Method for optimizing structure and improving strength of round steel for sucker rod

Abstract

The invention discloses a structure optimization and strength improvement method for round steel for a sucker rod, and belongs to the technical field of steel for oil exploitation. According to the method, the chemical components of the round steel are optimized, alloy elements and microalloy elements in a specific proportion are added, the round steel structure is optimized by combining a reasonable rolling process and a reasonable heat treatment process, and the strength, toughness, fatigue resistance and corrosion resistance of the round steel are remarkably improved. Experiments prove that all performance indexes of the round steel for the sucker rod prepared by the method are superior to those of traditional round steel, the use requirements of the sucker rod in a complex underground environment can be better met, the service life of the sucker rod is prolonged, and the round steel has high practical value and popularization prospects.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of steel materials for oil exploitation, and particularly relates to a structure optimization and strength improvement method for round steel for sucker rods. BACKGROUND

[0002] The sucker rod is an important component for connecting the pumping unit and the oil well pump in oil exploitation, and has a poor working environment, needs to bear alternating loads, stretching, bending and various forces, and is also eroded by corrosive media in the well. The performance of the sucker rod mainly depends on the quality of the round steel used. The traditional round steel for the sucker rod has defects in strength, toughness and fatigue resistance, and cannot meet the long-term use requirements in the complex downhole environment, and often has problems such as fracture and deformation, which shortens the service life of the sucker rod and increases the production cost of the oil field.

[0003] At present, the methods for improving the performance of the round steel for the sucker rod mainly focus on single component adjustment or process improvement. For example, the strength is improved by increasing the carbon content, but the toughness is reduced; the conventional rolling process is used, the internal grains of the round steel are coarse, and the structure is not dense enough, which affects the performance. These methods cannot realize the synergistic optimization of the structure and performance of the round steel, and are difficult to significantly improve the comprehensive performance of the round steel. Therefore, it is of great significance to develop a method which can optimize from the chemical composition, rolling process, heat treatment process and the like, realize the structure refinement and strength improvement of the round steel. SUMMARY

[0004] In order to overcome the problem of insufficient comprehensive performance of the round steel for the sucker rod in the prior art, the purpose of the present application is to provide a structure optimization and strength improvement method for round steel for sucker rods.

[0005] To solve the above technical problems, the technical scheme adopted by the present application is as follows:

[0006] The structure optimization and strength improvement method for round steel for sucker rods provided by the present application comprises the following steps:

[0007] (1) Chemical composition optimization: taking iron as the matrix, the chemical composition of the round steel is as follows in terms of mass percentage: C 0.35%-0.45%, Si 0.20%-0.40%, Mn 0.70%-1.00%, Cr 0.80%-1.20%, Ni 0.40%-0.60%, Mo 0.15%-0.25%, Nb 0.02%-0.06%, Ti 0.01%-0.03%, V 0.03%-0.05%, and the rest is Fe and inevitable impurities, wherein S≤0.015% and P≤0.020% in the impurities;

[0008] (2) Smelting and casting: ingredients are prepared according to the above chemical composition, smelting process of electric arc furnace + LF refining furnace is adopted, electric arc furnace smelting temperature is controlled at 1550-1600℃, LF refining furnace refining temperature is 1500-1550℃, refining time is 30-45 minutes, then continuous casting is carried out, continuous casting temperature is 1450-1500℃, and the blank drawing speed is 0.8-1.2 m / min, so as to obtain round billet;

[0009] (3) Rolling process optimization: the round billet is heated to 1150-1250℃, and then multi-pass rolling is carried out after 2-3 hours of heat preservation, the first pass reduction rate is controlled at 25%-30%, the reduction rate of each subsequent pass is gradually reduced and controlled at 15%-20%, the finish rolling temperature is 850-900℃, and after rolling, water mist cooling is adopted, the cooling speed is 10-15℃ / s, and the cooling is stopped after cooling to 300-400℃;

[0010] (4) Heat treatment process: the round steel after rolling is subjected to quenching and tempering heat treatment, first heated to 860-900℃, heat preservation for 40-60 minutes, then oil quenching, after quenching, heated to 550-600℃, heat preservation for 90-120 minutes, and then air cooling;

[0011] (5) Structure refining treatment: low temperature rolling is carried out on the round steel after heat treatment, rolling temperature is 200-300℃, reduction rate is 5%-10%, further refining grains and improving structure density. Low temperature rolling can further refine grains, increase dislocation density and improve structure density without changing the overall organization type of the steel, so as to further improve the strength and fatigue resistance of the round steel.

[0012] Further, the chemical composition of the round steel is as follows in terms of mass percentage: C 0.35%, Si 0.20%, Mn 0.70%, Cr 0.80%, Ni 0.40%, Mo 0.15%, Nb 0.02%, Ti 0.01%, V 0.03%, the rest is Fe and inevitable impurities, wherein S is 0.012%, and P is 0.018%.

[0013] Further, the chemical composition of the round steel is as follows in terms of mass percentage: C 0.40%, Si 0.30%, Mn 0.85%, Cr 1.00%, Ni 0.50%, Mo 0.20%, Nb 0.04%, Ti 0.02%, V 0.04%, the rest is Fe and inevitable impurities, wherein S is 0.010%, and P is 0.015%.

[0014] Further, the chemical composition of the round steel is as follows in terms of mass percentage: C 0.45%, Si 0.40%, Mn 1.00%, Cr 1.20%, Ni 0.60%, Mo 0.25%, Nb 0.06%, Ti 0.03%, V 0.05%, and the balance of Fe and inevitable impurities, wherein S is 0.008% and P is 0.012%.

[0015] Further, the round steel is prepared according to the following steps.

[0016] Smelting and casting: according to the component allocation, the arc furnace smelting temperature is 1550℃, the LF refining furnace refining temperature is 1500℃, the refining time is 30 minutes, the continuous casting temperature is 1450℃, and the blank drawing speed is 0.8m / min, so that the round billet is obtained.

[0017] Rolling process optimization: the round billet is heated to 1150℃ and kept for 2 hours, 5 passes of rolling are performed, the first pass reduction is 25%, the reduction of each subsequent pass is 15%, the finish rolling temperature is 850℃, water mist cooling is performed after rolling, the cooling speed is 10℃ / s, and the round steel is air cooled after being cooled to 300℃;

[0018] Heat treatment process: the round steel after rolling is heated to 860℃ and kept for 40 minutes, oil quenching is performed with a cooling speed of 50℃ / s, then the round steel is heated to 550℃ and kept for 90 minutes, and air cooling is performed;

[0019] Structure refining treatment: the round steel after heat treatment is subjected to low-temperature rolling by using a two-high reversible mill, the rolling temperature is 200℃, and the reduction is 5%;

[0020] Performance test result: the tensile strength is 920MPa, the yield strength is 780MPa, the elongation is 18%, and the impact toughness at-20℃ is 65J / cm 2 .

[0021] Further, the round steel is prepared according to the following steps.

[0022] Smelting and casting: according to the component allocation, the arc furnace smelting temperature is 1575℃, the LF refining furnace refining temperature is 1525℃, the refining time is 37 minutes, the continuous casting temperature is 1475℃, and the blank drawing speed is 1.0m / min, so that the round billet is obtained.

[0023] Rolling process optimization: the round billet is heated to 1200℃ and kept for 2.5 hours, 6 passes of rolling are performed, the first pass reduction is 27%, the reduction of each subsequent pass is 17%, the finish rolling temperature is 875℃, water mist cooling is performed after rolling, the cooling speed is 12℃ / s, and the round steel is air cooled after being cooled to 350℃;

[0024] Heat treatment process: the round steel after rolling is heated to 880℃ and kept for 50 minutes, oil quenching is performed with a cooling speed of 65℃ / s, then the round steel is heated to 575℃ and kept for 105 minutes, and air cooling is performed;

[0025] Structural refinement: The heat-treated round steel is rolled at a low temperature of 250℃ using a two-roll reversible rolling mill with a reduction rate of 7%.

[0026] Performance test results: tensile strength 980MPa, yield strength 820MPa, elongation 19%, impact toughness at -20℃ 70J / cm 2 .

[0027] Furthermore, specifically:

[0028] Smelting and casting: According to the ingredients, the electric arc furnace melting temperature is 1600℃, the LF refining furnace refining temperature is 1550℃, the refining time is 45 minutes, the continuous casting temperature is 1500℃, and the billet pulling speed is 1.2m / min to obtain round steel billets.

[0029] Rolling process optimization: The round steel billet is heated to 1250℃, held for 3 hours, and rolled in 7 passes. The reduction rate of the first pass is 30%, and the reduction rate of each subsequent pass is 20%. The final rolling temperature is 900℃. After rolling, the billet is cooled by water mist at a rate of 15℃ / s. After cooling to 400℃, it is air cooled.

[0030] Heat treatment process: Heat the rolled round steel to 900℃, hold for 60 minutes, oil quench, cool at a rate of 80℃ / s, then heat to 600℃, hold for 120 minutes, and air cool.

[0031] Structural refinement: The heat-treated round steel is rolled at a low temperature of 300℃ using a two-roll reversible rolling mill with a reduction rate of 10%.

[0032] Performance test results: tensile strength 1050MPa, yield strength 880MPa, elongation 17%, impact toughness at -20℃ 68J / cm 2 .

[0033] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0034] The round steel for sucker rods prepared by the method of this invention is superior to that prepared by traditional methods in terms of tensile strength, yield strength, elongation and impact toughness. This shows that the present invention can effectively optimize the structure and improve the strength of round steel, and can better meet the requirements of sucker rod use. Detailed Implementation

[0035] A method for structural optimization and strength enhancement of round steel for sucker rods includes five steps: chemical composition optimization, smelting and casting, rolling process optimization, heat treatment process, and structural refinement.

[0036] Chemical composition optimization: taking iron as the matrix, the chemical composition of the round steel is as follows in terms of mass percentage: carbon (C) 0.35%-0.45%, silicon (Si) 0.20%-0.40%, manganese (Mn) 0.70%-1.00%, chromium (Cr) 0.80%-1.20%, nickel (Ni) 0.40%-0.60%, molybdenum (Mo) 0.15%-0.25%, niobium (Nb) 0.02%-0.06%, titanium (Ti) 0.01%-0.03%, vanadium (V) 0.03%-0.05%, and the rest is iron (Fe) and inevitable impurities, among which sulfur (S) is ≤0.015% and phosphorus (P) is ≤0.020%.

[0037] Carbon element is a key element determining the strength of steel, and controlling it within 0.35%-0.45% can ensure higher strength while avoiding excessive decline in toughness. Silicon element, as a deoxidizer, can improve the strength and hardness of steel, and it is appropriate to control its content within 0.20%-0.40%. Manganese element can improve the hardenability and strength of steel and improve the processing performance of steel, and its content is 0.70%-1.00%. Chromium element can improve the corrosion resistance and strength of steel, nickel element can enhance the toughness and fatigue resistance of steel, and molybdenum element can improve the high-temperature strength and wear resistance of steel, and the three elements work together to further improve the comprehensive performance of round steel. Micro-alloy elements such as niobium, titanium, and vanadium can form fine carbonitride with carbon and nitrogen in steel, effectively refining the grains and improving the strength and toughness of steel. Strictly controlling the content of impurities such as sulfur and phosphorus can avoid their adverse effects on the performance of steel, and sulfur can cause steel to be hot brittle, and phosphorus can cause steel to be cold brittle.

[0038] Smelting and casting: according to the above chemical composition, the smelting process of electric arc furnace + LF refining furnace is adopted. During electric arc furnace smelting, the raw materials are added to the electric arc furnace, and the power is heated to 1550-1600℃ to fully melt the raw materials and remove part of the gas and impurities. Then the molten steel is transferred to the LF refining furnace, and the refining is carried out at 1500-1550℃ for 30-45 minutes. By adding slagging agent and other ways, the impurities and gas in the molten steel are further removed, and the composition of the molten steel is adjusted to the target range. After refining is completed, continuous casting is carried out, and the continuous casting temperature is controlled at 1450-1500℃, and the strand speed is 0.8-1.2 m / min to obtain round steel billets. Reasonable control of smelting and casting temperature and speed can ensure the quality of molten steel and reduce the defects such as pores and inclusions in round steel billets.

[0039] Rolling process optimization: heat the round billet to 1150-1250℃, and keep for 2-3 hours to make the temperature of the round billet uniform, which is convenient for rolling. Then, multi-pass rolling is carried out, and the number of passes is 5-7. During the rolling process, the first-pass reduction rate is controlled to be 25%-30%, a larger reduction is used to break the coarse as-cast grains, which lays a foundation for subsequent grain refinement; the reduction rate of the subsequent passes is gradually reduced and is controlled to be 15%-20%, so that the grains are further refined and uniform. The finish rolling temperature is controlled to be 850-900℃, and the rolling is completed at this temperature, which can ensure that the steel has good plasticity and toughness. After rolling, water mist cooling is used, the cooling speed is 10-15℃ / s, and then air cooling is carried out after fast cooling to 300-400℃, so that the grain growth is inhibited, and a fine structure is obtained.

[0040] Heat treatment process: the rolled round steel is subjected to quenching and tempering heat treatment. First, the round steel is heated to 860-900℃ and kept for 40-60 minutes to fully austenitize the steel structure. Then, oil quenching is carried out, and the cooling speed of oil quenching is 50-80℃ / s, so that the steel obtains a martensite structure, and the strength and hardness of the steel are improved. After quenching, the round steel is heated to 550-600℃ and kept for 90-120 minutes for tempering treatment, so as to eliminate quenching stress, improve the toughness of the steel, and make the steel have good strength-toughness matching. After tempering, air cooling is carried out to complete the heat treatment.

[0041] Structure refinement treatment: the round steel after heat treatment is subjected to low-temperature rolling, and a two-high reversible mill is used, the rolling temperature is 200-300℃, and the reduction rate is 5%-10%. The low-temperature rolling can further refine the grains, increase the dislocation density, and improve the structure density without changing the overall structure type of the steel, so as to further improve the strength and fatigue resistance of the round steel.

[0042] The application will be further described in detail in combination with specific embodiments.

[0043] Example 1:

[0044] A structure optimization and strength improvement method of a round steel for sucker rods, and the steps are as follows:

[0045] (1) Chemical composition optimization: the chemical composition of the round steel is as follows in terms of mass percentage: carbon 0.35%, silicon 0.20%, manganese 0.70%, chromium 0.80%, nickel 0.40%, molybdenum 0.15%, niobium 0.02%, titanium 0.01%, vanadium 0.03%, and the rest is iron and unavoidable impurities, wherein the sulfur is 0.012%, and the phosphorus is 0.018%.

[0046] (2) Smelting and casting: according to the above composition, the arc furnace smelting temperature is 1550℃, the LF refining furnace refining temperature is 1500℃, the refining time is 30 minutes, the continuous casting temperature is 1450℃, and the billet drawing speed is 0.8 m / min, so as to obtain a round billet.

[0047] (3) Rolling process optimization: heat the round billet to 1150℃, keep for 2 hours, carry out 5 passes of rolling, the first pass reduction rate is 25%, the subsequent pass reduction rate is 15%, the finish rolling temperature is 850℃, water mist cooling after rolling, cooling speed is 10℃ / s, air cooling after cooling to 300℃.

[0048] (4) Heat treatment process: heat the rolled round steel to 860℃, keep for 40 minutes, oil quenching (cooling speed is 50℃ / s), then heat to 550℃, keep for 90 minutes, air cooling.

[0049] (5) Structure refining treatment: use two high reversible rolling mill to carry out low temperature rolling on the heat treated round steel, rolling temperature is 200℃, reduction rate is 5%.

[0050] The performance test results of the round steel prepared in this embodiment show that the tensile strength is 920MPa, the yield strength is 780MPa, the elongation is 18%, and the impact toughness (-20℃) is 65J / cm 2 .

[0051] Example 2:

[0052] A structure optimization and strength improvement method of a round steel for sucker rod, the steps are as follows:

[0053] (1) Chemical composition optimization: the chemical composition of the round steel is as follows in mass percentage: carbon 0.40%, silicon 0.30%, manganese 0.85%, chromium 1.00%, nickel 0.50%, molybdenum 0.20%, niobium 0.04%, titanium 0.02%, vanadium 0.04%, and the rest is iron and unavoidable impurities, among which sulfur is 0.010% and phosphorus is 0.015%.

[0054] (2) Smelting and casting: according to the above composition, the electric arc furnace smelting temperature is 1575℃, the LF refining furnace refining temperature is 1525℃, the refining time is 37 minutes, the continuous casting temperature is 1475℃, and the billet drawing speed is 1.0m / min, to obtain a round billet.

[0055] (3) Rolling process optimization: heat the round billet to 1200℃, keep for 2.5 hours, carry out 6 passes of rolling, the first pass reduction rate is 27%, the subsequent pass reduction rate is 17%, the finish rolling temperature is 875℃, water mist cooling after rolling, cooling speed is 12℃ / s, air cooling after cooling to 350℃.

[0056] (4) Heat treatment process: heat the rolled round steel to 880℃, keep for 50 minutes, oil quenching (cooling speed is 65℃ / s), then heat to 575℃, keep for 105 minutes, air cooling.

[0057] (5) Structure refinement treatment: adopt two-roll reversible rolling mill to low-temperature rolling of the round steel after heat treatment, rolling temperature 250℃, reduction 7%.

[0058] Performance test results: tensile strength 980MPa, yield strength 820MPa, elongation 19%, impact toughness (-20℃) 70J / cm 2 .

[0059] Example 3:

[0060] A method for optimizing the structure and improving the strength of a sucker rod round steel, the steps are as follows:

[0061] (1) Chemical composition optimization: the chemical composition of the round steel is as follows in mass percentage: carbon 0.45%, silicon 0.40%, manganese 1.00%, chromium 1.20%, nickel 0.60%, molybdenum 0.25%, niobium 0.06%, titanium 0.03%, vanadium 0.05%, and the rest is iron and unavoidable impurities, among which sulfur is 0.008% and phosphorus is 0.012%.

[0062] (2) Smelting and casting: according to the above composition, the electric arc furnace smelting temperature is 1600℃, the LF refining furnace refining temperature is 1550℃, the refining time is 45 minutes, the continuous casting temperature is 1500℃, and the blank drawing speed is 1.2m / min, to get round billet.

[0063] (3) Rolling process optimization: heat the round billet to 1250℃, keep for 3 hours, roll for 7 passes, the first pass reduction is 30%, the subsequent pass reduction is 20%, the finish rolling temperature is 900℃, water mist cooling after rolling, cooling speed 15℃ / s, and air cooling after cooling to 400℃.

[0064] (4) Heat treatment process: heat the rolled round steel to 900℃, keep for 60 minutes, oil quenching (cooling speed 80℃ / s), then heat to 600℃, keep for 120 minutes, and air cool.

[0065] (5) Structure refinement treatment: adopt two-roll reversible rolling mill to low-temperature rolling of the round steel after heat treatment, rolling temperature 300℃, reduction 10%.

[0066] Performance test results: tensile strength 1050MPa, yield strength 880MPa, elongation 17%, impact toughness (-20℃) 68J / cm 2 .

[0067] Comparative experiment: the sucker rod round steel prepared by the traditional method has a tensile strength of 750MPa, a yield strength of 620MPa, an elongation of 15%, and an impact toughness (-20℃) of 45J / cm 2 .

[0068] By comparison, the round steel prepared by the method has better tensile strength, yield strength, elongation and impact toughness than the round steel prepared by the traditional method, which shows that the method can effectively realize the structure optimization and strength improvement of the round steel, and better meet the use requirements of the sucker rod.

[0069] The above-described embodiments are only used to describe the preferred modes of the present application, and do not limit the scope of the present application, and various modifications and improvements to the technical solutions of the present application made by those skilled in the art without departing from the design spirit of the present application shall fall within the protection scope of the present application defined by the claims.

Claims

1. A method for structural optimization and strength improvement of round steel for sucker rods, characterized in that, include: (1) Chemical composition optimization: Using iron as the matrix, the chemical composition of the round steel, by mass percentage, is as follows: C 0.35%-0.45%, Si 0.20%-0.40%, Mn 0.70%-1.00%, Cr 0.80%-1.20%, Ni 0.40%-0.60%, Mo 0.15%-0.25%, Nb 0.02%-0.06%, Ti 0.01%-0.03%, V 0.03%-0.05%, the remainder being Fe and unavoidable impurities, of which S≤0.015% and P≤0.020%; (2) Smelting and casting: The above chemical composition is used for batching, and the smelting process of electric arc furnace + LF refining furnace is adopted. The electric arc furnace melting temperature is controlled at 1550-1600℃, the LF refining furnace refining temperature is 1500-1550℃, the refining time is 30-45 minutes, and then continuous casting is carried out at a continuous casting temperature of 1450-1500℃ and a billet pulling speed of 0.8-1.2m / min to obtain round steel billets; (3) Rolling process optimization: Heat the round steel billet to 1150-1250℃, hold it for 2-3 hours, and then perform multi-pass rolling. During the rolling process, control the reduction rate of the first pass to 25%-30%, and gradually reduce the reduction rate of each subsequent pass to 15%-20%. The final rolling temperature is 850-900℃. After rolling, use water mist cooling at a rate of 10-15℃ / s. After cooling to 300-400℃, air cool. (4) Heat treatment process: The rolled round steel is subjected to quenching and tempering heat treatment. First, it is heated to 860-900℃ and held for 40-60 minutes. Then, it is oil quenched. After quenching, it is heated to 550-600℃ and held for 90-120 minutes. Then, it is air cooled. (5) Structural refinement treatment: The heat-treated round steel is rolled at a low temperature of 200-300℃ with a reduction rate of 5%-10% to further refine the grains and improve the structural density.

2. The method for structural optimization and strength improvement of round steel for sucker rods according to claim 1, characterized in that, The chemical composition of the round steel, by mass percentage, is as follows: C 0.35%, Si 0.20%, Mn 0.70%, Cr 0.80%, Ni 0.40%, Mo 0.15%, Nb 0.02%, Ti 0.01%, V 0.03%, with the remainder being Fe and unavoidable impurities, including S 0.012% and P 0.018%.

3. The method for structural optimization and strength improvement of round steel for sucker rods according to claim 1, characterized in that, The chemical composition of the round steel, by mass percentage, is as follows: C 0.40%, Si 0.30%, Mn 0.85%, Cr 1.00%, Ni 0.50%, Mo 0.20%, Nb 0.04%, Ti 0.02%, V 0.04%, with the remainder being Fe and unavoidable impurities, including S 0.010% and P 0.015%.

4. The method for structural optimization and strength improvement of round steel for sucker rods according to claim 1, characterized in that, The chemical composition of the round steel, by mass percentage, is as follows: C 0.45%, Si 0.40%, Mn 1.00%, Cr 1.20%, Ni 0.60%, Mo 0.25%, Nb 0.06%, Ti 0.03%, V 0.05%, with the remainder being Fe and unavoidable impurities, including S 0.008% and P 0.012%.

5. The method for structural optimization and strength improvement of round steel for sucker rods according to claim 2, characterized in that, Specifically: Smelting and casting: According to the ingredients, the electric arc furnace melting temperature is 1550℃, the LF refining furnace refining temperature is 1500℃, the refining time is 30 minutes, the continuous casting temperature is 1450℃, and the billet pulling speed is 0.8m / min to obtain round steel billets. Rolling process optimization: The round steel billet is heated to 1150℃, held for 2 hours, and rolled in 5 passes. The reduction rate of the first pass is 25%, and the reduction rate of each subsequent pass is 15%. The final rolling temperature is 850℃. After rolling, the billet is cooled by water mist at a rate of 10℃ / s. After cooling to 300℃, it is air cooled. Heat treatment process: Heat the rolled round steel to 860℃, hold for 40 minutes, oil quench, cool at a rate of 50℃ / s, then heat to 550℃, hold for 90 minutes, and air cool. Structural refinement: The heat-treated round steel is rolled at a low temperature of 200℃ and a reduction rate of 5% using a two-roll reversible rolling mill. Performance test results: tensile strength 920 MPa, yield strength 780 MPa, elongation 18%, impact toughness at -20℃ 65 J / cm² 2 .

6. The method for structural optimization and strength improvement of round steel for sucker rods according to claim 3, characterized in that, Specifically: Smelting and casting: According to the ingredients, the electric arc furnace melting temperature is 1575℃, the LF refining furnace refining temperature is 1525℃, the refining time is 37 minutes, the continuous casting temperature is 1475℃, and the billet pulling speed is 1.0m / min to obtain round steel billets. Rolling process optimization: The round steel billet is heated to 1200℃, held for 2.5 hours, and rolled in 6 passes. The reduction rate of the first pass is 27%, and the reduction rate of each subsequent pass is 17%. The final rolling temperature is 875℃. After rolling, the billet is cooled by water mist at a rate of 12℃ / s. After cooling to 350℃, it is air cooled. Heat treatment process: Heat the rolled round steel to 880℃, hold for 50 minutes, oil quench, cool at a rate of 65℃ / s, then heat to 575℃, hold for 105 minutes, and air cool. Structural refinement: The heat-treated round steel is rolled at a low temperature of 250℃ using a two-roll reversible rolling mill with a reduction rate of 7%. Performance test results: tensile strength 980MPa, yield strength 820MPa, elongation 19%, impact toughness at -20℃ 70J / cm 2 .

7. The method for structural optimization and strength improvement of round steel for sucker rods according to claim 4, characterized in that, Specifically: Smelting and casting: According to the ingredients, the electric arc furnace melting temperature is 1600℃, the LF refining furnace refining temperature is 1550℃, the refining time is 45 minutes, the continuous casting temperature is 1500℃, and the billet pulling speed is 1.2m / min to obtain round steel billets. Rolling process optimization: The round steel billet is heated to 1250℃, held for 3 hours, and rolled in 7 passes. The reduction rate of the first pass is 30%, and the reduction rate of each subsequent pass is 20%. The final rolling temperature is 900℃. After rolling, the billet is cooled by water mist at a rate of 15℃ / s. After cooling to 400℃, it is air cooled. Heat treatment process: Heat the rolled round steel to 900℃, hold for 60 minutes, oil quench, cool at a rate of 80℃ / s, then heat to 600℃, hold for 120 minutes, and air cool. Structural refinement: The heat-treated round steel is rolled at a low temperature of 300℃ using a two-roll reversible rolling mill with a reduction rate of 10%. Performance test results: tensile strength 1050MPa, yield strength 880MPa, elongation 17%, impact toughness at -20℃ 68J / cm 2 .