parts are divided, there is no limit to the number of divisions, and there are cases where division is not required depending on the flame spraying structure of the slit burner and the width of the steel plate.

[0046] An annealing furnace (RT furnace), a cooling zone, a hot-dip equipment, an alloying treatment equipment, etc. are arranged downstream of the direct-fired heating furnace. The RT furnace, the cooling zone, the hot-dip equipment, the alloying treatment equipment, etc. are not particularly limited and can be commonly used equipment. A preheating furnace can also be arranged upstream of the direct-fired heating furnace.

[0047] <Slot burner>

[0048] Figure 3 This is an explanatory diagram showing an image of an actual combustion heating state of a steel plate using the slot burner of the present invention. The slot burner will be described below based on the description thereof.

[0049] A slit burner is a slit burner having a rectangular burner flame ejection port whose length of the opening in the width direction of the steel plate S is longer than the length of the opening in the direction of travel of the steel plate S (also referred to as a slit gap B), and its detailed size is not particularly limited. As a reference, when the length of the opening in the direction of travel of the steel plate S, i.e., the short side, is set to B, the length of the opening in the width direction, i.e., the long side, is about 2B to about 200B in length. In the present invention, burners having such elongated rectangular (slit)-shaped flame ejection ports and the like that eject slit-shaped flames 5 are collectively referred to as "slit burners". Therefore, there are no particular restrictions on the internal structure and the ejection port. Moreover, the flame ejection port can control the ejection width of the flame 5 by dividing the ejection port in the width direction. By using the above-mentioned structure, the ejection width of the flame 5 can be adjusted according to the width of the target steel plate.

[0050] It is effective to arrange only one slot burner in the traveling direction of the steel sheet S, but reduction can be more effectively performed by arranging two or more slot burners in series. There is no limit to the arrangement interval when arranging in series, but when the interval is about 3B to about 10B, interference between the flames 5 and temperature unevenness are less likely to occur.

[0051] In addition, the flame injection port 2 attached to the slit burner can be arranged in a staggered manner on the surface and back of the steel plate in the traveling direction of the steel plate S, that is, it can be offset. By offsetting it, it is possible to prevent the flames 5 exposed from the ends of the steel plate from interfering with each other. Therefore, compared with the case of no offset, a larger range can be heated uniformly. The offset amount is based on the range of about B to about 3B. When the offset amount is too large, the heating temperature may change on the surface and back. In a vertical furnace, since the burners are arranged in the up and down direction, the flame 5 becomes unstable due to the interference of the flame 5 and combustion gas ejected by the burner located on the downstream side (the lower side of the furnace), resulting in a decrease in the temperature uniformity and stability in the width / length direction of the steel plate. In the case of a circular burner, the interference of the flame 5 and the combustion gas can be alleviated by staggered arrangement, but in the slit burner, there is no interruption of the flame 5 in the width direction, so the influence of the interference from the downstream side becomes stronger. Therefore, in the section where the slot burner is provided, a slot-shaped exhaust port 6 may be provided at the lower side of the slot burner for the purpose of releasing the combustion exhaust gas flow from the downstream side. It is preferred to draw the combustion exhaust gas from the exhaust port 6 provided at the lower side of the slot burner. If the equipment length and heating capacity meet the required performance, the exhaust port 6 may be provided in each slot burner provided. In addition, even if Figure 4 The connection parts of each burner group shown in the figure can also achieve sufficient effects. Specifically, the combustion exhaust gas refers to the high-temperature gas produced by the reaction of fuel and air, and refers to the gas composed mainly of carbon dioxide, water vapor and nitrogen contained in the air as reaction products, and trace components such as unreacted residual fuel components, oxygen and reaction intermediate products.

[0052] Regardless of 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 of the burner at the maximum combustion load by the amount of fuel gas actually introduced into the burner. When the burner burns at the maximum combustion load, the combustion rate is 100%. In the present invention, there is no special restriction on the combustion rate of the burner, but when the combustion load of the burner becomes lower, a stable combustion state cannot be obtained, so it is preferably set to above the following threshold. The predetermined threshold value of the combustion rate is the ratio of the amount of fuel gas at the lower limit of the combustion load that can ensure a stable combustion state to the amount of fuel gas at the maximum combustion load. The 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%.

[0053] <Air ratio between oxidation zone and reduction zone>

[0054] The burner groups 11 to 13 of the oxidation zone 1-1 freely select combustion or stop combustion for each burner group. During combustion, it is preferred to set the combustion rate to a predetermined setting value or more, and in order to stably oxidize the surface of the steel plate, it is necessary to operate under the condition that the air ratio of the oxidation zone 1-1 is 1 or more. It is preferred to operate under the condition that the air ratio of the oxidation zone 1-1 is 1.00 or more. It is more preferred to operate under the condition that the air ratio of the oxidation zone 1-1 is 1.05 or more, and it is further preferred to operate under the condition that it is 1.10 or more. In order to prevent the formation of excessive oxide film, the generation of nitrogen oxides, and the blowout of the flame, it is preferred to operate under the condition that the air ratio in the oxidation zone 1-1 is less than 1.50. It is more preferred to operate under the condition that the air ratio of the oxidation zone 1-1 is 1.40 or less, and it is further preferred to operate under the condition that it is 1.30 or less. The air ratio is the value obtained by dividing the amount of air actually introduced into the burner by the amount of air required for complete combustion of the fuel gas.

[0055] In addition, the slit burner of the burner group 14 of the reduction zone 1-2 needs to operate under the condition of an air ratio of less than 1, and it is more preferable to operate under the condition of making the air ratio greater than 0.70 and less than 1.00, and the combustion rate can also be controlled. In the burner group 14 of the reduction zone 1-2, by burning in the range of an air ratio greater than 0.70 and less than 1.00, the Fe oxide generated on the surface of the steel plate can be reduced to generate reduced Fe on the surface. Specifically, when the air ratio is less than 0.70, the fuel unit consumption is deteriorated and the steel plate is contaminated by soot. On the other hand, when the air ratio is greater than 1.00, the oxygen concentration in the combustion gas becomes high, resulting in oxidation of the steel plate. Since reduced Fe exists on the surface of the steel plate, when the steel plate S leaving the direct-fired heating furnace contacts the roller in the RT furnace, the oxide can be prevented from adhering to the roller, and the defects (roller marks) caused by the oxide adhering can be prevented. Therefore, the air ratio is preferably set to be greater than 0.70. The air ratio is more preferably set to be greater than 0.75, and more preferably set to be greater than 0.80. The air ratio is set to be less than 1, preferably 0.95 or less, and more preferably 0.90 or less.

[0056] For each steel sheet S that passes through the plate, the number of burner groups to be fired is determined in consideration of the heating load, the amount of generated oxides, etc., and regarding the burner groups to be fired, by setting the air ratio and the combustion rate to the values ​​within the above range, the plate temperature fluctuation in the running direction of the steel sheet S is reduced for each steel sheet S, and for example, a sufficient amount of Fe oxides required for internal oxidation of Si can be stably generated in the running direction of the steel strip S. The reduction of the plate temperature fluctuation in the running direction of the steel sheet S also contributes to the stabilization of the oxide reduction action in the burner group 14 of the subsequent reduction zone 1-2, and also contributes to the prevention of insufficient reduction of Fe oxides and internal oxidation of Si in the RT furnace, and also contributes to the suppression of oxide adhesion to the rollers of the RT furnace.

[0057] In this embodiment, the burner group 11 to 13 of the oxidation zone 1-1 is an oxidation burner operating under an air ratio of greater than 1.00, and the burner group 14 of the reduction zone 1-2 is a reduction burner operating under an air ratio of less than 1.00. The heating area of ​​the burner group 11 to 13 of the DFF oxidation zone 1-1 is the oxidation section, and the heating area of ​​the burner group 14 of the DFF reduction zone 1-2 is the reduction section.

[0058] When the length of the above-mentioned reduction zone is short, the Fe oxide film remains on the surface layer, and the roll mark prevention effect becomes insufficient. On the other hand, when the length of the reduction zone is long, a surface enrichment layer of Si and the like is formed on the surface layer of the steel sheet during the subsequent reduction annealing, thereby hindering the plating property. Therefore, the length of the reduction zone is preferably set as follows.

[0059] The length of the steel plate S in the traveling direction of the burner group 14 of the reduction zone 1-2 (reduction section length) is preferably 150 mm or more, and when the uniformity in the width direction is also considered, it is more preferably 300 mm or more. It is further preferably 500 mm or more, and most preferably 1000 mm or more. There is no particular upper limit on the length of the reduction section, but when it is too long, the temperature rise ΔTrd in the reduction section becomes larger, so it is necessary to reduce the temperature rise ΔTox in the oxidation section. In this way, an excessively long reduction section is not conducive to ensuring the oxidation amount, so it is preferably less than 10 m. It is more preferably less than 5 m, and further preferably less than 3 m. In addition, this is also advantageous in terms of cost. The length of the steel plate S in the traveling direction of the burner group 14 of the reduction zone 1-2 is the length of the steel plate S heating area of ​​the burner group from the burner located most upstream in the traveling direction of the steel plate S to the burner located most downstream of the burner group 14 of the reduction zone 1-2 ( Figure 1 in the text box).

[0060] In addition, the above oxidation section length is preferably a length that can ensure the necessary internal oxidation amount. However, since the oxidation amount varies depending on the steel type, temperature history, plate passing speed, and steel plate size, it is necessary to set the section length to ensure the necessary oxidation amount even under the least oxidizing conditions among production conditions.

[0061] In the present invention, in the direct-fire heating furnace 1, the steel plate S is oxidized and then reduced. The amount of oxidation formed in the oxidation section needs to be precisely controlled in the travel direction / width direction of the steel plate S. In order to control the amount of oxidation to an appropriate amount for various steel types, temperature histories, plate passing speeds, and sizes of steel plates passing through the plate, it is preferred that the burners arranged opposite to the surface of the steel plate S be divided into at least two groups in the travel direction of the steel plate S, and the combustion rate and air ratio can be independently controlled for each group. When determining the burner group, it is best not to mix the slit burner and the circular burner in one group, but to divide them into different groups and control them separately.

[0062] On the other hand, in the reduction section, even if the burners are controlled as a group, the desired effect of the present invention can be obtained. Therefore, in the present invention, the burners arranged opposite to the steel plate surface of the oxidation zone 1-1 are divided into two or more burner groups in the traveling direction of the steel plate S, in which the combustion rate and the air ratio can be independently controlled.

[0063] The thickness of the Fe-based oxide film formed in the oxidation zone 1-1 (oxidation zone 1-1 burner group 11 to 13) also varies according to the Si content, plate thickness, etc. of the target steel plate, and is preferably set to 100 to 500 nm. That is, when it is less than 100 nm, the function as a barrier layer to prevent Si from diffusing and enriching to the surface may become 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 further preferably 200 nm or more. On the other hand, even if the thickness is greater than 500 nm, the function as a barrier layer is almost unchanged, and it is also accompanied by the disadvantages of a longer heating time of the oxidation zone 1-1 and an increase in the use of fuel, 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 further preferably 400 nm or less.

[0064] In addition, the thickness of the reduced Fe formed in the reduction zone 1-2 (reduction zone 1-2 burner group 14) also varies according to the Si content, plate thickness, etc. of the target steel plate, and is preferably set to 1 to 30 nm. That is, when it is less than 1 nm, the roller mark prevention effect may become 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, even if the thickness is greater than 30 nm, due to excessive reduction of Fe, a surface enrichment layer of Si and the like is formed on the surface of the steel plate during the subsequent reduction annealing, the plating property is hindered. Therefore, the thickness of the reduced Fe is preferably less than 30 nm. The thickness of the reduced Fe is more preferably less than 25 nm, and further preferably less than 20 nm.

[0065] The thickness of the Fe-based oxide film and reduced Fe can be estimated relatively easily by monitoring the plate temperature at the input and output of the direct-fired heating furnace 1 and making corrections using the steel type, plate thickness, line speed, air ratio of the oxidation zone 1-1 / reduction zone 1-2, and combustion rate of the oxidation zone 1-1 / reduction zone 1-2. By mainly adjusting the combustion rate of the oxidation zone 1-1 / reduction zone 1-2 based on this value, stable oxidation and reduction conditions can be determined and ensured, thereby obtaining a steel plate without non-plating defects.

[0066] The steel sheet oxidized / reduced in the direct-fired heating furnace 1 is then reduced and annealed in an RT furnace, cooled, and then immersed in a hot-dip galvanizing bath for hot-dip galvanizing, or further alloyed as needed. Reduction annealing and subsequent steps may be conventional methods. The plating method is not particularly limited, and electrogalvanizing may be performed instead of hot-dip galvanizing.

[0067] In the direct-fired heating furnace 1, after an appropriate amount of Fe-based oxides are formed, their surface layer is reduced to contain reduced Fe, and in the subsequent reduction annealing step, all Fe-based oxides are reduced and Si is internally oxidized, and the oxides can be prevented from adhering to the rollers. Therefore, indentations caused by roller marks, surface layer enrichment of Si, and poor plating caused by insufficient reduction of Fe-based oxides do not occur.

[0068] The hot-dip galvanized steel sheet to be manufactured according to the present invention is effective when containing a large amount of metal elements such as Si that are more easily oxidized than Fe. Specifically, it is suitable for manufacturing a high-Si content hot-dip galvanized steel sheet containing 0.1 to 3.0 mass % Si.

[0069] Example

[0070] In the CGL equipped with a direct-fired heating furnace (DFF) 1, a DFF consisting of four burner groups 11 to 14 is used as a heating burner, and the three burner groups 11 to 13 on the upstream side in the running direction of the steel strip S are arranged in the oxidation zone 1-1, and the last burner group 14 is arranged in the reduction zone 1-2. Furthermore, two cases are divided into two cases: a case where the air-fuel ratio and the combustion rate of each burner group in the oxidation zone 1-1 are controlled separately, and a case where the burner groups 11 to 13 in the oxidation zone are controlled uniformly under the same conditions. Figure 1 An example of a burner configuration is shown in FIG. Figure 1 In the burner groups 11 to 13 in the oxidation zone, flame injection ports 3 attached to the round burners are arranged, and flame injection ports 2 attached to the slit burners are arranged in the burner group 14 in the reduction zone. The burner type was changed for each burner group according to the conditions and the test was conducted. The fuel gas of the burner used was a gas with a composition shown in Table 1. It should be noted that the length of each burner group ( Figure 1The "L" in the figure is set to 3m, and the slit gap B is set to 20mm.

[0071] Table 2 shows the steel component composition of the steel strip S used in the test.

[0072] Other test conditions were set as follows: plate thickness 1.0 mm, plate width 1000 mm, average plate temperature at DFF inlet side 200°C, average temperature at DFF outlet side 650°C, annealing temperature in RT furnace 850°C, plating bath temperature 463°C, Al plating concentration 0.135%, alloying temperature 550°C. Three levels of steel strip S speed (LS) of 60 mpm, 90 mpm, and 120 mpm were studied.

[0073] The evaluation was conducted on the rolling mark defects (roller marks) caused by overoxidation, the appearance of the coating layer, and the quality deviation in the running direction and the width direction. In any test, evaluations A and B were acceptable, and C was unacceptable.

[0074] The rolling mark defect (roll mark) caused by overoxidation was obtained by the same method as described in Patent Document 5. The rolling mark defect (roll mark) was measured by an optical surface defect meter at a depth of 1 m from the surface of the steel sheet randomly selected. 2 The above-mentioned surface defect meter can detect flaws with a diameter of 0.5 mm or more, and judges them as concave defects caused by contact with the roller mark, and is judged as a rolling mark defect here.

[0075] [Patent Document 5] Japanese Patent No. 6607339

[0076] A (good): every 1m 2 0 (no rolling mark defect)

[0077] B (generally good): every 1m 2 1 to 2 (slight rolling mark defects were observed sporadically)

[0078] C(difference): every 1m 2 More than 3 (with rolling mark defects)

[0079] The evaluation of the coating appearance is determined by measuring the deviation of the Fe concentration (an index of the alloying rate) in the coating on the surface of the steel sheet after the alloying treatment from the target value. The smaller the deviation of the Fe concentration in the coating from the target value, the better the coating appearance is judged to be. It should be noted that the Fe concentration is determined by the same method as the method described in the following patent document 6, that is, by a method of calculating the change in the diffraction peak angle of the alloy phase constituting the coating using an X-ray diffraction method.

[0080] [Patent Document 6] Japanese Patent No. 5962615

[0081] A (good): less than ±0.5% (no non-plating and uneven alloying)

[0082] B (generally good): less than ±1% (slight non-plating and / or slight alloying unevenness)

[0083] C (deterioration): ±1% or more (obvious non-plating and / or obvious alloying unevenness)

[0084] Evaluations A and B are qualified, and C is unqualified.

[0085] In addition, regarding the quality in the running direction and the width direction, three points in the running direction of the front end, the center, and the tail end of the steel strip S were selected, and samples of 1000 mm in length were cut in the width direction, and the evaluation results of the rolling marks and the appearance of the coating at the center of each width were evaluated. In addition, regarding the width direction, in the samples of width × 1000 mm cut from the center of the steel strip S, the evaluation was performed as follows based on the evaluation results of the rolling marks and the appearance of the coating at five points, namely, the center in the width direction, the 1 / 4 width and 3 / 4 width positions, and the two ends.

[0086] ◎: The evaluation of rolling marks and galvanizing properties under the same conditions was only A

[0087] ○: The evaluation of rolling mark and galvanizing property under the same conditions is A or B

[0088] △: The evaluation of rolling marks or galvanizing properties under the same conditions is only B

[0089] ×: The evaluation of the rolling mark or galvanizing property under the same conditions includes C

[0090] In the present invention, qualified samples are samples that have no C-rated parts in the rolling mark defect and coating appearance, and are judged as ◎, ○, and △ in the width direction and the running direction. In the judgment, if both the width direction and the running direction are △ or above, it is recorded as qualified (○), and the case containing × is recorded as unqualified (×).

[0091] Conditions No. 1 to 7 were produced under the condition that the conveying speed of the steel strip S was 60 mpm.

[0092] 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 is unstable in the width direction and the running direction, and rolling marks caused by overoxidation are sporadically observed, which is unqualified. Condition 2 is an example of the present invention in which a slit burner is used instead of a circular burner in the reduction zone burner group 14. Since the burner flame is uniform in the width direction and reduced Fe is uniformly obtained in the width direction and the running direction, a stable quality steel strip S without rolling marks caused by overoxidation can be obtained. In Condition 3, a slit burner is used, but the air ratio in the oxidation zone is 0.90, and the oxidizing power is insufficient, so the coating appearance is poor. Condition 4 is a case where the air ratio in the oxidation zone is excessive relative to Condition 3. According to Condition 3, the surface quality is stable and is within the acceptable range. Condition 5 uses a slit burner in the same way as Conditions 2 and 3, but the air ratio in the reduction zone is as high as 1.00, so rolling marks are sporadically observed. Condition 6 is an example in which the air ratio in the reduction zone is low relative to Condition 5. Compared with Condition 5, surface defects are reduced and are within the acceptable range. Condition 7 is an example of adjusting the air ratio in the reduction zone and further reducing the combustion rate in the reduction zone. Compared with Condition 5, the surface defects are reduced and are within the acceptable range.

[0093] Conditions 8 and 9 are examples in which the conveying speed of the steel strip S is 90 mpm. In condition 8, a round burner is used in the reduction zone burner group 14, and the quality is poor and unqualified as in condition 1. On the other hand, condition 9 is an example in which a slot burner is used. Therefore, the surface quality is improved compared with condition 8, and it is qualified.

[0094] Conditions 10, 11, and 12 are examples in which the conveying speed of the steel strip S is 120 mpm. Condition 10 uses a round burner as in Conditions 1 and 8, so the surface quality is unqualified. Condition 11 uses a slot burner, and similar to Condition 9, the surface quality is improved and stabilized, and is qualified.

[0095] Condition 12 is an example of applying slot burners to both the oxidation zone and the reduction zone. This results in high and stable surface quality.

[0096] Condition 13 is an example of applying a slit burner in the heating zone of a horizontal furnace. Among the four burner groups constituting the heating zone, the most downstream burner group 14 is used as a reduction zone with an air ratio of less than 1. Since it is a horizontal furnace, although the production efficiency is lower than that of other examples, the defects are maintained at a low level and are within the acceptable range. However, since it is a horizontal furnace, the atmosphere separation between the heating zone and the soaking zone is insufficient, and the uniformity in the width direction and the travel direction is reduced.

[0097] Condition 14 is an example in which an exhaust port is provided upstream of the burner group using the slot burner, in contrast to condition 2. By providing the exhaust port, interference between flames is prevented, uniformity in the width direction and the traveling direction is further improved, and surface quality is good and stable.

[0098] The above results confirm that the surface quality can be improved by introducing a slit burner in the reduction zone, and that even better surface quality can be obtained by optimizing the control method and combustion conditions.

[0099] [Table 1]

[0100]

[0101] [Table 2]

[0102]

[0103]

[0104] Explanation of symbols

[0105] 1Direct Fire Furnace (DFF)

[0106] 1-1 Oxidation Zone

[0107] 1-2 Restore Area

[0108] 2 Flame jet port attached to the slot burner

[0109] 3. Flame jets attached to the round burner

[0110] 4 Radiation Thermometer

[0111] 5. Flame

[0112] 6 Exhaust ports

[0113] S steel plate (steel strip)

[0114] L is the length of the steel strip heating area of ​​the burner group 14 from the burner located most upstream in the running direction of the steel strip to the burner located most downstream.

[0115] 11 Burner group in oxidation zone

[0116] 12 Burner group in oxidation zone

[0117] 13 Burner group in oxidation zone

[0118] 14 Burner group in reduction zone

[0119] B Slit gap

[0120] 20 Preheating Zone

[0121] 21 heating zones (direct fire heating)

[0122] 22. Heat balancing zone

[0123] 23Cooling Zone

[0124] 24 Cooling Zones

[0125] 25 Plating bath (zinc pot)

[0126] 26 Alloying zone

[0127] 27 Cooling Zone

Claims

1. A method for heating a steel plate, wherein: The surface side and back side of a steel plate passing through a direct-fired heating furnace having an oxidation zone operating at an air ratio of 1 or more and a reduction zone operating at an air ratio of less than 1 are heated at least during the period of passing through the reduction zone by flames ejected from one or more slit burners extending in the width direction of the steel plate.

2. The method for heating a steel plate according to claim 1, wherein: The direct-fired heating furnace conveys the steel plate in an up-and-down direction and sucks combustion exhaust gas from an exhaust port provided at a lower side of the slot burner.

3. The method for heating a steel plate according to claim 1 or 2, wherein: The air ratio of the oxidation zone is controlled to be greater than 1.00 and less than 1.50, and the air ratio of the reduction zone is controlled to be greater than 0.70 and less than 1.

00.

4. A method for manufacturing a plated steel sheet, wherein: A cold-rolled steel sheet is subjected to a heating treatment by the heating method according to any one of claims 1 to 3, and the cold-rolled steel sheet is further subjected to a plating treatment.

5. The method for producing a plated steel sheet according to claim 4, wherein: The plating treatment may be any one of electrogalvanizing, hot-dip galvanizing, and alloy hot-dip galvanizing.

6. A direct-fired heating furnace comprising an oxidation zone operating at an air ratio of 1 or more, a reduction zone operating at an air ratio of less than 1, and a control device capable of controlling the air ratio between the oxidation zone and the reduction zone, At least in a portion of the reduction zone, At least one slot burner extending in the width direction of the steel plate and spraying flames toward the steel plate passing through the oxidation zone and the reduction zone is provided on each of the front side and the back side of the steel plate.

7. The direct-fired heating furnace according to claim 6, wherein: The steel plate is conveyed in the up-down direction, and combustion exhaust gas is sucked from an exhaust port provided at a lower side of the slot burner.

8. The direct-fired heating furnace according to claim 6 or 7, wherein: The air ratio of the oxidation zone is controlled to be greater than 1.00 and less than 1.50, and the air ratio of the reduction zone is controlled to be greater than 0.70 and less than 1.

00.

9. A continuous hot-dip galvanizing facility comprising the direct-fired heating furnace according to any one of claims 6 to 8.

10. The continuous hot-dip galvanizing equipment according to claim 9, wherein: We also have alloying equipment for alloying the hot-dip galvanized layer.

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