Line pipe steel with excellent low-temperature toughness and manufacturing method thereof

Abstract

The present invention aims to provide a thick, high-strength API steel material that is excellent in low-temperature toughness and economical cost, and a method for producing the same. [Solution] The steel material for line pipe of the present invention contains, by weight, C: 0.030 to 0.100%, Si: 0.50% or less (excluding 0%), Mn: 0.50 to 2.50%, Nb: 0.070% or less (excluding 0%), V: 0.030% or less (excluding 0%), Mo: 0.05% or less (excluding 0%), Ni: 0.05% or less (excluding 0%), Cr: 0.10 to 0.30%, P: 0.03% or less (excluding 0%), S: 0.050% or less (excluding 0%), The steel sheet is characterized in that it is composed of 0.050% or less (excluding 0%), Al: 0.050% or less (excluding 0%), N: 0.010% or less (excluding 0%), Cu: 0.010% or less (excluding 0%), the balance being Fe and unavoidable impurities, and that the microstructure contains, in area %, 80% or more (including 100%) of ferrite, the balance being one or more selected from the group consisting of pearlite and bainite, and that the average crystal grain size in the surface layer in the thickness direction is 10 to 15 μm.

Description

[Technical Field]

[0001] The present invention relates to a high-strength API steel material having excellent low-temperature toughness and economical efficiency, and a manufacturing method thereof. More particularly, the present invention relates to a high-strength API steel material having excellent properties suitable for transporting crude oil, and a manufacturing method thereof. [Background technology]

[0002] API steel for line pipes, used to transport crude oil from mining to use, requires high strength and deformation stability to protect the structure from deformation due to external factors and external shocks such as earthquakes. Therefore, the API steel used for crude oil transportation has traditionally been thick steel plates made by minimizing impurities in the steel and adding large amounts of solid-solution strengthening elements such as C, Si, Mn, and Cr, or hardening elements such as Ni and Mo to strengthen the steel.

[0003] However, as the demand for high-strength thick plates has increased recently, more expensive elements such as Ni and Mo have been added to ensure the strength of thick plates. Therefore, in order to ensure the strength, the heating temperature, rolling temperature, and cooling end temperature have become very low, resulting in a decrease in productivity. Furthermore, as the cooling end temperature becomes lower, a quenched structure is formed in the surface layer, causing problems such as hard spots. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Korean Patent Publication No. 2014-0002273 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a thick, high-strength API steel material that is excellent in low-temperature toughness and economical efficiency, and a method for producing the same.

[0006] The object of the present invention is not limited to the above-mentioned content, and anyone having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention from the entire content of this specification. [Means for solving the problem]

[0007] The steel material for line pipe of the present invention is composed, by weight%, of C: 0.030 to 0.100%, Si: 0.50% or less (excluding 0%), Mn: 0.50 to 2.50%, Nb: 0.070% or less (excluding 0%), V: 0.030% or less (excluding 0%), Mo: 0.05% or less (excluding 0%), Ni: 0.05% or less (excluding 0%), Cr: 0.10 to 0.30%, P: 0.03% or less (excluding 0%), S: 0.050% or less (excluding 0%), Al: 0.050% or less (excluding 0%), N: 0.010% or less (excluding 0%), Cu: 0.010% or less (excluding 0%), the balance being Fe and inevitable impurities, The microstructure contains, by area%, 80% or more (including 100%) of ferrite, the remainder being one or more selected from the group consisting of pearlite and bainite, The average crystal grain size in the thickness direction of the surface layer is 10 to 15 μm.

[0008] Further, the method for producing a steel material for line pipe of the present invention comprises the steps of: reheating a slab consisting of, by weight, C: 0.030 to 0.100%, Si: 0.50% or less (excluding 0%), Mn: 0.50 to 2.50%, Nb: 0.070% or less (excluding 0%), V: 0.030% or less (excluding 0%), Mo: 0.05% or less (excluding 0%), Ni: 0.05% or less (excluding 0%), Cr: 0.10 to 0.30%, P: 0.03% or less (excluding 0%), S: 0.050% or less (excluding 0%), Al: 0.050% or less (excluding 0%), N: 0.010% or less (excluding 0%), Cu: 0.010% or less (excluding 0%), the balance being Fe and unavoidable impurities, at 1050 to 1250°C; A step of rough rolling the reheated slab and then finish rolling it at an austenite single phase temperature to obtain a thick steel plate. a primary cooling stage in which, after the finish rolling is completed, the thick steel plate is water-cooled to a temperature of 700 to 800°C at an average cooling rate of 5 to 25°C / s; A secondary cooling stage in which the above-mentioned primarily cooled steel is air-cooled; and The method is characterized by including a tertiary cooling step in which the secondarily cooled steel is water-cooled to a temperature of 300 to 500°C at an average cooling rate of more than 25°C / s and not more than 60°C / s. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a thick, high-strength API steel material that is excellent in low-temperature toughness and economy, and a method for producing the same.

[0010] The various yet significant advantages and effects of the present invention are not limited to the above, but can be more easily understood in the course of describing specific embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.

[0012] Meanwhile, the terms used in this specification are intended to describe specific embodiments and are not intended to limit the present invention. For example, the singular form used in this specification includes the plural form unless a related definition clearly indicates otherwise. Furthermore, the meaning of "comprises" used in this specification embodies a configuration and does not exclude the presence or addition of other configurations.

[0013] Hereinafter, one aspect of the present invention will be described in detail with respect to a steel material for line pipe (or API steel material).

[0014] First, the reasons for adding alloying elements to the API thick steel plate of the present invention and the range of their content will be described in detail. The content of each element described below is based on weight percent unless otherwise specified.

[0015] C:0.030~0.100% 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 content is set to 0.030%. On the other hand, if the carbon content is excessive, the strength may increase too much and ductility may deteriorate, so the upper limit of the carbon content is set to 0.100%. On the other hand, from the perspective of further improving the above-mentioned effects, the lower limit of the carbon content may be 0.040%, or the upper limit of the carbon content may be 0.090%.

[0016] Si: 0.50% or less (excluding 0%) Silicon (Si) is an element that contributes to increasing strength through deoxidation of molten steel and solid-solution strengthening. However, in the present invention, silicon is not intentionally added, and its absence does not significantly impair the physical properties. Taking into consideration the possibility that silicon may be unavoidably included during the manufacturing process, the lower limit of the silicon content is set at 0%. However, if the silicon content is excessive, red scale due to Si may form on the surface of the thick steel plate, degrading surface quality and weldability. Therefore, the silicon content is set at 0.50%. To further improve the above-mentioned effects, the lower limit of the silicon content may be 0.01%, or the upper limit of the silicon content may be 0.40%.

[0017] Mn: 0.50 to 2.50% Manganese (Mn) is an effective element for solid-solution strengthening of steel, and must be added in an amount of 0.50% or more to ensure adequate strength. However, since an excessive manganese content may cause center segregation during the continuous casting process, the upper limit of the manganese content is set to 2.5%. Meanwhile, to further improve the above-mentioned effects, the lower limit of the manganese content may be 0.60%, or the upper limit of the manganese content may be 2.40%.

[0018] Nb: 0.070% or less (excluding 0%) Niobium (Nb) is a precipitation-strengthening element that is effective in ensuring strength by refining crystal grains while forming NbC-series precipitates. However, excessive niobium content has the drawback of making it difficult to achieve a low yield ratio by increasing the yield strength relative to the tensile strength due to the grain refinement effect. Therefore, in the present invention, the niobium content is controlled to 0.070% or less. However, considering cases where Nb is unavoidably contained, the lower limit of the niobium content is excluded from 0%. To further improve the above-mentioned effects, the lower limit of the niobium content may be 0.010%, or the upper limit of the niobium content may be 0.060%.

[0019] V: 0.030% or less (excluding 0%) Vanadium (V) is also a precipitation-strengthening element, which is effective in ensuring the strength of steel. However, if the vanadium content is excessive, a large amount of precipitates will reduce low-temperature toughness and weldability, and the cost of the alloy will increase. Therefore, in the present invention, the vanadium content is controlled to 0.030% or less. However, in consideration of unavoidable inclusion, the lower limit of the vanadium content does not include 0%. Meanwhile, in order to further improve the above-mentioned effects, the lower limit of the vanadium content may be 0.010%, or the upper limit of the vanadium content may be 0.020%.

[0020] Mo: 0.05% or less (excluding 0%) Molybdenum (Mo) is a typical element that improves the hardenability of steel and significantly enhances the ability to form low-temperature structures even at low cooling rates. This makes it an effective element for forming low-temperature structures such as bainite to ensure the strength of steel. Molybdenum is not only a relatively expensive element, but excessive molybdenum content can also lead to a deterioration in toughness. Therefore, in the present invention, molybdenum is not added artificially for economic reasons, and the upper limit of the molybdenum content is set to 0.05%. However, considering cases where molybdenum is unavoidably included, the lower limit of the molybdenum content is set to 0.005%, or the upper limit of the molybdenum content may be set to 0.04%. To further improve the above-mentioned effects, the lower limit of the molybdenum content may be set to 0.005%, or the upper limit of the molybdenum content may be set to 0.04%.

[0021] Ni: 0.05% or less (excluding 0%) Nickel (Ni) serves to simultaneously improve the strength and toughness of steel, but in the present invention, the addition of nickel does not pose a significant problem in terms of ensuring physical properties, so it is not necessary to artificially add nickel. On the other hand, since an excessive nickel content reduces economic viability, the present invention specifies a nickel content of 0.05% or less. However, in consideration of cases where nickel is unavoidably included, 0% is excluded from the lower limit of the nickel content. On the other hand, in order to further improve the above-mentioned effects, the lower limit of the nickel content may be 0.005%, or the upper limit of the nickel content may be 0.04%.

[0022] Cr:0.10~0.30% Chromium (Cr) strengthens the solid solution of steel, delays the bainite transformation during cooling, and contributes to the formation of equiaxed ferrite. It also effectively enhances hardenability, especially when added together with Mo. In the present invention, 0.10% or more of Cr is preferably added to achieve these effects. However, excessive Cr content can deteriorate weldability and brittleness, so the Cr content is set to 0.30% or less in the present invention. To further improve the above-mentioned effects, the lower limit of the Cr content may be 0.12%, or the upper limit may be 0.20%.

[0023] P: 0.03% or less (excluding 0%) Phosphorus (P) is an impurity that is inevitably contained in steel, and it is preferable to control its content as low as possible. In particular, if the phosphorus content is excessive, there is a high risk of deterioration of weldability and embrittlement of the steel. Therefore, in the present invention, the phosphorus content is controlled to 0.03% or less. However, taking into consideration the case where it is unavoidable, 0% is excluded from the lower limit of the phosphorus content. On the other hand, in order to further improve the above-mentioned effects, the lower limit of the phosphorus content may be 0.001%, or the upper limit of the phosphorus content may be 0.025%.

[0024] S: 0.050% or less (excluding 0%) Sulfur (S) is an impurity that is inevitably contained in steel, and it is preferable to control its content as low as possible. In particular, if its content is excessive, it can combine with Mn and other elements to form nonmetallic inclusions, which may increase the risk of embrittlement in the steel. Therefore, in the present invention, the sulfur content is controlled to 0.050% or less. However, in consideration of cases where S is unavoidably contained, 0% is excluded from the lower limit of the sulfur content. On the other hand, in order to further improve the above-mentioned effects, the lower limit of the sulfur content may be 0.001%, or the upper limit of the sulfur content may be 0.045%.

[0025] Al: 0.050% or less (excluding 0%) Although aluminum (Al) contributes to the deoxidation of molten steel, it is not intentionally added in the present invention, and its absence does not significantly impair the maintenance of physical properties. However, since an excessive aluminum content can cause nozzle clogging during continuous casting, the aluminum content is controlled to 0.050% or less in the present invention. However, in consideration of cases where aluminum is unavoidably included, 0% is excluded from the lower limit of the aluminum content. To further improve the above-mentioned effects, the lower limit of the aluminum content may be 0.010%, or the upper limit of the aluminum content may be 0.040%.

[0026] N: 0.010% or less (excluding 0%) Nitrogen (N) contributes to improving the strength of steel, but is not intentionally added in the present invention, and its absence does not pose a significant problem in terms of ensuring physical properties. However, since excessive nitrogen content increases the risk of steel embrittlement, the present invention limits its content to 0.010% or less. However, in consideration of cases where unavoidable inclusion of nitrogen is taken into account, 0% is excluded from the lower limit of the nitrogen content. To further improve the above-mentioned effects, the lower limit of the nitrogen content may be 0.001%, or the upper limit of the nitrogen content may be 0.009%.

[0027] Cu: 0.010% or less (excluding 0%) Copper (Cu) plays a role in increasing strength by forming fine precipitates, but in the present invention, the addition of copper does not pose a significant problem in terms of ensuring physical properties. However, since excessive copper content can cause surface cracking and deteriorate room-temperature workability, the copper content is controlled to 0.010% or less in the present invention. However, in consideration of unavoidable copper content, 0% is excluded from the lower limit of the copper content. Meanwhile, to further improve the above-mentioned effects, the lower limit of the copper content may be 0.0001%, or the upper limit of the copper content may be 0.009%.

[0028] The remainder of the composition is Fe. However, since unintended impurities may be inevitably mixed in from raw materials or the surrounding environment during normal manufacturing processes, it is not possible to exclude them. Since these impurities are known to anyone skilled in the art, not all of them will be specifically mentioned in this specification, but typical impurities are as follows:

[0029] On the other hand, the carbon equivalent (Ceq) of the above steel material, as defined by the following relational expression 1, can be controlled to 0.45 or less (excluding 0), in order to ensure an appropriate level of weldability. [Equation 1] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5 (In the above Relational Formula 1, each of [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] represents the content (wt %) of the element in the parentheses.)

[0030] In the present invention, the microstructure of the base steel material is not particularly limited, but for example, the base steel material may include ferrite, pearlite, and bainite as the microstructure. In particular, according to one embodiment of the present invention, the steel material may include, as the microstructure, 80% or more (including 100%) of ferrite by area, with the remainder being one or more selected from the group consisting of pearlite and bainite. If the area ratio of ferrite is less than 80%, workability may be deteriorated, which may increase the possibility of cracks occurring during pipe expansion after pipe making.

[0031] According to one embodiment of the present invention, the steel may have an average grain size of 10 to 15 μm in the surface layer in the thickness direction. In one feature of the present invention, the thick steel plate, which is the base material, has a difference in grain size between the surface layer and the center due to multi-stage cooling, as described below. When the grain size distribution varies in the thickness direction of the steel, the nucleation and crack propagation speeds between fine grains and normal-sized grains are slowed, improving toughness. In this case, having a difference in distribution in the thickness direction is more advantageous for ensuring toughness than having fine grains overall. Therefore, the present invention aims to simultaneously ensure strength and toughness by refining the grain size in the surface layer in the primary cooling and ensuring appropriate grain size and a fine structure in the secondary cooling.

[0032] On the other hand, in order to further improve the above-mentioned effect, the lower limit of the average crystal grain size of the surface layer portion may be 11 μm, or the upper limit of the average crystal grain size of the surface layer portion may be 14.5 μm.

[0033] In this specification, the above-mentioned crystal grain size indicates an average size for a phase including ferrite, pearlite, and bainite, and the above-mentioned size indicates a circle-equivalent diameter.

[0034] Furthermore, according to one embodiment of the present invention, the definition of the surface layer portion is not particularly limited and can be determined by a method commonly used in the art. As one example, the surface layer portion may represent a region corresponding to 15% of the entire thickness in the thickness direction from at least one surface of the steel material. Therefore, it may refer to a region including 15% of the entire thickness in the thickness direction based on both surfaces of the steel material, i.e., a region corresponding to a total of 30%. Furthermore, the average grain size can be measured by measuring grain sizes at multiple points (e.g., n points; n is a positive constant) at equal intervals in the thickness direction of the surface layer portion and calculating the average value.

[0035] Although not particularly limited, according to one embodiment of the present invention, a steel material for line pipe having the above-mentioned alloying components has the advantage of being economical by reducing the content of expensive alloying elements such as nickel (Ni), molybdenum (Mo), etc., and according to one non-limiting example, the steel material of the present invention exhibits physical properties of a yield strength of 550 to 700 MPa, a tensile strength of 600 to 800 MPa, and an elongation of 30 to 60%, and can have a product thickness of 30 to 50 mm.

[0036] Although not particularly limited, according to one embodiment of the present invention, the average grain size in the thickness direction of the central portion may be larger than the average grain size in the surface layer portion. As described above, in the present invention, by controlling the grain sizes of the central portion and the surface layer portion to be different, the nucleation and crack propagation speed between fine grains and normal-sized grains is slowed, thereby improving toughness.

[0037] Here, the definition of the central portion is not particularly limited and can be determined by a method commonly used in the art. As an example, the central portion can refer to the remaining central region of the steel material in the thickness direction, excluding the surface layer portion with respect to the total thickness (i.e., the region corresponding to 70% of the total thickness in the thickness direction, excluding the surface layer portions, which are regions corresponding to 15% from each of the two surfaces (corresponding to a total of 30% based on both surfaces)). Meanwhile, the average grain size of the central portion can be measured in the same manner as the average grain size of the surface layer portion. For example, the average grain size can be measured by measuring the grain size at multiple points (e.g., n points; n is a positive constant) at equal intervals in the thickness direction of the central portion and calculating the average value.

[0038] On the other hand, according to one embodiment of the present invention, in order to further improve the above-mentioned effects, the average crystal grain size of the central portion may be more than 15 μm and not more than 30 μm, or the lower limit of the average crystal grain size of the central portion may be 16 μm, or the upper limit of the average crystal grain size of the central portion may be 25 μm.

[0039] Although not particularly limited, according to one embodiment of the present invention, the difference between the average grain size of the central portion and the average grain size of the surface layer may be controlled to exceed 5 μm. Controlling the difference in grain size between the central portion and the surface layer reduces the speed of nucleation and crack propagation between fine grains and normal-sized grains, thereby improving toughness. However, if the difference in size is too large, it may actually impair physical properties, so the difference should be controlled within an appropriate range. To further improve the above-mentioned effect, the difference between the average grain size of the central portion and the average grain size of the surface layer may be 6 μm or more or 10 μm or less.

[0040] The steel material of the present invention described above can be produced by various methods, and the method of production is not particularly limited. However, as a preferred example, it can be produced by the following method.

[0041] First, a slab having the above-mentioned composition is reheated at a temperature of 1050 to 1250°C. If the reheating temperature is less than 1050°C, the rolling load may become excessively large in the subsequent rolling process, and elements such as Nb, which form precipitates, may not be fully dissolved and remain, preventing them from contributing to precipitate formation in the subsequent process, resulting in reduced strength. On the other hand, if the reheating temperature exceeds 1250°C, abnormal growth of some austenite grains may cause local coarsening, resulting in a non-uniform grain size in the final microstructure. Meanwhile, in the present invention, the slab reheating time is not particularly limited, and may be any ordinary condition. As a non-limiting example, the slab reheating time may be 100 to 400 minutes.

[0042] Next, the reheated slab is rough rolled and then finish rolled at a temperature in the austenite single phase region to obtain a thick steel plate.

[0043] Here, rough rolling refers to a series of intermediate rolling processes carried out before finish rolling, and in the present invention, the specific conditions of rough rolling are not particularly limited, and ordinary conditions may be used. The rough rolling temperature can be set at a temperature high enough to ensure the finish rolling temperature.

[0044] Finish rolling is performed at a temperature in the austenite single-phase region to increase the uniformity of the structure. For example, the finish rolling temperature during rolling may be 800 to 950°C. If the finish rolling temperature is less than 800°C, the rolling load may increase, reducing productivity, and the crystal grains may become excessively refined or coarse unrecrystallized austenite may be formed. On the other hand, if the temperature exceeds 950°C, the austenite crystal grains in the slab may become excessively coarse, making it difficult to achieve the target strength.

[0045] Next, the steel plate is accelerated and cooled in three stages to produce fine ferrite on the surface and ferrite of an appropriate size in the center, thereby improving toughness by properly controlling the grain size ratio between the surface and center.

[0046] First, after the finish rolling is completed, the thick steel plate undergoes primary cooling, in which the plate is water-cooled to a temperature of 700 to 800°C at an average cooling rate of 5 to 25°C / s. Although not particularly limited, according to one embodiment of the present invention, the cooling start temperature for the primary cooling can be controlled within a range of 750 to 850°C. If the cooling start temperature exceeds 850°C, the bainite area fraction increases, increasing the likelihood of coarse bainite formation, which may result in a deterioration in toughness. On the other hand, if the cooling start temperature is less than 750°C, air-cooled ferrite structures may be formed, potentially resulting in a decrease in tensile strength.

[0047] The cooling end temperature during the primary cooling is preferably limited to a range of 700 to 800°C. At this temperature, most of the austenite in the surface layer is transformed to ferrite, while untransformed austenite remains in the center. If the cooling end temperature during the primary cooling exceeds 800°C, sufficient fine ferrite cannot be formed in the surface layer, resulting in a deterioration of low-temperature toughness. If the cooling end temperature during the primary cooling is less than 700°C, excessive ferrite is formed in the surface layer, making it difficult to ensure a sufficient proportion of bainite in the center, resulting in a decrease in strength.

[0048] Next, the primarily cooled steel material is subjected to secondary cooling by air cooling. After this, the secondary cooled steel material can be subjected to tertiary cooling by water cooling to a temperature of 300 to 500°C at an average cooling rate of more than 25°C / s and not more than 60°C / s. At this time, although not particularly limited, the steel material can be held under the air cooling conditions for 3 seconds or more during the secondary cooling.

[0049] The cooling end temperature during the tertiary cooling is controlled within a range of 300 to 500°C. At this temperature, the austenite remaining in the center after the primary cooling transforms into ferrite and bainite, and at this time, grain growth occurs after the primary cooling, resulting in the formation of ferrite of an appropriate size. If the cooling end temperature during the tertiary cooling exceeds 500°C, excessive ferrite growth occurs in the center, resulting in insufficient bainite formation and a decrease in strength. Furthermore, if the cooling end temperature is less than 300°C, sufficient ferrite cannot be secured, resulting in a decrease in low-temperature toughness. Meanwhile, according to one embodiment of the present invention, in order to further improve the above-mentioned effects, the average cooling rate during the tertiary cooling may be 30 to 60°C / s.

[0050] Although not particularly limited, the above-mentioned manufacturing method can improve profitability by reducing the content of expensive alloying elements such as nickel (Ni) and molybdenum (Mo), while reducing the process load by applying general management standards to the rolling and cooling processes.In addition, the linepipe steel of the present invention manufactured by the above-mentioned manufacturing method exhibits physical properties such as a yield strength of 550 to 700 MPa, a tensile strength of 620 to 820 MPa, an elongation of 30 to 60%, and a DWTT ductile fracture surface ratio of 95% or more at -30°C, making it suitable for use as a linepipe material. [Example]

[0051] The present invention will be described in more detail with reference to the following examples. However, it should be noted that the following examples are merely illustrative of the present invention and are not intended to limit the scope of the present invention, as the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred from them.

[0052] (Example) Slabs having the compositions shown in Tables 1 and 2 below were reheated to 1100°C for 200 minutes, and then rough-rolled and finish-rolled to satisfy the conditions described above to obtain hot-rolled steel. In all examples, the thickness of the rough-rolled slab was kept constant at 20% of the thickness of the reheated slab. The steel was then subjected to primary (water cooling), secondary (air cooling, 3 seconds), and tertiary (water cooling) cooling under the conditions shown in Table 3 below.

[0053] The microstructure of the manufactured steel was then analyzed using an SEM to measure the area ratio. Mechanical properties included tensile strength, yield strength, and elongation, all of which were measured using a room-temperature tensile tester. The fracture surfaces of the test specimens were measured at -30°C using a 100,000 J drop weight tester to measure the DWTT ductile fracture ratio. The results are shown in Table 4 below. For reference, in all examples, the remaining structure outside the ferrite was pearlite and / or bainite. The average grain size in the surface and central regions was measured in the same manner as described above in the specification.

[0054] [Table 1]

[0055] [Table 2]

[0056] [Table 3]

[0057] [Table 4] F: Ferrite, B: Bainite, P: Pearlite

[0058] As shown in Table 4 above, Examples 1 and 2, which satisfy all of the alloy compositions and manufacturing conditions proposed in the present invention, exhibited excellent strength, elongation, and low-temperature ductile fracture rate without adding expensive elements such as Ni.

[0059] In contrast, in the case of Comparative Examples 1 to 5, one or more of the alloy composition and manufacturing conditions were outside the conditions proposed in the present invention, and the strength was inferior to the specifications.

[0060] In particular, in the case of Comparative Example 6, although the material quality was satisfactory, excessive amounts of Ni and Mo were added, which showed a deterioration in economic efficiency.

[0061] As described above, the detailed description of the present invention has been given with reference to the preferred embodiment of the present invention, but it is obvious that various modifications can be made by a person skilled in the art without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiment, but should be defined by the claims below as well as equivalents thereof.

Claims

1. In weight percent, it consists of C: 0.030 to 0.100%, Si: 0.50% or less (excluding 0%), Mn: 0.50 to 2.50%, Nb: 0.070% or less (excluding 0%), V: 0.030% or less (excluding 0%), Mo: 0.05% or less (excluding 0%), Ni: 0.05% or less (excluding 0%), Cr: 0.10 to 0.30%, P: 0.03% or less (excluding 0%), S: 0.050% or less (excluding 0%), Al: 0.050% or less (excluding 0%), N: 0.010% or less (excluding 0%), Cu: 0.010% or less (excluding 0%), the balance being Fe and inevitable impurities, The microstructure contains, by area%, 80% or more (including 100%) of ferrite, the remainder being one or more selected from the group consisting of pearlite and bainite, A steel material for line pipes, characterized in that the average crystal grain size in the thickness direction of the surface layer is 10 to 15 μm.

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

3. 2. The steel material for line pipe according to claim 1, wherein the average grain size in the center portion in the thickness direction is more than 15 μm and not more than 30 μm.

4. 2. The steel material for line pipe according to claim 1, wherein the average crystal grain size of the surface layer portion is 11 to 14.5 μm.

5. 2. The steel material for line pipe according to claim 1, wherein the average grain size in the thickness direction at the center portion is 16 to 25 μm.

6. 3. The steel material for line pipe 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. 3. The steel material for line pipe 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 material for line pipe according to claim 1 , wherein the surface layer portion indicates a region corresponding to 15% of the entire thickness in a thickness direction from at least one surface of the steel material.

9. The surface layer portion represents a region corresponding to 15% of the entire thickness in a thickness direction from at least one surface of the steel material, The steel material for line pipe according to claim 2 , wherein the central portion indicates a remaining central region of the steel material in the thickness direction, excluding the surface layer portion, with respect to the entire thickness.

10. 2. The steel material for line pipe according to claim 1, characterized in that the carbon equivalent (Ceq) defined by the following relational expression 1 satisfies 0.45 or less (excluding 0): [Relationship 1] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5 (In the above Relational Formula 1, each of [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] means the content (wt %) of the element in the parentheses.)

11. a step of reheating a slab consisting of, by weight%, C: 0.030 to 0.100%, Si: 0.50% or less (excluding 0%), Mn: 0.50 to 2.50%, Nb: 0.070% or less (excluding 0%), V: 0.030% or less (excluding 0%), Mo: 0.05% or less (excluding 0%), Ni: 0.05% or less (excluding 0%), Cr: 0.10 to 0.30%, P: 0.03% or less (excluding 0%), S: 0.050% or less (excluding 0%), Al: 0.050% or less (excluding 0%), N: 0.010% or less (excluding 0%), Cu: 0.010% or less (excluding 0%), the balance being Fe and inevitable impurities, at 1050 to 1250°C; A step of rough rolling the reheated slab and then finish rolling it at an austenite single phase temperature to obtain a thick steel plate. a primary cooling step in which, after the finish rolling is completed, the thick plate steel is water-cooled to a temperature of 700 to 800 ° C. at an average cooling rate of 5 to 25 ° C. / s; A secondary cooling step of air-cooling the primarily cooled steel material; and The method for manufacturing a steel material for line pipes includes a tertiary cooling step of water-cooling the secondarily cooled steel material to a temperature of 300 to 500°C at an average cooling rate of more than 25°C / s and not more than 60°C / s.

12. The method for manufacturing a steel material for line pipe according to claim 11, wherein the average cooling rate in the tertiary cooling stage is 30 to 60°C / s.

13. 12. The method for manufacturing a steel material for line pipe according to claim 11, wherein the steel material is held under the air-cooling condition for 3 seconds or more in the secondary cooling step.

14. 12. The method for manufacturing a steel material for line pipe according to claim 11, wherein the cooling start temperature in the first cooling stage is 750 to 850°C.

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