Continuous annealing line and method for detecting surface defects of steel sheets in continuous annealing line

Abstract

To provide a continuous annealing line capable of suppressing erroneous detection caused by unevenness in glossiness and detecting surface defects that occur in processes prior to the continuous annealing line and also capable of detecting surface defects that occur in the continuous annealing line; and to provide a method for detecting steel sheet surface defects in the continuous annealing line.SOLUTION: A continuous annealing line 100 provided with an annealing furnace 21 for manufacturing a high tensile steel sheet by rapidly cooling a steel sheet 1 after a rolling process, comprises a first detection device 30 and a second detection device 31 for detecting surface defects of the steel sheet 1, and cleaning equipment 4 for cleaning the steel sheet 1. The first detection device 30 disposed on the upstream side of the annealing furnace 21 and on the downstream side of the cleaning equipment 4 in a direction of conveying the steel sheet 1, detects surface defects that occur to the steel sheet 1 in the rolling process, and the second detection device 31 disposed on the downstream side of the annealing furnace 21 in the direction of conveying the steel sheet 1, detects surface defects that occur to the steel sheet 1 in the annealing furnace 21.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a continuous annealing line and a method for detecting surface defects of a steel sheet in a continuous annealing line. [Background technology]

[0002] In recent years, demand for high-tensile steel sheets (sometimes referred to as high-tensile cold-rolled steel sheets) has increased, making rapid cooling technology, which is advantageous for producing high-tensile steel sheets, important. Among the methods for producing high-tensile steel sheets, water quenching, in which a heated steel sheet is immersed in cooling water, has the fastest cooling rate. Furthermore, water quenching ensures strength through martensitic transformation, allowing for the addition of fewer alloying elements to ensure strength. Therefore, it is possible to produce high-tensile steel sheets more inexpensively than other manufacturing methods. However, with water quenching, the steel sheet shrinks due to rapid cooling and expands due to martensitic transformation, causing buckling. This can lead to contact between the rolls in the annealing furnace and the steel sheet as it passes through the annealing furnace, resulting in uneven gloss.

[0003] In general, in continuous annealing lines producing high-tensile steel sheets, a surface inspection device is installed at the outlet of the annealing furnace, and the surface of water-quenched steel sheets is inspected using the surface inspection device. However, if the water-quenched steel sheet has the above-mentioned gloss unevenness, the gloss unevenness may cause the surface inspection device to erroneously detect surface defects in the steel sheet, potentially preventing the detection of surface defects in the steel sheet that require detection. Examples of the above-mentioned surface defects in steel sheets that require detection include surface defects that occur in the steel sheet in the continuous annealing line and surface defects that occur in the steel sheet in processes prior to the continuous annealing line. Examples of surface defects that occur in the steel sheet in the continuous annealing line (hereinafter referred to as surface defects in the current process) include mainly adhesion of foreign matter and uneven roll marks. Examples of surface defects that occur in the steel sheet in processes prior to the continuous annealing line (hereinafter referred to as surface defects in the previous process) include patterned scab defects and uneven scab defects.

[0004] An example of the above-mentioned surface inspection device is described in Patent Document 1. This device is configured to use polarized light to detect pattern-like surface defects that do not have significant unevenness. Specifically, polarized light is irradiated onto the surface of the steel plate, and the reflected light is received by a camera. Multiple analyzer angles are preset to optimize the contrast for each surface defect to be detected, and the device is configured to determine the presence or absence of a surface defect based on the intensity of the received polarized light. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-221391 Summary of the Invention [Problem to be solved by the invention]

[0006] The device described in Patent Document 1 can suppress the detection of gloss unevenness, which can cause false detection, by adjusting the analyzer angle, and can also detect surface defects in the current process. However, suppressing the detection of gloss unevenness also suppresses the detection of surface defects in the previous process. This is because the optical characteristics of surface defects in the current process and the optical characteristics of gloss unevenness are similar to each other.

[0007] The present invention has been made to solve the above-mentioned problems, and has an object to provide a continuous annealing line and a surface defect detection method for steel sheets in a continuous annealing line that can suppress erroneous detection due to gloss unevenness and detect surface defects that occur in a process prior to the continuous annealing line, and that can detect surface defects that occur in the continuous annealing line. [Means for solving the problem]

[0008] In order to achieve the above object, the present invention provides: [1] A continuous annealing line equipped with an annealing furnace that produces high-tensile steel plates by rapidly cooling steel plates that have undergone a rolling process, the continuous annealing line comprising a first detection device and a second detection device that detect surface defects in the steel plates, and cleaning equipment that cleans the steel plates, the first detection device being located upstream of the annealing furnace in the conveying direction of the steel plates and downstream of the cleaning equipment, and detecting surface defects that have occurred in the steel plates during the rolling process, and the second detection device being located downstream of the annealing furnace in the conveying direction of the steel plates, and detecting surface defects that have occurred in the steel plates in the annealing furnace. [2] The continuous annealing line according to the above item [1], wherein the second detection device determines surface defects that have occurred on the steel sheet in the annealing furnace based on first detection information detected by the first detection device and second detection information detected by the second detection device. [3] A continuous annealing line according to [1] or [2] above, wherein the first detection device is configured to detect at least pattern defects and uneven defects among the surface defects of the steel sheet, and the second detection device is configured to detect at least foreign matter adhering to the surface of the steel sheet and transportation defects that occur on the surface of the steel sheet when the steel sheet is transported in the annealing furnace among the surface defects of the steel sheet. [4] A method for detecting surface defects in steel plates in a continuous annealing line equipped with an annealing furnace that produces high-tensile steel plates by rapidly cooling steel plates that have undergone a rolling process, the method comprising: a first detection step in which surface defects that have occurred in the steel plate during the rolling process are detected by a first detection device that is provided downstream of a cleaning facility and upstream of the annealing furnace in the conveying direction of the steel plate; and a second detection step in which surface defects that have occurred in the steel plate in the annealing furnace are detected by a second detection device that is provided downstream of the annealing furnace in the conveying direction. [Effects of the Invention]

[0009] According to the present invention, a first detection device is provided downstream of the cleaning equipment and upstream of the annealing furnace in the conveying direction of the steel sheet, and a second detection device is provided downstream of the annealing furnace in the conveying direction. Therefore, the first detection device can detect a first surface defect that has occurred on the steel sheet in the rolling process without uneven gloss. Furthermore, the second detection device can detect a second surface defect that has occurred on the steel sheet in the annealing furnace. In this way, the present invention can suppress erroneous detection due to uneven gloss and detect surface defects that have occurred in processes prior to the continuous annealing line, and can also detect surface defects that occur in the continuous annealing line. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing an example of continuous annealing equipment according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will be specifically described below through embodiments of the present invention. The following embodiments are preferred examples of the present invention, and the present invention is not limited to these examples.

[0012] A continuous annealing line according to an embodiment of the present invention is a facility for producing a high-tensile steel sheet by water quenching a steel sheet that has undergone a rolling process and then rapidly cooling it. The steel sheet that has undergone the rolling process is in the form of a strip, and is transported in a rolled state to an entry facility of the continuous annealing line, where it is unwound from the roll and supplied to the continuous annealing line.

[0013] Fig. 1 is a diagram showing an example of continuous annealing equipment corresponding to the continuous annealing line according to this embodiment. The continuous annealing equipment 100 shown in Fig. 1 is broadly divided into an entry-side equipment 20, a furnace body equipment 21, and an exit-side equipment 24, which are arranged in this order from the upstream side in the conveyance direction of the steel sheet 1. The entry-side equipment 20 includes a payoff reel 2, a welding machine 3, a cleaning equipment 4, and an entry-side looper 5, which are arranged in this order from the upstream side in the conveyance direction of the steel sheet 1.

[0014] The payoff reel 2 is a facility that unwinds the steel sheet 1 wound in a roll and supplies it to the continuous annealing equipment 100, and in the example shown in Fig. 1, two payoff reels 2 are provided. The steel sheets 1 unwound from these payoff reels 2 are alternately transported to the welding machine 3, where the tail end of the leading steel sheet 1 and the front end of the trailing steel sheet 1 are joined together by welding.

[0015] The cleaning equipment 4 is a process prior to the continuous annealing equipment 100 (hereinafter referred to as the upstream process), and is equipment that removes grease adhering to the steel sheet 1, for example, during the rolling process, and may be, for example, an electrolytic cleaning device that removes grease from the surface of the steel sheet 1 by electrolytic degreasing.

[0016] The entry-side looper 5 is a facility for temporarily storing the steel sheets 1 in order to adjust the conveying speed of the steel sheets 1 in the furnace body facility 21 and the processing speed in the entry-side facility 20 .

[0017] The furnace body equipment 21 corresponds to the annealing furnace according to this embodiment, and is composed of an annealing equipment 22 and a reheating equipment 23. The annealing equipment 22 is provided upstream of the reheating equipment 23 in the transport direction of the steel sheet 1, and includes a heating zone 6, a soaking zone 7, and a cooling zone 8, which are arranged in this order from the upstream side in the transport direction of the steel sheet 1. The annealing equipment 22 may also include a preheating zone 9 provided upstream of the heating zone 6 in the transport direction.

[0018] Heating zone 6 is provided with heating equipment for raising the temperature of steel sheet 1, and this heating equipment is used to heat steel sheet 1 to a preset temperature within a temperature range of approximately 600°C to 900°C depending on the chemical composition of steel sheet 1. In heating zone 6, a direct flame or radiant combustion burner is used.

[0019] A device for maintaining the steel sheet 1 at a predetermined temperature is placed in the soaking zone 7. The device for maintaining the steel sheet 1 at a predetermined temperature has a heating capacity sufficient to compensate for the heat dissipated from the furnace body.

[0020] Cooling equipment is arranged in the cooling zone 8 to cool the steel sheet 1 to a predetermined temperature. Cooling methods used in this cooling equipment include liquid cooling, gas jet cooling, roll cooling, and mist cooling (sometimes referred to as gas-liquid mixed cooling). Liquid cooling is often performed by water cooling (sometimes referred to as water quenching). Water cooling is a cooling method in which the steel sheet 1 is immersed in an immersion water tank installed downstream of the soaking zone 7 in the conveying direction of the steel sheet 1 to cool it. Gas jet cooling is a cooling method in which gas is sprayed from a nozzle onto the surface of the steel sheet 1. Roll cooling is a cooling method in which the steel sheet 1 is cooled by contacting it with water-cooled rolls. Mist cooling is a cooling method in which water is sprayed in the form of a fine mist, and cooling is performed by the heat of vaporization. In mist cooling, the size of the sprayed water droplets is preferably approximately 0.1 mm or more and 1.0 mm or less.

[0021] The reheating equipment 23 is arranged downstream of the cooling zone 8 in the conveying direction of the steel sheet 1, and is equipment that reheats the steel sheet 1 to a predetermined temperature after it has been cooled to a predetermined temperature in the cooling zone 8. The reheating equipment 23 includes a reheating zone 10, an overaging zone 11, and a final cooling zone 12, which are arranged in this order from the upstream side in the conveying direction of the steel sheet 1.

[0022] In the reheating zone 10, an induction heating device is arranged as an example, and in the reheating zone 10, the steel sheet 1 is reheated to a temperature of about 300°C or more and 400°C or less using the induction heating device.

[0023] An over-aging zone 11 is provided downstream of the reheating zone 10 in the conveying direction of the steel sheet 1, where the reheated steel sheet 1 is held for a predetermined time for an over-aging treatment. A final cooling zone 12 is provided downstream of the over-aging zone 11 in the conveying direction of the steel sheet 1, where the over-aging-treated steel sheet 1 is finally cooled to near room temperature. Note that the reheating equipment 23 is not essential for the continuous annealing equipment 100, and some continuous annealing equipment does not include this equipment.

[0024] The delivery equipment 24 includes a delivery looper 13, a temper rolling equipment 14, an inspection table 15, and a tension reel 16, which are arranged in this order from the upstream side in the conveying direction of the steel sheet 1.

[0025] The outlet looper 13 is a facility for temporarily storing the steel sheet 1 in order to adjust the conveying speed of the steel sheet 1 in the furnace body facility 21 and the processing speed in the outlet facility 24 .

[0026] The temper rolling equipment 14 is a device that flattens the shape of the steel sheet 1 by imparting an elongation of approximately 0.1% to 3.0% to the steel sheet 1. In the example shown in FIG. 1 , it is arranged between the delivery looper 13 and the inspection table 15 in the conveying direction of the steel sheet 1. The work rolls used in the temper rolling equipment 14 need to be replaced at a predetermined timing. By arranging the temper rolling equipment 14 downstream of the delivery looper 13 in the conveying direction of the steel sheet 1, the delivery looper 13 can suppress changes in the speed of the steel sheet 1 in the furnace body equipment 21 when the work rolls are replaced. In other words, it is possible to secure the time required to replace the work rolls while suppressing changes in the speed of the steel sheet 1 in the furnace body equipment 21.

[0027] On the inspection table 15, the dimensional accuracy, surface quality, etc. of the steel sheet 1 are visually inspected. The tension reel 16 is a facility for winding the steel sheet 1 into a coil. The steel sheet 1 is wound into a coil by the tension reel 16. If the steel sheet 1 passes the quality inspection on the inspection table 15, it may be shipped as a product coil or sent to a surface treatment facility where the steel sheet 1 is plated for surface treatment. On the other hand, if the steel sheet 1 is wound into a coil by the tension reel 16 and is judged to have failed or been withheld on the quality inspection on the inspection table 15, it is sent to a recoil line (not shown), where the dimensions and weight of the steel sheet 1 are adjusted, samples are taken for quality assurance, shape and dimension inspections are performed, and the coil is rewound.

[0028] The continuous annealing equipment 100 shown in FIG. 1 also includes two detection devices for detecting surface defects of the steel sheet 1. The first detection device 30 is a device for detecting surface defects of the steel sheet 1 in a previous process, and is provided between the cleaning equipment 4 and the furnace body equipment 21 in the conveying direction of the steel sheet 1. Specifically, it is provided between the cleaning equipment 4 and the entry looper 5 in the conveying direction. This is to avoid the possibility of false detection of surface defects due to the oil and grease when attempting to detect surface defects of the steel sheet 1 when oil and grease are attached to the surface of the steel sheet 1. Examples of surface defects in the previous process include pattern-like scab defects and uneven scab defects. The pattern-like scab defects described above correspond to the pattern-like defect in the present invention, and the uneven scab defects correspond to the uneven defect in the present invention. The processing performed by the first detection device 30 corresponds to the first detection process in the present invention.

[0029] The second detection device 31 is a device that detects surface defects that have occurred on the steel sheet 1 in the furnace equipment 21 (hereinafter referred to as surface defects in the current process), and is provided downstream of the furnace equipment 21 in the conveying direction of the steel sheet 1. Specifically, it is provided between the temper rolling equipment 14 and the inspection table 15 in the conveying direction. Examples of surface defects in the current process include foreign matter adhering to the surface of the steel sheet 1 and roll marks that occur on the steel sheet 1 when a conveying roll (not shown) in the furnace equipment 21 comes into contact with the steel sheet 1. The above-mentioned roll marks correspond to conveying marks in the present invention. The process performed by the second detection device 31 corresponds to the second detection process in the present invention.

[0030] Each of the detection devices 30, 31 may be a device that detects surface defects of the steel sheet 1 using polarized light, and may be configured substantially similarly to the device described in Patent Document 1, for example. The detection devices 30, 31 are electrically connected to each other and are configured to output first detection information about surface defects of the steel sheet 1 detected by the first detection device 30 to the second detection device 31. The second detection device 31 is configured to identify surface defects of the steel sheet 1 in the current process based on the input first detection information and second detection information about surface defects of the steel sheet 1 detected by the second detection device 31. That is, each piece of detection information is accompanied by position information of the surface defect on the surface of the steel sheet 1, and thus, by combining and using each piece of detection information, surface defects in the previous process and surface defects in the current process can be identified. Furthermore, by combining the first detection information from the first detection device 30 and the second detection information from the second detection device 31, defects in the previous process and defects in the current process can be detected without omission, thereby improving the accuracy of quality assurance. The detection results of the surface defects of the steel sheet 1 by each of the detection devices 30, 31 may be configured to be output to a display means (not shown). Furthermore, when a surface defect is detected, the detection results may be configured to be output to a warning means (not shown).

[0031] Next, the operation and effect of the continuous annealing equipment 100 according to this embodiment will be described. The steel sheet 1 in a rolled state after the rolling process is unwound by the payoff reel 2 of the entry equipment 20 and transported to the cleaning equipment 4 via a welding machine 3. In the cleaning equipment 4, oils and grease adhering to the surface of the steel sheet 1 are removed, and then the steel sheet 1 is inspected for surface defects by the first detection device 30. In other words, the steel sheet 1 is inspected for surface defects by the first detection device 30 in a state in which there is no gloss unevenness. Therefore, it is possible to avoid or suppress erroneous detection of surface defects in the upstream process due to gloss unevenness, and it is possible to improve the accuracy of detecting surface defects in the upstream process.

[0032] After passing through the first detection device 30, the steel sheet 1 is transported to the furnace equipment 21, where it is water quenched. Specifically, the steel sheet 1 is water quenched in the cooling zone 8, and then transported to the outlet equipment 24 via the reheating equipment 23. When the steel sheet 1 is water quenched, buckling occurs in the steel sheet 1. Furthermore, when the steel sheet 1 passes through the furnace equipment 21, the steel sheet 1 may come into contact with a transport roll (not shown) in the furnace equipment 21, causing roll marks in the steel sheet 1. Gloss unevenness occurs at the locations where such buckling or roll marks have occurred. Here, the above-mentioned gloss unevenness means that the gloss of the surface of the steel sheet 1 is different from the gloss of its surroundings.

[0033] The steel sheet 1 that has passed through the furnace body equipment 21 is transported to the temper rolling equipment 14 of the outlet equipment 24 and flattened. Next, the second detection device 31 inspects the steel sheet 1 for surface defects in the current process. The surface of the steel sheet 1 that has been transported to the second detection device 31 may have gloss unevenness. However, even if gloss unevenness occurs in the steel sheet 1, the optical characteristics of the gloss unevenness and the optical characteristics of surface defects of the steel sheet 1 in the current process, specifically, foreign matter and roll marks adhering to the steel sheet 1, are different from each other. Therefore, even if gloss unevenness exists, surface defects in the current process can be accurately detected by using polarized light.

[0034] Furthermore, the first detection information from the first detection device 30 is output to the second detection device 31, and the second detection device 31 determines surface defects in the current process based on the first detection information and the second detection information. Combining the first detection information from the first detection device 30 and the second detection information from the second detection device 31 makes it possible to detect defects in both the previous process and the current process without omission. This also improves the detection accuracy of surface defects in the current process.

[0035] As a result, the continuous annealing equipment 100 according to this embodiment can avoid erroneous detection of surface defects in the upstream process due to gloss unevenness, thereby improving the overall accuracy of detecting surface defects in steel sheets compared to the prior art. Furthermore, because surface defects in the upstream process are detected in a state without gloss unevenness, even minor surface defects in the upstream process can be detected, thereby enabling the accuracy of quality assurance to be improved.

[0036] The present invention is not limited to the above-described embodiment. For example, gloss unevenness does not occur on the steel sheet 1 upstream of the furnace body equipment 21. Therefore, the first detection device 30 may be a surface defect detection device that uses a conventionally known general-purpose specular reflection optical system or diffuse reflection optical system instead of a device that uses polarized light to detect surface defects on the steel sheet 1. [Example]

[0037] Next, an example will be described that was conducted to confirm the operation and effect of the continuous annealing line and the method for detecting surface defects in a steel sheet in a continuous annealing line according to an embodiment of the present invention. In this example, a first detection device was installed upstream of the furnace body equipment in the steel sheet transport method, and a second detection device was installed downstream, and the detection results were combined to inspect the steel sheet for surface defects. On the other hand, in a comparative example, a second detection device was installed only downstream of the furnace body equipment in the steel sheet transport method to inspect the steel sheet for surface defects. Note that a surface defect detection device using a specular reflection optical system or a diffuse reflection optical system was used as the first detection device in the example. A detection device using polarized light was used as the second detection device.

[0038] The results of the surface defect inspection for the examples and comparative examples are summarized in Table 1.

[0039] [Table 1]

[0040] As shown in Table 1, in the comparative example in which the second detection device was located only downstream of the furnace equipment, only severe surface defects in the upstream process and severe surface defects in the current process could be detected. In contrast, in the example in which the detection devices were located both upstream and downstream of the furnace equipment, good results were obtained in that surface defects in both the upstream process and the current process, ranging from mild to severe, could be detected.

[0041] 1 steel plate 2 Payoff Reel 3. Welding machine 4. Cleaning equipment 5 Inlet looper 7 Heating Zone 7. Equal Temperature 8 Cooling Zone 9 Pre-tropical zone 10 Reheating Zone 11 Overaged zone 12 Final cooling zone 13 Exit looper 14 Temper rolling equipment 15 Examination table 16 Tension reel 20 Entrance equipment 21 Furnace equipment 22 Annealing equipment 23 Reheating equipment 24 Outlet equipment 30 First detection device 31 Second detection device 100 Continuous annealing equipment

Claims

1. A continuous annealing line equipped with an annealing furnace for producing high-tensile steel plates by rapidly cooling steel plates that have been through a rolling process that is a process prior to the continuous annealing line, a first detection device and a second detection device for detecting surface defects of the steel plate; and a cleaning facility for cleaning the steel plate, the first detection device is a surface defect detection device that uses a specular reflection optical system or a diffuse reflection optical system, and is provided upstream of the annealing furnace and downstream of the cleaning facility in the conveying direction of the steel sheet, and detects surface defects that have occurred on the steel sheet during the rolling process; The second detection device is a detection device that uses polarized light and is installed downstream of the annealing furnace in the conveying direction of the steel sheet, and detects surface defects that have occurred on the steel sheet in the annealing furnace.

2. 2. The continuous annealing line according to claim 1, wherein the second detection device determines surface defects that have occurred on the steel sheet in the annealing furnace based on first detection information detected by the first detection device and second detection information detected by the second detection device.

3. The first detection device is configured to detect at least pattern defects and uneven defects among the surface defects of the steel plate, 3. The continuous annealing line according to claim 1, wherein the second detection device is configured to detect, among the surface defects of the steel sheet, at least foreign matter adhering to the surface of the steel sheet and transportation defects that occur on the surface of the steel sheet when the steel sheet is transported in the annealing furnace.

4. A method for detecting surface defects in a steel plate in a continuous annealing line equipped with an annealing furnace for producing high-tensile steel plate by rapidly cooling a steel plate that has been through a rolling process that is a process preceding the continuous annealing line, comprising: a first detection step of detecting surface defects generated on the steel sheet in the rolling step by a first detection device provided downstream of the cleaning facility and upstream of the annealing furnace in the conveyance direction of the steel sheet; and thereafter, a second detection step of detecting surface defects occurring on the steel sheet in the annealing furnace by a second detection device provided downstream of the annealing furnace in the conveying direction, the first detection device is a surface defect detection device that uses a specular reflection optical system or a diffuse reflection optical system, The method for detecting surface defects of a steel plate in a continuous annealing line, wherein the second detection device is a detection device that uses polarized light.

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