Cladded steel sheet and method for producing same

Abstract

Provided is a high-strength clad steel sheet which has excellent ammonia SCC resistance and low-temperature toughness, and which is suitable for tanks or the like for transporting and storing liquid ammonia. The coating material is characterized in that the coating material has a CEB / CEC of 2.000 or more, and in the metallographic structure of the base material, the volume fraction of bainite is 90% or more, the average particle diameter of bainite is 25 [mu] m or less, the Vickers hardness of the coating material is 210 HV10 or less, tC1 is 2.0 mm or more, tC2 is 0.0 mm or more, and (tC1 + tC2) / (tB + tC1 + tC2) is 0.30 or less.

Description

Technical Field

[0001] The present invention relates to a clad steel plate and a method for manufacturing the same. In addition, the present invention particularly relates to a high-strength clad steel plate having excellent low-temperature toughness and ammonia SCC resistance, which is suitable as a structural component such as a tank used in a low-temperature and liquid ammonia environment. Background Art

[0002] In a liquid ammonia environment, carbon steel is concerned about stress corrosion cracking caused by liquid ammonia (hereinafter referred to as "ammonia SCC" (Stress Corrosion Cracking)). Therefore, for structures such as carbon steel pipes, storage tanks, tank trucks, and line pipes for handling liquid ammonia, steels with low ammonia SCC sensitivity are applied, or operating measures to suppress ammonia SCC are adopted.

[0003] For example, it is known that ammonia SCC is related to the strength and hardness of materials. 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, it is necessary to implement post-weld heat treatment based on overall annealing to adjust the tensile strength and other countermeasures.

[0004] However, liquid ammonia does not produce CO even when burned. 2 . Therefore, in recent years, liquid ammonia has attracted attention as a clean energy source, and large-scale demand is expected. Along with this, the large-scale of equipment for transporting and storing liquid ammonia is required.

[0005] Generally, when making a tank larger, from the viewpoints of weight reduction and construction cost reduction, thinning of the steel used is required, so it is desirable to use high-strength steel.

[0006] In addition, for the efficient operation of transportation equipment and storage equipment, these equipment are sometimes used for both liquid ammonia and LPG. Liquefied gases such as liquid ammonia and LPG are transported and stored at low temperatures. Therefore, steels used for these purposes need to have excellent low-temperature toughness.

[0007] As technologies related to steels used for the above purposes, for example, Patent Documents 1 to 3 are disclosed. Among them, Patent Document 1 describes a method for softening the surface of a steel material. In addition, Patent Documents 2 and 3 describe methods for manufacturing a clad steel plate having a mild steel layer on one side.

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: Japanese Patent Publication No. 55-30062

[0011] Patent Document 2: Japanese Patent Laid-Open No. 57-139493

[0012] Patent Document 3: Japanese Patent Laid-Open No. 8-269537 Summary of the Invention

[0013] However, in the method described in the above Patent Document 1, in order to uniformly and sufficiently soften the surface layer, long-term heat treatment is required, and it is difficult to control the strength of the center of the steel plate. Therefore, there is a problem in terms of strength.

[0014] In addition, in the methods described in the above Patent Documents 2 and 3, the clad layer is manufactured by a casting method, a surfacing method, or a continuous casting method. However, in the clad steel plate manufactured by the methods described in Patent Documents 2 and 3, it cannot be said that excellent low-temperature toughness, excellent ammonia SCC resistance, and high strength are achieved simultaneously. In addition, in the methods described in the above Patent Documents 2 and 3, there is also an economic problem that the cost of the equipment and energy used for their manufacture is large.

[0015] An object of the present invention is to solve the above problems and provide a high-strength clad steel plate having excellent ammonia SCC resistance and low-temperature toughness suitable for use in tanks for transporting and storing liquid ammonia, and a method for manufacturing the same.

[0016] In order to achieve the above object, the present inventors have repeatedly conducted in-depth studies on various factors related to the ammonia SCC resistance, low-temperature toughness, and tensile properties (strength properties) of steel plates. As a result, the following insights have been obtained.

[0017] That is, since ammonia SCC occurs inside the product (tank), the ammonia SCC resistance mainly depends on the properties of the surface layer of the steel plate that becomes the inside. Therefore, the present inventors have come up with a clad steel plate formed by joining a base material and a clad material, that is, 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 is used as the clad material from the viewpoint of improving the ammonia SCC resistance.

[0018] Moreover, the present inventors have found that in this clad steel plate, the ammonia SCC resistance, low-temperature toughness, and tensile properties are all excellent.

[0019] In addition, the present inventors have found that when the plate thickness of the clad material of the clad steel plate is too thin, wear caused by corrosion occurs, and the ammonia SCC resistance of the clad steel plate deteriorates. On the other hand, the present inventors have found that when the plate thickness of the clad material of the clad steel plate is too thick, the strength of the clad steel plate decreases.

[0020] It should be noted that compared with the clad steel plate using stainless steel or non-ferrous alloy as the clad material, the clad steel plate of the present invention can greatly suppress the alloy cost and the manufacturing cost.

[0021] The present invention has been completed based on the above insights, that is, the gist of the present invention is as follows.

[0022] 1. A clad steel plate having a clad material of carbon steel on at least one side of a base material,

[0023] The composition of the above base material contains, by mass%, C: 0.010 - 0.200%, Si: 0.01 - 1.00%, Mn: 0.50 - 2.50%, Al: 0.001 - 0.060%, P: 0.0200% or less, and S: 0.0100% or less, with the balance being Fe and unavoidable impurities.

[0024] And, CE B / CE C is 2.000 or more, where CE B and CE C are the carbon equivalent of the above base material and the carbon equivalent of the above clad material, respectively.

[0025] In the metal structure of the above base material, the volume fraction of bainite is 90% or more, and the average grain size of the above bainite is 25 μm or less.

[0026] The Vickers hardness of the above clad material is 210 HV10 or less.

[0027] 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, where t B is the plate thickness of the above base material, t C1 is the plate thickness of the clad material on one side of the above base material in the above clad material, and t C2 is the plate thickness of the clad material on the other side of the above base material in the above clad material.

[0028] 2. The clad steel plate according to the above 1, wherein the composition of the above base material further contains, by mass%, one or more 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.

[0029] 3. The clad steel sheet according to 1 or 2 above, wherein the composition of the cladding material contains C: 0.100% or less and Mn: 0.01 to 1.50% by mass, and further contains one or more 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%, and the balance is Fe and unavoidable impurities.

[0030] Moreover, the CR value obtained by the following formula (1) is 0.30 or more.

[0031] CR value = 2.3[Cu] + 2.8[Cr] + 7.3[Sb] + 3.6[Sn] ··· (1)

[0032] Here, [X] represents the content (mass%) of the element X in the composition of the cladding material.

[0033] 4. A method for manufacturing a clad steel sheet for manufacturing a clad steel sheet having a cladding material of carbon steel on at least one side surface of a base material. In this method,

[0034] A clad billet steel sheet is obtained by overlapping a base material billet steel sheet having the composition of the base material according to 1 or 2 above and the cladding material billet steel sheet of the carbon steel according to 1 above, and the clad billet steel sheet is heated to 1000 to 1250 °C.

[0035] Next, hot rolling is performed on the above clad billet steel sheet with a cumulative reduction ratio in the non-recrystallization temperature range of the above base material billet steel sheet of 20% or more and a rolling end temperature of Ar 3 above the phase transformation point to produce a hot-rolled steel sheet.

[0036] Next, the above hot-rolled steel sheet is cooled with a cooling start temperature: Ar 3 above the phase transformation point, an average cooling rate: 20 to 120 °C / s, and a cooling stop temperature: 500 °C or less.

[0037] 5. The method for manufacturing a clad steel sheet according to 4 above, wherein the composition of the cladding material billet steel sheet contains C: 0.100% or less and Mn: 0.01 to 1.50% by mass, and further contains one or more 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%, and the balance is Fe and unavoidable impurities.

[0038] Moreover, the CR value obtained by the following formula (1) is 0.30 or more.

[0039] CR value = 2.3[Cu] + 2.8[Cr] + 7.3[Sb] + 3.6[Sn] ··· (1)

[0040] Among them, [X] represents the content (mass%) of element X in the composition of the clad material blank steel plate.

[0041] 6. According to the manufacturing method of the clad steel plate described in 4 or 5 above, after the cooling, tempering is performed on the hot-rolled steel plate in a temperature range below 650°C.

[0042] According to the present invention, a high-strength clad steel plate with excellent ammonia SCC resistance and low-temperature toughness suitable for use in tanks for transporting and storing liquid ammonia can be obtained. In addition, the clad steel plate of the present invention does not use stainless steel or non-ferrous alloys for the clad material, and the manufacturing process is also simple, so it is extremely advantageous in terms of cost. Detailed implementation mode

[0043] The clad steel plate of an embodiment of the present invention has a clad material of carbon steel on at least one side of the base material.

[0044] Among them, the clad steel plate of an embodiment of the present invention has excellent ammonia SCC resistance and low-temperature toughness, so it is suitable for structural components such as tanks used in a liquid ammonia environment. It should be noted that the use environment is not limited to liquid ammonia, and it can also be other liquefied gases such as LPG and liquefied CO 2 etc. In the present disclosure, when referring to ammonia and the like, it means not only liquid ammonia, but also all other liquefied gases such as LPG and liquefied CO 2 etc.

[0045] In the clad steel plate of an embodiment of the present invention, the side with the clad material can be any side of the base material. However, when used for tanks and the like, the clad material is arranged on the side (hereinafter also referred to as the first side or inner surface of the base material) that should come into contact with ammonia and the like. Thus, as a tank (structure), ammonia SCC resistance and low-temperature toughness can be obtained. It should be noted that in the present disclosure, the clad material arranged on the first side of the base material is also referred to as the first clad material. In addition, the other side of the first side of the base material is referred to as the second side or outer surface of the base material. The clad material arranged on the second side of the base material is also referred to as the second clad material. Among them, the second clad material is arbitrary. That is, the clad steel plate of an embodiment of the present invention includes a clad steel plate having a first clad material on one side of the base material, and a clad steel plate having a first clad material on one side of the base material and a second clad material on the other side of the base material. In addition, the thickness of the base material can be set as t B and the thickness of the first clad material can be set as t C1 and the thickness of the second clad material can be set as t C2 .

[0046] Hereinafter, a clad steel sheet according to an embodiment of the present invention will be further specifically described. It should be noted that "%" indicating the content of each of the following elements means "% by mass" unless otherwise specified.

[0047] (1) Composition of the base material

[0048] C: 0.010 to 0.200%

[0049] C is the most effective element for increasing the strength of the clad steel sheet according to an embodiment of the present invention. To obtain this effect, the C content is made 0.010% or more. Further, from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost, the C content is preferably 0.030% or more. On the other hand, if the C content exceeds 0.200%, deterioration of toughness and weldability occurs. Therefore, the C content is made 0.200% or less. Further, from the viewpoint of toughness, the C content is preferably 0.170% or less.

[0050] Si: 0.01 to 1.00%

[0051] Si is added for deoxidation. To obtain this effect, the Si content is made 0.01% or more. Further, the Si content is preferably 0.03% or more. On the other hand, if the Si content exceeds 1.00%, deterioration of toughness and weldability occurs. Therefore, the Si content is made 1.00% or less. Further, from the viewpoint of toughness, the Si content is preferably 0.40% or less.

[0052] Mn: 0.50 to 2.50%

[0053] Mn is an element having 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 made 0.50% or more. Further, from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost, the Mn content is preferably 0.70% or more. On the other hand, if the Mn content exceeds 2.50%, the toughness decreases. Therefore, the Mn content is made 2.50% or less. Further, from the viewpoint of suppressing the decrease in toughness, the Mn content is preferably 2.30% or less.

[0054] Al: 0.001 to 0.060%

[0055] Al acts as a deoxidizer. To obtain this effect, the Al content is made 0.001% or more. On the other hand, if the Al content exceeds 0.060%, oxide-based inclusions increase and the cleanliness decreases. In addition, the toughness decreases. Therefore, the Al content is made 0.060% or less. Further, from the viewpoint of suppressing the decrease in toughness, the Al content is preferably 0.050% or less.

[0056] P: 0.0200% or less

[0057] P is an element contained as an inevitable impurity. In addition, P segregates at grain boundaries, causing adverse effects such as reduced toughness and weldability. Therefore, it is preferable to minimize the P content as much as possible. However, a P content of 0.0200% or less is acceptable. It should be noted that the lower limit of the P content is not particularly limited and can be 0%. In addition, usually P is an element inevitably contained in steel as an impurity, so the P content can exceed 0% industrially. In addition, excessive reduction of P will lead to an increase in refining costs. Therefore, the P content is preferably 0.0005% or more.

[0058] S: 0.0100% or less

[0059] S is an element contained as an inevitable impurity. In addition, S exists in steel in the form of sulfide-based inclusions such as MnS, causing adverse effects such as reducing toughness by serving as the starting point of fracture. Therefore, it is preferable to minimize the S content as much as possible. However, an S content of 0.0100% or less is acceptable. It should be noted that the lower limit of the S content is not particularly limited and can be 0%. In addition, usually S is an element inevitably contained in steel as an impurity, so the S content can exceed 0% industrially. In addition, excessive reduction of S will lead to an increase in refining costs. Therefore, from the perspective of cost, it is preferable to make the S content 0.0005% or more.

[0060] In addition, in the component composition of the base material of the clad steel sheet according to an embodiment of the present invention, the elements described below (hereinafter also referred to as optional addition elements) can be optionally contained. In addition, in the component composition of the base material of the clad steel sheet according to an embodiment of the present invention, the remaining part except the above elements and the following optional addition elements is Fe and inevitable impurities.

[0061] Cu: 1.00% or less

[0062] Cu is an element effective for improving the strength of the clad steel sheet. However, if the Cu content is less than 0.01%, the 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.

[0063] Ni: 2.00% or less

[0064] Ni is effective not only in improving the strength of the clad steel plate, but also in improving the toughness. However, if the Ni content is less than 0.01%, the 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 is saturated and the alloy cost increases. Therefore, when Ni is contained, the Ni content is preferably 2.00% or less.

[0065] Cr: 1.00% or less

[0066] Cr is an element effective in improving the strength of the clad steel plate. However, if the Cr content is less than 0.01%, the 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%, the toughness deteriorates. Therefore, when Cr is contained, the Cr content is preferably 1.00% or less.

[0067] Mo: 1.00% or less

[0068] Mo is an element effective in improving the strength of the clad steel plate. However, if the Mo content is less than 0.01%, the 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.

[0069] V: 0.500% or less

[0070] V is an element that has the effect of improving the strength of the clad steel plate. In order to obtain this effect, when V is contained, it is preferred that the V content be 0.005% or more. On the other hand, if the V content exceeds 0.500%, it leads to deterioration of weldability and increase in alloy cost. Therefore, when V is contained, the V content is preferably 0.500% or less. A more preferred lower limit of the V content is 0.010%. A more preferred upper limit of the V content is 0.100%.

[0071] Ti: 0.100% or less

[0072] Ti is an element that has a strong tendency to form nitrides and has the effect of fixing N and reducing solid-solution N. Therefore, by adding Ti, the toughness of the base material and the weld can be improved. In order to obtain 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 is reduced. 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.

[0073] Nb: 0.100% or less

[0074] Nb is an element that has the effect of reducing the grain size of the original austenite and improving toughness by precipitating in the form of carbonitride. 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, if the Nb content exceeds 0.100%, a large amount of NbC precipitates and the toughness decreases. 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.

[0075] Ca: 0.0200% or less

[0076] Ca is an element that has the function of combining with S to inhibit the formation of MnS, etc. that elongates in the rolling direction. That is, by containing Ca, the morphology of sulfide-based inclusions can be controlled to be spherical, improving the toughness of the welded part, etc. To obtain this effect, when Ca is contained, 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. The decrease in cleanliness leads to a decrease in toughness. Therefore, when Ca is contained, 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.

[0077] Mg: 0.0200% or less

[0078] Mg, like Ca, is an element that has the function of combining with S to inhibit the formation of MnS, etc. that elongates in the rolling direction. That is, by containing Mg, the morphology of sulfide-based inclusions can be controlled to be spherical, improving the toughness of the welded part, etc. To obtain this effect, when Mg is contained, 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. The decrease in cleanliness leads to a decrease in toughness. Therefore, when Mg is contained, 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.

[0079] REM: 0.0200% or less

[0080] REM (rare earth metal), like Ca and Mg, is an element that has the effect of combining with S to inhibit the formation of MnS and the like that elongate in the rolling direction. That is, by containing REM, the morphology of sulfide-based inclusions can be controlled to be spherical, improving the toughness of welded parts and the like. To obtain this effect, when REM is contained, 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. The decrease in cleanliness leads to a decrease in toughness. Therefore, when REM is contained, 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.

[0081] (2) Regarding the composition of the clad material

[0082] In the clad steel plate according to one embodiment of the present invention, the clad material uses CE B / CE C carbon steel of 2.000 or more.

[0083] It should be noted that carbon steel is an alloy of iron (Fe) and carbon (C), and is a steel having the following composition, that is, the C content is 2.2 mass% or less, and the total content of elements other than C and Fe (Si, Mn, P, S, etc. In addition, elements equivalent to inevitable impurities are also included) is 5.0 mass% or less, and the remaining part is Fe.

[0084] CE B / CE C : 2.000 or more

[0085] Make CE B / CE C be 2.000 or more. Among them, CE B and CE C are the carbon equivalent of the base metal and the carbon equivalent of the clad material, respectively. If CE B / CE C is less than 2.000, the carbon equivalent of the clad material is too large, and the hardness of the clad steel plate rises excessively, resulting in deterioration of the ammonia SCC resistance. CE B / CE C is preferably 2.050 or more, more preferably 2.100 or more. In addition, the upper limit of CE B / CE C is not particularly limited. CE B / CE C For example, it is preferably 5.000 or less.

[0086] It should be noted that CE B and CE C can be obtained by the following formulas, respectively.

[0087] CE B = [C] B + [Mn] B / 6 + [Si] B / 24 + [Ni] B / 40 + [Cr] B / 5 + [Mo] B / 4 + [V] B / 14

[0088] Among them, [X] B represents the content (mass%) of element X in the component composition of the base material. It should be noted that elements not contained can be calculated as "0". The same applies to the following formulas.

[0089] CE C = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14

[0090] Among them, [X] represents the content (mass%) of element X in the component composition of the clad material.

[0091] In addition, the preferred component composition of the clad material of the clad steel plate according to an embodiment of the present invention is as follows.

[0092] C: 0.100% or less

[0093] C is an element that increases the hardness of steel. The higher the hardness, the higher the susceptibility to liquid ammonia SCC. Therefore, the C content of the clad material is preferably 0.100% or less. On the other hand, the lower the C content of the clad material, the more preferable, but excessive reduction of C will lead to an increase in refining cost. Therefore, the C content is preferably 0.0005% or more.

[0094] Mn: 0.01 - 1.50%

[0095] Mn is an element that has the effect of increasing the hardenability of steel. Therefore, if the content of Mn increases excessively, the hardness of the clad material will increase excessively. If the hardness of the clad material increases excessively, it will lead to deterioration of the ammonia SCC resistance. Therefore, the Mn content is preferably 1.50% or less. The Mn content is more preferably 1.25% or less, and further preferably 1.00% or less. On the other hand, it requires a large cost to reduce Mn to less than 0.01%. Therefore, the Mn content is preferably 0.01% or more.

[0096] Cu, Cr, Sb, and Sn can further improve the ammonia SCC resistance of the steel plate. Therefore, it is preferable to contain one or more of them in the amounts described below, and to make the CR value obtained by the following formula (1) 0.30 or more.

[0097] It should be noted that the CR value is a formula designed to infer the ammonia SCC resistance based on the content of each element. The higher the CR value, the higher the ammonia SCC resistance. Therefore, by making the CR value 0.30 or more, stress corrosion cracking can be effectively suppressed in the liquid ammonia environment.

[0098] CR value = 2.3[Cu] + 2.8[Cr] + 7.3[Sb] + 3.6[Sn] ··· (1)

[0099] Among them, [X] represents the content (mass%) of element X in the composition of the cladding material. It should be noted that since the composition of the cladding material and the composition of the base plate of the cladding material blank are substantially the same, it can also be said that [X] represents the content (mass%) of element X in the composition of the base plate of the cladding material blank. Similarly, it can also be said that the above [X] B also represents the content (mass%) of element X in the composition of the base material blank plate.

[0100] Among them, by making the cladding material a low-hardness steel plate, ammonia SCC can be suppressed. However, if there are dents or scratches on the surface of the cladding material, stress concentration will occur at the location where the dents or scratches exist, and there is a concern about the deterioration of ammonia SCC resistance. In this regard, by making the CR value 0.30 or more, even when there are dents or scratches on the surface of the cladding material, the deterioration of ammonia SCC resistance can be suppressed. Therefore, the CR value is preferably 0.30 or more. The CR value is more preferably 0.32 or more, and further preferably 0.35 or more. In addition, the upper limit of the CR value is not particularly limited. The CR value is preferably 1.50 or less, for example.

[0101] In addition, Cu, Cr, Sb, and Sn have the effect of rapidly forming a protective corrosion product in the liquid ammonia environment and suppressing stress corrosion cracking. In order to obtain this effect, when Cu is contained, it is preferably made such that the Cu content is 0.01% or more. When Cr is contained, it is preferably made such that the Cr content is 0.01% or more. When Sb is contained, it is preferably made such that the Sb content is 0.01% or more. When Sn is contained, it is preferably made such that the Sn content is 0.01% or more.

[0102] On the other hand, if Cu, Cr, Sb, and Sn are added in excess, the weldability and toughness deteriorate. In addition, it is also disadvantageous from the viewpoint of alloy cost. Therefore, when Cu is contained, it is preferably made such that the Cu content is 0.50% or less. When Cr is contained, it is preferably made such that the Cr content is 0.50% or less. In addition, when Sb is contained, it is preferably made such that the Sb content is 0.50% or less. When Sn is contained, it is preferably made such that the Sn content is 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.

[0103] In the preferred composition of the clad material of the clad steel plate according to an embodiment of the present invention, the balance other than the above elements is Fe and inevitable impurities.

[0104] (3) Metallographic structure

[0105] [Metallographic structure of the base material: The volume fraction of bainite is 90% or more, and the average grain size of bainite is 25 μm or less]

[0106] In order to satisfy the tensile properties and low-temperature toughness of the base material, in the metallographic structure of the base material, it is necessary to make the volume fraction of bainite 90% or more. That is, if the volume fraction of bainite is less than 90%, the volume fractions of ferrite, island martensite, martensite, pearlite, and austenite other than this increase, and sufficient strength and low-temperature toughness cannot be obtained. The upper limit of the volume fraction of bainite is not particularly limited and may be 100%.

[0107] Among them, bainite includes structures called bainite ferrite and granular ferrite and structures obtained by tempering these structures. Among them, the structures called bainite ferrite and granular ferrite are structures formed during or after cooling during hot rolling that contribute to transformation strengthening.

[0108] It should be noted that the remaining structure with a volume fraction of 10% or less may include 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, and the remaining structure is preferably pearlite. The volume fraction of the remaining structure may be 0%.

[0109] In addition, in order to obtain excellent low-temperature toughness, the average grain size of bainite is made 25 μm or less. That is, if the average grain size of bainite exceeds 25 μm, the crack propagation resistance decreases, and sufficient low-temperature toughness cannot be obtained. The average grain size of bainite is preferably 22 μm or less. In addition, the lower limit of the average grain size of bainite is not particularly limited. The average grain size of bainite is preferably 1 μm or more, for example.

[0110] Among them, the volume fraction and average grain size of bainite can be measured by the method described in the examples below.

[0111] It should be noted that the metallographic structure of the clad material is not particularly limited. The metallographic structure of the clad material can be, for example, the metallographic structure of conventionally known carbon steels such as ferrite and bainite.

[0112] [Vickers hardness of the clad material: 210 HV10 or less]

[0113] The Vickers hardness of the clad material is 210 HV10 or less. If there is a high-hardness region on the surface layer of the clad steel plate, it promotes ammonia SCC. That is, if the Vickers hardness of the clad material exceeds 210 HV10, the required ammonia SCC resistance cannot be obtained. The Vickers hardness of the clad material is preferably 200 HV10 or less. The lower limit of the Vickers hardness of the clad material is not particularly limited. The Vickers hardness of the clad material is preferably 100 HV10 or more, for example.

[0114] It should be noted that the Vickers hardness can be measured by the method described in the following examples.

[0115] In addition, the Vickers hardness of the base material is not particularly limited and can be 210 HV10 or less, or can exceed 210 HV10.

[0116] (4) Thickness of the base material and the clad material

[0117] [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]

[0118] Make t C1 be 2.0 mm or more, make t C2 be 0.0 mm or more, and make (t C1 + t C2 ) / (t B + t C1 + t C2 )(hereinafter also referred to as "cladding ratio") be 0.30 or less. Among them, t B (mm) is the thickness of the base material, t C1 (mm) is the thickness of the clad material on one side of the base material in the clad material, that is, the thickness of the first clad material, t C2 (mm) is the thickness of the clad material on the other side of the base material in the clad material, that is, the thickness of the second clad material.

[0119] When t C1 is less than 2.0 mm, due to corrosion, the low-hardness region wears, and the ammonia SCC resistance deteriorates. t C1 is preferably 2.1 mm or more, more preferably 2.2 mm or more. t C1 is preferably 5.0 mm or less.

[0120] In addition, if the cladding ratio exceeds 0.30, the proportion of the low-hardness cladding material becomes too large relative to the base material providing strength, and sufficient strength cannot be obtained. The cladding ratio is preferably 0.20 or less. The cladding ratio is preferably 0.05 or more.

[0121] It should be noted that since the second cladding material may not be present (in other words, the second cladding material is optional), t C2 can be 0.0 mm or more. t C2 It is preferably 0.5 mm or more, more preferably 1.0 mm or more. t C2 It is preferably 5.0 mm or less.

[0122] In addition, the total thickness of the clad steel plate, usually t B +t C1 +t C2 is preferably 8 mm or more, more preferably 15 mm or more. The total thickness of the clad steel plate is preferably 50 mm or less, more preferably 40 mm or less.

[0123] (5) Manufacturing method

[0124] Next, the manufacturing method of the clad steel plate according to an embodiment of the present invention will be described. First, for example, a base material blank steel plate having the composition of the above-mentioned base material and a clad material blank steel plate of carbon steel (CE B / CE C being 2.000 or more) are overlapped to form a clad blank steel plate. The preparation methods of the base material blank steel plate and the clad material blank steel plate are not particularly limited. For example, the base material blank steel plate and the clad material blank steel plate can be prepared by using conventionally known manufacturing methods. Specifically, molten steel adjusted to a specified composition by a usual melting method (converter method, electric furnace method, etc.) is cast by a usual casting method (continuous casting method, ingot casting method) to form a slab blank. Then, the obtained slab blank is hot-rolled or the like to form a base material blank steel plate. Next, a clad material blank steel plate is overlapped on at least one side surface of the base material blank steel plate, particularly the surface that should come into contact with ammonia or the like, to prepare a two-layer clad blank steel plate. Alternatively, clad material blank steel plates are overlapped on both sides of the base material blank steel plate to prepare a three-layer clad blank steel plate (a clad blank steel plate in the order of clad material blank steel plate / base material blank steel plate / clad material blank steel plate).

[0125] Then, the clad blank steel plate is pressure-bonded and heat-treated under specified conditions to control the structure.

[0126] That is, the clad blank steel plate is heated to 1000 to 1250 °C,

[0127] Next, apply the cumulative reduction ratio in the non-recrystallization temperature range of the above-mentioned base material blank steel plate to the above-mentioned clad blank steel plate: 20% or more, and the rolling end temperature: Ar 3 Hot-roll at a temperature above the phase transformation point to produce a hot-rolled steel plate.

[0128] Next, apply the cooling start temperature to the above-mentioned hot-rolled steel plate: Ar 3 Cooling is carried out at a temperature above the phase transformation point, with an average cooling rate of 20 to 120 °C / s and a cooling stop temperature of 500 °C or lower. Thus, the clad steel plate of one embodiment of the present invention can be manufactured.

[0129] In addition, after the above cooling, the hot-rolled steel plate can be tempered in a temperature range of 650 °C or lower.

[0130] It should be noted that the temperature in each manufacturing condition is the temperature at the 1 / 2 thickness position of the base material or the base material blank steel plate. In addition, the temperature at this position can be directly measured. In addition, the temperature at this position can be obtained by, for example, differential calculation using a process computer based on the surface temperature of the clad steel plate or the clad blank steel plate measured by a radiation thermometer.

[0131] [Heating temperature: 1000 - 1250 °C]

[0132] If the heating temperature of the clad blank steel plate (hereinafter also referred to as the heating temperature) is less than 1000 °C, the solid solution of carbides is insufficient and the necessary strength cannot be obtained. In addition, from the perspective of the bonding property between the base material blank steel plate and the clad material blank steel plate, a high heating temperature is preferred. Therefore, the heating temperature is 1000 °C or higher. On the other hand, if the heating temperature exceeds 1250 °C, the toughness deteriorates due to the coarsening of the grains of the base material. Therefore, the heating temperature is 1250 °C or lower.

[0133] [Cumulative reduction ratio in the non-recrystallization temperature range of the base material blank steel plate: 20% or more]

[0134] By making the cumulative reduction ratio (hereinafter also simply referred to as the cumulative reduction) in the non-recrystallization temperature range of the base material blank steel plate 20% or more, deformation bands serving as nucleation sites are introduced into the austenite grains of the base material blank steel plate. As a result, the bainite formed during the phase transformation during cooling after hot rolling is refined, and the toughness of the clad steel plate is improved. Therefore, the cumulative reduction ratio is 20% or more. The cumulative reduction ratio is preferably 30% or more, more preferably 40% or more. In addition, the cumulative reduction ratio is preferably 85% or less, more preferably 80% or less.

[0135] Among them, the non-recrystallization temperature range of the base material blank steel plate is the temperature range below Tnr (°C). In addition, Tnr (°C) can be obtained by the following formula.

[0136] Tnr (°C) = 174 × log([Nb] B × ([C] B + 12 / 14 [N] B )) + 1444

[0137] where [X] B represents the content (mass %) of element X in the component composition of the base material slab steel plate. In addition, log is the common logarithm.

[0138] In addition, the cumulative reduction ratio can be obtained by the following formula.

[0139] [Cumulative reduction ratio (%)] = [Total thickness reduction amount (mm) of the base material slab steel plate in the non-recrystallization temperature region of the base material slab steel plate] ÷ [Thickness (mm) of the base material slab steel plate in the clad slab steel plate before the start of hot rolling] × 100

[0140] It should be noted that whether each pass of hot rolling is in the non-recrystallization temperature region of the base material slab steel plate (in other words, whether the thickness reduction amount of the base material slab steel plate in each pass of hot rolling is included in the total thickness reduction amount of the base material slab steel plate in the non-recrystallization temperature region of the base material slab steel plate) is judged according to the outlet side temperature of each pass.

[0141] In addition, the cumulative reduction ratio is based on the thickness of the base material slab steel plate in the clad slab steel plate because the toughness of the clad steel plate has a particularly large influence on the toughness of the base material.

[0142] [Rolling end temperature: Ar 3 phase transformation point or higher]

[0143] If the rolling end temperature of hot rolling is less than Ar 3 phase transformation point, the formed ferrite is affected by processing, so the toughness deteriorates. Therefore, the rolling end temperature is Ar 3 phase transformation point or higher. The upper limit of the rolling end temperature is not particularly limited. For example, the rolling end temperature is preferably (Ar 3 phase transformation point + 90 °C) or lower.

[0144] Ar 3 phase transformation point can be obtained by the following formula.

[0145] Ar 3 phase transformation point (°C) = 910 - 310 [C] B - 80 [Mn] B - 20 [Cu] B - 15 [Cr] B - 55 [Ni] B - 80 [Mo] B

[0146] wherein, [X] B represents the content (mass%) of element X in the component composition of the base material blank steel plate.

[0147] [Cooling start temperature: Ar 3 above the phase transformation point]

[0148] Cool the hot-rolled steel plate obtained after hot rolling from a temperature above the Ar 3 phase transformation point. If the cooling start temperature is less than Ar 3 phase transformation point, excessive ferrite is generated. The generated ferrite coexists with bainite and martensite, which have a large strength difference with ferrite. As a result, insufficient strength and deterioration of toughness occur. Therefore, the cooling start temperature after hot rolling is above the Ar 3 phase transformation point. The upper limit of the cooling start temperature is not particularly limited. For example, the cooling start temperature is preferably (Ar 3 phase transformation point + 70 °C) or less.

[0149] [Average cooling rate: 20 - 120 °C / s]

[0150] By making the average cooling rate 20 °C / s or more, a clad steel plate with high strength and high toughness can be obtained. In particular, by cooling at a faster rate, the strength increase effect brought about by transformation strengthening can be obtained. That is, if the average cooling rate is less than 20 °C / s, the grain size of bainite becomes larger. In addition, there is a risk of generating ferrite and pearlite, resulting in insufficient strength and deterioration of toughness. On the other hand, if the average cooling rate exceeds 120 °C / s, the volume fraction of martensite becomes excessive and the toughness decreases. Therefore, the average cooling rate is 20 °C / s to 120 °C / s.

[0151] It should be noted that the average cooling rate mentioned here is the average value of the cooling rates from the above cooling start temperature to the cooling stop temperature, based on the temperature at the 1 / 2 position of the thickness of the base material. For example, by using a process computer to perform a differential calculation based on the surface temperature at the start of cooling and the surface temperature at the end of cooling measured by a radiation thermometer, the temperatures at the 1 / 2 position of the thickness of the base material at the start of cooling and at the end of cooling are respectively obtained. Then, the average cooling rate can be calculated by the following formula.

[0152] [Average cooling rate (°C / s)] = ([Temperature at the 1 / 2 position of the thickness of the base material at the start of cooling (°C)] - [Temperature at the 1 / 2 position of the thickness of the base material at the end of cooling (°C)]) ÷ [Cooling time (s)]

[0153] [Cooling stop temperature: 500 °C or less]

[0154] By setting the cooling stop temperature to 500°C or lower, a specified volume fraction of bainite can be formed in the metal structure of the base material. If the cooling stop temperature exceeds 500°C, excessive ferrite and pearlite are generated, resulting in insufficient strength and deterioration of toughness. Therefore, the cooling stop temperature is 500°C or lower. On the other hand, the lower limit of the cooling stop temperature is not particularly limited. The cooling stop temperature can be, for example, room temperature, but from the viewpoint of production efficiency, etc., it is preferably 150°C or higher.

[0155] [Tempering temperature: 650°C or lower]

[0156] To restore the toughness of the base material, tempering can be carried out arbitrarily. Among them, if the tempering temperature, that is, the temperature of the clad steel plate during reheating caused by tempering (the temperature at the 1 / 2 thickness position of the base material), exceeds 650°C, there is a risk of dislocation recovery and reduction in the strength of the base material. Therefore, when tempering is carried out, the tempering temperature is 650°C or lower. On the other hand, from the viewpoint of restoring the toughness of the base material, the lower limit of the tempering temperature is preferably 350°C.

[0157] As described above, a clad steel plate according to an embodiment of the present invention can be manufactured. The clad steel plate according to an embodiment of the present invention thus obtained has excellent tensile properties and toughness.

[0158] Among them, excellent tensile properties refer to a yield strength YS (yield point YP when there is a yield point, 0.2% proof stress σ0.2 when there is no yield point) measured by a tensile test in accordance with JIS Z 2241 (2022): 490 MPa or more, and a tensile strength (TS): 610 MPa or more. In addition, excellent toughness refers to a ductile-brittle transition temperature (hereinafter also referred to as vTrs) measured by a Charpy impact test in accordance with JIS Z 2242 (2018) of -30°C or lower. Details are described in the examples below.

[0159] It should be noted that the conditions other than those described above are not particularly limited and can be carried out according to conventional methods.

[0160] Examples

[0161] [Example 1]

[0162] Table 1 shows the component composition of the base material (the remaining part is Fe and inevitable impurities). In the table, steel grades A to P are suitable steels that satisfy the component composition of the base material of the clad steel plate according to an embodiment of the present invention. On the other hand, steel grades Q to X are comparative steels outside the range of the component composition of the base material of the clad steel plate according to an embodiment of the present invention.

[0163] The base material blank steel plate having the component composition shown in Table 1 and the CE shown in Table 2 CClad material blanks are prepared by overlapping steel plates, and clad steel plates (No. 1 to 34) are manufactured under the conditions shown in Table 2. For the obtained clad steel plates, the volume fraction and average grain size of bainite in the metal structure of the base material, the Vickers hardness of the clad material, the evaluation of tensile properties and toughness, and the evaluation of ammonia SCC resistance in an ammonia environment are respectively carried out. The test methods are as follows.

[0164] [Measurement of the volume fraction of bainite in the metal structure of the base material]

[0165] Samples are taken with the center of the plate thickness (1 / 2 position of the plate thickness) of the base material of the clad steel plate as the observation surface. Then, the taken samples are mirror-polished and further etched with nitric acid alcohol. Then, using a scanning electron microscope (SEM), the 10 mm × 10 mm range of the samples is photographed at a magnification of 500 to 3000 times. The photographed images are analyzed using an image analysis device, and thus the volume fraction of bainite in the metal structure of the base material is obtained. It should be noted that when the anisotropy of the metal structure of the base material is small, the area fraction is equivalent to the volume fraction, and the area fraction is regarded as the volume fraction.

[0166] [Measurement of the average grain size of bainite in the metal structure of the base material]

[0167] The same samples as those used for the measurement of the volume fraction of bainite in the metal structure of the above-mentioned base material are also used for the measurement of the average grain size of bainite. First, the surface of the above-mentioned samples is mirror-polished. Then, using the Electron Back-Scattering Pattern (EBSP) device attached to the SEM, the crystal orientation is measured from the electron backscattering diffraction image. Specifically, within a 200 μm square area of the sample, the crystal orientation is measured at an interval of 0.3 μm. Then, the region surrounded by grain boundaries with a crystal orientation difference of 15° or more from adjacent grains is set as one grain, and for the grains judged to be bainite, the equivalent circle diameter of the grain is obtained based on the area of the grain. Then, the average value of the equivalent circle diameters of the grains judged to be bainite is used as the average grain size of bainite.

[0168] It should be noted that grains with lath-shaped ferrite having elongated growth in the grains are judged to be bainite.

[0169] [Measurement of the Vickers hardness of the clad material]

[0170] A sample is taken from the clad steel plate with a cross-section perpendicular to the rolling direction, a so-called T-section, as the measurement surface. Next, the sample is mirror-polished. Then, in accordance with JIS Z 2244 (2020), at the position of 1 / 2 of the plate thickness of the clad material, 20 points of Vickers hardness (HV10, measurement load: 10 kgf) are measured at 1 mm intervals in the direction perpendicular to rolling (the direction perpendicular to the rolling direction and the plate thickness direction). Then, their average value is taken as the Vickers hardness of the clad material. It should be noted that, for example, when the Vickers hardness measured under the condition of a measurement load of 10 kgf is 210, it is usually expressed as 210HV10.

[0171] [Tensile properties]

[0172] A test piece No. 1B of JIS Z 2241 (2022) is taken from the clad steel plate with the direction perpendicular to rolling as the long side direction, and a tensile test is carried out 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, 0.2% proof stress σ0.2 when there is no yield point) and the tensile strength (TS). Then, a steel plate with a yield strength of 490 MPa or more and a tensile strength of 610 MPa or more is evaluated as having excellent tensile properties. The initial strain rate is 1×10 ―3 / s.

[0173] [Toughness]

[0174] A V-notch test piece of JIS Z 2242 (2018) is taken from the base material of the clad steel plate with the rolling direction as the long side direction, and a Charpy impact test is carried out according to the procedures of JIS Z 2242 (2018) to measure vTrs. Then, a steel plate with vTrs of -30°C or lower is evaluated as having excellent toughness.

[0175] [Ammonia SCC resistance]

[0176] The ammonia SCC resistance is evaluated by a four-point bending anodic electrolysis test according to the following steps.

[0177] Take test pieces of 15 mm × 115 mm with a thickness of 5 mm from each clad steel plate with the first surface (inner surface) of the clad steel plate as the evaluation surface. Next, perform ultrasonic degreasing of the test pieces in acetone for 5 minutes. Next, apply a stress equivalent to the yield strength of each test piece to each test piece by four-point bending. Next, place each test piece in a test cell in a state where stress is applied. Next, fill the test cell with a test solution mixed with 12.5 g of ammonium carbamate and 1 L of liquid ammonia. Next, use a potentiostat to control the potential difference from the reference electrode to +2.0 V vs Pt and perform potentiostatic anodic electrolysis. The temperature of the test atmosphere is room temperature (25 °C). Then, maintain for 720 hours from the start of immersion (electrification) in this state. Then, visually confirm the test pieces after maintenance, and determine that the case where no cracking is found in the test pieces is excellent (qualified) in ammonia SCC resistance. On the other hand, the case where cracking occurs in the test pieces is determined as defective.

[0178] Record the evaluation results in Table 2 together.

[0179]

[0180]

[0181] As can be seen from Table 2, the inventive examples all have a yield strength YS of 490 MPa or more and a tensile strength of 610 MPa or more. In addition, the inventive examples all have a vTrs of -30 °C or less. Furthermore, the inventive examples are all excellent in ammonia SCC resistance. That is, the inventive examples are all excellent in ammonia SCC resistance and low-temperature toughness, and have high strength.

[0182] In contrast, for Comparative Example No. 17, the CE B / CE C and the Vickers hardness of the cladding material are outside the appropriate range. For Nos. 18 and 19, the t C1 and the cladding ratio are outside the appropriate range. In addition, for Nos. 20 to 26, since a part of the manufacturing conditions is outside the appropriate range, the desired metal structure of the base material cannot be obtained. As a result, in these comparative examples, any one of the yield strength YS, tensile strength TS, low-temperature toughness, and ammonia SCC resistance is poor.

[0183] In addition, for Nos. 27 to 34, since a part of the component composition of the base material is outside the appropriate range, any one of the yield strength YS, tensile strength TS, low-temperature toughness, and ammonia SCC resistance is poor.

[0184] [Example 2]

[0185] A clad billet steel plate was prepared by overlapping a base material billet steel plate having the composition shown in Table 1 (the balance being Fe and inevitable impurities) and a clad material billet steel plate having the composition shown in Table 3 (the balance being Fe and inevitable impurities), and clad steel plates (No. 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, the Vickers hardness of the clad material, and the evaluation of tensile properties and toughness were respectively carried out in the same manner as in Example 1. In addition, the evaluation of ammonia-resistant SCC resistance in an ammonia environment was carried out according to the following procedure.

[0186] [Ammonia-resistant SCC resistance]

[0187] In the same manner as in Example 1, two test pieces of 15 mm × 115 mm with a thickness of 5 mm were taken from each clad steel plate with the first surface (inner surface) of the clad steel plate as the test surface. Then, for one test piece, assuming the presence of a dent on the surface (first surface) of the clad steel plate, a notch with a depth of 0.3 mm and a diameter of 0.2 mm was provided on the test surface of the test piece. Hereinafter, the test piece without a notch is referred to as the first test piece, and the test piece with a notch is referred to as the second test piece. Next, using the first test piece and the second test piece, a four-point bending anodic electrolysis test was carried out in the same manner as in Example 1. Then, the test pieces after the test were visually confirmed, and the ammonia-resistant SCC resistance was evaluated according to the following criteria.

[0188] Excellent (qualified, particularly excellent): No cracking was found in both the first test piece and the second test piece.

[0189] Qualified (excellent): No cracking was found in the first test piece, but cracking was found in the second test piece.

[0190] Poor: Cracking was found in both the first test piece and the second test piece.

[0191] [Table 3]

[0192]

[0193]

[0194] As can be seen from Table 4, the inventive examples all have a yield strength YS of 490 MPa or more and a tensile strength of 610 MPa or more. In addition, the inventive examples all have a vTrs of -30°C or less. Furthermore, the inventive examples all have excellent ammonia-resistant SCC resistance. That is, the inventive examples all have excellent ammonia-resistant SCC resistance and low-temperature toughness, and have high strength. In particular, when the CR value of the clad material is 0.30 or more, it has particularly excellent ammonia-resistant SCC resistance.

Claims

1. A clad steel plate having a clad material of carbon steel on at least one side of a base material, wherein the composition of the base material contains, by mass%, C: 0.010 - 0.200%, Si: 0.01 - 1.00%, Mn: 0.50 - 2.50%, Al: 0.001 - 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, wherein, CE B and CE C are the carbon equivalent of the base material and the carbon equivalent of the clad 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 clad material is 210 HV10 or less, 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, where t B is the plate thickness of the base material, t C1 is the plate thickness of the cladding material on one side of the base material in the cladding material, t C2 is the plate thickness of the cladding material on the other side of the base material in the cladding material.

2. The clad steel plate according to claim 1, wherein, the composition of the base material further contains, by mass%, one or more 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.

3. The clad steel plate according to claim 1 or 2, wherein, the composition of the clad material contains, by mass%, C: 0.100% or less and Mn: 0.01 - 1.50%, and further contains one or more selected from Cu: 0.01 - 0.50%, Cr: 0.01 - 0.50%, Sb: 0.01 - 0.50%, and Sn: 0.01 - 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) wherein [X] represents the content of element X in the composition of the clad material in mass%.

4. A method for manufacturing a clad steel plate for manufacturing a clad steel plate having a clad material of carbon steel on at least one side of a base material, wherein, a base material blank steel plate having the composition of the base material according to claim 1 or 2 is overlapped with a clad material blank steel plate of the carbon steel according to claim 1 to obtain a clad blank steel plate, and the clad blank steel plate is heated to 1000 - 1250 °C, Next, hot rolling is performed on the clad billet steel plate at a cumulative reduction ratio of 20% or more in the non-recrystallization temperature range of the base billet steel plate and at a rolling end temperature above the Ar 3 transformation point to produce a hot-rolled steel plate. Next, the hot-rolled steel sheet is cooled with a starting temperature above the Ar 3 transformation point, an average cooling rate of 20 to 120 °C / s, and a stopping temperature of 500 °C or lower.

5. The method for manufacturing a clad steel plate according to claim 4, wherein, the composition of the clad material blank steel plate contains, by mass%, C: 0.100% or less and Mn: 0.01 - 1.50%, and further contains one or more selected from Cu: 0.01 - 0.50%, Cr: 0.01 - 0.50%, Sb: 0.01 - 0.50%, and Sn: 0.01 - 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) Among them, [X] represents the content of element X in the composition of the clad material blank steel plate in mass%.

6. The manufacturing method of the clad steel plate according to claim 4, wherein, after the cooling, tempering is performed on the hot-rolled steel plate in a temperature range below 650 °C.

7. The manufacturing method of the clad steel plate according to claim 5, wherein, after the cooling, tempering is performed on the hot-rolled steel plate in a temperature range below 650 °C.

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