Continuous annealing line, continuous hot-dip galvanizing line, and method of producing steel sheet

Abstract

We provide a continuous annealing device capable of manufacturing steel sheets with excellent hydrogen embrittlement resistance. The continuous annealing device (100) of the present invention comprises a payoff reel (10) for discharging a cold-rolled steel sheet (S) from a cold-rolled coil (C), and an annealing furnace (20) for passing the cold-rolled steel sheet (S) through and continuously annealing it, wherein a heating zone (22), a cracking zone (24), and a cooling zone (26) are located upstream from the direction of passage, and the cold-rolled steel sheet (S) is annealed in a reducing atmosphere containing hydrogen in the heating zone (22) and the cracking zone (24), and the cold-rolled steel sheet (S) is cooled in the cooling zone (26); a downstream facility (30) for continuously passing the cold-rolled steel sheet (S) discharged from the annealing furnace (20); a tension reel (50) for winding the cold-rolled steel sheet (S) passing through the downstream facility (30); and a device passing from the cooling zone (26) to the tension reel (50). With respect to the cold-rolled steel sheet (S), a magnetic field application device (60) that applies a normal magnetic field along the plate width direction of the cold-rolled steel sheet (S) is provided.

Description

Technology Field

[0001] The present invention relates to a continuous annealing apparatus, a continuous hot-dip galvanizing apparatus, and a method for manufacturing a steel sheet. In particular, the present invention relates to a continuous annealing apparatus, a continuous hot-dip galvanizing apparatus, and a method for manufacturing a steel sheet having excellent hydrogen embrittlement resistance with a low hydrogen content, suitable for use in fields such as automobiles, home appliances, and construction materials. Background Technology

[0002] For example, when manufacturing annealed steel sheets and hot-dip galvanized steel sheets in continuous annealing and continuous hot-dip galvanizing devices, respectively, the annealing of the steel sheets is performed under a reducing atmosphere containing hydrogen; consequently, hydrogen enters the steel sheets during this annealing process. The hydrogen inherent in the steel sheets degrades formability, such as ductility, bendability, and stretch flangeability. Furthermore, the hydrogen inherent in the steel sheets can cause embrittlement, leading to delayed fracture. Therefore, treatment to reduce the amount of hydrogen in the steel sheets becomes necessary.

[0003] For example, the hydrogen content in steel can be reduced by leaving product coils manufactured in continuous annealing and continuous hot-dip galvanizing units at room temperature. However, since it takes time for hydrogen to migrate from the interior of the steel sheet to the surface and detach from it at room temperature, leaving the steel for several weeks or more is required to sufficiently reduce the hydrogen content. Consequently, the space and time required for this dehydrogenation treatment become a problem in the manufacturing process.

[0004] In addition, Patent Document 1 discloses a method for reducing the amount of hydrogen in steel by maintaining a steel sheet after annealing, a hot-dip galvanized steel sheet, or an alloyed hot-dip galvanized steel sheet for 1,800 seconds or more and 43,200 seconds or less within a temperature range of 50°C or more and 300°C or less. Prior art literature

[0005] International Publication No. 2019 / 188642 The problem to be solved

[0006] However, in Patent Document 1, there is a concern regarding changes in mechanical properties such as an increase in yield strength or tempering embrittlement due to changes in the structure caused by heating.

[0007] Therefore, taking into account the above problem, the present invention aims to provide a continuous annealing apparatus and a continuous hot-dip galvanizing apparatus, and a method for manufacturing a steel sheet, which are capable of manufacturing a steel sheet with excellent hydrogen embrittlement resistance without compromising production efficiency or changing mechanical properties. means of solving the problem

[0008] The inventors, in order to solve the above problem, have repeatedly conducted research and discovered the following. Specifically, it was found that in a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL), hydrogen in a steel sheet can be reduced sufficiently efficiently by applying a steady magnetic field along the width direction of the steel sheet to the steel sheet that is continuously passing through during the cooling process from the annealing temperature to room temperature after annealing the steel sheet in a reducing atmosphere containing hydrogen. This is presumed to be due to the following mechanism. By applying a steady magnetic field to the steel sheet, the shape of the steel sheet changes due to the magnetostriction effect. At this time, since the steady magnetic field applied to the steel sheet follows the width direction of the steel sheet, the lattice spacing of the steel sheet expands inward along the width direction of the steel sheet. As a result, hydrogen inside the steel plate diffuses toward the main surface (surface) of the steel plate, which has low potential energy, and detaches from the said main surface.

[0009] That is, the present invention is based on the above understanding, and its gist is as follows.

[0010] [1] A payoff reel that outputs cold-rolled steel sheets from a cold-rolled coil, and

[0011] An annealing furnace for passing the above cold-rolled steel sheet through and continuously annealing it, wherein a heating zone, a soaking zone, and a cooling zone are located upstream of the direction of passage, and the cold-rolled steel sheet is annealed in a reducing atmosphere containing hydrogen in the heating zone and the soaking zone, and the cold-rolled steel sheet is cooled in the cooling zone;

[0012] A downstream facility for continuously passing the cold-rolled steel sheet discharged from the annealing furnace, and

[0013] A tension reel for winding the cold-rolled steel sheet passing through the downstream facility, and

[0014] A magnetic field application device that applies a normal magnetic field along the plate width direction of the cold-rolled steel sheet passing from the cooling zone to the tension reel,

[0015] A continuous annealing device having

[0016] [2] The magnetic field application device is a continuous annealing device described in [1], formed on the cooling bed.

[0017] [3] The magnetic field application device described in [1] or [2] is formed at a position where a magnetic field can be applied to the cold-rolled steel sheet passing through the downstream facility.

[0018] [4] The magnetic field application device described in any one of [1] to [3] includes an electromagnet located on the outer side of the widthwise end of the cold-rolled steel sheet, and the electromagnet has a magnetic pole face facing the widthwise cross-section of the cold-rolled steel sheet.

[0019] [5] The magnetic field application device described in any one of [1] to [3] comprises a pair of electromagnets each located on the outer side of both ends in the width direction of the cold-rolled steel sheet, and each pair of electromagnets has a magnetic pole face facing the cross-section in the width direction of the cold-rolled steel sheet, and one of the magnetic pole faces is the N pole and the other is the S pole.

[0020] [6] The magnetic field application device is a continuous annealing device described in any one of [1] to [5], which is set so that the magnetic flux density in the plate width direction of the cold-rolled steel sheet is 0.1 to 15 T.

[0021] [7] The continuous annealing device described in [1] above, and

[0022] As the above downstream facility, a molten zinc plating bath located downstream in the direction of the passing plate of the above annealing furnace, immersing the cold-rolled steel sheet and performing molten zinc plating on the cold-rolled steel sheet,

[0023] A continuous hot-dip galvanizing device having

[0024] [8] The magnetic field application device described in [7] is formed at a position where a magnetic field can be applied to the cold-rolled steel plate passing upstream of the molten zinc plating bath.

[0025] [9] The magnetic field application device described in [7] or [8] is formed at a position where a magnetic field can be applied to the cold-rolled steel plate passing downstream from the molten zinc plating bath.

[0026]

[10] The continuous hot-dip galvanizing apparatus described in [7], having an alloying furnace located downstream of the hot-dip galvanizing bath in the direction of passage, through which the cold-rolled steel sheet is passed, and which heats and alloys the hot-dip galvanizing.

[0027]

[11] The magnetic field application device described in

[10] is formed at a position where a magnetic field can be applied to the cold-rolled steel plate passing upstream of the molten zinc plating bath.

[0028]

[12] The magnetic field application device described in

[10] or

[11] is formed at a position where a magnetic field can be applied to the cold-rolled steel plate passing downstream from the molten zinc plating bath.

[0029]

[13] The magnetic field application device described in any one of claims [7] to

[12] comprises an electromagnet located on the outer side of the widthwise end of the cold-rolled steel sheet, and the electromagnet has a magnetic pole surface facing the widthwise cross-section of the cold-rolled steel sheet.

[0030]

[14] The magnetic field application device described in any one of [7] to

[12] comprises a pair of electromagnets each located on the outer side of both ends in the width direction of the cold-rolled steel sheet, and each pair of electromagnets has a magnetic pole face facing the cross-section in the width direction of the cold-rolled steel sheet, and one of the magnetic pole faces is the N pole and the other is the S pole.

[0031]

[15] The above magnetic field application device is a continuous hot-dip galvanizing device described in any one of [7] to

[14] , which is set so that the magnetic flux density in the plate width direction of the cold-rolled steel sheet is 0.1 to 15 T.

[0032]

[16] (A) A process of extruding cold-rolled steel sheets from cold-rolled coils by a payoff reel, and

[0033] (B) A process of performing continuous annealing in which the cold-rolled steel sheet is passed through an annealing furnace in which a heating zone, a cracking zone, and a cooling zone are located from the upstream side of the passing direction, (B-1) the cold-rolled steel sheet is annealed in a reducing atmosphere containing hydrogen in the heating zone and the cracking zone, and (B-2) the cold-rolled steel sheet is cooled in the cooling zone, and

[0034] (C) A process of continuously passing the cold-rolled steel sheet discharged from the annealing furnace, and,

[0035] (D) A process of winding the above cold-rolled steel sheet by a tension reel to form a product coil,

[0036] Have in this order,

[0037] A method for manufacturing a steel sheet comprising a magnetic field application process, wherein a normal magnetic field is applied along the width direction of the cold-rolled steel sheet being processed, after process (B-2) and also before process (D).

[0038]

[17] The above magnetic field application process is the method for manufacturing a steel plate as described in

[16] , which is performed in process (B-2).

[0039]

[18] The above magnetic field application process is the method of manufacturing a steel plate as described in

[16] or

[17] , which is performed in process (C).

[0040]

[19] A method for manufacturing a steel sheet as described in

[16] , comprising the process (C) of immersing the cold-rolled steel sheet in a molten zinc plating bath located downstream of the annealing furnace in the direction of the sheet, and performing molten zinc plating on the cold-rolled steel sheet.

[0041]

[20] The above magnetic field application process is performed before process (C-1), the method for manufacturing steel plates described in

[19] .

[0042]

[21] The above magnetic field application process is performed after process (C-1), the method for manufacturing a steel plate described in

[19] or

[20] .

[0043]

[22] The above process (C) is a method for manufacturing a steel sheet as described in

[19] , which, following the above process (C-1), (C-2) passes the cold-rolled steel sheet through an alloying furnace located downstream of the molten zinc plating bath and heats and alloys the molten zinc plating.

[0044]

[23] The above magnetic field application process is performed before process (C-1), the method for manufacturing steel plates described in

[22] .

[0045]

[24] The above magnetic field application process is performed after process (C-1), the method for manufacturing steel plates described in

[22] or

[23] .

[0046]

[25] A method for manufacturing a steel sheet as described in any one of

[16] to

[24] , wherein in the above magnetic field application process, the magnetic flux density in the width direction of the cold-rolled steel sheet is 0.1 to 15 T.

[0047]

[26] A method for manufacturing a steel sheet as described in any one of

[16] to

[25] , wherein the above magnetic field application process includes an electromagnet located on the outer side of the widthwise end of the cold-rolled steel sheet, and the normal magnetic field is applied by a magnetic field application device having a magnetic pole face facing the widthwise cross-section of the cold-rolled steel sheet.

[0048]

[27] A method for manufacturing a steel sheet described in any one of

[16] to

[25] , wherein the above magnetic field application process includes a pair of electromagnets each located on the outer side of both ends in the width direction of the cold-rolled steel sheet, and the pair of electromagnets each have a magnetic pole face facing the cross-section in the width direction of the cold-rolled steel sheet, and the normal magnetic field is applied by a magnetic field application device in which one of the magnetic pole faces is the N pole and the other is the S pole.

[0049]

[28] A method for manufacturing a steel plate as described in any one of

[16] to

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

[0050]

[29] The above cold-rolled steel sheet is, in mass %,

[0051] C: 0.030∼0.800%,

[0052] Si: 0.01∼3.00%,

[0053] Mn: 0.01∼10.00%,

[0054] P: 0.001∼0.100%,

[0055] S: 0.0001∼0.0200%,

[0056] N: 0.0005∼0.0100% and,

[0057] Al: Contains 0.001–2.000%,

[0058] A method for manufacturing a steel plate as described in any one of

[16] to

[28] , having a composition in which the remainder consists of Fe and unavoidable impurities.

[0059]

[30] The above composition of components, additionally, in mass%,

[0060] Ti: 0.200% or less,

[0061] Nb: 0.200% or less,

[0062] V: 0.500% or less,

[0063] W: 0.500% or less,

[0064] B: 0.0050% or less,

[0065] Ni: 1.000% or less,

[0066] Cr: 1.000% or less,

[0067] Mo: 1.000% or less,

[0068] Cu: 1.000% or less,

[0069] Sn: 0.200% or less,

[0070] Sb: 0.200% or less,

[0071] Ta: 0.100% or less,

[0072] Ca: 0.0050% or less,

[0073] Mg: 0.0050% or less,

[0074] Zr: 0.1000% or less and,

[0075] REM: 0.0050% or less

[0076] A method for manufacturing a steel plate as described in

[29] , containing at least one element selected from the group consisting of

[0077]

[31] The above cold-rolled steel sheet is, in mass %,

[0078] C: 0.001∼0.400%,

[0079] Si: 0.01∼2.00%,

[0080] Mn: 0.01∼5.00%,

[0081] P: 0.001∼0.100%,

[0082] S: 0.0001∼0.0200%,

[0083] Cr: 9.0∼28.0%,

[0084] Ni: 0.01∼40.0%,

[0085] N: 0.0005∼0.500% and,

[0086] Al: Contains 0.001–3.000%,

[0087] A method for manufacturing a steel plate as described in any one of

[16] to

[27] , wherein the remaining portion is a stainless steel plate having a composition of Fe and unavoidable impurities.

[0088]

[32] The above composition of components, additionally, in mass%,

[0089] Ti: 0.500% or less,

[0090] Nb: 0.500% or less,

[0091] V: 0.500% or less,

[0092] W: 2,000% or less,

[0093] B: 0.0050% or less,

[0094] Mo: 2,000% or less,

[0095] Cu: 3,000% or less,

[0096] Sn: 0.500% or less,

[0097] Sb: 0.200% or less,

[0098] Ta: 0.100% or less,

[0099] Ca: 0.0050% or less,

[0100] Mg: 0.0050% or less,

[0101] Zr: 0.1000% or less and,

[0102] REM: 0.0050% or less

[0103] A method for manufacturing a steel plate as described in

[31] , containing at least one element selected from the group consisting of

[0104]

[33] The above product coil has a diffusible hydrogen content of 0.50 mass ppm or less, a method for manufacturing a steel plate as described in any one of

[16] to

[32] . Effects of the invention

[0105] According to the continuous annealing device, the continuous hot-dip galvanizing device, and the method for manufacturing a steel sheet of the present invention, a steel sheet with excellent hydrogen embrittlement resistance can be manufactured without compromising production efficiency or changing mechanical properties. Brief explanation of the drawing

[0106] FIG. 1 is a schematic diagram of a continuous annealing device (100) according to one embodiment of the present invention. FIG. 2 is a schematic diagram of a continuous molten zinc plating apparatus (200) according to one embodiment of the present invention. FIG. 3 is a schematic diagram of a continuous molten zinc plating apparatus (300) according to another embodiment of the present invention. FIG. 4 is a schematic diagram showing the configuration of a pair of electromagnets (60A, 60B) as a magnetic field application device used in each embodiment of the present invention, and magnetic field lines generated from the electromagnets (60A, 60B). FIGS. 5(A) and 5(B) are schematic diagrams illustrating examples of the installation of a pair of electromagnets (60A, 60B) as magnetic field generating devices for a cold-rolled steel sheet (S) being processed in each embodiment of the present invention. Specific details for implementing the invention

[0107] (Form for carrying out the invention)

[0108] 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).

[0109] A method for manufacturing a steel sheet according to one embodiment of the present invention is realized by a Continuous Annealing Line (CAL) or a Continuous Hot-Dip Galvanizing Line (CGL).

[0110] Referring to FIG. 1, a continuous annealing device (CAL) (100) according to a first embodiment of the present invention comprises a payoff reel (10) for discharging a cold-rolled steel sheet (S) from a cold-rolled coil (C), an annealing furnace (20) for passing the cold-rolled steel sheet (S) through and continuously annealing it, a downstream facility (30) for passing the cold-rolled steel sheet (S) discharged from the annealing furnace (20) through, and a tension reel (50) for winding the cold-rolled steel sheet (S) passing through the downstream facility (30) to form a product coil (P). In the annealing furnace (20), a heating zone (22), a cracking zone (24), and a cooling zone (26) are located upstream of the plate direction. In the heating zone (22) and the cracking zone (24), a cold-rolled steel sheet (S) is annealed in a reducing atmosphere containing hydrogen, and in the cooling zone (26), the cold-rolled steel sheet (S) is cooled. Additionally, the annealing furnace (20) of the CAL (100) preferably has an overaging treatment zone (28) downstream of the cooling zone (26), but this is not mandatory. In the overaging treatment zone (28), an overaging treatment is performed on the cold-rolled steel sheet (S). In this embodiment, a product coil of cold-rolled annealed steel sheet (CR) is manufactured by the CAL (100).

[0111] Referring to FIG. 1, a method for manufacturing a steel sheet according to a first embodiment, realized by a continuous annealing device (CAL) (100), comprises: (A) a process of discharging a cold-rolled steel sheet (steel strip) (S) from a cold-rolled coil (C) by a payoff reel (10); (B) a process of passing the cold-rolled steel sheet (S) through an annealing furnace (20) in which a heating zone (22), a cracking zone (24), and a cooling zone (26) are located from the upstream side of the passing direction, (B-1) annealing the cold-rolled steel sheet (S) in a reducing atmosphere containing hydrogen in the heating zone (22) and the cracking zone (24), and (B-2) cooling the cold-rolled steel sheet (S) in the cooling zone (26), and performing continuous annealing; (C) a process of continuously passing the cold-rolled steel sheet (S) discharged from the annealing furnace (20); and (D) tension The process of winding a cold-rolled steel sheet (S) by a reel (50) to form a product coil (P) is performed in this order. In addition, in the continuous annealing process (B) by the annealing furnace (20) of the CAL (100), it is preferable to perform an over-aging treatment on the cold-rolled steel sheet (S) in an over-aging treatment zone (28) located downstream of the (B-3) cooling zone (26), but this process is not mandatory. This embodiment is a method for manufacturing a product coil of a cold-rolled annealed steel sheet (CR) by the CAL (100).

[0112] Referring to FIG. 2, a continuous hot-dip galvanizing apparatus (CGL) (200) according to a second embodiment of the present invention comprises a payoff reel (10) for discharging a cold-rolled steel sheet (S) from a cold-rolled coil (C), an annealing furnace (20) for passing the cold-rolled steel sheet (S) through and continuously annealing it, a downstream facility (30) for passing the cold-rolled steel sheet (S) discharged from the annealing furnace (20) through, and a tension reel (50) for winding the cold-rolled steel sheet (S) passing through the downstream facility (30) to form a product coil (P). In the annealing furnace (20), a heating zone (22), a cracking zone (24), and a cooling zone (26) are located upstream of the plate direction. In the heating zone (22) and the cracking zone (24), a cold-rolled steel sheet (S) is annealed in a reducing atmosphere containing hydrogen, and in the cooling zone (26), the cold-rolled steel sheet (S) is cooled. Additionally, the CGL (200) has a downstream facility (30) that includes a molten zinc plating bath (31) located downstream of the annealing furnace (20) in the plate direction, in which the cold-rolled steel sheet (S) is immersed and molten zinc plating is performed on the cold-rolled steel sheet (S), and an alloying furnace (33) located downstream of the molten zinc plating bath (31) in the plate direction, in which the cold-rolled steel sheet (S) is passed and the molten zinc plating is heated and alloyed. In this embodiment, a product coil of alloyed hot-dip galvanized steel sheet (GA) in which the zinc plating layer is alloyed is manufactured by CGL (200). Additionally, if the steel sheet (S) is passed through the alloying furnace (33) without performing heat alloying, a product coil of hot-dip galvanized steel sheet (GI) in which the zinc plating layer is not alloyed is manufactured.

[0113] Referring to FIG. 2, a method for manufacturing a steel sheet according to a second embodiment realized by a continuous hot-dip galvanizing apparatus (CGL) (200) comprises: (A) a process of discharging a cold-rolled steel sheet (steel strip) (S) from a cold-rolled coil (C) by a payoff reel (10); (B) a process of passing the cold-rolled steel sheet (S) through an annealing furnace (20) in which a heating zone (22), a cracking zone (24), and a cooling zone (26) are located from the upstream side of the passing direction, (B-1) annealing the cold-rolled steel sheet (S) in a reducing atmosphere containing hydrogen in the heating zone (22) and the cracking zone (24), and (B-2) cooling the cold-rolled steel sheet (S) in the cooling zone (26), and performing continuous annealing; (C) a process of continuously passing the cold-rolled steel sheet (S) discharged from the annealing furnace (20); and (D) cold rolling by a tension reel (50). The process of winding a steel plate (S) to form a product coil (P) is carried out in this order. Then, process (C) includes (C-1) a process of immersing a cold-rolled steel plate (S) in a molten zinc plating bath (31) located downstream of the passing direction of the annealing furnace (20) to perform molten zinc plating on the cold-rolled steel plate (S), and subsequently (C-2) a process of passing the cold-rolled steel plate (S) through an alloying furnace (33) located downstream of the passing direction of the molten zinc plating bath (31) to heat and alloy the molten zinc plating. This embodiment is a method for manufacturing a product coil of an alloyed molten zinc-plated steel plate (GA) in which the zinc plating layer is alloyed by the CGL (200).

[0114] Referring to FIG. 3, a continuous hot-dip galvanizing apparatus (CGL) (300) according to a third embodiment of the present invention has the same configuration as a CGL (200) except that it does not have an alloying furnace (33). In this embodiment, a product coil of hot-dip galvanized steel sheet (GI) in which the galvanizing layer is not alloyed is manufactured by the CGL (300).

[0115] That is, the method for manufacturing a steel sheet according to the third embodiment, in which process (C-1) is performed but process (C-2) is not performed, is realized, for example, by a CGL (300) that does not have an alloying furnace (33), and is also realized by a method in which the steel sheet (S) is passed through the alloying furnace (33) of the CGL (200) without performing heat alloying. This embodiment is a method for manufacturing a product coil of a hot-dip galvanized steel sheet (GI) in which the zinc plating layer is not alloyed by the CGL (200) or the CGL (300).

[0116] Each configuration of the CAL according to the first embodiment and the CGL according to the second and third embodiments is described in detail. In addition, each process of the method for manufacturing a steel plate according to the first, second, and third embodiments is described in detail.

[0117] [Payoff Reel and Equipment from Payoff Reel to Annealing Furnace]

[0118] [Process (A)]

[0119] Referring to FIGS. 1 to 3, the payoff reel (10) discharges a cold-rolled steel sheet (S) from a cold-rolled coil (C). That is, in process (A), the cold-rolled steel sheet (S) is discharged from the cold-rolled coil (C) by the payoff reel (10). The discharged cold-rolled steel sheet (S) passes through a welding machine (11), a cleaning facility (12), and an entry-side looper (13) and is supplied to an annealing furnace (20). However, the upstream facility between the payoff reel (10) and the annealing furnace (20) is not limited to the welding machine (11), the cleaning facility (12), and the entry-side looper (13), and may be any known or arbitrary device.

[0120] [Annealing Lo]

[0121] [Process (B)]

[0122] Referring to FIGS. 1 to 3, the annealing furnace (20) passes a cold-rolled steel sheet (S) through it and performs continuous annealing. In the annealing furnace (20), a heating zone (22), a cracking zone (24), and a cooling zone (26) are located upstream of the passage direction. In the heating zone (22) and the cracking zone (24), the cold-rolled steel sheet (S) is annealed in a reducing atmosphere containing hydrogen, and in the cooling zone (26), the cold-rolled steel sheet (S) is cooled. That is, in process (B), a cold-rolled steel sheet (S) is passed through the annealing furnace (20), in which the heating zone (22), the cracking zone (24), and the cooling zone (26) are located upstream of the passage direction, and continuous annealing is performed. The cooling zone (26) may be composed of a plurality of cooling zones. Additionally, a preheating zone may be located upstream of the passage direction of the heating zone (22). Additionally, the annealing furnace (20) of the CAL (100) shown in FIG. 1 preferably has an over-aging treatment zone (28) downstream of the cooling zone (26), but this is not mandatory. In FIG. 1 to 3, each zone is shown as a vertical furnace, but it is not limited thereto and may be a horizontal furnace. In the case of a vertical furnace, adjacent zones are connected through a throat (restriction portion) that connects the upper or lower parts of each zone.

[0123] (Heating zone)

[0124] In the heating zone (22), the cold-rolled steel sheet (S) can be heated directly using a burner, or the cold-rolled steel sheet (S) can be heated indirectly using a radiant tube (RT) or an electric heater. In addition, heating is possible using induction heating, roller heating, electric resistance heating, direct current heating, salt bath heating, electron beam heating, etc. It is preferable that the average temperature inside the heating zone (22) be 500 to 800°C. At the same time as gas from the cracking zone (24) is introduced into the heating zone (22), a separate reducing gas is supplied. As for the reducing gas, a H2-N2 mixed gas is typically used, and for example, a gas having a composition consisting of H2: 1 to 35 volume%, with the remainder being one or both of N2 and Ar and unavoidable impurities (dew point: about -60°C).

[0125] (Rift zone)

[0126] In the cracking zone (24), the cold-rolled steel sheet (S) can be indirectly heated using a radiant tube (RT). It is preferable that the average temperature inside the cracking zone (24) be 600 to 950°C. A reducing gas is supplied to the cracking zone (24). As the reducing gas, a H2-N2 mixed gas is typically used, and for example, a gas having a composition consisting of H2: 1 to 35 volume%, the remainder being one or both of N2 and Ar, and unavoidable impurities (dew point: about -60°C).

[0127] (Cooling zone)

[0128] In the cooling section (26), the cold-rolled steel sheet (S) is cooled by gas, a mixture of gas and water, or water. When the cold-rolled steel sheet (S) exits the annealing furnace (20), it is cooled to approximately 100–400°C in the CAL and to approximately 470–530°C in the CGL. In the cooling section (26), a plurality of cooling nozzles are formed along the steel sheet conveyance path. The cooling nozzles are circular pipes longer than the width of the steel sheet, for example, as described in Japanese Patent Publication No. 2010-185101, and are installed so that the direction of the circular pipes is parallel to the width direction of the steel sheet. In the circular pipes, a plurality of through holes are formed at predetermined intervals along the direction of the circular pipes in the part facing the steel sheet, and water inside the circular pipes is sprayed toward the steel sheet through said through holes. Cooling nozzles are installed in pairs facing the front and back surfaces of the steel plate, and additionally, a plurality of pairs (e.g., 5 to 10 pairs) of cooling nozzles are arranged at predetermined intervals along the steel plate conveying path to form one cooling zone. Furthermore, it is preferable to arrange about 3 to 6 cooling zones along the steel plate conveying path.

[0129] (Statute of Limitations Processing Unit)

[0130] Referring to FIG. 1, in the CAL (100), in the over-aging treatment unit (28), the cold-rolled steel sheet (S) that has exited the cooling unit (26) is subjected to at least one of isothermal holding, reheating, furnace cooling, and air cooling, and the cold-rolled steel sheet (S) is cooled to about 100 to 400°C in the step of exiting the annealing furnace (20).

[0131] [Downstream Facilities]

[0132] [Process (C)]

[0133] Referring to FIGS. 1 to 3, in process (C), cold-rolled steel sheets (S) discharged from an annealing furnace (20) are continuously passed to downstream equipment (30). Referring to FIG. 1, CAL (100) has an out-side looper (35) and a temper rolling mill (36) as downstream equipment (30). Referring to FIG. 2, CGL (200) has a molten zinc plating bath (31), a gas wiping device (32), an alloying furnace (33), a cooling device (34), an out-side looper (35), and a temper rolling mill (36) as downstream equipment (30). Referring to FIG. 3, the CGL (300) has a downstream facility (30) including a molten zinc plating bath (31), a gas wiping device (32), a cooling device (34), an exit looper (35), and a tempering rolling mill (36). However, the downstream facility (30) is not limited to these and may be any known or arbitrary device. For example, the downstream facility (30) may include a tension leveler, a chemical treatment facility, a surface conditioning facility, an oiling facility, and an inspection facility.

[0134] (Hot-dip galvanizing bath)

[0135] (Process (C-1))

[0136] Referring to FIGS. 2 and 3, the molten zinc plating bath (31) is located downstream of the annealing furnace (20) in the direction of the passage, and the cold-rolled steel sheet (S) is immersed to perform molten zinc plating on the cold-rolled steel sheet (S). That is, in process (C-1), the cold-rolled steel sheet (S) is immersed in the molten zinc plating bath (31) located downstream of the annealing furnace (20) in the direction of the passage, and molten zinc plating is performed on the cold-rolled steel sheet (S). The snout (29) connected to the lowermost section of the annealing furnace (cooling section (26) in FIGS. 2 and 3) is a member having a rectangular cross-section perpendicular to the direction of the passage that defines the space through which the cold-rolled steel sheet (S) passes, and its tip is immersed in the molten zinc plating bath (31), so that the annealing furnace (20) and the molten zinc plating bath (31) are connected. It is best to perform hot-dip galvanizing according to the prescribed method.

[0137] Gas can be sprayed onto the cold-rolled steel sheet (S) from a pair of gas wiping devices (32) positioned between the cold-rolled steel sheet (S) being lifted from the molten zinc plating bath (31), thereby adjusting the amount of molten zinc applied to both sides of the cold-rolled steel sheet (S).

[0138] (In alloying)

[0139] (Process (C-2))

[0140] Referring to FIG. 2, the alloying furnace (33) is located downstream of the molten zinc plating bath (31) and the gas wiping device (32) in the direction of passage, and passes a cold-rolled steel sheet (S) through it to heat and alloy the molten zinc plating. That is, in process (C-2), a cold-rolled steel sheet (S) is passed through the alloying furnace (33) located downstream of the molten zinc plating bath (31) and the gas wiping device (32) in the direction of passage, and heats and alloys the molten zinc plating. The alloying treatment may be performed according to the prescribed method. The heating means in the alloying furnace (33) is not particularly limited, and examples include heating by high-temperature gas or induction heating. However, the alloying furnace (33) is any equipment in the CGL, and the alloying process is any process in the method of manufacturing steel sheets using the CGL.

[0141] (Cooling device)

[0142] Referring to FIGS. 2 and 3, the cooling device (34) is located downstream of the gas wiping device (32) and the alloying furnace (33) in the direction of passage. The cold-rolled steel sheet (S) can be passed through the cooling device (34) to cool the cold-rolled steel sheet (S). The cooling device (34) cools the cold-rolled steel sheet (S) by water cooling, air cooling, gas cooling, mist cooling, etc.

[0143] [Tension Reel]

[0144] [Process (D)]

[0145] Referring to FIGS. 1 to 3, the cold-rolled steel sheet (S) that has passed through the downstream facility (30) is finally wound by a tension reel (50) as a winding device to become a product coil (P).

[0146] [Magnetic Field Application Device and Magnetic Field Application Process]

[0147] The CAL (100) of the first embodiment, the CGL (200) of the second embodiment, and the CGL (300) of the third embodiment need to have a magnetic field application device (60) that applies a normal magnetic field along the plate width direction of the cold-rolled steel sheet (S) passing from the cooling bed (26) to the tension reel (50). That is, the method of manufacturing a steel sheet according to the first, second, and third embodiments needs to include a magnetic field application process that applies a normal magnetic field along the plate width direction of the cold-rolled steel sheet (S) passing through the cold-rolled steel sheet (S) after process (B-2) and before process (D). Accordingly, hydrogen contained in the cold-rolled steel sheet (S) can be sufficiently and efficiently reduced by annealing, and a steel sheet with excellent hydrogen embrittlement resistance can be manufactured. In addition, since the application of a normal magnetic field is integrated into the manufacturing process (in line) of the steel plate by CAL (100), CGL (200) or CGL (300), production efficiency is not compromised. Also, since hydrogen is removed by the application of a normal magnetic field rather than by heating, there is no risk of changing the mechanical properties of the steel plate.

[0148] (Magnetic field application device (60))

[0149] Each embodiment of the present invention can be realized by installing a magnetic field application device (60) as shown in FIGS. 4 and FIGS. 5(A) and 5(B) in CAL (100), CGL (200), or CGL (300), and the magnetic field application process applies a normal magnetic field to a cold-rolled steel sheet (S) being passed through using the magnetic field application device (60). Referring to FIG. 4, the magnetic field application device (60) includes a pair of electromagnets (60A, 60B) each located on the outer side of both ends in the width direction of the cold-rolled steel sheet (S). Each electromagnet (60A, 60B) has an iron core (62A, 62B), a coil (64A, 64B) that winds the iron core (62A, 62B), and a driving power source (not shown) for flowing current through the coil (64A, 64B). By turning the driving power source ON and flowing a continuous DC current through the coil (64A, 64B), the electromagnets (60A, 60B) can be magnetized, thereby generating a steady magnetic field. The axial direction of the coil (64A, 64B) coincides with the plate width direction of the cold-rolled steel sheet (S). Each pair of electromagnets (60A, 60B) has a magnetic pole surface (66A, 66B) facing each other at a predetermined distance from the cross-section in the width direction of the cold-rolled steel sheet (S). By controlling the direction of the current flowing through the coils (64A, 64B), one magnetic pole surface (66A) can be made the N pole and the other magnetic pole surface (66B) can be made the S pole. A pair of magnetic pole surfaces (66A, 66B) are located at the same position in the direction of passage of the cold-rolled steel sheet (S) and also face each other with the cold-rolled steel sheet (S) in between. Because of this, as shown in FIG. 4, the steady magnetic field generated by a pair of electromagnets (60A, 60B) has its main magnetic flux directed from the magnetic pole surface (66A) (N pole) to the magnetic pole surface (66B) (S pole), and that direction coincides with the width direction of the cold-rolled steel sheet (S). Accordingly, a steady magnetic field can be uniformly applied along the width direction of the cold-rolled steel sheet (S).In addition, as shown in FIG. 5(A) and 5(B), by arranging a plurality of such pairs of electromagnets (60A, 60B) along the direction of the sheet, sufficient time can be secured to apply a magnetic field to the cold-rolled steel sheet (S). Furthermore, in this specification, "continuous direct current" means a direct current in which the current value is maintained continuously (preferably constant) without pulses. Also, in this specification, "steady magnetic field" means a magnetic field maintained continuously without pulses, and includes a magnetic field formed by a stationary magnet and a magnetic field formed by an electromagnet supplied with a continuous direct current.

[0150] It is important to apply a steady magnetic field, rather than a pulsed magnetic field, to the cold-rolled steel sheet (S). In a pulsed magnetic field, the expansion of the lattice spacing of the steel sheet due to the magnetostriction effect does not continue, so hydrogen cannot be efficiently removed from the cold-rolled steel sheet (S). In addition, it is important to apply the magnetic field along the width direction of the cold-rolled steel sheet (S). For example, if a magnetic field is applied along the thickness direction of the cold-rolled steel sheet (S), the lattice spacing of the steel sheet expands along the thickness direction of the steel sheet. In this case, hydrogen inside the steel sheet diffuses inward along the plane of the steel sheet and can be removed from the cross-section in the width direction of the steel sheet. However, since the cross-section in the width direction of the steel sheet has a small area, a sufficient hydrogen removal effect cannot be obtained. In contrast, if a magnetic field is applied along the width direction of the cold-rolled steel sheet (S), the lattice spacing of the steel sheet expands inward along the width direction of the steel sheet and into the main surface (surface surface) of the steel sheet. As a result, hydrogen inside the steel plate diffuses toward the main surface (surface) of the steel plate, which has a large surface area, and is detached from said main surface. Therefore, a sufficient hydrogen detachment effect can be obtained.

[0151] Additionally, while the installation method of the pair of electromagnets (60A, 60B) is preferably as described above, the installation method is not limited as long as a steady magnetic field with a magnetic flux component in the width direction of the cold-rolled steel sheet (S) is generated. Furthermore, the configuration of the magnetic field application device (60) is not limited to the pair of electromagnets (60A, 60B) as long as a steady magnetic field with a magnetic flux component in the width direction of the cold-rolled steel sheet (S) is generated. For example, the magnetic field application device (60) may consist of only one of the electromagnet (60A) and the electromagnet (60B). If the magnetic field formed by one of the electromagnets has a strength such that a magnetic field along the width direction is applied to the entire width of the cold-rolled steel sheet (S), then only one of them may be sufficient.

[0152] In the first, second, and third embodiments, the position of the magnetic field application device (60) is not limited as long as it can apply a magnetic field to the cold-rolled steel sheet (S) passing from the cooling bed (26) to the tension reel (50).

[0153] Referring to FIG. 1, in a first embodiment of manufacturing a product coil of cold-rolled annealed steel sheet (CR) with CAL (100), a suitable location of the magnetic field application device (60), that is, a suitable timing for the magnetic field application process, is described. As an example, the magnetic field application device (60) can be formed in the cooling bed (26). In this case, the magnetic field application process can be performed in process (B-2). Furthermore, the entire magnetic field application device (60) does not need to be located inside the cooling bed (26), but at least the electromagnets (60A, 60B) need to be located inside the cooling bed (26).

[0154] As another example, a magnetic field application device (60) can be formed at a location where a magnetic field can be applied to a cold-rolled steel sheet (S) passing through the downstream facility (30). In this case, the magnetic field application process can be performed in process (C). Specifically, a magnetic field application device (60) can be formed at least one location between (i) the over-aging treatment unit (28) and the exit looper (35), (ii) inside the exit looper (35), (iii) between the exit looper (35) and the temper rolling mill (36), and (iv) between the temper rolling mill (36) and the tension reel (50).

[0155] The magnetic field application device (60) may be formed at both ends of the cooling zone (26) and the downstream facility (30) at a location where a magnetic field can be applied to the cold-rolled steel sheet (S) being processed. That is, the magnetic field application process may be performed at both ends of process (B-2) and process (C). Additionally, the magnetic field application device (60) may be formed on the over-aging treatment zone (28) so that the magnetic field application process is performed during the over-aging treatment.

[0156] Next, with reference to FIG. 2, in a second embodiment for manufacturing a product of alloyed hot-dip galvanized steel sheet (GA) using CGL (200), a suitable location of the magnetic field application device (60), that is, a suitable timing for the magnetic field application process, will be explained. As an example, the magnetic field application device (60) can be formed at a first location where a magnetic field can be applied to a cold-rolled steel sheet (S) passing upstream of the hot-dip galvanizing bath (31). In this case, the magnetic field application process can be performed before process (C-1). Specifically, the magnetic field application device (60) can be formed in the cooling bed (26). Furthermore, the entire magnetic field application device (60) does not need to be located inside the cooling bed (26), but at least the electromagnets (60A, 60B) need to be located inside the cooling bed (26). In addition, at least electromagnets (60A, 60B) of the magnetic field application device (60) may be installed within the snout (29).

[0157] As another example, a magnetic field application device (60) can be formed at a second location downstream of the molten zinc plating bath (31) to apply a magnetic field to the cold-rolled steel sheet (S) being passed through. In this case, the magnetic field application process can be performed after process (C-1). Specifically, a magnetic field application device (60) can be formed in at least one of the following: (i) between the molten zinc plating 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) between the alloying furnace (33) and the cooling device (34); (v) between the cooling device (34) and the exit looper (35); (vi) inside the exit looper (35); (vii) between the exit looper (35) and the temper rolling mill (36); and (viii) between the temper rolling mill (36) and the tension reel (50). In particular, it is preferable to form a magnetic field application device (60) in the air cooling zone of (iv).

[0158] In terms of more sufficiently removing hydrogen from the steel plate, it is preferable to form the magnetic field application device (60) at the first position rather than at the second position. That is, it is preferable to perform the magnetic field application process before the process (C-1) rather than after the process (C-1). However, the magnetic field application device (60) may be formed at both the first position and the second position. That is, the magnetic field application process may be performed before and after the process (C-1).

[0159] Next, with reference to FIG. 3, in a third embodiment of manufacturing a product of a hot-dip galvanized steel sheet (GI) using a CGL (300), a suitable location of the magnetic field application device (60), that is, a suitable timing for the magnetic field application process, will be explained. As an example, the magnetic field application device (60) can be formed at a first location where a magnetic field can be applied to a cold-rolled steel sheet (S) passing upstream of the hot-dip galvanizing bath (31). In this case, the magnetic field application process can be performed before process (C-1). Specifically, the magnetic field application device (60) can be formed in the cooling bed (26). Furthermore, the entire magnetic field application device (60) does not need to be located inside the cooling bed (26), but at least the electromagnets (60A, 60B) should be located inside the cooling bed (26). In addition, at least electromagnets (60A, 60B) of the magnetic field application device (60) may be installed within the snout (29).

[0160] As another example, a magnetic field application device (60) can be formed at a second location downstream of the molten zinc plating bath (31) to apply a magnetic field to the cold-rolled steel sheet (S) being passed through. In this case, the magnetic field application process can be performed after process (C-1). Specifically, the magnetic field application device (60) can be formed at least one location between (i) the molten zinc plating bath (31) and the gas wiping device (32), (ii) the air cooling zone between the gas wiping device (32) and the cooling device (34), (iii) between the cooling device (34) and the exit looper (35), (iv) inside the exit looper (35), (v) between the exit looper (35) and the temper rolling mill (36), and (vi) between the temper rolling mill (36) and the tension reel (50). In particular, it is desirable to form a magnetic field application device (60) in the air-cooled zone of (ii).

[0161] In terms of more sufficiently removing hydrogen from the steel plate, it is preferable to form the magnetic field application device (60) at the first position rather than at the second position. That is, it is preferable to perform the magnetic field application process before the process (C-1) rather than after the process (C-1). However, the magnetic field application device (60) may be formed at both the first position and the second position. That is, the magnetic field application process may be performed before and after the process (C-1).

[0162] (Magnetic flux density)

[0163] From the perspective of promoting the diffusion of hydrogen and sufficiently removing hydrogen contained in the cold-rolled steel sheet (S), it is preferable that the magnetic flux density in the width direction of the cold-rolled steel sheet (S) be 0.1T or higher, more preferable that it be 0.2T or higher, and even more preferable that it be 0.5T or higher. On the other hand, considering the performance of a general magnetic field application device, it is preferable that the magnetic flux density in the width direction of the cold-rolled steel sheet (S) be 15T or lower, and more preferable that it be 14T or lower. The magnetic flux density in the width direction of the cold-rolled steel sheet (S) can be adjusted by adjusting the number of coil turns or the current value. The "magnetic flux density in the width direction of the cold-rolled steel sheet (S)" can be measured in-line by installing a Tesla meter near the cross-section in the width direction of the cold-rolled steel sheet (S) being passed through, and also near the magnetic field generation surface of the magnetic field application device (60). Alternatively, if the number of coil turns and the magnitude of the current value in the magnetic field application device (60) are determined, the “magnetic flux density in the plate width direction of the cold-rolled steel sheet (S)” can be determined in advance offline.

[0164] (Luculant field authorization time)

[0165] In the view of more sufficiently reducing hydrogen from the cold-rolled steel sheet (S), in the magnetic field application process, it is preferable that the magnetic field application time for the cold-rolled steel sheet (S) be 1 second or more, more preferable that it be 5 seconds or more, and even more preferable that it be 10 seconds or more. On the other hand, in the view of not hindering productivity, it is preferable that the magnetic field application time for the cold-rolled steel sheet (S) be 3600 seconds or less, more preferable that it be 1800 seconds or less, and even more preferable that it be 900 seconds or less. In this specification, "magnetic field application time for the cold-rolled steel sheet (S)" means the time during which a magnetic field is applied to each position in the plate width direction of the cold-rolled steel sheet (S), and if a magnetic field is applied to each position from a plurality of magnetic field application devices (60), it means the cumulative time. Referring to FIG. 4, when a pair of electromagnets (60A, 60B) are used, the part of the cold-rolled steel sheet (S) facing the pair of electromagnets (60A, 60B) can be considered to have a magnetic field applied. Accordingly, the accumulation of the time during which each part of the cold-rolled steel sheet (S) faces the pair of electromagnets (60A, 60B) can be considered as the magnetic field application time. The magnetic field application time can be adjusted by the passing speed of the cold-rolled steel sheet (S) and the position of the magnetic field application device (60) (for example, the number of passing directions of the pair of electromagnets (60A, 60B) shown in FIG. 4).

[0166] [Cold-rolled steel sheet]

[0167] In this embodiment, the cold-rolled steel sheet (S) supplied to CAL (100), CGL (200) and CGL (300) is not particularly limited. The cold-rolled steel sheet (S) is preferably less than 6 mm in thickness, and may be, for example, a high-strength steel sheet having a tensile strength of 590 MPa or more, or a stainless steel sheet.

[0168] [Composition of Cold-Rolled Steel Sheet: High-Strength Steel Sheet]

[0169] The composition of the cold-rolled steel sheet (S) when it is a high-strength steel sheet is described. Hereinafter, "mass%" is simply written as "%".

[0170] C: 0.030∼0.800%

[0171] C has the effect of increasing the strength of the steel sheet. To obtain this effect, the amount of C is 0.030% or more, preferably 0.080% or more. However, if the amount of C is excessive, the steel sheet becomes significantly embrittled regardless of the amount of hydrogen in the steel sheet. Therefore, the amount of C is 0.800% or less, preferably 0.500% or less.

[0172] Si: 0.01∼3.00%

[0173] Si has the effect of increasing the strength of the steel sheet. To obtain this effect, the Si content is set to 0.01% or more, preferably 0.10% or more. However, if the Si content is excessive, the steel sheet becomes embrittled, reducing ductility, or red scale occurs, causing surface properties to deteriorate or plating quality to deteriorate. Therefore, the Si content is set to 3.00% or less, preferably 2.50% or less.

[0174] Mn: 0.01∼10.00%

[0175] Mn has the effect of increasing the strength of the steel sheet through solid solution strengthening. To obtain this effect, the Mn content is set to 0.01% or more, preferably 0.5% or more. However, if the Mn content is excessive, non-uniformity in the steel structure is prone to occur due to Mn segregation, and hydrogen embrittlement starting from the non-uniformity may manifest. Therefore, the Mn content is set to 10.00% or less, preferably 8.00% or less.

[0176] P: 0.001∼0.100%

[0177] P is an element that has a solid solution strengthening effect and can be added according to the desired strength. To obtain this effect, the amount of P is 0.001% or more, preferably 0.003% or more. However, if the amount of P is excessive, weldability deteriorates, and if the zinc plating is alloyed, the alloying rate decreases, thereby damaging the quality of the zinc plating. Therefore, the amount of P is 0.100% or less, preferably 0.050% or less.

[0178] S: 0.0001∼0.0200%

[0179] S segregates at grain boundaries and embrittles the steel during hot working, and also exists as a sulfide, reducing local deformation capacity. Therefore, the S content is set to 0.0200% or less, preferably 0.0100% or less, and more preferably 0.0050% or less. On the other hand, due to production technology constraints, the S content is set to 0.0001% or more.

[0180] N: 0.0005∼0.0100%

[0181] N is an element that degrades the aging resistance of steel. Therefore, the amount of N is set to 0.0100% or less, and preferably 0.0070% or less. Although it is preferable for the amount of N to be as low as possible, due to production technology constraints, the amount of N is set to 0.0005% or more, and preferably 0.0010% or more.

[0182] Al: 0.001∼2.000%

[0183] Al acts as a deoxidizer and is an element effective for the cleanliness of steel. To obtain this effect, the amount of Al is 0.001% or more, preferably 0.010% or more. However, if the amount of Al is excessive, there is a possibility that steel billet breakage may occur during continuous casting. Therefore, the amount of Al is 2.000% or less, preferably 1.200% or less.

[0184] The remainder other than the above components is Fe and unavoidable impurities. However, it may optionally include at least one element selected from the following.

[0185] Ti: 0.200% or less

[0186] Ti contributes to an increase in the strength of steel sheets through precipitation strengthening of steel or fine-grain strengthening by inhibiting the elongation of ferrite grains. Therefore, when adding Ti, it is desirable to have a Ti content of 0.005% or more, and more desirable to have a Ti content of 0.010% or more. However, if the Ti content is excessive, a large amount of carbonitrides may precipitate, which may reduce formability. Therefore, when adding Ti, the Ti content should be 0.200% or less, and preferably 0.100% or less.

[0187] Nb: 0.200% or less, V: 0.500% or less, W: 0.500% or less

[0188] Nb, V, and W are effective for precipitation strengthening of steel. Therefore, when adding Nb, V, and W, it is preferable that the content of each element be 0.005% or more, and more preferable that it be 0.010% or more. However, if the content of each is excessive, a large amount of carbonitrides may precipitate, which may reduce formability. Therefore, when adding Nb, the amount of Nb should be 0.200% or less, and preferably 0.100% or less. When adding V and W, the content of each element should be 0.500% or less, and preferably 0.300% or less.

[0189] B: 0.0050% or less

[0190] B is effective for strengthening grain boundaries or increasing the strength of steel sheets. Therefore, when adding B, it is desirable to have a B content of 0.0003% or more. However, if the B content is excessive, formability may decrease. Therefore, when adding B, the B content should be 0.0050% or less, and preferably 0.0030% or less.

[0191] Ni: 1.000% or less

[0192] Ni is an element that increases the strength of steel through solid solution strengthening. Therefore, when adding Ni, it is desirable to have a Ni content of 0.005% or more. However, if the Ni content is excessive, the area ratio of hard martensite becomes excessive, and during tensile testing, microvoids at the grain boundaries of the martensite increase, and additionally, crack propagation proceeds, which may lead to a decrease in ductility. Therefore, when adding Ni, the Ni content should be 1.000% or less.

[0193] Cr: 1.000% or less, Mo: 1.000% or less

[0194] Cr and Mo have the effect of improving the balance between strength and formability. Therefore, when adding Cr and Mo, it is desirable to have a content of 0.005% or more of each element. However, if the content of each is excessive, the area ratio of hard martensite becomes excessive, and during tensile testing, microvoids at the grain boundaries of martensite increase, and furthermore, crack propagation proceeds, which may lead to a decrease in ductility. Therefore, when adding Cr and Mo, the content of each element is kept at 1.000% or less.

[0195] Cu: 1.000% or less

[0196] Cu is an element effective for strengthening steel. Therefore, when adding Cu, it is desirable to have a Cu content of 0.005% or more. However, if the Cu content is excessive, the area ratio of hard martensite becomes excessive, and during tensile testing, microvoids at the grain boundaries of tempered martensite increase, and additionally, crack propagation proceeds, which may lead to a decrease in ductility. Therefore, when adding Cu, the Cu content should be 1.000% or less.

[0197] Sn: 0.200% or less, Sb: 0.200% or less

[0198] Sn and Sb are effective in suppressing decarburization in the surface layer of the steel sheet, which occurs due to nitriding or oxidation of the steel sheet surface, and in ensuring strength and material stability. Therefore, when adding Sn and Sb, it is desirable to have a content of each element of 0.002% or more. However, if the content of each is excessive, toughness may decrease. Therefore, when adding Sn and Sb, the content of each element is 0.200% or less.

[0199] Ta: 0.100% or less

[0200] Ta, like Ti or Nb, contributes to high strength by forming alloy carbides or alloy carbonitrides. Furthermore, it is believed that by partially dissolving Ta into Nb carbides or Nb carbonitrides and forming complex precipitates such as (Nb, Ta)(C, N), it significantly suppresses the coarsening of precipitates, thereby stabilizing the contribution to strength through precipitation strengthening. Therefore, when Ta is added, it is desirable to have a Ta content of 0.001% or more. However, even if Ta is added in excess, the precipitate stabilization effect may become saturated, and alloy costs also increase. Therefore, when Ta is added, the Ta content is kept at 0.100% or less.

[0201] Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.1000% or less, REM (Rare Earth Metal): 0.0050% or less

[0202] Ca, Mg, Zr, and REM are effective elements for spheroidizing the shape of sulfides and improving the adverse effects of sulfides on moldability. When adding these elements, it is desirable to have a content of 0.0005% or more of each element. However, if the content of each is excessive, inclusions increase, and surface and internal defects may occur. Therefore, when adding these elements, the content of each element should be 0.0050% or less.

[0203] [Composition of Cold-Rolled Steel Sheet: Stainless Steel Sheet]

[0204] The composition of the cold-rolled steel sheet (S) when it is a stainless steel sheet is described. Hereinafter, "mass%" is simply written as "%".

[0205] C: 0.001∼0.400%

[0206] C is an indispensable element for obtaining high strength in stainless steel. However, during tempering in steel manufacturing, it combines with Cr to precipitate as carbides, which degrades the corrosion resistance and toughness of the steel. Sufficient strength is not obtained when the C content is less than 0.001%, and the above deterioration becomes significant when it exceeds 0.400%. For this reason, the C content is set to 0.001 to 0.400%.

[0207] Si: 0.01∼2.00%

[0208] Si is an element useful as a deoxidizer. To obtain this effect, the Si content should be 0.01% or more. However, if the Si content is excessive, the Si dissolved in the steel reduces the workability of the steel. Therefore, Si should be 2.00% or less.

[0209] Mn: 0.01∼5.00%

[0210] Mn has the effect of increasing the strength of steel. To obtain this effect, the amount of Mn is set to 0.01% or more. However, if the amount of Mn is excessive, the workability of the steel decreases. Therefore, the amount of Mn is set to 5.00% or less.

[0211] P: 0.001∼0.100%

[0212] P is an element that promotes intergranular fracture caused by grain boundary segregation. For this reason, it is preferable to keep the P content low, 0.100% or less, preferably 0.030% or less, and more preferably 0.020% or less. On the other hand, due to production technology constraints, the P content is set to 0.001% or more.

[0213] S: 0.0001∼0.0200%

[0214] S exists as sulfide inclusions such as MnS and reduces ductility and corrosion resistance. For this reason, it is preferable to keep the S content low, 0.0200% or less, preferably 0.0100% or less, and more preferably 0.0050% or less. On the other hand, due to production technology constraints, the S content is set to 0.0001% or more.

[0215] Cr: 9.0∼28.0%

[0216] Cr is a basic element that constitutes stainless steel and is also an important element that provides corrosion resistance. When considering corrosion resistance in harsh environments of 180°C or higher, sufficient corrosion resistance is not obtained when the Cr content is less than 9.0%, and when it exceeds 28.0%, the effect becomes saturated, causing problems in terms of economic feasibility. For this reason, the Cr content is set to 9.0 to 28.0%.

[0217] Ni: 0.01∼40.0%

[0218] Ni is an element that improves the corrosion resistance of stainless steel. Its effect is not fully exerted when the Ni content is less than 0.01%. On the other hand, if the Ni content is excessive, in addition to deteriorating formability, stress corrosion cracking becomes more likely to occur. For this reason, the Ni content is set to 0.01 to 40.0%.

[0219] N: 0.0005∼0.500%

[0220] N is an element detrimental to the improvement of the corrosion resistance of stainless steel. Therefore, the N content is set to 0.500% or less, and preferably 0.200% or less. Although it is preferable for the N content to be lower, due to production technology constraints, the N content is set to 0.0005% or more.

[0221] Al: 0.001∼3.000%

[0222] In addition to acting as a deoxidizer, Al has the effect of inhibiting the peeling of oxide scale. To obtain these effects, the amount of Al is set to 0.001% or more. However, if the amount of Al is excessive, a decrease in elongation and deterioration of surface quality occur. Therefore, the amount of Al is set to 3.000% or less.

[0223] The remainder other than the above components is Fe and unavoidable impurities. However, it may optionally include at least one element selected from the following.

[0224] Ti: 0.500% or less

[0225] Ti combines with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, and depth drawability. However, if the Ti content exceeds 0.500%, toughness deteriorates due to dissolved Ti. Therefore, when adding Ti, the Ti content should be 0.500% or less.

[0226] Nb: 0.500% or less

[0227] Like Ti, Nb combines with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, and deep drawability. In addition to improving processability and high-temperature strength, it also inhibits inter-hole corrosion and promotes repassivation. However, excessive addition leads to hardening and deteriorates formability. Therefore, when adding Nb, the amount of Nb should be 0.500% or less.

[0228] V: 0.500% or less

[0229] V inhibits inter-hole corrosion. However, excessive addition degrades formability. Therefore, when adding V, the amount of V should be 0.500% or less.

[0230] W: 2,000% or less

[0231] W contributes to the improvement of corrosion resistance and high-temperature strength. However, excessive addition leads to deterioration of toughness or increased costs during steel sheet manufacturing. Therefore, when adding W, the amount of W should be 2,000% or less.

[0232] B: 0.0050% or less

[0233] B improves the secondary processability of the product by segregating at grain boundaries. However, excessive addition leads to a decrease in processability and corrosion resistance. Therefore, when adding B, the amount of B should be 0.0050% or less.

[0234] Mo: 2,000% or less

[0235] Mo is an element that improves corrosion resistance and, in particular, inhibits interstitial corrosion. However, excessive addition degrades formability. Therefore, when adding Mo, the amount of Mo should be 2,000% or less.

[0236] Cu: 3,000% or less

[0237] Like Ni or Mn, Cu is an austenite-stabilizing element and is effective for grain refinement through phase transformation. It also inhibits inter-hole corrosion and promotes repassivation. However, excessive addition degrades toughness and formability. Therefore, when adding Cu, the amount of Cu should be 3,000% or less.

[0238] Sn: 0.500% or less

[0239] Sn contributes to the improvement of corrosion resistance and high-temperature strength. However, excessive addition may cause slab cracking during steel sheet manufacturing. Therefore, when adding Sn, the amount of Sn should be 0.500% or less.

[0240] Sb: 0.200% or less

[0241] Sb has the effect of increasing high-temperature strength by segregating at grain boundaries. However, excessive addition may cause cracking during welding due to Sb segregation. Therefore, when adding Sb, the amount of Sb should be 0.200% or less.

[0242] Ta: 0.100% or less

[0243] Ta combines with C or N to contribute to the improvement of toughness. However, excessive addition leads to saturation of the effect and an increase in manufacturing costs. Therefore, when Ta is added, the amount of Ta is limited to 0.100% or less.

[0244] Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.1000% or less, REM (Rare Earth Metal): 0.0050% or less

[0245] Ca, Mg, Zr, and REM are effective elements for spheroidizing the shape of sulfides and improving the adverse effects of sulfides on moldability. When adding these elements, it is desirable to have a content of 0.0005% or more of each element. However, if the content of each is excessive, inclusions may increase, and surface and internal defects may occur. Therefore, when adding these elements, the content of each element should be 0.0050% or less.

[0246] [Diffusible hydrogen amount]

[0247] In this embodiment, in order to ensure good ductility, it is preferable that the diffusible hydrogen content of the product coil be 0.50 mass ppm or less, more preferable that it be 0.30 mass ppm or less, and even more preferable that it be 0.20 mass ppm or less. In addition, although the lower limit of the diffusible hydrogen content of the product coil is not specifically specified, due to production technology constraints, the diffusible hydrogen content of the product coil may be 0.01 mass ppm or more.

[0248] Herein, the method for measuring the amount of diffusible hydrogen in a product coil is as follows. A test specimen with a length of 30 mm and a width of 5 mm is taken from the product coil. In the case of a product coil of hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet, the hot-dip galvanizing layer or alloyed hot-dip galvanizing layer of the test specimen is removed by grinding or alkali. Afterward, the amount of hydrogen released from the test specimen is measured by Thermal Desorption Spectrometry (TDS). Specifically, the test specimen is continuously heated from room temperature to 300°C at a heating rate of 200°C / h, then cooled to room temperature, and the amount of integrated hydrogen released from the test specimen from room temperature to 210°C is measured and taken as the amount of diffusible hydrogen in the product coil.

[0249] (Example)

[0250] Steel having a compositional composition consisting of the elements shown in Table 1, with the remainder being Fe and unavoidable impurities, was melted in a converter and formed into a slab by a continuous casting method. The obtained slab was hot-rolled and cold-rolled to obtain a cold-rolled coil. As shown in Table 2, at some levels, a product coil of cold-rolled annealed steel sheet (CR) was manufactured by the CAL shown in Fig. 1, at other levels, a product coil of hot-dip galvanized steel sheet (GI) was manufactured by the CGL shown in Fig. 2 without performing heat alloying, and at the remaining levels, a product coil of alloyed hot-dip galvanized steel sheet (GA) was manufactured by the CGL shown in Fig. 2.

[0251] At each level, a normal magnetic field was applied along the width direction of a cold-rolled steel sheet in transit using a magnetic field application device as shown in FIGS. 4 and FIGS. 5(A) and 5(B), under the conditions of magnetic flux density (width direction) and magnetic field application time shown in Table 2. The “magnetic field application location” in Table 2 indicates the area where the magnetic field application process was performed in the CAL or CGL, that is, the location where the magnetic field application device was installed.

[0252] “(B-2)” means that in CAL and CGL, a magnetic field application device is installed in the cooling zone, and a magnetic field application process is performed in the cooling zone of process (B-2).

[0253] “(C)” means that, in the CAL, a magnetic field application device is installed at a location where a magnetic field can be applied to a cold-rolled steel sheet passing through a downstream facility, and means that a magnetic field application device is installed at at least one location downstream from the cooling zone and upstream from the tension reel, 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 temper rolling mill (36), and (iv) between the temper rolling mill (36) and the tension reel (50). That is, “(C)” means that, in the CAL, a process (C), specifically, a magnetic field application process is performed at at least one of the above (i) to (iv).

[0254] “(C-1) Before” means that in the CGL, a magnetic field application device is installed at a location downstream of the cooling zone and upstream of the molten zinc plating bath, specifically at the snout (29), and the magnetic field application process is performed after process (B-2) and before process (C-1).

[0255] “(C-1) After” is a position downstream of the molten zinc plating bath and upstream of the tension reel in the CGL, specifically, (i) between the molten zinc plating 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) an air cooling zone between the alloying furnace (33) and the cooling device (34), (v) between the cooling device (34) and the exit looper (35), (vi) inside the exit looper (35), (vii) between the exit looper (35) and the temper rolling mill (36), and (viii) between the temper rolling mill (36) and the tension reel (50), at least one location where a magnetic field application device is installed, and after process (C-1), specifically, the above This means that a magnetic field application process was performed at at least one of (i) to (viii).

[0256] Samples of steel sheets were taken from product coils obtained at each level, and tensile properties and hydrogen embrittlement resistance properties were evaluated as follows, and the results are shown in Table 2.

[0257] Tensile testing was performed in accordance with JIS Z 2241 (2011) using a JIS No. 5 test specimen sampled such that the tensile direction was perpendicular to the rolling direction of the steel plate, and TS (tensile strength) and EL (total elongation) were measured.

[0258] The hydrogen embrittlement resistance characteristics were evaluated as follows from the above tensile test. When the value obtained by dividing the EL of the steel plate after the application of the magnetic field measured above by the EL' when the hydrogen content in the steel of the same steel plate is 0.00 mass ppm is 0.70 or higher, the hydrogen embrittlement resistance characteristics were judged to be good. In addition, EL' was measured by reducing the hydrogen in the steel by leaving the same steel plate in the atmosphere for a long time, and then performing a tensile test after confirming that the hydrogen content in the steel became 0.00 mass ppm by TDS.

[0259] The amount of diffusible hydrogen in the product coils obtained at each level was measured using the method described above, and the results are shown in Table 2.

[0260] In the present invention example, since a magnetic field application process was performed, a steel plate with excellent hydrogen embrittlement resistance could be manufactured.

[0261]

[0262]

[0263]

[0264] (Industrial Applicability)

[0265] According to the continuous annealing device, the continuous hot-dip galvanizing device, and the method for manufacturing a steel sheet of the present invention, a steel sheet with excellent hydrogen embrittlement resistance can be manufactured without compromising production efficiency or changing mechanical properties. Explanation of the symbols

[0266] 100: Continuous annealing device 200: Continuous hot-dip galvanizing unit 300: Continuous Hot-Dip Galvanizing Unit 10: Payoff Reel 11 : Welder 12: Cleaning facilities 13: Inlet-side looper 20 : Annealing 22 : Heating zone 24 : Rift zone 26 : Cooling plate 28 : Statute of Limitations Processing Unit 29 : Snout 30: Downstream facilities 31 : Hot-dip galvanizing bath 32: Gas wiping device 33: Alloying 34: Cooling device 35 : Out-side looper 36 : Tempering rolling mill 50: Tension Reel 60: Magnetic field application device 60A : Electromagnet (magnetic field application device) 60B: Electromagnet (magnetic field application device) 62A : Iron core 62B : Iron core 64A: Coil 64B: Coil 66A : Stimulation surface (North pole) 66B : Stimulation surface (South pole) C: Cold-rolled coil S: Cold-rolled steel sheet P: Product coil

Claims

Claim 1 A payoff reel for discharging a cold-rolled steel sheet from a cold-rolled coil, and an annealing furnace for passing the cold-rolled steel sheet and performing continuous annealing, wherein a heating zone, a cracking zone, and a cooling zone are located upstream of the passing direction, and the cold-rolled steel sheet is annealed in a reducing atmosphere containing hydrogen in the heating zone and the cracking zone, and the cold-rolled steel sheet is cooled in the cooling zone; a downstream facility for continuously passing the cold-rolled steel sheet discharged from the annealing furnace, a tension reel for winding the cold-rolled steel sheet passing through the downstream facility, and a magnetic field application device for applying a steady magnetic field along the plate width direction of the cold-rolled steel sheet to the cold-rolled steel sheet passing from the cooling zone to the tension reel, wherein the magnetic field application device is set such that the magnetic flux density in the plate width direction of the cold-rolled steel sheet is 0.1 to 15T, for continuous annealing Device. Claim 2 In claim 1, the magnetic field application device is a continuous annealing device formed on the cooling bed. Claim 3 In claim 1, the magnetic field application device is a continuous annealing device formed at a position where a magnetic field can be applied to the cold-rolled steel sheet passing through the downstream facility. Claim 4 A continuous annealing device according to any one of claims 1 to 3, wherein the magnetic field application device comprises an electromagnet located on the outer side of the widthwise end of the cold-rolled steel sheet, and the electromagnet has a magnetic pole face facing the widthwise cross-section of the cold-rolled steel sheet. Claim 5 A continuous annealing device according to any one of claims 1 to 3, wherein the magnetic field application device comprises a pair of electromagnets each located on the outer side of both ends in the width direction of the cold-rolled steel sheet, and each of the pair of electromagnets has a magnetic pole surface facing the cross-section in the width direction of the cold-rolled steel sheet, and one of the magnetic pole surfaces is a N pole and the other is a S pole. Claim 6 A continuous hot-dip galvanizing apparatus having a continuous annealing apparatus described in claim 1, and a hot-dip galvanizing bath, which is located downstream of the annealing furnace in the direction of the plate flow and immerses the cold-rolled steel sheet and performs hot-dip galvanizing on the cold-rolled steel sheet as a downstream facility. Claim 7 In claim 6, the magnetic field application device is a continuous hot-dip galvanizing device formed at a position capable of applying a magnetic field to the cold-rolled steel sheet passing upstream of the hot-dip galvanizing bath. Claim 8 In claim 6, the magnetic field application device is a continuous hot-dip galvanizing device formed at a position where a magnetic field can be applied to the cold-rolled steel sheet passing downstream from the hot-dip galvanizing bath. Claim 9 A continuous hot-dip galvanizing apparatus according to claim 6, wherein, as a downstream facility, it has an alloying furnace located downstream of the plate-passing direction of the hot-dip galvanizing bath, through which the cold-rolled steel sheet is passed, and which heats and alloys the hot-dip galvanizing. Claim 10 In claim 9, the magnetic field application device is a continuous hot-dip galvanizing apparatus formed at a position capable of applying a magnetic field to the cold-rolled steel sheet passing upstream of the hot-dip galvanizing bath. Claim 11 In claim 9, the magnetic field application device is a continuous hot-dip galvanizing device formed at a position where a magnetic field can be applied to the cold-rolled steel sheet passing downstream from the hot-dip galvanizing bath. Claim 12 A continuous hot-dip galvanizing apparatus according to any one of claims 6 to 11, wherein the magnetic field application device comprises an electromagnet located on the outer side of the widthwise end of the cold-rolled steel sheet, and the electromagnet has a magnetic pole surface facing the widthwise cross-section of the cold-rolled steel sheet. Claim 13 A continuous hot-dip galvanizing apparatus according to any one of claims 6 to 11, wherein the magnetic field application device comprises a pair of electromagnets each located on the outer side of both ends in the width direction of the cold-rolled steel sheet, and each of the pair of electromagnets has a magnetic pole surface facing the cross-section in the width direction of the cold-rolled steel sheet, and one of the magnetic pole surfaces is the N pole and the other is the S pole. Claim 14 (A) a process of extruding a cold-rolled steel sheet from a cold-rolled coil by means of a payoff reel; (B) a process of passing the cold-rolled steel sheet through an annealing furnace in which a heating zone, a cracking zone, and a cooling zone are located upstream of the passing direction; (B-1) annealing the cold-rolled steel sheet in a reducing atmosphere containing hydrogen in the heating zone and the cracking zone; (B-2) cooling the cold-rolled steel sheet in the cooling zone; a process of performing continuous annealing, wherein (C) a process of continuously passing the cold-rolled steel sheet discharged from the annealing furnace; and (D) a process of winding the cold-rolled steel sheet by means of a tension reel to form a product coil, wherein, after process (B-2) and also before process (D), a magnetic field application process is included in which a normal magnetic field is applied along the plate width direction of the cold-rolled steel sheet being passed through, and in the magnetic field application process, the magnetic flux density in the plate width direction of the cold-rolled steel sheet A method for manufacturing a steel plate with a thickness of 0.1 to 15T. Claim 15 In paragraph 14, the above magnetic field application process is a method for manufacturing a steel plate, which is performed in process (B-2). Claim 16 In paragraph 14, the above magnetic field application process is a method for manufacturing a steel plate, which is performed in process (C). Claim 17 In claim 14, the process (C) comprises the process of (C-1) immersing the cold-rolled steel sheet in a molten zinc plating bath located downstream of the annealing furnace in the direction of the sheet flow, and performing molten zinc plating on the cold-rolled steel sheet. Claim 18 In claim 17, the above magnetic field application process is performed before process (C-1), a method for manufacturing a steel plate. Claim 19 In claim 17, the above magnetic field application process is performed after process (C-1), a method for manufacturing a steel plate. Claim 20 A method for manufacturing a steel sheet according to claim 17, wherein the above process (C) comprises, following the above process (C-1), passing the cold-rolled steel sheet through an alloying furnace located downstream of the molten zinc plating bath in the direction of passage (C-2), and heating and alloying the molten zinc plating. Claim 21 In claim 20, the above magnetic field application process is performed before process (C-1), a method for manufacturing a steel plate. Claim 22 In claim 20, the above magnetic field application process is performed after process (C-1), a method for manufacturing a steel plate. Claim 23 A method for manufacturing a steel sheet according to any one of claims 14 to 22, wherein the magnetic field application process includes an electromagnet located on the outer side of the widthwise end of the cold-rolled steel sheet, and the normal magnetic field is applied by a magnetic field application device having a magnetic pole surface facing the widthwise cross-section of the cold-rolled steel sheet. Claim 24 A method for manufacturing a steel sheet according to any one of claims 14 to 22, wherein the magnetic field application process comprises a pair of electromagnets each located on the outer side of both ends in the width direction of the cold-rolled steel sheet, and the pair of electromagnets each have a magnetic pole face facing the cross-section in the width direction of the cold-rolled steel sheet, and the normal magnetic field is applied by a magnetic field application device in which one of the magnetic pole faces is a North pole and the other is a South pole. Claim 25 A method for manufacturing a steel sheet according to any one of claims 14 to 22, wherein the cold-rolled steel sheet is a high-strength steel sheet having a tensile strength of 590 MPa or more. Claim 26 A method for manufacturing a steel sheet according to any one of claims 14 to 22, wherein the cold-rolled steel sheet has a composition in mass% comprising 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 being Fe and unavoidable impurities. Claim 27 A method for manufacturing a steel sheet according to claim 26, wherein the above composition further comprises, 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. Claim 28 A method for manufacturing a steel sheet according to any one of claims 14 to 22, wherein the cold-rolled steel sheet is a stainless steel sheet having a composition in mass% comprising 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 being Fe and unavoidable impurities. Claim 29 A method for manufacturing a steel sheet according to claim 28, wherein the above composition further comprises, in mass%, 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. Claim 30 A method for manufacturing a steel plate according to any one of claims 14 to 22, wherein the product coil has a diffusible hydrogen content of 0.50 mass ppm or less. Claim 31 delete Claim 32 delete Claim 33 delete

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