Hot-rolled steel plates and steel pipes for ground reinforcement and their manufacturing methods

Abstract

The present invention relates to a hot-rolled steel plate and steel pipe for ground reinforcement, which are excellent in strength and formability, and a method for producing the same.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet and steel pipe for ground reinforcement, which are excellent in strength and formability, and a method for manufacturing the same.

Background Art

[0002] As facilities such as underground tunnels, underground transfer centers, and underground shopping centers for road undergrounding increase, the need for ground reinforcement materials as the foundation of such facilities is increasing. Along with this, a new standard for steel pipes for ground reinforcement (KS D 3872) has been established. Steel pipes for ground reinforcement used for the reinforcement of ground structures such as civil engineering and architecture must satisfy YS: 800 MPa, TS: 860 MPa, and EL: 10% or more in the longitudinal direction. In order to satisfy such physical properties, hot-rolled steel sheets are required to have high strength of YS: 700 MPa or more, TS: 750 MPa or more, and formability of EL: 15% or more. It is easy to obtain high strength by utilizing low-temperature structures such as bainite and martensite, but such structures have a disadvantage in that they cause softening of the welded part due to a slow cooling rate after welding and inhibit the physical properties of the welded part.

[0003] On the other hand, in order to manufacture a small-diameter steel pipe suitable for ground reinforcement, since work hardening during pipe manufacturing increases due to the small diameter of the steel pipe, it must have excellent formability, and in order to enable pipe manufacturing without shape defects, the microstructure must be uniform.

Summary of the Invention

Problems to be Solved by the Invention

[0004] One aspect of the present invention is to provide a hot-rolled steel sheet and steel pipe for ground reinforcement, which are excellent in strength and formability, and a method for manufacturing the same.

Means for Solving the Problems

[0005] One embodiment of the present invention provides a hot-rolled steel sheet for ground reinforcement that has excellent strength and formability, having a microstructure containing 90% or more ferrite by area, with the crystal grains of the ferrite having an average size of 15 μm or less, and containing 0.05% or more by weight of carbides containing Ti, Nb, and Mo, with the carbides having an average size of 20 nm or less. [Relationship 1] 0.002 ≤ (Ti / 48 + Mo / 96 + Nb / 93) ≤ 0.004 [Relationship 2] 0.002 ≤ (C / 12) - (Ti / 48 + Mo / 96 + Nb / 93) ≤ 0.006 (However, the content of each alloying element in the above relational formulas 1 and 2 refers to weight percent.)

[0006] Another embodiment of the present invention provides a steel pipe for ground reinforcement that has excellent strength and formability, manufactured using the above-mentioned hot-rolled steel sheet.

[0007] Another embodiment of the present invention provides a method for manufacturing a hot-rolled steel sheet for ground reinforcement that has excellent strength and formability, comprising the steps of: reheating a steel slab that satisfies the following relational formulas 1 and 2 in weight percent, C: 0.05~0.1%, Si: 0.1% or less (excluding 0%), Mn: 1.5~1.9%, Ti: 0.05~0.15%, Nb: 0.03~0.1%, Mo: 0.03~0.1%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.01% or less (excluding 0%), with the remainder being Fe and unavoidable impurities, in a temperature range of 1150~1300°C; finishing hot-rolling the reheated steel slab in a temperature range of 800~950°C to obtain a hot-rolled steel sheet; and winding the hot-rolled steel sheet in a temperature range of 550~700°C. [Relationship 1] 0.002 ≤ (Ti / 48 + Mo / 96 + Nb / 93) ≤ 0.004 [Relationship 2] 0.002 ≤ (C / 12) - (Ti / 48 + Mo / 96 + Nb / 93) ≤ 0.006 (However, the content of each alloying element in the above relational formulas 1 and 2 refers to weight percent.)

[0008] Another embodiment of the present invention provides a method for manufacturing a steel pipe for ground reinforcement that has excellent strength and formability, including the step of forming a steel pipe from a hot-rolled steel sheet manufactured by the above manufacturing method. [Effects of the Invention]

[0009] According to one aspect of the present invention, it is possible to provide hot-rolled steel plates and steel pipes for ground reinforcement that have excellent strength and formability, as well as methods for manufacturing these. [Modes for carrying out the invention]

[0010] The following describes a hot-rolled steel sheet according to one embodiment of the present invention. First, the alloy composition of the present invention will be described. The content of the alloy composition described below is in weight percent.

[0011] C: 0.05~0.1% Carbon (C) is added not only for its solid solution strengthening effect but also for carbide formation with Ti, Nb, and Mo, and is an element for ensuring tensile strength. To obtain the above effect, it is preferable to add 0.05% or more. However, if it exceeds 0.1%, carbide coarsening occurs, making it impossible to sufficiently secure the precipitation strengthening effect, and the pearlite fraction in the microstructure increases, making it impossible to secure 90% or more ferrite as intended in this invention. Therefore, it is preferable that the C content be in the range of 0.05 to 0.1%. The lower limit of the C content is more preferably 0.06%, even more preferably 0.065%, and most preferably 0.07%. The upper limit of the C content is more preferably 0.09%, even more preferably 0.085%, and most preferably 0.08%.

[0012] Si: 0.1% or less (excluding 0%) Si is not only useful for deoxidizing steel, but is also effective in ensuring strength through solid solution strengthening. However, if the Si content exceeds 0.1%, it has the disadvantage of forming silicon oxide, which makes plating difficult. Therefore, it is preferable that the Si content be 0.1% or less. It is more preferable that the Si content be 0.08% or less, even more preferable that be 0.065% or less, and most preferable that be 0.05% or less.

[0013] Mn: 1.5~1.9% Mn is added to provide a solid solution strengthening effect and to ensure the hardening ability of the weld during cooling after welding. To obtain the above effects, it is preferable to add 1.5% or more. However, if it exceeds 1.9%, Mn segregation increases, which may cause defects and material deviations during continuous casting. Therefore, it is preferable that the Mn content be in the range of 1.5 to 1.9%. The lower limit of the Mn content is more preferably 1.55%, even more preferably 1.6%, and most preferably 1.65%. The upper limit of the Mn content is more preferably 1.85%, even more preferably 1.8%, and most preferably 1.75%.

[0014] Ti: 0.05~0.15% Ti is added for precipitation strengthening and grain coarsening suppression. If the Ti content is less than 0.05%, it is difficult to obtain the high strength targeted in this invention, and if it exceeds 0.15%, coarse carbides are formed, making it ineffective for precipitation strengthening. Therefore, the Ti content is preferably in the range of 0.05 to 0.15%. The lower limit of the Ti content is more preferably 0.07%, even more preferably 0.08%, and most preferably 0.09%. The upper limit of the Ti content is more preferably 0.14%, even more preferably 0.13%, and most preferably 0.12%.

[0015] Nb: 0.03~0.1% Nb is added to suppress recrystallization during hot rolling and to obtain a finer grain size, along with a precipitation strengthening effect. If the Nb content is less than 0.03%, a sufficient precipitation strengthening effect cannot be obtained, and if it exceeds 0.1%, the strength may decrease due to the formation of coarse precipitates. Therefore, it is preferable that the Nb content be in the range of 0.03 to 0.1%. The lower limit of the Nb content is more preferably 0.035%, even more preferably 0.038%, and most preferably 0.04%. The upper limit of the Nb content is more preferably 0.08%, even more preferably 0.07%, and most preferably 0.06%.

[0016] Mo: 0.03~0.1% Mo is added to suppress the growth of precipitates. It also contributes to grain refinement by delaying ferrite formation, allowing ferrite to form at lower temperatures. If the Mo content is less than 0.03%, the above effects cannot be fully obtained. On the other hand, if it exceeds 0.1%, economic efficiency may decrease. Therefore, the Mo content is preferably in the range of 0.03 to 0.1%. 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 0.09%, even more preferably 0.08%, and most preferably 0.07%.

[0017] P: 0.02% or less (excluding 0%) Since phosphorus (P) segregates at grain boundaries as an impurity element, reducing toughness, it is preferable to omit it as much as possible, and in this invention, its upper limit is limited to 0.02%. The above P content is more preferably 0.018% or less, even more preferably 0.017% or less, and most preferably 0.015% or less.

[0018] S: 0.02% or less (excluding 0%) S is the main element that forms MnS as an impurity element. Since the formation of coarse MnS reduces toughness, in the present invention, its content is limited to 0.02% or less. It is more preferable that the above S content is 0.015% or less, further preferably 0.01% or less, and most preferably 0.005% or less.

[0019] N: 0.01% or less (excluding 0%) When the content of N exceeds 0.01% as an impurity element, it reacts with Ti and Nb at high temperatures to form nitrides, thus having the drawback of reducing the content of Ti and Nb that substantially contribute to precipitation strengthening and lowering the strength of the steel material. Therefore, it is preferable that the above N content is 0.01% or less. It is more preferable that the above N content is 0.008% or less, further preferably 0.007% or less, and most preferably 0.006% or less.

[0020] Moreover, the hot-rolled steel sheet of the present invention preferably satisfies the following relational expressions 1 and 2.

[0021] [Relational expression 1] 0.002 ≤ (Ti / 48 + Mo / 96 + Nb / 93) ≤ 0.004

[0022] The above relational expression 1 is a parameter for improving strength by controlling the contents of Ti, Mo, and Nb, which are precipitation strengthening elements. When the value of (Ti / 48 + Mo / 96 + Nb / 93) is less than 0.002, the amount of precipitates is too small to be effective in improving strength. On the other hand, when the value of (Ti / 48 + Mo / 96 + Nb / 93) exceeds 0.004, an effective precipitation strengthening effect may not be obtained due to coarsening of the precipitates.

[0023] [Relational expression 2] 0.002 ≤ (C / 12) - (Ti / 48 + Mo / 96 + Nb / 93) ≤ 0.006

[0024] The above relational equation 2 is a parameter that indicates the amount of carbon (C) used for the solid solution strengthening effect, excluding the amount of carbon used for the precipitation strengthening effect. If the value of (C / 12)-(Ti / 48+Mo / 96+Nb / 93) is less than 0.002, sufficient strength cannot be obtained in the ferrite phase, and therefore the high strength targeted in this invention cannot be achieved. On the other hand, if the value of (C / 12)-(Ti / 48+Mo / 96+Nb / 93) exceeds 0.006, the precipitate easily becomes coarse as the remaining carbon content increases excessively, and the target strength cannot be obtained. The pearlite fraction also increases, making it difficult to obtain the 90% or more ferrite targeted in this invention. Furthermore, as pearlite formation is promoted in the center of the steel plate where the cooling rate is relatively slow, a large difference in hardness occurs between the surface and the interior. As a result, a difference in hardness occurs in the thickness direction even in steel pipes. In addition, during the rolling process, specific parts may undergo excessive processing, leading to problems such as shape defects and crack formation. Rolling refers to the process of creating protrusions on the surface of a steel pipe.

[0025] In addition to the steel composition described above, the remainder may include Fe and unavoidable impurities. Since unavoidable impurities can be unintentionally introduced during the normal steel manufacturing process, it is impossible to completely eliminate them, and the implications of this can be easily understood by engineers in the field of normal steel manufacturing. Furthermore, this invention does not completely exclude the addition of other compositions besides the steel composition described above.

[0026] The hot-rolled steel sheet of the present invention preferably has a microstructure containing 90% or more ferrite by area. In the present invention, it is important to ensure 90% or more ferrite in order to ensure excellent formability. Theoretically, the microstructure of the present invention is preferably a single phase of ferrite, but due to the manufacturing process, one or more of pearlite, retained austenite, bainite, and martensite may inevitably be formed. However, if the amount of low-temperature transformation phases such as bainite or martensite increases, the formability decreases. Also, if pearlite is present on the surface of the steel pipe, cracks are likely to occur during rolling due to cementite, which is a hard phase. Therefore, it is preferable to have as little of the above-mentioned residual structure as possible. The ferrite fraction is more preferably 93% or more, and even more preferably 95% or more. On the other hand, the ferrite may be one or more of polygonal ferrite, bainite ferrite, and acicular ferrite.

[0027] In this case, it is preferable that the ferrite grains have an average size of 15 μm or less. If the ferrite grain size exceeds 15 μm, sufficient strength may not be obtained due to grain coarsening. It is more preferable that the ferrite grain size is 12 μm or less, and even more preferable that it is 10 μm or less.

[0028] The hot-rolled steel sheet of the present invention preferably contains 0.05% by weight or more of carbides containing Ti, Nb, and Mo individually or in combination, and preferably the carbides have an average size of 20 nm or less. By forming a large amount of fine carbides with an average size of 20 nm or less at a concentration of 0.05% by weight or more, an excellent strength improvement effect can be obtained without the fracture of the carbides. The fraction of the carbides is more preferably 0.07% by weight or more, and even more preferably 0.08% by weight or more. The average size of the carbides is more preferably 15 nm or less, and even more preferably 10 nm or less. On the other hand, in the present invention, the more carbides that are formed, the more advantageous it is, so there is no particular upper limit, but considering the Ti, Nb, and Mo content in the steel, it is difficult to exceed 0.2% by weight.

[0029] As described above, the hot-rolled steel sheet of the present invention has a yield strength (YS) of 700 MPa or more, a tensile strength (TS) of 750 MPa or more, and an elongation (EL) of 15% or more, ensuring excellent strength and formability.

[0030] On the other hand, the present invention can provide a steel pipe manufactured using the above-mentioned hot-rolled steel sheet.

[0031] The steel pipe of the present invention has a yield strength (YS) of 800 MPa or more, a tensile strength (TS) of 860 MPa or more, and an elongation (EL) of 10% or more, ensuring excellent strength and formability.

[0032] On the other hand, stable rolling is possible without defects caused by stress concentration during processing, provided that the steel pipe is uniformly deformed in the thickness direction during rolling. However, since the steel pipe of the present invention has a hardness deviation of 15% or less, it has the advantage of ensuring uniform hardness, which is beneficial for rolling. The above hardness deviation can be defined as [(maximum hardness value - minimum hardness value) / maximum hardness value × 100], based on the hardness values ​​measured at points 0.5 mm, t / 4, and t / 2 (t: steel pipe thickness) from the surface of the steel pipe in the thickness direction.

[0033] The following describes a method for manufacturing a hot-rolled steel sheet according to one embodiment of the present invention.

[0034] First, the steel slab satisfying the alloy composition and relational equations 1 and 2 described above is reheated in a temperature range of 1150 to 1300°C. The reason for reheating the steel slab in a temperature range of 1150 to 1300°C is to make the alloy composition and microstructure uniform. If the reheating temperature is below 1150°C, the precipitates formed on the slab will not be dissolved, and the optimal precipitation strengthening effect cannot be obtained in subsequent processes. Also, if the reheating temperature exceeds 1300°C, excessive grain growth will occur, making it difficult to secure the target material and quality. Therefore, it is preferable that the reheating temperature of the steel slab be in the range of 1150 to 1300°C. The lower limit of the reheating temperature is more preferably 1170°C, even more preferably 1180°C, and most preferably 1200°C. The upper limit of the reheating temperature is more preferably 1290°C, even more preferably 1270°C, and most preferably 1250°C.

[0035] Subsequently, the reheated steel slab is finished hot-rolled in a temperature range of 800 to 950°C to obtain a hot-rolled steel sheet. If the finishing hot-rolling temperature is less than 800°C, some of the austenite will transform into ferrite, resulting in uneven grain size in the final product. If it exceeds 950°C, scale defects and the like may occur. Therefore, it is preferable that the finishing hot-rolling temperature be in the range of 800 to 950°C. The lower limit of the finishing hot-rolling temperature is more preferably 820°C, even more preferably 825°C, and most preferably 850°C. The upper limit of the finishing hot-rolling temperature is more preferably 940°C, even more preferably 920°C, and most preferably 900°C.

[0036] After this, the hot-rolled steel sheet is wound up in a temperature range of 550 to 700°C. If the winding temperature is less than 550°C, not only will the microstructure desired by the present invention not be obtained, but carbide formation will not be sufficient, resulting in insufficient precipitation strengthening. If it exceeds 700°C, carbide coarsening will occur, and the target strength will not be obtained. Therefore, it is preferable that the winding temperature be in the range of 550 to 700°C. The lower limit of the winding temperature is more preferably 600°C, even more preferably 620°C, and most preferably 640°C. The upper limit of the winding temperature is more preferably 680°C, even more preferably 665°C, and most preferably 650°C. On the other hand, cooling from the finish hot rolling until winding can be performed on a runout table.

[0037] On the other hand, in the present invention, after obtaining a hot-rolled steel sheet by the process described above, a steel pipe can be obtained by forming the hot-rolled steel sheet into a pipe. In this case, electric resistance welding (ERW) or the like can be used as the welding method during pipe formation. Furthermore, in the present invention, a step of rolling the steel pipe may be further included after the step of obtaining the steel pipe. [Examples]

[0038] The present invention will be described in more detail below with reference to examples. However, such examples are merely illustrative of the implementation of the present invention and do not limit the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0039] (Examples) After preparing steel slabs having the alloy compositions listed in Table 1 below, hot-rolled steel sheets with a thickness of 2.8 to 6 mm were produced by reheating, finish hot-rolling, and winding under the conditions listed in Table 2 below. Subsequently, steel pipes were manufactured by forming these hot-rolled steel sheets. At this time, roll forming and electric resistance welding, which are common methods for manufacturing electric resistance welded steel pipes, were used, and the pipe-forming conditions (t / D) corresponding to the thickness of the material (t) and the diameter of the steel pipe (D) were set to 14% or higher. The microstructure, precipitates, and mechanical properties of the hot-rolled steel sheets and steel pipes produced in this manner were measured, and the hardness at different positions in the thickness direction was further measured for the steel pipes. The results are shown in Tables 3 and 4 below.

[0040] The types and fractions of microstructure were measured using an optical microscope (OM). Ferrite grain size was measured using the circular intercept method of ASTM E 112 after imaging with a scanning electron microscope (SEM).

[0041] The size of the precipitates was determined by collecting precipitates from the test specimens using the carbon replica method and measuring them using a transmission electron microscope (TEM). The fraction of the precipitates was determined by measuring the Ti, Nb, and Mo content using the residue extraction method.

[0042] Yield strength (YS), tensile strength (TS), and elongation (EL) were measured by tensile testing according to the KS B 0802 standard. For hot-rolled steel sheets, tensile test specimens were prepared using KS B 0801 standard No. 5 specimens, with the length aligned with the hot-rolling direction. For steel pipes, tensile tests were performed using KS B 0801 standard No. 11 specimens.

[0043] The hardness at different locations in the thickness direction was measured using a Vickers hardness tester with a 1 kg load at points 0.5 mm, t / 4, and t / 2 (t: steel pipe thickness) in the thickness direction from the outer surface of the steel pipe. At each location, five points were measured and the average value was calculated. The hardness deviation listed in Table 3 below was calculated from the hardness values ​​measured at each location using the formula [(maximum hardness value - minimum hardness value) / maximum hardness value × 100].

[0044] [Table 1]

[0045] [Table 2]

[0046] [Table 3]

[0047] [Table 4]

[0048] As can be seen from Tables 1 to 4 above, in the case of Invention Examples 1 to 16, which satisfy the alloy composition, relational formulas 1 and 2, and manufacturing conditions proposed by the present invention, it is clear that not only are excellent mechanical properties obtained by securing the microstructure and precipitates that the present invention aims to achieve, but the deviation of hardness at different positions in the thickness direction of the steel pipe is also at a low level.

[0049] In Comparative Example 1, although the alloy composition of the present invention was met, the low reheating temperature of the steel slab resulted in insufficient redissolution of the precipitation strengthening elements, and a sufficient precipitation strengthening effect could not be obtained. As a result, the high strength targeted by the present invention could not be secured.

[0050] In the case of Comparative Example 2, although the alloy composition of the present invention was satisfied, the low finish hot rolling temperature resulted in the formation of coarse precipitates during rolling, preventing a sufficient precipitation strengthening effect from being obtained. As a result, the high strength targeted by the present invention could not be secured.

[0051] In the case of Comparative Example 3, although the alloy composition of the present invention was satisfied, the high finishing hot rolling temperature caused the crystal grains to coarseen, making it impossible to secure the high strength targeted by the present invention.

[0052] In the case of Comparative Example 4, although the alloy composition of the present invention was satisfied, the low winding temperature prevented the acquisition of the microstructure intended by the present invention, as well as a sufficient precipitation strengthening effect, making it impossible to secure the high strength targeted by the present invention.

[0053] In the case of Comparative Example 5, although the alloy composition of the present invention was satisfied, the high winding temperature resulted in the formation of coarse carbides, making it impossible to secure the high strength targeted by the present invention.

[0054] In the case of Comparative Example 6, the low content of Ti and Nb prevented a sufficient precipitation strengthening effect from being obtained, and thus the high strength targeted by the present invention could not be secured.

[0055] In the case of Comparative Example 7, the high Ti content resulted in the formation of coarse carbides, making it impossible to achieve the high strength targeted by the present invention.

[0056] In the case of Comparative Example 8, the low Mn content resulted in a low solid solution strengthening effect, and the change in phase transformation conditions during cooling prevented a sufficient precipitation strengthening effect from being obtained, making it impossible to secure the high strength targeted by the present invention.

[0057] In the case of Comparative Example 9, the high Nb content resulted in the formation of coarse carbides, making it impossible to achieve the high strength targeted by the present invention.

[0058] In the case of Comparative Example 10, not only was it impossible to satisfy relational equation 2, but the low C content also prevented the securing of the high strength targeted by the present invention.

[0059] In the case of Comparative Example 11, the high carbon content resulted in a high solid-solution carbon content, which increased the perlite content. This prevented the acquisition of the microstructure intended by the present invention, and the relationship equation 2 was not satisfied. As a result, the high strength targeted by the present invention could not be secured, and the deviation of hardness at different positions in the thickness direction of the steel pipe also increased significantly.

[0060] In the case of Comparative Example 12, because relational equation 2 is not satisfied, the appropriate balance between precipitates and solid solution C proposed by the present invention cannot be obtained. As a result, not only was high strength not ensured, but the deviation of hardness at different positions in the thickness direction of the steel pipe became large.

[0061] In the case of Comparative Example 13, sufficient precipitation strengthening effect could not be obtained because relational formula 1 was not satisfied, and the high strength targeted by the present invention could not be secured.

Claims

1. In weight percent, it contains C: 0.05-0.1%, Si: 0.1% or less (excluding 0%), Mn: 1.5-1.9%, Ti: 0.05-0.15%, Nb: 0.03-0.10%, Mo: 0.03-0.1%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), and N: 0.01% or less (excluding 0%), with the remainder being Fe and unavoidable impurities. The following relational equations 1 and 2 are satisfied, It has a microstructure containing 90% or more ferrite by area percentage, The ferrite crystal grains have an average size of 15.0 μm or less. It contains 0.050% by weight or more of carbides containing Ti, Nb, and Mo individually or in combination. The carbide is a hot-rolled steel plate for ground reinforcement having an average size of 20 nm or less. [Relationship 1] 0.0020 ≤ (Ti / 48 + Mo / 96 + Nb / 93) ≤ 0.0040 [Relationship 2] 0.002 ≤ (C / 12) - (Ti / 48 + Mo / 96 + Nb / 93) ≤ 0.0060 (However, the content of each alloying element in relational formulas 1 and 2 above refers to weight percent.)

2. The hot-rolled steel sheet for ground reinforcement according to claim 1, wherein the microstructure comprises one or more of pearlite, retained austenite, bainite, and martensite as the remaining structure.

3. The hot-rolled steel sheet for ground reinforcement according to claim 1, wherein the hot-rolled steel sheet has a yield strength (YS) of 700 MPa or more, a tensile strength (TS) of 750 MPa or more, and an elongation (EL) of 15% or more.

4. A steel pipe for ground reinforcement, manufactured using a hot-rolled steel sheet as described in any one of claims 1 to 3.

5. The steel pipe for ground reinforcement according to claim 4, wherein the steel pipe has a yield strength (YS) of 800 MPa or more, a tensile strength (TS) of 860 MPa or more, and an elongation rate (EL) of 10% or more.

6. The steel pipe for ground reinforcement according to claim 4, wherein the steel pipe has a hardness deviation of 15% or less. (However, the hardness deviation is defined as [(maximum hardness value - minimum hardness value) / maximum hardness value × 100], calculated from the hardness values ​​measured at points 0.5 mm, t / 4, and t / 2 (t: steel pipe thickness) in the thickness direction from the surface of the steel pipe, and the unit of hardness is Hv.)

7. The process involves reheating a steel slab containing, by weight percent, C: 0.05-0.1%, Si: 0.1% or less (excluding 0%), Mn: 1.5-1.9%, Ti: 0.05-0.15%, Nb: 0.03-0.10%, Mo: 0.03-0.1%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), and N: 0.01% or less (excluding 0%), with the remainder being Fe and unavoidable impurities, satisfying the following relational formulas 1 and 2, in a temperature range of 1150-1300°C; The steps include: and obtaining a hot-rolled steel sheet by finishing hot-rolling the reheated steel slab at a temperature range of 800 to 950°C; and A method for manufacturing a hot-rolled steel sheet for ground reinforcement according to any one of claims 1 to 3, comprising the step of winding the hot-rolled steel sheet in a temperature range of 550 to 700°C. [Relationship 1] 0.0020 ≤ (Ti / 48 + Mo / 96 + Nb / 93) ≤ 0.0040 [Relationship 2] 0.002 ≤ (C / 12) - (Ti / 48 + Mo / 96 + Nb / 93) ≤ 0.0060 (However, the content of each alloying element in relational formulas 1 and 2 above refers to weight percent.)

8. A method for manufacturing steel pipes for ground reinforcement, comprising the step of forming a pipe from a hot-rolled steel sheet manufactured by the manufacturing method described in claim 7 to obtain a steel pipe.

9. A method for manufacturing a steel pipe for ground reinforcement according to claim 8, further comprising the step of rolling the steel pipe after the step of obtaining the steel pipe.

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