Corrosion-resistant rolled clad steel plate for medium-temperature equipment and manufacturing method therefor

Abstract

A corrosion-resistant rolled clad steel plate for medium-temperature equipment and a manufacturing method therefor. The clad steel plate comprises a base material, a cladding layer, and a transition layer between the base material and the cladding layer; in percentage by weight, the base material is prepared from the following components: C: 0.01%-0.15%, Si: 0.2%-0.7%, Mn: 0.1%-1.70%, Ti: 0.006%-0.025%, Als: 0.004%-0.015%, Nb: 0.0010%-0.050%, V: 0.15%-0.50%, B: 0.0005%-0.0050% N: 0.0015%-0.0050%, P ≤ 0.0165%, S ≤ 0.010%, O ≤ 0.005%, and the balance being Fe and other unavoidable impurities. By preventing or reducing the diffusion of carbon on a carbon steel side of the base material at an interface of the clad steel plate, the possibility of failure caused by intergranular corrosion of stainless steel at the cladding layer side is reduced, the corrosion resistance and the safety of the rolled clad steel plate are comprehensively improved, and the present invention is suitable for 300-450°C medium-temperature equipment in energy production fields such as petroleum refining and the coal chemical industry.

Description

A corrosion-resistant rolled composite steel plate for medium-temperature equipment and a manufacturing method thereof Technical Field

[0001] The present invention relates to corrosion-resistant steel and a manufacturing method thereof, and in particular to a corrosion-resistant rolled composite steel plate for medium-temperature equipment and a manufacturing method thereof, which is suitable for medium-temperature equipment with a temperature of 300 to 450°C in energy manufacturing fields such as petroleum refining and coal chemical industry. Background Art

[0002] Composite steel plates composed of low-alloy steel and various corrosion-resistant materials are frequently used in industries such as crude oil processing and the energy and chemical industry due to their high mechanical strength and strong corrosion resistance. With the rapid development of the national economy, the demand for various energy-related raw materials, such as petroleum and fertilizers, is increasing. To improve energy production efficiency, energy production and processing equipment must be larger, more highly parameterized (higher temperatures and pressures), and lighter. This demand for equipment upgrades places higher demands on the performance of the steel plates used in manufacturing equipment, requiring not only higher strength and hardness, but also excellent toughness, formability, and corrosion resistance.

[0003] Traditionally, composite steel plates are produced using explosive lamination. This process, however, poses significant challenges: the use of explosives for explosive lamination produces large amounts of harmful gases, adversely impacting the environment and making it non-compliant with environmental standards. Furthermore, the bond strength at the interface of the explosively laminated steel plates is low, sometimes failing to meet the required bond strength of 97% or higher. Using this type of steel in the manufacture of high-temperature, high-pressure, and large-scale vessels presents significant safety risks. If the composite plate joints separate, equipment failure may occur, significantly impacting the safety of personnel and property.

[0004] Compared to explosively bonded steel plates, rolled composite steel plates have a significantly higher bonding rate of over 99%, and the production process is more environmentally friendly than explosively bonded steel plates. Therefore, in recent years, the production scale and engineering use of rolled composite steel plates have been expanding year by year.

[0005] However, whether it is explosive composite steel plate or rolled composite steel plate, a certain transition layer will be produced at the interface between carbon steel and other corrosion-resistant materials. The composition of the transition layer is relatively complex. Especially when working for a long time under medium temperature conditions of 300-450℃, the alloy elements in the base carbon steel will migrate and diffuse to the composite material through the transition layer, causing the microstructure of the corrosion-resistant material to change, thereby adversely affecting the corrosion resistance of the composite corrosion-resistant material.

[0006] Therefore, how to reasonably design the alloy composition and production process of the carbon steel base layer, control the microstructure and precipitation phase of the carbon steel material, minimize the diffusion behavior of elements in the base carbon steel material, especially the carbon element under long-term high temperature conditions, and avoid the negative impact of the increase of carbon elements in the corrosion-resistant material, which leads to the formation of a large amount of carbides, resulting in intergranular corrosion of stainless steel and a decrease in the corrosion resistance of stainless steel, has become an urgent requirement for improving the comprehensive performance of rolled composite steel plates.

[0007] Existing technology, such as Chinese Patent Publication No. CN103934266A, discloses a "Method for Preparing a Copper / Aluminum Composite Strip with Reduced Interface Layer Thickness." This invention uses soft T2 copper strip as the copper raw material; soft aluminum-silicon alloy strip with a silicon content of 3.25-8.89% by mass as the silicon-aluminum alloy raw material; and a thickness ratio of 1:1 between the copper raw material and the silicon-aluminum alloy raw material. The strip is then rolled and laminated to form a rolled composite strip. The strip then undergoes a diffusion heat treatment, heating the strip to 200-400°C under an inert gas atmosphere, holding the temperature for 0.5-2 hours, and then cooling the strip to room temperature. The primary purpose of this method is to produce a copper / aluminum composite strip with high dimensional accuracy, a small composite interface, a strong bond, and good surface quality, while saving 50% of the copper material. Because the materials used in this invention are pure copper and aluminum-silicon alloy, whose properties differ significantly from those of low-alloy steel and stainless steel, the method for reducing the interface layer thickness described in this patent application is not suitable for rolling composite steel sheets made of low-alloy steel and stainless steel.

[0008] Chinese Patent Publication No. CN108116006A discloses "A Super Austenitic Stainless Steel Rolled Composite Steel Plate and Its Manufacturing Method." This patent application combines carbon steel and austenitic stainless steel slabs, followed by rolling. The resulting rolled composite steel plate consists of a carbon steel base layer and an austenitic cladding layer. However, the materials disclosed in this patent application do not address the impact of element diffusion at the interface on the corrosion resistance of the composite material.

[0009] Chinese Patent Publication No. CN108085585A discloses "A high-strength, corrosion-resistant composite patterned steel and its manufacturing method." The rolled composite steel plate comprises a carbon steel base layer and an austenitic stainless steel clad layer. The thickness of the composite plate is no more than 10 mm. The patent application directly combines carbon steel and austenitic stainless steel slabs, hot-rolls them, and coils them to form the rolled composite coils. While the patent application mentions the thickness of the transition layer, it does not address the diffusion of elements within the transition layer.

[0010] Based on the above patent applications, the existing rolled composite steel plate technology has given little consideration to the transition layer at the interface between the base layer and the composite material, especially the influence of the diffusion of carbon elements in the base carbon steel material through the interface to the composite material on the corrosion resistance of the composite material under long-term high temperature conditions. Therefore, the technical solutions of these patent applications have not taken targeted measures to reduce the possibility of weakening the corrosion resistance of stainless steel due to the precipitation of carbides at the interface of the composite material. Especially in the chemical and energy industries, the operating temperature of composite steel plate manufacturing equipment is often in the range of 300-450℃. If it is operated at high temperature for a long time, the elements at the material interface may diffuse again, especially the carbon elements on the carbon steel side will diffuse into the stainless steel composite layer, forming more carbides, which will further reduce the corrosion resistance of stainless steel and create accident hazards.

[0011] Summary of the Invention

[0012] The present invention aims to provide a corrosion-resistant rolled composite steel plate for medium-temperature equipment and a method for manufacturing the same. By blocking or reducing the diffusion of carbon elements from the carbon steel side of the substrate at the composite steel plate interface, the possibility of intergranular corrosion and failure of the stainless steel on the composite side is reduced, thereby comprehensively improving the corrosion resistance and safety of the rolled composite steel plate. The plate is suitable for medium-temperature equipment operating at 300-450°C in energy manufacturing fields such as petroleum refining and coal chemical industry. It should be understood that "medium temperature" in this application refers to a temperature range of 300-450°C.

[0013] To achieve the above object, the technical solution of the present invention is:

[0014] The composite steel plate of the present invention forms a large number of finely dispersed MC (M represents one or more of V, Nb, and Ti) precipitation phases with stable high-temperature performance in the base carbon steel. These stable carbide precipitation phases can fix the carbon element in the base carbon steel, thereby preventing the carbon element in the base from diffusing and migrating to the transition layer near the bonding interface of the rolled composite steel plate under working conditions of 300 to 450°C, resulting in an increase in the carbon element concentration in the stainless steel near the interface and the generation of a large number of carbide precipitation phases at the stainless steel grain boundaries, inducing intergranular corrosion, and ultimately weakening or even losing the corrosion protection function of the composite material.

[0015] Specifically, the corrosion-resistant rolled composite steel plate for medium-temperature equipment of the present invention is formed by composite rolling of carbon steel as a base material and stainless steel as a clad layer. Therefore, the corrosion-resistant rolled composite steel plate for medium-temperature equipment of the present invention comprises a base material layer, a clad layer, and a transition layer between the base material layer and the clad layer, wherein the composition weight percentage of the base material layer is as follows: C: 0.01% to 0.15%, Si: 0.20% to 0.70%, Mn: 0.10% to 1.70%, Ti: 0.006% to 0.025%, Als: 0.004% to 0.015%, Nb: 0.0010% to 0.050%, V: 0.15% to 0.50%, B: 0.0005% to 0.0050%, N: 0.0015% to 0.0050%, P≤0.0165%, S≤0.010%, O≤0.005%, and the balance includes Fe and other unavoidable impurities;

[0016] The surface microstructure of the substrate layer is 60-80% by area of ​​bainite + 20-40% by area of ​​ferrite + MC precipitation phase; the central microstructure of the substrate layer is 30-40% by area of ​​bainite and 60-70% by area of ​​ferrite + MC precipitation phase; wherein M represents one or more of Nb, V, and Ti, the average major axis length of the MC precipitation phase is less than 500 nm, and the volume density of the MC precipitation phase with a major axis size less than 500 nm is greater than 2.0×10 4 Pieces / mm 3 ;

[0017] The microstructure of the composite layer contains MC precipitates; wherein M represents one or more of Cr, Nb, Mo, and Ti, the average major axis length of the MC precipitates is less than 700 nm, and the volume density of the MC precipitates is less than 1.0×10 4 Pieces / mm 3 .

[0018] Furthermore, the balance of the base material layer is Fe and other inevitable impurities.

[0019] Furthermore, the substrate layer further contains at least one of the following chemical elements: <Ca≤0.0055%、0<Ni≤0.65%、0<Cu≤0.55%。

[0020] Preferably, the stainless steel described herein can be any of various stainless steels well known in the art, including but not limited to austenitic stainless steel, ferritic stainless steel, duplex stainless steel, or super stainless steel. The present invention can be implemented using readily available stainless steel products, or stainless steel can be prepared independently for use as the composite casting slab of the present invention.

[0021] In some embodiments, the weight percentages of the components of the stainless steel are: C: 0.01-0.05%; Si: 0.50-0.80%; Mn: 1.10-1.50%; Ni: 6.5-9.5%; Cr: 15.0-22.0%; N: 0.060-0.120%; P: ≤0.015%; S: ≤0.005%; O: ≤0.005%; the balance is Fe and other unavoidable impurities.

[0022] Preferably, in the base material layer of the composite steel plate, the MC precipitated phases with a major axis size of less than 500 nm account for more than 50% of the total MC precipitated phases.

[0023] Preferably, in the composite layer of the composite steel plate, the MC precipitated phase with a major axis size of less than 700 nm accounts for more than 50% of the total MC precipitated phase.

[0024] Furthermore, a very small amount of σ phase exists in the composite layer microstructure of the composite steel plate, and the σ phase ratio (area ratio) is ≤1.5%. Preferably, the σ phase ratio near the transition layer is ≤0.5%.

[0025] Preferably, the thickness of the base layer of the composite steel plate is ≥6 mm, and the thickness of the cladding layer is ≥2 mm; preferably, the thickness of the transition layer is ≤230 μm. In some embodiments, the thickness of the base layer is 6 to 200 mm. In some embodiments, the thickness of the base layer is 10 to 200 mm. In some embodiments, the thickness of the cladding layer is 2 to 20 mm. In some embodiments, the thickness of the transition layer is 2 to 230 μm. In some embodiments, the thickness of the transition layer is 2 to 150 μm. In some embodiments, the thickness of the transition layer is 2 to 50 μm. Herein, the thickness of the base layer and cladding layer is measured in accordance with GB / T 6396-2008, and the thickness of the transition layer is measured in accordance with GB / T 6462-2005.

[0026] The substrate layer of the composite steel plate of the present invention has a yield strength of ≥325 MPa, a tensile strength of ≥430 MPa, a cross-sectional shrinkage rate of ≥60%, an elongation of ≥19%, an impact absorption energy of ≥100 J at -30°C, and an impact absorption energy of ≥45 J at -50°C; the grain size grade of the substrate is not lower than grade 7.0; and the shear strength at the composite interface of the composite steel plate is ≥310 MPa.

[0027] In some embodiments, the yield strength of the substrate layer of the composite steel plate of the present invention is ≥360 MPa. In some embodiments, the yield strength of the substrate layer of the composite steel plate of the present invention is ≥380 MPa. In some embodiments, the yield strength of the substrate layer of the composite steel plate of the present invention is between 380 and 500 MPa.

[0028] In some embodiments, the tensile strength of the substrate layer of the composite steel plate of the present invention is ≥450 MPa. In some embodiments, the tensile strength of the substrate layer of the composite steel plate of the present invention is ≥480 MPa. In some embodiments, the tensile strength of the substrate layer of the composite steel plate of the present invention is between 480 and 600 MPa.

[0029] In some embodiments, the cross-sectional shrinkage of the substrate layer of the composite steel plate of the present invention is ≥70%. In some embodiments, the cross-sectional shrinkage of the substrate layer of the composite steel plate of the present invention is between 60% and 80% or between 70% and 80%.

[0030] In some embodiments, the elongation of the substrate layer of the composite steel plate of the present invention is ≥ 21%. In some embodiments, the elongation of the substrate layer of the composite steel plate of the present invention is between 19% and 25%.

[0031] In some embodiments, the -30°C impact absorbed energy of the substrate layer of the composite steel plate of the present invention is ≥ 150 J. In some embodiments, the -30°C impact absorbed energy of the substrate layer of the composite steel plate of the present invention is ≥ 180 J. In some embodiments, the -30°C impact absorbed energy of the substrate layer of the composite steel plate of the present invention is between 100 and 240 J, such as between 180 and 240 J.

[0032] In some embodiments, the -50°C impact absorbed energy of the substrate layer of the composite steel plate of the present invention is ≥80 J. In some embodiments, the -50°C impact absorbed energy of the substrate layer of the composite steel plate of the present invention is ≥100 J. In some embodiments, the -50°C impact absorbed energy of the substrate layer of the composite steel plate of the present invention is ≥140 J. In some embodiments, the -50°C impact absorbed energy of the substrate layer of the composite steel plate of the present invention is between 45 and 180 J, or between 80 and 180 J, or between 100 and 180 J, or between 140 and 180 J.

[0033] In some embodiments, the shear strength at the composite interface of the substrate layer of the composite steel plate of the present invention is ≥330 MPa. In some embodiments, the shear strength at the composite interface of the substrate layer of the composite steel plate of the present invention is between 310 and 420 MPa.

[0034] In the composition design of the composite steel plate substrate layer of the present invention:

[0035] C: C is an important strengthening element. To ensure that carbon steel plates have appropriate strength, the mass percentage of C in the steel must be controlled to be greater than or equal to 0.01%. Furthermore, to improve the weldability of the base carbon steel material, the C content in the carbon steel must be kept below a certain level. Furthermore, if the carbon content in the carbon steel substrate layer is high, the difference in carbon concentration gradients between the two materials can lead to large-scale carbon migration at the interface between the substrate layer and the stainless steel cladding, causing intergranular corrosion of the stainless steel. Therefore, the upper limit of carbon content is controlled to 0.15%. Accordingly, the present invention controls the C content to between 0.01% and 0.15%.

[0036] Si: Similar to carbon, Si is a commonly used strengthening element in low-alloy steels. Adding a certain amount of Si to steel can improve its strength. Si also has a deoxidizing function, assisting in deoxidation during the smelting process. Most of the SiO2 products formed after deoxidation are removed from the slag and have no impact on the steel's properties. Furthermore, adding a certain amount of Si to steel can reduce the solubility of carbon in austenite, thereby promoting carbon desolvation, promoting the formation of MC precipitation phases, and fixing carbon in the steel, preventing large-scale carbon diffusion. Therefore, a certain amount of Si is required in steel. However, excessive Si can reduce the steel's weldability. Because Si binds more strongly with oxygen than iron does, it easily forms low-melting-point silicates during welding. These increase the fluidity of the slag and molten metal, causing spattering and affecting the quality of the weld joint. Therefore, controlling the Si content within a certain range can help improve the weldability of steel. Therefore, the present invention controls the Si content to 0.20% to 0.70%.

[0037] Mn: Mn is also an important strengthening element that can effectively improve the strength of the base steel plate. At the same time, Mn is an effective austenite stabilizing element. Adding a certain amount of Mn can increase the hardenability of steel. Therefore, to ensure the strength of steel, the lower limit of Mn is 0.1%. However, Mn is very prone to central segregation in steel. Excessive addition of Mn will cause severe segregation at the center of the thickness of the steel plate, reducing the low-temperature toughness of the core of the steel plate. Therefore, the upper limit of the Mn content can be limited to 1.7%. Based on this, the present invention controls the Mn content to 0.10% to 1.7%.

[0038] Ti: Ti is both a deoxidizing element and a strong carbide- and nitride-forming element. In the present invention, adding an appropriate amount of Ti can, on the one hand, form a stable MC precipitate phase in the steel, fixing the carbon in the substrate layer and preventing it from diffusing to the composite material interface. Simultaneously, some of the Ti helps fix the free nitrogen in the steel, forming a TiN precipitate phase. This TiN precipitate phase inhibits the growth of austenite grains, refines the grains, and simultaneously improves the strength and toughness of the steel sheet. Ti fixes some of the free nitrogen, helping to ensure that the boron in the steel remains in a free state, improving the steel's hardenability. Furthermore, Ti deoxidizes the steel to form Ti2O3 particles, which promote the formation of intragranular ferrite and enhance the steel's low-temperature impact toughness. However, when the Ti content in the steel is too high, the resulting TiN and Ti2O3 particles become larger, no longer hindering austenite growth and the formation of intragranular ferrite. Instead, they become a source of crack initiation, reducing the steel's low-temperature impact toughness. Therefore, the upper limit of Ti is 0.025%. Based on this, the present invention controls the Ti content to 0.006% to 0.025%.

[0039] Als: Als is primarily an important deoxidizing element, forming aluminum oxide inclusions. Furthermore, Al can combine with nitrogen in steel to form AlN precipitates, reducing the free nitrogen content in the steel. This prevents the combination of boron and nitrogen in the steel, promotes an increase in the free boron content in the steel, improves the hardenability of the steel, and contributes to the strength of the steel plate. However, when the acid-soluble aluminum content in the steel is too high, the size of its deoxidation product, aluminum oxide, increases. Furthermore, due to the strong adsorption capacity between aluminum oxide inclusions, large aluminum oxide clusters form. These can clog the water inlet during steelmaking, affecting the pouring process of molten steel. If these large clustered inclusions enter the steel plate, they will cause the steel plate to fail inclusion ratings and flaw detection tests. Therefore, the present invention controls the Als content to 0.004% to 0.015%.

[0040] Nb: Nb is a strong carbide-forming element. It combines with carbon in steel to form MC precipitates, stabilizing the carbon in the steel and preventing it from diffusing to the bonding interface. Furthermore, adding an appropriate amount of Nb can increase the recrystallization temperature of the steel. When the steel is recrystallized and rolled in the austenite region, the austenite grains do not grow rapidly, resulting in fine-grained steel with higher strength and toughness. However, excessive Nb content in the steel can result in larger MC precipitates, which in turn reduces the toughness of the steel plate and worsens the toughness of the weld heat-affected zone. Therefore, the present invention controls the Nb content to between 0.0010% and 0.050%.

[0041] V: As a strong carbide-forming element, adding an appropriate amount of V can combine with free carbon in steel to form a stable VC precipitate. This not only fixes the carbon in the steel, but also creates a fine VC precipitate that strengthens the steel plate through precipitation strengthening. However, if too much V is added, it will dissolve excessively in the ferrite matrix, weakening the interatomic bonding. Therefore, in this invention, the V content is controlled to 0.15% to 0.50%.

[0042] B: The addition of an appropriate amount of B element is to compensate for the decrease in the content of solid-solution carbon in the steel caused by the fixation of carbon elements by carbides, which leads to a decrease in the strength performance of the steel plate. If the B element in the steel exists in a free state, some of the B will preferentially occupy the grain boundary position, thereby improving the hardenability of the steel plate. The premise for it to work is that B must exist in the steel in a free state. Therefore, in order to prevent B in the steel from combining with oxygen and nitrogen to form boron oxide and boron nitride, the oxygen content and nitrogen content in the steel must be well controlled, which is also a key point in the design of the alloy composition. However, it should be noted that if excessive B is added to the steel, the excessive B will easily form a large amount of segregation at the grain boundaries, which will be detrimental to the toughness of the steel. Therefore, the present invention controls the B content to 0.0005% to 0.0050%.

[0043] N: In the present invention, appropriate amounts of Ti and Al are added during the alloy composition design. These two elements can form TiN and AlN precipitates with the N element in the steel. When these precipitates are small and dispersed, they can refine the austenite grains, thereby improving the strength and toughness of the steel plate. However, when the N content in the steel is too high, the excess N element will combine with the B element in the steel to form BN, consuming the free B element in the steel and reducing the effect of the B element on improving the strength of the steel plate. At the same time, excessive N elements will have an adverse effect on the toughness of the steel, especially when the solid solution N content exceeds 0.005%, the low-temperature toughness of the steel plate will be significantly reduced. Therefore, in the present invention, the N content is controlled to 0.0015-0.0050%.

[0044] The composite steel plate substrate layer of the present invention may further contain at least one of the following chemical elements: 0<Ca≤0.0055%, 0<Ni≤0.65%, 0<Cu≤0.55%.

[0045] Ca: Ca is a strong deoxidizing element. Adding an appropriate amount of calcium can control the oxygen content in steel, thereby preventing the oxidation of element B. However, when the Ca content in steel exceeds 0.0055%, it is easy to form large inclusions of calcium oxide and calcium sulfide in the steel, which is detrimental to the toughness of the steel. Therefore, the present invention controls the Ca content to 0 <Ca≤0.0055%。

[0046] Ni: Ni is a typical austenite stabilizing element that can significantly improve the hardenability of steel plates and also improve the low-temperature toughness of steel plates. However, the price of Ni is relatively high. Considering the production cost of steel, it is not advisable to add excessive Ni to steel. Therefore, the present invention controls the Ni content to 0 <Ni≤0.65%。

[0047] Cu: Adding an appropriate amount of Cu element helps to improve the strength of steel and improve the corrosion resistance of steel. However, when the Cu content in steel is too high, the steel plate is prone to high temperature brittleness at high temperatures. Therefore, the present invention controls the Cu content to 0 <Cu≤0.55%。

[0048] It should be noted that the addition of the above-mentioned Ca, Ni and Cu elements will increase the production cost of the material. Taking into account the performance and cost control, in the technical solution of the present invention, at least one of the above-mentioned elements can be preferably added.

[0049] The inevitable impurities in the composite steel plate substrate layer of the present invention are P≤0.0165%, S≤0.010%, and O≤0.005%.

[0050] P: P is a harmful element for most steel plates. This is primarily because it easily segregates toward grain boundaries, reducing the bonding strength between atoms there and causing temper brittleness in the steel. This temper brittleness is particularly exacerbated when the steel is operated at high temperatures. Therefore, the present invention controls the P content to ≤ 0.0165%.

[0051] S: The S element in steel easily combines with the Mn element in the steel, forming MnS inclusions. The higher the S content, the larger the MnS inclusions, which negatively impacts the strength and toughness of the steel plate. Excessive S content also leads to central segregation in the slab and can easily cause hot cracking. Therefore, the present invention controls the S content to ≤ 0.010%.

[0052] O: Oxygen in steel primarily exists as oxides. Excessive O content in steel indicates excessive oxides, potentially leading to the presence of large inclusions, which can affect the strength and toughness of the steel plate. Therefore, the present invention controls the O content to 0.005% or less.

[0053] The surface microstructure of the composite steel plate's substrate layer is composed of 60-80% bainite, 20-40% ferrite, and MC precipitates. The core microstructure of the substrate layer is composed of 30-40% bainite and 60-70% ferrite and MC precipitates. The substrate's surface microstructure is primarily bainite, supplemented by ferrite, primarily to ensure strength and toughness. This is particularly true when the steel plate is subjected to long-term service in the 300-450°C temperature range, where some of the free carbon elements that contribute to solid solution strengthening transform into a secondary phase, reducing surface strength. Bainite strengthening ensures that material strength remains above the minimum design strength. The increased ferrite content in the core microstructure further mitigates the reduced toughness caused by core segregation in the continuous casting ingot at half the plate thickness.

[0054] The substrate layer of the present invention comprises a stable MC precipitate phase, where M represents one or more of Nb, V, and Ti. The average major axis length of the MC precipitate phase is less than 500 nm. Excessively large MC precipitates can adversely affect the mechanical properties of the substrate layer, so their average major axis length must be controlled to less than 500 nm. In the present invention, carbide precipitates are formed as small as possible in the substrate layer to ensure that the carbide's carbon fixation function is fully utilized without adversely affecting the mechanical properties of the steel sheet due to carbide precipitation. Therefore, the ratio of MC precipitates with a major axis length of less than 500 nm to the total MC precipitates is limited to greater than 50%.

[0055] Preferably, the volume density of the MC precipitated phase with a major axis length of less than 500 nm in the substrate layer is greater than 2.0×10 4 Pieces / mm 3 In order to prevent the free carbon element from diffusing into the cladding material, the free carbon element in the steel is fixed by forming MC precipitation phase in the substrate layer of the present invention. Therefore, in the present invention, it is necessary to ensure that the volume density of MC is greater than 2.0×10 4 Pieces / mm 3 Only in this way can the carbon element in the steel be effectively controlled and the large-scale diffusion of free carbon elements to the stainless steel side be avoided.

[0056] The composite steel plate cladding material of the present invention comprises a stable MC precipitate phase, wherein M represents one or more of Cr, Mo, Nb, and Ti, and the average major axis length of the MC precipitate phase is less than 700 nm. Larger carbide sizes in the cladding material have a greater impact on the corrosion resistance of stainless steel. Therefore, it is desirable for the carbide precipitate phase in the cladding material to be as small as possible. However, during the production process of the cladding material and the rolled composite steel plate, some carbide precipitates are inevitably generated. To ensure the corrosion resistance of the rolled composite steel plate, the present invention requires that the average major axis length of the MC precipitate phase in the cladding material be less than 700 nm.

[0057] The volume density of the MC precipitated phase in the composite material of the present invention is less than 1.0×10 4 Pieces / mm 3 From the perspective of controlling the amount of carbide precipitation phase in the composite material, it is hoped that the amount of carbide is as small as possible, that is, the smaller the volume density is, the better. However, some carbides will inevitably form during the heat treatment of the composite steel plate. The volume density of the carbide is controlled to be less than 1.0×10 4 Pieces / mm 3 It can ensure the excellent corrosion resistance of the composite material.

[0058] Preferably, in the composite material of the present invention, the MC precipitates with a major axis length of less than 700 nm account for greater than 50% of the total MC precipitates. In the present invention, the "volume density" of the MC precipitates is measured according to the method described in the YB / T 5320-2006 standard. To minimize the number of carbide precipitates in the composite material and increase the corrosion resistance of the composite material, the present invention limits the size and number of MC precipitates, namely, the number of MC precipitates with a major axis length of less than 700 nm accounts for greater than 50% of the total MC precipitates. Exceeding the limit on the number and size of carbide precipitates significantly reduces the corrosion resistance of the composite material, affecting the safe service performance of the rolled composite steel plate.

[0059] The cladding layer described in the present invention is made of stainless steel (such as austenitic stainless steel, ferritic stainless steel, duplex stainless steel, and super stainless steel). The σ phase in its microstructure is an intermetallic phase with very high hardness and very low plasticity. When it precipitates along grain boundaries, it significantly affects the plasticity of the steel, significantly reducing its impact toughness. Therefore, the σ phase ratio of stainless steel is ≤ 1.5%, and particularly ≤ 0.5% near the bonding interface (transition layer).

[0060] The present invention also provides a method for manufacturing a corrosion-resistant rolled composite steel plate for medium-temperature equipment, which comprises the following steps:

[0061] 1) Smelting and casting, preparation of base material and composite casting billet

[0062] The above composition is smelted and cast into a base material ingot, and during the smelting process, a deoxidizing agent Mn+Si, Al, and Ti alloy is added to the molten steel for deoxidation; Mn+Si and Al are added sequentially for pre-deoxidation, and the oxygen level of the molten steel after pre-deoxidation is 0.0020% to 0.0075%, and then Ti is added for final deoxidation; after deoxidation, B element and other alloys are added, and then the base material ingot is cast by the top pouring method; preparing or providing stainless steel for use as a composite ingot;

[0063] 2) Assembly

[0064] The base ingot and the composite ingot are surface treated, and then superimposed. The four sides of the superimposed slab are welded and sealed, vacuumed, and sealed again to form a composite slab; preferably, the vacuum degree after vacuuming is 8 to 60 Pa;

[0065] 3) Composite slab rolling

[0066] The composite slab is heated to 1050-1220° C., and then rolled in two stages, wherein the total reduction rate of the slab in the first stage is not less than 60% (e.g., 60-70%), and the total reduction rate of the slab in the second stage is not less than 20% (e.g., 20-35%), the reduction rate of the last pass is 8-18%, and the total reduction rate is not less than 80% (e.g., 80-98%), and the finishing temperature is above 850° C. (e.g., 850-960° C.);

[0067] 4) Heat treatment

[0068] The composite steel plate is subjected to quenching and tempering heat treatment.

[0069] Preferably, in step 2), the substrate and the composite cast slab are surface treated to ensure that the slab surface has no obvious oil stains, slag inclusions, cracks or other surface defects, and that the roughness of the processed slab surface is no greater than 2.0 Ra.

[0070] Preferably, in step 4), the quenching temperature is 935-975°C, the quenching holding time is T1=(1-1.2)H, and after the holding is completed, the product is taken out of the furnace and water-cooled to room temperature, wherein T1 is in min, and H is the thickness of the substrate layer in mm.

[0071] Preferably, in step 4), the tempering temperature is 650-725° C., the tempering holding time T2 is (1-1.5) H, and after the holding is completed, the product is taken out of the furnace and water-cooled to room temperature, wherein T2 is in min, and H is the thickness of the substrate layer in mm.

[0072] In the manufacturing method of the present invention,

[0073] The composite steel plate described in the present invention requires the formation of a large number of uniformly dispersed nanoscale MC precipitates within the substrate layer. These MC precipitates immobilize free carbon in the carbon steel substrate layer, preventing the carbon from diffusing extensively to the interface between the substrate layer and the composite material, which would otherwise cause carbides to form in the stainless steel and reduce its corrosion resistance. This improves the service safety of rolled composite steel plates for high-temperature equipment. However, free carbon in steel is a primary strengthening element in the carbon steel substrate layer. When most of the free carbon is immobilized in the form of MC, the strength of the carbon steel decreases. To compensate for the strength loss caused by carbon immobilization, the addition of element B is necessary to improve the hardenability of the steel plate. The hardenability of element B is primarily due to the distribution of free B at grain boundaries. Because element B is an extremely reactive element, it can combine with oxygen in the molten steel to form boron oxide. Furthermore, boron readily combines with nitrogen to form boron carbide. To this end, the deoxidation sequence of the molten steel and the timing of B addition are specifically designed in the technical solution of the present invention. That is, during deoxidation, Si+Mn is first used for pre-deoxidation, and most of the silicon oxide and manganese oxide will float to the top slag. Deoxidation with the strong deoxidizer Al can more accurately control the oxygen level (determined according to the method for measuring oxygen content in molten steel described in ISO 14284:1996) to 0.0020% to 0.0075%. Then, Ti is used for final deoxidation, forming a large amount of Ti oxide. At the same time, the added Ti can combine with the free nitrogen in the steel to form TiN. The addition of Ti further reduces the free oxygen concentration in the steel. Within a certain oxygen level range (0.0020% to 0.0075%), the appropriately sized Ti2O3 inclusions formed after deoxidation (2-5 μm) facilitate the formation of a partial acicular ferrite structure during solidification and subsequent phase transformation, improving the toughness of the steel plate. Furthermore, Ti combines with nitrogen in the steel to form some TiN, which, on the one hand, reduces the adverse effects of dissolved nitrogen on the toughness of the steel. On the other hand, TiN can precipitate at high temperatures during solidification, pinning the growth of austenite and refining the original austenite grains. Once the oxygen and titanium levels in the steel are properly controlled, an appropriate amount of B is added, ensuring that the added B is primarily present in the steel in a free state, ensuring the substrate has high hardenability and significantly improving the strength of the steel plate.

[0074] In step 3), the composite slab is heated to 1050-1220°C for rolling. This is because heating temperatures below 1050°C prevent the carbides and nitrides in the base cast from fully dissolving, reducing the carbides' pinning effect on the austenite grains during rolling. This temperature range also facilitates homogenization of the austenite in the composite material and complete dissolution of the carbides in the composite material. However, heating the slab above 1220°C can lead to rapid growth of the austenite grains in the base and composite layers, compromising the mechanical properties of the steel.

[0075] Accordingly, it is preferred that the total reduction rate of the slab in the first stage of rolling is not less than 60%, the total reduction rate in the second stage of rolling is not less than 20%, the reduction rate of the last pass is 8-18%, the total cumulative reduction rate of rolling is not less than 80%, and the final rolling temperature is above 850°C.

[0076] Through the first stage of high-temperature and high-reduction rolling, the material can be fully recrystallized during the rolling deformation process, the austenite grains can be continuously refined, and the formation of larger austenite grains in the steel can be inhibited, thereby improving the low-temperature toughness of the steel. When the total reduction rate in the first stage is less than 60%, the austenite grain size in the steel is larger, which is not conducive to the strength and toughness indicators of the steel plate.

[0077] The total reduction rate in the second stage of rolling is not less than 20%, which can further promote the refinement of austenite grains. The final rolling reduction rate is between 8% and 18%, which can increase the dislocation density in the steel and ensure the strength of the steel plate. The reduction rate is lower than 8%, and the dislocation density is low, which is not conducive to strength improvement. When the reduction rate is higher than 18%, the rolling torque is too large, which is not conducive to the rolling equipment. When the final rolling temperature is lower than 850℃, the deformation resistance of the steel plate increases, making it difficult to ensure a large final rolling reduction rate.

[0078] In step 4), the quenching temperature is 935-975°C, the quenching holding time is T1=(1-1.2)H, and after the holding is completed, the product is taken out of the furnace and water-cooled to room temperature, wherein T1 is in min, and H is the thickness of the substrate layer in mm.

[0079] It should be noted that during the quenching process, when the quenching temperature is below 935°C, the steel's austenite homogenization takes a long time, reducing heat treatment efficiency. If the quenching temperature is above 975°C, some of the austenite in the carbon steel tends to grow, easily forming a mixed crystal structure, which is detrimental to the uniformity of the steel's structure. Accordingly, when the steel is held at the quenching temperature for a holding time T1 of less than 1 hour, sufficient austenitization cannot occur. When the holding time T1 exceeds 1.2 hours, the steel exceeds the required austenitization time, reducing the efficiency of the quenching process.

[0080] In step 4), the tempering temperature is 650-725°C, and the tempering holding time is T2 = (1-1.5)H. After the holding is completed, the product is taken out of the furnace and water-cooled to room temperature, wherein T2 is in min and H is the thickness of the substrate layer in mm.

[0081] During the tempering process, when the tempering temperature of steel is below 650°C, a longer tempering time is required to remove residual quenching stresses in the steel, which reduces tempering efficiency and slows the formation of MC precipitates in carbon steel. When the tempering temperature is above 725°C, the MC precipitates in the steel are prone to Ostwald coarsening and aggregation, which is not conducive to the formation of fine and dispersed MC precipitates in the steel. Accordingly, when the tempering holding time T2 is less than 1 hour, the quenching stresses in the steel tend not to be completely removed, while when the tempering holding time T2 is greater than 1.5 hours, the MC precipitates in the steel tend to coarsen, reducing the production efficiency of the tempering process.

[0082] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0083] 1. The present invention utilizes fine, dispersed, spherical (2-5 μm) Ti2O3 oxides formed in the steel to serve as nucleation cores for MC carbide precipitation. Combined with optimized alloy composition and heat treatment process design, a large number of dispersed, nanoscale MC (M represents one or more of V, Nb, and Ti) precipitations can be formed in the substrate layer. This can immobilize free carbon in the substrate layer, reduce the diffusion and aggregation of carbon at the composite interface, and reduce the precipitation of carbides at the grain boundaries of stainless steel, thereby ensuring the corrosion resistance of the composite rolled steel plate. In particular, when the rolled composite steel plate is used in high-temperature equipment, the high-melting-point Ti2O3 oxides (melting point 2130°C) formed by the controlled smelting process can remain stably present in the steel under high-temperature operating conditions of 300-450°C, serving as precipitation cores for MC carbides and continuing to function. That is, some of the free carbon elements in the steel can still react with the alloying elements in the steel with Ti2O3 oxide as the nucleation core to form fine MC carbides, thereby further fixing the carbon elements, ensuring that the carbon elements in the carbon steel do not diffuse to the interface, and protecting the corrosion resistance of the rolled composite steel plate under long-term high-temperature service.

[0084] 2. The current production process of low-alloy, high-strength steel pursues purity, often using aluminum as a deoxidizer to minimize the oxygen content in the steel. While aluminum deoxidation offers the advantage of high deoxidation efficiency, its disadvantage is that the Al2O3 inclusions formed in the molten steel can adversely affect the flaw detection and performance of the steel plate. This is primarily because individual Al2O3 inclusions often exhibit sharp angular shapes, easily forming crack initiations in the steel. Furthermore, Al2O3 inclusions easily aggregate in the molten steel, forming large clusters that can block the pouring nozzle. The presence of large inclusions in the steel can easily cause fluctuations in the tensile and impact properties of the steel plate. Furthermore, Al2O3 inclusions cannot serve as nucleation sites for carbide precipitation.

[0085] In the present invention, by controlling the type, sequence, and amount of deoxidizer added during the steelmaking process, a fine, dispersed, spherical (2-5 μm) Ti2O3 oxide with a high melting point is ultimately formed in the steel. The Ti2O3 oxide is less likely to aggregate and grow in the molten steel, is inherently flat, and does not significantly impair the flaw detection and mechanical properties of the steel plate, thereby optimizing the steel plate's microstructure. By controlling the element content ratio, TiN precipitation is controlled, refining the austenite grains. By designing the order of addition of the steel alloy, the amount of oxidation and nitridation of the boron element can be effectively reduced, thereby maximizing the utilization of the added boron element, maximizing the hardenability of the steel plate, refining the grains, and ensuring the steel plate has high strength and excellent low-temperature toughness.

[0086] Furthermore, through optimal smelting process control, particularly by controlling the type, sequence, and amount of deoxidizer added, the fine, dispersed spherical (2-5 μm) Ti2O3 oxides formed in the steel can serve as nucleation cores for the precipitation of MC carbides. Combined with optimized alloy composition design and heat treatment process design, a large number of dispersed, nanoscale MC (M represents one or more of V, Nb, and Ti) precipitates can be formed in the substrate layer. This can immobilize free carbon in the substrate layer, reduce the diffusion and aggregation of carbon at the composite interface, and reduce the precipitation of carbides at the grain boundaries of the stainless steel, thereby ensuring the corrosion resistance of the composite rolled steel plate. In particular, when the rolled composite steel plate is used in high-temperature equipment, the high-melting-point Ti2O3 oxides (melting point of 2130°C) formed by the controlled smelting process can remain stably present in the steel under high-temperature operating conditions of 300°C to 450°C, serving as precipitation cores for MC carbides and continuing to function. That is, some of the free carbon elements in the steel can still react with the alloying elements in the steel with Ti2O3 oxide as the nucleation core to form fine MC carbides, thereby further fixing the carbon elements, ensuring that the carbon elements in the carbon steel do not diffuse to the interface, and protecting the corrosion resistance of the rolled composite steel plate under long-term high-temperature service.

[0087] The composite steel plate of the present invention has excellent corrosion resistance and can be effectively used in the energy and chemical industry as a manufacturing material for high-temperature reaction vessels at 300-450°C. It has very important practical significance and a very broad application prospect. DETAILED DESCRIPTION

[0088] The present invention will be further described below with reference to specific embodiments. However, this description does not constitute an undue limitation to the technical solution of the present invention.

[0089] The composition of the present invention's examples is shown in Table 1, with the remainder comprising Fe and other unavoidable impurities. The composite material of the rolled composite steel plates in the present invention's examples and comparative examples is illustrated using austenitic stainless steel as an example. The manufacturing process parameters for the present invention's examples are shown in Table 2. Commercially available austenitic stainless steel can be used as the composite slab.

[0090] The manufacturing method of the embodiment of the present invention comprises the following steps:

[0091] 1) Smelting and casting: smelting and continuous casting are carried out according to the chemical composition shown in Table 1 to produce ingots; the smelting equipment adopts a 500 kg vacuum induction furnace; first, 410 kg of industrial pure iron is added to the vacuum induction furnace, and a certain amount of CaO is added with the furnace according to the alloy composition ratio of the steel (the added CaO is used for slag formation during smelting); vacuum is evacuated, and smelting is carried out at a minimum vacuum degree of about 25 Pa. Argon gas is required for atmosphere protection during the smelting process; after the pure iron is melted, deoxidizers Si+Mn and Al are added in sequence for pre-deoxidation, and the amount of deoxidizer added is fine-tuned according to the online monitoring results of the molten steel composition and the oxygen level of free oxygen in the molten steel is controlled to 0.0020% to 0.0075%, and then an appropriate amount of Ti is added for final deoxidation; alloys such as Nb and V are added for alloying treatment, and finally B is added, and the ingot is cast by the top pouring method;

[0092] 2) Assembling: The surfaces of the base carbon steel slab and the clad stainless steel slab are processed. The roughness of the butt joint surface of the two materials is ≤2.0Ra. After surface inspection, it is ensured that there are no surface defects such as oil stains, slag inclusions, cracks, etc.; After welding around the slab, vacuum is drawn to a vacuum degree of 8-60Pa;

[0093] 3) rolling, heating the composite slab at 1050-1220°C, and then performing two-stage rolling, the total reduction rate of the slab in the first stage is not less than 60%, the total reduction rate in the second stage is not less than 20%, the reduction rate of the last pass is 8-18%, the total reduction rate is not less than 80%, and the final rolling temperature is above 850°C;

[0094] 4) Heat treatment, quenching and tempering the rolled composite steel plate; quenching the composite rolled steel plate at a quenching temperature of 935-975°C and a quenching holding time T1 of 1-1.2 hours. After the holding time is completed, the plate is removed from the furnace and water-cooled to room temperature; then tempering is performed at a tempering temperature of 650-725°C and a tempering holding time T2 of 1-1.5 hours. The plate is then air-cooled after being removed from the furnace; where H represents the thickness of the steel plate in mm.

[0095] In the present invention, the chemical composition design and related processes of the composite steel plate substrates of Examples 1 to 7 all meet the design specification requirements of the present invention.

[0096] In contrast, the rolled composite steel plates of Comparative Examples 1 to 3 are manufactured using the same process steps as Example 1, except that the alloy element content of the base carbon steel of Comparative Examples 1 to 2 exceeds the design range, while the alloy range of the base carbon steel of Comparative Example 3 is within the design range. However, in the smelting and pouring operations of the above-mentioned step (1), the order of adding the deoxidizer and the B alloy in Comparative Examples 1 to 3 is different; and the heat treatment and tempering processes are different.

[0097] Electrolytic samples were taken from the base carbon steel material and the clad stainless steel layer of the composite steel plates of Examples 1-7 and the control rolled composite steel plates of Comparative Examples 1-3. MC precipitates were extracted from the stainless steel using electrolytic extraction, and precipitates of varying size ranges were separated using filter membranes of varying particle sizes. The particle size distribution of the precipitates was measured using a laser particle size analyzer, and image analysis software was used to determine the quantity and size distribution of the MC precipitates (M represents one or more of Nb, V, Ti, and Cr) in the steel plates of each Example and Comparative Example. The data obtained from these observations and analyses are summarized in Table 3 (base material) and Table 4 (clad material at the interface).

[0098] As shown in Table 3, the total number of precipitated phases in the carbon steel base material of the rolled composite steel plates of Examples 1-7 is 582 to 791, which is much higher than the 340 to 432 precipitated phases in the carbon steel base material of the comparative example. This indicates that after the implementation of the technical solution of the present invention, a larger number of MC carbide precipitates can be formed in the base carbon steel material, thereby ensuring the effect of using the stable carbides in the base carbon steel to fix the free carbon in the carbon steel as designed in the technical solution. Moreover, the size of the MC precipitates in the carbon steel base material of the rolled composite steel plates of Examples 1-7 is relatively small, with their main distribution range being 200 to 600 nm. The average major axis length of the MC precipitates is less than 500 nm, and the volume density of the MC precipitates is greater than 2.0×10 4 Pieces / mm 3 The MC precipitates with a size of less than 500nm account for more than 50% of the total MC precipitates. When the precipitates are small in size, they can, to a certain extent, play a role in pinning the growth of austenite grains, thereby refining the grains and further improving the strength and toughness of the material.

[0099] In contrast, the MC precipitates in the carbon steel substrates of the rolled composite steel plates of Comparative Examples 1-3 are larger, with an average major axis length greater than 800 nm. The main precipitate size distribution range is above 800 nm, and precipitates larger than 1000 nm account for a high proportion. This poor pinning effect on the austenite grains results in larger grain sizes, impacting the strength and toughness of the steel plates. Due to the small number of stable precipitates formed in the carbon steel materials of the comparative examples, the precipitates reduce the amount of free carbon fixed by the carbon steel, causing a large amount of free carbon to segregate toward the bonding interface. This results in a large amount of grain boundary carbides precipitating in the composite material at the bonding interface, reducing the corrosion resistance of the composite material at the interface.

[0100] As shown in Table 4, in the composite materials of the composite steel plates of Examples 1-7 located near the transition layer, the number of MC precipitates detected was 146 to 182, which was much less than 403 to 411 in the comparative example. In addition, the volume density of the MC precipitates in the composite materials at the interface (transition layer) of the examples was less than 1×10 4 Pieces / mm 3 . This shows that the amount of carbon in the base carbon steel material of Examples 1-7 that diffuses into the cladding material through the interface is small, and the amount of MC formed is small, while in Comparative Examples 1-3, a large amount of carbon elements in the carbon steel material diffuses into the cladding material through the interface, resulting in a larger amount of carbides in the corresponding cladding material. At the same time, it can be seen from Table 4 that in the cladding material at the bonding interface (transition layer) of Examples 1-7, the average major axis length of the MC precipitate phase is less than 700nm, and the MC precipitate phase with a size less than 700nm accounts for more than 50% of the total MC precipitate phase; correspondingly, the average size of the MC precipitate phase obtained by inspection in the comparative example is greater than 900nm. This further illustrates that the carbide fixing effect on free carbon in the examples is much better than that in the comparative example.

[0101] After analyzing the carbide precipitation phase of the composite steel plates of Examples 1 to 7 and the steel plates of Comparative Examples 1 to 3, the mechanical properties (according to ASTM A370-24 and ASTM A264-12 (2019) standards) and corrosion resistance (according to ASTM A262-15 (2021) standard practice E method) of the steel plates of Examples 1 to 7 and Comparative Examples 1 to 3 were also evaluated. The evaluation results are shown in Table 5. As shown in Table 5, the yield strength and tensile strength of the carbon steel substrate in the comparative example are lower than those in the examples. This is primarily due to differences in the smelting process, alloy addition sequence, and heat treatment process of the carbon steel in the comparative example, which prevents the B element in the comparative steel from fully exerting its hardenability. Furthermore, the different precipitation phases in the steel have different effects on the microstructure of the steel plate. The surface microstructure of the composite steel plate substrate in the present invention is 60-80% bainite, 20-40% ferrite, and MC precipitation phases, while the central microstructure of the substrate is 30-40% bainite and 60-70% ferrite with MC precipitation phases. The smaller precipitation phases in the examples further refine the austenite grains, contributing to the improved strength and toughness of the steel plate.

[0102] Intergranular corrosion testing was conducted on the stainless steel near the interface (transition layer) of the rolled composite steel plates from the examples and comparative examples according to ASTM A262 Practice E. The test results are shown in Table 6. These results demonstrate that the stainless steel at the interface of the rolled composite steel plates produced according to the technical requirements of the present invention exhibits excellent corrosion resistance. Therefore, the corrosion resistance of the composite stainless steel plates from Examples 1 to 7 is significantly superior to that of the comparative examples.

[0103] In summary, the composite steel plate of the present invention can be produced through reasonable chemical composition design and optimized production process, especially by controlling the type of deoxidizer, deoxidation sequence and amount of deoxidizer added during the steel smelting process, and performing alloying treatment in a certain order, so as to generate a large number of dispersed nano-scale MC (M represents one or more of Nb, V, Ti, Cr) precipitation phases in the steel. These nano-scale MC precipitation phases can fix the free carbon elements in the base carbon steel, thereby preventing these carbon elements from diffusing in large quantities to the bonding interface (transition layer), resulting in the formation of a large number of carbide precipitation phases in the stainless steel at the bonding interface, causing the corrosion resistance of the stainless steel to decrease. Therefore, the use of the technical solution of the present invention can ensure the strength and toughness of the carbon steel substrate, and at the same time can greatly improve the corrosion resistance of the rolled composite steel plate.

[0104] In addition, it should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0105] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.

[0106] Table 6

Claims

1. A corrosion-resistant rolled composite steel plate for medium-temperature equipment, which is rolled by composite rolling of carbon steel as a substrate and stainless steel as a composite layer, comprising a substrate layer, a composite layer and a transition layer between the substrate layer and the composite layer, wherein the composition weight percentage of the substrate layer is: C: 0.01% to 0.15%, Si: 0.2% to 0.7%, Mn: 0.1% to 1.70%, Ti: 0.006% to 0.025%, Als: 0.004% to 0.015%, Nb: 0.0010% to 0.050%, V: 0.15% to 0.50%, B: 0.0005% to 0.0050%, N: 0.0015% to 0.0050%, P≤0.0165%, S≤0.010%, O≤0.005%, and the balance includes Fe and other inevitable impurities; The surface microstructure of the substrate layer is 60-80% bainite + 20-40% ferrite + MC precipitation phase in area percentage; the central microstructure of the substrate layer is 30-40% bainite and 60-70% ferrite + MC precipitation phase in area percentage; wherein, M represents one or more of Nb, V, and Ti, the average major axis length of the MC precipitates is less than 500 nm, and the volume density of the MC precipitates with a major axis length less than 500 nm is greater than 2.0×10 4 Pieces / mm 3 ; The microstructure of the multilayer contains MC precipitates; wherein M represents one or more of Cr, Nb, Mo, and Ti, the average major axis length of the MC precipitates is less than 700 nm, and the volume density of the MC precipitates is less than 1.0×10 4 Pieces / mm 3 .

2. The corrosion-resistant rolled composite steel plate for medium-temperature equipment according to claim 1, characterized in that: The substrate further contains at least one of the following chemical elements: 0<Ca≤0.0055%, 0<Ni≤0.65%, 0<Cu≤0.55%.

3. The corrosion-resistant rolled composite steel plate for medium temperature equipment according to claim 1 or 2, characterized in that: The balance of the composition of the base material layer is Fe and other inevitable impurities.

4. The corrosion-resistant rolled composite steel plate for medium temperature equipment according to claim 1, 2 or 3, characterized in that: In the substrate layer, the MC precipitated phases with a major axis length of less than 500 nm account for more than 50% of the total MC precipitated phases.

5. The corrosion-resistant rolled composite steel plate for medium temperature equipment according to claim 1, 2, 3 or 4, characterized in that: The stainless steel is austenitic stainless steel, ferritic stainless steel, duplex stainless steel or super stainless steel.

6. The corrosion-resistant rolled composite steel plate for medium-temperature equipment according to any one of claims 1 to 5, characterized in that: The weight percentage of the components of the stainless steel is: C: 0.01-0.05%; Si: 0.50-0.80%; Mn: 1.10-1.50%; Ni: 6.5-9.5%; Cr: 15.0-22.0%; N: 0.060~0.120%; P: ≤0.015%; S: ≤0.005%; O: ≤0.005%; the balance is Fe and other inevitable impurities.

7. The corrosion-resistant rolled composite steel plate for medium-temperature equipment according to any one of claims 1 to 6, characterized in that: In the multilayer, the MC precipitated phases with a major axis length of less than 700 nm account for more than 50% of the total MC precipitated phases.

8. The corrosion-resistant rolled composite steel plate for medium-temperature equipment according to any one of claims 1 to 7, characterized in that: The microstructure of the composite layer also contains a σ phase with an area ratio of ≤1.5%; preferably, the area ratio of the σ phase near the transition layer is ≤0.5%.

9. The corrosion-resistant rolled composite steel plate for medium-temperature equipment according to any one of claims 1 to 8, characterized in that: The thickness of the substrate of the composite steel plate is ≥6 mm, and the thickness of the composite layer is ≥2 mm; preferably, the thickness of the transition layer is ≤230 μm; preferably, the thickness of the substrate layer is 6 to 200 mm or 10 to 200 mm; preferably, the thickness of the composite layer is 2 to 20 mm; preferably, the thickness of the transition layer is 2 to 230 μm, 2 to 150 μm or 2 to 50 μm.

10. The corrosion-resistant rolled composite steel plate for medium-temperature equipment according to any one of claims 1 to 9, characterized in that: The yield strength of the substrate of the composite steel plate is ≥325MPa, the tensile strength is ≥430MPa, the cross-sectional shrinkage is ≥60%, the elongation is ≥19%, the -30°C impact absorption energy is ≥100J, and the -50°C impact absorption energy is ≥45J; the grain size grade of the substrate is not less than grade 7.0; the shear strength at the composite interface of the composite steel plate is ≥310MPa.

11. The corrosion-resistant rolled composite steel plate for medium-temperature equipment according to claim 10, characterized in that: The yield strength of the base material layer of the composite steel plate is ≥360MPa, ≥380MPa; preferably, the yield strength of the base material layer of the composite steel plate is between 380 and 500MPa; The tensile strength of the substrate layer of the composite steel plate is ≥450MPa or ≥480MPa; preferably, the tensile strength of the substrate layer of the composite steel plate is between 480 and 600MPa; The cross-sectional shrinkage of the substrate layer of the composite steel plate is ≥70%, or the cross-sectional shrinkage of the substrate layer of the composite steel plate is between 60% and 80% or between 70% and 80%; The elongation of the base material layer of the composite steel plate is ≥21%; or the elongation of the base material layer of the composite steel plate is between 19% and 25%; The -30°C impact absorption energy of the base material layer of the composite steel plate is ≥150J or ≥180J, or between 100 and 240J or between 180 and 240J; The -50°C impact absorption energy of the base material layer of the composite steel plate is ≥80J, ≥100J or ≥140J, or between 45 and 180J, or between 80 and 180J, or between 100 and 180J, or between 140 and 180J; The shear strength at the composite interface of the base material layer of the composite steel plate is ≥330MPa or 310-420MPa between.

12. The method for manufacturing a corrosion-resistant rolled composite steel plate for medium-temperature equipment according to any one of claims 1 to 11, characterized in that: The steps include: 1) Smelting and casting, preparation of base material casting billet According to the composition of claim 1, 2 or 3, the base material ingot is smelted and cast, and deoxidizers Mn+Si, Al, and Ti alloys are added to the molten steel for deoxidation during the smelting process; Mn+Si and Al are added in sequence for pre-deoxidation, and the oxygen level of the molten steel after pre-deoxidation is 0.0020% to 0.0075%, and then Ti is added for final deoxidation; after deoxidation, B element and other alloys are added, and then the base material ingot is cast by the top pouring method; Preparing or providing stainless steel for composite casting; 2) Assembly The substrate ingot and the composite ingot are surface treated, and then overlapped, and the overlapped ingots are welded and sealed around the edges, vacuumed, and sealed again to form a composite ingot; preferably, after the vacuum is exhausted, the vacuum degree is 8 to 60 Pa; 3) Composite slab rolling The composite slab is heated to 1050-1220°C, and then rolled in two stages, the total reduction rate of the slab in the first stage is not less than 60%, the total reduction rate in the second stage is not less than 20%, the reduction rate of the last pass is 8-18%, the total reduction rate is not less than 80%, and the final rolling temperature is above 850°C; 4) Heat treatment The composite steel plate is subjected to quenching and tempering heat treatment.

13. The method for manufacturing the corrosion-resistant rolled composite steel plate for medium-temperature equipment according to claim 12, wherein: In step 2), the substrate ingot and the composite ingot are surface treated to ensure that there are no obvious surface defects such as oil stains, slag inclusions, and cracks on the surface of the ingot, and at the same time ensure that the roughness of the processed surface of the ingot is not greater than 2.0 Ra.

14. The method for manufacturing the corrosion-resistant rolled composite steel plate for medium-temperature equipment according to claim 12, wherein: In step 4), the quenching temperature is 935-975°C, the quenching holding time is T1=(1-1.2)H, and after the holding is completed, the product is taken out of the furnace and cooled to room temperature with water, wherein T1 is in min, and H is the thickness of the substrate layer in mm.

15. The method for manufacturing a corrosion-resistant rolled composite steel plate for medium-temperature equipment according to any one of claims 12 to 14, characterized in that: In step 4), the tempering temperature is 650-725°C, the tempering holding time is T2=(1-1.5)H, and after the holding is completed, the product is taken out of the furnace and cooled to room temperature with water, wherein T2 is in min, and H is the thickness of the substrate layer in mm.

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