Layer-rolled steel sheet and its manufacturing method

Abstract

To provide: an overlapped rolled steel plate that can secure brittle crack propagation stop performance with a plate thickness of more than 70 mm or more, specifically, Kca at a service temperature of -10°C can stably secure 8000 N / mm1.5 or more, as well as, high strength excellent in host toughness and weld heat affected zone toughness; and a producing method thereof.SOLUTION: An overlapped rolled steel plate having a plate thickness of 70 mm or more comprises integrated two steel plates, which each have a specific composition, a specific surface integration degree, a Charpy fracture surface transition temperature (vTrs) at a center of a plate thickness being -50°C or less, the Charpy fracture surface transition temperature (vTrs) at a 1 / 4 position of the plate thickness of being -70°C or less, and the Charpy fracture surface transition temperature (vTrs) at a steel sheet surface being -80°C or less, and a position of a joining interface thereof is included within a positional range of 30 to 70% of a total plate thickness of the integrated two steel plates in a plate thickness direction.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a lap-rolled steel plate having a thickness of 70 mm or more, which is used for large structures such as ships, marine structures, low-temperature storage tanks, and architectural and civil engineering structures, and a method for manufacturing the same. [Background technology]

[0002] In large structures such as ships, marine structures, cryogenic storage tanks, and architectural and civil engineering structures, accidents caused by brittle fracture have a significant impact on society, the economy, and the environment. For this reason, improvements in safety are constantly being sought for these large structures, and the steel plates used as materials for these structures are required to have a high level of brittle crack arrestability at operating temperatures.

[0003] In ships such as container ships and bulk carriers, high-strength extra-thick steel plates are used for the outer hull plates due to their structure. Recently, as ships have become larger, even higher strength is required, and the thickness of the extra-thick steel plates used as materials has been increasing.

[0004] Generally, the brittle crack arrestability of steel plates tends to deteriorate as their strength or thickness increases. For this reason, the requirements for brittle crack arrestability of extra-thick steel plates used in large structures are becoming increasingly stringent.

[0005] Here, a method of increasing the Ni content in steel has been known as a means of improving the brittle crack propagation arrestability of steel plates. For example, 9% Ni steel is used on a commercial scale in storage tanks for liquefied natural gas (LNG).

[0006] However, an increase in the Ni content in steel inevitably leads to a significant increase in manufacturing costs, making it difficult to apply 9% Ni steel to applications other than LNG storage tanks.

[0007] On the other hand, for relatively thin steel plates with a thickness of less than 50 mm, such as those used in ships and line pipes, which do not reach the extremely low temperatures of LNG, the TMCP (Thermo Mechanical Control Process) method can be used to refine the microstructure of the steel plate and improve low-temperature toughness, thereby achieving excellent brittle crack propagation arrest properties.

[0008] Furthermore, Patent Document 1 proposes a steel sheet having an ultra-fine structure in the surface layer in order to improve brittle crack propagation arrestability without increasing alloy costs.

[0009] The steel sheet with excellent brittle crack arrestability described in Patent Document 1 was developed by focusing on the fact that shear lips (plastic deformation regions) that occur in the surface layer of the steel sheet when a brittle crack propagates are effective in improving brittle crack arrestability, and is characterized by absorbing the propagation energy of the propagating brittle crack by refining the crystal grains in the shear lip portion. Patent Document 1 also describes a process in which the surface layer is cooled to below the Ar3 transformation point by controlled cooling after hot rolling, and then the controlled cooling is stopped and the surface layer is heated to above the Ar3 transformation point. This process is repeated one or more times, and during this process, a rolling reduction is applied to the steel sheet, causing repeated transformation or work recrystallization, thereby generating an ultrafine ferrite structure in the surface layer.

[0010] Patent Document 2 describes that in order to improve the brittle crack propagation arrestability of a steel plate having a microstructure mainly composed of ferrite-pearlite, it is important to reduce the variation in ferrite grain size by forming a ferrite structure on both surfaces of the steel plate having ferrite grains with a circular equivalent grain size of 5 μm or less and an aspect ratio of 2 or more in at least 5% of the surface layer region in the plate thickness direction, and that a method of reducing this variation is to suppress local recrystallization by setting the maximum reduction rate per pass during finish rolling to 12% or less.

[0011] Patent Document 3 describes a technology that is an extension of TMCP, which not only refines ferrite grains but also controls the subgrains formed within the ferrite grains to improve brittle crack propagation arrestability.

[0012] Another known method is to improve the brittle crack arrestability by applying a reduction to the transformed ferrite during controlled rolling to develop a texture. This method increases the resistance to brittle fracture by generating separation on the fracture surface of the steel sheet in a direction parallel to the sheet surface and alleviating the stress at the tip of the brittle crack.

[0013] For example, Patent Document 4 describes that brittle fracture resistance can be improved by controlling the (110) plane X-ray intensity ratio to 2 or more through controlled rolling and by controlling the area ratio of coarse grains with a circle-equivalent diameter of 20 μm or more to 10% or less.

[0014] Patent Document 5 discloses a steel plate for welded structures that has an X-ray plane intensity ratio of 1.5 or more for the (100) plane on the rolled surface within the plate thickness, as a steel plate with excellent brittle crack propagation arrestability at joints, and describes that the excellent brittle crack propagation arrestability is achieved due to the angular deviation between the stress load direction and the crack propagation direction caused by the texture development.

[0015] Furthermore, Patent Document 6 describes a method for manufacturing a high-strength thick steel plate with excellent brittle crack propagation arrestability, in which a texture is developed in each part in the plate thickness direction (at least 1 / 3 of the total plate thickness including the plate center) by specifying the average reduction rate in controlled rolling.

[0016] Furthermore, recent large container ships exceeding 6,000 TEU use extra-thick steel plates with a plate thickness of 70 mm or more. Non-Patent Document 1 evaluates the brittle crack arrest properties of a steel plate with a plate thickness of 65 mm, and reports that brittle cracks were not arrested in a large-scale brittle crack arrest test on the steel plate. [Prior art documents] [Patent documents]

[0017] [Patent Document 1] Special Publication No. 7-100814 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-256375 [Patent Document 3] Patent No. 3467767 [Patent Document 4] Patent No. 3548349 [Patent Document 5] Patent No. 2659661 [Patent Document 6] Patent No. 5733425 [Non-patent literature]

[0018] [Non-Patent Document 1] Long and Large Brittle Crack Propagation Behavior in Thick Shipbuilding Steel, Proceedings of the Japan Society of Naval Architects and Ocean Engineers, No. 3, 2006, pp. 359-362 Summary of the Invention [Problem to be solved by the invention]

[0019] In the techniques described in Patent Documents 1 and 2, a specific structure is obtained by cooling only the surface layer of the steel sheet, then reheating it, and processing it during reheating. This makes it difficult to control on an actual production scale, and the process places a heavy load on rolling and cooling equipment, especially in the production of thick steel sheets with a thickness of 70 mm or more.

[0020] Furthermore, based on the manufacturing conditions and disclosed examples, the steel plates described in Patent Documents 1 to 6 are all primarily intended for plate thicknesses of approximately 50 mm to 70 mm. It is unclear whether the specified properties can be obtained when applied to thicker materials of 70 mm or more, and the crack propagation properties in the plate thickness direction, which are necessary for large structures, have not been verified at all.

[0021] Furthermore, in Non-Patent Document 1, the ESSO test of the test material showed that the Kca at a use temperature of -10°C was 3000N / mm 1.5This suggests that the technology of Non-Patent Document 1 cannot ensure sufficient safety in the case of large structures that use steel plates with a thickness of more than 50 mm.

[0022] Furthermore, structural steel materials used in fields such as shipbuilding are finished into structures of desired shapes by welding. Highly efficient, large-heat-input welding methods such as submerged arc welding, electrogas welding, and electroslag welding are used to weld thick steel materials, particularly those with a plate thickness of 70 mm or more. Therefore, the toughness of the weld heat-affected zone (HAZ) becomes an issue. However, none of the patent documents adequately examines the toughness of the HAZ of thick steel materials.

[0023] In view of the above circumstances, the present invention provides a steel sheet having a thickness of 70 mm or more and brittle crack propagation arrestability, specifically, a Kca of 8000 N / mm at a use temperature of -10°C. 1.5 The objective of this study is to provide a high-strength lap-rolled steel plate that can ensure the above and also has excellent base material toughness and weld heat-affected zone toughness, and a manufacturing method for the same. The strength level of the steel plate is 390 MPa or more in YS and 510 MPa or more in TS (so-called 50 kgf / cm2 steel or more), and the weld heat-affected zone toughness is 60 J or more when an impact test is conducted at -20°C on the weld bond. [Means for solving the problem]

[0024] In order to solve the above problems, the inventors conducted extensive research into high-strength extra-thick steel plates having excellent brittle crack arrestability, base metal toughness, and weld heat-affected zone toughness even at thicknesses of 70 mm or more, and a manufacturing method for reliably obtaining such steel plates. As a result, they discovered that excellent brittle crack arrestability can be obtained by having a texture in which the (211) plane integration degree on the rolled surface at the center of thickness of the steel plate is 1.2 or more and the (200) plane integration degree on the rolled surface at the steel plate surface (sometimes simply referred to as the "surface") is 1.7 or more, and by having a Charpy fracture appearance transition temperature (vTrs) (also referred to as vTrs) at the center of thickness, which are indicators of toughness, of -50°C or less, a Charpy fracture appearance transition temperature (vTrs) at the 1 / 4 thickness position of -70°C or less, and a Charpy fracture appearance transition temperature (vTrs) at the steel plate surface of -80°C or less. Furthermore, it was discovered that by finely dispersing composite sulfides of TiN, CaS, and MnS in the weld heat-affected zone, the grain growth when exposed to high temperatures during welding is suppressed, and by promoting intragranular transformation during the subsequent cooling process, the structure of the weld heat-affected zone at room temperature is refined, thereby obtaining excellent toughness in the weld heat-affected zone.

[0025] Furthermore, when attempting to obtain the above properties with steel plates 70 mm or thicker, if a conventional continuously cast slab is used as the raw material, the initial thickness of the slab must be large to ensure a sufficient rolling reduction, but there are cases where it is not possible to cast slabs with such large initial thicknesses due to equipment limitations on the continuous casting machine. Furthermore, slabs with large initial thicknesses generally have poor internal quality, and after rolling, internal defects such as lumps, in which unsolidified areas from the casting stage remain in the center of the plate thickness, can occur, which can adversely affect brittle crack propagation arrest properties.

[0026] In order to solve the above problems, in the present invention, two continuously cast slabs are reheated in advance, then rolled, and these slabs are then stacked and rolled. This makes it possible to achieve the following: (1) refinement of the steel sheet microstructure by reheating and rolling twice; (2) refinement of the steel sheet microstructure and development of texture by ensuring the rolling reduction rate by ensuring the initial thickness of the stacked slabs; and (3) ensuring internal quality by avoiding the location of center segregation and cracks in the center of the thickness of the stacked rolled steel sheet, and preventing the deterioration of brittle crack propagation arrestability that accompanies the deterioration of internal quality.

[0027] The present invention has been completed based on the above findings and further investigations. The gist and configuration of the present invention are as follows. [1] A steel sheet having two integrated steel plates and a joining interface between the two steel plates, Two steel plates each have the following mass%: C: 0.03~0.15%, Si: 0.50% or less, Mn: 0.5 to 2.2%, P: 0.015% or less, S: 0.0005 to 0.0040%, Ti: 0.005 to 0.030%, Al: 0.005 to 0.080%, N: 0.0035~0.0075%, Ca: 0.0005 to 0.0030%, O: Contains 0.0040% or less, a Ceq defined by the following formula (1) is 0.36 or more, and the contents of Ca, O, and S each satisfy the following formula (2), with the balance consisting of Fe and unavoidable impurities; and a (211) plane integration degree on the rolled surface at the center of the sheet thickness of the two integrated steel sheets is 1.2 or more, the (200) plane integration degree at the rolled surface on the surface of the two integrated steel plates is 1.7 or more, the Charpy fracture appearance transition temperature vTrs at the center of the plate thickness of the two integrated steel plates is -50°C or less, the Charpy fracture appearance transition temperature vTrs at a 1 / 4 position of the plate thickness of the two integrated steel plates is -70°C or less, and the Charpy fracture appearance transition temperature vTrs on the surface of the two integrated steel plates is -80°C or less, A lap-rolled steel plate having a thickness of 70 mm or more, wherein the position of the joining interface is included in a position that is 30 to 70% of the total thickness of the two integrated steel plates in the plate thickness direction. Ceq=C+Mn / 6+Cu / 15+Ni / 15+Cr / 5+Mo / 5+V / 5...(1) Here, C, Mn, Cu, Ni, Cr, Mo, and V in formula (1) are the contents of each element. (% by mass), and if not contained, it is set to 0. 0<{(Ca-(0.18+130×Ca)×O) / 1.25} / S≦0.8···(2) Here, Ca, O, and S represent the content (mass%) of each component. [2] The component composition is further expressed in mass% as follows: B: 0.0003~0.0030%, V: 0.2% or less, Nb: 0.003 to 0.030%, Ni: 1.0% or less, Cu: 1.0% or less, Cr: 0.7% or less, Mo: 0.7% or less Mg: 0.0005 to 0.0050%, Zr: 0.001 to 0.020%, The laminated rolled steel sheet according to [1], containing one or more of REM: 0.001 to 0.020%. [3] Two slabs having the chemical composition described in [1] or [2] are heated to a temperature range of 950 to 1100 ° C., then hot-rolled, cooled to room temperature, polished on one side of the surface of the two slabs, stacked so that the polished surfaces of the two slabs are in contact with each other, and fillet-welded at the joint between the two slabs to form a single stacked slab; A method for producing an overlap-rolled steel plate with a thickness of 70 mm or more, comprising the steps of: heating the stacked slabs to a temperature range of 1000 to 1150°C, and then hot-rolling the stacked slabs under conditions where the cumulative reduction rate when the temperature at the center of the thickness of the stacked slabs is in the austenite recrystallization temperature range is 10% or more, the cumulative reduction rate when the temperature at the center of the thickness of the stacked slabs is in the austenite non-recrystallization temperature range is 60% or more, and the cumulative reduction rate when the temperature at the surface of the stacked slabs is below the Ar3 transformation point and the temperature at the center of the thickness of the stacked slabs is above the Ar3 transformation point is more than 20%, and then cooling the stacked slabs at an average cooling rate of 0.5°C / s or more to a cooling stop temperature of 500°C or less, and then cooling the stacked slabs to room temperature at an average cooling rate of 0.5°C / s or more after cooling to the cooling stop temperature of 500°C or less, and then cooling the stacked slabs at an average cooling rate of 0.5°C / s or more to room temperature at an average cooling rate of 0.5°C / s or more. [4] Two slabs having the chemical composition described in [1] or [2] are heated to a temperature range of 950 to 1100 ° C., then hot-rolled, cooled to room temperature, polished on one side of the surface of the two slabs, stacked so that the polished surfaces of the two slabs are in contact with each other, and fillet-welded at the joint between the two slabs to form a single stacked slab; The stacked slabs are heated to a temperature range of 1000 to 1150°C, and then hot-rolled under conditions in which the cumulative reduction rate when the temperature at the center of the thickness of the stacked slabs is in the austenite recrystallization temperature range is 10% or more, the cumulative reduction rate when the temperature at the center of the thickness of the stacked slabs is in the austenite non-recrystallization temperature range is 60% or more, and the cumulative reduction rate when the temperature at the surface of the stacked slabs is in a temperature range in which the Ar3 transformation point is not more than the Ar3 transformation point and the temperature at the center of the thickness of the stacked slabs is not less than the Ar3 transformation point is more than 20%, and then cooled to a cooling stop temperature of 500°C or less at an average cooling rate of 0.5°C / s or more, A method for manufacturing an overlap-rolled steel sheet, wherein after cooling to the cooling stop temperature of 500°C or less, tempering is performed at a temperature at which the temperature at the center of the plate thickness of the overlap slab becomes lower than the Ac1 transformation point. [Effects of the Invention]

[0028] According to the present invention, since the texture in the plate thickness direction is appropriately controlled, even an extremely thick steel plate having a plate thickness of 70 mm or more has excellent brittle crack propagation arrestability, excellent base material toughness and weld heat-affected zone toughness, and high strength. Furthermore, according to the present invention, by optimizing the rolling conditions, it is possible to stably produce a lap-rolled steel plate using an industrially very simple process. For example, by applying the lap-rolled steel plate of the present invention to deck members joined to hatch side coamings in the strong deck structures of container ships and bulk carriers in the shipbuilding field, it contributes to improving the safety of ships and is extremely useful industrially. [Brief explanation of the drawings]

[0029] [Figure 1] This shows a cross-sectional view of a welded joint prepared under specified conditions to evaluate the toughness of the heat-affected zone (HAZ toughness). [Figure 2] The cross-sectional view of the welded joint shows the locations where Charpy test specimens were taken and the notch positions. DETAILED DESCRIPTION OF THE INVENTION

[0030] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

[0031] <Steel plate composition> Each component will be explained below. Note that "%" representing the content of a component means "% by mass."

[0032] C: 0.03 to 0.15% The C content is set to 0.03% or more to obtain the strength required for structural steel. On the other hand, the C content is set to 0.15% or less to suppress the formation of island martensite. The C content is preferably set to 0.10% or less.

[0033] Si:0.50% or less If the Si content exceeds 0.50%, the toughness of the weld heat affected zone will be deteriorated. Therefore, the Si content is set to 0.50% or less. Preferably, the Si content is set to 0.40% or less. There is no particular restriction on the lower limit, but since reducing the Si content improves the toughness of the weld heat affected zone, 0% may be sufficient.

[0034] Mn: 0.5 to 2.2% The Mn content must be 0.5% or more to ensure the strength of the steel plate. Preferably, the Mn content is 1.0% or more. On the other hand, if the Mn content exceeds 2.2%, the toughness of the welded joint deteriorates. Therefore, the Mn content is 2.2% or less. Preferably, the Mn content is 2.0% or less.

[0035] P:0.015% or less P is an unavoidable impurity, and if the P content exceeds 0.015%, it reduces the toughness of the steel plate and welds. For this reason, the P content is set to 0.015% or less. The lower limit is not particularly limited, but since the toughness of the steel plate and welded portion improves as the content is reduced, it may be 0%.

[0036] S: 0.0005 to 0.0040% The S content must be 0.0005% or more to produce the required CaS or MnS, but if it exceeds 0.0040%, the toughness of the steel plate will deteriorate, so the S content is set to 0.0040% or less.

[0037] Ti: 0.005 to 0.030% Ti precipitates as TiN when molten steel solidifies in the steelmaking process, suppresses the coarsening of austenite in the weld heat affected zone, and acts as a ferrite transformation nucleus, contributing to increased toughness. If the Ti content is less than 0.005%, this effect is small. For this reason, the Ti content is 0.005% or more. On the other hand, if it exceeds 0.030%, the TiN particles become coarse and the above effect cannot be obtained. Therefore, the Ti content is set to 0.030% or less. The Ti content is preferably 0.020% or less.

[0038] Al: 0.005 to 0.080% The Al content must be 0.005% or more to deoxidize the steel. Preferably, the Al content is 0.010% or more. On the other hand, an Al content exceeding 0.080% reduces the toughness of the steel plate and also deteriorates the toughness of the weld metal, so the Al content is set to 0.080% or less. Preferably, the Al content is 0.060% or less.

[0039] N: 0.0035 to 0.0075% N is an element necessary to ensure the required amount of TiN, and if the N content is less than 0.0035%, a sufficient amount of TiN cannot be obtained. Therefore, the N content is set to 0.0035% or more. On the other hand, if it exceeds 0.0075%, the amount of dissolved N in the region where TiN dissolves due to the welding heat cycle increases, significantly reducing the toughness of the weld heat affected zone. Therefore, the N content is set to 0.0075% or less. Preferably, the N content is 0.0060% or less.

[0040] Ca: 0.0005 to 0.0030% Ca is an element that improves the toughness of steel sheets by fixing S. To achieve this effect, at least 0.0005% Ca is contained. However, even if the Ca content exceeds 0.0030%, the effect saturates. For this reason, in the present invention, the Ca content is set to 0.0030% or less.

[0041] O: 0.0040% or less O precipitates as an oxide when molten steel solidifies in the steelmaking process. If the O content exceeds 0.0040%, the toughness of the steel plate and the weld heat-affected zone will decrease. Therefore, the O content is set to 0.0040% or less. Preferably, the O content is 0.0020% or less. Although there is no particular lower limit, the O content is preferably 0.0005% or more.

[0042] Ceq: 0.36 or higher In the lap-rolled steel sheet of the present invention, in addition to each component being within the above content range, Ceq, represented by the following formula (1), is adjusted to 0.36 or more. If Ceq is less than 0.36, it becomes difficult to increase the (211) plane concentration on the rolled surface at the center of the sheet thickness, and it also becomes difficult to ensure the strength of the lap-rolled steel sheet. From the viewpoint of ensuring the properties of the welded joint, Ceq is preferably 0.43 or less. Ceq=C+Mn / 6+Cu / 15+Ni / 15+Cr / 5+Mo / 5+V / 5...(1) Here, C, Mn, Cu, Ni, Cr, Mo, and V in formula (1) are the contents of each element. (% by mass), and if not contained, it is set to 0.

[0043] 0<{(Ca-(0.18+130×Ca)×O) / 1.25} / S≦0.8···(2) Here, Ca, O, and S represent the content of each component (mass %). Ca, O, and S must be contained so as to satisfy the relationship 0<{(Ca-(0.18+130×Ca)×O) / 1.25} / S≦0.8. When formula (2) is satisfied, a complex sulfide form is formed in which MnS precipitates on CaS. If the value of {(Ca-(0.18+130×Ca)×O) / 1.25} / S exceeds 0.8, most of the S is fixed by Ca, and MnS, which acts as a ferrite formation nucleus, does not precipitate on CaS, making it impossible to ensure toughness in the weld heat-affected zone. Furthermore, if the value of {Ca-(0.18+130×Ca)×O) / 1.25} / S is 0 or less, almost no CaS precipitates, making it impossible to ensure toughness in the weld heat-affected zone.

[0044] The above is the basic composition of the present invention, with the balance being Fe and unavoidable impurities.

[0045] In the present invention, in order to further improve the strength and toughness of the steel plate and the toughness of the weld heat-affected zone, in addition to the above-mentioned chemical composition, one or more of B, V, Nb, Ni, Cu, Cr, Mo, Mg, Zr, and REM may be contained.

[0046] B: 0.0003 to 0.0030% B is an element that generates BN in the weld heat affected zone, reduces solute N, and acts as a ferrite transformation nucleus, contributing to grain refinement. To achieve this effect, if B is contained, the B content must be 0.0003% or more. Preferably, the B content is 0.0008% or more. On the other hand, if the B content exceeds 0.0030%, the hardenability increases and the toughness of the weld heat affected zone deteriorates. Therefore, if B is contained, the B content is set to 0.0030% or less. Preferably, the B content is 0.0025% or less.

[0047] V: 0.2% or less V improves the strength and toughness of steel sheets and acts as a ferrite formation nucleus in the form of VN. However, if the V content exceeds 0.2%, it actually leads to a decrease in the toughness of the steel sheet. Therefore, when V is contained, the V content is set to 0.2% or less. Preferably, the V content is 0.05% or less. To obtain the above effects, when V is contained, the V content is preferably set to 0.01% or more.

[0048] Nb: 0.003 to 0.030% Nb is an element that is effective in ensuring the strength and toughness of steel plates and the strength of joints. However, if the Nb content is less than 0.003%, this effect is small. Therefore, if Nb is contained, the Nb content must be 0.003% or more. On the other hand, if the Nb content exceeds 0.030%, island martensite is formed in the weld heat affected zone, which deteriorates toughness. Therefore, if Nb is contained, the Nb content must be 0.030% or less.

[0049] Ni: 1.0% or less Ni increases the strength of the steel sheet while maintaining high toughness. However, when the Ni content exceeds 1.0%, this effect saturates, so when Ni is contained, the Ni content is set to 1.0% or less. To obtain the above effect, when Ni is contained, the Ni content is preferably set to 0.1% or more.

[0050] Cu: 1.0% or less Like Ni, Cu increases the strength of the steel sheet while maintaining its high toughness. However, if the Cu content exceeds 1.0%, hot embrittlement occurs, deteriorating the surface properties of the steel sheet. Therefore, if Cu is contained, the Cu content is set to 1.0% or less. Preferably, the Cu content is 0.5% or less. To obtain the above effects, if Cu is contained, the Cu content is preferably set to 0.1% or more.

[0051] Cr:0.7% or less Cr is an element effective in increasing the strength of steel sheets. However, a large amount of Cr can adversely affect the toughness of the steel sheets. Therefore, when Cr is contained, the Cr content is set to 0.7% or less. Preferably, the Cr content is set to 0.5% or less. To obtain the above effect, when Cr is contained, the Cr content is preferably set to 0.1% or more.

[0052] Mo: 0.7% or less Mo is an element effective in increasing the strength of steel sheets. However, a large amount of Mo can adversely affect the toughness of steel sheets. Therefore, when Mo is contained, the Mo content is set to 0.7% or less. Preferably, the Mo content is set to 0.5% or less. To obtain the above effect, when Mo is contained, the Mo content is preferably set to 0.1% or more.

[0053] Mg: 0.0005 to 0.0050% Mg is an element that improves the toughness of steel sheets by dispersing oxides. To achieve this effect, if Mg is contained, the Mg content must be at least 0.0005%. However, if the Mg content exceeds 0.0050%, the effect saturates. Therefore, if Mo is contained, the Mo content must be 0.0050% or less.

[0054] Zr: 0.001 to 0.020% Zr is an element that has the effect of improving the toughness of steel sheets by dispersing oxides. To achieve this effect, if Zr is contained, at least 0.001% Zr is contained. However, if Zr is contained in an amount exceeding 0.020%, the effect saturates. Therefore, if Zr is contained, the Zr content is set to 0.020% or less.

[0055] REM: 0.001 to 0.020% REM is an element that has the effect of improving the toughness of steel sheet by dispersing oxides. To exert this effect, if REM is contained, the REM content must be at least 0.001%. However, if the REM content exceeds 0.020%, the effect saturates. Therefore, if REM is contained, the REM content must be 0.020% or less.

[0056] <Collective organization> The lap-rolled steel sheet of the present invention is required to have a (211) plane integration degree on the rolled surface at the center of the thickness of the lap-rolled steel sheet of 1.2 or more in order to promote fine crack branching during brittle crack propagation, thereby slowing down and arresting the crack propagation. Furthermore, in order to cause the brittle crack to propagate while changing its propagation direction and to deviate the brittle crack propagation from the direction perpendicular to the stress load direction, the lap-rolled steel sheet has a texture in which the (200) plane integration degree on the rolled surface at the surface (within a range from the extreme surface to 1 mm below the surface) is 1.7 or more. More preferably, the (200) plane integration degree on the rolled surface at the surface (within a range from the extreme surface to 1 mm below the surface) is 2.0 or more. The lap-rolled steel sheet of the present invention can be obtained by employing the above-mentioned chemical composition and controlling the manufacturing conditions described below so that the texture satisfies the above range.

[0057] <Position of the joining interface: A position 30 to 70% of the total thickness of the two steel plates that have been integrated in the plate thickness direction> In the present invention, two slabs are stacked to form a single stacked slab, which is then rolled to form a stack-rolled steel sheet. The two integrated steel sheets refer to the stack-rolled steel sheet described above, which has a joining interface between the two steel sheets. The total thickness refers to the thickness of the stack-rolled steel sheet obtained by rolling a single stacked slab. In ordinary steel sheets, central segregation or pitting of Mn, P, etc. may occur in the center of the thickness, which can deteriorate the internal quality. However, in the stack-rolled steel sheet of the present invention, the central segregation or pitting can be avoided in the center of the thickness, ensuring internal quality and, as a result, preventing deterioration of brittle crack propagation arrestability. If the bonding interface of overlap-rolled steel plates, which is created by stacking two slabs, is less than 30% or more than 70% of the total thickness of the overlap-rolled steel plate in the thickness direction of the overlap-rolled steel plate, central segregation or cracking of one slab will be located near the center of the thickness of the overlap-rolled steel plate, and the toughness at the center of the thickness will deteriorate, thereby deteriorating the brittle crack propagation arrest properties. Therefore, the bonding interface should be located at a position between 30 and 70% of the total thickness of the overlap-rolled steel plate. The position of the bonding interface is the distance from the upper surface of the overlap-rolled steel plate to the bonding interface divided by the total thickness of the overlap-rolled steel plate.

[0058] In the lap-rolled steel sheet of the present invention, the Charpy fracture transition temperature (vTrs) at the center of the thickness of the two integrated steel sheets (lap-rolled steel sheet) is −50°C or lower, the Charpy fracture transition temperature (vTrs) at a quarter-thickness position of the two integrated steel sheets is −70°C or lower, and the Charpy fracture transition temperature (vTrs) at the surface of the steel sheet is −80°C or lower. Specifically, the Charpy fracture transition temperature (vTrs) at the center of the thickness of −50°C or lower can be achieved by suppressing the segregation of elements such as Mn and P at the center of the thickness compared to conventional materials (steel sheets manufactured from a single slab) and by controlling the manufacturing conditions. The Charpy fracture transition temperature (vTrs) at the quarter-thickness position of the two steel sheets and the Charpy fracture transition temperature (vTrs) at the surface of the steel sheet of −70°C or lower can be achieved by controlling the manufacturing conditions described below.

[0059] As described above, in the present invention, as long as the texture satisfies the above ranges by controlling the chemical composition and the manufacturing conditions described below, the lap-rolled steel sheet of the present invention has the effects of being excellent in strength, toughness, weld heat-affected zone toughness, and brittle crack propagation arrestability, even if the plate thickness is 70 mm or more.

[0060] <Manufacturing method> Molten steel with the above-mentioned chemical composition is melted in a converter or the like, formed into two slabs by continuous casting or the like, heated to a temperature of 950 to 1100°C (this temperature is the material heating temperature), hot rolled to the target thickness, cooled to room temperature, polished on one side of the two slabs, overlapped so that the polished surfaces are in contact, and fillet welded at the joint to form a single overlapping slab. Note that the above room temperature is -10°C to 50°C.

[0061] If the heating temperature of the raw material (two slabs) is less than 950°C, abnormal grain growth of austenite occurs during reheating after the production of the stacked slabs, resulting in mixed grains and a deterioration in toughness. Therefore, the heating temperature of the raw material (two slabs) is set to 950°C or higher, preferably 970°C or higher. On the other hand, if the heating temperature of the raw material (two slabs) exceeds 1100°C, the austenite grains become coarse, and the austenite also becomes coarse during reheating after the production of the stacked slabs, resulting in a decrease in toughness. Therefore, the heating temperature of the raw material (two slabs) is set to 1100°C or lower. From the viewpoint of improving the toughness of the steel plate, the heating temperature of the raw material (two slabs) is preferably 1050°C or lower. Note that the temperature of each of the two slabs means the temperature at the center of the plate thickness of each slab.

[0062] After heating, the sheet is hot rolled to a target thickness and then cooled to room temperature, but the conditions for this are not particularly specified. For example, accelerated cooling may be performed after hot rolling.

[0063] One side of two slabs is polished, and the polished surfaces are overlapped and fillet welded at the joint to form a single stacked slab. There are no specific fillet welding methods specified, but to prevent oxidation of the polished surfaces, it is preferable to use electron beam or laser welding in a vacuum. If stacked slabs are produced without polishing, poor bonding due to oxides will occur at the joint interface after rolling, leading to a decrease in internal quality and toughness, so polishing is essential.

[0064] If the heating temperature of the stacked slabs (this temperature is referred to as the product heating temperature) is less than 1000°C, it is not possible to ensure sufficient time for rolling in the austenite recrystallization temperature range. Therefore, the heating temperature of the stacked slabs is set to 1000°C or higher. On the other hand, if the product heating temperature of the stacked slabs exceeds 1150°C, the austenite grains become coarse, resulting in a decrease in toughness. Therefore, the product heating temperature of the stacked slabs is set to 1150°C or lower. From the viewpoint of improving the toughness of the steel plate, the preferred product heating temperature range is 1100°C or lower. Note that the temperature of the stacked slabs means the temperature at the center of the thickness of the steel plate (stacked slab).

[0065] In hot rolling, first, rolling is performed with a cumulative reduction of 10% or more when the temperature at the center of the thickness of the stacked slab is in the austenite recrystallization temperature range. By setting the cumulative reduction in this temperature range to 10% or more, the Charpy fracture transition temperature (vTrs) at the center of the thickness of the stacked rolled steel plate can be set to −50°C or less, and the Charpy fracture transition temperature (vTrs) at the ¼ position of the thickness of the stacked rolled steel plate can be set to −70°C or less. If the cumulative reduction is less than 10%, the austenite grains are not sufficiently refined, toughness is not improved, and these Charpy fracture transition temperatures (vTrs) cannot be achieved. There is no particular upper limit to the cumulative reduction, but if the cumulative reduction exceeds 45%, the effect of improving grain refinement is reduced, so it is preferably 45% or less. In the case of the component composition of the present invention, the austenite recrystallization temperature range is 1100 to 950°C, and the cumulative reduction when the temperature at the center of the thickness of the stacked slab is 1100 to 950°C is preferably 10% or more.

[0066] Furthermore, rolling is performed with a cumulative reduction of 60% or more when the temperature at the center of the thickness of the stacked slab is in the austenite non-recrystallization temperature range. By setting the cumulative reduction at 60% or more in this temperature range, a texture is formed in which the (211) plane integration degree on the rolled surface at the center of the thickness of the stacked rolled steel sheet is 1.2 or more, and the Charpy fracture transition temperature (vTrs) at the center of the thickness of the stacked rolled steel sheet is -50°C or less, and further, the Charpy fracture transition temperature (vTrs) at the quarter thickness position of the stacked rolled steel sheet is -70°C or less. If the cumulative reduction at this temperature range is less than 60%, a texture in which the (211) plane integration degree on the rolled surface at the center of the thickness of the stacked rolled steel sheet is 1.2 or more, or the desired Charpy fracture transition temperature (vTrs) at each thickness position cannot be achieved. There is no particular upper limit to the cumulative reduction, but it is preferably 75% or less so as not to impair rolling efficiency. In the case of the component composition of the present invention, the austenite non-recrystallization temperature range is 950 to 700°C, and the cumulative reduction when the temperature at the center of the plate thickness is 950 to 700°C is preferably 60% or more.

[0067] Furthermore, in the present invention, the cumulative reduction ratio during hot rolling is set to more than 20% when the surface temperature of the steel sheet is below the Ar3 transformation point and the temperature at the center of the thickness of the stacked slabs is above the Ar3 transformation point. This is an important requirement in the present invention, and hot rolling under these conditions can develop the (200) plane concentration at the rolled surface of the steel sheet. During this hot rolling, the surface of the steel sheet is in the two-phase region, and at the same time, the center of the thickness of the stacked slabs is in the temperature region of the austenite unrecrystallized region. By rolling with a cumulative reduction ratio of more than 20% under these conditions, it is possible to simultaneously create different textures near the surface and at the center of the thickness of the steel sheet. That is, a (200) plane texture develops near the surface of the steel sheet, and a (211) plane texture develops at the center of the thickness of the stacked rolled steel sheet. Here, both the (200) texture near the surface of the lap-rolled steel sheet and the (211) texture at the center of the thickness of the lap-rolled steel sheet are textures that make it difficult for cracks to propagate in the longitudinal direction of the steel sheet, and furthermore, the crack propagation directions of these textures are different. Therefore, the crack propagation direction is inevitably changed at the boundary between these textures, and the texture boundary further hinders crack propagation. Therefore, in the present invention, not only is a texture that makes it difficult for cracks to propagate, but also separate textures with different crack propagation directions are used, which further hinders crack propagation and results in improved crack propagation arrestability.

[0068] By performing hot rolling under these conditions, the (200) plane integration degree on the rolled surface of the lap-rolled steel sheet can be set to 1.7 or more, and the Charpy fracture transition temperature (vTrs) on the surface of the lap-rolled steel sheet can be set to -80°C or less. If the cumulative reduction rate is 20% or less when the steel sheet surface of the lap-rolled steel sheet is in this temperature range, the desired texture and (vTrs) cannot be obtained. Here, the Ar3 transformation point is expressed by the following formula (3). Ar3(℃)=910-310C-80Mn-20Cu-15Cr-55Ni-80Mo...(3) The element symbols in the above formula (3) represent the content (mass%) of each element, and those that are not included are represented as 0. The temperature range below the Ar3 transformation point that is suitable for rolling is the temperature range in which the temperature at the surface of the steel sheet is from the Ar3 transformation point to (Ar3 transformation point - 80)°C. Furthermore, the temperature range above the Ar3 transformation point that is suitable for rolling is the temperature range in which the temperature at the center of the sheet thickness is from (Ar3 transformation point + 80)°C to the Ar3 transformation point.

[0069] Furthermore, in the hot rolling of the present invention, rolling outside the temperature range specified above is not limited, and it is sufficient that rolling is performed at least in the temperature range specified above with the specified cumulative reduction rate.

[0070] After rolling, the steel sheet (layer-rolled steel sheet) is cooled to a cooling stop temperature of 500°C or less at an average cooling rate of 0.5°C / s or more. If the average cooling rate is less than 0.5°C / s, it is not possible to ensure that the (211) plane integration degree on the rolled surface at the center of the sheet thickness of the layer-rolled steel sheet is 1.2 or more. Furthermore, if the cooling stop temperature exceeds 500°C, it is not possible to obtain the desired layer-rolled steel sheet strength and texture. There is no particular upper limit to the average cooling rate, and the average cooling rate refers to the average cooling rate between 700°C and 550°C.

[0071] Furthermore, after cooling to a cooling stop temperature of 500°C or below, the material is cooled to room temperature by air cooling or at a cooling rate slower than that of air cooling. Cooling by air cooling or at a cooling rate slower than that of air cooling specifically means that the average cooling rate is 1°C / s or less. The average cooling rate mentioned here refers to the average cooling rate from the cooling stop temperature to room temperature.

[0072] Furthermore, when tempering is performed for the purpose of reducing residual stress in the lap-rolled steel sheet after cooling to a cooling stop temperature of 500°C or less, it is necessary to perform the tempering at a temperature below the Ac1 transformation point at the center of the thickness of the lap-rolled steel sheet. If the temperature at the center of the thickness during tempering is above the Ac1 transformation point, the texture developed during rolling will be lost. Note that when tempering is performed, there are no particular restrictions on the cooling conditions after cooling to a cooling stop temperature of 500°C or less. Here, the Ac1 transformation point is expressed by the following formula (4). Ac1(℃)=751-26.6C+17.6Si-11.6Mn-169Al-23Cu-23Ni+24.1Cr+22.5Mo+233Nb-39.7V-5.7Ti-895B...(4) The element symbols in formula (4) represent the content (mass%) of each element, and those that are not included are represented as 0.

[0073] In the above description, the temperature at the center of thickness is determined by heat transfer calculation from the steel sheet surface temperature measured with a radiation thermometer. The temperature condition in the cooling conditions after rolling is the temperature at the center of thickness, and the cooling rate means an average cooling rate of 700°C to 550°C calculated based on the temperature at the center of thickness.

[0074] In the lap-rolled steel plate of the present invention, the thicknesses of the two slabs used as raw materials are adjusted and controlled so that the position of the joining interface is 30 to 70% of the total thickness of the two integrated steel plates in the plate thickness direction.

[0075] Furthermore, the present invention can be carried out whether the two slabs used as raw materials have the same or different component compositions. [Example]

[0076] Next, an embodiment of the present invention will be described.

[0077] Molten steels with the chemical compositions shown in Table 1 were melted in a converter and continuously cast into slabs. These slabs were reheated, hot-rolled, and then cooled to room temperature. Two slabs with the same chemical composition were polished on one side, and the polished surfaces were overlapped and fillet-welded to form a single slab. After reheating, the slabs were hot-rolled to a thickness of 70–120 mm and further cooled to obtain the test steels (products) shown in Table 2. Table 2 shows the heating, hot-rolling, and cooling conditions. For specimens that were tempered after cooling, the tempering temperature is also shown. For specimens that were not tempered, all specimens were air-cooled to room temperature after cooling to the cooling stop temperature. The position of the joining interface was varied between 15 and 80% of the total plate thickness.

[0078] [Table 1]

[0079] [Table 2]

[0080] From the obtained lap-rolled steel sheets, JIS No. 14A test pieces with a diameter of 14 mm were taken from the 1 / 4 position of the sheet thickness so that the longitudinal axis of the test piece was parallel to the rolling direction, and a tensile test was performed to measure the yield strength (YS) and tensile strength (TS). Steel sheets with a YS of 390 MPa or more and a TS of 510 MPa or more were evaluated as having high strength (exceeding the target and passing).

[0081] JIS No. 4 impact test specimens were taken from the center and quarter positions of the thickness of the lap-rolled steel plate, and from the surface of the steel plate, with the longitudinal axis of the test specimen parallel to the rolling direction, and a Charpy impact test was performed to determine the Charpy fracture transition temperature (vTrs). Plates with a vTrs of -50°C or less at the center of the thickness, -70°C or less at the quarter position, and -80°C or less on the surface were evaluated as having good toughness (exceeding the target and passing).

[0082] In addition, to evaluate the texture of the lap-rolled steel sheets, the (211) plane integration degree on the rolled surface at the center of the sheet thickness of the lap-rolled steel sheets and the (200) plane integration degree on the rolled surface at the steel sheet surface of the lap-rolled steel sheets (the steel sheet surface refers to the range from the extreme surface to 1 mm below the surface) were measured.

[0083] The planar integration degree was measured using an X-ray diffractometer (manufactured by Rigaku Corporation) with a Mo radiation source, and the texture orientation integration degree was measured from the inverted pole figure intensity, and was calculated by taking the ratio of the X-ray diffraction intensity to that of annealed pure iron.

[0084] Next, to evaluate the brittle crack propagation arrestability, a temperature gradient ESSO test was conducted in accordance with WES2815, and Kca at -10°C (hereinafter referred to as Kca(-10°C)) was calculated. 1.5 The above results were considered to be good (exceeded the target and passed).

[0085] Furthermore, the toughness of the weld heat-affected zone (HAZ toughness) was evaluated. Figure 1 shows a cross-sectional view of a welded joint prepared under specified conditions to evaluate the toughness of the weld heat-affected zone (HAZ toughness), and Figure 2 shows the positions where Charpy test specimens were taken and notch positions in the cross-sectional view of the welded joint. In Figures 1 and 2, reference numeral 1 indicates the lap-rolled steel plate, reference numeral 2 indicates the plate thickness of the lap-rolled steel plate, reference numeral 3 indicates the ceramic backing, and reference numeral 4 indicates the water-cooled copper plate. To evaluate the toughness of the weld heat-affected zone (HAZ toughness), a joint was created using electrogas welding (EGW) with a single or multiple passes and a high heat input (450-700 kJ / cm) under the groove conditions shown in Figure 1. Charpy impact test specimens 5 and 6 were then taken from the boundary between the lap-rolled steel plate 1 and the weld metal 8, 1 mm from the front and 1 mm from the back, in the plate thickness direction, as shown in Figure 2. Using a Charpy impact test specimen with a notch 7 in the bond line, the absorbed energy (vE-20) at a test temperature of -20°C was determined. Six specimens (three each at 1 mm from the front and 1 mm from the back) with an average absorbed energy (vE-20) of 60 J or more at a test temperature of -20°C were evaluated as good (exceeding the target and passing).

[0086] Table 3 shows the results of these tests.

[0087] [Table 3]

[0088] As shown in Table 3, the lap-rolled steel sheets of the present invention have a texture in which the (211) plane integration degree on the rolled surface at the center of the thickness of the lap-rolled steel sheet is 1.2 or more, and the (200) plane integration degree on the rolled surface at the steel sheet surface is 1.7 or more, the Charpy fracture transition temperature (vTrs) at the center of the thickness is -50°C or less, the Charpy fracture transition temperature (vTrs) at the 1 / 4 position of the thickness is -70°C or less, and the Charpy fracture transition temperature (vTrs) at the steel sheet surface is -80°C or less, and the Charpy fracture transition temperature (vTrs) at the steel sheet surface is excellent in toughness, and the Kca (-10°C) is 8000N / mm 1.5 These results indicate that excellent brittle crack propagation arrest properties were obtained. Furthermore, the lap-rolled steel sheet of the present invention had an average Charpy absorbed energy (vE-20) of 60 J or more in the weld heat-affected zone for six specimens, and also had excellent toughness in the weld heat-affected zone. That is, the lap-rolled steel sheet of the present invention had a chemical composition and areal density within the ranges of the present invention, and exceeded the targets for YS, TS, Charpy fracture appearance transition temperature (vTrs) at the center of the plate thickness, Charpy fracture appearance transition temperature (vTrs) at the 1 / 4 position of the plate thickness, Charpy fracture appearance transition temperature (vTrs), Kca (-10°C), and (vE-20) on the steel plate surface, thereby passing the test.

[0089] On the other hand, the comparative examples outside the scope of the present invention did not satisfy any of YS, TS, plane integration degree (texture), vTrs, Kca (-10°C), and vE-20. [Explanation of symbols]

[0090] 1. Overlapped steel plate 2 Plate Thickness 3 Ceramic backing 4 Water-cooled copper plate 5 Charpy test specimen taken from the surface 1 mm in the thickness direction 6 Charpy test specimen taken from the back surface 1 mm in the thickness direction 7 Notch (Notch position) 8 Weld Metal

Claims

1. The steel sheet has two integrated steel plates and a joining interface between the two steel plates, The two steel plates are, by mass%, C: 0.03-0.15%, Si: 0.50% or less, Mn: 0.5-2.2%, P: 0.015% or less, S: 0.0005-0.0040%, Ti: 0.005-0.030%, Al: 0.005-0.080%, N: 0.0035-0.0075%, Ca: 0.0005-0.0030%, O: Contains 0.0040% or less, a Ceq defined by the following formula (1) is 0.36 or more, and the contents of Ca, O, and S satisfy the following formula (2), with the balance consisting of Fe and unavoidable impurities; and a (211) plane integration degree on the rolled surface at the center of the sheet thickness of the two integrated steel sheets is 1.2 or more, the (200) plane integration degree at the rolled surface on the surface of the two integrated steel plates is 1.7 or more, the Charpy fracture appearance transition temperature vTrs at the center of the plate thickness of the two integrated steel plates is -50°C or less, the Charpy fracture appearance transition temperature vTrs at a 1 / 4 position of the plate thickness of the two integrated steel plates is -70°C or less, and the Charpy fracture appearance transition temperature vTrs on the surface of the two integrated steel plates is -80°C or less, A lap-rolled steel plate having a plate thickness of 70 mm or more, wherein the position of the joining interface is included in a position of 30 to 70% of the total plate thickness of the two integrated steel plates in the plate thickness direction. Ceq=C+Mn / 6+Cu / 15+Ni / 15+Cr / 5+Mo / 5+V / 5...(1) Here, C, Mn, Cu, Ni, Cr, Mo and V in formula (1) represent the contents of each element. If no component is contained, it is set to 0. 0<{(Ca-(0.18+130×Ca)×O) / 1.25} / S≦0.8...(2) Here, Ca, O, and S represent the content (mass %) of each component.

2. The component composition is further expressed in mass % as follows: B: 0.0003 to 0.0030%, V: 0.2% or less, Nb: 0.003 to 0.030%, Ni: 1.0% or less, Cu: 1.0% or less, Cr: 0.7% or less, Mo: 0.7% or less, Mg: 0.0005-0.0050%, Zr: 0.001-0.020%, The lap-rolled steel sheet according to claim 1, further comprising one or more of REM: 0.001 to 0.020%.

3. A method for manufacturing a stack-rolled steel plate according to claim 1 or 2, comprising the steps of: heating two slabs having the above-mentioned composition to a temperature range of 950 to 1100°C, hot rolling them, cooling them to room temperature, polishing the surfaces of one side of the two slabs, stacking the two slabs so that the polished surfaces are in contact with each other, and fillet welding the joints of the two slabs to form a single stacked slab; The method for producing a stack-rolled steel plate having a thickness of 70 mm or more includes heating the stacked slabs to a temperature range of 1000 to 1150°C, and then hot rolling the stacked slabs under the conditions that the cumulative reduction rate when the temperature at the center of the thickness of the stacked slabs is in the austenite recrystallization temperature range is 10% or more, the cumulative reduction rate when the temperature at the center of the thickness of the stacked slabs is in the austenite non-recrystallization temperature range is 60% or more, and the cumulative reduction rate when the temperature at the surface of the stacked slabs is equal to or lower than the Ar3 transformation point and the temperature at the center of the thickness of the stacked slabs is equal to or higher than the Ar3 transformation point is more than 20%, and then cooling the stacked slabs at an average cooling rate of 0.5°C / s or more to a cooling stop temperature of 500°C or less, and then cooling the stacked slabs to room temperature at an air-cooling or a cooling rate slower than air-cooling.

4. A method for manufacturing a stack-rolled steel plate according to claim 1 or 2, comprising the steps of: heating two slabs having the above-mentioned composition to a temperature range of 950 to 1100°C, hot-rolling them, cooling them to room temperature, polishing the surfaces of one side of the two slabs, stacking the two slabs so that the polished surfaces are in contact with each other, and fillet-welding the joints of the two slabs to form a single stacked slab; The stacked slabs are heated to a temperature range of 1000 to 1150 ° C., and then hot-rolled under the conditions that the cumulative reduction rate when the temperature at the center of the thickness of the stacked slabs is in the austenite recrystallization temperature range is 10% or more, the cumulative reduction rate when the temperature at the center of the thickness is in the austenite non-recrystallization temperature range is 60% or more, and the cumulative reduction rate when the temperature at the surface of the stacked slabs is below the Ar3 transformation point and the temperature at the center of the thickness is above the Ar3 transformation point is more than 20%, and then cooled at an average cooling rate of 0.5 ° C. / s or more to a cooling stop temperature of 500 ° C. or less, After cooling to the cooling stop temperature of 500 ° C. or less, tempering is performed at a temperature at which the temperature at the center of the plate thickness of the stacked slab is lower than the Ac1 transformation point.

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