Steel for bolt as well as preparation method and application of steel

Abstract

The invention relates to bolt steel and a preparation method and application thereof, belongs to the technical field of bolt steel, and solves at least one of the problems of poor hardenability, insufficient strength, large surface hardness fluctuation and the like of bolt steel in the prior art. The invention provides steel for a bolt. The steel for the bolt comprises, by mass, 0.35%-0.45% of C, 0.17%-0.25% of Si, 0.50%-0.90% of Mn, 0.90%-1.20% of Cr, 0.20%-0.40% of Mo, 0.15%-0.35% of V, 0.02%-0.05% of Nb, smaller than or equal to 0.005% of P, smaller than or equal to 0.005% of S, smaller than or equal to 0.005% of N and the balance Fe and other inevitable impurities. The steel for the bolt meets the technical requirements of 12.9-grade M42 or above total cross-section mechanical stable bolts, Rm is larger than or equal to 1290 MPa, Rp0.2 is larger than or equal to 1260 MPa, the percentage elongation after fracture A is larger than or equal to 10%, and the percentage shrinkage after fracture Z is larger than or equal to 49%; the section hardness of the steel for the bolt is uniform, the minimum value of the section hardness is larger than or equal to 42 HRC, the hardness fluctuation is smaller than or equal to 2 HRC, and the critical through quenching diameter is larger than or equal to 70 mm; the method can be used for preparing 12.9-grade M42 or above total cross-section mechanically stable bolts with uniform mechanical properties.

Description

Technical Field

[0001] The present invention relates to the technical field of steel for bolts, and particularly to a steel for bolts, a preparation method thereof and an application thereof. Background Art

[0002] In recent years, with the popularization of clean energy, the wind power industry in China, especially large-capacity megawatt-level large wind turbines, has developed rapidly. High-strength fasteners used in wind power equipment have been in service in the wild for a long time, with harsh environments and poor maintenance conditions. This requires the materials for manufacturing bolts to have high mechanical stability.

[0003] As the capacity of the wind turbines increases continuously, the weight of the wind towers also continues to increase, which leads to the continuous increase in the application specifications of wind power bolts. Currently, the maximum diameter of bolts for large-capacity wind turbines is 72 mm (M72), and traditional bolt materials such as 42CrMo and B7 are usually used for manufacturing. Generally, the manufactured bolts are of grade 10.9. However, the critical hardenability diameter of 42CrMo bolt steel is about 40 mm. If 42CrMo steel continues to be used to manufacture bolts with a diameter of 42 mm and above, it cannot be fully hardened during the quenching and tempering process, and there is a large difference in performance between the surface and the core (the hardness fluctuation reaches 6 - 8 HRC). The increase in the bolt specification reduces the hardenability, which results in a decrease in the uniformity of the surface mechanical distribution after the bolt is tempered (the hardness fluctuation reaches 4 - 5 HRC). The large mechanical property fluctuation causes local overload of the bolt during service, which in turn leads to bolt failure and major accidents.

[0004] Currently, the existing steel for bolts above M42 level cannot well balance strength, hardenability and mechanical uniformity (mainly referring to hardness uniformity). Therefore, it is necessary to develop a steel for bolts with good hardenability, high strength and high hardness uniformity. Summary of the Invention

[0005] In view of the above analysis, the embodiments of the present invention aim to provide a steel for bolts, a preparation method thereof and an application thereof, so as to solve the problem that the existing steel for bolts above M42 level cannot well balance strength, hardenability and mechanical uniformity (mainly referring to hardness uniformity).

[0006] The present invention provides a steel for bolts. The specific composition of the steel for bolts is as follows by mass percentage: C 0.35 - 0.45%, Si 0.17 - 0.25%, Mn 0.50 - 0.90%, Cr 0.90 - 1.20%, Mo 0.20 - 0.40%, V 0.15 - 0.35%, Nb 0.02 - 0.05%, P ≤ 0.005%, S ≤ 0.005%, N ≤ 0.005%, and the rest is Fe and other inevitable impurities.

[0007] Specifically, the steel for bolts consists of lath martensite, which is formed by quenching austenite; the width of the martensite laths is 0.5 - 1.5 μm.

[0008] Furthermore, the martensite forms lath aggregates along the original austenite grain boundaries, and the size of the lath aggregates is 5.0 - 20 μm.

[0009] Specifically, the steel for bolts includes discrete NbC precipitation phases, VC precipitation phases, and VN precipitation phases;

[0010] Among them, the size of the NbC precipitation phase is 20 - 30 nm, and the size of the VC precipitation phase is 10 - 20 nm.

[0011] Specifically, the steel for bolts meets the technical requirements of fully-section mechanical stable bolts of grade 12.9 and above M42, with Rm ≥ 1290 MPa, Rp 0.2 ≥ 1260 MPa, elongation after fracture A ≥ 10%, reduction of area Z ≥ 49%; the hardness of the cross-section of the steel for bolts is uniform, the minimum value of the cross-section hardness ≥ 42 HRC, the hardness fluctuation ≤ 2 HRC, and the critical hardenability diameter ≥ 70 mm.

[0012] The present invention also provides a preparation method for the above-mentioned steel for bolts, including the following steps:

[0013] S1: Conduct preliminary batching according to a preset formula, and obtain a continuous casting billet that meets the composition of the preset formula after electric furnace smelting, LF refining, vacuum smelting, and continuous casting;

[0014] S2: Heat and hold the continuous casting billet, and naturally cool it to room temperature after rough rolling and finish rolling to obtain round steel with a specification of 48 - 65 mm;

[0015] S3: Quench the round steel, and then perform tempering heat treatment and air cooling to obtain the finished product of the steel for bolts.

[0016] Specifically, in step S1, the hydrogen content is measured to be ≤ 2.0 ppm after vacuum smelting.

[0017] Specifically, the specific operation of step S2 is as follows:

[0018] Uniformly heat the continuous casting billet to 1000°C - 1150°C and hold it, and the holding time ≥ 120 min;

[0019] In the rough rolling stage, the initial rolling temperature is 1000°C - 1150°C, the final rolling temperature ≥ 850°C, and the deformation amount is 25% - 35%;

[0020] In the finish rolling stage, the initial rolling temperature is 1000°C to 1050°C, the final rolling temperature is ≥800°C, in the high temperature stage of 950°C to 1050°C, the deformation amount is 15% to 25%; in the medium temperature stage of 880°C to 950°C, the deformation amount is 25% to 30%; in the low temperature stage of 800°C to 880°C, the deformation amount is 20% to 30%.

[0021] Specifically, in step S3, the quenching temperature is 870°C to 920°C, the holding time is ≥30 min, and it is water-cooled to room temperature;

[0022] The tempering heat treatment temperature is 550°C to 600°C, the holding time is ≥120 min, and then it is air-cooled to room temperature.

[0023] The present invention also provides an application of the steel for bolts, and the steel for bolts is used to prepare fully-section mechanically stable bolts above grade 12.9 and M42.

[0024] Compared with the prior art, the present invention can at least achieve one of the following beneficial effects:

[0025] 1. The steel for bolts provided by the present invention has high strength, good hardenability, and stable full-section mechanical properties (that is, uniform mechanical properties and small hardness fluctuation across the section).

[0026] Through multi-element alloying design, the present invention appropriately adds V and Nb elements, and refines the original austenite grain size (corresponding to refining the size of martensite aggregates) by means of the precipitated NbC phase of 20 - 30 nm, while improving the strength of the bolt steel, ensuring toughness; by regulating the content of Mn element, and strictly controlling the content of impurity elements such as S, P, and N in the bolt steel, aiming to reduce grain boundary segregation, prevent grain boundary embrittlement, reduce the generation of non-metallic inclusions, which helps to improve the toughness of the bolt steel and ensure the processing performance of the bolt steel; by controlling the content of Mo element and adding V element with strong secondary hardening effect at the same time, spherical or near-spherical fine VC phase of 10 - 20 nm is obtained in the tempering stage, so that the bolt steel has high strength at a higher tempering temperature, and the dispersed carbide ensures that the bolt steel has more uniform mechanical properties.

[0027] At the same time, the V element dissolved in the matrix of the bolt steel will segregate at the austenite grain boundary, which not only occupies the nucleation position of proeutectoid ferrite on the original austenite grain boundary, but also increases the formation energy of the new phase near the grain boundary, hindering the nucleation of ferrite; at the same time, the vanadium atoms segregated at the grain boundary will make the atomic arrangement around the interface more stable, which will reduce the interface energy at the grain boundary and slow down the growth rate of ferrite; in summary, the appropriately added V element in the bolt steel inhibits the nucleation and growth rate of ferrite, delays the pearlite phase transformation and promotes the martensite phase transformation, and at the same time improves the stability of supercooled austenite, thereby improving the hardenability of the material.

[0028] It should be noted that, with the preparation method provided by the present invention, the role of the multi-alloying design can be more fully exerted, so that the steel for bolts obtains more excellent properties. The steel for bolts meets the technical requirements of fully-section mechanical stable bolts above grade 12.9 and M42, with Rm≥1290 MPa, Rp 0.2 ≥1260 MPa, elongation after fracture A≥10%, reduction of area Z≥49%; the cross-section hardness of the steel for bolts is uniform, the minimum cross-section hardness≥42 HRC, hardness fluctuation≤2 HRC, and critical hardenability diameter≥70 mm.

[0029] 2. The preparation method provided by the present invention has a relatively simple process, easily available raw materials and equipment, and relatively mild process conditions, which are suitable for large-scale application and wide promotion; and the preparation method can better exert the performance of the steel for bolts.

[0030] In the present invention, the above technical solutions can also be combined with each other to achieve more preferred combination schemes. Other features and advantages of the present invention will be described in the subsequent specification, and some advantages can be made obvious from the specification, or understood by implementing the present invention. The purpose and other advantages of the present invention can be realized and obtained from the content specifically pointed out in the specification and the drawings. Description of the Drawings

[0031] The drawings are only for the purpose of showing specific embodiments, and are not considered as a limitation to the present invention. Throughout the drawings, the same reference signs represent the same components.

[0032] Figure 1 It is the hardenability graph (represented by end-quench hardness) of the steel for bolts in Example 1;

[0033] Figure 2 It is the hardenability graph (represented by end-quench hardness) of the steel for bolts in Comparative Example 6;

[0034] Figure 3 It is the cross-section hardness comparison graph of the steel for bolts in Example 1 and Comparative Example 6;

[0035] Figure 4 It is the microstructural photograph (austenite grain structure) of the steel for bolts in Example 1 after quenching;

[0036] Figure 5 It is the microstructural photograph (austenite grain structure) of the steel for bolts in Comparative Example 6 after quenching;

[0037] Figure 6 It is the microstructural photograph (including precipitation phase) of the finished steel for bolts in Example 1. Detailed Embodiments

[0038] The preferred embodiments of the present invention will be specifically described below in conjunction with the accompanying drawings. The accompanying drawings form a part of this application and are used together with the embodiments of the present invention to explain the principles of the present invention, rather than to limit the scope of the present invention.

[0039] The present invention provides a steel for bolts. The specific composition of the steel for bolts is as follows by mass percentage: C 0.35 - 0.45%, Si 0.17 - 0.25%, Mn 0.50 - 0.90%, Cr 0.90 - 1.20%, Mo 0.20 - 0.40%, V 0.15 - 0.35%, Nb 0.02 - 0.05%, P ≤ 0.005%, S ≤ 0.005%, N ≤ 0.005%, and the balance is Fe and other inevitable impurities.

[0040] The functions / synergies of each element and the basis for determining the content are as follows:

[0041] C: The carbon element is a necessary element for the hardenability to ensure the strength of high-strength bolt steel, and can also improve the strength and hardness of the material. To ensure the hardenability of the bolt steel, generally the carbon content needs to be ≥ 0.35%. The increase in the carbon content can significantly improve the strength of the bolt steel, but will also reduce its plasticity, toughness and cold working performance. At the same time, too high a carbon content will reduce the yield ratio. Therefore, the upper limit of the carbon content in the present invention cannot exceed 0.45%. Therefore, its content is controlled at 0.35 - 0.45%.

[0042] Si: The silicon element is an important reducing agent and deoxidizer during the smelting process, and mainly exists in the bolt steel in a solid solution form. Appropriate Si can increase the yield ratio of the bolt steel and can significantly improve the tempering stability of the material. However, Si will segregate at the grain boundaries, reducing the mechanical uniformity of the bolt steel and at the same time reducing its cold working performance. If the content exceeds 0.25%, the plasticity of the material will be reduced, and if the content is lower than 0.17, its deoxidation and solid solution effects will be reduced. Therefore, its content is controlled at 0.17 - 0.25%.

[0043] Mn: The manganese element is a good deoxidizer and desulfurizer. Appropriate Mn element can homogenize and refine the quenched and tempered structure of the bolt steel, which is beneficial to the mechanical uniformity of the bolt steel and reduces the hardness fluctuation. However, during the high-temperature tempering process, Mn and S are prone to form inclusions at positions such as grain boundaries, promoting temper brittleness. Therefore, appropriately reducing the Mn content is beneficial to the stability of the mechanical properties of the bolt steel, and it is controlled at 0.50 - 0.90%.

[0044] Cr: Chromium is a common alloying element in bolt steel. It can significantly improve the hardenability of bolt steel, enhance the corrosion resistance, and is beneficial to maintaining the high-temperature strength of the steel. When the content is ≤0.5%, it is difficult to achieve the above effects. However, when the content exceeds 1.2%, it will deteriorate the cold working performance of the steel. Therefore, the content is controlled at 0.90 - 1.20%.

[0045] Mo: Molybdenum element in bolt steel has the functions of strengthening grain boundaries, improving the hardenability of materials, enhancing the strength of materials, and enhancing tempering stability. If the content of Mo is less than 0.20%, the above effects cannot be achieved; if the content of Mo is higher than 0.40%, it will increase the cost of bolt steel and cannot achieve the best economic benefits. Therefore, the content is controlled at 0.20 - 0.40%.

[0046] V: Vanadium element can effectively refine grains, improve the hardenability of materials, and enhance the mechanical properties of materials. If the content of V is less than 0.15%, it is difficult to play the above roles; if the content of V is higher than 0.35%, it will instead reduce the mechanical properties of materials. Therefore, the content is controlled at 0.15 - 0.35%.

[0047] Nb: Niobium element can refine grains and improve the strength of materials. In bolt steel, the Nb element belongs to trace elements. Excessive Nb content not only makes it difficult to play the above roles, but also reduces the economy of bolt steel. Therefore, the content is controlled at 0.02 - 0.05%.

[0048] S: Sulfur element will increase the hot brittleness of bolt steel. Excessive S will form MnS inclusions with Mn, reducing the hot working performance of bolt steel. Therefore, the content is controlled ≤0.005%.

[0049] P: Phosphorus element will increase the cold brittleness of bolt steel. It is a harmful residual element. It forms microsegregation during the solidification of molten steel, significantly increasing the brittleness of bolt steel. Therefore, the content is controlled ≤0.005%.

[0050] N: Nitrogen element plays a role in solid solution strengthening and aging strengthening in bolt steel. At the same time, it will form fine nitrides with vanadium element in bolt steel, having the effect of refining grains. However, excessive N will form Fe4N, reducing the toughness of bolt steel and also reducing its cold working performance. Therefore, the content is controlled ≤0.005%.

[0051] Synergistic effect: Through the design of multi-element alloying, appropriate addition of V and Nb elements, and by means of the precipitated NbC phase with a size of 20 - 30 nm to refine the original austenite grain size (corresponding to refining the size of martensite aggregates), while increasing the strength of the bolt steel, the toughness is ensured; by regulating the content of Mn element and strictly controlling the content of impurity elements such as S, P, and N in the bolt steel, it aims to reduce the segregation at grain boundaries, prevent the embrittlement of grain boundaries, reduce the generation of non-metallic inclusions, which helps to improve the toughness of the bolt steel and ensure the processing performance of the bolt steel; by controlling the content of Mo element and adding V element with a strong secondary hardening effect at the same time, spherical or near-spherical VC phases with a size of 10 - 20 nm are obtained during the tempering stage, enabling the bolt steel to have high strength at a relatively high tempering temperature, and the carbides distributed in a dispersed manner ensure that the bolt steel has more uniform mechanical properties. Additionally, the V element dissolved in the bolt steel inhibits the nucleation and growth rate of ferrite, delays the pearlite phase transformation and promotes the martensite phase transformation, improves the stability of supercooled austenite, and thus improves the hardenability of the material.

[0052] It should be emphasized that the two main added elements, Nb and V, and other multi-element alloy components do not work alone. When Nb and V are added simultaneously and their contents are within an appropriate range, the effect of "1 + 1 > 2" will occur, which is difficult to predict. Based on theoretical analysis, combined with process practice and certain accidental factors, the alloy with the above composition was developed in this invention. These two core elements and other multi-element alloy elements have produced changes including but not limited to those described above (such as various precipitation phases and other possible synergistic effects that have not been fully studied), resulting in a significant improvement in the toughness, hardness, and mechanical uniformity of the steel for bolts (see Comparative Examples 2 - 5).

[0053] Specifically, the steel for bolts is composed of lath martensite, and the martensite is formed by quenching austenite; the width of the martensite laths is 0.5 - 1.5 μm.

[0054] Furthermore, the martensite forms martensite lath aggregates along the boundaries of the original austenite grains, and the size of the lath aggregates is 5.0 - 20 μm.

[0055] It should be noted that during the cooling process of austenite (face-centered cubic structure, FCC), due to the inhibition of carbon atom diffusion, its crystal structure will transform into martensite (body-centered cubic or body-centered orthorhombic structure). This process is diffusionless, that is, carbon atoms remain in the lattice. In this invention, by appropriately adding V and Nb elements, the original austenite grain size is refined by means of the precipitated NbC phase with a size of 20 - 30 nm. Therefore, the grains of the original austenite are significantly reduced compared to 42CrMo steel (such as Figure 4 and Figure 5As shown, the austenite size of the steel for bolts provided by the present invention is about 16 - 20 μm, and the austenite size of 42CrMo steel is about 30 - 45 μm. The martensite transformed from austenite will still be demarcated / aggregated along the original austenite grain boundaries, thereby obtaining martensite aggregates with smaller sizes.

[0056] It should be noted that the refinement of austenite grains in bolt steel represents an increase in the number of grain boundaries. During the movement of dislocations, more grain boundaries cut and hinder them. When dislocations want to continue moving, a greater stress is required to break free from the hindrance, thereby improving the strength of the material. At the same time, finer grains enable plastic deformation to be more evenly distributed within more grains, thus reducing the concentration of internal stress and ensuring the toughness of the material.

[0057] Furthermore, the refinement of austenite grains reduces the size of martensite lath width, which provides more precipitation sites for the precipitates during the tempering stage. More precipitates further improve the strength of the material.

[0058] Specifically, the steel for bolts includes discretely distributed NbC precipitates, VC precipitates, and VN precipitates;

[0059] Among them, the size of NbC precipitates is 20 - 30 nm, and the size of VC precipitates is 10 - 20 nm.

[0060] Specifically, the NbC phase can refine the original austenite grain size (corresponding to refining the size of martensite aggregates), ensuring toughness while increasing the strength of bolt steel. The 10 - 20 nm fine VC phase enables the bolt steel to have high strength at a relatively high tempering temperature, and at the same time, the dispersed carbides ensure that the bolt steel has more uniform mechanical properties. For the specific distribution state of the precipitates, see Figure 6 .

[0061] Specifically, the steel for bolts meets the technical requirements of fully-section mechanical stable bolts above grade 12.9 M42, with Rm ≥ 1290 MPa, Rp 0.2 ≥ 1260 MPa, elongation after fracture A ≥ 10%, reduction of area Z ≥ 49%; the cross-section hardness of the steel for bolts is uniform, the minimum cross-section hardness ≥ 42 HRC, hardness fluctuation ≤ 2 HRC, and critical hardenability diameter ≥ 70 mm.

[0062] The present invention also provides a preparation method for the above-mentioned steel for bolts, including the following steps:

[0063] S1: Conduct preliminary batching according to a preset formula, and obtain a continuous casting billet that meets the composition of the preset formula after electric furnace smelting, LF refining, vacuum smelting, and continuous casting;

[0064] S2: Heat and hold the continuous casting billet, then cool it naturally to room temperature after rough rolling and finish rolling to obtain round steel with a specification of 48 - 65 mm.

[0065] S3: Quench the round steel, then perform tempering heat treatment and air cooling to obtain the finished bolt steel.

[0066] Specifically, after vacuum smelting in step S1, the measured hydrogen content ≤ 2.0 ppm. Here, the hydrogen content refers to the hydrogen content in the solid state after the alloy liquid is cast into shape under vacuum conditions.

[0067] Specifically, the specific operation of step S2 is as follows:

[0068] Uniformly heat the continuous casting billet to 1000°C - 1150°C and hold, with the holding time ≥ 120 min;

[0069] In the rough rolling stage, the initial rolling temperature is 1000°C - 1150°C, the final rolling temperature ≥ 850°C, and the deformation amount is 25% - 35%;

[0070] In the finish rolling stage, the initial rolling temperature is 1000°C - 1050°C, the final rolling temperature ≥ 800°C. In the high - temperature stage of 950°C - 1050°C, the deformation amount is 15% - 25%; in the medium - temperature stage of 880°C - 950°C, the deformation amount is 25% - 30%; in the low - temperature stage of 800°C - 880°C, the deformation amount is 20% - 30%.

[0071] Specifically, in step S3, the quenching temperature is 870°C - 920°C, the holding time ≥ 30 min, and water - cool to room temperature;

[0072] The tempering heat treatment temperature is 550°C - 600°C, the holding time ≥ 120 min, and then air - cool to room temperature. For bolt steel, using a lower tempering temperature can effectively improve the strength of bolt steel, but it will increase the risk of delayed fracture. A higher tempering temperature can ensure the plasticity - toughness and hydrogen embrittlement resistance of the material.

[0073] Therefore, for 12.9 - grade high - strength bolts, there is a minimum tempering temperature. For 42CrMo steel, in the actual production process, the tempering temperature requirement is not lower than 550°C. However, a higher tempering temperature will soften the material matrix, coarsen the size of the precipitated phase, and reduce the strength of bolt steel. For bolt steel, when the tempering temperature exceeds 600°C, its strength decreases significantly and cannot meet the strength requirements of 12.9 - grade. Therefore, for the bolt steel provided by the present invention, the optimal tempering temperature is 550°C - 600°C.

[0074] The present invention also provides an application of the bolt steel, and the bolt steel is used to prepare fully - section mechanical stability bolts of 12.9 - grade and above M42.

[0075] The component compositions of each example and comparative example are shown in Table 1, where the balance is Fe and impurities.

[0076] Table 1 Component compositions of each example and comparative example (wt.%)

[0077]

[0078]

[0079] Example 1:

[0080] S1: Electric furnace smelting is adopted, followed by LF refining and vacuum smelting. After vacuum smelting, the hydrogen content is measured to be 1.8 ppm, and a 150 mm square continuous casting billet is produced by continuous casting;

[0081] S2: The heating temperature of the continuous casting billet during the soaking stage is 1000 °C to 1080 °C, and the holding time is 130 min;

[0082] Then, the continuous casting billet is subjected to rough rolling. The initial rolling temperature is 1140 °C, and the final rolling temperature is 860 °C. After rough rolling, the total deformation of the continuous casting billet is 26%;

[0083] The intermediate billet after rough rolling is subjected to finish rolling. The initial rolling temperature is 1040 °C, and the final rolling temperature is 810 °C. Among them, in the high-temperature stage (950 °C to 1040 °C), the deformation is 16%, in the medium-temperature stage (880 °C to 950 °C), the deformation is 26%, and in the low-temperature stage (810 °C to 880 °C), the deformation is 20%. Finally, it is rolled into a round steel with a specification of 44 mm;

[0084] S3: The round steel is subjected to quenching and tempering heat treatment. The quenching temperature is 870 °C, and it is held for 30 min, then water-cooled to room temperature. The tempering temperature is 600 °C, and it is held for 120 min, then air-cooled to room temperature to obtain the finished bolt steel.

[0085] Example 2:

[0086] S1: Electric furnace smelting is adopted, followed by LF refining and vacuum smelting. After vacuum smelting, the hydrogen content is measured to be 1.6 ppm, and a 150 mm square continuous casting billet is produced by continuous casting;

[0087] S2: The heating temperature of the continuous casting billet during the soaking stage is 1050 °C to 1120 °C, and the holding time is 130 min;

[0088] Then, the continuous casting billet is subjected to rough rolling. The initial rolling temperature is 1120 °C, and the final rolling temperature is 870 °C. After rough rolling, the total deformation of the continuous casting billet is 28%; the intermediate billet after rough rolling is subjected to finish rolling. The initial rolling temperature is 1035 °C, and the final rolling temperature is 815 °C. Among them, in the high-temperature stage (950 °C - 1035 °C), the deformation is 18%, in the medium-temperature stage (880 °C - 950 °C), the deformation is 27%, and in the low-temperature stage (815 °C - 880 °C), the deformation is 22%. Finally, it is rolled into round steel with a specification of 50 mm;

[0089] S3: The round steel is subjected to quenching and tempering heat treatment. The quenching temperature is 870 °C, holding for 30 min, water-cooled to room temperature, the tempering temperature is 600 °C, holding for 120 min, and air-cooled to room temperature to obtain the finished bolt steel.

[0090] Example 3:

[0091] S1: Electric furnace smelting is adopted, followed by LF refining and vacuum smelting. After vacuum smelting, the hydrogen content is measured to be 1.7 ppm, and a 150 mm square continuous casting billet is produced by continuous casting;

[0092] S2: The heating temperature of the continuous casting billet in the soaking stage is 1080 °C - 1150 °C, and the holding time is 130 min;

[0093] Then, the continuous casting billet is subjected to rough rolling. The initial rolling temperature is 1125 °C, and the final rolling temperature is 865 °C. After rough rolling, the total deformation of the continuous casting billet is 30%; the intermediate billet after rough rolling is subjected to finish rolling. The initial rolling temperature is 1040 °C, and the final rolling temperature is 820 °C. Among them, in the high-temperature stage (950 °C - 1040 °C), the deformation is 20%, in the medium-temperature stage (880 °C - 950 °C), the deformation is 25%, and in the low-temperature stage (820 °C - 880 °C), the deformation is 25%. Finally, it is rolled into round steel with a specification of 55 mm;

[0094] S3: The round steel is subjected to quenching and tempering heat treatment. The quenching temperature is 870 °C, holding for 30 min, water-cooled to room temperature, the tempering temperature is 600 °C, holding for 120 min, and air-cooled to room temperature to obtain the finished bolt steel.

[0095] Example 4:

[0096] S1: Electric furnace smelting is adopted, followed by LF refining and vacuum smelting. After vacuum smelting, the hydrogen content is measured to be 1.8 ppm, and a 150 mm square continuous casting billet is produced by continuous casting;

[0097] S2: The heating temperature of the continuous casting billet in the soaking stage is 1060 °C - 1130 °C, and the holding time is 130 min;

[0098] Then, the continuous casting billet is subjected to rough rolling. The initial rolling temperature is 1145 °C, and the final rolling temperature is 855 °C. After rough rolling, the total deformation of the continuous casting billet is 33%; the intermediate billet after rough rolling is subjected to finish rolling. The initial rolling temperature is 1035 °C, and the final rolling temperature is 820 °C. Among them, in the high-temperature stage (950 °C - 1035 °C), the deformation is 23%, in the medium-temperature stage (880 °C - 950 °C), the deformation is 27%, and in the low-temperature stage (820 °C - 880 °C), the deformation is 24%. Finally, it is rolled into round steel with a specification of 60 mm;

[0099] S3: The round steel is subjected to quenching and tempering heat treatment. The quenching temperature is 870 °C, holding for 30 min, water-cooled to room temperature, the tempering temperature is 600 °C, holding for 120 min, and air-cooled to room temperature to obtain the finished bolt steel.

[0100] Example 5:

[0101] S1: Electric furnace smelting is adopted, followed by LF refining and vacuum smelting. After vacuum smelting, the hydrogen content is measured to be 2.0 ppm, and a 150 mm square continuous casting billet is produced by continuous casting;

[0102] S2: The heating temperature of the continuous casting billet during the soaking stage is 1000 °C - 1110 °C, and the holding time is 130 min;

[0103] Then, the continuous casting billet is subjected to rough rolling. The initial rolling temperature is 1150 °C, and the final rolling temperature is 850 °C. After rough rolling, the total deformation of the continuous casting billet is 32%; the intermediate billet after rough rolling is subjected to finish rolling. The initial rolling temperature is 1050 °C, and the final rolling temperature is 825 °C. Among them, in the high-temperature stage (950 °C - 1050 °C), the deformation is 21%, in the medium-temperature stage (880 °C - 950 °C), the deformation is 28%, and in the low-temperature stage (825 °C - 880 °C), the deformation is 26%. Finally, it is rolled into round steel with a specification of 65 mm;

[0104] S3: The round steel is subjected to quenching and tempering heat treatment. The quenching temperature is 870 °C, holding for 30 min, water-cooled to room temperature, the tempering temperature is 600 °C, holding for 120 min, and air-cooled to room temperature to obtain the finished bolt steel.

[0105] Comparative Example 1

[0106] S1: Electric furnace smelting is adopted, followed by LF refining and vacuum smelting. After vacuum smelting, the hydrogen content is measured to be 1.7 ppm, and a 150 mm square continuous casting billet is produced by continuous casting;

[0107] S2: The heating temperature of the continuous casting billet during the soaking stage is 1060 °C - 1130 °C, and the holding time is 130 min;

[0108] Then the continuous casting billet is subjected to rough rolling. The initial rolling temperature is 1125°C, and the final rolling temperature is 865°C. After rough rolling, the total deformation of the continuous casting billet is 30%; the intermediate billet after rough rolling is subjected to finish rolling. The initial rolling temperature is 1040°C, and the final rolling temperature is 820°C. Among them, in the high-temperature stage (950°C - 1040°C), the deformation is 20%, in the medium-temperature stage (880°C - 950°C), the deformation is 25%, and in the low-temperature stage (820°C - 880°C), the deformation is 25%. Finally, it is rolled into round steel with a specification of 44 mm;

[0109] S3: The round steel is subjected to quenching and tempering heat treatment. The quenching temperature is 870°C, holding for 30 min, water-cooled to room temperature, the tempering temperature is 600°C, holding for 120 min, and air-cooled to room temperature to obtain the finished bolt steel.

[0110] Comparative Example 2

[0111] S1: Electric furnace smelting is adopted, followed by LF refining and vacuum smelting. After vacuum smelting, the hydrogen content is measured to be 1.7 ppm, and a 150 mm square continuous casting billet is produced by continuous casting;

[0112] S2: The heating temperature of the continuous casting billet during the soaking stage is 1000°C - 1110°C, and the holding time is 130 min;

[0113] Then the continuous casting billet is subjected to rough rolling. The initial rolling temperature is 1145°C, and the final rolling temperature is 855°C. After rough rolling, the total deformation of the continuous casting billet is 33%; the intermediate billet after rough rolling is subjected to finish rolling. The initial rolling temperature is 1035°C, and the final rolling temperature is 820°C. Among them, in the high-temperature stage (950°C - 1035°C), the deformation is 23%, in the medium-temperature stage (880°C - 950°C), the deformation is 27%, and in the low-temperature stage (820°C - 880°C), the deformation is 24%. Finally, it is rolled into round steel with a specification of 50 mm;

[0114] S3: The round steel is subjected to quenching and tempering heat treatment. The quenching temperature is 870°C, holding for 30 min, water-cooled to room temperature, the tempering temperature is 600°C, holding for 120 min, and air-cooled to room temperature to obtain the finished bolt steel.

[0115] Comparative Example 3

[0116] S1: Electric furnace smelting is adopted, followed by LF refining and vacuum smelting. After vacuum smelting, the hydrogen content is measured to be 1.7 ppm, and a 150 mm square continuous casting billet is produced by continuous casting;

[0117] S2: The heating temperature of the continuous casting billet during the soaking stage is 1050°C - 1120°C, and the holding time is 130 min;

[0118] Then, the continuous casting billet is subjected to rough rolling. The initial rolling temperature is 1140 °C, and the final rolling temperature is 860 °C. After rough rolling, the total deformation of the continuous casting billet is 26%; the intermediate billet after rough rolling is subjected to finish rolling. The initial rolling temperature is 1040 °C, and the final rolling temperature is 810 °C. Among them, in the high-temperature stage (950 °C - 1040 °C), the deformation is 16%, in the medium-temperature stage (880 °C - 950 °C), the deformation is 26%, and in the low-temperature stage (810 °C - 880 °C), the deformation is 20%. Finally, it is rolled into round steel with a specification of 55 mm;

[0119] S3: The round steel is subjected to quenching and tempering heat treatment. The quenching temperature is 870 °C, holding for 30 min, water-cooled to room temperature, the tempering temperature is 600 °C, holding for 120 min, and air-cooled to room temperature to obtain the finished bolt steel.

[0120] Comparative Example 4

[0121] S1: Electric furnace smelting is adopted, followed by LF refining and vacuum smelting. After vacuum smelting, the hydrogen content is measured to be 1.7 ppm, and a 150 mm square continuous casting billet is produced by continuous casting;

[0122] S2: The heating temperature of the continuous casting billet during the soaking stage is 1050 °C - 1120 °C, and the holding time is 130 min;

[0123] Then, the continuous casting billet is subjected to rough rolling. The initial rolling temperature is 1140 °C, and the final rolling temperature is 860 °C. After rough rolling, the total deformation of the continuous casting billet is 27%; the intermediate billet after rough rolling is subjected to finish rolling. The initial rolling temperature is 1035 °C, and the final rolling temperature is 810 °C. Among them, in the high-temperature stage (950 °C - 1035 °C), the deformation is 16%, in the medium-temperature stage (880 °C - 950 °C), the deformation is 27%, and in the low-temperature stage (810 °C - 880 °C), the deformation is 22%. Finally, it is rolled into round steel with a specification of 60 mm;

[0124] S3: The round steel is subjected to quenching and tempering heat treatment. The quenching temperature is 870 °C, holding for 30 min, water-cooled to room temperature, the tempering temperature is 600 °C, holding for 120 min, and air-cooled to room temperature to obtain the finished bolt steel.

[0125] Comparative Example 5

[0126] S1: Electric furnace smelting is adopted, followed by LF refining and vacuum smelting. After vacuum smelting, the hydrogen content is measured to be 1.6 ppm, and a 150 mm square continuous casting billet is produced by continuous casting;

[0127] S2: The heating temperature of the continuous casting billet during the soaking stage is 1050 °C - 1120 °C, and the holding time is 130 min;

[0128] Then, the continuous casting billet is subjected to rough rolling. The initial rolling temperature is 1130°C, and the final rolling temperature is 860°C. After rough rolling, the total deformation of the continuous casting billet is 26%. The intermediate billet after rough rolling is subjected to finish rolling. The initial rolling temperature is 1040°C, and the final rolling temperature is 820°C. Among them, in the high-temperature stage (950°C - 1040°C), the deformation is 17%, in the medium-temperature stage (880°C - 950°C), the deformation is 26%, and in the low-temperature stage (820°C - 880°C), the deformation is 21%. Finally, it is rolled into round steel with a specification of 65 mm.

[0129] S3: Quench and temper the round steel. The quenching temperature is 870°C, hold for 30 min, water-cool to room temperature, the tempering temperature is 600°C, hold for 120 min, and air-cool to room temperature to obtain the finished bolt steel.

[0130] The mechanical property parameters of each example and comparative example are shown in Table 2.

[0131] Table 2 Mechanical property parameters

[0132]

[0133]

[0134] From the comprehensive mechanical properties of the examples and comparative examples, the mechanical properties of the bolt steel examples of the present invention are superior to those of the comparative examples. The main reason is the change of alloying elements. The design of the process links and parameter optimization also have a certain promoting effect on the formation and properties of the alloy structure (the influence is smaller compared to the design of alloying elements). In the present invention, due to the addition of appropriate amounts of V and Nb elements, under the same preparation process and parameters, the Nb element can inhibit the growth of austenite grains and promote fine grain strengthening, thereby improving the processing performance and heat treatment effect of the steel. This makes the steel more stable during the rolling process and reduces the performance degradation caused by grain growth. The V element can increase the recrystallization temperature and expand the rolling temperature range, which ensures that the bolt steel has a high deformation ability at a lower rolling temperature and improves the stability and controllability of the bolt steel during the rolling process.

[0135] Compared with the comparative example, in the bolt steel of the present invention, appropriate amounts of V and Nb elements are added. In the quenching stage, the V element dissolved in the matrix inhibits the nucleation and growth rate of ferrite, significantly improving the hardenability of the bolt steel and thus increasing the strength of the bolt steel. The Nb element forms NbC precipitation phases, hindering the growth of the original austenite grains and refining the grains of the bolt steel, ensuring toughness while increasing the strength. In the tempering stage, the precipitated VC precipitation phases further increase the strength of the bolt steel, and the dispersed nanoscale precipitation phases help reduce the hardness fluctuation of the bolt steel cross-section and improve the mechanical uniformity.

[0136] Additionally, during the preparation of bolt steel, fluctuations in composition inevitably occur, and these compositional fluctuations are the main reason for the differences in mechanical properties in the examples. In terms of performance, within the designed composition range, as the contents of V and Nb elements increase, the strength of the examples also increases, and at the same time, the fluctuation of cross-sectional hardness gradually decreases, which fully demonstrates the influence of alloy composition on mechanical properties. However, it can also be seen that when the addition amounts of each element gradually approach the upper limit, there is no obvious change in hardness uniformity and the lower limit of hardness. Continuing to increase the element content will cause waste of resources and increase in cost.

[0137] It is worth emphasizing that according to Comparative Examples 2 to 5, it can be seen that the two main added elements, Nb and V, and other multi-element alloy components do not act alone. When Nb and V are added simultaneously and their contents are within an appropriate range, the effect of "1 + 1 > 2" will occur, and this effect is difficult to predict. Based on theoretical analysis, combined with process practice and certain accidental factors, the present invention has developed the alloy with the above composition. These two core elements and other multi-element alloy elements have produced changes including but not limited to those described above (such as various precipitation phases and other possible synergistic effects that have not been fully studied thoroughly), which have significantly improved the toughness, hardness, and mechanical uniformity of the steel for bolts. Therefore, Comparative Examples 2 to 5 respectively give the comparative examples of separately adding V and Nb additionally and the inappropriate addition amounts of both. It can be seen that although roughly adding elements can improve the strength and hardness values of the alloy to a certain extent (the ductility is even equal to or slightly better than that of the examples), the improvement amplitude is not large, and due to the unreasonable addition amounts, it will even have a negative impact on hardness uniformity.

[0138] The end-quench tests were respectively carried out on the semi-finished steel for bolts (not quenched) in Example 1 and 42CrMo steel (not quenched) of the same specification. The end-quench test was carried out in accordance with the national standard GB / T 225—2006 "Steel - End-Quench Test Method for Hardenability". Two mutually parallel planes with a depth of 0.4 - 0.5 mm parallel to the axis direction were ground at both ends of the quenched specimen. The Rockwell hardness values at positions 1.5 - 60 mm from the quenched end face were measured on the ground planes using an HP-250 type Rockwell hardness tester (loading load 150 kg), and then the hardness distribution curve of the test steel was drawn based on the obtained data. The test results are as Figure 1 、 Figure 2As shown, the critical hardenability diameter of the steel for bolts in Example 1 is approximately 70 - 80 mm, and the critical hardenability diameter of 42CrMo steel (Comparative Example 6) is approximately 40 mm. The principle for determining the critical hardenability diameter is as follows: First, determine the point where the trend of the hardness after quenching undergoes a sudden change (from a gentle change to a downward trend), and then multiply the distance from the quenching end corresponding to this point by 2 to obtain the critical hardenability diameter. In Example 1, the distance from the quenching end corresponding to the hardness mutation point of the steel for bolts is approximately 35 - 40 mm, and the distance from the quenching end corresponding to the hardness mutation point of 42CrMo steel is approximately 20 mm.

[0139] In summary, the steel for bolts meets the technical requirements of fully-section mechanical stable bolts of grade 12.9 and above M42, with Rm ≥ 1290 MPa, Rp0.2 ≥ 1260 MPa, elongation after fracture A ≥ 10%, reduction of area Z ≥ 49%; the cross-section hardness of the steel for bolts is uniform, the minimum cross-section hardness ≥ 42 HRC, the hardness fluctuation ≤ 2 HRC, and the critical hardenability diameter ≥ 70 mm (therefore, the round steel size is set not to exceed 65 mm to ensure complete hardening).

[0140] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A steel for bolts, characterized in that: The specific composition of the bolt steel is calculated by mass percentage as follows: C 0.35-0.45%, Si 0.17-0.25%, Mn 0.50-0.90%, Cr 0.90-1.20%, Mo 0.20-0.40%, V 0.15-0.35%, Nb 0.02-0.05%, P≤0.005%, S≤0.005%, N≤0.005%, and the rest is Fe and other inevitable impurities.

2. The bolt steel according to claim 1, characterized in that: The bolt steel is composed of lath-shaped martensite, which is formed by quenching austenite; the width of the martensite lath is 0.5-1.5 μm.

3. The bolt steel according to claim 2, characterized in that: The martensite forms martensite lath aggregates along the original austenite grain boundaries, and the size of the lath aggregates is 5.0-20 μm.

4. The bolt steel according to claim 1, characterized in that: The bolt steel comprises discretely distributed NbC precipitation phase, VC precipitation phase and VN precipitation phase; The size of the NbC precipitated phase is 20-30 nm, and the size of the VC precipitated phase is 10-20 nm.

5. The bolt steel according to claim 1, characterized in that: The bolt steel meets the technical requirements of full-section mechanically stable bolts above grade 12.9 M42, Rm≥1290MPa, Rp 0.2 ≥1260MPa, elongation after fracture A≥10%, shrinkage after fracture Z≥49%; the cross-section hardness of the bolt steel is uniform, the minimum cross-section hardness is ≥42HRC, the hardness fluctuation is ≤2HRC, and the critical hardening diameter is ≥70mm.

6. A method for preparing the bolt steel according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1: Preliminary batching is performed according to a preset formula, and after electric furnace smelting, LF refining, vacuum smelting, and continuous casting, a continuous casting billet that meets the preset formula is obtained; S2: heating and keeping the continuous casting billet warm, and naturally cooling it to room temperature after rough rolling and finish rolling to obtain round steel with a specification of 48 to 65 mm; S3: After the round steel is quenched, it is subjected to tempering heat treatment and air cooling to obtain a finished steel product for bolts.

7. The preparation method according to claim 6, characterized in that: After vacuum smelting in step S1, the hydrogen content is measured to be ≤2.0ppm.

8. The preparation method according to claim 6, characterized in that: The specific operations of step S2 are: The continuous casting billet is uniformly heated to 1000℃~1150℃ and kept warm for ≥120min; In the rough rolling stage, the initial rolling temperature is 1000℃~1150℃, the final rolling temperature is ≥850℃, and the deformation is 25%~35%; In the finishing rolling stage, the initial rolling temperature is 1000℃~1050℃, the final rolling temperature is ≥800℃, in the high temperature stage 950℃~1050℃, the deformation is 15%~25%; in the medium temperature stage 880℃~950℃, the deformation is 25%~30%; in the low temperature stage 800℃~880℃, the deformation is 20%~30%.

9. The preparation method according to claim 6, characterized in that: In step S3, the quenching temperature is 870°C to 920°C, the holding time is ≥30min, and the quenching temperature is cooled to room temperature by water; The tempering heat treatment temperature is 550℃~600℃, the holding time is ≥120min, and then air-cooled to room temperature.

10. Use of the bolt steel according to any one of claims 1 to 5 or the bolt steel prepared by the preparation method according to any one of claims 6 to 9, characterized in that: The bolt steel is used for preparing full-section mechanically stable bolts of grade 12.9 or above M42.

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