Manufacturing method of clad steel plate

Abstract

Provided is a manufacturing method for a clad steel plate that can produce a thick clad steel plate having excellent low-temperature toughness while ensuring a tensile strength of 535 MPa or more. The manufacturing method for a clad steel plate of the present invention includes the steps of laminating a first material plate having a predetermined composition with a second material plate made of a corrosion-resistant alloy to obtain a clad slab, heating the clad slab to a predetermined temperature, and then subjecting the clad slab to hot rolling under predetermined conditions to obtain a clad rolled plate in which the first material plate and the second material plate are joined to each other to become a base steel plate and a clad plate, respectively, allowing the clad rolled plate to cool, reheating the clad rolled plate to 850°C to 1050°C, cooling the clad rolled plate so that the average cooling rate of the base steel plate is 2°C / s to 20°C / s, and then tempering the clad rolled plate to 450°C to 650°C.

Description

[Technical field]

[0001] The present invention relates to a manufacturing method of a clad steel plate, and more particularly to a manufacturing method of a clad steel plate suitable for use as a material for line pipes. [Background technology]

[0002] Clad steel plate is a steel plate in which a clad plate made of a corrosion-resistant alloy is bonded to a base steel plate such as carbon steel. Clad steel plate has the advantage of being highly economical because it can ensure the same corrosion resistance as solid steel while reducing the amount of expensive alloy elements used.

[0003] In recent years, oil and natural gas development has expanded to areas where line pipes are exposed to severe corrosive environments. Accordingly, the demand for line pipes that enable economical transportation of oil and natural gas is expected to increase. And, due to their high corrosion resistance, clad steel plates, especially Ni-based alloy clad steel plates and austenitic stainless clad steel plates, are expected to be in demand for line pipes exposed to severe corrosive environments.

[0004] Furthermore, in recent years, against the backdrop of line pipe leak accidents in wet hydrogen sulfide environments, there has been a trend toward safer designs, and clad steel plates for line pipes are also required to have improved hydrogen induced cracking resistance (hereinafter also referred to as "HIC resistance") of the base steel plate.

[0005] Here, clad steel plates for line pipes are required to have predetermined mechanical properties, particularly excellent low-temperature toughness from the viewpoint of preventing brittle fracture of line pipes, in addition to the corrosion resistance of clad steel plates.

[0006] For example, Patent Document 1 describes such a clad steel plate as follows: "A clad steel plate having a Ni alloy as a cladding material and a low alloy steel as a base material, the chemical components of the base material being, in mass %, C: 0.020-0.100%, Si: 0.10-0.50%, Mn: 0.75-1.80%, P: 0.015% or less, S: 0.0030% or less, Cu: 0.01-0.50%, Ni: 0.01-0.45%, Cr: 0.01% or less, The document discloses a Ni alloy clad steel plate having excellent low temperature toughness and HAZ toughness of the base material and excellent corrosion resistance of the clad material, the Ni alloy clad steel plate being characterized by containing Cr: 0.01-0.50%, Mo: 0.01-0.50%, Nb: 0.005-0.080%, Ti: 0.005-0.030%, N: 0.0010-0.0060%, Al: 0.070% or less, Ca: 0.0010-0.0040%, with the balance being Fe and unavoidable impurities.

[0007] In addition, Patent Document 2 describes a clad steel plate having a Ni alloy as a cladding material and a low alloy steel as a base material, in which the chemical components of the base material are, in mass%, C: 0.020-0.100%, Si: 0.10-0.50%, Mn: 0.75-1.80%, P: 0.015% or less, S: 0.0030% or less, Cu: 0.01-0.50%, Ni: 0.01-0.45%, Cr: 0.01-0.50%, M The document discloses a Ni alloy clad steel plate having excellent low temperature toughness and HAZ toughness of the base material and excellent corrosion resistance of the clad material, the Ni alloy clad steel plate being characterized by containing 0.01-0.50% O, 0.005-0.080% Nb, 0.005-0.030% Ti, 0.0010-0.0060%, 0.070% or less Al, 0.0010-0.0040% Ca, with the balance being Fe and unavoidable impurities.

[0008] Furthermore, Patent Document 3 states that "in a clad steel plate having an austenitic stainless steel as a clad material and a low alloy steel as a base material, the base material contains, in mass%, C: 0.020 to 0.100%, Si: 0.10 to 0.50%, Mn: 0.75 to 1.80%, P: 0.015% or less, S: 0.0030% or less, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.45%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and The document discloses an austenitic stainless clad steel plate having excellent low-temperature toughness and HAZ toughness of the base material and excellent corrosion resistance of the clad material, the austenitic stainless clad steel plate containing 0.01-0.50%, 0.005-0.080%, 0.005-0.030%, 0.0010-0.0060%, 0.070% or less of Al, 0.0010-0.0040%, and the balance being Fe and unavoidable impurities.

[0009] Furthermore, Patent Document 4 describes a clad steel plate having a base steel plate and a cladding material made of a corrosion-resistant alloy bonded to one side thereof, the base steel plate containing, in mass %, C: 0.020-0.100%, Si: 0.05-0.50%, Mn: 0.75-1.80%, P: 0.015% or less, S: 0.0030% or less, Al: 0.010-0.070%, Nb: 0.005-0.080%, Ti: 0.005-0.030%, and N: 0.0010-0.0 060% of Cr and the balance consisting of Fe and unavoidable impurities, the base steel plate has a steel structure in which, at a 1 / 2 position in the plate thickness direction of the base steel plate, it has 94% or more of bainite and 6% or less of island martensite in terms of area ratio, the average crystal grain size of the bainite is 25 μm or less, and further, the joint interface shear strength between the base steel plate and the clad material is 300 MPa or more."

[0010] Furthermore, Patent Document 5 discloses, as a technology aimed at improving HIC resistance, "a clad steel plate which maintains a tensile strength of 535 MPa or more, further improves low-temperature toughness, and also has excellent HIC resistance," and "has a clad steel plate having a clad material made of a corrosion-resistant alloy bonded to one side of a base steel plate, the base steel plate having a component composition in which the ACR and PHIC values ​​are appropriately controlled, the base steel plate having a steel structure in which the area ratio of bainite is 94% or more at a 1 / 2 position in the plate thickness direction of the base steel plate and the average crystal grain size of the bainite is 25 μm or less, and further the joint interface shear strength between the base steel plate and the clad material is 300 MPa or more." [Prior art documents] [Patent documents]

[0011] [Patent Document 1] JP 2015-86422 A [Patent Document 2] JP 2015-117408 A [Patent Document 3] JP 2015-105399 A [Patent Document 4] International Publication No. 2018 / 181381 [Patent Document 5] International Publication No. 2020 / 004410 Summary of the Invention [Problem to be solved by the invention]

[0012] In the technologies of Patent Documents 1 to 3, by applying TMCP (Thermo-mechanical control process), it is possible to improve the corrosion resistance of cladding materials without compromising productivity, and to achieve plate thickness of 30 mm or less, tensile strength of 535 MPa or more, DWTTSA -20℃ Furthermore, in Patent Document 4, the clad slab is directly quenched and tempered after rolling, thereby realizing a mechanical property of DWTTSA. -30℃In Patent Document 5, similar to Patent Document 4, the clad slab is rolled and then directly quenched and tempered to achieve mechanical properties of DWTTSA. -30℃ A clad steel plate is disclosed that realizes mechanical properties of ≧85% and also has HIC resistance.

[0013] Here, DWTTSA -20℃ and DWTTSA -30℃ These are the ductile fracture surface area rates obtained in a DWTT (Drop Weight Tear Test) conforming to API-5L when the test temperatures are -20°C and -30°C, respectively.

[0014] In order to improve the low-temperature toughness, which is emphasized in the present invention, it is important to refine the microstructure of the base steel plate in the clad steel plate. In the techniques described in Patent Documents 1 to 5, a predetermined rolling reduction in a specific temperature range during rolling is ensured, and accelerated cooling is performed to avoid coarsening due to high-temperature transformation, thereby realizing appropriate transformation strengthening. However, when the plate thickness of the clad steel plate is large, for example, 25 mm or more, not only is it difficult to impart rolling strain to the inside of the steel plate, but also, in the accelerated cooling, since the cooling behavior of the steel plate is rate-limited by the thermal conductivity of the steel plate, it is difficult to achieve a desired cooling rate to the center of the plate thickness. As a result, it is not possible to achieve fine grains like in the case of thin clad steel plates, and there is still room for improvement in low-temperature toughness, so there has been a demand for a manufacturing technique that can achieve high strength and excellent low-temperature toughness in thick clad steel plates.

[0015] In view of the above problems, the first object of the present invention is to provide a method for manufacturing clad steel plate that is capable of producing thick clad steel plate having excellent low-temperature toughness while maintaining a tensile strength of 535 MPa or more.

[0016] A second object of the present invention is to provide a method for manufacturing clad steel plate that is capable of producing thick clad steel plate that has excellent low-temperature toughness and HIC resistance while maintaining a tensile strength of 535 MPa or more.

[0017] In addition, "excellent low-temperature toughness" refers to the ductile fracture surface area ratio (DWTTSA) obtained by the DWTT test (test temperature: -30°C) conforming to API-5L. -30℃ "Excellent HIC resistance" means that the crack area ratio (CAR) obtained in the HIC test in accordance with NACE Standard TM0284-2003 is 5.0% or less. [Means for solving the problem]

[0018] The inventors conducted various studies in order to develop a thick clad steel plate having improved low-temperature toughness while maintaining predetermined tensile properties, and obtained the following findings. (A) In order to obtain excellent low-temperature toughness, it is effective to simultaneously refine the structure of the base steel plate, which increases the brittle crack propagation resistance, and reduce the hard phase that can be the starting point of brittle cracks. For this purpose, it is preferable that the steel structure of the base steel plate is a structure mainly composed of bainite. Specifically, it is effective to set the area ratio of bainite at the 1 / 2 position in the plate thickness direction of the base steel plate to 94% or more and the average crystal grain size of bainite to 23 μm or less.

[0019] (B) In order to achieve the above-mentioned structural control, it is important to properly control the chemical composition and manufacturing conditions, and in particular, to apply reheating, quenching and tempering after rolling of the clad slab, and to properly control these conditions.

[0020] The present invention, which has been completed based on the above findings, has the following essential features. [1] In mass percent, C: 0.020% or more and 0.100% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.75% or more and 1.80% or less, P: 0.015% or less, S: 0.0030% or less, Al: 0.01% or more and 0.07% or less, Nb: 0.005% or more and 0.080% or less, Ti: 0.005% or more and 0.030% or less, N: 0.0010% or more and 0.0060% or less, and O (oxygen): 0.0030% or less and further comprising Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less V: 0.100% or less, and Ca:0.0040% or less preparing a first blank sheet having a composition in which Ceq defined by the following formula (1) is 0.40 or more and 0.50 or less, with the remainder being Fe and unavoidable impurities; providing a second blank plate made of a corrosion-resistant alloy; A step of stacking the first material plate and the second material plate to obtain a clad slab; Heating the clad slab to a temperature of 1050°C or more and 1200°C or less; Then, the clad slab is subjected to hot rolling with a rolling end temperature of 900 ° C. or more to obtain a clad rolled plate in which the first material plate and the second material plate are joined together to form a base steel plate and a clad plate, respectively; Cooling the clad rolled plate; Thereafter, reheating the clad rolled plate to 850°C or higher and 1050°C or lower; Thereafter, the clad rolled plate is cooled so that the average cooling rate of the base steel plate is 2 ° C. / s or more and 20 ° C. / s or less; Thereafter, tempering the clad rolled plate at 450°C or higher and 650°C or lower; The method for producing a clad steel plate includes the steps of: producing a clad steel plate by joining the cladding plate to one side of the base steel plate. Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 ···(1) The element symbols in formula (1) represent the content (mass%) of each element, and zero is substituted when the element is not contained.

[0021] [2] In the composition of the first blank plate, the Ca content is, by mass%, 0.0005% or more and 0.0040% or less, the ACR defined by the following formula (2) is 1.00 or more and 6.00 or less, and the P defined by the following formula (3) is HIC The method for producing a clad steel plate according to the above [1], wherein the modulus of elasticity is less than 1.250. ACR={Ca-(0.18+130Ca)×O} / (1.25S) ···(2) P HIC =4.46C+2.37Mn / 6+(1.18Cr+1.95Mo+1.74V) / 5+(1.74Cu+1.7Ni) / 15+22.36P ···(3) The element symbols in formulas (2) and (3) indicate the content (mass%) of each element, and zero is substituted when the element is not contained.

[0022] [3] The method for producing a clad steel plate according to the above [1] or [2], wherein the corrosion-resistant alloy is a Ni-based alloy selected from the group consisting of Alloy 825 and Alloy 625, or an austenitic stainless steel.

[0023] [4] The clad slab is formed by stacking the first material plate, the second material plate, the second material plate, and the first material plate in this order, and a release material is applied between the two second material plates, thereby obtaining two clad rolled plates by hot rolling; The method for producing a clad steel plate according to any one of the above [1] to [3], wherein after the tempering, the two rolled clad plates are peeled off via the peeling material. Effect of the Invention

[0024] According to the first embodiment of the present invention, it is possible to manufacture a thick clad steel plate that has excellent low-temperature toughness while ensuring a tensile strength of 535 MPa or more.

[0025] Moreover, according to the second embodiment of the present invention, it is possible to manufacture a thick clad steel plate that is excellent in both low-temperature toughness and HIC resistance while ensuring a tensile strength of 535 MPa or more. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] One embodiment of the present invention relates to a method for manufacturing a clad steel plate formed by bonding a clad plate to one side of a base steel plate. The method for manufacturing a clad steel plate according to this embodiment includes the steps of: laminating a first material plate having a predetermined composition with a second material plate made of a corrosion-resistant alloy to obtain a clad slab; heating the clad slab to a predetermined heating temperature; hot rolling the clad slab under predetermined conditions to obtain a clad rolled plate in which the first material plate and the second material plate are bonded to each other to become a base steel plate and a clad plate, respectively; cooling the clad rolled plate; reheating and quenching the clad rolled plate to a predetermined reheating temperature and then cooling it; and tempering the clad rolled plate to a predetermined tempering temperature. The first material plate is a material for the base steel plate, and the second material plate is a material for the clad plate.

[0027] The thickness of the clad steel plate manufactured in this embodiment (total thickness of the base steel plate and clad plate) is 25 mm or more, and preferably 30 mm or more. There is no particular upper limit to the thickness of the clad steel plate manufactured in this embodiment, but the thickness of the clad steel plate is preferably, for example, 50 mm or less. In addition, the thicknesses of the base steel plate and clad plate in the clad steel plate manufactured in this embodiment are preferably 8 mm or more and 46 mm or less and 3.0 mm or more and 4.0 mm or less, respectively.

[0028] 1. Composition of base steel sheet and first material sheet The composition of the base steel sheet and the first blank sheet will be described. Note that the unit of the content of elements in the composition is always "mass %", but hereinafter, unless otherwise specified, it will simply be represented by "%".

[0029] (1-1) First embodiment In the first embodiment, the composition of the base steel plate and the first blank plate is, in mass%, C: 0.020% or more and 0.100% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.75% or more and 1.80% or less, P: 0.015% or less, S: 0.0030% or less, Al: 0.01% or more and 0.07% or less, Nb: 0.005% or more and 0.080% or less, Ti: 0.005% or more and 0.030% or less. %, N: 0.0010% or more and 0.0060% or less, and O (oxygen): 0.0030% or less, and further containing one or more selected from the group consisting of Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.100% or less, and Ca: 0.0040% or less, with the balance being Fe and unavoidable impurities. This first embodiment has a composition necessary to achieve a tensile strength of 535 MPa or more and excellent low-temperature toughness.

[0030] C: 0.020% or more and 0.100% or less C is an element that contributes to precipitation strengthening as a carbide. If the C content is less than 0.020%, sufficient strength cannot be ensured in the base steel plate. Therefore, the C content is set to 0.020% or more, and preferably 0.030% or more. On the other hand, if the C content exceeds 0.100%, the low-temperature toughness and the toughness of the welded heat-affected zone of the base steel plate deteriorate, and the HIC resistance property also deteriorates. Therefore, the C content is set to 0.100% or less, and preferably 0.080% or less.

[0031] Si: 0.10% or more and 0.50% or less Silicon is an element added to ensure the strength of steel sheets by deoxidization and solid solution strengthening. From the viewpoint of obtaining this effect, the silicon content is set to 0.10% or more, preferably 0.15% or more, and more preferably 0.20% or more. On the other hand, if the silicon content exceeds 0.50%, the toughness and weldability deteriorate. Therefore, the silicon content is set to 0.50% or less, and preferably 0.40% or less.

[0032] Mn: 0.75% or more and 1.80% or less Mn is an element added to ensure strength and toughness. From the viewpoint of obtaining this effect, the Mn content is set to 0.75% or more, and preferably 1.00% or more. On the other hand, if the Mn content exceeds 1.80%, the weldability deteriorates. Therefore, the Mn content is set to 1.80% or less, and preferably 1.70% or less.

[0033] P:0.015% or less P is an inevitable impurity that deteriorates weldability. Therefore, the P content is set to 0.015% or less, and preferably 0.010% or less. There is no particular lower limit for the P content, and the P content can be 0.000% or more.

[0034] S: 0.0030% or less S is generally present in steel as sulfide-based inclusions and deteriorates ductility and toughness. Therefore, it is preferable to reduce S as much as possible, and the S content is set to 0.0030% or less, preferably 0.0010% or less. There is no particular lower limit for the S content, and the S content can be 0.0000% or more.

[0035] Al: 0.01% or more and 0.07% or less Al is an element added for deoxidation. From the viewpoint of obtaining this effect, the Al content is set to 0.01% or more, and preferably 0.02% or more. On the other hand, if the Al content exceeds 0.07%, alumina clusters are formed and ductility is deteriorated. Therefore, the Al content is set to 0.07% or less, and preferably 0.04% or less.

[0036] Nb: 0.005% or more and 0.080% or less Nb is effective in increasing the strength of steel plates by precipitation strengthening and increasing hardenability. It also has the effect of expanding the austenite non-recrystallization temperature range, and contributes to improving toughness by refining the structure. From the viewpoint of obtaining these effects, the Nb content is set to 0.005% or more, and preferably 0.010% or more. On the other hand, if the Nb content exceeds 0.080%, the toughness and HIC resistance characteristics of the weld heat affected zone deteriorate. Therefore, the Nb content is set to 0.080% or less, and preferably 0.060% or less.

[0037] Ti: 0.005% or more and 0.030% or less Ti suppresses the coarsening of austenite through the pinning effect of nitride formation, and contributes to ensuring the toughness of the base material and the welded heat affected zone. Ti is also an effective element for increasing the strength of steel plates through precipitation strengthening. From the viewpoint of obtaining these effects, the Ti content is set to 0.005% or more, and preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.030%, the nitrides become coarse and become the starting point of brittle fracture or ductile fracture. Therefore, the Ti content is set to 0.030% or less, and preferably 0.020% or less.

[0038] N: 0.0010% or more and 0.0060% or less N suppresses the coarsening of austenite through the pinning effect of nitride formation, and contributes to ensuring the toughness of the base material and the welded heat affected zone. Therefore, the N content is set to 0.0010% or more, and preferably 0.0020% or more. On the other hand, if the N content exceeds 0.0060%, the toughness of the welded heat affected zone deteriorates. Therefore, the N content is set to 0.0060% or less, and preferably 0.0050% or less.

[0039] In order to fully exert the pinning effect due to nitride formation, it is preferable to regulate the ratio of Ti content to N content. Specifically, when the Ti content / N content is less than 2.0 or exceeds 3.5 in mass%, the pinning effect due to nitride formation is not fully exerted, and austenite may become coarse and the toughness may deteriorate. Therefore, it is preferable to set the Ti content / N content to 2.0 or more and 3.5 or less.

[0040] O (oxygen): 0.0030% or less O generally exists in steel as oxides of Al and Ca. If O is contained in excess, the oxides in the steel increase, and the toughness and HIC resistance deteriorate. Therefore, the O content is set to 0.0030% or less, and preferably 0.0020% or less. The lower limit of the O content is not particularly limited, and the O content can be 0.0000% or more.

[0041] The composition of the base steel plate and the first material plate contains, in addition to the basic components described above, one or more elements selected from the group consisting of Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.100% or less, and Ca: 0.0040% or less.

[0042] Cu: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less Cu, Cr, and Mo are all elements that improve hardenability and contribute to increasing the strength of the base material and the welded heat affected zone. In order to obtain this effect, when Cu, Cr, and Mo are contained, the content of each element is 0.01% or more, preferably 0.05% or more. On the other hand, when the Cu, Cr, and Mo content each exceeds 0.50%, the toughness of the welded heat affected zone may deteriorate. Therefore, when Cu, Cr, and Mo are contained, the content of each element is 0.50% or less, preferably 0.40% or less.

[0043] Ni: 0.50% or less Ni is an element that improves hardenability and contributes to increasing the strength of the base material and the welded heat affected zone. From the viewpoint of obtaining this effect, when Ni is contained, the Ni content is set to 0.01% or more, and preferably 0.05% or more. On the other hand, since Ni is an expensive element, the inclusion of a large amount of Ni leads to an increase in costs. Therefore, when Ni is contained, the Ni content is set to 0.50% or less, and preferably 0.40% or less.

[0044] V:0.100% or less V contributes to increasing the strength of steel plate by precipitation strengthening. From the viewpoint of obtaining this effect, when V is contained, the V content is set to 0.010% or more. On the other hand, when the V content exceeds 0.100%, the toughness of the weld heat affected zone deteriorates. Therefore, when V is contained, the V content is set to 0.100% or less.

[0045] Ca:0.0040% or less Ca fixes S in steel and improves the toughness of the steel plate. From the viewpoint of obtaining this effect, when Ca is contained, the Ca content is set to 0.0010% or more. On the other hand, when the Ca content exceeds 0.0040%, the inclusions in the steel increase, and the toughness may deteriorate. Therefore, when Ca is contained, the Ca content is set to 0.0040% or less, and preferably 0.0030% or less.

[0046] Carbon equivalent Ceq defined by the following formula (1): 0.40 or more and 0.50 or less The carbon equivalent Ceq defined by the following formula (1) correlates with the strength of the steel sheet. In the case of the reheating, quenching and tempering process which is the manufacturing method of this embodiment, in order to obtain a high strength of tensile strength of 535 MPa or more, the carbon equivalent Ceq is set to 0.40 or more, preferably 0.42 or more. On the other hand, if the carbon equivalent Ceq is too large, there is a risk of deterioration of toughness. Therefore, the carbon equivalent Ceq is set to 0.50 or less, preferably 0.48 or less. Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 ···(1) In addition, the element symbols in formula (1) represent the content (mass%) of each element in the base steel plate or the first material plate, and zero is substituted when the element is not contained.

[0047] (1-2) Second embodiment Next, the composition of the base steel plate and the first blank sheet according to the second embodiment, which is necessary for the base steel plate to exhibit excellent HIC resistance in addition to a tensile strength of 535 MPa or more and excellent low-temperature toughness, will be described. Items different from the first embodiment and items added will be described below. Items not described below conform to the first embodiment.

[0048] P:0.010% or less P is an inevitable impurity that increases the hardness of the center segregation portion, thereby deteriorating the HIC resistance. Therefore, when HIC resistance is required, the P content is preferably 0.010% or less, and more preferably 0.006% or less. There is no particular lower limit for the P content, and the P content can be 0.000% or more.

[0049] S: 0.0010% or less S is generally present in steel as sulfide-based inclusions, which deteriorates toughness and HIC resistance. Therefore, when HIC resistance is required, the S content is preferably 0.0010% or less, and more preferably 0.0008% or less. The lower limit of the S content is not particularly limited, and may be 0.0000% or more.

[0050] Ca: 0.0005% or more and 0.0040% or less Ca fixes S in steel to improve the toughness and HIC resistance of the steel plate. From the viewpoint of obtaining these effects, the Ca content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more. On the other hand, if the Ca content exceeds 0.0040%, the inclusions in the steel increase, and the toughness and HIC resistance may deteriorate. Therefore, the Ca content is preferably 0.0040% or less, and more preferably 0.0030% or less.

[0051] When HIC resistance is required, the ACR and P defined by the following formulas (2) and (3), respectively, are HIC It is preferable to appropriately control the value of ACR={Ca-(0.18+130Ca)×O} / (1.25S) ···(2) P HIC =4.46C+2.37Mn / 6+(1.18Cr+1.95Mo+1.74V) / 5+(1.74Cu+1.7Ni) / 15+22.36P ···(3) In addition, the element symbols in formulas (2) and (3) represent the content (mass%) of each element in the base steel plate or the first material plate, and zero is substituted when the element is not contained.

[0052] ACR: 1.00 or more and 6.00 or less ACR is a numerical value that is an index of the effect of Ca in suppressing the formation of MnS. MnS in steel affects hydrogen-induced cracking, and in order to improve HIC resistance, it is effective to fix S in steel with Ca and suppress the formation of MnS. If ACR is 1.00 or more, the effect of suppressing the formation of MnS is fully exhibited. Therefore, ACR is preferably 1.00 or more, and more preferably 1.40 or more. On the other hand, if the ACR value exceeds 6.00, although the formation of MnS is suppressed, a large amount of Ca-based oxide is formed, and the HIC resistance property is deteriorated. Therefore, ACR is preferably 6.00 or less, and more preferably 3.30 or less.

[0053] P HIC : Less than 1.250 P HIC is a numerical value that indicates the hardness of the central segregation region, and the larger this numerical value, the higher the hardness of the central segregation region. Since the hardness of the central segregation region affects hydrogen-induced cracking, it is effective to suppress the increase in hardness of the central segregation region in order to improve HIC resistance. For this reason, P HIC is preferably less than 1.250, and more preferably 1.000 or less. HIC Although there is no particular lower limit for P, from the viewpoint of ensuring sufficient strength in the base steel sheet, HIC is preferably 0.750 or more.

[0054] 2. Microstructure of base steel sheet (preferable conditions) In the clad steel plate manufactured according to this embodiment, the base steel plate preferably has the following microstructure.

[0055] Area ratio of bainite at 1 / 2 position in the thickness direction of the base steel plate: 94% or more Bainite is an important structure for achieving both strength and low-temperature toughness. Bainite is also a transformation strengthening structure that effectively contributes to improving the strength of steel plate. For this reason, it is preferable that the steel structure of the base steel plate is a structure mainly composed of bainite. Specifically, it is preferable that the area ratio of bainite at the 1 / 2 position in the plate thickness direction of the base steel plate (hereinafter also simply referred to as the "1 / 2 position of plate thickness") is 94% or more. The area ratio of bainite is more preferably 97% or more, and may be 100%.

[0056] The steel structure of the base steel plate must be basically composed of bainite, but may contain trace amounts of island martensite (MA: martensite-austenite constituent), ferrite, cementite, etc. as remaining structures other than bainite. The total area ratio of the remaining structures at the 1 / 2 position of the plate thickness is acceptable if it is 6% or less. The area ratio of the remaining structures may be 0%. If the area ratio of the remaining structures exceeds 6%, the area ratio of bainite becomes small, which is not preferable because it may not be possible to achieve both strength and low-temperature toughness.

[0057] Here, the area ratio of each phase at the 1 / 2 sheet thickness position can be obtained as follows: After mirror polishing the L cross section (a cross section parallel to the rolling direction and parallel to the normal direction of the rolling surface) of the base steel sheet, it is etched with nital or electrolytic etching (electrolyte: 100 ml distilled water + 25 g sodium hydroxide + 5 g picric acid), and a scanning electron microscope (SEM) is used to measure the area ratio of 1.2 × 10 randomly selected areas at the 1 / 2 sheet thickness position on the L cross section at a magnification of 2000 times. -2 mm 2 This can be determined by observing the area and analyzing the images.

[0058] Average grain size of bainite at 1 / 2 thickness: 23μm or less Since the grain boundaries of bainite act as resistance to the propagation of brittle cracks, refining the grains increases the number of grain boundaries, which contributes to improving low-temperature toughness. Therefore, the average grain size of bainite at the 1 / 2 position of the plate thickness is preferably 23 μm or less, and more preferably 20 μm or less. There is no particular lower limit for the average grain size, but the average grain size may be 5 μm or more.

[0059] Here, the average grain size of bainite can be obtained as follows: After mirror-polishing the L-section (a section parallel to the rolling direction and parallel to the normal direction of the rolling surface) of the base steel sheet, 1.2 × 10 randomly selected grains at the 1 / 2 sheet thickness position on the L-section are subjected to electron backscattering pattern (EBSP) analysis. -2 mm 2 The crystal orientation of the region is measured, and the region where the angle difference between adjacent pixels is 15° or more is determined as the grain boundary by image analysis. area is the area occupied by each grain a i and the equivalent circle diameter of each grain d i It can be calculated using the following formula: d area =Σ(a i d i ) / Σa i

[0060] 3. Cladding plate and second material plate In this embodiment, the clad plate and the second blank plate are made of a corrosion-resistant alloy. The corrosion-resistant alloy is preferably a Ni-based alloy or an austenitic stainless steel. In particular, a Ni-based alloy is preferable because it exhibits high stress corrosion cracking resistance in a high hydrogen sulfide partial pressure environment (sour environment). The Ni-based alloy is, for example, Alloy 625 or Alloy 825, and from the viewpoint of stress corrosion cracking resistance, Alloy 625 is particularly preferable.

[0061] The following describes the preferred composition of the cladding plate and the second blank plate. Note that the unit of the content of elements in the composition is always "mass%", but hereinafter, unless otherwise specified, it will simply be represented by "%".

[0062] (3-1) Optimal composition of Alloy 625 Alloy625 is a Ni-based alloy equivalent to NCF625 of JIS G 4902:2019, and among them, a Ni-based alloy containing, in mass%, C: 0.030% or less, Si: 0.02 to 0.50%, Mn: 0.02 to 0.50%, P: 0.010% or less, S: 0.0010% or less, Cr: 20.0 to 23.0%, Mo: 8.00 to 10.00%, Fe: 5.0% or less, Al: 0.02 to 0.40%, Ti: 0.10 to 0.40%, and a total amount of Nb and Ta: 3.15 to 4.15%, with the balance being Ni and unavoidable impurities.

[0063] C: 0.030% or less C precipitates as carbides at grain boundaries during the thermal history during the manufacture of clad steel plate, degrading corrosion resistance. If the C content exceeds 0.030%, the precipitation of carbides is promoted, degrading corrosion resistance. Therefore, the C content is preferably 0.030% or less, and more preferably 0.020% or less. The lower limit of the C content is not particularly limited, and the C content can be 0.000% or more.

[0064] Silicon: 0.02 to 0.50% Si is added for deoxidation. From the viewpoint of obtaining this effect, the Si content is preferably 0.02% or more. On the other hand, if the Si content exceeds 0.50%, the corrosion resistance deteriorates. Therefore, the Si content is preferably 0.50% or less, and more preferably 0.20% or less.

[0065] Mn: 0.02 to 0.50% Mn is added for deoxidation. From the viewpoint of obtaining this effect, the Mn content is preferably 0.02% or more. On the other hand, if the Mn content exceeds 0.50%, the corrosion resistance deteriorates. Therefore, the Mn content is preferably 0.50% or less, and more preferably 0.15% or less.

[0066] P:0.010% or less P is an impurity element that segregates at grain boundaries and deteriorates corrosion resistance. Therefore, the P content is preferably 0.010% or less, and more preferably 0.005% or less. There is no particular lower limit for the P content, and the P content can be 0.000% or more.

[0067] S: 0.0010% or less Like P, S is an impurity element that segregates to grain boundaries and deteriorates corrosion resistance. Therefore, the S content is preferably 0.0010% or less, and more preferably 0.0005% or less. There is no particular lower limit for the S content, and the S content can be 0.0000% or more.

[0068] Cr: 20.0~23.0% Cr forms a highly protective oxide film on the surface of the metal, improving pitting corrosion resistance and intergranular corrosion resistance. In addition, Cr improves stress corrosion cracking resistance in sour environments when added in combination with Ni. From the viewpoint of obtaining these effects, the Cr content is preferably 20.0% or more, and more preferably 21.5% or more. However, from the viewpoint of the balance with Ni and other alloys, the Cr content is preferably 23.0% or less.

[0069] Mo: 8.00~10.00% Mo improves pitting corrosion resistance and crevice corrosion resistance. In addition, Mo improves stress corrosion cracking resistance in sour environments when added in combination with Ni. From the viewpoint of obtaining these effects, the Mo content is preferably 8.00% or more. However, from the viewpoint of the balance with Ni and other alloys, the Mo content is preferably 10.00% or less.

[0070] Fe:5.0% or less Fe is an impurity that is inevitably mixed in when ferrochromium, ferromolybdenum, etc. are used as raw materials, and if the Fe content exceeds 5.0%, the corrosion resistance deteriorates. Therefore, the Fe content is preferably 5.0% or less, and more preferably 3.5% or less. There is no particular lower limit for the Fe content, and the Fe content can be 0.0% or more.

[0071] Al: 0.02 to 0.40% Al is an effective deoxidizing element. From the viewpoint of obtaining this effect, the Al content is preferably 0.02% or more. On the other hand, if the Al content exceeds 0.40%, the stress corrosion cracking resistance deteriorates. Therefore, the Al content is preferably 0.40% or less, and more preferably 0.25% or less.

[0072] Ti: 0.10-0.40% Ti fixes C, which deteriorates corrosion resistance, by precipitating it as carbide. From the viewpoint of obtaining this effect, the Ti content is preferably 0.10% or more. On the other hand, if the Ti content exceeds 0.40%, Ti precipitates as an intermetallic compound, which deteriorates the bondability with the base steel sheet. Therefore, the Ti content is preferably 0.40% or less, and more preferably 0.30% or less.

[0073] Total content of Nb and Ta: 3.15 to 4.15% Both Nb and Ta are effective in fixing C. From the viewpoint of obtaining this effect, the total amount of Nb and Ta is preferably 3.15% or more. On the other hand, if the total amount of Nb and Ta exceeds 4.15%, Nb and Ta form an intermetallic compound with a low melting point, and the hot workability is deteriorated. Therefore, the total amount of Nb and Ta is preferably 4.15% or less.

[0074] The other components are Ni and unavoidable impurities. Ni is an element that improves corrosion resistance, and particularly improves stress corrosion cracking resistance in sour environments. For this reason, the Ni content is preferably 58% or more.

[0075] (3-2) Optimal composition of Alloy 825 Alloy825 is a Ni-based alloy equivalent to NCF825 in JIS G 4902:2019, and among them, a Ni-based alloy having a composition containing, in mass%, C: 0.020% or less, Si: 0.50% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Ni: 38.0 to 46.0%, Cr: 19.5 to 23.5%, Mo: 2.50 to 3.50%, Cu: 1.50 to 3.00%, Al: 0.01 to 0.20%, and Ti: 0.60 to 1.20%, with the balance being Fe and unavoidable impurities, is preferred.

[0076] C: 0.020% or less C precipitates as carbides at grain boundaries during the thermal history during the manufacture of clad steel plate, degrading corrosion resistance. If the C content exceeds 0.020%, the precipitation of carbides is promoted, degrading corrosion resistance. Therefore, the C content is preferably 0.020% or less, and more preferably 0.015% or less. The lower limit of the C content is not particularly limited, and the C content may be 0.000% or more.

[0077] Si:0.50% or less Silicon is added for deoxidation. However, if the silicon content exceeds 0.50%, silicon remains as nonmetallic inclusions, and corrosion resistance deteriorates. Therefore, the silicon content is preferably 0.50% or less, and more preferably 0.20% or less. There is no particular lower limit for the silicon content, but from the viewpoint of obtaining a sufficient deoxidation effect, the silicon content is preferably 0.02% or more.

[0078] Mn: 1.00% or less Mn is added for deoxidation. However, if the Mn content exceeds 1.00%, the corrosion resistance deteriorates. Therefore, the Mn content is preferably 1.00% or less, and more preferably 0.50% or less. There is no particular lower limit for the Mn content, but from the viewpoint of obtaining a sufficient deoxidation effect, the Mn content is preferably 0.02% or more.

[0079] P:0.030% or less P is an impurity element that segregates at grain boundaries and deteriorates corrosion resistance. Therefore, the P content is preferably 0.030% or less, and more preferably 0.020% or less. The lower limit of the P content is not particularly limited, and the P content can be 0.000% or more.

[0080] S: 0.0050% or less Like P, S is an impurity element that segregates at grain boundaries and deteriorates corrosion resistance. Therefore, the S content is preferably 0.0050% or less, and more preferably 0.0010% or less. There is no particular lower limit for the S content, and the S content can be 0.0000% or more.

[0081] Ni: 38.0~46.0% Ni is an element that improves corrosion resistance, and in particular significantly improves stress corrosion cracking resistance in sour environments. From the viewpoint of obtaining this effect, the Ni content is preferably 38.0% or more. However, since Ni is a very expensive element, adding a large amount of Ni increases costs. For this reason, from the viewpoint of the balance between the effect of improving corrosion resistance and costs, the Ni content is preferably 46.0% or less.

[0082] Cr: 19.5~23.5% Cr forms a highly protective oxide film on the surface of the metal, improving pitting corrosion resistance and intergranular corrosion resistance. In addition, Cr improves stress corrosion cracking resistance in sour environments when added in combination with Ni. From the viewpoint of obtaining these effects, the Cr content is preferably 19.5% or more, and more preferably 21.5% or more. However, from the viewpoint of the balance with Ni and other alloys, the Cr content is preferably 23.5% or less.

[0083] Mo: 2.50-3.50% Mo improves pitting corrosion resistance and crevice corrosion resistance. In addition, Mo improves stress corrosion cracking resistance in sour environments when added in combination with Ni. From the viewpoint of obtaining these effects, the Mo content is preferably 2.50% or more, and more preferably 3.00% or more. However, from the viewpoint of the balance with Ni and other alloys, the Mo content is preferably 3.50% or less.

[0084] Cu: 1.50-3.00% Cu improves general corrosion resistance. From the viewpoint of obtaining this effect, the Cu content is preferably 1.50% or more, and more preferably 1.80% or more. On the other hand, when the Cu content exceeds 3.00%, the effect is saturated, so that the Cu content is preferably 3.00% or less.

[0085] Al: 0.01 to 0.20% Al is an effective deoxidizing element. From the viewpoint of obtaining this effect, the Al content is preferably 0.01% or more, and more preferably 0.10% or more. On the other hand, if the Al content exceeds 0.20%, the stress corrosion cracking resistance deteriorates. Therefore, the Al content is preferably 0.20% or less, and more preferably 0.15% or less.

[0086] Ti: 0.60-1.20% Ti fixes C, which deteriorates corrosion resistance, by precipitating it as carbide. From the viewpoint of obtaining this effect, the Ti content is preferably 0.60% or more, and more preferably 0.70% or more. On the other hand, if the Ti content exceeds 1.20%, Ti precipitates as an intermetallic compound, which deteriorates the bondability with the base steel sheet, so the Ti content is preferably 1.20% or less.

[0087] The components other than those mentioned above are Fe and unavoidable impurities.

[0088] (3-3) Austenitic stainless steel A suitable chemical composition of the austenitic stainless steel contains, in mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.045% or less, S: 0.030% or less, Ni: 12.0 to 15.0%, Cr: 16.0 to 18.0%, and Mo: 2.00 to 3.00%, with the balance being Fe and unavoidable impurities.

[0089] C: 0.030% or less C precipitates as carbides at grain boundaries during the thermal history during the manufacture of clad steel plate, degrading corrosion resistance. If the C content exceeds 0.030%, the precipitation of carbides is promoted, degrading corrosion resistance. Therefore, the C content is preferably 0.030% or less, more preferably 0.020% or less, and even more preferably 0.015% or less. There is no particular lower limit for the C content, and the C content may be 0.000% or more.

[0090] Si: 1.00% or less If the Si content exceeds 1.00%, Si remains as a nonmetallic inclusion, and the corrosion resistance deteriorates. Therefore, the Si content is preferably 1.00% or less, and more preferably 0.75% or less. The lower limit of the Si content is not particularly limited, and the Si content can be 0.00% or more.

[0091] Mn: 2.00% or less If the Mn content exceeds 2.00%, the corrosion resistance deteriorates. Therefore, the Mn content is preferably 2.00% or less, more preferably 1.40% or less, and even more preferably 1.00% or less. The lower limit of the Mn content is not particularly limited, and the Mn content can be 0.00% or more.

[0092] P:0.045% or less P is an impurity element that segregates at grain boundaries and deteriorates corrosion resistance. Therefore, the P content is preferably 0.045% or less, and more preferably 0.030% or less. The lower limit of the P content is not particularly limited, and the P content can be 0.000% or more.

[0093] S: 0.030% or less Like P, S is an impurity element that segregates at grain boundaries and deteriorates corrosion resistance. Therefore, the S content is preferably 0.030% or less, and more preferably 0.010% or less. There is no particular lower limit for the S content, and the S content can be 0.000% or more.

[0094] Ni: 12.0-15.0% Ni is an element that improves corrosion resistance, and particularly significantly improves stress corrosion cracking resistance in sour environments. From the viewpoint of obtaining this effect, the Ni content is preferably 12.0% or more, and more preferably 12.5% ​​or more. However, since Ni is a very expensive element, adding a large amount of Ni increases costs. For this reason, from the viewpoint of the balance between the effect of improving corrosion resistance and costs, the Ni content is preferably 15.0% or less, and more preferably 14.5% or less.

[0095] Cr: 16.0~18.0% Cr forms a highly protective oxide film on the surface of the metal, improving pitting corrosion resistance and intergranular corrosion resistance. In addition, Cr improves stress corrosion cracking resistance in sour environments when added in combination with Ni. From the viewpoint of obtaining these effects, the Cr content is preferably 16.0% or more, and more preferably 16.5% or more. However, from the viewpoint of the balance with Ni and other alloys, the Cr content is preferably 18.0% or less, and more preferably 17.5% or less.

[0096] Mo: 2.00-3.00% Mo improves pitting corrosion resistance and crevice corrosion resistance. In addition, Mo improves stress corrosion cracking resistance in sour environments when added in combination with Ni. From the viewpoint of obtaining these effects, the Mo content is preferably 2.00% or more, and more preferably 2.20% or more. However, from the viewpoint of the balance with Ni and other alloys, the Mo content is preferably 3.00% or less, and more preferably 2.80% or less.

[0097] The components other than those mentioned above are Fe and unavoidable impurities.

[0098] 4. Manufacturing conditions of clad steel plate A manufacturing method of a clad steel plate according to one embodiment of the present invention includes the steps of stacking a first material plate having the above-mentioned composition and a second material plate made of a corrosion-resistant alloy to obtain a clad slab, heating the clad slab to 1050°C or more and 1200°C or less, and then subjecting the clad slab to hot rolling with a rolling end temperature of 900°C or more to obtain a clad rolled plate in which the first material plate and the second material plate are joined together to become a base steel plate and a clad plate, respectively, a step of cooling the clad rolled plate, a step of reheating the clad rolled plate to 850°C or more and 1050°C or less, a step of cooling the clad rolled plate so that the average cooling rate of the base steel plate is 2°C / s or more and 20°C / s or less, and a step of tempering the clad rolled plate to 450°C or more and 650°C or less.

[0099] In the present embodiment, all temperature conditions defined as manufacturing conditions relate to the steel sheet surface temperature, which can be measured using, for example, a radiation thermometer.

[0100] (4-1) Process for obtaining clad slabs First, a first material plate which is a base steel plate and a second material plate which is a clad plate are laminated to obtain a clad slab. -4 The first and second blank plates are temporarily joined together by, for example, electron beam welding or laser welding in an environment of a pressure of not more than torr, thereby obtaining a clad slab.

[0101] Also, it is preferable to form a clad slab by laminating the first blank plate, the second blank plate, the second blank plate, and the first blank plate in this order, with a release material applied between the two second blank plates. That is, the clad slab is formed by stacking two sets of laminates in which the first blank plate and the second blank plate are laminated, with the second blank plates facing each other. The clad slab is hot-rolled to obtain two clad rolled plates. After tempering, the two clad rolled plates are peeled off via the release material to obtain a clad steel plate that will be the product plate. In this case, symmetry in the plate thickness direction is maintained during rolling, so that the occurrence of warping in the vertical direction during rolling can be suppressed.

[0102] (4-2) Clad rolling process Clad slab heating temperature: 1050℃ to 1200℃ If the clad slab heating temperature is less than 1050°C, the reduction ratio cannot be sufficiently ensured in the rolling from the start of rolling after heating to the rolling end temperature described later, and the bondability between the base steel plate and the clad plate in the finally obtained clad steel plate is deteriorated. In addition, if the slab heating temperature is less than 1050°C, Nb and the like contained in the first material plate that becomes the base steel plate is not sufficiently dissolved, and it is difficult to ensure the strength of the base steel plate in the finally obtained clad steel plate. For this reason, the heating temperature of the clad slab is 1050°C or higher. On the other hand, if the clad slab heating temperature exceeds 1200°C, austenite coarsens in the first material plate that becomes the base steel plate, and the structure of the base steel plate cannot be sufficiently refined in the finally obtained clad steel plate, so that the toughness is deteriorated. Therefore, the clad slab heating temperature is 1200°C or lower, preferably 1100°C or lower.

[0103] Rolling end temperature: 900℃ or higher The bondability between the base steel plate and the clad plate in the clad steel plate is ensured by rolling in a high temperature range. That is, in rolling in a high temperature range, the deformation resistance of the base steel plate and the clad plate is reduced and a good bonded interface is formed, which facilitates mutual diffusion of elements at the bonded interface, thereby ensuring the bondability between the base steel plate and the clad plate. For this reason, the rolling end temperature is 900°C or higher, and preferably 950°C or higher. The upper limit of the rolling end temperature is not particularly limited, but the rolling end temperature may be 1000°C or lower.

[0104] (4-3) Cooling process After the above hot rolling is completed, the obtained clad rolled sheet is allowed to cool naturally. Here, cooling naturally refers to a process in which the clad rolled sheet after rolling is not forcibly cooled, but is placed in a predetermined storage area such as a yard in a factory and left as it is to wait for the temperature of the clad rolled sheet to drop. Therefore, the cooling rate is not particularly limited. The cooling stop temperature in the cooling naturally may be any temperature as long as it is a temperature at which the ferrite transformation is completely completed. In practice, it is often necessary to wait for the temperature to drop to near room temperature (e.g., 25°C), but it is not necessarily required that the temperature be dropped to near room temperature. The cooling stop temperature in the cooling naturally may be, for example, 100°C or less.

[0105] (4-4) Reheating and quenching (reheating and cooling) Next, reheating and quenching is carried out in which the clad rolled plate is reheated and then cooled.

[0106] Reheating temperature: 850℃ or higher and 1050℃ or lower First, in the reheating treatment, the naturally cooled clad rolled sheet is heated to a reheating temperature of 850°C or more and 1050°C or less. If the reheating temperature is less than 850°C, ferrite is generated in the base steel sheet prior to bainite transformation, and the target strength is not obtained and the Charpy absorbed energy is also reduced. Therefore, the reheating temperature is set to 850°C or more, preferably 900°C or more. On the other hand, if the reheating temperature exceeds 1050°C, coarsening of austenite grains occurs, and low-temperature toughness is significantly reduced. Therefore, the reheating temperature is set to 1050°C or less, preferably 1000°C or less.

[0107] Average cooling rate of base steel plate: 2℃ / s to 20℃ / s Next, the reheated clad rolled plate is subjected to a cooling treatment in which the average cooling rate in the base steel plate is 2°C / s or more and 20°C / s or less. If the average cooling rate is less than 2°C / s, ferrite transformation occurs in the base steel plate, the target strength cannot be obtained, and the low-temperature toughness is significantly reduced. Therefore, the average cooling rate is 2°C / s or more, preferably 5°C / s or more. From the viewpoint of refining the structure and reducing the hard phase that may be the starting point of brittle cracks, it is more preferable that the average cooling rate is 7°C / s or more. Although there is no particular limit to the upper limit of the average cooling rate, if the average cooling rate is too large, the strength of the transformed structure increases excessively, which may lead to a decrease in the Charpy absorbed energy. Therefore, from the viewpoint of preventing a decrease in the Charpy absorbed energy, it is preferable that the average cooling rate is 20°C / s or less. The average cooling rate is the value obtained by dividing the temperature difference from the cooling start temperature (i.e., the reheating temperature) to the cooling end temperature by the time required from the start of cooling to the end of cooling.

[0108] In order to prevent a decrease in the toughness of the base steel plate, the cooling stop temperature in the reheating and quenching is preferably 200° C. or less. The lower limit of the cooling stop temperature is not particularly limited, and the cooling stop temperature may be, for example, 25° C. or more.

[0109] (4-5) Tempering process Tempering temperature: 450℃ or higher and 650℃ or lower The cooled clad rolled plate is heated and tempered. If the tempering temperature is less than 450°C, the decomposition of island martensite and the recovery of dislocations are insufficient, and the toughness of the base steel plate deteriorates. Therefore, the tempering temperature is set to 450°C or higher. On the other hand, if the tempering temperature exceeds 650°C, precipitates such as cementite become coarse, and the toughness of the base steel plate deteriorates. In addition, the formation of precipitates may deteriorate the corrosion resistance of the clad plate. Therefore, the tempering temperature is set to 650°C or lower, and preferably 500°C or lower. EXAMPLES

[0110] A clad slab was prepared by laminating a first material plate (base steel plate material) having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) and a second material plate (clad plate material) of a corrosion-resistant alloy having the composition shown in Table 2 (the balance being Fe and unavoidable impurities) in the order of the first material plate, the second material plate, the second material plate, and the first material plate in this order, and the clad rolled plate was obtained by subjecting the clad rolled plate to clad rolling under the conditions shown in Table 3. Next, the obtained clad rolled plate was subjected to reheating quenching and tempering under the conditions shown in Table 3 to produce a clad steel plate with a plate thickness of 35 mm (base steel plate thickness: 32 mm, clad plate thickness: 3 mm). The cooling stop temperature for natural cooling was 30°C, and the cooling stop temperature for reheating quenching was 30°C.

[0111] From the thus obtained clad steel plate, test pieces for the tensile test and the DWTT test were taken. The tensile test was carried out in accordance with API-5L to determine the tensile strength TS and the yield strength YS. The DWTT test was carried out in accordance with API-5L at a test temperature of -30°C, and the ductile fracture surface area ratio DWTTSA -30℃ The results are shown in Table 4. Tensile strength and ductile fracture surface area ratio DWTTSA -30℃The target values ​​are as follows: Tensile strength: 535MPa or more Ductile fracture surface ratio: DWTTSA -30℃ : 85% or more

[0112] Furthermore, HIC tests were conducted in accordance with NACE Standard TM0284-2003 to evaluate the HIC resistance of the base steel plate by the crack area fraction (CAR). The target values ​​for the crack area fraction (CAR) are as follows: Crack area ratio (CAR): 5.0% or less (1.0% or less is particularly excellent) The crack area ratio (CAR) was measured by immersing three test pieces taken from the same steel plate in a 5 mass% NaCl + 0.5 mass% CH3COOH aqueous solution saturated with hydrogen sulfide for 96 hours, and then detecting defects on the test pieces using ultrasonic testing. The above evaluation was made based on the maximum crack area ratio (CAR) measured for the three test pieces. The results are shown in Table 4.

[0113] Furthermore, the area ratio and average grain size of bainite at the 1 / 2 position in the thickness direction of the base steel plate were measured by the above-mentioned method. These results are shown in Table 4.

[0114] From Table 4, all of the invention examples that satisfy claim 1 have a tensile strength of 535 MPa or more and a ductile fracture surface ratio DWTTSA -30℃ In addition, all of the invention examples that satisfy claim 2 have a tensile strength of 535 MPa or more and a ductile fracture surface ratio DWTTSA -30℃ On the other hand, in all of the comparative examples shown in Table 4, the desired properties were not obtained.

[0115] [Table 1]

[0116] [Table 2]

[0117] [Table 3]

[0118] [Table 4] [Industrial Applicability]

[0119] The clad steel plate produced according to the present invention exhibits excellent low-temperature toughness when applied to line pipes for transporting oil or natural gas, which are exposed to severe corrosive environments, and is therefore extremely useful industrially.

Claims

1. In mass percent, C: 0.020% or more and 0.100% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.75% or more and 1.80% or less, P: 0.015% or less, S: 0.0030% or less, Al: 0.01% or more and 0.07% or less, Nb: 0.005% or more and 0.080% or less, Ti: 0.005% or more and 0.030% or less, N: 0.0010% or more and 0.0060% or less, and O (oxygen): 0.0030% or less and further comprising Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.100% or less, and Ca: 0.0040% or less preparing a first material sheet having a composition in which Ceq defined by the following formula (1) is 0.40 or more and 0.50 or less, with the balance being Fe and unavoidable impurities; providing a second blank plate made of a corrosion-resistant alloy; A step of stacking the first material plate and the second material plate to obtain a clad slab; Heating the clad slab to a temperature of 1050° C. or more and 1200° C. or less; Then, the clad slab is subjected to hot rolling with a rolling end temperature of 900 ° C. or more to obtain a clad rolled plate in which the first material plate and the second material plate are joined together to form a base steel plate and a clad plate, respectively; Cooling the clad rolled plate; Thereafter, reheating the clad rolled plate to 850°C or higher and 1050°C or lower; Thereafter, cooling the clad rolled plate so that the average cooling rate of the base steel plate is 2 ° C. / s or more and 20 ° C. / s or less; Thereafter, tempering the clad rolled plate at 450°C or higher and 650°C or lower; The method for producing a clad steel plate includes the steps of: producing a clad steel plate by joining the cladding plate to one side of the base steel plate. Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5...(1) The element symbols in formula (1) represent the content (mass%) of each element, and zero is substituted when the element is not contained.

2. In the composition of the first material plate, the Ca content is, in mass%, from 0.0005% to 0.0040%, the ACR defined by the following formula (2) is from 1.00 to 6.00, and the P content defined by the following formula (3) is ＨＩＣ The method for producing a clad steel plate according to claim 1, wherein the tensile strength is less than 1.

250. ACR={Ca-(0.18+130Ca)×O} / (1.25S)...(2) P ＨＩＣ =4.46C+2.37Mn / 6+(1.18Cr+1.95Mo+1.74V) / 5+(1.74Cu+1.7Ni) / 15+22.36P ・・・(3) The element symbols in formulas (2) and (3) indicate the content (mass%) of each element, and zero is substituted when the element is not contained.

3. 3. The method for producing a clad steel plate according to claim 1, wherein the corrosion-resistant alloy is a Ni-based alloy selected from the group consisting of Alloy 825 and Alloy 625, or an austenitic stainless steel.

4. The clad slab is laminated in the order of the first material plate, the second material plate, the second material plate, and the first material plate, and a release material is applied between the two second material plates, thereby obtaining two clad rolled plates by hot rolling; The method for producing a clad steel plate according to claim 1 or 2, wherein after the tempering, the two clad rolled plates are peeled off via the peeling material.

5. The clad slab is laminated in the order of the first material plate, the second material plate, the second material plate, and the first material plate, and a release material is applied between the two second material plates, thereby obtaining two clad rolled plates by hot rolling; The method for producing a clad steel plate according to claim 3, wherein after the tempering, the two clad rolled plates are peeled off via the peeling material.