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 contains large amounts of Cr, Mo, Ni, and N, and is prone to the precipitation of intermetallic compounds called sigma phases and chromium nitrides. The temperature range in which these precipitate is approximately 950°C to 650°C, although this varies slightly depending on the composition. When sigma phases precipitate in duplex stainless steel, a chromium-depleted layer forms around them, reducing the corrosion resistance of the steel. Similarly, when chromium nitrides precipitate, a chromium-depleted layer forms around them, reducing the corrosion resistance of the steel. For this reason, standard duplex stainless steel solid sheets are manufactured by performing solution heat treatment at 1000°C or higher after rolling to dissolve the precipitates.

[0011] However, in clad steel plates, the base material and cladding material have different compositions and crystal structures, so their thermal expansion coefficients are significantly different. Therefore, when solution heat treatment is performed at temperatures above 1000°C, the difference in thermal expansion coefficients causes the plate to deform significantly, and correcting the flatness is very costly. Therefore, when manufacturing clad steel plates, heat treatment after rolling is often omitted.

[0012] Patent Document 1 discloses a technique for producing a duplex stainless clad steel sheet that has good corrosion resistance even without heat treatment by controlling hot rolling conditions. In Patent Document 1, after rolling at 900°C or higher, accelerated cooling is performed 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 and achieving good corrosion resistance. In addition to the above patent document, there are several patent documents that describe techniques for achieving good corrosion resistance by accelerating cooling within the sigma phase and chromium nitride precipitation temperature range using methods such as water cooling. However, when water cooling is performed to ensure the corrosion resistance of clad materials, the proportion of bainite and martensite in the base material structure increases, making it more likely to deteriorate workability. In particular, the cooling rate is extremely high in the surface layer of the base material, which is directly exposed to the cooling water, and the proportion of bainite and martensite increases compared to the interior of the base material, resulting in poor workability. Tempering to soften bainite and martensite is difficult because of the plate bending caused by the difference in expansion coefficients mentioned above.

[0013] 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.

[0014] Patent Document 3 discloses a technology for controlling the ferrite structure of the surface layer of solid steel plates made of ordinary steel by utilizing high-pressure water descaling. However, this patent document relates to solid steel plates and does not describe the effect of high-pressure water descaling on the clad material structure of clad steel plates. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Patent No. 6477735 [Patent Document 2] Patent No. 6573060 [Patent Document 3] Patent No. 3572756 Summary of the Invention [Problem to be solved by the invention]

[0016] As a result of extensive research, the present inventors have found the following problems to be solved. Patent Document 1 discloses a technology that achieves good corrosion resistance of clad materials even when heat treatment after rolling is omitted by accelerated cooling. Patent Document 2 discloses a technology that improves the workability of a base material by softening bainite or martensite in the surface layer of the base material by tempering. Patent Document 3 discloses a technology that controls the structure of the base material using a solid ordinary steel material. However, there is no disclosure or suggestion of a technology that achieves both the corrosion resistance of clad materials and the workability of the base material even when heat treatment after rolling is omitted in duplex stainless clad steel sheets.

[0017] 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 cladding material and excellent workability of the base material, and a method for manufacturing the same. [Means for solving the problem]

[0018] The inventors recognized that when manufacturing clad steel plates by heating, rolling, and cooling a material that is laminated in the order of base material, cladding material, cladding material, and base material, and then peeling it off in the center, good workability can be achieved by controlling the ferrite phase ratio to a certain value or more in the base material structure, particularly in the surface layer of the base material close to the surface of the base material that is directly hit by cooling water during water cooling. The inventors have also recognized that good corrosion resistance can be achieved by reducing the precipitation of sigma phase and chromium nitrides in the clad structure. From these results, the inventors have found that in order to achieve both the corrosion resistance of the cladding material and the workability of the base material even when heat treatment after rolling is omitted, the problem to be solved is to reduce the precipitates in the cladding material by optimizing the base material components and rolling conditions and to control the ferrite phase ratio in the surface layer of the base material.

[0019] 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] The clad steel plate according to [1], wherein the base material has a chemical composition 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 consisting of 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 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%, V: 0.001 to 0.100%, Ti: 0.00 The clad steel plate according to [2], containing one or more selected from the group consisting of Al: 0.1 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%.

[0020] [4] In the method for producing a clad steel plate according to any one of [1] to [3], the base material and the cladding material are laminated so that the bonding surface is vacuum, and the four circumferences of the bonding surface are sealed by welding to form a clad material, and the clad rolled material obtained by assembling two of the clad materials is subjected to a temperature calculation using the formula (2) T Nb A series of rolling passes starts at +70°C or less and 950°C or more, and for one or more passes including the final pass, the temperature of the surface of the rolling material during rolling is T Nbor less, and a total reduction in the temperature range is 5% or more, and after rolling, cooling is performed at an average cooling rate of 2°C / s or more for the entire thickness in the 900 to 650°C range. T Nb (℃)=815℃+720×√Nb...Equation (2) In the formula, Nb is the content (mass %) of Nb in the chemical composition of the base steel sheet. [Effects of the Invention]

[0021] 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

[0022] The present inventors have conducted the following investigations to address the above-mentioned problems. Specifically, they investigated the corrosion resistance of clad steel plates made of various duplex stainless steels by changing the cooling rate after rolling. As a result, they obtained the following findings (a) and (b).

[0023] (a) The less sigma phase and chromium nitride precipitation in the clad material, the better the corrosion resistance. Pitting corrosion initiation temperature evaluation can be used as a means of evaluating the corrosion resistance of the clad material portion of clad steel plate. 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 portion 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 portion 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.

[0024] (b) In order to suppress the precipitation of sigma phases and chromium nitrides and improve the corrosion resistance of clad materials, it is effective to perform accelerated cooling in which the clad material temperature at the end of rolling is 900°C or higher and the cooling rate in the temperature range of 900°C to 650°C after rolling is 2°C / s or higher.

[0025] An example of the investigation results that led to the findings (a) and (b) above will be described. As a specific example, a laminated material having the composition shown in E in Table 1 below was used. As a hot rolling method, the temperature at the end of rolling was set to 900°C or higher, and two average cooling rates were used for the entire thickness average in the temperature range of 900 to 650°C: 3.0°C / s (rapid cooling) and 1.0°C / s (slow cooling). Extraction residue analysis was performed to evaluate the amount of sigma phase and chromium nitride precipitation in the clad material after hot rolling. The extraction residue was electrolyzed using an electrolyte (10% acetylacetone-1% hydrochloric acid-methanol solution) and collected using a 0.2 μm pore polycarbonate and polyester filter. Then, ICP analysis was used to measure the mass percentage of the elements Fe, Cr, Mo, Nb, and V relative to the total extracted amount. Of the analytical values ​​for each element, Cr, which is present in both the sigma phase and chromium nitride and significantly affects the corrosion resistance of stainless steel, was used as an indicator of the amount of precipitation. Under slow cooling conditions, the Cr content in the residue was 0.08 mass%, while under rapid cooling conditions, the Cr content was 0.01 mass%, demonstrating that rapid cooling after hot rolling suppresses the precipitation of sigma phase and chromium nitride. Additionally, solution-treated samples were prepared by subjecting the clad materials manufactured under both the rapid cooling and slow cooling conditions to a 10-minute solution treatment at 1050°C. The samples without solution treatment are referred to as product samples. Each sample was evaluated using the ferric chloride CPT. The solution CPT was 40°C for both the rapid cooling and slow cooling conditions. Meanwhile, for the product samples without solution treatment, the product CPT for the rapid cooling condition was 40°C, resulting in a CPT difference of 0°C and equivalent corrosion resistance to the solution-treated samples. Meanwhile, the product CPT for the slow cooling condition was 25°C, resulting in a CPT difference of 15°C. It is clear that the adoption of rapid cooling conditions in hot rolling improves the corrosion resistance of clad materials, as evaluated by the ferric chloride CPT.

[0026] Furthermore, the present inventors conducted the following investigations to address the above-mentioned issues. Specifically, for clad steel plates using various types of ordinary steel or low-alloy steel as the base material, the metal structure of the surface layer of the base material was investigated by changing the composition of the base material, the surface layer temperature and reduction rate during rolling, and the cooling rate after rolling, and the relationship with workability was evaluated. As a result, the following findings (c) to (e) were obtained.

[0027] (c) The greater the ferrite fraction in the surface layer of the base material (1 mm from the surface of the base material in the thickness direction), the better the workability of the base material. Therefore, it is effective to make the ferrite fraction in the surface layer of the base material exceed 15%.

[0028] (d) To achieve good corrosion resistance in clad materials, it is important to maintain the clad material at a temperature of 900°C or higher at the end of rolling and to maintain an average cooling rate of 2°C / s or higher across the entire thickness in the temperature range of 900°C to 650°C, as described above in (b). However, increasing the cooling rate of the clad material simultaneously rapidly cools the surface of the base material, which is directly exposed to water and air on the opposite side of the clad material. This results in the formation of hard bainite and martensite near the surface layer of the base material, located 1 mm from the surface in the thickness direction. This can lead to, for example, fractures occurring in the hard, low-ductility surface areas during tensile tests, reducing ductility, and necking and cracking occurring from the surface of the base material during bending, thereby reducing workability. In particular, in base material chemical compositions that provide sufficient strength for structural applications, hard bainite and martensite are likely to form when the cooling rate is increased. The austenite to ferrite transformation is promoted by the residual strain accumulated in austenite when rolling at a temperature range where austenite recrystallization does not occur. Therefore, in order to suppress the formation of bainite and martensite even when rapid cooling is performed after rolling, it is effective to roll at a low temperature where recrystallization and recovery take time, and to leave residual strain when cooling after rolling. Specifically, as will be described in detail later, in the case of one or more passes of hot rolling including the final pass, the temperature of the surface of the rolled material during rolling is T NbBy ensuring that the total rolling reduction within this temperature range is 5% or more, the residual strain of the austenite phase in the surface layer of the base material increases, and even if rapid cooling is performed after rolling, the ferrite phase transformation in the surface layer of the base material is promoted. T Nb (℃)=815℃+720×√Nb...Equation (2)

[0029] (e)T Nb If a method of rolling by lowering the temperature of the entire plate by, for example, allowing it to cool during rolling is used as a means of achieving a total reduction of 5% or more within the temperature range below, the temperature of the clad material portion will drop at the same time as the surface, and particularly with thin plate, the temperature will drop during transport from the rolling mill to the cooling equipment, causing the clad material temperature to fall below 900°C when water cooling begins, which could prevent sufficient rapid cooling after rolling and reduce the corrosion resistance of the clad material. Therefore, in order to achieve both good corrosion resistance of the clad material and suppression of the formation of bainite and martensite in the surface layer of the base material, it is preferable to keep the temperature of the entire plate, including the clad material portion, high while rolling by lowering the temperature of the surface layer of the base material near the surface of the base material. To achieve a low temperature for recrystallization and recovery only in the surface layer of the base material, it is effective to place the base material surface on the surface of the rolled material and then utilize high-pressure water descaling, which is typically used to remove scale and prevent surface defects. Adjusting the water volume and rolling pass schedule during high-pressure water descaling can lower the temperature of only the surface layer of the base material, which is directly exposed to the water, thereby increasing residual strain. Furthermore, Nb is an element that raises the recrystallization temperature. Adding Nb to the base material increases the recrystallization temperature of the base material, thereby delaying recrystallization and recovery without significantly lowering the rolling temperature. In other words, adding Nb to the base material increases the recrystallization temperature of the base material to prevent the surface residual strain from being eliminated by the heat inside the plate during transport to the cooling equipment.

[0030] Therefore, in order to obtain a duplex stainless clad steel sheet having excellent corrosion resistance of the clad material and excellent workability of the base material, it is necessary to suppress the precipitation of sigma phases and chromium nitrides in the clad material and control the austenite-to-ferrite transformation in the base material surface layer by optimizing the base material composition, the temperature of the base material surface layer during rolling, 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.

[0031] 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%.

[0032] 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 the precipitation of sigma phases and chromium nitrides in the clad material and promote ferrite phase transformation in the surface layer of the base material.

[0033] 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.

[0034] 2-2. Ferrite phase ratio in the base material surface The ferrite phase ratio in the surface layer of the base material of a clad steel plate is set to be more than 15%. If it is 15% or less, 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. The interior of the base material is not directly exposed to cooling water and the cooling rate is slower than the surface layer of the base material, so the structure therein is not particularly specified, but like the surface layer of the base material, 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.

[0035] 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.

[0036] 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%.

[0037] 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%.

[0038] 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%.

[0039] 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.

[0040] 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.

[0041] Nb is an element that raises the recrystallization temperature, and its addition in an amount of more than 0.008% is preferable, and the addition of 0.010% or more is more preferable. However, if the Nb content exceeds 0.200%, weldability is impaired and the alloy cost increases. Therefore, the Nb content is set to 0.200% or less. The upper limit of Nb is preferably 0.100%. It is more preferably 0.010 to 0.050%, and even more preferably 0.030 to 0.050%.

[0042] N is an element that combines with Nb, V, Ti, etc. to precipitate nitrides or carbonitrides, and if the N content exceeds 0.010%, fine precipitates reduce workability and toughness. Therefore, the N content is set to 0.010% or less, 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.

[0043] 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 better the hardness and the worse the 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, and preferably 0.23 or more. If Ceq exceeds 0.50, weldability deteriorates, and interpass temperature control and post-heat treatment become necessary, increasing welding costs. Therefore, Ceq is set to 0.50 or less, and 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.

[0044] The chemical composition of the base material is further modified by replacing a portion of the Fe with, 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%, V: 0.001 to 0.100%, and 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%.

[0045] 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%.

[0046] 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%.

[0047] 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%.

[0048] 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%.

[0049] 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%.

[0050] 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%.

[0051] 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%.

[0052] 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%.

[0053] 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%.

[0054] 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%.

[0055] 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%.

[0056] 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%.

[0057] 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%.

[0058] 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 0.0100%, 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%.

[0059] 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%.

[0060] 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.

[0061] 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.

[0062] 4. Duplex stainless steel cladding material Next, we will explain duplex stainless steel cladding materials. Duplex stainless steel 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: 0 to 3.50%, Cu: 0 to 2.0%, N An example of a chemical composition is b: 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%, with the balance being Fe and unavoidable impurities, and the PREN_Mn value calculated by the following formula (i) is less than 45.0. This chemical composition is merely an example, and the present invention is not limited thereto. The reason for listing this chemical composition is as follows. In the following description, "%" in relation to the content means "% by mass." PREN_Mn value=Cr+3.3Mo+16N-Mn (i) However, the element symbols in the above formula (i) represent the content (mass %) of each element contained in the cladding material, and 0 is substituted when no element is contained.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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 chromium nitride precipitated in the weld metal and 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.

[0070] 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.

[0071] Mo: 0 to 3.50% Mo is an element that improves corrosion resistance, so it may be added as needed. However, adding 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 3.50% or less. To achieve the above effects, the Mo content is preferably 0.10% or more. Furthermore, the Mo content is preferably 2.50% or less, more preferably 1.00% or less, and even more preferably 0.60% or less.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] REM: 0 to 0.50% REM (rare earth elements) have the effect of improving hot workability and may be added as needed. However, adding a large amount of REM 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.

[0083] 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.

[0084] The balance of the chemical composition of the steel sheet of the present invention is Fe and impurities. Here, "impurities" refers 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.

[0085] PREN_Mn value: less than 45.0 The PREN_Mn value is a general index showing the pitting corrosion resistance of a stainless steel sheet, and is calculated from the chemical composition of the stainless steel using the following formula (i). PREN_Mn value=Cr+3.3Mo+16N-Mn (i) However, the element symbols in the above formula (i) represent the content (mass%) of each element contained in the steel, and 0 is substituted when the element is not contained. An increase in the PREN_Mn value may cause problems such as increased alloy cost due to the increased Cr and Mo contents and reduced corrosion resistance due to the precipitation of sigma phases. Furthermore, the generation of nitrogen bubbles due to the increased N content and decreased Mn content becomes a problem. Therefore, the PREN_Mn value is set to less than 45.0. A PREN_Mn value of less than 35.0 is preferred. While there is no particular need to specify a lower limit, to obtain corrosion resistance equivalent to that of SUS304, a PREN_Mn value of 18.0 or higher is preferred, and a PREN_Mn value of 20.0 or higher is even more preferred.

[0086] 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 above clad steel plate, the base material and cladding material are stacked so that the bonding surface is vacuum, and the four peripheries of the bonding surface are sealed by welding to form the clad material. Two clad materials are assembled so that the base material surface of each clad material becomes the surface of the clad rolled material to form the clad rolled material. For the assembled clad rolled material, one or more passes of rolling, including the final pass, raise the temperature of the rolled material surface to T, calculated by equation (2). NbA series of rolling passes is carried out so that the total reduction is 5% or more within the following temperature range. To control the temperature of the surface layer, the surface layer may be cooled by descaling or the like immediately before any rolling pass. After rolling, cooling is carried out at an average cooling rate of 2°C / s or more across the entire thickness in the 900 to 650°C range to produce a clad steel plate. T Nb (℃)=815℃+720×√Nb...Equation (2) In the formula, Nb is the content (mass %) of Nb in the chemical composition of the base steel sheet.

[0087] 5-1. Clad materials 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 limited, 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 clad material obtained is made by stacking two clad materials so that the base metal surface of each clad material faces the surface of the clad rolled material, applying a release agent between them, and assembling them to form the clad rolled material. To minimize plate warping during cooling, it is desirable that the base materials and clad materials of the two clad materials are of equal thickness. Of course, the assembly method does not have to be limited to the one described above.

[0088] 5-2.Hot rolling Next, the obtained clad rolling material is subjected to hot rolling. Here, a "series of rolling passes" in hot rolling will be explained. The first pass of the series of rolling passes is the pass when the surface temperature of the rolling material before rolling is T Nb ℃~T Nb The surface of the rolled material is cooled by descaling from +70°C and rolled, and the surface temperature is T Nb ℃ or more. Nb The earlier of the passes in which rolling was performed at 50°C or lower is considered the reduction rate. A series of rolling passes from the first pass to the last pass is performed so that the total reduction rate is 5% or more. A series of rolling passes may be only one pass, and the first pass and the last pass may be the same. To control the temperature of the surface layer of the base material, the surface of the rolled material may be cooled by descaling or the like immediately before any rolling pass. After rolling, cooling is performed at an average cooling rate of 2°C / s or more across the entire thickness in the 900 to 650°C range. T Nb (℃)= 815℃ + 720×√Nb ··· Formula (2) In the formula, Nb is the content (mass%) of the element in the composition of the base steel sheet. Nb is an empirically derived formula for the effect of Nb content on the recrystallization temperature of steel.

[0089] The first pass of a series of rolling passes is when the surface temperature of the rolled material before rolling is T Nb ℃~T Nb The surface of the rolled material is cooled by descaling from +70°C and rolled, and the surface temperature reaches T Nb ℃ or more. Nb In this case, the temperature of the surface of the rolled material at the first pass of a series of rolling passes is T NbThe temperature is as follows. The thermometer of the rolling mill is usually installed before the descaling equipment, and since water gets on the steel plate when descaling is performed, it is difficult to measure the surface temperature during actual rolling after descaling. However, if the surface is cooled with more than the amount of descaling water described below, the surface temperature will drop by more than 70°C, so T Nb It can be considered that rolling was carried out at a temperature below T Nb ℃~T Nb Even if the surface of the rolled material is cooled from +70°C by descaling, the surface temperature will rise to T Nb If the rolling temperature at the time of descaling can be measured by some technology or simulation, the temperature shall not be considered as the first pass of the series of rolling passes. Nb The pass at which the rolling temperature reaches or exceeds 100°C may be set as the starting pass of the series of rolling passes. A series of rolling passes Nb If the temperature is above +70°C, even if descaling is performed, the surface temperature will rise to T due to uneven cooling or heat recovery from inside the plate. Nb There may be some areas where it is not possible to reduce the temperature below T Nb +50°C or less. A series of rolling passes is preferably started when the surface temperature of the rolling material is 950°C or higher. Since the clad material is located at the center of the plate thickness, if the surface temperature of the rolling material is 950°C or higher at the start of a series of rolling passes, the clad material temperature will be higher, and with a normal rolling time and amount of descaling water, the clad material temperature at the end of rolling will not fall below 900°C. If a series of rolling passes is started at a temperature below 950°C, the clad material temperature will decrease during hot rolling and transport to the cooling equipment, making it difficult to sufficiently cool in the 900 to 650°C range described below, and sigma phases and chromium nitrides will precipitate, reducing corrosion resistance. A temperature of 970°C or higher is more preferable.

[0090] The total reduction rate of a series of rolling passes is 5% or more. Nb Rolling at temperatures above T causes recrystallization of the austenite phase during hot rolling and transportation to the cooling equipment, resulting in a small residual strain that does not contribute to the promotion of the subsequent ferrite phase transformation.Nb The total reduction ratio of a series of rolling passes performed within the following temperature range is specified: T Nb If the total reduction rate is less than 5% in the following temperature range, the residual strain in the austenite phase becomes small, and the subsequent ferrite phase transformation cannot be promoted. The lower the rolling temperature, the greater the residual strain, which promotes the ferrite phase transformation. Nb There is no lower limit for the rolling temperature in the following temperature range, but from the viewpoint of improving strength and toughness by refining the structure, A calculated by formula (3) r3 It is preferable that the value is equal to or greater than T Nb The higher the total reduction rate within the following temperature range, the greater the residual strain and the more accelerated the ferrite phase transformation, so no upper limit is set. However, from the viewpoint of the rolling time and the increase in defects due to low-temperature rolling, it is preferable that the total reduction rate be 15% or less. T Nb (℃)= 815℃ + 720×√Nb ··· Formula (2) A r3 (℃)==910℃-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo...Formula (3) In the formula, Nb, C, Mn, Cu, Cr, Ni, and Mo are the contents (mass%) of each element in the chemical composition of the base steel sheet.

[0091] For the above series of rolling passes, the temperature of the rolling material surface during rolling is T NbWhen rolling is performed so that the total reduction within this temperature range is 5% or more, the surface of the rolled material may be cooled by high-pressure water descaling or the like immediately before any rolling pass in order to control the temperature of the rolled material surface and enable cooling from 900°C or higher after the aforementioned rolling. Cooling only the surface of the rolled material by high-pressure water descaling increases the residual strain in the surface layer of the base material located on the surface of the rolled material, promoting the transformation of the ferrite phase, while maintaining the temperature inside the rolled material where the cladding material is located, thereby suppressing the precipitation of sigma phase and chromium nitrides during cooling after rolling. If the base material has a sufficiently high Nb content, it is possible to achieve both the ferrite fraction in the surface layer of the base material and the corrosion resistance of the cladding material without descaling. However, descaling is recommended because it effectively increases the ferrite fraction in the surface layer of the base material. When cooling the surface of the rolled material by high-pressure water descaling, a 0.05 m 3 / m 2 The water flow rate is preferably 0.1 m / min or more. 3 / m 2 / min. Not all of the water that hits the plate surface evaporates, but some floats on the plate and some flows down. There is no upper limit to the flow rate, but the water that directly contacts the surface is what mainly contributes to cooling the surface, and increasing the amount of water only increases costs and the cooling effect becomes saturated. There is also a concern that the temperature of the entire plate will drop during transport due to the temperature drop caused by the water on the plate, which could reduce the corrosion resistance of the cladding. For this reason, the water flow density is preferably 1.0 m 3 / m 2 / min or less. 3 / m 2 / min) is the amount of water that actually hits the plate surface. For example, the total flow rate (m 3 / min) ÷ area of ​​the area exposed to water (m 2 ) The effect of the plate threading speed can be ignored within the range of plate threading speeds used in practical rolling. Naturally, there is no impediment to carrying out high-pressure water descaling for the purpose of descaling, which is its original effect. T Nb(℃)=815℃+720×√Nb...Equation (2)

[0092] The heating temperature, heating time and 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.

[0093] 5-3. Cooling after rolling After rolling, the average cooling rate of the steel sheet averaged over the entire thickness in the 900 to 650°C range is desirably 2°C / s or more. At a cooling rate of less than 2°C / s, sigma phases and chromium nitrides precipitate in the cladding material, resulting in reduced corrosion resistance. It is preferably 4°C / s or more. If the cooling rate is too fast, bainite and martensite become dominant in the surface layer structure of the base material, resulting in reduced workability. Therefore, the cooling rate is preferably 30°C / s or less. More preferably, it is 10°C / s or less. From the viewpoint of the corrosion resistance of the cladding material, the cooling end temperature is preferably 650°C or less, and even more preferably 600°C or less. Furthermore, from the viewpoint of the workability of the base material, the cooling end temperature is preferably 350°C or more. Since it is necessary to specify the average cooling rate of the steel sheet averaged over the entire thickness in the 900 to 650°C range after rolling, the average temperature of the steel sheet averaged over the entire thickness at the end of rolling is 900°C or more. The steel sheet surface temperature can be measured as the steel sheet temperature during and after rolling. The average temperature of the steel plate throughout its thickness during cooling after rolling can be determined by heat transfer calculations based on the measured surface temperature of the steel plate. The average cooling rate of the steel plate throughout its thickness in the 900-650°C range can be calculated based on the time difference between when the average temperature of the steel plate throughout its thickness reaches 900°C and when it reaches 650°C.

[0094] 5-4.Heat treatment after rolling According to the present invention, a duplex stainless clad steel sheet can be obtained that has excellent corrosion resistance of the clad material and excellent workability of the base material, even without heat treatment after rolling. 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 sigma phases or chromium nitrides precipitate 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.

[0095] 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]

[0096] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0097] A clad material made of duplex stainless steel 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 clad steel plates with a thickness of 30 mm and base steel plates with a thickness of 130 mm. Using the resulting clad material and base steel, the base steel and clad material were laminated so that the bonding surface was vacuum-sealed, and the four peripheries of the bonding surface were sealed by welding to produce a clad material. Two clad materials were stacked in the order of base steel-clad material-release agent-clad material-base steel, with a release agent applied between them, and assembled as a clad rolled material. The obtained clad rolled material was hot-rolled under the hot-rolling conditions shown in Table 3, and then peeled at the release agent portion to produce a 16 mm-thick clad steel plate. The PREN_Mn value calculated from the chemical composition of the clad material using the above formula (i) is listed in the "PREN" column of Table 1. In Table 2, T Nb indicates the value calculated from the chemical composition of the base material using equation (2). T Nb (℃)=815℃+720×√Nb...Equation (2) In the formula, Nb is the content (mass %) of Nb in the chemical composition of the base steel sheet.

[0098] [Table 1]

[0099] [Table 2]

[0100] In rolling clad steel plates, the materials listed in Table 3 as base material No. and cladding material No. were used, and the manufacturing conditions listed in Table 3 were changed to examine the various characteristic values. The manufacturing conditions in Table 3 are explained below. In Table 3, "T1" indicates the temperature (°C) of the surface of the rolled material at the start of a series of rolling passes. "Descaling" indicates the number of times high-pressure water descaling was performed. In the present invention examples, T1 was T Nb ℃~T Nb The surface of the rolled material is cooled by descaling from +70°C and rolled, and the surface temperature is T Nb Passes that did not exceed ℃ or T without descaling Nb In this case, the temperature of the surface of the rolling material at the first pass of a series of rolling passes is T Nb "r" indicates the total reduction rate (%) in a series of rolling passes. "CR" indicates the average cooling rate (°C / s) for the entire thickness in the 900-650°C range after rolling.

[0101] The evaluation results shown in Table 3 will be explained. The "α ratio" column in Table 3 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.

[0102] 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.

[0103] 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 of Table 3. ○ indicates a CPT difference of 10°C or less, × indicates a CPT difference of more than 10°C.

[0104] The following test was conducted to evaluate workability. The clad steel plate was machined to remove the clad material portion, and a JIS No. 1A tensile test piece was taken from the base steel plate portion. A tensile test was conducted in accordance with JIS Z 2241 to determine the total elongation. A total elongation of 16.0% or more was evaluated as good (○), and a total elongation of less than 16.0% was evaluated as poor (×). The "Workability" column in Table 3 shows the results of the tensile test, with ○ indicating good workability and × indicating poor workability.

[0105] The manufacturing conditions and the above results are summarized in Table 3. 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.

[0106] [Table 3]

[0107] Samples 1 to 47 are examples of the present invention, satisfy the preferred manufacturing conditions, have good corrosion resistance of cladding materials, and have good workability with a ferrite fraction of more than 15% in the surface layer of the base material.

[0108] Samples 48 to 55 are comparative examples that do not satisfy the preferred manufacturing conditions, and the corrosion resistance of the cladding material is poor or the ferrite fraction in the surface layer of the base material is 15% or less, resulting in poor workability. Comparative Examples Nos. 45 and 51 are examples that deviate from the rolling conditions of the present invention, and the temperature T1 at the start of the rolling pass was intentionally set to T Nb Temperatures above +70°C, T Nb When the total reduction rate r was 5% or more in the temperature range including the temperature that did not fall below 15%, the ferrite fraction of the base material surface was 15% or less. Nb If the temperature exceeds +70°C, the temperature of the surface of the rolled material during rolling will increase to T due to uneven cooling during descaling and heat recovery from inside the plate. NbSince there were passes in which the ferrite fraction was equal to or greater than 15%, it is believed that the ferrite fraction in the surface layer of the base material cannot exceed 15% even if the total rolling reduction r in this temperature range is 5% or greater. Nb The total rolling reduction r in the following temperature range was less than 5%, and the ferrite phase fraction in the surface layer of the base material was 15% or less. In Comparative Examples 46, 48, 50, and 52, the average cooling rate CR averaged over the entire thickness in the 900 to 650°C range after rolling was less than 2°C / s, and the corrosion resistance of the clad material was poor.

[0109] 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]

[0110] 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 cladding material has a chemical composition, in mass %, of 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: 0 to 3.50%, Cu: 0 to 2.0%, Nb: 0 to 0.10%, Sn: 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%, and the balance being Fe and unavoidable impurities, and the PREN_Mn value calculated by the following formula (i) is less than 45.0, 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 plate. PREN_Mn value=Cr+3.3Mo+16N-Mn...(i) However, the element symbols in the above formula (i) 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%, V: 0.001 to 0.100%, A 2. The clad steel plate according to claim 1, containing one or more selected from the group consisting of Zr, Hf, Ta, Zr+Hf+Ta, Hf+Ta+Ta+Zr+Hf+Ta ...

3. In the method for manufacturing a clad steel plate according to claim 1 or 2, the base material and the cladding material are laminated so that the bonding surface is vacuum, and the four circumferences of the bonding surface are sealed by welding to form a clad material, and for the clad rolled material obtained by assembling two of the clad materials, the temperature of the surface of the rolled material during rolling is T Ｎｂ or less, and the total reduction in the temperature range is 5% or more, and after rolling, cooling is performed at an average cooling rate of 2°C / s or more for the entire thickness in the 900 to 650°C range. T Ｎｂ (℃) = 815℃ + 720 × √Nb ・・・Equation (2) In the formula, Nb is the content (mass%) of Nb in the chemical composition of the base steel plate.

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