Continuous annealing equipment for metal strips, and continuous annealing method for metal strips

Abstract

The present invention provides a continuous annealing apparatus for metal strips and a continuous annealing method for metal strips that can produce metal strips with excellent chemical treatment properties and corrosion resistance without reducing the manufacturing efficiency of the metal strips. [Solution] A continuous annealing apparatus for a metal strip, in which a direct-fired heating zone, a soaking zone, and a pickling facility are arranged in this order in the direction of transport of the metal strip, and having a control device that determines the heating conditions of the metal strip in the direct-fired heating zone and the soaking zone, and the pickling conditions of the metal strip in the pickling facility, the control device having a heating condition determination unit that determines the thickness of the reduced iron layer to be formed on the metal strip based on the material information and quality information of the metal strip and determines the heating conditions in the direct-fired heating zone and the soaking zone based on the thickness of the reduced iron layer, and a pickling condition determination unit that determines the pickling conditions in the pickling facility based on the thickness of the reduced iron layer formed on the metal strip, the material information and quality information of the metal strip.

Description

Technical Field

[0001] The present invention relates to a continuous annealing facility for a metal strip and a method for continuously annealing a metal strip.

Background Art

[0002] As a method for increasing the tensile strength of metal strips such as steel strips and steel plates, a solid solution strengthening method in which solid solution strengthening elements such as Si and Mn are added to the metal strip raw material is known. Si and Mn are more easily oxidized than Fe. Therefore, when annealing a metal strip, if the metal strip is heated in an Fe-reducing atmosphere, Si and Mn concentrate on the surface of the metal strip to form oxides. It is known that the chemical conversion treatment property of the metal strip deteriorates when oxides of Si and Mn are present on the surface of the metal strip.

[0003] Therefore, methods for improving the chemical conversion treatment property of the high-tensile metal strip have been conventionally studied. Patent Document 1 discloses a technique in which pickling is continuously performed twice on a continuously annealed steel strip to remove oxides of Si and Mn present on the surface of the steel strip. In the first pickling, the steel strip is continuously immersed in a mixed solution containing an oxidizing first acid and a non-oxidizing second acid. In the second pickling, the steel strip is continuously immersed in an acid solution containing a non-oxidizing third acid.

[0004] Patent Document 2 discloses forming an iron oxide layer on the surface of a high-Si cold-rolled steel sheet in a heating furnace to suppress the concentration of Si and Mn on the surface. Further, the iron oxide layer is reduced in a soaking furnace to form a reduced iron layer, and the oxides of Si and Mn inevitably formed on the surface are removed by electrolytic pickling of the reduced iron layer.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

[0006] In the technology disclosed in Patent Document 1, pickling is performed twice, which can lead to a longer time required to remove Si and Mn oxides and a decrease in manufacturing efficiency.

[0007] The technology disclosed in Patent Document 2 involves only one electrolytic pickling process for high-Si cold-rolled steel sheets, resulting in higher manufacturing efficiency compared to the technology disclosed in Patent Document 1. However, depending on the manufacturing conditions of the high-Si cold-rolled steel sheet, it may be difficult to achieve both chemical conversion treatment properties and corrosion resistance. Specifically, if a thin iron oxide layer is formed, the subsequently formed reduced iron layer will also be thin. Furthermore, depending on the electrolytic pickling conditions, even a portion of the reduced iron layer may dissolve and disappear, potentially exposing Si and Mn oxides on the surface of the high-Si cold-rolled steel sheet. In these areas where Si and Mn oxides are exposed, the chemical conversion treatment properties deteriorate. Conversely, if a thick iron oxide layer is formed on the surface of the high-Si cold-rolled steel sheet to retain the reduced iron layer after electrolytic pickling, oxygen may penetrate into the interior of the high-Si cold-rolled steel sheet, causing Si and Mn oxides to form at the grain boundaries, thus degrading the corrosion resistance of the high-Si cold-rolled steel sheet.

[0008] The present invention was made to solve the above problems, and aims to provide a continuous annealing apparatus for metal strips and a continuous annealing method for metal strips that can produce metal strips with excellent chemical treatment properties and corrosion resistance without reducing the manufacturing efficiency of the metal strips. [Means for solving the problem]

[0009] The means to solve the above problems are as follows: [1] A continuous annealing apparatus for a metal strip, wherein a direct-fired heating zone, a soaking zone, and a pickling facility are arranged in this order in the direction of transport of the metal strip, and the apparatus has a control device for determining the heating conditions of the metal strip in the direct-fired heating zone and the soaking zone, and the pickling conditions of the metal strip in the pickling facility, wherein the control device has a heating condition determination unit that determines the thickness of the reduced iron layer to be formed on the metal strip based on material information and quality information of the metal strip, and determines the heating conditions in the direct-fired heating zone and the soaking zone based on the determined thickness of the reduced iron layer, and a pickling condition determination unit that determines the pickling conditions in the pickling facility based on the thickness of the reduced iron layer formed on the metal strip, material information and quality information of the metal strip. [2] The pickling apparatus is configured to pickle the metal strip by at least one of immersing the metal strip in an acidic chemical solution and electrolyzing the metal strip with the metal strip as the anode, and the control device controls at least one of the concentration of the acidic chemical solution in the pickling apparatus and the current density in the electrolysis based on the pickling conditions, the continuous annealing apparatus for a metal strip as described in [1]. [3] The continuous annealing apparatus for a metal strip according to [1] or [2], wherein the direct-fired zone has at least one oxidation zone with an air ratio of 1.0 or more and at least one reduction zone with an air ratio of 0.7 or more and less than 1.0, and is arranged in the order of the oxidation zone and the reduction zone in the direction of transport of the metal strip. [4] Continuous annealing apparatus for metal strips according to any one of [1] to [3], wherein the dew point in the homogeneous tropical region is -30°C or lower. [5] A continuous annealing method for a metal strip, comprising performing a heating step, a soaking step, and a pickling step on the metal strip in this order, wherein in the heating step and the soaking step, the heating conditions in the heating step and the soaking step are determined based on the thickness of the reduced iron layer determined from the material information and quality information of the metal strip, and in the pickling step, the pickling conditions for the metal strip in the pickling step are determined based on the thickness of the reduced iron layer formed on the metal strip by heating based on the heating conditions, the material information and quality information of the metal strip. [6] The continuous annealing method for a metal strip according to [5], wherein the pickling step involves pickling the metal strip by at least one of immersing it in an acidic chemical solution and electrolyzing it with the metal strip as the anode, and controlling at least one of the concentration of the acidic chemical solution and the current density in the electrolysis based on the pickling conditions. [7] The continuous annealing method for a metal strip according to [5] or [6], wherein the heating step involves heating the metal strip in at least one oxidation zone where the air ratio is 1.0 or more and 1.5 or less, and then heating the metal strip in at least one reduction zone where the air ratio is 0.7 or more and less than 1.0. [Effects of the Invention]

[0010] According to the present invention, it is possible to manufacture a metal strip with excellent chemical treatment properties and corrosion resistance without reducing the manufacturing efficiency of the metal strip. [Brief explanation of the drawing]

[0011] [Figure 1] This figure shows a continuous annealing apparatus for metal strips and a continuous annealing apparatus to which the continuous annealing method for metal strips can be applied according to this embodiment. [Figure 2] This is a diagram showing a direct flame zone. [Figure 3] This is a view from arrow A, as shown in Figure 2. [Figure 4] This is a view taken along arrow B, as shown in Figure 2. [Figure 5] This is a diagram illustrating the configuration of the control device. [Modes for carrying out the invention]

[0012] The present invention will be described in detail below through embodiments of the present invention. The following embodiments are preferred examples of the present invention and are not limiting in any way.

[0013] FIG. 1 is a diagram showing a continuous annealing facility for a metal strip according to the present embodiment and a continuous annealing facility to which a continuous annealing method for a metal strip can be applied. The continuous annealing facility 100 shown in FIG. 1 is a facility for continuously annealing the metal strip 1, and is roughly classified into an inlet side facility 20, a furnace body facility 21, and an outlet side facility 24. The inlet side facility 20 includes a payoff reel 2, a welder 3, and an inlet side looper 4. The furnace body facility 21 is composed of an annealing facility 22 and a reheating facility 23. The annealing facility 22 includes a direct firing zone 6, a soaking zone 7, and a cooling zone 8, and may include a preheating zone 5 on the upstream side of the direct firing zone 6. The reheating facility 23 includes a reheating zone 9, an overaging zone 10, and a final cooling zone 11. The outlet side facility 24 includes a pickling facility 12, an outlet side looper 13, a temper rolling facility 14, an inspection table 15, and a tension reel 16.

[0014] Examples of the metal strip 1 include a high-tensile steel sheet in which Si and Mn are added to the raw material of the metal strip 1 to increase the tension. The metal strip 1 is wound into a coil in a previous process and attached to the payoff reel 2 in the form of a coil. As shown in FIG. 1, the metal strip 1 is unwound by the payoff reel 2 and conveyed along the conveyance direction of the metal strip 1 (the right direction in FIG. 1) and enters the inlet side looper 4. In the continuous annealing facility 100 shown in FIG. 1, two payoff reels 2 are provided. The metal strips 1 unwound from those payoff reels 2 are alternately conveyed to the welder 3, and in the welder 3, the trailing end of the preceding metal strip 1 and the leading end of the succeeding metal strip 1 are joined together by welding.

[0015] The inlet side looper 4 is a facility for securing the extra length of the metal strip 1 in order to continuously pass the metal strip 1 when joining the metal strips 1 together by the welder 3. After passing through the inlet side looper 4, the metal strip 1 enters the annealing facility 22 of the continuous annealing facility 100. In the example shown in FIG. 1, the preheating zone 5, the direct firing zone 6, the soaking zone 7, and the cooling zone 8 are arranged in this order from the upstream side in the conveyance direction of the metal strip 1.

[0016] (Preheating zone) The preheating zone 5 is a facility for preheating the metal strip 1 that enters the direct-firing zone 6. The preheating method of the metal strip 1 in the preheating zone 5 is not limited, but it may be a preheating method that utilizes the combustion exhaust gas generated in the direct-firing zone 6. In the preheating zone 5, for example, the metal strip 1 is preheated to about 300 °C.

[0017] (Direct-firing zone) In the direct-firing zone 6 shown in FIG. 1, heating equipment of the direct-firing heating method is arranged. The metal strip 1 is heated to a preset temperature according to the component composition of the metal strip 1 using the heating equipment. The combustion burner used in the direct-firing zone 6 is not limited, but in order to make the thickness of the oxide film formed on the surface of the metal strip 1 uniform in the longitudinal direction and the width direction of the metal strip 1, it is preferable to use a slit burner (hereinafter simply referred to as a burner). Note that the process of heating the metal strip 1 in the direct-firing zone 6 corresponds to the heating process of the present embodiment.

[0018] In order to form an iron oxide layer on the surface of the metal strip 1, it is preferable that the atmosphere around the metal strip 1 in the direct-firing zone 6 is an acidic atmosphere. This is because Si and Mn are less likely to diffuse in the iron oxide layer compared to in the steel. That is, by heating the metal strip 1 in an acidic atmosphere, an iron oxide layer is formed on the surface of the metal strip 1, suppressing the concentration of Si and Mn on the surface of the metal strip 1.

[0019] The thickness of the iron oxide layer is preferably 30 nm or more and 300 nm or less. This is because if the iron oxide layer is less than 30 nm, all of the reduced iron layer formed on the surface of the metal strip 1 during pickling may dissolve. Then, oxides of Si and Mn remaining inside the metal strip 1 may be exposed on the surface of the metal strip 1, and the formation treatment property may deteriorate. On the other hand, if the iron oxide layer exceeds 300 nm, oxides of Si and Mn may be formed at the grain boundaries inside the metal strip 1 when the iron oxide layer is formed, and the corrosion resistance may deteriorate. Note that the thickness of the iron oxide layer formed on the surface of the metal strip 1 is determined based on material information and quality information of the metal strip, etc. in a control device described later.

[0020] The thickness of the iron oxide layer formed on the surface of the metal strip 1 may be determined by calculation or by actual measurement. The method for calculating the thickness of the iron oxide layer is not limited, but for example, the thickness of the iron oxide layer corresponding to the air ratio in the direct-fired zone 6 and the temperature of the metal strip 1 at the exit of the direct-fired zone 6 may be determined experimentally in advance, and a table summarizing these relationships may be prepared. Based on this table, the actual air ratio in the direct-fired zone 6, and the actual temperature of the metal strip 1 at the exit of the direct-fired zone 6 and the heating time conditions, i.e., the line speed, the thickness of the iron oxide layer on the surface of the metal strip 1 may be calculated. Alternatively, the thickness of the iron oxide layer may be measured. In that case, a thickness gauge using interference spectroscopy (not shown) is installed behind the direct-fired zone 6 in the conveying direction of the metal strip 1. Light is shone onto the metal strip 1 using the thickness gauge, and the wavelength of the reflected light corresponding to the thickness of the iron oxide layer is measured. Furthermore, the wavelength of reflected light corresponding to the thickness of the iron oxide layer in a metal strip whose thickness is known in advance through experiments is measured using a thickness gauge, and the relationship between the thickness of the iron oxide layer and the wavelength measured by the thickness gauge is summarized in a table. The thickness of the iron oxide layer formed on the surface of the metal strip 1 is calculated by applying the wavelength measured by the thickness gauge to this table.

[0021] Figure 2 shows the direct-fired zone 6. Figure 3 is a view along arrow A in Figure 2. Figure 4 is a view along arrow B in Figure 2. As shown in Figures 2 to 4, the direct-fired zone 6 has at least one oxidation zone and at least one reduction zone in the direction of transport of the metal strip 1. In the example shown in Figures 2 to 4, the direct-fired zone 6 has seven oxidation zones 30 to 36 and one reduction zone 37. In the direction of transport of the metal strip 1, the zones are labeled from upstream as the first oxidation zone 30, the second oxidation zone 31, the third oxidation zone 32, the fourth oxidation zone 33, the fifth oxidation zone 34, the sixth oxidation zone 35, the seventh oxidation zone 36, and the reduction zone 37.

[0022] Each zone 30-37 has multiple burners 38, as shown in Figures 3 and 4. Each burner 38 generates a flame according to a heat pattern determined by the design based on the composition and material specifications of the metal strip 1. This raises the metal strip 1 to a predetermined temperature. Note that the number, diameter, and arrangement of burners shown in Figures 3 and 4 are examples only and are not limited thereto.

[0023] Each oxidation zone 30-36 is a region where the metal strip 1 is oxidized by burning fuel gas, forming an iron oxide layer on the surface of the metal strip 1. To create an acidic atmosphere in each oxidation zone 30-36, the air ratio in each oxidation zone 30-36 is preferably 1.0 or more and 1.5 or less. If the air ratio is less than 1.0, the atmosphere around the metal strip 1 in each oxidation zone 30-36 becomes a reducing atmosphere. Since Si and Mn oxidize more easily than Fe, if the air ratio is less than 1.0, an iron oxide layer will not form on the surface of the metal strip 1, and oxides of Si and Mn will form, worsening the chemical conversion treatment performance. On the other hand, if the air ratio exceeds 1.5, nitrogen oxides will be generated, and there is a concern that the flame will be extinguished. Therefore, the air ratio is preferably 1.0 or more and 1.5 or less, and more preferably 1.0 or more and 1.3 or less.

[0024] It is preferable that the heating end temperature of the metal band 1 in each oxidation zone 30 to 36 be between 500°C and 750°C. This is because the oxidation of the metal band 1 proceeds rapidly above 500°C. In other words, if the heating end temperature of the metal band 1 in each oxidation zone 30 to 36 is below 500°C, the oxidation of the metal band 1 will not proceed easily. Furthermore, if the heating end temperature of the metal band 1 in each oxidation zone 30 to 36 exceeds 750°C, oxides of Si and Mn will form at the grain boundaries within the metal band 1, resulting in a deterioration of corrosion resistance.

[0025] The reduction zone 37 is a region where the interior is a reducing atmosphere, and the surface of the iron oxide layers formed in each oxidation zone 30-36 is reduced to form a reduced iron layer. The iron oxide layer is easily peeled off, and when the iron oxide layer peels off, the peeled iron oxide layer may be picked up by a conveyor roll (not shown) and may also adhere to the surface of the conveyor roll. If this happens, the iron oxide layer adhering to the surface of the conveyor roll may cause indentations on the surface of the metal strip 1 due to the aforementioned picking. Therefore, the surface of the iron oxide layer is reduced to form a reduced iron layer to suppress the peeling of the iron oxide layer.

[0026] The air ratio in the reduction zone 37 is preferably 0.7 or higher and less than 1.0. This is because if the air ratio is less than 0.7, the fuel efficiency will deteriorate and the metal band 1 will become contaminated with soot, and if the air ratio is 1.0 or higher, the reduction zone 37 will become an oxidizing atmosphere and the metal band 1 will oxidize. The heating temperature of the metal band 1 in the reduction zone 37 is preferably 650°C or higher and 850°C or lower. This is because if the heating temperature of the metal band 1 in the reduction zone 37 is less than 650°C, the temperature rise required for recrystallization of the metal band 1 in the homogenization zone 7 will become excessively large, reducing the production efficiency, so this is to be avoided. Also, if the heating temperature of the metal band 1 in the reduction zone 37 exceeds 850°C, recrystallization of the metal band 1 will start in the direct-fire zone 6, so this is to be avoided.

[0027] (Syntropical) The uniforming zone 7 is equipment that promotes recrystallization and controls the material properties by maintaining the metal strip 1, heated in the direct-heat zone 6, at a predetermined temperature. The uniforming zone 7 is equipped with a device for maintaining the metal strip 1 at a predetermined temperature. This device generally controls the material properties of the metal strip 1 by uniformly heating it above the recrystallization temperature using indirect heating, but is not limited to this. The predetermined temperature mentioned above may be 800 to 900°C. The process of maintaining the metal strip 1 at a predetermined temperature in the uniforming zone 7 corresponds to the uniform heating process in this embodiment.

[0028] Furthermore, in the uniform environment 7, the dew point around the metal strip 1 is controlled to below -30°C. If there is an excess of moisture in the uniform environment 7, the Si and Mn inside the metal strip 1 will react with the moisture, and oxides of these Si and Mn will be formed inside the metal strip 1. The presence of Si and Mn oxides inside the metal strip 1 will worsen the corrosion resistance of the metal strip 1. In order to suppress the formation of Si and Mn oxides inside the metal strip 1, the dew point around the metal strip 1 is controlled to below -30°C.

[0029] Here, controlling the dew point means controlling the amount of moisture inside the uniform zone 7. In order to control the dew point to below -30°C, the dew point in the uniform zone 7 is measured, and if the dew point exceeds -30°C, a refiner (not shown) is provided in the uniform zone 7 to remove excess moisture.

[0030] The dew point of the uniform zone 7 is measured using a dew point meter (not shown) as specified in JIS Z 8806:2001. Dew point meters are installed before and after the uniform zone 7 in the direction of transport of the metal strip 1, and the dew point around the metal strip 1 inside the uniform zone 7 is calculated based on the dew points before and after the uniform zone 7 measured by these meters. If the dew point calculated in this way exceeds -30°C, excess moisture is removed using a refiner.

[0031] Furthermore, by creating a reducing atmosphere within the uniform zone 7, the iron oxide layer formed in the metal zone 1 in the direct-fire zone 6 is reduced to a reduced iron layer. To create a reducing atmosphere, for example, a mixed gas of hydrogen and nitrogen is supplied to the uniform zone 7.

[0032] The thickness of the reduced iron layer formed in the uniform zone 7 is preferably 10 nm or more. If the thickness of the reduced iron layer is less than 10 nm, oxides of Si and Mn present inside the metal strip 1 may be exposed on the surface, potentially worsening the chemical conversion treatment performance. The thickness of the reduced iron layer formed on the surface of the metal strip 1 is determined by the control device described later, based on material information and quality information of the metal strip.

[0033] The thickness of the reduced iron layer can be determined by calculation. While there are no limitations on the method for calculating the thickness of the reduced iron layer, for example, the emissivity corresponding to the thickness of the reduced iron layer can be determined experimentally beforehand, and a table summarizing the relationship between the thickness of the reduced iron layer and the emissivity can be prepared. The thickness of the reduced iron layer can then be calculated based on this table and the actual emissivity of the metal strip 1 at the exit of the uniform zone 7. The emissivity is measured using the simplified method for measuring normal emissivity with an infrared radiation thermometer (not shown) as specified in JIS Z 8704:1993.

[0034] Cooling equipment is installed in the cooling zone 8 to cool the metal strip 1 to a predetermined temperature. Cooling methods used in this cooling equipment include liquid cooling, gas jet cooling, roll cooling, and mist cooling (gas-liquid mixed cooling). Liquid cooling is often performed by water cooling (water quenching). Water cooling is a cooling method in which the metal strip 1 is immersed in an immersion water tank installed downstream of the uniform zone 7. Gas jet cooling is a cooling method in which gas is blown from a nozzle onto the surface of the metal strip 1. Roll cooling is a cooling method in which the metal strip 1 is cooled by contacting it with a water-cooled roll. Mist cooling is a cooling method in which water is sprayed in a fine mist and cooled by the absorption of heat of vaporization. In mist cooling, the size of the water droplets sprayed is often about 0.1 to 1.0 mm.

[0035] The reheating equipment 23 is located downstream of the cooling zone 8 in the direction of transport of the metal strip 1. In the reheating equipment 23, the reheating zone 9, the overaging zone 10, and the final cooling zone 11 are arranged in this order from upstream in the direction of transport of the metal strip 1. The reheating zone 9 is equipment that reheats the metal strip 1 to a temperature of approximately 300-400°C using an induction heating device or the like. The overaging zone 10 is equipment that performs overaging treatment on the reheated metal strip 1, holding it for a predetermined time. The final cooling zone 11 is equipment that cools the overaged metal strip 1 to near room temperature. Note that the reheating equipment 23 is not essential for continuous annealing equipment, and some continuous annealing equipment may not have it.

[0036] The exit equipment 24 is located downstream of the final cooling zone 11 in the direction of transport of the metal strip 1. In the exit equipment 24, the pickling equipment 12, exit looper 13, temper rolling equipment 14, inspection table 15, and tension reel 16 are arranged in this order from upstream in the direction of transport of the metal strip 1.

[0037] (pickling equipment) The pickling equipment 12 in this embodiment is an electrolytic pickling equipment. The pickling equipment 12 removes Si and Mn oxides from the surface of the metal strip 1 by utilizing the dissolving power of the chemical solution and electrolysis, the mechanical action of hydrogen and oxygen bubbles generated by the electrolysis of the chemical solution, and the reducing power of hydrogen. Although not shown in detail, the pickling equipment 12 has a pickling tank in which the metal strip 1 is immersed in the chemical solution, a chemical solution tank for storing the chemical solution, a diluent tank for storing a diluent for diluting the chemical solution, an anode group and a cathode group for electrolysis of the chemical solution, and a power source for supplying power to each electrode group. Furthermore, in order to control the removal capacity of Si and Mn oxides by the chemical solution and electrolysis, a control device, which will be described later, can control at least one of the concentration of the chemical solution in the pickling tank, the immersion time in the chemical solution, and the current density and current application time applied to each electrode group. For example, if a high surface quality is required for the metal strip 1, the control device increases the concentration of the chemical solution in the pickling tank and the immersion time in the chemical solution, as well as the current density and current application time applied to each electrode group. In the pickling equipment 12, the metal strip 1 is connected to the anode group, and the metal strip 1 is electrolyzed as an anode. The process of pickling the metal strip 1 in the pickling equipment 12 corresponds to the pickling process in this embodiment.

[0038] Examples of chemical solutions include aqueous nitric acid solution and aqueous sulfuric acid solution. When using aqueous nitric acid solution, a passivation layer may form on the surface of metal strip 1 depending on its material. Therefore, it is preferable to use aqueous sulfuric acid solution. The following explanation will use aqueous sulfuric acid solution as an example.

[0039] The concentration of the sulfuric acid aqueous solution is preferably, for example, 0.1 to 150 g / L. If the concentration of the sulfuric acid aqueous solution is less than 0.1 g / L, it may become difficult to dissolve the oxides of Si and Mn. If the concentration of the sulfuric acid aqueous solution exceeds 150 g / L, the surface of the metal strip 1 may be excessively dissolved. Therefore, by using the above concentration range, it is possible to remove the oxides of Si and Mn from the surface of the metal strip 1 while suppressing the excessive dissolution of the surface of the metal strip 1 and the formation of a new iron oxide layer.

[0040] Furthermore, the current density is 5.0 A / dm². 2 Preferably, the current density is 5.0 A / dm². 2 If this value is exceeded, the amount of dissolution of the surface of the metal strip 1 by electrolysis becomes excessive, and oxides of Si and Mn present inside the metal strip 1 may be exposed to the surface of the metal strip 1, potentially worsening the chemical conversion treatment performance.

[0041] The exit looper 13 is a device for temporarily storing the metal strip 1 in order to adjust the conveying speed of the metal strip 1 in the furnace equipment 21 and the processing speed in the exit equipment 24. The temper rolling equipment 14 is a device for adjusting the material dimensional accuracy of the metal strip 1. The temper rolling equipment 14 is located between the exit looper 13 and the inspection table 15. The work rolls used in the temper rolling equipment 14 need to be replaced at predetermined intervals. By locating the temper rolling equipment 14 downstream of the exit looper 13 in the conveying direction of the metal strip 1, changes in the speed of the metal strip 1 in the furnace equipment 21 when work rolls are replaced can be suppressed by the exit looper 13.

[0042] The inspection table 15 is equipment used to inspect the dimensional accuracy and surface quality of the metal strip 1. The tension reel 16 is equipment used to wind the metal strip 1 into a coil. The metal strip 1, inspected on the inspection table 15, is then wound into a coil by the tension reel 16.

[0043] Metal strips 1 that pass the quality inspection on inspection table 15 may be shipped as product coils, or they may be sent to a surface treatment facility for plating and chemical conversion treatment. Metal strips 1 that fail the quality inspection on inspection table 15 or are put aside are sent to a recoil line (not shown). On the recoil line, the dimensions and weight of the metal strips 1 are adjusted, samples are taken for quality assurance, shape and dimension inspections are performed, and the coils are rewound.

[0044] (Control device) The continuous annealing equipment 100 has a control device 40 that controls the various devices and equipment described above. Figure 5 is a diagram illustrating the configuration of the control device 40. The control device 40 shown in Figure 5 has an input unit 41, a calculation unit 42, a determination unit 43, an acquisition unit 44, a control unit 45, and a storage unit 46. The control device 40 is mainly composed of a microcomputer, for example. The control device 40 is configured to function as the input unit 41, determination unit 43, acquisition unit 44, and control unit 45 by executing a program read from the storage unit 46.

[0045] The storage unit 46 may be, for example, an updatable flash memory, a hard disk that is built-in or connected via a data communication terminal, an information recording medium such as a memory card, and a device for reading and writing such a medium. The storage unit 46 stores programs for the control device 40 to execute each function, as well as data used by such programs.

[0046] The memory unit 46 further stores a database 47. The database 47 stores various types of data that are input to the control device 40. Examples of data stored in the database 47 include the operating conditions of the continuous annealing equipment 100, past operating performance data, material information of the metal strip 1 produced by the continuous annealing equipment 100, and quality information required for the metal strip 1. Material information may include the type and material of the metal strip 1, and quality information may include chemical treatment properties and corrosion resistance. In addition, the data stored in the database 47 may include the aforementioned table used to calculate the thickness of the reduced iron layer. Operating performance data may include actual data on the thickness of the reduced iron layer of the metal strip 1 produced by the continuous annealing equipment 100, actual data on the dew point in the uniform tropical zone 7, and actual data on the pickling conditions in the pickling equipment 12.

[0047] The input unit 41 inputs the operating conditions of the continuous annealing equipment 100, as well as material and quality information of the metal strip 1, to the control device 40. For example, the input unit 41 is connected to a data input device (not shown) operated by an operator. The operator inputs the aforementioned operating conditions, material and quality information of the metal strip 1 from the data input device to the input unit 41, and also inputs it from the input unit 41 to the control device 40. Furthermore, the various conditions and information input from the input unit 41 are stored in the database 47 of the storage unit 46.

[0048] The calculation unit 42 calculates the thickness of the iron oxide layer and the reduced iron layer formed on the metal strip 1. For example, it reads the table for calculating the thickness of the iron oxide layer from the database 47 and calculates the thickness of the iron oxide layer on the surface of the metal strip 1 based on this table, the actual air ratio in the direct-fired zone 6, and the actual temperature of the metal strip 1 at the exit of the direct-fired zone 6. It also reads the table for calculating the thickness of the reduced iron layer from the database 47 and calculates the thickness of the reduced iron layer based on this table and the actual emissivity of the metal strip 1 at the exit of the uniform zone 7. Furthermore, the calculation unit 42 stores the calculated thicknesses of the iron oxide layer and the reduced iron layer in the database 47.

[0049] The determination unit 43 determines the heating conditions in the direct-fired zone 6 and the uniform zone 7 for forming an iron oxide layer and a reduced iron layer on the metal strip 1, as well as the pickling conditions in the pickling equipment 12. Specifically, the determination unit 43 includes a heating condition determination unit 48 that determines the heating conditions in the direct-fired zone 6 and the uniform zone 7, and a pickling condition determination unit 49 that determines the pickling conditions in the pickling equipment 12 based on the thickness of the reduced iron layer, material information of the metal strip 1, and quality information.

[0050] The heating condition determination unit 48 reads, for example, the material information and quality information of the metal strip 1 stored in the database 47 and calculates target values ​​for the thickness of the iron oxide layer and the reduced iron layer to be formed on the metal strip 1. For example, the appropriate thickness of the iron oxide layer and the reduced iron layer according to the material information and quality information of the metal strip 1 is determined by experimentation, and a table summarizing their relationships is prepared. The thickness of the iron oxide layer and the reduced iron layer are calculated based on this table and the material information and quality information of the metal strip 1. Then, the heating condition determination unit 48 determines the heating conditions in the direct-fire zone 6 and the uniform zone 7 based on the calculated target values ​​for the thickness of the iron oxide layer and the reduced iron layer. Examples of heating conditions include the air ratio and temperature in the direct-fire zone 6, and the dew point, uniform heating time, and temperature in the uniform zone 7. The heating condition determination unit 48 also stores the calculated target values ​​for the thickness of the iron oxide layer, the target values ​​for the thickness of the reduced iron layer, and the heating conditions in the database 47 of the storage unit 46.

[0051] The pickling condition determination unit 49 reads, for example, the material information and quality information of the metal strip 1 stored in the database 47, and determines the pickling conditions in the pickling equipment 12 based on this information and the calculated thickness of the reduced iron layer formed in the uniform tank 7. For example, appropriate pickling conditions corresponding to the material information, quality information, and thickness of the reduced iron layer of the metal strip 1 are determined by experimentation, and a table summarizing their relationships is prepared. The pickling conditions are calculated based on this table and the calculated material information, quality information, and thickness of the reduced iron layer of the metal strip 1. The pickling equipment 12 then stores the pickling conditions thus calculated in the database 47. Examples of pickling conditions include the concentration of the chemical solution in the pickling tank, the immersion time of the metal strip 1 in the chemical solution, the current density applied to each electrode group, and the current application time.

[0052] The acquisition unit 44 acquires material information, quality information, heating conditions, and pickling conditions of the metal strip 1 from the database 47, and outputs the acquired information and conditions to the control unit 45. Furthermore, the acquisition unit 44 acquires sensor information from various sensors (not shown) and stores it in the database 47 of the storage unit 46.

[0053] The control unit 45 controls the heating of the metal strip 1 in the direct-fire zone 6 and the uniform zone 7, and controls the pickling of the metal strip 1 in the pickling equipment 12, based on the various information and conditions input from the acquisition unit 44. For example, the control unit 45 controls the air ratio in the direct-fire zone 6, the temperature of the metal strip 1 in the direct-fire zone 6, the dew point in the uniform zone 7, and the temperature of the metal strip 1 in the uniform zone 7, based on the heating conditions. The control unit 45 also controls the concentration of the chemical solution in the pickling tank, the immersion time of the metal strip 1 in the chemical solution, and at least one of the current density applied to each electrode group or the current application time, or at least one of both, based on the pickling conditions. Furthermore, the control unit 45 controls the refiner installed in the uniform zone 7 based on the dew point detected by the dew point meter, and removes excess moisture in the uniform zone 7 with the refiner.

[0054] (Effects / Actions) According to this embodiment, target values ​​for the thickness of the iron oxide layer and the reduced iron layer are calculated based on the material information and quality information of the metal strip 1, and the heating conditions in the direct-fired zone 6 and the uniform zone 7 are determined based on these target values. Then, the heating of the metal strip 1 in the direct-fired zone 6 and the uniform zone 7 is controlled based on these heating conditions. In other words, in the direct-fired zone 6, an excessively thick layer is formed on the surface of the metal strip 1, thereby suppressing the formation of Si and Mn oxides at the grain boundaries and the deterioration of the corrosion resistance of the metal strip 1. Also, an excessively thin layer of iron oxide is formed, suppressing the concentration of Si and Mn oxides on the surface of the metal strip 1.

[0055] Furthermore, in the uniform tropical zone 7, it is possible to suppress the formation of an excessively thick reduced iron layer on the surface of the metal strip 1, which would worsen manufacturing efficiency. It is also possible to suppress the formation of an excessively thin reduced iron layer, which would then dissolve and disappear during subsequent pickling.

[0056] Furthermore, according to this embodiment, the pickling conditions in the pickling equipment 12 are determined based on the material information, quality information, and thickness of the reduced iron layer at the exit of the uniform zone 7 of the metal strip 1. Then, the pickling of the metal strip 1 in the pickling equipment 12 is controlled based on these pickling conditions. In other words, the concentration of the chemical solution, the immersion time in the chemical solution, the current density during electrolysis, or the current application time is controlled. As a result, it is possible to suppress excessive pickling of the metal strip 1, which would cause even a small portion of the reduced iron layer to dissolve and disappear, exposing Si and Mn oxides and worsening the chemical conversion treatment performance. Also, it is possible to suppress insufficient pickling, which would prevent the removal of Si and Mn oxides inevitably present in the reduced iron layer of the metal strip 1 and worsen the chemical conversion treatment performance.

[0057] As a result, according to this embodiment, it is possible to manufacture a metal strip 1 that is excellent in chemical conversion treatment and corrosion resistance without reducing the manufacturing efficiency of the metal strip 1.

[0058] Furthermore, the method of continuously annealing the metal strip 1 using the continuous annealing equipment 100 of this embodiment described above corresponds to the continuous annealing method of the metal strip 1 of this embodiment. [Examples]

[0059] High-tensile steel sheets (hereinafter simply referred to as "steel sheets") were annealed using a continuous annealing apparatus configured similarly to the continuous annealing apparatus 100 shown in Figure 1. The composition of the steel sheets contained 1.32% to 1.48% by mass of Si and 2.1% or more by mass of Mn. The target chemical conversion treatment properties and corrosion resistance were then evaluated, as well as the presence or absence of indentation defects caused by pickup.

[0060] Table 1 summarizes the manufacturing conditions and evaluation results for Examples 1-15 and Comparative Examples 1-4.

[0061] [Table 1]

[0062] (Method for evaluating chemical treatment suitability) Test specimens were taken from the leading edge in the longitudinal direction of steel plates manufactured using a continuous annealing apparatus configured similarly to the continuous annealing apparatus 100 shown in Figure 1. These test specimens were subjected to chemical conversion treatment using a chemical conversion agent (SD2800, manufactured by Nippon Paint Co., Ltd.) at a bath temperature of 42°C and a chemical conversion treatment time of 120 seconds. After drying, the test specimens were observed using a scanning electron microscope (SEM) to evaluate their chemical conversion properties. If chemical conversion crystals were formed over the entire surface of the test specimen, it was considered a pass and marked with "◎" in Table 1. If there were areas on the surface of the test specimen where chemical conversion crystals were not formed, but there were no quality issues, it was considered a pass and marked with "〇" in Table 1. If there were areas on the surface of the test specimen where chemical conversion crystals were not formed and the required quality was not met, it was considered a fail and marked with "×" in Table 1.

[0063] (Method for evaluating corrosion resistance) The aforementioned test specimens that underwent chemical conversion treatment were subjected to corrosion resistance tests in accordance with the salt spray test method specified in JIS Z2371:2015. After the test, the surface of the test specimens was observed. If no peeling of the chemical conversion coating formed on the surface of the test specimen was observed, it was considered a pass and marked with "◎" in Table 1. If some peeling of the chemical conversion coating formed on the surface of the test specimen was observed, but there was no problem in terms of quality, it was considered a pass and marked with "〇" in Table 1. If the chemical conversion coating formed on the surface of the test specimen was significantly peeled and did not meet the required surface quality, it was considered a fail and marked with "×" in Table 1.

[0064] (Method for evaluating dents / damage) Test specimens were taken from the leading edge in the longitudinal direction of steel plates manufactured using a continuous annealing apparatus configured similarly to the continuous annealing apparatus 100 shown in Figure 1. Surface observation was performed on these test specimens. If no indentation marks caused by pickup were observed on the surface of the test specimen, it was considered acceptable and marked with "◎" in Table 1. If indentation marks caused by pickup were observed on the surface of the test specimen, but there were no quality issues, it was considered acceptable and marked with "〇" in Table 1. If indentation marks caused by pickup were clearly visible on the surface of the test specimen and it did not meet the required surface quality, it was considered unacceptable and marked with "×" in Table 1.

[0065] (Example 1) Example 1 is an example in which material information and quality information of the steel sheet were input into the control device, and target values ​​for the thickness of the iron oxide layer and the reduced iron layer were calculated based on this information, and the heating conditions in the direct-fire zone and the uniform zone were determined based on each target value. In other words, an iron oxide layer was formed throughout the direct-fire zone, and a reduced iron layer was formed in the uniform zone. In Example 1, the thickness of the reduced iron layer formed in the uniform zone was measured, and the pickling conditions were determined based on the thickness of the reduced iron layer and the material information and quality information mentioned above. In Example 1, as shown in Table 1, some indentations caused by picking were observed that were not severe enough to result in product defects.

[0066] (Examples 2 and 3) In Examples 2 and 3, material information and quality information of the steel sheet were input into the control device. Based on this information, target values ​​for the thickness of the iron oxide layer and the reduced iron layer were calculated, and the heating conditions in the direct-heat zone and the uniform zone were determined based on these target values. Furthermore, an acidic atmosphere was used in the preceding stage of the direct-heat zone to form an iron oxide layer on the surface of the steel sheet, and a reduced iron layer was used in the succeeding stage to form a reduced iron layer on the surface of the iron oxide layer. The dew point in the uniform zone was set to above -30°C. In Examples 2 and 3, the pickling conditions were determined in the same way as in Example 1. As shown in Table 1, in Examples 2 and 3, the corrosion resistance was evaluated to a degree that did not result in product failure. This is thought to be because a small amount of Si oxide was formed inside the steel sheet due to the dew point of the uniform zone being set to above -30°C.

[0067] (Examples 4-6) In Examples 4-6, material information and quality information of the steel sheet were input into the control device. Based on this information, target values ​​for the thickness of the iron oxide layer and the reduced iron layer were calculated, and the heating conditions in the direct-fire zone and the uniform zone were determined based on these target values. Furthermore, an acidic atmosphere was created before the direct-fire zone to form an iron oxide layer on the surface of the steel sheet, and a reduced atmosphere was created after the direct-fire zone to form a reduced iron layer on the surface of the iron oxide layer. The dew point in the uniform zone was set to -30°C or lower. In Examples 4-6, the pickling conditions were determined in the same way as in Example 1. As shown in Table 1, Examples 4-6 passed the tests for chemical conversion treatmentability, corrosion resistance, and indentation resistance.

[0068] (Examples 7-15) In Examples 7-15, material information and quality information of the steel sheet were input into the control device. Based on this information, target values ​​for the thickness of the iron oxide layer and the reduced iron layer were calculated, and the heating conditions in the direct-fire zone and the uniform zone were determined based on these target values. An acidic atmosphere was created in the pre-direct-fire zone to form an iron oxide layer on the surface of the steel sheet, and a reducing atmosphere was created in the post-direct-fire zone to form a reduced iron layer on the surface of the iron oxide layer. The dew point in the uniform zone was set to -30°C or lower. In addition, pickling conditions were determined based on the material information and quality information of the steel sheet input into the control device, and the thickness of the reduced iron layer formed in the uniform zone. The steel sheet was then pickled according to these conditions. As shown in Table 1, Examples 7-15 passed the tests for chemical conversion treatmentability, corrosion resistance, and indentation resistance.

[0069] (Comparative Examples 1 and 2) In Comparative Examples 1 and 2, material information of the steel sheet was input into the control device, and target values ​​for the thickness of the iron oxide layer and the reduced iron layer were calculated based on this material information. Based on these target values, heating conditions in the direct-fire zone and the homogenized zone were determined. Furthermore, an iron oxide layer was formed on the surface of the steel sheet by creating an acidic atmosphere throughout the direct-fire zone, and a reduced iron layer was formed on the surface of the iron oxide layer in the homogenized zone. In addition, without linking the material information of the steel sheet with the thickness of the reduced iron layer formed in the homogenized zone, the pickling conditions were determined solely by predicting the thickness of the iron oxide layer formed in the direct-fire zone, and the steel sheet was pickled under these conditions. In Comparative Examples 1 and 2, the reduced iron layer completely dissolved and disappeared during pickling, and the chemical conversion treatment performance deteriorated, as shown in Table 1.

[0070] (Comparative Example 3) In Comparative Example 3, material information of the steel sheet was input into the control device, and target values ​​for the thickness of the iron oxide layer and the reduced iron layer were calculated based on this material information. Based on these target values, the heating conditions in the direct-fire zone and the homogenized zone were determined. Furthermore, an acidic atmosphere was created before the direct-fire zone to form an iron oxide layer on the surface of the steel sheet, and a reduced iron layer was created after the direct-fire zone to form a reduced iron layer on the surface of the iron oxide layer. In addition, without linking the material information of the steel sheet with the thickness of the reduced iron layer formed in the homogenized zone, the pickling conditions were determined solely by the thickness of the reduced iron layer, and the steel sheet was pickled under these conditions. In Comparative Example 3, similar to Comparative Examples 1 and 2, the reduced iron layer completely dissolved and disappeared during pickling, and the chemical conversion treatment performance deteriorated, as shown in Table 1.

[0071] (Comparative Example 4) In Comparative Example 4, the heating conditions in the direct-fired zone and the homogenized zone were determined based on past performance data of the same steel type, without inputting material information of the steel sheet into the control device. Furthermore, an acidic atmosphere was created before the direct-fired zone to form an iron oxide layer on the surface of the steel sheet, and a reduced atmosphere was created afterward to form a reduced iron layer on the surface of the iron oxide layer. The dew point in the homogenized zone was set to -30°C or lower. In addition, the material information of the steel sheet was linked to the thickness of the reduced iron layer formed in the homogenized zone, and the pickling conditions were determined solely by the thickness of the reduced iron layer. The steel sheet was then pickled under these conditions. In Comparative Example 4, the reduced iron layer completely dissolved and disappeared during pickling, resulting in a deterioration of the chemical conversion treatment performance, as shown in Table 1. This is because, in Comparative Example 4, past performance data of the same steel type was used without inputting information on the sheet width and thickness, and the pickling conditions were determined based on the measured thickness of the reduced iron layer. In other words, because the size of the steel sheet was not accurate information, the reduced iron dissolved during pickling, resulting in a deterioration of the chemical conversion treatment performance. [Explanation of Symbols]

[0072] 1 Metal strip 2 Payoffriel 3. Welding machine 4. Inlet Louver 5. Pre-tropical 6. Direct flame zone 7 Tropical 8 Cooling Zone 9 Reheating Zone 10 Overaged zone 11. Final Cooling Zone 12 Pickling equipment 13 Outlet Looper 14. Temper rolling equipment 15 Examination Tables 16 Tension Reels 20 Entrance equipment 21 Furnace equipment 22 Annealing equipment 23 Reheating equipment 24 Outlet equipment 30 First Oxidation Zone 31 Second Oxidation Zone 32 Third Oxidation Zone 33 Fourth Oxidation Zone 34. Fifth Oxidation Zone 35. Sixth Oxidation Zone 36. Seventh Oxidation Zone 37 Reduction Zone 38 Burner 40 Control device 41 Input section 42 Calculation Section 43 Decision Section 44 Acquisition Department 45 Control Unit 46 Memory section 47 Databases 48 Heating condition determination unit 49 Pickling condition determination section 100 Continuous Annealing Equipment

Claims

1. A continuous annealing apparatus for metal strips, in which a direct-fired heating chamber, a soaking chamber, and an pickling chamber are arranged in this order in the direction of transport of the metal strip, The system includes a control device that determines the heating conditions of the metal strip in the direct heating zone and the uniform heating zone, and the pickling conditions of the metal strip in the pickling equipment. The control device includes a heating condition determination unit that determines the thickness of the reduced iron layer to be formed on the metal strip based on the material information and quality information of the metal strip, and determines the heating conditions in the direct heating zone and the homogenized zone based on the determined thickness of the reduced iron layer, A continuous annealing apparatus for a metal strip, comprising a pickling condition determination unit that determines the pickling conditions in the pickling apparatus based on the thickness of the reduced iron layer formed in the metal strip, material information of the metal strip, and quality information.

2. The pickling apparatus is configured to pickle the metal strip by at least one of the following: immersing the metal strip in an acidic chemical solution and electrolyzing the metal strip with the metal strip as the anode. The continuous annealing apparatus for a metal strip according to claim 1, wherein the control device controls at least one of the concentration of the acidic chemical solution in the pickling apparatus and the current density in the electrolysis based on the pickling conditions.

3. The aforementioned direct-fire zone has at least one oxidation zone with an air ratio of 1.0 or more and 1.5 or less, and at least one reduction zone with an air ratio of 0.7 or more and less than 1.

0. The continuous annealing apparatus for a metal strip according to claim 1 or 2, wherein the oxidation zone and the reduction zone are arranged in that order in the conveying direction of the metal strip.

4. The continuous annealing apparatus for metal strips according to claim 1 or 2, wherein the dew point in the homogeneous zone is -30°C or lower.

5. The continuous annealing apparatus for a metal strip according to claim 3, wherein the dew point in the homogeneous zone is -30°C or lower.

6. A continuous annealing method for a metal strip, comprising heating, soaking, and pickling in that order, In the heating step and the soaking step, the heating conditions in the heating step and the soaking step are determined based on the thickness of the reduced iron layer determined from the material information and quality information of the metal strip. A method for continuous annealing of a metal strip, wherein the pickling conditions for the metal strip in the pickling step are determined based on the thickness of the reduced iron layer formed on the metal strip by heating based on the heating conditions, the material information of the metal strip, and the quality information of the metal strip.

7. The continuous annealing method for a metal strip according to claim 6, wherein the pickling step involves pickling the metal strip by at least one of immersing it in an acidic chemical solution and electrolyzing it with the metal strip as the anode, and the method controls at least one of the concentration of the acidic chemical solution and the current density in the electrolysis based on the pickling conditions.

8. The continuous annealing method for a metal strip according to claim 6 or 7, wherein in the heating step, the metal strip is heated in at least one oxidation zone where the air ratio is 1.0 or more and 1.5 or less, and thereafter, the metal strip is heated in at least one reduction zone where the air ratio is 0.7 or more and less than 1.0.

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