Corrosion-resistant steel with grade of 6.8 or above, pipe piece bolt and preparation method of corrosion-resistant steel

Abstract

The invention belongs to the technical field of fastening connection, and particularly relates to corrosion-resistant steel with the grade of 6.8 or above, a duct piece bolt and a preparation method of the duct piece bolt. By compounding Cu and Ni, a compact and stable rust layer is promoted to be formed on the surface of the material, further corrosion is prevented, and the corrosion resistance of the material is improved; through trace Sn, a compound of SnO2 and graphite can be formed on the surface of the material to serve as an anti-corrosion layer, and the corrosion resistance of the material is greatly enhanced; by optimizing the content of C and adding a trace amount of B, the hardenability of the steel is improved so as to meet the performance requirements of the large-specification duct piece bolt of the tunnel. According to the corrosion-resistant steel with the grade of 6.8 or above, specific elements with the specific content are adopted, the steel does not depend on a coating, the excellent corrosion resistance is achieved, the mechanical performance is good, and the comprehensive performance is excellent.

Description

Technical Field

[0001] The invention belongs to the technical field of fastening connections, and in particular relates to a corrosion-resistant steel material of grade 6.8 or above, a segment bolt and a preparation method thereof. Background Art

[0002] With the vigorous development of China's transportation construction, the amount of cross-river tunnels, undersea tunnels and mountain tunnels has increased year by year. Concrete segments and steel segments are often connected by segment bolts. The amount of segment bolts is large, and their corrosion resistance level is generally required to be above 6.8. Existing segment bolts are usually made of low-alloy steel (such as 20Cr, 40Cr) combined with surface treatment. However, due to the complex service environment of traffic tunnels, when the surface coating of low-alloy steel peels off, the exposed part of the segment bolt is easily exposed to corrosive ions in the environment (such as Cl - 、SO4 2- ) or gas undergoes chemical or electrochemical reactions, causing bolt corrosion. The corrosion of segment bolts has many effects on the shield lining structure. It not only reduces the cross-sectional area of ​​the bolts themselves, but also causes concrete to rust, expand, crack, and damage. In addition, it also causes the bonding performance between the bolts and concrete to decrease, seriously affecting the safety of the tunnel structure.

[0003] At present, there is no clear standard in China requiring the corrosion resistance of segment bolts, but anti-corrosion technology for large-size bolt materials or similar materials has emerged. However, its corrosion resistance is insufficient and cannot meet the segment connection requirements in complex tunnel structure environments. Summary of the invention

[0004] The purpose of the present invention is to provide a corrosion-resistant steel material of grade 6.8 or above, a segment bolt and a preparation method thereof. The corrosion-resistant steel grade provided by the present invention reaches grade 6.8 or above, is independent of coating, and has excellent corrosion resistance.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] The invention provides a corrosion-resistant steel of grade 6.8 or above, comprising the following chemical components in percentage by mass: C 0.18-0.24%, Si 0.15-0.23%, Mn 0.50-0.72%, Ni 0.10-0.36%, Cu 0.22-0.38%, Al 0.018-0.030%, Ti 0.01-0.05%, B 0.0008-0.0040%, Sn 0.005-0.100% and Fe balance.

[0007] Preferably, the corrosion-resistant steel of grade 6.8 or above further comprises a multi-element alloy co-penetration layer; the multi-element alloy co-penetration layer covers the surface of the corrosion-resistant steel of grade 6.8 or above; the thickness of the multi-element alloy co-penetration layer is not less than 80 μm, and the mass is not less than 11.2 g / m2 The multi-element alloy co-diffusion layer includes the following chemical components in percentage by mass: 75-80wt% zinc, 13-18wt% aluminum and the remainder chromium.

[0008] The present invention also provides a method for preparing the corrosion-resistant steel of grade 6.8 or above described in the above scheme, comprising the following steps:

[0009] The raw steel is subjected to a tempering heat treatment after hot upsetting, wherein the tempering heat treatment comprises quenching, water cooling, tempering and cooling in sequence, so as to obtain the corrosion-resistant steel of grade 6.8 or above; the raw steel comprises the following chemical components in percentage by mass: C 0.18-0.24%, Si 0.15-0.23%, Mn 0.50-0.72%, Ni 0.10-0.36%, Cu 0.22-0.38%, Al 0.018-0.030%, Ti 0.01-0.05%, B 0.0008-0.0040%, Sn 0.005-0.100% and Fe balance.

[0010] Preferably, the quenching temperature is 830-870° C., and the holding time is 45-60 min.

[0011] Preferably, the tempering temperature is 400-480° C., and the holding time is 60-90 min.

[0012] Preferably, the tempering heat treatment also includes surface treatment of the obtained product; the surface treatment is powder multi-component alloy co-infiltration surface treatment; the infiltration temperature of the powder multi-component alloy co-infiltration surface treatment is 300-350°C, and the infiltration layer thickness is not less than 80μm; the infiltration layer elements of the powder multi-component alloy co-infiltration surface treatment include 75-80wt% zinc, 13-18wt% aluminum and a chromium remainder.

[0013] The present invention also provides the use of the corrosion-resistant steel of grade 6.8 or above described in the above scheme or the corrosion-resistant steel of grade 6.8 or above obtained by the preparation method of the above scheme in fastening connectors.

[0014] The present invention also provides a segment bolt, which is prepared by using the corrosion-resistant steel of grade 6.8 or above described in the above scheme or the corrosion-resistant steel of grade 6.8 or above obtained by the preparation method described in the above scheme.

[0015] Preferably, the specification of the segment bolts is ≤M30, and the length is 200-550 mm.

[0016] The present invention also provides a method for preparing the segment bolts described in the above scheme, comprising the following steps:

[0017] The raw steel is subjected to hot upsetting and then to hot thread rolling and quenching and tempering heat treatment in sequence, wherein the quenching and tempering heat treatment includes quenching, water cooling, tempering and cooling in sequence to obtain the segment bolts; the raw steel includes the following chemical components in percentage by mass: C 0.18-0.24%, Si 0.15-0.23%, Mn 0.50-0.72%, Ni 0.10-0.36%, Cu 0.22-0.38%, Al 0.018-0.030%, Ti 0.01-0.05%, B 0.0008-0.0040%, Sn 0.005-0.100% and Fe remainder.

[0018] The present invention provides a corrosion-resistant steel of grade 6.8 or above. The present invention promotes the formation of a dense and stable rust layer on the surface of the material through the composite of Cu and Ni, hinders further corrosion, and improves the corrosion resistance of the material; the present invention can form a composite of SnO2 and graphite on the surface of the material as an anti-corrosion layer through a trace amount of Sn, greatly enhancing the corrosion resistance of the material; the present invention improves the hardenability of steel by optimizing the C content and adding a trace amount of B to meet the performance requirements of large-scale tunnel segment bolts. The corrosion-resistant steel of grade 6.8 or above provided by the present invention adopts a specific element with a specific content, does not rely on a coating, achieves excellent corrosion resistance, good mechanical properties, and excellent comprehensive performance. The results of the embodiment show that in a 72h accelerated corrosion test in a NaCl solution at a constant temperature of 30°C, a pH value of 0.85, and a concentration of 10wt%, the annual average corrosion rate (CR) is ≤0.3mm / a.

[0019] The present invention also provides a method for preparing the corrosion-resistant steel of grade 6.8 or above as described in the above scheme. The preparation method provided by the present invention has simple steps, convenient operation, high feasibility and low cost.

[0020] The present invention also provides the use of the corrosion-resistant steel of grade 6.8 or above described in the above scheme or the corrosion-resistant steel of grade 6.8 or above obtained by the preparation method of the above scheme in fastening connectors. The corrosion-resistant steel of grade 6.8 or above provided by the present invention has excellent corrosion resistance and is suitable for fastening connections, such as preparing M30-M36 segment bolts, and can especially meet the performance requirements of complex tunnel environments such as mountains, rivers or seabeds.

[0021] The present invention also provides a segment bolt. Compared with the prior art, the segment bolt provided by the present invention can significantly improve the corrosion resistance under the same bolt performance grade condition, the annual average corrosion rate in the accelerated corrosion test is lower than 0.3 mm / a, no red rust appears in the neutral salt spray test for 1400 hours, and no red rust appears in the salt spray test for 900 hours.

[0022] The present invention also provides a method for preparing the segment bolts of the above solution. The preparation method provided by the present invention has low labor cost, good safety, and is suitable for industrial-scale production. DETAILED DESCRIPTION

[0023] The invention provides a corrosion-resistant steel of grade 6.8 or above, comprising the following chemical components in percentage by mass: C 0.18-0.24%, Si 0.15-0.23%, Mn 0.50-0.72%, Ni 0.10-0.36%, Cu 0.22-0.38%, Al 0.018-0.030%, Ti 0.01-0.05%, B 0.0008-0.0040%, Sn 0.005-0.100% and Fe balance.

[0024] The corrosion-resistant steel of grade 6.8 or above provided by the present invention comprises 0.18-0.24% C, specifically 0.18%, 0.19%, 0.20%, 0.21%, 0.22%, 0.23% or 0.24%. The present invention improves the strength and hardness of the material by adding the above-mentioned C as the main strengthening element in the material, while ensuring the toughness and plasticity of the material.

[0025] The corrosion-resistant steel of grade 6.8 or above provided by the present invention comprises 0.15-0.23% Si, specifically 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.20%, 0.21%, 0.22% or 0.23%. The present invention adds Si as a reducing agent and a deoxidizing agent, and Si can also play a role of solid solution strengthening.

[0026] The corrosion-resistant steel of grade 6.8 or above provided by the present invention comprises 0.50-0.72% of Mn, specifically 0.50%, 0.52%, 0.54%, 0.56%, 0.58%, 0.60%, 0.62%, 0.64%, 0.66%, 0.68%, 0.70% or 0.72%. The present invention mainly uses Mn as a desulfurizing agent by adding Mn; the present invention controls the content of Mn within the above range, and can also avoid the tendency of Mn segregation to grain boundaries during tempering.

[0027] The corrosion-resistant steel of grade 6.8 or above provided by the present invention comprises 0.10-0.36% Ni, specifically 0.10%, 0.12%, 0.14%, 0.16%, 0.18%, 0.20%, 0.22%, 0.24%, 0.26%, 0.28%, 0.30%, 0.32%, 0.34% or 0.36%. By adding Ni, the present invention significantly increases the compactness of the inner rust layer of the material, shifts the corrosion potential of the material positively, and improves the acid, alkali and atmospheric corrosion resistance of the material; in addition, Ni can improve the toughness of the material, especially the low-temperature toughness.

[0028] The corrosion-resistant steel of grade 6.8 or above provided by the present invention comprises 0.22-0.38% Cu, specifically 0.22%, 0.24%, 0.26%, 0.28%, 0.30%, 0.32%, 0.34%, 0.36% or 0.38%. The present invention adds Cu, which plays the role of activating the cathode in the corrosion process, promotes the passivation of the material anode, thereby slowing down the corrosion; at the same time, it promotes the compactness and stability of the inner rust layer, and hinders the further occurrence of corrosion.

[0029] The corrosion-resistant steel of grade 6.8 or above provided by the present invention comprises 0.018% to 0.030% Al, specifically 0.018%, 0.020%, 0.022%, 0.024%, 0.026%, 0.028% or 0.030%. The present invention refines the grains, improves the anti-oxidation property of the material, and reduces the brittle transition temperature of the material by adding Al as a deoxidizing and nitrogen fixing agent.

[0030] The corrosion-resistant steel of grade 6.8 or above provided by the present invention comprises 0.01-0.05% Ti, specifically 0.01%, 0.02%, 0.03%, 0.04% or 0.05%. The present invention adds Ti as a carbide-forming element, which has a strong solid solution strengthening effect and can significantly refine the grains.

[0031] The corrosion-resistant steel of grade 6.8 or above provided by the present invention comprises 0.0008-0.0040% B, specifically 0.0008%, 0.0010%, 0.0015%, 0.0020%, 0.0025%, 0.0030%, 0.0035% or 0.0040%. By adding a trace amount of B, the hardenability of the material can be significantly improved.

[0032] The corrosion-resistant steel of grade 6.8 or above provided by the present invention comprises 0.005-0.100% Sn, specifically 0.005%, 0.010%, 0.020%, 0.030%, 0.040%, 0.050%, 0.070%, 0.090% or 0.100%. Sn has always been considered as a harmful impurity element in steel materials, which affects the quality of steel materials, especially the quality of continuous casting billets, causes hot brittleness and temper brittleness of steel, produces cracks and fractures, and affects the welding performance of steel. However, the present invention has found that by adding a trace amount of Sn, a dense SnO2 and graphite protective layer can be formed on the surface of the material, which hinders the further occurrence of corrosion reaction and significantly improves the corrosion resistance of the material.

[0033] The corrosion-resistant steel of grade 6.8 or above provided by the present invention includes Fe surplus. Corrosion-resistant materials inevitably contain impurities, and the present invention reduces the content of impurity elements P and S as much as possible, reduces their segregation on grain boundaries, and improves grain boundary strength.

[0034] The corrosion-resistant steel of grade 6.8 or above provided by the present invention preferably further comprises a multi-element alloy co-infiltration layer; the multi-element alloy co-infiltration layer is covered on the surface of the corrosion-resistant steel of grade 6.8 or above; the thickness of the multi-element alloy co-infiltration layer is preferably not less than 80 μm, and the mass is preferably not less than 11.2 g / m 2 The multi-element alloy co-diffusion layer preferably comprises the following chemical components in percentage by mass: 75-80wt% zinc, 13-18wt% aluminum and the remainder chromium.

[0035] In the present invention, the content of zinc is preferably 75-80wt%, specifically 75wt%, 76wt%, 77wt%, 78wt%, 79wt% or 80wt%.

[0036] In the present invention, the aluminum content is preferably 13-18wt%, specifically 13wt%, 14wt%, 15wt%, 16wt%, 17wt% or 18wt%.

[0037] The present invention also provides a method for preparing the corrosion-resistant steel of grade 6.8 or above described in the above scheme, comprising the following steps:

[0038] The raw steel is subjected to a tempering heat treatment after hot upsetting, wherein the tempering heat treatment comprises quenching, water cooling, tempering and cooling in sequence, so as to obtain the corrosion-resistant steel of grade 6.8 or above; the raw steel comprises the following chemical components in percentage by mass: C 0.18-0.24%, Si 0.15-0.23%, Mn 0.50-0.72%, Ni 0.10-0.36%, Cu 0.22-0.38%, Al 0.018-0.030%, Ti 0.01-0.05%, B 0.0008-0.0040%, Sn 0.005-0.100% and Fe balance.

[0039] The present invention performs hot upsetting on the raw steel. In the present invention, the raw steel is preferably round steel; the raw steel is preferably obtained by hot rolling of steel ingots; the starting rolling temperature of the hot rolling is preferably 1200-1350°C, specifically 1200°C, 1230°C, 1250°C, 1280°C, 1300°C, 1320°C or 1350°C, the final rolling temperature is preferably 1100-1150°C, specifically 1100°C, 1110°C, 1120°C, 1130°C, 1140°C or 1150°C; the steel ingot is preferably obtained by pouring molten steel into ingots.

[0040] In the present invention, the temperature of the hot upsetting is preferably 900-1050°C, specifically 900°C, 950°C, 1000°C or 1050°C; the pressure is preferably 400-520KN, specifically 400KN, 420KN, 450KN, 480KN, 500KN or 520KN; the hot upsetting time is preferably 2-3s, specifically 2.5s.

[0041] After hot upsetting, the present invention performs a tempering heat treatment on the obtained hot upsetting piece, and the tempering heat treatment includes quenching, water cooling, tempering and cooling in sequence to obtain the corrosion-resistant steel of grade 6.8 or above. In the present invention, the quenching temperature is preferably 830-870°C, specifically 830°C, 840°C, 850°C, 860°C or 870°C, and the holding time is preferably 45-60min, specifically 45min, 50min, 55min or 60min.

[0042] In the present invention, the cooling rate of the water cooling is preferably not more than 230°C / s, and can be specifically 230°C / s, 200°C / s, 180°C / s, 150°C / s, 120°C / s, 90°C / s or 80°C / s; the water cooling preferably includes a first water cooling above 600°C (high temperature section) and a second water cooling below 300°C (low temperature section) in sequence; the cooling rate of the first water cooling is preferably not more than 230°C / s; the cooling rate of the second water cooling is preferably 80-90°C / s, and can be specifically 85°C / s; the final temperature of the water cooling is preferably 40-45°C, and can be specifically 43°C.

[0043] In the present invention, the tempering temperature is preferably 400-480°C, specifically 400°C, 410°C, 420°C, 430°C, 440°C, 450°C, 460°C, 470°C or 480°C, and the holding time is preferably 60-90 min, specifically 60 min, 70 min, 80 min or 90 min.

[0044] In the present invention, the cooling is preferably air cooling; the cooling rate is preferably 250-350°C / min, specifically 250°C / min, 270°C / min, 280°C / min, 300°C / min, 320°C / min or 350°C / min, and the final temperature is preferably room temperature, specifically 25°C.

[0045] In the present invention, the tempering heat treatment preferably also includes surface treatment of the obtained product; the surface treatment is preferably powder multi-component alloy co-diffusion surface treatment; the infiltration temperature of the powder multi-component alloy co-diffusion surface treatment is preferably 300-350°C, specifically 300°C, 310°C, 330°C or 350°C, and the infiltration layer thickness is preferably not less than 80μm, specifically 80μm, 90μm, 100μm, 120μm, 150μm, 180μm or 200μm.

[0046] In the present invention, the diffusion layer elements of the powder multi-element alloy co-diffusion surface treatment preferably include 75-80wt% zinc, 13-18wt% aluminum and a remainder of chromium; the zinc content is preferably 75-80wt%, specifically 75wt%, 76wt%, 77wt%, 78wt%, 79wt% or 80wt%.

[0047] In the present invention, the aluminum content is preferably 13-18wt%, specifically 13wt%, 14wt%, 15wt%, 16wt%, 17wt% or 18wt%.

[0048] The present invention also provides the use of the corrosion-resistant steel of grade 6.8 or above described in the above scheme or the corrosion-resistant steel of grade 6.8 or above obtained by the preparation method of the above scheme in fastening connectors.

[0049] The corrosion-resistant steel of grade 6.8 or above provided by the present invention has excellent corrosion resistance and is suitable for fastening connections, such as preparing M30-M36 segment bolts, and can especially meet the performance requirements of complex tunnel environments such as mountains, rivers or seabeds.

[0050] The present invention also provides a segment bolt, which is prepared by using the corrosion-resistant steel of grade 6.8 or above described in the above scheme or the corrosion-resistant steel of grade 6.8 or above obtained by the preparation method described in the above scheme.

[0051] In the present invention, the specification of the segment bolts is preferably ≤M30, and the length is preferably 200-550 mm, specifically 200 mm, 250 mm, 300 mm, 350 mm, 400 mm, 450 mm, 500 mm or 550 mm.

[0052] The segment bolts provided by the present invention can significantly improve the corrosion resistance under the condition of the same bolt performance grade, with an average annual corrosion rate of less than 0.3 mm / a in an accelerated corrosion test, no red rust appearing in a neutral salt spray test for 1400 h, and no red rust appearing in a salt spray test for 900 h.

[0053] The present invention also provides a method for preparing the segment bolts described in the above scheme, comprising the following steps:

[0054] The raw steel is subjected to hot upsetting and then to hot thread rolling and quenching and tempering heat treatment in sequence, wherein the quenching and tempering heat treatment includes quenching, water cooling, tempering and cooling in sequence to obtain the segment bolts; the raw steel includes the following chemical components in percentage by mass: C 0.18-0.24%, Si 0.15-0.23%, Mn 0.50-0.72%, Ni 0.10-0.36%, Cu 0.22-0.38%, Al 0.018-0.030%, Ti 0.01-0.05%, B 0.0008-0.0040%, Sn 0.005-0.100% and Fe remainder.

[0055] In the present invention, the preparation parameters of the segment bolts are the same as the preparation parameters of the corrosion-resistant steel of grade 6.8 or above described in the above scheme, the difference being that the temperature of the hot thread rolling is preferably 600-700°C, specifically 600°C, 620°C, 640°C, 660°C, 680°C or 700°C.

[0056] In order to further illustrate the present invention, the scheme of the present invention is described in detail below in conjunction with embodiments, but they should not be understood as limiting the scope of protection of the present invention.

[0057] Example

[0058] A 100kg vacuum induction furnace was used to smelt 3 furnaces of corrosion-resistant steel of grade 6.8 or above of the present invention (Examples 1 to 3) and 4 furnaces of comparative steel materials (Comparative Examples 1 to 4). At the same time, 3 furnaces of typical bolt steel 45, 20Cr and weathering bolt steel NH6 were selected as comparative steel materials. The specific components are shown in Table 1. After the molten steel was cast into ingots, it was hot rolled into Φ30 round steel. The round steel was hot-forged (temperature was 950°C, pressure was 470KN, and hot upsetting time was 2.5s) to form. After tempering heat treatment, the surface was treated and processed into standard room temperature tensile specimens, standard V-mouth Charpy impact specimens and hanging sheet specimens (60mm×25mm×5mm). The specimens were subjected to tensile tests at room temperature, impact performance tests were carried out at -20°C, and accelerated corrosion tests were carried out for 72h. The conditions of the accelerated corrosion test are shown in Table 2. The corrosion weight loss and annual average corrosion rate of each specimen were evaluated. The specific conditions and test results are shown in Table 3.

[0059] Segment bolts were prepared using the round steel of Examples 1 to 3 and Comparative Examples 1 to 4, and hot thread rolling was performed before surface treatment. Other conditions were the same as above. The existing surface treatment process was used as a comparison. After different surface treatments, neutral salt spray tests (refer to GB / T 10125 "Artificial atmosphere corrosion test salt spray test") and wet heat tests (60±2°C, 95±3%RH, refer to GB / T2423.3 "Environmental testing Part 2: Test method Test Cab: Constant wet heat test") were performed. The corrosion resistance of the segment bolts was determined by the time when red rust appeared. The test results are shown in Table 4.

[0060] Table 1 Composition (wt%) of Examples 1 to 3, Comparative Examples 1 to 4 and Comparative Steel Materials

[0061]

[0062]

[0063] Table 2 Accelerated corrosion test conditions

[0064] Test solution NaCl (10 wt%) and HCl mixed solution, pH = 0.85 Test temperature 30℃ constant temperature Test time 72h Remark Replace the solution every 24 hours to keep the pH value stable

[0065] Table 3 Test results of Examples 1 to 3, Comparative Examples 1 to 4 and comparative steel materials

[0066]

[0067] It can be seen from Table 3 that the corrosion-resistant steel of grade 6.8 or above provided by the present invention has better strength and toughness as a whole, a tensile strength of more than 920 MPa, a yield strength ratio of more than 0.85, a low-temperature impact energy of -20°C KV2 of not less than 130 J, better corrosion resistance, and a comprehensive performance significantly better than that of the comparative example.

[0068] Table 4 Segment bolt test results of Examples 1 to 3 and Comparative Examples 1 to 4

[0069]

[0070] It can be seen from Table 4 that the segment bolts made of corrosion-resistant steel of grade 6.8 or above of the present invention have excellent corrosion resistance, and no red rust is observed on the surface of the specimen after a neutral salt spray test of 1400 h, and no red rust is observed on the surface of the specimen after a wet heat test of 900 h.

[0071] It can be seen from the above embodiments that the corrosion-resistant steel of grade 6.8 or above provided by the present invention has excellent corrosion resistance, good mechanical properties and excellent comprehensive performance.

[0072] Although the above embodiment describes the present invention in detail, it is only a part of the embodiments of the present invention, not all of the embodiments. Other embodiments can be obtained based on this embodiment without creativity, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A corrosion-resistant steel of grade 6.8 or above, characterized in that: The invention comprises the following chemical compositions in mass percentage: C 0.18-0.24%, Si 0.15-0.23%, Mn 0.50-0.72%, Ni 0.10-0.36%, Cu 0.22-0.38%, Al 0.018-0.030%, Ti 0.01-0.05%, B 0.0008-0.0040%, Sn 0.005-0.100% and Fe balance.

2. The corrosion-resistant steel of grade 6.8 or above according to claim 1, characterized in that: It also includes a multi-element alloy co-penetration layer; the multi-element alloy co-penetration layer is covered on the surface of the corrosion-resistant steel of grade 6.8 or above; the thickness of the multi-element alloy co-penetration layer is not less than 80 μm, and the mass is not less than 11.2 g / m 2 The multi-element alloy co-diffusion layer includes the following chemical components in percentage by mass: 75-80wt% zinc, 13-18wt% aluminum and the remainder chromium.

3. The method for preparing the corrosion-resistant steel of grade 6.8 or above according to any one of claims 1 to 2, characterized in that: The following steps are involved: The raw steel is subjected to a quenching and tempering heat treatment after hot upsetting, wherein the quenching and tempering heat treatment includes quenching, water cooling, tempering and cooling in sequence to obtain the corrosion-resistant steel of grade 6.8 or above; The raw steel comprises the following chemical components in percentage by mass: C 0.18-0.24%, Si 0.15-0.23%, Mn 0.50-0.72%, Ni 0.10-0.36%, Cu 0.22-0.38%, Al 0.018-0.030%, Ti 0.01-0.05%, B 0.0008-0.0040%, Sn 0.005-0.100% and Fe balance.

4. The preparation method according to claim 3, characterized in that: The quenching temperature is 830-870° C., and the holding time is 45-60 minutes.

5. The preparation method according to claim 3 or 4, characterized in that: The tempering temperature is 400-480° C., and the heat preservation time is 60-90 minutes.

6. The preparation method according to claim 3 or 4, characterized in that: The tempering heat treatment also includes surface treatment of the obtained product; The surface treatment is powder multi-element alloy co-penetration surface treatment; The infiltration temperature of the powder multi-element alloy co-infiltration surface treatment is 300-350°C, and the thickness of the infiltration layer is not less than 80 μm; The elements of the infiltration layer of the powder multi-element alloy co-infiltration surface treatment include 75-80wt% zinc, 13-18wt% aluminum and the balance of chromium.

7. Use of the corrosion-resistant steel with a grade of 6.8 or above as claimed in any one of claims 1 to 2 or the corrosion-resistant steel with a grade of 6.8 or above obtained by the preparation method according to any one of claims 3 to 6 in fastening connectors.

8. A segment bolt, characterized in that: The method is prepared by using the corrosion-resistant steel of grade 6.8 or above as described in any one of claims 1 to 2 or the corrosion-resistant steel of grade 6.8 or above obtained by the preparation method described in any one of claims 3 to 6.

9. The segment bolt according to claim 8, characterized in that: The specification of the segment bolts is ≤M30, and the length is 200-550mm.

10. The method for preparing the segment bolts according to any one of claims 8 to 9, characterized in that: The following steps are involved: The raw steel is subjected to hot upsetting and then hot thread rolling and quenching and tempering heat treatment in sequence, wherein the quenching and tempering heat treatment includes quenching, water cooling, tempering and cooling in sequence to obtain the segment bolts; The raw steel comprises the following chemical components in percentage by mass: C 0.18-0.24%, Si 0.15-0.23%, Mn 0.50-0.72%, Ni 0.10-0.36%, Cu 0.22-0.38%, Al 0.018-0.030%, Ti 0.01-0.05%, B 0.0008-0.0040%, Sn 0.005-0.100% and Fe balance.

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