Clad steel plate, welded joint and manufacturing method thereof

Abstract

The present invention provides 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, and a method for manufacturing the same. A clad steel plate having a base material with a specified chemical composition and a clad material with a specified chemical composition bonded to at least one surface (the surface that becomes the inner surface), in which 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, the maximum hardness of the clad material is 210 HV or less, and the bonding rate between the base material and the clad material is 70% or more.

Description

[Technical Field]

[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. [Background technology]

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

[0003] For example, ammonia SCC is known to correlate with the strength and hardness of materials. When using carbon steel, it is considered desirable to use materials with a tensile strength of less than 600 MPa. Ammonia SCC is also known to occur in weld heat-affected zones. Therefore, when using high-strength steel in a liquid ammonia environment, measures such as post-weld heat treatment using full annealing to adjust the tensile strength and hardness of the weld are necessary.

[0004] In recent years, liquid ammonia has been attracting attention as a clean energy source because it does not produce CO2 when burned, and large-scale demand is expected. This has led to a demand for larger facilities to transport and store liquid ammonia. Generally, when making a tank 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 same 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 in 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. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 55-30062 Summary of the Invention [Problem to be solved by the invention]

[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 was 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 and the like. 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 3.0 mm thick test piece 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 corresponding base metal is applied to each test piece by four-point bending. The four-point bending test piece is placed in a test cell. Next, the test cell is filled with a solution prepared by mixing 2 L of liquid ammonia with a purity of 99.999% or higher with 5.00 mass% ammonium carbamate, 1.000 bar O2, and 0.10 mass% water. Specifically, a predetermined amount of ammonium carbamate and water is placed in the test cell, and then O2 gas is blown in before liquid ammonia is added. The specific liquid volume, which is the ratio of the amount of immersion liquid to the surface area of ​​the immersed test specimen, is 42 mL / cm. 2 During the test, the solution was continuously stirred 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 voltage 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 inspected for cracks and evaluated. In this invention, a 504-hour immersion test was conducted on nine test specimens under one condition. If cracks of 1.5 mm or more were found in two or fewer test pieces, the ammonia SCC resistance was judged to be good (○), i.e., excellent, and if cracks were found in three or more test pieces, the ammonia SCC resistance was judged to be poor (×). Furthermore, excellent low-temperature toughness means that the absorbed energy is 47 J or more when a Charpy impact test is carried out at -40°C in accordance with JIS Z 2242 (2023). Furthermore, excellent bondability means that in a cross section including the base material and cladding material, the bonding rate, expressed by the following formula, is 70% or more. Bonding rate (%): 100 x bonding interface length (mm) / total measurement length (mm) Furthermore, "high strength" means that the tensile strength (TS) of the base material measured in accordance with JIS Z 2241 (2022) is 780 MPa or more. [Means for solving the problem]

[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 sheets, and as a result have obtained the following findings. In other words, since ammonia SCC occurs inside the product (tank), the characteristics of the steel plate surface layer, which is on the inside, are dominant in ammonia SCC resistance. Therefore, we came up with the idea of ​​using a steel plate with excellent strength and low-temperature toughness as the base material, and further joining a steel plate with excellent ammonia SCC resistance to the base material as a clad steel plate. The inventors have also found that by using such clad steel plate, excellent ammonia SCC resistance, low-temperature toughness and strength properties can all be obtained.

[0012] In addition, since the base material and clad material used in the present invention are both carbon steel, alloy costs and manufacturing costs can be significantly reduced compared to clad steel plates that use stainless steel or non-ferrous alloys as clad materials.

[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 cladding material on at least one side of a base material, The chemical composition of the base material is In mass%, C: 0.030~0.150%, Si: 0.05 to 0.55% Mn: 0.50~2.10% P: 0.020% or less, S: 0.010% or less, Al: 0.018~0.070%, Ni: 0.30~2.20% Ti: 0.005 to 0.020%, N: 0.0020~0.0080%, O (oxygen): 0.0050% or less and Ti / N is 2.2 or more and 6.5 or less, and further 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, Ca:0.0040% or less and the balance being Fe and unavoidable impurities, The chemical composition of the cladding material is, in mass%, C: 0.030~0.140%, Mn: 0.20 to 1.60% P: 0.020% or less, S: 0.010% or less, Al: 0.018~0.070%, N: 0.0020~0.0050%, O (oxygen): 0.0050% or less and the balance being Fe and unavoidable impurities, 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 47J or more, The maximum hardness of the cladding material is 210HV or less, Clad steel plate with a bonding rate of 70% or more between the base material and cladding material.

[0014] [2] The chemical composition of the cladding material is further expressed in mass%: Si: 0.55% or less, Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.10% or less, Mo: 0.10% or less Ti: 0.020% or less, Nb: 0.030% or less, V: 0.050% or less, Ca:0.0040% or less The clad steel plate according to [1] above, containing one or more of the following:

[0015] [3] A welded joint using the clad steel plate according to [1] or [2] above, The absorbed energy of the weld heat affected zone in a Charpy impact test at -40°C is 47J or more, A welded joint in which the maximum hardness of the weld heat-affected zone of the cladding material is 210HV or less.

[0016] [4] A method for producing a clad steel plate according to [1] or [2], The base material slab is heated to a surface temperature of 900°C or more and 1200°C or less, and then hot-rolled to a rolling end temperature of 700°C or more to form the base material. After heating the cladding material slab, it is hot rolled to make cladding material. The laminated slab obtained by laminating the base material and the cladding material is heated to a surface temperature of 1000°C or more and 1250°C or less, hot rolling is performed at a cumulative reduction rate of 60% or more and a rolling end temperature of the Ar3 transformation point or more and 1000°C or less to produce a rolled plate having a base material and a clad material; A method for producing a clad steel plate, which comprises subjecting the rolled plate to the following treatment (A) or (B): (A) After cooling the rolled plate after hot rolling, Reheat to between 800°C and 1000°C. Accelerated cooling is performed from a temperature above the Ar3 transformation point of the base material 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. After the accelerated cooling, tempering is carried out at a temperature of 550°C or higher and 700°C or lower. (B) After hot rolling, the rolled sheet is subjected to accelerated cooling from a temperature equal to or higher than the Ar3 transformation point of the base material at an average cooling rate of 3°C / s to 50°C / s to a cooling stop temperature of 500°C or lower, or further, after the accelerated cooling, is subjected to tempering at a temperature of 700°C or lower.

[0017] [5] A method for manufacturing a welded joint, which uses the clad steel plate according to [1] or [2] above and welds it under conditions of a heat input of 50 kJ / cm or less. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide a high-strength clad steel plate that has excellent ammonia SCC resistance, low-temperature toughness and joinability, and is suitable for structural members such as tanks used in a liquid ammonia environment. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention provides a clad steel plate having a base material and a cladding material made of carbon steel 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 cladding 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 be LPG, liquefied CO2, or other liquefied gases. 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. Furthermore, the method is not limited to assembled slabs produced by stacking base slabs (material for the base steel plate) and cladding 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 slabs in a gas atmosphere to form the chemical composition of the cladding material.

[0020] Hereinafter, embodiments of the present invention will be described more specifically. 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 is, 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: 0.00 20 to 0.0080%, O (oxygen): 0.0050% or less, with Ti / N being 2.2 or more and 6.5 or less, and further containing 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 chemical composition of the cladding material is, in mass%, C: 0.030 to 0.140%, Mn: 0.20 to 1.60%, P: 0.020% or less, S: 0.010% or less, Al: 0.018 to 0.070%, N: 0.0020 to 0.0050%, O (oxygen): 0.0050% or less, with the remainder being Fe and unavoidable impurities; 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, the maximum hardness of the cladding material is 210 HV or less, and the bonding rate between the base material and the cladding material is 70% or more. In addition, "%" representing the content of the following component elements means "% by mass" unless otherwise specified.

[0021] (1) Chemical composition of the base material C: 0.030 to 0.150% C is the most effective element for increasing the strength of the steel sheet produced by the cooling method 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 contents of other alloying elements and producing the steel sheet at lower cost, the C content is preferably 0.040% or more. On the other hand, if the C content exceeds 0.150%, the toughness and weldability of the steel plate will deteriorate. 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.

[0022] Si: 0.05 to 0.55% Si is added to improve the strength of the steel sheet and also 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. On the other hand, if the Si content exceeds 0.55%, the toughness and weldability deteriorate. Therefore, the Si content is specified to be 0.55% or less. Furthermore, the Si content is preferably 0.50% or less.

[0023] 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 contents of other alloying elements and achieving lower manufacturing costs, the Mn content is preferably 0.70% or more. On the other hand, if the Mn content exceeds 2.10%, it will cause 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.

[0024] 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 P content of 0.020% or less is acceptable. 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 more than 0%. Moreover, since excessive reduction of P leads to an increase in refining costs, the P content is preferably 0.001% or more.

[0025] S: 0.010% or less S is an element contained as an unavoidable impurity, and exists in steel as sulfide-based inclusions such as MnS, which have adverse effects such as acting as fracture initiation points 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. The lower limit of the S content is not particularly limited and may be 0%, but since S is an element that is usually unavoidably contained in steel as an impurity, industrially it may be greater than 0%. Moreover, since excessive reduction leads to an increase in refining costs, from the viewpoint of cost, it is preferable that the S content be 0.001% or more.

[0026] Al: 0.018 to 0.070% Al acts as a deoxidizer, and to obtain this effect, the Al content is set to 0.018% 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.

[0027] 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 metal 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.

[0028] 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 obtain this effect, the Ti content needs to be 0.005% or more. On the other hand, if the Ti content exceeds 0.020%, the toughness 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.

[0029] N: 0.0020~0.0080% N can improve the toughness of welds by forming TiN. To obtain this effect, the N content must be 0.0020% or more. On the other hand, if the N content exceeds 0.0080%, the toughness will actually decrease. 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.

[0030] O (oxygen): 0.0050% or less O is an element contained as an unavoidable impurity, and exists in steel as oxides such as Al2O3. It has adverse effects, such as acting as a fracture initiation point and reducing the toughness of the steel plate. Therefore, it is desirable to keep the O content as low as possible, but a content of 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, industrially it may be 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 that the O content be 0.0005% or more.

[0031] 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 Ti content to 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 to be in the range of 2.2 to 6.5. 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.

[0032] In addition to the above components, the base material of the present invention further contains one or more of the elements listed 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 present to the extent that they do not impair the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap. Examples of impurities include Co, Sn, Zn, Pb, As, Sb, Bi, H, and REM.

[0033] 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 preferably 0.10% or more. On the other hand, if the Cu content exceeds 0.50%, scratches will occur on the surface of the steel sheet. Therefore, if Cu is contained, the Cu content is set to 0.50% or less, and preferably to 0.45% or less.

[0034] Cr:1.60% or less Cr is an element effective in improving the strength of steel sheets. 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, if Cr is contained, the Cr content is specified to be 1.60% or less. The Cr content is preferably 1.50% or less.

[0035] 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 plate deteriorates. Therefore, if Mo is contained, the Mo content is set to 0.60% or less, and preferably to 0.55% or less.

[0036] 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, resulting in a decrease in toughness. Therefore, when Nb is contained, the Nb content is set to 0.030% or less. Furthermore, the Nb content is preferably set to 0.027% or less.

[0037] 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 plate deteriorates. Therefore, when V is contained, the V content is set to 0.100% or less. The V content is preferably set to 0.090% or less.

[0038] 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 plate deteriorates. Therefore, when B is contained, the B content is set to 0.0050% or less, and preferably to 0.0040% or less.

[0039] 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 so that they assume a spherical shape, 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, if Ca is contained, the Ca content is set to 0.0040% or less. The Ca content is preferably 0.0030% or less.

[0040] (2) Chemical composition of cladding materials C: 0.030 to 0.140% C is an element that increases the hardness of steel sheets, and the higher the hardness, the higher the liquid ammonia SCC susceptibility. Therefore, the C content of cladding materials is specified to be 0.140% or less. The C content is preferably 0.130% or less. On the other hand, the lower the C content of the cladding material, the better, but excessive reduction leads to an increase in refining costs. Therefore, the C content is specified to be 0.030% or more. Furthermore, the C content is preferably 0.040% or more.

[0041] Mn: 0.20 to 1.60% Mn is an element that increases the hardenability of steel. Therefore, if the Mn content is too high, the hardness of the steel sheet will increase too much. If the hardness of the steel sheet increases too much, it will lead to a deterioration in ammonia SCC resistance. Therefore, the Mn content is specified to be 1.60% or less. It is preferable that the Mn content be 1.55% or less. On the other hand, since it would be very costly to reduce the Mn content to less than 0.20%, the Mn content is set to 0.20% or more, and more preferably, the Mn content is set to 0.30% or more.

[0042] 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 P content of 0.020% or less is acceptable. 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 more than 0%. Moreover, since excessive reduction of P leads to an increase in refining costs, the P content is preferably 0.001% or more.

[0043] S: 0.010% or less S is an element contained as an unavoidable impurity, and exists in steel as sulfide-based inclusions such as MnS, which have adverse effects such as acting as fracture initiation points 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. The lower limit of the S content is not particularly limited and may be 0%, but since S is an element that is usually unavoidably contained in steel as an impurity, industrially it may be greater than 0%. Moreover, since excessive reduction leads to an increase in refining costs, from the viewpoint of cost, it is preferable that the S content be 0.0003% or more.

[0044] Al: 0.018 to 0.070% Al acts as a deoxidizer. To obtain this effect, the Al content is set to 0.018% or more. The Al content is preferably 0.020% or more. On the other hand, if the Al content exceeds 0.070%, oxide inclusions increase, reducing cleanliness and toughness, so the Al content is specified to be 0.070% or less.

[0045] N: 0.0020~0.0050% Nitrogen (N) is an element that increases the hardness of steel sheets, and the higher the hardness, the higher the susceptibility to liquid ammonia SCC. Therefore, the N content of cladding materials is specified to be 0.0050% or less. The N content is preferably 0.0045% or less. On the other hand, the lower the N content of the cladding material, the better, but excessive reduction leads to an increase in refining costs. Therefore, the N content is specified to be 0.0020% or more. Furthermore, the N content is preferably 0.0025% or more.

[0046] O (oxygen): 0.0050% or less O (oxygen) is an element contained as an unavoidable impurity. It exists in steel as oxides such as Al2O3 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 a content of 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 usually an element that is unavoidably contained in steel as an impurity, it may be industrially acceptable to have a content greater than 0%. Furthermore, excessive reduction in O content leads to increased refining costs, so from a cost perspective, it is preferable to set the O content to 0.0005% or more.

[0047] 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, manufacturing processes, manufacturing equipment, etc., and are allowed to be included to the extent that they do not impair the objectives 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.

[0048] By mass%, one or more of the following: Si: 0.55% or less, Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.10% or less, Mo: 0.10% or less, Ti: 0.020% or less, Nb: 0.030% or less, V: 0.050% or less, Ca: 0.0040% or less Si, Cu, Ni, Cr, Mo, and V can improve the strength of the steel sheet. To achieve this effect, it is preferable to set the Si content to 0.05% or more, the Cu content to 0.05% or more, the Ni content to 0.05% or more, the Cr content to 0.05% or more, the Mo content to 0.05% or more, and the V content to 0.005% or more. On the other hand, excessive Si content deteriorates weldability. Therefore, when Si is contained, the Si content is set to 0.55% or less. Furthermore, excessive Cu, Ni, Cr, Mo, and V content increases hardness and is also disadvantageous from the viewpoint of alloy cost. Therefore, when Cu is contained, the Cu content is set to 0.30% or less; when Ni is contained, the Ni content is set to 0.30% or less; when Cr is contained, the Cr content is set to 0.10% or less; when Mo is contained, the Mo content is set to 0.10% or less; and when V is contained, the V content is set to 0.050% or less. The V content is preferably set to 0.030% or less.

[0049] Ti and Nb can further improve the toughness of the steel sheet, and in order to obtain this effect, it is preferable to specify the Ti content to be 0.005% or more and the Nb content to be 0.005% or more. On the other hand, excessive addition of Ti and Nb increases the hardness and is also disadvantageous from the viewpoint of alloy cost. Therefore, when Ti is added, the Ti content is set to 0.020% or less, and when Nb is added, the Nb content is set to 0.030% or less.

[0050] 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 adding Ca, the morphology of sulfide-based inclusions is controlled so that they assume a spherical shape, thereby improving the toughness of welds and the like. To obtain this effect, the Ca content is preferably 0.0005% or more. On the other hand, if the Ca content exceeds 0.0040%, the cleanliness of the steel decreases. Therefore, when Ca is added, the Ca content is set to 0.0040% or less. The Ca content is more preferably 0.0010% or more. The Ca content is preferably 0.0030% or less. The Ca content is more preferably 0.0020% or less.

[0051] (3) Characteristics of clad steel plates [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 930 MPa or less.

[0052] [Charpy impact test energy absorption] 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) 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.

[0053] [Maximum hardness] In the clad steel plate of the present invention, the maximum hardness of the clad material is set to 210 HV or less. If a high-hardness region exists in the surface layer of the clad steel plate, ammonia SCC is promoted. In other words, if the hardness of the clad material exceeds 210 HV, the desired ammonia SCC resistance cannot be obtained.

[0054] [Joining rate] In clad steel plates, there are areas where the base material and clad material are bonded directly to each other via oxides or voids. The bond strength between the base material and oxide and the bond strength between the clad material and oxide are both smaller than the bond strength between the base material and clad material. Therefore, the higher the bonding rate between the base material and clad material, the higher the bond 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, and more preferably 85% or more. There is no particular upper limit to the bonding rate between the base material and clad material, and a higher bonding rate is preferable, and it may be 100%.

[0055] [Plate 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.

[0056] 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.

[0057] 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.

[0058] The clad ratio of the clad steel plate of the present invention is not particularly limited, but may be 0.01 or more. 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, but may be 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.

[0059] (4) Welded joints 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 any conventionally known steel plate welding method 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. In addition, in the welded joint, it is preferable that the maximum hardness of the weld heat affected zone of the cladding material is 210 HV or less.

[0060] [Heat input: 50kJ / 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. Furthermore, 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 an excessive reduction in the heat input leads to a decrease in welding efficiency, it is preferable that the heat input be 5.0 kJ / cm or more.

[0061] (5) Manufacturing conditions In the present invention, first, a base material slab and a cladding material slab having the aforementioned chemical composition are manufactured. There are no particular limitations on the manufacturing method of these base material slabs, and conventionally known slab manufacturing methods can be applied. That is, molten steel adjusted to the aforementioned chemical composition by a conventional melting method (such as a converter method or an electric furnace method) is cast by a conventional casting method (such as a continuous casting method or an ingot casting method) to obtain the base material slab. Next, the obtained base material slab is heated to a surface temperature of 900°C or more and 1200°C or less and then hot rolled. The hot rolling ends at a surface temperature of 700°C or more to produce a base material of predetermined dimensions. Furthermore, the obtained clad material slab is heated and then hot rolled to produce a clad material of predetermined dimensions.

[0062] [Heating temperature: 900℃ or higher and 1200℃ or lower] If the heating temperature of the base material slab in the clad steel plate of the present invention is less than 900°C, the solid solution of carbides will be insufficient and the required strength will not be obtained. Therefore, the heating temperature is set to a surface temperature of 900°C or higher. The heating temperature is preferably set to 920°C or higher. On the other hand, if the base material slab is heated to a temperature exceeding 1200°C, the amount of energy consumed increases. Therefore, the heating temperature is set to a surface temperature of 1200°C or less. The heating temperature is preferably set to 1150°C or less.

[0063] [Finishing temperature (FT): 700°C or higher] If the rolling end temperature of the hot rolling is less than 700°C, the generated ferrite will be affected by the working, resulting in a deterioration in toughness. Therefore, the rolling end temperature is set to 700°C or higher (surface temperature). Furthermore, the rolling end temperature is preferably set to 750°C or higher. Although there is no particular upper limit, the rolling end temperature is preferably 1000°C or less in terms of surface temperature.

[0064] Next, a clad material is layered on the surface of the obtained base material that comes into contact with ammonia or the like to form at least two layers, or three layers by layering a sacrificial material, a clad material, and a base material, or even four layers by layering a base material, a clad material, a clad material, and a base material, and the like, and the clad material is pressure-welded and bonded, and an appropriate heat treatment is performed to control the structure. In the tables of examples described later, the slab stacking method is referred to as the sandwich method when stacking from top to bottom in the order of base material slab, cladding material slab, cladding material slab, base material slab, or cladding material slab, base material slab, base material slab, and cladding material slab. Also, the method of stacking from top to bottom in the order of cladding material slab, base material slab, or base material slab, cladding material slab is referred to as the open method. Also, the method of stacking from top to bottom in the order of sacrificial material, cladding material slab, base material slab, or base material slab, cladding material slab, and sacrificial material is referred to as the sacrificial material method. The clad slab (laminated slab) made by overlapping the base material and cladding material is heated to a temperature range of 1000°C to 1250°C and then hot rolled. In the present invention, the hot rolling has a cumulative reduction rate of 60% or more and a rolling end temperature of the Ar3 transformation point or more and 1000°C or less. Furthermore, after the hot rolling, the following treatment (A) or (B) is carried out. (A) After cooling the hot-rolled sheet, it is reheated to 800°C to 1000°C and then accelerated 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 from a temperature above the Ar3 transformation point of the base material. After accelerated cooling, it is tempered at a temperature of 550°C to 700°C. The reheating of the rolled plate to 800° C. or more and 1000° C. or less is carried out at an average heating rate of, for example, 1° C. / s or more and 50° C. / s or less. (B) After hot rolling, the rolled sheet is accelerated cooled from a temperature equal to or higher than the Ar3 transformation point of the base material at an average cooling rate of 3°C / s to 50°C / s to a cooling stop temperature of 500°C or lower. Alternatively, after accelerated cooling, the sheet is tempered at a temperature of 700°C or lower.

[0065] [Heating temperature: 1000℃ or higher and 1250℃ or lower] If the heating temperature of the laminated slab, which is made by laminating the base material and the cladding material, is less than 1000°C, the carbide solid solution is insufficient and the required strength cannot 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, it is preferably set to 1020°C or higher. On the other hand, heating above 1250° C. increases energy consumption, so the heating temperature is set to 1250° C. or less. Furthermore, the heating temperature is preferably 1230° C. or less.

[0066] [Cumulative rolling reduction of laminated slabs: 60% or more] By rolling the laminated slabs to a cumulative reduction rate of 60% 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 rate of the laminated slabs is set to 60% or more. The cumulative reduction rate of the laminated slabs is preferably 65% ​​or more, and more preferably 70% or more. While there are no particular limitations on the upper limit of the cumulative reduction rate, it is preferably 95% or less from the viewpoint of rolling efficiency.

[0067] [Rolling finish temperature (FT): Ar3 transformation point or higher and 1000°C or lower] If the rolling end temperature of the hot rolling is lower than the Ar3 transformation point, ferrite will be generated, resulting in a deterioration in toughness. Furthermore, if the rolling end temperature is lower than the Ar3 transformation point, hot rolling will be performed at a low temperature, which will degrade the bondability of the clad steel from the perspective of diffusion bonding. Therefore, the rolling end temperature is set to the Ar3 transformation point or higher. Furthermore, the rolling end temperature is preferably set to the Ar3 transformation point + 20°C or higher. 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 less. Furthermore, the rolling end temperature is preferably set to 980°C or less.

[0068] (Step (A) above (accelerated cooling step after reheating)) In the above step (A), after cooling, the material is reheated to 800°C or higher and 1000°C or lower, and then accelerated cooling is performed from a temperature above the Ar3 transformation point of the base material at an average cooling rate of 1.0°C / s to 20.0°C / s to a cooling stop temperature of 350°C or lower. 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, the sheet may be cooled to room temperature (-5 to 50°C) before being reheated. The cooling rate at this time does not need to be particularly specified, but can be, for example, air cooling at a rate of 0.01 to 1°C / s.

[0069] [Reheating temperature: 800℃ or higher and 1000℃ 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. Also, 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, there is a risk of insufficient strength. Therefore, the reheating temperature is set to 1000°C or less. Furthermore, the reheating temperature is preferably set to 980°C or less.

[0070] [Cooling start temperature: above Ar3 transformation point] The reheated steel sheet is cooled from a temperature equal to or higher than the Ar3 transformation point of the base material. Here, a temperature equal to or higher than the Ar3 transformation point of the base material refers to the surface temperature of the base material. If the cooling start temperature is lower than the Ar3 transformation point of the base material, ferrite will be formed in excess and will coexist with martensite or bainite, which have large strength differences. This results in insufficient strength and a deterioration in toughness of the base material. 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. The Ar3 transformation point can be calculated using the following formula: Ar3(℃)=910-310×[C]-80×[Mn]-20×[Cu]-55×[Ni]-15×[Cr]-80×[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) when no element M is contained.

[0071] [Average cooling rate of steel plate: 1.0℃ / s or more and 20.0℃ / 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, and 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 will become large, and ferrite and pearlite will form, which may result in insufficient strength and a deterioration in toughness. Therefore, the average cooling rate should be 1.0°C / s or more. Preferably, it should be 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 becomes too large, which may result in a decrease in toughness. Furthermore, the bondability may also decrease. 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 1 / 2 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 between them 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.

[0072] [Cooling stop temperature: 350℃ or less] In the present invention, after reheating, cooling is performed under the above-mentioned conditions to a cooling stop temperature of 350°C or less, thereby achieving a predetermined volume fraction of bainite or martensite uniformly throughout the thickness center of the base material. However, if the cooling stop temperature exceeds 350°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 350°C or less. Preferably, the cooling stop temperature is 300°C or less. On the other hand, 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 and the like.

[0073] Furthermore, such clad steel sheet 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.

[0074] [Tempering temperature: 550℃ or higher and 700℃ or lower] In the present invention, tempering is performed to restore the toughness of the base material. If the average temperature of the steel sheet (the temperature at the 1 / 2 position of the steel sheet thickness) exceeds 700°C during reheating for tempering, dislocations may be restored, resulting in a decrease in the strength of the base material. Therefore, the tempering temperature is set to 700°C or less. Preferably, the tempering temperature is 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 set to 550°C or higher. The tempering temperature is preferably set to 600°C or higher.

[0075] (Step (B) above (accelerated cooling step)) In the above step (B), accelerated cooling is performed from a temperature equal to or higher than the Ar3 transformation point of the base material to a cooling stop temperature of 500°C or lower at an average cooling rate of 3°C / s to 50°C / s. Alternatively, after accelerated cooling, tempering is performed at a temperature of 700°C or lower. 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.

[0076] [Cooling start temperature: above Ar3 transformation point] If the cooling start temperature of the accelerated cooling is lower than the Ar3 transformation point, ferrite will be generated, resulting in a deterioration in toughness. Furthermore, if the cooling start temperature of the accelerated cooling is lower than the Ar3 transformation point, the bondability of the clad steel will be deteriorated from the viewpoint of diffusion bonding. Therefore, the cooling start temperature of the accelerated cooling is set to be equal to or higher than the Ar3 transformation point. The cooling start temperature of the accelerated cooling is preferably set to be equal to or higher than the Ar3 transformation point + 20°C. It is preferable that the cooling (accelerated cooling) here be carried out immediately after hot rolling, without air-cooling to room temperature or reheating the rolled sheet to 800°C or higher and 1000°C or lower.

[0077] [Average cooling rate of steel plate: 3℃ / s to 50℃ / s] Cooling at an average cooling rate of 3°C / s or more is an essential process for obtaining high-strength, high-toughness steel plates, and rapid cooling 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 deterioration in toughness. Therefore, the average cooling rate is set to 3°C / s or more. The average cooling rate is preferably 5°C / s or more, and more preferably 10°C / s or more. On the other hand, if the average cooling rate exceeds 50°C / s, the volume fraction of martensite becomes too large, which may result in a decrease in toughness. Furthermore, the bonding rate between the base material and the cladding 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 1 / 2 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 between them 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.

[0078] [Cooling stop temperature: 500℃ or less] In the present invention, by performing cooling under the above-mentioned conditions up to a cooling stop temperature of 500°C or less, it is possible to uniformly obtain 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 or pearlite structures may be formed, which may result in insufficient strength and deterioration of toughness. Therefore, the cooling stop temperature is specified to be 500°C or less. The cooling stop temperature is preferably 300°C or less. Note that, 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. On the other hand, the lower limit of the cooling stop temperature is not particularly limited and may be room temperature, but from the viewpoint of production efficiency and the like, the lower limit of the cooling stop temperature is preferably set to 150°C.

[0079] Furthermore, after the accelerated cooling in the treatment of step (B) above is performed on such clad steel plate 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.

[0080] [Tempering temperature: 700℃ or less] In the present invention, tempering can be performed as necessary to restore the toughness of the base material. If the average temperature of the steel sheet (the temperature at the 1 / 2 position of the steel sheet thickness) exceeds 700°C during reheating for tempering, dislocations may be restored, resulting in a decrease in the strength of the base material. Therefore, the tempering temperature is set to 700°C or less. Preferably, the tempering temperature is 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 550°C or higher, and more preferably 600°C or higher.

[0081] 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.

[0082] 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, stress corrosion cracking resistance, and excellent joinability.

[0083] In the production method according to the present invention, any item not described in this specification can be carried out in a conventional manner. [Example]

[0084] Table 1 shows the chemical composition of the base material (the balance being Fe and unavoidable impurities). In the table, steel types A to J and AA to AC are inventive examples that fall within the scope of the present invention. On the other hand, steel types K to Z are comparative examples in which one of the components is outside the scope of the present invention. Table 2 shows the chemical compositions of the cladding materials (the balance being Fe and unavoidable impurities). In the table, steel types a to i and t to v are inventive examples that fall within the scope of the present invention. On the other hand, steel types j to s are comparative examples in which one of the components is 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.

[0085] [Table 1]

[0086] [Table 2]

[0087] Clad steel plates (Nos. 1 to 84) 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 obtained steel plates, and welded joints were fabricated. The welding method was TIG welding, and multi-layer welding was performed from the base material side to the clad material side. The slabs were assembled using one of three methods: sandwich, open, or sacrificial material. In Tables 3-1, 3-2, and 3-3, the sandwich method involves stacking the slabs in the order of base material slab, cladding material slab, cladding material slab, and base material slab from top to bottom, the open method involves stacking the slabs in the order of cladding material slab, base material slab, and the sacrificial material method involves stacking the slabs in the order of sacrificial material, cladding material slab, and base material slab. The obtained clad steel plates were evaluated for strength and toughness, Vickers hardness, bonding rate, and ammonia SCC resistance in a liquid ammonia environment.

[0088] [Strength characteristics] The clad steel plate was thinned on the clad side, and a JIS Z 2201 No. 1B test piece was taken from the entire thickness of the base material. A tensile test was performed according to 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 were evaluated as having excellent tensile properties.

[0089] [Toughness] To evaluate the toughness of the base material of the clad steel plate, JIS Z 2202 V-notch test specimens were taken with the center at half the thickness of the base material of the clad steel plate. To evaluate the toughness of the welded joints of the clad steel plate, JIS Z 2202 V-notch test specimens were taken from the weld heat-affected zone of the base material of the clad steel plate in the welded joint, with the center at half the thickness of the base material of the clad steel plate. For each condition, three test specimens were subjected to Charpy impact tests at -50°C for the base material and -40°C for the heat-affected zone of the welded joint, as per JIS Z 2242 (2023), and the absorbed energy was measured. All three test specimens with absorbed energies of 47 J or greater were evaluated as having excellent toughness. The smallest absorbed energy of the three test specimens is listed in Tables 3-1, 3-2, and 3-3.

[0090] [Vickers hardness] Samples were taken so that the cross section perpendicular to the welding direction of the welded joint served as the measurement surface, and then mirror-polished. Next, in accordance with JIS Z 2244 (2020), the hardness of the cladding material's non-heat-affected zone and weld heat-affected zone was measured at 20 points each at positions 1.0 mm and 2.0 mm below the steel plate surface using a Vickers hardness tester. The maximum hardness values ​​of the non-heat-affected zone and weld heat-affected zone of the cladding material were determined and recorded as the maximum hardness values. For cladding materials with a plate thickness of less than 3.0 mm, the hardness of the non-heat-affected zone and weld heat-affected zone of the cladding material was measured at 20 points each at a position 1.0 mm below the steel plate surface using a Vickers hardness tester. The maximum hardness values ​​of the non-heat-affected zone and weld heat-affected zone of the cladding material were determined and recorded as the maximum hardness values.

[0091] [Joining rate] The bonding ratio at the bonding interface of the clad steel plate was determined as follows: A cross section of the clad steel plate including the base material and cladding material was mirror-polished, and a scanning electron microscope (SEM) was used to measure a length of 1.0 × 10 in the direction along the interface randomly selected on the cross section at a magnification of 1000. -1Ten fields of view were observed over an area of ​​1 mm. The length of the bonded interface was determined by image analysis. The bond ratio was then calculated using the following formula based on the measured total length and the bonded interface length. Bonding rate (%): 100 x bonding interface length (mm) / total measurement length (mm) The bonded interface length is the total length of the portion of the interface that was determined to be free of voids and oxides as a result of image analysis. Steel sheets with a bonding rate of 70% or more were evaluated as having excellent bondability.

[0092] [Ammonia SCC resistance] The ammonia SCC resistance in the present invention was evaluated by applying a potential to a test piece to which stress was applied by four-point bending in a test solution. Specifically, the following procedure was carried out. The clad steel plate according to the present invention is used so that the clad material comes into contact with ammonia and the like. Therefore, first, the clad steel plate was thinned from the base metal side to obtain test pieces measuring 1.5 to 3.0 mm thick x 15 mm x 115 mm from the clad material portion. When the clad material thickness exceeded 3.0 mm, a 3.0 mm thick test piece was obtained from the side of the clad material that was not joined to the base metal. The obtained test pieces were ultrasonically degreased 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 piece by four-point bending. The four-point bending test pieces were placed in a test cell. Next, the test cell was filled with a solution prepared by mixing 2 L of liquid ammonia with a purity of 99.999% or higher, 5.00 mass% ammonium carbamate, 1.000 bar O2, and 0.10 mass% water. Specifically, a predetermined amount of ammonium carbamate and water was placed in the test cell, and then O2 gas was blown in, followed by liquid ammonia. 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 placed in the test cell. The test solution temperature was set to 25°C. After adjusting the test solution temperature 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 the immersion test was initiated by controlling the potential to apply a potential of +0.5 V vs. Pt to the test specimen at that time. 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 cracking. In this example, a 504-hour immersion test was performed on nine test specimens under one condition. If cracks of 1.5 mm or more were found in two or fewer test pieces, the ammonia SCC resistance was judged to be good (○), and if cracks occurred in three or more test pieces, the ammonia SCC resistance was judged to be poor (×). The evaluation results obtained are shown in Tables 3-1, 3-2 and 3-3.

[0093] [Table 3-1]

[0094] [Table 3-2]

[0095] [Table 3-3]

[0096] As shown in Tables 3-1, 3-2, and 3-3, all of the invention examples Nos. 1 to 26 and Nos. 78 to 84 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, a maximum hardness of 210 HV or less, and a bonding rate of 70% or more between the base material and cladding material. It can also be seen that all of the invention examples have excellent ammonia SCC resistance.

[0097] In contrast, Nos. 27 to 52 have chemical compositions outside the scope of the present invention and are therefore inferior in at least one of tensile strength TS, low-temperature toughness, hardness, and ammonia SCC resistance. Further, although the chemical compositions of Nos. 53 to 77 are within the range of the present invention, the manufacturing conditions are outside the range of the present invention, and therefore, they are inferior in at least one of tensile strength TS, hardness, 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, The chemical composition of the base material is In mass%, C: 0.030-0.150%, Si: 0.05-0.55%, Mn: 0.50 to 2.10%, P: 0.020% or less, S: 0.010% or less, Al: 0.018-0.070%, Ni: 0.30-2.20%, Ti: 0.005 to 0.020%, N: 0.0020-0.0080%, O (oxygen): 0.0050% or less and Ti / N is 2.2 or more and 6.5 or less, and further 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, Ca: 0.0040% or less and the balance being Fe and unavoidable impurities, The chemical composition of the cladding material is, in mass%, C: 0.030-0.140%, Mn: 0.20-1.60%, P: 0.020% or less, S: 0.010% or less, Al: 0.018-0.070%, N: 0.0020-0.0050%, O (oxygen): 0.0050% or less and the balance being Fe and unavoidable impurities, 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 47J or more, The maximum hardness of the cladding material is 210 HV or less, A clad steel plate having a bonding rate of 70% or more between the base material and the clad material.

2. The chemical composition of the cladding material is further expressed in mass% as follows: Si: 0.55% or less, Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.10% or less, Mo: 0.10% or less, Ti: 0.020% or less, Nb: 0.030% or less, V: 0.050% or less, Ca: 0.0040% or less The clad steel plate according to claim 1, comprising one or more of the following:

3. A welded joint using the clad steel plate according to claim 1 or 2, The absorbed energy of the weld heat affected zone in a Charpy impact test at -40°C is 47J or more, A welded joint in which the maximum hardness of the weld heat affected zone of the cladding material is 210 HV or less.

4. The method for manufacturing a clad steel plate according to claim 1 or 2, The 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. After heating the cladding material slab, it is hot rolled to make cladding material. The laminated slab obtained by laminating the base material and the cladding material is heated to a surface temperature of 1000°C or more and 1250°C or less, The cumulative reduction rate is 60% or more, and the rolling end temperature is Ar ３ hot rolling is performed at a temperature of not less than the transformation point and not more than 1000°C to produce a rolled plate having a base material and a clad material; A method for producing a clad steel plate, which comprises subjecting the rolled plate to the following treatment (A) or (B): (A) After cooling the rolled plate after hot rolling, Reheating to 800°C or higher and 1000°C or lower, Base material Ar ３ Accelerated cooling is performed from a temperature equal to or higher than the transformation point to a cooling stop temperature of 350°C or lower at an average cooling rate of 1.0°C / s or higher and 20.0°C / s or lower, After the accelerated cooling, tempering is carried out at a temperature of 550°C or higher and 700°C or lower. (B) The rolled sheet after hot rolling is 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.

5. A method for manufacturing a welded joint, comprising welding the clad steel plate according to claim 1 or 2 under conditions of a heat input of 50 kJ / cm or less to manufacture a welded joint.

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