A method for manufacturing a multi-layer composite steel having a harmonic structure

Abstract

The application discloses a preparation method of a multilayer composite steel with a harmonic structure. The method selects austenitic steel with an FCC face-centered cubic structure and ferritic steel with a BCC body-centered cubic structure as metal materials with different crystal structures, adopts a vacuum hot rolling + cold rolling + heat treatment process, and obtains the multilayer composite steel with the harmonic structure through temperature control rolling. The strength and plasticity of the multilayer composite steel with the harmonic structure obtained by the application are greatly improved, the purpose of strengthening and toughening is achieved, and a new idea is provided for a layer / net coupling interface.

Description

Technical Field

[0001] This invention relates to the preparation of a multilayer composite steel with a harmonic structure, and to deformation processing technology, which is applicable to the field of metal hot working. Background Technology

[0002] In today's world, the performance requirements for materials are increasingly demanding, and materials with simple structures can no longer meet the needs of industrialization. Researchers have begun to explore high-performance structural materials with a balance of strength and toughness. Traditional strengthening methods for metallic materials mainly include solid solution strengthening, deformation strengthening, grain refinement strengthening, and second-phase strengthening. Among these, grain refinement strengthening can improve both the strength and ductility of materials. However, traditional strengthening mechanisms tend to result in an inverse strength-ductility relationship in metallic materials, meaning that higher strength often corresponds to lower ductility and toughness. The strength-ductility curve often exhibits a concave "banana" shape, compromising safety. Therefore, how to maintain high strength while ensuring good ductility is a pressing issue that needs to be addressed.

[0003] In recent years, the design concept of multi-level, multi-scale structures has become a guiding principle for optimizing the strength and plasticity of metallic structural materials. Among them, multi-layer composite steel and heterogeneous structural steel have become one of the current research hotspots. These materials mainly utilize controlled rolling and controlled cooling technology to construct ultrafine-grained and layered structures, thereby obtaining high strength and plasticity. Through delamination fracture at interlayer or phase interfaces, they effectively improve the fracture and impact toughness of steel materials. Straight interlayer interfaces have always been a goal pursued by researchers. However, in reality, with the increase of deformation, the work hardening behavior between the "hard phase" and the "soft phase" will show significant differences, often leading to the formation of wavy interfaces. For example, Damascus steel knives are constructed by folding and forging low-carbon and high-carbon steels to create a multi-layer composite structure, with the interface exhibiting complex Damascus patterns. Exploring the deformation coordination of interlayer interfaces and their role in strengthening and toughening metallic materials has become one of the current challenges in the research of multi-layer composite steel.

[0004] Inspired by ancient Damascus steel swords, researchers have developed layered metal composites by combining metals with high ductility and low strength with metals with high strength and low ductility, thereby achieving a dual improvement in strength and ductility. Therefore, layered metal composites are a very promising structural material that can effectively improve the strength-toughness mismatch in single materials, fully utilize the performance advantages of each layer component to improve the overall performance of the material, and are finding increasing applications in many important fields such as aerospace, transportation, and electronic information equipment.

[0005] Harmonic structure materials, also known as core-shell materials, are essentially bimodal structures composed of coarse-grained and nanocrystalline regions arranged in a specific three-dimensional periodic pattern. Hard, ultrafine-grained / nanocrystalline regions surround soft, coarse-grained regions in three dimensions, forming a continuously linked network structure. This improves dislocation plasticity within the particles by promoting dislocation accumulation, and simultaneously enhances the strength-plasticity relationship through heterogeneous deformation-induced strengthening. Harmonic structures are typically prepared using traditional powder metallurgy methods. Each unit initially consists of metal powder, which undergoes severe plastic deformation on its surface through ball milling, grinding, high-pressure gas jet milling, and shot peening. During this severe plastic deformation, dislocations are repeatedly introduced and grains are segmented until a nanoscale ultrafine-grained structure is formed. Finally, the harmonic structure is prepared by sintering these near-surface nanocrystals and the internal coarse grains. However, powder metallurgy cannot produce large-scale products because the flowability of powder is worse than that of molten metal, thus limiting its shape and size. Furthermore, ball milling has low efficiency, consumes a lot of energy, and results in significant frictional loss between the grinding media and the machine body. During shot peening, the compressive stress on the material surface is also high, which to some extent affects the material's molding and usability. In the final sintering process, the selection and control of temperature affect the quality of harmonic structure formation. Therefore, harmonic structures prepared by traditional powder metallurgy methods cannot be applied to large thin plates, while the method described in this paper successfully prepares composite materials with harmonic structures on thin plates. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of current technologies by providing a method for preparing multilayer composite steel with a harmonic structure. This method involves selecting two metallic materials with different crystal structures and distributing them alternately, employing a vacuum hot rolling + cold rolling + heat treatment process to obtain multilayer composite steel with a harmonic structure through temperature-controlled rolling. The multilayer composite steel with a harmonic structure obtained by this invention exhibits significantly improved strength and ductility, achieving the goal of enhanced strength and toughness, and also provides a new approach to the layer / network coupling interface.

[0007] The specific technical solution of the present invention is as follows:

[0008] A method for preparing multilayer composite steel with harmonic structure, the method comprising the following steps:

[0009] (1) Cut the FCC face-centered cubic austenitic steel and the BCC body-centered cubic ferritic steel into sheets respectively;

[0010] The thickness of the sheet is 0.5-2mm;

[0011] The FCC face-centered cubic austenitic steel is specifically SUS304 austenitic stainless steel, and the BCC body-centered cubic ferritic steel is specifically Q235, 45# carbon steel or Cr17.

[0012] The sheet material is preferably a round or rectangular sheet; its diameter or length is 60-80 mm.

[0013] (2) The sheet obtained in step (1) is sanded with SiC sandpaper;

[0014] (3) Place the sheet obtained in step (2) in acetone for ultrasonic treatment to remove surface oil and clean the surface;

[0015] (4) Stack the two different materials obtained in step (3) into a carbon steel box in an “AB…AB” pattern, with a total of 2n layers and n = 30 to 80.

[0016] (5) Seal the carbon steel box obtained in step (4) by electric welding;

[0017] (6) Vacuum treatment is applied to the sealed carbon steel box until the vacuum degree inside the billet reaches 10. -2 -10 -4 Pa;

[0018] (7) Place the billet obtained in step (6) into a box furnace and heat it to 1150-1200℃ for 40-60 minutes to ensure that the base material is completely austenitized. Then send the billet into a hot rolling mill for 3-5 passes until the thickness reaches 6-15mm. The total hot rolling reduction is 85-88%. Then air cool to room temperature.

[0019] (8) The hot-rolled sample was subjected to cold rolling deformation at room temperature, and the cold rolling deformation amount was controlled to be 80% to 90%.

[0020] (9) The cold-rolled sample was subjected to heat treatment experiment. The temperature was set at 500-800℃ and held for 5-10 minutes to obtain multi-layer composite steel with harmonic structure.

[0021] The essential features of this invention:

[0022] The two raw materials selected in this invention are metallic materials with different crystal structures: FCC face-centered cubic austenitic steel and BCC body-centered cubic ferritic steel. This is particularly important because the deformation incompatibility between ferrite and austenite during rolling deformation will cause interface bending, providing a basis for the formation of harmonic structures. Subsequently, these two raw materials are stacked in a cross-laminated manner to form 120 layers, placed in a billet box, and subjected to vacuum treatment to achieve a vacuum degree of 10. -2Pa is used to prevent oxide formation at the interface from affecting the interfacial bonding. The material is then placed in a heat treatment furnace at 1200℃ to ensure both raw materials are in the fully austenitic region. Hot rolling experiments are then performed to obtain a hot-rolled multilayer composite material with a flat interface. Cold rolling experiments are then conducted, and controlling the cold rolling reduction to 90% yields a layer / network coupled harmonic structure.

[0023] This invention involves controlling the degree of deformation by subjecting hot-rolled multilayer composite steel to large-deformation cold rolling below the recrystallization temperature. By controlling the cold rolling reduction to 80%-90%, a significant difference in work hardening behavior between the hard and soft phases in the multilayer composite steel is created, resulting in a wavy harmonic structure that significantly enhances the strength and plasticity of the multilayer composite material. The emergence of this harmonic structure improves the traditional strength-plasticity inversion relationship, increasing strength without sacrificing plasticity, and even enhancing plasticity. This has significant implications for structural materials.

[0024] The beneficial effects of this invention are:

[0025] (1) The raw materials selected in this invention are austenitic steel with FCC structure and ferritic steel with BCC structure. Since the microstructures of austenite and ferrite are completely different, the deformation synergy of the two materials is inconsistent during the deformation process, which provides a theoretical basis for the generation of harmonic structures.

[0026] (2) First, vacuum hot rolling is performed to ensure that there are no obvious interfacial inclusions and oxides at the interfaces between the layer components. When the vacuum degree reaches 10... -2 At Pa, the interface of the hot-rolled multilayer composite steel is free of inclusions, obvious pores, and microcracks. It is worth noting that when vacuum hot rolling is used for the first time, the rolling reduction should be controlled at about 85% to ensure good metallurgical bonding between the heterogeneous phases, thus preventing interface delamination during subsequent rolling processes.

[0027] (3) Austenitic stainless steel with FCC face-centered cubic structure exhibits severe work hardening behavior. During cold rolling, as the cold rolling reduction increases, the "softening" phenomenon of the harder phase austenitic stainless steel layer becomes more and more obvious, which leads to the generation of local necking. In other words, with the continuous increase of the reduction, the deformation of austenitic stainless steel is uneven, while the softer phase Q235 carbon steel layer is still in a high plastic deformation state, which will eventually lead to severe deformation incoordination and interface bending in multilayer composite steel. When the cold rolling reduction reaches 90%, a harmonic structure of "soft phase" covering "hard phase" is generated.

[0028] (4) Research on the generation of harmonic structures in steel materials is almost non-existent. However, this patent, which differs from traditional powder metallurgy and sintering methods, successfully prepares multilayer steel composite materials with harmonic structures, providing a universal preparation method for a series of steel materials. For example, in Example 1, SUS304 austenitic stainless steel and Q235 low-carbon steel were composited. Using the preparation method of this invention, multilayer composite steel with harmonic structures was successfully prepared in steel materials. The tensile test results were: tensile strength of 748 MPa and elongation after fracture of 25%. In Example 2, SUS304 austenitic stainless steel and medium carbon 45# steel were composited. Using the preparation method of this invention, multilayer composite steel with harmonic structures was successfully prepared. The tensile test results were: tensile strength of 1.0 GPa and elongation after fracture of 18%, significantly improving its strength and toughness. In Example 3, SUS304 austenitic stainless steel and Cr17 ferritic stainless steel were used, and a multilayer composite steel with a harmonic structure was successfully prepared using the present invention. Tensile results showed that the tensile strength after cold rolling and annealing at 650°C for 6 min was 960 MPa, and the elongation after fracture was 28%, achieving a good match between plasticity and toughness. Attached Figure Description

[0029] Figure 1 A process flow diagram for preparing multilayer composite steel with harmonic structure according to the present invention;

[0030] Figure 2 The SUS304 / Q235 multilayer composite steel with harmonic structure prepared in Example 1;

[0031] Figure 3 The SUS304 / 45# multilayer composite steel with harmonic structure prepared in Example 2;

[0032] Figure 4 The SUS304 / Cr17 multilayer composite steel with harmonic structure prepared in Example 3 is shown. Detailed Implementation

[0033] The process flow diagram for preparing multilayer composite steel with harmonic structure according to this invention is as follows: Figure 1 As shown, firstly, two raw materials with different crystal structures are cut into 0.5-2mm sheets using wire electrical discharge machining (EDM). The wire cutting marks are removed with sandpaper, and the sheets are cleaned. Then, the two raw materials are cross-layered in an "AB…AB" pattern and placed in a carbon steel billet box equipped with a vacuum tube. Teflon high-temperature insulating cloth is placed at the bottom and top to prevent the carbon steel box from sticking to the materials during subsequent hot rolling. The boxes are then sealed and vacuum-treated, and hot rolling experiments are conducted above the recrystallization temperature.

[0034] This invention effectively combines large-deformation rolling technology with heat treatment to prepare a multilayer composite steel with a harmonic structure by controlling the component composition. Due to the large deformation, elongated grains are observed in the rolling direction (RD), providing a certain degree of grain refinement and strengthening. Further increasing the deformation, due to the coordinated deformation of the hard and soft phases, the interface undergoes plastic instability, gradually transforming from a straight state to a curved state, and eventually into a network harmonic structure. This improves the processability, plastic deformation capacity, and comprehensive mechanical properties of the multilayer material to a certain extent, and provides a new solution for the topology optimization design of multilayer composite steel. This has significant implications for achieving ultra-high strength and toughness in the future steel industry.

[0035] The hard and soft phase materials involved in this invention are SUS304 austenitic stainless steel and low carbon steels Q235, 45# steel and Cr17 steel. The main chemical compositions are as follows: SUS304 steel: 68%-70% Fe, 18%-20% Cr, 8%-10% Ni, 0.020%-0.030% C; Q235 steel: 0.15-0.22% C, 0.35-0.6% Mn, S<0.30%, S<0.05%, P<0.045%; 45# steel: 0.42-0.48% C, 0.17-0.37% Si, 0.50-0.70% Mn, Cr≤0.25%, Ni≤0.30%; Cr17 steel: 78-82% Fe, 18-20% Cr, 0.3-0.5% Ni, 0.02-0.03% C, 0.4-0.55% Mn, P≤0.045%, S≤0.05%.

[0036] The carbon steel boxes described in the following embodiments are cylindrical or rectangular. The cylindrical carbon steel boxes have a diameter of 65-85mm and a height of 35-60mm; the rectangular carbon steel boxes have a length and width of 70-90mm, a height of 35-85mm, and a wall thickness of 2-3mm. It is important to note that during actual hot rolling, the height of the carbon steel box must not exceed its diameter or dimensions to prevent it from tipping over and being damaged.

[0037] Example 1 (SUS304 austenitic stainless steel / Q235 low carbon steel)

[0038] The hard and soft layers used in Example 1 are SUS304 austenitic stainless steel and Q235 low-carbon steel, which are commonly available on the market.

[0039] The two types of steel were cut into 60 circular pieces each, each 60mm in diameter and 0.5mm thick, using an EDM wire cutter. The cut steel was then polished to a bright finish using 60# and 240# sandpaper, ultrasonically cleaned in acetone, and dried. The treated SUS304 and Q235 steel were then stacked in an "AB…AB" pattern, forming 120 pieces with a height of 60mm. These were placed in a cylindrical carbon steel box with a diameter of 70mm and a height of 60mm and sealed using manual arc welding. It is important to place Teflon high-temperature insulating cloth at the bottom and top during sealing to ensure the outer billet is separated from the base material after hot rolling. A vacuum pump was then used to create a vacuum level of 10 ppm inside the billet box. -2 Pa, awaiting rolling.

[0040] The specific steps are as follows:

[0041] (1) Place the obtained billet into a box furnace and heat it to 1200℃. Then place the processed billet in the furnace and keep it warm for 40 minutes.

[0042] (2) Take out the billet and quickly feed it into the two-roll hot rolling mill for rolling test. It should be noted that the rolling direction should be kept in one direction, and the operator should wear protective gear and use clamps to fix the billet for rolling;

[0043] (3) The specific operation is as follows: the first pass reduction is set to 50%, that is, the thickness is reduced to 30mm after the first pass rolling; the second pass reduction is set to 40%, and the thickness is reduced to 18mm; the third pass reduction is set to 35%, and the thickness is reduced to 11.7mm; the fourth pass reduction is set to 31.6%, and the thickness is reduced to 8mm. After four passes rolling, SUS304 / Q235 multilayer composite steel with a reduction of 86.7% is finally obtained, so that the thickness is reduced from 60mm to 8mm. Then it is cooled in air to obtain hot-rolled multilayer composite steel.

[0044] (4) The hot-rolled multilayer composite steel was subjected to a cold rolling experiment at room temperature, resulting in a final rolled thickness of 0.8 mm and a cold rolling reduction of 90%, thus obtaining a multilayer composite steel with a harmonic structure. The metallographic structure is as follows: Figure 2 As shown in the figure, the thickness of the "soft phase" Q235 layer remains basically unchanged, while the thickness of the "hard phase" SUS304 layer is uneven due to multiple local necking. It is precisely because of the different crystal structures of the two phase layers that the work hardening behavior is different during cold rolling deformation, ultimately forming a harmonic structure multilayer composite steel with "soft phase" covering "hard phase".

[0045] (5) The cold-rolled sheet obtained in step (4) is annealed at a temperature of 650°C for 6 minutes. Through short-time annealing, a multilayer composite steel with excellent tensile mechanical properties and a harmonic structure is obtained. The tensile mechanical properties are shown in Table 1.

[0046] Example 2 (SUS304 austenitic stainless steel / 45# medium carbon steel)

[0047] The hard and soft layers used in Example 2 are SUS304 austenitic stainless steel and medium carbon steel 45# steel, which are commonly found on the market.

[0048] The two types of steel were cut into 60 rectangular pieces each, 60mm long and wide, and 0.5mm thick, using an EDM wire cutter. The cut steel was then polished to a bright finish using 60# and 240# sandpaper, ultrasonically cleaned in acetone, and dried. The treated SUS304 and 45# steel were then stacked in an "AB…AB" pattern, forming 120 pieces with a height of 60mm. These were placed in a rectangular carbon steel box, 70mm long and wide and 65mm high, and sealed using manual arc welding. Note that Teflon high-temperature insulating cloth was placed at the bottom and top during sealing to ensure the outer billet is separated from the base material after hot rolling. A vacuum pump was then used to create a vacuum of 10⁻⁶ inside the billet box. -2 Pa, awaiting rolling.

[0049] The specific steps are as follows:

[0050] (1) Place the obtained billet into a box furnace and heat it to 1200℃. Then place the processed billet in the furnace and keep it warm for 60 minutes.

[0051] (2) Take out the billet and quickly feed it into the two-roll hot rolling mill for rolling test. It should be noted that the rolling direction should be kept in one direction, and the operator should wear protective gear and use clamps to fix the billet for rolling;

[0052] (3) The specific operation is as follows: the first pass reduction is set to 50%, that is, the thickness is reduced to 30mm after the first pass rolling; the second pass reduction is set to 40%, and the thickness is reduced to 18mm; the third pass reduction is set to 35%, and the thickness is reduced to 11.7mm; the fourth pass reduction is set to 31.6%, and the thickness is reduced to 8mm. After four passes rolling, SUS304 / 45# multilayer composite steel with a reduction of 86.7% is finally obtained, which reduces the thickness from 60mm to 8mm. Then it is cooled in air to obtain hot-rolled multilayer composite steel.

[0053] (4) The hot-rolled multilayer composite steel was subjected to a cold rolling experiment at room temperature, resulting in a final rolled thickness of 0.85 mm and a cold rolling reduction of 89.38%, thus obtaining a multilayer composite steel with a harmonic structure. The metallographic structure is as follows: Figure 3 As shown in the figure, the thickness of the "soft phase" 45# steel layer remains constant, while the thickness of the "hard phase" SUS304 layer is uneven due to multiple local necking. It is precisely because of the different crystal structures of the two phase layers that the work hardening behavior during cold rolling deformation is different, ultimately forming a harmonic structure multilayer composite steel with "soft phase" covering "hard phase".

[0054] (5) The cold-rolled sheet obtained in step (4) is annealed at a temperature of 700℃ for 6 minutes. Through short-time annealing, a multilayer composite steel with excellent tensile mechanical properties and a harmonic structure is obtained. The tensile mechanical properties are shown in Table 1.

[0055] Example 3 (SUS304 austenitic stainless steel / Cr17 ferritic stainless steel)

[0056] The hard and soft layers used in Example 3 are SUS304 austenitic stainless steel and Cr17 ferritic stainless steel, which are commonly found on the market.

[0057] Sixty pieces of each type of steel were cut into circular sheets, each 60mm in diameter and 0.5mm thick, using an EDM wire cutter. The cut steel was then polished to a bright finish using 60# and 240# sandpaper, ultrasonically cleaned in acetone, and dried. The treated SUS304 and Cr17 steel were then stacked in an "AB…AB" pattern, with 120 sheets per sheet, each 60mm high. These sheets were placed in a cylindrical carbon steel box with a diameter of 70mm and a height of 65mm and sealed using manual arc welding. Note that Teflon high-temperature insulating cloth was placed at the bottom and top during sealing to ensure the outer billet is separated from the base material after hot rolling. A vacuum pump was then used to create a vacuum of 10⁻⁶ inside the billet box. -2 Pa, awaiting rolling.

[0058] The specific steps are as follows:

[0059] (1) Place the obtained billet into a box furnace and heat it to 1150°C. Then place the processed billet in the furnace and keep it warm for 40 minutes.

[0060] (2) Take out the billet and quickly feed it into the two-roll hot rolling mill for rolling test. It should be noted that the rolling direction should be kept in one direction, and the operator should wear protective gear and use clamps to fix the billet for rolling;

[0061] (3) The specific operation is as follows: the first pass reduction is set to 50%, that is, the thickness is reduced to 30mm after the first pass rolling; the second pass reduction is set to 40%, and the thickness is reduced to 18mm; the third pass reduction is set to 35%, and the thickness is reduced to 11.7mm; the fourth pass reduction is set to 31.6%, and the thickness is reduced to 8mm. After four passes rolling, SUS304 / Cr17 multilayer composite steel with a reduction of 86.7% is finally obtained, which reduces the thickness from 60mm to 8mm. Then it is cooled in air to obtain hot-rolled multilayer composite steel.

[0062] (4) The hot-rolled multilayer composite steel was subjected to a cold rolling experiment at room temperature, resulting in a final rolled thickness of 0.85 mm and a cold rolling reduction of 89.38%, thus obtaining a multilayer composite steel with a harmonic structure. The metallographic structure is as follows: Figure 4 As shown in the figure, the thickness of the "soft phase" Cr17 layer remains basically unchanged, while the thickness of the "hard phase" SUS304 layer is uneven due to multiple local necking. It is precisely because of the different crystal structures of the two phase layers that the work hardening behavior is different during cold rolling deformation, ultimately forming a harmonic structure multilayer composite steel with "soft phase" covering "hard phase".

[0063] (5) The cold-rolled sheet obtained in step (4) is annealed at a temperature of 650°C for 6 minutes. Through short-time annealing, a multilayer composite steel with excellent tensile mechanical properties and a harmonic structure is obtained. The tensile mechanical properties are shown in Table 1.

[0064] The metallographic images described in the examples were all etched with 4% nitric acid alcohol for 5-10 seconds. The white part that was not etched out is SUS304 austenitic stainless steel, and the blackish-gray part that was etched out is Q235, 45# steel and Cr17 steel.

[0065] The tensile mechanical property test results described in the examples are as follows:

[0066] Test conditions: The tensile mechanical properties described in this invention were all tested on an AGS-X50KN universal testing machine equipped with an Eplilon32 extensometer.

[0067] The tensile mechanical properties described in this invention are shown in Table 1:

[0068] Yield strength / MPa Tensile strength / MPa Elongation / % Example 1 597 748 25 Example 2 863 1024 18 Example 3 701 960 28

[0069] A multi-layered composite steel with excellent comprehensive mechanical properties was prepared using vacuum hot rolling, cold rolling, and heat treatment processes. Its structure is a layer / network coupled harmonic structure. This significantly improves tensile strength and impact toughness, achieving an excellent strength-toughness balance, and provides technical guidance and theoretical basis for the formation of harmonic structures in a series of steel materials.

[0070] As can be seen from the above embodiments, the present invention first involves stacking two materials with different crystal structures (face-centered cubic and body-centered cubic) in a cross-layered manner, and then encapsulating them in a carbon steel billet box according to an "AB…AB" pattern. It is important to note that a Teflon high-temperature insulating cloth is placed inside the carbon steel box parallel to the rolling direction to prevent the carbon steel box from sticking to the raw materials during hot rolling. The dimensions of the steel materials with the two different crystal structures and the dimensions of the billet box are controllable, and the shape of the billet box can be designed as cylindrical or rectangular, depending on the shape of the actual raw materials. Secondly, the carbon steel billet box is subjected to vacuum treatment to maintain a vacuum level of 10. -2 -10 -4 The purpose of Pa is to prevent oxide formation at the interlayer interface, which would degrade the interfacial bonding strength. Next, hot rolling is performed above the recrystallization temperature, with the billet temperature controlled at 1150-1200℃ and the holding time controlled at 40-60 minutes. Hot rolling is then carried out on a hot rolling mill with 3-5 passes, resulting in a hot rolling reduction of 85-88%, yielding a hot-rolled composite plate with straight interfaces and strong interfacial bonding. Subsequently, a cold rolling process below the recrystallization temperature allows for greater deformation of the multilayer composite material, further improving the interfacial bonding strength and obtaining ultrafine fiber grains in the rolling direction. However, cold rolling often increases the internal stress and dislocation density of the material, thus requiring heat treatment to further improve the material. The composite material prepared by this invention has comprehensive performance far superior to its individual components. Furthermore, the vacuum hot rolling process above the recrystallization temperature results in low deformation resistance, high interfacial bonding strength, and is simple to operate, low in cost, and suitable for large-scale production.

[0071] Matters not covered in this invention are common knowledge.

Claims

1. A method for preparing multilayer composite steel with harmonic structure, characterized in that: The method includes the following steps: (1) Cut the FCC face-centered cubic austenitic steel and the BCC body-centered cubic ferritic steel into sheets respectively; The thickness of the sheet is 0.5-2mm; (2) The sheet obtained in step (1) is polished with SiC sandpaper; (3) Place the sheet obtained in step (2) in acetone for ultrasonic treatment to remove surface oil and clean the surface; (4) Stack the two different materials obtained in step (3) in a carbon steel box with "AB…AB" intervals, with a total number of layers of 2n, n=30~80; (5) Seal the carbon steel box obtained in step (4) by electric welding; (6) Vacuum treatment is applied to the sealed carbon steel box until the vacuum degree inside the billet reaches 10. -2 -10 -4 Pa; (7) Place the billet obtained in step (6) into a box furnace and heat it to 1150-1200℃ for 40-60 minutes to ensure that the base material is completely austenitized. Then send the billet into a hot rolling mill for 3-5 passes until the thickness reaches 6-15mm. The total hot rolling reduction is 85-88%. Then air cool to room temperature. (8) The hot-rolled sample is subjected to room temperature cold rolling deformation on a cold rolling mill, and the cold rolling deformation is controlled to be 80%~90%; (9) The cold-rolled sample was subjected to heat treatment experiment. The temperature was set at 500~800℃ and held for 5~10 min to obtain multi-layer composite steel with harmonic structure. The sheet material is a round or rectangular sheet; its diameter or length is 60-80 mm. The FCC face-centered cubic austenitic steel is SUS304 austenitic stainless steel, and the BCC body-centered cubic ferritic steel is Q235, 45# carbon steel or Cr17.

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