Welded joints

Abstract

The objective is to provide welded joints with excellent resistance to ammonia stress corrosion cracking. [Solution] The present invention relates to a welded joint having two or more steel plates and a welded joint formed by welding the steel plates together, wherein the Vickers hardness of the weld metal at a position 1 mm from the surface of the weld is 230 HV10 or less, and the component composition at a position 1 mm from the surface of the weld metal is 0.28 or less in terms of Ceq, which is expressed by a predetermined relational formula.

Description

Technical Field

[0001] The present invention relates to a welded joint excellent in ammonia stress corrosion cracking resistance, which is suitable for structural members such as tanks used in a liquid ammonia environment.

Background Art

[0002] When carbon steel is used in a liquid ammonia environment, there is a concern about the occurrence of stress corrosion cracking due to liquid ammonia. Therefore, for structures such as carbon steel pipes, storage tanks, tank trucks, and line pipes that handle liquid ammonia, the application of steel materials excellent in ammonia stress corrosion cracking resistance and operational measures to suppress ammonia stress corrosion cracking have been taken.

[0003] It is known that ammonia stress corrosion cracking is correlated with the strength and hardness of materials. Therefore, when using high-strength steel in a liquid ammonia environment, measures such as performing post-weld heat treatment by overall annealing to adjust the hardness of the welded part are necessary.

[0004] On the other hand, in recent years, liquid ammonia has attracted attention as a clean energy because it does not generate CO2 even when burned, and a large demand is expected. Along with this, the enlargement of facilities for transporting and storing liquid ammonia is required.

[0005] Generally, when enlarging a tank, thinning of the steel material to be used is desired from the viewpoints of weight reduction and reduction of construction costs, so the use of high-strength steel is desired.

[0006] Also, in such steel structures, there are many welded joints at the joints. It is known that ammonia stress corrosion cracking also occurs in welded joints, and countermeasures are necessary.

[0007] As methods for achieving both the above high strength and excellent ammonia stress corrosion cracking resistance, Patent Documents 1 to 4 are disclosed.

[0008] Of these, Patent Document 1 describes a method for softening the surface of steel plates used in tanks.

[0009] Furthermore, Patent Documents 2 and 3 describe methods for manufacturing clad steel plates having a mild steel layer on one side, which are used as steel plates for tanks.

[0010] Furthermore, while stainless steel is known as a steel material that suppresses ammonia stress corrosion cracking, it has the problem of being expensive. Therefore, as shown in Patent Document 4, economically superior stainless steel clad steel sheets have been developed by using expensive stainless steel as the cladding material and relatively inexpensive ordinary steel or low-alloy steel as the base material. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Special Publication No. 55-30062 [Patent Document 2] Japanese Patent Application Publication No. 57-139493 [Patent Document 3] Japanese Patent Application Publication No. 8-269537 [Patent Document 4] Japanese Patent Publication No. 2015-105399 [Overview of the project] [Problems that the invention aims to solve]

[0012] However, the method described in Patent Document 1 above has the problem that the thickness of the softened layer on the surface of the steel plate is thin at 500 μm, making it difficult to form a uniform and stable softened layer across the entire surface of the steel plate.

[0013] Furthermore, the methods described in Patent Documents 2 and 3 above, which manufacture clad steel sheets using casting, build-up welding, and continuous casting methods, have economic problems such as the high cost of equipment and energy required for these processes.

[0014] Furthermore, the method described in Patent Document 4 above has an economic problem in that it is costly because it uses stainless steel, which is more expensive than mild steel, as the cladding material. In addition, because the cladding material is a high alloy compared to the base material of ordinary steel or low alloy steel, the deformation resistance during hot rolling at high temperatures, which has the greatest impact on the jointability, is much greater than that of ordinary steel or low alloy steel. This makes it difficult to ensure jointability.

[0015] Furthermore, these patent documents did not consider the ammonia stress corrosion cracking resistance of welded joints.

[0016] The present invention aims to solve the above problems and to provide a welded joint with excellent resistance to ammonia stress corrosion cracking, suitable for use in liquid ammonia transport and storage tanks, etc.

[0017] In this invention, "excellent resistance to ammonia stress corrosion cracking" means that an accelerated test is performed by immersing a test specimen in a test solution and anodic electrolysis to accelerate corrosion, and the maximum crack depth of the test specimen after 504 hours of immersion is evaluated, and the maximum crack depth is less than the maximum crack depth obtained when a conventional steel test specimen was subjected to the same test. This evaluation by accelerated test shall be performed according to the method described in the examples below. [Means for solving the problem]

[0018] To achieve the above objective, the inventors diligently investigated various factors affecting the ammonia stress corrosion cracking resistance of steel plates. As a result, they found that appropriately adjusting the component composition and hardness of the surface layer of the weld metal in the welded joint is effective in improving the ammonia stress corrosion cracking resistance.

[0019] The present invention is based on the above findings, and its gist is as follows. [1] A welded joint having two or more steel plates and a welded portion formed by welding the steel plates together, where the Vickers hardness at a position 1 mm from the surface of the weld metal of the welded portion is 230 HV10 or less, and the component composition at a position 1 mm from the surface of the weld metal is 0.28 or less in Ceq represented by the formula (1). A welded joint. Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 ···(1) Here, [X] shown in the formula (1) indicates the content (mass%) of the X element in the weld metal, and the content of the element not contained is zero. [2] The component composition at a position 1 mm from the surface of the weld metal is, in mass%, C: 0.100% or less, Si: 0.01 - 0.60%, Mn: 0.01 - 1.40%, P: 0.100% or less, S: 0.0500% or less, Al: 0.001 - 0.100%, and further contains Cu: 1.00% or less, Ni: 2.00% or less, Co: 2.00% or less, Cr: 2.00% or less, Sn: 0.50% or less, Sb: 0.50% or less, Mo: 0.50% or less, W: 0.50% or less, V: 0.200% or less, Nb: 0.100% or less, Ti: 0.100% or less, Zr: 0.100% or less, B: 0.0050% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less, N: 0.0300% or less, and O: 0.0300% or less It contains one or more selected from among them, The welded joint described in [1] above, wherein the remainder consists of Fe and unavoidable impurities. [3] The welded joint according to [1] or [2] above, wherein the Vickers hardness at a position 1 mm from the surface of the steel plate is 230 HV10 or less. [4] The steel sheet is a hot-rolled steel sheet, The component composition of the aforementioned hot-rolled steel sheet is, in mass%, C: 0.010~0.200%, Si: 0.01~1.00%, Mn: 0.20~2.50%, P: 0.020% or less, S: 0.0100% or less, Al: 0.010~0.100% N: 0.0010~0.0100%, O: 0.0100% or less It contains, and further, Cu: 1.00% or less, Ni: 2.00% or less, Co: 2.00% or less, Cr: 2.00% or less, Sn: 0.50% or less, Sb: 0.50% or less, Mo: 0.50% or less, W: 0.50% or less, V: 0.200% or less, Ti: 0.100% or less, Nb: 0.100% or less, Zr: 0.100% or less, B: 0.0100% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, REM: 0.0200 or less% It contains one or more selected from among them, The welded joint described in [3] above, wherein the remainder consists of Fe and unavoidable impurities. [5] The steel plate is a clad steel plate having a base material and a cladding material, The component composition of the aforementioned base material is, in mass%, C: 0.010~0.200%, Si: 0.01~1.00%, Mn: 0.20~2.50%, P: 0.020% or less, S: 0.0100% or less, Al: 0.010~0.100%, N: 0.0010~0.0100%, O: 0.0100% or less It contains, and further, Cu: 1.00% or less, Ni: 2.00% or less, Co: 2.00% or less, Cr: 2.00% or less, Sn: 0.50% or less, Sb: 0.50% or less, Mo: 0.50% or less, W: 0.50% or less, V: 0.200% or less, Ti: 0.100% or less, Nb: 0.100% or less, Zr: 0.100% or less, B: 0.0100% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, REM: 0.0200 or less% It contains one or more types selected from among them, The remainder consists of Fe and unavoidable impurities. The component composition of the aforementioned composite material is, in mass%, C: 0.160% or less, Si: 0.01~0.50%, Mn: 0.01~1.80%, P: 0.100% or less, S: 0.0500% or less, Al: 0.001~0.100%, N: 0.0010~0.0100%, O: 0.0100% or less It contains, and further, Cu: 1.00% or less, Ni: 2.00% or less, Co: 2.00% or less, Cr: 2.00% or less, Sn: 0.50% or less, Sb: 0.50% or less, Mo: 0.50% or less, W: 0.50% or less, V: 0.200% or less, Ti: 0.100% or less, Nb: 0.100% or less, Zr: 0.100% or less, B: 0.0005% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less It contains one or more selected from among them, The welded joint described in [3] above, wherein the remainder consists of Fe and unavoidable impurities. [6] The ratio H1 / H2, which is the average value H1 of the Vickers hardness over the entire thickness measured at 1 mm intervals in the thickness direction from the surface of the weld metal, to the Vickers hardness H2 at a position 1 mm from the surface of the weld metal, is greater than 1.0. Furthermore, the welded joint according to any one of the above [1] to [5], wherein the average value H1 of the Vickers hardness over the entire thickness is 180HV10 or higher. [Effects of the Invention]

[0020] According to the present invention, a welded joint is available that exhibits excellent resistance to ammonia stress corrosion cracking and is suitable for structural members such as tanks used in liquid ammonia environments. [Brief explanation of the drawing]

[0021] [Figure 1] Figure 1 is a partially enlarged view illustrating the welded portion and its surrounding area of ​​the welded joint of the present invention. [Figure 2] Figure 2 illustrates an example of the groove shape of a welding location in an embodiment of the present invention. [Figure 3]Figure 3 illustrates an example of a test specimen taken from a welded joint in an embodiment of the present invention. [Modes for carrying out the invention]

[0022] The following describes one embodiment of the present invention. However, the present invention is not limited to this embodiment. In the following description, "%" used to represent the content of constituent elements means "mass%" unless otherwise specified.

[0023] First, the welded joint of the present invention will be described with reference to Figure 1. Figure 1 is a cross-sectional view perpendicular to the welding direction illustrating the weld metal 2 and its surroundings in a welded joint 3, which is one embodiment of the present invention. As an example, Figure 1 shows a state in which the opposing ends of two adjacent steel plates 1 are joined together.

[0024] As shown in Figure 1, the present invention relates to a welded joint 3 formed by welding steel plates together. This welded joint 3 comprises two or more steel plates 1 and a welded joint where the steel plates are joined together. In the present invention, the hardness and component composition of the surface layer of the weld metal 2 in this welded joint are defined as follows.

[0025] (1) Hardness of the surface layer of the weld metal [Vickers hardness at a distance of 1 mm from the surface of the weld metal: 230HV10 or less] The presence of a high-hardness region on the surface of the weld metal promotes ammonia stress corrosion cracking. Specifically, if the hardness of the weld metal surface exceeds 230 HV10, the anodic reaction at the crack tip is accelerated, promoting cracking and preventing the achievement of the desired ammonia stress corrosion cracking resistance. Therefore, in this invention, the hardness of the weld metal surface is defined as the hardness at a position 1 mm from the surface of the weld metal.

[0026] The Vickers hardness at a position 1 mm from the surface of the weld metal shall be 230 HV10 or less. Preferably, the Vickers hardness at this position shall be 210 HV10 or less, and more preferably 200 HV10 or less. There is no particular lower limit for the Vickers hardness at this position. However, if the hardness of the surface layer of the weld metal is too low, it will lead to a decrease in the tensile strength of the weld metal, so preferably, the Vickers hardness at a position 1 mm from the surface of the weld metal shall be 160 HV10 or higher, and more preferably 170 HV10 or higher.

[0027] In this invention, the Vickers hardness at a position 1 mm from the surface of the weld metal is measured in accordance with JIS Z 2244 (2024). The hardness measurement surface is a cross section perpendicular to the welding direction at the center of the longitudinal direction of the weld bead, and the hardness measurement positions are 1 mm from the surface of the weld metal, with four points in the center of the weld metal and four points on each side to the left and right of the center, with a measurement interval of 2 mm. The average value of the Vickers hardness at the nine measurement points is then calculated. The obtained average value is taken as the Vickers hardness at a position 1 mm from the surface of the weld metal.

[0028] (2) Composition of the surface layer of weld metal [Ceq: 0.28 or less] When the Ceq of the surface layer of the weld metal exceeds 0.28, the hardness of the weld metal increases, the anodic reaction at the crack tip is promoted, and cracking is encouraged. As a result, resistance to ammonia stress corrosion cracking deteriorates. Therefore, the Ceq of the weld metal should be 0.28 or less. Preferably, the Ceq should be 0.24 or less. However, since a lower Ceq results in lower hardness and lower susceptibility to ammonia stress corrosion cracking, no lower limit is specifically defined. From the viewpoint of the strength of the weld metal, the Ceq is preferably 0.15 or higher, and more preferably 0.20 or higher. The above Ceq of the weld metal is the value at a position 1 mm from the surface of the weld metal.

[0029] Furthermore, Ceq follows the following formula (1). Ceq=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (1) Here, [X] in equation (1) represents the mass %) content of element X in the weld metal, and elements that are not present are treated as having a content of zero.

[0030] As described above, the welded joint of the present invention can achieve the ammonia stress corrosion cracking resistance targeted in the present invention by having the weld metal surface layer satisfy the above hardness and Ceq. Furthermore, in order to obtain this property more effectively, the steel plate and weld metal used in the welded joint of the present invention may have the following configurations.

[0031] (3) Composition of the surface layer of weld metal The weld metal surface layer of the present invention can achieve its effects as long as it has a component composition that satisfies the above Ceq. Therefore, the component composition of the weld metal is not particularly limited. Suitable weld metals include, for example, those containing the following elements within their respective numerical ranges. Note that the component composition of the weld metal surface layer is the value at a position 1 mm from the surface of the weld metal.

[0032] C: 0.100% or less Carbon (C) is an element that increases the hardness of weld metal, and higher hardness leads to a deterioration in resistance to ammonia stress corrosion cracking. Therefore, the C content should be 0.100% or less. It is more preferable that the C content be 0.08% or less. On the other hand, reducing excess C leads to a surge in refining costs. Therefore, it is preferable that the C content be 0.001% or more.

[0033] Si: 0.01~0.60% Si improves the cleanliness of the weld metal through deoxidation and has the effect of suppressing the occurrence of welding defects such as blowholes. To obtain this effect, the Si content should be 0.01% or more. A Si content of 0.03% or more is more preferable. On the other hand, if the Si content exceeds 0.60%, it remains as nonmetallic inclusions, and the resistance to ammonia stress corrosion cracking deteriorates. Therefore, the Si content should be 0.60% or less. A Si content of 0.40% or less is more preferable.

[0034] Mn: 0.01~1.40% Mn improves the cleanliness of the weld metal through deoxidation and has the effect of suppressing the occurrence of welding defects such as blowholes. To obtain this effect, the Mn content should be 0.01% or more. A Mn content of 0.30% or more is more preferable. On the other hand, if the Mn content exceeds 1.40%, the hardenability of the steel is excessively increased, leading to an increase in hardness and a deterioration of ammonia stress corrosion cracking resistance. Therefore, the Mn content should be 1.40% or less. A Mn content of 1.20% or less is more preferable.

[0035] P:0.100% or less P is an impurity element, and if it is present in amounts exceeding 0.100%, it segregates at grain boundaries, reducing toughness and weldability. Therefore, the P content should be 0.100% or less. It is more preferable that the P content be 0.05% or less. On the other hand, reducing excess P leads to increased refining costs. Therefore, it is preferable that the P content be 0.001% or more. It is more preferable that the P content be 0.010% or more.

[0036] S: 0.0500% or less S is an impurity element, and if present in amounts exceeding 0.0500%, it acts as a starting point for fracture in the form of sulfide-based inclusions such as MnS, reducing the toughness of the steel sheet. Therefore, the S content should be 0.0500% or less. It is more preferable that the S content be 0.0300% or less. On the other hand, reducing excess S leads to increased refining costs. Therefore, it is preferable that the S content be 0.0001% or more. It is more preferable that the S content be 0.0005% or more.

[0037] Al: 0.001~0.100% Al improves the cleanliness of the weld metal through deoxidation and has the effect of suppressing the occurrence of welding defects such as blowholes. To obtain this effect, the Al content should be 0.001% or more. More preferably, the Al content should be 0.010% or more. On the other hand, if the Al content exceeds 0.100%, the number of nonmetallic inclusions increases and the toughness deteriorates. Therefore, the Al content should be 0.100% or less. More preferably, the Al content should be 0.060% or less.

[0038] The remainder of the weld metal surface layer of the present invention, other than the above-mentioned components, consists of Fe and unavoidable impurities. Unavoidable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included in a range that does not hinder the objective of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap.

[0039] The properties desired by the present invention can be obtained with the basic component composition of the weld metal surface described above. Furthermore, in order to further improve the properties, in addition to this basic component composition, one or more elements selected from those listed below may be included as needed. Since each of these elements can be included arbitrarily, each component may be present at 0%.

[0040] Cu:1.00% or less Cu forms a protective film and improves resistance to ammonia stress corrosion cracking. Cu can be included as desired. However, if the Cu content exceeds 1.00%, the effect saturates and the alloy cost increases. Therefore, when Cu is included, it is preferable to keep the Cu content at 1.00% or less. A Cu content of 0.50% or less is more preferable. A Cu content of 0% or more is preferable, and 0.05% or more is more preferable.

[0041] Ni: 2.00% or less Ni is an element that improves the strength of weld metal and can be included as desired. However, if the Ni content exceeds 2.00%, it degrades the resistance to ammonia stress corrosion cracking. Therefore, when Ni is included, it is preferable to keep the Ni content at 2.00% or less. A Ni content of 1.00% or less is more preferable. A Ni content of 0% or more is preferable, and 0.05% or more is more preferable.

[0042] Co:2.00% or less Co is an element that improves the strength of the weld metal and can be included optionally. However, if the Co content exceeds 2.00%, the alloy cost increases. Therefore, when Co is included, it is preferable to keep the Co content at 2.00% or less. A Co content of 1.00% or less is more preferable. A Co content of 0% or more is preferable, and 0.05% or more is more preferable.

[0043] Cr:2.00% or less Cr forms a protective film and improves resistance to ammonia stress corrosion cracking. Cr can be included at will. However, if the Cr content exceeds 2.00%, its effect saturates, and the alloy cost increases. Therefore, when Cr is included, it is preferable to keep the Cr content below 2.00%. A Cr content of 1.00% or less is more preferable. A Cr content of 0% or more is preferable, and 0.10% or more is more preferable.

[0044] Sn: 0.50% or less Sn forms a protective film and improves resistance to ammonia stress corrosion cracking. Sn can be included as desired. However, if the Sn content exceeds 0.50%, the effect saturates and the alloy cost increases. Therefore, when Sn is included, it is preferable to keep the Sn content at 0.50% or less. A Sn content of 0.30% or less is more preferable. A Sn content of 0% or more is preferable, and 0.02% or more is more preferable.

[0045] Sb: 0.50% or less Sb forms a protective film and improves resistance to ammonia stress corrosion cracking. Sb can be included as desired. However, if the Sb content exceeds 0.50%, the effect saturates and the alloy cost increases. Therefore, when Sb is included, it is preferable to keep the Sb content at 0.50% or less. A Sb content of 0.30% or less is more preferable. A Sb content of 0% or more is preferable, and 0.02% or more is more preferable.

[0046] Mo: 0.50% or less Mo is an element that improves the strength of steel plates and can be included as desired. However, if the Mo content exceeds 0.50%, it deteriorates the resistance to ammonia stress corrosion cracking. Therefore, when Mo is included, it is preferable to keep the Mo content at 0.50% or less. A Mo content of 0.30% or less is more preferable. A Mo content of 0% or more is preferable, and 0.02% or more is more preferable.

[0047] W: 0.50% or less W forms a protective film and has the effect of improving resistance to ammonia stress corrosion cracking. W can be included as desired. However, if the W content exceeds 0.50%, the effect saturates and the alloy cost increases. Therefore, when W is included, it is preferable to keep the W content at 0.50% or less. A W content of 0.30% or less is more preferable. A W content of 0% or more is preferable, and 0.02% or more is more preferable.

[0048] V:0.200% or less V is an element that has the effect of improving the strength of the weld metal and can be included optionally. However, if the V content exceeds 0.200%, the hardness of the weld metal increases excessively, leading to a deterioration in ammonia stress corrosion cracking resistance. Therefore, when V is included, it is preferable to keep the V content to 0.200% or less. A V content of 0.100% or less is more preferable. A V content of 0% or more is preferable, and 0.010% or more is more preferable.

[0049] Nb: 0.100% or less Nb is an element that improves the strength of weld metal and can be included as desired. However, if the Nb content exceeds 0.100%, the hardness of the weld metal increases excessively, leading to a deterioration in ammonia stress corrosion cracking resistance. Therefore, when Nb is included, it is preferable to keep the Nb content at 0.100% or less. A Nb content of 0.050% or less is more preferable. A Nb content of 0% or more is preferable, and 0.005% or more is more preferable.

[0050] Ti:0.100% or less Ti is an element that has the effect of improving the strength of the weld metal and can be included as desired. However, if the Ti content exceeds 0.100%, the hardness of the weld metal increases excessively, leading to a deterioration in ammonia stress corrosion cracking resistance. Therefore, when Ti is included, it is preferable to keep the Ti content to 0.100% or less. A Ti content of 0.050% or less is more preferable. A Ti content of 0% or more is preferable, and 0.005% or more is more preferable.

[0051] Zr: 0.100% or less Zr is an element that improves the strength of weld metal and can be included as desired. However, if the Zr content exceeds 0.100%, the hardness of the weld metal increases excessively, leading to a deterioration in ammonia stress corrosion cracking resistance. Therefore, when Zr is included, it is preferable to keep the Zr content below 0.100%. A Zr content of 0.050% or less is more preferable. A Zr content of 0% or more is preferable, and 0.005% or more is more preferable.

[0052] B: 0.0050% or less B is an element that has the effect of improving the strength of the weld metal and can be included optionally. However, if the B content exceeds 0.0050%, it deteriorates the resistance to ammonia stress corrosion cracking. Therefore, when B is included, it is preferable to keep the B content to 0.0050% or less. A B content of 0.0030% or less is more preferable. A B content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0053] Ca:0.0100% or less In steel sheets, Ca is an element that combines with S and suppresses the formation of MnS and other compounds that elongate in the rolling direction. That is, by including Ca in the steel sheet, the morphology of sulfide inclusions can be controlled so that they appear spherical, thereby improving the toughness of the heat-affected zone of the weld. When Ca is included in the steel sheet, the weld metal also contains Ca due to base metal dilution, so the Ca content of the weld metal can be arbitrarily determined. However, if the Ca content of the weld metal exceeds 0.0100%, the cleanliness of the steel sheet decreases. A decrease in cleanliness leads to a decrease in the toughness of the steel sheet. Therefore, when the weld metal contains Ca, it is preferable to keep the Ca content at 0.0100% or less. A Ca content of 0.0050% or less is more preferable. A Ca content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0054] Mg: 0.0100% or less In steel sheets, Mg, like Ca, is an element that combines with S and suppresses the formation of MnS and other compounds that elongate in the rolling direction. That is, by including Mg in the steel sheet, the morphology of sulfide inclusions can be controlled so that they appear spherical, thereby improving the toughness of the heat-affected zone of the weld. When Mg is included in the steel sheet, the weld metal also contains Mg due to base metal dilution, so the weld metal can contain Mg arbitrarily. However, if the Mg content of the weld metal exceeds 0.0100%, the cleanliness of the steel sheet decreases. A decrease in cleanliness leads to a decrease in the toughness of the steel sheet. Therefore, when the weld metal contains Mg, it is preferable to keep the Mg content at 0.0100% or less. A Mg content of 0.0050% or less is more preferable. A Mg content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0055] REM: 0.0200% or less In steel sheets, REM (rare earth metals), like Ca and Mg, are elements that bond with S and suppress the formation of MnS and other elements that elongate in the rolling direction. That is, by including REM in the steel sheet, the morphology of sulfide inclusions can be controlled so that they appear spherical, thereby improving the toughness of the heat-affected zone of the weld. When REM is included in the steel sheet, the weld metal also contains REM due to base metal dilution, so the weld metal can contain REM as desired. However, if the REM content of the weld metal exceeds 0.0200%, the cleanliness of the steel sheet decreases. A decrease in cleanliness leads to a decrease in the toughness of the steel sheet. Therefore, when the weld metal contains REM, it is preferable that the REM content be 0.0200% or less. A REM content of 0.0100% or less is more preferable. A REM content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0056] Here, REM refers to the collective term for 17 elements, which are the 15 lanthanides plus Y and Sc. These elements can be included individually or in combination. Therefore, the REM content represents the total content of these elements.

[0057] N: 0.0300% or less N is an impurity element, and if its content exceeds 0.0300%, coarse nitrides are formed, becoming the starting point for fracture and reducing toughness. Therefore, the N content should be 0.0300% or less. It is more preferable that the N content be 0.0200% or less. On the other hand, reducing excess N leads to increased costs. Therefore, it is preferable that the N content be 0.0020% or more. It is more preferable that the N content be 0.0050% or more.

[0058] O: 0.0300% or less O is an element that forms oxides and improves toughness. However, if the content exceeds 0.0300%, the number density of inclusions that become fracture initiation points increases, reducing toughness. Therefore, the O content should be 0.0300% or less. It is more preferable that the O content be 0.0200% or less. On the other hand, reducing excess O leads to increased costs. Therefore, it is preferable that the O content be 0.0010% or more. It is more preferable that the O content be 0.0030% or more.

[0059] (4) Vickers hardness at a distance of 1 mm from the surface of the steel plate: 230HV10 or less As described above, from the viewpoint of further improving resistance to ammonia stress corrosion cracking, it is also effective to adjust the hardness of the surface layer of steel plates used as structural members for liquid ammonia transport and storage tanks. Therefore, in this invention, the hardness of the steel plate surface layer is defined as the hardness at a position 1 mm from the surface of the steel plate.

[0060] As will be explained in more detail later, the steel plates used as structural members in this invention include hot-rolled steel plates and clad steel plates. In other words, when clad steel plates are used, the above-mentioned "1 mm position from the surface of the steel plate" refers to the hardness of the cladding material, that is, "1 mm position from the surface of the cladding material."

[0061] If the Vickers hardness at a position 1 mm from the surface of the steel plate exceeds 230 HV10, the anodic reaction at the crack tip is accelerated, promoting cracking. As a result, the ammonia stress corrosion cracking resistance deteriorates. Therefore, it is preferable that the Vickers hardness at a position 1 mm from the surface of the steel plate be 230 HV10 or less. It is more preferable that the Vickers hardness at this position be 220 HV10 or less. There is no particular lower limit for the Vickers hardness at a position 1 mm from the surface of the steel plate. From the viewpoint of strength, it is preferable that the Vickers hardness at this position be 160 HV10 or more.

[0062] In this invention, the Vickers hardness at a position 1 mm from the surface of the steel plate is measured in accordance with JIS Z 2244 (2024). The hardness measurement surface is a cross section perpendicular to the welding direction at the center of the longitudinal direction of the weld bead, and the hardness measurement positions are 20 points on each side of the molten boundary line towards the base metal at a position 1 mm from the surface of the steel plate, with a measurement interval of 1 mm. The average value of the Vickers hardness at the 40 measurement points is then calculated. The obtained average value is taken as the Vickers hardness at a position 1 mm from the surface of the steel plate.

[0063] The Vickers hardness of clad steel sheets is measured using the same method as described above.

[0064] (5) Steel plate In this invention, examples of steel plates used for structural members include hot-rolled steel plates and clad steel plates. While the thickness of the steel plate is not specifically defined, it is preferable that the plate thickness be 6 mm or more, and more preferably 50 mm or less, as it is suitable for use as a steel material for liquid ammonia transport and storage tanks, etc. Furthermore, it is more preferable that the plate thickness be 8 mm or more, and more preferably 38 mm or less. (5-1) Hot rolled steel plate First, we will describe examples of suitable component compositions for hot-rolled steel sheets and methods for manufacturing the steel sheets when the steel sheets constituting the welded joint of the present invention are hot-rolled steel sheets.

[0065] [Component composition of hot-rolled steel sheet] C: 0.010~0.200% Carbon (C) is the most effective element for increasing the strength of steel sheets. To achieve this effect, it is preferable to have a C content of 0.010% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost, it is even more preferable to have a C content of 0.020% or more. On the other hand, if the C content exceeds 0.200%, it leads to a deterioration of the toughness and weldability of the steel sheet. Therefore, it is preferable to have a C content of 0.200% or less. Furthermore, from the viewpoint of toughness and weldability, it is even more preferable to have a C content of 0.170% or less.

[0066] Si: 0.01~1.00% Si acts as a deoxidizing agent. To obtain this effect, it is preferable to have a Si content of 0.01% or more, and more preferably 0.03% or more. On the other hand, if the Si content exceeds 1.00%, it leads to deterioration of the toughness and weldability of the steel sheet. Therefore, it is preferable to have a Si content of 1.00% or less. Furthermore, from the viewpoint of toughness and weldability, it is more preferable to have a Si content of 0.60% or less.

[0067] Mn: 0.20~2.50% Mn is an element that increases the hardenability of steel and is one of the elements effective in satisfying the high strength requirement as in the present invention. To obtain this effect, it is preferable to have a Mn content of 0.20% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost, it is even more preferable to have a Mn content of 0.50% or more. On the other hand, if the Mn content exceeds 2.50%, toughness and weldability decrease, and the alloy cost becomes excessively high. Therefore, it is preferable to have a Mn content of 2.50% or less. Furthermore, from the viewpoint of suppressing the decrease in toughness and weldability, it is even more preferable to have a Mn content of 2.30% or less.

[0068] P:0.020% or less P has adverse effects, such as reducing toughness and weldability by segregating at grain boundaries. Therefore, it is desirable to keep the P content as low as possible, but a value of 0.020% or less is acceptable. There is no particular lower limit to the P content, and it may be 0%, however, P is usually an element that is inevitably contained in steel as an impurity, and industrially it may be greater than 0%. Furthermore, since reducing excess P leads to increased refining costs, it is preferable to keep the P content at 0.001% or higher.

[0069] S: 0.0100% or less S (S) is present in steel as sulfide inclusions such as MnS, and is an element that has adverse effects, such as becoming the starting point for fracture and reducing the toughness of steel sheets. Therefore, it is desirable to keep the S content as low as possible, but it is acceptable if it is 0.0100% or less. There is no particular lower limit to the S content, and it may be 0%. Normally, S is an element that is inevitably present in steel as an impurity, and industrially it may be greater than 0%. Furthermore, reducing excess S leads to a rise in refining costs, so from a cost perspective, it is preferable to have an S content of 0.0001% or more.

[0070] Al: 0.010~0.100% Al acts as a deoxidizing agent. To obtain this effect, it is preferable to have an Al content of 0.010% or more. On the other hand, if the Al content exceeds 0.100%, oxide-based inclusions increase, reducing cleanliness and toughness. Therefore, it is preferable to have an Al content of 0.100% or less. Furthermore, from the viewpoint of preventing toughness degradation, it is even more preferable to have an Al content of 0.050% or less.

[0071] N: 0.0010~0.0100% N combines with Ti to precipitate as TiN, contributing to microstructure refinement and improving the toughness of the steel sheet. To obtain this effect, it is preferable to have an N content of 0.0010% or more, and more preferably 0.0020% or more. On the other hand, if the N content exceeds 0.0100%, it can actually lead to a decrease in toughness. Therefore, it is preferable to have an N content of 0.0100% or less. Furthermore, from the viewpoint of suppressing a decrease in toughness and weldability, it is more preferable to have an N content of 0.0080% or less.

[0072] O: 0.0100% or less Since oxygen (O) is an element that has adverse effects, such as forming oxides, becoming the starting point for fracture, and reducing the toughness of steel sheets, it is preferable to keep the O content at 0.0100% or less. It is more preferable to keep the O content at 0.0050% or less, and even more preferable to keep it at 0.0030% or less. On the other hand, there is no particular lower limit to the O content, and it may be 0%, however, O is an element that is usually inevitably contained in steel as an impurity, and industrially it may be greater than 0%. Furthermore, since reducing the excess O leads to a rise in refining costs, from a cost standpoint it is preferable to keep the O content at 0.0005% or more.

[0073] The remainder of the hot-rolled steel sheet in this invention, other than the above-mentioned components, consists of Fe and unavoidable impurities. The above-mentioned effects can be obtained with the basic component composition of the hot-rolled steel sheet. However, in order to further improve the properties, in addition to this basic component composition, one or more elements selected from the elements described below may be included as needed. Since each of these elements can be included arbitrarily, each component may be present at 0%.

[0074] Cu:1.00% or less Cu forms a protective film with appropriate properties and improves resistance to ammonia stress corrosion cracking. Cu can be included as desired. However, if the Cu content exceeds 1.00%, the effect saturates and the alloy cost increases. Therefore, when Cu is included, it is preferable to keep the Cu content at 1.00% or less. A Cu content of 0.50% or less is more preferable. A Cu content of 0% or more is preferable, and 0.05% or more is more preferable.

[0075] Ni: 2.00% or less Ni is an element that improves the strength and toughness of steel plates and can be included as desired. However, if the Ni content exceeds 2.00%, the resistance to ammonia stress corrosion cracking deteriorates. Therefore, when Ni is included, it is preferable to keep the Ni content at 2.00% or less. A Ni content of 1.00% or less is more preferable. A Ni content of 0% or more is preferable, and 0.05% or more is more preferable.

[0076] Co:2.00% or less Co is an element that improves the strength and toughness of steel sheets and can be included as desired. However, if the Co content exceeds 2.00%, the effect saturates, and the alloy cost increases. Therefore, when Co is included, it is preferable to keep the Co content at 2.00% or less. A Co content of 1.00% or less is more preferable. A Co content of 0% or more is preferable, and 0.05% or more is more preferable.

[0077] Cr:2.00% or less Cr forms a protective film with appropriate properties, improving resistance to ammonia stress corrosion cracking. However, if the Cr content exceeds 2.00%, the effect saturates, and the alloy cost increases. Therefore, when Cr is included, it is preferable to keep the Cr content at 2.00% or less. A Cr content of 1.00% or less is more preferable. A Cr content of 0% or more is preferable, and 0.10% or more is more preferable.

[0078] Sn: 0.50% or less Sn forms a protective film with appropriate properties, improving resistance to ammonia stress corrosion cracking. However, if the Sn content exceeds 0.50%, the effect saturates, and the alloy cost increases. Therefore, when Sn is included, it is preferable to keep the Sn content at 0.50% or less. A Sn content of 0.30% or less is more preferable. A Sn content of 0% or more is preferable, and 0.02% or more is more preferable.

[0079] Sb: 0.50% or less Sb forms a protective film with appropriate properties, improving resistance to ammonia stress corrosion cracking. However, if the Sb content exceeds 0.50%, the effect saturates, and the alloy cost increases. Therefore, when Sb is included, it is preferable to keep the Sb content at 0.50% or less. A Sb content of 0.30% or less is more preferable. A Sb content of 0% or more is preferable, and 0.02% or more is more preferable.

[0080] Mo: 0.50% or less Mo is an element that improves the strength of steel plates and can be included as desired. However, if the Mo content exceeds 0.50%, it deteriorates the resistance to ammonia stress corrosion cracking. Therefore, when Mo is included, it is preferable to keep the Mo content at 0.50% or less. A Mo content of 0.30% or less is more preferable. A Mo content of 0% or more is preferable, and 0.02% or more is more preferable.

[0081] W: 0.50% or less W forms a protective film with appropriate properties, improving resistance to ammonia stress corrosion cracking. However, if the W content exceeds 0.50%, the effect saturates, and the alloy cost increases. Therefore, when W is included, it is preferable to keep the W content at 0.50% or less. A W content of 0.30% or less is more preferable. A W content of 0% or more is preferable, and 0.02% or more is more preferable.

[0082] V:0.200% or less V is an element that has the effect of improving the strength of steel plates and can be included as desired. However, if the V content exceeds 0.200%, the hardness increases excessively, leading to a deterioration in resistance to ammonia stress corrosion cracking. Therefore, when V is included, it is preferable to keep the V content to 0.200% or less. A V content of 0.100% or less is more preferable. A V content of 0% or more is preferable, and 0.010% or more is more preferable.

[0083] Nb: 0.100% or less Nb is an element that improves toughness by reducing the size of the prior austenite grain size through precipitation as carbonitrides. However, if the Nb content exceeds 0.100%, a large amount of NbC precipitates, reducing toughness. Therefore, when Nb is included, it is preferable to keep the Nb content below 0.100%. A Nb content of 0.050% or less is more preferable. A Nb content of 0% or more is preferable, and 0.005% or more is more preferable.

[0084] Ti:0.100% or less Ti has a strong tendency to form nitrides, and is an element that has the effect of improving toughness by forming TiN, and can be included as desired. However, if the Ti content exceeds 0.100%, toughness will actually decrease. Therefore, when Ti is included, it is preferable to keep the Ti content to 0.100% or less. A Ti content of 0.050% or less is more preferable. A Ti content of 0% or more is preferable, and 0.005% or more is more preferable.

[0085] Zr: 0.100% or less Zr is an element that improves the strength of steel plates and can be included as desired. However, if the Zr content exceeds 0.100%, the hardness increases excessively, leading to a deterioration in resistance to ammonia stress corrosion cracking. Therefore, when Zr is included, it is preferable to keep the Zr content below 0.100%. A Zr content of 0.050% or less is more preferable. A Zr content of 0% or more is preferable, and 0.005% or more is more preferable.

[0086] B: 0.0100% or less B is an element that significantly improves hardenability even in trace amounts. In other words, it can improve the strength of steel plates. B can be included at will. However, if the B content exceeds 0.0100%, the weldability decreases. Therefore, when B is included, it is preferable to keep the B content to 0.0100% or less. A B content of 0.0030% or less is more preferable. A B content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0087] Ca:0.0200% or less Ca is an element that combines with S and suppresses the formation of MnS and other elements that elongate in the rolling direction. In other words, by including Ca, the morphology of sulfide inclusions can be controlled so that they appear spherical, thereby improving the toughness of the HAZ. Ca can be included at will. However, if the Ca content exceeds 0.0200%, the cleanliness of the steel decreases. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Ca is included, it is preferable to keep the Ca content at 0.0200% or less. A Ca content of 0.0050% or less is more preferable. A Ca content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0088] Mg: 0.0200% or less Like Ca, Mg is an element that combines with S and suppresses the formation of MnS and other compounds that elongate in the rolling direction. In other words, by including Mg, the morphology of sulfide inclusions can be controlled so that they appear spherical, thereby improving the toughness of the HAZ. Mg can be included at will. However, if the Mg content exceeds 0.0200%, the cleanliness of the steel decreases. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Mg is included, it is preferable to keep the Mg content at 0.0200% or less. A Mg content of 0.0050% or less is more preferable. A Mg content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0089] REM: 0.0200% or less Rare earth metals (REMs), like Ca and Mg, are elements that bond with sulfur and suppress the formation of MnS and other elements that elongate in the rolling direction. In other words, by including REMs, the morphology of sulfide inclusions can be controlled to be spherical, thereby improving the toughness of the heat-affected zone (HAZ). REMs can be included as desired. However, if the REM content exceeds 0.0200%, the cleanliness of the steel decreases. A decrease in cleanliness leads to a decrease in toughness. Therefore, when REMs are included, it is preferable that the REM content be 0.0200% or less. A REM content of 0.0100% or less is more preferable. A REM content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0090] Here, REM refers to the collective term for 17 elements, which are the 15 lanthanides plus Y and Sc. These elements can be included individually or in combination. Therefore, the REM content represents the total content of these elements.

[0091] [Manufacturing method for hot-rolled steel sheets] Here, an example of a preferred method for manufacturing the hot-rolled steel sheet used in the present invention will be described. This hot-rolled steel sheet is manufactured by heating a steel material (steel slab) having the above-mentioned component composition, hot-rolling the steel material, then accelerating cooling, and immediately applying heat treatment.

[0092] First, there is no particular need to limit the manufacturing conditions of the steel material, but it is preferable to, for example, melt molten steel having the above-mentioned component composition using a known melting method such as a converter, and then use a known casting method such as continuous casting to produce steel material such as slabs of a predetermined size. There is no problem in producing steel material such as slabs of a predetermined size by ingot-part rolling.

[0093] The steel material obtained in this way can be hot-rolled directly without cooling, or it can be hot-rolled after being reheated. After the hot-rolling, controlled rolling or accelerated cooling may be performed, or tempering may be performed. Alternatively, after the hot-rolling, the material may be cooled to room temperature, then reheated, quenched, and tempered again.

[0094] (5-2) Clad steel plate Next, we will describe a suitable base material composition and cladding material composition, as well as an example of a method for manufacturing the cladding steel sheet, when the steel sheet constituting the welded joint of the present invention is a clad steel sheet.

[0095] [Component composition of the base material of clad steel sheets] C: 0.010~0.200% Carbon (C) is the most effective element for increasing the strength of steel sheets. To achieve this effect, it is preferable to have a C content of 0.010% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost, it is even more preferable to have a C content of 0.030% or more. On the other hand, if the C content exceeds 0.200%, it leads to a deterioration of the toughness and weldability of the steel sheet. Therefore, it is preferable to have a C content of 0.200% or less. Furthermore, from the viewpoint of toughness, it is even more preferable to have a C content of 0.170% or less.

[0096] Si: 0.01~1.00% Si acts as a deoxidizing agent. To obtain this effect, it is preferable to have a Si content of 0.01% or more. It is more preferable to have a Si content of 0.03% or more. On the other hand, if the Si content exceeds 1.00%, it leads to deterioration of the toughness and weldability of the steel sheet. Therefore, it is preferable to have a Si content of 1.00% or less. Furthermore, from the viewpoint of toughness, it is more preferable to have a Si content of 0.40% or less.

[0097] Mn: 0.20~2.50% Mn is an element that increases the hardenability of steel. To obtain this effect, it is preferable to have a Mn content of 0.20% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost, it is even more preferable to have a Mn content of 0.50% or more. On the other hand, if the Mn content exceeds 2.50%, the toughness decreases. Therefore, it is preferable to have a Mn content of 2.50% or less. Furthermore, from the viewpoint of suppressing the decrease in toughness, it is even more preferable to have a Mn content of 2.30% or less.

[0098] P:0.020% or less P has adverse effects, such as reducing toughness and weldability by segregating at grain boundaries. Therefore, it is desirable to keep the P content as low as possible, but a value of 0.020% or less is acceptable. There is no particular lower limit to the P content, and it may be 0%, but P is usually an element that is inevitably present in steel as an impurity, and industrially it may be greater than 0%. Furthermore, since reducing excess P leads to a rise in refining costs, it is preferable to keep the P content at 0.001% or higher.

[0099] S: 0.0100% or less S (S) is present in steel as sulfide inclusions such as MnS, and is an element that has adverse effects, such as becoming the starting point for fracture and reducing the toughness of steel sheets. Therefore, it is desirable to keep the S content as low as possible, but it is acceptable if it is 0.0100% or less. There is no particular lower limit to the S content, and it may be 0%, but S is usually an element that is inevitably present in steel as an impurity, and industrially it may be greater than 0%. Furthermore, reducing excess S leads to a rise in refining costs, so from a cost perspective, it is preferable to have an S content of 0.0005% or more.

[0100] Al: 0.010~0.100% Al acts as a deoxidizing agent. To obtain this effect, it is preferable to have an Al content of 0.010% or more. On the other hand, if the Al content exceeds 0.100%, oxide-based inclusions increase, reducing cleanliness and toughness. Therefore, it is preferable to have an Al content of 0.100% or less. Furthermore, from the viewpoint of preventing toughness degradation, it is even more preferable to have an Al content of 0.050% or less.

[0101] N: 0.0010~0.0100% N combines with Ti to precipitate as TiN, contributing to microstructure refinement and improving the toughness of the steel sheet. To obtain this effect, it is preferable to have an N content of 0.0010% or more, and more preferably 0.0020% or more. On the other hand, if the N content exceeds 0.0100%, it can actually lead to a decrease in toughness. Therefore, it is preferable to have an N content of 0.0100% or less. Furthermore, from the viewpoint of suppressing a decrease in toughness and weldability, it is more preferable to have an N content of 0.0080% or less.

[0102] O: 0.0100% or less O is an element that has adverse effects, such as forming oxides, becoming the starting point for fracture, and reducing the toughness of steel sheets. For this reason, it is preferable that the O content be 0.0100% or less. It is more preferable that the O content be 0.0050% or less, and even more preferable that be 0.0030% or less. On the other hand, there is no particular lower limit to the O content, and it may be 0%, however, O is usually an element that is inevitably contained in steel as an impurity, and industrially it may be greater than 0%. Furthermore, since reducing excess O leads to a rise in refining costs, from a cost standpoint, it is preferable that the O content be 0.0005% or more.

[0103] The remainder of the base material of the clad steel sheet in this invention, other than the above-mentioned components, consists of Fe and unavoidable impurities. The above-mentioned effects can be obtained with the basic component composition of the clad steel sheet base material. However, in order to further improve the properties, in addition to this basic component composition, one or more elements selected from the elements described below may be included as needed. Since each of these elements can be included arbitrarily, each component may be present at 0%.

[0104] Cu:1.00% or less Cu is an element that has the effect of increasing the hardenability of steel and can be included as desired. On the other hand, if the Cu content exceeds 1.00%, the effect saturates, weldability deteriorates, and alloy costs increase. Therefore, when Cu is included, it is preferable to keep the Cu content at 1.00% or less. A Cu content of 0.50% or less is more preferable. A Cu content of 0% or more is preferable, and 0.05% or more is more preferable.

[0105] Ni: 2.00% or less Ni is an element that improves the strength and toughness of steel plates and can be included as desired. On the other hand, if the Ni content exceeds 2.00%, the weldability deteriorates and the alloy cost increases. Therefore, when Ni is included, it is preferable to keep the Ni content at 2.00% or less. A Ni content of 1.00% or less is more preferable. A Ni content of 0% or more is preferable, and 0.05% or more is more preferable.

[0106] Co:2.00% or less Co is an element that improves the strength and toughness of steel sheets and can be included as desired. However, if the Co content exceeds 2.00%, the effect saturates, and the alloy cost increases. Therefore, when Co is included, it is preferable to keep the Co content at 2.00% or less. A Co content of 1.00% or less is more preferable. A Co content of 0% or more is preferable, and 0.05% or more is more preferable.

[0107] Cr:2.00% or less Cr is an element that increases the hardenability of steel and can be included as desired. However, if the Cr content exceeds 2.00%, the effect saturates, and the alloy cost increases. Therefore, when Cr is included, it is preferable to keep the Cr content at 2.00% or less. A Cr content of 1.00% or less is more preferable. A Cr content of 0% or more is preferable, and 0.10% or more is more preferable.

[0108] Sn: 0.50% or less Sn forms a protective film with appropriate properties, improving resistance to ammonia stress corrosion cracking. However, if the Sn content exceeds 0.50%, the effect saturates, and the alloy cost increases. Therefore, when Sn is included, it is preferable to keep the Sn content at 0.50% or less. A Sn content of 0.30% or less is more preferable. A Sn content of 0% or more is preferable, and 0.02% or more is more preferable.

[0109] Sb: 0.50% or less Sb forms a protective film with appropriate properties, improving resistance to ammonia stress corrosion cracking. However, if the Sb content exceeds 0.50%, the effect saturates, and the alloy cost increases. Therefore, when Sb is included, it is preferable to keep the Sb content at 0.50% or less. A Sb content of 0.30% or less is more preferable. A Sb content of 0% or more is preferable, and 0.02% or more is more preferable.

[0110] Mo: 0.50% or less Mo is an element that improves the strength of steel sheets and can be included as desired. However, if the Mo content exceeds 0.50%, the effect saturates, and the alloy cost increases. Therefore, when Mo is included, it is preferable to keep the Mo content at 0.50% or less. A Mo content of 0.30% or less is more preferable. A Mo content of 0% or more is preferable, and 0.02% or more is more preferable.

[0111] W: 0.50% or less W is an element that has the effect of improving the strength of steel sheets and can be included as desired. On the other hand, if the W content exceeds 0.50%, the effect saturates, and the alloy cost increases. Therefore, when W is included, it is preferable to keep the W content at 0.50% or less. A W content of 0.30% or less is more preferable. A W content of 0% or more is preferable, and 0.02% or more is more preferable.

[0112] V:0.200% or less V is an element that has the effect of improving the strength of steel plates and can be included as desired. However, if the V content exceeds 0.200%, it leads to a deterioration of toughness. Therefore, when V is included, it is preferable to keep the V content to 0.200% or less. A V content of 0.100% or less is more preferable. A V content of 0% or more is preferable, and 0.010% or more is more preferable.

[0113] Ti:0.100% or less Ti has a strong tendency to form nitrides, and is an element that has the effect of improving toughness by forming TiN, and can be included as desired. However, if the Ti content exceeds 0.100%, toughness will actually decrease. Therefore, when Ti is included, it is preferable to keep the Ti content to 0.100% or less. A Ti content of 0.050% or less is more preferable. A Ti content of 0% or more is preferable, and 0.005% or more is more preferable.

[0114] Nb: 0.100% or less Nb is an element that improves toughness by reducing the size of the prior austenite grain size through precipitation as carbonitrides. However, if the Nb content exceeds 0.100%, a large amount of NbC precipitates, reducing toughness. Therefore, when Nb is included, it is preferable to keep the Nb content below 0.100%. A Nb content of 0.050% or less is more preferable. A Nb content of 0% or more is preferable, and 0.005% or more is more preferable.

[0115] Zr: 0.100% or less Zr is an element that improves the strength of steel sheets and can be included as desired. However, if the Zr content exceeds 0.100%, it leads to a deterioration of toughness. Therefore, when Zr is included, it is preferable to keep the Zr content to 0.100% or less. A Zr content of 0.050% or less is more preferable. A Zr content of 0% or more is preferable, and 0.005% or more is more preferable.

[0116] B: 0.0100% or less B is an element that significantly improves hardenability even in trace amounts. In other words, it can improve the strength of steel plates. B can be included at will. However, if the B content exceeds 0.0100%, the weldability decreases. Therefore, when B is included, it is preferable to keep the B content to 0.0100% or less. A B content of 0.0030% or less is more preferable. A B content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0117] Ca:0.0200% or less Ca is an element that combines with S and suppresses the formation of MnS and other elements that elongate in the rolling direction. In other words, by including Ca, the morphology of sulfide inclusions can be controlled so that they appear spherical, thereby improving the toughness of the HAZ. Ca can be included at will. However, if the Ca content exceeds 0.0200%, the cleanliness of the steel decreases. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Ca is included, it is preferable to keep the Ca content at 0.0200% or less. A Ca content of 0.0050% or less is more preferable. A Ca content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0118] Mg: 0.0200% or less Like Ca, Mg is an element that combines with S and suppresses the formation of MnS and other compounds that elongate in the rolling direction. In other words, by including Mg, the morphology of sulfide inclusions can be controlled so that they appear spherical, thereby improving the toughness of the HAZ. Mg can be included at will. However, if the Mg content exceeds 0.0200%, the cleanliness of the steel decreases. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Mg is included, it is preferable to keep the Mg content at 0.0200% or less. A Mg content of 0.0050% or less is more preferable. A Mg content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0119] REM: 0.0200% or less Rare earth metals (REMs), like Ca and Mg, are elements that bond with sulfur and suppress the formation of MnS and other elements that elongate in the rolling direction. In other words, by including REMs, the morphology of sulfide inclusions can be controlled to be spherical, thereby improving the toughness of the heat-affected zone (HAZ). REMs can be included as desired. However, if the REM content exceeds 0.0200%, the cleanliness of the steel decreases. A decrease in cleanliness leads to a decrease in toughness. Therefore, when REMs are included, it is preferable that the REM content be 0.0200% or less. A REM content of 0.0100% or less is more preferable. A REM content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0120] Here, REM refers to the collective term for 17 elements, which are the 15 lanthanides plus Y and Sc. These elements can be included individually or in combination. Therefore, the REM content represents the total content of these elements.

[0121] [Component composition of the laminated material for clad steel sheets] C: 0.160% or less Carbon (C) is an element that increases the hardness of steel plates, and higher hardness leads to a deterioration in resistance to ammonia stress corrosion cracking. Therefore, it is preferable to keep the C content at 0.160% or less. On the other hand, reducing excess C leads to a surge in refining costs. Therefore, it is preferable to keep the C content at 0.001% or more.

[0122] Si: 0.01~0.50% Si acts as a deoxidizing agent. To obtain this effect, it is preferable to have a Si content of 0.01% or more, and more preferably 0.03% or more. On the other hand, if the Si content exceeds 0.50%, it remains as nonmetallic inclusions, and the resistance to ammonia stress corrosion cracking deteriorates. Therefore, it is preferable to have a Si content of 0.50% or less.

[0123] Mn: 0.01~1.80% Mn acts as a deoxidizing agent. To obtain this effect, it is preferable to have a Mn content of 0.01% or more. On the other hand, if the Mn content exceeds 1.80%, it excessively increases the hardenability of the steel and increases its hardness, leading to a deterioration in its resistance to ammonia stress corrosion cracking. Therefore, it is preferable to have a Mn content of 1.80% or less.

[0124] P:0.100% or less P is an impurity element, and its presence exceeding 0.100% degrades resistance to ammonia stress corrosion cracking. Therefore, it is preferable to keep the P content below 0.100%. On the other hand, reducing excess P leads to increased refining costs. Therefore, it is preferable to keep the P content above 0.001%.

[0125] S: 0.0500% or less S is an impurity element, and its presence exceeding 0.0500% degrades resistance to ammonia stress corrosion cracking. Therefore, it is preferable to keep the S content below 0.0500%. On the other hand, reducing excess S leads to increased refining costs. Therefore, it is preferable to keep the S content above 0.0001%.

[0126] Al: 0.001~0.100% Al acts as a deoxidizing agent. To obtain this effect, it is preferable to have an Al content of 0.001% or more. On the other hand, if the Al content exceeds 0.100%, the number of nonmetallic inclusions increases, and the resistance to ammonia stress corrosion cracking deteriorates. Therefore, it is preferable to have an Al content of 0.100% or less.

[0127] N: 0.0010~0.0100% N combines with Ti to precipitate as TiN, contributing to microstructure refinement and improving the toughness of the steel sheet. To obtain this effect, it is preferable to have an N content of 0.0010% or more, and more preferably 0.0020% or more. On the other hand, if the N content exceeds 0.0100%, it can actually lead to a decrease in toughness. Therefore, it is preferable to have an N content of 0.0100% or less. Furthermore, from the viewpoint of suppressing a decrease in toughness and weldability, it is more preferable to have an N content of 0.0080% or less.

[0128] O: 0.0100% or less Since oxygen (O) forms oxides, acts as a starting point for fracture, and has adverse effects such as reducing the toughness of steel sheets, it is preferable to keep the O content below 0.0100%. It is more preferable to keep the O content below 0.0050%, and even more preferable to keep it below 0.0030%. On the other hand, there is no particular lower limit to the O content, and it may be 0%, however, O is usually an element that is inevitably contained in steel as an impurity, and industrially it may be greater than 0%. Furthermore, since reducing the excess O leads to a rise in refining costs, from a cost standpoint, it is preferable to keep the O content above 0.0005%.

[0129] The remainder of the clad steel sheet composite material in this invention, other than the above-mentioned components, consists of Fe and unavoidable impurities. The above-mentioned effects can be obtained with the basic component composition of the clad steel sheet composite material. However, in order to further improve the properties, in addition to this basic component composition, one or more elements selected from the elements described below may be included as needed. Since each of these elements can be included arbitrarily, each component may be present at 0%.

[0130] Cu:1.00% or less Cu forms a protective film with appropriate properties and improves resistance to ammonia stress corrosion cracking. Cu can be included as desired. However, if the Cu content exceeds 1.00%, the effect saturates and the alloy cost increases. Therefore, when Cu is included, it is preferable to keep the Cu content at 1.00% or less. A Cu content of 0.50% or less is more preferable. A Cu content of 0% or more is preferable, and 0.05% or more is more preferable.

[0131] Ni: 2.00% or less Ni is an element that improves the strength of steel plates and can be included as desired. However, if the Ni content exceeds 2.00%, it deteriorates the resistance to ammonia stress corrosion cracking. Therefore, when Ni is included, it is preferable to keep the Ni content at 2.00% or less. A Ni content of 1.00% or less is more preferable. A Ni content of 0% or more is preferable, and 0.05% or more is more preferable.

[0132] Co:2.00% or less Co is an element that improves the strength and toughness of steel sheets and can be included as desired. However, if the Co content exceeds 2.00%, the effect saturates, and the alloy cost increases. Therefore, when Co is included, it is preferable to keep the Co content at 2.00% or less. A Co content of 1.00% or less is more preferable. A Co content of 0% or more is preferable, and 0.05% or more is more preferable.

[0133] Cr:2.00% or less Cr forms a protective film with appropriate properties and improves resistance to ammonia stress corrosion cracking. Cr can be included at will. However, if the Cr content exceeds 2.00%, its effect saturates, and the alloy cost increases. Therefore, when Cr is included, it is preferable to keep the Cr content below 2.00%. A Cr content of 1.00% or less is more preferable. A Cr content of 0% or more is preferable, and 0.10% or more is more preferable.

[0134] Sn: 0.50% or less Sn forms a protective film with appropriate properties and improves resistance to ammonia stress corrosion cracking. Sn can be included as desired. However, if the Sn content exceeds 0.50%, its effect saturates, and the alloy cost increases. Therefore, when Sn is included, it is preferable to keep the Sn content at 0.50% or less. A Sn content of 0.30% or less is more preferable. A Sn content of 0% or more is preferable, and 0.02% or more is more preferable.

[0135] Sb: 0.50% or less Sb forms a protective film with appropriate properties and improves resistance to ammonia stress corrosion cracking. Sb can be included as desired. However, if the Sb content exceeds 0.50%, its effect saturates and the alloy cost increases. Therefore, when Sb is included, it is preferable to keep the Sb content at 0.50% or less. A Sb content of 0.30% or less is more preferable. A Sb content of 0% or more is preferable, and 0.02% or more is more preferable.

[0136] Mo: 0.50% or less Mo is an element that improves the strength of steel plates and can be included as desired. However, if the Mo content exceeds 0.50%, it deteriorates the resistance to ammonia stress corrosion cracking. Therefore, when Mo is included, it is preferable to keep the Mo content at 0.50% or less. A Mo content of 0.30% or less is more preferable. A Mo content of 0% or more is preferable, and 0.02% or more is more preferable.

[0137] W: 0.50% or less W forms a protective film and has the effect of improving resistance to ammonia stress corrosion cracking. W can be included as desired. However, if the W content exceeds 0.50%, the effect saturates and the alloy cost increases. Therefore, when W is included, it is preferable to keep the W content at 0.50% or less. A W content of 0.30% or less is more preferable. A W content of 0% or more is preferable, and 0.02% or more is more preferable.

[0138] V:0.200% or less V is an element that has the effect of improving the strength of steel plates and can be included as desired. However, if the V content exceeds 0.200%, the hardness increases excessively, leading to a deterioration in resistance to ammonia stress corrosion cracking. Therefore, when V is included, it is preferable to keep the V content to 0.200% or less. A V content of 0.100% or less is more preferable. A V content of 0% or more is preferable, and 0.010% or more is more preferable.

[0139] Ti:0.100% or less Ti is an element that improves the strength of steel plates and can be included as desired. However, if the Ti content exceeds 0.100%, the hardness increases excessively, leading to a deterioration in resistance to ammonia stress corrosion cracking. Therefore, when Ti is included, it is preferable to keep the Ti content at 0.100% or less. A Ti content of 0.050% or less is more preferable. A Ti content of 0% or more is preferable, and 0.010% or more is more preferable.

[0140] Nb: 0.100% or less Nb is an element that improves the strength of steel plates and can be included as desired. However, if the Nb content exceeds 0.100%, the hardness increases excessively, leading to a deterioration in resistance to ammonia stress corrosion cracking. Therefore, when Nb is included, it is preferable to keep the Nb content at 0.100% or less. A Nb content of 0.050% or less is more preferable. A Nb content of 0% or more is preferable, and 0.010% or more is more preferable.

[0141] Zr: 0.100% or less Zr is an element that improves the strength of steel plates and can be included as desired. However, if the Zr content exceeds 0.100%, the hardness increases excessively, leading to a deterioration in resistance to ammonia stress corrosion cracking. Therefore, when Zr is included, it is preferable to keep the Zr content below 0.100%. A Zr content of 0.050% or less is more preferable. A Zr content of 0% or more is preferable, and 0.010% or more is more preferable.

[0142] B: 0.0050% or less B is an element that has the effect of improving the strength of steel plates and can be included as desired. However, if the B content exceeds 0.0050%, it deteriorates the resistance to ammonia stress corrosion cracking. Therefore, when B is included, it is preferable to keep the B content to 0.0005% or less. A B content of 0.0030% or less is more preferable. A B content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0143] Ca:0.0100% or less Ca is an element that combines with S and suppresses the formation of MnS and other elements that elongate in the rolling direction. In other words, by including Ca, the morphology of sulfide inclusions can be controlled so that they exhibit a spherical shape, thereby improving the toughness of the HAZ. Ca can be included at will. However, if the Ca content exceeds 0.0100%, the cleanliness of the steel decreases. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Ca is included, it is preferable to keep the Ca content at 0.0100% or less. A Ca content of 0.0050% or less is more preferable. A Ca content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0144] Mg: 0.0100% or less Like Ca, Mg is an element that combines with S and suppresses the formation of MnS and other compounds that elongate in the rolling direction. In other words, by including Mg, the morphology of sulfide inclusions can be controlled so that they appear spherical, thereby improving the toughness of the HAZ. Mg can be included at will. However, if the Mg content exceeds 0.0100%, the cleanliness of the steel decreases. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Mg is included, it is preferable to keep the Mg content to 0.0100% or less. A Mg content of 0.0050% or less is more preferable. A Mg content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0145] REM: 0.0200% or less Rare earth metals (REMs), like Ca and Mg, are elements that bond with sulfur and suppress the formation of MnS and other elements that elongate in the rolling direction. In other words, by including REMs, the morphology of sulfide inclusions can be controlled to be spherical, thereby improving the toughness of the heat-affected zone (HAZ). REMs can be included as desired. On the other hand, if the REM content exceeds 0.0200%, the cleanliness of the steel decreases. A decrease in cleanliness leads to a decrease in toughness. Therefore, when REMs are included, it is preferable that the REM content be 0.0200% or less. A REM content of 0.0100% or less is more preferable. A REM content of 0% or more is preferable, and 0.0005% or more is more preferable.

[0146] Here, REM refers to the collective term for 17 elements, which are the 15 lanthanides plus Y and Sc. These elements can be included individually or in combination. Therefore, the REM content represents the total content of these elements.

[0147] [Manufacturing conditions for clad steel sheets] Here, we will describe an example of a preferred method for manufacturing the clad steel sheet used in the present invention.

[0148] First, the base material steel sheet and the composite material steel sheet are manufactured. There are no particular restrictions on the manufacturing method of these base material steel sheets, and conventionally known steel sheet manufacturing methods can be applied. That is, molten steel adjusted to the above-mentioned preferred component composition by a conventional melting method (e.g., converter method, electric furnace method, etc.) is cast by a conventional casting method (e.g., continuous casting method or ingot making method), and the resulting cast material is hot-rolled to produce steel sheets of predetermined dimensions.

[0149] Next, using these base material steel plates and cladding material steel plates, assembled slabs for clad rolling (hereinafter referred to as "assembled slabs") are assembled.

[0150] The assembled slab can be manufactured using known methods, and for example, a form in which two cladding materials are overlapped by sandwiching them between base materials on both sides is efficient in terms of manufacturing. Furthermore, in order to suppress warping during cooling after rolling, it is desirable that the base materials and cladding materials be of equal thickness. The assembled assembled slab for clad rolling is preferably tack-welded by electron beam welding in a vacuum chamber with controlled pressure. When assembled using the above method, it is preferable to apply a release agent such as Al2O3 or MgO between the two opposing cladding materials. However, it is not necessary to limit the assembly method to the above method. Here, the assembled slab for clad rolling is heated after assembly and then hot-rolled. The clad steel sheet used in the present invention may undergo controlled rolling or accelerated cooling after the hot-rolling described above in order to ensure the desired base material mechanical properties, or it may undergo tempering. Alternatively, after hot-rolling, it may be cooled to room temperature, then reheated and hardened, and then tempered.

[0151] (6) The ratio of the hardness of the weld metal surface to the hardness of the entire weld metal. [The ratio of the average Vickers hardness H1 across the entire thickness, measured at 1mm intervals in the thickness direction from the surface of the weld metal, to the Vickers hardness H2 at a position 1mm from the surface of the weld metal (i.e., H1 / H2): greater than 1.0] Furthermore, the average value of the Vickers hardness over the entire thickness, measured at 1 mm intervals from the surface of the weld metal in the thickness direction, is H1: 180HV10 or higher. If the overall hardness of the weld metal is low, the desired joint strength cannot be obtained. In other words, if the ratio (H1 / H2) of the average hardness H1 over the entire thickness to the hardness H2 at the 1 mm position is 1.0 or less, the desired joint strength cannot be obtained. Therefore, it is preferable that this ratio is greater than 1.0, and more preferably 1.2 or higher. There is no particular lower limit for this ratio. It is preferable that this ratio be 2.0 or less.

[0152] Furthermore, if the average Vickers hardness H1 across the entire thickness is less than 180HV10, the joint strength will be less than 570MPa, making it unsuitable for applications requiring high strength. It is more preferable that the average hardness H1 be 200HV10 or higher. From the viewpoint of workability and toughness, it is preferable that the average hardness H1 be 350HV10 or lower.

[0153] The hardness of the weld metal surface (i.e., the hardness at a position 1 mm from the surface of the steel plate) is measured using the method described above. The hardness of the entire weld metal (i.e., the hardness over the entire thickness) is measured at 1 mm intervals in the thickness direction, starting from a position 1 mm from the surface of the weld metal. The average value of the measured hardness is calculated and this value is taken as the hardness of the entire weld metal. Using the obtained values, the ratio (H1 / H2) between the hardness of the weld metal surface and the average hardness of the entire weld metal is calculated.

[0154] The welded joint of the present invention can achieve the above-described effects if the hardness of the surface layer of the weld metal and the hardness of the entire weld metal are controlled to satisfy the above conditions. As a method for controlling the hardness of the surface layer of the weld metal and the hardness of the entire weld metal, for example, a method of using different welding materials for the surface welding pass and the welding passes other than the surface layer can be used.

[0155] The welded joint of the present invention can be manufactured, for example, through a manufacturing process that includes at least a welding step. This welding step is a process of welding two or more prepared steel plates together under predetermined welding conditions. First, a groove is made on one side of the long edge of each steel plate to be welded, as shown in Figure 2. Next, the two plates are butted together and fixed with the grooves facing inward, and then submerged arc welding is performed. In this case, different welding materials may be used for the welding pass on the surface layer of the weld metal and the welding pass on the other layers. The welding conditions, such as the welding wire and welding materials, should be appropriately selected according to the joint characteristics and the steel plates constituting the welded joint. [Examples]

[0156] The present invention will be described below with reference to examples. However, the present invention is not limited to the following examples.

[0157] In this invention, hot-rolled steel sheets and clad steel sheets can be used for the steel sheets that make up the welded joint.

[0158] First, when using hot-rolled steel sheets, the hot-rolled steel sheets were manufactured as follows: Steel having the component composition shown in Table 1 was melted in a converter and used as a steel material by continuous casting. The heated steel material was then hot-rolled, accelerated cooling, and heat-treated to obtain a hot-rolled steel sheet with a thickness of 25 mm.

[0159] Furthermore, when using clad steel sheets, the clad steel sheets were manufactured as follows: Clad steel having the base material composition shown in Table 2 and the cladding material composition shown in Table 3 was melted, and the cast slab material was hot-rolled to obtain clad steel sheets with a cladding material thickness of 3 mm and a base material thickness of 25 mm.

[0160] Note that blank spaces in Tables 1 to 3 indicate intentional omission of elements, and include not only cases where elements are absent but also cases where elements are inevitably present. Furthermore, the Ceq for each steel sheet was calculated using formula (1) above, and the values ​​are shown in Tables 1 to 3, respectively. In addition, the surface hardness of each steel sheet was measured using the method described in (4) above, and the values ​​are shown in the "Hardness" column of Tables 1 and 3, respectively.

[0161] Next, the hot-rolled steel sheet shown in Table 1 and the clad steel sheet consisting of the base material shown in Table 2 and the cladding material shown in Table 3 were welded together using a welding material having the component composition shown in Table 4, with the surface pass and other passes of the steel sheet being welded.

[0162] Here, two hot-rolled steel plates and two clad steel plates, each measuring 25 mm thick x 250 mm short side x 1000 mm long side, were prepared, and a groove was made on one side of the long side of each of the two plates. The two plates were then joined together and fixed so that the grooves faced inward, and then welded. The groove 4 had the groove shape shown in Figure 2. The welding method used was two-electrode submerged arc welding. A commercially available molten flux was used. The wire diameter was 4.0 mm for both electrodes, and the welding conditions were 600A-30V for the first electrode, 550A-36V for the second electrode, and a welding speed of 56 cm / min.

[0163] Next, the properties of the resulting welded joints were evaluated. Resistance to ammonia stress corrosion cracking was evaluated by an accelerated test, in which the test specimen was immersed in a test solution and anodic electrolysis was performed to promote corrosion.

[0164] Specifically, the ammonia stress corrosion cracking resistance was evaluated using the following procedure.

[0165] As shown in the cross-sectional view of Figure 3, a test specimen 5 measuring 5 mm thick × 15 mm wide × 115 mm long was taken from the welded joint 3 for evaluation of ammonia stress corrosion cracking resistance. The sampling position was 1 mm from the joint surface in the plate thickness direction and included the weld metal.

[0166] Next, the test specimens were ultrasonically degreased in acetone for 5 minutes, and an external stress (100% YS) equal to the yield strength of each steel plate was applied by four-point bending. The test cell containing these four-point bent specimens was then filled with 5 mass% ammonium carbamate, air at 0.1 MPa, and 2 L of liquid ammonia. Subsequently, the corrosion potential of the specimens was measured using a potentiostat, and the test was started by controlling the potential to be 0.5 V relative to the corrosion potential after 1 hour. After 504 hours of immersion, the specimens were removed from the test cell, corrosion products were removed from the specimen surface, and cracks were observed visually on the specimen surface to evaluate the maximum crack length.

[0167] Here, if the maximum length of the crack (i.e., the maximum crack length) is less than 10 mm, the ammonia stress corrosion cracking resistance is judged to be good, and the symbol "○" is indicated in Tables 5 and 6. On the other hand, if the maximum crack length is 10 mm or more, it is judged to be poor, and the symbol "×" is indicated in Tables 5 and 6.

[0168] Furthermore, component analysis was performed on chips collected from 1 mm below the surface of the weld metal of the fabricated welded joints. Table 5 shows the component composition at 1 mm below the surface of the weld metal in test specimens fabricated using the hot-rolled steel sheet shown in Table 1 and the welding material having the component composition shown in Table 4. Table 6 shows the component composition at 1 mm below the surface of the weld metal in test specimens fabricated using clad steel sheet consisting of the base material from Table 2 and the cladding material from Table 3 and the welding material having the component composition shown in Table 4.

[0169] Furthermore, the Vickers hardness (H2) of the weld metal surface layer was measured using the method described in (1) above, and the average Vickers hardness (H1) of the entire weld metal was measured using the method described in (6) above. Using the obtained Vickers hardness H2 of the weld metal surface layer and the average Vickers hardness H1 of the entire weld metal, the "hardness ratio: H1 / H2" explained in (6) above was determined.

[0170] Furthermore, a tensile test specimen (No. 14A specimen) was taken from the center of the weld metal thickness of the main test specimen, and a tensile test was performed. The tensile test was conducted in accordance with JIS Z 2241 (2022). The measured values ​​are shown in the "TS" column of Tables 5 and 6. Here, a tensile strength (TS) of 570 MPa or higher of the weld metal of the welded joint was considered good.

[0171] The obtained evaluation results and measured values ​​are shown together in Tables 5 and 6.

[0172] As can be seen from Tables 5 and 6, all of the examples of the present invention exhibit excellent resistance to ammonia stress corrosion cracking and achieve sufficient tensile strength of the weld metal. On the other hand, the comparative examples were judged to have poor resistance to ammonia stress corrosion cracking and low tensile strength of the weld metal.

[0173] [Table 1]

[0174] [Table 2]

[0175] [Table 3]

[0176] [Table 4]

[0177] [Table 5]

[0178] [Table 6] [Explanation of Symbols]

[0179] 1 steel plate 2. Weld metal 3. Welded joints 4 Bevel 5. Test specimens for evaluating resistance to ammonia stress corrosion cracking.

Claims

1. A welded joint having two or more steel plates and a welded joint formed by welding the steel plates together, The Vickers hardness at a position 1 mm from the surface of the weld metal in the aforementioned weld is 230 HV 10 or less. A welded joint in which the component composition at a position 1 mm from the surface of the weld metal is 0.28 or less in terms of Ceq shown in formula (1). Ceq=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14...(1) Here, [X] in equation (1) represents the mass %) content of element X in the weld metal, and elements that are not present are represented as having a content of zero.

2. The component composition at a position 1 mm from the surface of the weld metal is, in mass%, C: 0.100% or less, Si: 0.01-0.60%, Mn: 0.01 to 1.40%, P: 0.100% or less, S: 0.0500% or less, Al: 0.001-0.100%, It contains, and further, Cu: 1.00% or less, Ni: 2.00% or less, Co: 2.00% or less, Cr: 2.00% or less, Sn: 0.50% or less, Sb: 0.50% or less, Mo: 0.50% or less W: 0.50% or less, V: 0.200% or less, Nb: 0.100% or less, Ti: 0.100% or less, Zr: 0.100% or less, B: 0.0050% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less, N: 0.0300% or less, and O: 0.0300% or less It contains one or more selected from among them, The welded joint according to claim 1, wherein the remainder consists of Fe and unavoidable impurities.

3. The welded joint according to claim 1, wherein the Vickers hardness at a position 1 mm from the surface of the steel plate is 230 HV10 or less.

4. The welded joint according to claim 2, wherein the Vickers hardness at a position 1 mm from the surface of the steel plate is 230 HV10 or less.

5. The aforementioned steel sheet is a hot-rolled steel sheet. The component composition of the aforementioned hot-rolled steel sheet is, in mass%, C: 0.010-0.200%, Si: 0.01-1.00%, Mn: 0.20-2.50%, P: 0.020% or less, S: 0.0100% or less, Al: 0.010-0.100% N: 0.0010-0.0100%, O: 0.0100% or less It contains, and further, Cu: 1.00% or less, Ni: 2.00% or less, Co: 2.00% or less, Cr: 2.00% or less, Sn: 0.50% or less, Sb: 0.50% or less, Mo: 0.50% or less W: 0.50% or less, V: 0.200% or less, Ti: 0.100% or less, Nb: 0.100% or less, Zr: 0.100% or less, B: 0.0100% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, REM: 0.0200% or less It contains one or more selected from among them, The welded joint according to claim 3, wherein the remainder consists of Fe and unavoidable impurities.

6. The aforementioned steel sheet is a hot-rolled steel sheet. The component composition of the aforementioned hot-rolled steel sheet is, in mass%, C: 0.010-0.200%, Si: 0.01-1.00%, Mn: 0.20-2.50%, P: 0.020% or less, S: 0.0100% or less, Al: 0.010-0.100%, N: 0.0010-0.0100%, O: 0.0100% or less It contains, and further, Cu: 1.00% or less, Ni: 2.00% or less, Co: 2.00% or less, Cr: 2.00% or less, Sn: 0.50% or less, Sb: 0.50% or less, Mo: 0.50% or less W: 0.50% or less, V: 0.200% or less, Ti: 0.100% or less, Nb: 0.100% or less, Zr: 0.100% or less, B: 0.0100% or less, Ca: 0.0200% or less, Mg: 0.0200% or less and REM: 0.0200% or less It contains one or more selected from among them, The welded joint according to claim 4, wherein the remainder consists of Fe and unavoidable impurities.

7. The steel plate is a clad steel plate having a base material and a cladding material. The component composition of the aforementioned base material is, in mass%, C: 0.010-0.200%, Si: 0.01-1.00%, Mn: 0.20-2.50%, P: 0.020% or less, S: 0.0100% or less, Al: 0.010-0.100%, N: 0.0010-0.0100%, O: 0.0100% or less It contains, and further, Cu: 1.00% or less, Ni: 2.00% or less, Co: 2.00% or less, Cr: 2.00% or less, Sn: 0.50% or less, Sb: 0.50% or less, Mo: 0.50% or less W: 0.50% or less, V: 0.200% or less, Ti: 0.100% or less, Nb: 0.100% or less, Zr: 0.100% or less, B: 0.0100% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, REM: 0.0200% or less It contains one or more types selected from among them, The remainder consists of Fe and unavoidable impurities. The component composition of the aforementioned composite material is, in mass%, C: 0.160% or less, Si: 0.01 to 0.50%, Mn: 0.01 to 1.80%, P: 0.100% or less, S: 0.0500% or less, Al: 0.001-0.100%, N: 0.0010-0.0100%, O: 0.0100% or less It contains, and further, Cu: 1.00% or less, Ni: 2.00% or less, Co: 2.00% or less, Cr: 2.00% or less, Sn: 0.50% or less, Sb: 0.50% or less, Mo: 0.50% or less W: 0.50% or less, V: 0.200% or less, Ti: 0.100% or less, Nb: 0.100% or less, Zr: 0.100% or less, B: 0.0005% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less It contains one or more selected from among them, The welded joint according to claim 3, wherein the remainder consists of Fe and unavoidable impurities.

8. The steel plate is a clad steel plate having a base material and a cladding material. The component composition of the aforementioned base material is, in mass%, C: 0.010-0.200%, Si: 0.01-1.00%, Mn: 0.20-2.50%, P: 0.020% or less, S: 0.0100% or less, Al: 0.010-0.100%, N: 0.0010-0.0100%, O: 0.0100% or less It contains, and further, Cu: 1.00% or less, Ni: 2.00% or less, Co: 2.00% or less, Cr: 2.00% or less, Sn: 0.50% or less, Sb: 0.50% or less, Mo: 0.50% or less W: 0.50% or less, V: 0.200% or less, Ti: 0.100% or less, Nb: 0.100% or less, Zr: 0.100% or less, B: 0.0100% or less, Ca: 0.0200% or less, Mg: 0.0200% or less and REM: 0.0200% or less It contains one or more types selected from among them, The remainder consists of Fe and unavoidable impurities. The component composition of the aforementioned composite material is, in mass%, C: 0.160% or less, Si: 0.01 to 0.50%, Mn: 0.01 to 1.80%, P: 0.100% or less, S: 0.0500% or less, Al: 0.001-0.100%, N: 0.0010-0.0100%, O: 0.0100% or less It contains, and further, Cu: 1.00% or less, Ni: 2.00% or less, Co: 2.00% or less, Cr: 2.00% or less, Sn: 0.50% or less, Sb: 0.50% or less, Mo: 0.50% or less W: 0.50% or less, V: 0.200% or less, Ti: 0.100% or less, Nb: 0.100% or less, Zr: 0.100% or less, B: 0.0005% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less It contains one or more selected from among them, The welded joint according to claim 4, wherein the remainder consists of Fe and unavoidable impurities.

9. The ratio H1 / H2, which is the average Vickers hardness H1 measured at 1 mm intervals in the thickness direction from the surface of the weld metal to the Vickers hardness H2 at a position 1 mm from the surface of the weld metal, is greater than 1.

0. Furthermore, the welded joint according to any one of claims 1 to 8, wherein the average value H1 of the Vickers hardness over the entire thickness is 180HV10 or more.

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