Continuous annealing apparatus and continuous hot-dip galvanizing apparatus, and methods for manufacturing steel plates.

Abstract

This invention provides a continuous annealing apparatus capable of manufacturing steel plates with excellent resistance to hydrogen embrittlement. The continuous annealing apparatus (100) of the present invention comprises: a reel (10) for releasing a cold-rolled steel sheet (S) from a cold-rolled coil (C); an annealing furnace (20) for continuously annealing the cold-rolled steel sheet (S) by passing it through, wherein a heating zone (22), a soaking zone (24), and a cooling zone (26) are provided on the upstream side of the through-plate direction, wherein the cold-rolled steel sheet (S) is annealed in the heating zone (22) and the soaking zone (24) in a reducing atmosphere containing hydrogen, and the cold-rolled steel sheet (S) is cooled in the cooling zone (26); a downstream device (30) for continuously passing the cold-rolled steel sheet (S) discharged from the annealing furnace (20); a tension coiler (50) for winding the cold-rolled steel sheet (S) passing through the downstream device (30); and an acoustic irradiation device (60) for irradiating the cold-rolled steel sheet (S) passing from the cooling zone (26) into the tension coiler (50) with acoustic waves.

Description

Technical Field

[0001] This invention relates to a continuous annealing apparatus, a continuous hot-dip galvanizing apparatus, and a method for manufacturing steel plates. This invention is particularly applicable to the automotive, home appliance, and building materials industries, and relates to a continuous annealing apparatus and a continuous hot-dip galvanizing apparatus for manufacturing steel plates with inherently low hydrogen content and excellent resistance to hydrogen embrittlement, as well as a method for manufacturing steel plates. Background Technology

[0002] For example, when annealed steel sheets and hot-dip galvanized steel sheets are manufactured in continuous annealing and continuous hot-dip galvanizing units, respectively, the annealing is carried out in a reducing atmosphere containing hydrogen. Therefore, hydrogen penetrates into the steel sheet during this annealing process. The hydrogen inherent in the steel sheet reduces its formability, such as ductility, bendability, and tensile flangeability. Furthermore, the hydrogen inherent in the steel sheet causes embrittlement, which can lead to delayed failure. Therefore, treatment to reduce the hydrogen content in the steel sheet is necessary.

[0003] For example, product coils manufactured using continuous annealing and continuous hot-dip galvanizing units can have their hydrogen content reduced by leaving them at room temperature. However, the movement of hydrogen from the interior to the surface of the steel sheet at room temperature and its subsequent detachment from the surface takes time; therefore, sufficient reduction of hydrogen content in the steel requires several weeks or more of placement. Consequently, the space and time required for such dehydrogenation treatment present challenges in the manufacturing process.

[0004] In addition, Patent Document 1 discloses a method for reducing the hydrogen content in steel by holding annealed steel sheets, hot-dip galvanized steel sheets, or alloyed hot-dip galvanized steel sheets in a temperature range of 50°C to 300°C for 1800 seconds to 43200 seconds.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: International Publication No. 2019 / 188642 Summary of the Invention

[0008] However, Patent Document 1 expresses concerns about changes in mechanical properties such as increased yield strength and temper embrittlement caused by heating.

[0009] Therefore, in view of the above-mentioned problems, the present invention aims to provide a continuous annealing apparatus and a continuous hot-dip galvanizing apparatus, as well as a method for manufacturing steel plates with excellent resistance to hydrogen embrittlement, which can manufacture steel plates without compromising production efficiency or changing mechanical properties.

[0010] To solve the aforementioned problems, the inventors conducted repeated and in-depth research and discovered the following phenomenon: In a continuous annealing line (CAL) or continuous hot-dip galvanizing line (CGL), after annealing a steel sheet in a reducing atmosphere containing hydrogen, continuously irradiating the steel sheet with sound waves during the cooling process from the annealing temperature to room temperature can effectively reduce the hydrogen content in the steel sheet. This is presumably due to the following mechanism: Irradiating the steel sheet with sound waves forces it to vibrate micro-vibrates, thereby subjecting the steel sheet to repeated bending deformation. As a result, the lattice spacing on the surface expands compared to the thickness of the center of the steel sheet. Hydrogen in the steel sheet diffuses towards the surface of the steel sheet, where the lattice spacing is wider and the potential energy is lower, and then escapes from that surface.

[0011] That is, the present invention was completed based on the above circumstances, and the main point is as follows.

[0012] [1] A continuous annealing apparatus, comprising:

[0013] The wire unloading reel releases cold-rolled steel sheets from the cold-rolled coil;

[0014] An annealing furnace is an annealing furnace that continuously anneals the aforementioned cold-rolled steel sheet by passing it through. Starting from the upstream side of the through plate direction, a heating zone, a soaking zone, and a cooling zone are provided. The aforementioned cold-rolled steel sheet is annealed in the aforementioned heating zone and the aforementioned soaking zone in a reducing atmosphere containing hydrogen, and the aforementioned cold-rolled steel sheet is cooled in the aforementioned cooling zone.

[0015] Downstream equipment allows the cold-rolled steel sheet discharged from the annealing furnace to pass continuously;

[0016] A tension coiler winds the aforementioned cold-rolled steel sheet through the downstream equipment described above.

[0017] The acoustic irradiation device irradiates the cold-rolled steel sheet as it passes from the cooling belt to the tension coiler with acoustic waves.

[0018] [2] According to the continuous annealing apparatus described in [1] above, the acoustic irradiation device is provided in the cooling zone.

[0019] [3] According to the continuous annealing apparatus described in [1] or [2] above, the acoustic irradiation device is provided at a position that can irradiate the cold-rolled steel sheet passing through the downstream equipment with acoustic waves.

[0020] [4] The continuous annealing apparatus according to any one of [1] to [3] above, wherein the intensity of the sound wave generated from the sound wave irradiation device and the position of the sound wave irradiation device are set such that the sound pressure level of the surface of the cold-rolled steel sheet is 30 dB or more.

[0021] [5] The continuous annealing apparatus according to any one of [1] to [4] above, wherein the acoustic irradiation device is capable of irradiating acoustic waves having a frequency of 10 to 100,000 Hz.

[0022] [6] The continuous annealing apparatus according to any one of [1] to [5] above, wherein the configuration of the sound wave irradiation device and the through speed of the cold-rolled steel sheet are set such that the irradiation time of the cold-rolled steel sheet by the sound wave is 1 second or more.

[0023] [7] A continuous hot-dip galvanizing apparatus, comprising:

[0024] The continuous annealing apparatus described above [1]; and

[0025] As the aforementioned downstream equipment, located downstream of the aforementioned annealing furnace in the direction of the through plate, it impregnates the aforementioned cold-rolled steel sheet in a hot-dip galvanizing bath to perform hot-dip galvanizing on the aforementioned cold-rolled steel sheet.

[0026] [8] According to the continuous hot-dip galvanizing apparatus described above [7], the acoustic irradiation device is provided at a position that can irradiate the cold-rolled steel sheet passing upstream of the hot-dip galvanizing bath with acoustic waves.

[0027] [9] According to the continuous hot-dip galvanizing apparatus described in [7] or [8] above, the acoustic irradiation device is provided at a position that can irradiate the cold-rolled steel sheet downstream of the hot-dip galvanizing bath with acoustic waves.

[0028]

[10] According to the continuous hot-dip galvanizing apparatus described above [7], the downstream equipment includes an alloying furnace located downstream of the hot-dip galvanizing bath in the direction of the through plate, through which the cold-rolled steel sheet passes and is heated and alloyed for hot-dip galvanizing.

[0029]

[11] According to the continuous hot-dip galvanizing apparatus described in

[10] above, the acoustic irradiation device is provided at a position that can irradiate the cold-rolled steel sheet passing upstream of the hot-dip galvanizing bath with acoustic waves.

[0030]

[12] In the continuous hot-dip galvanizing apparatus described in

[10] or

[11] above, the acoustic irradiation device is provided at a position that can irradiate the cold-rolled steel sheet passing downstream of the hot-dip galvanizing bath with acoustic waves.

[0031]

[13] The continuous hot-dip galvanizing apparatus according to any one of [7] to

[12] above, wherein the intensity of the sound wave generated from the sound wave irradiation device and the position of the sound wave irradiation device are set such that the sound pressure level of the surface of the cold-rolled steel sheet is 30 dB or more.

[0032]

[14] The continuous hot-dip galvanizing apparatus according to any one of [7] to

[13] above, wherein the acoustic irradiation device is capable of irradiating acoustic waves with a frequency of 10 to 100,000 Hz.

[0033]

[15] The continuous hot-dip galvanizing apparatus according to any one of [7] to

[14] above, wherein the configuration of the sound wave irradiation device and the through speed of the cold-rolled steel sheet are set such that the irradiation time of the cold-rolled steel sheet by the sound wave is 1 second or more.

[0034]

[16] A method for manufacturing a steel plate, comprising the following steps in sequence:

[0035] (A) The process of releasing cold-rolled steel sheets from cold-rolled coils using a pay-off reel;

[0036] (B) In an annealing furnace provided with a heating zone, a soaking zone and a cooling zone on the upstream side of the through plate, the above-mentioned cold-rolled steel sheet is passed through and the following continuous annealing process is carried out, namely, (B-1) the above-mentioned cold-rolled steel sheet is annealed in the above-mentioned heating zone and the above-mentioned soaking zone in a reducing atmosphere containing hydrogen, and (B-2) the above-mentioned cold-rolled steel sheet is cooled in the above-mentioned cooling zone.

[0037] (C) The process of continuing to pass the above-mentioned cold-rolled steel sheet discharged from the above-mentioned annealing furnace through;

[0038] (D) The process of using a tension coiler to wind the above-mentioned cold-rolled steel sheet to produce product coils.

[0039] A sound wave irradiation process in which the cold-rolled steel sheet in the through plate is irradiated with sound waves in such a way that the sound pressure level on the surface of the cold-rolled steel sheet is 30 dB or more. This process is performed after process (B-2) and before process (D).

[0040]

[17] In the steel plate manufacturing method described in

[16] above, the above-mentioned acoustic irradiation process is carried out in process (B-2).

[0041]

[18] In the steel plate manufacturing method described in

[16] or

[17] above, the above-mentioned acoustic irradiation process is carried out in process (C).

[0042]

[19] According to the steel plate manufacturing method described in

[16] above, step (C) includes (C-1) a step of hot-dip galvanizing the cold-rolled steel plate by immersing it in a hot-dip galvanizing bath located downstream of the annealing furnace in the plate direction.

[0043]

[20] In the steel plate manufacturing method described above

[19] , the above-mentioned acoustic irradiation process is performed before process (C-1).

[0044]

[21] In the steel plate manufacturing method described in

[19] or

[20] above, the above-mentioned acoustic irradiation process is performed after process (C-1).

[0045]

[22] According to the steel plate manufacturing method described above

[19] , the above-mentioned step (C) after the above-mentioned step (C-1) includes (C-2) a step of heating and alloying the above-mentioned hot-dip galvanizing by passing the above-mentioned cold-rolled steel plate through an alloying furnace located downstream of the hot-dip galvanizing bath in the through-plate direction.

[0046]

[23] In the steel plate manufacturing method described above

[22] , the above-mentioned acoustic irradiation process is performed before process (C-1).

[0047]

[24] In the steel plate manufacturing method described in

[22] or

[23] above, the above-mentioned acoustic irradiation process is performed after process (C-1).

[0048]

[25] The method for manufacturing a steel plate according to any one of

[16] to

[24] above, wherein the sound wave has a frequency of 10 to 100,000 Hz.

[0049]

[26] The method for manufacturing a steel plate according to any one of

[16] to

[25] above, wherein, in the above-mentioned acoustic irradiation process, the irradiation time of the above-mentioned cold-rolled steel plate by the acoustic wave is set to 1 second or more.

[0050]

[27] The method for manufacturing steel plate according to any one of

[16] to

[26] above, wherein the cold-rolled steel plate is a high-strength steel plate having a tensile strength of 590 MPa or more.

[0051]

[28] The method for manufacturing the steel plate according to any one of

[16] to

[27] above, wherein the cold-rolled steel plate has the following composition, containing, by mass %: C: 0.030 to 0.800%, Si: 0.01 to 3.00%, Mn: 0.01 to 10.00%, P: 0.001 to 0.100%, S: 0.0001 to 0.0200%, N: 0.0005 to 0.0100%, and Al: 0.001 to 2.000%, with the remainder consisting of Fe and unavoidable impurities.

[0052]

[29] According to the steel plate manufacturing method described above

[28] , wherein the above-mentioned composition further contains, by mass %, at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.500% or less, W: 0.500% or less, B: 0.0050% or less, Ni: 1.000% or less, Cr: 1.000% or less, Mo: 1.000% or less, Cu: 1.000% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ta: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.1000% or less, and REM: 0.0050%.

[0053]

[30] The method for manufacturing a steel plate according to any one of

[16] to

[26] above, wherein the cold-rolled steel plate is a stainless steel plate having the following composition, wherein the composition, in mass %, contains C: 0.001 to 0.400%, Si: 0.01 to 2.00%, Mn: 0.01 to 5.00%, P: 0.001 to 0.100%, S: 0.0001 to 0.0200%, Cr: 9.0 to 28.0%, Ni: 0.01 to 40.0%, N: 0.0005 to 0.500%, and Al: 0.001 to 3.000%, with the remainder consisting of Fe and unavoidable impurities.

[0054]

[31] According to the steel plate manufacturing method described in

[30] above, the above-mentioned composition further contains, by mass %, an element selected from at least one of Ti: 0.500% or less, Nb: 0.500% or less, V: 0.500% or less, W: 2.000% or less, B: 0.0050% or less, Mo: 2.000% or less, Cu: 3.000% or less, Sn: 0.500% or less, Sb: 0.200% or less, Ta: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.1000% or less, and REM: 0.0050% or less.

[0055]

[32] The method for manufacturing steel plate according to any one of

[16] to

[31] above, wherein the product coil has a diffusible hydrogen content of 0.50 ppm by mass or less.

[0056] According to the continuous annealing apparatus and continuous hot-dip galvanizing apparatus of the present invention, as well as the method for manufacturing steel plates, steel plates with excellent resistance to hydrogen embrittlement can be manufactured without compromising production efficiency or altering mechanical properties. Attached Figure Description

[0057] Figure 1This is a schematic diagram of a continuous annealing apparatus 100 according to one embodiment of the present invention.

[0058] Figure 2 This is a schematic diagram of a continuous hot-dip galvanizing apparatus 200 according to one embodiment of the present invention.

[0059] Figure 3 This is a schematic diagram of a continuous hot-dip galvanizing apparatus 300 according to another embodiment of the present invention.

[0060] Figure 4 This is a schematic diagram illustrating the configuration of the acoustic irradiation device 60 used in various embodiments of the present invention.

[0061] Figure 5 The diagrams schematically illustrate the positional relationship between the cold-rolled steel sheet S in the through plate and the horn 68 of the sound wave irradiation device in various embodiments of the present invention. (A) is a side view of the first example, (B) is a top view of the first example, and (C) is a top view of the second example.

[0062] Figure 6 (A) to (H) are schematic diagrams showing an example of the positional relationship between the cooling nozzle 26A and the acoustic irradiation device 60 when the acoustic irradiation device 60 is installed in the cooling zone 26. Detailed Implementation

[0063] One embodiment of the present invention relates to a continuous annealing line (CAL), and another embodiment of the present invention relates to a continuous hot-dip galvanizing line (CGL).

[0064] The steel plate manufacturing method of one embodiment of the present invention is carried out by a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL).

[0065] Reference Figure 1The first embodiment of the continuous annealing apparatus (CAL) 100 of the present invention includes a pay-off reel 10 for releasing cold-rolled steel sheet S from cold-rolled coil C; an annealing furnace 20 for continuously annealing the cold-rolled steel sheet S by passing it through; a downstream device 30 for continuously passing the cold-rolled steel sheet S discharged from the annealing furnace 20; and a tension coiler 50 for winding the cold-rolled steel sheet S passing through the downstream device 30 to form a product coil P. In the annealing furnace 20, a heating zone 22, a soaking zone 24, and a cooling zone 26 are provided upstream in the plate-passing direction. The cold-rolled steel sheet S is annealed in the heating zone 22 and the soaking zone 24 in a reducing atmosphere containing hydrogen, and the cold-rolled steel sheet S is cooled in the cooling zone 26. It should be noted that it is preferable that the annealing furnace 20 of the CAL 100 has an over-aging treatment zone 28 downstream of the cooling zone 26, but it is not necessary. The cold-rolled steel sheet S is over-aged in the over-aging treatment zone 28. In this embodiment, CAL100 is used to manufacture cold-rolled annealed steel sheet (CR) product coils.

[0066] Reference Figure 1 The steel sheet manufacturing method of the first embodiment implemented by the continuous annealing apparatus (CAL) 100 includes the following steps: (A) releasing cold-rolled steel sheet (strip) S from cold-rolled coil C using a pay-off reel 10; (B) performing continuous annealing: passing the cold-rolled steel sheet S through an annealing furnace 20 which is provided with a heating zone 22, a soaking zone 24 and a cooling zone 26 on the upstream side of the through plate direction; (B-1) annealing the cold-rolled steel sheet S in a reducing atmosphere containing hydrogen in the heating zone 22 and the soaking zone 24; (B-2) cooling the cold-rolled steel sheet S in the cooling zone 26; (C) continuously passing the cold-rolled steel sheet S discharged from the annealing furnace 20; and (D) winding the cold-rolled steel sheet S by a tension coiler 50 to form a product coil P. It should be noted that in the continuous annealing process (B) of the CAL100-based annealing furnace 20, it is preferable (B-3) to perform an over-aging treatment on the cold-rolled steel sheet S in an over-aging treatment zone 28 arbitrarily located downstream of the cooling zone 26; this process is not essential. This embodiment is a method for manufacturing product coils of cold-rolled annealed steel sheet (CR) using CAL100.

[0067] Reference Figure 2The second embodiment of the continuous hot-dip galvanizing apparatus (CGL) 200 of the present invention includes a grating reel 10 for releasing cold-rolled steel sheet S from cold-rolled coil C; an annealing furnace 20 for continuously annealing the cold-rolled steel sheet S by passing it through; a downstream device 30 for continuously passing the cold-rolled steel sheet S discharged from the annealing furnace 20; and a tension coiler 50 for winding the cold-rolled steel sheet S passing through the downstream device 30 to form a product coil P. In the annealing furnace 20, a heating zone 22, a soaking zone 24, and a cooling zone 26 are provided on the upstream side in the direction of the plate passage. The cold-rolled steel sheet S is annealed in the heating zone 22 and the soaking zone 24 in a reducing atmosphere containing hydrogen, and the cold-rolled steel sheet S is cooled in the cooling zone 26. Furthermore, the downstream device 30 in the CGL200 includes a hot-dip galvanizing bath 31 located downstream of the annealing furnace 20 in the plate-passing direction, for impregnating cold-rolled steel sheets S and subjecting the cold-rolled steel sheets S to hot-dip galvanizing; and an alloying furnace 33 located downstream of the hot-dip galvanizing bath 31 in the plate-passing direction, for heating and alloying the cold-rolled steel sheets S by passing them through the hot-dip galvanizing bath. In this embodiment, the CGL200 is used to manufacture product coils of alloyed hot-dip galvanized steel sheets (GA) with an alloyed galvanized layer. It should be noted that, in the case where the steel sheet S is simply passed through the alloying furnace 33 without heating and alloying, product coils of hot-dip galvanized steel sheets (GI) with an unalloyed galvanized layer are manufactured.

[0068] Reference Figure 2 The steel sheet manufacturing method of the second embodiment, implemented using a continuous hot-dip galvanizing apparatus (CGL) 200, sequentially includes the following steps: (A) releasing cold-rolled steel sheet (strip) S from cold-rolled coil C using a pay-off reel 10; (B) passing the cold-rolled steel sheet S through an annealing furnace 20, which is provided with a heating zone 22, a soaking zone 24, and a cooling zone 26 on the upstream side of the through-plate direction, and performing continuous annealing as follows: (B-1) annealing the cold-rolled steel sheet S in a reducing atmosphere containing hydrogen in the heating zone 22 and the soaking zone 24; (B-2) cooling the cold-rolled steel sheet S in the cooling zone 26; (C) continuously passing the cold-rolled steel sheet S discharged from the annealing furnace 20; and (D) winding the cold-rolled steel sheet S using a tension coiler 50 to form a product coil P. However, process (C) includes (C-1) immersing the cold-rolled steel sheet S in a hot-dip galvanizing bath 31 located downstream of the annealing furnace 20 in the plate-passing direction, thereby performing hot-dip galvanizing on the cold-rolled steel sheet S; followed by (C-2) passing the cold-rolled steel sheet S through an alloying furnace 33 located downstream of the hot-dip galvanizing bath 31 in the plate-passing direction, thereby performing a heating alloying process for the hot-dip galvanized steel sheet. This embodiment is a method for manufacturing alloyed hot-dip galvanized steel sheet (GA) product coils with an alloyed galvanized layer using CGL200.

[0069] Reference Figure 3The continuous hot-dip galvanizing apparatus (CGL) 300 of the third embodiment of the present invention does not have an alloying furnace 33, but has the same configuration as CGL 200. In this embodiment, CGL 300 is used to manufacture product coils of hot-dip galvanized steel sheet (GI) with an unalloyed galvanized layer.

[0070] That is, the steel sheet manufacturing method of the third embodiment, which involves performing process (C-1) and not performing process (C-2), can be implemented, for example, using a CGL300 without an alloying furnace 33. Alternatively, it can be implemented by simply passing the steel sheet S through the alloying furnace 33 of a CGL200 without heating for alloying. This embodiment is a method for manufacturing hot-dip galvanized steel sheet (GI) product coils with an unalloyed galvanized layer using a CGL200 or CGL300.

[0071] The components of CAL in the first embodiment and CGL in the second and third embodiments described above will be explained in detail. Furthermore, each step of the steel plate manufacturing method of the first, second, and third embodiments will be explained in detail.

[0072] [Pay-out reel and equipment from the pay-out reel to the annealing furnace]

[0073] [Process (A)]

[0074] Reference Figures 1-3 The pay-off reel 10 releases the cold-rolled steel sheet S from the cold-rolled coil C. That is, in process (A), the pay-off reel 10 releases the cold-rolled steel sheet S from the cold-rolled coil C. The released cold-rolled steel sheet S is supplied to the annealing furnace 20 via the welding machine 11, the cleaning equipment 12, and the inlet looper 13. The upstream equipment between the pay-off reel 10 and the annealing furnace 20 is not limited to the welding machine 11, the cleaning equipment 12, and the inlet looper 13, and can be any known or arbitrary device.

[0075] Annealing furnace

[0076] [Process (B)]

[0077] Reference Figures 1-3 The annealing furnace 20 continuously anneales the cold-rolled steel sheet S by passing it through its interior. The annealing furnace 20 has a heating zone 22, a soaking zone 24, and a cooling zone 26 arranged upstream of the through-plate direction. The cold-rolled steel sheet S is annealed in the heating zone 22 and soaking zone 24 in a reducing atmosphere containing hydrogen, and cooled in the cooling zone 26. That is, in process (B), the cold-rolled steel sheet S is continuously annealed by passing it through the annealing furnace 20, which has a heating zone 22, a soaking zone 24, and a cooling zone 26 arranged upstream of the through-plate direction. The cooling zone 26 can be composed of multiple cooling zones. Additionally, a preheating zone can be provided upstream of the heating zone 22 in the through-plate direction. Furthermore, Figure 1The annealing furnace 20 of the CAL100 shown preferably has an over-aging treatment zone 28 downstream of the cooling zone 26, but this is not necessary. Figures 1-3 In the illustration, each strip is shown as a vertical furnace, but it is not limited to this and can be a horizontal furnace. In the case of a vertical furnace, adjacent strips are connected by a throat (throttle valve) that connects the upper or lower parts of each strip to each other.

[0078] (Heating belt)

[0079] In the heating zone 22, the cold-rolled steel sheet S can be directly heated using a burner, or indirectly heated using a radiant tube (RT) or an electric heater. Alternatively, heating can be achieved through induction heating, roll heating, resistance heating, direct electric heating, salt bath heating, electron beam heating, etc. The average internal temperature of the heating zone 22 is preferably 500–800°C. Gas from the heat exchange zone 24 flows into the heating zone 22, and a reducing gas is also supplied. As the reducing gas, a mixture of H2 and N2 is typically used; for example, a gas with H2: 1–35% by volume, the remainder consisting of one or both of N2 and Ar, and unavoidable impurities (dew point: approximately -60°C) can be used.

[0080] (All tropical)

[0081] The cold-rolled steel sheet S can be indirectly heated in the heat exchanger 24 using a radiant heating tube (RT). The average internal temperature of the heat exchanger 24 is preferably 600–950°C. A reducing gas is supplied to the heat exchanger 24. As the reducing gas, a mixture of H2 and N2 is typically used, for example, a gas having an H2 content of 1–35% by volume, with the remainder consisting of one or both of N2 and Ar, as well as unavoidable impurities (dew point: around -60°C).

[0082] (Cooling zone)

[0083] In cooling zone 26, the cold-rolled steel sheet S is cooled using gas, a mixture of gas and water, or any one of the following: gas or water. As the cold-rolled steel sheet S exits the annealing furnace 20, it is cooled to approximately 100–400°C in CAL and to approximately 470–530°C in CGL. For example... Figure 6As shown in (A) to (H), a plurality of cooling nozzles 26A are provided along the steel plate conveying path in the cooling zone 26. The cooling nozzles 26A are, for example, circular tubes longer than the width of the steel plate, as described in Japanese Patent Application Publication No. 2010-185101, with the extension direction of the circular tube parallel to the width direction of the steel plate. At the portion of the circular tube opposite the steel plate, a plurality of through holes are provided at predetermined intervals along the extension direction of the circular tube, through which water inside the circular tube is sprayed toward the steel plate. The cooling nozzles are arranged in pairs facing each other on the surface and back of the steel plate, and multiple pairs (e.g., 5 to 10 pairs) of cooling nozzles are arranged at predetermined intervals along the steel plate conveying path, forming a cooling zone. Preferably, about 3 to 6 cooling zones are arranged along the steel plate conveying path.

[0084] (Expired processing zone)

[0085] Reference Figure 1 In CAL100, the cold-rolled steel sheet S leaving the cooling zone 26 in the over-aging treatment zone 28 is fed to at least one of the following treatments: isothermal holding, reheating, furnace cooling, and decooling. The cold-rolled steel sheet S is cooled to about 100 to 400°C in the stage of leaving the annealing furnace 20.

[0086] Downstream equipment

[0087] [Process (C)]

[0088] Reference Figures 1-3 In process (C), the cold-rolled steel sheet S discharged from the annealing furnace 20 is continuously passed through the downstream equipment 30. (Refer to...) Figure 1 CAL100, as downstream equipment 30, has an export looper 35 and a leveling mill 36. (Refer to...) Figure 2 The CGL200, as downstream equipment 30, includes a hot-dip galvanizing bath 31, a gas wiping device 32, an alloying furnace 33, a cooling device 34, an outlet looper 35, and a leveling mill 36. (Refer to...) Figure 3 The CGL300, as downstream equipment 30, includes a hot-dip galvanizing bath 31, a gas wiping device 32, a cooling device 34, an outlet looper 35, and a leveling mill 36. However, downstream equipment 30 is not limited to these and can be any known or arbitrary device. For example, downstream equipment 30 can include tension regulators, chemical forming equipment, surface conditioning equipment, oiling equipment, and inspection equipment.

[0089] (Hot-dip galvanizing bath)

[0090] (Process (C-1))

[0091] Reference Figure 2 , 3The hot-dip galvanizing bath 31 is located downstream of the annealing furnace 20 in the direction of the through plate, and the cold-rolled steel sheet S is immersed in it to perform hot-dip galvanizing. That is, in process (C-1), the cold-rolled steel sheet S is immersed in the hot-dip galvanizing bath 31 located downstream of the annealing furnace 20 in the direction of the through plate, and the cold-rolled steel sheet S is hot-dip galvanized. This is related to the downstreammost strip of the annealing furnace (…). Figure 2 , 3 The nozzle 29 connected to the intermediate cooling zone 26 is a rectangular component with a cross-section perpendicular to the direction of the cold-rolled steel sheet S, which divides the space for the cold-rolled steel sheet S to pass through. Its front end is immersed in the hot-dip galvanizing bath 31. Therefore, the annealing furnace 20 is connected to the hot-dip galvanizing bath 31. Hot-dip galvanizing can be carried out according to conventional methods.

[0092] Gas can be sprayed onto the cold-rolled steel sheet S from a pair of gas wiping devices 32 that are clamped between the cold-rolled steel sheet S pulled out of the hot-dip galvanizing bath 31, thereby adjusting the amount of molten zinc adhering to both sides of the cold-rolled steel sheet S.

[0093] (Alloying furnace)

[0094] (Process (C-2))

[0095] Reference Figure 2 The alloying furnace 33 is located downstream of the hot-dip galvanizing bath 31 and the gas wiping device 32 in the through-plate direction, allowing the cold-rolled steel sheet S to pass through and undergo heating and alloying for hot-dip galvanizing. That is, in process (C-2), the cold-rolled steel sheet S is passed through the alloying furnace 33 located downstream of the hot-dip galvanizing bath 31 and the gas wiping device 32 in the through-plate direction, undergoing heating and alloying for hot-dip galvanizing. The alloying treatment can be performed using conventional methods. The heating method in the alloying furnace 33 is not particularly limited; examples include heating with high-temperature gas and induction heating. The alloying furnace 33 is any equipment in the CGL, and the alloying process is any process in the manufacturing method of the steel sheet using the CGL.

[0096] (Cooling device)

[0097] Reference Figure 2 , 3 The cooling device 34 is located downstream of the gas wiping device 32 and the alloying furnace 33 in the direction of the through plate. It allows the cold-rolled steel sheet S to be cooled by passing through the cooling device 34. The cooling device 34 cools the cold-rolled steel sheet S through water cooling, air cooling, gas cooling, mist cooling, etc.

[0098] Tension winding machine

[0099] [Process (D)]

[0100] Reference Figures 1-3 The cold-rolled steel sheet S, processed by downstream equipment 30, is finally wound into product coil P by tension winding machine 50, which serves as a winding device.

[0101] [Sound irradiation device and sound irradiation process]

[0102] It is important that the CAL100 of the first embodiment, the CGL200 of the second embodiment, and the CGL300 of the third embodiment have an acoustic irradiation device 60 for irradiating the cold-rolled steel sheet S passing from the cooling strip 26 to the tension coiler 50. That is, it is important that the steel sheet manufacturing methods of the first, second, and third embodiments include an acoustic irradiation process for irradiating the cold-rolled steel sheet S in the through sheet after process (B-2) and before process (D). This allows for a sufficient and efficient reduction of hydrogen contained in the cold-rolled steel sheet S during annealing, enabling the manufacture of steel sheets with excellent resistance to hydrogen embrittlement. Furthermore, since acoustic irradiation is installed in the steel sheet manufacturing process (production line) of CAL100, CGL200, or CGL300, production efficiency is not compromised. Additionally, since hydrogen is removed by acoustic irradiation rather than heating, there is no concern about altering the mechanical properties of the steel sheet.

[0103] Various embodiments of the present invention can be achieved by using... Figure 4 The typical acoustic irradiation device 60 shown is implemented in CAL100, CGL200, or CGL300. The acoustic irradiation process is performed by irradiating sound waves onto the cold-rolled steel sheet S in the through-plate from the acoustic irradiation device 60. The acoustic irradiation device 60 includes a controller 61, an acoustic oscillator 62, a vibration transducer (loudspeaker) 64, an amplifier 66, a horn 68, and a sound level meter 69. The acoustic oscillator 62 converts a general frequency (e.g., 50Hz, 60Hz) electrical signal into an electrical signal of the desired frequency and transmits it to the vibration transducer 64. It should be noted that the voltage is typically AC200-240V, but it is amplified to nearly 1000V inside the acoustic oscillator 62. The electrical signal of the desired frequency transmitted from the acoustic oscillator 62 is converted into mechanical vibration energy by a piezoelectric element located inside the vibration transducer 64, and this mechanical vibration energy is transmitted to the amplifier 66. Amplifier 66 amplifies (or converts) the amplitude of the vibrational energy transmitted from vibrating transducer 64 to an optimal amplitude before transmitting it to horn 68. Horn 68 is a component that directs the vibrational energy transmitted from amplifier 66, allowing it to propagate as a directional sound wave through the air. Sound level meter 69 uses frequency-weighted characteristic C to measure the sound pressure level of the sound wave emitted from horn 68. Controller 61 compares the output value of sound level meter 69 with a set value, performs PID calculations on the deviation, and determines the current values ​​of vibrating transducer 64 and amplifier 66 to provide command values ​​to sound wave oscillator 62 in a manner that yields a specified frequency and sound pressure level.

[0104] As an example, from the viewpoint of irradiating directional sound waves onto cold-rolled steel sheet S, the horn 68 can be a cylindrical component. Furthermore, as... Figure 5 As shown in (A) and (B), a plurality of horns 68 of sound wave irradiation devices 60 are arranged along the width direction of the cold-rolled steel sheet S in the through plate at predetermined intervals. By irradiating sound waves from the horns 68 of each sound wave irradiation device 60 toward the main surface of the cold-rolled steel sheet S in the through plate, sound waves can be uniformly irradiated in the width direction of the main surface. Figure 5 As shown in (A), the preferred direction of propagation of the sound wave is along the thickness direction of the cold-rolled steel sheet S. Additionally, as... Figure 5 As shown in (B), by arranging a group of multiple devices consisting of multiple acoustic irradiation devices 60 arranged along the width direction of the steel plate along the plate direction, it is possible to ensure that the surface of the cold-rolled steel plate S is exposed to acoustic waves for a sufficient period of time.

[0105] As another example, such as Figure 5 As shown in (C), from the viewpoint of uniformly irradiating directional sound waves along the width direction of the cold-rolled steel sheet S, the horn 68 can be a component with a rectangular opening having a long side direction aligned with the width direction of the cold-rolled steel sheet S. Furthermore, the horn 68 of the sound wave irradiation device 60 is provided with the opening facing the main surface of the cold-rolled steel sheet S in the through-plate at a predetermined interval. By irradiating sound waves from the horn 68 of the sound wave irradiation device 60 toward the main surface of the cold-rolled steel sheet S in the through-plate, sound waves can be uniformly irradiated along the width direction of the main surface. Preferably, the main direction of sound wave propagation is along the thickness direction of the cold-rolled steel sheet S. Additionally, as... Figure 5 As shown in (C), by arranging multiple acoustic irradiation devices 60 along the through plate direction, it is possible to ensure that the surface of the cold-rolled steel sheet S is exposed to acoustic waves for a sufficient period of time.

[0106] In the first, second, and third embodiments, the position of the acoustic irradiation device 60 is not limited as long as it can irradiate the cold-rolled steel sheet S passing from the cooling belt 26 to the tension coiler 50 with acoustic waves.

[0107] Reference Figure 1 This section describes the preferred location of the acoustic irradiation device 60, i.e., the preferred timing of the acoustic irradiation process, in the first embodiment of manufacturing cold-rolled annealed steel sheet (CR) coils using CAL100. As an example, the acoustic irradiation device 60 can be placed in the cooling zone 26. In this case, the acoustic irradiation process can be performed in process (B-2). Specifically, the acoustic irradiation device 60 can be placed between multiple cooling zones arranged along the steel sheet conveying path, and between adjacent cooling nozzles along the steel sheet conveying path in each cooling zone. Figure 5 The device group shown in (A) and (B) consists of multiple acoustic irradiation devices 60 arranged along the width direction of the steel plate. Figure 5 The acoustic irradiation device 60 shown in (C) is an example of this. Figure 6 (A) to (H) show examples of the positional relationship between the cooling nozzle 26A and the acoustic irradiation device 60 when the acoustic irradiation device 60 is installed inside the cooling zone 26. It should be noted that the acoustic irradiation device 60 does not necessarily need to be located inside the cooling zone 26 as a whole, at least the horn 68 needs to be located inside the cooling zone 26.

[0108] As another example, the acoustic irradiation device 60 can be positioned to irradiate the cold-rolled steel sheet S passing through the downstream equipment 30 with acoustic waves. In this case, the acoustic irradiation process can be carried out in process (C). Specifically, the acoustic irradiation device 60 can be installed at least one of the following locations: (i) between the aging treatment strip 28 and the exit looper 35, (ii) within the exit looper 35, (iii) between the exit looper 35 and the leveling mill 36, and (iv) between the leveling mill 36 and the tension coiler 50.

[0109] The acoustic irradiation device 60 can be installed in both the cooling zone 26 and the location where acoustic waves can be irradiated onto the cold-rolled steel sheet S passing through the downstream equipment 30. That is, the acoustic irradiation process can be performed using both process (B-2) and process (C). Alternatively, the acoustic irradiation device 60 can be installed in the over-aging treatment zone 28, where the acoustic irradiation process is performed during the over-aging treatment.

[0110] Next, refer to Figure 2 The preferred location of the acoustic irradiation device 60, i.e., the preferred timing of the acoustic irradiation process, will be explained in the second embodiment of the product manufactured from CGL200 alloyed hot-dip galvanized steel sheet (GA). As an example, the acoustic irradiation device 60 can be positioned at a first location capable of irradiating the cold-rolled steel sheet S upstream of the hot-dip galvanizing bath 31 with acoustic waves. In this case, the acoustic irradiation process can be performed before process (C-1). Specifically, the acoustic irradiation device 60 can be positioned in the cooling zone 26. More specifically, it can be positioned between multiple cooling zones arranged along the steel sheet conveying path, and between cooling nozzles adjacent to each cooling zone along the steel sheet conveying path. Figure 5 The device group shown in (A) and (B) consists of multiple acoustic irradiation devices 60 arranged along the width direction of the steel plate. Figure 5 The acoustic irradiation device 60 shown in (C) is also applicable in this embodiment. Figure 6 Examples are shown in (A) to (H). Furthermore, it is not necessary for the entire acoustic irradiation device 60 to be located inside the cooling zone 26; at least the horn 68 only needs to be located inside the cooling zone 26. Alternatively, at least the horn 68 of the acoustic irradiation device 60 may be located inside the nozzle 29.

[0111] As another example, the acoustic irradiation device 60 can be positioned at a second location capable of irradiating the cold-rolled steel sheet S downstream of the hot-dip galvanizing bath 31 with acoustic waves. In this case, the acoustic irradiation process can be performed after process (C-1). Specifically, the acoustic irradiation device 60 can be installed in at least one of the following: (i) between the hot-dip galvanizing bath 31 and the gas wiping device 32; (ii) between the gas wiping device 32 and the alloying furnace 33; (iii) inside the alloying furnace 33; (iv) in the air cooling zone between the alloying furnace 33 and the cooling device 34; (v) between the cooling device 34 and the outlet looper 35; (vi) inside the outlet looper 35; (vii) between the outlet looper 35 and the leveling mill 36; and (viii) between the leveling mill 36 and the tension coiler 50. In particular, it is preferred to install the acoustic irradiation device 60 in the air cooling zone of (iv).

[0112] From the viewpoint of facilitating more complete removal of hydrogen from the steel plate, the acoustic irradiation device 60 is preferably located in the first position rather than the second position. That is, the acoustic irradiation process is preferably performed before process (C-1) rather than after process (C-1). However, the acoustic irradiation device 60 can be located in both the first and second positions. That is, the acoustic irradiation process can be performed before or after process (C-1).

[0113] Next, refer to Figure 3 The preferred location of the acoustic irradiation device 60, i.e., the preferred timing of the acoustic irradiation process, will be explained in the third embodiment of manufacturing products using CGL300 hot-dip galvanized steel sheet (GI). As an example, the acoustic irradiation device 60 can be positioned at a first location capable of irradiating the cold-rolled steel sheet S, which passes upstream of the hot-dip galvanizing bath 31, with acoustic waves. In this case, the acoustic irradiation process can be performed before process (C-1). Specifically, the acoustic irradiation device 60 can be positioned in the cooling zone 26. More specifically, it can be positioned between multiple cooling zones arranged along the steel sheet conveying path, and between cooling nozzles adjacent to each cooling zone along the steel sheet conveying path. Figure 5 The device group shown in (A) and (B) consists of multiple acoustic irradiation devices 60 arranged along the width direction of the steel plate. Figure 5 The acoustic irradiation device 60 shown in (C) is also applicable in this embodiment. Figure 6 Examples are shown in (A) to (H). Furthermore, it is not necessary for the entire acoustic irradiation device 60 to be located inside the cooling zone 26; at least the horn 68 only needs to be located inside the cooling zone 26. Alternatively, at least the horn 68 of the acoustic irradiation device 60 may be located inside the nozzle 29.

[0114] As another example, the acoustic irradiation device 60 can be positioned at a second location capable of irradiating the cold-rolled steel sheet S downstream of the hot-dip galvanizing bath 31 with acoustic waves. In this case, the acoustic irradiation process can be performed after process (C-1). Specifically, the acoustic irradiation device 60 can be installed in at least one of the following: (i) between the hot-dip galvanizing bath 31 and the gas wiping device 32; (ii) between the gas wiping device 32 and the cooling device 34; (iii) between the cooling device 34 and the outlet looper 35; (iv) inside the outlet looper 35; (v) between the outlet looper 35 and the leveling mill 36; and (vi) between the leveling mill 36 and the tension coiler 50. In particular, it is preferred to install the acoustic irradiation device 60 in the air cooling zone of (ii).

[0115] From the viewpoint of ensuring more complete removal of hydrogen from the steel plate, the acoustic irradiation device 60 is preferably located in the first position rather than the second position. That is, the acoustic irradiation process is preferably performed before process (C-1) rather than after process (C-1). However, the acoustic irradiation device 60 can be located in either the first or the second position. In other words, the acoustic irradiation process can be performed both before and after process (C-1).

[0116] (Sound pressure level)

[0117] To reliably apply vibration to the cold-rolled steel sheet S and promote hydrogen diffusion, it is important that the sound pressure level on the surface of the cold-rolled steel sheet S is 30 dB or more, preferably 60 dB or more, and more preferably 80 dB or more during the acoustic irradiation process. On the other hand, considering the performance of a typical acoustic irradiation device, it is preferable that the sound pressure level on the surface of the cold-rolled steel sheet S is 150 dB or less, more preferably 140 dB or less during the acoustic irradiation process. The sound pressure level on the surface of the cold-rolled steel sheet S can be adjusted by adjusting the intensity of the sound waves generated from the acoustic irradiation device 60 and the position of the acoustic irradiation device 60 (i.e., the distance between the acoustic irradiation device 60 and the cold-rolled steel sheet S). The "sound pressure level on the surface of the cold-rolled steel sheet S" can be measured on the production line by installing a sound pressure meter near the surface of the cold-rolled steel sheet S in the through-plate and directly below the acoustic irradiation device 60. Alternatively, if the intensity I of the sound wave generated by the sound wave irradiation device 60 and the distance D between the sound wave irradiation device 60 and the cold-rolled steel sheet S are determined, the "sound pressure level of the surface of the cold-rolled steel sheet S" can also be determined offline. That is, by setting a sound pressure meter at a distance D from the offline sound wave generating device that generates the sound wave of intensity I in the main direction of sound wave propagation, the "sound pressure level of the surface of the cold-rolled steel sheet S" can be determined.

[0118] (Frequency of sound waves)

[0119] From the viewpoint that vibration is not hindered by the rigidity of the cold-rolled steel sheet S, thus further promoting hydrogen diffusion, the frequency of the sound wave irradiating the cold-rolled steel sheet S is preferably 10 Hz or higher, more preferably 100 Hz or higher, even more preferably 500 Hz or higher, and most preferably 1000 Hz or higher. On the other hand, from the viewpoint that sufficient vibration is applied to the cold-rolled steel sheet S to promote hydrogen diffusion by suppressing the attenuation of sound waves in air, the frequency of the sound wave irradiating the cold-rolled steel sheet S is preferably 100,000 Hz or lower, more preferably 80,000 Hz or lower, and even more preferably 50,000 Hz or lower. It should be noted that the frequency of the sound wave emitted by the sound wave irradiation device 60 can be controlled by the current applied to the vibration transducer 64.

[0120] (Sound wave irradiation time)

[0121] From the viewpoint of more sufficiently reducing hydrogen in the cold-rolled steel sheet S, the irradiation time of the cold-rolled steel sheet S by sound waves in the acoustic irradiation process is preferably 1 second or more, more preferably 5 seconds or more, and even more preferably 10 seconds or more. On the other hand, from the viewpoint of not hindering productivity, the irradiation time of the cold-rolled steel sheet S by sound waves is preferably 3600 seconds or less, more preferably 1800 seconds or less, and even more preferably 900 seconds or less. In this specification, "irradiation time of the cold-rolled steel sheet S by sound waves" refers to the time that each location on the surface of the cold-rolled steel sheet S is exposed to sound waves, and when each location is exposed to sound waves from multiple acoustic irradiation devices 60, it refers to the cumulative time. The irradiation time can be determined by the passing speed of the cold-rolled steel sheet S, the position of the acoustic irradiation devices (e.g., ...), and the position of the acoustic irradiation devices (e.g., ...). Figure 5 (A) and (B) show the number of devices arranged along the width of the steel plate in the direction of the plate, which consists of multiple acoustic irradiation devices 60. Figure 5 (C) The number of the through-plate directions of the acoustic irradiation device 60 shown in the figure is adjusted.

[0122] Cold-rolled steel sheet

[0123] The cold-rolled steel sheets S supplied for CAL100, CGL200, and CGL300 in this embodiment are not particularly limited. The cold-rolled steel sheet S is preferably less than 6 mm thick, and examples include high-strength steel sheets and stainless steel sheets with a tensile strength of 590 MPa or more.

[0124] [Composition of cold-rolled steel sheet: high-strength steel sheet]

[0125] The composition of cold-rolled steel sheet S is explained when it is a high-strength steel sheet. Hereinafter, "mass %" will be abbreviated as "%".

[0126] C: 0.030~0.800%

[0127] Carbon (C) has the effect of increasing the strength of steel plates. From the viewpoint of achieving this effect, the C content is 0.030% or more, preferably 0.080% or more. However, when the C content is excessive, the steel plate becomes significantly embrittled regardless of the hydrogen content. Therefore, the C content is 0.800% or less, preferably 0.500% or less.

[0128] Si: 0.01~3.00%

[0129] Si has the effect of improving the strength of steel sheets. From the viewpoint of achieving this effect, the Si content is 0.01% or more, preferably 0.10% or more. However, if the Si content is excessive, the steel sheet will become brittle, resulting in reduced ductility, or red oxide scale will form, causing deterioration of surface properties, or the coating quality will be reduced. Therefore, the Si content is 3.00% or less, preferably 2.50% or less.

[0130] Mn: 0.01~10.00%

[0131] Mn has the effect of improving the strength of steel plates through solid solution strengthening. From the viewpoint of achieving this effect, the Mn content is 0.01% or more, preferably 0.5% or more. However, when the Mn content is excessive, Mn segregation easily leads to inhomogeneity in the steel microstructure, resulting in significant hydrogen embrittlement starting from this inhomogeneity. Therefore, the Mn content is 10.00% or less, preferably 8.00% or less.

[0132] P: 0.001~0.100%

[0133] P is an element that provides solid solution strengthening and can be added according to the desired strength. From the viewpoint of achieving this effect, the amount of P is 0.001% or more, preferably 0.003% or more. However, in cases of excessive P, weldability deteriorates, and in the case of alloying galvanizing, the alloying rate decreases, resulting in compromised galvanizing quality. Therefore, the amount of P is 0.100% or less, preferably 0.050% or less.

[0134] S: 0.0001~0.0200%

[0135] Sulfide (S) segregates at grain boundaries, causing steel to become embrittled during hot working, and also reduces local deformation capacity as a sulfide. Therefore, the amount of S is 0.0200% or less, preferably 0.0100% or less, and more preferably 0.0050% or less. On the other hand, considering the limitations of production technology, the amount of S is 0.0001% or more.

[0136] N: 0.0005~0.0100%

[0137] Nitrogen (N) is an element that degrades the aging resistance of steel. Therefore, the amount of N is 0.0100% or less, preferably 0.0070% or less. The lower the amount of N, the better. Considering the limitations of production technology, the amount of N is 0.0005% or more, preferably 0.0010% or more.

[0138] Al: 0.001~2.000%

[0139] Al acts as a deoxidizer and is effective in improving the cleanliness of steel. From the viewpoint of achieving this effect, the Al content is 0.001% or more, preferably 0.010% or more. However, if the Al content is excessive, steel sheet cracks may occur during continuous casting. Therefore, the Al content is 2.000% or less, preferably 1.200% or less.

[0140] The remainder, besides the components mentioned above, consists of Fe and unavoidable impurities. It may contain at least one element selected from the following.

[0141] Ti: below 0.200%

[0142] Ti contributes to the improvement of steel sheet strength through precipitation strengthening and fine-grain strengthening resulting from the inhibition of ferrite grain growth. Therefore, when adding Ti, the Ti content is preferably 0.005% or more, and more preferably 0.010% or more. However, when the Ti content is excessive, a large amount of carbonitrides may precipitate, reducing formability. Therefore, when adding Ti, the Ti content is set to 0.200% or less, preferably 0.100% or less.

[0143] Nb: below 0.200%, V: below 0.500%, W: below 0.500%

[0144] Nb, V, and W are effective for precipitation strengthening of steel. Therefore, when Nb, V, and W are added, the content of each element is preferably 0.005% or more, more preferably 0.010% or more. However, when the content of each element is excessive, a large amount of carbonitrides precipitates, sometimes reducing formability. Therefore, when Nb is added, the amount of Nb is 0.200% or less, preferably 0.100% or less. When V and W are added, the content of each element is 0.500% or less, preferably 0.300% or less.

[0145] B: Below 0.0050%

[0146] Boron (B) is effective in strengthening grain boundaries and increasing the strength of steel sheets. Therefore, when adding B, the amount of B is preferably 0.0003% or more. However, when the amount of B is excessive, the formability may sometimes decrease. Therefore, when adding B, the amount of B is 0.0050% or less, preferably 0.0030% or less.

[0147] Ni: below 1.000%

[0148] Ni is an element that enhances the strength of steel through solid solution strengthening. Therefore, when adding Ni, the Ni content is preferably 0.005% or more. However, if the Ni content is excessive, the area ratio of hard martensite becomes too large, leading to increased microporosity at the martensite grain boundaries during tensile testing, crack propagation, and sometimes reduced ductility. Therefore, when adding Ni, the Ni content is 1.000% or less.

[0149] Cr: less than 1.000%, Mo: less than 1.000%

[0150] Cr and Mo have a balancing effect on improving strength and formability. Therefore, when Cr and Mo are added, the content of each element is preferably 0.005% or more. However, when the content of each element is excessive, the area ratio of hard martensite becomes too large, and during tensile testing, the microporosity of the martensite grain boundaries increases, crack propagation occurs, and sometimes ductility decreases. Therefore, when Cr and Mo are added, the content of each element is 1.000% or less.

[0151] Cu: below 1.000%

[0152] Cu is an effective strengthening element for steel. Therefore, when adding Cu, the Cu content is preferably 0.005% or more. However, if the Cu content is excessive, the area fraction of hard martensite becomes too large, and during tensile testing, microporosity at the grain boundaries of tempered martensite increases, crack propagation occurs, and sometimes ductility decreases. Therefore, when adding Cu, the Cu content is 1.000% or less.

[0153] Sn: below 0.200%, Sb: below 0.200%

[0154] Sn and Sb are effective in suppressing decarburization in the surface layer of steel plates (approximately tens of μm) caused by nitriding and oxidation, thus ensuring strength and material stability. Therefore, when Sn and Sb are added, the content of each element is preferably 0.002% or more. However, excessive amounts of either element can sometimes reduce toughness. Therefore, when Sn and Sb are added, the content of each element is 0.200% or less.

[0155] Ta: below 0.100%

[0156] Like Ti and Nb, Ta forms alloy carbides and alloy carbonitrides, contributing to increased strength. Furthermore, it is believed that a portion of Ta dissolves in Nb carbides and Nb carbonitrides, forming composite precipitates such as (Nb,Ta)(C,N), which significantly suppresses precipitate coarsening and stabilizes the strength-enhancing effect of precipitation strengthening. Therefore, when Ta is added, the Ta content is preferably 0.001% or more. However, even with excessive Ta addition, the precipitate stabilization effect sometimes becomes saturated, and the alloy cost increases. Therefore, when Ta is added, the Ta content is 0.100% or less.

[0157] Ca: below 0.0050%, Mg: below 0.0050%, Zr: below 0.1000%, REM (Rare Earth Metal): below 0.0050%

[0158] Ca, Mg, Zr, and REM are effective elements for shaping sulfides into spherical forms and mitigating their negative impact on moldability. When these elements are added, the content of each element is preferably 0.0005% or more. However, excessive amounts of any of these elements can lead to an increase in inclusions and other defects, resulting in surface and internal defects. Therefore, when these elements are added, the content of each element is 0.0050% or less.

[0159] [Composition of cold-rolled steel sheet: Stainless steel sheet]

[0160] The composition of the cold-rolled steel sheet S is explained when it is stainless steel. Hereinafter, "mass %" is abbreviated as "%".

[0161] C: 0.001~0.400%

[0162] Carbon (C) is an essential element for achieving high strength in stainless steel. However, during tempering in steelmaking, it combines with chromium (Cr) and precipitates as carbides, which deteriorate the steel's corrosion resistance and toughness. If the C content is less than 0.001%, sufficient strength cannot be obtained; if it exceeds 0.400%, the aforementioned deterioration becomes significant. Therefore, the optimal C content is 0.001% to 0.400%.

[0163] Si: 0.01~2.00%

[0164] Si is a useful element as a deoxidizer. From the viewpoint of achieving this effect, the Si content should be 0.01% or higher. However, in cases of excessive Si content, the Si dissolved in the steel reduces the steel's workability. Therefore, the Si content should be 2.00% or lower.

[0165] Mn: 0.01~5.00%

[0166] Mn has the effect of increasing the strength of steel. From the viewpoint of achieving this effect, the Mn content should be 0.01% or more. However, if the Mn content is excessive, the workability of the steel decreases. Therefore, the Mn content should be 5.00% or less.

[0167] P: 0.001~0.100%

[0168] Phosphorus (P) is an element that promotes grain boundary destruction due to grain boundary segregation. Therefore, a lower P content is preferred, preferably 0.100% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. On the other hand, considering production technology constraints, a P content of 0.001% or more is preferable.

[0169] S: 0.0001~0.0200%

[0170] Sulfide (S) exists as an inclusion in sulfide systems such as MnS, reducing ductility and corrosion resistance. Therefore, a lower S content is preferred, preferably 0.0200% or less, more preferably 0.0100% or less, and even more preferably 0.0050% or less. On the other hand, considering production technology limitations, an S content of 0.0001% or more is acceptable.

[0171] Cr: 9.0–28.0%

[0172] Cr is a fundamental element in stainless steel and therefore a crucial element in demonstrating its corrosion resistance. When considering corrosion resistance in harsh environments above 180°C, insufficient corrosion resistance is achieved if the Cr content is less than 9.0%, while exceeding 28.0% results in saturation, posing an economic challenge. Therefore, the optimal Cr content is 9.0%–28.0%.

[0173] Ni: 0.01~40.0%

[0174] Ni is an element that improves the corrosion resistance of stainless steel. If the Ni content is less than 0.01%, this effect cannot be fully realized. On the other hand, excessive Ni content deteriorates formability and easily leads to stress corrosion cracking. Therefore, the Ni content should be between 0.01% and 40.0%.

[0175] N: 0.0005~0.500%

[0176] Nitrogen (N) is an element that negatively impacts the corrosion resistance of stainless steel. Therefore, the N content should be 0.500% or less, preferably 0.200% or less. Lower N content is preferred, but considering production technology constraints, an N content of 0.0005% or more is preferable.

[0177] Al: 0.001~3.000%

[0178] Al acts as a deoxidizer and also inhibits oxide scale peeling. From the viewpoint of achieving these effects, an Al content of 0.001% or higher is recommended. However, excessive Al content leads to decreased elongation and deterioration of surface quality. Therefore, an Al content of 3.000% or lower is preferred.

[0179] The remainder, besides the components mentioned above, consists of Fe and unavoidable impurities. It may contain at least one element selected from the following.

[0180] Ti: below 0.500%

[0181] Ti combines with C, N, and S to improve corrosion resistance, resistance to intergranular corrosion, and deep-drawing properties. However, when the Ti content exceeds 0.500%, the toughness deteriorates due to Ti solution treatment. Therefore, when adding Ti, the Ti content should be below 0.500%.

[0182] Nb: below 0.500%

[0183] Like Ti, Nb combines with C, N, and S to improve corrosion resistance, resistance to intergranular corrosion, and deep-drawing properties. In addition to improved machinability and high-temperature strength, it also promotes the suppression of crevice corrosion and repassivation. However, excessive addition leads to hardening and deteriorates formability. Therefore, when adding Nb, the amount should be below 0.500%.

[0184] V: Below 0.500%

[0185] V inhibits crevice corrosion. However, excessive addition degrades formability. Therefore, when adding V, the amount should be below 0.500%.

[0186] W: Below 2.000%

[0187] W contributes to improved corrosion resistance and high-temperature strength. However, excessive addition can lead to decreased toughness and increased costs during steel plate manufacturing. Therefore, when adding W, the amount should be below 2.000%.

[0188] B: Below 0.0050%

[0189] Boron (B) improves the secondary processability of products through grain boundary segregation. However, excessive addition leads to a decrease in processability and corrosion resistance. Therefore, when adding boron, the amount should be below 0.0050%.

[0190] Mo: 2.000% or less

[0191] Mo is an element that improves corrosion resistance, especially inhibiting crevice corrosion. However, excessive addition deteriorates formability. Therefore, when adding Mo, the amount should be below 2.000%.

[0192] Cu: below 3.000%

[0193] Like Ni and Mn, Cu is an austenite stabilizing element, and its enhanced phase transformation is effective in refining grain size. Furthermore, it promotes the suppression and re-passivation of interstitial corrosion. However, excessive addition deteriorates toughness and formability. Therefore, when adding Cu, the Cu content should be below 3.000%.

[0194] Sn: below 0.500%

[0195] Sn contributes to improved corrosion resistance and high-temperature strength. However, excessive addition may lead to slab cracking during steel sheet manufacturing. Therefore, when adding Sn, the amount should be below 0.500%.

[0196] Sb: below 0.200%

[0197] Sb has the effect of improving high-temperature strength by segregating at grain boundaries. However, excessive addition may cause cracks during welding due to Sb segregation. Therefore, when adding Sb, the amount should be below 0.200%.

[0198] Ta: below 0.100%

[0199] Ta combines with C and N to improve toughness. However, excessive addition saturates this effect, leading to increased manufacturing costs. Therefore, when adding Ta, the amount should be below 0.100%.

[0200] Ca: below 0.0050%, Mg: below 0.0050%, Zr: below 0.1000%, REM (Rare Earth Metal): below 0.0050%

[0201] Ca, Mg, Zr, and REM are effective elements in shaping sulfides into spherical forms and mitigating their negative impact on moldability. When these elements are added, the content of each element is preferably 0.0005% or more. However, excessive amounts of any of these elements can increase inclusions and sometimes cause surface and internal defects. Therefore, when these elements are added, the content of each element is 0.0050% or less.

[0202] [Diffusible hydrogen content]

[0203] In this embodiment, to ensure good flexibility, the diffusible hydrogen content of the product roll is preferably 0.50 ppm by mass or less, more preferably 0.30 ppm by mass or less, and even more preferably 0.20 ppm by mass or less. It should be noted that there is no particular lower limit for the diffusible hydrogen content of the product roll; considering production technology constraints, the diffusible hydrogen content of the product roll can be 0.01 ppm by mass or more.

[0204] Here, the method for determining the diffusive hydrogen content of the product coil is as follows. A test piece with a length of 30 mm and a width of 5 mm is taken from the product coil. In the case of product coils made of hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet, the hot-dip galvanized layer or alloyed hot-dip galvanized layer on the test piece is removed by grinding or alkali treatment. Then, the amount of hydrogen released from the test piece is determined by thermal desorption spectrometry (TDS). Specifically, the test piece is continuously heated from room temperature to 300°C at a heating rate of 200°C / h, then cooled to room temperature, and the cumulative amount of hydrogen released from the test piece from room temperature to 210°C is measured to obtain the diffusive hydrogen content of the product coil.

[0205] Example

[0206] (Example 1)

[0207] Steel with a composition comprising 0.21% C, 1.5% Si, 2.7% Mn, 0.02% P, 0.002% S, 0.03% Al, 0.003% N, and the remainder consisting of Fe and unavoidable impurities is smelted in a converter and produced into slabs using continuous casting. The resulting slabs are then hot-rolled and cold-rolled to obtain cold-rolled coils. As shown in Table 1, in some examples, using... Figure 1 The CAL shown is used to manufacture cold-rolled annealed steel sheet (CR) product coils. In other examples, CAL is used... Figure 2 The CGL shown is manufactured without heat alloying to produce hot-dip galvanized steel sheet (GI) coils. In the example shown, it utilizes... Figure 2 The product shown is a coil of CGL-manufactured alloyed hot-dip galvanized steel sheet (GA).

[0208] According to each level, use Figure 4 The typical acoustic irradiation device shown irradiates cold-rolled steel sheets in a through-plate with sound waves under the conditions of sound pressure level, frequency, and irradiation time shown in Table 1. The "Acoustic Irradiation Location" in Table 1 indicates the area where the CAL or CGL acoustic irradiation process is performed, i.e., the location where the acoustic irradiation device is installed.

[0209] "(B-2)" refers to the installation of a sound wave irradiation device in the cooling zone of CAL and CGL, and the use of the cooling zone of process (B-2) for sound wave irradiation.

[0210] "(C)" refers to the installation of an acoustic irradiation device in CAL at a location capable of irradiating cold-rolled steel sheets passing through downstream equipment. Specifically, the acoustic irradiation device is installed at at least one location downstream of the cooling zone and upstream of the tension coiler, namely (i) between the over-aging treatment zone 28 and the exit looper 35, (ii) inside the exit looper 35, (iii) between the exit looper 35 and the leveling mill 36, and (iv) between the leveling mill 36 and the tension coiler 50. The process (C) specifically refers to performing the acoustic irradiation process at at least one of the locations mentioned in (i) to (iv).

[0211] "(C-1) before" refers to the position in CGL that is downstream of the cooling zone and upstream of the hot-dip galvanizing bath. Specifically, an acoustic irradiation device is installed at nozzle 29, and the acoustic irradiation process is carried out after process (B-2) and before process (C-1).

[0212] "(C-1) after" refers to the position in CGL that is downstream of the hot-dip galvanizing bath and upstream of the tension coiler. Specifically, at least one of the following positions is provided: (i) between the hot-dip galvanizing bath 31 and the gas wiping device 32; (ii) between the gas wiping device 32 and the alloying furnace 33; (iii) inside the alloying furnace 33; (iv) the air cooling zone between the alloying furnace 33 and the cooling device 34; (v) between the cooling device 34 and the outlet looper 35; (vi) inside the outlet looper 35; (vii) between the outlet looper 35 and the leveling mill 36; and (viii) between the leveling mill 36 and the tension coiler 50. After process (C-1), specifically, the acoustic irradiation process is performed at at least one of the positions (i) to (viii) above.

[0213] The product rolls obtained in each example were evaluated below, and the results are shown in Table 1.

[0214] [Determination of Hydrogen Content in Steel Plates]

[0215] A method for determining the diffusive hydrogen content of a product roll material using the method described above.

[0216] [Determination of Tensile Strength (TS)]

[0217] Tensile tests were conducted according to JIS Z 2241. JIS No. 5 test pieces were taken from the obtained product coil with the long side perpendicular to the rolling direction of the steel sheet. Using this test piece, the crosshead displacement velocity was 1.67 × 10⁻⁶. -1 Tensile tests were conducted under conditions of mm / s to determine TS.

[0218] [Evaluation of tensile flange performance]

[0219] Tensile flange properties were evaluated based on a hole expansion test. The hole expansion test was conducted according to JIS Z 2256. A 100 mm × 100 mm sample was cut from the obtained product roll. A 10 mm diameter hole was punched into the sample with a gap of 12.5%. Using a film with an inner diameter of 75 mm, a tapered punch with a 60° apex angle was pressed into the hole while suppressing the area around the hole under a wrinkling pressure of 9 tons (88.26 kN) to determine the hole diameter at which crack initiation is limited. The limiting hole expansion rate λ (%) was calculated using the following formula, and the hole expansion property was evaluated based on the value of this limiting hole expansion rate.

[0220] Limiting porosity: λ(%) = ((D) f -D0) / D0)×100

[0221] In the above formula, the judgment of D is... f D0 is the pore diameter at crack initiation (mm), and D0 is the initial pore diameter (mm). When λ is greater than 20%, the tensile flange properties are excellent.

[0222] [Evaluation of flexibility]

[0223] The bending test was conducted according to JIS Z 2248. The axial direction of the bending test was determined by the direction parallel to the rolling direction of the obtained product coil and the steel plate, using a rectangular test piece with a width of 30 mm and a length of 100 mm. Then, under a compressive load of 100 kN and a holding time of 5 seconds, a bending test was performed at a bending angle of 90° using the V-block method. It should be noted that in this invention, during the 90° V-bending test, the ridge of the bending apex was observed using a 40x microscope (RH-2000: manufactured by HIROX Corporation). The minimum bending radius (R) was defined as the bending radius at which no crack with a length of 200 μm or more was visible. A value (R / t) of R divided by the plate thickness (t) of 5.0 or less was considered a good bending test result.

[0224] In this invention example, because an acoustic irradiation process is performed, the amount of hydrogen is low, resulting in a steel plate with excellent tensile flange properties (λ) and bending properties (R / t), which are indicators of resistance to hydrogen embrittlement.

[0225]

[0226] (Example 2)

[0227] Steel with the elements shown in Table 2, and the remainder consisting of Fe and unavoidable impurities, is smelted in a converter and produced into slabs using continuous casting. The resulting slabs are then hot-rolled and cold-rolled to obtain cold-rolled coils. As shown in Table 3, in some examples, using… Figure 1 The product coil shown is manufactured by CAL using cold-rolled annealed steel sheet (CR). In other examples, [the following is used]. Figure 2 The CGL shown does not undergo heat alloying to manufacture hot-dip galvanized steel sheet (GI) coils. In the other examples, [the process involves] utilizing [the following methods]. Figure 2 The product shown is a coil of CGL-manufactured alloyed hot-dip galvanized steel sheet (GA).

[0228] Based on each level, use Figure 4 The typical acoustic irradiation device shown irradiates the cold-rolled steel sheet in the through-plate with sound waves under the conditions of sound pressure level, frequency, and irradiation time shown in Table 3. The "Acoustic Irradiation Location" in Table 3 indicates the area where the acoustic irradiation process of CAL or CGL is performed, that is, the location where the acoustic irradiation device is installed.

[0229] "(B-2)" refers to the installation of a sound wave irradiation device in the cooling zone of CAL and CGL, and the use of the cooling zone of process (B-2) for sound wave irradiation.

[0230] "C" refers to the installation of an acoustic irradiation device in CAL at a location where acoustic waves can be irradiated onto cold-rolled steel sheets passing through downstream equipment. Specifically, the acoustic irradiation device is installed at at least one location downstream of the cooling zone and upstream of the tension coiler, specifically (i) between the over-aging treatment zone 28 and the exit looper 35, (ii) inside the exit looper 35, (iii) between the exit looper 35 and the leveling mill 36, and (iv) between the leveling mill 36 and the tension coiler 50. The process (C) specifically involves performing an acoustic irradiation process at at least one of the locations mentioned in (i) to (iv).

[0231] "(C-1) before" refers to the position in CGL that is downstream of the cooling zone and upstream of the hot-dip galvanizing bath. Specifically, an acoustic irradiation device is installed at nozzle 29, and the acoustic irradiation process is carried out after process (B-2) and before process (C-1).

[0232] "(C-1) after" refers to the position in CGL that is downstream of the hot-dip galvanizing bath and upstream of the tension coiler. Specifically, an acoustic irradiation device is installed at at least one of the following locations: (i) between the hot-dip galvanizing bath 31 and the gas wiping device 32; (ii) between the gas wiping device 32 and the alloying furnace 33; (iii) inside the alloying furnace 33; (iv) the air cooling zone between the alloying furnace 33 and the cooling device 34; (v) between the cooling device 34 and the outlet looper 35; (vi) inside the outlet looper 35; (vii) between the outlet looper 35 and the leveling mill 36; and (viii) between the leveling mill 36 and the tension coiler 50. The acoustic irradiation process is performed at at least one of the locations (i) to (viii) mentioned above after process (C-1).

[0233] Samples of steel sheets were taken from the product coils obtained from each example, as shown below. The tensile properties and resistance to hydrogen embrittlement were evaluated, and the results are shown in Table 3.

[0234] The tensile test was conducted using JIS 5 test pieces taken with the tensile direction perpendicular to the rolling direction of the steel plate, in accordance with JIS Z 2241 (2011), to determine TS (tensile strength) and EL (total elongation).

[0235] The hydrogen embrittlement resistance was evaluated based on the tensile test described above. A value of EL (elasticity) of the steel plate after acoustic irradiation (as measured above) divided by EL' (the hydrogen content in the same steel plate when it is 0.00 ppm by mass) was considered good if the result was 0.70 or higher. It should be noted that EL' was determined by placing the same steel plate in the atmosphere for an extended period to reduce the internal hydrogen content, confirming the hydrogen content to be 0.00 ppm by mass using TDS, and then performing a tensile test.

[0236] The diffusible hydrogen content of the product rolls obtained in each example was determined according to the method described above, and the results are shown in Table 3.

[0237] In this invention, due to the acoustic irradiation process, a steel plate with excellent resistance to hydrogen embrittlement can be manufactured.

[0238]

[0239]

[0240] Industrial availability

[0241] According to the continuous annealing apparatus and continuous hot-dip galvanizing apparatus of the present invention, as well as the method for manufacturing steel plates, steel plates with excellent resistance to hydrogen embrittlement can be manufactured without compromising production efficiency or altering mechanical properties.

[0242] Symbol Explanation

[0243] 100 Continuous Annealing Unit

[0244] 200 Continuous Hot-Dip Galvanizing Unit

[0245] 300 Continuous Hot-Dip Galvanizing Unit

[0246] 10 wire reel

[0247] 11 Welding Machine

[0248] 12 Cleaning Equipment

[0249] 13-entry loop

[0250] 20 Annealing Furnace

[0251] 22 heating belt

[0252] 24-hour tropical

[0253] 26 Cooling band

[0254] 26A cooling nozzle

[0255] 28 Over-aged processing belt

[0256] 29 nozzles

[0257] 30 Downstream Equipment

[0258] 31 Hot-dip galvanizing bath

[0259] 32 Gas Wiping Device

[0260] 33 alloying furnace

[0261] 34 Cooling device

[0262] 35 Export Loop

[0263] 36 leveling rolling mill

[0264] 50 tension winding machine

[0265] 60 Acoustic Irradiation Device

[0266] 61 controller

[0267] 62 Acoustic Oscillator

[0268] 64 Vibration Transducer

[0269] 66 enhancer

[0270] 68 loudspeakers

[0271] 69 sound level meter

[0272] C cold-rolled coil

[0273] S cold-rolled steel sheet

[0274] P Product Roll Material

Claims

1. A continuous annealing apparatus having: a payoff reel to pay out a cold-rolled steel sheet from a cold-rolled coil; an annealing furnace to perform continuous annealing by passing the cold-rolled steel sheet therethrough, the annealing furnace being provided with a heating zone, a soaking zone, and a cooling zone from an upstream side in a sheet passing direction, the cold-rolled steel sheet being annealed in a reducing atmosphere containing hydrogen in the heating zone and the soaking zone, and the cold-rolled steel sheet being cooled in the cooling zone; a downstream equipment to continuously pass the cold-rolled steel sheet discharged from the annealing furnace; a tension reel to wind up the cold-rolled steel sheet passed through the downstream equipment; and an acoustic wave irradiation apparatus to irradiate an acoustic wave having a frequency of 100 Hz or more to the cold-rolled steel sheet passed from the cooling zone to the tension reel in such a manner that a sound pressure level at a surface of the cold-rolled steel sheet is 30 dB or more and an irradiation time is 10 seconds or more, the acoustic wave irradiation apparatus being provided at a position at which the acoustic wave can be irradiated to the cold-rolled steel sheet passed through the downstream equipment. The acoustic wave irradiation apparatus is also provided at the cooling zone. The acoustic wave irradiation apparatus can irradiate an acoustic wave having a frequency of 100,000 Hz or less.

4. A continuous hot-dip galvanizing apparatus having: the continuous annealing apparatus according to claim 1; and a hot-dip galvanizing bath as a part of the downstream equipment, located downstream of the annealing furnace in the sheet passing direction, to dip the cold-rolled steel sheet and to perform hot-dip galvanizing on the cold-rolled steel sheet. The acoustic wave irradiation apparatus is provided at a position at which the acoustic wave can be irradiated to the cold-rolled steel sheet passed through a position downstream of the hot-dip galvanizing bath. The acoustic wave irradiation apparatus is also provided at a position at which the acoustic wave can be irradiated to the cold-rolled steel sheet passed through a position upstream of the hot-dip galvanizing bath. The continuous hot-dip galvanizing apparatus has an alloying furnace as a part of the downstream equipment, located downstream of the hot-dip galvanizing bath in the sheet passing direction, to pass the cold-rolled steel sheet and to perform heating alloying on the hot-dip galvanizing. The acoustic wave irradiation apparatus is also provided at a position at which the acoustic wave can be irradiated to the cold-rolled steel sheet passed through a position upstream of the hot-dip galvanizing bath. The acoustic wave irradiation apparatus can irradiate an acoustic wave having a frequency of 100,000 Hz or less.

9. A method of manufacturing a steel sheet, sequentially having: (A) a step of paying out a cold-rolled steel sheet from a cold-rolled coil by a payoff reel; (B) a step of passing the cold-rolled steel sheet through an annealing furnace provided with a heating zone, a soaking zone, and a cooling zone from an upstream side in a sheet passing direction, to perform continuous annealing as follows, that is, (B-1) annealing the cold-rolled steel sheet in a reducing atmosphere containing hydrogen in the heating zone and the soaking zone, and (B-2) cooling the cold-rolled steel sheet in the cooling zone; (C) a step of continuously passing the cold-rolled steel sheet discharged from the annealing furnace; (D) a step of winding up the cold-rolled steel sheet by a tension reel to produce a product coil, and an acoustic wave irradiation step of irradiating an acoustic wave having a frequency of 100 Hz or more to the cold-rolled steel sheet in the sheet passing direction in such a manner that a sound pressure level at a surface of the cold-rolled steel sheet is 30 dB or more and an irradiation time is 10 seconds or more, after the step (B-2) and before the step (D), the acoustic wave irradiation step being performed in the step (C). ​ ​ ​ ​ ​ 2. The continuous annealing apparatus according to claim 1, wherein ​ 3. The continuous annealing apparatus according to claim 1 or 2, wherein ​ ​ ​ ​ ​ 5. The continuous hot-dip galvanizing apparatus as claimed in claim 4, wherein ​ 6. The continuous hot-dip galvanizing apparatus set forth in claim 4, wherein ​ 7. The continuous hot-dip galvanizing apparatus as claimed in claim 6, wherein ​ 8. The continuous hot-dip galvanizing apparatus according to any one of claims 4 to 7, wherein ​ ​ ​ ​ ​ ​ ​ ​ 10. The method of producing a steel sheet according to claim 9, wherein The sound wave irradiation step is also performed in step (B-2).

11. The method of producing a steel sheet according to claim 9, wherein Step (C) includes (C-1) a step of subjecting the cold-rolled steel sheet to hot-dip galvanizing by immersing the cold-rolled steel sheet in a hot-dip galvanizing bath located downstream in a sheet passing direction of the annealing furnace, and the sound wave irradiation step is performed after step (C-1).

12. The method of producing a steel sheet according to claim 11, wherein The sound wave irradiation step is also performed before step (C-1).

13. The method of producing a steel sheet according to claim 11, wherein Step (C) includes (C-2) after step (C-1), a step of subjecting the hot-dip galvanizing to heating alloying by passing the cold-rolled steel sheet in an alloying furnace located downstream in a sheet passing direction of the hot-dip galvanizing bath.

14. The method of producing a steel sheet according to claim 13, wherein The sound wave irradiation step is also performed before step (C-1).

15. The method of producing a steel sheet according to any one of claims 9 to 14, wherein The sound wave has a frequency of 100,000 Hz or less.

16. The method of producing a steel sheet according to any one of claims 9 to 14, wherein The cold-rolled steel sheet is a high-strength steel sheet having a tensile strength of 590 MPa or more.

17. The method of producing a steel sheet according to any one of claims 9 to 14, wherein The cold-rolled steel sheet has a composition consisting of, in mass%, C: 0.030 to 0.800%, Si: 0.01 to 3.00%, Mn: 0.01 to 10.00%, P: 0.001 to 0.100%, S: 0.0001 to 0.0200%, N: 0.0005 to 0.0100%, and Al: 0.001 to 2.000%, the remainder consisting of Fe and unavoidable impurities.

18. The method of producing a steel sheet according to claim 17, wherein The composition further includes, in mass%, at least one element selected from the group consisting of Ti: 0.200% or less, Nb: 0.200% or less, V: 0.500% or less, W: 0.500% or less, B: 0.0050% or less, Ni: 1.000% or less, Cr: 1.000% or less, Mo: 1.000% or less, Cu: 1.000% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ta: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.1000% or less, and REM: 0.0050% or less.

19. The method of producing a steel sheet according to any one of claims 9 to 14, wherein The cold-rolled steel sheet is a stainless steel sheet having a composition consisting of, in mass%, C: 0.001 to 0.400%, Si: 0.01 to 2.00%, Mn: 0.01 to 5.00%, P: 0.001 to 0.100%, S: 0.0001 to 0.0200%, Cr: 9.0 to 28.0%, Ni: 0.01 to 40.0%, N: 0.0005 to 0.500%, and Al: 0.001 to 3.000%, the remainder consisting of Fe and unavoidable impurities.

20. The method of producing a steel sheet according to claim 19, wherein The ingredient group further contains at least one element selected from the group consisting of Ti: 0.500% or less, Nb: 0.500% or less, V: 0.500% or less, W: 2.000% or less, B: 0.0050% or less, Mo: 2.000% or less, Cu: 3.000% or less, Sn: 0.500% or less, Sb: 0.200% or less, Ta: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.1000% or less, and REM: 0.0050% or less, in mass %.

21. The method of producing a steel sheet according to any one of claims 9 to 14, wherein The product coil has a diffusible hydrogen amount of 0.50 mass ppm or less.

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