Low-alloy high-strength and high-toughness Q460C electric power angle steel for towers and a preparation method thereof

Abstract

The present application relates to the technical field of steel for power transmission tower, and particularly relates to a kind of low alloy high strength high toughness Q460C power angle steel for tower and a preparation method thereof.The preparation method at least includes the steps of heating billet, rolling forming and cooling;in the step of heating billet, the temperature of heating first section is 900-1030 DEG C, the temperature of heating second section is 1150-1250 DEG C, the temperature of soaking section is 1140-1240 DEG C, and the temperature of billet out of furnace is 1050-1160 DEG C;the step of rolling forming adopts double BD rolling+four continuous rolling of finish rolling, and the temperature of steel to be controlled before finish rolling, and the rolling temperature is 800-900 DEG C;the step of cooling adopts ultrafast cooling cooling device.The Q460C angle steel for tower finally obtained by using microalloying component system and specific controlled rolling and controlled cooling process can exceed the standard requirement in strength and impact toughness, has a larger margin, and greatly reduces the production cost.

Description

Technical Field

[0001] This invention relates to the field of steel technology for power transmission towers, specifically to a low-alloy high-strength and high-toughness Q460C angle steel for power transmission towers and its preparation method. Background Technology

[0002] Currently, the main steel materials used for transmission towers in my country are generally Q355 and Q420, while the diagonal bracing is mainly Q235 and Q355. These steels have relatively low strength values, resulting in a large steel consumption for transmission line towers. The increasing number of double-circuit towers, multi-circuit towers, and crossing towers, along with rising tower heights, leads to a continuous increase in tower loads. The application of multi-split conductors further exacerbates this increase. This trend places higher demands on the load-bearing capacity of transmission towers, while also requiring that the tower weight and corridor footprint be minimized as much as possible.

[0003] Improving the mechanical properties of steel used in power transmission towers is an effective means to enhance the load-bearing capacity and reduce the weight of the towers. Domestically and internationally, angle steel production employs conventional rolling processes without controlled rolling and cooling. The microstructure is generally ferrite + pearlite, and strength and toughness are primarily ensured by adding microalloying agents such as Nb, V, and Ti. This results in high production costs, and the low-temperature impact toughness cannot consistently meet standards, leading to extremely low rolling efficiency and persistently high smelting costs. Existing technology CN 109127724 A provides a production process for high-toughness angle steel. This method involves air-drying the steel plate after rough and medium rolling to reduce the temperature when the plate enters the finishing mill. This causes deformation-induced ferrite precipitation in medium and large-sized angle steel with a ∠10 or greater during finishing rolling, promoting grain refinement. This existing technology can improve the impact toughness of medium and large-sized angle steel, but it does not improve the yield strength or tensile strength of the angle steel. Summary of the Invention

[0004] In view of the fact that most of the current power transmission tower angle steel is of the QA235, Q355 and Q420 type, and its mechanical properties need to be improved, this invention provides a low alloy high strength and high toughness Q460C power transmission tower angle steel and its preparation method.

[0005] In a first aspect, the present invention provides a method for preparing low-alloy high-strength and high-toughness Q460C power angle steel for iron towers, which includes at least the steps of heating steel billets, rolling and forming, and cooling.

[0006] The chemical composition of the steel billet, by weight percentage, includes C 0.14%~0.18%, Si 0.25%~0.40%, Mn 1.35%~1.50%, V 0.04%~0.08%, P≤0.025%, S≤0.02%, Als 0.015%~0.03%, N≤0.015%, with the balance being Fe and unavoidable impurities;

[0007] In the billet heating process, the temperature of the first heating stage is controlled at 900~1030℃, the furnace pressure of the first heating stage is 20±10Pa, the temperature of the second heating stage is 1150~1250℃, the furnace pressure of the second heating stage is 25±10Pa, the temperature of the soaking stage is 1140~1240℃, the furnace pressure of the soaking stage is 30±10Pa, and the billet exit temperature is 1050~1160℃.

[0008] The rolling forming process adopts double BD rolling + four-pass finishing rolling. The initial rolling temperature of BD1 is 1050~1150℃, and it is rolled in 5 passes. BD2 is rolled in 3 passes, and the final rolling temperature of BD2 is 1000~1100℃. Before finishing rolling, the temperature of the steel is controlled. The initial rolling temperature of finishing rolling is 800~900℃, and it is rolled in 4 passes. The final rolling temperature of finishing rolling is 780~850℃.

[0009] The cooling process uses an ultra-fast cooling device, and the temperature of the angle steel after cooling is 640~685℃.

[0010] Furthermore, the steel billet is selected from square or rectangular cross-section steel billets, such as square steel billets with dimensions of 165mm×165mm, rectangular steel billets with cross-section dimensions of 180mm×220mm, or rectangular steel billets with cross-section dimensions of 220mm×290mm obtained by continuous casting.

[0011] Furthermore, the billet heating time is 50-60 minutes.

[0012] Furthermore, the temperature control time for steel before finishing rolling is 50~90s, and the temperature is reduced by 150~250℃.

[0013] Furthermore, the finishing rolling speed is 1~1.5m / s.

[0014] Furthermore, the main pipe pressure of the ultra-fast cooling device is 9~11 bar, and the flow rate is 800~900 m³ / h. 3 / h, frequency of 40~50Hz, ultrafast cooling roller speed of 1~1.5m / s.

[0015] Furthermore, it also includes the converter smelting and continuous casting steps. The converter smelting adopts efficient dephosphorization technology to improve the early oxidation of the converter and adopts the process technology of slag retention to quickly form early slag. The high-pulling and blowing process is adopted to improve the dephosphorization rate of the converter. The molten steel is refined in the LF furnace and deoxidized by diffusion using silicon carbide and ferrosilicon powder. After the free oxygen content in the molten steel is lower than 5ppm, the aluminum addition operation is carried out. The Al content in the molten steel is controlled at 0.015%~0.030% to create a high-basicity ternary slag system, which absorbs the calcium aluminate inclusions generated after aluminum addition. This can not only ensure the aluminum recovery rate, but also reduce the formation of Al2O3 inclusions, which is conducive to smooth continuous casting.

[0016] Continuous casting employs protective casting, compound flow control + automatic stopper rod control, electromagnetic stirring, and an ultra-weak cooling regime during the casting process;

[0017] Through the above-mentioned converter smelting and continuous casting steps, the resulting steel billet has high steel purity and few defects, and features low production cost, strong safety and high practicality.

[0018] Secondly, the present invention provides a low-alloy high-strength and high-toughness Q460C power tower angle steel produced by the above-mentioned preparation method.

[0019] Furthermore, the low-alloy high-strength and high-toughness Q460C power angle steel for towers has a grain size grade of 8.5 or higher, a yield strength between 490 and 545 MPa, a tensile strength between 600 and 655 MPa, a longitudinal impact toughness of 80 to 120 J at 0℃, and an elongation after fracture >22%.

[0020] The beneficial effects of this invention are as follows:

[0021] The present invention provides a micro-alloying composition system, combined with a specific controlled rolling and cooling process, to reduce the amount of alloying elements such as Mn and V added, saving expensive alloys. At the same time, the requirements for steel purity (mainly inclusions in the composition) are reduced. The resulting Q460C angle steel for iron towers has strength and impact toughness that exceed the standard requirements, with a large margin, and greatly reduces production costs.

[0022] The Q460C angle steel for power transmission towers of this invention has a reasonable composition and process design, and relaxed production control and process system, enabling stable production on the profile production line. Based on an annual production of 2000 tons of Q460C power transmission angle steel for power transmission towers, the profit per ton is over 600 yuan, resulting in an annual profit of over 1.2 million yuan, demonstrating high economic benefits. Simultaneously, the production process is simple, energy-efficient, and environmentally friendly, offering positive social benefits.

[0023] The billet heating step of this invention employs micro-positive pressure control, using a shorter heating time and a lower initial rolling temperature, which effectively controls the original austenite grain size of the billet. Both double-BD rolling processes are performed in the austenite recrystallization zone, and through repeated deformation-recrystallization, the finest possible austenite grains are obtained. The finishing rolling process promotes the precipitation of ferrite (VN) at austenite grain boundaries and within the grains by controlling the temperature during steel preparation. During the ultra-fast cooling process, a large amount of ferrite can be generated at austenite grain boundaries and within the grains, with VN particles as the nucleation core, increasing the nucleation driving force of ferrite during phase transformation, refining the final microstructure, and thus improving the strength and toughness of the steel. Detailed Implementation

[0024] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions in the embodiments of this invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0025] Example 1

[0026] A low-alloy, high-strength, high-toughness Q460C angle steel for power transmission towers is produced according to the following preparation method:

[0027] (1) Converter smelting

[0028] The process employs efficient dephosphorization technology to improve the early-stage oxidizing properties of the converter and utilizes slag retention operations to rapidly form early-stage slag. A high-pulling and supplementary blowing process is also employed to enhance the converter's dephosphorization rate. Molten steel is refined in an LF furnace, and silicon carbide and ferrosilicon powder are used for diffusion deoxidation. Once the free oxygen content in the molten steel falls below 5 ppm, an aluminum addition operation is performed to create a high-basicity ternary slag system. This system absorbs the calcium aluminate inclusions generated after aluminum addition, ensuring both aluminum recovery and reducing the formation of Al2O3 inclusions, which is beneficial for smooth continuous casting.

[0029] (2) Continuous casting

[0030] Using a protective casting process, a dual flow control system with automatic stopper rod control, electromagnetic stirring, and an ultra-weak cooling regime during casting, a high-purity Q460C qualified 220mm×290mm rectangular billet was obtained. The chemical composition was C 0.15%, Si 0.33%, Mn 1.35%, V 0.054%, P 0.018%, S 0.008%, Al 0.03%, N 0.011%, with the balance being Fe and unavoidable impurities.

[0031] (3) Heating of steel billet

[0032] The temperature of the first heating stage is controlled at 1010℃, the furnace pressure of the first heating stage is 18Pa, the temperature of the second heating stage is 1205℃, the furnace pressure of the second heating stage is 21Pa, the temperature of the soaking stage is 1150℃, the furnace pressure of the soaking stage is 22Pa, the billet is heated for 58 minutes, and the billet exit temperature is 1100℃.

[0033] (4) Rolling and forming

[0034] The rolling process for ∠200×20 equilateral angle steel employs a double BD rolling process followed by a four-pass finishing rolling process. The initial rolling temperature of BD1 is 1100℃, with five rolling passes and two 90° turning passes. BD2 is rolled in three passes, with a final rolling temperature of 1000℃. The BD2 extension roller table is temperature-controlled for 55 seconds. The entry temperature of the finishing rolling process is 900℃, with four rolling passes and a final finishing rolling temperature of 850℃. The rolling speed is controlled at 1m / s during the finishing rolling process.

[0035] (5) Cooling

[0036] An ultra-fast cooling device (SH00-00-00) is used, with the main pipe pressure controlled at 10.2 bar and the flow rate at 850 m³ / h. 3 / h, 2 pumps are running, the frequency is 43Hz, the ultra-fast cooling roller speed is 1.1m / s, and the temperature of the angle steel after cooling is 645℃.

[0037] The grain size and mechanical properties of the Q460C power angle steel for towers obtained in Example 1 were tested. The grain size grade of the angle steel was 9.0, the yield strength was 545 MPa, the tensile strength was 655 MPa, the longitudinal impact toughness at 0℃ was 144 J, and the elongation after fracture was 25%.

[0038] Observing the Q460C power angle steel for towers obtained in Example 1, after the controlled cooling step, the surface of the power angle steel forms a metallographic structure mainly composed of pearlite + ferrite + bainite, while the core of the power angle steel mainly consists of pearlite + ferrite.

[0039] Comparative Example 1

[0040] The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 does not undergo controlled cooling and is naturally cooled after rolling.

[0041] The angle steel obtained in Example 1 was tested for grain size and mechanical properties. The grain size grade of the angle steel was 7.5, the yield strength was 470 MPa, the tensile strength was 585 MPa, the longitudinal impact toughness at 0℃ was 85 J, and the elongation after fracture was 26%.

[0042] The coarser grain size of Comparative Example 1 compared to Example 1 is the main reason for its low impact toughness. The low performance is mainly due to the lack of controlled cooling process, the absence of bainite structure in the microstructure, and the presence of only pearlite and ferrite. Under the same composition conditions, the yield strength is reduced by 75 MPa and the tensile strength is reduced by 70 MPa compared to the embodiment of the present invention.

[0043] Comparative Example 2

[0044] The difference between Comparative Example 2 and Example 1 is that the converter smelting step in Comparative Example 2 does not involve aluminum addition. The chemical composition of the billet is C 0.15%, Si 0.33%, Mn 1.35%, V 0.054%, P 0.018%, S 0.008%, N 0.011%, with the balance being Fe and unavoidable impurities.

[0045] The angle steel obtained in Comparative Example 2 was tested for grain size and mechanical properties. The grain size grade of the angle steel was 8.0, the yield strength was 530 MPa, the tensile strength was 645 MPa, the longitudinal impact toughness at 0℃ was 52 J, and the elongation after fracture was 24%.

[0046] Comparative Example 2, due to the absence of Al element in its composition, has coarse grain size, which is the main reason for its low impact toughness. However, due to its controlled cooling process, its performance is basically the same as that of the embodiments of the present invention.

[0047] Comparative Example 3

[0048] The difference between Comparative Example 3 and Example 1 is that the finishing rolling temperature of Comparative Example 3 is 920°C, and the temperature of the angle steel after cooling is 500°C.

[0049] The angle steel obtained in Comparative Example 3 was tested for grain size and mechanical properties. The grain size grade of the angle steel was 9.5, the yield strength was 580 MPa, the tensile strength was 705 MPa, the longitudinal impact toughness at 0℃ was 135 J, and the elongation after fracture was 17%.

[0050] Comparative Example 3 shows that due to the high finishing rolling temperature and the low temperature after controlled cooling, the increased cooling intensity leads to an increase in the bainite content and hardened layer depth in the microstructure of the angle steel. Furthermore, tempered martensite and tempered sorbite appear in some locations, resulting in increased yield strength and tensile strength, and a sharp decrease in elongation after fracture. This is not conducive to the application of power angle steel in iron towers.

[0051] Example 2

[0052] A low-alloy, high-strength, high-toughness Q460C angle steel for power transmission towers is produced according to the following preparation method:

[0053] (1) Converter smelting

[0054] The process employs efficient dephosphorization technology to improve the early-stage oxidizing properties of the converter and utilizes slag retention operations to rapidly form early-stage slag. A high-pulling and supplementary blowing process is also employed to enhance the converter's dephosphorization rate. Molten steel is refined in an LF furnace, and silicon carbide and ferrosilicon powder are used for diffusion deoxidation. Once the free oxygen content in the molten steel falls below 5 ppm, an aluminum addition operation is performed to create a high-basicity ternary slag system. This system absorbs the calcium aluminate inclusions generated after aluminum addition, ensuring both aluminum recovery and reducing the formation of Al2O3 inclusions, which is beneficial for smooth continuous casting.

[0055] (2) Continuous casting

[0056] Using a protective casting process, a dual flow control system with automatic stopper rod control, electromagnetic stirring, and an ultra-weak cooling regime during casting, a high-purity Q460C qualified 220mm×290mm rectangular billet was obtained. The chemical composition was C 0.16%, Si 0.36%, Mn 1.36%, V 0.050%, P 0.024%, S 0.014%, Al 0.026%, N 0.012%, with the balance being Fe and unavoidable impurities.

[0057] (3) Heating of steel billet

[0058] The temperature of the first heating stage is controlled at 980℃, the furnace pressure of the first heating stage is 16Pa, the temperature of the second heating stage is 1180℃, the furnace pressure of the second heating stage is 18Pa, the temperature of the soaking stage is 1150℃, the furnace pressure of the soaking stage is 20Pa, the billet is heated for 60 minutes, and the billet exit temperature is 1050℃.

[0059] (4) Rolling and forming

[0060] The rolling process for ∠180×16 equilateral angle steel employs a double BD rolling process followed by a four-pass finishing rolling process. The initial rolling temperature of BD1 is 1050℃, with five rolling passes and two 90° turns. BD2 is rolled in three passes, with a final rolling temperature of 980℃. The BD2 extension roller table is temperature-controlled for 65 seconds. The entry temperature of the finishing mill is 890℃, with four rolling passes and a final finishing temperature of 850℃. The rolling speed is controlled at 1.3 m / s during the finishing rolling process.

[0061] (5) Cooling

[0062] An ultra-fast cooling device (SH00-00-00) is used, with the main pipe pressure controlled at 9.5 bar and the flow rate at 800 m³ / h. 3 / h, 2 pumps are running, the frequency is 41Hz, the ultra-fast cooling roller speed is 1.5m / s, and the temperature of the angle steel after cooling is 650℃.

[0063] The grain size and mechanical properties of the Q460C power angle steel for towers obtained in Example 2 were tested. The grain size grade of the angle steel was 9.0, the yield strength was between 530 MPa, the tensile strength was between 650 MPa, the longitudinal impact toughness at 0℃ was 96 J, and the elongation after fracture was 24%.

[0064] Observing the Q460C power angle steel for towers obtained in Example 2, after the controlled cooling step, the surface of the power angle steel forms a metallographic structure mainly composed of pearlite + ferrite + bainite, while the core of the power angle steel mainly consists of pearlite + ferrite.

[0065] Although the present invention has been described in detail by way of preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for preparing a low-alloy high-strength and high-toughness Q460C power angle steel for a tower, characterized in that, At least comprising billet heating, rolling forming and cooling steps; The chemical composition of the billet comprises, by weight percentage, C 0.14%-0.18%, Si 0.25%-0.40%, Mn 1.35%-1.50%, V 0.04%-0.08%, P≤0.025%, S≤0.02%, Als 0.015%-0.03%, N≤0.015%, and the balance of Fe and inevitable impurities; In the billet heating step, the first heating section temperature is 900-1030℃, the first heating section furnace pressure is 20±10Pa, the second heating section temperature is 1150-1250℃, the second heating section furnace pressure is 25±10Pa, the soaking section temperature is 1140-1240℃, the soaking section furnace pressure is 30±10Pa, and the billet discharge temperature is 1050-1160℃; The rolling forming step adopts double BD rolling+four continuous rolling in finishing rolling, the BD1 rough rolling temperature is 1050-1150℃, the rolling is 5 passes, the BD2 rolling is 3 passes, the BD2 finishing rolling temperature is 1000-1100℃, the steel is controlled in temperature before finishing rolling, the finishing rolling rough rolling temperature is 800-900℃, the rolling is 4 passes, and the finishing rolling finishing rolling temperature is 780-850℃; The cooling step adopts ultra-fast cooling device, and the angle steel temperature after cooling is 640-685℃; The power angle steel surface forms a metallographic structure mainly composed of pearlite+ferrite+beihai, and the core of the power angle steel is mainly composed of pearlite+ferrite.

2. The production method according to claim 1, wherein The billet is selected from square section billet or rectangular section billet.

3. The production method according to claim 1, wherein The billet heating time is 50-60min.

4. The production method according to claim 1, wherein The steel control temperature time before finishing rolling is 50-90s.

5. The production method according to claim 1, wherein The finishing rolling speed is 1-1.5m / s.

6. The production method according to claim 1, wherein The total pipe pressure of the ultrafast cooling device is 9-11 bar, the flow rate is 800-900 m 3 / h, the frequency is 40-50 Hz, and the ultrafast cooling roller speed is 1-1.5 m / s.

7. The production method according to claim 1, wherein The steps of converter smelting and continuous casting are further included, the molten steel is refined by LF furnace, silicon carbide and ferrosilicon powder are used for diffusion deoxidization, the free oxygen content in the molten steel is less than 5ppm after the aluminum addition operation, and the Als content in the molten steel is controlled at 0.015%-0.030%.

8. A low-alloy high-strength high-toughness Q460C power angle steel for iron tower produced by the preparation method of claim 1.

9. The low-alloy high-strength high-toughness Q460C electric power angle steel tower steel according to claim 8, characterized in that, The grain size grade reaches 8.5 or above, the yield strength is between 490-545MPa, the tensile strength is between 600-655MPa, the 0℃ longitudinal impact toughness is 80-120J, and the elongation after fracture is >22%.