เหล็กกล้าแผ่นหนาประกบผิวและวิธีการผลิตสิ่งนี้

Abstract

DEPCT68 การประดิษฐ์ปัจจุบันจัดให้มีเหล็กกล้าแผ่นหนาประกบผิว(cladsteelplate)ทนแรงสูงที่มี ความต้านทานSCCที่เหนี่ยวนำโดยแอมโมเนียและความเหนียวที่อุณหภูมิต่ำที่ดีมากและ ที่เหมาะสำหรับการใช้ในถัง,ฯลฯสำหรับการขนส่งและการเก็บแอมโมเนียเหลวในเหล็กกล้า แผ่นหนาประกบผิวCEB / CECคือ2.000หรือมากกว่านั้น,โลหะฐานมีโครงสร้างจุลภาคของโลหะ ซึ่งในนั้นเศษส่วนปริมาตรของเบไนต์คือ90เปอร์เซ็นต์หรือมากกว่านั้นและขนาดเกรนเฉลี่ยของ เบไนต์คือ25ไมโครเมตรหรือน้อยกว่านั้น,โลหะประกบผิวมีความแข็งวิกเกอร์ส210HV10 หรือน้อยกว่านั้น,tC1คือ2.0มิลลิเมตรหรือมากกว่านั้น,tC2คือ0.0มิลลิเมตรหรือมากกว่านั้น,และ (tC1+tC2) / (tB+tC1+tC2)คือ0.30หรือน้อยกว่านั้น;

Description

Clad steel plate and its manufacturing method

[0001] The present invention relates to a clad steel plate and a manufacturing method thereof. The present invention also relates to a high-strength clad steel plate with excellent low-temperature toughness and ammonia SCC resistance, which is particularly suitable for use as structural members for tanks and the like used in low-temperature and liquid ammonia environments.

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

[0003] For example, it is known that ammonia SCC correlates with the strength and hardness of a material. That is, when using carbon steel, it is desirable to use a material with a tensile strength of less than 600 MPa. Therefore, when using high-strength carbon steel (hereinafter also referred to as high-strength steel) in a liquid ammonia environment, measures such as adjusting the tensile strength by performing post-weld heat treatment using full annealing are necessary.

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

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

[0006] As technologies relating to steel materials used for the above applications, for example, Patent Documents 1 to 3 are disclosed. Of these, Patent Document 1 describes a method for softening the surface of a steel material. Furthermore, Patent Documents 2 and 3 describe methods for producing clad steel plates having a mild steel layer 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 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 in terms of strength.

[0009] In addition, in the methods described in Patent Documents 2 and 3, clads are manufactured by a casting method, an overlay welding method, or a continuous casting method. However, it cannot be said that the clad steel plates manufactured by the methods described in Patent Documents 2 and 3 simultaneously achieve excellent low-temperature toughness, excellent ammonia SCC resistance, and high strength. In addition, the methods described in Patent Documents 2 and 3 have an economic problem in that the costs for the equipment and energy required for manufacturing them are high.

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

[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 tensile properties (strength properties) of steel plates. As a result, they have obtained the following findings. That is, since ammonia SCC occurs inside a product (tank), ammonia SCC resistance is dominated by the properties of the surface layer of the steel plate, which is on the inside. Therefore, the present inventors conceived of a clad steel plate formed by bonding a base material and a clad material, i.e., a clad steel plate in which a steel plate having excellent strength and low-temperature toughness is used as the base material and a steel plate having a low hardness as the clad material from the viewpoint of improving ammonia SCC resistance. Then, the present inventors have found that such a clad steel plate has excellent ammonia SCC resistance, low-temperature toughness, and tensile properties.

[0012] In addition, the inventors have found that if the thickness of the clad steel plate cladding material is too thin, corrosion-induced wear occurs, deteriorating the ammonia SCC resistance of the clad steel plate, while if the thickness of the clad steel plate cladding material is too thick, the strength of the clad steel plate decreases.

[0013] The clad steel plate of the present invention can significantly reduce alloy costs and manufacturing costs compared to clad steel plates using stainless steel or non-ferrous alloys as cladding materials.

[0014] 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 base material and a cladding material of carbon steel on at least one side thereof, wherein the base material has a chemical composition, in mass %, of C: 0.010 to 0.200%, Si: 0.01 to 1.00%, Mn: 0.50 to 2.50%, Al: 0.001 to 0.060%, P: 0.0200% or less, and S: 0.0100% or less, with the balance being Fe and unavoidable impurities, and CE Ｂ / CE Ｃ is 2.000 or more, where CE Ｂ and C.E. Ｃare the carbon equivalents of the base material and the cladding material, respectively; in the metal structure of the base material, the volume fraction of bainite is 90% or more and the average grain size of the bainite is 25 μm or less; the Vickers hardness of the cladding material is 210 HV10 or less; t Ｃ１ is 2.0 mm or more, and t Ｃ２ is 0.0 mm or more, and (t Ｃ１ +t Ｃ２ ) / (t Ｂ +t Ｃ１ +t Ｃ２ ) is 0.30 or less, where t Ｂ is the thickness of the base material, and t Ｃ１ is the thickness of the cladding material on one side of the base material, and t Ｃ２ is the plate thickness of the clad material on the other side of the base material.

[0015] 2. The clad steel plate according to 1 above, wherein the composition of the base material further contains, in mass%, one or more elements selected from Cu: 1.00% or less, Ni: 2.00% or less, Cr: 1.00% or less, Mo: 1.00% or less, V: 0.500% or less, Ti: 0.100% or less, Nb: 0.100% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, and REM: 0.0200% or less.

[0016] 3. The clad steel sheet according to 1 or 2 above, wherein the cladding material has a chemical composition, in mass%, of C: 0.100% or less and Mn: 0.01 to 1.50%, and further contains one or more elements selected from Cu: 0.01 to 0.50%, Cr: 0.01 to 0.50%, Sb: 0.01 to 0.50%, and Sn: 0.01 to 0.50%, with the balance being Fe and unavoidable impurities, and wherein the CR value calculated by the following formula (1) is 0.30 or more: CR value = 2.3 [Cu] + 2.8 [Cr] + 7.3 [Sb] + 3.6 [Sn] (1), where [X] represents the content (mass%) of element X in the chemical composition of the cladding material.

[0017] 4. A method for manufacturing a clad steel plate having a carbon steel cladding material on at least one side of a base material, comprising: heating a clad steel plate obtained by stacking a base material steel plate having the composition of the base material described in 1 or 2 above and a carbon steel cladding material steel plate described in 1 above to 1000 to 1250°C; and then rolling the clad steel plate at a cumulative reduction rate of 20% or more in the non-recrystallization temperature range of the base material steel plate and at a rolling end temperature of Ar. ３ The hot-rolled steel sheet is subjected to hot rolling at a temperature equal to or higher than the transformation point to obtain a hot-rolled steel sheet, and then the hot-rolled steel sheet is cooled to a cooling start temperature of Ar. ３ A method for manufacturing a clad steel plate, comprising cooling to a transformation point or higher, an average cooling rate of 20 to 120°C / s, and a cooling stop temperature of 500°C or lower.

[0018] 5. The method for manufacturing a clad steel plate according to 4 above, wherein the chemical composition of the clad steel plate contains, in mass %, C: 0.100% or less and Mn: 0.01 to 1.50%, and further contains one or more elements selected from Cu: 0.01 to 0.50%, Cr: 0.01 to 0.50%, Sb: 0.01 to 0.50%, and Sn: 0.01 to 0.50%, with the balance being Fe and unavoidable impurities, and the CR value calculated by the following formula (1) is 0.30 or more: CR value = 2.3 [Cu] + 2.8 [Cr] + 7.3 [Sb] + 3.6 [Sn] (1) where [X] represents the content (mass %) of element X in the chemical composition of the clad steel plate.

[0019] 6. The method for producing a clad steel plate according to 4 or 5 above, wherein after the cooling, the hot-rolled steel plate is tempered in a temperature range of 650°C or less.

[0020] According to the present invention, a high-strength clad steel plate having excellent ammonia SCC resistance and low-temperature toughness can be obtained, which is suitable for use in tanks for transporting and storing liquid ammonia, etc. Furthermore, the clad steel plate of the present invention does not use stainless steel or non-ferrous alloys as cladding materials and the manufacturing process is simple, so it is also extremely advantageous in terms of cost.

[0021] A clad steel plate according to one embodiment of the present invention has a cladding material made of carbon steel on at least one surface of a base material. Here, the clad steel plate according to one embodiment of the present invention has excellent ammonia SCC resistance and low-temperature toughness, and is therefore suitable for structural members such as tanks used in a liquid ammonia environment. Note that the usage environment is not limited to liquid ammonia, and may also include LPG and liquefied CO2. ２ In the present disclosure, the term "ammonia" refers not only to liquid ammonia but also to LPG and liquefied CO ２ In addition to the above, the term "clad steel plate" refers to liquefied gases in general. In a clad steel plate according to one embodiment of the present invention, the side having the cladding material may be on either side of the base material. However, when used in a tank, the cladding material is placed on the side expected to come into contact with ammonia, etc. (hereinafter also referred to as the first surface or inner surface of the base material). This is because ammonia SCC resistance and low-temperature toughness are obtained as a tank (structure). In this disclosure, the cladding material placed on the first surface of the base material is also referred to as the "first cladding material." The surface opposite the first surface of the base material is also referred to as the "second surface" or outer surface of the base material. The cladding material placed on the second surface of the base material is also referred to as the "second cladding material." Here, the second cladding material is optional. That is, the clad steel plate according to one embodiment of the present invention includes a clad steel plate having a first cladding material on one surface of the base material and a clad steel plate having a first cladding material on one surface of the base material and a second cladding material on the other surface of the base material. In addition, the thickness of the base material is t Ｂ , the thickness of the first cladding material is t Ｃ１ , the thickness of the second cladding material is t Ｃ２ This can be done as follows.

[0022] A clad steel plate according to one embodiment of the present invention will be described in more detail below. Note that "%" representing the content of each element below means "mass %" unless otherwise specified. (1) Regarding the composition of the base metal: C: 0.010-0.200% C is the most effective element for increasing the strength of a clad steel plate according to one embodiment of the present invention. To achieve this effect, the C content is set to 0.010% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and achieving lower manufacturing costs, the C content is preferably set to 0.030% or more. On the other hand, a C content exceeding 0.200% leads to deterioration of toughness and weldability. Therefore, the C content is set to 0.200% or less. Furthermore, from the viewpoint of toughness, the C content is preferably set to 0.170% or less.

[0023] Si: 0.01 to 1.00% Si is added for deoxidation. To achieve this effect, the Si content is set to 0.01% or more. Furthermore, the Si content is preferably set to 0.03% 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 set to 1.00% or less. Furthermore, from the viewpoint of toughness, the Si content is preferably set to 0.40% or less.

[0024] Mn: 0.50 to 2.50% Mn is an element that has the effect of increasing the hardenability of steel. That is, Mn is one of the important elements for obtaining high strength. To obtain this effect, the Mn content is set to 0.50% or more. Furthermore, from the viewpoint of reducing the content of other alloy elements and manufacturing at lower cost, the Mn content is preferably set to 0.70% or more. On the other hand, if the Mn content exceeds 2.50%, toughness decreases. Therefore, the Mn content is set to 2.50% or less. Furthermore, from the viewpoint of suppressing the decrease in toughness, the Mn content is preferably set to 2.30% or less.

[0025] Al: 0.001 to 0.060% Al acts as a deoxidizer. To achieve this effect, the Al content is set to 0.001% or more. On the other hand, if the Al content exceeds 0.060%, oxide-based inclusions increase, reducing cleanliness. Furthermore, toughness decreases. Therefore, the Al content is set to 0.060% or less. Furthermore, from the viewpoint of suppressing a decrease in toughness, the Al content is preferably set to 0.050% or less.

[0026] P: 0.0200% or less P is an element contained as an unavoidable impurity. Furthermore, P segregates at grain boundaries, causing adverse effects such as reducing toughness and weldability. Therefore, it is desirable to reduce the P content as much as possible. However, a P content of 0.0200% or less is acceptable. The lower limit of the P content is not particularly limited and may be 0%. Furthermore, since P is typically an element that is unavoidably contained in steel as an impurity, the P content may be industrially greater than 0%. Furthermore, excessive reduction of P leads to increased refining costs. Therefore, the P content is preferably 0.0005% or more.

[0027] S: 0.0100% or less S is an element contained as an unavoidable impurity. Furthermore, S exists in steel as sulfide-based inclusions such as MnS, and has adverse effects such as becoming the origin of fracture and reducing toughness. Therefore, it is desirable to reduce the S content as much as possible. However, an S content of 0.0100% or less is acceptable. The lower limit of the S content is not particularly limited and may be 0%. Furthermore, since S is usually an element that is inevitably contained in steel as an impurity, the S content may exceed 0% industrially. Furthermore, excessive reduction of S leads to increased refining costs. Therefore, from a cost perspective, it is preferable to set the S content to 0.0005% or more.

[0028] The base metal of the clad steel plate according to one embodiment of the present invention may optionally contain the following elements (hereinafter also referred to as optional additional elements): The remainder of the base metal of the clad steel plate according to one embodiment of the present invention, other than the above elements and the following optional additional elements, is Fe and unavoidable impurities.

[0029] Cu: 1.00% or less Cu is an element effective in improving the strength of clad steel plates. However, if the Cu content is less than 0.01%, this effect is poor. Therefore, when Cu is contained, the Cu content is preferably 0.01% or more. On the other hand, if the Cu content exceeds 1.00%, the toughness deteriorates. Therefore, when Cu is contained, the Cu content is preferably 1.00% or less.

[0030] Ni: 2.00% or less Ni is not only effective in improving the strength of clad steel plates, but also in improving toughness. However, if the Ni content is less than 0.01%, this effect is poor. Therefore, when Ni is contained, the Ni content is preferably 0.01% or more. On the other hand, if the Ni content exceeds 2.00%, the effect saturates and the alloy cost increases. Therefore, when Ni is contained, the Ni content is preferably 2.00% or less.

[0031] Cr: 1.00% or less Cr is an element effective in improving the strength of clad steel plates. However, if the Cr content is less than 0.01%, this effect is poor. Therefore, when Cr is contained, the Cr content is preferably 0.01% or more. On the other hand, if the Cr content exceeds 1.00%, toughness deteriorates. Therefore, when Cr is contained, the Cr content is preferably 1.00% or less.

[0032] Mo: 1.00% or less Mo is an element effective in improving the strength of clad steel plates. However, if the Mo content is less than 0.01%, this effect is poor. Therefore, when Mo is contained, the Mo content is preferably 0.01% or more. On the other hand, if the Mo content exceeds 1.00%, the toughness deteriorates. Therefore, when Mo is contained, the Mo content is preferably 1.00% or less.

[0033] V: 0.500% or less V is an element that has the effect of improving the strength of clad steel plate. To obtain this effect, when V is contained, the V content is preferably 0.005% or more. On the other hand, if the V content exceeds 0.500%, it will lead to deterioration of weldability and an increase in alloy costs. Therefore, when V is contained, the V content is preferably 0.500% or less. More preferably, the lower limit of the V content is 0.010%. More preferably, the upper limit of the V content is 0.100%.

[0034] Ti: 0.100% or less Ti is an element that has a strong tendency to form nitrides and has the effect of fixing N and reducing solute N. Therefore, adding Ti can improve the toughness of the base metal and weld. To achieve this effect, when Ti is contained, the Ti content is preferably 0.005% or more. Furthermore, the Ti content is more preferably 0.007% or more. On the other hand, if the Ti content exceeds 0.100%, the toughness actually decreases. Therefore, when Ti is contained, the Ti content is preferably 0.100% or less. Furthermore, the Ti content is more preferably 0.090% or less.

[0035] Nb: 0.100% or less Nb is an element that has the effect of reducing the prior austenite grain size and improving toughness by precipitating as carbonitrides. In order to obtain this effect, when Nb is contained, the Nb content is preferably 0.005% or more. Furthermore, the Nb content is more preferably 0.007% or more. On the other hand, when the Nb content exceeds 0.100%, a large amount of NbC precipitates, reducing toughness. Therefore, when Nb is contained, the Nb content is preferably 0.100% or less. Furthermore, the Nb content is more preferably 0.060% or less.

[0036] Ca: 0.0200% or less Ca is an element that bonds with S and suppresses the formation of MnS and other elements that elongate in the rolling direction. That is, by including Ca, the morphology of sulfide-based inclusions is controlled to be spherical, thereby improving the toughness of welds and other structures. When Ca is included to achieve this effect, the Ca content is preferably 0.0005% or more. On the other hand, if the Ca content exceeds 0.0200%, the cleanliness of the steel decreases. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Ca is included, the Ca content is preferably 0.0200% or less. The Ca content is more preferably 0.0020% or more. The Ca content is more preferably 0.0100% or less.

[0037] Mg: 0.0200% or less Like Ca, Mg is an element that bonds with S and suppresses the formation of MnS and other compounds that elongate in the rolling direction. That is, by including Mg, 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 Mg is included, the Mg content is preferably 0.0005% or more. On the other hand, if the Mg content exceeds 0.0200%, the cleanliness of the steel decreases. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Mg is included, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0020% or more. The Mg content is more preferably 0.0100% or less.

[0038] REM: 0.0200% or less Like Ca and Mg, REM (rare earth metal) is an element that bonds with S and suppresses the formation of MnS and other elements that elongate in the rolling direction. In other words, by including REM, the morphology of sulfide-based inclusions is controlled to be spherical, thereby improving the toughness of welds and other structures. To achieve this effect, if REM is included, the REM content is preferably 0.0005% or more. On the other hand, if the REM content exceeds 0.0200%, the cleanliness of the steel decreases. A decrease in cleanliness leads to a decrease in toughness. Therefore, if REM is included, the REM content is preferably 0.0200% or less. The REM content is more preferably 0.0020% or more. The REM content is more preferably 0.0100% or less.

[0039] (2) Composition of cladding material In the clad steel plate according to one embodiment of the present invention, the cladding material contains CE Ｂ / CE Ｃ Carbon steel having a CE of 2.000 or more is used. Carbon steel is an alloy of iron (Fe) and carbon (C), and has a composition in which the C content is 2.2 mass% or less, the total content of elements other than C and Fe (such as Si, Mn, P, and S, and also including elements corresponding to unavoidable impurities) is 5.0 mass% or less, and the balance is Fe. Ｂ / CE Ｃ :2.000 or more CE Ｂ / CE Ｃ is set to 2.000 or more. Here, CE Ｂ and C.E. Ｃ are the carbon equivalents of the base material and the cladding material, respectively. Ｂ / CE Ｃ If CE is less than 2.000, the carbon equivalent of the cladding material becomes too large, causing the hardness of the clad steel plate to increase too much, resulting in a deterioration in ammonia SCC resistance. Ｂ / CE Ｃ is preferably 2.050 or more, more preferably 2.100 or more. Ｂ / CE Ｃ There is no particular upper limit for CE. Ｂ / CE Ｃis preferably 5,000 or less. Ｂ and C.E. Ｃ can be calculated using the following formulas: Ｂ = [C] Ｂ + [Mn] Ｂ / 6+[Si] Ｂ / 24+[Ni] Ｂ / 40+[Cr] Ｂ / 5+[Mo] Ｂ / 4 + [V] Ｂ / 14 where [X] Ｂ indicates the content (mass%) of element X in the composition of the base material. Elements that are not contained may be calculated as "0". The same applies to the following formulas. Ｃ = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 Here, [X] indicates the content (mass%) of element X in the chemical composition of the cladding material.

[0040] The preferred chemical composition of the clad steel plate of the clad steel plate according to one embodiment of the present invention is as follows: C: 0.100% or less C is an element that increases the hardness of steel, and the higher the hardness, the higher the liquid ammonia SCC susceptibility. Therefore, the C content of the clad steel is preferably 0.100% or less. On the other hand, the lower the C content of the clad steel, the better, but excessive reduction of C leads to an increase in refining costs. Therefore, the C content is preferably 0.0005% or more.

[0041] Mn: 0.01 to 1.50% Mn is an element that has the effect of increasing the hardenability of steel. Therefore, if the Mn content is too high, the hardness of the cladding material will increase too much. If the hardness of the cladding material increases too much, it will lead to a deterioration in ammonia SCC resistance. Therefore, it is preferable to set the Mn content to 1.50% or less. The Mn content is more preferably 1.25% or less, and even more preferably 1.00% or less. On the other hand, reducing Mn to less than 0.01% requires a great deal of cost. Therefore, it is preferable to set the Mn content to 0.01% or more.

[0042] Cu, Cr, Sb, and Sn can further improve the ammonia SCC resistance of the steel sheet. Therefore, it is preferable to contain one or more of these elements in the amounts described below and to set the CR value calculated by the following formula (1) to 0.30 or more. The CR value is a formula devised to estimate ammonia SCC resistance from the content of each element, and the higher the CR value, the better the ammonia SCC resistance. Therefore, by setting the CR value to 0.30 or more, it is possible to effectively suppress stress corrosion cracking in a liquid ammonia environment. CR value = 2.3 [Cu] + 2.8 [Cr] + 7.3 [Sb] + 3.6 [Sn] ... (1) Here, [X] represents the content (mass%) of element X in the composition of the cladding material. Note that the composition of the cladding material and the composition of the cladding material steel sheet are substantially the same, so [X] can also be said to represent the content (mass%) of element X in the composition of the cladding material steel sheet. Similarly, the above-mentioned [X] Ｂ It can also be said that the content (mass %) of element X in the chemical composition of the base material steel sheet.

[0043] Here, by using a low-hardness steel plate as the cladding material, it is possible to suppress ammonia SCC. However, if there are dents or scratches on the surface of the cladding material, stress concentration occurs at the locations of the dents or scratches, which may result in a deterioration of ammonia SCC resistance. In this regard, by setting the CR value to 0.30 or more, it is possible to prevent the deterioration of ammonia SCC resistance even when there are dents or scratches on the surface of the cladding material. Therefore, the CR value is preferably 0.30 or more. The CR value is more preferably 0.32 or more, and even more preferably 0.35 or more. Furthermore, there is no particular limitation on the upper limit of the CR value. For example, the CR value is preferably 1.50 or less.

[0044] Furthermore, Cu, Cr, Sb, and Sn have the effect of quickly forming protective corrosion products in a liquid ammonia environment and suppressing stress corrosion cracking. To achieve this effect, when Cu is contained, the Cu content is preferably 0.01% or more. When Cr is contained, the Cr content is preferably 0.01% or more. When Sb is contained, the Sb content is preferably 0.01% or more. When Sn is contained, the Sn content is preferably 0.01% or more.

[0045] On the other hand, excessive addition of Cu, Cr, Sb, and Sn deteriorates weldability and toughness. It is also disadvantageous from the viewpoint of alloy cost. Therefore, when Cu is contained, the Cu content is preferably 0.50% or less. When Cr is contained, the Cr content is preferably 0.50% or less. Furthermore, when Sb is contained, the Sb content is preferably 0.50% or less. When Sn is contained, the Sn content is preferably 0.50% or less. More preferably, the Cu content is 0.40% or less, the Cr content is 0.40% or less, the Sb content is 0.40% or less, and the Sn content is 0.40% or less.

[0046] In a preferred composition of the clad steel plate clad material according to one embodiment of the present invention, the balance other than the above elements is Fe and unavoidable impurities.

[0047] (3) Metallographic Structure [Base Metallographic Structure: Bainite Volume Fraction of 90% or More, and Bainite Average Grain Size of 25 μm or Less] In order to satisfy the tensile properties and low-temperature toughness of the base metal, the base metal must have a bainite volume fraction of 90% or more. In other words, if the bainite volume fraction is less than 90%, the volume fractions of other components, such as ferrite, island martensite, martensite, pearlite, and austenite, will increase, making it impossible to obtain sufficient strength and low-temperature toughness. The upper limit of the bainite volume fraction is not particularly limited, and may be 100%.

[0048] Here, bainite includes structures called bainitic ferrite and granular ferrite, as well as structures obtained by tempering these. Here, the structures called bainitic ferrite and granular ferrite are structures that are formed during or after cooling after hot rolling, which contribute to transformation strengthening. The remaining structure, which accounts for 10% or less in volume fraction, may contain martensite in addition to ferrite, pearlite, and austenite. The volume fraction of each structure in the remaining structure does not need to be particularly limited, but it is preferable that the remaining structure be pearlite. The volume fraction of the remaining structure may be 0%.

[0049] Furthermore, in order to obtain excellent low-temperature toughness, the average grain size of bainite is set to 25 μm or less. That is, if the average grain size of bainite exceeds 25 μm, crack propagation resistance decreases, and sufficient low-temperature toughness cannot be obtained. The average grain size of bainite is preferably 22 μm or less. Furthermore, there is no particular lower limit for the average grain size of bainite. For example, the average grain size of bainite is preferably 1 μm or more. Here, the volume fraction and average grain size of bainite can be measured by the methods described in the examples below.

[0050] The metal structure of the cladding material is not particularly limited, and may be a conventionally known metal structure of carbon steel, such as ferrite and bainite.

[0051] [Vickers hardness of cladding material: 210 HV10 or less] The Vickers hardness of the cladding material is 210 HV10 or less. If a high hardness region exists in the surface layer of the clad steel plate, ammonia SCC is promoted. In other words, if the Vickers hardness of the cladding material exceeds 210 HV10, the desired ammonia SCC resistance cannot be obtained. The Vickers hardness of the cladding material is preferably 200 HV10 or less. The lower limit of the Vickers hardness of the cladding material is not particularly limited. The Vickers hardness of the cladding material is preferably 100 HV10 or more, for example. The Vickers hardness can be measured by the method described in the Examples below. The Vickers hardness of the base material is not particularly limited and may be 210 HV10 or less or may exceed 210 HV10.

[0052] (4) Plate thickness of base material and cladding material [t Ｃ１ is 2.0 mm or more, t Ｃ２ is 0.0 mm or more, and (t Ｃ１ +t Ｃ２ ) / (t Ｂ +t Ｃ１ +t Ｃ２ ) is 0.30 or less] t Ｃ１ 2.0 mm or more, t Ｃ２ is 0.0 mm or more, and (t Ｃ１ +t Ｃ２ ) / (t Ｂ +t Ｃ１ +t Ｃ２ ) (hereinafter also referred to as "cladding ratio") is set to 0.30 or less. Ｂ (mm) is the thickness of the base material, and t Ｃ１ (mm) is the thickness of the cladding material on one side of the base material, that is, the thickness of the first cladding material, and t Ｃ２ (mm) is the thickness of the cladding material on the other side of the base material, that is, the thickness of the second cladding material. Ｃ１ If the thickness is less than 2.0 mm, the low hardness region is worn away by corrosion, and the ammonia SCC resistance deteriorates. Ｃ１ is preferably 2.1 mm or more, more preferably 2.2 mm or more. Ｃ１ is preferably 5.0 mm or less. If the clad ratio exceeds 0.30, the proportion of the low-hardness clad material relative to the base material that provides the strength becomes too large, and sufficient strength cannot be obtained. The clad ratio is preferably 0.20 or less. The clad ratio is preferably 0.05 or more. Note that the second clad material may not be necessary (in other words, the second clad material is optional), so t Ｃ２ may be 0.0 mm or more. Ｃ２ is preferably 0.5 mm or more, more preferably 1.0 mm or more. Ｃ２ The total thickness of the clad steel plate, usually t Ｂ +t Ｃ１ +t Ｃ２ The total thickness of the clad steel plate is preferably 50 mm or less, more preferably 40 mm or less.

[0053] (5) Manufacturing Method Next, a method for manufacturing a clad steel plate according to one embodiment of the present invention will be described. First, for example, a base material steel plate having the above-mentioned base material composition and the above-mentioned (CE Ｂ / CE Ｃ A clad material steel plate is prepared by overlapping a base material steel plate made of a carbon steel (whose composition is 2.000 or more) with a clad material steel plate having the aforementioned clad material composition. The method for preparing the base material steel plate and the clad material steel plate is not particularly limited. For example, the base material steel plate and the clad material steel plate can be prepared using a conventionally known manufacturing method. Specifically, molten steel adjusted to a predetermined composition by a conventional melting method (such as a converter or electric furnace) is cast by a conventional casting method (such as a continuous casting method or an ingot casting method) to prepare a slab material. The obtained slab material is then hot-rolled or the like to prepare a base material steel plate. A clad material steel plate is then overlapped on at least one surface of the base material steel plate, particularly the surface expected to come into contact with ammonia or the like, to prepare a two-layer clad material steel plate. Alternatively, a three-layer clad steel plate (a clad steel plate in the order clad steel plate / base steel plate / clad steel plate) is prepared by overlapping clad steel plates on both sides of a base steel plate.

[0054] The clad steel sheet is then pressure welded and heat treated under predetermined conditions to control the structure. That is, the clad steel sheet is heated to 1000 to 1250°C, and then the clad steel sheet is rolled at a cumulative reduction rate of 20% or more in the non-recrystallization temperature range of the base steel sheet, and at a rolling end temperature of Ar. ３ The hot-rolled steel sheet is subjected to hot rolling at a temperature equal to or higher than the transformation point to obtain a hot-rolled steel sheet, and then the hot-rolled steel sheet is cooled to a cooling start temperature of Ar. ３Cooling is performed to a temperature above the transformation point, with an average cooling rate of 20 to 120°C / s and a cooling stop temperature of 500°C or less. This allows the clad steel plate according to the embodiment of the present invention described above to be manufactured. After the cooling, the hot-rolled steel plate may be tempered in a temperature range of 650°C or less. The temperatures under each manufacturing condition are all temperatures at a position half the plate thickness of the base material or base material steel plate. The temperature at this position may also be measured directly. The temperature at this position may also be determined by performing a differential calculation using, for example, a process computer from the surface temperature of the clad steel plate or clad material steel plate measured with a radiation thermometer.

[0055] [Heating temperature: 1000 to 1250°C] If the heating temperature of the clad material steel plate (hereinafter also referred to as the heating temperature) is less than 1000°C, the solid solution of carbides is insufficient and the required strength cannot be obtained. In addition, from the viewpoint of the bondability between the base material steel plate and the cladding material steel plate, a high heating temperature is preferable. Therefore, the heating temperature is set to 1000°C or higher. On the other hand, if the heating temperature exceeds 1250°C, the crystal grains of the base material will become coarse, leading to a deterioration in toughness. Therefore, the heating temperature is set to 1250°C or lower.

[0056] [Cumulative reduction in the non-recrystallization temperature range of the base material steel plate: 20% or more] By setting the cumulative reduction in the non-recrystallization temperature range of the base material steel plate (hereinafter also simply referred to as cumulative reduction) to 20% or more, deformation bands that become nucleation sites are introduced into the austenite grains of the base material steel plate. This refines the bainite that is transformed and generated during cooling after hot rolling, improving the toughness of the clad steel plate. Therefore, the cumulative reduction is set to 20% or more. The cumulative reduction is preferably 30% or more, more preferably 40% or more. In addition, the cumulative reduction is preferably 85% or less, more preferably 80% or less. Here, the non-recrystallization temperature range of the base material steel plate is a temperature range below Tnr (°C). In addition, Tnr (°C) can be calculated using the following formula: Tnr (°C) = 174 × log([Nb] Ｂ × ([C] Ｂ +12 / 14[N] Ｂ )) + 1444 where [X] Ｂindicates the content (mass %) of element X in the chemical composition of the base material steel plate. Log is common logarithm. The cumulative rolling reduction can be calculated by the following formula: [Cumulative rolling reduction (%)] = [Total thickness reduction (mm) of base material steel plate in the non-recrystallization temperature range of the base material steel plate] ÷ [Thickness (mm) of base material steel plate in the clad material steel plate before the start of hot rolling] × 100. Whether each pass of hot rolling is in the non-recrystallization temperature range of the base material steel plate (in other words, whether the thickness reduction of the base material steel plate in each pass of hot rolling is included in the total thickness reduction of the base material steel plate in the non-recrystallization temperature range of the base material steel plate) is determined by the outlet temperature of each pass. The reason why the cumulative rolling reduction is based on the thickness of the base material steel plate among the clad material steel plates is that the toughness of the clad steel plate is particularly significantly affected by the toughness of the base material.

[0057] [Rolling end temperature: Ar ３ Transformation point or higher] If the rolling end temperature of the hot rolling is lower than the Ar3 transformation point, the generated ferrite will be affected by the working, and the toughness will deteriorate. Therefore, the rolling end temperature is set to be equal to or higher than the Ar3 transformation point. There is no particular upper limit to the rolling end temperature. For example, the rolling end temperature is preferably equal to or lower than (Ar3 transformation point + 90°C). Ar ３ The transformation point can be calculated by the following formula: Ar ３ Transformation point (℃) = 910-310 [C] Ｂ -80 [Mn] Ｂ -20[Cu] Ｂ -15 [Cr] Ｂ -55 [Ni] Ｂ -80 [Mo] Ｂ Here, [X] Ｂ indicates the content (mass %) of element X in the chemical composition of the base steel sheet.

[0058] [Cooling start temperature: Ar ３Transformation point or higher] The hot-rolled steel sheet obtained after hot rolling is cooled from a temperature equal to or higher than the Ar3 transformation point. If the cooling start temperature is lower than the Ar3 transformation point, excessive ferrite is generated. The generated ferrite coexists with bainite and martensite, which have a large difference in strength from ferrite. As a result, insufficient strength and deterioration of toughness are caused. Therefore, the cooling start temperature after hot rolling is set to be equal to or higher than the Ar3 transformation point. There is no particular upper limit to the cooling start temperature. For example, the cooling start temperature may be set to (Ar ３ The temperature is preferably the transformation point +70°C or lower.

[0059] [Average Cooling Rate: 20 to 120°C / s] By setting the average cooling rate to 20°C / s or more, a high-strength, high-toughness clad steel plate can be obtained. In particular, cooling at a fast rate can increase the strength through transformation strengthening. That is, an average cooling rate of less than 20°C / s results in a large bainite grain size. Furthermore, ferrite and pearlite may form, potentially resulting in insufficient strength and a decrease in toughness. On the other hand, an average cooling rate of more than 120°C / s results in an excessively high volume fraction of martensite, resulting in a decrease in toughness. Therefore, the average cooling rate is set to 20°C / s or more and 120°C / s or less. The average cooling rate here refers to the average cooling rate from the cooling start temperature to the cooling stop temperature, and is based on the temperature at the half-thickness position of the base material. For example, the temperatures at the half-thickness position of the base material at the start and end of cooling can be calculated using a process computer based on the surface temperatures at the start and end of cooling measured with a radiation thermometer. The average cooling rate can be calculated using the following formula: [Average cooling rate (°C / s)] = ([Temperature of base material at 1 / 2 the thickness at the start of cooling (°C)] - [Temperature of base material at 1 / 2 the thickness at the end of cooling (°C)]) / [Cooling time (s)]

[0060] [Cooling stop temperature: 500°C or less] By setting the cooling stop temperature to 500°C or less, it is possible to achieve a predetermined volume fraction of bainite in the metal structure of the base material. If the cooling stop temperature exceeds 500°C, ferrite and pearlite are excessively formed, resulting in insufficient strength and deterioration of toughness. Therefore, the cooling stop temperature is set to 500°C or less. On the other hand, there is no particular restriction on the lower limit of the cooling stop temperature. The cooling stop temperature may be, for example, room temperature, but is preferably 150°C or higher from the viewpoint of production efficiency, etc.

[0061] [Tempering temperature: 650°C or less] Tempering can be performed optionally for the purpose of restoring the toughness of the base material. Here, if the tempering temperature, i.e., the temperature of the clad steel plate during reheating by tempering (the temperature at 1 / 2 the plate thickness position of the base material) exceeds 650°C, dislocations may be restored, and the strength of the base material may decrease. Therefore, when tempering is performed, the tempering temperature is set to 650°C or less. On the other hand, the lower limit of the tempering temperature is preferably 350°C from the viewpoint of restoring the toughness of the base material.

[0062] As described above, a clad steel plate according to one embodiment of the present invention can be manufactured. The clad steel plate according to one embodiment of the present invention thus obtained has excellent tensile properties and toughness. Here, excellent tensile properties mean a yield strength YS (yield point YP when there is a yield point, or 0.2% proof stress σ0.2 when there is no yield point) of 490 MPa or more and a tensile strength (TS) of 610 MPa or more, as measured by a tensile test in accordance with JIS Z 2241 (2022). Furthermore, excellent toughness means a fracture appearance transition temperature (hereinafter also referred to as vTrs) of -30°C or less, as measured by a Charpy impact test in accordance with JIS Z 2242 (2018). Details are as described in the examples below.

[0063] The conditions other than those mentioned above are not particularly limited, and may be those according to conventional methods.

[0064] [Example 1] Table 1 shows the composition of the base material (the balance being Fe and unavoidable impurities). In the table, steel types A to P are suitable steels that satisfy the composition of the base material of the clad steel plate according to one embodiment of the present invention. On the other hand, steel types Q to X are comparative steels that fall outside the range of the composition of the base material of the clad steel plate according to one embodiment of the present invention. A base material steel plate having the composition shown in Table 1 and a CE shown in Table 2 were used. Ｃ Clad steel plates (Nos. 1 to 34) were prepared by overlapping a clad steel plate having a base material having a thickness of 1000 mm with a clad steel plate having a thickness of 1000 mm, and clad steel plates (Nos. 1 to 34) were manufactured under the conditions shown in Table 2. For the obtained clad steel plates, the volume fraction of bainite in the metal structure of the base material and the average grain size of bainite were measured, the Vickers hardness of the clad steel plate was measured, and the tensile properties and toughness of the clad steel plate were evaluated, as well as the ammonia SCC resistance in a liquid ammonia environment was evaluated. The test methods were as follows.

[0065] [Measurement of the volume fraction of bainite in the metallographic structure of the base material] A sample was taken so that the center of the plate thickness (half the plate thickness position) of the base material of the clad steel plate was the observation surface. The taken sample was then mirror-polished and further subjected to nital etching. Next, a 10 mm × 10 mm area of ​​the sample was photographed using a scanning electron microscope (SEM) at magnifications of 500 to 3000 times. The photographed image was analyzed using an image analyzer to determine the volume fraction of bainite in the metallographic structure of the base material. Note that when the anisotropy of the metallographic structure of the base material is small, the area fraction corresponds to the volume fraction, and therefore the area fraction was considered to be the volume fraction here.

[0066] [Measurement of the Average Grain Size of Bainite in the Metallographic Structure of the Base Material] The same sample as used to measure the volume fraction of bainite in the metallographic structure of the base material was used to measure the average grain size of bainite. First, the surface of the sample was mirror-polished. Next, an Electron Backscattering Pattern (EBSP) device attached to an SEM was used to measure the crystal orientation from an electron backscatter diffraction image. Specifically, the crystal orientation was measured at 0.3 μm intervals within a 200 μm square region of the sample. Next, a region surrounded by grain boundaries where the crystal orientation difference with adjacent grains was 15° or more was defined as one crystal grain, and for crystal grains determined to be bainite, the circle-equivalent diameter of the crystal grain was calculated from the area of ​​the crystal grain. The average circle-equivalent diameter of the crystal grains determined to be bainite was then used as the average grain size of bainite. Among the crystal grains, crystal grains containing elongated, lath-shaped ferrite were determined to be bainite.

[0067] [Measurement of Vickers hardness of clad material] A sample was taken from the clad steel plate so that the cross section perpendicular to the rolling direction, i.e., the so-called T-section, served as the measurement surface. The sample was then mirror-polished. Next, in accordance with JIS Z 2244 (2020), the Vickers hardness (HV10, measurement load: 10 kgf) was measured at 20 points at 1 mm intervals in the direction perpendicular to the rolling direction (direction perpendicular to the rolling direction and the plate thickness direction) at a position halfway through the plate thickness of the clad material. The average value of these values ​​was taken as the Vickers hardness of the clad material. For example, if the Vickers hardness measured under the condition of a measurement load of 10 kgf is 210, it is usually expressed as 210HV10.

[0068] [Tensile properties] Test pieces of JIS Z 2241 (2022) No. 1B were taken from the clad steel plate so that the direction perpendicular to the rolling direction was the longitudinal direction, and tensile tests were performed according to the procedures described in JIS Z 2241 (2022) to measure the yield strength YS (yield point YP when there is a yield point, and 0.2% proof stress σ0.2 when there is no yield point) and tensile strength (TS). Then, those with a yield strength of 490 MPa or more and a tensile strength of 610 MPa or more were evaluated as having excellent tensile properties. The initial strain rate was 1 × 10 ―３ / s.

[0069] [Toughness] A V-notch test piece according to JIS Z 2242 (2018) was taken from the base material of the clad steel plate so that the rolling direction was the longitudinal direction, and a Charpy impact test was performed according to the procedure of JIS Z 2242 (2018) to measure vTrs. Then, a test piece with a vTrs of −30° C. or less was evaluated as having excellent toughness.

[0070] [Ammonia SCC Resistance] Ammonia SCC resistance was evaluated by a four-point bending anodic electrolysis test according to the following procedure. A 15 mm × 115 mm test specimen with a thickness of 5 mm was taken from each clad steel plate, with the first surface (inner surface) of the clad steel plate serving as the evaluation surface. The test specimens were then subjected to ultrasonic degreasing in acetone for 5 minutes. A stress equivalent to the yield strength of each test specimen was then applied to each test specimen by four-point bending. Each test specimen was then placed in a test cell while still under stress. The test cell was then filled with a test solution containing 12.5 g of ammonium carbamate and 1 L of liquid ammonia. A potentiostat was used to control the potential difference with the reference electrode to +2.0 V vs. Pt, and constant-potential anodic electrolysis was performed. The temperature of the test atmosphere was set to room temperature (25°C). The specimens were then maintained in this state for 720 hours from the start of immersion (current application). The test pieces were then visually inspected after the test, and if no cracks were found in the test pieces, they were judged to have excellent ammonia SCC resistance (passed). On the other hand, if cracks were found in the test pieces, they were judged to be defective. The evaluation results are also shown in Table 2.

[0071]

[0072]

[0073] As can be seen from Table 2, all of the invention examples have a yield strength YS of 490 MPa or more and a tensile strength of 610 MPa or more. Furthermore, all of the invention examples have a vTrs of -30°C or less. Furthermore, all of the invention examples have excellent ammonia SCC resistance. That is, all of the invention examples have excellent ammonia SCC resistance and low-temperature toughness, as well as high strength.

[0074] In contrast, No. 17, which is a comparative example, is CE Ｂ / CEＣ The Vickers hardness of the cladding material is outside the appropriate range. Ｃ１ and clad ratio are outside the appropriate range. In addition, in Nos. 20 to 26, some of the manufacturing conditions are outside the appropriate range, so the desired base metal structure is not obtained. As a result, these comparative examples are inferior in any of yield strength YS, tensile strength TS, low-temperature toughness, and ammonia SCC resistance.

[0075] In addition, Nos. 27 to 34 have a part of the composition of the base metal outside the appropriate range, and therefore are inferior in any one of yield strength YS, tensile strength TS, low-temperature toughness, and ammonia SCC resistance.

[0076] Example 2 A clad material steel plate was prepared by overlapping a base material steel plate having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) with a clad material steel plate having the composition shown in Table 3 (the balance being Fe and unavoidable impurities), and clad steel plates (Nos. 1 to 17) were manufactured under the conditions shown in Table 4. For the obtained clad steel plates, the volume fraction and average grain size of bainite in the metal structure of the base material were measured, the Vickers hardness of the clad material was measured, and the tensile properties and toughness were evaluated in the same manner as in Example 1. In addition, ammonia SCC resistance in a liquid ammonia environment was evaluated in the following manner.

[0077] [Ammonia SCC Resistance] As in Example 1, two test specimens measuring 15 mm x 115 mm and 5 mm thick were taken from each clad steel plate, with the first surface (inner surface) of the clad steel plate serving as the test surface. A notch with a depth of 0.3 mm and a diameter of 0.2 mm was formed on the test surface of one of the test specimens, assuming the presence of a dent on the surface (first surface) of the clad steel plate. Hereinafter, the unnotched test specimen will be referred to as the first test specimen, and the notched test specimen will be referred to as the second test specimen. Next, a four-point bending anodic electrolysis test was performed using the first and second test specimens in the same manner as in Example 1. The test specimens were then visually inspected after the test, and the ammonia SCC resistance was evaluated according to the following criteria: Excellent (passed, particularly excellent): No cracks were observed in either the first or second test specimen. Pass (Excellent): No cracks observed on the first test piece, but cracks observed on the second test piece. Poor: Cracks observed on both the first and second test pieces.

[0078]

[0079]

[0080] As can be seen from Table 4, all of the inventive examples have a yield strength YS of 490 MPa or more and a tensile strength of 610 MPa or more. Furthermore, all of the inventive examples have a vTrs of -30°C or less. Furthermore, all of the inventive examples have excellent ammonia SCC resistance. That is, all of the inventive examples have excellent ammonia SCC resistance and low-temperature toughness, as well as high strength. In particular, when the CR value of the cladding material was 0.30 or more, they had particularly excellent ammonia SCC resistance.