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 capture carbon dioxide (CO2) and transport it to a storage site. As steel for the line pipes, API steel may be used. API steel refers to steel that meets the specifications defined by the American Petroleum Institute.

[0003] When capturing carbon dioxide and transporting it to a storage site, the material requires a high impact energy value to prevent the propagation of fracture when fracture occurs. In addition, high strength of the material is required to stably protect the structure against high-pressure transport environments and external impacts. Therefore, conventionally, as steel for API for CO2 transport, thick plate steel has been mainly used 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 prevalent, making it difficult to achieve high impact toughness and impact energy values ​​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 steel material having excellent low-temperature impact toughness while securing high strength 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 (excluding 0%), V: 0.03% or less (excluding 0%), 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] The average grain size (AGS) of the surface layer of the steel is 10㎛ or less, and

[0011] The difference in average grain size between the surface layer and the center of the steel can be 10 to 25 µm.

[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 to 15% of the length from the surface of the steel in the thickness direction to the center of the steel, and the microstructure of the above surface layer may include 50% or more of bainite and the remainder of acicular ferrite in terms of area fraction.

[0016] The microstructure of the above-mentioned center may contain at least 20% bainite and the remainder acicular ferrite in terms of area fraction.

[0017] The above steel material may have a thickness of 5 to 30 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 Charpy impact absorption energy of 400J or more at -30℃.

[0020]

[0021] 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 (excluding 0%), V: 0.03% or less (excluding 0%), 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℃;

[0022] A step of rough rolling the above steel slab, followed by finish rolling at the austenite single-phase temperature;

[0023] A step of primary cooling to a primary cooling end temperature of 700~900℃ after the above finishing rolling;

[0024] A step of air cooling for 3 to 5 seconds after the above first cooling; and

[0025] It may include a step of secondary cooling to a secondary cooling end temperature of 300 to 500°C after the above air cooling.

[0026] The heating of the above steel slab can be performed for 100 to 400 minutes.

[0027] The above finishing rolling temperature may be 800~950℃.

[0028] The difference between the above finishing rolling temperature and the first cooling end temperature may be 50 to 100°C.

[0029] First cooling can be performed within 1 to 5 seconds after the above finishing rolling.

[0030] The above first cooling rate may be 10 to 50℃ / sec.

[0031] The above secondary cooling rate may be 30~50℃ / sec.

[0032] According to the present invention, it is possible to provide a steel material for line pipes that has excellent low-temperature impact toughness while ensuring high strength. In particular, it may be suitable for line pipes for CO2 transport.

[0033] 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.

[0034] Embodiments of the present invention are provided to more fully explain the invention to those with average knowledge in the art. Meanwhile, the shapes and sizes of elements in the drawings may be exaggerated for clearer explanation.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.%).

[0039] The above steel contains, in weight%, C: 0.05~0.10%, Si: 0.5% or less (excluding 0%), Mn: 0.5~2.5%, Nb: 0.07% or less (excluding 0%), V: 0.03% or less (excluding 0%), 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.

[0040] Carbon (C): 0.05~0.10%

[0041] 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 0.50 to 0.100%.

[0042] Silicon (Si): 0.5% or less (excluding 0%)

[0043] 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.

[0044] Manganese (Mn): 0.5~2.5%

[0045] 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%.

[0046] Niobium (Nb): 0.07% or less (excluding 0%)

[0047] The above Nb is a precipitation-strengthening element that is effective in securing strength by refining the grain size 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. Preferably, it may be 0.070% or less.

[0048] Vanadium (V): 0.03% or less (excluding 0%)

[0049] 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. Preferably, it may be 0.030% or less.

[0050] Molybdenum (Mo): 0.2% or less

[0051] 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%.

[0052] Nickel (Ni): 0.3% or less

[0053] 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%.

[0054] Chrome (Cr): 0.1~0.3%

[0055] 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 desirable 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 desirable to include a content of 0.3% or less. Preferably, it may be 0.10 to 0.30%.

[0056] Phosphorus (P): 0.03% or less (excluding 0%)

[0057] 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.

[0058] Sulfur (S): 0.05% (excluding 0%)

[0059] 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.

[0060] Aluminum (Al): 0.05% or less (excluding 0%)

[0061] 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.

[0062] Nitrogen (N): 0.01% or less (excluding 0%)

[0063] 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.

[0064] Copper (Cu): 0.01% or less

[0065] 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%.

[0066] 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.

[0067] 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.

[0068] [Formula 1] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5

[0069] (Here, [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] represent the content (weight%) of the respective element)

[0070] Average grain size (G) in the surface layer of the above steel f The thickness of the surface layer may be 10㎛ or less. The surface layer may be up to 15% of the length from the surface to the center of the thickness of the steel plate. It is preferable that the thickness of the surface layer be 100㎛ or more. If the thickness of the surface layer is less than 100㎛, it is not easy to secure sufficient crack resistance. On the other hand, if the thickness of the surface layer is too thick, although there is no particular disadvantage, it may be 15% or less of the thickness length, as reverse transformation due to double heat may occur and grain refinement is difficult.

[0071] If the average grain size of the surface layer exceeds 10㎛, it is difficult to secure sufficient toughness. It is preferable that the average grain size of the surface layer be 3 to 10㎛.

[0072] The microstructure of the surface layer of the steel may comprise 50% or more of bainite and the remainder of acicular ferrite in terms of area percentage. In addition to the bainite and acicular ferrite, it may include structures that may inevitably occur during the manufacturing process. The remainder excluding the surface layer may form the core. The fraction of bainite in the surface layer is not specifically limited, but considering the composition and process of the present invention, it may be 90% or less.

[0073] The microstructure of the central part above may include 20% or more of bainite and the remainder of acicular ferrite in terms of area percentage. In addition to the bainite and acicular ferrite, it may include structures that may inevitably occur during the manufacturing process. In the present invention, the upper limit of the bainite structure in the central part is not specifically limited, but may be 60% or less.

[0074] Here, "other tissues" refers to phases that may be unintentionally formed during the manufacturing process.

[0075] Average grain size (G) of the center above s ) can be 20~30㎛.

[0076] The difference in average grain size between the central and surface layers (△G = G s - G f It is preferable that ) be 10 to 25 μm.

[0077] 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.

[0078] The above steel has a yield strength of 560 to 670 MPa, a tensile strength of 630 to 800 MPa, a total elongation of 30 to 60%, and a Charpy impact energy of 400 J or more at -30℃, and the steel may have a thickness of 5 to 30 mm.

[0079]

[0080] 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.

[0081] 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 local 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.

[0082] After rough rolling the above heated slab, steel is obtained by finish rolling at the austenite single-phase temperature.

[0083] The above rough rolling refers to a series of intermediate rolling processes performed prior to finish rolling. In the present invention, the specific conditions for rough rolling are not specifically limited, and ordinary conditions are acceptable. The rough rolling temperature may be set to a sufficiently high temperature to ensure the finish rolling temperature is achieved.

[0084] 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 Ac3 or higher, for example, 750 to 950°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 950°C, the austenite grains of the slab may become excessively coarse, making it difficult to secure the target strength.

[0085] Next, the steel is cooled after rolling. During this process, the cooling proceeds in multiple stages: primary cooling, air cooling, and secondary cooling. After fine ferrite is formed on the surface layer, reverse transformation occurs due to double heating during the air cooling phase leading up to secondary cooling, thereby refining the average grain size (AGS) of the surface layer. During secondary cooling, fine bainite and ferrite are formed from the refined austenite on the surface layer. By appropriately controlling the ratio of bainite to ferrite formed in the center, sufficient strength can be secured, and toughness and yield ratio can be improved.

[0086] The above-mentioned first cooling end temperature is preferably in the range of 700 to 900°C. If the first cooling end temperature exceeds 900°C, the likelihood of forming coarse bainite increases, leading to a high probability of deterioration in toughness properties. Conversely, if the cooling end temperature is below 700°C, subsequent reheating is not sufficiently achieved, making it difficult to secure fine austenite that has undergone reverse transformation up to the surface layer. Consequently, it becomes difficult to secure sufficient bainite during the subsequent cooling process, which may result in a decrease in tensile strength. The above-mentioned first cooling is preferably performed within 1 to 5 seconds after the end of rolling, and can be carried out using a proximity cooling device; if it exceeds 5 seconds, the grains may grow excessively. The cooling rate of the above-mentioned first cooling is preferably performed in the range of 10 to 50°C / sec. While the cooling rate is not specifically limited, an appropriate upper limit may exist due to the characteristics of the thick plate material.

[0087] It is preferable that the difference between the above finishing rolling temperature and the first cooling end temperature be 50 to 100℃.

[0088] After the first cooling mentioned above, air cooling is performed. Through the air cooling, reheating occurs from the inside, causing the bainite and ferrite transformed in the surface layer to undergo reverse transformation back into austenite. During this reverse transformation, fine austenite is formed in the surface layer. Although the air cooling period is not specifically limited, if the air cooling time is not within the specified range, insufficient reheating occurs, resulting in an insufficient amount of reverse transformation; conversely, if the air cooling time is long, grain growth occurs, making it difficult to produce fine grains, and the temperature decreases during air cooling, making it difficult to secure a sufficient temperature during the second cooling. For example, the air cooling time may be 3 to 5 seconds.

[0089] After the above air cooling, a secondary cooling is performed. It is preferable that the secondary cooling end temperature be 300 to 500°C. At this temperature, a mixed structure of fine bainite and ferrite transformed from fine austenite is formed in the surface layer, while ferrite and bainite of an appropriate size of approximately 20 to 30 µm are formed from untransformed austenite in the center. If the cooling end temperature exceeds 500°C, 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°C, a decrease in low-temperature toughness occurs because sufficient ferrite is not secured. The above secondary cooling can be performed at a cooling rate of 30 to 50°C / sec. If the cooling rate is less than 30°C / sec, it is difficult to secure sufficient strength, and if it exceeds 50°C / sec, the low-temperature transformation phase is excessively formed, making it difficult to secure toughness.

[0090] The process after the above secondary cooling is not separately limited, and processes commonly performed in the technical field to which the present invention belongs may be carried out.

[0091] 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.

[0092] (Example)

[0093] A steel slab having the composition of Table 1 below (the remainder being Fe and unavoidable impurities) was heated to 1100°C for 200 minutes, and then rough rolling and finish rolling were performed. The finish rolling temperature is shown in Table 2. Meanwhile, after finish rolling, a steel material was manufactured by performing first cooling, air cooling, and second cooling. The thickness of the rough-rolled steel slab was kept constant at 20% of the thickness of the heated steel slab, and the first and second coolings were performed at a cooling rate of 30°C / sec.

[0094] Classification Alloy Composition (Wt%) Ceq CS iMn Nb VPS AlNC rNiMoCu Steel Grade 10.05 90.25 1.8 30.04 40.00 10.01 0.00 40.025 0.00 50.25 0.21 0.08 0.00 10.444 Steel Grade 20.06 20.24 1.8 10.04 50.00 20.01 0.00 40.025 0.00 50.24 0.20.08 0.00 10.441 Steel Grade 30.16 0.25 1.3 0.03 0.00 10.01 0.00 40.025 0.00 50.15 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

[0095] Steel Grade Finish Rolling Temperature (°C) 1st Cooling End Temperature Air Cooling Time (sec) 2nd Cooling End Temperature (°C) Remarks Steel Grade 1880 830 4430 Invention Example 1880 820 8450 Comparative Example 1880 430 - (None) - Comparative Example 2 Steel Grade 2880 800 4430 Invention Example 2880 800 8450 Comparative Example 3880 430 - (None) - Comparative Example 4 Steel Grade 3880 830 4430 Comparative Example 5 Steel Grade 4880 830 4430 Comparative Example 6

[0096] Subsequently, the microstructure of the manufactured steel was analyzed, and the results are shown in Table 3. The mechanical properties were evaluated, and the results are shown in Table 4 below. In Table 3, the surface layer refers to the length from the surface to 15% of the center of the thickness in the thickness direction on the upper and lower surfaces. The rest refers to the center. In Table 3, the difference in average grain size indicates the difference in average grain size between the center and the surface layer.

[0097] In Table 3, the types and analysis of the microstructure were confirmed through image analysis of sub-images observed 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.

[0098] Meanwhile, the impact absorption energy was measured by a Charpy impact test on a standard specimen of 55 x 55 x 100 mm at -30°C. The DWTT ductile fracture rate was measured by a drop weight test at -20°C according to API RP 5L3 standards.

[0099] Steel Type Surface Layer Center Average Grain Size Difference (㎛) Remarks Bainite (Area %) Ferrite (Area %) Average Grain Size (㎛) Bainite (Area %) Ferrite (Area %) Average Grain Size (㎛) Steel Type 160 405 4060 2520 Invention Example 130 70232080241 Comparative Example 180 20156040227 Comparative Example 2 Steel Type 265 354 40602420 Invention Example 230 70224060231 Comparative Example 380 20166040226 Comparative Example 4 Steel Type 385 15785 152215 Comparative Example 5 Steel Type 435 65825752416 Comparative Example 6

[0100] Steel Grade Yield Strength (MPa) Tensile Strength (MPa) Total Elongation (%) Impact Absorbed Energy (-30℃, J) DWTT Ductile Fracture Rate (-20℃, %) Remarks Steel Grade 160 368 336 430 100 Invention Example 1570 627 48 450 95 Comparative Example 1620 710 312 80 40 Comparative Example 2 Steel Grade 2599 678 36 432 100 Invention Example 2571 625 48 45 595 Comparative Example 361 871 131 27 540 Comparative Example 4 Steel Grade 3650 720 212 1023 Comparative Example 5 Steel Grade 4550 610 52 450 100 Comparative Example 6

[0101] 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.

[0102] 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 (excluding 0%), V: 0.03% or less (excluding 0%), 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, The average grain size (AGS) of the surface layer of the steel is 10㎛ or less, and Steel having an average grain size difference of 10 to 25 μm 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 to 15% of the length from the surface of the steel to the center of the steel in the thickness direction, and the microstructure of the above surface layer comprises, in terms of area fraction, 50% or more of bainite and the remainder being acicular ferrite, steel.

4. In Paragraph 1, A steel in which the microstructure of the above-mentioned center comprises, by area fraction, 20% or more of bainite and the remainder of acicular ferrite.

5. In Paragraph 1, The above steel is a steel with a thickness of 5 to 30 mm.

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

7. In Paragraph 1, The above steel is a steel having a Charpy impact absorption energy of 400J or more at -30℃.

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 (excluding 0%), V: 0.03% or less (excluding 0%), 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, followed by finish rolling at the austenite single-phase temperature; A step of primary cooling to a primary cooling end temperature of 700~900℃ after the above finishing rolling; A step of air cooling for 3 to 5 seconds after the above first cooling; and A step of secondary cooling to a secondary cooling end temperature of 300~500℃ after the above air cooling; 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 800~950℃.

11. In Claim 8, A method for manufacturing steel in which the difference between the above-mentioned finishing rolling temperature and the first cooling end temperature is 50 to 100℃.

12. In claim 8, A method for manufacturing steel by performing primary cooling within 1 to 5 seconds after the above-mentioned finishing rolling.

13. In claim 8, A method for manufacturing steel in which the above first cooling rate is 10 to 50℃ / sec.

14. In Claim 8, A method for manufacturing steel in which the above secondary cooling rate is 30~50℃ / sec.