Steel plate for vibration control damper with excellent impact toughness and its manufacturing method

Abstract

The object of the present invention is to relate to a steel material for seismic dampers having excellent impact toughness and a manufacturing method thereof, and more particularly to a steel material for seismic dampers used to ensure the earthquake resistance of structures against earthquakes and a manufacturing method thereof. [Solution] The steel plate for seismic dampers of the present invention comprises a base steel plate and a scale layer formed on at least one surface of the base steel plate, wherein the base steel plate is composed of, by weight, C: 0.005-0.02%, Si: 0.05-0.2%, Mn: 0.1-0.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005-0.05%, N: 0.005% or less, Nb: 0.02-0.06%, Ti: 48 / 14 × [N] to 0.05%, with the balance being Fe and other unavoidable impurities, the base steel plate having a microstructure containing ferrite at an area fraction of 95% or more, and the scale layer having a total content of FeO and Fe2SiO4 of 2-5% by weight.

Description

[Technical Field]

[0001] The present invention relates to a steel plate for a seismic damper having excellent impact toughness and a manufacturing method thereof, and more particularly to a steel material for a seismic damper used to ensure the earthquake resistance of a structure against an earthquake and a manufacturing method thereof. [Background technology]

[0002] In the past, earthquake-resistant designs primarily used in Korea have mainly involved lowering the yield ratio of the steel used in columns and beams of structures during earthquakes, thereby delaying the point at which the structure collapses. However, earthquake-resistant designs using steel with such low yield ratios have had problems, such as not only being unable to reuse the steel used in the structure, but also not ensuring the stability of the structure itself, which means that the structure must be rebuilt.

[0003] In recent years, earthquake-resistant design technology has advanced, and the practical application of seismic control or seismic isolation structures has progressed. In particular, a variety of technologies have been developed to ensure earthquake resistance by absorbing the energy applied to a structure during an earthquake in specific areas. Seismic control dampers are used as devices to absorb this earthquake energy, and the steel material used for seismic control dampers has an extremely low yield point. By lowering the yield point of steel material for seismic control dampers compared to existing structural materials such as columns and beams, they yield earlier during an earthquake, absorbing the vibration energy caused by the earthquake, while maintaining other structural materials within their elastic range, thereby suppressing deformation of the structure.

[0004] However, conventional steel materials for seismic dampers use ultra-low carbon steel with a coarse ferrite structure, which means that they exhibit continuous yield behavior without a yield point phenomenon during tensile tests. Therefore, when they absorb the plastic deformation energy generated by an earthquake, work hardening occurs rapidly, resulting in a large increase in yield strength, which poses a problem that needs to be improved in order to be used as a steel material for seismic dampers to absorb earthquake energy.

[0005] However, the reality is that no technology has been developed to date that can meet such high demand. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Korean Patent Publication No. 2008-0088605 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a steel plate for a seismic damper which has a low yield strength and can be used to ensure the earthquake resistance of a structure, and a method for manufacturing the same.

[0008] Another object of the present invention is to provide a steel plate for a seismic damper which has low yield strength and excellent low-temperature impact toughness, and a method for manufacturing the same.

[0009] The object of the present invention is not limited to the above content. Anyone having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention from the entire content of the specification of the present invention. [Means for solving the problem]

[0010] The steel plate for a seismic damper of the present invention is A base steel sheet, a scale layer formed on at least one surface of the base steel sheet, The base steel sheet contains, by weight percent, C: 0.005 to 0.02%, Si: 0.05 to 0.2%, Mn: 0.1 to 0.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.05%, N: 0.005% or less, Nb: 0.02 to 0.06%, Ti: 48 / 14 × [N] to 0.05%, with the remainder being Fe and other unavoidable impurities; The base steel sheet has a microstructure containing ferrite at an area fraction of 95% or more, The scale layer is characterized in that the total content of FeO and Fe2SiO4 is 2 to 5 wt %.

[0011] Furthermore, the method for manufacturing a steel plate for a seismic damper of the present invention includes the steps of: the base steel sheet contains, by weight %, C: 0.005 to 0.02%, Si: 0.05 to 0.2%, Mn: 0.1 to 0.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.05%, N: 0.005% or less, Nb: 0.02 to 0.06%, Ti: 48 / 14 × [N] to 0.05%, the balance being Fe and other unavoidable impurities; and Rough rolling the reheated steel slab at a temperature of Tnr+50°C or higher to obtain a rough rolled bar; and hot rolling the rough rolled bar at a temperature equal to or higher than Tnr to obtain a hot rolled steel sheet. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a steel plate that can be suitably used for a seismic damper that is used to ensure the earthquake resistance of a structure, and a method for manufacturing the same.

[0013] Furthermore, according to the present invention, it is possible to provide a steel plate for a seismic damper having a small yield strength and excellent low-temperature impact toughness, and a method for manufacturing the same.

[0014] The various beneficial advantages and effects of the present invention are not limited to the above-mentioned contents, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0015] [Figure 1] 1 shows a photograph of the microstructure inside a steel sheet according to one aspect of the present invention, taken with an optical microscope. [Figure 2] 1 is a graph showing the change in yield strength and tensile strength depending on the ferrite grain size in the steel material according to the present invention. [Figure 3]1 is a graph showing the change in yield strength depending on the hot rolling finish temperature in the present invention. [Figure 4] 1 is a photograph showing the adhesion of a scale layer formed on the surface of a base steel sheet after rolling in the present invention, and showing the shape of the scale layer that has fallen off due to reduced adhesion. [Figure 5] FIG. 1 is a photograph showing a cross section of a scale layer formed on the surface of a base steel sheet after rolling in the present invention, and is an optical photograph showing the distribution of FeO+Fe2SiO4 in the scale layer formed on the upper layer of the base steel sheet. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.

[0017] Conventionally, steel materials used to ensure the earthquake resistance of structures have been produced using a technique that uses elements close to pure iron and performs additional heat treatment in the range of 910 to 960°C. However, this technique requires additional heat treatment at high temperatures of 900°C or higher after finish rolling, which can lead to problems with steel materials with extremely low yield points that do not contain Si, such as excessive scaling and resulting in defects, the formation of coarse Nb or Ti precipitates, and deterioration of impact toughness. Another problem is that the additional heat treatment process at high temperatures of 900°C or higher increases manufacturing costs.

[0018] Alternatively, there was a technology for using ultra-low carbon steel to create a coarse ferrite structure for conventional seismic damper steel, but this technology exhibits continuous yielding behavior in which the yield point phenomenon does not occur during tensile testing. Therefore, when absorbing the plastic deformation energy generated by an earthquake, work hardening occurs rapidly, which significantly increases the yield strength, creating a problem that needed to be improved for use in seismic dampers to absorb earthquake energy.

[0019] Therefore, the inventors conducted extensive research and developed a steel plate for seismic dampers that has low yield strength, excellent low-temperature impact toughness, and exhibits the yield point phenomenon. This has led to the completion of a technology that can suppress an increase in yield strength by reducing the rapid work hardening caused by plastic deformation when an earthquake occurs.

[0020] Specifically, a steel sheet for a seismic damper according to one aspect of the present invention includes a base steel sheet and a scale layer formed on at least one surface of the base steel sheet.

[0021] In this case, the base steel sheet contains, by weight, C: 0.005 to 0.02%, Si: 0.05 to 0.2%, Mn: 0.1 to 0.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.05%, N: 0.005% or less, Nb: 0.02 to 0.06%, Ti: 48 / 14 × [N] to 0.05%, with the remainder being Fe and other unavoidable impurities.

[0022] The reasons for adding each alloying element constituting the composition of the base steel sheet, which is one of the main features of the present invention, and the appropriate content ranges thereof will be explained below.

[0023] C: 0.005 to 0.02% C is an element that causes solid solution strengthening and, in its free state, binds to dislocations, increasing yield strength and reducing elongation. Therefore, for suitable use as a steel material for seismic dampers, the C content must be controlled to 0.005% or more; if the C content exceeds 0.02%, the strength may exceed the appropriate level for use as a seismic damper. Therefore, in the present invention, the C content is controlled to 0.005 to 0.02%. However, more preferably, the lower limit of the C content may be 0.011%, or the upper limit of the C content may be 0.018%.

[0024] Si: 0.05 to 0.2% Like C, Si is an element that causes solid solution strengthening, increases yield strength, and reduces elongation. Therefore, for steel materials suitable for use in seismic dampers, the lower the Si content, the better. However, if Si is not added in an appropriate amount, the adhesion of secondary scale generated during rolling will be poor, increasing the possibility that the scale will be pressed into the surface of the steel sheet during production, causing surface defects. Therefore, in the present invention, the Si content is controlled to 0.05% or more to ensure adhesion of secondary scale, and to 0.2% or less to ensure low yield strength. However, more preferably, the lower limit of the Si content may be 0.07%, or the upper limit of the Si content may be 0.15%.

[0025] Mn: 0.1 to 0.5% Like Si, Mn is an element that causes solid solution strengthening, increasing yield strength and decreasing elongation. Therefore, in order to suitably use the steel for seismic dampers, the Mn content is controlled to 0.1% or more from the viewpoint of ensuring appropriate strength, and the upper limit is controlled to 0.5% or less to avoid excessive solid solution strengthening. However, more preferably, the lower limit of the Mn content may be 0.18%, and the upper limit of the Mn content may be 0.35%.

[0026] P: 0.02% or less (excluding 0%) Although P is an element advantageous for improving strength and corrosion resistance, it can significantly impair impact toughness, so it is preferable to maintain the P content as low as possible. Therefore, in the present invention, the P content can be controlled to 0.02% or less, more preferably 0.013% or less. Furthermore, the lower limit of the P content may be set apart from 0%, taking into account unavoidable P contamination, and more preferably, the lower limit of the P content may be 0.0005%.

[0027] S: 0.01% or less (excluding 0%) Since S is an element that forms MnS and the like and significantly impairs impact toughness, it is preferable to maintain its content as low as possible. Therefore, in the present invention, the S content can be controlled to 0.01% or less, more preferably 0.004% or less. Furthermore, the lower limit of the S content may be set to 0% or less, taking into account cases where S is inevitably mixed in, and more preferably, the lower limit of the S content may be 0.0005% or more.

[0028] Al: 0.005 to 0.05% Al is an element that can inexpensively deoxidize molten steel, and from the viewpoint of ensuring impact toughness while sufficiently reducing yield strength, the upper limit of the Al content is controlled to 0.05%. Alternatively, more preferably, the upper limit of the Al content can be controlled to 0.035%, and from the viewpoint of ensuring minimum deoxidation performance, the lower limit of the Al content can be controlled to 0.005%. However, more preferably, the lower limit of the Al content may be 0.01%, and the upper limit of the Al content may be 0.035%.

[0029] N: 0.005% or less (excluding 0%) N is an element that causes solid solution strengthening and, in its free state, binds to dislocations, increasing yield strength and reducing elongation. Therefore, the lower the N content, the better. Therefore, to ensure low yield strength, the N content is controlled to 0.005% or less. However, the lower limit of the N content can be excluded from 0% in consideration of unavoidable N contamination, and more preferably, the lower limit of the N content can be set to 0.001%.

[0030] Nb: 0.02 to 0.06% Nb is an important element in the production of TMCP steel, and precipitates in the form of NbC or NbCN. Furthermore, Nb dissolved in solid solution during reheating at high temperatures has the effect of suppressing the recrystallization of austenite and refining the structure.

[0031] On the other hand, in order to introduce appropriate deformation-induced precipitates, it is preferable to add 0.02% or more Nb. Also, in order to prevent deterioration of impact toughness due to coarsening of precipitates, it is preferable to add 0.06% or less Nb. However, more preferably, the lower limit of the Nb content may be 0.03% and the upper limit of the Nb content may be 0.05%.

[0032] Ti: 48 / 14 × [N] ~ 0.05% Ti is an element that prevents N from attaching to dislocations by precipitating in the form of TiN. Therefore, in order to secure N in steel within an appropriate range, Ti must be added in an amount of 48 / 14 × [N]% or more, taking into account the added N content (wt%) (where [N] refers to the wt% content of N in the base steel sheet). On the other hand, if Ti is added in excess, precipitates may become coarse and impact toughness may deteriorate. Therefore, to ensure impact toughness, Ti is controlled to 0.05% or less. However, more preferably, the lower limit of the Ti content may be 0.02% and the upper limit of the Ti content may be 0.045%.

[0033] On the other hand, although not particularly limited, according to one aspect of the present invention, the base steel sheet satisfies the following relational expression 1.

[0034] [Equation 1] 0.001≦[C]-12 / 93×[Nb]-12 / 48×[A]≦0.01 (In the above relational expression 1, the above [C] represents the average weight percent content of C in the base steel sheet, the above [Nb] represents the average weight percent content of Nb in the base steel sheet, and the above [A] represents the value defined by the following relational expression 2.)

[0035] [Equation 2] [A] = [Ti] - 48 / 12 × [N] (In the above relational expression 2, [Ti] represents the average content by weight of Ti in the base steel sheet, and [N] represents the average content by weight of N in the base steel sheet.)

[0036] According to one aspect of the present invention, the value of free carbon, expressed by [C]-12 / 93 × [Nb]-12 / 48 × [A] in the above-mentioned relational expression 1, can be controlled to a range of 0.001 to 0.01%. If the above-mentioned value of free carbon is less than 0.001%, it may be difficult to realize the yield point phenomenon, while if it exceeds 0.01%, there is a risk that the strength suitable for use in seismic dampers may be exceeded. In other words, in the present invention, by satisfying the above-mentioned relational expression 1, it is possible to obtain a steel sheet that promotes the occurrence of the upper yield point and does not experience excessive work hardening during an earthquake.

[0037] Therefore, according to the present invention, it is possible to provide a steel plate for seismic dampers that has excellent low-temperature impact toughness, with a yield strength in the range of 205 to 245 MPa, a tensile strength of 300 MPa or more, and a Charpy impact transition temperature of -20°C or less.

[0038] In the present invention, the remaining component is Fe. However, since unintended impurities may be inevitably mixed in from raw materials or the surrounding environment during the normal manufacturing process, it is not possible to exclude them. Since such impurities are known to anyone skilled in the art, the entire contents of the impurities will not be mentioned in this specification.

[0039] According to one aspect of the present invention, the base steel sheet may have a microstructure containing ferrite in an area fraction of 95% or more (more preferably 99% or more), with the remainder being 5% or less (including 0%) of other phases such as pearlite. Alternatively, most preferably, the base steel sheet has a single ferrite structure (i.e., the base steel sheet has a microstructure containing ferrite in an area fraction of 100%). By satisfying this requirement, the steel sheet can effectively absorb energy during an earthquake and function as an earthquake damper.

[0040] Furthermore, although not particularly limited, according to one aspect of the present invention, the average grain size of the ferrite grains in the base steel sheet may be in the range of 20 to 50 μm, more preferably 30 to 50 μm. If the average grain size of the ferrite grains in the base steel sheet is less than 20 μm, the target yield strength for use in a seismic damper may be exceeded, whereas if it exceeds 50 μm, the coarse ferrite grain size may facilitate dislocation movement, resulting in continuous yield behavior.

[0041] The above-mentioned average grain size of ferrite crystal grains means the average value of the circle-equivalent diameters measured for the crystal grains based on a cut surface in the thickness direction of the steel material (i.e., the direction perpendicular to the rolling direction), and specifically, it is the average value of the measured grain size when a spherical particle is assumed, with the longest length penetrating the interior of the crystal grain being the grain size.

[0042] Meanwhile, according to the present invention, a scale layer may be formed on at least one surface of the base steel sheet. In this case, although not particularly limited thereto, the scale layer may refer to a layer made of FeO, Fe2SiO4, Fe2O3, Fe3O4, or oxides of other alloy elements, etc., depending on the conditions during the manufacturing process of the steel sheet.

[0043] According to one aspect of the present invention, the total content of FeO and Fe2SiO4 in the scale layer may be 2 to 5 wt. If the total content of FeO and Fe2SiO4 is less than 2 wt. % relative to the total content of the scale layer, the adhesion of the scale layer may be reduced, and irregular scale spalling may occur on the surface. On the other hand, if the total content of FeO and Fe2SiO4 exceeds 5% relative to the total content of the scale layer, a problem of yield strength exceeding 245 MPa may occur. From the viewpoint of further improving the above-mentioned effect, the lower limit of the total content of FeO and Fe2SiO4 relative to the total content of the scale layer may be 2.28%, or the upper limit of the total content of FeO and Fe2SiO4 relative to the total content of the scale layer may be 4%.

[0044] Meanwhile, according to one aspect of the present invention, to further improve the effect of providing a steel sheet for seismic dampers having low yield strength and excellent low-temperature impact toughness, exhibiting a yield point phenomenon, and ensuring adhesion of the scale layer, thereby providing excellent surface properties, the FeO content in the scale layer may be 0.5 to 2 wt % and / or the Fe2SiO4 content in the scale layer may be 1 to 4.5 wt %. Alternatively, from the viewpoint of maximizing the above-mentioned effects, the lower limit of the FeO content in the scale layer may be 0.79%, or the upper limit of the FeO content in the scale layer may be 1.5%. Alternatively, from the viewpoint of maximizing the above-mentioned effects, the lower limit of the Fe2SiO4 content in the scale layer may be 1.18%, or the upper limit of the Fe2SiO4 content in the scale layer may be 3.5%.

[0045] According to one aspect of the present invention, the ratio (W1 / W2) of the Fe2SiO4 content (W1) to the FeO content (W2) in the scale layer may be 1 to 9. If the W1 / W2 ratio in the scale layer is less than 1.0, the Fe2SiO4 content may be insufficient, which may result in a problem of weak scale adhesion, and if the W1 / W2 ratio exceeds 9, the problem of red scale may occur on the steel sheet surface. From the viewpoint of further improving the above-mentioned effect, the lower limit of the ratio (W1 / W2) may be 1.06, or the upper limit of the ratio (W1 / W2) may be 4.

[0046] According to one aspect of the present invention, the average thickness of the scale layer may be 10 to 100 μm. If the average thickness of the scale layer is less than 10 μm, problems such as weak adhesion of the scale may occur, and if it exceeds 100 μm, problems in processing may occur. On the other hand, from the viewpoint of further improving the above-mentioned effect, the lower limit of the average thickness of the scale layer may be 31 μm, or the upper limit of the average thickness of the scale layer may be 45 μm.

[0047] Hereinafter, a method for manufacturing a steel sheet for a seismic damper, which is yet another aspect of the present invention, will be described in detail. However, the method for manufacturing a steel sheet for a seismic damper of the present invention does not necessarily mean that the steel sheet should be manufactured by the manufacturing method described below.

[0048] Slab reheating stage A method for producing a steel material for seismic dampers according to one aspect of the present invention may include a step of reheating a steel slab satisfying the above-described composition, and the reheating may be carried out at a temperature range of 1050 to 1250°C. At this time, the heating temperature of the steel slab is controlled to 1050°C or higher in order to sufficiently dissolve the carbonitrides of Ti and / or Nb formed during casting. However, if the slab is heated to an excessively high temperature, there is a risk of austenite coarsening, and it takes an excessive amount of time for the surface temperature after rough rolling to reach the cooling start temperature of the surface layer, so it is preferable to heat the slab at 1250°C or lower.

[0049] Descaling stage after reheating stage During the above-described reheating of the slab, oxides generated in the heating furnace may penetrate into the surface of the steel slab, thereby deteriorating the adhesion of the scale layer that is ultimately formed. Therefore, in order to ensure good adhesion of the scale layer and improve surface quality, a descaling treatment can be performed by supplying high-pressure water at a pressure of 150 to 200 bar to the surface of the steel slab before the rough rolling step after the above-described reheating step.

[0050] Rough rolling stage According to one aspect of the present invention, the reheated steel slab may be subjected to a rough rolling step to adjust the shape of the slab before the finish rolling step described below. The rough rolling temperature may be controlled to a temperature (Tnr) at which austenite recrystallization stops + 50°C or higher. Rough rolling can have the effect of destroying structural structures such as dendrites formed during casting, and can also have the effect of reducing the size of austenite. Meanwhile, more preferably, the rough rolling can be performed at a temperature in the range of 999 to 1155°C.

[0051] Secondary scale removal stage after rough rolling stage Meanwhile, oxides formed on the surface of the rough-rolled bar not only in the slab reheating step but also in the rough rolling step may penetrate and affect the adhesion of the final scale layer. Therefore, in the present invention, in order to improve surface quality by ensuring good adhesion of the scale layer, a descaling treatment can be performed by selectively supplying high-pressure water at a pressure of 150 to 150 bar to the surface of the rough-rolled bar after the rough rolling step and before the hot rolling step. The pressure of the high-pressure water in the secondary descaling step can be controlled in the range of 1 to 1.2 times, more preferably 1.02 to 1.2 times, the pressure of the high-pressure water in the primary descaling step.

[0052] Hot rolling stage The method includes a step of hot rolling the above-mentioned rough rolled bar in a temperature range above Tnr, and after hot rolling, the bar can be cooled by air cooling.

[0053] If the hot rolling temperature is lower than Tnr, a large number of non-uniform deformation bands are introduced into the austenite grains, as shown in Figure 3. These bands act as ferrite nucleation sites, causing the transformation of fine ferrite, potentially resulting in a yield strength exceeding 245 MPa. That is, if the hot rolling temperature is lower than the non-recrystallization stop temperature (Tnr), the yield strength increases sharply, resulting in a yield strength exceeding 245 MPa. Therefore, the rolling end temperature must be higher than the non-recrystallization stop temperature (Tnr). Since the Tnr formula used for ordinary ultra-low carbon steels is similarly applicable, the Tnr is not separately defined in the present invention. Meanwhile, according to one aspect of the present invention, the hot rolling can be performed at a temperature range of 922 to 962°C.

[0054] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0055] [Experimental Example 1] Steel slabs were prepared having the alloy compositions and properties shown in Table 1 below. In Table 1, the content of each component is in wt %, with the remainder consisting of Fe and other inevitable impurities. That is, among the steel slabs shown in Tables 1 and 2 below, inventive steels A to D are examples whose alloy compositions fall within the range defined in the present invention, while comparative steels E to I are examples whose alloy compositions fall outside the range defined in the present invention.

[0056] The prepared steel slab was reheated to a temperature range of 1050 to 1250°C, and then steel was produced by performing slab reheating, rough rolling, and hot rolling under the conditions shown in Table 3. After reheating and before rough rolling, a primary descaling treatment was performed by supplying high-pressure water at a pressure of 150 bar to the surface of the slab, and after the rough rolling and before hot rolling, a secondary descaling treatment was performed by supplying high-pressure water at a pressure of 180 bar to the surface of the rough-rolled bar.

[0057] [Table 1]

[0058] [Table 2] [A]*=[Ti]-48 / 12×[N] Free C*=[C]-12 / 93×[Nb]-12 / 48×[A]

[0059] [Table 3]

[0060] Steel sheets were produced under the conditions shown in Table 3 above, and then the steel sheets thus obtained were polished and etched. The steel sheets were then observed under an optical microscope to confirm that they had a ferrite single-phase structure.

[0061] Furthermore, the average grain size of ferrite grains, yield strength (YS), tensile strength (TS), and Charpy impact transition temperature of the steel sheets obtained from each experimental example were measured, and the results are shown in Table 4. The target ranges of the yield strength and tensile strength, which correspond to the target strength characteristic ranges of the present invention, are shown in Figure 2 together with the ferrite grain size.

[0062] In addition, the scale layer was photographed using an optical microscope to allow observation, and the average thickness of the scale layer was measured and shown in Table 4 below. The contents of FeO and Fe2SiO4 in the scale layer were measured using a scanning electron microscope and EDS and are shown in Table 4 below.

[0063] The average grain size of ferrite grains was measured using the line measurement method, the point at which yielding occurred using a tensile tester was defined as the yield strength, and the strength at which necking occurred was defined as the tensile strength. The Charpy impact transition temperature was determined by measuring the impact absorption energy using a Charpy impact tester, and was shown as the temperature at which fracture transitioned from ductile to brittle.

[0064] Furthermore, to evaluate the surface properties of the steel sheets, the steel sheets obtained from each experiment were 2 The steel plate surface in each area was visually inspected, and the peeled area of ​​the scale layer was measured and evaluated according to the following criteria.

[0065] ○: The peeled area of ​​the scale layer is 20% or less △: The peeled area of ​​the scale layer is more than 20% and less than 40% ×: The peeled area of ​​the scale layer exceeds 40%

[0066] [Table 4]

[0067] [Table 5]

[0068] As shown in Table 5 above, the examples that satisfied both the steel composition and manufacturing conditions of the present invention exhibited the yield point phenomenon, and all of the physical properties of the steel materials satisfied the following: yield strength of 205 to 245 MPa, tensile strength of 300 MPa or more, and Charpy impact transition temperature of -20°C or less.

[0069] Furthermore, the steel sheets obtained in the examples of the present invention all had a scale layer with a total content of FeO and Fe2SiO4 ranging from 2 to 5 wt%, which resulted in no peeling of the scale layer and excellent adhesion, confirming excellent surface properties. This is believed to be because SiO2 formed at the boundary between the scale and the base material reacts with FeO to form Fe2SiO4 (Fayalite), which increases the bonding strength between the scale and the base material and results in a stable scale state.

[0070] In particular, a photograph of the microstructure of the steel sheet obtained in Example 1-1 above, taken with an optical microscope, is shown in Figure 1. As can be seen from Figure 1, the microstructure of the steel sheet was a single ferrite structure, and it was confirmed that the average size of the ferrite crystal grains was in the range of 20 to 50 μm.

[0071] Furthermore, the steel sheet obtained in Example 1-1 was manufactured so that the cross section in the thickness direction could be observed, and then a photograph taken with an optical microscope is shown in Figure 5. From this, it was confirmed that the scale layer formed on the base steel sheet contained FeO + Fe2SiO4.

[0072] On the other hand, in Comparative Example 1, the C content did not satisfy the lower limit specified in the present invention, and the value of free C was insufficient, which resulted in continuous yielding and a yield strength of less than 205 MPa.

[0073] In Comparative Example 2, the C content exceeds the content specified in the present invention, and the yield strength exceeds 245 MPa.

[0074] Comparative Example 3 is a case where Si is added in excess, and the yield strength exceeds 245 MPa.

[0075] Comparative Example 4 satisfied all the manufacturing conditions of the present invention, but the Ti content exceeded the upper limit specified in the present invention, and the Charpy impact transition temperature exceeded −20° C. due to the formation of coarse precipitates.

[0076] In Comparative Example 5, the Si content was insufficient as specified in the present invention, resulting in a yield strength that did not satisfy the 205 MPa requirement. Furthermore, the total content of FeO and Fe2SiO4 in the scale layer was less than 2 wt%, demonstrating poor surface properties. In particular, the state of scale layer peeling in Comparative Example 5 is shown in Figure 4.

[0077] Furthermore, in Reference Examples 1 to 4, which satisfy the steel composition of the present invention but do not satisfy the manufacturing conditions, the hot rolling finish temperature is lower than Tnr. In such Reference Examples 1 to 4, dislocations are introduced by rolling in the ferrite region, and continuous yield behavior is exhibited, with the yield strength exceeding 245 MPa.

Claims

1. A base steel sheet, a scale layer formed on at least one surface of the base steel sheet, The base steel sheet contains, by weight %, C: 0.005 to 0.02%, Si: 0.05 to 0.2%, Mn: 0.1 to 0.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.05%, N: 0.005% or less, Nb: 0.02 to 0.06%, Ti: 48 / 14×[N] to 0.05%, with the balance being Fe and other unavoidable impurities, FeO and Fe in the scale layer ２ SiO ４ The total content of is 2 to 5% by weight, The base steel sheet contains ferrite as a microstructure at an area fraction of 95% or more, The steel plate for seismic dampers is characterized in that the average grain size of the ferrite crystal grains is 20 to 50 μm. (Here, [N] indicates the average weight percent content of N in the base steel sheet.)

2. 2. The steel sheet for a seismic damper according to claim 1, wherein the base steel sheet satisfies the following relational expression 1: [Relationship 1] 0.001≦[C]-12 / 93×[Nb]-12 / 48×[A]≦0.01 (In the relational expression 1, the [C] represents the average content by weight of C in the base steel sheet, the [Nb] represents the average content by weight of Nb in the base steel sheet, and the [A] represents a value defined by the following relational expression 2.) [Relationship 2] [A]=[Ti]-48 / 12×[N] (In the relational expression 2, [Ti] represents the average content by weight of Ti in the base steel sheet, and [N] represents the average content by weight of N in the base steel sheet.)

3. 2. The steel plate for a seismic damper according to claim 1, wherein the content of FeO in the scale layer is 0.5 to 2 wt %.

4. Fe in the scale layer ２ SiO ４ 2. The steel plate for a seismic damper according to claim 1, wherein the content of is 1 to 4.5 wt %.

5. Fe in the scale layer ２ SiO ４ 2. The steel plate for seismic dampers according to claim 1, wherein the ratio (W1 / W2) of the content (W1) of WO to the content (W2) of FeO is 1-9.

6. 2. The steel plate for a seismic damper according to claim 1, wherein the average thickness of the scale layer is 10 to 100 μm.

7. 2. The steel plate for a seismic damper according to claim 1, wherein the yield strength of the steel plate is 205 to 245 MPa.

8. 2. The steel plate for a seismic damper according to claim 1, wherein the steel plate has a tensile strength of 300 MPa or more.

9. 2. The steel plate for a seismic damper according to claim 1, wherein the Charpy impact transition temperature of the steel plate is −20° C. or lower.

10. a step of reheating a steel slab containing, by weight, C: 0.005 to 0.02%, Si: 0.05 to 0.2%, Mn: 0.1 to 0.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.05%, N: 0.005% or less, Nb: 0.02 to 0.06%, Ti: 48 / 14×[N] to 0.05%, with the balance being Fe and other unavoidable impurities, at 1050 to 1250°C; Rough rolling the reheated steel slab at a temperature of Tnr+50°C or higher to obtain a rough rolled bar; The method for manufacturing a steel plate for a seismic damper according to any one of claims 1 to 9, further comprising: hot rolling the roughly rolled bar at a temperature equal to or higher than Tnr to obtain a hot-rolled steel plate. (Here, [N] indicates the average weight percent content of N in the base steel sheet.)

11. 11. The method of claim 10, further comprising the step of providing high-pressure water at a pressure of 150 to 200 bar to a surface of the steel slab to perform a primary scale removal treatment after the reheating step and before rough rolling.

12. After the rough rolling and before the hot rolling, the method further includes a step of performing a secondary descaling treatment by applying high-pressure water at a pressure of 150 to 200 bar to the surface of the rough-rolled bar; The method for manufacturing steel plate for seismic dampers according to claim 11, characterized in that the pressure of the high-pressure water in the secondary scale removal treatment step is controlled to be in the range of 1 to 1.2 times the pressure of the high-pressure water in the primary scale removal treatment step.

Images