Patent Application
20240425959
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.
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.
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.
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.
PTL 1: WO 2019/188642 A1 SUMMARY
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.
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.
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.
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.
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.
In the accompanying drawings:
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).
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).
With reference to
With reference to
With reference to
With reference to
With reference to
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.
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.
[Payoff Reel, and Line from Payoff Reel to Annealing Furnace]
With reference to
With reference to
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.
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.
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
With reference to
With reference to
With reference to
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.
With reference to
With reference to
With reference to
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.
Each embodiment of the present disclosure can be carried out by installing a vibration application device 60 as illustrated in
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
In
In
In
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.
The vibration detector 64 illustrated in
Each embodiment of the present disclosure can be carried out by installing a vibration application device 70 as illustrated in
Each vibration element 72 may be any general piezoelectric element without limitation on its shape and installation. However, for example, as illustrated in
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.
Each vibration detector 73 illustrated in
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.
With reference to
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.
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.
With reference to
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).
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).
With reference to
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).
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).
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. 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
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
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
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.
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%
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%
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%
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%
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%
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%
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%
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.
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
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
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
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
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
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
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
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
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
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.
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%
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%
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%
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%
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%
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%
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%
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%
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%
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
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
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
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
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
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
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
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
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
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
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
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.
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.
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.
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
For each case, vibration was applied to the cold-rolled steel sheet being passed using the electromagnetic vibration application device illustrated in
“(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).
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.
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.
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.
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.
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.
600000
120000
0.35
0.66
0.54
0.71
0.56
0.53
0.35
0.54
0.56
0.55
1) CR: cold-rolled steel sheet, GI: hot-dip galvanized steel sheet (without alloying treatment of galvanizing), GA: galvannealed steel sheet
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-116761 | Jul 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/020579 | 5/17/2022 | WO |
