KR20220082432A - Steel sheet for seismic damper having superior toughness property and manufacturing method of the same - Google Patents

Abstract

In the present invention, in wt%, 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] ~ 0.05% (here, [N] means the wt% content of nitrogen), Nb: 0.04 ~ 0.15%, the balance including Fe and other unavoidable impurities, ferrite single structure and, the average grain size of ferrite grains in the surface layer from the surface to the region of 30% of the total thickness is 150 to 500 μm, to provide a steel material for a vibration damper and a manufacturing method thereof.

Description

Steel material for vibration damping with excellent impact toughness and manufacturing method thereof

The present invention relates to a steel material for a vibration damper used to secure the seismic resistance of a structure from an earthquake, and a method for manufacturing the same.

The seismic design, which has been mainly used in Korea in the past, was mainly used in the case of an earthquake, by lowering the yield ratio of steel used for structures of columns or beams to delay the time of structural destruction. However, the seismic design using such a steel material with a resistance to yield ratio has a problem in that it is not possible to reuse the steel used in the structure, and the structure itself must be reconstructed due to the lack of stability.

In recent years, seismic design technology has been developed and a seismic isolation or seismic isolation structure is being put to practical use. In particular, various technologies for securing seismic performance by absorbing energy applied to a structure due to an earthquake in a specific area are being developed. A vibration damper is used as a device for absorbing such seismic energy, and in the case of a steel material for a vibration damping damper, it has the characteristic of an extreme resistance yield point. By lowering the yield point of structural materials of existing columns or beams, the steel for vibration damper absorbs vibration energy from earthquakes by causing yielding in the event of an earthquake. do.

Patent Publication No. 2008-0088605

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

Alternatively, another aspect of the present invention is to provide a steel material for a vibration damper that has low yield strength and excellent low-temperature impact toughness and a manufacturing method thereof.

The subject of the present invention is not limited to the above. Those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional problems of the present invention from the contents throughout the present specification.

One aspect of the present invention is

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] ~ 0.05% (where [N] means the wt% content of nitrogen), Nb: 0.04 ~ 0.15%, the balance contains Fe and other unavoidable impurities,

Ferrite has a single structure,

Provided is a steel material for a vibration damping damper having an average grain size of ferrite grains in the surface layer from the surface to the region of 30% of the total thickness of 150 to 500 μm.

In addition, another aspect of the present invention,

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] ~ 0.05% (here, [N] means the wt% content of nitrogen), Nb: 0.04 ~ 0.15%, the balance steel slab containing Fe and other unavoidable impurities at 1050 ~ 1250 ℃ heating to a temperature range;

Finish rolling the heated steel slab in a temperature range of Ar3-80°C or higher and Ar3 or lower; and

A shot blasting step on the surface of the finish-rolled steel;

including,

The step of the shot blasting treatment is carried out by rotating a metal ball or a non-metal ball at a speed of 1,500 to 2,500 rpm, and spraying it on the surface of the plate at a speed of 60 to 100 m/s. Provides a method of manufacturing a steel for vibration damper. .

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

Alternatively, according to another aspect of the present invention, it is possible to provide a steel material for a vibration damper having low yield strength and excellent low-temperature impact toughness, and a method for manufacturing the same.

Various and advantageous advantages and effects of the present invention are not limited to the above, and will be more easily understood in the course of describing specific embodiments of the present invention.

Figure 1a is a photograph taken with an optical microscope schematically showing the microstructure in the inner region other than the surface layer portion and the surface layer portion for the steel of the present invention. Also, FIG. 1B is an enlarged view of area A in FIG. 1A, FIG. 1C is an enlarged view of area B in FIG. 1A, and FIG. 1D is an enlarged view of area C in FIG. 1A.
2 is a graph showing a change in the recrystallization stop temperature (Tnr) according to the amount of Nb added for the steel of the present invention.
3 is a graph showing the average grain size in the surface layer portion for the steel of the present invention, and the change in yield strength according to the average grain size in the inner region other than the surface layer portion.
4 is a graph showing the change in the ratio of the thickness of the upper and lower surface layers to the total thickness of the steel material according to the LMP, which is a parameter expressed as a heat treatment temperature and time.
5 is a graph showing the change in yield strength according to the ratio of the thickness of the upper and lower surface layers to the thickness of the steel material.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiment of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided in order to more completely explain the present invention to those of ordinary skill in the art.

As a steel material used to secure the seismic resistance of structures from earthquakes, conventionally, a technique of performing additional heat treatment in a temperature range of 910 to 960° C. has been known using a component close to pure iron.

However, since this technique requires additional heat treatment at a high temperature of 900° C. or higher after finish rolling, excessive scale is generated in the case of extremely resistant double-point steel without Si added, causing defects, or coarse Nb or Ti precipitates This formed, and there was a problem that the impact toughness deteriorated. In addition, since an additional heat treatment process at a high temperature of 900° C. or higher is involved, there is also a problem of causing an increase in manufacturing cost.

Accordingly, the present inventors have studied diligently to solve the above problems, and as a result, by optimizing the composition of the steel, the microstructure of the surface layer and manufacturing conditions, etc., the yield strength is as low as 120 MPa or less and excellent low-temperature impact toughness steel for vibration damper It has been found that the present invention can be provided.

Hereinafter, [steel material for vibration damper] according to the present invention will be described in detail.

Specifically, the steel material for a vibration damper according to an aspect of the present invention, 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%, Nb: 0.04 to 0.15%, the balance has a composition including 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 an appropriate content range thereof will be first described.

C: 0.006% or less

C is an element that causes solid solution strengthening and is fixed to dislocations in a free state to increase yield strength and lower elongation. Therefore, in order to be suitably used as a steel material for a vibration damper, the lower the C content, the better. In addition, the C content may be 0.0005% or more.

Si: 0.05% or less

Like C, Si is an element that causes solid solution strengthening, and is an element that increases yield strength and lowers elongation. For proper use as a steel material for vibration damping, the lower the Si content is, the better. Therefore, in the present invention, the Si content can be controlled to 0.3% or less, more preferably 0.013% or less, in terms of securing low yield strength. In addition, the Si content may be 0.001% or more.

Mn: 0.3% or less

Mn is an element that causes solid solution strengthening like Si, and is an element that increases yield strength and lowers elongation. Therefore, in order to be suitably used as a steel material for a vibration damper, in the present invention, the Mn content can be controlled to 0.3% or less, and more preferably, to 0.2% or less in terms of securing low yield strength. In addition, the Mn content may be 0.06% or more.

P: 0.02% or less

Although P is an element advantageous for strength improvement and corrosion resistance, since it may greatly impair 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, and more preferably to be controlled to 0.013% or less. In addition, the P content may be 0.004% or more.

S: 0.01% or less

Since S is an element that greatly impairs impact toughness by forming MnS or the like, it is preferable to keep the 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. In addition, the S content may be 0.0005% or more.

Al: 0.005~0.05%

Al is an element that can inexpensively deoxidize molten steel, and controls the upper limit of the Al content to 0.05% in terms of securing impact toughness while sufficiently lowering the yield strength. Alternatively, more preferably, the upper limit of the Al content may be controlled to 0.035%, and the lower limit of the Al content may be controlled to 0.005% in terms of securing the minimum deoxidation performance.

N: 0.005% or less

N is an element that causes solid solution strengthening and is fixed to dislocations in a free state to increase yield strength and lower elongation. Therefore, since the lower the N content is, the better, the N content is controlled to 0.005% or less in terms of securing low yield strength. In addition, the N content may be 0.001% or more.

Nb: 0.04 to 0.15%

Nb is an important element in the manufacture of TMCP steel, and it is a very important element to prevent C from adhering 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 the effect of refining the structure.

On the other hand, in order to introduce the deformed organic precipitates, it is necessary to secure a wide non-recrystallization region. As can be seen in FIG. 2, in terms of securing a temperature region of 50° C. or higher between Ar3 and Tnr, adding 0.04% or more of Nb is desirable. In addition, in order to prevent the impact toughness from being deteriorated due to the coarsening of the precipitate, it is preferable to add Nb in an amount of 0.15% or less.

Specifically, FIG. 2 is a graph showing the change in the recrystallization stop temperature (Tnr) according to the addition amount of Nb for the steel 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 very high at about 890°C, and the change in Ar3 is insignificant. Therefore, since the change value of Ar3 becomes negligible, it can be expressed by fixing Ar3 to about 890°C as shown in FIG. 2, and only when the Nb content is added to 0.04 to 0.15%, the recrystallization stop temperature (Tnr) of ultra-low carbon steel is highly controlled. can do. Therefore, as in the present invention, by controlling the Nb content in the range of 0.04 to 0.15%, the difference between Tnr and Ar3 of the ultra-low carbon steel can be secured to 50° C. or more, and this causes the deformed organic precipitates to be minutely generated to reduce C It becomes possible to adhere to the precipitate.

Ti: 48/14×[N]~0.05%

Ti is an element serving to prevent N from adhering to dislocations by precipitation in the form of TiN. Therefore, in order to fix N in the steel in an appropriate range, 48/14 × [N]% or more of Ti must be added in consideration of the added N content (wt%) (here, [N] is expressed in wt% means the content of nitrogen), or 0.02% or more should be added. On the other hand, when Ti is excessively added, the precipitates are coarsened and there is a risk of deterioration of impact toughness. Therefore, in terms of securing impact toughness, Ti can be controlled to 0.05% or less, and more preferably to 0.04% or less. have.

That is, according to the present invention, by controlling the Ti content in the range of 48/14 × [N] to 0.05%, N in the steel can be fixed as precipitates, and by controlling the Nb content in the range of 0.04 to 0.15%, C in the steel is reduced It can be fixed as a precipitate. Therefore, the present invention can be controlled to form the deformable organic precipitates finely to an appropriate size by optimizing the Ti and Nb content, thereby effectively providing a steel material for a vibration damper with low yield strength and excellent low-temperature impact toughness. have.

Specifically, when C or N is in a free state, C or N adheres to the dislocation to cause an upper yield point phenomenon, which causes the yield strength to exceed 120 MPa. In addition, if coarse precipitates are present in the ferrite single structure, the impact toughness deteriorates. However, in the case of precipitation by deformation induction during rolling, it is possible to suppress deterioration of impact toughness due to its small size, and suppress the expression of the upper yield point to obtain an extremely resistant yield steel material. Therefore, according to the present invention, it is possible to provide a steel material having a very low yield strength of 120 MPa or less and excellent low-temperature impact toughness having a Sharpie impact transition temperature of -20°C or less.

On the other hand, according to one aspect of the present invention, although not particularly limited, the R1 value defined by the following relation 1 may satisfy 0.8 or more, or more preferably, the R1 value is 0.8~ 150 may be in the range. If 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. In addition, when the R1 value is 150 or less, Nb precipitates are formed finely, and thus more excellent impact toughness can be secured.

[Relational Expression 1]

R1 = [Nb]/[Si]

(In Relation 1, [Nb] represents the weight% content of Nb, and [Si] represents the weight% content of Si.)

Alternatively, according to an aspect of the present invention, the R2 value defined by the following relation 2 may satisfy 0.8 or more in the steel material for the vibration damping damper. 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. In addition, when the R2 value is 200 or less, Nb precipitates are formed finely, and more excellent impact toughness can be secured.

[Relational Expression 2]

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

(In Relational Expression 2, [Ti] represents the wt% content of Ti, [Nb] represents the wt% content of Nb, and [Si] represents the wt% content of Si.)

In the present invention, the remaining component is Fe. However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to those skilled in the art, all contents thereof are not mentioned herein.

According to one aspect of the present invention, the steel material for the vibration damper has a single ferrite structure. By fulfilling this, it is possible to effectively absorb energy when an earthquake occurs and serve as an earthquake damper.

In addition, according to one aspect of the present invention, the average particle diameter of the ferrite grains in the surface layer portion may be 150 ~ 500㎛, more preferably 200 ~ 300㎛. If the average grain diameter of the ferrite grains in the surface layer is less than 150㎛, there may be a problem that the yield strength exceeds the target yield strength, and if it exceeds 500㎛, the yield strength of the damper steel is lower than the target strength. can

In addition, in the present specification, the surface layer means a region up to 30% of the total thickness from the surface of the steel material. Therefore, the internal region other than the surface layer portion to be described later means a region excluding the surface layer portions (upper surface layer portion and lower surface layer portion) respectively present in the upper and lower portions in the thickness direction of the steel material.

According to an aspect of the present invention, the average grain size of ferrite grains in the surface layer portion may be larger than the average grain size of ferrite grains in the inner region other than the surface layer portion, and more preferably 150 than the average grain size of ferrite grains in the inner region. It may be larger than ㎛. By satisfying this, the effect of securing the desired yield strength can be exhibited.

Alternatively, according to one aspect of the present invention, the average grain size of ferrite grains in the inner region other than the surface layer portion may be in the range of 10-50 μm, more preferably in the range of 30-50 μm. If the average grain size of the ferrite grains in the inner region is less than 10 μm, a problem of exceeding the target yield strength may occur, and if it exceeds 50 μm, the yield strength of the entire damper may be lower than the target strength.

The above-described average grain diameter of ferrite grains refers to the average value of the values obtained by measuring the equivalent circle diameter of the grains based on the cut surface in the thickness direction of the steel (ie, the direction perpendicular to the rolling direction), and specifically, the When it is assumed that spherical particles drawn with the longest length penetrating the inside as the particle diameter, this is the average value of the measured particle diameters.

For the steel of Invention Example 1-2 to be described later corresponding to an example of the present invention, an optical photograph obtained by photographing the microstructure using an optical microscope is 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 inner region other than the surface layer portion.

In addition, according to one aspect of the present invention, although not particularly limited, based on the thickness direction of the steel (ie, the direction perpendicular to the rolling direction), the thickness (Ds) of the surface layer with respect to the total thickness (Dt) of the steel ) ratio (Ds/Dt) may range from 0.1 to 0.3. In this way, the thickness ratio (Ds/Dt) of the surface layer to the total thickness of the steel satisfies 0.1 to 0.3, as can be seen in FIG. can be provided effectively.

On the other hand, in the present invention, if the ratio (Ds/Dt) is less than 0.1, there may occur a problem that sufficient energy as a damper cannot be absorbed in excess of the target yield strength, and the ratio (Ds/Dt) is When it exceeds 0.3, as shown in FIG. 3, the yield strength is too low, which may cause a problem in performing safe support for the structure.

At this time, it is necessary to note that the surface layer part is a concept encompassing all the surface layer parts formed on each of the upper and lower parts of the steel material.

According to one aspect of the present invention, the yield strength (YS) of the above-described steel material for the damping damper may be 120 MPa or less, but is not particularly limited, but more preferably in the range of 80 to 120 MPa. If the yield strength of the steel material exceeds 120 MPa, there may be a problem that energy absorption may not be sufficient when an earthquake occurs, and if the yield strength of the steel material is less than 80 MPa, there may be a problem in stable maintenance of the structure.

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

heating stage of the slab

The method for manufacturing a steel material for a vibration damper according to an aspect of the present invention may include reheating a steel slab that satisfies the above-described composition, and the reheating may 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 dissolve the carbonitride of Ti and/or Nb formed during casting. However, when heating to an excessively high temperature, there is a risk of austenite coarsening, and excessive time is required for the surface temperature after rough rolling to reach the surface layer cooling start temperature. it is preferable

rough rolling stage

According to one aspect of the present invention, the heated steel slab may further include a step of performing rough rolling to adjust the shape of the slab before the finish rolling step to be described later, and the temperature of this rough rolling is determined by recrystallization of austenite. It can be controlled above the stopping temperature (Tnr). The effect of destroying structural structures such as dentrite formed during casting by rough rolling can be obtained, and also the effect of reducing the size of austenite can be obtained.

finish rolling step

It includes the step of finish rolling the above-mentioned heated steel slab (or rough-rolled bar) in a temperature range of Ar3-80 ℃ or more and Ar3 or less. Subsequently, after finishing rolling, if necessary, it may include a step of cooling, and the cooling may be air cooling.

On the other hand, when the temperature of the finish rolling is less than Ar3-80°C, there may be a problem in that the ferrite grain size inside the steel is too fine. In addition, when the temperature of the finish rolling exceeds Ar3, there may be a problem in that the ferrite grain size inside the steel is coarse.

Shot blasting steps

Comprising the step of shot blasting on the surface of the above-mentioned finish-rolled steel, the shot blasting is by rotating a metal ball or a non-metal ball at a speed of 1,500 ~ 2,500 rpm, 60 ~ 100 m / s on the surface of the plate material It can be carried out to spray. By performing the shot blasting treatment, coarse ferrite grains can grow in the surface layer of the steel, and the ratio of the thickness of the surface layer to the total thickness of the steel increases to lower the yield strength.

In the case of the shot blasting treatment, if the rotational speed of the metallic or non-metallic ball is less than 1,500 rpm, a sufficient speed may not be secured and a problem may occur that the size of the ferrite grains in the surface part cannot be secured. Problems can arise.

In addition, if the injection speed is less than 60 m/s, there may be a problem in that the desired physical properties cannot be secured due to insufficient stress application on the surface of the steel, and if it exceeds 100 m/s, deep grooves are generated on the surface of the steel, resulting in product defect may cause

According to an aspect of the present invention, a metal ball or a non-metal ball having an average diameter of 0.8 to 1.2 mm may be used during the shot blasting treatment. If the diameter of the ball is less than 0.8mm, a problem of insufficient energy to be transmitted to the surface of the steel may result, and if the diameter of the ball exceeds 1.2mm, a problem in which energy cannot be uniformly transmitted to the surface of the steel may result. have.

In addition, according to one aspect of the present invention, the shot blasting treatment may be performed for 10 to 30 minutes. If the shot blasting treatment time is less than 10 minutes, a problem of insufficient energy transferred to the surface of the steel may occur, and if the shot blasting treatment time is more than 30 minutes, a problem of causing defects in the surface quality of the steel may result have.

heat treatment step

According to one aspect of the present invention, although not particularly limited, after the shot blasting step, the step of heat treatment so that the LMP value defined by the following relation 3 meets the range of 23.5 to 24.5 may be further included.

[Relational Expression 3]

LMP = TХ[log(t)+20]/1000

(In Relational Expression 3, T represents the heat treatment temperature, and the unit is ° C. In addition, t represents the heat treatment time, and the unit is minutes.)

At this time, since the value of the following relation 3 is a numerical value obtained empirically, the unit may not be particularly determined. That is, in the following relational expression 3, it is sufficient to satisfy each unit of T and t described later.

According to one aspect of the present invention, the LMP value defined by the above-mentioned Relation 3 satisfies the range of 23.5 to 24.5, as can be seen in FIG. 4, the thickness ratio of the surface layer to the total thickness of the steel is controlled in the range of 0.1 to 0.3 and, thereby, it is possible to obtain a steel material satisfying the target yield strength of 120 MPa or less (more preferably, in the range of 80 to 120 MPa).

When a heat treatment is performed on a steel sheet that has been subjected to the shot blast treatment, coarse ferrite grows from the surface portion of the steel material due to the stress introduced to the surface layer portion. As such, by controlling the heat treatment conditions to form coarse ferrite as shown in FIG. 1 on the surface layer of the steel, it is possible to introduce a change in the yield strength of the steel.

In addition, according to one aspect of the present invention, although not particularly limited, the heat treatment may be performed in a range of 850 to 900°C. If the heat treatment temperature is less than 850 ° C., there may be a problem in that sufficiently coarse ferrite growth cannot be secured on the surface.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following serial examples are only for explaining the present invention by way of illustration and not for limiting the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

(Example)

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

After reheating the prepared steel slab in a temperature range of 1050 to 1250 °C, slab reheating-rough rolling-finish rolling was performed under the conditions shown in Table 2 below. Then, using a metal ball having a diameter of 1.0 m, after shot blasting for 15 minutes under the conditions of Table 3 below, heat treatment was performed to prepare steel.

[Table 1]

(In Table 1, Ti* represents a value of 48/14×N (wt%).)

[Table 2]

[Table 3]

After the steel materials were manufactured under the conditions shown in Tables 2 and 3, 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 average grain diameter, yield strength (YS), tensile strength (TS) and Sharpie impact transition temperature for each in the inner region other than the surface layer part and the surface layer part for the steel obtained from each experimental example are measured. Table 4 shown in

At this time, the average grain size of the grains was measured using the line measurement method, the point at which yielding occurred using a tensile tester was used as the yield strength, and the strength at the time of necking was used as the tensile strength. The Sharpie impact transition temperature was measured by measuring the shock absorption energy using a Sharpie impact tester, and represents the temperature at which fracture transitions from ductility to brittleness.

[Table 4]

In Table 4, Examples 1-1, 1-2, 2-1, 2-2, 3-1, 3-2, 4-1 and 4-2 show all of the steel composition and manufacturing conditions of the present invention. As a satisfactory case, the ratio of the thickness of the upper and lower surface layers to the total thickness of the steel was in the range of 0.1 to 0.3, and all of the physical properties of the steel satisfied the yield strength of 80 to 120 MPa and the Sharpie impact transition temperature of -20 ° C or less.

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

In addition, in Comparative Example 1, C exceeded the upper limit of the content prescribed in the present invention, and the yield strength exceeded 120 MPa. In Comparative Example 2, Si, a solid solution strengthening element, exceeded the upper limit of the content prescribed in the present invention, and the yield strength exceeded 120 MPa. Comparative Example 3 was a case in which Nb was added excessively, and the impact toughness was deteriorated by the formation of coarse precipitates, and the Sharpie 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 prescribed in the present invention, and the Sharpie impact transition temperature exceeds -20°C due to the generation of coarse precipitates. Comparative Example 5 satisfies all the manufacturing conditions of the present invention, but the content of Ti is less than the lower limit prescribed in the present invention. The phenomenon was developed, and the yield strength exceeded 120 MPa.

Claims (11)

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] ~ 0.05% (where [N] means the wt% content of nitrogen), Nb: 0.04 ~ 0.15%, the balance contains Fe and other unavoidable impurities,
Ferrite has a single structure,
A steel material for a vibration damping damper, wherein the average grain size of ferrite grains in the surface layer portion from the surface to the region of 30% of the total thickness is 150 to 500 µm.
The method according to claim 1,
The average grain diameter of ferrite grains in the inner region other than the surface layer portion is 10 to 60㎛, a steel material for a vibration damping damper.
The method according to claim 1,
The R1 value defined by the following relation 1 is in the range of 0.8 to 150, a steel material for a vibration damper.
[Relational Expression 1]
R1 = [Nb]/[Si]
(In Relation 1, [Nb] represents the weight% content of Nb, and [Si] represents the weight% content of Si.)
The method according to claim 1,
R2 value defined by the following relation 2 is in the range of 4 to 200, a steel material for a vibration damper.
[Relational Expression 2]
R2 = ([Ti]+[Nb])/[Si]
(In Relational Expression 2, [Ti] represents the wt% content of Ti, [Nb] represents the wt% content of Nb, and [Si] represents the wt% content of Si.)
The method according to claim 1,
The ratio (Ds/Dt) of the thickness (Ds) of the surface layer to the total thickness (Dt) of the steel is in the range of 0.1 to 0.3, the steel for vibration damper.
The method according to claim 1,
The yield strength of the steel is 120 MPa or less, a steel for vibration damper.
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 (wt%) ~ 0.05%, Nb: 0.04 ~ 0.15%, heating the steel slab containing the balance Fe and other unavoidable impurities to a temperature range of 1050 ~ 1250 ℃;
Finish rolling the heated steel slab in a temperature range of Ar3-80°C or higher and Ar3 or lower; and
A shot blast treatment step on the surface of the finish-rolled steel;
including,
The step of the shot blasting treatment is carried out by rotating a metal ball or a non-metal ball at a speed of 1500 to 2500 rpm, and spraying it on the surface of the plate at a speed of 60 to 100 m/s, a method of manufacturing a steel for vibration damper.
8. The method of claim 7,
The step of the shot blasting treatment is performed for 10 to 30 minutes, a method of manufacturing a steel material for a vibration damper.
8. The method of claim 7,
A method of manufacturing a steel material for a vibration damping damper using that the metal ball or non-metal ball has a diameter of 0.8 to 1.2 mm.
8. The method of claim 7,
After the shot blasting step, the method of manufacturing a steel material for a vibration damper further comprising the step of heat-treating so that the LMP value defined by the following relation 3 meets the range of 23.5 to 24.5.
[Relational Expression 3]
LMP = TХ[log(t)+20]/1000
(In Relational Expression 3, T represents the heat treatment temperature, and the unit is ° C. In addition, t represents the heat treatment time, and the unit is minutes.)
11. The method of claim 10,
The heat treatment step is performed in the range of 850 ~ 900 ℃, a method of manufacturing a steel material for a vibration damper.

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