High-strength thin-walled api steel material having excellent deformation stability and method for manufacturing the same

Abstract

One embodiment of the present invention provides a high-strength thin-wall API steel material having excellent deformation stability and a manufacturing method thereof, the thin-wall API steel material comprising, in weight %, C: 0.05-0.15 %, Si: 0.5 % or less (except 0 %), Mn: 0.5-2.5 %, Nb: 0.05 % or less (except 0 %), V: 0.004 % or less (except 0 %), Mo: 0.03-0.2 %, Cr: 0.1-0.3 %, P: 0.03 % or less (except 0 %), S: 0.015 % (except 0 %), Al: 0.05 % or less (except 0 %), N: 0.01 % or less (except 0 %), Fe and other inevitable impurities in a balance, and a microstructure comprising, in area %, 10-30 % of ferrite having an average grain size of 15-30 μm and a balance of bainite, the thin-wall API steel material comprising 3000 / μm 2 V-based precipitates below.

Description

Technical Field

[0001] This invention relates to a high-strength thin-walled API steel with excellent deformation stability and a method for manufacturing the same, and more specifically, to a high-strength thin-walled API steel with excellent deformation stability that can be used for transporting crude oil, etc., and a method for manufacturing the same. Background Technology

[0002] In the case of API steel used for line-pipes used to transport crude oil to its destination after extraction, high strength and deformation stability of the steel are required to safely protect the structure from external factors such as deformation or impacts like earthquakes. Therefore, hot-rolled steel has traditionally been the primary API material used for transporting crude oil. This hot-rolled steel is strengthened by 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 with minimal impurities.

[0003] However, in recent years, with the strengthening of seismic design, as a project that can evaluate the deformation stability against earthquakes, a low yield strength ratio is required. In the case of hot-rolled steel with a large amount of precipitation strengthening elements such as Ti, Nb, and V, the yield strength increases due to the effect of excessive precipitates. Therefore, it is difficult to achieve a low yield strength ratio while maintaining high strength. Summary of the Invention

[0004] Technical problems to be solved

[0005] One aspect of the present invention is to provide a high-strength thin-walled API steel with a low yield strength ratio and excellent deformation stability, and a method for manufacturing the same.

[0006] Technical solution

[0007] One embodiment of the present invention provides a high-strength thin-walled API steel with excellent deformation stability. By weight percent, the thin-walled API steel comprises: C: 0.05-0.15%, Si: less than 0.5% (excluding 0%), Mn: 0.5-2.5%, Nb: less than 0.05% (excluding 0%), V: less than 0.004% (excluding 0%), Mo: 0.03-0.2%, Cr: 0.1-0.3%, P: less than 0.03% (excluding 0%), S: 0.015% (excluding 0%), Al: less than 0.05% (excluding 0%), N: less than 0.01% (excluding 0%), with the balance being Fe and other unavoidable impurities. By area percent, the microstructure comprises 10-30% ferrite and the balance being bainite, the ferrite having an average grain size of 15-30 μm, and the thin-walled API steel containing 3000 grains / μm. 2 The following V-based precipitates.

[0008] Another embodiment of the present invention provides a method for manufacturing high-strength thin-walled API steel with excellent deformation stability, comprising the following steps: reheating a slab at 1200-1400°C, wherein the slab comprises, by weight %,: C: 0.05-0.15%, Si: less than 0.5% (excluding 0%), Mn: 0.5-2.5%, Nb: less than 0.05% (excluding 0%), V: less than 0.004% (excluding 0%), Mo: 0.03-0.2%, Cr: 0.1-0.3%, P: less than 0.03% (excluding 0%). Except for), S: 0.015% (excluding 0%), Al: less than 0.05% (excluding 0%), N: less than 0.01% (excluding 0%), with the balance being Fe and other unavoidable impurities; after rough rolling the reheated slab, it is finished rolled at an austenitic single-phase temperature to obtain hot-rolled steel; the hot-rolled steel is water-cooled to a temperature of 650-750°C at a rate of 40-60°C / second, and then air-cooled for 3-7 seconds; and the air-cooled hot-rolled steel is water-cooled to a temperature of 450-600°C at a rate of 30-50°C / second and then coiled.

[0009] Beneficial effects

[0010] According to one aspect of the present invention, a high-strength thin-walled API steel with a low yield strength ratio and excellent deformation stability, and a method thereof, can be provided.

[0011] The numerous advantages and effects of the present invention are not limited to the above description, and can be more easily understood in the process of describing the specific embodiments of the present invention.

[0012] Best practice

[0013] The following describes a high-strength thin-walled API steel with excellent deformation stability according to an embodiment of the present invention.

[0014] First, the alloy composition of the present invention will be described. Unless otherwise specified, the following alloy composition contents are expressed in weight percent.

[0015] C: 0.05-0.15%

[0016] Carbon (C) is the most economical and effective element for ensuring strength. When the C content is less than 0.05%, it is difficult to ensure the target strength even with the addition of precipitation strengthening elements such as Nb. On the other hand, when the C content exceeds 0.15%, the ductility may deteriorate due to excessive strength increase. Therefore, the C content is preferably in the range of 0.05-0.15%. 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%.

[0017] Si: less than 0.5% (except 0%)

[0018] Si helps to improve the strength of molten steel through deoxidation and solid solution strengthening, but it is not intentionally added in this invention, and even without the intentional addition of Si, it will not have a significant impact on ensuring physical properties. However, when the Si content exceeds 0.5%, a red oxide scale forms on the surface of the hot-rolled steel due to Si, thus reducing surface quality and potentially decreasing weldability. Therefore, the Si content is preferably 0.5% or less. More preferably, the Si content is 0.45% or less, further preferably 0.4% or less, and most preferably 0.35% or less.

[0019] Mn: 0.5-2.5%

[0020] Mn is an effective element for solid solution strengthening of steel, and to ensure appropriate strength, it is preferable to add 0.5% or more. However, when the Mn content exceeds 2.5%, there is a risk of generating center segregation in continuous casting processes. Therefore, the Mn content is preferably 0.5-2.5%. The lower limit of the Mn content is more preferably 0.8%, further preferably 1.0%, and most preferably 1.2%. The upper limit of the Mn content is more preferably 2.3%, further preferably 2.0%, and most preferably 1.8%.

[0021] Nb: less than 0.05% (except 0%)

[0022] Nitrogen (Nb) is a precipitation-strengthening element and an effective element for ensuring strength by refining grains through the formation of NbC-based precipitates. However, when the Nb content exceeds 0.05%, the grain refinement effect becomes excessive, increasing the yield strength to tensile strength ratio, thus making it difficult to achieve a low yield strength ratio. Therefore, in this invention, the Nb content is controlled to be below 0.05%. The upper limit of the Nb content is more preferably 0.045%, further preferably 0.04%, and most preferably 0.035%. The lower limit of the Nb content is more preferably 0.01%, further preferably 0.015%, and most preferably 0.02%.

[0023] V: Below 0.004% (except 0%)

[0024] V is also a precipitation-strengthening element and an effective element for ensuring the strength of steel. In particular, in the case of V-based precipitates, precipitation occurs at low temperatures compared to Nb-based precipitates, thus having the effect of finely forming precipitates during winding. However, as described above, the small size and uniform dispersion of the fine precipitates can lead to yield point phenomena and result in an increase in the yield strength ratio of the steel. Therefore, in this invention, it is sought to minimize the V content, and thus the V content is controlled to be 0.004% or less. The V content is more preferably 0.003% or less, and even more preferably 0.0025% or less.

[0025] Mo: 0.03-0.2%

[0026] Mo is a representative element for improving the hardenability of steel and significantly enhances the ability to form low-temperature phase transformation structures even at low cooling rates. Therefore, Mo is an effective element for forming low-temperature phase transformation structures such as bainite and ensuring the strength of steel. In this invention, to achieve the above-mentioned effects, the Mo content is preferably 0.03% or more. However, compared with other alloying elements, Mo is not only a relatively high-valence element, but its toughness may also deteriorate if its content is excessively increased; therefore, the Mo content is preferably 0.2% or less. Therefore, the Mo content is preferably in the range of 0.03-0.2%. The lower limit of the Mo content is more preferably 0.035%, further preferably 0.04%, and most preferably 0.045%. The upper limit of the Mo content is more preferably 0.18%, further preferably 0.15%, and most preferably 0.13%.

[0027] Cr: 0.1-0.3%

[0028] Cr strengthens steel through solid solution treatment and delays the bainitic phase transformation during cooling, thereby contributing to the formation of equiaxed ferrite. In particular, its hardenability is improved more effectively when added together with Mo. In this invention, to achieve the above effects, the Cr content is preferably 0.1% or more. However, when the Cr content exceeds 0.3%, weldability and brittleness may deteriorate. Therefore, the Cr content is preferably in the range of 0.1-0.3%. The upper limit of the Cr content is more preferably 0.27%, further preferably 0.25%, and most preferably 0.23%. The lower limit of the Cr content is more preferably 0.11%, further preferably 0.12%, and most preferably 0.13%.

[0029] P: Below 0.03% (except 0%)

[0030] Polymer (P) is an unavoidable impurity in steel, and it is preferable to control the P content to the lowest possible level. In particular, when the P content is excessive, the risk of decreased weldability and brittleness of the steel increases; therefore, in this invention, the P content is controlled to be below 0.03%. More preferably, the P content is below 0.025%, further preferably below 0.023%, and most preferably below 0.02%.

[0031] S: 0.015% (excluding 0%)

[0032] S is an unavoidable impurity in steel, and it is preferable to control the S content at the lowest possible level. In particular, when the S content is excessive, it may combine with Mn and other metals to form non-metallic inclusions, increasing the risk of steel brittleness. Therefore, in this invention, the S content is controlled to be below 0.015%. More preferably, the S content is below 0.013%, further preferably below 0.012%, and most preferably below 0.011%.

[0033] Al: Less than 0.05% (except 0%)

[0034] Al contributes to the deoxidation of molten steel, but it is not intentionally added in this invention. Even without adding Al, there is no significant impact on ensuring physical properties. However, when the Al content exceeds 0.05%, nozzle clogging may occur during continuous casting. Therefore, the Al content is preferably 0.05% or less. More preferably, the Al content is 0.047% or less, even more preferably 0.045% or less, and most preferably 0.04% or less.

[0035] N: Less than 0.01% (except 0%)

[0036] Nitrogen (N) contributes to the strength of steel, but it is not intentionally added in this invention, and even without its addition, there is little impact on ensuring physical properties. However, when the N content exceeds 0.01%, the risk of steel brittleness may increase. Therefore, the N content is preferably 0.01% or less. More preferably, it is 0.009% or less, even more preferably 0.007% or less, and most preferably 0.005% or less.

[0037] Apart from the aforementioned composition, the remainder is Fe. However, undesirable impurities inevitably contaminate the raw materials or the surrounding environment during normal manufacturing processes, and therefore these impurities cannot be excluded. These impurities are well known to those skilled in the art, and therefore not all of them are specifically mentioned in this specification, but representative impurities are described below.

[0038] Ni: less than 0.05% (except 0%)

[0039] Ni plays a role in simultaneously improving the strength and toughness of steel. However, in this invention, even without intentional addition of Ni, there is no significant impact on ensuring physical properties. However, compared to other alloying elements, Ni is a relatively expensive element, so economic efficiency may decrease when the Ni content exceeds 0.05%. Therefore, the Ni content is preferably 0.05% or less. More preferably, the Ni content is 0.04% or less, further preferably 0.035% or less, and most preferably 0.03% or less.

[0040] Cu: less than 0.05% (except 0%)

[0041] Cu enhances strength by forming fine precipitates, but in this invention, even without intentional addition of Cu, there is no significant impact on ensuring physical properties. However, when the Cu content exceeds 0.01%, thermal structural stability and room-temperature processability may deteriorate. Therefore, the Cu content is preferably 0.01% or less. More preferably, the Cu content is 0.04% or less, even more preferably 0.035% or less, and most preferably 0.03% or less.

[0042] Furthermore, the carbon equivalent (Ceq) of the steel of the present invention, as defined by the following [Formula 1], is preferably 0.4 or less. When the carbon equivalent exceeds 0.4, weldability may be difficult to ensure.

[0043] [Formula 1] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5

[0044] (Where, [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] represent the content (by weight%) of the corresponding elements.)

[0045] The fine structure of the present invention will be described below.

[0046] The fine microstructure of the steel of the present invention preferably comprises 10-30% ferrite and the balance bainite. The low yield strength ratio of the steel of the present invention is characterized by continuous yielding behavior achieved through sufficient mobile dislocations resulting from the volume change of the high fraction of bainite. Therefore, in the present invention, it is desirable to include bainite as the main microstructure to obtain the above-mentioned effects. The ferrite is a soft phase with excellent toughness and achieves the effect of ensuring elongation. When the fraction of ferrite is less than 10%, there is a disadvantage of excessively high strength and reduced toughness; when the fraction of ferrite exceeds 30%, there is a disadvantage of difficulty in ensuring strength. The upper limit of the ferrite fraction is more preferably 27%, and further preferably 25%. The lower limit of the ferrite fraction is more preferably 13%, and further preferably 15%. Furthermore, in the present invention, the formation of pearlite is not intentional, but it may inevitably occur during the manufacturing process. When a trace amount of pearlite is included, it has the effect of promoting the formation of mobile dislocations. However, when the pearlite content is too high, the toughness may deteriorate; therefore, the pearlite content is preferably 10% or less. More preferably, the pearlite content is 7% or less, further preferably 5% or less, and most preferably 3% or less.

[0047] The ferrite preferably has an average grain size of 15-30 μm. The relationship between the average grain size of the ferrite and the yield strength can be expressed by the Hall-Petch equation. If the average grain size of the ferrite is controlled at the level described above, the desired yield strength can be ensured. When the average grain size of the ferrite is less than 15 μm, the yield strength may become too high; when the average grain size of the ferrite exceeds 30 μm, the yield strength deteriorates, and therefore it may be difficult to obtain the desired strength. The upper limit of the average grain size of the ferrite is more preferably 27 μm, and even more preferably 25 μm. The lower limit of the average grain size of the ferrite is more preferably 17 μm, and even more preferably 20 μm.

[0048] Furthermore, according to one example, the bainite can have an average lath bundle size of 5-20 μm. The average lath bundle size of the bainite is a factor affecting toughness; when the average lath bundle size exceeds 20 μm, the toughness may deteriorate. On the other hand, when the average lath bundle size of the bainite is less than 5 μm, the yield strength may become excessively high. The upper limit of the average lath bundle size of the bainite is more preferably 17 μm, and even more preferably 15 μm. The lower limit of the average lath bundle size of the bainite is more preferably 8 μm, and even more preferably 10 μm.

[0049] One of the technical features of the steel of this invention is that it contains 3000 particles / μm. 2The following V-based precipitates. That is, the aim is to suppress the formation of a large number of said V-based precipitates as much as possible. When the number of said V-based precipitates per unit area exceeds 3000 / μm. 2 At this time, the numerous finely formed V-based precipitates hinder dislocation movement, leading to dislocation pile-up and thus increasing the yield point phenomenon. As the yield point phenomenon increases, the ratio of yield strength to tensile strength increases, making it difficult to ensure the desired low yield strength ratio. More preferably, the number of V-based precipitates per unit area is 2700 precipitates / μm. 2 The following is a further preferred value: 2500 particles / μm 2 The following is a further explanation. Additionally, in this invention, the specific type of the V-based precipitate is not particularly limited, but it can be, for example, VC, VN, or V(C,N).

[0050] According to one example, the V-based precipitates can have an average diameter of 5-10 nm and a maximum diameter of less than 20 nm. When the average diameter of the V-based precipitates is less than 5 nm, the V-based precipitates are formed at relatively low temperatures, making it difficult to ensure a sufficient quantity per unit area, and the yield point rise is not significant. On the other hand, when the average diameter of the V-based precipitates exceeds 10 nm or the maximum diameter exceeds 20 nm, the yield point rise is not significant due to the coarse precipitates. The maximum diameter of the V-based precipitates is more preferably less than 17 nm, and more preferably less than 15 nm. Furthermore, the average diameter refers to the average equivalent circular diameter of the V-based precipitates detected when observing the cross-section in the thickness direction of the steel, and the maximum diameter refers to the maximum equivalent circular diameter of the V-based precipitates detected when observing the cross-section in the thickness direction of the steel.

[0051] The steel according to one embodiment of the present invention, as described above, can have a low yield strength ratio while possessing high strength and excellent deformation stability. As an example, it can have a yield strength of 500-700 MPa, a tensile strength of 600-800 MPa, a yield strength ratio of 80-85%, an elongation of 20-30%, and an impact toughness of over 80 J at -30°C.

[0052] The steel of the present invention described above can be manufactured by various methods, and there are no particular limitations on the manufacturing method. However, as a preferred example, it can be manufactured by the following method.

[0053] The following describes a method for manufacturing high-strength thin-walled API steel with excellent deformation stability according to one embodiment of the present invention.

[0054] First, the slab with the aforementioned alloy composition is reheated at 1200-1400°C. When the reheating temperature is below 1200°C, the rolling load in the subsequent hot rolling process may increase excessively. Due to the refinement of the initial austenite size, the grain size of bainite and ferrite will also be refined in the subsequent process, thus increasing the yield strength. On the other hand, when the reheating temperature exceeds 1400°C, the grain size of the final fine structure may become uneven due to partial coarsening caused by the abnormal growth of some austenite grains. The lower limit of the reheating temperature is more preferably 1220°C, and further preferably 1250°C. The upper limit of the reheating temperature is more preferably 1350°C, further preferably 1320°C, and most preferably 1300°C. In addition, in this invention, the slab reheating time is not particularly limited, as long as the conditions are normal. As a non-limiting example, the slab reheating time can be 100-400 minutes. When the reheating time of the slab is less than 100 minutes, alloying elements such as Mo cannot be fully dissolved, thus reducing the contribution of hardenability during cooling and failing to adequately ensure grain size. This may result in an increase in yield strength. When the reheating time of the slab exceeds 400 minutes, the initial austenite size becomes excessively coarse, which may make it difficult to ensure sufficient strength. The lower limit of the reheating time is more preferably 120 minutes, further preferably 150 minutes, and most preferably 180 minutes. The upper limit of the reheating time is more preferably 350 minutes, further preferably 320 minutes, and most preferably 300 minutes.

[0055] Subsequently, after rough rolling the reheated slab, finish rolling is performed at an austenitic single-phase temperature to obtain hot-rolled steel. Rough rolling refers to a series of intermediate rolling processes performed before finish rolling. In this invention, the specific conditions for rough rolling are not particularly limited, as long as they are general conditions. As a non-limiting example, the thickness of the rough-rolled slab can be 10-25% of the thickness of the reheated slab, and the rough rolling temperature can be set to ensure a sufficiently high finish rolling temperature. The finish rolling is performed at an austenitic single-phase temperature to increase the uniformity of the microstructure. According to one example, the temperature during finish rolling can be 800-1000°C. When hot finish rolling is performed within the above temperature range, the austenitic microstructure of the hot-rolled steel has an average grain size of 10-40 μm. Furthermore, when the finishing rolling temperature is below 800°C, the hot rolling load increases, which may reduce productivity and cause the grains to become excessively fine. However, when the finishing rolling temperature exceeds 1000°C, the austenite grains in the slab become excessively coarse, making it difficult to ensure the target strength. The lower limit of the finishing rolling temperature is more preferably 830°C, and even more preferably 850°C. The upper limit of the finishing rolling temperature is more preferably 970°C, even more preferably 950°C, and most preferably 930°C.

[0056] The hot-rolled steel is then cooled. When cooling the hot-rolled steel using conventional continuous cooling, it is difficult to ensure the grains and lath bundles are of the desired size, thus potentially worsening the yield strength ratio. Therefore, in this invention, the hot-rolled steel is cooled in two stages: water-cooled at a rate of 40-60°C / second to a temperature of 650-750°C, followed by air-cooling for 3-7 seconds. The temperature of 650-750°C is the temperature at which the austenite-ferrite phase transformation occurs most rapidly, corresponding to the temperature at which ferrite can grow most effectively; this temperature range is referred to as the intermediate temperature in this invention. When the intermediate temperature exceeds 750°C or the air-cooling time exceeds 7 seconds, excessive ferrite growth occurs, potentially worsening the yield strength. Conversely, when the intermediate temperature is below 650°C or the air-cooling time is less than 3 seconds, excessive refinement of the ferrite size may occur, making it difficult to ensure the yield strength ratio. The lower limit of the intermediate temperature is more preferably 660°C, further preferably 670°C, and most preferably 680°C. The upper limit of the intermediate temperature is more preferably 740°C, further 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.

[0057] Subsequently, the air-cooled hot-rolled steel is water-cooled to a temperature of 450-600°C at a rate of 30-50°C / second and then coiled. When strength is ensured by Nb,V composite precipitates as a conventional method, hardenability is insufficient, and it is difficult to obtain sufficient bainite when cooling after intermediate temperatures. Therefore, in this invention, it is desirable to solve this problem by adding an appropriate amount of Mo. Within the above-mentioned coiling temperature range, bainite formation is most active due to Mo, so when coiling is performed within this temperature range, sufficient bainite structure is generated, thereby ensuring the desired strength. When the coiling temperature exceeds 600°C, the bainite structure cannot be sufficiently ensured, and therefore it is difficult to obtain the desired strength. On the other hand, when the coiling temperature is below 450°C, the shape of the sheet deteriorates during cooling, and shape defects may occur when manufacturing pipes. The lower limit of the coiling temperature is more preferably 470°C, and more preferably 500°C. The upper limit of the coiling temperature is more preferably 580°C, and more preferably 550°C. Detailed Implementation

[0058] The present invention will now be described in more detail through embodiments. However, these embodiments are merely illustrative of implementations of the invention, and the invention is not limited to these embodiments. This is because the scope of the invention is determined by the matters set forth in the claims and those reasonably deduced therefrom.

[0059] (Example)

[0060] A slab with the alloy composition described in Table 1 was reheated to 1280°C for 250 minutes. After rough rolling, hot-rolled steel was manufactured under the conditions described in Table 2. The thickness of the rough-rolled slab was kept constant at 20% of the thickness of the reheated slab. Comparative Examples 2 and 4 involved continuous cooling to the coiling temperature after finish rolling without cooling or air cooling to an intermediate temperature. The microstructure of the hot-rolled steel manufactured as described above was measured, and the results of the evaluation of its mechanical and physical properties are shown in Table 4.

[0061] Fine structures were observed using an optical microscope at a magnification of ×200, and the area fraction of each phase was measured using the point count method according to ASTM E562.

[0062] The average lath bundle 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 locations at a magnification of ×500, and the resulting data were averaged using the grain size procedure provided in the TSL OIM Analysis 6.0 software.

[0063] The fraction and average diameter of the precipitates were measured by carbon replication using transmission electron microscopy (TEM).

[0064] For mechanical and physical properties, tensile test pieces were taken in the width direction of each hot-rolled steel according to API standards. The yield strength, tensile strength and elongation were then measured at room temperature (about 25°C), and the impact toughness was measured by Charpy impact test at -30°C.

[0065] [Table 1]

[0066]

[0067] [Table 2]

[0068]

[0069] [Table 3]

[0070]

[0071] [Table 4]

[0072]

[0073] As can be seen from Tables 1 to 4, under the conditions of Invention Example 1 and Invention Example 2, which meet the alloy composition and manufacturing conditions proposed in this invention, the fine microstructure and precipitates desired by this invention were obtained, and thus the yield strength of 500-700 MPa, the tensile strength of 600-800 MPa, the yield strength ratio of 80-85%, and the elongation of 20-30% can be met.

[0074] On the other hand, it is known that in the case of Comparative Examples 1 to 4, which meet the alloy composition proposed in this invention but do not meet the manufacturing conditions, it is difficult to obtain the average grain size of ferrite that is desired in this invention, and therefore the yield strength ratio is at a poor level.

[0075] It is understood that in Comparative Examples 5 to 8, where the manufacturing conditions proposed in this invention are met but the alloy composition is not met, the fine microstructure or precipitate conditions of this invention cannot be obtained, and therefore the mechanical and physical properties desired by this invention cannot be ensured.

Claims

1. A high-strength, thin-walled API steel with excellent deformation stability, comprising, by weight percent: C: 0.05-0.15%, Si: less than 0.5% and excluding 0%, Mn: 0.5-2.5%, Nb: less than 0.05% and excluding 0%, V: less than 0.004% and excluding 0%, Mo: 0.03-0.2%, Cr: 0.1-0.3%, P: less than 0.03% and excluding 0%, S: 0.015% and excluding 0%, Al: less than 0.05% and excluding 0%, N: less than 0.01% and excluding 0%, with the balance being Fe and other unavoidable impurities. The microstructure, by area percent, comprises 10-30% ferrite and the balance bainite, wherein the ferrite has an average grain size of 15-30 μm. The thin-walled API steel contains 3000 particles / μm. 2 The following V-based precipitates.

2. The high-strength thin-walled API steel with excellent deformation stability according to claim 1, wherein, The unavoidable impurities include one or more of Ni and Cu, and the content of Ni and Cu is suppressed to Ni: less than 0.05% and excluding 0% and Cu: less than 0.05% and excluding 0%.

3. The high-strength thin-walled API steel with excellent deformation stability according to claim 1, wherein, The steel has a carbon equivalent Ceq of 0.4 or less, as defined by the following [Equation 1]. [Formula 1] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5 Where [C], [Mn], [Cu], [Ni], [Cr], [Mo], and [V] represent the content of the corresponding elements, and the unit of content is weight%.

4. The high-strength thin-walled API steel with excellent deformation stability according to claim 1, wherein, The fine structure further comprises less than 10% pearlite.

5. The high-strength thin-walled API steel with excellent deformation stability according to claim 1, wherein, The bainite has an average lath bundle size of 5-20 μm.

6. The high-strength thin-walled API steel with excellent deformation stability according to claim 1, wherein, The V-based precipitates have an average diameter of 5-10 nm.

7. The high-strength thin-walled API steel with excellent deformation stability according to claim 1, wherein, The V-based precipitate has a maximum diameter of less than 20 nm.

8. The high-strength thin-walled API steel with excellent deformation stability according to claim 1, wherein, The steel has a yield strength of 500-700MPa, a tensile strength of 600-800MPa, a yield-to-tensile ratio of 80-85%, an elongation of 20-30%, and an impact toughness of over 80J at -30℃.

9. A method for manufacturing high-strength thin-walled API steel with excellent deformation stability as described in claim 1, comprising the following steps: The slab is reheated at 1200-1400°C. The slab contains, by weight %,: C: 0.05-0.15%, Si: less than 0.5% and excluding 0%, Mn: 0.5-2.5%, Nb: less than 0.05% and excluding 0%, V: less than 0.004% and excluding 0%, Mo: 0.03-0.2%, Cr: 0.1-0.3%, P: less than 0.03% and excluding 0%, S: 0.015% and excluding 0%, Al: less than 0.05% and excluding 0%, N: less than 0.01% and excluding 0%, with the balance being Fe and other unavoidable impurities. After rough rolling the reheated slab, finish rolling is performed at a temperature in the austenitic single-phase region to obtain hot-rolled steel. The hot-rolled steel is water-cooled to a temperature of 650-750°C at a rate of 40-60°C / second, and then air-cooled for 3-7 seconds. as well as The air-cooled hot-rolled steel is water-cooled to a temperature of 450-600°C at a rate of 30-50°C / second and then coiled up.

10. The method for manufacturing high-strength thin-walled API steel with excellent deformation stability according to claim 9, wherein, The unavoidable impurities include one or more of Ni and Cu, and the content of Ni and Cu is suppressed to Ni: less than 0.05% and excluding 0% and Cu: less than 0.05% and excluding 0%.

11. The method for manufacturing high-strength thin-walled API steel with excellent deformation stability according to claim 9, wherein, The slab is reheated for 100-400 minutes.

12. The method for manufacturing high-strength thin-walled API steel with excellent deformation stability according to claim 9, wherein, The thickness of the rough-rolled slab is 10-25% of the thickness of the reheated slab.

13. The method for manufacturing high-strength thin-walled API steel with excellent deformation stability according to claim 9, wherein, The finishing rolling temperature is 800-1000℃.

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