Production method of hot rolled strip steel for high-frequency welded pipe with yield strength of 700MPa

Abstract

The invention provides a production method of hot-rolled strip steel for a high-frequency welded pipe with the yield strength of 700 MPa. The production method comprises the steps of steelmaking, continuous casting, heating, rolling, cooling, coiling and leveling. A secondary calcium treatment process is adopted in the steelmaking stage, and the content of nitrogen, sulfur and calcium in refined molten steel and the mass ratio of calcium to sulfur are controlled; and in cooperation with a subsequent continuous casting process, a multi-stage heating process, a rolling process and a three-stage cooling process, the finally obtained hot-rolled strip steel has excellent strength and plasticity, and the overall strength and plasticity stability are good. Wherein the yield strength of the hot-rolled strip steel is 740-780 MPa, the tensile strength of the hot-rolled strip steel is 820-850 MPa, the ductility of the hot-rolled strip steel is 22-25%, and the product of strength and elongation of the hot-rolled strip steel is 18-23 GPa.%. And meanwhile, the steel pipe prepared from the hot-rolled strip steel through high-frequency welding has the elongation rate of 20% or above, good flattening performance and flaring, no cracking phenomenon, low internal stress and good welding performance, and the squareness is not larger than 2 mm / 10 m.

Description

Technical Field

[0001] This application relates to the field of steel processing technology, and in particular to a method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700MPa. Background Technology

[0002] High-frequency welded pipe is a metal tube formed by heating the edges of a steel strip to a molten state using the skin effect and proximity effect generated by high-frequency current, and then pressing it together with extrusion rollers. It has advantages such as a small heat-affected zone and high production efficiency, and is widely used in the manufacture of structural components for new energy vehicles, such as battery frames, chassis, and crash beams.

[0003] High-strength steel with a yield strength of 700MPa is mainly used in the manufacture of cold-formed structural components such as automobile beams and bodies, and has been gradually applied to the high-frequency welded pipe industry in recent years. However, in the high-frequency welding process, there are problems such as low strip elongation, high internal stress, large strength fluctuation, poor welding performance, and poor straightness of welded pipes, which to some extent limit its further promotion in the high-frequency welded pipe industry.

[0004] The impact of strip steel quality on high-frequency welded pipe processing is mainly reflected in the following aspects: (1) Low strip steel elongation will lead to poor plasticity of finished steel pipe, making it difficult to meet the usage requirements, and may even cause cracking during pipe bending; (2) Strength fluctuations in the width direction and rolling direction of strip steel, stress distribution state inside strip steel and other factors will affect the welding accuracy, thereby affecting the weld quality and straightness of welded pipe; (3) The content of impurity elements and the morphology of inclusions in strip steel are also important aspects affecting the welding quality of strip steel, which can easily lead to welding cracks, flattening cracks and other problems.

[0005] To address the issue of large strength fluctuations in high-strength steel plates used in high-frequency welded pipes, some steel mills have adopted Mo microalloying. For example, patent publication number CN 118531312 A discloses an 800MPa-level nano-precipitated steel and its preparation method, which adds 0.10~0.40% Mo, resulting in a steel plate with a tensile strength greater than 800MPa. However, this significantly increases the alloy cost. Furthermore, it still does not solve the problem of low elongation in high-frequency welded pipes. Summary of the Invention

[0006] This application aims to address the common technical problems of low elongation and poor straightness in existing high-frequency welded pipes with a yield strength of 700MPa. It proposes a method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700MPa. This hot-rolled strip steel possesses excellent tensile strength and plasticity, while exhibiting low internal stress, making it easy to weld. Furthermore, the steel pipes prepared by high-frequency welding have high elongation and good straightness.

[0007] To achieve the above objectives, this application adopts the following technical solution:

[0008] A method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700 MPa includes the following steps:

[0009] (1) Steelmaking

[0010] The molten steel from the converter is subjected to LF refining, primary calcium treatment, RH treatment, and secondary calcium treatment to obtain refined molten steel; the refined molten steel has a nitrogen content ≤0.0050%, a sulfur content ≤0.0030%, a calcium content of 0.0015~0.0040%, and a calcium-to-sulfur mass ratio of 0.8~4.0;

[0011] (2) Continuous casting

[0012] The refined molten steel is continuously cast to obtain slabs. During the continuous casting process, the tundish temperature is T. L +10~25℃, pulling speed is 1.20±0.1m / min; crystallizer water flow rate is 3600~3800L / min on the wide side and 500~600L / min on the narrow side; electromagnetic stirring in the secondary cooling zone is used, with stirring current of 200~400A and frequency of 5~7Hz;

[0013] (3) Heating

[0014] The slab is heated to obtain an intermediate billet. The heating includes a preheating stage, a first heating stage, a second heating stage, and a homogenization stage. The initial temperature of the preheating stage is 400~700℃, the heating rate from the preheating stage to the second heating stage is 5.0~8.0℃ / min, the temperature of the second heating stage is 1280~1310℃, the temperature of the homogenization stage is 1270~1300℃, and the heating time of the homogenization stage is 50~700min.

[0015] (4) Rolling

[0016] The intermediate billet is subjected to rough rolling and finish rolling to obtain the finished plate shape. The thickness of the intermediate billet at the rough rolling exit is 35~38mm. The target value of the plate shape at the finish rolling exit is set based on the thickness t of the finished plate shape. When the thickness of the finished plate shape is 1.8≤t<2.5mm, the target value of the plate shape is set to 4~6IU; when the thickness of the finished plate shape is 2.5≤t<4.0mm, the target value of the plate shape is set to 3~5IU; when the thickness of the finished plate shape is 4.0≤t≤6.0mm, the target value of the plate shape is set to 1~3IU.

[0017] (5) Cooling and winding

[0018] A three-stage cooling mode of water-air-water cooling is used to cool the finished sheet shape before coiling the strip.

[0019] (6) Leveling

[0020] The leveling process uses a sinusoidal curve roll with a negative crown of 0.015mm, a rolling force of 300~350 tons, and a bending force of 30~60 tons.

[0021] Furthermore, the refined molten steel comprises the following components by mass percentage: C: 0.05~0.10%, Si: 0.10~0.30%, Mn: 1.50~1.70%, Nb: 0.020~0.050%, Ti: 0.10~0.13%, Al: 0.020~0.040%, Ca: 0.0015~0.0040%, P≤0.018%, S≤0.0030%, N: ≤0.0040%, with the remainder consisting of iron and unavoidable impurities.

[0022] The roles and dosage selection criteria for each element in the above-mentioned composition design of the refined steel are as follows:

[0023] Carbon (C): Carbon is one of the most important alloying elements in steel, and its content directly affects the steel's properties. This application mainly ensures the mechanical properties of steel through precipitation strengthening and grain refinement strengthening. Carbon forms dispersed precipitates with microalloying elements such as Nb and Ti in the steel, which can refine the grains and improve the strength of the steel. If the carbon content is too low, the strength of the steel will not meet the requirements; if the carbon content is too high, the plasticity and weldability of the steel will deteriorate. Therefore, the carbon content is selected as 0.05~0.10%.

[0024] Silicon (Si): Silicon does not form carbides in steel but exists as a solid solution in ferrite or austenite, which can improve the strength and hardness of steel. Excessive silicon content can lead to red iron oxide defects on the surface of the strip steel. In this invention, the silicon content is controlled at 0.10~0.30%.

[0025] Manganese (Mn): Manganese plays a role in solid solution strengthening in steel. It also stabilizes austenite and refines grains. Excessive manganese content can easily lead to segregation, affecting the cold workability of the steel. Therefore, the manganese content is selected to be 1.50~1.70%.

[0026] Phosphorus (P): Phosphorus is an impurity element in steel. High phosphorus content in steel can lead to poor low-temperature toughness, so the phosphorus content in steel needs to be controlled. However, too low a phosphorus content will also increase the smelting cost of steel. Therefore, the phosphorus content is selected to be ≤0.018%.

[0027] Sulfur (S): Sulfur is an impurity element in steel. It easily combines with Mn to form MnS inclusions. After rolling deformation, MnS forms elongated strips, disrupting the continuity of the matrix. In high-frequency welding, low-melting-point sulfide inclusions are one of the main causes of welding cracks. Therefore, the sulfur content is selected to be ≤0.0030%.

[0028] Aluminum (Al): Aluminum is a deoxidizing element in steel. Excessive aluminum can increase the number of Al2O3 inclusions in steel, affecting its cold working properties. Therefore, the aluminum content is selected to be 0.020~0.050%.

[0029] Calcium (Ca): The main purpose of calcium treatment in the steelmaking process is to improve the castability of molten steel and increase its purity. After calcium treatment in the steelmaking process described in this application, a certain calcium content will still be retained in the molten steel to control the calcium-sulfur ratio. The reaction between Ca and S in the steel will produce spherical CaS, thereby reducing low-melting-point, strip-shaped MnS inclusions and preventing the formation of welding cracks in high-frequency welding.

[0030] Microalloying elements (Nb and Ti): (1) The combined addition of Nb and Ti, coupled with appropriate rolling processes, inhibits austenite grain growth during heating; promotes the precipitation of fine and dispersed carbonitrides during rolling, thereby inhibiting recrystallization; and precipitates during cooling and coiling, thereby refining the steel structure and improving its strength. (2) The combined addition of Nb and Ti as strong carbide-forming elements to steel can reduce the activity of C in austenite, decrease the diffusion rate of C in steel, and increase the phase transformation undercooling. In addition, the dispersed microalloyed carbonitrides generated during rolling can provide more nucleation sites for phase transformation. Through the increase in phase transformation undercooling and nucleation sites, grain refinement is achieved, which, combined with subsequent laminar cooling processes, improves the plasticity of steel and enhances its cold working performance.

[0031] Furthermore, the vacuum level during the RH treatment is 0~2mBar, and the treatment time is 10~15min.

[0032] Furthermore, in the first calcium treatment, 130-150m of silicon-calcium wire is fed per furnace, and the soft stirring time is 5-7 minutes; in the second calcium treatment, 100-120m of silicon-calcium wire is fed per furnace, and the soft stirring time is 10-12 minutes.

[0033] Furthermore, in the continuous casting process, a tundish covering agent, a submerged entry nozzle, and an argon seal are used to fully protect the molten steel during casting. The depth of the submerged entry nozzle is 150~180mm, and the argon seal back pressure is 0.05~0.15Bar.

[0034] Furthermore, the roughing process employs 1+5 or 3+3 passes.

[0035] Furthermore, the inlet temperature of the finishing mill is 1080~1120℃, and the outlet temperature is 900~930℃.

[0036] Furthermore, in the three-stage cooling mode of water-air-water cooling, the first stage of water cooling is to cool to 650-680°C at a rate of 30-60°C / s, the second stage of air cooling is to cool for 3-5 seconds, and the third stage of water cooling is to cool to 590-620°C at a rate of 30-60°C / s for winding. The water-to-water ratio used in the first stage of water cooling and the third stage of water cooling is 0.8.

[0037] Furthermore, after winding, the wound strip is placed in an insulation pit for slow cooling, and the slow cooling time is 48-72 hours or more.

[0038] Furthermore, during the leveling process, the difference in roller diameter between the upper and lower working rollers of the leveling machine is 0~0.5mm.

[0039] The beneficial effects of this application are:

[0040] The hot-rolled strip steel prepared by the method of this application possesses both excellent strength and plasticity. The yield strength of the hot-rolled strip steel is 740~780MPa, the tensile strength is 800~850MPa, the elongation is 21~25%, and the strength-ductility product is 16~21.3GPa·%. Furthermore, it exhibits good overall strength and plasticity stability. Simultaneously, the steel pipes prepared from the hot-rolled strip steel by high-frequency welding can also achieve an elongation of over 20%, with good flattening performance and flaring without cracking. The steel pipes have low internal stress, good weldability, and a right angle not exceeding 2mm / 10m.

[0041] In the steelmaking process, during the solidification of molten steel, sulfur (S) tends to combine with manganese (Mn) to form manganese sulfide. During subsequent hot rolling, manganese sulfide inclusions evolve into elongated banded or spindle-shaped inclusions. In the rapid heating and cooling processes of high-frequency welding, low-melting-point sulfide inclusions can easily become potential crack initiation sites. Compared to Mn, calcium (Ca) has a stronger affinity for sulfur (S) and reacts more readily with it to form spherical, higher-melting-point CaS or (Ca,Mn)S composite inclusions, which can mitigate the impact of sulfur on the quality of high-frequency welding. Based on these factors, this application employs a secondary calcium treatment process in the steelmaking stage. By stabilizing and controlling the sulfur content, calcium content, and calcium-sulfur mass ratio in the molten steel, the generation of elongated banded or spindle-shaped manganese sulfide inclusions can be effectively suppressed, reducing the impact of sulfur on the subsequent high-frequency welding quality. Combined with subsequent rolling and flattening strategies, good welding quality is achieved.

[0042] In high-Ti steel, TiN precipitates in both liquid and solid phases. Liquid-phase TiN precipitation occurs at high temperatures and produces large particles, which not only harms the steel's properties but also reduces the effective titanium content, thus diminishing the strengthening effect of high-Ti steel. This application controls TiN precipitation behavior by adjusting the calcium treatment process, the nitrogen and calcium content in the molten steel, and the continuous casting mold water flow control process. A lower N content reduces the formation of coarse liquid-phase TiN, while the secondary calcium treatment effectively spheroidizes inclusions in the steel, promoting the flotation of large inclusions. The dispersed fine composite inclusions in the steel act as nucleation sites for fine TiN, forming fine, dispersed precipitates under the influence of the mold water flow during continuous casting. This achieves the goal of inhibiting the formation of large-sized TiN and increasing the density of fine TiN. On the one hand, reducing large TiN particles in the steel ensures its strength and ductility; on the other hand, the precipitation of fine TiN particles also helps improve weld quality. A single calcium treatment after LF or RH is unlikely to fully achieve this effect.

[0043] In the continuous casting process, protective casting is used to prevent molten steel from coming into contact with air during casting, thereby avoiding secondary oxidation and nitrogen absorption, improving the purity of the molten steel, and reducing the formation of large titanium nitride particles. The use of appropriate tundish superheat and casting speed, combined with electromagnetic stirring in the secondary cooling zone, aims to reduce core segregation and central cracks, and prevent the formation of banded manganese sulfide and titanium nitride inclusions at segregation and crack sites, thereby mitigating the impact of slab defects on the plasticity and strength of the steel.

[0044] In the heating process, this application sets the heating rate of the heating stage to 5.0~8.0℃ / min; the secondary heating stage temperature to 1280~1310℃; the soaking stage temperature to 1270~1300℃; and the soaking stage time to 50~70min. The soaking temperature is lower than the secondary heating temperature, and the soaking time is guaranteed. This effectively improves the temperature uniformity of the slab, thereby ensuring that microalloying elements at different positions in the thickness and length directions of the slab are fully dissolved, providing a temperature guarantee for subsequent precipitation strengthening. Combined with the cooling process, a more uniform precipitation strengthening effect is obtained. At the same time, it can also reduce uneven deformation during hot rolling, thereby improving the hot-rolled plate shape accuracy and avoiding uneven stress distribution in the cost steel plate.

[0045] In the rolling process, the rolling process of this application can effectively reduce the rolling load and reduce the cooling time of the strip in the air by setting the intermediate billet thickness at the roughing mill exit, the entry temperature of the finishing mill, and the exit temperature, thereby improving the temperature uniformity in the width and length directions of the plate and providing a basis for uniform phase transformation in the width and length directions of the plate during the subsequent cooling process; at the same time, the plate shape target value at the finishing mill exit is set based on the thickness t of the finished plate shape, which can compensate for the edge waviness trend caused by the inconsistent cooling of the edge and the middle during the laminar cooling process.

[0046] In the cooling process, this application employs a three-stage cooling mode. By coordinating the cooling rate, temperature, and cooling method, the phase transformation and microalloying precipitation processes of the steel are precisely controlled. The first stage involves rapid water cooling, preserving the austenitic hardened state of the rolled piece, cooling to 650-680℃, then stopping the rapid cooling and followed by air cooling, allowing for the rapid and uniform precipitation of nano-carbide. The third stage continues rapid cooling to 590-620℃, on the one hand inhibiting the growth and coarsening of precipitates, and on the other hand, obtaining a fine quasi-polygonal ferrite matrix through phase transformation strengthening. By controlling the water-to-water ratio in the water-cooling section, uniform cooling of the upper and lower surfaces of the steel is achieved, reducing cooling and phase transformation stress.

[0047] During the slow cooling stage, this application places the coiled strip in an insulation pit for slow cooling for 48-72 hours after coiling. On the one hand, this can slowly release the internal stress generated by cooling and phase transformation. On the other hand, slow cooling in the insulation pit can promote the full precipitation of microalloyed carbonitrides at the head and tail of the strip, thereby improving the performance of the head and tail of the strip.

[0048] During the leveling stage, in view of the characteristics of high strength and high internal stress of high-strength steel, this application adopts concave roller shape in the leveling process to avoid uneven deformation on the left and right sides caused by strip deviation; increases the leveling force of the leveling unit to improve the shape of high-strength strip; and reduces the difference in roller diameter of the leveling rollers to avoid uneven deformation of the upper and lower surfaces caused by the difference in speed of the upper and lower rollers. Attached Figure Description

[0049] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0050] Figure 1 This is a 500X metallographic image of the hot-rolled strip steel obtained in Example 1 of this application;

[0051] Figure 2 This is a 5000X scan topography image of hot-rolled strip steel obtained in Example 1 of this application;

[0052] Figure 3 This is a 20000X scan topography image of hot-rolled strip steel obtained in Example 1 of this application. Detailed Implementation

[0053] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of the embodiments will be described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application. In the description of this application, the experimental methods, unless otherwise specified, are conventional methods; the reagents and materials, unless otherwise specified, are all commercially available.

[0054] The following disclosure provides many different implementations or examples for carrying out this application. To simplify the disclosure of this application, specific examples of components and arrangements are described below. Of course, these are merely examples and are not intended to limit the scope of this application. Those skilled in the art will recognize the application of other processes and / or the use of other materials.

[0055] Example 1

[0056] This embodiment provides a method for producing hot-rolled strip steel for high-frequency welded pipes with a finished plate thickness t of 2.5 mm and a yield strength of 700 MPa, comprising the following steps:

[0057] (1) Steelmaking

[0058] Molten steel from the converter is refined through LF refining, a first calcium treatment, RH treatment, and a second calcium treatment after vacuum breaking to obtain refined molten steel. In the first calcium treatment, 140m / heat of silicon-calcium wire is fed in and the soft stirring time is 6 minutes. In the second calcium treatment, 110m / heat of silicon-calcium wire is fed in and the soft stirring time is 11 minutes. The RH treatment is performed under a vacuum of 1 mBar for 13 minutes. After vacuum treatment, the vacuum is broken, and then 160m of seamless silicon-calcium wire is fed in for calcium treatment. Finally, refined molten steel is obtained. The mass percentages of the various chemical components in the refined molten steel are shown in Table 1 below.

[0059] (2) Continuous casting

[0060] The refined steel was cast under full protective conditions using a tundish covering agent, a submerged entry nozzle, and argon sealing to obtain a slab; wherein the submerged entry nozzle depth was 160 mm, the argon sealing back pressure was 0.10 Bar, and the tundish temperature was T. L +20℃, actual value is 1536℃, pulling speed is 1.20m / min; water flow rate of the crystallizer is 3700L / min on the wide side and 550L / min on the narrow side; electromagnetic stirring in the secondary cooling zone is used, stirring current is 300A, frequency is 6Hz.

[0061] (3) Heating

[0062] The slab is placed into a heating furnace for heating to obtain an intermediate billet. The heating furnace includes a preheating stage, a first heating stage, a second heating stage, and a homogenization stage. The furnace entry temperature, i.e., the initial temperature of the preheating stage, is 550°C. The heating rate from the preheating stage to the second heating stage is 6.5°C / min. The temperature of the second heating stage is 1295°C. The temperature of the homogenization stage is 1285°C. The heating time of the homogenization stage is 60min.

[0063] (4) Rolling

[0064] The intermediate billet is subjected to rough rolling and finish rolling to obtain the finished sheet shape. The rough rolling process uses a 1+5 method. The thickness of the intermediate billet at the rough rolling exit is 37 mm. The inlet temperature of the finish rolling is 1100℃, and the outlet temperature is 915℃. The target value of the finished sheet shape at the finish rolling exit is set based on the thickness t of the finished sheet shape; in this embodiment, the thickness t of the finished sheet shape is 2.5 mm, and the target value of the finished sheet shape at the finish rolling exit is 4 IU.

[0065] (5) Cooling and winding

[0066] A three-stage cooling process—water cooling-air cooling-water cooling—is used to cool the finished strip. In this process, the first stage involves water cooling at a rate of 40-50°C / s to 670°C; the second stage involves air cooling for 4 seconds; and the third stage involves water cooling at a rate of 43°C / s to 610°C before coiling to obtain coiled strip. The coiled strip is then placed in an insulated pit for slow cooling for 60 hours.

[0067] The water cooling system used in the first stage and the third stage employs a water-to-water ratio of 0.8.

[0068] (6) Leveling

[0069] After the slowly cooled coiled strip is leveled, a hot-rolled strip for high-frequency welded pipes with a thickness of 2.5 mm and a yield strength of 700 MPa is obtained. The leveling process uses a sinusoidal curve roll with a negative crown of 0.015 mm, a rolling force of 330 tons, a bending roll force of 45 tons, and a diameter difference between the upper and lower work rolls of no more than 0.1 mm.

[0070] Example 2

[0071] This embodiment provides a method for producing hot-rolled strip steel for high-frequency welded pipes with a finished plate thickness t of 5.0 mm and a yield strength of 700 MPa, comprising the following steps:

[0072] (1) Steelmaking

[0073] Molten steel from the converter is subjected to LF refining, a first calcium treatment, RH treatment, and a second calcium treatment after vacuum breaking to obtain refined molten steel. The first calcium treatment involves feeding 130m / heat of a calcium silicate wire and soft stirring for 5 minutes. The second calcium treatment involves feeding 100m / heat of a calcium silicate wire and soft stirring for 10 minutes. The RH treatment is performed under a vacuum of 2 mBar for 10 minutes. The final refined molten steel is then obtained. The mass percentages of the various chemical components in the refined molten steel are shown in Table 1 below.

[0074] (2) Continuous casting

[0075] The refined steel was cast under full protective conditions using a tundish covering agent, a submerged entry nozzle, and argon sealing to obtain a slab; wherein the submerged entry nozzle depth was 150 mm, the argon sealing back pressure was 0.05 Bar, and the tundish temperature was T. L +25℃, actual value is 1545℃, pulling speed is 1.21m / min; crystallizer water flow rate is 3600L / min on the wide side and 500L / min on the narrow side; electromagnetic stirring in the secondary cooling zone is used, stirring current is 200A, frequency is 5Hz.

[0076] (3) Heating

[0077] The slab is placed into a heating furnace for heating to obtain an intermediate billet. The heating furnace includes a preheating stage, a first heating stage, a second heating stage, and a homogenization stage. The furnace entry temperature, i.e., the initial temperature of the preheating stage, is 400℃. The heating rate from the preheating stage to the second heating stage is 8.0℃ / min. The temperature of the second heating stage is 1280℃. The temperature of the homogenization stage is 1270℃. The heating time of the homogenization stage is 50min.

[0078] (4) Rolling

[0079] The intermediate billet is subjected to rough rolling and finish rolling to obtain the finished sheet shape. The rough rolling process uses 3+3 passes. The thickness of the intermediate billet at the rough rolling exit is 35 mm. The inlet temperature of the finish rolling is 1080℃, and the outlet temperature is 900℃. The target value of the sheet shape at the finish rolling exit is set based on the thickness t of the finished sheet shape; in this embodiment, the thickness t of the finished sheet shape is 5.0 mm, and the target value of the sheet shape at the finish rolling exit is 2 IU.

[0080] (5) Cooling and winding

[0081] A three-stage cooling process—water cooling-air cooling-water cooling—is used to cool the finished strip. In this process, the first stage involves water cooling at a rate of 30°C / s to 680°C; the second stage involves air cooling for 3 seconds; and the third stage involves water cooling at a rate of 30°C / s to 620°C before coiling to obtain coiled strip. The coiled strip is then placed in an insulated pit for slow cooling for 48 hours.

[0082] The water cooling system in the first stage and the water cooling system in the third stage uses a water-to-water ratio of 0.9.

[0083] (6) Leveling

[0084] After the slowly cooled coiled strip is leveled, a hot-rolled strip for high-frequency welded pipes with a thickness of 5.0 mm and a yield strength of 700 MPa is obtained. The leveling process employs a sinusoidal curve roll profile with a negative crown of 0.015 mm, a rolling force of 300-320 tons, a bending force of 30 tons, and a diameter difference between the upper and lower work rolls of no more than 0.5 mm.

[0085] Example 3

[0086] This embodiment provides a method for producing hot-rolled strip steel for high-frequency welded pipes with a finished plate thickness t of 2.0 mm and a yield strength of 700 MPa, comprising the following steps:

[0087] (1) Steelmaking

[0088] Molten steel from the converter is subjected to LF refining, a first calcium treatment, RH treatment, and a second calcium treatment after vacuum breaking to obtain refined molten steel. The first calcium treatment involves feeding 150m / heat of a calcium silicate wire and soft stirring for 7 minutes. The second calcium treatment involves feeding 120m / heat of a calcium silicate wire and soft stirring for 12 minutes. The RH treatment is performed under 0 mBar vacuum for 15 minutes. The final refined molten steel is then obtained. The mass percentages of the various chemical components in the refined molten steel are shown in Table 1 below.

[0089] (2) Continuous casting

[0090] The refined steel was cast under full protective conditions using a tundish covering agent, a submerged entry nozzle, and argon sealing to obtain a slab; wherein the submerged entry nozzle depth was 180 mm, the argon sealing back pressure was 0.15 Bar, and the tundish temperature was T. L +15℃, actual value is 1530℃, pulling speed is 1.19m / min; crystallizer water flow rate is 3800L / min on the wide side and 600L / min on the narrow side; electromagnetic stirring in the secondary cooling zone is used, stirring current is 400A, frequency is 7Hz.

[0091] (3) Heating

[0092] The slab is placed into a heating furnace for heating to obtain an intermediate billet. The heating furnace includes a preheating stage, a first heating stage, a second heating stage, and a homogenization stage. The furnace entry temperature, i.e., the initial temperature of the preheating stage, is 700℃. The heating rate from the preheating stage to the second heating stage is 5.0℃ / min. The temperature of the second heating stage is 1310℃. The temperature of the homogenization stage is 1300℃. The heating time of the homogenization stage is 70min.

[0093] (4) Rolling

[0094] The intermediate billet is subjected to rough rolling and finish rolling to obtain the finished sheet shape. The rough rolling process uses 1+5 passes. The thickness of the intermediate billet at the rough rolling exit is 38 mm. The inlet temperature of the finish rolling is 1120℃, and the outlet temperature is 930℃. The target value of the sheet shape at the finish rolling exit is set based on the thickness t of the finished sheet shape; in this embodiment, the thickness t of the finished sheet shape is 2.0 mm, and the target value of the sheet shape at the finish rolling exit is 5 IU.

[0095] (5) Cooling and winding

[0096] A three-stage cooling process—water cooling-air cooling-water cooling—is used to cool the finished strip. In this process, the first stage involves water cooling at a rate of 60°C / s to 680°C, the second stage involves air cooling for 5 seconds, and the third stage involves water cooling at a rate of 60°C / s to 590°C before coiling to obtain coiled strip. The coiled strip is then placed in an insulated pit for slow cooling for 108 hours.

[0097] The water cooling system used in the first stage and the third stage employs a water-to-water ratio of 0.7.

[0098] (6) Leveling

[0099] After the slowly cooled coiled strip is leveled, a hot-rolled strip for high-frequency welded pipes with a thickness of 3.0 mm and a yield strength of 700 MPa is obtained. The leveling process uses a sinusoidal curve roll with a negative crown of 0.015 mm, a rolling force of 350 tons, a bending roll force of 60 tons, and a diameter difference between the upper and lower work rolls of no more than 0 mm.

[0100] Comparative Example 1

[0101] This embodiment provides a method for producing hot-rolled strip steel with a finished plate thickness t of 2.5 mm, including the following steps:

[0102] (1) Steelmaking

[0103] Molten steel from the converter is refined through LF refining, calcium treatment, and RH treatment to obtain refined molten steel. The calcium treatment involves feeding 120m / furnace into a calcium silicate wire and soft stirring for 4 minutes. The RH treatment process uses a vacuum of 3 mBar and lasts for 9 minutes. The final refined molten steel is then obtained. The mass percentages of each chemical component in the refined molten steel are shown in Table 1 below.

[0104] (2) Continuous casting

[0105] The refined steel was cast under full protective conditions using a tundish covering agent, a submerged entry nozzle, and argon sealing to obtain a slab; wherein the submerged entry nozzle depth was 160 mm, the argon sealing back pressure was 0.10 Bar, and the tundish temperature was T. L +30℃, actual value is 1546℃, pulling speed is 1.25m / min; water flow rate of the crystallizer is 3500L / min on the wide side and 400L / min on the narrow side; electromagnetic stirring in the secondary cooling zone is not used.

[0106] (3) Heating

[0107] The slab is placed into a heating furnace for heating to obtain an intermediate billet. The heating furnace includes a preheating stage, a first heating stage, a second heating stage, and a homogenization stage. The furnace entry temperature, i.e., the initial temperature of the preheating stage, is 100℃. The heating rate from the preheating stage to the second heating stage is 10.0℃ / min. The temperature of the second heating stage is 1270℃. The temperature of the homogenization stage is 1250℃. The heating time of the homogenization stage is 40min.

[0108] (4) Rolling

[0109] The intermediate billet is subjected to rough rolling and finish rolling to obtain the finished sheet shape. The rough rolling process consists of 1+5 passes. The thickness of the intermediate billet at the rough rolling exit is 40 mm. The inlet temperature of the finish rolling is 1060℃, and the outlet temperature is 880℃. The target sheet shape value at the finish rolling exit is 1 IU.

[0110] (5) Cooling and winding

[0111] The finished strip is cooled to 570-590°C using a conventional front-end cooling method before being coiled to obtain coiled strip steel. During the cooling process, the water-to-water ratio is 1.0. After coiling, the coiled strip steel is stacked in air to cool.

[0112] (6) Leveling

[0113] The coiled strip is leveled to obtain a hot-rolled strip with a thickness of 2.5 mm. The leveling process uses flat rolls, with a rolling force of 250 tons, a bending roll force of 200 tons, and a diameter difference of 0.6 mm between the upper and lower work rolls.

[0114] Comparative Example 2

[0115] A method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700 MPa. The only difference between the method of this comparative example and the method of Example 1 is that the calcium treatment process is not used in the steelmaking process of this comparative example in LF refining and RH treatment. As a result, the content of Ca and S in the chemical composition of this comparative example and Example 1 are slightly different from that of Example 1. See Table 1 for details.

[0116] Comparative Example 3

[0117] A method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700 MPa. The difference between the method of this comparative example and the method of Example 1 is that in the steelmaking process of this comparative example, calcium treatment is performed only after the air-breaking process in the RH treatment. The calcium treatment process is to feed 100-120 m of silicon-calcium wire and stir for 11 min. That is, this comparative example only performs calcium treatment once, so the content of Ca and S in the chemical composition of this comparative example and Example 1 is also slightly different from that of Example 1. See Table 1 for details.

[0118] Comparative Example 4

[0119] A method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700MPa is basically the same as that in Example 1 in terms of chemical composition mass percentage, process steps and parameters. The only difference is that the target value of the finished roll exit shape described in this comparative example is not set based on the thickness t of the finished strip shape, and the target value of the strip shape is 0IU.

[0120] Comparative Example 5

[0121] A method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700MPa is basically the same as that in Example 1 in terms of chemical composition by mass percentage, process steps and parameters. The only difference is that the fine rolling inlet temperature is 1050℃ and the outlet temperature is 860℃ in this comparative example.

[0122] Comparative Example 6

[0123] A method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700MPa is basically the same as that in Example 1 in terms of chemical composition mass percentage, process steps and parameters. The only difference is that the cooling process described in this comparative example adopts conventional front-end cooling, and the water-to-water ratio during cooling is 1.0.

[0124] Comparative Example 7

[0125] A method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700MPa is basically the same as that in Example 1 in terms of chemical composition mass percentage, process steps and parameters. The only difference is that in the leveling process described in this comparative example, the difference in diameter between the upper and lower working rollers of the leveling machine is 1.0mm.

[0126] To better compare the refined steel composition of each embodiment and comparative example, the chemical composition of the refined steel of the above embodiments and comparative examples is summarized as follows:

[0127] Table 1. Comparison of the mass percentage of chemical composition of refined steel in each embodiment and comparative example.

[0128]

[0129] Performance testing test 1

[0130] To better compare the properties of the hot-rolled strip steel obtained in the above embodiments and comparative examples, samples were taken for tensile testing of the hot-rolled strip steel obtained in each embodiment and comparative example using the GB / T2975-2018 method. The tensile properties of each sample were then tested according to the GB / T228 method, and the test results are shown in Table 2 below. The hot-rolled strip steel obtained in each embodiment was welded into steel pipes according to the GB / T 40316-2021 standard, and the steel pipes were subjected to tensile, flattening, and flaring tests according to the GB / T 40316-2021 standard. The test results are shown in Table 2 below.

[0131] Table 2. Performance test results of hot-rolled strip steel and steel pipes welded from hot-rolled strip steel obtained in each embodiment and comparative example.

[0132]

[0133] As can be clearly seen from the test data of Examples 1-3 shown in Table 2, the hot-rolled strip steel prepared using the examples of this application has a yield strength of 740~780MPa, a tensile strength of 800~850MPa, an elongation of 21~25%, and a strength-ductility product of 16~21.3GPa·%. Furthermore, it exhibits good overall strength stability. The welded steel pipes also achieve an elongation of over 20%, with good flattening and flaring properties without cracking, good weldability, and right angles all less than 2mm / 10m.

[0134] The performance test data from Example 1 and Comparative Examples 2 and 3 clearly show that, compared with no calcium treatment or only one calcium treatment, the use of secondary calcium treatment in the steel refining stage can effectively ensure that the strip has excellent tensile strength while improving its plasticity. In addition, the strip has excellent weldability, and the welded steel pipe has high elongation and good quality.

[0135] The test data from Comparative Examples 4-7 show that the setting of the target value for the strip shape at the finishing mill exit, the setting of the finishing mill inlet temperature, the cooling method in the cooling process, and the difference in diameter between the upper and lower work rolls in the leveling process all have a significant impact on the weldability of the strip. Furthermore, the test data from Comparative Example 5 shows that the finishing mill inlet temperature also has a very significant impact on the strength-ductility product of the strip itself.

[0136] Performance testing test 2

[0137] The microstructure and metallographic morphology of the hot-rolled strip obtained in Example 1 were scanned and examined. The results are as follows: Figures 1 to 3 As shown. Among them. Figure 1 This is a 500X metallographic image of hot-rolled strip steel. Figure 2 This is a 5000X scan topography image of hot-rolled strip steel. Figure 3 This is a 20000X scan topographic image of hot-rolled strip steel. (Source: [Original Source Name]) Figure 1 and Figure 2 It can be seen that the matrix structure of the steel prepared in this application consists of quasi-polygonal ferrite + granular Mao islands, with fine and uniformly distributed grains. Figure 3 It can be seen that a large number of dispersed microalloyed carbonitrides are dispersedly precipitated inside the ferrite grains.

[0138] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0139] The above provides a detailed description of a production method for hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700MPa, as provided in the embodiments of this application. Specific examples have been used to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the technical solutions and core ideas of this application. Those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700 MPa, characterized in that, Includes the following steps: (1) Steelmaking The molten steel from the converter is subjected to LF refining, primary calcium treatment, RH treatment, and secondary calcium treatment to obtain refined molten steel; the refined molten steel has a nitrogen content ≤0.0050%, a sulfur content ≤0.0030%, a calcium content of 0.0015~0.0040%, and a calcium-to-sulfur mass ratio of 0.8~4.0; (2) Continuous casting The refined molten steel is continuously cast to obtain slabs. During the continuous casting process, the tundish temperature is T. L +10~25℃, pulling speed is 1.20±0.1m / min; crystallizer water flow rate is 3600~3800L / min on the wide side and 500~600L / min on the narrow side; electromagnetic stirring in the secondary cooling zone is used, with stirring current of 200~400A and frequency of 5~7Hz; (3) Heating The slab is heated to obtain an intermediate billet. The heating includes a preheating stage, a first heating stage, a second heating stage, and a homogenization stage. The initial temperature of the preheating stage is 400~700℃, the heating rate from the preheating stage to the second heating stage is 5.0~8.0℃ / min, the temperature of the second heating stage is 1280~1310℃, the temperature of the homogenization stage is 1270~1300℃, and the heating time of the homogenization stage is 50~700min. (4) Rolling The intermediate billet is subjected to rough rolling and finish rolling to obtain the finished plate shape. The thickness of the intermediate billet at the rough rolling exit is 35~38mm. The target value of the plate shape at the finish rolling exit is set based on the thickness t of the finished plate shape. When the thickness of the finished plate shape is 1.8≤t<2.5mm, the target value of the plate shape is set to 4~6IU; when the thickness of the finished plate shape is 2.5≤t<4.0mm, the target value of the plate shape is set to 3~5IU; when the thickness of the finished plate shape is 4.0≤t≤6.0mm, the target value of the plate shape is set to 1~3IU. (5) Cooling and winding A three-stage cooling mode of water-air-water cooling is used to cool the finished sheet shape before coiling the strip. (6) Leveling The leveling process uses a sinusoidal curve roll with a negative crown of 0.015mm, a rolling force of 300~350 tons, and a bending force of 30~60 tons.

2. The method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700 MPa as described in claim 1, characterized in that, The refined molten steel comprises the following components by mass percentage: C: 0.05~0.10%, Si: 0.10~0.30%, Mn: 1.50~1.70%, Nb: 0.020~0.050%, Ti: 0.10~0.13%, Al: 0.020~0.040%, Ca: 0.0015~0.0040%, P≤0.018%, S≤0.0030%, N:≤0.0040%, with the remainder consisting of iron and unavoidable impurities.

3. The method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700 MPa as described in claim 1, characterized in that: The vacuum level during RH treatment is 0~2mBar, and the treatment time is 10~15min.

4. The method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700 MPa as described in claim 1, characterized in that, The primary calcium treatment involves feeding 130-150m of silicon-calcium wire per furnace, with a soft stirring time of 5-7 minutes. The secondary calcium treatment involves feeding 100-120m of silicon-calcium wire per furnace, with a soft stirring time of 10-12 minutes.

5. The method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700 MPa as described in claim 1, characterized in that, In the continuous casting process, the refined molten steel is fully protected by a tundish covering agent, a submerged entry nozzle, and an argon seal. The depth of the submerged entry nozzle is 150~180mm, and the argon seal back pressure is 0.05~0.15Bar.

6. The method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700 MPa as described in claim 1, characterized in that: The roughing process uses 1+5 or 3+3 passes.

7. The method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700 MPa as described in claim 1, characterized in that: The inlet temperature of the finishing mill is 1080~1120℃, and the outlet temperature is 900~930℃.

8. The method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700 MPa as described in claim 1, characterized in that: In the three-stage cooling mode of water-air-water cooling, the first stage of water cooling is to cool to 650-680℃ at a rate of 30-60℃ / s, the second stage of air cooling is to cool for 3-5 seconds, and the third stage of water cooling is to cool to 590-620℃ at a rate of 30-60℃ / s for winding. The water-to-water ratio used in the first stage of water cooling and the third stage of water cooling is 0.

8.

9. The method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700 MPa as described in claim 1, characterized in that: After winding, the wound strip is placed in an insulated pit for slow cooling, and the slow cooling time is 48-72 hours or more.

10. The method for producing hot-rolled strip steel for high-frequency welded pipes with a yield strength of 700 MPa as described in claim 1, characterized in that: During the leveling process, the difference in roller diameter between the upper and lower working rollers of the leveling machine is 0~0.5mm.

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