Method for improving maximum tensile resistance stability of extra-high voltage insulator steel foot

Abstract

The invention belongs to the technical field of steel smelting, rolling and forging processing, and discloses a method for improving the maximum tensile resistance stability of an extra-high voltage insulator steel foot, the steel grade is 45Mn2, and the production process comprises the steps of converter smelting, LF refining, RH vacuum treatment, continuous casting, rolling, straightening, blanking, forging and cooling. By controlling the steel-making DiZ value, controlling the out-of-roundness and the dimensional tolerance through steel rolling, controlling the material bending degree through straightening and controlling the final forging temperature and the cooling rate after forging, the stability of the maximum tensile resistance of insulator steel feet of different batches can be greatly improved, and the fluctuation range of the maximum tensile resistance compared with a reference value is controlled within + / -4.5 KN.

Description

Technical Field

[0001] This invention belongs to the field of steel smelting, rolling and forging technology, and specifically relates to a method for improving the maximum tensile strength stability of steel feet of ultra-high voltage insulators. Background Technology

[0002] To improve the transmission capacity of power lines and reduce energy loss, countries around the world are researching and developing ultra-high voltage (UHV) lines. As the conductor diameter of UHV lines increases, so does their weight, necessitating the use of higher strength insulators to reduce the number of insulators used in the lines, lower tower loads, simplify line connections, and improve the safety factor of the lines. The development of UHV technology has placed higher demands on the insulator industry. Currently, 840 (760) strength grade insulators are the highest grade insulators both domestically and internationally.

[0003] As the core load-bearing connector between the insulator string and the tower in overhead transmission lines, the steel foot of the UHV insulator bears multiple mechanical stresses such as conductor tension, wind load, snow and ice load, and vibration fatigue for a long time. Its mechanical properties, especially the stability of the maximum tensile strength, are directly related to the safe operation of the entire line. The 840 (760) strength grade steel foot must have good toughness while ensuring sufficient strength, and its performance uniformity is crucial. The mechanical strength of the insulator parts, from high to low, is as follows: glass parts, adhesive, iron cap, and steel foot. The mechanical properties of the steel foot are the final mechanical properties that the insulator string can bear. At present, in production and application, the maximum tensile strength of this type of steel foot still fluctuates to a certain extent, which may lead to premature failure or performance redundancy of some components in actual service, becoming a potential safety hazard and quality control difficulty for the UHV power grid.

[0004] From both domestic and international perspectives, the manufacturing of high-end insulator steel feet is developing towards "high strength, high consistency, and high durability." Leading international companies have achieved narrow fluctuations in the mechanical properties of their products through refined material design and comprehensive process control. In contrast, domestic production still has room for improvement in the uniformity of basic material structure and process stability, particularly in how to systematically control the parameter dispersion at each stage from smelting and rolling to hot processing—a common challenge faced by the industry. Summary of the Invention

[0005] This invention provides a method to improve the stability of the maximum tensile strength of steel feet of 840(760) strength grade ultra-high voltage insulators. The aim is to control the performance fluctuation range within ±4.5KN by controlling the DiZ value, controlling the out-of-roundness and dimensional tolerance of the rolled steel, controlling the material curvature by straightening, and controlling the final forging temperature and cooling rate after forging, so as to meet the stringent requirements of ultra-high voltage power transmission projects for the reliability of key components.

[0006] The technical solution adopted to achieve the purpose of this invention is as follows: a method for improving the maximum tensile strength stability of steel feet of 840 (760) strength grade ultra-high voltage insulators, comprising the following steps performed in sequence: converter smelting, LF refining, RH vacuum treatment, continuous casting, rolling, straightening, blanking, forging, and cooling;

[0007] (1) There is no need to specifically control residual elements such as Cr, Ni, and Mo when tapping steel from the converter. There are no restrictions on the type of scrap steel. The ratio of scrap steel to molten iron can be freely matched according to the production rhythm. The DiZ value is controlled between 53.5 and 57.5 mm at the end of LF refining.

[0008] Where, DiZ value = 25.4*(0.265*[C]*[C]+0.001*[C]+0.171)*(0.7*[Si]+1)*(5.1*[Mn]-1.12)*(2.16*[Cr]+1)*(0.363*[Ni]+1)*(3*[Mo]+1)*(0.365*[Cu]+1)*(1.73*[V]+1)*(1+2.5*[Zr]), and [X] is the mass content of the metal element, where X represents the metal element.

[0009] LF refining allows for flexible adjustment of the main elements C, Mn, and Cr based on the residual element content of the molten steel, ensuring that the DiZ value at the LF outlet falls within the required range. This reduces the raw material requirements for the converter process, thereby lowering production costs and improving converter production efficiency.

[0010] (2) Before the RH vacuum treatment, the composition of the molten steel is sampled and tested. Based on the real-time composition and DiZ value, the DiZ is finely adjusted by feeding carbon wire and metallic manganese or extending the vacuum treatment time to control the target of 55.0~56.0mm.

[0011] Specifically, if the DiZ value of the molten steel before RH vacuum treatment is lower than 55.0, then carbon wire is fed or metallic manganese is added to increase the C or Mn content, thereby ensuring that the final DiZ value is within the range of 55.0 to 56.0. If the DiZ value of the molten steel before RH vacuum treatment is higher than 56, then the vacuum treatment time is extended, and the gas flow rate is adjusted and increased to achieve the effect of vacuum demanganese removal, reducing the Mn content and thus lowering the DiZ value, thereby ensuring that the DiZ value is within the range of 55.0 to 56.0. If the DiZ value is between 55.0 and 56.0, then no adjustment is required.

[0012] (3) The finished product dimensional accuracy of the rolling process is ±0.1mm, and the out-of-roundness is ≤0.15mm;

[0013] Excessive out-of-roundness or abrupt dimensional changes can easily generate uneven stress fields during machining, forming harmful residual stress concentration points. These points can become the origin of fatigue cracks or lead to deformation cracking, directly impairing performance stability.

[0014] (4) The straightening process shall control the bending degree of the steel to be ≤1.5mm / m;

[0015] Bar stock with good curvature results in smaller weight deviations after blanking, leading to more uniform deformation across different parts during subsequent forging processes. This avoids uneven internal structure (such as grain size and texture) caused by localized over-deformation or under-deformation, thus laying the foundation for consistent performance.

[0016] (5) The final forging temperature is controlled at 910~950℃;

[0017] This temperature range avoids the risk of abnormally coarse grains or even grain boundary oxidation. At the same time, it ensures that the final product at the end of forging is fine, deformed austenitic grains.

[0018] (6) Immediately after forging, the steel foot is placed in an insulated box. The insulated box includes a box body and a cover plate. The cover plate closes to the open end of the box body and has a through hole. The cooling rate of the insulated box is controlled at ≤62℃ / h until the temperature of the steel foot reaches 300℃. The box body and cover plate of the insulated box have a three-layer structure: the outermost layer is steel, the middle layer is a heat insulation layer, and the innermost layer is a heat-resistant layer. When the temperature reaches 300℃, the cover plate is opened for cooling.

[0019] Immediately after forging, slow cooling is performed with strictly controlled cooling rate. This transforms austenite into a mixed structure of pearlite (P) and ferrite (F). This structure has moderate hardness and good plasticity, greatly reducing the internal stress and hardness of the material, while maximizing the elimination of forging internal stress. Ultimately, this ensures that each insulator steel foot has highly stable and reliable strength.

[0020] This invention relates to a steel grade of 45Mn2, which comprises, by weight: C 0.42-0.49%, Si 0.17-0.37%, Mn 1.20-1.80%, Cr ≤0.30%, Ni ≤0.30%, Mo ≤0.10%, Cu ≤0.30%, V ≤0.05%, Zr ≤0.05%, with the balance being Fe and unavoidable impurities.

[0021] Compared with existing technologies, this invention's process, by controlling the DiZ value, eliminates the need for specific control of residual elements such as Cr, Ni, and Mo in the LF refining process. The DiZ value can be flexibly adjusted based on the residual element content of the molten steel to meet the required range, thus reducing the raw material requirements for the converter process, lowering production costs, and improving converter efficiency. In the RH process, the DiZ value is finely adjusted by feeding carbon wire, metallic manganese, or extending the vacuum treatment time, based on real-time composition and the DiZ value. Rolling controls out-of-roundness and dimensional tolerances, while straightening controls material curvature, ensuring minimal weight deviation in the blanked sections. This results in more uniform deformation across different parts during subsequent forging, laying the foundation for performance uniformity. After forging, controlling the final forging temperature and cooling rate ensures that each insulator steel leg has highly stable and reliable strength. Ultimately, this significantly improves the stability of the maximum tensile strength of insulator steel legs between different batches, controlling the fluctuation range of the maximum tensile strength from the reference value to within ±4.5 kN. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of an insulation box used for collecting insulator steel feet after forging, as shown in the example.

[0023] Figure 2 This is a schematic diagram of the insulation box body and cover plate structure for collecting insulator steel feet after forging, as shown in the example.

[0024] Figure 3 This is a schematic diagram of a standard collection box after the steel feet of an insulator have been forged for comparison.

[0025] In this design, 1 represents the housing, 2 represents the cover plate, and 3 represents the through hole on the cover plate. Detailed Implementation

[0026] This invention is not limited to the specific embodiments listed below. Those skilled in the art can implement this invention using various other specific embodiments based on the content disclosed herein. Any modifications or alterations made to the design structure and concept of this invention fall within the protection scope of this invention. It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other.

[0027] This embodiment and comparative example illustrate the comprehensive control effect of the present invention by describing the smelting, rolling and forging process of 840 (760) strength grade ultra-high voltage insulator steel feet.

[0028] The present invention will be further described in detail below with reference to the embodiments:

[0029] Example 1

[0030] (1) The residual Cr of 0.18%, residual Ni, Mo, etc. can be ignored when the steel is tapped from the converter;

[0031] (2) The measured values ​​of C 0.45%, Si 0.22%, Mn 1.38%, Cr 0.18%, Ni 0.01%, Mo 0.01%, V 0.003%, Cu 0.02%, and Zr 0.001% at the LF refining station were 56.9.

[0032] (3) The RH vacuum treatment process includes: increasing the gas flow rate by 80 Nm³. 3 / h, RH vacuum treatment time 10min, sample taken two minutes before rupture of vacuum to detect composition C 0.45%, Si 0.22%, Mn 1.37%, Cr 0.18%, Ni 0.01%, Mo 0.01%, V 0.003%, Cu 0.02%, Zr 0.001%, DiZ value at this time 56.4; extend total vacuum treatment time to 14min, rupture sample detected composition C 0.45%, Si 0.22%, Mn 1.36%, Cr 0.18%, Ni 0.01%, Mo 0.01%, V 0.003%, Cu 0.02%, Zr 0.001%, DiZ value at this time 55.9;

[0033] (4) The out-of-roundness of the rolled steel product is 0.12mm, and the size range is φ39.95~φ40.07mm, to obtain steel bars;

[0034] (5) The bending degree of the straightened steel is 1.0 mm / m;

[0035] (6) The final forging temperature after forging is 930℃;

[0036] (7) Immediately after forging, place the steel foot into the heat preservation box. The structure of the heat preservation box is as follows: Figure 1 , Figure 2 As shown, the box includes a housing and a cover. The cover fits over the opening of the housing and has a through hole. The housing and cover of the insulated box have a three-layer structure: the outermost layer is steel (Q235B), the middle layer is a heat insulation layer (slag wool), and the innermost layer is a heat-resistant layer (magnesia-carbon brick). The controlled cooling rate of the insulated box is ≤62℃ / h, and the actual cooling rate is 55℃ / h. When the temperature of the steel feet reaches 300℃, the cover is opened for cooling.

[0037] Example 2

[0038] (1) The residual Cr of 0.18%, residual Ni, Mo, etc. can be ignored when the steel is tapped from the converter;

[0039] (2) The measured values ​​of C, Si, Mn, Cr, Ni, Mo, V, Cu, and Zr at the LF refining station were 0.47%, 0.22%, 1.29%, 0.18%, 0.01%, 0.01%, 0.003%, 0.02%, and 0.001%, respectively. The DiZ value at this time was 53.6.

[0040] (3) The RH vacuum treatment process includes: increasing the gas flow rate by 80 Nm³. 3 / h, RH vacuum treatment time 10min. Two minutes before vacuum breaking, samples were taken for composition analysis: C 0.46%, Si 0.22%, Mn 1.28%, Cr 0.18%, Ni 0.01%, Mo 0.01%, V 0.003%, Cu 0.02%, Zr 0.001%. At this time, the DiZ value was 52.6. After 10min, normal vacuum breaking was performed. The composition of the vacuum-broken sample was then analyzed: C 0.46%, Si 0.22%, Mn 1.28%, Cr 0.18%, Ni 0.01%. The composition of the steel is as follows: C 0.46%, Mo 0.01%, V 0.003%, Cu 0.02%, Zr 0.001%. At this point, the DiZ value is still 52.6. After adding 70.6 kg of metallic manganese (total steel volume 130t), and after soft blowing for 10 min, a sample is taken to test the composition: C 0.46%, Si 0.22%, Mn 1.33%, Cr 0.18%, Ni 0.01%, Mo 0.01%, V 0.003%, Cu 0.02%, Zr 0.001%. At this point, the DiZ value is 55.0.

[0041] (4) The out-of-roundness of the rolled steel product is 0.12mm, and the size range is φ39.95~φ40.07mm, to obtain steel bars;

[0042] (5) The bending degree of the straightened steel is 1.0 mm / m;

[0043] (6) The final forging temperature after forging is 930℃;

[0044] (7) Immediately after forging, the steel foot is placed in an insulated box. The insulated box includes a box body and a cover plate. The cover plate is closed on the open end of the box body and has a through hole. The box body and cover plate of the insulated box have a three-layer structure. The outermost layer is steel (Q235B), the middle layer is a heat insulation layer (slag wool), and the inner layer is a heat-resistant layer (magnesia-carbon brick). The cooling rate of the insulated box is controlled at ≤62℃ / h, and the actual cooling rate is 55℃ / h. When the temperature of the steel foot reaches 300℃, the cover plate is opened for cooling.

[0045] Example 3

[0046] (1) When the steel is tapped from the converter, the residual Cr is 0.16%, the residual Mo is 0.04%, the residual Ni is 0.05%, and other residues can be ignored;

[0047] (2) The measured values ​​of C 0.45%, Si 0.22%, Mn 1.31%, Cr 0.16%, Ni 0.05%, Mo 0.04%, V 0.003%, Cu 0.02%, and Zr 0.001% at the LF refining station were 57.2.

[0048] (3) The RH vacuum treatment process includes: increasing the gas flow rate by 80 Nm³.3 / h, RH vacuum treatment time 10min, sample taken two minutes before rupture to detect composition C 0.45%, Si 0.22%, Mn 1.30%, Cr 0.16%, Ni 0.05%, Mo 0.04%, V 0.003%, Cu 0.02%, Zr 0.001%, DiZ value at this time 56.6; extend total vacuum treatment time to 15min, rupture sample detected composition C 0.45%, Si 0.22%, Mn 1.28%, Cr 0.16%, Ni 0.05%, Mo 0.04%, V 0.003%, Cu 0.02%, Zr 0.001%, DiZ value at this time 55.6;

[0049] Steps (4) to (7) are the same as in Example 1.

[0050] Example 4

[0051] (1) The residual Cr is 0.06% and the residual Mo is 0.04% when the steel is tapped from the converter. Other residues can be ignored.

[0052] (2) The measured values ​​of C, Si, Mn, Cr, Ni, Mo, V, Cu, and Zr at the LF refining station were 0.43%, 0.22%, 1.49%, 0.06%, 0.01%, 0.04%, 0.003%, 0.02%, and 0.001%, respectively. The DiZ value was 54.0 at this time.

[0053] (3) The RH vacuum treatment process includes: increasing the gas flow rate by 80 Nm³. 3 / h, RH vacuum treatment time 10min. Two minutes before vacuum breaking, samples were taken for composition analysis: C 0.42%, Si 0.22%, Mn 1.48%, Cr 0.06%, Ni 0.01%, Mo 0.04%, V 0.003%, Cu 0.02%, Zr 0.001%. At this time, the DiZ value was 53.0. After 10min, normal vacuum breaking was performed. The composition of the vacuum-broken sample was analyzed: C 0.42%, Si 0.22%, Mn 1.48%, Cr 0.06%, Ni 0.01%, Mo 0.04%. With V 0.003%, Cu 0.02%, and Zr 0.001%, the DiZ value is still 53.0. 28.2 kg of metallic manganese is added, and 120 meters of carbon wire (equivalent to 42 kg) is fed in (total steel volume 130t). After soft blowing for 10 minutes, a sample is taken to test the composition: C 0.45%, Si 0.22%, Mn 1.50%, Cr 0.06%, Ni 0.01%, Mo 0.04%, V 0.003%, Cu 0.02%, and Zr 0.001%. At this point, the DiZ value is 55.5.

[0054] Steps (4) to (7) are the same as in Example 1.

[0055] Example 5

[0056] In step (4), the out-of-roundness of the rolled steel product is 0.15mm, and the size range is φ39.94~φ40.09mm. The rest of the operation is the same as in Example 1.

[0057] Example 6

[0058] In step (5), the bending degree of the straightened steel is 1.5 mm / m, and the rest of the operation is the same as in Example 1.

[0059] Example 7

[0060] In step (6), the final forging temperature after forging is 950℃, and the remaining operations are the same as in Example 1.

[0061] Example 8

[0062] In step (7), the actual cooling rate after forging is 62℃ / h, and the rest of the operation is the same as in Example 1.

[0063] Comparative Example 1

[0064] (1) The residual Cr of 0.18%, residual Ni, Mo, etc. can be ignored when the steel is tapped from the converter;

[0065] (2) The measured values ​​of C, Si, Mn, Cr, Ni, Mo, V, Cu, and Zr at the LF refining station were 0.45%, 0.22%, 1.42%, 0.18%, 0.01%, 0.01%, 0.003%, 0.02%, and 0.001%, respectively. The DiZ value was 58.9 at this time.

[0066] (3) The RH vacuum treatment process includes: increasing the gas flow rate by 80 Nm³. 3 / h, RH vacuum treatment time 10min. Two minutes before vacuum breaking, samples were taken to detect the composition: C 0.45%, Si 0.22%, Mn 1.41%, Cr 0.18%, Ni 0.01%, Mo 0.01%, V 0.003%, Cu 0.02%, Zr 0.001%. At this point, the DiZ value was 58.4. The total vacuum treatment time was extended to 14min, and samples were taken again to detect the composition: C 0.45%, Si 0.22%, Mn 1.38%, Cr 0.18%. The composition of the sample was 0.01% Ni, 0.01% Mo, 0.003% V, 0.02% Cu, and 0.001% Zr, with a DiZ value of 56.9. The total vacuum treatment time was further extended to 20 minutes, and the composition of the sample after vacuum treatment was determined to be 0.45% C, 0.22% Si, 1.38% Mn, 0.18% Cr, 0.01% Ni, 0.01% Mo, 0.003% V, 0.02% Cu, and 0.001% Zr. The DiZ value remained at 56.9.

[0067] Steps (4) to (7) are the same as in Example 1.

[0068] Comparative Example 2

[0069] (1) The residual Cr of 0.18%, residual Ni, Mo, etc. can be ignored when the steel is tapped from the converter;

[0070] (2) The measured values ​​of C, Si, Mn, Cr, Ni, Mo, V, Cu, and Zr at the LF refining station were 0.45%, 0.22%, 1.30%, 0.18%, 0.01%, 0.01%, 0.003%, 0.02%, and 0.001%, respectively. The DiZ value was 53.0 at this time.

[0071] (3) The RH vacuum treatment process includes: increasing the gas flow rate by 80 Nm³. 3 / h, RH vacuum treatment time 10min, sample taken two minutes before rupture to detect composition C 0.45%, Si 0.22%, Mn 1.29%, Cr 0.18%, Ni 0.01%, Mo 0.01%, V 0.003%, Cu 0.02%, Zr 0.001%, DiZ value at this time 52.5, rupture normally after 10min, rupture sample detected composition C 0.45%, Si 0.22%, Mn 1.29%, Cr 0.18%, DiZ value at this time still 52.5, no further fine-tuning of composition by carbon wire and metallic manganese;

[0072] Steps (4) to (7) are the same as in Example 1.

[0073] Comparative Example 3

[0074] In Example 1, the phrase "the out-of-roundness of the rolled steel product is 0.12 mm, and the size range is φ39.95~φ40.07 mm" is modified to "the out-of-roundness of the rolled steel product is 0.20 mm, and the size range is φ39.95~φ40.15 mm", while other conditions remain the same as in Example 1.

[0075] Comparative Example 4

[0076] In Example 1, the phrase "the out-of-roundness of the rolled steel product is 0.12 mm, and the size range is φ39.95~φ40.07 mm" is modified to "the out-of-roundness of the rolled steel product is 0.22 mm, and the size range is φ39.85~φ40.07 mm", while other conditions remain the same as in Example 1.

[0077] Comparative Example 5

[0078] In Example 1, “the bending degree of the straightened steel is 1.0 mm / m” is modified to “the bending degree of the straightened steel is 2.0 mm / m”, and other conditions are the same as in Example 1.

[0079] Comparative Example 6

[0080] In Example 1, "final forging temperature after forging is 930℃" is modified to "final forging temperature after forging is 1000℃", and other conditions are the same as in Example 1.

[0081] Comparative Example 7

[0082] In Example 1, "final forging temperature after forging is 930℃" is modified to "final forging temperature after forging is 900℃", and other conditions are the same as in Example 1.

[0083] Comparative Example 8

[0084] The description in Example 1, "Immediately after forging, the steel foot is placed in a specially designed insulated box. The insulated box has a three-layer structure: the outermost layer is steel, the middle layer is a heat insulation layer, and the inner layer is a heat-resistant layer. The cover is a movable steel heat-insulating cover with round holes. The actual cooling rate is 55℃ / h. When the temperature of the steel foot reaches 300℃, the cover is opened for cooling," is modified to "After forging, it is placed in a common collection box (made of Q235B) for sheltered cooling." Other conditions are the same as in Example 1. The structure of the common collection box is as follows: Figure 3 As shown, the top is open and uncovered.

[0085] Comparative Example 9

[0086] The statement in Example 1, "Immediately after forging, the steel foot is placed in a heat-insulating box, which has a three-layer structure: an outermost steel layer, a middle heat-insulating layer, and an inner heat-resistant layer. The cover is a movable steel heat-insulating cover with round holes. The actual cooling rate is 55℃ / h. When the temperature of the steel foot reaches 300℃, the cover is opened for cooling," is modified to "Immediately after forging, the steel foot is placed in a heat-insulating box without a cover. The actual cooling rate is 90℃ / h." All other conditions remain the same as in Example 1.

[0087] The DiZ values ​​of the steel rods prepared in Examples 1-8 and Comparative Examples 1-9 and the maximum tensile strength of the steel feet of the finished insulators were tested. The test standard was GB / T 775.3, and the test results are shown in Table 1.

[0088] Table 1. DiZ values ​​of steel rods obtained in Examples 1-8 and Comparative Examples 1-9, and maximum tensile strength of finished insulator steel legs.

[0089]

[0090] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for improving the stability of the maximum tensile force of steel feet in ultra-high voltage insulators, characterized in that: The steel grade is 45Mn2, composed of the following mass percentages: C 0.42-0.49%, Si 0.17-0.37%, Mn 1.20-1.80%, Cr ≤0.30%, Ni ≤0.30%, Mo ≤0.10%, Cu ≤0.30%, V ≤0.05%, Zr ≤0.05%, with the balance being Fe and unavoidable impurities; the method includes the following steps: converter smelting, LF refining, RH vacuum treatment, continuous casting, rolling, straightening, blanking, forging, and cooling; In the LF process, there is no need to specifically control residual elements such as Cr, Ni, and Mo. In the final stage of LF refining, the DiZ value is controlled between 53.5 and 57.5 mm. The DiZ value is calculated as follows: DiZ value = 25.4 * (0.265 * [C] * [C] + 0.001 * [C] + 0.171) * (0.7 * [Si] + 1) * (5.1 * [Mn] - 1.12) * (2.16 * [Cr] + 1) * (0.363 * [Ni] + 1) * (3 * [Mo] + 1) * (0.365 * [Cu] + 1) * (1.73 * [V] + 1) * (1 + 2.5 * [Zr]), where [X] is the mass content of the metal element, and X represents the metal element. Before the vacuum treatment, the composition of the molten steel is sampled and analyzed. Based on the real-time composition and DiZ value, the DiZ value is finely adjusted by feeding carbon wire and metallic manganese, or by extending the vacuum treatment time, to control the target of 55.0~56.0mm. The dimensional accuracy of the finished product during the rolling process is ±0.1mm, and the out-of-roundness is ≤0.15mm; The straightening process controls the steel curvature to be ≤1.5mm / m; The final forging temperature is controlled at 910~950℃; Immediately after forging, the steel foot is placed in a covered heat-insulating box for cooling, with the cooling rate controlled at ≤62℃ / h, until the temperature of the steel foot reaches 300℃, at which point the cover is opened for further cooling.

2. The method for improving the maximum tensile strength stability of UHV insulator steel feet as described in claim 1, characterized in that: If the DiZ value of the molten steel before RH vacuum treatment is below 55.0, then feed carbon wire or add metallic manganese to increase the DiZ value to 55.0~56.0; if the DiZ value of the molten steel before RH vacuum treatment is above 56.0, then extend the vacuum treatment time and adjust the gas flow rate to decrease the DiZ value to 55.0~56.0; if the DiZ value is between 55.0 and 56.0, no adjustment is needed.

3. The method for improving the maximum tensile strength stability of UHV insulator steel feet as described in claim 1, characterized in that: The insulated box includes a box body and a cover plate. The cover plate covers the open end of the box body and has a through hole.

4. The method for improving the maximum tensile strength stability of UHV insulator steel feet as described in claim 1 or 3, characterized in that: The insulated box has a three-layer structure: the outermost layer is steel, the middle layer is a heat insulation layer, and the innermost layer is a heat-resistant layer.

5. The method for improving the maximum tensile strength stability of UHV insulator steel feet as described in claim 1, characterized in that: The steel feet of the ultra-high voltage insulator are of strength grade 840 (760).

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