Clad steel plate and its manufacturing method

Abstract

To provide a clad steel plate in which corrosion resistance of a cladding metal and workability of a base material are excellent, and a method for manufacturing the clad steel plate.SOLUTION: A stainless clad steel plate is provided which uses a two-phase stainless steel as a cladding material and a carbon steel or a low alloy steel as a base material, and which is excellent in corrosion resistance of the cladding metal and workability of the base material, wherein difference between a ferric chloride CPT of the cladding material and a ferric chloride CPT of a sample obtained by subjecting the cladding material to solid solution treatment is 10°C or less, and wherein a ferrite phase ratio of a base material surface layer (position of 1 mm from base material surface in plate thickness direction) exceeds 15%. Heat treatment for improving corrosion resistance and workability after manufacture is unnecessary, consequently a clad steel plate applicable to a complicated structure can be provided at low cost. The ferric chloride CPT means a corrosion generation temperature (°C) evaluated according to ASTM G48 E method.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a clad steel plate having excellent workability of the base material and excellent corrosion resistance of the clad material, and a method for manufacturing the same. [Background technology]

[0002] Stainless steel has excellent corrosion resistance, making it suitable for use in severely corrosive environments. Examples of such severely corrosive environments include oil well environments, high-chloride environments such as seawater or brackish water, and plant equipment and chemical tankers exposed to various acid solutions. In these severely corrosive environments, stainless steel is used in seawater desalination plants, flue gas desulfurization systems, chemical storage tanks, structural components for oil well tubular goods, pumps and valves, heat exchangers, and other applications.

[0003] On the other hand, stainless steel contains a lot of alloying elements such as Cr, Ni, and Mo to ensure corrosion resistance, and compared to carbon steel and low-alloy steel, the cost of materials as well as processing and welding is high. In addition, the price can fluctuate greatly due to the rising prices of alloying elements. Therefore, its use is sometimes restricted mainly due to cost.

[0004] As mentioned above, when considering the cost aspect, it is effective to use clad steel plate as a material from the viewpoint of processing and welding. Clad steel plate refers to a material in which two or more different types of metal are bonded together. Steel plate that is not bonded together will be referred to as "solid steel plate" hereinafter. Compared to solid steel plate made only of high-alloy steel, clad steel plate can reduce the amount of high-alloy steel used, thereby reducing material costs. In addition, since less dissimilar material welding is required, the cost of welding materials can also be reduced.

[0005] In addition, in clad steel plates made by bonding two types of metal together, one metal is referred to as the "base material," and the other metal (raw material) bonded to the base material is referred to as the "clad material." By bonding a material (clad material) with excellent properties to the base material, it is possible to obtain the excellent properties of both the clad material and the base material.

[0006] For example, a high-alloy steel having the properties (e.g., corrosion resistance) required for the application environment may be used for the cladding material, and a carbon steel or low-alloy steel having the toughness and strength required for the application environment may be used for the base material. In such a case, not only can costs be reduced as described above, but properties (e.g., corrosion resistance) equivalent to those of solid steel plates and strength and toughness equivalent to those of carbon steel and low-alloy steel can be ensured. This allows for both economical and functional compatibility.

[0007] Due to the above circumstances, the need for clad steel sheets using stainless steel as the cladding material has been increasing in recent years in various industrial fields. Traditionally, austenitic stainless steel has been used as the cladding material in most cases. There is a growing trend toward switching to cheaper duplex stainless steel for these applications, and there is also a potential demand for even cheaper clad steel sheets using duplex stainless steel as the cladding material.

[0008] As the applications of clad steel plates expand, they are increasingly being used in complex shapes such as chemical tankers, plants, and large structures. For these applications, the workability of the base material, which is evaluated, for example, by elongation in tensile tests and surface cracking in bending tests, is important. Therefore, there is a need for technology that can achieve both the corrosion resistance of the clad material and the workability of the base material for clad steel plates made of duplex stainless steel.

[0009] The present invention is directed to a clad steel plate in which a cladding material is bonded to only one surface of the base material. The surface of the base material on which the cladding material is not bonded (where the base material is exposed) is hereinafter referred to as the "base material surface," and the position 1 mm from the base material surface in the plate thickness direction is referred to as the "base material surface layer."

[0010] Duplex stainless steel is a stainless steel that has both austenite and ferrite phases in its structure. Compared to austenitic stainless steel, which generally has the same corrosion resistance, duplex stainless steel has a lower Ni content and higher strength, so it has attracted attention as a material that can be used at low alloying costs and with thinner walls.

[0011] There are many types of duplex stainless steel. Among them, duplex stainless steels containing moderate amounts of Cr, Ni, and Mo, such as SUS329J3L and ASTM S32205, are called general-purpose duplex stainless steels (M type). They have corrosion resistance equal to or better than the general-purpose austenitic stainless steel SUS316, and are sometimes used as replacements for it. Duplex stainless steels containing large amounts of Cr, Ni, and Mo, such as SUS327L1 and ASTM S32750, are called super duplex stainless steels (S type). They have corrosion resistance equal to or better than the highly corrosion-resistant austenitic stainless steel SUS312L, and are sometimes used as replacements for it.

[0012] A challenge in manufacturing these general-purpose duplex stainless steels and super duplex stainless steels is the reduction in corrosion resistance due to the precipitation of intermetallic compounds of Cr, Mo, and Fe called sigma phases. When sigma phases precipitate in duplex stainless steel, a chromium-depleted layer forms around them, reducing corrosion resistance. For this reason, conventional duplex stainless steel solid sheets are manufactured by performing solution heat treatment at 1000°C or higher after rolling to dissolve the precipitates.

[0013] However, in clad steel plates, the base material and cladding material have different chemical compositions and crystal structures, resulting in significantly different thermal expansion coefficients. Therefore, when heated above 1000°C for solution heat treatment, the difference in thermal expansion coefficients can cause the plate to deform significantly, resulting in significant costs for flatness correction. Furthermore, above 1000°C, ferrite-austenite transformation can occur in the base carbon steel or low-alloy steel, which can alter the structure engineered during rolling and potentially prevent the desired strength and toughness characteristics from being achieved. Therefore, post-rolling heat treatment is often omitted when manufacturing clad steel plates.

[0014] Patent Document 1 discloses a technique for producing a duplex stainless clad steel sheet that has good corrosion resistance even without heat treatment by rolling at 900°C or higher and then accelerating cooling in the 900°C to 750°C range, which is the sigma phase precipitation temperature range, thereby suppressing sigma phase precipitation in the clad material.In addition to the above patent document, there are several patent documents that describe techniques for achieving good corrosion resistance by accelerating cooling in the sigma phase precipitation temperature range using methods such as water cooling. However, when clad steel plates are water-cooled to ensure the corrosion resistance of the clad material, the proportion of bainite and martensite in the base material structure increases, which can lead to reduced workability. In particular, with conventional manufacturing methods, such as the open-type method in which the base material and clad material are layered together to form a rolled material, or the sandwich-type method in which the base material, clad material, (release agent), clad material, and base material are layered together to form a rolled material, the cooling rate in the surface layer of the base material, which is directly exposed to the cooling water, is extremely high. This increases the proportion of bainite and martensite compared to the interior of the base material, resulting in reduced workability. Furthermore, if slow cooling is used to reduce the proportion of bainite and martensite in the base material, the clad material will not be cooled sufficiently, resulting in the precipitation of sigma phase. Tempering to soften bainite and martensite or to dissolve the sigma phase is difficult due to the sheet bending caused by the aforementioned difference in expansion coefficients.

[0015] Patent Document 2 discloses a technology for clad steel sheets that achieves both strength and workability by controlling the surface and internal structures of the base material. However, this control of the base material structure is achieved by tempering, and there is no description of structure control without tempering.

[0016] Patent Document 3 discloses a manufacturing technology for clad steel plates, which, unlike ordinary sandwich-type manufacturing methods, involves stacking layers in the following order to form a rolled material: cladding material-base material-(release agent)-base material-cladding material. However, the problems solved by Patent Document 3 are bondability, warpage, and yield, and the examples of clad steel plates are those in which the base material is made of carbon steel (SS41) and the cladding material is made of stainless steel (SUS304). There is no description of the corrosion resistance when the cladding material is a duplex stainless steel plate, or the workability of the base materials, carbon steel or low-alloy steel, or how to achieve both. [Prior art documents] [Patent documents]

[0017] [Patent Document 1] Patent No. 6477735 [Patent Document 2] Patent No. 6573060 [Patent Document 3] Patent No. 3899709 Summary of the Invention [Problem to be solved by the invention]

[0018] Patent Document 1 discloses a technology that uses accelerated cooling to achieve good corrosion resistance in clad materials even when heat treatment after rolling is omitted. Patent Document 2 discloses a technology that softens the bainite or martensite in the surface layer of the base material by tempering to improve the workability of the base material. Patent Document 3 discloses a manufacturing technology for clad steel plate that, unlike ordinary sandwich-type manufacturing methods, layers the clad material-base material-(release agent)-base material-clad material in this order to form the rolling material. However, although there is a demand for a technology for duplex stainless clad steel sheets that can achieve both the corrosion resistance of the clad material and the workability of the base material even if heat treatment after rolling is omitted, such a technology has not been disclosed or suggested.

[0019] In view of the above-mentioned problems, the present invention aims to provide a duplex stainless clad steel plate having excellent corrosion resistance of the clad material and excellent workability of the base material, and a method for manufacturing the same. [Means for solving the problem]

[0020] The present inventors have discovered that by optimizing the assembly method of the rolled material and the rolling conditions, it is possible to reduce the sigma phase precipitation in the clad material and simultaneously control the ferrite phase ratio in the surface layer of the base material, thereby achieving both the corrosion resistance of the clad material and the workability of the base material even if heat treatment after rolling is omitted.The inventors then came up with the idea that a method for achieving this would require both increasing the cooling rate of the clad material and slowing the cooling rate of the base material, and have conducted extensive research.

[0021] The present invention has been made to solve the above-mentioned problems, and is summarized as the following clad steel plate and a method for manufacturing the same. [1] A clad steel plate comprising a base material and a clad material joined to the base material, the base material is made of carbon steel or low alloy steel, the cladding material is made of duplex stainless steel, the difference between the ferric chloride CPT of the cladding material of the solution-treated sample and the ferric chloride CPT of the cladding material is 10°C or less, The clad steel plate is characterized in that the ferrite phase ratio in the surface layer of the base material is more than 15%. Here, the surface layer of the base material refers to a position 1 mm from the surface of the base material in the thickness direction. Also, ferric chloride CPT means the pitting corrosion temperature (°C) evaluated in accordance with the ASTM G48E method. [2] In the clad steel plate according to [1], the chemical composition of the base material contains, in mass %, C: 0.020 to 0.200%, Si: 1.00% or less, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.050% or less, Nb: 0.200% or less, and N: 0.020% or less, and Ceq is 0.20 to 0.50, with the balance being Fe and impurities. Here, Ceq is defined by the following equation (1). Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5...(1) In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass%) of each element in the chemical composition of the base steel sheet. [3] The chemical composition of the base metal further contains, in mass%, Ni: 0.01 to 3.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 0.50%, W: 0.01 to 1.00%, Cu: 0.01 to 2.00%, Co: 0.01 to 0.50%, Se + Te: 0.01 to 0.10%, Al: 0.005 to 0.300%, V: 0.00 The clad steel plate according to [2], containing one or more selected from the group consisting of: Ti: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Ca: 0.0003 to 0.0100%, B: 0.0003 to 0.0030%, Mg: 0.0003 to 0.0100%, Zr+Hf+Ta: 0.0001 to 0.0100%, and REM: 0.0003 to 0.0100%. [4] A method for manufacturing a clad steel plate according to any one of [1] to [3], characterized in that the base material and clad material are stacked so that the bonding surface is vacuum, and the four peripheries of the bonding surface are sealed by welding to form a clad material, and two of the clad materials are overlapped so that the clad material is positioned on the outside, and the four peripheries of the interface between the base materials are welded together to form a rolled material, the temperature calculated from the chemical composition of the clad material using formula (1) is the sigma phase precipitation start temperature Tσ, and the temperature at the start of the final pass is rolled to be Tσ-50 (°C) or higher, and accelerated cooling is performed from Tσ-100°C or higher after rolling, and the surface temperature of the plate reheated after cooling is 650°C or lower, and the average cooling rate at the center of the plate thickness from the start of accelerated cooling to the end of reheating is 25°C / s or lower. Tσ=4Cr+25Ni+71(Mo+W)-11.4(Mo-1.3) 2 +5Si-6Mn-30N+569(℃)... Formula (2) Each element symbol in formula (2) indicates the content (mass%) of the corresponding element in the cladding material, and 0 is substituted if the element is not contained. [Effects of the Invention]

[0022] According to the present invention, a duplex stainless clad steel sheet having excellent corrosion resistance of the clad material and excellent workability of the base material can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0023] The sandwich-type manufacturing method is applied when manufacturing a clad steel plate by hot rolling. The material assembled into the sandwich mold before hot rolling is called the "rolled material," and the steel plate assembled into the sandwich mold after hot rolling the rolled material is called the "rolled steel plate." The inventors conceived the idea that the above-mentioned problem could be solved by stacking the clad steel plate - base material - (release agent) - base material - clad steel plate in this order to form the rolled material, rather than assembling the rolled material with the base material of the clad steel plate as the front side of the rolled material as in a conventional sandwich mold, and conducted the following study. Specifically, the inventors investigated the corrosion resistance of the clad steel plate and the workability of the base material by changing the stacking method, rolling conditions, and cooling conditions for clad steel plates using various duplex stainless steels as clad materials. As a result, they obtained the following findings (a) and (b). (a) By positioning the cladding material on the outside of the rolled material and controlling the rolling temperature, cooling temperature range, and cooling rate, the precipitation of sigma phase during rolling can be suppressed, and good corrosion resistance can be obtained without heat treatment. (b) By positioning the base material inside the rolled material and controlling the cooling temperature range and cooling rate at the center of the steel plate thickness after rolling, the formation of ferrite phase during rolling can be promoted, and good workability can be obtained without heat treatment.

[0024] (a) By positioning the cladding material on the outside of the rolled material and controlling the rolling temperature, cooling temperature range, and cooling rate, the precipitation of sigma phase during rolling can be suppressed, and good corrosion resistance can be obtained without heat treatment. The temperature calculated from the chemical composition of the cladding material using equation (2) is the sigma phase precipitation start temperature, Tσ. To suppress sigma phase precipitation and prevent a decrease in the corrosion resistance of the cladding material, it is effective to set the temperature of the surface of the rolled steel sheet (cladding material portion) at the end of rolling to (Tσ-50°C) or higher, and to cool the cladding material surface at a cooling rate of 2°C / s or higher from a temperature at which the surface temperature of the rolled steel sheet is (Tσ-100°C) or higher after rolling. In practice, if accelerated cooling is performed to the extent that the entire surface of a large rolled sheet can be cooled uniformly, a cooling rate of 2°C / s or higher can be achieved on the surface of the rolled steel sheet or directly below the sacrificial material. Tσ=4Cr+25Ni+71(Mo+W)-11.4(Mo-1.3) 2+5Si-6Mn-30N+569(℃)... Formula (2) In the formula 1, each element symbol indicates the content (mass%) of the element in the cladding material, and 0 is substituted if the element is not contained.

[0025] Pitting corrosion initiation temperature evaluation can be used as a means of evaluating the corrosion resistance of clad steel plate clad material parts. Pitting corrosion initiation temperature is evaluated in accordance with the ASTM G48E method. Hereinafter, this evaluation method will be referred to as "ferric chloride CPT." In practice, the ferric chloride CPT evaluated for the clad material part of a clad steel plate product that has not undergone solution heat treatment (hereinafter referred to as "product CPT") is compared with the ferric chloride CPT of a sample whose clad material part has undergone solution heat treatment (hereinafter referred to as "solution CPT"), and it is effective to evaluate the difference between the solution CPT and the product CPT (hereinafter referred to as "CPT difference") to be 10°C or less.

[0026] (b) By positioning the base material inside the rolled material and controlling the cooling temperature range and cooling rate at the center of the steel plate thickness after rolling, the formation of ferrite phase during rolling can be promoted, and good workability can be obtained without heat treatment. The austenite-to-ferrite transformation is a phenomenon that requires a certain amount of time, and the proportion of bainite and martensite phases is highly dependent on the cooling rate. To suppress the precipitation of bainite and martensite in the base material surface and achieve good workability, accelerated cooling, in which the average cooling rate is 25°C / s or less in the temperature range in which the austenite-to-ferrite transformation occurs, is effective. If the cooling rate in this temperature range increases, hard bainite and martensite form instead of soft ferrite near the base material surface, located 1 mm from the base material surface in the thickness direction. This reduces workability, such as reducing ductility in tensile tests and causing necking and cracking during bending. In particular, in a base material with a chemical composition that provides sufficient strength for structural applications, hard bainite and martensite are likely to form when the cooling rate increases. After hot rolling, when the steel plate undergoes accelerated cooling, the surface of the plate cools rapidly and then reheats after the accelerated cooling is complete. However, the temperature history of the center of the plate does not follow a "rapid cooling-reheating" cycle, but follows a monotonous temperature drop from the start of accelerated cooling to the end of reheating. Just before the start of accelerated cooling and at the end of reheating, there is a temperature difference of several degrees Celsius to several tens of degrees Celsius between the surface and center of the plate, depending on the thickness. However, the temperature difference between the surface and center of the plate is considered to be roughly the same at both points. Therefore, the cooling rate at the center of the plate can be approximately calculated from the cooling rate calculated from the surface temperature at the start of accelerated cooling and at the end of reheating. Therefore, by positioning the base material inside the rolled material, setting the temperature of the center of the thickness of the steel plate after rolling at the start of accelerated cooling after hot rolling to a temperature higher than the temperature zone in which austenite → ferrite transformation occurs, and setting the temperature of the center of the thickness of the steel plate after rolling at the end of reheating to a temperature lower than the temperature zone in which austenite → ferrite transformation occurs, and setting the average cooling rate between these two points to 25°C / s or less, it is possible to suppress the formation of bainite and martensite and promote the formation of ferrite phase.

[0027] A bending test can be used as a means of evaluating the workability of the surface layer of the base material of a steel plate. The bending test is performed in accordance with JIS Z 2248. The entire thickness of the obtained clad steel plate is bent 180 degrees, with the base material on the outside and the cladding material on the inside, and the bending radius is the same as the thickness of the clad steel plate. The test piece is then removed from the testing machine and the outside of the bent part (the base material side) is visually inspected, and those without cracks or other defects are considered good.

[0028] Therefore, in order to obtain a duplex stainless clad steel sheet with excellent corrosion resistance of the clad material and excellent workability of the base material, it is necessary to simultaneously suppress sigma phase precipitation in the clad material and control the austenite-to-ferrite transformation in the surface layer of the base material by optimizing the assembly method of the rolled material, the rolling finish temperature, and the cooling conditions after rolling. The present invention was made based on the above findings. Each requirement of the present invention will be described in detail below.

[0029] 1. Configuration of the present invention The clad steel plate according to the present invention comprises a base material and a clad material bonded to one side of the base material. The base material is made of carbon steel or low-alloy steel, as described below. The clad material is made of duplex stainless steel. The surface of the base material on the side to which the clad material is not bonded (where the base material is exposed) is referred to as the "base material surface," and the position 1 mm in the plate thickness direction from the base material surface is referred to as the "base material surface layer." Furthermore, the difference (the "CPT difference") between the ferric chloride CPT of a sample obtained by solution heat treating the clad material (the "solution heat treated CPT") and the ferric chloride CPT of the clad material of the product (the "product CPT") is 10°C or less, and the ferrite fraction of the base material surface layer is more than 15%.

[0030] 2. Corrosion resistance of cladding material and workability of base material To obtain a clad steel sheet that has excellent corrosion resistance in the clad material and excellent workability in the base material, it is necessary to simultaneously improve corrosion resistance by suppressing sigma phase precipitation in the clad material and promote ferrite phase transformation in the surface layer of the base material.

[0031] 2-1. The corrosion resistance of the clad steel plate according to the present invention will be described below. As mentioned above, the corrosion resistance of clad steel is determined by the difference (the "CPT difference") between the ferric chloride CPT of a sample obtained by solution heat treating the clad steel (the "solution heat treated CPT") and the ferric chloride CPT of the clad steel in the finished product (the "product CPT"), as described above. If the CPT difference exceeds 10°C, when selecting a clad steel suitable for the environment in which it will be used, a clad steel with a higher alloy and greater corrosion resistance will have to be selected, and the cost reduction effect of the clad steel, which is one of the advantages of clad steel plates, will not be achieved. Preferably, the CPT difference is 5°C or less, and more preferably 0°C or less. The smaller the CPT difference, the better, so no lower limit is set. Here, ferric chloride CPT means the pitting corrosion temperature evaluated in accordance with the ASTM G48E method as described above.

[0032] 2-2. Ferrite phase ratio in the base material surface The ferrite phase ratio in the surface layer of the base material of the clad steel plate is set to be more than 15%. If it is less than 15%, cracks may occur in a bending test. The higher the ferrite phase ratio in the surface layer of the base material, the better the workability of the base material, so no upper limit is set. It is preferably 20% or more, and more preferably 30% or more. The remainder other than the ferrite phase is made up of pearlite, bainite, martensite, or a mixed structure of two or more phases. There are no particular restrictions on the structure inside the base material, but like the surface layer, it is desirable that the ferrite phase ratio be more than 15%. It is preferably 20% or more, and more preferably 30% or more. The remainder is made up of pearlite, bainite, martensite, or a mixed structure of two or more phases. Here, the base material surface layer refers to a position 1 mm from the base material surface in the thickness direction, and the ferrite phase fraction refers to the area fraction where the KAM (Kernel Average Misorientation) in the EBSD test is 1° or less.

[0033] 3. Chemical composition of the base material The base material is made of carbon steel or low alloy steel. The preferred chemical composition of the base metal is a steel sheet containing, by mass%, C: 0.020 to 0.200%, Si: 1.00% or less, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.050% or less, Nb: 0.200% or less, and N: 0.020% or less, with Ceq of 0.20 to 0.50, and the balance being Fe and impurities, where Ceq is defined by the following formula (1): Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5...(1) In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass%) of each element in the chemical composition of the base material.

[0034] C is an element that improves the strength of steel, and a content of 0.020% or more provides sufficient strength. However, if it exceeds 0.200%, it will cause deterioration in weldability and toughness. Therefore, the C content is set to 0.020 to 0.200%, preferably 0.040% or more, and more preferably 0.050% or more. On the other hand, the upper limit is preferably 0.100% or less, and even more preferably 0.080% or less. A more preferable range is 0.040% to 0.100%, and an even more preferable range is 0.050% to 0.080%.

[0035] Silicon is effective for deoxidation and is an element that improves the strength of steel. However, if the silicon content exceeds 1.00%, it will deteriorate the surface quality and toughness of the steel. Therefore, the silicon content is set to 1.00% or less, and preferably 0.50% or less. Silicon may not be contained. The preferred lower limit of silicon content is 0.01%.

[0036] Mn is an element that increases the strength of steel, and this effect is manifested when the content is 0.10% or more. However, if the content exceeds 3.00%, weldability is impaired and the alloy cost increases. Therefore, the Mn content is set to 0.10 to 3.00%, preferably 0.50 to 2.00%, and more preferably 0.90 to 1.60%.

[0037] P is an impurity in steel, and if the content exceeds 0.050%, toughness deteriorates. Therefore, the P content is set to 0.050% or less, preferably 0.015% or less.

[0038] S is an impurity in steel, and if the content exceeds 0.050%, toughness deteriorates. Therefore, the S content is set to 0.050% or less, preferably 0.010% or less.

[0039] Nb is an element that raises the recrystallization temperature and greatly affects the rate of ferrite phase transformation. In order to stably maintain the ferrite phase ratio in the surface layer at 15% or more, it is preferable to add more than 0.008%, and more preferably 0.010% or more. However, if it exceeds 0.200%, weldability is impaired and the alloy cost increases. Therefore, the Nb content is set to 0.200% or less. The preferred upper limit of Nb is 0.100%. It is more preferably 0.010 to 0.050%, and even more preferably 0.030 to 0.050%. Nb may not be contained.

[0040] N is an element that combines with Nb, V, Ti, etc. to precipitate nitrides or carbonitrides. If the N content exceeds 0.010%, fine precipitates will reduce workability and toughness. Therefore, the N content is set to 0.020% or less, and preferably 0.006% or less. There is no particular lower limit, but due to production technology constraints, the N content is preferably set to 0.0005% or more.

[0041] Ceq (carbon equivalent) is a value used to estimate hardness and weldability from the chemical composition of steel, and is calculated using formula (1). The higher Ceq, the more likely bainite or martensitic transformation will occur instead of ferrite transformation, improving hardness and deteriorating weldability. If Ceq is less than 0.20, sufficient strength for a structure cannot be obtained. Therefore, Ceq is set to 0.20 or more, preferably 0.23 or more. If Ceq exceeds 0.50, a ferrite phase fraction of 15% cannot be consistently achieved in the surface layer, and weldability deteriorates, requiring interpass temperature control and post-heat treatment, which increases welding costs. Therefore, Ceq is set to 0.50 or less, preferably 0.40 or less, and more preferably 0.35 or less. Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5...(1) In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass%) of each element in the chemical composition of the base material.

[0042] The chemical composition of the base material contains the above components, with the balance being Fe and impurities. The chemical composition of the base material further contains, in mass %, the following elements in place of a portion of the Fe: Ni: 0.01-3.00%, Cr: 0.01-1.00%, Mo: 0.01-0.50%, W: 0.01-1.00%, Cu: 0.01-2.00%, Co: 0.01-0.50%, Se+Te: 0.01-0.10%, V: 0.001-0.100%, Ti: 0. It may contain one or more selected from: Al: 0.001 to 0.200%, Al: 0.005 to 0.300%, Ca: 0.0003 to 0.0100%, B: 0.0003 to 0.0030%, Mg: 0.0003 to 0.0100%, Zr+Hf+Ta: 0.0001 to 0.0100%, and REM: 0.0003 to 0.0100%.

[0043] Ni is an element that improves the hardenability of steel, and improves the strength and toughness of the steel after rolling. However, if it exceeds 3.00%, it causes deterioration of weldability and toughness. Therefore, when Ni is contained, the Ni amount is set to 3.00% or less. It is preferably 1.00% or less, more preferably 0.50% or less, and even more preferably 0.30% or less. The preferred lower limit of the Ni content is 0.01%.

[0044] Cr is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if the content exceeds 1.00%, it causes deterioration of weldability and toughness. Therefore, if Cr is contained, the Cr content should be 1.00% or less, preferably 0.50% or less, and more preferably 0.30% or less. The preferred lower limit of the Cr content is 0.01%.

[0045] Mo is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if it exceeds 0.50%, it causes deterioration of weldability and toughness. Therefore, when Mo is contained, the Mo content is set to 0.50% or less, preferably 0.30% or less, and more preferably 0.1% or less. The preferred lower limit of the Mo content is 0.01%.

[0046] W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. If the W content exceeds 1.00%, hot workability deteriorates, resulting in a decrease in productivity. Therefore, when W is contained, the W content is set to 1.00% or less, preferably 0.50% or less, and more preferably 0.30% or less. The preferred lower limit of the W content is 0.01%.

[0047] Cu is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if it exceeds 2.00%, it causes deterioration of weldability and toughness. Therefore, when Cu is contained, the Cu content is set to 2.00% or less. It is preferably 1.00% or less, more preferably 0.50% or less, and even more preferably 0.30% or less. The preferred lower limit of the Cu content is 0.01%.

[0048] Co is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if the content exceeds 0.50%, hot workability is impaired, resulting in a decrease in productivity. Therefore, when Co is contained, the Co content is set to 0.50% or less, preferably 0.30% or less, and more preferably 0.1% or less. The preferred lower limit of the Co content is 0.01%.

[0049] Se and Te inhibit easily oxidizable elements such as Mn, Si, and Al in steel sheet from diffusing to the steel sheet surface and forming oxides, thereby improving the surface quality and platability of the steel sheet. However, this effect saturates when the total content exceeds 0.10%. Therefore, when Se and Te are added, the total amount of Se and Te should be 0.10% or less, and more preferably 0.05% or less. The preferred lower limit of the Se + Te content is 0.01%.

[0050] Al is an element that is effective in deoxidizing steel. However, if the content exceeds 0.300%, it can cause a deterioration in the toughness of welds. Therefore, if Al is contained, the Al content should be 0.300% or less, preferably 0.100% or less. The preferred lower limit of the Al content is 0.005%.

[0051] V increases the strength of steel by forming carbonitrides. However, if it exceeds 0.100%, it causes deterioration of weldability and toughness. Therefore, if V is contained, the V content should be 0.100% or less, preferably 0.050% or less. The preferred lower limit of the V content is 0.001%.

[0052] Ti is an element that refines crystal grains and increases strength, and this effect is manifested when added at 0.001% or more. However, if it exceeds 0.200%, weldability is impaired and alloy costs increase. Therefore, the Ti content is set to 0.001 to 0.200%, preferably 0.005 to 0.100%, and more preferably 0.010 to 0.050%.

[0053] Ca is an element that refines the structure of the weld heat affected zone and improves toughness. However, if it exceeds 0.0100%, it forms coarse inclusions and deteriorates toughness. Therefore, when Ca is contained, the Ca content is set to 0.0100% or less, preferably 0.0050% or less, and more preferably 0.0030% or less. The preferred lower limit of the Ca content is 0.0003%.

[0054] B is an element that improves the hardenability of steel, improving the strength and toughness of the steel after rolling. However, if it exceeds 0.0030%, it causes deterioration of weldability and toughness. Therefore, when B is contained, the B content is set to 0.0030% or less, preferably 0.0015% or less. The preferred lower limit of the B content is 0.0003%.

[0055] Mg is an element that improves ductility and toughness by controlling the morphology of sulfide-based inclusions. However, if the content exceeds 0.0100%, the amount of nonmetallic inclusions increases, and ductility and toughness decrease. Therefore, when Mg is contained, the content is set to 0.0100% or less. It is preferably 0.0050% or less, and more preferably 0.0030% or less. The preferred lower limit of the Mg content is 0.0003%.

[0056] Zr, Hf, and Ta are elements that contribute to improving formability. If the total content of one or more of Zr, Hf, and Ta exceeds 0.01%, ductility may decrease. Therefore, when one or more of Zr, Hf, and Ta are contained, the total content is set to 0.0100% or less, preferably 0.0070% or less. The preferred total content of one or more of Zr, Hf, and Ta is 0.0001%.

[0057] REM refines the structure of the weld heat affected zone and improves toughness. However, if it exceeds 0.0100%, it forms coarse inclusions, degrading toughness. Therefore, if REM is contained, the REM content should be 0.0100% or less, preferably 0.005% or less. The preferred lower limit of the REM content is 0.0003%.

[0058] Here, REM is a collective term for 17 elements, including 15 lanthanoid elements plus Y and Sc. One or more of these 17 elements can be contained in a steel material, and the REM content refers to the total content of these elements.

[0059] The balance of the chemical composition of the base metal of the present invention is Fe and impurities. Here, "impurities" refer to components that are mixed in during industrial production of steel due to various factors in raw materials such as ores and scraps, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention.

[0060] 4. Duplex stainless steel cladding material Next, we will explain duplex stainless steel clad materials. Duplex stainless steel is a stainless steel whose structure consists of two phases, ferrite and austenite, and is specified as "austenitic-ferritic" in JIS G 4304, for example. Ferrite-austenite duplex stainless steel applicable to the cladding material of this embodiment has, for example, the following chemical composition in mass %: C: 0.10% or less, Si: 2.00% or less, Mn: 0.50 to 6.00%, P: 0.050% or less, S: 0.050% or less, Ni: 0.10 to 8.00%, Cr: 17.0 to 30.0%, N: 0.05 to 0.30%, Mo: 1.00 to 5.00%, Cu: 0 to 2.0 %, Nb: 0-0.10%, Sn: 0-1.00%, W: 0-1.00%, V: 0-1.00%, Ti: 0-0.05%, B: 0-0.0050%, Ca: 0-0.0050%, Mg: 0-0.0050%, Al: 0-0.05%, REM: 0-0.50%, the balance being Fe and unavoidable impurities, and the PREN_Mn value calculated by the following formula (3) is 25 or more. This chemical composition is merely an example, and the present invention is not limited thereto. The reason for giving this chemical composition is as follows. In the following description, "%" regarding the content means "% by mass." PREN_Mn value = Cr + 3.3(Mo + 0.5W) + 16N - Mn (3) However, the element symbols in the above formula (3) represent the content (mass %) of each element contained in the cladding material, and 0 is substituted if the element is not contained.

[0061] C: 0.10% or less C is an element that dissolves in the austenite phase to increase strength. However, if the C content exceeds 0.10%, the strength of the steel increases and workability deteriorates. Furthermore, it promotes the precipitation of Cr carbides, which leads to the occurrence of intergranular corrosion. Therefore, the C content is set to 0.10% or less. The C content may be 0.050% or less, or may be 0.040% or less. Furthermore, while a low C content is preferable from the viewpoint of corrosion resistance, reducing the C content to 0.002% or less with existing steelmaking equipment would result in a significant increase in costs. Therefore, the C content is preferably 0.002% or more.

[0062] Si:2.00% or less Si is sometimes used as a deoxidizing element or added to improve oxidation resistance. However, if the Si content exceeds 2.00%, the steel sheet becomes hard and its toughness and workability deteriorate. Therefore, the Si content is set to 2.00% or less. The Si content is preferably 1.50% or less, and more preferably 1.00% or less. Furthermore, reducing the Si content to a minimum level increases the cost of refining the steel. Therefore, the Si content is preferably 0.03% or more.

[0063] Mn: 0.50 to 6.00% Mn has the effect of increasing the austenite phase and increasing the solid solubility of nitrogen, thereby suppressing defects such as pores during manufacturing. However, a large amount of Mn content reduces corrosion resistance and hot workability. Therefore, the Mn content is set to 0.50 to 6.00%. The Mn content is preferably 1.00% or more, and more preferably 2.50% or more. Furthermore, the Mn content is preferably 4.00% or less.

[0064] P:0.050% or less P is an element that is inevitably mixed into steel and is also contained in raw materials such as Cr, making it difficult to reduce, but a large amount of P reduces formability. The lower the P content, the better, and it is set to 0.050% or less. The P content is preferably 0.040% or less. A lower P content is desirable, but reducing the P content significantly increases costs, so the P content may be 0.0005% or more.

[0065] S: 0.050% or less S is an element that is inevitably mixed into steel, and it can combine with Mn to form inclusions that can become the starting point for rust. Therefore, the S content is set to 0.050% or less. The lower the S content, the better the corrosion resistance, so it is preferably set to 0.0030% or less. Although a lower S content is desirable, reducing the S content would result in a significant increase in cost, so the S content may be 0.0001% or more.

[0066] Ni: 0.10 to 8.00% Ni is an austenite stabilizing element and is an important element for increasing the austenite phase ratio in the surface layer. Ni also has the effect of improving corrosion resistance. However, a large amount of Ni content increases raw material costs and may cause problems such as stress corrosion cracking. Therefore, the Ni content is set to 0.10 to 8.00%. The Ni content is preferably 1.00% or more. The Ni content is preferably 6.00% or less, more preferably 4.00% or less, and even more preferably 3.00% or less.

[0067] Cr: 17.0~30.0% Cr is an element necessary for ensuring corrosion resistance. However, a large amount of Cr can cause hot work cracking and increase the amount of sigma phase precipitation in the weld metal zone and the weld heat-affected zone. Therefore, the Cr content is set to 17.0 to 30.0%. The Cr content is preferably 20.0% or more, and more preferably 21.0% or more. The Cr content is preferably 25.0% or less, more preferably 23.0% or less, and even more preferably 22.0% or less.

[0068] N: 0.05 to 0.30% N is an element that dissolves in the austenite phase, improving strength and corrosion resistance and contributing to alloy saving. However, N has a significant effect on the precipitation of chromium nitrides during welding cooling. If the N content exceeds 0.30%, the amount of chromium nitrides precipitated in the weld metal and the weld heat-affected zone increases, resulting in a large difference in corrosion resistance between the base metal and the weld. Therefore, the N content is set to 0.05 to 0.30%. From the viewpoints of strength and corrosion resistance, the N content may be 0.08% or more, preferably 0.10% or more, and more preferably 0.15% or more. Furthermore, from the viewpoint of suppressing the precipitation of chromium nitrides, the N content is preferably 0.25% or less, and more preferably 0.20% or less.

[0069] Mo: 1.00-5.00% Mo is an element that improves corrosion resistance. However, a large amount of Mo increases raw material costs and causes problems such as reduced corrosion resistance due to the precipitation of sigma phases in welds. Therefore, the Mo content is set to 1.00% or more and 5.00% or less. To achieve the above effects, the Mo content is preferably 2.00% or more. Furthermore, the Mo content is preferably 4.00% or less.

[0070] Cu: 0 to 2.0% Cu is an element that is very effective in improving sulfuric acid resistance and may be added as needed. To achieve the above effect, the Cu content is preferably 0.1% or more. The Cu content is more preferably 0.3% or more. On the other hand, because Cu is an element that increases the activity of N and facilitates the precipitation of chromium nitride in the weld metal, the Cu content is set to 2.0% or less. The Cu content is preferably 1.5% or less, and more preferably 1.0% or less.

[0071] Nb: 0 to 0.10% Nb may be added as needed because it has the effect of suppressing the precipitation of chromium nitrides by forming a compound with N. However, a large amount of Nb reduces the workability of the steel sheet. Therefore, the Nb content is set to 0.10% or less. To obtain the above effect, the Nb content is preferably 0.01% or more, and more preferably 0.04% or more.

[0072] Sn: 0 to 1.00% Sn is an element that improves corrosion resistance, so it may be contained as needed. However, a large amount of Sn content deteriorates hot workability. Therefore, the Sn content is set to 1.00% or less. To obtain the above effects, the Sn content is preferably 0.010% or more.

[0073] W: 0 to 1.00% W is an element that improves corrosion resistance, so it may be contained as needed. However, if a large amount of W is contained, the load during rolling increases, making manufacturing defects more likely to occur. Therefore, the W content is set to 1.00% or less. To obtain the above effects, the W content is preferably 0.01% or more. Furthermore, the W content is preferably 0.50% or less.

[0074] V: 0 to 1.00% V is an element that improves corrosion resistance, so it may be contained as needed. However, if a large amount of V is contained, the load during rolling increases, making manufacturing defects more likely to occur. Therefore, the V content is set to 1.00% or less. To obtain the above effects, the V content is preferably 0.01% or more. Furthermore, the V content is preferably 0.50% or less.

[0075] Ti: 0 to 0.05% Like Nb, Ti prevents coarsening of the weld heat-affected zone and further has the effect of forming fine equiaxed crystals in the solidification structure, so it may be added as needed. However, a large amount of Ti reduces the uniform elongation and local elongation. Therefore, the Ti content is set to 0.05% or less. To obtain the above effects, the Ti content is preferably 0.005% or more.

[0076] B: 0 to 0.0050% B has the effect of improving hot workability and may be added as needed. However, if a large amount of B is added, corrosion resistance will be significantly deteriorated. Therefore, the B content is set to 0.0050% or less. To obtain the above effect, the B content is preferably 0.0003% or more. Furthermore, the B content is preferably 0.0030% or less.

[0077] Ca: 0 to 0.0050% Ca may be added as needed for desulfurization and deoxidation. However, a large amount of Ca content increases the likelihood of hot work cracking and reduces corrosion resistance. Therefore, the Ca content is set to 0.0050% or less. To achieve the above effects, the Ca content is preferably 0.0001% or more.

[0078] Mg: 0 to 0.0050% Mg may be added as needed, as it not only deoxidizes but also refines the solidification structure. However, adding a large amount of Mg increases the cost of the steelmaking process. Therefore, the Mg content is set to 0.0050% or less. To achieve the above effects, the Mg content is preferably 0.0001% or more.

[0079] Al: 0 to 0.05% Al may be added as needed for desulfurization and deoxidation. However, adding a large amount of Al increases the risk of manufacturing defects and increases raw material costs. Therefore, the Al content is set to 0.05% or less. To achieve the above effects, the Al content is preferably 0.0030% or more.

[0080] REM: 0 to 0.50% REM (rare earth elements) have the effect of improving hot workability and may be added as needed. However, a large amount of REM content impairs manufacturability and increases costs. Therefore, the REM content is set to 0.50% or less. To achieve the above effect, the REM content is preferably 0.005% or more. The REM content is preferably 0.020% or more and 0.20% or less.

[0081] REM is a general term for 17 elements, including Sc, Y, and 15 elements from La to Lu (lanthanoids), and the REM content refers to the total content of these elements. Industrially, lanthanoids are added in the form of misch metals.

[0082] In the chemical composition of the cladding material of the present invention, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in during industrial steel production due to various factors in raw materials such as ore and scrap, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention.

[0083] PREN_Mn value: 25 or higher The PREN_Mn value is a general index that indicates the pitting corrosion resistance of a stainless steel sheet, and is calculated from the chemical composition of the stainless steel using the following formula (3). PREN_Mn value=Cr+3.3Mo+16N-Mn (3) However, the element symbols in the above formula (3) represent the content (mass %) of each element contained in the cladding material, and 0 is substituted when no element is contained. If the PREN_Mn value is low, it may not be possible to obtain corrosion resistance suitable for the usage environment. In order to obtain corrosion resistance equivalent to that of SUS316, the PREN_Mn value is set to 25 or more. It is preferably 30 or more, and more preferably 35 or more. The higher the PREN_Mn value, the better the corrosion resistance, so there is no need to specify an upper limit. However, since problems may arise such as increased alloy costs and the generation of nitrogen bubbles due to an increase in the N content and a decrease in the Mn content, it is preferable to set the PREN_Mn value to less than 50, and more preferably less than 45.

[0084] 5. Manufacturing method The method for manufacturing a clad steel plate according to the present invention will now be described. As mentioned above, in order to obtain a clad steel plate with excellent corrosion resistance of the cladding material and excellent workability of the base material, it is necessary to control the metal structure. Such a metal structure can be achieved by combining the chemical composition of the steel with appropriate manufacturing conditions. In the clad steel plate described above, the base material and cladding material are laminated so that the bonding surface is vacuumed, and the four periphery of the bonding surface is sealed by welding to form the clad material. Two clad materials are assembled so that the cladding surface of each clad material becomes the surface of the rolled material to form the rolled material. The base material surface of each clad material is placed at the center of the thickness of the rolled material, with a release agent sandwiched between them. The assembled rolling material is hot rolled. The temperature calculated from the chemical composition of the cladding material using formula (1) is the sigma phase precipitation start temperature, Tσ. Rolling is carried out so that the temperature at the start of the final pass of hot rolling is Tσ-50 (°C) or higher, and after rolling, cooling is carried out in the temperature range of Tσ-100°C to 650°C, with the average cooling rate at the center of the plate thickness being 25°C / s or less. Tσ =4Cr+25Ni+71(Mo+W)-11.4(Mo-1.3) 2 +5Si-6Mn-30N+569(℃)... Formula (2) However, each element symbol in formula (2) indicates the content (mass%) of the corresponding element in the cladding material, and 0 is substituted if the element is not contained.

[0085] 5-1 Clad material, rolled material The clad material is manufactured by the method described below. Specifically, carbon steel and low-alloy steel, which serve as the base material, and duplex stainless steel, which serve as the clad material, are melted by a known method such as a converter, electric furnace, or vacuum melting furnace, and then slabs are produced by a continuous casting method or an ingot-blooming method. The resulting slabs are hot-rolled under commonly used conditions to produce hot-rolled sheets, which serve as the clad material and base material. The resulting hot-rolled sheets may be annealed, pickled, polished, etc., as necessary. The clad material is assembled by laminating the cladding material and base material so that the bonding surface is vacuum, and then sealing the four edges of the bonding surface by welding. An insert material such as Ni foil may be inserted between the cladding material and base material to improve adhesion and interfacial corrosion resistance. The method for vacuuming the bonding surface is not particularly specified, but examples include electron beam welding in a vacuum, or pre-drilling a vacuum hole, welding the four edges in air by arc welding or laser welding, and then vacuuming with a vacuum pump. A vacuum level (absolute pressure) of 0.1 Torr or less results in a good bonded interface with minimal interfacial oxides, and is preferably 0.05 Torr or less. Since a higher vacuum level (lower absolute pressure) tends to result in a better bonded interface, no lower limit is set. The obtained clad material is made by applying a release agent to the base material side, then stacking two clad materials in the following order: cladding material-base material-(release agent)-base material-cladding material, and then welding the interface between one base material and the other to form the rolled material. By assembling in this order, the surface layer of the base material of the product is located inside the rolled material during rolling, and does not come into direct contact with the cooling medium during cooling, which allows for a slower cooling rate for the surface layer of the base material. To prevent sigma phase precipitation due to a decrease in surface temperature caused by contact of the cladding material with the rolls during rolling, a sacrificial material made of, for example, carbon steel may be superimposed on the outer surface of the cladding material in advance, and the superimposed sacrificial material may be assembled and rolled, followed by peeling and removal of the sacrificial material after rolling. To facilitate removal of the sacrificial material after rolling, it is desirable to insert a release agent between the sacrificial material and the cladding material. Because adding sacrificial material increases the rolling cost, it is desirable for the thickness of the sacrificial material to be 3 mm or less after rolling. To minimize plate warpage during cooling, it is desirable that the two clad materials have the same thickness for both the base material and the clad material. As long as the clad material is on the surface side of the rolled material and the base material is on the inside, other conditions do not need to be limited to the assembly method described above.

[0086] 5-2.Hot rolling Next, the obtained rolled material is hot rolled. Here, the temperature calculated from the chemical composition of the cladding material using formula (2) is defined as the sigma phase precipitation start temperature Tσ. Rolling is performed so that the temperature T1 at the start of the final pass of hot rolling is Tσ-50 (°C) or higher. If T1 is less than Tσ-50°C, the sigma phase will precipitate in the cladding material during hot rolling and transportation to the cooling equipment, making it impossible to obtain good corrosion resistance. Preferably, T1 is Tσ°C or higher. The higher T1 is, the more the sigma phase precipitation is suppressed, so no upper limit is set. Tσ =4Cr+25Ni+71(Mo+W)-11.4(Mo-1.3) 2 +5Si-6Mn-30N+569(℃)... Formula (2) However, each element symbol in formula (2) indicates the content (mass%) of the corresponding element in the cladding material, and if the element is not contained, 0 is substituted.

[0087] The heating temperature, heating time, and hot rolling reduction ratio of the rolling material may be determined as appropriate, but preferred ranges are shown below from the viewpoint of properties other than corrosion resistance and workability, and manufacturability. The heating temperature is preferably 1050 to 1250°C. If the heating temperature is less than 1050°C, the hot workability deteriorates and the joining strength also deteriorates. Therefore, the heating temperature is preferably 1050°C or higher, and more preferably 1100°C or higher. On the other hand, if the heating temperature exceeds 1250°C, the steel billet may deform in the heating furnace or defects may easily occur during hot rolling. Therefore, the heating temperature is preferably 1250°C or lower, and more preferably 1220°C or lower. The heating time should be 30 minutes or more to ensure uniform temperature throughout the thickness of the plate. The reduction ratio, calculated as material thickness / product thickness, is preferably 3 or more and 15 or less. If the reduction ratio is less than 3, the interface bonding by rolling may be insufficient, resulting in low shear strength at the interface. A reduction ratio of 5 or more is more preferable. If the reduction ratio exceeds 15, the rolling time becomes long, the finishing temperature becomes too low, and the rolling cost increases. A reduction ratio of 10 or less is more preferable.

[0088] 5-3. Cooling after rolling After rolling, the steel sheet undergoes accelerated cooling. The temperature T2 at the start of accelerated cooling is set to Tσ-100°C or higher. Accelerated cooling is performed so that the surface temperature T3 of the rolled steel sheet reheated after the end of accelerated cooling is 650°C or lower. If the cooling start temperature T2 is lower than Tσ-100°C, sigma phases will precipitate in the cladding material located on the surface of the rolled steel sheet, and good corrosion resistance will not be obtained if heat treatment is not performed after hot rolling. The preferred cooling start temperature is Tσ-50°C or higher. The higher the temperature, the more the sigma phase precipitation is suppressed, so no upper limit is set. Furthermore, if the surface temperature of the sheet reheated after cooling exceeds 650°C, sigma phases will precipitate in the reheated cladding material, and good corrosion resistance will not be obtained.

[0089] The temperature at which the austenite-to-ferrite transformation begins in the base material is thermodynamically indicated as point A3 and varies depending on the steel's composition. However, the austenite-to-ferrite transformation is also a kinetic phenomenon, and at the cooling rate during manufacturing, it begins at a temperature approximately 100 to 200°C supercooled from point A3. The Tσ-100°C of the general-purpose duplex stainless steel and super duplex stainless steel used as cladding materials in this invention is sufficiently higher than the temperature at which the austenite-to-ferrite transformation begins in practice. Furthermore, although the temperature at which the austenite-to-ferrite transformation ends in the base material is lower than 650°C, the cooling rate at the center of the plate thickness during air-cooling after accelerated cooling is sufficiently slower than the cooling rate at which bainite or martensite forms.

[0090] As mentioned above, there is a temperature difference of several degrees Celsius to several tens of degrees Celsius between the surface and center of the thickness of a rolled steel sheet immediately before the start of accelerated cooling and at the end of recuperation, depending on the thickness. However, the temperature difference between the surface and center of the thickness is considered to be roughly the same at both times. Furthermore, the temperature history of a rolled steel sheet at the center of the thickness does not follow a "rapid cooling-recuperation" cycle, but follows a monotonically decreasing temperature history from the start of accelerated cooling to the end of recuperation. Therefore, the cooling rate at the center of the thickness can be approximately calculated from the cooling rate calculated from the surface temperatures at the start of accelerated cooling and the end of recuperation. Therefore, by starting accelerated cooling at Tσ-50°C or higher, setting the recuperation temperature after the end of accelerated cooling to 650°C or lower, and setting the average cooling rate between the start of accelerated cooling and the end of recuperation to 25°C / s or lower, the average cooling rate CR in the same temperature range at the center of the thickness can be kept to 25°C / s or lower. By setting the average cooling rate in the temperature range where austenite to ferrite transformation occurs in the base material to 25°C / s or less, the precipitation of bainite and martensite in the base material surface layer can be suppressed, and good workability can be obtained. If the average cooling rate at the center of the plate thickness exceeds 25°C / s, hard bainite and martensite, rather than soft ferrite, will form near the base material surface layer 1 mm from the base material surface in the plate thickness direction, making it impossible to obtain good workability. The rate is preferably 15°C / s or less.

[0091] Although the method of accelerated cooling that results in an average cooling rate CR at the center of the plate thickness of 25°C / s or less is not specified, examples include water cooling and forced air cooling.

[0092] The method for evaluating the average cooling rate CR at the center of the plate thickness will be explained below. The surface temperature of the plate measured before accelerated cooling is defined as the cooling start temperature T2 (°C). The surface temperature of the plate measured when the plate is reheated after cooling and the temperature rise on the surface stops is defined as the cooling stop temperature T3 (°C). Since the temperature difference between the surface and the center of the plate thickness is considered to be roughly the same immediately before the start of accelerated cooling and when reheating is complete, the difference between the cooling start temperature T2 (°C) and the cooling stop temperature T3 (°C) divided by the difference in the time (seconds) at which the temperatures were measured is defined as the average cooling rate CR at the center of the plate thickness.

[0093] 5-4.Heat treatment after rolling According to the present invention, it is possible to obtain a duplex stainless clad steel sheet that has excellent corrosion resistance of the clad material and excellent workability of the base material without heat treatment. As mentioned above, heat treatment of the clad steel sheet increases costs, but heat treatment can be performed as needed to improve other properties. If a sigma phase precipitates in the clad material during heat treatment, corrosion resistance will decrease, so the heat treatment temperature is 800°C or less, preferably 650°C or less.

[0094] According to the present invention, it is possible to obtain a clad steel plate having excellent corrosion resistance of the cladding material and excellent workability of the base material. The clad steel plate according to the present invention does not require additional heat treatment or the like to improve corrosion resistance or workability. Furthermore, since the clad steel plate has high workability and can be processed into complex shapes, there are no limitations on its use and it can be applied to structural members where solid steel plates have traditionally been used. Therefore, the clad steel plate significantly contributes to cost reduction. [Example]

[0095] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0096] A duplex stainless steel clad material with the chemical composition shown in Table 1 and a base material with the chemical composition shown in Table 2 were melted and formed into billets. These were then hot-rolled, annealed, and pickled to produce 30 mm-thick clad and 130 mm-thick base steel plates. The resulting clad and base materials were laminated together to create a vacuum at the bonded surfaces, and the four edges of the bonded surfaces were sealed by welding to create a clad material. Two clad materials were stacked in the following order: clad material-base material-release agent-base material-clad material, with release agent applied between the base materials. This was then assembled into a rolled material. The resulting rolled material was hot-rolled to a total thickness of 32 mm under the hot-rolling conditions shown in Tables 3 and 4, then transported to a water-cooling facility and water-cooled using accelerated cooling. The surface temperature of the plate measured before water-cooling was designated the cooling start temperature (T2), and the surface temperature measured after water-cooling, when the temperature rise on the plate surface stopped, was designated the cooling stop temperature (T3). The difference between the cooling start temperature T2 (°C) and the cooling end temperature T3 (°C) divided by the difference in the time (seconds) at which each temperature was measured was taken as the average cooling rate CR at the center of the plate thickness. The obtained rolled steel plate was peeled off at the release agent portion to produce a clad steel plate with a thickness of 16 mm. The PREN_Mn value calculated from the chemical composition of the cladding material using equation (3) above is shown in the "PREN_Mn" column of Table 1. In Table 2, Tσ represents the value calculated from equation (2) using the chemical composition of the cladding material. Tσ =4Cr+25Ni+71(Mo+W)-11.4(Mo-1.3) 2 +5Si-6Mn-30N+569(℃)... Formula (2) However, each element symbol in formula (2) indicates the content (mass%) of the corresponding element in the cladding material, and 0 is substituted if the element is not contained.

[0097] [Table 1]

[0098] [Table 2]

[0099] In rolling clad steel plates, the materials listed in Tables 3 and 4 were used as the base metal No. and cladding material No., and the manufacturing conditions listed in Tables 3 and 4 were varied to investigate the various property values. The manufacturing conditions in Tables 3 and 4 are explained below. In Tables 3 and 4, "T1" indicates the start temperature (°C) of the final rolling pass. "T2" indicates the temperature at which water cooling began. "T3" indicates the temperature at which reheating was completed after water cooling was stopped. "CR" indicates the average cooling rate (°C / s) at the center of the plate thickness.

[0100] The evaluation results shown in Tables 3 and 4 will be explained below. The "α ratio" column in Tables 3 and 4 indicates the ferrite fraction (%) of the base material surface. The ferrite fraction was evaluated using the KAM value, as described below. To measure the KAM value, a cross section perpendicular to the rolling direction of the sample was polished with colloidal silica. EBSD measurements were performed three times on the base material surface (1 mm from the base material surface in the depth direction) at a magnification of 500x, an area of ​​177 μm × 519 μm (the 177 μm side is the thickness direction, and the center of the area is 1 mm from the base material surface), and a measurement step of 1 μm. From the obtained data, the area fraction (%) with a KAM value of 1° or less was calculated, and the average was taken as the ferrite fraction of the base material surface. Note that these measurement conditions are merely an example and may be modified as appropriate depending on the metal structure of the sample, particularly the grain size.

[0101] Kernel Average Misorientation (KAM) is calculated for each pixel of measurement data by taking the average misorientation between the six neighboring pixels as the KAM value for that pixel. By performing this calculation without crossing the grain boundaries, it is possible to obtain a distribution map of strain based on local orientation changes within the grains. Ferrite, which forms at high temperatures, is primarily a diffusion transformation, and therefore has the characteristic of having smaller transformation strain than bainite or martensite. Based on this characteristic, a comparison with the structure observed after etching determined that a KAM value of 1° or less was ferrite, and the ferrite area fraction measured by EBSD was defined as the ferrite phase fraction in the base material surface.

[0102] The corrosion resistance was evaluated by the ferric chloride CPT, which was measured by a ferric chloride corrosion test in accordance with ASTM G 48 E method. The ferric chloride CPT was evaluated by evaluating the ferric chloride CPT of the clad material part of a two-phase stainless clad steel plate product (product CPT) and the ferric chloride CPT of a sample where the clad material part of the product had been solution heat treated (solution CPT).The corrosion resistance of the clad material was judged based on the difference between the solution CPT and the product CPT (CPT difference). We prepared two types of duplex stainless clad steel plates: the product duplex stainless clad steel plate and the solution-treated steel plate, which was solution-treated at 1050°C for 10 minutes. Measurements were performed on four test pieces, each measuring 2 mm thick, 25 mm long, and 50 mm wide, with the evaluation surfaces located 0.5 mm and 2.5 mm from the surface in the thickness direction. While the specimen sampling location was not specified, it was preferable to sample the specimen from the interior, at least 100 mm from the edge of the rolled material in terms of width and length, to avoid transient regions. More preferably, the specimen was sampled from the interior, at least 300 mm from the edge. The lowest pitting initiation temperature (°C) measured for each specimen was defined as the pitting initiation temperature. The pitting initiation temperature of the unsolution-treated product was defined as the "product CPT," and the pitting initiation temperature of the solution-treated sample was defined as the "solution-treated CPT." The "product CPT" was then subtracted from the measured "solution-treated CPT" to obtain the "CPT difference." The results are shown in the "CPT difference" column in Tables 3 and 4. ○ indicates a CPT difference of 10°C or less, × indicates a CPT difference of more than 10°C.

[0103] To evaluate workability, a bending test was conducted in accordance with JIS Z 2248. No. 1 test pieces with a total thickness of 16 mm were prepared from the clad steel plate. The bending radius was set to 16 mm, the same as the plate thickness, and the piece was bent 180 degrees with the base material on the outside and the clad material on the inside. The test pieces were then removed from the testing machine and the outside of the bent part (the base material side) was visually inspected. Those without cracks or other defects were rated as good (◯), and those with any defects were rated as poor (×). The "Workability" column in Tables 3 and 4 shows the results of the bending test, with ◯ indicating good workability and × indicating poor workability.

[0104] The manufacturing conditions and the above results are summarized in Tables 3 and 4. Items that fall outside the scope of the clad steel plate of the present invention and items that fall outside the scope of the preferred manufacturing method of the present invention are underlined.

[0105] [Table 3]

[0106] [Table 4]

[0107] Samples 1 to 47 are examples of the present invention, satisfying the preferred manufacturing conditions, with good corrosion resistance of the clad material and good workability due to the ferrite fraction of the base material surface layer exceeding 15%. Samples 48 to 55 are comparative examples, not satisfying the preferred manufacturing conditions, with poor corrosion resistance of the clad material or poor workability due to the ferrite fraction of the base material surface layer being 15% or less.

[0108] As described above, the clad steel sheets obtained in the examples of the present invention had excellent corrosion resistance of the clad material and workability of the base material. On the other hand, the comparative examples did not satisfy the preferred manufacturing conditions, and the corrosion resistance of the clad material was poor or the ferrite fraction of the surface layer was outside the range specified in the present invention, resulting in poor workability of the base material. [Industrial Applicability]

[0109] According to the present invention, a duplex stainless clad steel plate having excellent corrosion resistance of the clad material and excellent workability of the base material can be obtained at low cost, and is extremely useful industrially. The clad steel plate of the present invention may be applicable to corrosive environments such as high chloride environments such as seawater exposure, and corrosive environments such as those in plant facilities exposed to acid solutions such as phosphoric acid or sulfuric acid. Specific examples include seawater desalination plants, flue gas desulfurization equipment, chemical storage tanks, structural components such as oil well tubular goods, pumps and valves, and heat exchangers.

Claims

1. A clad steel plate comprising a base material and a clad material joined to the base material, the base material is made of carbon steel or low alloy steel, The base metal has a chemical composition containing, in mass%, C: 0.020 to 0.200%, Si: 1.00% or less, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.050% or less, Nb: 0.200% or less, and N: 0.020% or less, with Ceq being 0.20 to 0.50, and the balance being Fe and impurities; the cladding material is made of duplex stainless steel, The chemical composition of the cladding material is, in mass %, C: 0.10% or less, Si: 2.00% or less, Mn: 0.50 to 6.00%, P: 0.050% or less, S: 0.050% or less, Ni: 0.10 to 8.00%, Cr: 17.0 to 30.0%, N: 0.05 to 0.30%, Mo: 1.00 to 5.00%, Cu: 0 to 2.0%, Nb: 0 to 0.10%, S n: 0 to 1.00%, W: 0 to 1.00%, V: 0 to 1.00%, Ti: 0 to 0.05%, B: 0 to 0.0050%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Al: 0 to 0.05%, REM: 0 to 0.50%, the balance being Fe and unavoidable impurities, and the PREN_Mn value calculated by the following formula (3) is 25 or more, the difference between the ferric chloride CPT of the cladding material of the solution-treated sample and the ferric chloride CPT of the cladding material is 10°C or less, The clad steel plate is characterized in that the ferrite phase ratio of the surface layer of the base material is more than 15%. Here, the surface layer of the base material refers to a position 1 mm from the surface of the base material in the thickness direction. Furthermore, ferric chloride CPT means the pitting corrosion temperature (°C) evaluated in accordance with the ASTM G48E method. Here, Ceq is defined by the following equation (1). Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5...(1) In the formula, C, Mn, Cu, Ni, Cr, Mo, and V are the contents (mass%) of each element in the chemical composition of the base steel sheet, and 0 is substituted if the element is not contained. PREN_Mn value=Cr+3.3(Mo+0.5W)+16N−Mn (3) However, the element symbols in the above formula (3) represent the content (mass %) of each element contained in the cladding material, and 0 is substituted when no element is contained.

2. The chemical composition of the base metal further contains, in mass %, Ni: 0.01 to 3.00%, Cr: 0.01 to 1.00%, Mo: 0.01 to 0.50%, W: 0.01 to 1.00%, Cu: 0.01 to 2.00%, Co: 0.01 to 0.50%, Se+Te: 0.01 to 0.10%, Al: 0.005 to 0.300%, V: 0.001 to 0.50%, and 2. The clad steel plate according to claim 1, containing one or more selected from the group consisting of Zr, Hf, Ta, and Cu, Si, Al, Cu, Fe, Ni, Fe+Zn, Ni, Fe+Zn, Cu ...

3. 3. The method for manufacturing a clad steel plate according to claim 1 or 2, wherein the base material and the clad material are stacked so that the pressure-bonding surface is vacuum, and the four peripheries of the pressure-bonding surface are sealed by welding to form a clad material, and two of the clad materials are stacked so that the clad material is positioned on the outside, and the four peripheries of the interface between the base materials are welded together to form a rolled material, the temperature calculated from the chemical composition of the clad material using formula (2) is the sigma phase precipitation start temperature Tσ, and rolling is performed so that the temperature at the start of the final pass is Tσ-50 (°C) or higher, and accelerated cooling is performed from Tσ-100 °C or higher after rolling, and the surface temperature of the plate reheated after cooling is 650 °C or lower, and the average cooling rate at the center of the plate thickness from the start of accelerated cooling to the end of reheating is 25 °C / s or lower. Tσ = 4Cr + 25Ni + 71(Mo + W) - 11.4(Mo - 1.3) ２ + 5Si - 6Mn - 30N + 569(°C) ··· Formula (2) Each element symbol in formula (2) indicates the content (mass%) of the corresponding element in the cladding material, and 0 is substituted when the element is not contained.

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