Austenitic steel material and its manufacturing method

Abstract

Provided are austenitic steel materials with excellent ultra-low temperature toughness and a method for producing the same. [Solution] The austenitic steel of the present invention is composed, by weight%, of manganese (Mn): 10 to 45%, carbon (C): in a range satisfying 24 × [C] + [Mn] ≧ 25 and 33.5 × [C] - [Mn] ≦ 18, chromium (Cr): 10% or less (excluding 0%), one or more of Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), Mo: 5% or less (excluding 0%) and B: 0.5% or less (excluding 0%), with the remainder being iron (Fe) and unavoidable impurities, and the microstructure in the weld heat-affected zone contains, by area%, 95% or more (including 100%) austenite and 5% or less (including 0%) carbides, and the content of C solid-solved in the austenite is 60% or more of the average C content of the steel.

Description

[Technical Field]

[0001] The present invention relates to an austenitic steel material and a manufacturing method thereof, and more particularly to an austenitic steel material that can be suitably applied to structural materials used in ultra-low temperature environments such as liquefied gas storage tanks and liquefied gas transport facilities, and a manufacturing method thereof. [Background technology]

[0002] Liquefied gases such as liquefied hydrogen (boiling point: -253°C), liquefied natural gas (LNG, boiling point: -164°C), liquefied oxygen (boiling point: -183°C), and liquefied nitrogen (boiling point: -196°C) require cryogenic storage. Therefore, to store these gases, structures such as pressure vessels made of materials with sufficient toughness and strength at cryogenic temperatures are required.

[0003] Materials that can be used at low temperatures in liquefied gas atmospheres include Cr-Ni stainless steel alloys such as AISI 304, 9% Ni steel, and 5000-series aluminum alloys. However, aluminum alloys are limited in use due to their high alloy costs, low strength, which increases the design thickness of structures, and poor weldability. While Cr-Ni stainless steel and 9% nickel (Ni) steel have significantly improved the physical properties of aluminum, they contain large amounts of expensive nickel (Ni), making them undesirable from an economic standpoint. Summary of the Invention [Problem to be solved by the invention]

[0004] One aspect of the present invention is to provide an austenitic steel material and a method for producing the same.

[0005] A preferred aspect of the present invention is to provide an austenitic steel material having excellent ultra-low temperature toughness and a method for producing the same. [Means for solving the problem]

[0006] One embodiment of the present invention provides an austenitic steel material comprising, by weight%, manganese (Mn): 10 to 45%, carbon (C): in a range satisfying 24 × [C] + [Mn] ≧ 25 and 33.5 × [C] - [Mn] ≦ 18, chromium (Cr): 10% or less (excluding 0%), one or more of Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), Mo: 5% or less (excluding 0%), and B: 0.5% or less (excluding 0%), with the remainder being iron (Fe) and unavoidable impurities, and the microstructure in the weld heat affected zone contains, by area%, 95% or more (including 100%) austenite and 5% or less (including 0%) carbides, and the content of C solid-solved in the austenite is 60% or more of the average C content of the steel material.

[0007] The austenite may have an average crystal grain size of 10 to 200 μm.

[0008] The weld heat affected zone may have a lateral expansion of 0.32 mm or more after a Charpy impact test at -253°C.

[0009] The weld heat affected zone may have a Charpy impact energy of 27 J or more at -253°C.

[0010] Another embodiment of the present invention provides a method for producing an austenitic steel product, the method including the steps of heating a slab containing, in weight percent, 10 to 45% manganese (Mn), carbon (C) in a range satisfying 24 × [C] + [Mn] ≧ 25 and 33.5 × [C] − [Mn] ≦ 18, 10% or less (excluding 0%) of chromium (Cr), one or more of 5% or less (excluding 0%) of Cu, 5% or less (excluding 0%) of Si, 5% or less (excluding 0%) of Al, 5% or less (excluding 0%) of Mo, and 0.5% or less (excluding 0%) of B, with the remainder being iron (Fe) and inevitable impurities, at 1000 to 1300°C, finish hot rolling the heated slab at 800 to 1050°C to obtain a hot-rolled steel sheet, and welding the hot-rolled steel sheet. [Effects of the Invention]

[0011] According to one aspect of the present invention, an austenitic steel material and a method for producing the same can be provided.

[0012] According to a preferred aspect of the present invention, it is possible to provide an austenitic steel material having excellent ultra-low temperature toughness and a method for producing the same. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram showing the correlation between the carbon content and the manganese content of an austenitic steel according to one aspect of the present invention. [Figure 2] FIG. 1 is a diagram illustrating a method for measuring the transverse expansion value of an austenitic steel material according to one aspect of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] An austenitic steel material according to one embodiment of the present invention will be described below. First, the alloy composition will be described. The contents of the alloy composition described below are in weight percent unless otherwise specified.

[0015] Manganese (Mn): 10-45% Manganese is an element that plays an important role in stabilizing austenite. To stabilize austenite at ultralow temperatures, a manganese (Mn) content of 10% or more is preferred. If the manganese (Mn) content is less than this, a metastable epsilon martensite phase is formed, which easily transforms to alpha martensite through strain-induced transformation at ultralow temperatures, resulting in poor toughness. While increasing the carbon (C) content to suppress the formation of epsilon martensite can stabilize austenite, this can result in the precipitation of a large amount of carbides, which can rapidly deteriorate physical properties. Therefore, a manganese (Mn) content of 10% or more is preferred. A manganese (Mn) content of 15% or more is preferred, and a manganese (Mn) content of 18% or more is even more preferred. Excessive manganese (Mn) content not only reduces the corrosion rate of the steel but is also undesirable from an economic standpoint. Therefore, a manganese (Mn) content of 45% or less is preferred. A preferred manganese (Mn) content may be 40% or less, and a more preferred manganese (Mn) content may be 35% or less.

[0016] Carbon (C): 24 × [C] + [Mn] ≥ 25 and 33.5 × [C] - [Mn] ≤ 18 Carbon (C) stabilizes austenite and increases its strength. In particular, carbon (C) lowers the Ms or Md, the transformation point from austenite to epsilon- or alpha-martensite, during cooling or processing. Therefore, carbon (C) effectively stabilizes austenite. Insufficient carbon (C) content can lead to insufficient austenite stability at ultralow temperatures, preventing stable austenite from being obtained. External stress can easily induce strain-induced transformation to epsilon- or alpha-martensite, potentially reducing the toughness or strength of the steel. On the other hand, excessive carbon (C) content can rapidly degrade the toughness of the steel due to carbide precipitation, and can increase the steel's strength excessively, potentially reducing its workability.

[0017] The inventors of the present invention have conducted extensive research into the relative behavior of carbon (C) and manganese (Mn) contents with respect to carbide formation. As a result, they have concluded that by determining the relative content relationship of carbon (C) and manganese (Mn), it is possible to effectively stabilize austenite while effectively controlling the amount of carbide precipitation, as shown in Figure 1. Although carbides are formed by carbon (C), carbon (C) does not affect carbide formation independently, but rather acts in combination with manganese (Mn) to affect carbide formation.

[0018] To stabilize austenite, it is preferable to control the value of 24 × [C] + [Mn] (where [C] and [Mn] represent the contents of each component in weight percent) to 25 or greater, provided that the other components satisfy the ranges specified in this invention. This boundary refers to the inclined left boundary of the parallelogram region shown in Figure 1. If 24 × [C] + [Mn] is less than 25, the stability of austenite decreases, and deformation-induced transformation occurs upon impact at ultralow temperatures, which may result in a decrease in the impact toughness of the steel. On the other hand, to suppress the formation of carbides, it is preferable to control the value of 33.5 × [C] - [Mn] (where [C] and [Mn] represent the contents of each component in weight percent) to 18 or less, provided that the other components satisfy the ranges specified in this invention. If 33.5 × [C] - [Mn] exceeds 18, the addition of excess carbon (C) may cause carbide precipitation, resulting in a decrease in the low-temperature impact toughness of the steel. Therefore, in the present invention, carbon (C) is preferably added so as to satisfy the conditions 24 × [C] + [Mn] ≥ 25 and 33.5 × [C] - [Mn] ≤ 18. As can be seen from Figure 1, the lowest limit of the carbon (C) content within the range that satisfies the above formula is 0%.

[0019] Chromium (Cr): 10% or less (excluding 0%) Chromium (Cr) is an austenite-stabilizing element. Up to an appropriate addition amount, it stabilizes austenite, improving the low-temperature impact toughness of steel and dissolving in austenite to increase the strength of steel. Chromium (Cr) also effectively contributes to improving the corrosion resistance of steel. Therefore, the present invention adds chromium (Cr) as an essential element. A preferred lower limit of the chromium (Cr) content may be 1%, and a more preferred lower limit may be 2%. However, chromium (Cr) is a carbide-forming element, and it can form carbides, especially at austenite grain boundaries, thereby reducing the low-temperature impact toughness of steel. Furthermore, if the amount of chromium (Cr) added exceeds a certain level, excessive carbides may precipitate in the heat-affected zone (HAZ), potentially degrading the ultra-low temperature toughness. Therefore, the present invention limits the upper limit of chromium (Cr) to 10%. A preferred upper limit for the chromium (Cr) content may be 8%, and a more preferred upper limit for the chromium (Cr) content may be 7%.

[0020] One or more of Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), Mo: 5% or less (excluding 0%), and B: 0.5% or less (excluding 0%)

[0021] Cu: 5% or less (excluding 0%) Copper (Cu) has very low solid solubility in carbides and slow diffusion in austenite, so it concentrates at the interface between austenite and nucleated carbides. This effectively slows carbide growth by blocking carbon diffusion, ultimately suppressing carbide formation. Copper also stabilizes austenite and improves cryogenic toughness. However, because a Cu content exceeding 5% can degrade the hot workability of steel, it is preferable to limit the Cu content to 5% by weight. The preferred upper limit of the Cu content may be 3%, and more preferably 2%. The preferred lower limit of the Cu content may be 0.1%, and more preferably 0.3%.

[0022] Si: 5% or less (excluding 0%) Silicon (Si) improves the castability of molten steel. When added to austenitic steel, it dissolves in the steel, effectively increasing its strength. It also affects the activity of carbon in the steel, effectively suppressing the formation of carbides and increasing its toughness. However, if added in amounts exceeding 5%, it reduces stacking fault energy and promotes twinning, potentially resulting in a decrease in toughness due to increased strength. Therefore, the upper limit of the silicon (Si) content is preferably 5%. The upper limit of the silicon (Si) content may be preferably 3%, and more preferably 2.5%. The lower limit of the silicon (Si) content may be preferably 0.1%, and more preferably 0.3%.

[0023] Al: 5% or less (excluding 0%) Aluminum (Al) stabilizes austenite within an appropriate range of addition and lowers the Ms and Md, the transformation points from austenite to epsilon- or alpha-martensite during cooling or processing, thereby improving the toughness of steel. It also dissolves in steel to increase strength. It specifically affects the activity of carbon in steel, effectively suppressing carbide formation and increasing toughness. It is particularly well known for effectively increasing stacking fault energy and promoting slip. However, adding more than 5% aluminum can lead to problems such as the formation of oxides and nitrides, which can degrade the castability and surface quality of steel. Therefore, the upper limit of the aluminum (Al) content is preferably 5% by weight. The preferred upper limit of the aluminum (Al) content may be 3%, and more preferably 2.5%. The preferred lower limit of the aluminum (Al) content may be 0.2%, and more preferably 0.3%.

[0024] Mo: 5% or less (excluding 0%) Molybdenum (Mo) stabilizes austenite within an appropriate range of addition, lowering the Ms and Md, the transformation points from austenite to epsilon- or alpha-martensite during cooling or processing, thereby improving the toughness of steel. It also dissolves in steel to increase strength. In particular, it segregates at austenite grain boundaries, improving grain boundary stability and reducing energy, thereby suppressing grain boundary precipitation of carbonitrides. Molybdenum (Mo) is also well known for effectively increasing stacking fault energy and promoting slip. However, adding more than 5 wt% of Mo can lead to problems such as reduced economic viability due to its high cost and reduced toughness due to increased strength. Therefore, the upper limit of the Mo content is preferably 5 wt%. The preferred upper limit of the Mo content may be 4.5%, and more preferably 4%. A preferred lower limit of the molybdenum (Mo) content may be 0.1%, and a more preferred lower limit of the molybdenum (Mo) content may be 0.3%.

[0025] B: 0.5% or less (excluding 0%) Boron (B) is a representative element that improves the hardenability of steel. It preferentially segregates at austenite grain boundaries at high temperatures, lowering the energy of the austenite grain boundaries and stabilizing them. Therefore, it improves hardenability by preventing the formation of undesired structures such as ferrite and pearlite at the austenite grain boundaries during quenching of carbon steel. Boron (B) also segregates at the austenite grain boundaries of high-manganese steel, lowering the energy of the austenite grain boundaries and preventing carbon diffusion. This makes it an effective element for suppressing the formation of carbides. However, adding more than 0.5 wt% of boron (B) can lead to the formation of coarse boron nitrides, which can reduce physical properties. Therefore, the upper limit is preferably 0.5 wt%. The preferred upper limit of the boron (B) content is 0.4%, and more preferably 0.3%. A preferred lower limit of the boron (B) content may be 0.0005%, and a more preferred lower limit of the boron (B) content may be 0.001%.

[0026] Since structures are generally produced by processing and welding steel materials, even if the base material itself has sufficient cryogenic impact toughness, if the welded joint does not have sufficient cryogenic impact toughness, the safety of the structure itself may be significantly reduced. Therefore, an austenitic steel according to one aspect of the present invention aims to ensure not only the cryogenic impact toughness of the base material itself but also the cryogenic impact toughness of the weld heat-affected zone (HAZ). Therefore, the present invention controls the microstructure of not only the base material but also the weld HAZ within a specific range.

[0027] In an austenitic steel according to one aspect of the present invention, the microstructure of not only the base metal but also the heat-affected zone (HAZ) can contain 95 area % or more of austenite in order to ensure desired physical properties. The preferred austenite fraction can be 97 area % or more, including a case where the austenite fraction is 100 area %. Meanwhile, in an austenitic steel according to one aspect of the present invention, the carbide fraction can be actively suppressed to 5 area % or less to prevent a decrease in cryogenic impact toughness. The preferred carbide fraction can be 3 area % or less, including a case where the carbide fraction is 0 area %. In the present invention, the austenitic steel is not particularly limited in its measurement method, and can be easily determined by a person of ordinary skill in the art using a measurement method commonly used to measure microstructures and carbides.

[0028] The content of C dissolved in the austenite is preferably 60% or more of the average C content of the steel. When all of the C contained in the steel is dissolved in austenite, the content of C dissolved in the austenite is 100%, and when the C contained in the steel is precipitated as carbides, the content of C dissolved in the austenite is less than 100%. In this way, by dissolving a large amount of C in austenite, it is possible to suppress carbide precipitation and prevent a decrease in impact toughness. The content of C dissolved in the austenite relative to the average C content of the steel is more preferably 70% or more, even more preferably 85%, and most preferably 100%. The method for measuring the content of C dissolved in the austenite relative to the average C content of the steel is not particularly limited, and can be easily determined by a person of ordinary skill in the art using a measurement method commonly used to measure microstructures and carbides.

[0029] The austenite preferably has an average grain size of 10 to 200 μm. If the average grain size of austenite in the heat-affected zone (HAZ) is excessively small, the strength of the weld is improved, but localized reduction in ultra-low temperature impact toughness in the heat-affected zone (HAZ) may occur. Furthermore, because the primary precipitation site of carbides is the austenite grain boundary, excessively fine grain size may result in excessive precipitation of carbides, resulting in reduced impact toughness. Therefore, the average grain size of austenite in the heat-affected zone may be limited to 10 μm or more. On the other hand, a larger average grain size of austenite in the heat-affected zone is advantageous for ensuring the ultra-low temperature impact toughness of the weld, but may result in localized reduction in strength in the heat-affected zone. Therefore, the average grain size of austenite may be limited to 200 μm or less. The lower limit of the average grain size of austenite is more preferably 15 μm, and even more preferably 20 μm. The upper limit of the average austenite grain size is more preferably 180 μm, and even more preferably 150 μm.

[0030] When the austenitic steel according to one aspect of the present invention is used as a base metal and welded under normal welding conditions used for welding cryogenic structures, the weld heat-affected zone may have a lateral expansion value of 0.32 mm or more after a Charpy impact test at −253°C. However, since a higher lateral expansion value is more advantageous in the present invention, the upper limit is not particularly limited. However, the upper limit of the lateral expansion value may be 2.30 mm, for example.

[0031] The inventors of the present invention have found that for steel materials to be used in cryogenic environments, plastic deformation characteristics are a key factor in ensuring safety. That is, after extensive research, the inventors of the present invention have confirmed that for steel materials that satisfy the chemical composition proposed by the present invention, the lateral expansion value (mm) of the weld heat affected zone is a more important factor in ensuring the safety of the weld than the Charpy impact energy value (J) of the weld heat affected zone.

[0032] The transverse expansion value in the weld heat-affected zone refers to the average value of the transverse plastic deformation of a test piece subjected to a -253°C standard Charpy impact test. Figure 2 shows a photograph of a test piece subjected to a -253°C standard Charpy impact test. As shown in Figure 2, the transverse expansion value can be calculated by calculating the increase in transverse length near the fracture surface (△X1 + △X2). If the transverse expansion value in the weld heat-affected zone is 0.32 mm or more, it can be determined that the minimum low-temperature safety required for cryogenic structures is met.

[0033] According to the research results of the present inventors, it has been confirmed that the -253°C standard Charpy impact energy (J) and the lateral expansion value (mm) of the test piece generally show a tendency similar to that of the following relational expression 1, and that the lateral expansion value (mm) is preferably 0.32 mm or more. It has been found that the larger the lateral expansion value (mm), the better the low-temperature impact toughness, and a value of 0.72 to 1.4 mm is more effective.

[0034] [Equation 1] Lateral expansion value (mm) = 0.0088 x Charpy impact energy value (J) + 0.0893

[0035] When the austenitic steel material according to one aspect of the present invention is used as a base metal and welded under normal welding conditions used for welding structures for cryogenic use, the lateral expansion value of the weld heat-affected zone of a test piece subjected to a -253°C reference Charpy impact test is at a level of 0.32 mm or more, and therefore, when the steel material is used to fabricate a structure for cryogenic use, excellent structural safety can be ensured.

[0036] When the austenitic steel material of the present invention is used as a base metal and welded under normal welding conditions used for welding structures for cryogenic use, the weld heat-affected zone may have a Charpy impact energy of 27 J or more at -253°C. By ensuring such a high level of cryogenic Charpy impact energy, ductile fracture occurs when the structure is fractured, thereby ensuring the fracture stability of the structure for cryogenic use. Meanwhile, in the present invention, the higher the cryogenic Charpy impact energy, the more advantageous it is, so there is no particular restriction on the upper limit. However, the upper limit of the cryogenic Charpy impact energy may be 250 J, for example.

[0037] Hereinafter, a method for producing an austenitic steel material according to one embodiment of the present invention will be described.

[0038] First, a slab having the above-mentioned alloy composition is heated at 1000 to 1300°C. If the slab heating temperature is less than 1100°C, there are drawbacks, such as the alloy components not being redissolved and homogenized, or it taking a considerable amount of time for the center of the slab to reach the target temperature. If the slab heating temperature exceeds 1300°C, there are drawbacks, such as partial melting occurring in the alloy component segregation portion of the slab, or severe surface oxidation occurring. The lower limit of the slab heating temperature is more preferably 1030°C, even more preferably 1070°C, and most preferably 1100°C. The upper limit of the slab heating temperature is more preferably 1250°C, even more preferably 1230°C, and most preferably 1200°C.

[0039] The heated slab is then finish hot-rolled at 800 to 1050°C to obtain a hot-rolled steel sheet. If the finish hot-rolling temperature is less than 800°C, the high high-temperature strength of the material makes rolling difficult, and as the unrecrystallized region is rolled, the material's strength increases excessively, resulting in a decrease in impact toughness. If the finish hot-rolling temperature exceeds 1050°C, the austenite becomes coarse, resulting in a decrease in strength. The lower limit of the finish hot-rolling temperature is more preferably 820°C, even more preferably 850°C, and most preferably 870°C. The upper limit of the finish hot-rolling temperature of the slab is more preferably 1030°C, even more preferably 1000°C, and most preferably 980°C. Meanwhile, the reduction ratio during the hot rolling can be applied within an appropriate range depending on the target plate thickness. As a non-limiting example, the final thickness of the hot-rolled hot-rolled steel sheet may be 5 to 80 mm.

[0040] Thereafter, the hot-rolled steel sheet can be air-cooled to room temperature.

[0041] The hot-rolled steel sheets are then welded. In the present invention, the weld heat affected zone can be formed by welding under normal welding conditions used for welding cryogenic structures using a covered electrode, a flux-cored arc welding wire, a TIG welding electrode and wire, a submerged arc welding wire and flux, etc. [Example]

[0042] Hereinafter, an austenitic steel material and a manufacturing method thereof 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 define the scope of the present invention. The scope of the present invention can be determined by the matters set forth in the claims and matters that can be reasonably inferred therefrom.

[0043] (Example) A 250 mm thick slab having the alloy composition shown in Table 1 below was prepared, and then the slab was heated and hot rolled under the conditions shown in Table 2 below, and submerged arc welding was performed under normal conditions to produce a steel material having a weld heat affected zone.

[0044] The microstructure and physical properties of the weld heat affected zone of the steel material thus produced were measured, and the results are shown in Table 2 below.

[0045] The microstructure was measured using an optical microscope at room temperature.

[0046] The average austenite grain size was measured by taking a photograph of the microstructure using an optical microscope and then performing image analysis.

[0047] The amount of dissolved carbon in austenite relative to the average carbon content of the steel was measured by point analysis of the matrix and carbides using an energy dispersive X-ray spectrometer under a scanning electron microscope.

[0048] The Charpy impact energy was measured using a Charpy impact tester after the test piece was kept at -253°C for 15 minutes or more.

[0049] The lateral expansion was calculated by calculating the average value of the amount of lateral plastic deformation of test pieces subjected to a Charpy impact test at -253°C.

[0050] [Table 1]

[0051] [Table 2]

[0052] As can be seen from Tables 1 and 2 above, in the case of Examples 1 to 6, which satisfy the alloy composition and manufacturing conditions of the present invention, the content of C dissolved in austenite is secured relative to the average C content of the microstructure and steel material that are desired to be obtained by the present invention, and therefore, excellent physical properties are secured.

[0053] In the case of Comparative Examples 1 to 4, which do not satisfy the alloy composition of the present invention, it is found that the physical properties are at a low level because it is not possible to ensure the content of C dissolved in austenite relative to the microstructure or average C content of the steel material that the present invention aims to obtain.

Claims

1. The alloy contains, by weight, manganese (Mn): 10 to 45%, carbon (C): in a range satisfying 24 x [C] + [Mn] ≥ 25 and 33.5 x [C] - [Mn] ≤ 18, chromium (Cr): 10% or less (excluding 0%), one or more of Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), Mo: 5% or less (excluding 0%) and B: 0.5% or less (excluding 0%), with the remainder being iron (Fe) and inevitable impurities, The microstructure in the weld heat affected zone contains, by area%, 95% or more (including 100%) of austenite and 5% or less (including 0%) of carbides; The austenitic steel material is characterized in that the content of C dissolved in the austenite is 60% or more of the average C content of the steel material.

2. 2. The austenitic steel according to claim 1, wherein the austenite has an average grain size of 10 to 200 μm.

3. 2. The austenitic steel material according to claim 1, wherein the weld heat affected zone has a lateral expansion of 0.32 mm or more after a Charpy impact test at −253° C.

4. 2. The austenitic steel material according to claim 1, wherein the weld heat affected zone has a Charpy impact energy of 27 J or more at −253° C.

5. a step of heating a slab containing, in weight percent, manganese (Mn): 10 to 45%, carbon (C): in a range satisfying 24 x [C] + [Mn] ≥ 25 and 33.5 x [C] - [Mn] ≤ 18, chromium (Cr): 10% or less (excluding 0%), one or more of Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), Mo: 5% or less (excluding 0%), and B: 0.5% or less (excluding 0%), with the remainder being iron (Fe) and inevitable impurities, at 1000 to 1300°C; Finish hot rolling the heated slab at 800 to 1050°C to obtain a hot-rolled steel sheet; a step of welding the hot-rolled steel sheets.

Images