Steel material for seismic dampers with excellent impact toughness and method for manufacturing the same

Abstract

Low yield strength and excellent low-temperature impact toughness To provide a steel material for seismic dampers that can be used to ensure the earthquake resistance of structures against earthquakes, and a method for manufacturing the same. The present invention is characterized in that it contains, by weight%, C: 0.006% or less, Si: 0.05% or less, Mn: 0.3% or less, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.05%, N: 0.005% or less, Ti: 48 / 14 × [N] to 0.05% (where [N] means the weight % content of nitrogen), Nb: 0.04 to 0.15%, with the balance being Fe and other unavoidable impurities, has a single ferrite structure, and the average grain size of ferrite crystal grains in the surface layer portion from the surface to a region of 30% of the total thickness is 150 to 500 μm.

Description

Technical Field

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

Background Art

[0002] Conventionally, in seismic design mainly used in South Korea, a technique of lowering the yield ratio of steel materials used in column and beam structures during an earthquake to delay the point at which the structure fails has been mainly used. However, seismic design using such low-yield-ratio steel materials not only makes it impossible to reuse the steel materials used in the structure, but also fails to ensure the stability of the structure itself, resulting in the problem that reconstruction must be carried out.

[0003] In recent years, seismic design technology has developed and the practical application of seismic or aseismic structures has been promoted. In particular, various technologies have been developed to ensure seismic performance by absorbing the energy applied to a structure by an earthquake at specific sites. A seismic damper is used as a device for absorbing such earthquake energy, and steel materials for seismic dampers have characteristics of an extremely low yield point. By lowering the yield point compared to existing column and beam structural materials, the steel material for a seismic damper yields first during an earthquake to absorb the vibration energy caused by the earthquake, and other structural materials are maintained within the elastic range, thereby suppressing the deformation of the structure.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The objective of this invention is to provide a steel material for seismic dampers that has a low yield strength and can be used to ensure the seismic resistance of structures from earthquakes, as well as a method for manufacturing the same.

[0006] Furthermore, the present invention aims to provide a steel material for seismic dampers that has low yield strength and excellent low-temperature impact toughness, as well as a method for manufacturing the same.

[0007] The problems addressed by the present invention are not limited to those described above. Anyone with ordinary skill in the art to which the present invention pertains will have no difficulty understanding the further problems addressed by the present invention from the entirety of the specification. [Means for solving the problem]

[0008] The present invention In weight percent, it contains C: 0.006% or less, Si: 0.05% or less, Mn: 0.3% or less, P: 0.02% or less, S: 0.01% or less, Al: 0.005~0.05%, N: 0.005% or less, Ti: 48 / 14 × [N] ~0.05% (where [N] means the weight percent content of nitrogen), Nb: 0.04~0.15%, with the remainder being Fe and other unavoidable impurities. It has a ferrite monolithic structure, The present invention provides a steel material for seismic dampers in which the average grain size of ferrite crystal grains in the surface layer, from the surface to 30% of the total thickness, is 150 to 500 μm.

[0009] Furthermore, the present invention is The process involves heating a steel slab containing, by weight percent, C: 0.006% or less, Si: 0.05% or less, Mn: 0.3% or less, P: 0.02% or less, S: 0.01% or less, Al: 0.005~0.05%, N: 0.005% or less, Ti: 48 / 14×[N]~0.05% (where [N] means the weight percent content of nitrogen), Nb: 0.04~0.15%, with the remainder being Fe and other unavoidable impurities, to a temperature range of 1050~1250°C. The process involves finishing rolling a heated steel slab at a temperature range of Ar3-80°C or higher and Ar3 or lower, The process includes the step of shot blasting the surface of the finished-rolled steel material, The above shot blasting step is performed by rotating metal or non-metallic balls at a speed of 1,500 to 2,500 rpm and spraying them onto the surface of the plate at a speed of 60 to 100 m / s, providing a method for manufacturing steel materials for seismic dampers. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a steel material that can be suitably used as a seismic damper for ensuring the seismic resistance of structures from earthquakes, and a method for manufacturing the same.

[0011] Furthermore, this invention makes it possible to provide a steel material for seismic dampers with low yield strength and excellent low-temperature impact toughness, as well as a method for manufacturing the same.

[0012] The diverse and beneficial advantages and effects of the present invention are not limited to those described above and can be more easily understood in the process of describing specific embodiments of the present invention. [Brief explanation of the drawing]

[0013] [Figure 1a] This is a photograph taken with an optical microscope, schematically showing the microstructure of the surface layer and the internal region other than the surface layer of the steel material of the present invention. [Figure 1b] Figure 1a shows an enlarged view of area A. [Figure 1c] Figure 1a shows an enlarged view of area B. [Figure 1d] Figure 1a shows a magnified view of region C. [Figure 2] This graph shows the change in the recrystallization termination temperature (Tnr) of the steel material of the present invention depending on the amount of Nb added. [Figure 3] This graph shows the change in yield strength due to the average grain size in the surface layer and the average grain size in the internal region other than the surface layer of the steel material of the present invention. [Figure 4]It is a graph showing the change in the thickness ratio of the upper and lower surface layers with respect to the total thickness of the steel material according to the LMP, which is a parameter represented by the heat treatment temperature and time. [Figure 5] It is a graph showing the change in the yield strength according to the thickness ratio of the upper and lower surface layers with respect to the thickness of the steel material.

Embodiments for Carrying Out the Invention

[0014] 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. Also, the embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0015] Conventionally, as a steel material used to ensure the seismic resistance of a structure from earthquakes, a technique using a component close to pure iron and performing additional heat treatment in the temperature range of 910 to 960 °C has been known.

[0016] However, in such a technique, since it is necessary to perform additional heat treatment at a high temperature of 900 °C or higher after finish rolling, in the case of a steel material with an extremely low yield point without Si addition, excessive scale occurs and defects occur, or coarse Nb or Ti precipitates are formed, resulting in deterioration of impact toughness. Also, there is a problem of increasing the manufacturing cost due to the accompanying additional heat treatment process at a high temperature of 900 °C or higher.

[0017] Therefore, as a result of intensive studies to solve the above problems, the inventors have found that by optimizing the composition of the steel, the microstructure of the surface layer, and manufacturing conditions, etc., it is possible to provide a steel material for a seismic damping damper that has a low yield strength of 120 MPa or less and excellent low-temperature impact toughness, and thus the present invention has been completed.

[0018] Hereinafter, the [steel material for a seismic damping damper] according to the present invention will be described in detail.

[0019] Specifically, the steel material for seismic dampers of the present invention has a composition consisting of, by weight percent, C: 0.006% or less, Si: 0.05% or less, Mn: 0.3% or less, P: 0.02% or less, S: 0.01% or less, Al: 0.005~0.05%, N: 0.005% or less, Ti: 48 / 14×[N]~0.05%, Nb: 0.04~0.15%, with the remainder being Fe and other unavoidable impurities. Below, we will first explain the reasons for adding each alloy component that constitutes the steel composition, which is one of the main features of the present invention, and the appropriate content ranges for them.

[0020] C: 0.006% or less (excluding 0%) Carbon (C) is an element that causes solid solution strengthening, and in its free state, it binds to dislocations, increasing yield strength and reducing elongation. In order to ensure the aforementioned effects, the present invention excludes cases where the carbon content is 0% (i.e., the carbon content is greater than 0%). Therefore, for suitability as a steel material for seismic dampers, the lower the carbon content, the better, so the content can be controlled to 0.006% or less, and more preferably to 0.0045% or less. Furthermore, the above carbon content may be 0.0005% or more.

[0021] Si: 0.05% or less (excluding 0%) Si, like C, is an element that causes solid solution strengthening, increasing yield strength and decreasing elongation. To ensure the aforementioned effects, the Si content is not 0% (i.e., the Si content is greater than 0%). However, for suitability as a steel material for seismic dampers, a lower Si content is preferable. Therefore, in this invention, from the viewpoint of ensuring low yield strength, the Si content can be controlled to 0.03% or less, and more preferably to 0.013% or less. Furthermore, the above Si content may be 0.001% or more.

[0022] Mn: 0.3% or less (excluding 0%) Mn, like Si, is an element that causes solid solution strengthening, increasing yield strength and decreasing elongation. To ensure the aforementioned effects, the case where the Mn content is 0% is excluded (i.e., the Mn content is greater than 0%). However, in order to be suitably used as a steel material for seismic dampers, in this invention, from the viewpoint of ensuring low yield strength, the Mn content can be controlled to 0.3% or less, and more preferably to 0.2% or less. Furthermore, the above Mn content may be 0.06% or more, and more preferably 0.1% or more.

[0023] P: 0.02% or less (excluding 0%) Since phosphorus (P) is an element advantageous for improving strength and corrosion resistance, the P content is not 0% (i.e., the P content is greater than 0%) in order to ensure the aforementioned effects. However, since P can significantly impair impact toughness, it is preferable to keep the P content as low as possible. Therefore, in this invention, the P content can be controlled to 0.02% or less, and more preferably to 0.013% or less. Furthermore, the above P content may be 0.001% or more, and more preferably 0.004% or more.

[0024] S: 0.01% or less (excluding 0%) Since sulfur (S) is an element that significantly impairs impact toughness by forming MnS and the like, it is preferable to keep its content as low as possible. Therefore, in the present invention, the S content can be controlled to 0.01% or less, and more preferably to 0.004% or less. Furthermore, the above S content may be 0.0005% or more, and more preferably 0.001% or more.

[0025] Al: 0.005~0.05% Al is an element that can deoxidize molten steel inexpensively, and from the viewpoint of ensuring impact toughness while sufficiently lowering the yield strength, the upper limit of the Al content is controlled to 0.05%. Alternatively, the upper limit of the Al content can be controlled to 0.035%, and from the viewpoint of ensuring minimal deoxidation performance, the lower limit of the Al content can be controlled to 0.005%, and more preferably to 0.023%.

[0026] N: 0.005% or less (excluding 0%) N is an element that causes solid solution strengthening, and in its free state, it binds to dislocations, increasing yield strength and decreasing elongation. To ensure the aforementioned effects, the case where the N content is 0% is excluded (i.e., the N content is greater than 0%). However, since the lower the N content, the better, the N content is controlled to 0.005% or less from the viewpoint of ensuring low yield strength. Furthermore, the above N content may be 0.001% or more.

[0027] Nb: 0.04~0.15% Nb is an important element in the production of TMCP steel, and it is a crucial element that precipitates in the form of NbC or NbCN to prevent carbon from adhering to dislocations. Furthermore, when reheated to high temperatures, the dissolved Nb suppresses the recrystallization of austenite, resulting in a finer microstructure.

[0028] On the other hand, in order to introduce deformation-induced precipitates, it is necessary to secure a wide unrecrystallized region. As shown in Figure 2, from the viewpoint of securing a temperature range of 50°C or higher between Ar3 and Tnr, it is preferable to add 0.04% or more of Nb. Furthermore, in order to prevent deterioration of impact toughness due to the coarsening of precipitates, it is preferable to add 0.15% or less of Nb.

[0029] Specifically, Figure 2 shows a graph illustrating the change in recrystallization stopping temperature (Tnr) depending on the amount of Nb added to the steel material of the present invention. That is, in the case of ultra-low carbon steel, in which the carbon content is controlled to an extremely low level as in the present invention, Ar3 is very high at about 890°C, and the change in Ar3 is slight. Therefore, the change in Ar3 is at a negligible level, and as shown in Figure 2, Ar3 can be fixed at about 890°C, and the recrystallization stopping temperature (Tnr) of ultra-low carbon steel can be controlled to a high level only when the Nb content is added at 0.04 to 0.15%. Accordingly, as in the present invention, by controlling the Nb content in the range of 0.04 to 0.15%, a difference of 50°C or more between the Tnr and Ar3 of ultra-low carbon steel can be secured, which allows for the fine generation of deformation-induced precipitates and the fixation of C as precipitates. On the other hand, from the viewpoint of improving the aforementioned effects, it is more preferable that the lower limit of the Nb content be 0.07%, or the upper limit of the Nb content be 0.1%.

[0030] Ti: 48 / 14 × [N] ~ 0.05% Ti is an element that prevents nitrogen from adhering to dislocations by precipitating in the form of TiN. Therefore, in order to adhering nitrogen to an appropriate range in steel, Ti must be added at least 48 / 14 × [N]% (where [N] represents the nitrogen content expressed in weight percent), or at least 0.02%, taking into account the added nitrogen content (weight %). On the other hand, if Ti is added excessively, the precipitates may become coarser and the impact toughness may deteriorate. Therefore, from the viewpoint of ensuring impact toughness, the Ti can be controlled to 0.05% or less, and more preferably to 0.04% or less.

[0031] In other words, according to the present invention, by controlling the Ti content to a range of 48 / 14 × [N] ~ 0.05%, N in the steel can be fixed as precipitates, and by controlling the Nb content to a range of 0.04 ~ 0.15%, C in the steel can be fixed as precipitates. Therefore, in the present invention, by optimizing the Ti and Nb content, it is possible to control the formation of deformation-induced precipitates to an appropriate size and finely, thereby effectively providing a steel material for seismic dampers that has low yield strength while having excellent low-temperature impact toughness.

[0032] Specifically, when carbon (C) or nitrogen (N) is in a free state, it adheres to dislocations, causing the upper yield point phenomenon, which results in a yield strength exceeding 120 MPa. Furthermore, the presence of coarse precipitates in a ferrite monostructure degrades impact toughness. However, when precipitation occurs under deformation-induced conditions during rolling, the size of the precipitates becomes finer, suppressing the degradation of impact toughness and preventing the occurrence of the upper yield point, thus enabling the production of steel with an extremely low yield point. Therefore, according to the present invention, it is possible to provide steel with excellent low-temperature impact toughness, having a Charpy impact transition temperature of -20°C or lower, while having a very low yield strength of 120 MPa or less.

[0033] On the other hand, according to the present invention, although not particularly limited, the steel material for the seismic damper can satisfy an R1 value of 0.8 or more as defined by the following relational formula 1, or more preferably, the R1 value can be in the range of 0.8 to 150. When the R1 value is 0.8 or more, a steel material having a very low yield strength of 120 MPa or less can be provided more effectively. Furthermore, when the R1 value is 150 or less, fine Nb precipitates are formed, so better impact toughness can be ensured.

[0034] [Relationship 1] R1 = [Nb] / [Si] (In the above relational equation 1, [Nb] represents the weight percentage content of Nb, and [Si] represents the weight percentage content of Si.)

[0035] On the other hand, from the viewpoint of improving the aforementioned effects, more preferably, the lower limit of the R1 value defined by the above relational expression 1 may be 3.33, or the upper limit of the R1 value may be 90.

[0036] Alternatively, according to one aspect of the present invention, the steel material for the seismic damper can satisfy an R2 value of 0.8 or more, as defined by the following relational formula 2. More preferably, the R2 value may be in the range of 0.8 to 200, and most preferably in the range of 4 to 200. When the R2 value is 0.8 or more, a steel material having a low yield strength of 120 MPa or less can be provided more effectively. Furthermore, when the R2 value is 200 or less, Nb precipitates are finely formed, ensuring better impact toughness.

[0037] [Relationship 2] R2 = ([Ti] + [Nb]) / [Si] (In the above relational equation 2, [Ti] represents the weight percentage content of Ti, [Nb] represents the weight percentage content of Nb, and [Si] represents the weight percentage content of Si.)

[0038] On the other hand, from the viewpoint of improving the aforementioned effect, more preferably, the lower limit of the R2 value defined by the above relational expression 2 may be 4.33, and the upper limit of the R2 value may be 130.

[0039] In this invention, the remaining components are Fe and other unavoidable impurities. That is, the steel material for seismic dampers according to the present invention may inevitably contain unintended impurities from the raw materials or the surrounding environment during the normal manufacturing process, and therefore these cannot be eliminated. Since such impurities are recognizable to any ordinary engineer, this specification will not mention all of them.

[0040] According to the present invention, the steel material for the seismic damper has a ferrite monostructure. By satisfying this condition, it can effectively absorb energy when an earthquake occurs and fulfill its role as an earthquake damper.

[0041] Furthermore, according to the present invention, the average particle size of the ferrite crystal grains in the surface layer may be 150 to 500 μm. If the average particle size of the ferrite crystal grains in the surface layer is less than 150 μm, there is a possibility that the yield strength will exceed the target yield strength, and if it exceeds 500 μm, there is a possibility that the yield strength of the damper steel will be lower than the target strength. On the other hand, the lower limit of the average particle size of the ferrite crystal grains in the surface layer may more preferably be 175 μm, and most preferably be 200 μm. Alternatively, the upper limit of the average particle size of the ferrite crystal grains in the surface layer may more preferably be 310 μm, and most preferably be 300 μm.

[0042] In this specification, the above-mentioned surface layer refers to the area from the surface of the steel material up to 30% of its total thickness. Therefore, the internal area other than the surface layer, as described later, refers to the area excluding the surface layers (upper surface layer and lower surface layer) located at the top and bottom of the steel material in the thickness direction.

[0043] According to the present invention, the average grain size of ferrite crystal grains in the surface layer may be larger than the average grain size of ferrite crystal grains in the internal region other than the surface layer, and more preferably, it may be 150 μm or more larger than the average grain size of ferrite crystal grains in the internal region. By satisfying this condition, the effect of securing the desired yield strength can be achieved.

[0044] Alternatively, according to the present invention, the average grain size of the ferrite crystal grains in the internal region other than the surface layer may be in the range of 10 to 50 μm, and more preferably in the range of 30 to 50 μm. If the average grain size of the ferrite crystal grains in the internal region is less than 10 μm, there is a possibility that the yield strength will exceed the target strength, and if it exceeds 50 μm, there is a possibility that the yield strength of the entire damper will be lower than the target strength.

[0045] The average grain size of the ferrite crystal grains mentioned above refers to the average value of the equivalent circular diameter measured relative to the crystal grain, with respect to the cross-section in the thickness direction of the steel material (i.e., the direction perpendicular to the rolling direction). Specifically, it is the average value of the measured grain size when assuming a spherical particle whose grain size is represented by the longest length penetrating the interior of the crystal grain.

[0046] Figure 1 shows an optical photograph of the microstructure of a steel material described in Invention Example 1-2 below, which is an example of the present invention, taken using an optical microscope. As can be seen from Figure 1, it can be confirmed that the ferrite crystal grain size in the surface layer is larger than the ferrite crystal grain size in the internal region other than the surface layer.

[0047] Furthermore, according to the present invention, although not particularly limited, the ratio of the thickness of the surface layer (Ds / Dt) to the total thickness (Dt) of the steel material (Dt) may be in the range of 0.1 to 0.3, based on the thickness direction of the steel material (i.e., the direction perpendicular to the rolling direction). In this way, by satisfying the ratio of the thickness of the surface layer (Ds / Dt) to the total thickness of the steel material (Ds / Dt) of 0.1 to 0.3, as can be seen from Figure 5, it is possible to effectively provide a steel material for seismic dampers having a very low yield strength of 120 MPa or less, which is the target of the present invention.

[0048] On the other hand, in the present invention, if the above ratio (Ds / Dt) is less than 0.1, there is a possibility that the damper will not be able to absorb sufficient energy beyond the target yield strength, and if the above ratio (Ds / Dt) exceeds 0.3, as shown in Figure 3, the yield strength may become too low, which may cause problems in providing safe support to the structure.

[0049] While not particularly limited, from the viewpoint of improving the aforementioned effects, it is more preferable that the lower limit of the above ratio (Ds / Dt) be 0.14, or the upper limit of the above ratio (Ds / Dt) be 0.25.

[0050] It should be noted that the above-mentioned surface layer is a concept that encompasses all surface layers formed on both the upper and lower parts of the steel material.

[0051] According to the present invention, the yield strength (YS) of the steel material for the seismic damper described above may be 120 MPa or less, and is not particularly limited, but is more preferably in the range of 80 to 120 MPa. If the yield strength of the steel material exceeds 120 MPa, there is a possibility that it will not be able to absorb enough energy when an earthquake occurs, and if the yield strength of the steel material is less than 80 MPa, there is a possibility that it will be difficult to maintain the stability of the structure.

[0052] The following describes in detail the method for manufacturing steel materials for seismic dampers according to the present invention.

[0053] Slab heating stage The present invention's method for manufacturing steel materials for seismic dampers may include a step of reheating a steel slab that satisfies the aforementioned composition, and the reheating can be performed in the 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 solid dissolve the Ti and / or Nb carbonitrides formed during casting. However, if heated to an excessively high temperature, the austenite may coarseen, and it will take an excessive amount of time for the surface temperature to reach the cooling start temperature of the surface layer after rough rolling, so it is preferable to heat the slab at 1250°C or lower. On the other hand, although not particularly limited, from the viewpoint of improving the aforementioned effects, it is more preferable that the lower limit of the reheating temperature of the slab be 1075°C, or the upper limit of the reheating temperature of the slab be 1125°C.

[0054] Rough rolling stage According to the present invention, the heated steel slab may further include a step of rough rolling to adjust the shape of the slab before the finish rolling step described later, and the temperature of such rough rolling can be controlled to be above the temperature (Tnr) at which austenite recrystallization stops. Rough rolling can have the effect of destroying structural elements such as dendrites formed during casting, and can also have the effect of reducing the size of austenite. On the other hand, although not particularly limited, from the viewpoint of improving the above-mentioned effects, the lower limit of the rough rolling completion temperature may be 995°C, or the upper limit of the rough rolling completion temperature may be 1035°C.

[0055] Finish rolling stage The process includes a step of finish rolling the aforementioned heated steel slab (or roughly rolled bar) at a temperature range of Ar3-80°C or higher and Ar3 or lower. Subsequently, after finish rolling, a step of cooling may be included as needed, and the cooling may be done by air cooling.

[0056] On the other hand, if the finishing rolling temperature is below Ar3-80°C, there is a possibility that the ferrite grain size inside the steel material will become excessively fine. Also, if the finishing rolling temperature exceeds Ar3, there is a possibility that the ferrite grain size inside the steel material will become coarse. On the other hand, although not particularly limited, from the viewpoint of improving the aforementioned effects, the lower limit of the finishing rolling start temperature may be 955°C, or the upper limit of the finishing rolling start temperature may be 980°C. Also, the lower limit of the finishing rolling end temperature may be 860°C, or the upper limit of the finishing rolling end temperature may be 905°C.

[0057] Shot blasting stage The process includes a step of shot blasting the surface of the finished rolled steel material as described above. This shot blasting can be performed by rotating metal or non-metallic balls at a speed of 1,500 to 2,500 rpm and spraying them onto the surface of the plate at a speed of 60 to 100 m / s. Shot blasting allows coarse ferrite grains to grow on the surface of the steel material, and increases the ratio of the surface thickness to the total thickness of the steel material, thereby lowering the yield strength.

[0058] During the shot blasting process described above, if the rotation speed of the metal or non-metallic balls is less than 1,500 rpm, it may not be possible to secure sufficient speed, potentially resulting in a problem where the ferrite grain size on the surface cannot be secured. If the rotation speed exceeds 2,500 rpm, problems may occur in the stable operation of the machine. On the other hand, although not particularly limited, from the viewpoint of improving the aforementioned effects, the lower limit of the rotation speed may be 1,550 rpm, or the upper limit of the rotation speed may be 2,350 rpm.

[0059] Furthermore, if the injection speed is less than 60 m / s, there is a possibility that insufficient effective stress will be applied to the surface of the steel material, making it impossible to secure the desired physical properties. If it exceeds 100 m / s, deep grooves may form on the surface of the steel material, potentially causing product defects. On the other hand, although not particularly limited, from the viewpoint of improving the aforementioned effects, it is more preferable that the lower limit of the injection speed be 62 m / s, or that the upper limit of the injection speed be 94 m / s.

[0060] According to the present invention, during the shot blasting process, metal balls or non-metal balls with an average diameter of 0.8 to 1.2 mm can be used. If the diameter of the balls is less than 0.8 mm, there is a possibility that insufficient energy will be transferred to the surface of the steel material, and if the diameter of the balls exceeds 1.2 mm, there is a possibility that energy cannot be uniformly transferred to the surface of the steel material. On the other hand, although not particularly limited, from the viewpoint of improving the aforementioned effects, it is more preferable that the lower limit of the average diameter of the metal balls (or non-metal balls) be 0.9 mm, or that the upper limit of the average diameter of the metal balls (or non-metal balls) be 1.1 mm.

[0061] Furthermore, according to the present invention, the shot blasting treatment can be performed for 10 to 30 minutes. If the shot blasting treatment time is less than 10 minutes, there is a possibility that insufficient energy will be transferred to the surface of the steel material, and if the shot blasting treatment time exceeds 30 minutes, there is a possibility that defects will be induced in the surface quality of the steel material. On the other hand, although not particularly limited, from the viewpoint of improving the aforementioned effects, it is more preferable that the lower limit of the shot blasting treatment time be 15 minutes, or the upper limit of the shot blasting treatment time be 25 minutes.

[0062] Heat treatment stage According to the present invention, although not particularly limited, the process may further include a step of heat treatment after the shot blasting step, such that the LMP value defined by the following relational expression 3 satisfies the range of 23.5 to 24.5.

[0063] [Relationship Equation 3] LMP = T × [log(t) + 20] / 1000 (In the above relational equation 3, T represents the heat treatment temperature, and the unit is Kelvin (K) (Also, the above t indicates the heat treatment time, and the unit is minutes.)

[0064] In this case, the value of relational equation 3 below is an empirically obtained numerical value, so it is not necessary to specify a unit. That is, it is sufficient that relational equation 3 below satisfies the units of T and t described later.

[0065] According to the present invention, by satisfying the range of 23.5 to 24.5 for the LMP value defined by the aforementioned relational equation 3, the ratio of the surface layer thickness to the total thickness of the steel material can be controlled to a range of 0.1 to 0.3, as shown in Figure 4. This makes it possible to obtain a steel material that satisfies the target yield strength of 120 MPa or less (more preferably in the range of 80 to 120 MPa).

[0066] When a steel plate that has undergone shot blasting is heat-treated, coarse ferrite grows from the surface of the steel due to the stress introduced into the surface layer. In this way, by controlling the heat treatment conditions to form coarse ferrite on the surface of the steel as shown in Figure 1, it becomes possible to introduce a change in the yield strength of the steel.

[0067] On the other hand, although not particularly limited, from the viewpoint of improving the aforementioned effects, the lower limit of the LMP value defined by the above relational expression 3 may be 23.7, or the upper limit of the LMP value defined by the above relational expression 3 may be 24.3.

[0068] Furthermore, although not particularly limited, the heat treatment step can be performed in the range of 850 to 900°C according to the present invention. If the heat treatment temperature is less than 850°C, there is a possibility that the growth of sufficiently coarse ferrite on the surface will not be ensured, and if it exceeds 900°C, there is a possibility that ferrite grains that are excessively coarse than the target ferrite grains will be formed. On the other hand, although not particularly limited, from the viewpoint of improving the above-mentioned effects, the lower limit of the heat treatment temperature may be 855°C, or the upper limit of the heat treatment temperature may be 880°C.

[0069] Furthermore, according to the present invention, although not particularly limited, the heat treatment time may be in the range of 5 to 30 minutes. More preferably, the lower limit of the heat treatment time may be 10 minutes, or the upper limit of the heat treatment time may be 25 minutes.

[0070] 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.

[0071] (Examples) Steel slabs having the alloy composition and properties shown in Table 1 below were prepared. In Table 1, the content of each component is in weight percent, with the remainder being Fe and other unavoidable impurities. That is, in the steel slabs listed in Table 1 (the remainder being Fe), inventive steels A to D are examples that match the range of alloy composition defined in this invention, while comparative steels E to I are examples that fall outside the range of alloy composition defined in this invention. On the other hand, for the steel slabs listed in Table 1 below, the values ​​of Ar3 and Tnr were experimentally measured from the point where the stress changed due to temperature through high-temperature torsion experiments using ultra-low carbon steel.

[0072] After reheating the prepared steel slabs in the temperature range of 1050 to 1250°C, the slabs were reheated, roughly rolled, and finished rolled under the conditions described in Table 2 below. Subsequently, the slabs were shot blasted for 15 minutes using metal balls with an average diameter of 1.0 m under the conditions described in Table 3 below, and then heat-treated to produce the steel material.

[0073] [Table 1] (In Table 1 above, Ti* represents a value of 48 / 14 × N (weight %).)

[0074] [Table 2]

[0075] [Table 3]

[0076] After manufacturing the steel material under the conditions described in Tables 2 and 3 above, the resulting steel material was polished and etched, and then observed with an optical microscope to confirm that it had a single-phase ferrite structure.

[0077] Furthermore, the average grain size, yield strength (YS), tensile strength (TS), and Charpy impact transition temperature for the surface and internal regions of the steel obtained from each experimental example are shown in Table 4 below.

[0078] In this study, the average grain size was measured using the line measurement method. The yield strength was defined as the point at which yielding occurred using a tensile testing machine in accordance with ASTM standards, and the tensile strength was defined as the strength at which necking occurred. The Charpy impact transition temperature was determined by measuring the impact absorption energy using a Charpy impact testing machine, and it represents the temperature at which fracture transitioned from ductile to brittle.

[0079] [Table 4]

[0080] In Table 4 above, Examples 1-1, 1-2, 2-1, 2-2, 3-1, 3-2, 4-1, and 4-2 satisfy all of the steel composition and manufacturing conditions of the present invention, with the ratio of the thickness of the upper and lower surface layers to the total thickness of the steel being in the range of 0.1 to 0.3, and the physical properties of the steel all satisfy a yield strength of 80 to 120 MPa and a Charpy impact transition temperature of -20°C or lower.

[0081] On the other hand, Reference Examples 1 to 4 are cases where the steel composition of the present invention is satisfied, but the manufacturing conditions deviate from the present invention. Of these, Reference Examples 1 to 4 are cases where the LMP exceeds 24.5. In such cases as Reference Examples 1 to 4, the surface layer thickness ratio falls outside the range of 0.1 to 0.3, and the yield strength is all less than 80 MPa.

[0082] Furthermore, in Comparative Example 1, the C content exceeded the upper limit specified in the present invention, resulting in a yield strength exceeding 120 MPa. In Comparative Example 2, the Si, a solid solution strengthening element, exceeded the upper limit specified in the present invention, resulting in a yield strength exceeding 120 MPa. In Comparative Example 3, an excess of Nb was added, leading to deterioration of impact toughness due to the formation of coarse precipitates, and the Charpy impact transition temperature exceeded -20°C. In Comparative Example 4, although all the manufacturing conditions of the present invention were met, the Ti content exceeded the upper limit specified in the present invention, resulting in a Charpy impact transition temperature exceeding -20°C due to the formation of coarse precipitates. In Comparative Example 5, although all the manufacturing conditions of the present invention were met, the Ti content did not reach the lower limit specified in the present invention, resulting in insufficient Ti content to precipitate free N as nitride, causing the yield point phenomenon to occur and a yield strength exceeding 120 MPa.

Claims

1. In weight percent, it contains C: 0.006% or less, Si: 0.05% or less, Mn: 0.3% or less, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.05%, N: 0.005% or less, Ti: 48 / 14 × [N] to 0.05% (where [N] means the weight percent content of nitrogen), Nb: 0.04 to 0.15%, with the remainder being Fe and other unavoidable impurities. It has a ferrite monolithic structure, It consists of a surface region where the average grain size of the ferrite crystal grains is 150 to 500 μm and an internal region where the average grain size of the ferrite crystal grains is 10 to 50 μm. The steel material for seismic dampers is characterized in that the surface portion is the surface and the region from the surface to a certain depth, and the ratio of the thickness of the surface portion (Ds) to the total thickness (Dt) of the steel material (Ds / Dt) is in the range of 0.1 to 0.

3.

2. The steel material for seismic dampers according to claim 1, characterized in that the R1 value defined by the following relational expression 1 is in the range of 0.8 to 150. [Relationship 1] R1=[Nb] / [Si] (In the above relational formula 1, [Nb] represents the weight percentage content of Nb, and [Si] represents the weight percentage content of Si.)

3. The steel material for seismic dampers according to claim 1, characterized in that the R2 value defined by the following relational expression 2 is in the range of 4 to 200. [Relationship Equation 2] R2=([Ti]+[Nb]) / [Si] (In the above relational equation 2, [Ti] represents the weight percentage content of Ti, [Nb] represents the weight percentage content of Nb, and [Si] represents the weight percentage content of Si.)

4. The steel material for seismic dampers according to claim 1, characterized in that the yield strength of the steel material is 120 MPa or less.

5. A step of heating a steel slab containing, by weight percent, C: 0.006% or less, Si: 0.05% or less, Mn: 0.3% or less, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.05%, N: 0.005% or less, Ti: 48 / 14 × [N] to 0.05% (where [N] means the weight percent content of nitrogen), Nb: 0.04 to 0.15%, with the remainder being Fe and other unavoidable impurities, to a temperature range of 1050 to 1250°C. The steps include: finishing rolling a heated steel slab in a temperature range of Ar3-80°C or higher and Ar3 or lower; The process includes the step of shot blasting the surface of the finished-rolled steel material, The method for manufacturing steel materials for seismic dampers according to any one of claims 1 to 4, characterized in that the shot blasting step is performed by rotating metal balls or non-metal balls at a speed of 1500 to 2500 rpm and spraying them onto the surface of the plate material at a speed of 60 to 100 m / s.

6. The method for manufacturing steel materials for seismic dampers according to claim 5, characterized in that the shot blasting step is performed for 10 to 30 minutes.

7. The method for manufacturing steel materials for seismic dampers according to claim 5, characterized in that the diameter of the metal ball or non-metal ball is 0.8 to 1.2 mm.

8. The method for manufacturing steel material for seismic dampers according to claim 5, further comprising a step of heat treatment after the shot blasting step, such that the LMP value defined by the following relational formula 3 satisfies the range of 23.5 to 24.

5. [Relationship Equation 3] LMP=T×[log(t)+20] / 1000 (In the above relational equation 3, T represents the heat treatment temperature, and its unit is Kelvin (K). Also, t represents the heat treatment time, and its unit is minutes.)

9. The method for manufacturing steel material for seismic dampers according to claim 8, characterized in that the heat treatment step is performed in the range of 850 to 900°C.

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