Rare earth stress-corrosion-resistant high-strength bolt steel as well as preparation method and application thereof

Abstract

The invention provides rare earth 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. By adding trace rare earth, the corrosion resistance and stress corrosion resistance of the high-strength bolt steel in service in an industrial pollution environment can be remarkably improved, and the high-strength bolt steel has remarkable cost advantages and resource advantages; the rare earth element can convert a hydrogen-induced intergranular fracture of the bolt steel in an acid environment into a small number of cross-crystal fractures and a large number of dimple fractures, that is, typical intergranular brittle fracture is converted into ductile fracture, which indicates that the rare earth element has excellent hydrogen embrittlement resistance in the steel, so that the stress corrosion resistance of the steel can be improved; through microalloying of the rare earth elements, the grain boundary characteristics of the bolt steel can be improved, the grain boundary density is greatly increased, meanwhile, the large-angle grain boundary proportion is reduced, in addition, the rare earth elements can inhibit roughening of tempered carbide, nucleation sites generated by stress corrosion cracks are effectively reduced, crack propagation is hindered, and therefore the stress corrosion resistance of the bolt steel is improved.

Description

Technical Field

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

[0002] As the key connection and load-bearing structural parts of major engineering equipment and facilities (large steel structure bridges, high-speed railways, wind power and buildings, etc.), high-strength bolts are subjected to huge working and structural stresses. At the same time, they are also severely eroded by industrial atmospheric pollution or marine environments. During long-term service, they are prone to brittle fracture failures such as stress corrosion, causing serious catastrophic accidents. Engineering practice has also found many high-strength bolt steel fracture failure accidents caused by stress corrosion. Therefore, the corrosion resistance and stress corrosion resistance of high-strength bolt steel have gradually attracted people's attention.

[0003] At present, the 10.9 grade high-strength bolt steel widely used in China is mainly 20MnTiB, which is a high-performance cold-headed high-strength bolt steel developed in the 1980s. It has been used until now and is still the mainstream steel grade of high-strength bolt steel in China. It is widely used in railways, bridges and steel structure buildings. However, in the early research and development design of these high-strength bolt steels, the main focus was on their hardenability and toughness and other properties. The composition design lacked stress corrosion resistance design, resulting in frequent corrosion failure accidents of my country's high-strength bolt steel in the service environment, which seriously threatened the service safety of major engineering equipment. Therefore, the research and development of high-strength bolt steel with good toughness, corrosion resistance and stress corrosion resistance has important engineering practical value.

[0004] Therefore, how to improve the stress corrosion resistance of bolts has become a technical problem to be solved urgently in this field. Summary of the invention

[0005] The purpose of the present invention is to provide a rare earth stress corrosion resistant high-strength bolt steel and a preparation method and application thereof. The rare earth stress corrosion resistant high-strength bolt steel provided by the present invention has obvious stress corrosion resistance advantages while having good strength and toughness.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The invention provides a rare earth 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%, RE: 0.005-0.1% and the balance Fe.

[0008] Preferably, the alloy comprises the following chemical composition by mass percentage: C: 0.21%, Si: 0.28%, Mn: 1.43%, Ti: 0.02%, B: 0.002%, RE: 0.016% and the balance Fe.

[0009] Preferably, the alloy comprises the following chemical composition by mass percentage: C: 0.22%, Si: 0.29%, Mn: 1.48%, Ti: 0.05%, B: 0.002%, RE: 0.065% and the balance Fe.

[0010] Preferably, the alloy comprises the following chemical composition by mass percentage: C: 0.25%, Si: 0.27%, Mn: 1.43%, Ti: 0.07%, B: 0.002%, RE: 0.009% and the balance Fe.

[0011] Preferably, the RE is Ce, La or Y.

[0012] The present invention provides a method for preparing the rare earth stress corrosion resistant high-strength bolt steel described in the above technical solution, comprising the following steps:

[0013] (1) melting and casting the raw materials in sequence to obtain bolt steel ingots;

[0014] (2) forging and hot rolling the bolt steel ingot obtained in step (1) in sequence to obtain a rolled product;

[0015] (3) The rolled piece obtained in step (2) is subjected to quenching treatment and tempering treatment in sequence to obtain rare earth stress corrosion resistant high-strength bolt steel.

[0016] Preferably, in step (2), the starting temperature of hot rolling is 1100-1150°C, and the finishing temperature of hot rolling is 850-900°C.

[0017] Preferably, in step (3), the holding temperature before quenching treatment is 850-880° C., the holding time before quenching treatment is 45-75 min, and the medium for quenching treatment is water.

[0018] Preferably, the holding temperature of the tempering treatment in step (3) is 400-500° C., the holding time of the tempering treatment is 45-60 min, and the cooling method after the tempering treatment is water cooling to room temperature.

[0019] The present invention provides the rare earth stress corrosion resistant high-strength bolt steel described in the above technical solution or the rare earth 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.

[0020] The invention provides a rare earth 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%, RE: 0.005-0.1% and the balance Fe. The present invention can significantly improve the corrosion resistance and stress corrosion resistance of the existing high-strength bolt steel in an industrial pollution environment by adding a trace amount of rare earth. The alloy cost has significant cost and resource advantages compared with corrosion-resistant or hydrogen-resistant alloy elements such as Ni, Cr, Cu and Nb. The rare earth element can transform the hydrogen-induced intergranular fracture of the existing 20MnTiB steel in an acidic environment into a small amount of transgranular and a large amount of ductile dimple fractures, that is, transform the typical intergranular brittle fracture into a ductile fracture, indicating that the rare earth element has an excellent anti-hydrogen embrittlement effect in the steel, thereby improving the stress corrosion resistance in the acidic industrial pollution environment. The microalloying of the rare earth element can improve the grain boundary characteristics of the bolt steel, greatly increase the grain boundary density, and reduce the proportion of large-angle grain boundaries. In addition, the rare earth element can inhibit the coarsening of tempered carbides, thereby effectively reducing the nucleation sites for the initiation of stress corrosion cracks and hindering the crack propagation, thereby improving its stress corrosion resistance. The results of the embodiments show that the yield strength of the rare earth stress corrosion resistant high-strength bolt steel provided by the present invention reaches more than 1000 MPa, and the tensile strength is close to 1050 MPa. It also has high elongation and area reduction, high strength and high plasticity, and also has excellent stress corrosion resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The microstructure morphology of the bolt steel provided in Example 1;

[0022] Figure 2 The microstructure morphology of the bolt steel provided in Example 2;

[0023] Figure 3 The microstructure morphology of the bolt steel provided in Example 3;

[0024] Figure 4 The microstructure morphology of the bolt steel provided for Comparative Example 1;

[0025] Figure 5 EBSD grain boundary feature map of the bolt steel provided in Example 1;

[0026] Figure 6 EBSD grain boundary feature map of the bolt steel provided in Example 2;

[0027] Figure 7 EBSD grain boundary feature map of the bolt steel provided in Example 3;

[0028] Figure 8EBSD grain boundary feature map of the bolt steel provided for Comparative Example 1;

[0029] Fig. 9 The large and small grain boundary densities and the total grain boundary density of the bolt steel provided in Examples 1 to 3 and Comparative Example 1;

[0030] Fig.10 The stress-strain curves of the bolt steels provided for Examples 1 to 3 and Comparative Example 1;

[0031] Fig.11 SSRT tensile curves of the bolt steels provided for Examples 1 to 3 and Comparative Example 1;

[0032] Fig.12 The stress corrosion sensitivity index of the bolt steel provided in Examples 1 to 3 and Comparative Example 1;

[0033] Fig.13 A side morphology diagram of the stress corrosion fracture of the bolt steel provided in Example 1 in a simulated industrial pollution environment;

[0034] Fig.14 A side morphology diagram of the stress corrosion fracture of the bolt steel provided in Example 2 in a simulated industrial pollution environment;

[0035] Fig.15 A side morphology diagram of the stress corrosion fracture of the bolt steel provided in Example 3 in a simulated industrial pollution environment;

[0036] Fig.16 This is a side morphology of the stress corrosion fracture of the bolt steel provided for Comparative Example 1 in a simulated industrial pollution environment. DETAILED DESCRIPTION

[0037] The invention provides a rare earth 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%, RE: 0.005-0.1% and the balance Fe.

[0038] In terms of mass percentage, the rare earth stress corrosion resistant high-strength bolt steel provided by the present invention includes 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, the strength, hardness and hardenability will be subject to certain limitations; 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 prone to occur. Therefore, the present invention controls the content of C within the range of 0.17-0.30%. 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 0.30%.

[0039] The rare earth stress corrosion resistant high-strength bolt steel provided by the present invention includes Si: 0.17-0.37% by mass percentage. In the present invention, Si is a good deoxidizer for bolt steel, and the solid solution strength of bolt steel can be improved by adding Si, which is beneficial to increasing the tempering stability of bolt steel; when the content of Si is too high, the plasticity of bolt steel will be reduced, and when the content of Si is too low, the deoxidation and solid solution strengthening effects will be weak, so the content of Si is controlled within the range of 0.17-0.37% in the present invention. 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%.

[0040] In terms of mass percentage, the rare earth 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 effect of improving the hardenability of the material, improving the toughness of the material and making the structure uniform and refined; but if the Mn content is higher than 1.6%, it will lead to the increase of component segregation and temper brittleness of the bolt steel, and will also lead to the reduction of the corrosion resistance of the bolt steel, which cannot reach the best, and at the same time increase the production cost of the bolt steel, and if the manganese content is lower than 1.3%, it will make it 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%, 1.35%, 1.4%, 1.43%, 1.45%, 1.48%, 1.5%, 1.55% or 1.6%.

[0041] In terms of mass percentage, the rare earth 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, eliminate N in the steel, and avoid the formation of BN brittle phase. At the same time, Ti can refine the grain structure of the steel and improve the strength and toughness of the steel. However, if excessive titanium is added, coarse Ti (CN) may be formed, which affects the plasticity and toughness of the bolt steel. Therefore, the present invention controls the content of Ti 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%.

[0042] In terms of mass percentage, the rare earth 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 bolt steel. A trace amount of boron can significantly improve the hardenability of steel, but when the boron content exceeds 0.007%, it is easy to cause brittleness. Therefore, the present invention controls the content of B 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%.

[0043] The rare earth stress corrosion resistant high-strength bolt steel provided by the present invention comprises RE: 0.005-0.1% by mass percentage. In the present invention, the RE is preferably Ce, La or Y, more preferably Ce. The present invention can significantly improve the corrosion resistance and stress corrosion resistance of the existing high-strength bolt steel in an industrial pollution environment by adding a trace amount of rare earth. The alloy cost has significant cost and resource advantages compared with corrosion-resistant or hydrogen-resistant alloy elements such as Ni, Cr, Cu and Nb. The rare earth element can transform the hydrogen-induced intergranular fracture of the existing 20MnTiB steel in an acidic environment into a small amount of transgranular and a large amount of ductile dimple fractures, that is, transform the typical intergranular brittle fracture into a ductile fracture, indicating that the rare earth element has an excellent anti-hydrogen embrittlement effect in the steel, thereby improving the stress corrosion resistance in the acidic industrial pollution environment. The microalloying of the rare earth element can improve the grain boundary characteristics of the bolt steel, greatly increase the grain boundary density, and reduce the proportion of large-angle grain boundaries. In addition, the rare earth element can inhibit the coarsening of tempered carbides, thereby effectively reducing the nucleation sites for the initiation of stress corrosion cracks and hindering the crack propagation, thereby improving its stress corrosion resistance. As an embodiment of the present invention, the mass percentage of RE can be 0.005%, 0.009%, 0.01%, 0.016%, 0.02%, 0.025%, 0.03%, 0.035%, 0.04%, 0.045%, 0.05%, 0.055%, 0.06%, 0.065%, 0.07%, 0.075%, 0.08%, 0.085%, 0.09%, 0.095% or 0.1%.

[0044] In terms of mass percentage, the rare earth stress corrosion resistant high-strength bolt steel provided by the present invention preferably also includes P≤0.035% and S≤0.035%. In the present invention, P and S elements are impurity elements, and the less their content is controlled, the better.

[0045] In terms of mass percentage, the rare earth stress corrosion resistant high-strength bolt steel provided by the present invention includes a balance of Fe. In the present invention, Fe is a matrix element of the bolt steel.

[0046] In the present invention, the microstructure of the rare earth stress corrosion resistant high-strength bolt steel is preferably a tempered troostite structure or a tempered troostite structure.

[0047] The present invention also provides a method for preparing the rare earth stress corrosion resistant high-strength bolt steel described in the above technical solution, comprising the following steps:

[0048] (1) melting and casting the raw materials in sequence to obtain bolt steel ingots;

[0049] (2) forging and hot rolling the bolt steel ingot obtained in step (1) in sequence to obtain a rolled product;

[0050] (3) The rolled piece obtained in step (2) is subjected to quenching treatment and tempering treatment in sequence to obtain rare earth stress corrosion resistant high-strength bolt steel.

[0051] The invention sequentially smelts and casts raw materials to obtain bolt steel ingots.

[0052] The present invention has no particular limitation on the specific type and amount of the raw materials, and commercially available products known to those skilled in the art can be used as long as the composition of the bolt steel ingot meets the requirements. As an embodiment of the present invention, the raw materials can be single substances or intermediate alloys of carbon, silicon, manganese, titanium, boron, rare earth elements and iron.

[0053] In the present invention, the smelting method is preferably to first smelt carbon, silicon, manganese, titanium, boron and iron, and then add rare earth elements after they are completely melted. In the present invention, the smelting temperature is preferably 1600-1650°C, more preferably 1650°C; the vacuum degree of the smelting is preferably <50Pa; the smelting can be carried out in a vacuum electromagnetic induction furnace. In the present invention, the rare earth elements are added last and the vacuum degree of the smelting is controlled, which can increase the yield of rare earth elements.

[0054] The present invention has no special limitation on the specific operation and process parameters of the casting, and the casting operation well known to those skilled in the art can be adopted.

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

[0056] After obtaining the bolt steel ingot, the present invention sequentially performs forging and hot rolling on the bolt steel ingot 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 that the grains in the bolt steel ingot can be fully broken and the billet flow line can be ensured to be continuous and complete. The present invention can eliminate the structural defects generated in the casting process through forging, and fully break the grains of the bolt steel ingot to obtain fine internal grains. As an embodiment of the present invention, the forging temperature is 1100-1200°C, and can also be 1140-1160°C. In the present invention, when the forging temperature does not meet the above requirements, the present invention preferably heats the bolt steel ingot. The present invention has no special limitation on the heating temperature and time, and it is sufficient to make the forging temperature meet the requirements.

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

[0059] In the present invention, the starting rolling temperature of the hot rolling is preferably 1100-1150°C; the finishing rolling temperature of the hot rolling is preferably 850-900°C; and the cumulative reduction rate of the hot rolling is preferably ≥50%. The present invention can fully refine the grains through hot rolling. As an embodiment of the present invention, the starting rolling temperature of the austenite recrystallization zone rolling can be 1100°C, 1110°C, 1120°C, 1130°C, 1140°C or 1150°C.

[0060] After the rolled piece is obtained, the invention sequentially performs quenching treatment and tempering treatment on the rolled piece to obtain rare earth stress corrosion resistant high-strength bolt steel.

[0061] In the present invention, the holding temperature before quenching treatment is preferably 850-880°C; the holding time before quenching treatment is preferably 45-75min; the medium of quenching treatment is preferably water; the holding temperature of tempering treatment is preferably 400-500°C; the holding time of tempering treatment is preferably 45-60min; the cooling method after tempering treatment is preferably water cooling to room temperature. The present invention can form a tempered troostite structure or tempered troostite structure with fine and uniform grains in the bolt steel by quenching and tempering treatment, effectively improving the strength, toughness and stress corrosion resistance of the bolt steel. As an embodiment of the present invention, the holding temperature before quenching treatment can be 860-870°C; the holding time before quenching treatment can be 50-70min, or 60min; the holding temperature of tempering treatment can be 420-480°C, or 450°C; the holding time of tempering treatment can be 50-55min.

[0062] In the process of preparing bolt steel, the present invention first casts a bolt steel ingot, then forges the prepared bolt steel ingot to fully break the internal structure of the bolt steel ingot, and then performs austenite homogenization treatment on the forged square billet to eliminate the stress generated in the forging process and obtain a uniform austenite structure, thereby improving the strength and plasticity of the bolt steel. Then hot rolling can fully refine the grains, and finally quenching and tempering treatment can be used to form a tempered troostite structure or a tempered troostite structure with fine and uniform grains in the bolt steel, thereby effectively improving the strength, toughness and stress corrosion resistance of the bolt steel.

[0063] The present invention also provides the rare earth stress corrosion resistant high-strength bolt steel described in the above technical solution or the rare earth 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.

[0064] The present invention has no particular limitation on the specific manner of the application, and the application may be performed according to the technical common sense of those skilled in the art.

[0065] The rare earth 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 natural environmental corrosion, improves the service life of large-scale engineering equipment, and provides strong guarantee for the long-term stable operation of major engineering equipment. The bolt steel material component system provided by the present invention has a low precious metal content, high stress corrosion resistance, and excellent cost performance. At the same time, the preparation process of the bolt steel is simple, the application is convenient, and it is easy to promote on a large scale.

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

[0067] (1) The corrosion resistance and stress corrosion resistance of the high-strength bolt steel in service in industrial pollution environments can be significantly improved by adding trace amounts of rare earth elements. The alloy cost has significant cost and resource advantages compared to corrosion-resistant or hydrogen-resistant alloying elements such as Ni, Cr, Cu and Nb.

[0068] (2) Rare earth elements can transform the hydrogen-induced intergranular fracture of the current 20MnTiB steel in an acidic environment into a small amount of transgranular fracture and a large amount of ductile dimple fracture, that is, transform the typical intergranular brittle fracture into a ductile fracture, indicating that the rare earth elements added in the present invention have excellent anti-hydrogen embrittlement effect in the steel, and thus can improve the stress corrosion resistance in an acidic industrial pollution environment.

[0069] (3) Microalloying with rare earth elements can improve the grain boundary characteristics of bolt steel, greatly increase the grain boundary density, and reduce the proportion of large-angle grain boundaries. In addition, the elements can inhibit the coarsening of tempered carbides, thereby effectively reducing the nucleation sites for stress corrosion crack initiation and hindering crack propagation, thereby improving its stress corrosion resistance.

[0070] The technical solutions in the present invention will be described clearly and completely below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0071] Example 1

[0072] A rare earth stress corrosion resistant high-strength bolt steel, the chemical composition of which is, by mass percentage, 0.25% C, 0.27% Si, 1.43% Mn, 0.07% Ti, 0.002% B, 0.009% Ce and the balance Fe;

[0073] The preparation method of the rare earth stress corrosion resistant high-strength bolt steel is:

[0074] (1) mixing raw materials except Ce and smelting them in a vacuum electromagnetic induction furnace at a vacuum degree of less than 50 Pa and a temperature of 1650° C., adding Ce after the raw materials are completely melted, and then casting to obtain a bolt steel ingot; the chemical composition of the bolt steel ingot is the same as the chemical composition of the rare earth stress corrosion resistant high-strength bolt steel;

[0075] (2) forging the bolt steel ingot obtained in step (1), then heating it, and then hot rolling it to obtain a rolled product; the forging temperature is 1150° C.; the heating insulation temperature is 1200° C., and the heating insulation time is 2 h; the hot rolling start temperature is 1100° C., and the hot rolling finish temperature is 875° C.;

[0076] (3) The rolled piece obtained in step (2) is subjected to quenching treatment and tempering treatment in sequence, and finally cooled to room temperature with water to obtain rare earth stress corrosion resistant high-strength bolt steel; the holding temperature before quenching treatment is 850° C., the holding time before quenching treatment is 1 hour, and the quenching treatment medium is water; the holding temperature of tempering treatment is 400° C., and the holding time of tempering treatment is 1 hour.

[0077] Example 2

[0078] A rare earth stress corrosion resistant high-strength bolt steel, the chemical composition of which is, by mass percentage, 0.21% C, 0.28% Si, 1.43% Mn, 0.02% Ti, 0.002% B, 0.016% Ce and the balance Fe;

[0079] The preparation method of the rare earth stress corrosion resistant high-strength bolt steel is:

[0080] (1) mixing raw materials except Ce and smelting them in a vacuum electromagnetic induction furnace at a vacuum degree of less than 50 Pa and a temperature of 1650° C., adding Ce after the raw materials are completely melted, and then casting to obtain a bolt steel ingot; the chemical composition of the bolt steel ingot is the same as the chemical composition of the rare earth stress corrosion resistant high-strength bolt steel;

[0081] (2) forging the bolt steel ingot obtained in step (1), then heating it, and then hot rolling it to obtain a rolled product; the forging temperature is 1150° C.; the heating insulation temperature is 1200° C., and the heating insulation time is 2 h; the hot rolling start temperature is 1100° C., and the hot rolling finish temperature is 875° C.;

[0082] (3) The rolled piece obtained in step (2) is subjected to quenching treatment and tempering treatment in sequence, and finally cooled to room temperature with water to obtain rare earth stress corrosion resistant high-strength bolt steel; the holding temperature before the quenching treatment is 850° C., the holding time before the quenching treatment is 1 hour, and the quenching treatment medium is water; the holding temperature of the tempering treatment is 400° C., and the holding time of the tempering treatment is 1 hour.

[0083] Example 3

[0084] A rare earth stress corrosion resistant high-strength bolt steel, the chemical composition of which is, by mass percentage, 0.22% C, 0.29% Si, 1.48% Mn, 0.05% Ti, 0.002% B, 0.065% Ce and the balance Fe;

[0085] The preparation method of the rare earth stress corrosion resistant high-strength bolt steel is:

[0086] (1) mixing raw materials except Ce and smelting them in a vacuum electromagnetic induction furnace at a vacuum degree of less than 50 Pa and a temperature of 1650° C., adding Ce after the raw materials are completely melted, and then casting to obtain a bolt steel ingot; the chemical composition of the bolt steel ingot is the same as the chemical composition of the rare earth stress corrosion resistant high-strength bolt steel;

[0087] (2) forging the bolt steel ingot obtained in step (1), then heating it, and then hot rolling it to obtain a rolled product; the forging temperature is 1160° C.; the heating holding temperature is 1200° C., and the heating holding time is 2 h; the hot rolling start temperature is 1100° C., and the hot rolling finish temperature is 880° C.;

[0088] (3) The rolled piece obtained in step (2) is subjected to quenching treatment and tempering treatment in sequence, and finally cooled to room temperature with water to obtain rare earth stress corrosion resistant high-strength bolt steel; the holding temperature before the quenching treatment is 850° C., the holding time before the quenching treatment is 1 hour, and the quenching treatment medium is water; the holding temperature of the tempering treatment is 400° C., and the holding time of the tempering treatment is 1 hour.

[0089] Comparative Example 1

[0090] A bolt steel, the chemical composition of which is, by mass percentage, 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:

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

[0093] (2) forging the bolt steel ingot obtained in step (1), then heating it, and then hot rolling it to obtain a rolled product; the forging temperature is 1150° C.; the heating insulation temperature is 1200° C., and the heating insulation time is 2 h; the hot rolling start temperature is 1100° C., and the hot rolling finish temperature is 875° C.;

[0094] (3) The rolled piece obtained in step (2) is subjected to quenching treatment and tempering treatment in sequence, and finally cooled to room temperature with water to obtain bolt steel; the holding temperature before the quenching treatment is 850° C., the holding time before the quenching treatment is 1 hour, and the medium for the quenching treatment is water; the holding temperature of the tempering treatment is 400° C., and the holding time of the tempering treatment is 1 hour.

[0095] The metallographic structure morphology of the bolt steel provided in Examples 1 to 3 and Comparative Example 1 is as follows: Figures 1 to 4 As shown. Figures 1 to 4 It can be seen that the bolt steels provided in Examples 1 to 3 and Comparative Example 1 are all tempered troostite or tempered troostite structures.

[0096] The EBSD grain boundary distribution diagrams of the bolt steels provided in Examples 1 to 3 and Comparative Example 1 are as follows: Figures 5 to 8 As shown. Figures 5 to 8 It can be seen that the addition of rare earth element Ce will significantly increase the grain boundary density of bolt steel, especially the density of small-angle grain boundaries. Small-angle grain boundaries are not conducive to crack initiation and can hinder crack propagation, which is beneficial to improving the stress corrosion resistance of bolt steel.

[0097] 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 were obtained by EBSD data analysis. The results are as follows: Fig. 9 As shown. Fig. 9It can be seen that with the addition of rare earth element Ce, the large and small grain boundary density and the total grain boundary density of the bolt steel increased to varying degrees, indicating that the rare earth element Ce can improve the large and small grain boundary density and the total grain boundary density of the bolt steel; through the comparison of Examples 1 to 3, it can be seen that when the rare earth element content in the bolt steel is different, the increase in the large and small grain boundary density and the total grain boundary density of the bolt steel is also different, indicating that by controlling the content of rare earth elements, the stress corrosion resistance of the bolt steel can be controlled.

[0098] The mechanical properties of the bolt steels provided in Examples 1 to 3 and Comparative Example 1 were tested according to the national standard GB / T228-09, and the stress-strain curves of the bolt steels were obtained as shown in FIG. Fig.10 The mechanical properties of bolt steel are shown in Table 1:

[0099] Table 1 Mechanical properties of bolt steel provided by Examples 1 to 3 and Comparative Example 1

[0100] Yield strength / MPa Tensile strength / MPa Elongation after break / % Sectional shrinkage / % Comparative Example 1 1017 1059 9.2 35.3 Example 1 1013 1055 9.2 35.5 Example 2 1071 1101 9.6 36.9 Example 3 1080 1123 8.5 32.4

[0101] Depend on Fig.10 As can be seen from Table 1, the yield strength of the high-strength bolt steel provided by the present invention reaches more than 1000 MPa, the tensile strength is close to 1050 MPa, and it has high elongation and area reduction, excellent strength and plasticity. Compared with Comparative Example 1, the performance is basically not reduced, and even slightly improved, indicating that the technical solution of the present invention will not affect the mechanical properties of the bolt steel.

[0102] The stress corrosion resistance of the bolt steels provided in Examples 1 to 3 and Comparative Example 1 was tested by a slow strain rate tensile test method (SSRT). The test conditions were air pulling and industrial pollution simulation liquid (0.01 mol / L NaHSO3 solution), and the tensile rate was 0.0024 mm / min. The SSRT tensile curves of the bolt steels provided in Examples 1 to 3 and Comparative Example 1 were obtained. Fig.11 As shown. Fig.11 It can be seen that the elongation of the bolt steels provided by Examples 1 to 3 and Comparative Example 1 in the air is relatively close, but the elongation of the bolt steel provided by Comparative Example 1 drops sharply in the NaHSO3 solution, and the material undergoes brittle fracture near the yield point. The bolt steels provided by Examples 1 to 3 fracture after a certain uniform plastic strain, and the elongation is significantly improved compared with the comparative example, indicating that the bolt steels provided by Examples 1 to 3 of the present invention have excellent stress corrosion resistance in a simulated industrial acidic environment.

[0103] By calculating the elongation after fracture and the section shrinkage, the elongation loss I of different steel grades in solution relative to that in air is obtained. δ and area reduction loss I Ψlike Fig.12 As shown, the elongation loss after fracture is used δ and area reduction loss I Ψ As an index for evaluating stress corrosion (SCC) sensitivity. Fig.12 It can be seen that the bolt steel provided in Comparative Example 1 has a higher stress corrosion sensitivity in an acidic industrial environment, while the stress corrosion sensitivity of the high-strength bolt steel micro-alloyed with the rare earth element Ce provided in Examples 1 to 3 of the present invention is significantly reduced.

[0104] The side morphologies of the stress corrosion fractures of the bolt steels provided in Examples 1 to 3 and Comparative Example 1 in a simulated industrial pollution environment are as follows: Figures 13-16 As shown. Figures 13-16 It can be seen that the prototype steel has typical rock candy-like intergranular fracture characteristics on its fracture surface, and there are many intergranular secondary cracks, showing typical hydrogen-induced cracking characteristics, indicating that the bolt steel provided in Comparative Example 1 has a high sensitivity to hydrogen-induced stress corrosion in an acidic industrial pollution environment; compared with Comparative Example 1, after microalloying by adding 0.009%, 0.016% and 0.065% of rare earth element Ce in Examples 1 to 3 of the present application, the fracture surfaces of the bolt steels all show ductile fractures, with a small amount of brittle zones on the edges, and transgranular fractures, and the side surfaces of the fractures show uniform corrosion morphology, with basically no secondary cracks; the fracture morphologies of the two further indicate that rare earth element microalloying can significantly improve the stress corrosion resistance of high-strength bolt steels in acidic industrial environments.

[0105] It can be seen from the above embodiments and comparative examples that the rare earth element microalloying proposed in the present invention can significantly improve the stress corrosion resistance of 20MnTiB high-strength bolt steel in an acidic industrial environment.

[0106] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A rare earth 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%, RE: 0.005-0.1% and the balance Fe.

2. The rare earth stress corrosion resistant high-strength bolt steel according to claim 1, characterized in that: By mass percentage, it includes the following chemical compositions: C: 0.21%, Si: 0.28%, Mn: 1.43%, Ti: 0.02%, B: 0.002%, RE: 0.016% and the balance of Fe.

3. The rare earth stress corrosion resistant high-strength bolt steel according to claim 1, characterized in that: By mass percentage, it includes the following chemical compositions: C: 0.22%, Si: 0.29%, Mn: 1.48%, Ti: 0.05%, B: 0.002%, RE: 0.065% and the balance of Fe.

4. The rare earth stress corrosion resistant high-strength bolt steel according to claim 1, characterized in that: By mass percentage, it includes the following chemical compositions: C: 0.25%, Si: 0.27%, Mn: 1.43%, Ti: 0.07%, B: 0.002%, RE: 0.009% and the balance of Fe.

5. The rare earth stress corrosion resistant high-strength bolt steel according to any one of claims 1 to 4, characterized in that: The RE is Ce, La or Y.

6. The method for preparing the rare earth 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 rare earth 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 1100-1150°C, and the finishing temperature of hot rolling is 850-900°C.

8. The preparation method according to claim 6, characterized in that: In the step (3), the holding temperature before quenching treatment is 850-880° C., the holding time before quenching treatment is 45-75 min, 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 400-500° C., the holding time of the tempering treatment is 45-60 min, and the cooling method after the tempering treatment is water cooling to room temperature.

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

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