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 plating failure. 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; and at least during the period of passing through the oxidation zone, heating is performed by using flames ejected from more than one slit type combustor.

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 cost of addition 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] Conventionally, as a countermeasure for this, Patent Document 1 proposes a pre-plating method in which a steel sheet (original sheet) before plating is subjected to Fe-based 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 in advance to form an Fe-based oxide film on the surface, and then annealing / plating is performed in a reduction furnace.

[0005] However, when the former pre-plating method is adopted, it is necessary to install a plating facility closer to the inlet side of the annealing furnace in the continuous hot-dip galvanizing facility, which is difficult to implement in consideration of space and facility investment cost.

[0006] The latter redox method can be applied by adjusting the combustion atmosphere in existing non-oxidation furnace (NOF) or direct-fired furnace (DFF) hot-dip coating lines.

[0007] However, in the case of a burner having a circular burner nozzle outlet shape as described in Patent Documents 4 and 5, even if the burners are dispersed in a staggered manner, the thickness of the Fe-based oxide film during oxidation to ensure good coating properties cannot be uniformly controlled, resulting in non-coating defects.

[0008] On the other hand, Patent Document 6 proposes a method of using a slit burner whose shape of the burner nozzle outlet is parallel to the steel plate width direction in a horizontal furnace for uniformity in the plate width direction. However, even if a slit burner is provided in the oxidation furnace behind the non-oxidation furnace, the oxidation furnace atmosphere will flow into the non-oxidation furnace in the horizontal furnace, causing uneven plate temperature and uneven Fe-based oxide film, and the slit burner cannot achieve the effect of making the flame uniform in the width direction.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application Publication No. 2008-144264

[0012] Patent Document 2: Japanese Patent Application Laid-Open No. 4-202630

[0013] Patent Document 3: Japanese Patent Application Laid-Open No. 7-34210

[0014] Patent Document 4: Japanese Patent Application Laid-Open No. 62-29820

[0015] Patent Document 5: Japanese Patent Application Laid-Open No. 9-59753

[0016] Patent Document 6: Japanese Patent No. 3889019 Summary of the invention

[0017] Problems to be solved by the invention

[0018] However, even if the atmosphere in the furnace is adjusted during combustion in the conventional pre-plating method, the thickness of the oxide film is not uniform under the conventional burner nozzle shape and configuration. In addition, even if a slit burner is applied, when it is used in combination with a non-oxidizing furnace, an oxidizing furnace, a reducing furnace, and a subsequent horizontal furnace, the Fe-based oxide film is not uniformly formed, and even if a slit burner is used in an oxidizing furnace, the unevenness cannot be eliminated.

[0019] The present invention has been accomplished in view of the above-mentioned problems, and an object of the present invention is to manufacture a galvanized steel sheet of stable quality with no uncoated parts by a relatively easy method suitable for practical use.

[0020] Methods used to solve problems

[0021] The gist of the present invention made in order to solve the above-mentioned problems lies in the following configurations.

[0022] [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 slot burners at least during the period of passing through the oxidation zone.

[0023] [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.

[0024] [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.

[0025] [4] The method for heating a steel plate according to any one of [1] to [3], wherein the steel plate is heated by using at least one of the slot burners provided within a range where the temperature of the steel plate passing through the oxidation zone reaches 400° C. or higher.

[0026] [5] 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], and further subjecting the cold-rolled steel sheet to a plating treatment.

[0027] [6] The method for producing a plated steel sheet according to [5], wherein the plating treatment uses any one of electrogalvanizing, hot-dip galvanizing, and alloyed hot-dip galvannealing.

[0028] [7] 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 in a portion of the oxidation zone, one or more slit burners 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 are provided on the surface side and the back side of the steel plate respectively.

[0029] [8] The direct-fired heating furnace according to [7], 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.

[0030] [9] The direct-fired heating furnace according to [7] or [8], 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.

[0031]

[10] The direct-fired heating furnace according to any one of [7] to [9], wherein at least one of the slot burners is provided within a range where the temperature of the steel plate passing through the oxidation zone reaches 400° C. or higher.

[0032]

[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 slot burners capable of independently controlling the air ratio and the combustion rate.

[0033]

[12] A continuous hot-dip galvanizing facility comprising the direct-fired heating furnace described in any one of [7] to [9].

[0034]

[13] The continuous hot-dip galvanizing equipment according to

[12] , further comprising an alloying equipment for alloying the hot-dip galvanized layer.

[0035] Effects of the Invention

[0036] According to the present invention, an excellent galvanized steel sheet having a beautiful surface appearance without any ungalvanized surface 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 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

[0037] 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.

[0038] Figure 2 This is a schematic diagram of a continuous hot-dip galvanizing facility as one embodiment of the present invention.

[0039] 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.

[0040] Figure 4 This is a diagram showing an example of the structure of the direct-fired heating furnace of the present invention. DETAILED DESCRIPTION

[0041] 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, it is necessary to control the atmosphere of the direct-fire burner to be oxidizing to ensure an appropriate oxide film (Fe-based oxide) on the surface of the steel plate. After ensuring an appropriate amount of Fe-based oxide, the Si is internally oxidized by performing reduction annealing, which can improve the coating property of the high-Si steel.

[0042] However, in the case of burners with a circular burner nozzle outlet shape used in the past, even if the burners are dispersed in a staggered manner, the thickness of the Fe-based oxide film for ensuring good coating properties cannot be uniformly controlled in the travel direction / width direction of the steel plate, resulting in non-coating defects.

[0043] Therefore, in the present invention, a method of uniformly controlling the thickness of the Fe-based oxide film in the traveling direction and the width direction by using a slit burner instead of a circular burner was considered.

[0044] 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.

[0045] 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.

[0046] 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 11 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 round 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 moving direction of the steel strip S 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.

[0047] <Direct-fire heating furnace installed in continuous hot-dip galvanizing equipment>

[0048] 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.

[0049] The direct-fired heating furnace 1 of the present invention is envisioned as a heating furnace introduced into the heating zone 21 even in a continuous hot-dip galvanizing facility.

[0050] In this embodiment, more than two slit burners can also be controlled independently. The direct-fire 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 (sections) 11 to 13 in the direction of travel of the steel plate. In this example, a circular burner is set in the burner group 11 in the oxidation zone, and its flame injection port is represented by 3 in the figure. Slit burners are set in the (oxidation zone) burner groups 12 and 13, and their flame injection ports are represented by symbol 2 in the figure. The reduction zone is only provided with a burner group (burner group in the reduction zone) 14, which is a section in which a circular burner is set. The flame injection port of the circular burner is symbol 3 in the figure. Regarding the burner groups 11, 12, and 13 in the oxidation zone 1-1, the combustion rate and air ratio of the burner can be independently controlled for each burner group. The burner groups 11 to 13 in the oxidation zone burn under the condition that the combustion rate is a combustion rate above a predetermined threshold.

[0051] 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 sections and control each section as a group.

[0052] In addition, for example, Figure 4 As shown in the apparatus, not only the oxidation zone is provided with slot burners, but both the oxidation zone 1-1 and the reduction zone 1-2 are provided with slot burners.

[0053] 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 oxidation zone 1-1. 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 onto 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 depending on the flame injection structure of the slit burner and the width of the steel plate, there are cases where division is not required. On the other hand, the circular burner is dispersedly arranged opposite to the steel plate surface.

[0054] <Slot burner>

[0055] 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.

[0056] A slit burner is a slit burner having a rectangular burner flame jet 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 that eject slit-shaped flames such as flame jet ports in the shape of an elongated rectangle (slit) are collectively referred to as "slit burners". Therefore, there are no particular restrictions on the internal structure and the jet port. Moreover, the flame jet port can control the jet width of the flame 5 by dividing the jet port in the width direction. By using the above-mentioned structure, the jet width of the flame 5 can be adjusted according to the width of the target steel plate.

[0057] It is effective to arrange only one slot burner in the direction of the steel plate travel, but oxidation can be more effectively performed by arranging two or more in series. There is no limit to the arrangement interval when arranging in series, but when the interval is about 3B to about 10B, it is less likely to cause interference between the flames 5 and temperature unevenness.

[0058] In addition, since most of the Fe-based oxide film is generated in the range where the plate temperature reaches 400°C or more, at least one slot burner can be applied in the oxidation zone 1-1, especially where the plate temperature reaches the above range. In addition, it is further preferred to apply the slot burner in the range of 450°C or more. On the other hand, since the oxidation amount increases sharply at high temperatures such as higher than 650°C, it is preferred to apply the slot burner at a plate temperature of 650°C or less. It is further preferred to be below 600°C, and most preferably below 550°C. In order to use the slot burner for heating when the preferred temperature range is reached, as a method for predicting and grasping the plate temperature of the steel plate S during the DFF1 process, the plate temperature can be estimated in advance based on the steel type, plate thickness, plate width, line speed, air ratio, combustion rate, etc. In addition, radiation thermometers can be set at several locations in the plate passing direction of the oxidation zone 1-1 to measure the plate temperature.

[0059] According to the above situation, a slit burner can be applied to the downstream side of the plate passing direction of the oxidation zone 1-1 where the plate temperature becomes high. Although the slit burner can also be applied to all the burners in the oxidation zone 1-1, the slit gap B at the outlet of the slit burner is narrower than that of the circular burner. When the slit is blocked by foreign objects such as fragments of the burner brick or the slit is deformed due to the high temperature of the flame 5, regular maintenance is required. Therefore, the conventional circular burner can be arranged on the upstream side of the oxidation zone 1-1 where the plate temperature is low, and the slit burner can be arranged 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 collides with the steel plate vertically from the viewpoint of heating efficiency.

[0060] In addition, the arrangement of the flame injection ports 2 attached to the slit burner can be staggered on the surface and back of the steel plate S in the direction of travel 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 S from interfering with each other. Therefore, compared with the case of no offset, the steel plate S can be heated more 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 becomes unstable due to the interference of the flame 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 and combustion gas can be alleviated by staggered arrangement, but there is no interruption of the flame in the width direction in the slit burner, so interference from the downstream side cannot be avoided. Therefore, in the present invention, it is preferable to provide a slit-shaped exhaust port in the section where the slit burner is provided. By providing the slit-shaped exhaust port, interference between the flame and the combustion gas can be easily alleviated.

[0061] The exhaust port is preferably provided at least in one group on the inside and outside of the connecting part of each section. The exhaust port is preferably provided at the lower side of the slot burner, and the combustion exhaust gas is sucked from the exhaust port. Specifically, the combustion exhaust gas refers to the high-temperature gas generated by the reaction of fuel and air, and refers to the gas mainly composed 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.

[0062] If the equipment length and heating capacity meet the required performance, an exhaust port can also be set in the middle of each slot burner constituting each section.

[0063] 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%.

[0064] <Air ratio between oxidation zone and reduction zone>

[0065] The burner groups 11 to 13 of the oxidation zone 1-1 can 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 steel plate surface, 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 further preferred to operate under the condition that the air ratio of the oxidation zone 1-1 is 1.05 or more, and it is most preferred to operate under the condition that the air ratio of the oxidation zone 1-1 is 1.10 or more.

[0066] In order to prevent the formation of excessive oxide film, the generation of nitrogen oxides, and the blowout of flames, 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 further preferred to operate under the condition that the air ratio in the oxidation zone 1-1 is 1.40 or less, and it is most preferred to operate under the condition that it is 1.30 or less. The air ratio is a value obtained by dividing the amount of air actually introduced into the burner by the amount of air required for complete combustion of the fuel gas.

[0067] In addition, the circular burner of the burner group 14 of the reduction zone 1-2 needs to make the air ratio less than 1, and it is further preferred to make the air ratio greater than 0.70 and less than 1.00 to operate, 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, so the air ratio is preferably set to 0.70 or more. It is further preferred that the air ratio is greater than 0.75, and most preferably greater than 0.80. 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 1 contacts the roller in the RT furnace (annealing furnace), the oxide can be prevented from adhering to the roller, and the defects (roller marks) caused by the oxide adhesion can be prevented. Therefore, the air ratio is preferably less than 1.00. The air ratio is more preferably 0.95 or less, and most preferably 0.90 or less.

[0068] For various steel sheets S that pass through the plate, the number of burner groups for combustion is determined in consideration of the heating load, the amount of oxidation formed, etc. With respect to the burner groups for combustion, by setting the air ratio and the combustion rate to values ​​within the above range, the plate temperature fluctuation in the running direction of the steel sheet S is reduced for various steel sheets S. As a result, 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 sheet S. The reduction in 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 subsequent burner group 14 of the reduction zone 1-2. In addition, the reduction in the plate temperature fluctuation also contributes to preventing insufficient reduction of Fe oxides and internal oxidation of Si in the RT furnace, and also contributes to suppressing the adhesion of oxides to the rollers of the RT furnace.

[0069] In this embodiment, the burner groups 11 to 13 of the oxidation zone 1-1 are oxidation burners, and the burner group 14 of the reduction zone 1-2 is a reduction burner. The heating area of ​​the burner groups 11 to 13 of the oxidation zone 1-1 is the oxidation section, and the heating area of ​​the burner group 14 of the reduction zone 1-2 is the reduction section.

[0070] When the reducing atmosphere is short, the Fe oxide film remains on the surface, and the roll mark prevention effect becomes insufficient. On the other hand, when the reducing atmosphere is long, a surface enriched layer of Si and the like is formed on the surface of the steel sheet during the subsequent reduction annealing, thereby hindering the platability.

[0071] <Length of the restoration area>

[0072] 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. Therefore, 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 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 flame jet port 3 attached to the circular burner located most upstream in the traveling direction of the steel plate to the flame jet port 3 attached to the circular burner located most downstream of the burner group 14 of the reduction zone 1-2 ( Figure 1 It should be noted that even when a slot burner is used in the burner group 14 of the reduction zone 1-2, it is preferred that the reduction zone length be set to the above-mentioned length.

[0073] <Length of oxidation zone>

[0074] The length of the burner groups 11 to 13 of the oxidation zone 1-1 in the direction of travel of the steel plate (oxidation section length) should be a length that can ensure the necessary internal oxidation amount. However, since the oxidation amount varies depending on the type of steel passed through the plate, the temperature history, the plate passing speed, and the size of the steel plate, it is necessary to ensure a section length that can ensure the necessary oxidation amount even under the most difficult oxidation conditions among production conditions.

[0075] 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, the burners arranged relative to the surface of the steel plate S need to be divided into at least two groups in the travel direction of the steel plate, 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.

[0076] Regarding the reduction section, even if the burners are controlled as a group, the desired effects of the present invention can be obtained. Therefore, in the present invention, the burners arranged opposite to the surface of the steel sheet S in the oxidation zone 1-1 are divided into two or more burner groups in the traveling direction of the steel sheet S, in which the combustion rate and the air ratio can be independently controlled.

[0077] The thickness of the Fe-based oxide film formed in the oxidation zone 1-1 also varies according to the Si content, plate thickness, etc. of the steel plate S as the target, and is preferably 100 to 500 nm. 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 fuel used. Therefore, the thickness of the Fe-based oxide film is preferably 500 nm or less. The thickness of the Fe-based oxide film is more preferably 450 nm or less, and further preferably 400 nm or less.

[0078] The thickness of the Fe-based oxide film 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, and combustion rate of the oxidation zone 1-1. By mainly adjusting the combustion rate of the oxidation zone 1-1 based on this value, stable oxidation conditions can be determined and ensured, thereby obtaining a steel plate S without non-plating defects.

[0079] The steel sheet S 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 processes may be conventional methods. The plating method is not particularly limited, and electrogalvanizing may be performed instead of hot-dip galvanizing.

[0080] In the direct-fired heating furnace 1, after an appropriate amount of Fe-based oxide is formed, its surface layer is reduced and reduced Fe exists. Therefore, in the subsequent reduction annealing step, the Fe-based oxide is reduced and Si is internally oxidized, and the oxide can be prevented from adhering to the roller. Therefore, indentations caused by roller marks, surface enrichment of Si, and poor plating caused by insufficient reduction of the Fe-based oxide do not occur.

[0081] 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, the effect is particularly significant in the manufacture of high-Si content hot-dip galvanized steel sheets containing 0.1 to 3.0 mass % of Si.

[0082] 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 1. The annealing 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 1.

[0083] Example

[0084] In a CGL equipped with a direct-fired heating furnace (DFF) 1, a heating burner DFF1 consisting of four burner groups (11 to 14) was used, and the three burner groups (11 to 13) on the upstream side in the running direction of the steel strip S were set as the oxidation zone 1-1, and the final burner group (14) was set as the reduction zone 1-2. Furthermore, the experiment was conducted in two cases: a case where the air ratio and combustion rate of each burner group in the oxidation zone 1-1 were controlled separately (A) and a case where the burner groups 11 to 13 in the oxidation zone were uniformly controlled under the same conditions (B). It should be noted that the air ratio and combustion rate in the reduction zone were controlled separately from those in the oxidation zone. Figure 1 An example of a burner configuration is shown in FIG. Figure 1 In the burner group 11 of the oxidation zone and the burner group 14 of the reduction zone, flame injection ports 3 attached to the round burner are arranged, and flame injection ports 2 attached to the slit burner are arranged in the burner group 12 of the oxidation zone and the burner group 13 of the oxidation 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 1 The "L" in the figure is set to 3m, and the slit gap B is set to 20mm.

[0085] Table 2 shows the steel component composition of the steel strip S used in the test.

[0086] Other test conditions were set as follows: plate thickness 1.0 mm, plate width 1000 mm, average plate temperature at DFF1 inlet side 200°C, average temperature at DFF1 outlet side 650°C, annealing temperature (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. The burner was used with a combustion rate of 30% or more.

[0087] The evaluation included rolling mark defects (roller marks) caused by overoxidation, coating appearance, quality deviation in the running direction / width direction of the steel sheet, and temperature deviation in the running direction of the steel sheet. Evaluations A and B were acceptable, and C was unacceptable.

[0088] In the present invention, the same method as described in Patent Document 7 is used to determine the rolling mark defect (roller mark) caused by overoxidation. The rolling mark defect (roller mark) of the steel sheet surface of the steel strip S randomly selected is inspected by an optical surface defect meter. 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.

[0089] [Patent Document 7] Japanese Patent No. 6607339

[0090] A (good): every 1m 2 0 (no rolling mark defect)

[0091] B (generally good): every 1m 2 1 to 2 (slight rolling mark defects were observed sporadically)

[0092] C(difference): every 1m 2 More than 3 (with rolling mark defects)

[0093] The evaluation of the coating appearance is determined by measuring the deviation of the Fe concentration (alloying ratio) 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. It should be noted that the Fe concentration is determined by the same method as the method described in the following patent document 8, that is, by calculating the change in the diffraction peak angle of the alloy phase constituting the coating using an X-ray diffraction method.

[0094] [Patent Document 8] Japanese Patent No. 5962615

[0095] A (good): less than ±0.5% (no non-plating and uneven alloying)

[0096] B (generally good): less than ±1% (slight non-plating and / or slight alloying unevenness)

[0097] C (poor): ±1% or more (obvious non-plating and / or obvious alloying unevenness)

[0098] Evaluations A and B are qualified, and C is unqualified.

[0099] 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.

[0100] ◎: The evaluation of rolling marks and galvanizing properties under the same conditions was only A

[0101] ○: The evaluation of rolling mark and galvanizing property under the same conditions is A or B

[0102] △: The evaluation of rolling marks or galvanizing properties under the same conditions is only B

[0103] ×: The evaluation of the rolling mark or galvanizing property under the same conditions includes C

[0104] 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 (×).

[0105] Conditions Nos. 1 to 8 and 15 were produced under the condition that the conveying speed of the steel strip S was 60 mpm.

[0106] Condition 1 is a comparative example using only a circular burner. When the combustion rate is less than 30% while using a circular burner, the combustion state of the burner is unstable. In addition, the burner groups 11 to 13 that uniformly control the oxidation zone have large quality deviations in the width direction and the travel direction.

[0107] Condition 2 is a case where slit burners are used in burner groups 11 to 13, compared to the circular burners in condition 1. The combustion state of the burners is unstable as in condition 1, but due to the use of slit burners, the rolling mark defects and the coating appearance have become good, and the quality deviations in the width direction and the running direction have also been slightly improved.

[0108] Compared with conditions 1 and 2, in which the burner groups 11 to 13 are collectively controlled, condition 3 is a case where the air ratio and combustion rate can be controlled for each burner group. As a result, only necessary burners (in this case, only the burner group 13) can be operated. However, in condition 3, since the burner group 13 in operation is a circular burner, it is easy to cause uneven combustion, and there is a tendency for the surface quality to be worse than condition 1.

[0109] Condition 4 is an example in which each burner group is controlled in the same manner as Condition 3, but the shape of the burner is changed from a circular shape to a slot burner. This improves the surface quality and makes the quality in the width direction / travel direction more uniform.

[0110] Condition 5 is an example in which a slot burner is used under the same control as Condition 4, but the air ratio of the burner group 13 is reduced to 0.90. In this example, although uniformity in the width direction and the running direction is obtained compared with Condition 3, many defects are found and the sample is rejected.

[0111] Condition 6 is an example in which the air ratio of the burner group 13 is too large, 1.65, compared with Condition 5. The degree of the defect is reduced and is within the acceptable range.

[0112] Condition 7 is an example in which the air ratio of the reduction zone of the burner group 14 is as high as 1.00 compared to Condition 4. In this case, as in Condition 5, the uniformity is relatively good, but many defects occur and the result is unacceptable.

[0113] Condition 8 is an example in which the air ratio in the reduction zone is lower than that in Condition 7. Compared with Condition 7, the degree of defects is reduced and is within the acceptable range.

[0114] Conditions 9 and 10 are examples produced under the condition that the conveying speed of the steel strip S is 90 mpm, and both conditions control the air ratio and the combustion rate for each burner group.

[0115] In condition 9, a slot burner and a round burner were used in combination in the oxidation zone. However, since the slot burner was not used in the range where the steel plate temperature reached 400°C, although the surface quality was within the acceptable range, poor areas were generated.

[0116] In condition 10, the burner used in the oxidation zone is composed of only a slot burner, and the surface quality is better than that of condition 9.

[0117] Conditions 11 to 13 are examples of production under the condition that the conveying speed of the steel strip S is 120 mpm. Under any of the conditions, a slot burner is introduced into the oxidation zone.

[0118] In condition 11, control is not performed for each burner group but is performed collectively, and circular burners are used in some cases. However, similarly to condition 2, the surface quality is within the acceptable range due to the introduction of slit burners.

[0119] Condition 12 is an invention example in which the surface quality is better than that of Condition 11 because control is performed for each burner group.

[0120] Condition 13 is an example in which a slot burner is used in the reduction zone in addition to the oxidation zone. This further improves the surface quality.

[0121] Condition 14 is an example of introducing a slit burner into a horizontal annealing furnace and using it as an oxidation zone. Since it is a horizontal furnace, the conveying speed is reduced compared to the invention example, and the production is carried out at 30mpm. The slit burner is applied in the oxidation zone, and the overoxidation defects and the coating appearance are within the qualified range. However, since there is no mechanism that can convey the steel plate in the up and down direction, and there is no exhaust port, the exhaust gas partially flows into the upstream non-oxidation furnace, resulting in slight non-plating and alloying unevenness in the width direction and the length direction.

[0122] Condition 15 is an example of operating under the same conditions as Condition 2, but closing the exhaust ports provided between the sections (equivalent to a state without exhaust ports) and not sucking the combustion gas. As a result, the stability is reduced due to the interference between the flames, and the number of defects increases compared to Condition 2, but the surface quality is within the acceptable range.

[0123] The above examples confirm that the surface quality can be improved by introducing a slot burner in the oxidation zone, and that even better surface quality can be obtained by optimizing the control method and combustion conditions.

[0124] [Table 1]

[0125]

[0126] [Table 2]

[0127]

[0128]

[0129] Explanation of symbols

[0130] 1Direct Fire Furnace (DFF)

[0131] 1-1 Oxidation Zone

[0132] 1-2 Restore Area

[0133] 2 Flame jet port attached to the slot burner

[0134] 3. Flame jets attached to the round burner

[0135] 4 Radiation Thermometer

[0136] 5. Flame

[0137] 6 Exhaust ports

[0138] S Steel Plate

[0139] L is the length of the steel plate heating area of ​​the burner group 14 from the burner located most upstream in the steel strip running direction to the burner located most downstream.

[0140] 11 Burner group in oxidation zone

[0141] 12 Burner group in oxidation zone

[0142] 13 Burner group in oxidation zone

[0143] 14 Burner group in reduction zone

[0144] B Slit gap

[0145] 20 Preheating Zone

[0146] 21 heating zones (direct fire heating)

[0147] 22. Heat balancing zone

[0148] 23Cooling Zone

[0149] 24 Cooling Zones

[0150] 25 Plating bath (zinc pot)

[0151] 26 Alloying zone

[0152] 27 Cooling Zone

Claims

1. A method for heating a steel plate, wherein: The surface and back sides 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 by flames ejected from one or more slot burners at least during the period of passing through the oxidation zone.

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. The method for heating a steel plate according to any one of claims 1 to 3, wherein: The steel plate is heated by using at least one of the slot burners provided within a range where the temperature of the steel plate passing through the oxidation zone reaches 400° C. or higher.

5. 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.

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

7. 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 oxidation 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.

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

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

10. The direct-fired heating furnace according to any one of claims 7 to 9, wherein: At least one of the slot burners is provided within a range where the temperature of the steel plate passing through the oxidation zone reaches 400° C. or higher.

11. The direct-fired heating furnace according to any one of claims 7 to 9, wherein: The oxidation zone includes a burner group having two or more slot burners capable of independently controlling the air ratio and the combustion rate.

12. A continuous hot-dip galvanizing facility comprising the direct-fired heating furnace according to any one of claims 7 to 9.

13. The continuous hot-dip galvanizing equipment according to claim 12, wherein: We also have alloying equipment for alloying the hot-dip galvanized layer.

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