High-strength thin-wall API steel with excellent deformation stability and its manufacturing method

Abstract

One embodiment of the present invention is a steel sheet containing, by weight %, C: 0.05 to 0.15%, Si: 0.5% or less (excluding 0%), Mn: 0.5 to 2.5%, Nb: 0.05% or less (excluding 0%), V: 0.004% or less (excluding 0%), Mo: 0.03 to 0.2%, Cr: 0.1 to 0.3%, P: 0.03% or less (excluding 0%), S: 0.015% (excluding 0%), Al: 0.05% or less (excluding 0%), N: 0.01% or less (excluding 0%), the balance being Fe and other inevitable impurities, and the microstructure contains, by area %, 10 to 30% ferrite and the balance being bainite, the ferrite having an average grain size of 15 to 30 μm, and V-based precipitates having a density of 3,000 particles / μm 2 The present invention provides a high-strength, thin-gauge API steel material with excellent deformation stability, including the following, and a method for manufacturing the same.

Description

[Technical Field]

[0001] The present invention relates to a high-strength, thin-walled API steel material having excellent deformation stability and a manufacturing method thereof, and more particularly to a high-strength, thin-walled API steel material having excellent deformation stability that can be used for transporting crude oil, etc., and a manufacturing method thereof. [Background technology]

[0002] API steel for linepipes, used to transport crude oil from mining to use, requires high strength and deformation stability to safely protect the structure against deformation due to external factors and external shocks such as earthquakes. Therefore, the API material used for crude oil transportation has traditionally been hot-rolled steel, which is made by minimizing impurities in the steel and adding large amounts of solid-solution strengthening elements such as C, Si, Mn, and Cr, or precipitation strengthening elements such as Ti, Nb, and V to high-purity steel for enhanced strength.

[0003] However, with the recent strengthening of earthquake-resistant design, a low yield ratio is required as an item for evaluating deformation stability against earthquakes. However, in the case of hot-rolled steel with large additions of precipitation-strengthening elements such as Ti, Nb, and V, the yield strength increases due to the effect of excessive precipitates, making it difficult to achieve a low yield ratio while maintaining high strength. Summary of the Invention [Problem to be solved by the invention]

[0004] One aspect of the present invention is to provide a high-strength, thin-gauge API steel material that has a low yield ratio but is excellent in deformation stability, and a method for producing the same. [Means for solving the problem]

[0005] One embodiment of the present invention is a steel sheet containing, by weight %, C: 0.05 to 0.15%, Si: 0.5% or less (excluding 0%), Mn: 0.5 to 2.5%, Nb: 0.05% or less (excluding 0%), V: 0.004% or less (excluding 0%), Mo: 0.03 to 0.2%, Cr: 0.1 to 0.3%, P: 0.03% or less (excluding 0%), S: 0.015% (excluding 0%), Al: 0.05% or less (excluding 0%), N: 0.01% or less (excluding 0%), the balance being Fe and other unavoidable impurities, and the microstructure contains, by area %, 10 to 30% ferrite and the balance being bainite, the ferrite having an average grain size of 15 to 30 μm, and V-based precipitates having a density of 3,000 particles / μm 2 We provide high-strength, thin-gauge API steel with excellent deformation stability, including:

[0006] Another embodiment of the present invention is a slurry containing, by weight, C: 0.05 to 0.15%, Si: 0.5% or less (excluding 0%), Mn: 0.5 to 2.5%, Nb: 0.05% or less (excluding 0%), V: 0.004% or less (excluding 0%), Mo: 0.03 to 0.2%, Cr: 0.1 to 0.3%, P: 0.03% or less (excluding 0%), S: 0.015% (excluding 0%), Al: 0.05% or less (excluding 0%), N: 0.01% or less (excluding 0%), the balance being Fe and other unavoidable impurities. the hot-rolled steel material is water-cooled at a rate of 40-60°C / sec to a temperature of 650-750°C, and then air-cooled for 3-7 seconds; and the air-cooled hot-rolled steel material is water-cooled at a rate of 30-50°C / sec to a temperature of 450-600°C, and then coiled. [Effects of the Invention]

[0007] According to one aspect of the present invention, it is possible to provide a high-strength, thin-gauge API steel material that has a low yield ratio but is excellent in deformation stability, and a method for producing the same.

[0008] 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

[0009] Hereinafter, a high-strength, thin-gauge API steel material having excellent deformation stability according to one embodiment of the present invention will be described.

[0010] First, the alloy composition of the present invention will be described. The contents of the alloy composition described below are in weight percent unless otherwise specified.

[0011] C: 0.05 to 0.15% C is the most economical and effective element for ensuring strength. If the C content is less than 0.05%, it may be difficult to achieve the target strength even if precipitation strengthening elements such as Nb are added. On the other hand, if the C content exceeds 0.15%, the ductility may decrease due to an excessive increase in strength. Therefore, the C content is preferably in the range of 0.05 to 0.15%. The lower limit of the C content is more preferably 0.06%, and even more preferably 0.07%. The upper limit of the C content is more preferably 0.14%, even more preferably 0.12%, and most preferably 0.10%.

[0012] Si: 0.5% or less (excluding 0%) Although Si contributes to increasing strength through deoxidation of molten steel and solid solution strengthening, it is not intentionally added in the present invention, and the absence of Si does not pose a significant problem in terms of ensuring physical properties. However, if the Si content exceeds 0.5%, red scale due to Si may form on the surface of the hot-rolled steel, degrading surface quality and weldability. Therefore, the Si content is preferably 0.5% or less. The Si content is more preferably 0.45% or less, even more preferably 0.4% or less, and most preferably 0.35% or less.

[0013] Mn: 0.5 to 2.5% Mn is an element effective in solid-solution strengthening steel, and is preferably added in an amount of 0.5% or more to ensure appropriate strength. However, if the Mn content exceeds 2.5%, there is a risk of center segregation occurring during the continuous casting process. Therefore, the Mn content is preferably 0.5 to 2.5%. The lower limit of the Mn content is more preferably 0.8%, even more preferably 1.0%, and most preferably 1.2%. The upper limit of the Mn content is more preferably 2.3%, even more preferably 2.0%, and most preferably 1.8%.

[0014] Nb: 0.05% or less (excluding 0%) Nb is a precipitation-strengthening element that effectively ensures strength by forming NbC-series precipitates and refining crystal grains. However, if the Nb content exceeds 0.05%, the effect of refining crystal grains is excessive, which has the drawback of making it difficult to achieve a low yield ratio by increasing the yield strength relative to the tensile strength. Therefore, in the present invention, the Nb content is controlled to 0.05% or less. The upper limit of the Nb content is more preferably 0.045%, even more preferably 0.04%, and most preferably 0.035%. The lower limit of the Nb content is more preferably 0.01%, even more preferably 0.015%, and most preferably 0.02%.

[0015] V: 0.004% or less (excluding 0%) V is also a precipitation-strengthening element, which is effective in ensuring the strength of steel. In particular, V-based precipitates precipitate at lower temperatures than Nb-based precipitates, which has the effect of forming fine precipitates during coiling. However, the fine precipitates formed in this manner are small in size but uniformly dispersed, which causes a yield point phenomenon and increases the yield ratio of the steel. Therefore, in the present invention, the V content is minimized, and thus the V content is controlled to 0.004% or less. The V content is more preferably 0.003% or less, and even more preferably 0.0025% or less.

[0016] Mo: 0.03 to 0.2% Mo is a typical element that improves the hardenability of steel and significantly improves the ability to form low-temperature transformation structures even at low cooling rates. This makes it an effective element for forming low-temperature transformation structures such as bainite to ensure the strength of steel. In the present invention, to achieve the above effects, the Mo content is preferably 0.03% or more. However, since Mo is a relatively expensive element compared to other alloying elements and an excessively high Mo content can reduce toughness, the Mo content is preferably 0.2% or less. Therefore, the Mo content is preferably in the range of 0.03 to 0.2%. The lower limit of the Mo content is more preferably 0.035%, even more preferably 0.04%, and most preferably 0.045%. The upper limit of the Mo content is more preferably 0.18%, even more preferably 0.15%, and most preferably 0.13%.

[0017] Cr: 0.1 to 0.3% Cr strengthens the steel through solid solution and delays the bainite phase transformation during cooling, which helps form equiaxed ferrite. It can further effectively enhance hardenability, especially when added together with Mo. In the present invention, to achieve the above effects, the Cr content is preferably 0.1% or more. However, if the Cr content exceeds 0.3%, weldability and brittleness may be reduced. Therefore, the Cr content is preferably in the range of 0.1 to 0.3%. The upper limit of the Cr content is more preferably 0.27%, even more preferably 0.25%, and most preferably 0.23%. The lower limit of the Cr content is more preferably 0.11%, even more preferably 0.12%, and most preferably 0.13%.

[0018] P: 0.03% or less (excluding 0%) 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 P content is excessive, there is a high risk of deterioration of weldability and embrittlement of the steel. Therefore, in the present invention, the P content is controlled to 0.03% or less. The P content is more preferably 0.025% or less, even more preferably 0.023% or less, and most preferably 0.02% or less.

[0019] S: 0.015% (excluding 0%) 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 the S content is excessive, it can combine with Mn and the like to form non-metallic inclusions, which increases the risk of steel embrittlement. Therefore, in the present invention, the S content is controlled to 0.015% or less. The S content is more preferably 0.013% or less, even more preferably 0.012% or less, and most preferably 0.011% or less.

[0020] Al: 0.05% or less (excluding 0%) Although Al contributes to deoxidation of molten steel, it is not intentionally added in the present invention, and its absence does not significantly impair the physical properties. However, if the Al content exceeds 0.05%, nozzle clogging may occur during continuous casting. Therefore, the Al content is preferably 0.05% or less. The Al content is more preferably 0.047% or less, even more preferably 0.045% or less, and most preferably 0.04% or less.

[0021] N: 0.01% or less (excluding 0%) 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 the necessary physical properties. However, if the N content exceeds 0.01%, the steel is likely to become brittle. Therefore, the N content is preferably 0.01% or less. The N content is more preferably 0.009% or less, even more preferably 0.007% or less, and most preferably 0.005% or less.

[0022] The remainder of the composition is Fe. However, during normal manufacturing processes, unintentional impurities may be mixed in from raw materials or the surrounding environment, and this cannot be excluded. 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:

[0023] Ni: 0.05% or less (excluding 0%) Ni improves both the strength and toughness of steel, but in the present invention, intentionally not adding Ni does not pose a significant problem in terms of ensuring the necessary physical properties. However, because Ni is a relatively expensive element compared to other alloying elements, if its content exceeds 0.05%, economic efficiency may decrease. Therefore, the Ni content is preferably 0.05% or less. The Ni content is more preferably 0.04% or less, even more preferably 0.035% or less, and most preferably 0.03% or less.

[0024] Cu: 0.05% or less (excluding 0%) Cu plays a role in increasing strength by forming fine precipitates, but in the present invention, even if Cu is not intentionally added, there is no significant problem in terms of ensuring physical properties. 0.05 %, the hot structural stability and room temperature workability may be reduced. 0.05 % or less. The Cu content is more preferably 0.04% or less, further preferably 0.035% or less, and most preferably 0.03% or less.

[0025] On the other hand, the steel material of the present invention preferably has a carbon equivalent (Ceq) of 0.4 or less, as defined by the following [Equation 1]. If the carbon equivalent exceeds 0.4, it is difficult to ensure weldability. [Formula 1] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5 (Here, [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] mean the content (wt%) of the respective elements.)

[0026] The microstructure of the present invention will be described below.

[0027] The microstructure of the steel material of the present invention preferably contains 10 to 30% ferrite and the remainder bainite. The low yield ratio characteristics of the steel material of the present invention are characterized by the continuous yield behavior due to the sufficient mobile potential generated by the volume change of a high fraction of bainite. Therefore, in the present invention, the above effects are achieved by including bainite as the main microstructure. The ferrite is a soft phase that improves toughness and ensures elongation. A ferrite fraction of less than 10% has the disadvantage of excessively high strength and low toughness, while a ferrite fraction exceeding 30% has the disadvantage of making it difficult to ensure strength. The upper limit of the ferrite fraction is more preferably 27%, and even more preferably 25%. The lower limit of the ferrite fraction is more preferably 13%, and even more preferably 15%. Meanwhile, although pearlite is not intentionally formed in the present invention, it may inevitably be formed during the manufacturing process. When a trace amount of pearlite is included, it has the effect of promoting the generation of mobile potential. However, if the pearlite fraction is too high, toughness may be reduced, so the pearlite fraction is preferably 10% or less, more preferably 7% or less, even more preferably 5% or less, and most preferably 3% or less.

[0028] The ferrite preferably has an average crystal grain size of 15 to 30 μm. The relationship between the average crystal grain size of the ferrite and the yield strength can be expressed by the Hall-Petch equation, and when the average crystal grain size of the ferrite is controlled at the same level as above, the desired yield strength can be ensured. If the average crystal grain size of the ferrite is less than 15 μm, the yield strength may become very high, and if it exceeds 30 μm, the yield strength may decrease and it may be difficult to obtain the desired strength. The upper limit of the average crystal grain size of the ferrite is more preferably 27 μm, and even more preferably 25 μm. The lower limit of the average crystal grain size of the ferrite is more preferably 17 μm, and even more preferably 20 μm.

[0029] Meanwhile, in one example, the bainite may have an average packet size of 5 to 20 μm. The average bainite packet size is a factor that affects toughness, and if the average bainite packet size exceeds 20 μm, the toughness may decrease. On the other hand, if the average bainite packet size is less than 5 μm, the yield strength may become very high. The upper limit of the average bainite packet size is more preferably 17 μm, and even more preferably 15 μm. The lower limit of the average bainite packet size is more preferably 8 μm, and even more preferably 10 μm.

[0030] The steel material of the present invention has V-based precipitates at 3,000 particles / μm. 2 One of the technical features of the present invention is that the formation of a large amount of the above V-based precipitates is suppressed as much as possible. The number of the above V-based precipitates per unit area is 3,000 particles / μm 2 If the value exceeds , a large number of finely formed V-based precipitates will hinder the movement of the potential, causing the potential to pile up, which will increase the yield point phenomenon. If the yield point phenomenon increases, the yield strength relative to the tensile strength will increase, making it difficult to ensure the desired low yield ratio. The number of V-based precipitates per unit area is 2,700 / μm. 2 More preferably, it is 2,500 particles / μm or less. 2 It is more preferable that the V-based precipitates are as follows: On the other hand, in the present invention, the specific types of the V-based precipitates are not particularly limited, but may be, for example, VC, VN, or V(C,N).

[0031] For example, the V-based precipitates may have an average diameter of 5 to 10 nm and a maximum diameter of 20 nm or less. When the average diameter of the V-based precipitates is less than 5 nm, they are formed at a relatively low temperature, making it difficult to secure a sufficient number of precipitates per unit area, resulting in a small increase in yield point. On the other hand, when the average diameter of the V-based precipitates exceeds 10 nm or the maximum diameter exceeds 20 nm, the increase in yield point is small due to the presence of coarse precipitates. The maximum diameter of the V-based precipitates is preferably 17 nm or less, and more preferably 15 nm or less. The average diameter refers to the average equivalent circular diameter of the V-based precipitates detected by observing a cross section of the steel material in the thickness direction, and the maximum diameter refers to the maximum equivalent circular diameter of the V-based precipitates detected by observing a cross section of the steel material in the thickness direction.

[0032] The steel material according to one embodiment of the present invention provided as described above can have high strength and excellent deformation stability while having a low yield ratio, for example, a yield strength of 500 to 700 MPa, a tensile strength of 600 to 800 MPa, a yield ratio of 80 to 85%, an elongation of 20 to 30%, and an impact toughness of 80 J or more at -30°C.

[0033] 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.

[0034] Hereinafter, a method for producing a high-strength, thin-gauge API steel material having excellent deformation stability according to one embodiment of the present invention will be described.

[0035] First, a slab having the above-described alloy composition is reheated at 1200 to 1400°C. If the reheating temperature is less than 1200°C, the rolling load in the subsequent hot rolling process may be excessively large. The refinement of the initial austenite size may also refine the grain size of bainite and ferrite in the subsequent process, potentially resulting in an increased yield strength. On the other hand, if the reheating temperature exceeds 1400°C, abnormal growth of some austenite grains may result in partial coarsening, potentially resulting in an inhomogeneous grain size in the final microstructure. The lower limit of the reheating temperature is more preferably 1220°C, and even more preferably 1250°C. The upper limit of the reheating temperature is more preferably 1350°C, even more preferably 1320°C, and most preferably 1300°C. 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. If the slab reheating time is less than 100 minutes, alloy elements such as Mo do not dissolve sufficiently, resulting in a small contribution of hardening ability during cooling, an insufficient grain size, and high yield strength. If the time exceeds 400 minutes, the initial austenite size becomes significantly coarse, making it difficult to ensure sufficient strength. The lower limit of the reheating time is more preferably 120 minutes, even more preferably 150 minutes, and most preferably 180 minutes. The upper limit of the reheating time is more preferably 350 minutes, even more preferably 320 minutes, and most preferably 300 minutes.

[0036] The reheated slab is then rough-rolled and then finish-rolled at a temperature in the austenite single-phase region to obtain a hot-rolled steel material. Here, rough rolling refers to a series of intermediate rolling processes performed before finish rolling. The specific conditions for rough rolling are not particularly limited in the present invention, and typical conditions may be used. As a non-limiting example, the thickness of the rough-rolled slab may be 10 to 25% of the thickness of the reheated slab, and the rough rolling temperature may be set to a temperature high enough to ensure the finish rolling temperature. The finish rolling is performed at a temperature in the austenite single-phase region to increase the uniformity of the structure. For example, the temperature during the finish rolling may be 800 to 1000°C. During finish hot rolling within this temperature range, the austenite structure of the finish-rolled hot-rolled steel material has an average grain size of 10 to 40 μm. On the other hand, if the finish rolling temperature is less than 800°C, the hot rolling load increases, which may reduce productivity and may result in excessive grain refinement. On the other hand, if the temperature exceeds 1000°C, the austenite grains in the slab may become excessively coarse, making it difficult to ensure the target strength. The lower limit of the finish rolling temperature is more preferably 830°C, and even more preferably 850°C. The upper limit of the finish rolling temperature is more preferably 970°C, and even more preferably 950°C, and most preferably 930°C.

[0037] The hot-rolled steel is then cooled. When the hot-rolled steel is cooled using conventional continuous cooling, it is difficult to maintain the desired grain size and packet size, which can result in a decrease in yield ratio. Therefore, in the present invention, the hot-rolled steel is cooled using two-stage cooling, in which the hot-rolled steel is water-cooled at a rate of 40-60°C / sec to a temperature of 650-750°C, followed by air-cooling for 3-7 seconds. Here, the temperature of 650-750°C corresponds to the temperature at which austenite transforms most rapidly to ferrite and allows ferrite to grow most efficiently. In the present invention, this temperature range is referred to as the intermediate temperature. If the intermediate temperature exceeds 750°C or the air-cooling time exceeds 7 seconds, excessive ferrite growth may occur, resulting in a decrease in yield strength. On the other hand, if the intermediate temperature is less than 650°C or the air-cooling time is less than 3 seconds, the ferrite size may become too fine, making it difficult to maintain the yield ratio. The lower limit of the intermediate temperature is more preferably 660°C, even more preferably 670°C, and most preferably 680°C. The upper limit of the intermediate temperature is more preferably 740°C, even more preferably 730°C, and most preferably 720°C. The lower limit of the air-cooling time is more preferably 4 seconds. The upper limit of the air-cooling time is more preferably 6 seconds.

[0038] The air-cooled hot-rolled steel material is then water-cooled to a temperature of 450 to 600°C at a rate of 30 to 50°C / sec and then coiled. When strength is ensured through Nb and V composite precipitates, as is the case in the conventional method, hardening ability is insufficient, making it difficult to obtain sufficient bainite during cooling after the intermediate temperature. Therefore, the present invention solves this problem by appropriately adding Mo. Bainite formation is most active due to Mo within the above coiling temperature range. Therefore, when coiling is performed within the above temperature range, a sufficient bainite structure is formed, ensuring the desired strength. If the coiling temperature exceeds 600°C, a sufficient bainite structure cannot be obtained, making it difficult to obtain the desired strength. On the other hand, if the coiling temperature is less than 450°C, the sheet shape may be distorted during cooling, potentially resulting in poor shape during pipe formation. The lower limit of the coiling temperature is more preferably 470°C, and even more preferably 500°C. The upper limit of the coiling temperature is more preferably 580°C, and even more preferably 550°C. [Example]

[0039] The present invention will be described in more detail below through examples. However, the description of these examples is intended to illustrate the implementation of the present invention, and the present invention is not limited by the description of these examples. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred from them.

[0040] (Example) Slabs having the alloy compositions shown in Table 1 below were reheated at 1280°C for 250 minutes and rough rolled, and then hot-rolled steel materials were produced under the conditions shown in Table 2 below. The thickness of the rough-rolled slab was kept constant at 20% of the reheated slab thickness. On the other hand, in Comparative Examples 2 and 4, the slabs were continuously cooled to the coiling temperature after finish rolling without cooling to the intermediate temperature or air cooling. The microstructures of the hot-rolled steel materials produced in this manner were measured, and the mechanical properties were evaluated. The results are shown in Table 4.

[0041] The microstructure was observed using an optical microscope at a magnification of ×200, and the area fraction of each phase was measured using the point count method based on ASTM E 562 standard.

[0042] The average packet size of bainite and the average grain size of ferrite were measured using electron backscatter diffraction (EBSD). More specifically, EBSD was measured 10 times at random positions at 500x magnification, and the data obtained were averaged using the Grain size program provided by TSL OIM Analysis 6.0 software.

[0043] The fraction and average diameter of the precipitates were measured by the carbon replica method via transmission electron microscopy (TEM).

[0044] For the mechanical properties, tensile test pieces were taken from each hot-rolled steel material in the width direction according to API standards, and then the yield strength, tensile strength, and elongation were measured at room temperature (approximately 25°C), and the impact toughness was measured by a Charpy impact test at -30°C.

[0045] [Table 1]

[0046] [Table 2]

[0047] [Table 3]

[0048] [Table 4]

[0049] As can be seen from Tables 1 to 4 above, in the case of Invention Examples 1 and 2, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, the microstructure and precipitates that the present invention aims to obtain are obtained, and as a result, it is found that the yield strength: 500 to 700 MPa, tensile strength: 600 to 800 MPa, yield ratio: 80 to 85%, and elongation: 20 to 30% are satisfied.

[0050] On the other hand, in the case of Comparative Examples 1 to 4, which satisfy the alloy composition proposed by the present invention but do not satisfy the manufacturing conditions, the average grain size of ferrite that the present invention aims to obtain is not obtained, and therefore the yield ratio is at a poor level.

[0051] In the case of Comparative Examples 5 to 8, which satisfy the manufacturing conditions proposed by the present invention but do not satisfy the alloy composition, the microstructure or precipitate conditions of the present invention are not obtained, and it is clear that the present invention cannot ensure the desired mechanical properties.

Claims

1. The steel sheet contains, by weight, C: 0.05 to 0.15%, Si: 0.5% or less (excluding 0%), Mn: 0.5 to 2.5%, Nb: 0.05% or less (excluding 0%), V: 0.004% or less (excluding 0%), Mo: 0.03 to 0.2%, Cr: 0.1 to 0.3%, P: 0.03% or less (excluding 0%), S: 0.015% (excluding 0%), Al: 0.05% or less (excluding 0%), N: 0.01% or less (excluding 0%), with the balance being Fe and other unavoidable impurities; The microstructure contains, by area percentage, ferrite: 10 to 30%, pearlite: 10% or less, and the remainder bainite, The ferrite has an average grain size of 15 to 30 μm, V-based precipitates 3,000 pieces / μm ２ Including: The steel material for steel pipes, wherein the V-based precipitates have an average diameter of 5 to 10 nm.

2. 2. The steel material for steel pipes according to claim 1, wherein the inevitable impurities include one or more of Ni and Cu, and the contents thereof are suppressed to Ni: 0.05% or less (excluding 0%) and Cu: 0.05% or less (excluding 0%).

3. The steel material for steel pipes according to claim 1, wherein the steel material has a carbon equivalent (Ceq) defined by the following [Equation 1] of 0.4 or less. [Formula 1] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5 (Here, [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] mean the content (wt%) of the corresponding element.)

4. 2. The steel material for steel pipes according to claim 1, wherein the bainite has an average packet size of 5 to 20 μm.

5. 2. The steel material for steel pipes according to claim 1, wherein the V-based precipitates have a maximum diameter of 20 nm or less.

6. The steel material has a yield strength of 500 to 700 MPa, a tensile strength of 600 to 800 MPa, a yield ratio of 80 to 85%, an elongation rate of 20 to 30%, and an impact toughness at -30 ° C. of 80 J or more. Steel material for steel pipes according to claim 1.

7. reheating the slab containing, by weight, C: 0.05 to 0.15%, Si: 0.5% or less (excluding 0%), Mn: 0.5 to 2.5%, Nb: 0.05% or less (excluding 0%), V: 0.004% or less (excluding 0%), Mo: 0.03 to 0.2%, Cr: 0.1 to 0.3%, P: 0.03% or less (excluding 0%), S: 0.015% (excluding 0%), Al: 0.05% or less (excluding 0%), N: 0.01% or less (excluding 0%), with the balance being Fe and other unavoidable impurities, at 1200 to 1400°C; a step of rough rolling the reheated slab and then finish rolling it at an austenite single phase temperature to obtain a hot-rolled steel material; Water-cooling the hot-rolled steel material at a rate of 40 to 60°C / sec to a temperature of 650 to 750°C, and then air-cooling it for 3 to 7 seconds; and The method for producing a steel material for steel pipes according to any one of claims 1 to 6, comprising the step of water-cooling the air-cooled hot-rolled steel material to a temperature of 450 to 600°C at a rate of 30 to 50°C / sec, and then coiling the material.

8. 8. The method for producing a steel material for steel pipes according to claim 7, wherein the inevitable impurities include one or more of Ni and Cu, and the contents thereof are suppressed to Ni: 0.05% or less (excluding 0%) and Cu: 0.05% or less (excluding 0%).

9. The method for producing a steel material for steel pipes according to claim 7, wherein the slab is reheated for 100 to 400 minutes.

10. The method for producing a steel material for steel pipes according to claim 7, wherein the thickness of the roughly rolled slab is 10 to 25% of the thickness of the reheated slab.

11. The method for producing a steel material for steel pipes according to claim 7, wherein the finish rolling temperature is 800 to 1000°C.

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