Steel material for seismic damper having excellent impact toughness and method for manufacturing the same

Abstract

The present invention provides a steel material for a seismic damper and a method for manufacturing the same. The steel material includes, at most 0.006% by weight of C, at most 0.05% by weight of Si, at most 0.3% by weight of Mn, at most 0.02% by weight of P, at most 0.01% by weight of S, 0.005% to 0.05% by weight of Al, at most 0.005% by weight of N, 48 / 14 x [N] to 0.05% by weight of Ti (here, [N] is the nitrogen content in % by weight), 0.04% to 0.15% by weight of Nb, and the remainder of Fe and other unavoidable impurities, and has a ferrite single structure, and has a ferrite grain average particle diameter of 150 to 500 μm in a region of a surface layer portion corresponding to 30% of the total thickness from the surface of the steel material.

Description

TECHNICAL FIELD

[0001] The present invention relates to a steel material for a seismic damper for securing the seismic resistance of a structure against an earthquake and a manufacturing method thereof. BACKGROUND

[0002] In the past, a seismic design mainly used in Korea has mainly used a technique of lowering the yield ratio of a steel material for a column or beam structure to delay the time point at which the structure is destroyed during an earthquake. However, the seismic design using such a steel material having a low yield ratio has the following problems: it is impossible to reuse the steel material used in the structure, and the structure itself should be rebuilt due to the lack of stability.

[0003] Recently, with the development of a seismic design technique, the practical application of a seismic damper or vibration damper structure is progressing. In particular, various techniques of securing a seismic performance by absorbing energy applied to a structure by an earthquake to a specific portion of the structure are being developed. A seismic damper is used as a device for absorbing such seismic energy, and a steel material for a seismic damper has an ultra-low yield point characteristic. By further lowering the yield point of the steel material for a seismic damper compared to a structure material of an existing column or beam, the steel material first yields during an earthquake to absorb vibration energy generated by the earthquake, and deformation of the structure is suppressed by keeping other structure materials in an elastic range.

[0004] (Patent Document 1) Patent Publication No. 2008-0088605 SUMMARY

[0005] TECHNICAL PROBLEM

[0006] One aspect of the present disclosure is to provide a steel material for a seismic damper having a low yield strength and which can be used to secure the seismic resistance of a structure against an earthquake; and a manufacturing method thereof.

[0007] Alternatively, another aspect of the present disclosure is to provide a steel material for a seismic damper having both a low yield strength and excellent low-temperature impact toughness, and a manufacturing method thereof.

[0008] The present disclosure is not limited to the above description. The object of the present disclosure will be understood from the entire content of the present specification, and those skilled in the art to which the present disclosure pertains will not have difficulty understanding additional objects of the present disclosure.

[0009] TECHNICAL SOLUTION

[0010] According to one aspect of the present disclosure,

[0011] Provided is a steel material for a seismic damper, the steel material including: 0.006% by weight or less of C, 0.05% by weight or less of Si, 0.3% by weight or less of Mn, 0.02% by weight or less of P, 0.01% by weight or less of S, 0.005% to 0.05% by weight of Al, 0.005% by weight or less of N, 48 / 14x[N] to 0.05% by weight of Ti, where [N] refers to the content of nitrogen (% by weight), and 0.04% to 0.15% by weight of Nb, and the balance of Fe and other inevitable impurities,

[0012] having a ferrite single structure,

[0013] wherein the average ferrite grain size in the surface layer portion from the surface of the steel material to the region corresponding to 30% of the total thickness is 150 μm to 500 μm.

[0014] According to another aspect of the present disclosure,

[0015] Provided is a method for manufacturing a steel material for a seismic damper, the method including: heating a steel billet to a temperature in the range of 1050°C to 1250°C, the steel billet including: 0.006% by weight or less of C, 0.05% by weight or less of Si, 0.3% by weight or less of Mn, 0.02% by weight or less of P, 0.01% by weight or less of S, 0.005% to 0.05% by weight of Al, 0.005% by weight or less of N, 48 / 14x[N] to 0.05% by weight of Ti, where [N] refers to the content of nitrogen (% by weight), and 0.04% to 0.15% by weight of Nb, and the balance of Fe and other inevitable impurities;

[0016] subjecting the heated steel billet to finish rolling in a temperature range of Ar3-80°C or more and Ar3 or less; and

[0017] performing a shot blasting operation on the surface of the finish-rolled steel material,

[0018] wherein the shot blasting operation is performed such that the metal balls or non-metal balls are rotated at a rate of 1500 rpm to 2500 rpm and are sprayed on the surface of the plate material at a rate of 60 m / sec to 100 m / sec.

[0019] Advantages

[0020] As described above, according to one aspect of the present disclosure, it is possible to provide a steel material for a seismic damper that can be appropriately used to ensure the seismic resistance of a structure against earthquakes, and a method for manufacturing the same.

[0021] Alternatively, according to another aspect of the present disclosure, a steel material for a seismic damper having a low yield strength and excellent low-temperature impact toughness and a method of manufacturing the same can be provided.

[0022] The various and advantageous effects and features of the present disclosure are not limited to the above description and can be more easily understood through the process of describing specific exemplary embodiments of the present disclosure. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1a A photograph showing microstructures in a surface layer portion of the steel material of the present disclosure and an internal region other than the surface layer portion, taken with an optical microscope, is shown. Further, Figure 1b A photograph showing Figure 1a an enlarged view of the region A, Figure 1c A photograph showing Figure 1a an enlarged view of the region B, and Figure 1d A photograph showing Figure 1a an enlarged view of the region C.

[0024] Figure 2 A graph to show a change in recrystallization stop temperature (Tnr) according to an amount of Nb added to the steel of the present disclosure.

[0025] Figure 3 A graph to show a change in yield strength according to an average grain size in a surface layer portion of the steel material of the present disclosure and an average grain size in an internal region other than the surface layer portion.

[0026] Figure 4 A graph to show a change in a thickness ratio of an upper surface layer portion and a lower surface layer portion with respect to a total thickness of the steel material according to a parameter LMP represented by a heat treatment temperature and time.

[0027] Figure 5 A graph to show a change in yield strength according to a thickness ratio of an upper surface layer portion and a lower surface layer portion with respect to a thickness of the steel material. DETAILED DESCRIPTION

[0028] Hereinafter, preferred embodiments of the present disclosure will be described. However, the embodiments of the present disclosure can be modified in various forms, and the scope of the present disclosure should not be construed as being limited to the embodiments described below. The present embodiments are provided to a person skilled in the art to further elaborate the present disclosure in detail.

[0029] As a steel material for securing the seismic resistance of a structure against an earthquake, conventionally, a technique of using a composition close to pure iron and performing additional heat treatment in the range of 910℃ to 960℃ is known.

[0030] However, since the technology requires additional heat treatment at a high temperature of 900°C or more after finish rolling, in the case of a steel material having an ultra-low yield point without the addition of Si, excessive scale occurs, defects occur, or coarse Nb or Ti precipitates are formed, so there are problems in that impact toughness deterioration occurs. In addition, since the process of additional heat treatment at a high temperature of 900°C or more is included, there is also a problem in that manufacturing costs increase.

[0031] Therefore, in order to solve the above problems, as a result of the present disclosure, the inventors have developed a steel material that can provide a yield strength as low as 120 MPa or less and excellent low-temperature impact toughness by optimizing the composition and microstructure of the steel and the manufacturing conditions of the surface layer portion, and thus provide the present disclosure.

[0032] Hereinafter, the [Steel material for seismic damper] according to the present disclosure will be described in detail.

[0033] Specifically, the steel material for seismic damper has a composition comprising 0.006% by weight or less of C, 0.05% by weight or less of Si, 0.3% by weight or less of Mn, 0.02% by weight or less of P, 0.01% by weight or less of S, 0.005% to 0.05% by weight of Al, 0.005% by weight or less of N, 48 / 14 x [N] % by weight to 0.05% by weight of Ti, 0.04% to 0.15% by weight of Nb, and the balance of Fe and other inevitable impurities. Hereinafter, the reasons for adding each alloy component constituting the composition of the steel, which is one of the main features of the present disclosure, and the appropriate content range thereof will be described first.

[0034] C: 0.006% or less (excluding 0%)

[0035] C is an element that causes solid solution strengthening, and is fixed to dislocations in a free state to increase the yield strength and reduce the elongation. In order to ensure the above effects, in the present disclosure, the case where the C content is 0% is excluded (i.e., the C content exceeds 0%). Therefore, in order to be suitable for use as a steel material for seismic damper, the lower the C content, the better, so the C content is controlled to be 0.006% or less, and more preferably 0.0045% or less. In addition, more preferably, the C content can be 0.0005% or more.

[0036] Si: 0.05% or less (excluding 0%)

[0037] Si, like C, is an element to cause solid solution strengthening to increase the yield strength and reduce the elongation. To ensure the above effect, a case where the Si content is 0% is excluded (i.e., the Si content exceeds 0%). However, to be suitable for use as a steel material for a seismic damper, the Si content is better the lower. Therefore, in the present disclosure, the Si content can be controlled to be 0.03% or less, more preferably 0.013% or less, in terms of ensuring low yield strength. Further, the Si content can be 0.001% or more.

[0038] Mn: 0.3% or less (excluding 0%)

[0039] Mn, like Si, is an element to cause solid solution strengthening to increase the yield strength and reduce the elongation. To ensure the above effect, a case where the Mn content is 0% is excluded (i.e., the Mn content exceeds 0%). However, to be suitable for use as a steel material for a seismic damper, in the present disclosure, the Mn content can be controlled to be 0.3% or less, more preferably 0.2% or less, in terms of ensuring low yield strength. Further, the Mn content can be 0.06% or more, and more preferably 0.1% or more.

[0040] P: 0.02% or less (excluding 0%)

[0041] Since P is an element that is advantageous for strength improvement and corrosion resistance, a case where the P content is 0% is excluded (i.e., the P content exceeds 0%) to ensure the above effect. Since P can greatly impair impact toughness, it is preferable to keep the P content as low as possible. Therefore, in the present disclosure, the P content can be controlled to be 0.02% or less, more preferably 0.013% or less. Further, the P content can be 0.001% or more, and more preferably 0.004% or more.

[0042] S: 0.01% or less (excluding 0%)

[0043] Since S is an element that greatly impairs impact toughness by forming MnS or the like, it is preferable to keep the S content as low as possible. Therefore, in the present disclosure, the S content can be controlled to be 0.01% or less, more preferably 0.004% or less. Further, the S content can be 0.0005% or more, more preferably 0.001% or more.

[0044] Al: 0.005% to 0.05%

[0045] Al is an element capable of inexpensively deoxidizing molten steel, and an upper limit of the Al content is controlled to 0.05% in terms of ensuring impact toughness while sufficiently reducing yield strength. Alternatively, more preferably, the upper limit of the Al content can be controlled to 0.035%, and a lower limit of the Al content can be controlled to 0.005% in terms of ensuring minimum deoxidation performance, and more preferably 0.023%.

[0046] N: 0.005% or less (excluding 0%)

[0047] N is an element causing solid solution strengthening and is fixed to dislocations in a free state to increase yield strength and reduce elongation. In order to ensure the above effects, a case where the N content is 0% is excluded (i.e., the N content exceeds 0%). However, the lower the N content, the better, so the N content is controlled to 0.005% or less in terms of ensuring low yield strength. In addition, the N content can be 0.001% or more.

[0048] Nb: 0.04% to 0.15%

[0049] Nb is an important element in manufacturing TMCP steel, and is a very important element for preventing C from being fixed to dislocations by precipitating in the form of NbC or NbCN. In addition, Nb dissolved during reheating to a high temperature suppresses recrystallization of austenite, thereby exhibiting an effect of refining the structure.

[0050] Meanwhile, in order to introduce a deformed organic precipitate, it is necessary to ensure a wide non-recrystallization region. As can be seen in Figure 2 As can be seen in

[0051] Specifically, Figure 2 A graph showing a change in recrystallization stop temperature (Tnr) according to the amount of Nb added to the steel material of the present disclosure is shown. That is, in the case of ultra-low carbon steel in which the carbon content is controlled to an ultra-low amount as in the present disclosure, Ar3 is very high, at about 890°C, and the change in Ar3 is not significant. Therefore, since the change value of Ar3 becomes a negligible level, it is possible to Figure 2Ar3 is represented by fixing Ar3 to about 890°C, and the recrystallization stop temperature (Tnr) of the ultra-low carbon steel can be controlled to be high only when the Nb content is added at 0.04% to 0.15%. Thus, as in the present disclosure, by controlling the Nb content in the range of 0.04% to 0.15%, it is possible to ensure a difference of 50°C or more between the Tnr and Ar3 of the ultra-low carbon steel, and thus, it is possible to finely produce a deformed organic precipitate, and it is possible to fix C as a precipitate. Meanwhile, in terms of improving the above-described effects, more preferably, the lower limit of the Nb content can be 0.07%, or the upper limit of the Nb content can be 0.1%.

[0052] Ti: 48 / 14 x [N] % to 0.05%

[0053] Ti is an element precipitated in the form of TiN, for preventing N from being fixed to dislocations. Thus, in order to fix N in the steel in a suitable range, Ti should be added in an amount of 48 / 14 x [N] % or more, with [N] referring to the nitrogen content (wt%), or Ti should be added in an amount of 0.02% or more, considering the added N content (wt%). Meanwhile, when Ti is excessively added, there is a concern that the impact toughness can be deteriorated due to coarsening of the precipitates, so in terms of securing the impact toughness, Ti can be controlled to be 0.05% or less, and more particularly, Ti can be controlled to be 0.04% or less.

[0054] That is, according to the present disclosure, by controlling the Ti content in the range of 48 / 14 x [N] % to 0.05%, it is possible to fix N in the steel as a precipitate, and by controlling the Nb content in the range of 0.04% to 0.15%, it is possible to fix C in the steel as a precipitate. Thus, in the present disclosure, by optimizing the Ti content and the Nb content, it is possible to control a deformed organic precipitate to be finely formed in a suitable size, thereby effectively providing a seismic damper steel material having excellent low-temperature impact toughness while having a low yield strength.

[0055] Specifically, when C or N is in a free state, C or N is fixed to dislocations to cause an upper yield point phenomenon, resulting in a yield strength exceeding 120 MPa. In addition, when coarse precipitates exist in a ferrite single structure, the impact toughness is deteriorated. However, in the case of being precipitated by strain induction during rolling, the impact toughness can be inhibited due to the fine size, and the upper yield point can be inhibited from being exhibited, so it is possible to provide a steel material having an ultra-low yield point. Thus, according to the present disclosure, it is possible to provide a steel material having excellent low-temperature impact toughness while having a very low yield strength of 120 MPa or less, with a Charpy impact transition temperature of -20°C or lower.

[0056] Meanwhile, according to one aspect of the present disclosure, although the steel material for a seismic damper is not particularly limited, an R1 value defined by the following relation 1 can be 0.8 or more, 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 an ultra-low yield strength of 120 MPa or less can be more effectively provided. In addition, when R1 is 150 or less, Nb precipitates can be finely formed, so that more excellent impact toughness can be ensured.

[0057] [Relation 1]

[0058] R1 = [Nb] / [Si]

[0059] In relation 1, [Nb] denotes the content of Nb (wt%), and [Si] denotes the content of Si (wt%).

[0060] Meanwhile, in terms of improving the above-described effects, more preferably, the lower limit of the R1 value defined by relation 1 can be 3.33, or the upper limit of the R1 value can be 90.

[0061] Alternatively, according to one aspect of the present disclosure, an R2 value of the steel material for a seismic damper defined by the following relation 2 can satisfy 0.8 or more. Or, more preferably, the R2 value can 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 more effectively provided. In addition, when the R2 value is 200 or less, Nb precipitates can be finely formed, so that better impact toughness can be ensured.

[0062] [Relation 2]

[0063] R2 = ([Ti] + [Nb]) / [Si]

[0064] In relation 2, [Ti] denotes the content of Ti (wt%), [Nb] denotes the content of Nb (wt%), and [Si] denotes the content of Si (wt%).

[0065] Meanwhile, in terms of improving the above-described effects, more preferably, the lower limit of the R2 value defined by relation 2 can be 4.33, and the upper limit of the R2 value can be 130.

[0066] In the present disclosure, the remaining portion is Fe. That is, in the steel material for a seismic damper, since unintentional impurities can be inevitably incorporated from raw materials or the surrounding environment in a common manufacturing process, the components can not be excluded. Since these impurities are known to any skilled person in a common manufacturing process, the entire content thereof is not particularly mentioned in the present specification.

[0067] According to one aspect of the present disclosure, the steel material for a seismic damper has a ferrite single structure. By satisfying this, the steel material can be used as a seismic damper by effectively absorbing energy at the time of an earthquake.

[0068] Further, according to one aspect of the present disclosure, the average ferrite grain size in the surface layer portion can be 150 μm to 500 μm. When the average ferrite grain size in the surface layer portion is less than 150 μm, a problem of exceeding the target yield strength can occur, and when the average ferrite grain size exceeds 500 μm, a problem of the yield strength of the steel material for a damper being lower than the target strength can occur. Meanwhile, the lower limit of the average ferrite grain size in the surface layer portion can be more preferably 175 μm, and most preferably 200 μm. Alternatively, the upper limit of the average ferrite grain size in the surface layer portion can be more preferably 310 μm, and most preferably 300 μm.

[0069] Further, in the present specification, the surface layer portion refers to a region from the surface of the steel material to a region corresponding to 30% of the total thickness. Therefore, the inner region other than the surface layer portion described hereafter refers to a region excluding the surface layer portions (upper surface layer portion and lower surface layer portion) respectively provided in the upper portion and the lower portion in the thickness direction of the steel material.

[0070] According to one aspect of the present disclosure, the average ferrite grain size in the surface layer portion can be greater than the average ferrite grain size in the inner region other than the surface layer portion, and more particularly, the average ferrite grain size can be 150 μm or more greater than the average ferrite grain size in the inner region. By satisfying this, an effect of securing the target yield strength can be exhibited.

[0071] Alternatively, according to one aspect of the present disclosure, the average ferrite grain size in the inner region other than the surface layer portion can be in the range of 10 μm to 50 μm, more preferably in the range of 30 μm to 50 μm. When the average ferrite grain size in the inner region is less than 10 μm, a problem of exceeding the target yield strength can occur, and when the average ferrite grain size in the inner region exceeds 50 μm, a problem of the yield strength of the entire damper being lower than the target strength can occur.

[0072] Based on the cut surface in the thickness direction of the steel material (i.e., the direction perpendicular to the rolling direction), the above-described average ferrite grain size refers to the average of the values obtained by measuring the equivalent circle diameters of the grains, and assuming that a spherical particle drawn with the longest length passing through the inside of the grain is the particle diameter, the above-described average ferrite grain size is the average of the measured grain sizes.

[0073] For the steel materials of the inventive examples 1 to 2 to be described later, optical photographs of microstructures taken with an optical microscope corresponding to the examples of the present disclosure are shown in FIG. 1. As can be seen in FIG. 1, it can be confirmed that the ferrite grain size in the surface layer portion is larger than the ferrite grain size in the internal region other than the surface layer portion.

[0074] Further, according to one aspect of the present disclosure, although not particularly limited, the ratio (Ds / Dt) of the thickness Ds of the surface layer portion to the total thickness Dt of the steel material can be in the range of 0.1 to 0.3 based on the thickness direction (a direction perpendicular to the rolling direction) of the steel material. Thus, the ratio (Ds / Dt) of the surface layer portion to the total thickness of the steel material satisfies the range of 0.1 to 0.3, so as can be seen in Figure 5 in the present disclosure, the steel material for a seismic damper having a very low yield strength of 120 MPa or less desired in the present disclosure can be effectively provided.

[0075] Meanwhile, in the present disclosure, when the ratio (Ds / Dt) is less than 0.1, a problem that the target yield strength can be exceeded, and as a damper, sufficient energy can not be absorbed, and when the ratio (Ds / Dt) exceeds 0.3, as shown in Figure 3 , the yield strength is too low so that a problem that a structure can not be safely supported can occur.

[0076] Meanwhile, although not particularly limited, it can be more preferable in terms of improving the above-described effects that the lower limit of the ratio (Ds / Dt) can be 0.14, or the upper limit of the ratio (Ds / Dt) can be 0.25.

[0077] In this case, it should be noted that the surface layer portion is a concept including all surface layer portions formed on each of the upper and lower portions of the steel material.

[0078] According to one aspect of the present disclosure, the yield strength (YS) of the above-described steel material for a seismic damper can be 120 MPa or less, and although not particularly limited, it can be more preferable in the range of 80 MPa to 120 MPa. When the yield strength of the steel material exceeds 120 MPa, a problem that energy can not be sufficiently absorbed when an earthquake occurs can occur, and when the yield strength of the steel material is less than 80 MPa, a problem that a structure can not be stably maintained can occur.

[0079] Hereinafter, a manufacturing method for a steel material for a seismic damper according to the present disclosure will be described in detail.

[0080] Slab heating operation

[0081] The manufacturing method for steel material for seismic damper according to one aspect of the present disclosure can include the operation of reheating a steel billet satisfying the composition described above, and the reheating can be performed to a temperature in the range of 1050 to 1250°C. In this case, the heating temperature of the steel billet is controlled to 1050°C or more to sufficiently dissolve the carbonitride of Ti and / or Nb formed during casting. However, when heated to a temperature that is too high, there can be a concern of austenite coarsening, and it takes too much time to bring the surface temperature after rough rolling to the cooling start temperature of the surface layer portion, and thus it can be more preferable to heat the slab at a temperature of 1250°C or less. Meanwhile, although not particularly limited, in terms of improving the effects described above, more preferably, the lower limit of the reheating temperature of the slab can be 1075°C, or the upper limit of the reheating temperature of the slab can be 1125°C.

[0082] Rough rolling operation

[0083] According to one aspect of the present disclosure, before the finish rolling operation described later, the heated steel billet can include the operation of performing rough rolling to adjust the shape of the slab, and the temperature during rough rolling can be controlled to be higher than the temperature at which austenite recrystallization stops (Tnr). Through rough rolling, the effect of destroying the structural organization such as dendrite crystals formed during casting can be obtained, and the effect of reducing the austenite size can also be obtained. Meanwhile, although not particularly limited, in terms of improving the effects described above, more preferably, the lower limit of the rough rolling end temperature can be 995°C, or the upper limit of the rough rolling end temperature can be 1035°C.

[0084] Finish rolling operation

[0085] The operation of finish rolling the heated steel billet (or the rough-rolled bar) at a temperature range of Ar3-80°C or more and Ar3 or less is included. Subsequently, if necessary, a cooling operation can be included after finish rolling, and the cooling can be air cooling.

[0086] Meanwhile, when the finish rolling temperature is lower than Ar3-80°C, there can be a problem that the ferrite grain size inside the steel material becomes too fine. In addition, when the finish rolling temperature exceeds Ar3, there can be a problem that the ferrite grain size inside the steel material becomes coarse. Meanwhile, although not particularly limited, in terms of improving the effects described above, more preferably, the lower limit of the finish rolling start temperature can be 955°C, or the upper limit of the finish rolling start temperature can be 980°C. In addition, the lower limit of the finish rolling end temperature can be 860°C, or the upper limit of the finish rolling end temperature can be 905°C.

[0087] Shot peening operation

[0088] An operation of performing a shot blasting process on the surface of the above-mentioned finished steel material is included, in which the shot blasting process can be performed so that the metal balls or non-metal balls rotate at a rate of 1,500 to 2,500 rpm and are sprayed on the surface of the plate at a rate of 60 m / sec to 100 m / sec. By performing the shot blasting process, coarse ferrite grains can be grown on the surface layer portion of the steel material, and the ratio of the thickness of the surface layer portion to the total thickness of the steel material can be increased to reduce the yield strength.

[0089] During the shot blasting process, when the rotation speed of the metal balls or non-metal balls is less than 1,500 rpm, it can be difficult to ensure sufficient speed, resulting in a problem of not being able to ensure the ferrite grain size on the surface layer portion, and when the rotation speed of the metal balls or non-metal balls exceeds 2,500 rpm, a problem can occur in the stable operation of the machine. At the same time, although not particularly limited, in terms of improving the above-mentioned effects, more preferably, the lower limit of the rotation speed can be 1,550 rpm, or the upper limit of the rotation speed can be 2,350 rpm.

[0090] In addition, when the spraying speed is less than 60 m / sec, there can be a problem of not being able to ensure the desired physical properties due to a lack of effective stress application on the surface of the steel material, and when the spraying speed exceeds 100 m / sec, deep grooves are generated on the surface of the steel material, causing product defects. At the same time, although not particularly limited, in terms of improving the above-mentioned effects, more preferably, the lower limit of the spraying speed can be 62 m / sec, or the upper limit of the spraying speed can be 94 m / sec.

[0091] According to one aspect of the present disclosure, in the shot blasting process, metal balls or non-metal balls having an average diameter of 0.8 mm to 1.2 mm can be used. When the diameter of the balls is less than 0.8 mm, it can cause a problem of insufficient energy being transferred to the surface of the steel material, and when the diameter of the balls exceeds 1.2 mm, it can cause a problem of non-uniform energy being transferred to the surface of the steel material. At the same time, although not particularly limited, in terms of improving the above-mentioned effects, more preferably, the lower limit of the average diameter of the metal balls (or non-metal balls) can be 0.9 mm, or the upper limit of the average diameter of the metal balls (or non-metal balls) can be 1.1 mm.

[0092] In addition, according to one aspect of the present disclosure, the shot blasting process can be performed for 10 minutes to 30 minutes. When the shot blasting process time is less than 10 minutes, it can cause a problem of insufficient energy being transferred to the surface of the steel material, and when the shot blasting process time exceeds 30 minutes, it can cause a problem of defects in the surface quality of the steel material. At the same time, although not particularly limited, in terms of improving the above-mentioned effects, more preferably, the lower limit of the shot blasting process time can be 15 minutes, or the upper limit of the shot blasting process time can be 25 minutes.

[0093] Heat treatment operation

[0094] According to one aspect of the present disclosure, although not particularly limited, after the shot blasting operation, a heat treatment operation can also be included such that the LMP value defined by the following relation 3 satisfies the range of 23.5 to 24.5.

[0095] [Relation 3]

[0096] LMP = T x [log(t) + 20] / 1000

[0097] In relation 3, T denotes the heat treatment temperature, the unit of which is °C, and t denotes the heat treatment time, the unit of which is minutes.

[0098] In this case, since the value of relation 3 is an empirically obtained numerical value, the unit can not be particularly determined. That is, in relation 3, it is sufficient when the respective units of T and t described later are satisfied.

[0099] According to one aspect of the present disclosure, the LMP value defined by the above relation 3 can satisfy the range of 23.5 to 24.5, and thus, as can be shown in Figure 4 the thickness ratio of the surface layer portion with respect to the total thickness of the steel material can be controlled in the range of 0.1 to 0.3, so that a steel material having a target yield strength of 120 MPa or less (more preferably, in the range of 80 MPa to 120 MPa) can be obtained.

[0100] When the heat treatment is performed on the steel sheet subjected to the shot blasting, coarse ferrite grows from the surface layer portion of the steel material due to the stress introduced to the surface layer portion. Thus, by controlling the heat treatment conditions to form coarse ferrite as shown in FIG. 1 on the surface layer portion of the steel material, the change in the yield strength of the steel material can be introduced.

[0101] Meanwhile, although not particularly limited, in terms of improving the above-described effects, the lower limit of the LMP value defined by relation 3 can be 23.7, or the upper limit of the LMP value defined by relation 3 can be 24.3.

[0102] Further, according to one aspect of the present disclosure, although not particularly limited, the heat treatment operation can be performed in the range of 850°C to 900°C. When the heat treatment temperature is lower than 850°C, there can be a problem in that sufficient coarse ferrite growth cannot be ensured, and when the heat treatment temperature is higher than 900°C, there can be a problem in that ferrite grains much coarser than the target ferrite grains are formed. Meanwhile, although not particularly limited, in terms of improving the above-described effects, more preferably, the lower limit of the heat treatment temperature can be 855°C, or the upper limit of the heat treatment temperature can be 880°C.

[0103] Further, according to one aspect of the present disclosure, the heat treatment time can be in the range of 5 minutes to 30 minutes. Meanwhile, more preferably, the lower limit of the heat treatment time can be 10 minutes, or the upper limit of the heat treatment time can be 25 minutes.

[0104] Embodiment of Invention

[0105] Hereinafter, the present disclosure will be specifically described through the following examples. However, it should be noted that the following examples are only for describing the present disclosure by illustration, and are not intended to limit the scope of rights of the present disclosure. The reason is that the scope of rights of the present disclosure is determined by matters described in the claims and reasonably inferred therefrom.

[0106] (Example)

[0107] Steels having the alloy compositions and properties shown in Table 1 below were prepared. In this case, the content of each component in Table 1 below is in weight %, and the remainder is Fe and unavoidable impurities. That is, among the steels described in Table 1 below (the remainder is Fe), Inventive Steels A to D show examples matching the range of the alloy composition defined in the present disclosure, and Comparative Steels E to I show examples deviating from the range of the alloy composition defined in the present disclosure.

[0108] After the prepared steel billets were reheated to a temperature in the range of 1050°C to 1250°C, slab reheating-rough rolling-finish rolling were performed under the conditions shown in Table 2 below. Subsequently, after shot peening was performed for 15 minutes using metal balls having an average diameter of 1.0 m under the conditions of Table 3, heat treatment was performed to manufacture steel materials.

[0109] [Table 1]

[0110]

[0111] In Table 1, Ti* indicates the value (weight %) of 48 / 14 x N.

[0112] [Table 2]

[0113]

[0114] [Table 3]

[0115]

[0116] After the steel materials were manufactured under the conditions described in Tables 2 and 3 above, the steel plates thus obtained were polished and etched, and then observed with an optical microscope, thereby confirming that the steel materials had a ferrite single structure.

[0117] Further, results of measuring the average grain size, the yield strength (YS), the tensile strength (TS), and the Charpy impact transition temperature in the surface layer portion and in the internal region other than the surface layer portion of each steel material obtained from each experimental example are shown in Table 4 below.

[0118] In this case, the average grain size is measured using a line measurement method, and the point at which yielding occurs is set as the yield strength and the strength at which necking occurs is set as the tensile strength using a tensile testing machine according to the ASTM standard. For the Charpy impact transition temperature, the impact absorbed energy is measured using a Charpy impact testing machine and the temperature at which the fracture transition from ductile to brittle is shown.

[0119] 1Table 4

[0120]

[0121] In Table 4 above, in Examples 1-1, 1-2, 2-1, 2-2, 3-1, 3-2, 4-1, and 4-2 which satisfy both the steel composition and the manufacturing conditions of the present disclosure, the thickness ratio of the upper surface layer portion and the lower surface layer portion to the total thickness of the steel material is in the range of 0.1 to 0.3, and the physical properties of the steel material all satisfy the yield strength of 80 MPa to 120 MPa and the Charpy impact transition temperature of -20°C or lower.

[0122] Meanwhile, Reference Examples 1 to 4 show cases in which the steel composition of the present disclosure is satisfied but the manufacturing conditions are deviated. Among them, Reference Examples 1 to 4 show cases in which the LMP exceeds 24.5. Reference Examples 1 to 4 show cases in which the thickness ratio of the surface layer portion deviates from the range of 0.1 to 0.3, and the yield strength is all less than 80 MPa.

[0123] Further, in Comparative Example 1, C exceeds the upper limit of the content specified in the present disclosure, and the yield strength exceeds 120 MPa. In Comparative Example 2, the solid solution strengthening element Si exceeds the upper limit of the content specified in the present disclosure, and the yield strength exceeds 120 MPa. In Comparative Example 3, when Nb is excessively added, the impact toughness is deteriorated due to the formation of coarse precipitates, and the Charpy impact transition temperature exceeds -20°C. Comparative Example 4 shows a case in which all the manufacturing conditions of the present disclosure are satisfied, but the Ti content exceeds the upper limit specified in the present disclosure, and the Charpy impact transition temperature exceeds -20°C due to the formation of coarse precipitates. Comparative Example 5 shows a case in which all the manufacturing conditions of the present disclosure are satisfied, but the Ti content is less than the lower limit specified in the present disclosure, and in Comparative Example 5, free N is not sufficiently precipitated as nitride due to the insufficient Ti content, and the yield point phenomenon is exhibited, and the yield strength exceeds 120 MPa.

Claims

1. A steel material for a seismic damper, comprising: 0.006% by weight or less of C, 0.05% by weight or less of Si, 0.3% by weight or less of Mn, 0.02% by weight or less of P, 0.01% by weight or less of S, 0.005% to 0.05% by weight of Al, 0.005% by weight or less of N, 48 / 14 x [N] to 0.05% by weight of Ti, where [N] refers to the weight of nitrogen (% by weight), and 0.04% to 0.15% by weight of Nb, and the balance of Fe and other unavoidable impurities, having a ferrite single structure, wherein the steel material includes a surface layer portion having an average ferrite grain size of 150 μm to 500 μm and an internal region other than the surface layer portion having an average ferrite grain size of 10 μm to 50 μm, and wherein a ratio (Ds / Dt) of a thickness Ds of the surface layer portion to a total thickness Dt of the steel material is in a range of 0.1 to 0.

3.

2. The steel material for a seismic damper according to claim 1, wherein an R1 value defined by the following relation 1 is in a range of 0.8 to 150, [Relation 1] R1 = [Nb] / [Si] in the relation 1, [Nb] represents a content of Nb (% by weight), and [Si] represents a content of Si (% by weight).

3. The steel material for a seismic damper according to claim 1, wherein an R2 value defined by the following relation 2 is in a range of 4 to 200, [Relation 2] R2 = ([Ti] + [Nb]) / [Si] in the relation 2, [Ti] represents a content of Ti (% by weight), [Nb] represents a content of Nb (% by weight), and [Si] represents a content of Si (% by weight).

4. The steel material for a seismic damper according to claim 1, wherein a yield strength of the steel material is 120 MPa or less.

5. A manufacturing method for a steel material for a seismic damper, comprising: heating a steel billet to a temperature in a range of 1050°C to 1250°C, the steel billet including: 0.006% by weight or less of C, 0.05% by weight or less of Si, 0.3% by weight or less of Mn, 0.02% by weight or less of P, 0.01% by weight or less of S, 0.005% to 0.05% by weight of Al, 0.005% by weight or less of N, 48 / 14 x [N] to 0.05% by weight of Ti, where [N] refers to a content of nitrogen (% by weight), and 0.04% to 0.15% by weight of Nb, and the balance of Fe and other unavoidable impurities; subjecting the heated steel billet to finish rolling in a temperature range of Ar3-80°C or higher and Ar3 or lower; and performing a shot peening operation on a surface of the finish-rolled steel material, wherein the shot peening operation is performed such that metal balls or non-metal balls are rotated at a rate of 1500 rpm to 2500 rpm and are sprayed on a surface of the sheet material at a rate of 60 m / sec to 100 m / sec. ​ ​ ​ ​ 6. The manufacturing method for steel material for seismic damper according to claim 5, wherein the shot blasting operation is performed for 10 minutes to 30 minutes.

7. The manufacturing method for steel material for seismic damper according to claim 5, wherein the diameter of the metal ball or the non-metal ball is 0.8 mm to 1.2 mm.

8. The manufacturing method for steel material for seismic damper according to claim 5, further comprising, after the shot blasting operation: performing heat treatment such that the LMP value defined by the following relation 3 satisfies the range of 23.5 to 24.5, [relation 3] LMP = T x [log(t) + 20] / 1000 in which, in the relation 3, T denotes the heat treatment temperature, wherein the unit thereof is °C; and further, t denotes the heat treatment time, wherein the unit thereof is minutes.

9. The manufacturing method for steel material for seismic damper according to claim 8, wherein the heat treatment operation is performed in the range of 850 °C to 900 °C. ​ ​

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