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

Abstract

A corrosion-resistant rolled clad steel plate for high-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: 0.08%-0.18%, Si: 0.01%-0.6%, Mn: 0.30%-0.70%, Ti: 0.009%-0.015%, Als: 0.010%-0.015%, Nb: 0.0010%-0.050%, Cr: 1.85%-3.85%, Mo: 0.5%-1.55%, B: 0.0007%-0.0050% N: 0.0025%-0.0050%, P ≤ 0.010%, S ≤ 0.010%, O ≤ 0.004%, and the balance being Fe and other unavoidable impurities. The present invention fully accounts for interfacial characteristics of the clad steel plate, ensures the corrosion resistance of the cladding layer stainless steel, is suitable for high-temperature equipment at operating temperatures of 450-530°C, and is widely applicable in energy industries such as thermal energy, petroleum refining, and the chemical industry.

Description

Corrosion-resistant rolled composite steel plate for high-temperature equipment and 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 high-temperature equipment and a manufacturing method thereof, which is suitable for high-temperature equipment with an operating temperature of 450-530°C in energy manufacturing fields such as electricity and petrochemicals. Background Art

[0002] With the rapid development of my country's economy, the demand for electricity, oil, fertilizers, and other energy-related raw materials is increasing. To improve energy production efficiency, it is imperative that energy production and processing equipment be larger, more highly parameterized (higher temperatures and higher pressures), and lighter. This demand for equipment upgrades places higher demands on the performance of the steel plates used in manufacturing equipment. These plates must not only have higher strength and hardness, but also possess excellent toughness, formability, and, most importantly, better corrosion resistance. Stainless steel has excellent corrosion resistance, and carbon steel offers a good balance of strength and toughness. Combining the advantages of stainless steel and carbon steel creates a composite material that exhibits both high strength and high corrosion resistance. Composite steel plates made from low-alloy steel and various corrosion-resistant materials are examples of high-performance composite materials. Due to their high mechanical strength and strong corrosion resistance, they are often used in industries such as crude oil processing and the energy and chemical industries.

[0003] The traditional production process of composite steel plates is the explosive composite process. The main problem of this production process is that when explosives are used for explosive composite, the explosion will produce a large amount of harmful gases, which will have an adverse impact on the environment. That is, this type of production process does not meet environmental protection requirements; at the same time, the bonding rate at the interface of the explosive composite steel plate is low, and sometimes cannot meet the bonding rate requirement of more than 97%. When this type of steel plate is used to manufacture various high-temperature, high-pressure, and large-scale container equipment, there will be a major safety hazard. Once the composite steel plate joints are separated and the equipment fails, it will have a huge impact on the safety of personnel and property.

[0004] Compared with explosive clad steel plates, the bonding rate of rolled clad steel plates is greatly improved to over 99%, and the production process is more environmentally friendly than explosive cladding. Therefore, in recent years, the production scale and engineering application of rolled clad steel plates have been expanding year by year.

[0005] However, whether it is an explosive composite steel plate or a 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, which will have an adverse effect on the strength of the carbon steel base material and the corrosion resistance of the corrosion-resistant material. Among them, the carbon element in the base carbon steel material enters the stainless steel at the bonding interface through the transition layer in large quantities, forming a large amount of carbide, causing intergranular corrosion of the stainless steel. When the corrosion resistance of the stainless steel is greatly affected, the corrosion-resistant material will have a greater risk of failure, especially under high temperature conditions. Once the material fails and the equipment fails, it may cause a major industrial production accident, resulting in casualties and huge economic losses.

[0006] Therefore, how to reasonably design the alloy composition and production process of the carbon steel base layer to minimize the negative impact of the corrosion resistance of the corrosion-resistant material caused by the diffusion of carbon elements in the base carbon steel material through the transition layer to the stainless steel due to long-term high-temperature service has become an urgent requirement for improving the comprehensive performance of rolled composite steel plates.

[0007] Chinese patent application publication number CN108231273A discloses a method for improving the interface of a copper-aluminum composite material. The invention discloses a method for improving the interface of a copper-aluminum composite material. The main technical solution involves pre-coating or depositing graphene at the copper-aluminum interface, and then processing and laminating the copper, aluminum, and graphene. The coating or deposition method is one of painting, electroplating, and chemical vapor deposition; the laminating method is one of rolling lamination, extrusion lamination, and drawing lamination; and the graphene is in the form of a powder or film. In this technical solution, adding graphene to the copper-aluminum interface ensures interfacial bonding, prevents the formation of brittle and poorly conductive intermetallic compounds, and improves the interface's bonding and conductivity. Since the materials used in this invention are copper and aluminum, and the primary purpose of adding graphene to the interface is to improve the material's conductivity, the method for improving the interface properties of the composite material described in this patent application is not applicable to rolled clad steel sheets of low-alloy steel and stainless steel, as the properties of copper and aluminum differ significantly from those of low-alloy steel and stainless steel. Furthermore, carbon steel and stainless steel clad steel sheets do not require enhanced conductivity.

[0008] Chinese patent application publication number CN108239725A discloses a "high-shear-strength rolled composite steel plate and its manufacturing method." The high-shear-strength rolled composite steel plate comprises a carbon steel base layer and a stainless steel clad layer rolled onto the carbon steel base layer. The carbon steel base layer comprises the following chemical elements by mass: C: 0.17% to 0.25%; Si: 0.15% to 0.40%; Mn: 1.15% to 1.50%; Mo: 0.45% to 0.60%; Ni: 0.40% to 0.70%; Al: 0.020% to 0.040%; Ca: 0.0010% to 0.0030%; the remainder being Fe and other unavoidable impurities. The high-shear-strength rolled composite steel plate has a room temperature tensile strength of 570 to 690 MPa, a room temperature yield strength of 365 MPa or greater, a shear strength of 350 MPa or greater, and an elongation of 20% or greater. In this patent application, the inventors achieved high shear strength performance of the composite steel plate by controlling the microstructure ratio of the carbon steel material. However, their invention materials did not mention the control of the microstructure and precipitation at the interface.

[0009] Chinese patent application publication number 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 inventors directly assembled the carbon steel and austenitic stainless steel slabs, hot-rolled them, and coiled them to produce a rolled composite coil. The target steel grade in this patent application is a hot-rolled composite coil product with a thickness of less than 10 mm, typically operating at room temperature. The invention does not address the impact of high-temperature operating conditions on the corrosion resistance of the composite layer at the interface between the composite materials.

[0010] Based on the above patent applications, the existing rolled composite steel plate technology, especially the carbon steel and stainless steel composite plate production technology, has given little consideration to the transition layer at the interface between the base and the composite material. In particular, the diffusion of alloying elements, especially carbon elements, in the carbon steel material to the interface may cause a significant decrease in the corrosion resistance of the stainless steel, and ultimately there is a hidden danger of material failure, which is almost not addressed. Therefore, in the technical solutions of the published patent applications, basically no targeted measures have been taken to reduce the possibility of weakening the corrosion resistance of the stainless steel due to the precipitation of carbides at the grain boundaries of the stainless steel at the interface of the composite material. In fact, in the chemical and energy industries, the operating temperature of composite steel plate manufacturing equipment is often in the range of 450-530℃. If it is operated at such a 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 and larger carbides, which further reduces the corrosion resistance of the stainless steel and creates a potential accident hazard.

[0011] Summary of the Invention

[0012] The present invention aims to provide a corrosion-resistant rolled composite steel plate for high-temperature equipment and a method for manufacturing the same. This method fully considers the interfacial properties of the composite steel plate while ensuring the corrosion resistance of the stainless steel composite layer. The plate is suitable for high-temperature equipment operating at temperatures between 450°C and 530°C, and is widely applicable to energy industries such as thermal power, petroleum refining, and chemical engineering. The "high temperature" in this context refers to temperatures between 450°C and 530°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 Cr, Mo, 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 steel in the base carbon steel, thereby preventing the carbon element in the carbon steel from diffusing and migrating to the interface of the rolled composite steel plate under high-temperature working conditions of 450°C to 530°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, which induces intergranular corrosion and ultimately weakens or even loses the corrosion protection function of the composite material.

[0015] Specifically, the corrosion-resistant rolled composite steel plate for high-temperature equipment of the present invention is formed by composite rolling of carbon steel as a base material and stainless steel as a cladding layer. Therefore, the corrosion-resistant rolled composite steel plate for high-temperature equipment according to the present invention comprises a substrate layer, a composite layer, and a transition layer between the substrate layer and the composite layer; the components of the substrate layer are as follows by weight: C: 0.080% to 0.180%, Si: 0.01% to 0.60%, Mn: 0.30% to 0.70%, Ti: 0.009% to 0.015%, Als: 0.0100% to 0.0150%, Nb: 0.0010% to 0.050%, Cr: 1.85% to 3.85%, Mo: 0.5% to 1.55%, B: 0.0007% to 0.0050%, N: 0.0025% to 0.0050%, P≤0.010%, S≤0.010%, O≤0.004%, and the balance comprises Fe and other unavoidable impurities;

[0016] The surface microstructure of the substrate layer is bainite with an area ratio of 90-95% + martensite with an area ratio of 5-10% + MC precipitation phase, and the central microstructure of the substrate layer is bainite with an area ratio of 90-95% + martensite with an area ratio of 0-5% + ferrite with an area ratio of 0-5% + MC precipitation phase; wherein M represents one or more of Cr, Mo, Nb, and Ti, the average major axis length of the MC precipitation phase is less than 400 nm, and the volume density of the MC precipitation phase with a major axis length less than 400 nm is greater than 3.0×10 4 Pieces / mm 3 ;

[0017] The microstructure of the composite layer contains MC precipitates, wherein M represents one or more of Cr, Mo, Nb, 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 composite steel plate base material layer composition is Fe and other inevitable impurities.

[0019] Furthermore, the composite steel plate base material layer also contains at least one of the following chemical elements: 0<Ca≤0.0055%, 0<Ni≤0.65%, 0<Cu≤0.55%.

[0020] Preferably, the MC precipitates with a major axis length of less than 400 nm in the composite steel plate base material layer account for more than 50% of the total MC precipitates.

[0021] In some embodiments, the area percentage of ferrite in the central microstructure of the substrate layer is 1 to 5%, preferably 2 to 5%.

[0022] 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.

[0023] 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.

[0024] Preferably, in the composite steel plate cladding layer, the MC precipitates with a major axis length of less than 700 nm account for more than 50% of the total MC precipitates.

[0025] Furthermore, the microstructure of the composite steel plate layer may also contain a very small amount of σ phase, with the σ phase ratio (area ratio) being ≤1.5%. Preferably, the σ phase ratio near the transition layer is ≤0.5%.

[0026] Preferably, the thickness of the composite steel plate substrate is ≥6 mm, and the thickness of the cladding layer is ≥2 mm; preferably, the thickness of the transition layer is ≤250 μm. In some embodiments, the thickness of the substrate layer is 6 to 200 mm. In some embodiments, the thickness of the substrate layer is 10 to 200 mm or 15 to 150 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 1 to 250 μm. In some embodiments, the thickness of the transition layer is 1 to 150 μm. In some embodiments, the thickness of the transition layer is 1 to 50 μm or 2 to 30 μm. Herein, the thickness of the base layer and the 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.

[0027] The yield strength of the rolled composite steel plate substrate of the present invention is ≥455 MPa, the tensile strength is ≥535 MPa, the cross-sectional shrinkage is ≥65%, the elongation is ≥18%, the -40°C impact absorption energy is ≥120 J, and the -60°C impact absorption capacity is ≥70 J; the grain size grade of the substrate is not less than grade 7.0; and the shear strength of the rolled composite steel plate is ≥330 MPa.

[0028] In some embodiments, the yield strength of the substrate layer of the composite steel plate of the present invention is ≥470 MPa. In some embodiments, the yield strength of the substrate layer of the composite steel plate of the present invention is ≥480 MPa. In some embodiments, the yield strength of the substrate layer of the composite steel plate of the present invention is between 455 and 515 MPa.

[0029] In some embodiments, the tensile strength of the substrate layer of the composite steel plate of the present invention is ≥550 MPa. In some embodiments, the tensile strength of the substrate layer of the composite steel plate of the present invention is ≥570 MPa. In some embodiments, the tensile strength of the substrate layer of the composite steel plate of the present invention is between 535 and 635 MPa.

[0030] 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 65% and 75%.

[0031] In some embodiments, the elongation of the substrate layer of the composite steel plate of the present invention is ≥20%. In some embodiments, the elongation of the substrate layer of the composite steel plate of the present invention is between 18% and 25%.

[0032] In some embodiments, the -40°C impact absorbed energy of the substrate layer of the composite steel plate of the present invention is ≥ 150 J. In some embodiments, the -40°C impact absorbed energy of the substrate layer of the composite steel plate of the present invention is ≥ 180 J. In some embodiments, the -40°C impact absorbed energy of the substrate layer of the composite steel plate of the present invention is between 120 and 255 J, such as between 180 and 255 J.

[0033] In some embodiments, the -60°C impact absorbed energy of the substrate layer of the composite steel plate of the present invention is ≥ 100 J. In some embodiments, the -60°C impact absorbed energy of the substrate layer of the composite steel plate of the present invention is ≥ 130 J. In some embodiments, the -60°C impact absorbed energy of the substrate layer of the composite steel plate of the present invention is ≥ 150 J. In some embodiments, the -60°C impact absorbed energy of the substrate layer of the composite steel plate of the present invention is between 70 and 190 J, or between 100 and 190 J, or between 130 and 190 J, or between 150 and 190 J.

[0034] In some embodiments, the shear strength of the composite steel plate of the present invention is ≥350 MPa. In some embodiments, the shear strength of the composite steel plate of the present invention is ≥380 MPa. In some embodiments, the shear strength of the composite steel plate of the present invention is ≥400 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 330 and 470 MPa.

[0035] In the composition design of the composite steel plate substrate of the present invention:

[0036] C: C is an important alloying element. In order to make the carbon steel plate have appropriate strength, it is necessary to control the mass percentage of the C element in the steel to be greater than or equal to 0.08%. At the same time, in order to improve the welding performance of the base carbon steel material, it is necessary to control the upper limit of the C element content in the carbon steel. In addition, if the carbon element content in the carbon steel base material is high, due to the difference in the carbon element concentration gradient of the two materials, it will cause large-scale carbon migration at the interface between the base material and the stainless steel cladding material, causing intergranular corrosion of the stainless steel. Therefore, its upper limit is controlled to 0.18%. In addition, controlling the appropriate carbon element content can form a large amount of dispersed nano-scale carbides in the steel through the combination of composition and process, and can also play a precipitation strengthening effect to improve the strength of the base carbon steel. Based on this, the present invention controls the C content to 0.08~0.18%.

[0037] Si: Si is also 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 do not affect 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 to interfaces. Therefore, a certain amount of Si is required in steel. However, excessive Si content can reduce the steel's weldability. Furthermore, Si's ability to bind oxygen is stronger than that of iron, making it more likely to form low-melting-point silicates during welding. This increases the fluidity of the slag and molten metal, causing spattering and affecting the quality of the welded 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 0.01 to 0.60%. In some embodiments of the present invention, the Si content is controlled to be 0.05 to 0.60% or 0.20 to 0.60%.

[0038] Mn: Mn is also an important strengthening element, effectively increasing the strength of the base steel plate. Furthermore, Mn is an effective austenite stabilizer, and adding a certain amount of Mn can increase the hardenability of steel. Therefore, to ensure steel strength, the lower limit of Mn is 0.30%. However, Mn is highly susceptible to center segregation in steel. Excessive addition of Mn can cause severe segregation at the center of the steel plate's thickness, reducing the low-temperature toughness of the core. Therefore, the upper limit of Mn content is limited to 0.70%. Based on this, the present invention controls the Mn content to between 0.30% and 0.70%.

[0039] Ti: As a deoxidizing element, Ti participates in the deoxidation reaction during the smelting process, which facilitates the precise control of the oxygen level of the molten steel. After Ti deoxidation, Ti2O3 particles can be formed, thereby promoting the formation of intracrystalline ferrite and improving the low-temperature impact toughness of the steel. At the same time, Ti is also a strong carbide-forming element and nitride-forming element. In the present invention, adding an appropriate amount of Ti element can, on the one hand, form a stable MC precipitation phase in the steel, fix the carbon element in the substrate, and prevent it from diffusing to the interface of the composite material. In addition, part of the Ti element participates in fixing the free N element in the steel to form a TiN precipitation phase. The TiN precipitation phase can prevent the growth of austenite grains, refine the grains, and simultaneously improve the strength and toughness of the steel plate. After Ti fixes part of the free nitrogen element, it helps to avoid the combination of B element and N element to form BN precipitation phase, thereby ensuring that the B element in the steel exists in the steel in a free form, improving the hardenability of the steel, and ensuring that the steel plate has excellent strength properties. In the alloy system of the present invention, when the Ti content is less than 0.009%, the amount of TiN and Ti2O3 formed is relatively small, weakening the pinning effect on the austenite grains. Therefore, the Ti content in the steel is controlled to be no less than 0.009%. However, when the Ti content in the steel is too high, the particle size of the TiN and Ti2O3 formed becomes larger, so that these precipitates no longer have the ability to hinder the growth of austenite and the formation of intragranular ferrite. Instead, they easily become the source of crack initiation, reducing the low-temperature impact toughness of the steel. Therefore, the upper limit of Ti is 0.015%. Based on this, the present invention controls the Ti content to 0.009-0.015%.

[0040] Cr: Adding an appropriate amount of Cr can improve the hardenability of the steel plate, ensuring uniformity of properties across the thickness of the thick steel plate after quenching, particularly ensuring high strength and toughness in the core of the steel plate. Adding Cr to steel can enhance the steel plate's oxidation resistance when operating at high temperatures of 450-530°C. Furthermore, as a strong carbide-forming element, Cr in steel can combine with free carbon in the steel plate to form stable MC carbides, securing the free carbon in the carbon steel and preventing carbon from diffusing into the interface and affecting the corrosion resistance of the composite material. When the Cr content in steel is low, its effect on the hardenability of the thick steel plate is weakened. Simultaneously, fewer stable carbides are formed, weakening the free carbon fixation effect. Therefore, in the present invention, the Cr content is set to no less than 1.85%. However, higher Cr contents can reduce the steel plate's resistance to temper embrittlement, making it less suitable for long-term operation at high temperatures. Therefore, the upper limit of Cr is set at 3.85%. For these reasons, the present invention controls the Cr content to 1.85-3.85%.

[0041] Mo: Mo is a strong carbide-forming element. Mo can improve the high-temperature creep strength of steel plates by forming a composite solid solution with interstitial elements such as carbon and nitrogen. Adding an appropriate amount of Mo can also form small, stable MC carbide precipitation phases, which not only achieves the effect of precipitation strengthening, but also plays a role in fixing free carbon in carbon steel. When the Mo content in the alloy system is lower than 0.50, the carbide density is low, and the precipitation strengthening and carbon fixation effects are reduced. However, when the Mo content in the steel is high, the precipitation phase is prone to ripening and becomes larger in size, and its precipitation strengthening effect is also weakened. Based on this, the present invention controls the Mo content to 0.50-1.55%.

[0042] Acid-soluble Al2O3: Al is an important deoxidizing element that forms 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 B and nitrogen in the steel, promotes an increase in the free B content in the steel, improves the hardenability of the steel, and contributes to improving the strength properties 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, will increase. Furthermore, since aluminum oxide inclusions have strong adsorption capacity, large-sized aluminum oxide clusters will form. These can clog the water inlet during steelmaking, affecting the casting process of the molten steel. If these large-sized clustered inclusions enter the steel plate, they will cause the steel plate to fail inclusion ratings and flaw detection. Therefore, the present invention controls the acid-soluble Al2O3 content to 0.0100-0.0150%.

[0043] Nb: Nb is a strong carbide-forming element. Nb can combine with carbon in steel to form MC precipitation phase, stabilize the carbon in steel, and prevent carbon from diffusing to the interface. At the same time, adding an appropriate amount of Nb can increase the recrystallization temperature of steel. When the steel is recrystallized and rolled in the austenite region, the austenite grains will not grow rapidly, and the strength and toughness of fine-grained steel are higher. However, when the Nb content in the steel is too high, the size of the MC precipitation phase formed will be larger, which will reduce the toughness of the steel plate and worsen the toughness of the weld heat-affected zone. Therefore, the present invention controls the Nb content to 0.0010-0.050%. In some embodiments, the present invention controls the Nb content to 0.010-0.050%.

[0044] B: The addition of an appropriate amount of B element is to compensate for the fact that after the free carbon in the steel is fixed by stable carbides, the solid solution carbon content in the steel is reduced, resulting in 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 the 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 after excessive B is added to the steel, the excessive B is prone to form a large amount of segregation at the grain boundaries, which is detrimental to the toughness of the steel. Therefore, the present invention controls the B content to 0.0007~0.0050%.

[0045] N: The alloy composition of the present invention incorporates appropriate amounts of Ti and Al. These two elements react with the nitrogen in the steel to form TiN and AlN precipitates. When these precipitates are fine and dispersed, they refine the austenite grains, thereby improving the strength and toughness of the steel. However, when the nitrogen content in the steel is too high, the excess nitrogen combines with the boron in the steel to form BN, consuming the free boron in the steel and reducing its effect on improving the steel's strength. Furthermore, excessive nitrogen can negatively impact the steel's toughness, particularly when the dissolved nitrogen content exceeds 0.005%, significantly reducing the steel's low-temperature toughness. Therefore, the present invention controls the nitrogen content to between 0.0025% and 0.0050%.

[0046] Preferably, the rolled composite steel plate substrate for high-temperature equipment according to the present invention further contains at least one of the following chemical elements: 0 < Ca ≤ 0.0055%, 0 < Ni ≤ 0.65%, and 0 < Cu ≤ 0.55%.

[0047] In the above technical solution of the present invention, the elements Ca, Ni and Cu can further improve the performance of the steel plate for high-temperature equipment of the present invention.

[0048] 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%。

[0049] 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%。

[0050] Cu: In the rolled composite steel plate substrate for high-temperature equipment described in the present invention, adding an appropriate amount of Cu element helps to improve the strength of the steel and improve the corrosion resistance of the steel. However, when the Cu content in the 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%。

[0051] It should be noted that the addition of the above-mentioned Ca, Ni and Cu elements will increase the 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.

[0052] Inevitable impurities exist in the composite steel plate substrate of the present invention, with P≤0.010%, S≤0.010%, and O≤0.004%.

[0053] P, S and O are all impurity elements in steel. When technical conditions permit, in order to obtain steel with better performance and higher quality, the content of impurity elements in steel should be reduced as much as possible.

[0054] 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 tempering embrittlement is particularly exacerbated when the steel is subjected to high temperatures. Therefore, in the rolled composite steel plate substrate for high-temperature equipment described herein, the mass percentage of P is controlled to ≤ 0.010%.

[0055] 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 can also lead to central segregation in the slab and easily cause hot cracking. Therefore, in the rolled composite steel plate substrate for high-temperature equipment described herein, the mass percentage of S is controlled to ≤ 0.010%.

[0056] O: Oxygen in steel primarily exists as oxides. Excessive O content in steel indicates an excess of oxides, potentially leading to the presence of large inclusions and affecting the strength and toughness of the steel plate. Therefore, in the rolled composite steel plate substrate for high-temperature equipment described herein, the mass percentage of O is controlled to ≤ 0.004%.

[0057] The surface structure of the composite steel plate substrate layer of the present invention is 90-95% bainite structure and 5-10% martensite structure, and the central microstructure is 90-95% bainite (B) and 0-5% martensite (M) structure + 0-5% ferrite (F) structure. In order to ensure that the tensile strength and low-temperature impact performance of the composite steel plate substrate layer meet the requirements, it is necessary to ensure that 90-95% of the structure of the surface and center of the steel plate thickness is bainite structure; 5-10% martensite structure on the surface of the steel plate can further improve the strength of the surface steel plate to compensate for the reduction in strength after part of the free carbon elements are converted into carbides during subsequent high-temperature service. 0-5% ferrite in the central microstructure is beneficial to improve its low-temperature impact toughness to compensate for the problem of reduced toughness in the center of the plate thickness due to segregation.

[0058] The substrate layer 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 400 nm. Because larger precipitates can adversely affect the mechanical properties of the steel sheet, especially low-temperature impact toughness, the present invention requires that the average major axis length of the MC precipitate phase in the steel sheet be controlled to be less than 400 nm. In the present invention, carbide precipitates are formed as small as possible in the substrate layer to ensure that the carbide carbon fixation effect is fully utilized without adversely affecting the mechanical properties of the steel sheet due to carbide precipitation. Therefore, the number of MC precipitates with a major axis length of less than 400 nm is limited to greater than 50% of the total MC precipitates.

[0059] Preferably, the volume density of the MC precipitated phase with a major axis length of less than 400 nm in the substrate layer is greater than 3.0×10 4 Pieces / mm 3 The purpose of forming MC precipitates in steel is to fix the free carbon in the steel. Therefore, in the present invention, it is necessary to ensure that the volume density of MC with a major axis length of less than 400 nm is greater than 3.0×10 4 Pieces / mm 3 In order to effectively control the carbon element in the steel and avoid the large-scale diffusion of free carbon elements to the stainless steel side. In some embodiments, the volume density of the MC precipitate phase with a major axis length of less than 400 nm in the substrate layer is ≥3.5×10 4 Pieces / mm 3 In some embodiments, the volume density of the MC precipitated phase with a major axis length of less than 400 nm in the substrate layer is 3.5×10 4 Pieces / mm 3 to 4.8×10 4 Pieces / mm 3 .

[0060] Preferably, the MC precipitates with a major axis length of less than 400 nm in the substrate layer 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. In the present invention, carbide precipitates are formed as small as possible to ensure that the carbide's carbon fixation function is fully utilized without adversely affecting the mechanical properties of the steel plate due to carbide precipitation. Therefore, the MC precipitates with a major axis length of less than 400 nm account for greater than 50% of the total MC precipitates.

[0061] The composite steel plate cladding material of the present invention has 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. The larger the carbide size in the cladding material, the greater the impact on the corrosion resistance of stainless steel. Therefore, it is hoped that the carbide precipitate phase size in the cladding material is as small as possible. However, during the production process of the cladding material and the rolled composite steel plate, some carbide precipitates are inevitably produced. In order 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 is less than 700 nm. In some embodiments, the average major axis length of the MC precipitate phase in the cladding material of the present invention is ≤675 nm, typically within the range of 600 to 675 nm.

[0062] 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.

[0063] Preferably, in the clad 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 carbide precipitates in the clad material and increase the corrosion resistance of the clad material, the present invention limits the size and number of the MC precipitates, namely, the MC precipitates with a major axis length of less than 700 nm account for greater than 50% of the total MC precipitates. When the number and size of the carbide precipitates exceed the limit, the corrosion resistance of the clad material will be greatly reduced, affecting the safe service performance of the rolled composite steel plate. In some embodiments, in the clad material of the present invention, the MC precipitates with a major axis length of less than 700 nm account for greater than or equal to 55% of the total MC precipitates.

[0064] The composite layer described in the present invention is stainless steel (including austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, duplex stainless steel, super stainless steel, and other stainless steel products). The σ phase in stainless steel is an intermetallic phase with very high hardness and very low plasticity. When present in stainless steel, especially when precipitated along grain boundaries, it significantly affects the plasticity of the steel, resulting in a significant reduction in the impact toughness of the steel. Therefore, the σ phase ratio of stainless steel is ≤1.5%, and in particular, the σ phase ratio near the bonding interface (transition layer) is ≤0.5%.

[0065] The present invention also provides a method for manufacturing the rolled composite steel plate for high-temperature equipment, comprising the following steps:

[0066] 1) Smelting and casting, preparation of base material and composite casting billet

[0067] The above composition is smelted and cast into a base material ingot. During the smelting process, deoxidizers Si+Mn, Al, and Ti alloys are sequentially added to the molten steel for deoxidation. Si+Mn and Al are first added for pre-deoxidation. After the pre-deoxidation, the oxygen level of the molten steel is 0.0020% to 0.0075%. Ti is then added for final deoxidation. After deoxidation, element B is added, followed by other alloys. After homogenization, the molten steel is cast using an over-pouring method. Simultaneously, stainless steel is produced or provided as a composite ingot.

[0068] 2) Assembly

[0069] The substrate and the composite slab are surface treated, then stacked, the four sides of the stacked composite slab are welded and sealed, vacuumed, and sealed again; preferably, the vacuum degree after vacuuming is 8 to 60 Pa;

[0070] 3) Composite slab rolling

[0071] The composite slab is heated to 1080-1250° 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%), the total reduction rate of the slab in the second stage is not less than 20% (e.g., 20-30%), the reduction rate of the last pass is 8-18%, and the total cumulative reduction rate of the two stages is not less than 80% (e.g., 80-97%); the final rolling temperature is above 880° C. (e.g., 880-960° C.);

[0072] 4) Heat treatment: quenching and tempering the composite steel plate.

[0073] Preferably, in step 2), the surfaces of the substrate and the composite ingot are processed to ensure that there are no obvious surface defects such as oil stains, slag inclusions, cracks, etc. on the ingot surface, and the roughness of the processed ingot surface must be no greater than 2.0 Ra.

[0074] Preferably, in step 4), the quenching temperature is 945-985°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.

[0075] Preferably, in step 4), the tempering temperature is 690-735° 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.

[0076] In the manufacturing method of the present invention:

[0077] The composite steel plate substrate described in the present invention requires the formation of a large number of uniformly dispersed nanoscale MC precipitates. These MC precipitates immobilize free carbon in the carbon steel substrate, preventing the carbon from diffusing extensively to the interface between the substrate and the composite material, which would otherwise form carbides 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. 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 and Mn are 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 range (0.0020-0.0075%), the Ti2O3 inclusions formed after deoxidation are of appropriate size (2-5 μm), which facilitates 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, playing a role in pinning the growth of austenite and refining the original austenite grains. Once the oxygen and titanium elements in the steel are properly controlled, an appropriate amount of B is added to ensure that the added B element is primarily present in the steel in a free state, ensuring that the base steel plate has high hardenability and significantly improving the strength of the steel plate. Most importantly, the aforementioned deoxidizer addition sequence and oxygen level control techniques form a large number of uniformly dispersed, fine (2-5 μm), spherical Ti2O3 oxides in the steel. These oxides serve as nucleation cores for the subsequent carbide precipitation. Because these carbide nucleation cores are uniformly dispersed throughout the steel, the precipitated carbides are also uniformly dispersed throughout the steel, maintaining a large number and small size, without adversely affecting the mechanical properties of the substrate.

[0078] In step 2), the surfaces of the substrate and composite slab are processed to ensure that the slab surface is free of obvious surface defects such as oil stains, slag inclusions, and cracks. The roughness of the processed slab surface must be no greater than 2.0 Ra. After evacuation, the vacuum level is 8-60 Pa, which ensures the best rolling performance for the composite slab.

[0079] In step 3), the composite slab is heated to 1080-1250°C for rolling. This is because heating temperatures below 1080°C prevent the carbides and nitrides in the base material from fully dissolving, reducing the carbides' pinning effect on the austenite grains during rolling. Furthermore, this temperature range facilitates homogenization of the austenite in the composite material and complete dissolution of the carbides in the composite material. However, heating the composite slab above 1250°C can lead to rapid growth of the austenite grains in the base material and composite material, compromising the mechanical properties of the steel.

[0080] In addition, 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 880°C.

[0081] 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.

[0082] 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 reduction rate in the final rolling pass 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 880℃, the deformation resistance of the steel plate increases, making it difficult to ensure a large final rolling reduction rate.

[0083] During the quenching process, when the quenching temperature is below 945°C, the steel's austenite takes a long time to homogenize, reducing heat treatment efficiency. If the quenching temperature is above 985°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 less than 1 hour, sufficient austenitization cannot occur. When the quenching temperature holds for more than 1.2 hours, the required austenitization time is exceeded, reducing the efficiency of the quenching process.

[0084] During the tempering process, when the tempering temperature of steel is below 690°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 735°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 1.0h, the quenching stress in the steel tends to not be completely removed, while when the tempering holding time T2 is greater than 1.5h, the MC precipitates in the steel tend to coarsen, reducing the production efficiency of the tempering process.

[0085] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0086] The present invention forms a large number of dispersed nanoscale MC (M represents one or more of Cr, Mo, Nb, and Ti) precipitates in the steel by forming fine, dispersed spherical (2-5 μm) Ti2O3 oxides that serve as nucleation cores for MC carbide precipitation phases. This stabilizes free carbon in the carbon steel, prevents carbon from diffusing and aggregating at the composite interface (transition layer), reduces carbide precipitation at the grain boundaries of the stainless steel, and ensures the corrosion resistance of the composite rolled steel plate. In particular, when the rolled composite steel plate is used in high-temperature equipment, under high-temperature operating conditions of 450°C to 530°C, the high-melting-point Ti2O3 oxide (melting point of 2130°C) can still stably exist in the steel and continue to function as a precipitation core for MC carbides. 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] Furthermore, the most typical smelting technology used in the production of low-alloy, high-strength steel is aluminum-deoxidized steel, which controls the oxygen content in the steel within a target range through aluminum deoxidation. This process boasts high deoxidation efficiency, but it also leads to the formation of large and numerous Al2O3 inclusions. These inclusions tend to aggregate and form clusters within the steel, causing the steel to fail flaw detection, mechanical properties, or inclusion ratings. Furthermore, most Al2O3 inclusions are irregular, polyhedral, and blocky, which cannot serve as carbide nucleation cores to promote uniform carbide mass distribution. Consequently, in aluminum-killed steel, carbide precipitates tend to aggregate at grain boundaries and form large particles, adversely affecting both the conventional and high-temperature properties of the steel.

[0088] The present invention optimizes the production process, that is, by controlling the type of deoxidizer, the deoxidation sequence and the amount of deoxidizer added during the steel smelting process, so that the type of deoxidation product in the steel can be controlled to be a spherical (2-5μm) Ti2O3 oxide with a high melting point and fine dispersion. Compared with the Al2O3 inclusions formed in traditional aluminum-killed steel, this type of oxide is not easy to aggregate and grow, has no sharp edges, is harmless to the flaw detection and mechanical properties of the steel plate, and can optimize the structure type of the steel plate; through TiN precipitation, the austenite grains are refined; by designing the addition sequence of the steel alloy, the oxidation and nitridation amounts of the B element are reduced, and the added B element can be fully utilized to maximize the hardenability of the steel plate, refine the grains, and ensure that the steel plate has high strength and excellent low-temperature toughness.

[0089] Furthermore, through smelting process control, particularly by controlling the type, sequence, and amount of deoxidizers 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 and heat treatment process design, a large number of dispersed, nanoscale MC (M represents one or more of Cr, Mo, Nb, and Ti) precipitates can be formed in the steel. This immobilizes free carbon in the carbon steel, preventing carbon from diffusing and aggregating at the composite interface (transition layer), reducing the precipitation of carbides at the grain boundaries of the stainless steel, and 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 450-530°C, continuing to function as precipitation cores for MC carbides. 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.

[0090] The rolled composite steel plate for high-temperature equipment produced by 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 450-530°C. It has very important practical significance and a very broad application prospect. DETAILED DESCRIPTION

[0091] The present invention will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an improper limitation on the technical solution of the present invention.

[0092] The composition of the examples of the present invention is shown in Table 1, with the remainder comprising Fe and other unavoidable impurities. In the examples and comparative examples, austenitic stainless steel is used as the cladding material for the rolled composite steel plates. The manufacturing process parameters for the examples of the present invention are shown in Table 2. Commercially available austenitic stainless steel can be used as the cladding slab.

[0093] The manufacturing method of the embodiment of the present invention comprises the following steps:

[0094] 1) Smelting and casting: Smelting and continuous casting are carried out according to the chemical composition shown in Table 1 to produce slabs; the smelting equipment is a 500 kg vacuum induction furnace. First, 420 kg of industrial pure iron is added to the vacuum induction furnace. According to the alloy composition ratio of the steel, a certain amount of CaO is added with the furnace (the added CaO is used for slag formation during smelting); vacuum is evacuated and smelting is carried out at a minimum vacuum 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. Based on the online monitoring results of the molten steel composition, the amount of deoxidizer added is fine-tuned and the oxygen level of free oxygen in the molten steel is controlled to 0.0020% to 0.0075%. Then, an appropriate amount of Ti is added for final deoxidation; alloys such as Nb, Cr, and Mo are added for alloying treatment, and finally B is added; casting is performed using the top pouring method to form a cast slab;

[0095] 2) Assembling: The base carbon steel ingot and the clad stainless steel ingot are surface treated to ensure that the surface roughness of the two materials is ≤2.0Ra. After surface inspection, the surface is guaranteed to be free of surface defects such as oil stains, slag inclusions, and cracks. The base carbon steel ingot and the clad stainless steel ingot are superimposed to form a composite slab. The composite slab is welded all around and then vacuumed to a vacuum degree of 8-60Pa.

[0096] 3) rolling, heating the composite slab at 1080-1250°C, and then performing two-stage rolling, with the total cumulative rolling reduction rate not less than 80%, the total rolling reduction rate of the slab in the first stage not less than 60%, the total rolling reduction rate in the second stage not less than 20%, the final rolling reduction rate of 8-18%, and the final rolling temperature being above 880°C;

[0097] 4) Heat treatment, quenching and tempering the rolled composite steel plate; quenching the composite steel plate at a quenching temperature of 945-985°C, a quenching holding time T1 of 1-1.2 hours, and after the holding is completed, the composite steel plate is removed from the furnace and water-cooled to room temperature; then tempering is performed at a tempering temperature of 690-735°C, a tempering holding time T2 of 1-1.5 hours, and air-cooling is performed after removal from the furnace.

[0098] In the present invention, the chemical composition design and related processes of the composite steel plates of Examples 1 to 8 all meet the design specification requirements of the present invention.

[0099] Accordingly, the composite steel plates of Comparative Examples 1 to 5 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 4 exceeds the design range, and the alloy range of Comparative Example 5 is still within the design range. However, in the smelting and casting operations of the above step 1), the order of adding deoxidizers and the order of adding B alloys in Comparative Examples 1 to 5 are different; and the heat treatment and tempering processes are different.

[0100] Electrolytic samples were taken from the carbon steel substrate material and the stainless steel clad layer at the composite interface of the composite steel plates of Examples 1 to 8 of the present invention and the control rolled composite steel plates of Comparative Examples 1 to 5. MC precipitates were extracted from the carbon steel substrate and stainless steel clad layer using electrolytic extraction. Precipitates of varying size ranges were separated using filter membranes of varying particle sizes. The particle size distribution of the precipitates was determined 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, Ti, Cr, and Mo) in the steel plates of each embodiment and comparative example. The data obtained from these observations and analyses are summarized in Table 3 (substrate material) and Table 4 (interface, i.e., transition layer, clad layer material).

[0101] As shown in Table 3, the total number of precipitates in the carbon steel base material of the rolled composite steel plates of Examples 1 to 8 ranges from 747 to 908, which is much higher than the 467 to 547 precipitates in the carbon steel base material of Comparative Examples 1 to 5. This indicates that after the implementation of the technical solution of the present invention, a larger number of MC (M is one or more of Cr, Mo, Ti, and Nb) carbide precipitates can be formed in the carbon steel base material of Examples 1 to 8, thereby ensuring the effect of using the stable carbides in the carbon steel base material 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 to 8 is relatively small, with their main distribution range being 100 to 600 nm. The average major axis length of the MC precipitates is less than 400 nm, and the volume density of the MC precipitates is greater than 3.0×10 4 Pieces / mm 3 The MC precipitates with a size of less than 400 nm 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.

[0102] In contrast, the MC precipitates in the carbon steel substrates of the rolled composite steel plates of Comparative Examples 1-5 are larger in size, with an average major axis length greater than 800 nm. The size distribution of the main precipitates is above 800 nm, and the proportion of precipitates larger than 1000 nm is relatively high. This poor pinning effect on the austenite grains results in larger grain sizes, affecting the strength and toughness of the steel plates. Due to the small number of stable precipitates formed in the carbon steel materials of Comparative Examples 1-5, the precipitates reduce the fixation of free carbon in the carbon steel substrate, causing a large amount of free carbon to segregate toward the bonding interface (transition layer), thereby generating a large amount of grain boundary carbides in the composite material at the bonding interface, reducing the corrosion resistance of the composite material at the bonding interface.

[0103] As shown in Table 4, in the composite materials near the transition layer of the composite steel plates of Examples 1 to 8, the number of MC (M is one or more of Cr, Mo, Ti, and Nb) precipitates detected is 145 to 176, which is much less than 503 to 544 in Comparative Examples 1 to 5. Moreover, the volume density of the MC precipitates at the interface of the composite materials of Examples 1 to 8 is less than 1×10 4 Pieces / mm 3 The volume density of the MC precipitated phase of the composite material at the interface of Comparative Examples 1 to 5 is greater than 1.5×10 4 Pieces / mm 3 This indicates that the carbides in the carbon steel substrate of Examples 1 to 8 have a better effect on fixing the free carbon in the steel, and the amount of free carbon that diffuses into the cladding material through the bonding interface is small, so the amount of MC formed in the cladding material is small. In contrast, in Comparative Examples 1 to 5, since the carbide precipitation phase in the carbon steel substrate has a poor effect on fixing the carbon, the carbon in the carbon steel diffuses into the cladding material through the bonding interface in large quantities, resulting in a large amount of carbides in the corresponding cladding material, and a volume density greater than 1.5×10 4 Pieces / mm 3 Table 4 also shows that in the composite materials at the bonding interface of Examples 1-7, the average major axis length of the MC precipitates is less than 700 nm, and MC precipitates with a size less than 700 nm account for more than 50% of the total MC precipitates. Correspondingly, the average size of the MC precipitates observed in the comparative examples is greater than 900 nm. This further demonstrates that the carbides in Examples 1-8 have a far better effect on the fixation of free carbon than those in Comparative Examples 1-5.

[0104] After analyzing the carbide precipitation phase of the steel plates for high-temperature equipment of Examples 1 to 8 and the comparative steel plates of Comparative Examples 1 to 5, the mechanical properties (according to ASTM A370-24, ASTM A264-12 (2019) standards) and corrosion resistance (according to ASTM A262-15 (2021) practice E standard) of the steel plates of Examples 1 to 8 and Comparative Examples 1 to 5 were also tested. The test results are shown in Table 5.

[0105] As shown in Table 5, the composite steel plate substrate of the present invention has a yield strength of 455 MPa or greater, a tensile strength of 535 MPa or greater, a reduction of area of ​​65% or greater, an elongation of 18% or greater, an impact absorption capacity of 120 J at -40°C, and 70 J at -60°C. The shear strength of the composite steel plate is 330 MPa or greater.

[0106] The yield strength and tensile strength of the carbon steel substrate in the comparative example are lower than those in the examples. This is mainly because the smelting process, alloy addition sequence, and heat treatment process of the carbon steel in the comparative example differ from those in the examples. As a result, the hardenability of the B element in the comparative example steel cannot be fully exerted. In addition, 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 described in the present invention is 90-95% bainite + and 5-10% martensite + MC precipitation phases, and the central microstructure of the substrate is 90-95% bainite (B) and 0-5% martensite (M) structure + 0-5% ferrite (F) structure. The microstructural ratio of the carbon steel material in the comparative example is significantly different from that in the examples. The ratio of bainite and martensite is significantly reduced, and the ratio of ferrite is increased, which also affects the strength of the steel plate.

[0107] In addition, a large number of smaller precipitates in the carbon steel substrates of Examples 1 to 8 further refine the austenite grains. The grain size of the carbon steel substrates of the examples is not less than 7.5, while the grain size of the carbon steel substrates in the comparative examples is in the range of 5 to 5.5. It can be seen that the embodiments fully utilize the technical solution of the present invention to pin austenite and refine the grains, which plays a certain role in improving the strength and toughness of the steel plates.

[0108] Corrosion testing of the stainless steel composites at the interface of the rolled composite steel plates from the examples and comparative examples was conducted according to ASTM A262 Practice E. The test results are shown in Table 6. These results demonstrate that the stainless steel composites at the interface of the rolled composite steel plates produced according to the present invention exhibit excellent corrosion resistance. Therefore, the corrosion resistance of the composite stainless steel composites of Examples 1-8 is significantly superior to that of Comparative Examples 1-5.

[0109] In summary, the composite steel plate suitable for high-temperature equipment with an operating temperature of 450-530°C according to the present invention can be produced in the base steel by optimizing the composition design and production process, especially by controlling the type of deoxidizer, the deoxidation sequence and the amount of deoxidizer added during the steel smelting process, adding the B element in a timely and appropriate amount, and then coordinating with other production processes. A large number of dispersed nano-scale MC (M represents one or more of Cr, Mo, Nb, and Ti) precipitation phases can be generated. 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, resulting in the formation of a large number of carbide precipitation phases in the stainless steel, which reduces the corrosion resistance of the stainless steel. 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, thereby ensuring the intrinsic safety performance of the rolled composite steel plate when it is in high-temperature and long-term service.

[0110] In addition, it should be noted that the combination of the various technical features in the present invention 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.

[0111] 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.

[0112] Table 6

Claims

1. A corrosion-resistant rolled composite steel plate for high-temperature equipment, which is rolled by composite rolling of carbon steel as a substrate and stainless steel as a composite layer, and comprises a substrate layer, a composite layer and a transition layer between the substrate and the composite layer; the components of the substrate layer are as follows by weight: C: 0.080% to 0.180%, Si: 0.01% to 0.60%, Mn: 0.30% to 0.70%, Ti: 0.009% to 0.015%, acid-soluble Als: 0.0100% to 0.0150%, Nb: 0.0010% to 0.050%, Cr: 1.85% to 3.85%, Mo: 0.5% to 1.55%, B: 0.0007% to 0.0050%, N: 0.0025% to 0.0050%, P≤0.010%, S≤0.010%, O≤0.004%, and the balance comprises Fe and other inevitable impurities; The surface microstructure of the substrate is 90-95% bainite + 5-10% martensite + MC precipitation phase, and the central microstructure of the substrate layer is 90-95% bainite + 0-5% martensite + 0-5% ferrite + MC precipitation phase; wherein, M represents one or more of Cr, Mo, Nb, and Ti, the average major axis length of the MC precipitates is less than 400 nm, and the volume density of the MC precipitates with a major axis length less than 400 nm is greater than 3.0×10 4 Pieces / mm 3 ; The microstructure of the multilayer contains MC precipitates; wherein M represents one or more of Cr, Mo, Nb, 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 high-temperature equipment according to claim 1, characterized in that: The substrate layer 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 high-temperature equipment according to claim 1, characterized in that: In the substrate layer: The content of Si is 0.05-0.60% or 0.20-0.60%; and / or The content of Nb is 0.010 to 0.050%.

4. The corrosion-resistant rolled composite steel plate for high-temperature equipment according to any one of claims 1 to 3, characterized in that: In the base material layer, the balance is Fe and other inevitable impurities.

5. The corrosion-resistant rolled composite steel plate for high-temperature equipment according to any one of claims 1 to 4, characterized in that: The MC precipitated phases with a major axis length of less than 400 nm in the substrate layer account for more than 50% of the total MC precipitated phases.

6. The corrosion-resistant rolled composite steel plate for high-temperature equipment according to any one of claims 1 to 5, characterized in that: The stainless steel is austenitic stainless steel, ferritic stainless steel, duplex stainless steel or super stainless steel.

7. The corrosion-resistant rolled composite steel plate for high-temperature equipment according to any one of claims 1 to 6, 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.

8. The corrosion-resistant rolled composite steel plate for high-temperature equipment according to any one of claims 1 to 7, 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.

9. The corrosion-resistant rolled composite steel plate for high-temperature equipment according to any one of claims 1 to 8, characterized in that: The microstructure of the composite steel plate 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, ie, the bonding interface, is ≤0.5%.

10. The corrosion-resistant rolled composite steel plate for high-temperature equipment according to any one of claims 1 to 9, characterized in that: The thickness of the composite steel plate substrate is ≥6 mm, and the thickness of the composite layer is ≥2 mm; preferably, the thickness of the transition layer is ≤250 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 1 to 230 μm, 1 to 150 μm or 1 to 50 μm.

11. The corrosion-resistant rolled composite steel plate for high-temperature equipment according to any one of claims 1 to 10, characterized in that: The yield strength of the rolled composite steel plate substrate is ≥455MPa, the tensile strength is ≥535MPa, the cross-sectional shrinkage is ≥65%, the elongation is ≥18%, the -40°C impact absorption energy is ≥120J, and the -60°C impact absorption capacity is ≥70J; the substrate grain size is not less than grade 7.0; the shear strength of the composite steel plate is ≥330MPa; preferably: The yield strength of the base material layer of the composite steel plate is ≥470MPa or ≥480MPa, or between 455 and 515MPa; The tensile strength of the base material layer of the composite steel plate is ≥550MPa or ≥570MPa, or between 535 and 635MPa; The cross-sectional shrinkage of the base material layer of the composite steel plate is ≥70%, or between 65% and 75%; The elongation of the base material layer of the composite steel plate is ≥20%, or between 18% and 25%; The -40°C impact absorption energy of the substrate layer of the composite steel plate is ≥150 J or ≥180 J, or between 120 and 255 J, such as between 180 and 255 J; The -60°C impact absorption energy of the substrate layer of the composite steel plate is ≥100J, ≥130J or ≥150J, or between 70 and 190J, or between 100 and 190J, or between 130 and 190J, or between 150 and 190J; The shear strength of the composite steel plate is ≥350MPa, ≥380MPa or ≥400MPa, or between 330 and 470MPa.

12. The method for manufacturing a corrosion-resistant rolled composite steel plate for high-temperature equipment according to any one of claims 1 to 10, characterized in that: The steps include: 1) Smelting and casting, preparation of base material casting billet According to the composition of claim 1 or 2 or 3 or 4, the base material ingot is smelted and cast, and deoxidizers Si+Mn, Al, and Ti alloys are sequentially added to the molten steel during the smelting process for deoxidation, wherein Si+Mn and Al are first added for pre-deoxidation, and the oxygen level of the molten steel is 0.0020% to 0.0075% after the pre-deoxidation, and then Ti is added for final deoxidation; after the deoxidation is completed, B element is added, and then other alloys are added, and after homogenization, casting is performed using the top pouring method; Preparing or providing stainless steel as a composite casting slab; 2) Assembly The substrate ingot and the composite ingot are surface treated, and then stacked, and the four sides of the stacked composite slab are welded and sealed, vacuumed, and sealed again; preferably, after vacuuming, the vacuum degree is 8-60Pa; 3) Composite slab rolling The composite slab is heated to 1080-1250°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%, and the total cumulative reduction rate of the two-stage rolling is not less than 80%; the final rolling temperature is above 880°C; 4) Heat treatment: quenching and tempering heat treatment of the composite steel plate.

13. The method for manufacturing the corrosion-resistant rolled composite steel plate for high-temperature equipment according to claim 12, wherein: In step 2), the surfaces of the substrate ingot and the composite ingot are processed to ensure that there are no obvious surface defects such as oil stains, slag inclusions, cracks, etc. on the surface of the ingot, and at the same time, the roughness of the processed surface of the ingot must be no greater than 2.0 Ra.

14. The method for manufacturing the corrosion-resistant rolled composite steel plate for high-temperature equipment according to claim 12, wherein: In step 4), the quenching temperature is 945-985°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 high-temperature equipment according to any one of claims 12 to 14, characterized in that: In step 4), the tempering temperature is 690-735°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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