KR101797300B1 - Steel for construction having excellent surface flatness and method for manufacturing the same - Google Patents

Abstract

A preferred embodiment of the present invention is to provide a steel material for building structure having a high strength and a low yield ratio and excellent in plate width and flatness in the longitudinal direction, and a manufacturing method thereof.
A preferred embodiment of the present invention is a steel sheet comprising, by weight%, 0.02 to 0.08% of C, 0.01 to 0.6% of Si, 1.5 to 3.0% of Mn, 0.02% 0.005 to 0.5% of Al, 0.005 to 0.1% of B, 5 to 40 ppm of B, 0.005 to 0.1% of Ti, 15 to 150 ppm of N, 0.1 to 1.0% of Cr, 0.01 to 2.0% of Ni, , The remainder Fe and unavoidable impurities; The microstructure includes 60 to 90% of bainitic ferrite and 10 to 40% of granular bainite by area%; The maximum wave height of the surface is 15 mm or less; And having a thickness of 10 to 30 mm and excellent flatness, and a method of manufacturing the same.
According to the present invention, it is possible to provide an excellent structural property having a tensile strength of 800 MPa or more, a yield ratio of 0.85 or less, an impact absorbing energy of 100 J or more at -40 캜, and a plate shape satisfying the flatness standard allowance of KS D 3500 Steel can be provided.

Description

TECHNICAL FIELD [0001] The present invention relates to a steel material for an architectural structure having excellent flatness and a method of manufacturing the steel material.

TECHNICAL FIELD The present invention relates to a steel material for building construction usable in skyscraper buildings, and more particularly, to a steel material for building construction having excellent flatness and high strength and low resistance, and a method of manufacturing the same.

Recently, steel structure for building structure has been required to have higher strength compared with the existing structure, and the yield ratio is still required to be higher in order to improve the earthquake resistance. In addition, as the structure is becoming bigger, it is demanding a wide and long steel which reduces the total steel requirement compared to the conventional steel instead of using the high-performance steel for reasons such as cost reduction.

In addition to the physical properties and weldability of steel in the construction site, the most important consideration is the flatness of the steel. Like the shipbuilding steels, construction steels are also used in the form of columns or pipes after being welded in most cases in order to be used in actual structures.

In this case, when the steel material is not flat in the lengthwise or widthwise direction, it is impossible to weld between the plate and the plate, and when the stress is concentrated during the actual use, the welded portion is relatively weak. Therefore, in the case of construction steels used in fixed structures, the specifications are specified in the work specifications and are strictly controlled (KS D 3500 in Korea).

Generally, the technique of securing the flatness of the steel is mainly made by softening the material through the additional heat treatment after the plastic working, compressing the material by a strong press or the like to remove the bending. However, in the case of the heat treatment, a large apparatus such as a furnace capable of uniformly heat-treating the steel in its entirety is indispensable, and there are problems such as additional manufacturing cost and air delay.

A preferred embodiment of the present invention is to provide a steel material for building structure having a high strength and a low yield ratio and excellent in plate width and flatness in the longitudinal direction, and a manufacturing method thereof.

Another desirable aspect of the present invention is to provide a structural steel for building structure excellent in toughness of a weld heat affected zone and having high strength and low yield ratio and excellent flatness in the width and longitudinal direction of the plate and a method for manufacturing the same .

A preferred embodiment of the present invention is a steel sheet comprising, by weight%, 0.02 to 0.08% of C, 0.01 to 0.6% of Si, 1.5 to 3.0% of Mn, 0.02% 0.005 to 0.5% of Al, 0.005 to 0.1% of B, 5 to 40 ppm of B, 0.005 to 0.1% of Ti, 15 to 150 ppm of N, 0.1 to 1.0% of Cr, 0.01 to 2.0% of Ni, , The remainder Fe and unavoidable impurities;

The microstructure includes 60 to 90% of bainitic ferrite and 10 to 40% of granular bainite by area%;

The maximum peak height of the surface is 20 mm or less; And

Provided is a steel for building structure excellent in flatness with a thickness of 10 to 30 mm.

The microstructure may contain not more than 5% of statutory martensite (MA) in area%.

The steel material may have a carbon equivalent (Ceq.) Value of 0.60 or less as defined by the following relational expression (1) and a weld crack susceptibility index (Pcm.) Value as defined by the following relational expression (2) of 0.30 or less.

[Relation 1]

Carbon equivalent (Ceq.) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15

Here, C, Mn, Cr, Mo, V, Cu, and Ni are values indicating the content of each element in weight%

[Relation 2]

The weld crack susceptibility index (Pcm.) = C + (Mn + Cr + Cu) / 20 + Si / 30 + Ni / 60 + Ti / 10 + Mo /

(Where C, Mn, Cr, Cu, Si, Ni, Ti, Mo, and B represent the content of each element in weight%

The steel may further include one or more selected from the group consisting of Mo: 0.1 to 1.0%, Cu: 0.01 to 1.0%, and V: 0.005 to 0.3%.

Another preferred embodiment of the present invention is a ferritic stainless steel comprising: 0.02 to 0.08% of C, 0.01 to 0.6% of Si, 1.5 to 3.0% of Mn, 0.02% of P (excluding 0) 0.005 to 0.5% of Al, 0.005 to 0.1% of Nb, 5 to 40 ppm of B, 0.005 to 0.1% of Ti, 15 to 150 ppm of N, 0.1 to 1.0% of Cr, 0.01 to 2.0 %, The remainder being Fe and unavoidable impurities, at 1100 to 1200 ° C; Subjecting the reheated slab to rough rolling at 900 to 1100 ° C to obtain a bar;

Hot-rolling the bar to obtain a hot-rolled steel sheet having a thickness of 10 to 30 mm; and

And cooling the hot-rolled steel sheet at a cooling rate of 35 DEG C / s or higher to a cooling termination temperature equal to or lower than the Bs temperature (bainite transformation start temperature), thereby providing a method of manufacturing a steel for building structural excellent in flatness.

The steel slab may have a carbon equivalent (Ceq.) Value of 0.60 or less as defined by the following relational expression (1) and a weld crack susceptibility index (Pcm.) Value of 0.30 or less as defined by the following relational expression (2).

[Relation 1]

Carbon equivalent (Ceq.) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15

Here, C, Mn, Cr, Mo, V, Cu, and Ni are values indicating the content of each element in weight%

[Relation 2]

The weld crack susceptibility index (Pcm.) = C + (Mn + Cr + Cu) / 20 + Si / 30 + Ni / 60 + Ti / 10 + Mo /

(Where C, Mn, Cr, Cu, Si, Ni, Ti, Mo, and B represent the content of each element in weight%

The steel slab may further include one or more selected from the group consisting of Mo: 0.1 to 1.0%, Cu: 0.01 to 1.0%, and V: 0.005 to 0.3%.

The cooling start temperature for cooling the hot-rolled steel sheet may preferably be 700 to 850 ° C.

The cooling rate may preferably be 40 to 55 DEG C / s.

The cooling end temperature is preferably 400 ° C to 600 ° C, and more preferably, the cooling end temperature may be 450 ° C to 550 ° C.

According to a preferred embodiment of the present invention, it is possible to provide a steel for building structure having high strength and low yield ratio and excellent flatness in the width and length direction of the plate.

According to another preferred embodiment of the present invention, it is possible to provide a structural steel for building structure which is excellent in toughness of a weld heat affected zone, has high strength and low yield ratio, and is excellent in plate width and flatness in the longitudinal direction.

Fig. 1 is a schematic diagram for defining the maximum peak (phase difference) of the surface of a steel sheet.

The inventors of the present invention have conducted research and experiments to obtain a steel material having a high strength and a low yield ratio and a thickness of 10 to 30 mm which is excellent in the flatness in the width direction and the longitudinal direction of the plate, .

The present invention provides a steel material having a thickness of 10 to 30 mm, which has high strength and low yield ratio and excellent flatness in the width direction and the longitudinal direction by controlling the steel composition, texture and manufacturing conditions of the steel material.

The main concept of the present invention is as follows.

1) The hot-rolled steel sheet is cooled in the desired cooling conditions to optimize the steel composition to obtain the desired final structure.

By controlling the steel composition in this way, it is possible to minimize the thermal stress generated upon cooling the hot-rolled steel sheet while obtaining the desired final structure, thereby improving the flatness of the steel sheet while ensuring high strength and low resistance.

2) In particular, microstructure is controlled to secure high strength and low yield ratio.

By controlling the microstructure in this way, high strength and low yield ratio can be secured.

3) The cooling condition is controlled to obtain the desired microstructure while minimizing the thermal stress generated during cooling of the hot-rolled steel sheet.

4) Preferably, the carbon equivalent (Ceq.) And weld crack susceptibility index (Pcm.) Can be controlled to improve the toughness of the weld heat affected zone.

5) It is preferable to suppress the formation of amorphous martensite (MA) in order to improve low-temperature toughness.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a structural steel for building construction having excellent flatness, which is a preferred embodiment of the present invention, will be described in detail.

The structural steel having excellent flatness, which is a preferred embodiment of the present invention, contains 0.02 to 0.08% of C, 0.01 to 0.6% of Si, 1.5 to 3.0% of Mn, 0.02% of P (excluding 0) 0.005 to 0.5% of Al, 0.005 to 0.1% of Nb, 5 to 40 ppm of B, 0.005 to 0.1% of Ti, 15 to 150 ppm of N, 0.1 to 1.0% of Cr, S: 0.01% , Ni: 0.01 to 2.0%, and the balance of Fe and unavoidable impurities.

Hereinafter, the reason for limiting the components and the range of the components of the steel will be described.

C: 0.02 to 0.08% by weight (hereinafter referred to as "%")

C is a component that forms soft phase ferrite and bainite and improves strength.

If the content of C is less than 0.02%, the strength is markedly decreased. If the content of C is more than 0.08%, the MA structure is easily formed and the impact toughness of the material deteriorates. In addition, There is a possibility that the weldability is lowered.

Therefore, the content of C is preferably limited to 0.02 to 0.08%, more preferably 0.04 to 0.06%.

Si: 0.01 to 0.6%

Si is a component that plays a role in improving the role and strength of the deoxidizer.

If the content of Si is less than 0.01%, the deoxidizing effect and the strength improving effect are insufficient. If the Si content exceeds 0.6%, the low temperature toughness is lowered and the weldability is also deteriorated.

Therefore, the content of Si is preferably limited to 0.01 to 0.6%, more preferably 0.1 to 0.4%.

Mn: 1.5 to 3.0%

Mn is a useful component for enhancing strength by solid solution strengthening.

When the content of Mn is less than 1.5%, the effect of improving the strength is insufficient due to solid solution strengthening. When the content of Mn exceeds 3.0%, the toughness of the welded portion may be greatly deteriorated due to an excessive increase in hardenability.

Therefore, the Mn content is preferably limited to 1.5 to 3.0%.

A more preferable Mn content is 2.0 to 3.0%, and a more preferable Mn content is 2.2 to 2.7%.

P: 0.02% or less (excluding 0)

P is advantageous for strength improvement and corrosion resistance, but it is advantageous to lower the P content because it is a component adverse to impact toughness. Therefore, the upper limit of the P content is preferably 0.02%, and the more preferable content of P is 0.012% .

S: 0.01% or less (excluding 0)

Since S is an element which forms MnS or the like at the center of the thickness of the steel sheet and significantly lowers the impact toughness, it is advantageous to make it as low as possible. Therefore, the upper limit of the S content is preferably 0.01% 0.005% or less.

Al: 0.005-0.5%

Al acts as a deoxidizer and has an effect of refining particles at high temperatures.

If the Al content is less than 0.005%, the deoxidizing effect can not be sufficiently obtained. If the Al content is more than 0.5%, the nozzle may be clogged during continuous casting.

Therefore, the Al content is preferably limited to 0.005 to 0.5%.

Nb: 0.005 to 0.1%

Nb plays a role of improving the toughness by grain refinement of the texture and precipitates in the form of NbC or NbCN, which greatly improves the strength of the base material and welded part. In addition, it can form bainite even at low cooling rate when it is cooled after rough rolling.

In order to obtain the above-mentioned effect, the Nb content is preferably 0.005% or more. If the Nb content is more than 0.1%, there is a high possibility of causing a brittle crack at the edge of the steel material and a manufacturing cost is greatly increased.

Therefore, the Nb content is preferably limited to 0.005 to 0.1%.

B: 5 to 40 ppm

B is a component for improving the hardenability and enables the formation of bainite even at a slow cooling rate in cooling after rough rolling.

When the B content is less than 5 ppm, it is difficult to secure sufficient curing ability. When the B content is more than 40 ppm, the hardenability is lowered and the low temperature toughness is greatly lowered. Therefore, it is preferable to add 5 to 40 ppm. More preferred B content is 10 to 25 ppm.

Ti: 0.005 to 0.1%

Ti can greatly improve low temperature toughness by inhibiting grain growth during reheating of steel slabs and, for this, should be at least 0.005%. However, if it is contained in an amount exceeding 0.1%, the performance tends to be deteriorated due to clogging of the performance nozzle or crystallization of the center portion.

Therefore, the content of Ti is preferably limited to 0.005 to 0.1%.

N: 15 to 150 ppm

N increases the strength but decreases the toughness, so it is necessary to limit the content to 150 ppm or less. However, since controlling the N content to less than 15 ppm leads to an increase in the steelmaking load, the lower limit of the N content is preferably 15 ppm.

Cr: 0.1 to 1.0%

Cr is a component that increases the hardenability and increases the strength. In order to obtain such an effect, Cr is preferably contained in an amount of 0.1% or more.

However, when the Cr content exceeds 1.0%, the weldability is largely lowered, so the upper limit is preferably limited to 1.0%.

Ni: 0.01 to 2.0%

Ni is an element capable of simultaneously improving the strength and toughness of a base material. In order to obtain the effect, Ni should be contained in an amount of 0.01% or more. However, since Ni is a very expensive element, when it is contained in an amount exceeding 2.0%, the economical efficiency is remarkably lowered and the weldability is lowered. Therefore, the upper limit is preferably 2.0%.

In the present invention, one or more of the following alloying elements may be further included as necessary.

Mo: 0.1 to 1.0%

Since Mo has an effect of greatly improving the hardenability and inhibiting the formation of ferrite even by adding a small amount of Mo, the strength can be greatly improved. Therefore, Mo is preferably contained in an amount of 0.1% or more. However, when the content exceeds 1.0%, the hardness of the welded portion is excessively increased and the toughness is deteriorated. Therefore, the content is preferably limited to 1.0% or less.

Cu: 0.01 to 1.0%

Cu is an element capable of minimizing toughness deterioration of a steel material and increasing strength, and its content is preferably 0.01% or more. However, when Cu is also excessively contained as an extremely expensive element, the economical efficiency is remarkably lowered and the product surface quality is lowered. Therefore, the content of Cu is preferably limited to 1.0% or less.

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 the strength from dropping by precipitating in the weld heat affected zone, and its content is preferably 0.005% or more. However, when V is also a very expensive element, when it is contained in an amount of more than 0.3%, not only the economical efficiency is lowered but also the toughness is largely lowered, so that the content thereof is preferably limited to 0.005 to 0.3%.

The remainder other than the above components are Fe and unavoidable impurities.

The steel material may have a carbon equivalent (Ceq.) Value of 0.60 or less as defined by the following relational expression (1) and a weld crack susceptibility index (Pcm.) Value as defined by the following relational expression (2) of 0.30 or less.

[Relation 1]

Carbon equivalent (Ceq.) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15

Here, C, Mn, Cr, Mo, V, Cu, and Ni are values indicating the content of each element in weight%

[Relation 2]

The weld crack susceptibility index (Pcm.) = C + (Mn + Cr + Cu) / 20 + Si / 30 + Ni / 60 + Ti / 10 + Mo /

(Where C, Mn, Cr, Cu, Si, Ni, Ti, Mo, and B represent the content of each element in weight%

Because of the nature of materials for building construction, welding is essential, so carbon equivalence and weld crack susceptibility index must be considered in steel development. The larger the carbon equivalent and the weld crack susceptibility index, the more difficult it is to weld and the impact toughness characteristics at welds also deteriorate. The carbon equivalent (Ceq.) And weld crack susceptibility index (Pcm.) Are increased as the content of alloying elements such as Mn, Mo, Cr, etc. increases in addition to carbon. The carbon equivalent (Ceq.) Is preferably 0.6 or less, the weld crack susceptibility index (Pcm.) Is preferably 0.3 or less, the carbon equivalent (Ceq.) Is preferably 0.50 to 0.55, The crack susceptibility index (Pcm.) Is more preferably 0.20 to 0.22.

The steel microstructure of the present invention preferably contains 60 to 90% of bainitic ferrite and 10 to 40% of granular bainite in terms of area%.

The bainitic ferrite serves as a phase to ensure strength and impact toughness. When the area percentage is less than 60%, the impact toughness may deteriorate. When the area percentage exceeds 90%, the yield ratio exceeds the yield ratio. There is a concern.

The grain aspect ratio of the bainitic ferrite is preferably 0.3 to 0.4.

If the particle aspect ratio of the bainitic ferrite is less than 0.3, the yield ratio may be exceeded, and if it exceeds 0.4, the impact toughness may be deteriorated.

The granular bainite serves as a phase satisfying the resistance ratio at the same time as securing strength. When the area ratio is less than 10%, the yield ratio may exceed the yield ratio. When the area ratio exceeds 40% The impact toughness may be deteriorated.

The size of the granular bainite is preferably 30 to 60 mu m.

If the size of the granular bainite is less than 30 탆, the yield ratio may exceed. If it exceeds 60 탆, the tensile strength may be less than 800 MPa or the impact toughness may deteriorate.

The microstructure may contain no artificial martensite (MA), or may contain 5% or less by area%. It does not contain martensite (MA) or contains less than 5% of martensite (MA), so the low temperature toughness is improved.

For example, the steel has a maximum peak height (phase difference) of 20 mm or less as defined in Fig.

The surface top peak height (phase difference) of the steel is more preferably 15 mm or less.

The thickness of the steel material is 10 to 30 mm, preferably 15 to 30 mm, and more preferably 20 to 25 mm.

The steel material may have a tensile strength of 800 MPa or more, a yield ratio of 0.85 or less, and an impact absorption energy of 100 J or more even at a low temperature of -40 캜, and may satisfy the flatness standard allowance of the KS D 3500 standard.

Hereinafter, a method for manufacturing a structural steel for building structure having excellent flatness, which is another preferred embodiment of the present invention, will be described in detail.

First, the steel slab satisfying the above composition is reheated to 1100 to 1200 占 폚.

The reheated slab is subjected to rough rolling at 900 to 1100 DEG C to obtain a bar.

If the rough rolling temperature is lower than 900 캜, the austenite may be deformed in a state in which recrystallization does not occur, and particles may become coarsened. When the rough rolling temperature exceeds 1100 캜, recrystallization occurs and particles grow, There is a fear that the particles become coarse.

The above bars are hot-rolled to obtain a hot-rolled steel sheet having a thickness of 30 mm or less.

The thickness of the hot-rolled steel sheet may be 10 to 30 mm, preferably 15 to 30 mm, and more preferably 20 to 25 mm.

The finish rolling temperature in the hot rolling is preferably 700 to 950 占 폚.

If the finishing rolling temperature is lower than 700 캜, the temperature of the plate material is low, and a load may be generated in the rolling mill, which may result in failure of rolling to the final thickness, and if it exceeds 950 캜, recrystallization may occur during rolling.

The reduction ratio in the finish rolling is preferably set to 50 to 80%.

If the finish rolling reduction ratio is less than 50%, there is a risk of equipment accidents due to an increase in the load acting on the material during rolling. If the finish rolling reduction ratio exceeds 80%, the rolling pass number increases, There is a fear that it will not be able to do.

The hot-rolled steel sheet is cooled to a cooling termination temperature below the Bs temperature at a cooling rate of 35 ° C / s or higher to produce a steel material.

According to the present invention, it is possible to specify a steel component and a range of composition (steel composition) according to the present invention and to secure a target microstructure and a fraction thereof while minimizing thermal stress generated during cooling, It is important to cool down with the cooling conditions.

That is, when the present invention the composition of the steel specified as described above, the continuous cooling curve of the steel is specified, so as gihayeo a given continuous cooling curve desired by controlling the cooling rate and cooling termination temperature of the microstructure, i.e., the area% 60 to 90% of bainitic ferrite and 10 to 40% of granular bainite.

In the present invention, excellent flatness must be ensured in addition to securing a desired microstructure. From the viewpoint of ensuring excellent flatness, It is advantageous to shorten the cooling time of the hot-rolled steel sheet.

As the cooling time becomes longer, the temperature unevenness in the longitudinal direction of the hot-rolled steel sheet becomes worse as the length of the hot-rolled steel sheet becomes longer, and the thermal stress generated thereby increases. As a result, the steel sheet is distorted such as twisting of the steel sheet. If such a deformation of the steel sheet occurs, it becomes difficult to secure an excellent flatness.

As described above, in the present invention, in a steel material having a thickness of 10 to 30 mm, It is possible to secure a microstructure including 60 to 90% of bainitic ferrite and 10 to 40% of granular bainite by area%, and to obtain a maximum peak of a surface of 20 mm or less, preferably a maximum peak of 15 mm or less The steel component and the range of the component are specified and, after hot rolling and rolling The cooling rate and the cooling termination temperature of the hot-rolled steel sheet are controlled to be not less than 35 DEG C / s and Bs temperature, respectively.

If the cooling rate of the hot-rolled steel sheet is less than 35 ° C / s, the cooling time becomes longer, and the temperature unevenness in the longitudinal direction of the hot-rolled steel sheet is generated. As a result of this temperature unevenness, thermal stress is generated, It is difficult to secure a good flatness.

If the cooling rate of the hot-rolled steel sheet is less than 35 DEG C / s, a large amount of ground martensite (MA) may be formed.

Thus, the lower limit of the cooling rate of the hot-rolled steel sheet is preferably limited to 35 ° C / s in consideration of securing excellent flatness and suppressing formation of ground martensite (MA).

On the other hand, the larger the cooling rate of the hot-rolled steel sheet is, the more advantageous in terms of securing excellent flatness and suppressing formation of ground martensite (MA), and if the desired microstructure and its fraction can be ensured, And is not particularly limited.

More preferably, the cooling rate of the hot-rolled steel sheet is 35 to 65 ° C / s, and the cooling rate of the hot-rolled steel sheet is more preferably 40 to 55 ° C / s.

After hot finish rolling When the cooling end temperature during cooling of the hot-rolled steel sheet is too high, the phase transformation of bainitic ferrite, which is the main structure of the present invention, is not initiated and the upper limit of the cooling termination temperature is limited to the Bs temperature Do.

If the cooling termination temperature is too low, there is a fear of exceeding the strength due to the development of the martensite structure. When the martensite structure develops as a main structure, sufficient strength is ensured but impact toughness is significantly deteriorated, making it difficult to use as a structural material.

More preferably, the cooling end temperature of the hot-rolled steel sheet is from 400 ° C to 600 ° C, and more preferably the cooling end temperature of the hot-rolled steel sheet is from 450 ° C to 550 ° C.

The cooling start temperature for cooling the hot-rolled steel sheet may preferably be 700 to 850 ° C.

The cooling may be performed by pressing several tons of water on the hot-rolled steel sheet and sprinkling it. When the cooling is performed in this manner, the desired properties of the steel material can be obtained in the present invention without additional cost and time.

Hereinafter, embodiments of the present invention will be described in detail. The following examples are intended to illustrate the preferred embodiments of the present invention and are not intended to limit the scope of the present invention.

(Example)

A steel slab having a composition of the following Table 1 was reheated at 1150 占 폚 and roughly rolled at 1050 占 폚 to obtain a bar having a thickness of 100 mm and then hot rolled at 900 占 폚 to obtain a hot- ≪ / RTI > Thereafter, the hot-rolled steel sheet was cooled at 790 ° C and cooled at the cooling rate and cooling termination temperature shown in Table 2 below to produce a steel material.

The microstructure, tensile strength, yield ratio, Charpy impact absorption energy (CVN @ -40 ° C) and flatness of the steel were measured, and the results are shown in Table 2 below.

The flatness in Table 2 was evaluated on the basis of the KS D 3500 standard. The flatness standard was "excellent" in the case of 15 mm or less, "good" in the case of 15 mm or more and 20 mm or less and " Respectively.

Here, the flatness "excellent" can be used without any additional calibration process, and "good" can be used in addition to a subsequent calibration process. As the calibration process, a press calibration process or the like can be used.

division C Si Mn P S Al Cr Ni Mo Ti Nb B N Bs (占 폚) Inventive Steel A 0.051 0.144 2.16 0.007 0.001 0.038 0.204 0.410 0.295 0.019 0.040 0.0017 0.0033 568 Invention steel B 0.055 0.160 2.65 0.007 0.001 0.029 0.202 0.801 0.300 0.015 0.038 0.0013 0.0042 508 Inventive Steel C 0.050 0.155 2.20 0.008 0.001 0.027 0.202 0.395 0.150 0.015 0.039 0.0012 0.0037 577 Inventive Steel D 0.051 0.150 2.45 0.008 0.001 0.030 0.203 0.401 0 0.016 0.041 0.0012 0.0035 567 Comparative Steel E 0.12 0.21 1.53 0.014 0.003 0.034 0 0 0 0.013 0.03 0.0021 0.0035 689 Comparative Steel F 0.080 0.25 1.55 0.010 0.002 0.024 0 0 0 0.01 0.01 0.0002 0.0028 669

division
Microstructure (area%) Bs temperature
(° C) Cooling end temperature
(° C) Cooling rate
(° C / s) The tensile strength
(MPa) Yield ratio
CVN @ -40 C (J) flatness
(mm) Remarks BF GB MA Inventive Steel A

A-1 69.1 29.7 1.2
568

495 45.1 841 0.77 275 14.4 Great A-2 58.4 38.3 3.3 496 29.5 843 0.78 152 24.6 Bad A-3 86.7 12.6 0.7 505 50.1 835 0.79 241 6.3 Great A-4 65.4 32.0 2.6 551 60.6 819 0.74 217 18.3 Good Invention steel B
B-1 88.0 11.2 0.8
508
404 54.2 897 0.83 108 19.0 Good B-2 72.6 26.9 0.5 480 50.8 885 0.82 224 9.1 Great B-3 67.0 31.9 1.1 494 45.1 876 0.83 180 12.0 Great Inventive Steel C

C-1 61.3 35.8 2.9
577

527 25.4 833 0.79 124 21.6 Bad C-2 73.0 26.0 1.0 527 46.5 857 0.81 234 12.0 Great C-3 68.0 30.5 1.5 528 38.8 823 0.74 169 17.3 Good C-4 85.2 11.0 3.8 432 22.5 817 0.75 275 27.7 Bad Inventive Steel D

D-1 63.7 33.8 2.5
567

507 25.5 846 0.79 249 24.1 Bad D-2 67.2 31.1 1.7 481 37.4 859 0.76 214 19.0 Good D-3 79.3 20.5 0.2 498 54.7 865 0.74 237 8.8 Great D-4 76.1 23.2 0.7 489 45.4 866 0.78 196 12.6 Great Comparative Steel E

E-1 B: 93.1
M: 2.2
F: 3.1 1.6


689 498 25.5 787 0.71 108 25.0 Under-strength E-2 B: 96.5
F: 3.5 0 541 26.8 763 0.71 189 19.4 Under-strength E-3 B: 98.3
F: 1.7 0 538 47.3 777 0.70 131 13.2 Under-strength E-4 B: 93.6
F: 6.4 0 580 37.1 759 0.69 111 10.1 Under-strength Comparative Steel F
F-1 B: 86.2
M: 7.1
F: 5.7 1.0

669 485 26.4 774 0.71 184 13.8 Under-strength F-2 M: 98.7
F: 1.0 0.3 438 57.2 981 0.76 91 11.0 Under shock F-3 B: 95.4
F: 4.6 0 523 31.3 759 0.70 310 12.9 Under-strength

B: ferrite, B: bainite, M: martensite, Bs: bainite transformation initiation temperature) in the same manner as in Example 1 (BF: bainitic ferrite, GB: granular bainite, MA:

As shown in Table 2, the steel material produced according to the steel composition and manufacturing conditions in accordance with the present invention has high strength and high toughness [Charpy impact energy (CVN @ -40 ° C)], It can be understood that the allowable value is satisfied.

While the present invention has been particularly shown and described with reference to the particular embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims (12)

0.002 to 0.08% of C, 0.01 to 0.6% of Si, 1.5 to 3.0% of Mn, 0.02% of P, Nb: 0.005 to 0.1%, B: 5 to 40 ppm, Ti: 0.005 to 0.1%, N: 15 to 150 ppm, Cr: 0.1 to 1.0%, Ni: 0.01 to 2.0%, the balance being Fe and unavoidable impurities / RTI >
The microstructure consists of 60 to 90% of bainitic ferrite, 10 to 40% of granular bainite and 5% or less (inclusive of 0%) of ground martensite (MA) in area percentages;
The maximum peak height of the surface is 20 mm or less; And
Structural structural steel with excellent flatness with thickness of 10 ~ 30mm. delete The method according to claim 1,
Wherein the steel has a carbon equivalent (Ceq.) Value of 0.60 or less as defined by the following relational expression (1) and a weld crack susceptibility index (Pcm.) Value of 0.30 or less as defined by the following relational expression (2) Excellent structural steel for construction.

[Relation 1]
Carbon equivalent (Ceq.) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15
Here, C, Mn, Cr, Mo, V, Cu, and Ni are values indicating the content of each element in weight%
[Relation 2]
The weld crack susceptibility index (Pcm.) = C + (Mn + Cr + Cu) / 20 + Si / 30 + Ni / 60 + Ti / 10 + Mo /
(Where C, Mn, Cr, Cu, Si, Ni, Ti, Mo, and B represent the content of each element in weight% The steel according to claim 1, wherein the steel further comprises at least one selected from the group consisting of Mo: 0.1 to 1.0%, Cu: 0.01 to 1.0%, and V: 0.005 to 0.3% Excellent structural steel for construction. 0.002 to 0.08% of C, 0.01 to 0.6% of Si, 1.5 to 3.0% of Mn, 0.02% of P, Nb: 0.005 to 0.1%, B: 5 to 40 ppm, Ti: 0.005 to 0.1%, N: 15 to 150 ppm, Cr: 0.1 to 1.0%, Ni: 0.01 to 2.0%, the balance being Fe and unavoidable impurities Reheating the steel slab to 1100 to 1200 ° C; Subjecting the reheated slab to rough rolling at 900 to 1100 ° C to obtain a bar;
Hot-rolling the bar to obtain a hot-rolled steel sheet having a thickness of 10 to 30 mm; and
And cooling the hot-rolled steel sheet to a cooling end temperature not higher than the Bs temperature (bainite transformation start temperature) at a cooling rate of not less than 35 ° C / s, wherein the bainitic ferrite having a surface area of 60 to 90% Of granular bainite and 5% or less (inclusive of 0%) of ground martensite (MA). The method of producing a steel for building structure having excellent flatness is also provided. 6. The method of claim 5,
Wherein the steel slab has a carbon equivalent (Ceq.) Value of 0.60 or less as defined by the following relational expression (1) and a weld crack susceptibility index (Pcm.) Value as defined by the following relational expression (2) A method for manufacturing a structural steel for structural use.

[Relation 1]
Carbon equivalent (Ceq.) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15
Here, C, Mn, Cr, Mo, V, Cu, and Ni are values indicating the content of each element in weight%
[Relation 2]
The weld crack susceptibility index (Pcm.) = C + (Mn + Cr + Cu) / 20 + Si / 30 + Ni / 60 + Ti / 10 + Mo /
(Where C, Mn, Cr, Cu, Si, Ni, Ti, Mo, and B represent the content of each element in weight% 6. The method of claim 5,
The steel slab further comprises at least one selected from the group consisting of Mo: 0.1 to 1.0%, Cu: 0.01 to 1.0%, and V: 0.005 to 0.3%. ≪ / RTI > 6. The method of claim 5, wherein the finish rolling temperature during the hot rolling is 700 to 950 占 폚. 6. The method of claim 5,
Wherein the finish rolling reduction ratio during the hot rolling is 50 to 80%. 6. The method of claim 5,
Wherein the cooling start temperature during cooling of the hot-rolled steel sheet is 700 to 850 ° C. 6. The method of claim 5,
Wherein the cooling rate during cooling of the hot-rolled steel sheet is 40 to 55 ° C / s. 6. The method of claim 5,
Wherein the cooling end temperature during cooling of the hot-rolled steel sheet is 450 to 550 ° C.

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