Cladded steel sheet and production method for same

Abstract

Provided is a high-strength cladded steel sheet that has excellent ammonia SCC resistance and low-temperature durability and is suited to use in a liquid ammonia transport and storage tank or the like and a production method for the cladded steel sheet.　A cladded steel sheet according to the present invention comprises a parent material that has a prescribed component composition and a cladding material that has a prescribed component composition and is bonded to at least one surface (the surface that is to be an inside surface), the tensile strength of the cladded steel sheet being at least 780 MPa, and the energy absorbed by the parent material and a weld-heat-affected zone under Charpy impact testing at -40°C being at least 47 J.

Description

Clad steel sheet and method for manufacturing the same

[0001] This invention relates to a high-strength clad steel sheet exhibiting excellent low-temperature toughness and stress corrosion cracking resistance. In particular, it relates to a high-strength clad steel sheet suitable for structural components such as tanks used in liquid ammonia environments. Furthermore, it relates to a method for manufacturing such a high-strength clad steel sheet.

[0002] In a liquid ammonia environment, carbon steel is susceptible to stress corrosion cracking (hereinafter referred to as ammonia SCC) caused by liquid ammonia. Therefore, for structures such as carbon steel piping, storage tanks, tank cars, and line pipes that handle liquid ammonia, measures have been taken to use steel materials with low susceptibility to ammonia SCC and to implement operational measures 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. Furthermore, ammonia SCC is known to occur in the heat-affected zone of the weld. Therefore, when using high-strength steel in a liquid ammonia environment, measures such as adjusting the tensile strength and hardness of the weld by performing post-weld heat treatment are necessary. One example of post-weld heat treatment is annealing the entire structure.

[0004] In recent years, even when liquid ammonia burns, it produces CO2. ２ Because it does not produce waste, liquid ammonia is attracting attention as a clean energy source, and large-scale demand is expected. Consequently, there is a need for larger facilities for transporting and storing liquid ammonia. Generally, when enlarging tanks, the goal is to reduce weight and construction costs by using thinner steel, thus making the use of high-strength steel desirable.

[0005] Furthermore, in order to efficiently operate transportation and storage facilities, these facilities are sometimes used for both liquid ammonia and LPG. Since liquefied gases such as liquid ammonia and LPG are transported and stored at low temperatures, steel plates used for these applications require excellent low-temperature toughness.

[0006] Patent documents 1 to 3 disclose methods for achieving both high strength and excellent ammonia SCC resistance. Of these, Patent Document 1 describes a method for softening the surface of steel material. Patent Documents 2 and 3 describe methods for manufacturing clad steel sheets having low-carbon steel on one side.

[0007] Japanese Patent Publication No. 55-30062 Publication No. 139493 / 1983 Japanese Patent Publication No. 8-269537

[0008] However, the method described in Patent Document 1 above requires prolonged heat treatment to homogenize and sufficiently soften the surface layer, making it difficult to control the strength of the steel sheet's core. Therefore, there are problems in terms of the steel sheet's strength.

[0009] Furthermore, the methods described in the above-mentioned Patent Documents 2 and 3 manufacture cladding using casting, build-up welding, and continuous casting methods. However, these methods incur high costs for equipment and energy, so there has been a need to establish other new technologies.

[0010] The present invention aims to solve the above problems and provide a clad steel sheet and a method for manufacturing the same, which is suitable for use in tanks for transporting and storing liquid ammonia, and which has excellent ammonia SCC resistance and low-temperature toughness, as well as high strength.

[0011] Here, "high strength" refers to a tensile strength of 780 MPa or more for the base material (base steel sheet) of the clad steel sheet obtained in accordance with the procedure described in JIS Z 2241 (2022). "Excellent ammonia SCC resistance" refers to a test specimen measuring 5 mm thick x 15 mm x 115 mm including the cladding surface taken from the clad steel sheet, ultrasonically degreasing in acetone for 5 minutes, applying a stress of 100% YS of the yield strength of the specimen by four-point bending, filling a test cell in which the specimen is placed with a solution of 12.5 g of ammonium carbamate and 1 L of liquid ammonia, immersing it at room temperature, and showing no cracks after 720 hours. "Excellent low-temperature toughness" refers to an absorbed energy of 47 J or more in the base material and the weld heat-affected zone, measured by a Charpy impact test at -40°C in accordance with the procedure of JIS Z 2242 (2023). In this invention, the absorbed energy of the base material refers to the absorbed energy measured by performing a Charpy impact test on impact test specimens taken from the base material steel plate in the steady state portion of the clad steel plate, excluding the weld heat-affected zone. In this invention, the absorbed energy of the weld heat-affected zone refers to the absorbed energy measured by performing a Charpy impact test on impact test specimens taken from the base material (base material steel plate) in the weld heat-affected zone of the clad steel plate.

[0012] To achieve the above objective, the inventors diligently investigated various factors affecting the ammonia SCC resistance, low-temperature toughness, and strength characteristics of steel sheets. As a result, they obtained the following findings. Since ammonia SCC occurs in the part that comes into contact with liquid ammonia, i.e., the inside of the product (tank), the ammonia SCC resistance is predominantly determined by the properties of the surface layer of the steel sheet that is on the inside of the product (tank). Therefore, they conceived of using a clad steel sheet in which a steel sheet with excellent strength and low-temperature toughness is used as the base material, and furthermore, a steel sheet with excellent ammonia SCC resistance is joined to the base material as a cladding material to improve ammonia SCC resistance. They found that by using such a clad steel sheet, excellent values ​​can be obtained for ammonia SCC resistance, low-temperature toughness, and strength characteristics.

[0013] Furthermore, because the cladding material used in this invention is stainless steel, it offers superior corrosion resistance compared to clad steel sheets that use carbon steel as the cladding material.

[0014] This invention is based on the above findings, and the gist of this invention is as follows: 1. A clad steel sheet in which a cladding material is joined to at least one surface of a base material, wherein 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: 0.60 to 2.20%, Ti: 0.005 to 0.020%, N: 0.0020 to 0.0080%, O: 0.0050% 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. The composite material contains at least one selected from B: 0.0050% or less, Ca: 0.0040% or less, with the remainder being Fe and unavoidable impurities, and the chemical composition of the composite material is, in mass%, C: 0.001 to 0.080%, Si: 0.05 to 1.00%, Mn: 0.30 to 4.00%, P: 0.040% or less, S: 0.020% or less, Cr: 11.0 to 28.0%, and further optionally contains one or more selected from Ni: 30.0% or less, Cu: 3.00% or less, Mo: 7.00% or less, V: 0.100% or less, N: 0.300% or less, O: 0.0050% or less. The PRE value shown in formula (1) is 15 or higher, the remainder consists of Fe and unavoidable impurities, and the number of sulfide-based inclusions on the surface of the composite material is 2.5 × 10 ５ pieces / mm ２The following clad steel sheet has a tensile strength of 780 MPa or more, and the absorbed energy of the Charpy impact test at -40°C in the base material and weld heat-affected zone is 47 J or more: PRE = Cr% + 3.3 × Mo% + 30 × N% - Mn% ... (1) In equation (1), Cr%, Mo%, N%, and Mn% are, respectively, the Cr content (mass%), Mo content (mass%), N content (mass%), and Mn content (mass%) in the clad material.

[0015] 2. A method for manufacturing a clad steel sheet, comprising: heating a base material slab having the component composition described in 1 above to a surface temperature of 900°C to 1200°C; hot rolling the base material slab to a rolling completion temperature of 700°C or higher at a surface temperature to obtain a base steel sheet material; heating a cladding material slab having the component composition described in 1 above and manufactured by continuous casting with an average residence time in the tundish of 10 minutes or more, and then hot rolling it to obtain a cladding material; heating a clad slab formed by laminating the base steel sheet material and the cladding material to a surface temperature of 1000°C to 1250°C; hot rolling the clad slab to a cumulative reduction ratio of 65% or higher and a rolling completion temperature of 700°C to 1000°C to produce a rolled sheet having a base material and a cladding material; and subjecting the rolled sheet to the following treatment (A) or (B). (A) The rolled sheet after hot rolling is air-cooled to room temperature, and then the rolled sheet is reheated to 800°C to 1000°C, Cooling start temperature: Ar ３ (B) The rolled sheet after hot rolling is subjected to accelerated cooling under the following conditions: above the transformation point, average cooling rate: 1.0 to 20°C / s, cooling stop temperature: 350°C or lower, and then tempered at a temperature of 550°C to 700°C. (B) The rolled sheet after hot rolling is subjected to accelerated cooling under the following conditions: cooling start temperature: 650°C to 1000°C, average cooling rate: 5 to 50°C / s, cooling stop temperature: 300°C or lower, and then tempered at a temperature of 550°C to 700°C.

[0016] According to the present invention, it is possible to provide a high-strength clad steel sheet that is excellent in ammonia SCC resistance and low-temperature toughness, and is suitable for structural members such as tanks used in liquid ammonia environments.

[0017] The present invention relates to a clad steel sheet having a corrosion-resistant alloy cladding material on at least one side of the base material. Here, the clad steel sheet according to the present invention has excellent ammonia SCC resistance and low-temperature toughness, making it suitable for structural members such as tanks for storing liquid ammonia. The clad steel sheet according to the present invention is not limited to liquid ammonia, but also to LPG and liquefied CO2. ２ In addition to the above, it may also be applied to structural components such as tanks for storing liquefied gas.

[0018] The clad steel sheet of the present invention is a clad steel sheet in which a cladding material is joined to at least one surface of a base material, wherein 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: 0.60 to 2.20%, Ti: 0.0 It contains 0.5-0.020%, N: 0.0020-0.0080%, O: 0.0050% or less, and further contains at least one selected from 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, with the remainder being Fe and unavoidable impurities. The chemical composition of the composite material is as follows, by mass%, C: 0.001-0.080%, Si: 0.05-1.00%, Mn: 0.30-4.00%, P: 0.040% or less, S: 0.020% or less, Cr: 11.0-28.0%, and further optionally contains one or more elements selected from Ni: 30.0% or less, Cu: 3.00% or less, Mo: 7.00% or less, V: 0.100% or less, N: 0.300% or less, O: 0.0050% or less, the PRE value shown in formula (1) is 15 or more, the remainder consists of Fe and unavoidable impurities, and the number of sulfide-based inclusions on the surface of the composite material is 2.5 × 10⁻⁶. ５ pieces / mm ２The following conditions apply: the tensile strength is 780 MPa or higher, and the absorbed energy of the Charpy impact test at -40°C in the base material and the heat-affected zone of the weld is 47 J or higher. PRE = Cr% + 3.3 × Mo% + 30 × N% - Mn% ... (1) In equation (1), Cr%, Mo%, N%, and Mn% are, respectively, the Cr content (mass%), Mo content (mass%), N content (mass%), and Mn content (mass%) in the composite material.

[0019] Embodiments of the present invention will be described below. Unless otherwise specified, the "%" used to indicate the content of the constituent elements below refers to "mass percent".

[0020] (1) Chemical composition of the base material C: 0.030 to 0.150% C is the most effective element for increasing the strength of clad steel sheets (hereinafter also referred to as steel sheets) produced by cooling according to the present invention. To obtain this effect, the C content is specified to be 0.030% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and producing at a lower cost, it is preferable that the C content be 0.040% or more. On the other hand, if the C content exceeds 0.150%, it will lead to deterioration of the toughness and weldability of the steel sheet. Therefore, the C content is specified to be 0.150% or less. Furthermore, from the viewpoint of toughness, it is preferable that the C content be 0.140% or less.

[0021] Si: 0.05-0.55% Si is added to improve the strength of the steel plate and also for deoxidation. To obtain these effects, the Si content is specified to be 0.05% or more. Furthermore, it is preferable to have a Si content of 0.07% or more. On the other hand, if the Si content exceeds 0.55%, it will lead to deterioration of weldability. Therefore, the Si content is specified to be 0.55% or less. Furthermore, it is preferable to have a Si content of 0.50% or less.

[0022] Mn: 0.50-2.10% Mn is an element that increases the hardenability of steel and is one of the important elements that must be added to satisfy the high strength required in this invention. To obtain this effect, the Mn content is specified to be 0.50% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost, it is preferable that the Mn content be 0.70% or more. On the other hand, if the Mn content exceeds 2.10%, it leads to a deterioration of weldability. Therefore, the Mn content is specified to be 2.10% or less. Furthermore, it is preferable that the Mn content be 1.90% or less.

[0023] P: 0.020% or less. P is an element that is inevitably present as an impurity and has adverse effects such as reducing toughness and weldability by segregating at grain boundaries. Therefore, it is desirable to keep the P content as low as possible, but 0.020% or less is acceptable. Thus, the P content is set to 0.020% or less. Note that there is no particular lower limit to the P content and it may be 0%, but since P is usually an element that is inevitably present in steel as an impurity, it may be greater than 0% industrially. Furthermore, since reducing excess P leads to a rise in refining costs, it is preferable to set the P content to 0.001% or more.

[0024] S: 0.010% or less. S is an element that is inevitably present as an impurity. It exists in steel as sulfide-based inclusions such as MnS, and is an element that has adverse effects, such as becoming the starting point for fracture and reducing the toughness of the steel sheet. Therefore, it is desirable to keep the S content as low as possible, but it is acceptable if it is 0.010% or less. Thus, the S content is set to 0.010% or less. Note that there is no particular lower limit to the S content, and it may be 0%, but since S is usually an element that is inevitably present in steel as an impurity, it may be industrially greater than 0%. Furthermore, reducing excess S leads to a rise in refining costs, so from a cost perspective, it is preferable to have an S content of 0.001% or more.

[0025] Al: 0.018-0.070% Al acts as a deoxidizing agent. To obtain this effect, the Al content is specified to be 0.018% or higher. 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 lower. Furthermore, from the viewpoint of preventing deterioration of toughness, it is preferable to have an Al content of 0.060% or lower.

[0026] Ni: 0.60–2.20% Ni is not only effective in improving the strength of steel plates, but also has the effect of improving the toughness of the base material and the heat-affected zone of the weld. However, if the Ni content is less than 0.60%, the effect is poor, and if it exceeds 2.20%, defects will occur on the surface of the steel plate. Therefore, the Ni content is specified to be in the range of 0.60–2.20%. Furthermore, it is preferable that the Ni content be 0.70% or more. Also, it is preferable that the Ni content be 2.10% or less.

[0027] Ti: 0.005 to 0.020% Ti is an element that has a strong tendency to form nitrides and has the effect of fixing N and reducing solid-solution N. Therefore, the toughness of the weld can be improved by adding Ti. In order to obtain this effect, it is necessary to contain 0.005% or more Ti. On the other hand, if the Ti content exceeds 0.020%, the toughness will actually decrease. Therefore, the Ti content is specified in the range of 0.005 to 0.020%. Furthermore, it is preferable that the Ti content be 0.008% or more. Also, it is preferable that the Ti content be 0.017% or less.

[0028] N: 0.0020 to 0.0080% N can improve the toughness of the weld by forming TiN. To obtain this effect, it is necessary to contain 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 in the range of 0.0020 to 0.0080%. Furthermore, it is preferable that the N content be 0.0025% or more. Also, it is preferable that the N content be 0.0070% or less.

[0029] O: 0.0050% or less. O is an element contained as an unavoidable impurity, Al２ O ３ exists in steel as oxides such as this, and is an element that has an adverse effect, such as becoming a starting point for fracture and reducing the toughness of the steel plate. Therefore, it is desirable to make the O content as low as possible, but it is acceptable if it is 0.0050% or less. Thus, the O content is set to 0.0050% or less. Note that the lower limit of the O content is not particularly limited and may be 0%, but usually, O is an element that is inevitably contained in steel as an impurity, so industrially it may be more than 0%. Also, excessive reduction leads to an increase in refining costs, so from the perspective of cost, it is preferable to set the O content to 0.0005% or more.

[0030] In addition to the above components in the base material of the present invention, at least one selected from the following is contained: 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 is Fe and inevitable impurities. Inevitable impurities are impurities that inevitably混入 from raw materials, manufacturing processes, or manufacturing equipment, etc., and are allowed to be included within a range that does not inhibit the object of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap.

[0031] Cu: 0.50% or less Cu is an element effective for improving the strength of the steel plate. However, if the Cu content is less than 0.05%, the effect is poor, and it is preferable to make the Cu content 0.05% or more. On the other hand, if the Cu content exceeds 0.50%, flaws will occur on the surface of the steel plate. Therefore, when Cu is contained, the Cu content is defined within the range of 0.50% or less. Furthermore, it is preferable to make the Cu content 0.10% or more. Also, it is more preferable to make the Cu content 0.45% or less.

[0032] Cr: 1.60% or less. Cr is an element effective in improving the strength of the steel plate. However, if the Cr content is less than 0.05%, the effect is poor, and it is preferable to set the Cr content to 0.05% or more. On the other hand, if the Cr content exceeds 1.60%, the toughness of the steel plate deteriorates. Therefore, when containing Cr, the Cr content is defined within the range of 1.60% or less. Further, it is preferable to set the Cr content to 0.10% or more. Also, it is more preferable that the Cr content is 1.50% or less.

[0033] Mo: 0.60% or less. Mo is an element effective in improving the strength of the steel plate. However, if the Mo content is less than 0.05%, the effect is poor, and it is preferable to set the Mo content to 0.05% or more. On the other hand, if the Mo content exceeds 0.60%, the toughness of the steel plate deteriorates. Therefore, when containing Mo, the Mo content is defined within the range of 0.60% or less. Further, it is preferable to set the Mo content to 0.10% or more. Also, it is more preferable that the Mo content is 0.55% or less.

[0034] 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 carbides and nitrides. To obtain such an effect, when containing Nb, it is preferable to set the Nb content to 0.005% or more. Further, it is more preferable to set it to 0.007% or more. On the other hand, if the Nb content exceeds 0.030%, a large amount of NbC precipitates and the toughness decreases. Therefore, when containing Nb, the Nb content is set to 0.030% or less. Further, it is preferable that the Nb content is 0.027% or less.

[0035] V: 0.100% or less. V is an element effective in improving the strength of the steel plate. However, if the V content is less than 0.005%, the effect is poor, and it is preferable to set the V content to 0.005% or more. On the other hand, if the V content exceeds 0.100%, the toughness of the steel plate deteriorates. Therefore, when containing V, the V content is defined within the range of 0.100% or less. Further, it is preferable to set the V content to 0.030% or more. Also, it is preferable that the V content is 0.090% or less.

[0036] B: 0.0050% or less. B is an effective element for improving the strength of steel plates. However, if the B content is less than 0.0005%, its effect is poor, and it is preferable to have a B content of 0.0005% or more. On the other hand, if the B content exceeds 0.0050%, the toughness of the steel plate deteriorates. Therefore, when B is included, the B content is specified to be within the range of 0.0050% or less. Furthermore, it is preferable to have a B content of 0.0008% or more. Also, it is preferable to have a B content of 0.0040% or less.

[0037] Ca: 0.0040% or less. Ca is an element that combines with S and suppresses the formation of MnS and other elements that elongate in the rolling direction. That is, by adding Ca, the morphology of sulfide-based inclusions can be controlled to be spherical, thereby improving the toughness of welds and the like. To obtain this effect, when Ca is included, it is preferable that the Ca content be 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 included, the Ca content should be 0.0040% or less. The Ca content is more preferably 0.0010% or more. Also, the Ca content is preferably 0.0030% or less.

[0038] (2) Corrosion-resistant alloy of the composite material The corrosion-resistant alloy of the composite material of the present invention contains, by mass%, C: 0.001 to 0.080%, Si: 0.05 to 1.00%, Mn: 0.30 to 4.00%, P: 0.040% or less, S: 0.020% or less, Cr: 11.0 to 28.0%, and further optionally contains one or more selected from Ni: 30.0% or less, Cu: 3.00% or less, Mo: 7.00% or less, V: 0.100% or less, N: 0.300% or less, O: 0.0050% or less, has a PRE value of 15 or more as shown in formula (1), and the remainder consists of Fe and unavoidable impurities, and the number of sulfide-based inclusions on the surface of the composite material is 2.5 × 10 ５ pieces / mm ２The following is the formula: PRE = Cr% + 3.3 × Mo% + 30 × N% - Mn% ... (1) In formula (1), Cr%, Mo%, N%, and Mn% are, respectively, the Cr content (mass%), Mo content (mass%), N content (mass%), and Mn content (mass%) in the composite material. A detailed explanation follows.

[0039] C: 0.001 to 0.080% C is an effective element for increasing the strength of steel sheets manufactured by cooling according to the present invention. To obtain this effect, the C content is specified to be 0.001% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost, it is preferable that the C content be 0.010% or more. On the other hand, if the C content exceeds 0.080%, carbides will precipitate at the grain boundaries, causing stress corrosion cracking when in contact with liquid ammonia. Therefore, the C content is specified to be 0.080% or less. Furthermore, from the viewpoint of toughness, it is preferable that the C content be 0.050% or less.

[0040] Si: 0.05-1.00% Si is added to improve the strength of the steel plate and also for deoxidation. To obtain these effects, the Si content is specified to be 0.05% or more. Furthermore, it is preferable that the Si content be 0.07% or more. On the other hand, if the Si content exceeds 1.00%, it will lead to deterioration of toughness and weldability. Therefore, the Si content is specified to be 1.00% or less. Furthermore, it is preferable that the Si content be 0.80% or less.

[0041] Mn: 0.30-4.00% Mn is an element that increases the hardenability of steel, and is also one of the important elements that need to be added for deoxidation. To obtain this effect, the Mn content is specified to be 0.30% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost, it is preferable that the Mn content be 0.40% or more. On the other hand, if the Mn content exceeds 4.00%, it leads to a deterioration of weldability. Therefore, the Mn content is specified to be 4.00% or less. Furthermore, it is preferable that the Mn content be 3.00% or less.

[0042] P: 0.040% or less. P is an element that is inevitably present as an impurity and has adverse effects such as reducing toughness and weldability by segregating at grain boundaries. Therefore, it is desirable to keep the P content as low as possible, but 0.040% or less is acceptable. Thus, the P content is set to 0.040% or less. The P content is preferably 0.035% or less. The lower limit of the P content is not particularly limited and may be 0%, however, since P is an element that is inevitably present in steel as an impurity, it may be greater than 0% industrially. Furthermore, since reducing excess leads to a rise in refining costs, it is preferable that the P content be 0.001% or more.

[0043] S: 0.020% or less. S is an element that is contained as an unavoidable impurity. It exists in steel as sulfide-based inclusions such as MnS, and is an element that has adverse effects, such as becoming the starting point for fracture and reducing the toughness of the steel sheet. Therefore, it is desirable to keep the S content as low as possible, but it is acceptable if it is 0.020% or less. Thus, the S content is set to 0.020% or less. The S content is preferably 0.015% or less. The S content is more preferably 0.010% or less, and even more preferably 0.005% 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, it may be greater than 0% industrially. Furthermore, reducing excess S leads to a rise in refining costs, so from a cost standpoint, it is preferable to set the S content to 0.001% or more.

[0044] Cr: 11.0-28.0% Cr is an effective element for improving the corrosion resistance of stainless steel. To prevent stress corrosion cracking when in contact with liquid ammonia, the Cr content is specified to be 11.0% or more. Furthermore, it is preferable that the Cr content be 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 deteriorate. Therefore, the Cr content is specified to be 28.0% or less. Furthermore, it is preferable that the Cr content be 26.0% or less.

[0045] The remainder of the composite material of the present invention, other than the above-mentioned components, consists of Fe and unavoidable impurities. Unavoidable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included in a range that does not hinder the objective of the present invention. Examples of raw materials include iron ore, reduced iron, ferroalloys, various non-ferrous metal raw materials, or scrap. However, the elements listed below may be added as needed, and it is preferable to contain one or more of the elements listed below.

[0046] Ni: 30.0% or less. Ni is an effective element for improving the corrosion resistance of stainless steel to various acids. In addition to being effective in improving the strength of steel plates, it also has the effect of improving the toughness of the base material and the heat-affected zone of the weld. In order to exhibit this effect, when Ni is included, the Ni content is preferably 0.05% or more, and more preferably 0.10% or more. When the laminated material is austenitic stainless steel, the Ni content is preferably 6.0% or more, and more preferably 8.0% or more. When the laminated material is duplex stainless steel, the Ni content is preferably 3.0% or more, and more preferably 4.5% or more. On the other hand, since Ni is an expensive metal, the Ni content is specified as 30.0% or less from the viewpoint of alloy cost. When the laminated material is austenitic stainless steel, the Ni content is preferably 28.0% or less, and more preferably 15.0% or less. When the composite material is duplex stainless steel, the Ni content is preferably 8.0% or less, and more preferably 7.5% or less.

[0047] Cu: 3.00% or less. Cu is an effective element for improving the corrosion resistance and strength of steel sheets. To achieve these effects, when Cu is included, the Cu content is preferably 0.05% or more, and more preferably 0.10% or more. However, if the Cu content exceeds 3.00%, the hot workability decreases significantly, so when Cu is included, the Cu content is preferably 3.00% or less, and preferably 2.20% or less.

[0048] Mo: 7.00% or less Mo is an effective element for improving the pitting corrosion resistance of stainless steel. To achieve this effect, when Mo is included, the Mo content is preferably 0.05% or more, and more preferably 0.10% or more. On the other hand, since Mo is an expensive metal, from the viewpoint of alloy cost, when Mo is included, the Mo content is preferably 7.00% or less, and more preferably 5.50% or less. In the case of duplex stainless steel containing Mo in the composite material, depending on the cooling rate of the steel, the sigma phase may precipitate significantly, potentially causing a significant deterioration in corrosion resistance. Therefore, the Mo content is preferably 5.0% or less, and more preferably 4.0% or less.

[0049] V: 0.100% or less. Since V readily bonds with C compared to Cr, it is an effective element for suppressing corrosion resistance degradation due to Cr carbide formation. To achieve this effect, when V is included, the V content is preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, from the viewpoint of alloy cost, when V is included, the V content is preferably 0.100% or less, and more preferably 0.080% or less.

[0050] N: 0.300% or less. N is an effective element for improving the corrosion resistance and strength of stainless steel. To achieve this effect, when N is included, the N content is preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the N content exceeds 0.300%, the toughness will actually decrease. Therefore, when N is included, the N content should be 0.300% or less, and preferably 0.250% or less.

[0051] O: 0.0050% or less. O is an element that is inevitably contained in steel as an impurity, Al ２ O ３Oxygen (O) is an element that exists in steel as an oxide and has adverse effects, such as becoming the starting point for fracture and reducing the toughness of steel sheets. Therefore, it is desirable to keep the O content as low as possible, but it is acceptable if it is 0.0050% or less. Thus, if O is present, the O content should be 0.0050% or less. There is no particular lower limit to the O content, and it may be 0%, but since O is an element that is usually inevitably present in steel as an impurity, it may be industrially greater than 0%. Furthermore, since reducing excess O leads to a rise in refining costs, from a cost perspective, it is preferable to have an O content of 0.0005% or more.

[0052] PRE value: 15 or higher PRE = Cr% + 3.3 × Mo% + 30 × N% - Mn% ... (1) In equation (1), Cr%, Mo%, N%, and Mn% are, respectively, the Cr content (mass%), Mo content (mass%), N content (mass%), and Mn content (mass%) in the composite material. PRE is the pitting corrosion index and can be calculated from PRE = Cr% + 3.3 Mo% + 30 × N% - Mn%. The higher the PRE value, the lower the susceptibility to liquid ammonia SCC. Therefore, the PRE value of the composite material is specified to be 15 or higher. On the other hand, a higher PRE value of the composite material is preferable, but an excessive PRE value leads to increased costs. Therefore, a PRE value of 27 or lower is preferable.

[0053] Furthermore, various types of stainless steel can be used as the bonding material in this invention. From the viewpoint of improving the toughness of the heat-affected zone of the weld of the bonding material, it is preferable to use austenitic stainless steel or duplex stainless steel.

[0054] The number of sulfide-based inclusions on the surface of the composite material is 2.5 × 10 ５ pieces / mm ２ The following composite materials contain sulfide-based inclusions exceeding a predetermined amount (e.g., FeS, Fe ２ S, MnS, CuS, Cu ２ When sulfide inclusions (such as S and NiS) are present, the sulfide inclusions become the starting point for stress corrosion cracking in liquid ammonia, reducing ammonia resistance to SCC. In this invention, the number of such inclusions per unit area is controlled to 2.5 × 10⁻⁶. ５ pieces / mm ２The following applies. The above number is preferably 1.0 × 10 ５ pieces / mm ２ The following, and more preferably 2.5 × 10 ４ pieces / mm ２ The following applies: While fewer sulfide inclusions on the surface are desirable, removing all sulfide inclusions is practically difficult in manufacturing, therefore, 1.0 × 10⁻⁶ ２ pieces / mm ２ The above is preferable.

[0055] The number of sulfide inclusions can be determined by observing a 100 μm × 100 μm area of ​​the clad steel sheet surface at a magnification of 2000x using a scanning electron microscope (SEM), analyzing the chemical composition with a characteristic X-ray analyzer attached to the SEM to identify the sulfide inclusions, and calculating the number of inclusions per unit area. Here, if the length of the sulfide inclusions is less than 1.0 μm, even if the inclusions dissolve, the progression of pitting corrosion tends to stop due to repassivation; therefore, only sulfide inclusions with a length of 1.0 μm or more are counted.

[0056] (3) Characteristics of clad steel sheets [Tensile strength] The clad steel sheet of the present invention shall have a tensile strength of 780 MPa or more. In the present invention, the tensile strength of the clad steel sheet refers to a tensile strength of 780 MPa or more obtained by performing a tensile test on a tensile test piece taken from the base material (base steel sheet) of the clad steel sheet. There is no particular upper limit to the tensile strength, but the tensile strength of the high-strength steel sheet of the present invention may be 930 MPa or less.

[0057] [Charpy Impact Test Absorption Energy] The clad steel sheet of the present invention shall have an absorption energy of 47 J or more in a Charpy impact test at -40°C for the base material and the heat-affected zone of the weld. In the present invention, the absorption energy of the base material refers to the absorption energy measured by performing a Charpy impact test on an impact test specimen taken from the base material (base steel sheet) of the steady-state portion of the clad steel sheet, excluding the heat-affected zone of the weld. In the present invention, the absorption energy of the heat-affected zone of the weld refers to the absorption energy measured by performing a Charpy impact test on an impact test specimen taken from the base material (base steel sheet) of the heat-affected zone of the clad steel sheet. When used as a structural member such as a tank, PWHT (Post Weld Heat Treatment) treatment is performed, but depending on the plate thickness, PWHT may not be performed. In that case, the absorption energy required for the base material and heat-affected zone of the weld that have not undergone PWHT treatment is preferably 100 J or more, more preferably 170 J or more.

[0058] (4) Manufacturing conditions The manufacturing method of the clad steel sheet of the present invention will be described below. In the present invention, first, a base material slab and a cladding material slab having the above-mentioned preferred component composition and structure are manufactured. There are no particular restrictions on the manufacturing method of the base material slab, and conventionally known steel sheet manufacturing methods can be applied. That is, molten steel adjusted to the above-mentioned preferred component composition by a normal melting method (converter method, electric furnace method, etc.) is cast by a normal casting method (continuous casting method or ingot making method), and if necessary, the cast slab is further subjected to ingot rolling to obtain the material slab.

[0059] Furthermore, as a manufacturing condition for the corrosion-resistant alloy slab that will serve as the cladding material, the average residence time of the molten steel in the tundish during continuous casting of the slab should be 10 minutes or more. This promotes the floating of inclusions, ultimately reducing the amount of inclusions present in the cladding material of the clad steel sheet, and also reducing the amount of inclusions present on the surface of the cladding material of the clad steel sheet. Here, the average residence time is the value obtained by dividing the tundish capacity (tons) by the amount of molten steel discharged into the mold (also called throughput; the unit is tons / minute). As a method to achieve the average residence time in the tundish as defined in this invention, a large-capacity tundish may be used, or a weir may be installed in the tundish to lengthen the flow path of the molten steel. In this way, the material slab for the cladding material is obtained by casting the corrosion-resistant alloy that will serve as the cladding material using a continuous casting method.

[0060] Next, the obtained base material slab is heated to a surface temperature of 900°C to 1200°C and then hot-rolled, ending the rolling process at a surface temperature of 700°C or higher to obtain the base material steel plate of a predetermined size. Alternatively, the obtained laminate material slab is heated and then hot-rolled to obtain laminate material of a predetermined size. The heating conditions for the laminate material slab and the subsequent hot-rolling conditions are not particularly limited, but an appropriate thermal history can be adopted, selected according to the component composition of the laminate, to prevent the formation of harmful precipitates and intermetallic compounds during heating and hot-rolling. Alternatively, the obtained laminate material slab may be used as is as the laminate slab.

[0061] [Heating temperature of the base material slab: 900°C or higher and 1200°C or lower] When the base material slab is heated and hot-rolled, if the heating temperature is less than 900°C at the surface temperature, the solid solution of carbides will be insufficient and the required strength will not be obtained. Therefore, the heating temperature of the base material slab should be 900°C or higher. Furthermore, it is preferable that the heating temperature be 920°C or higher. On the other hand, if the base material slab is heated to a temperature exceeding 1200°C at the surface temperature, the crystal grains of the base material will coarseen, leading to deterioration of the toughness of the base material in the final clad steel sheet. Therefore, the heating temperature should be 1200°C or lower. Furthermore, it is preferable that the heating temperature be 1150°C or lower.

[0062] [Rolling end temperature of base material slab: 700°C or higher] When the base material slab is heated and hot-rolled, if the rolling end temperature is below 700°C (surface temperature), the generated ferrite will be affected by the processing, resulting in a deterioration of the toughness of the base material in the final clad steel sheet. Therefore, the rolling end temperature should be 700°C or higher. Furthermore, it is preferable to set it at 750°C or higher. There is no particular upper limit to the rolling end temperature, but the rolling end temperature may be 1100°C or lower (surface temperature) or 1050°C or lower.

[0063] The basic process for manufacturing clad steel sheets involves first assembling a clad slab by appropriately laminating the base steel sheet material and the cladding material, then heating the clad slab, and finally hot-rolling the heated clad slab to obtain a rolled sheet. The rolled sheet obtained by hot-rolling may be reheated and then subjected to accelerated cooling, or accelerated cooling may be performed on the rolled sheet immediately after rolling. Clad steel sheets are obtained by cutting or, if necessary, peeling the rolled sheet obtained in this way.

[0064] When assembling a clad slab, there are three main methods for overlapping the base steel sheet material and the cladding material: (1) Sandwich method In the sandwich method, the base steel sheet material / cladding material / cladding material / base steel sheet material are overlapped in this order. A release agent, such as oxide powder, is applied between the two cladding steel sheet materials. From a clad slab assembled using the sandwich method, two clad steel sheets are produced by separating the upper and lower parts of the rolled plate manufactured by rolling the clad slab. (2) Sacrificial material method (semi-sandwich method) In the sacrificial material method (semi-sandwich method), the sacrificial material material / cladding material / base steel sheet material are overlapped in this order. A release agent, such as oxide powder, is applied between the sacrificial material material and the cladding material. In the clad slab assembled using the sacrificial material method, a single clad steel plate is produced by removing the sacrificial material in the rolled body manufactured by rolling the clad slab. (3) Open sandwich method (open method) In the open sandwich method, the materials of a pair of cladding materials and the base steel plate are layered together. In the clad slab assembled using the open sandwich method, the rolled plate manufactured by rolling the clad slab itself becomes a single clad steel plate.

[0065] In this invention, the conditions under which the obtained clad slab is heated and hot-rolled, and then reheated and accelerated-cooled after hot-rolling, and the conditions under which it is accelerated-cooled immediately after hot-rolling, are described below.

[0066] First, we will explain the process of producing a rolled sheet having a base material and a cladding material by heating a clad slab and performing hot rolling.

[0067] [Heating temperature of clad slab: 1000°C or higher and 1250°C or lower] When heating and hot rolling a clad slab, if the heating temperature of the clad slab is less than 1000°C at the surface temperature, the solid solution of carbides will be insufficient and the required strength cannot be obtained. In addition, from the viewpoint of the joinability of the clad steel, a higher heating temperature is preferable. Therefore, the above heating temperature should be 1000°C or higher. Furthermore, it is preferable that the above heating temperature be 1020°C or higher. On the other hand, if the clad slab is heated above 1250°C, it will lead to deterioration of toughness due to the coarsening of the crystal grains of the base material. Therefore, the above heating temperature should be 1250°C or lower. Furthermore, it is preferable that it be 1230°C or lower.

[0068] [Cumulative reduction ratio of clad slab: 65% or more] When hot-rolling a heated clad slab, rolling is performed to achieve a cumulative reduction ratio of 65% or more. This promotes the recrystallization of austenite, introduces deformation zones that serve as nucleation sites within the austenite grains, and refines the bainite and martensite that are formed by the subsequent accelerated cooling under the conditions described below. As a result, the toughness of the clad steel sheet is improved. For this reason, the cumulative reduction ratio is set to 65% or more. Furthermore, it is preferable to set it to 70% or more. There is no particular upper limit to the cumulative reduction ratio, but from the viewpoint of rolling efficiency, it is preferable to set the cumulative reduction ratio to 95% or less.

[0069] [Rolling end temperature for clad slabs: 700°C or higher and 1000°C or lower] If the rolling end temperature of the hot rolling is below 700°C, the generated ferrite is greatly affected by the processing, resulting in a deterioration of toughness. Also, if the rolling end temperature is below 700°C, it becomes hot rolling at a low temperature, which is an unfavorable condition from the viewpoint of diffusion bonding, and the bondability of the clad steel deteriorates. Therefore, the rolling end temperature should be 700°C or higher. Furthermore, it is preferable that the rolling end temperature be 720°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 and martensite are not obtained, and the toughness of the clad steel sheet deteriorates. Therefore, the rolling end temperature should be 1000°C or lower. Furthermore, it is preferable that the rolling end temperature be 980°C or lower.

[0070] In this way, a rolled sheet having a base material and a bonding material is manufactured.

[0071] In this invention, the obtained rolled sheet is subjected to the following treatment (A) or (B): (A) The rolled sheet after hot rolling is air-cooled to room temperature, and then the rolled sheet is reheated to 800°C or higher and 1000°C or lower, with the cooling start temperature being Ar ３ (B) The rolled sheet after hot rolling is subjected to accelerated cooling under the conditions of being above the transformation point, average cooling rate: 1.0 to 20°C / s, and cooling stop temperature: 350°C or lower, and then tempered at a temperature of 550°C to 700°C. (A) The rolled sheet after hot rolling is subjected to accelerated cooling under the conditions of starting cooling temperature: 650°C to 1000°C, average cooling rate: 5 to 50°C / s, and cooling stop temperature: 300°C or lower, and then tempered at a temperature of 550°C to 700°C. Here, the average cooling rate in accelerated cooling is determined by dividing the difference between the starting cooling temperature and the stopping cooling temperature at the 1 / 2 thickness position in the thickness direction of the base material by the cooling time.

[0072] First, we will describe the method in (A) in which the rolled sheet is air-cooled to room temperature, and then reheated to perform accelerated cooling.

[0073] [Air Cooling to Room Temperature] In this embodiment, the rolled sheet obtained by hot rolling is air-cooled without any special treatment such as water injection or blowing air. After the rolled sheet has cooled to room temperature, it is reheated. Here, cooling to room temperature is not necessarily limited to cooling to room temperature, such as 20-30°C, but is sufficient as long as it has cooled to the point where the transformation of the microstructure of the base material is complete, and this includes cases where it has cooled to, for example, 100°C or below.

[0074] [Reheating temperature: 800°C or higher and 1000°C or lower] Next, the rolled sheet, which has been air-cooled to room temperature, is reheated. If the reheating temperature of the rolled sheet is below 800°C, it may result in insufficient strength. Therefore, the reheating temperature should be 800°C or higher. Furthermore, it is preferable to set it at 820°C or higher. On the other hand, if the reheating temperature exceeds 1000°C, the strength may become excessive and the toughness may not recover. Therefore, the reheating temperature should be 1000°C or lower. Furthermore, it is preferable to set the reheating temperature at 980°C or lower.

[0075] [Cooling start temperature: Ar ３ [Above the transformation point] In this embodiment, the steel sheet, after reheating, is cooled (accelerated cooling) from a temperature above the Ar3 transformation point of the base material. If the starting temperature for this cooling is below the Ar3 transformation point of the base material, excessive ferrite will be formed, and it will coexist with a martensitic structure or bainite with a large difference in strength. As a result, this leads to insufficient strength and deterioration of toughness of the base material. Therefore, the starting temperature for cooling after reheating should be above the Ar3 transformation point. Furthermore, it is preferable that the starting temperature for cooling after reheating be above the Ar3 transformation point + 20°C. There is no particular upper limit to the starting temperature for cooling after reheating, but it is preferable that the starting temperature for cooling be below the reheating temperature mentioned above. ３ The transformation point can be determined by the following equation (2): Ar ３ (°C) = 910 - 310 × [C] - 80 × [Mn] - 20 × [Cu] - 55 × [Ni] - 15 × [Cr] - 80 × [Mo] ... (2) Here, in equation (2), [M] refers to the content (mass%) of element M in the base material (base steel plate, base material slab).

[0076] [Average cooling rate: 1.0 to 20°C / s] In this embodiment, cooling at an average cooling rate of 1.0°C / s or higher is an essential process for obtaining high-strength and high-toughness steel sheets, as rapid cooling provides a strength-enhancing effect through transformation strengthening. If the average cooling rate is less than 1.0°C / s, the grain size of bainite and martensite increases, and ferrite and pearlite are formed, which may lead to insufficient strength and deterioration of toughness. Therefore, the average cooling rate should be 1.0°C / s or higher. Furthermore, it is preferable that the average cooling rate be 1.5°C / s or higher. On the other hand, if the average cooling rate exceeds 20°C / s, the volume fraction of martensite becomes too large, and toughness decreases. Therefore, the average cooling rate should be 20°C / s or lower. Furthermore, it is preferable that the average cooling rate be 15°C / s or lower. Here, the average cooling rate in accelerated cooling is determined by dividing the difference between the cooling start temperature and the cooling stop temperature at the 1 / 2 thickness position in the thickness direction of the base material by the cooling time.

[0077] [Cooling Stop Temperature: 350°C or less] In this embodiment, after the completion of rolling, cooling is performed under the above conditions to a cooling stop temperature of 350°C or less, thereby enabling the base material to uniformly contain bainite or martensite at a predetermined volume fraction up to the center of the plate thickness. If the cooling stop temperature exceeds 350°C, excessive ferrite and pearlite structures will be formed, leading to insufficient strength and deterioration of toughness. Therefore, the cooling stop temperature is specified as 350°C or less. Furthermore, it is preferable that the cooling stop temperature be 300°C or less. On the other hand, the lower limit of this cooling stop temperature is not particularly limited and may be room temperature, but it is preferable to set it at 100°C from the viewpoint of production efficiency, etc. After the hot-rolled plate is reheated in this manner, tempering is performed on the rolled plate that has undergone accelerated cooling. The conditions for tempering will be described later.

[0078] Next, we will describe the method in which accelerated cooling is performed on the rolled sheet after hot rolling, as described in (B).

[0079] [Cooling start temperature: 650°C or higher and 1000°C or lower] In this embodiment, if the cooling start temperature for hot rolling is less than 650°C, accelerated cooling is performed from a state in which ferrite is present, resulting in the retention of ferrite, which is softer than bainite and martensite, which may lead to a decrease in strength and deterioration of toughness. Also, if the cooling start temperature is less than 650°C, hot rolling will be performed at a low temperature, which may deteriorate the bondability of the clad steel from the viewpoint of diffusion bonding. Therefore, the cooling start temperature should be 650°C or higher. Furthermore, it is preferable that the cooling start temperature be 670°C or higher. On the other hand, if the cooling start temperature exceeds 1000°C, deformation bands that serve as nucleation sites are not introduced into the austenite grains, fine bainite and martensite cannot be obtained, and the toughness of the clad steel sheet deteriorates. Therefore, the cooling start temperature should be 1000°C or lower. Furthermore, it is preferable that the cooling start temperature be 980°C or lower.

[0080] [Average cooling rate: 5 to 50°C / s] In this embodiment, cooling at an average cooling rate of 5°C / s or higher is an essential process for obtaining high-strength and high-toughness steel sheets, as rapid cooling provides a strength-enhancing effect through transformation strengthening. Therefore, if the average cooling rate is less than 5°C / s, the grain size of bainite and martensite will increase, and ferrite and pearlite may be formed, potentially leading to insufficient strength and deterioration of toughness. Accordingly, the average cooling rate should be 5°C / s or higher. Furthermore, it is preferable that the average cooling rate be 10°C / s or higher. On the other hand, if the average cooling rate exceeds 50°C / s, the volume fraction of martensite becomes too large, reducing toughness. Accordingly, the average cooling rate should be 50°C / s or lower. Furthermore, it is preferable that the average cooling rate be 45°C / s or lower. Here, the average cooling rate in accelerated cooling is determined by dividing the difference between the cooling start temperature and the cooling stop temperature at the 1 / 2 thickness position in the thickness direction of the base material by the cooling time.

[0081] [Cooling Stop Temperature: 300°C or less] In this embodiment, after the completion of rolling, cooling is performed under the above conditions to a cooling stop temperature of 300°C or less, thereby enabling the base material to uniformly contain bainite or martensite at a predetermined volume fraction up to the center of the plate thickness. If the cooling stop temperature exceeds 300°C, excessive formation of ferrite and pearlite structures may occur, potentially leading to insufficient strength and deterioration of toughness. Therefore, the cooling stop temperature is specified as 300°C or less. Furthermore, it is preferable that the cooling stop temperature be 280°C or less. On the other hand, the lower limit of this cooling stop temperature is not particularly limited and may be room temperature, but from the viewpoint of production efficiency, it is preferable to set it at 150°C. The rolled plate that has been subjected to accelerated cooling after hot rolling is then tempered. The tempering conditions are described below.

[0082] The clad steel sheet, which has undergone accelerated cooling under conditions (A) or (B), is further tempered at a temperature of 550°C to 700°C. Unless otherwise specified, the temperature of the material and the clad steel sheet in this invention refers to the temperature at the 1 / 2 thickness point of the base material. While this temperature cannot be directly measured, a value obtained by, for example, performing a difference calculation using a process computer from the temperature of the steel sheet surface measured with a radiation thermometer may be used.

[0083] [Tempering temperature: 550°C or higher and 700°C or lower] In this invention, tempering is performed for the purpose of restoring the toughness of the base material. If the average temperature of the steel plate (temperature at half the thickness of the base material) exceeds 700°C during reheating after tempering, dislocations may recover, and the strength of the base material may decrease. Therefore, the tempering temperature should be 700°C or lower. Furthermore, it is preferable that the tempering temperature be 680°C or lower. On the other hand, if the average temperature of the steel plate is less than 550°C during reheating after tempering, there is a risk of insufficient toughness of the base material. Therefore, the tempering temperature should be 550°C or higher. Furthermore, it is preferable that the tempering temperature be 600°C or higher.

[0084] By subjecting the base material and cladding material slabs having the aforementioned component composition to the above-described manufacturing conditions, a clad steel sheet having the component composition and structure according to the present invention can be obtained. The high-strength clad steel sheet thus obtained according to the present invention will have excellent low-temperature toughness, stress corrosion cracking resistance, and excellent bonding properties.

[0085] In addition, in the manufacturing method according to the present invention, any items not described herein can be handled by conventional methods.

[0086] Clad steel plates (No. 1 to 69) were manufactured using base material with the chemical composition shown in Table 1 and cladding material slabs with the chemical composition shown in Table 2, under the manufacturing conditions shown in Table 3 (Tables 3-1, 3-2, and 3-3). 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 cladding material side. The slab assembly method was one of the following: sandwich method, open method, or sacrificial method. In Table 3, the sandwich method is in the order of base material slab, cladding material slab, cladding material slab, and base material slab from top to bottom; the open method is in the order of cladding material slab and base material slab from top to bottom; and the sacrificial method is in the order of sacrificial material, cladding material slab, and base material slab.

[0087] Table 1 shows the chemical composition of the base material (the remainder being Fe and unavoidable impurities). In the table, steel grades A to J are examples of inventions that fall within the scope of the present invention. On the other hand, steel grades K to X are comparative examples in which at least one component falls outside the scope of the invention. Table 2 shows the chemical composition of the composite material (the remainder being Fe and unavoidable impurities). In the table, steel grades a to h are examples of inventions that fall within the scope of the present invention. On the other hand, steel grade i is a comparative example in which the PRE value falls outside the scope of the invention.

[0088] [Number of sulfide inclusions] Observation test pieces with dimensions of 10 mm × 10 mm × total thickness of the clad steel sheet were taken from the clad steel sheet and the surface was mirror-polished. Then, five fields of view of a 100 μm × 100 μm area on the surface were observed at a magnification of 2000x using a scanning electron microscope (SEM). Sulfide inclusions were identified using a characteristic X-ray analyzer, and the number of sulfide inclusions with a length of 1.0 μm or more was counted. From this, the number of sulfide inclusions per unit area was calculated.

[0089] [Strength Characteristics] Tensile test specimens of type 1B according to JIS Z 2201 were taken from the base material portion of the clad steel sheet, excluding the cladding material portion. Specifically, the cladding material was removed by cutting or grinding from the clad steel sheet side, and tensile test specimens of the remaining base material portion were taken. The thickness of the tensile test specimen of the base material taken by the thickness reduction process may be smaller than the thickness of the base material (base steel sheet) of the clad steel sheet, but the difference is a maximum of 0.2 mm. Tensile tests were performed on the tensile test specimens taken in this manner according to the procedure described in JIS Z 2241 (2022), and the yield strength YS (yield point YP if there is a yield point, or 0.2% proof stress σ0.2 if there is no yield point) and tensile strength (TS) were measured. Steel sheets with a tensile strength of 780 MPa or higher were evaluated as having excellent tensile properties.

[0090] [Toughness] Charpy impact test specimens (V-notch specimens) were taken from the base material (not the cladding material) of the clad steel sheet, specifically from the steady-state portion excluding the weld heat-affected zone, and from the weld heat-affected zone of the welded joint. For base material thicknesses of 38 mm, 45 mm, or 60 mm, the Charpy impact test specimen thickness was set to 10 mm. For base material thickness of 10 mm, the Charpy impact test specimen thickness was set to 7.5 mm. Charpy impact tests were performed on these specimens at -40°C according to the procedure of JIS Z 2242 (2023), and the absorbed energy was measured. For both the base material and the weld heat-affected zone, specimens with an absorbed energy of 47 J or higher were evaluated as steel sheets with excellent toughness.

[0091] [Ammonia SCC Resistance] In this invention, ammonia SCC resistance was evaluated by performing a four-point bending test with a test solution and measuring the natural immersion potential. Specifically, the following procedure was followed: A 5 mm thick × 15 mm × 115 mm test piece was taken from the inner surface of the steel plate (the surface in contact with ammonia, etc.), ultrasonically degreased in acetone for 5 minutes, and then subjected to a stress of 100% YS of the actual yield strength of each test piece by four-point bending. After filling the test cell containing the four-point bent test piece with a solution of 12.5 g of ammonium carbamate and 1 L of liquid ammonia, the cell was immersed at room temperature (25°C). If no cracks were observed after 720 hours of immersion, the ammonia SCC resistance was judged to be good. On the other hand, if cracks occurred, it was judged to be poor. The obtained evaluation results are shown in Table 3.

[0092]

[0093]

[0094]

[0095]

[0096]

[0097]

[0098]

[0099]

[0100] As can be seen from Table 3, all of the inventive examples have a tensile strength of 780 MPa or more and exhibit excellent ammonia SCC resistance and low-temperature toughness. In contrast, the comparative examples do not obtain the necessary properties.

Claims

1. A clad steel sheet in which a cladding material is joined to at least one surface of a base material, wherein 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: 0.60 to 2.20%, Ti: 0.005 to 0.020%, N: 0.0020 to 0.0080%, O: 0.0050% 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. The composite material contains at least one selected from B: 0.0050% or less, Ca: 0.0040% or less, with the remainder being Fe and unavoidable impurities, and the chemical composition of the composite material is, in mass%, C: 0.001 to 0.080%, Si: 0.05 to 1.00%, Mn: 0.30 to 4.00%, P: 0.040% or less, S: 0.020% or less, Cr: 11.0 to 28.0%, and further optionally contains one or more selected from Ni: 30.0% or less, Cu: 3.00% or less, Mo: 7.00% or less, V: 0.100% or less, N: 0.300% or less, O: 0.0050% or less. The PRE value shown in formula (1) is 15 or higher, the remainder consists of Fe and unavoidable impurities, and the number of sulfide-based inclusions on the surface of the composite material is 2.5 × 10 ５ pieces / mm ２ The following clad steel sheet has a tensile strength of 780 MPa or more, and the absorbed energy of the Charpy impact test at -40°C in the base material and weld heat-affected zone is 47 J or more: PRE = Cr% + 3.3 × Mo% + 30 × N% - Mn% ... (1) In equation (1), Cr%, Mo%, N%, and Mn% are, respectively, the Cr content (mass%), Mo content (mass%), N content (mass%), and Mn content (mass%) in the clad material.

2. A method for manufacturing a clad steel sheet, comprising: heating a base material slab having the component composition described in claim 1 to a surface temperature of 900°C to 1200°C; subjecting the base material slab to hot rolling with a rolling completion temperature of 700°C or higher at a surface temperature to obtain a base steel sheet material; heating a cladding material slab having the component composition described in claim 1 and manufactured by continuous casting with an average residence time in the tundish of 10 minutes or more; subjecting the clad slab to hot rolling with a rolling completion temperature of 700°C to 1000°C to obtain a cladding material; heating a clad slab formed by laminating the base steel sheet material and the cladding material to a surface temperature of 1000°C to 1250°C; subjecting the clad slab to hot rolling with a cumulative reduction ratio of 65% or higher and a rolling completion temperature of 700°C to 1000°C to produce a rolled sheet having a base material and a cladding material; and subjecting the rolled sheet to the following treatment (A) or (B). (A) The rolled sheet after hot rolling is air-cooled to room temperature, and then the rolled sheet is reheated to 800°C to 1000°C, Cooling start temperature: Ar ３ (B) The rolled sheet after hot rolling is subjected to accelerated cooling under the following conditions: above the transformation point, average cooling rate: 1.0 to 20°C / s, cooling stop temperature: 350°C or lower, and then tempered at a temperature of 550°C to 700°C. (B) The rolled sheet after hot rolling is subjected to accelerated cooling under the following conditions: cooling start temperature: 650°C to 1000°C, average cooling rate: 5 to 50°C / s, cooling stop temperature: 300°C or lower, and then tempered at a temperature of 550°C to 700°C.

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