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. This is a clad steel plate having a base material with a specified chemical composition and a clad material with a specified chemical composition on at least one surface (the inner surface), in which the tensile strength of the base material is 620 MPa or more, the absorbed energy of the base material in a Charpy impact test at -50°C is 47 J or more, 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 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 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 inside 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 the 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. The corrosion products on the surface of the test specimen were removed, and the surface and cross section were visually inspected for cracks to evaluate them. In the present invention, a 504-hour immersion test is conducted on nine test specimens under one set of conditions. If two or fewer test specimens show cracks 1.5 mm or more in depth, the ammonia SCC resistance is judged to be good (◯), i.e., excellent; if cracks occur in three or more test specimens, the ammonia SCC resistance is judged to be poor (×). Furthermore, excellent low-temperature toughness means that the base material has an absorbed energy of 47 J or more when subjected to a Charpy impact test at -50°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 620 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 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 cladding material on at least one side of a base material, The chemical composition of the base material is In mass%, C: 0.030~0.170%, Si: 0.05 to 0.40% Mn: 0.05 to 1.60%, P: 0.020% or less, S: 0.010% or less, Al: 0.018~0.070%, N: 0.0020~0.0080%, O (oxygen): 0.0050% or less and further comprising Cu: 0.50% or less, Ni: 2.10% or less, Cr: 0.70% or less, Mo: 0.60% or less Nb: 0.040% or less, Ti: 0.020% or less, V: 0.100% or less, B: 0.0050% or less, Ca:0.0040% or less and Pcm represented by formula (1) is 0.16 or more and 0.28 or less, with the remainder being Fe and unavoidable impurities, The cladding material is The PRE value shown in formula (2) is 15 or more, The tensile strength of the base material is 620 MPa or more, The absorbed energy of the base material in a Charpy impact test at -50°C is 47J or more, Clad steel plate with a bonding rate of 70% or more between the base material and cladding material. Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B...(1) PRE=Cr%+3.3Mo%+30N%-Mn%...(2) In formula (1), the element symbol indicates the content (mass%) of the element in the base material, In formula (2), M% represents the content (mass%) of element M contained in the cladding material, and in each formula, when no element is contained, it is set to 0 (zero).

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

[0015] [3] The method for producing a clad steel plate according to [1], The base material slab is heated to a surface temperature of 900°C or higher and 1200°C or lower, and then hot-rolled to a rolling end temperature of the surface temperature equal to or higher than the Ar3 transformation point 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, hot rolling is performed at a cumulative reduction rate of 65% 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 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. (B) After cooling the rolled plate after hot rolling, Reheat to between 800°C and 1000°C. After accelerated cooling 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, Tempering is carried out at a temperature of 600°C or higher and 750°C or lower.

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

[0017] 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

[0018] The present invention provides a clad steel plate having a base material and a cladding material that is 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 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. In addition, the method is not limited to assembled slabs (laminated slabs) produced by stacking base material slabs (material for base steel plate) and cladding material slabs (material for 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 give the cladding material its chemical composition.

[0019] 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 components of the base material contain, in mass%, C: 0.030 to 0.170%, Si: 0.05 to 0.40%, Mn: 0.05 to 1.60%, P: 0.020% or less, S: 0.010% or less, Al: 0.018 to 0.070%, N: 0.0020 to 0.0080%, and O (oxygen): 0.0050% or less, and further containing one or more of Cu: 0.50% or less, Ni: 2.10% or less, Cr: 0.70% or less, Mo: 0.60% or less, Nb: 0.040% or less, Ti: 0.020% or less, V: 0.100% or less, B: 0.0050% or less, and Ca: 0.0040% or less, and having a Pcm value of 0.16 or more and 0.28 or less as shown in formula (1), with the balance being Fe and unavoidable impurities; The cladding material has a PRE value shown in formula (2) of 15 or more, The tensile strength of the base material is 620 MPa or more, the absorbed energy of the base material in a Charpy impact test at -50°C is 47 J or more, and the bonding rate between the base material and cladding material is 70% or more. Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B...(1) PRE=Cr%+3.3Mo%+30N%-Mn%...(2) In formula (1), the element symbol indicates the content (mass%) of the element in the base material, In formula (2), M% represents the content (mass%) of element M contained in the cladding material, and in each formula, when no element is contained, it is set to 0 (zero). In addition, "%" representing the content of the following component elements means "% by mass" unless otherwise specified.

[0020] (1) Chemical composition of the base material C: 0.030 to 0.170% 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, and more preferably 0.050% or more. On the other hand, if the C content exceeds 0.170%, the toughness and weldability of the steel plate will deteriorate. Therefore, the C content is specified to be 0.170% or less. Furthermore, from the viewpoint of toughness, it is preferably 0.160% or less, and more preferably 0.150% or less.

[0021] Si: 0.05 to 0.40% Si is added to improve the strength of steel sheet and also for deoxidation. To achieve this effect, the Si content is specified to be 0.05% or more. It is furthermore preferably 0.07% or more, and more preferably 0.10% or more. On the other hand, if the Si content exceeds 0.40%, it will result in deterioration of toughness and weldability. Therefore, the Si content is specified to be 0.40% or less, and more preferably 0.35% or less.

[0022] Mn: 0.05 to 1.60% 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 0.05% or more. Furthermore, from the perspective of reducing the contents of other alloying elements and achieving lower manufacturing costs, the Mn content is preferably 0.20% or more. On the other hand, if the Mn content exceeds 1.60%, it will cause deterioration of weldability. Therefore, the Mn content is specified to be 1.60% or less, and more preferably 1.55% 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 P 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 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, and more preferably 0.003% or more.

[0024] 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. It has adverse effects, such as acting as a fracture initiation point and reducing the toughness of the steel sheet. 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 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.

[0025] 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. Furthermore, the Al content is preferably set to 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, it is preferably 0.060% or less, and more preferably 0.050% or more.

[0026] N: 0.0020~0.0080% Nitrogen (N) can improve the toughness of welds by forming TiN. To achieve this effect, the N content must be 0.0020% or more. The N content is preferably 0.0025% or more. On the other hand, if the N content exceeds 0.0080%, 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 0.0070% or less, more preferably 0.0060% or less, and even more preferably 0.0050% or less.

[0027] O (oxygen): 0.0050% or less Oxygen (O) is an element contained as an unavoidable impurity and exists in steel as oxides such as Al2O3. It has adverse effects, such as being the starting point 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. 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, industrially it may be more than 0%. Moreover, since excessive reduction leads to an increase in refining costs, from the viewpoint of cost, the O content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0028] 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.

[0029] Cu: 0.50% or less Cu is an element effective in improving the strength of steel sheet. When Cu is contained, from the viewpoint of the strength of the steel sheet, 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. If the Cu content exceeds 0.50%, scratches will occur on the surface of the steel sheet. Furthermore, if the Cu content exceeds 0.50%, it will cause a deterioration in toughness. Therefore, if Cu is contained, the Cu content should be 0.50% or less. The Cu content is preferably 0.45% or less, and more preferably 0.40% or less.

[0030] Ni: 2.10% or less Ni is not only effective in improving the strength of steel plate, but also has the effect of improving the toughness of the base material and the weld heat affected zone. From the viewpoint of the strength and toughness of the steel plate, when Ni is contained, the Ni content is preferably 0.05% or more. The Ni content is more preferably 0.10% or more. If the Ni content exceeds 2.10%, scratches will occur on the surface of the steel sheet. Therefore, when Ni is contained, the Ni content is set to 2.10% or less. The Ni content is preferably set to 2.00% or less, more preferably set to 1.80% or less, and even more preferably set to 1.50% or less.

[0031] Cr:0.70% or less Cr is an element effective in improving the strength of steel sheet. When Cr is contained, from the viewpoint of the strength of the steel sheet, the Cr content is preferably 0.05% or more. Furthermore, the Cr content is more preferably 0.10% or more. If the Cr content exceeds 0.70%, the toughness of the steel plate deteriorates. Therefore, if Cr is contained, the Cr content is set to 0.70% or less, and preferably to 0.60% or less.

[0032] Mo: 0.60% or less Mo is an element effective in improving the strength of steel sheet. When Mo is contained, from the viewpoint of the strength of the steel sheet, the Mo content is preferably 0.05% or more, and more preferably 0.10% or more. 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.

[0033] Nb: 0.040% or less Nb is an element that has the effect of reducing the prior austenite grain size and improving toughness by precipitating as carbonitrides. When Nb is added to obtain such an effect, the Nb content is preferably 0.005% or more, and more preferably 0.007% or more. If the Nb content exceeds 0.040%, a large amount of NbC precipitates, resulting in a decrease in toughness. Therefore, when Nb is contained, the Nb content is set to 0.040% or less. The Nb content is preferably set to 0.035% or less. The Nb content is more preferably set to 0.030% or less, and even more preferably set to 0.025% or less.

[0034] Ti: 0.020% or less 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, the inclusion of Ti can improve the toughness of welds. To achieve this effect, the Ti content is preferably 0.005% or more. The Ti content is more preferably 0.008% or more. If the Ti content exceeds 0.020%, the toughness will actually decrease. Therefore, if Ti is contained, the Ti content should be 0.020% or less, and preferably 0.017% or less.

[0035] V:0.100% or less V is an element effective in improving the strength of a steel sheet. When V is contained, from the viewpoint of the strength of the steel sheet, the V content is preferably 0.005% or more. The V content is more preferably 0.030% or more. 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 0.080% or less, and more preferably set to 0.050% or less.

[0036] B: 0.0050% or less B is an element effective in improving the strength of steel sheet. From the viewpoint of the strength of steel sheet, the B content is preferably 0.0005% or more, and more preferably 0.0008% or more. 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. The B content is preferably set to 0.0040% or less, and more preferably set to 0.0030% or more.

[0037] Ca:0.0040% or less Ca is an element that bonds with S and inhibits the formation of MnS and other compounds that elongate in the rolling direction. That is, by adding Ca, the morphology of sulfide-based inclusions is controlled to be spherical, thereby improving the toughness of welds and other structures. To achieve this effect, when Ca is added, the Ca content is preferably 0.0005% or more. The Ca content is more preferably 0.0010% or more. 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.

[0038] Pcm: 0.16 or more and 0.28 or less Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B...(1) In formula (1), the element symbol indicates the content (mass %) of the element contained in the base material, and is set to 0 (zero) when the element is not contained. Pcm is an index for evaluating the limit of occurrence of weld cold cracking. If Pcm exceeds 0.28, the toughness and weldability of the base material will deteriorate significantly. Therefore, the upper limit of Pcm is set to 0.28. Pcm is preferably 0.26 or less. On the other hand, if Pcm is less than 0.16, the desired strength cannot be obtained. Therefore, the lower limit of Pcm is set to 0.16. Pcm is preferably 0.18 or more.

[0039] (2) About cladding materials PRE value: 15 or higher PRE=Cr%+3.3Mo%+30N%-Mn%...(2) In formula (2), M% represents the content (mass%) of element M contained in the cladding material, and is set to 0 (zero) when no element M is contained. 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 cladding materials is specified to be 15 or more. On the other hand, the higher the PRE value of the cladding material, the better, but an excessive PRE value will lead 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 cladding material and the weld heat-affected zone, 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 composition of the cladding material may further contain 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, as necessary. 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 sheet produced by the cooling method 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 contents of other alloy elements and producing the steel sheet 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 precipitate at the grain boundaries, which may cause stress corrosion cracking when the steel 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 cause deterioration in 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 contents of other alloying elements and achieving lower production costs, the Mn content is more preferably 0.40% or more. On the other hand, if the Mn content exceeds 4.00%, it may lead to deterioration of weldability. Therefore, the Mn content is preferably set to 4.00% or less. Furthermore, the Mn content is more preferably set to 3.00% or less.

[0043] P:0.045% or less P is an element contained as an unavoidable impurity, and its segregation at grain boundaries can have 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 0.045% 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 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.

[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 sheet. 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 greater than 0% industrially. 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 steels and Ni-based alloys against various acids. It is also effective in improving not only the strength of steel plates but also the toughness of the base material and the weld heat-affected zone. However, if the Ni content is less than 0.05%, this effect is poor. 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 is used as the cladding material instead of a Ni-based alloy, the Ni content may be 25.0% or less, 20.0% or less, 13.0% or less, or 10.0% or less.

[0046] Cr: 11.0~28.0% Cr is an element that is 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 welded joint 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 that is 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 be reduced. 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 exists in steel as oxides such as Al2O3. It is an element that 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 greater than 0% industrially. 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.

[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, 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 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, when Cu is contained, 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%, the hot workability may be significantly reduced. Therefore, if 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 element effective in improving the pitting corrosion resistance of stainless steel. However, if the content is less than 0.05%, this effect is poor. Therefore, if 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, if 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 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, if the V content is less than 0.005%, this effect is poor. Therefore, if 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, and 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, this effect is poor at less than 0.005%. 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, and 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 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, the Ti content is preferably 1.00% or less, and more preferably 0.90% or less.

[0055] B: 0.0050% or less B is an element effective in improving the strength of steel sheets. 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, and 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, which 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, when Ca is included, 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, and 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 if Ta is contained, it is preferable that the Ta content be 0.02% or more. On the other hand, if Ta is excessive, Ta forms an intermetallic compound with a low melting point, which reduces hot workability. Therefore, if Ta is contained, it is preferable that the Ta content be 3.00% or less.

[0060] (3) Characteristics of clad steel plates [Tensile strength] The clad steel plate of the present invention has a base metal tensile strength of 620 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 may be less than 780 MPa.

[0061] [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 -50°C of the base material, and an absorbed energy of 47 J or more in a Charpy impact test at -40°C of the weld heat affected zone of a welded joint using the clad steel plate of the present invention. 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 the absorbed energy of the base material is preferably 100 J or more, more preferably 170 J or more.

[0062] [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%.

[0063] [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.

[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 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.

[0067] (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.

[0068] [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 40 kJ / cm or less. The heat input is more preferably 30 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.

[0069] (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 to 1200°C and then hot rolled. The rolling end temperature of the hot rolling is set to a surface temperature equal to or higher than the Ar3 transformation point, and a base material of predetermined dimensions is obtained. In addition, the obtained clad material slab is heated and then hot rolled to obtain a clad material of predetermined dimensions.

[0070] [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 heating temperature exceeds 1200°C, the crystal grains of the base material will become coarse, resulting in a deterioration in toughness. 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.

[0071] [Finishing temperature (FT): Ar3 transformation point or higher] If the rolling end temperature of the hot rolling is lower than the Ar3 transformation point of the base material, the generated ferrite will be affected by the processing, resulting in a deterioration in toughness. Therefore, the rolling end temperature is set to a surface temperature equal to or higher than the Ar3 transformation point. The rolling end temperature is preferably set to be equal to or higher than the Ar3 transformation point + 50°C. Although there is no particular upper limit, the rolling end temperature is preferably 980°C or less in terms of surface temperature.

[0072] Next, a clad material is layered on the side 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 if necessary, 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 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 has a cumulative reduction of 65% or more and a rolling end temperature of the Ar3 transformation point or more and 1000°C or less. After this hot rolling is performed to obtain a rolled plate having the base material and clad material, the following treatment (A) or (B) is performed. (A) 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. (B) After cooling the rolled plate after hot rolling, Reheat to between 800°C and 1000°C. After accelerated cooling 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, Tempering is carried out at a temperature of 600°C or higher and 750°C or lower.

[0073] [Heating temperature: 1000℃ or higher and 1250℃ or lower] If the heating temperature of the laminated slab, which is formed by laminating the base material and the cladding material, is less than 1000°C, the carbide may not be dissolved sufficiently and the required strength may 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. The heating temperature is preferably 1020°C or higher. On the other hand, if the laminated slab is heated to a temperature exceeding 1250°C, the crystal grains of the base material will become coarse, resulting in a deterioration in toughness. Therefore, the heating temperature is set to 1250°C or less. The heating temperature is preferably 1230°C or less.

[0074] [Cumulative reduction rate of laminated slab: 65% or more] By rolling the laminated slabs to a cumulative reduction rate 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 rate of the laminated slabs is set to 65% or more. It is preferable that the cumulative reduction rate of the laminated slabs be 70% or more. While there is no particular upper limit to the cumulative reduction rate, it is preferable that the cumulative reduction rate of the laminated slabs be 95% or less from the viewpoint of rolling efficiency.

[0075] [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, the generated ferrite will be significantly affected by the processing, which may result in a deterioration in toughness. Also, if the rolling end temperature is lower than the Ar3 transformation point, the hot rolling will be performed at a low temperature, which will deteriorate 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. The rolling end temperature is preferably set to the Ar3 transformation point + 30°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. The rolling end temperature is preferably set to 980°C or less.

[0076] (Step (A) above (accelerated cooling step)) In the above step (A), accelerated cooling is performed from a temperature above the Ar3 transformation point of the base material to a cooling stop temperature of 500°C or below 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.

[0077] [Cooling start temperature: above Ar3 transformation point] The steel sheet after the hot rolling is cooled from a temperature 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 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 hot rolling is set to be equal to or higher than the Ar3 transformation point. The cooling start temperature is preferably set to be at least 20°C above the Ar3 transformation point. 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. 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.

[0078] [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 1 / 2 position in the thickness direction of the base material 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 1 / 2 position. The cooling time is the time during which cooling water is supplied to the steel plate, and is the time required for the 1 / 2 position to change from the cooling start temperature to the cooling stop temperature.

[0079] [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 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. Furthermore, 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 100°C.

[0080] Furthermore, after the accelerated cooling in the treatment of step (A) above is performed on such clad steel sheet to the cooling stop temperature, the clad steel sheet 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 (A), tempering does not have to be performed, but tempering may be performed at a temperature of 700°C or less after the accelerated cooling.

[0081] [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. The tempering temperature is preferably set to 680°C or less. Furthermore, the tempering temperature is preferably set to 550°C or more. The tempering temperature is more preferably set to 600°C or more.

[0082] (Step (B) above (accelerated cooling after reheating)) Instead of the above step (A), in step (B), after cooling, the material is reheated to 800°C or higher and 1000°C or lower, and 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, the temperature equal to or higher than the Ar3 transformation point of the base material refers to the surface temperature 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. 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.

[0083] [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.

[0084] [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, "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 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 taken as 0 (zero) when not contained.

[0085] [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 fast 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, the average cooling rate 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).

[0086] [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. 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 set to 350°C or less. The cooling stop temperature is preferably set to 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 set to 100°C from the viewpoint of production efficiency, etc.

[0087] Furthermore, such clad steel sheet is subjected to accelerated cooling in the treatment of the above step (B) up to the cooling stop temperature, and then further tempered at a temperature of 600°C or higher and 750°C or lower.

[0088] [Tempering temperature: 600℃ or higher and 750℃ or lower] In the present invention, tempering is carried out for the purpose of restoring 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 750°C during reheating for tempering, dislocations may recover, resulting in a decrease in the strength of the base material. Therefore, the tempering temperature is set to 750°C or less. The tempering temperature is preferably set to 730°C or less. On the other hand, if the average temperature of the steel sheet during reheating for tempering is less than 600°C, the toughness of the base material may be insufficient. Therefore, the tempering temperature is set to 600°C or higher. The tempering temperature is preferably set to 620°C or higher.

[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 produced 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. [Example]

[0092] Table 1 shows the chemical compositions of the base metal (the balance being Fe and unavoidable impurities). In the table, steel types A to T are inventive examples that fall within the scope of the present invention. On the other hand, steel types U to AK are comparative examples in which one or more of the components falls 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 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 whose 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] [Table 1]

[0094] [Table 2]

[0095] Clad steel plates (Nos. 1 to 98) were manufactured under the manufacturing conditions shown in Table 3 using a base material slab having the chemical composition shown in Table 1 and a clad material slab having the chemical composition 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 with the heat input shown in Table 3, and multi-layer welding was performed from the base metal side to the cladding side. The slabs were assembled using one of three methods: sandwich, open, or sacrificial material. In Tables 3-1 and 3-2, 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 characteristics and toughness, bonding rate, and ammonia SCC resistance in a liquid ammonia environment. Each test method was as follows.

[0096] [Strength characteristics] The clad steel plate was thinned on the cladding 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 620 MPa or more 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 at half the plate thickness of the base material of the clad steel plate. To evaluate the toughness of the welded joints of the clad steel plate, V-notch test specimens according to JIS Z 2202 were taken from the heat-affected zone of the welded joint of the clad steel plate, with the center at half the plate 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 an absorbed energy 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 and 3-2.

[0098] [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. -1 Ten 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 area where no voids or oxides were found to exist on the interface as a result of image analysis. Steel sheets with a bonding rate 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 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. 2 The 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 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 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 are shown in Table 3.

[0100] [Table 3-1]

[0101] [Table 3-2]

[0102] As can be seen from Table 3, all of the inventive examples have a tensile strength of 620 MPa or more, an absorbed energy of 47 J or more in a Charpy impact test of the base material at -50°C, and a bonding rate of 70% or more between the base material and cladding material. It can also be seen that all of the inventive examples have excellent ammonia SCC resistance.

[0103] In contrast, as can be seen from Table 3-2, Nos. 52 to 70 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, and ammonia SCC resistance. Additionally, Nos. 71 to 98 have chemical compositions within the scope of the present invention, but are produced under manufacturing conditions outside the scope of the present invention and are therefore inferior in at least one of tensile strength TS, low-temperature toughness, bondability, 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: 0.030-0.170%, Si: 0.05-0.40%, Mn: 0.05-1.60%, P: 0.020% or less, S: 0.010% or less, Al: 0.018-0.070%, N: 0.0020-0.0080%, O (oxygen): 0.0050% or less and further comprising Cu: 0.50% or less, Ni: 2.10% or less, Cr: 0.70% or less, Mo: 0.60% or less, Nb: 0.040% or less, Ti: 0.020% or less, V: 0.100% or less, B: 0.0050% or less, Ca: 0.0040% or less and Pcm represented by formula (1) is 0.16 or more and 0.28 or less, with the remainder being Fe and unavoidable impurities, The chemical composition of the cladding material is In mass%, C: 0.001-0.080%, Si: 0.05-1.00%, Mn: 0.30-4.00%, P: 0.045% or less, S: 0.030% or less, Ni: 0.05-80.0%, Cr: 11.0-28.0%, N: 0.005-0.400%, O (oxygen): Contains 0.0050% or less, Or even more so, 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, Ta: contains 3.00% or less, The PRE value shown in formula (2) is 15 or more, the balance being Fe and unavoidable impurities; The tensile strength of the base material is 620 MPa or more, The absorbed energy of the base material in a Charpy impact test at -50 ° C is 47 J or more, A clad steel plate having a bonding rate of 70% or more between the base material and the clad material. Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B...(1) PRE=Cr%+3.3Mo%+30N%-Mn%...(2) In formula (1), the element symbols indicate the contents (mass%) of the elements contained in the base material, In formula (2), M % represents the content (mass %) of element M contained in the cladding material, and in each formula, when no element is contained, it is set to 0 (zero).

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

3. The method for manufacturing a clad steel plate according to claim 1, The base material slab is heated to a surface temperature of 900°C or more and 1200°C or less, and then rolled to an end temperature of 1200°C. ３ The base material is obtained by hot rolling above the transformation point. 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 65% 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) 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. (B) After cooling the rolled plate after hot rolling, Reheating to 800°C or higher and 1000°C or lower, Base material Ar ３ After performing accelerated cooling 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 or more and 20.0°C / s or less, Tempering is carried out at a temperature of 600°C or higher and 750°C or lower.

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

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