Welded steel pipe for slurry transfer and method for manufacturing the same

Abstract

An object of the present invention is to provide a welded steel pipe having excellent wear resistance and low temperature toughness, minimizing hot cracking at welds, and suitable for transporting slurry, and a manufacturing method thereof. [Solution] The manufacturing method of the welded steel pipe suitable for transporting slurry of the present invention includes the steps of forming an austenitic steel plate into a tubular shape to form a butt joint, and welding the butt joint to provide a welded steel pipe so that the carbon content of all welded parts satisfies the following relational expression 1. [Equation 1] 0.03*[Mn]+0.04*[Cr]≦[C]≦0.05*[Mn]+0.26*[Cr] In the above Relational Formula 1, [C], [Mn] and [Cr] respectively mean the C, Mn and Cr contents (wt %) contained in the entire welded portion.

Description

[Technical Field]

[0001] The present invention relates to a welded steel pipe and a method for manufacturing the same, and more particularly, to a welded steel pipe suitable for slurry transportation and a method for manufacturing the same, which has excellent wear resistance and low-temperature toughness by optimizing the steel composition and microstructure of a steel pipe material and the steel pipe manufacturing process, and minimizes hot cracking in a welded portion. [Background Art]

[0002] Parts and structures used in wear-resistant environments require regular inspection and replacement to ensure work safety, work soundness, and the like. Since the operation of the relevant industrial facility must be temporarily interrupted to inspect and replace the parts and structures, frequent inspection and replacement of parts and structures leads to a decrease in productivity and economic efficiency. Accordingly, various attempts have been made to extend the service life of parts and structures used in wear-resistant environments, and various studies have been actively conducted to improve the wear resistance of the materials themselves constituting the parts and structures.

[0003] With the growth of the mining industry and the oil and gas industries, wear of welded steel pipes used in the processes of mining, transportation, refining and storage has become a major problem. On the other hand, development of oil sands has recently entered a full-scale phase, and it is known that welded steel pipes manufactured from API standard steel materials commonly used in the oil and gas industries cannot secure a level of wear resistance sufficient to transport slurry containing oil, rock, gravel, sand and the like for a long period of time. In addition, in the case of welded steel pipes used for slurry transportation, excellent low-temperature toughness is required, and it is known that API standard steel materials that are currently commonly used cannot secure low-temperature toughness meeting this requirement. Therefore, there is an urgent need for research on welded steel pipes for slurry pipes excellent in wear resistance and low-temperature toughness. [Summary of the Invention] [Problem to be Solved by the Invention]

[0004] The object of the present invention is to provide a welded steel pipe and a method for manufacturing the same that have excellent wear resistance and low-temperature toughness, minimize high-temperature cracking of the welded joint, and are suitable for slurry transfer.

[0005] The problems that the present invention addresses are not limited to those described above. A person of ordinary skill should have no difficulty understanding further problems that the present invention addresses from the entirety of this specification. [Means for solving the problem]

[0006] The present invention provides a method for manufacturing a welded steel pipe for slurry transfer, comprising the steps of forming an austenitic steel sheet into a tubular shape so that a butt joint is formed, and welding the butt joint so that the carbon content of the entire welded portion satisfies the following relational formula 1 to provide a welded steel pipe. [Relationship 1] 0.03*[Mn]+0.04*[Cr]≦[C]≦0.05*[Mn]+0.26*[Cr] In the above relational equation 1, [C], [Mn], and [Cr] represent the C, Mn, and Cr content (by weight %) included in the entire welded area, respectively.

[0007] The above austenitic steel sheet contains, by weight percent, C: 0.4-1.2%, Si: 1.0% or less (including 0%), Mn: 10-28%, Cr: 6.0% or less (including 0%), V: 0.5% or less (including 0%), P: 0.025% or less, and S: 0.025% or less, with the remainder being Fe and unavoidable impurities. The surface hardness of the above austenitic steel sheet satisfies the following relational equation 2. [Relationship 2] 18.7*[Mn]-10.3*[Cr]+1.7*[C]≦Surface hardness (Hv)≦22.1*[Mn]+18.3*[Cr]+2.4*[C] In the above relational equation 2, [Mn], [Cr], and [C] represent the Mn, Cr, and C content (by weight %) contained in the steel sheet, respectively. If the corresponding component is not present, 0 should be substituted.

[0008] The above austenitic steel sheet contains 80% or more austenite in its microstructure, and the average grain size of the austenite is 60 μm or less.

[0009] The above austenitic steel sheet contains twins of 20% or less by area, and the average size of these twins is 400 nm or less.

[0010] The forming method for the above-mentioned austenitic steel sheet into a tubular shape is selected from one of the following: spiral forming, UOE pressing, roll bending, and JCO forming.

[0011] The welding method for the butt joint described above is either one or more arc welding methods selected from SMAW (shield metal arc welding), GMAW (gas metal arc welding), GTAW (gas tungsten arc welding), FCAW (flux cored arc welding), and SAW (sub-merged arc welding), or electric resistance welding (ERW). The butt joint is welded with a heat input of 4.3 kJ / mm or less.

[0012] The welded steel pipe for slurry transfer of the present invention includes a welded steel pipe base material portion having austenite as its matrix structure, and a welded portion connecting both ends of the welded steel pipe base material portion, characterized in that the total carbon content of the welded portion included in the welded portion satisfies the following relational expression 1. [Relationship 1] 0.03*[Mn]+0.04*[Cr]≦[C]≦0.05*[Mn]+0.26*[Cr] In the above relational equation 1, [C], [Mn], and [Cr] represent the C, Mn, and Cr content (by weight %) included in the entire welded area, respectively.

[0013] The base material of the welded steel pipe described above contains, by weight percent, C: 0.4-1.2%, Si: 1.0% or less (including 0%), Mn: 10-28%, Cr: 6.0% or less (including 0%), V: 0.5% or less (including 0%), P: 0.025% or less, and S: 0.025% or less, with the remainder being Fe and unavoidable impurities. The surface hardness of the base material of the welded steel pipe described above satisfies the following relational equation 2. [Relationship 2] 18.7*[Mn]-10.3*[Cr]+1.7*[C]≦Surface hardness (Hv)≦22.1*[Mn]+18.3*[Cr]+2.4*[C] In the above relational equation 2, [Mn], [Cr], and [C] represent the Mn, Cr, and C content (by weight %) contained in the welded steel pipe base material, respectively. If the corresponding component is not present, 0 should be substituted.

[0014] The base material of the welded steel pipe described above contains austenite in a microstructure of 80 area or more, and the average grain size of the austenite is 60 μm or less.

[0015] The welded steel pipe base material contains twins of 20% area or less, and the average size of these twins is 400 nm or less.

[0016] The maximum length of hot cracks formed in the above-mentioned welded area is 0.5 mm or less.

[0017] The welded steel pipe base material described above has a tensile strength of 800 MPa or more, and the temperature at which the ductile fracture surface ratio in the DWTT test is 85% or more is -25°C or lower.

[0018] The means of solving the above problems do not enumerate all of the features of the present invention, and the various features of the present invention and the advantages and effects associated therewith can be understood in more detail by referring to the following specific examples. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide a welded steel pipe and a method for manufacturing the same that have excellent wear resistance and low-temperature toughness, minimize high-temperature cracking of the welded joint, and are suitable for slurry transfer.

[0020] The effects of the present invention are not limited to the matters described above, and can be interpreted to include matters that can be reasonably inferred by a person skilled in the art from the matters described in the present specification. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] [Figure 1] It is a cross-sectional observation photograph of a welded portion of test piece 1. [Figure 2] It is a cross-sectional observation photograph of a welded portion of test piece 2. MODE FOR CARRYING OUT THE INVENTION

[0022] The present invention relates to a welded steel pipe for slurry transfer and a method for producing the same, and preferred embodiments of the present invention will be described below. Embodiments of the present invention can be modified into various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to explain the present invention in more detail to a person having ordinary knowledge in the technical field to which the present invention pertains.

[0023] Hereinafter, the austenitic steel sheet used for producing the welded steel pipe of the present invention will be described in more detail. Hereinafter, unless otherwise specified, percentages stated for the content of a steel composition are based on weight, and percentages stated for the fraction of a microstructure are based on area.

[0024] An austenitic steel sheet according to one aspect of the present invention comprises, by weight%, C: 0.4 to 1.2%, Si: 1.0% or less (including 0%), Mn: 10 to 28%, Cr: 6.0% or less (including 0%), V: 0.5% or less (including 0%), P: 0.025% or less, S: 0.025% or less, with the balance being Fe and unavoidable impurities.

[0025] Carbon (C): 0.4 to 1.2% Carbon (C) is a component that effectively contributes to ensuring the strength and improving the wear resistance of steel. Furthermore, carbon (C) is a component that improves the hardening ability of steel and effectively contributes to the stabilization of austenite. This invention allows for limiting the carbon (C) content in austenitic steel sheets to 0.4% or more in order to improve the strength and austenitic safety of not only the base material of welded steel pipes but also the welded joints. A more preferable carbon (C) content can be 0.5% or more, and a more preferable carbon (C) content can be 0.6% or more. On the other hand, if carbon (C) is added excessively, a large amount of carbides may precipitate, reducing the wear resistance and elongation of the steel; therefore, this invention allows for limiting the upper limit of the carbon (C) content to 1.2%.

[0026] Silicon (Si): 1.0% or less (including 0%) Silicon (Si) not only acts as a deoxidizing agent but also effectively contributes to improving the strength of steel through solid solution strengthening. Therefore, the austenitic steel sheet of the present invention may contain silicon (Si) to achieve such effects. The lower limit of the preferred silicon (Si) content can be 0.1%, and the lower limit of the more preferred silicon (Si) content can be 0.2%. However, since silicon (Si) is a component that unfavorably affects high and low temperature formability, the present invention may limit the upper limit of the silicon (Si) content to 1.0%. The upper limit of the preferred silicon (Si) content can be 0.9%, and the upper limit of the more preferred silicon (Si) content can be 0.8%.

[0027] Manganese (Mn): 10-28% Manganese (Mn) is a component that effectively contributes to austenitic stability, increasing the degree of work hardening of steel while simultaneously improving strength, impact toughness, and wear resistance. Furthermore, manganese (Mn) not only effectively contributes to maintaining the non-magnetic properties of steel after processing, but also enables effective desulfurization by bonding with sulfur (S) in the steel to form MnS. The austenitic steel sheet of the present invention may contain 10% or more manganese (Mn) to achieve these effects. The lower limit of the preferred manganese (Mn) content can be 12%. On the other hand, if manganese (Mn) is added excessively, not only will the manufacturing cost inevitably increase, but the corrosion resistance of the steel may also decrease. Therefore, the present invention can limit the upper limit of the manganese (Mn) content to 28%. The upper limit of the preferred manganese (Mn) content can be 26%, and the upper limit of the more preferred manganese (Mn) content can be 24%.

[0028] Chromium (Cr): 6.0% or less (including 0%) Chromium (Cr) is a ferrite-stabilizing element, and its addition has the advantage of reducing the content of austenite-stabilizing elements. Furthermore, chromium (Cr) is used in MC or M 23Since chromium (Cr) is a core component in the formation of carbides such as C6, adding a certain amount or more of chromium (Cr) can result in a higher degree of precipitation hardening. Furthermore, since chromium (Cr) is a strong antioxidant component, adding a certain level of chromium (Cr) can effectively improve the corrosion resistance of steel. The austenitic steel sheet of the present invention can contain chromium (Cr) to achieve such effects. The lower limit of the preferred chromium (Cr) content can be 0.5%, and the lower limit of the more preferred chromium (Cr) content can be 1.0%. On the other hand, if chromium (Cr) is added excessively, it can lead to the formation of coarse-grained carbides, resulting in a decrease in wear resistance. Therefore, the upper limit of the chromium (Cr) content in the austenitic steel sheet of the present invention can be limited to 6.0%. The upper limit of the preferred chromium (Cr) content can be 5.0%, and the upper limit of the more preferred chromium (Cr) content can be 4.0%.

[0029] Vanadium (V): 0.5% or less (including 0%) Vanadium(V) is a component that effectively contributes to improving the strength of steel materials by forming fine precipitates. The austenitic steel sheet of the present invention can contain vanadium(V) to achieve this effect. A preferred vanadium(V) content can be 0.05% or more, and a more preferred vanadium(V) content can be 0.1% or more. On the other hand, since vanadium(V) is an expensive element, adding it in large quantities not only increases production costs, but its effect saturates when added above a certain level. Therefore, the austenitic steel sheet of the present invention can limit the upper limit of its vanadium(V) content to 0.5%. A preferred upper limit of its vanadium(V) content can be 0.45%, and a more preferred upper limit of its vanadium(V) content can be 0.4%.

[0030] Phosphorus (P): 0.025% or less (including 0%) Phosphorus (P) is an impurity that inevitably flows into the steel manufacturing process and is a component that is highly likely to induce secondary segregation; therefore, it is preferable to suppress its addition as much as possible. Theoretically, the most preferable phosphorus (P) content is 0%, but controlling the amount of phosphorus (P) added to 0% would cause excessive process load. Therefore, in this invention, considering the phosphorus (P) content that is inevitably added, the upper limit is limited to 0.025%.

[0031] Sulfur (S): 0.025% or less (including 0%) Sulfur (S) is an impurity that inevitably enters during the steel manufacturing process and is a component that promotes hot cracking during welding; therefore, it is preferable to suppress its addition as much as possible. Theoretically, the most preferable sulfur (S) content is 0%, but controlling the amount of sulfur (S) added to 0% would cause excessive process load. Therefore, in this invention, considering the unavoidable sulfur (S) content, the upper limit is limited to 0.025%.

[0032] An austenitic steel sheet according to one aspect of the present invention may consist of the remaining Fe and other unavoidable impurities in addition to the components described above. However, since unintended impurities are inevitably introduced from raw materials or the surrounding environment during the normal manufacturing process, it is not possible to completely eliminate them. These impurities are recognizable to anyone with ordinary skill in the art, and therefore, their full details are not specifically mentioned herein. Furthermore, the addition of additional effective components other than those described above is not entirely excluded.

[0033] An austenitic steel sheet according to one aspect of the present invention may contain austenite as a matrix structure. The austenite fraction may be 80 area% or more, and may include cases where it is 90 area% or more. The average grain size of the austenite may be 60 μm or less. An austenitic steel sheet according to one aspect of the present invention not only contains austenite as a matrix structure, but also limits the average grain size of the austenite to a certain level or less, thereby effectively ensuring the desired wear resistance, strength, and low-temperature toughness. On the other hand, the present invention does not completely exclude the formation of other structures that inevitably form in addition to austenite, and other remaining structures, including precipitates such as carbides, may be included in a range of 20 area% or less.

[0034] An austenitic steel sheet according to one aspect of the present invention may contain twins of 20 area percent or less, and the average size of the twins may be 400 nm or less. When a certain fraction of twins is present, wear resistance can be effectively improved by twin-induced hardening and microstructure refinement due to recrystallization. However, when the fraction of twins reaches a certain range, the effect saturates, so the present invention can limit the fraction of twins to a range of 20 area percent or less. On the other hand, as the average size of the twins decreases, the degree of orientation disorder in the recrystallized texture increases, which can effectively increase wear resistance, so the present invention can limit the average size of twins to 400 nm or less.

[0035] According to one aspect of the present invention, the surface hardness (Hv) of the austenitic steel sheet satisfies the following relational equation 2, thereby effectively ensuring wear resistance and formability. [Relationship 2] 18.7*[Mn]-10.3*[Cr]+1.7*[C]≦Surface hardness (Hv)≦22.1*[Mn]+18.3*[Cr]+2.4*[C] In the above relational equation 2, [Mn], [Cr], and [C] represent the Mn, Cr, and C content (by weight %) contained in the steel sheet, respectively. If the corresponding component is not present, 0 should be substituted.

[0036] An austenitic steel sheet according to one aspect of the present invention may have a tensile strength of 800 MPa or more, and the temperature at which the ductile fracture surface ratio in the DWTT test is 85% or more may be -25°C or lower.

[0037] The process for manufacturing the austenitic steel sheet of the present invention will be described in more detail below.

[0038] A method for manufacturing an austenitic steel sheet according to one aspect of the present invention may include the steps of: heating a slab containing, by weight %, C: 0.4-1.2%, Si: 1.0% or less (including 0%), Mn: 10-28%, Cr: 6.0% or less (including 0%), V: 0.5% or less (including 0%), P: 0.025% or less, S: 0.025% or less, with the remainder being Fe and unavoidable impurities, at a temperature range of 1100-1250°C for 180-320 minutes; hot rolling the heated slab at a finishing rolling temperature of 800°C or higher to provide a hot-rolled material; and cooling the hot-rolled material to room temperature at a cooling rate of 15°C / s or higher to provide a final material.

[0039] Slab heating Since the slab provided in the manufacturing method of the present invention corresponds to the steel composition of the austenitic steel sheet described above, the explanation of the steel composition of the slab will be replaced by the explanation of the steel composition of the austenitic steel sheet described above.

[0040] Slabs provided with the above steel composition can be heated in a temperature range of 1100 to 1250°C. If the heating temperature is below a certain range, problems may occur such as excessive rolling load during hot rolling or insufficient solid solution of alloy components. Therefore, the lower limit of the slab heating temperature can be limited to 1100°C. On the other hand, if the heating temperature exceeds a certain range, the crystal grains may grow excessively, reducing strength, or the temperature may exceed the solidus temperature of the steel, degrading hot-rollability. Therefore, the upper limit of the slab heating temperature can be limited to 1250°C.

[0041] Hot rolling The hot rolling process includes a rough rolling process and a finish rolling process, and the heated slab can be hot-rolled and provided as a hot-rolled material. At this time, it is preferable that the finish hot rolling be carried out in a temperature range of 800°C or higher. If the finish hot rolling temperature is excessively low, there is a concern about excessive rolling load; therefore, the finish hot rolling temperature can be limited to a range of 800°C or higher. Furthermore, there is no particular upper limit to the finish hot rolling temperature, but to prevent excessive grain growth, the upper limit can be limited to 950°C. There is no particular limit to the thickness of the hot-rolled material after hot rolling is completed, but as an example of non-limiting thickness, the thickness of the hot-rolled material can be in the range of 6 to 30 mm.

[0042] cooling Hot-rolled materials can be cooled to room temperature at a cooling rate of 15°C / s or higher. If the cooling rate is below a certain range, carbides precipitated at the grain boundaries during cooling may reduce the ductility of the steel and consequently degrade its wear resistance. Therefore, the cooling rate of hot-rolled materials can be limited to a range of 15°C / s or higher. However, while a faster cooling rate is advantageous in suppressing carbide precipitation, cooling rates exceeding 100°C / s are difficult to achieve under normal cooling conditions due to equipment characteristics. Therefore, the upper limit of the cooling rate can be limited to 100°C / s. The cooling method is not particularly limited, but accelerated cooling is preferably applied.

[0043] The austenitic steel sheet manufactured by the above-described manufacturing method may contain 80 area percent or more of austenite with an average grain size of 60 μm or less, and 20 volume percent or less of twins with an average size of 400 nm or less.

[0044] Furthermore, austenitic steel sheets manufactured by the above-described manufacturing method have a tensile strength of 800 MPa or higher, a temperature at which the ductile fracture surface ratio in the DWTT test is 85% or higher is -25°C or lower, and a surface hardness (Hv) that satisfies the following relational equation 2. [Relationship 2] 18.7*[Mn]-10.3*[Cr]+1.7*[C]≦Surface hardness (Hv)≦22.1*[Mn]+18.3*[Cr]+2.4*[C] In the above relational equation 2, [Mn], [Cr], and [C] represent the Mn, Cr, and C content (by weight %) contained in the steel sheet, respectively. If the corresponding component is not present, 0 should be substituted.

[0045] The following describes in more detail a method for manufacturing welded steel pipes for slurry transfer according to one aspect of the present invention.

[0046] A method for manufacturing a welded steel pipe for slurry transfer according to one aspect of the present invention may include the steps of forming an austenitic steel sheet into a tubular shape so that a butt joint is formed, and welding the butt joint so that the carbon content of the entire welded portion satisfies the following relational formula 1 to provide a welded steel pipe. [Relationship 1] 0.03*[Mn]+0.04*[Cr]≦[C]≦0.05*[Mn]+0.26*[Cr] In the above relational equation 1, [C], [Mn], and [Cr] represent the C, Mn, and Cr content (by weight %) included in the entire welded area, respectively.

[0047] Molding stage After preparing the austenitic steel sheet as described above, the austenitic steel sheet can be formed into a tubular shape. In this invention, the method of forming the austenitic steel sheet into a tubular shape is not particularly limited. Forming methods that are normally applied when manufacturing welded steel pipes can be applied, and any one of the forming methods selected from spiral forming, UOE pressing, roll bending, and JCO forming can be applied.

[0048] Furthermore, the process may include a step of improving the ends of the austenitic steel sheet that form the butt joint before forming the austenitic steel sheet into a tubular shape.

[0049] Welding stage A welded steel pipe can be provided by forming an austenitic steel sheet into a tubular shape and then welding the butt joint. The welding method applied in this invention is not particularly limited and any welding method that is normally applied during the manufacture of welded steel pipes can be applied without restriction. One or more arc welding or electric resistance welding (ERW) selected from SMAW (shield metal arc welding), GMAW (gas metal arc welding), GTAW (gas tungsten arc welding), FCAW (flux cored arc welding), and SAW (sub-merged arc welding) can be applied, and this can be interpreted as including cases where one or more of these are selectively combined and applied to welding.

[0050] The inventors of this invention have confirmed that the carbon (C), manganese (Mn), and chromium (Cr) content of the entire welded area is a major factor influencing the strength and low-temperature toughness of the weld, and have derived the following relational equation 1. [Relationship 1] 0.03*[Mn]+0.04*[Cr]≦[C]≦0.05*[Mn]+0.26*[Cr] In the above relational equation 1, [C], [Mn], and [Cr] represent the C, Mn, and Cr content (by weight %) included in the entire welded area, respectively.

[0051] The present invention allows for effective improvement of the strength and low-temperature toughness of the welded joint by adjusting the welding conditions and the amount of welding material mixed in so that the entire welded joint satisfies the following relational equation 1.

[0052] Furthermore, the inventors of the present invention have confirmed that the amount of heat input applied during welding is a major factor influencing the occurrence of high-temperature cracks in the welded area when welding is performed using steel plates that satisfy the component system limited by the present invention, and have derived the present invention. That is, the method for manufacturing welded steel pipes according to one aspect of the present invention controls the amount of heat input applied during welding of austenitic steel plates to a range of 4.3 kJ / mm or less, thereby effectively suppressing high-temperature cracks that occur in the welding line. A preferred amount of heat input can be 4.2 kJ / mm or less, and a more preferred amount of heat input can be 4.1 kJ / mm or less.

[0053] The following describes in more detail a welded steel pipe for slurry transfer according to one aspect of the present invention.

[0054] An austenitic welded steel pipe according to one aspect of the present invention includes a welded steel pipe base material portion in which austenite is the matrix structure, and a welded portion connecting both ends of the welded steel pipe base material portion, wherein the total carbon content of the welded portion included in the welded portion can satisfy the following relational expression 1. [Relationship 1] 0.03*[Mn]+0.04*[Cr]≦[C]≦0.05*[Mn]+0.26*[Cr] In the above relational equation 1, [C], [Mn], and [Cr] represent the C, Mn, and Cr content (by weight %) included in the entire welded area, respectively.

[0055] The welded steel pipe base material of the present invention has a steel composition and microstructure corresponding to the austenitic steel sheet described above, and therefore can have a tensile strength of 800 MPa or more, and a temperature of -25°C or lower at which the ductile fracture surface ratio in the DWTT test is 85% or more.

[0056] Since the entire welded portion included in the weld of the present invention satisfies the above relational equation 1, excellent weld strength and low-temperature toughness can be ensured. Furthermore, since the weld of the present invention is formed by applying a heat input of 4.3 kJ / mm or less, the occurrence of hot cracks in the weld can be suppressed to the greatest extent possible. That is, the formation of hot cracks on the molten line is suppressed to the greatest extent possible, and even if hot cracks occur, the maximum crack length of the hot cracks can be suppressed to a level of 0.5 mm or less.

[0057] Therefore, the welded steel pipe for slurry transfer according to one aspect of the present invention not only has excellent wear resistance and low-temperature toughness, but also minimizes the formation of high-temperature cracks at the welded joint, thus providing physical properties particularly suitable for slurry transfer.

[0058] The following describes in more detail, with specific examples, a welded steel pipe for slurry transfer and its manufacturing method according to one aspect of the present invention. It should be noted that the following examples are for understanding the present invention and not for defining the scope of 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. [Examples]

[0059] After preparing slabs having the alloy components listed in Table 1 below, test specimens were fabricated by applying the process conditions listed in Table 2 below.

[0060] [Table 1]

[0061] [Table 2]

[0062] After cutting each specimen, a mirror surface was prepared and etched using a 2 vol% nital etching solution. The microstructure was then observed using an optical microscope and an electron scanning microscope, and the rm results are shown in Table 3. Tensile tests were performed on each specimen using a tensile testing machine, and the results are shown in Table 3. After fracturing the specimens using a DWTT testing machine while lowering the temperature with liquid nitrogen, the ductile fracture surface ratio was measured, and the DWTT temperature was measured based on a ductile fracture surface ratio of 85%, as shown in Table 3.

[0063] [Table 3]

[0064] As shown in Tables 1 to 3, test specimens 1 to 3 can be confirmed to satisfy the alloy composition and microstructure conditions of the present invention and to possess excellent strength and low-temperature properties. [Examples]

[0065] After welding test specimens of invention steel 1 under the following welding conditions, the presence or absence of high-temperature cracking in the weld area was evaluated using an optical microscope.

[0066] Test specimen 1 was fabricated by SAW (sub-merged arc welding) welding using a 790A*38V power supply with a heat input of 4.4kJ / mm. High-temperature cracking exceeding 0.5mm in maximum crack length was observed in the weld of test specimen 1. On the other hand, test specimen 2 was fabricated by SAW (sub-merged arc welding) welding using a 650A*35V power supply with a heat input of 3.5kJ / mm. High-temperature cracking exceeding 0.5mm in maximum crack length was not observed in the weld of test specimen 2. Therefore, it can be seen that crack occurrence in the weld is effectively suppressed in the welded steel pipe according to one aspect of the present invention.

[0067] Although the present invention has been described in detail above with reference to examples, other forms of embodiments are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the examples.

Claims

1. A method for manufacturing a welded steel pipe for slurry transfer, The steps of forming an austenitic steel sheet into a tubular shape so that a butt joint is formed, and The step of providing a welded steel pipe is to weld the butt joint such that the carbon content of the entire welded portion satisfies the following relational expression 1. The surface hardness of the austenitic steel sheet satisfies the following relational equation 2: The method for manufacturing the austenitic steel sheet is to heat a slab containing, by weight percent, C: 0.4 to 1.2%, Si: 1.0% or less (including 0%), Mn: 10 to 28%, Cr: 6.0% or less (including 0%), V: 0.5% or less (including 0%), P: 0.025% or less, S: 0.025% or less, with the remainder being Fe and unavoidable impurities, at a temperature range of 1100 to 1250°C for 180 to 320 minutes. The process includes the steps of: hot-rolling the slab heated in the heating step at a finishing rolling temperature of 800°C or higher to provide a hot-rolled material; and cooling the hot-rolled material to room temperature at a cooling rate of 15°C / s or higher to provide a final material. The butt joint is welded with a heat input of 4.3 kJ / mm or less. The austenitic steel sheet contains 80% or more austenite in its microstructure. The average grain size of the austenite is 60 μm or less. The austenitic steel sheet contains twins of 20 area percent or less. A method for manufacturing welded steel pipes for slurry transfer, characterized in that the average size of the twin crystals is 400 nm or less. Here, the microstructure was measured using an optical microscope and / or an electron scanning microscope after preparing a mirror-like surface on the sample piece and etching it with a 2 vol% nital etching solution. [Relationship 1] 0.03*[Mn]+0.04*[Cr]≦[C]≦0.05*[Mn]+0.26*[Cr] In the above relational formula 1, [C], [Mn], and [Cr] represent the total C, Mn, and Cr content (by weight %) in the entire welded area, respectively. [Relationship Equation 2] 18.7*[Mn]-10.3*[Cr]+1.7*[C]≦Surface hardness (Hv)≦22.1*[Mn]+18.3*[Cr]+2.4*[C] In the above relational equation 2, [Mn], [Cr], and [C] represent the Mn, Cr, and C content (by weight %) contained in the steel sheet, respectively, and 0 is substituted if the corresponding component is not present.

2. The method for manufacturing a welded steel pipe for slurry transfer according to claim 1, characterized in that the austenitic steel sheet contains, by weight percent, C: 0.4 to 1.2%, Si: 1.0% or less (including 0%), Mn: 10 to 28%, Cr: 6.0% or less (including 0%), V: 0.5% or less (including 0%), P: 0.025% or less, S: 0.025% or less, with the remainder being Fe and unavoidable impurities.

3. The method for forming the austenitic steel sheet into a tubular shape is one selected from the spiral forming method, the UOE pressing method, the roll bending method, and the JCO forming method, as described in claim 1, for manufacturing a welded steel pipe for slurry transfer.

4. The welding method for the butt joint is as follows: SMAW (shield metal arc welding), GMAW (gas It is one or more arc welds selected from among metal arc welding, GTAW (gas tungsten arc welding), FCAW (flux-cored arc welding), and SAW (sub-merged arc welding). A method for manufacturing a welded steel pipe for slurry transfer according to claim 1, characterized in that it is electric resistance welding (ERW).

5. Austenite is the base material of the welded steel pipe, and A welded steel pipe including a welded portion that connects both ends of the base material portion of the welded steel pipe to each other, The total carbon content of the welded portion in the aforementioned weld satisfies the following relational equation 1. The surface hardness of the base material portion of the welded steel pipe satisfies the following relational equation 2. The base material portion of the welded steel pipe contains austenite in a fine structure of 80 area percentage or more. The average grain size of the austenite is 60 μm or less. The welded steel pipe base material portion contains twins of 20 area percent or less. A welded steel pipe for slurry transfer, characterized in that the average size of the twin crystals is 400 nm or less. Here, the microstructure was measured using an optical microscope and / or an electron scanning microscope after preparing a mirror-like surface on the sample piece and etching it with a 2 vol% nital etching solution. [Relationship 1] 0.03*[Mn]+0.04*[Cr]≦[C]≦0.05*[Mn]+0.26*[Cr] In the above relational formula 1, [C], [Mn], and [Cr] represent the total C, Mn, and Cr content (by weight %) in the entire welded area, respectively. [Relationship Equation 2] 18.7*[Mn]-10.3*[Cr]+1.7*[C]≦Surface hardness (Hv)≦22.1*[Mn]+18.3*[Cr]+2.4*[C] In the relational expression 2 described above, [Mn], [Cr], and [C] represent the Mn, Cr, and C content (by weight %) contained in the welded steel pipe base material, respectively, and 0 is substituted if the corresponding component is not present.

6. The aforementioned welded steel pipe base material portion is The welded steel pipe for slurry transfer according to claim 5, characterized in that it contains, by weight percent, C: 0.4 to 1.2%, Si: 1.0% or less (including 0%), Mn: 10 to 28%, Cr: 6.0% or less (including 0%), V: 0.5% or less (including 0%), P: 0.025% or less, S: 0.025% or less, with the remainder being Fe and unavoidable impurities.

7. The welded steel pipe for slurry transfer according to claim 5, characterized in that the maximum length of the hot crack formed in the welded portion is 0.5 mm or less.

8. The aforementioned welded steel pipe base material portion is The tensile strength is 800 MPa or more. The welded steel pipe for slurry transfer according to claim 5, characterized in that the temperature at which the ductile fracture surface ratio in the DWTT test is 85% or more is -25°C or lower.

Images