Steel sheet for seismic damper having excellent toughness characteristics and method for manufacturing the same

Abstract

The present invention relates to a steel plate for a seismic damper for providing a structure resistant to earthquakes having seismic resistance, and a method for manufacturing the same, and more particularly, to a steel plate for a seismic damper having excellent toughness characteristics, and a method for manufacturing the same.

Description

Technical Field

[0001] This disclosure relates to steel for seismic dampers used to ensure the seismic resistance of structures and methods for manufacturing the same. Background Technology

[0002] In the past, South Korea primarily used earthquake-resistant designs that employed techniques to reduce the yield strength ratio of steel used in column or beam structures in order to delay structural failure during earthquakes. However, earthquake-resistant designs using steel with such low yield strength ratios have the following problems: the steel used in the structure cannot be reused, and the structure itself must be rebuilt because its stability is not guaranteed.

[0003] Recently, with the development of seismic design technology, the practical application of seismic damping or vibration damping structures is progressing. In particular, various technologies are being developed to ensure seismic resistance by absorbing the energy applied to a structure by an earthquake to specific parts of it. Seismic dampers are used as devices to absorb such seismic energy, and the steel used in seismic dampers has an ultra-low yield point characteristic. By further reducing the yield point of the steel used in seismic dampers compared to existing column or beam structural materials, the steel yields first during an earthquake to absorb the vibrational energy generated by the earthquake, and suppresses structural deformation by keeping other structural materials within their elastic range.

[0004] However, conventional steels used in seismic dampers are ultra-low carbon steels with a coarse ferrite structure, exhibiting continuous yielding behavior in which no yield point phenomenon is observed during tensile testing. For this reason, rapid work hardening occurs when absorbing plastic strain energy generated by an earthquake, and the yield strength increases significantly. Therefore, there are issues that need improvement as steels for dampers used to absorb seismic energy.

[0005] However, no technology has yet been developed that can meet such high-end requirements.

[0006] (Patent Document 1) Patent Publication No. 2008-0088605 Summary of the Invention

[0007] Technical issues

[0008] One aspect of this disclosure is to provide a steel plate for a seismic damper having low yield strength and being used to ensure the seismic resistance of structures resisting earthquakes, and a method thereof for manufacturing the same.

[0009] Alternatively, one aspect of this disclosure is to provide a steel plate for seismic dampers that simultaneously possesses low yield strength and excellent low-temperature impact toughness, and a method for manufacturing the same.

[0010] One purpose of this disclosure is not limited to those described above. The purposes of this disclosure will be understood from the entirety of this specification, and other purposes of this disclosure will be readily apparent to those skilled in the art.

[0011] Technical solution

[0012] According to one aspect of this disclosure,

[0013] Steel plates for seismic dampers are provided, the steel plates comprising: a foundation steel plate; and

[0014] An oxide scale layer formed on at least one surface of the base steel plate.

[0015] The base steel plate contains, by weight percent: 0.005% to 0.02% C, 0.05% to 0.2% Si, 0.1% to 0.5% Mn, 0.02% or less P, 0.01% or less S, 0.005% to 0.05% Al, 0.005% or less N, 0.02% to 0.06% Nb, 48 / 14×[N]% to 0.05% Ti, and the balance Fe and other unavoidable impurities.

[0016] The base steel plate comprises 95% or more ferrite as its microstructure by area fraction.

[0017] The total content of FeO and Fe2SiO4 in the oxide layer is 2% to 5% by weight.

[0018] According to another aspect of this disclosure, a method for manufacturing steel plates for seismic dampers is provided, the method comprising:

[0019] The steel billet is reheated to a temperature in the range of 1050°C to 1250°C, the steel billet comprising, by weight %: 0.005% to 0.02% C, 0.05% to 0.2% Si, 0.1% to 0.5% Mn, 0.02% or less P, 0.01% or less S, 0.005% to 0.05% Al, 0.005% or less N, 0.02% to 0.06% Nb, 48 / 14×[N]% to 0.05% Ti, and the balance Fe and other unavoidable impurities;

[0020] The reheated steel billet is subjected to rough rolling at a temperature of Tnr+50°C or higher to obtain rough-rolled bars; and

[0021] Roughly rolled bars are hot rolled at Tnr or higher to obtain hot rolled steel sheets.

[0022] Beneficial effects

[0023] As described above, according to one aspect of this disclosure, steel plates suitable for use in seismic dampers to ensure the seismic resistance of structures resisting earthquakes, and methods for manufacturing the same, can be provided.

[0024] Alternatively, according to another aspect of this disclosure, a steel plate for seismic dampers having low yield strength and excellent low-temperature impact toughness, and a method for manufacturing the same, can be provided.

[0025] The various and beneficial advantages and effects of this disclosure are not limited to those described above, and can be more readily understood in the process of describing specific exemplary embodiments in this disclosure. Attached Figure Description

[0026] Figure 1 A photograph of the internal microstructure of a steel plate according to one aspect of this disclosure, taken with an optical microscope, is shown.

[0027] Figure 2 A graph showing the yield strength and tensile strength of steel according to this disclosure as a function of ferrite grain size.

[0028] Figure 3 A graph showing the change in yield strength as a function of the hot rolling end temperature in this disclosure is provided.

[0029] Figure 4 The photograph shows the adhesion of the oxide layer formed on the surface of the base steel plate after rolling, as disclosed in this disclosure, and shows the shape of the oxide layer falling off due to poor adhesion.

[0030] Figure 5 An optical photograph showing the distribution of FeO+Fe2SiO4 in the oxide layer of the base steel plate formed in this disclosure is provided, as a cross-section showing the oxide layer formed on the surface of the base steel plate after rolling is completed. Detailed Implementation

[0031] Preferred embodiments of this disclosure will be described below. However, embodiments of this disclosure may be modified in various ways, and the scope of this disclosure should not be construed as limited to the embodiments described below. These embodiments are provided to those skilled in the art to further elaborate on this disclosure.

[0032] As steel used to ensure the seismic resistance of structures, it is generally known to use a composition close to pure iron and undergo additional heat treatment in the range of 910°C to 960°C. However, because this technique requires additional heat treatment at 900°C or higher after finishing rolling, excessive oxide scale occurs in steels with ultra-low yield points without added Si, leading to defects or the formation of coarse Nb or Ti precipitates, resulting in deterioration of impact toughness. Furthermore, the additional heat treatment process at 900°C or higher also increases manufacturing costs.

[0033] Alternatively, as a conventional steel for seismic dampers, there is a technique that uses ultra-low carbon steel to control the microstructure to achieve a coarse ferritic structure. However, this technique exhibits continuous yielding behavior in which no yield point occurs during tensile testing. For this reason, rapid work hardening occurs when absorbing the plastic strain energy generated by an earthquake, resulting in a large increase in yield strength. This presents a problem requiring improvement as a steel plate for seismic dampers used to absorb seismic energy.

[0034] As a result of this, the inventors have developed a steel plate for seismic dampers that exhibits a yield point phenomenon, with low yield strength and excellent low-temperature impact toughness. As a result, a technique has been completed that can suppress the increase in yield strength by reducing the rapid work hardening caused by plastic deformation in the event of an earthquake.

[0035] Specifically, according to one aspect of this disclosure, the steel plate for the seismic damper includes a base steel plate and an oxide layer formed on at least one surface of the base steel plate.

[0036] In this case, the base steel plate may contain, by weight percent: 0.005% to 0.02% C, 0.05% to 0.2% Si, 0.1% to 0.5% Mn, 0.02% or less P, 0.01% or less S, 0.005% to 0.05% Al, 0.005% or less N, 0.02% to 0.06% Nb, 48 / 14×[N]% to 0.05% Ti, and the balance Fe and other unavoidable impurities.

[0037] In the following text, the reasons for the addition of one of the main features of the present invention to each alloy component constituting the composition of the base steel plate and the suitable content range thereof will be described first.

[0038] C: 0.005% to 0.02%

[0039] Carbon (C) is an element that induces solid solution strengthening and is fixed to dislocations in a free state to increase yield strength and reduce elongation. Therefore, for steel suitable for use as a seismic damper, the C content needs to be controlled to 0.005% or more, and when the C content exceeds 0.02%, it may exceed the suitable strength for use as a seismic damper. Therefore, in this disclosure, the C content is controlled to be from 0.005% to 0.02%. However, more preferably, the lower limit of the C content can be 0.011%, or the upper limit of the C content can be 0.018%.

[0040] Si: 0.05% to 0.2%

[0041] Like carbon (C), silicon (Si) is an element that causes solid solution strengthening and increases yield strength while decreasing elongation. Therefore, to suit steel used in seismic dampers, the Si content should be minimized as much as possible. However, if Si is not added in an appropriate amount, poor adhesion of the secondary oxide scale generated during rolling results in scale forming on the surface of the steel sheet during production, increasing the likelihood of surface defects. Therefore, in this disclosure, the Si content is controlled to 0.05% or more to ensure adhesion of the secondary oxide scale, and to 0.2% or less to ensure low yield strength. More preferably, the lower limit of the Si content can be 0.07%, or the upper limit can be 0.15%.

[0042] Mn: 0.1% to 0.5%

[0043] Like Si, Mn is an element that causes solid solution strengthening, thereby increasing yield strength and decreasing elongation. Therefore, in order to make it suitable for use as steel for seismic dampers, in this disclosure, the Mn content is controlled to 0.1% or more, and its upper limit is controlled to 0.5% or less, to ensure appropriate strength and avoid excessive solid solution strengthening effects. However, more preferably, the lower limit of the Mn content can be 0.18%, and the upper limit of the Mn content can be 0.35%.

[0044] P: 0.02% or less (excluding 0%)

[0045] Phosphorus (P) is beneficial for improving strength and corrosion resistance, but it can significantly impair impact toughness; therefore, the P content is preferably kept as low as possible. Thus, in this disclosure, the P content can be controlled to 0.02% or less, more preferably 0.013% or less. Furthermore, as a lower limit for P content, considering the unavoidable inclusion, 0% may not be included, and more preferably, the lower limit for P content may be 0.0005%.

[0046] S: 0.01% or less (excluding 0%)

[0047] Because sulfur (S) is an element that forms MnS and thus significantly impairs impact toughness, it is preferable to keep the S content as low as possible. Therefore, in this disclosure, the S content can be controlled to 0.01% or less, more preferably 0.004% or less. Furthermore, as a lower limit for the S content, considering the unavoidable inclusion, 0% may not be included, and more preferably, the lower limit for the S content may be 0.0005% or greater.

[0048] Al: 0.005% to 0.05%

[0049] Al is an element that can deoxidize molten steel inexpensively, and the upper limit of Al content is controlled at 0.05% to ensure impact toughness while sufficiently reducing yield strength. Alternatively, more preferably, the upper limit of Al content can be controlled at 0.035%, and the lower limit of Al content can be controlled at 0.005% to ensure minimum deoxidation performance. However, more preferably, the lower limit of Al content can be 0.01%, and the upper limit of Al content can be 0.035%.

[0050] N: 0.005% or less (excluding 0%)

[0051] Nitrogen (N) is an element that causes solid solution strengthening and is fixed to dislocations in a free state to increase yield strength and reduce elongation. Therefore, the lower the N content, the better, so the N content is controlled to 0.005% or less to ensure low yield strength. However, as a lower limit for N content, 0% may not be included, considering that it may be unavoidably included, and more preferably, the lower limit for N content may be 0.001% or greater.

[0052] Nb: 0.02% to 0.06%

[0053] Nitrogen (Nb) is an important element in the manufacture of TMCP steel, and it precipitates as NbC or NbCN. Furthermore, Nb dissolved during reheating to high temperatures inhibits the recrystallization of austenite, thus exhibiting a microstructure refinement effect.

[0054] Simultaneously, 0.02% or more of Nb is preferably added to promote the desired deformation of the organic precipitates. Furthermore, it is preferred to add Nb to 0.06% or less to prevent degradation of impact toughness due to coarsening of the precipitates. However, more preferably, the lower limit of the Nb content can be 0.03%, and the upper limit of the Nb content can be 0.05%.

[0055] Ti: 48 / 14 × [N]% to 0.05%

[0056] Ti is an element that prevents N from being fixed to dislocations by precipitating in the form of TiN. Therefore, in order to fix N in the steel within a suitable range, Ti should be added at an amount of 48 / 14 × [N]% or greater, taking into account the added N content (wt%), where [N] refers to the N content (wt%) in the base steel plate. However, when Ti is added in excess, there is a problem that impact toughness may deteriorate due to coarsening of the precipitates; therefore, Ti should be controlled to 0.05% or less to ensure impact toughness. More preferably, the lower limit of the Ti content can be 0.02%, and the upper limit of the Ti content can be 0.045%.

[0057] Meanwhile, although not specifically limited, according to one aspect of this disclosure, the base steel plate satisfies the following relational expression 1.

[0058] [Relational Expression 1]

[0059] 0.001≤[C]-12 / 93×[Nb]-12 / 48×[A]≤0.01

[0060] In relational expression 1, [C] represents the average C content (wt%) in the base steel plate, [Nb] represents the average Nb content (wt%) in the base steel plate, and [A] represents the value defined by the following relational expression 2.

[0061] [Relational Expression 2]

[0062] [A] = [Ti] - 48 / 12 × [N]

[0063] In relational expression 2, [Ti] represents the average content of Ti (by weight%) in the base steel plate, and [N] represents the average content of N (by weight%) in the base steel plate.

[0064] According to one aspect of this disclosure, the value of free C, expressed as [C]-12 / 93×[Nb]-12 / 48×[A], can be controlled within the range of 0.001% to 0.01%. When the aforementioned free C value is less than 0.001%, it may be difficult to exhibit the yield point phenomenon, while its value exceeds 0.01%, there is a risk of exceeding the appropriate strength suitable for the purpose of seismic dampers. That is, in this disclosure, by satisfying relational expression 1, a steel plate in which excessive work hardening does not occur during an earthquake by promoting the performance of the upper yield point can be obtained.

[0065] Therefore, according to this disclosure, steel plates for seismic dampers with excellent low-temperature impact toughness, yield strength in the range of 205 MPa to 245 MPa, tensile strength of 300 MPa or greater, and Charpy impact transition temperature of -20°C or lower can be provided.

[0066] In this disclosure, the remainder is Fe. However, due to the unintended introduction of impurities from raw materials or the surrounding environment during common manufacturing processes, the aforementioned components may not be excluded. Since these impurities in common manufacturing processes are known to any person skilled in the art, their full content is not specifically mentioned in this specification.

[0067] According to one aspect of this disclosure, the base steel plate may contain 95% or more (more preferably 99% or more) ferrite as a microstructure by area fraction, and the balance of 5% or less (including 0%) other phases such as pearlite. Alternatively, most preferably, the base steel plate has a single ferrite structure (i.e., the base steel plate contains 100% ferrite as a microstructure by area fraction). By satisfying this, energy can be effectively absorbed during an earthquake and it can be used as a seismic damper.

[0068] Furthermore, although not specifically limited, according to one aspect of this disclosure, the average ferrite grain size in the base steel plate can be in the range of 20 μm to 50 μm, more preferably 30 μm to 50 μm. In the base steel plate, if the average ferrite grain size is less than 20 μm, problems may arise where the target yield strength is exceeded for use as a seismic damper. In the base steel plate, when the average ferrite grain size exceeds 50 μm, dislocations can easily move due to the coarse ferrite grain size, leading to problems with continuous yielding behavior.

[0069] Based on the cut surface of the steel in the thickness direction (i.e., the direction perpendicular to the rolling direction), the above-mentioned average ferrite grain size refers to the average value obtained by measuring the equivalent circle diameter of the grain, and specifically, assuming that the longest length through the interior of the grain is used as the grain diameter for a spherical grain, the above-mentioned average ferrite grain size is the average value of the measured grain size.

[0070] Furthermore, according to this disclosure, an oxide scale layer can be formed on at least one surface of the base steel plate. In this case, although not particularly limited to this, the oxide scale layer can refer to a layer formed of oxides of FeO, Fe2SiO4, Fe2O3, Fe3O4, and other alloying elements, depending on the conditions during the manufacturing process of the steel plate.

[0071] According to one aspect of this disclosure, the total content of FeO and Fe2SiO4 in the oxide layer can be from 2% to 5% by weight. When the total content of FeO and Fe2SiO4 relative to the total content of the oxide layer is less than 2% by weight, the adhesion of the oxide layer may deteriorate, leading to irregular peeling of the oxide layer on its surface. On the other hand, when the total content of FeO and Fe2SiO4 relative to the total content of the oxide layer exceeds 5%, the yield strength may exceed 245 MPa. To further improve the above-mentioned effect, the lower limit of the total content of FeO and Fe2SiO4 relative to the total content of the oxide layer can be 2.28%, or the upper limit of the total content of FeO and Fe2SiO4 relative to the total content of the oxide layer can be 4%.

[0072] Meanwhile, although not specifically limited, according to one aspect of this disclosure, in order to further improve the steel plate for seismic dampers that ensures low yield strength and excellent low-temperature impact toughness and exhibits yield point phenomenon, and to ensure the adhesion of the oxide layer to achieve excellent surface properties, the FeO content in the oxide layer can be from 0.5% to 2% by weight, and / or the Fe2SiO4 content in the oxide layer can be from 1% to 4.5% by weight. Alternatively, to maximize the above-mentioned effects, the lower limit of the FeO content in the oxide layer can be 0.79%, or the upper limit of the FeO content in the oxide layer can be 1.5%. Alternatively, to maximize the above-mentioned effects, the lower limit of the Fe2SiO4 content in the oxide layer can be 1.18%, or the upper limit of the Fe2SiO4 content in the oxide layer can be 3.5%.

[0073] Furthermore, according to one aspect of this disclosure, the ratio (W1 / W2) of Fe2SiO4 content (W1) to FeO content (W2) in the oxide layer can be from 1 to 9. In the oxide layer, when the W1 / W2 ratio is less than 1.0, the adhesion of the oxide layer may be weakened due to insufficient Fe2SiO4 content, while when the W1 / W2 ratio exceeds 9, a red oxide layer may appear on the surface of the steel plate. To further improve the above-mentioned effects, the lower limit of the ratio (W1 / W2) can be 1.06, or the upper limit of the ratio (W1 / W2) can be 4.

[0074] Furthermore, according to one aspect of this disclosure, the average thickness of the oxide layer can be from 10 μm to 100 μm. When the average thickness of the oxide layer is less than 10 μm, the adhesion of the oxide layer may be weakened, while when the average thickness exceeds 100 μm, processing problems may occur. Meanwhile, to further improve the above-mentioned effects, the lower limit of the average thickness of the oxide layer can be 31 μm, or the upper limit of the average thickness of the oxide layer can be 45 μm.

[0075] The following describes in detail a method for manufacturing a steel plate for a seismic damper according to another aspect of this disclosure. However, the method for manufacturing a steel plate for a seismic damper according to this disclosure does not necessarily mean that it must be manufactured by the following methods.

[0076] Slab reheating operation

[0077] A method for manufacturing steel for seismic dampers according to one aspect of this disclosure may include reheating a steel billet that meets the above-described composition, wherein the reheating may be performed at a temperature in the range of 1050°C to 1250°C. In this case, the heating temperature of the steel billet is controlled at 1050°C or higher to sufficiently dissolve the carbonitrides of Ti and / or Nb formed during casting. However, when heated to excessively high temperatures, austenite coarsening may occur, and it takes an excessive amount of time for the surface temperature to reach the cooling start temperature of the surface layer after rough rolling. Therefore, it is preferable to heat the slab at 1250°C or lower.

[0078] Oxide removal after reheating

[0079] When the slab is reheated, the oxides generated in the furnace can penetrate to the surface of the slab and deteriorate the adhesion of the final oxide scale layer. Therefore, in order to improve surface quality by ensuring good adhesion of the oxide scale layer, high-pressure water at a pressure of 150 to 200 bar can be applied to the surface of the slab before the roughing operation and after the reheating operation to remove the oxide scale.

[0080] Roughing operation

[0081] According to one aspect of this disclosure, prior to the finishing rolling operation described later, the reheated steel billet may further include a rough rolling operation to adjust the shape of the slab, and the temperature during rough rolling may be controlled at the austenite recrystallization cessation temperature (Tnr) + 50°C or higher. Rough rolling can achieve the effect of disrupting structural structures formed during casting, such as dendrites, and can also reduce the austenite size. More preferably, rough rolling can be performed in the range of 999°C to 1155°C.

[0082] Secondary oxide scale removal operation after rough rolling

[0083] Furthermore, not only in the aforementioned slab reheating operation, but also in the rough rolling operation, oxides formed on the surface of the rough-rolled bar can penetrate into it and affect the adhesion of the final oxide scale layer. Therefore, in this disclosure, in order to improve surface quality by ensuring good adhesion of the oxide scale layer, before the hot rolling operation and after the rough rolling operation, high-pressure water at 150 bar can be selectively supplied to the surface of the rough-rolled bar to remove oxide scale, and the pressure of the high-pressure water in the secondary oxide scale removal operation can be controlled within the range of 1 to 1.2 times the pressure of the high-pressure water in the primary oxide scale removal operation. More preferably, the pressure can be controlled within the range of 1.02 to 1.2 times.

[0084] Hot rolling operation

[0085] The above-mentioned rough-rolled bars can be hot-rolled in a temperature range of Tnr or higher, and can be cooled by air cooling after hot rolling.

[0086] When the hot rolling temperature is lower than Tnr, such as Figure 3 As shown, numerous non-uniform deformation zones are introduced into the austenite grains to act as ferrite nucleation sites, and fine ferrite is transformed, allowing the yield strength to exceed 245 MPa. That is, when the hot rolling temperature is below the non-recrystallization stopping temperature (Tnr), the yield strength exceeds 245 MPa due to the rapid increase in yield strength. Therefore, the rolling end temperature should be higher than the non-recrystallization stopping temperature (Tnr). In this case, since the Tnr formula used in normal ultra-low carbon steel is equally applicable, Tnr is not separately defined in this disclosure. Furthermore, according to one aspect of this disclosure, hot rolling can be carried out in a temperature range of 922°C to 962°C.

[0087] Invention Embodiments

[0088] The present disclosure will be specifically described below through the following embodiments. However, it should be noted that the following embodiments are for illustrative purposes only and are not intended to limit the scope of the present disclosure. This is because the scope of the present disclosure is determined by the claims and matters reasonably inferred therefrom.

[0089] (Experimental Example 1)

[0090] Steel billets with the alloy composition and properties shown in Table 1 below were prepared. In this case, the content of each component in Table 1 is expressed in weight % and the remainder is Fe and unavoidable impurities. That is, among the steel billets described in Tables 1 and 2 below, inventive steels A to D show examples that match the range of alloy composition defined by this disclosure, while comparative steels E to I show examples that deviate from the range of alloy composition defined by this disclosure.

[0091] After reheating the prepared steel billet in a temperature range of 1050°C to 1250°C, the slab is reheated, rough rolled, and hot rolled under the conditions shown in Table 3 below to produce steel. In this case, before rough rolling and after reheating, high-pressure water at a pressure of 150 bar is applied to the surface of the slab for a primary oxide scale removal treatment, and before hot rolling and after rough rolling, high-pressure water at a pressure of 180 bar is applied to the surface of the rough-rolled bar for a secondary oxide scale removal treatment.

[0092] [Table 1]

[0093]

[0094] [Table 2]

[0095] steel type [A]* Free C* Tnr[℃] Invention Steel A 0.007 0.004 938 Invention Steel B 0.018 0.010 921 Invention Steel C 0.02 0.003 951 Invention Steel D 0.028 0.002 922 Comparison of steel E 0.010 <![CDATA[ -0.005 ]]> 937 Comparison of steel F 0.008 0.022 952 Comparison of steel G 0.029 0.006 932 Comparison of steel H 0.048 0.005 935 Comparison of steel I 0.014 0.006 931

[0096] [A]*=[Ti]-48 / 12×[N]

[0097] Free C* = [C] - 12 / 93 × [Nb] - 12 / 48 × [A]

[0098] [Table 3]

[0099]

[0100] After the steel sheet was manufactured under the conditions described in Table 3 above, the resulting steel sheet was polished and etched, and then observed using an optical microscope. Therefore, it was determined that the base steel sheet has a single ferritic microstructure.

[0101] Furthermore, the results of measurements of the average ferrite grain size, yield strength (YS), tensile strength (TS), and Charpy impact transformation temperature of the steel plates obtained from each experimental example are shown in Table 4 below. In this case, the target ranges of yield strength and tensile strength corresponding to the strength characteristic ranges desired in this disclosure, along with the ferrite grain size, are shown in... Figure 2 middle.

[0102] Furthermore, the average thickness of the oxide layer was measured by imaging it with an optical microscope, as shown in Table 4 below. Additionally, the contents of FeO and Fe2SiO4 in the oxide layer were measured using a scanning electron microscope and EDS, as shown in Table 4 below.

[0103] In this case, the average ferrite grain size was measured using an online measurement method, and the point at which yielding occurred using a tensile testing machine was set as the yield strength, while the strength at which necking occurred was set as the tensile strength. For the Charpy impact transition temperature, the impact absorbed energy was measured using a Charpy impact testing machine, and the fracture transition temperature from ductile to brittle was shown.

[0104] In addition, to evaluate the surface properties of the steel plate, an area of ​​1m² was observed with the naked eye. 2 The surface of the steel plate is examined, and then the area of ​​oxide layer peeling is measured and evaluated according to the following standards.

[0105] ○: The area of ​​oxide layer peeling is 20% or less.

[0106] △: The area of ​​oxide layer peeling off exceeds 20% and is 40% or less.

[0107] ×: More than 40% of the oxide layer has peeled off.

[0108] [Table 4]

[0109]

[0110] [Table 5]

[0111]

[0112] As can be seen in Table 5, the embodiments of steel composition and manufacturing conditions that satisfy the present disclosure exhibit yield point phenomenon, and the physical properties of the steel all meet the requirements of yield strength of 205 MPa to 245 MPa, tensile strength of 300 MPa or greater, and Charpy impact transformation temperature of -20°C or lower.

[0113] Furthermore, in all steel sheets obtained according to the embodiments of this disclosure, the total content of FeO and Fe2SiO4 in the oxide layer meets the range of 2% to 5% by weight, and therefore exhibits excellent adhesion without oxide layer peeling, thus confirming excellent surface properties. It has been determined that the reaction of SiO2 formed at the boundary between the oxide layer and the substrate with FeO to form Fe2SiO4 (ferroolitic) increases the bonding force between the oxide layer and the substrate, resulting in a stable oxide layer state.

[0114] In particular, regarding the steel plate obtained from Examples 1-1, photographs of its microstructure taken using an optical microscope are shown in [the image / image / etc.]. Figure 1As shown in [the image / document]. Figure 1 As can be seen, the microstructure of the steel plate is a single ferrite structure, and the average ferrite grain size can be determined to be in the range of 20μm to 50μm.

[0115] Furthermore, regarding the steel plate obtained from Examples 1-1, after it was manufactured such that its cross-section in the thickness direction could be observed, and thus the oxide layer could be observed, photographs taken with an optical microscope were obtained. Figure 5 As shown in the figure. Therefore, it is determined that the oxide scale formed on the base steel plate contains FeO+Fe2SiO4.

[0116] On the other hand, in Comparative Example 1, the C content was less than the lower limit specified in this disclosure, and the value of free C was insufficient, resulting in continuous yielding and a yield strength of less than 205 MPa.

[0117] In Comparative Example 2, the C content exceeded the specified content, resulting in a yield strength exceeding 245 MPa.

[0118] In Comparative Example 3, Si was added in excess, and the yield strength exceeded 245 MPa.

[0119] Comparative Example 4 shows a case where all the manufacturing conditions of this disclosure are met, but the Ti content exceeds the upper limit specified in this disclosure, and the Charpy impact transition temperature exceeds -20°C due to the formation of coarse precipitates.

[0120] In Comparative Example 5, the yield strength was less than 205 MPa due to insufficient Si content as specified in this disclosure, and the total content of FeO and Fe2SiO4 in the oxide layer was less than 2% by weight, indicating surface property deterioration. Specifically, the state of oxide layer peeling in Comparative Example 5 is shown in… Figure 4 middle.

[0121] Furthermore, in Reference Examples 1 to 4, which satisfy the steel composition of this disclosure but do not meet its manufacturing conditions, cases are shown where the hot rolling end temperature is below Tnr. In such Reference Examples 1 to 4, continuous yielding behavior is shown due to the introduction of dislocations in the ferrite region by hot rolling, and the yield strength of all of them exceeds 245 MPa.

Claims

1. A steel sheet for a seismic damper, comprising: a base steel sheet; and an oxide layer formed on at least one surface of the base steel sheet, wherein the base steel sheet contains, in mass%, 0.005% to 0.02% of C, 0.05% to 0.2% of Si, 0.1% to 0.5% of Mn, 0.02% or less of P, 0.01% or less of S, 0.005% to 0.05% of Al, 0.005% or less of N, 0.02% to 0.06% of Nb, 48 / 14×[N]% to 0.05% of Ti, and a balance of Fe and other inevitable impurities, wherein a total content of FeO and Fe2SiO4 in the oxide layer is 2% to 5% in mass%, wherein a microstructure of the base steel sheet is ferrite single structure, wherein an average ferrite grain size is 20 μm to 50 μm, and wherein a yield strength of the steel sheet is 205 MPa to 245 MPa. 2.The steel sheet for a seismic damper according to claim 1, wherein the base steel sheet satisfies the following relationship Expression 1, [Relationship Expression 1] 0.001 ≤ [C] - 12 / 93×[Nb] - 12 / 48×[A] ≤ 0.01 in the relationship Expression 1, wherein [C] represents an average content of C (mass%) in the base steel sheet, [Nb] represents an average content of Nb (mass%) in the base steel sheet, and [A] represents a value defined by the following relationship Expression 2, [Relationship Expression 2] [A] = [Ti] - 48 / 12×[N] in the relationship Expression 2, wherein [Ti] represents an average content of Ti (mass%) in the base steel sheet, and [N] represents an average content of N (mass%) in the base steel sheet. 3.The steel sheet for a seismic damper according to claim 1, wherein a content of FeO in the oxide layer is 0.5% to 2% in mass%. 4.The steel sheet for a seismic damper according to claim 1, wherein a content of Fe2SiO4 in the oxide layer is 1% to 4.5% in mass%. 5.The steel sheet for a seismic damper according to claim 1, wherein a ratio (W1 / W2) of a content (W1) of Fe2SiO4 to a content (W2) of FeO in the oxide layer is 1 to 9. 6.The steel sheet for a seismic damper according to claim 1, wherein an average thickness of the oxide layer is 10 μm to 100 μm. 7.The steel sheet for a seismic damper according to claim 1, wherein a tensile strength of the steel sheet is 300 MPa or more. 8.The steel sheet for a seismic damper according to claim 1, wherein a Charpy impact transition temperature of the steel sheet is -20℃ or less. 9.A method of manufacturing the steel sheet for a seismic damper according to any one of claims 1 to 8, comprising: ​ ​ ​ ​ ​ ​ a steel billet containing, in mass %: 0.005 to 0.02% of C, 0.05 to 0.2% of Si, 0.1 to 0.5% of Mn, 0.02% or less of P, 0.01% or less of S, 0.005 to 0.05% of Al, 0.005% or less of N, 0.02 to 0.06% of Nb, 48 / 14 x [N]% to 0.05% of Ti, and the balance of Fe and other inevitable impurities is reheated to a temperature in the range of 1050°C to 1250°C; subjecting the reheated steel billet to rough rolling at a temperature of Tnr + 50°C or higher to obtain a rough-rolled bar; and hot-rolling the rough-rolled bar at Tnr or higher to obtain a hot-rolled steel sheet.

10. The method of manufacturing a steel sheet for a seismic damper according to claim 9, further comprising: before the rough rolling, a descaling treatment of providing high-pressure water having a pressure of 150 to 200 bar to the surface of the steel billet is performed after the reheating operation.

11. The method of manufacturing a steel sheet for a seismic damper according to claim 10, further comprising: before the hot rolling, a secondary descaling treatment of providing high-pressure water having a pressure of 150 to 200 bar to the surface of the rough-rolled bar is performed after the rough rolling operation, wherein the pressure of the high-pressure water in the secondary descaling treatment is controlled to be in the range of 1 to 1.2 times the pressure of the high-pressure water in the primary descaling treatment.

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