Composite steel plate and its manufacturing method

Abstract

The present invention provides a composite steel plate comprising a base layer and a coating layer on the base layer, the composite steel plate having a thickness of 60 to 120 mm, the coating layer being made of stainless steel, the base layer being made of carbon steel, and the bond strength between the base layer and the coating layer being 500 MPa or more. The composite steel plate of the present invention exhibits excellent tensile properties, low-temperature impact properties, and interlayer bonding properties. The present invention also provides a manufacturing method for the composite steel plate.

Description

[Technical Field]

[0001] Technical Field The present invention relates to the technical field of clad steel plates, and more particularly to clad steel plates and methods for producing the same. [Background technology]

[0002] Background technology In harsh environments such as seawater with a high chloride ion content, flue gas pipelines with complex medium components, oil and gas pipelines subjected to pressure and temperature, and chemical transport containers, it is necessary to select stainless steel materials with excellent corrosion resistance.

[0003] By using clad steel plates, material design prioritizes the base carbon steel layer to ensure structural strength and toughness, achieving different strength and toughness requirements, while the stainless steel clad layer provides corrosion resistance. Thus, clad steel plates can simultaneously achieve the advantageous performance of two materials. Clad steel plate design allows for larger gauge clad steel plates, reducing the number of weld seams required to connect the steel plates and the risk of localized failures introduced during the welding process. Furthermore, the use of clad steel plates converts the previously required welding between carbon steel and stainless steel to welding between carbon steel and the carbon steel of the clad steel plate, or between carbon steel and the clad steel plate as a whole. This ensures that the primary material involved in fusion welding remains carbon steel, reducing costs and simplifying the control of weld quality. Production efficiency is therefore improved.

[0004] Dissimilar clad plates, which combine the characteristics of two or more materials, show wide application prospects, especially when the two sides of the steel plate are intended for different service environments, or when a single material cannot meet specific performance requirements. Dissimilar clad plates are an ideal choice in such cases. Compared with explosion cladding, which is limited by factors such as the explosion location and environmental contamination, weld-roll cladding achieves metallurgical bonding of materials through conventional hot rolling. At the same time, due to repeated rolling at high temperatures, the clad interface offers advantages over explosion cladding interfaces, such as stronger bond strength and fewer coating defects.

[0005] Traditional hydroelectric power steel plates, which meet the corrosion- or wear-resistant requirements while ensuring the overall structural strength of the equipment, typically use surfacing or riveting welding to metallurgically bond the corrosion- or wear-resistant layer metal to the base material. However, this approach can introduce welding defects and requires subsequent heat treatment of the processed parts to relieve stress, leading to relatively low overall production efficiency. The use of dissimilar clad plates can solve these problems. However, because the coated steel plates must be subjected to part processing such as bending and cutting, the clad plates must have high shear strength and bonding strength as well as excellent structural performance to ensure material workability.

[0006] Current production technologies for coating extra-thick plates mostly use homogeneous coating or adopt symmetric plate assembly methods to control plate flatness. For heterogeneous coatings, due to the difference or significant difference in physical properties between the cladding and the substrate, warpage problems are likely to occur during the rolling process, hindering smooth production. Furthermore, for hydroelectric steel plates, they must exhibit sufficient corrosion resistance and strength to withstand water flow corrosion during use, as well as sufficient shear strength to prevent material tearing during processing.

[0007] Chinese patent application CN201610203545.1 discloses a 160 mm thick extra-heavy steel plate with excellent low-temperature toughness and a method for manufacturing the same. The plate is obtained by using two 300 mm thick continuously cast slabs for homogeneous coating, followed by rolling under high pressure and normalizing.

[0008] Chinese patent application CN201710520635.8 discloses a low-alloy extra-heavy steel plate with excellent internal quality, low-temperature impact toughness, and resistance to lamellar cracking, produced by coating two slabs. By using a 370 mm or 450 mm thick continuous cast slab, electron beam welding, coating rolling, and normalizing treatment, an extra-heavy steel plate is obtained that meets defect detection requirements and has specific low-temperature toughness.

[0009] The above two patents fall within the homogeneous coating category.

[0010] Chinese patent application CN201610211275.9 discloses a manufacturing method for a stainless steel-carbon steel extra-thick heterogeneous clad plate. The stainless steel is embedded in a groove of the carbon steel, and carbon steel, stainless steel, stainless steel, and carbon steel are successively stacked. A separator is added between the two stainless steel layers to form a clad slab, which is then rolled to obtain a heterogeneous clad steel plate.

[0011] Chinese patent application CN201810890483.5 discloses an extra-thick clad steel plate with high shear strength for third-generation nuclear power plants and a manufacturing method thereof. Stainless steel, carbon steel, and stainless steel are successively stacked, with a separator between two carbon steel layers to form a clad slab. The clad slab is then sealed at all four corners and rolled, and then quenched and tempered to obtain a clad plate with a finished thickness of 29 to 100 mm for use in nuclear power plants.

[0012] Chinese patent application CN201810890483.5 discloses a manufacturing method for extra-thick stainless steel clad plates. Two stainless steel plates are nested and then clad with a base carbon steel for assembly welding, resulting in upper and lower dissimilar extra-thick stainless steel clad plates. This method is a dissimilar clad method, but due to the use of a multi-layer slab assembly method, there are certain limitations on the finished plate thickness. The specification does not provide a detailed description of the performance of the steel plate after clad, so the application scenario remains unclear.

[0013] The manufacturing methods for heavy-duty steel plates included in the above patent applications involve either homogeneous or heterogeneous symmetrical coating. Homogeneous coatings cannot solve the problem of different application scenarios for clad steel plate materials on two sides, thereby limiting their application environments. While heterogeneous symmetrical coatings can solve this problem, they use symmetrical top-bottom slab assembly, resulting in relatively large slab thicknesses, typically reaching 600-1000 mm. This approach places high demands on the rolling mill's equipment load and maximum rolling thickness, leading to poor production line adaptability and ultimately preventing mass production. At the same time, the multi-layer symmetrical slab assembly rolling method can cause problems such as insufficient core material cooling, which can cause carbide precipitation in the stainless steel and degrade corrosion resistance. Furthermore, performance adjustment cannot be achieved through online water cooling; it must be adjusted through a series of subsequent heat treatments, thereby increasing production costs and other issues.

[0014] WO 2020 / 134675 A1 discloses a corrosion-resistant marine clad steel plate and a method for manufacturing the same. The corrosion-resistant clad steel plate has a two-layer structure, with one layer being duplex stainless steel and the other being carbon steel. It is rolled using a double-barrier vacuum slab assembly process, exhibiting good structural strength, excellent corrosion resistance, and simultaneously reducing structural weight. However, the bond strength at the stainless steel-carbon steel interface still has room for further improvement.

[0015] This field still requires a clad steel plate that not only has excellent tensile performance and low-temperature impact performance but also exhibits excellent interlayer bonding strength.

Summary of the Invention

Means for Solving the Problems

[0016] Abstract To solve the above technical problems, a first aspect of the present invention is a clad steel plate including a base layer and a clad layer on the base layer, the clad steel plate having a thickness of 60 to 120 mm, the clad layer being made of stainless steel, and the stainless steel containing the following chemical components in mass percentage: 0 < C ≤ 0.08%, 0 < Si ≤ 1.0%, 0 < Mn ≤ 2.0%, Cr: 18.0 - 20.0%, Ni: 8.0 - 11.0%, 0 < S ≤ 0.03%, 0 < P ≤ 0.035%, and the balance being Fe and inevitable impurities; the base layer being made of carbon steel, and the carbon steel containing the following chemical components in mass percentage: 0 < C ≤ 0.24%, 0 < Si ≤ 0.55%, 0 < Mn ≤ 1.60%, 0 < P ≤ 0.035%, 0 < S ≤ 0.035%, 0 < Cr ≤ 0.30%, 0 < Ni ≤ 0.30%, 0 < Cu ≤ 0.40%, and the balance being Fe and inevitable impurities, and the bonding strength between the base layer and the clad layer being 500 MPa or more. A clad steel plate is provided.

[0017] By adopting the technical solution of the present invention, by providing a diffusion layer having a specific thickness, a clad steel plate having a large thickness (60 - 120 mm) achieves an extremely high bonding strength between the base layer and the clad layer, while also exhibiting excellent tensile performance and low-temperature impact performance.

[0018] The stainless steel and carbon steel used in the clad steel plate of the present invention can be conventional materials in the art. For example, the stainless steel can preferably be austenitic stainless steel, particularly 304L austenitic stainless steel from the viewpoints of high temperature resistance, processing performance and toughness, and its composition satisfies GB / T 4237-2015 "Hot-rolled stainless steel plates, sheets and strips". The carbon steel can be a low-alloy high-strength structural steel plate. From the viewpoints of high strength, high toughness, fatigue resistance, impact resistance and weldability, Q355C carbon steel is preferred, and its composition complies with GB / T 3274-2017 "Hot-rolled plates, sheets and strips of carbon structural steel and high-strength low-alloy structural steel".

[0019] Preferably, the clad layer contains the following chemical components in mass percentage: C: 0.02-0.045%, Si: 0.3-0.6%, Mn: 1.0-1.4%, Cr: 18.0-19.0%, Ni: 8.0-9.0%, 0<P≤0.03%, 0<S≤0.01%, and the balance is Fe and inevitable impurities.

[0020] Preferably, the base layer contains the following chemical components in mass percentage: C: 0.03-0.1%, Si: 0.1-0.3%, Mn: 1.0-1.5%, 0<P≤0.01%, 0<S≤0.005%, Cr: 0.24-0.30%, Ni: 0.10-0.25%, Cu: 0.1-0.3%, and the balance is Fe and inevitable impurities.

[0021] Preferably, the clad layer (stainless steel) has an austenite structure, and is rapidly cooled after rolling to prevent carbide precipitation and promote intergranular corrosion resistance; the base layer (carbon steel) has a structure of polygonal ferrite, bainite, and a small volume fraction of martensite-austenite islands smaller than either of the above polygonal ferrite or bainite, which enables the carbon steel to have both the toughness of ferrite and the strength of bainite, and at the same time, the small amount of martensite-austenite islands distributed in ferrite and bainite further improves the impact performance of the carbon steel.

[0022] Preferably, in the base layer, ferrite has a volume fraction of 40 to 60%, bainite has a volume fraction of 30 to 50%, and martensite-austenite islands have a volume fraction of 5 to 10%. More preferably, the ferrite in the carbon steel has a grain size grade of 5 or more.

[0023] Preferably, the diffusion layer is present between the base layer and the cladding layer, and the diffusion layer has an average thickness of 15 to 25 μm.

[0024] Preferably, in the clad steel plate, the base layer has a yield strength of 300 to 550 MPa, a tensile strength of 500 to 600 MPa, and an impact energy at 0°C of 200 to 400 J, preferably 300 to 400 J; and / or the clad layer has a yield strength of 400 to 500 MPa, a tensile strength of 650 to 750 MPa, and an impact energy at 0°C of 200 to 300 J.

[0025] Preferably, in the clad steel plate, the bond strength at the interface between the base layer and the clad layer is 550 to 650 MPa.

[0026] A second aspect of the present invention provides a method for producing the above clad steel plate, comprising the following steps in order: 1) Assembling the slab: preparing stainless steel and carbon steel, laminating the stainless steel and carbon steel after dimensional processing, vacuum sealing to obtain a clad slab, and evacuating the clad slab to a vacuum of 0.01 Pa or less; 2) Heating: Heat the clad slab to 1150-1250°C; 3) Rolling: Including first rolling and second rolling, wherein The first rolling is longitudinal rolling, the longitudinal direction of which is consistent with the longitudinal direction of the clad slab, and during rolling, the upper layer is stainless steel and the lower layer is carbon steel; Then, the clad slab is turned over so that the upper layer is carbon steel and the lower layer is stainless steel, and the second rolling is performed to obtain a rolled steel plate; 4) Cooling: First, the rolled steel plate is cooled in a first stage using compressed air or water, and then the rolled steel plate is cooled in layers to obtain the clad steel plate.

[0027] The first rolling is a multi-pass rolling, and each pass has a reduction of 5 to 10%.

[0028] The terms "first," "second," etc. are used for descriptive distinction only and should not be construed as indicating or implying relative importance.

[0029] In the present invention, atomic interdiffusion at the stainless steel-carbon steel interface is achieved by rolling to form a diffusion layer between the dissimilar clad and the base layer, thereby obtaining the desired clad steel plate with excellent bond strength.

[0030] A reduction of 10% or less avoids warping of the steel sheet during rolling, while multiple-pass rolling with a reduction of 5% or more ensures sufficient mixing of the structure at the carbon steel-stainless steel clad interface, thereby ensuring that the bond strength of the clad interface of the steel sheet after the first rolling is 500 MPa or more.

[0031] Preferably, the second rolling is a multi-pass rolling, each pass having a rolling reduction of 10 to 20%, and the finish rolling temperature is 850 to 1000°C, preferably 870 to 950°C, more preferably 890 to 920°C.

[0032] The second rolling can be performed using a relatively large reduction ratio to promote grain fragmentation and recrystallization. Controlling the finish rolling temperature prevents the formation of an abnormally coarse structure caused by rolling in the two-phase region. At the same time, rapid cooling to the phase transformation temperature after rolling further inhibits coarsening of the structure, which in turn refines the grain size and improves material strength and low-temperature impact toughness.

[0033] Preferably, the first-stage cooling has a starting cooling temperature of 830 to 1000°C, preferably 830 to 880°C, a cooling rate of 5 to 20°C / s, preferably 10 to 20°C / s, and a final cooling temperature of 500 to 650°C, preferably 500 to 580°C.

[0034] Preferably, in the above method, before laminating the austenitic stainless steel plate with the carbon steel or low-alloy steel plate and before vacuum sealing, the surfaces of the austenitic stainless steel and the carbon steel or low-alloy steel are subjected to a pretreatment, which includes removing oxide scale and contaminants, and evacuating the clad slab to a vacuum of 0.01 Pa or less.

[0035] In the present invention, the extra-thick clad steel plate manufactured by two-layer dissimilar cladding achieves a balance between the corrosion resistance performance of stainless steel and the mechanical performance of carbon steel. It can be widely applied to structural components used in corrosive environments such as hydroelectric power generation. Compared with the prior art, it offers the following beneficial effects: 1. Compared with traditional homogeneous cladding, the two-layer heterogeneous cladding slab assembly method has a wider application range for materials. Compared with the heterogeneous symmetric four-layer slab assembly or multi-layer slab assembly method, it requires smaller slab dimensions and lower load requirements on manufacturing equipment such as rolling mills, which improves the adaptability of the manufacturing line and facilitates mass production.

[0036] 2. By adopting a two-stage rolling strategy, during the first rolling process, multiple passes of deformation at low reduction ensure complete bonding at the clad interface and avoid unbonded areas caused by weld seams or interface mismatch due to large deformation. During the second rolling process, high reduction promotes fragmentation and recrystallization of the original grains, achieving grain refinement strengthening and meeting the performance requirements of the product.

[0037] 3. The positioning of the stainless steel layer as the bottom layer during the second rolling effectively avoids problems such as warping and water accumulation in the steel plate caused by faster cooling shrinkage when the stainless steel layer is installed as the top layer, which would otherwise deteriorate the flatness of the rolled plate. This significantly improves the flatness of such two-layer dissimilar clad steel plate after rolling.

[0038] 4. Because both the stainless steel and carbon steel layers are exposed during the post-rolling water-cooling process, adjusting the cooling rates of the upper and lower layers allows for independent control of the performance of the stainless steel and carbon steel layers. This method outperforms the process for dissimilar symmetric clad steel plates, where insufficient cooling of the core material requires subsequent heat treatment to improve performance, thereby improving production efficiency. At the same time, rapid cooling inhibits carbide precipitation, thereby suppressing the formation of chromium-depleted zones at grain boundaries and reducing the susceptibility of the stainless steel to intergranular corrosion caused by carbide precipitation.

[0039] 5. By adopting layered cooling in the slow cooling pit after rolling, the tendency of hydrogen generation at the interface of dissimilar materials is reduced, and the bonding strength of the interface is further enhanced.

[0040] The method for manufacturing clad steel plate of the present invention optimizes the rolling process to achieve excellent flatness in the steel plate after rolling, while ensuring that the final clad steel plate exhibits outstanding mechanical performance. [Brief explanation of the drawings]

[0041] [Figure 1] FIG. 1 shows a structural photograph of the clad steel plate of Example 1 of the present invention taken along its thickness direction. [Figure 2] FIG. 2 shows the results of an Fe element line scan along the arrow direction shown in FIG.

[0042] Detailed Description Embodiments of the present invention are further illustrated by the following examples: Unless otherwise specified, all percentages are by weight. 1. Clad steel plate The clad steel plate of the present invention has a base layer, a clad layer bonded to the surface of the base layer, and a diffusion layer between the base layer and the clad layer. 1. Composition of the base layer The substrate is made of carbon steel or low-alloy steel. The carbon steel or low-alloy steel may be a carbon structural steel or low-alloy steel commonly used in the fields of pressure pipelines, pressure vessels, and hydroelectric power (auxiliary) equipment (especially hydroelectric power generation), preferably conforming to "GB / T 4237-2015 Hot-rolled Stainless Steel Sheet, Plate, and Strip." Examples include Q355 steel, Q390 steel, etc., with Q355C steel being preferred in consideration of factors such as mechanical performance, workability, and cost.

[0043] The design principles for the preferred chemical composition of the base steel are explained below.

[0044] C: C is an austenite stabilizing element, which plays a role in solid solution strengthening in steel and significantly enhances the strength of steel. However, excessively high C content has a negative effect on weldability and toughness, and is also likely to form hard phase structures such as pearlite and martensite-austenite islands, which deteriorate the corrosion resistance of steel. Therefore, to balance the strength and toughness of steel plates and meet the weldability requirements for carbon steel materials, the C content in the carbon steel layer is controlled to 0.24% or less, preferably 0.03% or more and 0.10% or less.

[0045] Si: Si is a deoxidizing element. Furthermore, Si can be dissolved in ferrite and plays a role in solid solution strengthening, significantly improving the strength and hardness of the steel. The Si content in the carbon steel layer is controlled to be 0.55% or less, preferably 0.1% or more and 0.3% or less.

[0046] Mn: Mn retards pearlite transformation, reduces the critical cooling rate, and improves the hardenability of steel. It also acts as a solid solution strengthening element and functions as the main solid solution strengthening element in steel. However, excessively high Mn content can cause the formation of segregation bands and martensitic structures, which adversely affect the toughness of steel. At the same time, the segregation bands can reduce the corrosion resistance of steel. The Mn content in the carbon steel layer is controlled to be 1.6% or less, preferably 1.0% or more and 1.5% or less.

[0047] P, S: P and S are common impurity elements in carbon steel. Among them, P increases the hardness of the central segregation zone, causing a deterioration in HIC resistance, while S typically exists as sulfide inclusions in the steel, degrading toughness and HIC resistance. Therefore, the contents of P and S should preferably be limited to 0.035% or less, respectively. Preferably, P is 0.01% or less, and S is 0.005% or less.

[0048] Cr: Cr is an element that reduces the austenite phase field and is also a medium-strength carbide former, soluble in ferrite. Cr increases the stability of austenite and shifts the C-curve to the right, thereby lowering the critical cooling rate and improving the hardenability of the steel. Cr also lowers the austenite transformation temperature and precipitates various carbides, such as (Fe, Cr)3C, (Fe, Cr)7C3, and (Fe, Cr)23C7, at relatively low temperatures. This refines the structure and carbides, significantly increasing the strength and hardness of the steel. However, Cr has a negative effect on the toughness of the steel. Taking these factors into consideration, the technical solution of the present invention controls the Cr content in the carbon steel layer to within 0.30%, preferably 0.24% or more.

[0049] Ni: Ni in steel dissolves only in the matrix phase of ferrite and austenite without forming carbides. It has a strong austenite stabilizing effect and improves the low-temperature toughness of steel. Therefore, in the technical solution of the present invention, the addition of Ni in the carbon steel layer is controlled to be within 0.3%, preferably 0.1% or more and 0.25% or less.

[0050] Cu: Cu in steel exists primarily in solid solution and as precipitated elemental phases. Dissolved Cu contributes to solid solution strengthening; since the solubility of Cu in ferrite rapidly decreases with decreasing temperature, supersaturated Cu precipitates as elemental particles at low temperatures, providing precipitation strengthening. Furthermore, the addition of a small amount of Cu to the carbon steel layer significantly improves the atmospheric corrosion resistance of the clad steel plate. Therefore, in the technical solution of the present invention, the addition of Cu to the carbon steel layer is controlled to within 0.40%, preferably 0.1% or more and 0.3% or less.

[0051] In the present invention, appropriate amounts of Mo, Ni, V, Nb, Ti, Al, N and other elements may be added to the base steel as needed.

[0052] In the present invention, the tensile strength of the base steel is typically controlled within the range of 500 to 620 MPa. This is because if the tensile strength falls below the lower limit, the hardness and ductility of the steel may be impaired. On the other hand, if the tensile strength exceeds the upper limit, an increase in the C content is often required in terms of chemical composition, which reduces the plasticity of the steel. Generally, during welding, if the C content exceeds 0.4%, measures such as preheating before welding, slow cooling after welding, or heat treatment are required to reduce the hardening tendency of the steel plate during welding.

[0053] In the present invention, the yield strength of the base steel is 300 MPa or more, preferably 350 MPa or more, more preferably 400 MPa or more. Although no specific upper limit is provided, considering factors such as manufacturing cost, practical requirements, and balance with other performances, the yield strength is preferably 500 MPa or less. The tensile strength is 500 MPa or more. Although no specific upper limit is provided, considering factors such as manufacturing cost, practical requirements, and balance with other performances, the tensile strength is preferably 600 MPa or less. The impact energy (KV2) at 0 °C is 200 J or more, preferably 250 J or more, more preferably 300 J or more. Although no specific upper limit is provided, considering factors such as manufacturing cost, practical requirements, and balance with other performances, the impact energy at 0 °C is generally 400 J or less. 2. Cladding layer composition In the present invention, the clad layer preferably consists of austenitic stainless steel. This ensures that the outer surface of the clad steel plate exhibits corrosion resistance. The specific grade of austenitic stainless steel is not particularly limited, but preferably complies with "GB / T 3274-2017 Hot Rolled Stainless Steel Plates, Sheets and Strips". Examples include SUS304L, SUS316L, SUS317L, etc. Among these, SUS304L is preferred due to its low carbon content, excellent resistance to intergranular corrosion, and the fact that heat treatment is not required after welding.

[0054] The austenitic stainless steel contains the following chemical components in mass percentage: 0 < C ≤ 0.08%, 0 < Si ≤ 1.0%, 0 < Mn ≤ 2.0%, Cr: 18.0 - 20.0%, Ni: 8.0 - 11.0%, 0 < S ≤ 0.03%, 0 < P ≤ 0.035%, and the balance is Fe and inevitable impurities; preferably contains the following: C: 0.02 - 0.045%, Si: 0.3 - 0.6%, Mn: 1.0 - 1.4%, Cr: 18.0 - 19.0%, Ni: 8.0 - 9.0%, 0 < P ≤ 0.03%, 0 < S ≤ 0.01%, and the balance is Fe and inevitable impurities.

[0055] The design principles for the preferred chemical composition of the cladding layer are explained below.

[0056] C: C precipitates as carbides at grain boundaries during thermal processing in the production of clad plates, which deteriorates corrosion resistance. Therefore, the C content should be controlled to 0.08% or less, preferably 0.02% or more and 0.045% or less.

[0057] Si: Si is a deoxidizing element. If the Si content is too low, the effect is insufficient. However, if the Si content exceeds 1.0%, the corrosion resistance deteriorates. The Si content is preferably 0.30% or more and 0.60% or less.

[0058] Mn: Mn is an important element for improving the hardenability of steel and forming the austenitic structure. However, excessive Mn content adversely affects the toughness, weldability, HIC resistance, and corrosion resistance of steel. Therefore, the Mn content in the clad layer should be controlled to 2.0% or less, preferably 1.0% or more and 1.4% or less.

[0059] Cr: Cr improves the hardenability of steel and increases its strength and hardness. Furthermore, during tempering, Cr inhibits or slows the precipitation and aggregation of carbides, enhancing the tempering stability of steel. It also forms a highly protective oxide film on the metal surface, improving pitting and intergranular corrosion resistance. Furthermore, when combined with Ni, Cr enhances stress corrosion cracking resistance in acidic environments. Therefore, the Cr content should be 18.0% or more. Taking into account the balance with other alloy elements such as Ni, the Cr content should be limited to 20.0% or less, preferably 19.0% or less.

[0060] Ni: Ni is an element that strengthens corrosion resistance, and significantly improves stress corrosion cracking resistance, especially in acidic environments. However, due to the high cost of Ni, its content is restricted to preferably 8.0% or more and 11.0% or less, and more preferably 9.0% or less, taking into account the balance between corrosion resistance effect and cost.

[0061] P, S: P and S are impurity elements that degrade corrosion resistance through grain boundary segregation. Therefore, the P content in the cladding layer is preferably 0.035% or more, more preferably 0.030% or less. The S content in the cladding layer is preferably 0.03% or less, more preferably 0.01% or less.

[0062] In the present invention, appropriate amounts of Cu, Mo, Al, Ti, N and other elements can be added to the clad layer steel as needed.

[0063] In the present invention, the yield strength of the clad layer steel is preferably 400 MPa or more. Although no specific upper limit is set, the yield strength is generally 500 MPa or less, taking into consideration factors such as production costs, practical requirements, and the balance with other performances. The tensile strength is preferably 650 MPa or more. Although no specific upper limit is set, the tensile strength is preferably 750 MPa or less, taking into consideration factors such as production costs, practical requirements, and the balance with other performances. The impact energy (KV2) at 0°C is 200 J or more, preferably 250 J or more. Although no specific upper limit is set, the impact energy at 0°C is generally 300 J or less, taking into consideration factors such as production costs, practical requirements, and the balance with other performances. 3. Composition of clad steel plate In the clad steel plate of the present invention, the base layer made of carbon steel or low alloy steel has a thickness of approximately 40 to 110 mm. The clad layer made of austenitic stainless steel has a thickness of approximately 5 to 20 mm. The thickness of the clad steel plate is 60 to 120 mm, preferably 70 to 100 mm.

[0064] It should be noted that at the clad interface between the carbon steel substrate and the stainless steel clad layer, elements with higher concentrations diffuse toward the side with lower concentrations due to the difference in element content between the carbon steel substrate and the stainless steel clad layer in the thickness direction. This results in the formation of a diffusion layer that penetrates approximately 5 to 20 μm on both sides. The average composition of each element in the diffusion layer is between its respective contents in the substrate and the clad layer, exhibiting a gradient transition. For example, alloying elements such as Cr and Ni, which have a higher mass percentage in the stainless steel clad layer, diffuse toward the substrate, while C, which has a higher mass percentage in the substrate, diffuses toward the clad layer. The average composition of C, Cr, and Ni in the diffusion layer is between their respective concentrations in the stainless steel clad layer and the carbon steel substrate. In the present invention, the average thickness of the diffusion layer is typically between 15 and 25 μm.

[0065] The bond strength at the bonding interface between the base layer and the clad layer is 500 MPa or more, preferably 550 MPa or more, and more preferably 600 MPa or more. Although no specific upper limit is set, it is generally 650 MPa or less, taking into consideration factors such as manufacturing costs, practical requirements, and the balance with other performances.

[0066] The clad steel plates of Examples 1 to 7 of the present invention are manufactured through the following steps in order: 1) Assembling the slab: Select stainless steel and carbon steel with the required thickness and composition, process them to the required dimensions, and then remove oxide scale and contaminants from the surfaces of the stainless steel and carbon steel. The stainless steel and carbon steel are stacked, vacuum sealed, and the four corners of the stainless steel and carbon steel are joined to form a clad slab. The clad slab is evacuated to a vacuum of 0.01 Pa or less. The compositions and thicknesses of the selected stainless steel and carbon steel are shown in Table 1.

[0067] In the present invention, two of the above two-layer clad slabs can be stacked in the thickness direction with their exposed stainless steel surfaces facing each other, followed by welding at the periphery to obtain a four-layer clad slab.

[0068] 2) Heating: Heat the clad slab to 1150-1250°C for a total furnace time of 300-400 minutes. After heating, remove it from the furnace and transfer it to a rolling mill for rolling.

[0069] 3) Rolling: First rolling and second rolling, preferably including only two rolling passes. Here, the first rolling is longitudinal rolling along the length of the clad slab, during which the upper layer is stainless steel and the lower layer is carbon steel. After rolling, the stack is cooled; The clad slab is then turned over and reheated to 1150-1250°C, and then subjected to a second rolling process. During the rolling process, the upper layer is carbon steel and the lower layer is stainless steel, resulting in a rolled steel plate.

[0070] 4) Cooling: First, the rolled steel plate is cooled in the first stage using compressed air or water, and then the rolled steel plate is cooled in layers to obtain a clad steel plate.

[0071] In the present invention, the selection of the heating temperature takes into consideration the physical properties of the austenitic stainless steel and the carbon steel in a comprehensive manner, which allows for the complete or partial dissolution of carbides of elements such as niobium and titanium, promotes uniform distribution of segregated alloying elements that may be present in both the carbon steel and the austenitic stainless steel, facilitates complete metallurgical bonding, and ensures the formation of a uniform austenitic structure without coarsening of the austenitic grains.

[0072] In the present invention, atomic interdiffusion at the stainless steel-carbon steel interface is achieved by rolling to form a diffusion layer between the dissimilar clad and the base layer, thereby obtaining the desired clad steel plate with excellent bond strength.

[0073] In step 2, preferably, during the first rolling, the clad slab is rolled to 1.5 to 2.5 times the finishing thickness, more preferably 1.5 to 2.0 times the finishing thickness.

[0074] Preferably, during the first rolling, the clad slab is subjected to multiple pass rolling with a reduction of 5-10% per pass. A reduction of 10% or less prevents warping of the steel sheet during rolling, while multiple pass rolling with a reduction of 5% or more ensures sufficient structural mixing at the carbon steel-stainless steel clad interface, thereby achieving excellent bond strength at the steel sheet clad interface. Preferably, the second rolling is width expansion rolling, specifically, the clad slab is subjected to multiple pass rolling with a reduction of 10-20% per pass. The finish rolling temperature is 850-1000°C, preferably 870-950°C, more preferably 890-920°C.

[0075] The second rolling employs a relatively large reduction ratio to provide sufficient deformation energy storage, ensuring plastic deformation of the material and promoting particle fragmentation and recrystallization. Specifically, this allows the metal atoms at the composite interface to undergo interpenetration through diffusion under sufficient compressive stress, achieving atomic-level bonding and sufficient deformation at the interface. The interface undergoes extensive recrystallization, thereby enhancing the impact performance of the clad steel plate.

[0076] The finish rolling temperature is controlled within 850-1000°C to ensure that the rolling occurs above the carbide precipitation temperature of the cladding stainless steel and close to the recrystallization temperature range of the austenitic structure in the base layer.

[0077] In step 4, during the first stage cooling, preferably, the initial cooling temperature is 830 to 1000°C, more preferably 830 to 880°C, the cooling rate is 5 to 20°C / s, more preferably 10 to 20°C / s, and the final cooling temperature is 500 to 650°C, more preferably 500 to 580°C. The rolled steel sheet is then removed from the line and transferred to an annealing pit for intensive lamination cooling, and then removed from the pit and laminated after cooling to 200°C or less.

[0078] More preferably, the rolled steel sheet is first cooled using water, where the ratio of the amount of water in the upper and lower portions is 1:2 to 1:4, most preferably about 1:3.

[0079] Example The following provides a more specific description of the embodiments of the present invention through examples. However, the following examples are illustrative of the embodiments of the present invention and should not be construed as limiting the scope of the present invention in any way. Furthermore, all percentages used herein are by weight unless otherwise specified. Example 1 Carbon steel Q355C (thickness: 280 mm) from Baoshan Iron & Steel Co. Ltd. was used as the base layer slab, and stainless steel SUS304L (00Cr19Ni10; thickness: 50 mm) from Baoshan Iron & Steel Co. Ltd. was used as the clad layer slab. The chemical compositions of the carbon steel and stainless steel are shown in Table 1 below.

[0080] The clad slabs were processed according to the process conditions shown in Table 2 below, specifically as follows:

[0081] After surface processing to remove oxide scale and ensure surface roughness and flatness, the clad layer and base layer were bonded along all four corners by vacuum sealing, and then the bonded interface was evacuated to a vacuum level of less than 0.01 Pa to obtain a final clad slab thickness of 330 mm.

[0082] The clad slab was then heated and rolled. During the first rolling, the upper layer of the clad slab was stainless steel, and the lower layer was carbon steel. The clad slab was heated to 1220°C in a heating furnace for a total furnace time of 355 minutes. After heating, it was removed from the furnace and transferred to a rolling mill for rolling. During rolling, the reduction per pass was controlled at approximately 8%, and the clad slab was finally rolled to 220 mm to complete the first rolling. The clad slab was then subjected to layer cooling.

[0083] After cooling, the clad slab was flipped over, resulting in a carbon steel upper layer and a stainless steel lower layer. It was then heated in a furnace for a total furnace time of 300 minutes to the same heating temperature as the first rolling. After heating, it was removed from the furnace for rolling at a reduction of approximately 13% per pass, and the clad slab was finally rolled to approximately 80 mm. The finishing rolling temperature was controlled at 915°C.

[0084] After rolling, water cooling was performed with a top / bottom water ratio of approximately 1:3. The cooling rate was approximately 18°C / s, and the final cooling temperature was 612°C. The steel plates were then removed from the line and transferred to a slow cooling pit for concentrated lamination cooling, removed from the pit, and cooled to below 200°C before lamination. The properties of the resulting clad steel plates were determined, and the results are shown in Table 3 below.

[0085] Here, the grading of grain size was performed as follows: the ferritic structure in both carbon steel and stainless steel was evaluated for grain size using the intercept point method according to GB / T 6394-2017 "Determination of the estimation of the average grain size of metals".

[0086] The yield strength and tensile strength of carbon steel and stainless steel were measured according to GB / T 6396-2008 "Clad steel plates - Mechanical and technological tests" and GB / T 228-2010 "Metallic materials - Tensile tests - Test methods at room temperature."

[0087] The impact energy (KV2) (longitudinal direction) of carbon steel and stainless steel at 0°C was measured according to GB / T 6396-2008 "Clad steel plates - Mechanical and technological tests" and GB / T 229-2020 "Metallic materials - Charpy pendulum impact test method."

[0088] The bond strength between the clad and base layer of the clad steel plate was measured according to GB / T 6396-2008 "Clad steel plate - mechanical and technological tests".

[0089] Defect detection in clad steel plates was measured according to NB / T 47013.3-2015 "Non-destructive testing of pressure equipment - Part 3: Ultrasonic testing" using the standards specified in Amendment No. 1, New Tables 6 and 7, Grade I.

[0090] The intergranular corrosion resistance of stainless steel was measured according to GB / T 4334-2000 "Corrosion of metals and alloys - Test method for intergranular corrosion of stainless steel" (Method E).

[0091] Examples 2 to 7 and Comparative Examples 1 to 5 Following substantially the same process as in Example 1, clad steel plates of Examples 2 to 7 and Comparative Examples 1 to 5 were manufactured using the steels specified in Table 1, and their performance was determined. Specific process conditions and performance results are shown in Tables 2 and 3.

[0092] Table 1 shows the compositions (in wt.%) and thicknesses of selected stainless steel and carbon steel slabs, where the balance of the stainless steel composition and carbon steel composition is Fe and unavoidable impurities other than P and S.

[0093] [Table 1]

[0094] Table 2 shows the specific process parameters employed to manufacture the clad steel plates of Examples 1 to 7 and Comparative Examples 1 to 5, as well as the finished thickness of the clad steel plates.

[0095] In Table 2, the meaning of the numbering of the clad steel plates is, for example, that the numbering of the adopted clad steel plates from 1 to 3 refers to the selection of the stainless steel clad steel plate numbered 1 in Table 1 and the carbon steel clad steel plate numbered 3 in Table 1.

[0096] Table 3 shows the metallographic structure of the carbon steel layer and stainless steel layer, mechanical performance, and thickness of the diffusion layer for the clad steel plates of Examples 1 to 7 and Comparative Examples 1 to 5.

[0097] The stainless steel in the clad steel plate is composed of austenite, while the carbon steel has a structure of polygonal ferrite, bainite, and a small amount of martensite-austenite islands, with a volume fraction of approximately 40-60% polygonal ferrite, 30-50% bainite, and 5-10% martensite-austenite islands. Due to the significant difference in Fe content between the base layer and the clad layer, the Fe element on the two sides of the pre-clad interface shows a gradual change. After rolling and coating the steel plate of the present invention, the Fe element near the clad interface shows a continuous change, indicating Fe diffusion from the carbon steel to the stainless steel. As a result, the Fe content in the carbon steel near the interface becomes lower than the Fe content in the carbon steel matrix, and the Fe content in the stainless steel near the interface becomes higher than the Fe content in the stainless steel matrix. Therefore, the diffusion layer thickness is defined as the length of the Fe content transition zone near the clad interface. In the present invention, the first boundary of the diffusion layer is defined as the position in the carbon steel where the Fe content decreases by 5% compared to the matrix. The secondary boundary is defined as the location in the stainless steel where the Fe content increases by 5% compared to the matrix. The thickness of the diffusion layer is defined as the distance between the primary and secondary boundaries.

[0098] Figure 1 shows a photograph of the interface of the clad steel plate in Example 1 of the present invention. Figure 2 shows the results of an Fe element line scan along the arrow direction on the steel plate shown in Figure 1. The element count rate (counts per second, CPS) indicates the relative content of the element at each position, and the line scan results reflect the trend of element content change along a straight line. The results show that the average thickness of the diffusion layer in the clad steel plate of Example 1 is approximately 18 μm. The average diffusion layer thickness in the present invention is the average of the diffusion layer thicknesses obtained from Fe element line scans at 10 different positions within the field of view.

[0099] [Table 2]

[0100] [Table 3-1]

[0101] [Table 3-2]

[0102] As shown in Table 3, the total thickness of the clad steel plate obtained by the manufacturing process of the present invention is 60 to 120 mm, and the thickness of the diffusion layer is between 17 and 21 μm. The metallographic structure of the stainless steel in the steel plate is austenite, where the austenite has a grain size grade between 3 and 5, the yield strength is 400 to 600 MPa, and the tensile strength is 650 to 850 MPa. The metallographic structure of the carbon steel is polygonal ferrite + bainite + a small amount of martensite-austenite islands, where the ferrite has a grain size grade between 5 and 7, the yield strength is 300 to 500 MPa, the tensile strength is 500 to 600 MPa, and the impact energy at 0°C is 200 to 400 J. No defects / flaws were detected at the interface between the clad and the base layer (diffusion layer) in the clad steel plates of Examples 1 to 7, achieving a 100% defect detection pass rate. None of the stainless steels in Examples 1 to 7 exhibited intergranular corrosion.

[0103] Comparative Example 1 failed to meet the requirements for grain size, intergranular corrosion and bond strength because the reduction ratio and cooling rate during the first and second rolling were outside the ranges specified in the present invention.

[0104] Comparative Example 2 failed to meet the requirements for bond strength because the reduction during the second rolling was outside the range specified in the present invention.

[0105] Comparative Example 3 failed to satisfy the requirements for intergranular corrosion because the finish rolling temperature, final cooling temperature and cooling rate were outside the ranges specified in the present invention.

[0106] Comparative Example 4 failed to satisfy the impact performance requirements because the finish rolling temperature, the initial cooling temperature, and the final cooling temperature were outside the ranges specified in the present invention.

[0107] Comparative Example 5 failed to meet the requirements for grain size and bonding strength because the reduction during rolling was outside the range specified in the present invention.

[0108] By using the manufacturing method of the present invention, particularly by controlling the rolling and cooling processes, the bond strength at the interface between the base layer and the clad layer of the steel plate is 550-650 MPa. The clad steel plate exhibits excellent flatness, outstanding mechanical properties, and corrosion resistance.

[0109] The manufacturing method of the present invention is simple and practical, and is highly suitable for large-scale production of heterogeneous clad steel plates for hydroelectric power generation applications.

[0110] It should be noted that all technical features described in the present invention can be freely combined or integrated in any way as long as they are not mutually inconsistent. As will be apparent to those skilled in the art, various modifications and variations can be made to the present invention without departing from its scope. For example, features illustrated or described as part of one embodiment can be combined with another embodiment to yield a further embodiment. The present invention is intended to cover such modifications within the scope of the appended claims and their equivalents.

Claims

1. A clad steel plate comprising a base layer and a clad layer on the base layer, the clad steel plate having a thickness of 60 to 120 mm; The clad layer is made of stainless steel, and the stainless steel contains the following chemical components in mass percent: 0<C≦0.08%, 0<Si≦1.0%, 0<Mn≦2.0%, Cr: 18.0-20.0%, Ni: 8.0-11.0%, 0<S≦0.03%, 0<P≦0.035%, with the balance being Fe and unavoidable impurities; The substrate is made of carbon steel, and the carbon steel has the following chemical compositions in mass percent: 0<C≦0.24%, 0<Si≦0.55%, 0<Mn≦1.60%, 0<P≦0.035%, 0<S≦0.035%, 0<Cr≦0.30%, 0<Ni≦0.30%, 0<Cu≦0.40%, the balance being Fe and unavoidable impurities; and The bond strength between the base layer and the clad layer is 500 MPa or more. Clad steel plate.

2. the cladding layer has the following chemical composition in mass percent: C: 0.02-0.045%, Si: 0.30-0.60%, Mn: 1.0-1.4%, Cr: 18.0-19.0%, Ni: 8.0-9.0%, 0<P≦0.03%, 0<S≦0.01%, the balance being Fe and unavoidable impurities; and / or 2. The clad steel plate according to claim 1, wherein the base layer contains the following chemical components in mass percent: C: 0.03-0.10%, Si: 0.1-0.3%, Mn: 1.0-1.5%, 0<P≦0.01%, 0<S≦0.005%, Cr: 0.24-0.30%, Ni: 0.10-0.25%, Cu: 0.1-0.3%, and the balance being Fe and unavoidable impurities.

3. 2. The clad steel plate according to claim 1, wherein the clad layer has an austenite structure, and the base layer has a structure of polygonal ferrite, bainite, and martensite-austenite islands having a volume fraction smaller than either polygonal ferrite or bainite.

4. 4. The clad steel plate according to claim 3, wherein in the base layer, ferrite has a volume fraction of 40 to 60%, bainite has a volume fraction of 30 to 50%, and martensite-austenite islands have a volume fraction of 5 to 10%; and / or the ferrite has a grain size grade of 5 or more.

5. 2. The clad steel plate according to claim 1, wherein the diffusion layer is present between the base layer and the clad layer, and the diffusion layer has an average thickness of 15 to 25 μm.

6. the substrate has a yield strength of 300 to 550 MPa, a tensile strength of 500 to 600 MPa, and an impact energy at 0°C of 200 to 400 J; and / or the clad layer having a yield strength of 400-500 MPa, a tensile strength of 650-750 MPa, and an impact energy at 0°C of 200-300 J; The clad steel plate according to any one of claims 1 to 5.

7. The clad steel plate according to any one of claims 1 to 5, wherein the bond strength at the interface between the base layer and the clad layer is 550 to 650 MPa.

8. A method for manufacturing a clad steel plate according to any one of claims 1 to 7, comprising the following steps in order: 1) Assembling the slab: preparing stainless steel and carbon steel, laminating the stainless steel and carbon steel after dimensional processing, vacuum sealing to obtain a clad slab, and evacuating the clad slab to a vacuum of 0.01 Pa or less; 2) Heating: Heat the clad slab to 1150-1250°C; 3) Rolling: including first rolling and second rolling, The first rolling is longitudinal rolling, and during rolling, the upper layer is stainless steel and the lower layer is carbon steel; Then, the clad slab is turned over so that the upper layer is carbon steel and the lower layer is stainless steel, and the second rolling is performed to obtain a rolled steel plate; 4) Cooling: firstly, the rolled steel plate is cooled in a first stage using compressed air or water, and then the rolled steel plate is cooled in layers to obtain the clad steel plate; Here, the first rolling is a multi-pass rolling, and each pass has a reduction of 5 to 10%.

9. 9. The method according to claim 8, wherein the second rolling is a multi-pass rolling, each pass having a reduction of 10-20%, and the finish rolling temperature is 850-1000°C.

10. 10. The method according to claim 8 or 9, wherein the first stage cooling has a starting cooling temperature of 830 to 1000°C, a cooling rate of 5 to 20°C / s and a final cooling temperature of 500 to 650°C.

11. 11. The method of claim 10, wherein the first stage cooling has a starting cooling temperature of 830-880°C, a cooling rate of 10-20°C / s, and a final cooling temperature of 500-580°C.

12. 9. The method according to claim 8, wherein the cooling step uses water to perform a first-stage cooling of the rolled steel sheet with a water amount ratio of 1:2 to 1:4 between the top and bottom, and then the steel sheet is transferred to an annealing pit for concentrated lamination, and then cooled to 200°C or less before being removed from the pit for lamination.

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