Steel material and method for manufacturing same

Abstract

The present invention relates to a steel material for use in line pipes and the like, and a method for manufacturing same.

Description

Steel and its manufacturing method

[0001] The present invention relates to a steel material used for line pipes, etc., and a method for manufacturing the same.

[0002] Line pipes may be used to produce crude oil in cold regions and transport it to storage sites. As steel for these line pipes, API steel may be used. API steel refers to steel that meets the specifications defined by the American Petroleum Institute.

[0003] In the case of the aforementioned API steel, fracture toughness at low temperatures, particularly Drop Weight Tear Test (DWTT) characteristics, is required to prevent pipe fracture. Additionally, high material strength is required to stably protect the structure against high-pressure transport environments and external impacts. Therefore, conventionally, as such API materials for transport, thick plate steel has been mainly used, in which strength is enhanced by adding large amounts of solid solution strengthening elements such as C, Si, Mn, and Cr, or by adding hardenable elements such as Ni and Mo, to high-purity steel with minimized impurities (Patent Document 1).

[0004] However, as the demand for high-strength materials has increased recently, expensive elements such as Ni and Mo have been added in large quantities to secure the strength of thick plate materials. In addition, to secure strength, the slab heating temperature, rolling temperature, and cooling end temperature have been lowered significantly, resulting in a decrease in productivity. Furthermore, as the cooling end temperature is lowered, a rapid cooling structure is formed in the surface layer, causing problems such as the occurrence of hard spots.

[0005] In particular, due to the addition of hardenable elements, low-temperature structures such as lamellar martensite (MA) become more abundant, which reduces low-temperature toughness and makes it difficult to achieve excellent DWTT characteristics while maintaining high strength.

[0006] (Patent Document 1) Korean Published Patent No. 10-2011-0062903

[0007] According to one embodiment of the present invention, a high-strength steel with excellent low-temperature toughness and a method for manufacturing the same may be provided.

[0008] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0009] A steel material according to one embodiment of the present invention comprises, in weight percent, C: 0.05~0.10%, Si: 0.5% or less (excluding 0%), Mn: 0.5~2.5%, Nb: 0.07% or less, V: 0.03% or less, Mo: 0.2% or less, Ni: 0.3% or less, Cr: 0.1~0.3%, P: 0.03% or less (excluding 0%), S: 0.05% (excluding 0%), Al: 0.05% or less (excluding 0%), N: 0.01% or less (excluding 0%), Cu: 0.01% or less, and the remainder being Fe and unavoidable impurities.

[0010] Difference in average grain size between the center and the surface (△G = G s - G f ) is 10 μm or less, and

[0011] The grain aspect ratio of the surface layer and the center may be 1.5 or higher.

[0012] The above steel may have a carbon equivalent (Ceq) of 0.48 or less, as defined by the following [Equation 1].

[0013] [Formula 1] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5

[0014] (Here, [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] represent the content (weight%) of the respective element)

[0015] The above surface layer extends from the steel surface to a thickness t / 4 (t: steel thickness), and the average grain size (G) of the surface layer f ) can be 10㎛ or less.

[0016] The microstructure of the above-mentioned center may have bainite of 90% or less as an area fraction.

[0017] The above steel material may have a thickness of 20 to 50 mm.

[0018] The above steel may have a yield strength of 560 to 670 MPa, a tensile strength of 630 to 800 MPa, and a total elongation of 30 to 60%.

[0019] The above steel may have a DWTT ductile fracture rate of 85% or more at -45℃.

[0020] A method for manufacturing steel according to another embodiment of the present invention comprises the step of heating a steel slab, comprising, in weight percent, C: 0.05~0.10%, Si: 0.5% or less (excluding 0%), Mn: 0.5~2.5%, Nb: 0.07% or less, V: 0.03% or less, Mo: 0.2% or less, Ni: 0.3% or less, Cr: 0.1~0.3%, P: 0.03% or less (excluding 0%), S: 0.05% (excluding 0%), Al: 0.05% or less (excluding 0%), N: 0.01% or less (excluding 0%), Cu: 0.01% or less, and the remainder being Fe and unavoidable impurities, to a temperature range of 1050~1250℃;

[0021] A step of rough rolling the above steel slab, wherein the rough rolling is performed in a temperature range of 900 to 1050℃;

[0022] After the above rough rolling, a step of finish rolling at the austenite single-phase region temperature; and

[0023] After the above finishing rolling, it may include a step of accelerated cooling to a range of 300 to 500 ℃ at a cooling rate of 30 to 50 ℃ / sec.

[0024] The heating of the above steel slab can be performed for 100 to 400 minutes.

[0025] The above finishing rolling temperature may be 750 to 900°C.

[0026] The cooling rate between rolling passes during the above rough rolling can be performed at 5~10℃ / sec.

[0027] During the above rough rolling, the temperature reduction between rolling passes can be 5 to 30°C.

[0028] According to the present invention, it is possible to provide a steel material for line pipes that has excellent low-temperature toughness while ensuring high strength. In particular, it may be suitable for line pipes for crude oil transportation.

[0029] The various and beneficial advantages and effects of the present invention are not limited to those described above and will be more easily understood in the process of explaining specific embodiments of the present invention.

[0030] The embodiments of the present invention are provided to more fully explain the invention to those with average knowledge in the relevant technical field.

[0031] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0032] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0033] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0034] Hereinafter, a steel plate according to one embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. Unless otherwise noted, the unit % in the alloy composition described below refers to weight % (wt.%).

[0035] The above steel contains, in weight percent, C: 0.05~0.10%, Si: 0.5% or less (excluding 0%), Mn: 0.5~2.5%, Nb: 0.07% or less, V: 0.03% or less, Mo: 0.2% or less, Ni: 0.3% or less, Cr: 0.1~0.3%, P: 0.03% or less (excluding 0%), S: 0.05% (excluding 0%), Al: 0.05% or less (excluding 0%), N: 0.01% or less (excluding 0%), Cu: 0.01% or less, and the remainder is Fe and unavoidable impurities.

[0036] Carbon (C): 0.05~0.10%

[0037] The above C is the most economical and effective element for securing strength. If the carbon content is excessively low, it may be difficult to secure the target strength even if precipitation strengthening elements such as Nb are added. On the other hand, if the content is excessively high, ductility and impact toughness may deteriorate due to an excessive increase in strength. Preferably, it may be 0.050 to 0.100%.

[0038] Silicon (Si): 0.5% or less (excluding 0%)

[0039] The above Si contributes to the increase in strength through deoxidation of molten steel and solid solution strengthening, but it is not intentionally added in the present invention, and there is no significant impediment in terms of securing physical properties even if the above Si is not added. Meanwhile, if the content is excessively high, red scale caused by Si may form on the surface of the thick plate steel, which may degrade surface quality and weldability. Preferably, it may be 0.50% or less.

[0040] Manganese (Mn): 0.5~2.5%

[0041] The above Mn is an element effective for solid solution strengthening of steel, and may be included in an amount of 0.5% or more to ensure appropriate strength. However, if the content is excessively high, there is a risk of central segregation occurring during the continuous casting process. Preferably, it may be 0.50 to 2.50%.

[0042] Niobium (Nb): 0.07% or less

[0043] The above Nb is a precipitation-strengthening element that is effective in securing strength by refining the crystal grains while generating NbC series precipitates. However, since there is a disadvantage that weldability may be reduced if the content is excessively high, it is effective to keep the content 0.07% or less in the present invention. The above Nb may contain 0%. Preferably, it may be 0.070% or less.

[0044] Vanadium (V): 0.03% or less

[0045] The above V is also a precipitation-strengthening element and is an effective element for securing the strength of steel. However, if its content is excessively high, low-temperature toughness and weldability are reduced due to a large amount of precipitates, which has the disadvantage of increasing the cost of the alloy; therefore, in the present invention, its content is managed to be 0.03% or less. The above V may contain 0%.

[0046] Molybdenum (Mo): 0.2% or less

[0047] The above Mo is an element that improves the hardenability of steel and significantly enhances the ability to form low-temperature structures even at low cooling rates. As a result, it is an effective element for securing the strength of steel by forming low-temperature structures such as bainite. However, it is a relatively expensive element, and if the content becomes excessively high, toughness may deteriorate. The above Mo may contain 0%.

[0048] Nickel (Ni): 0.3% or less

[0049] The above Ni plays a role in simultaneously improving the strength and toughness of steel and is an effective element for securing low-temperature toughness by lowering the phase transformation temperature. However, since it is a relatively expensive element, it is preferable to include its content in the present invention at 0.3% or less. The above Ni may be included at 0%.

[0050] Chrome (Cr): 0.1~0.3%

[0051] The above Cr strengthens the steel through solid solution and delays the bainite phase transformation upon cooling, thereby aiding in the formation of equiaxed ferrite; in particular, when added together with Mo, it further enhances hardenability more effectively. In the present invention, it is preferable to add at least 0.1% to obtain these effects. On the other hand, if the content is excessive, weldability and brittleness deteriorate; therefore, in the present invention, it is preferable to include a content of 0.3% or less.

[0052] Phosphorus (P): 0.03% or less (excluding 0%)

[0053] The above P is an impurity inevitably contained in steel, and it is desirable to keep its content as low as possible. In particular, if its content is excessive, the risk of weldability deterioration and steel brittleness increases, and in the present invention, it is desirable to include a content of 0.03% or less.

[0054] Sulfur (S): 0.05% (excluding 0%)

[0055] The above S is an impurity inevitably included in steel, and it is desirable to keep its content as low as possible. In particular, if its content is excessive, it can combine with Mn, etc., to form non-metallic inclusions, and the risk of brittleness in steel increases; therefore, in the present invention, it is desirable to include its content at 0.015% or less.

[0056] Aluminum (Al): 0.05% or less (excluding 0%)

[0057] Although the above Al contributes to the deoxidation of molten steel, it is not intentionally added in the present invention, and there is no significant impediment to securing physical properties even without adding aluminum. Meanwhile, if the content is excessive, phenomena such as nozzle clogging may occur during continuous casting, so in the present invention, it is preferable to include the content at 0.05% or less.

[0058] Nitrogen (N): 0.01% or less (excluding 0%)

[0059] Although the above N contributes to improving the strength of the steel, it is not intentionally added in the present invention, and there is no significant impediment to securing physical properties even if aluminum is not added. Meanwhile, since the risk of brittleness occurring in the steel increases if the content is excessive, it is preferable to include the content at 0.01% or less in the present invention.

[0060] Copper (Cu): 0.01% or less

[0061] The above Cu plays a role in increasing strength by forming fine precipitates, but in the present invention, there is no significant impediment to securing physical properties even if Cu is not added. Meanwhile, if the content is excessive, cracks may occur on the surface and room-temperature processability may deteriorate; therefore, in the present invention, it is preferable to include a content of 0.01% or less. The above Cu may include 0%.

[0062] The remaining component is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any skilled person in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0063] In order to ensure an appropriate level of weldability, it is desirable for the above steel to contain a carbon equivalent (Ceq) defined by [Equation 1] below of 0.48 or less.

[0064] [Formula 1] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5

[0065] (Here, [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] represent the content (weight%) of the respective element)

[0066] The above steel is characterized by the fact that, during rough rolling, rolling is smooth to the center due to the transformation of the surface layer caused by cooling, and consequently, the difference in grain size between the surface layer and the center is small. Typically, in the case of thick materials, the grain size increases in the center relative to the cross-section of the steel, resulting in a disadvantage of inferior toughness, particularly in DWTT characteristics. However, the steel of the present invention allows the rolling force to propagate to the center without loss due to the transformation of the surface layer caused by cooling during rough rolling, thereby enabling the initial austenite grain size to be refined during rough rolling. Furthermore, as the difference in grain size in the thickness direction becomes small and fine, the crack propagation speed is slowed down if a crack is generated in the surface layer, thereby improving toughness.

[0067] Accordingly, the above steel has a difference in average grain size between the center and the surface layer in the cross-sectional thickness direction (△G = G s - G f It is preferable that ) be 10 μm or less.

[0068] The present invention is characterized by a difference in grain size and microstructure between the surface layer and the core. When the distribution of grain size differs in the thickness direction, the nucleation and crack propagation speeds between fine grains and normal-sized grains are slowed down, thereby improving impact toughness. The present invention is characterized by optimizing the steel composition, managing cooling after rolling, refining the average grain size (AGS) of the surface layer through reheating after cooling, and securing the core microstructure during secondary cooling, thereby simultaneously securing high strength and impact toughness.

[0069] Average grain size (G) in the surface layer of the above steel f) may be 10㎛ or less. The surface layer may extend from the surface to a thickness t / 4 (t: steel thickness) along the length from the center of the steel thickness to the surface. If the average grain size of the surface layer exceeds 10㎛, it is difficult to ensure sufficient toughness. It is preferable that the average grain size of the surface layer be 5 to 10㎛. The average grain size of the center (G s ) can be 5~15㎛.

[0070] Meanwhile, the average grain aspect ratio of the surface layer and the core of the above steel may be 1.5 or higher. In conventional rolling methods, the rolling effect in the core is insufficient, so acicular ferrite with an aspect ratio of 1.5 or higher may be obtained in the surface layer, while polygonal ferrite with an aspect ratio of 1.5 or lower may be obtained in the core. If polygonal ferrite is formed in the core, the frequency of crack occurrence at the microstructure boundary may increase, which may make it difficult to secure impact toughness. Grain size and aspect ratio can be determined by electron backscatter diffraction (EBSD). More specifically, EBSD can be measured 10 times at random locations at a magnification of 500x, and the average value can be obtained by taking the data obtained therefrom using the Grain Shape Aspect Ratio program provided by default in TSL OIM Analysis 6.0 software.

[0071] The microstructure of the above steel is not specifically limited, but may include, for example, ferrite, pearlite, and bainite, in which case the area fraction of bainite may be 90% or less. If the area fraction of bainite is 90% or more, the possibility of cracking occurring during expansion after tube forming may increase due to deterioration of workability.

[0072] The above steel has a yield strength of 560 to 670 MPa, a tensile strength of 630 to 800 MPa, an elongation of 30 to 60%, and a DWTT ductile fracture rate of 85% or more at -45℃, and can have a product thickness of 20 to 50 mm.

[0073]

[0074] Hereinafter, a method for manufacturing a steel plate according to an embodiment of the present invention will be described. The steel material of the present invention can be manufactured by various methods, and the manufacturing method is not particularly limited. However, as a preferred example, it can be manufactured by the following method.

[0075] First, a steel slab having the alloy composition described above is heated. It is preferable to heat the steel slab within a temperature range of 1050 to 1250°C. If the heating temperature is below 1050°C, the rolling load in the subsequent rolling process may become excessively large, and precipitate-forming elements such as Nb may not be sufficiently dissolved and may remain, failing to contribute to precipitate formation in the subsequent process, which may result in a decrease in strength. On the other hand, if the temperature exceeds 1250°C, there is a risk that the grain size of the final microstructure may not be homogeneous due to partial coarsening caused by abnormal growth of some austenite grains. Meanwhile, the present invention does not specifically limit the heating time of the steel slab, and normal conditions are acceptable. As an example that is not limited, the heating time of the steel slab may be 100 to 400 minutes.

[0076] After rough rolling the heated slab, steel is obtained by finishing rolling at the austenite single-phase temperature. The rough rolling refers to a series of intermediate rolling processes performed before finishing rolling. In the present invention, during rough rolling, cooling is performed at the rear end of each rolling pass, and then rolling is performed again. The total number of passes for rough rolling can be 5 to 10 passes. The rough rolling temperature can be set to a temperature at which the finishing rolling temperature can be secured, and it may be 900 to 1050°C. If the rough rolling temperature is below 900°C, the rolling load increases, which may reduce productivity, and if it exceeds 1050°C, the austenite grains become excessively coarsened, making it difficult to secure the target low-temperature toughness after rough rolling.

[0077] Typically, rough rolling is performed in 5 to 10 passes, and the cooling rate between rolling passes can be 5 to 10°C / sec. If the cooling rate between rolling passes is excessively fast or slow, it may be difficult to secure the desired surface layer of ferrite. In addition, it is desirable that the temperature drop due to cooling between rolling be 5 to 30°C.

[0078] The above finish rolling is performed at the austenite single-phase temperature to increase the uniformity of the microstructure. The above finish rolling temperature may be Ar3 or higher, for example, 750 to 900°C. If the finish rolling temperature is below 750°C, the rolling load increases, which may reduce productivity, and there is a risk that the grains may become excessively fine or coarse unrecrystallized austenite may be formed. On the other hand, if it exceeds 900°C, the austenite grains of the slab may become excessively coarse, making it difficult to secure the target strength.

[0079] Next, the steel is accelerated after rolling. The accelerated cooling rate may be 30 to 50°C / sec. To ensure sufficient strength, it is desirable to perform the process at a rate of 30°C / sec or higher; however, if the acceleration is excessive, there is a risk that the MA phase or low-temperature phase will be excessive, leading to a decrease in toughness.

[0080] It is desirable to limit the cooling end temperature to a range of 300 to 500 ℃. At this temperature, ferrite and bainite of an appropriate size of approximately 5 to 15 µm are formed from the fine austenite produced by rolling. If the cooling end temperature exceeds 500 ℃, excessive ferrite growth occurs in the center, and bainite formation is insufficient, resulting in a decrease in strength. Furthermore, if the cooling end temperature is below 300 ℃, insufficient ferrite is secured, leading to a decrease in low-temperature toughness.

[0081] The process following the completion of the above accelerated cooling is not separately limited, and processes commonly performed in the technical field to which the present invention belongs may be carried out.

[0082] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0083] (Example)

[0084] A steel slab having the composition of Table 1 below (the remainder being Fe and unavoidable impurities) was heated to 1140°C for 200 minutes, and then rough rolling and finish rolling were performed. The conditions for rough rolling and finish rolling are shown in Table 2. The thickness of the rough-rolled steel slab was kept constant at 20% of the thickness of the heated steel slab, and accelerated cooling after finish rolling was performed at a cooling rate of 30°C / sec.

[0085] Subsequently, the microstructure of the manufactured steel was analyzed and is shown in Table 3, and the mechanical properties were evaluated, with the results presented in Table 4 below. For reference, in all examples, the microstructure consists of bainite and ferrite. The portion labeled as the surface layer refers to the length from the surface to 1 / 4 of the way from the top and bottom surfaces in the thickness direction.

[0086] In Table 3, the types and analysis of the microstructure were confirmed through image analysis after observation with an optical microscope, and the tensile strength, yield strength, and elongation were determined by performing a room temperature tensile test under 0.5% strain conditions.

[0087] Meanwhile, the impact absorption energy was measured by a Charpy impact test on a standard specimen of 55×55×100 mm at -30℃. The DWTT ductile fracture rate was measured by a drop weight test at -45℃ according to API RP 5L3 standards.

[0088] Classification Alloy Composition (Wt%) Ceq CS iMn Nb VPS AlNC rNiMoCu Steel Grade 10.06 10.24 1.79 0.045 0.00 10.01 0.00 40.025 0.005 0.25 0.01 0.08 0.00 10.426 Steel Grade 20.05 90.26 1.82 0.045 0.002 0.01 0.004 0.025 0.005 0.24 0.01 0.08 0.00 10.427 Steel Grade 30.16 0.25 1.30.03 0.00 10.01 0.004 0.025 0.005 0.15 0.01 0.01 0.00 10.410 Steel Grade 40.040.241.80.040.0020.010.0040.0250.0050.140.20.080.0010.398

[0089] Steel Grade Rough Rolling Start Temperature (°C) Number of Rough Rolling Passes Cooling Rate Between Passes Temperature Decrease Between Passes Rough Rolling End Temperature (°C) Accelerated Cooling End Temperature (°C) Remarks Steel Grade 1 10 50 76 129 20 430 Invention Example 1 1 0 50 10 378 70 450 Comparative Example 1 1 0 50 10 37 10 20 430 Comparative Example 2 Steel Grade 2 1 0 50 77 139 10 430 Invention Example 2 1 0 50 12 378 70 450 Comparative Example 3 1 0 50 12 26 10 20 430 Comparative Example 4 Steel Grade 3 1 0 50 10 37 10 20 430 Comparative Example 5 Steel Grade 4 1 0 50 10 37 10 20 430 Comparative Example 6

[0090] Steel Type Surface Layer Center Average Grain Size Difference (㎛) Remarks Bainite (Area %) Ferrite (Area %) Average Grain Size (㎛) Aspect Ratio Bainite (Area %) Ferrite (Area %) Average Grain Size (㎛) Aspect Ratio Steel Type 140 60 51.8 35 65 61.71 Invention Example 170 30 51.8 40 60 20 1.315 Comparative Example 175 25 51.8 40 60 25 1.420 Comparative Example 2 Steel Type 235 65 61.7 35 65 61.70 Invention Example 265 35 61.7 40 60 21 1.315 Comparative Example 375 25 51.8 40 60 25 1.420 Comparative Example 4 Steel Type 390 10 71.7 80 20 140 1.1 33 Comparative Example 5 Steel Type 4505081.640601401.1132 Comparative Example 6

[0091] Steel Grade Yield Strength (MPa) Tensile Strength (MPa) Total Elongation (%) Impact Absorbed Energy (-30℃, J) DWTT Ductile Fracture Rate (-45℃, %) Remarks Steel Grade 160 368 336 430 100 Invention Example 1610 690 333 50 75 Comparative Example 1620 710 312 80 40 Comparative Example 2 Steel Grade 2599 678 36 432 100 Invention Example 2613 691 333 40 75 Comparative Example 3618 711 312 75 40 Comparative Example 4 Steel Grade 3580 6812 12 10 23 Comparative Example 5 Steel Grade 4550 610 52 450 100 Comparative Example 6

[0092] As can be seen in Table 4, when all the alloy composition and manufacturing conditions proposed in the present invention are satisfied, excellent strength can be secured without adding a large amount of expensive elements such as Ni, while excellent low-temperature impact toughness can be secured.

[0093] In contrast, when deviating from the alloy composition or manufacturing process, it was difficult to secure the mechanical properties required by the present invention.

Claims

1. In wt%, C: 0.05~0.10%, Si: 0.5% or less (excluding 0%), Mn: 0.5~2.5%, Nb: 0.07% or less, V: 0.03% or less, Mo: 0.2% or less, Ni: 0.3% or less, Cr: 0.1~0.3%, P: 0.03% or less (excluding 0%), S: 0.05% (excluding 0%), Al: 0.05% or less (excluding 0%), N: 0.01% or less (excluding 0%), Cu: 0.01% or less, and the remainder comprises Fe and unavoidable impurities, Difference in average grain size between the center and the surface (△G = G s - G f ) is 10 μm or less, and Steel having a grain aspect ratio of 1.5 or higher between the surface layer and the center.

2. In Paragraph 1, The above steel is a steel having a carbon equivalent (Ceq) of 0.48 or less as defined by the following [Equation 1]. [Formula 1] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5 (Here, [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] represent the content (weight%) of the respective element) 3. In Paragraph 1, The above surface layer extends from the steel surface to a thickness t / 4 (t: steel thickness), and the average grain size (G) of the surface layer f ) is steel material with a thickness of 10㎛ or less.

4. In Paragraph 1, The above-mentioned central microstructure is a steel material in which bainite is 90% or less in area fraction.

5. In Paragraph 1, The above steel is a steel with a thickness of 20 to 50 mm.

6. In Paragraph 1, The above steel has a yield strength of 560 to 670 MPa, a tensile strength of 630 to 800 MPa, and a total elongation of 30 to 60%.

7. In Paragraph 1, The above steel is a steel having a DWTT ductile fracture rate of 85% or more at -45℃.

8. A step of heating a steel slab containing, in wt%, C: 0.05~0.10%, Si: 0.5% or less (excluding 0%), Mn: 0.5~2.5%, Nb: 0.07% or less, V: 0.03% or less, Mo: 0.2% or less, Ni: 0.3% or less, Cr: 0.1~0.3%, P: 0.03% or less (excluding 0%), S: 0.05% (excluding 0%), Al: 0.05% or less (excluding 0%), N: 0.01% or less (excluding 0%), Cu: 0.01% or less, and the remainder being Fe and unavoidable impurities, to a temperature range of 1050~1250℃; A step of rough rolling the above steel slab, wherein the rough rolling is performed in a temperature range of 900 to 1050℃; After the above rough rolling, a step of finish rolling at the austenite single-phase region temperature; and A step of accelerating cooling to a range of 300~500℃ at a cooling rate of 30~50℃ / sec after the above finishing rolling; A method for manufacturing steel materials including 9. In Claim 8, A method for manufacturing steel in which the above steel slab heating is performed for 100 to 400 minutes.

10. In claim 8, A method for manufacturing steel in which the above finishing rolling temperature is 750~900℃.

11. In Claim 8, A method for manufacturing steel in which the cooling rate between rolling passes during the above rough rolling is 5~10℃ / sec.

12. In claim 8, A method for manufacturing steel in which the temperature drop between rolling passes during the above rough rolling is 5 to 30℃.