JP2023554297A - Steel material for vibration damper with excellent impact toughness and its manufacturing method - Google Patents

Abstract

The present invention provides a steel material for seismic dampers that has low yield strength and excellent low-temperature impact toughness and can be used to ensure earthquake resistance of structures from earthquakes, and a method for manufacturing the same. In the present invention, 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 x [N] to 0.05% (here, [N] means the weight% content of nitrogen), Nb : Contains 0.04 to 0.15%, the remainder consists of Fe and other unavoidable impurities, has a single ferrite structure, and contains ferrite crystal grains in the surface layer from the surface to a region of 30% of the total thickness. It is characterized by an average particle size of 150 to 500 μm. [Selection diagram] Figure 1a

Description

The present invention relates to a steel material for seismic dampers with excellent impact toughness and a method for manufacturing the same, and more specifically, a steel material for seismic dampers with excellent impact toughness used to ensure the seismic resistance of structures from earthquakes. and its manufacturing method.

Traditionally, seismic design, which has been mainly used in Korea, mainly uses technology that lowers the yield ratio of the steel materials used in structures such as columns and beams to delay the point at which the structure will fail in the event of an earthquake. It's here. 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 does not ensure the stability of the structure itself. The problem was that it had to be rebuilt.

In recent years, seismic design technology has developed and the practical use of seismic control or seismic isolation structures is progressing.In particular, there are various technologies that ensure seismic performance by absorbing the energy applied to structures due to earthquakes in specific parts. has been developed. A seismic damper is used as a device to absorb such seismic energy, and the steel material for the seismic damper has the characteristic of an extremely low yield point. Steel materials for seismic dampers have a lower yield point than existing structural materials for columns and beams, allowing them to yield first in the event of an earthquake and absorb the vibration energy caused by the earthquake, while maintaining other structural materials within their elastic range. By doing so, deformation of the structure can be suppressed.

Korean Published Patent No. 2008-0088605

An object of the present invention is to provide a steel material for a seismic damper that has a low yield strength and can be used to ensure the earthquake resistance of a structure from an earthquake, and a method for manufacturing the same.

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

The object of the present invention is not limited to the above-mentioned contents. Anyone having ordinary knowledge in the technical field to which the present invention pertains will have no difficulty in understanding the further objects of the present invention from the content throughout the specification of the present invention.

The present invention
In 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~ 0.05%, N: 0.005% or less, Ti: 48/14×[N] ~ 0.05% (here, [N] means the weight% content of nitrogen), Nb: 0.04 ~0.15%, with the remainder consisting of Fe and other unavoidable impurities,
Has a single ferrite structure,
Provided is a steel material for a seismic damper, in which the average grain size of ferrite crystal grains in the surface layer region from the surface to 30% of the total thickness is 150 to 500 μm.

Moreover, the present invention
In 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~ 0.05%, N: 0.005% or less, Ti: 48/14×[N] ~ 0.05% (here, [N] means the weight% content of nitrogen), Nb: 0.04 heating a steel slab containing ~0.15% Fe and other unavoidable impurities to a temperature range of 1050~1250°C;
Finish rolling the heated steel slab in a temperature range of Ar3-80°C or higher and Ar3 or lower;
shot blasting the surface of the finish-rolled steel material;
The shot blasting step is performed by rotating the metal balls or non-metal balls at a speed of 1,500 to 2,500 rpm and spraying them onto the surface of the plate material at a speed of 60 to 100 m/s. Provided is a method for manufacturing steel materials for seismic dampers.

According to the present invention, it is possible to provide a steel material that can be suitably used as a seismic damper used to ensure the earthquake resistance of a structure from an earthquake, and a method for manufacturing the same.

Further, according to the present invention, it is possible to provide a steel material for a seismic damper that has low yield strength and excellent low-temperature impact toughness, and a method for manufacturing the same.

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.

1 is a photograph taken with an optical microscope, schematically showing a microstructure in a surface layer and an internal region other than the surface layer of the steel material of the present invention. This is an enlarged view of area A in FIG. 1a. This is an enlarged view of region B in FIG. 1a. An enlarged view of region C in FIG. 1a is shown. It is a graph showing changes in recrystallization stop temperature (Tnr) depending on the amount of Nb added for the steel material of the present invention. 2 is a graph showing changes in yield strength depending on the average grain size in the surface layer and the average grain size in the internal region other than the surface layer for the steel material of the present invention. It is a graph showing changes in the thickness ratio of the upper and lower surface layer parts to the total thickness of the steel material according to LMP, which is a parameter expressed by heat treatment temperature and time. It is a graph showing the change in yield strength depending on the thickness ratio of the upper and lower surface layer parts to the thickness of the steel material.

Preferred embodiments of the present invention will be described below. 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. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

Conventionally, steel materials used to ensure the seismic resistance of structures against earthquakes have been known to use compositions close to pure iron, and to perform additional heat treatment in the temperature range of 910 to 960 degrees Celsius. .

However, such technology requires additional heat treatment at a high temperature of 900°C or higher after finish rolling, which can lead to excessive scale and failure in the case of steel materials with extremely low yield points that do not contain Si. There were problems in that the impact toughness deteriorated due to the formation of coarse Nb or Ti precipitates. Furthermore, since an additional heat treatment step at a high temperature of 900° C. or higher is involved, there is also a problem of an increase in manufacturing costs.

Therefore, as a result of intensive studies to solve the above-mentioned problems, the inventors of the present invention achieved a yield strength of 120 MPa or less by optimizing the composition of the steel, the microstructure of the surface layer, manufacturing conditions, etc. The present inventors have discovered that it is possible to provide a steel material for seismic dampers that has excellent low-temperature impact toughness, although it is low, and has completed the present invention.

Below, the [steel material for seismic damper] according to the present invention will be explained in detail.

Specifically, the steel material for a seismic damper of the present invention has, in 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%, Nb: 0.04 to 0.05%. 15%, with the remainder consisting of Fe and other unavoidable impurities. Hereinafter, the reason for adding each alloy component constituting the steel composition, which is one of the main features of the present invention, and the appropriate content range thereof will be explained first.

C: 0.006% or less (excluding 0%)
C is an element that causes solid solution strengthening and, in a free state, fixes to dislocations, increases yield strength, and lowers elongation. In order to ensure the above effects, the present invention excludes the case where the C content is 0% (ie, the C content exceeds 0%). Therefore, in order to suitably use the steel material for seismic dampers, the lower the C content, the better, so the content should be controlled to 0.006% or less, more preferably 0.0045% or less. I can do it. More preferably, the C content may be 0.0005% or more.

Si: 0.05% or less (excluding 0%)
Like C, Si is an element that causes solid solution strengthening, increases the yield strength, and decreases the elongation rate. That is, the Si content exceeds 0%). However, in order to suitably use the steel material for seismic dampers, the lower the Si content, the better. Therefore, in the present invention, from the viewpoint of ensuring low yield strength, the Si content can be controlled to 0.03% or less, more preferably 0.013% or less. Further, the Si content may be 0.001% or more.

Mn: 0.3% or less (excluding 0%)
Like Si, Mn is an element that causes solid solution strengthening, and is an element that increases yield strength and decreases elongation. In order to ensure the above effect, except when the Mn content is 0% (ie, the Mn content is more than 0%). However, in order to suitably use the steel material for seismic dampers, in the present invention, the Mn content is controlled to 0.3% or less, more preferably 0.2% or less, from the viewpoint of ensuring low yield strength. can do. Further, the Mn content may be 0.06% or more, more preferably 0.1% or more.

P: 0.02% or less (excluding 0%)
Since P is an element that is advantageous for improving strength and corrosion resistance, in order to ensure the above-mentioned effects, the case where the P content is 0% is excluded (that is, the P content is more than 0%). However, since P can greatly inhibit impact toughness, it is preferable to keep 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. Further, the P content may be 0.001% or more, more preferably 0.004% or more.

S: 0.01% or less (excluding 0%)
Since S is an element that forms MnS and the like and greatly inhibits impact toughness, 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, more preferably 0.004% or less. Further, the S content may be 0.0005% or more, more preferably 0.001% or more.

Al: 0.005-0.05%
Al is an element that can deoxidize molten steel at low cost, and the upper limit of the Al content is controlled to 0.05% from the viewpoint of ensuring impact toughness while sufficiently lowering yield strength. Alternatively, more preferably, the upper limit of the Al content can be controlled to 0.035%, and from the viewpoint of ensuring minimum deoxidizing performance, the lower limit of the Al content can be controlled to 0.005%, and more preferably Preferably, it may be 0.023%.

N: 0.005% or less (excluding 0%)
N is an element that causes solid solution strengthening and, in a free state, fixes to dislocations, increases yield strength, and decreases elongation. To ensure the above effects, except when the N content is 0% (ie, the N content is more than 0%). However, the lower the N content, the better; therefore, from the viewpoint of ensuring low yield strength, the N content is controlled to 0.005% or less. Further, the N content may be 0.001% or more.

Nb: 0.04-0.15%
Nb is an important element in the production of TMCP steel, and is a very important element that precipitates in the form of NbC or NbCN and prevents C from sticking to dislocations. Furthermore, Nb dissolved in solid solution during reheating to a high temperature has the effect of suppressing recrystallization of austenite and making the structure finer.

On the other hand, in order to introduce deformation-induced precipitates, it is necessary to secure a wide unrecrystallized region, but as shown in Figure 2, a temperature region of 50°C or higher is secured between Ar3 and Tnr. From this viewpoint, it is preferable to add 0.04% or more of Nb. Further, in order to prevent impact toughness from deteriorating due to coarsening of precipitates, it is preferable to add 0.15% or less of Nb.

Specifically, FIG. 2 graphically shows changes in recrystallization stop temperature (Tnr) depending on the amount of Nb added for 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 amount as in the present invention, Ar3 is extremely high at about 890°C, and the change in Ar3 is small. Therefore, since the change value of Ar3 becomes a negligible level, it can be expressed by fixing Ar3 at about 890°C as shown in Figure 2, and only when the Nb content is added from 0.04 to 0.15%. The recrystallization stop temperature (Tnr) of ultra-low carbon steel can be highly controlled. Therefore, by controlling the Nb content in the range of 0.04 to 0.15% as in the present invention, it is possible to maintain a difference of 50°C or more between Tnr and Ar3 in ultra-low carbon steel, and thereby Fine deformation-induced precipitates are generated, and C can be fixed as the precipitates. On the other hand, from the viewpoint of improving the above-mentioned effects, more preferably, the lower limit of the Nb content may be 0.07%, or the upper limit of the Nb content may be 0.1%.

Ti: 48/14×[N] ~ 0.05%
Ti is an element that plays a role of preventing N from sticking to dislocations by precipitating in the form of TiN. Therefore, in order to fix N in steel within an appropriate range, Ti must be added at 48/14 x [N]% or more, taking into account the added N content (wt%) (here, [N] means the nitrogen content expressed in weight percent), or 0.02% or more must be added. On the other hand, if Ti is added excessively, the precipitates may become coarse and impact toughness may deteriorate. Therefore, from the viewpoint of ensuring impact toughness, it is more preferable to control Ti to 0.05% or less. can be controlled to 0.04% or less.

That is, according to the present invention, by controlling the Ti content within the range of 48/14×[N] to 0.05%, N in the steel can be fixed as a precipitate, and the Nb content can be reduced to 0.05%. By controlling the content within the range of 0.04 to 0.15%, C in the steel can be fixed as a precipitate. Therefore, in the present invention, by optimizing the Ti and Nb contents, it is possible to control the formation of deformation-induced precipitates finely to an appropriate size, thereby achieving good low-temperature impact toughness while having low yield strength. It is possible to effectively provide excellent steel materials for vibration control dampers.

Specifically, when C or N becomes free, C or N sticks to dislocations and causes an upper yield point phenomenon, resulting in the yield strength exceeding 120 MPa. Furthermore, the presence of coarse precipitates in a single ferrite structure deteriorates impact toughness. However, when precipitation occurs due to deformation during rolling, its size becomes fine and deterioration of impact toughness can be suppressed, and the development of the upper yield point can be suppressed to obtain steel materials with extremely low yield points. become. Therefore, according to the present invention, it is possible to provide a steel material with excellent low-temperature impact toughness and a Charpy impact transition temperature of -20° C. or less, although the yield strength is very low at 120 MPa or less.

On the other hand, according to the present invention, although not particularly limited, the above-mentioned steel material for seismic damper can satisfy the R1 value defined by the following relational expression 1 of 0.8 or more, or more preferably, The R1 value may be in the range of 0.8 to 150. When the R1 value is 0.8 or more, it is possible to more effectively provide a steel material having a very low yield strength of 120 MPa or less. Further, when the R1 value is 150 or less, fine Nb precipitates are formed, so that better impact toughness can be ensured.

[Relational expression 1]
R1=[Nb]/[Si]
(In the above relational expression 1, [Nb] indicates the weight % content of Nb, and [Si] indicates the weight % content of Si.)

On the other hand, from the viewpoint of improving the above-mentioned 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 above R1 value may be 90. good.

Alternatively, according to one aspect of the present invention, the above-mentioned steel material for a seismic damper can satisfy an R2 value of 0.8 or more as defined by the following relational expression 2. Alternatively, more preferably, the R2 value may be in the range of 0.8 to 200, most preferably in the range of 4 to 200. When the R2 value is 0.8 or more, it is possible to more effectively provide a steel material having a low yield strength of 120 MPa or less. Further, when the R2 value is 200 or less, fine Nb precipitates are formed, and more excellent impact toughness can be ensured.

[Relational expression 2]
R2=([Ti]+[Nb])/[Si]
(In the above relational expression 2, [Ti] indicates the weight% content of Ti, [Nb] indicates the weight% content of Nb, and [Si] indicates the weight% content of Si.)

On the other hand, from the viewpoint of improving the above-mentioned effects, 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 above R2 value may be 130.

In the present invention, the remaining components are Fe and other unavoidable impurities. That is, in the steel material for a seismic damper according to the present invention, unintended impurities may inevitably be mixed in from raw materials or the surrounding environment during the normal manufacturing process, and this cannot be excluded. Since such impurities are known to those of ordinary skill in the art, they will not be discussed in their entirety in this specification.

According to the present invention, the steel material for a seismic damper has a single ferrite structure. By meeting this requirement, it can effectively absorb energy during an earthquake and serve as an earthquake damper.

Further, according to the present invention, the average grain size of the ferrite crystal grains in the surface layer portion may be 150 to 500 μm. If the average grain size of the ferrite crystal grains in the surface layer is less than 150 μm, a problem may occur in which the yield strength exceeds the target yield strength, and if it exceeds 500 μm, the yield strength of the damper steel material may exceed the target yield strength. The problem may arise that the strength is lower than the actual strength. On the other hand, the lower limit of the average grain size of the ferrite crystal grains in the surface layer portion may be more preferably 175 μm, and most preferably 200 μm. Alternatively, the upper limit of the average grain size of the ferrite crystal grains in the surface layer portion may be more preferably 310 μm, and most preferably 300 μm.

In addition, in this specification, the said surface layer part means the area|region from the surface of a steel material to 30% of the total thickness. Therefore, the internal region other than the surface layer described below means the region excluding the surface layer portions (upper surface layer portion and lower surface layer portion) that are present at the upper and lower portions of the steel material in the thickness direction.

According to the present invention, the average grain size of the ferrite crystal grains in the surface layer portion may be larger than the average grain size of the ferrite crystal grains in the internal region other than the surface layer portion, and more preferably, the average grain size of the ferrite crystal grains in the internal region. It may be 150 μm or more larger than the average particle size of the particles. By satisfying this requirement, it is possible to exhibit the effect of ensuring the desired yield strength.

Alternatively, according to the present invention, the average grain size of the ferrite crystal grains in the internal region other than the surface layer portion 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 target yield strength will be exceeded, and if it exceeds 50 μm, the yield strength of the entire damper will be lower than the target strength. There may be a problem of lowering.

The average grain size of the ferrite crystal grains mentioned above is the average value of the equivalent circle diameters measured for the crystal grains, based on the cut plane in the thickness direction of the steel material (i.e., perpendicular to the rolling direction). Specifically, it is the average value of the measured values of the particle size, assuming a spherical particle whose particle size is the longest length that penetrates the inside of a crystal grain.

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

Further, according to the present invention, although not particularly limited, the surface layer portion of the steel material relative to the total thickness (Dt) of the steel material is based on the thickness direction of the steel material (that is, the direction perpendicular to the rolling direction). The thickness (Ds) ratio (Ds/Dt) may range from 0.1 to 0.3. In this way, by satisfying the thickness ratio (Ds/Dt) of the surface layer to the total thickness of the steel material from 0.1 to 0.3, as can be confirmed from FIG. It is possible to effectively provide a steel material for seismic dampers having a very low yield strength.

On the other hand, in the present invention, if the ratio (Ds/Dt) is less than 0.1, there is a possibility that the target yield strength is exceeded and sufficient energy cannot be absorbed as a damper. When (Ds/Dt) exceeds 0.3, as shown in FIG. 3, the yield strength becomes too low, which may cause problems in safely supporting the structure.

Although not particularly limited, from the viewpoint of improving the above-mentioned effect, the lower limit of the ratio (Ds/Dt) may be more preferably 0.14, or the lower limit of the ratio (Ds/Dt) may be more preferably 0.14. ) may be 0.25.

At this time, it is necessary to keep in mind that the above-mentioned surface layer section is a concept that includes all the surface layer sections formed on the upper and lower parts of the steel material.

According to the present invention, the yield strength (YS) of the above-mentioned steel material for a seismic damper may be 120 MPa or less, and although not particularly limited, it may be more preferably in the range of 80 to 120 MPa. If the yield strength of the above-mentioned steel exceeds 120 MPa, there may be a problem that energy cannot be absorbed sufficiently in the event of an earthquake, and if the yield strength of the above-mentioned steel is below 80 MPa, it may be difficult to maintain the structure stably. may cause problems.

Below, the [method for manufacturing a steel material for vibration control damper] according to the present invention will be explained in detail.

Heating step of slab The method for manufacturing a steel material for a seismic damper according to the present invention can include a step of reheating a steel slab satisfying the above-mentioned composition, and the reheating can be performed at a temperature 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 dissolve Ti and/or Nb carbonitrides formed during casting. However, if the slab is heated to an excessively high temperature, there is a risk that the austenite will become coarse 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. It is preferable to carry out the reaction at a temperature below ℃. On the other hand, although not particularly limited, from the viewpoint of improving the above-mentioned effects, more preferably, the lower limit of the reheating temperature of the slab may be 1075°C, or the upper limit of the reheating temperature of the slab. may be 1125°C.

Rough Rolling Stage According to the present invention, the heated steel slab may further include a stage of rough rolling for shape adjustment of the slab before the finish rolling stage described below, and such rough rolling The temperature can be controlled to be higher than the temperature at which austenite recrystallization stops (Tnr). 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. On the other hand, although not particularly limited, from the viewpoint of improving the above-mentioned effects, more preferably, the lower limit of the rough rolling end temperature may be 995°C, or the upper limit of the rough rolling end temperature may be 1035°C. It may be ℃.

Finish rolling step This includes a step of finish rolling the heated steel slab (or rough rolled bar) described above at a temperature range of Ar3-80° C. or higher and Ar3 or lower. Next, after finish rolling, a step of cooling may be included if necessary, and the cooling may be air cooling.

On the other hand, if the temperature of the finish rolling is less than Ar3-80°C, a problem may arise in that the ferrite grain size inside the steel material becomes excessively fine. Moreover, if the temperature of the finish rolling exceeds Ar3, a problem may arise in that the ferrite grain size inside the steel material becomes coarse. On the other hand, although not particularly limited, from the viewpoint of improving the above-mentioned effects, more preferably, the lower limit of the finish rolling start temperature may be 955°C, or the upper limit of the finish rolling start temperature may be 980°C. It may be ℃. Further, the lower limit of the finish rolling finish temperature may be 860°C, or the upper limit of the finish rolling finish temperature may be 905°C.

Shot blasting step The shot blasting step includes a step of shot blasting the surface of the finished rolled steel material described above, and the shot blasting process involves rotating a metal ball or a non-metal ball at a speed of 1,500 to 2,500 rpm, and It can be carried out such that it is sprayed onto the surface of the plate at a speed of 100 m/s. By performing shot blasting, coarse ferrite crystal grains can grow in the surface layer of the steel material, and the ratio of the thickness of the surface layer to the total thickness of the steel material increases, reducing the yield strength. .

If the rotational speed of the metal ball or non-metal ball is less than 1,500 rpm during the above shot blasting process, it may not be possible to secure a sufficient speed and the problem may occur that the ferrite crystal grain size on the surface cannot be secured. If the speed exceeds 2,500 rpm, problems may arise in stable operation of the machine. On the other hand, although not particularly limited, from the viewpoint of improving the above-mentioned 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. There may be.

In addition, if the above-mentioned injection speed is less than 60 m/s, there is a possibility that the effective application of stress to the surface of the steel material will be insufficient, resulting in the inability to secure the desired physical properties. If this occurs, deep grooves may occur on the surface of the steel material, which may cause product defects. On the other hand, although not particularly limited, from the viewpoint of improving the above-mentioned effects, the lower limit of the injection speed may be more preferably 62 m/s, or the upper limit of the injection speed may be 94 m/s. There may be.

According to the present invention, metal balls or non-metal balls having an average diameter of 0.8 to 1.2 mm can be used during the shot blasting process. If the diameter of the ball is less than 0.8 mm, there may be a problem that the energy transmitted to the surface of the steel material is insufficient, and if the diameter of the ball exceeds 1.2 mm, energy may be transferred to the surface of the steel material. There may be a problem that the information cannot be transmitted uniformly. On the other hand, although not particularly limited, from the viewpoint of improving the above-mentioned effects, more preferably, the lower limit of the average diameter of the metal balls (or non-metal balls) may be 0.9 mm, or The upper limit of the average diameter of the metal balls (or non-metal balls) may be 1.1 mm.

Further, according to the present invention, the shot blasting treatment can be performed for 10 to 30 minutes. If the above-mentioned shot blasting time is less than 10 minutes, there may be a problem that the energy transmitted to the surface of the steel material is insufficient, and if the above-mentioned short blasting time exceeds 30 minutes, the surface quality of the steel material may be deteriorated. There is a possibility that the problem of inducing On the other hand, although not particularly limited, from the viewpoint of improving the above-mentioned effects, more preferably, the lower limit of the shot blasting time may be 15 minutes, or the upper limit of the shot blasting time may be 25 minutes. It may be a minute.

Heat Treatment Step According to the present invention, although not particularly limited, after the shot blasting step, heat treatment is performed so that the LMP value defined by the following relational expression 3 satisfies the range of 23.5 to 24.5. The method may further include the step of:

[Relational expression 3]
LMP=T×[log(t)+20]/1000
(In the above relational expression 3, the above T indicates the heat treatment temperature, and the unit is °C. Also, the above t indicates the heat treatment time, and the unit is minutes.)

At this time, since the value of Relational Expression 3 below is a numerical value obtained empirically, there is no need to specify a unit in particular. That is, in the following relational expression 3, each unit of T and t, which will be described later, may be satisfied.

According to the present invention, since the LMP value defined by the above-mentioned relational expression 3 satisfies the range of 23.5 to 24.5, as shown in FIG. The ratio can be controlled within the range of 0.1 to 0.3, thereby making 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).

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

On the other hand, although not particularly limited, from the viewpoint of improving the above-mentioned effects, more preferably, the lower limit of the LMP value defined by the above relational expression 3 may be 23.7, or the above relational expression The upper limit of the LMP value defined by 3 may be 24.3.

Further, according to the present invention, the heat treatment step can be performed at a temperature of 850 to 900° C., although it is not particularly limited. If the above heat treatment temperature is less than 850°C, there is a possibility that it will not be possible to ensure the growth of sufficiently coarse ferrite on the surface. A problem may arise with the formation of ferrite grains. On the other hand, although not particularly limited, from the viewpoint of improving the above-mentioned effects, more preferably, 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. Good too.

Further, according to the present invention, the heat treatment time may be in the range of 5 to 30 minutes, although it is not particularly limited. 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.

Hereinafter, the present invention will be explained in more detail with reference to Examples. However, it should be noted that the following examples are for explaining the present invention through illustrations, and are not intended to limit the scope of the present invention. The scope of rights to the present invention is determined by the matters stated in the claims and matters reasonably inferred therefrom.

(Example)
A steel slab having the alloy composition and properties shown in Table 1 below was prepared. At this time, in Table 1 below, the content of each component is expressed in weight %, and the remainder is Fe and other unavoidable impurities. That is, in the steel slabs listed in Table 1 below (the remainder is Fe), invention steels A to D are examples that match the range of alloy composition defined in the present invention, and comparative steels E to I are examples defined in the present invention. This is an example outside the alloy composition range. Meanwhile, for the steel slabs listed in Table 1 below, Ar3 and Tnr values were experimentally measured from a point where the stress changes due to temperature through a high-temperature torsion experiment using ultra-low carbon steel.

After reheating the prepared steel slab in a temperature range of 1050 to 1250°C, slab reheating, rough rolling, and finish rolling were performed under the conditions listed in Table 2 below. Next, shot blasting was performed for 15 minutes using metal balls having an average diameter of 1.0 m under the conditions shown in Table 3 below, followed by heat treatment to produce steel materials.

（上記表１において、Ｔｉ＊は４８／１４×Ｎ（重量％）の値を示す。）

(In Table 1 above, Ti* indicates the value of 48/14×N (wt%).)

After manufacturing steel materials under the conditions listed in Tables 2 and 3 above, the steel materials thus obtained were polished and etched, and then observed with an optical microscope to confirm that they had a ferrite single-phase structure.

In addition, the results of measuring the average grain size, yield strength (YS), tensile strength (TS), and Charpy impact transition temperature in the surface layer and internal regions other than the surface layer of the steel materials obtained from each experimental example are shown below. It is shown in Table 4 below.

At this time, the average grain size of the crystal grains was measured using a line measurement method, and the point at which yielding occurred was defined as the yield strength using a tensile tester according to ASTM standards, and the strength at which necking occurred was defined as the tensile strength. The Charpy impact transition temperature is the temperature at which the fracture transitions from ductile to brittle by measuring the impact absorption energy using a Charpy impact tester.

In Table 4 above, Examples 1-1, 1-2, 2-1, 2-2, 3-1, 3-2, 4-1 and 4-2 have all the steel compositions and manufacturing conditions of the present invention. In the case where the thickness ratio of the upper and lower surface layers 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 are both yield strength of 80 to 120 MPa and Charpy impact transition temperature - The temperature is below 20℃.

On the other hand, Reference Examples 1 to 4 are cases in which the steel composition of the present invention is satisfied, but the manufacturing conditions deviate from the present invention. Among these, Reference Examples 1 to 4 are cases where LMP exceeds 24.5. In the case of Reference Examples 1 to 4, the thickness ratio of the surface layer part falls outside the range of 0.1 to 0.3, and the yield strengths are all less than 80 MPa.

Furthermore, in Comparative Example 1, C exceeds the upper limit of the content specified in the present invention, and the yield strength exceeds 120 MPa. In Comparative Example 2, the content of Si, which is a solid solution strengthening element, exceeds the upper limit of the content specified in the present invention, and the yield strength exceeds 120 MPa. In Comparative Example 3, excessive Nb was added, the impact toughness deteriorated due to the formation of coarse precipitates, and the Charpy impact transition temperature exceeded -20°C. Comparative Example 4 satisfies all the manufacturing conditions of the present invention, but the Ti content exceeds the upper limit specified by the present invention, and the Charpy impact transition temperature is -20°C due to the formation of coarse precipitates. exceeded. Comparative Example 5 is a case in which all the manufacturing conditions of the present invention are met, but the Ti content does not reach the lower limit specified by the present invention, and the Ti content is insufficient and Free N is insufficient to precipitate as nitrides, a yield point phenomenon occurs, and the yield strength exceeds 120 MPa.

Claims (11)

In 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~ 0.05%, N: 0.005% or less, Ti: 48/14×[N] ~ 0.05% (here, [N] means the weight% content of nitrogen), Nb: 0.04 ~0.15%, with the remainder consisting of Fe and other unavoidable impurities,
Has a single ferrite structure,
A steel material for a seismic damper, characterized in that the average grain size of ferrite crystal grains in the surface layer from the surface to a region of 30% of the total thickness is 150 to 500 μm. The steel material for a seismic damper according to claim 1, wherein the average grain size of the ferrite crystal grains in the internal region other than the surface layer portion is 10 to 60 μm. The steel material for seismic damper 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.
[Relational expression 1]
R1=[Nb]/[Si]
(In the above relational expression 1, [Nb] indicates the weight% content of Nb, and [Si] indicates the weight% content of Si.) The steel material for seismic damper 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.
[Relational expression 2]
R2=([Ti]+[Nb])/[Si]
(In the above relational expression 2, [Ti] indicates the weight% content of Ti, [Nb] indicates the weight% content of Nb, and [Si] indicates the weight% content of Si.) The damping device according to claim 1, wherein the ratio (Ds/Dt) of the thickness (Ds) of the surface layer to the total thickness (Dt) of the steel material is in the range of 0.1 to 0.3. Steel material for dampers. The steel material for a seismic damper according to claim 1, wherein the steel material has a yield strength of 120 MPa or less. In 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~ 0.05%, N: 0.005% or less, Ti: 48/14×[N] ~ 0.05% (here, [N] means the weight% content of nitrogen), Nb: 0.04 heating a steel slab containing ~0.15% Fe and other unavoidable impurities to a temperature range of 1050~1250°C;
Finish rolling the heated steel slab in a temperature range of Ar3-80°C or higher and Ar3 or lower;
shot blasting the surface of the finish-rolled steel material;
The shot blasting step is characterized in that the metal balls or non-metal balls are rotated at a speed of 1500 to 2500 rpm and sprayed onto the surface of the plate material at a speed of 60 to 100 m/s. A method for manufacturing steel materials for seismic dampers. The method of manufacturing a steel material for a seismic damper according to claim 7, wherein the shot blasting step is performed for 10 to 30 minutes. 8. The method of manufacturing a steel material for a seismic damper according to claim 7, characterized in that the metal balls or non-metal balls have a diameter of 0.8 to 1.2 mm. 8. The method according to claim 7, further comprising the step of heat-treating after the shot blasting step so that the LMP value defined by the following relational expression 3 satisfies a range of 23.5 to 24.5. A method for manufacturing steel materials for seismic dampers.
[Relational expression 3]
LMP=T×[log(t)+20]/1000
(In the above-mentioned relational expression 3, the above-mentioned T indicates the heat treatment temperature, and the unit is °C. Also, the above-mentioned t indicates the heat treatment time, and the unit is minutes.) The method of manufacturing a steel material for a seismic damper according to claim 10, wherein the heat treatment step is performed at a temperature in the range of 850 to 900°C.

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