Welded steel pipe for transporting slurry and its manufacturing method

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 manufacturing method thereof, and more particularly to a welded steel pipe having excellent wear resistance and low-temperature toughness and minimizing hot cracking at welds by optimizing the steel composition and microstructure of a steel pipe material and the steel pipe manufacturing process, and suitable for slurry transport, and a manufacturing method thereof. [Background technology]

[0002] Parts and structures used in abrasive environments require periodic inspection and replacement to ensure work safety and soundness. In order to inspect and replace parts and structures, the operation of the industrial facility must be temporarily suspended, and frequent inspection and replacement of parts and structures leads to reduced productivity and economic efficiency. Therefore, various attempts have been made to extend the life of parts and structures used in abrasive environments, and various research projects are being actively conducted to improve the wear resistance of the materials themselves that constitute the parts and structures.

[0003] As the mining industry and the oil and gas industry grow, wear of welded steel pipes used in the processes of mining, transportation, refining, and storage has become a major issue. Meanwhile, development of oil sands has recently been in full swing, but it is known that welded steel pipes made of API standard steel materials commonly used in the oil and gas industry cannot ensure a level of wear resistance to transport slurries containing oil, rocks, gravel, sand, etc. for a long period of time. In addition, welded steel pipes used for slurry transportation are required to have excellent low-temperature toughness, but it is known that the currently commonly used API standard steel materials cannot ensure the corresponding low-temperature toughness. Therefore, research into welded steel pipes for slurry pipes with excellent wear resistance and low-temperature toughness is urgently needed. Summary of the Invention [Problem to be solved by the invention]

[0004] 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.

[0005] The object of the present invention is not limited to the above-mentioned contents, and a person skilled in the art will have no difficulty in understanding further object of the present invention from the entire contents of this specification. [Means for solving the problem]

[0006] The method for producing a welded steel pipe for transporting slurry of the present invention is characterized by comprising 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 such that the carbon content of all welded parts satisfies the following Relational Formula 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.

[0007] The austenitic steel sheet contains, 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 remainder being Fe and unavoidable impurities, and the surface hardness of the austenitic steel sheet satisfies the following Relational Formula 2. [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 Formula 2, [Mn], [Cr] and [C] respectively mean the Mn, Cr and C contents (wt%) contained in the above steel plate, and 0 is substituted when the corresponding components are not contained.

[0008] The austenitic steel sheet contains austenite in a fine structure at 80% by area or more, and the average crystal grain size of the austenite is 60 μm or less.

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

[0010] The forming method for forming the austenitic steel sheet into a tubular shape is any one selected from the group consisting of a spiral forming method, a UOE pressing method, a roll bending method, and a JCO forming method.

[0011] The welding method of the butt joint is one or more arc welding methods selected from the group consisting of shield metal arc welding (SMAW), gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), flux cored arc welding (FCAW), and sub-merged arc welding (SAW), 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 transporting slurry of the present invention comprises a welded steel pipe base material portion having an austenite base structure, and a welded portion connecting both ends of the welded steel pipe base material portion, and is characterized in that the carbon content of all welded portions included in the welded portion satisfies the following Relational Formula 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.

[0013] The base metal part of the welded steel pipe contains, 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 remainder being Fe and unavoidable impurities, and the surface hardness of the base metal part of the welded steel pipe satisfies the following relational expression 2. [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 Formula 2, [Mn], [Cr] and [C] respectively mean the Mn, Cr and C contents (wt%) contained in the base material of the welded steel pipe, and 0 is substituted when the corresponding components are not contained.

[0014] The base material of the welded steel pipe contains austenite in a fine structure of 80% by area or more, and the average crystal grain size of the austenite is 60 μm or less.

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

[0016] The maximum length of hot cracks formed in the weld is 0.5 mm or less.

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

[0018] The above solutions to the problems are not intended to be exhaustive, and the various features of the present invention and their associated advantages and effects can be more fully understood by reference to the following specific examples. Effect of the Invention

[0019] According to the present invention, it is possible to provide a welded steel pipe having excellent wear resistance and low temperature toughness, minimizing hot cracking at a welded portion, and suitable for transporting slurry, and a manufacturing method thereof.

[0020] The effects of the present invention are not limited to the above-mentioned matters, but can be interpreted as including matters that a person of ordinary skill in the art can reasonably infer from the matters described in this specification. [Brief description of the drawings]

[0021] [Figure 1] 1 is a photograph showing a cross section of a welded portion of test piece 1. [Diagram 2] 1 is a cross-sectional observation photograph of a welded portion of test piece 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The present invention relates to a welded steel pipe for transporting slurry and a manufacturing method thereof, and preferred embodiments of the present invention will be described below. The embodiments of the present invention may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments are provided to further explain the present invention in detail to those skilled in the art to which the present invention pertains.

[0023] The austenitic steel plate used to manufacture the welded steel pipe of the present invention will be described in more detail below. Unless otherwise specified, the percentages given for the contents of the steel compositions are based on weight, and the percentages given for the fractions of the microstructures are based on area.

[0024] An austenitic steel sheet according to one aspect of the present invention contains, 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, and the remainder may be Fe and unavoidable impurities.

[0025] Carbon (C): 0.4~1.2% Carbon (C) is a component that effectively contributes to ensuring the strength of steel and improving its wear resistance. Carbon (C) is also a component that improves the hardening ability of steel and effectively contributes to stabilizing austenite. In the present invention, the carbon (C) content in the austenitic steel plate can be limited to a range of 0.4% or more in order to improve the strength and austenite safety of the welded joint as well as the base material of the welded steel pipe. 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 be precipitated, which may reduce the wear resistance and elongation of the steel, so the present invention can limit the upper limit of the carbon (C) content to 1.2%.

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

[0027] Manganese (Mn): 10-28% Manganese (Mn) is a component that effectively contributes to austenite stability, and is a component that effectively improves the strength, impact toughness, and wear resistance while increasing the work hardening degree of steel. In addition, manganese (Mn) is a component that effectively contributes to maintaining the non-magnetic properties of steel after processing, and is also a component that can effectively desulfurize because sulfur (S) in steel is bonded to form MnS. The austenitic steel sheet of the present invention can contain 10% or more manganese (Mn) to achieve such effects. A preferable lower limit of the manganese (Mn) content can be 12%. On the other hand, if manganese (Mn) is added excessively, not only is an increase in manufacturing costs inevitable, but the corrosion resistance of the steel may also be reduced. Therefore, the present invention can limit the upper limit of the manganese (Mn) content to 28%. A preferable upper limit of the manganese (Mn) content can be 26%, and a more preferable upper limit of the manganese (Mn) content can be 24%.

[0028] Chromium (Cr): 6.0% or less (0% included) Chromium (Cr) is a ferrite stabilizing element, and has the advantage that the content of austenite stabilizing elements can be reduced as chromium (Cr) is added. Chromium (Cr) is also an important element in the MC or M steels. 23Since chromium (Cr) is a key component for the formation of carbides such as C6, when a certain amount or more of chromium (Cr) is added, a higher level of precipitation hardening can be obtained. Furthermore, since chromium (Cr) is a strong oxidation-resistant component, when a certain level of chromium (Cr) is added, the corrosion resistance of the steel can be effectively improved. The austenitic steel sheet of the present invention can contain chromium (Cr) to achieve such an effect. A preferred lower limit of the chromium (Cr) content may be 0.5%, and a more preferred lower limit of the chromium (Cr) content may be 1.0%. On the other hand, when chromium (Cr) is added excessively, coarse grain carbides may be formed, resulting in a decrease in wear resistance, so the upper limit of the chromium (Cr) content of the austenitic steel sheet of the present invention may be limited to 6.0%. A preferred upper limit of the chromium (Cr) content may be 5.0%, and a more preferred upper limit of the chromium (Cr) content may be 4.0%.

[0029] Vanadium (V): 0.5% or less (including 0%) Vanadium (V) is a component that forms fine precipitates and effectively contributes to improving the strength of steel materials. The austenitic steel sheet of the present invention may contain vanadium (V) to achieve such effects. The preferred vanadium (V) content may be 0.05% or more, and the more preferred vanadium (V) content may be 0.1% or more. On the other hand, vanadium (V) is an expensive element, and when added in large amounts, it not only acts as a factor in increasing production costs, but also saturates its effect when added at a content above a certain level, so that the austenitic steel sheet of the present invention may limit the upper limit of the vanadium (V) content to 0.5%. The preferred upper limit of the vanadium (V) content may be 0.45%, and the more preferred upper limit of the vanadium (V) content may be 0.4%.

[0030] Phosphorus (P): 0.025% or less (including 0%) Phosphorus (P) is an impurity that inevitably flows into the steel during the manufacturing process and is an element that has a high possibility of inducing secondary segregation, so it is preferable to suppress its addition amount as much as possible. Theoretically, the most preferable phosphorus (P) content is 0%, but since controlling the phosphorus (P) content to 0% causes excessive process load, in the present invention, taking into consideration the unavoidably added phosphorus (P) content, the upper limit is limited to 0.025%.

[0031] Sulfur (S): 0.025% or less (including 0%) Sulfur (S) is also an impurity that inevitably flows into the steel during the manufacturing process, and is an ingredient that promotes hot cracks during welding, so it is preferable to suppress its addition amount as much as possible. Theoretically, the most preferable sulfur (S) content is 0%, but controlling the sulfur (S) content to 0% causes excessive process load, so in the present invention, taking into consideration the unavoidably added sulfur (S) content, the upper limit is limited to 0.025%.

[0032] The austenitic steel sheet according to one aspect of the present invention may contain the remaining Fe and other inevitable impurities in addition to the above-mentioned components. However, since unintended impurities may be inevitably mixed in from raw materials or the surrounding environment during normal manufacturing processes, it is not possible to completely exclude them. Since these impurities are known to anyone having ordinary skill in the art, the entire contents of the impurities will not be specifically mentioned in this specification. Furthermore, the addition of additional effective components other than the above-mentioned components is not completely excluded.

[0033] The austenitic steel sheet according to one aspect of the present invention may include austenite as a matrix structure. The austenite fraction may be 80 area % or more, and may include 90 area % or more. The average grain size of the austenite may be 60 μm or less. The austenitic steel sheet according to one aspect of the present invention not only includes austenite as a matrix structure, but also limits the average grain size of the austenite to a certain level or less, so that the intended wear resistance, strength, and low temperature toughness can be effectively ensured. Meanwhile, the present invention does not completely exclude the formation of other structures that are inevitably formed in addition to austenite, and other remaining structures including precipitates such as carbides may be included in the range of 20 area % or less.

[0034] According to one aspect of the present invention, the austenitic steel sheet may include 20% or less by area of ​​twins, and the average size of the twins may be 400 nm or less. When a certain percentage of twins is included, the wear resistance can be effectively improved by twin-induced hardening and microstructural refinement due to recrystallization. However, when the percentage of twins reaches a certain range, the effect is saturated, so the present invention may limit the percentage of twins to a range of 20% or less by area. Meanwhile, as the average size of twins becomes smaller, the degree of orientation disorder of the recrystallized texture increases, which can act more effectively to increase the wear resistance, so the present invention may limit the average size of twins to 400 nm or less.

[0035] The surface hardness (Hv) of the austenitic steel sheet according to one aspect of the present invention satisfies the following Relational Expression 2, and therefore the wear resistance and formability can be effectively ensured. [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 Formula 2, [Mn], [Cr] and [C] respectively mean the Mn, Cr and C contents (wt%) contained in the above steel plate, and 0 is substituted when the corresponding components are not contained.

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

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

[0038] A method for producing an austenitic steel sheet according to one aspect of the present invention includes the steps of heating a slab containing, by weight%, 0.4-1.2% C, 1.0% or less (including 0%) Si, 10-28% Mn, 6.0% or less (including 0%) Cr, 0.5% or less (including 0%) V, 0.025% or less P, 0.025% or less S, 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 finish rolling temperature of 800°C or more to provide a hot-rolled material, and cooling the hot-rolled material to room temperature at a cooling rate of 15°C / s or more to provide a final material.

[0039] Slab heating The slab provided in the manufacturing method of the present invention corresponds to the steel composition of the austenitic steel plate described above, so the description of the steel composition of the slab is substituted for the description of the steel composition of the austenitic steel plate described above.

[0040] The slab 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 such as excessive rolling load during hot rolling or insufficient solid solution of alloy components may occur. 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, crystal grains may grow excessively, resulting in a decrease in strength, or the solidus temperature of the steel may be exceeded, resulting in deterioration of hot rollability, so 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 is hot rolled to provide a hot rolled material. At this time, the finish hot rolling is preferably performed at a temperature range of 800°C or more. If the finish hot rolling temperature is too low, excessive rolling load may be concerned, so the finish hot rolling temperature may be limited to a range of 800°C or more. In addition, the upper limit of the finish hot rolling temperature is not particularly limited, but the upper limit may be limited to 950°C in order to prevent excessive growth of crystal grains. The thickness of the hot rolled material after hot rolling is not particularly limited, but as a non-limiting example, the thickness of the hot rolled material may satisfy the range of 6 to 30 mm.

[0042] cooling The hot-rolled hot material may be cooled to room temperature at a cooling rate of 15°C / s or more. If the cooling rate is below a certain range, carbides precipitated at grain boundaries during cooling may cause problems such as reduced ductility of the steel material and deterioration of wear resistance, so the cooling rate of the hot-rolled material may be limited to a range of 15°C / s or more. However, although a faster cooling rate is more advantageous in terms of suppressing carbide precipitation, a cooling rate exceeding 100°C / s is difficult to achieve in normal cooling due to equipment characteristics, so the upper limit of the cooling rate may be limited to 100°C / s. The cooling method is not particularly limited, but accelerated cooling may be preferably applied.

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

[0044] In addition, the austenitic steel plate manufactured by the above-mentioned manufacturing method has a tensile strength of 800 MPa or more, a temperature at which the ductile fracture area ratio in a DWTT test is 85% or more is -25°C or less, and a surface hardness (Hv) that satisfies the following Relational Expression 2. [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 Formula 2, [Mn], [Cr] and [C] respectively mean the Mn, Cr and C contents (wt%) contained in the above steel plate, and 0 is substituted when the corresponding components are not contained.

[0045] Hereinafter, a method for producing a welded steel pipe for transporting slurry according to one aspect of the present invention will be described in more detail.

[0046] A method for manufacturing a welded steel pipe for transporting slurry according to one aspect of the present invention includes 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 such that the carbon content of all welded parts satisfies Relational Formula 1 below. [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.

[0047] Molding stage After preparing the above-mentioned austenitic steel sheet, the austenitic steel sheet can be formed into a tubular shape. In the present invention, the method of forming the austenitic steel sheet into a tubular shape is not particularly limited. A forming method commonly applied in the manufacture of welded steel pipes can be applied, and any one of a spiral forming method, a UOE press method, a roll bending method, and a JCO forming method can be applied.

[0048] Also, the method may further include a step of improving both ends that form butt joints before forming the austenitic steel plate into a tubular shape.

[0049] Welding stage A welded steel pipe can be provided by forming an austenitic steel plate into a tubular shape and welding the butt joint. The welding method applied in the present invention is not particularly limited, and any welding method that is generally applied in manufacturing a welded steel pipe can be applied without limitation. At least one 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 it can be interpreted as including a case where at least one of them is selectively combined and applied to welding.

[0050] The inventors of the present invention have confirmed that the carbon (C), manganese (Mn) and chromium (Cr) contents of the entire weld are the factors that have the main effect on the strength and low temperature toughness of the weld, and have derived the following Relational Formula 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.

[0051] In the present invention, by adjusting the welding conditions and the amount of welding material mixed in, welding is performed so that the entire welded portion satisfies the following Relational Expression 1, so that the strength and low temperature toughness of the welded portion can be effectively improved.

[0052] In addition, the inventors of the present invention have confirmed that when welding is performed using steel plates that satisfy the composition system restricted by the present invention, the amount of welding heat applied during welding is a factor that has a major effect on whether hot cracks occur in the welded portion, and have thus devised the present invention. That is, a method for manufacturing a welded steel pipe according to one aspect of the present invention controls the amount of welding heat input applied during welding of austenitic steel plates to a range of 4.3 kJ / mm or less, thereby effectively suppressing hot cracks that occur in the welding line. A preferred amount of welding heat input may be 4.2 kJ / mm or less, and a more preferred amount of welding heat input may be 4.1 kJ / mm or less.

[0053] Hereinafter, a welded steel pipe for transporting slurry according to one aspect of the present invention will be described in more detail.

[0054] According to one aspect of the present invention, there is provided an austenitic welded steel pipe including a welded steel pipe base metal part having an austenite matrix structure, and a welded part connecting both ends of the welded steel pipe base metal part, and the carbon content of all welded parts included in the welded part can satisfy 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.

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

[0056] Since all of the welds included in the weld of the present invention satisfy the above-mentioned Relational Expression 1, it is possible to ensure excellent weld strength and low-temperature toughness. In addition, since the weld of the present invention is formed by applying a heat input of 4.3 kJ / mm or less, it is possible to minimize the occurrence of hot cracks in the weld. In other words, the formation of hot cracks on the fusion line is minimized, and even if hot cracks do occur, the maximum crack length can be suppressed to a level of 0.5 mm or less.

[0057] Therefore, the welded steel pipe for transporting slurry according to one aspect of the present invention not only has excellent wear resistance and low temperature toughness, but also minimizes hot crack formation at the welded portion, thereby providing physical properties particularly suitable for transporting slurry.

[0058] Hereinafter, a welded steel pipe for transporting slurry and a method for manufacturing the same according to one aspect of the present invention will be described in more detail with reference to specific examples. It should be noted that the following examples are provided for the purpose of understanding the present invention and are not intended to specify 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 a slab having the alloy composition shown in Table 1 below, test pieces were produced by applying the process conditions shown in Table 2 below.

[0060] [Table 1]

[0061] [Table 2]

[0062] After cutting each test piece, a mirror surface was made and it was etched using 2 vol% nital etching solution, after which the microstructure was observed using an optical microscope and an electron scanning microscope, and the rm results are shown in Table 3. A tensile test was performed on each test piece using a tensile testing machine, and the results are shown in Table 3. The test piece was broken using a DWTT testing machine while lowering the temperature using liquid nitrogen, and then the ductile fracture ratio was measured, and the DWTT temperature was measured based on a ductile fracture ratio of 85%, and is shown in Table 3.

[0063] [Table 3]

[0064] As shown in Tables 1 to 3, it can be confirmed that Test Pieces 1 to 3 satisfy the alloy composition and microstructural conditions of the present invention and have excellent strength and low-temperature properties. EXAMPLES

[0065] Test pieces of the invention steel 1 were welded under the following welding conditions, and the welds were then evaluated using an optical microscope for the presence or absence of hot cracking.

[0066] Test piece 1 was produced by performing SAW (sub-merged arc welding) welding with a heat input of 4.4 kJ / mm using 790 A*38 V, and hot cracks with a maximum crack length exceeding 0.5 mm were confirmed in the welded part of test piece 1. Meanwhile, test piece 2 was produced by performing SAW (sub-merged arc welding) welding with a heat input of 3.5 kJ / mm using 650 A*35 V, and hot cracks with a maximum crack length exceeding 0.5 mm were not confirmed in the welded part of test piece 2. Therefore, it can be seen that the welded steel pipe according to one aspect of the present invention effectively suppresses the occurrence of cracks in the welded part.

[0067] Although the present invention has been described in detail with reference to the embodiments, other embodiments may be possible, and therefore the spirit and scope of the claims set forth below should not be limited to the embodiments.

Claims

1. forming an austenitic steel sheet into a tubular shape so as to form a butt joint; and a step of welding the butt joints to provide a welded steel pipe such that the carbon content of the entire welded portion satisfies the following Relational Formula 1: [Relationship 1] 0.03*[Mn]+0.04*[Cr]≦[C]≦0.05*[Mn]+0.26*[Cr] In the above-mentioned relational expression 1, [C], [Mn] and [Cr] respectively represent the C, Mn and Cr contents (wt %) contained in the entire weld.

2. The austenitic steel plate 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, and the remainder being Fe and unavoidable impurities; 2. The method for producing a welded steel pipe for transporting slurry according to claim 1, wherein the surface hardness of the austenitic steel plate satisfies the following relational expression 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 Formula 2, [Mn], [Cr] and [C] respectively mean the Mn, Cr and C contents (wt%) contained in the steel plate, and 0 is substituted when the corresponding components are not contained.

3. The austenitic steel sheet contains 80% or more by area of ​​austenite in a microstructure, 2. The method for producing a welded steel pipe for transporting a slurry according to claim 1, wherein the average crystal grain size of the austenite is 60 μm or less.

4. The austenitic steel sheet contains twins (Twin) in an amount of 20% or less by area, 2. The method for manufacturing a welded steel pipe for transporting a slurry according to claim 1, wherein the average size of the twin crystals is 400 nm or less.

5. 2. The method for manufacturing a welded steel pipe for transporting slurry according to claim 1, wherein the forming method for forming the austenitic steel plate into a tubular shape is any one selected from the group consisting of a spiral forming method, a UOE press method, a roll bending method, and a JCO forming method.

6. The method for welding the butt joints includes the steps of: The arc welding is at least one selected from the group consisting of shield metal arc welding (SMAW), gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), flux cored arc welding (FCAW) and sub-merged arc welding (SAW); 2. The method of claim 1, wherein the welded steel pipe is made by electric resistance welding (ERW).

7. 2. The method for producing a welded steel pipe for transporting slurry according to claim 1, wherein the butt joint is welded with a heat input of 4.3 kJ / mm or less.

8. A welded steel pipe base material having an austenite base structure, and A welded steel pipe including a welded portion connecting both ends of the welded steel pipe base material portion to each other, A welded steel pipe for transporting slurry, characterized in that the carbon content of all welded portions contained 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-mentioned relational expression 1, [C], [Mn] and [Cr] respectively represent the C, Mn and Cr contents (wt %) contained in the entire weld.

9. The welded steel pipe base material portion is By weight percent, it contains 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, and the remainder is Fe and unavoidable impurities, 9. The welded steel pipe for transporting slurry according to claim 8, wherein the surface hardness of the base metal portion of the welded steel pipe satisfies the following Relational Expression 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 Formula 2, [Mn], [Cr] and [C] respectively mean the Mn, Cr and C contents (wt%) contained in the welded steel pipe base material, and 0 is substituted when the corresponding components are not contained.

10. The welded steel pipe base material portion has a fine structure containing 80% or more by area of ​​austenite, 9. The welded steel pipe for transporting slurry according to claim 8, wherein the average grain size of the austenite is 60 μm or less.

11. The welded steel pipe base material portion contains twins (Twin) of 20% or less by area, 9. The welded steel pipe for transporting slurry according to claim 8, wherein the average size of the twin crystals is 400 nm or less.

12. 9. The welded steel pipe for transporting slurry according to claim 8, wherein the maximum length of hot cracks formed in the welded portion is 0.5 mm or less.

13. The welded steel pipe base material portion is The tensile strength is 800 MPa or more, 9. The welded steel pipe for transporting slurry according to claim 8, characterized in that the temperature at which the ductile fracture area ratio in the DWTT test is 85% or more is −25° C. or lower.

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