Steel material for pipeline having excellent low-temperature toughness and method for manufacturing same

Abstract

The present invention relates to a high-strength API steel material having excellent low-temperature toughness and cost effectiveness, and a method for manufacturing the same, and more particularly, to a high-strength API steel material which has outstanding characteristics and can be used for transporting crude oil, and a method for manufacturing the same.

Description

Technical Field

[0001] The present disclosure relates to high-strength API steel materials having excellent low-temperature toughness and cost-effectiveness, and high-strength API steel materials having outstanding properties that can be used for transporting crude oil and a method for manufacturing the same. Background Art

[0002] In the case of API steel materials for pipelines used when transporting crude oil after extraction to a place of use, high strength and deformation stability of the material are required to stably protect the structure from deformation caused by external factors and external impacts such as earthquakes. Therefore, as API materials for transporting crude oil, thick plate steel materials used in the related art are mainly used, and the strength is enhanced by adding a large amount of solid solution strengthening elements such as C, Si, Mn, Cr, etc. to high-purity steel having the least impurities in the steel or by adding hardenable elements such as Ni, Mo thereto.

[0003] However, with the recent increase in demand for thick high-strength materials, expensive elements such as Ni, Mo, etc. have been added in large amounts to ensure the strength of thick plate materials. Therefore, in order to ensure the foregoing strength, the heating temperature, rolling temperature, cooling end temperature, etc. are greatly reduced, resulting in a decrease in productivity, and since the cooling end temperature is reduced, a rapid cooling structure is generated on the surface layer, resulting in problems such as hard spots.

[0004] [Prior Art Documents]

[0005] [Patent Documents]

[0006] (Patent Document 1) Korean Patent Publication No. 2014-0002273 Summary of the Invention

[0007] Technical Problem

[0008] One aspect of the present disclosure is to provide thick high-strength API steel materials having excellent low-temperature toughness and economic benefits and a method for manufacturing the same.

[0009] The tasks of the present disclosure are not limited to the above. Those of ordinary skill in the technical field to which the present disclosure pertains will not have difficulty understanding additional tasks of the present disclosure based on the content throughout the specification of the present disclosure.

[0010] Technical Solution

[0011] According to one aspect of the present disclosure, the steel for pipes comprises: C: 0.030% to 0.100% by weight, Si: 0.50% or less and excluding 0%, Mn: 0.50% to 2.50%, Nb: 0.070% or less and excluding 0%, V: 0.030% or less and excluding 0%, Mo: 0.05% or less and excluding 0%, Ni: 0.05% or less and excluding 0%, Cr: 0.10% to 0.30%, P: 0.03% or less and excluding 0%, S: 0.050% or less and excluding 0%, Al: 0.050% or less and excluding 0%, N: 0.010% or less and excluding 0%, and Cu: 0.010% or less and excluding 0%, and Fe and inevitable impurities as the balance,

[0012] wherein the steel comprises: ferrite selected from 80% or more and including 100% as the microstructure, and at least one of pearlite and bainite as the balance, in area%, and

[0013] a surface layer portion having an average grain size of 10 μm to 15 μm in the thickness direction.

[0014] Furthermore, according to another aspect of the present disclosure, a method for manufacturing steel for pipes comprises:

[0015] an operation of reheating a slab at 1050°C to 1250°C, the slab comprising: C: 0.030% to 0.100% by weight, Si: 0.50% or less and excluding 0%, Mn: 0.50% to 2.50%, Nb: 0.070% or less and excluding 0%, V: 0.030% or less and excluding 0%, Mo: 0.05% or less and excluding 0%, Ni: 0.05% or less and excluding 0%, Cr: 0.10% to 0.30%, P: 0.03% or less and excluding 0%, S: 0.050% or less and excluding 0%, Al: 0.050% or less and excluding 0%, N: 0.010% or less and excluding 0%, and Cu: 0.010% or less and excluding 0%, and the balance of Fe and inevitable impurities;

[0016] an operation of rough rolling the reheated slab and then finish rolling at the austenite single-phase temperature to obtain thick plate steel;

[0017] a first cooling operation of water-cooling the thick plate steel at an average cooling rate of 5°C / second to 25°C / second to a temperature of 700°C to 800°C after finish rolling;

[0018] a second cooling operation of air-cooling the steel after the first cooling; and

[0019] A third cooling operation of water-cooling the second-cooled steel to a temperature of 300°C to 500°C at an average cooling rate greater than 25°C / second and less than or equal to 60°C / second.

[0020] Beneficial effects

[0021] As one of many effects of the present disclosure, thick high-strength API steel and a method for manufacturing the same with excellent low-temperature toughness and economic efficiency can be provided.

[0022] The various and beneficial advantages and effects of the present disclosure are not limited to the above, and will be more easily understood during the process of describing the detailed embodiments of the present disclosure. Detailed embodiments

[0023] Hereinafter, preferred embodiments of the present disclosure will be described. However, the embodiments of the present disclosure can be modified in various other forms, and the scope of the present disclosure is not limited to the embodiments described below. In addition, the embodiments of the present disclosure are provided to allow those of ordinary skill in the relevant technical field to more fully understand the present disclosure.

[0024] In addition, the terms used in this specification are for the purpose of describing specific embodiments and are not intended to limit the present disclosure. For example, unless the relevant definition clearly indicates the contrary meaning, the singular forms used in this specification also include the plural forms. In addition, the meaning of "including / containing" used in this specification designates a configuration and does not exclude the existence or addition of other configurations.

[0025] Hereinafter, one aspect of the present disclosure regarding steel for pipelines (or, API steel) will be described in detail.

[0026] First, the reasons for adding alloy components and the content limitation ranges to the API thick plate steel of the present disclosure will be described in detail. It should be noted in advance that unless otherwise specified, the content of each component described below is based on wt%.

[0027] C: 0.030% to 0.100%

[0028] Carbon (C) is the most economical and effective element for ensuring strength. If the carbon content is too low, it may be difficult to ensure the target strength even if precipitation strengthening elements such as Nb are added, so the lower limit of the carbon addition amount is set to 0.030%. On the other hand, if the carbon content is excessive, ductility may deteriorate due to excessive strength increase, so the upper limit of the carbon addition amount is set to 0.100%. At the same time, for further improving the foregoing effects, the lower limit of the carbon content can be 0.040%, or the upper limit of the carbon content can be 0.090%.

[0029] Si: 0.50% or less and excluding 0%

[0030] Silicon (Si) is an element that contributes to an increase in strength through the deoxidation of molten steel and solid solution strengthening. However, in the present disclosure, silicon is not intentionally added, and even without adding silicon, there are no significant problems in ensuring physical properties. Considering the case where this element is inevitably contained during the manufacturing process, the lower limit of the silicon content is not set at 0%. On the other hand, if the silicon content is excessively high, red scale due to Si is formed on the surface of thick plate steel, which may deteriorate the surface quality and weldability. Therefore, the content is set at 0.50%. At the same time, for further improving the aforementioned effects, the lower limit of the silicon content can be 0.01%, or the upper limit of the silicon content can be 0.40%.

[0031] Mn: 0.50% to 2.50%

[0032] Manganese (Mn) is an effective element for strengthening steel through solid solution strengthening, and it is necessary to add 0.50% or more to ensure appropriate strength. However, if the manganese content is too high, there is a risk of central segregation occurring during the continuous casting process. Therefore, the upper limit of the manganese content is set at 2.5%. At the same time, for further improving the aforementioned effects, the lower limit of the manganese content can be 0.60%, or the upper limit of the manganese content can be 2.40%.

[0033] Nb: 0.070% or less and excluding 0%

[0034] Niobium (Nb) is a precipitation strengthening element that is effective in ensuring strength by forming NbC series precipitates and refining grains. On the other hand, if the niobium content is excessively high, the disadvantage is that due to the effect of grain refinement, it is difficult to achieve a low yield ratio by increasing the yield strength compared to the tensile strength. Therefore, in the present disclosure, the niobium content is controlled to be 0.070% or less. However, considering the case where this element is inevitably contained, 0% is excluded from the lower limit of the niobium content. At the same time, for further improving the aforementioned effects, the lower limit of the niobium content can be 0.010%, or the upper limit of the niobium content can be 0.060%.

[0035] V: 0.030% or less and excluding 0%

[0036] Vanadium (V) is also a precipitation strengthening element and is an effective element for ensuring the strength of steel. However, if the vanadium content is excessively high, due to a large number of precipitates, the low-temperature toughness and weldability are reduced, which increases the cost of the alloy. Therefore, in the present disclosure, the vanadium content is controlled to be 0.030% or less. However, considering the case where this element is inevitably contained, 0% is excluded from the lower limit of the vanadium content. At the same time, for further improving the foregoing effects, the lower limit of the vanadium content can be 0.010%, or the upper limit of the vanadium content can be 0.020%.

[0037] Mo: 0.05% or less and excluding 0%

[0038] Molybdenum (Mo) is a representative element for improving the hardenability of steel and greatly improves the ability to produce low-temperature structures even at low cooling rates. This is an effective element for ensuring the strength of steel by forming low-temperature structures such as bainite, etc. Molybdenum is not only a relatively expensive element, but also if its content is too high, the toughness may deteriorate. Therefore, in the present disclosure, for economic reasons, molybdenum is not artificially added, and the upper limit of the molybdenum content is set to 0.05%. However, considering the case where this element is inevitably contained, 0% is excluded from the lower limit of the molybdenum content. At the same time, for further improving the foregoing effects, the lower limit of the molybdenum content can be 0.005%, or the upper limit of the molybdenum content can be 0.04%.

[0039] Ni: 0.05% or less and excluding 0%

[0040] Nickel (Ni) plays a role in improving both the strength and toughness of steel, but in the present disclosure, even without adding nickel, there are no significant problems in ensuring physical properties, so there is no need to artificially add nickel. On the other hand, if the nickel content is excessive, the economic benefits are reduced, so in the present disclosure, the content is 0.05% or less. However, considering the case where this element is inevitably contained, 0% is excluded from the lower limit of the nickel content. At the same time, for further improving the foregoing effects, the lower limit of the nickel content can be 0.005%, or the upper limit of the nickel content can be 0.04%.

[0041] Cr: 0.10% to 0.30%

[0042] Chromium (Cr) strengthens steel by solid solution and delays the bainite phase transformation during cooling, thereby contributing to the formation of equiaxed ferrite, and especially when added together with Mo, it effectively improves the hardenability. In the present disclosure, it is preferably added at 0.10% or more to obtain this effect. On the other hand, if the chromium content is excessive, the weldability and brittleness deteriorate, so in the present disclosure, the content is set to 0.30% or less. At the same time, for further improving the foregoing effects, the lower limit of the chromium content can be 0.12%, or the upper limit of the chromium content can be 0.20%.

[0043] P: 0.03% or less and excluding 0%

[0044] Phosphorus (P) is an impurity inevitably contained in steel, and it is desirable to control its content as low as possible. In particular, if the phosphorus content is excessive, the weldability deteriorates and the risk of steel brittleness increases. Therefore, in the present disclosure, the phosphorus content is controlled to 0.03% or less. However, considering the inevitable inclusion of this element, 0% is excluded from the lower limit of the phosphorus content. At the same time, for further improving the aforementioned effects, the lower limit of the phosphorus content can be 0.001%, or the upper limit of the phosphorus content can be 0.025%.

[0045] S: 0.050% or less and excluding 0%

[0046] Sulfur (S) is an impurity inevitably contained in steel, and it is desirable to control its content as low as possible. In particular, if the content is excessive, it may combine with Mn or the like to form non-metallic inclusions, and the risk of steel brittleness increases. Therefore, in the present disclosure, the sulfur content is controlled to 0.050% or less. However, considering the inevitable inclusion of this element, 0% is excluded from the lower limit of the sulfur content. At the same time, for further improving the aforementioned effects, the lower limit of the sulfur content can be 0.001%, or the upper limit of the sulfur content can be 0.045%.

[0047] Al: 0.050% or less and excluding 0%

[0048] Aluminum (Al) contributes to the deoxidation of molten steel, but aluminum is not intentionally added in the present disclosure, and there are no significant problems in ensuring physical properties even without adding aluminum. At the same time, if the aluminum content is excessive, nozzle blockage or the like may occur during continuous casting. Therefore, in the present disclosure, the aluminum content is controlled to 0.050% or less. However, considering the inevitable inclusion of this element, 0% is excluded from the lower limit of the aluminum content. At the same time, for further improving the aforementioned effects, the lower limit of the aluminum content can be 0.010%, or the upper limit of the aluminum content can be 0.040%.

[0049] N: 0.010% or less and excluding 0%

[0050] Nitrogen (N) helps to improve the strength of steel. However, in the present disclosure, nitrogen is not intentionally added, and even without adding nitrogen, there are no significant problems in ensuring physical properties. At the same time, if the nitrogen content is excessive, the risk of steel brittleness increases. Therefore, in the present disclosure, the content is controlled to be 0.010% or less. However, considering the situation where this element is inevitably contained therein, 0% is excluded from the lower limit of the nitrogen content. At the same time, for further improving the aforementioned effects, the lower limit of the nitrogen content can be 0.001%, or the upper limit of the nitrogen content can be 0.009%.

[0051] Cu: 0.010% or less and excluding 0%

[0052] Copper (Cu) forms fine precipitates to improve strength. However, in the present disclosure, even without adding copper, there are no significant problems in ensuring physical properties. On the other hand, if the content is excessive, cracks appear in the surface part and the workability at room temperature deteriorates. Therefore, in the present disclosure, the content is controlled to be 0.010% or less. However, considering the situation where this element is inevitably contained therein, 0% is excluded from the lower limit of the copper content. On the other hand, for further improving the aforementioned effects, the lower limit of the copper content can be 0.0001%, or the upper limit of the copper content can be 0.009%.

[0053] Except for the above components, the remainder is Fe. However, since it is impossible to exclude non-intended inevitable impurities from raw materials or the surrounding environment that may inevitably be mixed in during the conventional manufacturing process. Since these impurities are known to those of ordinary skill in the art, not all of them are specifically mentioned in this specification, but representative impurities are as follows.

[0054] At the same time, in the steel, the carbon equivalent (Ceq) defined by the following relational expression 1 can be controlled to be 0.45 or less but not including 0 to ensure an appropriate level of weldability.

[0055] [Relational expression 1]

[0056] Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5

[0057] (In the above relational expression 1, each of [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] represents the content (wt%) of the element in the corresponding parentheses.)

[0058] In the present disclosure, the microstructure of the steel is not particularly limited. For example, the steel may include ferrite, pearlite, and bainite as the microstructure. In particular, according to one embodiment of the present disclosure, the steel may include ferrite selected in an area percentage of 80% or more and including 100%, and at least one of the remaining pearlite and bainite as the microstructure. If the area ratio of ferrite is less than 80%, the possibility of cracks occurring during the expansion after tube forming may increase due to the deterioration of workability.

[0059] According to one embodiment of the present disclosure, the average grain size in the surface layer portion in the thickness direction of the steel may be 10 μm to 15 μm. The thick plate steel has a characteristic that due to the following multi-stage cooling, the grain sizes of the surface layer portion and the central portion are different. When the grain size distribution of the steel is different in the thickness direction, the nucleation and crack propagation speeds between the fine grains and the normal-sized grains slow down, thereby improving toughness. At this time, ensuring toughness is more favorable when the grains are differently distributed in the thickness direction than when the grains are overall fine. Therefore, the present disclosure aims to simultaneously ensure strength and toughness by refining the grain size of the surface layer portion in the first cooling and ensuring appropriate grains and microstructure in the second cooling.

[0060] Meanwhile, for further improving the aforementioned effects, the lower limit of the average grain size of the surface layer portion may be 11 μm, or the upper limit of the average grain size of the surface layer portion may be 14.5 μm.

[0061] Meanwhile, in this specification, the grain size represents the average size of the phases including ferrite, pearlite, and bainite, and the size represents the equivalent diameter of a circle.

[0062] Furthermore, according to one embodiment of the present disclosure, the definition of the surface layer portion is not particularly limited and can be specified by a conventional method in the relevant technical field. As one embodiment, the surface layer portion may represent a region corresponding to 15% of the total thickness in the thickness direction starting from at least one surface of the steel. Therefore, it may mean a region corresponding to a total of 30% (including 15% of the total thickness in the thickness direction based on the two surfaces of the steel). In addition, the average grain size can be measured by separately measuring the grain sizes at several equally spaced points (for example, n points; n is a positive constant) in the thickness direction of the above-mentioned surface layer portion and calculating their average value.

[0063] Although there is no particular limitation, according to an embodiment of the present disclosure, the steel for pipelines having the foregoing alloy components has the advantage of excellent economic efficiency by reducing the content of expensive alloy elements such as nickel (Ni), molybdenum (Mo), etc. And according to a non-limiting example, the steel of the present disclosure exhibits physical properties such as a yield strength of 550 MPa to 700 MPa, a tensile strength of 600 MPa to 800 MPa, and an elongation of 30% to 60%, and the product thickness can be 30 mmt to 50 mmt.

[0064] Although there is no particular limitation, according to an embodiment of the present disclosure, the average grain size of the central portion in the thickness direction can be larger than the average grain size of the surface layer portion. As described above, in the present disclosure, by controlling the grain sizes of the central portion and the surface layer portion differently, the nucleation and crack propagation rates between fine grains and normal-sized grains are slowed down, thereby exerting the effect of improving toughness.

[0065] At this time, there is no particular limitation on the definition of the central portion, and it can be specified by a conventional method in the relevant technical field. As an example of an embodiment, the central portion can represent the central region of the steel for the entire thickness in the thickness direction except for the surface layer portion (for example, corresponding to the region of 70% of the entire thickness in the thickness direction except for the surface layer portion, and the surface layer portion is the region corresponding to 15% from the respective two surfaces (corresponding to 30% in total based on the two surfaces)). At the same time, when measuring the average grain size of the above-mentioned central portion, it can be measured in the same manner as measuring the average grain size of the above-mentioned surface layer portion. And for example, it can be measured by respectively measuring the grain sizes of several points (for example, n points; n is a positive constant) equally spaced in the thickness direction with respect to the above-mentioned central portion and calculating their average value.

[0066] At the same time, according to an embodiment of the present disclosure, for further improving the foregoing effect, the average grain size of the central portion can be larger than 15 μm and 30 μm or less, or the lower limit of the average grain size of the central portion can be 16 μm, or the upper limit of the average grain size of the central portion can be 25 μm.

[0067] Although there is no particular limitation, according to an embodiment of the present disclosure, the difference between the average grain size of the central portion and the average grain size of the surface layer portion can be controlled to exceed 5 μm. By controlling the grain sizes of the central portion and the surface layer portion differently, the nucleation and crack propagation rates between fine grains and normal-sized grains are slowed down, enabling improvement in toughness. However, if the size difference is too large, it may instead impair the physical properties, so it can be controlled within an appropriate range. For further improving the aforementioned effect, the difference between the average grain size of the central portion and the average grain size of the surface layer portion can be 6 μm or more, or can be 10 μm or less.

[0068] The steel materials of the present disclosure described above can be manufactured in various ways, and the manufacturing method is not particularly limited. However, as a preferred example, it can be manufactured by the following method.

[0069] First, the slab having the aforementioned composition is reheated to a temperature of 1050°C to 1250°C. If the reheating temperature is lower than 1050°C, the rolling load may become too large during the subsequent rolling process, and precipitation-forming elements such as Nb may not dissolve sufficiently but remain, such that they cannot contribute to the formation of precipitates in the subsequent process, which may lead to a reduction in strength. On the other hand, if the reheating temperature exceeds 1250°C, there is a problem that the grain size of the final microstructure may be non-uniform due to partial coarsening caused by abnormal growth of some austenite grains. Meanwhile, in the present disclosure, the reheating time of the slab is not particularly limited, and any normal conditions can be sufficient. As a non-limiting example, the reheating time of the slab can be 100 minutes to 400 minutes.

[0070] Next, the reheated slab is rough-rolled and then finish-rolled at the austenite single-phase temperature to obtain thick plate steel materials.

[0071] Here, rough rolling refers to a series of intermediate rolling processes performed before finish rolling. In the present disclosure, the detailed conditions of rough rolling are not particularly limited, and any normal conditions can be sufficient. The rough rolling temperature can be set to a sufficiently high temperature at which the finish rolling temperature can be ensured.

[0072] Finish rolling is performed at the austenite single-phase temperature, which aims to improve the uniformity of the microstructure. As an example, during rolling, the finish rolling temperature can be 800°C to 950°C. If the finish rolling temperature is lower than 800°C, the rolling load may increase, which may reduce the productivity, and there is a problem that the grains may become too fine or coarse unrecrystallized austenite may be produced. On the other hand, if it exceeds 950°C, the austenite grains of the slab may become too coarse, making it difficult to ensure the target strength.

[0073] Next, the thick plate steel is cooled at an accelerated rate. The cooling operation is carried out in three stages to produce fine ferrite in the surface layer and ferrite of appropriate size in the central part. At this time, when the grain size ratio of the surface layer part and the central part is appropriately adjusted, the toughness can be improved.

[0074] First, after finish rolling, the thick plate steel is water-cooled to a temperature of 700°C to 800°C at an average cooling rate of 5°C / second to 25°C / second for the first cooling. At this time, although there is no particular limitation, according to one embodiment of the present disclosure, the cooling start temperature of the first cooling at which cooling starts can be controlled within the range of 750°C to 850°C. If the cooling start temperature starts at a temperature exceeding 850°C, the possibility of forming coarse bainite increases due to the increase in the bainite area ratio, and thus there is a high possibility that the toughness characteristics will deteriorate. On the other hand, if the cooling start temperature starts at a temperature below 750°C, an air-cooled ferrite structure may be formed, resulting in a decrease in the tensile strength.

[0075] Preferably, the cooling end temperature during the first cooling is limited to the range of 700°C to 800°C. At the corresponding temperature, most of the austenite in the surface layer part is transformed into ferrite, and untransformed austenite still exists in the central part. At this time, if the cooling end temperature during the first cooling exceeds 800°C, insufficient micro-ferrite is formed in the surface layer part, resulting in deterioration of the low-temperature toughness. In addition, if the cooling end temperature of the first cooling is below 700°C, excessive ferrite is formed in the surface layer part, making it difficult to ensure a sufficient bainite fraction in the central part, and thus resulting in a decrease in strength.

[0076] Next, the second cooling is carried out to cool the steel cooled in the first cooling in air. Then, the third cooling can be carried out at an average cooling rate greater than 25°C / second and 60°C / second or lower to cool the steel cooled in the second cooling in water to a temperature of 300°C to 500°C. At this time, although there is no particular limitation, the steel can be held under air-cooling conditions for 3 seconds or longer during the second cooling.

[0077] Control the cooling end temperature during the third cooling within the range of 300°C to 500°C. At the corresponding temperature, austenite remaining in the central portion after the first cooling transforms into ferrite and bainite, and at this time, grain growth occurs after the first cooling, resulting in the formation of ferrite of appropriate size. At this time, if the cooling end temperature during the third cooling exceeds 500°C, excessive ferrite growth occurs in the central portion, and bainite formation is insufficient, resulting in a decrease in strength. In addition, if the cooling end temperature is lower than 300°C, sufficient ferrite is not ensured, resulting in a decrease in low-temperature toughness. Meanwhile, according to an embodiment of the present disclosure, for further improving the aforementioned effects, the average cooling rate in the third cooling can be 30°C / second to 60°C / second.

[0078] Although not particularly limited, according to the aforementioned manufacturing method, the content of expensive alloy elements such as nickel (Ni) and molybdenum (Mo) can be reduced, thereby improving profitability, while reducing the load of the process by applying general management standards to the rolling and cooling processes. In addition, the pipeline steel material of the present disclosure manufactured according to the above manufacturing method exhibits physical properties such as a yield strength of 550 MPa to 700 MPa, a tensile strength of 620 MPa to 820 MPa, an elongation of 30% to 60%, and a DWTT ductile fracture rate of 95% or more at -30°C, and can therefore be suitably used as a pipeline material.

[0079] Embodiments of the invention

[0080] Hereinafter, the present disclosure will be described more specifically by way of examples. However, it should be noted that the following examples are only intended to illustrate the present disclosure by way of examples, and are not intended to limit the scope of the rights of the present disclosure. This is because the scope of the rights of the present disclosure is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0081] (Example)

[0082] Slabs having the compositions shown in Tables 1 and 2 below were reheated at 1100°C for 200 minutes, and then rough rolling and finish rolling were performed to meet the above conditions in the specification to obtain hot-rolled steel materials. At this time, in all examples, the thickness of the rough-rolled slab was kept constant at 20% compared to the thickness of the reheated slab. Thereafter, the steel materials were subjected to first cooling (water cooling), second cooling (air cooling for 3 seconds), and third cooling (water cooling) under the conditions described in Table 3 below.

[0083] After that, the microstructure of the manufactured steel was analyzed by SEM to measure the area ratio, and as mechanical properties, the tensile strength, yield strength, and elongation were measured using a room-temperature tensile testing machine, and the DWTT ductile fracture rate was measured by measuring the fracture surface of the specimen at -30 °C using a drop-weight testing machine with a capacity of 100,000 J. The results are shown in Table 4 below. For reference, in all examples, the remaining structure other than ferrite is pearlite and / or bainite. At this time, the average grain size in the surface layer part and the center part was measured in the same manner as described above in the specification.

[0084] [Table 1]

[0085]

[0086] [Table 2]

[0087]

[0088] [Table 3]

[0089]

[0090]

[0091] [Table 4]

[0092]

[0093] F: Ferrite, B: Bainite, P: Pearlite

[0094] As can be seen from Table 4 above, in the case of Invention Examples 1 and 2 that satisfy both the alloy composition and manufacturing conditions proposed in the present disclosure, excellent strength, elongation, and low-temperature ductile fracture rate are exhibited even without adding expensive elements such as Ni.

[0095] In contrast, in the case of Comparative Examples 1 to 5, at least one of the alloy composition and manufacturing conditions deviates from the conditions proposed in the present disclosure, and thus the strength is inferior to the specification.

[0096] In particular, in the case of Comparative Example 6, the material is satisfactory, but Ni and Mo are added in excessive amounts, resulting in poor economic efficiency.

[0097] As described above, in the detailed description of the present disclosure, preferred embodiments of the present disclosure are described, but obviously, those skilled in the art to which the present disclosure pertains can make various modifications within the scope of the present disclosure. Therefore, the scope of the rights of the present disclosure should not be limited to the described embodiments, but should be defined not only by the appended claims but also by their equivalents.

Claims

1. A steel for pipes, comprising: C by weight %: 0.030% to 0.100%, Si: 0.50% or less and excluding 0%, Mn: 0.50% to 2.50%, Nb: 0.070% or less and excluding 0%, V: 0.030% or less and excluding 0%, Mo: 0.05% or less and excluding 0%, Ni: 0.05% or less and excluding 0%, Cr: 0.10% to 0.30%, P: 0.03% or less and excluding 0%, S: 0.050% or less and excluding 0%, Al: 0.050% or less and excluding 0%, N: 0.010% or less and excluding 0%, and Cu: 0.010% or less and excluding 0%, and Fe and inevitable impurities as the balance, wherein the steel comprises: ferrite selected as the microstructure of 80% or more and including 100% thereof, and at least one of pearlite and bainite as the balance, by area %, and a surface layer portion having an average grain size of 10 μm to 15 μm in the thickness direction.

2. The steel according to claim 1, wherein the average grain size of the central portion of the steel in the thickness direction is greater than the average grain size of the surface layer portion.

3. The steel according to claim 1, wherein the average grain size of the central portion of the steel in the thickness direction is greater than 15 μm and less than or equal to 30 μm.

4. The steel according to claim 1, wherein the average grain size of the surface layer portion is 11 μm to 14.5 μm.

5. The steel according to claim 1, wherein the average grain size of the central portion of the steel in the thickness direction is 16 μm to 25 μm.

6. The steel according to claim 2, wherein the difference between the average grain size of the central portion and the average grain size of the surface layer portion exceeds 5 μm.

7. The steel according to claim 2, wherein the difference between the average grain size of the central portion and the average grain size of the surface layer portion is 6 μm or more.

8. The steel according to claim 1, wherein the surface layer portion represents a region corresponding to 15% of the total thickness of the steel in the thickness direction starting from at least one surface of the steel.

9. The steel according to claim 2, wherein the surface layer portion represents a region corresponding to 15% of the total thickness of the steel in the thickness direction starting from at least one surface of the steel, and the central portion represents the remaining central region of the steel in the thickness direction excluding the surface layer portion.

10. The steel according to claim 1, wherein the carbon equivalent (Ceq) defined by the following relational expression 1 satisfies 0.45 or less but excluding 0, [Relational expression 1] Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5, Each of [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] represents the content (weight %) of the element in the corresponding brackets.

11. A method for manufacturing steel for pipes, comprising: a reheating operation of a slab at 1050°C to 1250°C, the slab comprising, by weight %: C: 0.030% to 0.100%, Si: 0.50% or less and excluding 0%, Mn: 0.50% to 2.50%, Nb: 0.070% or less and excluding 0%, V: 0.030% or less and excluding 0%, Mo: 0.05% or less and excluding 0%, Ni: 0.05% or less and excluding 0%, Cr: 0.10% to 0.30%, P: 0.03% or less and excluding 0%, S: 0.050% or less and excluding 0%, Al: 0.050% or less and excluding 0%, N: 0.010% or less and excluding 0%, and Cu: 0.010% or less and excluding 0%, and the balance of Fe and inevitable impurities; a rough rolling operation on the reheated slab, followed by finish rolling at the austenite single-phase temperature to obtain a thick plate steel; a first cooling operation of water-cooling the thick plate steel at an average cooling rate of 5°C / second to 25°C / second to a temperature of 700°C to 800°C after completion of the finish rolling; a second cooling operation of air-cooling the steel after the first cooling; and a third cooling operation of water-cooling the steel after the second cooling at an average cooling rate greater than 25°C / second and less than or equal to 60°C / second to a temperature of 300°C to 500°C.

12. The method for manufacturing steel for pipes according to claim 11, wherein the average cooling rate in the third cooling is 30°C / second to 60°C / second.

13. The method for manufacturing steel for pipes according to claim 11, wherein in the second cooling operation, the steel is held under the air-cooling condition for 3 seconds or longer.

14. The method for manufacturing steel for pipes according to claim 11, wherein the cooling start temperature in the first cooling operation is 750°C to 850°C.

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