KR101260065B1 - Steel for structure and manufacturing method of it - Google Patents

Abstract

The present invention relates to a structural steel material and a method of manufacturing the same, and more particularly, to a structural steel material for improving the low temperature toughness of a weld heat affected zone by suppressing the growth of austenite grains during large heat welding .

The structural steel according to the present invention is characterized by containing 0.03 to 0.09% of C, 0.3 to 2.5% of Mn, 0.01 to 0.4% of Si, 0.01 to 2.0% of Ni, 0.005 to 0.10% of Nb, 0.005 to 0.10% of Ti, 0.005 B: 5 to 40 ppm, N: 15 to 50 ppm, Ca: 5 to 50 ppm, S: 0.01% or less, Mg: 0.001 to 0.03%, Al: 0.002 to 0.03% 5 to 30 ppm and Zr: 0.005 to 0.03%, and contains at least one of Zr-Al-Ti-O-based and Al-Mg-Ti-O-based oxides. It is preferable that at least one of the Zr-Al-Ti-O-based and Al-Mg-Ti-O-based oxides is used as a nucleus and at least one of sulfide and nitride is contained in the periphery of the oxide, M < 2 > are distributed in the number of 400 / mm < 2 > or more.

According to another aspect of the present invention, there is provided a method of manufacturing a structural steel, comprising the steps of preparing molten steel, deoxidizing the molten steel by injecting Mn and Si into the molten steel, and further deoxidizing the molten steel by injecting Al into the molten steel Adding titanium to the molten steel to produce titanium oxide, and injecting an alloy containing Mg into the molten steel to produce a plurality of nuclei. Further, the step of solidifying the molten steel after the step of generating the plurality of nuclei to precipitate a composite inclusion containing at least one of sulfide and nitride in the plurality of nuclei.

(HAZ), high heat input welding, austenite, toughness,

Description

Technical Field [0001] The present invention relates to a structural steel and a manufacturing method thereof,

The present invention relates to a structural steel material and a method of manufacturing the same, and more particularly, to a structural steel material for improving the low temperature toughness of a weld heat affected zone by suppressing the growth of austenite grains during large heat welding .

In recent years, as construction structures such as buildings and bridges have become higher and larger, structural steels used in these structures are required to have high strength in order to withstand high loads. At the same time, the proportion of used steel is increasing.

In case of welding such a steel sheet, a highly efficient welding method is indispensably required, and an electrogas and an electro slag welding method are widely used. In particular, a high heat input welding method, which enables one pass welding, is applied to a construction structure having a large proportion of use of a steel material. In the case of large-volume heat welding, as the amount of heat input increases, the amount of welding can be increased and the number of welding passes can be reduced.

However, in case of welding the steel material of the construction structure by the large heat welding method, since the heat affected zone (HAZ) of the steel is influenced for a long time in the high temperature environment of 1400 ° C. or more, the low temperature toughness ) Is lowered. That is, the weld heat affected zone near the fusion boundary has austenite grains coarsened as the heat input increases, and then the upper bainite and the malleable martensite The microstructure of martensite austenite constituent (MA) is formed. Therefore, it has been difficult to secure the reliability of the construction structure.

Conventional technologies for improving the toughness of the weld heat affected zone in order to solve such a problem are as follows.

First, as a method of appropriately distributing Ti-based carbonitride or the like on a steel material and delaying the growth of grain growth of the weld heat affected zone, toughness of ferrite is ensured through TiN precipitates to ensure toughness. At this time, the nitride does not dissolve in welding, and contributes to miniaturization of the austenite grains through the effect of pinning.

However, the prior art of this type has a problem in that the nitride is easily dissolved in the base material during the heat-up welding process in which the weld heat affected zone is exposed to a high temperature environment of 1400 占 폚 or more for a long time. In addition, conventional structural steel improves strength by adding B, but in the prior art, it is impossible to secure solubility B within the content range of N, and the added B precipitates as BN, causing a disadvantage of lowering the strength of the steel .

Another conventional technique is a method of inducing a phase transformation in the austenite grains during welding to finely maintain the austenite grains so that the shape, composition and composition of inclusions composed of oxides, sulfides or nitrides stable at high temperatures The number was appropriately controlled and distributed to the steel. However, there is a problem in that the shape, composition, etc. of the inclusion particles are not specifically shown, or only the number of inclusions is presented in a certain range, thereby practically implementing the steel in the structural steel.

In addition, when the conventional method for inducing the phase transformation is applied to the structural steel as it is, when the amount of the alloy element is increased, granular bainite and bainitic ferrite are more likely to be produced than acicular ferrite, ferrite microstructure is first generated. That is, the oxide, sulfide, or nitride distributed in the austenite grains does not act properly as nucleation accelerator for the needle-shaped ferrite in order to homogeneously produce the needle-shaped ferrite.

Another conventional technique is to suppress the growth of the austenite grains and to induce phase transformation of the acicular ferrite through the inclusions in the austenite grains to miniaturize the effective grains of the austenite in order to miniaturize the structure of the weld heat affected zone during welding to be. Specifically, Mg and Ti composite oxides which are stable at high temperatures are used to inhibit the growth of austenite grains while simultaneously reacting Mg and Ti composite oxides with a size of several micrometers as transformation nuclei of the needle-like ferrite And the crystal grains of the weld heat affected zone are refined at the time of welding.

Generally, the production and growth of oxides in molten steel are carried out by a nucleation step by reaction of dissolved oxygen and deoxidized elements present in molten steel, a step of nucleation by diffusion of dissolved oxygen, and a step of coalescence by wettability of molten steel Lt; / RTI > However, in the conventional technology, since oxides are made of nano-sized oxides, it is difficult to keep the oxides in a fine state while suppressing the growth and coalescence of the oxides during the manufacturing process of the steel material which takes a long time from the refining step to the continuous casting step. Further, the nano-sized oxide is not captured by dendrite when solidification proceeds, and remains in the liquid phase region, which causes a problem of uneven distribution in the steel.

If the effective grain size of the austenite in the weld heat affected zone is made finer, the fraction of the effective grain size in competition with the distribution fraction of the inclusions in the austenite grain is increased. Therefore, the grain boundary ferrite, the ferrite side plate, or the coarse upper bainite are placed in a state in which the ferrite side plate or the coarse upper bainite is preferentially transformed. As a result, phase transformation of the needle-shaped ferrite starting from the inclusions in the austenite grains is generated, and it is difficult to miniaturize the crystal grains through the needle-shaped ferrite.

As described above, the conventional techniques have been difficult to improve or improve the toughness of a steel material to which a large heat-welding method is applied for a long period of time at a high temperature of 1400 DEG C or higher.

In order to solve the above-mentioned problems, the present invention provides a method of manufacturing a semiconductor device, which comprises using a Zr-Al-Ti-O-based oxide and / or an Al-Mg- / Or (N, N) N nitrides in a number of more than 400 / mm 2 per unit area in a welded heat affected zone (HAZ) To thereby improve the low temperature toughness of the weld heat affected zone, and a method of manufacturing the same.

In order to achieve the above object, the present invention provides a structural steel comprising, by weight, 0.03 to 0.09% of C, 0.3 to 2.5% of Mn, 0.01 to 0.4% of Si, 0.01 to 2.0% of Ni, 0.005 to 0.10% of Ti, 0.002 to 0.03% of Al, 0.002 to 0.03% of Al, 0.0010 to 0.01% of O, 0.02% or less of P, 5 to 40 ppm of B, 15 to 50 ppm of N, At least one of Zr-Al-Ti-O system and Al-Mg-Ti-O system oxide containing S: 0.01% or less, Mg: 5 to 30 ppm and Zr: 0.005 to 0.03% On the other hand, when at least one of the Zr-Al-Ti-O-based and Al-Mg-Ti-O-based oxides is used as a nucleus and at least one of sulfide and nitride is contained in the periphery of the oxide, M < 2 > are distributed in the number of 400 / mm < 2 > or more.

According to another aspect of the present invention, there is provided a structural steel comprising at least one of Zr-Al-Ti-O-based and Al-Mg-Ti-O-based oxides as nuclei, The composite inclusions containing at least one of the nitrides and having an average particle size of 0.5 to 5 mu m are distributed in a number of 400 / mm < 2 > or more.

According to another aspect of the present invention, there is provided a method of manufacturing a structural steel comprising the steps of preparing molten steel, deoxidizing the molten steel, adding Ti to the molten steel to produce titanium oxide, And adding an alloy containing Mg to the molten steel to generate a plurality of nuclei. The step of deoxidizing the molten steel may include the steps of deoxidizing the molten steel by injecting Mn and Si into the molten steel, and further deoxidizing the molten steel by injecting Al into the molten steel.

Meanwhile, the step of forming the plurality of nuclei includes solidifying the molten steel to precipitate a composite inclusion containing at least one of sulfide and nitride in the plurality of nuclei.

(Mn, Cu) S sulfide and / or a (Ti, O) -sulfide and / or a (Zr-Al-Ti-O) based oxide on the periphery of the oxide, It is possible to suppress the growth of the austenite grains in the weld heat affected zone during the bulk heat welding which provides a high temperature environment of 1400 ° C or higher by distributing the composite inclusion containing Nb nitride.

Therefore, the above-mentioned steel material can improve the efficiency of the welding work by enabling one pass welding at the time of large heat welding, and can be suitably used for a construction structure requiring one pass welding.

That is, welding productivity can be improved through the steel material according to the present invention, and the reliability of the construction structure can be secured.

Hereinafter, embodiments according to the present invention will be described in more detail with reference to the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of other various forms of implementation, and that these embodiments are provided so that this disclosure will be thorough and complete, It is provided to let you know completely. Like reference numerals refer to like elements throughout.

The structural steel according to one embodiment of the present invention may contain 0.03 to 0.09% of C, 0.3 to 2.5% of Mn, 0.01 to 0.4% of Si, 0.01 to 2.0% of Ni, 0.005 to 0.10% of Nb, B: 5 to 40 ppm, N: 15 to 50 ppm, Ca: 5 to 50 ppm, S: 0.01% or less, and more preferably 0.005 to 0.03% Ti, 0.002 to 0.03% 5 to 30 ppm of Mg, and 0.005 to 0.03% of Zr, and at least one of Zr-Al-Ti-O-based and Al-Mg-Ti-O-based oxides. Of course, the remainder of the composition of the steel consists of Fe and other inevitable impurities.

The steel according to an embodiment of the present invention may have at least one of Zr-Al-Ti-O-based and Al-Mg-Ti-O-based oxides as nuclei and at least one of sulfides and nitrides The composite inclusions having an average particle size of 0.5 to 5 占 퐉 are distributed in a number of 400 / mm < 2 > or more.

In general, the inhibition of the growth of austenite grains by the size and number of fine grains can be confirmed by the following Equation 1.

……… (1)

... ... ... (One)

는 결정립의 평균 직경,

는 평균 입자 크기,

는 입자의 용적(volume) 비율이다.From here,

The average diameter of the crystal grains,

Average particle size,

Is the volume fraction of the particles.

In addition, the volume ratio of the particles in the formula (1) is expressed by the following formula (2).

………(2)

... ... ... (2)

는 단위 용적당 입자의 개수를 나타낸다.From here,

Represents the number of particles per unit volume.

)는 평균 입자 크기(

)에 비례하고, 입자의 용적 비율(

)에 반비례한다. 따라서, 결정립의 성장을 억제하기 위해서는 평균 입자 크기(

)를 작게하거나 또는 입자의 용적 비율(

)를 늘리면 된다. 한편, [식 2]에서와 같이 입자의 용적 비율(

)이 일정하다고 가정한다면 단위 용적당 입자의 개수(

)가 많아질수록 결정립의 성장 억제에 효과적이다.Referring to Equation 1, the average diameter of the grains (

) Has an average particle size (

), And the volume ratio of particles (

). Therefore, in order to suppress the growth of the crystal grains,

) Or the volume ratio of particles (

). On the other hand, as shown in [Equation 2], the volume ratio of particles

) Is constant, the number of particles per unit volume (

) Is more effective in inhibiting the growth of crystal grains.

)를 크게 증가시키지 않고서도 오스테나이트 결정립의 성장을 억제할 수 있는 입자의 용적 비율(

)을 유지할 수 있다. 즉, 오스테나이트 결정립에서 차지하는 복합 개재물의 분율 비율을 극대화시킬 수 있어서 용접열영향부에서의 오스테나이트 결정립의 성장을 억제하는 것이 가능해진다.In the present invention, composite inclusions having an average particle size of 0.5 to 5 탆 and distributed in a number of 400 / mm 2 to 900 / mm 2 per unit area were uniformly distributed in a steel material. That is, by forming the average size of particles constituting the complex inclusions within the range of 0.5 to 5 mu m, the number of particles per unit volume (

The volume ratio of particles capable of suppressing the growth of the austenite grains (

). That is, it is possible to maximize the fraction ratio of the composite inclusions in the austenite grains, thereby making it possible to suppress the growth of the austenite grains in the weld heat affected zone.

On the other hand, the composite inclusion has at least one of Zr-Al-Ti-O-based and Al-Mg-Ti-O-based oxides which are not dissolved in the base material even at a high temperature of 1400 ° C or higher as nuclei, and sulfides and nitrides And at least one of them is contained. At this time, the sulfide is composed of (Mn, Cu) S and the nitride is composed of (Ti, Nb) N.

Hereinafter, the reasons for the addition of each alloy component constituting the composition of the structural steel according to the present invention and the appropriate content range thereof will be described in detail.

(Basic alloy element)

C: 0.03 to 0.09% (hereinafter, the content of each component means weight%)

C should be contained within an appropriate range as an element that determines the strength of the steel. When the content of C is less than 0.03%, the tensile strength becomes insufficient due to the decrease of the fraction of martensite austenite constituent (MA). When the content of C exceeds 0.09%, the fraction of martensite increases, And deteriorates the weldability, so that the content range of C is 0.03 to 0.09%.

Si: 0.01 to 0.4%

Si is used as a deoxidizer and improves the strength. That is, Si improves the stability of the martensite and improves the strength of the martensite because a large amount of martensite can be formed even at a low C content. However, when the content of Si is 0.4% or more, the low temperature toughness is lowered and the weldability is deteriorated. In addition, when Si is contained at 0.01% or less, the deoxidation effect is insufficient. Therefore, the content range of Si is set to 0.01 to 0.4%.

Mn: 0.3 to 2.5%

Mn is used together with Si as a deoxidizing agent and at the same time, 0.3% or more as an element for improving the strength by solid solution strengthening. However, when the content of Mn exceeds 2.5%, the toughness of the weld heat affected zone is greatly lowered due to an increase in the hardenability, so the content of Mn is in the range of 0.3 to 2.5%.

P: not more than 0.02%

P is an element which improves strength and corrosion resistance, but it is advantageous to contain as little as possible because it is an element which lowers the impact toughness, and the upper limit of the content is set to 0.02%.

S: not more than 0.01%

S is an element which forms MnS or the like to greatly deteriorate the impact toughness. Therefore, it is advantageous to contain as little as possible, and the upper limit is preferably 0.01%.

Al: 0.002 to 0.03%

Since Al is an element capable of inexpensively deoxidizing molten steel, it is preferable to add 0.002% or more of Al. However, if it is added in an amount of 0.03% or more, the number of fine oxides is decreased and nozzle clogging occurs in continuous casting The content range of Al is set to 0.002 to 0.03%.

Nb: 0.005 to 0.1%

Nb precipitates in the form of NbC or NbCN, which greatly improves the strength of the base material and weld heat affected zone. In addition, Nb dissolved at the time of reheating at a high temperature has the effect of suppressing austenite recrystallization and suppressing transformation of ferrite or bainite to make the structure finer. Therefore, it is preferable that Nb is added in an amount of 0.005% or more, but when it is added in an amount of 0.1% or more, the possibility of brittle cracks is increased at the edge of the steel, so the content of Nb is preferably 0.005-0.1% do.

B: 5 to 40 ppm

B is a very low-cost additive element and exhibits strong hardenability, and addition of a small amount greatly improves the strength. It is preferable to add Fe 2 O 3 (CB) ₆ in an amount of at least 5 ppm or more, and when it is added excessively, Fe 23 (CB) ₆ is formed to lower the hardenability and lower the low temperature toughness. Therefore, the content range of B is 5 to 40 ppm.

Ti: 0.005 to 0.1%

Ti bonds with dissolved oxygen in molten steel to form Ti oxide. In addition, at the time of reheating, there is an effect of improving the low-temperature toughness by binding with N and forming fine TiN precipitates to suppress the growth of crystal grains. In order to exhibit such an effect, Ti content should be added in an amount of 0.005% or more. In addition, when the content is excessively increased to 0.1% or more, clogging of the performance nozzle or low temperature toughness due to centering may occur, so that the content of Ti is in the range of 0.005 to 0.1%.

Ni: 0.01 to 2.0%

Ni is an element capable of simultaneously improving the strength and toughness of a base material, and enhances the solubility of Mg in molten steel. In order to exhibit the effect of increasing the solubility of Mg, it is required to be added in an amount of 0.01% or more. However, since Ni is an expensive element, if it is added in an amount of 2.0% or more, economical efficiency decreases. Therefore, Ni has a content range of 0.01 to 2.0%.

N: 15 to 50 ppm

Addition of N increases the strength, while it decreases the toughness significantly, so it is necessary to limit the content to 50 ppm or less. However, when it is contained at 15 ppm or less, the lower limit of the N content is set at 15 ppm to increase the steelmaking load. That is, N has a content range of 15 to 50 ppm.

Ca: 5 to 50 ppm

Ca has an effect of controlling the shape of coarse MnS during steel refining. If it exceeds 50 ppm, large inclusions and clusters are formed to decrease the toughness of the steel. Therefore, the range of 5 to 50 ppm is required.

Mg: 5 to 30 ppm

Mg is used as a deoxidizer, and Ti oxide is reduced together with Zr to increase the number of fine oxides. In order to exhibit the effect of Mg, it is preferable to add 5 ppm or more. If it exceeds 30 ppm, a coarse Mg oxide is formed to lower the toughness of the weld heat affected zone, so the content range is set to 5 to 30 ppm.

Zr: 0.005 to 0.03%

Zr is used as a deoxidizer, and Ti oxide is reduced together with Mg to increase the number of fine oxides. Zr is preferably added in an amount of 0.005% or more because of the effect of increasing the strength. When the content exceeds 0.03%, the toughness of the weld heat affected zone is lowered, so that the content range is 0.005 to 0.03%.

O: 0.001 to 0.01%

O is an element which reacts with a deoxidizing agent such as Ti, Mg and Zr in the molten steel to form an oxide. When the content is less than 0.001%, the oxide is not produced in a sufficient number. When the content is more than 0.01% It is difficult to control the oxide due to floating separation. Therefore, the content range of O is set to 0.001 to 0.01%.

The structural steel according to the present invention can exhibit the effect of improving the low-temperature toughness of the weld heat affected zone by merely including the alloy element having the above-mentioned content range in the steel material. However, Additional additional alloying elements may be added within a suitable range. Only one kind of the additional alloying elements described later may be added, or two or more kinds of the additional alloying elements may be added at the same time.

(Additional alloying element)

Cr: 0.05 to 1.0% (hereinafter, the content of each component means weight%)

Cr has a great effect on increasing the hardenability by increasing the hardenability and therefore, it is necessary to add 0.05% or more to obtain the effect. However, when it is added in an amount of 1.0% or more, the weldability is greatly lowered, so that it is preferable to be limited to 1.0% or less. Therefore, the content of Cr is set to 0.05 to 1.0%.

Mo: 0.01 to 1.0%

Since Mo can greatly improve the hardenability and inhibit the formation of ferrite by only adding a small amount of Mo, it is necessary to add Mo at a content of 0.01% or more. If Mo is added at a content of 1.0% or more, Excessively increases and inhibits toughness. Therefore, Mo has a content range of 0.01 to 1.0%.

Cu: 0.01 to 1.0%

Cu can increase the strength while minimizing toughness deterioration of the base material. In order to exhibit such an effect, it should be added in an amount of 0.01% or more, and if it is added in an amount exceeding 1.0%, the surface quality of the steel may be significantly deteriorated. Therefore, Cu has a content range of 0.01 to 1.0%.

V: 0.005 to 0.3%

V has a low temperature to be employed as compared with other fine alloys and has an effect of preventing precipitation in the weld heat affected portion and thus lowering the strength. It is preferably added in an amount of 0.005% or more, and when it is added in excess of 0.3%, the toughness is lowered, so that the content of V is 0.005 to 0.3%.

Hereinafter, a method of manufacturing a structural steel according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

1 is a flowchart illustrating a method of manufacturing a structural steel according to an embodiment of the present invention.

Referring to FIG. 1, a molten steel containing certain alloying elements is first prepared to produce a structural steel. During the refining process, the molten steel that is introduced in the first converter contains the basic alloying elements of C, O, P, N and S in the proper amount range, and then other basic alloying elements such as Nb and Ni are inputted. Further, one or more alloying elements selected from the additional alloying elements of Cu, V, Mo, and Cr are injected into the molten steel (S110)

Then, Mn and Si used as a deoxidizer are added as molten steel to make the dissolved oxygen in the molten steel into a spoil acid state. (S120) In this embodiment, the spoil acid state is formed in a range of 40 to 70 ppm. As Mn and Si are added, MnO-SiO ₂ oxide is produced in the molten steel. In this way, dissolved oxygen in the molten steel is further reduced by addition of Al to the molten steel in the spoil-acid state. (S130) At this time, in this embodiment, dissolved oxygen in the molten steel due to the addition of Al is in the range of 20 to 40 ppm. In addition, the MnO ₂ -SiO ₂ -based oxide produced by the addition of Mn and Si is partially reduced by using the property of Al, which agglomerates and coarsens the oxide, thereby promoting floating separation. In this embodiment, since the amount of dissolved oxygen in the molten steel can be further lowered, the amount of expensive Ti added thereafter can be minimized, thereby ensuring economical efficiency.

Then, Ti is injected into the molten steel to form a TiO _x oxide (S140). In general, oxides nucleate and grow oxides by diffusion of dissolved oxygen. The addition of Ti is delayed by the additional deoxidation of Al, which is different from the fact that the dissolved oxygen in the molten steel is merely spoiled by the introduction of Mn and Si, thereby delaying the time during which the fine TiO _x oxide is separated. That is, the TiO _x oxide formed in accordance with the input of Ti is not easily separated in the molten steel and is finely dispersed in the molten steel.

Thereafter, the input of the Mg in the form of a Ni-Zr-Mg-based alloy. (S150) one, Ni-Zr-Mg-based alloy In is completed, a, a TiO _x oxide finely dispersed Zr and Mg at the same time partial reduction of At least one kind of Zr-Al-Ti-O system and Al-Mg-Ti-O system oxide is formed into a plurality of nuclei.

In the present embodiment, the introduction of the Ni-Zr-Mg alloy is performed within a time period of less than 10 minutes after the Ti is introduced and the TiO _x oxide is finely dispersed in the molten steel. Of course, it is preferable that the Ni-Zr-Mg alloy is injected as soon as possible even within the time range of less than 10 minutes.

The reason why the Ni-Zr-Mg alloy is added as soon as possible within a time period of less than 10 minutes is because the TiO _x oxide formed over time grows and coheres to become coarse and easy to float, The reduction amount of the oxide produced by the addition of the Ni-Zr-Mg based alloy does not change greatly.

Mg is added to the molten steel in the form of a Ni-Zr-Mg alloy after the deoxidation step of Ti, since the specific gravity of Mg is 1.74, the melting point is 649 ° C and the boiling point is 1090 ° C. For the reason that it is difficult to inject directly. For this reason, when it is added under an atmospheric pressure, it is added in the form of a Ni-Zr-Mg alloy having a high specific gravity. On the other hand, in Ni-Zr-Mg alloys, since Ni has a high solubility of Mg, it is effective to increase the rate of occurrence of Mg in molten steel by using an alloy containing Ni at the same time. Zr was added to maximize the simultaneous addition effect with Mg.

Conventionally, when the amount of Mg is excessively increased in the course of Mg addition, a large amount of Mg vapor and a vigorous oxidation exothermic reaction cause operational problems, and a large amount of Mg is added to increase the cost. Since the Ni-Zr-Mg alloy having a high density is used as in the present embodiment, the amount of Mg used can be reduced by increasing the rate of realization of Mg. In addition, Zr-Al-Ti-O Based oxide and Al-Mg-Ti-O-based oxide can be smoothly generated as nuclei, thereby reducing Mg charging cost. That is, it is possible to reduce the load during the steelmaking process and to secure economical efficiency and operability.

Through the above steps, molten steel containing at least one kind of Zr-Al-Ti-O system and Al-Mg-Ti-O system oxide as a plurality of nuclei is solidified through a continuous casting process to form a steel material. As the molten steel solidifies, any one of MnS, CuS sulfides, and nitrides of TiN and NbN is contained in the vicinity of a plurality of nuclei having at least one kind of Zr-Al-Ti-O system and Al-Mg- Thereby precipitating the composite inclusions as shown in Fig. 2. (S160)

2 is a view showing a complex inclusion according to an embodiment of the present invention using a scanning electron microscope (FE-SEM) and energy dispersive X-ray analysis (EDS).

Referring to FIG. 2, the molten steel is solidified to form a composite inclusion having a spherical or distorted spherical shape. The composite inclusion includes at least one of a Zr-Al-Ti-O-based oxide and an Al-Mg-Ti-O-based oxide, wherein the oxide is locally located in a spherical or distorted spherical shape. That is, the oxide particle may be located at the inner center portion of the composite inclusion, and may be located partially exposed at one side of the outer circumferential surface.

In the present embodiment, the composite inclusions are formed within the range of 0.5 to 5 mu m in average particle size, and are precipitated in a number of 400 / mm < 2 > per unit area (the size and shape of the composite inclusions are more detailed )

At least one nucleus of the Zr-Al-Ti-O-based and Al-Mg-Ti-O-based oxides contained in the molten steel does not just precipitate the composite inclusions in the continuous casting process, And serves as nuclei for precipitation of composite inclusions in the subsequent cooling process or reheating-hot working.

On the other hand, in the present embodiment, the continuous casting method is used for casting molten steel to form a steel material because the continuous casting method has a high cooling rate. Therefore, if the cooling rate is high, the molten steel can be solidified in a short time, and the precipitate formed in the molten steel can be finely dispersed. Various kinds of techniques such as control rolling and control cooling known in the art can be applied according to the user's use in the hot rolling process, and the heat treatment is applied to improve the mechanical properties. It is possible.

Hereinafter, the present invention will be described in more detail by way of examples.

[Table 1] shows the chemical composition of the rolled plate material subjected to the preliminary treatment-converter refining-second refining-performance step-rolling step in turn. Table 2 shows the number of composite inclusions per unit area, Average particle size and Charpy test results are shown.

[Table 1]

The remainder: iron (Fe) and inevitable impurities

[Table 2]

The specimens were taken from each of the rolled plates having the chemical composition shown in Table 1 (inventive steels 1 to 7, comparative steels 8, 7 and 9), and subjected to one-pass substitution heat welding (heat input: 800 KJ / cm, Arc welding) was performed to obtain a welded joint. Then, Charpy impact test specimens were taken from the weld heat affected zone (HAZ) welded from these weld seams to evaluate the toughness of the weld heat affected zone. As the evaluation method, the Charpy impact test was performed on each of three test pieces under the environmental condition of -20 ° C, and the absorbed energy was measured three times, and the average value for three measurements was obtained. Here, the average value for the absorbed energy represents impact toughness (vE _{-20 ° C} : J) of the weld heat affected zone (HAZ).

As shown in Table 2, it was evaluated that the toughness of the weld heat affected zone was excellent for the test object having an average value of absorbed energy exceeding 200J. The shape of the composite inclusion can be confirmed as shown in FIG. 2 by observing the weld heat affected zone of each test piece using a scanning electron microscope (FR-SEM), and the energy dispersive X-ray analysis (EDS) The composition of the complex inclusions was confirmed. As a result, at least one of the Zr-Al-Ti-O system and the Al-Mg-Ti-O system oxide is used as a nucleus, and at least (Mn, Cu) S sulfide and Composite inclusions containing one are formed within a range of 0.5 to 5 탆 in average particle size and have a shape of a distorted spherical shape similar to a spherical or spherical shape. The number of complex inclusions was found by point counting method through an electron microscope and converted to the number per unit area through an image analyzer. As a result, it was confirmed that the number of complex inclusions was 400 / ㎟ to 900 / ㎟.

From the above results, the following results are obtained.

First, the rolling steels satisfying the requirements of the present invention (inventive steels 1 to 7 of Table 1) are compared with comparative steels (comparative steels 8, 9 and 10 of Table 1) , The number of complex inclusions per unit area is formed, and the number of composite inclusions is remarkably increased as compared with the comparative material. Therefore, it can be confirmed that the density according to the number of composite inclusions is distributed so that the growth of the austenite grains is suppressed in the weld heat affected zone, and the toughness of the weld heat affected zone is improved.

On the other hand, it can be confirmed that good toughness of the weld heat affected zone can not be obtained in the rolled plate materials (comparative steels 8, 9 and 10 in Table 1) in which any of the requirements specified in the present invention are insufficient. Specifically, in the comparative steel 8, Mg exceeds the optimum content range (5 to 30 ppm), and in the comparative steel 9, Zr exceeds the optimum content range (0.005 to 0.03%) to produce coarse Mg oxide and Zr oxide , And then flotation separation occurred due to the collision between the oxides. As a result, the density of the composite inclusions was lowered according to the number of the inclusions, and it was impossible to suppress the growth of the austenite grains in the weld heat affected zone. Therefore, low toughness appeared in the weld heat affected zone.

On the other hand, in the comparative steel 10, Al, which is a deoxidizing element, exceeded the appropriate content range (0.002 to 0.03%) and dissolved oxygen in the molten steel was kept extremely low, so that the oxide could not be properly produced. In addition, the coarsened Al oxide is formed by the collision between the oxides and the coalescence process, so that the number of the complex inclusions is reduced and the growth of the austenite grains in the weld heat affected zone can not be suppressed, resulting in low toughness .

As described above, the structural steel according to one embodiment of the present invention and the manufacturing method thereof can uniformly distribute the composite inclusions, which can withstand at high temperatures of 1400 ° C. or higher, to the steel material, so that the growth of austenite grains . Therefore, it can be used as a backing steel which can be applied to a construction structure, and it is possible to perform one pass welding, thereby improving the efficiency of the welding work. That is, the reliability of the construction structure can be secured through the structural steel material and the manufacturing method thereof according to the present invention.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. Accordingly, those skilled in the art will appreciate that various modifications and changes may be made thereto without departing from the spirit of the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart illustrating a method of manufacturing a structural steel according to an embodiment of the present invention; FIG.

FIG. 2 is a diagram showing complex inclusions according to an embodiment of the present invention using a scanning transfer microscope (FE-SEM) and energy dispersive X-ray analysis (EDS).

Claims (13)

delete The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.03 to 0.09% of C, 0.3 to 2.5% of Mn, 0.01 to 0.4% of Si, 0.01 to 2.0% of Ni, 0.005 to 0.10% of Nb, 0.005 to 0.03% 0.001 to 0.01% of O, 0.02% or less of P, 5 to 40 ppm of B, 15 to 50 ppm of N, 5 to 50 ppm of Ca, 0.01% or less of S, 5 to 30 ppm of Mg, 0.03%, and at least one of Zr-Al-Ti-O-based and Al-Mg-Ti-O-based oxides. 3. The method of claim 2, Wherein at least one of the Zr-Al-Ti-O-based and Al-Mg-Ti-O-based oxides is used as a nucleus and at least one of sulfide and nitride is contained in the periphery of the oxide, Composite inclusions are distributed in the number of more than 400 / ㎟ ~ 900 / ㎟. The method according to claim 2 or 3, Wherein the steel further comprises at least one selected from the group consisting of Cu: 0.010 to 1.0, V: 0.005 to 0.3%, Mo: 0.01 to 1.0%, and Cr: 0.05 to 1.0%. The method of claim 3, Wherein the sulfide is composed of (Mn, Cu) S and the nitride is composed of (Ti, Nb) N. Preparing molten steel; Deoxidizing the molten steel; Introducing Ti into the molten steel to produce titanium oxide; And Adding an Mg-containing alloy to the molten steel to generate a plurality of nuclei; Of the structural steel. The method according to claim 6, The step of deoxidizing the molten steel comprises: Deoxidizing the molten steel by injecting Mn and Si into the molten steel; Further deoxidizing the molten steel by injecting Al into the molten steel; Of the structural steel. The method according to claim 6, And solidifying the molten steel after the step of generating the plurality of nuclei to deposit a composite inclusion containing at least one of sulfide and nitride in the plurality of nuclei. 8. The method of claim 7, Method for manufacturing a structural steel which comprises the step of putting the Al, the Al is by partial reduction of the MnO-SiO ₂ based oxide produced in the molten steel to promote the floatation. The method according to claim 6, Wherein the Mg-containing alloy has a specific gravity larger than that of Mg in the step of adding an Mg-containing alloy to the molten steel. 11. The method of claim 10, Wherein the Mg-containing alloy is a Ni-Zr-Mg-based alloy. 12. The method of claim 11, Wherein the Ni-Zr-Mg alloy is charged into the molten steel within a time period of less than 10 minutes after the step of producing the titanium oxide. 9. The method of claim 8, Wherein the molten steel is cooled through a continuous casting step in the step of precipitating the composite inclusions.

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