Steel material and its manufacturing method

Abstract

The present invention provides an extra-thick steel material that has excellent strength in the center, impact toughness, and internal quality, and a method for producing the same. [Solution] The steel sheet contains, by weight, 0.03-0.08% carbon (C), 0.1-0.5% silicon (Si), 1.0-1.6% manganese (Mn), 0.01% or less phosphorus (P), 0.003% or less sulfur (S), 0.01-0.05% aluminum (Al), 0.015-0.035% niobium (Nb), 0.3% or less (excluding 0%) chromium (Cr), 0.1-0.5% nickel (Ni), 0.01-0.02% titanium (Ti), 0.002-0.01% nitrogen (N), with the remainder being Fe and unavoidable impurities. The microstructure contains, by area fraction, 60-85% ferrite, 10-20% bainite, and the remainder being pearlite and unavoidable structures. The steel sheet is characterized by having an average size of effective crystal grains having high-angle grain boundaries of 15° or more of 20 μm or less.

Description

[Technical Field]

[0001] The present invention relates to a steel material and a manufacturing method thereof, and more particularly to a steel material that can be used for wind power generators having a monopile or jacket structure, and a manufacturing method thereof. [Background technology]

[0002] Recently, as the height of structures used in onshore and offshore wind turbines has gradually increased, there has been an increasing demand for extra-thick, high-strength steel materials with excellent load resistance, and there has also been a demand for impact toughness at low temperatures and aging impact toughness when deformation such as bending is involved.

[0003]

[0003] In order to ensure high strength as well as excellent low-temperature impact toughness and aging impact toughness, a grain refinement process is essential. However, when manufacturing extra-thick steel products through a rolling process, there is a drawback in that sufficient rolling force is not transmitted to the center of the thickness, limiting the grain refinement. In addition, voids present in the steel slab are often not sufficiently compressed during the rolling process, and segregation zones present in the center of the thickness of the steel slab after continuous casting often result in deterioration of internal quality even after the production of the final hot-rolled steel sheet.

[0004] As the steel used in the structure of wind turbines becomes thicker, it is becoming more difficult to ensure the physical properties of the center of the thickness mentioned above. However, there are actually an increasing number of cases where the strength of the center of the thickness, low-temperature impact toughness, and excellent internal quality are required.

[0005] In order to ensure the physical properties and internal quality of the center of the thickness, it is necessary to appropriately add alloying elements that are advantageous for grain refinement and to manufacture steel slabs with sound internal structures by optimizing the continuous casting process. It is also necessary to optimize the rolling and accelerated cooling processes to control the type and fraction of microstructures so that the target strength and low-temperature impact toughness can both be achieved.

[0006] Patent Document 1 proposes a method for ensuring the quality of the central portion by increasing the reduction rate per pass during rough rolling to 10% or more so that sufficient rolling force is applied to the central portion of the hot-rolled steel sheet, and by providing a sufficient waiting time for air cooling before finish rolling, when the surface portion hardened by the temperature drop is rolled, further rolling force is applied to the central portion. However, this method has the drawback of being limited in terms of ensuring the quality of the central portion when segregation defects or large voids exist inside the steel slab. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Korean Patent Publication No. 10-2023-0102791 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention aims to provide an extra-thick steel material that is excellent in strength at the center, impact toughness, and internal quality, and a method for producing the same.

[0009] The object of the present invention is not limited to the above-mentioned matters. Further object of the present invention is described in the entire content of the specification, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the further object of the present invention from the content described in the specification of the present invention. [Means for solving the problem]

[0010] The steel material of the present invention contains, 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 consisting of Fe and inevitable impurities, The microstructure, in terms of area fraction, is 60-85% ferrite, 10-20% bainite, and the remainder is pearlite and unavoidable structures. The average size of effective crystal grains having high-angle grain boundaries of 15° or more is 20 μm or less.

[0011] The steel material is 100 mm 2 The number of residual voids having the above size may be one or less.

[0012] The steel material may have a yield strength of 320 MPa or more at points t / 4 and t / 2 of the thickness (t).

[0013] The steel material may have a tensile strength of 430 MPa or more and 590 MPa or less at points t / 4 and t / 2 of the thickness (t).

[0014] The steel material may have a Charpy impact absorption energy of 100 J or more at -50°C.

[0015] The method for producing a steel material of the present invention contains, 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. The method includes the steps of producing a steel slab containing 0.02%, nitrogen (N): 0.002-0.01%, and the remainder being Fe and unavoidable impurities, heating the steel slab in a temperature range of 1000-1080°C, rough rolling the heated steel slab in a temperature range of 900-1000°C, finish rolling the slab after the rough rolling in a temperature range of Ar3 or more and Tnr or less, and cooling to a temperature of 350-500°C at a cooling rate of 2-5°C / s.

[0016] The steel slab is manufactured by continuous casting of molten steel, and a light reduction (unit: mm) of 3 mm to 6 mm is applied during the continuous casting, and after solidification, an additional reduction of 3 mm to 10 mm is applied to manufacture a steel slab with a thickness of 290 mm to 397 mm.

[0017] The hydrogen concentration in the molten steel may be 1.5 ppm or less.

[0018] In producing the steel slab, a degassing process (RH process) of molten steel can be carried out before continuous casting, and the RH process can be carried out under conditions of 2 torr or less for 15 minutes or more. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide an extra-thick steel material for wind power structures that has excellent strength in the center, low-temperature impact toughness, and aging impact toughness.

[0020] The various beneficial advantages and effects of the present invention are not limited to the above-mentioned contents, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is an optical microscope photograph of the microstructure at the t / 2 point in the thickness direction of a steel material having the composition of Example 1 and a thickness of 120 mm. [Figure 2] 1 shows the EBSD results of observing the microstructure at the t / 2 point in the thickness direction of a steel material having the composition of Example 1 and a thickness of 120 mm. DETAILED DESCRIPTION OF THE INVENTION

[0022] The terminology used herein is for the purpose of describing the invention and is not intended to limit the invention. Furthermore, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the relevant definition clearly dictates otherwise.

[0023] The meaning of "comprises" as used in the specification embodies features and does not exclude the presence or addition of other features.

[0024] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted as having a meaning consistent with the relevant technical literature and the present disclosure.

[0025] The inventors of the present invention recognized the need for a method to ensure the required physical properties of steel materials used in wind turbine structures, such as monopiles and jackets, as they become larger and require stricter internal quality. In particular, they conducted extensive research into a method to ensure the strength of the core as well as low-temperature impact toughness and aging impact toughness in steel materials for wind turbine structures with a certain thickness or greater. As a result, they confirmed that it is possible to provide steel materials for wind turbine structures with the desired physical properties by controlling the relationship between the chemical composition and certain components in alloy design and optimizing manufacturing conditions, leading to the completion of the present invention. As an example, steel slabs with excellent internal integrity can be manufactured using the Thermo Mechanical Controlled Process (TMCP) method.

[0026] First, an example of the steel material of the present invention will be described in detail. The alloy composition and range of the steel material will be described below. The content of the alloy composition described below means weight % unless otherwise specified.

[0027] The steel material is characterized by 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%, and nitrogen (N): 0.002 to 0.01%.

[0028] Carbon (C):0.03~0.08% Carbon (C) is an effective element for increasing the hardenability of steel and improving its strength. To fully achieve this effect, the C content should be 0.03% or more. However, if the C content exceeds 0.08%, the high hardenability can lead to a high fraction of bainite and the presence of coarse carbides within it, which can significantly reduce impact toughness. If the C content is less than 0.03%, it is insufficient to ensure strength. Therefore, the C content should be 0.03-0.08%, and more preferably 0.03-0.05%.

[0029] Silicon (Si): 0.1 to 0.5% Silicon (Si) is not only used as a deoxidizer, but is also an element advantageous in improving the strength and toughness of steel. To fully obtain the above effects, the Si content can be 0.1% or more. However, if the Si content exceeds 0.5%, there is a risk of excessive formation of island martensite (MA), which may result in a deterioration of low-temperature impact toughness. Therefore, the Si content can be 0.1 to 0.5%.

[0030] Manganese (Mn): 1.0-1.6% Manganese (Mn) is an element advantageous for improving the strength of steel through its solid solution strengthening effect. To fully obtain this effect, Mn can be contained in an amount of 1.0% or more. However, if the Mn content exceeds 1.6%, it bonds 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, Mn can be contained in an amount of 1.0 to 1.6%, and more preferably 1.4 to 1.6%.

[0031] Phosphorus (P): 0.01% or less Phosphorus (P) is an element advantageous for improving the strength and ensuring corrosion resistance of steel, but it may significantly impair the impact toughness of steel, so it is preferable to limit its content as low as possible. In the present invention, since the target physical properties can be achieved even if a maximum of 0.01% of P is contained, its content can be limited to 0.01% or less. However, 0% can be excluded in consideration of the level of unavoidable addition.

[0032] Sulfur (S): 0.003% or less Sulfur (S) is an element that significantly impairs the hydrogen-induced cracking resistance and impact toughness of steel by combining with Mn in steel to form MnS and other compounds. Therefore, it is advantageous to limit the S content to as low as possible. In the present invention, since the target physical properties can be achieved even if the S content is contained at a maximum of 0.003%, the S content can be limited to 0.003% or less. However, 0% can be excluded in consideration of the level of unavoidable addition.

[0033] Aluminum (Al): 0.01 to 0.05% Aluminum (Al) is an element that can inexpensively deoxidize molten steel, and to fully obtain the above-mentioned effects, the Al content can be 0.01% or more. However, an excessive Al content exceeding 0.05% is not preferable because it not only induces nozzle clogging during continuous casting but also may significantly reduce impact toughness due to the formation of Al-based oxidized inclusions. Therefore, the Al content can be 0.01 to 0.05%.

[0034] Niobium (Nb): 0.015-0.035% Niobium (Nb) precipitates in the form of NbC or Nb(C,N), significantly improving the strength of the base metal. Furthermore, when reheated at high temperatures, the dissolved Nb inhibits the recrystallization of austenite and the transformation of ferrite or bainite, thereby achieving a refined microstructure. However, excessive Nb content causes undissolved Nb to form TiNb(C,N), which can lead to poor UT and reduced low-temperature impact toughness. Therefore, the upper limit of Nb content is preferably set to 0.035%. Therefore, in the present invention, Nb can be contained in an amount of 0.015 to 0.035%, more preferably 0.025 to 0.03%.

[0035] Nickel (Ni): 0.1 to 0.5% Nickel (Ni) is an element that can simultaneously improve the strength and low-temperature impact toughness of the base material, but it is an expensive element, and if its content exceeds 0.5%, there is a problem that its economic viability is significantly reduced. Therefore, Ni can be contained at 0.5% or less. However, to maximize the effects of Ni, it is preferable to add at least 0.1% or more.

[0036] Titanium (Ti): 0.01-0.02% Titanium (Ti), when added together with N, forms TiN, which reduces the occurrence of surface cracks due to the formation of AlN precipitates, so it is preferable to add at least 0.01%. However, if the Ti content exceeds 0.02%, coarse TiN is formed during reheating of the steel slab, which acts as a factor that reduces low-temperature impact toughness. Therefore, the Ti content is preferably 0.01 to 0.02%, and more preferably 0.01 to 0.015%.

[0037] Nitrogen (N): 0.002-0.01% Nitrogen (N), when added together with Ti, is an element that is advantageous in that it forms TiN and suppresses grain growth due to heat effects during welding. To fully obtain the above-mentioned effects when Ti is added, 0.002% or more of N can be contained. However, if the N content exceeds 0.01%, coarse TiN is formed, which impairs low-temperature impact toughness, which is undesirable. Therefore, the N content is preferably 0.002 to 0.01%.

[0038] Chromium (Cr): 0.3% or less (excluding 0%) Chromium (Cr) is an effective element that not only increases the hardenability of steel and increases the proportion of low-temperature phases such as bainite, but also improves strength by forming fine Cr-based carbides. However, if the Cr content exceeds 0.3%, the coarse bainite and the carbides present therein may significantly reduce low-temperature impact toughness and aging impact toughness. Therefore, it is preferable that the Cr content be 0.3% or less.

[0039] The remaining component is iron (Fe), and may contain some unintentional and unavoidable impurities mixed in during the manufacturing process. These impurities are known to anyone skilled in the art of ordinary manufacturing processes, and therefore not all of them are specifically mentioned in this specification.

[0040] The steel material may contain, in area %, 60 to 85% ferrite, 10 to 20% bainite, and the remainder pearlite and unavoidable structures.

[0041] The steel preferably has an average size of effective crystal grains having high-angle grain boundaries of 15° or more of 20 μm or less.

[0042] If the ferrite fraction is less than 60% and the bainite fraction exceeds 20%, the impact toughness may be significantly reduced due to excessively high strength. Conversely, if the ferrite fraction exceeds 85% and the bainite fraction is too low at less than 10%, the strength may be insufficient.

[0043] On the other hand, if the average size of effective grains with high-angle grain boundaries exceeds 20 μm, impact toughness may be significantly reduced. One way to measure effective grains with high-angle grain boundaries of 15° or more is to use EBSD (Electron Backscatter Diffraction).

[0044] In addition, the steel is 100 mm 2 It is preferable that the number of residual voids having a size of 1 or more be one or less. Residual voids affect the internal quality. Steel can be measured by ultrasonic flaw detection. Residual voids are usually voids that exist in the center of the slab and are not transmitted to the center by sufficient rolling force during rolling, so they generally exist in a thickness of 1 / 4t to the center of the hot-rolled steel plate.

[0045] 100mm 2If there is more than one residual void of a size equal to or larger than 1, there is a possibility that an accident such as a fracture may occur due to an internal defect when the hot-rolled steel plate is used as a structure. 2 It is preferable to have as few residual voids as possible having a size equal to or larger than this, so it is preferable that there be one or less.

[0046] On the other hand, it is preferable that the residual voids present inside the steel material are 30 mm or less based on the long side.

[0047] The thickness of the steel may be between 100mm and 150mm.

[0048] The steel has a yield strength of 320 MPa or more, a tensile strength of 430 MPa to 590 MPa, and a Charpy impact energy (CVN) value of 100 J or more, evaluated perpendicular to the rolling direction at t / 4 and t / 2 points in the thickness direction (where t means the thickness of the steel (mm)). The Charpy impact energy (CVN) value evaluated in the rolling direction at -50°C is 100 J or more, and thus has excellent strength and low-temperature impact toughness.

[0049] The steel material was subjected to 8% tensile deformation perpendicular to the rolling direction at t / 4 and t / 2 points in the thickness direction (where t means the thickness (mm) of the steel material), and then aging heat treatment was performed at 250°C for 60 minutes. For the test specimen, the Charpy impact energy (CVN) value at -50°C was 50J or more, and the steel material exhibited excellent aging impact toughness.

[0050] Next, an example of the method for producing a steel material according to the present invention will be described in detail.

[0051] The manufacturing method includes the steps of manufacturing a steel slab containing the above-mentioned alloy composition, heating the steel slab, rolling the heated steel slab, and cooling the same, as will be described in detail below.

[0052] Steel slab manufacturing In the present invention, it is preferable to obtain a steel slab with excellent internal quality in order to ensure the center strength, low-temperature impact toughness, aging impact toughness, and internal quality of the final steel material. As an example of a method for producing a steel slab, a steel slab can be produced by continuously casting molten steel that has been subjected to a steelmaking process.

[0053] As a preferred example, during the steelmaking process, after lifting a molten steel ladle, a vacuum is created inside the vessel. A reflux gas is then introduced into the vessel through an immersion tube, and the molten steel is refluxed to perform a vacuum degassing process (RH process) to reduce the hydrogen concentration in the molten steel to 1.5 ppm or less. The RH process is preferably performed for 15 minutes or longer at a pressure of 2 torr or less. If the RH process is performed for less than 15 minutes at a pressure of 2 torr or less, sufficient degassing is not achieved, resulting in an excessively high hydrogen concentration, which may leave large voids inside the final steel and adversely affect the internal quality. While there is no specific upper limit, it is preferable to perform the RH process within 30 minutes because exceeding 30 minutes may impose a burden on the manufacturing process.

[0054] Steel slabs can be produced by continuous casting molten steel with a hydrogen concentration of 1.5 ppm or less. During the continuous casting process, a soft reduction of 3 mm to 6 mm can be performed in the solid / liquid coexistence region. Soft reduction minimizes segregation in the center of the steel slab. Electromagnetic Stirring (EMS) can be selectively used to suppress dendrite growth and induce the formation of equiaxed crystals to suppress center segregation. If the soft reduction is less than 3 mm, the Mn segregation zone in the center of the steel slab may not be fully extruded into the liquid phase, resulting in residual segregation. On the other hand, if the soft reduction is excessive (more than 6 mm), cracks may occur during slab production.

[0055] Meanwhile, after soft reduction and solidification are completed, additional reduction of 3mm to 10mm can be performed to minimize the size of voids present inside the steel slab. As a result, the remaining voids in the final steel material can be reduced to 100mm in area. 2 It is preferable that the residual voids do not exceed 30 mm in terms of the long side, and that there is no more than one residual void with a size exceeding this. If the additional reduction after solidification is less than 3 mm, the size of the voids present inside the steel slab cannot be sufficiently reduced, which may have a negative impact on the internal quality of the final steel material, while if the additional reduction is more than 10 mm, although it is very effective in reducing voids, the excessive reduction force may act and induce surface cracks.

[0056] Steel slab heating It is preferable to heat the steel slab and perform a homogenization treatment, preferably in the temperature range of 1000 to 1080°C. If the heating temperature of the steel slab is less than 1000°C, the precipitates (carbonitrides) formed in the slab will not be sufficiently redissolved, and the formation of precipitates will decrease in the process after hot rolling, ultimately making it difficult to achieve the yield strength and tensile strength proposed in the present invention. On the other hand, if the temperature exceeds 1080°C, the austenite grains will become coarse, which may impair the physical properties of the steel.

[0057] On the other hand, the heating time is preferably 2 hours or more. If the heating time of the steel slab is less than 2 hours, the time required to sufficiently heat the center of the slab will be insufficient, and although there is no upper limit, it is preferable that the maximum heating time be less than 6 hours in consideration of the load on the manufacturing process.

[0058] hot rolling Hot-rolled steel sheets can be produced by hot-rolling heated steel slabs. After rough rolling at a temperature between 900 and 1000°C, finish rolling begins at a temperature below Tnr, and finish hot rolling can be performed at Ar3 or higher.

[0059] The above Tnr and Ar3 can be defined as follows:

[0060] Tnr(℃)=887+(464×C)+((6445×Nb)-(644×Nb 0.5 )) + (890 × Ti) + (363 × Al) - (357 × Si) Ar3(℃)=910-310×C-80×Mn-55×Ni+124×Ti-18×Nb+179×Al (Here, each element means its weight content.)

[0061] If the temperature during rough rolling is less than 900°C, the temperature during the subsequent finish hot rolling will be too low. If finish rolling is started above the Tnr temperature, even if the subsequent end temperature is below the Tnr temperature, there is a disadvantage that coarse and fine crystal grains may coexist, which may intermittently reduce the individual values ​​of impact toughness. If the finish hot rolling temperature is less than Ar3, the rolling load will be large, which may result in quality defects such as surface cracks.

[0062] cooling After hot rolling, the material 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 to ensure strength, but due to the characteristics of extra-thick material, excessive oscillation work will be required, which may cause serious strain in the manufacturing process. On the other hand, if the cooling end temperature exceeds 500°C, it will be difficult to ensure strength. If the cooling rate during cooling is less than 2°C / s, the proportion of polygonal ferrite will increase excessively, making it difficult to ensure strength. If it exceeds 5°C / s, it may be advantageous to ensure strength, but the increased flow rate during water cooling will severely overload the manufacturing process, making it difficult to apply to mass production. A cooling rate of 2.0 to 5.0°C / s may be preferable. [Example]

[0063] Hereinafter, examples of the present invention will be described. It goes without saying that those skilled in the art will be able to make various modifications to the following examples without departing from the scope of the present invention. The following examples are provided for the purpose of understanding the present invention, and the scope of the present invention should not be limited to the following examples, but should be defined by the claims below as well as equivalents thereof.

[0064] (Example) Steel slabs were produced by continuous casting molten steel having the alloy composition (wt %, the remainder being Fe and unavoidable impurities) and hydrogen concentration shown in Table 1 below. The units for each component are wt %, the remainder being Fe and unavoidable impurities, and Tnr and Ar3 were calculated using the following formula.

[0065] Tnr(℃)=887+(464×C)+((6445×Nb)-(644×Nb 0.5 )) + (890 × Ti) + (363 × Al) - (357 × Si) Ar3(℃)=910-310×C-80×Mn-55×Ni+124×Ti-18×Nb+179×Al (Here, each element means its content (wt%).)

[0066] When producing the steel slabs, a soft reduction of 4.5 mm was applied, and the additional reduction after solidification was started under the conditions shown in Table 2 below. In invention examples 1 and 2 and comparative examples 1 to 3, rough rolling was carried out at 900 to 1000°C, and in comparative example 4, rough rolling was carried out at over 1000°C. The produced steel slabs were heated, hot rolled and cooled under the conditions shown in Table 2 to produce steel materials.

[0067] [Table 1]

[0068] [Table 2]

[0069] The microstructure of the manufactured steel was then observed and its mechanical properties were evaluated. The microstructure was observed using EBSD and an optical microscope, followed by an image analysis program. This allowed the average size of effective grains, including high-angle grains with grain boundary angles of 15° or more, to be measured. High-angle grains with misorientation of 5° or less were classified as ferrite, and irregular sub-boundaries within the grains were classified as bainite. Pearlite was defined as the black area appearing in the optical microscope image.

[0070] The microstructure was measured at the t / 2 point (t: thickness, mm) in the thickness direction of each steel, and the results are shown in Table 3 below. The mechanical properties of each steel were then evaluated, and the results are shown in Table 4 below. Tensile test specimens were taken at the t / 2 point (t: thickness, mm) in the thickness direction perpendicular to the rolling direction, and tensile strength (TS), yield strength (YS), and elongation (El) were measured at room temperature. Impact test specimens were taken from JIS No. 4 test specimens at the t / 2 point in the thickness direction parallel to the rolling direction. Impact toughness (Charpy impact energy, CVN) at -50°C was measured three times, and the average value and the minimum and maximum ranges are shown in parentheses. Furthermore, test specimens were subjected to 8% tensile deformation perpendicular to the rolling direction and then heat-treated at 250°C for 60 minutes. Impact toughness (aging impact toughness) at -50°C was evaluated using the JIS No. 4 impact test specimen, and the results are also shown in Table 4 below.

[0071] In addition, residual voids were measured by ultrasonic testing at the t / 2 (t: thickness, mm) point in the thickness direction, and 2 The number of residual voids having a size greater than 100 mm is also shown in Table 4.

[0072] [Table 3]

[0073] As shown in Table 3, Examples 1 and 2, which were manufactured using the proposed alloy composition, component relationships, and manufacturing conditions, satisfy the ferrite, bainite, and pearlite fractions and effective grain size proposed by the present invention. On the other hand, Comparative Examples 1 to 3 satisfy the ferrite, bainite, and pearlite fractions but do not satisfy the effective grain size, and Comparative Example 4 deviates from the values ​​proposed by the present invention not only in the ferrite and bainite fractions but also in the effective grain size.

[0074] [Table 4]

[0075] Table 4 shows the yield strength, tensile strength, elongation, low-temperature impact toughness, aging impact toughness, and the thickness of the hot-rolled steel sheet with an area of ​​100 mm 2 The figure shows the number of residual voids having a size of over 1000. It can be seen that all of Inventive Examples 1 and 2 and Comparative Examples 1 to 4 easily meet the tensile properties proposed in the present invention. Specifically, Inventive Examples 1 and 2 exhibited excellent properties not only in tensile properties but also in the average and individual values ​​of low-temperature impact toughness and aging impact toughness. It was also confirmed that the additional reduction process after solidification eliminated residual voids, ensuring excellent internal quality.

[0076] On the other hand, in Comparative Example 1, the steel was not added with Ni, which is useful for reducing the effective grain size. It was confirmed that the minimum value of impact toughness deteriorated according to the coarse effective grain size, and in particular, the aged impact toughness was significantly reduced. In Comparative Example 2, due to the excessive addition of C, the yield / tensile strength satisfied the values ​​proposed in the present invention, but the minimum values ​​of impact toughness and aged impact toughness did not satisfy the values ​​proposed in the present invention. In Comparative Example 3, the steel met the chemical composition and rolling / cooling conditions proposed in the present invention, but no additional reduction was applied after solidification during the steel slab manufacturing process. Although the tensile properties were satisfied, it was confirmed that the minimum values ​​of impact toughness and aged impact toughness deteriorated. It was also confirmed that residual voids were not sufficiently compressed and remained in a coarse form. Similarly, in the case of Comparative Example 4, the composition satisfies the ranges proposed by the present invention, but the high slab heating temperature causes the austenite grains to coarsen, increasing the hardenability and the bainite fraction, and the finish rolling start temperature is applied above the Tnr temperature, preventing sufficient grain refinement. As a result, although the strength is improved, the minimum values ​​of impact toughness and aging impact toughness are significantly reduced.

[0077] FIG. 1 is an optical microscope photograph of the microstructure at the t / 2 point of the steel material having the composition of Example 1 and a thickness of 120 mm, and it can be seen that the structure is a mixture of ferrite, bainite, and some pearlite.

[0078] FIG. 2 is a photograph of the EBSD microstructure observed at the t / 2 point of the thickness of a steel material having the composition of Example 1 and a thickness of 120 mm, where (a) shows ferrite and (b) shows bainite.

Claims

1. The alloy contains, by weight, 0.03 to 0.08% carbon (C), 0.1 to 0.5% silicon (Si), 1.0 to 1.6% manganese (Mn), 0.01% or less phosphorus (P), 0.003% or less sulfur (S), 0.01 to 0.05% aluminum (Al), 0.015 to 0.035% niobium (Nb), 0.3% or less chromium (Cr) (excluding 0%), 0.1 to 0.5% nickel (Ni), 0.01 to 0.02% titanium (Ti), 0.002 to 0.01% nitrogen (N), and the remainder being Fe and inevitable impurities; The microstructure contains, in area fraction, 60 to 85% ferrite, 10 to 20% bainite, and the remainder pearlite and unavoidable structures. A steel material characterized in that the average size of effective crystal grains having high-angle grain boundaries of 15° or more is 20 μm or less.

2. The steel material is 100 mm ２ 2. The steel material according to claim 1, wherein the number of residual voids having the above size is one or less.

3. 2. The steel material according to claim 1, wherein the steel material has a yield strength of 320 MPa or more at points t / 4 and t / 2 of the thickness (t).

4. The steel material according to claim 1, wherein the steel material has a tensile strength of 430 MPa or more and 590 MPa or less at points t / 4 and t / 2 of the thickness (t).

5. 2. The steel material according to claim 1, wherein the steel material has a Charpy impact absorption energy of 100 J or more at −50° C.

6. a step of producing a steel slab containing, in weight percent, 0.03-0.08% carbon (C), 0.1-0.5% silicon (Si), 1.0-1.6% manganese (Mn), 0.01% or less phosphorus (P), 0.003% or less sulfur (S), 0.01-0.05% aluminum (Al), 0.015-0.035% niobium (Nb), 0.3% or less (excluding 0%) chromium (Cr), 0.1-0.5% nickel (Ni), 0.01-0.02% titanium (Ti), 0.002-0.01% nitrogen (N), and the remainder being Fe and unavoidable impurities; heating the steel slab at a temperature in the range of 1000 to 1080°C; Rough rolling the heated steel slab at a temperature in the range of 900 to 1000°C; After the rough rolling, a step of finish rolling in a temperature range of Ar3 or more and Tnr or less; A method for producing a steel material, comprising a step of cooling to a temperature of 350 to 500°C at a cooling rate of 2 to 5°C / s.

7. The steel slab is produced by continuous casting of molten steel, 7. The method for producing steel material according to claim 6, wherein a soft reduction (unit: mm) of 3 mm to 6 mm is applied during the continuous casting, and an additional reduction of 3 mm to 10 mm is applied after solidification to produce a steel slab having a thickness of 290 mm to 397 mm.

8. 8. The method for producing a steel material according to claim 7, wherein the hydrogen concentration in the molten steel is 1.5 ppm or less.

9. 8. The method for producing a steel material according to claim 7, wherein a degassing process (RH process) of molten steel can be performed before continuous casting when producing the steel slab, and the RH process is performed for 15 minutes or more under conditions of 2 torr or less.

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