400mpa grade composite steel bar for marine environment and preparation method thereof

Abstract

The application discloses a 400MPa-grade composite reinforcing steel for marine environment and a preparation method thereof. The composite reinforcing steel is a three-layer coaxial structure, and sequentially comprises a core part, an intermediate transition layer and an outer layer from inside to outside. The core part is made of carbon steel; the intermediate transition layer is made of pure nickel; and the outer layer is made of stainless steel. The three-layer material is metallurgically combined to form a composite round billet through a horizontal centrifugal casting process, and then is normalized and rolled into a reinforcing steel. The reinforcing steel has excellent mechanical properties and corrosion resistance, and effectively inhibits carbon diffusion through the pure nickel transition layer, avoids the formation of brittle phases, and significantly improves the durability in the marine chloride ion environment. Compared with traditional alloyed corrosion-resistant reinforcing steel or physical cladding reinforcing steel, the application has significant advantages in cost, performance and process stability, and can be widely applied to reinforcing steel for marine environment.

Description

Technical Field

[0001] This invention relates to the field of metal composite materials and steel reinforcement manufacturing technology, and more specifically, to a 400MPa grade composite steel reinforcement for marine environments and its preparation method. Background Technology

[0002] Marine engineering infrastructure (such as cross-sea bridges, port terminals, and offshore platforms) is subjected to harsh corrosive environments with high chloride ion concentrations and high humidity for extended periods, placing extremely high demands on the corrosion resistance of steel reinforcement in reinforced concrete structures. Traditional carbon steel reinforcement is prone to corrosion in marine environments, leading to cracking and spalling of concrete structures, severely impacting project safety and service life. Composite steel reinforcement is a new type of corrosion-resistant steel reinforcement formed by metallurgically bonding a cladding layer with a carbon steel core. If stainless steel is used for the cladding, the outer surface of the reinforcement has excellent corrosion resistance, blocking contact between external corrosive agents and the carbon steel core, thus protecting the carbon steel core from corrosion and improving the durability of the reinforced concrete structure. Furthermore, the cladding thickness of composite steel reinforcement is thinner, with the load-bearing capacity primarily provided by the carbon steel core. This significantly reduces costs while meeting load-bearing capacity and improving structural durability. For reinforced concrete structures operating in corrosive environments, compared to traditional carbon steel and stainless steel reinforcement, composite steel reinforcement offers significant advantages in durability and overall cost, thus possessing broad engineering application prospects.

[0003] Chinese patent CN202510711761.6 discloses a method for manufacturing bimetallic composite steel bars of stainless steel and carbon steel. It utilizes a stainless steel round tube billet and a carbon steel core to manufacture a stainless steel bimetallic composite intermediate billet, and then uses this intermediate billet to manufacture the stainless steel bimetallic composite steel bars. The production process is relatively complex and requires sophisticated rolling equipment. Chinese patent CN202111231833.5 discloses a method for preparing stainless steel composite steel bars. This process involves vacuum interfacial casting of a carbon steel round billet and a stainless steel tube to create a composite billet with a carbon steel core and a stainless steel coating, which is then rolled to obtain the steel bars. This composite method suffers from poor interfacial bonding and is prone to defects such as wrinkling and tearing of the inner and outer layers during rolling. Chinese patent CN202210821512.9 discloses a stainless steel composite ingot and its vertical centrifugal casting process. This process uses high-strength carbon steel for both the transition layer and the core, but does not consider the interface integration between the carbon steel and the outer stainless steel layer. Furthermore, vertical centrifugal casting cannot meet the requirements for longer billets. Chinese patent CN201610677162.8 discloses a method for preparing stainless steel composite threaded steel bars by cladding and rolling. This method involves deforming the cladding material to cover the core material, welding the joints, and then reheating the ends before rolling. This method requires high precision in core material surface cleaning, composite end sealing, and rolling deformation. Chinese patent CN202011121918.3 discloses a high-strength, high-toughness, and corrosion-resistant multi-layer composite steel bar and its manufacturing method. This method uses a combination of cladding material, an isolation layer, and a core material, with welding of the cladding surface joints and hot rolling of the composite billet. This method is prone to delamination and cracking during the rolling of different materials. Summary of the Invention

[0004] This invention addresses the problems of existing technologies, such as complex processes, high equipment requirements, poor interface bonding leading to defects, inability to meet the needs of long-sized billets, and insufficient mechanical properties, corrosion resistance, and service life. It provides a 400MPa grade composite steel bar for marine environments and its preparation method. The composite round billet is produced by centrifugal casting and employs a three-layer composite structure, consisting of a carbon steel core, a pure nickel intermediate layer, and a stainless steel outer layer from the inside out. Through heat treatment and optimized rolling, a composite steel bar with excellent mechanical properties and marine corrosion resistance is prepared, overcoming the shortcomings of existing technologies and improving the service life and safety of the steel bar.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A 400MPa grade composite steel bar for marine environments, wherein the composite steel bar has a three-layer coaxial structure, comprising a core, an intermediate transition layer and an outer layer from the inside out; the core is made of carbon steel; the intermediate transition layer is made of pure nickel; and the outer layer is made of stainless steel.

[0007] A method for preparing 400MPa grade marine environment composite steel bars as described above includes the following steps: S1. Centrifugal Casting of Composite Round Billet: A horizontal centrifugal casting machine is used. The mold is preheated to 250~350℃. First, the outer layer is poured at a temperature of 1500~1550℃ and a rotation speed of 830~850 r / min. After an interval of 30~60 seconds, when the temperature of the inner surface of the outer layer drops to 1400℃ and a solid-liquid coexistence state is formed, the intermediate transition layer is poured at a temperature of 1480~1520℃ and a rotation speed of 600~650 r / min. After another interval of 20~40 seconds, the intermediate transition layer solidifies, and the core is poured at a temperature of 1560~1600℃, with the mold rotation speed controlled at 500~550 r / min. After pouring, the mold continues to rotate for 2~5 minutes, and then air-cooled to room temperature to obtain the composite round billet. S2. Normalizing treatment: The composite round billet is heated to 900~920℃ and held for 1.5~2.5h, then air-cooled to room temperature to obtain the normalized composite round billet; S3. Hot rolling: The normalized composite round billet is heated to 1100~1150℃ and held for 1~3h; then it is rolled in multiple passes, with an initial rolling temperature of 1050~1100℃ and a final rolling temperature of 850~900℃; the total rolling compression ratio is controlled at (30~50):1, and after rolling into the target specification threaded steel bar, the cooling rate is controlled at 0~5℃ / s.

[0008] Implementing the embodiments of the present invention will have the following beneficial effects: The composite reinforcing steel of this invention adopts a three-layer coaxial structure: the outer layer is made of stainless steel, the middle transition layer is made of pure nickel, and the core is made of carbon steel. A horizontal centrifugal casting process is used to metallurgically bond the three layers to form a composite round billet, which is then normalized and rolled into reinforcing steel. This reinforcing steel possesses both excellent mechanical properties and corrosion resistance. Furthermore, the pure nickel transition layer effectively inhibits carbon diffusion, preventing the formation of brittle phases and significantly improving durability in marine chloride ion environments. Compared with traditional alloyed corrosion-resistant reinforcing steel or physically coated reinforcing steel, this application has significant advantages in cost, performance, and process stability, and can be widely used in reinforcing steel applications in marine environments. Specific effects are as follows: (1) Three-layer structure innovation: from the inside out, it includes a carbon steel core, a pure nickel transition layer and a stainless steel outer layer. For the first time, pure nickel is used as a transition layer in a steel composite structure, which effectively solves the problem of carbon diffusion when dissimilar metals are combined. The stainless steel outer layer provides corrosion resistance, and the carbon steel core ensures mechanical properties, thus achieving an optimal balance between function and cost.

[0009] (2) Process innovation: For the first time, centrifugal casting process was applied to the preparation of composite steel billet, and centrifugal force was used to achieve uniform distribution and good bonding of the three layers of metal; the interface metallurgical bonding quality was ensured by precisely controlling the pouring temperature, interval time and casting speed; and the comprehensive performance of composite steel was optimized by combining normalizing treatment and controlled rolling process.

[0010] (3) Excellent mechanical properties and corrosion resistance: The outer layer of stainless steel can form a stable passivation film in the marine environment, and its resistance to chloride ion corrosion is far superior to that of ordinary steel bars; the pure nickel transition layer prevents carbon diffusion from causing stainless steel sensitization and maintains long-term corrosion resistance; the composite steel bar made by normalizing and controlled rolling of carbon steel core has a yield strength ≥400MPa, tensile strength ≥540MPa, and elongation after fracture ≥18%. Interface bonding strength: The interfaces between the outer layer and the intermediate transition layer and between the intermediate transition layer and the core are metallurgically bonded, and the interface shear strength is ≥300MPa.

[0011] (4) Excellent corrosion resistance: Electrochemical testing was conducted using simulated concrete pore fluid. The critical chloride ion concentration threshold was determined by potentiodynamic scanning. Electrochemical testing was performed using the M398 electrochemical corrosion testing system. The electrochemical test adopted a three-electrode system, with a 304 stainless steel coated working electrode, a platinum wire mesh as the auxiliary electrode, and a saturated calomel electrode as the reference electrode. The test temperature was 25℃, and the test solution was simulated concrete pore fluid (saturated Ca(OH)2 solution, NaCl mass fraction of 3%). The polarization curve test range was from -250mV (relative to open circuit potential) in the forward scan, with a scan rate of 0.33mV / s. Through the above test evaluation, the corrosion rate was less than 0.049mm / a, and its corrosion resistance was improved by more than 15 times compared with HRB400E steel bars.

[0012] (5) Cost advantage: Compared with pure stainless steel steel bars, the cost is reduced by 40%~50%; compared with alloyed corrosion-resistant steel bars, the cost increases by only 15%~20%, but the corrosion resistance is significantly improved, and its corrosion resistance is more than 15 times that of ordinary steel bars.

[0013] (6) Process stability: The centrifugal casting process is mature and easy to scale up production; through process parameter optimization, the yield of composite billets can reach more than 90%. Detailed Implementation

[0014] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0015] This invention discloses a 400MPa grade composite steel bar for marine environments. The composite steel bar has a three-layer coaxial structure, consisting of a core, an intermediate transition layer, and an outer layer from the inside out. The core is made of carbon steel; the intermediate transition layer is made of pure nickel; and the outer layer is made of stainless steel.

[0016] In one specific embodiment, the three-layer coaxial structure is a three-layer coaxial cylindrical structure.

[0017] In one specific embodiment, the core material is HRB400E carbon steel core to ensure that the mechanical properties of the steel bar meet the requirements of national standards; the core contains, by mass percentage: C: 0.22%~0.25%, Si: 0.50%~0.60%, Mn: 1.40%~1.60%, P≤0.025%, S≤0.025%, and the remainder is Fe and unavoidable impurities.

[0018] In one specific embodiment, the mass percentage of Ni in the intermediate transition layer is ≥99.5%. Specifically, the main functions of the pure nickel intermediate layer include: inhibiting carbon diffusion, preventing carbon atoms in the carbon steel core from migrating to the stainless steel outer layer, avoiding the formation of chromium carbides at the interface, thereby preventing a decrease in the corrosion resistance of the stainless steel outer layer; relieving thermal stress, as the thermal conductivity of nickel is between that of stainless steel and carbon steel, which can alleviate thermal stress caused by the difference in the coefficients of thermal expansion between the two; and improving interfacial bonding, as nickel has high miscibility with iron and can form good metallurgical bonds with the outer layer and the core, respectively.

[0019] In one specific embodiment, the outer layer is made of 304 austenitic stainless steel, which provides excellent resistance to chloride ion corrosion. In the outer layer, the mass percentage of C is ≤0.07%, the mass percentage of Cr is 18%~20%, and the mass percentage of Ni is 8%~10.5%, which can form a dense Cr2O3 passivation film on the surface, effectively resisting corrosion in the marine environment.

[0020] In one specific embodiment, the outer layer thickness is 10% to 15% of the composite steel bar radius, and the intermediate transition layer is 3% to 5% of the composite steel bar radius.

[0021] In one specific embodiment, the composite steel bar has a yield strength ≥400MPa, a tensile strength ≥540MPa, and an elongation after fracture ≥18%.

[0022] In one specific embodiment, the interfaces between the outer layer and the intermediate transition layer, as well as between the intermediate transition layer and the core, are metallurgically bonded; the interface shear strength is ≥300MPa.

[0023] This invention also discloses a method for preparing the 400MPa grade marine environment composite steel bar as described above, comprising the following steps: S1. Centrifugal Casting of Composite Round Billet: A horizontal centrifugal casting machine is used. The mold is preheated to 250~350℃. First, the outer layer is poured at a pouring temperature of 1500~1550℃, and evenly distributed on the inner wall of the mold under centrifugal force at a rotation speed of 830~850 r / min. After an interval of 30~60s, when the temperature of the inner surface of the outer layer drops to 1400℃ and a solid-liquid coexistence state is formed, the intermediate transition layer is poured at a pouring temperature of 1480~1520℃ at a rotation speed of 600~650 r / min. After another interval of 20~40s, the intermediate transition layer solidifies, and the core is poured at a pouring temperature of 1560~1600℃ at a mold rotation speed controlled at 500~550 r / min to achieve preliminary metallurgical bonding in the liquid state. After pouring, the core is rotated for another 2~5 minutes and then air-cooled to room temperature to obtain the composite round billet.

[0024] S2. Normalizing treatment: Heat the composite round billet to 900~920℃ and hold for 1.5~2.5h to homogenize the microstructure and eliminate casting stress. Then air cool to room temperature to obtain the normalized composite round billet.

[0025] S3. Hot rolling: Heat the normalized composite round billet to 1100~1150℃ and hold for 1~3 hours; then perform multi-pass rolling, with an initial rolling temperature of 1050~1100℃ and a final rolling temperature of 850~900℃; control the total rolling compression ratio to (30~50):1, and control the cooling rate to 0~5℃ / s after rolling into the target specification threaded steel bar.

[0026] In one specific embodiment, the microstructure of the outer layer is an austenitic matrix, with grains distributed in a fibrous streamline pattern along the rolling direction; annealing twins are visible within the grains; due to large deformation and dynamic recrystallization, the grains are refined, with an average grain size of 15~30μm.

[0027] In one specific embodiment, the microstructure of the intermediate transition layer is a single-phase austenitic equiaxed grain structure; rolling and recrystallization significantly refine its grains, with a grain size of 9.0 grade or higher, resulting in a uniform structure and no second phase precipitation.

[0028] In one specific embodiment, the microstructure of the core consists of ferrite and pearlite. The ferrite grains are elongated along the rolling direction, and the pearlite lamellar structure is clearly defined.

[0029] In one specific embodiment, the interface between the outer layer and the intermediate transition layer is a metallurgical bond. The interface is straight, the bond is dense, and there are no cracks, pores, or continuous oxides. Scanning electron microscopy shows that at this interface, the concentrations of Fe, Cr, and Ni elements transition smoothly in a transition region spanning a width of 5-10 μm, forming a stable solid solution metallurgical bond.

[0030] In one specific embodiment, the interface between the intermediate transition layer and the core is metallurgically bonded. The interface bonding is good, and the EDS line scan clearly shows that the C element concentration drops sharply when it reaches the interface and is maintained at a level of C≤0.02% in the intermediate transition layer, which reflects the excellent carbon inhibition function of the nickel layer. There is no continuous brittle carbide layer in the narrow interface region with a width of 2~6μm.

[0031] In one specific embodiment, in step S3, the total rolling compression ratio is the ratio of the cross-sectional area of ​​the round billet to the cross-sectional area of ​​the finished steel bar.

[0032] The following are specific embodiments. Example 1 The Φ25mm composite threaded steel bar in this embodiment has a three-layer coaxial structure, consisting of an HRB400E carbon steel core, a pure nickel transition layer, and a 304 austenitic stainless steel outer layer, from the inside out. The interfaces between the 304 austenitic stainless steel outer layer and the pure nickel transition layer, as well as between the pure nickel transition layer and the HRB400E carbon steel core, are metallurgically bonded.

[0033] HRB400E carbon steel core: Smelted according to GB / T 1499.2 requirements, with the following composition by mass percentage: C: 0.23%, Si: 0.55%, Mn: 1.50%, P: 0.018%, S: 0.015%, with the remainder being Fe and unavoidable impurities. The microstructure consists of ferrite and pearlite.

[0034] Pure nickel transition layer: Ni mass percentage ≥ 99.5%, main impurities Cu ≤ 0.05%, Fe ≤ 0.05%. Microstructure is single-phase austenitic equiaxed crystal structure; no second phase precipitation.

[0035] The outer layer of 304 austenitic stainless steel conforms to GB / T 20878 standard. Before casting, the composition of the melt is adjusted by mass percentage as follows: C ≤ 0.07%, Cr: 18.2%, Ni: 8.5%. The microstructure is an austenitic matrix with fibrous flow lines distributed along the rolling direction; annealed twins are visible within the grains.

[0036] The outer layer of 304 stainless steel is 10% of the radius of the composite steel bar, the intermediate transition layer is 3% of the radius of the composite steel bar, and the remaining part is composed of the HRB400E carbon steel core.

[0037] The method for preparing composite steel bars in this embodiment includes the following steps: S1. Centrifugal Casting of Composite Round Billet: A horizontal centrifugal casting machine is used. The mold is preheated to 300℃. First, the outer layer of 304 stainless steel is cast at a temperature of 1520℃ and a rotation speed of 830 r / min. After a 30-second interval, when the temperature of the inner surface of the outer layer drops to 1400℃ and a solid-liquid coexistence state is formed, the pure nickel transition layer is cast at a temperature of 1480℃ and a rotation speed of 600 r / min. After another 30-second interval, the pure nickel transition layer solidifies, and the HRB400E carbon steel core is cast at a temperature of 1580℃, with the mold rotation speed controlled at 500 r / min. After casting, the mold continues to rotate for 2 minutes, and then air-cooled to room temperature to obtain a Φ150mm composite round billet. S2. Normalizing treatment: The composite round billet is heated to 910℃ and held for 1.5h, then air-cooled to room temperature to obtain the normalized composite round billet. S3. Hot rolling: The normalized composite round billet is heated to 1120℃ and held for 1.5h; then it is rolled in multiple passes with an initial rolling temperature of 1080℃ and a final rolling temperature of 880℃; 11 passes are rolled, and the total rolling compression ratio is controlled at 36:1. After rolling into the target specification of threaded steel bar, the cooling rate is controlled at 0~5℃ / s.

[0038] Example 2 The Φ32mm layered composite threaded steel bar in this embodiment has a three-layer coaxial structure, consisting of an HRB400E carbon steel core, a pure nickel transition layer, and a 304 austenitic stainless steel outer layer, from the inside out. The interfaces between the 304 austenitic stainless steel outer layer and the pure nickel transition layer, as well as between the pure nickel transition layer and the HRB400E carbon steel core, are metallurgically bonded.

[0039] HRB400E carbon steel core: Smelted according to GB / T 1499.2 requirements, the composition by mass percentage includes: C: 0.22%, Si: 0.57%, Mn: 1.45%, P: 0.016%, S: 0.012%, with the remainder being Fe and unavoidable impurities. The microstructure consists of ferrite and pearlite.

[0040] Pure nickel transition layer: Ni mass percentage ≥ 99.5%, main impurities Cu ≤ 0.05%, Fe ≤ 0.05%. The microstructure of the transition layer is single-phase austenitic equiaxed crystal structure, with no second phase precipitation.

[0041] The outer layer of 304 austenitic stainless steel conforms to GB / T 20878 standard. Before casting, the composition of the melt is adjusted by mass percentage as C ≤ 0.07%, Cr: 18.5%, and Ni: 9%. The microstructure is an austenitic matrix with fibrous flow lines distributed along the rolling direction; annealed twins are visible within the grains.

[0042] The outer layer of 304 stainless steel is 12% of the radius of the composite steel bar, the intermediate transition layer is 3% of the radius of the composite steel bar, and the remaining part is composed of HRB400E carbon steel core.

[0043] The method for preparing composite steel bars in this embodiment includes the following steps: S1. Centrifugal Casting of Composite Round Billet: A horizontal centrifugal casting machine is used. The mold is preheated to 260℃. First, a 304 stainless steel outer layer is cast at a temperature of 1510℃ and a rotation speed of 840 r / min. After a 30-second interval, when the temperature of the inner surface of the outer layer drops to 1400℃ and a solid-liquid coexistence state is formed, a pure nickel transition layer is cast at a temperature of 1490℃ and a rotation speed of 610 r / min. After another 30-second interval, the pure nickel transition layer solidifies, and the HRB400E carbon steel core is cast at a temperature of 1570℃, with the mold rotation speed controlled at 510 r / min. After casting, the mold continues to rotate for 3 minutes, and then air-cooled to room temperature to obtain a Φ180mm composite round billet. S2. Normalizing treatment: The composite round billet is heated to 915℃ and held for 1.6h, then air-cooled to room temperature to obtain the normalized composite round billet. S3. Hot rolling: The normalized composite round billet is heated to 1130℃ and held for 2.2h; then it is rolled in multiple passes with an initial rolling temperature of 1070℃ and a final rolling temperature of 870℃; 13 passes are rolled, and the total rolling compression ratio is controlled at 31.6:1. After rolling into the target specification of threaded steel bar, the cooling rate is controlled at 0~5℃ / s.

[0044] Example 3 The Φ36mm composite threaded steel bar in this embodiment has a three-layer coaxial structure, consisting of an HRB400E carbon steel core, a pure nickel transition layer, and a 304 austenitic stainless steel outer layer, from the inside out. The interfaces between the 304 austenitic stainless steel outer layer and the pure nickel transition layer, as well as between the pure nickel transition layer and the HRB400E carbon steel core, are metallurgically bonded.

[0045] HRB400E carbon steel core: Smelted according to GB / T 1499.2 requirements, the composition by mass percentage includes: C: 0.24%, Si: 0.57%, Mn: 1.50%, P: 0.017%, S: 0.010%, with the remainder being Fe and unavoidable impurities. The microstructure consists of ferrite and pearlite.

[0046] Pure nickel transition layer: Ni mass percentage ≥ 99.5%, main impurities Cu ≤ 0.05%, Fe ≤ 0.05%. Microstructure is single-phase austenitic equiaxed crystal structure; no second phase precipitation.

[0047] The outer layer of 304 austenitic stainless steel conforms to GB / T 20878 standard. Before casting, the composition of the melt is adjusted by mass percentage as C ≤ 0.07%, Cr: 19%, Ni: 9%. The microstructure is an austenitic matrix with fibrous flow lines distributed along the rolling direction; annealed twins are visible within the grains.

[0048] The outer layer of 304 stainless steel is 13% of the radius of the composite steel bar, the intermediate transition layer is 3.5% of the radius of the composite steel bar, and the remaining part is composed of HRB400E carbon steel core.

[0049] The method for preparing composite steel bars in this embodiment includes the following steps: S1. Centrifugal Casting of Composite Round Billet: A horizontal centrifugal casting machine is used. The mold is preheated to 320℃. First, a 304 stainless steel outer layer is cast at a temperature of 1530℃ and a rotation speed of 835 r / min. After a 30-second interval, when the temperature of the inner surface of the outer layer drops to 1400℃ and a solid-liquid coexistence state is formed, a pure nickel transition layer is cast at a temperature of 1500℃ and a rotation speed of 630 r / min. After another 35-second interval, the pure nickel transition layer solidifies, and the HRB400E carbon steel core is cast at a temperature of 1580℃, with the mold rotation speed controlled at 530 r / min. After casting, the mold continues to rotate for 2 minutes, and then air-cooled to room temperature to obtain a Φ200mm composite round billet. S2. Normalizing treatment: The composite round billet is heated to 915℃ and held for 2 hours, then air-cooled to room temperature to obtain the normalized composite round billet. S3. Hot rolling: The normalized composite round billet is heated to 1130℃ and held for 1.8h; then it is rolled in multiple passes with an initial rolling temperature of 1090℃ and a final rolling temperature of 885℃; 15 passes are rolled, and the total rolling compression ratio is controlled at 30.8:1. After rolling into the target specification of threaded steel bar, the cooling rate is controlled at 0~5℃ / s.

[0050] Example 4 The Φ40mm composite threaded steel bar in this embodiment has a three-layer coaxial structure, consisting of an HRB400E carbon steel core, a pure nickel transition layer, and a 304 austenitic stainless steel outer layer, from the inside out. The interfaces between the 304 austenitic stainless steel outer layer and the pure nickel transition layer, as well as between the pure nickel transition layer and the HRB400E carbon steel core, are metallurgically bonded.

[0051] HRB400E carbon steel core: Smelted according to GB / T 1499.2 requirements, the composition by mass percentage includes: C: 0.25%, Si: 0.59%, Mn: 1.55%, P: 0.010%, S: 0.008%, with the remainder being Fe and unavoidable impurities. The microstructure consists of ferrite and pearlite.

[0052] Pure nickel transition layer: Ni mass percentage ≥ 99.5%, main impurities Cu ≤ 0.05%, Fe ≤ 0.05%. Microstructure is single-phase austenitic equiaxed crystal structure; no second phase precipitation.

[0053] The outer layer of 304 austenitic stainless steel conforms to GB / T 20878 standard. Before casting, the composition is adjusted by mass percentage as follows: C ≤ 0.07%, Cr: 19.5%, Ni: 10%. The microstructure is an austenitic matrix with fibrous streamline distribution of grains along the rolling direction; annealed twins are visible within the grains.

[0054] The outer layer of 304 stainless steel is 14% of the radius of the composite steel bar, the intermediate transition layer is 4.5% of the radius of the composite steel bar, and the remaining part is composed of HRB400E carbon steel core.

[0055] The method for preparing composite steel bars in this embodiment includes the following steps: S1. Centrifugal Casting of Composite Round Billet: A horizontal centrifugal casting machine is used. The mold is preheated to 300℃. First, the outer layer of 304 stainless steel is cast at a temperature of 1540℃ and a rotation speed of 845 r / min. After a 50-second interval, when the temperature of the inner surface of the outer layer drops to 1400℃ and a solid-liquid coexistence state is formed, the pure nickel transition layer is cast at a temperature of 1510℃ and a rotation speed of 645 r / min. After another 30-second interval, the pure nickel transition layer solidifies, and the HRB400E carbon steel core is cast at a temperature of 1590℃, with the mold rotation speed controlled at 540 r / min. After casting, the mold continues to rotate for 4.5 minutes, and then air-cooled to room temperature to obtain a Φ220mm composite round billet. S2. Normalizing treatment: The composite round billet is heated to 920℃ and held for 2.5h, then air-cooled to room temperature to obtain the normalized composite round billet; S3. Hot rolling: The normalized composite round billet was heated to 1145℃ and held for 2.8 hours; then, it was rolled in multiple passes with an initial rolling temperature of 1090℃ and a final rolling temperature of 890℃; 15 passes were rolled, controlling the total rolling compression ratio at 30.3:1, and the cooling rate was controlled at 0~5℃ / s after rolling into the target specification of threaded steel bars. The mechanical properties of Examples 1~4 and the comparative examples are shown in Table 1. The corrosion properties and metallographic structure of Examples 1~4 and the comparative examples are shown in Table 2.

[0056] Table 1 Mechanical properties of steel bars in Examples 1-4 and comparative examples

[0057] Table 2 Corrosion properties and metallographic structure of the reinforcing bars in Examples 1-4 and Comparative Examples

[0058] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A 400MPa grade composite steel bar for marine environments, characterized in that, The composite steel bar has a three-layer coaxial structure, which includes a core, an intermediate transition layer and an outer layer from the inside to the outside. The core material is carbon steel; The intermediate transition layer is made of pure nickel. The outer layer is made of stainless steel.

2. The 400MPa grade marine environment composite steel bar according to claim 1, characterized in that, The core material is HRB400E carbon steel core; The core comprises, by mass percentage: C: 0.22%~0.25%, Si: 0.50%~0.60%, Mn: 1.40%~1.60%, P≤0.025%, S≤0.025%, with the remainder being Fe and unavoidable impurities.

3. The 400MPa grade marine environment composite steel bar according to claim 1, characterized in that, In the intermediate transition layer, the mass percentage content of Ni is ≥99.5%.

4. The 400MPa grade marine environment composite steel bar according to claim 1, characterized in that, The outer layer material is 304 austenitic stainless steel; In the outer layer, the mass percentage of C is ≤0.07%, the mass percentage of Cr is 18%~20%, and the mass percentage of Ni is 8%~10.5%.

5. The 400MPa grade marine environment composite steel bar according to claim 1, characterized in that, The outer layer thickness is 10% to 15% of the radius of the composite steel bar, and the intermediate transition layer is 3% to 5% of the radius of the composite steel bar.

6. The 400MPa grade marine environment composite steel bar according to claim 1, characterized in that, The composite steel reinforcement has a yield strength ≥400MPa, a tensile strength ≥540MPa, and an elongation after fracture ≥18%. The interfaces between the outer layer and the intermediate transition layer, as well as between the intermediate transition layer and the core, are all metallurgically bonded. Interfacial shear strength ≥300MPa.

7. A method for preparing 400MPa grade marine environment composite steel bars as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Centrifugal Casting of Composite Round Billet: A horizontal centrifugal casting machine is used. The mold is preheated to 250~350℃. First, the outer layer is poured at a temperature of 1500~1550℃ and a rotation speed of 830~850 r / min. After an interval of 30~60 seconds, when the temperature of the inner surface of the outer layer drops to 1400℃ and a solid-liquid coexistence state is formed, the intermediate transition layer is poured at a temperature of 1480~1520℃ and a rotation speed of 600~650 r / min. After another interval of 20~40 seconds, the intermediate transition layer solidifies, and the core is poured at a temperature of 1560~1600℃, with the mold rotation speed controlled at 500~550 r / min. After pouring, the mold continues to rotate for 2~5 minutes, and then air-cooled to room temperature to obtain the composite round billet. S2. Normalizing treatment: The composite round billet is heated to 900~920℃ and held for 1.5~2.5h, then air-cooled to room temperature to obtain the normalized composite round billet; S3. Hot rolling: The normalized composite round billet is heated to 1100~1150℃ and held for 1~3h; then it is rolled in multiple passes, with an initial rolling temperature of 1050~1100℃ and a final rolling temperature of 850~900℃; the total rolling compression ratio is controlled at (30~50):1, and after rolling into the target specification threaded steel bar, the cooling rate is controlled at 0~5℃ / s.

8. The method for preparing 400MPa grade marine environment composite steel bars according to claim 7, characterized in that, The microstructure of the outer layer is an austenitic matrix with grains distributed in a fibrous streamline pattern along the rolling direction; annealed twins are visible within the grains; the average grain size is 15~30μm. The microstructure of the intermediate transition layer is a single-phase austenitic equiaxed crystal structure; the grain size is above 9.0, and no second phase precipitates. The microstructure of the core consists of ferrite and pearlite.

9. The method for preparing 400MPa grade marine environment composite steel bars according to claim 7, characterized in that, The interface between the outer layer and the intermediate transition layer is a metallurgical bond. At this interface, the concentrations of Fe, Cr and Ni elements transition smoothly in a transition region with a width of 5~10μm. The interface between the intermediate transition layer and the core is metallurgically bonded. The C element concentration decreases when it reaches the interface and is maintained at a level of C≤0.02% within the intermediate transition layer. There is no continuous brittle carbide layer within the narrow interface region with a width of 2~6μm.

10. The method for preparing 400MPa grade marine environment composite steel bars according to claim 7, characterized in that, In step S3, the total rolling compression ratio is the ratio of the cross-sectional area of ​​the round billet to the cross-sectional area of ​​the finished steel bar.

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