Hot-rolled steel sheet, method for manufacturing same, and steel pipe using same

Abstract

Disclosed are a hot-rolled steel sheet, a method for manufacturing same, and a steel pipe using same. The hot-rolled steel sheet, according to the present invention, comprises, by weight%: C: 0.12-0.3%, Si: more than 0% but not more than 0.5%, Mn: 0.3-2.0%, Al: more than 0% but not more than 0.105%, N: more than 0% but not more than 0.015%, and the balance being Fe and inevitable impurities, wherein the hot-rolled steel sheet has a weight ratio of Al to N (Al / N) in a range of 1 to 7, contains V, Nb, and Ti each in an amount of either 0% or less than 0.01% by weight, and has a microstructure that includes ferrite in an area fraction of 50% or more.

Description

Hot-rolled steel sheet, manufacturing method thereof, and steel pipe using the same

[0001] The present invention relates to a hot-rolled steel sheet capable of reducing the increase in yield strength after forming and a method for manufacturing the same.

[0002] In addition, the present invention relates to a steel pipe in which the increase in yield strength is not large compared to that of a hot-rolled steel plate.

[0003] In conventional hot-rolled steel, yield point elongation has been developed as a technology to suppress surface patterns such as warping due to uneven elongation.

[0004] In particular, in steels containing less than 1.0 wt% carbon (C), yield point elongation occurs easily due to the correlation between the dissolved carbon and dislocations in the metal, and this leaves a surface pattern, so much research has been conducted to reduce yield point elongation (e.g., patent document 1).

[0005] However, in steels containing 1.2 wt% or more of carbon, the dissolved carbon is precipitated as pearlite or cementite, so the yield point elongation does not appear well, and therefore there is no need to develop technology to reduce the yield point elongation.

[0006] However, with the recent increase in social safety issues, it is necessary to suppress the increase in yield strength after pipe manufacturing by limiting the yield strength of the steel pipe after pipe manufacturing to the same as the raw material in steel containing 1.2 wt% or more of carbon, for example, to ensure a yield ratio of 85% or less before and after pipe manufacturing. In particular, even if the raw material before pipe manufacturing has a low yield ratio, the yield strength can increase significantly due to work hardening of the steel pipe body during pipe manufacturing, so suppressing this increase in yield strength after pipe manufacturing is necessary.

[0007] The problem to be solved by the present invention is to provide a hot-rolled steel sheet and a manufacturing method thereof capable of suppressing an increase in yield strength during forging by controlling alloy components and microstructure.

[0008] In addition, the problem to be solved by the present invention is to provide a steel pipe having a small difference in yield strength from that of the raw material.

[0009] In order to achieve the above task, a hot-rolled steel sheet according to an embodiment of the present invention contains, in wt%, C: 0.12-0.3%, Si: more than 0% and 0.5% or less, Mn: 0.3-2.0%, Al: more than 0% and 0.105% or less, N: more than 0% and 0.015% or less, and the remainder of Fe and unavoidable impurities, and has a microstructure in which Al / N is 1-7 in wt%, V, Nb, and Ti contents are 0% or less than 0.01 wt% each, and ferrite is included in an area ratio of 50% or more.

[0010] The above hot-rolled steel sheet may have a microstructure in which the average crystal grain size of ferrite is 20㎛ or less at a point 1 / 4 in the thickness direction from the surface.

[0011] The above hot-rolled steel sheet may have an average grain size of ferrite on the surface / an average grain size of ferrite at a point 1 / 4 in the thickness direction from the surface of 1 or less.

[0012] The above hot-rolled steel sheet may have a microstructure including at least one of pearlite, bainite, martensite, and cementite in addition to the above ferrite.

[0013] The content of the above N may be 0.004-0.012% by weight.

[0014] The content of the above Al may be 0.01-0.03% by weight.

[0015] In order to achieve the above object, a method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention comprises the steps of: reheating a steel material containing, in wt%, C: 0.12-0.3%, Si: more than 0% and 0.5% or less, Mn: 0.3-2.0%, Al: more than 0% and 0.105% or less, N: more than 0% and 0.015% or less, and the remainder of Fe and inevitable impurities, and having an Al / N ratio of 1-7 and contents of V, Nb, and Ti of 0% or less than 0.01 wt% each, at 1100-1300°C; hot-rolling the reheated steel material at an Ar3 temperature or higher; and coiling the hot-rolled steel material at a temperature of 660°C or lower.

[0016] In the above hot rolling step, the finishing rolling temperature may be in the range of 830-900°C.

[0017] In the above winding step, the winding temperature may be 560-660°C.

[0018] The content of the above N may be 0.004-0.012% by weight.

[0019] The content of the above Al may be 0.01-0.03% by weight.

[0020] In order to achieve the above object, a steel pipe according to an embodiment of the present invention includes a steel pipe body and a hollow portion surrounded by the steel pipe body, and the steel pipe body contains, in wt%, C: 0.12-0.3%, Si: more than 0% and 0.5% or less, Mn: 0.3-2.0%, Al: more than 0% and 0.105% or less, N: more than 0% and 0.015% or less, and the remainder of Fe and unavoidable impurities, and the outer diameter of the steel pipe is D (mm), the thickness of the steel pipe body is t (mm), and the yield point elongation of the steel pipe body is El YP When (%), (t / D) × 100 is 2 or more, and (t / D) × 100 - El YP ≤ 50 / (3.67 × (t / D) × 100 + 10).

[0021] In the above steel pipe body, the weight ratio of Al / N may be 1-7.

[0022] In the above steel pipe body, the content of N may be 0.004-0.012% by weight, and the content of Al may be 0.01-0.03% by weight.

[0023] In the above steel pipe body, the contents of V, Nb and Ti may be 0% or less than 0.01 wt% each.

[0024] The above steel pipe body may have a microstructure containing ferrite at an area ratio of 50% or more.

[0025] The above steel pipe body may have a microstructure including at least one of pearlite, bainite, martensite, and cementite other than the above ferrite.

[0026] The above steel pipe body may have a microstructure in which the average crystal grain size of ferrite is 20㎛ or less at a point 1 / 4 in the thickness direction from the surface.

[0027] The above steel pipe body may have an average crystal grain size of ferrite on the surface / an average crystal grain size of ferrite at a point 1 / 4 in the thickness direction from the surface of 1 or less.

[0028] In the case of the hot-rolled steel sheet according to the present invention, yield point elongation can be induced even when the carbon content is 0.12 wt% or more through control of the alloy composition and microstructure. In particular, the hot-rolled steel sheet according to the present invention can exhibit yield point elongation of approximately 1-5% by controlling the aluminum and nitrogen contents and forming a fine, homogeneous microstructure on the surface and inside. As a result of this intended yield point elongation, the increase in yield strength when manufacturing the hot-rolled steel sheet according to the present invention can be reduced.

[0029] In addition to the effects described above, specific effects of the present invention are described below while explaining specific details for carrying out the invention.

[0030] Figure 1 schematically illustrates an example of a strain-stress curve showing yield point elongation.

[0031] The advantages and features of the present invention, and the methods for achieving them, will become clearer with reference to the embodiments described in detail below. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms. These embodiments are provided solely to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the scope of the invention. The present invention is defined solely by the scope of the claims.

[0032] Hereinafter, a hot-rolled steel sheet, a manufacturing method thereof, and a steel pipe using the same according to a preferred embodiment of the present invention will be described in detail.

[0033] The present invention relates to a hot-rolled steel sheet that can be used in various ways, such as for construction, bridges, and ground support, and a method for manufacturing the same. The hot-rolled steel sheet according to the present invention can be utilized in various forms, such as plates, H-beams, and steel pipes. In particular, the hot-rolled steel sheet according to the present invention relates to a hot-rolled steel sheet that can prevent excessive increase in yield strength due to work hardening during pipe manufacturing, for example, in steel having a yield strength of 275 MPa or more and a yield ratio of 85% or less, and a method for manufacturing the same.

[0034] The inventors of the present invention, through long-term research, have found that when a yield point elongation of about 1-5% occurs in a hot-rolled steel sheet having a carbon content of 0.12 wt% or more, the increase in yield strength during forming decreases.

[0035] Yield point elongation can be determined from the stress-strain curve during a tensile test of a steel. Specifically, it is defined as the serration at the lower yield point, as shown in the example in Fig. 1. The yield point elongation is defined as the length corresponding to this serration, i.e., the elongation (%). If there is no serration immediately after the yield point, the yield point elongation can be considered to be 0%, i.e., no yield point elongation has occurred.

[0036] Yield-point elongation is generally known to occur with the addition of interstitial elements such as nitrogen (N). Additionally, yield-point elongation can occur when conditions such as a decrease in mobile dislocation density through low-temperature structure reduction, grain refinement, and surface and internal grain homogenization are met. Of course, excessively high yield-point elongation, exceeding approximately 6-7%, can cause surface defects such as wrinkles and undulations, so it is preferable to maintain yield-point elongation at approximately 5% or less.

[0037] The present invention provides a hot-rolled steel sheet capable of exhibiting a yield point elongation of about 1-5% and a method for manufacturing the same.

[0038] hot rolled steel sheet

[0039] A hot-rolled steel sheet according to an embodiment of the present invention contains, in wt%, C: 0.12-0.3%, Si: more than 0% and 0.5% or less, Mn: 0.3-2.0%, Al: more than 0% and 0.105% or less, N: more than 0% and 0.015% or less, and the remainder of Fe and unavoidable impurities, and the weight ratio of Al / N is 1-7.

[0040] Hereinafter, the role and content of each component included in the hot-rolled steel sheet according to the present invention will be described.

[0041] Carbon (C): 0.12-0.3 wt%

[0042] Carbon is the most effective element for securing strength. The present invention contains at least 0.12 wt% carbon to ensure strength. However, if the carbon content exceeds 0.3 wt%, the steel may become excessively brittle and its weldability may deteriorate, thereby reducing its utility as a steel pipe.

[0043] From this point of view, the appropriate carbon content is 0.12-0.3 wt%, more preferably 0.15-0.25 wt%.

[0044] Silicon (Si): 0 wt% or more and 0.5 wt% or less

[0045] Silicon has a high affinity for oxygen and is utilized for deoxidation. It is an element that stably forms ferrite and, when dissolved in ferrite, can enhance its strength. However, excessive silicon additions, exceeding 0.5 wt%, can degrade weldability during electric resistance welding (ERW) and increase the austenite transformation temperature, coarsening the ferrite grain size and hindering yield-point elongation.

[0046] From this point of view, the appropriate silicon content is 0.5 wt% or less, preferably 0.3 wt% or less, more preferably 0.01-0.09 wt%, and most preferably 0.04-0.08 wt%.

[0047] Manganese (Mn): 0.3-2.0 wt%

[0048] Manganese combines with sulfur, an impurity, to improve steel purity. Manganese also contributes to increased strength and hardness through solid solution strengthening. To achieve these benefits, manganese must be added at a level of at least 0.3 wt%. However, high manganese content, exceeding 2.0 wt%, can lead to center segregation and excessively increase the carbon equivalent, impairing weldability.

[0049] From this point of view, the appropriate manganese content is 0.3-2.0 wt%, more preferably 0.3-1.7 wt%, and even more preferably 0.5-1.3 wt%.

[0050] Aluminum (Al): 0 wt% or more and 0.105 wt% or less

[0051] Aluminum has a high affinity for oxygen and is used for deoxidation. Furthermore, when precipitate-forming elements such as niobium, titanium, and vanadium are substantially absent, it combines with nitrogen during the hot-rolled steel sheet manufacturing process to form AlN, thereby suppressing grain growth. Aluminum can also be added to achieve a solid solution strengthening effect.

[0052] Excessive Al content exceeding 0.105 wt% may cause slab cracking during the casting process and may cause intergranular oxidation in the final steel product.

[0053] From this point of view, the appropriate aluminum content is 0.105 wt% or less, preferably 0.09 wt% or less, and more preferably 0.01-0.03 wt%.

[0054] Nitrogen (N): More than 0 wt% and less than 0.015 wt%

[0055] Nitrogen (N) can suppress grain growth by combining with aluminum to form AlN. In particular, in the present invention, nitrogen is an interstitial element and contributes to the yield point elongation of steel.

[0056] However, if the nitrogen content is excessive, exceeding 0.015 wt%, the toughness of the steel may be significantly reduced.

[0057] From this point of view, the appropriate nitrogen content is 0.015 wt% or less, more preferably 0.004-0.012 wt%.

[0058] Al / N content ratio: 1-7

[0059] In conventional hot-rolled steel sheets, the Al / N content ratio between Al and N is maintained at approximately 10. For example, in conventional hot-rolled steel sheets, Al is approximately 0.02 wt%, and N is approximately 0.002 wt%. In this case, it is difficult to obtain the yield point elongation effect due to N, an interstitial element.

[0060] Accordingly, in the present invention, the Al / N content ratio is controlled to be in the range of 1-7, more preferably in the range of 1-6, even more preferably in the range of 1.5-5.5, and most preferably in the range of 2-5, so that the yield point elongation effect due to N, an interstitial element, is sufficiently achieved. If the Al / N content ratio exceeds 7, the N content is insufficient, making it difficult to obtain the yield point elongation effect. On the other hand, if the Al / N content ratio is less than 1, the Al content is too small, making it difficult to obtain deoxidation and grain refinement effects, or the properties of the steel may deteriorate due to excessive N content.

[0061] In the hot-rolled steel sheet according to the present invention, it is preferable that the contents of V, Nb, and Ti be 0% or less than 0.01 wt% each so as not to affect the effect of reducing the increase in yield strength during forming due to yield point elongation.

[0062] The hot-rolled steel sheet according to the present invention may contain the remaining iron (Fe) and unavoidable impurities in addition to the composition described above.

[0063] Unavoidable impurities may be unintentionally introduced during the normal manufacturing process, and thus cannot be ruled out. For example, the hot-rolled steel sheet according to the present invention may contain phosphorus (P) and sulfur (S) as impurities.

[0064] Phosphorus (P) is an impurity that is inevitably contained in steelmaking. Since it causes brittleness through segregation, it is desirable to manage its content to as low as possible, below 0.03 wt%.

[0065] Sulfur (S) is an impurity that is inevitably contained in steelmaking. It may form inclusions or form FeS compounds with a low melting point, causing grain boundary embrittlement during hot rolling. Therefore, it is desirable to manage the content to be as low as possible, at 0.02 wt% or less.

[0066] The hot-rolled steel sheet according to the present invention has a microstructure containing ferrite at an area ratio of 50% or more through the alloy composition as described above and the process control described below. In addition, the hot-rolled steel sheet according to the present invention may contain a microstructure containing one or more of pearlite, bainite, martensite, and cementite at an area ratio of 50% or less in addition to the ferrite. The ferrite structure has a lower mobile dislocation density than low-temperature structures such as bainite, and thus is prone to yield point elongation.

[0067] In addition, the hot-rolled steel sheet according to the present invention may have a microstructure in which the average grain size of ferrite is 20 µm or less at a point 1 / 4 of the way from the surface in the thickness direction. Fine ferrite grains are advantageous for the occurrence of yield point elongation, and conversely, when the average grain size exceeds 20 µm, it is difficult for yield point elongation to occur.

[0068] In addition, the hot-rolled steel sheet according to the present invention may have an average grain size of ferrite on the surface / an average grain size of ferrite at a point 1 / 4 in the thickness direction from the surface (hereinafter, 1 / 4t point) of 1 or less. In general, since the cooling rate of the surface is faster after hot rolling, the average grain size of ferrite on the surface is smaller than the average grain size of ferrite at the point 1 / 4t. However, for example, when the finishing rolling temperature is lower than the Ar3 temperature, a mixed grain structure is generated, so that the average grain size of ferrite on the surface is larger than the average grain size of ferrite at the point 1 / 4t, and the average grain size of ferrite on the surface / an average grain size of ferrite at a point 1 / 4 in the thickness direction from the surface (hereinafter, 1 / 4t point) may exceed 1. In this case, the microstructures of the surface and the interior are not homogeneous, making it difficult for yield point elongation to occur. Accordingly, in the present invention, by controlling process conditions such as the finishing rolling temperature to suppress the mixed grain structure, the average crystal grain size of ferrite on the surface / the average crystal grain size of ferrite at the 1 / 4 point in the thickness direction from the surface (hereinafter, the 1 / 4t point) is set to 1 or less, thereby allowing the yield point elongation to occur well.

[0069] In addition, the hot-rolled steel sheet according to the present invention can have a yield strength of 275 MPa or more through the above-described alloy composition and process condition control, and the yield strength increase after forming can be 50 MPa or less.

[0070] Hot rolled steel sheet manufacturing method

[0071] A method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention includes a step of reheating a steel material having the above-described alloy component at 1100-1300°C, a step of hot-rolling the reheated steel material at a temperature higher than Ar3, and a step of coiling the hot-rolled steel material at a temperature lower than 660°C.

[0072] First, steel materials such as slabs having the aforementioned alloy composition are reheated at a temperature of 1100°C or higher and 1300°C or lower for a period of 270 minutes or less. Reheating can promote homogenization of the steel.

[0073] Homogenized steel slabs are hot-rolled at temperatures above Ar3. If hot-rolling is performed at temperatures below Ar3, yield-point elongation of the hot-rolled steel sheet is difficult to achieve due to the formation of mixed grain structures. On the other hand, excessively high finishing rolling temperatures can lead to grain coarsening and deterioration of surface quality. In this regard, hot-rolling is preferably performed at a finishing rolling temperature of 830-900°C, more preferably 840-880°C.

[0074] After hot rolling, water cooling is usually performed within 5 seconds, and the cooling speed during water cooling can be approximately 50℃ / sec or less.

[0075] In hot-rolled steel sheets, the coiling temperature determines the steel sheet's microstructure, strength, and other properties. In the present invention, the coiling temperature is preferably 560-660°C, more preferably 580-640°C. Coiling at a temperature lower than 560°C is highly unlikely to result in yield-point elongation due to increased mobile dislocation density. Conversely, coiling at a temperature higher than 660°C can lead to excessive scaling, potentially degrading surface quality.

[0076] steel pipe

[0077] A steel pipe according to an embodiment of the present invention is manufactured by forming a steel plate, and includes a steel pipe body and a hollow portion surrounded by the steel pipe body. The steel pipe body is a steel plate having the above-described alloy composition. Preferably, the steel pipe body is a hot-rolled steel plate having the above-described alloy composition and microstructural characteristics. That is, in the steel pipe according to an embodiment of the present invention, the steel pipe body includes, in wt%, C: 0.12-0.3%, Si: more than 0% and 0.5% or less, Mn: 0.3-2.0%, Al: more than 0% and 0.105% or less, N: more than 0% and 0.015% or less, and the remainder Fe and unavoidable impurities. In terms of generating a yield point elongation, the Al / N content ratio of the steel pipe body may be in the range of 1-7. More preferably, the content of N may be 0.004-0.012% by weight, and the content of Al may be 0.01-0.03% by weight. In addition, in the steel pipe body, the contents of V, Nb, and Ti may be 0%, or each may be less than 0.01% by weight. In addition, the steel pipe body may have a microstructure including ferrite in an area ratio of 50% or more. At this time, in terms of generating a yield point elongation, the steel pipe body may have a microstructure in which the average grain size of ferrite at a point 1 / 4 from the surface in the thickness direction is 20㎛ or less. In addition, in terms of generating a yield point elongation, the steel pipe body may have an average grain size of ferrite at the surface / an average grain size of ferrite at a point 1 / 4 from the surface in the thickness direction is 1 or less.

[0078] Meanwhile, the outer diameter of the steel pipe is D (mm), the thickness of the steel pipe body is t (mm), and the yield point elongation of the steel pipe body is El YP (%), when the ratio of the thickness of the steel pipe body to the outer diameter (D) of the steel pipe as a percentage (t / D) × 100(%) is 2% or more, more preferably 3% or more, (t / D) × 100 - El YPIt is desirable to satisfy ≤ 50 / (3.67 × (t / D) × 100 + 10). The inventor of the present invention studied the correlation between the outer diameter of the steel pipe, the thickness of the steel pipe body, and the yield point elongation of the steel pipe body, and confirmed that when the above equation is satisfied, the yield strength increase after pipe manufacturing is 50 MPa or less. In the above equation, for reference, (3.67 × (t / D) × 100 + 10) is a regression equation that reflects the fact that the yield strength increase rate differs depending on t / D.

[0079] For example, if the thickness of the steel pipe body is 3 mm and the outer diameter of the steel pipe is 100 mm, the yield point elongation needs to be at least about 0.62% to satisfy the above equation. As another example with a larger deformation, if the thickness of the steel pipe body is 5 mm and the outer diameter of the steel pipe is 100 mm, the yield point elongation needs to be at least about 3.24% to satisfy the above equation. In other words, the greater the deformation of the steel pipe body during pipe manufacturing, the greater the yield point elongation required.

[0080] As a result of actual experiments, it was confirmed that when the outer diameter of the steel pipe, the body thickness, and the yield point elongation of the steel plate constituting the body satisfy the above formula, the increase in yield strength after pipe manufacturing is 50 MPa or less (see Table 3).

[0081] In the case of the present invention, since the thickness and yield point elongation of the hot-rolled steel sheet can be known, and only the outer diameter of the steel pipe that can satisfy the above formula is selected accordingly, the increase in yield strength after pipe manufacturing can be suppressed to 50 MPa or less, and therefore, it can be considered desirable for guaranteeing the steel pipe specification.

[0082] Example

[0083] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the examples described below are intended only to illustrate and further concretize the present invention and are not intended to limit the scope of the present invention.

[0084] Table 1 shows the components of the steel specimens of the present invention. The remaining components, other than those listed in Table 1, are iron and unavoidable impurities. There was no intentional addition of V, Nb, or Ti. In Table 1, Specimens 1, 2, and 16 had Al / N content ratios exceeding 7.0.

[0085] A steel slab having the components shown in Table 1 was reheated at 1250°C, hot-rolled under the finish rolling temperature (FDT) conditions shown in Table 2, and then cooled at an average cooling rate of approximately 15°C / sec, and coiled at the coiling temperature (CT) shown in Table 2 to manufacture a hot-rolled steel sheet specimen.

[0086] [Table 1] (Unit: weight%)

[0087]

[0088] Table 2 shows the microstructure and tensile test results of the manufactured hot-rolled steel sheet specimens.

[0089] In Table 2, the microstructure analysis was obtained through microscopic observation, and the tensile test was performed using JIS No. 5 specimens. The yield point elongation (%) was calculated as the length corresponding to the sawtooth section at the lower yield point in the strain-stress curve obtained from the tensile test results.

[0090] In the microstructure fractions in Table 2, F represents ferrite, P represents pearlite, C represents cementite, B represents bainite, M represents martensite, and % represents area%.

[0091] In addition, in Table 2, 1 / 4t FGS (Ferrite Grain Size) means the average ferrite grain size at 1 / 4 point in the thickness direction from the surface of the hot-rolled steel sheet specimen, and surface FGS means the average ferrite grain size on the surface of the hot-rolled steel sheet specimen.

[0092] [Table 2]

[0093]

[0094] Table 3 shows the results of the experiments on the yield point elongation (%) and the increase in yield strength after pipemaking according to t / D (%) of each specimen. The yield strength after pipemaking was obtained through a longitudinal tensile test of the steel pipe.

[0095] [Table 3]

[0096]

[0097] Equation 1: (t / D) × 100 - El YP ≤ 50 / (3.67 × (t / D) × 100 + 10)

[0098] Referring to Tables 2 and 3, for specimens 3-6, 9, 11-13, and 15 that satisfy the alloy composition and hot rolling conditions presented in the present invention, Equation 1 was satisfied at least under the condition that the ratio of the steel pipe body thickness to the steel pipe outer diameter (D) (t / D) × 100 was 2% and 3%, respectively, and as a result, the yield strength increase rate after pipe manufacturing was 50 MPa or less. In some specimens, the yield strength increase rate after pipe manufacturing was 50 MPa or less even under the condition that the ratio of the steel pipe body thickness to the steel pipe outer diameter (D) (t / D) × 100 was 5%.

[0099] Meanwhile, referring to Tables 2 and 3, specimens 1, 2, and 16 that did not satisfy the range of Al / N content ratio of 7 or less did not experience yield point elongation, and thus did not satisfy Equation 1 above under the condition that the ratio of the steel pipe body thickness to the steel pipe outer diameter (D) (t / D) × 100 was 3% or more, and as a result, the yield strength increase after pipe manufacturing exceeded 50 MPa.

[0100] In addition, in the case of specimens 17 and 18 in which finishing rolling was performed under conditions where the FDT temperature was lower than the Ar3 temperature, a mixed grain structure occurred on the surface, and the average grain size of ferrite on the surface / the average grain size of ferrite at a point 1 / 4 (1 / 4t) in the thickness direction from the surface did not satisfy 1 or less, so that the yield point elongation did not occur, and accordingly, under the condition that the ratio of the pipe body thickness to the outer diameter (D) of the steel pipe (t / D) × 100 was 3% or more, the above equation 1 was not satisfied, and as a result, the increase in yield strength after pipe manufacturing exceeded 50 MPa.

[0101] Specimens 19 and 20 had a CT temperature exceeding 660℃, and the average ferrite grain size at the 1 / 4t point exceeded 20㎛. As a result, yield point elongation did not occur in Specimens 19 and 20, and accordingly, Equation 1 was not satisfied under the condition that the ratio of the pipe body thickness to the outer diameter (D) of the pipe (t / D) × 100 was 3% or more, and as a result, the yield strength increase after pipe making exceeded 50MPa.

[0102] It is judged that specimens 7, 8, 10, and 14 did not exhibit yield point elongation due to an increase in the operating dislocation density at temperatures below CT 560℃, and as a result, they did not satisfy Equation 1 above under the condition that the ratio of the pipe body thickness to the outer diameter (D) of the pipe (t / D) × 100 was 3% or more, and as a result, the increase in yield strength after pipe manufacturing exceeded 50 MPa.

[0103] In the present invention, t / D(%) can be applied in various ways depending on the purpose of the steel pipe, and is not necessarily limited to the range of t / D(%) 1-5% shown in the examples, and may also allow a strength increase of 50 MPa or more as needed.

[0104] While the embodiments of the present invention have been described above, it is clear that the present invention is not limited to the embodiments disclosed herein, and that various modifications may be made by those skilled in the art within the scope of the technical concept of the present invention. Furthermore, even if the operational effects according to the configuration of the present invention were not explicitly described and explained while describing the embodiments of the present invention, it is natural that the effects predictable by the corresponding configuration should also be acknowledged.

Claims

1. Contains, in weight%, C: 0.12-0.3%, Si: more than 0% but less than 0.5%, Mn: 0.3-2.0%, Al: more than 0% but less than 0.105%, N: more than 0% but less than 0.015%, and the remainder of Fe and unavoidable impurities. The weight ratio of Al / N is 1-7, The contents of V, Nb and Ti are 0% or less than 0.01 wt% each, A hot-rolled steel sheet having a microstructure containing ferrite at an area ratio of 50% or more.

2. In paragraph 1, The above hot-rolled steel sheet is a hot-rolled steel sheet having a microstructure in which the average crystal grain size of ferrite at a point 1 / 4 in the thickness direction from the surface is 20㎛ or less.

3. In paragraph 1, The above hot-rolled steel sheet is a hot-rolled steel sheet having an average grain size of ferrite on the surface / an average grain size of ferrite at 1 / 4 of a point in the thickness direction from the surface of 1 or less.

4. In paragraph 1, The above hot-rolled steel sheet is a hot-rolled steel sheet having a microstructure including at least one of pearlite, bainite, martensite, and cementite in addition to the above ferrite.

5. In paragraph 1, A hot-rolled steel sheet having a content of N of 0.004 to 0.012 wt%.

6. In paragraph 1, A hot-rolled steel sheet having an Al content of 0.01 to 0.03 wt%.

7. A step of reheating a steel material containing, by weight%, C: 0.12-0.3%, Si: more than 0% and 0.5% or less, Mn: 0.3-2.0%, Al: more than 0% and 0.105% or less, N: more than 0% and 0.015% or less, and the remainder of Fe and unavoidable impurities, with a weight ratio of Al / N of 1-7, and contents of V, Nb, and Ti of 0% or less than 0.01 wt% each, at 1100-1300℃; A step of hot rolling the above reheated steel at a temperature higher than Ar3; and A method for manufacturing a hot-rolled steel sheet, comprising the step of coiling the hot-rolled steel at a temperature of 660°C or lower.

8. In paragraph 7, A method for manufacturing a hot-rolled steel sheet, wherein in the above hot rolling step, the finishing rolling temperature is in the range of 830-900°C.

9. In paragraph 7, A method for manufacturing a hot-rolled steel sheet, wherein in the above-mentioned coiling step, the coiling temperature is 560-660℃.

10. In paragraph 7, A method for manufacturing a hot-rolled steel sheet, wherein the content of N is 0.004-0.012% by weight.

11. In paragraph 7, A method for manufacturing a hot-rolled steel sheet, wherein the content of Al is 0.01 to 0.03 wt%.

12. As a steel pipe, It comprises a steel pipe body and a hollow portion surrounded by the steel pipe body, The above steel pipe body contains, in weight %, C: 0.12-0.3%, Si: more than 0% and less than 0.5%, Mn: 0.3-2.0%, Al: more than 0% and less than 0.105%, N: more than 0% and less than 0.015%, and the remainder of Fe and unavoidable impurities. The outer diameter of the above steel pipe is D (mm), the thickness of the steel pipe body is t (mm), and the yield point elongation of the steel pipe body is El YP When (%) is said, (t / D) × 100 is greater than or equal to 2, (t / D) × 100 - El YP Steel pipe satisfying ≤ 50 / (3.67 × (t / D) × 100 + 10).

13. In paragraph 12, A steel pipe having an Al / N weight ratio of 1 to 7 in the above steel pipe body.

14. In paragraph 13, A steel pipe, wherein the content of N in the above steel pipe body is 0.004-0.012% by weight and the content of Al is 0.01-0.03% by weight.

15. In paragraph 12, A steel pipe, wherein the contents of V, Nb and Ti in the above steel pipe body are 0% or less than 0.01 wt% each.

16. In paragraph 12, The above steel pipe body is a steel pipe having a microstructure containing ferrite at an area ratio of 50% or more.

17. In paragraph 12, The above steel pipe body is a steel pipe having a microstructure in which the average crystal grain size of ferrite is 20㎛ or less at 1 / 4 point in the thickness direction from the surface.

18. In paragraph 12, The above steel pipe body is a steel pipe, wherein the average crystal grain size of ferrite on the surface / the average crystal grain size of ferrite at 1 / 4 point in the thickness direction from the surface is 1 or less.

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