Clad steel plate and producing method thereof

Abstract

To provide a high strength clad steel plate excellent in stress corrosion cracking resistance and low temperature toughness and a producing method thereof.SOLUTION: A clad steel plate comprises a base material and a laminating material formed on the base material, wherein the laminating material has a specific composition, and when the laminating material thickness exceeds 2 mm, the Vickers hardness at a position 1 mm deep from the surface not joined to the base material is 210 or less; when the laminating material thickness is 2 mm or less, the Vickers hardness at the halfway position through the thickness of the laminating material is 210 or less; the maximum value of the highest hardness test evaluated according to JIS Z 3101:1990 is 210 or less in Vickers hardness; the base material has a specific composition, a yield strength of 620 MPa or more, a tensile strength of 700 MPa or more, and an absorbed energy of 47 J or more in a Charpy impact test conducted at -40°C; the clad steel plate has a yield strength of 620 MPa or more and a tensile strength of 700 MPa or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a clad steel plate formed by rolling two types of steel plates with different chemical compositions together, and in particular to a clad steel plate having excellent stress corrosion cracking resistance, excellent low-temperature toughness, and high strength, and a method for producing the same. [Background technology]

[0002] The use of ammonia is expanding in response to carbon neutrality. Traditionally, ammonia was mainly used as a fertilizer raw material, but its use as a hydrogen carrier or directly as a fuel is on the rise. As the use of ammonia expands, the tanks used to store it are also becoming larger, with progress being made in the design and construction of large tanks up to around 100,000 tons, up from the conventional 10,000-20,000 ton class. In order to increase the size of the tanks, it is necessary to increase the thickness of the steel plates used or to increase the strength of the steel plates, and in particular, it is hoped that higher strength will be promoted in order to reduce the weight of the steel.

[0003] Ammonia liquefies when cooled to -35°C or below or under high pressure, and is usually stored in a liquefied state in tanks. However, liquefied ammonia is known to corrode steel and cause stress corrosion cracking, and the higher the strength of the steel, the greater the susceptibility to this stress corrosion cracking. This makes it difficult to increase the strength of the steel plates used for the aforementioned liquefied ammonia tanks, and 400 MPa grade (JIS G3126:2022 SLA325A, etc.) is mainly used.

[0004] Therefore, studies have been conducted on steel plates that are resistant to stress corrosion cracking caused by liquefied ammonia. For example, Patent Document 1 discloses a method for producing steel plates that have a strength of 400 MPa or more, low-temperature toughness at -60°C, and excellent toughness in the heat-affected zone of high-heat-input welds, by reducing the amount of C in the steel and optimally controlling the amounts of Ti and N. Patent Document 2 discloses a method for producing a steel plate having a strength of 530 MPa or more while realizing a low yield ratio by reducing the amount of C in the steel and applying accelerated cooling. Furthermore, Patent Document 3 discloses a method for producing a steel sheet having a strength of 570 MPa or more while suppressing an increase in hardness in the surface layer of the steel sheet by reducing the amount of C in the steel and applying accelerated cooling. In addition, in Patent Document 4, a decarburized layer is formed on the surface of a steel sheet, and the resulting strength is 590 MPa or more (60 kgf / mm 2 The present invention discloses a method for manufacturing a steel sheet that has a strength of 1000 MPa or more and also ensures excellent stress corrosion cracking resistance. Furthermore, Patent Document 5 discloses a method for manufacturing a steel sheet that ensures excellent stress corrosion cracking resistance by subjecting the surface of the steel sheet to a softening treatment to reduce the surface hardness to 190 or less in Vickers hardness. Patent Documents 6 and 7 disclose methods for manufacturing steel sheets that ensure stress corrosion cracking resistance by optimally controlling the ferrite structure fraction and crystal grain size of the steel sheets and controlling the yield strength to 440 MPa or less. Patent Document 8 discloses a method for manufacturing a steel plate that ensures stress corrosion cracking resistance by optimizing the ferrite structure fraction in the center of the steel plate thickness while suppressing the hardness of the surface layer of the steel plate to 210 or less. Patent Document 9 discloses a method for manufacturing a steel plate that ensures stress corrosion cracking resistance by optimizing the morphology of the surface layer of the steel plate and optimizing the ferrite structure fraction in the center of the steel plate thickness. Patent Document 10 discloses a method for manufacturing a steel plate suitable for a tank for transporting liquefied ammonia, in which the yield ratio of the steel plate is reduced by minimizing the structure of ferrite and the like at the quarter thickness position of the steel plate. Furthermore, Patent Document 11 discloses a method for producing a steel plate that ensures excellent stress corrosion cracking resistance by rapidly cooling the steel plate in two stages during quenching or direct quenching to generate a soft ferrite structure. Furthermore, Patent Document 12 discloses a method for manufacturing a clad steel plate that ensures excellent stress corrosion cracking resistance by joining a clad material having a low carbon and manganese content to the surface of the steel plate by cast cladding or overlay welding. In Patent Document 13, the yield strength of the weld heat affected zone is 440 MPa or less (45 kgf / mm 2 The present invention discloses a method for manufacturing a clad steel plate that ensures excellent stress corrosion cracking resistance by joining clad steel sheets (hereinafter referred to as "clad steel sheets") together. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 11-131178 [Patent Document 2] Japanese Patent Application Publication No. 10-195533 [Patent Document 3] Japanese Patent Application Laid-Open No. 2001-115233 [Patent Document 4] Japanese Patent Application Publication No. 61-279631 [Patent Document 5] Japanese Patent Application Publication No. 50-085516 [Patent Document 6] Patent No. 7323090 [Patent Document 7] Patent No. 7323091 [Patent Document 8] Patent No. 7323088 [Patent Document 9] International Publication No. 2021 / 106368 [Patent Document 10] Patent Publication No. 2021-88753 [Patent Document 11] Japanese Patent Publication No. 156228 / 1983 [Patent Document 12] Japanese Patent Application Publication No. 57-149425 [Patent Document 13] Japanese Patent Application Publication No. 50-085546 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the techniques described in Patent Documents 1 to 4 are aimed at strengths up to 590 MPa, which is insufficient for manufacturing large tanks, which are in demand these days. Furthermore, Patent Document 5 does not take into consideration the low temperature toughness of the base material at all. The techniques described in Patent Documents 6 to 10 are based on the premise that the steel sheet contains a soft ferrite structure, and are therefore unable to produce the high-strength steel sheet for large tanks that is the subject of the present invention. The technique described in Patent Document 11 uses two-stage cooling, which requires special water cooling equipment. The technique described in Patent Document 12 requires cast cladding or overlay welding, which requires special equipment and has problems in productivity. The technology described in Patent Document 13 can suppress stress corrosion cracking in the weld heat affected zone. However, when rolling and heat treatment conditions are selected to increase the strength of the clad base material in order to manufacture a large tank, the hardness of the clad material before welding increases, and stress corrosion cracking performance cannot be suppressed outside the weld heat affected zone.

[0007] Furthermore, as a countermeasure against stress corrosion cracking, stainless steel plates or clad steel plates made of stainless steel plates can be used, but this has the problem of being significantly more expensive than using low-alloy steel plates.

[0008] As described above, none of the methods proposed so far has disclosed a manufacturing method that can ensure excellent stress corrosion cracking resistance in high-strength steel plates exceeding 590 MPa, which are used for large tanks, without using special equipment. Furthermore, no method other than Patent Document 1 has been disclosed for ensuring the toughness at temperatures of −35° C. or below, which is necessary when storing liquefied ammonia.

[0009] In view of the above-mentioned problems of the conventional technology, the present invention aims to provide a steel plate having a high strength exceeding 590 MPa for use in large tanks and having excellent stress corrosion cracking resistance and low temperature toughness, specifically, to provide a clad steel plate having a yield strength of 620 MPa or more, a tensile strength of 700 MPa or more, and having excellent stress corrosion cracking resistance and low temperature toughness, and a method for manufacturing the same.

[0010] Here, the yield strength and tensile strength are the yield strength and tensile strength of the clad steel plate and the base material of the clad steel plate measured in accordance with JIS Z 2241:2022. In addition, excellent stress corrosion cracking resistance means that the Vickers hardness of the clad steel plate's clad material is 210 or less at the plate thickness cross section at a depth of 1 mm from the clad material surface, measured in accordance with JIS Z 2244:2009, and the maximum value of the maximum hardness test evaluated in JIS Z 3101:1990 is 210 or less in Vickers hardness. In addition, excellent low-temperature toughness refers to a base material having an absorbed energy of 47J or more when a Charpy impact test is conducted at -40°C in accordance with JIS Z 2242:2018. [Means for solving the problem]

[0011] The present inventors have conducted extensive research to solve the above problems, and have come to the following conclusions: First, they investigated the conditions for suppressing liquefied ammonia stress corrosion cracking in low-alloy steel, and found that it is necessary to set the Vickers hardness of the steel plate on the surface that comes into contact with liquefied ammonia to 210 or less. Furthermore, it was found that the Vickers hardness of the weld heat affected zone after welding must also be kept at 210 or less. It has been said that it is appropriate to carry out the maximum hardness test specified in JIS Z 3101:1990 in order to evaluate the hardness of the weld heat affected zone of steel plates. On the other hand, it is said that keeping the yield strength of steel below 440 MPa is also effective in suppressing liquefied ammonia stress corrosion cracking, but it has been found that cracks will occur if the Vickers hardness of the steel plate on the surface that comes into contact with liquefied ammonia exceeds 210.

[0012] Next, we considered ways to achieve a yield strength of 620 MPa or more and a tensile strength of 700 MPa or more across the entire thickness of the steel plate while satisfying the above conditions.As a result, we found that even if hot rolling and heat treatment were performed on a slab with a normal single component composition, it would be difficult to achieve both the desired surface hardness (the hardness of the surface that comes into contact with liquefied ammonia and the hardness of the weld heat-affected zone when welding is performed) and strength across the entire thickness. Therefore, as a result of further investigation, it was found that it is possible to achieve both the desired surface hardness and strength throughout the entire thickness by using a clad steel plate that uses a high-strength steel plate as the base material and combines it with a clad material that has a low content of elements and can maintain low hardness even after hot rolling and heat treatment to ensure the strength of the base material. Furthermore, it was found that in order to ensure productivity, it is more effective to apply rolled clad, which has better production efficiency than cast clad or overlay welding.

[0013] The present invention was completed through further investigation 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 clad material formed on the base material, The cladding material is, in mass%, C: 0.01 to 0.08%, Si: 0.01 to 0.60%, Mn: 0.10 to 1.60% P:0.030% or less, S: 0.030% or less, Al: 0.001 to 0.060% Contains And Pcm represented by the following formula (1) is 0.14 or less, The balance is Fe and unavoidable impurities, and If the thickness of the cladding material is more than 2 mm, the Vickers hardness at a position 1 mm deep from the surface where the cladding material is not joined to the base material is 210 or less. When the thickness of the cladding material is 2 mm or less, the Vickers hardness at the 1 / 2 thickness position in the thickness direction of the cladding material is 210 or less, The maximum value of the highest hardness test evaluated by JIS Z 3101:1990 is 210 or less in Vickers hardness, The base material is, in mass%, C: 0.05~0.15%, Si: 0.01 to 0.60%, Mn: 1.00~2.20%, P: 0.020% or less, S: 0.005% or less, Al: 0.001 to 0.060% Contains And Pcm represented by the following formula (1) is 0.20 or more and 0.32 or less, The balance is Fe and unavoidable impurities, and The base material is The yield strength is 620 MPa or more, The tensile strength is 700 MPa or more, Charpy impact absorption energy at -40°C is 47J or more, The clad steel plate is The yield strength is 620 MPa or more, Clad steel plate with a tensile strength of 700 MPa or more. Here, Pcm is defined by the following equation (1). Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B...(1) In formula (1), the element symbols indicate the content (mass%) of each element in the composition of the cladding material, and indicate the content (mass%) of each element in the composition of the base material, with elements that are not contained being represented as 0. [2] The component composition of the cladding material is further, in mass%, Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.30% or less, Mo: 0.30% or less Nb: 0.05% or less, V: 0.05% or less, Ti: 0.05% or less, Ca: 0.005% or less, Mg: 0.005% or less The clad steel plate according to [1] above, containing one or more selected from the following: [3] The composition of the base material is, in mass%, Cu: 1.00% or less, Ni: 2.50% or less, Cr: 1.00% or less, Mo: 1.00% or less Nb: 0.10% or less, V: 0.10% or less, Ti: 0.10% or less, B: 0.0050% or less, Ca: 0.005% or less, Mg: 0.005% or less The clad steel plate according to [1] or [2] above, containing one or more selected from the following: [4] A method for producing a clad steel plate according to any one of [1] to [3], The slab, which is formed by overlapping and welding the base material and cladding material, is heated to a temperature of 1000°C to 1200°C. Next, hot rolling is performed at a finishing temperature of 700°C to 1000°C, a first treatment of accelerated cooling from 650°C to 400°C at an average cooling rate of 10°C / s or more to a cooling stop temperature of 200°C to 400°C; A method for manufacturing clad steel plate, which comprises either air-cooling to room temperature, then heating to 850°C or higher and 1050°C or lower, and a second treatment of accelerated cooling from 650°C to 400°C at an average cooling rate of 10°C / s or higher to a cooling stop temperature of 20°C or higher and 400°C or lower. [5] The method for producing a clad steel plate according to [4], wherein after the first treatment or the second treatment, tempering is performed at 500°C or higher and 680°C or lower. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a high-strength clad steel plate having excellent stress corrosion cracking resistance and low-temperature toughness, which is suitable for large tanks for storing liquefied ammonia and the like. DETAILED DESCRIPTION OF THE INVENTION

[0015] The clad steel plate of the present invention will be specifically described below. The clad steel plate of the present invention has a base material and a cladding material formed on the base material, and the base material and the cladding material have the following chemical compositions. Specifically, the clad steel sheet of the present invention has a cladding material containing, by mass%, C: 0.01 to 0.08%, Si: 0.01 to 0.60%, Mn: 0.10 to 1.60%, P: 0.030% or less, S: 0.030% or less, and Al: 0.001 to 0.060%, and the Pcm represented by the following formula (1) is 0.14 or less, with the balance being Fe and unavoidable impurities. The base material has a composition containing, in mass%, C: 0.05 to 0.15%, Si: 0.01 to 0.60%, Mn: 1.00 to 2.20%, P: 0.020% or less, S: 0.005% or less, and Al: 0.001 to 0.060%, and has a component composition in which Pcm, represented by the following formula (1), is 0.20 or more and 0.32 or less, with the remainder being Fe and unavoidable impurities. Here, Pcm is defined by the following equation (1). Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B...(1) In formula (1), the element symbols indicate the content (mass%) of each element in the composition of the cladding material, and indicate the content (mass%) of each element in the composition of the base material, with elements that are not contained being represented as 0. First, the reasons for limiting the composition of the steel (base material and cladding material) in the present invention will be explained for each component. Note that "%" regarding the composition means "mass %" unless otherwise specified.

[0016] [Component composition of cladding material] C: 0.01 to 0.08% C is the element that has the greatest effect on hardness. That is, if the C content exceeds 0.08%, the hardness increases and the resistance to liquefied ammonia stress corrosion cracking deteriorates, so the C content is set to 0.08% or less. Preferably, the C content is 0.06% or less. On the other hand, if the C content is less than 0.01%, the steelmaking cost will increase significantly, so the C content is set to 0.01% or more, and preferably, the C content is 0.02% or more.

[0017] Si: 0.01 to 0.60% Since Si is an element that is inevitably contained in pig iron and a large amount thereof reduces weldability, the Si content is set to 0.60% or less, and preferably 0.45% or less. On the other hand, reducing the Si content to less than 0.01% would require significant steelmaking costs, so a Si content of 0.01% or more is acceptable. Therefore, the Si content is set to 0.01% or more. Preferably, the Si content is 0.05% or more.

[0018] Mn: 0.10 to 1.60% Mn is an element that is inevitably contained in pig iron, and if it is contained in large amounts, it reduces weldability and causes the hardness of the weld heat affected zone to exceed the desired value after welding. Therefore, the Mn content is set to 1.60% or less, and preferably, the Mn content is set to 1.40% or less. On the other hand, reducing the Mn content to less than 0.10% would require significant steelmaking costs, so a Mn content of 0.10% or more is allowed. Therefore, the Mn content is set to 0.10% or more. Preferably, the Mn content is 0.50% or more.

[0019] P:0.030% or less P is an element that has a large embrittlement effect, and if contained in large amounts, it reduces the ductility and toughness of steel. Therefore, the P content is set to 0.030% or less. It is further preferable that the P content be set to 0.025% or less. On the other hand, since the less P the better, there is no particular lower limit for the P content. However, since an excessively low P content increases the refining time and costs, the P content is preferably 0.001% or more.

[0020] S: 0.030% or less S is an element that has a large embrittlement effect, and if contained in large amounts, it reduces the ductility and toughness of steel. Therefore, the S content is set to 0.030% or less. Furthermore, the S content is preferably set to 0.015% or less. On the other hand, since the less S there is, the better, there is no particular lower limit for the S content. However, since excessively low S content increases the refining time and costs, the S content is preferably 0.0001% or more.

[0021] Al: 0.001 to 0.060% Al is effective as a deoxidizer. To obtain this effect, the Al content must be 0.001% or more. Preferably, the Al content is 0.005% or more. On the other hand, if the Al content exceeds 0.060%, the cleanliness of the steel material or steel plate decreases, resulting in a decrease in ductility and toughness.The Al content is preferably 0.055% or less.

[0022] Pcm: 0.14 or less Pcm is an index for evaluating the limit of occurrence of weld cold cracking, and is highly correlated with the hardness of the weld heat affected zone. If Pcm exceeds 0.14, the Vickers hardness of the weld heat affected zone cannot be controlled to 210 or less, so the upper limit of Pcm is set to 0.14. Preferably, the upper limit of Pcm is set to 0.13. There is no particular lower limit, but to ensure strength through the entire thickness, Pcm is preferably 0.05 or more. More preferably, Pcm is 0.06 or more.

[0023] The clad steel plate cladding material of the present invention has the basic composition as described above, but may further contain one or more elements selected from the group consisting of Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.30% or less, Mo: 0.30% or less, Nb: 0.05% or less, V: 0.05% or less, Ti: 0.05% or less, Ca: 0.005% or less, and Mg: 0.005% or less, as necessary, in order to ensure tensile strength throughout the entire thickness and improve weldability.

[0024] Cu:0.30% or less Cu is an element that improves strength, and in order to obtain this effect, the Cu content is preferably 0.05% or more. If the Cu content exceeds 0.30%, the hardness of the weld heat affected zone increases and liquid ammonia stress corrosion cracking becomes more likely to occur, so the upper limit is set to 0.30%. Therefore, if Cu is contained, the Cu content is set to 0.30% or less. Preferably, the Cu content is 0.10% or less.

[0025] Ni: 0.30% or less Ni is an element that improves strength, and in order to obtain this effect, the Ni content is preferably 0.05% or more. If the Ni content exceeds 0.30%, the hardness of the weld heat affected zone increases and liquid ammonia stress corrosion cracking becomes more likely to occur, so the upper limit is set to 0.30%. Therefore, if Ni is contained, the Ni content is set to 0.30% or less. Preferably, the Ni content is 0.10% or less.

[0026] Cr:0.30% or less Cr is an element that improves strength, and in order to obtain this effect, the Cr content is preferably set to 0.05% or more. If the Cr content exceeds 0.30%, the hardness of the weld heat affected zone increases and liquid ammonia stress corrosion cracking becomes more likely to occur, so the upper limit is set to 0.30%. Therefore, if Cr is contained, the Cr content is set to 0.30% or less. Preferably, the Cr content is 0.20% or less.

[0027] Mo: 0.30% or less Mo is an element that improves strength, and in order to obtain this effect, the Mo content is preferably 0.05% or more. If the Mo content exceeds 0.30%, the hardness of the weld heat affected zone increases and liquid ammonia stress corrosion cracking becomes more likely to occur, so the upper limit is set to 0.30%. Therefore, if Mo is contained, the Mo content is set to 0.30% or less. Preferably, the Mo content is 0.15% or less.

[0028] Nb: 0.05% or less Nb is an element that improves strength, and in order to obtain this effect, the Nb content is preferably 0.01% or more. If the Nb content exceeds 0.05%, the hardness of the weld heat affected zone increases and liquid ammonia stress corrosion cracking becomes more likely to occur, so the upper limit is set to 0.05%. Therefore, if Nb is contained, the Nb content is set to 0.05% or less. Preferably, the Nb content is 0.03% or less.

[0029] V:0.05% or less V is an element that improves strength, and in order to obtain this effect, the V content is preferably 0.01% or more. On the other hand, if the V content exceeds 0.05%, the hardness of the weld heat affected zone increases and liquid ammonia stress corrosion cracking becomes more likely to occur, so the upper limit is set to 0.05%. Therefore, when V is contained, the V content is set to 0.05% or less. Preferably, the V content is 0.03% or less.

[0030] Ti: 0.05% or less Ti is an element that reduces surface defects in Nb-added steel sheets, and to obtain this effect, the Ti content is preferably 0.01% or more. If the Ti content exceeds 0.05%, TiC precipitates, increasing the hardness of the weld heat affected zone and making it more susceptible to liquefied ammonia stress corrosion cracking, so the upper limit is set to 0.05%. Therefore, if Ti is contained, the Ti content is set to 0.05% or less. Preferably, the Ti content is 0.03% or less.

[0031] Ca: 0.005% or less Ca is an element that suppresses the formation of MnS and improves the internal quality of the steel sheet, and in order to obtain this effect, the Ca content is preferably 0.001% or more. If the Ca content exceeds 0.005%, Ca clusters and deteriorates the internal quality of the steel sheet, so the upper limit is set to 0.005%. Therefore, if Ca is contained, the Ca content is set to 0.005% or less. Preferably, the Ca content is 0.003% or less.

[0032] Mg: 0.005% or less Mg is an element that suppresses the formation of MnS and improves the internal quality of the steel sheet, and in order to obtain this effect, the Mg content is preferably 0.001% or more. If the Mg content exceeds 0.005%, Mg clusters and deteriorates the internal quality of the steel sheet, so the upper limit is set to 0.005%. Therefore, if Mg is contained, the Mg content is set to 0.005% or less. Preferably, the Mg content is 0.003% or less.

[0033] The cladding material used in the clad steel plate of the present invention has the above-mentioned composition, with the balance being Fe and unavoidable impurities.

[0034] [Base material composition] C: 0.05 to 0.15% C is the element that has the greatest effect on the strength and toughness of the base material. That is, if the C content exceeds 0.15%, the base material toughness deteriorates, so the C content is set to 0.15% or less. Preferably, the C content is 0.14% or less. On the other hand, if the C content is less than 0.05%, the desired base metal strength cannot be achieved, so the C content is set to 0.05% or more, and preferably 0.06% or more.

[0035] Si: 0.01 to 0.60% Since Si is an element that is inevitably contained in pig iron and a large amount thereof reduces the toughness and weldability of the base material, the Si content is set to 0.60% or less, and preferably 0.55% or less. On the other hand, reducing the Si content to less than 0.01% would require significant steelmaking costs, so a Si content of 0.01% or more is acceptable. Therefore, the Si content is set to 0.01% or more. Preferably, the Si content is 0.05% or more.

[0036] Mn: 1.00 to 2.20% Mn is an element that is inevitably contained in pig iron, and if contained in large amounts, it reduces the toughness and weldability of the base material, so the Mn content is set to 2.20% or less, and preferably 2.00% or less. On the other hand, if the Mn content is less than 1.00%, the desired base metal strength cannot be obtained, so the lower limit of the Mn content is set to 1.00%.The Mn content is preferably 1.10% or more.

[0037] P:0.020% or less P is an element that has a large embrittlement effect, and if contained in large amounts, it reduces the ductility and toughness of steel. Therefore, the P content is set to 0.020% or less. Furthermore, the P content is preferably set to 0.015% or less. On the other hand, since the less P the better, there is no particular lower limit for the P content. However, since an excessively low P content increases the refining time and costs, the P content is preferably 0.001% or more.

[0038] S: 0.005% or less S is an element that has a large embrittlement effect, and if contained in large amounts, it reduces the ductility and toughness of steel. Therefore, the S content is set to 0.005% or less. Furthermore, the S content is preferably set to 0.003% or less. On the other hand, since the less S there is, the better, there is no particular lower limit for the S content. However, since excessively low S content increases the refining time and costs, the S content is preferably 0.0001% or more.

[0039] Al: 0.001 to 0.060% Al is effective as a deoxidizer. To obtain this effect, the Al content must be 0.001% or more. Therefore, the Al content is set to 0.001% or more. Preferably, the Al content is 0.005% or more. On the other hand, if the Al content exceeds 0.060%, the cleanliness of the steel material or steel plate decreases, resulting in decreased ductility and toughness. Therefore, the Al content is set to 0.060% or less. The Al content is preferably 0.055% or less.

[0040] Pcm: 0.20 or more and 0.32 or less Pcm is an index for evaluating the limit of weld cold cracking, and is highly correlated with the base material strength and toughness. If Pcm exceeds 0.32, the base material toughness and weldability deteriorate, so the upper limit of Pcm is set to 0.32. The preferred upper limit of Pcm is 0.28. On the other hand, if Pcm is less than 0.20, the desired strength cannot be obtained, so the lower limit of Pcm is set to 0.20, and Pcm is preferably 0.22 or more.

[0041] The base material of the clad steel plate of the present invention has the basic composition as described above, but in order to ensure tensile strength throughout the entire thickness and improve weldability, it may further contain one or more elements selected from the group consisting of Cu: 1.00% or less, Ni: 2.50% or less, Cr: 1.00% or less, Mo: 1.00% or less, Nb: 0.10% or less, V: 0.10% or less, Ti: 0.10% or less, B: 0.0050% or less, Ca: 0.005% or less, and Mg: 0.005% or less, as necessary.

[0042] Cu: 1.00% or less Cu is an element that improves strength. To obtain this effect, the Cu content is preferably 0.10% or more, and more preferably 0.15% or more. On the other hand, if the Cu content exceeds 1.00%, defects occur on the surface of the steel sheet, so when Cu is contained, the upper limit is set to 1.00%. Preferably, the Cu content is 0.30% or less.

[0043] Ni: 2.50% or less Ni is an element that improves strength and toughness. To obtain this effect, the Ni content is preferably 0.10% or more, and more preferably 0.15% or more. On the other hand, if the Ni content exceeds 2.50%, scratches occur on the surface of the steel sheet, so when Ni is contained, the upper limit of the Ni content is set to 2.50%, and preferably the Ni content is 0.30% or less.

[0044] Cr:1.00% or less Cr is an element that improves strength. To obtain this effect, the Cr content is preferably 0.10% or more, and more preferably 0.15% or more. On the other hand, if the Cr content exceeds 1.00%, the toughness and weldability of the base material decrease, so when Cr is contained, the upper limit of the Cr content is set to 1.00%, and preferably the Cr content is 0.50% or less.

[0045] Mo: 1.00% or less Mo is an element that improves strength. To obtain this effect, the Mo content is preferably 0.05% or more, and more preferably 0.15% or more. On the other hand, if the Mo content exceeds 1.00%, the toughness and weldability of the base material decrease, so when Mo is contained, the upper limit is set to 1.00%. Preferably, the Mo content is 0.50% or less.

[0046] Nb: 0.10% or less Nb is an element that improves strength and toughness, and in order to obtain this effect, the Nb content is preferably 0.01% or more. On the other hand, if the Nb content exceeds 0.10%, the base material toughness and weldability deteriorate, so when Nb is contained, the upper limit of the Nb content is set to 0.10%.Moreover, the Nb content is more preferably set to 0.06% or less.

[0047] V: 0.10% or less V is an element that improves strength. To obtain this effect, the V content is preferably 0.01% or more, and more preferably 0.02% or more. On the other hand, if the V content exceeds 0.10%, the base material toughness and weldability decrease, so when V is contained, the upper limit of the V content is set to 0.10%. The V content is preferably set to 0.05% or less.

[0048] Ti: 0.10% or less Ti is an element that improves surface defects and strength, and to obtain this effect, the Ti content is preferably 0.01% or more. On the other hand, if the Ti content exceeds 0.10%, TiC precipitates, reducing the toughness of the base material, so when Ti is contained, the upper limit of the Ti content is set to 0.10%, and the Ti content is preferably set to 0.03% or less.

[0049] B: 0.0050% or less B significantly improves the hardenability of the steel sheet and improves its strength and toughness. To obtain this effect, the B content is preferably 0.0005% or more. On the other hand, if the B content exceeds 0.0050%, BN precipitates, reducing the toughness of the base material, so when B is contained, the upper limit of the B content is set to 0.0050%. Furthermore, the B content is preferably set to 0.0020% or less.

[0050] Ca: 0.005% or less Ca is an element that suppresses the formation of MnS and improves the internal quality and weldability of the steel sheet. To obtain this effect, the Ca content is preferably 0.001% or more. On the other hand, if the Ca content exceeds 0.005%, Ca clusters and deteriorates the internal quality and weldability of the steel sheet, so when Ca is contained, the upper limit of the Ca content is set to 0.005%, and the Ca content is preferably set to 0.003% or less.

[0051] Mg: 0.005% or less Mg is an element that suppresses the formation of MnS and improves the internal quality and weldability of the steel sheet. To obtain this effect, the Mg content is preferably 0.001% or more. On the other hand, if the Mg content exceeds 0.005%, Mg clusters and deteriorates the internal quality and weldability of the steel sheet, so when Mg is contained, the upper limit of the Mg content is set to 0.005%.Moreover, the Mg content is preferably set to 0.003% or less.

[0052] The base material used in the clad steel plate of the present invention has the above-mentioned composition, with the balance being Fe and unavoidable impurities.

[0053] [Mechanical properties of cladding material] If the thickness of the cladding material is more than 2 mm, the Vickers hardness at a position 1 mm deep from the surface where the cladding material is not joined to the base material: 210 or less When the thickness of the cladding material is 2 mm or less, the Vickers hardness at the 1 / 2 thickness position in the thickness direction of the cladding material is 210 or less The liquefied ammonia stress corrosion cracking properties of cladding materials are examined, and when the cladding material thickness exceeds 2 mm, the Vickers hardness at a position 1 mm deep from the surface of the cladding material where it is not joined to the base material is to be kept to 210 or less. Furthermore, when the cladding material thickness is 2 mm or less, the Vickers hardness at a position 1 / 2 the thickness in the plate thickness direction of the cladding material is to be kept to 210 or less. These measures can prevent the occurrence of stress corrosion cracking. The above Vickers hardness is preferably 200 or less.

[0054] Maximum hardness test value evaluated by JIS Z 3101:1990: Vickers hardness 210 or less When the liquefied ammonia stress corrosion cracking properties of the weld heat affected zone of the cladding material were examined, it was found that the occurrence of stress corrosion cracking can be suppressed by suppressing the maximum hardness of the weld heat affected zone of the cladding material to 210 or less in Vickers hardness, and therefore the upper limit of the Vickers hardness is set to 210. The Vickers hardness is preferably 200 or less. Furthermore, when the maximum hardness of the weld heat affected zone generated by various types of welding applied to actual welded joints was compared with the maximum hardness evaluated by JIS Z 3101:1990, it was found that JIS Z 3101:1990 is stricter, so the maximum hardness of the weld heat affected zone of clad materials will be evaluated by JIS Z 3101:1990. Note that the maximum hardness evaluated in JIS Z 3101:1990 is determined only by the chemical composition of the steel, so the test is usually carried out on scrap material taken before the clad material is layered on the base material to create a slab. The test can also be carried out after clad rolling, but in that case, if the thickness of the clad material is thin and the weld metal reaches the base material, making it impossible to measure the hardness of the weld heat-affected zone at the bottom of the weld metal as per the standard, the hardness of the weld heat-affected zone at the center of the plate thickness of the clad material should be measured.

[0055] [Mechanical properties of base material] Yield strength: 620 MPa or more, tensile strength: 700 MPa or more The large-scale liquefied ammonia tank that is the primary target of this invention is assumed to be a PC tank in the 90,000 to 110,000 ton class. The strength required for this large-scale liquefied ammonia tank, when a tensile test is conducted on a cut-out base material portion alone, requires a yield strength of 620 MPa or more and a tensile strength of 700 MPa or more. Preferably, the yield strength is 665 MPa or more and the tensile strength is 760 MPa or more. More preferably, the yield strength is 680 MPa or more and the tensile strength is 780 MPa or more. In tank strength design, there are cases where the strength and thickness of the entire clad thickness are used, and cases where the strength and thickness of only the clad base material and only the base material are used. Therefore, in this invention, both requirements are satisfied.

[0056] Absorbed energy when conducting Charpy impact test at -40℃: 47J or more Ammonia liquefies at -35°C under normal pressure. Therefore, the low-temperature toughness of steel must be evaluated at temperatures lower than -35°C, and testing is conducted at -40°C. The test temperature is preferably -50°C, and even more preferably -55°C. The absorbed energy is 47J or greater, consistent with JIS G 3115:2022, the primary standard used for high-strength tanks, and is the average value obtained from three test specimens. The absorbed energy is preferably 47J average, with individual test values ​​of 27J or greater. While no special impact test method is specified, the same method as JIS G 3115:2022, the primary standard used for high-strength tanks, is typically used, with test specimens taken from 1 / 4 the thickness of the base material, perpendicular to the rolling direction. The 1 / 4 thickness of the base material is defined as the point originating from the surface opposite the surface joined to the clad material.

[0057] [Thickness of clad steel plate] Clad steel plate thickness: 8mm or more (optimal conditions) In the case of large liquefied ammonia tanks, which are the main target of this invention, that is, when clad steel plates are used for tanks, the plate thickness will not be less than 8 mm, so the lower limit of the plate thickness of clad steel plates when used for tanks is 8 mm. The plate thickness of clad steel plates is more preferably 9 mm or more. Furthermore, the plate thickness of clad steel plates is more preferably 60 mm or less. Although not particularly limited, the thickness of the cladding material is preferably 1 mm or more, and more preferably 5 mm or less. Although not particularly limited, the thickness of the base material is preferably 6 mm or more, and more preferably 60 mm or less.

[0058] [Mechanical properties of clad steel plates] Yield strength: 620 MPa or more, tensile strength: 700 MPa or more The large-scale liquefied ammonia tank that is the primary target of this invention is assumed to be a PC tank in the 90,000 to 110,000 ton class. The strength required for this large-scale liquefied ammonia tank, when a tensile test is conducted across the entire thickness of the clad steel plate, requires a yield strength of 620 MPa or more and a tensile strength of 700 MPa or more. Preferably, the yield strength is 665 MPa or more and the tensile strength is 760 MPa or more. More preferably, the yield strength is 680 MPa or more and the tensile strength is 780 MPa or more. In terms of tank strength design, there are cases where the strength and plate thickness of the entire clad thickness are used, and cases where the strength and plate thickness of only the clad base material are used. Therefore, in this invention, both are satisfied.

[0059] [Manufacturing method] Next, a method for producing a clad steel plate according to the present invention will be described. A slab is used that is integrated by overlapping and welding the cladding material and the base material. The hot rolling conditions for producing the base material and cladding material are not particularly specified. Two methods are used: sandwich cladding, which involves preparing two slabs of overlapping cladding material and base material, and applying a release agent between the two slabs to enable separation after hot rolling. Sacrificial cladding, which involves preparing a slab of overlapping cladding material and base material, and a steel plate (sacrificial material) that will not be used in the final product, and applying a release agent between the two slabs to enable separation after hot rolling, is also used. In sandwich cladding and sacrificial cladding, the width and length of the cladding material are slightly smaller than the base material. A steel plate called a spacer is inserted to fill the gap between the cladding material and the base material, and the spacer is then welded to the base material. Welding can be performed in either air or vacuum. However, if welding is performed in air, the base material and cladding material must be polished and then immediately overlapped and welded to prevent oxidation between them.

[0060] In the manufacturing method of clad steel plate of the present invention, a slab formed by overlapping and welding a base material having the aforementioned chemical composition and a cladding material having the aforementioned chemical composition is heated to a temperature of 1000°C or higher and 1200°C or lower, and subsequently hot-rolled at a finishing temperature of 700°C or higher and 1000°C or lower, followed by accelerated cooling from 650°C to 400°C at an average cooling rate of 10°C / s or higher to a cooling stop temperature of 200°C to 400°C or lower (a first process), or air-cooling to room temperature, followed by heating to 850°C to 1050°C or lower, and accelerated cooling from 650°C to 400°C at an average cooling rate of 10°C / s or higher in the plate thickness direction to a cooling stop temperature of 20°C to 400°C or lower (a second process). In the clad steel plate of the present invention, the cladding material is preferably formed on one side of the base material. Hereinafter, in the present invention, the temperature refers to the surface temperature of the slab or steel plate unless otherwise specified.

[0061] Reheating temperature: 1000℃ or higher and 1200℃ or lower The reheating temperature of the slab affects the toughness of the base material and the bondability of the clad interface. If the reheating temperature is below 1000°C, poor bonding occurs at the clad interface. In addition, the strength of the base material decreases. Therefore, the lower limit of the reheating temperature is set at 1000°C. On the other hand, if the reheating temperature exceeds 1200°C, the toughness of the base metal decreases, so the upper limit of the reheating temperature is set to 1200°C. The reheating temperature is preferably 1050 to 1200°C.

[0062] Hot rolling finishing temperature: 700℃ to 1000℃ The finishing temperature of hot rolling (hot rolling end temperature) affects the toughness of the base material and the bondability of the clad interface. If the finishing temperature is below 700°C, poor bonding occurs at the clad interface. In addition, the strength of the base material decreases. Therefore, the lower limit of the finishing temperature is set to 700°C. The finishing temperature is preferably 750°C or higher. If the finishing temperature exceeds 1000° C., the toughness of the base material decreases, so the upper limit of the finishing temperature is set to 1000° C. The finishing temperature is preferably 950° C. or lower.

[0063] After hot rolling, either (1) the first treatment, in which accelerated cooling is performed immediately after hot rolling, or (2) the second treatment, in which accelerated cooling is performed after hot rolling by air cooling to room temperature, reheating, and then accelerated cooling. The accelerated cooling is performed using water.

[0064] (1) First treatment Accelerated cooling rate: Average cooling rate from 650°C to 400°C (average cooling rate in the thickness direction): 10°C / s or more The cooling rate of the accelerated cooling carried out immediately after hot rolling affects the strength of the steel sheet. The cooling rate correlated with these performance characteristics is defined as the average cooling rate from 650°C to 400°C (average cooling rate in the plate thickness direction). The average cooling rate is determined by heat conduction calculations based on the relationship between the actual cooling start temperature (650°C) and cooling stop temperature (400°C) and the time between them. If the average cooling rate is below 10°C / s, the desired steel plate strength cannot be obtained, so the lower limit of the average cooling rate is set to 10°C / s. The average cooling rate is preferably 15°C / s or higher. Although there is no particular upper limit, the average cooling rate may be 100°C / s or less. In the first treatment, "accelerated cooling is carried out immediately after hot rolling" means that accelerated cooling is carried out without air-cooling to room temperature after hot rolling.

[0065] Accelerated cooling stop temperature: 200°C to 400°C The cooling stop temperature of accelerated cooling performed immediately after hot rolling affects the strength and toughness of the base material. The cooling stop temperature is the temperature at which the plate is reheated to a degree that makes the temperature distribution in the thickness direction uniform by air cooling after accelerated cooling. If the cooling stop temperature is below 200°C, the toughness of the base material cannot be ensured, so the lower limit of the cooling stop temperature is set to 200°C. The cooling stop temperature is preferably 220°C or higher. On the other hand, if the cooling stop temperature exceeds 400°C, the strength of the steel sheet cannot be ensured, so the upper limit of the cooling stop temperature is set to 400°C. The cooling stop temperature is preferably 380°C or lower. After accelerated cooling, the steel sheet may be air-cooled to room temperature or immediately tempered to the desired temperature.

[0066] (2) Secondary processing Heating temperature after cooling to room temperature: 850℃ to 1050℃ After hot rolling, the steel is cooled (air cooling: 0.01 to 1°C / s) to room temperature (-5 to 50°C), and the heating temperature of the subsequent heat treatment affects the strength and toughness of the base material. If the heating temperature is below 850°C, the desired base material strength cannot be obtained, so the lower limit of the heating temperature is set to 850°C. The heating temperature is preferably 880°C or higher. If the heating temperature exceeds 1050°C, the base material toughness cannot be obtained, so the upper limit of the heating temperature is set to 1050°C. The heating temperature is preferably 1000°C or lower.

[0067] Accelerated cooling rate: Average cooling rate from 650°C to 400°C: 10°C / s or more After the heating (reheating), accelerated cooling is immediately carried out, preferably within 300 seconds after heating. The cooling rate after air cooling and reheating following hot rolling affects the strength of the base material. The cooling rate that correlates with these performance characteristics is defined as the average cooling rate in the thickness direction from 650°C to 400°C. The average cooling rate is determined by heat conduction calculations based on the relationship between the actual cooling start temperature (650°C) and cooling stop temperature (400°C) and the time between them. If the average cooling rate is below 10°C / s, the desired base material strength cannot be obtained, so the lower limit of the average cooling rate is set to 10°C / s. The average cooling rate is preferably 15°C / s or higher. Although there is no particular upper limit, the average cooling rate may be 100°C / s or less.

[0068] Accelerated cooling stop temperature: 20℃ to 400℃ The cooling stop temperature after air cooling and reheating after hot rolling affects the strength and toughness of the base material. The cooling stop temperature is the temperature at which the plate is reheated to a level where the temperature distribution in the thickness direction becomes uniform through air cooling after accelerated cooling. If the cooling stop temperature is below 20°C, the toughness of the base material cannot be ensured, so the lower limit of the cooling stop temperature is set to 20°C. On the other hand, if the cooling stop temperature exceeds 400° C., the strength of the base material cannot be ensured, so the upper limit of the cooling stop temperature is set to 400° C. The cooling stop temperature is preferably 20 to 300° C. After accelerated cooling, the material may be air-cooled to room temperature, or immediately tempered to the desired temperature.

[0069] The steel sheet that has been subjected to accelerated cooling or reheating followed by cooling (the steel sheet that has been subjected to the first treatment or the second treatment) can be further subjected to tempering treatment.

[0070] Tempering temperature: 500℃ or higher and 680℃ or lower The tempering temperature affects the strength and toughness of the base material. If the tempering temperature is below 500°C, the desired base material strength may not be obtained, so the lower limit of the tempering temperature is set to 500°C. The tempering temperature is preferably 550°C or higher. If the tempering temperature exceeds 680°C, the base material toughness may not be obtained, so the upper limit of the tempering temperature is set to 680°C. The tempering temperature is preferably 650°C or lower. Tempering may be performed by reheating using a method such as induction heating immediately after cooling is stopped, or by air-cooling to room temperature after cooling is stopped and then reheating in a heating furnace, etc. In the case of induction heating, the surface of the steel sheet is heated rapidly, and the temperature at which the temperature becomes uniform in the thickness direction after heating is completed is the tempering temperature. [Example]

[0071] Next, the present invention will be described in more detail based on examples. The following examples are intended to illustrate preferred examples of the present invention, and the present invention is not limited to these examples in any way.

[0072] First, by continuous casting, slabs for cladding materials having the chemical compositions shown in Table 1 and slabs for base materials having the chemical compositions shown in Table 2 were produced. The blank spaces in Tables 1 and 2 indicate that the element is not contained or is contained as an unavoidable impurity.

[0073] [Table 1]

[0074] [Table 2]

[0075] Next, the obtained slabs for the base material and the cladding material were used to produce base material materials and cladding material materials of various thicknesses by hot rolling. These base material materials and cladding material materials were cut to the same length and width, and the surface scale was removed by polishing. The base material materials and cladding material materials thus obtained were stacked to be assembled using either the sacrificial clad or sand clad method. Then, the materials were placed in a vacuum chamber of 10 -4 ~10 -5 The interface between the base material and the cladding material was welded with an electron beam around the entire periphery in a Torr chamber to form an integrated slab. Note that in Table 3, when a sacrificial clad material was used, it is listed as "sacrificial material," and when a sand clad material was used, it is listed as "sand." The obtained slabs were then used to produce clad steel plates under the conditions shown in Table 3. Specifically, the produced slabs were reheated and hot-rolled, and then some of the steel plates were immediately subjected to accelerated cooling (first treatment: in Table 3, the timing of accelerated cooling is indicated as "immediately after hot-rolling"). In addition, some of the steel plates were air-cooled to room temperature (25°C), then reheated and subjected to accelerated cooling (second treatment: in Table 3, the timing of accelerated cooling is indicated as "after air-cooling to room temperature").

[0076] Some of the steel sheets were tempered after accelerated cooling (water cooling). In Table 3, the tempering method is indicated as "induction heating immediately after cooling was stopped" for steel sheets that were reheated by induction heating immediately after cooling was stopped, and as "air-cooled to room temperature, then furnace heating" for steel sheets that were reheated in a heating furnace after cooling was stopped.

[0077] [Table 3]

[0078] For the manufactured steel plates, the following material test pieces were taken from the clad material, the base material and the entire clad thickness and tested. The results are summarized in Table 4.

[0079] [Vickers hardness of cladding material] The test surface was measured from the center position in the longitudinal and width directions of the obtained clad steel plate as follows. <1> or <2> A sample was taken from the cross section perpendicular to the rolling direction so that the position was . The surface of the test piece was mirror-polished, and then the Vickers hardness was measured in accordance with JIS Z 2244:2009. A load of 10 kgf was used for the measurement, and the test was performed at five points, and the maximum value was taken as the Vickers hardness. In Table 4, this is shown as the Vickers hardness of the clad material. <1> If the cladding material thickness is over 2 mm, the position 1 mm deep from the surface where the cladding material is not joined to the base material <2> If the cladding material thickness is 2 mm or less, the position is 1 / 2 the thickness of the cladding material in the thickness direction

[0080] [Maximum hardness of cladding material] The maximum hardness test according to JIS Z 3101:1990 was conducted on scrap material taken before the laminated material was stacked on the base material to create a slab. The welding material used was LB26 manufactured by Kobe Steel, Ltd., and the test was conducted with a welding current of 170±10A and a welding speed of 150±10mm / s.

[0081] [Base material yield strength, tensile strength] Test pieces were taken from the center positions in the longitudinal and transverse directions of the obtained clad steel plate, and the cladding material was removed by milling, and JIS Z 2241:2022 No. 5 tensile test pieces were machined from only the base metal portion. The test pieces were taken perpendicular to the rolling direction. A tensile test was conducted on the taken test pieces to measure the yield strength and tensile strength. The upper yield point was taken as the yield strength, and if the upper yield point could not be obtained, the 0.2% proof stress was taken as the yield strength.

[0082] [Impact test of base material] Test pieces were taken from the center positions in the longitudinal and transverse directions of the obtained clad steel plate, and the cladding material was removed by milling. 2 mm V-notch Charpy impact test pieces according to JIS Z 2242:2018 were machined from only the base metal portion. The test pieces were taken perpendicular to the rolling direction and from a position at or close to 1 / 4 of the thickness in the plate thickness direction. 10 mm x 10 mm test pieces were used when the base metal thickness was 12 mm or more, and 10 mm x 7.5 mm test pieces were used when the base metal thickness was less than 12 mm. The test was conducted at -40°C, and the absorbed energy was measured.

[0083] [Yield strength and tensile strength of clad steel plate throughout its thickness] Test pieces were taken from the center positions in the longitudinal and transverse directions of the obtained clad steel plate and processed into No. 5 tensile test pieces according to JIS Z 2241:2022. The test pieces were taken perpendicular to the rolling direction. A tensile test was conducted on the taken test pieces to measure the yield strength and tensile strength. The upper yield point was taken as the yield strength, and if the upper yield point could not be obtained, the 0.2% proof stress was taken as the yield strength.

[0084] [Table 4]

[0085] It was found that the clad steel plates of the present invention had high strength, with both the base material and the clad steel plates having a yield strength of 620 MPa or more and a tensile strength of 700 MPa or more. Furthermore, when the clad steel plate of the present invention has a cladding thickness of more than 2 mm, the Vickers hardness at a position 1 mm deep from the surface of the cladding material where it is not joined to the base material is 210 or less. When the cladding material thickness is 2 mm or less, the Vickers hardness at a position 1 / 2 the thickness in the plate thickness direction of the cladding material is 210 or less. Furthermore, the maximum value of the maximum hardness test evaluated by JIS Z 3101:1990 is 210 or less in Vickers hardness. These findings demonstrate that the cladding steel plate has excellent stress corrosion cracking resistance. Furthermore, when the clad steel plate of the present invention was subjected to a Charpy impact test at -40°C, the absorbed energy of the base material was 47 J or more, demonstrating excellent low-temperature toughness. On the other hand, it was found that the clad steel plates of the comparative examples were inferior in at least one of strength, stress corrosion cracking resistance, and low-temperature toughness.

Claims

1. A clad steel plate having a base material and a clad material formed on the base material, The cladding material is, in mass %, C: 0.01-0.08%, Si: 0.01-0.60%, Mn: 0.10 to 1.60%, P: 0.030% or less, S: 0.030% or less, Al: 0.001-0.060% Contains and Pcm represented by the following formula (1) is 0.14 or less, The remainder is Fe and unavoidable impurities, and When the thickness of the cladding material is more than 2 mm, the Vickers hardness at a position 1 mm deep from the surface of the cladding material that is not joined to the base material is 210 or less, When the cladding material thickness is 2 mm or less, the Vickers hardness at the position of 1 / 2 thickness in the plate thickness direction of the cladding material is 210 or less, The maximum value of the maximum hardness test evaluated by JIS Z 3101:1990 is 210 or less in Vickers hardness, The base material comprises, in mass %, C: 0.05-0.15%, Si: 0.01-0.60%, Mn: 1.00-2.20%, P: 0.020% or less, S: 0.005% or less, Al: 0.001-0.060% Contains and Pcm represented by the following formula (1) is 0.20 or more and 0.32 or less, The remainder is Fe and unavoidable impurities, and The base material is The yield strength is 620 MPa or more, The tensile strength is 700 MPa or more, The Charpy impact absorption energy at -40°C is 47J or more, The clad steel plate is The yield strength is 620 MPa or more, A clad steel plate having a tensile strength of 700 MPa or more. Here, Pcm is defined by the following equation (1). Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B...(1) In formula (1), the element symbols indicate the content (mass%) of each element in the composition of the cladding material, and indicate the content (mass%) of each element in the composition of the base material, with elements that are not contained being represented as 0.

2. The component composition of the cladding material is further, in mass%, Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.30% or less, Mo: 0.30% or less, Nb: 0.05% or less, V: 0.05% or less, Ti: 0.05% or less, Ca: 0.005% or less, Mg: 0.005% or less The clad steel plate according to claim 1, comprising one or more selected from the following:

3. The composition of the base material is, in mass%, Cu: 1.00% or less, Ni: 2.50% or less, Cr: 1.00% or less, Mo: 1.00% or less, Nb: 0.10% or less, V: 0.10% or less, Ti: 0.10% or less, B: 0.0050% or less, Ca: 0.005% or less, Mg: 0.005% or less The clad steel plate according to claim 1, comprising one or more selected from the following:

4. The composition of the base material is, in mass%, Cu: 1.00% or less, Ni: 2.50% or less, Cr: 1.00% or less, Mo: 1.00% or less, Nb: 0.10% or less, V: 0.10% or less, Ti: 0.10% or less, B: 0.0050% or less, Ca: 0.005% or less, Mg: 0.005% or less The clad steel plate according to claim 2, comprising one or more selected from the following:

5. The method for producing a clad steel plate according to any one of claims 1 to 4, The slab, which is integrated by overlapping and welding the base material and the cladding material, is heated to a temperature of 1000°C or more and 1200°C or less; Subsequently, hot rolling is performed at a finishing temperature of 700°C or more and 1000°C or less, and then A first treatment in which the average cooling rate from 650°C to 400°C is 10°C / s or more and accelerated cooling is performed to a cooling stop temperature of 200°C to 400°C, and A method for manufacturing clad steel plate, which comprises either air-cooling to room temperature, then heating to 850°C or higher and 1050°C or lower, and a second treatment of accelerated cooling from 650°C to 400°C at an average cooling rate of 10°C / s or higher to a cooling stop temperature of 20°C to 400°C.

6. The method for producing a clad steel plate according to claim 5, wherein tempering is performed at 500°C or higher and 680°C or lower after the first treatment or the second treatment.

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