Clad steel plate, its manufacturing method and welded structure

Abstract

To provide a clad steel sheet having an excellent fracture toughness value of a joined surface and its manufacturing method.SOLUTION: There is provided a clad steel sheet excellent in fracture toughness value of a joined surface, the clad steel sheet having a width of a region having a nano-hardness of 7 GPa or more of 5 μm or less and a CTOD value of the clad interface of 0.15 mm or more, in an interface between a cladding material and a base material of the clad steel sheet where the cladding material is stainless steel or Ni-based alloy and the base material is carbon steel or low-alloy steel. Since a martensite region with low toughness is small on the joined surface of the clad steel sheet, interfacial separation can be prevented even if a weld defect occurs at the interface during welding.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a clad steel plate having excellent fracture toughness at the joining surface, a method for manufacturing the same, and a welded structure. [Background technology]

[0002] Stainless steels and Ni-based alloys have excellent corrosion resistance, making them suitable for use in severely corrosive environments. Examples of such corrosive environments include oil well environments, high-chloride environments such as those exposed to seawater or brackish water, and plant equipment and chemical tankers exposed to various acid solutions. In these corrosive environments, stainless steels and Ni-based alloys are used in seawater desalination plants, flue gas desulfurization systems, chemical storage tanks, structural components such as oil well tubular goods (OCTG), pumps and valves, and heat exchangers.

[0003] On the other hand, stainless steels and Ni-based alloys contain large amounts of alloying elements such as Cr, Ni, and Mo to ensure corrosion resistance, and compared to carbon steels and low-alloy steels, the costs of not only the material but also processing and welding are high. Furthermore, prices can fluctuate significantly due to rising prices of alloying elements. Therefore, their use is sometimes restricted, mainly due to cost.

[0004] As mentioned above, when considering the cost aspect, it is effective to use clad steel plate as a material from the viewpoint of processing and welding. Clad steel plate is a material made by bonding two or more different types of metals together. Compared to steel plate made only of high alloy steel (hereinafter referred to as "solid steel plate"), clad steel plate can reduce the amount of high alloy steel used, thereby reducing material costs. In addition, since less dissimilar material welding is required, the cost of welding materials can also be reduced.

[0005] Furthermore, in clad steel plates, by bonding a material with excellent properties to the base material (hereinafter, the bonded material will be referred to as the "clad material"), it is possible to obtain the excellent properties of both the clad material and the base material.

[0006] For example, a high-alloy steel having the properties required for the application environment may be used for the cladding material, and a carbon steel or low-alloy steel having the toughness and strength required for the base material may be used. In this case, not only can costs be reduced as described above, but properties equivalent to those of a solid steel plate and strength and toughness equivalent to those of carbon steel and low-alloy steel can be ensured. This allows for both economical and functional compatibility.

[0007] For these reasons, the need for clad steel plates made of stainless steel or Ni-based alloys has been increasing in recent years in various industrial fields. However, when using clad steel plates, it is important to prevent delamination at the joint between the clad material and the base material. If delamination occurs between the clad material and the base material during use, desired properties such as corrosion resistance and strength may not be achieved. Furthermore, there may be a risk of holes or collapse of the structure, for example.

[0008] In clad steel plates made of stainless steel or Ni-based alloys, Cr and Ni diffuse from the clad material to the base material, and C diffuses from the base material to the clad material during heating during rolling, creating a diffusion layer of elements at the interface. The concentration of each element in the diffusion layer gradually changes, but depending on the element concentration, martensitic transformation may occur during cooling after rolling in a region where the temperature at which martensitic transformation begins is high and the critical cooling rate for martensitic transformation is slow.

[0009] Patent Document 1 discloses a technique for suppressing sensitization near the interface of a duplex stainless clad steel sheet by controlling the thickness of the carbon diffusion layer at the interface, but does not disclose any description of the martensite phase at the interface.

[0010] Patent Document 2 discloses a technology for preventing delayed fracture of austenitic stainless clad steel sheets by softening the martensite at the interface by specifying the temperature and time of tempering after rolling. However, since the addition of a tempering process leads to increased costs, a technology for improving the fracture toughness of the martensite at the interface without tempering is practically required, but no solution to this problem is disclosed or suggested.

[0011] In normal use, the martensite at the interface does not affect interfacial delamination. However, if a defect occurs in the martensite at the interface, for example, through welding, stress is generated at the interface due to structural stress, deformation during welding, and transformation of the base material near the weld, and even if the defect is minute, the combination of these factors may cause large brittle cracks.

[0012] When strength is required that will not cause peeling failure at the bonding interface between the clad material and the base material against such acting forces, a strength evaluation test is used that targets the bonding interface of clad steel plates.

[0013] The most basic test is the shear strength test specified in JIS standard G0601 (Testing methods for clad steel) (Non-Patent Document 1). Although this test is very simple and easy to carry out, it has been pointed out to have many problems, such as the fact that the loading pattern in actual welded structures is not necessarily shear mode, the evaluation value obtained is insensitive to the interface properties, and the test is strongly affected by deformation at the loading point (side surface of the clad material), and therefore it is not a highly reliable evaluation method.

[0014] The peel test described in the JIS standard G0601, for example, solves the problem of the loading method in the shear strength test. In this test, if appropriate test specimen dimensions are used, the interfacial peel strength against a tensile load in the thickness direction can be obtained. However, because shear can occur not only at the clad joint interface but also at the clad material itself, it is necessary to conduct tests with various dimensions in advance to find the appropriate test specimen dimensions.

[0015] Furthermore, both of the above JIS standard G0601 tests are strength tests in which defect-free material is pulled. However, actual welded structures may have small weld defects that are overlooked and deemed to pass non-destructive testing. Furthermore, interfacial delamination may occur during welding construction. To prevent these construction defects from causing fatal damage during use, it is useful to manage allowable defects using fracture mechanics, which is where interfacial fracture toughness becomes necessary.

[0016] Crack Tip Opening Displacement (CTOD) evaluation is known as one of the tests for determining the fracture toughness (fracture resistance) of materials with cracks. The CTOD test is a test for determining the crack tip opening (critical CTOD value) at which unstable fracture occurs. As a test for evaluating the fracture toughness of the bonding interface of clad steel plates, for example, as shown in Non-Patent Document 2, a supplementary member is attached to each of the clad material and the base material by multi-layer welding, and then a fracture toughness test piece, typified by a CT test piece, is taken and evaluated. [Prior art documents] [Patent documents]

[0017] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-209688 [Patent Document 2] Japanese Patent Application Publication No. 6-7803 [Non-patent literature]

[0018] [Non-Patent Document 1] Japanese Industrial Standards, JIS G0601:2012 "Test methods for clad steel" [Non-patent document 2] Junichi Katsuta et al., "Fracture toughness and fatigue crack propagation characteristics of bonded interfaces of duplex stainless clad steel plates for chemical tankers," Pressure Technology, Vol. 57, No. 1, 2019, pp. 4-12 Summary of the Invention [Problem to be solved by the invention]

[0019] As a result of extensive research, the present inventors have found the following problems to be solved. When CTOD evaluation is applied to evaluate the fracture toughness of the joint interface of clad steel plates using the method described in Non-Patent Document 2, the welding residual stress distribution introduced by multi-layer welding is large, and the length of the fatigue pre-crack introduced before the fracture toughness test varies significantly depending on the thickness position of the CT test specimen. Furthermore, because the test specimen has welding residual stress as an internal force, it is difficult to accurately calculate the fracture toughness value. Furthermore, fatigue pre-cracks aimed at the clad interface often deviate toward the base material, resulting in the fracture toughness of the base material rather than the clad interface. As a result, the true fracture toughness value of the clad interface cannot be obtained.

[0020] In order to evaluate the fracture toughness at the interface between the clad steel clad material and the base metal, it is first necessary to weld a supplementary member to obtain the dimensions of the fracture toughness test specimen, such as a CT test specimen. When multi-layer welding is performed using conventional steel plate welding methods (e.g., shielded arc welding, gas-shielded arc welding, submerged arc welding, TIG welding, etc.), a large welding residual stress distribution occurs around the welding. As a result, residual stresses that vary significantly depending on the thickness position of the test specimen are also generated in the fracture toughness evaluation region (the interface between the clad steel clad material and the base metal). As a result, the length of the fatigue pre-crack introduced before the fracture toughness test varies significantly depending on the thickness position of the test specimen. Furthermore, since the test piece has welding residual stress as an internal force, it is difficult to accurately calculate the fracture toughness value from the external load. Therefore, it is necessary to join the clad steel plate and the supplementary member using a joining method that reduces the distribution of welding residual stress in the vicinity of the fracture toughness evaluation portion.

[0021] Furthermore, in clad steel plates, the hardness of the clad material and the base material generally do not match perfectly, resulting in differences in hardness at each location. In this case, even if a fatigue precrack is introduced through a mechanical notch at the clad joint interface, the fatigue precrack will deviate toward the softer side. In many cases, the carbon steel side of the base material is softer than the stainless steel side of the clad material, and the fatigue precrack deviates toward the carbon steel, resulting in the fracture toughness value of the carbon steel. Therefore, measures must be taken to prevent the fatigue precrack from deviating from the clad joint interface. Therefore, first, we clarified the CTOD evaluation method for evaluating the fracture toughness of the bonded surface of clad steel plates.

[0022] The inventors have recognized that in the fracture toughness test of the bonded surface clarified above, the lower the hardness of martensite, the higher the fracture toughness, and further, that the larger the width of martensite in the diffusion layer, the higher the risk that a small crack will lead to large interfacial delamination. Furthermore, the inventors have found that the problem to be solved in order to suppress interfacial delamination of the clad is to control the hardness and width of the martensite at the interface.

[0023] In view of the above-mentioned problems, the present invention aims to provide a clad steel plate having excellent fracture toughness at the joining interface, a manufacturing method thereof, and a welded structure. [Means for solving the problem]

[0024] The inventors recognized that in order to evaluate fracture toughness at the bond interface between the clad steel plate and the base metal, it is necessary to avoid multi-layer welding when welding a supplementary member to the clad steel plate and use a single-pass full-penetration weld to minimize the residual stress distribution in the thickness direction of the fracture toughness test specimen, and to provide appropriate side grooves on the side of the test specimen along the clad bond interface so that fatigue pre-cracks do not deviate from the clad bond interface. Furthermore, when cutting out a fracture toughness test specimen from the welded joint, it is preferable to reduce the thickness of the test specimen to minimize the residual stress distribution in the thickness direction of the test specimen.

[0025] The present invention has been made to solve the above-mentioned problems, and is summarized as follows: a clad steel plate, a manufacturing method thereof, and a welded structure. [1] A clad steel plate comprising a base material and a clad material joined to the base material, the base material is made of carbon steel or low alloy steel, the cladding material is made of a corrosion-resistant alloy, At the interface between the base material and the clad material of the clad steel plate (hereinafter referred to as the "clad interface"), the width in the plate thickness direction of the region where the nanohardness is 7 GPa or more is 5 μm or less, A clad steel plate characterized in that, in evaluating the fracture toughness of the clad interface, a supplementary member is welded in one pass by electron beam or laser welding, and the CTOD value of the clad interface measured using a CT test piece in which a side groove is provided along the clad interface is 0.15 mm or more at 0°C. [2] The clad steel plate according to [1], wherein the base metal has a chemical composition containing, by mass%, C: 0.020 to 0.200%, Si: 1.00% or less, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.050% or less, Nb: 0.001 to 0.200%, and N: 0.020% or less, and has a Ceq of 0.20 to 0.50, with the balance being Fe and impurities, where Ceq is defined by the following formula (1): Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5...(1) In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass%) of each element in the chemical composition of the base steel sheet. [3] The chemical composition of the base metal further contains, in mass%, Ni: 0.01 to 3.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 0.50%, W: 0.01 to 1.00%, Cu: 0.01 to 2.00%, Co: 0.01 to 0.50%, Se + Te: 0.01 to 0.10%, V: 0.001 to 0.100%, Ti: 0.00 The clad steel plate according to [2], containing one or more selected from the group consisting of Al: 0.1 to 0.200%, Al: 0.005 to 0.300%, Ca: 0.0003 to 0.0100%, B: 0.0003 to 0.0030%, Mg: 0.0003 to 0.0100%, Zr+Hf+Ta: 0.0001 to 0.0100%, and REM: 0.0003 to 0.0100%. [4] The clad steel plate according to any one of [1] to [3], wherein the clad steel plate has a cladding material which is a stainless steel or a nickel-based alloy containing 10% or more of Cr by mass.

[0026] [5] In the clad steel plate according to any one of [1] to [4], a base material and a cladding material are laminated so that the bonding surface is vacuum, and the four peripheries of the bonding surface are sealed by welding to form a clad material, and a clad rolled material assembled from one or more of the clad materials is subjected to heating and hot rolling such that d calculated by formula (2) is 1 to 9, where d is the maximum heating temperature T (°C) in the heating furnace, the time t (minutes) from the time the heating temperature in the heating furnace reaches the maximum heating temperature T-20°C until the heating furnace extraction, and the rolling reduction ratio r is calculated by the material thickness / product thickness, and after rolling, the material is cooled at an average cooling rate of 2°C / s or more in the TA3 (°C) to 650°C range of the material calculated by formula (3), and the width in the plate thickness direction of the region where the nanohardness at the interface between the base material and the cladding material is 7 GPa or more is 5 μm or less, and the CTOD value at the clad interface is 0.15 mm or more at 0°C. d=2.2×10 5 ×(√(exp(-3.2×10 4 / (T+273))×t)) / r...Equation (2) TA3(℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (3) In the formula, C, Si, Mn, Ni, Ti, Nb, and Al are the contents (mass%) of each element in the chemical composition of the base steel sheet.

[0027] [6] A welded structure made using the clad steel plate according to any one of [1] to [4]. [Effects of the Invention]

[0028] According to the present invention, it is possible to obtain a clad steel plate having a good fracture toughness value of the bonded surface. [Brief explanation of the drawings]

[0029] [Figure 1] 1A and 1B are diagrams showing the arrangement of the raw material (steel billet) of the test piece of the present invention, where (A) is a side view and (B) is a front view. [Figure 2]1A and 1B are diagrams showing the position where one full-pass welding is performed in a welded joint, where (A) is a side view and (B) is a front view. [Figure 3] 1A and 1B are diagrams showing positions from which test pieces of the present invention (CT test pieces) are cut out of a welded joint, where (A) is a side view and (B) is a front view. [Figure 4] 1A and 1B are diagrams showing a CT test piece, where (A) is a side view and (B) is a front view. DETAILED DESCRIPTION OF THE INVENTION

[0030] The present inventors have conducted the following investigations to address the above-mentioned problems. Specifically, the inventors have investigated the element diffusion and metal structure at the interface of clad steel plates made of various stainless steels and Ni-based alloys by changing the heating temperature, heating time, reduction ratio, and cooling rate after rolling, and evaluated the relationship with the fracture toughness value of the interface. As a result, the following findings (a) to (c) have been obtained.

[0031] (a) The thinner the region of the clad steel plate where the nanohardness at the interface is 7 GPa or more, the higher the fracture toughness value tends to be. Therefore, it is effective to make the region of 7 GPa or more 5 μm or less.

[0032] (b) In the rolled material of clad steel plate, the base material, carbon steel or low-alloy steel, is in contact with the cladding material, stainless steel or Ni-based alloy. The alloying element profile at the interface could be organized by the material heating temperature and time and the rolling reduction ratio. Furthermore, when cladding materials containing 10% or more Cr by mass were used, it was confirmed that the Cr diffusion width corresponded to the martensite phase width. This is because Cr diffuses the fastest among the main alloying elements and is an element that also enhances hardenability. Therefore, martensitic transformation occurs in regions where only the Cr content is high and the content of austenite-stabilizing elements such as Ni is low.

[0033] (c) The hardness of the martensite at the interface is affected by the cooling rate after rolling. The mechanism is thought to be as follows. If the cooling rate after rolling is slow and carbon is expelled and diffused during the austenite to ferrite transformation or austenite to ferrite + pearlite transformation, the carbon dissolved in the austenite phase will concentrate on the cladding material side, which contains a lot of Cr and has a low carbon activity coefficient. In this case, the degree of concentration will be greater if the cladding material side is the austenite phase. Due to this mechanism, if the cooling rate after rolling is slow, a region with a high carbon concentration will form near the interface. If this region overlaps with an area where martensite phase can form, a hard martensite phase will form at the interface of the clad steel plate, reducing the fracture toughness of the interface.

[0034] Therefore, in order to obtain a clad steel plate with excellent fracture toughness at the joining surface, it is necessary to control the Cr diffusion during heating and the C diffusion during cooling after rolling. The present invention was made based on the above findings. Each requirement of the present invention will be described in detail below.

[0035] 1. Configuration of the present invention The clad steel plate according to the present invention comprises a base material and a clad material bonded to the base material. The base material is made of carbon steel or low-alloy steel as described below. The clad material is made of a corrosion-resistant alloy, such as stainless steel containing 10% or more of Cr or a Ni-based alloy. Furthermore, the width of a region at the bonding surface (clad interface) between the base material and the clad material where the nano-hardness is 7 GPa or more is 5 μm or less, and the CTOD value of the clad interface at 0°C is 0.15 mm or more.

[0036] 2. Characteristics of the cladding interface The interfacial characteristics of the clad steel plate according to the present invention will now be described. In order to obtain a clad steel plate having a good fracture toughness value at the bonded surface, it is necessary to suppress the formation of a hard martensite phase at the clad interface.

[0037] 2-1. Nano-hardness of clad interface The width of the region with a nanohardness of 7 GPa or more at the clad interface is 5 μm or less. If the width in the plate thickness direction of the region with a nanohardness of 7 GPa or more exceeds 5 μm, the region of hard martensite with low fracture toughness is large, and when welding defects and stress in the welded structure are combined, the interface may peel off. It is preferably 3 μm or less, and more preferably 1 μm or less. The smaller the region with a nanohardness of 7 GPa or more, the higher the fracture toughness, so no lower limit is set. Here, nanohardness means the hardness of a material evaluated in accordance with the instrumented indentation hardness test (also called nanoindentation test) specified in ISO 14577.

[0038] 2-2. CTOD value of cladding interface In the present invention, the CTOD value of the clad interface shown below is used as an evaluation method for evaluating the fracture toughness value of the bonded surface. The CTOD value at the clad interface is 0.15 mm or more at 0° C., preferably 0.20 mm or more, and more preferably 0.30 mm or more. The higher the CTOD value, the better the fracture toughness, so no upper limit is set. Here, the CTOD value of the cladding interface is a value measured using a CT test piece in the following form when evaluating the CTOD (see Figs. 1 to 4).

[0039] In order to minimize the residual stress distribution in the thickness direction of the fracture toughness test piece (CT test piece 12), the supplementary members (4, 5) are welded to the clad steel plate 1 by single-pass full-penetration welding, avoiding multi-layer welding. Specifically, electron beam welding or laser welding is used.

[0040] In addition, to prevent fatigue pre-cracks from diverging from the clad bond interface, appropriate side grooves (side grooves 8) are provided on the side of the CT specimen 12 along the clad interface 10. The side grooves 8 also serve to prevent shear fractures on the side of the specimen.

[0041] Furthermore, when cutting out a fracture toughness test piece (CT test piece 12) from the welded joint, it is preferable to reduce the thickness of the test piece in order to reduce the residual stress distribution in the thickness direction of the test piece. However, an excessively thin test piece will be in a plane stress state, resulting in a shear-type fracture mode, and fracture toughness will not be obtained. It is preferable to have an appropriate test piece thickness so that the opening-type fracture mode becomes dominant. For this purpose, the test piece thickness (width W) is preferably 10 mm or more and 15 mm or less.

[0042] FIG. 1 shows the layout of the material (steel billet) for the test piece of the present invention. Clad steel plate 1 consists of clad material 2 of thickness H2 and base material 3 of thickness H3, which is cut to a length L x width W. A supplementary member 4 of height H4 made of the same material as base material 3 is butted against base material 3 with an area (length L x width W) roughly the same as the surface of base material 3. A supplementary member 5 of height H5 made of the same material as clad material is also butted against clad material 2 with an area (length L x width W) roughly the same as the surface of base material 3. A single-pass weld is performed from the side to penetrate width W, resulting in a welded joint 13 as shown in FIG. 2. Here, weld line 6 is the single-pass full-fill weld.

[0043] A fracture toughness test specimen is cut out from the welded joint 13 along the CT test specimen cutting position 7 in Figure 3, and the CT test specimen 12 in Figure 4 is taken. At this time, the cutting is performed so that the center line of the mechanical notch 11 is aligned with the clad interface 10. The thickness (width W) of the CT test specimen 12 is preferably set to 10 mm or more and 15 mm or less.

[0044] Furthermore, side grooves 8 are machined on both sides of the CT test piece 12 along the cladding interface 10 to obtain the CT test piece 12 shown in Figure 4. The dimensions of the side grooves 8 are, for example, an angle of 45 degrees, a curvature radius at the bottom of 0.25 mm, and a depth of 1 mm.

[0045] Then, a fatigue pre-crack 9 is introduced from the bottom of the mechanical notch 11 along the clad interface 10 to produce a fracture toughness test piece (CT test piece 12) for evaluation of the clad steel plate bonding interface. Although the details of the conditions for introducing the fatigue pre-crack 9 are not limited, it is preferable that the conditions comply with ISO standard 15653, for example.

[0046] As described above, in the present invention, when evaluating the fracture toughness of the clad interface 10, the supplementary members (4, 5) are welded in one pass by electron beam or laser welding, and the CTOD value of the clad interface is measured using a CT test piece 12 in which a side groove 8 is provided along the clad interface 10.

[0047] 3. Chemical composition of the base material The base material is made of carbon steel or low-alloy steel. The base material preferably has a chemical composition containing, by mass%, C: 0.020 to 0.200%, Si: 1.00% or less, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.050% or less, Nb: 0.001 to 0.200%, and N: 0.020% or less, with Ceq being 0.20 to 0.50, and the balance being Fe and impurities. Here, Ceq is defined by the following formula (1): Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5...(1) In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass%) of each element in the composition of the base material.

[0048] C is an element that improves the strength of steel, and a content of 0.020% or more provides sufficient strength. However, if it exceeds 0.200%, it will cause deterioration in weldability and toughness. Therefore, the C content is set to 0.020 to 0.200%, preferably 0.040% or more, and more preferably 0.050% or more. On the other hand, the upper limit is preferably 0.100% or less, and even more preferably 0.080% or less. A more preferable range is 0.040% to 0.100%, and an even more preferable range is 0.050% to 0.080%.

[0049] Silicon is effective for deoxidation and is an element that improves the strength of steel. However, if the silicon content exceeds 1.00%, it will deteriorate the surface quality and toughness of the steel. Therefore, the silicon content is set to 1.00% or less, and preferably 0.50% or less. Silicon may not be contained. The preferred lower limit of silicon content is 0.01%.

[0050] Mn is an element that increases the strength of steel, and this effect is manifested when the content is 0.10% or more. However, if the content exceeds 3.00%, weldability is impaired and the alloy cost increases. Therefore, the Mn content is set to 0.10 to 3.00%, preferably 0.50 to 2.00%, and more preferably 0.90 to 1.60%.

[0051] P is an impurity in steel, and if the content exceeds 0.050%, toughness deteriorates. Therefore, the P content is set to 0.050% or less, preferably 0.015% or less.

[0052] S is an impurity in steel, and if the content exceeds 0.050%, toughness deteriorates. Therefore, the S content is set to 0.050% or less, preferably 0.010% or less.

[0053] Nb is an element that raises the recrystallization temperature, and this effect is manifested when added at 0.001% or more. However, if it exceeds 0.200%, weldability is impaired and the alloy cost increases. Therefore, the Nb content is set to 0.001 to 0.200%, preferably 0.005 to 0.100%, and more preferably 0.010 to 0.050%.

[0054] N is an impurity in steel, and if the content exceeds 0.020%, ductility and toughness deteriorate. Therefore, the N content is set to 0.020% or less. Since the lower the N content, the better, there is no particular lower limit for the content. However, excessively low N content leads to increased costs, so the N content is preferably 0.0005% or more.

[0055] Ceq (carbon equivalent) is a value used to estimate hardness and weldability from the chemical composition of steel, and is calculated using formula (1). The higher Ceq, the better the hardness and the worse the weldability. If Ceq is less than 0.20, sufficient strength for a structure cannot be obtained. Therefore, Ceq is set to 0.20 or more, and preferably 0.23 or more. If Ceq exceeds 0.50, weldability deteriorates, and interpass temperature control and post-heat treatment become necessary, increasing welding costs. Therefore, Ceq is set to 0.50 or less, and preferably 0.40 or less, and more preferably 0.35 or less. Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5...(1) In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass%) of each element in the chemical composition of the base material.

[0056] The composition of the base metal further includes, in mass %, replacing a portion of the Fe, Ni: 0.01 to 3.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 0.50%, W: 0.01 to 1.00%, Cu: 0.01 to 2.00%, Co: 0.01 to 0.50%, Se+Te: 0.01 to 0.10%, V: 0.001 to 0.100%, Ti: It contains one or more selected from the group consisting of Zn: 0.001 to 0.200%, Al: 0.005 to 0.300%, Ca: 0.0003 to 0.0100%, B: 0.0003 to 0.0030%, Mg: 0.0003 to 0.0100%, Zr+Hf+Ta: 0.0001 to 0.0100%, and REM: 0.0003 to 0.0100%.

[0057] Ni is an element that improves the hardenability of steel, and improves the strength and toughness of the steel after rolling. However, if it exceeds 3.00%, it causes deterioration of weldability and toughness. Therefore, when Ni is contained, the Ni amount is set to 3.00% or less. It is preferably 1.00% or less, more preferably 0.50% or less, and even more preferably 0.30% or less. The preferred lower limit of the Ni content is 0.01%.

[0058] Cr is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if the content exceeds 1.00%, it causes deterioration of weldability and toughness. Therefore, if Cr is contained, the Cr content should be 1.00% or less, preferably 0.50% or less, and more preferably 0.30% or less. The preferred lower limit of the Cr content is 0.01%.

[0059] Mo is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if it exceeds 0.50%, it causes deterioration of weldability and toughness. Therefore, when Mo is contained, the Mo content is set to 0.50% or less, preferably 0.30% or less, and more preferably 0.1% or less. The preferred lower limit of the Mo content is 0.01%.

[0060] W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. If the W content exceeds 1.00%, hot workability deteriorates, resulting in a decrease in productivity. Therefore, when W is contained, the W content is set to 1.00% or less, preferably 0.50% or less, and more preferably 0.30% or less. The preferred lower limit of the W content is 0.01%.

[0061] Cu is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if it exceeds 2.00%, it causes deterioration of weldability and toughness. Therefore, when Cu is contained, the Cu content is set to 2.00% or less. It is preferably 1.00% or less, more preferably 0.50% or less, and even more preferably 0.30% or less. The preferred lower limit of the Cu content is 0.01%.

[0062] Co is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if the content exceeds 0.50%, hot workability is impaired, resulting in a decrease in productivity. Therefore, when Co is contained, the Co content is set to 0.50% or less, preferably 0.30% or less, and more preferably 0.1% or less. The preferred lower limit of the Co content is 0.01%.

[0063] Se and Te inhibit easily oxidizable elements such as Mn, Si, and Al in steel sheet from diffusing to the steel sheet surface and forming oxides, thereby improving the surface quality and platability of the steel sheet. However, this effect saturates when the total content exceeds 0.10%. Therefore, when Se and Te are added, the total amount of Se and Te should be 0.10% or less, and more preferably 0.05% or less. The preferred lower limit of the Se + Te content is 0.01%.

[0064] Al is an element that is effective in deoxidizing steel. However, if the content exceeds 0.300%, it can cause a deterioration in the toughness of welds. Therefore, if Al is contained, the Al content should be 0.300% or less, preferably 0.100% or less. The preferred lower limit of the Al content is 0.005%.

[0065] V increases the strength of steel by forming carbonitrides. However, if it exceeds 0.100%, it causes deterioration of weldability and toughness. Therefore, if V is contained, the V content should be 0.100% or less, preferably 0.050% or less. The preferred lower limit of the V content is 0.001%.

[0066] Ti is an element that refines crystal grains and increases strength, and this effect is manifested when added at 0.001% or more. However, if it exceeds 0.200%, weldability is impaired and alloy costs increase. Therefore, the Ti content is set to 0.001 to 0.200%, preferably 0.005 to 0.100%, and more preferably 0.010 to 0.050%.

[0067] Ca is an element that refines the structure of the weld heat affected zone and improves toughness. However, if it exceeds 0.0100%, it forms coarse inclusions and deteriorates toughness. Therefore, when Ca is contained, the Ca content is set to 0.0100% or less, preferably 0.0050% or less, and more preferably 0.0030% or less. The preferred lower limit of the Ca content is 0.0003%.

[0068] B is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if it exceeds 0.0030%, it causes deterioration of weldability and toughness. Therefore, when B is contained, the B content is set to 0.0030% or less, preferably 0.0015% or less. The preferred lower limit of the B content is 0.0003%.

[0069] Mg is an element that improves ductility and toughness by controlling the morphology of sulfide-based inclusions. However, if the content exceeds 0.0100%, the amount of nonmetallic inclusions increases, and ductility and toughness decrease. Therefore, when Mg is contained, the content is set to 0.0100% or less. It is preferably 0.0050% or less, and more preferably 0.0030% or less. The preferred lower limit of the Mg content is 0.0003%.

[0070] Zr, Hf, and Ta are elements that contribute to improving formability. If the total content of one or more of Zr, Hf, and Ta exceeds 0.01%, ductility may decrease. Therefore, when one or more of Zr, Hf, and Ta are contained, the total content is set to 0.0100% or less, preferably 0.0070% or less. The preferred total content of one or more of Zr, Hf, and Ta is 0.0001%.

[0071] REM refines the structure of the weld heat affected zone and improves toughness. However, if it exceeds 0.0100%, it forms coarse inclusions, degrading toughness. Therefore, if REM is contained, the REM content should be 0.0100% or less, preferably 0.005% or less. The preferred lower limit of the REM content is 0.0003%.

[0072] Here, REM is a collective term for 17 elements, including 15 lanthanoid elements plus Y and Sc. One or more of these 17 elements can be contained in a steel material, and the REM content refers to the total content of these elements.

[0073] The balance of the chemical composition of the base metal of the present invention is Fe and impurities. Here, "impurities" refer to components that are mixed in during industrial production of steel due to various factors in raw materials such as ores and scraps, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention.

[0074] 4. The corrosion-resistant alloy is a stainless steel or nickel-based alloy containing 10% or more Cr. The cladding material of the present invention is made of a corrosion-resistant alloy. As described above, the corrosion-resistant alloy contains a large amount of Cr, and the diffusion of this Cr improves the hardenability of the cladding interface, facilitating transformation to martensite. At the same time, carbon from the base material diffuses to the cladding material, forming a hard martensite phase at the base material interface, which reduces the fracture toughness of the bonding interface. In other words, the effects of the present invention are achieved when a corrosion-resistant alloy with a large amount of Cr is used. When the Cr content of the cladding material is 10% or more, the effects of applying the present invention are significantly enhanced. When the Cr content is 15% or more, the effects are even more significantly enhanced.

[0075] The present invention relates to a clad steel plate with excellent fracture toughness at the bonded interface due to control of the bonded interface structure, and a manufacturing method thereof. The type of clad material is not particularly specified, but examples of clad materials include stainless steel and nickel-based alloys. Stainless steels include austenitic stainless steel, ferritic stainless steel, and duplex stainless steel, while nickel-based alloys include various alloy compositions under the trade names Inconel, Incoloy, Hastelloy, etc.

[0076] 5. Manufacturing method The method for manufacturing a clad steel plate according to the present invention will now be described. As mentioned above, in order to obtain a good fracture toughness value at the bonding surface, it is necessary to control the metal structure. Such a metal structure can be achieved by combining the chemical composition of the steel with appropriate manufacturing conditions. In the above-mentioned clad steel plate, the base material and cladding material are stacked so that the bonding surface is vacuum, and the four periphery of the bonding surface is sealed by welding to form a clad material, and the clad rolled material assembled from one or more clad materials is subjected to heating and hot rolling such that d calculated by equation (2) is 1 or more and 9 or less, based on the maximum heating temperature T (°C) in the heating furnace, the time t (minutes) from the time the heating temperature in the heating furnace reaches the maximum heating temperature T-20°C to the time of removal from the heating furnace, and the rolling reduction ratio r calculated by material thickness / product thickness, and after rolling, the material is cooled at an average cooling rate of 2°C / s or more in the TA3 (°C) to 650°C range of the material calculated by equation (3), to produce a clad steel plate. d=2.2×10 5 ×(√(exp(-3.2×10 4 / (T+273))×t)) / r...Equation (2) TA3(℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (3)

[0077] 5-1. Clad materials The clad material is manufactured by the method described below. Specifically, carbon steel and low-alloy steel as the base material and a corrosion-resistant alloy as the cladding material are melted by a known method such as a converter, electric furnace, or vacuum melting furnace, and then slabs are produced by a continuous casting method or an ingot casting-blooming method. The resulting slabs are hot-rolled under commonly used conditions to produce hot-rolled sheets as the cladding material and base material. The resulting hot-rolled sheets may be annealed, pickled, polished, etc., as necessary. The clad material is assembled by laminating the cladding material and base material so that the bonding surface is vacuum, and then sealing the four periphery of the bonding surface by welding. An insert material such as Ni foil may be inserted between the cladding material and base material to improve adhesion and interfacial corrosion resistance. The method for vacuuming the bonding surface is not particularly specified, but examples include electron beam welding in a vacuum, or pre-drilling a vacuum hole, welding the four periphery in air by arc welding or laser welding, and then vacuuming with a vacuum pump. A vacuum level of 0.1 Torr or less results in a good bonded interface with minimal interfacial oxides, and a vacuum level of 0.05 Torr or less is more preferable. Since a higher vacuum level tends to result in a better bonded interface, no lower limit is set. The obtained clad material may be subjected to hot rolling as it is as a clad rolling material, or two clad materials may be stacked together with a release agent applied between them, and then the stacked clad material may be subjected to hot rolling. When stacking two materials, it is desirable that the base materials and clad materials are of equal thickness to reduce plate warpage during cooling. Of course, the assembly method does not have to be limited to the one described above.

[0078] 5-2.Hot rolling Next, the obtained clad rolled material is heated and hot rolled such that d, calculated using Equation (3) based on the maximum heating temperature T (°C) in the heating furnace, the time t (minutes) from when the heating temperature in the heating furnace reaches the maximum heating temperature T-20°C until the material is removed from the heating furnace, and the rolling reduction ratio r calculated as the material thickness / product thickness, is 1 to 9. If d exceeds 9, the element diffusion distance at the product interface becomes long, increasing the width of the region where martensitic transformation can occur and reducing the fracture toughness of the interface. Preferably, d is 7 or less. If d is less than 1, element diffusion at the interface is too little, making it impossible to obtain sufficient bonding strength. Preferably, d is 3 or more. d=2.2×10 5 ×(√(exp(-3.2×10 4 / (T+273))×t)) / r...Equation (2)

[0079] The maximum heating temperature T (°C) in the heating furnace, the time t (minutes) from when the heating temperature in the heating furnace reaches the maximum heating temperature T-20°C until the material is removed from the heating furnace, and the reduction ratio r calculated by dividing the material thickness by the product thickness can be appropriately determined so that d falls within the above-mentioned ranges. However, from the viewpoint of properties other than the fracture toughness of the interface and manufacturability, preferred ranges are shown below. The maximum heating temperature T in the heating furnace is preferably 1050 to 1250°C. If the maximum heating temperature T is less than 1050°C, the hot workability deteriorates and the bonding strength also deteriorates. Therefore, the maximum heating temperature T is preferably 1050°C or higher, and more preferably 1100°C or higher. On the other hand, if the maximum heating temperature T exceeds 1250°C, the steel billet may be deformed in the heating furnace, defects may easily occur during hot rolling, and diffusion at the interface may become rapid. Therefore, the maximum heating temperature T is preferably 1250°C or lower, and more preferably 1220°C or lower. The shorter the time t (minutes) from when the heating temperature in the heating furnace reaches the maximum heating temperature T-20°C until the material is removed from the heating furnace, the shorter the element diffusion distance at the interface, so no lower limit is set, but heating for 30 minutes or more is desirable to ensure a uniform temperature up to the center of the plate thickness. The reduction ratio r, calculated by dividing the material thickness by the product thickness, is preferably 3 or more and 15 or less. If the reduction ratio r is less than 3, the interface bonding by rolling may be insufficient, resulting in a low shear strength at the interface. A reduction ratio of 5 or more is more preferable. Furthermore, if the reduction ratio exceeds 15, the rolling time becomes long, increasing the rolling cost. A reduction ratio of 10 or less is more preferable.

[0080] As mentioned above, the size of the martensite phase region at the interface is primarily affected by Cr diffusion. Although Cr diffusion occurs at temperatures above several hundred °C, the diffusion distance increases exponentially with increasing temperature. Therefore, the substantial diffusion occurs during the heating period when the material is held near the maximum temperature. Furthermore, the sheet temperature drops rapidly during rolling and cooling, so diffusion is negligible. Therefore, the Cr diffusion distance in the product can be considered to be the diffusion distance during heating, reduced by the reduction ratio. We have measured the size of the martensite phase region at the interface using thin-film TEM observations of clad products with various heating temperatures, times, and reduction ratios. We confirmed that the value d calculated using Equation (2) based on the maximum heating temperature T (°C), the time t (minutes) from the time the heating temperature reached the maximum heating temperature T-20°C to the time of extraction from the furnace, and the reduction ratio r, accurately corresponds to the size of the martensite phase region.

[0081] 5-3. Cooling after rolling The average cooling rate of the material in the TA3 (°C) to 650°C range calculated from equation (3) after rolling is desirably 2°C / s or higher. At a cooling rate of less than 2°C / s, carbon diffuses and concentrates in the austenite region at the interface that can become martensite due to the austenite to ferrite transformation or austenite to ferrite + pearlite transformation, increasing the width of the region where the nanohardness is 7 GPa or higher. A cooling rate of 4°C / s or higher is preferable. There is no particular upper limit, but a cooling rate of 10°C / s or lower is desirable because a martensite structure becomes dominant at a fast rate, resulting in the base material becoming too strong or having poor toughness. TA3(℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (3) In the formula, C, Si, Mn, Ni, Ti, Nb, and Al are the contents (mass%) of each element in the chemical composition of the base steel sheet.

[0082] According to the present invention, it is possible to obtain a clad steel plate having excellent fracture toughness at the joining surface. The clad steel plate according to the present invention and a welded structure formed using the clad steel plate according to the present invention do not require measures to prevent peeling during welding or additional heat treatment. Furthermore, the clad steel plate has no limitations on its use and can be applied to structural members where solid steel plates have traditionally been used. Therefore, the clad steel plate significantly contributes to cost reduction. [Example]

[0083] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0084] A clad material with the chemical composition shown in Table 1 and a base material with the chemical composition shown in Table 2 were melted and formed into billets. These were then hot-rolled, annealed, and pickled to produce clad steel plates with a thickness of 30 mm and base steel plates with a thickness of 130 mm. Using the resulting clad material and base steel, the base steel and clad material were laminated so that the bonding surface was vacuum-sealed and the four peripheries of the bonding surface were sealed by welding to produce a clad material. Two clad materials were stacked in the following order: base steel-clad material-release agent-clad material-base steel, with a release agent applied between them, to form a clad rolling material. The resulting clad rolling material was hot-rolled under the hot-rolling conditions shown in Table 3, and then peeled at the release agent portion to produce clad steel plates with thicknesses of 53 mm (reduction ratio 3) to 12 mm (reduction ratio 13).

[0085] [Table 1]

[0086] [Table 2]

[0087] The rolling conditions for clad steel plates were varied as listed in Table 3, and each characteristic value was investigated. The manufacturing conditions in Table 3 are explained below. In Table 3, T indicates the maximum heating temperature (°C) in the heating furnace before rolling, and t indicates the time (minutes) from when the heating temperature in the heating furnace reached the maximum heating temperature T-20°C until the plate was removed from the heating furnace. r indicates the reduction ratio calculated by dividing the material thickness by the product thickness. d indicates the value calculated from the above T, t, and r using formula (2). TA3 indicates the value (°C) calculated from the chemical composition of the base material using formula (3). CR indicates the average cooling rate (°C / s) from TA3 (°C) to 650°C. L indicates the width (μm) of the region near the interface where the nanohardness is 7 GPa or more. The CTOD value (mm) is the result of the CTOD test. d=2.2×10 5 ×(√(exp(-3.2×10 4 / (T+273))×t)) / r...Equation (2) TA3(℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (3) In the formula, C, Si, Mn, Ni, Ti, Nb, and Al are the contents (mass%) of each element in the chemical composition of the base steel sheet.

[0088] Nanohardness measurements were performed in accordance with the instrumented indentation hardness test specified in ISO 14577. Nanohardness was measured at 0.5 μm intervals over a 10 μm range from the interface on both the cladding and base metal sides in the thickness direction. While the nanohardness measurement conditions can be selected as appropriate, an example is to perform measurements three times at each position using a 1000 μN load, 5 seconds to indent the specified load, 0 seconds of hold, and 5 seconds of return, and then calculate the average value as the nanohardness. The range where the nanohardness was 7 GPa or greater was read and designated L. If an insert material such as Ni foil was inserted between the cladding and base metal, measurements should be performed at both the interface between the cladding and insert material and the interface between the insert and base metal.

[0089] The following CTOD test was conducted to evaluate fracture toughness (see Figures 1 to 4). In Figures 1 to 4, dot hatching is used to indicate components having the chemical composition of the clad material. The obtained clad steel plate 1 was cut to a length L of 170 mm and a width W of 15 mm, and a supplementary member 5 having the same chemical composition as the clad material 2 was welded to the clad material side in the plate thickness direction, and a supplementary member 4 having the same chemical composition as the base material 3 was welded to the base material side in a single pass with an electron beam to form a welded joint 13. The electron beam welding was performed downward at a vacuum of 0.1 torr, a voltage of 60 kV, a current of 80 Am, an object distance of approximately 300 mm, and a speed of 90 mm / min.

[0090] A CT test piece 12 was taken from approximately the center of the above-mentioned welded joint 13 as a fracture toughness test piece. At this time, the center line of the machine notch 11 was cut out along the clad interface 10. The thickness of the CT test piece 12 was set to 10 mm. Furthermore, side grooves 8 with an angle of 45 degrees, a bottom curvature radius of 0.25 mm, and a depth of 1 mm were machined on both side surfaces of the CT test piece 12 along the clad interface 10.

[0091] The CT test piece 12 was placed in a hydraulic servo-type fatigue testing machine (not shown), and a fatigue pre-crack 9 approximately 1.5 mm long was introduced from the bottom of the machine notch 11. The CT test piece 12 was cooled to 0°C and pulled between pins at a displacement rate of 0.01 mm / sec (not shown), and the fracture characteristics from the tip of the fatigue pre-crack (critical CTOD (mm)) were obtained. Hereinafter, this will be referred to as the "CTOD value."

[0092] [Table 3]

[0093] Samples Nos. 1 to 44 are examples of the present invention, satisfying the preferred manufacturing conditions, with the length L of the region where the nanohardness is 7 GPa or more being 5 μm or less, and the CTOD value of the clad interface being 0.15 mm or more. Samples Nos. 45 to 50 are comparative examples, not satisfying the preferred manufacturing conditions, with the length L of the region where the nanohardness is 7 GPa or more being more than 5 μm, and the CTOD value of the clad interface being less than 0.15 mm.

[0094] As described above, the examples of the present invention achieved good fracture toughness at the bonded interface. On the other hand, the comparative examples did not satisfy the preferred manufacturing conditions, and the length of the region where the nanohardness was 7 GPa or more was outside the range specified in the present invention, so the CTOD value at the clad interface was outside the range specified in the present invention. [Industrial Applicability]

[0095] According to the present invention, a clad steel plate having a good fracture toughness value at the bonding surface can be obtained, which is extremely useful industrially. When a corrosion-resistant alloy is used as the clad material, the clad steel plate of the present invention may be applicable to corrosive environments such as high-chloride environments such as seawater exposure, and corrosive environments such as those in plant facilities exposed to acid solutions such as phosphoric acid or sulfuric acid. Specific examples include seawater desalination plants, flue gas desulfurization equipment, chemical storage tanks, structural components such as oil country tubular goods, pumps and valves, and heat exchangers. [Explanation of symbols]

[0096] 1: Clad steel plate 2: Cladding material 3: Base material 4: Supplementary material (material equivalent to base material) 5: Supplementary material (same material as cladding material) 6: Welding seam 7: Cutting position of CT test piece 8: Side groove 9: Fatigue pre-crack 10: Cladding interface 11: Machine notch 12: CT test piece 13: Welded joints L: Length W: Width H2: Thickness of cladding material H3: Thickness of base material H4: Height of supplementary member 4 H5: Height of supplementary member 5

Claims

1. A clad steel plate comprising a base material and a clad material joined to the base material, the base material is made of carbon steel or low alloy steel, The base metal has a chemical composition containing, in mass%, C: 0.020 to 0.200%, Si: 1.00% or less, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.050% or less, Nb: 0.001 to 0.200%, and N: 0.020% or less, with Ceq being 0.20 to 0.50, and the balance being Fe and impurities; the cladding material is made of a corrosion-resistant alloy, The clad steel plate is made of a stainless steel or nickel-based alloy containing 10% or more Cr by mass%, At the interface between the base material and the clad material of the clad steel plate (hereinafter referred to as the "clad interface"), the width in the plate thickness direction of the region where the nanohardness is 7 GPa or more is 5 μm or less, A clad steel plate characterized in that, in evaluating the fracture toughness of the clad interface, a supplementary member is welded in one pass by electron beam or laser welding, and the CTOD value of the clad interface measured using a CT test piece in which a side groove is provided along the clad interface is 0.15 mm or more at 0°C. Here, Ceq is defined by the following equation (1). Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass%) of each element in the chemical composition of the base steel plate.

2. The chemical composition of the base metal further contains, in mass %, Ni: 0.01 to 3.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 0.50%, W: 0.01 to 1.00%, Cu: 0.01 to 2.00%, Co: 0.01 to 0.50%, Se+Te: 0.01 to 0.10%, V: 0.001 to 0.100%, Ti: 0.001 to 0.100%, and 2. The clad steel plate according to claim 1, containing one or more selected from the group consisting of Al: 0.005 to 0.300%, Ca: 0.0003 to 0.0100%, B: 0.0003 to 0.0030%, Mg: 0.0003 to 0.0100%, Zr+Hf+Ta: 0.0001 to 0.0100%, and REM: 0.0003 to 0.0100%.

3. 3. The clad steel plate according to claim 1 or claim 2, wherein the base material and cladding material are laminated so that the bonding surface is vacuum, and the four periphery of the bonding surface is sealed by welding to form a clad material, and the clad rolled material assembled from one or more of the clad materials is subjected to heating and hot rolling such that d calculated by formula (2) is 1 or more and 9 or less, based on the maximum heating temperature T (°C) in the heating furnace, the time t (minutes) from the time the heating temperature in the heating furnace reaches the maximum heating temperature T-20°C to the time of extraction from the heating furnace, and the rolling reduction ratio r calculated by the material thickness / product thickness, and the material is subjected to heating and hot rolling such that d calculated by formula (2) is 1 or more and 9 or less, and the material is cooled at an average cooling rate of 2°C / s or more in the TA3 (°C) to 650°C range calculated by formula (3), and the width in the plate thickness direction of the region where the nanohardness at the interface between the base material and the cladding material is 7 GPa or more is 5 μm or less, and the CTOD value at the clad interface is 0 °C or more. d = 2.2 × 10 ５ ×(√(exp(-3.2×10 ４ / (T+273))×t)) / r ・・・Formula (2) TA3 (℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (3) In the formula, C, Si, Mn, Ni, Ti, Nb, and Al are the contents (mass%) of each element in the chemical composition of the base steel plate.

4. A welded structure made using the clad steel plate according to claim 1 or 2.

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