Clad steel plate, its manufacturing method and welded structure

Abstract

To provide a clad steel plate in which a joined surface (interface between base material and cladding metal) is excellent in hydrogen embrittlement resistance and workability, and a method for manufacturing the clad steel plate.SOLUTION: A stainless clad steel plate excellent in hydrogen embrittlement resistance and workability of a joined surface of base material and cladding metal is provided in which carbon steel or low alloy steel is used as the base material and a stainless steel or a Ni group alloy is used as the cladding metal, wherein width of a region with nano-hardness of 7 GPa or more is 5 μm or less and a ferrite phase rate of a base material surface layer is more than 15%, on an interface between the cladding metal and the base material. In the joined surface of the clad steel plate, a region of martensite having high hydrogen embrittlement sensitivity is small, therefore interfacial peeling can be prevented even when welding in which hydrogen is contained in welded gas is performed, and the stainless clad steel plate has large high ferrite phase rate of the base material surface layer and high workability, and the stainless clad steel plate can be applied to a complicated structure.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a clad steel plate having excellent hydrogen embrittlement resistance at the joining surface, a manufacturing method thereof, and a welded structure manufactured using the clad steel plate in a manufacturing process including welding or gouging using a gas containing hydrogen. [Background technology]

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

[0003] On the other hand, stainless steels and Ni-based alloys contain a lot of alloying elements such as Cr, Ni, and Mo to ensure corrosion resistance, and compared to carbon steels and low-alloy steels, the cost of materials, as well as the costs of processing and welding, are high. Prices can also fluctuate greatly due to rising prices of alloying elements. For this reason, their use may be 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 in which two or more different metals are bonded together. In addition, steel plate that is not bonded is hereinafter referred to as "solid steel plate". Compared with solid steel plate made of only high alloy steel, clad steel plate can reduce the amount of high alloy steel used, reducing material costs, and also reduces dissimilar material welding, thereby reducing the cost of welding materials.

[0005] In addition, in a clad steel plate made by bonding two kinds of metals together, one metal is referred to as the "base material" and the other metal (material) bonded to the base material is referred to as the "clad material." By bonding a material (clad material) with excellent properties to the base 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 (corrosion resistance, etc.) required in the usage environment may be used for the cladding material, and a carbon steel or low alloy steel having the toughness and strength required in the usage environment may be used for the base material. In such a case, not only can the cost be reduced as described above, but the properties (corrosion resistance, etc.) equivalent to those of a solid steel plate and the strength and toughness equivalent to those of carbon steel and low alloy steel can be ensured. This makes it possible to achieve both cost-effectiveness and functionality.

[0007] In light of the above, the need for clad steel plates using stainless steel or Ni-based alloys has been increasing in various industrial fields in recent years. However, when using clad steel plates, it is important to prevent peeling at the joint between the clad material and the base material. If the clad material and the base material peel off during use, the desired properties such as corrosion resistance and strength may not be obtained. In addition, there may be a risk of holes being formed in structures or the collapse of structures.

[0008] In addition, as the range of applications for clad steel plates expands, they are being used in complex shapes such as chemical tankers, plants, large structures, etc. In these applications, the workability and bendability of the base material become important.

[0009] In the present invention, the subject is a clad steel plate in which a cladding material is bonded to only one side of the surface of the base material. The surface of the base material on which the cladding material is not bonded (where the base material is exposed) is hereinafter referred to as the "base material surface," and the position 1 mm from the base material surface in the plate thickness direction is referred to as the "base material surface layer." After rolling clad steel plate, water cooling is sometimes performed to prevent deterioration of the corrosion resistance of the clad material. However, if the cooling rate is fast, the ratio of bainite and martensite in the base material structure increases, which tends to reduce workability. In particular, the cooling rate is fast in the surface layer of the base material close to the surface of the base material where the cooling water directly hits, so the ratio of bainite and martensite increases compared to the inside of the base material, which reduces workability.

[0010] Tempering may be performed to soften bainite or martensite, but tempering has problems from the viewpoint of cost, such as increased heat treatment costs and increased correction costs due to the difference in thermal expansion coefficient between the clad steel plate and the base material, which can cause the plate to warp.

[0011] 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 anything about the martensite phase at the interface.

[0012] Patent Document 2 discloses a technique for preventing delayed fracture of martensite at the interface by specifying the temperature and time of tempering after rolling for an austenitic stainless clad steel plate. However, this technique is for preventing delayed fracture during manufacturing, and does not disclose any technique for preventing delayed fracture in welded structures.

[0013] Patent Document 3 discloses a technology for controlling the surface and internal structures of a base material for a clad steel plate to achieve both strength and workability. However, this control of the base material structure is achieved by a tempering treatment, and there is no description of structure control without tempering treatment.

[0014] Patent Document 4 discloses a technology for controlling the ferrite structure of the surface layer of solid steel plates made of ordinary steel by utilizing high-pressure water descaling. However, this patent is related to solid steel plates, and there is no description of the effect of high-pressure water descaling on the interface structure of clad steel plates.

[0015] In addition, Non-Patent Document 1 evaluates the hydrogen embrittlement susceptibility of martensite at the interface of clads of SUS316L and Inconel 625. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] JP 2013-209688 A [Patent Document 2] Japanese Patent Application Publication No. 6-7803 [Patent Document 3] Patent No. 6573060 [Patent Document 4] Patent No. 3572756 [Non-patent literature]

[0017] [Non-Patent Document 1] Kushida et al., Iron and Steel, Vol. 75 (1989), p. 1508 Summary of the Invention [Problem to be solved by the invention]

[0018] As a result of intensive research, the present inventors have found the following problems to be solved. 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, resulting in a diffusion layer of elements at the interface between the base material and the clad material (hereinafter simply referred to as 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 at which martensitic transformation occurs is slow.

[0019] When clad steel plates are normally used, the martensite at the interface does not affect interfacial peeling. However, for example, when welding is performed using hydrogen as a welding gas, hydrogen enters the martensite, and stress is generated at the interface due to structural stress, deformation during welding, and transformation of the base material near the weld, and it is thought that the combined effect of these factors may cause hydrogen embrittlement.

[0020] Patent Document 2 discloses a technique for preventing delayed fracture of martensite at the interface by tempering. Patent Document 3 discloses a technique for improving the workability of the base material by softening the bainite and martensite in the surface layer of the base material by tempering. However, since an increase in the tempering process leads to an increase in costs, a technique for improving the hydrogen embrittlement resistance of the martensite at the interface and the workability of the base material even if tempering is omitted in practice is required, but no solution to this problem has been disclosed or suggested. Non-Patent Document 1 describes a method for evaluating the hydrogen embrittlement susceptibility of martensite at the interface. However, in actual clad steel, the width of the diffusion layer is presumed to vary depending on the heating temperature and rolling reduction ratio, but there is no description or suggestion of the relationship between the width of the diffusion layer and hydrogen embrittlement susceptibility.

[0021] The inventors recognized that at the interface between a base material and a cladding material, the harder the martensite at the interface, the more susceptible it is to hydrogen embrittlement, and further, that the greater the width of the martensite in the diffusion layer at the interface, the greater the risk that minute hydrogen embrittlement will lead to large interfacial peeling. The present inventors also recognized that good workability can be achieved by controlling the ferrite fraction to a certain value or more, particularly in the surface layer of the base material close to the surface of the base material that is directly hit by cooling water during water cooling. From these results, the inventors realized that in order to suppress interfacial delamination of clad steel plate due to hydrogen embrittlement during welding while achieving workability of the base material, the problem to be solved is to control the hardness and width of martensite at the interface and to control the ferrite phase ratio in the surface layer of the base material.

[0022] Hereinafter, the joint surface between the base material and the clad material of the clad steel plate may be simply referred to as the "joint surface". In view of the above-mentioned problems, the present invention aims to provide a clad steel plate having excellent hydrogen embrittlement resistance and workability at the joining surface (the joining surface between the base material and the clad material), a manufacturing method thereof, and a welded structure. [Means for solving the problem]

[0023] The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is the following clad steel plate, its manufacturing method, and welded structure. [1] A clad steel plate having a base material and a clad material joined to one side of 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, A clad steel plate characterized in that at the interface between the base material and the clad material of the clad steel plate, the width in the plate thickness direction of an area having a nano-hardness of 7 GPa or more is 5 μm or less, and the ferrite phase fraction of the surface layer of the base material is more than 15%. Here, the base material surface layer refers to a position 1 mm from the base material surface in the plate thickness direction. [2] The clad steel plate according to [1], wherein the chemical composition of the base material contains, by mass%, C: 0.020-0.200%, Si: 1.00% or less, Mn: 0.10-3.00%, P: 0.050% or less, S: 0.050% or less, Ceq is 0.20-0.40, and the balance is Fe and impurities, where Ceq is defined by the following formula (1): Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5...Equation (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. [3] The clad steel plate according to [2], wherein the composition of the base metal further contains, in mass%, one or more selected from Ni: 0.01-1.00%, Cr: 0.01-1.00%, Mo: 0.01-0.50%, Cu: 0.01-1.00%, Co: 0.01-0.50%, Se+Te: 0.01-0.10%, V: 0.001-0.100%, Ti: 0.001-0.200%, Nb: 0.010-0.200%, Al: 0.005-0.300%, Ca: 0.0003-0.0050%, B: 0.0003-0.0030% and REM: 0.0003-0.0100%, in place of a portion of the Fe. [4] The clad steel plate according to any one of [1] to [3], characterized in that a clad material of the clad steel plate is a stainless steel or a nickel-based alloy containing, by mass%, 10% or more of Cr.

[0024] [5] In the clad steel plate according to any one of [1] to [4], the base material and the clad material are laminated so that the bonding surface is vacuum, and the four circumferences of the bonding surface are sealed by welding to form a clad material, and two of the clad materials are assembled so that the surfaces of the base materials are both surfaces of the clad rolled material. For the clad rolled material, high-pressure water descaling is performed before the first or second pass before the final pass of rolling at 1030°C or less and before the start of hot rolling, and the rolling finishing temperature is 980°C or less, and after rolling, the T calculated by the formula (2) is obtained. A3 The method for producing a clad steel plate according to any one of [1] to [4], characterized in that the average cooling rate in the range from 100°C to 650°C is 2°C / s or more, and the width in the plate thickness direction of the region where the nano-hardness of the interface between the base material and the clad material is 7 GPa or more is 5 μm or less and the ferrite fraction of the surface layer of the base material exceeds 15%. T A3 (℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (2) 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.

[0025] [6] A welded structure made using the clad steel plate according to any one of [1] to [4]. [7] The clad steel plate according to any one of [1] to [4], characterized in that the clad steel plate is used for welding using hydrogen as a welding gas. Effect of the Invention

[0026] According to the present invention, it is possible to obtain a clad steel plate having excellent hydrogen embrittlement resistance and workability at the bonding surface. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] As mentioned above, the present invention is directed to a clad steel plate in which a cladding material is bonded to only one side of the surface of the base material. The surface of the base material on the side to which the cladding material is not bonded (where the base material is exposed) is called the "base material surface," and the position 1 mm from the base material surface in the plate thickness direction is called the "base material surface layer."

[0028] The present inventors conducted the following study to address the above-mentioned problems. Specifically, in clad steel plates made of various stainless steels and Ni-based alloys, the cooling rate after rolling was changed to investigate element diffusion and metal structure at the interface, and the relationship with hydrogen embrittlement resistance at the interface was evaluated. As a result, the following findings (a) to (c) were obtained.

[0029] (a) The thinner the region of the clad steel plate interface where the nano-hardness is 7 GPa or more, the lower the susceptibility of martensite to hydrogen embrittlement. For this reason, it is effective to make the region of 7 GPa or more 5 μm or less.

[0030] (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 profile of alloying elements at the interface could be organized by the temperature and time of material heating and the rolling reduction ratio. It was also confirmed that when cladding materials containing 10% or more Cr by mass were used, the diffusion width of Cr corresponded to the width of the martensite phase. This is because Cr diffuses the fastest among the main alloying elements and is an element that increases hardenability, so martensitic transformation occurs in areas where only the Cr content is high and the content of austenite stabilizing elements such as Ni is low.

[0031] (c) The hardness of the martensite at the interface is affected by the cooling rate after rolling. This mechanism is thought to be as follows. If the cooling rate after rolling is slow and carbon is expelled and diffused due to austenite → ferrite transformation or austenite → 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, if the cladding material side is an austenite phase, the degree of concentration will be greater. Due to this mechanism, if the cooling rate after rolling is slow, a region with high carbon concentration will be formed near the interface, and if this region overlaps with a region where martensite can form, hard martensite will form at the interface of the clad steel plate, and the hydrogen embrittlement resistance of the interface will decrease.

[0032] The present inventors have also carried out the following study to address the above-mentioned problems. Specifically, for clad steel plates using various ordinary steels and low alloy steels as base materials, the metal structure of the surface layer of the base material was investigated by changing the composition of the base material, the presence or absence of high-pressure water descaling during rolling, the rolling finish temperature, and the cooling rate after rolling, and the relationship with workability was evaluated. As a result, the following findings (d) to (f) were obtained.

[0033] (d) The greater the ferrite fraction in the surface layer of the base material, the better the workability. For this reason, it is effective to make the ferrite fraction in the surface layer of the base material exceed 15%.

[0034] (e) In order to thin the hard region at the interface, the T at which the austenite to ferrite transformation starts during cooling after rolling is determined based on (c) above. A3 It is important to cool the vicinity of the interface at a high cooling rate from 0.25 °C or higher. On the other hand, if the cooling rate near the interface between the base material and the cladding material is increased, the surface of the base material on the opposite side of the interface, where the water directly hits, is also rapidly cooled at the same time, and hard bainite and martensite are formed, especially near the surface layer of the base material located 1 mm from the base material surface in the plate thickness direction. For example, in tensile tests, fracture occurs from the hard, low-ductility part of the surface, reducing workability. In particular, in the composition of the base material that provides sufficient strength for a structure, hard bainite and martensite are likely to form when the cooling rate is increased. The austenite to ferrite transformation is promoted by residual strain. Therefore, in order to suppress the formation of bainite or martensite even when rapid cooling is performed, it is effective to perform rolling at a low temperature where recrystallization and recovery take time, and to leave residual strain when cooling after rolling. However, when rolling at a low temperature, the temperature drops during transportation from the rolling machine to the cooling equipment, and the T A3 If the temperature falls below 100°C, carbon may become concentrated near the interface between the base material and the clad material, resulting in a thicker hard region. Therefore, in order to simultaneously reduce the thickness of the hard region near the interface and suppress the formation of bainite and martensite in the surface layer of the base material, it is necessary to perform rolling by lowering the temperature of the surface layer of the base material close to the surface of the base material while keeping the temperature near the interface between the base material and the clad material high.

[0035] (f) In order to lower the temperature of only the base metal surface layer close to the base metal surface to a low temperature at which recrystallization and recovery take time, it is effective to utilize high-pressure water descaling, which is usually used to remove scale and prevent surface defects. By adjusting the amount of water for scaling with high-pressure water and the rolling pass schedule, it is possible to lower the temperature of only the base metal surface layer close to the base metal surface that is directly exposed to water, thereby increasing the residual strain.

[0036] Therefore, in order to obtain a clad steel plate having excellent hydrogen embrittlement resistance and workability at the joining surface, it is necessary to control the austenite to ferrite transformation in both the base metal surface layer and the interface between the base metal and the clad material by optimizing the temperature of the base metal surface layer during rolling and the cooling conditions after rolling. The present invention has been made based on the above findings. Each requirement of the present invention will be described in detail below.

[0037] 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 one side of 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, and examples of the corrosion-resistant alloy include stainless steel containing 10% or more of Cr and Ni-based alloys. The surface of the base material on the side to which the clad material is not bonded (where the base material is exposed) is called the "base material surface." Furthermore, the width of the region at the 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 ferrite fraction of the surface layer of the base material is more than 15%.

[0038] 2.Interface characteristics and surface structure of clad steel plate The interface characteristics and surface structure of the clad steel plate according to the present invention will be described. In order to obtain a clad steel plate with excellent workability and excellent hydrogen embrittlement resistance at the joining surface, it is necessary to simultaneously suppress the generation of hard martensite phase at the interface between the base material and the clad material and promote ferrite phase transformation at the base material surface (1 mm from the base material surface in the plate thickness direction).

[0039] 2-1. Nano-hardness of the interface between base material and cladding material The width of the region with a nano-hardness of 7 GPa or more at the interface between the base material and the cladding material is set to 5 μm or less. If the width in the plate thickness direction of the region with a nano-hardness of 7 GPa or more exceeds 5 μm, the region of martensite that is hard and highly susceptible to hydrogen embrittlement is large, so the interface may peel off when welding is performed using a welding gas containing hydrogen. It is preferably 3 μm or less, and more preferably 1 μm or less. The smaller the region with a nano-hardness of 7 GPa or more, the lower the susceptibility to hydrogen embrittlement, so no lower limit is set. Here, nanohardness refers to the hardness of a material evaluated in accordance with the instrumented indentation hardness test (also called nanoindentation test) specified in ISO 14577.

[0040] 2-2. Ferrite phase ratio in the base material surface The ferrite phase ratio in the surface layer of the base material of the clad steel plate is set to be more than 15%. If it is less than 15%, cracks may occur in a bending test. Since the higher the ferrite phase ratio, the better the workability, no upper limit is set. It is preferably 20% or more, and more preferably 30% or more. Here, the base material surface layer refers to the position 1 mm from the base material surface in the sheet thickness direction, and the ferrite phase ratio refers to the area ratio where the KAM (Kernel Average Misorientation) in the EBSD test is 1° or less.

[0041] 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 of C: 0.020-0.200%, Si: 1.00% or less, Mn: 0.10-3.00%, P: 0.050% or less, S: 0.050% or less, Ceq is 0.20-0.40, and the balance is 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.

[0042] C is an element that improves the strength of steel, and sufficient strength is achieved by including it in an amount of 0.020% or more. 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 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%.

[0043] Silicon is effective for deoxidization and is an element that improves the strength of steel. However, if the content exceeds 1.00%, it will cause deterioration of the surface properties and toughness of the steel. Therefore, the amount of silicon is set to 1.00% or less, and preferably 0.50% or less. Silicon may not be contained. The preferable lower limit of the silicon content is 0.01%.

[0044] Mn is an element that increases the strength of steel, and this effect is manifested by including it in an amount of 0.10% or more. However, if it exceeds 3.00%, the 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%.

[0045] 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, and preferably 0.015% or less.

[0046] 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, and preferably 0.010% or less.

[0047] Ceq (carbon equivalent) is a value used to estimate hardness and weldability from the chemical composition of steel, and is calculated by formula (1). The higher Ceq, the higher the hardness and the worse the weldability. If Ceq is less than 0.20, sufficient strength cannot be obtained for a structure. Therefore, Ceq is set to 0.20 or more, and preferably 0.23 or more. If Ceq exceeds 0.40, weldability deteriorates, and interpass temperature control and post heat treatment become necessary, increasing welding costs. Therefore, Ceq is set to 0.40 or less, and preferably 0.35 or less. Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5...Equation (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] The composition of the base material may further contain, in mass %, one or more selected from Ni: 0.01-1.00%, Cr: 0.01-1.00%, Mo: 0.01-0.50%, Cu: 0.01-1.00%, Co: 0.01-0.50%, Se+Te: 0.01-0.10%, V: 0.001-0.100%, Ti: 0.001-0.200%, Nb: 0.010-0.200%, Al: 0.005-0.300%, Ca: 0.0003-0.0050%, B: 0.0003-0.0030% and REM: 0.0003-0.0100%, in place of a portion of the Fe.

[0049] Ni is an element that improves the hardenability of steel, and improves the strength and toughness of steel after rolling. However, if it exceeds 1.00%, it causes deterioration of weldability and toughness. Therefore, when Ni is contained, the Ni amount is 1.00% or less. It is preferably 0.50% or less, and more preferably 0.30% or less. The preferable lower limit of the Ni content is 0.01%.

[0050] Cr is an element that improves the hardenability of steel, and improves the strength and toughness of the steel after rolling. However, if it exceeds 1.00%, it causes deterioration of weldability and toughness. Therefore, when Cr is contained, the Cr amount 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 Cr content is 0.01%.

[0051] Mo is an element that improves the hardenability of steel, and improves the strength and toughness of 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. It is preferably 0.30% or less, and more preferably 0.1% or less. The preferred lower limit of the Mo content is 0.01%.

[0052] Cu is an element that improves the hardenability of steel, and improves the strength and toughness of steel after rolling. However, if it exceeds 1.00%, it causes deterioration of weldability and toughness. Therefore, when Cu is contained, the Cu content is set to 1.00% or less. It is preferably 0.50% or less, and more preferably 0.30% or less. The preferred lower limit of the Cu content is 0.01%.

[0053] Co is an element that improves the hardenability of steel, and improves the strength and toughness of the steel after rolling. However, if it exceeds 0.50%, hot workability is impaired and productivity decreases. Therefore, when Co is contained, the Co amount is set to 0.50% or less. It is preferably 0.30% or less, and more preferably 0.1% or less. The preferred lower limit of the Co content is 0.01%.

[0054] Se and Te suppress easily oxidizable elements such as Mn, Si, and Al in the steel sheet from diffusing to the steel sheet surface to form oxides, improving the surface properties and galvanic properties of the steel sheet. However, this effect saturates when the total amount exceeds 0.10%. Therefore, when Se and Te are added, the total amount of Se and Te is set to 0.10% or less, and more preferably 0.05% or less. The preferred lower limit of the Se+Te content is 0.01%.

[0055] Al is an element that is effective in deoxidizing steel. However, if it exceeds 0.300%, it causes deterioration of the toughness of the welded part. Therefore, if Al is contained, the Al content is set to 0.300% or less, and preferably 0.100% or less. The preferable lower limit of the Al content is 0.005%.

[0056] 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 is set to 0.100% or less, preferably 0.050% or less. The preferred lower limit of the V content is 0.001%.

[0057] 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 the alloy cost increases. 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%.

[0058] Nb is an element that increases the strength of steel and delays recrystallization after rolling to increase the ferrite phase ratio, and it is preferable to add 0.010% or more. However, if it exceeds 0.200%, the weldability is impaired and the alloy cost increases. Therefore, the Nb content is set to 0.200% or less. The upper limit of Nb is preferably 0.100%, more preferably 0.010 to 0.050%, and further preferably 0.030 to 0.050%.

[0059] Ca refines the structure of the weld heat affected zone and improves toughness. However, if it exceeds 0.0050%, it forms coarse inclusions and deteriorates toughness. Therefore, when Ca is contained, the Ca content is set to 0.0050% or less, preferably 0.0030% or less. The preferred lower limit of the Ca content is 0.0003%.

[0060] B is an element that improves the hardenability of steel, and improves 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, and preferably 0.0015% or less. The preferred lower limit of the B content is 0.0003%.

[0061] REM 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, if REM is contained, the REM content is set to 0.0100% or less, preferably 0.005% or less. The preferred lower limit of the REM content is 0.0003%.

[0062] 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 means the total content of these elements.

[0063] In the chemical composition of the base metal of the present invention, the balance is Fe and impurities. Here, the term "impurities" refers to components that are mixed in due to various factors in raw materials such as ores and scraps and manufacturing processes during industrial production of steel materials, and are permissible within a range that does not adversely affect the present invention.

[0064] 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 Cr increases the hardenability of the cladding interface and makes it easier to transform into martensite, while carbon on the base metal side diffuses to the cladding material side, forming a hard martensite phase at the base metal side interface, which causes a decrease in hydrogen embrittlement resistance of the joint surface. That is, the effect of the present invention is exhibited when a corrosion-resistant alloy containing a large amount of Cr is used. If the Cr content of the cladding material is 10% or more, the effect of applying the present invention is remarkable. If the Cr content is 15% or more, the effect can be exhibited more remarkable.

[0065] The present invention relates to a clad steel plate with excellent hydrogen embrittlement resistance at the bonding surface by controlling the structure of the bonding interface (the interface between the base material and the cladding material), and a manufacturing method thereof. The type of steel used for the cladding material is not particularly specified, but examples of cladding materials include stainless steel and nickel-based alloys. Stainless steel includes austenitic stainless steel, ferritic stainless steel, and duplex stainless steel, while nickel-based alloys include various alloy components under the trade names of Inconel, Incoloy, Hastelloy, and the like.

[0066] 5. Manufacturing method The manufacturing method of the clad steel plate according to the present invention will be described. As described above, in order to obtain a clad steel plate having excellent hydrogen embrittlement resistance and workability of the joining surface, it is necessary to control the metal structure. Such a metal structure can be realized by combining the chemical composition of the steel with appropriate manufacturing conditions. In the above clad steel plate, the base material and cladding material are laminated so that the bonding surface is vacuum, and the four circumferences of the bonding surface are sealed by welding to form the clad material. Two clad materials are assembled to form the clad rolled material, and combined so that the base material surface of each clad material becomes the surface of the clad rolled material. The assembled clad rolled material is subjected to high-pressure water descaling at 1030°C or less and before starting the first or second pass before the final rolling pass, and hot rolling is performed with a rolling finishing temperature of 980°C or less. After rolling, the T calculated by formula (2) is obtained. A3 Cooling is carried out at an average cooling rate of 2°C / s or more in the range from 100°C to 650°C to produce a clad steel plate. T A3 (℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (2) 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.

[0067] 5-1 Clad Material 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, an electric furnace, or a vacuum melting furnace, and then a slab is made by a continuous casting method or an ingot casting-blotting method. The obtained slab is hot rolled under normally used conditions to obtain a cladding material and a base material as a hot-rolled sheet. The obtained hot-rolled sheet may be annealed, pickled, polished, etc., as necessary. The clad material is assembled by laminating the above cladding material and base material so that the bonding surface is vacuumed, and sealing the four circumferences 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 of vacuuming the bonding surface is not particularly specified, but examples include a method of electron beam welding in a vacuum, a method of opening a hole for vacuuming in advance, welding the four circumferences by arc welding or laser welding in the atmosphere, and then vacuuming with a vacuum pump. If the degree of vacuum (absolute pressure) is 0.1 Torr or less, a good bonding interface with less oxides at the interface can be obtained, and more preferably 0.05 Torr or less. Since the higher the degree of vacuum (the lower the absolute pressure), the better the bonding interface tends to be, no lower limit is set. The obtained clad material is preferably assembled by applying a release agent between two clad materials and stacking them as a clad rolling material, which is then subjected to hot rolling. When stacking the two, it is desirable that the base materials and the clad materials are of equal thickness to reduce plate warping during cooling. Of course, there is no need to be limited to the assembly method described above. As mentioned above, the clad materials are assembled so that the surfaces of the base materials become the surfaces of the clad rolling material.

[0068] 5-2.Hot rolling Next, the obtained clad rolled material is subjected to high-pressure water descaling before the final pass of rolling at 1030°C or less and before starting the rolling, and hot rolling is performed with a rolling finish temperature of 980°C or less. After rolling, the T A3 Cooling is carried out at an average cooling rate of 2°C / s or more in the range from 100°C to 650°C. T A3 (℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (2) In the formula, C, Si, Mn, Ni, Ti, Nb and Al are the contents (mass%) of each element in the composition of the base steel sheet. A3 is a known equation for the relationship between the A3 transformation temperature and the element contents in steel.

[0069] High-pressure water descaling is performed at 1030°C or less and before starting the rolling pass one or two passes before the final rolling pass. If the temperature exceeds 1030°C, the temperature of the base material surface and the base material surface layer will rise due to reheating even if high-pressure water descaling is performed, and recrystallization and recovery will occur in the base material surface layer, reducing residual strain and making it impossible to promote subsequent ferrite transformation. The lower the temperature, the greater the residual strain, so no lower limit is set, but from the viewpoint of suppressing the formation of a hard layer at the interface between the base material and the clad material, it is preferable to set the temperature at 930°C or more, and more preferably at 980°C or more. High-pressure water descaling is performed at the rolling pass one or two passes before the final rolling pass. If high-pressure water descaling is performed before the third pass before the final pass, too much time has passed since the plate was reheated, and the effect of increasing the residual strain in the surface layer cannot be obtained. High-pressure water descaling is performed at a temperature of 0.5 m in order to sufficiently cool the plate surface. 3 The flow rate is preferably 1.0 m 3 / min. Not all of the water that hits the plate surface evaporates, but some floats on the plate and some flows down. There is no upper limit on the flow rate, but since the main contributor to cooling the surface is the water that comes into direct contact with the surface, increasing the amount of water only increases costs and the cooling effect becomes saturated, so the flow rate is preferably 3.0 m 3 / min or less. The flow rate of the high pressure water descaling (m 3 / min) is the amount of water that actually hits the plate surface. For example, the total flow rate (m 3 It is calculated by: speed (m / min) × strip width (m) ÷ nozzle width (m). The effect of speed can be ignored within the range of strip threading speeds used in practical rolling. Naturally, there is no impediment to carrying out high-pressure water descaling before the third pass before the final pass for the purpose of descaling, which is the original effect of high-pressure water descaling.

[0070] The finishing temperature of the rolling is 980°C or less. If it exceeds 980°C, the temperature of the surface and the surface layer of the base material will rise due to reheating after rolling, causing recrystallization and recovery, reducing residual strain, and making it impossible to promote the subsequent ferrite transformation. Since the lower the temperature, the greater the residual strain, no lower limit is set, but from the viewpoint of suppressing the generation of a hard layer at the interface between the base material and the cladding material, it is preferable to set the temperature at 880°C or more, and more preferably at 930°C or more.

[0071] The heating temperature, heating time and rolling reduction ratio of the material may be determined as appropriate, but from the viewpoint of properties other than hydrogen embrittlement resistance and workability at the interface and manufacturability, preferred ranges are exemplified below. The heating temperature is preferably 1050 to 1250°C. If the heating temperature is less than 1050°C, the hot workability deteriorates and the joining strength also deteriorates. Therefore, the heating temperature is preferably 1050°C or higher, and more preferably 1100°C or higher. On the other hand, if the heating temperature exceeds 1250°C, the steel billet is likely to deform in the heating furnace and defects are likely to occur during hot rolling. Therefore, the heating temperature is preferably 1250°C or lower, and more preferably 1220°C or lower. The shorter the heating time, 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 achieve a uniform temperature up to the center of the plate thickness. The reduction ratio, calculated by material thickness / product thickness, is preferably 3 to 15. If the reduction ratio is less than 3, the interface bonding by rolling may be insufficient, and the shear strength of the interface may be low. A reduction ratio of 5 or more is more preferable. If the reduction ratio exceeds 15, the rolling time becomes long, the finishing temperature becomes too low, and the rolling cost increases. A reduction ratio of 10 or less is more preferable.

[0072] 5-3. Cooling after rolling After rolling, T calculated from equation (2) A3The average cooling rate in the range from 0°C to 650°C is desirably 2°C / s or more. At a cooling rate of less than 2°C / s, carbon diffuses and concentrates in the austenite region that can become martensite at the interface between the base material and the cladding material due to austenite → ferrite transformation or austenite → ferrite + pearlite transformation, and the width of the region where the nanohardness is 7 GPa or more increases. The cooling rate is preferably 4°C / s or more. If the cooling rate is too fast, bainite and martensite become the main components in the base material surface structure, deteriorating workability, so the cooling rate is desirably 10°C / s or less. T A3 (℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (2) 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.

[0073] 5-3.Heat treatment after rolling According to the present invention, it is possible to obtain a clad steel plate having excellent hydrogen embrittlement resistance and workability of the joint surface without heat treatment. Although the heat treatment of the clad steel plate increases the cost as described above, it is not prevented from performing the heat treatment as required for other properties. If a ferrite to austenite transformation occurs during the heat treatment, there is a possibility that a hard region will be generated at the interface due to the diffusion of C at the interface. Therefore, the heat treatment temperature is set to T calculated by the formula (2). A3 It is desirable to keep the temperature at or below T A3 - Below 100℃. T A3 (℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (2)

[0074] According to the present invention, it is possible to obtain a clad steel plate having excellent hydrogen embrittlement resistance and workability of the joining surface. The clad steel plate according to the present invention and a welded structure made using the clad steel plate according to the present invention do not require measures against peeling during welding or additional heat treatment. Furthermore, the clad steel plate has high workability and can be made into complex shapes, so there is no limit to the applications of use, and it can be applied to structural members for which solid steel plates have been used in the past. Therefore, the clad steel plate greatly contributes to cost reduction. The welded structure made using the clad steel plate according to the present invention can be a welded structure manufactured by a manufacturing process including welding or gouging using a gas containing hydrogen.

[0075] The clad steel plate of the present invention has excellent hydrogen embrittlement resistance and workability, and therefore does not suffer from hydrogen embrittlement even when used in welding using hydrogen as the welding gas, and can be applied to complex structures. EXAMPLES

[0076] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0077] The clad material with the chemical composition shown in Table 1 and the base material with the chemical composition shown in Table 2 were melted and made into steel billets, which were then hot-rolled, annealed, and pickled to produce steel plates with a thickness of 30 mm for the clad material and 130 mm for the base material. Using the obtained clad material and base material as materials, the base material and the clad material were laminated so that the bonding surface was in a vacuum, and the four circumferences of the bonding surface were sealed by welding to produce a clad material. Two clad materials were layered with a release agent applied between the clad materials so that the order was base material-clad material-release agent-clad material-base material, and assembled as a clad rolling material. The obtained clad rolling material was hot-rolled under the hot rolling conditions shown in Table 3, and then peeled at the release agent part to produce a clad steel plate with a thickness of 16 mm.

[0078] [Table 1]

[0079] [Table 2]

[0080] In the rolling of the clad steel plate, the conditions shown in Table 3 were changed and each property value was examined. The manufacturing conditions in Tables 2 and 3 are explained below. In Table 2, T A3 indicates the value (℃) calculated from the chemical composition of the base material using formula (2). In Table 3, "descaling" indicates whether high-pressure water descaling was performed or not and the pass performed, and the number indicates how many passes before the final pass that it was performed before the start of rolling. T1 indicates the temperature of the pass in which descaling was performed. T2 indicates the finishing temperature of rolling. CR is T A3 The graph shows the average cooling rate (℃ / s) from 0°C to 650°C. L shows the width (μm) of the region near the interface where the nanohardness is 7 GPa or more. The α ratio shows the ferrite area ratio (%) of the base material surface layer. Hydrogen resistance is the result of a hydrogen embrittlement resistance evaluation test, with ○ indicating good hydrogen embrittlement resistance and × indicating poor. Workability is the result of a tensile test, with ○ indicating good workability and × indicating poor workability. T A3 (℃)=937.2-436.5C+56Si-19.7Mn-26.6Ni+136.3Ti-19.1Nb+198.4Al...Formula (2) 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.

[0081] The nanohardness was measured in accordance with the instrumented indentation hardness test specified in ISO 14577, and the nanohardness was measured in a 10μm range from the interface in the thickness direction on the cladding side and the base material side at 0.5μm intervals. The conditions for the nanohardness measurement can be selected as appropriate, but for example, a measurement can be performed three times at each position with a load of 1000μN, 5 seconds to the specified indentation load, 0 seconds to hold, and 5 seconds to return, and the average value is taken as the nanohardness. The range where the nanohardness is 7GPa or more was read and recorded as L. If an insert material such as Ni foil is inserted between the cladding material and the base material, the interface between the cladding material and the insert material and the interface between the insert material and the base material should be measured separately.

[0082] To measure the KAM value, a cross section perpendicular to the rolling direction of the sample was polished with colloidal silica, and EBSD measurements were performed three times on the base material surface (1 mm from the base material surface in the depth direction) under the following measurement conditions: 500x magnification, 177μm×519μm area (the 177μm side is the thickness direction, and the center of the area is 1mm from the base material surface), and measurement step of 1μm. From the obtained data, the area ratio where the KAM value was 1° or less was calculated, and the average was taken as the ferrite phase ratio of the base material surface. Note that these measurement conditions are only an example, and may be changed as appropriate depending on the metal structure of the sample, especially the crystal grain size.

[0083] Kernel Average Misorientation (KAM) calculates the KAM value of each pixel by averaging the misorientation between the six neighboring pixels for each pixel of the measured data. By performing this calculation so as not to exceed the grain boundaries, a distribution map of strain based on local orientation changes within the grains can be obtained. Ferrite formed at high temperatures has a characteristic that the transformation strain is smaller than that of bainite or martensite, since the main transformation mechanism is diffusion transformation. Based on this characteristic, a comparison with the structure observed by etching was made, and a KAM value of 1° or less was defined as ferrite, and the ferrite area ratio measured by EBSD was defined as the ferrite phase ratio of the base material surface layer.

[0084] The following tests were carried out to evaluate hydrogen embrittlement resistance. To ensure the length in the plate thickness direction, the test specimens were prepared by welding the same steel type as the clad material to the clad material side of the clad steel plate, and the same steel type as the base material to the base material side, with the parallel part including the clad interface being 4φ×20mm, and a 60°, ρ=0.1mm notch was added to the clad interface to give a 3φ round bar test specimen. To suppress the thermal effects of welding, electron beam welding was selected as the welding method, which has a small heat input and can reduce the width of the weld metal, and grinding was carried out after welding. The cross-section of the test specimen was also observed to confirm that the weld metal was at least 2mm away from the interface. Before tension, the prepared test specimen was immersed in a 3 mass% NaCl + 3 g / L NHSCN aqueous solution at a current density of 10 (A / m 2) × 72(hr), then in a 3%NaCl+3g / L NHSCN aqueous solution at 10(A / m 2 ) Strain rate of parallel section while cathodically charging: 1×10 -3 A separate test was conducted in which the specimen was pulled without cathodic charging before and during the pulling process, and the strokes up to the break were compared. If the stroke of the charged specimen / the stroke of the uncharged specimen was 0.25 or more, it was evaluated as good (○), and if it was less than 0.25, it was evaluated as poor (×).

[0085] The following test was carried out to evaluate the workability. The clad steel plate was machined to remove the clad material, and a JIS No. 1A tensile test piece was taken from the base steel plate. A tensile test was carried out in accordance with JIS Z 2241 to determine the total elongation. If the total elongation was 16.0% or more, it was evaluated as good (○), and if it was less than 16.0%, it was evaluated as poor (×).

[0086] The manufacturing conditions and the above results are shown in Table 3. Items that fall outside the scope of the clad steel plate of the present invention and items that fall outside the scope of the preferred manufacturing method of the present invention are underlined.

[0087] [Table 3]

[0088] Samples 1 to 44 are examples of the present invention, which satisfy the preferred manufacturing conditions, have a length L of 5 μm or less where the nanohardness is 7 GPa or more, a ferrite fraction of the base material surface layer of more than 15%, and have good hydrogen embrittlement resistance and good workability of the joint surface. Samples 45 to 52 are comparative examples, which do not satisfy the preferred manufacturing conditions, have a length L of 5 μm or more where the nanohardness is 7 GPa or more, or a ferrite fraction of the base material surface layer of less than 15%, and have poor hydrogen embrittlement resistance or workability of the joint surface.

[0089] As described above, the examples of the present invention provided clad steel plates with excellent hydrogen embrittlement resistance and workability at the joining surface. On the other hand, the comparative examples did not satisfy the preferred manufacturing conditions, and the length of the region with a nano-hardness of 7 GPa or more was outside the range specified in the present invention, or the ferrite fraction of the base metal surface was outside the range specified in the present invention, so the hydrogen embrittlement resistance or workability of the joining surface was poor. [Industrial Applicability]

[0090] According to the present invention, a clad steel plate having excellent hydrogen embrittlement resistance and workability at the bonding surface can be obtained at a low cost, and is extremely useful in industry. Since a corrosion-resistant alloy is used as a clad material, the clad steel plate of the present invention may be applicable to corrosive environments such as high chloride environments exposed to seawater, and corrosive environments in plant facilities exposed to acid solutions such as phosphoric acid or sulfuric acid. Specifically, the clad steel plate may be used in seawater desalination plants, flue gas desulfurization equipment, storage tanks for chemicals, structural members such as oil well tubular goods, pumps and valves, heat exchangers, etc.

Claims

1. A clad steel plate comprising a base material and a clad material joined to one side of 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, Ceq being 0.20 to 0.40, and the balance being Fe and impurities; The cladding material is made of a corrosion-resistant alloy, A clad steel plate characterized in that, at the interface between the base material and the clad material of the clad steel plate, the width in the plate thickness direction of an area having a nano-hardness of 7 GPa or more is 5 μm or less, and the ferrite phase fraction of the surface layer of the base material is more than 15%. Here, the surface layer of the base material refers to a position 1 mm from the surface of the base material in the sheet thickness direction. Ceq is defined by the following equation (1). Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5...Equation (1)In the equation, C, Mn, Cu, Ni, Cr, Mo and V are the contents (mass%) of each element in the composition of the base steel plate. For elements that are not contained, 0 is substituted in the formula.

2. The composition of the base material further contains, in mass%, one or more selected from Ni: 0.01 to 1.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 0.50%, Cu: 0.01 to 1.00%, Co: 0.01 to 0.50%, Se + Te: 0.01 to 0.10%, V: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Nb: 0.010 to 0.200%, Al: 0.005 to 0.300%, Ca: 0.0003 to 0.0050%, B: 0.0003 to 0.0030% and REM: 0.0003 to 0.0100%, in place of a portion of the Fe. Clad steel plate according to claim 1.

3. 3. The clad steel plate according to claim 1, wherein a clad material of the clad steel plate is a stainless steel or a nickel-based alloy containing, by mass%, 10% or more of Cr.

4. In the clad steel plate according to any one of claims 1 to 3, the base material and the clad material are laminated so that the bonding surface is vacuum, and the four circumferences of the bonding surface are sealed by welding to form a clad material, and two of the clad materials are assembled so that the surfaces of the base materials are both surfaces of the clad rolled material. The clad rolled material is assembled so that the surfaces of the base materials are both surfaces of the clad rolled material, and the clad rolled material is subjected to high-pressure water descaling at 1030°C or less and before starting one or two passes before the final pass of rolling, and hot rolling is performed with a rolling finishing temperature of 980°C or less, and the T calculated by the formula (2) after rolling is obtained. Ａ３ The method for producing a clad steel plate according to any one of claims 1 to 3, characterized in that the average cooling rate in the range from 650°C to 2°C / s is 2°C / s or more, and the width in the plate thickness direction of the region where the nanohardness of the interface between the base material and the clad material is 7 GPa or more is 5 μm or less and the ferrite fraction of the surface layer of the base material is more than 15%. T Ａ３ (℃) = 937.2-436.5C + 56Si - 19.7Mn - 26.6Ni + 136.3Ti - 19.1Nb + 198.4Al ..... Equation (2) In the formula, C, Si, Mn, Ni, Ti, Nb and Al are the contents (mass%) of each element in the composition of the base steel plate. For elements that are not contained, 0 is substituted in the formula.

5. A welded structure made using the clad steel plate according to any one of claims 1 to 3.

6. The clad steel plate according to any one of claims 1 to 3, characterized in that the clad steel plate is used for welding using hydrogen as a welding gas.

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