Composite steel sheet and method for manufacturing the same

Abstract

This invention provides a composite steel plate comprising a base layer and a cladding layer on top of the base layer. The composite steel plate has a thickness of 60-120 mm, the cladding layer is made of stainless steel, the base layer is made of carbon steel, and the bond strength between the base layer and the cladding layer is above 500 MPa. The composite steel plate of this invention exhibits excellent tensile properties, low-temperature impact resistance, and interlayer bonding performance. This invention also provides a method for preparing the above-mentioned composite steel plate.

Description

Technical Field

[0001] This invention relates to the field of composite steel plate technology, and particularly to a composite steel plate and its manufacturing method. Background Technology

[0002] In harsh environments, such as seawater with high chloride ion content, flue gas pipelines with complex media composition, oil and gas pipelines with temperature and pressure, and containers for transporting chemicals, stainless steel materials with excellent corrosion resistance are required.

[0003] By adopting a composite steel plate design approach, material design allows for the use of carbon steel as the primary base layer to ensure structural strength and toughness, meeting varying strength and toughness requirements; while a stainless steel cladding provides corrosion resistance. Therefore, composite steel plates can simultaneously achieve the superior properties of both materials. Furthermore, the composite steel plate design enables the provision of larger-sized composite steel plates, reducing the number of welds at plate joints and lowering the risk of localized failures during welding. Additionally, due to the composite steel plate design, the welds that previously required butt joints between carbon steel and stainless steel are now replaced by welds between carbon steel and the carbon steel of the composite steel plate, or even integral welding of the carbon steel and composite steel plate. The primary material used in the welding remains carbon steel, which not only reduces costs but also simplifies weld quality control. Production efficiency can be improved.

[0004] Heterogeneous composite plates, possessing the characteristics of two or more materials, have a very broad application prospect, especially when the two sides of the steel plate have different service environments or when a single material cannot meet certain performance requirements. In these cases, heterogeneous composite plates become an ideal choice. Compared to explosive bonding, which is limited by factors such as explosion sites and environmental pollution, welded rolling bonding can achieve metallurgical bonding of materials through traditional hot rolling. Furthermore, because the composite interface is repeatedly rolled at high temperatures, it has the advantages of stronger bonding and fewer composite defects compared to explosive bonding interfaces.

[0005] Traditionally, steel plates used in hydropower projects are metallurgically bonded to the base material using methods such as overlay welding or riveting to meet corrosion or wear resistance requirements while ensuring the overall structural strength of the equipment. However, this can introduce welding defects and necessitates heat treatment of the processed parts to relieve stress, resulting in low overall production efficiency. Using heterogeneous composite plates can solve these problems. However, since the steel plates need to be processed after composite bonding, such as bending and cutting, the composite plates must possess excellent structural properties, as well as good shear strength and bond strength to ensure the material's machinability.

[0006] Current composite production technologies for extra-thick plates mostly employ homogeneous composites or use symmetrical plate assembly to control plate shape. For heterogeneous composites, due to the different, even significant, physical properties of the cladding and base layers, the steel plates are prone to warping during rolling, affecting smooth production. Furthermore, for plates used in hydropower projects, they must possess sufficient corrosion resistance and strength to withstand water corrosion during use, as well as sufficient shear strength to prevent tearing during processing.

[0007] Chinese patent application CN201610203545.1 discloses a 160mm ultra-thick steel plate with excellent low-temperature toughness and its production method. The ultra-thick steel plate with good low-temperature toughness is obtained by homogeneously combining two 300mm thick continuously cast billets, rolling them under high reduction, and then normalizing them.

[0008] Chinese patent application CN201710520635.8 discloses a low-alloy extra-thick steel plate with excellent internal quality, low-temperature impact toughness, and resistance to lamellar tearing, produced by a two-slab composite process. By using 370mm or 450mm thick continuously cast slabs, and through electron beam welding composite rolling followed by normalizing treatment, an extra-thick steel plate that meets flaw detection requirements and possesses certain low-temperature toughness is obtained.

[0009] The two patents mentioned above fall under the category of homogeneous composites.

[0010] Chinese patent application CN201610211275.9 discloses a method for producing an extra-thick stainless steel-carbon steel heterogeneous composite plate. The method involves sequentially stacking carbon steel, stainless steel, stainless steel, and carbon steel by embedding stainless steel into grooves in carbon steel, and coating a release agent between two layers of stainless steel to obtain a composite billet. The composite billet is then rolled to obtain the heterogeneous composite steel plate.

[0011] Chinese patent application CN201810890483.5 discloses a high-shear-strength, extra-thick composite steel plate for third-generation nuclear power plants and its manufacturing method. The method involves sequentially stacking stainless steel, carbon steel, carbon steel, and stainless steel, and coating a release agent between two layers of carbon steel to obtain a composite billet. The composite billet is then subjected to four-sided sealing and rolling, followed by quenching and tempering treatment to obtain a finished composite plate for nuclear power plants with a thickness of 29-100 mm.

[0012] Chinese patent application CN201810890483.5 discloses a method for producing extra-thick stainless steel composite plates. The method involves placing two stainless steel sheets within a nested structure, then covering them with upper and lower carbon steel substrates and welding them together to obtain two dissimilar extra-thick stainless steel composite plates. While this is a dissimilar composite process, the use of a multi-layer assembly method limits the thickness of the finished product. The specification does not provide a detailed description of the properties of the composite steel plate, therefore, its application scenarios are unclear.

[0013] The above patent applications all involve the production methods of extra-thick plates using either homogeneous composite or heterogeneous symmetrical composite methods. Homogeneous composite cannot solve the problem of different application scenarios for the two sides of the composite steel plate, limiting its application environment. Heterogeneous symmetrical composite can solve the above problems, but due to the use of symmetrical billet assembly, the billet size is relatively thick, typically reaching 600-1000mm. This places high demands on the load on the equipment and the maximum rolling thickness of the rolling mill, resulting in poor applicability to production lines and hindering mass production. Furthermore, the use of multi-layer symmetrical billet assembly during rolling can lead to issues such as insufficient cooling of the core material, which can easily cause the precipitation of stainless steel carbides, affecting corrosion performance. Performance cannot be adjusted through online water cooling, requiring subsequent heat treatment processes, increasing production costs.

[0014] WO2020 / 134675A1 discloses a corrosion-resistant marine composite steel plate and its manufacturing method. The corrosion-resistant composite 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 billet method, which has good structural strength and excellent corrosion resistance, while also achieving structural weight reduction. However, there is still room for further improvement in the bonding strength between the stainless steel and carbon steel interfaces.

[0015] There is still a need in the field for a composite steel plate that has excellent tensile properties, low-temperature impact properties, and even better interlayer bonding properties. Summary of the Invention

[0016] To solve the above-mentioned technical problems, a first aspect of the present invention provides a composite steel plate, comprising a base layer and a cladding layer on the base layer, wherein the thickness of the composite steel plate is 60-120 mm.

[0017] The cladding is made of stainless steel, whose chemical composition, by mass percentage, contains: 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.

[0018] The base layer is made of carbon steel, whose chemical composition, by mass percentage, contains: 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%, with the balance being Fe and unavoidable impurities.

[0019] The bonding strength between the base layer and the overlay is above 500 MPa.

[0020] By employing the technical solution of this invention, by providing a diffusion layer of a certain thickness, the base layer and the cladding layer of a composite steel plate with a large thickness (60-120mm) can exhibit extremely high bonding strength, while also enabling the composite steel plate to have excellent tensile properties and low-temperature impact properties.

[0021] The stainless steel and carbon steel used in the composite steel plate of this invention can be steels commonly used in the art. For example, the stainless steel can be austenitic stainless steel, especially 304L austenitic stainless steel, which is preferred from the perspective of high temperature resistance, processing performance and toughness, and whose composition meets the GB / T 4237-2015 standard "Hot-rolled stainless steel plates and strips". The carbon steel can be low-alloy high-strength structural steel plate. Q355C carbon steel is preferred from the perspective of high strength, high toughness, fatigue resistance, impact resistance and weldability, and its composition meets the GB / T 3274-2017 standard "Hot-rolled carbon structural steel and low-alloy structural steel plates and strips".

[0022] Preferably, the chemical composition of the coating, by mass percentage, contains: 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%, with the balance being Fe and unavoidable impurities.

[0023] Preferably, the chemical composition of the base layer, by mass percentage, contains: C: 0.03-0.1%, Si: 0.1-0.3%, Mn: 1.0-1.5%, 0 < P ≤ 0.01%, 0 < S ≤ 0.005%, Cr: 0.24-0.3%, Ni: 0.10-0.25%, Cu: 0.1-0.3%, with the balance being Fe and unavoidable impurities.

[0024] Preferably, the cladding (stainless steel) has an austenitic structure, and rapid cooling after rolling prevents carbide precipitation and improves resistance to intergranular corrosion; the base layer (carbon steel) has polygonal ferrite, bainite and a small amount of Mao island structure with a volume fraction smaller than the above-mentioned polygonal ferrite and bainite, so that the carbon steel has both the toughness of ferrite and the strength of bainite, while the small amount of Mao islands distributed on the ferrite and bainite can further improve the impact performance of carbon steel.

[0025] Preferably, in the base layer, the volume fraction of ferrite is 40-60%, the volume fraction of bainite is 30-50%, and the volume fraction of Mao islands is 5-10%. More preferably, the ferrite grain size in the carbon steel is grade 5 or higher.

[0026] Preferably, a diffusion layer exists between the base layer and the overlay, and the average thickness of the diffusion layer is 15-25 μm.

[0027] Preferably, in the above-mentioned composite steel plate, the yield strength of the base layer is 300-500MPa, the tensile strength is 500-600MPa, and the impact energy at 0℃ is 200-400J, preferably 300-400J; and / or, the yield strength of the cladding layer is 400-500MPa, the tensile strength is 650-750MPa, and the impact energy at 0℃ is 200-300J.

[0028] Preferably, in the above-mentioned composite steel plate, the bonding strength at the interface between the cladding and the base layer is 550-650 MPa.

[0029] A second aspect of the present invention provides a method for manufacturing the above-mentioned composite steel plate, comprising the following steps performed sequentially:

[0030] 1) Billet assembly: Prepare stainless steel and carbon steel, process the dimensions, stack the stainless steel and carbon steel together, and perform vacuum sealing welding to obtain a composite billet. Perform vacuum treatment on the composite billet to make the vacuum degree below 0.01Pa.

[0031] 2) Heating: Heat the composite blank to 1150-1250℃;

[0032] 3) Rolling: This includes the first rolling and the second rolling, wherein,

[0033] The first rolling process uses longitudinal rolling, with the longitudinal direction being the length of the composite billet. During rolling, the upper layer is stainless steel and the lower layer is carbon steel.

[0034] The composite billet is then flipped over, and during rolling, the upper layer is carbon steel and the lower layer is stainless steel. A second rolling process is then performed to obtain the rolled steel plate.

[0035] 4) Cooling: First, compressed air or water is used to cool the rolled steel plate in the first stage, and then the rolled steel plate is stacked to obtain composite steel plate.

[0036] The first rolling process described above is a multi-pass rolling process, with a reduction rate of 5-10% for each pass.

[0037] The terms “first”, “second”, etc., are used only to distinguish descriptions and should not be interpreted as indicating or implying relative importance.

[0038] In this invention, the atoms at the interface between stainless steel and carbon steel diffuse into each other through rolling, forming a diffusion layer between the heterogeneous cladding and the base layer, thereby obtaining the desired composite steel plate with excellent bonding strength.

[0039] A reduction rate of less than 10% can prevent the steel plate from warping during rolling, while a reduction rate of more than 5% for multiple rolling passes can ensure that the microstructure of the carbon steel-stainless steel composite interface is fully mixed, thereby ensuring that the bonding strength of the steel plate composite interface is above 500MPa after the first rolling.

[0040] Preferably, the second rolling is a multi-pass rolling, with a reduction rate of 10-20% per pass and a final rolling temperature of 850-1000℃, preferably 870-950℃, and more preferably 890-920℃.

[0041] The second rolling process can promote grain breakage and recrystallization by using a larger reduction rate. Controlling the final rolling temperature can avoid abnormally coarse structures caused by rolling in the two-phase region. At the same time, it can be rapidly cooled to the phase transformation temperature after rolling, further inhibiting the growth of the structure. By refining the grains, the strength and low-temperature impact toughness of the material can be improved.

[0042] Preferably, the starting temperature of the first stage of cooling is 830-1000℃, more preferably 830-880℃, the cooling rate is 5-20℃ / s, more preferably 10-20℃ / s, and the final cooling temperature is 500-650℃, more preferably 500-580℃.

[0043] Preferably, in the above method, before stacking the austenitic stainless steel sheet with the carbon steel or low alloy steel sheet and performing vacuum sealing welding, the surfaces of the austenitic stainless steel and the carbon steel or low alloy steel are pretreated. The pretreatment includes removing oxide scale and dirt, and the vacuuming reduces the vacuum degree to below 0.01 Pa.

[0044] In this invention, an extra-thick composite steel plate that combines the corrosion resistance of stainless steel with the mechanical properties of carbon steel is obtained through a double-layer heterogeneous composite process. This plate can be widely used in structural components used in corrosive environments such as hydropower plants. Compared with existing technologies, it has the following advantages:

[0045] 1. The double-layer heterogeneous composite billet assembly method has a wider range of application scenarios compared with the traditional homogeneous composite method. Compared with the heterogeneous symmetrical four-layer billet assembly method or multi-layer billet assembly method, the required billet size is smaller, the load requirements on production equipment such as rolling mills are lower, the applicability to production lines is stronger, and it is more conducive to mass production.

[0046] 2. By employing a two-stage rolling strategy, the first rolling process uses multiple passes with small reductions to ensure full bonding of the composite interface, avoiding unbonded areas caused by weld seams and interface misalignment due to large deformation. The second rolling process uses large reductions to promote the breakage and recrystallization of the original grains, resulting in grain refinement and strengthening, thus ensuring the product's performance requirements.

[0047] 3. Placing the stainless steel layer at the bottom during the second rolling process can effectively avoid the problem of water accumulation and deterioration of the rolled plate shape caused by the faster cooling and contraction of the stainless steel when it is placed on the top layer. This can significantly improve the post-rolling plate shape of this type of double-layer heterogeneous composite steel plate.

[0048] 4. Since both the stainless steel and carbon steel layers are exposed during the water cooling process after rolling, their properties can be controlled separately by adjusting the cooling rates of the upper and lower layers. This is superior to the process path of insufficient cooling of the core material after rolling in heterogeneous symmetrical composite steel plates, which requires subsequent heat treatment to improve performance, thus improving production efficiency. Simultaneously, rapid cooling can suppress carbide precipitation, thereby inhibiting the formation of chromium-depleted regions at grain boundaries and reducing the tendency of stainless steel to undergo intergranular corrosion due to carbide precipitation.

[0049] 5. By using a slow cooling pit after rolling to reduce the tendency of hydrogen evolution at the interface of heterogeneous materials, the bonding strength of the interface is further improved.

[0050] The composite steel plate preparation method of the present invention optimizes the rolling process to obtain rolled steel plates with excellent plate shape, and at the same time makes the produced composite steel plates have excellent mechanical properties. Attached Figure Description

[0051] Figure 1 A photograph showing the microstructure of the composite steel plate of Embodiment 1 of the present invention along the thickness direction is shown;

[0052] Figure 2 Show along Figure 1 The result of the Fe element line scan in the direction of the arrow in the image. Detailed Implementation

[0053] The embodiments of the present invention will be described in detail below with reference to specific examples. Unless otherwise specified, all percentages are by mass.

[0054] I. Composite steel plate

[0055] The composite steel plate of the present invention comprises a base layer and a cladding layer bonded to the surface of the base layer, and a diffusion layer between the base layer and the cladding layer.

[0056] 1. Basic Structure

[0057] The base layer is made of carbon steel or low-alloy steel. The carbon steel or low-alloy steel can be carbon structural steel or low-alloy steel commonly used in this field (especially in the hydropower sector) for pressure pipelines, pressure vessels, and hydropower (auxiliary) equipment, preferably conforming to the standard GB / T 4237-2015 Stainless Steel Hot-Rolled Plates and Strips. Examples include Q355 steel and Q390 steel, among which Q355C steel is preferred considering mechanical properties, processability, and cost.

[0058] The design principles for the optimal chemical composition of the base steel are explained below.

[0059] C: C is an austenite stabilizing element that plays a role in solid solution strengthening in steel, which can significantly improve the strength of steel. However, if the C content is too high, it will be detrimental to the weldability and toughness, and will also make it easier to increase the pearlite structure and hard phase structure such as Mao islands, which will have an adverse effect on the corrosion resistance of steel. Therefore, considering the strength and toughness matching of steel plates and the weldability requirements of carbon steel materials, the C content in the carbon steel layer is controlled below 0.24%, preferably above 0.03% and below 0.10%.

[0060] Si: Si is a deoxidizing element. Additionally, Si is soluble in ferrite, playing a role in solid solution strengthening and significantly improving the strength and hardness of steel. The Si content in the carbon steel layer is controlled below 0.55%, preferably above 0.1% and below 0.3%.

[0061] Mn: Mn can delay the pearlite transformation, reduce the critical cooling rate, and improve the hardenability of steel. It also has a solid solution strengthening effect on steel, making it a major solid solution strengthening element in steel. However, excessively high Mn content can easily lead to segregation bands and martensite structures, which negatively impact the toughness of the steel. Furthermore, the presence of segregation bands can reduce the corrosion resistance of the steel. The Mn content in the carbon steel layer is controlled below 1.6%, preferably above 1.0% and below 1.5%.

[0062] P and S: Both P and S are common impurity elements in carbon steel. P increases the hardness of the central segregation zone, thus deteriorating the HIC resistance. S generally exists in steel as sulfide inclusions, which deteriorate toughness and HIC resistance. Therefore, it is advisable to suppress the content of P and S, preferably to keep the content of P and S below 0.035%, more preferably P below 0.01% and S below 0.005%.

[0063] Cr: Cr is an element that shrinks the austenite phase region and is also a medium-strong carbide element, soluble in ferrite. Cr improves the stability of austenite, shifting the C-curve to the right, thus reducing the critical cooling rate and improving the hardenability of steel. Cr also lowers the austenite transformation temperature, causing various carbides such as (Fe,Cr)3C, (Fe,Cr)7C3, and (Fe,Cr)23C7 to precipitate at lower temperatures, resulting in a finer microstructure and carbides, which can significantly improve the strength and hardness of steel. However, Cr has an adverse effect on the toughness of steel. Considering the above factors, the technical solution of this invention controls the Cr content in the carbon steel layer to within 0.30%, preferably above 0.24%.

[0064] Ni: Ni dissolves only in the matrix phases ferrite and austenite in steel and does not form carbides. It has a very strong stabilizing effect on austenite. In addition, Ni can also improve the low-temperature toughness of steel. Therefore, in the technical solution of this invention, the amount of Ni added to the carbon steel layer is controlled within 0.3%, preferably above 0.1% and below 0.25%.

[0065] Cu: In steel, Cu mainly exists in solid solution and as a single-phase precipitate. Solid solution Cu provides solid solution strengthening. Since the solid solubility of Cu in ferrite decreases rapidly with decreasing temperature, at lower temperatures, supersaturated Cu precipitates as elemental, providing precipitation strengthening. Furthermore, adding a small amount of Cu to the carbon steel layer can significantly improve the atmospheric corrosion resistance of the composite steel plate. Therefore, in the technical solution of this invention, the amount of Cu added to the carbon steel layer is controlled to be less than 0.40%, preferably more than 0.1% and less than 0.3%.

[0066] In this invention, appropriate amounts of elements such as Mo, Ni, V, Nb, Ti, Al, and N may be added to the base steel as needed.

[0067] In this invention, the tensile strength of the base steel is typically controlled within the range of 500-620 MPa. This is because if the tensile strength is below the lower limit, the hardness and ductility of the steel may be problematic, while if it is above the upper limit, from a chemical composition perspective, the carbon content often needs to be increased, which reduces the plasticity of the steel. Generally, during welding, if the carbon content exceeds 0.4%, preheating before welding, slow cooling after welding, or heat treatment are required to reduce the hardening tendency of the steel plate during welding.

[0068] In this invention, the yield strength of the base steel is above 300 MPa, preferably above 350 MPa, and more preferably above 400 MPa. There is no particular limit to its upper limit, but considering the manufacturing cost, practical needs, and balance with other properties, the yield strength is preferably below 500 MPa; the 0℃ impact energy (KV2) is above 200 J, preferably above 250 J, and more preferably above 300 J. There is no particular limit to its upper limit, but considering the manufacturing cost, practical needs, and balance with other properties, the 0℃ impact energy is generally below 400 J.

[0069] 2. Coating composition

[0070] In this invention, the cladding layer is preferably made of austenitic stainless steel. This provides corrosion resistance to the outer surface of the composite steel plate. There are no particular restrictions on the type of austenitic stainless steel, but it is preferred to meet the standard GB / T 3274-2017 Hot-rolled Stainless Steel Plates and Strips. Examples include SUS304L, SUS316L, and SUS317L. SUS304L is preferred due to its low carbon content, excellent resistance to intergranular corrosion, and the fact that it does not require heat treatment after welding.

[0071] The chemical composition of the austenitic stainless steel, by mass%, contains: 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; preferably, it contains 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%, with the balance being Fe and unavoidable impurities.

[0072] The design principles for the optimal chemical composition of clad steel will be explained below.

[0073] C: During the thermal runaway process of composite board manufacturing, C precipitates at grain boundaries in the form of carbides, which degrades corrosion resistance. Therefore, it is advisable to keep the C content below 0.08%, preferably above 0.02% and below 0.045%.

[0074] Si: Si is a deoxidizing element. However, if the Si content is too low, the effect will be insufficient. If it exceeds 1.0%, the corrosion resistance will deteriorate. The preferred Si content is 0.30% or higher and 0.60% or lower.

[0075] Mn: Mn can improve the hardenability of steel and is a key element in the formation of austenitic structure. However, if the Mn content is too high, it will affect the toughness, weldability, HIC resistance and corrosion resistance of steel. Therefore, the Mn content in the coating should be below 2.0%, preferably above 1.0% and below 1.4%.

[0076] Cr: Cr can improve the hardenability of steel, as well as its strength and hardness. Furthermore, Cr can prevent or slow down the precipitation and aggregation of carbides during tempering, improving the tempering stability of steel. It can also form a highly protective oxide film on the metal surface, improving resistance to pitting corrosion and intergranular corrosion. Additionally, Cr, when added in combination with Ni, can improve resistance to stress corrosion cracking in acidic environments. Therefore, the Cr content should preferably be above 18.0%, and from the perspective of balancing with other alloys such as Ni, the Cr content should preferably be below 20.0%, preferably below 19.0%.

[0077] Ni: Ni is an element that improves corrosion resistance, especially significantly enhancing resistance to stress corrosion cracking in acidic environments. However, due to the high price of Ni, considering the balance between corrosion resistance and cost, the Ni content is preferably above 8.0% and below 11.0%, and more preferably below 9.0%.

[0078] P and S: P and S are impurity elements that segregate at grain boundaries and degrade corrosion resistance. Therefore, the P content in the coating is preferably 0.035% or less, more preferably 0.030% or less. The S content in the coating is preferably 0.03% or less, more preferably 0.01% or less.

[0079] In this invention, depending on the need, appropriate amounts of elements such as Cu, Mo, Al, Ti, and N can also be added to the cladding steel.

[0080] In this invention, the yield strength of the cladding steel is preferably above 400 MPa, with no particular upper limit. However, considering manufacturing costs, practical requirements, and the balance with other properties, the yield strength is generally below 500 MPa. The tensile strength is preferably above 650 MPa, with no particular upper limit. However, considering manufacturing costs, practical requirements, and the balance with other properties, the tensile strength is preferably below 750 MPa.

[0081] 3. Composite steel plate structure

[0082] In the composite steel plate of the present invention, the thickness of the base layer made of carbon steel or low alloy steel is approximately 40-110 mm. The thickness of the cladding layer made of austenitic stainless steel is approximately 5-20 mm. The thickness of the composite steel plate is 60-120 mm, preferably 70-100 mm.

[0083] It should be noted that, at the interface between the carbon steel base layer and the stainless steel cladding, due to the difference in elemental content between the two layers in the thickness direction, elements with higher content diffuse towards the side with lower content, thus forming a transition layer approximately 5-20 μm deep on both sides. The average composition of each element in the transition layer is intermediate between the corresponding base layer and cladding components, exhibiting a gradient transition. For example, alloying elements with a higher mass percentage on the stainless steel cladding side, such as Cr and Ni, diffuse towards the base layer, while carbon elements with a higher mass percentage in the base layer diffuse towards the cladding side. The average composition of C, Cr, and Ni elements in the transition layer is intermediate between that in the stainless steel cladding and the carbon steel base layer. In this invention, the average thickness of the diffusion layer is typically between 15 and 25 μm.

[0084] The bonding strength at the interface between the base layer and the cover layer is above 500 MPa, preferably above 550 MPa, and more preferably above 600 MPa. There is no particular upper limit, but considering manufacturing costs, practical needs, and the balance with other performance characteristics, it is generally below 650 MPa.

[0085] The composite steel plates of Examples 1-7 of this invention are obtained by performing the following steps in sequence:

[0086] 1) Billet assembly: Select stainless steel and carbon steel with the required thickness and composition, respectively, and process them to the required dimensions. Then remove the oxide scale and dirt from the surface of the stainless steel and carbon steel, and then stack the stainless steel and carbon steel together and perform vacuum sealing welding to bond the stainless steel and carbon steel on four sides to obtain a composite billet. The composite billet is then subjected to vacuum treatment with a vacuum degree of less than 0.01 Pa. The composition and thickness of the selected stainless steel and carbon steel are shown in Table 1.

[0087] In this invention, two composite blanks with the above-mentioned double-layer structure can also be stacked facing each other in the thickness direction with the exposed stainless steel side facing each other, and then sealed around the perimeter to form a four-layer composite blank.

[0088] 2) Heating: The composite billet is heated to 1150-1250℃, and the total time in the furnace is 300-400 minutes. After heating, it is taken out of the furnace and rolled into the rolling mill.

[0089] 3) Rolling: This includes a first rolling and a second rolling, preferably only two rolling processes. Among them,

[0090] The first rolling process uses longitudinal rolling, with the longitudinal direction being the length of the composite billet. During rolling, the upper layer is stainless steel and the lower layer is carbon steel; the billet is then cooled after rolling.

[0091] The composite billet is then flipped over and reheated to 1150-1250℃ for 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.

[0092] 4) Cooling: First, compressed air or water is used to cool the rolled steel plate in the first stage, and then the rolled steel plate is stacked to obtain the composite steel plate.

[0093] In this invention, the selection of heating temperature takes into account the physical properties of austenitic stainless steel and carbon steel, which can dissolve all or part of the carbides of niobium, titanium, etc., and make the segregated alloying elements that may exist in carbon steel and austenitic stainless steel have a better uniform distribution, which is conducive to complete metallurgical bonding and ensures that a uniform austenitic structure is obtained without the growth of austenitic grains.

[0094] In this invention, the atoms at the interface between stainless steel and carbon steel diffuse into each other through rolling, forming a diffusion layer between the heterogeneous cladding and the base layer, thereby obtaining the desired composite steel plate with excellent bonding strength.

[0095] In step 2, preferably, during the first rolling, the composite billet is rolled to 1.5-2.5 times the thickness of the finished product, more preferably about 1.5-2.0 times.

[0096] Preferably, the composite billet is rolled in multiple passes during the first rolling, with a reduction rate of 5-10% per pass. A reduction rate below 10% can prevent warping of the steel plate during rolling, while multiple passes with a reduction rate above 5% can ensure thorough mixing of the microstructure of the carbon steel-stainless steel composite interface, thereby giving the steel plate composite interface good bonding strength. Preferably, the second rolling is a widening rolling, specifically, the composite billet is rolled in multiple passes, with a reduction rate of 10-20% per pass. The final rolling temperature is 850-1000℃, preferably 870-950℃, and more preferably 890-920℃.

[0097] The second rolling process employs a larger reduction rate to provide sufficient energy for deformation storage, ensuring plastic deformation of the material and promoting grain breakage and recrystallization. Specifically, under sufficient compressive stress, the metal atoms at the composite material interface diffuse and interpenetrate, enabling the interface to achieve atomic bonding and sufficient deformation. This allows for several recrystallization processes at the interface, thereby improving the impact resistance of the composite steel plate.

[0098] The final rolling temperature is controlled at 850-1000℃ in order to roll within a range that is higher than the carbide precipitation temperature of the cladding stainless steel and close to the recrystallization temperature of the base austenite.

[0099] In step 4, preferably, during the first stage of cooling, the initial cooling temperature is 830-1000℃, more preferably 830-880℃, the cooling rate is 5-20℃ / s, more preferably 10-20℃ / s, and the final cooling temperature is 500-650℃, more preferably 500-580℃. Subsequently, the rolled steel plates are removed from the production line and placed in a slow cooling pit for centralized stacking. After cooling to below 200℃, they are removed from the pit and stacked.

[0100] More preferably, the rolled steel plate is cooled in the first stage by water, wherein the ratio of water volume between the upper and lower layers is approximately 1:2 to 1:4, and the most preferred ratio is approximately 1:3.

[0101] Example

[0102] The embodiments of the present invention will be described in more detail below. However, the following embodiments are merely specific descriptions of the embodiments of the present invention and should not be considered as any limitation on the scope of the present invention. In addition, unless otherwise stated, all percentages used in this specification are by mass.

[0103] Example 1

[0104] Carbon steel Q355C (thickness: 280mm) produced by Baoshan Iron & Steel Co., Ltd. was used as the base layer steel billet, and stainless steel SUS304L (00Cr19Ni10; thickness: 50mm) produced by Baoshan Iron & Steel Co., Ltd. was used as the cladding steel billet. The chemical compositions of the carbon steel and stainless steel are shown in Table 1 below.

[0105] The composite preform was processed under the conditions shown in Table 2. Details are as follows.

[0106] After surface machining to remove oxide scale and ensure surface roughness and flatness, the coating is bonded to the four sides of the base layer using vacuum sealing welding. The bonding surface is then vacuumed to ensure a vacuum degree of less than 0.01 Pa, resulting in a final composite blank thickness of 330 mm.

[0107] The composite billet was then heated and rolled. In the first rolling pass, the upper layer of the composite billet was made of stainless steel and the lower layer of carbon steel. The billet was heated in a furnace to 1220°C for a total furnace time of 355 minutes. After heating, it was removed from the furnace and sent to a rolling mill. During the rolling process, the reduction rate was controlled at approximately 8% per pass, ultimately rolling to a thickness of 220 mm, completing the first rolling pass. The composite billet was then subjected to a stacking cooling operation.

[0108] After cooling, the composite billet is flipped so that the upper layer is carbon steel and the lower layer is stainless steel. It is then heated in a furnace at the same temperature as the first rolling, with a total furnace time of 300 minutes. After heating, it is rolled, with the reduction rate controlled at about 13% per pass, until the composite billet is rolled to about 80 mm. The final rolling temperature is controlled at 915℃.

[0109] After rolling, the steel plates are cooled with water. The ratio of water volume between the top and bottom layers is approximately 1:3, the cooling rate is approximately 18℃ / s, and the final cooling temperature is 612℃. They are then removed from the production line and placed in a slow-cooling pit for centralized storage. Once cooled to below 200℃, they are removed from the pit and stored. The various properties of the resulting composite steel plates were measured and are shown in Table 3 below.

[0110] The grain size rating is conducted as follows: according to GB / T 6394-2017 "Method for Determination of Average Grain Size of Metals", the intercept point method is used to rate the grain size of ferrite structures in stainless steel and carbon steel respectively.

[0111] The yield strength and tensile strength of carbon steel and stainless steel were measured in accordance with GB / T 6396-2008 "Mechanical and Technological Properties of Composite Steel Plates" and GB / T 228-2010 "Metallic Materials - Tensile Testing at Room Temperature".

[0112] The impact energy (KV2) (longitudinal) at 0℃ for carbon steel and stainless steel was measured in accordance with GB / T 6396-2008 "Mechanical and Technological Properties of Composite Steel Plates" and GB / T 229-2020 "Charpy Pendulum Impact Test Method for Metallic Materials".

[0113] The bond strength between the cladding and the base layer of the composite steel plate was measured in accordance with GB / T 6396-2008 "Methods for Mechanical and Technological Properties of Composite Steel Plates".

[0114] The flaw detection of composite steel plates shall be carried out in accordance with the new Tables 6 and 7 of Amendment No. 1 to NB / T 47013.3-2015 "Non-destructive testing of pressure equipment - Part 3: Ultrasonic testing" for Class I standards.

[0115] Intergranular corrosion of stainless steel is measured in accordance with GB / T4334-2000 "Corrosion of metals and alloys - Test method for intergranular corrosion of stainless steel" (Method E).

[0116] Examples 2-7 and Comparative Examples 1-5

[0117] Following essentially the same steps as in Example 1, composite steel plates of Examples 2-7 and Comparative Examples 1-5 were prepared using the steels shown in Table 1 below, and their properties were measured. Specific process conditions and performance results are shown in Tables 2 and 3 below.

[0118] Table 1 shows the billet composition (in wt.%) and thickness of the selected stainless steel and carbon steel, wherein the balance of the stainless steel and carbon steel components is Fe and unavoidable impurities other than P and S.

[0119] Table 1

[0120]

[0121] Table 2 shows the specific process parameters used to obtain the composite steel plates of Examples 1-7 and Comparative Examples 1-5, as well as the finished thickness of the composite steel plates.

[0122] The meaning of the composite steel plate numbers shown in Table 2 is as follows: for example, if the composite steel plate number is 1-3, it refers to a composite steel plate made of stainless steel (number 1) and carbon steel (number 3) from Table 1.

[0123] Table 3 shows the metallographic structure and mechanical properties of carbon steel and stainless steel in the composite steel plates of Examples 1-7 and Comparative Examples 1-5, as well as the thickness of the diffusion layer.

[0124] In the composite steel plate, the stainless steel is composed of austenite, while the carbon steel has a microstructure of polygonal ferrite and bainite, with a small amount of Mao islands. The volume fraction of polygonal ferrite is approximately 40-60%, bainite approximately 30-50%, and Mao islands approximately 5-10%. Due to the significant difference in Fe element composition between the base layer and the cladding layer, the Fe element content on both sides of the interface before composite bonding should exhibit a step-like change. However, after the steel plate of this invention undergoes rolling and composite bonding, the Fe element content near the composite interface shows a continuous changing trend, indicating Fe diffusion from carbon steel to stainless steel. This results in the Fe element content in the carbon steel layer near the interface being lower than that in the carbon steel matrix, and the Fe element content in the stainless steel layer being higher than that in the stainless steel matrix. Therefore, the diffusion layer is defined as the length of the transitional region of Fe element content near the composite interface. In this invention, the location where the Fe content in the carbon steel is 5% lower than that in the matrix is ​​defined as the first boundary of the diffusion layer, and the location where the Fe content in the stainless steel is 5% higher than that in the matrix is ​​defined as the second boundary of the diffusion layer. The distance between the first and second boundaries is defined as the thickness of the diffusion layer.

[0125] Figure 1 This image shows a photograph of the interface of the composite steel plate in Embodiment 1 of the present invention. Figure 2 Showing the Figure 1 The image shows the results of a Fe element line scan of the steel plate along the arrow direction. The element count rate (in CPS) indicates the content of the element at that location, and the line scan results reflect the content variation trend of the element along the straight line. The results show that the average thickness of the diffusion layer of the composite steel plate in Example 1 is approximately 18 μm. In this invention, the average thickness of the diffusion layer is the average value obtained by performing Fe element line scans at 10 different locations in the field of view.

[0126]

[0127]

[0128] As shown in Table 3, the total thickness of the composite steel plate obtained using the preparation process of this invention is 60-120 mm, with the diffusion layer thickness between 17-21 μm. The stainless steel in the steel plate has an austenitic microstructure with a grain size of 4-6, a yield strength of 400-600 MPa, and a tensile strength of 650-850 MPa. The carbon steel has a polygonal ferrite + bainite + a small amount of Mao islands, with a ferrite grain size of 5-7, a yield strength of 300-500 MPa, a tensile strength of 500-600 MPa, and an impact energy of 200-400 J at 0℃. No defects were detected at the junction (diffusion layer) of the cladding and base layer in the composite steel plates of Examples 1-7, and the flaw detection pass rate was 100%. No intergranular corrosion was observed in the stainless steel in Examples 1-7.

[0129] Comparative Example 1 did not meet the requirements for grain size, intergranular corrosion, and bond strength because the first and second rolling reduction rates and cooling rates used were not within the ranges specified in this invention.

[0130] In Comparative Example 2, the bonding strength could not meet the requirements because the second rolling reduction rate used was not within the range specified in this invention.

[0131] Comparative Example 3, because the final rolling temperature, final cooling temperature, and cooling rate used were not within the ranges specified in this invention, could not meet the requirements for intergranular corrosion.

[0132] Comparative Example 4 did not meet the requirements for impact performance because the final rolling temperature, initial cooling temperature, and final cooling temperature used were not within the ranges specified in this invention.

[0133] Comparative Example 5 did not meet the requirements for grain size and bond strength because the rolling reduction rate used was not within the range specified in this invention.

[0134] Through the preparation method of the present invention, especially the control of rolling and cooling processes, the bonding strength of the interface between the base layer and the cladding layer in the steel plate is 550-650 MPa, and the composite steel plate exhibits good plate shape, excellent mechanical properties and corrosion resistance.

[0135] The preparation method of the present invention is simple and easy to implement, and is very suitable for mass production of heterogeneous composite steel plates for hydropower.

[0136] It should be noted that all technical features described in this invention can be freely combined or integrated in any manner, unless they contradict each other. Various modifications and variations can be made to this invention without departing from its scope, as will be apparent to those skilled in the art. For example, features shown or described as part of one embodiment can be used with another embodiment to produce yet another embodiment. Therefore, this invention is intended to cover these modifications that fall within the scope of the appended claims and their equivalents.

Claims

1. A composite steel plate comprising a base layer and a cladding layer over the base layer, wherein, The thickness of the composite steel plate is 60-120 mm. The cladding is made of stainless steel, the chemical composition of which, by mass percentage, contains: 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 base layer is made of carbon steel, and the chemical composition of the carbon steel, by mass percentage, contains: 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%, with the balance being Fe and unavoidable impurities. The cladding has an austenitic structure, and the base layer has polygonal ferrite, bainite, and a Mao island structure with a volume fraction less than either of the polygonal ferrite and bainite; a diffusion layer exists between the base layer and the cladding, the average thickness of the diffusion layer is 15-25 μm, and the bond strength between the base layer and the cladding is above 500 MPa.

2. The composite steel plate according to claim 1, wherein, The chemical composition of the coating, by mass percentage, contains: 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%, with the balance being Fe and unavoidable impurities; and / or The chemical composition of the base layer, by mass percentage, contains: 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%, with the balance being Fe and unavoidable impurities.

3. The composite steel plate according to claim 1, wherein, In the base layer, the volume fraction of ferrite is 40-60%, the volume fraction of bainite is 30-50%, the volume fraction of Mao islands is 5-10%; and / or, the ferrite grain size is grade 5 or above.

4. The composite steel plate according to any one of claims 1-3, wherein, The base layer has a yield strength of 300-500 MPa, a tensile strength of 500-600 MPa, and an impact energy of 200-400 J at 0℃; and / or, The coating has a yield strength of 400-500 MPa, a tensile strength of 650-750 MPa, and an impact energy of 200-300 J at 0℃.

5. The composite steel plate according to any one of claims 1-3, wherein, The bonding strength between the coating and the base layer is 550-650 MPa.

6. A method for manufacturing a composite steel plate according to any one of claims 1-5, comprising performing the following steps in sequence: 1) Assembly: Prepare stainless steel and carbon steel, process the dimensions, stack the stainless steel and carbon steel together, and perform vacuum sealing welding to obtain a composite billet. Perform vacuum treatment on the composite billet to make the vacuum degree below 0.01 Pa. 2) Heating: The composite blank is heated to 1150-1250℃; 3) Rolling: This includes the first rolling and the second rolling, wherein, The first rolling process is a longitudinal rolling process, in which the upper layer is the stainless steel and the lower layer is the carbon steel. The composite billet is then flipped over so that the upper layer is the carbon steel and the lower layer is the stainless steel, and then rolled a second time to obtain the rolled steel plate. 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 by stacking to obtain the composite steel plate; The first rolling process is a multi-pass rolling process, with a reduction rate of 5-10% per pass.

7. The method according to claim 6, wherein, The second rolling process is a multi-pass rolling process, with a reduction rate of 10-20% per pass and a final rolling temperature of 850-1000℃.

8. The method according to claim 6 or 7, wherein, The initial cooling temperature for the first stage is 830-1000℃, the cooling rate is 5-20℃ / s, and the final cooling temperature is 500-650℃.

9. The method according to claim 8, wherein, The initial cooling temperature for the first stage is 830-880℃, the cooling rate is 10-20℃ / s, and the final cooling temperature is 500-580℃.

10. The method according to claim 6, wherein, The cooling step involves using water to cool the rolled steel plate in the first stage, with a water volume ratio of 1:2 to 1:

4. The steel plate is then placed in a slow cooling pit and stacked until it cools to below 200°C before being removed from the pit and stacked.

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