CONTINUOUS ANNEALING APPARATUS, CONTINUOUS HOT-DIP GALVANIZATION APPARATUS, AND STEEL SHEET MANUFACTURING METHOD

Abstract

 Provided is a continuous annealing line capable of producing a steel sheet excellent in hydrogen embrittlement resistance. A continuous annealing line 100 includes: a payoff reel 10 configured to uncoil a cold-rolled coil C to feed a cold-rolled steel sheet S; an annealing furnace 20 configured to pass the cold-rolled steel sheet S therethrough to continuously anneal the cold-rolled steel sheet S and including a heating zone 22, a soaking zone 24, and a cooling zone 26 that are arranged from an upstream side in a sheet passing direction, the cold-rolled steel sheet S being annealed in a reducing atmosphere containing hydrogen in the heating zone 22 and the soaking zone 24, and cooled in the cooling zone 26; a downstream line 30 configured to continuously pass the cold-rolled steel sheet S discharged from the annealing furnace 20 therethrough; a tension reel 50 configured to coil the cold-rolled steel sheet S being passed through the downstream line 30; and a vibration application device 60 or 70 configured to apply vibration to the cold-rolled steel sheet S being passed from the cooling zone 26 to the tension reel 50 so that the cold-rolled steel sheet is caused to vibrate at a frequency of 100 Hz or more and 100,000 Hz or less and a maximum amplitude of 10 nm or more and 500 µm or less.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a continuous annealing line, a continuous hot-dip galvanizing line, and a steel sheet production method. The present disclosure particularly relates to a continuous annealing line, a continuous hot-dip galvanizing line, and a steel sheet production method for producing a steel sheet that has low hydrogen content in steel and excellent hydrogen embrittlement resistance and is suitable for use in the fields of automobiles, home electric appliances, building materials, etc.

BACKGROUND

[0002] For example, when producing an annealed steel sheet in a continuous annealing line and when producing a hot-dip galvanized steel sheet in a continuous hot-dip galvanizing line, a steel sheet is annealed in a reducing atmosphere containing hydrogen. During this annealing, hydrogen enters into the steel sheet. Hydrogen present in the steel sheet lowers the formability of the steel sheet, such as ductility, bendability, and stretch flangeability. Hydrogen present in the steel sheet also embrittles the steel sheet, and can cause a delayed fracture. A treatment for reducing the hydrogen content in the steel sheet is therefore needed.

[0003] For example, by leaving, at room temperature, a product coil produced in a continuous annealing line or a continuous hot-dip galvanizing line, the hydrogen content in the steel can be reduced. However, at room temperature, it takes time for hydrogen to move from the inside to the surface of the steel sheet and desorb from the surface. Accordingly, the product coil needs to be left at room temperature for at least several weeks, in order to sufficiently reduce the hydrogen content in the steel. The space and time required for such dehydrogenation treatment pose a problem in the production process.

[0004] WO 2019/188642 A1 (PTL 1) discloses a method of reducing the hydrogen content in steel by holding an annealed steel sheet, a hot-dip galvanized steel sheet, or a galvannealed steel sheet in a temperature range of 50 °C or more and 300 °C or less for 1,800 seconds or more and 43,200 seconds or less.

CITATION LIST

Patent Literature

[0005] PTL 1: WO 2019/188642 A1

SUMMARY

(Technical Problem)

[0006] With the method described in PTL 1, however, there is concern that microstructural changes by heating may cause changes in mechanical properties such as yield stress increase and temper embrittlement.

[0007] It could therefore be helpful to provide a continuous annealing line, a continuous hot-dip galvanizing line, and a steel sheet production method capable of producing a steel sheet excellent in hydrogen embrittlement resistance without changing the mechanical properties and without impairing the production efficiency.

(Solution to Problem)

[0008] Upon careful examination, we discovered the following: After annealing a steel sheet in a reducing atmosphere containing hydrogen in a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL), by applying vibration at a predetermined frequency and predetermined maximum amplitude to the steel sheet being continuously passed in a cooling process from the annealing temperature to room temperature, hydrogen in the steel sheet can be reduced sufficiently and efficiently. Specifically, it was found that microvibration of the steel sheet at a high frequency and small maximum amplitude can sufficiently and efficiently reduce hydrogen in the steel sheet. This is presumed to be due to the following mechanism: By forcibly microvibrating the steel sheet, the steel sheet undergoes repeated bending deformation. As a result, the lattice spacing of the surface expands as compared with the mid-thickness part of the steel sheet. Hydrogen in the steel sheet diffuses toward the surface of the steel sheet with wide lattice spacing and low potential energy, and desorbs from the surface.

[0009] The present disclosure is based on these discoveries. We thus provide:

[1] A continuous annealing line comprising: a payoff reel configured to uncoil a cold-rolled coil to feed a cold-rolled steel sheet; an annealing furnace configured to pass the cold-rolled steel sheet therethrough to continuously anneal the cold-rolled steel sheet and including a heating zone, a soaking zone, and a cooling zone that are arranged 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 cooled in the cooling zone; a downstream line configured to continuously pass the cold-rolled steel sheet discharged from the annealing furnace therethrough; a tension reel configured to coil the cold-rolled steel sheet being passed through the downstream line; and a vibration application device configured to apply vibration to the cold-rolled steel sheet being passed from the cooling zone to the tension reel so that the cold-rolled steel sheet is caused to vibrate at a frequency of 100 Hz or more and 100,000 Hz or less and a maximum amplitude of 10 nm or more and 500 µm or less.

[2] The continuous annealing line according to [1], wherein the vibration application device is located in the cooling zone.

[3] The continuous annealing line according to [1] or [2], wherein the vibration application device is located at a position that enables applying vibration to the cold-rolled steel sheet being passed through the downstream line.

[4] The continuous annealing line according to any one of [1] to [3], wherein an arrangement of the vibration application device and a sheet passing speed of the cold-rolled steel sheet are set so that a vibration application time for the cold-rolled steel sheet will be 1 second or more.

[5] The continuous annealing line according to any one of [1] to [4], wherein the vibration application device comprises an electromagnet having a magnetic pole surface spaced from and facing a surface of the cold-rolled steel sheet, and the vibration application device is configured to cause the cold-rolled steel sheet to vibrate in response to an external force exerted by the electromagnet on the cold-rolled steel sheet.

[6] The continuous annealing line according to any one of [1] to [4], wherein the vibration application device comprises a vibration element configured to contact the cold-rolled steel sheet, and the vibration application device is configured to cause the cold-rolled steel sheet to be vibrated by the vibration element.

[7] A continuous hot-dip galvanizing line comprising: the continuous annealing line according to [1]; and a hot-dip galvanizing bath located, as the downstream line, downstream of the annealing furnace in the sheet passing direction, and configured to immerse the cold-rolled steel sheet therein to apply a hot-dip galvanized coating onto the cold-rolled steel sheet.

[8] The continuous hot-dip galvanizing line according to [7], wherein the vibration application device is located at a position that enables applying vibration to the cold-rolled steel sheet being passed upstream of the hot-dip galvanizing bath.

[9] The continuous hot-dip galvanizing line according to [7] or [8], wherein the vibration application device is located at a position that enables applying vibration to the cold-rolled steel sheet being passed downstream of the hot-dip galvanizing bath.

[10] The continuous hot-dip galvanizing line according to [7], comprising an alloying furnace located, as the downstream line, downstream of the hot-dip galvanizing bath in the sheet passing direction, and configured to pass the cold-rolled steel sheet therethrough to heat and alloy the hot-dip galvanized coating.

[11] The continuous hot-dip galvanizing line according to [10], wherein the vibration application device is located at a position that enables applying vibration to the cold-rolled steel sheet being passed upstream of the hot-dip galvanizing bath.

[12] The continuous hot-dip galvanizing line according to [10] or [11], wherein the vibration application device is located at a position that enables applying vibration to the cold-rolled steel sheet being passed downstream of the hot-dip galvanizing bath.

[13] The continuous hot-dip galvanizing line according to any one of [7] to [12], wherein an arrangement of the vibration application device and a sheet passing speed of the cold-rolled steel sheet are set so that a vibration application time for the cold-rolled steel sheet will be 1 second or more.

[14] The continuous hot-dip galvanizing line according to any one of [7] to [13], wherein the vibration application device comprises an electromagnet having a magnetic pole surface spaced from and facing a surface of the cold-rolled steel sheet, and the vibration application device is configured to cause the cold-rolled steel sheet to vibrate in response to an external force exerted by the electromagnet on the cold-rolled steel sheet.

[15] The continuous hot-dip galvanizing line according to any one of [7] to [13], wherein the vibration application device comprises a vibration element configured to contact the cold-rolled steel sheet, and the vibration application device is configured to cause the cold-rolled steel sheet to be vibrated by the vibration element.

[16] A steel sheet production method comprising, in the following order: a step (A) of uncoiling a cold-rolled coil to feed a cold-rolled steel sheet by a payoff reel; a step (B) of passing the cold-rolled steel sheet through an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged from an upstream side in a sheet passing direction, to continuously anneal the cold-rolled steel sheet by a step (B-1) of annealing the cold-rolled steel sheet in a reducing atmosphere containing hydrogen in the heating zone and the soaking zone and a step (B-2) of cooling the cold-rolled steel sheet in the cooling zone; a step (C) of continuously passing the cold-rolled steel sheet discharged from the annealing furnace; and a step (D) of coiling the cold-rolled steel sheet by a tension reel to obtain a product coil, wherein the steel sheet production method comprises a vibration application step of applying vibration to the cold-rolled steel sheet being passed in or after the step (B-2) and before the step (D) so that the cold-rolled steel sheet is caused to vibrate at a frequency of 100 Hz or more and 100,000 Hz or less and a maximum amplitude of 10 nm or more and 500 µm or less.

[17] The steel sheet production method according to [16], wherein the vibration application step is performed in the step (B-2).

[18] The steel sheet production method according to [16] or [17], wherein the vibration application step is performed in the step (C).

[19] The steel sheet production method according to [16], wherein the step (C) includes a step (C-1) of immersing the cold-rolled steel sheet in a hot-dip galvanizing bath located downstream of the annealing furnace in the sheet passing direction to apply a hot-dip galvanized coating onto the cold-rolled steel sheet.

[20] The steel sheet production method according to [19], wherein the vibration application step is performed before the step (C-1).

[21] The steel sheet production method according to [19] or [20], wherein the vibration application step is performed after the step (C-1).

[22] The steel sheet production method according to [19], wherein the step (C) includes, following the step (C-1), a step (C-2) of passing the cold-rolled steel sheet through an alloying furnace located downstream of the hot-dip galvanizing bath in the sheet passing direction to heat and alloy the hot-dip galvanized coating.

[23] The steel sheet production method according to [22], wherein the vibration application step is performed before the step (C-1).

[24] The steel sheet production method according to [22] or [23], wherein the vibration application step is performed after the step (C-1).

[25] The steel sheet production method according to any one of [16] to [24], wherein in the vibration application step, a vibration application time for the cold-rolled steel sheet is 1 second or more.

[26] The steel sheet production method according to any one of [16] to [25], wherein in the vibration application step, the cold-rolled steel sheet is caused to vibrate in response to an external force exerted by an electromagnet on the cold-rolled steel sheet, the electromagnet having a magnetic pole surface spaced from and facing a surface of the cold-rolled steel sheet.

[27] The steel sheet production method according to any one of [16] to [25], wherein in the vibration application step, the cold-rolled steel sheet is caused to be vibrated by a vibration element configured to contact the cold-rolled steel sheet.

[28] The steel sheet production method according to any one of [16] to [27], wherein the cold-rolled steel sheet is a high strength steel sheet having a tensile strength of 590 MPa or more.

[29] The steel sheet production method according to any one of [16] to [28], wherein the cold-rolled steel sheet has a chemical composition containing (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 %, with the balance being Fe and inevitable impurities.

[30] The steel sheet production method according to [29], wherein the chemical composition further contains, 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.

[31] The steel sheet production method according to any one of [16] to [27], wherein the cold-rolled steel sheet is a stainless steel sheet having a chemical composition containing (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 %, with the balance being Fe and inevitable impurities.

[32] The steel sheet production method according to [31], wherein the chemical composition further contains, 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.

[33] The steel sheet production method according to any one of [16] to [32], wherein the product coil has a diffusible hydrogen content of 0.50 mass ppm or less.

(Advantageous Effect)

[0010] It is thus possible to provide a continuous annealing line, a continuous hot-dip galvanizing line, and a steel sheet production method capable of producing a steel sheet excellent in hydrogen embrittlement resistance without changing the mechanical properties and without impairing the production efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In the accompanying drawings:

FIG. 1 is a schematic view of a continuous annealing line 100 according to one embodiment of the present disclosure;

FIG. 2 is a schematic view of a continuous hot-dip galvanizing line 200 according to one embodiment of the present disclosure;

FIG. 3 is a schematic view of a continuous hot-dip galvanizing line 300 according to another embodiment of the present disclosure;

FIG. 4 is a schematic view illustrating the structure of a vibration application device 60 used in each embodiment of the present disclosure;

FIG. 5A is a schematic view illustrating an example of installation of electromagnets 63 of the vibration application device 60 relative to a cold-rolled steel sheet S being passed in each embodiment of the present disclosure;

FIG. 5B is a schematic view illustrating another example of installation of electromagnets 63 of the vibration application device 60 relative to a cold-rolled steel sheet S being passed in each embodiment of the present disclosure;

FIG. 6A is a schematic view illustrating how the magnetic field is generated from an electromagnet 63 in each embodiment of the present disclosure;

FIG. 6B is a schematic view illustrating how the magnetic field is generated from an electromagnet 63 in each embodiment of the present disclosure;

FIG. 7A is a schematic view illustrating the structure of a vibration application device 70 used in each embodiment of the present disclosure;

FIG. 7B is a schematic view illustrating an example of installation of a vibration element 72 of the vibration application device 70 relative to a cold-rolled steel sheet S being passed;

FIG. 8A is a schematic view illustrating an example of the positional relationship between cooling nozzles 26A and vibration application devices 60 or 70 in the case where the vibration application devices 60 or 70 are installed in a cooling zone 26; and

FIG. 8B is a schematic view illustrating another example of the positional relationship between cooling nozzles 26A and vibration application devices 60 or 70 in the case where the vibration application devices 60 or 70 are installed in a cooling zone 26.

DETAILED DESCRIPTION

[0012] One embodiment of the present disclosure relates to a continuous annealing line (CAL), and another embodiment of the present disclosure relates to a continuous hot-dip galvanizing line (CGL).

[0013] A steel sheet production method according to one embodiment of the present disclosure is implemented by a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL).

[0014] With reference to FIG. 1, a continuous annealing line (CAL) 100 according to Embodiment 1 of the present disclosure comprises: a payoff reel 10 configured to uncoil a cold-rolled coil C to feed a cold-rolled steel sheet S; an annealing furnace 20 configured to pass the cold-rolled steel sheet S therethrough to continuously anneal the cold-rolled steel sheet S; a downstream line 30 configured to continuously pass the cold-rolled steel sheet S discharged from the annealing furnace 20 therethrough; and a tension reel 50 configured to coil the cold-rolled steel sheet S being passed through the downstream line 30 to obtain a product coil P. In the annealing furnace 20, a heating zone 22, a soaking zone 24, and a cooling zone 26 are arranged from the upstream side in the sheet passing direction. In the heating zone 22 and the soaking zone 24, the cold-rolled steel sheet S is annealed in a reducing atmosphere containing hydrogen. In the cooling zone 26, the cold-rolled steel sheet S is cooled. The annealing furnace 20 in the CAL 100 preferably includes an overaging treatment zone 28 downstream of the cooling zone 26, although the overaging treatment zone 28 is not essential. In the overaging treatment zone 28, the cold-rolled steel sheet S is subjected to an overaging treatment. In this embodiment, the CAL 100 produces a product coil of a cold-rolled and annealed steel sheet (CR).

[0015] With reference to FIG. 1, a steel sheet production method according to Embodiment 1 implemented by the continuous annealing line (CAL) 100 comprises, in the following order: a step (A) of uncoiling a cold-rolled coil C to feed a cold-rolled steel sheet (steel strip) S by the payoff reel 10; a step (B) of passing the cold-rolled steel sheet S through the annealing furnace 20 in which the heating zone 22, the soaking zone 24, and the cooling zone 26 are arranged from the upstream side in the sheet passing direction, to continuously anneal the cold-rolled steel sheet S by a step (B-1) of annealing the cold-rolled steel sheet S in a reducing atmosphere containing hydrogen in the heating zone 22 and the soaking zone 24 and a step (B-2) of cooling the cold-rolled steel sheet S in the cooling zone 26; a step (C) of continuously passing the cold-rolled steel sheet S discharged from the annealing furnace 20; and a step (D) of coiling the cold-rolled steel sheet S by the tension reel 50 to obtain a product coil P. In the continuous annealing step (B) by the annealing furnace 20 in the CAL 100, it is preferable to perform a step (B-3) of subjecting the cold-rolled steel sheet S to an overaging treatment by the overaging treatment zone 28 optionally located downstream of the cooling zone 26, although this step is not essential. This embodiment is a method of producing a product coil of a cold-rolled and annealed steel sheet (CR) by the CAL 100.

[0016] With reference to FIG. 2, a continuous hot-dip galvanizing line (CGL) 200 according to Embodiment 2 of the present disclosure comprises: a payoff reel 10 configured to uncoil a cold-rolled coil C to feed a cold-rolled steel sheet S; an annealing furnace 20 configured to pass the cold-rolled steel sheet S therethrough to continuously anneal the cold-rolled steel sheet S; a downstream line 30 configured to continuously pass the cold-rolled steel sheet S discharged from the annealing furnace 20 therethrough; and a tension reel 50 configured to coil the cold-rolled steel sheet S being passed through the downstream line 30 to obtain a product coil P. In the annealing furnace 20, a heating zone 22, a soaking zone 24, and a cooling zone 26 are arranged from the upstream side in the sheet passing direction. In the heating zone 22 and the soaking zone 24, the cold-rolled steel sheet S is annealed in a reducing atmosphere containing hydrogen. In the cooling zone 26, the cold-rolled steel sheet S is cooled. The CGL 200 further comprises, as the downstream line 30: a hot-dip galvanizing bath 31 located downstream of the annealing furnace 20 in the sheet passing direction and configured to immerse the cold-rolled steel sheet S therein to apply a hot-dip galvanized coating onto the cold-rolled steel sheet S; and an alloying furnace 33 located downstream of the hot-dip galvanizing bath 31 in the sheet passing direction and configured to pass the cold-rolled steel sheet S therethrough to heat and alloy the hot-dip galvanized coating. In this embodiment, the CGL 200 produces a product coil of a galvannealed steel sheet (GA) whose galvanized layer is alloyed. In the case where the steel sheet S is simply passed through the alloying furnace 33 without being heated and alloyed, a product coil of a hot-dip galvanized steel sheet (GI) whose galvanized layer is not alloyed is produced.

[0017] With reference to FIG. 2, a steel sheet production method according to Embodiment 2 implemented by the continuous hot-dip galvanizing line (CGL) 200 comprises, in the following order: a step (A) of uncoiling a cold-rolled coil C to feed a cold-rolled steel sheet (steel strip) S by the payoff reel 10; a step (B) of passing the cold-rolled steel sheet S through the annealing furnace 20 in which the heating zone 22, the soaking zone 24, and the cooling zone 26 are arranged from the upstream side in the sheet passing direction, to continuously anneal the cold-rolled steel sheet S by a step (B-1) of annealing the cold-rolled steel sheet S in a reducing atmosphere containing hydrogen in the heating zone 22 and the soaking zone 24 and a step (B-2) of cooling the cold-rolled steel sheet S in the cooling zone 26; a step (C) of continuously passing the cold-rolled steel sheet S discharged from the annealing furnace 20; and a step (D) of coiling the cold-rolled steel sheet S by the tension reel 50 to obtain a product coil P. The step (C) includes: a step (C-1) of immersing the cold-rolled steel sheet S in the hot-dip galvanizing bath 31 located downstream of the annealing furnace 20 in the sheet passing direction to apply a hot-dip galvanized coating onto the cold-rolled steel sheet S; and a step (C-2) of, following the step (C-1), passing the cold-rolled steel sheet S through the alloying furnace 33 located downstream of the hot-dip galvanizing bath 31 in the sheet passing direction to heat and alloy the hot-dip galvanized coating. This embodiment is a method of producing a product coil of a galvannealed steel sheet (GA) whose galvanized layer is alloyed, by the CGL 200.

[0018] With reference to FIG. 3, a continuous hot-dip galvanizing line (CGL) 300 according to Embodiment 3 of the present disclosure has the same structure as the CGL 200 except that the alloying furnace 33 is not included. In this embodiment, the CGL 300 produces a product coil of a hot-dip galvanized steel sheet (GI) whose galvanized layer is not alloyed.

[0019] That is, a steel sheet production method according to Embodiment 3 that includes the step (C-1) but does not include the step (C-2) is, for example, implemented by the CGL 300 not including the alloying furnace 33 or by a method that simply passes the steel sheet S through the alloying furnace 33 in the CGL 200 without heating and alloying it. This embodiment is a method of producing a product coil of a hot-dip galvanized steel sheet (GI) whose galvanized layer is not alloyed, by the CGL 200 or the CGL 300.

[0020] Each component in the CAL according to Embodiment 1 and the CGLs according to Embodiments 2 and 3 will be described in detail below. Moreover, each step in the steel sheet production methods according to Embodiments 1 to 3 will be described in detail below.

[0021] [Payoff reel, and line from payoff reel to annealing furnace]

[Step (A)]

[0022] With reference to FIGS. 1 to 3, the payoff reel 10 uncoils the cold-rolled coil C to feed the cold-rolled steel sheet S. That is, in the step (A), the cold-rolled coil C is uncoiled to feed the cold-rolled steel sheet S by the payoff reel 10. The cold-rolled steel sheet S fed is passed through a welder 11, a cleaning line 12, and an entry looper 13 and supplied to the annealing furnace 20. The upstream line between the payoff reel 10 and the annealing furnace 20 is, however, not limited to the welder 11, the cleaning line 12, and the entry looper 13, and may be a known line or any line.

[Annealing furnace]

[Step (B)]

[0023] With reference to FIGS. 1 to 3, the annealing furnace 20 passes the cold-rolled steel sheet S therethrough to continuously anneal the cold-rolled steel sheet S. In the annealing furnace 20, the heating zone 22, the soaking zone 24, and the cooling zone 26 are arranged from the upstream side in the sheet passing direction. In the heating zone 22 and the soaking zone 24, the cold-rolled steel sheet S is annealed in a reducing atmosphere containing hydrogen. In the cooling zone 26, the cold-rolled steel sheet S is cooled. That is, in the step (B), the cold-rolled steel sheet S is passed through the annealing furnace 20 in which the heating zone 22, the soaking zone 24, and the cooling zone 26 are arranged from the upstream side in the sheet passing direction, to continuously anneal the cold-rolled steel sheet S. The cooling zone 26 may be composed of a plurality of cooling zones. A preheating zone may be provided upstream of the heating zone 22 in the sheet passing direction. The annealing furnace 20 in the CAL 100 illustrated in FIG. 1 preferably includes the overaging treatment zone 28 downstream of the cooling zone 26, although the overaging treatment zone 28 is not essential. Although each zone is illustrated as a vertical furnace in FIGS. 1 to 3, the zone is not limited to such, and may be a horizontal furnace. In the case of a vertical furnace, adjacent zones communicate with each other through a throat (restriction portion) that connects the upper parts or lower parts of the respective zones.

(Heating zone)

[0024] 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. Heating by induction heating, roll heating, electrical resistance heating, direct resistance heating, salt bath heating, electron beam heating, etc. is also possible. The average temperature inside the heating zone 22 is preferably 500 °C to 800 °C. The gas from the soaking zone 24 flows into the heating zone 22, and simultaneously a reducing gas is supplied to the heating zone 22. As the reducing gas, a H₂-N₂ mixed gas is usually used, such as a gas (dew point: about -60 °C) having a composition containing H₂: 1 vol% to 35 vol% with the balance being one or both of N₂ and Ar and inevitable impurities.

(Soaking zone)

[0025] In the soaking zone 24, the cold-rolled steel sheet S can be indirectly heated using a radiant tube (RT). The average temperature inside the soaking zone 24 is preferably 600 °C to 950 °C. A reducing gas is supplied to the soaking zone 24. As the reducing gas, a H₂-N₂ mixed gas is usually used, such as a gas (dew point: about -60 °C) having a composition containing H₂: 1 vol% to 35 vol% with the balance being one or both of N₂ and Ar and inevitable impurities.

(Cooling zone)

[0026] In the cooling zone 26, the cold-rolled steel sheet S is cooled by gas, a mixture of gas and water, or water. The cold-rolled steel sheet S is cooled to about 100 °C to 400 °C in the CAL and about 470 °C to 530 °C in the CGL, at the stage of leaving the annealing furnace 20. As illustrated in FIGS. 8A to 8B, a plurality of cooling nozzles 26A are arranged in the cooling zone 26 along the steel sheet conveyance path. For example, each of the cooling nozzles 26A is a circular pipe longer than the width of the steel sheet as described in JP 2010-185101 A, and is installed so that the extending direction of the circular pipe will be parallel to the transverse direction of the steel sheet. The circular pipe has, in a part facing the steel sheet, a plurality of through-holes at certain intervals in the extending direction of the circular pipe, and the water inside the circular pipe is jetted from the through-holes toward the steel sheet. A plurality of cooling nozzle pairs (for example, five to ten pairs) each of which are located to face the front and back of the steel sheet are arranged at certain intervals along the steel sheet conveyance path to form one cooling zone. Approximately three to six cooling zones are preferably arranged along the steel sheet conveyance path.

(Overaging treatment zone)

[0027] With reference to FIG. 1, in the overaging treatment zone 28 in the CAL 100, the cold-rolled steel sheet S that has left the cooling zone 26 is subjected to at least one treatment out of isothermal holding, reheating, furnace cooling, and natural cooling. The cold-rolled steel sheet S is cooled to about 100 °C to 400 °C at the stage of leaving the annealing furnace 20.

[Downstream line]

[Step (C)]

[0028] With reference to FIGS. 1 to 3, in the step (C), the cold-rolled steel sheet S discharged from the annealing furnace 20 is continuously passed through the downstream line 30. With reference to FIG. 1, the CGL 100 includes an exit looper 35 and a temper mill 36 as the downstream line 30. With reference to FIG. 2, the CGL 200 includes the hot-dip galvanizing bath 31, a gas wiping device 32, the alloying furnace 33, a cooling device 34, the exit looper 35, and the temper mill 36 as the downstream line 30. With reference to FIG. 3, the CGL 300 includes the hot-dip galvanizing bath 31, the gas wiping device 32, the cooling device 34, the exit looper 35, and the temper mill 36 as the downstream line 30. The downstream line 30 is, however, not limited to such, and may be a known line or any line. Examples of the downstream line 30 include a tension leveler, a chemical conversion treatment line, a surface control line, an oiling line, and an inspection line.

(Hot-dip galvanizing bath)

(Step (C-1))

[0029] With reference to FIGS. 2 and 3, the hot-dip galvanizing bath 31 is located downstream of the annealing furnace 20 in the sheet passing direction, and immerses the cold-rolled steel sheet S therein to apply a hot-dip galvanized coating onto the cold-rolled steel sheet S. That is, in the step (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 sheet passing direction, to apply a hot-dip galvanized coating onto the cold-rolled steel sheet S. A snout 29 connected to the most downstream zone (the cooling zone 26 in FIGS. 2 and 3) of the annealing furnace is a member having a rectangular cross section perpendicular to the sheet passing direction and defining the space through which the cold-rolled steel sheet S passes. The tip of the snout 29 is immersed in the hot-dip galvanizing bath 31, thereby connecting the annealing furnace 20 and the hot dip galvanizing bath 31. The hot-dip galvanizing may be performed according to a usual method.

[0030] A pair of gas wiping devices 32 arranged so that the cold-rolled steel sheet S pulled up from the hot-dip galvanizing bath 31 will be interposed therebetween blow a gas onto the cold-rolled steel sheet S, with it being possible to adjust the coating weight of molten zinc on both sides of the cold-rolled steel sheet S.

(Alloying furnace)

(Step (C-2))

[0031] With reference to FIG. 2, the alloying furnace 33 is located downstream of the hot-dip galvanizing bath 31 and the gas wiping device 32 in the sheet passing direction, and passes the cold-rolled steel sheet S therethrough to heat and alloy the hot-dip galvanized coating. That is, in the step (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 sheet passing direction, to heat and alloy the hot-dip galvanized coating. The alloying treatment may be performed according to a usual method. The heating means in the alloying furnace 33 is not limited, and examples include heating with high-temperature gas and induction heating. The alloying furnace 33 is an optional line in the CGL, and the alloying step is an optional step in the steel sheet production method using the CGL.

(Cooling device)

[0032] With reference 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 sheet passing direction, and passes the cold-rolled steel sheet S therethrough 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, or the like.

[Tension reel]

[Step (D)]

[0033] With reference to FIGS. 1 to 3, the cold-rolled steel sheet S that has passed through the downstream line 30 is eventually coiled into a product coil P by the tension reel 50 as a coiler.

[Vibration application device and vibration application step]

[0034] It is important that each of the CAL 100 according to Embodiment 1, the CGL 200 according to Embodiment 2, and the CGL 300 according to Embodiment 3 comprises a vibration application device 60 or 70 configured to apply vibration to the cold-rolled steel sheet S being passed from the cooling zone 26 to the tension reel 50. That is, it is important that each of the steel sheet production methods according to Embodiments 1 to 3 comprises a vibration application step of applying vibration to the cold-rolled steel sheet S being passed in or after the step (B-2) and before the step (D). Moreover, it is important that the vibration applied to the cold-rolled steel sheet S by the vibration application device 60 or 70 be such that the cold-rolled steel sheet S is caused to vibrate at a frequency of 100 Hz or more and 100,000 Hz or less and a maximum amplitude of 10 nm or more and 500 µm or less. Consequently, hydrogen contained in the cold-rolled steel sheet S as a result of annealing can be reduced sufficiently and efficiently, and a steel sheet excellent in hydrogen embrittlement resistance can be produced. Since the application of vibration is incorporated in the steel sheet production process by the CAL 100, the CGL 200, or the CGL 300 (inline), the production efficiency is not impaired. Moreover, since hydrogen is desorbed not by heating but by application of vibration, there is no concern that the mechanical properties of the steel sheet may be changed.

(Vibration application device 60)

[0035] Each embodiment of the present disclosure can be carried out by installing a vibration application device 60 as illustrated in FIG. 4 in the CAL 100, the CGL 200, or the CGL 300. The vibration application step can be carried out by applying vibration to the cold-rolled steel sheet S being passed using the vibration application device 60. With reference to FIG. 4, the vibration application device 60 includes a controller 61, an amplifier 62, an electromagnet 63, a vibration detector 64, and a power supply 65. With reference to FIGS. 6A and 6B, the vibration application device 60 has an electromagnet 63 that includes a magnet 63A and a coil 63B wound around the magnet 63A. The electromagnet 63 has a magnetic pole surface 63A1 that is spaced from and facing a surface of the cold-rolled steel sheet S. The vibration application device 60 is configured to cause the cold-rolled steel sheet S to vibrate in response to an external force (attractive force) exerted by the electromagnet 63 on the cold-rolled steel sheet S.

[0036] The shape and installation of the electromagnet 63 is not limited as long as the electromagnet 63 has a magnetic pole surface 63A1 that is spaced from and facing a surface of the cold-rolled steel sheet S. As illustrated in FIGS. 6A and 6B, this setup makes the direction of the magnetic field lines perpendicular to the cold-rolled steel sheet S and enables an attractive force to be applied to the cold-rolled steel sheet S. The shape and installation of such electromagnets can be seen, for example, in FIGS. 5A and 5B.

[0037] In FIG. 5A, rectangular-shaped electromagnets 63 are spaced at a predetermined distance from a surface of the cold-rolled steel sheet S and extend along the sheet transverse direction. This setup allows an external force (attractive force) to be applied uniformly in the width direction of the surface of the cold-rolled steel sheet S, thus achieving uniform vibration in the width direction. Then, arranging a plurality of such electromagnets 60 in the sheet passing direction allows sufficient time to apply vibration to the cold-rolled steel sheet S. As seen from FIG. 5A, each electromagnet 63 has a magnet 63A and a coil 63B wound around it, and the axial direction of the coil 63B is aligned with the thickness direction of the cold-rolled steel sheet S. In this case, depending on the direction of the current flowing in the coil 63B, the magnetic pole surface 63A1 facing the cold-rolled steel sheet S becomes N-pole as illustrated in FIG. 6A, or S-pole as illustrated in FIG. 6B.

[0038] In FIG. 5B, a plurality of cylindrical electromagnets 63 are arranged at predetermined intervals along the width direction of the cold-rolled steel sheet S so that their bottom pole surfaces are spaced apart and facing a surface of the cold-rolled steel sheet S. This setup allows an external force (attractive force) to be applied uniformly in the width direction of the surface of the cold-rolled steel sheet S, thus achieving uniform vibration in the width direction. Then, arranging multiple rows of such cylindrical-shaped electromagnets 63 along the sheet passing direction allows sufficient time to apply vibration to the cold-rolled steel sheet S. As seen from FIG. 5B, each electromagnet 63 has a cylindrical magnet and a coil wound around it, and the axial direction of the coil is aligned with the thickness direction of the cold-rolled steel sheet S. In this case, depending on the direction of the current flowing in the coil, the magnetic pole surface 63A1 facing the cold-rolled steel sheet S becomes N-pole as illustrated in FIG. 6A, or S-pole as illustrated in FIG. 6B.

[0039] In FIGS. 6A and 6B, the cold-rolled steel sheet S is subjected to an external force (attractive force) when an electric current is applied to the electromagnets 63. The current applied to the electromagnets 63 is either a pulsed direct current or a continuous alternating current. When a pulsed direct current is applied to the electromagnets 63, the cold-rolled steel sheet S vibrates due to the intermittent attractive force exerted on the cold-rolled steel sheet S. When a continuous alternating current is applied to the electromagnets, each time the direction of the current changes, each magnetic pole surface 63A1 facing the cold-rolled steel sheet S switches between the N and S poles, yet an external force (attractive force) is always exerted on the cold-rolled steel sheet S. In the case of alternating current, the magnitude of the external force (attractive force) exerted on the cold-rolled steel sheet S varies with changes in the current value over time, causing the cold-rolled steel sheet S to vibrate.

[0040] It is suffice for the electromagnets 63 to be installed to face one surface of the cold-rolled steel sheet S, yet the electromagnets may be installed so as to face both the front and back surfaces of the cold-rolled steel sheet. However, in such cases, it is preferable to shift the height positions of the electromagnets so that the electromagnets on one side are not at the same height position as the electromagnets on the other side.

[0041] The vibration detector 64 illustrated in FIG. 4 is a laser displacement meter or laser Doppler vibrometer positioned at a predetermined distance from the surface of the cold-rolled steel sheet S, and is capable of measuring the frequency and amplitude of vibration of the cold-rolled steel sheet S. By placing the vibration detector 64 at the same height as the electromagnets 63 relative to the cold-rolled steel sheet S, the maximum amplitude of vibration of the cold-rolled steel sheet S can be measured with the vibration detector 64. The frequency and maximum amplitude detected by the vibration detector 64 are output to the controller 61. The controller 61 receives the frequency and maximum amplitude values output from the vibration detector 64, compares them with the set values, performs operations such as PID operations on the deviations to determine the frequency (frequency of the pulsed direct current or continuous alternating current) and current value for the electromagnets 63 such that the cold-rolled steel sheet S is caused to vibrate at a predetermined frequency and maximum amplitude, as well as the current value to be provided to the amplifier 62 in consideration of the amplification rate of the amplifier 62, and provides command values to the power supply 65. The power supply 65 is a power supply for passing current through the coils of the electromagnets 63. The power supply 65 receives the command values input from the controller 61 and provides a current at a predetermined frequency and current value to the amplifier 62. The amplifier 62 amplifies the current values provided by the power supply 65 at a predetermined amplification rate and provides the command values to the electromagnets 63. In this way, a current at the predetermined frequency and current value flows through the electromagnets 63, enabling the cold-rolled steel sheet S to vibrate at the predetermined frequency and maximum amplitude.

(Vibration Application Device 70)

[0042] Each embodiment of the present disclosure can be carried out by installing a vibration application device 70 as illustrated in FIG. 7A in the CAL 100, the CGL 200, or the CGL 300. The vibration application step can be carried out by applying vibration to the cold-rolled steel sheet S being passed using the vibration application device 70. With reference to FIG. 7A, the vibration application device 70 includes a controller 71, vibration elements 72, and vibration detectors 73. The vibration application device 70 has the vibration elements 72 configured to contact the cold-rolled steel sheet S, and the vibration application device 70 is configured to cause the cold-rolled steel sheet S to be vibrated by the vibration elements 72.

[0043] Each vibration element 72 may be any general piezoelectric element without limitation on its shape and installation. However, for example, as illustrated in FIG. 7B, each vibration element 72 may be a planner vibration element with its length in the sheet transverse direction that is brought into surface contact with the cold-rolled steel sheet S to vibrate the cold-rolled steel sheet S.

[0044] It is suffice for the vibration elements 72 to be provided so as to contact one surface of the cold-rolled steel sheet S, yet the vibration elements 72 may be installed so as to face both the front and back surfaces of the cold-rolled steel sheet. However, in such cases, it is preferable to shift the height position of the vibration elements 72 so that the vibration elements on one side are not at the same height position as the vibration elements on the other side.

[0045] Each vibration detector 73 illustrated in FIG. 7A is a laser displacement meter or laser Doppler vibrometer positioned at a predetermined distance from the surface of the cold-rolled steel sheet S, and is capable of measuring the frequency and amplitude of vibration of the cold-rolled steel sheet S. By placing the vibration detectors 73 at the same height as the vibration elements 72 relative to the cold-rolled steel sheet S, the maximum amplitude of vibration of the cold-rolled steel sheet S can be measured with the vibration detectors 73. The frequency and maximum amplitude detected by the vibration detectors 73 are output to the controller 71. The controller 71 receives the frequency and maximum amplitude values output from the vibration detectors 73, compares them with the set values, performs operations such as PID operations on the deviations to determine the frequency and current value of a pulsed direct current to flow through the vibration elements 72 such that the cold-rolled steel sheet S is caused to vibrate at a predetermined frequency and maximum amplitude, and controls the power supply (not illustrated) to provide a pulsed direct current at a predetermined frequency and current value to the vibration elements 72. In this way, the vibration elements 72 are caused to vibrate at the predetermined frequency and amplitude, enabling the the cold-rolled steel sheet S to vibrate at the predetermined frequency and maximum amplitude.

[0046] In Embodiments 1, 2, and 3, the position of the vibration application device 60 or 70 is not limited as long as the cold-rolled steel sheet S being passed from the cooling zone 26 to the tension reel 50 can be applied with vibration.

[0047] With reference to FIG. 1, examples of the preferred position of the vibration application device 60 or 70, i.e. examples of the preferred timing of the vibration application step, in Embodiment 1 in which the CAL 100 produces a product coil of a cold-rolled and annealed steel sheet (CR) will be described below. As an example, the vibration application device 60 or 70 can be provided in the cooling zone 26. In this case, the vibration application step can be performed in the step (B-2). Specifically, the electromagnets 63 illustrated in FIG. 4 or the vibration elements 72 illustrated in FIGS. 7A and 7B can be installed between the plurality of cooling zones arranged along the steel sheet conveyance path or between adjacent cooling nozzles arranged along the steel sheet conveyance path in each cooling zone. FIGS. 8A and 8B each illustrate an example of the positional relationship between cooling nozzles 26A and vibration application devices 60 or 70 in the case where the vibration application devices 60 or 70 are installed in the cooling zone 26. Here, the vibration application devices 60 or 70 need not be entirely located inside the cooling zone 26 as long as at least the electromagnets 63 or vibration elements 72 are located inside the cooling zone 26.

[0048] As another example, the vibration application device 60 or 70 can be provided at a position that enables applying vibration to the cold-rolled steel sheet S being passed through the downstream line 30. In this case, the vibration application step can be performed in the step (C). Specifically, the vibration application device 60 or 70 can be provided in at least one of the following locations: (i) between the overaging treatment zone 28 and the exit looper 35, (ii) in the exit looper 35, (iii) between the exit looper 35 and the temper mill 36, and (iv) between the temper mill 36 and the tension reel 50.

[0049] The vibration application device 60 or 70 may be provided both in the cooling zone 26 and at a position that enables applying vibration to the cold-rolled steel sheet S being passed through the downstream line 30. That is, the vibration application step may be performed in both the step (B-2) and the step (C). The vibration application device 60 or 70 may be provided in the overaging treatment zone 28 to perform the vibration application step during the overaging treatment.

[0050] With reference to FIG. 2, examples of the preferred position of the vibration application device 60 or 70, i.e. examples of the preferred timing of the vibration application step, in Embodiment 2 in which the CGL 200 produces a product of a galvannealed steel sheet (GA) will be described below. As an example, the vibration application device 60 or 70 can be provided at a first position that enables applying vibration to the cold-rolled steel sheet S being passed upstream of the hot-dip galvanizing bath 31. In this case, the vibration application step can be performed before the step (C-1). Specifically, the vibration application device 60 or 70 can be provided in the cooling zone 26. In detail, the electromagnets 63 illustrated in FIG. 4 or the vibration elements 72 illustrated in FIGS. 7A and 7B can be installed between the plurality of cooling zones arranged along the steel sheet conveyance path or between adjacent cooling nozzles arranged along the steel sheet conveyance path in each cooling zone. The examples illustrated in FIGS. 8A and 8B apply in this embodiment, too. Here, the vibration application devices 60 or 70 need not be entirely located inside the cooling zone 26 as long as at least the the electromagnets 63 or vibration elements 72 are located inside the cooling zone 26. At least the electromagnets 63 or vibration elements 72 of the vibration application devices 60 or 70 may be installed in the snout 29.

[0051] As another example, the vibration application device 60 or 70 can be provided at a second position that enables applying vibration to the cold-rolled steel sheet S being passed downstream of the hot-dip galvanizing bath 31. In this case, the vibration application step can be performed after the step (C-1). Specifically, the vibration application device 60 or 70 can be provided in 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) in 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 exit looper 35, (vi) in the exit looper 35, (vii) between the exit looper 35 and the temper mill 36, and (viii) between the temper mill 36 and the tension reel 50. It is particularly preferable to provide the vibration application device 60 or 70 in the air cooling zone (iv).

[0052] The first position is more preferable than the second position as the position of the vibration application device 60 or 70, from the viewpoint of desorbing hydrogen from inside the steel sheet more sufficiently. That is, it is more preferable to perform the vibration application step before the step (C-1) than after the step (C-1). The vibration application device 60 or 70 may be provided at both the first position and the second position. That is, the vibration application step may be performed both before and after the step (C-1).

[0053] With reference to FIG. 3, examples of the preferred position of the vibration application device 60 or 70, i.e. examples of the preferred timing of the vibration application step, in Embodiment 3 in which the CGL 300 produces a product of a hot-dip galvanized steel sheet (GI) will be described below. As an example, the vibration application device 60 or 70 can be provided at a first position that enables applying vibration to the cold-rolled steel sheet S being passed upstream of the hot-dip galvanizing bath 31. In this case, the vibration application step can be performed before the step (C-1). Specifically, the vibration application device 60 or 70 can be provided in the cooling zone 26. In detail, the electromagnets 63 illustrated in FIG. 4 or the vibration elements 72 illustrated in FIGS. 7A and 7B can be installed between the plurality of cooling zones arranged along the steel sheet conveyance path or between adjacent cooling nozzles arranged along the steel sheet conveyance path in each cooling zone. The examples illustrated in FIGS. 8A and 8B apply in this embodiment, too. Here, the vibration application devices 60 or 70 need not be entirely located inside the cooling zone 26 as long as at least the electromagnets 63 or vibration elements 72 are located inside the cooling zone 26. At least the electromagnets 63 or vibration elements 72 of the vibration application devices 60 or 70 may be installed in the snout 29.

[0054] As another example, the vibration application device 60 or 70 can be provided at a second position that enables applying vibration to the cold-rolled steel sheet S being passed downstream of the hot-dip galvanizing bath 31. In this case, the vibration application step can be performed after the step (C-1). Specifically, the vibration application device 60 or 70 can be provided in at least one of the following locations: (i) between the hot-dip galvanizing bath 31 and the gas wiping device 32, (ii) in 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) in the exit looper 35, (v) between the exit looper 35 and the temper mill 36, and (vi) between the temper mill 36 and the tension reel 50. It is particularly preferable to provide the vibration application device 60 or 70 in the air cooling zone (ii).

[0055] The first position is more preferable than the second position as the position of the vibration application device 60 or 70, from the viewpoint of desorbing hydrogen from inside the steel sheet more sufficiently. That is, it is more preferable to perform the vibration application step before the step (C-1) than after the step (C-1). The vibration application device 60 or 70 may be provided at both the first position and the second position. That is, the vibration application step may be performed both before and after the step (C-1).

(Frequency of Vibration)

[0056] It is important that the frequency of vibration of the cold-rolled steel sheet S be 100 Hz or more from the viewpoint of facilitating the diffusion of hydrogen. If the frequency is less than 100 Hz, the effect of desorbing the hydrogen contained in the cold-rolled steel sheet S cannot be obtained.

[0057] From this perspective, the frequency is 100 Hz or higher, preferably 500 Hz or higher, and more preferably 1,000 Hz or higher. The cold-rolled steel sheet S naturally vibrates during the sheet passage process, or it vibrates, for example, when exposed to gas from the gas wiping device 32. However, in these cases, the frequency of vibration of the cold-rolled steel sheet S is at most about 20 Hz, and the effect of desorbing the hydrogen contained in the cold-rolled steel sheet S cannot be obtained. On the other hand, if the frequency is excessively high, sufficient time to expand the lattice spacing in the steel sheet cannot be ensured, and the effect of desorbing hydrogen cannot be obtained. From this perspective, it is important to keep the frequency at or below 100,000 Hz, preferably at or below 80,000 Hz, and more preferably at or below 50,000 Hz. The frequency of vibration of the cold-rolled steel sheet S can be measured by the vibration detector 64 illustrated in FIG. 4 or the vibration detectors 73 illustrated in FIG. 7A. The frequency of vibration of the cold-rolled steel sheet S can be adjusted by controlling the frequency of the pulsed direct current or continuous alternating current in the case of the vibration application device 60 illustrated in FIG. 4, or by controlling the frequency of vibration of the vibration elements 72 in the case of the vibration application device 70 illustrated in FIGS. 7A and 7B.

(Maximum amplitude of vibration)

[0058] If the maximum amplitude of the vibration is less than 10 nm, the lattice spacing on the surface of the cold-rolled steel sheet S does not sufficiently expand and the diffusion of hydrogen is not sufficiently facilitated, and thus the effect of desorbing the hydrogen contained in the cold-rolled steel sheet S is not achieved. Therefore, it is important that the maximum amplitude of the cold-rolled steel sheet S be 10 nm or more, preferably 100 nm or more, and more preferably 500 nm or more. If the maximum amplitude of the cold-rolled steel sheet S is more than 500 µm, the strain on the surface of the steel sheet increases and plastic deformation occurs, which ends up trapping hydrogen. Accordingly, the effect of desorbing the hydrogen contained in the cold-rolled steel sheet S is not achieved. From this perspective, it is important that the maximum amplitude of the cold-rolled steel sheet S be 500 µm or less, preferably 400 µm or less, and more preferably 300 µm or less. The cold-rolled steel sheet S naturally vibrates during the sheet passage process, or it vibrates, for example, when exposed to gas from the gas wiping device 32. However, in these cases, the maximum amplitude of the cold-rolled steel sheet S is at least more than 0.5 mm, and the effect of desorbing the hydrogen contained in the cold-rolled steel sheet S cannot be obtained. The maximum amplitude of the cold-rolled steel sheet S can be measured by the vibration detector 64 illustrated in FIG. 4 or the vibration detectors 73 illustrated in FIG. 7A. The maximum amplitude of the cold-rolled steel sheet S can be adjusted by controlling the amount of current flowing through the electromagnets 63 in the case of the vibration application device 60 illustrated in FIG. 4, or by controlling the amplitude of vibration of the vibration elements 72 in the case of the vibration application device 70 illustrated in FIGS. 7A and 7B.

(Vibration application time)

[0059] The vibration application time for the cold-rolled steel sheet S in the vibration application step is preferably 1 second or more, more preferably 5 seconds or more, and further preferably 10 seconds or more, from the viewpoint of more sufficiently reducing hydrogen in the cold-rolled steel sheet S. The vibration application time for the cold-rolled steel sheet S is preferably 3,600 seconds or less, more preferably 1,800 seconds or less, and further preferably 900 seconds or less, from the viewpoint of not hampering the productivity. Herein, the expression "vibration application time for the cold-rolled steel sheet S" denotes the time during which vibration is applied to each position on the surface of the cold-rolled steel sheet S. In the case where vibration is applied to each position by a plurality of vibration application devices 60 or 70, the term denotes the cumulative time. With reference to FIGS. 6A and 6B, in the case of using the vibration application device 60, a part of the surface of the cold-rolled steel sheet S that is facing the electromagnet 63 can be regarded as vibrating. Accordingly, the cumulative time during which each part of the cold-rolled steel sheet S is facing the electromagnet 63 can be used as the vibration application time. In the case of using the vibration application device 70 illustrated in FIGS. 7A and 7B, the cumulative time during which each part of the cold-rolled steel sheet S is in contact with any of the vibration elements 72 can be considered as the vibration application time. The vibration application time can be adjusted using the sheet passing speed of the cold-rolled steel sheet S and the position of the vibration application device 60 or 70 (for example, the number of electromagnets 63 arranged in the sheet passing direction as illustrated in FIG. 4, or the number of vibration elements 72 arranged in the sheet passing direction as illustrated in FIGS. 7A and 7B).

[Cold-rolled steel sheet]

[0060] The cold-rolled steel sheet S supplied to each of the CAL 100, the CGL 200, and the CGL 300 according to the foregoing embodiments is not limited. The cold-rolled steel sheet S is preferably less than 6 mm in thickness. Examples of the cold-rolled steel sheet S include a high strength steel sheet having a tensile strength of 590 MPa or more and a stainless steel sheet.

[Chemical composition of cold-rolled steel sheet: high strength steel sheet]

[0061] The chemical composition in the case where the cold-rolled steel sheet S is a high strength steel sheet will be described below. In the following description, "mass%" is simply expressed as "%".

C: 0.030 % to 0.800 %

[0062] C has an effect of increasing the strength of the steel sheet. From the viewpoint of achieving this effect, the C content is 0.030 % or more, and preferably 0.080 % or more. If the C content is excessively high, the steel sheet embrittles significantly irrespective of the hydrogen content in the steel sheet. The C content is therefore 0.800 % or less, and preferably 0.500 % or less.

Si: 0.01 % to 3.00 %

[0063] Si has an effect of increasing the strength of the steel sheet. From the viewpoint of achieving this effect, the Si content is 0.01 % or more, and preferably 0.10 % or more. If the Si content is excessively high, the steel sheet embrittles, causing a decrease in ductility. Moreover, red scale and the like form, as a result of which the surface characteristics degrade and the coating quality decreases. The Si content is therefore 3.00 % or less, and preferably 2.50 % or less.

Mn: 0.01 % to 10.00 %

[0064] Mn has an effect of increasing the strength of the steel sheet by solid solution strengthening. From the viewpoint of achieving this effect, the Mn content is 0.01 % or more, and preferably 0.5 % or more. If the Mn content is excessively high, the steel microstructure tends to be not uniform due to segregation of Mn, and hydrogen embrittlement originating from such nonuniformity may emerge. The Mn content is therefore 10.00 % or less, and preferably 8.00 % or less.

P: 0.001 % to 0.100 %

[0065] P is an element that has a solid solution strengthening action and can be added depending on the desired strength. From the viewpoint of achieving this effect, the P content is 0.001 % or more, and preferably 0.003 % or more. If the P content is excessively high, the weldability degrades. In the case of alloying the galvanizing, the alloying rate decreases and the galvanizing quality is impaired. The P content is therefore 0.100 % or less, and preferably 0.050 % or less.

S: 0.0001 % to 0.0200 %

[0066] S segregates to grain boundaries and embrittles the steel in hot working, and also exists as sulfide and causes a decrease in local deformability. The S content is therefore 0.0200 % or less, preferably 0.0100 % or less, and more preferably 0.0050 % or less. The S content is 0.0001 % or more under manufacturing constraints.

N: 0.0005 % to 0.0100 %

[0067] N is an element that degrades the aging resistance of the steel. The N content is therefore 0.0100 % or less, and preferably 0.0070 % or less. The N content is desirably as low as possible. Under manufacturing constraints, however, the N content is 0.0005 % or more, and preferably 0.0010 % or more.

Al: 0.001 % to 2.000 %

[0068] Al is an element that acts as a deoxidizer and is effective for the cleanliness of the steel. From the viewpoint of achieving this effect, the Al content is 0.001 % or more, and preferably 0.010 % or more. If the Al content is excessively high, slab cracking is likely to occur in continuous casting. The Al content is therefore 2.000 % or less, and preferably 1.200 % or less.

[0069] The balance other than the components described above is Fe and inevitable impurities. The chemical composition may optionally further contain at least one element selected from the following.

Ti: 0.200 % or less

[0070] Ti contributes to higher strength of the steel sheet by strengthening the steel by precipitation or by grain refinement strengthening through growth inhibition of ferrite crystal grains. Accordingly, in the case of adding Ti, the Ti content is preferably 0.005 % or more, and more preferably 0.010 % or more. If the Ti content is excessively high, carbonitride precipitates in a large amount, as a result of which the formability may decrease. Accordingly, in the case of adding Ti, the Ti content is 0.200 % or less, and preferably 0.100 % or less.

Nb: 0.200 % or less, V: 0.500 % or less, W: 0.500 % or less

[0071] Nb, V, and W are effective in strengthening the steel by precipitation. Accordingly, in the case of adding Nb, V, and W, the content of each element is preferably 0.005 % or more, and more preferably 0.010 % or more. If the content of each element is excessively high, carbonitride precipitates in a large amount, as a result of which the formability may decrease. Accordingly, in the case of adding Nb, the Nb content is 0.200 % or less, and preferably 0.100 % or less. In the case of adding V and W, the content of each element is 0.500 % or less, and preferably 0.300 % or less.

B: 0.0050 % or less

[0072] B is effective in strengthening grain boundaries and strengthening the steel sheet. Accordingly, in the case of adding B, the B content is preferably 0.0003 % or more. If the B content is excessively high, the formability may decrease. Accordingly, in the case of adding B, the B content is 0.0050 % or less, and preferably 0.0030 % or less.

Ni: 1.000 % or less

[0073] Ni is an element that increases the strength of the steel by solid solution strengthening. Accordingly, in the case of adding Ni, the Ni content is preferably 0.005 % or more. If the Ni content is excessively high, the area ratio of hard martensite is excessively high. In a tensile test, microvoids at the crystal grain boundaries of martensite increase and crack propagation progresses, as a result of which the ductility may decrease. Accordingly, in the case of adding Ni, the Ni content is 1.000 % or less.

Cr: 1.000 % or less, Mo: 1.000 % or less

[0074] Cr and Mo have an action of improving the balance between the strength and the formability. Accordingly, in the case of adding Cr and Mo, the content of each element is preferably 0.005 % or more. If the content of each element is excessively high, the area ratio of hard martensite is excessively high. In a tensile test, microvoids at the crystal grain boundaries of martensite increase and crack propagation progresses, as a result of which the ductility may decrease. Accordingly, in the case of adding Cr and Mo, the content of each element is 1.000 % or less.

Cu: 1.000 % or less

[0075] Cu is an element effective in strengthening the steel. Accordingly, in the case of adding Cu, the Cu content is preferably 0.005 % or more. If the Cu content is excessively high, the area ratio of hard martensite is excessively high. In a tensile test, microvoids at the crystal grain boundaries of tempered martensite increase and crack propagation progresses, as a result of which the ductility may decrease. Accordingly, in the case of adding Cu, the Cu content is 1.000 % or less.

Sn: 0.200 % or less, Sb: 0.200 % or less

[0076] Sn and Sb are effective in suppressing decarburization of regions of about several tens of µm of the steel sheet surface layer caused by nitridization or oxidation of the steel sheet surface and ensuring the strength and the material stability. Accordingly, in the case of adding Sn and Sb, the content of each element is preferably 0.002 % or more. If the content of each element is excessively high, the toughness may decrease. Accordingly, in the case of adding Sn and Sb, the content of each element is 0.200 % or less.

Ta: 0.100 % or less

[0077] Ta forms alloy carbide or alloy carbonitride and contributes to higher strength, as with Ti and Nb. Ta is also considered to have an effect of, by partially dissolving in Nb carbide or Nb carbonitride and forming composite precipitate such as (Nb, Ta)(C, N), significantly suppressing the coarsening of precipitate and stabilizing the contribution of precipitation to higher strength. Accordingly, in the case of adding Ta, the Ta content is preferably 0.001 % or more. If the Ta content is excessively high, the precipitate stabilizing effect is likely to be saturated, and also the alloy costs increase. Accordingly, in the case of adding Ta, the Ta content is 0.100 % or less.

Ca: 0.0050 % or less, Mg: 0.0050 % or less, Zr: 0.1000 % or less, REM (rare earth metal): 0.0050 % or less

[0078] Ca, Mg, Zr, and REM are elements effective for spheroidizing sulfide and improving the adverse effect of the sulfide on the formability. In the case of adding these elements, the content of each element is preferably 0.0005 % or more. If the content of each element is excessively high, inclusions and the like increase, as a result of which surface and internal defects may occur. Accordingly, in the case of adding these elements, the content of each element is 0.0050 % or less.

[Chemical composition of cold-rolled steel sheet: stainless steel sheet]

[0079] The chemical composition in the case where the cold-rolled steel sheet S is a stainless steel sheet will be described below. In the following description, "mass%" is simply expressed as "%".

C: 0.001 % to 0.400 %

[0080] C is an element essential for achieving high strength in the stainless steel. However, C combines with Cr and precipitates as carbide during tempering in steel production, which causes degradation in the corrosion resistance and toughness of the steel. If the C content is less than 0.001 %, sufficient strength cannot be obtained. If the C content is more than 0.400 %, the degradation is significant. The C content is therefore 0.001 % to 0.400 %.

Si: 0.01 % to 2.00 %

[0081] Si is an element useful as a deoxidizer. From the viewpoint of achieving this effect, the Si content is 0.01 % or more. If the Si content is excessively high, Si dissolved in the steel decreases the workability of the steel. The Si content is therefore 2.00 % or less.

Mn: 0.01 % to 5.00 %

[0082] Mn has an effect of increasing the strength of the steel. From the viewpoint of achieving this effect, the Mn content is 0.01 % or more. If the Mn content is excessively high, the workability of the steel decreases. The Mn content is therefore 5.00 % or less.

P: 0.001 % to 0.100 %

[0083] P is an element that promotes grain boundary fractures due to grain boundary segregation. Accordingly, the P content is desirably as low as possible. The P content is 0.100 % or less, preferably 0.030 % or less, and more preferably 0.020 % or less. The P content is 0.001 % or more under manufacturing constraints.

S: 0.0001 % to 0.0200 %

[0084] S exists as a sulfide-based inclusion such as MnS and causes decreases in ductility, corrosion resistance, and the like. Accordingly, the S content is desirably as low as possible. The S content is 0.0200 % or less, preferably 0.0100 % or less, and more preferably 0.0050 % or less. The S content is 0.0001 % or more under manufacturing constraints.

Cr: 9.0 % to 28.0 %

[0085] Cr is a basic element constituting stainless steel, and is an important element that develops the corrosion resistance. Considering the corrosion resistance in a harsh environment of 180 °C or more, if the Cr content is less than 9.0 %, the corrosion resistance is insufficient, and if the Cr content is more than 28.0 %, the effect is saturated and the economic efficiency is poor. The Cr content is therefore 9.0 % to 28.0 %.

Ni: 0.01 % to 40.0 %

[0086] Ni is an element that improves the corrosion resistance of the stainless steel. If the Ni content is less than 0.01 %, the effect is insufficient. If the Ni content is excessively high, the formability degrades, and stress corrosion cracking tends to occur. The Ni content is therefore 0.01 % to 40.0 %.

N: 0.0005 % to 0.500 %

[0087] N is an element detrimental to improving the corrosion resistance of the stainless steel. The N content is therefore 0.500 % or less, and preferably 0.200 % or less. The N content is desirably as low as possible, but is 0.0005 % or more under manufacturing constraints.

Al: 0.001 % to 3.000 %

[0088] Al acts as a deoxidizer, and also has an effect of suppressing exfoliation of oxide scale. From the viewpoint of achieving these effects, the Al content is 0.001 % or more. If the Al content is excessively high, the elongation decreases and the surface quality degrades. The Al content is therefore 3.000 % or less.

The balance other than the components described above is Fe and inevitable impurities. The chemical composition may optionally further contain at least one element selected from the following.

Ti: 0.500 % or less

[0089] Ti combines with C, N, and S and improves the corrosion resistance, the intergranular corrosion resistance, and the deep drawability. If the Ti content is more than 0.500 %, solute Ti degrades the toughness. Accordingly, in the case of adding Ti, the Ti content is 0.500 % or less.

Nb: 0.500 % or less

[0090] Nb combines with C, N, and S and improves the corrosion resistance, the intergranular corrosion resistance, and the deep drawability, as with Ti. Nb also improves the workability and the high-temperature strength, and suppresses crevice corrosion and facilitates repassivation. If the Nb content is excessively high, however, the formability degrades due to hardening. Accordingly, in the case of adding Nb, the Nb content is 0.500 % or less.

V: 0.500 % or less

[0091] V suppresses crevice corrosion. If the V content is excessively high, however, the formability degrades. Accordingly, in the case of adding V, the V content is 0.500 % or less.

W: 2.000 % or less

[0092] W contributes to improved corrosion resistance and high-temperature strength. If the W content is excessively high, however, the toughness degrades in steel sheet production, and the costs increase. Accordingly, in the case of adding W, the W content is 2.000 % or less.

B: 0.0050 % or less

[0093] B segregates to grain boundaries to improve the secondary workability of the product. If the B content is excessively high, however, the workability and the corrosion resistance decrease. Accordingly, in the case of adding B, the B content is 0.0050 % or less.

Mo: 2.000 % or less

[0094] Mo is an element that improves the corrosion resistance and in particular suppresses crevice corrosion. If the Mo content is excessively high, however, the formability degrades. Accordingly, in the case of adding Mo, the Mo content is 2.000 % or less.

Cu: 3.000 % or less

[0095] Cu is an austenite stabilizing element as with Ni and Mn, and is effective in crystal grain refinement by phase transformation. Cu also suppresses crevice corrosion and facilitates repassivation. If the Cu content is excessively high, however, the toughness and the formability degrade. Accordingly, in the case of adding Cu, the Cu content is 3.000 % or less.

Sn: 0.500 % or less

[0096] Sn contributes to improved corrosion resistance and high-temperature strength. If the Sn content is excessively high, however, slab cracking is likely to occur in steel sheet production. Accordingly, in the case of adding Sn, the Sn content is 0.500 % or less.

Sb: 0.200 % or less

[0097] Sb has an action of segregating to grain boundaries and increasing the high-temperature strength. If the Sb content is excessively high, however, cracking is likely to occur in welding due to Sb segregation. Accordingly, in the case of adding Sb, the Sb content is 0.200 % or less.

Ta: 0.100 % or less

[0098] Ta combines with C and N and contributes to improved toughness. If the Ta content is excessively high, however, the effect is saturated, and the production costs increase. Accordingly, in the case of adding Ta, the Ta content is 0.100 % or less.

Ca: 0.0050 % or less, Mg: 0.0050 % or less, Zr: 0.1000 % or less, REM (rare earth metal): 0.0050 % or less

[0099] Ca, Mg, Zr, and REM are elements effective for spheroidizing sulfide and improving the adverse effect of the sulfide on the formability. In the case of adding these elements, the content of each element is preferably 0.0005 % or more. If the content of each element is excessively high, inclusions and the like increase, as a result of which surface and internal defects may occur. Accordingly, in the case of adding these elements, the content of each element is 0.0050 % or less.

[Diffusible hydrogen content]

[0100] In this embodiment, the diffusible hydrogen content in the product coil is preferably 0.50 mass ppm or less, more preferably 0.30 mass ppm or less, and further preferably 0.20 mass ppm or less, in order to ensure favorable bendability. Although no lower limit is placed on the diffusible hydrogen content in the product coil, the diffusible hydrogen content in the product coil may be 0.01 mass ppm or more under manufacturing constraints.

[0101] The method of measuring the diffusible hydrogen content in the product coil is as follows: A test piece of 30 mm in length and 5 mm in width is collected from the product coil. In the case of a product coil of a hot-dip galvanized steel sheet or a galvannealed steel sheet, the hot-dip galvanized layer or the galvannealed layer of the test piece is removed by grinding or alkali. After this, the amount of hydrogen released from the test piece is measured 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 and then cooled to room temperature, and the cumulative amount of hydrogen released from the test piece from room temperature to 210 °C is measured and taken to be the diffusible hydrogen content in the product coil.

EXAMPLES

[0102] Steels each having a chemical composition containing the elements listed in Table 1 with the balance being Fe and inevitable impurities were each obtained by steelmaking using a converter, and continuously cast into a slab. The obtained slab was subjected to hot rolling and cold rolling to obtain a cold-rolled coil. As seen from Table 2, a product coil of a cold-rolled and annealed steel sheet (CR) was produced by the CAL illustrated in FIG. 1 in some cases, a product coil of a hot-dip galvanized steel sheet (GI) was produced without heating and alloying by the CGL illustrated in FIG. 2 in some other cases, and a product coil of a galvannealed steel sheet (GA) was produced by the CGL illustrated in FIG. 2 in the remaining cases.

[0103] For each case, vibration was applied to the cold-rolled steel sheet being passed using the electromagnetic vibration application device illustrated in FIGS. 4 to 6, under the conditions of the maximum amplitude, the frequency, and the vibration application time listed in Table 2. In Table 2, "Vibration application location" indicates the region in the CAL or the CGL where the vibration application step was performed, i.e. the installation location of the vibration application device.

"(B-2)" denotes that the vibration application device was installed in the cooling zone in the CAL or the CGL and the vibration application step was performed in the cooling zone of the step (B-2).

"(C)" denotes that the vibration application device was installed at a position that enables applying vibration to the cold-rolled steel sheet being passed through the downstream line in the CAL, that is, the vibration application device was installed at a position downstream of the cooling zone and upstream of the tension reel, specifically, at least one location out of (i) between the overaging treatment zone 28 and the exit looper 35, (ii) in the exit looper 35, (iii) between the exit looper 35 and the temper mill 36, and (iv) between the temper mill 36 and the tension reel 50. That is, "(C)" denotes that the vibration application step was performed in the step (C) in the CAL, specifically, in at least one location out of the foregoing (i) to (iv).

"Before (C-1)" denotes that the vibration application device was installed at a position downstream of the cooling zone and upstream of the hot-dip galvanizing bath in the CGL, specifically, in the snout 29, and the sound vibration application step was performed after the step (B-2) and before the step (C-1).

"After (C-1)" denotes that the vibration application device was installed at a position downstream of the hot-dip galvanizing bath and upstream of the tension reel in the CGL, specifically, at least one location out of (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) in 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 exit looper 35, (vi) in the exit looper 35, (vii) between the exit looper 35 and the temper mill 36, and (viii) between the temper mill 36 and the tension reel 50, and the vibration application step was performed after the step (C-1), specifically, in at least one location out of the foregoing (i) to (viii).

[0104] A steel sheet sample was collected form the product coil obtained in each case, and the tensile property and the hydrogen embrittlement resistance were evaluated as follows. The results are shown in Table 2.

[0105] A tensile test was conducted in accordance with JIS Z 2241 (2011) using a JIS No. 5 test piece collected so that the tensile direction would be perpendicular to the rolling direction of the steel sheet, and the tensile strength (TS) and the total elongation (EL) were measured.

[0106] The hydrogen embrittlement resistance was evaluated from the foregoing tensile test as follows: In the case where the value obtained by dividing EL in the steel sheet after the application of vibration measured in the foregoing test by EL' when the hydrogen content in the steel of the same steel sheet was 0.00 mass ppm was 0.70 or more, the hydrogen embrittlement resistance was determined as favorable. Here, EL' was measured by leaving the same steel sheet in the air for a long time to reduce hydrogen in the steel and, after determining that the hydrogen content in the steel had reached 0.00 mass ppm by TDS, conducting a tensile test.

[0107] The diffusible hydrogen content in the product coil obtained in each case was measured by the method described above. The results are shown in Table 2.

[0108] In each Example, the vibration application step was performed under the conditions of a predetermined frequency and maximum amplitude, so that a steel sheet excellent in hydrogen embrittlement resistance was able to be produced.

Table 2

No.	Steel sample ID	Vibration application location	Maximum amplitude (nm)	Frequency (Hz)	Vibration application time (see)	Product coil 1)	TS	EL	EL'	Hydrogen embrittlement resistance EL/EL'	Diffusible hydrogen content	Classification
							(MPa)	(%)	(%)		(mass ppm)
1	A	(B-2)	1650	1250	60	GA	1518	9.3	9.6	0.97	0.12	Example
2	B	after (C-1)	1000	8000	90	GA	1022	22.3	24.9	0.90	0.32	Example
3	C	(B-2)	3500	1000	180	CR	1010	24.1	26.0	0.93	0.30	Example
4	D	(B-2) + before (C-1) + after (C-1)	9600	90000	600	GI	2216	5.7	5.8	0.98	0.02	Example
5	E	(B-2)	800	42000	90	CR	602	21.2	25.9	0.82	0.20	Example
6	F	(B-2)	15000	2500	90	CR	1052	11.0	14.0	0.79	0.39	Example
7	G	(B-2)	16000	4000	60	GI	1833	5.9	8.1	0.73	0.42	Example
8	H	(B-2)	72000	10000	60	GA	1173	9.5	10.3	0.92	0.14	Example
9	I	(B-2) + before (C-1)	24000	500	240	GI	1521	20.7	21.0	0.99	002	Example
10	J	(B-2)	44000	1000	90	GA	1020	20.8	23.2	0.90	0.17	Example
11	K	after (C-1)	8000	1200	90	GI	1055	33.5	45.2	0.74	0.44	Example
12	L	(B-2) + before (C-1)	30000	1000	180	GA	986	17.8	19.1	0.93	0.05	Example
13	M	(C)	24000	5000	240	CR	785	23.7	23.9	0.99	002	Example
14	N	(B-2) + after (C-1)	10000	5000	180	GI	1341	11.3	12.5	0.90	0.33	Example
15	O	before (C-1)	92000	5000	30	GI	922	200	24.8	0.81	0.27	Example
16	P	before (C-1)	8000	4000	15	GA	1278	12.2	13.0	0.94	0.09	Example
17	Q	before (C-1) + after (C-1)	4800	30000	1200	GI	947	28.1	29.6	0.95	002	Example
18	R	(B-2) + (C)	6600	50000	600	CR	1156	12.7	13.6	0.93	0.17	Example
19	S	after (C-1)	14000	2000	120	GA	1478	10.3	12.5	0.82	0.36	Example
20	T	before (C-1)	8000	1000	15	GI	1001	14.0	18.7	0.75	0.49	Example
21	U	(B-2) + before (C-1) + after (C-1)	13200	1250	300	GA	1324	12.1	12.8	0.95	0.17	Example
22	V	before (C-1)	130000	25000	15	GA	1303	10.6	13.0	0.82	0.32	Example
23	w	(B-2)	125000	50000	30	GA	1228	14.8	16.8	0.88	0.18	Example
24	X	(B-2)	70000	8000	60	GI	1488	9.7	10.1	0.96	0.15	Example
25	Y	(B-2) + before (C-1)	88000	10000	60	GA	1534	12.1	12.5	0.97	0.06	Example
26	Z	(B-2)	46000	500	120	CR	1037	13.7	14.6	0.94	0.04	Example
27	AA	(B-2)	148000	1000	30	GA	1559	15.3	16.2	0.94	0.13	Example
28	AB	(C)	40000	1500	90	CR	1007	22.8	27.8	0.82	0.20	Example
29	AC	before (C-1)	328000	2000	15	GI	1343	13.9	15.0	0.93	0.09	Example
30	AD	(B-2) + before (C-1)	24000	2500	120	GA	1327	12.2	14.2	0.86	0.26	Example
31	AE	(B-2) + (C)	9000	2000	600	CR	619	48.0	48.5	0.99	0.05	Example
32	AF	(B-2) + (C)	16800	1500	300	CR	602	27.5	28.2	0.98	0.08	Example
33	AG	(B-2)	200	1750	150	CR	596	21.5	22.3	0.96	0.10	Example
34	AH	(B-2) + (C)	2000	1350	3000	CR	589	24.2	24.5	0.99	0.00	Example
35	Al	(B-2) + (C)	150	2500	900	CR	593	22.9	23.6	0.97	0.03	Example
36	AJ	(B-2)	32000	12000	180	CR	596	21.0	21.2	0.99	0.01	Example
37	A	-	-	-	-	CR	1521	3.7	10.9	0.35	0.66	Comparative example
38	A	(B-2) + before (C-1)	5	5000	60	GA	1550	7.6	14.2	0.54	0.71	Comparative example
39	A	(B-2)	600000	10000	90	GA	1492	4.8	8.6	0.56	0.53	Comparative example
40	A	(B-2) + before (C-1)	5000	50	240	GI	1513	3.2	9.2	0.35	0.54	Comparative example
41	A	(B-2) + before (C-1)	2000	120000	180	GA	1520	5.5	9.8	0.56	0.55	Comparative Example
42	A	(B-2)	150	3000	90	GA	1267	7.5	10.5	0.71	0.29	Example
43	A	(B-2) + before (C-1)	750	2200	120	GA	1309	8.3	100	0.83	0.15	Example
44	A	(B-2) + before (C-1)	380000	1300	150	GA	1290	10.2	10.3	0.99	0.02	Example
45	A	(B-2)	250000	650	90	GA	1575	10.3	12.4	0.83	0.11	Example
46	A	(B-2)	124000	95000	30	GA	1621	7.1	8.5	0.85	0.19	Example
47	A	before (C-1) + after (C-1)	30000	120	180	GI	1496	13.1	13.2	0.98	0.04	Example
48	A	(B-2) + before (C-1) + after (C-1)	320	75000	300	GA	1486	14.3	14.6	0.99	0.03	Example
49	A	(B-2) + before (C-1) + after (C-1)	1800	1250	3200	GI	1588	100	100	0.99	002	Example

Underlined if outside the scope of the disclosure. 1) CR: cold-rolled steel sheet, GI: hot-dip galvanized steel sheet (without alloying treatment of galvanizing), GA: galvannealed steel sheet

INDUSTRIAL APPLICABILITY

[0109] It is thus possible to provide a continuous annealing line, a continuous hot-dip galvanizing line, and a steel sheet production method capable of producing a steel sheet excellent in hydrogen embrittlement resistance without changing the mechanical properties and without impairing the production efficiency.

REFERENCE SIGNS LIST

[0110] 

100

continuous annealing line

200

continuous hot-dip galvanizing line

300

continuous hot-dip galvanizing line

10

payoff reel

11

welder

12

cleaning line

13

entry looper

20

annealing furnace

22

heating zone

24

soaking zone

26

cooling zone

26A

cooling nozzle

28

overaging treatment zone

29

snout

30

downstream line

31

hot-dip galvanizing bath

32

gas wiping device

33

alloying furnace

34

cooling device

35

exit looper

36

temper mill

50

tension reel

60

vibration application device

61

controller

62

amplifier

63

electromagnet

63A

magnet

63A1

magnetic pole surface

63B

coil

64

vibration detector

65

power supply

70

vibration application device

71

controller

72

vibration element

73

vibration detector

C

cold-rolled coil

S

cold-rolled steel sheet

P

product coil

Claims

1. A continuous annealing line comprising:

a payoff reel configured to uncoil a cold-rolled coil to feed a cold-rolled steel sheet;

an annealing furnace configured to pass the cold-rolled steel sheet therethrough to continuously anneal the cold-rolled steel sheet and including a heating zone, a soaking zone, and a cooling zone that are arranged 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 cooled in the cooling zone;

a downstream line configured to continuously pass the cold-rolled steel sheet discharged from the annealing furnace therethrough;

a tension reel configured to coil the cold-rolled steel sheet being passed through the downstream line; and

a vibration application device configured to apply vibration to the cold-rolled steel sheet being passed from the cooling zone to the tension reel so that the cold-rolled steel sheet is caused to vibrate at a frequency of 100 Hz or more and 100,000 Hz or less and a maximum amplitude of 10 nm or more and 500 µm or less.

2. The continuous annealing line according to claim 1, wherein the vibration application device is located in the cooling zone.

3. The continuous annealing line according to claim 1 or 2, wherein the vibration application device is located at a position that enables applying vibration to the cold-rolled steel sheet being passed through the downstream line.

4. The continuous annealing line according to any one of claims 1 to 3, wherein an arrangement of the vibration application device and a sheet passing speed of the cold-rolled steel sheet are set so that a vibration application time for the cold-rolled steel sheet will be 1 second or more.

5. The continuous annealing line according to any one of claims 1 to 4, wherein the vibration application device comprises an electromagnet having a magnetic pole surface spaced from and facing a surface of the cold-rolled steel sheet, and the vibration application device is configured to cause the cold-rolled steel sheet to vibrate in response to an external force exerted by the electromagnet on the cold-rolled steel sheet.

6. The continuous annealing line according to any one of claims 1 to 4, wherein the vibration application device comprises a vibration element configured to contact the cold-rolled steel sheet, and the vibration application device is configured to cause the cold-rolled steel sheet to be vibrated by the vibration element.

7. A continuous hot-dip galvanizing line comprising:

the continuous annealing line according to claim 1; and

a hot-dip galvanizing bath located, as the downstream line, downstream of the annealing furnace in the sheet passing direction, and configured to immerse the cold-rolled steel sheet therein to apply a hot-dip galvanized coating onto the cold-rolled steel sheet.

8. The continuous hot-dip galvanizing line according to claim 7, wherein the vibration application device is located at a position that enables applying vibration to the cold-rolled steel sheet being passed upstream of the hot-dip galvanizing bath.

9. The continuous hot-dip galvanizing line according to claim 7 or 8, wherein the vibration application device is located at a position that enables applying vibration to the cold-rolled steel sheet being passed downstream of the hot-dip galvanizing bath.

10. The continuous hot-dip galvanizing line according to claim 7, comprising
an alloying furnace located, as the downstream line, downstream of the hot-dip galvanizing bath in the sheet passing direction, and configured to pass the cold-rolled steel sheet therethrough to heat and alloy the hot-dip galvanized coating.

11. The continuous hot-dip galvanizing line according to claim 10, wherein the vibration application device is located at a position that enables applying vibration to the cold-rolled steel sheet being passed upstream of the hot-dip galvanizing bath.

12. The continuous hot-dip galvanizing line according to claim 10 or 11, wherein the vibration application device is located at a position that enables applying vibration to the cold-rolled steel sheet being passed downstream of the hot-dip galvanizing bath.

13. The continuous hot-dip galvanizing line according to any one of claims 7 to 12, wherein an arrangement of the vibration application device and a sheet passing speed of the cold-rolled steel sheet are set so that a vibration application time for the cold-rolled steel sheet will be 1 second or more.

14. The continuous hot-dip galvanizing line according to any one of claims 7 to 13, wherein the vibration application device comprises an electromagnet having a magnetic pole surface spaced from and facing a surface of the cold-rolled steel sheet, and the vibration application device is configured to cause the cold-rolled steel sheet to vibrate in response to an external force exerted by the electromagnet on the cold-rolled steel sheet.

15. The continuous hot-dip galvanizing line according to any one of claims 7 to 13, wherein the vibration application device comprises a vibration element configured to contact the cold-rolled steel sheet, and the vibration application device is configured to cause the cold-rolled steel sheet to be vibrated by the vibration element.

16. A steel sheet production method comprising, in the following order:

a step (A) of uncoiling a cold-rolled coil to feed a cold-rolled steel sheet by a payoff reel;

a step (B) of passing the cold-rolled steel sheet through an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged from an upstream side in a sheet passing direction, to continuously anneal the cold-rolled steel sheet by a step (B-1) of annealing the cold-rolled steel sheet in a reducing atmosphere containing hydrogen in the heating zone and the soaking zone and a step (B-2) of cooling the cold-rolled steel sheet in the cooling zone;

a step (C) of continuously passing the cold-rolled steel sheet discharged from the annealing furnace; and

a step (D) of coiling the cold-rolled steel sheet by a tension reel to obtain a product coil,

wherein the steel sheet production method comprises

a vibration application step of applying vibration to the cold-rolled steel sheet being passed in or after the step (B-2) and before the step (D) so that the cold-rolled steel sheet is caused to vibrate at a frequency of 100 Hz or more and 100,000 Hz or less and a maximum amplitude of 10 nm or more and 500 µm or less.

17. The steel sheet production method according to claim 16, wherein the vibration application step is performed in the step (B-2).

18. The steel sheet production method according to claim 16 or 17, wherein the vibration application step is performed in the step (C).

19. The steel sheet production method according to claim 16, wherein the step (C) includes a step (C-1) of immersing the cold-rolled steel sheet in a hot-dip galvanizing bath located downstream of the annealing furnace in the sheet passing direction to apply a hot-dip galvanized coating onto the cold-rolled steel sheet.

20. The steel sheet production method according to claim 19, wherein the vibration application step is performed before the step (C-1).

21. The steel sheet production method according to claim 19 or 20, wherein the vibration application step is performed after the step (C-1).

22. The steel sheet production method according to claim 19, wherein the step (C) includes, following the step (C-1), a step (C-2) of passing the cold-rolled steel sheet through an alloying furnace located downstream of the hot-dip galvanizing bath in the sheet passing direction to heat and alloy the hot-dip galvanized coating.

23. The steel sheet production method according to claim 22, wherein the vibration application step is performed before the step (C-1).

24. The steel sheet production method according to claim 22 or 23, wherein the vibration application step is performed after the step (C-1).

25. The steel sheet production method according to any one of claims 16 to 24, wherein in the vibration application step, a vibration application time for the cold-rolled steel sheet is 1 second or more.

26. The steel sheet production method according to any one of claims 16 to 25, wherein in the vibration application step, the cold-rolled steel sheet is caused to vibrate in response to an external force exerted by an electromagnet on the cold-rolled steel sheet, the electromagnet having a magnetic pole surface spaced from and facing a surface of the cold-rolled steel sheet.

27. The steel sheet production method according to any one of claims 16 to 25, wherein in the vibration application step, the cold-rolled steel sheet is caused to be vibrated by a vibration element configured to contact the cold-rolled steel sheet.

28. The steel sheet production method according to any one of claims 16 to 27, wherein the cold-rolled steel sheet is a high strength steel sheet having a tensile strength of 590 MPa or more.

29. The steel sheet production method according to any one of claims 16 to 28, wherein the cold-rolled steel sheet has a chemical composition containing, 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 %,

with the balance being Fe and inevitable impurities.

30. The steel sheet production method according to claim 29, wherein the chemical composition further contains, 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.

31. The steel sheet production method according to any one of claims 16 to 27, wherein the cold-rolled steel sheet is a stainless steel sheet having a chemical composition containing, 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 %,

with the balance being Fe and inevitable impurities.

32. The steel sheet production method according to claim 31, wherein the chemical composition further contains, 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.

33. The steel sheet production method according to any one of claims 16 to 32, wherein the product coil has a diffusible hydrogen content of 0.50 mass ppm or less.

Drawing