Stress-corrosion-resistant high-strength bolt steel as well as preparation method and application thereof

Abstract

The invention provides stress-corrosion-resistant high-strength bolt steel as well as a preparation method and application thereof, and belongs to the technical field of metal materials. According to the method, the Cu element and the Nb element are independently or compositely added into the bolt steel, Cu is easy to separate out to form a nano Cu-rich phase, through the dispersed nano Cu precipitated phase, the obdurability of the bolt steel can be improved, the corrosion uniformity can be improved, local corrosion and stress corrosion crack initiation can be inhibited, and the atmospheric corrosion resistance of the bolt steel is remarkably improved; nb can refine grains and form an NbC nano precipitated phase, and can be used as a hydrogen trap to reduce hydrogen embrittlement sensitivity, improve grain boundary characteristics, increase grain boundary density, increase the proportion of small-angle grain boundaries, reduce crack initiation nucleation sites, and hinder crack propagation through high-density large-angle and small-angle grain boundaries, so that the stress corrosion resistance of the bolt steel is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of metal materials, and particularly relates to a high-strength bolt steel resistant to stress corrosion, a preparation method thereof, and an application thereof. Background Art

[0002] In recent years, with the development of industrialization, various major engineering equipment and facilities have been widely used. As a key load-bearing structural component of major engineering equipment / facilities (such as offshore platforms, cross-sea bridges, high-speed rails, and wind power generation, etc.), the corrosion resistance and reliability of high-strength bolt steel are directly related to the service safety of the entire engineering equipment / facility. During service, high-strength bolt steel not only has to face the erosion of natural environments such as marine environments / industrial pollution, but also has to withstand complex stress loads generated by its own weight, sea winds and waves, and vehicle traffic, etc., and is extremely prone to brittle fracture accidents caused by stress corrosion and corrosion fatigue. At the same time, due to the polluting gas SO 2 dissolved in the liquid film on the metal surface, the liquid film on the metal surface becomes acidic, making the high-strength bolts in a harsh corrosion environment. Corrosion not only reduces the effective cross-sectional area of the bolts, but also the hydrogen atoms generated by the corrosion reaction will penetrate into the material, resulting in an increase in the local hydrogen concentration. All of these will lead to stress corrosion cracking of the bolts, greatly affecting the safe service of the steel structure. Engineering research cases in recent years have also shown that a large number of stress corrosion fracture failure accidents have occurred in high-strength bolt steel. The reason is that the mainstream high-strength bolt steels currently in service (such as 20MnTiB and 35VB, etc.) lack the component design of corrosion resistance and corrosion cracking resistance in the initial R & D design. Therefore, it has important engineering practical value to develop high-strength bolt steel with excellent stress corrosion resistance.

[0003] Therefore, how to improve the stress corrosion resistance of bolts has become an urgent technical problem in this field. Summary of the Invention

[0004] The purpose of the present invention is to provide a high-strength bolt steel resistant to stress corrosion, a preparation method thereof, and an application thereof. The high-strength bolt steel resistant to stress corrosion provided by the present invention has obvious stress corrosion resistance advantages on the premise of having good strength and toughness.

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

[0006] The present invention provides a high-strength bolt steel resistant to stress corrosion, which includes the following chemical components by mass percentage: C: 0.17 - 0.3%, Si: 0.17 - 0.37%, Mn: 1.3 - 1.6%, Ti: 0.02 - 0.1%, B: 0.0008 - 0.0035%, M: 0.02 - 0.9%, and the balance is Fe, wherein M is Cu and / or Nb.

[0007] Preferably, by mass percentage, the stress corrosion resistant high-strength bolt steel comprises the following chemical components: C: 0.17 - 0.3%, Si: 0.17 - 0.37%, Mn: 1.3 - 1.6%, Ti: 0.02 - 0.1%, B: 0.0008 - 0.0035%, Cu: 0.2 - 0.8% and the balance Fe.

[0008] Preferably, by mass percentage, the stress corrosion resistant high-strength bolt steel comprises the following chemical components: C: 0.17 - 0.3%, Si: 0.17 - 0.37%, Mn: 1.3 - 1.6%, Ti: 0.02 - 0.1%, B: 0.0008 - 0.0035%, Nb: 0.02 - 0.1% and the balance Fe.

[0009] Preferably, by mass percentage, the stress corrosion resistant high-strength bolt steel comprises the following chemical components: C: 0.17 - 0.3%, Si: 0.17 - 0.37%, Mn: 1.3 - 1.6%, Ti: 0.02 - 0.1%, B: 0.0008 - 0.0035%, Cu: 0.2 - 0.8%, Nb: 0.02 - 0.1% and the balance Fe.

[0010] Preferably, the microstructure of the stress corrosion resistant high-strength bolt steel is tempered sorbite structure or tempered troostite structure.

[0011] The present invention provides a method for preparing the stress corrosion resistant high-strength bolt steel according to the above technical solution, which comprises the following steps:

[0012] (1) Melting and casting the raw materials in sequence to obtain a bolt ingot;

[0013] (2) Forging and hot rolling the bolt ingot obtained in step (1) in sequence to obtain a rolled piece;

[0014] (3) Quenching and tempering the rolled piece obtained in step (2) in sequence to obtain the stress corrosion resistant high-strength bolt steel.

[0015] Preferably, in step (2), the starting rolling temperature of the hot rolling is ≥1100 °C, and the final rolling temperature of the hot rolling is ≥875 °C.

[0016] Preferably, in step (3), the holding temperature before the quenching treatment is 850 - 900 °C, the holding time before the quenching treatment is 0.5 - 1 h, and the quenching medium is water.

[0017] Preferably, in step (3), the holding temperature of the tempering treatment is 300 - 500 °C, the holding time of the tempering treatment is 0.5 - 1 h, and the cooling method after the tempering treatment is water cooling to room temperature.

[0018] The present invention provides the stress corrosion resistant high-strength bolt steel described in the above technical solution or the stress corrosion resistant high-strength bolt steel prepared by the preparation method described in the above technical solution, which is applied to offshore platforms, cross-sea bridges, high-speed railways and wind power generation facilities.

[0019] The invention provides a stress corrosion resistant high-strength bolt steel, which comprises the following chemical composition by mass percentage: C: 0.17-0.3%, Si: 0.17-0.37%, Mn: 1.3-1.6%, Ti: 0.02-0.1%, B: 0.0008-0.0035%, M: 0.02-0.8% and the balance Fe, wherein M is Cu and / or Nb. The present invention adds Cu and Nb elements to the bolt steel alone or in combination, so that Cu is easily precipitated to form a nano Cu-rich phase. The dispersed nano Cu precipitated phase can not only improve the strength and toughness of the bolt steel, but also improve the corrosion uniformity, inhibit the initiation of local corrosion and stress corrosion cracks, significantly improve the atmospheric corrosion resistance of the bolt steel, and solve the problem of insufficient corrosion resistance of the existing 20MnTiB high-strength bolt steel; Nb can refine the grains and form NbC nano precipitated phases, which can act as a hydrogen trap to reduce the sensitivity to hydrogen embrittlement, improve the grain boundary characteristics, increase the grain boundary density, and at the same time increase the small-angle grain boundary ratio, reduce the crack initiation nucleation site, and hinder the crack propagation through high-density large and small-angle grain boundaries, thereby And improve the stress corrosion resistance of bolt steel; there is a synergistic effect when Cu and Nb are added together. The addition of Nb can greatly promote the precipitation of nano Cu particles, and the precipitation of Cu particles inhibits the coarsening of Nb (C, N). The synergistic precipitation of the two not only improves the strength and toughness of steel, but also greatly improves the hydrogen trap capacity, thereby improving the stress corrosion resistance of bolt steel in industrial pollution environments; in addition, Cu and Nb are not easy to burn during smelting, the composition is easy to control, and compared with precious alloying elements such as Ni and Cr, the cost is lower. A small amount or trace addition can obtain significant corrosion resistance and stress corrosion resistance effects, effectively reducing the production cost of bolt steel and improving the market competitiveness of the product. The results of the embodiment show that the yield strength of the stress corrosion resistant high-strength bolt steel provided by the present invention reaches more than 1000MPa, the tensile strength is close to 1100MPa, and it has high elongation and area reduction, high strength and high plasticity, and excellent stress corrosion resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The microstructure morphology of the bolt steel provided in Example 1;

[0021] Figure 2 The microstructure morphology of the bolt steel provided in Example 2;

[0022] Figure 3 The microstructure morphology of the bolt steel provided in Example 3;

[0023] Figure 4 Microstructure morphology diagram of the bolt steel provided in Comparative Example 1;

[0024] Figure 5 EBSD grain boundary characteristic diagram of the bolt steel provided in Example 1;

[0025] Figure 6 EBSD grain boundary characteristic diagram of the bolt steel provided in Example 2;

[0026] Figure 7 EBSD grain boundary characteristic diagram of the bolt steel provided in Example 3;

[0027] Figure 8 EBSD grain boundary characteristic diagram of the bolt steel provided in Comparative Example 1;

[0028] Figure 9 Large and small grain boundary densities and total grain boundary density of the bolt steel provided in Examples 1-3 and Comparative Example 1;

[0029] Figure 10 Stress-strain curves of the bolt steel provided in Examples 1-3 and Comparative Example 1;

[0030] Figure 11 SSRT tensile curves of the bolt steel provided in Examples 1-3 and Comparative Example 1;

[0031] Figure 12 Stress corrosion susceptibility index of the bolt steel provided in Examples 1-3 and Comparative Example 1;

[0032] Figure 13 Side morphology diagram of the stress corrosion fracture of the bolt steel provided in Example 1 in a simulated industrial pollution environment;

[0033] Figure 14 Side morphology diagram of the stress corrosion fracture of the bolt steel provided in Example 2 in a simulated industrial pollution environment;

[0034] Figure 15 Side morphology diagram of the stress corrosion fracture of the bolt steel provided in Example 3 in a simulated industrial pollution environment;

[0035] Figure 16 Side morphology diagram of the stress corrosion fracture of the bolt steel provided in Comparative Example 1 in a simulated industrial pollution environment. Detailed implementation manners

[0036] The present invention provides a high-strength bolt steel resistant to stress corrosion, which comprises the following chemical components by mass percentage: C: 0.17 - 0.3%, Si: 0.17 - 0.37%, Mn: 1.3 - 1.6%, Ti: 0.02 - 0.1%, B: 0.0008 - 0.0035%, M: 0.02 - 0.9% and the balance Fe, wherein M is Cu and / or Nb.

[0037] By mass percentage, the high-strength bolt steel resistant to stress corrosion provided by the present invention comprises C: 0.17 - 0.30%. In the present invention, C can improve the strength, hardness and hardenability of the bolt steel; when the content of C is less than 0.17%, although the bolt steel has good toughness and plasticity, its strength, hardness and hardenability will be limited to a certain extent; when the content of C is greater than 0.30%, the toughness and plasticity of the bolt steel will be greatly reduced, and brittle fracture is likely to occur. Therefore, the present invention controls the content of C within the range of 0.17 - 0.3%. As an embodiment of the present invention, the mass percentage of C can be 0.17%, 0.18%, 0.19%, 0.2%, 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, 0.29% or 0.3%.

[0038] By mass percentage, the high-strength bolt steel resistant to stress corrosion provided by the present invention comprises Si: 0.17 - 0.37%. In the present invention, Si is a good deoxidizer for the bolt steel. By adding Si, the solid solution strength of the bolt steel can be improved, which is beneficial to increasing the tempering stability of the bolt steel; when the content of Si is too high, the plasticity of the bolt steel will be reduced, while when the content of Si is too low, its deoxidation and solid solution strengthening effects will be weak. Therefore, the present invention controls the content of Si within the range of 0.17 - 0.37%. As an embodiment of the present invention, the mass percentage of Si can be 0.17%, 0.18%, 0.19%, 0.2%, 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, 0.29%, 0.3%, 0.31%, 0.32%, 0.33%, 0.34%, 0.35%, 0.36% or 0.37%.

[0039] By mass percentage, the stress corrosion resistant high-strength bolt steel provided by the present invention includes Mn: 1.3-1.6%. In the present invention, Mn has the functions of improving the hardenability of the material, improving the strength and toughness of the material, and making the structure uniform and refined; however, when the Mn content is higher than 1.6%, it will lead to the segregation of the bolt steel composition and the increase of temper brittleness, and also lead to the reduction of the corrosion resistance of the bolt steel, which cannot reach the optimum, and at the same time increases the production cost of the bolt steel. When the manganese content is lower than 1.3%, it will be difficult to ensure that the strength of the bolt steel reaches the target value. Therefore, the present invention controls the Mn content within the range of 1.3-1.6%. As an embodiment of the present invention, the mass percentage of Mn can be 1.3%, 1.4%, 1.5% or 1.6%.

[0040] By mass percentage, the stress corrosion resistant high-strength bolt steel provided by the present invention includes Ti: 0.02-0.1%. In the present invention, Ti is mainly used to combine with N to eliminate N in the steel and avoid the formation of the brittle phase BN. At the same time, Ti can refine the grain structure of the steel and improve the strength and toughness of the steel. However, if an excessive amount of titanium is added, it may form coarse Ti(CN), which affects the plasticity and toughness of the bolt steel. Therefore, the present invention controls the Ti content within the range of 0.02-0.1%. As an embodiment of the present invention, the mass percentage of Ti can be 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09% or 0.1%.

[0041] By mass percentage, the stress corrosion resistant high-strength bolt steel provided by the present invention includes B: 0.0008-0.0035%. In the present invention, B is mainly used to improve the hardenability of the bolt steel. A small amount of boron can significantly improve the hardenability of the steel. However, when the boron content exceeds 0.007%, it is easy to cause brittleness. Therefore, the present invention controls the B content within the range of 0.0008-0.0035%. As an embodiment of the present invention, the mass percentage of B can be 0.0008%, 0.001%, 0.0015%, 0.002%, 0.0025%, 0.003% or 0.0035%.

[0042] By mass percentage, the stress corrosion resistant high-strength bolt steel provided by the present invention includes M: 0.02-0.9%. In the present invention, M is Cu and / or Nb.

[0043] In the present invention, when M is Cu, by mass percentage, the stress corrosion resistant high-strength bolt steel comprises Cu: 0.2-0.8%. In the present invention, Cu has beneficial effects such as improving hardenability, strength and toughness, and corrosion resistance, and can inhibit the stress corrosion process by suppressing the anodic dissolution of steel, generating insoluble products to hinder the intrusion of chloride ions, etc.; if the Cu content is lower than 0.2%, it has little effect on corrosion resistance and stress corrosion resistance, but if the Cu content is higher than 0.8%, the bolt steel is prone to hot brittleness during forging and rolling processes. Therefore, the content of Cu in the present invention is controlled within the range of 0.2-0.8%. As an embodiment of the present invention, the mass percentage of Cu can be 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7% or 0.8%.

[0044] In the present invention, when M is Nb, by mass percentage, the stress corrosion resistant high-strength bolt steel comprises Nb: 0.02-0.1%. In the present invention, Nb can precipitate in the form of fine and dispersed Nb(C,N), which can act as a deep hydrogen trap to reduce the hydrogen embrittlement sensitivity, and play a pinning role in the migration of grain boundaries, hindering the growth of grains. After phase transformation, the ferrite grain size inherits the original austenite grain size, so as to obtain a relatively fine and uniform microstructure, thereby improving the strength and toughness, and corrosion resistance of the steel; at the same time, Nb can improve the grain boundary characteristics, increase the proportion of small-angle grain boundaries, reduce the crack initiation and nucleation sites, and further improve the stress corrosion resistance of the bolt steel; if the Nb content is lower than 0.02%, the grain refinement and precipitation effects are not obvious enough, and the influence on the stress corrosion resistance is not significant enough; if the Nb content is higher than 0.1%, not only the cost increases significantly, but also the hot plasticity of the steel becomes worse, increasing the risk of transverse cracks during forging or rolling processes. Therefore, the content of Nb in the present invention is controlled within the range of 0.02-0.1%. As an embodiment of the present invention, the mass percentage of Nb can be 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09% or 0.1%.

[0045] In the present invention, when M is Cu and Nb, by mass percentage, the stress corrosion resistant high-strength bolt steel comprises Cu: 0.2-0.8% and Nb: 0.02-0.1%. In the present invention, there is also a synergistic effect between Cu and Nb. The addition of Nb can greatly promote the precipitation of nano-Cu particles, and at the same time, the precipitation of Cu particles inhibits the coarsening of Nb(C,N). The synergistic precipitation of the two not only improves the strength and toughness of the steel, but also can greatly improve the hydrogen trap ability, thereby improving the stress corrosion resistance of the bolt steel in the industrial pollution environment.

[0046] By mass percentage, the stress corrosion resistant high-strength bolt steel provided by the present invention comprises the balance of Fe. In the present invention, Fe is the matrix element of the bolt steel.

[0047] In the present invention, the microstructure of the stress corrosion resistant high strength bolt steel is preferably tempered sorbite structure or tempered troostite structure.

[0048] The present invention also provides a method for preparing the stress corrosion resistant high strength bolt steel according to the above technical solution, comprising the following steps:

[0049] (1) Smelting and casting the raw materials in sequence to obtain a bolt ingot;

[0050] (2) Forging and hot rolling the bolt ingot obtained in step (1) in sequence to obtain a rolled piece;

[0051] (3) Performing quenching treatment and tempering treatment on the rolled piece obtained in step (2) in sequence to obtain the stress corrosion resistant high strength bolt steel.

[0052] In the present invention, the raw materials are smelted and cast in sequence to obtain a bolt ingot.

[0053] The present invention has no special limitation on the specific types and dosages of the raw materials. Using commercially available products well-known to those skilled in the art, it is only necessary to make the composition of the bolt ingot meet the requirements. As an embodiment of the present invention, the raw materials may be carbon, silicon, manganese, titanium, boron, copper, niobium and iron.

[0054] The present invention has no special limitation on the specific operations and process parameters of the smelting and casting. Using the operations of smelting and casting well-known to those skilled in the art is sufficient. As an embodiment of the present invention, the temperature of the smelting may be 1650 °C; the smelting may be carried out in a vacuum electromagnetic induction furnace.

[0055] In the present invention, the chemical composition of the bolt ingot is preferably the same as that of the stress corrosion resistant high strength bolt steel.

[0056] After obtaining the bolt ingot, the present invention forges and hot rolls the bolt ingot in sequence to obtain a rolled piece.

[0057] The present invention has no special limitation on the specific operation of the forging. It is determined according to the technical common sense of those skilled in the art, and it is only necessary to make the grains in the bolt ingot fully broken and ensure the continuous integrity of the billet streamline. The present invention can eliminate the tissue defects generated during the casting process through forging and make the grains of the bolt ingot fully broken to obtain fine internal grains. As an embodiment of the present invention, the temperature of the forging is 1140-1160 °C. In the present invention, when the temperature of the forging does not meet the above requirements, the present invention preferably heats the bolt ingot. The present invention has no special limitation on the temperature and time of the heating, and it is only necessary to make the temperature of the forging meet the requirements.

[0058] The present invention preferably heats the forged product after forging. In the present invention, the holding temperature of the heating is preferably 1150-1250 °C, more preferably 1200 °C; the holding time of the heating is preferably 1.5-2 h, more preferably 2 h. By heating, the present invention can eliminate the internal stress generated during forging and obtain a uniform austenite structure at the same time.

[0059] In the present invention, the starting rolling temperature of the hot rolling is preferably ≥1100 °C; the finishing rolling temperature of the hot rolling is preferably ≥875 °C; the cumulative reduction ratio of the hot rolling is preferably ≥50%. By hot rolling, the present invention can fully refine the grains. As an embodiment of the present invention, the starting rolling temperature of the austenite recrystallization zone rolling can be 1100-1200 °C, or can also be 1100-1150 °C.

[0060] After obtaining the rolled piece, the present invention sequentially performs quenching treatment and tempering treatment on the rolled piece to obtain stress corrosion resistant high strength bolt steel.

[0061] In the present invention, the holding temperature before the quenching treatment is preferably 850-900 °C; the holding time before the quenching treatment is preferably 0.5-1 h; the medium for the quenching treatment is preferably water; the holding temperature of the tempering treatment is preferably 300-500 °C; the holding time of the tempering treatment is preferably 0.5-1 h; the cooling method after the tempering treatment is preferably water cooling to room temperature. By performing quenching treatment and tempering treatment, the present invention can form a tempered sorbite structure or a tempered troostite structure with fine and uniform grains in the bolt steel, effectively improving the strength and toughness of the bolt steel and the stress corrosion resistance. As an embodiment of the present invention, the holding temperature of the tempering treatment can be 350, 400 °C or 450 °C.

[0062] In the process of preparing bolt steel in the present invention, a bolt steel ingot is first cast, and then the prepared bolt steel ingot is forged to fully break the internal structure of the bolt steel ingot. Then, the forged billet is subjected to austenite homogenization treatment to eliminate the stress generated during forging and obtain a uniform austenite structure at the same time, thereby improving the strength and plasticity of the bolt steel. Then, hot rolling can fully refine the grains. Finally, by using quenching treatment and tempering treatment, a tempered sorbite structure or a tempered troostite structure with fine and uniform grains can be formed in the bolt steel, effectively improving the strength and toughness of the bolt steel and the stress corrosion resistance.

[0063] The present invention also provides the application of the stress corrosion resistant high strength bolt steel described in the above technical solution or the stress corrosion resistant high strength bolt steel prepared by the preparation method described in the above technical solution in offshore platforms, cross-sea bridges, high-speed railways and wind power generation facilities.

[0064] The present invention does not have any special limitations on the specific implementation of the application, and it can be implemented according to the common technical knowledge of those skilled in the art.

[0065] The stress corrosion resistant high strength bolt steel provided by the present invention can provide strong support, fixation and connection for large-scale engineering equipment / facilities. The bolt steel has excellent stress corrosion resistance and corrosion resistance, effectively resists stress corrosion fracture accidents caused by the corrosion of the natural environment, improves the service life of large-scale engineering equipment, and provides a strong guarantee for the long-term stable operation of major engineering equipment; the bolt steel material composition system provided by the present invention has less precious metal content, high stress corrosion resistance, excellent cost performance, and at the same time, the preparation process of the bolt steel is simple, the application is convenient, and it is convenient for large-scale popularization.

[0066] Compared with the prior art, the beneficial technical effects of the present invention are mainly as follows:

[0067] (1) Cu is one of the typical alloying elements resistant to atmospheric corrosion. The solubility of Cu in steel is small and it is easy to precipitate to form nano Cu-rich phases. Therefore, through the dispersed nano Cu precipitation phases, not only the strength and toughness of the bolt steel can be improved, but also its atmospheric corrosion resistance can be significantly improved, solving the problem of insufficient corrosion resistance of the existing 20MnTiB high strength bolt steel. From the results of the stress corrosion test, Cu alloying can improve the uniformity of corrosion and inhibit the initiation of local corrosion and stress corrosion cracks.

[0068] (2) The addition of Nb can refine the grains, form NbC nano precipitation phases, act as hydrogen traps to reduce the hydrogen embrittlement sensitivity, and can improve the grain boundary characteristics, increase the grain boundary density, at the same time increase the proportion of small angle grain boundaries, reduce the nucleation sites of crack initiation, and hinder the crack propagation through the high density of large and small angle grain boundaries, thereby improving the stress corrosion resistance of the high strength bolt steel. Therefore, the present invention improves the strength, toughness and stress corrosion resistance of the bolt steel by adding Cu and Nb alone or in combination.

[0069] (3) The alloying elements Cu and Nb added in the present invention are not easily burned out during melting, the composition is easy to control, and the cost is lower compared with precious alloying elements such as Ni and Cr. A small amount or trace addition can obtain significant corrosion resistance and stress corrosion resistance effects.

[0070] Next, the technical solutions in the present invention will be clearly and completely described in conjunction with the embodiments in the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0071] Example 1

[0072] A high-strength bolt steel resistant to stress corrosion, by mass percentage, has a chemical composition of: C: 0.28%, Si: 0.29%, Mn: 1.43%, Ti: 0.04%, B: 0.003%, Cu: 0.527% and the balance Fe;

[0073] The preparation method of the high-strength bolt steel resistant to stress corrosion is as follows:

[0074] (1) Mix the raw materials and melt them in a vacuum electromagnetic induction furnace at 1650 °C, then carry out casting to obtain a bolt ingot; the chemical composition of the bolt ingot is the same as that of the high-strength bolt steel resistant to stress corrosion;

[0075] (2) Forge the bolt ingot obtained in step (1), then carry out heating, and then carry out hot rolling to obtain a rolled piece; the forging temperature is 1150 °C; the holding temperature for heating is 1200 °C, and the holding time for heating is 2 h; the starting rolling temperature for hot rolling is 1100 °C, and the final rolling temperature for hot rolling is 875 °C;

[0076] (3) Subject the rolled piece obtained in step (2) to quenching treatment and tempering treatment in sequence, and finally water-cool to room temperature to obtain the high-strength bolt steel resistant to stress corrosion; the holding temperature before quenching treatment is 850 °C, the holding time before quenching is 1 h, and the quenching medium is water; the holding temperature for tempering treatment is 400 °C, and the holding time for tempering treatment is 1 h.

[0077] Example 2

[0078] A high-strength bolt steel resistant to stress corrosion, by mass percentage, has a chemical composition of: C: 0.21%, Si: 0.26%, Mn: 1.39%, Ti: 0.03%, B: 0.002%, Nb: 0.059% and the balance Fe;

[0079] The preparation method of the high-strength bolt steel resistant to stress corrosion is as follows:

[0080] (1) Mix the raw materials and carry out melting in a vacuum electromagnetic induction furnace at 1650 °C, then carry out casting to obtain a bolt ingot; the chemical composition of the bolt ingot is the same as that of the high-strength bolt steel resistant to stress corrosion;

[0081] (2) Forge the bolt ingot obtained in step (1), then carry out heating, and then carry out hot rolling to obtain a rolled piece; the forging temperature is 1150 °C; the holding temperature for heating is 1200 °C, and the holding time for heating is 2 h; the starting rolling temperature for hot rolling is 1100 °C, and the final rolling temperature for hot rolling is 875 °C;

[0082] (3) Quench and temper the rolled piece obtained in the step (2) in sequence, and finally cool it to room temperature with water to obtain the high-strength bolt steel resistant to stress corrosion. The holding temperature before quenching is 850 °C, the holding time before quenching is 1 h, and the quenching medium is water. The holding temperature for tempering is 400 °C, and the holding time for tempering is 1 h.

[0083] Example 3

[0084] A high-strength bolt steel resistant to stress corrosion, by mass percentage, the chemical composition is: C: 0.22%, Si: 0.28%, Mn: 1.45%, Ti: 0.03%, B: 0.002%, Cu: 0.537%, Nb: 0.061% and the balance Fe;

[0085] The preparation method of the high-strength bolt steel resistant to stress corrosion is as follows:

[0086] (1) Mix the raw materials and melt them in a vacuum electromagnetic induction furnace at 1650 °C, and then cast to obtain a bolt steel ingot. The chemical composition of the bolt steel ingot is the same as that of the high-strength bolt steel resistant to stress corrosion;

[0087] (2) Forge the bolt steel ingot obtained in the step (1), then heat it, and then perform hot rolling to obtain a rolled piece. The forging temperature is 1160 °C. The holding temperature for heating is 1200 °C, and the holding time for heating is 2 h. The starting rolling temperature for hot rolling is 1100 °C, and the final rolling temperature for hot rolling is 880 °C;

[0088] (3) Quench and temper the rolled piece obtained in the step (2) in sequence, and finally cool it to room temperature with water to obtain the high-strength bolt steel resistant to stress corrosion. The holding temperature before quenching is 850 °C, the holding time before quenching is 1 h, and the quenching medium is water. The holding temperature for tempering is 400 °C, and the holding time for tempering is 1 h.

[0089] Comparative Example 1

[0090] A bolt steel, by mass percentage, the chemical composition is: C: 0.22%, Si: 0.27%, Mn: 1.44%, Ti: 0.03%, B: 0.002% and the balance Fe;

[0091] The preparation method of the bolt steel is as follows:

[0092] (1) Mix the raw materials and melt them in a vacuum electromagnetic induction furnace at 1650 °C, and then cast to obtain a bolt steel ingot. The chemical composition of the bolt steel ingot is the same as that of the bolt steel;

[0093] (2) Forge the bolt ingot obtained in step (1), then heat it, and then perform hot rolling to obtain a rolled piece; the forging temperature is 1150 °C; the holding temperature for heating is 1200 °C, and the holding time for heating is 2 h; the starting rolling temperature for hot rolling is 1100 °C, and the final rolling temperature for hot rolling is 875 °C;

[0094] (3) Quench and temper the rolled piece obtained in step (2) in sequence, and finally water-cool it to room temperature to obtain bolt steel; the holding temperature before quenching is 850 °C, the holding time before quenching is 1 h, and the quenching medium is water; the holding temperature for tempering is 400 °C, and the holding time for tempering is 1 h.

[0095] The microstructural morphology diagrams of the bolt steels provided in Examples 1 to 3 and Comparative Example 1 are as follows in sequence Figures 1 - 4 shown. It can be Figures 1 - 4 seen that the microstructures of the bolt steels are all tempered sorbite structures, and the addition of Cu and Nb can significantly refine the tempered cementite / carbides, which is beneficial to improving the strength, toughness and corrosion resistance of the steel.

[0096] The EBSD grain boundary characteristic diagrams of the bolt steels provided in Examples 1 to 3 and Comparative Example 1 are as follows in sequence Figures 5 - 8 shown. By analyzing the EBSD data, the large and small grain boundary densities and the total grain boundary density of the bolt steels provided in Examples 1 to 3 and Comparative Example 1 are obtained, and the results are as follows in Figure 9 shown. It can be Figure 9 seen that the addition of Cu has little effect on the large and small angle grain boundary densities, while the addition of Nb can significantly increase the grain boundary density, especially the small angle grain boundary density. When Cu and Nb are added in combination, the large and small angle grain boundary densities can also be significantly increased, and the proportion of small angle grain boundaries is further increased. Since small angle grain boundaries have an inhibitory effect on crack initiation and propagation, the stress corrosion resistance of the steel can be improved.

[0097] Perform mechanical property tests on the bolt steels provided in Examples 1 to 3 and Comparative Example 1. According to the national standard GB / T228-09 for testing, the stress-strain curves of the bolt steels are as follows in Figure 10 shown, and the mechanical properties of the bolt steels are shown in Table 1:

[0098] Table 1 Mechanical properties of the bolt steels provided in Examples 1 to 3 and Comparative Example 1

[0099]

[0100]

[0101] It can be Figure 10As can be seen from Table 1, the yield strength of the bolt steel provided by the present invention reaches above 1000 MPa, the tensile strength is close to 1100 MPa, and at the same time, it has a relatively high elongation rate and reduction of area rate, indicating that the bolt steel provided by the present invention has excellent strength and plasticity; through the comparison between the examples and the comparative examples, it can be seen that the bolt steel obtained by adding Cu and Nb has improved strength and plasticity compared with that without addition.

[0102] For the stress corrosion resistance test of the bolt steels provided in Examples 1 to 3 and Comparative Example 1, the test method is the slow strain rate tensile test method (SSRT), and the test conditions are air drawing and an industrial pollution atmosphere simulated solution (0.01 mol / L Na 2 SO 4 solution, pH = 5), the tensile rate is 0.0024 mm / min, and the SSRT tensile curves of the bolt steels provided in Examples 1 to 3 and Comparative Example 1 are obtained, as shown in Figure 11 ; Using the loss of elongation after fracture and the loss of reduction of area under the solution condition relative to the air drawing condition as the index for evaluating the stress corrosion (SCC) sensitivity, the stress corrosion sensitivity indices of the bolt steels provided in Examples 1 to 3 and Comparative Example 1 are obtained, and the results are as shown in Figure 12 . It can be seen from Figure 11 and Figure 12 that the addition of Cu and Nb alone can improve the stress corrosion resistance of the steel to a certain extent, and the improvement effect is more obvious when Cu and Nb are added in combination.

[0103] The side morphologies of the stress corrosion fractures of the bolt steels provided in Examples 1 to 3 and Comparative Example 1 in the simulated industrial pollution environment are shown in Figures 13 - 16 in turn. It can be seen from Figures 13 - 16 that from the side of the fracture, the side of the bolt steel provided in Comparative Example 1 has more secondary cracks, indicating that stress corrosion is prone to initiation. The side of the bolt steel with a small amount of Cu added (Example 1) has almost no secondary cracks, and the surface shows a uniform corrosion morphology; for the steel grade with a trace amount of Nb added (Example 2), the size and number of surface secondary cracks are reduced; for the steel grade with Cu and Nb added in combination (Example 3), there are basically no secondary cracks on the surface and only a small number of flat and shallow corrosion pits.

[0104] Based on the above, it can be seen that the high-strength bolt steel provided by the present invention has obvious stress corrosion resistance advantages compared with the comparative examples on the premise of ensuring its good strength and toughness.

[0105] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and retouches can be made, and these improvements and retouches should also be regarded as the protection scope of the present invention.

Claims

1. A stress corrosion resistant high-strength bolt steel, comprising the following chemical composition by mass percentage: C: 0.17-0.3%, Si: 0.17-0.37%, Mn: 1.3-1.6%, Ti: 0.02-0.1%, B: 0.0008-0.0035%, M: 0.02-0.9% and the balance Fe, wherein: M is Cu and / or Nb.

2. The stress corrosion resistant high-strength bolt steel according to claim 1, characterized in that: By mass percentage, it includes the following chemical compositions: C: 0.17~0.3%, Si: 0.17~0.37%, Mn: 1.3~1.6%, Ti: 0.02~0.1%, B: 0.0008~0.0035%, Cu: 0.2~0.8% and the balance Fe.

3. The stress corrosion resistant high-strength bolt steel according to claim 1, characterized in that: By mass percentage, it includes the following chemical compositions: C: 0.17~0.3%, Si: 0.17~0.37%, Mn: 1.3~1.6%, Ti: 0.02~0.1%, B: 0.0008~0.0035%, Nb: 0.02~0.1% and the balance Fe.

4. The stress corrosion resistant high-strength bolt steel according to claim 1, characterized in that: By mass percentage, it includes the following chemical compositions: C: 0.17~0.3%, Si: 0.17~0.37%, Mn: 1.3~1.6%, Ti: 0.02~0.1%, B: 0.0008~0.0035%, Cu: 0.2~0.8%, Nb: 0.02~0.1% and the balance Fe.

5. The stress corrosion resistant high-strength bolt steel according to any one of claims 1 to 4, characterized in that: The microstructure of the stress corrosion resistant high-strength bolt steel is a tempered troostite structure or a tempered troostite structure.

6. The method for preparing the stress corrosion resistant high-strength bolt steel according to any one of claims 1 to 5, comprising the following steps: (1) melting and casting the raw materials in sequence to obtain bolt steel ingots; (2) forging and hot rolling the bolt steel ingot obtained in step (1) in sequence to obtain a rolled product; (3) The rolled product obtained in step (2) is subjected to quenching treatment and tempering treatment in sequence to obtain stress corrosion resistant high-strength bolt steel.

7. The preparation method according to claim 6, characterized in that: In the step (2), the starting temperature of hot rolling is ≥1100°C, and the finishing temperature of hot rolling is ≥875°C.

8. The preparation method according to claim 6, characterized in that: In the step (3), the holding temperature before quenching treatment is 850-900° C., the holding time before quenching treatment is 0.5-1 h, and the medium for quenching treatment is water.

9. The preparation method according to claim 6, characterized in that: The holding temperature of the tempering treatment in the step (3) is 300-500° C., the holding time of the tempering treatment is 0.5-1 h, and the cooling method after the tempering treatment is water cooling to room temperature.

10. The stress corrosion resistant high-strength bolt steel according to any one of claims 1 to 5 or the stress corrosion resistant high-strength bolt steel prepared by the preparation method according to any one of claims 6 to 9 is used in offshore platforms, cross-sea bridges, high-speed railways and wind power generation facilities.

Images