Clad steel sheet, welded joint, and method for manufacturing these

Abstract

Provided are: a high-strength clad steel sheet having excellent ammonia SCC resistance, low temperature toughness, and bondability, and that is suitable for liquid ammonia transport, storage tanks, or the like; a welded joint; and a method for manufacturing these.　The clad steel sheet comprises: a base material having prescribed chemical components; and a cladding material having prescribed chemical components and bonded to at least one surface (surface serving as an inner surface). In the base material, the tensile strength is 780 MPa or more, the absorption energy according to the Charpy impact test at -40°C is 47 J or more, and the bonding rate of the base material and the cladding material is 70% or more.

Description

Clad steel plate, welded joint and manufacturing method thereof

[0001] The present invention relates to a high-strength clad steel plate having excellent low-temperature toughness and stress corrosion cracking resistance, and a welded joint obtained using the high-strength clad steel plate. In particular, the present invention relates to a high-strength clad steel plate suitable for structural members such as tanks used in a liquid ammonia environment, and a welded joint obtained using the high-strength clad steel plate. The present invention also relates to a method for manufacturing such a high-strength clad steel plate and a welded joint.

[0002] In a liquid ammonia environment, there is a concern that carbon steel may experience stress corrosion cracking due to liquid ammonia (hereinafter referred to as ammonia SCC). Therefore, for structures such as piping, storage tanks, tank cars, and line pipes made of carbon steel that handle liquid ammonia, steel materials with low ammonia SCC susceptibility have been applied, and operational measures have been taken to suppress ammonia SCC.

[0003] For example, it is known that ammonia-induced SCC correlates with the strength and hardness of a material. When using carbon steel, it is considered desirable to use a material with a tensile strength of less than 600 MPa. It is also known that ammonia-induced SCC occurs in the heat-affected zone of a weld. Therefore, when using high-strength steel in a liquid ammonia environment, measures such as performing post-weld heat treatment by full annealing to adjust the tensile strength and hardness of the weld are necessary.

[0004] In recent years, liquid ammonia has been found to produce CO ２ Because it does not produce CO2, it has attracted attention as a clean energy source, and large-scale demand is expected. This has led to a demand for larger facilities to transport and store liquid ammonia. Generally, when making tanks larger, thinner steel is used to reduce weight and construction costs, and therefore the use of high-strength steel is desirable.

[0005] Furthermore, for efficient operation of transportation and storage facilities, the facilities may be used for both liquid ammonia and LPG. Because liquefied gases such as liquid ammonia and LPG are transported and stored at low temperatures, steel plates used for such applications are required to have excellent low-temperature toughness.

[0006] A method for achieving both high strength and excellent ammonia SCC resistance is disclosed in Patent Document 1. Patent Document 1 describes a method for softening the surface of a steel material.

[0007] Special Publication No. 55-30062

[0008] However, the method described in Patent Document 1 requires a long period of heat treatment to uniformly and sufficiently soften the surface layer, making it difficult to control the strength of the center of the steel sheet, which results in problems with the strength of the steel sheet.

[0009] The present invention aims to solve the above problems and to provide a high-strength clad steel plate and welded joint that are excellent in ammonia SCC resistance, low-temperature toughness, and joinability and are suitable for use in tanks for transporting and storing liquid ammonia, as well as methods for manufacturing the same.

[0010] Here, whether or not a material has excellent ammonia SCC resistance is determined by the following procedure. The clad steel plate according to the present invention is used so that the clad material comes into contact with ammonia, etc. Therefore, first, a test piece measuring 1.5 to 3.0 mm thick x 15 mm x 115 mm is taken from the clad material portion by reducing the thickness from the base metal side of the clad steel plate. If the clad material thickness exceeds 3.0 mm, a test piece measuring 3.0 mm thick is taken from the side of the clad material that is not joined to the base metal. The taken test piece is subjected to ultrasonic degreasing in acetone for 5 minutes. A stress of 100% of the actual yield strength YS of the base metal corresponding to each test piece is applied to each test piece by four-point bending. The four-point bending test piece is placed in a test cell. Next, 2 L of liquid ammonia with a purity of 99.999% or higher is added to a solution containing 5.00 mass% ammonium carbamate, 0 ２A test cell is filled with a solution containing a mixture of ammonium carbamate and water at 1.000 bar and 0.10 mass % of water. ２ After gas is blown in, liquid ammonia is poured in. The specific liquid volume, which is the ratio of the amount of immersion liquid to the surface area of ​​the immersed test piece, is 42 mL / cm. ２ During the test, stirring was performed continuously at 10 rpm using a stirrer placed in the test cell. The test solution temperature was set to 25°C. After adjusting the temperature of the test solution to 25°C, the corrosion potential of the test specimen was measured using a potentiostat. Potential measurement and application using the potentiostat were performed using a three-electrode method, with platinum electrodes used as both the reference electrode and counter electrode. The potential was determined to have stabilized one hour after the start of corrosion potential measurement, and at that point, the immersion test was initiated by controlling the potential so that a potential of +0.5 V vs. Pt was applied to the test specimen. 504 hours after the start of the immersion test, the test specimen was removed from the test cell. Corrosion products on the surface of the test specimen were removed, and the surface and cross section were visually observed for cracks and evaluated. In the present invention, a 504-hour immersion test was performed using nine test specimens for each condition. Among these, if two or fewer test pieces were found to have cracks 1.5 mm or more deep, the ammonia SCC resistance was judged to be good (◯), i.e., excellent; if cracks occurred in three or more test pieces, the ammonia SCC resistance was judged to be poor (×). Furthermore, "excellent low-temperature toughness" refers to an absorbed energy of 47 J or more when a Charpy impact test was conducted at -40°C in accordance with JIS Z 2242 (2023). Furthermore, "excellent bondability" refers to a bond ratio of 70% or more in a cross section including the base material and cladding material, as expressed by the following formula: Bond ratio (%): 100 × bond interface length (mm) / total measured length (mm). Furthermore, "high strength" refers to a tensile strength (TS) of the base material measured in accordance with JIS Z 2241 (2022) of 780 MPa or more.

[0011] In order to achieve the above object, the present inventors have conducted extensive research into various factors affecting the ammonia SCC resistance, low-temperature toughness, and strength properties of steel plates. As a result, they have obtained the following findings. That is, since ammonia SCC occurs inside a product (tank), ammonia SCC resistance is dominated by the properties of the surface layer of the steel plate, which is on the inside. Therefore, they conceived of using a steel plate with excellent strength and low-temperature toughness as a base material, and further bonding a steel plate with excellent ammonia SCC resistance to the base material as a clad steel plate. They have then found that by using such a clad steel plate, excellent ammonia SCC resistance, low-temperature toughness, and strength properties can all be obtained.

[0012] In the present invention, the clad material used is stainless steel, and therefore the corrosion resistance is superior to that of clad steel plates using carbon steel as the clad material.

[0013] The present invention has been made based on the above findings, and the gist of the present invention is as follows. [1] A clad steel plate having a clad material on at least one side of a base material, wherein the chemical composition of the base material contains, in mass%, C: 0.030 to 0.150%, Si: 0.05 to 0.55%, Mn: 0.50 to 2.10%, P: 0.020% or less, S: 0.010% or less, Al: 0.018 to 0.070%, Ni: 0.30 to 2.20%, Ti: 0.005 to 0.020%, N: 0.0020 to 0.0080%, O (oxygen): 0.0050% or less, and Ti / N is 2.2 to 6.5, and further contains Cu: 0.50% or less, Cr: 1.60% or less, Mo: 0.60% or less, Nb: 0.030% or less, A clad steel plate containing one or more of V: ​​0.100% or less, B: 0.0050% or less, and Ca: 0.0040% or less, with the balance being Fe and unavoidable impurities, wherein the clad material has a PRE value of 15 or more as shown in formula (1), a tensile strength of the base material of 780 MPa or more, an absorbed energy of the base material in a Charpy impact test at -40°C of 47 J or more, and a bonding rate between the base material and the clad material of 70% or more. PRE = Cr% + 3.3Mo% + 30N% - Mn% (1) In formula (1), M% represents the content (mass%) of element M in the clad material, and elements that are not contained are represented as 0 (zero).

[0014] [2] A welded joint using the clad steel plate according to [1] above, wherein the absorbed energy of the welded heat affected zone in a Charpy impact test at -40°C is 47 J or more.

[0015] [3] A method for manufacturing a clad steel plate according to the above [1], wherein a base material slab is heated to a surface temperature of 900°C or more and 1200°C or less, and then subjected to hot rolling with a rolling end temperature of 700°C or more to obtain a base material; a clad material slab is heated and then subjected to hot rolling to obtain a clad material; a laminated slab obtained by laminating the base material and the clad material is heated to a surface temperature of 1000°C or more and 1250°C or less, and then subjected to hot rolling with a cumulative reduction rate of 65% or more and a rolling end temperature of Ar ３A method for producing a clad steel plate, comprising: hot rolling the plate to a temperature between a transformation point and 1000°C to produce a rolled plate having a base material and a clad material; and subjecting the rolled plate to the following treatment (A) or (B). (A) After cooling the rolled plate after hot rolling, the plate is reheated to a temperature between 800°C and 1000°C, and the Ar of the base material is removed. ３ (B) The rolled sheet after hot rolling is subjected to accelerated cooling at an average cooling rate of 1.0°C / s to 20.0°C / s from a temperature above the transformation point to a cooling stop temperature of 350°C or less, and after the accelerated cooling, the sheet is tempered at a temperature of 550°C to 700°C. ３ Accelerated cooling is performed from a temperature above the transformation point at an average cooling rate of 3°C / s to 50°C / s to a cooling stop temperature of 500°C or less, or further, tempering is performed at a temperature of 700°C or less after the accelerated cooling.

[0016] [4] A method for manufacturing a welded joint, which uses the clad steel plate according to [1] above and welds it under conditions of a heat input of 50 kJ / cm or less.

[0017] According to the present invention, it is possible to provide a high-strength clad steel plate which has excellent ammonia SCC resistance, low-temperature toughness and bondability and is suitable for structural members such as tanks used in a liquid ammonia environment.

[0018] The present invention relates to a clad steel plate having a base material and a clad material made of a corrosion-resistant alloy on at least one surface of the base material. That is, the clad steel plate of the present invention has a base material and a clad material formed on at least one surface of the base material. Here, since the present invention has excellent ammonia SCC resistance and low-temperature toughness, it is suitable for structural members such as tanks used in a liquid ammonia environment. However, such an environment is not limited to liquid ammonia, and may also be used in LPG or liquefied CO2 environments. ２In addition to the above, liquefied gas may also be used. In the present invention, the surface having such cladding material may be on either side of the base material, but when used as a clad steel plate, it is at least the side that comes into contact with the ammonia, etc. This is because the ammonia SCC resistance and low-temperature toughness of the present invention can be obtained. In the present invention, the surface that comes into contact with the ammonia, etc. is also referred to as the inner surface. In addition, the method is not limited to assembled slabs (laminated slabs) produced by laminating base material slabs (material for the base steel plate) and cladding material slabs (material for the cladding steel plate), and may also be, for example, a method for producing an assembled slab in which elements are added to one or both surfaces of such base material slabs in a gas atmosphere to form the chemical composition of the cladding material.

[0019] The clad steel plate of the present invention is a clad steel plate having a cladding material on at least one side of a base material, and the chemical composition of the base material contains, in mass %, C: 0.030 to 0.150%, Si: 0.05 to 0.55%, Mn: 0.50 to 2.10%, P: 0.020% or less, S: 0.010% or less, Al: 0.018 to 0.070%, Ni: 0.30 to 2.20%, Ti: 0.005 to 0.020%, N: 0.0020 to 0.0080%, and O (oxygen): 0.0050% or less, and Ti / N is 2.2 to 6.5, and further The alloy contains one or more of Cu: 0.50% or less, Cr: 1.60% or less, Mo: 0.60% or less, Nb: 0.030% or less, V: 0.100% or less, B: 0.0050% or less, and Ca: 0.0040% or less, with the balance being Fe and unavoidable impurities. The cladding material has a PRE value shown in formula (1) of 15 or more. The tensile strength of the base material is 780 MPa or more, the absorbed energy of the base material in a Charpy impact test at -40°C is 47 J or more, and the bonding rate between the base material and the cladding material is 70% or more. PRE = Cr% + 3.3Mo% + 30N% - Mn% (1) In formula (1), M% represents the content (mass%) of element M in the cladding material, and elements that are not contained are represented as 0 (zero). In the following, "%" representing the content of each component element means "% by mass" unless otherwise specified.

[0020] (1) Chemical Composition of Base Metal C: 0.030-0.150% C is the most effective element for increasing the strength of steel plate produced by cooling according to the present invention. To achieve this effect, the C content is specified to be 0.030% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and achieving lower production costs, the C content is preferably 0.040% or more, and more preferably 0.050% or more. On the other hand, a C content exceeding 0.150% leads to deterioration of the toughness and weldability of the steel plate. Therefore, the C content is specified to be 0.150% or less. Furthermore, from the viewpoint of toughness, the C content is preferably 0.140% or less, and more preferably 0.130% or less.

[0021] Si: 0.05 to 0.55% Si is added to improve the strength of steel sheet and for deoxidation. To achieve this effect, the Si content is specified to be 0.05% or more. Furthermore, the Si content is preferably 0.07% or more, and more preferably 0.10% or more. On the other hand, a Si content exceeding 0.55% leads to deterioration of toughness and weldability. Therefore, the Si content is specified to be 0.55% or less. Furthermore, the Si content is preferably 0.50% or less, and more preferably 0.40% or less.

[0022] Mn: 0.50 to 2.10% Mn is an element that has the effect of increasing the hardenability of steel, and is one of the important elements that must be added to achieve the high strength required in the present invention. To achieve this effect, the Mn content is specified to be 0.50% or more. Furthermore, from the perspective of reducing the content of other alloying elements and achieving lower manufacturing costs, the Mn content is preferably 0.70% or more. On the other hand, a Mn content exceeding 2.10% leads to deterioration of weldability. Therefore, the Mn content is specified to be 2.10% or less. Furthermore, the Mn content is preferably 1.90% or less.

[0023] P: 0.020% or less P is an element contained as an unavoidable impurity, and its segregation at grain boundaries has adverse effects such as reducing toughness and weldability. Therefore, it is desirable to keep the P content as low as possible, but a content of 0.020% or less is acceptable. Furthermore, the P content is preferably 0.015% or less. The lower limit of the P content is not particularly limited and may be 0%, but since P is usually an element inevitably contained in steel as an impurity, industrially, it may be greater than 0%. Furthermore, because excessive reduction leads to increased refining costs, the P content is preferably 0.001% or more, and more preferably 0.003% or more.

[0024] S: 0.010% or less S is an element contained as an unavoidable impurity. It exists in steel as sulfide-based inclusions such as MnS, and has adverse effects such as becoming the origin of fracture and reducing the toughness of the steel plate. Therefore, it is desirable to keep the S content as low as possible, but a content of 0.010% or less is acceptable. Furthermore, the S content is preferably 0.005% or less, and more preferably 0.003% or less. The lower limit of the S content is not particularly limited and may be 0%, but since S is usually an element inevitably contained in steel as an impurity, it may exceed 0% industrially. Furthermore, excessive reduction leads to increased refining costs, so from a cost perspective, the S content is preferably 0.001% or more.

[0025] Al: 0.018 to 0.070% Al acts as a deoxidizer. To achieve this effect, the Al content is specified to be 0.018% or more. Furthermore, the Al content is preferably 0.025% or more. On the other hand, if the Al content exceeds 0.070%, oxide-based inclusions increase, reducing cleanliness and toughness. Therefore, the Al content is specified to be 0.070% or less. Furthermore, from the viewpoint of preventing deterioration of toughness, the Al content is preferably 0.060% or less, and more preferably 0.050% or more.

[0026] Ni: 0.30 to 2.20% Ni is not only effective in improving the strength of steel sheets, but also in improving the toughness of the base material and the weld heat-affected zone. However, if the Ni content is less than 0.30%, this effect is poor, and if it exceeds 2.20%, scratches occur on the surface of the steel sheet. Therefore, the Ni content is specified to be in the range of 0.30 to 2.20%. Furthermore, the Ni content is preferably 0.60% or more, and more preferably 0.70% or more. Furthermore, the Ni content is preferably 2.10% or less, more preferably 1.80% or less, and even more preferably 1.50% or less.

[0027] Ti: 0.005 to 0.020% Ti is an element that has a strong tendency to form nitrides and has the effect of fixing N and reducing the amount of solute N. Therefore, adding Ti can improve the toughness of welds. To achieve this effect, the Ti content must be 0.005% or more. On the other hand, if the Ti content exceeds 0.020%, the toughness actually decreases. Therefore, the Ti content is specified to be in the range of 0.005 to 0.020%. Furthermore, the Ti content is preferably 0.008% or more. Furthermore, the Ti content is preferably 0.017% or less.

[0028] N: 0.0020 to 0.0080% N can improve the toughness of welds by forming TiN. To achieve this effect, the N content must be 0.0020% or more. On the other hand, if the N content exceeds 0.0080%, toughness actually decreases. Therefore, the N content is specified to be in the range of 0.0020 to 0.0080%. Furthermore, the N content is preferably 0.0025% or more. Furthermore, the N content is preferably 0.0070% or less, more preferably 0.0060% or less, and even more preferably 0.0050% or less.

[0029] O (oxygen): 0.0050% or less O is an element contained as an unavoidable impurity, and Al ２ O ３O is an element that exists in steel as oxides such as Cr, Fe, and Cr, and has adverse effects such as becoming the origin of fracture and reducing the toughness of the steel sheet. Therefore, it is desirable to keep the O content as low as possible, but 0.0050% or less is acceptable. Furthermore, the O content is preferably 0.0040% or less, and more preferably 0.0030% or less. The lower limit of the O content is not particularly limited and may be 0%, but since O is an element that is usually unavoidably contained in steel as an impurity, it may be industrially greater than 0%. Furthermore, excessive reduction of O content leads to an increase in refining costs, so from a cost perspective, the O content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0030] Ti / N: 2.2 or more and 6.5 or less The formation of TiN can improve the toughness of the weld. To achieve this effect, the correlation between the Ti and N contents is important. If Ti / N, the ratio of the Ti content to the N content, is less than 2.2 or exceeds 6.5, the crystal grains become coarse and the toughness deteriorates. Therefore, Ti / N is specified in the range of 2.2 or more and 6.5 or less. Furthermore, Ti / N is preferably 2.3 or more. Furthermore, Ti / N is preferably 3.9 or less, and more preferably 3.8 or less.

[0031] In addition to the above components, the base material of the present invention further contains one or more of the elements described below, with the balance being Fe and inevitable impurities. Inevitable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, manufacturing equipment, etc., and are allowed to be included to the extent that they do not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, scrap, etc. Examples of impurities include Co, Sn, Zn, Pb, As, Sb, Bi, H, and REM.

[0032] Cu: 0.50% or less Cu is an element effective in improving the strength of steel sheet. However, if the Cu content is less than 0.05%, this effect is poor. Therefore, the Cu content is preferably 0.05% or more. Furthermore, the Cu content is more preferably 0.10% or more, and even more preferably 0.15% or more. On the other hand, if the Cu content exceeds 0.50%, scratches will occur on the surface of the steel sheet. Therefore, when Cu is contained, the Cu content is set to 0.50% or less. The Cu content is preferably 0.45% or less, and more preferably 0.40% or less.

[0033] Cr: 1.60% or less Cr is an element effective in improving the strength of steel plate. However, if the Cr content is less than 0.05%, this effect is poor. Therefore, the Cr content is preferably 0.05% or more. Furthermore, the Cr content is preferably 0.10% or more. On the other hand, if the Cr content exceeds 1.60%, the toughness of the steel plate deteriorates. Therefore, when Cr is contained, the Cr content is specified to be 1.60% or less. The Cr content is preferably 1.50% or less, and more preferably 1.40% or less.

[0034] Mo: 0.60% or less Mo is an element effective in improving the strength of steel sheet. However, if the Mo content is less than 0.05%, this effect is poor. Therefore, the Mo content is preferably 0.05% or more. Furthermore, the Mo content is preferably 0.10% or more. On the other hand, if the Mo content exceeds 0.60%, the toughness of the steel sheet deteriorates. Therefore, when Mo is contained, the Mo content is set to 0.60% or less. The Mo content is preferably set to 0.55% or less.

[0035] Nb: 0.030% or less Nb is an element that has the effect of reducing the prior austenite grain size and improving toughness by precipitating as carbonitrides. To obtain this effect, the Nb content is preferably 0.005% or more. Furthermore, the Nb content is more preferably 0.007% or more. On the other hand, if the Nb content exceeds 0.030%, a large amount of NbC precipitates, reducing toughness. Therefore, when Nb is contained, the Nb content is set to 0.030% or less. Furthermore, the Nb content is preferably 0.027% or less, more preferably 0.025% or less.

[0036] V: 0.100% or less V is an element effective in improving the strength of steel sheet. However, if the V content is less than 0.005%, this effect is poor. Therefore, the V content is preferably 0.005% or more. Furthermore, the V content is preferably 0.030% or more. On the other hand, if the V content exceeds 0.100%, the toughness of the steel sheet deteriorates. Therefore, when V is contained, the V content is set to 0.100% or less. The V content is preferably 0.080% or less, and more preferably 0.050% or less.

[0037] B: 0.0050% or less B is an element effective in improving the strength of steel sheet. However, if the B content is less than 0.0005%, this effect is poor. Therefore, the B content is preferably 0.0005% or more. Furthermore, the B content is preferably 0.0008% or more. On the other hand, if the B content exceeds 0.0050%, the toughness of the steel sheet deteriorates. Therefore, when B is contained, the B content is set to 0.0050% or less. The B content is preferably 0.0040% or less, and more preferably 0.0030% or less.

[0038] Ca: 0.0040% or less Ca is an element that bonds with S and has the effect of suppressing the formation of MnS and the like that elongate in the rolling direction. That is, by including Ca, the morphology of sulfide-based inclusions is controlled to be spherical, thereby improving the toughness of welds and the like. To achieve this effect, the Ca content is preferably 0.0005% or more. The Ca content is preferably 0.0010% or more. On the other hand, if the Ca content exceeds 0.0040%, the cleanliness of the steel decreases. Therefore, when Ca is included, the Ca content is set to 0.0040% or less. The Ca content is preferably 0.0030% or less.

[0039] (2) Cladding Material PRE Value: 15 or More PRE = Cr% + 3.3Mo% + 30N% - Mn% (1) In formula (1), M% represents the content (mass%) of element M in the cladding material. PRE is a pitting corrosion resistance index, calculated as PRE = Cr% + 3.3Mo% + 30N% - Mn%. The higher the PRE value, the lower the liquid ammonia SCC susceptibility. Therefore, the PRE value of the cladding material is set to 15 or more. On the other hand, the higher the PRE value of the cladding material, the better, but an excessive PRE value leads to increased costs. Therefore, the PRE value is preferably 55 or less, and more preferably 27 or less. The cladding material is preferably stainless steel. The structure of the stainless steel is not important, but from the viewpoint of improving the toughness of the weld heat-affected zone in the cladding material, the stainless steel is preferably austenitic stainless steel or duplex stainless steel. The cladding material may be a Ni-based alloy. The chemical composition of the cladding material is not particularly limited as long as it satisfies a PRE value of 15 or more. More specifically, the chemical composition of the cladding material preferably contains, in mass%, C: 0.001 to 0.080%, Si: 0.05 to 1.00%, Mn: 0.30 to 4.00%, P: 0.045% or less, S: 0.030% or less, Ni: 0.05 to 80.0%, Cr: 11.0 to 28.0%, N: 0.005 to 0.400%, O (oxygen): 0.0050% or less, with the balance being Fe and unavoidable impurities. Furthermore, the chemical components of the cladding material may further contain, as necessary, Cu: 3.00% or less, Mo: 5.50% or less, V: 0.100% or less, Nb: 3.00% or less, Ti: 1.00% or less, B: 0.0050% or less, Ca: 0.0040% or less, Al: 0.50% or less, Co: 1.20% or less, and Ta: 3.00% or less. Hereinafter, embodiments of the present invention will be specifically described.

[0040] C: 0.001 to 0.080% C is the most effective element for increasing the strength of the steel plate produced by cooling according to the present invention. To achieve this effect, the C content is preferably 0.001% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and producing at lower cost, the C content is more preferably 0.010% or more. On the other hand, if the C content exceeds 0.080%, carbides may precipitate at the grain boundaries, which may cause stress corrosion cracking when the steel plate comes into contact with liquid ammonia. Therefore, the C content is preferably 0.080% or less. Furthermore, from the viewpoint of toughness, the C content is more preferably 0.050% or less.

[0041] Si: 0.05 to 1.00% Si is added to improve the strength of the steel sheet and also for deoxidation. To achieve this effect, the Si content is preferably 0.05% or more. Furthermore, the Si content is more preferably 0.07% or more. On the other hand, if the Si content exceeds 1.00%, it may lead to deterioration of toughness and weldability. Therefore, the Si content is preferably 1.00% or less. Furthermore, the Si content is more preferably 0.80% or less.

[0042] Mn: 0.30 to 4.00% Mn is an element that increases the hardenability of steel, and is also one of the important elements that must be added for deoxidation. To achieve this effect, the Mn content is preferably 0.30% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and achieving lower manufacturing costs, the Mn content is more preferably 0.40% or more. On the other hand, a Mn content exceeding 4.00% may result in deterioration of weldability. Therefore, the Mn content is preferably 4.00% or less. Furthermore, the Mn content is more preferably 3.00% or less.

[0043] P: 0.045% or less P is an element contained as an unavoidable impurity, and may have 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 P content of 0.045% or less is acceptable. The P content is more preferably 0.040% or less. The lower limit of the P content is not particularly limited and may be 0%, but since P is an element that is usually unavoidably contained in steel as an impurity, industrially, it may be greater than 0%. Furthermore, since excessive reduction leads to increased refining costs, the P content is preferably 0.001% or more.

[0044] S: 0.030% or less S is an element contained as an unavoidable impurity. It exists in steel as sulfide-based inclusions such as MnS, and may have adverse effects, such as becoming the origin of fracture and reducing the toughness of the steel plate. Therefore, it is desirable to keep the S content as low as possible, but a content of 0.030% or less is acceptable. The S content is more preferably 0.025% or less. The lower limit of the S content is not particularly limited and may be 0%, but since S is usually an element that is unavoidably contained in steel as an impurity, it may be industrially greater than 0%. Furthermore, excessive reduction in S content leads to increased refining costs, so from a cost perspective, it is preferable to set the S content to 0.001% or more.

[0045] Ni: 0.05 to 80.0% Ni is an element effective in improving the corrosion resistance of stainless steel and Ni-based alloys against various acids. It is not only effective in improving the strength of steel plates, but also in improving the toughness of the base metal and the weld heat-affected zone. However, a Ni content of less than 0.05% results in little effect. Therefore, the Ni content is preferably 0.05% or more. Furthermore, the Ni content is more preferably 0.10% or more. On the other hand, Ni is an expensive metal, and from the viewpoint of alloy cost, the Ni content is preferably 80.0% or less. Furthermore, the Ni content is more preferably 46.0% or less. When stainless steel, rather than a Ni-based alloy, is used as the cladding material, the Ni content may be 25.0% or less, 20.0% or less, 13.0% or less, or 10.0% or less.

[0046] Cr: 11.0 to 28.0% Cr is an element effective in improving the corrosion resistance of stainless steel. To prevent stress corrosion cracking when in contact with liquid ammonia, the Cr content is preferably 11.0% or more. Furthermore, the Cr content is more preferably 13.0% or more. On the other hand, if the Cr content exceeds 28.0%, the toughness of the duplex stainless steel and the corrosion resistance of the weld may deteriorate. Therefore, the Cr content is preferably 28.0% or less. Furthermore, the Cr content is more preferably 26.0% or less.

[0047] N: 0.005 to 0.400% N is an element effective in improving the corrosion resistance and strength of stainless steel. To achieve this effect, the N content is preferably 0.005% or more. Furthermore, the N content is more preferably 0.010% or more. On the other hand, if the N content exceeds 0.400%, toughness may actually decrease. Therefore, the N content is preferably 0.400% or less. Furthermore, the N content is more preferably 0.300% or less.

[0048] O (oxygen): 0.0050% or less O is an element contained as an unavoidable impurity, and Al ２ O ３O is an element that exists in steel as oxides such as Cr, Fe, and Cr, and has adverse effects such as becoming the origin of fracture and reducing the toughness of the steel plate. Therefore, it is desirable to keep the O content as low as possible, but 0.0050% or less is acceptable. The lower limit of the O content is not particularly limited and may be 0%, but since O is an element that is usually unavoidably contained in steel as an impurity, it may be industrially greater than 0%. Moreover, excessive reduction of O content leads to an increase in refining costs, so from the viewpoint of cost, it is preferable to set the O content to 0.0005% or more.

[0049] In addition to the above components, the cladding material of the present invention may further contain the following components as necessary. The balance is Fe and inevitable impurities. Inevitable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, manufacturing equipment, etc., and are allowed to be included to the extent that they do not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, scrap, etc. Examples of impurities include Sn, Zn, Pb, As, Sb, Bi, H, and REM.

[0050] Cu: 3.00% or less Cu is an element effective in improving the corrosion resistance and strength of steel sheets. However, if the content is less than 0.05%, this effect is poor. Therefore, the Cu content is preferably 0.05% or more. Furthermore, the Cu content is more preferably 0.10% or more. If the Cu content exceeds 3.00%, hot workability may be significantly reduced. Therefore, when Cu is contained, the Cu content is preferably 3.00% or less. Furthermore, the Cu content is more preferably 2.20% or less.

[0051] Mo: 5.50% or less Mo is an effective element for improving the pitting corrosion resistance of stainless steel. However, if the content is less than 0.05%, this effect is poor. Therefore, when Mo is contained, the Mo content is preferably 0.05% or more. Furthermore, the Mo content is more preferably 0.10% or more. If the Mo content exceeds 5.50%, for example, in the case of duplex stainless steel, depending on the cooling rate of the steel sheet, sigma phase precipitation may be promoted, resulting in a significant deterioration of corrosion resistance. Therefore, when Mo is contained, the Mo content is preferably 5.50% or less. Furthermore, the Mo content is more preferably 5.00% or less.

[0052] V: 0.100% or less V is an element that is effective in suppressing deterioration of corrosion resistance due to the formation of Cr carbides because it bonds more easily with C than Cr. However, if the V content is less than 0.005%, this effect is poor. Therefore, when V is contained, the V content is preferably 0.005% or more. Furthermore, the V content is more preferably 0.010% or more. On the other hand, from the viewpoint of alloy cost, when V is contained, the V content is preferably 0.100% or less. Furthermore, the V content is more preferably 0.080% or less.

[0053] Nb: 3.00% or less Nb is an element that is effective in suppressing deterioration of corrosion resistance due to the formation of Cr carbides because it bonds more easily with C than Cr. However, if the content is less than 0.005%, this effect is poor. Therefore, when Nb is contained, the Nb content is preferably 0.005% or more. Furthermore, the Nb content is more preferably 0.010% or more. On the other hand, from the viewpoint of alloy cost, when Nb is contained, the Nb content is preferably 3.00% or less. Furthermore, the Nb content is more preferably 2.50% or less.

[0054] Ti: 1.00% or less Ti is an effective element for suppressing deterioration of corrosion resistance due to the formation of Cr carbides because it bonds more easily with C than Cr. However, this effect is poor at a content of less than 0.01%. Therefore, when Ti is contained, the Ti content is preferably 0.01% or more. Furthermore, the Ti content is more preferably 0.05% or more, and even more preferably 0.07% or more. On the other hand, from the viewpoint of alloy cost, when Ti is contained, the Ti content is preferably 1.00% or less. Furthermore, the Ti content is more preferably 0.90% or less.

[0055] B: 0.0050% or less B is an element effective in improving the strength of steel sheet. However, if the B content is less than 0.0005%, this effect is poor. Therefore, when B is contained, the B content is preferably 0.0005% or more. The B content is more preferably 0.0008% or more. If the B content exceeds 0.0050%, the toughness of the steel sheet may deteriorate. Therefore, when B is contained, the B content is preferably 0.0050% or less. The B content is more preferably 0.0040% or less.

[0056] Ca: 0.0040% or less Ca is an element that bonds with S and has the effect of suppressing the formation of MnS and the like that elongate in the rolling direction. That is, by containing Ca, the morphology of sulfide-based inclusions is controlled to be spherical, thereby improving the toughness of welds and the like. To achieve this effect, when Ca is contained, the Ca content is preferably 0.0005% or more. The Ca content is more preferably 0.0010% or more. On the other hand, if the Ca content exceeds 0.0040%, there is a concern that the cleanliness of the steel may decrease. Therefore, when Ca is contained, the Ca content is preferably 0.0040% or less. The Ca content is more preferably 0.0030% or less.

[0057] Al: 0.50% or less Al is an effective deoxidizing element. This effect is fully exhibited when the Al content is 0.02% or more, so when Al is contained, the Al content is preferably 0.02% or more. However, when the Al content exceeds 0.50%, stress corrosion cracking resistance deteriorates. Therefore, when Al is contained, the Al content is preferably 0.50% or less, and more preferably 0.30% or less.

[0058] Co: 1.20% or less Co is also an element that improves corrosion resistance, and this effect is manifested when the content is 0.01% or more. Therefore, the Co content is preferably 0.01% or more. However, if the content exceeds 1.20%, the alloy price increases. Therefore, when Co is contained, the Co content is preferably 1.20% or less. The Co content is more preferably 0.05% or more. The Co content is more preferably 1.00% or less.

[0059] Ta: 3.00% or less Ta is an element effective in fixing C. This effect is fully exhibited when Ta is 0.02% or more, so when Ta is contained, the Ta content is preferably 0.02% or more. On the other hand, when Ta is excessive, Ta forms an intermetallic compound with a low melting point, which reduces hot workability. Therefore, when Ta is contained, the Ta content is preferably 3.00% or less.

[0060] (3) Properties of the clad steel plate [Tensile strength] The clad steel plate of the present invention has a base metal tensile strength of 780 MPa or more. There is no particular upper limit to the tensile strength, but the tensile strength of the high-strength steel plate of the present invention can be 1000 MPa or less.

[0061] [Absorbed energy in Charpy impact test] The clad steel plate of the present invention has an absorbed energy of 47 J or more in a Charpy impact test at -40°C of the base material. When used as a structural member for a tank or the like, PWHT (Post Weld Heat Treatment) treatment is performed, but this may not be performed depending on the plate thickness. In this case, the absorbed energy required of the base material is higher, so it is preferably 100 J or more, and more preferably 170 J or more.

[0062] [Bonding rate] In clad steel plates, there are portions where the base material and clad material are bonded via oxides or voids, and portions where the base material and clad material are directly bonded. The bonding strength between the base material and oxide and the bonding strength between the clad material and oxide are both smaller than the bonding strength between the base material and clad material. Therefore, the higher the bonding rate between the base material and clad material, the higher the bonding strength of the clad steel plate, and the lower the possibility of peeling problems occurring during processing into products. The clad steel plate of the present invention has a bonding rate between the base material and clad material of 70% or more. The bonding rate between the base material and clad material is preferably 80% or more, more preferably 85% or more. The upper limit of the bonding rate between the base material and clad material is not particularly limited, and a higher bonding rate is preferable, and it may be 100%.

[0063] [Thickness, Clad Ratio] The clad steel plate of the present invention is not particularly limited, but may have a thickness of 7 mm or more. The clad steel plate of the present invention preferably has a thickness of 18 mm or more, more preferably 23 mm or more. The clad steel plate of the present invention may have a thickness of 66 mm or less. The clad steel plate of the present invention preferably has a thickness of 53 mm or less, more preferably 48 mm or less, and even more preferably 41 mm or less.

[0064] The thickness of the base material of the clad steel plate of the present invention may be 6 mm or more. The thickness of the base material of the clad steel plate of the present invention is preferably 15 mm or more, more preferably 20 mm or more. The thickness of the base material of the clad steel plate of the present invention may be 60 mm or less. The thickness of the base material is preferably 50 mm or less, more preferably 45 mm or less, and even more preferably 38 mm or less.

[0065] The thickness of the clad steel plate clad material of the present invention may be 1.0 mm or more. The thickness of the clad steel plate clad material of the present invention is preferably 2.0 mm or more, more preferably 2.5 mm or more. The thickness of the clad steel plate clad material of the present invention may be 6.0 mm or less. The thickness of the clad steel plate clad material of the present invention is preferably 5.0 mm or less, more preferably 4.0 mm or less.

[0066] The clad steel plate of the present invention may have a clad ratio of 0.01 or more, although it is not particularly limited thereto. The clad ratio of the clad steel plate of the present invention is preferably 0.03 or more, more preferably 0.05 or more. The clad ratio of the clad steel plate of the present invention is not particularly limited thereto, although it may have a clad ratio of 0.50 or less. The clad ratio of the clad steel plate of the present invention is preferably 0.40 or less, more preferably 0.30 or less.

[0067] (4) Regarding the welded joint: In the present invention, the above-mentioned clad steel plate is welded to manufacture a structure such as a tank. There are no particular limitations on the type of welding, and conventionally known steel plate welding methods can be applied, but TIG welding is preferred. In the welded joint, it is preferable that the absorbed energy of the weld heat-affected zone in a Charpy impact test at -40°C is 47 J or more.

[0068] [Heat input: 50 kJ / cm or less] When manufacturing a welded joint using the clad steel plate of the present invention, the heat input is specified to be 50 kJ / cm or less. The heat input is preferably 30 kJ / cm or less. The heat input is more preferably 20 kJ / cm or less. By satisfying this heat input range, the above-mentioned characteristics can be satisfied. There is no particular lower limit for the heat input, and since excessive reduction leads to a decrease in welding efficiency, the heat input is preferably 5.0 kJ / cm or more.

[0069] (5) Manufacturing Conditions In the present invention, first, a base material slab and a clad material slab having the aforementioned chemical compositions are manufactured. There are no particular limitations on the manufacturing methods for these base material slabs, and conventionally known slab manufacturing methods can be applied. That is, molten steel adjusted to the aforementioned chemical compositions by a conventional melting method (such as a converter or electric furnace) is cast by a conventional casting method (such as a continuous casting method or an ingot casting method) to obtain a base material slab. Next, the obtained base material slab is heated to a surface temperature of 900°C or higher and 1200°C or lower, and then hot-rolled. The hot-rolling end temperature is set to a surface temperature of 700°C or higher, and a base material slab of predetermined dimensions is obtained. The obtained clad material slab is heated and then hot-rolled to obtain a clad material slab of predetermined dimensions.

[0070] [Heating temperature: 900°C or higher and 1200°C or lower] If the heating temperature of the base material slab in the clad steel plate of the present invention is lower than 900°C, the carbides will not be dissolved sufficiently and the required strength will not be obtained. Therefore, the heating temperature is set to a surface temperature of 900°C or higher. Furthermore, the heating temperature is preferably set to 920°C or higher. On the other hand, if the base material slab is heated above 1200°C, the amount of energy consumption will increase. Therefore, the heating temperature is set to a surface temperature of 1200°C or lower. The heating temperature is preferably set to 1150°C or lower.

[0071] [Finishing Temperature (FT): 700°C or Higher] If the finishing temperature of the hot rolling is lower than 700°C, the generated ferrite will be affected by the working, resulting in a deterioration in toughness. Therefore, the finishing temperature of the rolling is set to 700°C or higher in terms of surface temperature. Furthermore, the finishing temperature of the rolling is preferably set to 750°C or higher. Although there is no particular upper limit, the finishing temperature of the rolling is preferably set to 1000°C or lower in terms of surface temperature.

[0072] Next, the obtained base material is laminated on the side that contacts ammonia or the like with a cladding material to form at least two layers, or three layers by laminating a sacrificial material, a cladding material, and a base material, or even four layers by laminating a base material, a cladding material, a cladding material, and a base material, and the cladding material is pressure-bonded, and an appropriate heat treatment is performed to control the structure. Note that, as for the slab lamination method, in the tables of the examples described below, a method of laminating base material slab, cladding material slab, cladding material slab, and base material slab from top to bottom is referred to as the sandwich method. Also, a method of laminating cladding material slab, base material slab, or base material slab, cladding material slab, and sacrificial material from top to bottom is referred to as the open method. Also, a method of laminating sacrificial material, cladding material slab, base material slab, or base material slab, cladding material slab, and sacrificial material from top to bottom is referred to as the sacrificial material method. The clad slab (laminated slab) obtained by laminating the base material and the clad material is heated to a surface temperature in the range of 1000°C to 1250°C and then hot rolled. In the present invention, the hot rolling is performed with a cumulative rolling reduction of 65% or more and a rolling end temperature of Ar. ３ After the hot rolling, the following treatment (A) or (B) is carried out. (A) After the hot rolling, the rolled sheet is cooled and then reheated to 800°C or higher and 1000°C or lower, and the Ar of the base material is removed. ３ Accelerated cooling is performed from a temperature above the transformation point to a cooling stop temperature of 350°C or less at an average cooling rate of 1.0°C / s to 20.0°C / s. Next, the rolled sheet is reheated to 800°C to 1000°C at an average rate of, for example, 1°C / s to 50°C / s. After accelerated cooling, tempering is performed at a temperature of 550°C to 700°C. (B) The rolled sheet after hot rolling is reheated to the Ar of the base material. ３ Accelerated cooling is performed from a temperature above the transformation point at an average cooling rate of 3°C / s to 50°C / s to a cooling stop temperature of 500°C or less. Alternatively, after accelerated cooling, tempering is performed at a temperature of 700°C or less.

[0073] [Heating temperature: 1000°C or higher and 1250°C or lower] If the heating temperature of the laminated slab formed by laminating the base material and the cladding material is lower than 1000°C, the carbides will not dissolve sufficiently and the required strength will not be obtained. In addition, from the viewpoint of the bondability of the clad steel, a higher heating temperature is preferable. Therefore, the heating temperature is set to 1000°C or higher. Furthermore, the heating temperature is preferably set to 1020°C or higher. On the other hand, heating above 1250°C increases energy consumption. Therefore, the heating temperature is set to 1250°C or lower. Furthermore, the heating temperature is preferably set to 1230°C or lower.

[0074] [Cumulative reduction ratio of laminated slabs: 65% or more] By rolling the laminated slabs to a cumulative reduction ratio of 65% or more, austenite recrystallization is promoted and deformation bands that become nucleation sites are introduced within the austenite grains. Subsequently, accelerated cooling under the conditions described below refines the bainite and martensite that are transformed, improving the toughness of the clad steel plate. Therefore, the cumulative reduction ratio of the laminated slabs is set to 65% or more. It is preferable that the cumulative reduction ratio of the laminated slabs is set to 70% or more. While there is no particular upper limit to the cumulative reduction ratio of the laminated slabs, it is preferable that the cumulative reduction ratio of the laminated slabs is set to 95% or less from the viewpoint of rolling efficiency.

[0075] [Rolling finish temperature (FT): Ar ３ Transformation point or higher and 1000°C or lower] The rolling end temperature of the hot rolling is Ar ３ If the rolling finish temperature is lower than the transformation point, ferrite is generated, resulting in a deterioration in toughness. ３ If the temperature is lower than the transformation point, the hot rolling will be performed at a low temperature, which will deteriorate the bondability of the clad steel from the viewpoint of diffusion bonding. ３ The rolling end temperature is set to be equal to or higher than the transformation point. ３ It is preferable to set the rolling end temperature at 20°C or higher than the transformation point. On the other hand, if the rolling end temperature exceeds 1000°C, deformation bands that serve as nucleation sites are not introduced into the austenite grains, fine bainite or martensite cannot be obtained, and the toughness of the clad steel plate deteriorates. Therefore, the rolling end temperature is set to 1000°C or lower. Furthermore, it is preferable to set the rolling end temperature to 980°C or lower.

[0076] (Step (A) above (Accelerated Cooling Step after Reheating)) In the step (A) above, after cooling, the material is reheated to 800°C or more and 1000°C or less, and the Ar of the base material is ３ Accelerated cooling is performed from a temperature above the transformation point at an average cooling rate of 1.0°C / s to 20.0°C / s to a cooling stop temperature of 350°C or less. Here, "from a temperature above the Ar3 transformation point of the base material" means that the surface temperature of the base material at the start of accelerated cooling is a temperature above the Ar3 transformation point of the base material. After hot rolling and before reheating, cooling may be performed to room temperature (-5 to 50°C). The cooling rate at this time does not need to be particularly specified, but can be, for example, air cooling at 0.01 to 1°C / s.

[0077] [Reheating temperature: 800°C or higher and 1000°C or lower] If the reheating temperature of the rolled plate having the base material and clad material is less than 800°C, the strength may become excessive and the toughness may not be restored. In addition, the joining rate may decrease. Therefore, the reheating temperature is set to 800°C or higher. Furthermore, the reheating temperature is preferably set to 820°C or higher. On the other hand, if the reheating temperature exceeds 1000°C, the strength may be insufficient. Therefore, the reheating temperature is set to 1000°C or lower. Furthermore, the reheating temperature is preferably set to 980°C or lower.

[0078] [Cooling start temperature: Ar ３ Transformation point or higher] The reheated steel plate is cooled from a temperature equal to or higher than the Ar3 transformation point of the base material. Here, "from a temperature equal to or higher than the Ar3 transformation point of the base material" means that the surface temperature of the base material at the start of accelerated cooling is equal to or higher than the Ar3 transformation point of the base material. If the cooling start temperature is lower than the Ar3 transformation point of the base material, ferrite will be produced in excess and will coexist with martensite or bainite, which have large strength differences. As a result, the base material will have insufficient strength and a deterioration in toughness. Therefore, the cooling start temperature after reheating is set to be equal to or higher than the Ar3 transformation point. Furthermore, the cooling start temperature is preferably set to be equal to or higher than the Ar3 transformation point + 20°C. Ar ３ The transformation point can be calculated by the following formula: Ar ３(°C) = 910 - 310 x [C] - 80 x [Mn] - 20 x [Cu] - 55 x [Ni] - 15 x [Cr] - 80 x [Mo] In the above formula, [M] represents the content (mass%) of element M in the steel sheet (base material), and is set to 0 (zero) if not contained.

[0079] [Average Cooling Rate of Steel Plate: 1.0°C / s or More and 20.0°C / s or Less] Cooling at an average cooling rate of 1.0°C / s or more is an essential process for obtaining high-strength, high-toughness steel plates. Cooling at a high rate can increase strength through transformation strengthening. If the average cooling rate is less than 1.0°C / s, the grain size of bainite and martensite increases, and ferrite and pearlite may form, potentially resulting in insufficient strength and reduced toughness. Therefore, the average cooling rate is set to 1.0°C / s or more. Furthermore, the average cooling rate is preferably set to 1.5°C / s or more. On the other hand, if the average cooling rate exceeds 20.0°C / s, the volume fraction of martensite may become too high, potentially reducing toughness. Furthermore, the joinability may be reduced. Therefore, the average cooling rate is set to 20.0°C / s or less. Furthermore, the average cooling rate is preferably set to 15.0°C / s or less. Here, the average cooling rate in accelerated cooling is determined by dividing the difference (°C) between the cooling start temperature and the cooling stop temperature at the half-thickness position in the thickness direction of the base steel plate by the cooling time (s). The cooling start temperature and cooling stop temperature at the half-thickness position in the thickness direction of the base steel plate can be obtained by measuring the temperature of the steel plate surface with a radiation thermometer and calculating the difference to determine the temperature at the half-thickness position. The cooling time is the time during which cooling water is supplied to the steel plate, and is the time required for the half-thickness position to change from the cooling start temperature to the cooling stop temperature.

[0080] [Cooling Stop Temperature: 350°C or Less] In the present invention, after reheating is completed, cooling is performed under the above-described conditions to a cooling stop temperature of 350°C or less, thereby making it possible to uniformly achieve a predetermined volume fraction of bainite or martensite throughout the thickness center of the base metal. If the cooling stop temperature exceeds 350°C, ferrite and pearlite structures are excessively formed, resulting in insufficient strength and a deterioration in toughness. Therefore, the cooling stop temperature is specified to be 350°C or less. The cooling stop temperature is preferably 300°C or less. Meanwhile, the lower limit of the cooling stop temperature is not particularly limited, and may be room temperature, but is preferably 100°C from the viewpoint of production efficiency, etc.

[0081] Furthermore, such clad steel plate is subjected to accelerated cooling in the treatment of the above step (A) up to the cooling stop temperature, and then further tempered at a temperature of 550°C or higher and 700°C or lower.

[0082] [Tempering temperature: 550°C or higher and 700°C or lower] In the present invention, tempering is performed for the purpose of restoring the toughness of the base material. If the average temperature of the steel sheet (the temperature at 1 / 2 the steel sheet thickness position) exceeds 700°C during reheating for tempering, dislocations may be restored, and the strength of the base material may decrease. Therefore, the tempering temperature is set to 700°C or lower. The tempering temperature is preferably set to 680°C or lower. On the other hand, if the average temperature of the steel sheet during reheating for tempering is less than 550°C, the toughness of the base material may be insufficient. Therefore, the tempering temperature is set to 550°C or higher. The tempering temperature is preferably set to 600°C or higher.

[0083] (Step (B) (Accelerated Cooling Step)) In the step (B), the base material is cooled to Ar. ３ Accelerated cooling is performed from a temperature above the transformation point at an average cooling rate of 3°C / s to 50°C / s to a cooling stop temperature of 500°C or less. Alternatively, after accelerated cooling, tempering is performed at a temperature of 700°C or less. Here, the temperature above the Ar3 transformation point of the base material refers to the surface temperature of the base material.

[0084] [Cooling start temperature: Ar ３ The cooling start temperature of the accelerated cooling is Ar ３ If the temperature is lower than the transformation point, ferrite is generated, which deteriorates the toughness.３ If the temperature is lower than the transformation point, the bondability of the clad steel deteriorates from the viewpoint of diffusion bonding. ３ The cooling start temperature for accelerated cooling is Ar ３ The temperature is preferably the transformation point +20°C or higher. The cooling (accelerated cooling) here is preferably carried out immediately after the hot rolling without air-cooling to room temperature or reheating the rolled sheet to 800°C or higher and 1000°C or lower.

[0085] [Average Cooling Rate of Steel Plate: 3°C / s or More and 50°C / s or Less] Cooling at an average cooling rate of 3°C / s or more is an essential process for obtaining high-strength, high-toughness steel plates. Cooling at a high rate can increase strength through transformation strengthening. If the average cooling rate is less than 3°C / s, the grain size of bainite and martensite may increase, or ferrite and pearlite may form, potentially resulting in insufficient strength and a decrease in toughness. Therefore, the average cooling rate is set to 3°C / s or more. The average cooling rate is preferably set to 5°C / s or more, and more preferably set to 10°C / s or more. On the other hand, if the average cooling rate exceeds 50°C / s, the volume fraction of martensite may become too high, potentially reducing toughness. Furthermore, the bonding ratio between the base material and the clad material may decrease. Therefore, the average cooling rate is set to 50°C / s or less. Furthermore, the average cooling rate is preferably set to 45°C / s or less. Here, the average cooling rate in accelerated cooling is determined by dividing the difference (°C) between the cooling start temperature and the cooling stop temperature at the half-thickness position in the thickness direction of the base steel plate by the cooling time (s). The cooling start temperature and cooling stop temperature at the half-thickness position in the thickness direction of the base material are obtained by measuring the temperature of the steel plate surface with a radiation thermometer and calculating the difference to determine the temperature at the half-thickness position. The cooling time is the time during which cooling water is supplied to the steel plate, and is the time required for the half-thickness position to change from the cooling start temperature to the cooling stop temperature.

[0086] [Cooling Stop Temperature: 500°C or Less] In the present invention, by performing cooling under the above-described conditions up to a cooling stop temperature of 500°C or less, it is possible to uniformly achieve a predetermined volume fraction of bainite or martensite throughout the thickness center of the base material. If the cooling stop temperature exceeds 500°C, excessive ferrite and pearlite structures are formed, resulting in insufficient strength and a deterioration in toughness. Therefore, the cooling stop temperature is specified to be 500°C or less. The cooling stop temperature is preferably 300°C or less. When tempering, which will be described later, is performed following the accelerated cooling process, the cooling stop temperature is preferably 300°C or less, and more preferably 280°C or less. Meanwhile, the lower limit of the cooling stop temperature is not particularly limited and may be room temperature, but from the viewpoint of production efficiency, etc., the lower limit of the cooling stop temperature is preferably 150°C.

[0087] Furthermore, after the accelerated cooling in the treatment of step (B) above is performed to the cooling stop temperature, the clad steel plate may be further tempered at a temperature of 700° C. or less, if necessary. That is, after the accelerated cooling in the treatment of step (B), tempering does not have to be performed, but tempering may be performed at a temperature of 700° C. or less after the accelerated cooling.

[0088] [Tempering temperature: 700°C or less] In the present invention, tempering can be performed as needed to restore the toughness of the base material. If the average temperature of the steel sheet (the temperature at 1 / 2 the steel sheet thickness position) exceeds 700°C during reheating for tempering, dislocations may be restored, and the strength of the base material may decrease. Therefore, the tempering temperature is set to 700°C or less. The tempering temperature is preferably set to 680°C or less. On the other hand, if the average temperature of the steel sheet during reheating for tempering is less than 550°C, the toughness of the base material may be insufficient. Therefore, the tempering temperature is preferably set to 550°C or more. The tempering temperature is more preferably set to 600°C or more.

[0089] In the present invention, the temperature of the base steel plate or clad steel plate refers to the temperature at 1 / 2 the plate thickness of the base material, and a value obtained by performing a differential calculation using, for example, a process computer from the temperature of the steel plate surface measured with a radiation thermometer may be used.

[0090] The clad steel plate according to the present invention can be obtained by subjecting the base material and cladding material steel plates having the above-mentioned chemical compositions to the above-mentioned manufacturing conditions. The high-strength clad steel plate according to the present invention thus obtained has excellent low-temperature toughness, ammonia SCC resistance, and excellent joinability.

[0091] In the production method according to the present invention, any item not described in this specification can be carried out in a conventional manner.

[0092] Table 1 shows the chemical composition of the base material (the balance is Fe and unavoidable impurities). In the table, steel types A to O are invention examples that fall within the scope of the present invention. On the other hand, steel types P to AE are comparative examples in which one of the components is outside the scope of the present invention. Table 2 shows the chemical composition of the cladding material (the balance is Fe and unavoidable impurities). In the table, steel types a to m are invention examples that fall within the scope of the present invention. On the other hand, steel types n to p are comparative examples in which the PRE values ​​are outside the scope of the present invention. The blank spaces in Tables 1 and 2 indicate that the element is not contained or is contained as an unavoidable impurity.

[0093]

[0094]

[0095] Clad steel plates (Nos. 1 to 91) were manufactured under the manufacturing conditions shown in Table 3 using base material slabs having the chemical compositions shown in Table 1 and clad material slabs having the chemical compositions shown in Table 2. Test plates for joints were taken from the resulting steel plates, and welded joints were fabricated. TIG welding was used, with multi-layer welding performed from the base material side to the clad material side. The slabs were assembled using either the sandwich method, the open method, or the sacrificial material method. In Tables 3-1 and 3-2, the sandwich method involves stacking the slabs in the order of base material slab, clad material slab, clad material slab, and base material slab from top to bottom. The open method involves stacking the slabs in the order of clad material slab, base material slab, and base material slab from top to bottom. The sacrificial material method involved stacking the slabs in the order of sacrificial material, clad material slab, and base material slab. The resulting clad steel plates were evaluated for strength properties and toughness, joining rate, and ammonia SCC resistance in a liquid ammonia environment. The test methods were as follows:

[0096] [Strength properties] The clad steel plate was thinned on the clad material side, and a JIS Z 2201 No. 1B test piece was taken from the entire thickness of the base material. A tensile test was carried out in accordance with the procedure described in JIS Z 2241 (2022), and the yield strength YS (yield point YP (lower yield point YP) when there was a yield point, and 0.2% proof stress σ0.2 when there was no yield point) and tensile strength (TS) were measured. Steel plates with a tensile strength of 780 MPa or more of the base material were evaluated as having excellent tensile properties.

[0097] [Toughness] To evaluate the toughness of the base material of the clad steel plate, V-notch test specimens according to JIS Z 2202 were taken with the center of the thickness direction at the half-thickness position of the base material of the clad steel plate. To evaluate the toughness of the welded joint of the clad steel plate, V-notch test specimens according to JIS Z 2202 were taken from the welded joint in the heat-affected zone of the base material of the clad steel plate, with the center of the thickness direction at the half-thickness position of the base material of the clad steel plate. Three test specimens for each condition were subjected to a Charpy impact test at -40°C according to JIS Z 2242 (2023), and the absorbed energy was measured. All three test specimens with an absorbed energy of 47 J or more were evaluated as having excellent toughness. In Tables 3-1 and 3-2, the smallest absorbed energy of the three test specimens is listed.

[0098] [Bonding rate] The bonding rate at the bonding interface of the clad steel plate was determined as follows: A cross section including the base material and cladding material of the clad steel plate was mirror-polished, and a scanning electron microscope (SEM) was used to measure a length of 1.0 × 10 in a direction along the interface randomly selected on the cross section at a magnification of 1000. －１ Ten fields of view were observed, each measuring 1 / 2 mm. The bonded interface length was determined by image analysis. The bonded interface ratio was calculated based on the measured total length and the bonded interface length using the following formula: Bonded interface ratio (%): 100 × bonded interface length (mm) / measured total length (mm). Here, the bonded interface length is the total length of the portion where, as a result of image analysis, it was determined that no voids or oxides existed on the interface. Steel sheets with a bonded interface ratio of 70% or more were evaluated as having excellent bondability.

[0099] [Ammonia SCC Resistance] The ammonia SCC resistance in the present invention was evaluated by applying a potential to a test specimen that had been stressed by four-point bending in a test solution. Specifically, the following procedure was performed. The clad steel plate according to the present invention is used so that the clad material comes into contact with ammonia, etc. Therefore, first, a test specimen measuring 1.5 to 3.0 mm thick x 15 mm x 115 mm was taken from the clad material portion by reducing the thickness from the base metal side of the clad steel plate. When the clad material thickness exceeded 3.0 mm, a test specimen measuring 3.0 mm thick was taken from the side of the clad material that was not joined to the base metal. The taken test specimens were subjected to ultrasonic degreasing in acetone for 5 minutes. A stress of 100% of the actual yield strength YS of the corresponding base metal was applied to each test specimen by four-point bending. The four-point bending test specimens were placed in a test cell. Next, 2 L of liquid ammonia with a purity of 99.999% or more was mixed with ammonium carbamate: 5.00 mass%, O ２ A test cell was filled with a solution containing 1.000 bar of ammonium carbamate and 0.10 mass% of water. ２ After the gas was blown in, liquid ammonia was poured in. The specific liquid volume, which is the ratio of the amount of immersion liquid to the surface area of ​​the immersed test specimen, was 42 mL / cm. ２The test was performed continuously at 10 rpm using a stirrer installed in the test cell during the test. The test solution temperature was set to 25°C. After adjusting the temperature of the test solution to 25°C, the corrosion potential of the test specimen was measured using a potentiostat. Potential measurement and application using the potentiostat were performed using a three-electrode method, with platinum electrodes used as the reference electrode and counter electrode. The potential was determined to have stabilized one hour after the start of corrosion potential measurement, and at that point, the immersion test was initiated by controlling the potential to apply a potential of +0.5 V vs. Pt to the test specimen. 504 hours after the start of the immersion test, the test specimen was removed from the test cell. Corrosion products on the surface of the test specimen were removed, and the surface and cross section were visually observed for cracks to evaluate the cracks. In this example, a 504-hour immersion test was performed on nine test specimens under one condition. When cracks of 1.5 mm or more were observed in two or less of these test pieces, the ammonia SCC resistance was judged to be good (◯), and when cracks were observed in three or more test pieces, the ammonia SCC resistance was judged to be poor (×). The obtained evaluation results are also shown in Table 3.

[0100]

[0101]

[0102] As can be seen from Table 3-1, all of the invention examples have a tensile strength of 780 MPa or more, an absorbed energy of 47 J or more in a Charpy impact test at -40°C, and a bonding rate between the base material and cladding material of 70% or more. It can also be seen that all of the invention examples have excellent ammonia SCC resistance.

[0103] In contrast, as can be seen from Table 3-2, Nos. 47 to 65 have chemical compositions of either the base material or the cladding material that are outside the scope of the present invention, and therefore are inferior in at least one of tensile strength TS, low-temperature toughness, and ammonia SCC resistance. Furthermore, Nos. 66 to 91 have chemical compositions of the base material and the cladding material that are within the scope of the present invention, but the manufacturing conditions are outside the scope of the present invention, and therefore are inferior in at least one of tensile strength TS, low-temperature toughness, bonding rate, and ammonia SCC resistance.

Claims

1. A clad steel plate having a cladding material on at least one side of a base material, wherein the chemical composition of the base material contains, in mass %, C: 0.030 to 0.150%, Si: 0.05 to 0.55%, Mn: 0.50 to 2.10%, P: 0.020% or less, S: 0.010% or less, Al: 0.018 to 0.070%, Ni: 0.30 to 2.20%, Ti: 0.005 to 0.020%, N: 0.0020 to 0.0080%, O (oxygen): 0.0050% or less, and the Ti / N ratio is 2.2 to 6.5, and further contains Cu: 0.50% or less, Cr: 1.60% or less, Mo: 0.60% or less, A clad steel plate containing one or more of Nb: 0.030% or less, V: 0.100% or less, B: 0.0050% or less, and Ca: 0.0040% or less, with the balance being Fe and unavoidable impurities, wherein the clad material has a PRE value of 15 or more as shown in formula (1), a tensile strength of the base material of 780 MPa or more, an absorbed energy of the base material in a Charpy impact test at -40°C of 47 J or more, and a bonding rate between the base material and the clad material of 70% or more. PRE = Cr% + 3.3Mo% + 30N% - Mn% (1) In formula (1), M% represents the content (mass%) of element M in the clad material, and elements that are not contained are represented as 0 (zero).

2. A welded joint using the clad steel plate according to claim 1, wherein the absorbed energy of the weld heat affected zone in a Charpy impact test at -40°C is 47 J or more.

3. A method for manufacturing clad steel plate according to claim 1, comprising: heating a base material slab to a surface temperature of 900°C or more and 1200°C or less, and then hot rolling to a rolling end temperature of 700°C or more to obtain a base material; heating a clad material slab and then hot rolling to obtain a clad material; heating a laminated slab obtained by laminating the base material and the clad material to a surface temperature of 1000°C or more and 1250°C or less, and then hot rolling the laminated slab to a surface temperature of 1000°C or more and 1250°C or less, and then hot rolling the laminated slab to a rolling end temperature of 65% or more, and ３ A method for producing a clad steel plate, comprising: hot rolling the plate to a temperature between a transformation point and 1000°C to produce a rolled plate having a base material and a clad material; and subjecting the rolled plate to the following treatment (A) or (B). (A) After cooling the rolled plate after hot rolling, the plate is reheated to a temperature between 800°C and 1000°C, and the Ar of the base material is removed. ３ (B) The rolled sheet after hot rolling is subjected to accelerated cooling at an average cooling rate of 1.0°C / s to 20.0°C / s from a temperature above the transformation point to a cooling stop temperature of 350°C or less, and after the accelerated cooling, the sheet is tempered at a temperature of 550°C to 700°C. ３ Accelerated cooling is performed from a temperature above the transformation point at an average cooling rate of 3°C / s to 50°C / s to a cooling stop temperature of 500°C or less, or further, tempering is performed at a temperature of 700°C or less after the accelerated cooling.

4. A method for manufacturing a welded joint, which comprises using the clad steel plate according to claim 1 and welding the same under conditions of a heat input of 50 kJ / cm or less to manufacture a welded joint.

Images