Steel and method for manufacturing same

Abstract

The present invention relates to: steel that can be used for a wind power generator having a monopile or jacket structure; and a method for manufacturing same.

Description

Steel and its manufacturing method

[0001] The present invention relates to a steel material that can be used in a wind turbine having a monopile or jacket structure, and a method for manufacturing the same.

[0002] As the height of structures used in onshore and offshore wind turbines has increased, the demand for ultra-thick, high-strength steels with excellent load-resisting capabilities has increased. At the same time, impact toughness at low temperatures and aging impact toughness are also required when deformation such as bending is involved.

[0003] To achieve high strength and excellent low-temperature impact toughness and aging impact toughness, a process of refining grains is essential. However, when manufacturing ultra-thick steels through the rolling process, there is a limitation in grain refinement because sufficient compressive force is not transmitted to the center of the thickness. Furthermore, the voids existing in the steel slab are often not sufficiently compressed during the rolling process, and segregation zones existing in the center of the steel slab thickness after continuous casting frequently result in a deterioration in internal quality even after the final hot-rolled steel sheet is manufactured.

[0004] Due to the thickening of steel used for the structure of wind turbines, it is difficult to secure the properties of the center of the thickness mentioned above, while cases requiring the strength of the center of the thickness, low-temperature impact toughness, and excellent internal quality are actually increasing.

[0005] In order to secure the properties and internal quality of the central portion of the thickness, it is necessary to appropriately add alloying elements that are advantageous for grain refinement and optimize the continuous casting process to manufacture a steel slab with sound internal structure. In addition, it is necessary to control the type and fraction of microstructure so that both the target strength and low-temperature impact toughness can be satisfied by optimizing rolling and accelerated cooling.

[0006] Patent Document 1 proposes a method of increasing the reduction ratio per pass during rough rolling to at least 10% to ensure sufficient pressure is applied to the center of the hot-rolled steel sheet, and then allowing sufficient air cooling time before finishing rolling to apply additional pressure to the center when the surface area hardened by the temperature drop is rolled, thereby ensuring the quality of the center. However, this method has the disadvantage of having limitations in ensuring the quality of the center when segregation defects or large voids exist within the steel slab.

[0007] (Patent Document 1) Korean Patent Publication No. 10-2023-0102791

[0008] One aspect of the present invention is to provide a steel having excellent strength in the center, impact toughness and internal quality in an extremely thick steel, and a method for manufacturing the same.

[0009] The objectives of the present invention are not limited to the above-described matters. Additional objectives of the present invention are described throughout the specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the contents described in the specification.

[0010] An example of the present invention comprises, in wt%, carbon (C): 0.03 to 0.08%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 1.0 to 1.6%, phosphorus (P): 0.01% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.01 to 0.05%, niobium (Nb): 0.015 to 0.035%, chromium (Cr): 0.3% or less (excluding 0%), nickel (Ni): 0.1 to 0.5%, titanium (Ti): 0.01 to 0.02%, nitrogen (N): 0.002 to 0.01%, and the remainder comprises iron and inevitable impurities.

[0011] The microstructure is composed of 60-85% ferrite, 10-20% bainite, and the remainder pearlite and unavoidable structures in area fraction.

[0012] This relates to steel having an average size of effective grains of 20㎛ or less and having high-angle grain boundaries of 15° or more.

[0013] The above steel is 100mm 2 The number of residual pores having the above size may be 1 or less.

[0014] The above steel may have a yield strength of 320 MPa or more at points t / 4 and t / 2 of thickness (t).

[0015] The above steel may have a tensile strength of 430 MPa or more and 590 MPa or less at points t / 4 and t / 2 of thickness (t).

[0016] The above steel may have a Charpy impact absorption energy of 100J or more at -50℃.

[0017]

[0018] Another example of the present invention is a step for manufacturing a steel slab comprising, in wt%, carbon (C): 0.03 to 0.08%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 1.0 to 1.6%, phosphorus (P): 0.01% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.01 to 0.05%, niobium (Nb): 0.015 to 0.035%, chromium (Cr): 0.3% or less (excluding 0%), nickel (Ni): 0.1 to 0.5%, titanium (Ti): 0.01 to 0.02%, nitrogen (N): 0.002 to 0.01%, and the remainder being iron and inevitable impurities;

[0019] A step of heating the above steel slab in a temperature range of 1000 to 1080°C;

[0020] A step of rough rolling the above-mentioned heated steel slab at a temperature range of 900 to 1000°C;

[0021] A step of finishing rolling at a temperature range of Ar3 or higher and Tnr or lower after the above rough rolling; and

[0022] Cooling step at a cooling rate of 2 to 5°C / s to a temperature of 350 to 500°C

[0023] It relates to a method for manufacturing steel including .

[0024] The above steel slab is manufactured by continuous casting of molten steel, and during the continuous casting, a light pressure (unit: mm) of 3 mm or more and 6 mm or less is applied, and after solidification, an additional pressure of 3 mm or more and 10 mm or less is applied, thereby manufacturing a steel slab having a thickness of 290 mm or more and 397 mm or less.

[0025] The hydrogen concentration in the above molten steel may be 1.5 ppm or less.

[0026] When manufacturing the above steel slab, a molten steel degassing process (RH process) can be performed before continuous casting, and the RH process can be performed for 15 minutes or more under conditions of 2 torr or less.

[0027] According to one aspect of the present invention, it is possible to provide an extremely thick steel material for wind power structures having excellent low-temperature impact toughness and aging impact toughness in addition to the strength of the central portion.

[0028] The various advantageous and beneficial advantages and effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0029] Figure 1 is an optical microscope photograph of the microstructure of a steel material having a thickness of 120 mm and a component of Invention Example 1 at a point t / 2 in thickness.

[0030] Figure 2 is an EBSD result observing the microstructure at a point of thickness t / 2 of a steel material having a thickness of 120 mm and a component of Invention Example 1.

[0031] The terminology used herein is for the purpose of describing the present invention and is not intended to limit the present invention. Furthermore, the singular forms used herein also include the plural forms, unless the context clearly dictates otherwise.

[0032] The meaning of "comprising" as used in the specification is to specify a configuration and not to exclude the presence or addition of other configurations.

[0033] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in the dictionary are to be interpreted to have meanings consistent with the relevant technical literature and the present disclosure.

[0034] The inventors of the present invention recognized the need for methods to secure the required properties of wind turbine structural steels, such as monopiles and jackets, as they became larger and required more stringent internal quality. In particular, they conducted in-depth research into methods for securing core strength, low-temperature impact toughness, and aging impact toughness in wind turbine structural steels exceeding a certain thickness. As a result, they confirmed that by controlling the composition and the relationship between certain components in the alloy design while optimizing manufacturing conditions, they could provide wind turbine structural steels with the desired properties, leading to the completion of the present invention. For example, a steel slab with excellent internal integrity can be manufactured using the Thermo-Mechanical Controlled Process (TMCP) method.

[0035] First, an example of the steel material of the present invention will be described in detail. Below, the alloy composition and range of the steel material will be described. Unless otherwise specified, the alloy composition content described below refers to weight percent.

[0036] The above steel may contain, in wt%, carbon (C): 0.03 to 0.08%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 1.0 to 1.6%, phosphorus (P): 0.01% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.01 to 0.05%, niobium (Nb): 0.015 to 0.035%, chromium (Cr): 0.3% or less (excluding 0%), nickel (Ni): 0.1 to 0.5%, titanium (Ti): 0.01 to 0.02%, and nitrogen (N): 0.002 to 0.01%.

[0037] Carbon (C): 0.03~0.08%

[0038] Carbon (C) is an effective element for enhancing the hardenability of steel and thus improving its strength. To fully achieve this effect, the C content may be 0.03% or more. However, if the content exceeds 0.08%, the high hardenability may result in the presence of a high fraction of bainite and coarse carbides within it, which may significantly reduce the impact toughness. In addition, if the content is less than 0.03%, it is insufficient to secure strength. Therefore, the C content may be 0.03 to 0.08%, and more advantageously, it may be 0.03 to 0.05%.

[0039] Silicon (Si): 0.1~0.5%

[0040] Silicon (Si) is not only used as a deoxidizer, but is also an element that is beneficial for improving the strength and toughness of steel. To fully achieve the aforementioned effects, the Si content may be 0.1% or more. However, if the content exceeds 0.5%, there is a risk of excessive formation of martensite (MA), which may deteriorate low-temperature impact toughness. Therefore, the Si content may be 0.1 to 0.5%.

[0041] Manganese (Mn): 1.0~1.6%

[0042] Manganese (Mn) is an element that is advantageous in improving the strength of steel through its solid solution strengthening effect. To fully achieve this effect, the Mn content may be 1.0% or more. However, if the content exceeds 1.6%, it combines with sulfur (S) in the steel to form MnS, which significantly reduces the low-temperature impact toughness and aging impact toughness of the core. Therefore, the Mn content may be 1.0 to 1.6%, and more advantageously, 1.4 to 1.6%.

[0043] Phosphorus (P): 0.01% or less

[0044] Phosphorus (P) is an element that is beneficial for improving the strength and corrosion resistance of steel. However, it can significantly impair the impact toughness of steel, so it is desirable to limit its content to the lowest possible. In the present invention, even if P is contained at a maximum of 0.01%, the desired physical properties can be secured without difficulty, and thus the content can be limited to 0.01% or less. However, considering the level of unavoidable addition, 0% may be excluded.

[0045] Sulfur (S): 0.003% or less

[0046] Sulfur (S) is an element that significantly reduces the hydrogen-induced cracking resistance and impact toughness of steel by combining with manganese (Mn) in steel to form MnS, etc. Therefore, it is advantageous to limit the S content to the lowest possible. In the present invention, even if the S content is contained up to 0.003%, the target physical properties can be secured without any problem, so the content can be limited to 0.003% or less. However, considering the level of unavoidable addition, 0% can be excluded.

[0047] Aluminum (Al): 0.01~0.05%

[0048] Aluminum (Al) is an element that can deoxidize molten steel inexpensively. To sufficiently obtain the above-described effect, Al may be included in an amount of 0.01% or more. However, if the content is excessive and exceeds 0.05%, it is not only undesirable because it may cause nozzle clogging during continuous casting, but also significantly reduce impact toughness due to the formation of Al-based oxidizing inclusions. Therefore, Al may be included in an amount of 0.01 to 0.05%.

[0049] Niobium (Nb): 0.015–0.035%

[0050] Niobium (Nb) precipitates in the form of NbC or Nb(C,N), which significantly improves the strength of the base material, and when reheated at a high temperature, the dissolved Nb suppresses recrystallization of austenite and transformation of ferrite or bainite, thereby achieving a structure refinement effect. However, if the content is excessive, undissolved Nb is formed in the form of TiNb(C,N), which becomes a factor that deteriorates UT defects and low-temperature impact toughness, so it is preferable to limit the upper limit of the Nb to 0.035%. Therefore, in the present invention, the Nb may be included in an amount of 0.015 to 0.035%, and more advantageously, it may be included in an amount of 0.025 to 0.03%.

[0051] Nickel (Ni): 0.1~0.5%

[0052] Nickel (Ni) is an element that can simultaneously improve the strength and low-temperature impact toughness of a base material. However, as it is an expensive element, if its content exceeds 0.5%, economic feasibility is significantly reduced. Therefore, Ni may be included at 0.5% or less. However, to achieve the effects of Ni, it is preferable to add at least 0.1%.

[0053] Titanium (Ti): 0.01~0.02%

[0054] Titanium (Ti) is preferably added at least 0.01%, as it forms TiN when added together with N, thereby reducing the occurrence of surface cracks due to the formation of AlN precipitates. However, if the content exceeds 0.02%, coarse TiN is formed during reheating of the steel slab, which acts as a factor that lowers the low-temperature impact toughness. Therefore, the Ti content is preferably 0.01 to 0.02%, and more preferably 0.01 to 0.015%.

[0055] Nitrogen (N): 0.002~0.01%

[0056] Nitrogen (N), when added together with Ti, forms TiN, which is an element that is advantageous in suppressing grain growth due to heat influence during welding. In order to sufficiently obtain the above-described effect when adding Ti, N may be included in an amount of 0.002% or more. However, if the content exceeds 0.01%, coarse TiN is formed, which is not desirable as it deteriorates low-temperature impact toughness. Therefore, the N content is preferably 0.002 to 0.01%.

[0057] Chromium (Cr): 0.3% or less (excluding 0%)

[0058] Chromium (Cr) is an effective element that not only increases the hardenability of steel, thereby increasing the fraction of low-temperature phases such as bainite, but also enhances strength by forming fine Cr-based carbides. However, if its content exceeds 0.3%, coarse bainite and the carbides present within it can significantly deteriorate low-temperature impact toughness and aging impact toughness. Therefore, it is preferable that the Cr content be 0.3% or less.

[0059] The remaining components are iron (Fe), and may contain some unintended and unavoidable impurities introduced during the manufacturing process. Since these impurities are readily apparent to anyone skilled in the art of manufacturing, their full details are not specifically mentioned in this specification.

[0060] The above steel may contain, in area %, 60 to 85% ferrite, 10 to 20% bainite, and the remainder pearlite and unavoidable structures.

[0061] The above steel is 15 o It is preferable that the average size of effective crystal grains having the above high-angle grain boundaries be 20㎛ or less.

[0062] If the above ferrite fraction is less than 60% and the bainite fraction exceeds 20%, the impact toughness may be significantly reduced due to excessively high strength, and conversely, if the ferrite fraction exceeds 85% and the bainite fraction is too low, less than 10%, the strength may be insufficient.

[0063] Meanwhile, if the average size of the effective crystal grains having the high-angle grain boundaries exceeds 20 ㎛, the impact toughness may be significantly reduced. 15 o As an example of measuring effective grains with high-angle grain boundaries, this can be confirmed through EBSD (Electron Backscatter Diffraction).

[0064] Meanwhile, the above steel is 100mm 2 It is preferable that the number of residual pores having the above size be 1 or less. The residual pores affect the internal quality. The steel can be measured through ultrasonic testing. The residual pores are usually present in the center of the slab, but they remain when sufficient pressing force is not transmitted to the center during rolling. Therefore, they are generally present in the center of the hot-rolled steel sheet from 1 / 4t in thickness.

[0065] 100mm above 2 If there is more than one residual void with the above size, accidents such as breakage due to internal defects may occur when the hot-rolled steel plate is used as a structure. Therefore, the above 100 mm 2 It is desirable that there be no residual pores with the above size, so it is desirable that there be no more than one.

[0066] Meanwhile, it is preferable that the residual void existing inside the steel material be 30 mm or less based on the long side.

[0067] The thickness of the above steel material may be 100 mm or more and 150 mm or less.

[0068] The above steel may have excellent strength and low-temperature impact toughness, with a yield strength of 320 MPa or more, a tensile strength of 430 MPa or more and 590 MPa or less, and a Charpy impact absorption energy (CVN) value of 100 J or more, evaluated in the rolling direction at points t / 4 and t / 2 in the thickness direction (wherein t represents the steel thickness (mm)).

[0069] The above steel can have excellent aging impact toughness with a Charpy impact absorption energy (CVN) value of 50 J or more at -50°C for a specimen that was subjected to tensile strain of 8% perpendicular to the rolling direction at points t / 4 and t / 2 in the thickness direction (where t represents the steel thickness (mm)) and then aged at 250°C for 60 minutes.

[0070] Next, an example of the method for manufacturing the steel of the present invention will be described in detail.

[0071] The above manufacturing method may include the steps of manufacturing a steel slab including the above-described alloy composition, heating the steel slab, rolling the heated steel slab, and cooling the steel slab. This will be described in detail below.

[0072] Steel slab manufacturing

[0073] In the present invention, in order to secure the core strength, low-temperature impact toughness, aging impact toughness, and internal quality of the final steel material, it is desirable to secure a steel slab with excellent internal quality. As an example of a method for manufacturing the steel slab, the steel slab can be manufactured by continuously casting molten steel that has undergone a steelmaking process.

[0074] As a desirable example, during the steelmaking process, after lifting the molten steel ladle, a vacuum degassing process (RH process) is performed in which the inside of the vessel is vacuumized and then a reflux gas is introduced into the vessel through an immersion tube to reflux the molten steel to perform degassing or decarburization, etc., so that the hydrogen concentration in the molten steel is 1.5 ppm or less. It is desirable that the RH process be performed for 15 minutes or more under conditions of 2 torr or less. If the process is performed for less than 15 minutes under conditions of 2 torr or less, sufficient degassing treatment will not be performed, resulting in an excessively high hydrogen concentration, which may leave coarse pores inside the final steel product, potentially adversely affecting its internal quality. On the other hand, although there is no specific upper limit, it is desirable to perform the RH process for less than 30 minutes, as exceeding 30 minutes may cause a load on the manufacturing process.

[0075] A steel slab can be manufactured by continuous casting of molten steel having a hydrogen concentration of 1.5 ppm or less. During the continuous casting process, a pressure reducing process of 3 mm or more and 6 mm or less can be performed in a solid / liquid coexistence region. By performing the pressure reducing process, segregation in the center of the steel slab can be minimized. At this time, it is possible to selectively perform a process of applying EMS (Electro Magnetic Stirring) to suppress the growth of dendrites and induce the formation of equiaxed crystals to suppress segregation in the center. If the pressure reducing process is less than 3 mm, the Mn segregation zone existing in the center of the steel slab may not be sufficiently pushed out into the liquid phase, which may cause a problem in which the segregation zone remains. On the other hand, if the pressure reducing process is excessive, exceeding 6 mm, cracks may occur during slab manufacturing.

[0076] Meanwhile, after the above-mentioned pressure reduction and solidification are completed, the size of the voids inside the steel slab can be minimized through additional pressure reduction of 3 mm to 10 mm. Through this, the residual voids in the final steel material can be reduced to 100 mm in area. 2It is desirable to make it below. In addition, the residual voids should not exceed 30 mm based on the long side, and it is desirable that the number of residual voids exceeding this size is no more than 1. If the additional pressing force after the above solidification is less than 3 mm, the size of the voids existing inside the steel slab may not be sufficiently reduced, which may have a negative effect on the internal quality of the final steel material. In addition, if the additional pressing force exceeds 10 mm, although it is effective in reducing the voids, excessive pressing force may be applied, which may cause surface cracks.

[0077] steel slab heating

[0078] It is preferable to go through a process of heating and homogenizing the above steel slab, and it is preferable to heat it in the temperature range of 1000 to 1080℃. If the heating temperature of the above steel slab is less than 1000℃, the precipitates (carbon and nitride) formed within the slab are not sufficiently re-dissolved, so that the formation of precipitates is reduced in the process after hot rolling, and ultimately, it becomes difficult to satisfy the yield strength and tensile strength proposed in the present invention. On the other hand, if the temperature exceeds 1080℃, there is a concern that the austenite grains may become coarser, which may deteriorate the physical properties of the steel.

[0079] Meanwhile, it is preferable that the heating time be 2 hours or longer. If the heating time of the steel slab is less than 2 hours, there is not enough time for the center of the slab to be sufficiently heated. Although there is no upper limit, it is preferable that it be less than 6 hours at most, taking into account the load in the manufacturing process.

[0080] hot rolling

[0081] The above-mentioned heated steel slab can be hot rolled to produce a hot-rolled steel sheet. The above-mentioned heated steel slab can be rough-rolled at a temperature range of 900 to 1000°C, and then finish-rolled at a temperature below Tnr, and then finished hot-rolled at Ar3 or higher.

[0082] The above Tnr and Ar3 can be defined as follows.

[0083] Tnr(℃)=887+(464×C)+((6445×Nb)-(644×Nb 0.5 ))+(890×Ti)+(363×Al)-(357×Si)

[0084] Ar3(℃)=910-310×C-80×Mn-55×Ni+124×Ti-18×Nb+179×Al

[0085] (Here, each element represents its weight content.)

[0086] If the temperature during the above rough rolling is below 900°C, there is a problem that the temperature during the subsequent finishing hot rolling becomes too low. If the above finishing rolling starts above the Tnr temperature, there is a disadvantage that coarse and fine grains may coexist, intermittently lowering the individual values ​​of impact toughness, even if the subsequent finishing temperature is below the Tnr temperature. If the above finishing hot rolling temperature is below Ar3, there is a concern that the rolling load will increase, which may cause quality defects such as surface cracks.

[0087] cooling

[0088] After hot rolling, it can be cooled to a temperature range of 350 to 500°C at a cooling rate of 2 to 5°C / s. If the cooling end temperature is less than 350°C, it may be advantageous in securing strength, but due to the nature of the extremely thick material, excessive oscillation work may be performed, which may cause a serious load in the manufacturing process. On the other hand, if the cooling end temperature exceeds 500°C, it is difficult to secure strength. If the cooling rate is less than 2°C / s during the cooling, the fraction of polygonal ferrite increases excessively, making it difficult to secure strength. If it exceeds 5°C / s, it may be advantageous in securing strength, but the increased flow rate during water cooling may cause a severe overload in the manufacturing process, making it difficult to apply to mass production. It may be preferably 2.0 to 5.0°C / s.

[0089] Hereinafter, embodiments of the present invention will be described. It should be apparent to those skilled in the art that various modifications to the following embodiments may be made without departing from the scope of the present invention. The following embodiments are intended to facilitate understanding of the present invention, and the scope of the present invention should not be limited to the following embodiments, but should be determined not only by the claims set forth below but also by their equivalents.

[0090] (Example)

[0091] A steel slab was manufactured by continuous casting of molten steel having the alloy composition (weight %, the remainder being Fe and unavoidable impurities) and hydrogen concentration in the molten steel shown in Table 1 below. The unit of each component is weight %, the remainder being Fe and unavoidable impurities, and Tnr and Ar3 were derived from the following equations.

[0092] Tnr(℃)=887+(464×C)+((6445×Nb)-(644×Nb 0.5 ))+(890×Ti)+(363×Al)-(357×Si)

[0093] Ar3(℃)=910-310×C-80×Mn-55×Ni+124×Ti-18×Nb+179×Al

[0094] (Here, each element represents the content (weight %).)

[0095] When manufacturing the above steel slab, a pressure of 4.5 mm was applied, and the additional pressure after solidification was disclosed under the conditions of Table 2 below. In Inventive Examples 1 and 2 and Comparative Examples 1 to 3, rough rolling was performed at 900 to 1000°C, and in Comparative Example 4, rough rolling was performed at over 1000°C. The manufactured steel slab was heated, hot rolled, and cooled under the conditions disclosed in Table 2 to manufacture steel.

[0096] steel grade CSiMnPSAlNbCrNiTiNHTnrAr310.0400.151.550.0080.0020.0250.0300.1500.4500 .0120.00351.195475420.0350.201.550.0080.0020.0250.0250.1000.4000.0120. 00351.291175930.0650.201.550.0080.0020.0300.0250.1000.0000.0120.00351.192777240.0850.151.450.0080.0020.0250.0350.1500.3000.0120.00351.3998756

[0097] Classification Steel grade Slab thickness (mm) Additional pressure after solidification (mm) Slab heating temperature ( o C) Cumulative pressure reduction (%) Finishing rolling start temperature ( o C) Finishing rolling end temperature ( o C) Cooling start temperature ( o C) Cooling end temperature ( o C) Cooling rate ( o C / s) Invention example 1129551050408077917804302.739551080308107957904502.5 Invention example 2229551050408007907884502.739551080308057927904502,5 Comparative example 1329551050408107907804302.739551080308047897854502.5 Comparative example 2429551050408007907854302.739551080308057957854502.5 Comparative example 3130001050408107907804302.740001080308047897854502.5 Comparative example 41295511104010209209004303.2395511003010259409204502.8

[0098] Afterwards, the microstructure of the manufactured steel was observed and the mechanical properties were evaluated. The microstructure was observed using EBSD and an optical microscope, and then an image analysis program was used. Through this, the effective average grain size of high-angle grains with grain boundary angles of 15° or more was measured. Ferrite was classified as a phase with misorientation of 5° or less within the high-angle grains, and bainite was classified as a phase with irregular sub-boundaries within the grains. Pearlite was defined as the black portion in the optical microscope image.

[0099] At this time, the microstructure was measured at the point t / 2 (t: thickness, mm) in the thickness direction of each steel material, and the results are shown in Table 3 below. In addition, the mechanical properties of each steel material were evaluated, and the results are shown in Table 4. At this time, the tensile specimen was taken at the point t / 2 (t: thickness, mm) in the thickness direction in the direction perpendicular to the rolling direction, and the tensile strength (TS), yield strength (YS), and elongation (El) were measured at room temperature, and the impact specimen was taken from the JIS No. 4 standard test piece at the point t / 2 in the thickness direction in the rolling direction, and the impact toughness (Charpy impact toughness energy, CVN) at -50℃ was measured three times, and the average value and the minimum and maximum ranges were indicated in parentheses. In addition, the impact toughness (aging impact toughness) at -50℃ was evaluated using an impact specimen of JIS No. 4 standard for a specimen that was heat-treated at 250℃ for 60 minutes after applying 8% tensile strain perpendicular to the rolling direction, and the results are also shown in Table 4 below.

[0100] In addition, the residual void was measured through ultrasonic testing at the point t / 2 (t: thickness, mm) in the thickness direction, 100 mm 2 The number of residual pores with excess size is also shown in Table 4.

[0101] Note Steel grade Steel plate thickness (mm) Ferrite (%) Bainite (%) Pearlite (%) Effective grain average size (㎛) Invention example 111207020101415075151016 Invention example 221207020101515075151016 Comparative example 131207020102115075151024 Comparative example 241205535102215055301523 Comparative example 311207020101415075151016 Comparative example 4112055405221505540524

[0102] As shown in Table 3 above, inventive examples 1 to 2 manufactured by the alloy composition, component relationship, and manufacturing conditions proposed in the present invention satisfy the fractions of ferrite, bainite, and pearlite and the effective grain size proposed in the present invention. On the other hand, comparative examples 1 to 3 satisfy the fractions of ferrite, bainite, and pearlite, but do not satisfy the effective grain size, and it can be confirmed that comparative example 4 deviates from the values ​​proposed in the present invention in terms of not only the fractions of ferrite and bainite but also the effective grain size.

[0103] Classification Steel grade Steel plate Thickness (mm) YS (MPa) TS (MPa) El. (%) Low temperature impact toughness (J@-50 o C) Age-related impact toughness (J@-50) oC) Number of residual pores (units) Invention example 1112041551233345 (319~362)315 (297~334)015039850135322 (289~355)295 (284~308)0 Invention example 2212040150834355 (323~372)309 (288~324)015039249736330 (291~360)282 (272~299)0 Comparative example 1312042352432124 (14~209)89 (10~124)015041951933112 (7~165)100 (13~145)0 Comparative example 2412043755528142 (12~212)121 (9~156)015043054630145 (11~184)84 (10~126)0Comparative Example 3112041551233215 (17~342)188 (10~312)415039850135322 (289~355)295 (284~308)7Comparative Example 4112044253327154 (11~269)114 (8~209)015043553029146 (15~195)98 (10~184)0

[0104] Table 4 shows the yield strength, tensile strength, elongation, low-temperature impact toughness, aging impact toughness, and the area of ​​100 mm inside the hot-rolled steel sheet for Invention Examples 1 and 2 and Comparative Examples 1 to 4. 2 The number of residual pores with an excess size was shown. In the case of Invention Examples 1 and 2 and Comparative Examples 1 to 4, it can be confirmed that the tensile properties presented in the present invention are satisfied without difficulty. Specifically, in the case of Invention Examples 1 and 2, not only the tensile properties but also the average and individual values ​​of low-temperature impact toughness and aging impact toughness showed excellent characteristics, and it was confirmed that excellent internal quality could be secured because there were no residual pores due to the additional pressing process after solidification.

[0105] On the other hand, in the case of Comparative Example 1, it can be confirmed that the minimum value of impact toughness is inferior according to the coarse effective grain size, and in particular, it can be confirmed that the aging impact toughness is greatly reduced as it is a steel material to which Ni, which is useful for reducing the effective grain size, is added. In the case of Comparative Example 2, due to the excessive addition of C, the yield / tensile strength may satisfy the values ​​presented in the present invention, but it can be confirmed that the minimum values ​​of impact toughness and aging impact toughness do not satisfy the values ​​presented in the present invention. In the case of Comparative Example 3, although both the components and rolling / cooling conditions presented in the present invention are satisfied, it is a case where additional reduction after solidification during the steel slab manufacturing process is not applied, and although the tensile properties are satisfactory, it can be confirmed that the minimum values ​​of impact toughness and aging impact toughness are inferior, and it can be confirmed that the residual pores are not sufficiently compressed and remain in a coarse form. In the case of Comparative Example 4, the components also satisfy the range proposed in the present invention, but due to the high slab heating temperature, the austenite grains become coarser and the hardenability increases, so the fraction of bainite increases, and since the rolling start temperature is applied above the Tnr temperature, the grains cannot be refined to a sufficient level, so the strength is improved, but it can be confirmed that the minimum values ​​of the impact toughness and the aging impact toughness are greatly reduced.

[0106] Figure 1 is an optical microscope photograph of the microstructure at a point t / 2 of the thickness of a steel material having a thickness of 120 mm and a component of Invention Example 1, and it can be confirmed that it is a structure in which ferrite, bainite, and some pearlite are mixed.

[0107] Figure 2 is a photograph showing the EBSD microstructure at a point t / 2 of the steel material having a thickness of 120 mm and a component of Invention Example 1, where (a) shows ferrite and (b) shows bainite.

Claims

1. Contains, in wt%, carbon (C): 0.03 to 0.08%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 1.0 to 1.6%, phosphorus (P): 0.01% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.01 to 0.05%, niobium (Nb): 0.015 to 0.035%, chromium (Cr): 0.3% or less (excluding 0%), nickel (Ni): 0.1 to 0.5%, titanium (Ti): 0.01 to 0.02%, nitrogen (N): 0.002 to 0.01%, and the remainder includes iron and inevitable impurities. The microstructure is composed of 60-85% ferrite, 10-20% bainite, and the remainder pearlite and unavoidable structures in area fraction. Steel having an average effective grain size of 20㎛ or less and a high-angle grain boundary of 15° or more.

2. In claim 1, The above steel is 100mm 2 Steel having one or less residual pores having the above size.

3. In claim 1, The above steel is a steel having a yield strength of 320 MPa or more at points t / 4 and t / 2 with a thickness (t).

4. In claim 1, The above steel is a steel having a tensile strength of 430 MPa or more and 590 MPa or less at points t / 4 and t / 2 of thickness (t).

5. In claim 1, The above steel is a steel having a Charpy impact absorption energy of 100J or more at -50℃.

6. A step for manufacturing a steel slab containing, by weight%, carbon (C): 0.03 to 0.08%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 1.0 to 1.6%, phosphorus (P): 0.01% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.01 to 0.05%, niobium (Nb): 0.015 to 0.035%, chromium (Cr): 0.3% or less (excluding 0%), nickel (Ni): 0.1 to 0.5%, titanium (Ti): 0.01 to 0.02%, nitrogen (N): 0.002 to 0.01%, and the remainder being iron and inevitable impurities; A step of heating the above steel slab in a temperature range of 1000 to 1080℃; A step of rough rolling the above heated steel slab at a temperature range of 900 to 1000°C; A step of finishing rolling at a temperature range of Ar3 or higher and Tnr or lower after the above rough rolling; and Cooling step at a cooling rate of 2~5℃ / s to a temperature of 350~500℃ A method for manufacturing steel including:

7. In claim 6, The above steel slab is manufactured by continuous casting of molten steel. A method for manufacturing steel, comprising: applying a pressure reduction (unit: mm) of 3 mm to 6 mm during the continuous casting, and performing an additional pressure reduction of 3 mm to 10 mm after solidification, thereby manufacturing a steel slab having a thickness of 290 mm to 397 mm.

8. In claim 7, A method for manufacturing steel having a hydrogen concentration in the molten steel of 1.5 ppm or less.

9. In claim 7, A method for manufacturing steel in which, when manufacturing the above steel slab, a molten steel degassing process (RH process) can be performed before continuous casting, and the RH process is performed for 15 minutes or more under conditions of 2 torr or less.

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