A 12.9 grade non-quenched and tempered long rod bolt steel and a preparation method thereof

Abstract

The application provides a 12.9-grade non-quenched long rod bolt steel and a preparation method thereof, and belongs to the field of fastener steel and manufacturing technology. The 12.9-grade non-quenched long rod bolt steel provided by the application comprises the following chemical components in percentage by mass: C: 0.15-0.28%, Si: <=0.30%, Mn: 1.2-2.0%, P: <=0.020%, S: <=0.020%, Cr: 0.1-0.6%, Ni: <=0.25%, Mo: <=0.30%, V: 0.04-0.14%, Nb: 0.02-0.07%, Ti: <=0.06%, Al: 0.01-0.05%, O <=0.002%, N <=0.025% and the balance of Fe.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fastener steel and manufacturing technology, and particularly relates to a 12.9-grade non-quenched and tempered long rod bolt steel and a preparation method thereof. BACKGROUND

[0002] Non-quenched and tempered steel is a structural steel obtained by adding appropriate amounts of alloy elements such as V, Nb and Ti on the basis of carbon manganese steel, and can reach the mechanical properties of quenched and tempered steel after forging or hot rolling by adopting controlled rolling and controlled cooling processes, so that the manufacturing cost is reduced by more than 20% compared with quenched and tempered steel.

[0003] At present, general 8.8-grade and above long bolts generally need to be quenched and tempered, but there are two main problems: first, quenching will cause deformation, which needs to be straightened, and the straightness cannot be guaranteed, resulting in a high scrap rate; second, the work efficiency is low and the energy consumption is high, so the manufacturing cost is large. When the non-quenched and tempered steel is used to manufacture long bolt rods, the quenching deformation problem is avoided, the part straightness is high, and the superior product rate is greatly improved, which has good economic and social benefits.

[0004] Application No. CN202110146484.0 discloses a non-quenched and tempered steel long rod bolt and a manufacturing method thereof, but due to the material limitation, the strength grade is only 8.8-grade. There are public literatures reporting the development of 9.8-grade and 10.9-grade non-quenched and tempered steel fastener raw materials, but there is no report on its use in fastener production. There is no public data report on 12.9-grade and higher strength non-quenched and tempered steel fasteners.

[0005] Therefore, it is a technical problem to be solved in the field to provide a 12.9-grade non-quenched long rod bolt steel. SUMMARY

[0006] The purpose of the present application is to provide a 12.9-grade non-quenched long rod bolt steel and a preparation method thereof. The toughness of the long rod bolt prepared by using the 12.9-grade non-quenched long rod bolt steel provided by the present application meets the requirements of existing 12.9-grade quenched and tempered bolts.

[0007] In order to achieve the above-mentioned purposes, the present application provides the following technical solutions:

[0008] The application provides a 12.9-grade non-quenched long rod bolt steel, which comprises the following chemical components in percentage by mass: C: 0.15-0.28%, Si: ≤0.30%, Mn: 1.2-2.0%, P: ≤0.020%, S: ≤0.020%, Cr: 0.1-0.6%, Ni: ≤0.25%, Mo: ≤0.30%, V: 0.04-0.14%, Nb: 0.02-0.07%, Ti: ≤0.06%, Al: 0.01-0.05%, O ≤0.002%, N ≤0.025% and the balance of Fe.

[0009] The chemical components of the 12.9-grade non-quenched long rod bolt steel satisfy 12.80 < G < 16.0; wherein G = 20 * C% + 1.3 Si% + 3.3 Mn% + 2.5 (Cr% + Ni% + Mo%) + 16 V% + 8.3 Nb% + 50 Ti% + 50 N%.

[0010] Preferably, the 12.9-grade non-quenched long rod bolt steel comprises the following chemical components in percentage by mass: C: 0.18-0.25%, Si: ≤0.30%, Mn: 1.4-1.9%, P: ≤0.020%, S: ≤0.020%, Cr: 0.15-0.45%, Ni: ≤0.25%, Mo: ≤0.30%, V: 0.08-0.12%, Nb: 0.02-0.07%, Ti: ≤0.06%, Al: 0.01-0.05%, O ≤0.002%, N ≤0.025% and the balance of Fe.

[0011] Preferably, the chemical components of the 12.9-grade non-quenched long rod bolt steel satisfy carbon equivalent Ceq < 0.7, wherein Ceq = C% + Mn% / 6 + (Cr% + V% + Mo%) / 5 + (Cu% + Ni%) / 15.

[0012] Preferably, the base metallographic structure of the 12.9-grade non-quenched long rod bolt steel is bainite structure.

[0013] The application provides a preparation method of the 12.9-grade non-quenched long rod bolt steel.

[0014] (1) smelting raw materials to obtain a cast ingot;

[0015] (2) sequentially performing hot rolling, drawing and aging treatment on the cast ingot obtained in the step (1) to obtain the 12.9-grade non-quenched long rod bolt steel.

[0016] Preferably, the smelting mode in the step (1) is converter / electric furnace + refining + VD / RH three-process.

[0017] Preferably, the hot rolling in step (2) is performed at a starting temperature of 970-1100°C and a finishing temperature of 780-900°C.

[0018] Preferably, the drawing in step (2) is performed at a reduction ratio of 18-45%.

[0019] Preferably, after the drawing in step (2), the drawn product is subjected to straightening, rolling, wire rolling and surface treatment.

[0020] Preferably, the aging treatment in step (2) is performed at a temperature of 240-300°C for 1-5h.

[0021] The application provides a 12.9-grade non-quenched long rod bolt steel, which comprises the following chemical components in percentage by mass: C: 0.15-0.28%, Si: <=0.30%, Mn: 1.2-2.0%, P: <=0.020%, S: <=0.020%, Cr: 0.1-0.6%, Ni: <=0.25%, Mo: <=0.30%, V: 0.04-0.14%, Nb: 0.02-0.07%, Ti: <=0.06%, Al: 0.01-0.05%, O <=0.002%, N <=0.025% and the balance of Fe; the chemical components of the 12.9-grade non-quenched long rod bolt steel satisfy 12.80 < G < 16.0; wherein G = 20*C% + 1.3Si% + 3.3Mn% + 2.5(Cr% + Ni% + Mo%) + 16V% + 8.3Nb% + 50Ti% + 50N%. The C element in the application is the main strengthening element of the non-quenched steel, which helps to improve the base body strength and hardenability; the Si element is a deoxidizer, which can be dissolved in ferrite to achieve solid solution strengthening, strengthens the ferrite phase and thus improves the tensile and yield strength of the steel; the Mn element is an effective element for deoxidization and desulfurization, which improves the hardness and strength of the steel, and the Mn element can improve the stability of the austenite structure and also can delay the pearlite phase change and promote the bainite phase change; the content of the P element and the S element is controlled, which can reduce the damage to the uniformity of the steel structure and performance, avoids the thermal embrittlement of the steel; the Cr element is a ferrite solid solution strengthening element, which has similar effects with the Si and Mn elements, in addition, the Cr element can effectively improve the hardenability of the steel and delay the bainite phase change to obtain the required high strength, and through solid solution strengthening, the bainite ferrite hardness can be significantly improved, and the Cr element can also reduce the activity of the C element, reduces the tendency of the steel surface decarburization during the heating, hot rolling and forging processes and has high fatigue resistance; the Cr element can greatly reduce the Bs point, can obtain a certain amount of bainite in the air cooling process and can also prevent cracking caused by too fast cooling speed; the C and Cr elements have a synergistic effect, further reduce the Bs point, avoid the appearance of granular bainite and upper bainite as much as possible in the air cooling process, obtain the lower bainite structure and refine the bainite size, thereby increasing the strength and toughness of the bainite ferrite base body; the Ni element can effectively improve the core toughness of the steel, reduce the ductile-brittle transition temperature, improve the low-temperature impact performance and is beneficial to the fatigue strength of the steel; the Mo element can significantly improve the hardenability of the steel and promote the bainite transformation; the Mo element can form strong carbides, hinders the diffusion of atoms, the movement of dislocations and the grain boundary migration, effectively prevents the recrystallization of the deformed austenite, and the carbide particles of the Mo element are small and will not cause stress concentration of the microstructure structure, which is beneficial to the impact toughness of the steel.The V element is a strengthening element, V has strong affinity with C and N, can form VC and V(CN), and forms a dispersed distribution in the hot rolling stage of the steel and the low-temperature aging stage after the bolt upsetting forming, has strong precipitation strengthening effect, thereby improving the strength, and the solid-solution V element can significantly inhibit the diffusion of C element in the process of bainite phase change, can play a role in refining bainite ferrite, and can improve the strength and toughness of the steel; the Nb element is similar to the V element, also has strong affinity with C and N, forms NbC and Nb(CN), forms a dispersed distribution in the hot rolling stage of the steel and the low-temperature aging stage after the bolt upsetting forming, and improves the strength; Ti has strong affinity with C, O and N, in the process of steel smelting and hot rolling, the TiN and TiC phases combined and precipitated by Ti, C and N can slow down the growth rate of austenite grain size, play a role in refining the grain, and the V-Ti composite addition has better effect on grain refinement; the Al element is an effective deoxidizer, and can form AlN to refine the grain; N can form compounds with V, Ti and Al, can promote the precipitation of V, reduce the amount of V, mainly plays a role in strengthening the precipitation strengthening effect and refining the grain in non-adjusted steel, and reduces the cost; controlling the content of O element can reduce the content of oxide inclusions, avoid damaging the continuity of the steel matrix, thereby further improving the toughness, plasticity, fatigue strength and corrosion resistance of the steel; by controlling the chemical composition of the non-quenched and tempered long bolt steel to meet 12.80 DETAILED DESCRIPTION

[0022] The application provides a 12.9-grade non-quenched and tempered long bolt steel, which contains the following chemical components in percentage by mass: C: 0.15-0.28%, Si: ≤0.30%, Mn: 1.2-2.0%, P: ≤0.020%, S: ≤0.020%, Cr: 0.1-0.6%, Ni: ≤0.25%, Mo: ≤0.30%, V: 0.04-0.14%, Nb: 0.02-0.07%, Ti: ≤0.06%, Al: 0.01-0.05%, O: ≤0.002%, N: ≤0.025% and the balance of Fe.

[0023] The chemical composition of the 12.9-grade non-quenched and tempered long bolt steel meets 12.80

[0024] The chemical composition of the 12.9 grade non-quenched long rod bolt steel provided by the application includes, in terms of mass percentage, C: 0.15-0.28%, preferably 0.18-0.25%, and more preferably 0.20-0.23%. In the application, the C element is the main strengthening element of the non-quenched steel, and the increase of the content of the C element helps to improve the base body strength and hardenability; by controlling the content of the C element, the insufficient strength caused by the too low content of the C element can be avoided, and at the same time, the deterioration of the plasticity and toughness and the cold working performance of the steel caused by the too high content of the C element can also be avoided.

[0025] The chemical composition of the 12.9 grade non-quenched long rod bolt steel provided by the application includes, in terms of mass percentage, Si: ≤0.30%, preferably 0.05-0.30%, and more preferably 0.06-0.15%. In the application, the Si element is a deoxidizer, and the Si element can be solid-solved in ferrite for solid-solution strengthening, thereby improving the tensile and yield strength of the steel; by controlling the content of the Si element, the insufficient adding effect caused by the too low content of the Si element can be avoided, and at the same time, the sharp increase of the deformation resistance of the steel caused by the too high content of the Si element, which greatly increases the consumption of the die during cold drawing and cold heading, can also be avoided.

[0026] The chemical composition of the 12.9 grade non-quenched long rod bolt steel provided by the application includes, in terms of mass percentage, Mn: 1.2-2.0%, preferably 1.4-1.9%, and more preferably 1.45-1.8%. In the application, the Mn element is an effective element for deoxidization and desulfurization, which improves the hardness and strength of the steel, and the Mn element can improve the stability of the austenite structure, and at the same time, can delay the pearlite phase transition and promote the bainite phase transition; by controlling the content of the Mn element, the bainite structure can be easily generated in the steel, and at the same time, the decrease of the plasticity and toughness of the steel and the excessive residual austenite after phase transition caused by the too high content of the Mn element, which deteriorates the fatigue and delayed fracture performance of the non-quenched steel, can also be avoided.

[0027] The chemical composition of the 12.9 grade non-quenched long rod bolt steel provided by the application includes, in terms of mass percentage, P: ≤0.020%, preferably ≤0.018%, and more preferably ≤0.013%. In the application, P has a strong segregation tendency, and is prone to segregate at the grain boundary when heated at high temperature, which increases the cold brittleness of the steel and reduces the plasticity, and is harmful to the uniformity of the structure and performance of the steel; by controlling the content of the P element to be less than or equal to 0.020%, the damage of the P element to the cold brittleness and plasticity of the steel can be greatly reduced.

[0028] The chemical composition of the 12.9 grade non-quenched long rod bolt steel provided by the application includes S: ≤0.020%, preferably ≤0.015%, and more preferably ≤0.010% by mass. In the application, the S element is also an inevitable harmful element in the steel. Sulfur is easy to form MnS inclusions with manganese in the steel, which causes the steel to be hot brittle. At the same time, the MnS inclusions have a certain damage to the toughness and fatigue performance of the steel, so it is necessary to control the content of the S element to be less than 0.020%.

[0029] The chemical composition of the 12.9 grade non-quenched long rod bolt steel provided by the application includes Cr: 0.1-0.6%, preferably 0.15-0.45%, and more preferably 0.20-0.30% by mass. In the application, the Cr element acts as a ferrite solid solution strengthening element similar to Si and Mn. In addition, the Cr element can effectively improve the hardenability of the steel and delay the bainite phase transition to obtain the required high strength, and can also significantly improve the bainite ferrite hardness through solid solution strengthening. At the same time, the Cr element can also reduce the activity of C, reduce the tendency of decarburization on the surface of the steel during heating, hot rolling and forging, and obtain high fatigue resistance. The Cr element can also greatly reduce the Bs point, so that a certain amount of bainite can be obtained during air cooling, and cracking caused by too fast cooling speed can also be prevented. The C element and the Cr element work together to further reduce the Bs point, ensure that granular bainite and upper bainite are avoided as much as possible during air cooling, obtain lower bainite structure and refine the bainite size, thereby increasing the strength and toughness of the bainite ferrite matrix.

[0030] The chemical composition of the 12.9 grade non-quenched long rod bolt steel provided by the application includes Ni: ≤0.25%, preferably ≤0.20%, and more preferably ≤0.15% by mass. In the application, the Ni element can effectively improve the toughness of the core of the steel, reduce the ductile-brittle transition temperature, and improve the low-temperature impact performance, which is beneficial to the fatigue strength of the steel material. By controlling the content of the Ni element, the steel cost can be reduced while the corresponding modification effect is achieved.

[0031] The chemical composition of the 12.9 grade non-quenched long rod bolt steel provided by the application includes Mo: ≤0.30%, preferably ≤0.26%, and more preferably ≤0.15% by mass. In the application, the Mo element can significantly improve the hardenability of the steel and promote the bainite transformation. The Mo element can form strong carbides to hinder the diffusion of atoms, the movement of dislocations, and the migration of grain boundaries, effectively preventing the recrystallization of deformed austenite. The Mo element carbide particles are small and will not cause stress concentration of the microstructure, which is beneficial to improving the impact toughness of the steel. By controlling the content of the Mo element, the steel cost can be reduced while the corresponding modification effect is achieved.

[0032] The chemical composition of the 12.9-grade non-quenched long rod bolt steel provided by the application includes V: 0.04-0.14%, preferably 0.08-0.12%, and more preferably 0.09-0.11% by mass. In the application, V is a strengthening element in the steel, and V has a strong affinity with C and N to form VC and V(CN), which are dispersedly distributed in the hot rolling stage of the steel and the low-temperature aging stage after the bolt is formed by upsetting, thereby having a strong precipitation strengthening effect to improve the strength; on the other hand, the V element in solid solution can significantly inhibit the diffusion of C element in the process of bainite transformation, and can play a role in refining bainite ferrite to improve the strength and toughness of the steel; by controlling the content of V element, the effect of too low content of V element can be avoided, and the cost increase caused by too high content of V element can also be avoided.

[0033] The chemical composition of the 12.9-grade non-quenched long rod bolt steel provided by the application includes Nb: 0.02-0.07%, preferably 0.03-0.06%, and more preferably 0.04-0.05% by mass. In the application, Nb element and V element are similar, and have a strong affinity with C and N to form NbC and Nb(CN), which are dispersedly distributed in the hot rolling stage of the steel and the low-temperature aging stage after the bolt is formed by upsetting, thereby improving the strength; by controlling the content of Nb element, the effect of too low content of Nb element can be avoided, and the cost increase caused by too high content of Nb element after the effect is saturated can also be avoided.

[0034] The chemical composition of the 12.9-grade non-quenched long rod bolt steel provided by the application includes Ti: ≤0.06%, preferably ≤0.04% by mass. By controlling the content of Ti element, the mechanical properties of the steel can be further improved in the application. In the application, Ti has a strong affinity with C, O and N, and the TiN and TiC phases combined and precipitated during the smelting and solidification of the steel and the hot rolling process slow down the growth rate of austenite grain size, thereby playing a role in refining the grain, and the effect of grain refinement is better when V-Ti is added together; by controlling the content of Ti element, the TiN point inclusions can be avoided when the content of Ti is too high, thereby reducing the impact toughness and fatigue life of the material.

[0035] The chemical composition of the 12.9-grade non-quenched long rod bolt steel provided by the application includes Al: 0.01-0.05%, preferably 0.02-0.04%, and more preferably 0.03% by mass. In the application, A1 element is an effective deoxidizer and can form A1N to refine the grain; by controlling the content of A1 element, the effect of too low content of A1 element can be avoided, and the deterioration of plasticity and toughness of the steel caused by the formation of coarse inclusions when the content of A1 element is too high can also be avoided.

[0036] The 12.9-grade non-quenched long rod bolt steel provided by the application contains, in percentage by mass, O≤0.002%. In the application, the O element almost exists in the form of oxide inclusions in the steel, which is easy to destroy the continuity of the steel matrix and often becomes the starting point of cracks under the action of dynamic load and static load, and greatly harms the toughness, plasticity, fatigue strength and corrosion resistance of the steel, so the oxygen content is controlled to be ≤0.002%.

[0037] The 12.9-grade non-quenched long rod bolt steel provided by the application contains, in percentage by mass, N≤0.025%, preferably ≤0.02%, and more preferably ≤0.015%. In the application, the N element can form compounds with V, Ti and Al, and can promote the precipitation of the V element, reduce the amount of V element, mainly play the role of strengthening the precipitation strengthening effect and refining the grains in the non-quenched steel, and reduce the cost; by controlling the amount of N element, the formation of bubbles and other continuous casting defects caused by too high free nitrogen can be avoided, and the reduction of the performance of the steel caused by the combination of Ti to form liquid-precipitated TiN inclusions can be avoided.

[0038] The 12.9-grade non-quenched long rod bolt steel provided by the application contains, in percentage by mass, N≤0.025%, preferably ≤0.02%, and more preferably ≤0.015%. In the application, the N element can form compounds with V, Ti and Al, and can promote the precipitation of the V element, reduce the amount of V element, mainly play the role of strengthening the precipitation strengthening effect and refining the grains in the non-quenched steel, and reduce the cost; by controlling the amount of N element, the formation of bubbles and other continuous casting defects caused by too high free nitrogen can be avoided, and the reduction of the performance of the steel caused by the combination of Ti to form liquid-precipitated TiN inclusions can be avoided.

[0039] In the application, the chemical composition of the 12.9-grade non-quenched long rod bolt steel satisfies 12.80

[0040] In the application, the chemical composition of the 12.9-grade non-quenched long rod bolt steel preferably satisfies carbon equivalent Ceq<0.7, wherein Ceq=C%+Mn% / 6+(Cr%+V%+Mo%) / 5+(Cu%+Ni%) / 15. In the application, C, Mn, Cr, Mo, V, Ni and Cu are the mass percentages of the elements in the steel. By controlling the parameter Ceq of the carbon equivalent to satisfy the above condition, the application can avoid the increase in the difficulty of material processing, and even the problem of fracture.

[0041] In the application, the base metallographic structure of the 12.9-grade non-quenched long rod bolt steel is preferably a bainite structure; the content of the bainite structure is preferably more than 95% of the total area of the base metallographic structure. The application can make the steel have higher mechanical properties by controlling the metallographic structure, thereby meeting the technical requirements of the 12.9-grade non-quenched long rod bolt steel.

[0042] The C element in the application is a main strengthening element of the non-quenched and tempered steel, which helps to improve the base body strength and hardenability; the Si element is a deoxidizer, which can be dissolved in ferrite for solid solution strengthening, strengthens the ferrite phase, thereby improving the tensile and yield strength of the steel; the Mn element is an effective element for deoxidization and desulfurization, which improves the hardness and strength of the steel, and the Mn element can improve the stability of the austenite structure, and can also delay the pearlite phase change and promote the bainite phase change; the content of P element and S element is controlled, which can reduce the damage to the uniformity of the steel structure and performance, and avoid the hot brittleness of the steel; the Cr element is a ferrite solid solution strengthening element, which has similar effects with Si and Mn, in addition, the Cr element can effectively improve the hardenability of the steel and delay the bainite phase change, so as to obtain the required high strength, and through solid solution strengthening, the bainite ferrite hardness can be significantly improved, and the Cr element can also reduce the activity of the C element, reduce the tendency of surface decarburization of the steel during heating, hot rolling and forging, and obtain high fatigue resistance; the Cr element can greatly reduce the Bs point, so that a certain amount of bainite can be obtained in the air cooling process, and cracking caused by too fast cooling speed can also be prevented; the C and Cr elements have a synergistic effect, further reduce the Bs point, avoid the appearance of granular bainite and upper bainite as much as possible in the air cooling process, obtain lower bainite structure and refine the bainite size, thereby increasing the strength and toughness of the bainite ferrite base body; the Ni element can effectively improve the core toughness of the steel, reduce the ductile-brittle transition temperature, improve the low-temperature impact performance, and is beneficial to the fatigue strength of the steel; the Mo element can significantly improve the hardenability of the steel and promote the bainite transformation; the Mo element can form strong carbides, hinder the diffusion of atoms, the movement of dislocations and the migration of grain boundaries, effectively prevent the recrystallization of deformed austenite, and the Mo element carbide particles are small and will not cause stress concentration of the microstructure structure, which is beneficial to improve the impact toughness of the steel; the V element is a strengthening element, the V element has strong affinity with C and N, can form VC and V(CN), and forms a dispersed distribution in the hot rolling stage of the steel and the low-temperature aging stage after the bolt upsetting forming, has strong precipitation strengthening effect, thereby improving the strength, and during the bainite transformation, the solid solution V element can significantly inhibit the diffusion of C element in the bainite phase change process, can refine the bainite ferrite, and can improve the strength and toughness of the steel; the Nb element is similar to the V element, also has strong affinity with C and N, forms VC and V(CN), forms a dispersed distribution in the hot rolling stage of the steel and the low-temperature aging stage after the bolt upsetting forming, and improves the strength; the Al element is an effective deoxidizer, and can form AlN to refine the grains; the N element can form compounds with V, Ti and Al, the N element can promote the precipitation of V, reduce the amount of V, mainly plays a role in strengthening the precipitation strengthening effect and refining the grains in the non-adjusted steel, and reduces the cost; the content of O element is controlled, which can reduce the content of oxide inclusions, avoid the damage to the continuity of the steel base body, thereby further improving the toughness, plasticity, fatigue strength and corrosion resistance of the steel, etc.By controlling the chemical composition of the non-quenched long rod bolt steel to meet 12.80 < G < 16.0, the content of each element in the steel can be increased to reach the 12.9 grade non-quenched long rod bolt steel.

[0043] The application provides a preparation method of the 12.9 grade non-quenched long rod bolt steel.

[0044] (1) smelting raw materials to obtain a cast ingot;

[0045] (2) sequentially performing hot rolling, drawing and aging treatment on the cast ingot obtained in the step (1) to obtain the 12.9 grade non-quenched long rod bolt steel.

[0046] The application smelts raw materials to obtain a cast ingot. The application does not have special limitations on the specific types and amounts of the raw materials, and can only make the components in the cast ingot meet the chemical composition requirements of the 12.9 grade non-quenched long rod bolt steel.

[0047] In the application, the smelting mode is preferably a converter / electric furnace + refining + VD / RH three-process sequence. The application does not have special limitations on the equipment used in the smelting process and the smelting parameters, which can be determined according to the technical knowledge of those skilled in the art and can only meet the chemical composition requirements of the 12.9 grade non-quenched long rod bolt steel.

[0048] In the application, the state of the cast ingot is preferably a coil. The application does not have special limitations on the parameters of the coil, which can be determined according to the technical needs of those skilled in the art. The application prepares the cast ingot into a coil shape, which is convenient for subsequent hot rolling and drawing processes, so as to obtain a steel material meeting the specifications.

[0049] After obtaining the cast ingot, the application sequentially performs hot rolling, drawing and aging treatment on the cast ingot to obtain the 12.9 grade non-quenched long rod bolt steel.

[0050] The application preferably performs heat preservation treatment on the cast ingot before hot rolling. In the application, the temperature of the heat preservation treatment is preferably 1000-1200 DEG C. The application does not have special limitations on the time of the heat preservation treatment, which can be determined according to the technical knowledge of those skilled in the art. The application performs heat preservation treatment on the cast ingot, which can make the cast ingot be heated uniformly inside and outside, thereby being beneficial to subsequent hot rolling.

[0051] In the present application, the hot rolling opening temperature is preferably 970-1100℃, more preferably 1000-1050℃; the hot rolling wire drawing temperature is preferably 780-900℃, more preferably 800-850℃. In the present application, the hot rolling wire diameter is preferably 7-12, more preferably 8-10. The present application does not have special limitation on the deformation of the hot rolling, which can be determined according to the common technical knowledge in the field. The present application can make the non-quenched and tempered steel wire rod obtain the microstructure with bainite as the matrix, and the bainite structure accounts for more than 95% of the total matrix surface area through the hot rolling process.

[0052] After the hot rolling, the present application preferably cools the hot rolling product to 500-670℃ at a cooling speed of 0.3-2℃ / s, collects and bales, and then cools to room temperature in air.

[0053] In the present application, the drawing reduction is preferably 18%-45%, more preferably 20-40%, and further preferably 25-35%. The present application can make the wire strength after drawing be 86%-94% of the final product strength by controlling the drawing reduction.

[0054] After the drawing, the present application preferably further includes straightening, rolling, wire rolling and surface treatment of the drawn product. The present application does not have special limitation on the specific operation of the straightening, rolling, wire rolling and surface treatment, which can be performed according to the common operation known to the person skilled in the art. The present application can improve the surface quality of the steel wire through the above process.

[0055] In the present application, the aging treatment temperature is preferably 240-300℃, more preferably 250-280℃; the aging treatment holding time is preferably 1-5h, more preferably 2-4h. Before the aging treatment, the matrix structure of the steel material is the local high-density dislocation bainite structure after the drawing work hardening, and the high-density dislocation at this time makes the material have higher strength but poorer plasticity; after the aging treatment, on the one hand, the nanoscale carbon and nitrogen compounds are dispersedly precipitated in the bainite matrix, the precipitates pin the dislocations, and secondary hardening and strengthening effect is produced, on the other hand, the dislocation density at the local high-density dislocation site is reduced, and subgrains are formed, thereby improving the plasticity and toughness; by controlling the aging treatment temperature, it can not only avoid that the heating temperature is too low, the precipitation rate of the dispersed nanoscale carbon and nitrogen compounds is low, and the number of precipitates is small, and the strengthening effect is not produced, but also avoid that the heating temperature is too high, the precipitation rate of the carbon and nitrogen compounds is fast, the precipitates are easy to grow and aggregate, the pinning effect of the dislocations is not produced, and the strengthening effect is weakened; by controlling the aging treatment time, it can not only avoid that the holding time is short, the number of precipitates is small, and the strengthening effect is not produced, but also avoid that the holding time is too long, the precipitates grow, and the strengthening effect is weakened.

[0056] The application adopts the multi-element alloy design idea, comprehensively designs the component of bainite organization strengthening, Nb, V and Al fine grain strengthening, and the low temperature aging precipitation strengthening of Nb and V precipitation strengthening elements after hot rolling and bolt upsetting forming, so as to obtain higher strength and toughness.

[0057] The technical solutions in the application will be clearly and completely described below in combination with the embodiments in the application. Obviously, the described embodiments are only some of the embodiments of the application, not all the embodiments. Based on the embodiments in the application, all the other embodiments obtained by those skilled in the art without creative work belong to the protection scope of the application.

[0058] Examples 1-9 and Comparative Examples 1-8

[0059] The chemical components of the non-quenched long rod bolt steels prepared in Examples 1-9 and Comparative Examples 1-8 are shown in Tables 1 and 2:

[0060] Table 1 Chemical components of non-quenched long rod bolt steels prepared in Examples 1-9 and Comparative Examples 1-8

[0061]

[0062]

[0063] Table 2 Chemical components of non-quenched long rod bolt steels prepared in Examples 1-9 and Comparative Examples 1-8

[0064] Mo % V％ Nb % Ti N％ G Carbon equivalent Ceq Example 1 0.3 0.14 0.03 0.013 0.02 15.97 0.65 Example 2 0.29 0.12 0.06 0.02 0.01 15.10 0.61 Example 3 0.26 0.11 0.1 0 0.005 13.26 0.57 Example 4 0.05 0.08 0.06 0.01 0.005 14.80 0.63 Example 5 0.09 0.08 0.06 0 0.005 14.00 0.62 Example 6 0.15 0.09 0.04 0.04 0.005 14.64 0.56 Example 7 0.13 0.08 0.02 0.01 0.0046 12.80 0.56 Example 8 0.18 0.04 0.06 0.0012 0.005 14.06 0.67 Example 9 0.3 0.08 0.06 0 0.005 14.60 0.69 Comparative Example 1 - 0.06 0.03 0.013 0.005 13.17 0.54 Comparative Example 2 - 0.06 0.03 0.013 0.005 12.71 0.53 Comparative Example 3 - 0.07 0.04 0.013 0.0045 14.42 0.61 Comparative Example 4 - 0.05 0.03 0.013 0.005 14.63 0.68 Comparative Example 5 0.3 0.07 0.04 0.013 0.0045 15.26 0.71 Comparative Example 6 0.09 0.09 0.06 0.013 0.005 12.76 0.58 Comparative Example 7 0.29 0.08 0.06 0.013 0.005 16.16 0.73 Comparative Example 8 - 0.10 0.03 0.013 0.02 15.64 0.61

[0065] The preparation method of the non-quenched long rod bolt steel provided in Examples 1-9 and Comparative Examples 1-8 consists of the following steps:

[0066] (1) After the raw materials are smelted through the converter + refining + VD three-link process, a cast ingot is obtained;

[0067] (2) After the cast ingot obtained in the step (1) is heat rolled at 1100℃, it is first cooled to 600℃ at a cooling rate of 1℃ / s, collected and baled, and then cooled to room temperature in air, and then sequentially subjected to drawing, straightening, rolling, wire rolling, surface treatment and aging treatment, to obtain a non-quenched long rod bolt; the temperature of the aging treatment is 300℃, and the holding time of the aging treatment is 4h.

[0068] The performance of the non-quenched long rod bolts prepared in Examples 1-9 and Comparative Examples 1-8 is tested, and the test standard is GB / T228.1-2021, and the results are shown in Table 3:

[0069] Table 3 Performance of non-quenched long rod bolts prepared in Examples 1-9 and Comparative Examples 1-8

[0070]

[0071] As can be seen from examples 1-9 in table 3, the long rod bolt prepared from the non-quenched and tempered long rod bolt steel provided by the application has a tensile strength of ≥1220MPa and a surface reduction of ≥46%, and the performance meets the requirements of the existing 12.9 grade quenched and tempered bolt.

[0072] As can be seen from comparative examples 1-8 in table 3, in comparative example 1, the G value and carbon equivalent are moderate, the mechanical properties meet the requirements, but the silicon content exceeds the relevant industry standard; in comparative example 2, the G value is small and the carbon equivalent is moderate, and the finished product strength is lower than the 12.9 grade bolt standard; in comparative example 3, the G value and carbon equivalent are moderate, the mechanical properties meet the requirements, but the Mn exceeds the standard, and the cold working cracking rate is high; in comparative example 4, the G value and carbon equivalent are moderate, the mechanical properties meet the requirements, but the S exceeds the standard, and the cold working cracking rate is high; in comparative example 5, the G value is moderate, the carbon equivalent is too large, the mechanical properties meet the requirements, but the wire breaking rate of the material during drawing is high; in comparative example 6, the G value is small and the carbon equivalent is moderate, and the finished product strength is lower than the lower limit of the standard; in comparative example 7, the G value and carbon equivalent are large, and the material processing process is difficult, even broken; in comparative example 8, the G value and carbon equivalent are moderate, but the carbon content exceeds the design range, and the wire breaking rate of the material during drawing is high.

[0073] The above only describes the preferred embodiments of the present application, and it should be noted that for ordinary skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements should be considered as the protection scope of the present application.

Claims

1. A steel for 12.9 grade non-quenched and tempered long rod bolt, comprising the following chemical components in percentage by mass: C: 0.15-0.18%, Si: ≤0.15%, Mn: 1.2-1.9%, P: ≤0.020%, S: ≤0.020%, Cr: 0.1-0.45%, Ni: ≤0.15%, Mo: ≤0.30%, V: 0.04-0.14%, Nb: 0.02-0.07%, Ti: ≤0.06%, Al: 0.01-0.05%, O: ≤0.002%, N: ≤0.025%, and the balance of Fe. The chemical composition of the 12.9 grade non-quenched long rod bolt steel satisfies 12.80 < G < 16.0; wherein, G = 20 * C% + 1.3Si% + 3.3Mn% + 2.5(Cr% + Ni% + Mo%) + 16V% + 8.3Nb% + 50Ti% + 50N%; A method for preparing the steel for 12.9 grade non-quenched and tempered long rod bolt, comprising the following steps: (1) smelting raw materials to obtain a cast ingot; (2) sequentially performing hot rolling, drawing and aging treatment on the cast ingot obtained in the step (1) to obtain the steel for 12.9 grade non-quenched and tempered long rod bolt; The aging treatment in the step (2) is performed at a temperature of 240-300℃ for a holding time of 1-5h.

2. The 12.9 grade non-quenched and tempered long rod bolt steel according to claim 1, characterized in that, The steel for 12.9 grade non-quenched and tempered long rod bolt comprises the following chemical components in percentage by mass: C: 0.18%, Si: ≤0.15%, Mn: 1.4-1.9%, P: ≤0.020%, S: ≤0.020%, Cr: 0.15-0.45%, Ni: ≤0.15%, Mo: ≤0.30%, V: 0.08-0.12%, Nb: 0.02-0.07%, Ti: ≤0.06%, Al: 0.01-0.05%, O: ≤0.002%, N: ≤0.025%, and the balance of Fe.

3. The 12.9 grade non-quenched and tempered long rod bolt steel according to claim 1 or 2, characterized in that, The chemical components of the steel for 12.9 grade non-quenched and tempered long rod bolt satisfy a carbon equivalent Ceq < 0.7, wherein Ceq = C% + Mn% / 6 + (Cr% + V% + Mo%) / 5 + (Cu% + Ni%) / 15.

4. The 12.9 grade non-quenched and tempered long rod bolt steel according to claim 1 or 2, characterized in that, The base metallographic structure of the steel for 12.9 grade non-quenched and tempered long rod bolt is bainite structure. 5.A method for preparing the steel for 12.9 grade non-quenched and tempered long rod bolt according to any one of claims 1-4, comprising the following steps: (1) smelting raw materials to obtain a cast ingot; (2) sequentially performing hot rolling, drawing and aging treatment on the cast ingot obtained in the step (1) to obtain the steel for 12.9 grade non-quenched and tempered long rod bolt; The aging treatment in the step (2) is performed at a temperature of 240-300℃ for a holding time of 1-5h.

6. The preparation method according to claim 5, characterized in that, The smelting in the step (1) is performed by a converter / electric furnace + refining + VD / RH three-process sequence.

7. The preparation method according to claim 5, characterized in that, The hot rolling in the step (2) is performed at a starting temperature of 970-1100℃ and a wire-rod temperature of 780-900℃.

8. The preparation method according to claim 5, characterized in that, The drawing in the step (2) is performed at a reduction of area of 18%-45%.

9. The preparation method according to claim 5, characterized in that, After the drawing in the step (2), the drawn product is further subjected to straightening, rolling, thread rolling and surface treatment.

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