Method for manufacturing composite steel sheet

Abstract

Provided is a method for producing a clad steel sheet, with which it is possible to produce a thick clad steel sheet having excellent low-temperature toughness while ensuring a tensile strength of 535 MPa or more. This method for manufacturing a composite steel sheet comprises: a step for obtaining a composite slab by laminating a first blank sheet having a prescribed component composition and a second blank sheet comprising a corrosion-resistant alloy; a step for heating the composite slab to a predetermined temperature; a step in which, after that, the composite slab is hot-rolled under predetermined conditions, the first blank plate and the second blank plate are joined to obtain a composite rolled plate in which the first blank plate and the second blank plate are each used as a base steel plate and a bonded plate; cooling the composite rolled plate; a step in which the composite rolled sheet is then reheated to 850 DEG C to 1050 DEG C; a step in which the composite rolled sheet is then cooled such that the average cooling rate in the base steel sheet is 2-20 DEG C / s; and a step for tempering the composite rolled sheet to 450 DEG C to 650 DEG C.

Description

TECHNICAL FIELD

[0001] The present application relates to a manufacturing method of a clad steel plate, and particularly relates to a manufacturing method of a clad steel plate suitable for a base material of a line pipe. BACKGROUND

[0002] A clad steel plate refers to a steel plate obtained by joining a clad plate composed of a corrosion-resistant alloy to a base material steel plate such as carbon steel. The clad steel plate has an advantage of high economy because it can ensure corrosion resistance equivalent to that of a solid wood material while suppressing the usage amount of expensive alloy elements.

[0003] In recent years, the development of oil and natural gas has been expanded to regions where line pipes are exposed to severe corrosion environments. Consequently, it is expected that the demand for line pipes that can economically transport oil and natural gas will increase. Moreover, for Ni-based alloy clad steel plates and austenitic stainless clad steel plates among clad steel plates, the demand for use as line pipes exposed to severe corrosion environments is expected due to their high corrosion resistance.

[0004] In addition, in recent years, based on the background of a leakage accident of a line pipe in a wet hydrogen sulfide environment, the demand for improvement of the hydrogen-induced cracking resistance (hereinafter, also referred to as "HIC resistance") of a base material steel plate is required for a clad steel plate for a line pipe in order to direct a safer design.

[0005] Here, for a clad steel plate for a line pipe, in addition to the corrosion resistance of a clad plate, it is required to ensure prescribed mechanical properties, and particularly, from the viewpoint of preventing brittle fracture of a line pipe, it is required to ensure excellent low-temperature toughness.

[0006] As such a clad steel plate, for example, Patent Literature 1 discloses "a Ni alloy clad steel plate having excellent low-temperature toughness of a base material, HAZ toughness, and corrosion resistance of a clad material, characterized in that, in a clad steel plate in which a Ni alloy is used as a clad material and a low-alloy steel is used as a base material, the chemical composition of the above 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%, Nb: 0.005 to 0.080%, Ti: 0.005 to 0.030%, N: 0.0010 to 0.0060%, Al: 0.070% or less, Ca: 0.0010 to 0.0040%, and the remainder is composed of Fe and inevitable impurities".

[0007] In addition, Patent Literature 2 discloses "A Ni alloy clad steel sheet having excellent low-temperature toughness of base material and HAZ toughness and corrosion resistance of a clad material, characterized in that, in a clad steel sheet in which a low alloy steel is used as a base material and a Ni alloy is used as a clad 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%, Nb: 0.005 to 0.080%, Ti: 0.005 to 0.030%, N: 0.0010 to 0.0060%, Al: 0.070% or less, Ca: 0.0010 to 0.0040%, and the remainder consists of Fe and inevitable impurities."

[0008] Further, Patent Literature 3 discloses "An austenitic stainless steel clad steel sheet having excellent low-temperature toughness of base material and HAZ toughness and corrosion resistance of a clad material, characterized in that, in a clad steel sheet in which an austenitic stainless steel is used as a clad material and a low alloy steel is used 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%, Nb: 0.005 to 0.080%, Ti: 0.005 to 0.030%, N: 0.0010 to 0.0060%, Al: 0.070% or less, Ca: 0.0010 to 0.0040%, and the remainder consists of Fe and inevitable impurities."

[0009] Further, in Patent Literature 4, there is disclosed "a clad steel sheet in which a cladding material composed of a corrosion-resistant alloy is joined to one side surface of a base steel sheet, and having a composition consisting of, in mass%, C: 0.020 to 0.100%, Si: 0.05 to 0.50%, Mn: 0.75 to 1.80%, P: 0.015% or less, S: 0.0030% or less, Al: 0.010 to 0.070%, Nb: 0.005 to 0.080%, Ti: 0.005 to 0.030%, and N: 0.0010 to 0.0060%, with the remainder consisting of Fe and inevitable impurities; and further, the base steel sheet has a steel structure having, at a position of 1 / 2 of the sheet thickness in the sheet thickness direction of the base steel sheet, 94% or more of bainite and 6% or less of island-shaped martensite in terms of area ratio, the average crystal grain diameter of the bainite being 25 μm or less, and the shear strength of the joint interface between the base steel sheet and the cladding material being 300 MPa or more".

[0010] Further, in Patent Literature 5, as a technique for improving HIC resistance, and as "a clad steel sheet in which the low-temperature toughness is further improved while ensuring a tensile strength of 535 MPa or more, and the HIC resistance is also excellent", there is disclosed "a clad steel sheet in which a cladding material composed of a corrosion-resistant alloy is joined to one side surface of a base steel sheet, in which the base steel sheet has a composition in which the values of ACR and PHIC are appropriately controlled, and further, the base steel sheet has, at a position of 1 / 2 of the sheet thickness in the sheet thickness direction of the base steel sheet, 94% or more of bainite in terms of area ratio, and the average crystal grain diameter of the bainite is 25 μm or less, and the shear strength of the joint interface between the base steel sheet and the cladding material is 300 MPa or more".

[0011] Prior Art Documents

[0012] Patent Literature

[0013] Patent Literature 1: Japanese Patent Application Publication No. 2015-86422

[0014] Patent Literature 2: Japanese Patent Application Publication No. 2015-117408

[0015] Patent Literature 3: Japanese Patent Application Publication No. 2015-105399

[0016] Patent Literature 4: International Publication No. 2018 / 181381

[0017] Patent Literature 5: International Publication No. 2020 / 004410 SUMMARY

[0018] In the technologies of Patent Documents 1 to 3, by applying TMCP (Thermo-mechanical control process), the plate thickness: 30 mm or less, the tensile strength: 535 MPa or more, the DWTT SA －20℃ of 85% or more of mechanical properties are realized together with the corrosion resistance of the clad material without affecting the productivity. Further, in Patent Document 4, by directly applying quenching and tempering after rolling the clad slab, the DWTT SA －30℃ of 85% or more of mechanical properties are realized. In Patent Document 5, a clad steel sheet is disclosed, which, like Patent Document 4, realizes the DWTT SA －30℃ of 85% or more of mechanical properties, and also has HIC resistance.

[0019] Here, the DWTT SA －20℃ and the DWTT SA －30℃ are the ductile fracture ratios obtained at the test temperatures of -20°C and -30°C, respectively, in the DWTT test (Drop Weight Tear Test) according to API-5L.

[0020] In order to improve the low-temperature toughness which is the focus of the present application, it is essential to refine the microstructure of the base steel sheet in the clad steel sheet. In the technologies described in Patent Documents 1 to 5, by ensuring the reduction rate within a specific temperature range to a prescribed degree at the time of rolling, and by performing accelerated cooling, coarsening accompanying high-temperature phase transformation is avoided, and appropriate phase transformation strengthening is realized. However, if the clad steel sheet is thick, for example, 25 mm or more, not only is it difficult to apply rolling strain to the inside of the steel sheet, but also because the cooling behavior of the steel sheet is limited by the thermal conductivity of the steel sheet, even if accelerated cooling is performed, it is difficult to achieve the desired cooling rate in the central portion of the plate thickness. As a result, the fine-graining as in the case of a thin clad steel sheet cannot be achieved, and there is room for improvement in the low-temperature toughness, and therefore, a manufacturing technique capable of realizing high strength and excellent low-temperature toughness in a thick-walled clad steel sheet is required.

[0021] In view of the above problems, a first object of the present application is to provide a manufacturing method of a clad steel sheet capable of manufacturing a thick-walled clad steel sheet excellent in low-temperature toughness while ensuring a tensile strength of 535 MPa or more.

[0022] Further, a second object of the present application is to provide a manufacturing method of a clad steel sheet capable of manufacturing a thick-walled clad steel sheet excellent in low-temperature toughness and HIC resistance while ensuring a tensile strength of 535 MPa or more.

[0023] Note that "excellent low-temperature toughness" means that the ductile fracture surface ratio DWTTSA obtained by a DWTT test according to API-5L (test temperature: -30°C) is 85% or more. In addition, "excellent HIC resistance" means that the crack area ratio (CAR) obtained by a HIC test according to NACE Standard TM0284-2003 is 5.0% or less. －30℃ In addition, "excellent HIC resistance" means that the crack area ratio (CAR) obtained by a HIC test according to NACE Standard TM0284-2003 is 5.0% or less.

[0024] The inventors have repeatedly conducted various researches in order to develop a thick-walled clad steel sheet which further improves low-temperature toughness while ensuring prescribed tensile properties, and have obtained the following insights.

[0025] (A) In order to obtain excellent low-temperature toughness, it is effective to simultaneously perform micro- structuring of the structure which increases the resistance to brittle crack propagation and reducing the hard phase which can become the starting point of brittle cracks in the base steel sheet. Therefore, it is preferable to set the steel structure of the base steel sheet to a structure in which bainite is the main body. Specifically, it is effective to make the area ratio of bainite at the 1 / 2 position of the sheet thickness in the sheet thickness direction of the base steel sheet 94% or more and make the average crystal grain size of the bainite 23 μm or less.

[0026] (B) In order to perform the above structure control, it is important to appropriately control the component composition and the manufacturing conditions, and in particular, to apply reheating quenching and tempering after rolling of the clad blank and appropriately control the conditions thereof.

[0027] The gist of the present application completed on the basis of the above insights is configured as follows.

[0028] [1] A manufacturing method of a clad steel sheet which manufactures a clad steel sheet in which the above-mentioned base steel sheet is joined to the above-mentioned clad sheet on one side surface, having the following steps:

[0029] a step of preparing a first blank sheet having the following component composition,

[0030] a step of preparing a second blank sheet composed of a corrosion-resistant alloy,

[0031] a step of laminating the above-mentioned first blank sheet and the above-mentioned second blank sheet to obtain a clad blank,

[0032] a step of heating the above-mentioned clad blank to 1050°C to 1200°C,

[0033] then, a step of performing hot rolling in which the rolling end temperature is 900°C or more to the above-mentioned clad blank to join the above-mentioned first blank sheet and the above-mentioned second blank sheet to obtain a clad rolled sheet in which they are respectively the base steel sheet and the clad sheet,

[0034] a step of cooling the above-mentioned clad rolled sheet,

[0035] Then, a step of reheating the above composite rolled plate to 850°C to 1050°C,

[0036] Then, a step of cooling the above composite rolled plate at an average cooling rate of 2°C / s to 20°C / s in the above base material steel plate,

[0037] Then, a step of tempering the above composite rolled plate to 450°C to 650°C,

[0038] The above component composition contains 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, Al: 0.01% to 0.07%, Nb: 0.005% to 0.080%, Ti: 0.005% to 0.030%, N: 0.0010% to 0.0060%, and O (oxygen): 0.0030% or less, in mass%,

[0039] Further contains one or more selected from 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,

[0040] And, Ceq defined by formula (1) is 0.40 to 0.50, and the remainder is composed of Fe and inevitable impurities,

[0041] Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5... (1)

[0042] The element symbols in formula (1) refer to the content (mass%) of each element, and when the element is not contained, zero is substituted.

[0043] [2] The manufacturing method of a composite steel plate according to the above [1], wherein, in the above component composition of the above first base material plate, the Ca content is 0.0005% to 0.0040% in mass%, ACR defined by the following formula (2) is 1.00 to 6.00, P HIC Less than 1.250.

[0044] ACR = {Ca - (0.18 + 130Ca) x O} / (1.25S)... (2)

[0045] P HIC = 4.46C + 2.37Mn / 6 + (1.18Cr + 1.95Mo + 1.74V) / 5 + (1.74Cu + 1.7Ni) / 15 + 22.36P... (3)

[0046] The element symbols in formula (2) and formula (3) refer to the content (mass %) of each element, and when the element is not included, zero is substituted.

[0047] [3] The manufacturing method of the clad steel sheet according to the above [1] or [2], wherein the corrosion-resistant alloy is a Ni-based alloy of either one of Alloy 825 and Alloy 625, or an austenitic stainless steel.

[0048] [4] The manufacturing method of the clad steel sheet according to any one of the above [1] to [3], wherein the clad slab is formed by sequentially stacking the first slab, the second slab, the second slab, and the first slab, and applying a peeling material between the two second slabs, so that the two clad rolled sheets can be obtained by hot rolling,

[0049] After the above tempering, the two clad rolled sheets are peeled via the peeling material.

[0050] According to the first embodiment of the present application, a thick-walled clad steel sheet having excellent low-temperature toughness can be manufactured while ensuring a tensile strength of 535 MPa or more.

[0051] In addition, according to the second embodiment of the present application, a thick-walled clad steel sheet having excellent low-temperature toughness and HIC resistance can be manufactured while ensuring a tensile strength of 535 MPa or more. DETAILED DESCRIPTION

[0052] One embodiment of the present application relates to a manufacturing method of a clad steel sheet in which a clad plate is joined to one side of a base steel sheet. The manufacturing method of the clad steel sheet of the present embodiment has the following steps: a step of laminating a first slab having a prescribed composition and a second slab composed of a corrosion-resistant alloy to obtain a clad slab, a step of heating the clad slab to a prescribed heating temperature, then, a step of subjecting the clad slab to hot rolling under prescribed conditions to join the first slab and the second slab, thereby obtaining a clad rolled sheet in which they are respectively a base steel sheet and a clad plate, a step of allowing the clad rolled sheet to cool, then, a reheating quenching step of reheating the clad rolled sheet to a prescribed reheating temperature and then cooling it, and a step of tempering the clad rolled sheet to a prescribed tempering temperature. The first slab is a slab of the base steel sheet, and the second slab is a slab of the clad plate.

[0053] The thickness of the composite steel sheet manufactured in the present embodiment (total thickness of the base steel sheet and the clad sheet) is 25 mm or more, and preferably 30 mm or more. The upper limit of the thickness of the composite steel sheet manufactured in the present embodiment is not particularly limited, and the thickness of the composite steel sheet is preferably, for example, 50 mm or less. In addition, the thickness of the base steel sheet and the clad sheet in the composite steel sheet manufactured in the present embodiment is preferably 8 mm to 46 mm and 3.0 mm to 4.0 mm, respectively.

[0054] 1. Composition of the base steel sheet and the first clad sheet

[0055] The composition of the base steel sheet and the first clad sheet will be described. Note that the content of each element in the composition is expressed in "mass%" and hereinafter, simply expressed as "%" unless otherwise specified.

[0056] (1-1) First Embodiment

[0057] In the first embodiment, the base steel sheet and the first clad sheet have the following composition, containing, 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, Al: 0.01% to 0.07%, Nb: 0.005% to 0.080%, Ti: 0.005% to 0.030%, N: 0.0010% to 0.0060%, and O (oxygen): 0.0030% or less, and further containing one or more selected from 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. The remainder is constituted by Fe and inevitable impurities. This first embodiment is a composition required to achieve a tensile strength of 535 MPa or more and excellent low-temperature toughness.

[0058] C: 0.020% to 0.100%

[0059] C is an element that contributes to precipitation strengthening as carbides. When the C content is less than 0.020%, sufficient strength cannot be ensured in the base steel sheet. 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 of the base steel sheet, the toughness of the weld heat-affected zone, and the HIC resistance are deteriorated. Therefore, the C content is set to 0.100% or less, and preferably 0.080% or less.

[0060] Si: 0.10% to 0.50%

[0061] Si is an element added to secure the strength of the steel sheet by deoxidation and solid solution strengthening. From the viewpoint of obtaining this effect, the Si 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 Si content exceeds 0.50%, the toughness and weldability deteriorate. Therefore, the Si content is set to 0.50% or less, and preferably 0.40% or less.

[0062] Mn: 0.75% to 1.80%

[0063] Mn is an element added to secure the 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.

[0064] P: 0.015% or less

[0065] P is an unavoidable impurity that deteriorates the weldability. Therefore, the P content is set to 0.015% or less, and preferably 0.010% or less. The lower limit of the P content is not particularly limited, and the P content can be 0.000% or more.

[0066] S: 0.0030% or less

[0067] S generally exists in steel as sulfide inclusions, deteriorating 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, and preferably 0.0010% or less. The lower limit of the S content is not particularly limited, and the S content can be 0.0000% or more.

[0068] Al: 0.01% to 0.07%

[0069] 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%, aluminum oxide clusters are formed, deteriorating ductility. Therefore, the Al content is set to 0.07% or less, and preferably 0.04% or less.

[0070] Nb: 0.005% to 0.080%

[0071] Nb is effective for increasing the strength of the steel sheet by utilizing precipitation strengthening and an increase in quenching property. In addition, it has an effect of expanding the austenite unrecrystallization temperature region, and is useful for improving the toughness by microstructure refinement. 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 of the weld heat-affected zone deteriorate. Therefore, the Nb content is set to 0.080% or less, and preferably 0.060% or less.

[0072] Ti: 0.005% to 0.030%

[0073] Ti produces a pinning effect by forming nitrides, suppresses the coarsening of austenite, and is useful for ensuring the toughness of the base material and the weld heat-affected zone. In addition, Ti is an effective element for increasing the strength of the steel sheet by 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 coarsen, and become the starting point of brittle fracture and ductile fracture. Therefore, the Ti content is set to 0.030% or less, and preferably 0.020% or less.

[0074] N: 0.0010% to 0.0060%

[0075] N produces a pinning effect by forming nitrides, suppresses the coarsening of austenite, and is useful for ensuring the toughness of the base material and the weld 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 weld heat-affected zone deteriorates. Therefore, the N content is set to 0.0060% or less, and preferably 0.0050% or less.

[0076] It should be noted that, in order to sufficiently exert the pinning effect by forming nitrides, it is preferable to limit the ratio of the Ti content to the N content. Specifically, if the Ti content / N content is less than 2.0 or more than 3.5, the pinning effect produced by forming nitrides cannot be sufficiently exerted, and the austenite can coarsen and the toughness can deteriorate. Therefore, the Ti content / N content is preferably 2.0 to 3.5.

[0077] O (oxygen): 0.0030% or less

[0078] O is generally present in steel as an oxide of Al and Ca. If too much O is contained, 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. It should be noted that the lower limit of the O content is not particularly limited, and the O content can be 0.0000% or more.

[0079] In the component composition of the base steel sheet and the first base material sheet, in addition to the above basic components, 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 are further contained.

[0080] Cu: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less

[0081] Cu, Cr, and Mo are all elements that improve hardenability, and contribute to high strength of the base material and the weld heat-affected zone. In order to obtain this effect, when Cu, Cr, and Mo are contained, the content of each element is set to 0.01% or more, and preferably 0.05% or more. On the other hand, if the content of Cu, Cr, and Mo exceeds 0.50% respectively, the toughness of the weld heat-affected zone is sometimes deteriorated. Therefore, in the case where Cu, Cr, and Mo are contained, the content of each element is set to 0.50% or less, and preferably 0.40% or less.

[0082] Ni: 0.50% or less

[0083] Ni is an element that improves hardenability, and contributes to high strength of the base material and the weld heat-affected zone. From the viewpoint of obtaining this effect, in the case where Ni is contained, the content of Ni is set to 0.01% or more, and preferably 0.05% or more. On the other hand, since Ni is an element that is expensive, if a large amount of Ni is contained, the cost increases. Therefore, in the case where Ni is contained, the content of Ni is set to 0.50% or less, and preferably 0.40% or less.

[0084] V: 0.100% or less

[0085] V contributes to high strength of the steel sheet through precipitation strengthening. From the viewpoint of obtaining this effect, in the case where V is contained, the content of V is set to 0.010% or more. On the other hand, if the content of V exceeds 0.100%, the toughness of the weld heat-affected zone is deteriorated. Therefore, in the case where V is contained, the content of V is set to 0.100% or less.

[0086] Ca: 0.0040% or less

[0087] Ca fixes S in the steel and improves the toughness of the steel sheet. From the viewpoint of obtaining this effect, in the case where Ca is contained, the content of Ca is set to 0.0010% or more. On the other hand, if the content of Ca exceeds 0.0040%, the inclusions in the steel increase, and sometimes the toughness is deteriorated. Therefore, in the case where Ca is contained, the content of Ca is set to 0.0040% or less, and preferably 0.0030% or less.

[0088] Carbon equivalent Ceq defined by the following formula (1): 0.40 to 0.50

[0089] The carbon equivalent Ceq defined by the following formula (1) has a correlation with the strength of the steel sheet. In the case of the reheat quenching-tempering step of the manufacturing method of the present embodiment, in order to obtain a high strength of tensile strength 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, it can lead to deterioration of toughness. Therefore, the carbon equivalent Ceq is set to 0.50 or less, preferably 0.48 or less.

[0090] Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5... (1)

[0091] Note that the element symbols in formula (1) refer to the content (mass %) of each element in the base material steel sheet or the first blank sheet, and when the element is not contained, zero is substituted.

[0092] (1-2) Second Embodiment

[0093] Next, the composition of the base material steel sheet and the first blank sheet of the second embodiment required for the base material steel sheet to exhibit excellent HIC resistance in addition to the tensile strength of 535 MPa or more and excellent low-temperature toughness will be described. Hereinafter, items different from the above-described first embodiment, additional items will be described. Items not described below follow the first embodiment.

[0094] P: 0.010% or less

[0095] P is an unavoidable impurity that deteriorates the HIC resistance due to an increase in the hardness of the center segregation portion. Therefore, in the case where HIC resistance is required, the P content is preferably 0.010% or less, more preferably 0.006% or less. The lower limit of the P content is not particularly limited, and the P content can be 0.000% or more.

[0096] S: 0.0010% or less

[0097] S deteriorates the toughness and HIC resistance by existing in the steel as sulfide inclusions. Therefore, in the case where HIC resistance is required, the S content is preferably 0.0010% or less, more preferably 0.0008% or less. The lower limit of the S content is not particularly limited, and can be 0.0000% or more.

[0098] Ca: 0.0005% to 0.0040%

[0099] Ca fixes S in the steel to improve the toughness and HIC resistance of the steel sheet. From the viewpoint of obtaining these effects, the Ca content is preferably 0.0005% or more, more preferably 0.0010% or more, and further 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 sometimes deteriorate. Therefore, the Ca content is preferably 0.0040% or less, and more preferably 0.0030% or less.

[0100] In the case where HIC resistance is required, ACR and P HIC defined by the following formulas (2) and (3), respectively, are preferably controlled appropriately.

[0101] ACR = {Ca - (0.18 + 130Ca) x O} / (1.25S)...(2)

[0102] P HIC = 4.46C + 2.37Mn / 6 + (1.18Cr + 1.95Mo + 1.74V) / 5 + (1.74Cu + 1.7Ni) / 15 + 22.36P...(3)

[0103] Note that the element symbols in formulas (2) and (3) refer to the contents (mass%) of the respective elements in the base steel sheet or the first billet sheet, and zero is entered when the element is not contained.

[0104] ACR: 1.00 to 6.00

[0105] ACR is a value that is an index of the effect of suppressing the generation of MnS by Ca. Hydrogen-induced cracking is affected by MnS in the steel, and in order to improve the HIC resistance, it is effective to suppress the generation of MnS by fixing S in the steel with Ca. If ACR is 1.00 or more, the effect of suppressing the generation of MnS is sufficiently exerted. Therefore, ACR is preferably 1.00 or more, and more preferably 1.40 or more. On the other hand, if the value of ACR exceeds 6.00, although the generation of MnS is suppressed, a large amount of Ca-based oxides is produced, and the HIC resistance deteriorates. Therefore, ACR is preferably 6.00 or less, and more preferably 3.30 or less.

[0106] P HIC : less than 1.250

[0107] P HIC is a value that is an index of the hardness of the center segregation portion, and the larger the value, the higher the hardness of the center segregation portion. Since hydrogen-induced cracking is affected by the hardness of the center segregation portion, in order to improve the HIC resistance, it is effective to suppress the increase in the hardness of the center segregation portion. Therefore, P HIC is preferably less than 1.250, and more preferably 1.000 or less. PHIC There is no specific lower limit for P, but from the perspective of ensuring sufficient strength in the base steel plate, HIC The preferred value is 0.750 or higher.

[0108] 2. Microstructure of the base steel plate (preferred conditions)

[0109] In the composite steel sheet manufactured by this embodiment, the base steel sheet preferably has the following microstructure.

[0110] The area ratio of bainite at the 1 / 2 thickness position in the thickness direction of the base steel plate is over 94%.

[0111] Bainite is an important microstructure that balances strength and low-temperature toughness. Furthermore, bainite is a transformation-strengthening microstructure, which helps to effectively improve the strength of the steel sheet. Therefore, the microstructure of the base steel sheet is preferably predominantly bainitic. Specifically, the area fraction of bainite at the 1 / 2 thickness position (hereinafter referred to as "1 / 2 thickness position") in the thickness direction of the base steel sheet is preferably 94% or more. More preferably, the area fraction of bainite is 97% or more, and may also be 100%.

[0112] The microstructure of the base steel plate should be primarily composed of bainite. However, the remaining microstructure, in trace amounts, may include island martensite (MA), ferrite, cementite, etc. A total area fraction of the remaining microstructure at the half-thickness position is acceptable if it is below 6%. The area fraction of the remaining microstructure can also be 0%. When the area fraction of the remaining microstructure exceeds 6%, the area fraction of bainite becomes smaller, which may compromise both strength and low-temperature toughness, and is therefore not preferred.

[0113] Here, the area ratio of each phase at the 1 / 2 thickness position can be calculated as follows. That is, after mirror polishing the L-section (the section parallel to the rolling direction and the normal direction of the rolling surface) of the base steel plate, it is etched using nitric acid ethanol or electrolytic etching (electrolyte: 100ml distilled water + 25g sodium hydroxide + 5g picric acid). Using a scanning electron microscope (SEM), the area at the 1 / 2 thickness position of the L-section is observed at a randomly selected 1.2×10⁻⁶ magnification (2000x). －2 mm 2 The region can be determined through image analysis.

[0114] Average grain size of bainite at the 1 / 2 thickness position: below 23 μm

[0115] Since the grain boundary of the bainite becomes a resistance to propagation of brittle cracks, the refinement of the grain increases the grain boundary, which contributes to the improvement of the low-temperature toughness. Therefore, the average crystal grain size of the bainite at the position of 1 / 2 of the thickness of the plate is preferably 23 μm or less, and more preferably 20 μm or less. The lower limit of the average crystal grain size is not particularly limited, and the average crystal grain size can be 5 μm or more.

[0116] Here, the average crystal grain size of the bainite can be calculated as follows. That is, 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 material steel plate, the crystal orientation of a region of 1.2 x 10 －2 mm 2 selected at random at the position of 1 / 2 of the thickness of the L section is measured by the electron beam backscattering diffraction method (EBSP), and the region in which the angle difference of the adjacent pixels is 15° or more is regarded as the grain boundary, and is calculated by image analysis. Note that the average crystal grain size d area may be calculated from the area a i occupied by each grain and the equivalent circle diameter d i of each grain by the following equation.

[0117] d area =∑(a i • d i ) / ∑a i

[0118] 3. Bonding plate and second base material plate

[0119] In the present embodiment, the bonding plate and the second base material plate are composed of a corrosion-resistant alloy. The corrosion-resistant alloy is preferably a Ni-based alloy or an austenitic stainless steel. In particular, the Ni-based alloy is preferred in that it exhibits high stress corrosion cracking resistance in an environment having a high hydrogen sulfide partial pressure (sulfur-containing environment). The Ni-based alloy is, for example, Alloy 625 or Alloy 825, and Alloy 625 is particularly preferred from the viewpoint of stress corrosion cracking resistance.

[0120] Hereinafter, a suitable composition of the bonding plate and the second base material plate will be described. Note that the unit of the content of each element in the composition is "mass%", and hereinafter, it is simply indicated as "%" unless otherwise specified.

[0121] (3-1) Suitable composition of Alloy 625

[0122] Alloy 625 is a Ni-based alloy corresponding to NCF625 of JIS G 4902:2019, in which a Ni-based alloy having the following composition is preferable: 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 the total amount of Nb and Ta: 3.15 to 4.15% by mass, with the remainder consisting of Ni and unavoidable impurities.

[0123] C: 0.030% or less

[0124] C precipitates as carbides at grain boundaries during the heat process at the time of manufacturing the clad steel sheet, degrading corrosion resistance. If the C content exceeds 0.030%, precipitation of carbides is promoted, degrading corrosion resistance. Therefore, the C content is preferably 0.030% or less, 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.

[0125] Si: 0.02 to 0.50%

[0126] Si is added for deoxidization. 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%, corrosion resistance deteriorates. Therefore, the Si content is preferably 0.50% or less, more preferably 0.20% or less.

[0127] Mn: 0.02 to 0.50%

[0128] Mn is added for deoxidization. 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%, corrosion resistance deteriorates. Therefore, the Mn content is preferably 0.50% or less, more preferably 0.15% or less.

[0129] P: 0.010% or less

[0130] P is an impurity element that segregates at grain boundaries and degrades corrosion resistance. Therefore, the P content is preferably 0.010% or less, more preferably 0.005% or less. The lower limit of the P content is not particularly limited, and the P content can be 0.000% or more.

[0131] S: 0.0010% or less

[0132] S and P are also impurity elements that segregate at grain boundaries and deteriorate corrosion resistance. Therefore, the S content is preferably 0.0010% or less, more preferably 0.0005% or less. The lower limit of the S content is not particularly limited, and the S content can be 0.0000% or more.

[0133] Cr: 20.0 to 23.0%

[0134] Cr forms a protective oxide film on the surface of the metal, improving pitting corrosion resistance and grain boundary corrosion resistance. In addition, Cr improves stress corrosion cracking resistance in a sulfur-containing environment by being added in combination with Ni. From the viewpoint of obtaining these effects, the Cr content is preferably 20.0% or more, more preferably 21.5% or more. Of these, from the viewpoint of balance with Ni and other alloys, the Cr content is preferably 23.0% or less.

[0135] Mo: 8.00 to 10.00%

[0136] Mo improves pitting corrosion resistance and crevice corrosion resistance. In addition, Mo improves stress corrosion cracking resistance in a sulfur-containing environment by being added in combination with Ni. From the viewpoint of obtaining these effects, the Mo content is preferably 8.00% or more. Of these, from the viewpoint of balance with Ni and other alloys, the Mo content is preferably 10.00% or less.

[0137] Fe: 5.0% or less

[0138] In the case where chromium iron, molybdenum iron, or the like is used as a raw material, Fe is an unavoidable impurity, and if the Fe content exceeds 5.0%, corrosion resistance deteriorates. Therefore, the Fe content is preferably 5.0% or less, more preferably 3.5% or less. The lower limit of the Fe content is not particularly limited, and the Fe content can be 0.0% or more.

[0139] Al: 0.02 to 0.40%

[0140] 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%, stress corrosion cracking resistance deteriorates. Therefore, the Al content is preferably 0.40% or less, more preferably 0.25% or less.

[0141] Ti: 0.10 to 0.40%

[0142] Ti fixes C by precipitating C as a carbide, which deteriorates corrosion resistance. 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, and the jointability with the base steel sheet deteriorates, so the Ti content is preferably 0.40% or less, more preferably 0.30% or less.

[0143] Total amount of Nb and Ta: 3.15 to 4.15%

[0144] Both Nb and Ta are effective for 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 intermetallic compounds having a low melting point, and hot workability is reduced. Therefore, the total amount of Nb and Ta is preferably 4.15% or less.

[0145] The components other than the above are Ni and inevitable impurities. Note that Ni is an element that improves corrosion resistance, and in particular, significantly improves stress corrosion cracking resistance in a sulfur-containing environment. Therefore, the Ni content is preferably 58% or more.

[0146] (3-2) Suitable component composition of Alloy 825

[0147] Alloy 825 is a Ni-based alloy corresponding to NCF825 of JIS G 4902:2019, and a Ni-based alloy having the following component composition is preferable: 0.020% or less of C, 0.50% or less of Si, 1.00% or less of Mn, 0.030% or less of P, 0.0050% or less of S, 38.0 to 46.0% of Ni, 19.5 to 23.5% of Cr, 2.50 to 3.50% of Mo, 1.50 to 3.00% of Cu, 0.01 to 0.20% of Al, and 0.60 to 1.20% of Ti, the remainder consisting of Fe and inevitable impurities, in terms of mass%.

[0148] C: 0.020% or less

[0149] C precipitates as carbides at grain boundaries during the heat process at the time of manufacturing the clad steel sheet, and deteriorates corrosion resistance. If the C content exceeds 0.020%, precipitation of carbides is promoted, and corrosion resistance is deteriorated. 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 can be 0.000% or more.

[0150] Si: 0.50% or less

[0151] Si is added for deoxidization. However, if the Si content exceeds 0.50%, Si remains as a non-metallic inclusion, and corrosion resistance is deteriorated. Therefore, the Si content is preferably 0.50% or less, and more preferably 0.20% or less. The lower limit of the Si content is not particularly limited, and from the viewpoint of sufficiently obtaining the effect of deoxidization, the Si content is preferably 0.02% or more.

[0152] Mn: 1.00% or less

[0153] Mn is added for deoxidization. However, if the Mn content exceeds 1.00%, the corrosion resistance deteriorates. Therefore, the Mn content is preferably 1.00% or less, more preferably 0.50% or less. The lower limit of the Mn content is not particularly limited, but from the viewpoint of sufficiently obtaining the effect of deoxidization, the Mn content is preferably 0.02% or more.

[0154] P: 0.030% or less

[0155] P is an impurity element that segregates at the grain boundaries and deteriorates the corrosion resistance. Therefore, the P content is preferably 0.030% or less, 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.

[0156] S: 0.0050% or less

[0157] S, like P, is an impurity element that segregates at the grain boundaries and deteriorates the corrosion resistance. Therefore, the S content is preferably 0.0050% or less, more preferably 0.0010% or less. The lower limit of the S content is not particularly limited, and the S content can be 0.0000% or more.

[0158] Ni: 38.0 to 46.0%

[0159] Ni is an element that improves the corrosion resistance, and in particular, greatly improves the stress corrosion cracking resistance in a sulfur-containing environment. From the viewpoint of obtaining this effect, the Ni content is preferably 38.0% or more. However, since Ni is a very expensive element, the addition of a large amount of Ni leads to an increase in cost. Therefore, from the viewpoint of the balance between the improvement effect of the corrosion resistance and the cost, the Ni content is preferably 46.0% or less.

[0160] Cr: 19.5 to 23.5%

[0161] Cr forms an oxide film that is protective on the surface of the metal, and improves the pitting corrosion resistance and the grain boundary corrosion resistance. In addition, Cr, by being added in combination with Ni, improves the stress corrosion cracking resistance in a sulfur-containing environment. From the viewpoint of obtaining these effects, the Cr content is preferably 19.5% or more, 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.

[0162] Mo: 2.50 to 3.50%

[0163] Mo improves the pitting resistance and the crevice corrosion resistance. In addition, Mo improves the stress corrosion cracking resistance in a sulfur-containing environment by being 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 balance with Ni and other alloys, the Mo content is preferably 3.50% or less.

[0164] Cu: 1.50 to 3.00%

[0165] Cu improves the 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, if the Cu content exceeds 3.00%, the effect saturates, and thus the Cu content is preferably 3.00% or less.

[0166] Al: 0.01 to 0.20%

[0167] 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. Thus, the Al content is preferably 0.20% or less, and more preferably 0.15% or less.

[0168] Ti: 0.60 to 1.20%

[0169] Ti fixes C by precipitating C as a carbide, which deteriorates the corrosion resistance. 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, and the jointability with the base steel sheet deteriorates, and thus the Ti content is preferably 1.20% or less.

[0170] The components other than the above are Fe and inevitable impurities.

[0171] (3-3) Austenitic Stainless Steel

[0172] The suitable component composition of the austenitic stainless steel contains, in terms of 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 remainder being constituted by Fe and inevitable impurities.

[0173] C: 0.030% or less

[0174] C precipitates as carbide at the grain boundary during the heat process at the time of manufacturing the clad steel sheet, deteriorating the corrosion resistance. If the C content exceeds 0.030%, the precipitation of carbide is promoted, deteriorating the corrosion resistance. Therefore, the C content is preferably 0.030% or less, more preferably 0.020% or less, and further preferably 0.015% or less. The lower limit of the C content is not particularly limited, and the C content can be 0.000% or more.

[0175] Si: 1.00% or less

[0176] If the Si content exceeds 1.00%, Si remains as a non-metallic inclusion, deteriorating the corrosion resistance. 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.

[0177] Mn: 2.00% or less

[0178] 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 further 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.

[0179] P: 0.045% or less

[0180] P is an impurity element that segregates at the grain boundary, deteriorating the 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.

[0181] S: 0.030% or less

[0182] S is also an impurity element that segregates at the grain boundary, deteriorating the corrosion resistance. Therefore, the S content is preferably 0.030% or less, and more preferably 0.010% or less. The lower limit of the S content is not particularly limited, and the S content can be 0.000% or more.

[0183] Ni: 12.0 to 15.0%

[0184] Ni is an element that improves the corrosion resistance, and particularly greatly improves the stress corrosion cracking resistance in a sulfur-containing environment. 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, the addition of a large amount of Ni leads to an increase in cost. Therefore, from the viewpoint of the balance between the effect of improving the corrosion resistance and the cost, the Ni content is preferably 15.0% or less, and more preferably 14.5% or less.

[0185] Cr: 16.0 to 18.0%

[0186] Cr forms a protective oxide film on the surface of the metal, improving the resistance to pitting corrosion and the resistance to intercrystalline corrosion. In addition, Cr improves the resistance to stress corrosion cracking in a sulfur-containing environment by being 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 balance with Ni and other alloys, the Cr content is preferably 18.0% or less, and more preferably 17.5% or less.

[0187] Mo: 2.00 to 3.00%

[0188] Mo improves the resistance to pitting corrosion and the resistance to intercrystalline corrosion. In addition, Mo improves the resistance to stress corrosion cracking in a sulfur-containing environment by being 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 balance with Ni and other alloys, the Mo content is preferably 3.00% or less, and more preferably 2.80% or less.

[0189] The components other than the above are Fe and unavoidable impurities.

[0190] 4. Manufacturing conditions of the clad steel sheet

[0191] The manufacturing method of the clad steel sheet of one embodiment of the present application has the following steps: a step of laminating a first base material sheet having the above-described component composition and a second base material sheet composed of a corrosion-resistant alloy to obtain a clad sheet; a step of heating the clad sheet to 1050°C to 1200°C; then, a step of performing hot rolling of the clad sheet with a finish rolling temperature set to 900°C or higher to join the first base material sheet and the second base material sheet, thereby obtaining a clad rolled sheet in which they are respectively a base steel sheet and a clad sheet; a step of cooling the clad rolled sheet; then, a step of reheating the clad rolled sheet to 850°C to 1050°C; then, a step of cooling the clad rolled sheet at an average cooling rate in the base steel sheet of 2°C / s to 20°C / s; and then, a step of tempering the clad rolled sheet to 450°C to 650°C.

[0192] In the present embodiment, the temperature conditions prescribed as the manufacturing conditions are all related to the steel sheet surface temperature. The steel sheet surface temperature can be measured, for example, using a radiation thermometer.

[0193] (4-1) Step of obtaining a clad sheet

[0194] First, the first base material sheet as a base steel sheet and the second base material sheet as a clad sheet are laminated to obtain a clad sheet. In a vacuum (negative pressure) environment, specifically, 10 －4The first material sheet and the second material sheet are temporarily welded by, for example, electron beam welding or laser welding in an environment of a pressure of 10"3torr or less, whereby a composite sheet blank can be obtained.

[0195] Further, a composite sheet blank in which the first material sheet, the second material sheet, the second material sheet, and the first material sheet are sequentially stacked and a peeling material is applied between the second material sheets is preferable. That is, two sets of the laminated body in which the first material sheet and the second material sheet are stacked are overlapped with the second material sheets facing each other, whereby a composite sheet blank is formed. By hot-rolling the composite sheet blank, two composite rolled sheets can be obtained. After tempering, the two composite rolled sheets are peeled via the peeling material, whereby a composite steel sheet as a product sheet can be obtained. In this case, the symmetry in the sheet thickness direction at the time of rolling is maintained, and thus the generation of warping in the up-down direction at the time of rolling can be suppressed.

[0196] (4-2) Composite rolling step

[0197] Composite sheet blank heating temperature: 1050°C to 1200°C

[0198] When the composite sheet blank heating temperature is less than 1050°C, the reduction ratio cannot be sufficiently ensured in the rolling from the heating to the rolling end temperature described later, and the joining property of the base material sheet to the clad sheet in the finally obtained composite steel sheet deteriorates. Further, when the sheet blank heating temperature is less than 1050°C, Nb and the like contained in the first material sheet as the base material sheet are not sufficiently solid-solved, and the strength of the base material sheet in the finally obtained composite steel sheet cannot be ensured. Therefore, the heating temperature of the composite sheet blank is set to 1050°C or higher. On the other hand, if the composite sheet blank heating temperature exceeds 1200°C, the austenite in the first material sheet as the base material sheet coarsens, and the structure of the base material sheet in the finally obtained composite steel sheet also cannot be sufficiently refined, and thus the toughness deteriorates. Therefore, the composite sheet blank heating temperature is set to 1200°C or lower, and preferably 1100°C or lower.

[0199] Rolling end temperature: 900°C or higher

[0200] The joining property of the base material sheet to the clad sheet in the composite steel sheet is ensured by rolling in a high temperature region. That is, in the rolling in the high temperature region, the deformation resistance of the base material sheet to the clad sheet decreases and a good joining interface is formed, and thus the elements at the joining interface easily diffuse into each other, and thus the joining property of the base material sheet to the clad sheet is ensured. Therefore, the rolling end temperature is set to 900°C or higher, and preferably 950°C or higher. The upper limit of the rolling end temperature is not particularly limited, and the rolling end temperature can be 1000°C or lower.

[0201] (4-3) Cooling step

[0202] After the above hot rolling, the resultant composite-rolled plate is left to cool. Here, the leaving to cool means a process in which the composite-rolled plate after the rolling is not subjected to forced cooling but is left to stand naturally at a prescribed place such as a factory yard, waiting for the temperature of the composite-rolled plate to drop. Therefore, the cooling rate is not particularly limited. The cooling stop temperature in the leaving to cool can be any temperature as long as it is a temperature at which the ferrite transformation is completed. In practice, although it is generally necessary to wait until the temperature drops to the vicinity of room temperature (for example, 25°C), it is not necessary to lower the temperature to the vicinity of room temperature. The cooling stop temperature in the leaving to cool can be, for example, 100°C or lower.

[0203] (4-4) Reheating Quenching (Reheating and Cooling)

[0204] Next, reheating quenching is performed, and the composite-rolled plate is cooled after being reheated.

[0205] Reheating temperature: 850°C to 1050°C

[0206] First, as the reheating process, the composite-rolled plate after the leaving to cool is heated to a reheating temperature of 850°C to 1050°C. If the reheating temperature is less than 850°C, ferrite is generated prior to the bainite transformation in the base material steel plate, the target strength is not obtained, and the Charpy absorbed energy is also reduced. Therefore, the reheating temperature is set to 850°C or higher, and preferably 900°C or higher. On the other hand, if the reheating temperature exceeds 1050°C, the austenite grains are coarsened, and the low-temperature toughness is significantly reduced. Therefore, the reheating temperature is set to 1050°C or lower, and preferably 1000°C or lower.

[0207] Average cooling rate in the base material steel plate: 2°C / s to 20°C / s

[0208] Next, the composite-rolled plate after the reheating is subjected to a cooling process at an average cooling rate of 2°C / s to 20°C / s in the base material steel plate. If the average cooling rate is less than 2°C / s, the ferrite transformation is induced in the base material steel plate, the target strength is not obtained, and the low-temperature toughness is significantly reduced. Therefore, the average cooling rate is set to 2°C / s or higher, and preferably 5°C / s or higher. From the viewpoint of the refinement of the structure and the reduction of hard phases that can be the starting point of brittle fracture, the average cooling rate is further preferably 7°C / s or higher. The upper limit of the average cooling rate is not particularly limited, and if the average cooling rate is excessively large, the strength of the phase transformation structure is excessively high, which can result in a reduction in the Charpy absorbed energy. Therefore, from the viewpoint of preventing a reduction in the Charpy absorbed energy, the average cooling rate is preferably 20°C / s or lower. Note that the average cooling rate is a 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 the cooling to the end of the cooling.

[0209] To prevent a decrease in the toughness of the base steel sheet, the cooling stop temperature in the reheating quenching is preferably 200°C or lower. The lower limit of the cooling stop temperature is not particularly limited, and for example, the cooling stop temperature can be 25°C or higher.

[0210] (4-5) Tempering step

[0211] Tempering temperature: 450°C or higher and 650°C or lower

[0212] Tempering treatment in which the cooled clad-rolled sheet is heated is performed. If the tempering temperature is less than 450°C, the decomposition of the island-shaped martensite and the recovery of dislocations are not sufficient, and the toughness of the base steel sheet 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 coarsen, and the toughness of the base steel sheet deteriorates. In addition, the corrosion resistance of the clad sheet can also deteriorate due to the formation of precipitates. Therefore, the tempering temperature is set to 650°C or lower, and preferably 500°C or lower.

[0213] Example

[0214] A first blank sheet (material of the base steel sheet) having the composition shown in Table 1 (the remainder is Fe and inevitable impurities) and a second blank sheet (material of the clad sheet) having the composition shown in Table 2 (the remainder is an Fe and inevitable impurities corrosion-resistant alloy) were stacked in the order of the first blank sheet, the second blank sheet, the second blank sheet, and the first blank sheet, and a clad sheet obtained thereby was subjected to clad rolling under the conditions shown in Table 3, to produce a clad-rolled sheet. Next, the obtained clad-rolled sheet was subjected to reheating quenching and tempering under the conditions shown in Table 3, to produce a clad steel sheet having a sheet thickness of 35 mm (sheet thickness of the base steel sheet: 32 mm, sheet thickness of the clad sheet: 3 mm). The cooling stop temperature upon cooling after standing was set to 30°C, and the cooling stop temperature in the reheating quenching was set to 30°C.

[0215] In this way, test pieces for a tensile test and test pieces for a DWTT test were taken from the obtained clad steel sheet. The tensile test was performed in accordance with API-5L, and the tensile strength TS and the yield strength YS were calculated. The DWTT test was performed in accordance with API-5L at a test temperature of -30°C, and the ductile fracture ratio DWTTSA was calculated. －30℃ These results are shown in Table 4. The target values of the tensile strength and the ductile fracture ratio DWTTSA －30℃ are as follows.

[0216] Tensile strength: 535 MPa or higher

[0217] Ductile fracture ratio: DWTTSA －30℃ : 85% or higher

[0218] Further, HIC test according to NACE Standard TM0284-2003 was carried out, and the HIC resistance of the base steel sheet was evaluated by crack area ratio (CAR). Here, the target value of the crack area ratio (CAR) was as follows.

[0219] Crack area ratio (CAR): 5.0% or less (particularly preferably 1.0% or less)

[0220] Note that the crack area ratio (CAR) was measured by immersing three test pieces taken from the same steel sheet in a 5 mass% NaCl + 0.5 mass% CH3COOH aqueous solution saturated with hydrogen sulfide for 96 hours, and then detecting defects of the test pieces by ultrasonic flaw detection test. Further, the above evaluation was carried out based on the maximum value of the crack area ratio (CAR) measured in the three test pieces. The results are shown in Table 4.

[0221] Further, by the above method, the area ratio and average crystal grain diameter of the bainite at the position of 1 / 2 of the sheet thickness in the sheet thickness direction of the base steel sheet were measured. These results are shown in Table 4.

[0222] According to Table 4, the inventive examples satisfying the claim 1 all obtained tensile strength: 535 MPa or more and ductile fracture ratio DWTT SA －30℃ : 85% or more. Further, the inventive examples satisfying the claim 2 all obtained tensile strength: 535 MPa or more, ductile fracture ratio DWTT SA －30℃ : 85% or more and CAR: 5.0% or less. On the other hand, the comparative examples shown in Table 4 all did not obtain the desired properties.

[0223]

[0224]

[0225] [Table 3]

[0226] [Table 4]

[0227]

[0228] Industrial applicability

[0229] The composite steel sheet produced by the present application is extremely useful in industry when applied to a pipeline for transporting oil and natural gas exposed to a severe corrosion environment, since it exhibits excellent low-temperature toughness.

Claims

1. A method for manufacturing a composite steel plate, comprising the following steps: (The method describes the process of bonding a bonding plate to one side of a base steel plate.) The process of preparing a first blank sheet having the following components, The process of preparing a second blank sheet made of a corrosion-resistant alloy. The process of stacking the first blank plate and the second blank plate to obtain a composite blank. The process of heating the composite slab to 1050℃~1200℃ Then, the composite slab is subjected to hot rolling at a rolling finish temperature of 900°C or higher, so that the first slab and the second slab are joined together to obtain a composite rolled plate with them as the base steel plate and the bonding plate, respectively. The process of cooling the composite rolled plate Then, the composite rolled plate is reheated to 850°C to 1050°C. Then, the composite rolled plate is cooled at an average cooling rate of 2°C / s to 20°C / s in the base steel plate. Then, the composite rolled plate is tempered to 450°C to 650°C. The composition, by mass%, contains C: 0.020%–0.100%, Si: 0.10%–0.50%, Mn: 0.75%–1.80%, P: less than 0.015%, S: less than 0.0030%, Al: 0.01%–0.07%, Nb: 0.005%–0.080%, Ti: 0.005%–0.030%, N: 0.0010%–0.0060%, and O (oxygen): less than 0.0030%. It further contains one or more components selected from 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. Furthermore, Ceq, as specified by equation (1), is 0.40–0.50, with the remainder consisting of Fe and unavoidable impurities. Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5···(1) The element symbols in formula (1) refer to the content of each element in terms of mass %; if the element is not present, substitute zero.

2. The method for manufacturing the composite steel plate according to claim 1, wherein, In the composition of the first blank plate, the Ca content is 0.0005% to 0.0040% by mass, the ACR is 1.00 to 6.00 as specified by formula (2), and the P content is specified by formula (3). HIC 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 equations (2) and (3) refer to the content of each element in terms of mass %; if the element is not present, substitute zero.

3. The method for manufacturing the composite steel plate according to claim 1 or 2, wherein, The corrosion-resistant alloy is either a Ni-based alloy of Alloy 825 and Alloy 625 or an austenitic stainless steel.

4. The method for manufacturing the composite steel plate according to any one of claims 1 to 3, wherein, The composite slab is formed by sequentially stacking the first slab, the second slab, the second slab, and the first slab, and applying a release material between the two second slabs, thereby obtaining two composite rolled plates through hot rolling. After the tempering, the two composite rolled plates are peeled off using the peeling material.

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